Abstract

Gold ore at the Dixie Comstock mine, Churchill County, Nevada, is composed of quartz breccia, quartz stockwork, incipiently crushed gabbro, and minor fault gouge. The ore zone forms a mullion within a range-bounding normal fault, the Dixie Comstock mine fault, which originated in the middle Miocene and separates Jurassic gabbro and Tertiary volcanic rocks of the Stillwater Range to the west from Quaternary colluvium of Dixie Valley to the east. Within the mullion, a potentially bulk mineable resource of several million tons grading approximately 0.06 oz of gold per ton has been defined by drilling.Within the fault zone, clasts of gabbro in quartz breccia, quartz stockwork gabbro, and incipiently crushed gabbro are altered to quartz, sericite, chlorite, montmorillonite, and sulfides. Gold occurs as electrum in quartz that cements quartz breccia, in quartz stockwork veins with quartz, pyrite, chalcopyrite, and montmorillonite in breccia clasts, and in veins in incipiently crushed gabbro. Fluid inclusion microthermometry and stable isotope analyses of mineralized quartz matrix and two later stages of quartz and calcite show that ore fluid was 180 degrees C, near-boiling meteoric water.An inverse stratigraphy of thermal spring silica detritus in hanging-wall colluvium, microthermometry and isotopic compositions of fluid inclusion water from quartz breccia, a subjacent geothermal reservoir, and radiometric ages of hydrothermal minerals in the fault cumulatively support a mid-Pleistocene age for gold mineralization. The lower average displacement rate on the Dixie Comstock mine fault that is required by the age and thickness of eroded thermal spring sinter and underlying quartz breccia and stockwork, compared to higher recent displacement rates along range-bounding faults on the western side of Dixie Valley, suggests that the current pattern of temporal clustering of fault displacements began by the mid-Pleistocene.

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