Los sistemas geotermales del Pirineo Central. III. Evaluación de las condiciones en profundidad y evolución de las soluciones hidrotermales durante su ascenso

Abstract

Reservoir temperature and physicochemical conditions are determined by modelling the heating of alkaline waters in Caldas de Bohi, Arties, Luchon, Cauterets, Panticosa and Benasque geothermal systems (Central Pyrenees). Kaolinite reequilibrium occurs during the ascent of these thermal waters and therefore simulations are performed in open system conditions with kaolinite.
Calculations indicate that thermal waters are in equilibrium with a common mineral assemblage constituted by albite, microcline, quartz, kaolinite, some calcium aluminosilicate and calcite, at temperatures between 90 and 125° C and pH values lower than at spring conditions in the reservoirs of Caldas, Arties, Luchon and Cauterets systems. These results are in agreement with those previously obtained by classical geothermometry (SiO₂-quartz and Na-K geothermometers). Similar mineral assemblage is predicted for the Panticosa system but with chalcedony instead of quartz, at temperatures around 80° C. And, finally, a disequilibrium situation is obtained in the reservoir of Benasque system for any of the mineral assemblages considered (with quartz or chalcedony) at temperatures lower than 85° C.
Geochemical modelling improves the results of classical geothermometry, mainly for the Panticosa and Benasque systems. In spite of their lower Na and K concentrations, Na-K geothermometer provides similar temperatures and equilibrium conditions to those of the other systems, given wrong results.
Kaolinite precipitation (between 1,5 . 10^-5 y 5 . 10^-5 mol/l, depending on the system) during the ascent of thermal solutions has important effects on the evolution of aluminosilicate saturation indexes. But the related silica mass transfer does not affect the path of quartz (or chalcedony) saturation indexes. The pH increase of thermal waters (at rates of 0.009 to 0.015 pH units per degree centigrade) during cooling are mainly controlled by reequilibrium of the active acid-base pairs (H⁺/OH^-, HC0₃-/C0₃⁼, H₄Si0₄°/H₃Si0₄^-, etc.) in the solution.