Overview of Regional Geology and Tectonic Setting of the Osgood Mountains Region, Humboldt County, Nevada
Elizabeth Jones Crafford, 2000. "Overview of Regional Geology and Tectonic Setting of the Osgood Mountains Region, 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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Paleozoic and Mesozoic rocks in the Osgood Mountains region can be grouped into five terranes based on distinct lithologic, age, and structural characteristics. These terranes are: the Lower Paleozoic Osgood block, the Lower Paleozoic Roberts terrane, the Cambrian and Devonian Dutch Flat terrane, the Mississippian to Permian Golconda terrane, and the Triassic to Jurassic Jungo terrane. Each of these terranes is structurally bounded by moderately to steeply dipping fault zones or melange belts and has a distinct internal structural fabric.
Geologic evidence exposed in the Osgood Mountains provides support for a revised model for the Paleozoic tectonic history of the region. Paleozoic tectonic events in Nevada can be reinterpreted in a new framework that portrays the traditional Antler and Sonoma orogenies as complex, transpressive episodes of tectonism along the Paleo-Pacific North American plate margin. Recognizing the accreted Paleozoic terranes of Nevada as tectonic blocks that have experienced significant translational displacement and deformation relative to each other and to the continental margin explains many of the geologic observations that have not been accounted for in other models of the tectonic history of Nevada.
Several world-class gold deposits in Nevada, notably the Carlin area deposits, the Getchell region, and Pipeline, among others, are located close to inferred high-angle fault boundaries that can be related to these accreted and dislocated terranes. It is proposed that these terrane boundaries are a first-order control for subsequent gold mineralization in these regions. During younger (Tertiary) mineralizing events, these boundaries served as the preexisting deep crust/upper mantle deep-sourcing conduits, circulating large quantities of auriferous fluids through prospective host rocks.
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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.