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 composi- tions, which demonstrate their meteoric origin and interactions with Upper Triassic evaporites and the metamor- phic 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 interconnect- ed 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.