Geochemistry of coal mine drainage, groundwater, and brines from the Ibbenbüren mine, Germany: A coupled elemental-isotopic approach

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 SO₄^2-. Br/Cl and Na/Cl ratios in deep Na-Cl-type water indicate halite dissolution as the common source of salinity. δ³⁴S_SO4 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 δ²H_H2O and δ¹⁸O_H2O 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 δ³⁴S_SO4 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 δ¹⁸O_H2O values between −6.9 and −8.65‰ (VSMOW) is the primary water source for brines, groundwater, and mine drainage.