Metamorphic facies and hydrothermal alteration characteristics of the metamorphic-rock hosted gold at Gunung Botak and Gogorea, Buru island, Indonesia

This paper discusses a recent study on metamorphic facies, hydrothermal alteration mineral assemblages, as well as geochemical characteristics (ore grades) of the metamorphic rock-hosted gold deposit at Gunung Botak and Gogorea, Buru Island, Indonesia. Based on the petrographic study of the metamorphic rocks, it was identified that the rock types are dominantly schist and less phyllite. The mineral assemblages include chlorite, muscovite, quartz, biotite, clay, cordierite, and opaque. Based on these metamorphic rock and mineral assemblages, it is inferred that metamorphic facies of the host rock is greenschist, with the formation temperature ranges from about 250 to 500oC and the pressure of about 2 to 10 Kbar, or in intermediate metamorphic grade. Based on direct observation in the field, the ore mineralizations are generally surrounded by argillic and chloritic alterations. Hydrothermal alteration minerals identified from XRD analysis include muscovite, chlorite and quartz. Seven ore samples were analyzed using AAS (fire assay) method, and indicates a grade range of Au 0.23-5.90 g/t. Other important elements were also assayed using ICP-OES method, mostly high-grade ore samples also indicate significant concentrations of As and Sb, whereas Ag, Cu, Pb and Zn are generally low