Abstract:
The Khamkeut Saen Oudom (KSO) gold deposit is located at Bolikhamxai district, central Lao PDR. It lies within the Truong Son Fold Belt where there are several significant mineral deposits particularly gold and copper. At KSO deposit, the mineralization occurs as quartz-carbonate-sulfides veins, minor stockworks and breccias. Major veins are trending almost E-W where it forms as single ore zone in the west and splays to three narrow veins at the eastern part of area. Several ore lenses are present along this E-W structure (e.g. Houay Keh, Nam Pan-east, Nam Pan-west). The gold mineralized veins are mainly hosted by meta-sedimentary rocks (e.g. meta-sandstone, meta-siltstone and slate). At least three stages of mineralization have been identified namely; Stage 1, microcrystalline quartz - arsenopyrite - pyrite; Stage 2, quartz ± calcite - sulfides (arsenopyrite - pyrite - sphalerite - chalcopyrite - galena-pyrrhotite) - gold and Stage 3, quartz - chlorite - calcite. Gold is closely associated with sulfide minerals (e.g., pyrite, arsenopyrite, chalcopyrite, sphalerite and galena), quartz and calcite. Alteration associated with gold mineralization is less pervasive and extensive and mainly characterized by 1) quartz - calcite - sericite - chlorite and 2) chlorite - sericite - calcite assemblages. EPMA analyzes of gold reveals that at Nam Pan ore lens it occurs as electrum with gold fineness range from 637 to 715 whereas, at Houay Keh ore lens it occurs native gold with gold fineness ranges from 827 to 866. EPMA analyzes of light-color sphalerite from Nam Pan has FeS content of 5.00 to 8.05 mole %. In contrast, dark-color sphalerite from Houay Keh ore lens has values of 8.49 to 16.08 mole %. Using FeS content of sphalerite, T and P of gold mineralization at KSO were calculated to be 320 to 450°C and 7.17 to 15.08 kbar respectively. Based on the above evidences (e.g., metamorphic host rocks, weak alteration, mineral assemblages, gold fineness and FeS content of sphalerite), the KSO gold deposit is likely to be orogenic deposit.
