Abstract
Reservoir temperatures are calculated for six geothermal systems in the Central Pyrenees (Arties, Caldas de Bohi, Benasque, Panticosa, Luchon and Cauterets) using different geothermometrical methods. In general, the selected hot springs have similar geochemical features belonging to the group of alkaline thermal systems installed in granitic environments. More in detail, however, it is important to note that Na and K concentrations of Panticosa and Benasque thermal springs are the lowest found in this water group to date. Both classical geothermometers and modeling calculations agree within a few degrees for the Arties, Caldas and Luchon geothermal systems: thermal waters are in equilibrium in their reservoirs with a common mineral association (quartz, albite, K-feldspar, caolinite and some calc-aluminum silicate) at 103, 107 and 127° C, respectively. Cauterets, Panticosa and Benasque thermal waters appear to be equilibrated with the same mineral association at 98, 107 and 94° C, respectively as deduced from silica and Na-K geothermometers. Na-K-Ca and Ca-K geothermometers provide lower temperatures for these systems, probably because of some exchange of calcium between rock and water during the ascent of solutions. Nevertheless, the modeling calculations fail to represent the composition of Panticosa and Benasque thermal springs: these waters are out of equilibrium with the selected mineral association at the temperatures given by Na-K and SiO 2 -quartz geothermometers Therefore, the agreement between these geothermometers appear to be fortuitous, specially for Benasque thermal waters.
Highlights
Reservoir temperatures are calculated for six geothermal systems in the Central Pyrenees (Arties, Caldas de Bohí, Benasque, Panticosa, Luchon and Cauterets) using different geothermometrical methods
The modeling calculations fail to represent the composition of Panticosa and Benasque thermal springs: these waters are out of equilibrium with the selected mineral association at the temperatures given by Na-K and SiOTquartz geothermometerso the agreement between these geotermometers appear to be fortuitous, specially for Benasque thermal waters
Los sistemas hidrotermales instalados en los materiales graníticos del Pirineo constituyen un conjunto de manifestaciones con rasgos composicionales homogéneos, propios del grupo de aguas termales alcalinas o sulfuroso-sódicas
Summary
Los sistemas geotermales estudiados están situados en la zona axial del Pirineo Central español, en las provincias de Huesca (Panticosa y Benasque) y Lérida (Arties y Caldas de Bohí), próximos a la frontera francesa. Por tanto, de sistemas con una escasa separación en la horizontal entre sus áreas de recarga y de descarga, y con circuitos hidrotermales en forma de U, caracteres similares a los normalmente definidos para este tipo de sistemas pirenaicos (Soulé, 1990; Chevalier-Lemire el al., 1990). Los rasgos composicionales de las soluciones de estos manantiales sin trazas de mezcla son análogos, globalmente, a los establecidos para el resto de sistemas geotermales pirenaicos (Tabla 1): muy baja mineralización, valores de pH elevados, sodio como catión dominante, ausencia de un anión en concentraciones preponderantes sobre los demás, caracteres sulfurosos y muy bajas concentraciones de magnesio (Auqué el al., 1996). Y de esta forma, las soluciones de los manantiales de Panticosa y Benasque muestran las más bajas concentraciones de sodio y potasio de todas las detectadas hasta el momento en los manantiales termales alcalinos del Pirineo o en sistemas análogos de otras áreas geográficas (Auqué el al., 1996). Ello implica que la evolución de las soluciones termales durante su ascenso tiene lugar en sistema abierto, con procesos de reequilibrio continuo respecto a ese mineral (Michard y Fouillac, 1980; Michard y Roekens, 1983; Auqué, 1993; Auqué et al, 1996), circunstancia que constituye uno de los aspectos más destacados de la evolución geoquímica de este tipo de soluciones
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