Geochemical characterisation and modelling of the Luchon hydrothermal system (Central Pyrenees, France) and lessons learnt for the use of geochemical modelling techniques in granite-hosted alkaline thermal waters

Abstract

Alkaline hydrothermal systems hosted in granitic rocks have been extensively investigated as natural analogues for radioactive waste disposal and for the geological storage of CO2. Thereby, their geochemical characterisation provides useful information for the long-term performance assessment predictions. The geochemical modelling of one of these alkaline hydrothermal systems has been performed in Luchon (France), together with the application of different geothermometrical techniques to determine the temperature, pH and mineral equilibrium conditions at depth. The modelling results show that the main processes controlling the hydrogeochemical evolution of this system are: (1) the mixing between deep thermal and cold surface waters, (2) the conductive cooling and (3) the external input of CO2. Other important results are that the most alkaline thermal waters are characterised by a high pH-buffering capacity during the mixing processes, and that the high pH values that characterise these thermal waters are not only inherited from the deep reservoir but strongly enhanced by conductive cooling. The reservoir temperature predicted by the geothermometrical modelling is in the range of 117 ± 8 °C, in good agreement with the temperatures ranging from 108 to 133 °C predicted by the classical geothermometers. The results of these calculations indicate as well that the thermal solutions have reached equilibrium with quartz, albite, potassium feldspar, zoisite and prehnite, and that a re-equilibirum with kaolinite and calcite is reached during their ascent to the surface. Another important outcome is that the precipitation of calcite in the deep reservoir could take place as a CO2 mineral trapping mechanism in similar systems. Further, the sharp influence of the surface waters on the deep thermal waters reveals a high susceptibility of the system to potential contamination processes.