Aplicación de la modelización geoquímica al estudio de pautas evolutivas en las salmueras del Salar de Llamará (Chile). Aproximación de método inverso

Abstract

Neutral brines of the Na-Cl type are strongly concentrated by evaporation under and arid climate in the «salar de Llamará» saline system (Chile). The PHRQPITZ geochemical code (which incorporates a chemical model based on Pitzer's equations) has been used for calculating ionic activities and saturation indexes, in orden to evaluate the mineral sequence controlling the geochemical evolution of brines. It seems to exist a disequilibrium between brines and atmosphere, since the partial pressure of CO₂ calculated by the code shows a progressive increase over the standard mean atmospheric value. All the samples are in equilibrium with both calcite and gypsum. Water activity conditions determine gypsum to be the more stable phase in the Ca-SO₄-H₂O system. Glauberite reaches saturation at late stage in the brine evolution. The more soluble saline phases do not attain saturation anytime, though more concentrated brines are almost in equilibrium with halite, mirabilite, and thenardite. The results of this thermodynamic treatment are in very good agreement with field observations. Moreover, mass-balance calculations by means of NETPATH code indicate that glauberite precipitation occurs simultaneously with gypsum dissolution.