Different crustal sources for Au-rich and Au-poor ores of the Grasberg Cu–Au porphyry deposit

The Grasberg is a porphyry copper deposit that is crosscut by a second stage mineralization which is greatly enriched in gold. Sulfides from the porphyry-type event yield a 2.9±0.3 Ma Re–Os isochron that agrees with published geochronology. The initial 187Os/188Os ratio of the isochron is 0.56±0.02, and implies a significant crustal component for the source of Os and by inference the other base metals. The samples from the crosscutting secondary event do not form an isochron, but form a mixing line with the older porphyry style mineralization as one of the end members and a very radiogenic end member possibly shale. The initial 187Os/188Os ratios from samples of the second event range from 0.81 to 1.26 and correlate with gold content. The most radiogenic samples have the highest gold. The Re–Os isotope data indicate different crustal sources for the ore-forming elements at the Grasberg Cu–Au deposit and support a model in which gold is derived from sedimentary protoliths, that may have been pre-enriched by sedimentary processes. The gold derived from these sources is concentrated by magmatic/hydrothermal systems. The requirement of a sedimentary source for gold to produce hydrothermal gold deposits has been debated for decades and this study provides strong support for the model.