Iron isotope fractionation by biogeochemical processes in mine tailings

• Iron isotope ratios were determined for the pore water, the 1M HCl / 1M 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 d56Fe 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 have the potential for becoming a tool for the prediction and control of acid mine drainage.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Nyckelord

iron isotopes

mine tailings

pyrite oxidation

Acidithiobacillus ferrooxidans

Earth sciences

Geovetenskap

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