Permeability and hydraulic conductivity from field and fluid inclusions data in exhumed geothermal systems

The understanding of the relationships between geothermal fluid flow and geological structures represents one of the main task to improve exploration and exploitation of geothermal resources. A contribute to this issue derives from analogous exhumed examples of geothermal systems developed in continental (Elba Island, Italy) and oceanic settings (Geitafell, Iceland).

The adopted methodology follows the classical approach of structural geology, petrography and fluid inclusion studies. Eastern Elba Island is characterized by a widespread Fe-oxides, Fehydroxides and sulphides deposits exploited for centuries and deriving from the hydrothermal activity occurring during the mid-crustal Porto Azzurro monzogranite (5.9 Ma) cooling. Hydrothermal circulation is accompanied by extensional tectonics, developing low-angle normal faults and NE-trending oblique-slip to normal shear zones, defining parallel sets of localized deformation, each other connected by linkage structures.

The field mapping highlighted that the damage zones and the slip-surfaces of both low-angle normal faults and NE-trending shear zones are characterized by extensional and shear veins filled up with the same hydrothermal minerals, thus indicating a contemporaneous faults activity. Fluid inclusions studies were carried out at different structural levels. At the deepest outcropping levels (tourmaline+quartz veins in Paleozoic micaschist), the results indicate occurrence of fluids with T of about 370-650°C and salinity encompassed between 19% and 48% wt. NaCl eq.; differently, at shallow structural levels (hematite+quartz veins in Triassic quartzite), T tends to decrease (T<350°C) whereas isotope analyses and salinity values (0-25 wt. % NaCl eq.) indicate a progressive contribute of meteoric water, leaching of the Late Triassic evaporitic level and/or mixing with magmatic fluids. Estimated fracture permeability from field data resulted in the range 1*10-13 -1*10-17 m2.

Fluid inclusions provided also fluid density values, representing the other parameter to estimate hydraulic conductivity that is encompassed between 1*10-8 and 1*10-13 m/s. The Geitafell volcano (5-6Ma) is an extinct central volcano that is believed to have been active in the rift zone during Late Miocene, and later migrated southeastwards as a consequence of the accretionary crustal processes at divergent margins. The magmatic chamber is represented by a gabbro pluton emplaced in Miocene flood-basalts host-rocks.

The field activity was based on collection of: (a) field-samples for laboratory analyses; (b) structural and kinematic data from the surrounding of the gabbro; (c) core-samples for petrographic and fluid inclusion studies from seven slim- boreholes (down to 30 m, as a maximum) drilled at the boundary between the gabbro and its hosting rock. Field mapping highlighted the occurrence of two systems of faults, NE- and NW-trending respectively, and dissecting both the gabbro and its hosting rocks. NE-trending faults are dominantly oblique-slip faults, whereas the NW-trending faults are mostly with a significant normal component.

Their damage zones and slip surfaces are characterized by shear veins with synkinematic hydrothermal minerals (andradite, epidote and quartz, mostly) indicating their coeval activity during hydrothermal fluid flow. Estimated fracture permeability from field data resulted in the range 10-13-10-17 m2. Fluid inclusions studies, carried out in samples from boreholes, clearly indicate that the hydrothermal fluids were characterized by boiling at T around 270-280°C and very low salinity (<1 wt. % NaCl). As a result of passive enrichment in solute species due to boiling, salinity up 10 wt. % NaCl eq. was locally registered in fluid trapped by calcite, and minor in quartz.

In garnet, fluid inclusions indicate fluids with T up to 420°C. Concluding, the whole collected data indicate that meteoric fluids were channeled to depth through the structural paths defined by the fault zones; the continuous interaction between meteoric water and cooling rocks determined an open geothermal system characterized by convective heat transfer, without fluids deriving from a magma source.

Prof. Domenico Liotta