Field Trip Day Four: Buena Vista Hills, Humboldt Mafic Complex, Western Nevada
David A. Johnson, Mark D. Barton, 2000. "Field Trip Day Four: Buena Vista Hills, Humboldt Mafic Complex, Western 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 purpose of this portion of the field trip is to examine the spatial and temporal distribution of sodium-rich hy-drothermal alteration and iron oxide mineralization generated by a saline hydrothermal system driven by the Middle Jurassic Humboldt mafic complex. The Humboldt system is of particular interest because it represents a basaltic mag-matic end member of intrusion-driven hydrothermal systems and, in this case, one where the fluids are largely, perhaps entirely, externally derived brines. Outcrops of Jurassic rocks within the complex record multiple, mutually crosscutting magmatic and hydrothermal events at different structural levels as exposed by mid-Tertiary extension. Mapping of selected areas across this large igneous complex allows definition of the relationships among the magmatic, structural, and hy-drothermal features. In turn, these enable an interpretation of the overall temporal and spatial evolution of a large intrusion-driven hydrothermal system (Johnson and Barton, 2000). From geology and geochemistry, one can estimate that upwards of 15 billion tonnes of iron and 35 million tonnes each of copper and zinc were moved by the hydrothermal system (Johnson, 2000). What happened to these metals and what are the implications for other systems? These are among the issues to consider.
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.