Petrology and Geochemistry of the Emigrant Pass Volcanics, Nevada: Implications for a Magmatic-Hydrothermal Origin of the Carlin Gold Deposits

摘要

The Emigrant Pass volcanics (EPV) are a 38.3 to 36.4 Ma calc-alkaline volcanic center that erupted andesite and dacite, and a late series of felsic dikes along the south flank of the Carlin trend in north-central Nevada. The EPV includes dacite and rhyolite porphyry dikes indistinguishable from porphyry dikes associated with mineralization in the trend. The volcanic rocks likely represent eruptive equivalents of deeper, unexposed Eocene plutons that may be the source of heat, fluids, and metals of Carlin-type gold deposits. New petrologic and geochemical data of the EPV are presented to test this hypothesis. The EPV is exposed over a 30 x 15 km area, has an estimated volume of 100 to 200 km~3, and is divided into the early Primeaux lavas (38.1-38.3 Ma), the Mack Creek lavas and related intrusions (37.1-38.4 Ma), and late eruptive and hypabyssal rocks (Henry and Faulds, 1999). The Primeaux is a> 500-m-thick sequence of pyroxene and hornblende andesite to dacite lavas and intrusions; minor pyroxene, hornblende and biotite dikes; and volcanic conglomerates. The Mack Creek is a 180 m thick succession of porphyritic dacite lava flows, domes, and intrusions. Late, small-volume units include the Bob Creek basaltic andesite lava (37.7 Ma) and rhyolitic to dacitic dikes (36.4-36.7 Ma). Primeaux andesite crystallized early clinopyroxene, orthopy-roxene, plagioclase, and late hornblende, which suggests moderate water contents (~3 wt%). Mack Creek dacite and late rhyolite-dacite dikes crystallized hornblende and biotite early, attesting to high water contents (> 4 wt%). Early Primeaux andesites contain abundant magnetite and sparse ilmenite, whereas Mack Creek dacite and late rhyolite-dacite dikes contain magnetite and local titanite. This mineralogy is consistent with the evolution of magmas from early moderately oxidized conditions to late strongly oxidized conditions (fO_2≥NNO+2) similar to porphyry copper magmas (Dilles, 1987). Carlin plagioclase-biotite-hornblende dacite porphyry dikes associated with gold ores at Betze-Post yielded two SHRIMP-RG U/Pb zircon ages (38.7±0.5, 38.8±0.4 Ma) and four LA-ICP-MS U/Pb zircon ages (36.7±1.8, 38.8±1.7, 39.2±1.6, 39.4±1.5 Ma), overlapping in age with the earliest EPV eruptions. All analyzed Carlin porphyries contain zircons with Eu_N/Eu_N* versus Hf arrays and Eu_N/Eu_N*> 0.4 similar to porphyry copper plutons (Dilles et al., 2015) and the 36 Ma Battle Mountain granodiorite porphyry associated with Cu(Au) ores (Farmer, 2013). Abundant partially melted granite xenoliths in EPV rocks indicate assimilation of crust during magma generation, while incompletely mixed magmas with differing mineralogy and sieved plagioclase and resorbed amphibole indicate magma mixing processes.