THE ISOTOPIC AND CHEMICAL-COMPOSITION OF CO2-RICH THERMAL WATERS IN THE MONT-DORE REGION (MASSIF-CENTRAL, FRANCE)

Chemical and isotope compositions of fluid samples, collected between 1974 and 1986 from 52 springs or shallow boreholes located in the Mont -Dore region (Massif Central, France), were examined. Some springs and wells were sampled several times during this period. The fluids emerg e from Quaternary volcanic rocks or Paleozoic granite at temperatures between 4 and 62 degrees C, and the origin of the H2O is meteoric. The waters can be classified into three groups: bicarbonate fluids, mixed bicarbonate-chloride fluids (with a mineralization up to 8 g/l), and acid-sulfate fluids. Only two fluids contain sufficient Cl- to be cons idered as 'mature' waters. Previous work has demonstrated that they al l contain partly mantle-derived CO2 gas, and that the CO2-rich gas pha se and bicarbonate-chloride waters are separated at substantial depth. Mineralized fluids circulate at depth and undergo several processes, such as cooling or dilution with recent freshwater, during their ascen t to the surface. Therefore, the CO2-rich gas phase can be partly diss olved in the freshwater, or in deep fluids after their dilution. This process leads to the dissolution of surrounding rocks; such dissolutio n is discussed on the basis of major-element concentrations (Na, K, Ca , Mg), as well as the Sr 87/86 isotope ratio. Dissolution of S-bearing minerals has also been demonstrated. The presence of the CO2-rich gas phase also leads to isotope exchange between CO2 and H2O. Some minera lized fluids are less affected by these processes than others, in whic h case they display the chemical and isotopic characteristics of the o riginal deep fluids. It was shown that the applicability of geothermom eter calculations for these waters is hampered by several processes th at modify the chemical composition. However, some geothermometers can be used for estimating the temperature of the deep fluids using the ch emical composition of the less modified fluids. They indicate that flu ids emerging from volcanic rocks in the Dordogne valley reach temperat ures of around 100- 130 degrees C at depth, while the temperature of t he fluid that issues from the granite at Saint-Nectaire is 160-175 deg rees C at depth. (C) 1997 Elsevier Science Ltd.