Field Trip Day Two SEG Getchell Region: Road Log, Twin Creeks Mine Turnoff to Getchell Mine
2000. "Field Trip Day Two SEG Getchell Region: Road Log, Twin Creeks Mine Turnoff to Getchell 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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0.0 Log begins at yield sign at Getchell/Twin Creeks mine Road fork: head due north on the mine road. See Day One for log up to the turnoff.
Looking west, the south end of the Summer Camp oxide gold open pit is visible behind the Summer Camp waste dump further north. The southern Getchell property boundary is 500 ft south of the open pit. The Summer Camp orebody was discovered in 1984, and mined in 1989–1990. It produced 2.2 M tons grading 0.034 opt. Ore-grade sulfide material (0.2 opt Au) is still exposed on the pit floor and makes up a small satellite resource. Gold ore is hosted in silicified and mineralized hornfelsed Comus formation carbonaceous mudstones and calcareous mudstones along the north-south, east-dipping Getchell fault and intersecting northeast- and southwest-striking faults.
Looking northwest, the Riley open pit and underground workings are visible along the range front. Riley is the largest of the tungsten mines that occur along the margins of the Osgood Mountain stock. Scheelite was discovered here in 1917 but was not mined until 1943. The Riley pits expose stratabound and discordant scheelite in garnetite and clinopyroxene (diopside) skarn in limestone protoliths along the intrusive contact of the Cretaceous (92 m.y.) Osgood Mountain stock. From here, the layering you can see in the highwalls of the pits is relic bedding of the stratigraphy that forms dip slopes among the open pits. The open pits are in the footwall of the Getchell fault, well exposed near the mine shack. The Getchell fault dips 45° E, essentially parallel to the bedded exposures in the pit highwalls. The historical Riley shaft head frame is just visible at the north end of the workings near the range front.
Figures & Tables
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.