by Roland Gotthard, Director of Playa One Pty Ltd
For geological units that amount to little more than muddy ponds, salt lakes are perhaps some of the most economically important geomorphological features on the planet. This is because of the presence in the Andean Cordillera, of brines containing lithium within some salt lake systems. Some ten million tonnes of lithium reserves are available in the salt lakes of Chile and Argentina, representing about USD 0.75 trillion in lithium carbonate equivalent at today’s prices. This is about 400 million ounces of gold equivalent. Ten years ago, however, the same lakes were worth 1/8th as much, or less.
With the economic importance of salt lakes demonstrated, it’s time to pay a bit more attention to these geological formations that, for a lot of geologists and explorers, are something you drill through to get to the ore, or drive right around on the way to drill for ore. Salt lakes clearly deserve far more of our attention, in Australia and worldwide, if we are to meet sustainable and circular economic models in the future. This article will serve as a brief introduction to their geology and economic potential.
Geology and Hydrology
Salt lakes are evaporitic systems formed in endorheic basins – watersheds which have no exit – also known as saline sumps. Salt lakes can be several meters across, to vast areas such as Salar de Uyuni at over 10 000 km2 (3861 mi2). Whilst sodium chloride salt is a ubiquitous component, salt lakes are highly variable in mineralogy, with almost no two lakes identical.
Salt lakes themselves can also vary meter by meter in mineralogy, chemistry and in terms of water chemistry. It is not unknown for super acidic pH 1.5 water to exist several meters away from pH 11 groundwater.
Water that falls into the drainage basin, or enters via groundwater discharge, leaves only via evaporation, and when evaporation rates outweigh rainfall, this will result in salt lakes of some form or another.
Water interaction with the rocks of the drainage basin controls groundwater chemistry. As bedrock is weathered to clay, alkalis (sodium, calcium, potassium) and other soluble elements (magnesium, chlorine, sulphur) are carried into the sump via subsurface flow through the soil and aquifers.
Dilute groundwater is concentrated by evaporation, forming minerals. The chemistry of the brine solution determines a series of minerals which precipitate, with the first usually being calcite. Thereafter, Mg+Ca:SO4 ratios determine if magnesium minerals or gypsum precipitate next, and this determines the evolution into alkaline, sulphate or calcium brine end-members.
The evolution of brine mineralogy is somewhat regular and predictable, with diagnostic mineral assemblages precipitated in the drainage basin allowing brine chemistry to be deduced from the salt and sediments. These brine trajectories also control the potential of a groundwater tract to contain economic mineralization.
Economic potential
Economic components of salt lake systems occur in three parts: the aquifers wherein salts are carried dissolved in brines; salt encrustations and efflorescences at the surface; and the solid residuum within the sedimentary succession, including salts, muds, clays and minerals within and around the lake beds.
Brines within salt lake systems contain one of the most important elemental concentrations on the planet – lithium in brines hosted within salt lake aquifers. However, other important elements are extracted from brines, such as sulphate of potash (K2SO4), muriate of potash (KCl), sodium carbonate and sulphates, borates, and table salt.
Important by-products of brine production, in certain areas, include bromine and iodine. The world production of bromine is dominated by the Dead Sea, in Israel. Metal is obtained from brines, either as magnesium produced from bitterns, and sodium and chlorine which is obtained by electrolysis of sodium salt.
Brine extraction involves pumping the aquifer brine into evaporation ponds, where the sun is used to evaporate the brine to saturation with salt (normally NaCl) which is then removed, and the resulting brine concentrated to precipitate the next salt such as potassium sulphate, with the residual bitterns enriched in magnesium. The same process is used to concentrate lithium brines.
Salt encrustations, usually dominated by simple table salt (NaCl) are mined from the surface of natural and artificial salt lake systems. Artificial salt lakes, such as the Dampier, Shark Bay and the proposed Mardie salt operation, evaporate ocean water to produce salt, however in some areas salt is mined from natural accumulations within salt lakes.
The solid residuum of saline groundwater systems and salt lakes can contain significant mineralization in the clays, fringing aprons of carbonates, and even in dunes of salt blown off the surface of the lakes.
Western and South Australia’s world-class endowment of high-quality gypsum mineralization is produced by ablation of salt efflorescences (gypsum crusts and crystals) formed on lake surfaces during hot summer months, with prevailing winds pushing the fine gypsum crystals into complex kopi dune systems. These accumulate on the down-wind margins of the salt pans, and can constitute hundreds of thousands of tonnes of 99% purity gypsum.
Within the fringing aprons of carbonate minerals that occur in the upstream and upper watersheds of salt lakes, calcrete and dolocrete may form significant cementation within sediments. Magnesite can form within the salt lake uplands as the groundwater chemistry evolves, and can even form evaporite crusts in certain alkaline lakes.
Calcretes can host mineralization, such as uranium and vanadium. Here the groundwater evolution of the aquifer from oxidized to reduced conditions occurs in concert with deposition of calcium carbonate; uranium and/or vanadium in the palaeochannel is then deposited within the pore space. Australia’s vast, ancient regolith systems dominated by uranium-rich granite, which feed into palaeochannels and eventually salt lakes, contain world class uranium deposits.
Clays within salt lake systems may also be important targets for economic mineralization. Groundwater compositions within salt lake systems can reach extremes unknown elsewhere in nature, such as extreme acidity or alkalinity, and extreme salinity. These are highly unusual fluid conditions which can result in alteration to unusual mineralogy. Two prime examples of the influence of extreme fluid compositions in saline environments are the mineral polymorphs halloysite, and palygorskite. Halloysite, a tubular form of kaolin, forms in acidic groundwater conditions, and deposits are known underneath acidic salt lakes in South Australia. Palygorskite, a tubular morphology of attapulgite-type clay, forms in highly Mg, Ca and Cl enriched saline lake and palaeochannel systems.
Exploration opportunities
Salt lakes contain 75% of the world’s lithium endowment and will remain an important source of the metal for decades to come. Exploration for lithium in brines worldwide is relatively mature but commissioning and proofing of evaporation processes is technically complex, and much work remains to exploit known but uneconomic brines that have not yet been proven up.
Brine abstraction and evaporation is a small but growing source of potassium sulphate in Australia and globally. Massive resources of potassium sulphate are reported from the Danakhil Depression in Ethiopia with more than 200 years of resources identified.
The exploitation of magnesium from brines in saline systems looks set to have a renaissance after the 2021 magnesium metal supply crunch. Magnesium brines in the USA, Canada and elsewhere are capable of producing magnesium chemical precursor salts as feedstocks for magnesium metal with lower carbon footprints than the current carbon-intensive Chinese smelting. Significant potential also exists for magnesium by-products in the Danakil Depression, with millions of tonnes of MgCl identified.
The economic potential of the solid residuum of salt lakes is poorly understood and woefully underappreciated. The chemical evolution of brines, saline groundwaters, and both alkaline and acidic chemistries, provides opportunities for concentration of many valuable elements, from transuranic elements, chalcophile elements, to those sensitive to eH and pH changes. Analogues of other clay-hosted deposits of lithium, magnesium, potassium and other critical elements are all potentially extractable from salts, clays, and carbonate aprons around salt lakes. Thorough understanding of the co-evolution of brine chemistry and mineralogy of sediments shows potential for identifying critical minerals – an example being halloysite and palygorskite.
Environmental considerations
Salt lakes are relatively sterile environments due to the harsh chemical environment, with extreme salinity, acidity and alkalinity possible – often wildly varied even within the same lake-bed. They are also, relatively speaking, quite durable environments capable of literally swallowing up almost any disturbance humanity wishes to inflict upon them – including whole bulldozers at times. On this basis, they can be prime locations for extracting minerals with lower ecological costs than other landforms.
Salt lakes and evaporite deposits within them are also quite young and replenish relatively quickly. The age of residuum in Australian salt lakes is from less than 2000 to 12 000 years, postdating the last glacial maximum. Evaporitic deposits have formed quickly and will replenish quickly if left to do so.
However, salt lakes are sensitive environments particularly regarding flux of water. One needs only look to the Great Salt Lake of Utah, where excessive use of surface and groundwater threatens to dry out the salt lake entirely, which may precipitate an almost unimaginable disaster. This threatens the survival of brine shrimp and migratory bird populations that rely upon them. Dusts from the lake bed carry heavy metals, and if they blow into the urban areas, catastrophic damage could be inflicted. Examples of salt lakes gone bad include the Salton Sea, in California, which is now a horrific alkaline wasteland due to over-extraction of water for agriculture.
Lakes are also not completely sterile. Extremophile bacteria thrive, and one only needs to visit Lake Hillier in Western Australia, to understand that salt lakes can be extraordinarily striking places to visit. Balancing tourism, environmental preservation and mineral extraction is always delicate. However, there are over 11 000 lakes in Western Australia alone, so there are plenty to choose from.
It is clear that one should visit salt lakes, and take what one needs, but they are environments which should be managed carefully, because they are such extreme environments, and such extreme concentrations of chemicals, salts and metals. Sustainable mineral extraction in these environments requires care and cannot occur without a careful understanding of hydrogeology, hydrology, and competing water uses.
Conclusions
Salt lakes are clearly extraordinary reservoirs of economic potential that, unlike other ore bodies, one can quite literally stumble across and walk upon billions of dollars of minerals – and you may not realise it. Salt lakes contain the key to a low-carbon future, both in terms of their endowment in lithium, but also in lower-carbon magnesium, uranium and are important sources of halides, borates and potassium.
Responsibly handled, mining on salt lakes can contribute materials for our modern society and future needs. We have only just scratched the surface of these fantastic formations in Western Australia and elsewhere.
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