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.

Automated summary

High-temperature geothermal areas are influenced by regional faults, and the geochemical and isotopic signature of discharged fluids provides important information about the pathways and fluid-rock interaction in geothermal reservoirs. The study of the Larderello geothermal area and surroundings reveals the control of transfer and normal faults on geothermal circulation, including the influence of a cooling magmatic intrusion. The results suggest the presence of deep regional pathways and interaction with shallow aquifers in the peripheral zone emissions.