The origin of the alkaline, NA-HC03 thermal waters of Rio Valdez, Isla Grande de Tierra del Fuego (Argentina).

CAPACCIONI, B.; MENICHETTI, M.; RENZULLI, A.; TASSONE A.

Torino

Congreso; Geoitalia. VIII Forum Italiano di Scienze della Terra; 2011

Federación Italiana de Ciencias de la Tierra

The small geothermal área of Rio Vaidez is located in the central portion of the Isla Grande de Tierra del Fuego (South Argentina), ten kilometres south of the south-eastern sector of Fagnano Lake. It is characterized by a series of thermal springs dlstrlbuted in an área of less than 1 km2, with low discharge rates ana temperatures in the range of 20-33°C. Thermal waters can be defined as partlally equilibrated, alkaline. Na-HC03 waters with low salinity but relatively high fluorine contents. Tnermal waters are characterized by slow circulation at depth, possibly driven by deep tectonic discontinuities connected with the Magallanes-Fagnano Fault (MFF) system. According to geothermometric calculations, thermal waters reach temperatures in the range of 100-150°C and an almost complete chemical equilibraron with the alkali-feldspars of the rocks they interacted with.  The relatively high fluorine contents can be explained by the slow ascent and cooling of deep groundwaters, followed by a progressive re-equilibratlon with F-bearing, hydrated Mg-sllicates, such as chlonte, which has been recognized as an abundant mineral in the meta-volcanics of the Lemalre Formation. As a matter of fact, deep rock-water nteractions with the alkaline intruslves belonging to the Fuegian Potassic Magmatism may aiso have contrlbuted to the high fluorine contents of the thermal waters.   Finally, the isotopic composition of the investigated samples is consístent with the infiltración of meteoric water from local snowmelt at altitudes in the range of 610-770 m asi.