The Bagni di Lucca thermal waters (Lucca province, Tuscany, Italy) have been well known since the Middle Age for their alleged curative properties. In the present work, the waters have been analysed for major and trace components and for O, H, S and Sr isotopes. The δ2H and δ18O values indicate a meteoric origin of the waters and the tritium content suggests long-term (more than 40 years) circulation at depth. The δ34S values in dissolved sulphate of the warmest waters (Doccione, Paolina, Bernabò, Demidoff, Cova) indicate interaction with Triassic evaporites; however, the δ34S values as well as the 87Sr/86Sr ratios in the waters appear to be a little higher (on average 17.6±0.4‰ and 0.70834±0.00006, respectively) than in Triassic sulphate minerals from the Northern Apennines (16.0±0.5‰ and 0.70778±0.00011). The waters are Ca-sulphate and characterised by high Na/Cl (about 2) and low Ca/SO4 (about 0.7) mole ratios, high Sr (≤ 12 mg/L), F (≤ 4.2 mg/L), As (≤ 80 mg/L), Cs (≤ 70 mg/L), Rb (≤ 107 mg/L) and salinity (up to 3087 mg/L of total dissolved solids), low alkalinity, and pH in the range 6.3–7.5. Assuming equilibrium of the waters with anhydrite and chalcedony, the evaluated temperature and pressure are about 70–75 °C and 200–300 bar; considering the pressure as hydrostatic, the evaluated depth of equilibration approaches that of the metamorphic basement (about 3.0–3.8 km) underlying the carbonate-evaporite formations occurring in the area. Thermodynamic computations indicate that the high Na/Cl ratio and the low Ca/SO4 ratio in the waters may be due to progressive dissolution of albite, anhydrite, dolomite and carbon dioxide accompanied by calcite precipitation. The linear correlations between the major and trace components, including 2H and 18O, demonstrate that thermal springs undergo dilution by low-salinity shallow waters which are recharged at low elevation.
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