Theoretical geothermometers andPCO2 indicators for aqueous solutions coming from hydrothermal systems of medium-low temperature hosted in carbonate-evaporite rocks. Application to the thermal springs of the Etruscan Swell, Italy

In order to deriveT-PCO2 geoindicators suitable for thermal waters coming from the hydrothermal systems of medium-low temperature which are hosted in carbonate-evaporite rocks, the theoretical concentration of Ca, Mg, HCO3, SO4, F and their complexes in aqueous solutions in equilibrium with a mineral assemblage made up of calcite, dolomite, anhydrite and fluorite has been calculated at temperatures between 0 and 150°C, atPco2; in the 0.1–100 bar interval and for total molality of mobile species Na and CI ranging between 0.0001 and 0.3. The results of such calculations indicated that: (1) the Ca, Mg, S04 and F contents and theS04(F)2 andCaMg ratios are potential geothermometers; (2) the(HC03)2SO4 ratio is a Pco, indicator; (3) theHC03F ratio and the HC03 content arc indicators of bothPco2 andT. A first geothermometric and geobarometric evaluation of the thermal springs of the Etruscan Swell generally shows equilibrium temperatures in the interval 50–100°C and Pco, from about 1 bar to bars. These estimations are consistent with the general knowledge of the hydrothermal systems of medium-low temperature present in the region.