Iron isotope fractionation by biogeochemical processes in mine tailings
Iron isotope ratios were determined for the pore water, the 1 M HCl/1 M hydroxylamine hydrochloride (HAH)-extractable solid phase, and the total extractable solid phase from sulfidic mine tailings in Impoundment 1, Kristineberg mine, northern Sweden. Within the tailings, pyrite oxidation occurs in a distinct Fe-depleted oxidation zone, and the greatest number of Fe(II)-oxidizing bacteria in the profile occur close to the boundary between oxidized and unoxidized tailings. Above the oxidation front in the oxidized tailings, a large iron isotope fractionation (−1.3 to −2.4‰) is measured between the pore water and the HAH-extractable solid phase. This isotope fractionation is explained by aqueous Fe(II)–Fe(III) equilibrium, microbial Fe(II) oxidation, and Fe(III) oxyhydroxide precipitation. The data suggests that pyrite in the tailings is enriched in 56Fe relative to Fe-rich silicates in the same material, such that pyrite oxidation results in a decrease in the mean δ56Fe value for the bulk tailings in the oxidized zone: a change in isotope composition that is not attributable to isotope fractionation. Iron isotope analyses yield valuable information on iron cycling in mine wastes, and they have the potential for becoming a tool for the prediction and control of acid mine drainage.