The La Laja spring system in the Argentine Precordillera: A conceptual model based on geochemical and isotopic data

Abstract

The La Laja Spring System (LLSS) (31.34°S; 68.48°W) in the Argentine Precordillera is well-known for its low enthalpy thermal baths since pre-Columbian times and its surroundings full of travertine fields. This work focuses on geochemical and isotopic analysis of the thermal waters and their hydrological implications. We found a system with partially equilibrated waters with dominant Cl−–Na+ and subordinated HCO3−Ca2+(Mg2+) and SO42−Ca2+compositions, which seem affected to the basin fill composed of thick Cenozoic synorogenic deposits overlying early Paleozoic carbonate successions. Several cationic geothermometers suggest a deep reservoir with average temperatures reaching ~170 °C, while the silica geothermometers suggest a shallow reservoir at 91 °C. The average temperature of the springs is 28 °C and the low-geothermal gradients (~27 °C/km) in the region indicate a relatively deeper (~5500 m) reservoir and another shallower (~2500 m). The source of heat is related to the orogenic belt because this region is on top of the flat-slab subduction of the South-Central Andes, with an absence of active volcanism. Hence, this spring system is mainly affected by the relatively low-geothermal gradient within the Eastern Precordillera. The geochemical and isotopic characteristics O18/O16 as well as H2/H1 ratios of waters indicate meteoric source associated with evaporation in relation to strong aridity and secondary processes modifying the chemistry of waters is likely to be erased by any thermal end-member signature. Our conceptual model for the LLSS indicates the N–S trending Villicum-Zonda regional thrust and the minor accommodation faults within the Eastern Precordillera domain seem to behave as the preferential zone for recharging and latter thermal water rising.