Geochemical controls on the formation of lithium brines in closed-basins of the Lithium Triangle

Sustainable lithium mining is critical to the renewable energy transition. Closed-basin brines are a major source of lithium yet the processes governing lithium enrichment remain poorly understood. In the Lithium Triangle (LT) of South America, hypersaline brines display anomalously high lithium concentrations including at the Salar de Uyuni (SDU) in Bolivia. Using new geochemical and isotopic data from the SDU, Bolivia, we update the accepted conceptual model of evaporative concentration and sequential mineral precipitation based on the formation of calcite, gypsum, and halite. Here we identify ulexite (Na-Ca-borate) precipitation as a previously overlooked but key process in the evaporative evolution of inflow waters that fundamentally alters brine chemistry prior to halite saturation. Additionally, we reveal that surficial brines are largely disconnected from the major lithium inflow, and instead their chemistries are controlled by cyclic halite dissolution-precipitation, leading to the conservative enrichment of solutes like lithium, boron, and magnesium. We suggest that deep brines exploited for lithium extraction are fossil and reflect different stages of evaporation, while modern processes make little contribution to the solute and lithium balance. This new conceptual model revises the classic geochemical pathway and has broad implications for lithium brines and resource sustainability across the LT.