Noble gas data from Goldfield and Tonopah epithermal Au-Ag deposits, ancestral Cascades Arc, USA: Evidence for a primitive mantle volatile source

The He, Ne, and Ar isotopic composition of fluid inclusions in ore and gangue minerals were analyzed to determine the source of volatiles in the high-grade Goldfield and Tonopah epithermal Au-Ag deposits in southwestern Nevada, USA. Ar and Ne are mainly atmospheric, whereas He has only a minor atmospheric component. Corrected ³He/⁴He ratios (with atmospheric He removed) range widely from 0.05 to 35.8 times the air ³He/⁴He ratio (R_A), with a median of 1.43 R_A. Forty-one percent of measured ³He/⁴He ratios are ≥4 R_A, corresponding to ≥50% mantle He assuming a mantle ratio of 8 R_A. These results suggest that mafic magmas were part of the magmatic-hydrothermal system underlying Goldfield and Tonopah, and that associated mantle-sourced volatiles may have played a role in ore formation. The three highest corrected ³He/⁴He ratios of 17.0, 23.7, and 35.8 R_Aindicate a primitive mantle He source and are the highest yet reported for any epithermal-porphyry system and for the Cascades arc region. Compiled ³He/⁴He measurements from epithermal-porphyry systems in subduction-related magmatic arcs around the world (n = 209) display a statistically significant correlation between ³He/⁴He and Au-Ag grade. The correlation suggests that conditions which promote higher fluid inclusion ³He/⁴He ratios (abundance of mantle volatiles and focused upward volatile transport) have some relation to conditions that promote higher Au-Ag grades (focused flow of metal-bearing fluids and efficient chemical traps). Results of this and previous investigations of He isotopes in epithermal-porphyry systems are consistent with the hypothesis posed in recent studies that mafic magmas serve an important function in the formation of these deposits.