Deep Regional Fluid Pathways in an Extensional Setting: The Role of Transfer Zones in the Hot and Cold Degassing Areas of the Larderello Geothermal System (Northern Apennines, Italy).

High‐temperature geothermal areas are often characterized by widespread surficial manifestations, whose location is strictly controlled by sets of faults of regional relevance. The geochemical and isotopic signature of the discharged fluids can reveal key information on the geothermal fluids pathway, shedding light on the sources and fluid‐rock interaction within the geothermal reservoirs. In this paper, a geochemical and structural data set from the Larderello geothermal area and surroundings is presented and discussed. We constrain the role of transfer and normal faults in controlling the geothermal circulation enhanced by a cooling magmatic intrusion underneath the Lago area (SW of Larderello). 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, was accompanied by high amounts of steam, and suffers intense shallow fractionation processes. The latter areas emit medium‐to‐low normalized‐CO2‐degassing rates (<270 t d−1 km−2) when compared to the extremely high values occurring in the peripheral sectors (up to 1,300 t d−1 km−2) of the Larderello geothermal systems, possibly suggesting an incipient propagation of such a system, likely wider than previously thought. Plain Language Summary: High‐temperature geothermal areas are specific zones on Earth where the geothermal gradient is greater than the average global value (∼30°C km−1). This is due to the existence of cooling magma source(s) at depth, providing heat that is transmitted to fluid reservoir(s) located at intermediate levels continuously and naturally fed by recharging fluids, and sealing rock(s) at shallower levels that maintain reservoir temperature and pressure. These geothermal areas commonly show steam‐dominated manifestations at the surface, accompanied by relevant degassing of carbon dioxide likely originated from different feeding systems (biogenic, thermometamorphic, and mantellic). Defining sources, processes, and transport mechanisms governing the CO2 emissions from soils is challenging but pivotal to understand the geothermal fluid origin, dynamics, and relation with the geological structures. To unravel these processes, we combined geochemical and structural measurements performed at the Larderello‐Travale‐Radicondoli (LTR) geothermal field (Italy). Distinct geothermal sectors were investigated according to their geochemical characteristics, CO2 degassing rates, and geological‐structural features, to understand how CO2 is transported and modified during its journey from the deep geothermal reservoir(s) to the surface, exploiting the permeability of fault zones and/or fractured rocks. Results suggest that the LTR geothermal system might be wider than previously thought, indicating a higher geothermal potential. Key Points: Extensive CO2 soil degassing measurements and carbon isotopic composition in the Larderello geothermal system were carried outHydrothermalized hot‐ and cold‐degassing areas are controlled by structural settingRegional transfer zones enhance the circulation of deep‐seated geothermal fluids [ABSTRACT FROM AUTHOR]