Mineralogy and Fluid Inclusion Microthermometry of Epithermal Gold-Base Metal Mineralization at Anggai, Obi Island, Indonesia

Epithermal gold-base metal mineralization at Anggai village, Obi island, Indonesia has been identified through an exploration program by Broken Hill Proprietary Company during 1995 to 1996. This paper describes a preliminary study on the prospect which focused on its ore and gangue mineralogy, hydrothermal alteration, fluid inclusion microthermometry as well as geochemistry, to elucidate the formation conditions and ore grades. Fresh and altered rocks and mineralization samples were collected from field work to be studied using petrography and ore microscopy, X-ray diffraction analysis, atomic absorption spectrometry, and fluid inclusion microthermometry. The study resulted that host rocks of the mineralization are fine-grained porphyritic basalt and andesite of the Oligocene to Early Miocene Bacan Volcanics. Hydrothermal alteration mineral assemblages include quartz, chlorite, epidote, albite, less biotite, sericite, clay, and pyrite. Mineralization styles are crustiform quartz vein and dissemination, where sulphides clustered and disseminated in closed-space both in quartz gangue and in strongly altered host rocks. Under the microscope, main hypogene ore minerals identified include galena, sphalerite, chalcopyrite, and pyrite which hosts fine-grained gold. Fluid inclusion microthermometric study on the vein quartz resulted in homogenization temperature of 220 to 230oC and salinity of 2.0 to 2.5 wt.% NaCl equivalent. Chemical analysis of nine selected samples resulted in highest grades of 98 g/t Au, 275 g/t Ag, 0.61% Cu, 48% Pb, and 5.35% Zn.