Petrology of Volcanic Rocks Associated with Silver-Gold (Ag-Au) Epithermal Deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada

Scientific Investigations Report 2019-5024
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  • Document: Report (1.68 MB pdf)
  • Related Work: Data Series 1099 — Petrographic, Geochemical, and Geochronologic Data for Cenozoic Volcanic Rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada
  • Data Release: USGS data release — Data to accompany U.S. Geological Survey Data Series 1099: Petrographic, geochemical and geochronologic data for Cenozoic volcanic rocks of the Tonopah, Divide, and Goldfield Mining Districts, Nevada
  • Open Access Version: Publisher Index Page
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Abstract

Miocene calc-alkaline volcanic rocks, part of the southern segment of the ancestral Cascades magmatic arc, are spatially, temporally, and likely genetically associated with precious metal epithermal deposits in the Tonopah, Divide, and Goldfield Districts of west-central Nevada. In the Tonopah mining district, volcanic rocks include the Mizpah Trachyte, Fraction Tuff, and Oddie Rhyolite; in the Divide mining district, they include the Heller Tuff, Brougher Rhyolite, trachyandesite of Red Mountain, Divide Andesite, and volcanics of Donovan Peak (which includes rhyolite, dacite, and rhyodacite units); in the Goldfield mining district they include the Milltown Andesite, an unnamed porphyritic andesite, and latite. All these rocks are porphyritic and contain phenocryst assemblages that include plagioclase, pyroxene, hornblende, biotite, quartz, alkali feldspar, and olivine. These mostly subalkaline, metaluminous, calc-alkalic, and magnesian rocks range from basaltic trachyandesite to rhyolite and contain 54 to 78 weight percent silicon dioxide.

In the Divide mining district, the Divide Andesite and the volcanics of Donovan Peak are compositionally distinct from volcanic rocks in the other two mining districts. These rocks define a somewhat more restricted range of silicon dioxide content; are more alkalic; have greater titanium dioxide, sodium oxide, barium, hafnium, lanthanum, niobium, tantalum, yttrium, ytterbium, and zirconium abundances; and lower magnesium oxide, strontium, and vanadium abundances. Elevated zirconium contents are particularly characteristic of these rocks, which are also distinctly younger than most of the rocks in the other two mining districts. The alkalic character (principally higher sodium oxide abundances) and elevated zirconium contents characteristic of the Divide Andesite and the volcanics of Donovan Peak suggest that distinctive sources and (or) processes contributed to the petrogenesis of these rocks.

In the Tonopah, Divide, and Goldfield mining districts the geochemistry of Oligocene and Miocene volcanic rocks constrain the processes that contributed to the petrogenesis of these rocks. Specifically, major oxide compositional variation among these rocks is consistent with crystallization and fractionation of the observed phenocryst minerals. In addition, these rocks have negatively sloping rare-earth element patterns consistent with partial melting in a high-pressure, garnet stable regime. Elevated strontium concentrations and small negative europium anomalies are consistent with partial melting in a plagioclase-unstable setting. However, larger negative europium anomalies among the more silica-rich volcanic rocks indicates a progressively greater role for plagioclase fractionation among these rocks. The importance of hornblende in the petrogenesis of these rocks is reflected in subtly U-shaped middle rare-earth element pattern segments. Increasing lead/cerium and decreasing phosphorus pentoxide/potassium oxide with increasing silicon dioxide are characteristic of these volcanic rocks. These characteristics and their distinctive pre-Cenozoic xenolith content suggest a significant role for crustal contamination in their petrogenesis. Diagnostic textural features preserved by phenocrysts, especially plagioclase, constitute additional evidence that open-system behavior, including reservoir-scale mixing, recharge, and assimilation, was critical to the petrogenesis of volcanic rocks in the Tonopah, Divide, and Goldfield mining districts.

Suggested Citation

du Bray, E.A., John, D.A., Colgan, J.P., Vikre, P.G., Cosca, M.A., and Morgan, L.E., 2019, Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield mining districts, Nevada: U.S. Geological Survey Scientific Investigations Report 2019–5024, 22 p., https://doi.gov/10.3133/sir20195024.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Abstract
  • Introduction
  • Geologic Setting of Ag-Au Deposits in the Tonopah, Divide, and Goldfield Mining Districts
  • Analytical Methods
  • Petrographic Characteristics
  • Major Oxide Geochemistry
  • Trace Element Geochemistry
  • Petrogenesis of Volcanic Rocks in the Tonopah, Divide, and Goldfield Mining Districts
  • Conclusions
  • References Cited

Additional publication details

Publication type Report
Publication Subtype USGS Numbered Series
Title Petrology of volcanic rocks associated with silver-gold (Ag-Au) epithermal deposits in the Tonopah, Divide, and Goldfield Mining Districts, Nevada
Series title Scientific Investigations Report
Series number 2019-5024
DOI 10.3133/sir20195024
Year Published 2019
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center, Central Mineral and Environmental Resources Science Center, Western Mineral and Environmental Resources Science Center
Description 22 p.
Country United States
State Nevada
Online Only (Y/N) Y