HOT AND COLD CO2-RICH MINERAL WATERS IN CHAVES GEOTHERMAL AREA (NORTHERN PORTUGAL)

In order to update the geohydrologic characterisation of Chaves geothe rmal area, coupled isotopic and chemical studies have been carried out on hot and cold CO2-rich mineral waters discharging, in northern Port ugal, along one of the major regional NNE-trending faults (the so-call ed ''Verin-Chaves-Penacova Depression''). Based upon their location, a nd chemical and isotopic composition, the analysed waters can be divid ed 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 sp ring waters of Vilarelho da Raia. The delta D and delta(18)O values sh ow that these waters are of meteoric origin. The lack of an O-18 shift indicates that there is no evidence of water/rock interaction at high temperatures. The southern group includes the cold spring waters of C ampilho/Vidago and Sabroso/Pedras Salgadas. Their chemistry is similar to that of the northern group but their heavier delta D and delta(18) O values could be attributed to different recharge altitudes. Mixing b etween deep mineralised waters and dilute superficial waters of meteor ic origin might explain the higher H-3 activity found in the Vidago an d Pedras Salgadas mineral waters. Alternatively, they could be mainly related to shallow underground flowpaths. The delta(13)C values suppor t a deep-seated origin for the CO2. The delta(37)Cl is comparable in a ll the mineral waters of the study area, indicating a common origin of Cl. The Sr-87/Sr-86 ratios in waters seem to be dominated by the diss olution of plagioclases of granitic rocks. (C) 1998 CNR. Published by Elsevier Science Ltd. All rights reserved.