Deep hydrothermal circulation in a granite intrusion beneath Larderello geothermal area (Italy): constraints from mineralogy, fluid inclusions and stable isotopes

A shallow (∼2 km) granite body has been intersected by the MONT-4 well in new producing areas of the Larderello geothermal field. The granite at 2.2–2.5 km and below 3 km underwent pervasive propylitic alteration that is dominated by the assemblage chlorite+adularia+quartz+illite+calcite+albite±epidote±sphene. Fluid inclusion studies indicate two stages of alteration. The first one is characterised by high-temperature fluids that, at 2218 m, were trapped under boiling conditions at about 340°C and 15 MPa, whereas at 3520 m evidence of boiling is not revealed and the temperature of the fluid was calculated to be about 350°C. The second alteration stage shows a cooling of the system, particularly evident in the shallower portion of the intrusion (∼235°C at 2218 m and ∼315°C at 3520 m). The temperature of homogenisation for subsequent cooler inclusions approximates present-day down-hole temperatures, and may be considered as representative of the fluid actually circulating in the granite in recent times. Calculated δ18O values of 0±1‰, for fluids in equilibrium at 340°C (2218 m) and 350°C (3520 m) with feldspar, epidote and chlorite, and δD values of −51 to −61‰, for fluids in equilibrium with chlorite, together with directly measured δD values of −33 to −61‰ for fluid inclusions in quartz, indicate derivation from meteoric waters. The original meteoric signature of the fluids has been altered through water–rock interaction, but remains clearly distinguished from a primary magmatic signature, for which no isotopic evidence was found. Active fracturing and the high structural relief of granite emplacement promoted hydrothermal circulation in the granitic body drilled by MONT-4 well, suggesting that such characteristics may be a pre-requisite for such circulation at Larderello.