COPPER ISOTOPE SYSTEMATICS OF THE VARIOUS TYPES OF NATIVE COPPER MINERALIZATION

The copper isotope variations of magmatic, hydrothermal, and supergene native copper from different localities have been measured by LA-MC-ICP-MS. The δ⁶⁵Cu (δ⁶⁵Cu = [(⁶⁵Cu/⁶³Cu)_sample/ (⁶⁵Cu/⁶³Cu)_NIST-SRM976-1] × 1000) values of peridotite (whole rock) and magmatic native copper grains from an orogenic massif (Horoman peridotite complex, Japan) are in a relatively narrow range (-0.03 to 0.14‰), implying that there is no significant copper isotope fractionation during high-temperature magmatic processes. Copper isotope compositions 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 genesis of this at conditions of moderate temperature. Primary native copper grains affected by serpentinization (Horoman peridotite) and low temperature alteration (Caledonia mine) have lower δ⁶⁵Cu values (-0.67 to -0.41‰, Horoman; -0.25 to -0.10‰, Caledonia) than pre-alteration. In contrast, secondary supergene native copper (δ⁶⁵Cu = 1.43 to 1.71‰) from the Besshi-type volcanogenic massive sulfide deposits (Mio mine, Japan) has heavier copper isotopic values as compared to precursor minerals (chalcopyrite, δ⁶⁵Cu = -0.34 to -0.13‰). These variations can be explained by redox-controlled isotope fractionation (preferential incorporation of heavy copper isotope in secondary Cu(II) solutions) at low temperature conditions. These data also suggest that negative isotopic fractionation accompanies the reduction of Cu (II) in solution to native copper (Cu (0)). Copper isotope geochemistry will be used to further understand the formation processes involved in these native copper occurrences.