Where Does Lithium Come From?
There are three main sources of lithium globally: brines, pegmatites and clays. Around 51% of current production is from lithium brines with the remaining 49% from pegmatites. There is potential for lithium-bearing clays as a lithium source, but there has been no commercial production as of yet.
How do Lithium Brines Form?
Brines are formed in seasonally flooded dry lakes, called “salars” in South America. Salars are considered to be mature when they are dominated by evaporate minerals and it is these deposits that dominated lithium production prior to 2000. Sediment in immature salars is dominated by clay, silt, sand and gravel, and these deposits are dominating the new generation of would-be lithium producers.
A salar must have a source of lithium in order to contain lithium. Typically this is volcanic rock of high-silica composition that is commonly associated with hot springs. Other important features are topographic control for thick sedimentation and an arid climate where annual evaporation is greater than seasonal rainfall. Potentially economic lithium-bearing brine is formed when all favourable conditions exist.
How to explore a Salar?
Exploration begins at the surface. Pit sampling is inexpensive and provides information on lithium grade at the surface of the salar. It does not provide assurances that grade with continue at depth. Pit sampling is good for initial-stage exploration, providing an impetus to explore further.
After surface sampling, exploration turns to the sub-surface through geophysics, drilling and laboratory testing. Large salars can be assessed efficiently through various geophysical techniques. The thickness of the sediments in the salar can be assessed using gravity surveys due to the lower density of the sediments relative to basement rocks underlying the salar. Seismic techniques are useful in assessing the basement contact and the internal structure of the sediment package. Electrical techniques can help delineate the limits of the conductive brine bodies and to some extent the basement topography.
Drilling is the next logical step after surface sampling and geophysical work. Drilling provides information on lithium grade at depth but also importantly, it allows assessment of salar maturity, sediment composition, porosity and permeability. Sampling of brines at various depths will determine the extent of lithium grade variation and it can be achieved through bailer sampling or by pumping from specific intervals.
Drilling can be completed with diamond drilling, which provides the highest quality sample but with variable recovery. Sonic drilling provides the best sample but is the most expensive method. Aircore is fast and cheap, but sample quality is reduced. Rotary drilling is used to install production boreholes.
Pump testing from drill holes is the final step, to determine the longer-term performance, drawdown and grade.
Potential for Commercial Production
In addition to lithium grade and quantity, other factors are important in determining the potential for commercial lithium production from a salar. The ratio of lithium to magnesium (Mg) and sulphate (SO4) is equally important as it impacts brine processing; low Mg and SO4 relative to Li is a favourable characteristic. Porosity and permeability is also of critical importance for production.
Brine chemistry is variable within the South American salars and has important implications for processing. The extremely large Salar de Uyuni in Bolivia has a lithium grade in excess of 400 mg/l, but the Mg/Li ratio is 18.6, which is three times higher than the smaller Salar de Atacama in Chile. As a result, the Bolivian example is unsuitable for conventional processing whereas the Chilean example is producing today.