Widespread base- and precious-metal anomalies, altered porphyry intrusions and oxidized veins occur in a portion of the Sulphur Springs Range, Nevada (adjacent to the Au-producing Carlin Trend). Some of the Eocene-Oligocene intrusions and cogenetic volcanic rocks in the range exhibit evidence of magma mixing and invite comparisons with other mineralized, Eocene mixed magma systems like the Bingham porphyry Cu deposit 300 km farther to the east. The Sulphur Springs igneous suite ranges compositionally through rhyolite, dacite, andesite and basaltic andesite but is less alkaline than the Bingham volcanic suite. However, the alkali content of the Sulphur Springs suite is similar to other Eocene igneous rocks along the Carlin Trend. The unusual geochemical signature of the Bingham igneous suite, enrichment in Cr, Ni, and Ba, is generally not found in the unaltered Sulphur Springs suite, with the exception of a set of altered mafic and intermediate dikes found in the core of the Sulphur Springs Range. The Bingham and Sulphur Springs volcanic suites both show extensive mixing of mafic magma with more silicic magma to create magma with intermediate compositions. The Bingham suite demonstrates mixing mineralogically by the presence of altered olivine and pyroxene in intermediate composition rocks. One of the disequilibrium Sulphur Springs rocks vividly expresses magma mixing as “andesite" - containing plagioclase, biotite, clinopyroxene, orthopyroxene, olivine, and amphibole coexisting with heavily resorbed megacrysts of quartz and K-feldspar. The Sulphur Springs mixed magma also contains abundant late-stage accessory magnetite and resorbed and oxidized garnet. The most likely parental magmas for this rock are a garnet-bearing quartz porphyry and olivine-bearing basaltic andesite which are both present in the range. Questions these data raise include: 1) Was there an unusual tectonic setting during the Eocene of the western United States that promoted both magma mixing and base- and precious-metal mineralization? 2) How vital might mixing processes and mafic magma be in delivering large amounts of S and chalcophile metals from deeper magmas to the shallow crust and eventual ore deposits?



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Physical and Mathematical Sciences; Geological Sciences



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economic geology, magma mixing, igneous rock, Nevada



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Geology Commons