phd part 1

THE MINERALOGICAL CHARACTERISATION AND INTERPRETATION OF A PRECIOUS METAL-BEARING FOSSIL GOSSAN, LAS

CRUCES, SPAIN

Volume 1

Text and References

A thesis submitted to the University of Cardiff in Candidature for the degree of Doctor of Philosophy

Christopher Blake

Department of Earth, Ocean and Planetary SciencesCardiff University

2008

Declaration and Statements

DECLARATION

This work has not previously been accepted in substance for any degree and is

not concurrently submitted in candidature for any degree.

Signed …………………………………………………………. (candidate)

Date …………………………

STATEMENT 1

This thesis is being submitted in partial fulfilment of the requirements for the

degree of PhD


Date …………………………

STATEMENT 2

This thesis is the result of my own independent work/investigation, except where

otherwise stated.

Other sources are acknowledged by explicit references.


Date …………………………

STATEMENT 3

I hereby give consent for my thesis, if accepted, to be available for photocopying

and for inter-library loan, and for the title and summary to be made available to

outside organisations.


Date …………………………

ii

Abstract

Abstract

The Las Cruces VMS deposit lies on the southern margins of the Iberian Pyrite Belt, Spain. The primary base metal massive sulphide is overlain by a supergene enriched zone and precious metal gossan that remains well preserved under approximately 150 metres of Tertiary marl.

The mineralogy, mineral textures and associations of five boreholes containing precious metal gossan mineralisation were characterised using a combination of optical microscopy, SEM and XRD techniques.

The mineralogy and geochemical profile of the gossan suggests that it was formed under near-surface weathering conditions, resulting in the development of the supergene zone and a mature gossan profile characterised by elevated levels of Au and Ag. The Au and Ag probably remobilised as chloride complexes under strongly acid, oxidising conditions, precipitating as high fineness Au and discrete Ag-bearing phases lower in the gossan profile.

The original Fe-oxyhydroxide dominated gossan mineral assemblage has subsequently been extensively replaced by later stages of siderite, greigite, galena and high fineness Au mineralisation that reflect marked changes in the depositional environment relative to the original gossan mineral assemblage. Fluctuating oxidising and reducing conditions, coupled with biogenic processes within the Niebla Posadas aquifer, situated directly above the present day Las Cruces gossan, provide a suitable mechanism for the formation of the extensive siderite and greigite mineralisation as well as precious metal remobilisation as a thiosulphate complex under near-neutral to alkaline conditions. Strongly negative δ13C stable isotope values for the siderite are consistent with biogenic processes involving Fe3+ and/or sulphate reducing bacteria as well as a significant influence from the oxidation of methane.

iii

Acknowledgement

Acknowledgement

I would never have started my thesis if it were not for the encouragement of my

mentor, Dr. Ivan Reynolds. Many thanks for your time and support during the

past 17 years of my career as a mineralogist with Rio Tinto.

A great deal of support has also been received from Dr. Hazel Prichard, helping

me through the maze of preparing a thesis and posting me the occasional

photocopy to save me the long trek into Cardiff. Many thanks for spending

endless hours reading through my thesis.

Finally, to my sister and parents. Thanks for providing me the love and support

of a great family.

About the Author

The author graduated with a B.Sc. honours degree in Geology and Geography

from the Cheltenham and Gloucester College of Higher Education in July 1991.

Following a vacation job with Rio Tinto's Anamet Services in the summer of 1990,

the author returned to Anamet Services as a technician/trainee mineralogist

under the guidance of Dr. Ivan Reynolds in November 1991. Anamet Services

closed in late 1997 and the mineralogy department was relocated to Clevedon

where, after a few years, the authors PhD project was approved and supported

by Rio Tinto. The author continued to work full time with Rio Tinto as Senior

Mineralogist in the Clevedon laboratory, working on his PhD during evenings,

weekends and vacation time. The Clevedon laboratory closed in December of

2008. The author now works as a consultant mineralogist.

v

Dedication

Dedication

To my mum and dad

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List of Contents

List of Contents

VOLUME 1: TEXT AND REFERENCES

Declaration and Statements iiAbstract iiiAcknowledgement vAbout the Author vDedication viList of Contents viiList of Figures xvList of Tables xix

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Aims of Study 1

1.3 Thesis Outline 3

2 GEOLOGY 4

2.1 Regional Geological Setting - The Iberian Peninsula 4

2.2 The Iberian Pyrite Belt 7

2.3 The Guadalquivir Basin 11

2.4 Las Cruces - Exploration History 13

2.5 Las Cruces - Geology and Mineralogy 15

2.5.1 Introduction 152.5.2 Gossan 172.5.3 Secondary Massive Sulphide 182.5.4 Primary Massive Sulphide 19

2.6 Las Cruces - Evolutionary History 20

2.7 Sample Suite 30

3 GOSSANS 34

3.1 Introduction 34

3.2 The Gossan Forming Process 36

3.3 Influences On Gossan Formation 41

3.3.1 Introduction 413.3.2 Effect of Primary Geology on pH 423.3.3 Effects of Climate on Gossan Formation 433.3.4 Effects of Geomorphology on Gossan Formation 45

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List of Contents

3.4 Element Mobility and Gossan Profiles 46

3.4.1 Introduction 463.4.2 Fe 473.4.3 Au and Ag Element Mobility 503.4.4 Au and Ag Mineralogy and Geochemical Profiles 553.4.5 Cu 593.4.6 Pb 613.4.7 As and Sb 643.4.8 Si, Sn and Ti 653.4.9 Other metals 69

3.5 Ancient Seafloor Weathering 71

3.5.1 Introduction 713.5.2 Ochres 713.5.3 Umbers 73

3.6 Modern Seafloor Weathering 76

3.6.1 Introduction 763.6.2 Modern Seafloor Fe-Oxide and Oxyhydroxide Deposits of Secondary Origin 763.6.3 Modern Seafloor Fe-Mn-Si Oxide and Oxyhydroxide Deposits of Primary Origin 79

3.7 Comparing Modern and Ancient Deposits 81

4 METHODS OF INVESTIGATION 83

4.1 Introduction 83

4.2 Sample Preparation 83

4.3 Microscopy 85

4.3.1 Transmitted Light 854.3.2 Reflected Light 85

4.4 Scanning Electron Microscopy 86

4.4.1 Qualitative Methods 864.4.2 SEM Image Collection and Enhancement 864.4.3 Image Analysis Techniques 894.4.4 Quantitative Methods 92

4.5 X-Ray Powder Diffraction 93

4.6 Fluid Inclusion Analyses 94

4.7 Isotope Analyses 95

4.8 Geochemical Whole Rock Analyses 96

5 BOREHOLE CR194 – SAMPLE DESCRIPTIONS 97

5.1 Introduction 97

5.2 Borehole CR194 - Chemistry 98

5.2.1 Introduction 985.2.2 Geochemical Profile 100

5.3 Borehole CR194 – Gossan 103

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List of Contents

5.3.1 Introduction 1035.3.2 Quartz 1035.3.3 Siderite 1055.3.4 Limonite 1075.3.5 Fe-Clay 1085.3.6 Accessory Transparent Gangue Minerals 1095.3.7 Fe-Sulphides 1095.3.8 Galena and Pb-Bearing Sulphides 1105.3.9 Secondary Pb-bearing Phases 1115.3.10 Amalgam and Hg-Bearing Phases 1125.3.11 Precious Metal Mineralisation 1135.3.12 Accessory Minerals 114

5.4 Borehole CR194 – Gossan Contact with Massive Sulphide 115

5.4.1 Introduction 1155.4.2 163.75 to 164.60m Sample Interval - Upper Portion 1155.4.3 163.75 to 164.60m Sample Interval - Middle Portion 1165.4.4 163.75 to 164.60m Sample Interval - Lower Portion 117

5.5 Borehole CR194 – Massive Sulphide Contact with Gossan 121

5.5.1 Introduction 1215.5.2 Clay-Rich Layer 1225.5.3 Galena-Rich Layer 1225.5.4 Leached Pyrite-Rich Layer 1285.5.5 Lower Core 129

5.6 Borehole CR194 – Massive Sulphide 130

5.6.1 Introduction 1305.6.2 Massive Sulphide 1305.6.3 Massive Sulphide/Shale 132

5.7 Borehole CR194 – Shale 134

5.7.1 Introduction 1345.7.2 Mineralogy 134

5.8 Borehole CR194 – Summary Diagram 135


6.1 Introduction 136



6.3 Borehole CR149 - Tertiary Sand 143

6.3.1 Introduction 1436.3.2 General Mineralogy 143

6.4 Borehole CR149 - Gossan 145

6.4.1 Introduction 1456.4.2 Quartz 1466.4.3 Siderite 1466.4.4 Limonite 147

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List of Contents

6.4.5 Accessory Transparent Gangue Minerals 1486.4.6 Fe-Sulphides 1486.4.7 Galena and Pb-Bearing Sulphides 1496.4.8 Accessory Minerals 1496.4.9 Precious Metal Mineralisation 150

6.5 Borehole CR149 - Gossan/Massive Sulphide Contact 151

6.5.1 Introduction 1516.5.2 Transparent Gangue 1516.5.3 Pyrite and other Fe-Sulphides 1526.5.4 Galena 1526.5.5 Accessory Minerals 1526.5.6 Precious Metal Mineralisation 152

6.6 Borehole CR149 - Massive Sulphide 154







7.3 Borehole CR038 - Quartz Replaced Tuffs 161

7.3.1 Introduction 1617.3.2 Transparent Gangue Mineralogy 1617.3.3 Ore Mineralogy 1637.3.4 Precious Metal Mineralisation 164

7.4 Borehole CR038 - Quartz Replaced Tuff/Partial Massive Sulphide Contact165

7.4.1 Introduction 1657.4.2 Transparent Gangue Mineralogy 1657.4.3 Ore Mineralogy 1657.4.4 Precious Metal Mineralisation 166

7.5 Borehole CR038 - Partial Massive Sulphide 167

7.5.1 Introduction 1677.5.2 Transparent Gangue Mineralogy 1677.5.3 Ore Mineralogy 167






8.3 Borehole CR191- Tertiary Polymict Conglomerate/ Gossan Contact 174

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List of Contents


8.4 Borehole CR191 - Upper Gossan 176

8.4.1 Introduction 1768.4.2 Gangue Mineralogy 1768.4.3 Ore Mineralogy 1778.4.4 Precious Metal Mineralisation 178

8.5 Borehole CR191 - Middle Gossan 179


8.6 Borehole CR191 - Lower Gossan 182


8.7 Borehole CR191- Partial Massive Sulphide 185






9.2.1 Geochemical Profile 1919.3 Borehole CR123- Tertiary Polymict Conglomerate 192

9.3.1 Introduction 1929.3.2 Gangue Mineralogy 1929.3.3 Ore Mineralogy 1939.3.4 Accessory Mineralogy 193

9.4 Borehole CR123 - Upper Siderite Gossan 194

9.4.1 Introduction 1949.4.2 General Mineralogy 1949.4.3 Precious Metal Mineralisation 195

9.5 Borehole CR123 - Middle Calcite Gossan 196


9.6 Borehole CR123 - Lower Siderite Gossan 198


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List of Contents

9.7 Borehole CR123- Gossan/Shale Conglomerate Contact 201


9.8 Borehole CR123 – Shale Conglomerate/Gossan Contact 203

9.8.1 Introduction 2039.8.2 Transparent Gangue 2039.8.3 Pyrite 2049.8.4 Cinnabar and Sulphosalt Minerals 2049.8.5 Precious Metal Mineralisation 204

9.9 Borehole CR123 – Partial Massive Sulphide/Shale 206



10 ENVIRONMENT AND FORMATIONAL MECHANISMS 208


10.2 Siderite Formational Environment 209

10.2.1 Introduction 20910.2.2 Oxic Zone (Berner, 1981) 21210.2.3 Sulphate Reduction Zone (Curtis et al., 1986; Irwin et al., 1977) 21310.2.4 'Methanic' or methanogenic zone (e.g. Berner 1981, Curtis et al., 1986) 21410.2.5 Methane Oxidation 21610.2.6 Fe3+ Reduction 21810.2.7 Nitrate Reduction 22110.2.8 Abiotic reactions - Thermally induced decarboxylation 222

10.3 Formation of Fe-sulphides 223

10.3.1 Introduction 22310.3.2 Formation Mechanisms 22310.3.3 The Role of Biological Processes 227

10.4 Mineral stability fields 229

10.4.1 Siderite 22910.4.2 Fe-sulphides 23010.4.3 Siderite/Fe-Sulphide Relationships 232

11 MINERALOGY: KEY FEATURES AND PARAGENESIS 234


11.2 Quartz 235

11.2.1 Relative Abundance 23511.2.2 Grain Size, Shape and Texture 23511.2.3 Fluid Inclusion and Isotope Analysis 240

11.3 Siderite 242

11.3.1 Relative Abundance 24211.3.2 Grain Size, Shape and Textures 242

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List of Contents

11.3.3 Associations 24811.3.4 Mineral Chemistry 25011.3.5 Isotope Analysis 25211.3.6 Fluid Inclusion Analysis 253

11.4 Galena 254

11.4.1 Relative Abundance 25411.4.2 Grain Size and Shape 25411.4.3 Associations 256

11.5 Fe-Sulphide Phases 259

11.5.1 Introduction 25911.5.2 Relative Abundance 26011.5.3 Reflected Light Characterisation 26111.5.4 Optical Properties and Occurrences of the Fe-sulphides 268

11.6 Au-Bearing Phases 271

11.6.1 Relative Abundance 27111.6.2 Grain Size and Shape 27211.6.3 Associations 27411.6.4 Chemistry 275

11.7 Gossan Paragenesis 276

11.7.1 Introduction 276

12 DISCUSSION AND CONCLUSIONS 281


12.2 Seafloor Gossan Formation 282

12.3 Sub-Aerial Gossan Formation 284

12.4 Gossan reworking 288

12.5 Marine incursion and seawater alteration 290

12.6 Deep burial by Tertiary sediments 291

12.7 Modern day gossan and aquifer 293

12.7.1 Introduction 29312.7.2 Siderite and Greigite 29312.7.3 Pb-bearing sulphides 29812.7.4 Precious metals 299

12.8 Conclusions 303

12.9 Future Investigations 306

REFERENCES 307

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List of Contents

VOLUME 2: APPENDICES

Appendix 1: List of Mineral Formulae A1Appendix 2: Sample List A3Appendix 3: Assay Data A6Appendix 4: XRD Data A15Appendix 5: SEM Analyses A28Appendix 6: Borehole CR194 Illustrations A49Appendix 7: Borehole CR149 Illustrations A108Appendix 8: Borehole CR038 Illustrations A133Appendix 9: Borehole CR191 Illustrations A152Appendix 10: Borehole CR123 Illustrations A175

xiv

List of Figures

List of Figures

Figure 2.1 - a) A geological map of Spain and Portugal showing the relative positions of the Central Iberian Zone (CIZ), the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the Pulo do Lobo (PL) and the Southern Portuguese Zone (SPZ). Figure 2.1b is a more detailed map of the area marked by a red rectangle on Figure 2.1a....................................................................................................................................6Figure 2.2 - The main lithostratigraphic units in the Iberian Pyrite Belt.............................9Figure 2.3 - A general map of the Betic Cordillera showing the position of the Guadalquivir Basin and Las Cruces.................................................................................11Figure 2.4 - Summary of the main lithostratigraphic units at Las Cruces (Knight, 2000). .........................................................................................................................................16Figure 2.5 – An idealised, simplified N-S cross-section through the Las Cruces orebody that is based on the interpretation of drill core data and block modelling information performed by Rio Tinto consultants..................................................................................17Figure 2.6 - Stage 1 - formation of the Las Cruces primary massive sulphide deposit during a primary hydrothermal event with waxing and waning thermal history................21Figure 2.7 - Stage 2 - Sub-marine oxidation and secondary Cu-sulphide enrichment during the waning stages of hydrothermal activity...........................................................23Figure 2.8 - Stage 3 - Sustained volcanism and sedimentation leading to the burial of the massive sulphide beneath ~1000m Palaeozoic Culm sediments..............................24Figure 2.9 - Stage 4 - Tilting of the primary massive sulphide occurred during the Hercynian, with uplift and erosion being followed by sub-aerial weathering and the development of the gossan, silica cap and supergene Cu-sulphides..............................25Figure 2.10 - Stage 5 - Reworking of the gossan and silica cap possibly prior to and following the onset of the marine incursion during the Miocene......................................26Figure 2.11 - Stage 6 - Burial and preservation of the Las Cruces deposit under up to 1000 metres Tertiary sediments.......................................................................................27Figure 2.12 – a) A map of the Las Cruces deposit illustrating the extent of the Au mineralisation (solid yellow line), supergene Cu-sulphide mineralisation (solid blue line) and the positions of the boreholes selected for examination during this investigation.....31Figure 3.1 – Diagram illustrating the zones of weathering in terms of Eh and pH according to Sato (1960)..................................................................................................38Figure 3.2 – Eh/pH diagram at 25oC and 1 atmosphere total pressure, illustrating the relationships between groundwater position and mineral stability ranges.......................39Figure 3.3 - Idealised zones in the weathering profile of a VHMS Zn-Pb-Cu deposit that has been weathered to produce a mature gossan profile................................................47Figure 3.4 – Eh/pH diagram illustrating the stability relations between iron oxides and iron sulphides in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10-6................................................................................................................................49Figure 3.5 – Eh/pH diagram illustrating the stability relations of some Au compounds in water at 25oC and 1 atmosphere total pressure at total dissolved chloride species of 100

and sulphur activity of 10-1................................................................................................51

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List of Figures

Figure 3.6 – Eh/pH diagram illustrating the stability relations of some Cu minerals in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10 -1, CO3

activity of 10-3....................................................................................................................60Figure 3.7 – Eh/pH diagram illustrating the stability relations of Pb compounds in water at 25oC and 1 atmosphere total pressure. Total dissolved sulphur of 10-1, pCO2 of 10-4..62Figure 3.8 - An illustration of the field relationships of a typical small umber hollow related to seafloor faulting, Troodos Massif, Cyprus........................................................75Figure 4.1 - A monochrome backscattered electron image illustrating a rather complex Fe-oxide-rich sample with fine intergrowths of galena.....................................................88Figure 4.2 - The monochrome backscattered electron image has been false coloured and permits the reader to readily distinguish the mineral species. Galena (white) occurs as fine skeletal aggregates. Limonite fragments (yellow-brown shades) exhibit a wide range in brightness that reflects degrees of hydration. Darker browns represent more hydrated Fe-oxides (e.g. goethite). The darkest brown/black portions of the image represent areas of high porosity.......................................................................................88Figure 4.3 - A typical backscattered electron image as captured by the image analysis system..............................................................................................................................90Figure 4.4 - The system recognises the range of grey shades of interest (red areas), depending on criteria set by the operator.........................................................................90Figure 4.5 - Each grain of interest is recognised by the electron microscope and selected for analysis/measurement..................................................................................91Figure 4.6 - An example of an EDX spectrum captured using a very rapid (typically 200msec) EDX analysis of each grain.............................................................................91Figure 5.1 - Illustrating the chemistry variation in borehole CR194.................................99Figure 5.61 - Diagram illustrating the key mineralogical features for the 'Gossan', 'Gossan/Massive Sulphide Contact', 'Massive Sulphide/Gossan Contact', 'Massive Sulphide', 'Massive Sulphide/Shale' and 'Shale'............................................................135Figure 6.1 - Illustrating the chemistry variations in borehole CR149............................139Figure 6.27 - Diagram illustrating the key mineralogical features for the 'Tertiary Sand', 'Gossan', 'Gossan /Massive Sulphide Contact' and 'Massive Sulphide'.........................155Figure 7.1 - Diagram illustrating chemistry variations in borehole CR038...................158Figure 7.21 - Diagram illustrating the key mineralogical features for the 'Quartz Replaced Tuffs', 'Quartz Replaced Tuff/Partial Massive Sulphide Contact' and 'Partial Massive Sulphide'.........................................................................................................................168Figure 8.1 - Diagram illustrating chemistry variations in borehole CR191....................171Figure 8.25 - Diagram illustrating the key mineralogical features for the 'Tertiary Conglomerate/Gossan Contact', ‘Upper Gossan', 'Middle Gossan', 'Lower Gossan’ and ‘Partial Massive Sulphide’..............................................................................................187Figure 9.1 - Diagram illustrating chemistry variations in borehole CR123....................190Figure 9.35 - Diagram illustrating the key mineralogical features for the ‘Tertiary Polymict Conglomerate’, ‘Upper Siderite Gossan', 'Middle Calcite Gossan', 'Lower Siderite Gossan’, ‘Gossan/Shale Conglomerate Contact’, ‘Shale Conglomerate/Gossan Contact’ and ‘Partial Massive Sulphide/Shale’.............................................................................207Figure 10.1 – A diagram illustrating the three distinct biogeochemical environments that mark the boundaries between regimes of aerobic and anaerobic metabolism..............211

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List of Figures

Figure 10.2 – Eh/pH diagram illustrating the stability of hematite, magnetite and siderite at 25oC and 1 atmosphere total pressure and pCO2 = 10-2 atmosphere with total activity of dissolved species = 10-6.............................................................................................229Figure 10.3 - Pe/pH diagrams illustrating the stability relations for iron sulphides in seawater at 25oC, 1 atmosphere total pressure. A) Iron activity 10−6, sulphur activity 10−2.551, C(IV) activity 10−3.001, troilite and pyrrhotite suppressed. B) Same as A with pyrite also suppressed. C) Same as B with marcasite suppressed. D) Same as C with greigite and mackinawite suppressed. E) Same as C but solution changed to world average river water with iron activity 10−6, sulphur activity 10−3.902, C(IV) activity 10−3.06. F) Same as C but iron activity 10−3 and C(IV) activity 10−2.5..................................................................231Figure 10.4 – Eh/pH diagram illustrating the stability relations between iron oxides, sulphides and carbonates in water at 25oC and 1 atmosphere total pressure at ΣCO2 of 100 and ΣS of 10-6...........................................................................................................232Figure 11.1 - Borehole CR123 - A colour transmitted light photomicrograph of fibrous quartz (white and grey shades) developed around the margins of pyrite crystals (black)........................................................................................................................................238Figure 11.2 - Borehole CR038 - A colour transmitted light photomicrograph illustrating more coarsely crystalline quartz fragments that are cemented by fine-grained, partially recrystallised chalcedony...............................................................................................239Figure 11.3 - Borehole CR149 - A colour, crossed polarised transmitted light photomicrograph from the gossan/massive sulphide contact illustrating a cavity that has been filled by fibrous chalcedony...................................................................................240Figure 11.4 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of angular siderite ‘fragments’ in a matrix of quartz and oxidised siderite..............................................................................................................243Figure 11.5 - Borehole CR194 – False colour backscattered electron images illustrating a) a siderite ‘fragment’ that actually represents a cavity filling. b) Compositionally zoned siderite filling a euhedral cavity in quartz. c) Siderite that appears to have extensively replaced barite. d) Siderite filling cavities in botryoidal limonite.....................................244Figure 11.6 - Borehole CR194 – a digitised photograph showing apparent ‘fragments' of siderite............................................................................................................................245Figure 11.7 - Borehole CR194 - False coloured backscattered electron image illustrating the presence of galena replacing siderite along grain boundaries and highlighting different generations of siderite mineralisation...............................................................246Figure 11.8 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of late-stage, unoxidised siderite filling a cavity in an oxidised, opaque siderite matrix.....................................................................................247Figure 11.9 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of early-formed siderite crystals that have formed in a cavity..............................................................................................................................247Figure 11.10 - Borehole CR194 – False colour backscattered electron image illustrating the presence of siderite and galena filling and partially filling cavities in hematite.........249Figure 11.11 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of skeletal galena in euhedral Fe-sulphide crystals..................................250Figure 11.12 - Borehole CR191 – False colour backscattered electron image illustrating the selective leaching of compositional zones within siderite crystals...........................251

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List of Figures

Figure 11.13 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of a fine-grained galena aggregate that is being progressively replaced from the upper left to lower right by siderite...................................................................256Figure 11.14 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of associations between galena and other minerals..........................................................................................................................258Figure 11.15 - An X-ray diffractogram clearly illustrating the presence of greigite........260Figure 11.16 – Colour, reflected light photomicrograph illustrating Type 1 Fe-sulphide, consisting of feathery, colloidal radiating aggregates of Fe-sulphide.............................261Figure 11.17 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals with marcasite/pyrite inclusions forming overgrowths on a colloidal Fe-sulphide aggregate.......................................................................................................................262Figure 11.18 – Colour, reflected light photomicrograph illustrating colloidal radiating aggregates of Fe-sulphide and finely disseminated euhedral Fe-sulphide crystals in siderite............................................................................................................................263Figure 11.19 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals in siderite...........................................................................................................264Figure 11.20 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals with marcasite/pyrite inclusions in siderite........................................................264Figure 11.21 - Colour, reflected light photomicrograph illustrating feathery, strongly anisotropic Fe-sulphide with cubic overgrowths of Fe-sulphide crystals in siderite.......265Figure 11.22 - Colour, reflected light photomicrograph illustrating platelets of an anisotropic Fe-sulphide phase with minor pyrite/marcasite in a matrix of siderite ........266Figure 11.23 - Platy textures in pyrite that has pseudomorphously replaced Type 4 Fe-sulphide in siderite..........................................................................................................267Figure 11.24 - Marcasite extensively replaces the strongly anisotropic Fe-sulphide phase that is partially filling a resin-filled euhedral cavity in quartz ...............................267Figure 11.25 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of morphologies and associations of the Au and Au-bearing grains...........................................................................................................273Figure 11.26 – Montage of false colour backscattered electron images illustrating partial and complete paragenetic sequences observed during this investigation.....................277Figure 12.1 - Diagram illustrating a) Primary massive sulphide and seafloor gossan preserved under culm sediments produced by continued volcanic activity. b) Tilting of the deposit during the Hercynian would have resulted in a steeply dipping primary massive sulphide and preserved seafloor gossan quite distinct from the sub-aerially derived, horizontal gossan, silica cap and supergene mineralisation..........................................283Figure 12.2 – A schematic illustrating the distinct biogeochemical and abiotic environments that mark the boundaries between regimes of aerobic and anaerobic metabolism and subsequent carbonate and/or sulphide mineral precipitation..............295Figure 12.3 – Diagram illustrating an idealised cross section through the Las Cruces deposit............................................................................................................................303

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List of Tables

List of Tables

Table 11.1 - Siderite 13C and 18O Ratios.....................................................................252Table 12.1 - Comparison of Mature Gossans and Las Cruces Gossan Mineralogy......287

xix

Chapter 1 Introduction

1 INTRODUCTION

1.1 Introduction

The Las Cruces Volcanogenic Massive Sulphide (VMS) deposit is situated within

the Iberian Pyrite Belt (IPB), one of the world’s largest massive sulphide

provinces. The IPB is approximately 250Km long and up to 70Km wide and

hosts more than 80 known mines including Aznalcollar-Los Frailes and Rio Tinto

in Spain and Neves Corvo in Portugal (Leistel et al., 1998). Las Cruces was

discovered by Rio Tinto in 1994.

The Las Cruces primary massive sulphide is essentially similar to other VMS

deposits within the IPB. However, unlike other VMS deposits within the region,

the gossan and supergene mineralisation at Las Cruces is undisturbed by

historical mining activity or erosion being extremely well preserved under

approximately 150 metres of Tertiary deposits.

Early mineralogy reports on the Las Cruces gossan conducted by Rio Tinto

Limited at their Anamet Services laboratory (R2643, 1996; R2644, 1996; R2696,

1997) confirmed that the mineralogy is markedly different from other VMS derived

gossans described in the literature. The Las Cruces gossan consists

predominantly of siderite, galena and subordinate amounts of Fe-sulphides

whereas most sub-aerially derived gossans typically consist of Fe-oxyhydroxide

and metal sulphates.

1.2 Aims of Study

The main focus of this investigation is the Las Cruces gossan. A thorough

characterisation of the mineralogy, the mineral associations and textures through

five sections of precious metal gossan provide the basis of this study. The

mineralogy is used to identify sequences of events in the history of the gossan to

help understand the processes that have resulted in the mineralogical

assemblage. Particular attention is given to the nature and mode of occurrence

of siderite, greigite and galena, the dominant gossan minerals, and to Au and Ag,

the only elements likely to be worthy of economic interest within the gossan.

Page 1


The mineralogy and geochemical profiles developed in the gossan are compared

and contrasted with the mineralogy and geochemical profiles developed in

gossans described in the literature, with the aim of interpreting, as far as

possible, the formational history of the gossan.

Reflected and transmitted light microscopy, X-ray powder diffraction (XRD) and

scanning electron microscopy (SEM) are used to identify and illustrate the

different mineral species present in Las Cruces. Modern SEM-based image

analysis techniques are also used to locate large numbers of precious metal-

bearing grains, with a large number of backscattered electron images being

prepared to illustrate textural information and mineral associations.

The Las Cruces deposit remains buried and as yet unexploited for its mineral

wealth. Removal of the overburden has commenced by the deposits’ current

owners, MK Resources Company, and it is expected that mining of the

supergene Cu ore will begin in 2008. The Las Cruces VMS deposit is considered

to be one of the highest grade Cu deposits in the world.

With only limited information currently available on the Las Cruces gossan, this

investigation provides significant detail on the mineralogy, geochemistry and

styles of mineralisation that may be used as a basis for future investigations.

The final data collection for this thesis took place on 28th September 2007.

Page 2


1.3 Thesis Outline

Chapter 2 describes the local and regional geology of Las Cruces and the Iberian

Pyrite Belt and exploration history of the Las Cruces orebody. The locations of

the samples selected for examination during this investigation are also discussed

in Chapter 2. Chapter 3 includes a literature review on gossans, the processes

involved in gossan formation and predominant influences on gossan formation.

The geochemical profiles, mineralogy and element mobility are discussed. Sub-

marine gossan formation in modern and ancient deposits is also discussed.

The methodologies employed during this investigation, including sample

preparation techniques, reflected and transmitted light microscopy, scanning

electron microscopy and X-ray powder diffraction are given in Chapter 4.

Chapters 5 through to 9 include the major and minor element geochemistry and

geochemical profiles of the Las Cruces gossan, together with detailed

descriptions of each of the boreholes selected for examination. The illustrations

are provided in Appendices 6 to 10.

Chapter 10 describes the environment and formational mechanisms for siderite

and greigite. Chapter 11 summarises the key mineralogical features of the Las

Cruces gossan.

Chapter 12 discusses the evidence presented in the previous chapters and

compares how the mineralogy and geochemistry of the Las Cruces gossan fit

with the model of formation described by Knight (2000), the only other significant

academic work on this deposit to date. Conclusions are drawn from the evidence

presented in the previous chapters. This chapter concludes with a brief

discussion on how future investigations may be focussed.

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2 GEOLOGY

2.1 Regional Geological Setting - The Iberian Peninsula

The Iberian Peninsula is largely underlain by a Hercynian belt of approximately

750 km in length, extending in a NW-SE direction (Figure 2.1a, blue). The

Hercynian belt consists of a number of discrete zones or terranes that were

progressively accreted during the Pan-African/Cadomian and Hercynian

Orogenies. These zones are the Cantabrian Zone (CZ), West Asturian-Leonese

Zone (WALZ), Galicia Tras-os-Montes Zone (GTZ), Central Iberian Zone (CIZ),

the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the

Pulo do Lobo (PL), the Southern Portuguese Zone (SPZ) and the Porto Tomar

Shear Zone (Figure 2.1a). Precambrian and Palaeozoic sequences in Alpine

belts are shown in red (Leistel et al., 1998).

The Central Iberian Zone belongs to the Iberian Autochthon onto which the other

zones were accreted (Ribeiro et al., 1990; Quesada, 1991). The Badajoz-

Cordoba Shear Zone is a major suture formed during the Pan-African and

Hercynian Orogenies (Quesada, 1991). The CIZ and accreted OMZ underwent a

passive margin type evolution in the northern margin of Gondwana until the onset

of the Hercynian orogeny in early to mid-Devonian (Leistel et al., 1998). The

Pulo do Lobo Zone is a complex ophiolite sequence formed as a result of the

subduction of oceanic lithosphere at the outer margin and underneath the OMZ

(Leistel et al., 1998).

The collision of the Southern Portuguese Zone with the Ossa-Morena Zone

resulted in the lateral escape of units that coincided with bimodal magmatism,

hydrothermal circulation and ore deposition (Leistel et al., 1998; Oliveira, 1990

and Quesada et al., 1991). These marginal units represent what is known today

as the Iberian Pyrite Belt. The tectonic setting was extensional and epicontinental

and developed during the Hercynian plate convergence, culminating in thin-

skinned deformation and accretion of the South Portuguese terrane to the Iberian

Palaeozoic continental block (Leistel et al., 1998) (Figure 2.1a).

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Chapter 2 Geology

Additional divisions of the Southern Portuguese Zone include the Baixo Alentejo

Flysch Domian and the SW Portugal Domain. These sub divisions are related to

the breakdown of a Devonian platform resulting from the continent-continent

collision that occurred throughout the early Carboniferous (Saez et al., 1996).

The northern sector of the Southern Portuguese Zone consists of siltstones of

Precambrian to upper Palaeozoic age that have been metamorphosed to

greenschist facies. These rocks are thought to be the source of the clastic

materials of the Iberian Pyrite Belt sediments (Strauss and Madel, 1974). The

central sector of the Southern Portuguese Zone makes up the Iberian Pyrite Belt.

The southern sector comprises marine sandstones, shales and limestones of

Devonian to Carboniferous age (Oliveira, 1983).

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Chapter 2 Geology

Figure 2.1 - a) A geological map of Spain and Portugal showing the relative positions of the Central Iberian Zone (CIZ), the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the Pulo do Lobo (PL) and the Southern Portuguese Zone (SPZ). The Hercynian orogenic belt is shown in blue. Precambrian and Palaeozoic sequences in Alpine belts are shown in red. Figure 2.1b is a more detailed map of the area marked by a red rectangle on Figure 2.1a. This map shows the locations of the main Volcanogenic Massive Sulphide (VMS) deposits, including the Las Cruces deposit, positioned toward the southeast of the region under the post Palaeozoic cover. (Modified from Quesada, 1991)

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2.2 The Iberian Pyrite Belt

The Las Cruces deposit lies within the Iberian Pyrite Belt (IPB), in the south west

part of the Iberian Peninsula (Figure 2.1b). Situated within late Devonian to

middle Carboniferous rocks, covered in places by Tertiary-Quaternary terrace

and alluvial deposits, the IPB is 250Km long and 25-70Km wide.

The IPB hosts a huge quantity of volcanic-hosted massive sulphide (VMS)

mineralisation with more than 80 known mines totalling 1700Mt of sulphides and

containing 14.6Mt Cu, 13.0Mt Pb, 34.9Mt Zn, 46100t Ag and 880t Au (Leistel

et al., 1998). Mining of the outcropping deposits in the IPB dates back to the

Chalcolithic era (5000–3000BC) with Tartassians, Phoenicians and Romans

extracting Cu, Au and Ag from oxide and supergene zones overlying the massive

sulphide orebodies (Strauss et al., 1990).

With recent cessation of the use of pyrite for sulphuric acid production, large

scale mining in the IPB belt is now limited. Only five mines remain active in the

belt today, namely Soteil-Migollas, Aznalcollar-Los Frailes, Rio Tinto and Tharsis

in Spain and Neves Corvo in Portugal (Leistel et al., 1998). The locations of

these deposits are shown in Figure 2.1b. With the discovery of the Neves-Corvo

Cu-Sn deposit in 1977, renewed interest in exploration for deep, 'blind' VMS

deposits resulted in a number of new discoveries, Las Cruces being one of the

more recent and significant additions.

Throughout the Phanerozoic, Europe has been subjected to three continuous

compressional and extensional periods of geotectonic activity, namely the

Caledonian Orogeny (circa 600–350Ma), Variscan Orogeny (circa 550-250Ma)

and Alpine Orogeny (circa 250-0Ma) (Rickard, 1999). The Iberian Pyrite Belt was

formed during the Variscan Orogeny, during the development of pull-apart basins

alongside continental margins. The Variscan Orogeny resulted from the closure

of the pre-Mediterranean Tethys Ocean, climaxing at around 300Ma and is partly

synonymous with the Hercynian Orogeny of Northern Europe (Rickard, 1999). All

the sequences of the Pyrite Belt were deformed during the Hercynian orogeny,

which was accompanied by low-grade regional metamorphism, ranging from

zeolite to lower greenschist facies (Munha, 1983).

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The massive sulphide deposits of the IPB exhibit specific features that aid in their

identification and characterisation, including mineralogy and geochemistry, Pb

isotope data, hydrothermal alteration and structure. The geochemistry of a large

part of the basic lavas associated with the IPB are comparable to those of

mantle-derived basalts emplaced in extensional tectonic settings and the

associated acidic rocks were produced by melting of a basic crustal protolith at

low to medium pressures and a steep geothermal gradient (Leistel et al., 1998).

The IPB consists of an extremely complex succession of Late Devonian to Middle

Carboniferous rocks resulting from several facies variations and intense tectonic

deformation overlain by Tertiary to Quaternary sediments (Oliveira, 1990).

The stratigraphy of the IPB has classically been sub-divided into three principle

units, the Phyllitic Quartzite (PQ) formation, the Volcano-Siliceous (VS,

Devonian-Carboniferous) complex and the Culm (or Flysch, Upper

Carboniferous) group (Schermerhorn, 1971). The main lithostratigraphic units in

the Iberian Pyrite Belt are illustrated in Figure 2.2.

The PQ formation, estimated to be greater than 1000m in thickness (Strauss,

1970), consists of Late Devonian shale, quartz sandstone and rare conglomerate

that essentially form the base of the IPB. The depositional environment is

thought to be a shallow epicontinental sea (Leistel et al., 1998). Dating of the

upper 30m thick sequence of carbonates and bioclastic lenses indicate late

Famennian age (late Devonian circa. 367–362Ma) (Van den Boogard et al.,

1980). Towards the top of the unit, the uniform nature of the PQ formation

changes and is marked by high energy sedimentary deposits registering the

tectonic evolution of the IPB basins (Moreno et al., 1996).

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Chapter 2 Geology

Figure 2.2 - The main lithostratigraphic units in the Iberian Pyrite Belt. 1. Shales and greywackes 2. Black shales, siliceous shales and tuffites 3. Exhalites (mostly jaspers) 4. Shales, greywackes, quartzwackes and quartzites 5. Polymetallic massive sulphides and stockworks 6. Felsic volcanic rocks, mostly tuffs 7. Mafic rocks (spilites and dolerites) 8. Phyllites and quartzites (modified from Carvalho 1999).

The VS complex dates from late Famennian to middle Visean (circa 342–339Ma)

(Oliveira, 1990) and varies in thickness between 100 and 600m (Leistel et al.,

1998). Exposure of the VS complex is restricted to the IPB. Although somewhat

variable between zones in the IPB, the VS complex essentially consists of

alternating felsic and mafic, sub-aerial to sub-marine volcanics within detrital and

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Chapter 2 Geology

chemically derived sediments (Saez et al., 1996). The most complete VS

sequence, evident in some units of the southern branch of the belt are (Leistel et

al., 1998):-

1. A lowermost rhyolitic sequence (VA1), with fine to coarse-grained

pyroclastics and lavas

2. A second rhyolitic sequence (VA2), with pyroclastics and lavas

3. A third rhyolitic sequence (VA3), mainly reworked tuffs and siliceous shale.

4. Basic lava, locally pillowed, intercalated between VA1 and VA3; basic

dykes and sills injected into the lower part of the complex (possibly feeder

zones).

5. A purple shale situated directly below VA3.

6. A pelite-black shale and sandstone containing beds of jasper and rare

limestone, interstratified with VA1 to VA2 volcanics.

The Culm facies or Baixo Alentejo flysch group is a thick turbidite formation

forming a south-westward prograding detrital cover that is diachronous over the

underlying VS complex. The thickness of this facies is estimated to be up to

3000m (Strauss and Madel, 1974).

Moreno (1993) describes three stratigraphic units for the Culm facies:-

1. The Basal Shaly Formation (BSF), consisting of volcanic and non-volcanic

sediments, marking the end of volcanic activity in the region and the

beginning of autochthonous sedimentation of pelagic clay.

2. The Culm Facies Turbidite Formation (CFTF), consisting of turbidite

sequences of sandstones, shales and minor conglomerates.

3. The Shallow-Platform Sandy Unit (SPSU) consisting of shales and

sandstones reworked and redeposited following the erosion of volcanic

uplands.

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Chapter 2 Geology

2.3 The Guadalquivir Basin

The Guadalquivir Basin is situated along the eastern end of the IPB. Las Cruces

lies along the western margin of the Guadalquivir Basin and is buried under

approximately 150m of Tertiary sediments. This relatively flat lying area ranges

from 15 to 50 metres above sea level. The Guadalquivir Basin lies between the

Iberian Foreland to the north and the Betic Cordillera to the south (Figure 2.3).

The Betic Cordillera is the northern segment of an arcuate orogen that extends

over 600Km westward across the Gibraltar Arc into the Rif Chain. The inner part

of this orogen is occupied by the extensional basin of the Alboran Sea. The

cordillera contains numerous Neogene basins, including the Sado Basin to the

NW and the Guadalquivir Basin in the SE (Sanz de Galdeano and Vera, 1992).

Figure 2.3 - A general map of the Betic Cordillera showing the position of the Guadalquivir Basin and Las Cruces (modified from Gomez et al., 2003).

The Guadalquivir Basin was formed during the Alpine orogeny (Miocene to

Recent) as the African Plate continued to collide with the Eurasian Plate. Dewey

et al. (1989) determined that this area experienced in the order of 200Km of N-S

convergence between the mid-Oligocene (circa 30Ma) and late Miocene (circa

6Ma), followed by approximately 50Km of WNW-directed oblique convergence in

the late Miocene to recent times. The evolution of the Guadalquivir Basin ended

in the Messinian (circa 6Ma) when the basin was partially filled by Miocene

sediments, consisting predominantly of marine marls (Fernandez et al., 1998).

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Chapter 2 Geology

The basement of the Guadalquivir Basin consists predominantly of PQ, VS and

Culm Palaeozoic sediments that dip gently in a SSE direction. The Miocene

sediments increase in thickness towards the south, reaching a maximum

thickness of ~15Km. Fernandez et al. (1998) suggest that the Iberian Massif to

the north of the Guadalquivir Basin provided clastic infill to the basin. These

clastics were subsequently redistributed along the ENE-WSW axis of the

basement by turbiditic currents.

The external zones of the Betic Cordillera provide a gravitational infill to the

south, producing sedimentary deposits known as 'olistostromes', consisting

predominantly of chaotic mixtures of Triassic evaporites, clays, limestones and

Upper Cretaceous to Palaeozoic limestones (Fernandez et al., 1998). There has

been significant erosion of the Miocene sediments in recent times and the Las

Cruces deposit may at one time have been buried by as much as 1000m of

sediment (Knight, 2000).

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Chapter 2 Geology

2.4 Las Cruces - Exploration History

The Las Cruces volcanogenic massive sulphide deposit is situated on the

western margin of the Guadalquivir basin, in the southern region of the Iberian

Pyrite Belt in the Seville Province of Andalucia approximately 15km northwest of

the city of Seville (Figures 2.1 and 2.3). After an absence from the Pyrite Belt of

several years, in 1990, Rio Tinto began the exploration programme that led to the

Las Cruces discovery. The block of ground that contains Las Cruces

(Faralaes II) is covered almost entirely by recent Tertiary sediments and was

applied for in 1991 on the supposition that Palaeozoic rocks prospective for base

metals were concealed below the Tertiary sediments. The presence of Boliden's

Aznalcóllar orebody in exposed Palaeozoic rocks 12.5km to the west gave weight

to this supposition.

The initial exploration method used by Riomin Exploraciones (Rio Tinto Mining

and Exploration) was gravity surveying. This is a costly technique and in 1992

Rio Tinto departed from traditional practice by reducing the density of the survey

points from 50 - 100/km² to 11 - 15/km² thus allowing more extensive and faster

survey coverage of all available prospective ground. This was largely

responsible for the discovery of Las Cruces. The key steps in the discovery of

the Las Cruces deposit were:-

Early 1994, a gravity anomaly of exceptional size and strength was

detected at Las Cruces.

The first scout hole to investigate the anomaly was drilled in May 1994,

showing that the cause was a large concealed sulphide deposit (the first

hole returned insignificant amounts of base metal, the sulphide consisting

almost entirely of pyrite).

September 1994, the first promising secondary Cu mineralisation was

intersected grading 3.77 per cent Cu over 42 metres.

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Chapter 2 Geology

June 1995, very high-grade secondary Cu mineralisation was intersected

close to the central part of the main orebody, grading 19.49 per cent Cu

over 17 metres.

June 1995 to October 1996, progressive delineation of secondary Cu-rich

mineralisation at shallow depth continued, together with the confirmation of

thick zones of primary Zn-Cu mineralisation.

October 1996, Las Cruces moved to project status, with a first phase

feasibility study being completed in September 1998.

The Las Cruces deposit was sold to MK Gold Company (now MK

Resources Company), a subsidiary of US-based Leucadia National

Corporation, in 1999. Inmet Mining Corporation is now the majority owner

of the project after it acquired 70 percent from MK Resources in August

2005.

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Chapter 2 Geology

2.5 Las Cruces - Geology and Mineralogy

2.5.1 Introduction

The mineralisation occurs in volcano-sedimentary rocks of Devonian to

Carboniferous age. Similar associated deposits include Neves Corvo in Portugal

and the Rio Tinto, Soteil and Aznalcollar mines in Spain. The Las Cruces deposit

is covered by a thick layer of Tertiary sediments of Miocene age, dating from

circa 6Ma. These sediments have prevented the erosion of the Las Cruces

orebody and have resulted in a high degree of preservation of the supergene

massive sulphide mineralisation and associated gossan.

Although Las Cruces lies outside of the confines of the IPB, the basement rocks

are the same as those described for the IPB and consist of the Phyllitic Quartz

formation, the Volcano-Sedimentary sequence and the Culm facies. The host

rock lithology for Las Cruces is typical for the IPB. The footwall is dominated by

highly deformed acid volcaniclastics, with interbedded shales becoming

increasingly important to the west (Knight, 2000). The main lithostratigraphic

units at Las Cruces are illustrated in Figure 2.4.

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Chapter 2 Geology

Figure 2.4 - Summary of the main lithostratigraphic units at Las Cruces (Knight, 2000). The massive sulphides lie within an approximately 80 metre thick sequence of black shales and consist of gossan, secondary Cu, primary Cu/Zn and stockwork zones.

The volcaniclastics include high level intrusive lavas and tuffs which, in part, have

been altered due to seawater interaction (Knight, 2000). Hydrothermal alteration

is prevalent in the footwall sequence, with some zoning around the stockwork

and chloritic and sericitic alteration throughout. Kaolinitic alteration is evident in

the centre of the footwall, with some late-stage carbonate replacement and

silicification also being evident (Knight, 2000).

The massive sulphide deposit is hosted within an approximately 80 metre thick

sequence of black shales with some volcanic material. The black shale host is

no more pyritic than other shale units in this sequence (Knight, 2000). The

hangingwall consists predominantly of shales with subordinate interbedded

volcaniclastics which, in places, are almost indistinguishable from the footwall

volcaniclastics, although some extensive zones of brecciation are present

(Knight, 2000).

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Chapter 2 Geology

The Las Cruces orebody consists of a gossan cap that overlies secondary and

primary massive sulphide mineralisation. The massive sulphide orebody consists

of a number of discrete primary and secondary sub-lenses comprising HC (HCH

and HCL), CZ (primary Zn and primary Cu), C4 and CB (Figures 2.4 and 2.5).

These lenses are described in greater detail in the following section.

Figure 2.5 – An idealised, simplified N-S cross-section through the Las Cruces orebody that is based on the interpretation of drill core data and block modelling information performed by Rio Tinto consultants (R2795, 1998). CB = Cu lens Barren, C4 = covellite zone, CZ = primary Cu/Zn, HCF = High Cu Footwall, HC = High Cu.

2.5.2 Gossan

The gossan is situated within the Carboniferous hangingwall, directly above the

HC secondary massive sulphide orebody and is developed in the pre-Tertiary

oxidising zone. The gossan ranges from between 0 and 20 metres in thickness

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Chapter 2 Geology

(R2795, 1998). In 1997, the gold bearing gossan resource was estimated to be

133,000 tonnes averaging 6.7ppm Au (R2703, 1998).

The mineralogy of the gossan is dominated by the presence of siderite and

quartz together with the more typical assemblage of Fe-oxides and Fe-

hydroxides. Sulphide minerals are also abundant throughout the gossan and

include galena and Fe-sulphides. Textural evidence suggests that the gossan

has been subjected to some degree of reworking and mechanical transportation.

The gossan is markedly enriched in Au and Ag relative to the underlying massive

sulphide mineralisation. The gossan is the main focus of this thesis and is

described in greater detail in Chapters 5 to 9.

2.5.3 Secondary Massive Sulphide

The HC (High Cu) sub-lens represents the secondary supergene enriched zone

and contains the bulk of the economic mineralisation. It is a flat-lying tabular unit

with a strong undulating footwall and flatter hangingwall. The bulk of the

secondary mineralisation occurs in the central and eastern part of the deposit.

Two thicker areas of secondary massive sulphide in the SW and NE are linked by

a thinner region of secondary massive sulphide forming a dumbbell shaped lens

(Knight, 2000).

The HC lens is divided into HCH (High Cu, High density) and HCL (High Cu, Low

density). The HCH lens is interpreted as the supergene replacement of massive

sulphides and the HCL is interpreted as the supergene replacement of partial

massive sulphides and associated wallrocks. The HCF (High Copper Footwall) is

a low tonnage, discontinuous, disseminated sulphide lens occurring just below

the HC footwall (R2795, 1998).

Localised E-W trending faults have produced permeable zones within the

orebody, increasing the depth of penetration of the supergene fluids, resulting in

an increase in thickness of the supergene enrichment (R2795, 1998). The

mineralogy and textures observed in the secondary massive sulphide reflect the

nature of the primary mineralisation and the degree of supergene alteration. The

HC zone consists of pyrite and digenite together with subordinate amounts of

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Chapter 2 Geology

chalcocite, covellite, chalcopyrite, bornite, tetrahedrite-tennantite, enargite and

galena (Knight, 2000). Gangue minerals include quartz, barite, calcite and

alunite. The C4 sub-lens is a small secondary massive sulphide lens at the base

of the primary sulphide orebody dominated by pyrite and covellite.

By 2005, the present owners of the deposit, Inmet Mining estimated that at a 1.0

percent Cu cutoff, the measured plus indicated resource for the combined HCH,

HCL and C4 lenses is 15.6 million tonnes averaging 6.89 percent copper, with an

additional inferred resource of 0.360 million tonnes averaging 8.66 percent

copper.

2.5.4 Primary Massive Sulphide

The CZ (Cu-Zn) sub-lens represents the main primary massive sulphide orebody

and consists of a tabular structure dipping to the north at an angle of

approximately 35o, flattening towards the west of the deposit. The upper portion

of this zone is typically Zn-rich relative to the lower Cu-rich portion (R2795, 1998).

The relative proportions of the dominant ore, gangue and accessory minerals

vary significantly, largely reflecting the primary depositional processes. The ore

is typically fine-grained and exhibits a range of textural features reflecting

variations in the degree of recrystallisation (Knight, 2000). In 1997, the CZ

resource was estimated to be 13.9 million tonnes at 2.2 per cent Cu, 0.9 per cent

Pb and 2.5 per cent Zn (R2703, 1998).

Pyrite is the dominant mineral together with subordinate amounts of chalcopyrite,

sphalerite and galena. Accessory minerals include tennantite-tetrahedrite,

arsenopyrite, enargite, cassiterite and Bi-bearing sulphosalts. Gangue minerals

include quartz, barite, clays, dolomite and calcite (Knight, 2000).

The CB (Cu lens, Barren) is a barren sub-lens within the primary massive

sulphide orebody. The footwall rocks are volcano-sedimentary silicate-rich rocks

containing some mineralised stockwork structures. The hangingwall rocks

consist predominantly of unmineralised Carboniferous volcano-sedimentary

silicate-rich rocks that are overlain by a thick sequence of Tertiary sand and marl

sediments (R2795, 1998).

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Chapter 2 Geology

2.6 Las Cruces - Evolutionary History

The emplacement of the Las Cruces orebody has many similarities to other

massive sulphide deposits of the IPB. This similarity ends with the events that

post date the emplacement of the primary massive sulphide and resulted in the

preserved supergene mineralisation and gossan that is observed today.

The only significant works to date on the formation of the massive sulphide

orebody is by Knight (2000), who produced a model for the development of the

primary and secondary mineralisation based on the mineralogy, stable isotopes,

fluid inclusions and noble gas data. Knight (2000) concludes that the paragenetic

sequence that resulted in the formation of the present day deposit at Las Cruces

included seven distinct events (Figures 2.6 to 2.10):-

Stage 1 - A primary hydrothermal event with waxing and waning thermal

history resulting in temporal and spatial mineralogical zoning (Figure 2.6).

Knight (2000) proposes that a suite of primary ore facies developed under

characteristic hydrothermal conditions whereby cycles of volcanic activity and

episodes of diffuse flow lead to focussed fluid discharge and the formation of

massive sulphide deposits over time. At Las Cruces, this initially resulted in the

primary precipitation within, and replacement of, the host black shales. This was

followed by diffuse flow of mixed hydrothermal and seawater fluids, leading to the

replacement and overgrowth of different generations of pyrite.

Saez et al. (1999) also suggest that interaction between the black shales and

hydrothermal fluids highlights one of the main differences between southern IPB

massive sulphides and other VMS deposits.

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Chapter 2 Geology

Figure 2.6 - Stage 1 - formation of the Las Cruces primary massive sulphide deposit during a primary hydrothermal event with waxing and waning thermal history (modified from Knight, 2000).

Saez et al. (1999) note that Pb isotope data suggest a single (or homogenised)

metal source derived from both the volcanic piles and the underlying Devonian

rocks. The authors also consider that the IPB deposits had magmatic activity as

the heat source, but the environment was not strictly volcanogenic, with many of

the evolutionary stages possibly occurring in conditions similar to those of

sediment hosted massive sulphides.

Saez et al. (1999) suggest that dispersion of hydrothermal fluids may have been

restricted and therefore focussed by the black shales, with massive sulphides

subsequently forming by deposition and replacement processes (citing

Almodovar et al, 1998).

The mineralogy and chemistry of the massive sulphide mound was modified over

a period of hundreds or thousands of years by cycles of hydrothermal diagenesis,

with each hydrothermal cycle involving a waxing stage, in which prograde

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Chapter 2 Geology

diagenesis occurs, a period of peak hydrothermal conditions and a waning stage,

in which retrograde diagenesis occurs (Knight 2000, citing Knott, 1994).

At Las Cruces, early diagenetic conditions are represented by a prograde

assemblage, which developed as a result of hydrothermal insulation within the

mound. Increased massive sulphide thickness provided both thermal and

chemical insulation of the hydrothermal fluids from the surrounding seawater.

Increased intensity of the hydrothermal system resulted in the development of the

Zn-Fe-Pb-(Cu) sulphides (Knight, 2000).

The peak hydrothermal stage is associated with pervasive, focussed, high

temperature mineralisation, with hydrothermal fluid temperatures >300oC,

resulting in the development of a high temperature, chalcopyrite-rich core with a

cooler, outer margin rich in sphalerite (Figure 2.7). These conditions are

analogous to the conditions in the central conduit of a black smoker chimney

(Knight, 2000).

Stage 2 - Oxidation during the waning stages of the hydrothermal system

resulting in the formation of secondary Fe oxides/hydroxides and

secondary Cu sulphides (Figure 2.7).

During the waning stages of hydrothermal activity, long term, low temperature

(~100oC–300oC) fluid circulation and diffuse venting of white smoker chimneys

replaced those of the focussed high temperature activity. These changes

resulted in the late overgrowth of silica, minor sphalerite, galena, barite and

covellite (Knight, 2000).

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Chapter 2 Geology

Figure 2.7 - Stage 2 - Sub-marine oxidation and secondary Cu-sulphide enrichment during the waning stages of hydrothermal activity (modified from Knight, 2000).

The retrograde hydrothermal conditions also lead to increasing conductive

cooling and seawater mixing, generating a low pH and oxidation. It is likely that

some oxidation of the massive sulphide orebody occurred during the waning

stages of sub-marine hydrothermal activity, similar to that described for modern

seafloor sulphide deposits. Knight (2000) provides evidence of Fe-oxide dustings

in silica samples suggesting the oxidation may have taken place at a similar time

to the late-stage silicification event that is also strongly correlated to a phase of

secondary Cu-sulphide mineralisation. This event, which took place at

temperatures of <200oC, produced characteristic isotope and fluid inclusion

signatures.

The secondary Cu-sulphides exhibit a slightly enriched 34S isotope signature most

likely caused by the addition of reduced seawater sulphate. Fluid inclusion and

oxygen isotope data for the associated quartz also confirm modified seawater

type solutions (Knight, 2000). This evidence supports the theory of oxidation and

supergene enrichment during the waning stages of hydrothermal activity. Due to

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Chapter 2 Geology

successive secondary events, however, evidence of the original seafloor

enrichment may be overprinted by later enrichment (Knight, 2000).

Stage 3 - Burial by a thick sequence of Culm sediments (up to c. 1500m)

during the late Carboniferous resulting in recrystallisation of the primary

massive sulphides (Figure 2.8).

Figure 2.8 - Stage 3 - Sustained volcanism and sedimentation leading to the burial of the massive sulphide beneath ~1000m Palaeozoic Culm sediments (modified from Knight, 2000).

Sustained sedimentation and volcanism led to the burial of the massive sulphide

beneath a thick succession of Palaeozoic sediments. Burial at this depth would

have resulted in an increase in the geothermal gradient and recrystallisation of

the massive sulphide deposit. Alteration of the secondary Cu-sulphides formed

during seafloor oxidation may also have occurred (Knight, 2000).

Stage 4 - Sub-aerial supergene enrichment following uplift and erosion.

This extensive period of sub-aerial weathering resulted in the development

of gossan sequences, a silica cap and pervasive supergene Cu sulphide

enrichment (Figure 2.9).

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Chapter 2 Geology

Figure 2.9 - Stage 4 - Tilting of the primary massive sulphide occurred during the Hercynian, with uplift and erosion being followed by sub-aerial weathering and the development of the gossan, silica cap and supergene Cu-sulphides (modified from Knight, 2000).

Palaeozoic sedimentation eventually ceased and tectonic uplift resulted in the

erosion of much of the Culm sequence. No further deposition occurring until the

Miocene. Regional tectonics deformed, folded and faulted the mineralised

sequence. This resulted in a hinge zone that effectively separates the steeply

dipping primary mineralisation from the largely horizontal secondary

mineralisation. The essentially horizontal secondary mineralisation reflects the

development of the weathering profile below a palaeo-surface that is similarly

oriented to the present day surface (Knight, 2000).

Partial exposure of the palaeo-surface before and during the Tertiary resulted in

the development of a mature gossan profile. The climate was warm with high

rainfall (Knight, 2000, citing Sanz de Galdeano and Vera, 1992; Moreno, 1993),

creating ideal conditions for oxidation and the development of a deep weathering

profile. Downward percolating groundwater caused oxidation and leaching of the

more mobile elements above the water table, with metal ions, notably Cu being

precipitated as sulphides in the reducing environment below (Knight, 2000).

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Chapter 2 Geology

The secondary sulphide mineralisation resulting from sub-aerial enrichment is

more pervasive than seafloor secondary mineralisation with extensive enrichment

and replacement of primary Cu-sulphides, notably chalcopyrite, by secondary Cu-

sulphides, (largely digenite). The sulphur isotope and fluid inclusion data support

the theory that the formation of the secondary sulphide mineralisation occurred

from modified meteoric fluids at ambient temperature, with 34S enrichment of the

Cu-sulphides resulting in response to dissolution of the primary sulphates,

notably anhydrite (Knight, 2000).

Stage 5 - Reworking of the sub-aerial gossan by seawater during the

Miocene (Figure 2.10).

Figure 2.10 - Stage 5 - Reworking of the gossan and silica cap possibly prior to and following the onset of the marine incursion during the Miocene (modified from Knight, 2000).

The gossan, enriched in less mobile elements, including Fe, Si and Au, contains

both sub-aerial and sub-aqueous derived gossanous materials in addition to

materials reworked during and after the marine incursion that followed. The

variability of the gossan types may reflect the topography of the palaeo-surface

with erosion evident on palaeo-topographic highs and accumulation of gossanous

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Chapter 2 Geology

materials in palaeo-lows (Knight, 2000). Knight (2000) and Doyle et al. (2003)

also note the presence of gossanous clasts in the Tertiary conglomerate,

suggesting that reworking occurred during the Tertiary marine incursion.

Reworking of the gossan may also have occurred prior to the marine incursion.

Similar reworked textures are observed in the Rio Tinto gossan, possibly

resulting from the activities of flash floods during periods of high rainfall

(Kosakevich et al., 1993).

Stage 6 - Burial by a thick sequence of Miocene sediments resulting in an

increase in geothermal gradient and the development of retrograde

mineralogical sequences and a high degree of preservation of the

supergene ore (Figure 2.11).

Figure 2.11 - Stage 6 - Burial and preservation of the Las Cruces deposit under up to 1000 metres Tertiary sediments (modified from Knight, 2000).

The marine incursion during the Miocene resulted in the Las Cruces deposit

being buried by as much as 1000 metres or more of Tertiary sediments. This not

only protected the gossan and supergene zones from further weathering and

erosion, but also had an additional impact resulting from interactions with

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Chapter 2 Geology

seawater and subsequent retrograde reactions resulting from burial. These

retrograde, re-sulphidation reactions are evident in the supergene Cu-sulphide

mineralogy in which digenite is replaced by bornite which, in turn is replaced by

chalcopyrite (Knight, 2000). Knight (2000) also notes that re-sulphidation of the

gossan may have occurred as a result of a rise in the water table immediately

prior to inundation and burial by the Tertiary sediments.

Stage 7?? - In addition, although not discussed in detail, Knight (2000)

also suggested that the present day water table may also have had some

effect on, in particular, the secondary sulphide zone.

The Las Cruces deposit is sandwiched between the top of the Palaeozoic

basement rocks and an overlying sequence of Miocene sands, conglomerates

and limestones which vary from less than 1m thick in the vicinity of the deposit to

over 50m thick further to the east under the Huelva River. The sands are in turn

overlain by a 140m thick sequence of marls (R2795, 1998).

Groundwater movement within the Miocene sand unit occurs by porous medium

flow in the lesser consolidated sand layers and by fracture flow in the

conglomerates and limestones. There is vertical hydraulic connection between

the Palaeozoic rocks and the overlying conglomerates, limestones and sands.

The sand sequence is termed locally the Niebla Posadas aquifer and provides

groundwater for local agricultural and industrial use and also for emergency back-

up water supplies for Seville (R2795, 1998).

The Niebla Posadas aquifer therefore lies directly above the gossan at Las

Cruces. Significant scope exists for marked changes in pH and Eh in and around

the gossan due to the influence of the aquifer. The aquifer is likely to carry

dissolved CO2, oxygen and chlorine (particularly during dryer periods) and could

also act as a transporting medium for dissolved metal ions. Although

groundwater quality is good in the recharge area, water quality deteriorates as it

moves downdip and the groundwater in the deeper parts of the basin is more

typically a sodium chloride type. Low level occurrences of trace metals and

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Chapter 2 Geology

increased concentrations of sulphate occur in some areas of the Palaeozoic

rocks, particularly in the Las Cruces area (R2795, 1998).

The Las Cruces gossan exhibits a high degree of porosity and the presence of an

aquifer is therefore likely to have a significant impact on the mineralogy of the

gossan and may go some way to explaining the distinctive mineralogy. The

impact of the aquifer is considered later in this thesis.

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Chapter 2 Geology

2.7 Sample Suite

The initial selection of core samples was based on whole rock geochemical

assay data provided by the Rio Tinto exploration geologists. In excess of 220

individual boreholes had been drilled into the precious metal and massive

sulphide mineralisation at the time of sample selection, with core material being

stored in warehouse facilities close to the Iberian exploration offices in Seville.

Access to these materials was limited. The list of samples used in this thesis

together with Rio Tinto whole rock assay data are provided in Appendices 2 and

3 respectively.

Five boreholes were selected for detailed mineralogical characterisation. The

relative positions of the boreholes are provided in Figure 2.12. Previous

mineralogical reports on the precious metal mineralogy of the Las Cruces gossan

(R2643, 1996; R2644, 1996; R2696, 1997) revealed that a significant proportion

of the Au mineralisation is sub-microscopic or extremely fine-grained in nature.

Therefore, selection of boreholes for examination was based on samples that

contained significant Au contents (>5ppm) over several metres of core, thereby

improving the chances of locating and identifying any Au-bearing phases.

Some consideration was given to the spatial distribution of the boreholes.

Boreholes CR149, CR194 and CR123 are situated due south of the main

supergene enriched massive sulphide mineralisation. The gossan is

mechanically and chemically reworked and may also occur some distance from

the original source. Boreholes CR038 and CR191 provided information on the

nature of the precious metal mineralisation away from the main supergene

orebody and it was predicted that these might differ somewhat from those in

direct contact with the underlying massive sulphide orebody. The Las Cruces site

is situated approximately 30 to 35 metres above sea level.

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Chapter 2 Geology

Figure 2.12 – a) A map of the Las Cruces deposit illustrating the extent of the Au mineralisation (solid yellow line), supergene Cu-sulphide mineralisation (solid blue line) and the positions of the boreholes selected for examination during this investigation. The contours represent gravity survey data. The red and purple contours represent areas of high gravity (relative to the surrounding areas shown in yellow, green and blue, scale unknown). The region of high gravity in the central left hand portion of the map represents the supergene enriched massive sulphide deposit and the central upper region of high gravity represents the primary massive sulphide orebody. Boreholes CR194, CR123 and CR038 are vertical holes and boreholes CR149 and CR191 are inclined holes. The grid spacing is in units of 60 metres. (Modified diagram courtesy of Rio Tinto Limited.)

Borehole CR194 exhibits extensive Au mineralisation with grades in excess of

14ppm Au in the gossan and in excess of 13ppm Au within the supergene

enriched massive sulphide. The Ag mineralisation is extensive, particularly

towards the base of the gossan, with grades exceeding 1100ppm. The gossan in

borehole CR194 lies directly above the supergene enriched massive sulphide

mineralisation. The supergene massive sulphide contains elevated Cu values in

addition to deleterious elements (from a mining perspective), including As, Bi, Hg

and Sb. The supergene massive sulphide mineralisation lies above a Cu-

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Chapter 2 Geology

enriched shale. This borehole was selected for examination due to the extensive

precious metal mineralisation and the central position relative to the underlying

massive sulphide and supergene Cu sulphide mineralisation.

The gossan in borehole CR149 also lies directly above the supergene enriched

massive sulphide mineralisation with Au contents ranging between 0.67 and

48.54ppm Au between 170.20 and 190.00 metres down hole. This borehole is an

inclined hole, the angle of dip being approximately 60 degrees. Therefore, the

depths are not representative of the vertical extent of the mineralisation. The Au

mineralisation is confined to the gossan with elevated Cu values occurring in the

underlying massive sulphides. The Ag content of this borehole is relatively low

with a significant increase in the Ag content (~730ppm) occurring at the contact

between the gossan and massive sulphide. Relatively high levels of As, Bi, Hg,

Sb and Sn occur throughout the gossan and massive sulphide. This borehole

was selected for examination because of the extensive precious metal

mineralisation and the central position relative to the underlying massive sulphide

and supergene Cu sulphide mineralisation.

The Au lens/gossan zone in borehole CR038 occurs between 150.80 and 157.25

metres and exhibits extensive precious metal mineralisation (1.33–11.31ppm Au,

3.8–1240ppm Ag). This borehole lies towards the margins of the precious metal

mineralisation for the Las Cruces orebody and away from the main massive

sulphide zone. The underlying geology is that of partial massive sulphide that

largely represents pyrite-rich shales and wall rocks that exhibit some degree of

supergene enrichment. This borehole was selected for examination because of

the extensive precious metal mineralisation and the marginal location relative to

the massive sulphide mineralisation.

Borehole CR191 is also extensively mineralised with respect to Au (0.61–

12.04ppm) with the Ag content (5.3–58.6ppm) being less significant than

previous boreholes. The Au zone occurs between 137.95 and 153.85 metres.

However, this borehole is an inclined hole, the angle of dip being approximately

70 degrees. Therefore, the depths are not representative of the vertical extent of

the mineralisation. Borehole CR191 was selected for examination because of the

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Chapter 2 Geology

extensive Au mineralisation and the marginal position relative to the main

supergene massive sulphide mineralisation.

The gossan zone of boreholes CR123 occurs between 152.40 and 172.85

metres. The Au content of the core is relatively high (1.47–56.55ppm), with

moderate amounts of Ag (13.6–175.3ppm) also being present. This borehole lies

on the margin of the Au and secondary Cu mineralisation, towards the southern

most region of the Las Cruces orebody. The gossan lies above partially

supergene enriched pyritised shales and wall rocks. Borehole CR123 was

selected for examination because of the extensive Au mineralisation and the

marginal position relative to the main supergene massive sulphide mineralisation.

Of the five boreholes selected for the current study, only borehole CR038 had

been examined previously (R2644, 1996). This earlier investigation revealed that

the bulk of the precious metal mineralisation occurred in the form of relatively

coarse native Au grains, with discrete grains commonly exceeding 25µm in

maximum dimension. Reports R2643 (1996), R2644 (1996) and R2696 (1997)

also provided some initial mineralogical information on the nature and mode of

occurrence of the precious metal mineralisation in other boreholes from the Las

Cruces gossan. However, only limited information was available on the textures

and association of the precious metal mineralisation, with the bulk of the

investigation being based on crushed reject materials from assay sampling.

The five boreholes consist of several hundred metres of core, with the upper 100

metres typically consisting of marl and unmineralised overburden. The field

geologists often discarded this material, as it had no commercial value with only

mineralised intersections (with respect to Au and/or Cu), and material

immediately above or below the mineralisation being retained for examination. It

was therefore not possible to examine material from all sample intervals within

each borehole. Subsequently, the samples selected for investigation consist

predominantly of Au-bearing gossan and material directly above and directly

below the Au lens. However, borehole CR194 contains significant Au values

within the massive sulphide zone and the sample suite therefore also included

this Au-bearing material.

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Chapter 3 Gossans

3 GOSSANS

3.1 Introduction

Gossans have been a source of great interest since ancient times, with the

earliest prospectors recognising gossans as the surface expressions of base and

precious metal-bearing orebodies. Recently, gossan evaluation has been

focussed on characterising the mineralogy and geochemistry of these oxidised

outcrops, with the aim of differentiating between barren and fertile gossans and

ironstones. This has become particularly important in the field of exploration

geology, because in many mineralised terrains, gossans provide the only visible

indication of potentially economic ore hidden at depth.

Some of the more notable work has been by Blain and Andrew (1977) and

Andrew (1978, 1984), with reviews on gossan typology, mineralogy and

geochemistry for both base and precious metal-bearing orebodies.

Recent literature on gossans is somewhat limited relative to those produced on

the underlying orebodies. This may be related to the degree of economic interest

in gossans. Although many gossans contain economic quantities of metals, their

value is often less than that in the underlying orebody. The bulk of detailed

papers on gossans have focussed on pathfinder geochemistry, identifying

economic sub-surface mineralisation.

Many gossans, particularly those in the Iberian Pyrite Belt, in which this study is

focussed, have been mined since before Roman times and much of the gossan

has long been removed. English language publications of Iberian Pyrite Belt

massive sulphide deposits are common, but information on the nature of the

respective gossans is scarce, being confined largely to Spanish and Portuguese

research papers held in university and research departments.

Limited information is available on gossans and massive sulphides in the Rio

Tinto mine, Spain (Kosakevitch et al., 1993 and Williams, 1933-34 and 1950).

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Chapter 3 Gossans

Nickel (1984), Taylor and Sylvester (1982), Taylor and Appleyard (1983) and

Scott et al. (2001) have produced detailed accounts of gossan profiles associated

with both barren (base/precious metal-poor) and fertile (base/precious metal-rich)

orebodies.

Boyle (1995) examined the gossan of the Murray Brook Deposit, New Brunswick.

Hannington et al. (1986, 1988) and Herzig et al. (1991) provide examples of the

weathering and formation of gossans in present day seafloor sulphides.

The geochemistry of gossan forming processes is reviewed by Blain and Andrew

(1977) and Andrew (1978, 1984). Thornber (1975, 1976) and Thornber and

Wildman (1984) provide experimental data on the chemical and electrochemical

processes of gossan formation, and associated formation of carbonates,

sulphates and oxide minerals. Mann (1984) and Webster and Mann (1984)

studied the mechanisms of precious metal mobilisation effects of climate and

geomorphology on gossan formation.

The following section consists of a literature review of gossan forming processes,

gossan geochemistry and geochemical profiles, element mobility, gossan

typology and mineralogy, especially those developed above polymetallic, pyrite

hosted Cu-Pb-Zn massive sulphide deposits and/or Au-bearing, sulphide-rich

orebodies, similar to the Las Cruces massive sulphide.

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Chapter 3 Gossans

3.2 The Gossan Forming Process

Most hypogene sulphide minerals are unstable under near-surface weathering

conditions, particularly in the presence of weathering agents such as water-

dissolved oxygen, carbon dioxide and ionic species. These cause the sulphide

body to re-equilibrate electrochemically (Blain and Andrew, 1977) and the

sulphide minerals oxidise to form sulphates and the metal-sulphur bonds are

broken, releasing metal cations that are either dissolved in the co-existing

groundwaters or precipitated as insoluble oxidate minerals. This gives give rise

to more stable secondary sulphide and oxide mineral assemblages.

The residues of Fe-bearing minerals and varying amounts of introduced silica,

are commonly the most abundant constituents of a gossan above massive

sulphides (Blain and Andrew, 1977). As sulphide minerals corrode to stable

oxide, carbonate and sulphate phases near the water table, they become

disconnected from the main sulphide ore zone, are poor conductors and no

longer contribute to the major electrochemical corrosion processes acting on the

orebody (Blain and Andrew, 1977).

At the water table, a dramatic increase in Eh results in decomposition of

Fe-sulphides, producing goethite and a low pH environment (Taylor and

Sylvester, 1982):-

4FeS2 + 10H2O + 15O2 → 4FeOOH + 8SO42– + 16H+

Thornber and Wildman (1984) compare the results of reacting different ore types

under varying conditions over a wide pH range. They highlight high and low pH

processes of Fe hydrolysis, where Fe is a major metal being released from a

sulphide (e.g. pyrite and/or Fe-S-hosted orebodies).

1. The high pH process (pH>7). Base metals, including ferrous Fe will be

hydrolysed and mixed Fe-Cu hydroxycarbonates and hydroxysulphates

form for Cu, and mixed Fe-Pb hydrocarbonates form for Pb. The Fe is

located in these initial compounds as a green rust where it is effectively

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Chapter 3 Gossans

bound as ferric hydroxide. Subsequent oxidation of this hydroxide

produces no further acid:-

4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3

2. The low pH process (pH<7). Even though some of the ferrous Fe will have

precipitated as an equivalent to Fe(OH)2, the solubility at low pH is such

that sufficient Fe2+ will remain in solution:-

4Fe2+ + O2 + 10H2O → Fe(OH)3 + 8H+

The dissociation of the water molecule during oxidation of Fe2+ and subsequent

H+ production is known as ferrolysis. During this reaction, the pH will fall even

further, so that the gossan forming environment will be at a pH less than 5 and

may be as low as 3. At these low pH values, the base metals are soluble and

only elements such as Se, As, Mo and Sb, are likely to be bound into gossans

(Thornber and Wildman, 1984).

The Eh and pH ranges in the sulphide oxidation zones were first measured by

Sato (1960), who deduced the limiting conditions of natural environments in

terms of Eh and pH functions (Figure 3.1). Sato (1960) attempted to define the

ranges of Eh and pH by direct measurement of mine waters in the vicinity of

oxidising orebodies, deducing the limiting conditions of the environment based on

geological observations.

The results of these investigations define a flattened, wedge-shaped area on the

Eh-pH diagram (Figure 3.1), portraying the chemical conditions under which

sulphide orebodies commonly oxidise. However, Sato’s measurements were

made on waters exposed to atmosphere and it is likely that contamination

occurred, resulting in higher Eh values than were present before mining

commenced.

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Chapter 3 Gossans

Figure 3.1 – Diagram illustrating the zones of weathering in terms of Eh and pH according to Sato (1960).

Anderson (1990) illustrates the Eh/pH environment in terms of the position of

groundwater and describes, in general terms, the associated mineralogy (Figure

3.2). Anderson’s illustration follows a similar trend to that described by Sato, with

the oxidation and enrichment of sulphides being dependant on the oxidation

potential (pO2) of the environment. Anderson (1990) notes that the highest pO2

obtained is that of air, with pO2 decreasing with increasing depth towards the

groundwater table. Within the aerated and recharge zones, sulphide minerals

oxidise to form metal oxides and sulphates. Below the groundwater table marks

the transition between oxidising and reducing conditions where metal sulphides

remain stable.

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Chapter 3 Gossans

Figure 3.2 – Eh/pH diagram at 25oC and 1 atmosphere total pressure, illustrating the relationships between groundwater position and mineral stability ranges (Anderson, 1990).

As well as the oxidation of Fe-sulphides in the massive sulphide deposit,

oxidation of associated transparent gangue minerals and the subsequent

weathering of surrounding wall rocks will also have an impact on the local

mineralogy. Trescases (1992) describes how interstitial fluids produce secondary

minerals and classifies as follows:-

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Chapter 3 Gossans

1. Dissolution – particular affects minerals with a high solubility, including

salts, sulphates and carbonates.

2. Oxidation - an increase in oxidation state, typically through the introduction

of oxygen through percolating groundwaters.

3. Hydrolysis - the reaction of minerals with water.

4. Transformation - solid-state reaction in which the organisation of oxygen

and most silicon ions is retained.

5. Alkalinolysis - weathering in very alkaline environments.

6. Acidolysis - weathering in very acid environments.

These processes may be linked in the weathering environment. Morris and

Fetcher (1987) note that the solubility of quartz increases following the ferrous-

ferric Fe reaction (oxidation). Trescases (1992) remarks that oxidation is a

process that accompanies or follows reaction with water (hydrolysis). The

hydrolysis of common silicate minerals may result in the formation of distinctive

solid residues including gibbsite (allitization), kaolinite (monosiallitization) and

smectitic (bisiallitization) (Trescases, 1992).

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Chapter 3 Gossans

3.3 Influences On Gossan Formation

3.3.1 Introduction

The pH of the environment is the most important parameter in determining the

initial minerals that form in the gossan. A high pH may result from buffering by

wall-rock minerals, a low Fe content and a high metal to sulphur ratio in the

sulphide.

Blain and Andrew (1977) note that the weathering environment is complex, and is

governed by climate, geology, geomorphology and groundwater movement.

Depth of sulphide oxidation, depth and stability of the water table, degree of

profile erosion and groundwater salinity also influence the composition of the

base metal gossans (Andrew, 1984).

Studies by Mann (1984) on pH and the effect of acid buffering on gossans within

the Yilgarn Block, Western Australia suggest that the ferrolysis reaction produces

very acid pallid zone profiles. Ground water seepages with pH values less than

2.5 are common. If the bicarbonate ion (e.g. from weathering of basic rocks) is

present this acid production may be neutralised (Mann, 1984). The Eh-pH

environment in the oxide zone and activities of radicals such as carbonate,

sulphate and silicate dictate the stability of secondary oxidate minerals in the

gossans (Andrew, 1984).

The effects of climate, geomorphology and groundwater movement on the

mobility of Au and Ag through the weathering profile of the Upper Ridges mine

Papua New Guinea, and at Westonia, Western Australia are discussed in detail

by Webster and Mann (1984).

Boyle (1995) describes the weathering profile of the Murray Brook deposit, New

Brunswick and lists factors affecting the rate of oxidation of the orebody and Au

and Ag mobility, including:-

Position of the sulphide body in the hydrologic regime

Diffusion rate of atmospheric O2 into the sulphide body

Flow characteristics of O2-CO2 bearing groundwater through sulphides

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Chapter 3 Gossans

Strength of electrochemical processes through the sulphide body

Primary and secondary porosity and permeability

Type and abundance of sulphides

Type and abundance of gangue minerals

Grain size distributions (reactive surface area)

Degree and types of microbiological activity

Climate

Ambient and internal temperatures

Nature and composition of surrounding wall rocks

Geometry and structure of the sulphide body

Microbiological activity aside, these factors may be attributed to one of three

criteria affecting pH, namely primary geology, climate and geomorphology.

3.3.2 Effect of Primary Geology on pH

Primary ore composition affects the ability to generate acid during near-surface

weathering conditions, particularly the Fe content, metal to sulphur ratio and the

grain size or reactive surface area of the sulphide minerals. Also, the gangue

minerals, particularly carbonates, may buffer the acid solutions and their grain

size and composition will determine the degree of acid buffering that will occur.

For example, calcite will react more rapidly as an acid buffer than siderite.

Low pH and the redistribution of Au and Ag in lateritic weathering profiles appear

to be more common over granitic and gneissic basement and may be inhibited by

carbonate in the weathering zone of basic rock sequences (Mann, 1984). The

leaching effect of pyrite-rich ore during oxidation is inhibited by reactive gangue

such as carbonate or mafic silicates (Andrew, 1984).

At Sierra de Cartagena, SE Spain, the presence of pyrite and marcasite produces

low pH conditions (~3, as indicated by the presence of jarosite), resulting in a

greater development of Fe- and Mn-oxides. Conversely, in ores where most of

the Fe is in the form of magnetite and siderite metal leaching is less pronounced

(Lopez Garcia et al., 1988).

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Chapter 3 Gossans

The low Fe-sulphide content of a Zn-Pb lode at Dugald River, North-West

Queensland limits the acidic solutions formed by oxidation reactions. Any acid is

rapidly buffered by reaction with dolomite and calcite in the gangue and wall

rocks. Oxidation of sphalerite, galena and the Fe sulphides has produced

sulphate-rich, mildly acidic solutions. Elements that are normally leached are

only partially leached, and occur in secondary sulphate minerals (Taylor and

Appleyard, 1983).

The geometry of the orebody and the presence of faulting, folding and other

structural features in the primary ore will also have a significant influence on the

diffusion rates of oxygenating groundwaters and atmospheric O2 as will the

primary and secondary porosity developed during early leaching of reactive

phases such as Fe-sulphides and carbonates. Removal of reactive minerals at

the early stage of oxidation may result in a significant increase in porosity and

rapidly accelerate the gossan forming process.

3.3.3 Effects of Climate on Gossan Formation

The most important climatic factor in the formation of gossans is the volume of

moisture derived from rain, mist and less commonly snow (Butt and Zeegers,

1992). Rainfall affects the position/depth of the water table, flow characteristics

of O2- and CO2-bearing groundwater, groundwater salinity, acidity (pH) of metal-

bearing solutions and overall groundwater composition (Boyle, 1995). Moderate

intensity rainfall is more effective at entering the soil than intense rainfall. High

intensity rainfall has a tendency to impact the surface of the soil and may runoff

rather than infiltrate, particularly if vegetation is sparse or a surface crust has

developed. Conversely, light rainfall may be trapped by vegetation and not reach

the weathering profile (Butt and Zeegers, 1992).

The Rio Tinto gossan and associated secondary mineralisation formed during the

late Miocene in a near sub-tropical climate with short periods of seasonal high

rainfall (Leistel et al., 1994) producing a gossan and supergene zone of between

80-120 metres thick (Almodovar et al., 1997). Here, Kosakevitch et al. (1993)

observed gossans 'transportado' formed by flash floods during periods of high

rainfall.

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Chapter 3 Gossans

Today, mine waters at Broken Hill are carbonate-sulphate waters of low salinity

and near-neutral pH. However, the region has recently emerged from a period of

aridity when the water table was at least 1000 feet lower than it is today. The

effect of aridity on groundwater is normally to greatly increase the concentration

of dissolved salts, especially chlorides (Taylor, 1958). The presence of chlorides

substantially increases the mobility of Pb, Au and Ag. The oxidation of sulphide

ores in arid regions is known to occur as deep as 800m below the present

surface (Blain and Andrew, 1977).

At the Upper Ridges mine in Papua New Guinea, heavy rainfall in a region of

rugged relief dilutes groundwater and prohibits the development of acid

conditions on a regional scale. At Westonia, Western Australia an arid climate

combined with low relief has brought about the formation of acidic, saline

groundwaters in the Fe-rich laterite profiles, creating ideal conditions for Au

transportation as AuCl4– (Webster and Mann, 1984).

Temperature influences the rate of chemical reactions and moisture has

significance as a reagent and transport medium for other reagents and reaction

products. The rate of reaction increases by a factor of 2 for every 10oC (Butt and

Zeegers, 1992).

Temperature is greatest in tropical areas. Cloud and vegetation cover reduce the

effectiveness of solar radiant energy, but act as insulators during the night, so

temperature extremes are less likely. Deserts and less well-vegetated areas

often experience extremes of temperature. Temperature declines with altitude.

Temperature will also have an impact on groundwater movement through

evaporation, including laterite and evaporite formation (Butt and Zeegers, 1992).

3.3.4 Effects of Geomorphology on Gossan Formation

Geomorphology impacts on gossan formation by controlling the drainage. By

affecting the movement of groundwater, it has a direct impact on pH and

therefore the gossan forming process.

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Chapter 3 Gossans

The geomorphology may affect the diffusion of atmospheric oxygen into the

orebody and subsequently affect the depth of oxidation. Highly erratic and

undulating oxide/sulphide interfaces may reflect irregular surface topography

prior and during oxidation. In stable areas, the depth of zonation commonly

relates to an even land surface and a present-day water table (Blain and Andrew,

1977).

Geomorphology may provide an indication of previous climatic events, such as

glaciation. Pleistocene glaciation, dominant in the higher latitudes, may have

destroyed near-surface gossans. Conversely, gossans in lower latitudes may

reflect sub-aerial weathering over tens or hundreds of millions of years (Butt and

Zeegers, 1992).

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Chapter 3 Gossans

3.4 Element Mobility and Gossan Profiles

3.4.1 Introduction

Hydromorphic dispersion is the mobilisation of elements by groundwaters during

weathering and is affected by the presence of other dissolved species, the

interactions between the solutions and mineral surfaces and the oxidation state of

the elements being taken into solution (Thornber, 1992).

During gossan formation, metals which are more mobile than Fe are significantly

leached from the system. A proportion of the original metal content is commonly

fixed in the oxide zone as stable oxidate minerals, or as adsorbed or co-

precipitated metal on finely crystalline goethite, metal-rich colloids or even on gel

silica (Blain and Andrew, 1977).

Some elements behave comparably during near surface weathering conditions

and distinct geochemical profiles may occur, resulting in a characteristic

mineralogy. These are discussed by Andrew (1984), Leistel et al. (1998), Garcia

Palomero et al. (1986), Nunez et al. (1987) and Taylor and Sylvester (1982).

Sulphide deposits may have been subjected to complex weathering histories.

Generally, cations are far more mobile at low pH and anions at high pH.

Therefore, the resultant geochemical and mineralogical profiles may differ

considerably (Blain and Andrew, 1977). Scott et al. (2001) refer to this complex

history as gossan maturity and compare and contrast several orebodies from the

Lachlan Fold Belt, NSW, Australia.

The ideal mature gossan profile above weathered sulphides consists of a zone of

supergene sulphides overlain by zones of secondary sulphate minerals,

carbonate minerals and phosphate minerals, before moving into the Fe-oxide

dominated surficial material (Scott et al., 2001) (Figure 3.3).

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Chapter 3 Gossans

Figure 3.3 - Idealised zones in the weathering profile of a VHMS Zn-Pb-Cu deposit that has been weathered to produce a mature gossan profile (Scott et al., 2001).

If these zones are not present, then the profile is considered to be immature.

These are typically enriched in Cu, Pb and Zn but depleted in As and Sn (Scott et

al., 2001). Immature gossans are discussed by Nickel (1984) and Thornber and

Wildman (1984) who conclude that pH plays a key role in metal dissolution,

mobilisation and reprecipitation.

Gangue minerals contribute trace elements as a result of acid-buffering reactions

or residual enrichment. Silica, supplied by the hydrolysis of silicate gangue, plays

an important role in the formation of gossans (Andrew, 1984). During supergene

oxidation of the sulphide minerals, metals such as Cu, Ni, Zn, Co and Pb are

initially released into solution at high concentrations (Thornber and Wildman,

1984). Patterns in element behaviour during oxidation, together with the

associated mineralogy are discussed in greater detail in this section.

3.4.2 Fe

Fe is the most abundant and probably most important element associated with

gossans formed during weathering of pyrite-rich orebodies and because of its

particular hydrolysis properties, is the least mobile of base metals in oxygenating

waters (Thornber and Wildman, 1984).

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Chapter 3 Gossans

At extremely low pH, the retardation in the oxidation of ferrous to ferric ion and

the low activity of hydroxyl prevent the in situ precipitation of goethite, and the

ferrous ion may be transported varying distances before it precipitates as ferric

oxides (Blain and Andrew, 1977).

Figure 3.4 illustrates the stability relations for some of the common Fe minerals at

25oC and the conditions under which Fe2+ and Fe3+ are stable in solution.

Pyrrhotite is stable under strongly reducing conditions at near neutral pH. Below

pH 7, the Fe-sulphides are relatively unstable, with the area of solubility

increasing markedly at lower pH. Fe2+ is soluble under a wide range of Eh

conditions at low pH. Fe3+ is only soluble under the most extreme oxidising, acid

conditions. Under strongly reducing conditions and high pH, hematite may be

reduced to magnetite. Hematite is the stable phase over a wide range of Eh

conditions, particularly at moderate to high pH.

Goethite (ideally -Fe3+O(OH)) and hematite (ideally Fe2O3) are the dominant Fe-

bearing minerals in all gossans examined during the literature review, including

Rio Tinto, Spain (Williams, 1950; Vinals et al., 1995), Lagoa Salgada, Portugal

(Oliveira et al., 1998), Teutonic Bore, Australia (Nickel, 1984), Mt Lyell, Tasmania

(Solomon, 1967), Murray Brook, Canada (Boyle, 1995), 18 gossans of the

Lachlan Fold Belt, Australia (Scott et al., 2001), Mugga Mugga, Australia (Taylor

and Sylvester, 1982) and lateritised gossans from Brasil (Angelica et al., 1996).

Jarosite (ideally KFe3(SO4)2(OH)6) is a common accessory mineral at Rio Tinto,

Murray Brook and the Lachlan Fold Belt.

Goethite and hematite typically occur as massive botryoidal aggregates

deposited in cavities (Kosakevitch et al., 1993; Williams, 1950; Vinals et al., 1995;

Nickel, 1984 and Solomon, 1967). Boxwork textures in hematite and goethite are

rare or absent at Rio Tinto (Williams, 1950), Lagoa Salgada (Oliveira et al., 1998)

and Flambeau (Ross, 1997). Boxwork textures are more common in the less

mature (medium-high pH) gossan profiles of Teutonic Bore (Nickel, 1984) and

Murray Brook (Boyle, 1995). In mature (low pH) gossan profiles, boxwork

textures are often destroyed as the Fe is taken into solution and reprecipitated as

botryoidal aggregates (Williams, 1950; Scott et al., 2001).

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Chapter 3 Gossans

Figure 3.4 – Eh/pH diagram illustrating the stability relations between iron oxides and iron sulphides in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10-6. Boundaries of solids are for total ionic activity of 10-6 (Garrels and Christ, 1965).

Fe may be mechanically and/or chemically transported out of the sites of the

original sulphide grains. Breccias commonly form by the mechanical transport of

clasts and are subsequently recemented by a chemically transported Fe-oxide

matrix (Blain and Andrew, 1977). The Rio Tinto gossan ‘transportado’ consists of

a jumbled aggregate of silicified rock fragments cemented by goethite (Williams,

1950). The gossan is composed of layered precipitates of colloidal Fe-oxides with

clastic material and indications of an old pedogenesis (soil development) at the

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Chapter 3 Gossans

top of the gossan (Kosakevitch et al., 1993). Pebbles of chert and botryoidal

masses of hematitic occur within a gossan conglomerate at Mt Lyell (Solomon,

1967).

3.4.3 Au and Ag Element Mobility

Native Au is almost inert under the physio-chemical conditions prevailing in most

weathering environments and Au only becomes geochemically mobile where

complexing anions are present in groundwaters (Gray et al., 1992) (see Figure

3.5). Garrels and Christ (1965) note that the Eh/pH diagram for Au in oxygenated

water is simple, in that only native Au appears, with no ions exceeding 10-6.

The mobility of Au and Ag in the weathering environment has been well

documented and a number of distinct mechanisms for Au and Ag dissolution,

mobilisation and reprecipitation are recognised. The mobility of Au and Ag is of

particular interest in this study as these precious metals form the main focus of

economic interest in the Las Cruces gossan.

Several ligands have been proposed and investigated for complexing with Au in

low-temperature, oxidising groundwaters (Ross, 1997). Although many ligands

have the ability to bond with Au+, only a few occur in natural groundwater

systems in sufficient quantities to induce Au mobilisation (Vassopoulos and

Wood, 1990).

Acidic, oxidising, saline groundwaters in an Fe-rich environment are the ideal

conditions for Au transportation as AuCl4- (Webster and Mann, 1984; Koshman

and Yugay, 1973; Williams, 1933-34; Mann, 1984). Au solubility as a chloride

complex has been demonstrated to occur in a restricted pH-Eh environment;

pH<5.5; Eh >0.9V; aCl->10-3.2 (Cl- activity) (Webster and Mann, 1984). Figure 3.5

illustrates that in the presence of high chloride, Au is soluble in certain acid,

oxidising solutions.

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Chapter 3 Gossans

Figure 3.5 – Eh/pH diagram illustrating the stability relations of some Au compounds in water at 25oC and 1 atmosphere total pressure at total dissolved chloride species of 100 and sulphur activity of 10-1. Boundaries of solids are for total ionic activity of 10-6 (Garrels and Christ, 1965).

Experimental evidence suggests that very acid chloride solutions generated by

ferrolysis are responsible for the dissolution of Au and Ag (Mann, 1984). Natural

waters in near-surface conditions will be oxidised by the atmosphere and can

contain abundant chlorine from the dissolution of salts (Ross, 1997). The

dissolution of Au to form a Au-chloride complex is expressed in the chemical

reaction below.

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Chapter 3 Gossans

4Au0 + 16Cl- + 3O2 + 12H+ → 4AuCl4- + 6H2O

The reaction requires the presence of oxygen, acid (H+), and a notably large

concentration of chloride ions (Mann, 1984). Such chlorine-rich groundwaters

have been sampled at Kalgoorlie, Western Australia, where Cl concentrations

ranged between 21,000 to 107,000mg/L (Grey et. al., 1992).

Further examples of Au and Ag believed to have been remobilised as chloride

complexes under oxidising conditions include Westonia and Yilgarn Block,

Western Australia (Webster and Mann, 1984, Mann, 1984) where dissolution of

Ag and/or Au is a relatively frequent occurrence along the rim of nuggets,

especially when there are adjoining Fe-oxides.

Secondary Au precipitated by reduction at the site of Fe oxidation is often of

higher fineness (lower Ag content) than the primary Au (Webster and Mann,

1984; Mann, 1984 and Saunders, 1991). This feature can be readily explained

by the nature of Au- and Ag-chloride complexes and their behaviour under near

surface weathering conditions. Following the release of Au and Ag from primary

Au and electrum grains as chloride complexes, the supergene solutions migrate

downward through the weathering profile and reducing conditions are

encountered. The Au-chloride is subsequently re-precipitated by reduction of the

AuCl4- ion with Fe2+ (Mann, 1984).

4AuCl4- + 3Fe2+ + 6H2O → Au0 + 3FeOOH + 4Cl- + 9H+

This reaction is thought to occur near the water table, where Fe2+ would be

present in the weathering profile (Mann, 1984).

Ag-chloride complexes are not initially affected by the encounter with reducing

conditions, because of the relative redox potentials of Fe2+/Fe3+ and Ag/AgCl0

(Mann 1984) and will remain in solution, migrating downward (Saunders, 1991).

The solubility of Ag is comparatively high as a chloride complex with respect to

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Chapter 3 Gossans

Au (Webster, 1986) and therefore the refinement of Au in supergene process is

the product of the different stabilities of Au- and Ag-chloride complexes

(Saunders, 1991).

The proposed mechanism of Au and Ag transportation as chloride complexes has

also been suggested for gossans associated with modern day seafloor sulphides.

Hannington et al. (1988) have documented precious metal-bearing grains from

supergene zones from the Mid Atlantic Ridge and note the relatively high purity of

the native Au grains.

Au and Ag mobilisation as chloride complexes is not the only viable mechanism

and a number of other possibilities are discussed in the literature. Garrels and

Christ (1965) suggest that under neutral to alkaline reducing conditions, Au is

soluble as a AuS- complex (Figure 3.5). More recent experimental works by

Vlassopoulos and Wood (1990) show that in groundwaters circulating through

oxidising orebodies, Au(S2O3)23- (thiosulphate), AuHS0 and Au(HS)2

- are the

stable solution species. Webser and Mann (1984) also suggest that thiosulphate

complexes are the stable species under alkaline oxidising conditions, citing

examples including the Upper Ridges Mine, Papua New Guinea, where, under

neutral to basic, moderately oxidising conditions, found in the vicinity of the

weathering carbonate veins, Au and Ag may be complexed by thiosulphate to

form Au(S2O3)23- and Ag(S2O3)2

3- or a mixed complex. Au of low fineness (high Ag

content) is re-precipitated by reduction at the water table, as, unlike the chloride

complexes, both the Au and Ag thiosulphate complexes destabilise under similar

pH and Eh conditions (Webser and Mann, 1984).

Thornber (1992) comments, however, that although sulphate complexes are

more stable than chloride complexes, because most natural waters have higher

activities of chloride than sulphate, chloride complexes are more important for

geochemical mobility.

Boyle (1995), on the Murray Brook precious metal-bearing gossans, notes that

during progressive oxidation and physico-chemical erosion of the gossan zone,

Au was transported downward in the groundwaters, probably as an Au0 colloid

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Chapter 3 Gossans

complex, to be concentrated in the lower horizons of the gossan profile. Boyle's

hypothesis was based on leaching experiments and microprobe analyses, which

indicate that Au is present in the gossan as sub micron composite sols of Au-Ag-

silica.

A number of other mechanisms for Au and Ag mobility are mentioned in the

literature. These include organic ligands, such as humic acid, cyanide complexes

CN- or SCN-, which can form locally from biogenic processes (Webster and

Mann, 1984). New thermodynamic data and theoretical calculations for gold

hydrolysis demonstrate that in conditions prevailing for most supergene waters

the complex that should control the solubility is AuOH(H2O)0 rather than AuCl4-

(Vlassopoulos and Wood, 1990).

Although it is generally accepted that only one complexing agent is active in a

deposit (Mann, 1984), Angelica et al. (1996) suggest that more than one

complexing agent may have been active at different stages of gossans

development. Angelica et al. (1996) describe a lateritised gossan in Brasil and

suggest the most accepted model for the dissolution of Au during the gossan

formation in this case is through thiosulphate complexes, in oxidising, neutral to

alkaline environments. In a second stage, however, the authors suggest that

during the laterisation of pre-existing gossan, other physiochemical conditions

may have prevailed in a more oxygenated environment, resulting in a new

remobilisation of Au through the combination of humates, thiocyanates and also

H2O-OH complexes.

Recent studies (Lengke and Southam, 2005; Reith and McPhail, 2006) have

shown that bacteria in the natural environment may play an important role in both

the mobilisation and reprecipitation of Au and other metals. Experimental studies

by Reith and McPhail (2006) have shown that aerobic and anaerobic microbiota

in auriferous soils from the Tomakin Park Gold Mine, New South Wales, Australia

are capable of dissolving finely disseminated Au bound within the soil fractions.

In the anoxic experiment, the maximum concentrations of solubilised Au were

lower than that of the oxic experiment. The authors show that Au can be

solubilised in in vitro studies with heterotrophic bacteria and found that Au amino

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Chapter 3 Gossans

acid complexes dominated. When the amino acids are utilized more rapidly than

they are produced, the Au-ions, if present in solution, are left without complexing

ligands and become unstable in solution, precipitating and/or re-adsorbing to the

solid soil phases.

Southam and Beveridge (1996) have shown that octahedral gold was formed

through indirect bacteria involvement when organic acids were released from

dead bacteria, which then formed complexes with gold in solution and finally

transformed to crystalline octahedral gold.

In carbon limited system such as quartz/Au veins, the resident microbiota also

released Au, but the Au release appears to be linked to a different microbially

mediated Au solubilisation process, probably Fe or sulphide oxidation. Fe- and

sulphur-oxidising bacteria such as strains of Acidithiobacillus sp. and

Leptospirillum sp. have been observed to mediate the release Au by breaking

down the sulphides in sulphidic Au ore (Reith and McPhail, 2006). The organisms

use Fe2+ and sulphide as electron donors in their metabolisms and oxidise them

to Fe3+, thiosulphate, and sulphate respectively.

Reith and McPhail (2006) note that despite the studies undertaken to date little is

known about the mobility of Au and its interactions with microorganisms in a

complex natural environment. The species or groups of bacteria and other

microorganisms that are important in affecting Au mobility need to be identified

more specifically and the speciation of Au needs to be identified.

3.4.4 Au and Ag Mineralogy and Geochemical Profiles

The review of selected gossans in the literature reveals that similarities occur in

both the precious metal mineralogy and the resultant profiles developed within

the gossan. The mineralogy and profiles are therefore discussed in greater detail

in this section.

Williams (1933-34) describes the Rio Tinto gossan in detail, and, although limited

analytical techniques were available at the time, significant detail on the nature of

the precious metal mineralogy and geochemical profile was obtained. One of the

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Chapter 3 Gossans

key features noted by Williams was the development of a precious metal layer at

the base of the gossan.

Williams describes this enrichment as being due to a concentration of the traces

of Au and Ag that were originally present in the sulphide deposits. Williams

comments that jarosite is the dominant mineral in this earthy, precious metal

bearing layer and as well as Au and Ag, this layer is also marked by enrichment

in Pb, Sb, Bi and Se.

Ag has been identified as cerargyrite and is also probably present as acanthite

(Williams, 1933-34). Much of William's work has been verified by Vinals et al.

(1995) who confirm that Ag is present in a number of forms, including members

of the beudantite-jarosite group of minerals, cerargyrite (plus or minus some

bromide and iodide), acanthite and Hg/Ag sulpho-halides. Vinals et al. (1995)

also confirm the presence of micrometre-sized native Au grains and note that the

majority of the Au contained in the ore is probably submicroscopic.

In the Salomon-Cerro Colorado area of Rio Tinto, Spain, the base of the oxide

zone is 10 to 40m deep and the contact is generally sharp. An earthy precious

metal layer (1 to 2m vertical interval) below the oxide zone, overlies a thin horizon

of leached pyrite. A well developed zone of secondary sulphide enrichment (30

to 40m vertical interval) grades into the hypogene ore (Blain and Andrew, 1977).

At Lagoa Salgada an increase in precious metals (Au and Ag) is evident in its

supergene enrichment zone, with Au contents reaching a maximum value of

2.38ppm. Ag occurs, at least in part, as relatively coarse grains of amalgam (Ag-

Hg alloy) that may be visible in hand specimen (Oliveira et al., 1998).

Lopez Garcia et al. (1988) on Sierra de Cartagena, southeastern Spain note that

in horizon 1, derived from the oxidation of a magnetite and siderite primary

assemblage, Ag occurs mainly as cerargyrite and native metal. In horizon 2,

derived from the oxidation of a pyrite and marcasite primary assemblage, it

occurs principally in jarosite and as native Ag. These differences in Ag

mineralogy are largely controlled by acid generation during oxidation of different

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Chapter 3 Gossans

primary geologies. A contributing factor to the formation of cerargyrite was the

proximity of this area to the sea, which resulted in an important source of wind-

borne chlorine (Lopez Garcia et al., 1988).

In the Eastern Lachlan Fold Belt, NSW, Australia, Scott et al. (2001) describe the

Au and Ag distribution and associations for a number of deposits within the

region. The authors note that for Woodlawn and Currawang, Ag may be severely

depleted in the gossanous outcrop relative to the original ore but substantial

enrichment may occur in the supergene sulphide, sulphate and carbonate zones.

Au is also significantly enriched in the carbonate zone of the gossan, relative to

the primary sulphide.

Scott et al. (2001) also comment that Ag is retained in high concentrations in the

gossans of Kangiara, Lewis Ponds, Peelwood and Mt Costigan and note

correlations between Ag-Pb and Ag-Sb contents. This may relate to gossan

maturity and different weathering susceptibilities of the primary Ag-bearing

phases, as Ag may be present in more than one phase within the primary

orebody. Ag initially concentrates in Cu-rich secondary sulphides, although with

continued weathering, they are concentrated in the Pb-bearing alunite-jarosite

minerals and to a lesser extent the Fe-oxides (Scott et al., 2001).

Au contents typically increase with Ag content in the gossans of the Lachlan Fold

Belt, except in four Ag-rich deposits. Au contents also increase with As except in

the five high As gossans. The Au grades of the gossans commonly represent a

significant enrichment relative to the primary ores, although the immature

gossans are less likely to show the extreme enrichment of some mature gossans

(Scott et al. 2001).

At the Murray Brook deposit, New Brunswick, Boyle (1995) notes a strong

correlation between Au, Sn and Si. However, much of Boyle's microscopic

interpretations appear to be based on a single occurrence of Au in the gossan as

a small grain of Au-Ag-silica gel-like material. Because halide minerals are not

present in the Murray Brook gossan or other gossans in the Bathurst Camp area,

it is unlikely that Au was transported as a halide complex (Boyle, 1995).

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Chapter 3 Gossans

Boyle suggests that the precipitation of silica may have had a controlling effect on

Au concentration and cites Fujii and Haramura (1976) and Fujii et al. (1977) who

have shown that colloidal silica is a good precipitating agent of Au sols, and that

acid silica solutions act as a reducing media for Au3+. As Si solubility decreases

with decreasing pH, groundwaters moving down into the oxidising pyrite-rich

zones would precipitate silica during the oxidation of pyrite (Boyle, 1995).

Boyle's hypothesis for the close association between Au, Sn and Si are to some

degree corroborated by microscopic interpretation of the primary ore. Boyle

remarks that in the primary ore, Sn is concentrated mainly in the pyrite-rich zones

and, because cassiterite has been shown to be very resistant to weathering

processes, the Au-rich zones in the gossan represent the former positions of

primary pyrite-rich zones. Boyle (1995) observed that native Ag in these ores

have a physical appearance similar to physically re-worked grains (e.g. from a

placer), however, much of the Ag occurs in jarosite group minerals with some Ag

in the pyrite-quartz sand occurring as acanthite.

Costa et al. (1999) and Angelica et al. (1996) have studied a number of lateritised

gossans from South America and conclude that Au mineralisation is closely

associated with the Fe oxyhydroxides in the gossans, with a great range of Au

compositions in the different parts of the profile. The higher Au values coincide

with the respectively greater goethite and hematite contents of the profile

(Angelica et al 1996). Ag was detected only in the upper part of the profile with

Cu, Mo, Sn and As also present in high values and exhibiting a good correlation

with Au (Angelica et al. 1996).

Costa et al. (1999) reveal that the gossan elements (Au, As, B, Cu, Mn, Mo, Ni,

Pb, Sn, W, Y and Zn) display good correlations and these persist in laterite,

latosols and colluvium. The authors suggest that this behaviour reinforces the

primary nature of these materials, controlled mainly by minerals that are still

preserved as resistates in the supergene materials. The most important are

dravite (B), wolframite (W), cassiterite (Sn), and Au (Costa et al. 1999).

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Chapter 3 Gossans

3.4.5 Cu

Cu, Pb, As and Sb often exhibit a close correlation in weathering profiles and

their presence and/or absence may also be indicative of the degree of profile

maturity and subsequently reflect conditions under which the gossan has formed.

This is evident in many of the gossan profiles studied in the literature.

In mature gossan profiles, Cu is typically depleted in the uppermost portions of

the gossan, but may be concentrated in the lower gossan within the supergene-

enriched zone in the form of secondary Cu sulphides. These secondary Cu

sulphide minerals may also host a significant proportion of As and Sb as well as

Ag and include chalcocite, enargite/luzonite and chalcanthite (Scott et al., 2001).

Cu is largely absent from the upper part of most mature gossan profiles, including

those of Rio Tinto, Lagoa Salgada and Murray Brook. Angelica et al. (1996)

revealed the presence of bornite, cuprite, malachite, chalcocite, native Cu, azurite

and chrysocolla in the secondary sulphide enrichment zone of a lateritised

gossan in the Amazon region.

The absence of Cu in mature gossan profiles is largely a result of the relatively

high solubility of Cu under the acid, oxidising conditions that often prevail during

the weathering of massive sulphide orebodies (Figure 3.6). Under oxidising,

near-neutral pH conditions, Cu-sulphates may form (e.g. chalcanthite), but their

high solubility often results in rapid redissolution and reprecipitation with Fe

oxides and hydroxides (Anderson, 1990). At high Eh and pH, Cu-oxides are the

stable species. The Cu-sulphides dominate under strongly reducing conditions

(Figure 3.6)

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Chapter 3 Gossans

Figure 3.6 – Eh/pH diagram illustrating the stability relations of some Cu minerals in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10-1, CO3 activity of 10-3

(Anderson, 1990).

Less mature gossans may contain elevated levels of Cu. Cu is particularly

abundant in the surficial gossan of the immature profile at Currawang where it is

present, partly, as malachite, although substantial amounts are also retained in

the Fe-oxides and plumbojarosite (Scott et al., 2001). At Mugga Mugga, Cu is

depleted at the base of the weathered zone, with levels increasing up the profile.

The Cu is incorporated into the hematite structure as well as being adsorbed by

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Chapter 3 Gossans

goethite (Taylor and Sylvester, 1982). The authors also note a similar trend for

Ag in this deposit.

Similarly, the Teutonic Bore contains between 500 and 1000ppm Cu in the upper

part of the gossan, the bulk of which has co-precipitated with Fe, presumably in

the form of Fe-oxyhydroxides. Nickel (1984) refers to Thornber and Wildman

(1984) noting that the coprecipitation of cations is favoured by a high pH, in the

case of the Teutonic Bore, probably resulting from the high level of carbonates in

the groundwater and partly dissolved carbonate species from the ore (Nickel,

1984). At the Dulgald River Lode, Taylor and Appleyard (1983) note that Cu

appears in relatively high concentrations within the bulk of the gossan profile,

indicative of an immature gossan profile developed during near neutral to alkaline

conditions.

3.4.6 Pb

Pb is one of the least mobile metals and is commonly observed throughout a

large number of the gossans in a variety of forms. Figure 3.7 illustrates the

stability fields for Pb compounds under conditions that resemble near-surface

weathering conditions. This illustration serves to confirm that Pb is soluble only

under the most extreme acid or alkaline conditions. Galena is the stable phase

under most reducing conditions, with anglesite and cerussite dominating under

acid and alkaline oxidising conditions respectively (Garrels and Christ, 1965).

Scott et al. (2001) comment that Pb is strongly retained in both mature and

immature gossans of Woodlawn and Currawang respectively. Despite its

immobility, the mineral hosts for Pb change significantly during weathering. The

great bulk of the Pb in the primary ores reviewed here occurs in the form of

galena. However, the respective gossans typically exhibit a wide variety of Pb-

rich species. Nickel (1984) notes that for the Teutonic Bore, Pb has been found

as a major component in twelve secondary minerals, the chief ones being

cerussite and plumbogummite.

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Chapter 3 Gossans

Figure 3.7 – Eh/pH diagram illustrating the stability relations of Pb compounds in water at 25oC and 1 atmosphere total pressure. Total dissolved sulphur of 10-1, pCO2 of 10-4. Boundaries of solids shown are for total ionic activity of 10-6 (Garrels and Christ, 1965).

Scott et al. (2001) note that high Pb gossans (Pb >4%) are typically immature,

probably due to the lesser abundance of pyrite and hence less acid conditions

during weathering. The authors identify a close association between Pb-As and

Pb-Sb in some gossans but not in the primary ore suggesting that As and Sb are

distributed between several phases in the ore but become associated with Pb in

alunite-jarosite during prolonged weathering. However, in many immature

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Chapter 3 Gossans

gossans, the Pb-As and Pb-Sb associations have not had time to develop and

the Pb is mainly present in oxidate phases like cerussite that typically contain

very low contents of As and Sb (Scott et al., 2001).

At Woodlawn, the carbonate zone contains acicular crystals of cerussite and the

sulphate zone anglesite. The supergene sulphide zone contains anglesite and

relict galena. At Currawang gossans retain boxwork textures, indicating that the

profile is immature, and contains cerussite. The sulphate zone material consists

of dark Fe-oxides with a basic Pb sulphate. Alunite-jarosite minerals are also

present, intergrown with the Fe-oxides (Scott et al., 2001).

At the Dulgald River Lode gossan, Pb minerals include plumbian jarosite,

plumbogummite and anglesite (Taylor and Appleyard, 1983). At the immature

Mugga Mugga gossan, Pb is retained and even concentrated in the lower part of

the profile, where it occurs as secondary sulphate, arsenate and phosphate

minerals of the alunite-jarosite series. Pb is, however, depleted in the upper part

of the profile (Taylor and Sylvester 1982). Pb has co-precipitated with Fe-oxides

in the immature gossans of the Teutonic Bore (Nickel, 1984).

Oliveira et al. (1998) note that the Lagoa Salgada gossan contains high Pb and

As values in the form of mimetite crystals. Williams (1933-34) notes a marked

enrichment in Pb at the base of the Rio Tinto gossan associated with the

precious metal layer and Vinals et al. (1995), revealed that Pb occurs as solid

solutions of beudantite-plumbojarosite-potassium jarosite. Pb was also detected,

but only occasionally, as anglesite associated with gangue species (Vinals et al.,

1995). Cerussite may also be present in minor amounts (Williams, 1933-34).

Lopez Garcia et al. (1988) on Sierra de Cartagena, south-eastern Spain,

comment that Pb was leached from the primary ores and precipitated as Pb- and

Ag-bearing jarosites, anglesite, cerussite, Pb-bearing coronadite and goethite.

The Pb bearing minerals of the Sierra de Cartagena gossan differ depending on

the composition of the primary ore. Anglesite, cerussite, Mn-oxides and goethite

occur in horizon 1, a gossan formed under weakly acid conditions resulting from

a low Fe-sulphide content in the primary ore and acid buffering from associated

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carbonates. Pb occurs in Mn-oxides and jarosite in horizon 2, a primary geology

rich in pyrite, forming strongly acidic conditions during weathering. In addition,

the ore textures also differ from one horizon to the other with pseudomorphic

textures frequently observed in horizon 1, but in horizon 2 primary textural

features have largely been obliterated (Lopez Garcia et al. 1988).

Supergene galena occurs throughout the transition zone of Broken Hill, Northern

Zimbabwe. Taylor (1958) relates this considerable migration of Pb in the zone of

weathering to a former period of aridity and increased salinity of the groundwater.

Cerussite is abundant in the oxide ore. Taylor highlights the abundance of

pyromorphite as an indication that the chloride ion is present. Blain and Andrew

(1977) also conclude that solutions enriched in chloride and bicarbonate ions

favour the dissolution of galena, thus enhancing the Pb content of the solutions

from which secondary sulphides may subsequently precipitate.

3.4.7 As and Sb

Arsenic and Sb are often closely associated in ores and gossans. In the

Currawang and Woodlawn deposits, the As and Sb occurs predominantly in

tetrahedrite-tennantite and enargite-luzonite solid solution series in the primary

ore and in alunite-jarosite minerals in the gossan, whereas in more As-rich

primary ores of the Lachlan fold belt, As is largely present as arsenopyrite,

occurring as scorodite in the profiles of immature gossans (Scott et al., 2001).

The gossan of the Dulgald River Lode contains elevated levels of As and Sb

possibly introduced as a result of leaching of the surface gossan (Taylor and

Appleyard, 1983). Similarly, in the Mugga Mugga massive sulphide deposit of

the Yilgarn Block, Taylor and Sylvester (1982) note that the anomalous

concentrations of As and Sb in the surface gossan result from the precipitation of

secondary Pb-bearing minerals of the alunite-jarosite series. The authors note

that the concentration of As immediately above the water table in secondary Pb

minerals is followed by a trend of slightly increasing As content up the profile.

This distribution reflects the low mobility of As in weakly acidic solutions and its

ready co-precipitation with Fe-oxides (citing Boyle and Jonasson, 1973).

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Similarly, at Murray Brook, Boyle (1995) notes that Pb, As, Sb and Bi correlate

strongly and occur within specific horizons. As with Dulgald River, Currawang

and Woodlawn, these elements are typically associated with the precipitation of

jarosite-group minerals within the gossan. Boyle also notes that borehole

sections rich in these metals contain lower than average Au contents, indicating

that precipitation of these hydroxyl-sulphate-oxide minerals has had little control

on the localisation of Au.

Williams (1933-34) notes enrichment in Sb associated with the precious metal

layer at the gossan/sulphide contact of the Rio Tinto deposit. Vinals et al., (1995)

comment that As was detected in members of the beudantite-plumbojarosite-

potassium jarosite solid solutions, appearing as powdery aggregates of zoned

and skeletal crystals, which could suggest a formation through successive

crystallisation re-dissolution processes. Sb was observed as fine-grained oxides

of the stibiconite-bindheimite group (Vinals et al., 1995).

Vink (1996) predicts that under both acid and alkaline oxidising conditions, Sb is

highly mobile as SbO3-(aq) and as Sb2O4

2-(aq) under strongly reducing alkaline

conditions. In the absence of sulphur, As is highly mobile under almost all

conditions, with native As only occurring under very strongly reducing conditions.

The high mobility of As means that arsenate and arsenite ionic species are widely

available for forming precipitates with many types and combinations of cations,

hence the wide variety of As-bearing species often observed in gossans (Vink,

1996).

3.4.8 Si, Sn and Ti

The breakdown of silicate minerals during the gossan forming process may result

in the supersaturation of SiO2 in the mineralising solutions. Below pH 9, silica is

in solution as the uncharged molecule Si(OH)4 and above pH 9 as Si(OH)4-

(Thornber, 1985).

Si is a common constituent of the gossans reviewed during this investigation,

occurring predominantly as quartz. Quartz is essentially a resistate phase and

exhibits a close correlation with other resistate phases, including cassiterite (Sn)

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and TiO2 in a number of the gossans. A significant proportion of the Si content of

these gossans is, however, present in the form of remobilised Si that appears to

have formed as a result of the dissolution of wall rocks and associated Si-rich

gangue minerals. This is typically followed by the subsequent reprecipitation of

the Si, largely as chert/jaspers in specific zones in the gossan. Blain and Andrew

(1977) note that it is quite likely that the acid buffering, hydrolysis reactions of

silicate wall rocks account for the release of silica.

The dissolution and mobilisation of Si is evident in the sub-rounded nature of

quartz grains in the gossan of the Flambeau mine, Wisconsin, U.S.A. Ross

(1997) suggests that rounding the quartz grains occurs during dissolution of

quartz grain edges by acidic supergene alteration fluids. Citing Morris and

Fletcher (1987), Ross (1997) also proposes a hypothesis that a reaction between

ferrous Fe in solution and quartz may have formed a thin layer of ferrous silicate

that would subsequently oxidise to form a hydrous Fe oxide (goethite), while

rapidly releasing silica into solution. Thus, the presence of ferrous Fe would

greatly increase the solubility of quartz, as opposed to the solubility of quartz in a

solution devoid of ferrous Fe (Ross, 1997). May (1977) describes a 5 metre thick

gossan and siliceous cap that is in sharp contact with the massive sulphide of

Flambeau.

Boyle (1995) describes the dissolution, reprecipitation and subsequent

accumulation of silica in the Murray Brook gossan. The bulk of the silica is

present as euhedral quartz and in lesser amounts, as amorphous silica. The

author notes that most of the quartz exhibits a chalky white appearance due to

attack by acidic solutions. During oxidation, the silicate minerals, and to a much

lesser extent, primary quartz, are dissolved by acidic solutions to form cation

complexes, silicic acid, and colloidal silica. During changes in pH and electrolyte

composition with depth, the colloidal silica becomes unstable in solution and

precipitates lower in the oxidising profile as amorphous silica (Boyle, 1995).

In Mugga Mugga, Taylor and Sylvester (1982) note that there has been an

absolute accumulation of silica immediately above the sulphide zone, occurring in

the form of a massive and slightly ferruginous chert derived from rock weathering

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Chapter 3 Gossans

and deposition from groundwaters. The authors also note that generally SiO2

and Fe2O3 contents vary antipathetically with an overall decrease of SiO2 and

increase in Fe2O3 up the profile.

Solomon (1967) describes cherts associated with the fossil gossan of Mt. Lyell,

Tasmania and compares them to the goethite-veined cherts overlying pyritic ore

on the Cerro Colorado at Rio Tinto, Spain (citing Williams, 1933-34). Oliveira et

al. (1998) note the gossan associated with Lagoa Salgada is more siliceous near

to the base. Taylor and Appleyard (1983) indicate that Si and Sn are essentially

immobile in the weathered zone of the Dulgald River Lode, illustrating the

resistate nature of the primary minerals within which these elements occur.

The presence of remobilised and reprecipitated cherts is also recognised at

Skouriotissa, Cyprus, where Constantinou and Govett (1972) note that the cherts

are common but are restricted to the ochre (oxide) sulphide contact. The authors

also confirm that their deposition was probably controlled by pH conditions, both

directly in their effect on the precipitation of colloidal silica (optimum pH 4.5),

(citing Okamoto et al., 1957) and indirectly as they affected the precipitation of Fe

and aluminium hydroxides (Constantinou and Govett, 1972).

An alternative mechanism for the formation of siliceous materials within gossans

is discussed by Hannington et al. (1986, 1988, 1991a, b, c and d) where

hydrothermal activity has continued intermittently during weathering of modern

seafloor sulphide mounds, resulting in localised silicification.

Sn2+ and Sn4+ are important in the aqueous gossan environment. As a cation,

Sn2+ is quite soluble below pH 5, occurring as the anion Sn(OH)3- above pH 9,

with some solubility of Sn(OH)20 at intermediate pH. Sn4+ is essentially insoluble.

In the primary ore, Sn4+, usually in the form of cassiterite, is highly resistate in

nature and will usually remain in the gossan. Sn2+, often occurring in the sulphide

minerals, may coprecipitate with the Fe-oxides, but will eventually oxidise to Sn4+

and form cassiterite (Thornber, 1985).

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Sn is a common accessory component of several of the massive sulphide

associated gossans described in the literature. Scott et al. (2001), referring to

Woodlawn and Currawang, note that in the primary ore, Sn is present as

cassiterite and stannite (ideally Cu2FeSnS4), with the latter breaking down to

additional cassiterite during weathering. Sn is residually concentrated in mature

gossans (Scott et al., 2001). Oliveira et al. (1998) on Lagoa Salgada note that

the values of Sn are relatively high, especially in the gossans with the bulk of the

Sn occurring as cassiterite (Oliveira et al., 1998).

At Murray Brook, Sn, as cassiterite, is conservative and correlates strongly with

Au, and Si (Boyle, 1995). At Mugga Mugga there is some concentration of Sn

above the water table. There is some suggestion of slight concentration of Sn in

the kaolinite zones indicating possible derivation from the amphibolite rather than

the sulphide mineralisation (Taylor and Sylvester, 1982). At Rio Tinto, Sn was

detected as anhedral grains (5-100um) of cassiterite commonly associated with

Sb oxides (Vinals et al., 1995).

Titanium has very low mobility under almost all environmental conditions, mainly

due to the high stability of TiO2 under all but the most acidic of conditions

(Brookins, 1988). Rutile, brookite and anatase are the naturally occurring

polymorphs of TiO2, with rutile being the most common, particularly in the primary

massive sulphide ores. These forms of TiO2 are highly resistate phases that are

largely retained and often concentrated during the gossans forming process. TiO2

therefore often exhibits a close correlation with other resistate phases, notably

quartz (Si) and cassiterite (Sn). Taylor and Sylvester (1982) comment that at

Mugga Mugga, Ti appears to be concentrated low in the weathering profile,

partially as a result of residual concentration.

Ti also occurs as a minor constituent of other less resistate phases, notably

amphibole and biotite, phases that may be predominantly leached from

surrounding wall rocks. Dimanche and Bartholome (1976) suggest that Ti is not

entirely immobile during weathering. Skrabal (1995) notes that Ti may exist in a

fully hydrated form, TiO(OH)2, in water above pH 2, being transported in a

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Chapter 3 Gossans

colloidal state rather than as a dissolved ion. Hutton et al. (1972) suggest that

Ti4+ is mobilised more readily in the presence of organic acids at pH <4.5.

3.4.9 Other metals

A number of other elements of interest are mentioned briefly in the literature and

are worthy of note. Hg is present as an accessory element in a number of the

gossans studied, probably occurring in solid solution in pyrite and sulphosalt

minerals within the primary ores. Boyle (1995), notes that in the Murray Brook

deposit, Hg is concentrated in cinnabar, the only stable secondary sulphide in the

massive sulphide gossan, and correlates strongly with Au, Sn and Si. At Mugga

Mugga, there is an enrichment of Hg close to the water table but throughout the

remainder of the weathered zone, Hg levels are approximately an order of

magnitude lower than in the fresh sulphide rock (Taylor and Sylvester, 1982).

Scott et al. (2001) on various deposits of the Lachlan fold belt comment that in

the primary ore, Bi can be hosted by pyrite, galena and/or native Bi and

bismuthinite and that Bi in different phases appears to behave differently during

weathering. However, according to the authors, Bi as well as Sn and Au may be

significantly enriched in the carbonate zone of the gossan, relative to the primary

sulphide zone

Because of their extreme solubility in acid solutions, Zn and Cd are generally

highly mobile during gossan formation and are therefore largely absent from even

the immature gossans. Both the mature and immature gossans of Woodlawn

and Currawang are severely depleted in Zn and Cd (Scott et al., 2001). Taylor

and Sylvester (1982) note that for Mugga Mugga, because of its extreme

solubility in acid solutions, Zn is depleted in the weathering zone although some

Zn may be fixed in goethite. Similarly, for Dulgald River, Zn and Cd have been

depleted by between 85 and 90 per cent relative to the primary ore, although

some high Zn is associated with secondary Pb minerals (Taylor and Appleyard,

1983).

Scott et al. (2001) on the eastern Lachlan fold belt note that Ba is mainly present

as barite and is highly variable within the profiles. In addition, secondary barite

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Chapter 3 Gossans

may occur as a result of remobilisation during weathering (from both primary

barite and/or Ba-bearing feldspars). Such secondary barite at Lewis Ponds

contains 2.1% Pb. Other elements and compounds described as highly mobile

under acidic, gossan forming conditions include MgO, CaO, K2O, MnO, S, CO2,

Cu, Co, Ni, Tl, Na and Sr (Taylor and Sylvester 1982; Taylor and Appleyard.

1983).

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3.5 Ancient Seafloor Weathering

3.5.1 Introduction

Thick gossanous Fe-oxides, overlain by Cretaceous pillow lavas and sediments

were found to cap the sulphide deposits on Cyprus (Robertson and Boyle, 1983).

This relationship indicates that some gossans formed on the ancient seafloor,

where massive sulphide deposits were exposed to oxidising seawater.

Fe-Mn-oxide sediments have been recognised in association with the massive

sulphide deposits of the Troodos Massif, Cyprus. Constantinou and Govett

(1972) describe the mineralogy and geochemistry of 'ochres' and 'umbers'

associated with the three major sulphide ores at Skouriotissa, Mousoulos and

Mathiati. The authors suggest that the primary ore, consisting of pyrite with trace

quantities of chalcopyrite and sphalerite contemporaneously or subsequently

underwent considerable oxidative leaching and secondary enrichment under sub-

marine conditions on the Cretaceous ocean floor. Ravizza et al. (2001) suggest

that the Skouriotissa mound may have been exposed to the seafloor for millions

of years.

Robertson and Boyle (1983) describe the Fe- and Mn-rich oxide sediments of the

Troodos Massif in the context of seafloor spreading in a small ocean basin during

the Upper Cretaceous. This produced major cupriferous sulphides and both Fe-

rich (ochres) and Fe-Mn-rich (umbers) oxide sediments. The authors conclude

that field, isotope and fluid inclusion data show that the sulphide ores were

derived by deep leaching of mafic ophiolitic rocks with an important component

derived from seawater. The sulphides formed from solutions released along

major faults located close to sites of lava extrusions near the spreading axis.

Robertson and Boyle examine several ancient deposits from the Mesozoic

Tethys Ocean, including the Troodos Massif (Cyprus), Semail Nappe (UAE) and

Baer-Bassit (Syria).

3.5.2 Ochres

Constantinou and Govett (1972) describe the ochre as a Mn-poor, Fe-bearing

sediment commonly containing sulphides as bands and fragments, enriched in

Cu and Zn, with varying proportions of interbedded chert, tuffaceous material and

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Chapter 3 Gossans

limestone. The ochre, which in some places is separated from the

stratigraphically higher umber by pillow lavas, is restricted to the immediate

surface of the sulphide bodies, and owes its origin directly to oxidation of the

sulphide ore.

The pyrite in the Skouriotissa ochre is detrital, intensely corroded and not in

equilibrium with the enclosing rocks. Pyrite in the ochre is unzoned, whereas

zoned pyrite predominates in the underlying ore. The graded bedding of the

ochres is presumably a detrital feature (Constantinou and Govett, 1972).

Constantinou and Govett (1972) conclude a marine environment of ochre

deposition, postulated on the basis of the interlayering of ochre and limestone at

Mathiati, where the limestone contains no terrestrial microfossils but has algal

filaments and liquid hydrocarbons and the presence of pillow lavas of presumed

submarine origin. The ochres at Mousoulos and Mathiati appear to have been

deposited in a quiet environment, probably in deep water at Mousoulos and in

relatively shallow water at Mathiati. The ochre at Skouriotissa was probably

deposited in a high-energy environment, as testified by graded bedding, fine

banding, pyrite washouts and the dispersal of Fe oxides for up to half a mile from

the orebody (Constantinou and Govett, 1972).

The ores of the Troodos Massif, Cyprus have also been described by Robertson

and Boyle (1983) and placed in context not only with other ancient deposits

formed in the Mesozoic Tethys Ocean, but also with the modern day seafloor

sulphides. The authors describe the ochres as brightly coloured ferruginous, Mn

poor metalliferous oxide-sediments restricted to immediately around and above

the massive sulphide orebodies. The ochres are distinct from sub-aerially formed

gossans. Robertson and Boyle (1983) recognise four distinct types of ochre:-

1. Massive and pseudo-conglomeratic ochre (Mathiati and Mousoulos)

formed by sedimentary transport and collapse upon oxidation, up to

several metres thick above the massive sulphide.

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Chapter 3 Gossans

2. Brown, grey or orange ochreous metalliferous siltstones, mudstones, clays

and volcaniclastics, with nodules of chalcedonic chert (Skouriotissa),

containing detrital sulphides and sedimentary structure indicative of

transport over surface of mineralised lavas and sulphides. Consists of

goethite, minor quartz, feldspar (probably authigenic), mixed layer clays

and some opaline silica. Enriched in Fe, Cu and Zn.

3. Finely laminated brown or grey oxide sediments, often with veins of Se-

bearing gypsum, found either above massive sulphides (Mathiati) or

interbedded with mineralised lavas (Skouriotissa). Consists of goethite,

gypsum, minor quartz and smectite. The Mn content is higher relative to

types 1 and 2.

4. Orange or red ferruginous veins and interstitial oxide sediments within

mineralised pillow lavas, consisting of goethite and hematite, with low Mn

and trace metal contents.

Robertson and Boyle conclude that the Cyprus ochres formed by seafloor

oxidation of sulphides, by erosion of ores and adjacent lavas, and by precipitation

of Mn-depleted ferruginous sediments from hydrothermal solutions released

during and soon after sulphide precipitation. Any Mn precipitated at this time was

remobilised during later stages of volcanism and hydrothermal activity.

More recent studies by Herzig et al. (1991) conclude that the abundance of

jarosite in the Skouriotissa ochres is a clear indication that they were derived

from sulphide oxidation, and their occurrence within a sequence of Cretaceous

pillow lavas and sediments strongly suggest that the oxidation took place on the

sea floor.

3.5.3 Umbers

Constantinou and Govett (1972) describe umbers as Mn- and Fe-rich sediment

essentially devoid of sulphides. The authors conclude that the umber owes little

of its character to the sulphide orebodies, being essentially a degradation product

of the pillow lavas.

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This interpretation of the origins of umbers by Constantinou and Govett predates

our current understanding of black smokers in modern seafloor sulphide deposits.

It is now understood that the umbers are formed by precipitation of Mn from

seawater. The Mn remains in solution in seawater as it exits the black smokers,

often being transported some distance before being precipitated in seafloor

hollows as Mn-rich umbers.

Robertson and Boyle (1983) describe the typical Fe-Mn umber as a finely

laminated oxide-sediment that is distributed in small fault-bound hollows in the

pillow lava surfaces (Figure 3.8). The lavas immediately below the umbers often

comprise of fault talus and are generally highly decomposed, chemically altered

and impregnated with Fe-oxide sediment.

Basal umbers, up to several centimetres thick, are generally bright orange and

overlain by finely laminated darker umber. Higher in the succession, the umber,

often several metres thick, may contain siliceous mudstone intercalations, small

black manganiferous concretions and nodules of chalcedonic quartz. The

umbers then typically pass upward into radiolarian cherts or argillaceous

sediments, then into bentonitic clays (Robertson and Boyle, 1983).

At Skouriotissa, the umbers directly overlie sulphides and lavas and pass

conformably upwards into manganiferous umber. The umbers typically consist of

poorly crystalline goethite, quartz, mixed layer clays and opaline silica. The

umbers are typically FeMn-rich and are particularly enriched in Ba, Co, Cu, Ni.

Pb, V and Zr relative to pelagic clays (Robertson and Boyle, 1983).

Robertson and Boyle (1983) conclude that the umbers precipitated within and

above pillow lavas that were erupted on the flanks of the spreading axis. Fe

precipitated first to form the basal Fe-rich umbers, with Fe-Mn umbers rapidly

accumulating as Mn oxidised. Mn was oxidised more slowly than Fe and

remained in solution, eventually precipitating in the overlying umbers after

volcanism ended.

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Chapter 3 Gossans

Figure 3.8 - An illustration of the field relationships of a typical small umber hollow related to seafloor faulting, Troodos Massif, Cyprus (after Robertson and Boyle, 1983).

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Chapter 3 Gossans

3.6 Modern Seafloor Weathering

3.6.1 Introduction

The presence of Fe- and/or Mn-rich oxide deposits have been recognised in

association with modern seafloor sulphides and are described in the literature by

Hannington et al. (1986, 1988, 1991a, 1991b), Herzig et al. (1991), Knott et al.

(1995), Rona et al. (1993), Hekinian et al. (1993) and Binns et al. (1993),

amongst others. These oxide sediments have been attributed to both primary

and secondary processes, with Fe-rich, Mn-poor deposits resulting from the

alteration and weathering of the seafloor sulphides and Fe- and Mn-rich deposits

resulting from primary deposition from hydrothermal vents.

3.6.2 Modern Seafloor Fe-Oxide and Oxyhydroxide Deposits of Secondary Origin

These deposits relate most closely with the 'ochres' described by Constantinou

and Govett (1972) and Roberston and Boyle (1983) for the Troodos Massif,

Cyprus and consist predominantly of Fe-rich oxide deposits that are present

either in direct contact with, or adjacent to the associated massive sulphides.

Seafloor gossans display a long history of submarine weathering (up to 40,000

years) which is recognized in a complex suite of Fe-oxide assemblages

(Hannington et al., 1991a). These include Fe-oxides with secondary sulphides,

Fe-oxide-atacamite assemblages, Fe-oxide-jarosite assemblages, in situ Fe-

oxide crusts, resedimented Fe-oxide debris and manganiferous Fe-oxide umbers.

Hannington et al. (1991b) describes the major components of the gossanous

sediments at the TAG hydrothermal mount as consisting of coarse sulphide and

Fe-oxide fragments up to several centimetres across, sand-sized detrital Fe-

oxides and pyrite and red or red-brown to yellow Fe-oxide muds. The Fe-oxide

mineralogy consists predominantly of amorphous oxyhydroxides and goethite,

with minor lepidocrocite, akaganeite, and hematite with late-stage Mn seeps

locally replacing pre-existing Fe-oxide gossan, producing ferromanganiferous

umbers. The sand fraction consists of atacamite, siliceous fragments

(amorphous silica, +/- quartz, +/- cristobalite), detrital Mn-oxides, basaltic glass,

and carbonate forams. The silt and mud fractions consist predominantly of Fe-

oxides, jarosite and fine-grained sulphides. Hydrothermal activity has continued

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Chapter 3 Gossans

intermittently during weathering of the mounds, resulting in local silicification of

gossanous material and the formation of ferruginous chert (Hannington et al.,

1991b).

Hannington et al. (1991a) note that mass wasting of the gossans has produced

abundant clastic Fe-oxide debris that is weakly lithified and cemented. The

authors note that coarse debris at the top of the sequence grade downward into

mixed sulphide-oxide sand with fine-grained oxidized material occurring at the

bottom of the cores. The authors conclude that the detrital Fe-oxides are derived

largely from weathered sulphide chimneys with the pyrite being derived from

mechanically weathered chimneys and from large blocks of chemically weathered

anhydrite.

Knott et al. (1995) note that at the Galapagos Rift, massive sulphides occur as

upstanding edifices up to 3m high, projecting through a cover of gossanous

sediments consisting of Fe oxyhydroxides, sulphides and silica. Chalcopyrite was

altered on grain edges fractures to covellite, with trace amount of bornite and

idaite. Amorphous Fe-oxyhydroxides and hydrous Fe-sulphates precipitate

around the sulphides and are preserved as a dusting within silica. Pervasive

oxidation appears to predate silica precipitation.

Knott et al. (1995) associate the oxidation with the retrograde stage as

hydrothermal activity declines, during which, minor oxidation of sulphides by

mixed seawater-hydrothermal solutions generate a low pH. At temperatures

below 250oC, metals, including Cu and Au, become increasingly mobile. Herzig

at al. (1991) recognises this initial stage of weathering by an abundance of

secondary Cu-sulphides and the production of jarosite.

Rona et al. (1993) describe localised extensive weathering of pyrite-rich talus that

has formed along the margins of the TAG hydrothermal mount. This has resulted

in oxidation of the pyrite and the formation of gossans composed of Fe-oxides,

secondary sulphides, atacamite and jarosite with minor covellite and digenite.

The authors note that low temperature hydrothermal Mn-oxide replacement of

existing Fe-oxide gossan has also occurred. Mass wasting of partially oxidised

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Chapter 3 Gossans

sulphides and Fe gossans has also produced abundant metalliferous sulphide-

oxide sediment which is deposited at the base of the talus slope. The main

components of the sediment are coarse sulphide and Fe-oxide fragments, detrital

pyrite and bright red to yellow Fe-oxide mud, with thin layers of Fe-oxides often

protecting the interior of the sulphides against rapid oxidation.

Hekinian et al. (1993) describe the Fe and Si oxyhydroxide deposits of the South

Pacific intraplate volcanoes and East Pacific Rise and note four distinct types of

deposit. Types 1, 3 and 4 are primary in origin and are described in Section 4.6.3.

The ochreous, type 2 oxyhydroxide deposits are of secondary origin and are

associated with sulphides, occurring either coating the exterior or plugging the

interior of sulphide chimneys. They also form powdery ochreous products

resulting from the alteration of massive Fe sulphides, form the cement of sulphide

breccias or occur as loose powdery deposits containing traces of sulphides. This

type of deposits is more commonly associated with dead, off-axis vents and is

commonly coated with a Fe-Mn crust.

Rona et al. (1993) suggest that supergene reactions of older sulphides with

seawater have produced secondary Au enrichment at the TAG hydrothermal

mount. Hannington et al. (1988) revealed the presence of supergene Au grains

up to 15µm in size within the seafloor gossans. The Au grains contained little or

no detectable Ag, although some Cu was present. The authors suggest that Au

may be dissolved from the primary sulphides during oxidation of sulphide

minerals and subsequently transported as Au-chloride complexes (AuCl4-), with

re-precipitation of the Au occurring as a result of increasing pH or reduction by

ferrous Fe.

The supergene enrichment zones of modern and ancient seafloor sulphides and

sub-aerially weathered sulphide bodies are essentially similar, with digenite,

covellite and bornite replacing primary chalcopyrite. Amorphous Si may also be

abundant within the supergene zones. The Fe-oxides may, however, differ from

those described in gossans derived from aerial weathering, with Fe-oxides

commonly found coating primary and secondary sulphide grains. Boxwork

textures have not been observed (Hannington et al., 1988).

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Chapter 3 Gossans

3.6.3 Modern Seafloor Fe-Mn-Si Oxide and Oxyhydroxide Deposits of Primary Origin

Although it is clear that oxidation of seafloor sulphides produce Fe-bearing

oxyhydroxide deposits, research by Hannington et al. (1986, 1988, 1991(a),

1991(b)), Herzig et al. (1991), Knott et al. (1995), Rona et al. (1993), Hekinian et

al. (1993) and Binns et al. (1993) describe Fe-bearing oxyhydroxide deposits of

primary origin that may well represent the dominant Fe-oxyhydroxide deposits in

modern seafloor sulphide environments. These typically MnFe-oxyhydroxide

deposits are akin to the 'umbers' described by Constantinou and Govett (1972)

and Robertson and Boyle (1983).

Hekinian et al. (1993) describe types 1, 3 and 4 Fe and Si oxyhydroxides that are

primary low temperature (<70oC) hydrothermal precipitates, forming edifices,

mounds and flat lying deposits. Type 1 or ‘purple-red’ Fe oxyhydroxide

precipitates form mounds up to 15m in height and consist predominantly of

amorphous Fe oxyhydroxides with subordinate amounts of disseminated

hexagonal globules or close packed lamellae of goethite. The clay-rich type 3

Fe-Si oxyhydroxides are characterised by alternating lamellae and layers of light

greenish yellow nontronite and purple-red limonite-goethite, coated by a dark

brown Fe-Mn crust. Type 3 deposits form concentric concretions of hydrothermal

edifices, flat lying powdery precipitates and irregular slabs. These Fe-Si

oxyhydroxides are coated with Mn-oxide and are associated with inactive

hydrothermal mounds. Type 4 deposits consist of soft, lightweight, gel-like

precipitates of amorphous silica, quartz and amorphous Fe oxide. These opaline

Si-Fe oxyhydroxides form edifices similar to type 1 Fe oxyhydroxides.

Rona et al. (1993) note that presence of low temperature venting and

precipitation of Fe and Mn oxide deposits at TAG hydrothermal mount. Knott et

al. (1995) note that at the Galapagos Rift, present day low temperature

hydrothermal activity also forms Fe and Mn oxide and silica deposits. The

authors recognise that venting of low temperature (<17oC) fluids is precipitating

Fe-Si-Mn oxyhydroxides. Within these deposits, amorphous silica is abundant,

occurring as late, 20µm coatings of globular habit, lining all cavities. Elsewhere,

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Chapter 3 Gossans

the silica may completely coat Fe-Cu sulphides. The authors suggest that

microbial action may play an important role in the precipitation of metals, with

filamentous silica in cavities probably representing coatings on filamentous

bacteria.

Binns et al. (1993) describe Fe-Si-Mn-oxyhydroxide/nontronite deposits of

hydrothermal exhalative origins at the Franklin Seamount, noting that the first

material to form is filamentous silica, likely nucleated on or within microbes. Fe

oxyhydroxide platelets form next, overgrowing or partly replacing the filamentous

silica. Simultaneously or later, Mn-oxides form where hydrothermal fluids and

seawater interact, with much of the Mn-oxide escaping to the deposit surface,

forming crusts as it is oxidised by seawater.

On metal mobility within these primary deposits, Binns et al. (1993), citing

Hannington (1991c and d), note the repeated association of Au and Ag with As,

Sb, Pb and Zn, ascribed to the fact that aqueous sulphur complexes rather than

chlorocomplexes are the transport mechanism in moderate temperature (~250oC)

hydrothermal fluids. The nature and behaviour of Au in the primary deposits of

modern seafloor sulphides is described by Hannington et al. (1986), noting that

the most substantial Au enrichment occurs within the late, low temperature

precipitates associated with the more mature massive sulphide deposits. The

authors describe the close association between Pb, As, Sb and Ag in Au-rich

samples, attributing this to the precipitation of Au with the sulphosalts of these

elements.

Hannington et al. (1986) comment that the bulk composition of the 'Devil's Mud'

gossanous cap found in the ophiolite-hosted massive sulphide deposits of Cyprus

is similar to that of the late, hydrothermal caps associated with the modern

seafloor sulphides. The Devil's Mud is described as containing up to 1 weight

percent Pb together with sulphates of Cu, Fe and Zn as well as up to 50 volume

percent of white friable silica. Prichard and Maliotis (1998) note a strong

correlation between Au and Si in low temperature, off-axis fluids associated with

the ancient Troodos ophiolite.

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Chapter 3 Gossans

3.7 Comparing Modern and Ancient Deposits

Since the onset of detailed mapping and seafloor exploration in the late 1970's, a

greater understanding of the relationships between modern seafloor sulphides

and their ancient equivalents has been achieved. Published works by

Hannington et al. (1986, 1988, 1991(a), 1991(b)) and Herzig et al. (1991)

describe gossan development and associated mineralogy of the modern day

deposits and compare these to the ancient massive sulphide ores of the Troodos

complex.

Constantinou and Govett's paper on the Cyprus ores was written out of context

with modern seafloor sulphides and Fe-bearing oxyhydroxides, as it predated the

discovery of the seafloor sulphides during the late 1970's. Robertson and Boyle

(1983) compared the ancient metalliferous sediments of the Mesozoic Tethys

Ocean with modern seafloor sulphide deposits, noting similarities in mineralogy

and geochemistry of the Fe-Mn-oxide sediments associated with the massive

sulphides and volcanics.

Robertson and Boyle (1983) note that the manganiferous ores in the highly

faulted Ligurian Apennine ophiolite can be compared closely with the TAG

hydrothermal field in the mid-Atlantic Ridge at 26oN, exhibiting similar Mn-rich

crusts and comparable chemistry. The Troodos is similar to the East Pacific Rise

at Juan de Fuca, where discoveries of sulphide- and oxide-sediments have been

made. Similarly at 00o45'N, 86o07'W on the Galapagos spreading axis, Fe-oxide

vents are reported as well as large volumes of dispersed Fe- and Mn-oxide

precipitates. The Fe-oxide 'vents' are similar to the dispersed Fe-oxide ochre in

the Troodos ophiolite (Robertson and Boyle, 1983).

Discovery of both black and white 'smokers' at the East Pacific Rise and the

associated sulphide chimneys are compared with similar structures observed at

the Troodos Mavravouni orebody. Robertson and Boyle (1983) suggest that

ochreous conglomerates associated with sulphides in the Semail lavas could be

collapsed oxidised chimneys.

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Chapter 3 Gossans

Robertson and Boyle (1983) conclude that comparisons of modern oceanic

sediments indicate that high temperature discharge from less rifted, fast

spreading axes produced major stratiform cupriferous sulphide orebodies and Fe-

Mn oxide sediments (umbers). Rifting and slower spreading allowed greater

seawater penetration and favoured formation of small stratiform cupriferous

sulphides and Fe-poor, Mn-oxide sediments. The metals of the Mesozoic

Tethyan rifts and passive margin precipitated from more dilute lower temperature

thermal springs, with varying degrees of trace element scavenging from

seawater. The end product was the condensed Fe-Mn nodules and crusts which

slowly accumulated on sediment-starved seamounts and subsiding platforms.

More recent studies by Hannington et al. (1986, 1988, 1991a, 1991b), Herzig et

al. (1991), Knott et al. (1995), Rona et al. (1993), Hekinian et al. (1993) and Binns

et al. (1993) indicate that although secondary Fe-bearing oxyhydroxides of

secondary origin are a common feature of modern seafloor sulphide deposits,

primary, low temperature Fe-bearing oxyhydroxides from hydrothermal vents are

also abundant and should be considered when characterising ancient massive

sulphides and the weathering processes.

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Chapter 4 Methods of Investigation

4 METHODS OF INVESTIGATION

4.1 Introduction

Characterisation of the Las Cruces mineralogy was achieved using a combination

of transmitted and reflected light microscopy, scanning electron microscopy and

X-ray powder diffraction. The extremely fine-grained nature of the mineralogy

and often the poor polish taken by the fine-grained and porous samples

hampered mineral identification and some of the more exotic mineral

assemblages could not be positively identified.

A small number of reflected and transmitted light photomicrographs were

captured using a Buehler Omnimet ‘Enterprise’ image analysis system and JVC

digital camera. By far the bulk of the illustrations used in this thesis are

backscattered electron images taken on a Leo 440 SEM. The illustrations form

the basis of the borehole descriptions (Chapters 5 to 9) and are provided in

Appendices 6 to 10.

4.2 Sample Preparation

The sections of drill core were examined macroscopically after which one or more

thin slices were cut parallel to the length of the core using a diamond saw. The

resultant slices were examined using a binocular microscope and representative

mineralised areas of each were selected for the preparation of polished sections.

The selected areas were cut out using a diamond saw then mounted in epoxy

resin and polished prior to examination using reflected light microscopy and

SEM-based techniques. A number of slices were also used for the preparation of

thin sections for microscopic examination using transmitted light methods where

appropriate.

Due to the highly friable nature of much of the core it was not always possible to

cut slices using the diamond saw. In these cases, the rubble-like material was

washed and wet screened. A selection of sized materials and larger fragments

were subsequently mounted in epoxy resin and polished in preparation for

examination using reflected light microscopy and SEM-based techniques.

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Due to the high degree of porosity of a number of the drill core samples, the more

friable materials were mounted in a low viscosity resin and then placed in a

vacuum impregnation unit to aid resin penetration into the sample material. A

total of 434 polished sections were prepared from the five boreholes selected for

examination. Each of the polished sections was initially ground both on a

diamond cup wheel and on several grades of silicon carbide paper prior to

automated polishing using 14, 6, 3 and 1µm diamond suspensions with a hand

finish on 1/4um. The author carried out all polished section sample preparation.

Camborne School of Mines Associates prepared the thin sections from slices

selected by the author.

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4.3 Microscopy

4.3.1 Transmitted Light

Each of the thin sections was systematically examined using a Zeiss Axioskop

transmitted light microscope, providing information on the textural relationships

between the transparent gangue and ore minerals as well as deformation,

recrystallisation and secondary alteration phenomena. A total of 20 thin sections

were prepared from the five boreholes.

4.3.2 Reflected Light

Each of the 434 polished sections was systematically examined using a Zeiss

Axioplan reflected light microscope. Each polished section from the Au-bearing

sample intervals was also systematically searched for the presence of discrete

Au-bearing phases using relatively high power magnification (typically 20x air

objective) to ensure the observation of any tiny Au grains (typically <5µm) in

addition to the larger grains that are readily observed at lower power

magnification. Due to the extremely fine-grained and complex nature of much of

the gossan mineralisation, a 100x oil immersion objective was also used for

mineral identification purposes.

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4.4 Scanning Electron Microscopy

4.4.1 Qualitative Methods

All qualitative and quantitative Scanning Electron Microscope (SEM) analyses

were performed using a LEO 440 scanning electron microscope, fitted with a

high-resolution Oxford Instruments Germanium Energy Dispersive X-ray (EDX)

detector and a Microspec 400 Wavelength Dispersive X-ray (WDX) detector.

4.4.2 SEM Image Collection and Enhancement

After initial examination of the polished sections by optical microscope-based

techniques, a representative selection of the sections was carbon coated and

placed in the scanning electron microscope in preparation for examination and

characterisation using backscattered electron (BSE) imaging techniques.

Carbon coating is required for non-conductive samples so as to provide an

effective 'earth' for the electron beam. The Oxford Instruments 'Tetra' BSE

detector allows the operator to distinguish between discrete mineral phases

based on compositional variations, which result in mineral species appearing in

different shades of grey on the SEM monitor. The BSE images are particularly

efficient at distinguishing between gangue and ore minerals, based on the

differences in the mean atomic number of each phase. The lower limits of

resolution are also particularly good, with discrete grains of 1µm or less being

readily resolved using this technique.

The BSE detector captures electrons that are backscattered from the surface of

the polished section and an image of the section surface is created from these

electrons. Contrast and brightness adjustments allow for the differentiation of

discrete mineral phases. Minerals that exhibit a high mean atomic number (e.g.

galena, Au) will backscatter more electrons than those minerals that exhibit a low

mean atomic number (e.g. quartz, calcite). The brightness of the BSE image is a

factor of the number of electrons that are backscattered from the polished section

surface and, as a result, Au and galena may appear 'bright' on the image, relative

to quartz and calcite. This allows for the operator of the SEM to readily

differentiate mineral species and, for example, mineral zoning, based on subtle

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variations in brightness of the image. This was the main technique used for

documentation of the five borehole samples, with 621 images being collected in

total, the bulk of which were BSE images. Not all images were used in the final

thesis. The selection of the images used was based on providing a broad

understanding of the nature of the Las Cruces gossan mineralisation.

Borehole CR194 was the first borehole to be examined and the images collected

documented both 'typical' features and more unusual textures and associations

observed during the investigation. The selection of polished sections for

examination by SEM was based on providing a thorough range of materials that

were seen as typical of each sample interval based on the optical microscope

examination. Because of the time consuming nature of SEM operation and the

large number of polished sections prepared for this study (434 sections), not all

sections were examined.

Due to the often fine-grained and complex nature of the Las Cruces mineralogy,

the grey-scale BSE images were enhanced and false coloured using CorelDraw

and Corel Photopaint prior to incorporation into the main body of the text. These

bitmap and vector graphics packages allowed for the colouring of the grey-scale

images, with a key being added to distinguish between the mineral phases. The

relative brightness of the colours used typically reflects the relative brightness of

the grey shade in the original image.

Consistency of colours for the same minerals was also used throughout the

thesis where possible. The use of colour not only permits the rapid differentiation

of minerals, but other characteristic features may be highlighted. These include

porosity, compositional zoning, fine-scale intergrowths, oxidation and topography

(reflecting different polishing hardness of the minerals). These features are

described, where relevant, in the figure caption of the illustrations. An example of

how colour is used in this way is provided in Figures 4.1 and 4.2.

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Figure 4.1 - A monochrome backscattered electron image illustrating a rather complex Fe-oxide-rich sample with fine intergrowths of galena. Differences in brightness reflect different mineral species, variations in mineral chemistry, including oxidation, hydration and compositional zoning, porosity and variations in polishing hardness.

Figure 4.2 - The monochrome backscattered electron image has been false coloured and permits the reader to readily distinguish the mineral species. Galena (white) occurs as fine skeletal aggregates. Limonite fragments (yellow-brown shades) exhibit a wide range in brightness that reflects degrees of hydration. Darker browns represent more hydrated Fe-oxides (e.g. goethite). The darkest brown/black portions of the image represent areas of high porosity.

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4.4.3 Image Analysis Techniques

Due to the complex and fine-grained nature of the precious metal mineralogy

contained in the Las Cruces samples, modern computing methods were

combined with the backscatter electron imaging and EDX analysis to provide a

rapid, automated method of locating and identifying Au-bearing grains. This

technique is referred to as image analysis. Native Au grains and Au-bearing

amalgam were successfully located using an Oxford Instruments IMQuant–X

image analysis system incorporated into the SEM analytical software.

Many of the Las Cruces gossan samples contained abundant fine-grained galena

and other Pb-bearing phases that resulted in the backscatter electron images

commonly containing many hundreds or thousands of very bright grains in each

field of view, even at relatively high magnifications (~500x). As a result, the

presence of native Au and/or other Au-bearing phases was masked by the

presence of large numbers of other high mean atomic number phases.

Despite these complications, the image analysis system successfully located

many hundreds of precious metal grains. The polished sections subjected to this

automated searching were typically scanned at 500x magnification. This allowed

for the identification of precious metal-bearing grains to an effective lower size

limit of ~0.5µm. Using an analysis time of 200 milliseconds, five grains per

second could be analysed. Nonetheless, each polished section could typically

contain in excess of 10,000 high mean atomic number grains at the magnification

selected for the scans and each polished section could take in excess of 24

hours to search.

Once a grain of interest was located, further information, including chemical

composition, grain size and coordinates within the polished section, was

recorded. Grains of interest were subsequently examined and documented using

backscattered electron imaging where appropriate. The basic steps involved in

the image analysis process are illustrated in Figures 4.3 to 4.6.

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Figure 4.3 - A typical backscattered electron image as captured by the image analysis system. Differences in brightness of the mineral phases in the image reflect variations in mineral chemistry. The white areas consist of high mean atomic number phases and may include native Au or Au-bearing grains. The light grey areas are Fe-sulphides and the darker grey background is siderite. Pore spaces are black.

Figure 4.4 - The system recognises the range of grey shades of interest (red areas), depending on criteria set by the operator. Each bright phase (or phase of interest) is automatically analysed by the electron microscope using the EDX analyser.

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Figure 4.5 - Each grain of interest is recognised by the electron microscope and selected for analysis/measurement. Each grain is assigned a random colour.

Figure 4.6 - An example of an EDX spectrum captured using a very rapid (typically 200msec) EDX analysis of each grain. This is adequate to recognise the presence or absence of Au. In this example, the grain is a Sb-bearing Pb(Sb)-sulphide, recognised by the presence of Pb, S and minor Sb.

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4.4.4 Quantitative Methods

Quantitative microbeam analyses were performed on native Au and Au-bearing

amalgam grains, siderite and various sulphides to provide information on their

compositional ranges and to aid in their characterisation. Mineral stoichiometry

was also calculated to confirm the mineral identifications and to provide an

additional check on the quality of the analyses. Due to the presence of a carbon

coat on the polished sections and the inability of the SEM to accurately determine

the C content of the siderite, CO2 was calculated by difference.

Where suitable standards were available, the SEM EDX system was calibrated

using certified reference materials provided by Micro Analysis Consultants.

Where suitable standards were not available, the 'virtual standards' (standardless

data) supplied with the EDX system were utilised. In all cases, beam current

variations were corrected prior to analysis using a cobalt standard. Detection

limits for EDX analysis are in the order of 0.5 weight percent. Calibrations and

analyses were performed for 30 seconds using a detector dead time of ~30 per

cent, a beam current of ~5nA and 20Kv accelerating voltage. The results of the

EDX analyses and the mineral recalculations are provided in Appendix 5.

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4.5 X-Ray Powder Diffraction

A selection of handpicked mineral grains, magnetic separates and rock fragments

were subjected to X-ray powder diffraction (XRD) analysis. Suitably prepared

samples were irradiated using monochromated Cu K-α radiation and appropriate

instrumental settings to ensure the optimum resolution of reflections. The

resultant data were captured automatically using dedicated Panalytical

‘Highscore Plus’ software that also allowed for the measurement of peak

positions and the calculation of both d-spacings and intensities. The resultant

diffractograms and peak measurements were checked visually and the major and

minor phases were identified using a computer-based search and identify

program.

The identities of the phases were confirmed by comparison of the measured d-

spacings with the standard ASTM data sets using methods recommended by the

Joint Committee on Powder Diffraction Standards (JCPDS). The detection limits

for the identification of crystalline materials using x-ray powder diffraction

techniques is typically between 5 and 10 per cent by volume. All XRD analyses

were performed using a Panalytical PW3040/60 X’Pert diffractometer and

X’celerator detector.

XRD analysis was extensively utilised for the positive identification of both major

and minor phases. The results of the analyses are provided in Appendix 4. A

single sample of poorly crystalline clay was submitted to the British Geological

Survey, Keyworth, for XRD analysis. The result of this analysis is provided in

Appendix 4.

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4.6 Fluid Inclusion Analyses

Two samples of siderite-bearing material were submitted to Dr. Jamie Wilkinson

of the Royal School of Mines, Imperial College, London for fluid inclusion

analysis. Inclusions that were of sufficient size were subjected to conventional

microthermometry using a Linkam MDS600 motorised heating-freezing stage

mounted on a Nikon Eclipse 600 binocular microscope equipped with a x50 long-

working distance LCD objective, digital camera and image-grabbing software.

Stage calibration was carried out at –56.6, 10.0, 30.6 and 294°C using an in-

house synthetic fluid inclusion standard. Accuracy and precision of temperature

measurements is ±0.1°C at sub-ambient temperature and ±1°C at elevated

temperature.

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4.7 Isotope Analyses

Three samples of siderite were submitted to Dr. Steve Crowley of the University

of Liverpool for isotope analysis. Carbon dioxide was prepared for isotope ratio

measurement by reacting 6-7mg of finely powdered sample with anhydrous

phosphoric acid at 50oC following the method of McCrea (1950). The CO2 and

H2O released by the reaction were separated cryogenically, and H2S generated

by acid decomposition of galena was removed by reaction with AgPO4. The

resultant clean CO2 was subsequently analysed by conventional stable isotope

ratio mass spectrometry using a VG SIRA10 mass spectrometer. Isotope ratios

were corrected for 17O effects following the procedures of Craig (1957) and

oxygen isotope data were adjusted for isotopic fractionation between siderite and

H3PO4 using a fractionation factor () of 1.01046 (Rosenbaum and Sheppard,

1986). Isotopic ratios are reported in conventional delta () notation in per mille

(o/oo) relative to the VPDB (Vienna Pee Dee Belemnite) international standard,

e.g.

18Osiderite(o/oo) = [(18O/16Osiderite – 18O/16Ostd)/(18O/16Ostd)] x 1000

Analytical precision for carbon and oxygen isotope ratios is better than 0.2o/oo (1)

and 0.3o/oo (1) for 13C and 18O respectively (Crowley, pers. comms.).

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4.8 Geochemical Whole Rock Analyses

All geochemical whole rock analyses were carried out by the former Rio Tinto

laboratory Anamet Services, located in Avonmouth, Bristol, UK. Au analyses

were determined using fire assay with an AAS or ICP finish. Cu, Pb, Zn, Fe and

Ag were determined using Atomic Absorption Spectrometry (AAS). Sulphur was

determined using a Leco sulphur analyser and As, Sb and Sn were determined

using X-Ray Fluorescence (XRF). Whole rock assay data, together with details

of the methods and equipment used and information on the precision and

accuracy of the results are included, courtesy of Rio Tinto, in Appendix 3.

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Chapter 5 Borehole CR194 - Sample Descriptions

5 BOREHOLE CR194 – SAMPLE DESCRIPTIONS

5.1 Introduction

Chapter 5 provides a detailed description of the chemistry and mineralogy of

borehole CR194. Details of the field geologists' core log lithocodes and lens

descriptions are provided in the Appendix 2. Section 5.2 describes the major and

minor element chemistry of the borehole with particular emphasis given to their

relative abundance and degree of correlation.

Many of the sample intervals exhibit similar bulk mineralogy. Therefore the

mineralogical description is provided in five main sections, describing the

mineralogy of the ‘gossan’ (Section 5.3), ‘gossan contact with massive sulphide’,

(Section 5.4) ‘massive sulphide contact with gossan’, (Section 5.5), the ‘massive

sulphide’ (Section 5.6) and 'shale' (Section 5.7). A summary diagram of the

mineralogy is provided in Section 5.8. Each section is extensively illustrated.

The illustrations are provided in Appendix 6 in the order that they are described in

the main body of the text and are therefore not necessarily in depth order.

Tertiary conglomerate was unavailable for characterisation. The mineralogical

characterisation is focussed on the Au and/or Ag-rich samples gossan samples.

The gossan and massive sulphide core exhibits a high degree of preservation of

the sample intervals and lithologies. Sample selection extended into the first of

the underlying shale samples, below which precious metal, base metal and

deleterious metal content were significantly depleted.

Borehole CR194 intersects the fossil gossan at a depth of 149.80 metres. A thin

cap of Tertiary polymict conglomerate overlies the gossan, above which is

predominantly Tertiary marl. This borehole is relatively central to the supergene

massive sulphide ore and intersects the sulphide at a depth of 164.60 metres.

The supergene massive sulphide continues for approximately 15 metres at which

point the borehole intersects the underlying shales. The characterisation of

borehole CR194 is based on the preparation and examination of 100 polished

sections and 3 thin sections.

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Chapter 5 Borehole CR194 – Sample Descriptions

5.2 Borehole CR194 - Chemistry

5.2.1 Introduction

The major and minor element chemistry data are provided in Appendix 3. The

major element chemistry exhibits significant variation reflecting major changes in

the mineralogy, marking the prominent boundaries between the gossan, massive

sulphide and underlying shales.

Borehole CR194 is characterised by the presence of variable, but significant

minor elements including precious metals (Au and Ag) and those considered as

deleterious (As, Bi, Hg, Sb) from a mining perspective. The minor/trace element

chemistry also exhibits significant variation with some correlation between

elements also being recognised. In order to facilitate ease of interpretation, these

were plotted on several graphs and combined in a diagram showing the position

of each sample interval (Figure 5.1). The diagram representing the borehole has

been colour coded to show the position of the Tertiary conglomerate, gossan,

sulphide and underlying shales. The borehole depths represent depth down hole

and are approximately equivalent to depth from surface, with CR194 being a

vertical hole.

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Figure 5.1 - Illustrating the chemistry variation in borehole CR194. Each sample interval is displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. The Tertiary conglomerate was not available for examination. The variation in chemistry with increasing depth is displayed on four graphs in the centre of the illustration. The major, precious and deleterious element chemistry clearly exhibits a significant degree of variation that reflects an equally wide variation in the mineralogy of each interval. TCP - Tertiary Polymict Conglomerate, GHS - Strong Hematitic Gossan, GBM - Moderate Hematite Magnetic, MMP - Massive Sulphide, QXM - Massive Quartz/Shale, SXM - Massive Shale.

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5.2.2 Geochemical Profile

The Cu content of the core is relatively low within the gossan but exhibits a

marked increase within the supergene massive sulphide. The increase in Cu

content of the sulphide ore is associated with a similar marked increase in the S

content within the sulphide zone. The Cu content of the massive sulphide

remains relatively consistent with no prominent peaks or troughs occurring.

Similar consistencies are also evident in the Fe, S and As contents in the

massive sulphide, possibly indicating some close associations between these

elements and possible consistencies in the nature of the mineralogy throughout

the massive sulphide.

The Pb content of this borehole is highly variable, with a number of prominent

peaks occurring, notable at or near the contact between the gossan and massive

sulphide and less significantly within the massive sulphide in a mixed

sulphide/shale zone (lithocoded MMP/QXM). The Pb content of the upper

gossan region is relatively low, and increases significantly, peaking to a

maximum (7.9%) at the contact between the gossan and sulphide. The Pb

content of the bulk of the sulphide and shale zones is, however, relatively low.

The increase in Pb at the gossan/massive sulphide contact is associated with a

dramatic increase in the Au, Sn, Ag, Bi, Sb and As contents and to a lesser

extent the Hg content. Within the mixed massive sulphide/shale zone, the less

significant increase in Pb content is again associated with a marked increase in

the Au, Sn, Ag, Bi and Sb contents although the As content appears to be

relatively unaffected in this region of the core.

The Fe content of the gossan is relatively high and exhibits a marginal but

progressive increase towards the lower portion of the gossan. The Fe content

exhibits a moderate decrease at the gossan/sulphide contact zone, which is

associated with a marked increase in the S and Cu contents. The Fe content of

the sulphide zone is relatively consistent and exhibits a marked decrease at the

contact with the underlying shales. The Fe content of the shales is consistently

low.

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The S content is relatively low but variable within the gossan reflecting the

presence of minor amounts of Fe-sulphide, galena and other Pb-bearing

sulphosalts. The S content of the massive sulphide is high, occurring

predominantly in pyrite and supergene copper sulphide mineral. The S content

exhibits a marked decrease at the contact with the underlying shales.

The Ag content is typically low within the bulk of the core, but exhibits a marked

increase at the contact between the gossan and the massive sulphide. The

marked increase in the Ag content at this contact zone is also accompanied by a

marked increase in the Pb, Au, Bi, Hg, Sb and Sn content. The As content peaks

slightly higher in the gossan profile. The Ag content also exhibits a less

pronounced increase in the mixed sulphide/shale zone (lithocoded MMP/QXM).

This is again accompanied by an increase in the Pb, Au, Bi, Hg, Sb and Sn

content and a marginal decrease in the Fe and S content.

The geochemical profile for Au follows a very similar pattern to that of Ag and

also exhibits similar associations with other minor elements, including Pb, Bi, Hg,

Sb and Sn. The most prominent increases in the Au content of the core occur at

the gossan/sulphide interface and in the mixed sulphide/shale zone. The

distribution of Au within the gossan is, however, somewhat more erratic, with a

marginal increase in the Au content being observed in the 155.75 to 156.70

metre sample interval. This peak in the Au content is associated with a slight

increase in the Pb and S content of the core. The increase in the Au content of

the gossan/sulphide interface also occurs slightly higher in the gossan profile

than that observed for Ag and Hg and may be more closely aligned with that of

Sn and Bi.

The As content of the upper gossan is low, but increases significantly towards the

gossan/sulphide contact zone, reaching a maximum slightly higher in the gossan

profile than that observed for Pb, Au, Bi, Ag, Sn, Hg and Sb. The As content of

the massive sulphide remains consistent with increasing depth and decreases

significantly in the underlying shales.

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The geochemical profiles for Bi, Sb and Sn are essentially similar, occurring in

relatively minor amounts in the upper gossan, but increasing significantly with

increasing depth in the lower gossan and peaking at the contact with the massive

sulphide. The mixed sulphide/shale zone also exhibits elevated levels of Bi, Sb

and Sn.

The Hg profile is similar to that described for Pb, Au, Sn, Bi and Sb, peaking at

the gossan/sulphide interface and in the mixed sulphide/shale zone. However,

the Hg profile is more similar to that of Ag, as the increase in Hg levels are not

observed as high in the gossan profile as those observed for Pb, Au, Sn, Bi and

Sb.

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5.3 Borehole CR194 – Gossan

5.3.1 Introduction

The 14 sample intervals from 149.80 metres to 163.75 metres are described

collectively as 'gossan'. The gossan is characterised by the presence of variable

amounts of Fe (27.06–62.74%), S (0.34–7.90%), Cu (0.01–0.34%) and Pb (0.95–

7.92%). The gossan also contains significant but highly variable amounts of Ag

(1.9–178.5ppm) and Au (0.07–7.39ppm) and an abundance of deleterious

elements including As (329–17897ppm), Bi (25–1388ppm), Hg (0.2–69.0ppm),

Sb (252–3375ppm) and Sn (32–450ppm).

The bulk of the gossan consists of fragmented quartz-rich rock fragments that

have been partially and/or extensively replaced by siderite (Figure 5.2). The

presence of siderite gives the core a characteristic reddish-brown colour that is

clearly observed in hand specimen. The gossan is highly variable in nature,

reflecting variations in the relative proportions of siderite, quartz, limonite and

sulphide minerals. The degree of porosity of the core also has a marked effect

on its physical properties with the more porous sections of core often being highly

friable in nature.

X-ray powder diffraction analysis confirms the presence of quartz, siderite,

hematite, goethite, anglesite, barite and anatase.

5.3.2 Quartz

Quartz is the dominant Si-bearing phase present in the gossan and, apart from

subordinate amounts of an Fe-rich clay, no other significant Si-bearing phases

were located. Unfortunately, SiO2 analyses of the core are not available, as

these would have provided a quantitative indication of the quartz distribution.

The proportion of quartz varies considerably within and between sample intervals

and ranges in abundance from <1 percent of the sample to >60 percent by

volume. Quartz is most abundant in the upper portion of the gossan (Figure 5.2),

but gradually decreasing with increasing depth towards the contact with the

underlying massive sulphide.

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The quartz is typically present as fragments that range in size from a few

micrometres to several millimetres (Figure 5.2). Examination of the quartz-rich

rock fragments in thin section confirms that they are polycrystalline in nature and

the size of the crystallites ranges from a few micrometres to tens or hundreds of

micrometres. The fine and coarse crystallites may occur in close association

within the same quartz fragments. Conversely, many fragments consist almost

entirely of either coarse or fine-grained crystallites. The majority of the quartz

fragments contain crystallites that are extremely fine-grained and polycrystalline,

possibly reflecting partially recrystallised chalcedony.

The bulk of the quartz fragments within the gossan matrix exhibits highly irregular

morphologies (Figures 5.3a, 5.6a and 5.7b), probably indicative of some degree

of dissolution. The larger quartz fragments may also exhibit fracturing, resulting

in a very angular morphology (Figures 5.3a and 5.7a). These fractures are often

filled with siderite. The finer quartz groundmass may exhibit more rounded

features that are also possibly a result of chemical dissolution. A subordinate

portion of the quartz occurs as elongated fragments, possibly representing

fragments of former quartz veinlets. Euhedral cavities are also locally present in

the quartz, reflecting the presence of former minerals that have been removed

during oxidation and/or dissolution.

The quartz crystallites often exhibit a 'grain-flattening' fabric that simply reflects

growth into an open cavity or fracture. These textures have clearly not formed in

situ and therefore cannot be interpreted in context with the surrounding

mineralogy. A subordinate but significant portion of the quartz crystallites exhibit

a fibrous texture that is commonly associated with euhedral pore spaces in the

quartz fragments. The euhedral pore spaces have often been filled by siderite.

The wide variety of textures exhibited by the quartz crystallites suggests multiple

sources for the quartz fragments.

The quartz is largely free from intergrowths and inclusions. Rare and often

micrometre-sized inclusions of pyrite are locally present. There appears to be no

direct association between the presence of quartz and the relative abundance of

other phases in the gossan. Quartz also occurs as fine-grained and porous

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shale-like rock fragments. These ferruginised fragments are clearly visible in

hand specimen (Figure 5.2). The shale fragments appear to have been

extensively leached of their primary components, including phyllosilicate minerals

with only the chemically resistate components of the original shale host

remaining, including quartz, some TiO2 (largely anatase) and carbonaceous

materials. Siderite may partially fill some of the pore spaces present in the shale

fragments.

5.3.3 Siderite

Siderite is the dominant mineral in the gossan and often exceeds 50 percent of

the sample by volume. The siderite commonly occurs as millimetre-sized

‘fragments’ (Figures 5.2 and 5.7a) and as a cement that replaces the fine-

grained, quartz-rich matrix (Figures 5.3b, 5.5b and to a lesser extent 5.4a and

5.6a). The gossan is essentially a conglomerate consisting of quartz-rich rock

fragments in a fine-grained, quartz-rich matrix with extensive replacement by

siderite (Figure 5.2).

The siderite ‘fragments’ actually consist of cavity fillings and late-stage

pseudomorphous replacements (Figures 5.2, 5.3b, 5.7a, 5.8 and 5.12b). The

cavity-filling nature of these ‘fragments’ is further evidenced by the presence of

euhedral crystals that develop into the open cavities (Figures 5.7a, 5.13a and

5.14a). Euhedral siderite crystals are a relatively common feature throughout the

gossan and they often exhibit less oxidation than the surrounding siderite matrix

(Figure 5.12a) due to their late-stage nature.

The siderite matrix is a late-stage cement that may replace the quartz (Figures

5.2 and 5.7a). Late-stage, unoxidised siderite also occurs as veinlets that

traverse the gossan (Figure 5.23a) or cements earlier stages of siderite

mineralisation (Figure 5.22b).

Dissolution of siderite is evident in the gossan, with rhomb-shaped voids often

being leached of their original carbonate content (Figure 5.28b), leaving empty

cavities or skeletal galena.

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Examination of the late-stage, unoxidised siderite in thin section reveals that it

consists of granular aggregates of siderite that are typically coarse grained

compared to the surrounding matrix, with discrete siderite crystallites exceeding

tens or even hundreds of micrometres in size. The coarse grained siderite is less

reactive than the fine-grained siderite and is therefore more resistant to oxidation

and subsequent replacement by limonite. The late-stage siderite veinlets also

consist of relatively coarse-grained crystallites.

Cavity-filling siderite (Figures 5.3b, 5.5b and 5.27b) clearly post-dates the

reworking of the quartz-rich rock fragments, as it infills spaces in the reworked

conglomerate. These later stages of siderite mineralisation are commonly

associated with sulphide minerals, including an Fe-sulphide assemblage (Figure

5.5b) and galena (Figures 5.3a, 5.27b and 5.28b).

A discrete, needle-like Pb(SbAs)-bearing sulphide is also present in some of the

late siderite (Figure 5.5b). This phase was too fine-grained for a quantitative

SEM analysis, however, semi-quantitative analysis confirmed that it contains in

excess of 5 weight percent Sb and similar levels of As. The presence of

significant As, which does not typically occur in such high quantities in galena,

and the acicular morphology, which again is uncommon in cubic minerals,

suggest that this phase probably represents a discrete sulphosalt mineral as

opposed to Sb- or As-bearing galena.

Galena commonly lines the margins of siderite veinlets and cavities (Figures

5.3b, 5.22b and 5.23a) and skeletal galena may partially fill cavities associated

with the siderite (Figures 5.4a and 5.6a). Skeletal galena also forms complex

textures within the late siderite (Figures 5.12b and 5.13b). Although the siderite

appears to be replaced by the galena in this case, SEM examination confirms

that compositional zoning in the siderite overprints/cross-cuts the galena

mineralisation and therefore probably represents a cavity filling.

As well as replacing the quartz-rich rock fragments and fine quartz matrix, the

late-stage siderite also extensively replaces relict barite (Figures 5.3a, 5.8 and

5.11b), fine-grained and porous Fe-clay (Figures 5.4b and 5.28a) and the

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limonite-rich matrix (Figures 5.13a and 5.26a). There is some compositional

variation in the siderite. The extensive oxidation of much of the early-formed

siderite has, however, significantly masked the composition of earlier stages of

mineralisation. A number of quantitative SEM analyses were performed on the

less oxidised siderite, the results of which are provided in Appendix 5.

The compositional variations largely reflect changes in the relative proportions of

Mg and Ca at the expense of Fe. Other elements are below the detection limits

for this technique (~0.5%). The MgO and CaO contents of the siderite ranges

between an effective lower limit of less than 0.5 per cent to the highest values

that may exceed 1.9 per cent and 5.6 per cent respectively. These variations

reflect compositional zoning within the late-stage siderite rather than discrete

stages of siderite mineralisation.

The siderite exhibits varying degrees of oxidation and replacement by limonite

that often highlight different stages of mineralisation, with the latest stages of

siderite typically exhibiting little or no oxidation.

5.3.4 Limonite

The term ‘limonite’ is used here to describe the Fe-oxide and Fe-hydroxide

assemblage that is intimately associated with the gossan. XRD analysis confirms

that the bulk of the ‘limonite’ consists of hematite, with goethite being present in

relatively minor amounts. The presence of hematite gives the core a distinctive

blood-red appearance in hand specimen (Figure 5.2).

Limonite is most abundant in the fine-grained matrix (Figures 5.10b and 5.12b),

probably reflecting the more reactive nature of the fine-grained siderite and, to

some degree, the partial oxidation of associated Fe-sulphide. The oxidation of

the siderite results in an increase in the porosity of the matrix due to volume

changes that occur during the oxidation/hydration process. This porosity is

clearly illustrated in Figure 5.5a.

The scale of the replacement textures between siderite and limonite ranges from

relatively coarse caries texture (Figures 5.8a and 5.8b) to the development of

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microscopic and sub-microscopic needles and botryoidal aggregates of limonite

on a scale that is on the limit of resolution for the electron microscope (Figure

5.7b).

Locally, limonite is abundant and occurs as delicately banded botryoidal

aggregates (Figures 5.25b, 5.26a and 5.26b). The botryoidal limonite may

exhibit fracturing (Figure 5.27a) and is often extensively replaced by late-stage,

unoxidised siderite. Late stage siderite and galena may also fill or partially fill the

cavities within the botryoidal limonite aggregates (Figures 5.26, 5.27 and 5.28b).

The fine-grained and more reactive siderite-rich matrix typically exhibits a higher

degree of oxidation relative to the less reactive, coarser-grained siderite (Figures

5.10b and 5.12b).

5.3.5 Fe-Clay

The gossan is characterised by the presence of variable amounts of a poorly

crystalline Fe-clay. XRD analysis confirmed this mineral is nontronite (ideally

Na0.3Fe3+2(Si,Al)4O10(OH)2.nH2O, see Appendix 4).

This clay-like phase is typically fine-grained and porous in nature (Figures 5.4b

and 5.28a) but also forms radiating and concretionary textures (Figures 5.18a

and 5.23a). Characteristic dehydration cracks are also typically present (Figures

5.21b and 5.30a)

This phase occurs throughout the gossan in relatively minor amounts and is often

associated siderite (Figures 5.4b and 5.28a), occurring as a cavity filling.

Nontronite is also occasionally developed along the margins of the siderite

veinlets (Figure 5.23a). Rarely, the Fe-clay is replaced by a AgSb(As)-sulphide

(possibly pyrargyrite, ideally Ag3SbS3) (Figure 5.23b). Qualitative SEM analysis

of the clay confirms that it consists predominantly of Fe, Si and O together with

subordinate but variable amounts of Na, Mg, Al, P, Ca, K and S, some of which

are likely adsorbed species.

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5.3.6 Accessory Transparent Gangue Minerals

A number of gangue minerals are present in the gossan in minor to moderate

amounts. Barite is a common accessory phase and occurs in localised patches

throughout the gossan (Figures 5.10b and to a lesser extent 5.3a and 5.4a). The

barite is typically extensively replaced by siderite (Figure 5.11b).

Another excellent example of replacement of barite by siderite is illustrated in

Figures 5.8a and 5.8b. In this association, the tabular, euhedral barite crystals

have been pseudomorphously replaced by siderite, with only minor amounts of

the original barite crystals remaining. Note the highly irregular morphology and

corroded appearance of the barite in Figure 5.8b. The siderite has been

subsequently oxidised and partially replaced by limonite. Apatite was also

observed in the gossan in minor amounts, occurring as small rounded grains in

the siderite-rich matrix (Figure 5.6b).

5.3.7 Fe-Sulphides

Fe-sulphides are a common accessory and account for the magnetic nature of

the core. These phases are described in greater detail in Chapter 10. Their very

fine-grained nature made optical identification difficult. However, they likely

consist of one or more of amorphous FeS, mackinawite, greigite, pyrrhotite and

marcasite/pyrite. For simplicity, they are simply described as Fe-sulphide in this

chapter.

Fe-sulphide is most abundant in the fine-grained matrix of the fragmented gossan

(Figures 5.3b and 5.13a) and within pore spaces (Figures 5.4b and 5.12a)

associated with late-stage siderite. The Fe-sulphides often occur as aggregates

of platelets that may exceed several hundred micrometres in size (Figures 5.4b

and to a lesser extent 5.3b). Discrete Fe-sulphide platelets may exceed 50µm in

length.

Fe-sulphide also occurs as fine-grained and porous granular aggregates (Figures

5.13a and 5.14a) with discrete grains rarely exceeding 20µm in size (Figure

5.6b). The granular Fe-sulphides may also be finely disseminated throughout the

siderite- and limonite-rich matrix. Euhedral crystals of Fe-sulphide feature within

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some of the late-stage siderite-rich veinlets where they appear to line the margins

of former cavities (Figure 5.5b) or are disseminated throughout the unoxidised

siderite (Figure 5.17b). Detailed examination of the Fe-sulphides at high power

magnification revealed the presence of minor amounts of intergrown Pb(SbAs)-

bearing sulphides (Figures 5.5b and 5.17b). Fine-grained galena is also

commonly present in the cores of the Fe-sulphide aggregates (Figures 5.5a and

5.12a).

5.3.8 Galena and Pb-Bearing Sulphides

Galena hosts the bulk of the Pb content of the gossan, typically ranging between

1 and 5 per cent by volume. The galena typically occurs as euhedral crystals

(Figures 5.6b, 5.23a and 5.23b) and skeletal aggregates (Figures 5.4a, 5.25b,

5.28a and 5.29a) with discrete crystals rarely exceeding a few micrometres in

size.

Well-defined skeletal textures may be observed in the galena locally (Figures

5.20a and 5.25b). These skeletal aggregates commonly occur within cavities in

the siderite- and limonite-rich core (Figures 5.25b and 5.20a). Several stages of

galena and galena + siderite mineralisation are often evident (Figures 5.27b,

5.28a, 5.28b, 5.4a and 5.6a). These textures are extremely common features of

the gossan.

The skeletal galena may also fill or partially fill rhombohedral voids (Figure

5.28b). These voids probably reflect the presence of former siderite, which has

subsequently been leached, leaving behind the skeletal galena that was present

as a component of the siderite + galena mineralisation.

Tiny euhedral galena crystals may also line the margins of former cavities (Figure

5.3b) or fractures (Figure 5.23a) that have subsequently been filled by siderite.

The galena may also be more randomly disseminated throughout the siderite-

and limonite-rich core (Figure 5.6b) or form thin veneer-like aggregates along the

margins of the late-stage siderite (Figure 5.10b).

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Galena may form complex, fine-grained and porous aggregates associated with

one or more of Fe-sulphides (Figures 5.13a and 5.13b), cerussite (Figure 5.17a)

and anglesite and may replace mimetite (ideally Pb5(AsO4)3Cl) (Figures 5.18b,

5.19a and 5.19b) and a CuFe-sulphide phase (Figures 5.11a, 5.14a and 5.16b).

Minor amounts of As and Sb are also often present in the galena aggregates,

possibly indicating the presence of discrete PbSbAs-sulphides or possibly some

Sb in solid solution with the galena. Due to the extremely fine-grained nature of

these aggregates, it was not possible to positively identify any discrete PbSbAs-

sulphides phases.

The galena-rich aggregates typically occur within the most porous regions of the

core, along grain boundaries and within former cavities (Figures 5.12b, 5.13a and

5.13b). The Pb-rich aggregates therefore often occur within the fine-grained,

porous matrix of the gossan (Figures 5.12b, 5.13a and 5.13b). Complex galena

textures are observed in a number of the siderite aggregates (Figures 5.12b and

5.13b). These complex textures represent the partial replacement of galena by

late-stage siderite leaving relict, skeletal galena.

A discrete PbAs-bearing sulphosalt phase is also present in the gossan,

occurring with Fe-sulphide and siderite (Figures 5.5b and 5.17b). This phase

typically occurs as finely disseminated, micrometre-sized needle-like grains and

was too fine-grained for a positive identification. XRD analysis confirms that the

galena also exhibits some degree of oxidation and replacement by anglesite.

5.3.9 Secondary Pb-bearing Phases

The gossan is characterised the presence of a number of secondary Pb-bearing

phases that are typically present in relatively minor amounts. These secondary

Pb-bearing phases are often present as components of the fine-grained galena-

rich aggregates (Figures 5.12b to 5.17a). Pyromorphite (ideally Pb5(PO4)3Cl) and

to a lesser extent mimetite (ideally Pb5(AsO4)3Cl) are minor accessory phases in

the gossan. Pyromorphite may occur as discrete euhedral crystals and granular

aggregates that exceed 100m in size (Figure 5.15b). Pyromorphite may also

form pseudo-hexagonal skeletal crystals (Figures 5.8a and 5.9a).

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Cerussite may be locally abundant where it typically occurs as fine-grained

aggregates and acicular crystals that are intimately associated with galena and,

to a lesser extent, anglesite (Figure 5.17a). Discrete cerussite crystals may

exceed 50m in maximum dimension.

5.3.10 Amalgam and Hg-Bearing Phases

Amalgam is absent throughout the bulk of the gossan, but becomes increasingly

abundant towards the contact with the massive sulphide. Although only present

in minor amounts, amalgam hosts the bulk of the Hg in the gossan. The

amalgam typically occurs within the siderite- and limonite-rich matrix.

Although amalgam aggregates may exceed 100µm in size (Figures 5.24a and

5.24b), the bulk of the amalgam occurs as finely disseminated grains that are

significantly finer. The amalgam aggregates commonly exhibit a highly irregular

morphology (Figures 5.24a and 5.24b) that may reflect some degree of

dissolution (Figure 5.24b). The amalgam exhibits some degree of replacement

along the margins by cinnabar (ideally HgS) (Figure 5.24a) and possibly limonite

(Figure 5.24b). It is possible that the amalgam may have initially formed under

conditions where the Ag-Hg alloy was stable, but subsequent changes in the

environment have resulted in the partial dissolution and/or replacement of the

amalgam by cinnabar. Quantitative SEM analyses of the amalgam are provided

in Appendix 5 (analyses #1 to #3). The analyses of the amalgam are relatively

consistent and confirm that it consists predominantly of Ag (54.2-56.4%) and Hg

(44.2-46.7%).

Aggregates consisting of one or more of acanthite, amalgam, cinnabar, (Figures

5.26b and 5.29b), iodargyrite (ideally AgI), a AgSe-sulphide (possibly aguilarite,

ideally Ag4SSe) and a Ag-selenide (possibly naumanite, ideally Ag2Se) were also

recognised and typically occur within botryoidal limonite aggregates.

Fine-grained and often granular native arsenic is also occasionally present within

the amalgam.

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5.3.11 Precious Metal Mineralisation

A small number of native Au grains were observed in the gossan and typically

occur as micrometre-sized grains that are present within the nontronite and

limonite-rich matrix (Figures 5.20b, 5.21a, 5.21b, 5.22a, 5.25a, 5.30a and 5.30b).

None of the grains exceeded 15µm in maximum dimensions. The morphology of

the grains varied from euhedral (Figure 5.30a) to subhedral (Figures 5.30b and

5.25a) to more irregularly shaped grains (Figures 5.20b, 5.21a, 5.21b and 5.22a).

The native Au grains may be intergrown or closely associated with one or more of

the CuFe-sulphide phase, native Bi, Fe-sulphides, bismuthinite and Pb-bearing

sulphides (Figures 5.20b, 5.21a, 5.21b and 5.22a). Other grains were associated

with siderite and limonite (Figures 5.25a and 5.30b). A single, euhedral native Au

grain was observed within a cavity in botryoidal limonite (Figure 5.30a). The

cavity was filled by nontronite. The native Au grain is approximately 5m in

maximum dimension and is compositionally zoned, with a fine rim of electrum

developed on the margins.

All of the native Au grains were too fine-grained for a quantitative SEM analysis.

Qualitative SEM analysis of the native Au grains confirms that they consist

predominantly of Au together with minor (close to detection limits, ~0.5 wt.%) to

moderate (typically ~5-15 wt.% percent) amounts of Ag being detected in a

number of grains.

The lower portion of the gossan is characterised by elevated levels of Ag.

Amalgam is by far the most common of the Ag-bearing phases. A AgSb(As)-

sulphide, possibly a member of the proustite-pyrargyrite solid solution series

(ideally Ag3AsS3–Ag3SbS3) (Figure 5.23b) occurs locally within the nontronite.

Other discrete Ag-bearing phases observed in very minor to trace amounts

include a Bi-Ag-sulphide, Ag-bearing chalcopyrite and a Ag-sulphide (possibly

acanthite) (Figures 5.26a and 5.29b). The AgSb(As)-sulphide was too fine-

grained for a positive identification by SEM analysis.

The systematic searching of the polished sections prepared from the gossan

samples failed to identify the presence of a significant number of native Au or Au-

bearing phases. The bulk of the native Au grains that were observed were

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extremely fine-grained in nature and it envisaged that a significant proportion of

the Au in the gossan is present in the form of sub-microscopic grains.

5.3.12 Accessory Minerals

The gossan contains a wide range of accessory minerals that are present in the

samples in relatively minor amounts. The accessory minerals include zircon,

TiO2 and cassiterite that are typically present as sub-rounded grains and angular

fragments that rarely exceed 10m in size. Qualitative SEM analysis confirmed

the presence of minor amounts of vanadium within a number of the cassiterite

grains.

A discrete CuFe-sulphide phase may be intimately associated with the galena-

rich aggregates (Figures 5.14a, 5.14b and 5.15a). Chalcopyrite was confirmed

by optical examination, although other CuFe-sulphides may be present.

A small number of tiny, micrometre-sized native bismuth grains were recognised

in the gossan and typically occur within the galena-rich aggregates and the fine-

grained nontronite and siderite/limonite matrix. The native bismuth grains are

also commonly associated with a discrete Bi-sulphide phase that probably

represents the mineral bismuthinite (ideally Bi2S3) (Figure 5.21a). Other phases

identified include a discrete BiAg-sulphide and a AgFe-sulphide (possibly

sternbergite, ideally AgFe2S3).

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5.4 Borehole CR194 – Gossan Contact with MassiveSulphide

5.4.1 Introduction

The lowest portion of the gossan occurs from 163.75 to 164.60 metres and is

lithocoded as Strong Hematitic Gossan. This final interval of gossan marks the

contact with the underlying massive sulphide zone and is extremely complex. It

is therefore described separately from the other gossan samples. In particular, it

is the last 10 to 15 centimetres of the gossan in this sample interval that exhibits

the most distinctive mineralogy and chemistry. The gossan and massive sulphide

components of the contact zone are illustrated in Figure 5.31. Figure 5.31a

shows an approximately 1:1 scale digitised image of a section of core taken from

the lower 10 centimetres of gossan from the 163.75 to 164.60 metre sample

interval.

The 163.75 to 164.60 metres sample interval contains significant amounts of Fe

(58.87%) together with moderate amounts of Pb (7.31%), minor S (0.88%) and

traces of Cu (0.04%). This sample is characterised by the presence of significant

amounts of Ag (1114.4ppm) and Au (14.42ppm). This sample also contains

significant amounts of As (4550ppm), Sb (5135ppm), Bi (1629ppm), Sn (626ppm)

and Hg (645.9ppm). X-ray powder diffraction analysis confirms the presence of

quartz, siderite, hematite, goethite, anglesite and galena.

Macroscopic examination of the 163.75 to 164.60 metre interval confirms that it

consists of three distinct zones, exhibiting a different bulk and trace mineralogy.

These are described separately as 'upper', 'middle' and 'lower' in the following

section of the thesis.

5.4.2 163.75 to 164.60m Sample Interval - Upper Portion

This portion of the core is essentially similar to the previous gossan samples and

consists of extensively oxidised siderite and fine-grained nontronite. The limonite

(oxidised siderite) and nontronite are intimately associated and typically occur as

fine-grained and porous aggregates. The limonite may occur as delicately

banded, botryoidal aggregates.

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Galena is abundant and occurs as finely disseminated, micrometre-sized grains,

skeletal crystals and as fine, granular aggregates. The galena may also exhibit

some degree of oxidation with the development of cerussite rims. Pb(AsSb)-

bearing sulphides are also typically finely intergrown with the galena.

Accessory phases include quartz, cassiterite, TiO2 and barite. A small number of

AuAgHg grains were also identified. These grains are typically <2m in size and

are intimately intergrown with galena, a Pb-arsenate and a CuSbAg-sulphide

(possibly Ag-bearing tetrahedrite, ideally Cu12Sb4S13).

5.4.3 163.75 to 164.60m Sample Interval - Middle Portion

The middle portion of this sample interval exhibits a distinctive yellow colour and

no fragmented gossan textures survive. X-ray powder diffraction analysis

confirms the presence of goethite (ideally -Fe3+O(OH)) and hematite. The

goethite and hematite are fine-grained and porous in nature and are intimately

associated with siderite. Galena is also abundant and typically occurs as finely

disseminated grains, and as fine, granular aggregates that form discrete, narrow

veinlets. Qualitative SEM analysis of a number of the galena grains confirmed

the presence of Pb and S together with minor amounts of Ag.

Detailed examination of the fine-grained galena-rich aggregates also revealed the

presence of a number of discrete, Au, Hg, Ag, Bi and S-bearing phases. These

phases are typically <5m in size and include a AuHgAg alloy (Au-bearing

amalgam), Ag-sulphide, amalgam, cinnabar and native bismuth (Figures 5.32a,

b, c and d). Pyromorphite is also present locally within these aggregates. Minor

amounts of a secondary Cu-sulphide (possibly covellite) were also recognised

within a siderite veinlet. Quantitative SEM analyses of the Au-bearing amalgam

are provided in Appendix 5 (Au-amalgam analyses #1 to #4) . The Au-bearing

amalgam grains exhibit some degree of compositional variation and typically

contain minor amounts of Fe (2.0-2.8%) that probably reflects the presence of

associated limonite. The Au content is low and somewhat variable (5.4–11.6%)

and appears to reflect similar variations in the Hg content (38.0–42.6%). The Ag

content is relatively consistent (48.5–50.7%).

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5.4.4 163.75 to 164.60m Sample Interval - Lower Portion

This portion of the sample interval is complex and exhibits a marked change in

the bulk and trace mineralogy and chemistry relative to the overlying gossan

samples. This sample represents the last ~10 centimetres of core prior to the

contact with the underlying massive sulphide. Figure 5.31a illustrates a typical

section through this portion of the core. The core exhibits a distinctive reddish

brown colour that reflects the presence of siderite and limonite. Numerous black

bands of nontronite also traverse the core. Within the clay and siderite-rich

areas, millimetre-sized amalgam grains are evident in hand specimen.

Extensively oxidised siderite is dominant (Figure 5.33a). Less extensively

oxidised siderite often exhibits replacement by hematite along grain boundaries

and fractures (Figures 5.33a and 5.35a). The siderite is notably fine-grained, with

large fragments of coarsely crystalline siderite, similar to those observed in the

overlying gossan samples, being essentially absent.

The oxidised and porous siderite/limonite aggregates are traversed by unoxidised

siderite veinlets that represent a later stage of mineralisation. The unoxidised

siderite veinlets significantly reduce the porosity of the sample locally (Figure

5.33b). These siderite veinlets may contain micrometre-sized euhedral crystals

of galena (Figure 5.34b) and less commonly, granular aggregates of anglesite

(Figure 5.33b). Radiating crystals of Fe-sulphide may also occur within the

siderite (Figure 5.34a). The siderite veinlets may be fractured, particularly within

the clay-rich areas, probably as a result of the dehydration and shrinkage

associated with the nontronite (Figure 5.34b). Quantitative SEM analysis of the

late-stage siderite confirms that it consists predominantly of Fe, C and O with Ca

and Mg typically being below detection limits for this technique (see Appendix 5).

Nontronite is particularly abundant in the final few centimetres of gossan before it

intersects with the underlying massive sulphide. The bulk of the nontronite

occurs within discrete bands within the extensively oxidised siderite (Figures

5.34b and 5.36a) and exhibits a delicately banded texture and marked

dehydration cracking (Figures 5.34b, 5.36a and 5.36b).

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Fine-grained galena (Figures 5.34b, 5.36b and 5.37a) and pyromorphite are

finely disseminated throughout the nontronite. The pyromorphite and galena

often exhibit a preferred orientation that reflects the marked lamination within the

clay (Figures 5.34b and 5.36b). Granular aggregates of amalgam and Au-

bearing amalgam are typically present within the clay layers (Figures 5.36a,

5.36b and 5.37a).

Galena is abundant and occurs as finely disseminated grains (Figures 5.33a and

5.36b), as discrete euhedral crystals in siderite veinlets (Figure 5.34b) and as

complex, fine-grained and often porous aggregates (Figures 5.33b, 5.35b, and

5.37a). The galena aggregates commonly occur along the margins of the

relatively unoxidised siderite linings of former cavities that have been

subsequently filled by the siderite.

Pyromorphite and galena are particularly abundant within the clay (Figures 5.34b

and 5.36b). Pyromorphite is a mineral of secondary origin that forms as a result

of the interaction between phosphoric acid and galena or cerussite. Galena

cores are commonly present in the larger pyromorphite aggregates (Figure

5.35a) and it is presumed that at least part of the pyromorphite has formed as a

result of the oxidation of galena. Pyromorphite forms a solid solution series with

mimetite.

Qualitative SEM analysis of the galena confirms that it consists predominantly of

Pb and S but also consistently contains minor amounts of Sb. Detailed

examination of the galena aggregates revealed the presence of a discrete PbSb-

sulphide (Figure 5.35b). However, due to the extensive replacement of the

PbSb-sulphide by galena, it was not possible to obtain a quantitative analysis of

this mineral for identification. Optical examination of this mineral confirms that it

is strongly anisotropic and exhibits grey-green colours in reflected light,

properties exhibited by a number of sulphosalt minerals, further inhibiting a

positive identification.

Aggregates of amalgam are present within the fine-grained, extensively oxidised

limonite/siderite and also within discrete clay layers. The amalgam grains exhibit

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a wide range in grain size with discrete grains commonly exceeding 1mm. The

amalgam grains typically exhibit a highly irregular morphology (Figures 5.36a,

5.36b and 5.37b), with rare subhedral to euhedral crystals also being observed

(Figure 5.37a).

A narrow zone of high porosity is also commonly developed along the margins of

the amalgam possibly indicative of some degree of dissolution (Figures 5.36b

and 5.37a). The amalgam aggregates commonly exhibit complex textural

relationships with one or more of Ag-sulphide and cinnabar. These phases are

typically developed along the margins of the amalgam grains and may represent

alteration products. Quantitative SEM analyses of the amalgam are provided in

Appendix 5.

The amalgam analyses are relatively consistent. Analyses #4 to #6 (52.9 -

54.6% Ag; 45.1 - 46.7% Hg) were performed on amalgam grains within a veinlet

several millimetres above the amalgam grains represented by analyses #7 to #10

(58.1 - 59.9% Ag; 39.7 - 42.9% Hg). The variation in Ag and Hg is therefore

relatively consistent within discrete horizons but suggests that some degree of

variation may occur within the sample as a whole. It is interesting to note that the

composition of the amalgam within the upper part of this sample (analyses #4 to

#6) is similar in composition to those grains described in Section 5.3 (analyses #1

to #3).

This portion of the gossan also contains Au-bearing amalgam that appears to be

confined to the clay bands that are traversed by siderite veinlets (Figure 5.38a, b,

c and d). The Au-bearing amalgam is typically present as highly irregular grains

that are rimmed by fine-grained aggregates of galena (Figures 5.38a, b, c and d).

The Au-bearing amalgam grains rarely exceed 20m in size. Au-bearing

amalgam hosts the bulk of the Au in this portion of the gossan. Quantitative SEM

analyses of the Au-bearing amalgam are provided in Appendix 5 (analyses #5 to

#11).

The composition of the Au-bearing amalgam grains is highly variable and exhibit

significantly higher Au contents (27.7-56.5%) than the amalgam grains observed

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in the middle portion of the 163.75 to 164.60 metre sample interval (5.4-11.6 %

Au).

Accessory minerals include a AgFe-sulphide (possibly sternbergite), native

bismuth, cassiterite (Figure 5.37b) and native arsenic. The native bismuth may

be intimately associated with the amalgam with discrete grains exceeding 20m

in size (Figure 5.37b).

Cassiterite typically occurs within fine-grained aggregates along the margins of

the amalgam (Figure 5.37b). These aggregates may also contain significant

amounts of Bi, Pb, As and Fe and possibly represent fine-grained intergrowths

between several discrete Pb, As, Bi and Fe-bearing phases. Cassiterite grains

may exceed 30m in size. Qualitative SEM analysis confirmed the presence of

minor amounts of vanadium within a number of the cassiterite grains.

Fe-sulphides are present in this sample in minor amounts and typically occur as

radiating tabular crystals and granular aggregates (Figure 5.34a). The Fe-

sulphide is often intergrown with minor amounts of magnetite and sphalerite.

Qualitative SEM analysis confirmed the presence of minor amounts of Hg within

a number of the sphalerite grains. Zircon was also observed during this

investigation.

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5.5 Borehole CR194 – Massive Sulphide Contact withGossan

5.5.1 Introduction

The massive sulphide contact zone occurs within the 164.60 to 165.80 metre

interval and is lithocoded as Massive Sulphide. This contact zone between the

sulphides and overlying gossan is extremely complex. It is therefore described

separately to the other massive sulphide samples. The first 10 to 15 centimetres

of the 164.60 to 165.80 metres sample interval exhibits the most complex and

distinctive mineralogy and chemistry.

The massive sulphide contact with the gossan is illustrated in Figure 5.31b.

Figure 5.31b shows an approximately 1:1 scale digitised image of a section of

core taken from the upper 10-15 centimetres of massive sulphide from the 164.60

to 165.80 metres sample interval.

The 164.60 to 165.80 metres sample interval contains significant amounts of Fe

(37.74%) and S (45.89%) together with moderate amounts of Pb (5.75%) and Cu

(7.42%). This portion of core is characterised by the presence of significant

amounts of Ag (546.4ppm) and Au (5.43ppm) and also contains significant

amounts of As (4892ppm), Sb (927ppm), Bi (517ppm), Sn (203ppm) and Hg

(69.4ppm). X-ray powder diffraction analysis confirms the presence of quartz,

siderite, hematite, goethite, pyrite, tennantite, melanterite, chalcopyrite, anglesite

and galena.

The upper 10-15cm of this sample interval consists of a narrow (~1cm) layer of

nontronite and finely disseminated galena (Figures 5.31b and 5.39a). This

extends into a more galena-rich layer of approximately 2cm in depth that exhibits

some delicate banding (Figures 5.31b and 5.39a). Within this layer, euhedral

crystals and angular fragments of quartz are often present (Figures 5.39a and

5.39b).

Below the galena-rich layer is an extensively leached pyrite zone (Figure 5.31b).

Galena and siderite are also common in the leached pyrite zone (Figures 5.47a

and 5.47b). Due to their complexity, the upper 10-15cm of core was examined in

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greatest detail. The mineralogy of the 164.60 to 165.80 metres sample interval is

described in four parts, describing the mineralogy of the clay-rich layer, galena-

rich layer and leached pyrite-rich layer. The remaining (~1m) of core consists

predominantly of pyrite and is essentially similar to the underlying massive

sulphide. This portion of core is therefore discussed briefly as Lower Core in

Section 5.5.5.

5.5.2 Clay-Rich Layer

This layer is essentially similar in appearance to the clay-rich layers described in

the lower portion of the previous sample interval. It consists predominantly of

poorly crystalline nontronite together with subordinate amounts of fine-grained

galena and pyromorphite (Figure 5.39a). The nontronite exhibits a marked

lamination and is traversed by numerous fractures that probably represent

dehydration cracks. Sb-bearing galena is finely disseminated throughout the clay

and may exhibit a preferred orientation that is developed parallel to the lamination

of the clay. Narrow, galena-rich veinlets are also present within this layer and

appear to traverse the clay-rich layer.

5.5.3 Galena-Rich Layer

This highly complex layer consists of fine-grained, delicately banded and

deformed aggregates that consist predominantly of one or more of galena,

amalgam, nontronite, chalcopyrite, tennantite, tetrahedrite, PbSb-sulphide, quartz

and siderite (Figures 5.39a, 5.39b, 5.41a, 5.41b, 5.44a and 5.44b).

Quartz is common and occurs as angular fragments and euhedral crystals

(Figure 5.39a). The smaller fragments typically exhibit highly irregular margins,

often with concave faces, indicative of dissolution. The overall grain size of the

quartz fragments is finer than that observed in the gossan, typically ranging

between a few micrometres and a few hundred micrometres in size.

Examination of the quartz in thin section confirms that it exhibit a variety of

textures and crystallite sizes. The polycrystalline quartz aggregates are typically

fine to medium grained. Some fibrous quartz is also present. A small number of

narrow quartz veinlets also traverse the sample. These veinlets consist of

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extremely fine-grained quartz crystallites that probably represent partially

recrystallised chalcedony.

Siderite is common and becomes increasingly abundant towards the contact with

the leached pyrite zone (Figure 5.39b). The increase in abundance is marked by

a similar decrease in the presence of quartz. The siderite fills former cavities that

are lined by euhedral sulphosalt minerals (Figures 5.39b, 5.40a, 5.40b, 5.43a and

5.43b).

Quantitative SEM analysis of the late-stage siderite confirms that it consists

predominantly of Fe, C and O with Ca and Mg typically being below detection

limits for this technique (Appendix 5). Minor amounts of nontronite are also

typically present within the galena-rich matrix.

Galena occurs as fine-grained, granular aggregates that are intimately associated

with one or more of amalgam, Au-bearing amalgam, nontronite, TiO2, cassiterite,

native bismuth, chalcopyrite, tennantite, tetrahedrite, Cu-arsenides, PbSb-

sulphide, quartz, native arsenic and siderite (Figures 5.39a, 5.39b, 5.40b and

5.43b). The galena also forms rims on other minerals (Figures 5.40b) including

tetrahedrite–tennantite and chalcopyrite, reflecting partial and extensive

replacement relationships. Qualitative SEM analysis also revealed the presence

of minor amounts of Se that are occasionally present in the galena. This

probably represents the solid solution between galena and the Pb-selenide

clausthalite (ideally PbSe). XRD confirms the localised oxidation of galena to

anglesite.

Tetrahedrite and tennantite are common accessory phases and are particularly

abundant within this galena-rich layer. The tennantite (As-rich end member) is

confined largely to the upper portion of the galena-rich layer, adjacent to the Fe-

rich clay. Tetrahedrite (Sb-rich end member) appears to be more abundant

within the lower portion of the galena layer. Tetrahedrite and tennantite may also

occur within the fine-grained, galena-rich matrix (Figure 5.41b). The tennantite

and tetrahedrite may be intimately intergrown with chalcopyrite, galena and

amalgam (Figures 5.40a, 5.40b and 5.41b).

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The tennantite and tetrahedrite also occurs along the margins of former cavities

that are subsequently filled by siderite (Figures 5.39a, 5.40a and 5.40b). The

tennantite and tetrahedrite may exhibit euhedral morphologies, indicative of

growth into open pores (Figures 5.40a and 5.40b). Galena and amalgam exhibit

replacement relationships with the tetrahedrite and tennantite (Figures 5.40a and

5.40b).

Quantitative analyses of the tennantite and tetrahedrite grains are provided in

Appendix 5. The analyses confirm that the tennantite and tetrahedrite are close

to the theoretical end member compositions. The tennantite exhibits a small

range in Cu (40.2 - 45.8%), Fe (5.8 - 7.9%), Sb (0.0 - 1.4%), As (20.6 - 22.2%)

and S (27.8 - 29.8%) contents, with Zn being below detection limits (~0.5%). The

tetrahedrite exhibits a small range in Cu (35.7 - 36.8%), Fe (4.8 - 5.8%), Zn (1.5 -

2.5%), Sb (29.0 - 30.5%), As (0.4 - 1.6%) and S (25.2 - 26.0%) contents.

The galena-rich layer is characterised by the presence of a number of discrete

Cu-rich arsenides. The Cu-arsenides occur as narrow veinlets (Figure 5.42a)

and as radiating tabular or prismatic crystals that are present within siderite-filled

cavities (Figures 5.43a and 5.43b). The euhedral crystals probably reflect the

pseudomorphous replacement of arsenopyrite (pers. comms. Rob Ixer). Galena

and amalgam exhibit replacement relationships with the Cu-arsenide crystals.

Three discrete Cu-arsenides are evident. Quantitative analyses are provided in

Appendix 5. One of the Cu-arsenides (Cu 59.1 - 59.7%; As 34.9 - 36.7%

analyses #1 to #3) occurs as narrow veinlets (Figure 5.42a, lower of the two

veinlets). The veinlet commonly contains cores of amalgam. The amalgam cores

exhibit highly irregular and corroded morphologies. The amalgam may represent

a relict phase that has been largely replaced by the Cu-arsenide. The Cu-

arsenide phase exhibits a moderate reflectivity in reflected light and appears

steel-grey in colour. Minor amounts of Ag (4.3 - 4.5%) are also typically present.

The semi-quantitative analyses and optical properties are consistent with that of

the mineral novakite (ideally (Cu,Ag)21As10).

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The novakite may also be intimately associated with a Cu-arsenide phase that

exhibits a distinctive crimson red or cerise colour in reflected light (Figure 5.42b).

This phase is present in relatively minor amounts and typically occurs along the

margins of amalgam grains. These grains rarely exceed a few micrometres in

size. The SEM analytical totals (analyses #4 to #6) are relatively low due to the

very fine grain size of the Cu-arsenide grains. The results are, however,

consistent and suggest that this phase may represent a Ag-poor variety of the

mineral novakite (Cu 59.4 - 60.1%; As 35.0 - 37.8%). The distinctive crimson

colour of this phase is not consistent with that of novakite, however, rapid

tarnishing and the iridescence sometimes observed in novakite may, at least in

part, explain the anomalous colours.

The third Cu-arsenide phase also commonly occurs as veinlets that appear to

partially replace the amalgam (Figure 5.42a, upper of the two veinlets). This Cu-

arsenide phase is distinguished from the previous Cu-arsenides by its bluish grey

colour in reflected light. The quantitative SEM analyses and the appearance of

this phase in reflected light suggest that it represents the mineral koutekite

(ideally Cu5As2) (Cu 64.9%; As 33.3 - 33.4%). Novakite and koutekite are

relatively common accessory minerals within this sample interval and may also

occur within the fine-grained galena and amalgam-rich matrix.

Chalcopyrite is abundant in the galena-rich layer and typically occurs as granular

aggregates that are complexly intergrown with the fine-grained galena and

amalgam (Figures 5.41a and 5.41b). Chalcopyrite may occur within the

tetrahedrite-tennantite-rich aggregates, particularly within the siderite-filled

cavities (Figure 5.40a). The chalcopyrite aggregates typically exhibit highly

irregular morphologies and may be rimmed and replaced by galena (Figure

5.41a). Amalgam is also often intimately associated with the chalcopyrite (Figure

5.44b). Discrete chalcopyrite aggregates may exceed 150µm in size (Figure

5.41a).

Amalgam is abundant and hosts the bulk of the Ag and Hg content of this

sample. The amalgam grains exhibit a wide range in grain size with discrete

grains commonly exceeding several hundred micrometres (Figures 5.39b, 5.44a,

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5.44b and 5.45a). The amalgam grains commonly exhibit highly irregular grain

boundaries, possibly indicative of some degree of dissolution (Figures 5.44b and

5.45a). Some of the amalgam aggregates exhibit more rounded morphologies

(Figure 5.44a). The fine-grained galena-rich matrix typically exhibits some

degree of deformation around the margins of the larger amalgam grains (Figures

5.39b and 5.44a).

The larger amalgam aggregates are typically elongated and exhibit a preferred

orientation parallel to the lamination observed in hand specimen. The larger,

elongated amalgam aggregates also commonly occur within specific horizons

within the galena-rich zone and may have originally occurred in the form of

narrow veinlets that have been fragmented during dissolution or replacement

(Figures 5.39b, 5.44a and 5.45a). Finely disseminated amalgam also typically

exhibits highly irregular morphologies that may represent the remnants of larger

grains that have been partially replaced by other phases, notably galena (Figures

5.42b and 5.44b). Quantitative SEM analyses of the amalgam are provided in

Appendix 5 (analyses #4 to #10). The amalgam exhibits minor variations in Ag

(52.9 - 59.9%) and Hg (39.7 - 46.7%) contents.

A small number of Au-bearing amalgam grains were also observed during the

detailed examination of this sample (Figures 5.46a, b, c and d). The Au-bearing

amalgam is also typically present as highly irregular (Figures 5.46a, b and c) and

subhedral grains (Figures 5.46b and d). Quantitative SEM analyses of the Au

amalgam are provided in Appendix 5 (analyses #12 to #18). The analyses of the

amalgam are relatively consistent and exhibit only minor variations in Ag (45.7 -

48.1%), Au (16.4 - 21.0%) and Hg (34.1 - 36.8%) contents.

Examination of this sample at relatively low power magnification confirms that the

Au-bearing amalgam is present within a narrow (~1mm wide) band that also

contains significant amounts of angular quartz fragments. This quartz-rich band

is evident in Figure 5.39a, although the Au-bearing amalgam grains are not

resolved in this illustration. Examination of the fine-grained galena-rich matrix

both above and below this quartz-rich band failed to reveal the presence of any

other Au-bearing grains.

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A single, relatively large Au-bearing amalgam grain was also observed (Figure

5.46d). This Au-bearing grain was the largest observed in this suite of samples

and exceeded 100m in length. This amalgam grain is also compositionally

zoned and exhibits a wide range in compositional variation that largely reflects

the Au-rich nature towards the margins of the grain.

Quantitative SEM analyses were performed on the compositionally zoned grain to

illustrate the degree of variability (Appendix 5, analyses #19 to #24). The core of

the compositionally zoned grain is typically Au-poor (8.3 - 11.3%) and Ag-rich

(49.9 - 50.7%). Conversely, the margin of the grain is typically Au-rich (15.4 -

31.9%) and Ag-poor (31.6 - 41.9%). This suggests that the margins of the grain

may have been subjected to some degree of leaching of the Ag or that the Au-

rich amalgam was precipitated at a later date.

Cassiterite is a common accessory phase that typically occurs as small, angular

fragments that rarely exceed 30m in size (Figure 5.45b). Zircon is also present

in very minor amounts and also typically occurs as small, angular fragments

within the fine-grained, galena-rich matrix. Quartz, cassiterite and zircon are

extremely resilient to chemical and physical weathering and it is therefore

possible that these represent resistate phases that have originated from

elsewhere in the orebody. A discrete PbSb-sulphide and PbSbCl-oxide phase

(probably nadorite, ideally PbSbO2Cl) were also observed in very minor amounts.

Native bismuth is also moderately common and occurs within the fine-grained

galena-rich matrix, often associated with amalgam.

Native arsenic may be intimately associated with amalgam (Figure 5.43a). This

often forms rims on the amalgam grains and may also occur within the fine-

grained galena-rich matrix. It appears to have been subjected to some degree of

oxidation to form a discrete As-oxide phase (possibly arsenolite or claudetite,

both ideally As2O3). In addition, the fine-grained matrix may also contain minor

amounts of a discrete Pb(Fe)-arsenate phase. This phase is fine-grained and

porous in nature and could therefore not be positively identified. The Pb-rich

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arsenate becomes increasingly abundant towards the lower part of the galena-

rich layer and may contain fine-grained Ag-sulphide (Figure 5.45b).

5.5.4 Leached Pyrite-Rich Layer

Directly below the galena-rich layer is a layer of extensively leached pyrite

(Figures 5.47a, 5.47b, 5.48a and 5.48b) that extends for a depth of a few

centimetres, after which the degree of leaching is less prominent. The underlying

massive sulphide typically exhibits a low degree of porosity and the pyrite is

granular in nature (Figure 5.51).

The pyrite in the leached layer was probably subjected to oxidation. The

resultant goethite and/or jarosite oxidation products that would normally be

formed during this process have, however, been completely leached resulting in

a highly porous, pyrite-rich aggregate.

The extensive leaching of the pyrite was superseded by the precipitation of

galena, which lines the margins of the bulk of the pyrite grains (Figures 5.47a,

5.47b, 5.48a and 5.48b). The galena also appears to partially replace the pyrite,

evident by the progressive penetration of galena along narrow fractures in many

of the pyrite aggregates (Figure 5.47b). Detailed examination of the galena rims

confirms that they consist of tiny, euhedral galena crystals and botryoidal

aggregates.

The bulk of the porosity created during the leaching of the pyrite has been filled

by siderite (Figures 5.47a and 5.47b). The siderite may exhibit euhedral

morphologies, indicative of growth in open space (Figure 5.48b). Euhedral

crystals of pyromorphite (Figure 5.48b) are also a common feature. Discrete

pyromorphite crystals may exceed 100m in size.

Minor amounts of tetrahedrite (Figures 5.48a and 5.48b), chalcopyrite, Ag-

sulphide, Ag(As,Sb)-sulphide (probably proustite-pyrargyrite), a PbBa-sulphate

and a fine-grained and porous Pb-arsenate (Figure 5.48b) are also typically

developed within the open pore spaces and appear to post-date the deposition of

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the fine galena rims. The Pb-arsenate is extremely porous and variable in nature

and may be intimately intergrown with minor amounts of nontronite.

Amalgam is notably absent from this layer. Minor amounts of native As were

observed. Chalcopyrite (Figures 5.48a and 5.47a) replaces pyrite along

crystallographic planes and probably forms a component of the secondary Cu-

sulphide mineralogy.

5.5.5 Lower Core

This portion of the core consists of extensively fractured pyrite together with

subordinate amounts of a fine-grained, Al and Si-rich clay (probably kaolinite,

ideally Al2Si2O5(OH)4) (Figures 5.49a, 5.49b and 5.50a), chalcopyrite (Figure

5.49b) and enargite (Figures 5.49a and 5.49b). The kaolinite typically occurs

along grain boundaries and fractures developed within the pyrite (Figures 5.49a,

5.49b and 5.50a).

The kaolinite clay may be intimately associated with chalcopyrite, enargite

(Figures 5.49a and 5.49b) and an unidentified Ba-Al-silicate (Figure 5.50a).

Enargite typically forms discrete narrow veinlets within the fractured pyrite grains

(Figure 5.49a). Quantitative SEM analyses of the enargite are provided in

Appendix 5 (analyses #1 to #3) and confirm that the enargite is close to the

theoretical end member composition, consisting predominantly of Cu (46.1 -

46.5%), As (18.5 - 19.9%) and S (32.8 - 33.7%) and only minor Fe (0.9 - 1.3%).

The Sb content (0.2 - 0.4%) is below detection limits for this technique. Minor

amounts of supergene Cu-sulphide (Figure 5.50b), compositionally zoned

tetrahedrite and Se-bearing galena are also present locally within the pyrite-rich

core.

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5.6 Borehole CR194 – Massive Sulphide

5.6.1 Introduction

The sample intervals from 165.80 to 178.50 metres consist predominantly of

massive sulphide ore. The massive sulphide ore is intersected by a shale-rich

horizon that is characterised by its highly friable, quartz-rich nature and a marked

increase in the precious metal content. This shale-rich interval occurs at a depth

of between 172.50 and 174.50 metres. The 165.80 to 178.50 metre sample

intervals are described as two discrete components entitled ‘massive sulphide’

and ‘massive sulphide/shale’.

5.6.2 Massive Sulphide

The 165.80 to 172.50 metres and 174.50 to 178.50 metres sample intervals are

essentially similar in bulk mineralogy and are logged by the field geologists as

‘massive sulphide’. Pyrite is the dominant mineral and typically occurs as

granular aggregates that have been subjected to a significant degree of fracturing

(Figures 5.51a, 5.51b, 5.52b and 5.53b).

Detailed SEM examination of the pyrite confirms that it exhibits complex

compositional zoning that reflects minor variations the presence of As in solid

solution in the pyrite crystal structure (Figures 5.51a and 5.51b). XRD analysis

confirms the presence of pyrite and djurleite (ideally Cu1.9S). Supergene Cu-

sulphides are common in the massive sulphide and may consist of several

discrete phases, including djurleite, chalcocite and covellite, although only

djurleite was confirmed by XRD analysis. These phases are therefore broadly

described as ‘Cu-sulphides’, unless optical properties, XRD or chemical

composition indicate otherwise.

Supergene Cu-sulphide occurs along grain boundaries and in fractures within the

pyrite (Figures 5.50b). The Cu-sulphide often exhibits a prominent cleavage and

may contain abundant inclusions of bornite (Figure 5.52a). The Cu-sulphide may

occur as discrete euhedral crystals locally, indicative of growth in open space.

Galena is abundant and occurs as fine-grained aggregates and narrow veinlets

that may line the margins of small cavities or be complexly and intimately

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intergrown with the Cu-sulphide and other Cu-bearing sulphide minerals (Figures

5.52a, 5.52b and 5.53a). Minor amounts of a BaAl-silicate (Figure 5.52b) occur

along the grain boundaries of the granular pyrite aggregates and in fractures.

Insufficient material was available for identification by XRD.

Chalcopyrite (Figure 5.53b), enargite (Figure 5.54a) and members of the

tetrahedrite–tennantite solid solution series (Figures 5.54a, 5.54b, 5.55a and

5.55b) are also present in subordinate amounts along the pyrite grain boundaries

and in fractures. Relatively simple supergene enrichment textures are evident in

the chalcopyrite and bornite associations observed in the massive sulphide

sample (Figures 5.53a and 5.53b). These associations typically consist of relict

chalcopyrite that has been partially replaced by bornite, Cu-sulphide and galena.

The chalcopyrite probably represents a component of the primary Cu-sulphide

mineralisation.

The associations observed between Cu-sulphide, enargite and tetrahedrite are

complex and represent the partial replacement of tetrahedrite/tennantite and

enargite by later stages of supergene Cu-sulphide. Figure 5.54a, 5.54b and 5.55b

illustrate the presence of relict enargite crystals that are fractured and partly

replaced by tetrahedrite. The tetrahedrite is subsequently replaced by later

stages of Cu-sulphide mineralisation. The tetrahedrite/tennantite associated with

the enargite contains variable levels of Hg.

The tetrahedrite-tennantite also exhibits complex compositional zoning that

largely reflects variations in the As-Sb ratios (Figure 5.55a). The Hg content of

the tetrahedrite and tennantite may also exhibit some degree of variation.

Quantitative SEM analyses were performed on the Hg-bearing tetrahedrite-

tennantite grains and the results are provided in Appendix 5 (analyses #1 to #5).

The analyses confirm that the tetrahedrite-tennantite exhibits a wide variation in

composition, with the bulk of the analyses consisting of intermediate members of

the solid solution series. The Hg content (5.5 to 12.8%) is variable and

associated with equally variable levels of Cu (34.5 - 41.4%), Zn (1.8 - 4.0%), Sb

(9.2 - 22.2%) and As (3.2 - 12.8%). The S content (23.8 - 25.8%) is relatively

consistent and the Fe content is low (0.8 - 1.5%).

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5.6.3 Massive Sulphide/Shale

The massive sulphide/shale sample intervals occur at a depth of between 172.50

and 174.50 metres. This highly porous and friable layer consists predominantly

of quartz-rich rock fragments and massive sulphide fragments in a fine-grained,

quartz- and pyrite-rich matrix (Figure 5.56a and 5.56b). The relative proportions

of sulphide and transparent gangue vary considerably over relatively short

distances. XRD analysis confirms the presence of pyrite, djurleite, quartz and

galena.

The quartz in the massive sulphide/shale sample intervals consists of millimetre-

sized fragments in a matrix of micrometre-sized grains and every range of grain

size in between these extremes (Figure 5.56a). The quartz-rich rock fragments

are often porous and the quartz typically exhibits a fabric or preferred orientation

that is typical of the leached shale fragments observed in the gossan samples.

The shale-like rock fragments in the massive sulphide/shale sample intervals also

appear to have been leached of the phyllosilicate minerals that were presumably

present as a component of the original shale. Minor amounts of TiO2 (Figure

5.56b) and carbon are also present in the shale fragments and also in the fine-

grained quartz-rich matrix.

More compact quartz fragments are also present in these sample intervals,

possibly representing fragments of vein quartz (Figure 5.56a). The quartz grains

within the matrix are typically fine-grained in nature and exhibit highly irregular

morphologies (Figures 5.57a, 5.57b and 5.59a) possibly indicating dissolution.

Fibrous quartz is also developed in places along the margins of the pyrite and

within fractures.

Pyrite is the dominant sulphide mineral and the textures are similar to those

described for the massive sulphide sample interval. Pyrite is present as granular

aggregates (Figure 5.56a) and as euhedral crystals within the fine-grained matrix

(Figures 5.56b and 5.57b). The pyrite crystals do not exhibit signs of reworking,

fracturing, alteration or rounding. Locally, a later-stage, porous pyrite overgrowth

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is present on the earlier formed pyrite (Figure 5.58a). The pyrite exhibits

collomorphic textures and contains detectable (>0.5wt.%) Cu and Co.

The euhedral pyrite crystals and granular aggregates may exhibit complex

compositional zoning that reflects the presence of variable amounts of As (Figure

5.58b). Quantitative SEM analyses were performed on the pyrite (Appendix 5)

and confirm that the As content may exceed 2 per cent within discrete zones.

Complex enargite and tetrahedrite aggregates are a common feature and are

often replaced by Cu-sulphide (Figures 5.57a and 5.58b). Very minor amounts of

native Bi were also observed within a number of the tetrahedrite. The tetrahedrite

is Hg-rich and a single quantitative SEM analyses is provided in Appendix 5

(analyses #6). The analysis confirms that it is close to the theoretical end

member composition for tetrahedrite. The tetrahedrite contains extremely high

levels of Hg (21.7%).

Enargite is extensively fractured and replaced by the tetrahedrite, reflecting a

relative enrichment of Sb in the supergene ore. Quantitative SEM analyses are

provided in Appendix 5 (analyses #4 to #7) and confirm that the enargite is close

to the theoretical end member composition, consisting predominantly of Cu (47.5

- 50.0%), As (17.7 - 20.9%) and S (31.5 - 32.3%).

Djurleite is the dominant Cu-bearing phase as confirmed by XRD. The Cu-

sulphide occurs within the massive sulphide fragments (Figures 5.57a, 5.58b and

5.59b) and as euhedral crystals (pseudomorphous after arsenopyrite) and

granular aggregates within the fine-grained and porous quartz-rich matrix (Figure

5.57b). The Cu-sulphide commonly forms rims on the pyrite crystals and

aggregates (Figures 5.57b, 5.58a and 5.59a). Galena is also commonly

associated with the Cu-sulphide, occurring as fine-grained, skeletal intergrowths

and as rims (Figure 5.57b). Minor amounts of arsenopyrite (Figure 5.59a) are

also present within the fine-grained matrix, occurring as euhedral crystals that

exhibit replacement by Hg-tetrahedrite and Cu-sulphide.

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5.7 Borehole CR194 – Shale

5.7.1 Introduction

Borehole CR194 sample intervals that are classified as ‘massive sulphide’ cease

at a depth of 178.50 metres. Below this level, the core is classified by the field

geologists as ‘massive shale’ and is characterised by decreasing levels of base

and precious metals with increasing depth. A single sample from the 178.50 to

180.00 metres sample interval was collected for examination.

5.7.2 Mineralogy

The mineralogy of this sample is relatively simple. Although this sample is

classified as massive shale, the phyllosilicate minerals that would be expected in

a shale-like rock have been extensively leached. The quartz-rich rock is highly

porous and the quartz is largely fine-grained in nature (Figure 5.60b). The quartz

also exhibits a preferred orientation that would be consistent with shale (Figure

5.60b). The preferred orientation is also evident in hand specimen. Locally the

quartz appears more robust in nature and porosity is significantly reduced. This

is particularly apparent around the margins of pyrite aggregates (Figures 5.60a

and 5.60b).

Accessory minerals observed in minor amounts include TiO2 (Figure 5.60a) and

carbon. The TiO2 and carbon are finely disseminated throughout the quartz-rich

matrix of the shale.

Pyrite occurs as framboids typically ranging between 10 µm and 50µm in size

(Figures 5.60a and 5.60b). Framboidal pyrite is a common feature of shales and

other sedimentary rocks. The framboids may be partially replaced by Cu-

sulphide (probably djurleite) and to a lesser extent galena (Figure 5.60a). Pyrite

is also present as euhedral crystals that may exceed 200µm in size (Figure

5.60b). The framboidal and euhedral pyrite crystals are typically associated with

a region of low porosity quartz (Figure 5.60). The pyrite framboids and euhedral

crystals exhibit a preferred orientation parallel to the lamination developed within

the porous, quartz-rich matrix.

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5.8 Borehole CR194 – Summary Diagram

Section 5.3: The Gossan is characterised by a high Fe and low S content with moderate amounts of Pb. Base and precious metal content is low. Siderite, limonite and quartz are the dominant minerals together with subordinate amounts of Fe-clay, galena and Fe-sulphide. Accessory minerals include PbSbAs-sulphides, TiO2, zircon, cassiterite, apatite, Ag(Sb,As)-sulphides, barite, Ag-sulphide, cinnabar, bismuthinite, pyromorphite and amalgam. Microscopic native Au grains are rare.

Section 5.4: The Gossan/Massive Sulphide contact is characterised by a high Fe and low S content and elevated levels of Pb, Au, Sn, Ag, Bi, Hg, Sb and As. Siderite, limonite and Fe-clay are the dominant phases. Marked by an increase in the abundance of galena, pyromorphite, Ag-bearing sulphides, Au-bearing amalgam and amalgam. Accessory minerals include quartz, sternbergite, cassiterite, native Bi, Fe-sulphide, zircon, anglesite and PbSb-sulphides.

Section 5.5: The Massive Sulphide/Gossan contact is characterised by high Fe and S contents and elevated levels of Cu, Pb, Au, Sn, Ag, Bi, Hg, Sb and As. Dominated by the presence of galena, quartz, Fe-clay and siderite, with subordinate amounts of amalgam, tetrahedrite/tennantite, Cu-arsenides and chalcopyrite. Accessory minerals include Au-amalgam, cassiterite, zircon, pyromorphite and AlSi-clay. Pyrite becomes increasingly abundant with increasing depth.

Section 5.6: The Massive Sulphide contains high Fe, S and Cu contents and is dominated by pyrite and secondary Cu-sulphide with subordinate amounts of chalcopyrite, enargite, tetrahedrite, bornite, BaAl-silicate and galena.

Section 5.6.2: The Massive Sulphide/Shale is a highly porous zone that exhibits a slight decrease in the Fe and S content relative to the massive sulphide and elevated levels of Au, Ag, Pb, Sn, Bi, Hg and Sb. Pyrite and fine-grained quartz are dominant, together with subordinate amounts of Cu-sulphide, tetrahedrite, enargite and galena and minor TiO2, carbon, calcite and arsenopyrite.

Section 5.6: Massive Sulphide.

Section 5.7: The Shale exhibits a marked decrease in the precious and base metal content. Dominated by the presence of quartz and subordinate amounts of pyrite. Accessory minerals include Cu-sulphide, galena, TiO2 and carbon.

Figure 5.61 - Diagram illustrating the key mineralogical features for the 'Gossan', 'Gossan/Massive Sulphide Contact', 'Massive Sulphide/Gossan Contact', 'Massive Sulphide', 'Massive Sulphide/Shale' and 'Shale'.

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6.1 Introduction

Chapter 6 describes the chemistry and mineralogy of borehole CR194. The

sample list is provided in Appendix 2. Section 6.2 describes the major and minor

element chemistry of the borehole including their relative abundance and degree

of correlation.

The mineralogical description is grouped into four separate sections,

summarising the mineralogy of the ‘Tertiary Sand' (Section 6.3)’, ‘gossan’

(Section 6.4), ‘gossan/massive sulphide contact’ (Section 6.5) and the ‘massive

sulphide’ (Section 6.6) respectively. A summary diagram of the geochemistry

and mineralogy is provided in Section 6.7.

This borehole was selected for examination as a result of the extensive precious

metal mineralisation and the relatively central position relative to the underlying

massive sulphide and supergene copper sulphide mineralisation. The location of

borehole CR149 is illustrated and described in detail in Chapter 3. This borehole

is an inclined hole, the angle of dip being approximately 60 degrees.

The detailed mineralogical characterisation of this borehole is focussed on the

gossan samples, and in particular on the Au and/or Ag-rich intersections. The

sample intervals are extensively illustrated. These illustrations are provided in

Appendix 7. The nature of the core is highly variable (Figures 6.2 and 6.3).

In places, the core is competent in nature and exhibits high recoveries during

drilling. However, a significant proportion of the core is friable in nature and only

poor core recoveries were achieved.

The contact between the gossan and massive sulphide was not well preserved

due to the highly friable nature of the gossanous material directly above the

massive sulphide. However, marked changes in the mineralogy of the contact

zone are evident in the rubble-like material that was selected from this zone.

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Sample selection starts in the Tertiary sand and extends into the first two

intersections of the partial massive sulphide, below which precious metal content

was significantly depleted. Borehole CR149 intersects the fossil gossan at a

depth of 170.90 metres. Tertiary sand overlies the gossan. The massive

sulphide ore is intersected at a depth of 190.00 metres.

The characterisation of borehole CR149 was based on the preparation and

examination of 105 polished sections and 5 thin sections.

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6.1.1 Introduction

Borehole CR149 exhibits a wide range in chemistry that largely reflects distinct

changes in the mineralogy of the core and marks the prominent boundary

between the gossan and massive sulphide. The minor/trace element chemistry is

also variable with some correlation between elements. The major and minor

element chemistry data are provided in Appendix 3. These were plotted on

several graphs and combined with a diagram showing the position of each

sample interval (Figure 6.1).

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Figure 6.1 - Illustrating the chemistry variations in borehole CR149. Each sample interval is displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. The sample intervals examined from borehole CR149 consist of the Tertiary sand, gossan, gossan/massive sulphide contact and massive sulphide mineralisation. The variation in chemistry with increasing depth is displayed to the right of the borehole schematic. The major, precious and deleterious element chemistry clearly exhibits a significant degree of variation that reflects an equally wide variation in the mineralogy of each sample interval. The borehole depths represent depth down hole and are therefore not equivalent to depth from surface, with CR149 being an inclined hole. TSA - Tertiary Sand, GHS - Strong Hematitic Gossan, GMS - Strong Magnetic Gossan, GEM - Moderately Leached Gossan, GLM - Moderate Limonitic Gossan, GEW - Weakly Leached Gossan, GHM - Moderate Hematitic Gossan, GLS - Strong Limonitic Gossan, GLW - Weak Limonitic Gossan, MMP - Massive Sulphide.

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The Cu content is low within the Tertiary sand and gossan exhibiting a slight

increase within the partial massive sulphide. Due to the relatively low Cu content

of the core, no correlations are particularly evident with other elements discussed

in this section.

The Pb content of this borehole is also relatively low, with a marginal increase in

the Pb content of the core occurring in the lower portion of the gossan, close to

the contact with the massive sulphide. The increase in Pb at the gossan/massive

sulphide contact is associated with a more marked increase in the Au and As

content.

The Fe content of the Tertiary sand is moderate. The Fe content of the gossan is

highly variable, but remains relatively high throughout the gossan intersections.

The contact between the Tertiary sand and the gossan contains significant Fe,

but with increasing depth the Fe content reduces markedly. The central gossan

region then exhibits a marked increase in the Fe content. The relatively high Fe

content of the Tertiary sand contact and central portion of the gossan is

associated with a similar pattern of abundance to that observed for Au, Bi, As and

Sb.

This pattern is not, however, repeated for the gossan/massive sulphide contact,

where a marked increase in the Au, Bi and As content is associated with a

decrease in the Fe content. The Fe content of the massive sulphide is high and

remains constant with increasing depth. The Fe content of the massive sulphide

exhibits a strong correlation with that of the S content.

The S content is relatively low throughput the Tertiary sand and gossan, but

slightly increases towards the base of the gossan, close to the contact with the

massive sulphide. The S content of the massive sulphide is high and exhibits a

strong correlation with the Fe content.

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The Ag content is typically low throughout the core, but exhibit a marginal

increase at the contact between the gossan and the massive sulphide. The

increase in Ag at the gossan/massive sulphide contact is associated with a more

marked increase in the Au and As content and a slight increase in the Bi and Hg

content.

The variation in Au content down hole is highly variable, with numerous peaks

and troughs occurring throughout the gossan. The Tertiary sand is essentially

devoid of significant Au with the upper portion of the gossan containing significant

Au values.

The elevated Au content of the upper gossan region is associated with a similar

increase in the Fe, Bi, As and Sb content and very slight increases in the Ag and

Hg content. The high Au content in the upper gossan is characterised by a rapid

decrease and then significant increase in Au levels. This second 'spike' in the Au

content, which occurs in the fourth gossan sample interval (lithocoded

GLM/GEW) is somewhat spurious, and does not correlate with any other of the

elements described during this investigation.

The central gossan region is also characterised by the presence of another

significant increase in Au. This 'spike' in the Au content is associated with very

strongly correlated increases in the Fe, Bi, As and Sb content. The Au continues

to vary somewhat down hole and then increases markedly at the contact with the

massive sulphide. The increase in Au content at the gossan/sulphide contact is

associated with only relatively small increases in the Ag, Bi and Hg content and a

more marked increase in the As content. The Au content of the massive sulphide

is consistently low.

The As content of the upper gossan is moderate, but variable and exhibits a

similar trend to that observed for Fe, Pb, Bi, Sn and Sb. The As content

increases significantly in the central portion of the gossan and again towards the

gossan/sulphide contact zone, reaching a maximum slightly higher in the gossan

profile than that observed for Au. The As content of the massive sulphide

appears to increase with increasing depth.

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The geochemical profiles for Bi and Sb are essentially similar, exhibiting a

cyclical increase then decrease in the upper gossan, peaking in the middle

portion of the gossan. This profile is also recognised in Fe, Au and As, and to a

lesser extent, Sn. The Bi and Sb contents also exhibit marginally elevated

abundance at the gossan/massive sulphide contact, which is associated with

more pronounced increases in the Au and As content and a decrease in the Fe

content. The Bi content of the massive sulphide is low. The Sb content of the

massive sulphide exhibits a pronounced peak or spike several metres below the

gossan contact that is strongly associated with a similar spike in the Sn content.

The Hg content of this borehole is consistently low, but exhibits marginal

increases in the upper portion of the gossan and at the contact between the

gossan and massive sulphide. The Sn content is somewhat more variable,

exhibiting a similar profile to that of Fe, Bi, As and Sb in the upper gossan, but

then peaking very markedly, several metres above the gossan/sulphide contact.

This marked increase in the Sn content is associated with a small, and

significantly less pronounced increase in the S, Au and As content and a

decrease in the Fe content. The Sn profile also peaks several metres below the

gossan/sulphide interface, similar to that observed for Sb.

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6.2 Borehole CR149 - Tertiary Sand

6.2.1 Introduction

The Tertiary sand is characterised by the presence of moderate amounts of Fe

(10.95%) together with minor to trace amounts of S (0.75%), Cu (0.03%) and Pb

(0.16%). The Ag (3.6ppm) and Au (1.57ppm) contents of this sample are

relatively low. Minor to trace amounts of deleterious elements are also present

including As (113ppm), Bi (55ppm), Hg (0.3ppm), Sb (261ppm) and Sn (77ppm).

The Tertiary sand was taken at a depth of between 170.20 and 170.90 metres

close to the contact with the underlying gossan. XRD analysis confirms the

presence of quartz, plagioclase feldspar, glauconite, siderite, anatase, galena

and chlorite.

6.2.2 General Mineralogy

The Tertiary sand is dominated by the presence of rounded, millimetre-sized

aggregates of glauconite (ideally (K,Na)(Fe3+,Al,Mg)2(Si,Al)4(OH)2). The bulk of

the quartz fragments in the Tertiary sand range from between 10µm and 100µm

in size and consist predominantly of monocrystalline aggregates with fine-grained

polycrystalline aggregates occurring in lesser amounts. The crystallite size

typically ranges between 10µm and 100µm.

The quartz fragments are typically angular to sub-angular and do not exhibit the

highly irregular morphologies often seen in the gossans. No fibrous or

deformation textures were observed. The quartz is largely free from inclusions

and intergrowths and presumably forms a component of the original sediment.

Angular fragments of plagioclase and K-feldspar are also present (Figure 6.4b).

Siderite is common and typically occurs as euhedral crystals that exhibit

compositional zoning (Figure 6.4a). The compositional zoning reflects variations

in the Fe, Ca and Mg content. Quantitative SEM analyses are provided in

Appendix 5. The cores are typically FeO-poor (43.0-45.5%) and CaO and MgO-

rich (5.3-7.6% and 4.9-6.2% respectively) relative to the rims that are more FeO-

rich (49.7-51.1%) and CaO and MgO poor (3.3-3.6% and 2.7-3.4% respectively).

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The euhedral nature of the siderite suggests that it has grown in situ and does

not represent a component of the original sediment.

Subordinate amounts of fine-grained glauconite and chlorite are present along

the margins of the glauconite spheroids. This fine-grained glauconite and chlorite

matrix also hosts a number of accessory minerals including galena, pyrite,

sphalerite, ilmenite and chromite. The pyrite typically occurs as fine-grained

framboidal aggregates (Figure 6.4a). Qualitative SEM analysis of the ilmenite

confirms that it also contains minor amounts of Mn. The ilmenite may exhibit

some degree of replacement by sphene. No discrete precious metal-bearing

grains were observed in this sample.

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6.3 Borehole CR149 - Gossan

6.3.1 Introduction

The gossan occurs at a depth of between 170.90 and 190.00 metres and is

highly variable in both its nature (Figures 6.2 and 6.3) and composition. The

188.90-190.00 metre interval marks the contact between the overlying gossan

and massive sulphide and exhibits a markedly different mineralogy compared to

the bulk of the gossan and is therefore described separately in Section 6.5.

The gossan is characterised by the presence of highly variable amounts of Fe

(4.34–44.15%), S (0.57–8.87%), Cu (0.01–0.24%) and Pb (0.31–3.70%). The

sample intervals also contain significant but highly variable amounts of Ag (3.3–

71.7ppm) and Au (0.67–48.54ppm) and an abundance of minor elements

including As (63–4892ppm), Bi (178–3497ppm), Hg (0.6–14.5ppm), Sb (1112–

7556ppm) and Sn (68–7326ppm). The great bulk of the gossan consists of

extensively fragmented quartz-rich rock fragments that have been partially and/or

extensively replaced by reddish-brown siderite that is clearly observed in hand

specimen (Figures 6.2a and 6.3c). XRD analysis confirms the presence of

quartz, siderite, hematite, goethite, anglesite, rutile, anatase, lepidocrocite, native

sulphur, greigite, pyrite, marcasite, cassiterite and calcite.

Core recovery is particularly poor in many parts of the gossan, largely due to the

friable nature of much of the material. This is due partly to the extensive

oxidation of the siderite (Figure 6.3b), which results in an increase in porosity and

less competent core. The oxidation of the siderite to form limonite also results in

characteristic colour changes in the core (Figure 6.3b). A subordinate, but

significant portion of the gossan consists of fine-grained and porous quartz-rich

sediments that exhibit little evidence of replacement by siderite (Figures 6.2b,

6.2c and 6.3a). These fine-grained, quartz-rich sediments may exhibit some

degree of fracturing and/or brecciation and appear to consist predominantly of

extensively leached, fine-grained, quartz-rich fragments.

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6.3.2 Quartz

The quartz content of the gossan varies significantly and often occurs in close

association with siderite. The relative proportions of quartz and siderite vary

considerably throughout the core, ranging from samples that consist almost

entirely of quartz (Figures 6.2b, 6.2c and 6.3a), to more siderite-rich sample

intervals that contain relatively minor amounts of quartz (Figures 6.2a and 6.3c).

Quartz is the dominant phase in the upper portion of the gossan and decreases

in abundance towards the middle portion of the gossan and this is reflected by

the relative increase in Fe content (siderite). The quartz content increases again

in the lower portion of the gossan.

The quartz largely represents the relicts of extensively leached rock fragments

(Figures 6.5a, 6.5b, 6.6a, 6.6b, 6.7a and 6.7b). The gossan commonly exhibits a

fragmental nature with large, millimetre-sized quartz fragments occurring within a

fine-grained quartz-rich matrix (Figures 6.5b and 6.6b). The large quartz-rich rock

fragments and fine-grained matrix are occasionally cemented by later stages of

chalcedony that often form narrow veinlets (Figure 6.7a). Elsewhere in the core,

the fragmental nature is less evident, with the great bulk of the quartz occurring

as grains that rarely exceed 100m in size (Figures 6.5a and 6.6a).

In the siderite-rich portions of the core, the quartz typically occurs as angular

fragments that may exceed several millimetres in size. Fine-grained quartz is

also often disseminated throughout the siderite-rich portions of the core. The

discrete quartz grains commonly exhibit highly irregular morphologies that reflect

dissolution (Figure 6.5b). Fibrous textures were not observed in the quartz from

the gossan.

6.3.3 Siderite

Siderite is locally abundant and accounts for the characteristic red and reddish

brown colour observed in certain portions of the core (Figures 6.2a and 6.3c).

The proportions of siderite relative to quartz are more clearly reflected in the Fe

content of the gossan samples, as illustrated in Figure 6.1.

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Siderite commonly replaces and cements the porous, quartz-rich sample intervals

(Figures 6.5a, 6.5b, 6.6a, 6.6b, 6.7a and 6.7b). The great bulk of the siderite

occurs as granular aggregates that may also be associated with one or more Pb-,

and Fe-sulphides (Figures 6.5a and 6.6a). A subordinate portion of the siderite

occurs as discrete euhedral crystals that may exceed several hundred

micrometres in size (Figure 6.10a). These euhedral crystals may be extensively

oxidised and/or replaced by limonite.

Several stages of siderite mineralisation are clearly evident in these ores, with

early-formed siderite often highlighted by partial and/or extensive oxidation and

replacement by limonite (Figures 6.8a and 6.10a). Figure 6.10a illustrates the

presence of partially oxidised euhedral siderite crystals that are surrounded by a

later stage of unoxidised siderite.

Quantitative SEM analyses (Appendix 5) confirm that the siderite exhibits a

relatively uniform composition and consist predominantly of FeO (49.0-51.3%)

together with subordinate amounts of CaO (3.5-4.9%) and MgO (3.3-4.2%).

Compositional zoning is evident locally (Figure 6.21).

6.3.4 Limonite

XRD analysis confirms that the bulk of the ‘limonite’ consists of hematite, with

goethite being present in relatively minor amounts. Limonite typically occurs as

an oxidation product of siderite (Figures 6.8a and 6.9b). The oxidation of the

siderite to limonite results in a volume change that may increase the porosity and

friability of the core (Figure 6.3b).

Tiny, micrometre-sized limonite platelets (specular hematite) may also occur

within the more porous quartz-rich aggregates (Figure 6.7b). Limonite typically

replaces siderite along the margins and within growth zones or grain boundaries

(Figure 6.15b). Goethite occurs largely as a result of the hydration of hematite.

Lepidocrocite (ideally γ-Fe3+O(OH)) occurs locally as an oxidation product of the

Fe-sulphide assemblage and is often associated with native sulphur.

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6.3.5 Accessory Transparent Gangue Minerals

A number of transparent gangue minerals are present in the gossan in minor,

occurring largely in rock fragments. These include K-feldspar, plagioclase and

microcline. Barite is also locally abundant and may occur as inclusions in quartz-

rich rock fragments, or as fractured grains and crystals that have been

extensively replaced by siderite (Figure 6.10b).

TiO2 (rutile and anatase) is a common accessory and typically occurs as angular

grains and fragments within the fine-grained quartz-rich matrix. TiO2 grains rarely

exceed 30µm in size and probably represent resistate phases. Minor amounts of

fine-grained nontronite clay may also occur locally in the gossan (Figures 6.18a

and 6.18b) and is often intimately associated with siderite. The clay is particularly

abundant within the Au-rich horizons. Apatite was also recognised in the gossan

in very minor amounts.

6.3.6 Fe-Sulphides

Fe-sulphides are a common accessory and account for the magnetic nature of

the core. Greigite, marcasite and pyrite were positively identified by XRD,

although other Fe-sulphides are possibly present. These phases are described

in greater detail in Chapter 10. The Fe-sulphides typically occur as granular

aggregates and euhedral crystals (Figures 6.8b, 6.9b and 6.12b) with discrete

Fe-sulphide crystals rarely exceeding 50µm in size.

Fe-sulphides may be locally abundant, with granular aggregates exceeding

several hundred micrometres in size (Figures 6.9b, 6.11a and 6.11b). The Fe-

sulphides may also exhibit concretionary textures (Figures 6.11a and 6.11b) that

typically exhibit less well-developed morphologies than the discrete Fe-sulphide

crystals. These aggregates are often porous in nature and may exhibit complex

intergrowths with siderite and galena. Plate-like crystals of Fe-sulphide are also

present in the gossan, with discrete crystals rarely exceeding 50µm in length.

Fe-sulphides are often intimately associated with siderite, limonite (Figures 6.8b,

6.9b and 6.11b) and galena-rich aggregates (Figures 6.11a and 6.12a). The Fe-

sulphides occur predominantly along fractures and within the fine-grained quartz-

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rich matrix along mineral grain boundaries (Figures 6.7b and 6.8b). Oxidation

and replacement of the Fe-sulphides by lepidocrocite is also evident locally.

6.3.7 Galena and Pb-Bearing Sulphides

The bulk of the galena occurs within fine-grained aggregates that typically occur

along grain boundaries and within cavities in the quartz (Figures 6.6b, 6.10a,

6.11a and 6.12b). The textures observed in the galena are similar to those

described for borehole CR194, with discrete euhedral crystals (Figure 6.13a) and

skeletal aggregates (Figure 6.13b) being recognised. Minor amounts of As and

Sb are typically present in the galena aggregates, probably reflecting intimately

intergrown sulphosalts and/or some degree of Sb solid solution. Galena often

forms overgrowths on native Au grains with some of the best examples illustrated

in Figures 6.19b, 6.19c and 6.19d. XRD confirms the localised oxidation of

galena to anglesite.

Locally, euhedral crystals of discrete PbSb-sulphides are intimately associated

with siderite and galena (Figures 6.5a, 6.6a, 6.14a and 6.14b). These phases

are anisotropic and therefore differ from the PbSb-sulphide recognised in

borehole CR194. These phases were not positively identified.


Cassiterite is present in the gossan in variable amounts. Fine-grained,

micrometre-sized cassiterite is finely disseminated throughout the quartz and

siderite aggregates. Discrete cassiterite grains rarely exceed 10µm in size.

Cassiterite may be locally abundant where it occurs as fine-grained granular

aggregates within the quartz-rich core (Figure 6.15a). The cassiterite grains

typically exhibit a rounded morphology (Figure 6.15b) and may represent a

resistate phase.

Native bismuth (Figure 6.14b) and bismuthinite are present in minor amounts and

typically occur within the Pb-rich aggregates and/or in association with native Au

(Figure 6.22). Mimetite was also recognised during this investigation, occurring

close to the base of the gossan (Figure 6.13a), forming botryoidal aggregates

associated with galena and cerussite. The mimetite may partly represent the

Page 149


oxidation products of galena and/or other Pb-bearing sulphides. Barite is a

common accessory mineral that occurs throughout the gossan (Figure 6.10b) and

typically exhibits highly irregular morphologies indicative of extensive

replacement by siderite (Figure 6.10b).


The gossan is characterised by the presence of variable but significant amounts

of Au (0.67–48.54ppm). The systematic examination of the polished sections

prepared from this borehole revealed the presence of a large number of precious

metal-bearing grains (in excess of 100). The great bulk of the Au content of the

gossan appears to be present in the form of extremely fine-grained native Au

grains that rarely exceed a few micrometres in size (Figures 6.15b to 6.23b). The

morphology of the Au grains is highly variable and ranges from subhedral

(Figures 6.17b, 6.17c and 6.20a, for examples) to irregular (Figures 6.17a and

6.19a).

The distribution of the native Au grains is variable. The Au grains may be locally

abundant (Figures 6.18a and 6.18b), or occur as relatively isolated grains within

the porous, fine-grained quartz-rich matrix (Figures 6.19a, 6.19b, 6.19c, 6.20a,

6.20b and 6.20d) and within siderite (Figures 6.16 and 6.17). Galena is often

present as rims on the native Au grains (Figures 6.19b, 6.19c, 6.19d and 6.23).

A single occurrence of a bismuthinite rim on native Au was also observed (Figure

6.22). Rarely, the Au occurs as inclusions in Fe-sulphide (Figure 6.15b).

Qualitative SEM examination of the native Au grains confirms that they consist

predominantly of Au with the Ag content typically being below detection limits

(~0.5%). A small number of native Au grains contain detectable amounts of Ag.

The Ag content of the gossan is relatively low (3.3–71.7ppm). The Ag exhibits no

direct association with Au and this is confirmed by the high fineness of the native

Au grains. No discrete Ag-bearing phases were recognised in the gossan and it

is envisaged that the bulk of the Ag may be present in solid solution within the

more common sulphide minerals, notably the Pb-bearing sulphides.

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6.4 Borehole CR149 - Gossan/Massive Sulphide Contact

6.4.1 Introduction

The 188.90-190.00 metre interval marks the contact between the gossan and

massive sulphide and exhibits a markedly different mineralogy compared to the

bulk of the gossan. The gossan/massive sulphide contact is characterised by a

marked increase in the Au (42.75ppm), Ag (735.8ppm) and Hg (83.5ppm)

content. Macroscopic examination of the core also revealed a distinctive change

in the colour relative to the overlying gossan, exhibiting a mottled white and dark

grey colour. Siderite is notably absent in this portion of the core. The contact is

relatively poorly preserved due to the highly friable nature of the core. XRD

analysis confirms the presence of pyrite, quartz, galena, anglesite, greigite,

calcite and native sulphur.

6.4.2 Transparent Gangue

The transparent gangue mineralogy of the contact between the gossan and

massive sulphide is dominated by the presence of quartz and calcite (Figure

6.24a). The relative proportions of quartz and calcite are highly variable, with

quartz typically occurring in subordinate amounts. Quartz may be locally

abundant, where it appears to contain numerous cavities that have subsequently

been filled by pyrite and calcite. The quartz may also occur as angular fragments

that have been extensively fractured. Examination of the quartz in thin section

confirms that it consists of fragments and cryptocrystalline chalcedony that

typically infills former cavities (Figure 6.25b). These patches of chalcedony rarely

exceed 200µm in size. A single occurrence of native Au in a chalcedony-filled

cavity was observed (Figure 6.25b).

Calcite is abundant and gives the core a distinctive milky white appearance in

hand specimen. This is contrasted by the dark grey appearance of pyrite and

galena that typically fill fractures in the calcite. Calcite is locally abundant, where

it exhibits extensive fracturing and replacement by one or more of pyrite, galena

and a AgFe-sulphide mineral that is confirmed by optical properties as being

sternbergite (ideally AgFe2S3) (Figure 6.24a).

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6.4.3 Pyrite and other Fe-Sulphides

Pyrite is the dominant sulphide mineral and occurs as granular and porous

aggregates that occur along fractures in calcite (Figure 6.24a). Pyrite may also

occur as fine needle-like aggregates that probably represent the replacement

products of former marcasite or pyrrhotite aggregates. The pyrite aggregates are

often complexly intergrown with one or more of sternbergite, galena, native Au

and Hg (Figures 6.24a, 6.24b and 6.25a). Other Fe-sulphides (probably largely

greigite) are present within the pyrite-rich aggregates. These Fe-sulphides are

typically extensively replaced by the pyrite.

6.4.4 Galena

Galena is a common accessory mineral and appears to be present almost

exclusively as complex skeletal aggregates and euhedral crystals within the

pyrite and sternbergite (Figures 6.24b and 6.25a). The galena may be intimately

associated with native Au (Figure 6.25a), Hg and Se. XRD confirms the localised

oxidation of galena to anglesite.


A number of accessory minerals were observed including a Bi-bearing sulphosalt,

bismuthinite and a AgSb-sulphide (probably pyrargyrite). These phases were

intimately associated with the sternbergite and precious metal-bearing

aggregates.


Sternbergite is the dominant precious metal-bearing mineral and is intimately

associated with the pyrite (Figures 6.24a, 6.24b and 6.25a). The sternbergite

may exhibit complex textural relationships with skeletal galena and native Au

(Figure 6.25a). The sternbergite may exhibit some degree of oxidation locally,

with the development of a FeAg-sulphate phase.

Discrete native Au grains are rare, with the great bulk of the Au occurring within

the fine-grained Pb- and Hg-rich aggregates that are associated with the

sternbergite. The Au content of this sample is relatively high. As only a few

occurrences of microscopic native Au were recognised, it is envisaged that the

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bulk of the Au may be present in a sub-microscopic form, probably within the

sternbergite. A discrete native Au grain was observed within a chalcedony-filled

cavity in calcite, associated with a fine-grained, Cu Fe-sulphide aggregate (Figure

6.25b).

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6.5 Borehole CR149 - Massive Sulphide

6.5.1 Introduction

Borehole CR149 intersects the massive sulphide at a depth of 190.00 metres.

The massive sulphide extends for several metres below the gossan. Due to the

relatively low precious metals content of the core, only the first two sample

intervals were selected for examination.

The massive sulphide directly below the gossan/massive sulphide contact is

characterised by a marked decrease in the Au (1.04ppm) and Ag (83.0ppm)

content relative to the overlying gossan. Due to the highly friable nature of the

core, the contact zone between the massive and overlying gossan was not well

preserved. The massive sulphide consists predominantly of pyrite together with

subordinate amounts of quartz and galena.


The massive sulphide directly below the gossan consists predominantly of pyrite

(Figure 6.26a and 6.26b). The pyrite occurs as euhedral crystals, granular

aggregates and as fine-grained aggregates that often exhibit primary textural

features (Figure 6.26b). The pyrite appears relatively fresh and does not exhibit

any evidence of dissolution and/or oxidation. The pyrite is, however, extensively

fractured (Figures 6.26a and 6.26b). Fine rims of galena commonly occur along

the margins of the pyrite grains and in fractures (Figure 6.26a). Quartz is the

dominant gangue mineral and may fill or partially fill the fractures in the pyrite

grains and aggregates (Figures 6.26a and 6.26b). Examination of the thin section

prepared from the massive sulphide confirms that the quartz exhibits a medium to

coarse-grained crystallite size.

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Section 6.3: The Tertiary Sand is characterised by a low Cu, Pb and S content and moderate amounts of Fe. The trace and precious metal content is moderately low. This sample interval is dominated by the presence of glauconite-like clay together with subordinate amounts of siderite, chlorite, quartz and feldspar. Accessory minerals include galena, pyrite, sphalerite, ilmenite and chromite.

Section 6.4: The Gossan contains highly variable amounts of Fe, largely reflecting variations in the relative proportions of the dominant minerals, siderite and quartz. Galena and Pb(SbAs)-sulphides are present in minor amounts throughout the gossan. The variable S content largely reflects the presence of Fe- and Pb-bearing sulphides. Fine-grained native Au grains are disseminated throughout the gossan and are particularly abundant in the upper and middle portions of the core. Accessory minerals include cassiterite, native bismuth, bismuthinite and mimetite.

Section 6.5: Calcite and quartz are the dominant minerals in the Gossan/Massive Sulphide Contact, which is also characterised by an elevated Au, Ag and Hg content. Fine-grained and complex native Au and native Hg are present in sternbergite, pyrite and galena aggregates.

Section 6.6: The uppermost portion of the Massive Sulphide is high in Fe and S and consists predominantly of pyrite together with subordinate amounts of quartz and galena.

Figure 6.27 - Diagram illustrating the key mineralogical features for the 'Tertiary Sand', 'Gossan', 'Gossan /Massive Sulphide Contact' and 'Massive Sulphide'.

Page 155



7.1 Introduction


samples selected for examination are provided in Appendix 2. Section 7.2

describes the major and minor element chemistry of the borehole including their

relative abundance and degree of correlation.

The mineralogical descriptions are grouped into three separate sections,

summarising the mineralogy of the ‘Quartz Replaced Tuffs' (Section 7.3), ‘Quartz

Replaced Tuff/Partial Massive Sulphide Contact’ (Section 7.4) and ‘Partial

Massive Sulphide’ (Section 7.5) respectively. A summary diagram of the

geochemistry and mineralogy is provided in Section 7.6.

This borehole was selected for examination because of the extensive precious

metal mineralisation and the marginal location relative to the massive sulphide

mineralisation. The location of borehole CR038 is illustrated and described in

further detail in Chapter 3. This borehole is a vertical hole.

Samples selected for examination consist predominantly of the quartz-replaced

tuffs (as described by the field geologists) that lie directly above more sulphide-

rich materials. Borehole CR038 does not exhibit the characteristic reddish-brown

gossan-style characteristics that were logged by the field geologists in the

previous two boreholes. The Au-envelope of quartz-replaced tuffs extends

between 150.80 and 157.25 metres, where they intersect partial massive

sulphide with clay.

The core is often friable in nature and consists of fine-grained sand-like materials

together with angular rock fragments that rarely exceed a few centimetres in

maximum dimensions. The core varies in colour from light grey to light yellow-

brown in the quartz replaced tuffs to a darker grey colour in the semi-massive

sulphide (Figure 7.2). The sulphide-rich material is also typically friable in nature.

Page 156


Core recoveries were relatively poor and contact zones are not well preserved.

Marked changes in the mineralogy of the core are, however, clearly recognised in

the polished section prepared from these materials. The friable samples were

wet screened into a number of size fractions and mounted in preparation for

examination using reflected light microscopy and SEM based techniques. A

small number of whole-rock polished sections were also prepared.

The characterisation of borehole CR038 was based on the examination of 84

polished sections and 5 thin sections. Samples of Au-bearing material from

borehole CR038 have been examined previously and the results presented in

R2644 (1996).

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7.1.1 Introduction

The variable chemistry exhibited in borehole CR038 reflects changes in the

mineralogy of the core. Only limited assay data is available for this borehole.

The assay data are provided in Appendix 3. A diagram combining the list of

selected samples together with the major and minor element chemistry is

illustrated in Figure 7.1.

Figure 7.1 - Diagram illustrating chemistry variations in borehole CR038. The sample intervals examined from borehole CR038 consist of quartz replaced massive tuffs and partial massive sulphide. The sample intervals are displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The variation in chemistry is displayed to the right of the borehole schematic. Distinct compositional zones are clearly evident, particularly at the tuff/sulphide contact, largely reflecting variations in the mineralogy of each sample interval. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. CR038 is a vertical hole. QTM - Quartz Replacement of Massive Tuff, MSPCL - Partial Massive Sulphide with Clay, MPS - Partial Massive Sulphide.

Page 158



The Cu content of the quartz tuffs is low, with marginally elevated Cu values

occurring in the underlying partial massive sulphide. The Pb content of this

borehole is also consistently low. Assay data for the Fe and S content of this

borehole were not available.

The Ag content of the tuff and sulphide intersection is low, with the exception of

the tuff/sulphide contact, where the Ag content exhibits a marked increase. This

increase is associated with a similar increase in the As content. The Hg and Au

content also increases significantly towards the base of the quartz-replaced tuff,

although slightly higher in the profile relative to the position of the elevated Ag

and As values.

Borehole CR038 exhibits highly elevated Au values close to the tuff/sulphide

contact zone. Although the Au content of the tuff intersections examined during

this investigation all exhibit significant Au values, the Au content peaks a few

metres above the contact zone. The elevated Au values are associated with a

similarly marked increase in the Sn content, with Ag, As and Hg also increasing

slightly lower in the geochemical profile.

The As content of the upper portion of the quartz replaced tuff is relatively low,

but increases significantly towards the base of the tuff, close to the contact with

the underlying sulphides. This increase in As content is closely associated with a

similar increase in the Ag content. The As content of the partial massive sulphide

is consistently high.

The Hg content of this borehole is similar to that described for Ag, occurring in

relatively minor amounts in the tuff and sulphide, but increasing significantly

above the tuff/sulphide interface.

The Bi content remains fairly constant with increasing depth, but increases

marginally with depth in the underlying sulphide intersections. The Sb content is

highly variable throughout the borehole, with no distinct zones or association with

other elements being apparent.

Page 159


The Sn content increases markedly, several metres above the tuff/sulphide

interface. This increase in Sn content is associated with a similar increase in the

Au content and occurs slightly higher in the profile relative to marked increases in

Ag, As and Hg. The Sn content also exhibits another marked increase several

metres below the tuff/sulphide interface.

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7.2 Borehole CR038 - Quartz Replaced Tuffs

7.2.1 Introduction

The quartz replaced tuffs occur at a depth of between 150.80 and 157.25 metres.

The 156.30 to 157.25 metre sample interval marks the contact between the tuffs

and underlying sulphide and is described separately in Section 7.4. The quartz

replaced tuffs contain only very minor to trace amounts of Cu (0.00–0.02%) and

Pb (0.06–0.58%). The Ag (3.8–22.0ppm) and Au (1.71–11.31ppm) contents of

these sample intervals are moderate but variable. Minor elements include As (0–

54ppm), Bi (19–40ppm), Hg (0.2–90.1ppm), Sb (36–252ppm) and Sn (144–

756ppm). The macroscopic examination of the core confirms that the quartz

replaced tuffs are typically friable in nature and consist predominantly of light

cream, grey and brown coloured quartz-rich rock fragments that may exhibit

some degree of replacement by siderite (Figure 7.2). XRD analysis confirms the

presence of quartz, pyrite, anatase, galena, goethite and cassiterite.

7.2.2 Transparent Gangue Mineralogy

Quartz is the dominant mineral, comprising in excess of 90 per cent of the core

locally. The bulk of the quartz is present as loosely aggregated, irregular shaped

fragments that range from a few micrometres to several millimetres in size

(Figures 7.4a and 7.6a). The highly irregular morphology of the fragments

suggests that a significant proportion of the quartz has been subjected to some

degree of dissolution, particularly the fine-grained quartz-rich matrix (Figures

7.8a, 7.8b and 7.16a).

Locally, the quartz may occur as larger, more coherent massive crystalline

aggregates (Figures 7.3a, 7.3b and 7.6b). These larger quartz fragments

commonly contain euhedral pore spaces that appear to represent the presence of

former pyrite crystals that have been subsequently oxidised and leached from the

quartz (Figures 7.3a and 7.3b). One or more of fine-grained quartz fragments

and siderite may partially or extensively fill these euhedral pore spaces (Figures

7.3a, 7.3b, 7.8a and 7.8b). Detailed examination of the quartz in thin section

confirms that it exhibits a highly variable grain size (Figures 7.4a, 7.4b, 7.5a,

7.5b, 7.6a to 7.6d and 7.7a to 7.7d).

Page 161


The size of the quartz crystallites is also highly variable and ranges from a few

micrometres to in excess of 100µm. The quartz also exhibits fibrous textures that

have developed along the margins of the euhedral pore spaces (Figures 7.4a and

7.4b).

The highly fragmented and irregular morphology of the quartz suggests a history

of brecciation and dissolution (Figures 7.6a and 7.6b). Many larger, more

coherent quartz-rich rock fragments consist of very fine-grained, fragmented

quartz fragments that have been cemented by later stages of chalcedony (Figure

7.4a, 7.4b, 7.5a, 7.5b, 7.7a and 7.7b). The chalcedony exhibits varying degrees

of recrystallisation.

With the exception of ring-like structures of anatase that occur in the quartz, the

bulk of the quartz is free from inclusions and intergrowths. The presence of

siderite, sulphide minerals and the precious metal mineralisation is strongly

influenced by the porosity of the quartz, with the euhedral cavities and fine-

grained, porous regions of quartz acting as conduits for the mineralisation.

Siderite is common and occurs along fractures and infilling cavities in the quartz-

rich rock fragments (Figures 7.3a, 7.3b, 7.8a, 7.8b, 7.9a and 7.9b). The bulk of

the siderite is relatively unweathered and exhibits little or no evidence of oxidation

and replacement by limonite. Several stages of siderite mineralisation are

evident, with euhedral crystals often being overgrown by later stages of siderite

(Figure 7.8a). The earlier formed siderite may also contain needle-like crystals of

a PbSb-sulphide that are either absent or less abundant in the later overgrowths

(Figure 7.8a). The later siderite overgrowths often exhibit anhedral

morphologies, despite being developed in cavities, possibly indicating that they

have been subjected to dissolution. Cassiterite (Figure 7.8b), anatase (Figure

7.9a), Fe-sulphides (Figure 7.9b) and native Au (Figure 7.16a) are closely

associated with the siderite mineralisation. Quantitative SEM analyses confirm

that the siderite contains variable amounts of CaO (5.8-9.2%) and MgO (5.5-

6.2%). Compositional zoning is also evident within some of the siderite.

Page 162


Minor amounts of Fe-rich clay (nontronite) were also observed along fractures in

the quartz (Figure 7.11a). A number of Au grains were locate within the

nontronite.

7.2.3 Ore Mineralogy

Fe-sulphides occur in minor amounts throughout the quartz replaced tuffs. The

Fe-sulphides occur as euhedral crystals and as needle-like grains that typically

form radiating aggregates within fractures and cavities in the fine-grained quartz-

rich matrix (Figures 7.9a, 7.9b and 7.11b). The Fe-sulphides may also be

intimately associated with the precious metal-bearing grains (Figures 7.12b,

7.12d, 7.13b, 7.14b, 7.15c and 7.16b).

Galena is the dominant Pb-bearing sulphide mineral and typically occurs as finely

disseminated aggregates within pore spaces and fractures. Discrete galena

grains rarely exceed a few micrometres in size. The galena is often intimately

associated with native Au (Figures 7.11b, 7.12c, 7.13a and 7.13c) and PbSb-

bearing sulphides. The PbSb-sulphides may also occur as rhomb-shaped

crystals and aggregates of radiating acicular crystals (Figures 7.8a, 7.8b and

7.11b) that rarely exceed 10µm in size. The PbSb-sulphides were not positively

identified due to their very fine grain size.

Galena may partially replace quartz along fractures and grain boundaries

(Figures 7.10a, 7.10b and 7.10c). Minor amounts of a Pb-selenide mineral

(probably clausthalite, ideally PbSe), was also identified.

Cassiterite hosts the bulk of the Sn content and occurs as granular aggregates

and euhedral crystals that typically occur within cavities and fractures in the fine-

grained quartz-rich matrix (Figures 7.8b, 7.13d and 7.14c). The cassiterite is

rarely associated with native Au (Figures 7.13d, 7.14a and 7.14c). Cassiterite

aggregates and crystals commonly exceed 20µm in size.

Anatase is a common accessory mineral and occurs as angular grains and

euhedral crystals that typically occur within the fine-grained quartz-rich matrix

(Figure 7.9a). The anatase often forms ring-like structures within the quartz that

Page 163


clearly represent neoformation of TiO2 species in cavities and are evidence of Ti

dissolution and reprecipitation (Figures 7.15c and 7.15d). The cavities within

which the anatase formed are largely filled by later stages of chalcedony

mineralisation. Numerous native Au grains were also observed as inclusions and

in close association with the anatase (Figures 7.15a, 7.15b and 7.15d).

Anatase and cassiterite are common resistate phases and may reflect

components of the primary mineralisation that have survived the extensive

reworking and dissolution that have affected these ores. However, the euhedral

nature of some of the cassiterite and anatase and the intimate nature of some of

the intergrowths with native Au clearly indicate that these phases formed in situ.


The Ag content of the quartz replaced tuffs is moderately low (3.8–22.0ppm).

Minor amounts of native Ag, sternbergite, Ag(SbAs)-sulphide (Figure 7.12a) and

iodargyrite (ideally AgI) were recognised and typically occur in association with

one or more of native Au, native Bi, galena, PbSb-sulphides and Fe-sulphides.

A large number of native Au grains were observed during this investigation. The

bulk of the native Au grains rarely exceed a few micrometres in size (Figures

7.10, 7.11 and 7.12), but may exceed 30µm locally (Figure 7.14d). The

morphology of the native Au grains is highly variable and ranges from subhedral

to irregular (Figures 7.10 through to 7.16).

The native Au typically occurs within cavities and along fractures in the fine-

grained quartz-rich matrix (Figure 7.16a), often occurring within euhedral cavities

(Figures 7.14a, b, c and d). This emphasises their secondary origin, as the bulk

of the Au present in the primary ores is present in a sub-microscopic form. There

is an intimate association between native Au and galena/PbSb-sulphosalts

(Figures 7.10, 7.11b, 7.12c, 7.13a and 7.13c), Fe-sulphides (Figures 7.12b,

7.12d, 7.13b, 7.14b, 7.15c, 7.15d and 7.16b), anatase and cassiterite (Figures

7.13d, 7.14a and 7.15a, b, c and d). Qualitative SEM analysis of the native Au

grains confirms that they are typically Au-rich, with the Ag content commonly

being below the detection limits for Energy Dispersive X-ray analysis (~0.5%).

Page 164


7.3 Borehole CR038 - Quartz Replaced Tuff/Partial MassiveSulphide Contact

7.3.1 Introduction

The contact between the quartz replaced massive tuffs and massive sulphide

occurs at a depth of between 156.30 and 157.25 metres and is characterised by

a marked change in the mineralogy of the core. The Cu (0.04%) and Pb (0.29%)

contents remain relatively low. The Ag (1240ppm) content exhibits a marked

increase. The Au content (1.33ppm) is moderate. Minor elements include As

(918ppm), Bi (26ppm), Hg (16.8ppm), Sb (126ppm) and Sn (216ppm). The

contact zone appears essentially similar to the silicified massive tuffs exhibiting a

light grey colour in hand specimen. The core is highly friable and rubble-like in

nature. The contact was not well preserved. XRD analysis confirms the

presence of pyrite and quartz.


The transparent gangue mineralogy is essentially similar to that described for the

quartz replaced tuffs. Quartz is the dominant gangue mineral and exhibits similar

textures to those described previously.

The quartz may contain euhedral cavities that represent the presence of former

pyrite crystals (Figure 7.17a). The quartz is often extensively fractured and the

highly irregular morphology of the quartz fragments may be indicative of some

degree of dissolution.


Pyrite typically occurs along the margins of the quartz grains and in fractures

(Figures 7.17a, 7.17b and 7.18b). With increasing depth, the core becomes

progressively more pyrite-rich and quartz-poor (Figures 7.19a and 7.19b).

Siderite is largely absent from the contact zone. The pyrite aggregates may be

fractured with quartz and/or goethite partially filling the fractures (Figure 7.19b).

The pyrite may also exhibit compositional zoning that largely reflects the

presence of minor amounts of arsenic.

Page 165


Goethite is common in the pyrite-rich portions of the contact zone (Figures 7.19a

and 7.19b). Euhedral pyrite does not oxidise as readily as other types of pyrite

(collomorphic overgrowths for example). The possibility exists that later stages

pyrite overgrowths once existed on the euhedral crystals, and these were

oxidised to produce the interstitial goethite. The fine galena rims that form

around the margins of the pyrite crystals provide further evidence for this theory

(Figure 7.19a). These rims appear suspended within the goethite and do not

touch the euhedral pyrite crystals. The fine galena rims may have originally

formed on a later stage pyrite overgrowth that has subsequently been oxidised.

During oxidation, galena is one of the last sulphide minerals to be oxidised, and

would therefore survive the partial oxidation of the pyrite.

Qualitative SEM analysis of the goethite also revealed the presence of minor

amounts of Si. This may represent minor amounts of quartz or FeSi-rich clay that

is present within the goethite.


Only a single, micrometre-sized native Au inclusion in pyrite was identified. A

significant proportion of the Au may therefore be present in a sub-microscopic

form, possibly in association with the Ag-bearing sulphide minerals. A HgAg-

sulphide (possibly imiterite) is present in the pyrite in very minor amounts (Figure

7.17b). A Ag-sulphide (possibly acanthite) was also recognised during this

investigation.

Sternbergite and proustite/pyrargyrite are the dominant Ag-bearing minerals

(Figures 7.17a, 7.17b, 7.18a and 7.18b). The sternbergite and

proustite/pyrargyrite are intimately associated with pyrite (Figures 7.17a, 7.18a

and 7.18b). Quantitative SEM analysis of a number of proustite/pyrargyrite

grains confirms that they are largely As-rich (14.6 - 16.7%) with the Sb content

being below detection limits and are therefore close to the proustite end member

composition (Appendix 5, analyses #1 to #6). A number of the

proustite/pyrargyrite grains are, however, compositionally zoned (Figure 7.18a)

with pyrargyrite (analysis #7, 19.1% Sb and 1.9% As) being intimately associated

with the proustite.

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7.4 Borehole CR038 - Partial Massive Sulphide

7.4.1 Introduction

The partial massive sulphide occurs at a depth of 157.25 metres. A single sample

of the partial massive sulphide ore from the 157.25 to 158.25 metre sample

interval was selected for characterisation. The Cu (0.21%) and Pb (0.05%)

contents are low. The Ag (9.9ppm) and Au (0.23ppm) contents are low. The As

(360ppm) content is moderate. The Bi (21ppm), Hg (7.9ppm) and Sb (54ppm)

contents are relatively low. The Sn (216ppm) content is moderate. This sample

exhibits a medium-dark grey colour and is highly friable. XRD analysis confirms

the presence of pyrite and quartz.


Quartz is the dominant gangue mineral and occurs along fractures and grain

boundaries associated with extensively fractured pyrite (Figures 7.20a and

7.20b). The quartz exhibits a medium to coarse-grained crystallite size. Fibrous

quartz is also present. The quartz is largely free from inclusions and

intergrowths. Minor amounts of TiO2 are also present.


Pyrite is the dominant sulphide mineral, is granular in nature and has been

extensively fractured. The pyrite may exhibit compositional zoning reflecting the

presence of minor amounts of As. With increasing depth, supergene Cu-

sulphides may also occur along the fractures in the pyrite (Figure 7.20b). The Cu-

sulphides are often highly porous in nature.

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Section 7.3: The Quartz Replaced Tuffs consist predominantly of quartz fragments and subordinate amounts of siderite. Accessory minerals include galena, PbSb-sulphide, Fe-sulphides, cassiterite and anatase. Ag-bearing minerals include native Ag, sternbergite, Ag(SbAs)-sulphides and iodargyrite. Abundant, fine-grained native Au grains occur in cavities and along the margins of the quartz fragments.

Section 7.4: The Quartz Replaced Tuff/Partial Massive Sulphide Contact consists predominantly of quartz fragments and subordinate amounts of pyrite and goethite. Accessory minerals include proustite/pyrargyrite and sternbergite, which host the bulk of the Ag content. Trace amounts of galena and AgHg-sulphide are also present. Native Au is rare.

Section 7.5: The Partial Massive Sulphide consists predominantly of pyrite and quartz together with subordinate amounts of secondary Cu-sulphides. Minor amounts of galena and TiO2 are also present.

Figure 7.21 - Diagram illustrating the key mineralogical features for the 'Quartz Replaced Tuffs', 'Quartz Replaced Tuff/Partial Massive Sulphide Contact' and 'Partial Massive Sulphide'.

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8.1 Introduction


sample list is provided in Appendix 2. Section 8.2 describes the major and minor

element chemistry of the borehole and includes details on their relative

abundance and any apparent correlation.

Section 8.3 describes the mineralogy of the ‘Tertiary Polymict

Conglomerate/Gossan Contact'. The mineralogy of the gossan is described in

Sections 8.4 (Upper Gossan), 8.5 (Middle Gossan) and 8.6 (Lower Gossan).

Section 8.7 describes the mineralogy of the ‘Partial Massive Sulphide'. A

summary diagram of the geochemistry and mineralogy is provided in Section 8.8.

Borehole CR191 is situated away from the main supergene ore body but contains

high levels of Au over a depth of more than 10 metres. The location of borehole

CR191 is illustrated and described in Chapter 3. This borehole is an inclined hole,

the angle of dip being approximately 70 degrees.

The mineralogical characterisation of this borehole is particularly focussed on the

Au and/or Ag-rich intersections. The field logs describe much of the gossan as

'extensively leached'. The core is often friable and exhibits a wide range in

textures, colour and associated mineralogy (Figure 8.2). The core ranges in

colour from pale grey and cream, through to red, dark reddish brown, dark grey

and black. The light coloured rock fragments are typically quartz-rich, with the

red and reddish brown colours typically reflecting the presence of siderite and/or

limonite. The dark grey and black core typically contains a larger proportion of

galena and Fe-sulphides. The Tertiary conglomerate situated directly above the

conglomerate/gossan contact zone was not available for characterisation.

The contact zones were often not clearly preserved. However, marked changes

in the mineralogy of the contact zones are evident. Sample selection starts at the

contact between the Tertiary conglomerate and the underlying gossan and

extends into the first three intersections of the supergene enriched massive

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sulphide, below which the precious metal content was significantly depleted.

Borehole CR191 intersects the fossil gossan at a depth down hole of 134.25

metres. The massive sulphide ore is intersected at a depth of 153.85 metres

down hole. The characterisation of borehole CR191 was based on the

preparation and examination of 85 polished sections and 7 thin sections.

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8.1.1 Introduction

Variation in the element chemistry marks the prominent boundary between the

gossan and massive sulphide with significant variations in chemistry within the

gossan reflecting areas of extensive leaching and/or element mobility. The assay

data are provided in Appendix 3.

Figure 8.1 - Diagram illustrating chemistry variations in borehole CR191. The sample intervals are displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The variation in chemistry is displayed to the right of the borehole schematic. Distinct compositional zones are clearly evident at the upper and lower portions of the gossan. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. Borehole CR191 is an inclined hole. TCP – Tertiary Polymict Conglomerate, GHW - Weak Hematitic Gossan, GEM - Moderately Leached Gossan, GMS - Strong Magnetic Gossan, GES - Strongly Leached Gossan, MMPXM - Massive Sulphide with Shale.

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The geochemical profile of borehole CR191 is illustrated in Figure 8.1. The Cu

content of the Tertiary conglomerate and gossan intersections is very low, with

marginally higher Cu values occurring in the underlying massive sulphide. Lead

occurs in relatively minor amounts in the Tertiary units, but exhibits a marked

increase, reaching a maximum in the upper portion of the gossan, several metres

below the Tertiary/gossan contact zone. The spike in the Pb value is strongly

correlated with a similar spike in the Ag, Bi, Sb and As values. Although other

elements may occur in significant amounts in this region, they peak at slightly

different positions in the geochemical profile. Notably, the increase in Pb content

is associated with a significant decrease in the Fe content. The Pb content is

significantly depleted in the middle portion of the gossan, but increases slightly

towards the base of the gossan. The Pb content of the underlying sulphides is

relatively low.

The geochemical profile for Fe is essentially similar to that for S, and the two

elements exhibit a strong correlation. The Fe and S content of the Tertiary

intersections is relatively low, but increases significantly in the upper portion of

the gossan. The elevated levels of Fe and S are also associated with similar

increases in the Pb, Au, Ag, Bi, Hg, Sn, Sb and As values. The central portion of

the gossan is characterised by consistently low levels of Fe and S. The Fe and S

content increases markedly towards the base of the gossan and into the

underlying massive sulphide intersections.

The geochemical profile for Ag is characterised by two prominent peaks,

occurring in the upper and lower portion of the gossan. The peak in Ag in the

upper gossan is strongly correlated with Pb, Bi, Sb and As. The Ag content of

the middle gossan region is low, but increases markedly, reaching a maximum at

the contact with the massive sulphide. At this point, the Fe and S content

increases significantly, but none of the minor elements appear to strongly

correlate with the elevated Ag values.

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Au exhibits a very strong correlation with Sn, and this correlation is evident in the

upper portion of the gossan and closer to the base of the gossan, approximately

two metres above the contact with the massive sulphide where minor increases

in the Ag, Hg and Sb contents are also recognised. The elevated levels of Au in

the upper gossan region are also associated with elevated levels of most of the

other elements, although their positions in the geochemical profile are slightly

different.

The As content of the Tertiary units is low, but increases significantly in the upper

portion of the gossan exhibiting a strong correlation with the other minor

elements, in particular Pb, Ag, Sb and Bi. The As content of the middle and

lower gossan is relatively low but increases to more significant levels in the

underlying massive sulphides.

The geochemical profile for Bi is closely associated with that of Ag, Pb, Sb and

As, peaking in the upper portion of the gossan. The Bi content also increases

very slightly towards the base of the gossan, where it is more closely associated

with increases in Au and Sn.

The Hg content of the gossan is consistently low, but very slightly increases in

the upper portion of the gossan, possibly associated with an increase in the Fe

content. A more prominent increase occurs close to the base of the gossan

where an association between Hg, Au and Sn may be evident. The elevated Hg

levels occur throughout the gossan/sulphide contact zone and into the upper

portion of the underlying sulphides, decreasing with increasing depth.

The Sb profile in the upper portion of the gossan is strongly correlated with high

levels of other elements, in particular Ag, Pb, Bi and As. Very minor increases in

the Sb content are also evident in the lower portion of the gossan, where a

correlation between Sb, Au and Sn may occur. The Sb content of the Tertiary

units, middle gossan and massive sulphide is low. The Sn content is strongly

associated with the Au content, occurring in elevated amounts in the upper

gossan and lower gossan regions.

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8.2 Borehole CR191- Tertiary Polymict Conglomerate/Gossan Contact

8.2.1 Introduction

The contact zone between the Tertiary conglomerate and gossan occurs at a

depth of between 134.25 and 135.25 metres. No sample material above this zone

was available for examination. The Fe (13.26%) content is moderate and reflects

the presence of siderite and/or limonite. The S content is very low (0.09%), with

minor amounts of galena/PbAsSb-sulphide and barite being observed. The Pb

content is moderately low (0.73%). The Cu (0.01%) content is low and no

discrete Cu-bearing minerals were recognised. The Ag (0.8ppm), Au

(<0.01ppm), As (81ppm), Bi (18ppm), Hg (0.7ppm), Sb (171ppm) and Sn

(41ppm) contents are also low.


This sample exhibits a pale cream colour and contains numerous reddish brown

veinlets. Transmitted and reflected light microscopy and SEM based techniques

confirm that this sample consists predominantly of extensively fractured

polycrystalline quartz aggregates together with interstitial siderite and limonite

(Figure 8.3a).

The quartz fragments range from only a few micrometres to several millimetres in

size (Figures 8.3a and 8.3b). The larger quartz aggregates typically exhibit a

medium-grained crystallite size of between 50µm and 300µm. The smaller

fragments typically exhibit a finer crystallite size with the quartz crystallites rarely

exceeding 100µm in size.

The quartz crystallites in the Tertiary conglomerate/gossan contact often exhibit

sutured grain boundaries, particularly in the smaller fragments that contain the

finer crystallites. This may partly relate to the stresses involved during the

extensive fracturing. The bulk of the quartz in the larger fragments exhibits

simple grain boundary relationships. The larger quartz crystals may exhibit

growth zoning that is marked by rows of tiny fluid inclusions.

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The fractures in the quartz may be partially filled by one or more of siderite,

galena and limonite. Siderite is present along the margins of the quartz fragments

and appears to partially replace the fine-grained quartz-rich matrix (Figure 8.3b).

The siderite exhibits a wide variation in the degree of oxidation and replacement

by limonite. Quantitative SEM analysis of the siderite, as provided in Appendix 5,

confirms that it typically contains minor amounts of CaO (3.4-5.8%) and MgO

(1.2-2.2%).

Locally, minor amounts of nontronite clay are present. Fine-grained aggregates

of galena are relatively common, occurring along the margins of the quartz

fragments associated with the siderite mineralisation (Figure 8.3b). Minor

amounts of a fine-grained and porous PbCa-phosphate (possibly a member of

the crandallite group of minerals) and TiO2 were also identified.

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8.3 Borehole CR191 - Upper Gossan

8.3.1 Introduction

The bulk mineralogy of the upper, middle and lower gossans for borehole CR191

is essentially similar to that described for borehole CR038. Variations in the

major elements in the gossan largely reflect the relative proportions of quartz,

siderite and limonite, together with the presence of accessory phases that host

the bulk of the trace elements and precious metals.

The upper gossan occurs at a depth of between 135.70 and 141.65 metres.

Examination of the core confirms that it is highly friable in nature and exhibits a

wide range in colour from dark reddish brown through to dark grey/black (Figure

8.4).

The Cu content of the upper gossan is consistently low (0.01–0.02%). No

discrete Cu-bearing phases were observed. The Pb (0.86–17.52%), Fe (12.00–

36.26%) and S (0.25–9.75%) contents are significantly higher than the middle

and lower gossans and largely reflect the presence of galena, Fe-sulphide and

siderite. The upper gossan is characterised by an elevated precious metal

content, exhibiting particularly high Au (0.01–12.04ppm) and moderate Ag (1.2–

58.6ppm) contents relative to the middle gossan, which is comparatively barren

(Figure 8.1). The As (99–16700ppm), Bi (65–3227ppm), Sb (342–5963ppm) and

Sn (54–9072ppm) contents are also high. The Hg content (0.2–9.8ppm) is

moderate. XRD analysis confirms the presence of quartz, goethite, rutile,

lepidocrocite, native sulphur, siderite, anglesite, galena, greigite, marcasite and

calcite.

8.3.2 Gangue Mineralogy

Quartz is common, but generally subordinate in abundance to siderite (Figure

8.6b). The quartz typically occurs as angular or irregularly shaped fragments that

may exhibit some degree of replacement by siderite. The quartz fragments

exhibit a wide range in grain size from millimetre-sized fragments to micrometre-

sized interstitial grains. The quartz fragments contain both medium- and fine-

grained polycrystalline aggregates.

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The upper portion of the gossan is particularly siderite-rich (Figures 8.5a and

8.5b). The siderite is similar to that described for previous samples and typically

contains minor, but variable amounts of CaO (4.2-4.6%) and MgO (4.6-5.5%).

Minor amounts of Mn are also rarely present in the siderite. The siderite may

exhibit some degree of oxidation and replacement by limonite (Figures 8.5a)

typically resulting in an increase in porosity.

The siderite may exhibit botryoidal textures (Figure 8.5b), or occur as discrete

euhedral crystals (Figure 8.5a). The siderite is often intimately associated with

galena (Figures 8.5b, 8.6a and 8.6b). Late stages of siderite mineralisation

reduce the porosity of the gossan locally.


Fe-sulphides are common and accounts for the magnetic properties of the core.

Only greigite and marcasite were positively identified by XRD analysis. The Fe-

sulphides are both fine-grained and complex and are therefore described in

greater detail in Chapter 10. The Fe-sulphides typically occur as granular

aggregates that are developed within cavities and are often intimately associated

with siderite and galena (Figure 8.5a). Fe-sulphides are locally abundant (Figure

8.7a) and often exhibit partial and extensive oxidation to limonite. XRD analysis

confirms that the oxidation of the Fe-sulphides typically results in the formation of

goethite, lepidocrocite and native sulphur as oxidation products (Figures 8.7a and

8.8a).

Galena typically occurs as fine-grained, skeletal crystals that are disseminated

throughout the siderite (Figures 8.6a and 8.6b). Skeletal galena may fill euhedral

cavities formed as a result of the dissolution of siderite (Figure 8.7b). Galena is

locally abundant, occurring as more coherent aggregates that occur along the

margins of siderite (Figure 8.5b). In the lower part of the upper gossan, where the

bulk of the Fe-sulphide has been oxidised to goethite, anglesite (ideally PbSO4) is

present (Figures 8.8a and 8.8b). Anglesite, is a common oxidation product of

galena. The anglesite often retains the skeletal textures of the former galena

crystals. Subordinate amounts of a PbSb-sulphide are also present locally.

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Fine-grained, TiO2 (largely rutile) and cassiterite are common accessory minerals

(Figures 8.8a and 8.8b) and are particularly abundant in the lower portion of the

upper gossan, where the degree of oxidation also appears to be high. The

cassiterite and TiO2 typically occurs as sub-rounded grains that probably

represent resistate phases. Discrete cassiterite and TiO2 grains rarely exceed

20µm in size (Figures 8.8a and 8.8b). Qualitative SEM analysis revealed the

presence minor amounts of Sn and V in the TiO2 grains as well as minor Ti in the

cassiterite, further highlighting the close association between these Sn- and Ti-

bearing phases.


Fifteen occurrences of native Au grains were located in the upper gossan

(Figures 8.8a to 8.10d). The grains rarely exceed a few micrometres in size and

exhibit subhedral and anhedral morphologies (Figures 8.8a to 8.10d). Ten native

Au grains were located in samples of the extensively oxidised and porous lower

portion of the upper gossan relative to only 5 grains in the upper portion.

The native Au grains may be firmly encapsulated within siderite (Figures 8.9a,

8.9b, 8.9c, 8.9d and 8.10b), or occur within cavities in the partially and

extensively oxidised matrix where they were probably originally associated with

Fe-sulphides (Figures 8.8a, 8.8b, 8.10a and 8.10c). A single occurrence of Au in

quartz (Figure 8.10d) was also observed. Native Au grains may be intimately

associated with galena (Figure 8.9b). The Ag content of the Au grains was below

the detection limit for EDX analysis (~0.5%). No discrete Ag-bearing grains were

located.

Due to the relatively small number of Au grains located during this study, it is

likely that a significant proportion of the Au content of this core may be present in

a sub-microscopic form.

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8.4 Borehole CR191 - Middle Gossan

8.4.1 Introduction

This precious and trace metal-depleted horizon occurs at a depth of between

141.65 and 149.15 metres. The core is friable in nature but typically exhibits a

pale grey colour with only very localised ferruginisation (Figure 8.11). The middle

section of the gossan is distinctively pale relative to the upper gossan (Figure

8.4). This section of core was logged by the field geologist as ‘leached gossan’.

The Cu content of the middle gossan is low (<0.01–0.03%). No discrete Cu-

bearing phases were observed. The Pb (0.17–1.87%), Fe (1.99–5.97%) and S

(0.30–2.69%) contents are consistently lower than that of the upper gossan. The

Au (0.60–4.96ppm) and Ag (5.3–12.8ppm) contents are also significantly lower

than the upper gossan (Figure 8.1). The As (117–1269ppm), Bi (38–208ppm),

Hg (0.5–7.4ppm), Sb (252–1107ppm) and Sn (1017–3209ppm) contents are

moderate but also notably lower compared to the upper gossan. XRD analysis

confirms the presence of quartz, siderite, rutile, anatase, galena, greigite, calcite,

marcasite, native sulphur and lepidocrocite.


The gangue mineralogy is essentially similar to the quartz-replaced tuffs of

CR038 and consists of a jumble of millimetres-sized quartz fragments in a finer-

grained matrix (Figures 8.12a, 8.12b and 8.13b). The aggregates of quartz are

commonly cemented by later stages of chalcedony that give the quartz a more

massive appearance when observed using SEM based techniques. The majority

of the quartz aggregates are angular or irregularly shaped. The quartz crystallite

size ranges from a few hundred micrometres to a few micrometres.

A subordinate portion of the quartz fragments are rounded. These rounded

'clasts' of quartz consist of polycrystalline aggregates that typically exhibit a

medium crystallite size. The rounded nature of these clasts suggests that they

have travelled some distance from their original source. Siderite may also

partially cement the angular quartz fragments and may partially replace the fine-

grained quartz-rich matrix (Figures 8.13a and 8.13b). Quartz veinlets may

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traverse the fine-grained and porous quartz-rich portions of the core (Figure

8.12b). Euhedral pore spaces are common in the quartz (Figures 8.12a and

8.12b), reflecting the presence of former pyrite crystals that have been

subsequently oxidised and leached. Siderite, Fe-sulphides and Pb-rich

aggregates may fill or partially fill the pores (Figure 8.12a).

Siderite is locally abundant and is often associated with Fe-sulphides, galena,

PbSb-sulphides, cassiterite and TiO2 (Figure 8.13a). The siderite typically

exhibits varying degrees of oxidation and replacement by limonite (Figure 8.14a).

The siderite also occurs as euhedral crystals that have developed in cavities

(Figure 8.14b). The siderite crystals commonly exhibit compositional zoning that

largely reflects variations in FeO (46.7-53.2%) and CaO (0.7-6.6%) with the MgO

content being more consistent (4.1-4.8%). The quantitative SEM analyses are

provided in Appendix 5.


Fe-sulphides are present in relatively minor amounts and occur as granular

aggregates and euhedral crystals that are present within cavities between the

quartz fragments (Figure 8.14b) often in close association with siderite (Figure

8.13a). The Fe-sulphides may also exhibit some degree of oxidation and

replacement by goethite and lepidocrocite (Figure 8.14b). Marcasite and greigite

were positively identified by XRD analysis, although it is likely that other Fe-

sulphides are present.

Galena is a common accessory mineral and occurs as fine skeletal grains,

euhedral crystals and as porous aggregates that are disseminated throughout the

core. The galena typically contains minor amounts of Sb and As and may reflect

the presence of discrete Pb(SbAs)-sulphides, although the fine-grained nature of

the intergrowths inhibited positive identification. The galena-rich aggregates

typically occur in cavities and within siderite-rich aggregates (Figures 8.12a,

8.12b and 8.13a). Fine, needle-like crystals of a PbSb-sulphide are also

occasionally present in the siderite (Figure 8.13a).

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Granular cassiterite and TiO2 are common accessory minerals, with discrete

grains rarely exceeding 50µm in size. The cassiterite and TiO2 occur within

siderite-rich aggregates and within the fine-grained and porous quartz-rich matrix

(Figures 8.13a and 8.13b). The cassiterite and TiO2 occur as rounded, sub-

rounded and irregularly shaped grains that probably represent both resistate and

neoformed phases (Figure 8.13b). Rutile and anatase were positively identified

by XRD techniques.


Only two native Au grains were located. This is not surprising considering the

low Au content of this portion of the core. The Au grains were present as

inclusions in quartz and are Au-rich, with Ag being below detection limits (~0.5%)

(Figure 8.14a).

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8.5 Borehole CR191 - Lower Gossan

8.5.1 Introduction

The lower gossan occurs at a depth of between 149.15 and 153.85 metres. The

core is friable in nature and exhibits a pale grey colour with some localised

ferruginisation (Figure 8.15). The lower gossan is similar in overall appearance

to the middle gossan. This section of core was logged by the field geologist as

‘leached gossan’.

The Cu content of the lower gossan is low (<0.01–0.01%). No discrete Cu-

bearing phases were observed. The Pb (0.37–0.93%) is also low relative to the

middle and upper gossans. The Fe (2.51–6.26%) and S (1.91–5.66%) contents

are notably lower than the upper gossan. The Au (0.85–10.74ppm) and Ag

(10.6–19.6ppm) contents are elevated relative to the middle gossan. The As

(396–495ppm), Bi (102–364ppm) and Sb (446–1301ppm) are similar to that of

the middle gossan. The Hg (7.7–22.5ppm) and Sn (882–6809ppm) contents are

relatively high. XRD analysis confirms the presence of quartz, galena, rutile,

anatase, marcasite, greigite, pyrite, native sulphur, lepidocrocite, jarosite and

szomolnokite, which is an oxidation product of pyrite.


The lower gossan is quartz-rich. Transmitted light microscopy confirms the

quartz consists of angular, irregularly shaped and less commonly rounded grains

that are often cemented by later stages of chalcedony (Figures 8.18b, 8.18c,

8.19a and 8.19b). The wide variety of textures, size and morphology observed in

the quartz suggests that the fragments have derived from several sources. The

quartz fragments range in size from several millimetres (Figure 8.19a) to a few

micrometres.

The quartz commonly contains abundant, euhedral cavities that appear to

represent former sulphide minerals (Figures 8.16b, 8.17a and 8.17b). The

cavities may be partially filled by Fe-sulphides, galena and siderite (Figures

8.16a, 8.16b, 8.17a and 8.17b). Quartz veinlets also traverse the core (Figure

8.17a).

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Fibrous textures are also a common feature of the quartz (Figure 8.18a). These

textures are particularly abundant in the large, irregularly shaped quartz

fragments (Figures 8.19a and 8.19b) and are also evident in the quartz veinlets.

Siderite is present in moderate amounts and represents a late-stage phase that

partially fills pores within the quartz-rich rocks (Figure 8.16a). Quantitative SEM

analyses (Appendix 5) confirm that the siderite typically contains minor amounts

of CaO (2.1-2.5%) and MgO (3.3-3.9%).


Fe-sulphides are present in minor amounts (Figure 8.16b) and occur as euhedral

crystals and granular aggregates within cavities in the quartz-rich rock fragments.

XRD and optical analysis confirms that the Fe-sulphides consist predominantly of

greigite that is often partially replaced by marcasite, although the nature of these

phases is fine-grained and complex. The Fe-sulphides may exhibit some degree

of oxidation and replacement by limonite (largely lepidocrocite). The Fe-

sulphides are often intimately intergrown with galena (Figure 8.16b). Pyrite

becomes increasingly abundant with increasing depth, largely at the expense of

greigite and marcasite (Figures 8.17a and 8.17b). Pyrite is also typically present

within cavities in the quartz fragments, or finely disseminated throughout the less

porous quartz-rich rocks (Figures 8.17a and 8.17b).

The cassiterite and TiO2 are finely disseminated throughout the core, occurring in

the fine-grained, quartz-rich matrix and throughout the more coherent quartz-rich

areas (Figures 8.17b, 8.20a, 8.20b and 8.21a). The cassiterite and TiO2 grains

range in size from a few micrometres to tens or hundred of micrometres (Figures

8.17b, 8.20a and 8.20b). The cassiterite may also occur as radiating aggregates

of euhedral crystals (Figure 8.21a). Cassiterite and TiO2 may be intimately

intergrown, possibly indicating neoformation of these phases (Figure 8.20b). The

cassiterite may also contain minor amounts of Ti in solid solution.

There is a common association between quartz, cassiterite, TiO2 and to a lesser

extent, zircon (Figures 8.20a and 8.20b). Examination of the quartz in thin

section confirms that the cassiterite and TiO2 are a component of the fine-grained

Page 184


matrix that has been cemented by later stages of chalcedony, giving the

appearance that the cassiterite and TiO2 are inclusions. The quartz, cassiterite

and TiO2 appear to have a resistate component and a neoformed component

precipitated from solution.

Minor amounts of fine-grained cinnabar are also present in the lower gossan, and

typically occur within the less porous quartz fragments as finely disseminated

grains. Cinnabar may exhibit replacement relationships with cassiterite (Figure

8.20a). Barite is present in minor amounts.


Only 3 microscopic native Au grains were located (Figures 8.21b, 8.22a and

8.22b). The largest Au grain exceeded 15µm in size (Figure 8.21b). The Au

grains exhibit subhedral (Figure 8.21b) and anhedral morphologies (Figures

8.22a and 8.22b) and are present in cavities in the quartz-rich aggregates

(Figures 8.21b, 8.22a and 8.22b). Minor amounts of sternbergite and

proustite/pyrargyrite were also present in close association with the largest Au

grain (Figure 8.21b). The Au grains were Ag-poor with only one of the grains

containing detectable amounts of Ag (Figure 8.21b).

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8.6 Borehole CR191- Partial Massive Sulphide

8.6.1 Introduction

The partial massive sulphide occurs at a depth of 153.85 metres and is

characterised by a marked increase in the Fe (26.55-30.29%) and S (31.11-

35.50%) content relative to the overlying leached gossan. The Cu (0.33-1.18%),

As (2408-3965ppm) and Sn (603-1130ppm) contents are moderate. The Ag

content (3.9-91.5ppm) exhibits a marked increase at the upper end of the

massive sulphide, close to the contact with the gossan. The Au (0.37-2.73ppm),

Pb (0.05-0.17%), Bi (44-63ppm), Hg (9.2-10.8ppm) and Sb (239-369ppm)

content is relatively low. XRD analysis confirms the presence of quartz, pyrite,

tennantite and chalcopyrite.


Quartz is the dominant gangue mineral (Figures 8.23a, 8.23b, 8.24a and 8.24b).

The quartz is present along fractures in the larger pyrite aggregates (Figure

8.23a) and also occurs as granular aggregates that form porous, rubble-like

intergrowths with pyrite (Figures 8.23b, 8.24a and 8.24b). The quartz fragments

rarely exceed 500µm in size. The quartz often exhibit euhedral morphologies

indicative of growth within open pores. Examination of the quartz in transmitted

light confirms that it exhibits a medium to coarse-grained crystallite size. Fibrous

quartz is also present along the margins of the pyrite and within fractures.

Pyrite is the dominant sulphide mineral and occurs as granular aggregates

(Figures 8.23a and 8.24a), angular fragments (Figure 8.23b) and euhedral

crystals (Figure 8.24b). The pyrite may be extensively fractured (Figures 8.23a

and 8.24a). The pyrite may also exhibit primary textural features that typically

exhibit some degree of recrystallisation. Enargite is a common accessory

mineral and typically occurs along the margins of the quartz and pyrite grains

(Figure 8.24a). Quantitative SEM analyses of the enargite are provided in

Appendix 5 (analyses #8 to #11) and confirm that the enargite consists

predominantly of Cu (40.4 - 42.2%), As (15.9 - 18.5%) and S (31.8 - 32.4%).

However, the enargite differs from those analysed in previous samples as it

contains moderate but significant amounts of Fe (7.0 - 7.8%) and Sb (0.3 - 3.7%).

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Tennantite and chalcopyrite were also detected using XRD techniques. Granular

cassiterite aggregates are common in the partial massive sulphide and are often

intimately associated with pyrite (Figure 8.24b). TiO2 is finely disseminated

throughout the porous, rubble-like quartz and pyrite aggregates (Figure 8.24b).

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Section 8.3: The Tertiary Conglomerate/Gossan Contact is characterised by very low levels of Cu, Pb, S, Ag, Au, As, Bi, Hg, Sb and Sn and consists predominantly of fragmented quartz together with subordinate amounts of siderite and galena-rich aggregates.

Section 8.4: The Upper Gossan is characterised by elevated of Pb, S, Fe, Ag, Au, As, Bi, Hg, Sb and Sn consists predominantly of siderite together with subordinate amounts of limonite, Fe-sulphides and galena/PbSb-sulphide. Cassiterite is disseminated throughout the upper gossan and accounts for the high Sn content. The small number of very fine-grained native Au grains suggest that the bulk of the Au is present in a sub-microscopic forms.

Section 8.5: The Middle Gossan is depleted in Cu, Pb, Fe, S, Ag, Au, As, Bi, Hg, Sb and Sn reflecting the quartz-rich nature of the core and relative paucity of those minerals that occur in abundance in the upper gossan.

Section 8.6: The Lower Gossan is characterised by elevated levels of Au, Hg, Ag and Sn. Quartz is the dominant mineral, together with subordinate amounts of Fe-sulphide, siderite and galena. Accessory minerals include cassiterite, TiO2, cinnabar and sternbergite. The bulk of the Au is present as microscopic and possibly sub-microscopic grains.

Section 8.7: The Partial Massive Sulphide consists predominantly of pyrite and quartz together with subordinate amounts of cassiterite, TiO2, enargite and tennantite.

Figure 8.25 - Diagram illustrating the key mineralogical features for the 'Tertiary Conglomerate/Gossan Contact', ‘Upper Gossan', 'Middle Gossan', 'Lower Gossan’ and ‘Partial Massive Sulphide’.

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9.1 Introduction


sample suite, field geologists' core log lithocodes and lens descriptions are

provided in Appendix 2. Section 9.2 describes the major and minor element

chemistry. Section 9.3 describes the mineralogy of the ‘Tertiary Polymict

Conglomerate'. The mineralogy of the gossan is described in Sections 9.4

(Upper Siderite Gossan), 9.5 (Middle Calcite Gossan) and 9.6 (Lower Siderite

Gossan). The ‘Gossan Contact with Shale Conglomerate’ and ‘Shale

Conglomerate Contact with Gossan’ are described in Sections 9.7 and 9.8

respectively. Section 9.9 describes the mineralogy of the ‘Partial Massive

Sulphide/Shale’. A summary diagram is provided in Section 9.10.

Borehole CR123 was selected for examination because of the extensive Au

mineralisation and the marginal position relative to the main supergene massive

sulphide mineralisation. The location of borehole CR123 is illustrated in Chapter

3. This borehole is a vertical hole. The precious metal mineralisation extends for

a depth of approximately 20 metres. The core is often friable in nature and

recoveries are poor.

Sample selection begins in the Tertiary conglomerate overlying the gossan and

extends into the first three intersections of the underlying shale. Borehole CR123

intersects the fossil gossan at a depth of 153.95 meters. The pyritised shale is

intersected at a depth of 176.00 metres. The characterisation was based on the

examination of 60 polished sections and 5 thin sections.

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The chemistry of borehole CR123 is variable, marking the prominent boundary

between the Tertiary conglomerate, gossan and shale. The assay data are

provided in Appendix 3. A diagram of borehole CR123 and the major and minor

element graphs are provided in Figure 9.1.

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Figure 9.1 - Diagram illustrating chemistry variations in borehole CR123. The sample intervals examined from borehole CR123 consist of Tertiary polymict conglomerate, gossan and quartz-rich shales that have been partially replaced by pyrite. The sample intervals are displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. A marked change in the chemistry of the borehole is evident at the contact between the shales and the gossan. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. The borehole depths are equivalent to depth from surface. TCP - Tertiary Polymict Conglomerate, GMS - Strong Magnetic Gossan, QXM - Quartz Replacement of Massive Shale, SXM - Massive Shale, EQU – Quartz Vein.

Page 191



The Cu content of the Tertiary conglomerate and gossan is low. The Cu values

are marginally higher in the underlying shales The Pb content of the Tertiary

conglomerate is low The available information suggests that the Pb content

varies significantly over relatively short distances down hole and peaks in the

central portion of the gossan. The elevated levels of Pb are associated with a

similar peak in the Fe content, with a less prominent rise in the S and Sb contents

and a decrease in the As content. The Pb content of the core rises again at the

contact between the gossan and underlying shale. This increase is associated

with a similar increase in the Au, Ag, Bi, Sn, Sb and Hg content. The Pb content

of the shale is low.

The Fe content of the Tertiary conglomerate is relatively high. The gossan

contains variable amounts of Fe. Some relationship between Fe and S is also

evident. The Fe content of the shale is also highly variable and increases

significantly in the middle portion of the shale, exhibiting a strong correlation with

S. Sulphur follows a similar pattern to Fe throughout the borehole, occurring in

moderate but variable amounts in the Tertiary conglomerate, gossan and shale.

Silver is present in minor amounts throughout the Tertiary conglomerate, gossan

and shale. The geochemical profile of Ag, although not particularly clear from the

diagram, follows a similar pattern to that of Hg. The Ag content reaches a

maximum value at the contact between the gossan and shale. Au is also present

in moderate amounts throughout the lower portion of the Tertiary conglomerate

and into the gossan, where it reaches a maximum value at the contact between

the gossan and shale. The Au content of the shale is low.

The As content of the Tertiary marl is low, but occurs in significant amounts in the

gossan and moderate amounts in the shale. Arsenic is not strongly correlated

with the other analysed elements. Bismuth follows a similar pattern of

abundance to Au, Ag, Sn, Hg and Sb. Bismuth is present in moderate amounts

in the lower portion of the shale.

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9.2 Borehole CR123- Tertiary Polymict Conglomerate

9.2.1 Introduction

The Tertiary conglomerate is situated directly above the gossan and occurs in the

upper portion of the 152.40 to 153.95 metres sample interval. This sample is

relatively poorly mineralised and contains minor amounts of Pb (1.12%) and Cu

(0.08%). The Fe (13.95%) and S (7.74%) contents are moderate. Minor but

significant amounts of Au (2.77ppm) and Ag (2.5ppm) are present. The As

(558ppm), Bi (148ppm), Sb (270ppm) and Sn (86ppm) contents are moderate.

The Hg content is low (0.8ppm). XRD analysis confirms the presence of quartz,

calcite, pyrite, glauconite, plagioclase and rutile.


The Tertiary polymict conglomerate (Figure 9.2a) consists predominantly of dark

green, rounded, poorly crystalline glauconite aggregates (ideally (K,Na)

(Fe3+,Al,Mg)2(Si,Al)4(OH)2) and angular quartz and feldspar fragments in a calcite-

rich matrix (Figures 9.2b and 9.3b). The glauconite exhibits a size range of

between 200µm and 60µm (Figure 9.2b and 9.3b).

Quartz is abundant and occurs as angular and irregularly shaped grains that

range in size from a few micrometres to several millimetres (Figures 9.2a, 9.3a

and 9.3b). The bulk of the quartz fragments are monocrystalline, with a small

proportion consisting of polycrystalline aggregates with discrete crystallites of

only a few micrometres in size. The quartz is typically cemented by calcite

(Figures 9.2a, 9.3a and 9.3b). The quartz presumably forms a component of the

original sediment, along with the glauconite, feldspars and shell debris.

Plagioclase and K-feldspar are subordinate in abundance to quartz and also

typically occur as angular and irregularly shaped grains that range in size from a

few micrometres to fragments that exceed 200µm (Figures 9.4a and 9.4b).

Calcite occurs as angular fragments that exhibit similar grain size and textures as

the quartz and feldspar (Figure 9.3a). Calcite also occurs in the form of shell and

coral fragments that commonly exceed 500µm in size (Figure 9.3a). A significant

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portion of the calcite occurs as fine-grained, crystalline cement that binds the

glauconite, quartz, calcite and feldspar fragments (Figures 9.2b, 9.3a, 9.3b, 9.4a

and 9.4b). Qualitative SEM analysis confirms that the calcite is often

compositionally zoned, representing the presence of minor amounts of Fe, Mn

and Mg.


Pyrite is the dominant sulphide mineral and occurs as granular aggregates and

as partially recrystallised framboidal aggregates (Figures 9.3a, 9.3b and 9.4a).

The pyrite also occurs as acicular crystals that probably represent pseudomorphs

after marcasite or pyrrhotite (Figure 9.4a). The pyrite typically occurs along the

margins of the glauconite, quartz, feldspar and calcite fragments and may

develop within cavities or form overgrowths on the common gangue minerals

(Figures 9.3a, 9.3b and 9.4a). The pyrite exhibits compositional zoning, reflecting

the presence of minor amounts of As. Galena is present in very minor amounts

and occurs as finely disseminated euhedral crystals (Figures 9.3a, 9.4a and 9.4b)

and as porous aggregates.

9.2.4 Accessory Mineralogy

Minerals observed in minor amounts include apatite, chlorite, a CaAl-phosphate

(probably crandallite, ideally CaAl3(PO4)2(OH)5.H2O) and TiO2. A single grain of

native Au was also observed. This grain is present along the margins of an albite

fragment, within the calcite-rich cement (Figure 9.4b). Qualitative SEM analysis

confirmed the presence of minor amounts of Ag and Cu within the native Au

grain.

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9.3 Borehole CR123 - Upper Siderite Gossan

9.3.1 Introduction

The contact between the gossan and Tertiary conglomerate was not well

preserved. The upper siderite gossan commences in the lower portion of the

152.40 metre interval and extends into the 154.85 metre interval. The gossan is

characterised by a marked increase in the Pb (16.49%) content and a significant

increase in the Au (4.47ppm), Ag (47.9ppm), As (2075ppm), Bi (338ppm), Sb

(1319ppm), Hg (66.3ppm) and Sn (388ppm) contents relative to the poorly

mineralised Tertiary conglomerate. The Fe (10.79%) and S (2.76%) contents are

lower than the Tertiary conglomerate. The Cu content of this sample is low

(0.2%). XRD analysis confirms the presence of siderite, anglesite, cerussite,

hematite, galena, pyrite, calcite, greigite and native sulphur.


This sample interval is similar to the siderite-rich core described for previous

boreholes and exhibits a distinctive reddish brown colour in hand specimen. It is

porous and friable and consists predominantly of siderite together with

subordinate amounts of Fe-sulphide and galena (Figures 9.5b and 9.6a). The

siderite is often extensively oxidised and replaced by limonite (largely hematite)

(Figures 9.5b, 9.6a and 9.8a). The oxidation of the siderite has resulted in a

volume change and increase in porosity of the core. Later stages of relatively

unoxidised siderite mineralisation are present locally. Quantitative SEM analyses

confirm that the siderite typically contains moderate amounts of CaO (6.7-8.0%)

and lesser MgO (3.0-4.8%). Minor amounts of MnO are also present locally in

the siderite (maximum 0.8% MnO).

Fe-sulphides are the dominant sulphide and occur predominantly as aggregates

of plate-like crystals (basal sections) within cavities and along the margins of the

siderite grains and aggregates (Figures 9.6a and 9.8b). The Fe-sulphide crystals

rarely exceed a few micrometres in thickness but may exceed 100µm in length

(Figures 9.6b and 9.8b). Granular aggregates and euhedral crystals of Fe-

sulphide are also present locally. The Fe-sulphides, at least in part, are strongly

Page 195


magnetic. Greigite and pyrite were identified by XRD techniques. The Fe-

sulphides are described in greater detail in Chapter 10.

Galena occurs as fine-grained and porous aggregates and tiny stringer-like

veinlets in cavities and along the margins of the porous siderite aggregates

(Figures 9.7a, 9.7b, 9.8a and 9.8b). XRD confirms the localised oxidation of

galena to anglesite and cerussite.


A small number of native Au grains were located (Figures 9.6b, 9.7a, 9.7b, 9.8a

and 9.8b) and are typically present in the porous, oxidised siderite in close

association with galena. The Au grains are typically subhedral and fine-grained

in nature, with the largest grain exceeding 25µm (Figure 9.7b). The native Au

grains exhibit a deep yellow colour when observed in reflected light (Figure 9.6b),

indicative of a low Ag content. Qualitative SEM analysis confirms that the Ag

content is typically less than 0.5 weight percent.

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9.4 Borehole CR123 - Middle Calcite Gossan

9.4.1 Introduction

This zone is situated directly below the siderite-rich gossan and occurs at a depth

of 157.05 metres, extending for approximately 3 metres where the core once

again becomes progressively more siderite-rich. The core consists of both

competent and more friable, rubble-like material. The core exhibits a distinctive

dark grey/black metallic appearance that is largely due to the presence of

abundant galena and Fe-sulphides. The presence of Fe-sulphides accounts for

the magnetic properties of the core. Core recoveries were extremely poor.

The Cu content of the core is low (0.03%-0.04%). The Pb (5.46-9.20%), Fe

(11.24-13.75%) and S (5.29-7.11%) contents are moderate and largely reflect the

presence of galena and Fe-S. The Ag (13.6-35.6ppm), Au (2.08-2.27ppm), As

(5136-5839ppm), Bi (348-403ppm), Sb (1413-1809ppm) and Sn (382-471ppm)

contents are also moderate. The Hg (8.8-20.0ppm) content is low. XRD analysis

confirms the presence of pyrite, galena, calcite, marcasite, harmotome, cerussite

and anglesite.


Calcite is the dominant gangue mineral and occurs as angular fragments and

granular aggregates that are intimately intergrown with galena and pyrite (Figures

9.9a through to 9.14b). The calcite occurs predominantly as angular fragments

that have been cemented or partially cemented by later stages of calcite

mineralisation (Figures 9.9a, 9.9b and 9.10b). The fragmented nature of the

calcite is more evident where galena and pyrite occur along the margins of the

calcite fragments (Figures 9.9a, 9.9b, 9.10b and 9.13a). Discrete calcite

fragments exceed 500µm (Figure 9.10b).

A subordinate portion of the calcite occurs as narrow veinlets that traverse the

core (Figure 9.10a). These veinlets may be several hundred micrometres in

width. These textures are similar to those observed between galena and siderite

in previous boreholes.

Page 197


The zeolite mineral harmotome (ideally (Ba,K)(SiAl)9O16.6H2O) is locally abundant

and occurs as radiating aggregates of acicular crystals in calcite. The

harmotome aggregates commonly exceed several hundred micrometres in size

(Figures 9.13a, 9.13b, 9.14a and 9.14b). The harmotome is often partially

replaced by pyrite (Figure 9.14a). The presence of harmotome was confirmed by

XRD. Quartz is present in very minor amounts in the calcite-rich gossan,

occurring as irregularly shaped grains that are disseminated throughout the

calcite (Figure 9.11b).


Galena is the dominant Pb-bearing sulphide mineral and occurs as fine-grained,

porous and skeletal aggregates that occur within cavities, along the margins of

the calcite grains and in calcite veinlets (Figures 9.9 and 9.10). XRD confirms the

localised oxidation of galena to anglesite and cerussite. A single occurrence of

native Bi was observed within the galena.

Pyrite is abundant and typically occurs as granular and/or porous aggregates that

are present along the margins of the calcite grains and partially fill cavities

(Figures 9.9a, 9.10a and 9.10b). The pyrite appears to replace porous Fe-

sulphide aggregates that are too fine grained and poorly crystalline for a positive

identification (Figures 9.12a and 9.12b). Fe-sulphides (excluding pyrite) are

present in subordinate amounts to the pyrite and occur as extremely porous

aggregates that are present along the margins of the calcite grains.


No discrete Au-bearing grains were located during this investigation, suggesting

that the Au may be present in a sub-microscopic form.

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9.5 Borehole CR123 - Lower Siderite Gossan

9.5.1 Introduction

This zone occurs at a depth of approximately 160.2 metres and extends for

approximately 3 metres. The core recovery below this zone was extremely poor

and no information is available on the nature of the core between 163.4 and

168.2 metres.

The core consists of centimetre-sized dark grey/black metallic fragments that are

extensively fractured and cemented by distinctive red-brown siderite (Figure

9.15). These fragments are essentially similar to the calcite/sulphide mineral

assemblage described for the calcite-rich gossan.

The Cu content is low (0.01–0.12%). The Pb (8.90–27.23%) and Fe (19.63–

32.64%) contents exhibit a marked increase relative to the overlying core. The S

(2.87–8.46%) content is also marginally higher than the calcite-rich gossan. The

Au (1.47–2.14ppm) and Ag (16.4–20.4ppm) contents are similar to the calcite

gossan, suggesting that the Au mineralisation is not directly associated with the

late-stage siderite that cements the calcite-rich fragments in this core. The Bi

(275–374ppm), Hg (5.4–9.1ppm), Sb (1481–2138ppm) and Sn (270–356ppm)

contents are also similar to the calcite gossan although the As (396–495ppm) is

significantly lower. XRD analysis confirms the presence of siderite, calcite,

galena, pyrite, greigite, marcasite, anglesite and native sulphur. The native

sulphur appears to be intimately associated with the oxidation of greigite.


Siderite is abundant and cements the fractured calcite/sulphide fragments

(Figures 9.15 and 9.16a). The siderite exhibits a high degree of crystallinity and

may also exhibit some localised degree of oxidation and replacement by limonite

(Figure 9.19a). The siderite is often intimately associated with fine-grained,

skeletal galena (Figure 9.17a and 9.17b) as well as a number of other Pb-bearing

phases, including cerussite (ideally PbCO3) and mimetite (ideally Pb5(AsO4)3Cl)

(Figures 9.18a and 9.18b). The siderite also appears to partially replace barite

(Figures 9.18a and 9.19a).

Page 199


Siderite may also form delicately banded botryoidal aggregates, indicative of

precipitation within open cavities. At least two stages of siderite mineralisation

are evident, with early-formed, often oxidised siderite occurring in close

association with later stages of unoxidised siderite. This is particularly evident in

Figure 9.19a, where siderite has extensively replaced barite and subsequently

been fragmented, reworked or subjected to dissolution, possibly more than once,

and recemented by later siderite mineralisation. The rounded morphology of the

siderite/barite ‘clasts’ may be indicative of dissolution rather than mechanical

transportation.

Compositional zoning is relatively common in the siderite, particularly in the later

stage mineralisation, notably the veinlets (Figure 9.18b). Quantitative SEM

analysis of the siderite confirms that it contains minor amounts of CaO (0.9-1.1%)

and MgO (1.2-1.7%) (see Appendix 5).

Calcite is common, but subordinate in abundance relative to siderite, occurring

predominantly as granular aggregates that are intimately associated with galena

and Fe-sulphides (largely pyrite) (Figures 9.16a and 9.16b). Nontronite is a

common accessory, occurring as a cavity filling, closely associated with siderite

(Figures 9.17a and 9.17b).

Barite is relatively common, forming coarse-grained aggregates that may exceed

several hundred micrometres in size (Figure 9.19a) The barite exhibits highly

irregular morphologies that appear to be the result of extensive dissolution and

replacement by siderite (Figures 9.18a and 9.19a).


Galena is the dominant sulphide mineral and occurs within the metallic grey/black

fragments associated with calcite, Fe-sulphides/pyrite (Figures 9.16a, 9.16b and

9.19b) and siderite (Figures 9.17a, 9.17b, 9.18a, 9.18b and 9.19a). The galena

typically occurs along the margins of the calcite grains as fine-grained and porous

aggregates of skeletal crystals, often infilling cavities (Figures 9.17a, 9.17b, 9.18a

and 9.18b). Discrete galena grains rarely exceed a few micrometres in size.

Galena may exhibit localised oxidation to anglesite.

Page 200


Fe-sulphides are abundant, are typically fine-grained and porous in nature and

also occur as granular aggregates (Figures 9.16a and 9.19b). The Fe-sulphides

exhibit less replacement by pyrite than the calcite-rich gossan. The Fe-sulphides,

at least in part, account for the magnetic properties of this portion of the core.


No discrete precious metal-bearing phases were located during this investigation

and it is assumed that the bulk of the Au may be present in a sub-microscopic

form.

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9.6 Borehole CR123- Gossan/Shale Conglomerate Contact

9.6.1 Introduction

This sample interval occurs at a depth of 168.20 to 169.00 metres and consists of

varying proportions of fine-grained galena and Fe-sulphide, together with

subordinate amounts of transparent gangue (Figures 9.20a to 9.23b). The core

is porous in nature. The gossan/shale conglomerate contact is characterised by

a marked increase in the Ag (69.7ppm) and Au (31.85ppm) contents. The Bi

(1578ppm), Hg (1160ppm), Sb (4536ppm) and Sn (1100ppm) contents are

markedly higher than the previous sample intervals. The As (2002ppm) and Cu

(0.09%) contents are similar to the siderite-rich gossan. The bulk of the S

(4.12%), Pb (6.4%) and Fe (9.60%) are present in the form of galena and Fe-

sulphides. XRD analysis confirms the presence of calcite, anglesite, galena,

pyrite, gypsum, quartz, cerussite, marcasite, greigite and barite. Gypsum

appears to represent a reaction/oxidation product associated with the Fe-

sulphides and calcite.


The contact zone consists of fragmented calcite aggregates that exhibit partial

and extensive replacement by galena and Fe-sulphides (Figures 9.20a, 9.22a

and 9.22b). Cerussite is also common and typically occurs as extensively

corroded aggregates that are intimately intergrown with galena and Fe-sulphide

(Figure 9.20b). The irregular morphology of the cerussite is indicative of

dissolution. Qualitative SEM analysis also revealed the presence of minor

amounts of Sr in the cerussite. Subordinate amounts of dolomite are also

present in this portion of the core (Figure 9.20b).

Galena is the dominant Pb-bearing phase and may extensively replace the

transparent gangue (largely calcite) (Figure 9.20a). The galena typically occurs

as fine-grained and porous aggregates (Figures 9.20a, 9.20b, 9.22a and 9.23b)

and as micrometre-sized skeletal crystals (Figure 9.22b). The skeletal galena

also occurs in narrow veinlets that traverse the core (Figures 9.21a and 9.21b).

Siderite is notably absent in the core. The galena may exhibit localised oxidation

Page 202


to anglesite. Qualitative SEM analysis revealed minor amounts of Sb within a

number of the galena aggregates.

Fe-sulphides are common and occur as acicular crystal and as porous and

granular aggregates that typically exhibit partial replacement by pyrite and

marcasite (Figure 9.22a). Greigite, marcasite and pyrite were identified by XRD.

A proportion of the Fe-sulphides are magnetic.

Sternbergite is the dominant Ag-bearing mineral and is intimately intergrown with

galena and Fe-sulphides (Figures 9.21a, 9.21b, 9.22a and 9.23b). Unresolved

intergrowths of Ag, Sn, Se, Pb, As, Hg and S-bearing phases are also present in

this core (Figures 9.23a and 9.23b)

Two native Au grains were located (Figure 9.23a), occurring as fine-grained

(<20µm) subhedral grains that are intimately associated with the galena

aggregates. Semi quantitative SEM analysis of these grains confirmed that they

contain approximately 20 weight percent Ag. Due to the paucity of Au grains

relative to the high Au content, it is assumed that the bulk of the Au is present in

a sub-microscopic form, probably associated with galena.

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9.7 Borehole CR123 – Shale Conglomerate/Gossan Contact

9.7.1 Introduction

This sample interval occurs at a depth of between 169.00 and 172.85 metres and

is markedly different from the gossan/shale conglomerate contact. This zone

contains significant amounts of Pb (6.94-13.95%), Fe (9.59-16.18%) and S

(16.03-21.75%). The Ag (175.3-181.0ppm) and Hg (3061-9525ppm) contents

are high and exhibit a marked increase relative to the overlying gossan. The Au

content is high (11.68-56.55ppm), although very few Au grains were located. The

Bi (758-1920ppm), Sb (801-4536ppm) and Sn (623-1437ppm) contents are

similar to the previous sample. The As content (174-210ppm) is notably lower

than the previous sample, whereas the Cu content (0.45-1.89%) is markedly

higher. XRD analysis confirms the presence of calcite, galena, anglesite,

gypsum and pyrite.

9.7.2 Transparent Gangue

The core consists of angular, millimetre-sized fragments of quartz and to a lesser

extent calcite that are present in a matrix of fine-grained calcite, subordinate

quartz and a host of sulphide minerals (Figures 9.24a, 9.24b, 9.24c, 9.25a and

9.26a). Examination of the quartz-rich clasts in transmitted light confirms that

they typically contain fine-grained crystallites that exhibit some degree of

replacement by calcite (Figure 9.27a).

A proportion of the quartz is also associated with a pyritisation event, where

euhedral crystals of pyrite and fibrous quartz replace the fine-grained and porous

quartz-rich matrix (Figures 9.27b, 9.28a and 9.28b). The fibrous quartz has been

encountered in most of the boreholes examined during this investigation,

occurring predominately as reworked fragments associated with euhedral voids.

The quartz fragments in the upper part of the conglomerate are typically pale

grey in colour (Figures 9.24a and 9.24b). The quartz within the lower part of the

conglomerate is typically dark grey, similar in appearance to the underlying

shales (Figure 9.24c). The pale colour is probably largely due to acid leaching of

the quartz.

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9.7.3 Pyrite

The relative proportions of pyrite and transparent gangue vary considerably. The

pyrite occurs along the margins of the calcite grains and is often associated with

one or more of cinnabar, proustite/pyrargyrite, pyrite and sternbergite (Figures

9.25b, 9.30a and 9.30b). The pyrite may be locally abundant, occurring as

massive sulphide fragments that exhibit partial replacement by galena, secondary

Cu-sulphides and Hg-tetrahedrite (Figures 9.29a and 9.29b).

9.7.4 Cinnabar and Sulphosalt Minerals

Cinnabar is a common accessory and typically occurs within the fine-grained

calcite matrix exhibiting a highly irregular morphology, probably indicating

dissolution and replacement (Figures 9.25a, 9.25b, 9.26a, 9.26b, 9.30a and

9.30b). Cinnabar aggregates may exceed 1mm (Figure 9.26). The cinnabar

aggregates are often intimately intergrown with proustite/pyrargyrite, pyrite and

sternbergite (Figures 9.25a, 9.25b, 9.26a, 9.26b, 9.30a and 9.30b). Minor

amounts of stibnite (Figure 9.26b) and native Au (Figures 9.26b and 9.31a) were

also observed in the cinnabar.

A host of other very fine-grained Ag, Pb, Cu, Hg, Se, Sn, Bi and As-bearing

phases were located but not positively identified, including a CuBi-sulphide

(possibly wittichenite, ideally Cu3BiS3) and a Hg-Se-sulphide (possibly

metacinnabar, ideally Hg(Se,S)). These phases typically occur within the calcite

matrix and within cavities associated with the massive sulphide fragments.


Proustite/pyrargyrite and sternbergite host the bulk of the Ag content, occurring

within the fine-grained calcite matrix associated with cinnabar and pyrite (Figures

9.25b, 9.26b, 9.30a and 9.30b). Quantitative SEM analyses (Appendix 5,

analyses #8 to #10) of the proustite (Ag 66.8%; Sb <0.5%; As 14.0% and S

18.1%) and pyrargyrite (Ag 60.1 - 61.1%; Sb 16.1 - 18.3%; As 5.1%; S 16.4 -

16.7%) confirm that they typically exhibit near end-member compositions.

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Only two discrete native Au grains were observed, including one occurrence

within a large cinnabar aggregate (Figures 9.26b and 9.31a) and a one in

association with calcite, galena and Hg-tetrahedrite (Figure 9.31b). The native

Au grains are less than 20µm in size and exhibit anhedral morphologies.

Semi-quantitative SEM analysis of the Au confirmed approximately 15 weight

percent Ag in the grain associate with cinnabar and approximately 30 weight

percent Ag in the grain associated with galena and tetrahedrite. The paucity of

microscopically visible Au grains suggests the bulk of the Au is probably in a

submicroscopic form.

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9.8 Borehole CR123 – Partial Massive Sulphide/Shale

9.8.1 Introduction

This sample interval occurs at a depth of 176.00 metres and extends for

approximately 5 metres to a depth of 181.50 metres, below which the shale is

largely unmineralised. The Cu (0.58-2.56%) content is variable, but significant.

The Fe (3.67-21.51% and S (4.11-26.98%) contents are variable and largely

reflect local variations in the degree of pyrite replacement of the shale. The Pb

(0.22-0.72%) content is low. The Au (<0.01-1.49ppm) content is significantly

lower than the previous sample interval. The Ag (1.4-115.1ppm) content is

moderately high. The As (63-846ppm), Bi (24-236ppm), Hg (0.8-134.2ppm), Sb

(171-509ppm) and Sn (38-135ppm) contents are all markedly lower than the

previous sample interval. XRD analysis confirms the presence of quartz, galena,

pyrite and covellite.


The upper portion of this sample interval is fragmented and consists of dark

grey/black shale-like rock fragments that have been partially replaced by pyrite

and late-stage quartz. The core becomes less fragmented with increasing depth

and the degree of pyritisation generally increases. The shale typically exhibits a

high degree of porosity (Figure 9.32a). The porosity decreases in areas of pyrite

and quartz replacement (Figures 9.32a and 9.33a). Examination of the quartz in

transmitted light confirms that it is fibrous in nature (Figure 9.33b). The bulk of

the fine-grained quartz matrix exhibits primary sedimentary layering and

represents an original component of the shale.

Pyrite is the dominant sulphide mineral and occurs as euhedral crystals and

granular aggregates that are disseminated throughout the shale. The pyrite is

typically intergrown with fibrous quartz and commonly exhibits a preferred

orientation (Figure 9.32b). Partially recrystallised framboidal pyrite is also a

common feature (Figure 9.34a). The pyrite may exhibit some degree of

replacement by galena and secondary Cu-sulphides (largely covellite) (Figures

9.32a and 9.34a). Minor amounts of TiO2 and carbon are also present (Figure

9.34b).

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Section 9.3: The Tertiary Polymict Conglomerate is poorly mineralised and consists of glauconite, quartz and feldspar and minor amounts of pyrite and galena. Native Au grains are rare.

Section 9.4: The Upper Siderite Gossan exhibits a marked increase in the Pb, Au, Ag, As, Bi, Sb, Hg and Sn contents and consists predominantly of extensively oxidised siderite and subordinate amounts of Fe-sulphide and galena. Native Au grains are rare.

Section 9.5: The Middle Calcite Gossan is characterised by high Pb, Fe and S contents, largely reflecting the presence of galena and Fe-sulphides. Calcite is the dominant gangue mineral. Accessory minerals include harmotome, quartz and bismuth. The Ag, Au, As, Bi, Sb and Sn contents are moderate. The Cu and Hg contents are low. No discrete Au grains were located.

Section 9.6: The Lower Siderite Gossan consists of fragments of calcite-rich gossan that have been extensively fragmented and cemented by later stage siderite. Siderite, Fe-sulphides and galena are abundant and host the bulk of the Fe, S and Pb content. Calcite is common. The Bi, Hg, Sb, Sn, Ag and Au contents are similar to the calcite-rich gossan. The As content is significantly lower. Accessory minerals include nontronite, barite, mimetite and cerussite. The Cu content is low. No discrete Au grains were located

Section 9.7: The Gossan/Shale Conglomerate Contact is essentially similar to the calcite-rich gossan and is characterised by elevated Ag, Au, Bi, Hg, Sb and Sn contents. Galena and Fe-sulphides host the bulk of the S, Pb and Fe. The As content is moderate and the Cu content is low. Accessory minerals include cerussite, dolomite, sternbergite and Ag-Sn-Se-Pb-As-Ag-Hg sulphosalts. Native Au grains are rare.

Section 9.8: The Shale Conglomerate/Gossan Contact consists of angular, millimetre-sized fragments of massive sulphide, pyritised shale, quartz and calcite in a matrix of fine-grained calcite and quartz. The bulk of the Pb, Fe and S occur in pyrite and galena. The high Ag and Hg contents reflect the presence of sternbergite, proustite/pyrargyrite, Hg-tetrahedrite and cinnabar. The Bi, Sb and Sn contents are similar to the gossan component. Accessory minerals include secondary Cu-sulphides, stibnite and Cu-Bi-Hg-Se-bearing sulphosalts. Native Au grains are rare.

Section 9.9: The Partial Massive Sulphide/Shale exhibits variable Fe and S contents reflecting pyritisation and silicification of porous quartz-rich shales. The Cu and Pb contents are low, reflecting the presence of minor amounts of secondary Cu-sulphides and galena. The low Au, As, Bi, Hg, Sb and Sn contents reflect the marked decrease in accessory sulphosalt minerals that typically host these elements. Accessory minerals include graphitic carbon and TiO2. No discrete precious metal grains were located.

Figure 9.35 - Diagram illustrating the key mineralogical features for the ‘Tertiary Polymict Conglomerate’, ‘Upper Siderite Gossan', 'Middle Calcite Gossan', 'Lower Siderite Gossan’, ‘Gossan/Shale Conglomerate Contact’, ‘Shale Conglomerate/Gossan Contact’ and ‘Partial Massive Sulphide/Shale’.

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Chapter 10 Environment and Formational Mechanisms

10 ENVIRONMENT AND FORMATIONAL MECHANISMS

10.1 Introduction

The presence of abundant siderite and subordinate amounts of associated galena

and Fe-sulphide (greigite) separate the Las Cruces gossan from other VMS

gossans described in the literature. Understanding the environment and

mechanisms behind the formation of the siderite and greigite in the Las Cruces

gossan is key to understanding the genesis of the present day deposit.

Siderite and greigite are unstable in oxidising environments and are not likely to

have formed during near-surface weathering conditions. In the presence of O2,

siderite is oxidised to goethite or hematite. The formation of stable siderite

requires an anoxic environment where reduced Fe (Fe2+) can exist in solution

(Berner, 1981). Similarly, greigite and associated Fe-sulphides only form under

very restricted set of Eh/pH conditions. The nature of the environment and

mechanisms of formation of siderite and Fe-sulphides, notably greigite, are

therefore summarised in this chapter. The references have been selected as

they have bearing on the processes that are being proposed in this thesis for the

formation of this unusual gossan mineral assemblage.

The literature covering the low temperature formation of siderite and Fe-sulphides

is focussed largely on sedimentary marine deposits and burial diagenesis.

However, other environments where these minerals are formed include aquifers,

lakes, swamps, soils and waste ponds. The mechanisms of formation largely

involve the microbial metabolism of organic matter.

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10.2 Siderite Formational Environment

10.2.1 Introduction

Organic matter is modified by several processes operating at different depths

during burial diagenesis (Irwin et al., 1977). Anaerobic (anoxic/reducing)

conditions occur in sedimentary environments when organic matter is deposited

at a rate exceeding the supply of dissolved oxygen. This depth, below which

there is no dissolved oxygen, marks the boundary between regimes of aerobic

and anaerobic metabolism (Claypool and Kaplan, 1974).

The interrelationships between sedimentological and ecological factors bring

about three distinct biogeochemical environments (Claypool and Kaplan, 1974).

These three distinct zones have been described by a number of authors, notably

Berner (1981), Curtis (1986) and Chapelle and Lovely (1992), with some

ambiguity and discrepancies among them. However, it is generally considered

that these zones consist of the following:-

1. Oxic zone

2. Sulphate reduction zone

3. Methanogenic zone

These zones are illustrated in Figure 10.1. Discrepancies in the nomenclature

used between authors are also described in this chapter. These three zones are

dominated by bacterial processes operating at low temperatures. At higher

temperatures (>50oC) and greater depths (>1000m), a fourth, abiotic zone

referred to as the 'decarboxylation zone' may dominate. Decarboxylation is also

described briefly in this chapter.

As sediments are buried, they pass successively through zones 1 to 3, within

which organic matter is being altered and carbon dioxide produced. The carbon

dioxide produced by all these reactions dissolves readily in porewater to increase

bicarbonate concentrations, often resulting in the precipitation of carbonate

minerals with distinctive carbon isotope values. The carbon isotopic composition

of the precipitated carbonate minerals, however, must be anticipated to be very

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different from that of marine reservoir bicarbonate (δ13C 0%o PDB) since the

source in each case is organic matter (δ13C -25%o PDB) (Irwin et al., 1977;

Curtis et al., 1986).

Reduction of Fe3+ to Fe2+ is also considered to be one of the most important

geochemical reactions in anaerobic aquatic sediments because of its many

consequences for the organic and inorganic chemistry of these environments

(Coleman et al., 1993). δ13C, Mn2+ and Fe2+ (Mn is largely absent in the Las

Cruces gossan) are the parameters most likely to record interpretable changes in

original pore water chemistry independent of carbonate mineralogy (Curtis et al.,

1986). Stable carbon isotopes and the behaviour of Fe during siderite formation

are therefore examined in detail in this Chapter.

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Figure 10.1 – A diagram illustrating the three distinct biogeochemical environments that mark the boundaries between regimes of aerobic and anaerobic metabolism. The schematic illustrates the approximate depths that the oxic, sulphate reducing and methanogenic zones occur, together with the typical δ13C values associated with the CO2 generated from the decomposition of organic matter (modified from Irwin et al., 1977 and Claypool and Kaplan, 1974).

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10.2.2 Oxic Zone (Berner, 1981)

This zone (Figure 10.26), also referred to as the 'aerobic zone' (Claypool and

Kaplan, 1974) or the 'bacterial oxidation zone' (Irwin et al., 1977; Spiro et al.,

1993) occurs in the uppermost part of the sediment, close or at the

sediment/water interface. The oxic zone is relatively open to downward diffusion

of O2 and tends to be no more than a few millimetres to centimetres thick (Curtis

et al., 1986). However, the overall depth of this zone is determined by the extent

of downward diffusion of O2 from overlying waters (Irwin et al., 1977).

This zone is distinguished from other zones by the absence of organic matter,

which has been completely decomposed by aerobic microorganisms prior to

burial (Berner, 1981). Berner (1981) also notes that ferric (Fe3+) oxide minerals

such as hematite are not necessarily an indication of an oxic environment, as

they can persist stably at relatively low levels of O2.

Aerobic respiration using organic matter is the most efficient energy-yielding

metabolic process (Claypool and Kaplan, 1974). The process of aerobic

respiration (the process of degradation of organic matter by aerobic organisms)

produces CO2 and may be expressed by the following formula (Claypool and

Kaplan, 1974):-

CH2O + O2 CO2 + H2O

Bicarbonate activities sufficient to cause carbonate super-saturation are unlikely

to be reached in the oxic zone because of upward diffusion into depositional

waters (Irwin et al., 1977) and as a result, siderite precipitation is unlikely.

Irwin et al. (1977) suggest that bacterial oxidation of the organic matter seem to

impose little fractionation of the C during aerobic respiration, such that very light

bicarbonate is to be anticipated, indicating δ13C of approximately -25%o (i.e. the

same as that of the original organic matter). Spiro et al. (1993) indicate a

moderately heavier range, of between -8 and -13%o for δ13C.

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10.2.3 Sulphate Reduction Zone (Curtis et al., 1986; Irwin et al., 1977)

This is referred to as the 'anoxic sulphidic zone' by Berner (1981) (Figure 10.1).

Within this zone, sulphidic conditions are brought about almost entirely by the

bacterial reduction of sulphate accompanying organic matter decomposition. The

reaction may be expressed by the following formula (Irwin et al., 1977; Claypool

and Kaplan 1974):-

2CH2O + SO42– S2- + 2CO2 + 2H2O

This process can proceed only under anoxic conditions after all dissolved oxygen

has been consumed by aerobes and therefore only takes place in anoxic basins

at or below the sediment/water interface in organic-rich sediments deposited in

oxygenated water.

Berner (1981) notes that O2 is rapidly depleted upon deposition in sediments rich

in organic carbon due to exhaustive aerobic decay. Even if the overlying water is

oxygenated, aerobic decay within the upper few millimetres of organic-rich

sediment will maintain anoxic reducing conditions. Given anoxic conditions and

reduced Fe, the primary factor that determines whether Fe-sulphides or siderite

will form is the presence of sulphide (H2S and HS-). The overall reaction for pyrite

formation is given by (Berner, 1981):-

3H2S + S + Fe2O3 → 2FeS2 +3H2O

Sulphate reduction is very common in marine waters because of the abundance

of dissolved sulphate in seawater. The first minerals formed as a result of the

reaction between hydrogen sulphide and detrital Fe minerals are a number of

monosulphide phases, chiefly mackinawite (ideally Fe1+xS) and greigite (ideally

Fe3S4). In the presence of excess H2S, these minerals are unstable and are

eventually converted to pyrite. However, if non-sulphidic conditions are attained

by the exhaustion of all sulphate and sulphide, the monosulphides can persist for

long periods of time and therefore may accompany siderite and vivianite (ideally

Fe3(PO4)2.8H2O) (Berner, 1981).

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The sulphate reduction zone extends down to depths of 1 to 10 metres in

sediments containing significant organic matter (Curtis et al., 1986).

Irwin et al. (1977) note that sulphate reduction seem to impose little fractionation

such that very light bicarbonate is to be anticipated, indicating a δ13C of

approximately -25%o. Above and within the sulphate reduction zone, the organic

matter decomposition adds 12C, steadily decreasing δ13C. Often, δ13C reaches a

minimum of ~ -20%o near the base of sulphate reduction (Malone et al., 2002).

10.2.4 'Methanic' or methanogenic zone (e.g. Berner 1981, Curtis et al., 1986)

This zone is also referred to as the bacterial fermentation zone by Irwin et al.

(1977) and Spiro et al. (1993) (Figure 10.1).

When the sulphate concentration of the water buried with the sediment is low, as

in brackish to fresh water environments, or in marine sediments below the zone of

sulphate reduction, carbonate or CO2 reduction replaces sulphate reduction as

the preferred process of anaerobic respiration (Claypool and Kaplan, 1974).

Berner (1981) suggests that non-marine sediments may more readily produce the

environments for formation of methanic siderite because the initial sulphate

content is on average 100 times less than that of seawater. Methane production

appears to occur immediately after sulphate reduction ceases, possibly because

the bacteria cannot tolerate dissolved sulphides (Irwin et al., 1977).

If sufficient reducible Fe is present as detrital minerals, all H2S formed from

sulphate reduction is precipitated to form Fe-sulphides in the sulphate reduction

zone. Consequently, continued Fe reduction at depth results in the build up of

Fe2+ in the interstitial water because insufficient H2S is present to precipitate it due

to a lack of interstitial sulphate, the primary source of the H2S. Ultimately,

depending on conditions, saturation with siderite is attained (Berner, 1981).

Accompanying the build-up of Fe2+, continued organic matter decomposition

results in the formation of dissolved methane. In this way the formation of siderite

is accompanied by methane formation (Berner, 1981).

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The sulphide minerals and the bacteria that produce H2S in sediments cannot

tolerate traces of oxygen without conversion to oxide minerals and death

respectively. In addition, H2S and O2 cannot coexist with one another in solution

as their coexistence is unfavourable thermodynamically and kinetically they

rapidly react (Berner, 1981).

Siderite is also inhibited from forming in marine environments because Ca2+

reacts preferentially with bicarbonate at normal marine concentrations. The

Fe2+/Ca2+ ratio in normal marine waters is two orders of magnitude too small to

permit siderite precipitation (Matsumoto, 1981). The presence of siderite is

therefore indicative of the absence of sulphate and the presence of organics and

is more commonly associated with freshwater (Berner, 1981).

Berner (1981) suggests that siderite forms through the combined effects of Fe

reduction and bacterial methanogenesis of organic carbon compounds. This

process can be expressed by the following reaction (Curtis et al., 1986):-

7CH2O + 2Fe2O3 3CH4 + 4FeCO3 + H2O

As with pyrite, the source of Fe is the reduction of detrital Fe oxides in a strongly

reducing, organic-rich sedimentary environment (Curtis et al., 1986).

Claypool and Kaplan (1974) believed that reduction of CO2 by biologically

produced hydrogen is the single most important mechanism. Borowski et al.

(1999) suggest that within marine sediments, microbially mediated methane

production generally occurs through two distinct pathways:-

1. CO2 reduction (CO2 + 4H2 CH4 + 2H2O) and

2. acetate fermentation (CH3COOH CH4 +CO2)

The methanic or methanogenic zone is not limited by external supply of oxidants

and may descend some hundreds of metres into the sediment column (Curtis et

al., 1986).

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Malone et al. (2002) note that methanogenesis in the marine environment

proceeds by CO2 reduction, typically producing CH4 with δ13C between -60 and

-80%o. Consequently, the residual δ13C increases throughout the zone of

methanogenesis, often reaching positive values of ~ +10%o.

Irwin et al., (1977) also suggest that methanogenesis imposes a very large

fractionation with bacterial methane (CH4) values of -75%o commonplace.

Consequently, carbon dioxide produced in this reaction must be heavy and is

estimated by Irwin et al., (1977), Curtis et al., (1986) and Spiro et al., (1993) to be

in the region of +15%o.

10.2.5 Methane Oxidation

Two additional processes of potentially significant relevance to the Las Cruces

gossan are methane oxidation and Fe reduction. These processes also have a

significant impact on the distinct biogeochemical environments under which

diagenetic carbonate minerals may form. Of potentially less significance,

although they should not be discounted, are the processes of nitrate reduction

and the higher temperature process of decarboxylation.

Malone et al. (2002) suggest that the upward transport of CH4 from the zone of

methanogenesis into the sulphate reduction zone and its subsequent oxidation

produces 12C-rich Dissolved Inorganic Carbon (DIC) and as a result the δ13C can

be much less than -20%o near the sulphate/methane interface.

The anaerobic oxidation of methane at the sulphate/methane interface (SMI) is

also noted by Borowski et al. (1999). Borowski et al. (1999) propose that this

reaction, involving sulphate, occurs within a localised horizon at the interface

between the sulphate reduction zone and methanic zone. Here, sulphate and

methane are consumed at the base of the sulphate reduction zone by the net

reaction:

CH4 + SO42– HCO3

– + HS– + H2O

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in a process called anaerobic methane oxidation.

Irwin et al. (1977) and Spiro et al. (1993) propose that methane oxidation

produces very isotopically light δ13C of between -70%o and -75%o. The anaerobic

oxidation of methane produces bicarbonate, increasing carbonate alkalinity and

saturation with respect to carbonate minerals. However, this zone is considered

by Borowski et al. (1999) to be a localised horizon and the precipitation of more

extensive volumes of very isotopically light carbonate (with respect to δ13C), such

as that encountered in the Las Cruces gossan, may be the result of other factors

such as the oxidation of methane derived from external sources.

Malone et al. (2002) describes highly negative δ13C values for calcite (as low as

-41.7%o) from the New Jersey shelf that must have formed from waters with a

large component of dissolved inorganic carbon derived from methane oxidation.

The authors suggest that the methane may have been oxidised or vented from

shelf sediments, perhaps during sea-level fluctuations during the Neogene.

Claypool and Kaplan (1974) and Malone et al. (2002) recognise that modern

continental shelves contain abundant methane produced through the bacterial

degradation of organic matter.

Lundegard (1994) suggests that highly negative δ13C compositions of calcite

cement in the Oseberg Formation, Norway, resulted from methane migration from

adjacent shale deposits, with the methane being derived from shallow biogenic

processes (methanogenesis). Lundegard (1994) concludes that considering the

probable burial depths at the time of calcite cementation (<500m), it is highly

unlikely that significant HCO3– would have been produced by thermal

decarboxylation of kerogen in adjacent mudrocks or that methane could have

been produced in adjacent mudrocks by thermogenic processes.

Lundegard (1994) notes that several additional factors indicate that methane was

oxidised during anaerobic sulphate reduction. The concentrations of Fe and Mn

in the calcite indicate that precipitation took place under reducing conditions. The

ubiquitous association of small amounts of pyrite and siderite with the calcite

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cement not only confirm that pore water was reducing but indicate that dissolved

sulphide was present at the time of calcite precipitation. Pyrite from calcite

cemented zones has sulphur isotopic compositions suggestive of an association

with sulphate reduction.

Larrasoaña et al. (2007) describe greigite and pyrrhotite formed as a by product

of microbially-mediated reactions in the sulphate, anaerobic oxidation of methane

(AOM), and the methanic/gas hydrate zones in diagenetically modified sediments.

They suggest that greigite formed in the strongly reducing conditions below the

sulphate zone as a result of the metabolic activity of microorganisms whose

populations are enhanced in the presence of gas hydrate. The authors conclude

that geochemical conditions favourable for formation and preservation of greigite

are a limited source of sulphide, so that pyritisation reactions are not driven to

completion.

10.2.6 Fe3+ Reduction

Berner (1981) describes not three, but four distinctive biogeochemical zones in

his original classification scheme. The fourth zone described by Berner is named

the 'Post Oxic' zone. With the exception of Spiro et al. (1993), who describes this

zone as 'suboxic', this zone is generally not ascribed a separate zone

classification as it contains several other distinctive processes within it, including

nitrate, Fe and Mn reduction (e.g. Malone et al., 2002; Curtis et al., 1986;

Coleman et al., 1993; Chapelle and Lovley 1992).

Nitrate, Fe and Mn reduction may be prevalent in the sediments of freshwater and

brackish water environments or other similar situations where availability of

dissolved sulphate may be the key limiting factor to sulphate reduction. In marine

environments, where dissolved sulphate is generally abundant, sulphate reducing

bacteria (SRB) produce H2S, which can reduce Fe oxyhydroxides to form Fe

sulphides. However, in non-marine environments, or where availability of

sulphate is low, nitrate, Fe and Mn reduction may dominate.

Of potentially greatest relevance to the Las Cruces gossan is the process of

organic matter oxidation by Fe3+ reduction (due to the abundance of available Fe).

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Mn reduction, although thermodynamically more favourable than Fe reduction

(and therefore naturally precedes Fe reduction) is likely to be a very limited

process in the Las Cruces gossan due to the low availability of reducible Mn. The

two processes are, however, essentially similar. Only Fe reduction is considered

here.

Curtis et al. (1986) note that Fe reduction is a major oxidant of organic matter

(especially in non-marine environments), where reduction of Fe3+ releases Fe2+ to

the diagenetic pore waters. The authors conclude that Fe3+ reduction may not be

mutually exclusive with other zones and may overlap with deeper diagenetic

reactions. Curtis et al. (1986) also note that aerobic oxidation, sulphate reduction

and methanogenesis reactions increase pore water bicarbonate activity and also

acidity, whereas Fe3+ reduction raises Fe activity but decreases acidity.

Combinations of these two reactions (Fe reduction with sulphate reduction or

methanogenesis) will therefore favour sulphide and/or carbonate precipitation

respectively (Curtis et al., 1986; Spiro et al., 1993). The authors express the

reduction of Fe3+ to Fe2+ with the following formula:-

2Fe2O3 + CH2O + 3H2O HCO3- + 4Fe2+ + 7OH-

The generation of Fe2+, bicarbonate and hydroxyl ions increases alkalinity and

carbonate mineral precipitation is favoured (Curtis et al.,1986; Coleman et al.,

1993):-

Fe2+ + OH- + HCO3- FeCO3 +H2O

Examples of Fe reduction in the literature are numerous. The formation of

siderite in organic-rich marine mudrocks has previously been associated with the

degradation of organic matter by anaerobic, methanogenic bacteria (Coleman et

al., 1993). Carbonate resulting from this process has a characteristic positive

isotope signature of between 10 and 15%o. Coleman et al. (1993) describe

geochemical and microbiological studies that suggest that contemporary

formation of siderite concretions in a salt marsh sediment resulting from the

activity of SRB. Instead of reducing Fe3+ indirectly through the production of Fe

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sulphides, some of these bacteria can reduce Fe3+ directly through an enzymatic

mechanism, producing siderite rather than Fe sulphides. The siderite concretions

are found in sediments that have been deposited in the last 50 years.

The results suggest that a high proportion of the micro-organisms living in the

concretion survive by anaerobic respiration such as Fe3+ or sulphate reduction

(Coleman et al., 1993). The authors also conclude that SRB may be an important

catalyst for Fe3+ reduction in other sedimentary environments as they are

abundant in the Fe3+ reduction zones of deep aquifers in the Atlantic Coast Plain

of the USA. As there is no apparent sulphate reduction in these zones and the

SRB cannot have been recently introduced to these deeply isolated

environments, the authors speculate that the SRB must survive by reducing Fe3+.

Lovley et al. (1990) investigate the possibility that microorganisms are catalysing

the ongoing reduction of Fe3+ in the sediments of deep (20-250m) aquifers in the

Atlantic Coastal Plain. The Fe3+ reducing microorganisms were capable of

reducing ferric oxides present in deep subsurface sediments. The authors

conclude that acetate-oxidising, Fe3+ reducing microorganisms were present in

sediments from all sites where Fe3+ reduction was still active, with

methanogenesis and sulphate reduction being the predominant terminal electron-

accepting processes in sites where microbially reducible Fe3+ had been depleted.

Lovley et al. (1990) propose that the enzymatic reduction of Fe3+ by

microorganisms reported here is the first mechanism of any kind actually shown

to have the potential to couple the oxidation of organic matter to carbon dioxide

with the reduction of Fe3+ under environmental conditions typically found in deep

aquifers. The authors also comment that despite its age (Late Cretaceous in this

case), organic matter in deep sediments can be slowly metabolised by anaerobic

microorganisms.

Chapelle and Lovley (1992) describe competitive behaviour between Fe3+

reducing bacteria and SRB, where the Fe reducing bacteria in zones of high Fe in

the Middendorf aquifer (South Carolina) maintain levels of dissolved hydrogen,

formate and acetate in the groundwater at levels lower than thresholds required

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by SRB. Where Fe is less abundant, the process is reversed and the levels of

hydrogen, formate and acetate increase to levels that allow SRB to become

active. There is a direct correlation between Dissolved Inorganic Carbon (DIC)

and dissolved Fe2+ within the high Fe zone, indicating that organic matter is being

oxidised to carbon dioxide during the reduction of Fe3+.

The authors conclude that based on their field evidence and laboratory

experiments, the results indicate that the activity and interaction of Fe3+ and

sulphate reducing micro-organisms in the Middendorf aquifer are responsible for

the development and localisation of high Fe groundwater. The results also

indicate that microbial activity is strongly influenced by hydrological factors such

as direction of groundwater flow and geological factors such as distribution of

sedimentary depositional environments in the aquifer (Chapelle and Lovley,

1992).

Murphy et al. (1992) comment that micro-organisms that utilise Fe3+ as a terminal

electron acceptor would have to come into direct contact with Fe3+ to reduce it.

This is because most Fe3+ in sediments is in solid form because of its low

solubility, noting that the less crystalline forms of Fe, such as amorphous Fe

hydroxides, are reduced most easily.

Irwin et al. (1977) found no information relating to fractionation during oxidation by

ferric compounds and have assumed that none occurs. Spiro et al. (1993)

propose a δ13C of approximately -25%o (i.e. the same as that for organic matter).

10.2.7 Nitrate Reduction

The degradation of organic matter through nitrate reduction is often considered

only briefly in the literature, presumably because, although it is a very efficient

energy yielding metabolic process (and therefore favoured by bacteria), the

availability of significant nitrate for the reduction process is generally limited and

other processes would therefore more likely dominate.

Claypool and Kaplan (1974) express the process of organic matter oxidation by

nitrate reduction with the following formula:-

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5CH2O + 4NO3– + 4H+ 2N2 + 5CO2 + 7H2O

10.2.8 Abiotic reactions - Thermally induced decarboxylation

At high temperatures (>50oC) (Claypool and Kaplan, 1974) and greater depths

(>500m; >1000m) (Lundegard, 1994; Irwin et al., 1977 respectively) the early

stages of kerogen maturation (catagenesis) liberate CO2 (Curtis et al., 1986) and

methane and other hydrocarbons are produced by non-biological reactions

(Claypool and Kaplan, 1974). In this decarboxylation zone δ13C values are

negative, ranging between -10 and -25%o (Curtis et al., 1986).

Evidence presented within this thesis suggests that siderite and greigite formation

is active in the present day gossan as a result of interactions with the Niebla

Posadas aquifer. This is discussed in further detail in Chapter 12. Maximum

temperatures measured within the aquifer are typically less than 40oC (Knight,

2000) at burial depths of approximately 150 metres. Thermally induced

decarboxylation should therefore be considered as an unlikely source of methane

and subsequent siderite formation.

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10.3 Formation of Fe-sulphides

10.3.1 Introduction

In ambient aqueous systems the iron sulphides constitute a diverse group of

minerals, the most abundant of which are amorphous Fe2+S (FeSam), mackinawite

(tetragonal FeS), greigite (cubic Fe2+Fe3+2S4), pyrrhotite (Fe1-xS) and

pyrite/marcasite (cubic/orthorhombic FeS2 respectively) (Rickard et al. 2001).

FeS(am), mackinawite and greigite are collectively known as Acid Volatile

Sulphides (AVS).

The interrelationships between the various forms of iron sulphide have been the

subject of extensive investigations as all but pyrite and pyrrhotite are metastable

at ambient temperatures (Rickard et al., 2001).

10.3.2 Formation Mechanisms

Schoonen and Barnes (1991) discuss pyrite and marcasite formation by

replacement of an FeS precursor as the probable mechanism in low-temperature

environments. Rickard et al. (2001) suggest that the FeS(am) precursor rapidly

transforms to metastable mackinawite. The formation of pyrite and marcasite by

precursor FeS is also discussed by Lennie et al., (1997), (citing Berner, 1964)

who consider that pyrite and marcasite are formed in sedimentary and

hydrothermal systems by the reaction sequence:

mackinawite → greigite → pyrite/marcasite

thus allowing the relatively complex crystal structures of pyrite and marcasite to

nucleate readily at low temperatures from the simpler structures of the precursors

mackinawite or amorphous FeS.

Oxidised surfaces of precursors FeS or of pyrite seeds speed up the

transformation (Benning et al., 2000). The formation of marcasite over pyrite is

thought to be related to crystal growth kinetics and pH, with marcasite often

forming at pH<5. Classical nucleation theory predicts that nucleation will be

faster for the more soluble mineral. In a solution supersaturated with respect to

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amorphous FeS and pyrite, the highly soluble FeS(am) rapidly nucleates,

preventing the nucleation of pyrite. The presence of pyrite seeds, however, lifts

the nucleation barrier. Little effort is made in the literature to distinguish between

crystal growth mechanisms and nucleation kinetics, despite their differences, and

this area of investigation remains largely unresolved. However, investigations

suggest that pyrite can grow in the presence of pyrite seeds without the presence

of an FeS precursor (Schoonen, 2004).

Schoonen and Barnes (1991) and Lennie et al. (1997) propose that the iron

monosulphide precursor (either amorphous FeS(am) or mackinawite) is formed by

reaction of aqueous hydrogen sulphide ions with aqueous Fe2+ ions according to

the reaction:

Fe2+(aq) + H2S(aq) → FeS(am or mackinawite) + 2H+

The hydrogen sulphide needed for formation of iron sulphides in anoxic

environments is produced via bacterial reduction of sulphate (SO42-), which

enables biogenic oxidation of organic matter by anaerobic microorganisms. As

discussed previously, hydrogen sulphide can also be produced during anaerobic

oxidation of methane (AOM). Molecular, isotopic, and molecular biological

approaches have revealed that AOM is performed by methanotrophic archaea

(methane-loving organisms) and sulphate reducing bacteria. The coupled

reaction is proposed to proceed according to the following equation (Van Dongen

et al., 2007):

CH4 (methane) + SO42- (sulphate) → HCO3

– (bicarbonate) + HS- (hydrogen sulphide) + H2O

The rate of hydrogen sulphide formation via sulphate reduction is constrained by

the availability of metabolisable organic matter. In turn, the rate of hydrogen

sulphide formation in relation to the availability of reactive iron exerts an important

control on the initial AVS/pyrite ratio (Schoonen, 2004).

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The conversion of FeS(am) to pyrite requires an electron acceptor to oxidise S2- to

S-. In addition, the Fe/S ratio must decrease via either the loss of Fe or the gain

of S. A number of potential mechanisms of pyrite transformation at low

temperatures have been proposed (Neretin et al., 2004; Schoonen, 2004):

1. Polysulfide pathway: FeS conversion via sulphur addition, with the added

sulphur acting as the electron acceptor

2. Ferrous iron loss pathway: FeS conversion via Fe loss combined with an

electron acceptor

3. H2S pathway: FeS conversion via sulphur addition, with a non-sulphur

electron acceptor

Polysulphide Pathway

Benning et al. (2000) suggest that the dominant formation path of pyrite is by

reactions between a precursor monosulphide and zero-valent sulphur species via

the ‘polysulphide pathway’. This reaction pathway is rooted in experimental

studies which demonstrate that precipitation of amorphous FeS in the presence of

native S produces pyrite, according to the reaction: (Schoonen, 2004)

FeS + S →FeS2

Schoonen (2004) notes that this mechanism has been called into question, and

that polysulphide species produced by the hydrolysis of sulphur or reactions

between sulphur and H2S are more likely the reactants in this process.

Metastable sulphur oxyanions have also been suggested as reactants.

Ferrous Fe Loss Pathway

Wilkin and Barnes (1996) indicate that under certain conditions, iron loss may

dominate over sulphur addition. Wilkin and Barnes (1996) show that iron

disulphide nucleation may proceed via loss of ferrous iron from the precursor

monosulphide rather than via addition of zero-valent sulphur according to the

following reaction:

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8FeS + 2H2O + 3O2 → 4FeS2 + 4FeO(OH)

Experimental work by Lennie et al. (1997) shows that mackinawite can readily

transform to greigite via iron loss, but the transformation to pyrite has not been

demonstrated.

H2S Pathway

Schoonen (2004) notes that in most anoxic sediments, hydrogen sulphide is by

far the most abundant source of dissolved sulphur. Wachtershauser (1988 and

1993) propose that H2S could also oxidise monosulphides. This

thermodynamically favourable pathway involving reactions between H2S and

possibly HS- to produce pyrite is also considered by Rickard (1997), according to

the reaction:

FeS + H2S → FeS2 + H2

However, this reaction has consistently failed to produce pyrite during

experimental conditions with only mackinawite being produced. This mechanism

continues to be an area of ongoing research and controversy. Observations in

nature indicate that in the deep, anoxic, sulphidic sediments the conversion of

AVS to pyrite proceeds slowly, with estimated rates of decades to centuries. It is

possible that the conversion proceeds via a direct reaction between AVS and

H2S, producing H2, but it is also possible that the reaction takes place with H2S as

the sulphur source and a non-sulphur electron acceptor (e.g. bicarbonate).

Despite significant progress in understanding the mechanisms behind pyrite

formation, some aspects are yet to be resolved, particularly those involving the

interactions between H2S and FeS2. This may partly be due to the slow

conversion of precursor FeS to FeS2 and the difficulties in reproducing the

reaction in laboratory experiments (Schoonen, 2004).

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10.3.3 The Role of Biological Processes

The formation of Fe-sulphides is intimately related to biological processes,

because the overwhelming source of sulphide in natural environments is bacterial

sulphate reduction. This close association with biological processes has been

recognised through their association with the early evolution of life and their role

in the intracellular processes of some micro-organisms (Rickard et al., 2001).

The two most common magnetic iron sulphide minerals are greigite and

monoclinic pyrrhotite (Fe7S8) (Larrasoaña et al., 2007). Bazylinski and Moskowitz

(1997) suggest that bacteria mediate the formation of the magnetic Fe-sulphides

in two fundamentally different ways:

1) Biologically Induced Mineralisation (BIM) - the mineralisation is not

controlled by the organism and the mineral grains form extracellularly.

2) Biologically Controlled Mineralisation (BCM) – the organisms control the

mineralisation to a high degree and the mineral grains are normally formed

intracellularly and exhibit a very restricted size range and range of

morphologies. Examples include magnetotactic bacteria.

As cellular textures are not observed in the Las Cruces magnetic Fe-sulphides,

BCM is not considered further here.

The biomineralisation associated with BIM occurs indirectly as a result of

metabolic activity and subsequent chemical reactions. The mineral formed may

be poorly crystalline and exhibit a wide range in grain size and lacking in specific

crystalline morphology. Examples include Fe- and S-reducing bacteria that use

Fe and S as terminal electron acceptors for energy generation and produce

greigite and magnetite. Sulphate reducing bacteria (SRB) respire with sulphate

anaerobically, releasing H2S which reacts with excess Fe present in the

environment producing mackinawite, greigite, pyrrhotite, pyrite and marcasite.

Two common examples of other minerals formed by BIM are siderite and vivianite

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(ideally Fe3(PO4)2.8H2O). There is no apparent function to the minerals formed by

BIM (Bazylinski and Moskowitz, 1997).

AVS conversion studies have largely been abiotic and consideration of the role of

SRB in pyrite formation has largely been restricted to the production of H2S.

Bacteria may also play an important role in the conversion of organic sulphur

compounds to reactants that can take part in pyrite formation. However, the

complexity of the natural system inhibits meaningful experimental studies.

Nonetheless, biotic studies corroborate the abiotic studies in highlighting the

important role that FeS precursors play in pyrite formation (Schoonen, 2004).

Experimental studies by Rickard et al. (2001) have suggested that in the

presence of the organic aldehydic carbonyl group of compound, FeSam is oxidised

in a solid state reaction to form greigite which retains the original morphology of

the FeS precursor. The authors note that in the absence of the organic

compound, FeSam is transformed to pyrite via an aqueous FeS complex, with no

greigite being formed.

Work by Donald and Southam (1999) showed rapid pyrite formation and

incorporation of sulphur initially added to the system as the sulphur-bearing

amino acid cysteine. Donald and Southam (1999) proposed that the cysteine was

possibly converted to H2S before it became incorporated, indicating addition of

sulphur to FeS. Wilkin and Barnes (1996) evaluated cysteine as a sulphur source

in a strictly abiotic system and found no evidence for incorporation of cysteine-

bound sulphur in pyrite. The work by Donald and Southam (1999) therefore points

to the role bacteria can play in converting organic sulphur compounds to

reactants that can take part in pyrite formation and indicate that it is increasingly

evident that SRB play a more important role than simply providing H2S for the

reaction.

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10.4 Mineral stability fields

10.4.1 Siderite

Chapter 4 describes how, in natural environments, under near surface weathering

conditions, the stable Fe-sulphides typically oxidise to form Fe-oxides, hydroxides

and sulphates. The presence of siderite in the Las Cruces gossan therefore

suggests that the Eh/pH conditions and chemistry of the mineralising fluids differ

markedly from those normally encountered in sub-aerially weathered gossans.

-

Figure 10.2 – Eh/pH diagram illustrating the stability of hematite, magnetite and siderite at 25oC and 1 atmosphere total pressure and pCO2 = 10-2 atmosphere with total activity of dissolved species = 10-6 (Garrels and Christ, 1965).

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Garrels and Christ (1965) consider the influence of CO2 on the stability of the Fe-

oxides. The authors note that at pCO2 of 10-3.5 atmosphere, the partial pressure

of CO2 in the earth’s atmosphere, siderite has only a small field of stability and

only if the total dissolved carbonate increases does the siderite field expand.

Given a total activity of dissolved species of 10-6 with pCO2 of about 10-1.4, the

magnetite stability field is completely displaced by siderite. The authors therefore

consider pCO2 of 10-2 (Figure 10.26). Figure 10.26 confirms that under these

conditions, siderite is indicative of a strongly reducing environment, near neutral

to alkaline pH and the presence of CO2 in amounts greater than atmosphere.

Under decreasing pH, siderite is unstable and Fe2+ will be released into solution,

particularly under strongly reducing conditions. At moderate to high pH and

increasing Eh, siderite will oxidise to form either magnetite or hematite,

depending on the pCO2.

10.4.2 Fe-sulphides

Schoonen (2004) describes pe/pH diagrams for the Fe-sulphides and notes that

pyrite is the stable iron sulphide in anoxic, low temperature environments (Figure

10.27A). Therefore the Fe-monosulphides identified in this study in the Las

Cruces gossan are predicted to be stable under very limited Eh/pH conditions.

Marcasite is metastable with respect to pyrite under all pressures and

temperatures and therefore does not show up on stability diagrams unless pyrite

is excluded (Figure 10.27B). The stability field areas do not change when pyrite

is excluded due to the very subtle difference in free energy between pyrite and

marcasite (~2kJ/mole). Only by exclusion of pyrite, marcasite and the stable iron

monosulphides pyrrhotite and troilite are the metastability fields for amorphous

FeS, mackinawite and greigite seen (Figures 10.27C, D and E). The shape and

size of the stability field is dependent on solution composition. Figure 10.27C

gives stability fields for world average seawater composition. The equivalent

pe/pH diagram for world average river water shows a decrease in the sulphur

activity and a subsequent reduction of the metastability fields for greigite,

mackinawite and FeS(am) (Figure 10.27E). Increasing the carbon and Fe activity

produces a stability field for siderite (Figure 10.27F).

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Figure 10.3 - Pe/pH diagrams illustrating the stability relations for iron sulphides in seawater at 25oC, 1 atmosphere total pressure. A) Iron activity 10−6, sulphur activity 10−2.551, C(IV) activity 10−3.001, troilite and pyrrhotite suppressed. B) Same as A with pyrite also suppressed. C) Same as B with marcasite suppressed. D) Same as C with greigite and mackinawite suppressed. E) Same as C but solution changed to world average river water with iron activity 10−6, sulphur activity 10−3.902, C(IV) activity 10−3.06. F) Same as C but iron activity 10−3 and C(IV) activity 10−2.5

(Schoonen, 2004).

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On the basis of these thermodynamic considerations, one would expect only

pyrite to form in low temperature environments if equilibrium was to be

maintained. However, this is clearly not the case as marcasite and Fe-

monosulphide precursors are widely reported in a range of environments

(Schoonen, 2004).

10.4.3 Siderite/Fe-Sulphide Relationships

Figure 10.4 – Eh/pH diagram illustrating the stability relations between iron oxides, sulphides and carbonates in water at 25oC and 1 atmosphere total pressure at ΣCO2 of 100 and ΣS of 10-6

(Garrels and Christ, 1965).

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Garrels and Christ (1965) consider the influence of a combination of CO2 and

sulphur on iron-water-oxygen relations. Figure 10.28 illustrates the relations at

ΣCO2 of 100 and ΣS of 10-6 and confirms that if siderite is to have a considerable

field of stability then ΣCO2 must be very high and ΣS very low. If the total

dissolved CO2 is reduced to 10-2, siderite does not appear on the Eh/pH diagram.

Under the conditions illustrated in Figure 10.28, siderite is indicative of both very

strongly reducing conditions and moderate reducing conditions. The field of

pyrrhotite is eliminated. The presence of siderite in ores may therefore be

indicative of the absence of appreciable divalent sulphur and the presence of

relatively large amounts of dissolved carbonate.

The mineral paragenesis, fluid geochemistry and relevance of Eh/pH conditions

with respect to the Las Cruces gossan mineral assemblage are discussed in

greater detail in Chapters 11 and 12.

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Chapter 11 Mineralogy: Key Features and Paragenesis

11 MINERALOGY: KEY FEATURES AND PARAGENESIS

11.1 Introduction

This chapter aims to highlight the key mineralogical features of the gossan,

including the dominant gangue, ore and precious metal-bearing minerals and

their paragenesis. Although differences exist between the five boreholes

examined during this investigation, a common mineral assemblage has been

ascertained for the gossan.

The present day Las Cruces gossan is defined by the presence of a reddish

brown coloured mineral assemblage dominated by siderite and associated

weathering products. The base of the gossans, at the contact with the underlying

massive sulphide is characterised by an absence of siderite and the presence of

secondary pyrite together with sternbergite and proustite/pyrargyrite. Although

there is clearly an overlap between these two mineral assemblages, the latter

suite is essentially a component of the supergene zone and is not discussed in

significant detail here.

The common gossan assemblage consists of resistate quartz that exhibits

varying degrees of replacement by a late-stage siderite-rich assemblage

containing galena, Fe-sulphides and precious metal mineralisation. This

assemblage is described in greater detail in this chapter.

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11.2 Quartz

11.2.1 Relative Abundance

Quartz is the dominant Si-bearing phase present in the Las Cruces gossan and,

apart from subordinate amounts of Fe-rich clay occurring locally, no other

significant Si-bearing phases were located during this investigation.

There is a strong correlation between the abundance of quartz in the gossan and

the lateral distance from the supergene massive sulphide. Boreholes CR194,

CR149 and CR123 are the least quartz-rich gossans, consisting predominantly of

siderite together with accessory amounts of quartz. These boreholes are also the

closest to the supergene massive sulphide ore, which itself contains only minor

amounts of quartz and is also the source of the remobilised Fe. Conversely,

boreholes CR191 and CR038, which appear to represent ferruginised wall rocks,

are the most quartz-rich gossans, containing only accessory amounts of siderite,

and are the most distant boreholes from the supergene massive sulphide and the

original source of the remobilised Fe. The distance from source and mobility of

Fe plays a key role in the relative abundance of quartz in the gossans.

Quartz is often found in greater abundance in the lower portions of the gossan,

close to the contact with the massive sulphide. This vertical concentration

appears to relate partly to the concentration of resistate quartz grains during

mass wasting of the massive sulphide (e.g. in the galena-rich layer of borehole

CR194) and also partly to the precipitation of secondary silica (chalcedony)

towards the base of the gossan profile (e.g. borehole CR191).

11.2.2 Grain Size, Shape and Texture

The grain size determinations include both the size of quartz fragments and also

information on grain size of the crystallites that make up fragments of rock or the

rock as a whole. The former is based largely on SEM examination and the latter

is determined by transmitted light microscopy. Grain shape is used here to

describe the shape of the quartz fragments. Grain shape may provide

information on the degree of reworking, sorting and evidence of chemical

dissolution. Where quartz is present in its primary form, such as a quartz vein or

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as a component of a tuff or shale it is described in terms of ‘crystallite shape’ or

‘texture’. Crystallite texture provides an indication of the source of the quartz

fragments and also helps correlate the occurrences of quartz between the

different boreholes. Texture is also important in determining evidence of

deformation. The texture of the quartz crystallites is based on transmitted light

microscope examination.

The grain size of the quartz fragments is highly variable throughout the gossans

and highlights the relatively poorly sorted nature of the quartz. The larger,

millimetre-sized quartz fragments typically exhibit angular morphologies or highly

irregular margins. The textures in the quartz fragments associated with the

gossans are varied and indicate a number of different and distinct sources. The

angular nature of the fragments is indicative of extensive fracturing and also

suggests the fragments have not been transported far from their original source.

The highly irregular morphologies exhibited by some of the fragments may be

indicative of dissolution and/or replacement, which is particularly evident in close

association with the siderite. The fine-grained quartz is more reactive than larger

grains and is significantly more prone to replacement by siderite.

The variable fragment size, angular shape and varied source has implications for

the nature of deposition of the quartz fragments and is suggestive of an erratic,

high-energy environment such as that described by Kosakevitch et al. (1993) for

the Rio Tinto deposit. The theory of Kosakevitch et al. (1993) is based on

regional climatic conditions at the time of gossan formation that would have also

impacted on the Las Cruces deposit.

Boreholes CR038 and CR191 consist predominantly of fine-grained quartz and

fibrous fragments that have been recemented by later stages of chalcedony.

These rocks appear bleached and are largely devoid of sulphide minerals that

appear to have been oxidised and leached, leaving euhedral voids. The primary

mineralogy, particularly of the associated shales, has also been extensively

leached of other silicate components that would have presumably been present

prior to oxidation and leaching. This would have resulted in a very porous and

possibly unstable matrix that may well have collapsed under the weight of the

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overlying rocks. The result would have been a jumbled mass of fine-grained

quartz from the shale fragments together with the fibrous quartz that formed as

part of the massive sulphide replacement of the shale, much like that observed in

the core in the present day. The jumbled mass of quartz has subsequently been

cemented by later stages of chalcedony mineralisation and lesser siderite.

The poorly sorted nature of the quartz may therefore have resulted from a

combination of exposure to a high energy environment, and collapse of the

groundmass under pressures formed during oxidation and leaching of sulphide

and gangue components by highly acidic Fe-rich solutions.

Rounded, pebble-like grains of quartz are evident locally. These are often

associated with angular and irregularly shaped fragments of quartz and provide

further evidence of poor sorting, multiple sources and greater distance of travel

than the bulk of the quartz in these ores.

Probably the most characteristic texture observed in the quartz is that of the

fibrous aggregates. These fibrous aggregates are also often associated with

euhedral voids. The source of these fibrous aggregates appears to be the

underlying shales (Figure 11.1), where the fibrous quartz aggregates form around

the margins of pyrite. Minor amounts of fibrous quartz are also associated with

the massive sulphide.

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Figure 11.1 - Borehole CR123 - A colour transmitted light photomicrograph of fibrous quartz (white and grey shades) developed around the margins of pyrite crystals (black). The surrounding matrix is fine-grained quartz and pore spaces. This image was taken in crossed polarised light from the shale. The width of view is approximately 1100µm.

These textures have been documented and described by Williams (1933-34),

occurring at the Rio Tinto deposit, Spain. Williams (1933-34) describes pyrite

crystals enwrapped in fibrous quartz, which is drawn out parallel to the schistosity

in a manner suggesting that it may have crystallised under the influence of a

direct stress.

The development of the fibrous quartz aggregates along the margins of pyrite is

well documented in the more recent literature, including Ramsay and Huber

(1983) and Passchier and Trouw (1996). These textures are described as

antitaxial pressure fringes. These studies suggest that the quartz has developed

around the margins of the rigid body of the pyrite as a result of pressure solution.

The textures developed within these fibrous growths often indicate the pressure

history of the surrounding wall rocks.

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It is clear that the fibrous quartz aggregates in the shales are essentially similar to

those observed in the overlying gossans. However, the quartz and pyrite have

developed in situ in the shales, whereas the fibrous quartz in the overlying

gossans is clearly reworked and often cemented and/or partially replaced by one

or more of chalcedony, siderite and, to a lesser extent, calcite. The fine-grained

quartz that is present as the matrix in gossans may well be derived from the

shale, however, the textures are less distinctive than that of the fibrous quartz and

their origins are therefore less clear.

A subordinate portion of the quartz in the gossan occurs in the form of partially

recrystallised chalcedony that often cements the surrounding matrix (Figure 11.2).

The partially recrystallised chalcedony may be locally abundant and appears

massive in nature when observed in hand specimen. A subordinate portion of the

chalcedony occurs as a cavity filling and typically exhibits delicately banded,

fibrous textures (Figure 11.3).

Figure 11.2 - Borehole CR038 - A colour transmitted light photomicrograph illustrating more coarsely crystalline quartz fragments (white and grey shades, far left and far right) that are cemented by fine-grained, partially recrystallised chalcedony (mottled grey/black shades centre of field). This image was taken in crossed polarised light from the quartz replaced tuff. The width of view is approximately 1100µm.

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Figure 11.3 - Borehole CR149 - A colour, crossed polarised transmitted light photomicrograph from the gossan/massive sulphide contact illustrating a cavity (centre field) that has been filled by fibrous chalcedony (white/grey shades). The surrounding matrix is predominantly calcite (pinkish brown shades). The width of view is approximately 2mm.

The fine, medium and coarse-grained polycrystalline quartz aggregates that occur

in the gossans are less characteristic than the fibrous aggregates and their

origins are less readily determined. Similar textures are, however, observed in

the quartz that is present in the underlying massive sulphide and it is likely that

they have derived, at least in part, from the reworking of surrounding wall rocks or

from oxidation and weathering of the massive sulphides.

11.2.3 Fluid Inclusion and Isotope Analysis

Knight (2000) suggested that the microthermometric data and δ18O signatures for

the silica cap, have indicated that a significant proportion of the quartz formed

during late-stage or retrograde activities as conditions declined in the seafloor

hydrothermal system. However, the abundance of monophase fluid inclusions

indicative of ambient temperatures and low salinities consistent with meteoric

waters suggest that a subordinate portion of the quartz in the silica cap appears

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to have formed conventionally in response to gossan formation during pre-

Tertiary, sub-aerial weathering. These conflicting data have been explained by

Knight (2000) as resulting from two separate processes, with the conventionally

formed, sub-aerial silica cap being reworked during the Miocene and with

seawater penetrating the more permeable parts of the deposit.

Petrographic studies during this investigation have, however, shown that the

quartz 'cap', or silica enrichment at the base of the gossan unit comprises largely

reworked shales that have subsequently been cemented by late-stage, low

temperature quartz and chalcedony. The quartz in this horizon can therefore be

characterised as two discrete types;

A mechanically reworked, resistate quartz component derived from

extensive reworking and leaching of Palaeozoic sediments, characterised

by higher temperatures of formation and higher salinities.

Chemically precipitated low salinity, low temperature quartz/chalcedony

cement associated with the downward migration of silica resulting from

dissolution of quartz and other silicate minerals during gossan formation.

This accounts for the unexpected range of temperatures (ambient to a maximum

of ~200oC), salinities (1.7 to 6.1 wt.% NaCl) and δ18O values observed by Knight

(2000).

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11.3 SIDERITE


Siderite is a common accessory mineral in the Tertiary sand of borehole CR149,

where it occurs as compositionally zoned euhedral crystals. Siderite was not

observed in the Tertiary deposits in any of the other boreholes examined during

this investigation and is essentially confined to the gossan. Siderite dominates

the gossan of boreholes CR194 and CR149. Siderite is less abundant in the

quartz dominated gossans of boreholes CR038, CR191 and CR123. The relative

proportions of siderite, limonite and quartz vary locally within the boreholes.

A significant portion of the siderite in borehole CR194 occurs as ‘fragments’.

These are less common in other boreholes, where the bulk of the siderite occurs

as a cavity filling, cement and less commonly along fractures. Significant

porosity is often developed as a result of the oxidation of the earlier stages of

siderite mineralisation. Later stages of unoxidised siderite typically replace the

oxidised matrix, resulting in a significant decrease in the porosity of the core. The

relative abundance of the siderite appears to be controlled largely by the porosity

of the gossan and distance from the original source of Fe, namely the massive

sulphide orebody.

11.3.2 Grain Size, Shape and Textures

The grain size, as described in this thesis, includes the size of siderite 'fragments'

as well as information on the size of the crystallites associated with the

fragments, veinlets and void fillings. The latter is determined by transmitted light

microscopy, whereas the more general description of the size of the siderite

fragments is based largely on SEM examination. Unfortunately, due to the

extensive oxidation of much of the siderite, crystallite size is not always readily

observed. Therefore, the bulk of the information on crystallite size is based on

the less oxidised, and often later stages of siderite mineralisation. Grain shape

describes the shape of the siderite fragments. The siderite ‘fragments’ appear to

represent reworked materials and these fragments have often been subjected to

oxidation and replacement that may mask their original morphology.

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Fragments of siderite are most evident in the gossan of borehole CR194. These

fragments typically exhibit and angular morphology and commonly exceed

several millimetres in size. They are set in a poorly sorted siderite- and quartz-

rich matrix (Figure 11.4). A small proportion of the siderite fragments are

relatively unoxidised and consist of polycrystalline siderite aggregates with

discrete crystallites exceeding 100µm (Figure 11.4). This crystallite size is typical

for all five boreholes.

Figure 11.4 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of angular siderite ‘fragments’ (pinkish white) in a matrix of quartz (light and dark grey shades) and oxidised siderite (black). The siderite fragments are medium-grained, with discrete crystallites exceeding 100µm in size. The width of view is approximately 2mm.

The siderite ‘fragments’ become less apparent with depth in borehole CR194.

This is largely as a result of the oxidation and replacement of the siderite by

limonite and a gradual destruction of the textures. Although the siderite often

occurs as clast-like fragments (Figure 11.4), the morphology of the siderite

aggregates is pseudomorphous and related to the morphology of former rock and

mineral fragments that the siderite has replaced or the cavities within which it has

precipitated (Figures 11.5a, b and c).

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Figure 11.5 - Borehole CR194 – False colour backscattered electron images illustrating a) a siderite ‘fragment’ that actually represents a cavity filling. The width of view is approximately 2mm. b) Compositionally zoned siderite filling a euhedral cavity in quartz. The width of view is approximately 600µm. c) Siderite that appears to have extensively replaced barite (light grey). The width of view is approximately 450µm. d) Siderite filling cavities in botryoidal limonite. The width of view is approximately 250µm. Voids are black.

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An excellent example of this pseudomorphous replacement can be seen in the

replacement of former quartz fragments by siderite (Figure 11.6). The presence

of siderite as mechanically 'transported' clasts would have a significant impact on

the interpretation of the timing and hence the mechanisms behind the

precipitation of siderite. However, the replacement/cavity filling textures imply that

the bulk of the siderite in the Las Cruces gossan appears to have been

precipitated in situ and is chemically rather than mechanically transported.

Figure 11.6 - Borehole CR194 – a digitised photograph showing apparent ‘fragments' of siderite (brown, outlined in red). These clasts are pseudomorphs after quartz-rich rock fragments (white/light grey). An example of a quartz-rich rock fragment partially replaced by siderite is outlined in black. The width of core is approximately 50mm.

The siderite aggregates also commonly exhibit highly irregular margins that are

interpreted here as evidence of dissolution (Figure 11.7). Galena often lines the

margins of the siderite aggregates and allows different stages of siderite

mineralisation to be recognised (Figure 11.7).

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Figure 11.7 - Borehole CR194 - False coloured backscattered electron image illustrating the presence of galena (white) replacing siderite along grain boundaries and highlighting different generations of siderite mineralisation. Limonite (light brown, red arrow) is also present. The width of view is approximately 2.3mm.

The later stages of unoxidised siderite in all five boreholes exhibit a medium

grained crystallite size that typically ranges between 50µm and 150µm. The

crystallites in each of the five boreholes are essentially similar and typically

exhibit simple grain boundary relationships that do not show any evidence of

deformation (Figures 11.4 and 11.8).

Where cavities have not been completely filled by siderite, euhedral crystals are

often present (Figures 11.5a and c). These euhedral crystals are a relatively

common feature in the gossans and the majority range between 20µm and 70µm

with the largest exceeding 100µm. They typically exhibit evidence for at least two

stages of growth with early-formed, oxidised crystals being overgrown by later

stages of unoxidised siderite (Figure 11.9).

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Figure 11.8 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of late-stage, unoxidised siderite (light and dark grey-brown shades) filling a cavity in an oxidised, opaque siderite matrix (black). Tiny skeletal galena crystals (white arrow, black) are often present in the siderite. The siderite also exhibits growth zoning (red arrows). The width of view is approximately 4mm.

Figure 11.9 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of early-formed siderite crystals (dark brown) that have formed in a cavity (dark grey). The early formed siderite crystals have been oxidised and replaced by hematite and then overgrown by later stages of unoxidised siderite (white). The width of view is approximately 1100µm.

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11.3.3 Associations

Several other phases are often present in close association with the siderite. This

provides an insight into the nature and composition of the transporting fluids from

which the siderite precipitated. The associations observed between siderite and

other minerals in the gossan are extensive. There are both direct associations

where siderite may well have been precipitated along with other phases, notably

the Pb- and Fe-bearing sulphides, and indirect associations, whereby siderite

appears to have partially replaced earlier formed and relict minerals, notably

quartz and barite.

Siderite is often intimately associated with Pb-sulphide mineralisation, notably

galena. The siderite veinlets and siderite-filled voids commonly contain abundant

fine-grained skeletal galena (Figure 11.13) and, to a lesser extent, micrometre-

sized, acicular crystals of Pb(AsSb)-sulphides.

The relationships between the siderite and galena are complex but the strong

association between these two minerals is clear. It appears that galena and

siderite mineralisation occurred cyclically and in multiple stages. Figure 11.10

shows the presence of galena partially filling cavities in botryoidal Fe-oxides both

with and without the presence of siderite. The absence of siderite in a number of

these cavities is probably a result of localised dissolution, a feature that is

common throughout the gossan. Numerous examples of galena replacing

siderite (e.g. Figure 11.7), highlight the multiple stages evident between these two

phases. A similar association is evident between siderite and the Fe-sulphide

mineralisation, with euhedral crystals of Fe-sulphide occurring within siderite-filled

cavities. Siderite replacement of Fe-sulphide is also evident locally (Figure

11.11).

Nontronite also exhibits a close association with the late-stage siderite

mineralisation. This commonly occurs along the interface between the oxidised

siderite/hematite matrix and the late stage siderite mineralisation. The

nontronite/siderite association is most evident in borehole CR194. Siderite may

also replace the porous, fine-grained nontronite.

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Figure 11.10 - Borehole CR194 – False colour backscattered electron image illustrating the presence of siderite (dark brown) and galena (white) filling and partially filling cavities in hematite (light brown shades). The galena exhibits characteristic skeletal textures. Siderite is only present filling some of the cavities in this sample and appears to have been leached from the galena-filled cavities in the lower left portion of this image. The width of view is approximately 310µm.

Siderite appears to partially and often extensively replace the quartz fragments

and the fine-grained quartz-rich matrix associated with the gossan and may also

cement the fine-grained quartz-rich matrix, particularly in boreholes CR149,

CR191 and CR038. Relict barite is also often extensively replaced by siderite.

The siderite often exhibits pseudomorphous textures after barite.

A close association also exists between hematite and siderite. Oxidation and

replacement of siderite by hematite is a common feature of the gossan and is

particularly evident in the finer-grained (and hence more reactive) siderite matrix.

Nonetheless, the coarse-grained siderite also exhibits some degree of oxidation,

particularly along grain boundaries and/or the margins of the siderite aggregates.

Siderite-anglesite and siderite-cerussite veinlets are also occasionally present in

the gossan.

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Figure 11.11 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of skeletal galena (pale grey) in euhedral Fe-sulphide crystals (cream-white). Siderite (dark grey/black background) is selectively replacing the Fe-sulphide. The width of view is approximately 85µm.

11.3.4 Mineral Chemistry

The results of the quantitative SEM analyses of the siderite are provided in

Appendix 5. The siderite exhibits some degree of compositional variation both

within and between the boreholes. There is no evidence to suggest any

correlation exists between the compositions of the siderite between boreholes.

Indeed, the compositional variations highlight different stages of siderite

mineralisation, of which there appear to be many.

Compositional zoning of the siderite is also evident throughout the gossan. The

changes in composition largely reflect minor variations in the relative proportions

of Fe, Mg and Ca, with most other elements being present below detection limits

(0.5%). The MgO and CaO content of the siderite ranges between an effective

lower limit of less than 0.5 per cent to the highest values that may exceed 8.00

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per cent. It should be noted that the majority of semi-quantitative and qualitative

EDX analyses were performed on the relatively late stage siderite mineralisation,

with the bulk of the earlier siderite being too oxidised to provide suitable material

for analysis.

The zoned siderites in the Tertiary sand of borehole CR149 exhibit compositions

that are largely chemically indistinct to those of the gossan samples. However,

the very prominent two-stages of zoning exhibited by these siderites, with a

CaMg-rich core and Fe-rich rim is a feature that distinguishes them from the

majority of the gossan siderites, which typically exhibit either relatively uniform

compositions or a more varied compositional zoning. Siderite dissolution is

common and the degree of dissolution appears to be affected by the composition

of the siderite, with selective leaching occurring within compositionally zoned

siderite crystals (Figure 11.12).

Figure 11.12 - Borehole CR191 – False colour backscattered electron image illustrating the selective leaching of compositional zones within siderite crystals (brown shades). The siderite is present along margins of quartz fragments (mauve) and within voids (black). Minor galena (white) and Fe-sulphide (light khaki) are also present. The width of view is approximately 310µm.

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Locally, minor amounts of Pb are present in the siderite. The orthorhombic Pb-

carbonate mineral cerussite (ideally PbCO3) does not exhibit solid solutions with

rhombohedral siderite and the presence of minor amounts of Pb therefore most

likely reflects the presence of sub-microscopic grains of cerussite and/or galena

within the siderite. The presence of sub-microscopic Pb-rich zones within the

siderite is further evidence for a strong association between the siderite and Pb

mineralisation.

11.3.5 Isotope Analysis

The presence of siderite as the dominant Fe-bearing mineral in the Las Cruces

gossan clearly indicates that this deposit has been subjected to processes that

are not typical of sub-aerially formed gossans, which are more commonly

dominated by the presence of Fe-oxides, hydroxides and sulphates. In order to

provide additional evidence to aid in the interpretation of mineral textures

observed in this deposit, three small, carefully selected samples of siderite were

submitted to Dr. Steve Crowley of the University of Liverpool for isotope analysis.

Due to the extensive oxidation of much of the siderite, sample selection was, to a

large degree, based on the ability to select relatively clean mineral separates.

Fresh, relatively unweathered siderite was carefully hand picked from wet

screened, sized mineral fractions of the gossan material from boreholes CR194

and CR123. These materials were ground in a pestle and mortar and checked for

purity using XRD analysis. XRD confirmed that the materials consisted

predominantly of siderite together with subordinate amounts of galena. No other

carbonate minerals were present confirming that the samples were suitable for

isotope analysis. Analysis of the siderite samples from boreholes CR194 and

CR123 confirm that the 13C and 18O ratios are as follows:-

Table 11.1 - Siderite 13C and 18O Ratios

Sample 13C 18O

CR123 -35.5 -3.4

CR194 #1 -33.0 -4.9

CR194 #2 -41.7 -5.1

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The carbon isotope ratios are particularly low, strongly indicating the involvement

of methane oxidation as an important source of HCO3. Crowley (pers. comms.)

warns of placing too much emphasis on the oxygen isotope data, noting that too

little is known about the temperature dependence of siderite-H2O oxygen isotope

fractionation. Professor Max Coleman from the University of Reading, suggests

that the oxygen isotope value may well be more negative than equilibrium if

produced by rapid microbial activity (Coleman, Pers. comms.). Coleman

suggests that estimating local groundwater at -7 SMOW would give formation

temperatures for the siderite at below 20oC. It must be emphasised, however,

that this is only indicative and information on the nature of the groundwater in this

region was unavailable.

11.3.6 Fluid Inclusion Analysis

Two samples from boreholes CR194 and CR123 respectively were analysed.

The main aim of this analysis was to provide constraints on the nature of the

fluids responsible for siderite precipitation.

Two doubly polished wafers approximately 100µm thick were prepared from the

two polished sections. Difficulties in identifying the presence of fluid inclusions in

the wafers were encountered due to the extremely fine-grained and oxidised

nature of much of the sample. Fluid inclusions were identified and described with

respect to their host mineral and any textural features present in the samples.

No fluid inclusions were observed in the oxidized material. Very small inclusions

forming cloudy, inclusion-rich growth bands occur in the first stage siderite

overgrowing opaque, oxide-rich material. This is overgrown by more coarsely

crystalline, transparent siderite that is inclusion-free and contains disseminated

opaque grains. Some vapour-rich inclusions appear to be present but these show

signs of possibly having leaked and are therefore considered unreliable.

Insufficient data were obtained from the fluid inclusion measurements to provide

any reliable information on the formation temperature. Wilkinson (pers. comms.)

notes that although the absence or paucity of fluid inclusions is not necessarily

diagnostic of a low temperature of formation, it might tend to be the case.

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11.4 Galena


Galena is the dominant Pb-bearing mineral in the Las Cruces Gossan. The Pb

content of the sample intervals provides a good measure of the abundance of

galena, with other Pb-bearing phases occurring in relatively minor amounts.

Galena is a very common accessory mineral in the gossans with the exception of

Borehole CR038, where the Pb content rarely exceeds 0.5 per cent. The relative

proportion of galena increases towards the base of the gossans of boreholes

CR194 and CR149, but is enriched at the top of the gossan of borehole CR191.

A number of galena-rich horizons exist in the gossan of borehole CR123, with the

Pb content reaching a maximum of 27 per cent in one sample interval, equivalent

to approximately 34 per cent galena.

Galena remains a common accessory mineral in the uppermost portion of the

massive sulphide of boreholes CR194, CR149, but decreases markedly with

depth relative to the overlying gossan for all of the boreholes examined during

this investigation.

11.4.2 Grain Size and Shape

Locally, the galena occurs as discrete euhedral crystals that rarely exceed a few

tens of micrometres in size. The bulk of the galena is present as fine-grained,

massive aggregates that largely consist of densely packed micrometre-sized

skeletal crystals or porous aggregates that do not exhibit any discrete crystal

form. Collomorphic textures are also locally present in the fine-grained and

porous galena aggregates.

Fine-grained, skeletal galena is one of the most striking and prominent textures

observed in the galena, occurring in all five boreholes examined during this

investigation. The most prominent and well-developed skeletal crystals occur in

the gossan/massive sulphide contact zone of borehole CR149.

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The fine-grained nature of the galena, the presence of skeletal crystals and

collomorphic textures and the absence of large, euhedral crystals are indicative of

rapid precipitation.

Skeletal crystals form by rapid mineral growth along corners and edges of the

crystal due to greater exposure to the mineralising solutions. Skeletal crystals

that form more extensive tree-like intergrowths are referred to as dendrites. The

rapid crystallisation may result from rapid cooling of the mineralising fluids or as a

result of supersaturation of the solution. Often, after the first rapid growth of

skeletal crystals, the skeletal texture is infilled by further crystallisation of galena

and the skeletal nature of the crystals may only be evident after chemical etching

or natural selective leaching (Ramdohr, 1980).

The skeletal galena in the Las Cruces gossan appears to have formed as a result

of rapid crystallisation followed by continued precipitation of less well developed,

porous galena aggregates. Later stages of siderite and to a lesser extent calcite

mineralisation selectively replace the fine-grained and porous galena aggregates,

leaving only the well formed skeletal galena which appears to be more resistant

to replacement by carbonate. There are several clear examples of this

replacement process (Figure 11.13). The resistance to replacement of the

skeletal galena is also seen in Figure 11.11.

The development of more coarsely crystalline galena was also probably inhibited

by a combination of fluid chemistry, notably the lack of available Pb and/or S for

further crystal growth, formation temperature and rate of precipitation.

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Figure 11.13 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of a fine-grained galena aggregate (pale cream) that is being progressively replaced from the upper left to lower right by siderite (dark grey background). The siderite is selectively replacing the finer-grained galena, with only the skeletal galena surviving as a relict phase. The width of view is approximately 375µm.

11.4.3 Associations

A range of typically associations between galena and other minerals are

illustrated in Figures 11.14a to k). The bulk of the galena in the gossan exhibits a

close association with siderite and Fe-sulphides (Figures 11.14a, b and c). The

nature of these associations is complex and suggests cyclical stages of galena

and siderite precipitation with various stages of dissolution and replacement also

being evident. Similar cyclical replacement and precipitation textures are

observed between galena and calcite in borehole CR123.

The fine-grained and porous galena aggregates typically contain variable (up to

several percent) amounts of Sb and As, that may reflect the presence of

unresolved PbAsSb-sulphides. Mimetite and cerussite are locally present in

close association with the galena.

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Galena is also closely associated with the precious metal mineralisation in the

gossan and may form micrometre-sized rims on the native Au and Au-amalgam

(Figure 11.14e). This feature is evident in all five boreholes. In borehole CR194,

there is a marked increase in the abundance of galena in the base of the gossan,

which, in turn, is associated with a marked increase in the precious metal content

with discrete Au-bearing grains being common in the galena-rich aggregates.

However, elevated levels of Pb in the core are not always associated with

elevated levels of Au and there is conflicting evidence to suggest that galena and

native Au-bearing grains formed from the same mineralising fluids. It is likely that

there have been several stages of galena mineralisation, not all of which have

been associated with precious metal mineralisation. Fe-sulphides are commonly

intergrown with galena in all five boreholes and complex replacement

associations may exist between these two minerals (Figure 11.11).

Towards the base of the gossan in borehole CR194, close to the underlying

massive sulphide, chalcopyrite and members of the tetrahedrite-tennantite solid

solution series are also intimately associated with the galena. The galena

appears to partially replace these phases along the margins of the sulphide-rich

aggregates and along grain boundaries (Figure 11.14i). At the contact between

the gossan and underlying massive sulphide in borehole CR149, euhedral galena

is present in sternbergite (Figure 11.14g). Similarly in borehole CR123, narrow

veinlets/fractures containing skeletal galena are also partially filled by

sternbergite. In the uppermost portions of the massive sulphide in boreholes

CR194, CR149, CR038 and CR191, galena forms thin rims on the primary pyrite

and also appears to replace the pyrite along grain boundaries and fractures

(Figure 11.14a).

Minor amounts of galena are also typically associated with the secondary Cu-

sulphide mineralisation in the supergene enriched massive sulphides associated

with each borehole.

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Figure 11.14 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of associations between galena and other minerals (various scales). a) Replacement of partially leached, relict primary pyrite by late-stage galena. b) Fine-grained and porous galena aggregates with intergrown Fe-sulphide. c) Partial replacement of siderite by vermicular galena. d) Au-bearing grains in galena. e) Fine galena rims on Au-bearing grains. f) Galena overgrowth on native Au. g) Euhedral galena crystals in sternbergite, Au and pyrite. h) Skeletal galena in cerussite, mimetite and siderite-bearing vein. i) Galena replacing tetrahedrite. j) Galena replacing quartz along grain boundaries. k) Galena replacing calcite along margins of fragments with later calcite and Fe-sulphide filling pore spaces (black).

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11.5 Fe-Sulphide Phases

11.5.1 Introduction

The Las Cruces gossan is characterised by a fine-grained Fe-sulphide

assemblage that is intimately associated with the siderite and galena

mineralisation. The Fe-sulphides, as described here, refer to Fe-sulphides other

than marcasite and pyrite, unless otherwise stated.

Optical microscopy and textural interpretation of the Las Cruces gossan Fe-

sulphide mineral assemblage, as presented in this Chapter, has recognised four

discrete Fe-sulphide phases that exhibit varying degrees of replacement by

marcasite and pyrite:

Type 1 - Colloidal, isotropic Fe-sulphide, probably amorphous FeS.

Type 2 - Colloidal/feathery, anisotropic Fe-sulphide, probably mackinawite.

Type 3 - Euhedral, isotropic Fe-sulphide, probably greigite.

Type 4 - Platy, anisotropic Fe-sulphide, probably pyrrhotite.

The Fe-sulphide assemblage is, at least in part, magnetic. Due to the paucity and

extremely fine-grained nature of the Fe-sulphides, positive identification was

considerably hampered. Nonetheless, XRD has confirmed the presence of

greigite (Figure 11.15), marcasite and pyrite. Mackinawite and pyrrhotite have

not been identified by XRD, although a detailed examination of the assemblage

using reflected light microscopy suggests that both mackinawite and pyrrhotite

may be present in these samples. The presence of a

mackinawite/greigite/marcasite/pyrite assemblage is also consistent with the

literature review on the formation of Fe-sulphides.

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Figure 11.15 - An X-ray diffractogram clearly illustrating the presence of greigite (peaks donated with blue vertical lines). The unlabelled peaks reflect the presence of quartz, lepidocrocite and sulphur. The lepidocrocite and sulphur represent the oxidation products of the Fe-sulphide assemblage. The broad peak width for greigite is indicative of a poorly crystalline nature.


The Fe-sulphide phases are confined to the gossan and show increased

replacement by pyrite and marcasite lower in the profile toward the contact with

the underlying massive sulphide. The Fe-sulphides rapidly oxide to form

lepidocrocite and native sulphur and are therefore largely confined to less

oxidised portions of the gossans and the later stages of siderite/galena

mineralisation. Due to their strongly magnetic nature, the presence of Fe-

sulphides is often evident in hand specimen and the magnetic nature of the core

has been logged by the field geologists (Appendix 2). Fe-sulphides are most

abundant in the middle portion of the gossan of CR194, the upper and lower

portions of the CR149 gossan and throughout the CR123 gossan. Fe-sulphides

are sparingly present throughout the quartz replaced tuffs of CR038 and are a

common accessory in the upper gossan of borehole CR191.

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11.5.3 Reflected Light Characterisation

Detailed examination of the Fe-sulphide phases by reflected light microscopy has

revealed additional information on the nature of this fine-grained mineral

assemblage and four discrete Fe-sulphide mineral types are evident.

Fe-sulphide ‘Type 1’ consists of radiating feathery aggregates that appear to be

colloidal, possibly amorphous in nature (Figure 11.16). This phase appears to be

anisotropic although the optical properties are somewhat masked by the

extremely fine-grained nature. This phase, tentatively described as FeS(am),

exhibits a distinctive pinkish brown colour in reflected light relative to pyrite and

marcasite. Although the polished section from which this material was located

exhibits strong magnetic properties, other forms of Fe-sulphide are intimately

present and it is not possible to identify the individual magnetic species.

Figure 11.16 – Colour, reflected light photomicrograph illustrating Type 1 Fe-sulphide, consisting of feathery, colloidal radiating aggregates of Fe-sulphide (FeSam or mackinawite/nanoparticulate mackinawite of Wolthers et al. (2003)). This sample is magnetic. Borehole CR194, 156.70m (upper). 100x oil, ppl, width of view 150um.

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Locally, anisotropic effects are observed in these feathery masses, particularly

along the margins of the aggregates where the feathery crystals exhibit a more

coarse-grained texture (Figure 11.17). These aggregates most likely consist of

amorphous FeS (FeS(am)) or nanoparticulate mackinawite (Wolthers et al., 2003)

that has been replaced by mackinawite or subjected to some degree of

recrystallisation and grain coarsening. This phase is Fe-sulphide ‘Type 2’.

Figure 11.17 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals (Type 3 Fe-sulphide, circled) with marcasite/pyrite inclusions (paler yellow-white, white arrow) forming overgrowths on a colloidal Fe-sulphide aggregate (Type 1). The colloidal Fe-sulphide exhibits paler coloured, feathery intergrowths, towards the margins which appear strongly anisotropic (Type 2, possibly mackinawite, very weakly defined, red arrows). This polished section is magnetic. Borehole CR149, 151.75m. 100x oil, 50% zoom ppl, width of view 105um.

As well as the colloidal, radiating aggregates of Fe-sulphide (Type 1), fine-

grained, idiomorphic crystals are disseminated throughout the late-stage siderite

(Figures 11.18, and 11.19). These ‘Type 3’ euhedral Fe-sulphide crystals are

anisotropic and commonly contain inclusions of pyrite/marcasite. The idiomorphic

crystals also appear to replace the colloidal, radiating aggregates of Type 1 Fe-

sulphide (Figure 11.18).

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Figure 11.18 – Colour, reflected light photomicrograph illustrating colloidal radiating aggregates of Fe-sulphide (Type 1, white arrow) and finely disseminated euhedral Fe-sulphide crystals (Type 3, light grey, yellow arrow) in siderite (dark brown transparent gangue). The euhedral Fe-sulphides may form as a replacement or recrystallisation product of the colloidal Fe-sulphide (white circle). The black regions within the centre of these aggregates are voids. This sample is magnetic. Borehole CR194, 156.70m (upper). 100x oil, ppl, width of view 150um.

The euhedral Fe-sulphide crystals often exhibit porous textures, possibly

indicative of volume changes during replacement (Figure 11.20). The euhedral

crystals appear to either replace the colloidal aggregates, or form as a result of

recrystallisation. The presence of pyrite/marcasite inclusions probably reflects

replacement of these euhedral Fe-sulphide crystals. The euhedral Fe-sulphide

crystals appear to represent greigite, as they are isotropic and are associated

with magnetism in the core (Figure 11.21). Greigite is, however, cubic and these

crystals rarely exhibit cubic morphologies. Nonetheless, greigite is known to

pseudomorphously replace mackinawite.

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Figure 11.19 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals (Type 3, cream/yellow) in siderite (black). The porous cores of the crystals may have resulted from volume changes during replacement or be relicts of recrystallisation/replacement of colloidal aggregates. This sample is magnetic. Borehole CR194, 156.70m (upper). 100x oil, ppl, width of view 150um.

Figure 11.20 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals (Type 3, pale pinkish brown) with marcasite/pyrite inclusions (paler yellow-white, white arrow) in siderite (black background). The crystals exhibit a marked porosity (red arrow), possibly indicative of volume changes during replacement. This sample is magnetic. Borehole CR194, 151.75m. 100x oil, 100% zoom, ppl, width of view 85um.

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Figure 11.21 - Colour, reflected light photomicrograph illustrating feathery, strongly anisotropic Fe-sulphide (Type 2, pinkish cream shades, mackinawite?) with cubic overgrowths of Fe-sulphide crystals (Type 3, greigite?, circled) in siderite (dark brown/black background). Cores of pyrite/marcasite are present (pale yellow). This sample is magnetic. Borehole CR149, 151.75m. 100x oil, 100% zoom, ppl, width of view 85um.

In addition to the key textural features described above, platy crystals of Fe-

sulphide also occur (Figure 11.22). These crystals, described here as ‘Type 4’

Fe-sulphide, consist of thin, strongly anisotropic platelets that appear to be

magnetic. This phase may represent monoclinic pyrrhotite, the other common

magnetic Fe-sulphide. The presence of pyrrhotite could not be confirmed by XRD

and due to the presence of other Fe-sulphides in the polished section, it could not

be confirmed that the magnetic nature of the material was directly associated with

this phase. Although this phase occurs in association with Types 1 to 3 Fe-

sulphide locally, it is largely present as a very separate phase of mineralisation,

whereas Types 1 to 3 are often intimately associated with each other. The platy

Fe-sulphide exhibits almost identical textures to the pyrrhotite recognised by

Larrasoaña et al. (2007) in marine sediments from Cascadia Margin, offshore

Oregon.

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Figure 11.22 - Colour, reflected light photomicrograph illustrating platelets of an anisotropic Fe-sulphide phase (Type 4) with minor pyrite/marcasite (poorly resolved, white arrow) in a matrix of siderite (dark brown background). This section is magnetic. Borehole CR123, 152.40m (lower). 100x oil, 100% zoom, ppl, width of view 85um.

Lower in the gossan profiles, towards the contact with the underlying massive

sulphides, the Fe-sulphides typically show a greater degree of pseudomorphous

replacement by marcasite and pyrite (Figures 11.23 and 11.24).

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Figure 11.23 - Platy textures in pyrite (pale yellow) that has pseudomorphously replaced Type 4 Fe-sulphide (pyrrhotite? darker pinkish brown, white arrow) in siderite (dark brown/black background). This section is weakly magnetic, probably due to the presence of disseminated greigite crystals in the siderite (not present in this image). Borehole CR149, 187.40m (middle). 100x oil, 100% zoom, ppl, width of view 85um.

Figure 11.24 - Marcasite (pale yellow, white arrow) extensively replaces the strongly anisotropic Fe-sulphide phase (probably mackinawite +/- greigite, darker pinkish yellow, red arrow) that is partially filling a euhedral cavity in quartz (undifferentiated black background). This section is magnetic. Borehole CR191, 150.10m. 40x air, ppl, width of view 375um.

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11.5.4 Optical Properties and Occurrences of the Fe-sulphides

Greigite is not a mineral commonly described in gossans, however, greigite

appears to be one of the dominant Fe-sulphide phases, contributing significantly,

if not entirely to the magnetic properties of the Las Cruces gossan. Therefore an

understanding of how it forms will help to identify the processes of formation of

this rather unusual gossan.

Greigite is a relatively recently discovered mineral (Skinner et al., 1964), and its

poor stability and often fine-grained nature make it difficult to identify. Failure to

identify greigite or incorrect identification is undoubtedly widespread with XRD

probably being one of the more reliable methods for positive identification of this

phase.

Greigite is a thiospinel, which shares the same crystal structure as magnetite and

is therefore strongly ferromagnetic (Roberts and Weaver, 2005). It often occurs

as tiny grains and crystals (Lennie et al., 1997), rarely as cubes, and as balls of

intergrown octahedral (known as framboids) with curved faces up to 0.5 mm in

size (Anthony et al., 1990). Greigite is pale creamy white in reflected, plane

polarised light, is isotropic and exhibits no internal reflections (Uytenbogaardt and

Burke, 1971). It is often formed in lacustrine beds and hydrothermal vein deposits

(Anthony et al., 1990) and may, partly, be biogenic in origin, being found as

inclusions in magnetotactic bacteria. Greigite forms authigenically in anoxic

sedimentary environments as a precursor to pyrite (FeS2) in association with

chemical reactions driven by bacterial degradation of organic matter (Roberts and

Weaver, 2005).

In nature, mackinawite typically occurs as a poorly crystalline precipitate (Lennie

et al., 1997), as well-formed thin tabular crystals, to 1 mm and as fine-feathery

massive aggregates (Anthony et al., 1990). It sometimes occurs as idiomorphic

crystals and due to its perfect basal cleavage, sometimes flakes like graphite.

Mackinawite exhibits a pinkish grey colour in reflected, plane polarised light,

similar to pyrrhotite, and a moderate to strong bireflectance, very strong

anisotropy and no internal reflections (Uytenbogaardt and Burke, 1971). It is

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formed by hydrothermal activity in mineral deposits, during serpentinisation of

peridotites, and in the reducing environment of river bottom muds. Mackinawite

may be produced by magnetotactic and sulphate-reducing bacteria (SRB).

Mackinawite occurs rarely in iron and carbonaceous chondrite meteorites

(Anthony et al., 1990).

Pyrite is the most widespread of the sulphide minerals. It is cubic, isotropic and

exhibits a pale brass-yellow in reflected light (Anthony et al., 1990). The crystal

form of pyrite is usually idiomorphic but may also be granular, colloidal,

concretionary and cryptocrystalline (Uytenbogaardt and Burke, 1971). Pyrite

forms in a wide variety of environments, including hydrothermal deposits and as

diagenetic deposits in sediments (Anthony et al., 1990).

Marcasite is orthorhombic and is distinguished from isotropic pyrite by its strong

anisotropy and strong bireflectance (Uytenbogaardt and Burke, 1971). Marcasite

typically forms under low temperature, highly acidic conditions including

sedimentary environments and hydrothermal veins. Marcasite typically occurs as

idiomorphic crystals, often as laths and aggregates of radiating crystals, or

colloform aggregates (Anthony et al., 1990).

Pyrrhotite is monoclinic or hexagonal. Pyrrhotite occurs as granular aggregates,

or commonly as tabular or platy crystals that may form rosettes. In reflected light

pyrrhotite is pinkish brown, exhibiting strong bireflectance and strong anisotropy

(Uytenbogaardt and Burke, 1971). Although pyrrhotite is largely found in mafic

igneous rocks, it is also associated with hydrothermal veins, sedimentary and

metamorphic rocks and meteorites (Anthony et al., 1990). Monoclinic pyrrhotite is

magnetic.

Optical investigations suggest that Type 1 likely represents FeS(am) or

nanoparticulate mackinawite and Type 2 is likely to be mackinawite. The

relationship between Type 1 and 2 is likely to be one of recrystallisation and/or

replacement. Type 3 is tentatively identified as greigite. Greigite is known to be

present due to confirmation by XRD techniques. The morphology of the greigite

is not necessarily consistent with a cubic mineral, but the greigite may well be

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pseudomorphous after mackinawite. The optical properties of Type 4 are

consistent with pyrrhotite, and the textures observed are almost identical to those

recognised by Larrasoaña et al. (2007).

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11.6 Au-Bearing Phases


The Au content of the gossan is variable, but is high relative to the overlying

Tertiary deposits and the underlying massive sulphides. In borehole CR194, the

Au content increases towards the middle portion of the gossan with discrete

native Au grains being observed. The Au content increases markedly towards

the base of the gossan at the contact with the underlying massive sulphide where

the dominant Au-bearing phase is Au-amalgam.

In boreholes CR149 and CR191, the Au content is relatively high in the upper

gossan, decreasing towards the middle gossan and then increasing again

towards the base of the gossan at the contact with the underlying massive

sulphide. Elevated levels of Au are also present throughout the quartz-replaced

tuffs of borehole CR038 and at the base of the gossan in borehole CR123.

Native Au is the dominant Au-bearing phase in all but borehole CR194, where

both Au and Au-amalgam is present.

The Au content of the massive sulphide is low in all five boreholes, but increases

markedly in the massive sulphide/shale horizon in borehole CR194. The increase

in Au content in this highly porous zone has resulted from the penetration of

supergene solutions. The Au content of the shale in borehole CR194 is very low.

There is enrichment of Au close to the base of the gossan at the contact with the

massive sulphide in all of the boreholes. These two features are consistent with a

model of supergene Au enrichment during oxidation and mass wasting of the

massive sulphide deposit. Other Au-rich horizons are, however, also developed

in the middle and upper portions of the gossan in some boreholes. This suggests

that other processes may be causing Au mobilisation and precipitation.

Alternatively, these horizons may represent relict supergene zones that once

represented contact zones between the gossan and massive sulphide, with

subsequent oxidation events resulting in a lowering of the water table and deeper

oxidation of the sulphides.

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11.6.2 Grain Size and Shape

The grain size of the native Au grains in the gossan is generally very fine, with

discrete grains rarely exceeding a few micrometres in maximum dimensions.

This and the general paucity of microscopically visible grains suggest that a

significant portion of the Au may be present in a sub-microscopic form. In

particularly Au-rich sample intervals, native Au grains may occasionally exceed

30µm (e.g. CR038 and CR123).

Within the galena-rich layer of borehole CR194, the Au-bearing amalgam may

exceed 100µm in size, although the majority of grains are typically <20µm. In

borehole CR149, rhythmically precipitated Au grains in sternbergite range from a

few micrometres in size to below the limits of optical resolution (see Figure

11.14g), indicating that at least some of the Au may be present in a sub-

microscopic form in the Ag-rich sulphides. Despite the relatively high Au content,

no discrete Au-bearing grains were recognised in the massive sulphide/shale

zone of borehole CR194, and the bulk of the Au is therefore likely to be present in

a sub-microscopic form.

The native Au grains in the gossans range from euhedral to highly irregular

(Figures 11.25a to 11.25i). Grains that are typically less than a few micrometres

in size more commonly exhibit rounded morphologies. The irregularly shaped,

cuspate margins of some of the Au/Au-amalgam grains associated with galena

suggests they may have been subjected to some degree of dissolution and/or

replacement (Figures 11.25a, 11.25b and 11.25c). A single, rounded and

compositionally zoned native Au grain was also located in the gossan of borehole

CR194. The bulk of the microscopic Au grains located in the gossan/massive

sulphide contact zone of borehole CR149 occurs as very fine-grained and

irregularly shaped grains in sternbergite, although the grains are often on the

limits of resolution (~1µm) (Figure 11.25i). The very minor amounts of fine-

grained native Au located in the lower gossan of borehole CR191 exhibits highly

irregular morphologies. This, at least in part, reflects the irregular shape of the

cavities in which the Au has precipitated, occurring largely along the margins of

angular and irregularly shaped quartz fragments.

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Figure 11.25 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of morphologies and associations of the Au and Au-bearing grains (various scales). a) An aggregate of irregularly shaped Au-amalgam grains in galena. b) Two native Au grains rimmed and possibly replaced by galena. c) Au-amalgam grains with cuspate margins that appear to have been extensively replaced by galena. d) Irregularly shaped native Au grains in cinnabar. e) A euhedral native Au grain in lepidocrocite. f) A euhedral native Au grain in siderite. g) Anhedral native Au in siderite and Fe-sulphide. h) Native Au in a euhedral cavity (black) in quartz. i) Euhedral native Au in galena replacing quartz. j) Au in Fe-sulphide. k) Euhedral Au with cassiterite. l) Au in Fe-sulphide and anatase.

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11.6.3 Associations

A striking feature of the native Au and Au-amalgam observed throughout the

gossan is the intimate association with galena (Figures 11.25a, b, c and i) which

commonly rims Au-bearing grains (Figure 11.25b). Less common are Pb(SbAs)-

sulphide rims on native Au. Similarly, native Au inclusions and intergrowths with

Fe-sulphides were also observed in lesser amounts but throughout the gossan

(Figures 11.25g, j and l), especially in borehole CR038.

Native Au grains are commonly present in euhedral cavities and along the

margins of the quartz-rich matrix, particularly in borehole CR038, highlighting the

importance of porosity to fluid migration (Figure 11.25h). Later stages of

mineralisation commonly fill or partially fill the cavities within which the Au is

present. Siderite and chalcedony are examples of these later stages of cavity-

filling mineralisation. Less common associations include nukundamite and

bismuthinite, both of which have been observed in close association with native

Au (Appendices 6 to 10).

The gossan contact with the massive sulphide in borehole CR149 contains

extremely fine-grained and complex aggregates of native Au that occur within

sternbergite (Figure 11.14g). Au-sternbergite associations were also recognised

in other boreholes in this suite. Borehole CR038 exhibited some associations

that were not observed in any of the other boreholes in this suite, including two

discrete occurrences of Au with cassiterite (Figure 11.25k) and numerous

examples of fine-grained native Au in anatase (Figure 11.25l).

Native Au grains were also located in a cinnabar aggregate in borehole CR123

(Figure 11.25d). These grains exhibit an irregular morphology and cuspate grains

boundaries that may indicate partial replacement of the Au. Although no discrete

microscopic grains of native Au were located in the porous massive

sulphide/shale of borehole CR194, the high levels of Au associated with this zone

coincide with the elevated levels of Ag, Bi, Hg and Sb associated with the

presence of supergene tetrahedrite. It is therefore likely that the Au is present in

a submicroscopic form associated with the supergene mineralisation.

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11.6.4 Chemistry

The majority of the native Au grains located in the gossans contain less than five

weight per cent Ag, with the majority of grains containing less than 0.5 weight

percent Ag. A single, spheroidal grain with a Ag-rich rim was located in borehole

CR194 and several Ag-rich Au grains (electrum), were identified within the

galena-rich aggregates. The low Ag content of the bulk of the native Au grains is

consistent with supergene Au precipitated from solution in an acidic and oxidising

environment. The presence of at least some electrum grains suggests that the

conditions under which the Au was mobilised may have been quite variable.

The gossan contact with the massive sulphide in borehole CR194 is

characterised by the presence of Au-amalgam grains. No native Au grains were

located during the examination of the polished sections from this sample interval

and it is presumed that the bulk of the Au is present as Au-amalgam. The Au, Ag

and Hg contents between grains are quite variable (Appendix 5). The Au-

amalgam grains in the middle portion of this contact zone typically contain

subordinate amounts of Au (range 5.4–11.6%). The Au content of the Au-

amalgam grains located in the lower portion of this contact zone typically contain

more significant amounts of Au (range 27.7–56.5%).

In the galena-rich layer of borehole CR194 the Au content of Au-amalgam grains

exhibits a more restricted compositional range (16.4–21.0%). A single,

compositionally zoned grain, depleted of Ag towards the margins, is possibly

indicative of leaching. Au-amalgam was not observed in any other of the

boreholes selected for examination during this investigation and may reflect

restricted Hg migration away from the central supergene sulphide orebody.

Elevated levels of Hg and Se were associated with native Au in borehole CR149.

A small number of Ag-bearing native Au grains were located in the gossan/shale

conglomerate contact of borehole CR123. The Ag content of the native Au grains

ranges from between approximately 15 and 30 per cent.

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11.7 Gossan Paragenesis

11.7.1 Introduction

The dominant gossan mineral paragenesis is described here. The paragenesis

has been determined by a detailed examination of mineral associations and

textural observations documented in this thesis. These observations also aid in

the understanding of the geochemistry of the mineralising fluids. The paragenetic

sequence for the Las Cruces gossan is as follows:

Quartz (resistate) → Native Au (± Au amalgam) → Galena (± Pb(SbAs)-

sulphosalts) → Fe-sulphides → Siderite

This mineral paragenesis is rarely observed in the gossan as a complete

sequence. This appears to be due to a number of factors including:

Multiple stages of mineralisation/changes in fluid chemistry

Oxidation, particularly of Fe-monosulphides and siderite

Replacement (e.g. siderite replaces Fe-monosulphides and galena)

Dissolution (e.g. of siderite)

Partial sequences are observed consistently in all five boreholes examined during

this investigation, examples of which are illustrated in Figure 11.26. Mineralogical

examination confirms that the bulk of the quartz is resistate in nature and is

therefore not strictly part of the siderite, sulphide and precious metal

mineralisation that forms the dominant gossan assemblage.

Native Au is commonly found in relative isolation within cavities and along grain

boundaries associated with relict quartz (Figure 11.26a). The native Au may well

have originally precipitated with galena (Figure 11.26b), Fe-monosulphides

(Figure 11.26e) and siderite, but extensive dissolution, replacement and oxidation

has, in many cases, removed these phases. Native Au is extremely inert and the

least mobile of the phases in this paragenetic sequence. It is resistant to

oxidation and dissolution and its survival in relative isolation is therefore not

unexpected.

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Figure 11.26 – Montage of false colour backscattered electron images illustrating partial and complete paragenetic sequences observed during this investigation. a) Au (yellow) is frequently located in isolation along the margins of relict quartz grains (mauve). b) Au with siderite (brown) cementing relict quartz. c) Au with overgrowths of galena (pale blue/white) cementing relict quartz. d) Au with overgrowths of galena (pale blue/white) and siderite cementing relict quartz. e) Au inclusion in Fe-monosulphide (light grey/brown) in quartz. f) Au with euhedral Fe-monosulphide crystals and siderite cement. g) A rare example of a complete paragenetic sequence consisting of Au → galena → Fe-monosulphide → siderite.

In the absence of complexing ions, native Au is insoluble across the entire Eh/pH

range. In acid, oxidising environments, Au is generally considered to be soluble

as a Au-chloride complex, but only under relatively high chloride concentrations.

In the presence of free sulphur, under near neutral to alkaline reducing

conditions, Au is most likely mobile as a thiosulphate complex. Subtle changes in

Eh, pH and solution chemistry would therefore likely result in the rapid

precipitation of native Au from solution. Au-chloride complexes may also be

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reduced in the presence of Fe2+. Au-amalgam was only observed in borehole

CR194 and is therefore not considered as part of the dominant mineral

assemblage.

Native Au is observed in close and direct association with siderite (Figure

11.26b). Examples illustrated in Figure 11.26d, 11.26f and 11.26g demonstrate

that siderite represents the final stage of mineralisation in this sequence.

However, the example illustrated in Figure 11.26b may be the result of two

discrete mineralisation events involving the initial precipitation of native Au (±

other phases removed via oxidation, replacement e.t.c.) followed by a later,

discrete stage of siderite mineralisation.

The most common and widespread association with native Au (or Au-amalgam) is

that of galena. Galena clearly forms later overgrowths on native Au (Figures

11.26c and d). Pb is extremely immobile under most Eh/pH conditions, only

becoming soluble as Pb2+aq and HPbO2

-aq under extreme acid and alkaline

oxidising conditions respectively. Galena is the stable phase under strongly

reducing conditions and rapidly oxidises to form anglesite and cerussite under

acid and alkaline oxidising conditions. The early precipitation of galena from

solution is therefore not unexpected due to the highly immobile nature of Pb. The

galena would have precipitated under strongly reducing conditions over a wide

pH range.

Locally within the gossan, anglesite and cerussite are present with the siderite

mineralisation. A small proportion of the anglesite, and to a lesser extent the

cerussite, has clearly formed as an oxidation product of the galena. However,

some of the cerussite and anglesite has formed from the same mineralising

solutions as the siderite, indicating elevated Eh conditions relative to those under

which the galena has precipitated. The precipitation of cerussite may also occur

if the sulphur activity of the solution is insufficient (relative to the CO2 activity) to

precipitate galena. Several, discrete Pb(Sb,As)-bearing sulphosalts are present

in the gossan. These, however, appear to be localised occurrences, probably

derived from more As and Sb-rich fluids than those directly responsible for the

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galena mineralisation and are rarely observed directly associated with the

precious metal mineralisation.

The precipitation of galena would affect the mineralising fluid chemistry by

reducing the Pb and S activity to an extent that galena mineralisation would

eventually cease. If the Pb activity became the first limiting factor for galena

precipitation, Fe-sulphides may then become the stable phase. If the S activity

was the limiting factor, then cerussite might form. Siderite is commonly observed

in association with the Au/galena intergrowths (Figure 11.26d), possibly as a

result of the lowering of Pb and S activity (due to the precipitation of galena), or a

later and discrete stage of siderite mineralisation.

The second most common native Au association is that of Au and Fe-

monosulphides (Figures 11.26e and f). A complete paragenetic sequence

consisting of native Au → galena → Fe-monosulphides → siderite is extremely

rare (Figure 11.26g). However, Au → Fe-monosulphide associations are

common and may occur either with or without later stages of siderite (Figures

11.26e and f respectively). Due to the greater mobility of Fe2+ relative to Pb2+,

particularly under medium to low pH conditions, any Pb in the mineralising

solutions would be expected to precipitate as galena prior to the formation of Fe-

sulphides. A decrease in Pb activity resulting from the precipitation of galena

may produce solutions that still contain a sufficiently high S activity for the

formation of Fe-monosulphides. Galena → Fe-monosulphide associations are

relatively common and are illustrated throughout Chapters 5 to 9. The formation

mechanisms for Fe-sulphide formation are discussed in detail in Chapter 10. A

significant portion of the Fe-monosulphides have been replaced by marcasite and

pyrite.

Siderite is the final stage of mineralisation observed in the Las Cruces gossan.

The siderite may occur with or without galena and Fe-monosulphides. Multiple

stages of siderite mineralisation are evident by the presence and absence of

associated phases and also by the variation in mineral chemistry. The siderite

may be compositionally zoned. The chemistry of the different siderite generations

is not characteristic enough to provide any genetic links between the different

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stages of mineralisation either within or between boreholes. Siderite will only

precipitate once the S activity of the mineralising solutions has been significantly

reduced by the precipitation of galena or Fe-monosulphides as siderite will only

have a significant field of stability if the CO2 activity is very high and the S activity

very low.

It is clear that some stages of mineralisation were relatively Pb-poor and

occurrences of Au → Fe-monosulphide and Au → Fe-monosulphide → siderite

mineralisation without galena are relatively common. Similarly, some stages of

mineralisation consist almost entirely of galena that often replaces earlier stages

of siderite and/or Fe-sulphide mineralisation. These differences in fluid chemistry

further highlight localised variations in solution chemistry and the multiple stages

of mineralisation that are evident in the Las Cruces gossan. These variations

may, in part, reflect the differences in mobility of the elements/stability of the

discrete mineral species. This difference in stability of the mineral species is

most clearly observed between the relatively unstable siderite and acid volatile

sulphides (greigite, mackinawite e.t.c.) and the relatively stable galena and Au.

Siderite will only form under a very restricted set of Eh/pH conditions and its

stability is very dependant on Eh and pH conditions. Under reducing conditions,

a very slight decrease in pH may result in the dissolution of siderite. The AVS are

also very prone to dissolution under these conditions. However, galena is stable

at moderate to low pH under reducing conditions and is often retained in the

gossan where selective leaching of siderite and Fe-sulphides is clearly evident.

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Chapter 12 Discussion and Conclusions

12 DISCUSSION AND CONCLUSIONS

11.8 Introduction

This Chapter compares and contrasts the information gathered during the

literature review on gossans with that of the mineralogy of the Las Cruces

gossan. Information gathered on the Eh/pH of the dominant gossan mineral

assemblage, paragenesis and formational mechanisms and environment are also

discussed in terms of the genetic history of the gossan and a model of formation

is described.

Knight (2000) concluded that, based on mineralogical and textural evidence,

stable isotopes, noble gas geochemistry and fluid inclusion studies, the formation

of Las Cruces included seven distinct events:

1. A primary hydrothermal event

2. Oxidation during the waning stages of the hydrothermal system

3. Burial by a thick sequence of culm sediments

4. Sub-aerial supergene enrichment following uplift and erosion

5. Reworking of the sub-aerially gossan by seawater during the Miocene.

6. Burial by a thick sequence of Miocene sediments

7. Possible interactions by the present day water table

The current investigation is focussed predominantly on the formation of the

gossan. Knight's model for the formation of the massive sulphide, together with

the limited geological information provided in the internal Rio Tinto reports is

therefore used as a basis for events that predated the formation of the gossan.

The evidence collected during this investigation suggests that the Las Cruces

gossan has formed as a result of the processes described in Sections 12.2 to

12.7.

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11.9 Seafloor Gossan Formation

Knight (2000) suggests that oxidation, similar to that described for modern

seafloor sulphide deposits, occurred during the waning stages of the

hydrothermal system resulting in the formation of secondary

Fe-oxides/hydroxides and secondary Cu sulphides on the ancient seafloor.

Supporting evidence for the theory that the present day Las Cruces gossan

formed as a result of seafloor oxidation, includes:

greater mobility of Pb and Au in Cl-rich environments

presence of Cl-bearing minerals mimetite, pyromorphite and rare

nadorite

Fe-oxide dustings on silica (Knight, 2000)

This, however, is by far outweighed by the evidence against seafloor oxidation as

a dominant formation mechanism.

Knight (2000) suggests that burial of the Las Cruces deposit during the late

Carboniferous was followed by tilting during the Hercynian, with uplift and erosion

being followed by sub-aerial weathering and the development of the gossan,

silica cap and supergene Cu-sulphides. This resulted in a hinge zone that

effectively separates the steeply dipping primary mineralisation from the largely

horizontal secondary mineralisation (Figure 12.1).

The essentially horizontal gossan and secondary mineralisation is oriented to the

post Hercynian palaeo-surface and present day surface (and water table). This

effectively discounts ancient seafloor weathering as having a significant influence

on the present day gossan, as the ancient seafloor gossan would be present as a

steeply dipping ore zone along the upper edge of the massive sulphide zone,

quite separate from the present day gossan and associated supergene zone

(Figure 12.1). To date, no such zone has been recognised.

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Figure 12.1 - Diagram illustrating a) Primary massive sulphide and seafloor gossan preserved under culm sediments produced by continued volcanic activity. b) Tilting of the deposit during the Hercynian would have resulted in a steeply dipping primary massive sulphide and preserved seafloor gossan quite distinct from the sub-aerially derived, horizontal gossan, silica cap and supergene mineralisation (modified from Knight, 2000).

The mature geochemical profile of the Las Cruces gossan is more akin to the

extensive weathering of a massive sulphide deposit under sub aerial, near

surface weathering conditions. The absence of low temperature Mn-rich

hydrothermal deposits, marine fossils, pillow lavas, the lack of graded bedding of

the gossan fragments and the absence of clastic sulphides and Fe-oxide debris

are also an indication that the Las Cruces gossan did not form on the seafloor.

Carvalho (1999) notes that the striking difference between most Iberian Pyrite

Belt deposits and present day seafloor sulphide deposits is the lack of significant

oxidation and sedimentary dilution. The IPB VMS deposits typically consist of

truly massive sulphides and therefore differ markedly from the oxidised sulphide

rubble mixed with sediment and rock fragments that are commonly observed on

the seafloor (citing Rona and Scott, 1993). Carvalho (1999) suggests that this

may be due to the formation of IPB massive sulphides below the palaeo-seafloor,

protected by a thin cap of impermeable sediments.

Siderite, galena and greigite, the dominant mineral assemblage in the Las Cruces

gossan, can only form under strongly reducing conditions, and the high sulphate

content of seawater would favour the precipitation of Fe-sulphides, not siderite,

which is more likely to form in freshwater environments.

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11.10 Sub-Aerial Gossan Formation

CR194 is the only borehole examined during this investigation that exhibits

evidence of relict Fe-oxyhydroxides that are typical of a mature, sub-aerially

derived gossan. The bulk of the present day gossan has been extensively

replaced by a siderite and sulphide dominated assemblage. Despite the lack of

'typical' relict sub-aerial gossan, significant evidence remains to suggest that the

Las Cruces gossan was initially formed as a result of sub-aerial weathering,

including the geological and climatic history of area, the local geology of gossan

and supergene zone, the presence of a relict/resistate mineralogy, the primary

geology of massive sulphide, the presence of a large Fe dispersion halo around

the massive sulphide orebody and the nature of precious metal mineralisation.

The geological history of the area indicates that the massive sulphide was

partially exposed before and during the Tertiary. The climate at the time was

warm with high rainfall, creating ideal conditions for oxidation and the

development of a deep weathering profile.

The local geology also indicates that the orientation of the present day gossan

and supergene zone indicates that they formed after the tilting, uplift and

subsequent erosion that occurred during the Hercynian.

The vertical extent of the present day gossan and supergene zone, the degree of

metal leaching and concentration lower in the profile is indicative of a mature

gossan profile that is only known to occur under strongly acidic conditions

resulting from near-surface weathering. The concentration of precious metals

and relict resistate phases such as quartz, TiO2 and cassiterite and the

reprecipitation of secondary chalcedony (from leached quartz grains and other

silicates), cassiterite (derived from Sn2+ from oxidised sulphides) and anatase (Ti

from leached oxides, micas, amphiboles and TiO2 e.t.c.) at the base of the

gossan profile are further evidence of extensive leaching of primary gangue and

ore minerals and a high degree of mass wasting as a result of the oxidation of the

original massive sulphides.

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The relict pyrite at the contact between the gossan and massive sulphide zone of

borehole CR194 exhibits highly irregular morphologies that are typical of partial

oxidation and dissolution under highly acidic conditions. The relict quartz and

ferruginised host rocks in all five boreholes also exhibit evidence of extensive acid

leaching. The absence of boxwork textures in the gossan is also characteristic of

a low pH environment.

The primary geology of the Las Cruces massive sulphide deposit is pyrite-rich

and acid-buffering gangue-poor. Extensive oxidation of the massive pyrite body

would have resulted in low pH conditions due to the high Fe-sulphide content and

the absence of significant acid buffering minerals. This would have resulted in a

significant degree of dissolution and remobilisation of Fe, together with base and

precious metals from the massive sulphide deposit and silica from surrounding

wall rocks. As a result, a geochemical profile, typical of a mature gossan profile,

would have developed.

Blain and Andrew (1977) note that the mobility of Fe is pH dependent, with

greater Fe mobility occurring under very low pH conditions. The gossan

developed at Las Cruces appears to have undergone not only mechanical

dispersion, but also significant degrees of chemical dispersion, with the gossan

halo developed some distance from the central massive sulphide orebody. This

may, however, at least in part, reflect the oxidation of more disseminated sulphide

mineralisation that is present in the associated shales and other wallrocks.

The extremely fine-grained nature of the precious metal mineralogy and the

apparent in situ growth of precious metal-bearing grains within cavities and

fractures are common features of secondary Au. Recent experimental works by

Vlassopoulos and Wood (1990) show that in groundwaters circulating through

oxidising orebodies, Au(S2O3)23-, AuHS0 and Au(HS)2

- are the stable solution

species. Bacteria in the natural environment may play an important role in both

the mobilisation and reprecipitation of Au and other metals (Lengke and Southam,

2005; Reith and McPhail, 2006). However, it is generally considered (Webster

and Mann, 1984; Koshman and Yugay, 1973; Williams, 1933-34; Mann, 1984;

Ross, 1997) that under strongly acidic, oxidising conditions the most likely

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mechanism for Au and Ag mobilisation during the initial, near-surface weathering

of sulphide deposits would be that of chloride complexes, with high fineness

native Au precipitating lower in the gossan profile following reduction by Fe2+.

The Ag chloride complex is typically precipitated further down in the gossan

profile due to the comparatively high solubility of the Ag chloride relative to Au

chloride. Numerous high fineness Au grains were observed in the Las Cruces

gossan, with Ag typically present as discrete phases, notably members of the

proustite-pyrargyrite solid solutions series and sternbergite lower in the profile. It

should be noted, however, that not all of the Au in the Las Cruces gossan exhibits

a high fineness, with Ag-bearing Au and Au-amalgam being present locally.

Several factors would have had the effect of slowing the rate of oxidation of the

Las Cruces orebody including:-

Recrystallisation of the pyrite in the primary orebody.

Buffering of acidic solutions by surrounding wallrocks.

The low porosity of the primary massive sulphide, limiting the diffusion rate

of oxygenating groundwaters.

The absence of highly reactive pyrrhotite.

Despite these limiting factors, the evidence suggests that the original Las Cruces

gossan formed under near surface weathering conditions, developing a mature

gossan profile under low pH conditions.

The key differences between the Las Cruces gossan and mature gossans

described in the literature relate to the mineralogy, which largely reflect the

reducing conditions that are prevalent at Las Cruces. These differences are

summarised in Table 12.1.

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Table 12.1 - Comparison of Mature Gossans and Las Cruces Gossan Mineralogy

Mature Gossans Las Cruces Gossan

Fe Mineralogy Goethite, hematite, jarosite Siderite, hematite, greigite

Au Mineralogy High fineness Au High fineness Au, Ag-bearing Au, Au-amalgam

Ag Mineralogy Ag-halides, acanthite, Ag-jarosite

Proustite, pyrargyrite, amalgam, sternbergite (largely in supergene zone)

Pb Mineralogy Anglesite, cerussite, Pb-sulphates

Galena

As-Sb Mineralogy PbSbAs-sulphates PbSbAs-sulphides

The siderite dominated mineral assemblage seen in the Las Cruces gossan

clearly represents a late stage of mineralisation that extensively replaces the

original gossan mineral assemblage. The bulk of the limonite in the Las Cruces

gossan represents the oxidation product of siderite. It is evident that the relict

limonite dominated, sub-aerial gossan that remains partially evident in borehole

CR194, formed under a very different environment relative to the later, siderite-

dominated environment, which could not have formed under oxidic, near surface

weathering conditions. A significant portion of the Au is also present associated

with the siderite/galena/Fe-sulphide assemblage and therefore did not precipitate

under oxidising conditions.

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11.11 Gossan reworking

Mechanically reworked gossans are a common feature of the Iberian Pyrite Belt

and are known locally as gossan transportado. If it is assumed that the dominant

stages of sulphide weathering at Las Cruces occurred prior to burial by the

Tertiary marl, then the climate immediately preceding the Tertiary was relatively

warm and wet (Knight, 2000; citing Sanz de Galdeano and Vera, 1992 and

Moreno, 1993). Kosakevich et al. (1993) describe the Rio Tinto gossan

transportado as comprising Fe oxide precipitates that have been brecciated,

reworked and redeposited in more recent sediments by solution transport.

Kosakevich et al. (1993) suggest that the bedding of these deposits indicates a

sudden discharge, possibly indicative of erosion under a warm wet

Mediterranean-type climate, with brief, violent seasonal rainfall.

The initial reworking of the Las Cruces gossan appears to have occurred prior to

burial by the Tertiary conglomerate, with the contact between the gossan and

marl being sharply defined in all of the boreholes examined during this

investigation. Some fragments of gossan are found in the Tertiary conglomerate

in places (Knight, 2000), but these are generally rare. Minor amounts of siderite

are present in the Tertiary conglomerate, but the siderite is relatively rare and

differs somewhat from the gossan mineralisation due to the nature of the very

distinctive two-stage zoning with particularly Ca- and Mg-rich cores and Fe-rich

rims. The Tertiary conglomerate siderite may therefore be unrelated to the

gossan mineralisation.

The Rio Tinto gossan transportado exhibits remarkably similar larger scale

textural features to the Las Cruces gossan, with assorted shale-like rock

fragments occurring within a distinctive red coloured matrix. The main difference

is that the Rio Tinto gossan is dominated by jarosite and Fe-oxyhydroxides,

whereas the Las Cruces gossan is dominated by the presence of siderite.

However, the relict Fe-oxyhydroxide gossan of borehole CR194 also exhibits the

characteristic reworked/fragmented texture similar to that described for the Rio

Tinto gossan, suggesting that the Las Cruces gossan may well have been

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essentially similar in mineral composition and texture to the Rio Tinto gossan,

prior to extensive replacement by siderite.

The relationship and timing between the gossan reworking and siderite

mineralisation has important implications for the paragenesis of the gossan.

Initial examination of the gossan suggested that the siderite had been subjected

to reworking as it typically exhibits a 'clast-like' appearance. This would suggest

that the siderite precipitated prior to the pre-Tertiary reworking event. Further

examination confirms that the siderite 'clasts' actually consist of cavity infills and

the pseudomorphous replacement of former rock fragments. This suggests that

the siderite mineralisation could post-date the reworking of the gossan.

Textural evidence suggests that continued cycles of dissolution and

reprecipitation of the siderite, together with the oxidation of disseminated

sulphides may have resulted in fragmentation of the gossan as these events

would have left large voids and an unstable structure that may have collapsed

under the weight of the overlying rocks. Therefore, some degree of reworking of

the gossan may have occurred post burial, suggesting two very discrete stages of

gossan reworking, one pre Tertiary and one post Tertiary.

The gossan reworking has not significantly affected the geochemical profile

developed during near-surface weathering conditions and the concentration of

resistate phases and precious metals near the base of the gossan appears

largely preserved. This possibly indicates that oxidation and/or geochemical

mobilisation continued during and after the period of reworking, with some

reworking of sulphide-rich fragments also possibly occurring during this time.

These sulphide-rich fragments would have subsequently been oxidised during

later stages of oxidation.

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11.12 Marine incursion and seawater alteration

Major climate change resulted in a significant rise in sea level during the Tertiary

and subsequent burial of the Las Cruces deposit by glauconite sand and marl,

eventually sealing the gossan from further near-surface weathering. The

incursion of significant volumes of seawater into the region may have played a

major role in the alteration of the Fe-oxyhydroxide dominated, sub-aerially formed

gossan. The possible oxidising effects of sea water, the introduction of other

elements into the gossan environment, including chlorine and CO2 may, at least

in part, provide some explanation for the complex mineralogy found in the modern

day gossan. However, the mineralogy of the Lagoa Salgada gossan, an Iberian

Pyrite Belt deposit also buried under the Tertiary marl, contains predominantly

goethite and hematite (Oliveira et al., 1998), and no siderite.

As discussed previously, seafloor gossans are dominated by the presence of Fe-

oxides and hydroxides, with siderite being rare or absent. The evidence of minor

amounts of Cl-bearing minerals, including pyromorphite and mimetite is by far

outweighed by the evidence against seawater alteration as an important

formation mechanism, with the dominant Las Cruces gossan mineral assemblage

of siderite, galena and greigite only forming under strongly reducing conditions.

Siderite is also inhibited from forming in marine environments because the

Fe2+/Ca2+ ratio in normal marine waters is two orders of magnitude too small to

permit siderite precipitation.

During the seawater incursion, the environment would have remained an

oxidising one, with goethite and hematite remaining the stable and dominant Fe-

bearing species. Continued oxidation of the sulphides and subsequent element

mobilisation may have occurred and would have been enhanced in the strongly

oxidising, Cl-rich environment. This may have resulted in the precipitation of

discrete Cl-rich species, including atacamite, pyromorphite and mimetite. Some

Cl-rich species are, however, often extremely soluble (e.g. atacamite) and

evidence of their formation may well have been destroyed by later stages of

mineralisation or circulating groundwater.

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11.13 Deep burial by Tertiary sediments

The presence of approximately 150 metres of Tertiary marine sediments

overlying the Las Cruces gossan is a clear indication that the deposit was

covered by a marine transgression that lasted for a significant period of time.

Knight (2000), estimates that during the Miocene, up to 1000 metres of marl may

have covered the Las Cruces deposit, prior to uplift and erosion to the present

day position and suggests a temperature in and around the deposit during burial

of ~100oC, although this would assume a relatively steep geothermal gradient.

Knight (2000) proposes that the increase in geothermal gradient that occurred

during burial has brought about a marked change in the sulphide mineral

assemblage of the secondary Cu zone, with retrograde replacement of digenite

by bornite and chalcopyrite (Knight, 2000). In addition, the burial of the gossan

by the Tertiary sediments resulted in both a high degree of preservation of the

oxide zone and a marked change in Eh.

The burial of the Las Cruces gossan, elevated temperatures and a shift from a

high to low Eh environment may have resulted in the marked changes in the

gossan mineralogy. However, although the reduction of anglesite might form

galena under such conditions, siderite and greigite form under very specific

conditions that burial alone cannot explain. Under decreasing Eh conditions,

goethite may dehydrate to form hematite and hematite may be reduced to form

magnetite. Magnetite is essentially absent from the Las Cruces gossan,

suggesting conditions were unsuitable for magnetite formation. Therefore, the

Fe-oxyhydroxides are unlikely to be significantly affected by burial except perhaps

under more extreme reducing conditions or significantly elevated temperatures.

This lack of alteration of the gossan minerals is evident at Lagoa Salgada,

another buried gossan in the IPB, where the gossan is dominated by the

presence of hematite.

The burial of the gossan does not explain the cyclical oxidation, reduction events

associated with the multiple stages of metal mobilisation and siderite dissolution

and reprecipitation that is evident in the core. Although the burial of the gossan

may have created suitable reducing conditions for siderite formation, other factors

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must have been active for the formation of the siderite, galena and greigite

dominated assemblage observed in the present day gossan. Burial would also

not explain the strong indication that bacterial Fe- and/or sulphate-reduction has

played an important role in both the siderite and greigite formation.

There is, however, significant evidence to suggest that the present day aquifer

provides the ideal environment for the formation of the mineral assemblage that is

currently observed in the Las Cruces gossan. The most significant of these

changes is the potential for the replacement of the primary, oxidic gossan

minerals by the carbonate- and sulphide-dominated assemblage, consisting of

siderite, galena and greigite.

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11.14 Modern day gossan and aquifer

11.14.1 Introduction

There is compelling evidence to suggest that the Niebla Posadas aquifer plays a

key role in the formation of the siderite/greigite/galena/precious metal mineral

assemblage currently observed in the Las Cruces gossan.

The aquifer lies directly above the Las Cruces gossan, within the Tertiary

conglomerate/glauconite sand unit. The water in this regionally important aquifer

is mainly a calcium bicarbonate type, probably as a result of the large volumes of

marl within the area from which, at least in part, the aquifer drains. In the deeper

parts of the basin, water quality decreases and is more typically a sodium chloride

type, with trace amounts of metals and sulphate also occurring in places,

suggesting metal mobilisation (R2795, 1998). Elevated water temperatures of

~40oC have been recorded in the aquifer (Knight, 2000).

Mineralogical evidence suggests that there have been several stages of

siderite/greigite/galena/precious metal mineralisation resulting from fluctuating

Eh/pH conditions, where the dominant environment is that of a reducing one,

induced by consumption of O2 and production of CO2 by biogenic processes.

Fluctuations in the level of the aquifer during periods of drought and high rainfall

and/or bacterially driven processes of oxidation and reduction might explain the

changes in Eh and pH in the region of the gossan, resulting in changes in O2,

chlorine, CO2 and metal content of the water.

11.14.2 Siderite and Greigite

The processes of siderite and greigite formation described in Chapter 10 has

significant implications for the formation of the Las Cruces gossan as they are

intimately linked to biological, anaerobic mechanisms that occur at or below the

water table. A detailed review of the literature, together with limited stable isotope

analyses, have shown that biogenic processes within the aquifer are the likely

mechanisms behind the extensive siderite mineralisation, with a significant

influence from the biogenic anaerobic oxidation of methane.

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These biological processes, including bacterial sulphate reduction,

methanogenesis, methane oxidation and Fe-reduction produce the key

components for carbonate and sulphide formation, including CO2/HCO3, CH4 and

H2S/HS-. These processes are summarised in Figure 12.2.

In summary, the oxidation of organic matter to CO2 occurs whenever oxygen is

present, however, once it is consumed, other, less energy producing substances

are utilised by anaerobic bacteria. This gives rise to the following succession in

the processes of organic matter decomposition:

Oxygen consumption (respiration)

Sulphate reduction

Methanogenesis

In each of these zones, the dominant microbial population exploits the

environment, creating a new environment that favours other species. Thus, the

transition from aerobic sediment, to anaerobic sulphate-reducing sediment, to

anaerobic methane-producing sediment is (at least in part) geochemical

consequences of species induced environmental changes (Claypool and Kaplan,

1974).

The aerobic oxidation of organic matter is a CO2 producing reaction. However,

siderite precipitation is unlikely in this zone because carbonate activities sufficient

to cause carbonate super-saturation are unlikely to be reached due to upward

diffusion into depositional waters. Siderite and greigite are also only stable under

reducing conditions.

Siderite and greigite formation are therefore only likely in the anaerobic zone of

sulphate reduction and methanogenesis, with a significant influence from

methane oxidation and Fe-reduction.

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Figure 12.2 – A schematic illustrating the distinct biogeochemical and abiotic environments that mark the boundaries between regimes of aerobic and anaerobic metabolism and subsequent carbonate and/or sulphide mineral precipitation. The schematic illustrates the approximate depths, changes in temperature and typical δ13C values associated with the CO2 generated from the decomposition of organic matter. In addition, the competitive and/or complementary processes of nitrate reduction and Fe-/Mn-reduction are also included (modified from Irwin et al., 1977 and Claypool and Kaplan, 1974).

Page 296


The δ13C values of -33, -36 and -42%o for the Las Cruces siderite indicate that

methane oxidation was a source of at least some of the carbon, diluted by

isotopically heavier carbon, probably produced by one or more of sulphate

reduction, Fe-reduction and methanogenesis. Methane oxidation is considered to

play a key role in the formation of the Las Cruces siderite as the upward transport

of CH4 from the zone of methanogenesis into the sulphate reduction zone

typically produces CH4 with δ13C between -60 and -80%o. No other processes

produce such isotopically light δ13C values.

The anaerobic oxidation of methane is performed by methanotrophic archaea

and sulphate reducing bacteria where sulphate and methane are consumed at

the base of the sulphate reduction zone. The anaerobic oxidation of methane

produces bicarbonate, increasing carbonate alkalinity and saturation with respect

to carbonate minerals and siderite may form. The reduction of sulphate will also

favour the precipitation of sulphides.

Although the source of the methane in the Las Cruces gossan has not been

determined during this investigation, the aquifer provides the ideal environment

for shallow biogenic processes to predominate.

The bacterial reduction of sulphate accompanied by organic matter

decomposition produces S2- and CO2. The carbon dioxide produced by this

reaction dissolves readily in pore water to increase bicarbonate concentrations,

often resulting in the precipitation of carbonate minerals with distinctive carbon

isotope values (δ13C typically -25%o).

The presence of significant levels of sulphur will inhibit the precipitation of siderite

and sulphides will preferentially form. Hence, sulphide precipitation, namely that

of galena and greigite, always precedes siderite mineralisation in the Las Cruces

gossan, assuming sufficient sulphur activity of the mineralising fluids

The presence of galena and greigite +/- other Fe-monosulphides is therefore

indicative of sulphate reduction. Crystal growth kinetics is considered to play an

important role in the initial formation of Fe monosulphides over pyrite and

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marcasite. The fact that greigite persists in the gossan and exhibits only partial

replacement by marcasite and pyrite is likely an indication that non-sulphidic

conditions are attained by the exhaustion of all sulphate and sulphide, so that

pyritisation reactions are not driven to completion. A limited source of sulphate is

typical of brackish to fresh water environments.

Methanogenesis is essentially a microbial process involving the production of

methane. Microbially mediated methane production generally occurs via CO2

reduction and/or acetate fermentation. Berner (1981) suggests that siderite

forms through the combined effects of Fe reduction and bacterial

methanogenesis of organic carbon compounds. δ13C values of +10 to +15%o are

typical of siderite formed as a result of methanogenesis.

Where the availability of Fe3+ outweighs that of sulphate, Fe-reduction will

predominate, releasing Fe2+ to the diagenetic pore waters. Fe reduction may

occur in conjunction with sulphate reduction and methanogenesis and the

subsequent generation of Fe2+, bicarbonate and hydroxyl ions increases alkalinity

and siderite precipitation is favoured.

Mineralogical evidence suggests that there have been several stages of late-

stage siderite/sulphide/precious metal mineralisation within the gossan, with

cyclical oxidising and reducing events and subsequent changes in pH, consistent

with what might be expected in a fluctuating water table associated with a semi-

arid, Mediterranean climate. The aquifer would not create a suitable environment

for the development of the mature gossan profile seen in the Las Cruces gossan

due to strong buffering of acid solutions by siderite and the relatively low metal

sulphide content of the gossan. This suggests that the original mature gossan

profile developed under near-surface weathering conditions eventually being

replaced by the siderite/secondary sulphide mineralisation during interaction with

the aquifer.

Siderite has formed as a result of chemical transportation of Fe. Porosity of the

host rocks has significantly influenced Fe migration within the gossan. Siderite

occurs as a late-stage phase that commonly forms along the margins of the

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friable, residual silica-rich rock fragments and within open pore spaces, further

indication of chemical transportation. The aquifer provides an ideal mechanism

for the chemical dispersion of Fe that is observed within the gossan.

Siderite is the final stage of mineralisation observed in the Las Cruces gossan.

Multiple stages of siderite mineralisation are evident by the presence and

absence of associated phases, notably galena and Fe-monosulphides, the

variation in siderite chemistry, evidence of overgrowths and degrees of oxidation.

The siderite may be compositionally zoned. The chemistry of the different

siderite generations is not characteristic enough to provide any genetic links

between the different stages of mineralisation either within or between boreholes.

Given the mechanisms behind siderite and greigite formation and the strong

influence that biological processes have on creating the ideal Eh/pH conditions

and products for siderite and greigite formation, the aquifer remains the only likely

environment throughout the history of the Las Cruces deposit for the formation of

this late-stage mineral assemblage.

11.14.3 Pb-bearing sulphides

Pb is mobile as Pb2+aq and HPbO2

-aq under extreme acid and alkaline oxidising

conditions respectively. At low Eh, galena is the stable phase, with anglesite and

cerussite occurring under oxidising acid and alkaline conditions respectively. The

presence of galena in the Las Cruces gossan mineral assemblage is therefore

further indication of strongly reducing conditions.

The close association between galena and the siderite/greigite mineralisation

suggest that Pb mobilisation and precipitation may be related to bacterial

processes within the gossan. Recent studies (Wu et al., 2006; Jensen-Spaulding

et al., 2004; Lui et al., 2008) show that bacteria may pose both positive and

negative impacts on the mobility of heavy metals. The abundance of galena in

the Las Cruces gossan and the close association with the siderite/greigite

mineralisation suggest that galena precipitation may have occurred as a result of

bacterial sulphate reduction within the aquifer.

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The mobility of Pb, one of the least mobile elements in the gossan profile is

enhanced by the presence of chlorine in the transport medium. The presence of

accessory mimetite and pyromorphite also provide evidence for the presence of

chlorine, although these phases are largely confined to the relict gossan of

borehole CR194 and may relate to the near-surface weathering of the primary

sulphide. Anglesite pseudomorphs after galena are evident locally within the

core, indicative of localised oxidising conditions. Increased chlorine levels and

localised oxidation/remobilisation of Pb are consistent with what might be

expected in a cyclical oxidising/reducing environment brought about by a

fluctuating water table.

It is likely that increased mobility of Pb within the gossan occurred as a result of

localised oxidising conditions resulting from either a decrease in the level of the

aquifer and/or by the actions of sulphur oxidising bacteria. The subsequent

decrease in pH and probable increase in the concentration of dissolved salts,

including chlorine, would have resulted in the dissolution of galena from the

supergene zone and/or primary ore. Contact with diluting groundwater and

sulphate reducing bacteria resulted in the rapid precipitation of Pb2+aq as galena.

The rapid precipitation of galena may also account to some degree for the

extremely fine-grained nature of this phase within the gossan.

The presence of anglesite and cerussite within some siderite veinlets is further

evidence of local variations in Eh during precipitation of the late-stage mineral

assemblage. The precipitation of cerussite may also occur if the sulphur activity

of the mineralising solution is insufficient, relative to the CO2 activity, to precipitate

galena. Several, but minor amounts of discrete Pb(Sb,As)-bearing sulphosalts

are present in the gossan and are probably derived from more As and Sb-rich

fluids than those directly responsible for the galena mineralisation.

11.14.4 Precious metals

A large number of Au and Ag-bearing grains were identified in the Las Cruces

gossan, providing a significant amount of information on the nature and mode of

occurrence of the precious metal mineralisation. Several potential mechanisms

of Au and Ag mobilisation have been identified and although Mann (1984)

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suggests that it is generally accepted that only one of these mechanisms may be

operating in a single deposit, there is evidence at Las Cruces that several

mechanisms of Au dissolution, remobilisation and reprecipitation may have

occurred during different stages of weathering.

Au is one of the least mobile of elements in the gossan and as such, the aquifer

may have had little effect on the high fineness Au that precipitated under near

surface weathering conditions. However, a portion of the Au is alloyed with Ag

and is also present as micrometre- and sub-micrometre grains that are intimately

associated with the siderite/galena/Fe-sulphide mineralisation. The low fineness

of the Au and the close association with the siderite, Fe-sulphide and galena

mineralisation indicates that remobilisation and reprecipitation of precious metals

has occurred since the initial near surface weathering event that formed the

Fe-oxyhydroxide dominated gossan.

Although cyclical fluctuations in the aquifer level may explain the various stages

of oxidation and reduction evident in the Las Cruces gossan, the close

association between Au and the siderite/greigite mineralisation suggests that

bacteria may play an important role in the mobilisation and precipitation of Au

within this assemblage. Although bacterially mediated Au mobilisation and

precipitation remains and area of ongoing research, Reith and McPhail (2006),

Lengke and Southam (2005) and Southam and Beveridge (1996) have shown

that bacteria in the natural environment play an important role in the mobilisation

and reprecipitation of Au.

In carbon poor environments, such as the Las Cruces gossan, Au release

appears to be linked to Fe or sulphide oxidation. Bacterially mediated Au

mobilisation could proceed by via number of mechanisms, including initial release

of Au from the supergene zone via sulphide oxidation and subsequent

mobilisation as Au-thiosulphate or Au-organic complex. Subsequent precipitation

of the Au may occur via bacterially mediated sulphate and/or Fe reduction. This

theory allies closely with the bacterially mediated precipitation of siderite and

greigite discussed previously in this study. In addition, Au has been shown to play

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a functional role in the oxidation of methane (Levchenko et al., 2002), a process

strongly associated with the siderite mineralisation at Las Cruces.

Similarly, localised oxidising conditions within the gossan resulting from cyclical

fluctuations in the water table may have resulted in localised oxidation of the

sulphides associated with the supergene mineralisation. The nature and

mechanisms behind the mobilisation of Au and Ag under such conditions would

be in marked contrast to those conditions under which the initial, sub-aerial

weathering took place. Instead of strongly acidic conditions, resulting from the

oxidation of a pyrite-dominated orebody, the remobilisation of Au in the siderite-

dominated gossan would be under near-neutral to alkaline conditions and may

therefore proceed via thiosulphate or AuOH(H2O)0 complexes rather than AuCl4- .

The co-precipitation of Au and Ag is greatest under near neutral to alkaline

conditions, particularly in the presence of free sulphur. This may account for the

presence of abundant Au-bearing amalgam and the presence of Ag-rich rims and

Ag-rich Au grains that are also observed locally in the present day gossan.

Vlassopoulos and Wood (1990), Webser and Mann (1984) and Thornber (1992)

have observed that in the presence of incompletely oxidised sulphides,

thiosulphate is the stable species under alkaline oxidising conditions and Au of

low fineness is re-precipitated by reduction at the water table.

Garrels and Christ (1965) suggest that, given sufficient sulphur activity, Au is

mobile as a Au-sulphur complex under reducing conditions and near neutral-

alkaline pH. It is pertinent to consider that although localised oxidation within the

gossan (whether biotic or abiotic) may have resulted in the release of metals into

the surrounding groundwaters, the fluids from which this assemblage is

associated are predominantly associated with reducing conditions. As such, it is

likely that the Au, mobile as a thiosulphate complex, precipitated along with

galena and Fe-sulphides as a result of a decrease in sulphur activity of the

mobilising fluid as well as possibly reduction to native Au by interactions with

Fe2+.

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The Ag content of the siderite/galena/Fe-sulphide gossan assemblage is

relatively low compared to the sternbergite and proustite/pyrargyrite dominated

supergene zone. Although the sternbergite and proustite/pyrargyrite are

considered as part of the supergene zone, there is a distinct possibility that this

Ag-rich assemblage has also formed as a result of interactions with the aquifer,

an area for possible future investigations.

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11.15 Conclusions

Figure 12.3 illustrates an idealised cross section through the Las Cruces gossan,

supergene zone and underlying primary massive sulphide. The gossan is

overlain by approximately 100-150m of Tertiary sediments, the lower portion of

which contains the Niebla Posadas aquifer within a Tertiary sand unit. The

aquifer lies in direct contact with the porous gossan zone.

Tertiary marine deposits: consisting of rounded glauconite aggregates, quartz and feldspar fragments, calcite cement and shell debris and accessory pyrite. This unit hosts the aquifer, which extends into the porous gossan zone. Vertical extent: 100-150m.

Gossan: Concentration of chemically and physically resistate Au, quartz, cassiterite and TiO2 has occurred due to mass wasting during near surface weathering. Biogenic activity within the Niebla Posadas aquifer resulted in extensive replacement of quartz-rich wall rocks and relict sub-aerial Fe-oxyhydroxide gossan by siderite, greigite, galena and Au mineralisation. Vertical extent: 0-20m.

Gossan/Sulphide contact: Some boreholes exhibit a pyrite, sternbergite, proustite/pyrargyrite assemblage with some native Au that may represent a biogenic supergene zone, distinct from the underlying supergene Cu-sulphide. Vertical extent: 0-10cm.

Supergene Cu-sulphide: Developed during sub-aerial weathering in the pre-Tertiary and consists predominantly of secondary Cu-sulphides and pyrite. Accessory minerals include tetrahedrite/tennantite and enargite. Vertical extent: 40-60m.

Primary Massive Sulphide: Pyrite dominated massive sulphide with minor but significant amounts of chalcopyrite, galena and sphalerite. Accessory minerals include tetrahedrite/tennantite quartz and calcite. Vertical extent: undetermined.

Figure 12.3 – Diagram illustrating an idealised cross section through the Las Cruces deposit.

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The gossan overlying the supergene massive sulphide typically consists of

reworked relict Fe-oxyhydroxide and quartz-rich rock fragments that exhibit

extensive replacement by a late-stage siderite, greigite, galena and Au mineral

assemblage. The gossan mineralisation also extends away from the massive

sulphide where the chemically precipitated siderite-rich assemblage typically

replaces quartz-rich wall rocks. The base of the gossan is often marked by a

narrow Ag-rich supergene zone consisting of secondary pyrite, sternbergite,

proustite/pyrargyrite and native Au.

The secondary Cu zone consists of supergene-enriched massive sulphide or

supergene-enriched wall rocks. The primary massive sulphide zone is a tabular

structure dipping to the north at an angle of approximately 35o and consists

predominantly of pyrite together with accessory galena, chalcopyrite and

sphalerite.

There is no evidence to suggest that the present day Las Cruces gossan formed

as a result of seafloor oxidation during the waning stages of massive sulphide

mineralisation.

Local geology, notably the alignment of the gossan to the present day water table

indicates that the gossan developed after the tilting that occurred during the

Hercynian orogeny. Following uplift during the Hercynian, a mature gossan profile

developed under low pH and high Eh conditions as a result of extensive near-

surface weathering of the primary massive sulphide orebody. This resulted in

significant acid leaching and mobilisation of the more mobile elements and a

fixing of Fe as Fe-oxyhydroxides above the water table. Mass wasting resulted in

a concentration of chemically immobile elements and physically resistate

minerals, notably Au, Si (quartz), Sn (cassiterite) and Ti (TiO2). Au mobilisation

likely occurred as a Au-chloride complex.

Some gossan reworking probably occurred before and during the Tertiary prior to

burial by Tertiary marine sediments resulting in a jumbled mass of Fe-

oxyhydroxides, shale debris and quartz-rich rock fragments.

Page 305


The original sub-aerially weathered, Fe-oxyhydroxide dominated gossan has

been extensively replaced by siderite, greigite, galena and Au mineralisation.

Gossan mineral paragenesis occurred in the order Au→galena→Fe-

sulphides→siderite. Early precipitation of Au and galena was controlled largely

by their highly immobile nature. Siderite precipitation only proceeded once the

CO2 activity of the mineralising fluids was sufficiently high and the S activity was

reduced via the precipitation of galena and greigite.

Siderite/greigite mineralisation is intimately linked to anaerobic bacterial

processes. Microbial Fe and sulphur oxidation, Fe- and sulphate reduction and

methane oxidation coupled with a fluctuating water table within the Niebla

Posadas aquifer provide the mechanisms for cyclical metal release and

subsequent siderite and greigite mineralisation. The aquifer provides the

mechanism for the hydromorphic dispersion of Fe and other metals within the Las

Cruces region.

The Eh/pH conditions and processes involved in metal mobilisation/precipitation

within the carbonate-rich and sulphide poor gossan would be in marked contrast

to those involved in the initial, sub-aerial gossan forming process.

Although little is known about the bacterial mobilisation of Au and Pb in the

complex natural environment, experimental studies and the close association

between Au/galena and the siderite/greigite mineralisation suggests that the

mobilisation and precipitation of these metals is also either directly or indirectly

associated with bacterial processes within the gossan.

Greigite exhibits partial and extensive replacement by marcasite and pyrite,

particularly with increasing depth in the gossan. This replacement process

extends down to the contact with the supergene Cu-sulphide zone and is often

marked by the presence of supergene pyrite, sternbergite, proustite/pyrargyrite,

Au mineralisation. This Ag-rich zone is marked by the absence of siderite, but

may also be intimately linked to bacterial activity within the aquifer. Possible

interactions between microbiota within the aquifer and the underlying supergene

Cu mineralisation should be examined in any future investigations.

Page 306


11.16 Future Investigations

The Las Cruces deposit, and in particular the gossan mineralisation, is clearly

complex in nature and significant further studies would be required to gain a more

comprehensive understanding of the processes that have resulted in the

formation of the modern day deposit. The interpretation of larger scale textural

features, in particular, those of the gossan, would become more apparent once

mining of the deposit has commenced. This investigation has focussed on the

detailed documentation of the siderite, greigite, galena and precious metal

mineralisation of the gossan. This information provides a sound base for any

future investigations.

There is a distinct possibility that the aquifer and associated bacterial activity has

also had a significant impact on the supergene mineralisation, in particular the

sternbergite, pyrite and proustite/pyrargyrite assemblage. This area provides and

exciting opportunity for future investigation.

The more traditional mineralogical techniques, including optical microscopy, x-ray

powder diffraction and electron microscopy have been extensively used during

the current study. A significant degree of information may be gained by focussing

future investigations on the analysis of specific phases and/or stages of

mineralisation, possibly incorporating additional fluid inclusion studies and stable

isotope analyses to gain a greater understanding of the nature of the fluids that

resulted in the formation of siderite and the late-stage sulphide mineralisation.

In addition, analysis of the aquifer in the Las Cruces area may also provide useful

information on the nature of dissolved species, in particular, dissolved cations,

chloride content, CO2, pH and Sulphate Reducing Bacteria (SRB) activity,

together with the possibility that the aquifer is still active in the dissolution,

oxidation, replacement and alteration of the Las Cruces gossan.

Page 307

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Appendix 1 List of Mineral Formulae

Appendix 1: List of mineral formulae

AcanthiteAguilariteAluniteAmalgamAnataseAnglesiteApatiteArgentojarosite ArsenopyriteAtacamiteBariteBeudantiteBindheimiteBismuthiniteBorniteCalciteCassiteriteCerussiteChalcociteChalcopyriteChlorargyriteCinnabarClausthaliteCoronaditeCovelliteCrandalliteCristobaliteDigeniteDjurleiteEnargiteGalenaGeocroniteGlauconiteGoethiteGreigiteHarmotomeHematiteIdaiteImiteriteIodargyriteJarositeJordaniteKaoliniteKesteriteKoutekiteLaffittiteLaurioniteLepidocrociteLudlockiteLuzoniteMackinawiteMarcasiteMetacinnabarMimetiteNadoriteNaumanite

Ag2SAg4SSeKAl3(SO4)2(OH)6 AgHg-alloyTiO2

PbSO4

Ca5(PO4)3(F,Cl,OH) AgFe3+

3(SO4)2(OH)6

FeAsSCu2Cl(OH)2

BaSO4

PbFe3(AsO4)(SO4)(OH)6

Pb2Sb2O6(O,OH)Bi2S3

Cu5FeS4

CaCO3

SnO2

PbCO3

Cu2SCuFeS2

AgClHgSPbSePb(Mn4+,Mn2+)8O16

CuSCaAl3(PO4)2(OH)5.H2O SiO2

Cu1.805S Cu1.9SCu3AsS4 PbSPb14(Sb,As)6S23 (K,Na)(Fe3+,Al,Mg)2(Si,Al)4(OH)2

α-Fe3+O(OH)Fe2+Fe2

3+S4

(Ba,K)(SiAl)9O16.6H2OFe2O3

Cu4FeS6

Ag2HgS2

AgIKFe3(SO4)2(OH)6

Pb14(As,Sb)6S23

Al2Si2O5(OH)4

Cu2(Zn,Fe)SnS4

Cu5As2

AgHgAsS3

PbClOHγ-Fe3+O(OH)(Fe,Pb)As2O6

Cu3AsS4

Fe9S8

FeS2

Hg (Se,S)Pb5(AsO4)3Cl PbSbO2ClAg2Se

A1

Appendix 1 List of Mineral Formulae

NontroniteNovakiteNukundamitePhilipsbornitePlumbogummitePlumbojarositeProustitePyrargyritePyritePyromorphitePyrrhotiteQuartzRhodochrositeRobinsoniteRutileScoroditeSideriteSphaleriteStanniteSternbergiteStibiconiteTennantiteTetrahedriteWitticheniteZinkeniteZircon

Na0.3Fe3+2(Si,Al)4O10(OH)2.nH2O

(Cu,Ag)21As10

(Cu,Fe)4S4

PbAl3(AsO4)2(OH)5.H2OPbAl3(PO4)2(OH)5.H2OPbFe6(SO4)4(OH)12

Ag3AsS3

Ag3SbS3

FeS2

Pb5(PO4)3ClFe1-xSSiO2

MnCO3

Pb4Sb6S13

TiO2

FeAsO4.2H2OFeCO3

ZnSCu2FeSnS4 AgFe2S3

Sb3+Sb5+2O6(OH)

(Cu,Fe)12As4S13

Cu12Sb4S13 Cu3BiS3

Pb9Sb22S42

ZrSiO4

A2

Appendix 2 Sample List

Appendix 2: Sample List (Data courtesy of Rio Tinto Limited)

Borehole CR194

Depth (m) LITHOCODE/DESCRIPTION LENS

From To149.80 150.80 GHS - Strong Hematitic Gossan150.80 151.75 GHS - Strong Hematitic Gossan HWL (Hangingwall)151.75 152.70 GHS - Strong Hematitic Gossan HWL (Hangingwall)152.70 153.70 GHS - Strong Hematitic Gossan HWL (Hangingwall)153.70 154.75 GHS - Strong Hematitic Gossan HWL (Hangingwall)154.75 155.75 GBM - Moderate Hematite Magnetic HWL (Hangingwall)155.75 156.70 GBM - Moderate Hematite Magnetic AU1 (Au)156.70 157.30 GBM - Moderate Hematite Magnetic HWP (Hangingwall Pb)157.30 158.70 GBM/GHS - Moderate/Strong Hematitic Magnetic HWP (Hangingwall Pb)158.70 159.75 GHS - Strong Hematitic Gossan HWP (Hangingwall Pb)159.75 160.75 GHS - Strong Hematitic Gossan AU (Au)160.75 161.75 GHS - Strong Hematitic Gossan AU (Au)161.75 162.75 GHS - Strong Hematitic Gossan AU (Au)162.75 163.75 GHS - Strong Hematitic Gossan AU (Au)163.75 164.60 GHS - Strong Hematitic Gossan AU (Au)164.60 165.80 MMP - Massive Sulphide HCH (Secondary Cu)165.80 166.80 MMP - Massive Sulphide HCH (Secondary Cu)166.80 167.75 MMP - Massive Sulphide HCH (Secondary Cu)167.75 168.70 MMP - Massive Sulphide HCH (Secondary Cu)170.70 171.60 MMP - Massive Sulphide HCH (Secondary Cu)171.60 172.50 MMP - Massive Sulphide HCH (Secondary Cu)172.50 173.50 MMP/QXM - Massive Sulphide/Quartz/Shale HCH (Secondary Cu)173.50 174.50 QXM/MMP - Quartz/Shale/Massive Sulphide HCH (Secondary Cu)174.50 175.45 MMP - Massive Sulphide HCH (Secondary Cu)175.45 176.35 MMP - Massive Sulphide HCH (Secondary Cu)176.35 177.20 MMP - Massive Sulphide HCH (Secondary Cu)177.20 178.50 MMP - Massive Sulphide HCH (Secondary Cu)178.50 180.00 SXM - Massive Shale MCL (Secondary Cu)

A3


Borehole CR149

Depth LITHOCODE/DESCRIPTION LENS

From To

170.20 170.90 TSA - Tertiary Sand AU (Au)170.90 172.40 GHS - Strong Hematitic Gossan AU (Au)172.40 174.10 GHS/GMS - Strong Hematitic/Strong Magnetic AU (Au)174.10 175.10 GEM - Moderately Leached Gossan AU (Au)175.10 175.90 GEM - Moderately Leached Gossan AU (Au)175.90 176.90 GLM/GEW - Moderate Limonitic /Weakly Leached AU (Au)176.90 178.05 GHM/GLS - Moderate Hematitic/Strong Limonitic AU (Au)178.05 179.00 GEM - Moderately Leached Gossan AU (Au)179.00 180.35 GEM/GLS - Moderately Leached/Strong Limonitic AU (Au)180.35 182.00 GLS/GHS - Strong Limonitic/Strong Hematitic AU (Au)182.00 182.85 GHS - Strong Hematitic Gossan AU (Au)182.85 183.90 GHS - Strong Hematitic Gossan AU (Au)183.90 185.40 GMS - Strong Magnetic Gossan AU (Au)185.40 186.80 GLW - Weak Limonitic Gossan AU (Au)186.80 187.40 GHS - Strong Hematitic Gossan AU (Au)187.40 188.90 GMS - Strong Magnetic Gossan AU (Au)188.90 190.00 GMS - Strong Magnetic Gossan AU (Au)190.00 190.90 MMP - Massive Sulphide HCH (Secondary Cu)190.90 191.90 MMP - Massive Sulphide HCH (Secondary Cu)

Borehole CR038


From To

150.80 151.45 QTM - Quartz Replacement of Massive Tuff AU (Au)151.45 152.40 QTM - Quartz Replacement of Massive Tuff AU (Au)152.40 153.20 QTM - Quartz Replacement of Massive Tuff AU (Au)153.20 154.20 QTM - Quartz Replacement of Massive Tuff AU (Au)154.20 155.20 QTM - Quartz Replacement of Massive Tuff AU (Au)155.20 156.30 QTM - Quartz Replacement of Massive Tuff AU (Au)156.30 157.25 QTM/MSPCL - Quartz/Tuff/Sulphide/Clay AU (Au)157.25 158.25 MSPCL - Partial Massive Sulphide with Clay E (Envelope)

A4


Borehole CR191


From To

134.25 135.25 TCP/GHW - Conglomerate/Weak Hematitic Gossan HWL (Hangingwall)135.25 135.70 GHW/GEM - Weak Hematitic/Moderately Leached HWL (Hangingwall)135.70 136.85 GEM - Moderately Leached Gossan HWL (Hangingwall)136.85 137.95 GMS - Strong Magnetic Gossan HWP (Hangingwall Pb)137.95 138.90 GMS - Strong Magnetic Gossan AU1 (Au)138.90 139.85 GMS - Strong Magnetic Gossan AU1 (Au)139.85 141.00 GMS - Strong Magnetic Gossan AU1 (Au)141.00 141.65 GES - Strongly Leached Gossan AU1 (Au)141.65 142.65 GES - Strongly Leached Gossan AU1 (Au)142.65 143.60 GES - Strongly Leached Gossan AU1 (Au)143.60 144.70 GES - Strongly Leached Gossan144.70 145.75 GES - Strongly Leached Gossan145.75 146.90 GES - Strongly Leached Gossan146.90 148.15 GES - Strongly Leached Gossan148.15 149.15 GES - Strongly Leached Gossan149.15 150.10 GES - Strongly Leached Gossan AU (Au)150.10 150.90 GES - Strongly Leached Gossan AU (Au)150.90 151.75 GES - Strongly Leached Gossan AU (Au)151.75 153.85 GES - Strongly Leached Gossan AU (Au)153.85 155.35 MMPXM - Massive Sulphide with Shale HCL (Secondary Cu)155.35 156.25 MMPXM - Massive Sulphide with Shale HCL (Secondary Cu)156.25 157.20 MMPXM - Massive Sulphide with Shale HCL (Secondary Cu)

Borehole CR123


From To

152.40 153.95 TCP - Tertiary Polymict Conglomerate AU (Au)153.95 154.85 GMS - Strong Magnetic Gossan AU (Au)154.85 157.05 No Core Recovery157.05 158.65 GMS - Strong Magnetic Gossan AU (Au)158.65 160.20 GMS - Strong Magnetic Gossan AU (Au)160.20 161.40 GMS - Strong Magnetic Gossan AU (Au)161.40 161.80 GMS - Strong Magnetic Gossan AU (Au)161.80 163.40 GMS - Strong Magnetic Gossan AU (Au)163.40 168.20 No Core Recovery168.20 169.00 GMS - Strong Magnetic Gossan AU (Au)169.00 169.65 QXM - Quartz Replacement of Massive Shale AU (Au)169.65 172.85 QXM - Quartz Replacement of Massive Shale AU (Au)172.85 176.00 No Core Recovery176.00 178.35 SXM - Massive Shale MCL (Secondary Cu)178.35 180.00 QXM - Quartz Replacement of Massive Shale MCL (Secondary Cu)180.00 181.50 SXM - Massive Shale MCL (Secondary Cu)

A5

Appendix 3 Assay Data

Appendix 3: Assay Data

Au Analysis – Fire assay with AAS/ICP finish

The sample is roasted and mixed with a suitable flux, transferred to a fireclay crucible and fused. Lead oxide in the flux is reduced, and the lead globules formed collect the precious metals. The lead button is cleaned and cupelled, the precious metal prill dissolved in aqua regia and the analytes of interest are analysed by AAS (Thermo Jarrell Ash Atomic Absorption Spectrophotometer model Video 12 E) or ICP (Philips PV8060 simultaneous ICP-emission spectrometer) against reagent matched standards.

Examination of the analysis results of MA2 and SARM-7 indicates that the precision and accuracy appears to be concentration and matrix dependent. Accuracy of the method was determined by examining analysis results of certified reference materials MA2 (1.86g/t Au) and SARM-7 (0.31g/t). A random series of analytical results from 24 month analyses of MA2 and SARM-7 were used as a basis for the determinations. MA2 is accurate to 1.1% and 6.4% for SARM-7. The uncertainty for MA2 at the 95% confidence level is 1.6%, while for a lower grade material SARM-7 the uncertainty at the 95% confidence level increases to 4.7%.

Cu, Pb, Zn, Fe, Ag - Atomic Absorption Spectrometry

The sample is digested by acid treatment and the solution evaporated to incipient dryness. The residue is then re dissolved in hydrochloric acid, and ammonium acetate solution and diluted to volume. The solution is then analysed by atomic absorption spectrometry (Thermo Jarrell Ash Atomic Absorption Spectrophotometer model Video 12 E) using flame atomisation.

The accuracy and precision (at 95% confidence levels) of the method for the elements Cu, Pb, Zn, Fe, and Ag was determined from the results of the analysis of the Certified Reference Standard MP-1a and CCU-1b.

Precision and accuracy of AAS analytical method

Element AccuracyCCU1b

AccuracyMP-1a

PrecisionCCU1b

PrecisionMP-1a

Cu +0.25% ±1.4% +0.71% ±0.86%

Ag +1.52% ±0.3% +0.72% ±1.3%

Fe +0.35% ±3.9% +0.98% ±1.6%

Pb +5.94% ±0.4% +1.09% ±0.44%

Zn +0.16% ±0.8% +0.85% ±1.66%

A6


Sulphur Analysis – Leco

An appropriate weight of sample is ignited in a stream of oxygen. The sulphur present in the sample is converted to sulphur dioxide and absorbed into dilute hydrochloric acid and starch solution. This solution is then titrated against potassium iodate solution.The precision and accuracy of this method is dependent on the photoelectric cell which controls the titration rate. Without regular cleaning and with time the photoelectric cell will deteriorate, thus reducing recovery and therefore reducing accuracy. Results should agree to within 0.1% for low levels of sulphur and within 0.2% for high levels of sulphur.

As, Sb, Sn - XRF

The powdered sample is mixed with a diluent and a binder in fixed proportions, and the mixture is pressed at 25 KN. The pellets are analysed by X-ray fluorescence spectroscopy using a Philips PW1400 and Philips PW2400. Precision and accuracy of this method is equipment, concentration and matrix dependant. Additional details of typical analyses of standards are provided below.

Certified reference standards MRG-1, SO-1, SO-2, SY-2, SY-3 and internal standard STD5 have been used to validate the Work Instruction for the PW1400 and PW2400.

Standard MRG-1

MRG-1

MRG-1

SO-1 SO-1 SO-1 SO-2 SO-2 SO-2

Element As (ppm)

Sb (ppm)

Sn (ppm)

As (ppm)

Sb (ppm)

Sn (ppm)

As (ppm)

Sb (ppm)

Sn (ppm)

Theoretical

values

1 1 4 2 1 3 1 1 3

No of values 52 52 52 53 53 53 106 106 106

Max 3 3 7 8 5 7 4 4 7

Min -3 -3 1 1 -2 -2 -3 -3 0

Mean 0 1 4 3 1 3 1 0 3

S.D. 1.540 1.572 1.607 1.674 1.420 2.057 1.531 1.656 1.733

%R.S.D. 153.98 157.18 40.18 83.69 141.98 68.57 153.11 165.60 57.77

Uncertainty 0.43 0.44 0.45 0.46 0.39 0.57 0.30 0.32 0.34

%Uncertainty 92.53 43.59 10.26 15.72 41.35 18.04 34.27 110.00 9.91

A7


Standard SY-2 SY-2 SY-2 SY-3 SY-3 SY-3 STD5 STD5 STD5

Element As (ppm)

Sb (ppm)

Sn (ppm)

As (ppm)

Sb (ppm)

Sn (ppm)

As (ppm)

Sb (ppm)

Sn (ppm)

Theoretical

values

17 1 6 19 1 7 1000 1000 1000

No of values 108 108 108 52 52 52 50 50 50

Max 22 7 12 30 2 12 1093 1064 1090

Min 13 -3 2 15 -3 6 948 945 983

Mean 18 1 7 20 0 8 1016 1015 1033

S.D. 2.764 1.838 2.549 4.299 1.488 1.935 39.958 35.626 30.122

%R.S.D. 16.26 183.76 42.48 22.63 148.84 27.64 3.70 3.56 3.01

Uncertainty 0.53 0.35 0.49 1.19 0.41 0.54 10.45 10.08 8.52

%Uncertainty 2.92 26.90 7.55 5.92 2146.59 6.69 1.03 0.99 0.82

Hg – AFS Cold vapour method

The sample is digested in an acid mixture and then placed in a water-bath at approximately 60oC for 1.5 to 2 hours. On cooling the sample is then oxidised with KMnO4 and then further oxidised with (NH

4)2S2O8. At this stage the process can be left

overnight.

Boric acid is then added to neutralise the HF in the solution. Finally, just prior to analysis, hydroxylammonium sulphate/sodium chloride solution is added to decolorise the solution and to dissolve any precipitated MnO2. A suitable aliquot of sample solution is diluted if

required and reduced with stannous chloride using a PSA vapour generator. The mercury vapour produced is then determined by Atomic Fluorescence Spectrophotometry using a PSA Merlin mercury fluorescence detector.

When analysing for mercury using this Work Instruction against the designated certified reference standards the following results were obtained:

Certified reference standard CCU-1b CPB-1

Certified value (ppm) 72 +6 5.5

Range found 71.17 - 79.92 5.63 - 6.75

Mean 76.7 5.9

Standard deviation 2.797 0.419

Number of measurements 12 12

Uncertainty at 95% confidence level +1.78 +0.27

% Uncertainty at 95% confidence level

+2.3% +4.5%

A8


Major Element Assay Data for Borehole CR194

Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)149.80 150.80 0.06 0.95 33.62 5.01 GHS150.80 151.75 0.03 1.33 27.06 2.51 GHS HWL151.75 152.70 0.03 1.41 31.49 1.54 GHS HWL152.70 153.70 0.10 1.56 28.02 1.56 GHS HWL153.70 154.75 0.02 1.39 31.47 1.34 GHS HWL154.75 155.75 0.01 1.64 40.22 6.14 GBM HWL155.75 156.70 0.03 5.31 41.20 7.40 GBM AU1156.70 157.30 0.34 3.79 40.80 7.90 GBM HWP157.30 158.70 0.02 3.40 44.83 3.43 GBM/GHS HWP158.70 159.75 0.09 2.78 52.87 2.28 GHS HWP159.75 160.75 0.01 3.01 62.74 0.34 GHS AU160.75 161.75 0.01 3.95 61.13 0.70 GHS AU161.75 162.75 0.04 6.36 51.31 0.97 GHS AU162.75 163.75 0.03 7.92 53.49 1.11 GHS AU163.75 164.60 0.04 7.31 58.87 0.88 GHS AU164.60 165.80 7.42 5.75 37.74 45.89 MMP HCH165.80 166.80 16.91 4.27 35.67 44.51 MMP HCH166.80 167.75 13.65 1.04 36.64 47.74 MMP HCH167.75 168.70 12.25 1.63 43.92 47.02 MMP HCH170.70 171.60 19.40 0.21 37.93 39.51 MMP HCH171.60 172.50 17.04 0.45 31.24 45.55 MMP HCH172.50 173.50 18.35 2.00 28.01 38.79 MMP/QXM HCH173.50 174.50 15.58 0.44 28.66 39.92 QXM/MMP HCH174.50 175.45 20.09 0.17 39.20 46.79 MMP HCH175.45 176.35 19.03 0.36 36.82 42.61 MMP HCH176.35 177.20 15.27 0.14 40.18 43.52 MMP HCH177.20 178.50 20.03 0.10 31.91 45.33 MMP HCH178.50 180.00 12.85 0.16 7.37 12.47 SXM MCL

A9


Minor/Trace Element Assay Data for Borehole CR194

Depth Ag Au As Bi Hg Sb Sn Lithocode LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

149.80

150.80 5.6 0.32 774 68 0.9 486 59 GHS

150.80

151.75 4.2 0.26 329 84 0.2 495 50 GHS HWL

151.75

152.70 3.5 0.21 504 73 0.2 437 50 GHS HWL

152.70

153.70 3.2 0.31 626 49 0.9 293 41 GHS HWL

153.70

154.75 3.3 0.18 887 86 0.2 252 32 GHS HWL

154.75

155.75 7.9 0.14 1476 25 0.9 306 50 GBM HWL

155.75

156.70 6.4 2.23 1391 55 0.9 558 68 GBM AU1

156.70

157.30 9.4 0.12 1625 133 4.2 1125 95 GBM HWP

157.30

158.70 6.2 0.08 2066 252 0.7 1670 185 GBM/GHS HWP

158.70

159.75 4.3 0.07 2633 336 0.9 1710 189 GHS HWP

159.75

160.75 1.9 5.91 5126 400 0.2 2025 221 GHS AU

160.75

161.75 5.0 5.29 10224 592 2.8 1625 239 GHS AU

161.75

162.75 22.5 5.03 17897 800 21.3 2295 225 GHS AU

162.75

163.75 178.5 7.39 10305 1388 69.0 3375 450 GHS AU

163.75

164.60 1114.4 14.42 4550 1629 645.9 5135 626 GHS AU

164.60

165.80 546.4 5.43 4892 517 69.4 927 203 MMP HCH

165.80

166.80 236.9 0.67 3213 65 206.5 450 <3 MMP HCH

166.80

167.75 149.9 0.52 2844 60 72.2 302 <3 MMP HCH

167.75

168.70 150.2 0.82 3609 61 86.2 360 <3 MMP HCH

170.70

171.60 205.0 0.84 2867 116 162.1 437 <3 MMP HCH

171.60

172.50 273.3 2.23 3416 140 834.2 428 <3 MMP HCH

172.50

173.50 495.3 13.03 2826 795 1817.9 1364 99 MMP/QXM HCH

173.50

174.50 108.7 2.19 3488 88 597.8 338 <3 QXM/MMP HCH

174.50

175.45 91.4 1.25 4419 106 133.7 311 <3 MMP HCH

175.45

176.35 71.8 1.78 4095 256 153.8 500 <3 MMP HCH

176.35

177.20 45.9 1.80 3695 110 112.4 333 <3 MMP HCH

177.20

178.50 51.4 1.14 3933 83 136.0 369 <3 MMP HCH

178.50

180.00 27.4 0.72 2894 64 238.1 513 <3 SXM MCL

A10


A11



Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)170.20 170.90 0.03 0.16 10.95 0.75 TSA AU170.90 172.40 0.11 2.85 22.75 2.99 GHS AU172.40 174.10 0.11 2.65 20.03 2.27 GHS/GMS AU174.10 175.10 0.64 0.48 9.05 3.34 GEM AU175.10 175.90 0.01 0.31 6.09 1.11 GEM AU175.90 176.90 0.02 0.45 4.34 0.57 GLM/GEW AU176.90 178.05 0.05 1.04 8.10 1.41 GHM/GLS AU178.05 179.00 0.10 1.28 11.28 2.50 GEM AU179.00 180.35 0.85 0.91 19.24 4.58 GEM/GLS AU180.35 182.00 0.02 2.79 36.06 3.02 GLS/GHS AU182.00 182.85 0.03 1.25 44.15 1.60 GHS AU182.85 183.90 0.05 2.14 41.39 5.86 GHS AU183.90 185.40 0.04 2.37 20.01 8.87 GMS AU185.40 186.80 0.05 3.70 26.45 4.73 GLW AU186.80 187.40 0.02 1.64 36.76 2.81 GHS AU187.40 188.90 0.24 3.70 17.07 7.54 GMS AU188.90 190.00 0.07 5.02 17.41 12.86 GMS AU190.00 190.90 0.12 3.35 42.48 52.19 MMP HCH190.90 191.90 0.32 0.41 42.64 50.18 MMP HCH

Minor/Trace Element Assay Data for Borehole CR149

Depth Ag Au As Bi Hg Sb Sn Code LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)170.20

170.90 3.6 1.57 113 55 0.3 261 77 TSA AU

170.90

172.40 15.7 21.79 711 586 0.6 2088 248 GHS AU

172.40

174.10 35.2 48.54 864 756 10.2 2925 396 GHS/GMS AU

174.10

175.10 7.0 47.67 239 238 14.5 2399 122 GEM AU

175.10

175.90 3.3 24.82 63 178 5.5 1922 68 GEM AU

175.90

176.90 6.9 47.99 86 191 8.9 1269 270 GLM/GEW AU

176.90

178.05 29.5 9.17 356 700 3.4 2993 981 GHM/GLS AU

178.05

179.00 20.0 7.11 855 537 3.7 1922 540 GEM AU

179.00

180.35 28.0 2.83 765 581 4.6 1755 734 GEM/GLS AU

180.35

182.00 71.7 20.51 3420 3497 7.4 7556 1071 GLS/GHS AU

182.00

182.85 35.1 2.23 536 921 1.8 2250 716 GHS AU

182.85

183.90 64.0 1.75 468 1052 2.5 2853 1409 GHS AU

183.90

185.40 34.9 8.51 1035 1052 4.0 2268 4262 GMS AU

185.40

186.80 24.3 9.93 2007 244 8.0 1112 7326 GLW AU

186.80

187.40 32.7 0.67 572 236 1.5 1413 428 GHS AU

187.4 188.90 69.2 2.66 4892 1066 14.5 2129 1206 GMS AU

A12


0188.90

190.00 735.8 42.75 882 1414 83.5 1967 1440 GMS AU

190.00

190.90 83.0 1.04 981 331 5.9 585 243 MMP HCH

190.90

191.90 48.4 1.00 1580 329 1.5 405 261 MMP HCH

A13


Assay Data for Borehole CR038

Depth Cu Pb Ag Au As Bi Hg Sb SnFrom To (%) (%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)150.8

0151.4

5<0.01 0.58 6.3 3.37 36 23 0.7 252 216

151.45

152.40

<0.01 0.09 3.8 2.94 36 40 0.5 90 252

152.40

153.20

<0.01 0.09 7.5 4.19 54 29 0.2 54 144

153.20

154.20

0.02 0.09 13.5 11.08 54 19 1.3 108 666

154.20

155.20

<0.01 0.07 17.7 11.31 <5 22 1.7 36 756

155.20

156.30

0.01 0.06 22.0 1.71 36 22 90.1 108 396

156.30

157.25

0.04 0.29 1240 1.33 918 26 16.8 126 216

157.25

158.25

0.21 0.05 9.9 0.23 360 21 7.9 54 216

158.25

159.25

0.94 0.07 28.0 0.22 1098 38 8.5 180 198

159.25

160.25

0.86 0.08 7.8 0.16 936 32 2.4 72 216

160.25

161.25

1.18 0.05 12.5 0.27 1008 37 0.8 54 198

161.25

162.25

1.57 0.06 9.6 0.15 864 31 3.0 72 216

162.25

163.25

1.38 0.08 8.2 0.18 1170 34 0.6 198 378

163.25

164.25

1.64 0.23 8.5 0.22 1404 48 3.3 126 666

164.25

165.35

0.91 0.2 7.0 0.17 972 42 0.2 198 936

165.35

166.30

1.86 0.22 9.0 0.16 1242 72 1.3 180 468

166.30

167.30

0.89 0.19 7.1 0.16 1188 74 0.4 144 216


Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)133.25 134.25 0.01 0.73 24.61 0.45 TCP HWL134.25 135.25 <0.01 0.67 13.26 0.09 TCP/GHW HWL135.25 135.70 0.02 0.78 14.06 0.25 GHW/GEM HWL135.70 136.85 0.01 0.86 12.00 0.31 GEM HWL136.85 137.95 0.01 2.82 36.26 6.34 GMS HWP137.95 138.90 0.02 13.37 34.00 7.54 GMS AU1138.90 139.85 0.01 17.52 19.37 7.20 GMS AU1139.85 141.00 0.01 10.51 32.27 9.75 GMS AU1141.00 141.65 0.01 1.41 32.68 7.37 GES AU1141.65 142.65 <0.01 1.87 2.11 0.60 GES AU1142.65 143.60 0.02 1.07 2.62 0.37 GES AU1143.60 144.70 0.01 0.58 1.99 0.30 GES144.70 145.75 0.03 0.60 3.08 0.37 GES145.75 146.90 0.01 0.17 4.79 1.56 GES146.90 148.15 <0.01 0.38 4.63 0.43 GES

A14


148.15 149.15 <0.01 0.32 4.57 0.70 GES149.15 150.10 0.01 1.39 5.97 2.69 GES AU150.10 150.90 0.01 0.56 2.51 1.91 GES AU150.90 151.75 <0.01 0.93 6.26 5.66 GES AU151.75 153.85 0.01 0.37 3.05 2.75 GES AU153.85 155.35 0.33 0.13 30.29 35.50 MMPXM HCL155.35 156.25 0.69 0.05 29.51 32.15 MMPXM HCL156.25 157.20 1.18 0.17 26.55 31.11 MMPXM HCL157.20 158.15 3.44 1.02 29.19 33.76 MMPXM HCL158.15 159.05 3.66 0.45 21.98 27.21 MMPXM HCL159.05 159.60 3.52 0.16 23.53 27.37 MMPXM HCL

A15


Minor Element Assay Data for Borehole CR191

Depth Ag Au As Bi Hg Sb Sn Code LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)133.25

134.25 6.4 0.07 554 119 0.2 365 293 TCP HWL

134.25

135.25 0.8 <0.01 81 18 0.7 171 41 TCP/GHW HWL

135.25

135.70 1.2 0.01 99 65 0.2 342 54 GHW/GEM HWL

135.70

136.85 2.1 0.01 221 145 0.7 666 149 GEM HWL

136.85

137.95 4.5 0.22 1733 785 8.4 2588 1098 GMS HWP

137.95

138.90 9.1 2.39 10305 1501 9.8 4838 1809 GMS AU1

138.90

139.85 58.6 2.89 16700 3227 7.9 5963 2993 GMS AU1

139.85

141.00 25.3 12.04 1836 1092 4.2 4050 9072 GMS AU1

141.00

141.65 15.0 0.82 666 386 3.0 1656 4874 GES AU1

141.65

142.65 9.9 4.96 1269 39 6.0 792 1706 GES AU1

142.65

143.60 12.8 2.94 446 38 3.3 441 1215 GES AU1

143.60

144.70 9.2 0.61 230 39 1.4 356 1017 GES

144.70

145.75 5.5 0.99 248 46 1.9 324 1494 GES

145.75

146.90 5.7 0.60 144 58 0.5 252 1364 GES

146.90

148.15 5.3 0.66 221 123 0.9 275 1251 GES

148.15

149.15 6.3 0.90 117 156 0.7 419 1476 GES

149.15

150.10 11.3 3.61 873 208 7.4 1107 3209 GES AU

150.10

150.90 10.6 0.85 396 120 7.7 819 882 GES AU

150.90

151.75 19.6 10.74 419 364 20.4 1301 6809 GES AU

151.75

153.85 11.8 5.01 495 102 22.5 446 3569 GES AU

153.85

155.35 91.5 2.73 2408 63 9.3 270 918 MMPXM HCL

155.35

156.25 10.9 0.91 3695 56 10.8 369 1130 MMPXM HCL

156.25

157.20 3.9 0.37 3965 44 9.2 239 603 MMPXM HCL

157.20

158.15 7.1 0.81 4320 126 4.0 432 117 MMPXM HCL

158.15

159.05 6.2 0.37 3236 43 4.0 284 225 MMPXM HCL

159.05

159.60 4.5 0.30 2363 40 2.0 293 513 MMPXM HCL


A16


Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)152.40 153.95 0.08 1.12 13.95 7.74 TCP AU153.95 154.85 0.02 16.49 10.79 2.76 GMS AU154.85 157.05 No Core Recovery157.05 158.65 0.04 9.20 11.24 5.29 GMS AU158.65 160.20 0.03 5.46 13.75 7.11 GMS AU160.20 161.40 0.01 27.23 28.69 8.46 GMS AU161.40 161.80 0.02 8.90 19.63 2.87 GMS AU161.80 163.40 0.12 25.63 32.64 6.66 GMS AU163.40 168.20 No Core Recovery168.20 169.00 0.09 6.40 9.60 4.12 GMS AU169.00 169.65 0.45 13.95 9.59 16.03 QXM AU169.65 172.85 1.89 6.94 16.18 21.75 QXM AU172.85 176.00 No Core Recovery176.00 178.35 1.33 0.53 5.11 5.95 SXM MCL178.35 180.00 2.38 0.22 19.75 26.93 QXM MCL180.00 181.50 2.56 0.61 21.51 26.98 SXM MCL181.50 183.00 1.1 0.70 6.46 8.20 SXM MCL183.00 184.40 0.58 0.72 3.67 4.11 QXM MCL184.40 185.70 0.76 0.12 1.04 1.46 EQU E185.70 187.60 0.57 0.43 1.17 5.23 SXM E

A17


Minor Element Assay Data for Borehole CR123

Depth Ag Au As Bi Hg Sb Sn Code LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)152.40 153.95 2.5 2.77 558 148 0.8 270 86 TCP AU153.95 154.85 47.9 4.47 2075 338 66.3 1319 388 GMS AU154.85 157.05 No Core Recovery157.05 158.65 13.6 2.27 5839 348 20.0 1809 471 GMS AU158.65 160.20 35.6 2.08 5136 403 8.8 1413 382 GMS AU160.20 161.40 16.4 1.81 1454 374 5.4 2138 270 GMS AU161.40 161.80 20.4 1.47 2488 275 9.1 1481 356 GMS AU161.80 163.40 18.1 2.14 2728 362 5.7 1701 331 GMS AU163.40 168.20 No Core Recovery168.20 169.00 69.7 31.85 2002 1578 1160 4536 1100 GMS AU169.00 169.65 181.0 56.55 174 1920 9525 4536 1437 QXM AU169.65 172.85 175.3 11.68 210 758 3061 801 623 QXM AU172.85 176.00 No Core Recovery176.00 178.35 115.1 1.49 624 83 134.2 171 38 SXM MCL178.35 180.00 1.4 <0.01 63 24 2.3 207 77 QXM MCL180.00 181.50 82.1 0.75 99 201 112.8 509 135 SXM MCL181.50 183.00 1.8 0.12 122 236 0.8 203 63 SXM MCL183.00 184.40 1.5 0.05 846 129 1.3 216 41 QXM MCL184.40 185.70 1.6 <0.01 693 357 2.4 95 45 EQU E185.70 187.60 0.8 <0.01 482 147 0.5 23 5 SXM E

A18

Appendix 4 XRD Data

Appendix 4: XRD Data

A19

Appendix 4 XRD Data

A20

Appendix 4 XRD Data

A21

Appendix 4 XRD Data

A22

Appendix 4 XRD Data

A23

Appendix 4 XRD Data

A24

Appendix 4 XRD Data

BoreholeFrom (m)

To(m) Description ID Major ID Minor ID Trace

CR194 149.80 150.80 Limonite Yellow Clast Qz, Goeth Gal, Sid Angle, AnatCR194 149.80 150.80 Matrix Qz, Sid, Goeth Gal AnatCR194 149.80 150.80 Sid Clast Qz, Sid Hem, Gal Goeth, possible PyrrhotiteCR194 150.80 151.75 Sid-rich Area Magnetic Qz, Sid Hem, GalCR194 150.80 151.75 Sid-rich Area Qz, Sid Hem, Gal Barite, GoethCR194 151.75 152.70 Green-Grey Matrix Qz, Sid Gal Angle, GoethCR194 154.75 155.75 Whole Rock Strong Magnetic Sid Qz Gal, GoethCR194 154.75 155.75 Whole Rock Weak Magnetic Sid, Qz Gal Hem, GoethCR194 155.75 156.70 Whole Rock Strong Magnetic Gal, Sid Qz AngleCR194 155.75 156.70 Whole Rock Weak Magnetic Gal, Sid, Qz Hem CR194 157.30 158.70 Whole Rock 'Red' Sid Sid Hem, GalCR194 158.70 159.75 Whole Rock Sid, Gal Hem GoethCR194 159.75 160.75 Whole Rock Hem Gal, Sid, AngleCR194 161.75 162.75 Whole Rock Sid, Hem Gal, AngleCR194 163.75 164.60 Black Clay Band Sid, Gal Hem, Qz, AngleCR194 163.75 164.60 Upper Whole Rock Hem, Gal Sid, AngleCR194 163.75 164.60 Middle Whole Rock Yellow Goeth, Sid, Hem Gal, AngleCR194 164.60 165.80 Upper Contact Black Layer Gal Sid, QzCR194 164.60 165.80 Upper Contact Black Layer Lwr Gal Py, Angle, SidCR194 164.60 165.80 Upper Contact Yellow Layer Goeth, Gal Angle, SidCR194 164.60 165.80 Whole Rock Py Tennantite, Chalcopyrite, Melanterite,CR194 165.80 166.80 Whole Rock Py DjurleiteCR194 168.70 169.70 Whole Rock Py DjurleiteCR194 172.50 173.50 Lower Whole Rock Py, Gal, Qz, DjurleiteCR194 173.50 174.50 Whole Rock Py, Gal, Qz Djurleite

Abbreviations: Qz = Quartz, Goeth = Goethite, Sid = Siderite, Gal = Galena, Angle = Anglesite, Anat = Anatase, Hem = Hematite, Py = Pyrite, Glauc = Glauconite, Rut = Rutile,

S = Sulphur (native), Lepid = Lepidocrocite, Greig = Greigite, Marc = Marcasite, Calc = Calcite, Ceruss = Cerussite, Gyp = Gypsum.

A25

Appendix 4 XRD Data

BoreholeFrom (m)


CR149 170.20 170.90 Whole Rock Qz, Albite, Glauc Sid Anat, Gal, ChloriteCR149 170.90 172.40 Lower Green/Grey Matrix Qz, Gal, Angle SidCR149 170.90 172.40 Upper Whole Rock Qz, Hem, Sid GalCR149 172.40 174.10 Middle Whole Rock Qz Sid, Gal, poorly crystalline clayCR149 175.10 175.90 Lower Whole Rock Chalky White Qz Rut, Gal, SidCR149 175.10 175.90 Upper Whole Rock Chalky White Qz Rut, SidCR149 176.90 178.00 Lower Whole Rock Qz, Goeth Sid Gal CR149 179.00 180.35 Lower Whole Rock Qz, Goeth Sid, GalCR149 180.35 182.00 Upper Whole Rock Sid, Qz Gal, Hem, Goeth RutCR149 182.00 182.85 Lower Whole Rock Sid Qz, Hem, GalCR149 182.00 182.85 Upper Whole Rock Sid Qz, Hem GalCR149 183.90 185.40 Lower Whole Rock Qz Py, S, Lepid, GreigCR149 183.90 185.40 Upper Magnetic Qz Py, S, Lepid, Greig, Goeth SidCR149 185.40 186.80 Upper Whole Rock Qz, Sid Goeth Rut, CassiteriteCR149 187.40 188.90 Lower Whole Rock Qz Angle, Gal Lepid, S, Anat, RutCR149 187.40 188.90 Middle Fe-Sulphide Qz Lepid, S, Py, Marc, Gal, Goeth, Greig CalcCR149 188.90 190.00 Lower Whole Rock Py, Qz, Gal AngleCR149 188.90 190.00 Fe-Sulphide HCl Leached Py, Qz Greig, SCR149 188.90 190.00 Fe-Sulphide plus AgFe-Sulphide Py, Qz, Calc GreigCR149 188.90 190.00 Upper Whole Rock Calc, Qz Py possible GreigCR149 190.00 190.90 Upper Whole Rock Py, Qz Gal AngleCR149 190.90 191.90 Upper Whole Rock Py, Qz, Gal Angle



A26

Appendix 4 XRD Data

BoreholeFrom (m)


CR038 150.80 151.45 Upper Whole Rock Qz Py, Sid Gal, Goeth, AnatCR038 151.45 152.40 Upper Whole Rock Qz Sid AnatCR038 152.40 153.20 Upper Whole Rock Qz Sid AnatCR038 153.20 154.20 Cyclosizer Heavy Mineral Conc. Qz, Anat, Sid Py CassiteriteCR038 153.20 154.20 Upper Whole Rock Qz Sid AnatCR038 154.20 155.20 Upper Whole Rock Qz Sid, AnatCR038 155.20 156.30 Upper Whole Rock Qz Sid, AnatCR038 156.30 157.25 Upper Whole Rock Qz Py AnatCR038 157.25 158.25 Upper Whole Rock Qz Py Anat



A27

Appendix 4 XRD Data

BoreholeFrom (m)


CR191 135.20 135.70 Whole Rock Qz, Goeth RutCR191 136.85 137.95 Whole Rock Sid Gal, Goeth, AngleCR191 137.95 138.90 Lower Whole Rock Gal, Sid QzCR191 137.95 138.90 Magnetic Sid S, Gal, Goeth, Lepid Greig CR191 137.95 138.90 Upper Whole Rock Sid, Qz S, Angle, Goeth, LepidCR191 137.95 138.90 Whole Rock Sid, Qz S, Angle, LepidCR191 138.90 139.85 Whole Rock Gal, Angle, Sid QzCR191 139.85 141.00 Lower Whole Rock Sid, Qz Gal, Lepid Angle, S, RutCR191 139.85 141.00 Upper Whole Rock Gal, Sid Qz Angle, RutCR191 141.00 141.65 Whole Rock Qz, Sid Gal Marc, Rut, Calc, possible CassiteriteCR191 141.65 142.65 Whole Rock Qz Rut, Anat, Gal, SidCR191 143.60 144.70 Whole Rock Qz Rut, Anat, Gal, SidCR191 145.75 146.90 Whole Rock Qz Sid, S, Marc, Lepid Greig, CalcCR191 148.15 149.15 Whole Rock Qz, Sid Gal, MarcCR191 150.10 150.90 Whole Rock Qz Gal Rut, Marc, Greig, S, LepidCR191 150.90 151.75 Lower Whole Rock Qz Cassiterite, Gal Rut, Anat CR191 150.90 151.75 Upper Whole Rock Qz Py, S, Anat, Lepid JarositeCR191 151.75 153.85 Lower Whole Rock Qz Cassiterite, Py, AnatCR191 151.75 153.85 Upper Whole Rock Qz Szomolnokite (oxidation product of Py), Py Anat, Rut, CassiteriteCR191 153.85 155.35 Whole Rock Qz, Py CR191 155.35 156.25 Whole Rock Qz, Py Tennantite Chalcopyrite



A28

Appendix 4 XRD Data

BoreholeFrom (m)


CR123 152.40 153.95 Lower Whole Rock Sid Angle, Gal Greig, SCR123 152.40 153.95 Upper Whole Rock Qz, Calc, Py,Glauc, Albite RutCR123 153.95 154.85 Lower Whole Rock Gal, Py, Calc Angle, CerussCR123 153.95 154.85 Upper Whole Rock Sid, HemCR123 157.05 158.65 Upper Whole Rock Gal, Py, Calc QzCR123 158.65 160.20 Lower Low Mag Frantz Calc, Gal Py HarmotomeCR123 158.65 160.20 Lower Magnetic Py, Gal, Calc Angle possible MarcCR123 158.65 160.20 Upper Whole Rock Calc, Gal, Angle Py, Gyp CerussCR123 160.20 161.40 Upper Whole Rock Gal, Py, Calc MarcCR123 161.80 163.40 Upper Metallic Clasts Gal, Sid AngleCR123 161.80 163.40 Lower Magnetic Sid, Gal Angle Greig, SCR123 161.80 163.40 Upper Whole Rock Sid, GalCR123 168.20 169.00 Lower Gangue Calc, Angle, Gal Gyp, PyCR123 168.20 169.00 Lower Magnetic Calc, Angle, Gal Py Sid, GypCR123 168.20 169.00 Lower Massive Pb-Sulphide Ceruss, Gal Calc QzCR123 168.20 169.00 Upper Massive Fe-Sulphide Gal, Py, Calc Greig, Marc, Gyp Barite, possible pyrrhotiteCR123 168.20 169.00 Upper Massive Pb-Sulphide Ceruss, Gal, Calc QzCR123 169.00 169.65 Upper Green/Grey Matrix Calc, Angle, Gal Gyp CR123 169.00 169.65 Upper Massive Sulphide Gal, Py AngleCR123 180.00 181.50 Whole Rock Qz, Py, Gal Covellite



A29

Appendix 4 XRD Data

5 10 15 20 25 30 352Theta (°)

0

1000

2000

3000

4000

5000

6000Inte

nsity

(co

unts

)

air dry 001; 15.0

glycol-solvated 001; 16.9

heated 001; 9.99

CR194 - Black Fe-rich Clay: Oriented Si crystal mount, whole powder scans (black -air dry, red - ethylene glycol-solvated, green - heated 550degC/2 hours) which clearly indicate a smectite-group mineral due to c.15ang 001 peak (air dry) swelling to 16.9ang on glycolation and collapsing to c.10ang on heating.

A30

Appendix 4 XRD Data

71 72 73 74 75 76 77 78 792Theta (°)

0

1000

2000

3000

4000

5000

Inte

nsity

(co

unts

)

?nontronite 060; 1.52

CR194 - Black Fe-rich Clay: Random powder Si crystal mount, scan over the diagnostic 060 diffraction band possibly suggests a spacing of 1.52ang - indicative of nontronite.

A31

Appendix 5 SEM Analyses

Appendix 5: SEM Analyses: Borehole CR194: Siderite

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14Compound Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)

FeO 52.9 52.8 55.2 56.8 53.2 52.6 53.6 55.3 59.4 54.1 54.0 53.3 59.1 58.1MgO 1.6 1.6 1.2 1.0 1.7 1.8 1.9 1.2 0.5 1.3 1.1 1.1 0.3 0.2CaO 3.5 3.5 2.4 1.4 3.8 4.4 3.4 2.8 0.4 4.8 4.3 5.6 0.2 0.1CO2 42.0 42.0 41.1 40.9 41.3 41.2 41.1 40.7 39.7 39.8 40.6 40.1 40.4 41.7

TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0At %

C 21.0 21.0 20.8 20.8 20.8 20.7 20.7 20.7 20.5 20.3 20.6 20.4 20.8 21.2Mg 0.9 0.9 0.7 0.6 1.0 1.0 1.0 0.7 0.3 0.7 0.6 0.6 0.2 0.1Ca 1.4 1.4 1.0 0.6 1.5 1.7 1.4 1.1 0.1 1.9 1.7 2.2 0.1 0.0Fe 16.2 16.2 17.1 17.7 16.4 16.2 16.5 17.2 18.8 16.9 16.8 16.6 18.6 18.1O 60.5 60.5 60.4 60.4 60.4 60.4 60.4 60.3 60.3 60.1 60.3 60.2 60.4 60.6

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

C 1.1 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.1Mg 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0Ca 0.1 0.1 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.0 0.0Fe 0.8 0.8 0.9 0.9 0.8 0.8 0.8 0.9 0.9 0.8 0.8 0.8 0.9 0.9O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 0.9 0.9 0.9 0.9 0.9 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 0.9

C+O 4.1 4.1 4.1 4.1 4.1 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.1 4.1TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

A32


Borehole CR194: Siderite - Analysis Locations

AnalysisFrom(m)

To(m) Location Comments

#1 149.80 150.80 Gossan FeS-lined cavity filling,

#2 149.80 150.80 Gossan Galena-lined cavity filling

#3 149.80 150.80 Gossan Galena-lined cavity filling

#4 149.80 150.80 Gossan Veinlet

#5 149.80 150.80 Gossan ‘Fragment’

#6 149.80 150.80 Gossan Euhedral cavity filling

#7 149.80 150.80 Gossan Cavity filling


#9 155.75 156.70 Gossan ‘Fragment’, zoned

#10 155.75 156.70 Gossan Cavity filling



#13 163.75 164.60Gossan contact with massive sulphide Late veinlet with anglesite

#14 164.60 165.80 Massive sulphide contact with gossan Cavity filling

A33


Borehole CR149: Siderite

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12

CompoundWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

FeO 43.0 51.1 45.5 50.1 44.7 49.7 50.3 51.3 49.0 51.2 51.0 49.9MgO 6.0 2.7 4.9 3.4 6.2 3.1 3.3 3.3 4.2 3.4 3.8 3.5CaO 7.6 3.3 5.4 3.6 5.3 3.6 3.8 3.5 4.3 4.2 4.8 4.9CO2 43.4 42.9 44.2 42.9 43.9 43.5 42.6 41.9 42.5 41.2 40.5 41.7

TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0At %

C 20.9 21.2 21.3 21.1 21.1 21.3 21.0 20.8 20.9 20.6 20.3 20.7Mg 3.1 1.4 2.6 1.8 3.3 1.7 1.8 1.8 2.3 1.9 2.1 1.9Ca 2.9 1.3 2.0 1.4 2.0 1.4 1.5 1.4 1.7 1.6 1.9 1.9Fe 12.7 15.5 13.4 15.1 13.1 14.9 15.2 15.6 14.7 15.6 15.6 15.2O 60.4 60.6 60.7 60.6 60.5 60.7 60.5 60.4 60.4 60.3 60.1 60.4

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

C 1.0 1.1 1.1 1.1 1.1 1.1 1.1 1.0 1.0 1.0 1.0 1.0Mg 0.2 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1Ca 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Fe 0.6 0.8 0.7 0.8 0.7 0.7 0.8 0.8 0.7 0.8 0.8 0.8O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 1.0 1.0 1.0

C+O 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.0 4.0 4.0TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

A34


Borehole CR149: Siderite – Analysis Locations

AnalysisFrom(m)


#1 170.20 170.90Tertiary conglomerate Zoned crystal - core

#2 170.20 170.90Tertiary conglomerate Zoned crystal - rim





#7 175.10 175.90 GossanAssociated with Fe-S and PbSb-sulphide, replacing quartz matrix






A35



Analysis #1 #2 #3 #4 #5 #6 #7

CompoundWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

FeO 44.7 42.8 43.0 46.5 44.0 44.8 46.4MgO 5.6 6.2 5.5 5.6 5.7 5.9 5.8CaO 7.9 8.7 9.5 5.8 9.2 7.5 6.4CO2 41.8 42.3 42.1 42.0 41.0 41.8 41.4

TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0At %

C 20.4 20.5 20.5 20.5 20.1 20.4 20.3Mg 3.0 3.3 2.9 3.0 3.1 3.1 3.1Ca 3.0 3.3 3.6 2.2 3.5 2.9 2.5Fe 13.4 12.7 12.8 13.9 13.2 13.4 14.0O 60.2 60.2 60.2 60.3 60.1 60.2 60.2

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

C 1.0 1.0 1.0 1.0 1.0 1.0 1.0Mg 0.2 0.2 0.1 0.2 0.2 0.2 0.2Ca 0.2 0.2 0.2 0.1 0.2 0.1 0.1Fe 0.7 0.6 0.6 0.7 0.7 0.7 0.7O 3.0 3.0 3.0 3.0 3.0 3.0 3.0

TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 1.0 1.0 1.0 1.0 1.0 1.0 1.0

C+O 4.0 4.0 4.0 4.0 4.0 4.0 4.0TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0

A36



AnalysisFrom(m)


#1 150.80 151.45Quartz replaced tuff Euhedral cavity filling

#2 150.80 151.45Quartz replaced tuff Euhedral crystal with Pb(SbAs)-sulphide needles

#3 150.80 151.45Quartz replaced tuff

Later anhedral overgrowth on #2 without Pb(SbAs)-sulphide needles

#4 150.80 151.45Quartz replaced tuff Euhedral crystal, zoned

#5 150.80 151.45Quartz replaced tuff Euhedral crystal with Pb(SbAs)-sulphide needles

#6 150.80 151.45Quartz replaced tuff Later anhedral overgrowth without Pb(SbAs)-sulphide needles

#7 150.80 151.45Quartz replaced tuff Later anhedral overgrowth without Pb(SbAs)-sulphide needles

A37



Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14

CompoundWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

FeO 50.8 53.6 53.1 51.7 48.8 48.6 47.7 53.2 46.7 53.2 50.9 52.3 51.5 51.9MgO 1.2 2.0 2.2 2.1 5.5 4.6 4.8 4.8 4.8 4.1 3.9 3.8 3.6 3.3CaO 5.8 3.4 4.1 4.1 4.2 4.6 5.5 0.7 6.6 0.7 2.5 2.2 2.1 2.5CO2 42.3 41.0 40.7 42.1 41.6 42.3 42.0 41.3 42.0 42.0 42.7 41.7 42.8 42.3

TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0At %

C 21.1 20.7 20.5 21.0 20.5 20.8 20.6 20.6 20.6 20.9 21.1 20.8 21.1 21.0Mg 0.6 1.1 1.2 1.2 2.9 2.5 2.6 2.6 2.6 2.2 2.1 2.0 2.0 1.8Ca 2.2 1.3 1.6 1.6 1.6 1.8 2.1 0.3 2.5 0.3 1.0 0.9 0.8 1.0Fe 15.5 16.5 16.4 15.8 14.7 14.6 14.4 16.3 14.0 16.2 15.4 16.0 15.6 15.8O 60.5 60.3 60.3 60.5 60.2 60.4 60.3 60.3 60.3 60.4 60.5 60.4 60.6 60.5

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

C 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.1 1.0 1.1 1.0Mg 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Ca 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0Fe 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.8 0.7 0.8 0.8 0.8 0.8 0.8O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 0.9 1.0 1.0 0.9 1.0 0.9 1.0 1.0 1.0 0.9 0.9 0.9 0.9 0.9

C+O 4.1 4.0 4.0 4.1 4.0 4.1 4.0 4.0 4.0 4.1 4.1 4.1 4.1 4.1TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

A38



AnalysisFrom(m)


#1 134.25 135.25Conglomerate contact with gossan Massive siderite




#5 139.85 141.00 Upper gossan Euhedral crystals

#6 139.85 141.00 Upper gossan Euhedral crystals

#7 141.00 141.65 Middle gossanCompositional zoning




#11 150.10 150.90 Lower gossan Cavity infilling




A39



Analysis #1 #2 #3 #4 #5 #6 #7 #8

CompoundWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

FeO 45.8 46.0 48.1 47.8 56.7 55.0 54.6 55.2MgO 4.8 3.9 3.0 3.5 1.5 1.7 1.7 1.2CaO 6.7 8.0 7.7 7.7 1.1 1.1 1.2 0.9MnO 0.8 0.6 0.1 0.3 bdl bdl bdl bdlCO2 41.9 41.5 41.1 40.8 40.7 42.2 42.5 42.7

TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0At %

C 20.6 20.5 20.5 20.3 20.7 21.2 21.3 21.4Mg 2.6 2.1 1.6 1.9 0.8 0.9 0.9 0.7Ca 2.6 3.1 3.0 3.0 0.4 0.4 0.5 0.3Mn 0.3 0.2 0.0 0.1 bdl bdl bdl bdlFe 13.8 13.9 14.7 14.6 17.7 16.9 16.7 16.9O 60.2 60.2 60.2 60.1 60.4 60.6 60.6 60.7

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

C 1.0 1.0 1.0 1.0 1.0 1.1 1.1 1.1Mg 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0Ca 0.1 0.2 0.2 0.2 0.0 0.0 0.0 0.0Mn 0.0 0.0 0.0 0.0 bdl bdl bdl bdlFe 0.7 0.7 0.7 0.7 0.9 0.8 0.8 0.8O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0

TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms

Mg+Ca+Fe+Mn 1.0 1.0 1.0 1.0 1.0 0.9 0.9 0.9C+O 4.0 4.0 4.0 4.0 4.0 4.1 4.1 4.1

TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

bdl = below detection limits (~0.5%)


AnalysisFrom(m)


#1 153.95 154.85Upper siderite gossan

Massive siderite, Mn-bearing

#2 153.95 154.85Upper siderite gossan

Massive siderite, Mn-bearing

#3 153.95 154.85Upper siderite gossan Massive siderite

#4 153.95 154.85Upper siderite gossan Massive siderite

#5 161.80 163.40Lower siderite gossan Massive siderite, trace Pb




A40


Boreholes CR194 and CR191: Enargite

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11

ElementWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Cu 46.1 46.1 46.5 49.4 47.5 50.0 48.1 40.6 40.7 40.4 42.2Fe 0.9 1.3 0.9 0.0 0.0 0.0 0.0 7.3 7.8 7.5 7.0Sb 0.4 0.4 0.2 0.0 0.0 0.0 0.0 3.7 1.9 2.2 0.3As 18.5 18.5 19.9 19.6 20.9 17.7 19.9 15.9 18.5 18.2 18.4S 33.7 32.8 33.0 31.5 32.3 31.6 32.2 32.3 31.8 32.4 32.4

TOTAL 99.6 99.0 100.6 100.5 100.7 99.3 100.2 99.8 100.7 100.7 100.2At %Cu 35.5 35.9 35.8 38.5 36.8 40.3 37.4 31.6 31.5 31.1 32.4Fe 0.8 1.1 0.8 0.0 0.0 0.0 0.0 6.5 6.8 6.6 6.1Sb 0.2 0.2 0.1 0.0 0.0 0.0 0.0 1.5 0.8 0.9 0.1As 12.1 12.2 13.0 13.0 13.7 11.1 13.1 10.5 12.2 11.9 12.0S 51.4 50.6 50.4 48.6 49.5 48.6 49.5 49.9 48.8 49.5 49.3

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

Cu 2.8 2.9 2.9 3.1 2.9 3.2 3.0 2.5 2.5 2.5 2.6Fe 0.1 0.1 0.1 0.0 0.0 0.0 0.0 0.5 0.5 0.5 0.5Sb 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.0As 1.0 1.0 1.0 1.0 1.1 0.9 1.0 0.8 1.0 1.0 1.0S 4.1 4.0 4.0 3.9 4.0 3.9 4.0 4.0 3.9 4.0 3.9

TOTAL 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0No. Atoms

Cu+Fe 2.9 3.0 3.0 3.1 2.9 3.2 3.0 3.0 3.1 3.0 3.1As+Sb 1.0 1.0 1.0 1.0 1.1 0.9 1.0 1.0 1.0 1.0 1.0

S 4.1 4.0 4.0 3.9 4.0 3.9 4.0 4.0 3.9 4.0 3.9TOTAL 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

A41


Boreholes CR194 and CR191: Enargite – Analysis Locations

Analysis BoreholeFrom(m)

To(m) Location

#1 CR194 164.60 165.80Massive sulphide contact with gossan



#4 CR194 173.50 174.50 Massive sulphide/shale




#8 CR191 155.35 156.25 Partial massive sulphide




A42


Borehole CR194: Hg-tetrahedrite/tennantite

Analysis #1 #2 #3 #4 #5 #6

ElementWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Cu 41.2 36.7 41.4 35.4 34.5 32.6Fe 0.8 0.8 1.0 1.1 1.5 0.4Hg 7.7 7.9 5.5 11.8 12.8 21.7Zn 2.0 4.0 3.6 2.7 1.8 0.1Sb 9.2 21.5 10.6 22.2 22.1 23.4As 12.8 4.1 11.6 3.2 3.4 0.5S 25.8 24.6 25.5 23.8 23.8 22.3

TOTAL 99.4 99.6 99.2 100.2 99.7 101.0At %Cu 36.4 34.2 36.4 33.9 33.3 33.7Fe 0.8 0.8 1.0 1.2 1.6 0.4Hg 2.2 2.3 1.5 3.6 3.9 7.1Zn 1.7 3.6 3.0 2.5 1.7 0.1Sb 4.2 10.5 4.9 11.1 11.1 12.6As 9.6 3.2 8.7 2.6 2.8 0.4S 45.1 45.4 44.5 45.1 45.6 45.6

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0No Atm

Cu 10.6 9.9 10.6 9.8 9.7 9.8Fe 0.2 0.2 0.3 0.3 0.5 0.1Hg 0.6 0.7 0.4 1.0 1.1 2.1Zn 0.5 1.0 0.9 0.7 0.5 0.0Sb 1.2 3.0 1.4 3.2 3.2 3.7As 2.8 0.9 2.5 0.8 0.8 0.1S 13.1 13.2 12.9 13.1 13.2 13.2

TOTAL 29.0 29.0 29.0 29.0 29.0 29.0No. Atoms

Cu+Fe+Zn+Hg 11.9 11.9 12.2 12.0 11.8 12.0As+Sb 4.0 4.0 3.9 4.0 4.0 3.8

S 13.1 13.2 12.9 13.1 13.2 13.2TOTAL 29.0 29.0 29.0 29.0 29.0 29.0

Borehole CR194: Hg-tetrahedrite/tennantite – Analysis Locations

AnalysisFrom(m)

To(m) Location

#1 170.70 171.60 Massive sulphide





#6 173.50 174.50Massive sulphide/shale

A43


Borehole CR194: Tetrahedrite/tennantite

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11

ElementWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Cu 40.6 40.5 40.4 40.2 40.4 44.4 45.8 36.8 36.3 35.7 36.0Fe 7.9 7.5 7.6 7.6 7.6 6.2 5.8 5.6 4.9 5.8 4.8Zn 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.5 2.1 2.5 1.7Sb 1.0 0.6 0.9 1.4 1.1 0.1 0.0 29.2 29.0 30.5 30.0As 22.2 21.8 21.2 21.9 21.3 21.6 20.6 1.6 1.1 0.4 0.9S 28.4 29.8 28.4 28.5 28.7 28.3 27.8 25.3 25.6 25.2 26.0

TOTAL 100.0 100.0 98.4 99.5 99.0 100.6 100.0 99.9 99.0 100.0 99.4At %Cu 32.4 31.9 32.6 32.3 32.4 35.3 36.6 33.1 32.8 32.2 32.4Fe 7.2 6.7 7.0 6.9 6.9 5.6 5.3 5.7 5.1 5.9 5.0Zn 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.3 1.9 2.2 1.5Sb 0.4 0.2 0.4 0.6 0.5 0.1 0.0 13.7 13.7 14.4 14.1As 15.0 14.6 14.5 14.9 14.5 14.6 14.0 1.2 0.9 0.3 0.6S 44.9 46.5 45.5 45.3 45.7 44.5 44.1 45.0 45.7 45.0 46.4

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

A44


Borehole CR194: Tetrahedrite/tennantite (continued)

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11No Atm

Cu 9.4 9.3 9.5 9.4 9.4 10.2 10.6 9.6 9.5 9.3 9.4Fe 2.1 1.9 2.0 2.0 2.0 1.6 1.5 1.7 1.5 1.7 1.4Zn 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.5 0.6 0.4Sb 0.1 0.1 0.1 0.2 0.1 0.0 0.0 4.0 4.0 4.2 4.1As 4.4 4.2 4.2 4.3 4.2 4.2 4.1 0.3 0.2 0.1 0.2S 13.0 13.5 13.2 13.1 13.3 12.9 12.8 13.1 13.3 13.1 13.5

TOTAL 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0No. Atoms Cu+Fe+Zn 11.5 11.2 11.5 11.4 11.4 11.9 12.2 11.6 11.5 11.7 11.3

As+Sb 4.5 4.3 4.3 4.5 4.3 4.2 4.1 4.3 4.2 4.3 4.3S 13.0 13.5 13.2 13.1 13.3 12.9 12.8 13.1 13.3 13.1 13.5

TOTAL 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0

A45


Borehole CR194: Tetrahedrite/tennantite – Analysis Locations

AnalysisFrom(m)

To(m) Location

#1 164.60 165.80Massive sulphide contact with gossan











A46


Boreholes CR038 and CR123: Proustite/pyrargyrite

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10

ElementWt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Wt (%)

Ag 67.4 66.2 65.9 67.5 66.9 65.4 62.3 60.1 66.8 61.1Sb 0.0 0.0 0.3 0.0 0.0 0.3 19.1 18.3 0.4 16.1As 14.8 15.8 15.6 14.6 15.1 16.7 1.9 5.1 14.0 5.1S 18.8 18.2 18.2 18.4 18.6 18.2 17.5 16.4 18.1 16.7

TOTAL 100.9 100.2 100.0 100.5 100.6 100.5 100.8 100.0 99.2 99.0At %Ag 44.4 44.1 44.0 44.9 44.2 43.3 44.3 43.2 45.1 44.0Sb 0.0 0.0 0.2 0.0 0.0 0.2 12.0 11.7 0.2 10.3As 14.0 15.1 15.0 14.0 14.4 16.0 2.0 5.3 13.6 5.3S 41.6 40.8 40.8 41.1 41.4 40.5 41.8 39.8 41.1 40.5

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

Ag 3.1 3.1 3.1 3.1 3.1 3.0 3.1 3.0 3.2 3.1Sb 0.0 0.0 0.0 0.0 0.0 0.0 0.8 0.8 0.0 0.7As 1.0 1.1 1.0 1.0 1.0 1.1 0.1 0.4 0.9 0.4S 2.9 2.9 2.9 2.9 2.9 2.8 2.9 2.8 2.9 2.8

TOTAL 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0No. Atm

Ag 3.1 3.1 3.1 3.1 3.1 3.0 3.1 3.0 3.2 3.1As+Sb 1.0 1.1 1.1 1.0 1.0 1.1 1.0 1.2 1.0 1.1

S 2.9 2.9 2.9 2.9 2.9 2.8 2.9 2.8 2.9 2.8TOTAL 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0

Boreholes CR038 and CR123: Proustite/pyrargyrite – Analysis Locations

Analysis BoreholeFrom(m)

To(m) Location

#1 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide







#8 CR123 169.00 169.65 Shale/gossan contact



A47


Borehole CR194: Cu-arsenides

Analysis #1 #2 #3 #4 #5 #6 #7 #8Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)

Cu 59.1 59.1 59.7 59.6 60.1 59.4 64.9 64.9As 36.7 34.9 35.6 35.0 35.1 37.8 33.3 33.4Ag 4.4 4.3 4.5 0.0 0.2 0.0 0.0 0.0

TOTAL 100.2 98.3 99.8 94.6 95.4 97.2 98.2 98.3At %Cu 63.7 64.8 64.5 66.7 66.8 64.9 69.7 69.6As 33.6 32.5 32.7 33.3 33.1 35.1 30.3 30.4Ag 2.8 2.8 2.9 0.0 0.1 0.0 0.0 0.0

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm

Cu 19.7 20.1 20.0 20.7 20.7 20.1 4.9 4.9As 10.4 10.1 10.1 10.3 10.2 10.9 2.1 2.1Ag 0.9 0.9 0.9 0.0 0.0 0.0 0.0 0.0

TOTAL 31.0 31.0 31.0 31.0 31.0 31.0 7.0 7.0No. Atoms

Cu+Ag 20.6 20.9 20.9 20.7 20.8 20.1 4.9 4.9As 10.4 10.1 10.1 10.3 10.2 10.9 2.1 2.1

TOTAL 31.0 31.0 31.0 31.0 31.0 31.0 7.0 7.0

Borehole CR194: Cu-arsenides – Analysis Locations

AnalysisFrom(m)



Steel grey colour, identified as novakite






Crimson tarnish, identified as novakite






Blue-grey colour, identified as koutekite


Blue-grey colour, identified as koutekite

No. AtomsTheoreticalNovakite

TheoreticalKoutekite

Cu+Ag 21.0 5.0

As 10.0 2.0

TOTAL 31.0 7.0

A48


Borehole CR194: As-bearing pyrite

Analysis #1 #2 #3 #4

Element Wt (%) Wt (%) Wt (%) Wt (%)

Fe 46.2 44.2 44.3 45.6

As 2.4 2.7 1.8 1.1

S 52.3 53.5 53.6 54.0

TOTAL 101.0 100.4 99.6 100.7At %Fe 33.2 31.7 31.9 32.5As 1.3 1.5 1.0 0.6S 65.5 66.8 67.2 67.0

TOTAL 100.0 100.0 100.0 100.0No Atm

Fe 1.0 1.0 1.0 1.0As 0.0 0.0 0.0 0.0S 2.0 2.0 2.0 2.0

TOTAL 3.0 3.0 3.0 3.0No.

AtomsFe 1.0 1.0 1.0 1.0

As+S 2.0 2.0 2.0 2.0TOTAL 3.0 3.0 3.0 3.0

Borehole CR194: As-bearing pyrite – Analysis Locations

AnalysisFrom(m)

To(m) Location





A49


Borehole CR194: Amalgam

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13

Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)

Ag 54.7 54.2 56.4 54.6 53.9 52.9 59.8 59.0 59.9 58.1 56.4 55.2 55.8

Hg 45.5 46.7 44.2 45.1 46.7 46.6 40.8 41.4 39.7 42.9 44.3 44.9 44.2

TOTAL 100.2 100.9 100.5 99.7 100.6 99.5 100.7 100.4 99.6 101.0 100.7 100.1 100.0

AnalysisFrom(m)

To(m) Location

#1 161.75 162.75 Gossan

#2 161.75 162.75 Gossan

#3 161.75 162.75 Gossan

#4 163.75 164.60Gossan contact with massive sulphide - Lower Portion







#11 164.60 165.80 Massive sulphide contact with gossan



A50


Borehole CR194: Au-Amalgam

Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16

Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)

Fe 2.8 2.5 2.0 2.1 nd nd nd nd nd nd nd nd nd nd nd nd

Ag 48.5 50.5 50.7 50.6 42.9 35.3 27.1 29.8 33.0 22.8 24.8 47.5 46.4 46.1 46.2 48.1

Au 11.6 8.5 6.5 5.4 27.7 37.2 48.6 47.0 44.0 56.5 46.0 18.5 16.4 19.4 18.8 17.0

Hg 38.0 39.3 40.5 42.6 28.9 27.1 23.8 21.6 23.6 20.5 30.0 34.9 36.8 35.0 33.8 34.3

TOTAL 100.9 100.8 99.7 100.8 99.5 99.7 99.5 98.4 100.6 99.8 100.8 100.9 99.7 100.5 98.8 99.4

Analysis #17 #18 #19 #20 #21 #22 #23 #24

Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)

Ag 45.7 46.0 50.7 49.9 34.4 41.9 31.9 31.6

Au 21.0 19.0 11.3 8.3 17.0 15.4 31.9 22.9

Hg 34.1 34.7 38.4 41.3 47.6 43.0 35.7 45.2

TOTAL 100.8 99.8 100.3 99.5 99.1 100.3 99.5 99.7

A51


Borehole CR194: Au-Amalgam Locations

AnalysisFrom(m)

To(m) Location Analysis

From(m)

To(m) Location

#1 163.75 164.60Gossan contact with massive sulphide - Middle #14 164.60 165.80 Massive sulphide contact with gossan




#5 163.75 164.60 Gossan contact with massive sulphide - Lower #18 164.60 165.80 Massive sulphide contact with gossan

#6 163.75 164.60 Gossan contact with massive sulphide - Lower #19 164.60 165.80 Massive sulphide/gossan contact, zoned grain (core)

#7 163.75 164.60 Gossan contact with massive sulphide - Lower #20 164.60 165.80 Massive sulphide/gossan contact , zoned grain (core)

#8 163.75 164.60 Gossan contact with massive sulphide - Lower #21 164.60 165.80Massive sulphide/gossan contact, zoned grain (margin)






A52

phd part 1

Documents

las cruces geology

present day las cruces

mature gossan profile

thesis outline

degree of phd

candidate date statement

precious metal remobilisation

dedication dedication