Insights into geochemical characteristics of the geothermal fluids of northwestern algeria: An updated conceptual model

The vigorous tectonic activity that affected the Northwestern part of Algeria has generated a Mio-Plio-Quaternary volcanic activity and led to the emergence of several thermal resurgences. These latter consist of thermal springs, travertine deposits and some hydrothermal alteration zones. The chemistry of twenty-one hot spring water samples highlight the heterogeneity of the water mineralization process in NW Algeria. Four major types of water are described. Na⁺-Cl⁻ and Na⁺-SO₄²⁻ with a high salinity up to 3794 mg/L accompanied by higher concentrations of F⁻, B, Li⁺, Br⁻ and Cs. These are probably derived from halite and gypsum rich evaporites from the surrounding environment. The other two types are Ca²⁺-HCO₃⁻ and Na⁺-HCO₃⁻. Chloride rich waters are associated with local carbonate formations. The stable isotopes (δ¹⁸O and δ² H) reveal a fossil system since a large depletion of both isotopes from present-time meteoric waters cannot only be plausibly explained in terms of "inland" and "altitude" effects, we may suppose their recharge under climatic conditions much colder than the present ones taking place probably during the glacial period. The gas mixing models of N_2[sbnd]He*1000-Ar*100 with O_2[sbnd]CO_2[sbnd]He*1000 demonstrate an unarguable meteoric origin for the thermal waters, displaying air-saturated water (ASW) signatures. The collected thermal waters are ‘immature’ while the deep geothermal reservoir temperatures were estimated to be about at 133 °C. The updated geochemical conceptual model indicates two main processes in the flow path of the fossil meteoric water. 1) In the Bouhdjar geothermal zone, the infiltrated meteoric waters are conductively heated at depth because of the high heat flow (∼120 mW/m²), during their ascent, along hydrothermal conduits and towards the ground surface, they have possibly been influenced by heat advection from the underlying magma chambers and/or a percolation with intrusive bodies beneath spring vents. 2) The meteoric waters are recharged from high altitude areas of the Atlasic lands and are deeply circulated and heated at greater depths, hot waters probably mix with cooler Mg-rich waters and/or leach Mg²⁺ from the surrounding rocks when rising to the surface. Since the locations of the hot springs are close to populated areas, a direct use of this resource is feasible to supply some energy demands that include electricity generation using binary cycles or direct uses of heat.