Extended Abstract

Introduction

Playas (daqs), as closed sedimentary environments in arid regions, play an essential role in the accumulation of valuable mineral resources. Globally, these basins are rich sources of evaporite minerals, including sulfates, carbonates, halite, nitrates, barites, bromine, iodine, and strategic elements such as lithium. For example, about 400,000 tons of sodium sulfate, worth $40 million, are extracted annually from the playas of western Canada, and the Silver Peak brines in the United States are considered one of the most considerable lithium resources in the world. Playas, closed sedimentary environments in arid regions, play an essential role in concentrating valuable mineral elements, especially the strategic element lithium. The Taherabad and Bojestan playas in Khorasan Razavi Province have a high potential for the formation of lithium reserves due to their specific geomorphological, climatic, and hydrological conditions.

Methodology

In this study, an integrated approach combining spatial data, remote sensing, field studies, laboratory analyses, and statistical modeling was used to investigate the role of geomorphological factors in controlling lithium mineral potential in the Taherabad and Bojestan playas. The basic data included 1:25,000 and 1:50,000 topographic maps, 1:100,000 geological maps, and the SRTM digital elevation model, which were used to extract morphometric parameters in ArcGIS 10.4. To analyze spatio-temporal changes in the playas, Landsat 8, Sentinel 2, and ASTER multispectral images were processed, and related spectral indices were extracted. Grid sampling of sediments and groundwater was conducted. Geochemical analyses (ICP-MS and XRF) were performed to determine lithium and associated element concentrations, and mineralogical studies (XRD and SEM) were conducted to identify lithium-bearing phases. Geophysical (ERT and IP) and hydrogeological data were also used as complementary layers in mineral potential modeling. Finally, using multivariate statistical analyses and FDA, GLM, and GBM models, lithium potential zoning was performed, and the models' performance was evaluated using statistical indices and ROC curves.

Results and Discussion

The results of geomorphological analysis show that geomorphology and topography play a decisive role in the lithium concentration pattern in both basins. The Bajestan basin, with a higher elevation range, steeper slope, and rougher terrain, acts as a transport-oriented system in which weathering, release, and lithium transport from highlands to lowlands and evaporative areas are dominant. In contrast, the Taherabad playa, with its lower elevation, gentle slopes, and lower roughness, represents a low-energy, accumulation-oriented system that provides suitable conditions for lithium storage and concentration. The analysis of the curvature and flow accumulation paths indicates that the lowlands and converging areas in both basins are the primary focus of runoff concentration and subsurface flows, which, by increasing water residence time, enhance rock-water reactions and lithium absorption by clays.

Geochemical results from the Taherabad playa show significant depth variations in lithium and associated element concentrations, reflecting the influence of dissolution processes, ionic migration, and chemical concentration under evaporative conditions. In the Bajestan basin, the spatial pattern of lithium is highly correlated with the location of watercourses, bedrock, and fine-grained sediment transport processes. PCA results indicate that the first factor (Na, Cl, Mg, Sr) represents evaporative-saline control, but lithium is not significantly correlated with this factor. The second factor, characterized by Li₂O, Al₂O₃, K₂O, Fe₂O₃, and TiO₂, highlights the key role of clay minerals and felsic origin in lithium concentration; a factor that is more dominant in Taherabad.

The results of FDA, GLM, and GBM modeling in both playas show high convergence in identifying zones with high to very high potential. In Bajestan, the central, southern, and southeastern areas were identified as the primary exploration target zones, reflecting the interaction of upland release and ultimate accumulation in low-energy environments. In Taherabad, the concentration of high-potential zones in the central, southern, and southeastern parts of the playa indicates the operation of an accumulation-driven system with firm control of fine-grained sediments, intense evaporation, and high fluid residence times. Quantitative results show that all three models FDA, GLM, and GBM have acceptable power in predicting areas prone to lithium concentration, but the FDA model performs superiorly with the highest accuracy; with AUC = 0.845, Kappa = 0.63, and ACC = 0.815, it has higher resolution and agreement than GLM (AUC = 0.796) and GBM (AUC = 0.754). The high values of SST (0.82) and SPF (0.81) in FDA indicate the high power of this model in correctly identifying real zones with lithium potential and appropriately controlling false-positive error. In contrast, the simultaneous decrease of these indices in GLM and GBM indicates increased uncertainty and a higher risk of missing promising zones.

Conclusion

The results of this study show that the lithium concentration in the playas of Bajestan and Taherabad is the outcome of a direct, structured interaction between geomorphological and geochemical-mineralogical processes. The playa of Bajestan, with its higher altitude (up to about 1100 m) and high topographic roughness, provides effective conditions for the intensification of weathering, the release, and the transport of Li⁺ ions from source rocks, due to its high geomorphic energy and its role as a high-energy transport-oriented system. The positive correlation of lithium with Al₂O₃, K₂O, and TiO₂ indicates the dominant role of silicate-clay minerals of felsic origin in lithium production. In contrast, the playa of Taherabad, with its lower altitude, gentle slopes, lower roughness, and convergent curvature, although having a lower intensity of initial lithium release, provides more stable conditions for the storage and chemical concentration of lithium due to the high residence time of fluids, closed environments, and fine-grained sediments; So that the lithium concentration reaches about 68 ppm in some areas.