SOURCES OF HOST ROCKS AND HYDROTHERMAL MINERAL COMPONENTS IN THE MIOCENE GOLDFIELD, TONOPAH, AND COMSTOCK LODE AU-AG-(CU) DISTRICTS, NV
At Goldfield early quartz-alunite-pyrite assemblages equilibrated with meteoric water, based on fi and mineral δD and δ18O values, whereas fi in hydrothermal quartz in ore structures reflect mixtures of meteoric and magmatic waters. δ34S values of 3-11‰ in WR and smi in Mp allow partial derivation of host rock S from Neoproterozoic (δ34S sedimentary pyrite mostly 10-40‰) or older strata, whereas δ34S values of gn, pyrite (1 to -3‰) and alunite (20 to 14‰) are consistent with S isotope fractionation by SO2 disproportionation at 400-300°C for Σδ34S=0‰. In the Tonopah and Comstock Lode districts δ34S values of smi in Mp of 7-8‰, and gn of -2 to -6‰, also suggest different S sources.
At Goldfield, Au and Te in high-grade ore (commonly107 times crust-mantle abundances) could have been derived from bulk decomposition of smi in Miocene subvolcanic intrusions. The smi contain metal ratios and abundances similar to ore. The characteristic ore texture, multiple encrustations of Au-Cu-As-Sb-Bi-Te-S and other minerals on fault breccia clasts, could reflect precipitation from pulsed metal-rich fluids in fault zones. However, decomposition of mantle sulfides might better explain Te enrichment and δ34S compositions.