Geochemistry of water discharges at Mt. Amiata volcanic complex in relation with the regional tectonics

The Mt. Amiata geothermal area (southern Tuscany, central Italy) is located in an extended continental crust, where extensional structures developed from the Middle Miocene to the Quaternary, whilst collisional structures occurred from the Cretaceous to the Early Miocene. The recent extensional stages are divided in: a) Middle–Late Miocene, when widespread extensional segmentation, mainly affected the Tuscan Nappe via normal faults with staircase geometry, produced space accommodation of the sediments filling the neighboring Neogene basins; b) Pliocene–Quaternary, when marine sediments filled the relict Miocene structural depressions, which were affected by intense faulting activity. In the Pliocene a magmatic body was emplaced in the thinned continental crust. The observed high heat flow is due to this magmatic body which is located about 6–7 km below sea-level and not completely cooled down. The body is responsible for a negative Bouguer anomaly and localized uplift of around 3,000 m centered on the Mt. Amiata volcano. At the top of the convex structural culmination, a crustal fissure striking N50° produced the eruption of the Mt. Amiata volcano during the Late Pleistocene. Hydrothermal circulation typifying the Bagni di S. Filippo and Bagnore areas is mainly concentrated at the intersection between this crustal fissure and two normal faults striking N160° and 180°. Mt. Amiata is the most recent (3-200 ka) and the largest volcanic apparatus of the Tuscan magmatic Province. The Amiata geothermal system is characterized by the presence of two reservoirs: the shallower one, having a temperature of 200–230 °C, is at depth of 500–1000 m in the carbonate– evaporite formations, whereas the deeper one (depth > 3000 m) is within the metamorphic basal complex where temperature are up to 350 °C. Both reservoirs are bi-phase systems and used for the production of electrical energy by ENEL. In this work, a relatively large data-set of chemical and isotopic compositions of discharging fluids was acquired in order to investigate if the deep-seated fluids may interact with the shallower volcanic aquifer. Thermal and cold springs associated to a CO 2 -rich gas phase are mainly located in the north-eastern and south-western sectors of the Mt. Amiata volcanic edifice. These types of fluid emergences are practically missing in other sectors of the volcano. The studied waters are chemically characterized by i) a Ca–SO 4 composition, ii) relatively high TDS (total dissolved solids) values, iii) an associated CO 2 -rich gas phase and iv) anomalously high concentrations of B, Li, Cs, HS and NH 4 . A Ca-HCO 3 shallow aquifer, having low TDS being directly recharged by meteoric water, is hosted within the fractured volcanic sequences, above the low permeable Ligurian Units that prevent a hydraulic connection with the deep hydrothermal aquifers. This aquifer fed cold springs that can abundantly be recognized all around the volcano. Accordingly, the spatial distribution of the CO 2 -rich Ca–SO 4 thermo-mineral waters confirms that they are controlled by regional faults and tectonic structures in the area.