Migration of hot to super-hot palaeo-fluids in an extensional setting (eastern Elba Island, Italy)

Understanding how hot (250°–350 °C) to super-hot (>350 °C) fluids circulate and interact with host rocks in fault-controlled upper continental crust is a challenging task. In this paper, we present the results of a structural, fluid inclusions, Sr isotope, petrographic and mineral study on an exhumed geothermal system to describe how hot to super-hot fluids were controlled by the increased permeability resulting from the interplay between low-angle normal faults and high-angle oblique-slip faults, the latter being part of a regional transfer zone. The study area is located on the eastern Elba Island (northern Tyrrhenian Sea, Italy), where the emplacement of a Messinian felsic intrusion has allowed hydrothermal fluids to circulate in relation to a fault-related permeability of the order of 10−15 to 10−13 m2. Magmatic fluids reacted with the host rocks (carbonate and phyllite) to form Fe-ore deposits hosted within the fault zones that controlled the fluid flow. Fluid inclusions trapped in quartz crystals indicate that the fluids consisted of saline solutions (salinity from 18.0 to 26.6 wt% NaCl eq.), which later mixed with low salinity waters of probably meteoric origin. The fluid temperature during the main hydrothermal stage ranged between about 300° and 360 °C under an estimated maximum hydrostatic pressure of 28–30 MPa. In contrast, the maximum possible temperature was about 400 °C at a lithostatic pressure of 89 MPa. The hydraulic breccias formed as a result of the transient fluid pressurisation up to lithostatic values, due to the self-sealing process and the subsequent pressure drop, down to hydrostatic values, as framed in the hydrofracturing processes. The results provide useful inputs for the identification of geothermal and ore-body targets in those geological settings where fluids of magmatic and meteoric origin are controlled by active faults in an extensional tectonic regime.