Hot and cold CO2-rich mineral waters in Chaves geothermal area (Northern Portugal)

In order to update the geohydrologic characterisation of Chaves geothermal area, coupled isotopic and chemical studies have been carried out on hot and cold CO2-rich mineral waters discharging, in northern Portugal, along one of the major regional NNE-trending faults (the so-called "Verin-Chaves- Penacova Depression"). Based upon their location, and chemical and isotopic composition, the analysed waters can be divided into two groups. The northern group belongs to the HCO3/Na/CO2-rich type, and consists of the hot spring waters of Chaves and the cold spring waters of Vilarelho da Raia. The 6D and 6180 values show that these waters are of meteoric origin. The lack of an 1So shift indicates that there is no evidence of water/rock interaction at high temperatures. The southern group includes the cold spring waters of Campilho/ Vidago and Sabroso/Pedras Salgadas. Their chemistry is similar to that of the northern group but their heavier 6D and 6180 values could be attributed to different recharge altitudes. Mixing between deep mineralised waters and dilute superficial waters of meteoric origin might explain the higher 3H activity found in the Vidago and Pedras Salgadas mineral waters. Alternatively, they could be mainly related to shallow underground flowpaths. The 613C values support a deep-seated origin for the CO2. The 637CI is comparable in all the mineral waters of the study area, indicating a common origin of CI. The S7Sr/S6Sr ratios in waters seem to be dominated by the dissolution ofplagioclases of granitic rocks. © 1998 CNR. Published by Elsevier Science Ltd. All rights reserved