Characterization of lithium cycling in the Salar de Olaroz, Central Andes, using a geochemical and isotopic approach

Abstract

The Salar de Olaroz is one of the main Li operations in Argentina. In order to identify sources and to understand the dynamics of Li in this salt pan, chemical and isotopic analysis (δ7Li and 87Sr/86Sr) of brines, thermal and river waters, as well as ignimbrites and sediments that outcrop in the basin were performed. The Li concentrations in shallow brines (i.e., ∼0.6 m below surface) range from 260 to 890 mg L−1 (average = 841 mg L−1, n = 16), while in deep brines (i.e, 200–450 m b.s.) concentrations are higher (average = 993 mg L−1, n = 16) and range from 690 to 1490 approximately. δ7Li values range from +5.9 to +7.2 ‰ and from +8.1 to +10.2 ‰ in shallow and deep brines respectively, while δ7Li values of the regional rocks and sediments vary between -13.8 and +0.5‰. The 87Sr/86Sr values indicate that the chemical composition of brines in Olaroz is the result of mixing between solutes that are originated from bedrock weathering by shallow meteoric water and water that has a hydrothermal contribution. Due to evaporation, Li concentrations in shallow brines increase by two or three orders of magnitude when compared with the values determined in the main tributaries and in the discharge of thermal waters. This process does not produce measurable isotopic fractionation of Li, thus the isotopic signature of shallow brines and halite crystals in contact with these waters are similar. As brine density increases, water descends through layers of silt and as the brine ages at depth, the formation of secondary minerals enhance the preferential adsorption of 6Li onto clay minerals and Fe (hydr)oxides resulting in higher δ7Li values in deep brines. Nevertheless, Li concentrations remain nearly constant or slightly increase with depth, which suggests that the removal of this element from the water due to adsorption is offset by Li contributions derived from processes that do not produce any isotopic fractionation. Two processes may account for this in the study system: 1) the mixing with ancient Li-rich fluids trapped in the deeper evaporitic layers that preserved the isotopic signature of their contemporaneous brines; and 2) the release of loosely- bound Li from the exchangeable sites of smectites that compose the clastic fraction of the salt flat sediments as they acquire a more crystalline nature over time. Consequently, it is expected that sediments accumulated below the salt crust may provide an important additional reservoir of Li to the brine, but further experimental studies will be necessary to investigate the mechanisms that control the partition of Li between the aqueous and solid phases and its isotopic fractionation in extremely arid and saline environments.