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.