COPPER PARTITIONING IN MID-MIOCENE STEENS BASALT LAVAS FROM SOUTHEASTERN OREGON (USA): IMPLICATIONS FOR CU BEHAVIOR IN MAFIC MAGMAS

It is generally accepted that beneath flood basalt provinces, Cu-Ni-PGE sulfide deposits may be found (Ridley, 2013). The focus of this study is the Steens Basalt, a mid-Miocene flood basalt and the basal member of the Columbia River Basalt Group, that crops out across southeastern Oregon, in the northern Great Basin (USA). Steens Basalt lavas contain between ~5-400 ppm copper and are often characterized by large plagioclase phenocrysts, some of which can contain primary inclusions of Cu despite the chalcophile nature of Cu (e.g., Oregon sunstones - Hofmeister and Rossman, 1985; Johnston et al., 1991). The purpose of this project is to identify the distribution of Cu among coexisting phases in Steens Basalt, test whether plagioclase crystals in Steens Basalt lavas can host Cu, even when Cu is not visible, and test whether sulfide minerals/droplets are present in Steens Basalt samples with low Cu concentrations. Samples of Steens Basalt were examined for sulfide minerals via reflected light microscopy, Raman spectroscopy, and X-ray diffraction with a molybdenum tube. Using an electron microprobe, silicate minerals, oxides, glass, and sulfides were analyzed for their Cu concentration, as well as other major and trace element chemistry. Glass did not contain detectable Cu which precluded partition coefficient (K_d) calculations. Based on average Cu concentration for the non-sulfides, magnetite contains the most Cu, followed by (forsteritic) weathered olivine, pyroxene, olivine, plagioclase, and ilmenite. Cu sulfides were discovered in samples MB97-19 and MB97-76C with additional non-Cu sulfides in MB97-76B. Some of these Cu sulfides exhibited evidence of prior quenched immiscible liquids based on textural comparisons with published studies (Barnes et al., 2017; Ray et al., 2017). Other textural evidence gained from the electron microprobe demonstrates that some Steens Basalt lavas were undercooled based on comparison with Shea and Hammer (2013). In conclusion, these results lay the groundwork for further investigation into potential Cu sulfide reserves in the magma plumbing of Steens Basalt as with other flood basalt packages linked to economically important mineral deposits.