Geochemical Investigations of Two Hydrothermal Systems in an Alpine External Crystalline Massif (Piedmont, Western Italy)

Abstract

The thermal springs of Bagni di Vinadio and Terme di Valdieri are located in the south of Piedmont, just 100km far away from Turin, in the Argentera Massif, the southernmost of the external Crystalline Massifs of the Alps. These waters have been geochemically studied to characterize the water path from the infiltration of cold meteoric waters to the discharge as thermal waters with temperatures up to 70°C. Water samples have been collected during two sampling campaigns, chemical and isotopic analysis were carried out to gather information about the chemical composition of cold and hot waters, the maximal equilibrium temperature reached at depth, the elevation of the recharge zone and the residence time. Thermal waters at Vinadio show a mixture of NaCl and NaSO4 compositions and a wide variability in terms of Temperature and TDS while at Valdieri are very similar to each other showing a NaSO4 chemical composition. Once cold meteoric waters infiltrate at more than 1700m a.s.l. through the faults they reach an average temperature of 120°C at Bagni di Vinadio and 100°C at Terme di Valdieri at depth. The circulation of the waters might take thousands of years to occur. Some hypothesis were developed to estimate the water rock interactions and the mixing processes in relation to natural fluid pathways inferred from the geological and structural settings of the study areas. Thermal waters interact with the rocks of the massif, in particular with granites and gneisses, that are involved in the tectonic activity of the massif. The chemical composition could suggest two different paths for the thermal waters discharging at the two thermal sites and also that mixing processes occur at Vinadio between a deep and hot NaCl end member and a NaSO4 groundwater while at Valdieri the mixing process is reflected just in terms of diluition of NaSO4 thermal with cold groundwaters. More information about the circulation pathways will be inferred from following geophysical and geological surveys that will help to better understand the very complex geological structures that control the circulation of these thermal waters and to define the most probable depth of the reservoir and eventually plan deeper investigations for the exploitation of these fluids.