Résumé
Geochemical reactions in hydrothermal aquifers are interdependent; all elements likely to have a quantitative influence on dissolution or precipitation reactions have to be considered. Geochemical constraints, the number of which equals the number of elements, were determined. In the study of this aquifer 12 elements are considered (Al, C, Cl, Ca, F, H, K, Mg, Na, O, S, Si). Three constraints apply to water activity, neutrality and conservation of chloride in aqueous solutions; determination of the remainder was based on saturation indices, stability diagrams of minerals and PCO (sub 2) . The present-day solutions are at equilibrium with respect to nine minerals (albite, anhydrite, chalcedony, calcite, dolomite, fluorite, kaolinite, K-feldspar and illite or chlorite) so it was shown that the composition of these solutions (including computed PCO (sub 2) ) is only a function of T and chloride content. It was also possible to test the validity of the geochemical system by computing its speciation and comparing the theoretical results with actual chemical analyses (pH and concentrations). Geochemical simulations were used to predict the quantities dissolved or precipitated, as T or chlorinity varied. The influence of aluminosilicates could not be ignored. This approach is useful for the study of processes related to fluid flow.