Salar de Atacama, Chile
Salar de Atacama is the lowest drainage sink in an endorheic basin in the pre-Andean depression of the Atacama Desert. It contains more than a kilometre-thick succession of interlayered halite and siliciclastics. At an elevation of 2,300 m above sea level with an area of 3,000 km2, it is the third-largest saline pan in the world (after Salar de Uyuni in Bolivia (10,582 km2) and Salinas Grandes in Argentina (6,000 km2)). The lake fill sediments are an excellent barometer of Pleistocene climate change in the southern hemisphere.
Mountains surround the Atacama drainage sump. The Andes' main chain lies to the east, while to the west lies a secondary Andean mountain range known as Cordillera de Domeyko. Large volcanoes dominate the landscape, including the Tumisa, Lejía, Miñiques, Licancabur, Acamarachi, Aguas Calientes and the Láscar; the latter is one of the most active volcanoes in Chile. All are located along the Salar de Atacama's eastern side, forming a generally north-south trending line of volcanoes that separate it from other hypersaline endorheic basins (Figure).
Today the San Pedro River forms an ephemeral stream feed along the western margin of the northern sector of the salar. It defines the edge of a saline mudflat dominated by sulphate, carbonate and siliciclastic sediments. The mudflat surface is dominated by brown halite-rich crusts and is underlain by silty clays with sand, displacive halite and locally organic-rich mud. A "sulphate marginal zone" dominated by ephemeral streams, eolian dune fields, sand sheets, dry mudflats and spring-fed ponds and marshes, forms the eastern edge of the salar and lies to the east of the saline mudflat. This northern region captures the majority of the siliciclastics entering Salar de Atacama.
View to the east from sulphate-rich marginal zone of Salar de Aracama with the Tumisa, Lejía and Miñiques volcanoes along the eastern skyline (image from Wikipedia - Francesco Mocellin)
Salar de Atacama, Chile. A) Saline pan facies associated with the San Pedro River's influx along the northeast side of the salar, the sulphate-rich marginal zone along the east and northeast of the salar and the salt-crusted halite facies dominating the southern portions of the salar. B) Laguna de la Piedra in the gypsum flat. Shows circular doline features indicating these zones of perennial brine ponding are karst associated. C) Laguna Tebiniquiche, the cuspate outline of the perennial water body, indicates dissolution karst feature. D) some of the potash/lithium pans on the halite nucleus and the brinefield wells and pipelines that feed the pans. Image scaled in MapInfo from a Bing® upload.
Lithium carbonate from salar brines
Salar de Atacama is the world's largest and purest active lithium source, containing 27% of the world's lithium reserve base (see Salty Matters, 30 July 2017). High lithium concentration in its brine (2,700 parts per million), a high rate of evaporation (3,500 mm per year), and extremely low annual rainfall (<30 mm average per year) make Atacama's finished lithium carbonate easier and cheaper to produce than from neighbouring salars.
Salar de Atacama's evaporation rate is the highest in the lithium industry, followed by Puna de Atacama, Argentina (2,600 mm per year), and the Salar de Uyuni (1,300–1,700 mm per year). The extraction of lithium-rich brines is causing conflict with water use by local communities and is possibly damaging the ecosystem, including the Andean flamingo.
Boron is also extracted from Atacama brines as boric acid (up to 0.85 g/l as B). The natural removal of boron and lithium from present-day brines possibly occurs as ulexite and lithium-sulphate, the latter as double or triple salts.
Production at Salar de Atacama, Chile. A) View of a brine field where saline brine is pumped to the surface and into nearby solar evaporation pans. B) Solar evaporation pans, pans showing yellow-green coloration where liquours are approaching lithium carbonate saturation (≈6000ppm) and are ready to be pumped to the brine processing plant.
Hydrology across the salar
Typically, the watertable is between tens of centimetres and several meters below the deflationary salar surface. Uppermost sulphate zone sediments are generally covered by and cemented with efflorescent sugary gypsum and minor halite. When trenched, these gypsum crusts form discontinuous bowl-shaped concave-upward layers tens of centimetres in diameter. A deep core (No. 2005) through the halite nucleus facies recovered some 100 meters of evaporitic sediment and sampled some 100,000 years of salar sedimentation. Deposition was dominated by arid conditions similar to today, with two significant wetter intervals. From 100 m to 62.8 m, the core is composed of clear interlocking halite with patches of millimetre-scale sugary halite. These textures are near identical to those forming at the surface today and indicate dry, mostly subaerial accumulations. From 62.8-47.0 m (75.7-60.7 ka), there is a vertical succession passing from mudflat gypsum to mudflat/shallow pond to subaerial halite and mudflat deposits that represent a progressive increase, then a decrease and finally an increase in the water supply to Salar de Atacama. This occurred within what were moderately wetter conditions (saline lakes and saline mudflats) than the subaerial conditions of today. A similar somewhat wetter interval occurred from 53.4 to 15.3 ka, with the wettest perennial lake interval from 26.7 to 16.5 ka. Short relatively wet periods (chevron halite) also occurred in the Holocene from 11.4 to 10.2 ka and 6.2 to 3.5 ka. The remaining sections are dominated by halite textures similar to today's subaerial-dominated capillary fringe conditions (see Warren, 2016; Chapter 3 for detailed literature compilation).
Interactions between the Salar de Atacama fault zone and halite sedimentation A) Interpreted seismic line ig 022 across the Salar reverse fault showing monoclinal flexure and thickening of beds across the fault. Fault trace dies out before reaching the current halite surface. B) Summary of halite facies in sequence 8, documented in SQM cores. Boreholes 2031 and 2005 are located west of the Salar fault system, whereas borehole 2002 is located east of the Salar fault system. U-series dates from boreholes 2005 and 2002, and comparisons of halite depositional environments at the three sites allow temporal correlation among the cores. The top of the perennial saline lake facies, dated as ca. 16.5 ka, is readily identifiable in all three locations. That horizon occurs at similar depths in cores 2005 and 2031 but is vertically displaced ≈28 m across the Salar fault system. C) Conceptual model of enhanced halite accumulation east of the Salar fault system, which leads to a horizontal depositional surface across the fault trace. Under the conditions of a desiccated surface environment, halite accumulates because of the evaporation of groundwater-fed brines. A drop in permeability at the fault, (either due to juxtaposition of lower permeability rocks in the western fault block or due to properties of the fault zone itself) blocks the westward flow of topographically driven groundwater, forcing a high rate of upwelling in the downthrown eastern fault block.
Seismic collected across Salar de Atacama shows a north-northwest–striking fault system that extends for at least 30 km across the salar and can be seen intersecting the lower part of the halite nucleus (Jordan et al., 2002). In the southern sector of the salar, there is at least 700 m of Pliocene–Quaternary down-to-the-east reverse motion across the fault, but locally the salar fault system deforms the lower halite unit into a monocline with 400 m of structural relief. Rates of displacement throughout the Quaternary have been on the order of 0.1– 2.0 mm/yr. And yet, despite these large displacements and high displacement rates so evident in the seismic, no modern topographic scarp exists in the halite nucleus. Seismically-defined stratal geometries and thicknesses reveal variation between former times when there was a topographic scarp at the deformation zone and times, like today, with no topographic expression. The available borehole data suggest that both the times with and without a topographic scarp were likely dominated by halite accumulation adjacent to the salar fault system.
Jordan et al. (2002) propose that unique sedimentation processes in a groundwater-fed evaporite environment are responsible for the lack of topographic expression of a major fault system that intersects the salt. During desiccated times, groundwater derived from the topographically-driven flow on the east side of the salar would have been forced upward by a barrier created at the fault zone, enhancing the supply of solutes to the region immediately east of the fault. This mechanism generates a higher rate of accumulation of halite east of the Salar fault system than to its west. The enhanced deposition on the eastern fault block is sufficient to prevent scarp formation during times of moderate displacement by the Salar fault system, though during times of unusually high rates of offset, this depositional mechanism was not sufficient to prevent the temporary formation of scarps. It is not yet known how common ‘‘truly blind’’ faults are beneath the dry salt pans of other salars, but they are not unique to the Atacama basin. Similar occurrences of blind faults beneath the depositional surface of the ephemeral saline pan facies characterise; Salar de Antofalla and Salar de Hombre Muerto in the Altiplano, along with the Badwater salt flat in Death Valley, California and the Great Salt Lake in Utah (Jordan et al., 2002). Like Salar de Atacama, they indicate the rapid deposition/accumulation rates that typify brine-saturated salt accumulating depressions.
Abandoned saltpetre processing plant, Atacama Desert