Petrology and geochemistry of the Emigrant Pass volcanics, Nevada : implications for a magmatic-hydrothermal origin of the Carlin gold deposits Public Deposited

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  • 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 gold district in north-central Nevada. The EPV includes dacite and rhyolite porphyry dikes indistinguishable from porphyry dikes associated with mineralization within the district. 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 are exposed over a 30 x 15 km area, have an estimated volume of 100 to >200 km³ and are 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 lavas are 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 lavas are a 180 m thick succession of porphyritic dacite lava flows, domes, and intrusions. Late small-volume units include the Bob Creek basaltic andesite to 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 (~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 dacites 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 (fO2 ≥ NNO+2) similar to porphyry copper magmas (Dilles, 1987). Carlin plagioclase-biotite-hornblende dacite porphyry dikes associated with gold ores yielded four new 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. Two additional U/Pb zircon ages were obtained on a coarsely porphyritic dacite lava of the Mack Creek sequence (37.10±0.54 Ma), and a porphyritic dacite lava (148.2±2.3 Ma) likely of the Frenchie Creek Volcanics at the southern terminus of the EPV. All analyzed Carlin porphyries contain zircons with EuN/EuN* versus Hf arrays and EuN/EuN* > 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, whereas incompletely mixed magmas with differing mineralogy along with abundantly 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, interpreted to indicate mixing, the MgO and Cr of most samples abruptly decrease with increased silica consistent with crystal fractionation. Dacites of the Mack Creek have elevated MgO and Cr compared to Primeaux andesites and Bob Creek basaltic-andesites, 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 820-890°C and 100-250 MPa (4-10 km depth), whereas high-Al amphibole crystallized at about 900-980°C and possibly greater depths. Electron microprobe studies of magmatic sulfide inclusions indicate monosulfide solid solution (MSS, pyrrhotite at low temperatures) was the dominant magmatic sulfide species with minor (n=4 of 119 analyses) intermediate solid solution (ISS, chalcopyrite at low temperatures). Sulfide contents decrease systematically as the magmas evolved from andesite-dacite-rhyolite, supporting the observation of increasing oxidation state and magmatic sulfate/sulfide ratio. Additionally, all phases of the EPV are highly depleted in whole rock Cu (<10ppm), suggesting Cu was efficiently removed during volatile exsolution at depth or alternatively was sequestered in magmatic sulfide at depth. Experimental studies of Au partitioning in intermediate composition melts indicate that the predominance of MSS over sulfide liquid or ISS is a prerequisite for forming Au-rich melts as MSS preferentially incorporates Cu over Au, thereby further enriching the Au/Cu ratio of the remaining melt (Muntean et al., 2011; Botcharnikov et al., 2013; Yi and Audetat, 2013). Muntean and others (2011) proposed a magmatic-hydrothermal fluid and metal source of the Carlin gold deposits and suggested source magmas were fundamentally enriched in Au relative to Cu as a result of MSS fractionation over ISS or sulfide liquid, which is supported by these new data on EPV magmatic sulfide inclusions.
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