A VLF Survey in Vinadio Geothermal Area

In the Argentera Massif, evidences of active thermal circulations are provided by groups of low-enthalpy springs whose presence has been reported since Roman times.
The first aspect to consider is that the structures that are likely to play a key role in controlling the thermal flow within this fault zone are only those formed under frictional regime, that are the strike-slip cataclastic faults. Structures that formed under frictional-to-viscous conditions (i.e. the Qz-Chl slickensides) and under viscous conditions (i.e. the mylonitic shears) have acted at deeper crustal conditions as important conduits for fluid flows.

Traces of rock alteration phenomena and ore deposits indicate the occurrence of past thermal circulations. Shaly and prophylitic alterations are present in two different forms: diffuse alterations of great rock volumes and alterations localized along shear zones, both early and late alpine in age. An example of the first type is found in the AM cover succession in the Stura-Gesso area. Here rock alteration in marbles is probably related to fluids expelled during deformation at the cover/basement interface. Many brittle NW and NE trending brittle shear zones are characterised by metasomatic processes and deposition of sulphide minerals.

The most common minerals are white mica, quartz, fluorite adularia, chlorite, calcite, tourmaline and sulphides. Many small sulphide ore deposits, related to these localized alterations are present inside the massif and its covers. These mineralizations are often constituted by galena and sphalerite with minor chalcopyrite, pyrrotite and arsenopyrite, while gangue is usually composed of quartz, fluorite, chlorite and calcite.

The study of the structural and geometrical characters of the BFZ, coupled with the statistical analysis of fault-rock thickness distribution, performed by Baietto (2009) on well-log data, furnishes a basis for the description of the BFZ hydraulic architecture.
The permeability architecture of the BFZ cataclastic fault zones can be described in terms of a combined conduit-barrier. Basically, this model recognizes the role of two high-permeability damage zones, sited at the borders of a fault core, acting as conduits for along fault flow, and at the borders of a low-permeability fault core acting as a barrier for cross-fault flow.

In this sense, the faults display multiple cataclastic-gouge strands that envelope lens-shaped blocks of breccias and fractured rocks. Therefore, many damage zones that at meter-scale seem to behave as potential conduits for fluid flow, at hectometer-scales result to be bounded by impermeable fault cores that prevent hydraulic connection with the bounding regions. Owing to these characters, it is suggested that these faults are likely to act as a complex combined conduit-barrier system, hosting a number of possible fluid flow pathways at several scales.
Thus, the numerous fractured blocks and layers that are present within strands of low permeable material (i.e. the gouges and the foliated cataclasites) can store fluids or let fluids circulating along the faults according to their location within the fault zone.

Another important aspect that arises developed within the granites and the migmatites is that the former are more suitable to act as conduits for fluid flow while the latter are more likely to act as combined conduit-barriers. In fact, the position of the granites, placed underneath the migmatites, implies that the granites can constitute a reservoir for fluids at regional scale, involving significant outcomes for the ore distribution and the present-day thermal circulations.