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

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


IKEHATA, Kei1, CHIDA, Kohsuke1, BORNHORST, Theodore J.2, ISHIBASHI, Junichiro3 and HIRATA, Takafumi4, (1)Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, 305-8572, Japan, (2)A. E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, MI 49931, (3)Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka, 812-8581, Japan, (4)Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan, ikkei@geol.tsukuba.ac.jp

Native copper (Cu = ~100wt.%), zero-valance state copper mineral, occurs in a diversity of environments such as igneous rock, clastic sediment and oxidized ore (Cornwall, 1956). These geneses roughly can be divided into three types; magmatic, hydrothermal and secondary (supergene).

Copper isotope geochemistry has been recognized as a potential tool for understanding copper sources and geochemical processes of copper transport and deposition in ore-forming systems (e.g., Mathur et al., 2009). In this study, we measured copper isotopic compositions for magmatic, hydrothermal, and secondary native copper from various localities by using a femtosecond-LA-MC-ICP-MS (Ikehata et al., 2008). The δ65Cu (δ65Cu = [(65Cu/63Cu)sample/ (65Cu/63Cu)NIST-SRM976-1] × 1000) values of lherzolite (whole rock) and magmatic native copper grains in it from an orogenic massif (Horoman peridotite complex, Japan) are in a relatively narrow range (-0.03 to 0.14‰), indicating that there is no significant copper isotopic fractionation during high-temperature magmatic processes. Copper isotopic ratios of hydrothermal native copper from the Caledonia mine in the Keweenaw region of Lake Superior, USA are homogeneous (δ65Cu = 0.27 to 0.34‰). These values are slightly higher than terrestrial igneous copper which is the probable source (δ65Cu = -0.27 to 0.27‰; Ikehata and Hirata, 2012). This suggests isotopic fractionation has occurred during formation of this at conditions of moderate temperature. The δ65Cu values of primary sulfide minerals (e.g., chalcopyrite) from the Besshi-type volcanogenic massive sulfide deposits (Mio mine, Japan) and the moderately high-temperature vein deposit (Okorogawa mine, Japan) are ~0‰. Native copper grains from the serpentinized Horoman peridotite have lighter δ65Cu values (-0.67 to -0.41‰) than before alteration. Hydrothermal native copper rimmed by malachite (Caledonia mine) has lighter δ65Cu values (-0.25 to -0.10‰) whereas the δ65Cu values of supergene native copper grains from Mio and Okorogawa deposits are significantly heavier (1.43 to 1.71‰, 1.60±0.10‰, respectively; Ikehata et al., 2011 and Ikehata and Hirata, 2012). Secondary low-temperature redox-controlled isotope fractionation controls secondary isotopic values.