We evaluated the hydrogeochemical characteristics of water from the old flooded Sierra Almagrera mines to determine the possible origin of its geothermal fluids, to establish a geological-geochemical model of the geothermal system, and evaluate the site’s geothermal potential. The mine water contained high concentrations of chloride (59.6 g/L), Na (28 g/L), K (1.75 g/L), Ca (7.2 g/L), Mg (0.63 g/L), and Li (66 mg/L), especially in water from the old dewatering system. Metal concentrations were especially elevated in the old mine shafts, with high amounts of Fe (1990 mg/L), Mn (600 mg/L), Zn (460 mg/L), Pb (4 mg/L), and Ni (11.4 mg/L). The Cl/Br molar ratios of the water was high, which may indicate the possible leaching of natural halite from the evaporite deposits in the aquifer recharge area. The mine water had the most elevated temperatures and are, possibly, representative of the extent of equilibration in most of the reservoir. The estimated mean temperatures in the geothermal reservoir, based on the triangular (Giggenbach) Na-K-Mg diagram, was 190ºC for equilibrated waters, which may justify the development of this geothermal resource. The geothermal characteristics imply convection of groundwater to 2500-3000 m below sea level, in agreement with the hydrodynamic model proposed.

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