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

Mid-Miocene epithermal Au-Ag ores of the northern Great Basin USA are related to magmatism associated with the inception of the Yellowstone hotspot. The chemical connection between these ores and spatially and temporally related volcanism is not well understood. The use of copper isotopes in ore deposit research to shed light on this connection is a new and evolving field. δ⁶⁵Cu values from electrum, naumannite, and chalcopyrite from the mineralization range between -2.1‰ to +3.2‰ (Saunders et al., 2015-MD) and for comparison, a suite of mid-Miocene northern Great Basin and Oregon Plateau volcanic rocks were analyzed for their δ⁶⁵Cu compositions. These rocks include basalt, trachybasalt, basaltic andesite, and basaltic trachyandesite compositions that are representative of regional flood basalt magmatism (e.g., Steens Basalt and locally erupted equivalents) These samples have all been previously well-characterized and include eruptions from the Santa Rosa-Calico volcanic field, NV (e.g., Buckskin-National district), the Owyhee Mountains, ID (e.g., Silver City District), from the Midas, NV region, near Jarbidge, NV, and proximal to Steens Mountain, OR. We also analyzed two Pleistocene basalt lavas from the central Snake River plain (e.g., McKinney Basalt and Basalt of Flat Top Butte), as well as one Eocene basalt from the Owyhee Mountains that is unrelated to the Yellowstone hotspot. The basalts span a wide range of δ⁶⁵Cu values from -0.84‰ to +2.61‰ and there is no relationship between their δ⁶⁵Cu composition and with wt. % SiO₂. These rocks have Cu concentrations up to 250 ppm and when plotted vs. δ⁶⁵Cu, a linear relationship exists where samples with higher Cu concentrations have lower δ⁶⁵Cu values. Furthermore, the rock samples directly associated with coeval mid-Miocene mineralization have the highest Cu elemental concentrations and generally lower δ⁶⁵Cu values. Other positive linear relationships exist between δ⁶⁵Cu values and Zn and Ni concentrations for the rocks that are spatially and temporally associated with mineralization. Fractionation processes associated with the mineralization are a likely explanation for the observed relationships between δ⁶⁵Cu and Cu, Zn, and Ni concentrations.