Abstract
Abstract Strontium isotopes and other geochemical signatures are used to determine the relationships between CO 2 -rich thermal (Chaves: 76 °C) and mineral (Vilarelho da Raia, Vidago and Pedras Salgadas: 17 °C) waters discharging along one of the major NNE–SSW trending faults in the northern part of mainland Portugal. The regional geology consists of Hercynian granites (syn-tectonic-310 Ma and post-tectonic-290 Ma) intruding Silurian metasediments (quartzites, phyllites and carbonaceous slates). Thermal and mineral waters have 87 Sr/ 86 Sr isotopic ratios between 0.716713 and 0.728035. 87 Sr/ 86 Sr vs. 1/Sr define three end-members (Vilarelho da Raia/Chaves, Vidago and Pedras Salgadas thermal and mineral waters) trending from rainfall composition towards that of the CO 2 -rich thermal and mineral waters, indicating different underground flow paths. Local granitic rocks have 87 Sr/ 86 Sr ratios of 0.735697–0.789683. There is no indication that equilibrium was reached between the CO 2 -rich thermal and mineral waters and the granitic rocks. The mean 87 Sr/ 86 Sr ratio of the thermal and mineral waters (0.722419) is similar to the Sr isotopic ratios of the plagioclases of the granitic rocks (0.71261–0.72087). The spatial distribution of Sr isotope and geochemical signatures of waters and the host rocks suggests that the thermal and mineral waters circulate in similar but not the same hydrogeological system. Results from this study could be used to evaluate the applicability of this isotope approach in other hydrogeologic investigations.
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