Copper isotope fractionation during sulfide-magma differentiation in the Tulaergen magmatic Ni-Cu deposit, NW China

摘要

Although it has been recently demonstrated that Cu isotope fractionation during mantle melting and basaltic magma differentiation is limited, the behavior of Cu isotopes during magmatic differentiation involving significant sulfide segregation remains unclear. Magmatic Ni-Cu deposits, which formed via sulfide segregation from basaltic or picritic magmas, are appropriate targets to address this issue. Here we report Cu isotope data for sulfides (chalcopyrite) from the Tulaergen Ni-Cu sulfide deposit in Xinjiang, NW China. Sulfides, including sparsely disseminated (hosted by hornblende gabbro), moderately disseminated (hosted by hornblende olivine websterite), densely disseminated (hosted by hornblende Iherzolite) and massive sulfides (sandwiched between country rocks and mafic-ultramafic rocks), were collected from adits at 1050 m,1100 m and 1150 m levels. The sparsely and moderately disseminated sulfides on 1150 m and 1050 m levels have a restricted range of delta Cu-65 values from -0.38 parts per thousand to 0.15 parts per thousand, whereas disseminated and massive sulfides on 1100 m level have delta Cu-65 values ranging widely from -1.98 parts per thousand to -0.04 parts per thousand and from -1.08 parts per thousand. to -0.52 parts per thousand, respectively, The delta Cu-65 values of disseminated sulfides are negatively correlated with whole-rock S and Cu concentrations, and sulfides formed at later stages have heavier delta Cu-65 values. These observations suggest significant Cu isotope fractionation during sulfide-magma differentiation above 600 degrees C. During the formation of the Tulaergen magmatic Ni-Cu deposit, sulfide segregation and crystallization of olivine and pyroxene caused the increase of Fe3+ contents in the residual magmas, which would move the redox reaction Cu+ + Fe3+ = Fe2+ + Cu2+ toward larger amounts of Cu2+ in the melt. The presence of Cu2+ in melt allowed redox transformation to happen during sulfide segregation. The residual magmas are enriched in heavy Cu isotopes due to the removal of Cu-65-depleted sulfides, and sulfides formed at later stages will have heavier delta Cu-65 values, accounting for the large and systematic Cu isotopic variation as observed. Our results therefore demonstrate that Cu isotopes could be significantly fractionated during high temperature sulfide-magma differentiation (up to 2 parts per thousand.), and Cu isotopes may be used to trace the mineralization process of Ni-Cu (-PGE) deposits and indicate the movement direction of sulfide-enriched magmas. (C) 2017 Elsevier B.V. All rights reserved.