The geochemical features of fluids accompanying the first stages of geothermal activity linked to magmatic intrusions have been documented for the Larderello geothermal system (Italy). Deep drilling has provided samples which preserve evidence of this early geothermal activity. Four wells (San Pompeo 2, Monteverdi 7, Sasso 22, and Serrazzano, VC 11) penetrated the deeper parts of the Larderello system, located in a metamorphic basement underlying the Tertiary nappe complex which constitutes the main aquifer at Larderello. The drill holes terminated close to the inferred roof of a granitic complex thought to be responsible for geothermal activity. Fluid inclusion data were obtained from recrystallized quartz lenses and quartz veins in samples displaying high temperature assemblages (plagioclase-actinolite-biotite-tourmaline; clinopyroxene +/- andradite-wollastonite) and also from magmatic quartz in a leucogranite dike. The inclusions are mainly secondary in origin, oriented in fluid inclusion planes (FIP) related to hydrothermal circulation in the Larderello system. Several generations of high temperature fluids were trapped and include: (1) H2O-CO2 dominated vapors displaying variable but significant contents of CH4 and N2; (2) aqueous vapors containing LiCl, with variable salinity; (3) aqueous LiCl brine, often oversaturated with respect to halite at room temperature; (4) complex brine, always oversaturated at room temperature with respect to two (halite and sylvite) or more (n less-than-or-equal-to 4) salts. The presence of LiCl was confirmed by identification of the salt hydrate (LiCl-5H2O) at very low temperature using Raman spectroscopy. Bulk salinities could be roughly estimated at around 30 wt% eq. LiCl for the LiCl brine. Geometric and chronologic relationships between FIP reveal close relationships and mutual contamination between the H2O-CO2 vapors and LiCl brine, indicating synchronism in their trapping. These fluids were generated and trapped at pressures of 100-130 MPa, nearly 23 MPa above the estimated present-day lithostatic pressure. This implies a denudation rate between 0.2 and 0.5 mm . a-1 since the onset of hydrothermal activity, compatible with the setting of Larderello in a young (Tortonian) collision belt. Fluid inclusion trapping temperatures (425-650-degrees-C) show a monotonous increase towards the inferred granite, and are around 100-200-degrees-C higher than the highest present-day temperatures. The results are interpreted as recording the interaction between magmatic and contact metamorphic fluids in the early Larderello system. The H2O-CO2 vapors resulted from the reheating of the basement metamorphic series (often C-rich) under relatively high temperatures during contact metamorphism. Li-rich fluids expelled from an underlying Li-rich leucogranite migrated through the metamorphic series and the already cooled granite dikes and experienced local boiling. The fluid inclusion data demonstrate the involvement of magmatic fluids during the initial development of this high energy geothermal field.
