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 1σ level):ΔCu(II)–CuS=0.26±0.02×106T−2+0.08±0.25. gh 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.