Understanding the migration of hydrothermal fluids represents a continuous task for<br>successful exploration of geothermal resources. Contributions to better constrain the<br>hydrogeological models in geothermal areas can derive from field and laboratory studies on<br>fossil geothermal systems, evidenced by the concentration of ore deposits in wide areas.<br>This work presents an integrated study based on fluid inclusion and structural analyses on a<br>Pliocene-Pleistocene fossil hydrothermal system, located to the south of the present active<br>Larderello geothermal field. Mineralization, mainly made up of quartz and pyrite, is<br>widespread distributed in the damage zone of the Pliocene Boccheggiano normal fault and,<br>far from it, in the older cataclastic levels, deriving from previous deformational events.