Bioprecipitation of As4S4 polymorphs in an abandoned mine adit

Abstract

Opportunities to study the anthropogenic cementation zones within permanently flooded mine works, where toxic elements accumulate by processes not fully described, are generally very rare. We examined macroscopic arsenic sulfide deposits inside a decomposed timbering of an abandoned mine adit, which had been flooded for 75 years. Bonazziite (β-As4S4) was always the dominant mineral in the deposits (up to 96.5%), while realgar and the ZnS phase were present in minor amounts. These findings differ from near-surface environments, where the most common biogenic As sulfide is realgar and other polymorphs are usually present in trace amounts only. Bonazziite was chemically highly pure with an average As/S molar ratio 0.9. It was 34S depleted compared to both aqueous sulfate and primary sulfides, which is consistent with biogenic sulfur reduction. Sulfide encrustation was confined to individual wood tracheids, indicating the extent of microenvironments that were oversaturated with respect to these minerals. The sulfides were accompanied by older gypsum and younger arsenian schwertmannite indicating complex succession of the redox conditions within the timbering. Authigenic electrum was found in one sample. Analysis of microbial communities revealed the dominance of sulfate reducers and fermenting microorganisms within the timber, similar to realgar-precipitating spots in wetland soils as previously documented. An anoxic environment in the mine water was maintained by autotrophic iron oxidation. The results suggest that a highly reduced microenvironment embedded in anoxic, but relatively oxidized surroundings is the key feature of the realgar and bonazziite-precipitating systems.