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

Abstract

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 lherzolite) and massive sulfides (sandwiched between country rocks and mafic–ultramafic rocks), were collected from adits at 1050m, 1100m and 1150m levels. The sparsely and moderately disseminated sulfides on 1150m and 1050m levels have a restricted range of δ65Cu values from −0.38‰ to 0.15‰, whereas disseminated and massive sulfides on 1100m level have δ65Cu values ranging widely from −1.98‰ to −0.04‰ and from −1.08‰ to −0.52‰, respectively. The δ65Cu values of disseminated sulfides are negatively correlated with whole-rock S and Cu concentrations, and sulfides formed at later stages have heavier δ65Cu values. These observations suggest significant Cu isotope fractionation during sulfide-magma differentiation above 600°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 65Cu-depleted sulfides, and sulfides formed at later stages will have heavier δ65Cu 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‰), 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.