Fluid speciation controls of low temperature copper isotope fractionation applied to the Kupferschiefer and Timna ore deposits

Abstract

Mass-balance among reduced and oxidized solution species exerts a strong control on the isotopic composition of sulphide minerals. We explore this issue for copper isotopic fractionation in sedimentary copper sulfide deposits through fluid speciation-isotopic Eh–pH calculations based on the thermodynamic stability of copper chloride aqueous solution complexes and experimentally measured copper isotope fractionation factors. Applying these speciation diagrams to MC-ICP-MS copper isotope data on the Kupferschiefer (SW part of the Lubin-Sieroszowice Copper District) and Timna (S. Israel) Sediment-hosted Stratiform Copper deposits (SSC), identifies differences in ore formation redox conditions. Timna Valley copper sulfides have light Cu-isotopic compositions of δ 65Cu = − 2.04 ± 0.44‰ 2σ (relatively to SRM 976 copper standard), which are shown to correspond to Eh values of 0.5 to 0.6 V at formation conditions (T = 40 °C; pH < 6). These Eh values indicate precipitation at relatively oxidized conditions where the Cu(I) solution complex (CuCl 3 2−) is < 10% of the total solution species. In contrast, the Kupferschiefer Cu-sulfides analyzed in this work dominantly show significantly higher δ 65Cu values = − 0.39 ± 0.36‰, corresponding to Eh values of 0.4 to 0.5 V at formation conditions (T = 100 °C; pH = 6.3). In these Eh conditions, most copper in solution occurs as CuCl 3 2− complexes (~ 80%). The above observations are in accord with field relations showing that Cu(II) minerals dominate the Timna system, but Cu-sulfides are the major minerals of the Kupferschiefer deposits. Fluid speciation–isotopic calculations show that copper isotopes can be used as effective tracers of redox conditions and this approach can be potentially applied to various hydrothermal ore deposits, such as black smokers and volcanic-hosted massive sulphide deposits, and to other metallic isotope systems. However, the isotopic fractionation may also be strongly influenced by the types of ligand bonding of copper ions and these effects need to be fully evaluated before the isotope geochemistry of copper ores can be fully understood.