Mineralogical and geochemical study of clay and calcite veins in a shear zone related to a sulphur spring (NW Portugal)

In the Terras de Bouro area, in Northwest Portugal, the role of water–rock interaction, either by the mineralization of the sulphur spring, or by the nature of the granite secondary minerals, was studied as a complement to exploration for mineral water. This approach is justified by the peculiar mineral paragenesis found, which is very different from those of the regional weathering products. Calcite and montmorillonite as veins infilling and pyrite in the wall rock are the secondary minerals characteristic of mineral water occurrence. Sulphur water composition, alkaline pH and low redox potential are some factors that may favour the formation of montmorillonite, calcite and essentially pyrite, in the spring zone. Isotopic signatures in δ2H and δ18O of sulphur water show that its origin is dominantly meteoric. Isotopic signatures δ13C and δ18O of calcite and of the inorganic carbon in the water suggest that calcite precipitates from carbonate species in the sulphur water at ∼17 °C.