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Coal mine drainage; Pyrite oxidation; Brine origin; Sulfur isotopic composition; Isotope proxies; Hydrogeochemistry
Abstract:
A coupled elemental-isotopic approach is applied to reconstruct the origin and chemical evolution of mine drainage, groundwater, and brines from the Carboniferous anthracite coal mine in Ibbenbüren, Germany. All solutions are characterized by an increase in salinity with depth, as well as by an increase in 34S/32S isotopic ratios of dissolved SO42-. Br/Cl and Na/Cl ratios in deep Na-Cl-type water indicate halite dissolution as the common source of salinity. δ34SSO4 values increase up to +21.1‰ (VCDT), linking the salinity to the migration of groundwater from the surrounding Mesozoic sediments. 87Sr/86Sr ratios between 0.7108 and 0.7135 and elevated alkali concentrations indicate ongoing water-rock interaction of the evaporite-derived brines with the Carboniferous siliciclastic rocks of the mine. A positive correlation of 87Sr/86Sr ratios with δ2HH2O and δ18OH2O values suggests mixing of the brines with isotopically heavy formation water within the Carboniferous bedrock. The oxidation of pyrite is the dominant sulfate source in shallow mine drainage and groundwater with a relatively low ionic strength (I < 0.035), as indicated by δ34SSO4 values between −8.3 and + 0.3‰ (VCDT). Intermediate water compositions are the result of the dilution of brines with shallow water. In any case, modern meteoric water with δ18OH2O values between −6.9 and −8.65‰ (VSMOW) is the primary water source for brines, groundwater, and mine drainage.
