Epidote-bearing hydrothermal ductile shear zones: evidence of tectonically driven fluid flow in the Larderello geothermal field (Northern Apennines, Italy)

The Larderello geothermal system (Northern Apennines) represents the result of a large-scale thermal anomaly due to emplacement of shallow-level intrusive rocks of Pliocene age. Diffuse networks of cross-cutting veins filled by hydrothermal mineral assemblages have been regarded as evidence of fluid circulation through interconnecting fracture systems related to cyclic events of brittle deformation. We report the occurrence of epidote-bearing shear zones associated with metasomatised skarn levels. The partitioning of ductile deformation and mineral reactions within a small volume of rocks (some metres) suggest the occurrence of channelised circulation of hot fluids within a ductile shear zone. The occurrence of such structures in the subsurface of the geothermal field represents an important feature which might explain the development of thick thermal aureoles and the diffusion of metasomatic reactions up to shallow levels.