Epithermal gold deposits; Part I

Abstract

The Goldfield district is an epithermal prec1ous metal depos1t of the quartz-alunrte type. Gold ore bod1es are assoc1ated spatially and temporally wrth a calc-alkalic volcanic center of earty M1ocene age Flows, tuffs, and brecc1as of this center overtap a small caldera of Oligocene age SiliCIC domes of Oligocene age and porphyritiC rhyodacite domes of earty M1ocene age both 1ntrude the caldera nng-fracture zone. Most of the rocks in and around the caldera are hydrothermally altered, and gold ore bod1es are clustered along the caldera marg1n, hosted both by Oligocene rocks cut by the ring-fracture zone and M1ocene rocks that 1ntrude or overtle the ring-fracture zone. The ore bodies formed at relatively shallow depths from meteonc water at about 200 to 300 °C.The d1strict has produced more than 4 2 mllhon ounces (130 metnc tons) of gold and 1 45 million ounces (45 metnc tons) of sliver, mostly before Wortd War I, and has modest reserves and inferred resources of gold M1nor intermittent production continues.

Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada

“…These rocks are cut by a dense network of west-northwest and north-to northeast-striking normal faults. Ashley (1990a) reports that silicic ash-flow tuff and flows (the Vindicator Rhyolite and the Morena and Sandstorm Rhyolites of Ransome, 1909) are associated with eruptions from a 6-kmdiameter caldera delineated by a series of poorly defined, presumed ring fractures. However, geophysical data suggest that these fractures are related to pluton emplacement rather than caldera collapse (Blakely and others, 2007).…”

“…Miocene volcanic rocks and pre-Tertiary rocks adjacent to mineralized faults have been extensively altered to quartz, alunite, kaolinite, pyrite, illite and other aluminosilicate and aluminosulfate minerals. The mineralized faults are localized in the southwestern part of a 40 square kilometer (km 2 ) area of intensely altered rocks (Ransome, 1909;Ashley and Albers, 1975;Ashley, 1990a;Vikre and Henry, 2011). About 4.19 Moz of gold and 1.45 Moz of silver were produced from main mining district fault zones and adjacent wall rocks, mostly from 1903-1940 (Albers and Stewart, 1972).…”

“…1). Significant amounts of silver (Ag) and gold (Au) were produced from epithermal vein deposits in the Tonopah (Nolan, 1935;Bonham and Garside, 1974;Ashley, 1990a), Divide (Bonham and Garside, 1979;Erdman and Barabas, 1996), and Goldfield (Ransome, 1909;Ashley, 1974;1990b;Ashley and Silberman, 1976) mining districts. These deposits are spatially,…”

“…Geochemical, petrographic, and geochronologic data for volcanic rocks in the Tonopah, Divide, and Goldfield mining districts were systematically compiled by du Bray and others (2019b). Some of the data for volcanic rocks in the Tonopah and Divide mining districts pertain to samples collected during geologic mapping in these areas and analyzed by Bonham and Garside (1979), whereas data pertinent to volcanic rocks in the Goldfield mining district were, in part, derived from samples collected and analyzed by Ashley (1974;1990b). In 2013, new petrographic data were derived from systematic re-examination of hundreds of thin sections originally prepared as part of investigations carried out by Bonham and Garside (1979) in the Tonopah and Divide mining districts and by Ashley (1974;1990b) in the Goldfield mining district.…”

“…Some of the data for volcanic rocks in the Tonopah and Divide mining districts pertain to samples collected during geologic mapping in these areas and analyzed by Bonham and Garside (1979), whereas data pertinent to volcanic rocks in the Goldfield mining district were, in part, derived from samples collected and analyzed by Ashley (1974;1990b). In 2013, new petrographic data were derived from systematic re-examination of hundreds of thin sections originally prepared as part of investigations carried out by Bonham and Garside (1979) in the Tonopah and Divide mining districts and by Ashley (1974;1990b) in the Goldfield mining district. Additional petrographic, geochemical, and geochronologic data were obtained for samples collected by the authors between 2012-2017, as part of the project titled "Magmatictectonic history and component sources of major precious metal deposits in the southern Walker Lane" funded by the U.S. Geological Survey's (USGS) Mineral Resources Program.…”