The Larderello and Mt Amiata geothermal fields in Tuscany are large active 
geothermal systems which overlie young plutonic rocks.  The features of the 
two geothermal systems are similar. The geothermal fields of Larderello and Mt. 
Amiata are located in the inner part of the Northern Apennines, characterized 
asthenosphere uplift and delamination of the crustal lithosphere or 
underplating. The heat source both at Larderello and Mt. Amiata can be 
ascribed to the presence of shallow igneous intrusions, and there is evidence 
of an early contact metamorphism related with the intrusion of the granitesThe 
heat flow data for the area surrounding both the Larderello and Mt. Amiata 
geothermal fields show a comparable areal extension and similar values (up to 
200-300 mW/m2  ). Cap rocks and reservoirs are also similar: both fields have 
shallow vapor-dominated sedimentary, and deep metamorphic reservoirs. At 
Larderello super-heated steam is present in both reservoirs, to depth of more 
than 3.5 km, whereas the deep reservoir of the Mt. Amiata geothermal fields is 
likely water-dominated. In both fields the upper reservoir is present below the 
flysch units forming the cap rocks. Permeability is due to rock fracturing, even at 
depths of about 4 km and temperatures as high as 350°C. Pressure greater 
than hydrostatic and a supercritical fluid can occur in the deepest part of the 
geothermal fields.  The water stable isotope values of the steam discharged by 
the geothermal wells at Larderello indicate a meteoric origin of the recharge. A 
geochemical regional study on the thermal waters and gases of the Mt. Amiata 
area indicates that the geothermal reservoirs originated from a meteoric fluid, 
mainly stored in a regional Mesozoic dolomite-anhydrite unit, and evolved in a 
Na-Cl, CO2 gas-reach reservoir by interaction with calcite-bearing metamorphic 
rocks. The high temperatures existing in correspondence of a deep seismic 
reflector suggest the occurrence of a deep-seated unconventional geothermal 
resource (UGR), which can be possibly exploited. The heat could be mined from 
silica-rich rocks close to a plastic state, but where fracturing can be induced by 
fluid overpressure and abrupt high strain rates. This geothermal resource is 
very important,  requires a re-assessment of the geothermal resources in Italy, 
considering the possibility of the exploitation of the new reservoirs.

Campus SIAF, SP del Monte Volterrano
Localita' Il Cipresso
Volterra, Italy