Geochemical processes affecting the composition of mineral waters in the South Portuguese Zone, Portugal

The Iberian Pyrite Belt is an important sector of the southernmost geostructural domain of the Iberian Peninsula Palaeozoic Massif. the South Portuguese Zone (SPZ). It is composed of metamorphic rocks of volcano-sedimentary origin, overlain by metasediments of the Mertola Formation. both formed in a submarine environment. In this sector, several highly mineralised sulphide waters occur, having all a Na-Cl matrix. In order to evaluate the geochemical processes affecting the composition of these waters, forward and inverse geochemical modelling were used. The results show that the final composition could be the result of water/rock interaction of meteoric water with the following phases: calcite, dolomite, pyrite, goethite, NaCl, CO2 gas and cationic exchange. Minerals like calcite, dolomite, pyrite and NaCl are dissolving in an open system, as indicated by the large amounts of O-2 consumed during the evolution, accompanied by the precipitation of goethite.