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Miocene Epithermal Au-Ag Vein Mineralization, Dixie Claims, Midas District, North-Central Nevada; Characteristics and Controls

F.G. Smith Fluid Inclusion Laboratory Department of Geology, University of Toronto Toronto, Ontario, Canada, M5S 3B1

The Dixie Zone Au-Ag deposit is a low sulfidation (adularia-sericite) epithermal system located 1.4 km from the producing Ken Snyder mine (7.0 M oz Au equivalent reserve/resource), Midas Mining District, north-central Nevada. This WNW striking, steep south dipping, fault system is hosted by mid-Miocene (17 Ma to 14 Ma) felsic volcanic rocks that were erupted from a vent(s) to the northeast. The system was mineralized by approximately 250 ± 20°C (n = 86), near-neutral pH, ascending fluids of predominantly meteoric origin [volatile corrected salinity = 0.52 ± 0.35 eq.wt% NaCl (n = 86)]. Bulk analytical fluid inclusion gas chromatography identified a mineralizing fluid volatile composition of 99.68 ± 0.18 mole % H₂O, 0.16 ± 0.09 mole % CO₂, 0.11 ± 0.05 mole percent N₂, 0.05 ± 0.04 mole % CH₄, and 0.01 ± 0.01 mole % C_nH_n+i (n = 7). Sulfur species were not analyzed due to absorption within the chromatography system; however, a H₂S concentration of 16 ppm to 160 ppm molar is estimated. The volatile content contributes approximately 50% of the microthermometrically measured freezing point depression. Cl^– concentrations up to approximately 9000 ppm suggest an additional, probably magmatic, fluid component.

Ascending fluids were channelled along approximately NW striking faults related to the same primary extension (17 Ma to 14 Ma) responsible for Nevada basin and range physiography. Boiling induced by CO₂ effervescence led to precipitation of Au and Ag over a 150 m vertical interval. At the time, this interval was about 460 m below the water table (approximately 38 bars), but because of erosion, it is now within 90 m of the surface. A Ag:Au ratio of 25:1 has been calculated for the Dixie Zone.