Geochemical evolution of brines in the Salar of Uyuni, Bolivia

Abstract

Recent analyses of brines from the Salars of Uyuni and Coipasa have been compared with published chemical data for Lakes Titicaca and Poopo in order to evaluate solute compositional trends in these remnants of two large Pleistocene lakes once connected by overflow from the northern to the southern part of the Bolivian Altiplano. From Titicaca to Poopo the water chemistry shows an increase in concentration of Cl and Na somewhat greater than the total solutes, suggesting preferential inputs of these elements or significant losses of other constitutents downstream from Lake Titicaca proper. In contrast, Ca and SO 4 increase to a lesser extent than do total dissolved solids, and carbonate species are relatively constant, suggesting solute losses most readily related to the precipitation of calcite and gypsum. Between Poopo and Coipasa the proportions of Ca, SO 4 and CO 3 continue to decrease. At Coipasa and Uyuni, the great salars frequently evaporate to halite saturation and widespread halite crystallization is accompanied by an increased proportion of K, Mg and SO 4 in residual brines. Notably high concentrations of Li and B in brines from the south end of Uyuni, near the mouth of the Rio Grande de Lipez, are clearly in excess of that expected for the solute concentration trends for inflow from Coipasa and the northern Bolivia Altiplano, as shown by comparison with Mg and K. This supports other indications that the major source of Li and B are the recent rhyolitic volcanic rocks prevalent in the drainage of the Rio Grande.