FRACTIONATION OF CHALCOPHILE AND SIDEROPHILE ELEMENTS IN SULFIDE-SATURATED LITHOSPHERIC MAGMAS OF THE WESTERN GREAT BASIN: RELATION TO MAJOR AU-AG DEPOSITS

Western Great Basin Miocene-Holocene basalts contain sulfide-saturated xenoliths (peridotite, pyroxenite, and gabbroic cumulate), xenocrysts and phenocrysts (olivine, pyroxene, spinel, amphibole, plagioclase). Sulfide occurs as spherical to ovoidal Fe-Ni-Cu-S inclusions in juvenile olivine, pyroxene, and spinel, and in metasomatized peridotite as inclusions and asymmetrical aggregates with olivine, pyroxene, spinel, amphibole and plagioclase. Sulfide minerals in gabbroic cumulate are mostly equant and lobate Fe-Cu-S aggregates in pyroxene, spinel, amphibole, and plagioclase. Aspects, compositions, and δ³⁴S values of inclusions and aggregates reflect sulfide liquid ± crystal entrapment during asthenosphere crystallization and trans-Moho metasomatism. Equant-lobate aggregates in cumulate represent sulfide saturation of magmas derived from partial melting of mid-crust components by basalt.

Whereas inclusions and aggregates of coexisting Fe-Ni-S and Fe-Cu-S minerals in upper mantle-lower crust xenoliths and xenocrysts attest to early sulfide saturation, the predominance of Fe-Cu-S inclusions and aggregates in mid-crust cumulate reflects Ni-Cu fractionation (Ni/Cu 10->1000) during crystallization of Fe-Mg silicate-oxide minerals. Weathering of rocks composed of these Ni-enriched minerals provides most global Ni supplies.

S, Pb, and Sr isotope values and element concentrations in Fe-Cu-S minerals in cumulate xenoliths and in epigenetic minerals of Goldfield district Au-Ag deposits support sourcing of chalcophile elements in ores (Cu, Au, Ag, As, Sb, Sn, Te, S, Se) from mid-crust (~15 km), sulfide-saturated, mantle-derived basalt magma that assimilated Mojave crust. Epigenetic minerals in Tonopah Ag-Au deposits were likely derived from shallow (~5 km) sulfide-saturated, peraluminous rhyolite magmas that evolved in Sierra Nevada plutons, fractionating Ag/Au to ~100. S, Pb and Sr isotope values of Comstock Lode epigenetic minerals are nearly identical to those of mantle-derived basalt, suggesting little to no crust assimilation. Syn-mineralization andesite magmas contain few Fe-Cu-S inclusions and no cumulate due to degassing or decompression; they provided thermally fluxed conduits that enabled transfer of elements from decomposed sulfide cumulate to deposit sites.