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Structural and stratigraphic controls
on mineralization at the
George Fisher Zn-Pb-Ag Deposit,
Northwest Queensland, Australia
Thesis submitted by
Travis E. Murphy
B.App.Sc. (Hons) University of Technology, Sydney
in October, 2004,
for the degree of Doctor of Philosophy
in the School of Earth Sciences,
James Cook University
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Statement of Access
I, the undersigned, author of this thesis, understand that the following restriction placed
by me on this thesis will not extend beyond the date specified.
I wish to place restriction on access to this thesis for a period of two years as per the
confidentiality agreement with MIM Ltd (now Xstrata)
After this period has elapsed I understand that James Cook University will make this
thesis available for use within the University library and make this thesis available via the
Australian Digital These network, for use elsewhere. All users consulting this thesis will
have to sign the following statement:
In consulting this thesis I agree not to copy or closely paraphrase it in whole or in part
without written consent of the author, and to make proper written acknowledgement for
any assistance which I obtained from it
_____________________________ __________________
Travis E. Murphy Date
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Statement of Sources
Declaration
I declare that this thesis is my own work and has not been submitted in any form foranother degree or diploma at any university or other institution of tertiary education.
Information derived from the published or unpublished work of others has been
acknowledged in the text and a list of referees is given.
_____________________________ __________________
Travis E. Murphy Date
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Electronic Copy
I, the undersigned, author of this work, declare that the electronic copy of this thesis
provided to the James Cook University Library is an accurate copy of the print thesis
submitted, within the limits of the technology available.
_____________________________ __________________
Travis E. Murphy Date
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Acknowledgements
This project has been possible thanks to financial support from MIM/Xstrata who have
provided access to required data, drill-core, facilities, and the mine at George Fisher.
The initial concept of this study was developed by Alice Clark and Tim Bell. Andrew
Allibone, Tom Blenkinsop, and Tim Bell are thanked for their supervision of respective
parts of this thesis. Andrew Allibone, in particular, is thanked for guiding this research
and for enthusiastic editing of several drafts of the thesis.
Myles Johnston, Ann Mawdesley, Alison Morley, Brad Cox and other geologists from
both the Mt Isa and George Fisher offices are thanked for providing logistical support
during my stays in Mt Isa on fieldwork. Communication with Allen Shaw proved
invaluable due to his intimate knowledge of the stratigraphy at George Fisher.
Paul Whelan (Metech) kindly provided the MineSight software package for use in this
study. Andrew Allibone, Roger Taylor, Kylie Prendergast, and Gavin Clark donated
some of the samples used in Part C of this study and are thanked for discussions related to
the same. Alan Chappell, Gordon Warria, and Kevin Blake provided supervision and
support of various stages of laboratory work in the Advanced Analytical Centre at JCU.
Rhenium-Osmium isotopic analysis was possible thanks to funding and support from the
PMD-CRC and the efforts of Lucy Chapman were integral in making this happen. Reid
Keays, Bruce Schaeffer, Rachelle Hubregtse and other VIEPS personnel conducted this
analytical work, and Reid Keays is thanked for his role in this collaborative work.
Finally, I would like to thank Rebecca, my wife, for encouragement to undertake this PhD
study, support during my candidature, and patience while I came to blows with this
computer whilst producing the thesis.
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ABSTRACT
Sediment-hosted stratiform Zn-Pb-Ag deposits have been interpreted as forming during
exhalative hydrothermal activity synchronous with the deposition of host rocks and
during either syn-diagenetic or syn-metamorphic veining and replacement of the host-
rocks later in their orogenic history. This study analyzes the distribution metal and ore-
types at the George Fisher deposit and investigates spatial and temporal relationships
between high-grade mineralization and structures at all scales throughout the deformation
history. The study aims to determine whether remobilization and upgrading of syn-
sedimentary and/or syn-diagenetic proto-ore or primary syntectonic mineralization are
involved in the formation of economic mineralization. Analysis of the kinematic controls
on any Zn-Pb-Ag mineralization which is structurally controlled may enable other
prospective structures in the near-mine region to be identified.
The George Fisher Zn-Pb-Ag deposit is located 22km north of Mt Isa in Queensland,
Australia; and is hosted by Proterozoic sedimentary rocks of similar age and lithology to
the Mt Isa and adjacent Hilton Zn-Pb-Ag deposits. The host-rocks at George Fisher
preserve a structural history comprising four distinct ductile deformations with
concomitant faulting and younger brittle faults which cut the ore-bearing sequence. The
first, D1 is manifest as an open fold (F1) with an approximately east-west striking axial
plane and was overprinted by F2 folding and a pervasive slaty/solution cleavage (S2)
during the regionally extensive D2 episode. An episode of sub-vertical shortening/sub-
horizontal extension (D3) followed this main phase of sub-horizontal shortening which
formed folds with sub-horizontal axial planes and gently-dipping crenulations of the S2
cleavage and bedding. D3 features are overprinted by sub-vertical crenulations (S4) and
minor folds (F4) with sub-vertical axial planes.
Ore shoots that comprise high-grade and thicker mineralization plunge parallel to the F1
fold axes and are largely confined to the short-limb of this fold. Subsidiary ore shoots are
coincident with areas of more intense F2 folding, and trend parallel to both F2 and F4 fold
axes in longitudinal projection.
Vein-hosted sphalerite and medium-grained galena breccia are the main sources of Zn
and Pb metal in the deposit, respectively. Logged widths of each form shoots of thicker
mineralization which are broadly coincident with high-grade shoots defined by the assay
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data. Sulphide textural studies and vein - host-rock fabric cross-cutting relationships
suggest that both postdate D2.
Empirical relationships between ore shoot geometry and the structural framework of the
deposit imply a D4 control on metal distribution. This is supported by the interpretation
of a dominantly syn-D4 relative timing of the vein-hosted sphalerite and medium-grained
galena breccia based on development of these mineralization-types in unique structural
settings and the apparent lack of deformation of their constituent sulphides. Potential pre-
F2 mineralization types include some disseminated sphalerite and fine-grained sphalerite-
galena breccias which do not currently constitute economic mineralization and account
for ca. 10% of the Zn+Pb in the deposit. Remobilization of proto-mineralization issupported by the Zn assay data which indicates that more than one population of Zn
grades exists and that a higher grade population is unique to the economic ore-horizons.
However, this qualitative observation does not discriminate between upgrading of a pre-
F2 or syn-/post-F2 sulphide accumulation during D4.
Re-Os isotopic analysis of sphalerite and galena at George Fisher define an isochron
whose slope indicates an age of 1423130Ma indicating closure of the Re-Os system
postdates host-rock deposition by ~100-360Ma.
A mantle source of Pb and Zn is interpreted from the Re-Os isotopic analysis based on the
initial187
Os/188
Os ratio of 0.0770.071. This differs from previous studies of Proterozoic
Zn-Pb-Ag deposits which infer scavenging of metal from within the sedimentary basin or
from the basement rocks immediately underlying the sedimentary basin. Proximity to a
regional fault zone such as the Mount Isa-Paroo Fault system, interpreted to be part of a
fault-network linked to a major mid-crustal shear zone, is considered necessary to bring
metal-bearing fluids from depth into contact with prospective host-lithologies at GeorgeFisher.
At the George Fisher deposit, it is inferred that the F1 fold focussed diagenetic
hydrothermal activity and alteration and also acted as a heterogeneity focussing dilation
and final sites of mineralization later in the deformation history. The F1 fold may be the
upper level expression of a reactivated basement fault thereby accounting for the
longevity of the hydrothermal system responsible for alteration and mineralization at
George Fisher. It is possible that diagenetic processes prepared the host-rock for latermineralization at George Fisher.
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TABLE OF CONTENTS
Statement of Access i
Statement of Sources ii
Statement of Submission of Electronic Copy iii
Acknowledgements iv
Abstract v
Table of Contents vii
List of Tables xiii
List of Figures xiii
INTRODUCTION 1
Thesis Structure 4
__________________________________________________________________
PART A. STRUCTURAL DEVELOPMENT OF HOST ROCKS TO ZN-PB-AG
MINERALIZATION AT THE GEORGE FISHER MINE, NORTHWEST QUEENSLAND,
AUSTRALIA.
Abstract 7
1. Introduction 8
2. Geological Setting 9
2.1 Regional Geology 9
2.2 Local Geology 16
3. Primary Layering Bedding 18
4. Deformation History 22
4.1 Description and interpretation of D1 structures 22
4.2 Description and interpretation of D2 structures 29
4.2.1 Thermal and stress history of the D2 deformation 37
4.3 Description and interpretation of D3 structures 47
4.4 Description and interpretation of D4 structures 55
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4.5 Description and interpretation of Post-D4 minor fabrics 69
4.6 Description and interpretation of late kink folding 71
4.7 Description and interpretation of brittle faulting 71
4.7.1 North-South reverse faults with oblique slip 75
4.7.2 East-West striking faults 75
4.7.3 North-northeast striking sub-vertical reverse faults 77
4.7.4 North-northwest striking, west-dipping sinistral faults 77
4.7.5 Northeast striking, dextral-normal faults 80
4.7.6 Bedding-parallel faults 80
5. Discussion 815.1 Comparison with previous structural interpretations 81
5.2 Discrete episodic foliation development vs conjugate crenulations 89
5.3 Origin and significance of D3 structures 90
6. Conclusions 94
References 96
__________________________________________________________________PART B. DISTRIBUTION OF ZN-PB-AG MINERALIZATION AND ITS
RELATIONSHIP TO STRUCTURAL DOMAINS AT THE GEORGE FISHER MINE,
NORTHWEST QUEENSLAND, AUSTRALIA.
Abstract 102
1. Introduction 103
2. Metal Distribution 1052.1 Methods for analyzing metal grade distributions 105
2.2 Deposit-wide correlations between metal grades and
structure in plan and section 109
2.3 Distribution of metal ratios 117
2.4 Mineralization and grade distribution in C ore-horizon 122
2.5 Mineralization and grade distribution in D ore-horizon 124
2.6 Mineralization and grade distribution in G ore-horizon 126
3. Deposit-scale sulphide distribution 127
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3.1 C ore-horizon 129
3.2 D ore-horizon 131
3.3 G ore-horizon 134
3.4 Combined C+D and C+D+G data 135
4. Discussion 135
4.1 Correlations between the geometry, orientation, and
location of ore shoots and structural features 135
4.2 Comparison with the structural setting of ore shoots
within the Mt Isa and Hilton Zn-Pb-Ag deposits 141
4.3 Kinematics controlling ore-sulphide concentration : Implicationsfor extensional and near-mine exploration 145
5. Conclusions 153
References 155
__________________________________________________________________
PART C. THE RELATIONSHIP BETWEEN THE FRACTAL DIMENSION OF
SPHALERITE GRAIN BOUNDARIES AND SULPHIDE DEFORMATION: APPLICATION
OF THE COASTLINE METHOD IN MICROSTRUCTURAL STUDIES.
Abstract 157
1. Introduction 158
2. Method 160
2.1 Methods of grain boundary characterization 160
2.2 Sample Preparation 1622.3 Procedure 162
3. Samples Studied 166
4. Sulphide Microstructures 166
4.1 Class A 169
4.2 Class B 169
4.3 Class C 171
4.4 Class D 171
4.5 Class E 173
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5. Results of fractal analysis of sphalerite grain boundaries 173
6. Discussion 179
6.1 Sulphide microstructural studies 179
6.2 Implications for ore deposit analysis 186
7. Conclusions 186
References 188
__________________________________________________________________
PART D. ORE CHARACTERISTICS, CROSS-CUTTING RELATIONSHIPS, AND
TIMING RELATIVE TO DEFORMATION AT THE GEORGE FISHER DEPOSIT,
NORTHWEST QUEENSLAND, AUSTRALIA.
Abstract 193
1. Introduction 194
2. Description and interpretation of sphalerite-dominant ore-types 199
2.1 Vein-hosted sphalerite 199
2.1.1 Evidence for replacive sphalerite 204
2.1.2 Sphalerite occurring as breccia-infill and replacement
of pre-existing veins 212
2.1.3 Sphalerite occurring synchronous with vein development 215
2.1.4 Implications of cross-cutting relationships and
palaeostress estimation 225
2.1.5 Summary and interpretation 237
2.2 Massive sphalerite 2382.3 Sphalerite breccia 240
2.4 Disseminated sphalerite 247
2.5 Fine-grained sphalerite-galena breccia 251
2.6 Some microstructural evidence for sphalerite mobility during D4 259
3. Description and interpretation of galena-dominant ore-types 262
3.1 Fine-grained galena-sphalerite breccia 262
3.2 Medium-grained galena breccia 264
3.2.1 Relative timing of medium-grained galena breccia
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formation 269
3.3 Coarse-grained galena breccia 280
4. Discrimination of remobilized vs syntectonic mineralization based on
analysis of grade populations 288
5. Discussion 292
5.1 Relative timing of ore-types integration with the structural history 292
5.2 Pre-/early-D2 mineralization at George Fisher?: Remobilization vs
syntectonic orebody formation 302
6. Conclusions 306
References 308
__________________________________________________________________
PART E. RHENIUM-OSMIUM ISOTOPIC ANALYSIS OF MINERALIZATION AT THE
GEORGE FISHER DEPOSIT, NORTHWEST QUEENSLAND, AUSTRALIA:
IMPLICATIONS FOR THE ABSOLUTE AGE OF MINERALIZATION AND THE SOURCE
OF METALS.
Abstract 314
1. Introduction 315
2. Methodology 318
3. Results 321
4. Discussion 326
4.1 Implications for the timing of mineralization at George Fisher 326
4.2 Source of metal at the George Fisher Zn-Pb-Ag deposit:comparison with previous models of Zn-Pb-Ag metallogeny in
the Western Fold Belt of the Mt Isa Inlier 330
5. Conclusions 332
References 334
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__________________________________________________________________
PART F. TECTONOSTRATIGRAPHIC SETTING AND CONTROLS ON STRATIFORM
BASE METAL MINERALIZATION IN THE WESTERN FOLD BELT OF THE MT ISA
INLIER, AUSTRALIA: COMPARISON OF THE MT ISA, GEORGE FISHER - HILTON,
LADY LORETTA, AND CENTURY ZN-PB-AG DEPOSITS.
Abstract 341
1. Introduction 342
2. Geological Evolution of the Western Fold Belt 342
3. Characteristics of the setting of the Mt Isa, George Fisher Hilton,
Lady Loretta, and Century Zn-Pb-Ag deposits 350
3.1 Mt Isa 350
3.2 George Fisher Hilton 353
3.3 Lady Loretta 358
3.4 Century 361
4. Discussion 364
4.1 Common and contrasting features in the setting of the Zn-Pb-Ag
deposits of the Western Fold Belt and implications for exploration 364
4.2 Significance of D1 (approximately east-west oriented) structures
and implications for Zn-Pb-Ag metallogenesis in the Western Fold
Belt of the Mt Isa Inlier 369
5. Conclusions 371
References 372
__________________________________________________________________CONCLUSIONS 382
__________________________________________________________________
APPENDICES
APPENDIX I. SAMPLE CATALOGUE 388
APPENDIX II. STRUCTURAL DATA 392
APPENDIX III. ELECTRON MICROPROBE ANALYSES 398
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APPENDIX IV. (A) FRACTAL ANALYSIS METHOD 400
(B) DETAILED DATA FROM FRACTAL ANALYSIS 401
LIST OF TABLES
PART A.Table 1. Fabric catalogue of recognized ductile deformation features 21
Table 2. Previous structural interpretations in the George Fisher - Lake
Moondarra Mt Isa area 82
PART C.Table 1. Sample locations and type for fractal analysis 168
Table 2. Descriptive statistics for the results of the fractal analysis 174
PART D.Table 1. Paragenetic sequence of mineralization and alteration 197
PART E.Table 1. Re-Os isotopic data and sample information 322
LIST OF FIGURES
PART A.
Figure 1. Map of the Proterozoic McArthur and Mt Isa Inliers 10
Figure 2. Geological map of the George Fisher Lake Moondarra area 12
Figure 3. Map of the Sybella Batholith and surrounding faults 13
Figure 4. Simplified geological map of the area southwest from George
Fisher and west of the Mt Isa fault 15
Figure 5. Surface geological map above the George Fisher Mine 17
Figure 6. Stratigraphic units/ore-horizons in plan and sectional view 19
Figure 7. Bedding measurements and photomicrographs of the early
stylolitic compaction related fabric 21
Figure 8. Photomicrographs and line drawing illustrating the preservation
of earlier fabrics adjacent to carbonate nodules 23
Figure 9. Bedding form-line maps illustrating the s-shaped F1 flexure 24
Figure 10. Photographs and mapping indicating that the flexure is unrelated
to late brittle faulting 26
Figure 11. Photomicrographs of the S2 foliation 30
Figure 12. S2 and F2 data displayed on stereonets and projected onto 12 Level
of the George Fisher Mine 31Figure 13. Variation in the intensity of the S2 foliation 32
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Figure 14. Mapping of 739 cross-cut on level 12C of the George Fisher Mine
indicating the change in the sense of shear on the S2 cleavage
around folds thereby interpreted as F2. 34
Figure 15. Photograph and line-drawing of rotated F2 folds in D-orebody 35Figure 16. Mapping of F2 folds in the northern part of level 12C 37
Figure 17. Graph illustrating the relationship of the fractal dimension of
quartz grain boundaries with deformation temperature and a
photomicrograph of an analyzed grain boundary from the Lake
Moondarra area 40
Figure 18. Determination of deformation temperature from calcite twin
geometry 41
Figure 19. Image of sample with cross-cutting vein relationships used for
palaeostress estimation 43
Figure 19 Electron microprobe analyses of vein and matrix carbonates 44
(contd).Figure 20. Palaeostress estimation graphs 45
Figure 21. Photomicrographs and line-drawing of S3 crenulations 47
Figure 22. S3 and F3 data displayed on stereonets and projected onto 12 Level
of the George Fisher Mine 49
Figure 23. Variation in the intensity of the S3 foliation 50
Figure 24. Method of calculation of the extension direction in the plane of
the S3 foliation 52
Figure 25. Photomicrographs and line-drawing of subtle D3 reactivation of
bedding and S2. 53
Figure 26. Fold vergence relationships observed at George Fisher 54
Figure 27. Syn-D3 quartz-carbonate fibre growth in S2 parallel veins 56
Figure 28. Photomicrographs and line-drawing of S4 crenulations 57
Figure 29. Photograph of foliation-bedding intersection lineations 58
Figure 30. Photomicrograph and line-drawing of an S3 crenulation
decrenulated at the intersection with a zone of D4 strain 60
Figure 31. S4 and F4 data displayed on stereonets and projected onto 12 Level
of the George Fisher Mine 61
Figure 32. Variation in the intensity of the S4 foliation 62
Figure 33. Contours of the angle of intersection between S0 and S4 63
Figure 34. Method of calculation of the extension direction in the plane of
the S4 foliation 64Figure 35. Overprinting fold relationships in cross-cut mapping of
D-orebody 66
Figure 36. Photomicrograph and line-drawing of foliation relationships in
barren siltstone 68
Figure 37. Photomicrograph and line-drawing of the foliations S2-S6 70
Figure 38. Mapping of east-west trending kink-like folds in the hangingwall
of C orebody 72
Figure 39. Map of main fault sets at George Fisher 73
Figure 40. Cataclasite from the Paroo Fault 74
Figure 41. Mapping of east-west striking south-dipping faults 76
Figure 42. East-dipping faults with minor east-side-up displacement 78Figure 43. Orientation data for northwest and northeast striking faults 79
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Figure 44. Line-drawing of a bedding-parallel fault and associated folding
and veining 79
Figure 45. District-scale map displaying form-lines of S2/F2 and S4/F4 84
Figure 46. Implications of S2/S4 asymmetry swap between Mt Isa andGeorge Fisher 87
Figure 47. Possible explanations for observed D3 strain at George Fisher 91
PART B.
Figure 1. Broader stratigraphic control on mineralization 104
Figure 2. Explanation of longitudinal projection and block-model
construction 106
Figure 3. Simplified stratigraphic column 108
Figure 4. Stacked sections displaying Zn grade 110
Figure 5. Stacked sections displaying Pb grade 111Figure 6. Stacked sections displaying Ag grade 112
Figure 7. Zn grade distribution in plan view 114
Figure 8. Pb grade distribution in plan view 115
Figure 9. Ag grade distribution in plan view 116
Figure 10. Zn vs Pb graphs of assay data 118
Figure 11. Distinction of high-Zn and high-Pb populations 120
Figure 12. Zn vs Pb graphs for the respective ore-horizons at George Fisher 121
Figure 13. C ore-horizon metal grades in longitudinal projection 123
Figure 14. D ore-horizon metal grades in longitudinal projection 125
Figure 15. G ore-horizon metal grades in longitudinal projection 128
Figure 16. C ore-horizon ore-sulphide thickness in longitudinal projection 130
Figure 17. D ore-horizon ore-sulphide thickness in longitudinal projection 132
Figure 18. G ore-horizon ore-sulphide thickness in longitudinal projection 133
Figure 19. C+D and C+D+G sulphide thickness in longitudinal projection 136
Figure 20. Generalized ore shoot geometry and relationship to
structural orientations 138
Figure 21. Comparison of geology and mineralization between the near surface
and 12 Level of the George Fisher Mine 140
Figure 22. Ore-fluid migration parallel to fold axes 143
Figure 23. Isometric block diagrams illustrating kinematics during the
successive deformations and possible sites for dilation 146Figure 24. Illustration of dilation in the F1 short-limb during D4 reactivation
of bedding 148
Figure 25. Illustration of potential net-dilation occurring during D4
reactivation of bedding 150
Figure 26. Maps illustrating the spatial relationship between more intense
S3 and S4 development and higher Pb grades 151
PART C.
Figure 1. Graphical representation of the algorithm used in the fractal
analysis 161Figure 2. Photomicrographs illustrating etched vs non-etched polished
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sections 163
Figure 3. Analysis of the sensitivity of the fractal method to digitizing
error 165
Figure 4. Explanation of the step-effect observed in plots of
Log P vs Log L 167
Figure 5. Photomicrographs of representative microstructures in sphalerite 170
Figure 6. Photomicrograph and line-drawing of sphalerite grains interpreted
as having recrystallized and subsequently deformed 172
Figure 7. Examples of representative grain boundary outlines used in the
fractal analysis 175
Figure 8. Graphs illustrating the lines of best fit to data used in the fractal
analysis 177
Figure 9. Frequency histogram displaying results from analysis of
undeformed, deformed, and recrystallized sphalerite 178Figure 10. Data for sphalerite grain boundaries plotted onto a graph of fractal
dimension vs log-grain diameter 180
Figure 11. Potential deformation/recrystallization paths indicated by trends
in the data on the D vs grain diameter plot 183
Figure 12. Data grouped according to deformation temperature 184
PART D.
Figure 1. Hand-specimens of vein-hosted sphalerite mineralization 201
Figure 2. Photomicrographs of vein-hosted sphalerite mineralization 203
Figure 3. Vein-hosted sphalerite grain boundary data plotted onto a
graph of fractal dimension vs grain diameter 205
Figure 4. Photomicrographs of sphalerite replacing gangue minerals in
vein-hosted sphalerite 207
Figure 5. Photomicrograph and line drawing illustrating brecciation and
replacement of host rock during vein-hosted sphalerite
mineralization 208
Figure 6. Vein-hosted mineralization terminating on a cross-cutting vein 210
Figure 7. Vein-hosted mineralization terminating on a cross-cutting vein 211
Figure 8. Photomicrographs of subhorizontal quartz-carbonate fibres
associated with vein-hosted sphalerite mineralization 214Figure 9. Photomicrographs illustrating relict deformed quartz clasts in
vein-hosted sphalerite 216
Figure 10. Photomicrograph and line-drawing of vein-hosted sphalerite
occurring in veins opening due to competency contrast 217
Figure 11. Hand-specimen and photomicrographs of vein-hosted sphalerite
heterogeneously distributed around a mesoscale fold 219
Figure 12. Photograph and line drawings of unfolding of an early fold creating
bedding-parallel sphalerite+quartz+carbonate veins 221
Figure 13. Heterogeneous development of vein-hosted sphalerite mineralization
around an F2 fold 222
Figure 14. Photomicrographs of an isoclinal microfold in a vein-hostedsphalerite layer 224
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Figure 15. Implications of palaeostress estimates on vein paragenesis 226
F3 microfolds and associated veining in vein-hosted sphalerite 227
Figure 16. Graph of strength vs temperature for sphalerite and carbonate
rock types 231Figure 17. Comparison of sphalerite textures in planar and folded strata 233
Figure 18. Fractal analysis of grains from samples in Figure 17 234
Figure 19. Distribution of vein-hosted sphalerite thickness in longitudinal
projection 236
Figure 20. Hand-specimen and photomicrographs of massive sphalerite 239
Figure 21. Distribution of massive sphalerite thickness in longitudinal
projection 241
Figure 22. Hand-specimen and photomicrographs of sphalerite breccia 242
Figure 23. Hand-specimen and photomicrographs of sphalerite breccia 243
Figure 24. Distribution of sphalerite breccia thickness in longitudinal
projection 246Figure 25. Photomicrographs of disseminated sphalerite 248
Figure 26. Photomicrograph of disseminated sphalerite concentrated in
fold hinges 250
Figure 27. Fine-grained sphalerite breccia in hand-specimen and
photomicrograph 252
Figure 28. Fine-grained sphalerite breccia in hand-specimen and
photomicrograph 253
Figure 29. Fractal analysis of relict deformed grains from fine-grained
breccia 255
Figure 30. Distribution of fine-grained sphalerite+galena breccia in
longitudinal projection 257
Figure 31. Photomicrographs illustrating the S4 and host-rock control on
sphalerite deposition 260
Figure 32. Photomicrographs illustrating D4 reactivation of bedding causing
dilation and minor sphalerite precipitation in S3 crenulations 261
Figure 33. Photomicrographs of fine-grained galena 263
Figure 34. Photographs of medium-grained galena breccia in hand-specimen 265
Figure 35. Photomicrographs of medium-grained galena breccia 266
Figure 36. Photomicrographs of galena tension gashes 268
Figure 37. Fold overprinting relationships and their implications on
medium-grained galena breccia timing 270Figure 38. Cross-cut mapping illustrating post-D3 timing for medium-grained
galena breccia 271
Figure 39. Mapping of a large galena-dominant bedding-discordant tension
vein 273
Figure 40. Mapping of a cross-cut illustrating the timing of galena breccia
development relative to folding episodes 275
Figure 41. Mapping, hand-specimen photograph, and photomicrograph of
thickening and deformation of galena in a fold 276
Figure 42. Medium-grained galena breccia hosted by a boudin-neck 277
Figure 43. Distribution of medium-grained galena breccia thickness in
longitudinal projection 279Figure 44. Coarse-grained galena breccia in hand-specimen and
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photomicrograph 281
Figure 45. Coarse-grained galena breccia in hand-specimen and
photomicrograph 282
Figure 46. Photograph and line drawing illustrating post-D3 timing of
coarse-grained galena breccia 284
Figure 47. Photograph of a relict fold hinge in a coarse-grained galena breccia 285
Figure 48. Coarse-grained galena breccia in an apparent boudin-neck formed
during bedding parallel extension 285
Figure 49. Distribution of coarse-grained galena breccia thickness in
longitudinal projection 287
Figure 50. Frequency and cumulative frequency histogram for D ore-horizon
data 289
Figure 51. Histograms and normal-probability plots for George Fisher data 291
Figure 52. Summary of the relative timing of ore-types 294Figure 53. Graph of Pb isotope data and interpretations for the major Pb
deposits in Australia 305
PART E.
Figure 1. Samples used in the Re-Os isotopic analysis 320
Figure 2. Results plotted on a187
Os/188
Os vs187
Re/188
Os graph 324
Figure 3. Explanation of resetting of host-rock Re-Os signature 328
PART
F.
Figure 1. Tectonostratigraphic divisions of the Mt Isa Inlier and
simplified geology of the Western Fold Belt 344
Figure 2. Stratigraphic column for the Western Fold Belt 345
Figure 3. Schematic illustration of reactivation of basement faults 348
Figure 4. Resource information for the major Zn-Pb-Ag deposits in the
Mt Isa Inlier 348
Figure 5. Illustrations of the location and structural control on Zn-Pb-Ag
orebodies at Mt Isa 351
Figure 6. Illustrations of the geology and orebody locations at the George
Fisher and Hilton deposits 354Figure 7. Controls on ore shoots at the George Fisher deposit 356
Figure 8. Distribution of calcite alteration at George Fisher and relationship
to the F1 short-limb 357
Figure 9. Geological map and section of the Lady Loretta deposit 359
Figure 10. Geological map of the setting of the Century deposit 363
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INTRODUCTION
The George Fisher Zn-Pb-Ag deposit is located 22 kilometres north of Mt Isa in the
north-western region of Queensland, Australia. Mineralization at George Fisher is hosted
by Proterozoic sedimentary rocks within the Mt Isa Inlier and shows many similarities to
the adjacent Hilton (2 kilometres south from George Fisher) and nearby Mt Isa base metal
deposits. Mineralization is dominantly stratiform and stratabound (Valenta, 1994;
Chapman, 2004) and is hosted exclusively by the Urquhart Shale unit which comprises
pyritic siltstones and carbonaceous siltstones interbedded with dolomitic mudstones
(Chapman, 2004). Some galena has been remobilized during deformation (Chapman,
2004). George Fisher has subtle differences from the Hilton and Mt Isa deposits such as
its relatively Zn-rich, Ag-poor resource and the absence of significant copper
mineralization. The Mt Isa deposit is relatively Pb-rich and has lower Zn grades than
George Fisher. Significant copper mineralization is located adjacent to the Pb-Zn
orebodies at Mt Isa. The George Fisher deposit therefore represents a Zn-rich, Cu and
Ag-poor end-member of the three world-class lead-zinc-silver deposits in the Mt Isa
district (Chapman, 2004).
World class base metal deposits can be divided into two categories: giant and super-giant
(Singer, 1995). Giant Zn deposits (largest 10% of deposits) comprise >1.7 Mt of Zn and
super-giant (largest 1% of deposits) >12 Mt of contained metal (Singer, 1995). Of the
eight super-giant Zn deposits in the world, five are in Australia; and include George
Fisher and the adjacent Hilton deposits (Large et al., 2002). The George Fisher deposit
had a pre-mining resource of 108 Mt grading 11.1% Zn, 5.4% Pb, and 93g/t Ag (MIM
Ltd : Report to shareholders 1998) and qualifies as a giant deposit (Singer, 1995) for
both Pb and Ag.
Stratiform lead-zinc deposits are, by definition, conformable with their bedded
sedimentary host-rocks. At the broader scale this can be interpreted as indicating
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concurrent sedimentation and processes of mineralization, however, textural analysis of
the mineralization-types often reveals that replacement and veining are important
mechanisms of sulphide deposition, therefore indicating mineralization subsequent to
deposition of the host-rock sequence. This may occur during diagenesis or later in the
deformation history of the rocks. Determination of the timing of mineralization with
respect to host-rock formation, diagenesis, and episodes of subsequent deformation has
significant implications on the types of exploration models employed in order to discover
further Zn-Pb-Ag resources.
Several genetic models have been developed for the sedimentary-hosted Zn-Pb-Ag
deposits of the Mt Isa Inlier. The syn-sedimentary or SEDEX genetic model involves
hydrothermal fluids carrying metals in solution being expelled from a vent, which are
typically inferred to be faults active during rift-associated extension, and sulphides
deposited on the seafloor contemporaneous with sediment deposition (e.g. Russell et al.,
1981; Sawkins, 1984; Hancock and Purvis, 1990; Cooke et al., 2000). The conformity of
mineralization within the sedimentary sequence and stacked geometry of orebodies has
been interpreted as a result of episodic expulsion of ore-fluid and precipitation punctuated
by periods of sedimentation during which little or no sulphides were formed (Sawkins,
1984; Valenta, 1994). Zn-Pb-Ag mineralization at Mt Isa and Hilton/George Fisher has
also been interpreted as having formed during diagenesis (Neudert and Russell, 1981;
Valenta, 1988, 1994, Chapman, 1999, 2004). This differs from SEDEX models in that
mineralization occurs below the seafloor and occurs as replacement and cavity infill of
specific sedimentary sequences. Chapman (1999) interpreted chemically distinct phases
of carbonate alteration associated with Zn-Pb-Ag mineralization as diagenetic in origin.
Precipitation of sulphides during syntectonic replacement of lithified and deformed
sediments has been inferred by Blanchard and Hall (1942), Perkins (1997, 1998), and
Perkins and Bell (1998). The spatial correlation and parallelism of high-grade
mineralization and fold axes at Mt Isa have been interpreted as supporting the syntectonic
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genetic model for Zn-Pb-Ag mineralization (Wilkinson, 1995; Perkins, 1997; Davis,
2004).
Previous interpretations of the George Fisher/Hilton deposit infer that the Zn-Pb-Ag
mineralization records the same deformation history as the host-rocks (Valenta 1988,
1994) and initial mineralization has been interpreted as predating much of the
deformation history. Chapman (1999, 2004) inferred that the George Fisher Zn-Pb-Ag
deposit formed during diagenesis with remobilization of galena in the later stages of the
deformation history. The key observations linking mineralization with diagenesis
include:
low temperature bitumens interpreted to be of diagenetic origin and cogenetic
with sphalerite,
spatial coincidence of the Zn-Pb-Ag deposit within a carbonate alteration
system interpreted as pre-stylolitization and diagenetic, and
isotopic evidence that the interpreted fluids responsible for Zn-Pb-Ag
mineralization are distinct from the Cu (D4) mineralizing fluids.
This study benefits from the wealth of studies into the stratigraphy, structure, alteration
and mineralization processes in the Mt Isa George Fisher Lake Moondarra area.
Fewer studies have been undertaken on the George Fisher deposit itself, principally due to
the lack of mine development pre-1998. The key studies of the George Fisher deposit and
environs include comprehensive analysis of the mineralization and alteration paragenesis
(Chapman, 1999) and structural analysis of the adjacent Hilton deposit (Valenta, 1988).
The purpose of this study is to make detailed structural observations of the George Fisher
deposit host-rocks and mineralization from thin-section to deposit-scale and assess
whether areas of higher-grade and thicker mineralization are systematically related to
deformational features or a unique structural setting. This requires analysis of grade and
sulfide distribution and interpretation of the relative timing of textural varieties of
mineralization within the framework of the structural history of the deposit. The results
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of this analysis are likely to be significant guides for within-mine definition and near-
mine/extensional exploration for Zn-Pb-Ag mineralization at George Fisher. Absolute
ages from rhenium-osmium isotopic data collected from the sulfide samples will assist in
refining the timing of mineralization relative to deformation episodes and the isotopic
signature can indicate the source of metals which comprise the deposit. Comparison of
the interpreted controls on the setting of the George Fisher Zn-Pb-Ag deposit with other
significant deposits of the Western Fold Belt in the latter part of this study aims to add to
the knowledge base on metallogenic processes in the Mt Isa Inlier and is intended as an
aid to exploration for further Zn-Pb-Ag mineralization.
Thesis Structure
This thesis is presented as six sections (A-F), each written in journal article format and
summarized below:
Part A. The structural features observed from mine-scale through to micro-scale at the
George Fisher deposit are described and their overprinting relationships demonstrated.
Emphasis is placed on micro-scale observations as the foliations are not mappable at the
exposure-scale. Areas which have unique geometric relationships of overprinting
structures and more intense foliation and/or fold development are indicated.
Part B. Grade distribution of constituent metals and the cumulative thickness of
mineralization is evaluated enabling definition of ore-shoot orientations and geometry.
Ore-shoot locations and orientations correlate with identified structures and domains in
Part A and is suggestive of some mine-scale remobilization/syntectonic mineralization.
Part C. Criteria for determining the deformation history of sphalerite through fractal
analysis of grain boundary geometry is established. Undeformed, deformed, and
recrystallized sphalerite can be distinguished using these criteria and conventional
microscopic analysis.
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Part D. Distinction and relative timing of mineralization-types is established through
mine-scale distribution, mapping at exposure to hand-specimen scale, and microtextural
analysis. Interpretation of the range of mineralization types includes both pre- and post-
deformation sulphide deposition.
Part E. Re-Os isotopic analysis of the ore-sulphides and some host-rocks suggests that
Zn and Pb have a common source from a mantle-derived fluid. An age estimate is
obtained but does not indicate a specific deformation episode controlling later
mineralization as the uncertainty is large.
Part F. Utilizing interpretations from Parts A-E, comparison is made with other large
Zn-Pb-Ag deposits in the Western Fold Belt to determine whether there are consistent
controls on the location of these deposits.
References
Blanchard, R. and Hall, G., 1942. Mt Isa ore deposition: Economic Geology, v. 32, p.
1042-1057.
Chapman, L.H., 1999. Geology and genesis of the George Fisher Zn-Pb-Ag deposit,
Mount Isa, Australia. (Unpublished PhD thesis James Cook University,
Townsville).
Chapman, L.H., 2004. Geology and mineralization styles of the George Fisher Zn-Pb-Ag
deposit, Mount Isa, Australia: Economic Geology, v. 99, p. 233-255.
Cooke, D.R., Bull, S.W., Large, R.R., and McGoldrick, P.J., 2000. The importance of
oxidized brines for the formation of Australian proterozoic stratiform sediment-
hosted Pb-Zn (Sedex) deposits: Economic Geology, v. 95, p. 1-18.
Davis, T.P., 2004. Mine-scale structural controls on the Mount Isa Zn-Pb-Ag and Cu
orebodies: Economic Geology, v. 99, p. 543-559.
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Hancock, M.C. and Purvis, A.H., 1990. Lady Loretta Silver-Lead-Zinc Deposit, In:
Hughes, F.E. (Ed.), Geology of the mineral deposits of Australia and Papua New
Guinea. p. 943-948, (The Australasian Institute of Mining and Metallurgy:
Melbourne).
Large, R., Cooke, D., and Davidson, G., 2002, What is a world class deposit?: Abstracts
of the 16th
Australian Geological Convention, p. 266.
Neudert, M.K. and Russel, R.E., 1981. Shallow water and hypersaline features from the
middle Proterozoic Mount Isa sequence: Nature, v. 293, p. 284-286.
Perkins, W.G., 1997. Mount Isa lead-zinc orebodies: Replacement lodes in a zoned
syndeformational copper-lead-zinc system?: Ore Geology Reviews, v. 12, p. 61-
110.
Perkins, W.G., 1998. Timing of formation of Proterozoic stratiform fine-grained pyrite:
Post-diagenetic cleavage replacement at Mount Isa?: Economic Geology, v. 93, p.
1153-1164.
Russell, M.J., Solomon, M., and Walshe, J.C., 1981. The genesis of sediment-hosted,
exhalative zinc and lead deposits: Mineralium Deposita, v. 16, p. 113-127.
Sawkins, F.J., 1984. Ore genesis by episodic dewatering of sedimentary basins:
Application to giant Proterozoic lead-zinc deposits: Geology, v. 12, p. 451-454.
Singer, D.H., 1995, World Class Base and Precious Metal Deposits A Quantitative
Analysis: Economic Geology, v. 90, p. 88-104.
Valenta, R.K., 1988. Deformation, fluid flow and mineralization in the Hilton area, Mt
Isa, Australia. (Unpublished PhD thesis Monash University, Melbourne).
Valenta, R.K., 1994, Deformation in the host rocks and stratiform mineralization in the
Hilton Mine area, Mt Isa: Australian Journal of Earth Sciences, v. 41, p. 429-443.
Wilkinson, D.L., 1995. Structural termination of 8 orebody and its implications for the
genesis of lead-zinc-silver mineralisation, Mount Isa, Queensland: Unpublished
Hons thesis, James Cook University, Townsville, Australia.
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