Geochemical and isotopic controls of carbon and sulphur in calcium-sulphate waters of the western Meso-Cenozoic Portuguese border (natural mineral waters of Curia and Monte Real

Abstract

The groundwater chemistry and stable isotopic composition of aqueous C- and S-bearing species were determined for mineral waters from Monte Real and Curia Spas. The results support dedolomitization -dolomite dissolution occurs as the gypsum dissolves and calcite precipitates- as a principal geochemical process controlling Ca - SO4 facies of these waters. Soil CO2 of atmospheric origin and carbonate weathering are the primary sources of dissolved inorganic carbon (DIC). However, dissolution / precipitation of carbonates, through incongruent dissolution of dolomite, is the controlling factor for the evolution of the C isotopic composition of DIC, as shown by d13C values of -11,8‰ (Monte Real) and -9,4‰ (Curia). When present, the dissolved sulphide species (d34SH2S = -36,1‰ for Monte Real) appears to be derived from biogenic reduction of dissolved sulphate (d34SSO4 = +16,1‰ for Monte Real and +14,8‰ at Curia), itself derived from dissolution of Hettangian evaporites gypsum (d34SGy = +14,4‰).