Geology and resources of Salars in the central Andes

Salars of the central Andes constitute one of the world's major continental evaporite complexes that contain significant portions of the known resources of lithium and boron.At least 100 salars and saline lakes having areas ranging from a few square kilometers to about 9,000 km^ are found in individual closed basins within a 400,000-km2 area of internal drainage in northwestern Argentina, western Bolivia, northern Chile, and southern Peru.An arid climate persisted in this region during most of late Tertiary and Quaternary time.Compared to glaciation in the Andes mountains to the north and south, Pleistocene glaciation in this region was scanty.Major Andean uplift took place during the late Tertiary and Quaternary, and this period was a time of intense volcanism.The basins in which the salars and saline lakes are found were formed both by faulting and by volcanic eruptions.The volcanic rocks are the major sources of the saline materials in the salars.Most of the salars formed by desiccation of saline lakes during the Holocene and by subsequent deposition of saline materials from evaporation of spring water, ground water, and annual or intermittent flood water.Some closed basins have had complex histories dating back to the late Tertiary or even to the late Cretaceous.The salars consist of hard to soft saline crusts that overlie sequences of lacustrine sediments.The saline crusts, generally not more than a few meters thick, contain a variety of saline minerals, of which halite and gypsum are most abundant.Mirabilite, thenardite, and ulexite are found in moderate to minor amounts in nearly all salars.Several other saline minerals are present in minor amounts in many salars and some of these are moderately abundant in a few salars.Five general types of salars can be recognized: (1) zoned salars consisting of an interior halite zone surrounded by a sulfate zone; (2) gypseous salars consisting chiefly of gypsum and lesser amounts of other sulfate minerals, halite, and ulexite; (3) salars having crusts of silty halite that formed chiefly by capillary evaporation of near-surface ground water or brine in lacustrine sediments; (4) salars consisting chiefly of mud flats subject to annual flooding and having thin ephemeral crusts of halite or sulfate minerals during dry seasons; and (5) Salar Grande, in the coastal region of northern Chile, which is a 40-km-long valley filled with coarsely crystalline halite to depths of as much as 160 m.Although the salars probably have been exploited for rock salt since Pre-Columbian time, systematic exploitation did not start until the 19th century when several salt quarries were opened in Salar Grande, Chile, which prior to the 1880's was in Peruvian territory.Ulexite was mined in several salars before the end of the.19th century.Small amounts of NaN03 were recovered from salars in the coastal region of northern Chile during the late 19th century and early 20th century.In the early 1940's, several thousand tonnes of KC1 and KN03 were produced from the K-rich saline crust of Salar de Bellavista, in the coastal region of northern Chile.Since the 1950's, the salars have been exploited as sources of rock salt, borate minerals, and Na2S04.Production of Li 203, using Li-rich brines from Salar de Atacama, Chile, began in 1984.