Experimental study of the copper isotope fractionation between aqueous Cu(II) and covellite, CuS

We report the results of a study of the geologically important copper isotope (⁶⁵Cu/⁶³Cu) fractionation between aqueous Cu(II) and copper sulphide. The experiments were made in anoxic conditions by precipitating covellite (CuS) from excess Cu as aqueous CuSO₄ through the addition of aqueous Na₂S. Isotopic measurements were collected on a Nu Instruments MC-ICP-MS using sample-standard bracketing and mass bias correction using a nickel internal standard. The results of a series of experiments at 20 °C give a mean fractionation of Δ⁶⁵Cu(Cu(II)_aq-CuS)=3.06 ± 0.14‰. Additional experiments made at 2°, 10° and 40 °C show that the fractionation factor varies inversely with temperature according to the relation (errors at 1 σ level): Δ_Cu(II)-CuS = 0.26±0.02 × 10⁶T^-2 + 0.08±0.25. Although the question of whether complete equilibration occurs during the precipitation reaction cannot be resolved, the measured fractionation factors provide a proxy for the natural fractionation processes involving abiogenic covellite formation by low-temperature precipitation. The ∼ 3‰ Cu(II)_aq-CuS fractionation compares with small experimentally measured Cu(II)_aq-malachite fractionation (0.20‰ to 0.38 ± 0.04‰ at 30 °C) reported by Maréchal and Sheppard [2002. Isotopic fractionation of Cu and Zn between chloride and nitrate solutions and malachite or smithsonite at 30° and 50 °C. Goldschmidt 2002 Conference, Davos, Geochim. Cosmochim. Acta. 66, (15A), A484.] and a similar fractionation measured in this study for Cu(II) hydroxide precipitation from Cu(II)_aq solution. The large Cu(II)_aq-CuS isotopic fractionation supports the conclusion that covellite is a Cu(I)S(-I) compound and that redox state is potentially a significant control of abiogenic ⁶⁵Cu/⁶³Cu fractionations in low-temperature geological environments.