Geological setting, geochemistry and genesis of the Sepon gold and copper deposits, Laos

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 280vîvѬ¥6 and 297vîvѬ¥7 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 287vîvѬ¥1 and 280vîvѬ¥1 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-TevîvѬ¥Au, (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 vîvÖ¬ß34S values from -11.6 to +33‚ÄövÑ‚àû and...