Origins of the Mg/Li ratio in closed-basin brines of the Lithium Triangle: The relative importance of high- and low-temperature water-rock interactions

The Mg/Li ratio is widely used to evaluate the extraction potential of lithium-rich brines, yet the processes controlling its variability remain poorly constrained. Across the Lithium Triangle of the Central Andes in South America, closed-basin brines exhibit Mg/Li ratios spanning more than three orders of magnitude (from <1 to >500) despite broadly similar geology, evaporative evolution, and major-ion chemistry. Here we present a regional synthesis of Mg/Li ratios in spring waters and closed-basin brines, drawing on geochemical data for hundreds of springs (n = 577) and brines (n = 1078) from more than 100 closed-basins to develop a framework for interpreting Mg/Li variability. We show that Mg/Li ratios in inflow waters are primarily governed by the temperature of water-rock interactions: high-temperature geothermal systems produce low Mg/Li ratios, whereas low-temperature, near-surface water-rock interactions produce water with higher Mg/Li ratios, resembling the ratios in source rocks. At the basin scale, brine Mg/Li ratios reflect mixing-weighted integration of high- and low-temperature derived inflows. Evaporative concentration, mineral precipitation, and water-sediment interactions modify absolute solute concentrations but generally preserve the integrated Mg/Li ratios of the inflows under the circumneutral pH conditions typical of Lithium Triangle brines. Important exceptions occur in alkaline systems, where Mg-silicate precipitation alters the inflow Mg/Li signature. These results demonstrate that Mg/Li ratios record the integrated thermal history of solute sources and provide a context-dependent indicator of geothermal versus near-surface fluid contributions to closed-basin brines. This framework refines interpretation of Mg/Li ratios in lithium exploration and has broader implications for continental brine systems worldwide.

Duke Scholars

Author Avner Vengosh Earth and Climate Sciences