Time-Space Development of an External Brine-Dominated, Igneous-Driven Hydrothermal System: Humboldt Mafic Complex, Western Nevada
David A. Johnson, Mark D. Barton, 2000. "Time-Space Development of an External Brine-Dominated, Igneous-Driven Hydrothermal System: 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 Humboldt mafic complex, west-central Nevada, is a large composite Middle Jurassic basaltic-composition volcano-plutonic center that has exceptionally extensive (>900 km3), intense (nearly complete leaching of many elements) sodium-rich hydrothermal alteration. Mapping of exposures at multiple structural levels allows assessment of the time-space development of hydrothermal alteration and cogenetic magnetite and hematite ± copper sulfide mineralization.
Alteration varies from early, deep and proximal marialitic scapolite-hornblende to shallow and distal albite-actinolite-chlorite and chlorite-carbonate assemblages. These associations reflect large compositional changes in host rocks (mass-transfer), whereas distal and deep propylitc assemblages are less intensely modified. Substantial quantities of iron are present in massive, breccia-form, and stratabound magnetite and hematite bodies at intermediate and shallow depths. Lesser amounts of copper, cobalt, and other metals are sporadically enriched at shallow levels.
Field, petrological, and geochemical constraints require that the fluids were dominantly or entirely non-magmatic, external brines that circulated in response to the heat and permeability increases associated with repeated basaltic intrusion. The Humboldt system represents a mafic end-member among iron oxide-rich copper-bearing hydrothermal systems (Barton and Johnson, 1996) and, in the larger context, and an end-member in the spectrum of igneous-related hydrothermal systems.
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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.