TRANSFER OF SULFUR IN THE MIOCENE COMSTOCK LODE AU-AG SYSTEM (NEVADA, USA) FROM MANTLE TO METEORIC WATER

Sulfur (S) isotope values of 14 Ma vein sulfide minerals, 18-14 Ma andesite host rocks, and xenolithic basalt in the Comstock Lode district support derivation of vein S from lithospheric mantle, with no S contribution from crustal sources. In high-grade Au-Ag deposits of the Comstock Lode (6.8 Moz Au @ 0.7 opt; 143 Moz Ag @ 15 opt) δ³⁴S values of vein sulfides are -3.0 to -8.7‰, whereas values of host rocks are ~3 to 9‰. Vein sulfide values approach isotopic equilibrium (pyrite > sphalerite ≈ chalcopyrite ≈ H₂S > galena > acanthite) in 350-250° C aqueous fluid with H₂S/SO₄^2- >5 and δ³⁴S_ΣS ≈ -2.5‰. S in McClellan Peak basalt (Comstock Lode district), and other Quaternary basalts in Nevada, occurs in Fe-Cu-Ni-S-O inclusions in pyroxene and olivine phenocrysts, glass, and peridotite xenoliths. Inclusion δ³⁴S values range from -2.3 ± 1.1‰ to -1.8 ± 1.4‰, suggesting little δ³⁴S variation in the lithospheric mantle beneath Nevada.

Comstock Lode vein sulfide values support derivation of S by decomposition of sulfide liquid represented by sulfide inclusions in McClellan Peak basalt and xenoliths. Low temperature H₂S-mineral fractionations, and small amounts of aqueous SO₄^2- account for the consistent differences in δ³⁴S values between vein sulfide minerals and sulfide inclusions in basalt and xenoliths. Vein sulfide δ³⁴S values, production Ag/Au, and microthermometry, δD_H2O and δ¹⁸O_H2O values of fluid inclusions in vein quartz are consistent with (1) condensation in meteoric water of H₂S, chalcophile elements, and water derived from sulfide liquid and volatiles in the parent basalt magma of host rock andesites, and (2) hydrothermal mineral precipitation by cooling of the mixed fluid. Existing andesite magma conduits were utilized by basalt-sourced volatiles that encountered meteoric water in Comstock Lode faults at depths ≥ 2 km.

In other high-grade Miocene Au-Ag deposits in western Nevada and eastern California (Tonopah, Goldfield, and Bodie), δ³⁴S values of vein sulfide minerals and of sulfide inclusions in andesite, dacite, and rhyolite host rocks are, in part, similar to those of the Comstock Lode. However, relatively high δ³⁴S values of some vein sulfide minerals, and Pb, Sr, and Nd isotope compositions, indicate that hydrothermal mineral components in these districts were substantially derived from the crust.