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Chapter 1 – Introduction
1
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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Chapter 1 – Introduction
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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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Chapter 1 – Introduction
3
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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Chapter 1 – Introduction
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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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Chapter 1 – Introduction
5
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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Chapter 1 – Introduction
6
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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Chapter 1 – Introduction
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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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Chapter 1 – Introduction
8
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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Chapter 1 – Introduction
9
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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Chapter 1 – Introduction
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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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Chapter 1 – Introduction
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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.
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Chapter 2 – Regional Geological Setting
12
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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Chapter 2 – Regional Geological Setting
13
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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Chapter 2 – Regional Geological Setting
14
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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Chapter 2 – Regional Geological Setting
15
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
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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.
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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.
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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
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Chapter 2 – Regional Geological Setting
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
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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).
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Chapter 2 – Regional Geological Setting
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
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Chapter 2 – Regional Geological Setting
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
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Chapter 2 – Regional Geological Setting
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).
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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
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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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
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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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.
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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.
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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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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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
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Chapter 3 – District-Scale Geological Setting of the SMD
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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.
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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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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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.
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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
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Chapter 3 – District-Scale Geological Setting of the SMD
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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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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;
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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).
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Chapter 3 – District-Scale Geological Setting of the SMD
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)
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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
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Chapter 3 – District-Scale Geological Setting of the SMD
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.
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Chapter 3 – District-Scale Geological Setting of the SMD
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).