Geological and Geochemical Indicators of Lithium Prospectivity in Northern Gondwana Sedimentary-Basin Brines

1. Introduction

Lithium (Li) is a crucial element that has emerged over the past decade as an essential component in sustainable energy solutions. Traditionally, Li has been sourced from conventional mining operations, primarily from spodumene-bearing pegmatites [1] and surface brines, particularly in South America (Andean salars: [2]). Recognizing the limitations of conventional and salar-based Li extraction, there is a growing interest in exploring alternative brine resources, such as sedimentary basin brines and geothermal fluids which can contain significant Li concentrations [3].

Li can be introduced to and concentrated in pore waters and sedimentary basin brines by a variety of mechanisms (Figure 1). These mechanisms include deposition of volcanic aerosols and ash into surface waters (marine or lake) subjected to evaporation, water-rock interaction, kerogen maturation and Li-rich hydrothermal fluids derived from deep crystalline lithologies [4,5,6,7,8,9,10]. Once Li is introduced into surface or subsurface brines, it tends to stay dissolved in aqueous solution due to its small size, ionic stability and high mobility.

The aim of this paper is threefold: 1) to review processes of introduction and enrichment of Li into sedimentary brines, 2) to develop a conceptual, yet comprehensive, workflow of geological, hydrogeological and geochemical factors that influence Li concentrations in brines, and 3) to investigate genetic relationships between Li and routinely reported water-chemistry data in published Li-bearing brines, thereby providing an analogue-based screening tool for basins where direct Li measurements are absent. We apply this conceptual workflow to the Northern Gondwanan Region (NGR; North Africa, Arabia, and part of Iran) to evaluate Li prospectivity using sedimentary formation-brine data [10,11,12,13,14,15,16,17,18]. Relationships reviewed here and discerned from our evaluations may also be applicable to other basins with similar geological and hydrogeological settings. The workflow is intended to prioritize direct analysis and does not predict Li concentration or establish a resource.

2. Sources of Li in Sedimentary Brines

The initial step to enrich sedimentary basins brines with Li requires the release of Li into the water phase through processes of volcanism, weathering of igneous rocks, and leaching from clay-rich sediments (Figure 1; [19,20,21]). Li can be introduced directly into the surface waters by volcanic aerosols and ash from felsic (i.e., rhyolitic) eruptions [4]. The effectiveness of this mechanism depends on the concentration of Li in the volcanic rock, distance between volcanic source and sink, explosiveness of the volcanic eruption, atmospheric transport patterns, and ash-particle size [22]. Over geological timescales, weathering of volcanic ash and other volcanic rock forms clay minerals such as kaolinite, smectite, and montmorillonite [10]. This process releases Li ions from the primary volcanic minerals, trapping them loosely into clay mineral structures, from which they can be easily leached out into the water phase [4,23].

Li is also present in other felsic rocks such as granites and granitic pegmatites within minerals such as spodumene, lepidolite, petalite and tourmaline [24]. Surface weathering can release Li ions from these minerals and from other felsic-derived clastic sediments into surface and groundwater [25]. Additionally, organic matter such as kerogen, which is typically deposed in reducing environments, can also be a significant source of Li in sedimentary basins (Figure 1; [4,6,9,19,26]). This relationship is indicated by positive correlations between total organic carbon (TOC) and Li in hydrocarbon source rocks in several US petroleum basins [27,28,29].

3. Key Mechanisms for Li Enrichment in Sedimentary Brines

3.1. Evaporative Enrichment

The formation of Li-rich subsurface brines has long been associated with evaporation of surface waters in hydrologically closed basins, particularly under arid climatic conditions (see Figure 1; [7,26,30,31,32,33,34,35]). For example, extreme evaporation in closed basins can enhance Li concentration in seawater by approximately 110 fold (e.g., from 0.18 ppm to 20 ppm; [31]). Therefore, this mechanism alone is not a prerequisite for economic accumulation of Li.

3.2. Water-Rock Interaction

Water-rock interaction is an effective mechanism for Li enrichment in brines through, desorption, ion exchange, hydrothermal alteration, and mineral replacement [36,37,38,39,40,41,42,43,44,45,46,47]. The effectiveness of Li enrichment is tightly linked to water-rock reaction rates, which are pressure and temperature dependent. A principal driver of subsurface pressure and temperature variations is basin subsidence and tectonics, and their impact on the rocks and fluids within them translates into an impact on Li enrichment. For instance, the transformation of low-temperature, Li-rich clay minerals (such as smectite and montmorillonite) into the higher-temperature mineral illite can release significant quantities of water-soluble Li and B to ambient pore water/brine [4,5,26]. Additionally, fractures of Li-rich crystalline or sedimentary rocks can increase the pores surface area, enhancing the capacity of hot formation water to leach Li [48].

3.3. Migration of High-Temperature, Deep-Sourced Fluids

Fault and fracture corridors represent major conduits for vertical fluid flow in the subsurface under suitable stress and pore-pressure conditions [49,50]. Several of these deformational events are interpreted to have occurred through reactivation of pre-existing, basement-involved faults [51,52]. Basement-involved faulting and fracturing can increase basement-rock permeability [53], providing conduits for the migration of Li-rich hydrothermal fluids to shallower sedimentary aquifers [8,54,55,56]. Due to the conductive nature of fluids, the existence of large-scale pathways in the form of faults and fracture corridors can results in not only the enrichment of Li concentration, but also the introduction of Li into an otherwise Li-poor brines [48].

3.4. Organic Matter Maturation

Increasing temperature associated with burial organic-rich lithologies can drive the release of Li and B during maturation and hydrocarbon expulsion [4,19]. Maturation-driven release of Li is controlled by TOC, kerogen type, maceral fabric, and mineral-matrix interactions [9]. Hydrous pyrolysis on organic-rich mudrocks has shown that organic-matter structure and mineral association can significantly influence petroleum-generation kinetics and reaction timing, thereby affecting the release of Li [57].

4. Materials and Methods

4.1. Study Design and Data Basis

This study applies a retrospective, indicator-based comparison to archived water-chemistry data from three NGR carbonate-hosted aquifers and to published regional and global analogues. The aquifers are identified by stratigraphic age as Triassic (n = 6), Jurassic (n = 6) and Paleogene (n = 3). The evaluated dataset comprises total dissolved solids, major ions, K, B, Br, δ18O and δ2H where available (Table A1). Li concentrations are not included in the evaluated aquifer dataset. No Li concentration is estimated or imputed.

4.2. Hydrochemical and Isotope Evaluation

Major-ion compositions were evaluated using a Piper diagram to identify hydrochemical facies. Chloride and bromide concentrations were compared with the seawater-evaporation trend of Carpenter [31] and with published sedimentary-basin brines to distinguish residual evaporative components from halite dissolution and subsequent water–rock interaction. Stable-isotope compositions were compared with the Global Meteoric Water Line and a seawater-evaporation trajectory to assess evaporative evolution, meteoric mixing and possible fluid-rock exchange. Concentrations reported in the source datasets were expressed in mg/l; values were used as reported and missing variables were not imputed.

4.3. Analogue-Based Prospectivity Screening

Geological evidence for possible Li sources and enrichment mechanisms was integrated with the hydrochemical observations. K, B and Br were treated as process and brine-evolution indicators rather than as universal proxies for Li. The B–K compositions of the studied aquifers were compared with published Smackover Formation brines and other sedimentary-basin datasets for which Li has been reported. The comparison is qualitative and is intended to rank aquifers for direct Li analysis. It is not a regression model, does not predict Li concentration and does not demonstrate an economic resource. Interpretations were required to be consistent with basin age, lithology, evaporative history, burial and water–rock interaction, organic-matter maturation and possible fault-controlled hydrothermal circulation.

5. Geological and Hydrogeochemical Results for NGR Sedimentary Brines

NGR has undergone a long history of tectonics, magmatic activity, sedimentary deposition and erosion from Precambrian basement formation to Cenozoic collision and mountain building (Figure 2; [58]). During the Late Neoproterozoic, the supercontinent Gondwana formed from the amalgamation of stable cratons and more juvenile crustal terranes following the suturing of west (Africa-South America) and east (India-Antarctica-Australia) Gondwana landmasses (Figure 2; [58,59]). Different rock types characterize the collisional zones spanning metamorphic, metasedimentary and metavolcanic units, as well as syn-orogenic, post-orogenic and anorogenic igneous intrusions.

The amalgamated terranes were affected by extensional collapse and transcurrent tectonics during the Late Neoproterozoic [61]. During much of the Paleozoic and Mesozoic eras, large sedimentary basins developed within relatively passive hosting thick successions of clastic, carbonate and evaporitic lithologies [62,63]. The passive margin settings were at times punctuated by extensional to compressional tectonic events that reactivated deep basement structures. Furthermore, several magmatic events affected these basins. The passive margins that characterized much of NGR history have created suitable conditions for the formation of numerous extensive shallow and deep aquifer systems [11,13,16,64] characterized by high total dissolved solids (TDS), including elevated Li concentrations. The conditions leading to these high salinities were often driven by global tectonic activity and the associated climate shifts. The widespread presence of evaporite strata, particularly those rich in halite and anhydrite, serves as a geological indicator of past arid conditions and high evaporation rates within closed basins throughout the NGR [62,65,66,67]. These evaporites form precisely when salts dissolved in the closed basin waters become so concentrated during evaporation that they precipitate out of solution to form thick evaporite layers.

5.1. Geological Context and Candidate Sources of Li

The first step to evaluate the potential for Li enrichment in the NGR brines is to identify original sources of Li. The tectonic and magmatic evolution of the region has resulted in the emplacement of a wide range of lithologies, the weathering and alteration of which have the potential to release significant volumes of Li to both surface water and subsurface brines. Among these are the Neoproterozoic granites and granitic pegmatites of the Arabian-Nubian Shield (ANS) (Figure 3), which are enriched in lithophile elements including Nb, Ta, Sn, Rb, and Li [68,69,70]. For example, Li measurements in the Asir region of the southwest ANS range between 4,727 and 7,865 ppm, indicating a major source of Li, primarily hosted by the Li-bearing mineral lepidolite [71]. Other areas of the Shield contain clusters and belts of felsic plutonic rocks that are reported to have Li concentrations up to 121 ppm [72]. Specifically, the plumasitic-type rocks, which are predominantly leucocratic alkali-feldspar granites, are characterized by high concentrations of Li, and are thought to have formed from the differentiation of plutons of the alkali-feldspar granite association [72]. Furthermore, in the Ghadara area of the central ANS, monzogranites hosting gold mineralization have also been found to contain Li concentrations up to 81 ppm [73]. The ANS is also considered a primary sediment source for the nearby clastic cover, including some Li-rich Early Triassic shaly units [74].

The Phanerozoic features several volcanic episodes that produced a diverse range of igneous lithologies, including potentially Li-rich rocks (Figure 3). Continental extension along the African part of northern Gondwana during the Middle Ordovician was associated with magmatism that produced plutonic rocks and deposited volcanic bentonite strata [75]. Furthermore, the break-up of Gondwana was associated with several phases of magmatism, including possible mantle-plume activity and localized intraplate rift-related magmatic events [76]. Late Permian andesite and rhyodacite lavas and pyroclastic deposits (Figure 3) have been observed in Oman [77]. Later during the Jurassic, a more widespread phase of magmatism affected several parts of the NGR (Figure 3). These included rhyolitic volcanism and microsyenite intrusions in north Libya [78] and subduction-related volcanism in the Zagros region of Iran (extending to the Paleogene) [79]. Where contemporaneous with brine-generating basins, volcanic ash, aerosols, and volcaniclastic material constitute plausible Li sources that can be evaluated within the proposed workflow.

The basins straddling the NGR have accumulated large thicknesses of sedimentary units, which due to subsidence and burial, led to the generation and accumulation of the region’s massive hydrocarbon resources and may also have released significant volumes of Li to associated oil-field brines. Hydrous-pyrolysis and kinetic modeling of Type II-S source rocks from the Arabian Basin demonstrate that maturation products can evolve differentially during generation and migration, reinforcing the need to couple brine-enrichment models with source-rock kinetics and burial-history reconstruction [80].

5.2. Evidence for Evaporative Brine Evolution

Brines with residual evaporitic components are reported from the NGR, including parts of southwest Asia and north Africa, consistent with a history of several prolonged periods of aridity and basin restriction [81,82,83]. Following the identification of potential hard-rock sources of Li, we assess the type of brine and degree of evaporation from three geological formations of ages spanning Triassic to Paleogene through a re-analysis of historical data (Table S1, supplement). The aquifers are mainly composed of carbonate (limestone and dolomite) rocks. The salinity of brine samples varied across the three aquifers, with a Triassic aquifer exhibiting a range of 231 to 299 g/L, a Jurassic aquifer brine ranging from 321 to 351 g/L, and a Paleogene aquifer brine ranging from 85 to 124 g/L TDS. Chloride concentrations also varied, with the Triassic aquifer brine exhibiting a range of 133 to 185 g/L, the Jurassic aquifer brine ranging from 183 to 192 g/L, and the Paleogene aquifer brine ranging from 51 to 76 g/L (Table S1, supplement).

Based on theoretical considerations of water-rock interaction, and given the lithologies of the aquifers (i.e., limestone and dolomite), the samples were expected to plot on the Ca-Mg-SO4 or Ca-Mg-HCO3 field of the Piper diagram [84]. However, hydrochemical analysis revealed a consistent Na-Cl water type across all brine aquifers, regardless of depth and location (Figure 4), suggesting a common salinity source unrelated to acquisition of solutes from host lithologies through water-rock interaction. For comparison purposes, data from other brines from the NGR [11,12,13,14,15,16,18] were plotted on the same Piper diagram, revealing the same Na-Cl water type (Figure 4).

Major sources of Na-Cl-type brine salinity include evaporation of seawater and dissolution of halite, which can be distinguished using chloride/bromide (Cl/Br) ratios (Figure 5; [13,31,64,85,86,87,88]). The Jurassic aquifer brine plots mainly along the Seawater Evaporation Trend (SET), suggesting an evaporated seawater origin, while samples from the Triassic and Paleogene brines plot to the right and below the SET, indicating mixing or dilution involving evaporated seawater end-members. Samples from the Paleogene aquifer brine exhibit a larger downward displacement from the SET, likely due to flushing and mixing with fresh paleometeoric water during the Late Pleistocene and Early Holocene [64].

Other published data from several NGR aquifers suggest a similar evaporated seawater mechanism of salinization in brines (Figure 5; Ordovician in Algeria [14]; Permo-Triassic in Iran [13]; Middle-Upper Jurassic in Arabia [18]; Oligocene-Lower Miocene in Iran [11]. An exception is noted from the Lower Cretaceous in Iraq, where the sample was plotted in the halite dissolution by evaporated seawater brines zone potentially due to moving upward through a halite layer along faults cutting Jurassic to Cretaceous units [15].

The SET in Figure 5 shows the related saturation stage for specific minerals, including the calcite, gypsum, halite, epsomite, sylvite, carnallite, and bischofite stages [89]. The plotted data from different brine samples are consistent with genesis from progressive evaporation of seawater beyond the point of gypsum precipitation and before epsomite precipitation. This demonstrates that seawater evaporation operated extensively, both spatially over much of the NGR and temporally at several extended geological intervals, as an important mechanism for concentrating dissolved constituents, potentially including Li. Bromide is particularly useful in this context as an indicator of residual brine evolution and halite precipitation or dissolution; it should not be treated as a universal positive predictor of Li.

5.3. Evidence of Hydrothermal Water-Rock Interaction

The history of episodic subsidence and basement-involved faulting of the NGR created environments conducive for enhanced water-rock interaction [91,92,93] potentially enriching hot brines residing in deep, overpressured strata with Li and other valuable commodities [4,5]. For example, studies of diagenesis and fluid flow have shown that hydrothermal fluids actively migrated up-faults from deeper sources during Late Cretaceous to Cenozoic tectonic activities [93,94,95,96].

To evaluate the effect of hydrothermal fluids on Li enrichment, we utilize oxygen (δ18O) and hydrogen (δ2H) isotopes as environmental tracers that can distinguish pristine meteoric water from water affected by evaporation and/or water-rock interaction [97,98,99]. For instance, δ18O and δ2H have been utilized to provide valuable insights into the origin of water [100,101], understanding migration pathways and mixing trends [48], evaluating temperature and circulation depths [102], and paleoclimates [103].

Figure 6 illustrates the isotopic composition of samples from the three brine aquifers in comparison with the Global Meteoric Water Line (GMWL) and the seawater evaporation curve [104]. The Triassic and Jurassic samples are enriched in both δ18O and δ2H, which suggests the evaporation of seawater as a dominant process to explain the current water composition consistent with the evaporative component of the proposed brine-enrichment model. Furthermore, the isotopic compositions deviate from the expected seawater evaporation curve (i.e., δ18O enrichment without affecting δ2H ratios). Such deviation can occur due to oxygen isotope exchange with bedrock minerals through high-temperature mineral water-rock interaction [48,105], indicating the potential for hydrothermal fluids to have intruded the Triassic and Jurassic aquifers.

Based on published data from the Smackover Formation (Gulf Coast, USA; [107]), a well-known example of a sedimentary brine system enriched in Li, we use the δ18O:δ2H plot to evaluate whether the studied brines experienced processes comparable to those associated with Li enrichment in Smackover (Figure 6). Samples from the Triassic and Jurassic brine aquifers follow a trend consistent with more progressive water-rock interaction. This similarity identifies water-rock interaction as a plausible component of their Li-prospectivity model, but isotopic displacement is not a Li concentration scale and cannot demonstrate that their Li concentrations equal or exceed those of Smackover. Similar enriched δ18O and δ2H values were observed for other NGR samples, including the Permo-Triassic successions in Iran [13], Late Cretaceous in Iraq [16], and Oligocene-Early Miocene in Iran [11].

In contrast, samples from the Paleogene brine aquifer are probably composed of two water types: evaporated seawater and Pleistocene meteoric water [64] (Figure 6). This indicates that any earlier evaporative and water-rock-interaction signature in the Paleogene aquifer is likely modified by mixing between these two end-members. Similar observations were made for the Middle-Late Jurassic in Arabia [18], the Ordovician in Algeria [14], and the Lower-Middle Cretaceous in Kuwait [108]. Samples from those aquifers plot close to the evaporated seawater curve, indicating a dominant evaporative origin, but show a slight deviation towards more enriched δ18O values, which is typically an indication of water-rock interaction. This deviation is not as pronounced as in the Triassic and Jurassic brine aquifers and the Smackover Formation, suggesting that water-rock interaction in these aquifers was less intense.

6. Discussion: Geochemical Indicators for Li-Prospectivity Screening

In the absence of direct Li measurements, which is common in historical water-chemistry reports, we propose that concentrations and ratios of routinely reported elements such as boron (B), potassium (K), bromide (Br), and chloride (Cl) be used as strategic indicators of processes that may favour Li enrichment in sedimentary brines. High concentrations of Li and B in Andean salar brines have been attributed to weathering of nearby Li- and B-rich felsic volcanic rocks, such as ignimbrites [32]. K remains in residual bittern until late stages of seawater evaporation [31], but can also be modified by feldspar alteration, clay-mineral reactions, and late evaporite precipitation. Br is excluded relative to Cl during halite precipitation and therefore records residual brine evolution, whereas halite dissolution produces comparatively Br-poor Na-Cl brines. Consequently, B and K concentrations, interpreted together with Br, Cl/Br, TDS, isotopes, lithology, burial history, and evidence for magmatic or hydrothermal activity, can indicate conditions analogous to those found in some Li-bearing basins. These variables are process indicators rather than unique or universally conservative proxies for Li.

This relationship can be examined by plotting K and B concentrations of Jurassic Smackover Formation brines along the Gulf Coast of the United States [107]. The Smackover sedimentary brines, with Li concentrations exceeding 400 mg/L, have a substantial residual evaporitic-brine component [30,35,107]. As shown in Figure 7, a plot of B versus K for these brines reveals a positive association between the two elements, with published Li concentrations represented by symbol colour. Lithium in Smackover brines has also been reported to correlate with K, possibly reflecting interaction with feldspathic minerals or clays [107]. Comparable co-occurrence is reported from Devonian Leduc and Swan Hills brines in the Alberta Basin, where Li-rich waters contain elevated K, B, and Br and are interpreted to involve alteration of Li- and K-bearing silicates [43,46]. A recent Alberta exploration workflow likewise selected K, together with Na, Mg, Ca, and TDS, among routinely available variables used to prioritize brines lacking Li analyses [109]. However, behaviour is not universal: Qianjiang Formation brines show only a weak Li-salinity relationship and a negative relationship between Li and Cl/Br [110]. These comparisons support integrated, basin-specific use of K, B, Br, and Cl/Br rather than any single-element proxy.

When this approach is applied to the analyzed samples from this study, the Triassic and Jurassic aquifers occupy the region of the B-K plot populated by published brines with higher Li concentrations (Figure 7). Together with their residual-evaporative character, stable-isotope evidence for water-rock interaction, basin burial history, and the regional occurrence of potentially contemporaneous magmatic and hydrothermal activity, this similarity indicates a possibility of substantive Li enrichment analogous to Smackover and Alberta basin brines. It does not predict a Li concentration or demonstrate a resource. In contrast, samples from the Paleogene aquifer occupy a lower-priority region of the B-K plot, consistent with dilution by meteoric water that partially recharged those aquifers during the more humid Pleistocene and Holocene in the region [64]. This analogue-based approach provides a practical means of prioritizing sedimentary brines for direct Li analysis.

However, this indicator-based approach should be framed as a screening workflow rather than a substitute for direct Li measurements. Integrated organic and inorganic geochemical datasets improve process-based interpretations of sedimentary systems, particularly where elemental indicators are used to infer depositional and diagenetic controls [111]. The approach should therefore combine B and K with Cl/Br, TDS, stable isotopes, geological age, host lithology, burial history, potential volcanic or magmatic sources, and structural evidence for fluid migration. Direct Li analysis is required to test the resulting ranking.

7. Conclusions

This study employed a multifaceted approach to evaluate the Li prospectivity of sedimentary basin brines along the NGR and other Gondwana-derived terranes, moving beyond conventional hard-rock and Andean salar brine sources. The resulting workflow is intended to prioritize Li exploration efforts in the region.

A comprehensive analysis of the unique geological setting of the NGR reveals a long and complex geological history, characterized by the development of extensive sedimentary basins containing highly saline formation waters. Importantly, the presence of significant potential primary sources of Li is well established. These include Neoproterozoic granites and pegmatites, as well as felsic igneous rocks generated during Paleozoic and Mesozoic magmatism in the region. Their presence establishes source plausibility, although source-to-aquifer connection must be evaluated for individual basins.

The geological history of the study area is also marked by extensive periods of arid climate, which created suitable conditions for intense evaporation in hydrologically isolated basins. Li supplied to these basins by volcanic ashfall, aerosols, and/or low-temperature weathering of volcanic and crystalline rocks could have become progressively concentrated in residual evaporitic brines. The widespread occurrence of evaporite strata and evidence for trapped paleoseawater within extensive brine aquifers support long-standing conditions capable of concentrating mobile solutes, potentially including Li.

This study also identifies potential mechanisms for Li transport from source rocks to sedimentary basin brines. Basin subsidence and associated tectonic activity likely played important roles in transferring Li from Li-bearing clastic sediments and organic matter to brines through temperature- and fluid-flow-enhanced water-rock interaction, clay reactions, and kerogen maturation.

Finally, published fluid-inclusion and diagenetic studies across the NGR support episodes of up-fault migration of hot fluids in parts of the sedimentary basins. These studies demonstrate plausible pathways by which deep fluids could interact with crystalline or sedimentary sources and migrate into shallower reservoir lithologies; they do not by themselves establish Li enrichment.

In conclusion, the convergence of potential primary Li sources, evaporative brine evolution, water-rock interaction, organic-matter maturation, possible transport pathways, and favourable conditions for brine preservation indicates that sedimentary basin brines along the NGR and other Gondwana-derived terranes represent a plausible frontier for Li exploration (Figure 1). The Triassic and Jurassic aquifers show a combination of K-B chemistry, residual-brine indicators, and isotopic evidence for water-rock interaction comparable to processes documented in published Li-bearing basins, whereas the Paleogene aquifer records greater meteoric dilution. This integrated geological and geochemical workflow identifies the former aquifers as higher priorities for direct Li analysis, but does not predict concentration, continuity, recoverability, or economic significance.