Critical mineral potential of Indian phosphorites: Evidence of REE enrichment in Ediacaran sediments from the Cuddapah Basin

Phosphorites constitute a potential non-traditional source of rare earth elements (REE), although their resource viability is still being evaluated globally. This study presents the first documentation of REE-enriched sedimenthosted phosphorites from India, and evaluates the processes driving REE mobilization, enrichment and accumulation. The Ediacaran phosphatic beds from the Owk Shale Formation, Kurnool Group of the Cuddapah basin, interbedded with shales and tuffs, were analysed using optical and SEM imaging, whole rock geochemical, in-situ fluorapatite (EPMA, LA-ICP-MS) and Raman spectroscopic analyses. Two apatite phases were identified: primary authigenic (Ap-1), fluorine-poor variety and secondary/reprecipitated REE-enriched fluorapatite (Ap-2). Wholerock REE + Y concentrations reach up to 1075 ppm, with characteristic middle REE (MREE) enriched patterns, also recorded in fluorapatite, interpreted as a result of diagenetic uptake from sediment porewaters. A strong correlation between REE + Y and redox tracer, Mn indicates initial MREE scavenging by Mn-(oxy)hydroxides and organic matter, followed by MREE release during Mn-reductive dissolution. While diagenesis enhanced REE enrichment, post-diagenetic hydrothermal reworking associated with igneous intrusions diluted REE concentrations through the formation of abundant Fe-Mg chlorite (chamosite) and REE and P redistribution within the interbedded shales. Sequence stratigraphic analysis suggests that these pristine phosphorites were deposited during marine regression, without subsequent reworking, preventing the formation of high grade phosphorite deposit, emphasizing the role of sea-level changes in controlling phosphogenesis, formation of a phosphorite giant and thus, REE enrichment. A temporal analysis of global phosphorites further reveals that progressive seawater influence, modulated by biogeochemical recycling and episodic weathering, together controlled the REE enrichment patterns in phosphorites through Earth's history.