COPPER ISOTOPE COMPOSITIONS OF CENOZOIC MAFIC-INTERMEDIATE ROCKS OF THE NORTHERN GREAT BASIN AND SNAKE RIVER PLAIN (USA)

Mid-Miocene epithermal Au-Ag ores of the northern Great Basin (NGB) USA are related to magmatism associated with the inception of the Yellowstone hotspot. A geochemical connection between these ores and spatially and temporally related volcanism is not well understood, but has been suggested (Kamenov, 2007; Saunders et al., 2015). These Cu- and Pb-isotope studies show that the ore and associated gangue minerals have different sources of Pb, which supports evidence that the metal(loids) originate from a deep magmatic source (Saunders et al., 2008). Cu isotopes as a tool for exploring linkages between ore deposits and potentially related volcanic rocks is a new and evolving field. A suite of mid-Miocene NGB and Oregon Plateau volcanic rocks were analyzed by aquaregia leach for their δ⁶⁵Cu compositions. These rocks include basalts, trachybasalt, basaltic andesites, and basaltic trachyandesites that are representative of regional flood basalt magmatism and younger basalt eruptions in central Idaho. Included are rocks from the Santa Rosa-Calico volcanic field, NV (e.g., Buckskin-National district); Owyhee Mtns, ID (Silver City District); Midas, NV region, near Jarbidge, NV; and a locality proximal to Steens Mtn, OR. Also included are two Pleistocene basalts from the central Snake River plain unequivocally related to Yellowstone hotspot volcanism (McKinney Basalt and Basalt of Flat Top Butte), and one Eocene basalt from the Owyhee Mtns that is related to pre-hotspot arc volcanism.

Our new δ⁶⁵Cu data greatly expands the range of known Cu isotopic compositions for basalts, with values ranging from -0.84‰ to +2.61‰. These values overlap with the δ⁶⁵Cu of regional ores, suggesting a link between the source(s) of the ores and the NGB rocks. The range of δ⁶⁵Cu values also overlaps with mantle rock values, suggesting that the Cu isotopic composition may be a signature derived from the mantle source. Fractionation mechanisms that cause such a broad range in Cu isotopes are still unclear but liquid-vapor transitions and mantle metasomatism are being explored. The aquaregia leach technique, although suitable for studying the Cu isotope compositions of ore minerals may not be suitable for studying volcanic rocks. Based on δ⁶⁵Cu values of international rock standards, whole rock dissolution may be a preferable technique.