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

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

Nov 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000e%26psl.183....7m&link_type=abstract

Earth and Planetary Science Letters, Volume 183, Issue 1-2, p. 7-14.

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Scientific paper

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.

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