Hot and cold degassing areas unravel deep regional fluids pathways in an extensional tectonic setting: insights from the Larderello geothermal system (Northern Apennines, Italy)

High-temperature geothermal areas are often characterized by widespread surficial manifestations, which location is strictly controlled by sets of faults of regional relevance. In this context, the geochemical and isotopic signature of the discharged fluids can reveal key information on the geothermal fluid pathways, shedding light on the sources and fluid-rock interaction within the geothermal reservoirs. In this work, geochemical and structural data collected at the Larderello geothermal area (Tuscany, Italy) and surroundings are presented and discussed. A total amount of 648 CO2 flux measurements was carried out in nine different areas of the Larderello geothermal field and neighboring zones, following the accumulation chamber method. In specific degassing areas, interstitial gas samples (n.77) at 20 cm depth, for the determination of the δ13C-CO2 (expressed as ‰ vs. V-PDB) values, were also collected and coupled with previously published data from the Monterotondo Marittimo area. The role of transfer and normal faults in controlling the geothermal circulation driven by a cooling magmatic intrusion underneath the Lago area (SW of Larderello) was constrained. The structural control on the fluids circulation is highlighted by both the location of the CO2 emissions along the fault segments – where permeability is enhanced –and their degassing rates, which increase moving away from the core of the Larderello geothermal system. The main results unravel the presence of deep regional pathways along which endogenous fluids circulate before being discharged in the investigated areas. The peripheral zone emissions are affected by interaction with shallow aquifers and condensation processes whereas the CO2 emitted from the central areas, located near the core of the geothermal system, are accompanied by high amounts of steam, and suffers intense shallow fractionation processes. The latter areas emit normalized CO2-degassing rates lower than 270 t d-1 km-2, which can be considered medium-to-low values when compared to the extremely high ones computed for the peripheral sectors (up to 1,300 t d-1 km-2) of the Larderello geothermal systems. The high degassing rates, the geochemical characteristics of the fluids discharged at the periphery of the geothermal area, and the structural setting possibly suggest an incipient propagation of such a system, which might be wider than previously thought.