Geothermometrical calculations of thermal waters affected by secondary processes: The case of Sierra Elvira (Spain)

Geothermometrical calculations in thermal systems are often limited by the presence of secondary processes that modify the chemistry of the deep reservoir fluid during the ascent to the surface (e.g., mixing, degasification). The effect of secondary processes can be avoided by applying some techniques to reconstruct the chemical equilibrium at depth and the fluid original composition. However, the reconstruction of thermal waters mixed with cold surficial waters is complicated when the dilution factors are unknown. For that case, here, we propose to use an approach consisting of the simulation of a concentration process that removed different amounts of water from the thermal solutions until the equilibrium temperatures of anhydrite and quartz converge for the waters affected by mixing in unknown proportions. Using classical geothermometers and geothermometrical modeling, including the water removal process and CO₂ degasification, a temperature range of 78 ± 9 °C at depth has been established for the Sierra Elvira geothermal system whose waters are in chemical equilibrium with respect to calcite, dolomite, anhydrite, quartz, illite, pyrophyllite and beidellite-K. The good agreement in the temperatures obtained for the different thermal fluids of the system suggests a common reservoir for all of them. The methodology used in this study can be applied to other geothermal systems in carbonate rocks affected by mixing.