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Chapter 1 – Introduction

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CHAPTER 1: INTRODUCTION

1.1 INTRODUCTION

This research documents the Sepon Mineral District (SMD) gold and copper deposits

that occur in the Sepon Basin, along the southern boundary of the NW-trending Truong Son

fold belt in south-central Laos. The central section of the SMD is located at longitude 105o59’E

and latitude 16o58’N and is 40 km north of the town of Sepon and 130 km east of the provincial

centre of Savannakhet in Savannakhet Province (Fig. 1.1).

Mining of and exploration for gold and copper resources in the SMD occurs in a

1250 km2 Mineral Exploration and Production Agreement area (MEPA) referred to as the

Sepon Project that is 100% operated by Lane Xang Minerals Limited (LXML), a local

subsidiary formerly owned by OZ Minerals Limited (Manini et al., 2001; Manini and Albert,

2003; Smith et al., 2005; Cannell and Smith, 2008; Fig. 1.1). The SMD was purchased from OZ

Minerals Limited in June 2009 by the Metals and Mining Group (MMG) owned by China

Minmetals Non-ferrous Co. Ltd, a division of the China Minmetals Corporation.

The known SMD gold and copper deposits and prospects occur in an E-W trending corridor,

approximately 40 km long by 10 km wide. At least three broad mineralisation styles are

recognised in the SMD: sedimentary rock-hosted Au (SHGD); Cu-Au skarn, and quartz

stockwork porphyry Cu-Mo (Loader 1999; Manini et al., 2001; Cromie et al., 2004a, b; Cromie

et al., 2006a, b; Smith et al., 2005; Olberg et al., 2006; Cannell and Smith, 2008).

Fig.1.1. Location of the Sepon Project area in south-central Laos (adapted from Smith et al., 2005)


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1.2 GEOGRAPHY AND ACCESS

Laos is located in central Indochina and has a population of 6.2 million people in a

landlocked country 236,800 km2 in area that shares borders with Thailand, Myanmar, China,

Vietnam and Cambodia (Vilaihongs et al., 1997). Vientiane is the capital city of Laos and the

main government administrative centre (Fig. 1.1). Daily international flights are available to

Vientiane from Thailand, China and Vietnam. The Mekong River forms most of the eastern

boarder of Laos and is a transportation route along the length of the country (Fig. 1.1).

The Sepon district is at an elevation of 250 m above sea level in moderately undulating

topography near the border of Vietnam (Fig. 1.1). The geographic setting of the SMD is shown

in Fig. 1.2A-E. Some patches of primary rain forest remain in areas dominated by secondary

forest mainly along ridges, and rice paddy fields occupy the valleys. Slash and burn agricultural

methods are used in a district, mostly to support rice and cotton farming (Fig. 1.2A-E). The

average annual rainfall in the Sepon District is 2200mm, associated with a monsoonal climate

with three main seasons: a cool, dry period from October to February; a hot, humid period from

March to June; and monsoonal rains from July to September (Smith et al., 2005).

The LXML SMD mining operations and exploration base in the Sepon District are

accessed either by a direct 90 minute company charter flight from Vientiane 6-times per week,

or by road transport starting from the provincial town of Savannakhet requiring a 2-hour drive

east along a sealed highway to the town of Sepon followed by a 1-hour drive north to site on an

unsealed all-weather public access road (Fig. 1.1). Access to the Sepon district can be difficult

during the monsoonal months of July to September, when roads are subject to flooding and

charter flights are scheduled during the mornings to avoid afternoon thunderstorms (Smith et

al., 2005; Ekins 2005). Accommodation during this study was provided at the permanent Padan

campsite where LXML also have their exploration office and drill core farm (Fig. 1.2D).

The historically infamous southern section of the Ho Chi Minh Trail travelled through

the Sepon District and was used as a major North Vietnamese supply route into southern

Vietnam during the Vietnam War, from 1964 to 1975. Consequently, heavy aerial

bombardments by US forces during this period primarily targeted the flow of North Vietnamese

forces and equipment into southern Vietnam, scarring the landscape along this supply route

during the war. Remnant ordinance from this campaign still contaminates areas within the

SMD (Fig. 1.2E). Effective safety systems have been developed by LXML for the clearance of

ordinance well ahead of operating areas on a daily basis (Smith et al., 2005).


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Fig.1.2. Photos showing the geographic setting of the Sepon Mineral District, Lao PDR. (A) View of the copper and gold processing operation at the Sepon mine site (foreground), operated by LXML (MMG), showing haul roads to the satellite gold deposits (right), and the hills of Thengkham (horizon). (B) Discovery outcrops of silicified calcareous shale containing >3.6 g/t Au that were identified by CRA (Rio Tinto) during 1990 along the Namkok River, adjacent to the present day Discovery Colluvial gold deposit. (C) An example of resource drilling in the Sepon mining area at Vang Ggang using a reverse circulation (RC) drill rig. (D) Accommodation units at the Padan campsite in Sepon mining area. (E) Aerial view of rice farming and forestry areas in the Sepon Mineral District, near the Sepon mine area. The small rounded dams in this picture are remnants of aerial bombardment that scared the landscape along the Ho Chi Min Trail during the Vietnam War.

A

B C

D E


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1.3 SMD EXPLORATION HISTORY AND MINING DEVELOPMENT

The discovery history of gold and copper resources in the SMD was documented by

Loader (1999), Manini et al. (2001), Manini and Albert (2003) and Smith et al. (2005).

Towards the end of the cold war during the late 1980s, Laos began seeking foreign investment,

which in turn enabled CRA Exploration to undertake reconnaissance visits during 1990 to

assess the technical and commercial opportunities in Laos. During October 1990, a CRA

literature study of the mineral resources of Laos, using United Nations Development Programs

(UNDP) information and data archived at the Department of Geology and Mines (DGM) in

Vientiane, identified the Sepon District as one of three high priority gold reconnaissance areas

selected for follow-up (Manini et al., 2001; Manini and Albert, 2003).

Before the 1990s, sporadic artisanal gold panning by local people occurred along

numerous river valleys in the Sepon area, with records at the DGM also describing alluvial gold

mining operations supported by Russia in the Sepon District from 1983 to 1985. Russian

geologists during the 1980s observed gold mineralisation in silicified and sericite altered rocks

within metasomatic contact zones of sub-volcanic intrusive complexes near the Namkok River,

north of Nongkadeng Village in the Sepon District. However, no serious evaluation of the hard-

rock potential was undertaken by the Russian geologists (Manini et al., 2001).

A reconnaissance visit by CRA geologists to the Namkok River north of Nongkadeng

Village during December 1990 (Gregory 1991) confirmed earlier observations of stockwork

quartz veining and altered porphyry intrusions reported by Russian geologists. CRA obtained

encouraging results during this visit, with 18 rock samples collected from silicified stringer

veined sedimentary rocks reporting between 3.6 and 55.9 g/t Au (Figs. 1.2B and 1.3; Manini et

al., 2001). The district-scale potential of the Sepon area, recognised by CRA after this first

visit, was compelling, with the district containing encouraging gold assay results from

extensive gold occurrences, the presence of kilometre-scale alteration and the association with

porphyritic intrusions (Manini et al., 2001; Manini and Albert 2003; Smith et al. 2005).

Subsequently, two years of pioneering negotiations with the Lao Government secured mineral

rights for CRA Exploration over the Sepon Project area under a 5000 km2 MEPA (Fig. 1.1).

CRA conducted intensive exploration programs in the SMD from 1993 to 1999

resulting in the initial discovery of 3.5 million ounces of gold in six separate gold deposits and

1.2 million tones of contained copper metal in the Khanong deposit (Fig. 1.3; Loader 1999;

Manini et al., 2001). In 1993, regional exploration programs in the MEPA commenced and

consisted of airborne radiometric surveys, aerial photograph and LANDSAT interpretation,

detailed regional stream sediment and rock geochemical surveys and geological mapping.


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During 1994, a district-scale, high-order multi-element geochemical signature was

identified by stream sediment geochemistry along an E-W trending zone that became known as

the SMD and subsequently became the main focus area for exploration (Fig. 1.4; Smith et al.,

2005). Although several exploration techniques and technologies were implemented in the

SMD, a strong geological focus during field programs resulted in discovery, mostly using basic

prospecting of anomalies, follow-up geological mapping of priority prospects, and drill testing

of highly prospective targets (Manini et al., 2001; Smith et al., 2005).

Oxiana Limited acquired an 80% share in the Sepon Project during 2000, with the

vendor Rio Tinto retaining a 20% shareholding. Oxiana immediately commenced geological

resource feasibility, environmental and social impact studies for a two stage development of the

Sepon gold and copper deposits (Fig. 1.3). The Sepon mine poured the first gold in late

December 2002 and produced 165,255 oz in the first twelve months of operation. During 2004,

the remaining 20% shareholding of the Sepon project held by RioTinto was purchased by

Oxiana Limited to increase its ownership to 100% (Manini et al., 2001; Manini and Albert,

2003). Mining of copper ores from the Khanong copper deposit commenced in late 2004 and

production of solution extraction electrowinning (SX-EW) copper cathode was commissioned

in 2005, with 30,000 tonnes of copper metal produced from the SMD in 2005 (Fig. 1.3; Oxiana,

2005). During 2008, Oxiana Limited and Zinifex merged to form OZ Minerals Limited, the

former owners of the Sepon Project until the purchase by MMG during 2009.

Fig. 1.3. (A) Map showing the location of gold and copper areas in the SMD (Oxiana Limited, 2005). (B) Aerial photograph showing the SMD gold and copper production operations, gold (Au) deposit open pits and the Khanong copper (Cu) deposit open pit.

Khanong (Cu)

Discovery West (Au)

Nalou (Au)

Discovery East (Au) Discovery Colluvial (Au)

Namkok West (Au) Vang Ngang (Au)

SMD gold and copper operations

A

B


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Fig. 1.4. Geochemical images of the SMD. Stream sediment geochemical images for gold (A) and copper (B) in the Sepon MEPA (i.e. the area located within the white boundary line) showing both gold and copper anomalies in the E-W trending SMD. The red rectangular area shown in Figs. 1.4A and B outlines the boundary of the SMD. Soil geochemical images of the SMD showing gold anomalies (C) coincident with and/or adjacent to copper anomalies (D). Image provided by Oxiana Limited from Manini et al. (2001).

A B

C

D


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1.4 GOLD AND COPPER RESOURCES IN THE SMD

Previous studies concluded that the gold mineralisation in the SMD SHGD is

predominantly micro-disseminated, closely associated with pyrite and having affinities with

Carlin-type gold deposits in Nevada, USA (Sillitoe, 1994a, b; Manini et al., 2001; Smith et al.,

2005). Tables 1.1 and 1.2 summarize the grades and tonnages of the currently known gold and

copper resources in the SMD, respectively. Extensive exploration and near mine resource

development programs in the SMD by Oxiana Limited (2000 – 2005) significantly upgraded

the indicated and inferred resources to 82.7 Mt @ 1.8 g/t Au for 4.75 million ounces of gold (at

0.5 g/t Au cut-off) in 8 separate but adjacent SHGD, as well as a gold resource of 18 Mt @ 0.76

g/t occurring in an ironstone horizon over the Khanong copper deposit (Table 1.1; Smith et al.,

2005). A comparison of the average grade and combined tonnage of the SMD SHGD with

other Carlin-type deposits is shown in Fig. 1.5. The SMD also has a significant combined

resource of 1.336 million tonnes (Mt) of copper metal contained in four separate supergene

copper deposits, namely the Khanong, Thengkham North, Thengkham South and Phabing

deposits (Table 1.2; Smith et al., 2005; Oxiana 2005, Cannell and Smith, 2008).

Table 1.1. Pre-mining and current gold resources in the SMD at a 0.5 g/t Au cut-off grade (compiled from Smith et al., 2005; Oxiana, 2005)

Gold Deposits Tonnes ore

(Mt) Grade

(Au g/t) Ounces

(gold oz) % of Total

gold oz

Nalou 30.24 1.69 1,643083 35

Discovery West 13.40 2.20 947,804 20

Discovery Main 8.79 2.87 811,076 17

Khanong (Au cap.) 18.24 0.76 445,686 9

Namkok West 4.24 2.48 338,071 7

Discovery Colluvial 2.94 2.87 271,282 6

Namkok East 3.22 1.19 123,195 3

Vang Ngang 1.01 2.86 92,871 2

Luang 0.59 4.26 80,808 2

Total 82.67 1.79 4,753,876 100

Table 1.2. Pre-mining and current supergene copper resources in the SMD (compiled from Cannell and Smith, 2008)

Copper Deposits Tonnes ore

(Mt) Grade (Cu %)

Tonnes (Mt) Cu metal

% of Total Cu metal

Khanong 25.5 3.52 0.893 67

Thengkham North 10.4 2.17 0.226 17

Thengkham South 10.7 1.39 0.149 11

Phabing 2.0 3.37 0.068 5

Total 48.6 2.61 1.336 100


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Fig. 1.5. Comparison of the average ore grade (g/t Au) versus metric tonnes ore for individual Carlin-type gold deposits in Nevada, USA (small black diamonds) and the combined known gold resources of the Sepon Mineral District (SMD) SHGD (red square). Deposits containing greater than 5 Moz gold (open squares) and major districts (open triangles) are shown also. Abbreviations: AR = Alligator Ridge District, CCT = central Carlin-Trend, CTZ = Cortez district in Battle Mountain-Eureka trend (BMET), EU = Eureka district in BMET, GB = Gold Bar district in BMET, GT = Getchell trend, JC = Jerritt Canyon district, NCT = north Carlin trend, SCT = south Carlin-trend. This figure is adapted from Cline et al. (2005) using the gold resource figure for the SMD SHGD from Smith et al. (2005).

1.5 PREVIOUS STUDIES

Company supported research on mineral deposits in the SMD was initiated during 1990

and focussed on geological documentation and petrological studies to guide exploration and

resource development projects. Knowledge of the stratigraphic formations in the Sepon mining

area was developed by Morris (1995, 1997) with further age constraints on the stratigraphy

established by dating fossil assemblages in a BSc (Hons) project at UTAS by Ekins (2005).

Preliminary SHRIMP U-Pb dating of zircon from a rhyodacite porphyry sample intruding the

SMD stratigraphy was conducted in AMIRA project P390A (Khin Zaw et al., 1999a, b) and the

age published in Loader (1999). Structural analysis of the Sepon Basin by Marten (1998a, b, c);

Coller (1999) and Smith (2003) provided insights into pull-apart basin development during

regional sinistral strike-slip transpression of the Truong Son Fold Belt. Consulting work by

Sillitoe (1994a, b, 1995, 1998) identified geological similarities between the SMD SHGD and

the Carlin-type gold deposits in Nevada. The SMD was also interpreted to host an intrusion-

related hydrothermal system, showing both mineral and metal content zonation patterns, with

distal sedimentary rock-hosted gold mineralisation localised by faults (Sillitoe, 1994a, b).


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Agnew (1998) highlighted the stream geochemical trace element associations and anomalies in

the SMD and Hackman (1998) investigated the down-hole trace element associations with gold.

Rio Tinto initially published the results of the exploration discovery and geological

setting of the Khanong copper deposit in Loader (1999) and Oxiana Limited reported on the

exploration history and geological setting for the Discovery, Nalou and Namkok old deposits in

Manini et al. (2001) and Manini and Albert (2003). Smith et al. (2005) and Olberg et al. (2006)

described the geological setting of the Sepon gold deposits and their comparison with the

Carlin-type gold deposits in the Great Basin, USA. Prior to 2006 there had been little research

published or company internal studies on the geochronology, mineral paragenesis, and ore-fluid

and isotope geochemistry to explain the genesis of the SMD gold and copper deposits.

1.6 AIMS

The main aims of this research thesis were to gain a better understanding of the SMD

gold and copper deposits by:

1) Investigating the temporal, geochemical and genetic relationships between hypogene gold

and copper mineralisation in the SMD by studying and documenting: (a) the geological

characteristics; (b) the paragenesis of ore-assemblages; (c) the timing of mineralisation;

(d) the ore-stage trace element associations; and; (e) the ore-stage isotope and fluid

chemistry;

2) Develop a genetic model to explain the geological and metallogenic evolution of gold in

the SMD gold and copper deposits;

3) Explaining the types and distribution of economic mineralisation in the Sepon basin

that could be used to develop exploration criteria that can be applied to predictive

targeting of copper and gold resources in the SMD and/or Asia region.

1.7 RESEARCH METHODS

The research sponsor for this thesis was OZ Minerals Limited (i.e. formerly Oxiana

Limited), then MMG after June 2009. The author’s research was supervised through the

CODES ARC Centre of Excellence in Ore Deposits at the University of Tasmania (UTAS) by

Dr. Khin Zaw (Principal Supervisor), Dr. David Cooke (Co-supervisor) and Dr. Noel White

(Associate-supervisor). The Society of Economic Geologists awarded a student research grant

to the author during 2006 towards the Re-Os dating of a single molybdenite sample at the

AIRIE molybdenite laboratory, Department of Earth Resources at Colorado State University

(CSU) under the directorship of Dr. Holly Stein (Chapter 5.0). The CSIRO also provided

funding through a Post-graduate Scholarship and access to the Proton Induced X-ray Emission

(PIXE) and Nuclear Microprobe (NMP) for trace element analyses and imaging in this thesis

and was supervised by Dr. Chris Ryan at the University of Melbourne (Chapter 6).


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1.7.1 Field investigation methods

During 2003, a preliminary field investigation and data scoping study for this thesis

was undertaken in the SMD (Cromie, 2003). Subsequent field seasons to the SMD to collect

research data was conducted by the author and documented in reports during three visits in:

(1) September and October 2003 (Cromie, 2004a), (2) May and June 2004 (Cromie, 2004b)

and, (3) April and May 2005 (Cromie, 2005). A total of 7796 m of diamond drill core from 70

drill holes was reviewed and sampled during field investigations by the author, from which 34

diamond drill holes were selected and logged in detail for a combined drill hole total core

length of 4190 m (Appendix 1.1). The drill holes listed in Appendix 1.1 were primarily chosen

along 25 N-S oriented cross-sections across the gold and copper deposits in the SMD to

provide the subsurface geological information presented in this thesis. Previous drilling by Rio

Tinto and Oxiana from 1993 to 2005 was predominantly conducted through the oxide and

partial oxide zones in the SMD. Sampling was required along a large number of cross-sections

during this thesis project to obtain suitable coherent rock-types for petrological and

geochemical analyses. Appendix 1.2 lists the samples collected during this study.

District- and deposit-scale geological maps and sections were provided for this study

by the LXML exploration and mine geology groups at Sepon. Detailed 1:500 scale open-pit

mapping and structural measurements was conducted by the author in accessible areas at the

Nalou (East), Discovery West, Discovery Colluvial and Discovery Main gold deposits. All of

the local SMD maps, sections and sample locations presented in this thesis are all referenced to

the UTM India 1960 datum that is used by LXML exploration at Sepon.

1.7.2 Laboratory research methods

Paragenetic and ore-petrography studies were conducted by the author at CODES and

supplementary petrographic data was obtained from internal Rio Tinto and Oxiana Limited

Company reports that were principally undertaken by Comsti (1995, 1996, 1997, 1998a, b) and

APS (2004a, b, 2005). Whole rock geochemical analyses (XRF), laser ablation (LA) trace-

element analyses (LA-ICP-MS) on sulphides, LA radiogenic lead isotope analyses on pyrite

and fluid inclusion analyses were conducted at CODES. U-Pb dating of zircon from SMD

intrusions was carried out at CODES using the LA-ICP-MS method. Laser ablation and

conventional sulphur isotope analyses were conducted at the Central Science Laboratory

(CSL), UTAS. Conventional carbon and oxygen isotope analyses were also undertaken at CSL.

Comparative laser ablation (LA) multi-collector lead isotope analyses were conducted at the

School of Earth Sciences, University of Melbourne. Preliminary research results were provided

to sponsors and supervisors as progress reports in Cromie (2003, 2004a, b; 2005, 2006, 2007).


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1.8 THESIS STRUCTURE AND CONVENTIONS

In addition to this introductory chapter, the thesis has been divided into the following

chapters:

Chapter 2 introduces (a) the interpreted tectonic setting of Indochina with emphasis on

the Truong Son Fold Belt, and (b) the regional geology of Laos.

Chapter 3 describes the district-scale geological setting of the Sepon Mineral District

(SMD). This chapter also provides thirteen new U-Pb age dates from zircons that

constrain the emplacement and timing of rhyodacite-porphry (RDP) intrusions in the

SMD and also two new U-Pb age dates from zircons extracted from granite samples

collected adjacent to the SMD. The associated whole rock and trace element chemistry

of RDP and granite intrusions investigated during this study are also presented.

Chapter 4 describes the deposit-scale geological setting and controls on mineralisation

for the gold and copper deposits studied in the SMD. Additional detailed mapping and

structural measurements produced during this study from the Discovery -Main,

-Colluvial, -West gold deposits and the Nalou gold deposit are also presented.

Chapter 5 provides new detailed descriptions of the alteration and ore-mineral

paragenesis of the gold and copper deposits in the SMD. The first direct dating of

sulphide mineralisation occurring in the SMD is also presented from new Re-Os

geochronology analyses conducted to determine the age of molybdenite, constraining

the timing of intrusion related retrograde skarn Cu-Mo mineralisation. The chapter

concludes with a comparison of the mineralogical similarities and differences between

the SMD gold and copper deposits.

Chapter 6 presents the results of LA-ICP-MS and PIXE NMP trace element

investigations used to constrain where gold occurs and concludes with a discussion of

the trace element associations in the SMD gold and copper systems.

Chapter 7 describes the physicochemical environment of ore deposition in the SMD

gold and coper deposits, based on the results of stable isotope (sulphur, oxygen, and

hydrogen), radiogenic isotope (lead) and fluid inclusion studies. The nature and

characteristic of the SMD ore fluid chemistry is also compared and contrasted with

other SHDG systems such as the Carlin and Chinese deposits in this chapter.

Chapter 8 concludes the thesis by presenting a genetic model for the formation of gold

and copper deposits in the SMD. In particular, the genesis of the SMD sedimentary

rock-hosted gold deposits (SHGD) and skarn-related copper deposits are discussed in

the model. Implications for exploration based on aspects of the genetic model are also

discussed, including recommendations for further research.

Chapter 2 – Regional Geological Setting

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CHAPTER 2: REGIONAL GEOLOGICAL SETTING

2.1 INTRODUCTION

The SMD is located in the Sepon Basin and occurs along the NW-trending Truong Son

Fold Belt on the NE-margin of the Indochina Terrane in south-central Laos (Figs. 1.1 and

2.2.1). In this Chapter, a literature review of the regional geological setting is presented to

provide a geological framework for the SMD district-scale geology presented in Chapter 3.

The principal aims of Chapter 2 are to outline: (a) the tectonic setting of the Indochina Terrane

and Truong Son Fold Belt, and (b) the regional geology of Laos.

2.2 TECTONIC SETTING

The tectonic setting of mainland Southeast Asia including the Indochina Terrane was

described by Bunopas and Vella (1983), Hutchinson (1989), Charusiri et al. (2002), Metcalfe

(1996a, 1996b, 1999), Stokes et al. (1996), Zhao et al. (1996), Lepvrier et al. (1997, 2004),

Wakita and Metcalfe (2005) and Sone and Metcalfe (2008). Fold belts comprised of Palaeozoic

sedimentary and volcanic rocks surrounding the Precambrian Indochina Terrane are reported to

host a range of hydrothermal deposits, especially porphyry, epithermal, skarn and sedimentary-

rock hosted ore-deposit types (Khin Zaw et al., 1999; Khin Zaw et al., 2007).

2.2.1 Principal tectonic components of Mainland Southeast Asia

The tectonic framework of mainland Southeast Asia is characterised by three major

allochthonous micro-plates (terranes) comprised of Precambrian and Phanerozoic rocks.

The Indochina Terrane (also called Indosinia) forms the eastern region of Southeast Asia (i.e.

encompassing Vietnam, Cambodia, Lao PDR, central and eastern Thailand) and is bound to the

north by the South China Terrane (also known as Cathaysia) and to the west by the Shan-Thai

Terrane (Fig. 2.2.1, Metcalfe, 1996a, 1996b; Zhao et al., 1996). The Shan-Thai Terrane is also

referred to as the Sibumasu Terrane and covers a broad area of Myanmar, western Thailand,

Malaysia and Sumatra (Metcalfe, 1999). The Indochina, Shan-Thai and South China Terranes

are separated by suture zones representing the closure of former oceans, identified by the

presence of ophiolites, major tectonic lineaments, accreted volcanic arcs and mobile belts

(Stokes and Smith, 1990). Both the Indochina and Shan-Thai Terranes are interpreted to have

originally been derived from the northwest margin of the Gondwana during the Early

Phanerozoic and later amalgamated with the rest of Asia during the Early Mesozoic

(Metcalfe, 1996a, 1996b, 1999).


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Fig. 2.2.1. Map showing the tectonic setting of mainland SE Asia and the present location of continental terranes (adapted from Hada et al., 1999; Metcalf, 1999; Singharaajwarapan and Berry, 2000). The South China, Indochina and Shan-Thai terranes are separated respectively by the sinistral Red River Fault, the Nan-Uttaradit, Sa Kaeo-Chathaburi and Bentong-Raub ophitic zones, and the dextral Sagaing Fault. The Sepon Mineral District (SMD) is shown as a red boxed area that is located along the Truongson Fold Belt on the NE margins of the Indochina Terrane.


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2.2.2 Indochina Terrane

The Indochina Terrane is an elongate cratonic block which can be subdivided into a

number of smaller geologically distinct terranes, namely (a) the Kontum Massif, is a Middle

Permian metamorphic core complex which contains high-grade gneiss and schist that are

mostly exposed in Vietnam and southern Laos (Workman, 1975; Leprivier et al., 2008), (b) the

Truong Son Fold Belt comprising a Palaeozoic volcano-sedimentary sequence along the north-

eastern margins of the Indochina Terrane in Laos, Vietnam and NE Cambodia (Hutchinson,

1989) and, (c) the Loei Fold Belt occurring on the western edges of the Indochina Terrane and

contains Permian and Triassic volcanics (Fig. 2.2.1; Hutchinson, 1989). These terranes are also

overlain by Mesozoic continental deposits belonging to the Khorat Group, mainly in Thailand,

Laos and NE Cambodia. The Song Ma Suture Zone, also referred to as the Ailao Shan-Red

River Fault Zone, is a major regional NW-trending sinistral strike-slip fault zone that bounds

the Indochina and South China Terranes along the northern margins of the Truong 1Son Fold

belt (Fig. 2.2.1; Leloup et al., 1995; Lan et al., 2001; Garnier et al., 2005). The Nan-(Uttaradit-

Sra Keo) and Bentong-Raub ophitic suture zones trend NNE along the Loei-Sukhothai Fold

belts and form the western boundary in Thailand between the Indochina and Shan-Thai

Terranes (Fig. 2.2.1; Hutchinson, 1989; Stokes et al., 1996; Metcalf, 1999).

2.2.2.1 Kontum Massif

Publications by Workman (1975), Fontaine and Workman (1978), Hutchinson (1989),

Lepvrier et al. (1997); Lepvrier et al. (2004), Maluski et al. (2005) and Lepvrier et al. (2008)

summarise the geology of the Kontum Massif. The Massif is composed of at least three main

metamorphic complexes, namely the: Kannack and Ngoc Linh complexes and the Poko

Formation (Fontaine and Workman, 1978; Hutchinson, 1989; Lepvrier et al., 2004). Basement

rocks occupying the central and southern portions of the Kontum Massif are represented by the

Kannack metamorphic complex, comprised of two-pyroxene gneiss, cordierite-sillimanite

gneiss, migmatite, charnockitic rocks, calc-silicate rocks and marble. The Ngoc Linh complex

is widely distributed in the Kontum Massif and mostly composed of low- to intermediate

pressure amphibolite metamorphic facies rocks (Hutchinson, 1989). It is composed of biotite-

sillimanite gneiss, amphibolite, biotite schist, migmatite schist, mica schist and marble. The

northern and eastern sections of the Kontum Massif are represented by the Poko Formation

composed of greenschist facies metaquartzite, sericitic shale and dolomite (Hutchinson, 1989;

Lepvrier et al., 2004).


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2.2.2.2 Truong Son Fold Belt

The Truong Son Fold Belt, also known as the Anamite Cordillera, consists of an

elongated mountain system comprised of Palaeozoic sedimentary and volcanic rocks in a NW-

trending belt that extends along the eastern margins of the Indochina Terrane, from central

Vietnam to northern Laos (Fig. 2.2.1; Hutchinson, 1989; Leprivier et al., 1997). The Palaeozoic

regional geology of this fold belt is comprised of conglomerate, arkosic and feldspathic

sandstone, tuffaceous sandstone, shale, variably calcareous and carbonaceous siltstone and

variably dolomitised limestone. Plutonic bodies of granite, granodiorite, monzodiorite, quartz

porphyry, and rhyodacite porphyry, including sub-volcanic andesite porphyry, also intrude the

belt and are emplaced along EW- to WNW-trending major structures (Hutchinson, 1989).

The Truong Son Fold Belt is interpreted to have been developed from a Palaeozoic

volcano-sedimentary arc in the Palaeo-Tethys Ocean (Fontaine and Workman, 1978;

Hutchinson, 1989). During the Early Palaeozoic to Devonian, deep-water marine sedimentary

rocks up to 10,000m thick were deposited in a region named the Truong Son mobile zone

(Fontaine and Workman, 1978). A volcanic arc also occupied the Truong Son mobile zone

during the Ordovician to Devonian, contributing volcanic rocks (andesite and tuffs) to the

associated sedimentary rocks. Silurian volcanic rocks and Devonian plutonic rocks (granite)

along the Truong Son mobile zone are inferred (Hutchinson, 1989) to have been derived from a

subduction zone dipping south-westerly under the Indochina Terrane. Both I and S type

plutonic rocks intrude Ordovician-Devonian rocks along the Truong Son Fold Belt with

magmatic periods reported by Hutchinson (1989) at 377 Ma (Devonian) and 330 Ma (Early

Carboniferous).

Closure of the Truong Son mobile zone between the Indochina and the South China

Terranes during the Carboniferous is interpreted by Fontaine and Workman (1979) to have

resulted in uplift and the development of folded sedimentary sequences, forming a regional

NW-trending anticlinorium referred to as the Truong Son Fold Belt that occurs along the

present day border between Vietnam and Lao PDR (Fig. 2.2.1). During the Permian, shallow

marine areas along the Truongson Fold Belt were dominated by limestone deposition with

intercalations of andesitic volcanic rocks, overlain by shallow marine clastic rocks of Early

Triassic age (Hutchinson, 1989).

2.2.2.3 The Loei and Sukhothai Fold Belts

The boundary between the Shan-Thai and Indochina Terranes is marked by two

parallel north-trending fold belts in central and western Thailand, namely the Loei Fold belt,

and the Sukhothai Fold belt. The Nan (-Uttaradit) and Sra Kaeo (-Chanthaburi) ophiolite zone

form a sutured boundary between these two fold belts (Fig. 2.2.1; Hutchinson, 1989; Stokes et

al., 1996). Jurassic continental red beds belonging to the Khorat Group unconformably overlie


16

both these fold belts (Hutchinson, 1989). In central Thailand, the Loei Fold Belt occurs along

the western margin of the Indochina Terrane and is composed of a Devonian volcano-

sedimentary sequence overlain by deep water chert, which in turn is covered by Carboniferous

to Permian age fossiliferous limestone (Fig. 2.2.1; Bunopas and Vella, 1983). Permian volcanic

rocks are known along this fold belt and older volcanic and sedimentary rocks are intruded by

Triassic granite, granodiorite and diorite with 235 Ma 40Ar/39Ar ages (Hutchinson, 1989).

The Sukhothai Fold Belt is composed of a Cambrian to Triassic age sequence of

sedimentary and volcanic rocks, occurring along the eastern margins of the Shan-Thai Terrane

in a north-trending belt in central and western Thailand (Fig. 2.2.1). Cambro-Ordovician age

conglomerate, metasandstone and shale are overlain by Ordovician limestone, dolomite and

calcareous shale. Deformed sequences of Permo-Carboniferous rocks include volcanic and

volcanoclastic rocks intercalated with meta-greywacke and minor phyllite (Bunopas and Vella,

1983; Hutchinson, 1989). Post-tectonic Triassic age granite, granodiorite and diorite intrude

older sequences along this fold belt (Singharawarapan, 1994).

2.2.3 Tectonic evolution of the Indochina Terrane

At present, the tectonic evolution of plate geometry and the timing of suturing in

Southeast Asia is poorly understood, in particular for the Indochina Terrane. Published

stratigraphic, palaeobiological and geochronology data indicate that both the Indochina and

Shan-Thai Terranes were derived from the Australia-India margin of Gondwanaland during the

Lower Palaeozoic (Metcalf, 1996a, 1996b, 1999; Bunopas and Vella, 1983; Hutchinson, 1989;

Burrett et al., 1990). During the Carmbrian-Ordovician, the Indochina and Shan-Thai Terranes

along with the North and South China, Tarim and Qiadam Terranes, collectively known as the

Southeast Asian Terranes, are interpreted to have formed along the northern margin of

Gondwana (Metcalfe, 1999).

The first main period of rifting and continental separation forming a Palaeo-Thethys

Ocean between the Southeast Asian Terranes and Gondwana during the Devonian is recorded

by a widely distributed Late Devonian–Early Carboniferous unconformity on most of the

Southeast Asian Terranes (Metcalfe, 1999). Alternatively, Hutchinson (1989) and Charusiri et

al. (2002) proposed that rifting of the Asian Terranes from Gondwana may have been initiated

earlier during the Silurian-Devonian period (Fig. 2.2.2). However, distributions of vertebrate

fossils also record the proximity of the Asian terranes to Gondwana during the Devonian (Long

and Burrett, 1989; Metcalfe, 1999).


17

Fig. 2.2.2. Sections through time showing tectonic development of the Indochina Terrane during the Silurian to Permian period (adapted from Hutchinson, 1989).


18

The absence of Gondwana-related fossils in the stratigraphy of the Tibetan and Shan-

Thai Terranes, which were located at equatorial latitudes during the Carboniferous-Permian,

indicate that rifting and separation of these two terranes from Gondwanaland occurred during

this period (Figs. 2.2.2 and 2.2.3; Metcalfe 1999; Sashida and Igo, 1999). Faunal assemblages

of Late Palaeozoic and Mesozoic ages that occur on both the Indochina and Southern China

Terranes indicate amalgamation of these two terranes at equatorial latitudes by the Early

Carboniferous (Fig. 2.2.3; Metcalfe, 1999). Geochronology (40Ar/39Ar) and Nd isotopic data

collected from plutonic rocks along the Song Ma suture indicate closure of the Palaeo-Tethys

Ocean and amalgamation of the Indochina and South China Terranes by the Early Triassic (250

Ma) during the Indosinian Orogeny (Fig. 2.2.3; Leprivier, 1997; Lan et al., 2001).

After suturing, the Indochina Terrane and in particular the Vietnam region was affected

by extensive intra-plate magmatism during the Late Jurassic to Cretaceous (145-75 Ma), which

also corresponds to a period of lithospheric relaxation and extension recorded in the South

China Terrane during the Yanshanian Orogeny (Lan et al., 2001; Hutchinson, 1989). During the

Early Tertiary (Himalayan Orogeny) the Indochina Terrane is interpreted to have commenced

moving southeast along the Song Ma suture zone, with a left-lateral fault movement of ~600

km, as determined from the displacement of Mesozoic and Tertiary magmatic suites exposed

along the suture zone (Fig. 2.2.1; Lan et al., 2001).

Fig. 2.2.3. Reconstruction maps showing the Phanerozoic positions of the Indochina Terrane (red coloured area), commencing in the Early Carboniferous to Late Triassic (modified from Metcalfe, 2006). Abbreviations: I=Indochina, KAZ=Kazakhstan, L=Lhasa, NC=North China, S=Shan-Thai (Sibumasu), QI=Qiangtang, QS=Qamdo-Simao, SC=South China, T=Tarim, WB=West Burma, WC=Western Cimmerian.


19

2.2.4 Mineralisation epochs along the margins of the Indochina Terrane

A diverse array of deposits and prospects containing gold and/or copper dominant

mineralisation occur within both the Loei and Truong Son Fold Belts, along the margins of the

Indochina Terrane (Fig. 2.2.5). Mineralisation styles along these two fold-belts are varied and

include: copper- and gold-rich porphyry types with associated skarn styles, low-sulphidation

epithermal and sedimentary-rock hosted gold deposits with Carlin-type affinities (Table 2.1).

At least four main metallogenic epochs of mineralisation have been reported by Chausiri

(1989), Khin Zaw et al. (1999a,b), Salam et al. (2004), Khin Zaw et al. (2007) and Meffre and

Khin Zaw (2007), with mineralisation peaks in the Early Permian, Early-Late Triassic, Late

Triassic-Early Jurassic, and after the Early Jurassic periods. Only one mineralisation event has

been recorded in the literature for the Truong Son Fold Belt by Loader (1999) who reported an

Early Permian age for intrusions inferred to be associated with copper mineralisation at Sepon.

A brief description of the Phanerozoic mineralisation events surrounding the Indochina Terrane

follows, with the location of deposits, prospects and associated geological characteristics

summarised in Table 2.1, Figs. 2.2.4 and 2.2.5.

2.2.4.1 Early Permian mineralisation (300 - 250 Ma)

Mineralisation that developed along the Indochina Terrane margins during the Permian

period consisted of Au-rich porphyry Cu-, skarn- and epithermal styles. The Phu Kham Cu-Au

deposit, located along the north-easterly extent of the Loei Fold Belt and near the junction with

the Truong Son Fold Belt in northern Laos, is an example of porphyry- and skarn-associated

Cu-Au mineralisation hosted by Early Permian porphyry intrusions with ages ranging from 295

+ 4 Ma to 275 + 6 Ma (Fig. 2.2.4; Table 2.1; Backhouse, 2004). The timing of these Permian

intrusions at Phu Kham also overlaps with the 290 Ma emplacement age of a rhyodacite-

porphyry intrusion in the SMD reported by Loader (1999).

2.2.4.2 Late Permian to Late Triassic (250 - 220 Ma)

The Early to Late Triassic period in central Thailand is characterised by Au-, Cu-, Fe-

porphyry and skarn-style mineralisation, including epithermal Au-Ag and vein-hosted Cu-Pb-

Zn mineralisation in association with oxidised granite (I-type). A Late Permian to Early

Triassic age was reported for the intermediate felsic volcanics (250 + 6 Ma) hosting low-

sulphidation epithermal Au mineralisation at the Chatree gold deposit in the Petchabun region,

Loei Fold Belt, central Thailand (Fig. 2.2.5; Diemar and Diemar, 1999; Cumming et al., 2007;

Salam et al., 2007; Meffre et al., 2008). Intrusion-associated Triassic mineralisation is reported

for the Phu Thap Fah (244 + 4 Ma), Phu Lon (244 + 3 Ma) and Puthep (248 + 6 Ma) deposits

located in the Loei Fold Belt (Fig. 2.2.5; Sitthithaworn, 1993; Kamvong et al., 2006a, 2006b).

The Wang Yai gold Prospect in central Thailand studied by De Little (2005) is an example of

epithermal mineralisation in this period (Fig. 2.2.5; Table 2.1).


20

2.2.4.3 Late Triassic to Jurassic (220 - 200 Ma)

Skarn-style Cu-Fe-Au (+Pb-Zn) is typically associated with Late Triassic to Middle

Jurassic deposits along the Loei Fold Belt in Thailand (Meffre and Khin Zaw, 2007). The

Frenchmen Mine Au-Cu skarn deposit investigated by Muller (1999) is associated with diorite

intrusions (Fig. 2.2.5). Charusiri (1989) also reported Sn-W (REE) mineralisation associated

with reduced granites in the Shan-Thai Terrane, western Thailand.

2.2.4.4 Post Jurassic (<200 Ma)

During the Late Cretaceous (80-65 Ma) reduced granite plutons are reported to intrude

the Shan-Thai Terrane and host Sn-W mineralisation, and are also associated with Sb-W-Au

mineralisation along the Sukhothai Fold Belt (Charusiri, 1989). Younger and lesser known

mineralisation periods occurring in the Shan-Thai Terrane, Sukhothai Fold Belt and Loei Fold

Belt mentioned by Charusiri (1989) include Palaeocene to Eocene (60-50 Ma) reduced granites

with pegmatite dikes containing Sn-Ta-Nb (and W), and Middle Eocene to Early Miocene

(45-20 Ma) reduced granites containing associated W-Sn dominant mineralisation.

Fig. 2.2.4. Location map of known mineral deposits along the margins of the Indochina Terrane (adapted after Kamvong et al., 2006). Deposit abbreviations: SMD = Sepon Mineral District, BM = Bong Mieu, PS = Phuoc Son, PK = Phu Kham, LCT = Long Chien Track, BH = Ban Houayxai, PL = Phu Lon, PT1 = Puthep 1, PT2 = Phuthep 2, PF = Phu Thap Fah, WY = Wang Yai, CT = Chat Tree, FM = French Man.


21

De

po

sit

Lo

cati

on

Ho

st r

oc

ks /

(ag

es)

Intr

us

ion

s / (

ages

)A

lter

atio

n t

ypes

Ore

Typ

e /

Min

eral

og

yT

on

nag

eR

efs

Sep

on

(SM

D)

(A

u, C

u)

Laos

:

16o

58'

N,

105

o 5

9' E

Sa

ndst

one

, mu

dsto

ne,

calc

are

ous

shal

e,

limes

tone

, dol

omite

(O

rdo

vici

an -

Car

bon

ifero

us)

Rhy

odac

ite P

orph

yry

(Pe

rmia

n -

Car

boni

fero

us)

(A)

Sed

imen

tary

roc

ks:

dec

alci

ficat

ion,

sili

cific

atio

n,

dol

omiti

satio

n, s

karn

,

(B)

Intr

usio

ns:

pota

ssic

, p

ropy

littic

, ph

yllic

, ska

rn

(A)

SH

GD

: pyr

ite, g

old

, sp

hale

rite,

ga

lena

(B

) S

karn

: cha

lcoc

ite,

chal

copy

rite

, bor

nite

4.7

5 M

oz A

u fo

r 8

3 M

t @

1.

8 g/

t A

u;

2

Mt

Cu

met

al

(2);

(7);

(9);

(1

3)

Bon

g M

ieu

(B

M)

Vie

tnam

(c

entr

al),

Q

uan

g

Nam

P

rovi

nce

Me

tam

orph

ose

d fa

cies

:

se

rici

te-b

iotit

e s

chis

t,

bi

otite

-sili

man

ite g

neis

s,

quar

tz-f

eld

spar

-bio

tite

sch

ist.

Gan

ite a

nd p

egm

atite

(U

ndi

ffer

ent

iate

d)P

otas

sic,

phy

llic,

ska

rn

(pro

gra

de a

nd r

etro

grad

e)

Intr

usio

n-as

soci

ated

sk

arn

: pyr

ite,

chal

cop

yrite

, py

rrho

tite,

sph

aler

ite,

gal

ena

, he

mat

ite,

mag

netit

e, b

ism

uth

0.6

1 M

oz A

u fo

r 5

Mt @

>

10

g/t

Au

(19)

;

Phu

oc S

on (

PS

)

Vie

tnam

(c

entr

al),

Q

uan

g

Nam

P

rovi

nce

Gre

ens

chis

t fa

cie

s m

etas

edim

ents

and

inte

rcal

ated

m

etav

olca

nic

s

(Pre

cam

bria

n-C

ambr

ian)

Gra

nite

s (D

evo

nia

n,

Pe

rmia

n, T

riass

ic);

G

abb

ro a

nd d

iorit

e (P

han

eoro

zoic

)

(A)

Met

apel

ite r

ocks

:

se

rici

te-b

iotit

e-al

bite

;

(B

) M

etab

asite

s: c

hlo

rite-

alb

ite-a

ctin

olite

-epi

dote

Intr

usio

n-as

soci

ated

: p

yrrh

otit

e, p

yrite

, bis

mut

h,

spha

lerit

e, g

ale

na,

tellu

rium

, silv

er,

gold

0.2

1 M

oz A

u fo

r 0.

5 M

t @

>

12

g/t

Au

(20)

;

Phu

Kha

m (

PK

)

(C

u, A

u)

Laos

:

18o

55'

N,

102

o 5

5' E

Vol

can

icla

stic

s an

d

inte

rbed

ded

lim

esto

ne

(C

arbo

nife

rous

- E

arly

Per

mia

n)

Dio

rite

por

phyr

y

(Ear

ly P

erm

ian)

Pot

assi

c, p

hylli

c, s

karn

(p

rog

rade

and

ret

rogr

ade)

Po

rph

yry-

rela

ted

skar

n

(oxi

dis

ed):

cha

lcop

yrite

, b

orni

te,

tetr

ahe

drite

, gol

d

108

Mt @

0.

8% C

u,

0

.3 g

/t A

u

(1);

(5);

(1

1);

Ban

Hou

ayxa

i (B

H)

(A

u-A

g)

Laos

:

18o

56'

N,

102

o 5

0' E

Silt

ston

e a

nd v

olc

anic

last

ics

(Ear

ly P

erm

ian)

Fe

ldsp

ar p

orp

hyr

y

(E

arly

Per

mia

n)Q

uart

z, s

eric

ite, c

hlor

ite,

pyrit

e,

clac

ite,

adul

aria

Epi

the

rma

l: el

ectr

um

, ch

alco

pyrit

e, te

trah

edrit

e sp

hale

rite,

ga

lena

No

Da

ta(1

7);

Lon

g C

hien

g T

rack

(LC

T)

(A

u)

Laos

:

18o

56'

N,

102

o 5

3' E

Vo

lca

nicl

astic

s an

d

in

terb

edde

d li

mes

tone

(Ca

rbo

nife

rous

- P

erm

ian)

Mon

zoni

te p

orph

yry

(E

arly

Per

mia

n)Q

uart

z, s

eric

ite, c

hlor

ite,

pyrit

e,

clac

ite,

adul

aria

Epi

the

rma

l: el

ectr

um

, ch

alco

pyrit

e, te

trah

edrit

e sp

hale

rite,

ga

lena

0.62

Mt @

0.

96 g

/t A

u(1

7);

Phu

Lon

(P

L)

(C

u, F

e, A

u)

Tha

iland

:

18o

12'

N,

102

o 0

8' E

Lim

esto

ne

and

Vol

cani

clas

tics

(L

ate

Dev

oni

an)

Dio

rite

and

qua

rtz

mon

zoni

te

porp

hyry

(T

riass

ic)

Pot

assi

c, p

ropy

litic

,

ph

yllic

, ska

rn

(p

rog

rade

and

ret

rogr

ade)

Por

phy

ry-r

elat

ed s

karn

(o

xid

ised

): c

halc

opyr

ite,

born

ite, m

agn

etite

, gol

d

5.4

Mt @

2.4%

Cu,

0.64

g/t

Au

(in

ferr

ed)

(6);

(1

0);

(14)

TA

BL

E 2

.1A

. Ge

olo

gic

al c

har

act

eri

sti

cs o

f kn

ow

n m

ine

ral

de

po

sit

s o

ccu

rrin

g a

lon

g t

he

ma

rgin

s o

f th

e In

do

chin

a T

err

ane

Ref

eren

ce to

num

bers

(R

efs)

sho

wn

in T

able

2.1

: (1)

Bac

khou

se (

2004

); (

2) C

rom

ie e

t al.

(200

6);

(3)

Die

mar

and

Die

mar

(19

99);

(4)

Khi

n Z

aw e

t al.

(200

7); (

5) K

amvo

ng e

t al.

(200

6a);

(6)

Kam

vong

et a

l. (2

006b

); (

7) M

anin

i et a

l. (2

001)

; (8)

Mul

ler

(199

9); (

9) S

mit

h et

al.

(200

5); (

10)

Sit

hith

awor

n (1

993)

; (11

) T

ate

(200

5); (

12)

Rod

man

ee (

2000

); (

13)

Olb

erg

et a

l. (2

006)

; (1

4) M

eine

rt e

t al.

(200

5); (

15)

Sal

am e

t al.

(200

7); (

16)

Cum

min

g et

al.

(200

7); (

17)

Man

aka

(200

8); (

18)

De

Lit

tle

(200

5); Q

uynh

et a

l. (2

004)

; Ban

ks e

t al.

(200

4).


22

Dep

osi

tL

oc

atio

nH

ost

ro

cks

/ (a

ges

)In

tru

sio

ns

/ (a

ges

)A

lter

atio

n t

ypes

Ore

Typ

e / M

iner

alo

gy

To

nn

age

Ref

s

Put

hep

(PT

1)

(Cu)

Tha

iland

:

17o

28'

N,

101o

52'

E

Sili

cicl

astic

s a

nd L

imes

tone

(C

arbo

nife

rous

)D

iorit

e a

nd q

uart

z m

onzo

nite

p

orph

yry

(Tria

ssic

)P

otas

sic,

pro

pyl

itic,

phy

llic,

sk

arn

(pro

grad

e an

d re

trog

rade

)

Por

phyr

y-re

late

d sk

arn

(oxi

dise

d):

chal

copy

rite,

py

rite,

ma

gnet

ite

85

Mt @

0.4

% C

u,

(in

ferr

ed)

(5);

Put

hep

(PT

2)

(Cu)

Tha

iland

:

17o

26'

N,

101o

46'

E

Sili

cicl

astic

s a

nd L

imes

tone

(C

arbo

nife

rous

)D

iorit

e a

nd q

uart

z m

onzo

nite

p

orph

yry

(Tria

ssic

)P

otas

sic,

pro

pyl

itic,

phy

llic,

sk

arn

(pro

grad

e an

d re

trog

rade

)

Por

phyr

y-re

late

d sk

arn

(oxi

dise

d):

chal

copy

rite,

py

rite,

ma

gnet

ite

36

Mt @

0.43

% C

u,

(in

ferr

ed)

(5);

Ph

u T

hap

Fha

(P

F)

(A

u)

Tha

iland

:

17o

56'

N,

101o

40'

E

Sili

cicl

astic

s a

nd L

imes

tone

(P

erm

ian)

Gra

nod

iorit

e

(Tria

ssic

)P

rogr

ade

and

retr

ogra

de

skar

n as

sem

bla

ges

Ska

rn (

redu

ced)

: el

ect

rum

, Au

, bis

mut

h,

tellu

ride,

cha

lcop

yrite

, py

rrho

tite

0.7

Mt @

7.9

7 g/

t Au,

(in

dica

ted)

(4);

(12)

Cha

tree

(C

T)

(A

u)

Tha

iland

:

16o

17'

N,

100o

39'

E

Vo

lca

nicl

ast

ics

(Per

mia

n -

Ea

rly T

riass

ic)

Hor

nble

nde

dio

rite

dyk

e

(E

arly

Tria

ssic

)Q

uart

z, p

yrite

, ca

laci

te,

adul

aria

chl

orite

, se

ricite

Epi

ther

mal

: ele

ctru

m,

min

or c

halc

opyr

ite1

.8 M

oz A

u

(1.8

g/t

Au)

(3);

(4

);

(15)

; (1

6)

Wan

g Y

ai (

WY

)

(A

u)

Tha

iland

:

16o

22'

N,

100o

38'

E

Vol

cani

clas

tics,

rhy

olite

(E

arly

Tria

ssic

)D

iori

te

(Ear

ly T

riass

ic)

Qua

rtz,

pyr

ite, c

ala

cite

,

ad

ula

ria c

hlor

ite,

seric

iteE

pith

erm

al: e

lect

rum

, ar

gent

ite, c

halc

opyr

iteN

o D

ata

(18)

;

Fre

nchm

en (

FM

)

(A

u, C

u)

Tha

iland

:

13o

57'

N,

101o

49'

E

Vo

lcan

ocla

stic

s an

d

in

terb

edde

d li

mes

tone

(Low

er P

erm

ian)

Gra

nod

iorit

e

(La

te T

rias

sic)

Pro

grad

e an

d re

trog

rade

sk

arn

asse

mbl

age

s

Por

phyr

y-re

late

d sk

arn

(oxi

dise

d):

chal

copy

rite,

p

yrite

, m

olyb

den

iteN

o D

ata

(8);

TA

BL

E 2

.1B

. G

eolo

gic

al c

har

acte

rist

ics

of

kno

wn

min

era

l dep

os

its

occ

urr

ing

alo

ng

th

e m

arg

ins

of

the

Ind

och

ina

Te

rra

ne

Ref

eren

ce to

num

bers

(R

efs)

sho

wn

in T

able

2.1

: (1)

Bac

khou

se (

2004

); (

2) C

rom

ie e

t al.

(200

6);

(3)

Die

mar

and

Die

mar

(19

99);

(4)

Khi

n Z

aw e

t al.

(200

7); (

5) K

amvo

ng e

t al.

(200

6a);

(6)

Kam

vong

et a

l. (2

006b

); (

7) M

anin

i et a

l. (2

001)

; (8)

Mul

ler

(199

9); (

9) S

mit

h et

al.

(200

5); (

10)

Sit

hith

awor

n (1

993)

; (11

) T

ate

(200

5); (

12)

Rod

man

ee (

2000

); (

13)

Olb

erg

et a

l. (2

006)

; (1

4) M

eine

rt e

t al.

(200

5); (

15)

Sal

am e

t al.

(200

7); (

16)

Cum

min

g et

al.

(200

7); (

17)

Man

aka

(200

8); (

18)

De

Lit

tle

(200

5); Q

uynh

et a

l. (2

004)

; Ban

ks e

t al.

(200

4).


23

Fig. 2.2.5. Summary time-space plot showing stratigraphic columns with the currently known representative sequences of volcano-sedimentary and igneous rocks that occur in the Central Laos, Loei, Petchabun-Pitchit, Lopburi and Srae Keo regions (modified from Khin Zaw et al., 2007). The Central Laos stratigraphy also contains the Truong Son Fold Belt volcano-sedimentary sequence. (A) The SMD rhyodacite intrusions occurring along the Truong Son Fold Belt with associated copper-skarn associated mineralisation are also shown to occur at 290 Ma (Early Permian), as reported by Loader (1999). Other deposits in SE Asia containing intrusions formed during the Early Permian comprise: Phu Kham (PK), Ban Houxai (BH) and Long Chieng Track (LCT; Fig. 2.2.4 and Tables 2.1A-B). (B) The Early Triassic intrusions associated with mineralisation at deposits, include: Chat Tree (CT) and Wang Yai (WY; Fig. 2.2.4 and Tables 2.1A-B). (C) Deposits associated with Middle Triassic intrusions include: Phu Thap Fah (PF), Phu Lon (PL) and Puthep 1 and -2 (PT1 and PT2, respectively; Fig. 2.2.4 and Tables 2.1A-B). (D) Mineralisation formed at the French Man deposit is associated with a Late Triassic intrusion (Table 2.1B).


24

2.3 REGIONAL GEOLOGY OF LAOS

The regional geology of Laos is the least known of any country in the Indochina

region, but is reported by Workman (1990) to contain Proterozoic through to Quaternary aged

rock types that share similarities with those occurring in neighbouring countries, in particular

Vietnam, eastern Thailand and northern Cambodia. An introduction to the Laos regional

geology is presented in this section, with information predominantly sourced from publications

(Workman, 1975; Fontaine and Workman, 1978; Hutchinson, 1989; Workman, 1990; Lepvrier

et al., 1997; Vilaihongs et al., 1997; Bunyongkul and Charusiri, 2000; Fontaine, 2002; Lepvrier

et al., 2004; Maluski et al., 2005; Lepvrier et al., 2008). Insights into the Palaeozoic

stratigraphy of Laos are also sourced from petroleum exploration reports about the Savannakhet

Basin located southwest of the Sepon Basin in south-central Laos by Wilson and Glover

(1990); Stokes and Smith (1990), Cullen et al. (1990), Martin (1992), and Birch and Cullen

(1996). The published regional geology maps of Laos are poorly constrained by the paucity of

detailed mapping of the individual time periods during the Phanerozoic. Much of Laos

currently has areas mapped as geological time groupings and includes examples like Cambrian-

Devonian, Cambrian-Ordovician and Ordovician-Silurian (Fig. 2.3.1). However, the published

literature does describe the individual time periods during the Phanerozoic that are herein

presented.

2.3.1 Precambrian and Phanerozoic metamorphic rocks

Precambrian metamorphic rocks in Laos are reported to occur in two basement

complexes, namely the (1) Song Ma Massif, and (2) the Pak Lay (Chiang Saen) Massif

(Fig. 2.3.1; Stokes and Smith, 1990; Vilaihongs et al., 1997). The Song Ma Massif in north-

eastern Laos is a NW-trending belt of Late Proterozoic low-grade metamorphic rocks

comprised of mica schist, quartz-chlorite-sericite schist intercalated with marble and quartzite

(Fig. 2.3.1; Workman, 1975; Stokes and Smith, 1990). The Pak Lay Massif, located in north-

west Laos near the border with Myanmar, extends in a southerly direction into northern

Thailand (Fig. 2.3.1). The geochronology of the Pak Lay Massif is unconstrained, but is

composed of non-foliated leucocratic granite, tracts of gneiss and minor bands of biotite-

amphibolite schist (Stokes and Smith, 1990).

The Kontum Massif contains Early Triassic rocks comprising paragneiss and

orthogneiss with associated biotite-garnet-staurolite schist intercalated with amphibolite,

quartzite, marble and migmatite (Lepvrier et al., 1997; Maluski et al., 2005). Exposures of the

Kontum Massif are located near the western border of Vietnam in south-central Laos, extend

NW towards Sepon and occur south of the Sepon-Thakhek regional transform fault (Fig. 2.3.1;

Hutchinson, 1989; Stokes and Smith, 1990; Lepvrier et al., 1997; Lepvrier, 2004; Gatinsky,

2005; Maluski et al., 2005).


25

Fig. 2.3.1. Regional geology map of Laos showing the location of the SMD (blue rectangle). Map information was provided as a MAPINFO digital format courtesy of OZ Minerals Limited. Abbreviations for the Proterozoic basement complexes outlined by red dashed lines, as follows: KM = Kontum Massif; PL = Pak Lay Massif; SM = Song Ma Massif. The Sepon - Thathek Fault is represented by the black dashed line (S-T).

S-T

PL

KM

SM


26

2.3.2 Palaeozoic sedimentary rocks

2.3.2.1 Cambrian

Early Palaeozoic Cambrian rocks (Pz1) occur in the north-east of Laos, in the valley of

Song Ma near the Vietnamese border (Fig. 2.3.1; Fontaine and Workman, 1978). The Middle

Cambrian Song Ma Formation is a 1300m thick sequence comprised of conglomerate,

micaceous quartzose schist, amphibolite schist, quartzite and limestone. Late Cambrian rocks

are represented by the Samneua Formation, an 1100 m thick sequence comprised of mudstone,

shale and limestone (Workman, 1975; Stokes and Smith, 1990).

2.3.2.2 Ordovician

The Ordovician rocks (Pz1) in Laos are comprised of shale, sandstone and in places

limestone, attaining a total sequence thickness of up to 3000m and conformably overlying

Cambrian rocks (Fontaine and Workman, 1978). The Ordovician rocks occur mostly in

northern Laos, especially in Xieng Khouang Province and also near the border with Vietnam in

the east and south-east of Laos (Fig. 2.3.1). Fromaget (1927) divided the Ordovician

stratigraphy in the Nape district and the valley of Nam Nhuong, eastern Laos, into three

sequences: (1) Early: non-fossiliferous black shales; (2) Middle: fossiliferous shales containing

echinoderms and trilobites, and; (3) Late: sandstone containing large trilobites.

2.3.2.3 Silurian

Silurian sequences occurring along the northern and eastern margins of the Indochina

Terrane are primarily composed of shales and can attain a total thickness of up to 5000m in

eastern Laos and central Vietnam (Fontaine and Workman, 1978). The Silurian sequence

conformably overlies the Ordovician rocks, and trilobite dominant beds belonging to the Late

Ordovician sandstone define the boundary. The Late Silurian conformably underlies the Early

Devonian, but the boundary is not clear due to a paucity of fossil marker horizons. In

northwestern Laos in the area of the Plain of Jars near Ban Ban, the Silurian stratigraphy is

typically composed of shale, sandstone, and greywacke containing crinoids, trilobites and

brachiopods (Spirifer sulcatus) indicating Llandovery to Wendlockian age of deposition

(Fontaine and Workman, 1978; Stokes and Smith, 1990).

2.3.2.4 Devonian

Devonian stratigraphy (Pz2) occurs mostly in the north, east and south of Laos and can

attain up to 4000 m thick fossiliferous marine sequences at some localities (Fontaine and

Workman, 1978). The Precambrian Kontum Massif is interpreted to have remained as an

emergent island mass during a period of marine transgression, commencing in the Early

Devonian (Stokes and Smith, 1990). Subsequently, two main regions of marine deposition

developed in Laos during this period: (1) marine shelf facies within the NW-SE trending

Truongson mobile zone, along the eastern margins of the Kontum Massif, particularly in


27

Attapu, Saravan, Savannakhet and Khammouan Provinces, and; (2) basinal flysch facies along

a NE-SW trending zone in northern Laos from Pak Lay Province to Xieng Khouang Province

(Fig. 2.3.1; Stokes and Smith, 1990).

In eastern Laos, Early Devonian shallow brackish-water marine facies preserved along

the present day Truongson Fold Belt comprise sandstone, shale, calcareous shale, marl and

minor intercalated sandy limestone and limestone containing brachiopod and coral fossils.

Middle Devonian stratigraphy occurring in a NE-SW trending belt from Pak Lay northwards

comprises sandy shale, shale, with intercalations of calcareous shale, chert and minor limestone

containing stromatoporoids, corals and brachiopods (Fontaine and Workman, 1978; Workman,

1990; Stokes and Smith, 1990). At several locations, Devonian limestone units are reported by

Workman (1990) to be metamorphosed and recrystallized to marble.

2.3.2.5 Carboniferous

Carboniferous rocks (Pz3) mostly occur in northern and eastern Laos and are

predominantly discordant with the underlying Devonian marine facies rocks, where the

boundary is marked terrestrial facies deposited during a marine regression period in the

Carboniferous (Fontaine and Workman, 1978). During the Carboniferous, the Kontum Massif

is interpreted to have remained emergent, forming the Proto-Indosinia continental fragment

with variable compositions of sandstone, shale, chert, coal and limestone (Stokes and Smith,

1990). In eastern Laos the Carboniferous stratigraphy attains thicknesses of between 1000 m

and 2000 m and commonly contains foraminifera fossils (Fontaine and Workman, 1978).

Terrestrial deposits with Carboniferous coal beds occur mostly near Vientiane and Saravane in

northern Laos (Workman, 1990).

2.3.2.6 Permian

Permian (Pz3) stratigraphy conformably overlies post-Cambrian sequences in northern

and central Laos, but covers large areas due to Early Permian marine transgressions beyond the

original Carboniferous basin boundaries (Fig. 2.3.1; Fontaine and Workman, 1978). Early to

Middle Permian sedimentary rocks are variable and include limestone, chert, shale, sandstone,

conglomerate, coal and in places include intercalations of andesite lavas and tuffs. Sequences of

Permian rocks can attain up to 2,500 m thickness. Permian coal beds predominantly occur in

the far north at Phong Saly and limestone karsts are commonly present in the west and clastic

rocks occur mostly in the south of Laos (Fig. 2.3.1). Abundant fossils occur in the Permian

limestone sequences and include: corals, algae, brachiopods and foraminifera. The Late

Permian sequences are marked by marine regression facies, during which period bauxite

occurrences in Laos are interpreted to have developed in hot and humid conditions (Fontaine

and Workman, 1978; Stokes and Smith, 1990; Workman, 1990).


28

2.3.3 Mesozoic sedimentary rocks

Mesozoic rocks occur throughout Indochina and consist of two major facies:

(1) Middle Triassic to Early Jurassic non-oxidised marine facies developed in restricted

sedimentary basins and; (2) Triassic to Cretaceous continental facies referred to as red-bed

facies (Workman and Fontaine, 1978; Hutchinson, 1989; Stokes and Smith, 1990). During the

Mesozoic there is an overall vertical transition from marine to continental facies with inter-

fingering of the two facies types, especially towards the Late Triassic sequences where

continental facies predominate. Mesozoic sequences overlying the Kontum Massif are

collectively known as the Khorat Group and occur mostly in eastern Thailand and southern

Laos (Fig. 2.3.1; Fontaine and Workman, 1978). Stratigraphic discontinuities between basal

Permian sequences and Middle to Late Triassic sequences mark the base of Mesozoic rocks

occurring in Laos (Stokes and Smith, 1990).

2.3.3.1 Triassic

Middle- to Late Triassic marine sequences comprised of limestone, sandstone and

siltstone mostly occur in northern Laos, particularly in the Sam Nua district (Fig. 2.3.1;

Workman, 1990). During the Late Triassic, folding and uplift along the northern margins of the

Indochina Terrane resulted in Marine regression. Subsequently, the erosion of mountain ranges

commenced the formation of continental red-bed facies composed of sandstone and

conglomerate that covered southern Laos during the Late Triassic to Cretaceous (Fig. 2.3.1;

Fontaine and Workman, 1978; Workman, 1990).

2.3.3.2 Jurassic to Cretaceous

Early Jurassic (Mz2) rocks occur in southern and central Laos and are mostly

continental red-bed facies known as Terrane Rouge, comprising purplish-red sandy shales,

sandstone, conglomerate, and in places, intercalations of minor limestone lenses and gypsum

beds (Fig. 2.3.1; Fontaine and Workman, 1978; Stokes and Smith, 1990). Calcareous shales

interbedded with red-beds of Early Jurassic age containing horizons yielding Plesiosaur fossils

have also been reported in Laos, especially near the Sepon district (Stokes and Smith, 1990).

The youngest known marine sequences in Laos occur along the Sekong Valley near the border

with Cambodia and are Early Jurassic in age (Fig. 2.3.1; Workman, 1990).

2.3.3.3 Cretaceous

Early Cretaceous sequences conformably overly Jurassic rocks in Laos and can be up

to 2000m thick (Stokes and Smith, 1990). During the Cretaceous (Mz3) widespread deposition

of red-coloured continental facies occurred mostly in northern and central Laos, comprising

mud, silt and fine sands inter-dispersed with evaporate units (Fig. 2.3.1; Workman, 1990).

Occurrences containing 100 - 300m thick units of gypsum and rock-salt interbedded with

Cretaceous sedimentary rocks are known in the Savannakhet Basin, southern Laos and also


29

near Vientiane in northern Laos (Stokes and Smith, 1990). Cretaceous dinosaur fossil remains

are also reported to be preserved at several localities in southern Laos, in particular:

Mandchurosaurus, Titanosaurus and Hadrosaurus, and also include theropods, turtules and

crocodiles (Stokes and Smith, 1990). The Cretaceous period marks the waning stages of red-

bed deposition for the Indochina Terrane (Workman, 1990).

2.3.4 Cenozoic

Early Tertiary rocks belonging to the Palaeogene have not been reported in Laos. Late

Tertiary sequences formed during the Neogene occur mostly in northern Laos and are derived

from terrestrial freshwater deposits located in small inter-montane valleys, comprising

conglomerate, sandstone, shale, carbonaceous mudstone, marl and lignite. Deposition of fluvial

sands and gravels occurred along and towards the Mekong River in western Laos during Late

Tertiary uplift and the subsequent erosion of highland areas. During the Quaternary, fluvial

terraces comprised of gravels, sands and silts, including loess and ash deposits developed in the

northern Laos valleys and along the Mekong River (Workman, 1990; Stokes and Smith, 1990).

2.3.5 Volcanic Activity

Three periods of Late Palaeozoic to Mesozoic volcanism are recognised in Laos: (1)

Silurian to Carboniferous, (2) Permian to Triassic, and (3) Cenozoic (Fontaine and Workman,

1979; Stokes and Smith, 1990). Silurian to Carboniferous volcanic rocks occur mostly in Pak

Lay Province, northern Laos, are contemporaneous with sedimentary rocks and comprise

bedded tuffs, dolerite dikes and some lava flows (Stokes and Smith, 1990).

Sequences of Permian age volcanic rocks comprising andesite, dacite, rhyolite and

basalt considered to be associated with subduction related volcanism, occur mostly in northern

Laos, especially in (1) the Pak lay – Luang Prabang region; (2) north-western Laos, towards the

border with Myanmar, and also (3) south-central Laos, Truong Son Fold Belt (Fig. 2.3.1;

Workman, 1990; Stokes and Smith, 1990; Loader 1999). Rhyolite and dacite rocks of probable

Triassic age are reported by Workman (1990) to occur in the San Nua region, north-eastern

Laos. Triassic volcanic sequences of up to 200 m thickness, comprised of andesite and trachyte

interbedded with tuff and agglomerate conformably underlie Late Triassic red-bed sequences in

the Pak Lay region, northern Laos (Stokes and Smith, 1990). In southern Laos along the

Sekong Valley and the Cambodian border region, inferred Triassic age rhyolite and tuff are

known to form an extensive plateau area (Fig. 2.3.1; Fontaine and Workman 1978).


30

Cenozoic basalt flows of probable Quaternary age overlying Mesozoic sandstone

sequences cover vast areas of the Bolvens Plateau near Pakse in southern Laos (Fig. 2.3.1). At

least two cycles of olivine- and pyroxene-dominant basaltic volcanism are known for the

Bolvens Plateau area: (1) an older cycle at 1-2 Ma, and (2) a young cycle at 0.6 to 0.7 Ma

(Fontaine and Workman, 1978). Small areas of similar Quaternary age basalts also occur in

northern Laos near Ban Houei Sai (Workman, 1990).

2.3.6 Igneous Intrusions

Stokes and Smith (1990) and Fontaine and Workman (1978) reported at least four main

magmatic cycles of igneous rocks in Laos, ranging from the Late Proterozoic through to the

Mesozoic. The earliest igneous rocks known are the Early to Late Proterozoic cycle (PR3)

comprising: gneiss, granodiorite, granite, migmatite and pegmatite intrusions that occur in the

eastern part of southern Laos (Kontum Massif) and also in Samneua and Xieng Khouang

Provinces, northern Laos (Fig. 2.3.1; Stokes and Smith, 1990). The geochronology of

Proterozoic igneous rocks in Laos is poorly constrained, with gneissic rocks from the Kontum

Massif reporting a range of ages from (1) 2300 Ma, determined from radiometric Pb isochrons

(Hutchinson, 1989), (2) 1650-1810 Ma using K/Ar and 1400-1600 Ma using Rb/Sr (Lepvrier et

al., 2004), and (3) 1400 Ma U-Pb zircon ages (Nam et al., 2001).

Early to Middle Palaeozoic intrusions (PZ1-2) represent the second cycle of magmatism

that occurs mostly along deep-seated faults in the NE and NW fold-belts of Laos (Fig. 2.3.1).

Intrusion compositions range from granodiorite, granite and plagiogranite through to ultramafic

rocks composed of dunite and serpentinite (Stokes and Smith, 1990). Meffre et al. (2005)

reported Silurian U-Pb zircon ages obtained from a granite intrusion (434 Ma) and felsic dikes

(433 to 434 Ma) occurring to the east of the Phu Kham deposit, along the Loei Fold Belt in

northern Laos. Mineralised intrusive rocks at Phu Kham have younger Early Permian ages

ranging from 292 + 10 to 298 + 5 Ma (Meffre et al., 2004). The Phu Kham mineralised

intrusion ages are also similar to those reported by Loader (1990) for a rhyodacite porphyry

(RDP) sample from Sepon, south-central Laos yielding a 290 Ma SHRIMP determined age.

Follow-up U-Pb zircon age dating completed on 13 RDP samples from Sepon during this

research project confirm the earlier SHRIMP age of 290 Ma reported by Loader (1999) with

results presented in section 2.5 of this chapter. Late Permian granodiorite (264 + 10 Ma) and

monzonite (255 + 10 Ma) intrusions age determined by the K/Ar method are also reported in

northern Laos and occur in a north-trending belt from Pak Lay to Luang Prabung Provinces

(Fontaine and Workman, 1978).


31

During the Late Palaeozoic to Early Mesozoic cycle (PZ3-MZ1), intrusion emplacement

occurred mostly in north-central Laos and comprises (a) Carboniferous age batholiths of

monzonite, diorite, granodiorite, granite and aplite, and (b) Triassic quartz-diorite, plagiogranite

and granite porphyry. Triassic ultramafic rocks also occur in Laos and predominantly consist of

dunite, peridotite and serpentinite that were structurally emplaced locally along faults within

the fold-belts (Fig. 2.3.1; Stokes and Smith, 1990; Workman, 1990). The geochronology of

intrusion phases in Laos during this period is poorly constrained. Late Mesozoic to Tertiary age

(MZ3-KZ) igneous rocks were also emplaced along the fold belts in northern Laos. This fourth

cycle of magmatic intrusions comprise (a) Cretaceous age granite, porphyry and biotite

muscovite granite, (b) Palaeogene age granosyenite porphyry, and, (3) Pliocene–Pleistocene

age gabbro-dolerite (Stokes and Smith, 1990; Workman, 1990). The geochronology of

intrusions during this period is also poorly constrained in Laos.

2.3.7 Regional structure of Laos

The present day structural framework of Laos is broadly defined by regions that exhibit

remnant orogenic features, as shown in Fig. 2.3.2. Fold belts in Laos are interpreted to have

developed in zones of crustal deformation around metamorphosed Phanerozoic proto-cores

forming the Indochina Terrane, during orogenic periods in the Palaeozoic and Early Mesozoic

(Workman, 1975; Fontaine and Workman, 1978; Stokes and Smith, 1990; Lepvrier et al., 1997;

Maluski et al., 2005). At least three main orogenic periods of folding are classified by Fontaine

and Workman (1978), namely (1) Variscan (Middle Carboniferous), (2) Indosinian I (Permian-

Early Triassic), and (3) Indosinian II (Late Triassic). Pre-Palaeozoic orogenic folding may have

also occurred, but preserved evidence is lacking in Laos (Stokes and Smith, 1990).

The Variscan (Hercynian) orogeny is interpreted to have commenced during the

Middle Carboniferous collision of the Indochina Terrane with the South China Terrane

resulting in a NW-SE trending structural grain formed sub-parallel to the Song Ma suture

(Fig. 2.3.2; Fontaine and Workman, 1978). Regional compression and uplift resulting from

terrane collision along the Song Ma suture zone developed a large NW-SE trending

anticlinorium in north-eastern Laos forming the north-central Variscides which are also known

as the Truong Son Fold Belt (Fig. 2.3.2). Large-scale regional thrust faults that formed along

the Truong Son Fold Belt during this period were directed south-westerly towards the Kontum

Massif foreland (Workman, 1975; Fontaine and Workman, 1978).

The collision of the Indochina-South China Terranes with the Shan-Thai Terrane

during the Indosinian orogeny (Permian-Late Triassic) is marked by uplift, folding and faulting

along the margins of the Indochina Terrane (Fontaine and Workman, 1978; Stokes and Smith,

1990). During this period, the Loei Fold Belt is interpreted to have formed along the north-

western margin of the Indochina Terrane, resulting in the formation of a NNE-trending fold belt


32

with a structural grain of sub-parallel fold hinges and faults (Fig. 2.3.2; Fontaine and Workman,

1978). Both the NNE-trending Loei Fold Belt and the NW-trending Truongson Fold Belt also

converged during this period towards latitude 103o E in northern Laos, where both fold-belts

were aligned sub-parallel to the margins of the Xieng Khouang Massif (Fig. 2.3.2; Fontaine and

Workman, 1978; Workman, 1990).

Fig. 2.3.2. Regional structural geology map of Laos showing the major fault zones (black dashed lines). Abbreviations: SMD = Sepon Mineral District (black rectangle area), LP-DBP = Luang Prah Bung - Dien Bien Phu fault, SC = Song Ca fault, SM = Song Ma fault, TK-S = Tha Khek - Sepon fault. This figure is modified from Workman (1990) and Lepvrier et al. (2004).

LP-DBP

SM

TK-S

SMD

SC


33

The Da Nang-Khe Sanh regional splay fault in central Vietnam, splitting off the south-

eastern end of the main NW-trending Sepon-Thakhek regional fault in Laos, has syn-kinematic

metamorphic mineral indicators dated by 40Ar-39Ar methods at 244-245 + 2 Ma, confirming

regional dextral strike-slip movements along the southern margins of the Truong Son Fold Belt

during the Early Triassic (Fig. 2.3.2; Lepvrier et al., 2004). These dates also occur in the range

of U-Pb dates from zircon in syn-collisional granites emplaced at 250 Ma along the Song Ma

Fault zone (Lepvrier et al., 2004). Based on geochronology and kinematic evidence, both Carter

et al. (2001) and Lepvrier et al. (2004) proposed that the onset of an Indosinian thermo-

tectonisim event resulting from the oblique collision of the Indochina-South China and Shan-

Thai Terranes during the Early Triassic induced at least three tectonic characteristics,

comprising (1) dextral strike-slip movements along NW-trending and E-W faults in the Truong

Son Fold Belt, (2) sinistral shearing along N-S faults in northern Laos and north-central

Vietnam, and (3) probably the unroofing of the Kontum metamorphic core complex

(Fig. 2.3.2).

Since the Middle Jurassic, the Indochina Terrane is interpreted to have remained stable

to the present day (Workman, 1975). However, the timing and origins of the dextral south-

easterly rotational movement of the Indochina Terrane along the Song Ma suture since the

Cenozoic is currently not constrained (Tapponnier et al., 1990; Huchon et., 2001; Sato et al.,

2001; Lepvrier et al., 2004). The structural grain of Laos has largely remained the same since

the Middle Jurassic, with pre-existing faults reactivated during the Cenozoic, especially on the

eastern margin of the Indochina Terrane (Fig. 2.3.2; Lepvrier et al., 2004).

Chapter 3 – District-Scale Geological Setting of the SMD

34

CHAPTER 3: DISTRICT-SCALE GEOLOGICAL

SETTING OF THE SEPON MINERAL DISTRICT

3.1 INTRODUCTION

The Sepon Basin is a small-scale clastic-carbonate sedimentary basin approximately

20 km long by 8 km wide and belonging to a group of Palaeozoic successor basins that occur

along the Truong Son Fold Belt. These basins have poorly constrained stratigraphic ages that

possibly range from Ordovician to Devonian (Manini et al., 2001; Smith, 2003; Ekins, 2005).

The geometry of the Sepon Basin was noted by Marten (1999) and Coller (1999) to strike

anomalously E–W within the overall NW-trend of the Truong Son Fold Belt, where the western

margins of the basin pinch out against the regional NW-trending Tha Khek-Sepon Fault, also

referred to by Coller (1999) as the Truong Son Fault (Fig. 3.1). The SMD is located in the

Sepon Basin, and the SMD covers an area 40 km long by 10 km wide (Figs. 3.1 and 3.2).

In this Chapter, the district-scale geological setting of the SMD will be described and

discussed in order to assist with developing a genetic model for the formation of gold and

copper mineralisation in the SMD. The SMD district-scale geology information presented in

this Chapter has mostly been based on the author’s own field investigations and drill core

logging, together with data from reports by Sillitoe (1994a, b; 1995; 1998); Morris (1996;

1997a, b; 1998); Marten (1998a, b, c); Loader et al. (1999); Norris (1999); Coller (1999), and

Smith (2003), and also in published papers by Loader (1999), Manini et al. (2001), Manini and

Albert (2003), Smith et al. (2005), Ekins (2005) and Olberg et al. (2006). This Chapter also

presents a geochronological framework for the timing of sedimentation and intrusions in the

SMD and provides new zircon U-Pb LA-ICPMS age data to constrain the timing of rhyodacite-

porphry (RDP) intrusion in the SMD.

3.2 SEPON BASIN STRATIGRAPHY

Previous detailed geological investigations in the central sector of the Sepon Basin

were conducted mostly near the current Sepon mining area and have contributed towards the

development of the present understanding of the Sepon stratigraphy, primarily through studies

by Morris (1996, 1997, 1998) and subsequently by Sillitoe (1997, 1998), Coller (1999), Loader

et al. (1999), Smith (2003) and Ekins (2005).

Metamorphosed basement to the south and south-east of the Sepon Basin is reported to

consist of Late Proterozoic gneisses, orthogneisses and schists that are interpreted to belong to

the Indochina Block (Norris, 1999). Schists occur as narrow wedges intercalated with gneisses


35

along major WNW-trending lineaments to the south of the Sepon Basin and vary in mineralogy

from quartz-albite-muscovite schist; quartz-albite-biotite-epidote schist, and; quartz-albite-

actinolite schist (Fig. 3.1). Gneisses are interpreted to have been derived from pelitic or

volcano-sedimentary rocks, and exhibit augen textures and are composed of quartz, feldspar

and chloritised mica enveloping feldspar porphyoblasts. Foliated biotite granite intrusions occur

as potassium-feldspar rich orthogneisses (Norris, 1999).

The Sepon Basin contains interbedded sequences of Phanerozoic continental fluvial

and shallow to deep marine sedimentary rocks intruded by Late Palaeozoic rhyodacite-

porphyry (RDP) dikes and sills (Morris, 1997; Manini et al., 2001; Smith et al., 2005).

Ordovician to Silurian clastic rocks comprising grey-green sandstone, siltstone and shale with

thin interbeds of lithic tuff, limestone, and calcareous shale are interpreted as unconformably

overlying Upper Proterozoic basement rocks in the Sepon Basin (Norris, 1999).

Fig. 3.1. Regional-scale geology map of the southern Truong Son Fold Belt in Laos (from Loader, 1999). The Sepon Mineral District (SMD) is located in the Sepon Basin (blue rectangle area) and shown in Fig. 3.2.

Fig. 3.2. District-scale geology map of the Sepon Mineral District (SMD) showing the location of the main gold and copper deposits (provided courtesy of OZ Minerals Limited).

36


37

The SMD stratigraphy comprises at least ten formations, commencing with the basal

Palat Formation through to uppermost Nan Kian Formation with three informal member units

belonging to the Nalou Formation (Table 3.2.1 and Fig. 3.3). Prior to 2006, the SMD

stratigraphy comprised eight formations, named from basal Formation 1 to Formation 8

(Table 3.2.1; Morris, 1996, 1997a,b, 1998; Manini et al., 2001; Smith et al., 2005; Ekins,

2005). Revisions to the SMD stratigraphy by Feldman (2006) and Morris (2006) replaced the

original formation numbering system with formal names, cancelled Formation 8 and included

four new formations, namely the Payee, Houay Bang, Vang Ngang and Namphuc Volcanics

(Table 3.2.1).

The SMD stratigraphy grades from older siliciclastic rocks through to younger

carbonate rocks (Fig. 3.3). Fossil ages obtained during a study by Ekins (2005) indicated that

the Sepon stratigraphy ranges in age from Ordovician (Payee Formation) through to Devonian

(Nan Kian Formation). The stratigraphic relationships observed for the SMD formations also

appear to occur away from the Sepon gold and copper mining areas into the outer parts of the

Sepon Basin, but they are not well-constrained due to the lack of geological mapping and

drilling information (Smith et al., 2005; Ekins, 2005; Morris, 2006). Descriptions of the Sepon

stratigraphy pertaining to the current SMD formations are presented here, based on the author’s

own observation and previous studies by Morris (1996, 1997a, b, 1998, 2006), Ekins (2005)

and Feldman (2006).

Table 3.2.1. Stratigraphy comparison of former and current Formation names in the SMD, Lao PDR.

Original SMD Stratigraphy New SMD Formations Current SMD Stratigraphy

(Pre 2006) (Introduced in 2006) (Post 2006)

Formation 8 (Redundant)

Formation 7 Nan Kian Formation

Formation 6 Discovery Formation

Formation 5 Nalou Formation

Kengkeuk Formation

Namphuc Volcanics Namphuc Volcanics

Formation 3b Vang Ngang Formation

Formation 3a Nampa Formation

Formation 2b Houay Bang Formation

Formation 2a Payee Formation

Formation 1 Palat Formation

Formation 2

Formation 4

Formation 3

Table 3.2.1 above is compiled from the publications by Manini et al. (2001) and Smith et al. (2005) and also the LXML company reports by Morris (1997a, b, 1998, 2006) and Feldman (2006).


38

Fig. 3.3. Stratigraphic column from the basal Payee Formation to the upper Nam Kian Formation in the SMD (courtesy of OZ Minerals Limited and adapted from Feldman, 2006 and Morris, 2006).


39

3.2.1 Palat Formation

The basal section of the Sepon stratigraphy unconformably overlies Late Proterozoic

basement rocks and is represented by the Palat Formation (previously Formation 1), a thin unit

(<100m thick) comprised of polymictic conglomerate with basement-derived metamorphic and

volcanic clasts, overlain by conglomerate with carbonate-rich clasts and interbeds of calcilutite

and calcarenite (Fig. 3.3). The depositional age of the Palat Formation has not been

constrained. The Palat Formation basal conglomerate containing rounded andesite clasts and

sub-angular carbonate clasts was intersected in drill holes DD95VNG044 and DD95VNG045 at

the Vang Ngang gold deposit, towards the southern margins of the Sepon Basin (Figs. 3.4A and

B). A basal sequence of unmetamorphosed red-brown and grey-green claystone and carbonate

rocks unconformably underlies the Palat Formation (Morris, 1997a, b, 2006; Feldman, 2006).

3.2.2 Payee Formation

The Payee Formation (Formation 2a) is up to 250m in thickness and dominantly a

siliciclastic sequence conformably overlying the Palat Formation (Fig. 3.3). It is comprised

mostly of yellow to green-grey massive bedded, strongly bioturbated, graded medium- to fine-

grained quartz-lithic sandstone and lacks primary organic detritus (Figs. 3.4C and D; Morris,

1997a,b, 2006; Loader et al., 1999; Feldman, 2006). The basal sections of the Payee Formation

commonly contain finely laminated olive-green claystone (Morris, 1997a,b). The depositional

environment for the Payee Formation was described by Morris (2006) to be low-gradient below

storm wave base marine with the anoxic interface below the sediment water interface as

indicated from the even grain size of sandstone, the paucity of primary organic material and the

high degree of bioturbation. Conodont age determinations by Ekins (2005) indicate deposition

of the Payee Formation during the Middle to Late Ordovician (470-458 Ma).

3.2.3 Houay Bang Formation

The Houay Bang Formation (Formation 2b) is up to 200m thick and reported by

Feldman (2006) and Morris (2006) to comprise two sequences of carbonates with intervening

sandstone (Fig. 3.3). Poorly bioturbated grey laminated calcareous fine grained sandstone and

siltstone containing thin interbeds of black carbonaceous mudstone characterise the two

carbonate intervals (Figs. 3.4E and F). In contrast, the grey-green well sorted medium to fine

grained sandstone interbedded unit is bioturbated and similar to the underlying Payee

Formation, but differ by containing carbonate in their matrix (Feldman, 2006; Morris, 2006).

The depositional environment for the Payee Formation was described by Morris (2006) to be

similar to the Payee Formation, but most likely above the sediment water interface due to the

paucity of bioturbation in the carbonate dominant intervals.


40

Fig. 3.4. Photographs showing lithological features of the Palat, Payee and Houay Bang Formations occurring in the SMD, Laos. (A) and (B) Palat Formation basal conglomerate unit comprised of sub-rounded basement derived metamorphic and andesite volcanic clasts (And) with contributions of sub-angular light grey to white limestone clasts (Lst) observed in Vang Gnang drill hole DD95VNG044 from 40m depth. (C) Payee Formation bioturbated and laminated fine-medium grained sandstone (Sst) from drill hole DD05HYB011 (from Feldman, 2006). (D) Fine-grained sandstone (Sst) from the Payee Formation with most of the laminations disrupted during bioturbation, example from drill hole DD05HYB06 (from Feldman, 2006). (E) Houay Bang Formation olive green massive calcareous sandstone (Sst) interval with minor bioturbation in drill hole DD05HYB016 (from Feldman, 2006). (F) Interval from the Houay Bang Formation in drill hole DD05HYB016 comprising grey laminated calcareous fine-grained sandstone (Cst) and siltstone with thin interbeds of black carbonaceous mudstone (from Feldman, 2006).


41

3.2.4 Nampa Formation

The Nampa Formation (Formation 3a) conformably overlies the Houay Bang

Formation with a gradational contact; it comprises a >300m thick sequence mainly composed

of grey-green to brown finely laminated illitic claystone and siltstone in the basal sections,

grading upwards to black organic-rich pyritic mudstone towards the top of the section (Figs. 3.3

and 3.5A; Morris, 2006). Olive claystone with stylo-nodular calcilutite mark the base portions

of this formation (Morris, 2006). A distal deep water facies depositional environment is

interpreted for the Nampa Formation (Morris, 1997a, b, 2006).

3.2.5 Vang Ngang Formation

The Vang Ngang Formation (Formation 3b) conformably overlies the Nampa

Formation and comprises a >250m thick sequence (Fig. 3.3). The basal section of the Vang

Ngang Formation is predominantly composed of sandstone and minor siltstone interbeds

overlain by variably dolomitised limestone. Overlying the limestone, minor interbeds of

nodular calcareous calcilutite and dark-grey to light green-grey laminated chert occur in the

upper sections of the Vang Ngang Formation (Fig. 3.3 and Fig. 3.5B; Morris, 2006). Outcrops

of the Nampa Formation chert sections were observed along the access road to the Vang Ngang

gold deposit, where interbeds of black pyritic mudstone contain graptolites that yielded Late

Ordovician to Early Silurian depositional ages from 444 - 437 Ma (Fig. 3.5B; Ekins, 2005).

3.2.6 Namphuc Volcanics

The Namphuc Volcanics is a volcano-sedimentary formation in the SMD occurring

between the underlying Nampa Formation and the overlying Kengkeuk Formation (Fig. 3.3).

Andesite with minor inclusions of country rock characterises the Namphuc Volcanics

comprising pyroclastics, intrusives, flows, agglomerate and conglomerate (Fig. 3.5C; Feldman,

2006; Morris, 2006). Exploration drilling by LXML in the eastern sector of the SMD identified

the Namphuc Volcanics during 2006, post the author’s field research work in the SMD. LXML

geologists interpret the Namphuc Volcanics as being formed by submarine volcanic processes

that were also eroded at a storm wave base level, producing sub-rounded to rounded poorly

sorted conglomerate clasts (Morris, 2006). The monomict andesitic conglomerates of the

Namphuc Volcanics differ from those of the interpreted older Palat Formation polymictic

conglomerates that comprise andesite and carbonate clasts (Morris, 2009).


42

3.2.7 Kengkeuk Formation

The Kengkeuk Formation (Formation 4) represents the lower-most carbonate-dominant

sequence in the stratigraphy; it is poorly exposed and has been shown from drilling information

collected in the Sepon mining area to be up to100m thick. This formation is predominantly

composed of dark grey-black finely laminated and in parts nodular, carbonaceous, calcareous

siltstone and mudstone (Figs. 3.3 and 3.5D). Stylolites and bedding conformable veinlets occur

throughout this formation, including thin lenses of fossiliferous debris and ooids that increase

towards the top of the Kengkeuk Formation (Morris, 1997; Coller, 1999; Smith, 2005; Ekins,

2005). Conodont fossils obtained from core drilled through the Kengkeuk Formation at

Kengkeuk Prospect and dated by Ekins (2005) yielded Early-Middle Silurian ages (428 Ma).

Marine regression is interpreted for the deposition of this formation off a platform slope and

below the storm wave base (Morris, 1997a, b, 2006; Loader et al., 1999).

3.2.8 Nalou Formation

The Nalou Formation (Formation 5) conformably overlies the Nampa Formation and

consists of a variably dolomitised bioclastic carbonate sequence up to 120m thickness that is

subdivided into three members (Morris, 1997a, b, 1998, 2006; Smith et al., 2005; Ekins, 2005).

Member 1 is a bioturbated and dolomitised basal dark grey bioclastic fine to medium grained

calcarenite (<30m thick) comprising fossilised reef fauna of tabulate corals, stromatoporoids,

gastropods and brachiopods (Fig. 3.3). The contact between the underlying Kengkeuk

Formation and Member 1 is gradational (Morris, 2006). Member 2 is characterised by light-

grey algal laminated stomatolitic limestone and dolomite with minor interbeds of fine- to

medium-grained dark grey bioclastic grainstone (Fig. 3.5E). The contact between Member 2

and the underlying Member 1 is gradational. A disconformity occurs between the upper

stomatolitic algal laminate sections of Member 2 and the overlying Member 3 sedimentary

rocks composed of dark grey bioclastic medium-grained dolarenite and dolorudite containing

shelly fauna and rugose corals (Fig. 3.5F). Fossil ages have not been constrained for the Nalou

Formation. Its environment of deposition is interpreted as fore-reef and platform for Members 1

and 3, and a lagoonal inter-reef or back-reef environment for Member 2 (Morris, 1997a, b,

1998, 2006; Ekins, 2005).


43

Fig. 3.5. Photographs showing in turn the lithological features of the Nampa Formation, Vang Ngang Formation, Namphuc Volcanics, Kengkeuk Formation and Nalou Formation occurring in the SMD. (A). Nampa Formation tan to light-green claystone (CST) comprising both massive and laminated sections marked by dolomitsation towards the base of this formation. The drill core in this picture is from hole DD05HYB010 at 45m depth (adapted from Feldman, 2006). (B). Vang Ngang Formation grey bedded chert (CHE) and minor siltstone (SLT) in the upper sections of this formation. Photo taken along the road to the Vang Ngang gold deposit. (C). Namphuc Volcanics interval in drill core comprising coarse-textured andesite lava (AND) with small pyroclastic intervals comprised of rounded andesite clasts and minor country rock (from Feldman, 2006). (D). Kengkeuk Formation dark grey-black finely laminated calcareous siltstone (CSH) also containing minor nodular and carbonaceous intervals. Drill core sample collected from hole NKK014 at 27.5m depth. (E). Close up view of Nalou Formation algal laminated dolomite (Member 2) from drill hole DIS021 @ 126.2m depth at the Discovery Main gold deposit. Note the algal dolomite (ADM) is represented by white laminations, in turn cut by later thin white calcite veins. (F). Close up view of bioclastic dolomite (BDM) with rugose corals and shelly fauna belonging to the Nalou Formation (Member 3), from drill hole DIS021 @ 127.0m depth at the Discovery Main gold deposit.


44

3.2.9 Discovery Formation

The Discovery Formation (Formation 6) predominantly consists of a black calcareous,

carbonaceous (organic-rich) nodular mudstone that grades upwards to laminated calcareous

carbonaceous mudstone, attaining a thickness of up to 230m (Fig. 3.3). A gradational contact

over a 10 to 20m interval occurs between the Discovery Formation and the underlying Nalou

Formation bioclastic sequence, where there is an increase of bioclastic debris down stratigraphy

within the transition zone (Morris, 1997a, b, 1998, 2006; Smith et al., 2005; Ekins, 2005).

Concentrations of carbonaceous matter occur along pressure solution features that are

developed in the Discovery Formation bedding-sub-parallel laminations (Smith 2003; Smith et

al., 2005). Within the upper sections of the Discovery Formation, remnants of crinoid,

brachiopod and gastropod fauna grade upwards and become less abundant (Morris, 1997a, b,

2006; Smith et al. 2005; Ekins, 2005). Conodont age determinations by Ekins (2005) indicate

an Early to Middle Devonian age for the deposition of the Discovery Formation. The main

carbonate packages in the Sepon Basin are represented by the Kengkeuk, Nalou and Discovery

Formations, with the Discovery Formation deposited during marine regression (Morris, 1997a,

b, 2006; Smith et al., 2005). The Discovery Formation is the main host-rock to the known

SHGD in the SMD and is variably decarbonatised and silicified during gold mineralisation

(Figs. 3.6.1A to C; Manini et al., 2001; Smith et al., 2005). Chapter 5 presents the SMD SHGD

mineral assemblage paragenesis stages hosted by the Discovery Formation.

Fig. 3.6.1. Photographs showing lithological features of the Discovery Formation occurring in the SMD. (A) West-looking view of the open pit wall at the Discovery Colluvial gold deposit. The Discovery Formation (DCF) calcareous shale (grey) is faulted against weathered rhyodacite porphyry (RDP). Note the tan coloured weathered profile of the Discovery Formation (DCF) calcareous shale underlying a dark-orange to brown coloured iron-rich soil profile. (B) Medium to dark-grey Discovery Formation (DCF) carbonaceous nodular calcareous shale (CSH) at the Discovery Colluvial gold deposit. Note the orange limonite developed on the exposed rock surfaces after the weathering of finely disseminated gold bearing pyrite (<1mm diameter) hosted by CSH. (C) Textural characteristics of dark-grey carbonaceous Discovery Formation calcareous shale (CSH) containing gold-bearing pyrite (py). The average grade of this drill core sample is 23 g/t in hole NLU0060300 from the Nalou gold deposit.


45

3.2.11 Nan Kian Formation

The Nan Kian Formation (Formation 7) is a siliciclastic sequence of up to 630m

thickness, comprised mostly of dark grey-black interbedded organic-rich carbonaceous, pyritic,

finely laminated non-calcareous mudstone and chert that is poorly bioturbated, similar to the

upper sections of the Vang Ngang Formation (Fig. 3.3; Morris, 1997a, b, 2006; Smith et al.,

2005). The upper and lower contacts of the Nan Kian Formation are gradational, with the lower

contact between the Discovery Formation and the Nan Kian Formation being conformable and

also comprised of a 10m transitional zone with alternating interbeds of dark grey chert and

calcareous shale (Figs. 3.3 and 3.6.2A and B; Morris, 1997a, b; Smith et al., 2005). At least

eight different species of tentaculid fauna were identified during a study of the Nan Kian

Formation rock samples by Ekins (2005), indicating Late Devonian deposition for this

formation, with ages ranging from 386 Ma to 360 Ma. A deep water depositional environment

is interpreted by Morris (1997) for the Nan Kian Formation.

Fig. 3.6.2. Photographs showing the Nan Kian Formation and the underlying transitional zone between the Discovery Formation and the Nan Kian Formation. (A) Open pit wall exposure at the Discovery West gold deposit (DSW) showing the contact relationships between rhyodacite porphyry (RDP, tan colour, left) intruding a high angle structure cutting a transition zone of interbedded Discovery Formation (DCF) calcareous shale and Nan Kian Formation (NKF) dark grey chert underlying a thicker and gently folded sequence of bedded Nan Kian Formation chert. (B) Close up view of the transition zone at DSW shown in Fig. 3.7A with thinly bedded gently dipping and oxidised Discovery Formation (DCF) calcareous shale (tan-orange colour) in the basal section progressing upwards to a thinly bedded chert dominant sequence belonging to the Nan Kian Formation (NKF).

3.2.12 Mesozoic Khorat Group

Jurassic to Cretaceous age continent-derived fluvial sedimentary rocks belonging to the Khorat

Group occur as remnants that unconformably overly the Palaeozoic sequences of the Payee

Formation through to the Nam Kian Formation in the eastern and central sectors of the SMD

(Fig. 3.3; Loader, 1999; Norris, 1999). This formation is divided into upper and lower members

in the SMD. The lower member unconformably overlies the Nan Kian Formation, exhibits

folds with moderate to steep dips, and is composed of red quartz-rich sandstone interbedded

with minor pebbly polymictic conglomerate. The upper member is composed of similar rock

types but forms thick flat-lying to gently dipping sequences in the SMD central areas (Norris,

1999). The sedimentary rocks of the Khorat Group are also widely distributed in the adjacent

areas of central and northwestern Thailand (Hutchinson, 1989).


46

3.3 SMD IGNEOUS ROCKS

Three types of intrusions occur in the SMD, namely (a) rhyodacite-porphyry (RDP)

stocks and dykes that predominantly occur within the Padan to Thengkham corridor, also

locally referred to as the P-T sector (Loader, 1999; Loader et al., 1999; Manini et al., 2001;

Smith et al., 2005), (b) later stage small-scale fine-grained mafic dykes that cut sedimentary

and RDP rocks in the P-T sector, such as at the Discovery Colluvial gold deposit (LXML,

1994), and (c) granite stocks along the southern margins of the SMD in the Bansopmi –

Kengkhup area (Cromie, 2005; Fig. 3.7). A description of these three types of intrusions is

presented here, including results from whole-rock and rare earth element analyses that were

undertaken to determine the geochemical characteristics of the igneous rocks prior to the

geochronology studies presented in Section 3.4.

3.3.1 Rhyodacite-porphyry

3.3.1.1 Occurrence

Rhyodacite porphyry (RDP) intrudes the Palaeozoic sedimentary sequence from the

Palat Formation to the Nan Kian Formation in the SMD P-T sector and predominantly along

steep E-W and NW structural trends, as shown in Fig. 3.2 (Marten, 1998a; Loader et al., 1999;

Loader, 1999; Manini et al., 2001; Smith et al., 2005). Pepperite margins have not been

confirmed along the contact between the RDP intrusions and sedimentary rocks in the SMD,

but the occurrence is not ruled out (Smith et al., 2005). Smith (2003) reported that only a small

number of the SMD RDP dikes observed in drill core and pit mapping showed chilled margins,

mostly 10 to 30 cm wide. Most of the sedimentary rock - RDP contacts are described as being

marked by shears, with the RDP dikes interpreted to having been affected by district-scale

faults that form the predominant structural fabric of the SMD (Smith, 2003; Smith et al., 2005).

Smith (2003) also noted that RDP dominantly occurs as dykes in the SMD, due to their

crosscutting the stratigraphy. Emplacement of the RDP intrusions is therefore interpreted to be

due to late-syn tectonic processes and before the cessation of dextral compression (Marten,

1998a; Loader et al., 1999; Smith et al., 2005).

The Padan and Thengkham RDP intrusions are the two main stocks in the SMD (Smith

et al., 2005). Emanating from these two stocks there are large RDP dykes with dominant ENE-

trends and also minor NW-trending dykes interpreted to have been intruded along pre-existing

major faults, as can be observed in outcrop from the Discovery East to Discovery West SHGD

and also at the Nalou and Namkok SHGD (Fig. 3.2; Marten, 1998a; Loader et al., 1999).

A brief description of the characteristics of known SMD RDP intrusions is provided from

Loader et al. (1999) and Norris (1999) in Table 3.3.1; their locations are shown in Fig. 3.2 and

Fig. 3.7. Most of the RDP intrusions listed in Table 3.3.1 yielded Permo-Carboniferous ages

from geochronology studies, the results of which are presented in Section 3.4.


47

Fig. 3.7. Satellite image map with K-Th radiometric data showing the outline of RDP intrusions in the SMD (green outline with radiometric data yellow colour fill) occurring along an E-W trending corridor in the Padan-Thengkham (P-T) sector. The SW corner of this figure shows red-orange colours for the Proterozoic sequence along the margins of the NW-trend Truongson Fold Belt sequence of Palaeozoic sedimentary rocks (light-blue colours). The red square box represents the Sepon gold and copper processing plant site. The numbers shown in grey circles represent the intrusion numbers that are linked to the names shown in Table 3.3.1 below, comprising: (1) Padan, (2) Khanong, (3) Discovery, (4) Nalou, (5) Thengkham, (6) Boung, (7) Banmai and (8) Nakachan. The satellite and GIS data for this figure was provided by Oxiana Limited.

Table 3.3.1 Summary of the known RDP intrusions occurring in the SMD, Laos

RDP Intrusion Name Type Orientation Intrusion characteristics

(1) Padan (PDN) Stock E - WA long RDP porphyry stock (>700m) with a >170m thick silica alteration cap grading to intermediate argillic and potassic alteration at depth, hosting minor Mo-Cu.

(2) Khanong (KHN) Dyke ENE - WSWShallow northerly dipping RDP dykes intruding along low-angle extensional faults. Skarn associated Cu reported to occur along RDP margins.

(3) Discovery (DIS) Dyke ENE - WSW

A long ENE-trending RDP dike of up to 120m thickness, from the Discovery-Colluvial to -Main SHGD, intruding the Nalou-, Discovery- and Nan Kian Formations and often along the footwall of steep faults.

(4) Nalou (NLU) Dykes ENE - WSW

Two RDP dykes are reported at Nalou, with: (a) an Upper RDP dyke of up to 60m thickness cuts the Discovery Formation, and (b) a Lower RDP dyke of up to 60m thickness cuts the Nalou Formation.

(5) Thengkham (TKM) Stock + Dykes E - W

An E-W trending zone of RDP interpreted to be a central stock with steeply dipping dykes extending along the same zone. Skarn associated Cu and minor Mo mineralisation reported to occur along RDP margins at Thengkham-South, -North, and -West.

(6) Boung (BNG) Dyke E - W An E-W trending semi-continuous RDP dyke with zircons yielding a 300 Ma age using SHRIMP (Khin Zaw et al., 1999b; Loader et al, 1999)

(7) Banmai (MAI) Dykes E - W Strongly sheared steeply dipping E-W trending RDP dykes cutting the Nampa Formation.

(8) Nakachan (NAK) Dyke NW NW-trending RDP dyke occurring sub-parallel to the Truongson Foldbelt.

References used in this Table include: Loader et al. (1999), Loader (1999) and Norris (1999)

1 2

3 4

5

6

7

8


48

3.3.1.2 RDP petrology

RDP in the SMD is mostly weathered in outcrop and to depths >5m, commonly

showing small coarse relict grains of the original framework phenocrysts comprising quartz and

plagioclase feldspar in a white clay-dominant matrix (Figs. 3.8A-D). Deep drill core

intersections generally show more fresh porphyritic textured RDP intrusions that typically

comprise large (>5 mm) phenocrysts of: (a) rounded, elongate and embayed quartz (>5 to <15

modal %) that is locally referred to as peanut-textured quartz with individual phenocryst sizes

varying from <10mm length x >5mm width; (b) plagioclase feldspar (>5 to <25 modal %) in

the form of euhedral and sub-rounded elongate laths (<10mm x 5mm in size), (c) orthoclase

(<5 modal %) as subhedral crystals (<5 mm diameter), and; (d) minor euhedral green

hornblende up to 2mm in diameter. Textural characteristics of the quartz and altered feldspar

phenocrysts occurring in RDP samples are shown in Figs. 3.8E-H.

Petrographic studies of hand specimen and polished thin section samples showed that

the SMD RDP samples are generally comprised of the following modal abundance percentages

of minerals: (a) framework containing large phenocrysts of quartz, and plagioclase feldspar,

and minor orthoclase feldspar and trace hornblende forming up to 30 % of the total phenocryst

mineral components (Figs. 3.8E-H); (b) equigranular fine-grained groundmass consisting of an

interlocking mosaic of quartz (<10 modal %), feldspar (<40 modal %) and amphibole (<10

modal %); (c) alteration minerals dominated by mica and calcite (<13 modal %).

Both phenocrysts and the groundmass in the RDP samples investigated typically

showed the following paragenetic sequence of alteration minerals in dykes distal to the main

RDP stocks: (i) early sericite replacement of feldspar and filling of voids in the groundmass

(<15 modal %) and weak chlorite alteration of amphibolite phenocrysts (trace), (ii) later stage

thin quartz veins (<2mm; <2 modal %) with trace euhedral pyrite (<1mm, <1 modal %), and;

(iii) late stage calcite veins (<2mm; <3 modal %). In general, the primary textures of RDP

exhibit alteration mineral assemblages, typically comprising: (a) sericite and quartz alteration

of RDP dykes in the SHGD; (b) both prograde garnet and retrograde chlorite-epidote skarn

alteration proximal to the main RDP stocks, and; (c) retrograde alteration of RDP stocks,

comprising potassium feldspar, chlorite, epidote and sericite alteration assemblages. The

detailed mineralogy paragenesis of these mineral assemblages will be presented in Chapter 4.

Based on petrographic observations, both Smith (2003) and this study have observed

that most RDP intrusions investigated appear to be similar throughout the SMD and typically

show homogeneity in their mineral composition, phenocryst assemblage and distribution, and

groundmass texture. However, due to the weakly altered nature of the SMD RDP intrusions,

whole rock analyses were also conducted to confirm the classification of these rocks using

immobile trace element chemistry (Section 3.3.1.3).


49

Fig. 3.8. SMD rhyodacite porphyry (RDP) represented in outcrop, drill core and thin section. (A) Photograph showing white to light grey weathered RDP (right) intruding Discovery Formation calcareous shale (dark grey, left) at the Discovery Colluvial (DSC) gold deposit. (B) Photograph showing malachite stained base metal veins (<5 cm wide) containing galena-sphalerite-tetrahedrite-pyrite cutting RDP at DSC. (C) Photograph of an RDP sill (light orange-yellow rocks, left) overlying Discovery Formation calcareous shale (black, right) at the Nalou gold deposit. (D) Photograph showing weathered massive RDP exposed in the open pit wall at Nalou showing coarse texture with large feldspar phenocrysts (<2 cm wide) cut by thin limonite stained veins (orange-brown). (E and F) Photographs of drill core from hole DIS015 @ 61m depth in the Discovery Main (DSM) gold deposit containing RDP with a low degree of weathering and showing porphyritic textured framework of sub-euhedral light-pink feldspars and minor sub-rounded peanut textured quartz (light grey) in a fine sericite altered matrix. (G) Photomicrograph of sericite altered RDP from the Nalou gold deposit. Note sub-rounded quartz (white, left) and sub-euhedral rectangular shaped feldspar (dark brown). (H) Photomicrograph of a late stage calcite vein (pink) cutting a sub-rounded quartz phenocryst (white) in a sericite altered RDP at Nalou.

2 mm 2 mm


50

3.3.1.3 RDP whole rock geochemistry

Whole rock geochemical analyses were conducted on RDP samples at CODES, UTAS,

to provide detailed compositional information with the primary aim of assisting in their rock-

type classification, and petrogenesis. A total of fourteen RDP samples were analysed,

comprising six drill core samples and eight outcrop samples (Appendix 3.3.1). The XRF

analytical technique was used to obtain both major and trace element geochemical data for the

RDP samples summarised in Table 3.3.2, with the analytical data provided in Appendix 3.3.1.

Table 3.3.2. Whole rock data for a total of fourteen RDP intrusions investigated from the SMD.

Sam

ple

#

KH

N013

0785

PD

N002

2740

NLU

0610

460

TK

W05

3115

7

5410

861

PC

NA

K030

01

TK

W05

3130

7

PC

DS

W030

01

PC

TK

M03

001

PC

DS

W030

10

DIS

0250

860

PC

DS

M03

007

DS

C55

3141

4

PC

TK

M03

003

Det

ectio

n Lim

it

Major (wt%)

SiO2 55.69 62.44 62.92 63.05 63.30 63.59 64.27 69.82 71.07 71.60 71.79 72.08 73.75 80.44

TiO2 0.29 0.32 0.27 0.28 0.34 0.33 0.31 0.31 0.30 0.26 0.24 0.37 0.23 0.22

Al2O3 14.97 16.52 15.02 15.35 17.22 15.95 15.51 17.05 15.64 16.51 15.96 17.85 15.85 10.59

Fe2O3 3.47 5.52 3.23 2.6 3.90 3.30 2.62 3.49 2.42 2.30 2.15 1.01 1.28 2.12

MnO 0.14 0.13 0.08 0.1 0.13 0.10 0.04 0.01 0.05 <0.01 <0.01 <0.01 0.01 <0.01

MgO 2.59 1.06 1.83 2.14 1.83 0.50 1.31 0.68 1.07 0.98 0.72 0.63 0.81 0.58

CaO 6.71 0.73 4.24 3.75 3.72 4.69 4.31 <0.01 0.12 <0.01 0.01 0.01 0.04 <0.01

Na2O 0.07 3.03 0.08 0.06 4.92 0.15 2.27 0.13 0.33 0.06 0.05 0.50 0.03 0.23

K2O 4.05 5.66 3.64 3.94 2.58 3.57 3.83 4.39 4.54 4.76 4.15 4.23 4.60 3.04

P2O5 0.16 0.15 0.10 0.15 0.17 0.15 0.12 0.10 0.02 0.05 0.02 0.05 0.03 0.05

Loss (inc. S-) 11.3 4.06 8.50 8.45 1.92 7.44 4.93 3.97 4.03 3.40 4.94 3.71 3.29 2.47

Total (-S) 99.45 99.62 99.92 99.87 100.02 99.76 99.52 99.94 99.59 99.92 100.04 100.43 99.92 99.74

S 2.32 0.26 1.81 0.27 0.06 0.02 0.88 0.01 0.01 0.01 1.52 0.01 0.71 0.01

Trace (ppm)

As 7 <3 44 107 <3 8 459 214 13 42 16 20 8 10 3

Ba 1551 571 207 10 745 970 4 525 135 161 373 358 436 40 4

Bi 6 <2 <2 9 <2 <2 9 <2 <2 <2 8 <2 <2 10 2

Ce 32 33 39 21 32 34 523 35 21 20 26 54 36 13 4

Cr 41 66 87 7 13 7 8 8 8 7 36 7 37 30 1

Cu 167 53 51 15 15 17 14 11 529 14 1293 5 16 163 1

La 12 14 14 36 15 19 27 10 9 4 10 22 12 9 2

Nb 10 9 8 12 9.3 9.8 13 8.4 7.4 8.7 7 9.4 8 4.6 1

Nd 14 14 13 3 16 16 3 12 7 5 10 23 14 11 2

Ni 5 10 4 36 5 5 35 3 4 2 5 1 3 2 1

Pb 15 9 209 49 14 9 75 65 4 797 39 414 429 218 1.5

Rb 170 166 161 81 85 142 102 175 166 178 161 135 199 107 1

Sc 8 6 8 14 11 9 3 9 8 5 6 7 5 6 2

Se 3 <1 <1 17 <1 <1 16 1 3 7 2 <1 <1 7 1

Sr 57 200 41 166 710 54 134 37 13 8 14 61 8 5 1

Th 4 5 9 56 7 7 337 4 3 7 3 4 10 2 1.5

U 2 2 2 8 3 3 10 2 <1.5 2 3 <1.5 <1.5 2 1.5

V 92 110 77 <2 82 71 <2 77 77 50 59 81 51 80 1.5

Y 11 13 11 <1 16 17 2.0 8 7 7 7 11 9 7 1

Zn 592 324 250 99 51 65 81 19 14 5 105 7 73 11 1

Zr 90 87 101 3 105 116 3 105 93 116 99 112 96 64 1

The whole rock analytical results for SiO2 contents for the RDP samples listed in Table

3.3.2 above generally ranged from 62% to 74 wt%, with an altered sample reporting the lowest

value of 55 wt% SiO2 (sample KHN0130785). A single RDP sample (PCTKM03003)

contained the highest value of 80.4 wt% SiO2 that may be due to overprinting micro-veinlets of

quartz providing the additional SiO2 reported in the bulk analysis.


51

An investigation of the variation of immobile elements plotted from Table 3.3.2 against

SiO2 indicated that Zr, Ti and Nb formed single clustered populations (Figs. 3.9A-C). However,

with increasing Si02 levels; the elements: Y, V and Sc indicated a broad linear decrease in

concentration in the RDP samples (Figs. 3.9D-F).

Fig. 3.9. Plot of immobile elements Zr, Ti, Nb, Sc, V and Y versus SiO2 using whole-rock analysis data from the eleven samples listed in Table 3.3.2

Using the data in Table 3.3.2, the plot of SiO2 versus Zr/TiO2 shown in Fig. 3.10A

indicates that the SMD RDP samples occur in the known fields for rhyodacite/dacite, based on

the classification by Winchester and Floyd (1997). The plot of immobile elements Zr/Y and Zr

in Fig. 3.10B shows that the RDP samples occur in the field for continental arcs established by

Pearce et al. (1984). Smith (2003) noted that the mineralogy of the SMD RDP intrusions is

similar to metaluminous I-type magmatic rocks based on the absence of both primary garnets

and primary mica. This observation was also confirmed during this study using a plot of Rb

versus Y+Nb, which indicates that the SMD RDP intrusions most likely occur in the field

established for I-type granites by Pearce et al. (1984) as shown in Fig. 3.11A.


52

Fig. 3.10. Geochemical data plotted from the SMD RDP samples listed in Table 3.3.2 used for rock type classification (red dots). A. Igneous rock classification diagram using SiO2 versus Zr/TiO2 adapted from Winchester and Floyd (1977) and showing SMD RDP occurring in the field for Rhyodacite/Dacite. B. Diagram using Zr/Y versus Zr to show that SMD RDP also plots in the field reported for Continental Arc rocks by Pearce et al. (1984).

Fig. 3.11. Comparison of geochemical data from the SMD RDP samples listed in Table 3.3.2 using the classification of Pearce et al. (1984) based on four main felsic rock groups: volcanic arc granites (VAG); collision granites (COLG); within plate granites (WPG), and; ocean ridge granites (ORG). Both diagrams A and B show SMD RDP in the fields for I-type granites.


53

3.3.2 Granite

3.3.2.1 Occurrence

LXML (1998) and Loader (1999) reported observations of both felsic and intermediate

igneous intrusions along the south-eastern margins of the Sepon Basin, comprising granite,

granodiorite, syenite and monzodiorite (Fig. 3.12). Previous petrographic descriptions of these

rocks are limited, and their distribution, age of emplacement, timing relationships and

classification are also poorly constrained.

During this study, two granite samples were collected (numbers BSK5531425 and

BSK5531426) along the southern margins of the Sepon Basin at Ban Sopmi (1) and Ban

Kengkok (2), collectively called the Ban Sopmi-Kengkok (BSK) area that is located

approximately 15 km south-east of the Sepon mining operations (Fig. 3.12). The primary aim

of analysing these two samples was to: (a) provide new petrographic and geochemical

information for rock type classification purposes, and; (b) to determine the age of granite

emplacement to ultimately assist with developing an understanding of the timing relationships

with regards to RDP in the SMD (Section 3.4). The description of granite occurring near the

SMD in this section is primarily based on the two samples from BSK (Table 3.3.3).

Fig. 3.12. Regional geology map of showing the location of the SMD (green boxed area) and granite samples collected along the margins of the Truongson Fold Belt at Ban Kengkok (1) and Ban Sopmi (2), both located within the blue boxed area.. The Sepon mine site is represented by the yellow boxed area.


54

3.3.2.2 Granite petrology

The BSK granite are porphyritic textured and in outcrop appear to have intruded light

grey-green lower greenschist facies Proterozoic metasedimentary rocks (Fig. 3.13A). Large

elongated phenocrysts of pink-orange feldspars (<15mm long by >10mm wide) and sub-hedral

quartz (>10 mm diameter) occur in a foliated groundmass, suggesting shearing during or after

emplacement of the granite intrusions in this area (Fig. 3.13B).

Petrographic studies of hand specimen and polished thin sections observed that the

BSK granite samples are primarily composed of the following phenocryst modal abundances

and crystal dimensions, respectively: (a) pink-orange coloured elongated potash feldspar

(30-35 modal %; <15mm long x >10mm width); (b) subhedral plagioclase (<20 modal %;

>10mm diameter), and; (c) elongate subhedral quartz (30-35 modal %; <15mm x <10mm).

Trace amounts of euhedral and lozenge-shaped pyrite (<1mm diameter) were also observed to

be incorporated into quartz and feldspar phenocrysts. The groundmass is highly sheared and

shows a foliated fabric occurring between larger framework phenocrysts comprising: (a) fine-

interlocking elongated and sheared quartz crystals (<10 modal %; <0.5mm diameter);

(b) sheared potash feldspar (<5 modal %, <1 mm diameter), and (c) interstitial fibrous biotite

(<5 modal %; <2mm long). Early minor sericite alteration was observed along feldspar margins

and later stage chlorite alteration of biotite occurs and also fills late stage fractures cutting

feldspars (Figs. 3.13C and D).

3.3.2.3 Granite whole rock geochemistry

The XRF analytical technique was used to obtain both major and trace element

geochemical data for the two granite samples summarised in Table 3.3.3, with the analytical

method and data provided in more detail in Appendix 3.3.1. Using the data from Table 3.3.3,

the diagram of Na2O+K2O versus SiO2 shown in Fig. 3.3.14A indicates that the BSK samples

occur in the known field for granite, based on the classification by Cox et al. (1979). The plot

of immobile elements Zr/Y and Zr in Fig. 3.3.14B also indicated that the SMD RDP samples

listed in Table 3.3.3 occur in the field for continental arcs established by Pearce et al. (1984).

A diagram of the results for Rb versus Y+Nb shows that the BSK granite intrusions: (a) occur

in the same fields for the SMD RDP, and; (b) most likely occur in the known field established

for I-type granites by Pearce et al. (1984), shown in Fig. 2.4.15.


55

Table 3.3.3. Whole rock XRF data for two granite intrusions from the margins of the Sepon Basin.

Sample # SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Loss (+S) Total (-S) S

BSK5531424 67.99 0.47 15.38 4.26 0.08 1.51 0.15 3.62 3.76 0.10 2.78 100.10 <0.01

BSK5531425 66.98 0.44 15.22 3.82 0.08 1.45 0.81 4.43 4.59 0.26 1.50 99.59 <0.01

As Ba Bi Ce Cr Cu La Nb Nd Ni Pb Rb Sc

BSK5531424 <3 1404 <2 73 85 23 57 24 41 12 24 123 8

BSK5531425 <3 1436 <2 92 100 24 54 14 38 12 25 140 8

Detection Limit 3 4 2 4 1 1 2 1 2 1 1.5 1 2

Se Sr Th U V Y Zn Zr

BSK5531424 <1 443 14 2 95 25 66 219

BSK5531425 <1 639 16 2 82 19 61 181

Detection Limit 1 1 1.5 1.5 1.5 1 1 1

Fig. 3.13. Photographs showing lithological features of granite in the Ban Sopmi-Kengkok (BSK) area, located approximately 15 km south-east of the Sepon mining operations (Fig. 3.12). (A) The BSK granite body in outcrop. This porphyritic textured granite (orange-light brown rocks) intrudes light grey-green lower greenschist facies Proterozoic metasedimentary rocks. (B) Outcrop with large phenocrysts of elongated pink-orange feldspar (<15mm long by >10mm wide) and subhedral quartz (>10 mm diameter) in a foliated groundmass suggesting shearing during or after emplacement of the granite intrusions in the BSK area. (C) Hand specimen from the BSK area composed of pink-orange coloured elongated potash feldspar, subhedral plagioclase and elongate subhedral quartz. (D) Photomicrograph using transmitted light showing sheared groundmass with foliated fabric between larger framework phenocrysts comprising fine-interlocking elongated and sheared quartz crystals, sheared potash feldspar and interstitial fibrous biotite (Sample number BSK5531424).

Granite


56

Fig. 3.14. Diagrams showing rock type classification, using geochemical data for the two granite samples listed in Table 3.3.3. (A) Igneous rock classification diagram adapted from Cox et al. (1979) using Na2O+K2O versus SiO2

that shows the BSK samples in the field for granite (blue dots). (B) Diagram using Zr/Y versus Zr plot indicating that both the SMD RDP (red dots) and BSK granite (blue dots) also plot into the field reported for Continental Arc rocks by Pearce et al. (1984).

Fig. 3.15. Diagrams showing comparison of geochemical data from the SMD RDP and BSK granite samples listed in Tables 3.3.2 and 3.3.3 respectively. These two diagrams use the classification by Pearce et al. (1984) based on their four main felsic rock groups: volcanic arc granites (VAG), collision granites (COLG), within plate granites (WPG), and ocean ridge granites (ORG). Diagrams A and B both show the SMD RDP (red dots) and BSK granite samples (blue dots) occurring in the fields for I-type granites.


57

3.3.3 Mafic intrusions

The occurrence of mafic dykes with basaltic and andesitic compositions in the Sepon

Basin and the surrounding region is briefly mentioned in Loader (1999) and Manini et al.

(2001). Norris (1999) reported that mafic intrusions in the region are rare, generally small-scale

and have mostly been observed: (a) as dark coloured dykes of probable basaltic to andesitic

compositions that cut older granite along the Sepon Basin margins in the Leloy sector near

BSK (Fig. 3.12), and (b) as dark green feldspar-phyric or hornblende-phyric mafic dykes

intersected in some SMD drill-holes. However, the distribution, age of emplacement and timing

relationships, petrography, geochemistry and classification of mafic intrusions in and around

the Sepon Basin remains poorly constrained.

A rare example of a small (<3m wide) fine-grained dark green mafic dyke cutting RDP

was intersected by LXML during 1994 in exploration drill hole DIS001 at the Discovery

Colluvial gold deposit (Fig. 3.16). The RDP intrusion shown in Fig. 3.16 also contained base

metal veins with pyrite-galena-sphalerite-tetrahedrite-quartz, suggesting that the cross-cutting

mafic dyke containing no sulphides was emplaced during or after base metal mineralisation. A

brecciated RDP contact was also observed but breccia fragments are absent from the mafic

dyke, suggesting that the RDP was possibly fractured earlier and the mafic dyke was then later

emplaced along the faulted contact. A dark green-brown fine-crystalline chilled margin

(<10mm wide) can be observed along the contact between RDP and the mafic dyke (Fig. 3.16).

Petrographic investigations of a polished thin section from the mafic dyke in drill hole

DIS001 at 94.4m down hole depth (DIS0010944) observed a framework with large carbonate-

altered phenocrysts of acicular plagioclase (<20 % modal abundance; <3 mm long), rounded

anhedral olivine (<10 modal %; <2mm diameter) and minor pyroxene (Fig. 3.16).

The groundmass is composed of a fine chlorite-altered crystalline matrix dominantly composed

of acicular plagioclase (<45 modal %; <0.25 mm long), pyroxene (<10 modal %) and minor

olivine and ilmenite. No zircons were observed. Thin carbonate veins (<3 mm wide) cut both

framework and groundmass minerals and contain traces of pyrite. Early carbonate alteration

replacing phenocrysts is interpreted to be coeval with the carbonate vein stage that also has

associated disseminated pyrite (<3 modal %) and minor fine-equigranular quartz. Late stage

chlorite forms thin reaction zones (<0.5mm wide) around phenocrysts and fills fractures (Fig.

3.16). This sample generally exhibits a doleritic texture and mineral assemblage. The whole

rock major element data for sample DIS0010944 is shown in Table 3.3.4. The overall

observations from this study and previous workers suggest that the cross-cutting mafic dykes in

the SMD postdate emplacement of both RDP and granite in this district.


58

Table 3.3.4. Whole rock geochemical data for a mafic dyke with doleritic composition, collected from drill hole DD94DIS001 at the Discovery Colluvial gold deposit (DSC), as represented in Fig. 3.16 (Sample numbered DIS0010944). The detection limits (DL) for all of the XRF analyses are also shown..

Sample # SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Loss (+S) Total (-S) S

43.05 1.27 13.11 8.67 0.13 7.12 12.47 3.22 0.78 1.04 9.07 99.91 0.04

As Ba Bi Ce Cr Cu La Nb Nd Ni Pb Rb Sc<3 1139 <2 147 328 62 91 35 51 254 9 23 18

Detection Limit 3 4 2 4 1 1 2 1 2 1 1.5 1 2

Se Sr Th U V Y Zn Zr<1 1330 19 <1.5 177 19 85 196

Detection Limit 1 1 1.5 1.5 1.5 1 1 1

Fig. 3.16. Textural features of a mafic dike cutting rhyodacite porphyry (RDP) at the Discovery Colluvial deposit. (A) Photograph showing a 3m thick dark-green mafic dyke (dolerite) cuts light-grey RDP at the DSC gold deposit, intersected in drill hole DD94DIS001 at 94.4 m depth. (B) Photograph showing a contact between RDP and dolerite dyke at 94.4m depth. Note an irregular and chilled intrusion margin preserved in the dolerite dyke. (C) Fresh dolerite dyke preserved in diamond drill core from hole DD94DIS001 at 94.8 m depth. Note thin carbonate veins (<3 mm wide, white) cut both framework and groundmass minerals and contain traces of pyrite. (D and E) Photomicrographs showing a framework with large carbonate altered phenocrysts of acicular plagioclase (<20 % modal abundance; <3 mm long), rounded anhedral olivine (<10 modal %; <2mm diameter) and minor pyroxene (Transmitted light). The groundmass is composed of a fine chlorite altered crystalline matrix dominantly composed of acicular plagioclase (<45 modal %; <0.25 mm long), pyroxene (<10 modal %) and minor olivine and ilmenite. Early carbonate alteration replacing phenocrysts is interpreted to be coeval with the carbonate vein stage that also has associated disseminated pyrite (<3 modal %) and minor fine equigranular quartz. Late stage chlorite also forms thin reaction zones (<0.5mm wide) around phenocrysts and fills fractures.


59

3.4 GEOCHRONOLOGY OF SMD IGNEOUS ROCKS

3.4.1 Introduction

Previous geochronology investigations undertaken to determine the age of intrusions in

the SMD consist of one study by Barley and Khin Zaw (1999) as part of the AMIRA P390A

Project conducted by CODES through UTAS. The analyses were conducted by Rak (1999)

under the supervision of Mark Barley at the University of Western Australia (UWA) using the

SHRIMP U-Pb zircon dating method for only one RDP sample (5410861), which was collected

by RioTinto geologists from Boung Prospect, located near Thengkham (Fig. 3.7). The results

from Barley and Khin Zaw (1999) yielded a U-Pb zircon age of 290 5 Ma and were

subsequently published in Loader (1999). Before this study, no other U-Pb zircon age

determinations are reported for the RDP intrusions or granites in the vicinity of the SMD. To

further constrain the ages of RDP and granite intrusions occurring in the SMD, the aims of the

U-Pb age determination on zircons obtained during this study, were to:

1. Investigate the range of RDP ages occurring in the SMD gold and copper deposits;

2. Establish the age of: (a) RDP intrusions associated with mineralised skarn, and

(b) RDP intrusions cutting mineralised rocks, to bracket the age of skarn-associated

mineralisation observed by LXML at Thengkham Prospect, and;

3. Determine the age of granites that occur along the southern margins of the Sepon Basin

to investigate if these ages are similar to the RDP ages established for the SMD.

3.4.2 U-Pb analytical methodology used for SMD zircon geochronology

The Laser Ablation–Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS)

method was applied to determine the U-Pb ages of zircons from both RDP and granite intrusion

samples collected in the SMD. The LA-ICPMS method is well established and has been used to

measure U, Th and Pb isotopic data during previous geochronology studies of zircon by Fryer

et al. (1993), Black et al. (2003), Black et al. (2004), Harris et al. (2004), Jackson et al. (2004),

and, Meffre et al. (2007). The analyses in this study were undertaken at CODES, UTAS using

the U-Pb zircon analytical method described by Meffre et al. (2007).

Zircons from 1 kg samples of intrusive rocks collected from the SMD, were first

separated at CODES, UTAS, using a conventional gravity and magnetic heavy mineral

separation method that is outlined in Appendix 3.4.2 and based on a similar version to the

method published in Meffre et al. (2007). A Cr-steel ring mill was used first to mill individual

rock samples, which were then sieved to obtain <180 micron zircons. A gold panning technique

followed by use of a Fe-Be-Nd hand magnet was applied to the <180 micron sample fraction to

separate the magnetic from the non-magnetic heavy minerals. Zircons were then hand picked

from the remaining heavy mineral concentrate using a microscope with cross-polarised

transmitted light. Separated zircons were set onto double-sided sticky tape and then a 2.5 cm

diameter round polished mount mould was placed over the area of zircons selected. Epoxy resin


60

was then poured into the moulds and allowed to dry for 12 hours before being polished with

clean sandpaper followed by a clean polishing lap to expose the zircons at the surface. Distilled

water in an ultrasonic bath was then used to wash and clean the samples for 5 minutes (Meffre

et al., 2007). Before U-Pb analyses, images of the zircons in each sample were produced using

cathodoluminescence (CL) imaging on an electron microprobe at the CSL, UTAS to identify

the individual zircon rim and core zones for testing (Fig. 3.17, Appendix 3.4.2).

A Hewlett Packard 4500 Quadrupole ICPMS with a 213 nm new wave solid state laser

was used to collect the primary LA-ICPMS U-Pb data from a minimum of 12 zircons in each

individual sample set (Table 3.4.1; Appendix 3.4.2). Individual LA-ICPMS U-Pb zircon

analyses commenced with 30 second blank gas measurement followed by switching the laser

on for a further 30 seconds of analytical time. A 35 micron laser beam diameter operating at

5 Hz and a density of approximately 12 J/cm2 was used to (a) sample the rims of zircons to

determine the age of zircon growth during emplacement of intrusions, and (b) test cores to

determine the age of inherited zircons. Particles liberated by the laser were subsequently carried

out of the chamber by a flow of helium carrier gas at a rate of 0.95 litres/minute and mixed with

argon gas before being carried to the ICPMS plasma torch. Elements measured sequentially for

0.14 seconds include 96Zr, 146Nd, 178Hf, 202Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th and 238U, with

longer counting time on Pb isotopes compared to other elements (Meffre et al., 2007). Routine

analyses involved: (a) first testing four primary standard zircons (Temora standard of Black et

al., 2003), (b) then measuring two secondary standard zircons (91500 standard of Wiendenbeck

et al., 1995), then (c) testing a set of twelve zircons from an individual sample, involving a total

of one hour to complete (Meffre et al., 2007).

Data reduction was conducted by Sebastian Meffre using the method outlined in

Appendix 1 of Meffre et al. (2007), which is modified from the method of Black et al. (2004) to

suit the LA-ICPMS at UTAS. The method by Kosler (2001) was used to calculate element

abundances using Zr as the internal standard element, assuming stoichiometric proportions and

using the secondary standard 91500 to correct for mass bias (Meffre et al., 2007).

3.4.3 Zircon petrology

Zircons from the analysed SMD RDP samples are clear to light yellow or light pink in

colour and vary from prismatic euhedral to sub-rounded stubby euhedral crystals that are

generally <200 microns long (Fig. 3.17). Oscillatory zonation of zircons was observed under

the microscope using transmitted light and was confirmed using CL-imaging (Fig. 3.17A).

Inheritance of geometrically complex older zircon cores surrounded by later oscillatory zircon

zonation was also revealed via CL-imaging in some samples (Figs. 3.17B-F).


61

Fig. 3.17. Photomicrographs showing textural features of SMD zircons from RDP and granite intrusions for age dateing using U-Pb LA-ICPMS at CODES, UTAS. (A) Transmitted light showing prismatic elongated clear to pink coloured zircons from Padan Prospect RDP sample PDN002208. (B) Cathodoluminescence (CL) image showing tabular zoned zircons from Boung prospect RDP sample 5410861. Some zircons show complex inherited cores with irregular shapes. (C) Prismatic and tabular shaped zoned zircons with some inherited cores shown by CL imaging of Discovery Main RDP sample DIS02500860. (D) CL image showing fragments of tabular zircons with inherited cores from Nalou RDP sample NLU0611460. (E) Thengkham RDP sample PCTKM03001 containing tabular zircons with complex inherited cores (CL image). (F) Concentric zonation in a prismatic zircon from Thengkham West RDP sample TKW531307. (G) Pink zircons from granite sample BSK5531425 under transmitted light. (H). CL image of zircons with complex inherited cores in granite sample BSK551424.


62

Both granite samples from BSK mostly contained stubby euhedral clear to light pink-

orange zircon crystals that were generally <250 microns long and showed oscillatory zonation

under transmitted microscopic light (Fig. 3.17G - H). Cathodoluminescence imaging revealed

that the BSK zircon populations also contained a high proportion of morphologically complex

inherited older cores surrounded by later oscillatory zircon growth banding (Fig. 3.17H).

3.4.4 Geochronology results

A total of thirteen SMD RDP and two BSK granite intrusion samples were dated using

the LA-ICPMS U-Pb zircon method at CODES, UTAS. The results from this study are

summarised in Table 3.4.1 and Figs. 3.18 to 3.19, with the full results including Concordia

plots of U-Pb data provided in Appendix 3.4.2. A total of 12 to 24 zircon grains per sample

were analysed (Table 3.4.1). From the 11 RDP samples submitted for dating, a total of 198

zircons were analysed with 47 zircons being rejected from this population (Table 3.4.3). A total

of 30 zircons were also analysed from the BSK granite samples listed in Table 3.4.3, from

which 14 zircons were rejected due to yielding inherited core ages (Appendix 3.4.2). Reasons

for rejecting zircons were mostly associated with (a) the intersection of older cores during laser

analyses, or (b) excessive lead loss from individual zircons that were not conducive to yield an

age (Appendix 3.4.2). A histogram of U-Pb data for all of the RDP zircon grains analysed

shows the range of ages determined during this study, including inherited cores (Fig. 3.18).

Table 3.4.1. Summary of LA-ICPMS U-Pb isotopic ages for zircons occurring in SMD RDP intrusions and granite intrusions from along the margins of the Sepon Basin.

Sample # Area E N Lithology Age Dev (Ma) % Error No. of No. Comments

(India 1960) (India 1960) (Ma) (2 ) (MSWD) analyses rejected

PCNAK03001 NAK 584500 1877250 RDP 289.8 5.7 2.0 18 6 Outcrop

TKW0531307 TKW 597102 1873810 RDP 297.0 4.0 1.3 24 11Drill core (@ -130.7m): Pre-syn mineralisation

TKW0531157 TKW 597102 1873810 RDP 283.0 2.0 0.8 18 2Drill core (@ -115.7m): Post-mineralisation

PCTKM03001 TKM 600027 1874282 RDP 287.8 2.3 0.8 12 2 Outcrop

PCTKM03003 TKM 600251 1874353 RDP 287.8 2.6 0.9 12 1 Outcrop

BNG5410861 BNG 601500 1877000 RDP 288.5 2.1 0.7 18 2 Outcrop

NLU061046.0 NLU 603760 1874791 RDP 280.0 6.0 2.0 18 9 Drill core (@ - 46m)

PCDSW03001 DSW 604371 1875769 RDP 284.4 4.5 1.6 12 4 Outcrop

PCDSW03010 DSW 604455 1875685 RDP 282.7 5.6 2.0 12 2 Outcrop

DSC5531414 DSC 606005 1875930 RDP 290.0 6.0 2.2 12 1 Outcrop

DIS0250600 DSM 607600 1876220 RDP 286.0 3.0 1.1 18 4 Drill core (@ - 60m)

KHN0130785 KHN 608470 1876119 RDP 283.0 3.0 1.0 12 1 Drill core (@ - 78.5m)

PDN0022740 PDN 610500 1875700 RDP 287.0 2.0 0.7 12 2 Drill core (@ - 274m)

BSK5531425 BSK 617096 1857714 GRANITE 243.0 3.0 1.1 12 3 Outcrop

BSK5531424 BSK 616814 1858015 GRANITE 247.0 4.0 1.5 18 11 Outcrop Abbreviations: BNG = Boung, BSK = Ban Sopmi-Ban Kengkok, DSC = Discovery Colluvial, DSM = Discovery Main, DSW = Discovery West, KHN = Khanong, NLU = Nalou, PDN = Padan,, TKM = Thengkham,, TKW = Thengkham West. Note: Co-ordinates are expressed in UTM using the India 1960 datum as a reference.


63

200 400 600 800 1000 1200 1400

AGE (Ma)

Rela

tive

Pro

babili

ty

Fig. 3.18. A relative probability histogram of the SMD RDP zircon ages obtained from the 198 zircons listed in Table 3.4.3 and Appendix 3.4.2. The youngest age of RDP intrusions is at 280 Ma. This diagram also shows the age of inherited cores ranging from 350 Ma to older inherited cores up to 1390 Ma.

Plot of SMD U-Pb Zircon Ages vs Easting Locations

230.0240.0250.0260.0270.0280.0290.0300.0310.0

577000 587000 597000 607000 617000

Easting (mE)

Ag

e (M

a)

NAK

TKM

BNG

NLU

DSW

DSC

DSM

KHN

PDN

BSK

TKW

Fig. 3.19. Plot of the range of LA-ICPMS U-Pb zircon isotopic ages determined for the SMD samples listed in Table 3.4.3 against their easting location in the SMD. Each sample shows the associated analytical error bars. The SMD RDP samples ranged from a minimum of 280 + 6 Ma through to a maximum of 297 + 7 Ma and hence bracket RDP emplacement in the SMD, but most RDP samples occurred in the range 282.7 + 5.6 Ma through to 290 + 6 Ma. Abbreviations for RDP sample locations: NAK = Nakachan, TKM = Thengkham South, TKW = Thengkham West, BNG = Boung, NLU = Nalou, DSW = Discovery West, DSC = Discovery Colluvial, DSM = Discovery Main, KHN = Khanong, PDN = Padan. BSK = Bansopmi-Kengkup (granite samples).


64

3.4.5 SMD RDP geochronology results

The eleven individual SMD RDP samples listed in Table 3.4.3 yielded new LA-ICPMS

U-Pb zircon ages that ranged from a minimum of 280 + 6 Ma through to a maximum of 297 7

Ma and hence bracket RDP emplacement in the SMD during this period (Fig. 3.19). However,

most RDP samples occurred in the range 282.7 5.6 Ma to 290 6 Ma (Table 3.4.3).

Comparing different dating methods, and taking into consideration the associated statistical

errors, the single test sample from Boung prospect (BNG5410861) yielded a LA-ICPMS U-Pb

zircon age of 288.5 + 2.1 Ma in this study, which compares favourably with the SHRIMP U-Pb

zircon age of 290 + 2 Ma that was obtained during the earlier study by Barley (1999) on the

same sample (Table 3.4.3). Overall, the remaining new LA-ICPMS U-Pb zircon results also

confirm the 290 + 2 Ma RDP age reported by Barley and Khin Zaw (1999).

Prior to all LA-ICPMS U-Pb analyses undertaken during this study, CL-imaging

proved to be an essential requirement to show the SMD RDP zircons with suitable oscillatory

zircon rims to target for age dating, and to highlight inherited older cores in zircons that needed

to be avoided during analyses. Figures 3.17B and 3.17E show examples of CL-images that

revealed several zircons with large inherited cores that needed to be avoided during LA-ICPMS

analyses, as results from the core zones would have produced older non-representative RDP

intrusion ages. Precise positioning of the LA beam along zircon rims was also assisted by the

use of CL-images.

The LA-ICPMS U-Pb zircon dating method was also used to constrain the timing of

Cu-skarn associated mineralisation in RDP at Thengkham West (TKW) in the eastern central

western sector of the SMD (Figs. 3.2 and 3.7). Two RDP samples were forwarded from the

TKW Prospect area by LXML and comprised a sulphide mineralised RDP intrusion collected at

130.7m depth in drill hole TKW053 (sample TKW0531307), that was cut by a later stage

unmineralised RDP intrusion at 115.7m (sample TKW053115.7). The first sample is inferred to

have been emplaced before or during mineralisation, whereas the second is inferred to be

emplaced after the mineralisation (James Cannell, pers com., 2006). Petrographic investigations

showed that sample TWW0531307 contained thin veinlets (<2 mm wide) of quartz and pyrite

and traces of chalcopyrite cutting quartz and feldspar phenocrysts in the RDP host rock (Figs.

3.20A-B), and sample TKW0531157 contained no visible evidence of sulphide mineralisation

(Fig. 3.20C-D). LA-ICPMS U-Pb zircon analysis of both these samples indicated that sample

TKW0531307 is the oldest RDP intrusion in the SMD, yielding an age of 297 + 4 Ma, and

sample TKW0531157 yielded a younger age of 283 + 2 Ma (Table 3.4.3, Fig. 3.20). The results

of this investigation broadly bracket the Cu-skarn associated mineralisation at TKW between

297 + 4 and 283 + 2 Ma.


65

Fig. 3.20. Thengkham West RDP examples analysed by LA-ICP-MS for their U-Pb zircon ages: (A) Photograph of mineralised (TKW0531307) hand specimen showing a vein with quartz and pyrite cutting RDP; (B) Photomicrograph of TKW0531307 (A) showing pyrite and quartz cutting phenocrysts of feldspar and quartz (TML+RFL+XPL); (C) Photograph of unmineralised RDP (TKW0531157) hand specimen; (D) Photomicrograph of TKW0531157 (C) showing rounded quartz eyes and biotite in a sericite altered quartz and feldspar matrix, with no sulphides present (TML+XPL).

3.4.6 BSK granite geochronology results

The 2 individual BSK Granite samples yielded new LA-ICPMS U-Pb zircon ages that

ranged from 243 + 3 Ma to 247 + 4 Ma, indicating that granite emplacement occurred near the

southern margins of the Sepon Basin during the Early Triassic (Table 3.4.1; Fig. 3.12). Several

zircons in the BSK samples analysed also contained complex inherited cores with ages ranging

from 260 to 1161 Ma (Appendix 3.4.2). No other zircon ages are known from the BSK area or

the surrounding region in Savannakhet Province, southern Laos, for comparison. However, in

comparison with the nearby SMD RDP intrusions ages obtained during this study using the

same LA-ICPMS U-Pb zircon dating method, the two BSK granite intrusions analysed are 37

to 54 Ma younger than the 280 to 297 Ma SMD RDP intrusions (Table 3.4.3 Fig. 3.19).


66

3.4.7 SMD mafic dyke geochronology

During this study attempts were made to determine the age of a late mafic dyke of

doleritic composition (sample DIS0010944, Table 3.3.4) cutting an RDP intrusion with

sulphide mineralisation at the Discovery Colluvial gold deposit (Fig. 3.16). No zircons could be

extracted from DIS0010994, however, apatite crystals were obtained from the sample but did

not contain enough uranium to yield a suitable LA-ICPMS U-Pb age. Small ilmenite crystals

(<100m in length) were also extracted, but were too thin to allow enough material to be

collected during a 30 second laser ablation interval to determine an isotopic age using the LA-

ICPMS U-Pb method. No further age dating attempts were conducted and the age of mafic

dykes occurring in the SMD remain unconstrained except that it was after Early Permian RDP

emplacement as they cut those intrusions (Fig. 3.16).

3.4.8 Comparison of SMD zircon data to previous regional geochronology studies

Before the geochronology investigations by Barley and Khin Zaw (1999), no other

U-Pb zircon age determinations were reported or published for the RDP intrusions or granites

in the vicinity of the SMD. There is also no published information about U-Pb zircon ages for

intrusions from neighbouring parts of Vietnam and Cambodia.

The U-Pb geochronology results obtained from the SMD zircons in this study provide

new information on the timing of intrusion emplacement along the Truongson Foldbelt, in

particular (a) indicate that RDP emplacement in the SMD ranges from 280 ± 6 Ma to 297

± 7 Ma ; (b) confirm the single U-Pb zircon age of 290 ± 5 Ma for RDP sample 5410861 from

the SMD Boung prospect obtained by Barley (1999); (c) broadly bracket the timing of Cu-

skarn associated mineralisation at Thengkham west (TKW) between 297 ± 4 Ma and 283 ± 2

Ma; and (d) indicate Early Triassic (243 ± 3 Ma to 247 ± 4 Ma) emplacement for the Ban

Sopmi-Kengkok granites located near the southern margins of the Sepon Basin. In comparison,

published Ar-Ar and K-Ar data from Leprivier et al. (1997) indicate that dextral shear fabrics in

paragneiss along the southern section of the Truongson Foldbelt near Da Nang and Khe Sahn in

Vietnam yield 40Ar-39Ar plateau cooling ages of late Triassic (241 Ma to 245 Ma) that are

similar in age to the BSK U-Pb zircon dates obtained in this study.


67

3.5 DISTRICT-SCALE STRUCTURAL SETTING OF THE SMD

The district-scale structural setting of the Sepon Basin is summarised in this section.

Structural information pertaining to the SMD has mostly been sourced from Oxiana company

reports by Marten (1997, 1998a, b, 1999), Coller (1999), Loader et al. (1999), Norris (1999)

and Smith (2003). Company memoranda by Marten (1997) and Vanderhor (1997), and

publications by Loader (1999), Manini et al. (2001) and Smith et al. (2005) have also been used

to provide a description of the SMD structural setting.

3.5.1 Architecture of the Sepon Basin

The present day geometry of the Sepon Basin was interpreted by Coller (1999) to be an

inverted E-W trending basin defined by a folded and faulted Palaeozoic sedimentary package of

carbonate and siliciclastic rocks. An early rift basin setting is interpreted for the deposition of

Palaeozoic sequences in the Sepon Basin, with primary controls by inherited WNW-trending

basement faults that form the opposing margins of the basin (Marten, 1998a; Coller, 1999;

Smith, 2003). Stratigraphy within the Sepon Basin is bounded to the north by the E–W trending

Northern Fault, to the south by an interpreted north-trending shoreline and to the west by the

regional NW-trending Truongson Fault (Fig. 3.21). The Northern Fault is interpreted by Marten

(1999a) to form a main bounding growth fault zone for the half graben structure that controlled

the deposition of north-dipping beds in the Sepon Basin.

Based on the observations by Marten (1998a, b, c), Coller (1999) and Smith (2003),

steep-sided faults divide the SMD into at least five main structural blocks or domains that are

summarised here, and shown in Fig. 3.21:

1) Phu Xo-Nampa siliciclastic block (PNSB) is predominantly comprised of a thick sequence

of sandstone and siltstone belonging to the Houay Bang and Nampa Formation

respectively, with NW-, NE- and E-W oriented fold axes. This block occurs from the

eastern extent of the SMD through to the contact with the Truongson Fault in the south-

western areas of the SMD;

2) Southern carbonate block (SCB) consists of a NW-trending calcareous shale Vang Gngang

and dolomitised limestone (Nalou Formation) package that is bounded to the south by the

Truongson Fault and to the north by the Thengkham-Ban Non Block;

3) Thengkham-Ban Non siliciclastic block (TBSB) contains a shale and siltstone E-W oriented

siliciclastic package in the centre of the Sepon Basin, and extending from the Thengkham

area through to Pha Vat and Ban Non areas;

4) Northern carbonate block (NCB) is an E-W oriented carbonate-dominant package

comprised of calcareous shale (Discovery Formation) and dolomite (Nalou Formation) that

is bounded to the south by the TBSB and to the north by the Northern siliciclastic block

(NSB); and;


68

5) Northern siliciclastic block (NSB) is mostly comprised of sandstone that occurs to the

north of the North Bounding Fault and extending the entire northern length of the SMD

from the east to the west (Fig. 3.21).

Overall, the Sepon Basin geology consists of two ENE-oriented carbonate packages

wedged between three siliciclastic blocks of variable orientation. The boundaries between these

blocks are interpreted to be controlled by the original basin-forming extensional faults. Most of

the contacts between formations in the SMD are dominantly structural and only two contacts

between formations appear to be depositional with known transition zones, namely between the

Nalou and Discovery Formation and also the Discovery Formation and the Nam Kian

Formation (Smith, 2003; Smith et al., 2005).

3.5.2 Major faults in the SMD

3.5 Two main fault-set directions occur within the SMD, comprising: (1) NW- and WNW-

trending faults with steep NE dips that mostly occur parallel to the Truongson Fault,

and (2) E-W and ENE-trending faults with subvertical dips and also steep north

or south dips that dominantly occur parallel to the basin bounding Northern Fault (Fig.

3.21; Coller, 1999; Smith, 2003). Both Coller (1999) and Smith (2003) interpret a

linked fault system occurring in the SMD, with (a) the original basin-forming

extensional faults represented by E-W and ENE – trending faults, and (b) reactivated

basin-forming transfer zones represented by WNW- to NW–trending faults (Figs. 3.21

and 3.22).

Fig. 3.21. Broadly defined basin architecture of the SMD showing the five main structural blocks, basinal NW-trending extensional faults and interpreted ENE-trending transfer faults shown in light green (from Smith, 2003).


69

Major E-W and ENE-oriented faults in the SMD include the Discovery, Thengkham-

Nalou, Nampa and Vang Ngang Faults, the characteristics of which are summarised in Table

3.5.1 and locations shown in Figs. 3.21 and 3.23. Examples of major WNW to NW-trending

structures are represented by the Truongson, Ban Vieng, Muang Luang and North-western

Faults (Table 3.5.1; Figs. 3.21 and 3.23). The intersection of these two major fault trends is

considered important, especially in localising and focussing RDP intrusions and hydrothermal

mineralisation in the SMD (Manini et al., 2001; Smith, 2003; Smith et al., 2005).

A transpressional structural model for the early formation of the Sepon Basin was

proposed by Marten (1997, 1998a), Coller (1999) and Smith (2003), involving E-W

compression resulting in major sinistral strike-slip faults and development of a N-S trending

pull-apart basin (Figs. 3.21 and 3.22). The timing of early sinistral transpression is not

constrained along the Truongson Foldbelt, but is interpreted to be before the deposition of

Ordovician sediments belonging to (Ekins, 2005). During the Early Triassic (Indosinian) at

245 Ma, regional-scale dextral movements reported by Lepvrier et al. (1997) overprinted all

earlier structural fabrics along the Truongson Foldbelt, and the Sepon Basin is interpreted to

have been inverted by N-S compression (Coller, 1999; Smith, 2003).

Fig. 3.22. Simplified model for the development of the SMD in an E-W oriented pull-apart basin in a NW-directed sinistral transpresional zone along the Truongson Fold Belt (from Smith, 2003).


70

Table 3.5.1. Summary of the major faults in the SMD, Lao PDR

Main fault number and name Fault Strike / Dip Fault characteristics Refs

(1) Truongson Fault (TSF) NW / unknown dip

NW- trending regional-scale strike-slip transfer fault, bounding the Sepon Basin along the south-western margin, with: (a) an early sinistral transpressional movement with E-W shortening, creating an interpreted rift setting and the subsequent onset of sedimentation during the Ordovician, and; (b) later dextral movement creating N-S shortening and inversion, most likely during the Indosinian (Leprivier et al., 1997).

(1) (2) (4)

(2) Nakachan Fault (NCF) NW / unknown dip NW-trending dextral strike-slip fault parallel to the TSF (2)

(3) Pha Vat Transfer (PVT) NW / unknown dip NW-trending strike-slip transfer fault (1)

(4) Houay Yeng Transfer (HYT) NW / unknown dip NW-trending strike-slip transfer fault (1)

(5) Lat Deng Transfer (LDT) NW / unknown dip NW-trending strike-slip transfer fault (1)

(6) Ban Vieng Transfer (BVT) NW / sub-vertical NW-trending dextral strike-slip transfer fault (1)

(7) Muang Luang Fault (MLF) NW / Steep NENW-trending steeply dipping normal fault that is interpreted to splay off the ENE-trending North Bounding Fault (NBF)

(1) (2) (3)

(8) North Western Transfer (NWT) NW / unknown dipNW-trending strike-slip transfer fault, that is interpreted to splay off the North Bounding Fault (NBF)

(1) (2)

(9) Discovery West Fault (DWF) NW / sub-vertical NW-trending dextral strike-slip transfer fault(1) (4)

(10) Thengkham - Nalou - Namkok -Nampa Fault (TNNN)

WNW / Steep NE WNW-trending steeply dipping normal fault (1) (2)

(11) North Bounding Fault (NBF) ENE / Steep N

ENE-trending steeply dipping dextral strike-slip fault with a >1100m offset, juxtaposing the Houay Bang Formation with the Discovery Formation and the Nan Kian Formation in the NE. Forms the northern bounding fault for the Sepon Basin.

(1) (2) (3)

(12) Ban Mai Fault (BMF) WNW / Moderate N WNW-trending dextral strike-slip fault(1) (4)

(13) Ban Non Fault (BNF) WNW / Steep S WNW-trending dextral strike-slip fault(1) (4)

(14) Nam Khun Fault (NKF) ENE / Steep N ENE-trending reverese dip slip fault(1) (2)

(15) Phavat South Fault (PVF) ENE / unknown dip ENE-trending strike-slip fault (1)

(16) Samliam Fault (SLF) ENE / unknown dip ENE-trending strike-slip fault (1)

(17) Discovery Fault (DCF) ENE / Steep NENE-trending reverse dip slip fault along margins of the Discovery Colluvial and Discovery Main SHGD

(1)

(18) Vang Ngang Fault (VNF) ENE / Steep N ENE-trending reverse dip slip fault (2)

References used in this table: 1 = Smith (2003); 2 = Loader et al. (1999); 3 = Loader (1999); 4 = Marten (1998)

Fig. 3.23. District-scale geology map of the Sepon Mineral District (SMD) showing the location of major fault zones and gold and copper deposits (provided courtesy of OZ Minerals Limited). Abbreviations used from Table 3.5.1: (1) = Truongson Fault, (2) Nakachan Fault, (3) = Pha Vat Transfer, (4) = Houay Yeng Transfer, (5) = Lat Deng Transfer, (6) = Ban Vieng Transfer, (7) = Muang Luang Fault, (8) = North-Western Transfer, (9) = Discovery West Fault, (10) = Thengkham-Nalou-Namkok-Nampa Fault, (11) = North Bounding Fault, (12) = Ban Mai Fault, (13) = Ban Non Fault, (14) = Nam Khun Fault, (15) = Pha Vat South Fault, (16) = Samliam Fault, (17) = Discovery Fault, (18) Van Ngang Fault. 71( ) , ( ) y , ( ) g g


72

3.5.3 District-scale folding

Coller (1999) observed that the district-scale folds in the north-eastern and central

sectors of the SMD have predominantly gentle ENE-trending axes, indicating generation by

NNW- to-SSE oriented compression (Fig. 3.23). Loader et al. (1999) reported that broad open

folds generally occur in the SMD, with siliciclastic rock sequences typically showing fold

wavelengths ranging from 300 to 600m and carbonate rock sequences exhibiting wavelengths

of between 500 to 1000m. Smaller folds are reported by Loader et al. (1999) to be more

dominant towards contacts with major faults, such as the chevron style folds that can be seen at

the Discovery West and Nalou gold deposits (Fig. 3.24). Towards the western sector of the

SMD, folds dominantly have NW-trending axes that are interpreted by Coller (1999) and Smith

(2003) to be reoriented during a period of dextral movement along the Truongson Fault, such as

the 245 Ma dextral movements reported by Lepvrier et al. (1997). No significant areas of

overturned stratigraphy are currently known in the SMD (Smith, 2003).

Fig. 3.24. Photographs of small-scale folds in the SMD. (A) Gentle folding of calcareous shale (black) under a RDP sill at the Nalou gold deposit. (B) Chevron-like folds around silicified calcareous shale preserved along anticline fold axes at the Nalou gold deposit. (C) Small parasitic M-type folds in chert and calcareous shale at the Discovery West gold deposit.


73

3.5.4 District-scale fault history summary

The timing of the Sepon Basin structural evolution is poorly constrained, however, the

following points summarise the currently interpreted history of the Sepon Basin, based on the

reports by Marten (1997, 1998a), Coller (1999) and Smith (2003), and shown in Fig. 3.25

(adapted from Smith, 2003):

(1) Pre- to syn-Ordovician, early sinistral movement along the Truongson Fold Belt developed

the Sepon Basin through E-W shortening and the subsequent onset of N-S basinal rifting;

(2) Major faults developed during the early sinistral movement comprise steep ENE-trending

normal faults, and NW- and WNW-trending strike-slip transfer faults;

(3) ENE-trending fold axes were preserved in the central SMD carbonate packages, indicating

formation during later NNW-SSE trending compression;

(4) NNS-SSE directed compression is interpreted to have developed during a later dextral

movement along the Truongson Foldbelt, possibly during the 245 Ma event reported by

Lepvrier et al. (1997), resulting in the inversion of the Sepon Basin along pre-existing

faults (Fig. 3.25).

Fig. 3.25. Interpreted models for the structural evolution of the Sepon Basin, including the SMD. (A) Summary model for the interpreted structural evolution of the Sepon Basin adapted from Smith (2003). (B) Interpreted development of a dextral strike-slip model for the Sepon Basin involving NNW-SSE directed compression along the NW-trending Truongson Foldbelt (adapted from Coller, 1998 and Smith, 2003).

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