AV俱乐部

Best Thesis Presentation Award

From 1992 to 1997 Bre-X Minerals Ltd. alleged to have found one of the richest gold deposits on earth in central Kalimantan, Indonesia. In 1997 AV俱乐部 professor Dr. G.C. Milligan was invited to visit Busang and collect samples from the highest grade zones. Initial study of these samples in Halifax failed to detect any gold. In 1997 independent evaluations determined that Busang samples had been systematically salted, unearthing one of the worst mining hoaxes in history.
The suite collected by Dr. Milligan includes 12 samples of drillcore from the Busang Southeast Zone. Here they have been re-studied petrographically, analyzed for major and trace elements, and microthermometry of quartz-hosted fluid inclusions has been carried out on mineralized veinlets.
The samples are volcanic and subvolcanic basaltic andesite-andesite. Their geochemistry suggests calc-alkaline magmas generated in an island arc typical of the host rocks in the Kalimantan Gold Belt. Alteration consists of silicification, calcitization, sericitization, and chloritization. In quartz-calcite (卤gypsum/anhydrite) veinlets, ore-microscopy recognizes pyrite, sphalerite, galena, bournonite-seligmannite, lesser chalcopyrite, and minor tetrahedrite-tennantite, but no discrete Au or Ag minerals. Pyrite is ubiquitous throughout the altered rocks as dendritic veinlets and clots. Fluid inclusions in veinlet-hosted quartz are small (<20 渭m), rare, and irregular with dominantly two-phases (aqueous liquid + vapour), and high temperatures of homogenization (198-330oC). Most are unclassified however some likely represent primary assemblages as they occur along primary growth zones in quartz.
In conclusion, the suite represents a moderately mineralized system with base metals, generated by hydrothermal fluids of predominantly meteoric origin, which were likely heated by an intrusion. As such, it is compatible with location at the fringe of a low-sulfidation epithermal precious metal system. However, the absence of a significant gold anomaly (8-139 ppb Au), the lack of promising textures, and the relatively high temperatures of the hydrothermal fluids are incompatible with the suite representing a rich ore deposit. It is likely that these samples came from deep in an epithermal system, and that any associated precious metal mineralization has been removed by erosion.

Keywords: Gold, Bre-X, Busang, Indonesia, epithermal, hydrothermal, mineralogy, geochemistry, fluid inclusions

Pages: 185
Supervisor: Marcos Zentilli / Jacob Hanley (SMU)

听