Hydrogeochemical and isotope geochemical study of northwestern Algerian thermal waters

Northwestern Algeria is characterized by a large number of thermal waters and volcanic eruptions and belongs to the Alpine-Magrebide belt. The geothermal reservoirs that feed these reservoirs are mainly hosted by a fractured Jurassic limestone and dolomite sequence. Seven samples were collected from thermal springs of near-neutral pH (6.2 to 7.56) with discharge temperatures between 42.9 and 66.1 °C. Hydrogeochemical analyses of the thermal waters reveal four types (Na⁺-Ca²⁺-Cl⁻, Na⁺-Ca²⁺-Cl⁻-HCO₃ ⁻, Na⁺-Ca²⁺-Cl⁻-SO₄ ²⁻, and Na⁺-HCO₃ ⁻-Cl⁻) and show high total dissolved solids up to 4002 mg/L. Stable isotopic results (δ¹⁸ O and δ D) indicate that the thermal waters are of meteoric origin deeply infiltrated and heated by advective heat anomalies and raised up to the surface through deep-seated faults acting as hydrothermal conduits. The estimated reservoir temperatures using silica geothermometers and fluid-mineral equilibria overlap between 66 and 125 °C, while Na/K and K/Mg geothermometers give much higher and lower results, respectively, and are mainly influenced by mixing with cooler Mg groundwaters as indicated by the Na-K-Mg plot in the immature water field and in silica and chloride mixing models. Thermal waters deeply circulated and heated at a depth of 2 km were supplied by the higher geothermal gradients, which can reach 42.8 °C km⁻¹ due to the complex geological setting.