Overview of the Yerington Porphyry Copper District: Magmatic to Nonmagmatic Sources of Hydrothermal Fluids, Their Flow Paths, Alteration Affects on Rocks, and Cu-Mo-Fe-Au Ores
John H. Dilles, Marco T. Einaudi, John Proffett, Mark D. Barton, 2000. "Overview of the Yerington Porphyry Copper District: Magmatic to Nonmagmatic Sources of Hydrothermal Fluids, Their Flow Paths, Alteration Affects on Rocks, and Cu-Mo-Fe-Au Ores", 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 Yerington district, Nevada, hosts at least four porphyry copper deposits and several small Fe oxide-copper-gold lodes within a middle Jurassic batholith and its volcanic cover. The contact aureole of the batholith contains early garnet-pyroxene hornfels and endoskarn, later copper-bearing andradite skarn deposits, and latest-stage large Fe oxide-copper-gold replacement deposits. The Jurassic host rocks have been faulted and tilted 60° to 90° W by Cenozoic normal faulting (Proffett, 1977) so that the modern exposures represent cross sections of a complex paleohydrothermal system from the volcanic environment to about 7 km depth.
This paper summarizes field, petrologic, and geochemical data that support the origin of hydrothermal wall-rock alteration and ore deposition due to two different types of fluids. Magmatic brines were derived from the crystallization of the youngest equigranular intrusion of the Yerington batholith, the Luhr Hill granite. Brines separated from the granite and were emplaced upward together with granite porphyry dikes to produce copper-iron sulfdes and associated K silicate alteration in the porphyry copper deposits and copper skarns. In the upper part of the hydrothermal system, magmatic fluids are an important source of acids and sulfur that produced sericitic and advanced argillic alteration.
A second type of ore fluid is brine derived from formation waters trapped in the Triassic-Jurassic sedimentary section intruded by the batholith. These fluids were heated by the batholith and circulated through its crystalline parts. Hornfels and endoskarn were produced along the contact of an early intrusion. Following intrusion of the porphyry dikes, sedimentary brines circulated up to 3 km into the batholith and upon heating produced sodic-calcic alteration there. Ascent of these brines, particularly after the waning of magmatic fluid input, may have caused shallow-level chlorite-dominated alteration in igneous host rocks and Fe oxide-Cu-Au lodes and replacement deposits in the batholith and its contact aureole, respectively.