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Fluid Inclusion Noble Gas Data From Goldfield and Tonopah Epithermal Au-Ag Deposits

July 26, 2017

Twenty-five ore and gangue mineral separates from the Miocene-age Goldfield and Tonopah epithermal Au-Ag deposits in southwestern Nevada were analyzed to determine the helium, neon, and argon (He, Ne, and Ar) isotopic compositions contained in fluid inclusions. Four mineral separates from the Butte Main Stage vein deposit and two from the Bingham pyrite-enargite vein deposits were also analyzed. Fifteen separates are from hand samples collected from underground mine workings and the remaining 16 are from mine tailings piles and pits excavated within the past 100 years. The separates consist dominantly of pyrite, enargite, and quartz, with lesser amounts of sphalerite, galena, potassium feldspar, bismuthinite, marcasite, alunite, chalcocite, chalcopyrite, rhodonite, and gold. Fluid inclusion gases were extracted by thermal decrepitation at 300 to 350C, with the exception of samples CL-480 and M2763A which were crushed under ultrahigh vacuum. Ar and Ne isotopic ratios are similar to atmospheric compositions while He isotopic ratios (helium-3/helium-4 [3He/4He]) range widely from 0.03 to 34.12 times the atmospheric 3He/4He value (R/RA, measured 3He/4He normalized to the atmospheric value of 1.384 x 10^-6).