Sulfur and oxygen isotopic systematics of dissolved sulfate and relative contributions of sulfide oxidants in the Erdenet mining region, Mongolia.

Dissolved sulfate, a major contaminant in mining areas, is generally produced by the oxidation of sulfides by either molecular oxygen or Fe(III). Because Fe(III) oxidizes sulfides much more rapidly, it contributes more effectively to elevated SO₄^2- concentrations. This study analyzed δ³⁴S_SO4, δ¹⁸O_SO4, δ¹⁸O_H2O, and δ²H_H2O in groundwater and surface water from the Erdenet mining region, Mongolia. The potential fraction of Fe(III) as the oxidant of pyrite was inferred based on δ¹⁸O_SO4 and δ¹⁸O_H2O. Theoretical and previously reported minimum fractions appear to constrain the lower range of Fe(III) contributions, whereas sulfate reduction and gypsum dissolution may reduce the apparent fractions in some samples. In addition, gypsum precipitation appears to limit the upper range of SO₄^2- concentrations, while the dissolution of efflorescent salts may increase SO₄^2- concentrations. After excluding samples potentially influenced by these processes, a positive relationship between the potential fraction of Fe(III) as the oxidant and SO₄^2- concentration remains evident. These results suggest the potential importance of Fe(III)-mediated oxidation in controlling sulfate concentrations in mining environments. Furthermore, when additional geochemical processes are appropriately accounted for, the combined use of δ¹⁸O_SO4 and δ¹⁸O_H2O provides a systematic framework for elucidating the mechanisms of sulfide oxidation in such environments. Additionally, variations in the relationship between the potential fraction of Fe(III) as the oxidant and SO₄^2- concentration across mining environments may provide evidence for site-specific geochemical processes, such as gypsum dissolution and sulfate reduction.