Hydrothermal alteration and gold mineralization of the Awak Mas metasedimentary rock-hosted gold deposit, Sulawesi, Indonesia

Abstract

The Awak Mas gold deposit is hosted by phyllites and schists. Indicated and infered resource are around 38.4 Mt at 1.41 g/t Au, equaling to total resource of 1.74 Moz gold. Hydrothermal alteration zones proximal to the gold-bearing veins, are characterized by the mineral assemblage albite-ankerite-pyrite and marked by elevated Na/Al ratios and depleted 3*K/Al ratios. These changes are manifested by the replacement of muscovite by albite. Carbonate alteration with ankerite and siderite occured simultaneous to albitization proximal to the veins. Distal alteration zones are characterized by chlorite-albite in phyllites, and albite-quartz in schists.There are two types of Au-mineralizing veins in the deposit, namely quartz-albite-ankerite veins and quartz-ankerite-siderite veins. Gold occurs in pyrite as sub-microscopic inclusions and as fracture fillings with a gold fineness of 925–935 and Au/Ag ratios between 12:1 and 14:1.The aqueous ore fluid was CO2-poor and of low salinity with homogenization temperatures between 275 and 325 °C. Fluid inclusion leachates have Br/Cl ratios between 1.0 and 2.0 × 10−3 and I/Cl ratios of 5.6 to 18.3 × 10−5 suggesting that the ore fluids were derived from metamorphic dewatering of organic-rich marine sedimentary rocks. On the other hand, Br/Cl ratios between 1.6 and 3.0 × 10−3 and I/Cl ratios from 5.1 to 10.6 × 10−5 in the non-mineralized veins indicate an input of magmatic hydrothermal fluids.The results of this study show that the Awak Mas deposit is an epigenetic, orogenic gold deposit, which developed at the transition between the epizone and the mesozone.