Groundwater: Past Achievements and Future Challenges, Sililo et al. (eds) © 2000 Balkema, Rotterdam, ISBN 90 5809 159 7 Geochemical processes affecting the composition of mineral waters in the South Portuguese Zone (Portugal) A.Chambel Dep. Geociências, Universidade de Évora, Portugal C.Almeida Dep. Geologia, Fac. Ciências, Universidade de Lisboa, Portugal ABSTRACT: 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 by metamorphic rocks of volcano-sedimentary origin, followed by the metassediments of the Mértola 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 association: calcite, dolomite, pyrite, goethite, NaCl, CO 2 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. 1 INTRODUCTION The South Portuguese Zone (SPZ) is the southern- most geostructural domain of the Iberian Massif. The presence of a Pyrite Belt (Iberian Pyrite Belt), of extraordinary economic importance for Portugal and Spain, and associated lithologies is related to several occurrences of highly mineralised sulphide waters (Calado & Chambel 1999). The aim of this paper is to evaluate the geo- chemical processes controlling the composition of these waters, using forward and inverse geochemical models and considering the final composition as the result of the interaction between meteoric water and rocks. 2 GEOLOGICAL SETTING The Iberian Massif is an important geological unit of the Iberian Peninsula and is sub-divided into several geostructural domains. The southernmost of these domains is the SPZ, characterised by the presence of three main units that are the result of the collision of the “middle European” and “North Europe/South Iberian” continents (Bard et al. 1980) accompanied by the closing of a Palaeozoic ocean just north of the SPZ (Fig. 1): − the Pulo do Lobo Sub-Zone, in the northern part − the Pyrite Belt − the Baixo Alentejo Flysch Group, in the southern part Figure 1. Tectono-stratigraphic unit of the South Portuguese Zone, in South Portugal, and its sub-divisions. The rectangle represents the area enlarged in Figure 2. 471