Reduction of oxidized sulfur in the formation of the Grasberg porphyry copper-gold deposit, Papua, Indonesia

The reduction of oxidized sulfur is essential in porphyry copper deposits whose mineralization predominantly comprises copper sulfides, whereas their source magmas are oxidized with most of their sulfur as SO 4 2− and with exsolved fluids having SO 2 >>H 2 S. To estimate the redox state of sulfur, we examined drill cores that intersect potassic and unaltered intrusive rocks in the deeper levels of the Grasberg porphyry copper-gold deposit. Magmatic oxybarometers such as the amphibole-titanite-magnetite-quartz assemblage, anhydrite, and amphibole consistently show that Grasberg ore-forming magmas were oxidized with f O 2 > FMQ+3. Initial hydrothermal events formed sulfide-free, anhydrite-rich K-feldspar, and biotite alteration, followed by successive vein stages of (1) magnetite, (2) biotite, (3) quartz, (4) anhydrite-chalcopyrite, (5) chalcopyrite ± sericite selvages, and (6) pyrite-chalcopyrite-quartz + sericite selvages. The δ 34 S values of sulfide-sulfate mineral pairs indicate SO 2 -derived SO 4 2− and H 2 S in SO 4 2− /H 2 S molar proportions of ~ 4:1 to ~ 3:1 at > 550 °C. The hydrothermal fluid then likely followed a rock-buffered trajectory and became more reduced at