Bromine geochemistry of salar de Uyuni and deeper salt crusts, Central Altiplano, Bolivia

The salar of Uyuni, in the central Bolivian Altiplano, is probably the largest salt pan in the world (10 000 km2). A 121 m deep well drilled in the central salar disclosed a complex evaporitic sequence of 12 salt crusts separated by 11 mud layers. In the lower half of the profile, thick halite beds alternate with thin mud layers. The mud layers thicken upwards and show clear lacustrine features. The thickness of the salt beds decreases markedly from the base upward. omine content of the halite ranges from 1.3 to 10.4 mg/kg. The halite does not originate from the evaporation of the dilute inflow waters of the Altiplano, which would lead to Br content of tens of mg/kg. The presence of halite of very low Br content (2 mg/kg) in a gypsum diapir strongly suggests that most of the halite deposited at Uyuni originated from the leaching of ancient salt formations associated with the numerous gypsum diapirs of the Altiplano. The deep and thick halite beds were probably deposited in a playa lake, as suggested by their very low and fairly constant Br content (1.6–2.3 mg/kg) and by the abundance of detrital minerals. Thereafter, perennial salt lakes of increasing duration flooded the central Altiplano. Increasing amounts of dilute inflow waters of low Cl−/Br− ratio and the exhaustion of diapiric halite lead to an increase of both the thickness of the lacustrine layers and of the bromine content of the salt crusts (5–10 mg/kg) deposited as the lakes dried up. Paleolake levels in the central Altiplano rose as those in the northern Altiplano (the Titicaca basin) were simultaneously falling. The progressive fluvial erosion of the threshold between the northern and the central Altiplano lowered the levels of the northern lakes and allowed more dilute waters of low Cl–/Br– ratio to flood the central Altiplano. Therefore, both the thickness of lacustrine sediments and the bromine content of the salt crust increased to the present. The three upper crusts show a decrease in bromine content which may reflect a recent modification of the inflow regime to the last lakes. lt crust deposited by the last salt lake, 10 000 years ago, shows a bromine distribution that unexpectedly increases with depth. As the lake dried, the salt crust was probably initialy distributed over a wider area than the present flat crust, covering the topographic surface up to the level of the lake where halite reached saturation. The dissolution of the initial salt of low Br content from the margins of the lake to the center is probably responsible for the observed Br distribution in the surface salt, as in a playa. During the Holocene, a severe drought hit the Altiplano and drastically lowered the intracrustal brine table within the salt crust, leaving a thin layer of high-concentrated residual brine at the bottom of the crust. Bromine-rich halite replaced primary halite by a temperature-activated mechanism in the lower crust, further increasing the Br gradient with depth.