ASSOCIATION OF SILVER-DOMINANT PRECIOUS METAL DEPOSITS WITH PERALUMINOUS, SULFIDE-SATURATED AND SULFIDIZED IGNEOUS ROCKS, WESTERN US

Silver-dominant precious metal districts (Ag/Au>80) in the western U.S. include 50+ Moz. deposits (Ag production + resources) at Rochester, Candelaria, and Tonopah, NV, and numerous smaller deposits (<20 Moz.) in NV, CA, and UT. Deposits are associated in space and time with peraluminous felsic igneous rocks (ASI>1.1), including sulfide-saturated and sulfidized rhyolites. Most Ag and Au were recovered from Ag-Sb-S minerals, electrum, and weathered derivatives.

At Rochester, large volume vein and stockwork deposits occur in Early Triassic peraluminous rhyolites regionally altered to quartz-sericite-pyrite. Elevated vein temperatures (>300⁰ C), mineral assemblages, and isotope values indicate that rhyolites were sulfidized at 5+ km depths beneath Jurassic Fencemaker allochthons and mineralized during Late Cretaceous magmatism. At Candelaria, stratabound vein and stockwork deposits occur in shear zones in Permian-Triassic black shales intruded by Mesozoic peraluminous felsic dikes and stocks. Elevated vein temperatures (360-460⁰ C) and stable isotope values imply that mineralization occurred beneath stacked allochthons and deposit S was derived from subjacent crust. At Tonopah, Fraction Tuff (FT) magmas, mostly peraluminous and sulfide saturated, evolved from partial melting of Precambrian siliciclastic rocks and Sierra Nevada plutons at ~740° C and 5-7 km, nearly simultaneously with Ag-Au vein formation at ~20 Ma. The similar S and Pb isotope values of vein and magmatic sulfide minerals in FT and host rock andesite correspond to crust and mantle S and Pb.

Pyrite and pyrrhotite disseminated in peraluminous rhyolites contain up to hundreds of ppm Ag and Sb, and ppm Au, corresponding to vein mineralogy. Experimental Ag solubility in melts with ASI ≥ 1.2 and <1 mol% Fe exceeds 50 ppm and is >10 times Au solubility. The small experimental D^{silicate melt}_Au and large D^{sulfide/silicate melt}_Ag (~400) in rhyolite melts are consistent with the measured concentrations in Fe-S of sulfide saturated peraluminous rhyolite magmas and production Ag/Au at Rochester and Tonopah. Given the small D^{vapor/silicate melt} of Ag and Au in rhyolite magmas, transfer of Ag, Sb, and Au to aqueous fluid by hydrolysis of cumulate and disseminated Fe-S is a more efficient process than magmatic degassing for forming high Ag/Au deposits.