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Abstract

RESUMENLas salmueras superficiales del salar de Llamará (Chile) experimentan un proceso de concentración por evaporación a causa de la aridez del clima al que se ven sometidas. Estas salmueras son del tipo clorurado-sódico y presentan un pH aproximadamente neutro. El tratamiento de los datos hidroquímicos mediante el código de modeli~ación PHRQPITZ (que incorpora un modelo químico basado en las ecuaciones de Pitzer) ha permitido realizar el cálculo de actividades iónicas e índices de saturación mineral; estos parámetros se han usado para evaluar la secuencia de precipitación mineral que controla la evolución geoquímica de las salmueras. La presión parcial de CO, en solución calculada por el código experimenta un ascenso progresivo (siempre por encima del valor atmosférico) conforme avanza el proceso, lo que parece indicar la existencia de un desequilibrio entre las salmueras y la atmósfera. Todas las muestras tratadas se encuentran en equilibrio con calcita y yeso, siendo esta última la fase más estable en el sistema Ca-SO4-H,O para los valores de actividad del agua calculados. Del resto de fases salinas de interés sólo la glauberita alcanza el equilibrio en una etapa más avanzada del proceso, manteniéndose las demás en un estado más o menos marcado de subsaturación, si bien halita, mirabilita y thenardita llegan prácticamente al equilibrio en las últimas muestras recogidas. Los resultados del tratamiento termodinámico son consistentes con la mineralogía de las sales analizadas en los depósitos del salar. Por otra parte, los cálculos de balance de masa realizados mediante el código NETPATH confirmar que la precipitación de glauberita en los últimos estadios conlleva la disolución concomitante del yeso precipitado anteriormente en el salar.Palabras clave: Salmueras, evaporación, rnodelización geoquímica, actividad, índice de saturación, equilibrio mineral. ABSTRACTNeutral 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 C 0 2 calculated by the code shows a progressive increase over the standard mean atmospheric value. Al1 the samples are in equilibrium with both calcite and gypsum. Water activity conditions determine gypsum to be the more stable phase in the Ca-SO,-H,O 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

Hydrochemical and Mineralogical Evolution through Evaporitic Processes in Salar de Llamara Brines (Atacama, Chile)

“…Cl concentration in samples collected in this study (blue) and in previous − studies (red). A clear seasonal variation is observed with a maximum concentration (corresponding to maximum evaporation) in December and a minimum concentration around September.…”

“…No top/bottom gradients have been taken into account previously for diagenesis purposes − but are clearly observed in our data taken in situ. This result is relevant for studies, such as isotopic analysis, in which the homogeneity or stratification of the waters play a relevant role.…”

“…No seasonal changes have been reported in the previous literature, − which worked with data collected in August and December only. Our data were collected in October and March and fortunately can be used to complement the previous data.…”

“…A comprehensive report on the geology and hydrology of the Pampa del Tamarugal, where the Salar de Llamara is located, can be found in ref and references therein. The most relevant studies of the hydrochemistry and mineralogy of the Huatacondo ponds were published by refs − . They show that the brines in these ponds are subject to a strong evaporation process that leads to the precipitation of calcite and gypsum, and they also show the evolution of the chemical composition of the brines with evaporation, modeling the equilibrium precipitation of mineral phases beyond the evaporation level of current puquı́o brines.…”

“…They show that the brines in these ponds are subject to a strong evaporation process that leads to the precipitation of calcite and gypsum, and they also show the evolution of the chemical composition of the brines with evaporation, modeling the equilibrium precipitation of mineral phases beyond the evaporation level of current puquı́o brines. Other than calcite and gypsum, they predict the precipitation of glauberite and the dissolution of gypsum in the more concentrated brines . The mineralogical analysis of salt crusts around the ponds, containing more soluble salts, shows the further precipitation of salts close to complete desiccation and from brines reaching the crust surface by capillarity…”