Using sulfur and oxygen isotope data for sulfide oxidation assessment in the Freiberg polymetallic sulfide mine

The isotopic and chemical composition and dissolved SO42- of mine water from a mined and backfilled ore vein at the polymetallic sulfide deposit in Freiberg was used to study the SO42- sources and mixing processes. The main SO42- sources in mine water are SO42- in soil seepage water and groundwater, and sulfide oxidation. Using O- and S-isotope analysis, it has been calculated that about 50% (±10%) of SO42- in the mine water at the deepest accessible level of the ore vein originates from sulfide oxidation. A decrease in δ34SSO4 and δ18OSO4 values from 1997 to 2000/2002, in combination with lower SO42- concentrations, is attributed to a lower SO42- input from soil seepage water and ground water. Anthropogenic sources may be responsible for mine water with anomalous stable isotope compositions and concentrations of anions, and metals. Hydrochemical and stable isotope compositions of waters in the central Freiberg and Morgenstern mines were determined to evaluate whether the same trends as those described for the backfilled ore vein are observed in the mines, and what additional processes may operate on a larger geological scale. The δ34SSO4 and δ18OSO4 values of most flowing mine waters follow the mixing trend found for the backfilled ore vein. In addition to these SO42- sources, anthropogenic sources, such as sewage water and industrial wastewater, were observed to be of importance. Judging from isotope values and element concentrations, no obvious decrease in the intensity of sulfide oxidation in the non-flooded part of the Freiberg mine has occurred during the last 10 a. The dual isotope approach to determine both sulfate O- and S-isotopic compositions is useful to better constrain the sources of SO42- in mine waters and can also be used for pollution assessment (portion of SO42- formed by sulfide oxidation) in long-term observation studies.