Low enthalpy Na-chloride waters from the Lunigiana and Garfagnana grabens, Northern Apennines, Italy: Tracing fluid connections and basement interactions via chemical and isotopic compositions

The Na-Cl waters from NW Tuscany (central Italy) record similar water isotopic and major chemical compositions, which demonstrate their meteoric origin and interactions with Upper Triassic evaporites and the metamorphic units of the Paleozoic basement. Slight differences are found in the deep temperature-pressure conditions of the Lunigiana graben (39-42 °C/143-145 bar) and the Garfagnana graben (73-78 °C/250-256 bar). In particular, the thermal fluids outpouring from Garfagnana are probably related to a common deep reservoir or interconnected fluids. Their differences are mainly evidenced by strontium isotopic ratio data (87Sr/86Sr), which combined with previously published sulfur isotope ratios (34S/32S) demonstrate the involvement of vein barites in water-rock interactions. Most likely, these minerals formed during Upper Oligocene-Miocene tectogenesis due to the mixing of fluids from the Verrucano Group and Upper Triassic units. The results of this hydrogeochemical study of the deep Na-Cl fluids could better clarify the distribution of the Verrucano Group within this area and the related discrepancies in the stratigraphic interpretations of the Palaeozoic-Mesozoic transition. Furthermore, the possible presence of an interconnected reservoir could be used to help interpret data produced by the local geochemical monitoring of seismic activity.