This work presents geochemistry and structural geology data concerning
the low enthalpy geothermal circuits of the Argentera crystalline Massif
in northwestern Italian Alps. I
n this area some thermal springs (50-60 degreesC), located in the small
Bagni di Vinadio village, discharge mixtures made up of a Na-Cl
end-member and a Na-SO4 component. The latter is also discharged by the
thermal springs of Terme di Valdieri located some kilometres apart
within the same tectonic complex. Both end-members share the same
meteoric origin and the same reservoir temperature, which is close to
150 degreesC. Explanations are thus required to understand how they
reach the surface and how waters of the same origin and circulating in
similar rocks can attain such different compositions.
Sodium-sulphate waters discharged at both sites, likely represent the
common interaction product of meteoric waters with the widespread
granitic-migmatitic rocks of the Argentera Massif, whereas Na-CI waters
originate through leaching of mineralised cataclastic rocks, which are
rich in phyllosilicatic minerals and fluid inclusions, both acting as
Cl- sources.
Due to the relatively low inferred geothermal gradient of the region,
-25C/km, meteoric waters have to descend to depths of 5.5-6 km to attain
temperatures of similar to 150 degreesC. These relevant depths can be
reached by descending meteoric waters, due to the recent extensional
stress field, which allows the development of geothermal circulations at
greater depths than in other sectors of the Alps by favouring a greater
fractures aperture. The ascent of the thermal waters rakes place along
brittle shear zones. In both sites, the thermal waters emerge at the
bottoms of the valleys, close to either the lateral termination of a
brittle shear zone at Terme di Valdieri, or a step-over between two
en-echelon brittle shear zones at Bagni di Vinadio. These observations
attest to a strong control operated on the location of outlet regions by
both brittle tectonics and the minima in hydraulic potential inside the
fractured massif.