The geochemical properties of the geothermal fluids of Northwestern Algeria: A revised conceptual framework

Abstract

The intense tectonic activity in northwestern Algeria has given rise to Mio-Plio-Quaternary volcanism and triggered the development of numerous thermal manifestations. Two of these— Geochemical analyses of 21 thermal spring water samples reveal considerable variability in water mineralization across the region, identifying four major water types. Na?–Cl? and Na?–SO?²?—are characterized by high salinity levels (up to 3794 mg/L) and elevated concentrations of F?, B, Li?, Br?, and Cs, likely sourced from halite- and gypsum-rich evaporites in the surrounding lithology. The other two types—Ca²?–HCO?? and Na?–HCO??—are linked to interactions with local carbonate formations.
Stable isotope analyses (?¹?O and ?²H) suggest the waters are of fossil origin, as their significant isotopic depletion relative to modern meteoric waters cannot be fully explained by "inland" or "altitude" effects alone. These findings imply recharge under colder climatic conditions. Gas composition modeling (N?–He1000–Ar100 vs. O?–CO?–He*1000) supports a meteoric origin, showing air-saturated water (ASW) signatures.
The samples are classified as ‘immature’, with estimated deep geothermal reservoir temperatures reaching approximately 133 °C. The updated geochemical conceptual model proposes two main flow paths for the fossil meteoric waters. (1) In the Bouhdjar geothermal zone, meteoric waters are conductively heated at depth due to high heat flow (~120 mW/m²). During their ascent through hydrothermal conduits, the fluids may be further influenced by advective heat transfer from underlying magmatic sources or by interactions with intrusive bodies. (2) In other zones, meteoric waters originate from high-altitude recharge areas of the Atlas Mountains, undergo deep circulation and heating, and may mix with cooler, Mg-rich waters or mobilize Mg²? through water-rock interactions during their ascent.