USING COPPER ISOTOPIC COMPOSITION TO DISTINGUISH NATIVE COPPER FORMATION MECHANISMS

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.