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

Abstract

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, Fetot 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 (Δ34SSO4-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 (Δ34SSO4-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 Δ34SSO4-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 δ18OSO4 values were enriched than those of background water in tailings sites with the similar Δ18OSO4–H2O values (+8.9 to +11‰). Based on field data revealing enhanced galena and/or sphalerite oxidation relative to pyrite, the observed Δ18OSO4–H2O (avg., 9‰) values coupled with the relationship between δ34SSO4 values and sulfate concentration can be reflective of oxidation of monosulfides in contrast to disulfide minerals (e.g, pyrite) within polymetallic rich AMD systems.