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At least 307 records · Page 17Linked to original sources
USGS1976

Fluid-Inclusion Petrology - Data from Porphyry Copper Deposits and Applications to Exploration

Fluid-inclusion studies of 37 porphyry copper deposits, mainly in the United States, demonstrate that all but 3 evolved through a hydrothermal stage characterized by very high salinities, generally in excess of about 35 weight percent NaCI equivalent. Temperatures of these fluids ranged from about 250 deg to 700 deg C for various stages and deposits. Most systems boiled. High salinities, shown by halite-bearing inclusions, and boiling, suggested by coexisting gas and liquid-rich inclusions, are considered to be diagnostic of epizonal intrusions which are the most favorable parents for porphyry copper mineralization. Depth of emplacement of many copper-bearing stocks is deduced from fluid inclusions to have been about 6,000 to 10,000 feet (1,800 to 3,000 metres); fluid pressures during mineralization are interpreted generally to be less than 500 bars. Moderate-salinity (less than about 12 percent) and moderate-temperature <350 deg C) fluids are noted in all porphyry copper deposits and were responsible for the deposition of most copper and molybdenum in deposits, such as Bagdad, Esperanza, Mineral Park, Morenci, Ray, Sierrita, in the Southwestern United States and several in southern British Columbia. However, with only three exceptions, highly saline fluids apparently were present at an early stage and also deposited metals. The relative amounts and economic importance of copper and molybdenum deposited from high- and moderate-salinity fluids varies within the porphyry deposit class. There is compelling geologic and geochemical evidence that chloride is important for transport of metals, but the porphyry coppers stand out as a class associated with fluids of especially high salinity during at least one stage of their formation. Halite cubes in fluid inclusions are an effective, although rough, indicator of those salinities and can be conveniently monitored during petrographic study of thin sections. Thus, fluid inclusions can be used to characterize favorable intrusions, metal anomalies, and caprocks as an additional method of detecting possible disseminated porphyry copper metallization. The presence of halite-bearing inclusions, especially coexisting with gas-rich inclusions, is considered to be a favorable fluid anomaly in the search for porphyry ore.

Professional Paper↗
USGS2017

A field trip guide to the petrology of Quaternary volcanism on the Yellowstone Plateau

The Yellowstone Plateau is one of the largest manifestations of silicic volcanism on Earth, and marks the youngest focus of magmatism associated with the Yellowstone Hot Spot. The earliest products of Yellowstone Hot Spot volcanism are from ~17 million years ago, but may be as old as ~32 Ma, and include contemporaneous eruption of voluminous mafic and silicic magmas, which are mostly located in the region of northwestern Nevada and southeastern Oregon. Since 17 Ma, the main locus of Yellowstone Hot Spot volcanism has migrated northeastward producing numerous silicic caldera complexes that generally remain active for ~2–4 million years, with the present-day focus being the Yellowstone Plateau. Northeastward migration of volcanism associated with the Yellowstone Hot Spot resulted in the formation of the Snake River Plain, a low relief physiographic feature extending ~750 kilometers from northern Nevada to eastern Idaho. Most of the silicic volcanic centers along the Snake River Plain have been inundated by younger basalt volcanism, but many of their ignimbrites and lava flows are exposed in the extended regions at the margins of the Snake River Plain .

Wyoming↗
USGS2019

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

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.

Nevada↗