The Geology and Gold Mineralization of the Twin Creeks Gold Deposits, Humboldt County, Nevada
Ronald F. Thoreson, Mark E. Jones, Fred J. Breit,, Jr., Micheline A. Doyle-Kunkel, Lori J. Clarke, 2000. "The Geology and Gold Mineralization of the Twin Creeks Gold 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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THE NEWMONT MINING Corporation Twin Creeks mine is located 45 mi northeast of the city of Winnemucca, Nevada in Humboldt County. Access to the property is east from Win-nemucca, along Interstate 80, 16 mi to Golconda then northeast out of Golconda along State Route 789 for 17 mi. At the Midas-Getchell intersection follow the well-maintained dirt road for 12 mi past the Pinson and Getchell mines to the Twin Creeks property (Fig. 1). The Twin Creeks mine is the third largest primary gold-producing mine in North America and includes the Mega, Vista and West pit disseminated Carlin-type gold deposits. In 1999, the mine produced 760,574 oz from an oxide and sulfide milling and oxide leaching operation. Twin Creeks currently employs approximately 650 employees.
Oxide ores at Twin Creeks are processed at a rate of 2,800 tpd through the Juniper Mill. The leach-grade material is processed through three leach pads. Sulfide ores at Twin Creeks are processed through the $250 million Sage Mill, which incorporates two of the largest autoclaves used in the gold industry. Start-up of the first 4,000 tpd autoclave occurred in May of 1997; the second autoclave, of the same size, began operation in November of 1997. The sulfide project enables Twin Creeks to process refractory ores that contain approximately 4.05 Moz of gold, about 59% of the total Twin Creeks’ reserves. Current reserves at Twin Creeks are 87,112,000 tons at an average grade of 0.079 oz/st and contains 6,857,000 oz of gold.
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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.