Abstract

The epithermal gold-silver deposit at Round Mountain occurs in variably welded, rhyolitic, ash-flow tuff of Round Mountain, erupted at 26.7 Ma from the Mount Jefferson caldera 6 km to the northeast. The ore-forming hydrothermal system was established less than 2 Ma after eruption. Upwelling hydrothermal fluids, guided by northwest-striking faults and joints, permeated laterally into the lower poorly welded part of the tuff and locally flowed toward the surface in zones of steeply dipping, sheeted fractures within the middle, densely welded part. Based on pressure-depth relations and temperatures inferred from fluid inclusions, outcrops on top of Round Mountain represent minimum depths of about 400 m below the coeval paleosurface. Gold and silver were deposited over a depth range of at least 750 m below the top of Round Mountain.More than 310,000 kg (10,000,000 oz) of gold (including 277,000,000 metric tons ore at 1.2 ppm Au as open-pit reserves plus past open-pit production) occur in tuff overprinted by both early, pervasive propylitic alteration (quartz-adularia-albite-chlorite-calcite-pyrite + or - epidote) and later, more fracture-controlled potassic alteration (quartz-adularia-white mica-calcite-pyrite). Temporal transition between propylitic and potassic alteration in densely welded and propylitized tuff is marked by fracture coatings (quartz-adularia-pyrite-gold + or - albite + or - calcite + or - epidote) in which hydrothermal quartz and feldspar occur only as overgrowths on intersected quartz and feldspar phenocrysts, respectively. The abundance of overgrowth veins correlates directly with the grade of bulk-minable gold ore. Fluid inclusions that formed throughout the transition are liquid dominated, and vapor bubbles in all fluid inclusions contract upon crushing. The final melting point of ice in all fluid inclusions is mostly 0.0 + or - 0.1 degrees C. Above and beyond the limits of intense propylitic alteration in densely welded tuff, potassic alteration occurs as envelopes on microbreccia veins (quartz-pyrite + or - gold + or - chalcedony).Silicic (quartz-adularia-pyrite) and intermediate argillic (quartz-illite/smectite-pyrite, with relict K feldspar) alteration developed at high levels in the system late in the hydrothermal history and is related in intensity to cockscomb quartz veins (quartz-adularia + or - pyrite + or - calcite + or - gold). Microbreccia and cockscomb veins locally contain high grades of gold and were selectively mined in the past, but they contain less than ten percent of total gold in the system. Alunite occurs locally in altered rocks and postdates the ore-forming hydrothermal system by 10 to 15 Ma.Paragenetic and spatial relations of alteration and mineralization types, in conjunction with phase equilibria and fluid inclusion microthermometry, suggest that the zone of strongest propylitic alteration represents peak hydrothermal temperatures in the upwelling hydrothermal plume of 250 degrees to 265 degrees C. Potassic alteration and deposition of gold occurred upon fracturing of the host rocks and consequent mixing of the hydrothermal fluid with ambient ground water flowing laterally in the tuff, during which temperature declined to below 200 degrees C. An initial hydrothermal fluid at 250 degrees C, salinity of 0.0 to 0.2 equiv wt percent NaCl, pH of 7.5, log f (sub O 2 ) = -36, and with log activities of Sigma S = -3, Sigma C -- -3.4, K (super +) = -3, and Ca (super +2) = -5, is consistent with data from fluid inclusions and mineral-solution equilibria at Round Mountain and from analogous active geothermal systems. This initial fluid, transporting gold as the bisulfide complex, could have deposited all the gold at Round Mountain while producing the observed sequence of mineral assemblages during cooling at water/rock ratios of 8 or less.

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