Vein Textures, Mineralization and Hydrothermal Alteration of the Spring Valley Deposit, Pershing County, Nevada

摘要

The Spring Valley deposit is a vein-hosted gold- and base metal deposit located on the eastern flank of the Humboldt Range in the Spring Valley mining district of Pershing County, Nevada. The deposit is hosted in the Permo-Triassic Rochester and Limerick Formations and in rhyolite dikes of the overlying Weaver Formation. Gold and associated pyrite, sphalerite, galena and chalcopyrite are present in a series of crosscutting quartz-carbonate ±pyrite ± tourmaline veins of clear mesozonal origin, including pre-ore ataxial veins and ore-stage syntaxial elongate-blocky and blocky quartz veins. Pre-ore ataxial veins are characterized by fibrous quartz with distinctly serrated grain boundaries and are associated with restricted silicification. Early ore-stage syntaxial veins formed by the crack-seal process and are composed of elongate-blocky quartz. They may host late carbonate, tourmaline, gold- and base-metal sulfide minerals that did not form during the crack-seal process. Elongate-blocky veins are crosscut by blocky syntaxial quartz veins with early to late carbonate or tourmaline and late gold- and base- metal sulfide mineralization. Ore stage veins are primarily brittle features, with internal textures indicative of formation during extension, including relatively planar, matching vein walls, open-space filling quartz textures, growth of elongated grains perpendicular to vein walls, and evidence of vein-opening direction, such as inclusion bands in crack-seal veins. Gold mineralization may be present in all vein stages, particularly where proximal to late Au-bearing veins. Gold mineralization is primarily associated with pyrite, though it may occur without pyrite and/or with late sphalerite and galena. Sulfide content is generally less than 2 vol. %, and only trace amounts of copper sulfide minerals-primarily chalcopyrite and tet-rahedrite-are present. Ore mineralogy is much simpler than that in nearby deposits, with virtually no silver and volumetrically minor sulfide mineralization. Proximal alteration for all ore-stage veins is characterized by a quartz-sericite-carbonate-pyrite ± tourmaline assemblage, which may grade outward into intermediate to distal quartz-hematite and quartz-specularite assemblages. Although the Spring Valley deposit was initially classified as a porphyry gold deposit with an associated mineralized diatreme, its ore mineralogy, alteration mineralogy, alteration zoning, and vein textures are inconsistent with a porphyry interpretation. Two alternative interpretations are thus proposed: (1) orogenic or (2) reduced intrusion-related gold (RIRG). The Spring Valley deposit shares many similarities with orogenic gold deposits, including: (1) mineralogy and consistency of alteration assemblages and zoning; (2) mineralogy and simplicity of gold and associated base-metal mineralization; and (3) elongate-blocky and blocky quartz textures that are indicative of formation at mesothermal depths. However, vein geometry is generally inconsistent with an orogenic interpretation. In addition to vein textures, which can be similar for both deposit types, several lines of evidence suggest Spring Valley is a reduced intrusion-related gold deposit: (1) mineralization is spatially and possibly temporally associated with the Late Cretaceous magmatism; (2) mineralization is low grade and irregularly distributed; (3) quartz-vein geometries are more in line with those of proximal RIRG deposits; and (4) metal zoning in the Humboldt Range is strikingly similar to metal zoning in established RIRG districts. However, RIRG deposits typically are associated with highly complex ore and alteration mineralogy, and no evidence of a genetic relationship between the various deposits in the Humboldt Range has been established.