Geochemical features and geothermometric modeling of Les Escaldes geothermal system (Andorra)

摘要

The geochemical characteristics of the thermal waters from Les Escaldes (Andorra) are similar to other geothermal systems in the Pyrenees, belonging to the group of granite-related alkaline thermal waters (high pH, low total dissolved solids, very low magnesium concentration, and sodium as the dominant cation).Because of the high pH of these waters (pH = 8.95), they have a very low CO2 partial pressure, and silica is partially ionized as H3SiO4-. We have carried out a geothermometric modeling exercise taking into account kaolinite re-equilibrium during the ascent of these thermal waters. Results show that the waters are in equilibrium with albite, K-feldspar, quartz, kaolinite and some calcium-aluminosilicate at a temperature of 90 ± 10 °C in the reservoir. The pH increases at a rate of 0.0125 pH-units/°C during ascent; the rate is not affected by the kaolinite re-equilibrium process, and it depends on the active acid-base pairs present in the thermal waters.