Origin and development of the geothermal fluids of the Baden-Baden area (SW-Germany): implications for geothermal systems of granitic reservoirs

Abstract The Baden-Baden thermal springs, among the hottest in Germany, emerge on the south-eastern slope of Florentinerberg, c. 25 m above the Oos valley floor. These Na-Cl-rich waters, with temperatures up to 69 °C, originate from a deep reservoir within the crystalline basement, likely the Nordschwarzwald batholith. New investigations, including hydrochemistry, isotopic data, geothermometry, and structural geological data reveal a deep origin for these waters with reservoir temperatures of c. 176 °C as constrained from various methods. Although the thermal waters (springs, boreholes) emerge from different lithologies (granites, metamorphic schists, Upper Carboniferous rocks), major and trace element concentrations are very similar implying no or only little impact of the different lithologies. The thermal waters are of Holocene age, recharged by meteoric water in c. 900 m asl. Holocene age is backed up by recalculated 14 C data yielding a mean residence time of 10,800 years. Hydrochemical and isotopic data indicate interaction with granitic rocks in the subsurface. NE-trending structures in Baden-Baden likely act as hydraulic barriers forcing the thermal waters to the surface. While none of the faults in the thermal spring area is favorably oriented in the stress field it is suggested that the steep SE-plunging intersection of their damage zones facilitate their ascent.