Copper isotope fractionation in acid mine drainage

Abstract

We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ 65Cu values (based on 65Cu/ 63Cu) of enargite (δ 65Cu = −0.01 ± 0.10‰; 2 σ) and chalcopyrite (δ 65Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ < δ 65Cu < 1‰). These mineral samples show lower δ 65Cu values than stream waters (1.38‰ ⩽ δ 65Cu ⩽ 1.69‰). The average isotopic fractionation (Δ aq-min = δ 65Cu aq − δ 65Cu min, where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14‰ and 1.60 ± 0.14‰ 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 65Cu relative to chalcopyrite (1.37 ± 0.14‰) and enargite (0.98 ± 0.14‰) 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 o = - 0.57 ± 0.14 ‰ , where min o refers to the starting mineral) and no apparent fractionation for enargite ( Δ aq-min o = 0.14 ± 0.14 ‰ ). Abiotic fractionation is attributed to preferential oxidation of 65Cu + at the interface of the isotopically 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 TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ 65Cu 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.