The origin of alkaline, Na-HCO3 thermal waters from Río Valdez, Isla Grande de Tierra del Fuego (Argentina)

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

JOURNAL OF HYDROLOGY

ELSEVIER SCIENCE BV

Año: 2011 p. 1 - 24

0022-1694

The small geothermal area of Rio Valdez is located in the central portion of the Isla Grande de Tierra del Fuego (South Argentina), 10 km south of the south-eastern sector of the Fagnano Lake. A series of thermal springs is distributed within an area of less than 1 km2, with low discharge rates and surface temperatures in the range of 20-33°C. Thermal waters are partially equilibrated, alkaline, of Na-HCO3 type with low salinity but relatively high fluorine contents. The thermal fluids are characterized by a slow circulation through depth, possibly driven by deep tectonic discontinuities connected with the Magallanes-Fagnano Fault (MFF) system. According to geothermometric calculations, thermal waters reach reservoir temperatures in the range of 100-150°C and an almost complete chemical equilibration by interaction with alkali-feldspars from host rocks.  The relatively high fluorine contents can be explained by the slow ascent and cooling of deep groundwaters, followed by a progressive re-equilibration with F-bearing, hydrated Mg-silicates, such as chlorite, which has been recognized as an abundant mineral in meta-volcanics of the Lemaire Formation. Deep rock-water interactions with alkaline intrusives belonging to the Fuegian Potassic Magmatism may also have contributed to the high fluorine content of thermal waters. The isotopic composition of thermal springs can be resonstructed through the infiltration of meteoric water from local snowmelt at altitudes from 610-770 m asl.