Chemical and isotopic characterization of the thermomineral water of Terme Sibarite springs (Northern Calabria, Italy)

The Terme Sibarite and nearby thermomineral water discharges have Ca-SO4 to Ca-SO4-HCO3 composition, near neutral pH value of 6.9 to 7.6, outlet temperatures of 16° to 25.5°C, variable redox potentials (-0.23 to 0.2 V) and total dissolved solids from 20 to 35 meq L-1. The total flow rate of the Terme Sibarite springs is about 130 L s-1. Solubility of chalcedony and the K-Mg geothermometer show a full equilibrium reservoir temperature of 33°C. The δ34S values of dissolved sulfate are constrained by mixing of thermomineral waters with cold waters, both interacting with Upper Triassic carbonate-evaporite rocks, as well as occurrence of bacterial sulfate reduction. In the light of the geological and hydrogeological framework of the study area, this chemical and isotopic evidence suggests that the thermal circuit develops entirely into the Upper Triassic sedimentary sequence, without any interaction with the Messinian evaporites which are possibly present below the Upper Triassic sedimentary sequence. The thermomineral waters are meteoric precipitations infiltrating in the Pollino Massif, at average altitudes of 950-1090 m a.s.l, as indicated by the δ18O values of water. These waters descend to maximum depths of 600 m below the Sibari Plain, where the geothermal reservoir is situated. Circulating into it, waters extract heat from reservoir rocks, attaining thermo-chemical equilibrium at the temperature suggested by chemical geothermometers. Then, the thermomineral waters locally rise relatively quickly to the surface, along subvertical faults and fractures, preserving part of their physical and chemical characteristics. The methodological approach utilized in this research may be applied to other fault-controlled, low-temperature geothermal systems in other zones of Italy and other nations.