Field Trip Day One SEG Getchell Region: Road Log from Golconda to Pinson Mine
2000. "Field Trip Day One SEG Getchell Region: Road Log from Golconda to Pinson Mine", 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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HU0.0: I80 EXIT 194 GOLCONDA. At the top of the off ramp, turn north (left). Proceed 0.1 mi to the stop sign at the junction with SR789. County mileposts begin here. Turn east (right) on SR789. Steam from active hot springs can sometimes be seen on the north and northeast sides of the town of Golconda. The springs range from 109° to 165°F, are anomalously radioactive, and are actively depositing travertine (Garside and Schilling, 1979). Metals in the spring waters include As (0.02 ppm), Cu (0.05 ppm), Li (0.36 ppm), Mn (0.10 ppm), and Hg (0.0001 ppm).
The gravel road to the south at the Interstate off ramp leads to the Adelaide mining district. The district was discovered in 1866 and has intermittently produced small amounts of gold, silver, lead, zinc, copper, tungsten, and manganese. Total gold production has been approximately 40,000 oz. About 25,000 oz of gold were recovered during 1897–1910 from the Adelaide mine (copper-gold skarn), and about 10,000 oz were produced during the 1930–1940s and 1990–1991 at the Adelaide Crown mine (quartz-adularia veins). The ore occurred in three banded cryptocrystalline to chal-cedonic quartz veins, the Crown, the Recovery, and the Margharita. These veins averaged 30 ft in width. The remainder of the gold was recovered from placers mostly in the very early days. The original Adelaide mine was once owned by President Herbert Hoover. Franco-Nevada Mining Corporation is the major property owner in the district (Pete Maciulaites, pers. commun., 2000).
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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.