On the common occurrence of sulphate with elevated δ34S in European mine waters: Sulphides, evaporites or seawater?

The stable isotopic (δ34S, δ18O, δ2H) composition of mine waters has been examined from coal and metal mines, of varying depth, in Spain (Asturias), the UK (north Derbyshire/Yorkshire) and Poland (Bytom, Upper Silesia). δ18O and δ2H data confirm a meteoric origin for all waters. The mine waters have elevated sulphate concentrations relative to recent meteorically-derived groundwaters and have elevated SO42−/Cl− ratios relative to marine water. In the shallower mines and Markham No. 3, the dissolved sulphate δ34S is low, suggesting the sulphate is derived from the oxidation of sulphide minerals. In most of the deeper mines, the dissolved sulphate δ34S is too high (>15‰), to be derived from simple pyrite oxidation. In the deep Polish and Spanish mine waters, the source of sulphate may be evaporite mineral dissolution, but such a mechanism is problematic in the case of the Caphouse (UK) mine. Fractionation associated with sulphate reduction may have contributed to elevated dissolved sulphate δ34S. Elevated sulphate in deep coal mine waters may not always be simply derived from pyrite oxidation. The consistent observation of elevated dissolved sulphate δ34S in deep coal mines suggests a distinct genetic pathway, the origin of which as yet remains equivocal. •Mine water sulphate δ34S characterised in three European coal mining regions.•Shallow mine waters typically exhibit δ34S characteristic of sulphide oxidation.•High δ34S in deeper mine waters suggests evaporite/marine derivation.•Sulphate reduction fractionation may have contributed to elevated sulphate δ34S.•Is deep mine-/groundwater a dynamic sulphate reservoir analogous to ocean water?