2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 206-10
Presentation Time: 9:00 AM-6:30 PM


MAYNARD, Annastacia L.1, BRUESEKE, Matthew E.1 and MATHUR, Ryan2, (1)Department of Geology, Kansas State University, Manhattan, KS 66506, (2)Department of Geology, Juniata College, 1700 Moore Street, Huntingdon, PA 16652, mayn1093@ksu.edu

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.