Nature and role of CO2 in some hot and cold HCO3/Na/CO2-rich Portuguese mineral waters: a review and reinterpretation

At the northern part of the Portuguese mainland, the upflow zone of several hot and cold HCO3/Na/CO2-rich mineral waters is mainly associated with imp ortant NNE-SSW faults. Several geochemical studies have been carried out on thermal and non-thermal hydromineral manifestations that occur along or ne ar these long tectonic alignments. The slight chemical differences that exi st between these meteoric hot and cold HCO3/Na/CO2-rich mineral waters seem to be mainly caused by CO2. delta C-13((TIDC)) values observed in these gr oundwaters range between -6.00 and -1.00 parts per thousand versus V-PDB (V denotes Vienna, the site of the International Atomic Energy Agency; PDB or iginates from the CaCO3 of the rostrum of a Cretaceous belemnite, Belemnite lla americana, collected in the Peedee formation of South Carolina, USA) in dicating a deep-seated (mantle) origin for most of the CO2. Nevertheless, i n the case of the heavier delta C-13((TIDC)) values, the contribution of me tamorphic CO2 or the dissolution of carbonate rock levels at depth cannot b e excluded. Concerning the hot waters, the lack of a positive O-18-shift sh ould be attributed to water-rock interaction in a low temperature environme nt, rather than to the isotopic influence of CO2 on the delta O-18-value of the waters.