SOURCES OF HOST ROCKS AND HYDROTHERMAL MINERAL COMPONENTS IN THE MIOCENE GOLDFIELD, TONOPAH, AND COMSTOCK LODE AU-AG-(CU) DISTRICTS, NV

Isotope and minor element compositions of host rocks, hydrothermal minerals, sulfide melt inclusions (smi), and fluid inclusions (fi) in the 22-20 Ma Goldfield, 21-19 Ma Tonopah, and 16-13 Ma Comstock Lode districts (14.4 Moz Au, 368 Moz Ag), which formed in discrete subduction-related Miocene volcanic fields, show that host rocks and components of hydrothermal minerals were derived from several sources. At Goldfield ƐNd<-4 in whole rocks (WR) and hydrothermal alunite, and ^6/4Pb≥19.15 with iSr≥0.706 in WR, plagioclase phenocrysts in Miocene host rocks (Mp), and ore galena (gn), alunite, barite and gypsum support hydrothermal acquisition of Pb, Sr, K, Ba, and Ca from pervasively hydrolyzed host rocks and subjacent continental crust. At Tonopah, 50 km N of Goldfield, lower ^6/4Pb and iSr, consistent with higher ƐNd values in Mp, WR, and gn, reflect contributions from both continental and oceanic crust. At the Comstock Lode, 280 km NW of Goldfield, Pb in Mp and gn was derived entirely from oceanic crust. This geographic trend locates the continental-oceanic crust boundary (iSr=0.706) near the latitude of Tonopah.

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.