SULFIDE PETROLOGY OF THE WESTERN GREAT BASIN LITHOSPHERE, USA; LINKS TO MAJOR AG-AU DEPOSITS

Xenoliths, xenocrysts, and phenocrysts derived from trans-Moho igneous rocks and magmas and entrained in Cenozoic basalt flows in the western Great Basin contain Fe-Cu-Ni-S phases that reflect sulfide liquid saturation, fractionation, and separation in the lower crust-upper mantle, and sub-solidus exsolution and decomposition in the mid- to upper crust. Sulfide phases occur as spherical to lobate intra-mineral and interstitial inclusions in and between pyroxene, olivine, spinel, plagioclase, and amphibole and in metasomatic aggregates of these minerals. Most have compositions near pyrrhotite, chalcopyrite, cubanite, and pentlandite; some are partially decomposed to Fe-Cu-Ni oxides. Inclusions in basalt glass derived from partial melting of asthenospheric peridotite, and abnormally large inclusions in basalt phenocrysts attest to separation of sulfide liquid during partial melting of the mantle and lower to middle crust. All inclusions, regardless of composition, mineral association, or geographic location, have δ³⁴S values of -3 to -1‰ with modes of ~ -2‰.

Sulfide inclusions in granular and cumulate xenoliths derived from lower crust, based on texture, composition, and amphibole thermobarometry, consist of the same Fe-Cu-Ni-S phases and fractionated NiS, Ni₃S₂, and Cu₅FeS₄; interstitial aggregates consist of pyrite, chalcopyrite, and pyrrhotite. These phases have δ³⁴S values of ~0 to 10‰, like inclusions in host and associated andesites and rhyolites in the Comstock Lode, Tonopah, and Goldfield Ag-Au districts. The δ³⁴S values of chalcogenide minerals in Comstock Lode deposits are -7.5 to -2.5‰ and closely correlate with experimental mineral-H₂S fractionations at 350-200° C and δ³⁴S_H2S = -2‰. These and other stable (D, O) and radiogenic (Pb, Sr) isotopic values of epigenetic minerals and fluids support thermal gradients and mixing of meteoric and magmatic water among deposits, and derivation of S, Pb, and Sr primarily from mantle reservoirs. Larger proportions of S and other epigenetic mineral components in Tonopah and Goldfield deposits were derived from middle and upper crust. Decomposition (in water at ~500° C) of sulfide phases in basalt and crust to H₂S, condensation of H₂S in meteoric water at paleodepths of ≤2 km, and combination with aqueous metal species formed chalcogenide ores.