Geochemical investigations of the geothermal systems from the Island of Sicily (southern Italy)

Sicily hosts many natural manifestations that include thermal waters, gas discharges and mud volcanoes. Due to the significant geodynamic and geological differences, the fluid discharges along a NE-WS-oriented transect that run from the Peloritani Mts. to the Sciacca Plain shows a large variability in water and gas chemical and isotopic compositions. The studied waters are characterized by Ca-HCO3, Ca(Mg)-SO4, Ca-Cl and Na-Cl compositions produced by distinct geochemical processes such as water-rock-gas interactions, mixing between deep and shallow aquifers and seawater and direct and reverse ion exchanges. The gas chemistry is dominated by CO2 to the east and CO2-N2 to the west of the study area, whereas the central part shows mud volcanoes discharging CH4-rich gases. Water isotopes suggest that the thermal waters are fed by a meteoric recharge, although isotopic exchange processes between thermal fluids and host rocks at temperature >150oC are recognized. Accordingly, liquid geothermometry suggests equilibrium temperatures up to 220oC. The carbon in CO2 and helium isotopes of the emissions from the westernmost sector of Sicily indicate that these two gases consists of up to 40 % of a mantle component, the latter decreasing to the east down to 10% where CO2 of thermometamorphic origin dominates. Accordingly, conceptual models of the fluid circulation for the western, central and eastern sectors are proposed. The regional geothermal reservoir, hosted in carbonates in the western sector and locally outcropping, is of low to medium temperature. Higher temperature conditions (up to 200-220oC) are suggested by geothermometry and probably related to deeper levels of the system. Sicily can be regarded as a potentially suitable area for future investigations to evaluate specific activities aimed at exploiting the geothermal resource.