Water-rock interaction responsible for the origin of high pH mineral waters (S. Portugal)

摘要

Water-rock interaction studies have greatly contributed to the conceptual hydrogeological circulation model of the Cabeco de Vide mineral waters. The studied mineral waters belong to the Na-Cl/Ca-OH-type waters (pH≈11.5) and issue from the intrusive contact between the mafic/ultramafic rocks and a local carbonate sequence. Mg-HCO_3-type waters constitute most of the shallow groundwaters discharging from local serpentinized ultramafic rocks. Similar δ~2H and δ~(18)O values found in these two types of waters suggest a common meteoric origin and the evolution of the Mg-HCO_3 waters towards Cabeco de Vide-type mineral waters. The Mg-HCO_3-type waters seem to be generated in an initial step under open CO_2 conditions by water-serpentinite interaction in a shallow environment; the Cabeco de Vide mineral waters could be produced in a second step under closed-system conditions as the result of water-dunite interaction at depth.