Origin of thermal waters in the Fiambalá basin (Argentina): Preliminary insights from hydrochemistry and isotopic tracers

Abstract

The hydrochemistry and isotopic signature of waters (i.e. rivers, groundwater and thermal springs) from the Geothermal System of the FiambalÁ Basin (GSFB), located in Catamarca province (Argentina) were studied. This work provides, for the first time, a conceptual model that explains both the hydrogeological functioning and the heat source of the geothermal system. The anionic evolution of springs with an increasing trend in Cl- and SO4 2- , accompanied by an increase in pH and TDS contents, follows the probable groundwater flow direction (i.e. north-south direction). According to the hydrogeological system proposed for the region, and based on isotopic composition, it is possible to identify that two distinct aquifers (i.e. hydrogeological units) feed the thermal springs. Saujil (~23◦C discharge temperature) and La Aguadita (~31◦C discharge emperature) thermal springs (δ18O ~-5.5 ‰ and δ2H ~-28 ‰ and 222Rn concentration ~15.0 Bq L- 1) are fed by one or more shallow aquifers, composed of Quaternary non-consolidated materials (Hydrogeological Unit 3). Groundwater from the La Ramadita well is also likely contained in this aquifer. Conversely, Fiambalá Spring, which shows a discharge temperature of ~50◦C, a more depleted stable isotopic composition (δ18O and δ2H ~-7.3 ‰ and ~-44 ‰, respectively), a 222Rn concentration 3 times higher (~46.865 Bq L- 1), and high F- contents (7.44 mg L- 1), is probably fed by a Paleozoic granitic/metamorphic fractured aquifer (Hydrogeological Unit 1). The stable isotopicsignature indicates that waters have a meteoric origin with no mixing with magmatic waters. The aquifers’ direct recharge zones are located in the alluvial slopes of the mountain ranges that border the basin, at elevations higher than 4,000 m a.s.l. Indirect recharge also occurs in the shallow aquifers through river water infiltration. Thermal springs emerge through secondary structures originated due to old structures reactivation during the Andean orogeny. Deep water circulation, possibly exceeding 4 km in depth, provides the main source of heating, assuming a typical geothermal gradient of 26◦C/km for the Northern Sierras Pampeanas and considering the thickness of the Neogene sedimentary deposits overlying the crystalline basement. Thermal spring waters areimmature and are not in equilibrium conditions, as revealed by the hydrochemistry. The geothermometer calculations suggest reservoir temperatures below ~100◦C. These results allow to classify the GSFB as a lowenthalpy (or low-temperature) geothermal system.