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• Chemical and isotopic data reveal distinct groups among the studied springs. • Chaudes-Aigues reservoir reaches 190 °C, confirming and refining previous study. • Springs originate from meteoric recharge on the southern edge of the granite. • Different geothermal reservoirs have been identity within the Margeride Granite. Granitic-hosted hydrothermal systems offer valuable insight into deep fluid circulation mechanisms in amagmatic contexts. This study focuses on the Chaudes-Aigues hydrothermal system in the French Massif Central, which hosts one of the hottest natural springs in Europe (Par Spring, 80–82 °C). Our work aims to better constrain the functioning of the system, including reservoir temperature, recharge zone, and fluid pathways. A combined approach of water geochemistry (major elements, isotopes) and thermo-hydraulic numerical modelling was applied. Using 22 thermal springs distributed across the Margeride Granitic Complex (MGC) and the Cantal Volcanic Complex (CVC) the temperature of Chaudes-Aigues reservoir has been estimated and is thought to reach 190 ± 10 °C, validating and refining previous estimates. Isotopic and geochemical tracers point to a meteoric origin, with the recharge area most likely located along the southern edge of the MGC, connected to the emergence zone via a thick N150E fault zone. This study redefines the Chaudes-Aigues system as fully amagmatic and structurally controlled. Other geothermal reservoirs have also been identified within the Margeride Granitic Complex, with temperatures exceeding 100 °C, indicating a broader geothermal potential of the area. Furthermore, this study highlights the potential for identifying similar hidden geothermal systems in other fractured granites with limited surface manifestations, offering new perspectives for high-temperature geothermal exploration in Europe.
Penhoët et al. (Sat,) studied this question.