Structurally-controlled permeability evolution and mineralization in geothermal caprocks: the Fenice Capanne mining district (Southern Tuscany, Italy)

The Neogene mineralized region of the Colline Metallifere in southern Tuscany (Italy) provides a natural laboratory to investigate feedback between fluid–rock interaction, structurally controlled fluid flow, and hydrothermal ore mineralization. This study focuses on the Fe–Cu–Pb–Zn deposit of Fenice Capanne, located south of the active Larderello–Travale geothermal system, where marly–limestone caprocks of the Liguride Complex preserve evidence of permeability creation and destruction above a regional geothermal reservoir. A multidisciplinary approach was applied to reconstruct the evolution of the hydrothermal system. Two main alteration stages were identified. An early prograde, high-temperature skarn metasomatism was associated with the growth of clinopyroxene–garnet assemblages and characterized by substantial gains in Si, Fe, Mn, and Ca, resulting in a large volume increase (up to ~400%). Reaction-induced fracturing generated secondary permeability, which superimposed on primary permeability related to bedding and lithological anisotropies, enhancing hydraulic connectivity. A subsequent retrograde stage, below ~300°C, was driven by mixing between magmatic and meteoric fluids and resulted in renewed fracturing, brecciation, and extensive quartz–sulphide veining. These processes recorded cyclic variations in fluid composition, redox conditions, salinity, and boiling associated with transient pressure drops during hydraulic fracturing. Overall, the Ligurian marly–limestone sequence evolved from a low-permeability sedimentary seal into a reactive, mineralized system capable of sustaining transient hydrothermal circulation. Structural connectivity controlled by faulting and reaction-induced fracturing governed both permeability evolution and mineralization. These results demonstrate that sedimentary caprocks can behave as dynamic components of the hydrothermal systems, with important implications for geothermal fluid flow, metal transport, and reservoir evolution.