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 SO42− and with exsolved fluids having SO2>>H2S. 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 fO2 > 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 δ34S values of sulfide-sulfate mineral pairs indicate SO2-derived SO42− and H2S in SO42−/H2S 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 < 550 °C. Hydrothermal biotite that replaces igneous amphibole and biotite has a phlogopitic composition, suggesting that Fe2+ was liberated from igneous mafic minerals and oxidized by reaction with SO42− to form magnetite, resulting in sulfide formation by the simplified reaction 12FeO + SO42− + 2H+ → 4Fe3O4 + H2S.