A CRITICAL EVALUATION OF THE ROLE OF THERMAL WATERS IN THE FORMATION OF LITHIUM DEPOSITS IN THE SALT FLATS AT LAGUNA VERDE IN THE CHILEAN ANDES

Despite lithium (Li) present in the brines of the salt flats often being associated with geothermal systems, most salt flats do not have a present day active geothermal system to evaluate the association between Li-enrichment of the brines and geothermal waters, making this association somewhat speculative. However, a salt flat in the Chilean Puna region of the Central Andes with a hypersaline lake – Laguna Verde (LV) – within a closed basin encircled by a series of stratovolcanoes, an active geothermal system with numerous surface manifestations, and outcrops of the potential Li source rocks, provides an excellent opportunity to evaluate this association. The study area has outcrops of mostly andesitic-dacitic rocks with the limited occurrence of rhyolitic rocks (sequences with ignimbrites and glass-rich ash), reported in the boreholes drilled in the LV area for mineral exploration. From these primary sources, Li is released through leaching by geothermal fluids, this process being active intermittently from the Miocene to the present day. The LV geothermal system consists of an outflow zone on the lake's southern margin, where more than 20 hot springs with temperatures up to 47°C are located. This system's upflow area is located 25 km SSW of LV, and is manifested by fumaroles located in the Nevado Ojos del Salado volcano's central crater. The considerable distance between upflow and outflow zones (i.e., the geothermal system's extension) causes requilibration(s) during the long-distance travel of the thermal water, leading to underestimation of reservoir temperature (85 and 90°C with Na-K-Mg and silica geothermometers, respectively). Whereas Li concentration in the hot springs is from 2.76 to 4.46 mg/L, in Peñas Blancas River that feeds LV after passing through the volcanic environment (rocks rich in Li), thus getting enriched in Li, it is 0.59 to 0.76 mg/L. Li contribution over six times the non-thermal water envisages the importance of the thermal waters in Li release and transport from the source rock to its eventual concentration in LV.