A hydro-thermo-haline numerical approach of the groundwater flow to explain the extreme Li-enrichment in the Salar de Atacama (NE Chile)

Salt flats (salars) are endorheic hydrogeological systems associated with arid to hyperarid climates. The brines of salt flats account the 80 % of the world’s reserves of Li highly demanded by modern industry. About 40 % of the worldwide Li is extracted from the brine that fills the pores and cavities of the Salar de Atacama. However, the origin of the extreme Li-enrichment of these brines is still unknown.

The thick accumulation of salts and brines in salt flats results from the groundwater discharge (phreatic evaporation) near the land surface for thousands to millions of years. The strong evaporation contributes the enrichment in major cations and anions as well as other rare elements (e.g. Li, B, Ba, Sr, Br, I and F) which are very attractive for mining exploitation. However, only evaporation cannot explain by itself the extreme concentrations of some of these elements and the strong decoupling between the most evaporated brines and the most Li-enriched brines in the Salar de Atacama. Several hypotheses have been proposed to explain the extreme Li-enrichment of the salt flat brines: (a) concentrated brines leaking down from salt flats located in the Andean Plateau, (b) leaching of hypothetical ancient salt flats buried among volcanic rocks, and (c) rising of hydrothermal brines from deep reservoirs through faults. However, none of them has been able probed neither validated by a numerical model till the date.

The objective of this work is to discuss the feasibility of the different hypotheses proposed until now to explain the formation of the world's largest lithium reserve. To achieve this objective, two sets of numerical simulations of a 2D vertical cross-section of the entire Salar de Atacama basin are carried out to define (1) the origin and evolution of a salt flat and how climate cycles can affect the location of the most Li-concentrated brines by evaporation and (2) the establishment of the hydro-thermo-haline circulation of a mature salt flat basin.