Epithermal gold mineralization in the Velvet District, Pershing County, Nevada

Gold mineralization in the Velvet District occurs in an eastward dipping sequence of Miocene tuffs, flows, and tuffaceous sediments on the west flank of the Trinity Range in Pershing County, Nevada. Numerous north-northeast trending normal faults extend through the district. These faults served as conduits for ascending hydrothermal fluids which deposited gold and silver along poorly defined zones of brecciation, argillic alteration, and quartz veining. Concentration of gold does not exceed a few parts per million and is highest in zones of intersecting fractures. The hydrothermal solutions which deposited the gold were the near-surface expression of a larger geothermal system. Meteoric water leached gold, silver, arsenic, antimony, and other metals from the surrounding rock as it percolated downward towards a deep heat source. The hot fluids escaped upward along fracture zones and precipitated precious metals in veins near the surface. Fluid inclusion studies indicate that the quartz veins were deposited in the temperature range 218-264°C from fluids which had salinities equivalent to 0.2-0.8 weight percent NaCl. δ¹⁸0 of quartz veins varies from -2.5 to +6.7 %, and the low values appear to define a zone of concentrated fluid flow and potential subsurface mineralization in the southeast part of the district. The lowest value, -2.5 %, indicates that the ore fluid must have been Tertiary meteoric water with δ¹⁸0 equal to -13 %. Reaction of hydrothermal fluids with wall rock produced an alteration assemblage of illite-kaolinite-quartz which underlies most of the shafts, adits, and prospect pits in the district. The illite-kaolinite-quartz assemblage is laterally gradational into a kaolinite (or dickite)-smectite-alunite-quartz assemblage which is indicative of lower temperatures. Pyrite and other sulfides are rare, but limonite and jarosite are widespread in surface outcrops. Alteration of tuffs released abundant silica which precipitated as quartz veins and discharged in hot springs at the bottom of lakes, where thick deposits of diatomaceous earth accumulated.