Executive Summary
Lithium is forecast to experience a 40-fold demand increase by 2050 (Figure 1); predominantly because of the increased demand for this metal in electromobility. Lithium is mined either from brine or ore; the latter is predominantly from pegmatites. Sedimentary deposits, however, account for approximately 8% of the known global lithium resources and are currently relatively under-explored. Similar to other mineral systems, the processes and elements required for the formation of non-marine, sedimentary lithium deposits are well understood (Figure 2) and therefore sites for accumulation are predictable. Given this and the ever-increasing demand for lithium, Getech has developed a mineral systems approach to the initial global screening process to determine locations where conditions were favourable for the accumulation of lithium-rich sedimentary deposits in onshore environments. This novel method relies heavily on the input of Getech’s GlobeTM, which is a global database housing ~40,000 data layers from the Permian to the Present Day (see Figure 3 for example data). The layers used from GlobeTM include, structural elements, depositional systems, uplift events, digital elevation models, and palaeoclimate models (biomes, precipitation minus evaporation, atmospheric wind data and run off) and derivative layers. For example, tracking volcanic ash particles using the uplift events and atmospheric wind data, and understanding the timing of the last uplift in millions of years from the time slice of interest. The methodology was applied over twelve timeslices for the Cenozoic (see Figure 10 for representative result); the predicted accumulations were then compared against known deposits in the geological record with positive results. This study confirms that combining Getech’s in-house global datasets and sophisticated analytics to evaluate already covered search spaces, or currently unknown exploration targets, is a good way of screening for palaeo, non-marine sedimentary accumulations. Furthermore, this methodology is quick.
Introduction
To ensure that the goals of the Paris Agreement are met, there needs to be a reduction on the reliance on petroleum usage and a prominent increase in renewable energies. The latter relies on critical minerals such as copper, nickel and lithium for energy storage1,2. Lithium in particular is commonly used in rechargeable batteries for a multitude of electronics, most notably, electric vehicles3. Ideally, electromobility needs to be the dominant transport type in the future as is vital for the much-needed reduction of transport-related emissions. In 2020, 34% of the lithium demand was for electric car batteries, however, this is expected to rise by 2030 to 75%1. In fact, the demand of lithium globally is set to increase 40-fold by 2050 (Figure 1;2). Finding new resources of lithium is time-critical as it only takes a few years (two to three) to build and optimise production at an EV plant, but it takes longer (five to ten years) to build and optimise a mine1. Used lithium can be recycled; however, there is no standardised policy for this as yet4 and we are decades away from this being accomplished on a tangible scale. Clearly, there is an immediate requirement for lithium as a primary commodity, which is already reflected in the recent increase(s) in lithium prices, and these prices are expected to remain high for the foreseeable future1,3.
Mined lithium is either from ore or brine. Lithium ore is predominantly from pegmatites. However, about 8% of known global lithium resources are from sedimentary deposits5. As with most sedimentary resources, understanding the pathway to its accumulation is the key to predicting where any lithium accumulations will occur. In light of this, Getech has adopted a novel mineral systems approach to predict the favourability of sedimentary, non-marine, lithium-rich accumulations globally in preparation for meeting the lithium demand. The method relies on using data obtained from Getech’s Globe™ knowledge base which is a global geodatabase comprising ~40,000 data layers from the Permian to the Present Day.
Sedimentary Lithium Formation
The lithium required for sedimentary lithium deposits is predominantly derived from brines, salars, pegmatites, or existing sediments. Sedimentary lithium is found in clay deposits (e.g., hectorite) which are typically preserved in lacustrine sediments that are cogenetic with lithium-rich evaporites derived from brines6. The common characteristics of all currently commercially viable present-day lithium brine deposits in the world are an arid climate, a hydrologically closed basin hosting a playa lake, tectonically driven subsidence, associated igneous or geothermal activity, a suitable lithium source (typically volcanic lithologies), and time to concentrate brine7,8. The range of lithium sources, pathways, and sinks within closed basin systems are outlined in Figure 2.
The lithium in sedimentary deposits is typically sourced from the dissolution of primary lithium-bearing igneous rocks6,8. Intrusive lithologies such as pegmatites and extrusive felsic volcanic lithologies (e.g. rhyolite, trachyte) contain lithium-rich minerals such as spodumene (LiAlSi2O6), lepidolite (K(Li,Al)3(Al,Si,Rb)4O10(F,OH)2), or petalite (LiAlSi4O10) which provide lithium ions from subaerial erosion and weathering as well as directly from ashfall9,10. Once released from the primary lithologies, lithium ions are transported by water, often in the form of lithium-containing brines. Interaction of basin-fill sediments with geothermal and hydrothermal fluids is also shown to be an important process in the development of lithium-rich brines and subsequent deposition11.
The formation of sedimentary lithium deposits is closely linked to the presence of hydrologically closed (endorheic) basins, which are essential for the accumulation of lithium-rich brines8,10. These basins effectively trap the brine and, where appropriate arid climatic conditions are present, allow for evaporation to occur. In this scenario, as the brine becomes increasingly concentrated, lithium salts such as lithium carbonate can precipitate, and lithium ions can become absorbed onto lacustrine clays in these basins6,11.
Extraction of lithium from sedimentary deposits requires less energy than pegmatite deposits and are typically higher grade than brines and intrusive deposits. They are, however, currently less well known and less frequently explored for.
Getech’s Globe
Getech has developed Globe™, a geoscience knowledgebase for ArcGIS that elucidates Earth’s geological evolution over the past 300 million years. The sedimentary lithium screening utilises information contained within Globe’s palaeo-atlases; these data are available for every Geological Stage from the Permian to the Present Day (58 in total). For each timeslice, the atlases comprise (amongst other layers), the tectonic plate configuration, the structural framework, palaeoenvironments, drainage networks (rivers and basins), digital elevation models (DEMs) and the results from the Earth System Modelling (see Figure 3 for example data). The palaeoenvironments comprise Tectonophysiographic Terranes (TPTs) and gross depositional systems. The TPTs are related to a specific tectonic regime, defined by a series of mantle and crustal processes, or driving geodynamic forces, and may be active or inactive. A general circulation model uses Getech’s DEMs as the boundary conditions resulting in climate layers such as, atmospheric modelling, wind speed and direction, surface temperature, ocean circulation, and the dominant biozones, to name but a few.
As the formation of sedimentary lithium is (broadly) well-constrained, we can apply this knowledge and look for favourable conditions using the GlobeTM layers (or their derivatives). Therefore, the GlobeTM layers can be used as the sole input for the lithium global screening to produce rapid results.
Lithium Sources and Transport
As discussed above, both intrusive and extrusive igneous rocks can contain lithium-bearing minerals. Lithium concentrations are highest in felsic rocks, followed by intermediate rocks, and mafic/ultramafic rocks are lithium poor3. Volcanic ash and tuff, ignimbrites, and lavas (e.g. rhyolite) all locally increase the lithium content within the sedimentary basin via meteoric waters6,9. The denudation of old volcanic landscapes can also locally increase the lithium into a basin via drainage networks8. Additionally, if there are drainage networks which travel across active faults, then any lithium-rich magmatic fluids which have been upwelled from depth will also locally increase the lithium concentration12. The transported lithium needs to be within an isolated basin (under evaporitic conditions; see Lithium Accumulation) for it to amass, ideally draining into playa lakes / salars. As the salars are buried over time, the lithium can be recycled within the sediment and indeed also migrate upwards via active faults (as illustrated in Figure 2).
Clearly, determining the location of volcanic sources is crucial. In the absence of an igneous rock database however, which for it to be valuable requires detail on information such as timings, extent, weathering, and mineralogy etc, then the palaeoenvironments can be repurposed by extrapolating the type of volcanics from the tectonophysiographic terranes (TPTs). This is because felsic volcanics are more prevalent in certain TPTs, settings such as continental-oceanic collision (e.g. Andean margin), or continental–continental collision (e.g. the Himalayas) and are globally mapped as standard for the GlobeTM product. Therefore, each TPT has been classified into likelihood of lithium enrichment (Figure 4).
Rivers have the potential to transport lithium over great distances12. The TPT volcanic classification, therefore, has been used in conjunction with the drainage network to determine where there are likely pathways for lithium transport. We modelled the potential increase of lithium from drainage networks by comparing the spatial locations of rivers and active volcanic terranes and where these intersected, we traced the rivers downstream to the depositional areas (Figure 5).
As discussed above and shown in Figure 5, active structures are also a potential source of upward migrating lithium-rich fluids. Any rivers that intersect these structures can transport the lithium long-distances, therefore, a similar approach to the eroding volcanics and rivers was adopted for the rivers and the structures (Figure 6).
Lithium derived from proximal volcanic ash fallout contributes to the local basin and, depending on atmospheric circulation, can contribute over much larger, certainly continental, distances8,9. The Earth System Modelling data contains atmospheric wind datasets (velocity and direction), and when combined with the volcanic layers, and the Particle Track Tool (ArcGIS), areas of potential ash fall are modelled (Figure 7). Although dependant on the size of the particles, the explosiveness of the volcano, and other factors, there is still an assumption that the more proximal to the volcanic source the more chance of having increased lithium form the volcanic ashfall, therefore we split the results into three categories based on distance (< 250 km, 250–500 km and 500–750 km; Figure 7).
An additional layer that is derived from the palaeogeographies is the last activity map (Figure 8). This determines how long ago (in millions of years) when an area was last affected by an uplift. This is used as a proxy for heat production at depth, a potential driver for circulating and expelling lithium enriched fluids from depth12. Classification is based on lithospheric thermal time constraints assumed to be 60 Ma or less13.
Lithium Accumulation
Several factors influence the accumulation of non-marine evaporitic deposits. Of these, climatic conditions play a crucial role. Evaporite deposition invariably occurs in regions with high evaporation rates, low rainfall, and limited runoff. This is evidenced in Present-Day known evaporite-hosted lithium deposits, which are always encountered in cool arid environments7. Directly using palaeoclimate data layers, a series of inputs have been created which indicate:
- Locations where net evaporitic climate regimes enable concentration of brines and precipitation of lithium salts (Figure 9: left).
- Areas where limited total runoff is observed. This lowers the risk of diluting ponding brines whilst maintaining sufficient flux to replenish lithium and enable thick evaporite accumulations.
- Regions where overall climatic conditions are favourable for non-marine evaporite accumulation (e.g. arid steppes and desert7) using the Köppen Classification scheme which divides global climates into groups based on seasonal precipitation and temperature patterns14 (Figure 9: right).
Figure 9: Example palaeoclimate data layers from the upper Cenozoic used to constrain potential accumulation regions; net evaporation (left) and Köppen Classification (right).
Endorheic basins, where water cannot easily drain away, are common sites for lithium-rich evaporite and sediment accumulation. These internally draining basins trap water and allow evaporation to concentrate dissolved minerals, resulting in the precipitation and accumulation of evaporite sequences. Using paleogeographic reconstructions and palaeo-drainage networks, internally draining basins can be mapped through geological time, providing the location and extent of potential sites of non-marine evaporite accumulation.
Additionally, tectonically-driven subsidence creates accommodation space for the accumulation of thick sequences of basin sediments and brines, with many non-marine evaporite deposits accumulating in active continental rift and strike-slip basins15. Using Getech’s global structural datasets with known activation histories enables determination of active tectonic systems (normal, strike-slip and transtensional fault structures) and proximal regions where accommodation space for lithium-rich sediment accumulation through time could be generated.
Input Data
As outlined above, many inputs govern the enrichment of sedimentary lithium. The GlobeTM data and any derivatives used for the modelling are shown in Table 1.
Results: Lithium Play Map
Employing Exploration Analyst, a geospatial risk mapping software developed by Exprodat (part of the Getech Group), and using the input layers, a global ‘play map’ displaying the relative favourability for sedimentary lithium accumulation for each geological timeslice is produced (Figure 10). These maps are validated using known and dated sedimentary lithium occurrences in the geological record to provide further confidence in the results, with the weighting of input parameters modified accordingly. Using this method, the favourability for sedimentary lithium accumulation at a given point on Earth through geological time from the Pleistocene to the start of the Permian can be determined.
Conclusions and Further Work
This mineral systems approach study demonstrates the global screening for non-marine sedimentary accumulations in the geological past. Sophisticated analytics coupled with Getech’s in-house, comprehensive global dataset has allowed for a system where existing targets can be re-evaluated, and future exploration can be guided to help fulfil the ever-increasing demand for lithium. Furthermore, post any initial screening, regional, detailed studies can be applied to further enhance and reduce exploration risk. The workflow could include a higher resolution structural framework, a detailed, fully attributed volcanic layer, and more integration of the surrounding timeslice(s) for determining: 1) the potential of lithium from older rock units; 2) a better model input for the palaeo-heatflow and; 3) potential preservation of palaeo-lithium accumulations.
Written By David Lee, Meg Galsworthy and Howard Golden
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