Faults controlling geothermal fluid flow in low permeability rock volumes: An example from the exhumed geothermal system of eastern Elba Island (northern Tyrrhenian Sea, Italy)

Geothermal fluid circulation and storage within rock volumes are essential conditions for an effective geothermal system. Although several studies examine fluid migration through permeable rock volumes, few papers deal with fluid flow and fluid-rock interaction within the cap rocks of magma-driven geothermal systems. In this paper, I present the results of an integrated study focused on the circulation of geothermal fluids and fluid-rock interaction with clayey-carbonate sedimentary rocks and interbedded tectonic slices of serpentinite, representing the cap of an exhumed geothermal system located in the south-eastern Elba Island (Acquarilli-Norsi area, Tuscan Archipelago). Structural and kinematic data highlight that the prevalent geothermal fluid pathways correspond to NE-trending normal to oblique-slip faults. A Fe-amphibole + quartz + calcite ± chlorite ± epidote ± titanite mineral assemblage filled the fracture networks associated with two different fault generations. Fluid-rock interaction gave rise to skarnoids having three distinct mineralised zones, locally, crossed by the faults. Fluid inclusions data on quartz and calcite coexisting with the amphibole in the first fault generation and calcite of the second fault generation suggest that at least two paleo-geothermal fluids permeated through the fault zones, at a maximum P of about 0.8 kbar. The sequence was (i) first an H2O-rich fluid enriched by a volatile phase mainly made up of CO2 derived from the decarbonation and dehydration reactions of the hosting metacarbonate and metapelitic rocks. This fluid characterised by temperature up to 495 °C and salinity between 1.0 and 6.4 wt.% NaClequiv, records the residual intra-formational fluid (derived from dehydration of phyllosilicate and serpentinite bodies) already present within the host rocks. This fluid was successively mobilised during the development of the fault zones and progressively cooled and CO2-depleted with time; (ii) a second fluid likely of meteoric origin, devoid of CO2 and characterised by relatively low-temperature from 146.5 to 254 °C and salinity between 1.9 and 2.9 wt.% NaClequiv related to the last fluid circulation event. The main conclusions are that: (i) geothermal fluid circulation occurred within the cap rocks of the paleo-geothermal system, along fault zones; (ii) fluid circulation occurred within restricted fault-damaged volumes, developed in an extensional setting; iii) faults dissected rocks that were affected by high-temperature metamorphism induced by the cooling of a magmatic intrusion. The super-hot fluid was produced by metamorphic reactions and only in the latest stage, meteoric water infiltrated in the geothermal system due to the progressive faulting and exhumation.