Copper isotope fractionation in acid mine drainage

摘要

We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a min_eralized watershed (Colorado, USA). The δ~65С_u values (based on ~65С_u/(63)С_u) of enargite (δ~(65)C_u = _0.01 ± 0.10%0; 2_a) and chalcopyrite (δ~65C_u = 0.16 ± 0.10%0) are within the range of reported values for terrestrial primary Cu sulfides (-1%0 __ δ~65C_u __ 1%,). These mineral samples show lower δ~65C_u values than stream waters (1.38%0 __ δ~65C_u __ 1.69%0). The average isotopic fractionation (~A_aq-min = δ~65С_u_aq _ δ~65Cu_min, where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14%0 and 1.60 ± 0.14%0 for chalcopyrite and enargite, respectively. To interpret this field survey, we leached chalcopyrite and enargite in batch experiments and found that, as in the field, the leachate is enriched in ~65C_u relative to chalcopyrite (1.37 ± 0.14%0) and enargite (0.98 ± 0.14%0) when microorganisms are absent. Leaching of minerals in the presence of Acidithiobacillus ferrooxidans results in smaller average fractionation in the opposite direction for chalcopyrite (Δ_aq-min° = _0.57 ± 0.14%0, where min° refers to the starting mineral) and no apparent fractionation for enargite (Δ_aq-min° = 0.14 + 0.14%0). Abiotic fractionation is attributed to preferential oxidation of ~65C_u+ at the interface of the isoto_pically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of ~65Cu_aq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under ТЕМ to occur in precipitates around bacteria and in intracel_lular polyphosphate granules. Thus, the values of δ~65C_u in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.