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. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6779 |
| Date | 11 December 2009 |
| Creators | Masterson, Wilmer Dallam |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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