S and O isotope ratios of sulfate suggest enhanced monosulfides oxidation in a historical Pb–Zn mine site

Aqueous and solid tailings geochemistry of two contrasting depositional sites (water-saturated and subaerial) in a historical Pb–Zn mine tailings in Balya, Turkey were chemically and isotopically (δ³⁴S and δ¹⁸O) characterized to trace source of sulfate and elucidate oxidation pathways of sulfide mixtures. Aqueous geochemistry data of water-soluble fractions (n = 15) obtained from the batch leaching tests applied on the subaerial mine tailings (SA-MT) revealed three geochemically distinct zones: i) oxidized zone (OXZ, 10–75 cm), ii) transition zone (TZ, 75–125 cm) and iii) unoxidized zone (U-OXZ, 125–350 cm). The OXZ is characterized by a sharp decrease in sulfur and metals contents of the tailing rocks while high sulfate and metals concentrations in their water-soluble fractions. In contrast, the TZ and U-OXZ zones are mainly characterized by low sulfate but relatively high metal concentrations with slightly acidic pH values (4.9–6.2). Compared to the SA-MT tailings geochemical profiles of the pore waters of the water-saturated tailings (S-MT) showed remarkably high sulfate, Fe_tot and metals concentrations with strong acidic pH values (1.9–3.8). The δ³⁴S and δ¹⁸O values of sulfate suggest that galena and/or sphalerite are the main source of sulfate and metals at the tailings site. The differences in sulfur isotope values (Δ³⁴S_SO4-sulfide) appear to be reflective of the extend of oxidation of monosulfides (e.g. galena) under two contrasting conditions of the tailings. The large sulfur isotope differences (Δ³⁴S_SO4-sulfides ~ -4.5 ± 0.2‰) imply incomplete oxidation of monosulfides to sulfate with aqueous geochemistry record of low sulfate, high metal and slightly acidic pH values. In contrast, the small Δ³⁴S_SO4-sulfide (<1 ± 0.2‰) values indicate the tailings site where complete oxidation of monosulfides to sulfate occurred resulting in acidic condition having with high sulfate, metal concentrations. The δ¹⁸O_SO4 values were enriched than those of background water in tailings sites with the similar Δ¹⁸O_SO4–_H2O values (+8.9 to +11‰). Based on field data revealing enhanced galena and/or sphalerite oxidation relative to pyrite, the observed Δ¹⁸O_SO4–_H2O (avg., 9‰) values coupled with the relationship between δ³⁴S_SO4 values and sulfate concentration can be reflective of oxidation of monosulfides in contrast to disulfide minerals (e.g, pyrite) within polymetallic rich AMD systems.