The Gold Deposits of Pinson Mining Company: A Review of the Geology and Mining History through 1999, Humboldt County, Nevada
Colin D. Mclachlan, Eric M. Struhsacker, Warren F. Thompson, 2000. "The Gold Deposits of Pinson Mining Company: A Review of the Geology and Mining History through 1999, 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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Open-pit mining by Pinson Mining Company from 1980 through 1999 yielded a total of 33,750 kg (1,085,105 oz) gold. Oxide ores were mined from sedimentary rock-hosted gold deposits at three locations along the Getchell trend in Humboldt County, Nevada.
Most Pinson Mining Company gold production (30,709 kg or 987,348 oz) came from the several deposits of the Pinson mine, located 35 km (21.7 mi) northeast of Golconda. These ores were extracted from carbonates and argillites of the Upper Cambrian to Upper Ordovician Comus formation. The deposits exhibit both strati-graphic and structural control and lie along several structural orientations. Silicification, decalcification, argillization, and fracture-controlled iron oxidation are the principal alteration types associated with these deposits.
The Preble mine, located 14.8 km (9.2 mi) northeast of Golconda produced 2,807 kg (90,249 oz) of gold. Ore was mined from interbedded carbonaceous shales, calcareous shales, and silty limestones of the middle member of the Lower Cambrian to Lower Ordovician Preble formation. Mineralization is localized within a broad northeast-striking, southeast-dipping shear zone locally flanked by massive limestone beds. Silicification, phyl-losilicate alteration, and iron oxidation are spatially associated with gold.
The Kramer Hill mine, located 3.2 km (2 mi) south of Golconda yielded 234 kg (7,508 oz) of gold. Most of this ore was mined from shattered, interbedded phyllitic shales and impure quartzites of the Twin Canyon member of the Precambrian to Cambrian Osgood Mountain quartzite within the hanging wall of a north-northeast-striking, west-dipping normal fault. Argillization, silicification, and fracture-controlled oxidation are the most evident types of alteration.
Gold in the three mines is very fine grained, typically <5 microns in size. Gold mineralization is associated with anomalous Hg, As, Sb, and Tl.
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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.