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

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, orthopyroxene, plagioclase, and late hornblende, which suggests moderate water contents (similar to 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. The andesites and dacites have high-K to shoshonitic compositions, similar to subduction-related arc magmas. Whereas most major elements display simple linear correlations with silica, the MgO and Cr of most samples abruptly decrease with increased silica, consistent with crystal fractionation, while some dacites have elevated MgO and Cr consistent with mixing of primitive basaltic andesite or andesite with rhyolite. Enrichment in Rb, Ba, and Cs indicate crustal additions compared to average arc magmas, whereas elevated V/Sc (1.4-17.3; Mean: 8.9) and Sr/Y (16.6-49.6; Mean: 36.1) ratios are similar to mineralizing porphyry Cu (Au) magmas (Loucks, 2014). Hornblende compositions from four samples are bimodal and include both low-Al and high-Al amphiboles now commonly recognized in arc suites (Aucanquilcha, Chile: Walker et al., 2013; Yanacocha, Peru: Chambefort et al., 2013). Using the Ridolfi et al. (2010) formulation, the low-Al hornblende crystallized at 770-890 degrees C and 100-250 MPa (4-10 km depth), whereas high-Al amphibole crystallized at about 900-980 degrees C and possibly greater depths. All phases of the EPV are highly depleted in Cu (< 10ppm), suggesting Cu, possibly along with Au, was efficiently removed during volatile exsolution at depth or alternatively was sequestered in magmatic sulfide.