Chemical relationship between discharging fluids in the Siena-Radicofani graben and the deep fluids produced by the geothermal fields of Mt Amiata, Torre Alfina and Latera (Central Italy)

Abstract

The thermal springs discharging in the Siena-Radicofani basin and the deep fluids within the geothermal systems of Piancastagnaio (Mt Amiata), Torre Alfina and Latera (Vulsini Mts) have a common origin. The chemical composition and evolution towards the low enthalpy of the springs as compared to the high enthalpy of the geothermal fluids are affected by both the structural setting of the region and the deep hydraulic conditions. Recharge of both the shallow thermal aquifer and the deep geothermal systems takes place in the outcrop areas of Mesozoic carbonate rocks, which constitute the main potential geothermal reservoir in central Italy. The waters of meteoric origin are heated at depth, as a consequence of anomalous heat flow in the region; these waters acquire a CO 2-rich rising gas phase, equilibrate equilibrate with the reservoir rocks and, finally, assume their CaHCO 3SO 4 composition. If these waters discharge rapidly from the border fault systems of the Siena-Radicofani basin they maintain their original composition. If, instead, they emerge from the inner faults of the graben, their temperature and dissolved solids increase so that they become NaCl with a high content of NH 4, H 3BO 3 and Li +. The chemical evolve towards high enthalpy. In this case the deep fluids again acquire an NaCl composition similar to carbonate reservoir is at a structural high, buried by both allochthonous flyschoid series (“Ligurids”) and by Neogenic pelitic formations, a slower recharge occurs in the reservoir, and consequently the fluids evolve towards high enthalpy. In this case the deep fluids again acquire an NaCl composition similar to that of the springs emerging inside the graben, since part of the reservoir recharge also occurs through the leakage of saline fossil waters stored within the Neogenic sedimentary formations at the border of the geothermal systems.