Mineral and geochemical variability of the phosphorite deposits in the Duwi Formation, Western Desert, Egypt: Insights into paleoenvironment and physicochemical conditions

Abstract This study explores the phosphorite deposits from the Lower Member of the Duwi Formation in Egypt, which forms part of the Mediterranean phosphorite belt extending from Africa to Asia. Detailed petrographical, mineralogical, and geochemical investigations were carried out to shed light on the prevailing physicochemical conditions and diagenetic history of these deposits. They are classified into three east-west trending belts, with significant economic occurrences located in the central facies belt, particularly along the Nile Valley and Red Sea shoreline. Geochemical analyses indicated that these phosphorites are predominantly composed of CaO (avg. 35.83 wt%) and P2O5 (avg. 25.8 wt%) with high average of detrital oxides like SiO2 (9.08 wt%), and Al2O3 (1.74 wt%) relative to seawater- derived phosphorite. The PAAS-normalized REEs plot shows that the examined phosphorites are characterized by slight negative Ce and positive Eu anomalies, with relatively enrichment of MREEs over LREEs and HREEs. These patterns and anomalies are inconsistent with typical seawater-derived phosphorites and instead point to detrital heavy mineral input. Sea-level changes during the Campanian-Maastrichtian periods are identified as the likely mechanism that the redeposition of pre-existing authigenic phosphorites into the current deposits within an oxic and dysoxic basin setting, as evidenced by their distinct chemical features. From an economic perspective, the high ∑REE + Y content (avg. 969 ppm) in these phosphorites is significant, as it surpasses the concentrations typical of seawater-derived deposits. Their economic potential is further highlighted by strong enrichment in light REEs (avg. 646 ppm) with substantial concentrations of both critical REEs (e.g., Nd = 153.89 ppm and Y = 182 ppm) and non-critical REEs (e.g., La = 154.36 ppm).