Geological setting, geochemistry and genesis of the Sepon gold and copper deposits, Laos
Cromie, PW (2010) Geological setting, geochemistry and genesis of the Sepon gold and copper deposits, Laos. PhD thesis, University of Tasmania.
This study documents the geology, mineralogy, geochronology and geochemistry of the
Sepon Mineral District (SMD) gold and copper deposits. The SMD is located in the Sepon Basin
along the Truong Son Fold Belt on the NE margins of the Indochina Terrane in south-eastern Laos.
The geology of the SMD is dominated by Ordovician, Silurian and Devonian-aged continental
fluvial and shallow to deep marine sedimentary rocks that were deposited in a half graben basin.
Intrusion of rhyodacite porphyry (RDP) mainly occurred along pre-existing faults during the Early
Permian, constrained by U-Pb dating of zircons to between 2806 and 2977 Ma.
Three main hypogene mineralisation styles are recognised in the SMD, comprising distal
sedimentary-rock hosted gold (SHGD), proximal skarn (Cu+Au) and central porphyry (Cu-Mo).
Exploration programs in the SMD conducted by CRA/RioTinto (1993-1999), Oxiana Limited/OZ
Minerals Limited (2000-2008) resulted in the discovery of a mineral district containing resources of
83 Mt @ 1.8 g/t Au for 4.75 million ounces of gold in seven separate but adjacent SHGD, and
supergene copper at three deposits, namely the Khanong (27 Mt @ 4.3 % Cu), Thengkham North
(11.4 Mt @ 2.7 % Cu) and Thengkham South (9.8 Mt @ 2.3 % Cu) deposits.
Gold in the SMD is predominantly hosted by Devonian Discovery Formation calcareous
shale with lesser amounts reported in turn for the underlying Devonian Nalou Formation bioclastic
dolomite, the Silurian-Devonian Kengkeuk Formation calcareous shale and the Ordovician-Silurian
Nampa Formation claystone and siltstone sequence. The Nalou Formation bioclastic dolomite and
the Kengkeuk Formation calcareous shale mainly host the known SMD copper deposits.
Hypogene gold and copper ore-types occurring in the SMD deposits are epigenetic and
often occur along steep faults and/or veins cutting all of the Ordovician to Middle Devonian aged
carbonate and siliciclastic rocks and Early Permian RDP dykes and sills. The principal structural
trends controlling and hosting gold mineralisation in the SMD SHGD comprise WNW-striking
normal faults with steep dips and high-angle ENE-striking normal and reverse faults, cutting all of
the Ordovician to Devonian-aged carbonate and siliciclastic rocks and Early Permian RDP dykes
and sills. Gold ore zone geometries in the SMD include (1) ribbon-like bodies that are strike
continuous, moderate to shallow dipping sheets that are not always connected to faults, and fault
controlled steep sheet-like bodies.
Common major sulphide minerals in the SMD gold and copper deposits include, pyrite,
arsenic-rich pyrite, chalcopyrite, and minor sphalerite, galena, bornite and stibnite, but no realgar,
orpiment or cinnabar have been observed. Alteration types occurring in the SMD gold deposits
include (a) variable decarbonatisation of carbonate units, (b) silicification (jasperoid formation)
along permeable horizons and faults, (c) locally developed argillisation along RDP contacts, and (d)
variable dolomitsation of carbonate units. Primary sulphide zones hosting copper mineralisation in
skarns proximal to RDP intrusions are associated with sericite alteration along intrusion margins, in
addition to (a) early prograde garnet and pyroxene skarn alteration cut by later retrograde chloriteepidote
alteration of carbonate host units, and (b) hornfels in non-calcareous sedimentary rocks. At least 5 main paragenetic mineral assemblage stages were observed across the SMD; they
are collectively grouped together as SMD Stages 1 to 5 based on the gold stage paragenetic
observations from this study: (a) the adjacent to distal type SHGD, (b) the proximal Cu (-Au) skarn
deposits underlying supergene copper, and (c) the porphyry Cu (-Mo) deposits. The pre-main gold
ore stages include SMD Stages 1, 2 and 3A. Main gold ore comprises SMD Stages 3B, 3C, 4A and
4B, and SMD Stage 5 is post-main gold deposition.
The SMD Stage 1 assemblage is interpreted to form early in the diagenesis of the Sepon
Basin (i.e. syn- to post-sedimentation); it does not contain any known gold and mainly comprises
rare disseminations of framboidal pyrite (Pyrite 1) hosted by calcareous shale (CSH) rocks, with
minor ferroan dolomite occurring along cleavage and stylolites in the CSH rocks, which are in turn
cut by small late stage milky white calcite veins.
The SMD Stage 2 consists of three types of gold-poor diagenetic pyrite, comprising:
(a) semi-massive nodular-shaped pyrite (Pyrite 2A); (b) euhedral spongy-textured pyrite (Pyrite
2B), and (c) euhedral angular pyrite (Pyrite 2C). These pyrite types contain very low levels of gold,
generally <0.3 ppm Au in the cores and <0.1 ppm in the rims. Characteristic pink calcite (Calcite 2)
filled fractures cutting SMD Stage 2 pyrite and also cuts both cleavages and stylolites containing
Stage 1 ferroan dolomite. The SMD Stage 2 is interpreted to form post-sedimentation and late in the
diagenesis of the Sepon Basin.
The SMD Stage 3 represents the combined main base metal dominant group of
assemblages in the SMD and consists of three sub-stages, namely: (a) the SMD Stage 3A: early
carbonate-hosted pyrite-galena-sphalerite-dolomite veins (i.e. in the SMD SHGD), followed by (b)
the SMD Stage 3B: low grade gold-bearing RDP intrusion-hosted early retrograde veins with
pyrite-sphalerite-galena-quartz-dolomite (SMD SHGD and Cu skarn deposits), and (c) the SMD
Stage 3C: late low grade gold-bearing RDP intrusion-hosted retrograde veins with quartzchalcopyrite+
bornite+molybdenite (SMD Cu skarn and porphyry Cu-Mo deposits). The SMD Stage
3B mineral assemblage contains low grades of gold ranging from 0.13 to <3 ppm Au that mainly
occur in veins containing pyrite and sphalerite, which were observed in all three deposit types,
namely: (a) central porphyry (in Pyrite 3B), (b) proximal skarn (in Pyrite SKN1 and sphalerite), and
(c) distal SHGD (in Pyrite 3B1 and sphalerite). SMD Stage 3C typically comprises chalcopyrite
with inclusions containing low grade gold ranging from 0.06 to 0.9 ppm Au in the central porphyry
and proximal skarn settings.
The SMD Stage 4 is the main high grade gold phase in the SMD. At least two sub-stages
have been observed, namely: (a) the SMD Stage 4A pyrite comprising high grades of gold
concentrated along (i) the growth rims of pyrite cores that also occur along fractures cutting SMD
Stage 3 sulphides in the SMD SHGD, or (ii) associated with rough-textured pyrite cutting SKN
Stage 3C assemblages in the SMD copper deposits, and (b) the SMD Stage 4B Hg-Au telluride
filling fractures in the SMD Stage 3 and SMD Stage 4A sulphides. Both the SMD Stage 4A pyrite
types observed in the SMD SHGD and the SMD proximal copper skarn deposits contained gold
values ranging from >1 and up to 293 ppm Au.
The post-main high grade gold stage assemblages are grouped into SMD Stage 5 and
comprises at least three vein assemblages, namely: (a) the SMD Stage 5A: quartz-stibnite-dolomite,
(b) the SMD Stage 5B: quartz-dolomite, and (c) the SMD Stage 5C: calcite-quartz-fluorite calcitequartz.
No gold grades were observed in the SMD Stage 5 sulphides. The timing of the Cu-Mo mineralisation at SMD is constrained by Re-Os dating of Stage
3C molybdenite which is coeval with Stage 3C chalcopyrite to be between 2871 and 2801 Ma at
the Thengkham South deposit and at Padan Prospect. The absolute timing of the SMD gold
mineralization is not well constrained but the distal nature of the SMD Stage 4A main gold ore to
Pyrite SKN1 suggest that the age of the SMD gold mineralization is broadly similar or slightly
younger than the SMD skarn (Cu+Au) formation.
Detailed textural, paragenetic and LA-ICPMS trace element investigations of sulphides
indicate that within the SMD SHGD, primary gold is not visible, but is associated with pyrite and
occurs in: (1) SMD Stage 2B Pyrite 2B (diagenetic), which contains <0.5 ppm Au in pyrite cores,
(2) SMD Stage 3B Pyrite 3B (in base metal veins), which contains <0.3 to 3 ppm Au as inclusions,
(3) SMD Stage 4A Pyrite 4A, which contains >3 to 200 ppm Au as overgrowths on As-rich rims,
and (4) SMD Stage 4B sulphosalts (in veins), which contain Hg-Au-telluride. The later high grades
of gold in Pyrite 4A could have been derived from the early SMD Stage 2 diagenetic pyrites in the
SMD and/or from the later SMD Stages 3B and 3C pyrite types during syn- to post-emplacement of
the SMD RDP intrusions.
Gold in the SMD copper deposits is not visible, it is within pyrite and occurs in: (1) SMD
Stage 3B Pyrite SKN1 (in retrograde veins), which contains <0.7 ppm Au as inclusions, (2) SMD
Stage 3C Chalcopyrite 3C (in retrograde veins), which contains <0.9 ppm Au as inclusions, and
(3) SMD Stage 4A Pyrite SKN2 (fractures), which contains >1 to 293 ppm Au in As-rich pyrite.
Gold deposited in the SMD Stage 3B and 3C pyrites syn- to post-emplacement of the SMD RDP
intrusions provides potential gold sources for the high grade gold formed in Pyrite SKN2.
Both the LA-ICPMS and PIXE NMP analytical methods established that the pyrite types in
the distal SMD SHGD comprise the following trace element signatures: (a) SMD Stage 2B Pyrite
2B (pre-main gold ore): Pb-Ni-Co-As-Ti, (b) SMD Stage 3A Pyrite 3A (pre-main gold ore): As-Cu-
Ni-Pb, and (c) SMD Stage 4A Pyrite 4A1 to 4A4 (main gold ore): Au-As-Sb-Pb-Cu-Ni-Ti-Zn-Mn-
Ag-Tl. In contrast, pyrite types from the proximal skarn mineral assemblages in the SMD copper
deposits contain the following trace element signatures: (a) SMD Stage 3B Pyrite SKN1 (pre-main
gold ore): Cu-Zn-Co-Se-Ni-Bi-Mn-TeAu, (b) SMD Stage 3C Chalcopyrite 3C (pre-main gold
ore): Cu-Zn-Se-Bi+Au, and (c) SMD Stage 4A Pyrite SKN2 (main gold ore): Au-Cu-As-Bi-Co-Se-
Pb-Zn-Ag. The presence of Cu-Zn-Se-Bi in primary copper ore stage Chalcopyrite 3C (SMD Stage
3C) probably implies derivation from magmatic sources during RDP intrusion emplacement.
The SMD Stage 2B diagenetic pyrite yielded a wide range of 34S values from -11.6 to
+33‰ and these results are comparable to the known ranges of 34S values for sedimentary pyrite
types in other Carlin-type SHGD in Nevada, USA and China. In contrast, the SMD Stage 3 group
base metal sulphides have 34S compositions that are more similar to those derived from deep
magmatic or metamorphic sources centred at 05‰. Furthermore, the light 34S values for SMD
Stage 4A pyrite (Pyrite 4A) ranging from -28.9 to 2.5‰ are comparable to 34S values reported for
(a) late gold ore stage pyrite in the Nevada Carlin-type gold depositsand (b) main gold ore stage
pyrite in the Chinese Carlin-type SHGD. However, the post-main gold ore SMD Stage 5 stibnite has
34S values more similar to magmatic systems.
Fluid inclusion studies combined with textural cathodoluminescence investigation
indicate that main primary copper ore stage quartz (SMD Stage 3C) yielded minimum homogenisation temperatures ranging from 175 to 260oC. The presence of primary hematite
with this mineral assemblage suggests oxidising fluid conditions during mineralisation, thereby
possibly enabling copper to be transported at these lower temperatures. The quartz from the
main primary copper ore (SMD Stage 3C) yielded moderate salinity values ranging from 7.7 to
11.2 wt % NaCl equiv. Bi-variate plots of the SMD 18O and D compositions from the SMD
Stages 3B and 3C pre-main gold ore assemblages have ore fluid characteristics similar to
metamorphic and magmatic waters. Overall, the SMD Stage 3C ore fluids in quartz associated
with Chalcopyrite 3C are probably derived from magmatic sources considering (a) their close
relationship with the SMD RDP intrusions, (b) their isothermal mixing characteristics, and
(c) the presence of CO2.
The main gold ore SMD Stage 4A quartz yielded homogenisation temperature (Th)
values ranging from 180 to 290oC; these results are similar to Th values in the distal
disseminated gold deposit (DDGD) types, but are hotter than those in typical Nevada Carlintype
deposits. Salinity values from the main gold ore SMD Stage 4A quartz yielded two groups
at 4.3 to 8.6 wt % NaCl equiv. and 12.1 to 13.7 wt % NaCl equiv., indicating the possible
presence of two fluids. Additionally, the SMD Stage 4A main-gold ore fluids in quartz yielded
calculated 18O values ranging from 9.3 to 13.8 ‰, which are in the range for metamorphic and
magmatic waters. Furthermore, surface fluid dilution trends confirmed the presence of at least
two saline fluids present in fluid inclusions from the SMD Stage 4A quartz associated with main
gold stage Pyrite 4A, indicating the involvement of ore fluids that are similar to (1) evolved
meteoric fluids for those with low-moderate salinities <9 wt % NaCl equiv., and (2) magmatic
fluids comprising moderate-high salinities >9 wt % NaCl equiv., although involvement of
amagmatic (i.e. metamorphic) fluids and/or connate brines cannot be ruled out.
All of the RDP, galena and pyrite Pb isotope results from the SMD imply a crustal Pb
source. The pre-main gold ore-stage mineralisation shows evidence for at least two distinct Pb
isotope sources, comprising (a) less radiogenic Pb in SMD Stage 3A galena associated with a
paragenetically early carbonate-hosted style of base metal mineralisation mainly hosted by
Devonian Nalou Formation bioclastic dolomite, and (b) paragenetically later but also more
radiogenic Pb occurring in SMD Stage 3B galena and SMD RDP samples that are hosted by
younger Early Permian RDP intrusions. However, The main gold ore-stage Pyrites 4A (SMD Stage
4A) contain a wide range of Pb isotopic sources that are interpreted to be derived from both (a)
early less radiogenic sedimentary Pb sources and (b) later more radiogenic Pb from RDP intrusions.
The overall geological setting, mineral assemblages and geochemistry of the SMD SHGD
share several broad characteristics with those in the Nevada (USA) and Dian-Qian-Gui (Southern
China) Carlin-type gold deposits and also some with the Nevada DDGD-types. In conclusion,
geochemical data from the SMD gold and copper deposits suggests a genetic model involving fluid
mixing processes, comprising (1) early magmatic and minor associated metamorphic fluids, mainly
contributing Cu, Au, Pb and S during the formation of the SMD copper skarn and porphyry deposits
(SMD Stage 3C), and (2) later interaction of the early deep magmatic and minor associated
metamorphic fluids with circulating evolved meteoric fluids, possibly also sourcing S, Pb, Au and
associated trace elements from sedimentary rocks for the gold deposits (SMD Stage 4A).
|Item Type:||Thesis (PhD)|
|Additional Information:||Copyright 2010 the Author - Embargoed until June 2013|
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|Deposited On:||28 Apr 2011 15:04|
|Last Modified:||02 Jul 2013 12:45|
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