Mean recharge times and chemical modelling transfers from shallow groundwater to mineralized thermal waters at Montrond-les-Bains, Eastern Massif Central, France

International audienceA re-appraisal of interactions between basinal waters and hydrothermal fluids within the silici-clastic series of the intracontinental Cenozoic Forez Basin is proposed using major element and isotope geochemistry together with thermodynamic models. Recharge processes and shallow sedimentary units in the Forez Basin were compared using major elements and isotopic data (87Sr/86Sr, δ18O, δ2H, δ13C, Tritium and 14C) for waters, sedimentary carbonates and Hercynian basement rocks. Waters derived from Late Pleistocene to Early Holocene surface recharge followed by mixing and chemical evolution through percolation into deep groundwater horizons are investigated. Isotopic ratios and chemical speciation computed for deeper waters from 0 to 200 m depth indicate that mineralised waters were produced by initial surficial dissolution of calcite in calcareous-mudstone providing its Sr isotopic signature. At about 200 m depth, Na–H3O+ cation-exchange with clayey-sand rich levels provides additional Ca, Na, and OH contents, for carbonate-saturated water. Na–H3O+ cation exchange can not contribute large Na–HCO3 concentrations. Geochemical changes such as increasing δ13CDIC values observed from 200 to 500 m depth were modelled by addition of thermal water and geogenic CO2 in increasing proportions with depth (500 mbs). Thermodynamic calculations of mineral proportions indicate water–rock interactions with primary minerals in the metamorphic basement and secondary minerals observed in the basin. Water temperature and its meteoric origin suggest a basin-wide recharge system with water–rock interaction driven by heat flow rather than through deep crustal fractures