Critical Raw Materials in Paleocene Karst Bauxites of the Northwestern Himalayas, Pakistan: Grades, Host Minerals, and Economic Implications

The increasing demand for critical raw materials (CRMs) utilized in green energy systems, batteries, and electronic devices has enhanced the exploration of unconventional ore systems, such as karst-type bauxites, which are host to economically significant Rare Earth Elements (REEs) and associated critical metals (e.g., Li, Ga, Zr, and Sc). Karst bauxite deposits have been documented in Paleocene strata of the northwestern Himalayan foreland basins,  Pakistan. However, the resource potential and beneficiation restrictions have not been adequately assessed yet. The present study measures grade variability, host phases, distribution, and enrichment of REEs and associated CRMs to assess their economic values. Ten bauxite outcrop sections were studied in the northwestern Himalayan fold and thrust belt of Pakistan, including the Hazara–Kashmir Syntax (HKS; n = 2), the Attock–Cherat Ranges (ACR; n = 3), the Trans-Indus Ranges (TIR; n = 2), and the Salt Range (SR; n = 3). The bauxite horizons are overlain by the Paleocene Hangu Formation and underlain by Cretaceous units (HKS, ACR, TIR) and Cambrian to Permian strata (SR). An integrated workflow consisting of fieldwork, XRD, SEM-EDS, XRF, and ICP-MS was used for mineralogical and geochemical characterization. The geochemical data reveal that the ores are primarily bauxitic clays with low to moderate ΣREE concentrations that vary regionally. The average concentrations of ΣREE are 174 ppm (SR), 287 ppm (TIR), 344 ppm (ACR), and 66 ppm (HKS). Compared to the Upper Continental Crust (UCC), Al₂O₃, Fe₂O₃, and TiO₂ are enriched, while SiO₂ is depleted. The CRM-relevant trace elements (Ga, Zr, V, Hf, Nb, Ta, Th, and U) show positive anomalies, indicating widespread but generally low enrichment. The SEM-EDS results reveal that ultrastable accessory minerals, especially zircon, tourmaline, and rutile, are the common carriers of REEs in the studied bauxites.  This implies that the ultrastable detrital phases host many REEs, which may have an impact on extractability. However, cerianite (CeO₂) is an additional important REE-bearing phase that is compatible with a positive Ce anomaly in the Salt Range. In the TIR, fluorapatite contributes to REE hosting (notably Ce, Nd, La, and Y); moreover, minor concentrations of REEs like Yb, La, Eu, and Ce also occur as trace, finely dispersed components within the matrix. The CRM distribution is mostly controlled by the strength of lateritization, which is normally weak to moderate but increases locally in Fe-rich horizons. Unlike other REEs, Ce was probably mobilized during intense ferrilitic weathering of primary REE minerals and then redistributed as cerianite (CeO₂), resulting in a positive Ce anomaly. From an economic perspective, the grade heterogeneity and the predominance of REEs in ultrastable minerals suggest that the prospectivity of CRM relies on identifying enriched layers and determining whether REEs are locked in resistant detrital hosts or occur in processable authigenic phases (cerianite, fluorapatite, monazite, or bastnäsite).