Geologic Overview of the Getchell Gold Mine Geology, Exploration, and Ore Deposits, Humboldt County, Nevada
Vic Chevillon, Eric Berentsen, Mark Gingrich, Bill Howald, Elizabeth Zbinden, 2000. "Geologic Overview of the Getchell Gold Mine Geology, Exploration, and Ore Deposits, Humboldt County, Nevada", Part I. Contrasting Styles of Intrusion-Associated Hydrothermal Systems: Part II. Geology & Gold Deposits of the Getchell Region, John H. Dilles, Mark D. Barton, David A. Johnson, John M. Proffett, Marco T. Einaudi, Elizabeth Jones Crafford
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THIS SUMMARY focuses on the past history of the Getchell property, its general geology, and current published information relative to gold mineralization. It should provide background and a geologic context for the field tour of Getchell. The Getchell property dates to 1934 gold discoveries, and its history includes some of the earliest open-pit oxide and sul-fide gold production in Nevada. In retrospect, the early gold discoveries were some of the first discoveries of what is now known as sediment hosted, micron gold, Carlin-type deposits. Several other open-pit mines were developed on similar deposits in what is now Nevada’s Getchell gold trend.
Current Getchell Gold Corporation land holdings in the Getchell Trend cover 50 mi2 at the northern end of the Os-good Mountains and the Dry Hills. With the 1991 discovery of the high-grade underground orebodies from the Getchell main (sulfide) open pit and the 1994 discovery and initial shaft development of the underground Turquoise Ridge Deposit, the Getchell Trend is entering a new era of bulk tonnage underground gold mining (Table 1).
Placer Dome merged with the Getchell Gold Corporation in the spring of 1999. Getchell Gold is the mine operator and is currently optimizing the Turquoise Ridge and Getchell mines through predevelopment work for bulk tonnage underground production. Placer Dome is conducting exploration aimed at reserve and resource expansion and the discovery of new deposits in outlying areas of the property.
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Part I. Contrasting Styles of Intrusion-Associated Hydrothermal Systems: Part II. Geology & Gold Deposits of the Getchell Region
Intrusion-related hydrothermal systems represent a large variety of geologic environments that in some cases form large metallic mineral deposits. The deposits examined in this trip represent the spectrum from systems dominated by magmatic fluid (Birch Creek, California and Yerington, Nevada) to those systems in which intrusions serve as heat engines to drive convectively circulating brines derived from sedimentary rocks (Hum-boldt, Nevada). In these examples, nonmagmatic fluids are largely excluded from more deeply emplaced intrusions in a compressive environment, and the hydrothermal composition and ores (e.g., granite W-F, Cu porphyry and skarn) are dictated by the composition of the magma and its mechanism of crystallization and aqueous fluid generation. Magmatic fluids are less important in the shallow crustal ore environment, but apparently contribute to acidic alteration zones located vertically above source intrusions. Using Humboldt as an example, we propose that the Fe oxide Cu-Au ores in the shallow environment require an abundant source of sedimentary brines (typical of evaporitic environments), high fracture permeability (promoted by an exten-sional setting) to allow aqueous fluid flow and dike emplacement, and shallowly emplaced intrusions to serve as heat sources.
IGNEOUS-RELATED hydrothermal systems constitute the most varied type of geologic environment, ranging in tectonic setting from spreading centers to collisional belts, in depth from the surface to the deep crust, and in sources of materials from purely magmatic to largely external. They comprise perhaps the single most important ore-forming environment, yet most igneous systems lack economically significant mineralization. This variety is attributable to igneous factors such as volatile content and its evolution from the intrusion, and to external factors that include depth of emplacement, host rocks, tectonic environment, and structural setting, which control permeability and access of external fluids to the crystallized intrusion and its contact aureole.
This field trip examines three large but markedly different intrusion-centered hydrothermal systems in the western Great Basin of California and Nevada (Fig. 1, Table 1). Each example represents a major group of these systems worldwide. The field emphasis will be on examining mass transfer features—such as mechanisms for igneous emplacement, degassing of magmatic-aqueous fluids, and fracturing and ductile deformation—that allow variation from near-lithostatic to hydrostatic conditions, incursion of nonmagmatic fluids into the high-temperature environment, and hydrothermal alteration, vein deposition, and wall-rock replacement via aqueous fluids. The broader questions of metallogenic provinces and processes will be raised as a context for the specific sites examined.
The overall emphasis of this trip will be on documenting and understanding the dynamics of igneous-related hydrothermal systems.