Geochemistry of Late Cretaceous phosphorites in Egypt: Implication for their genesis and diagenesis

Phosphorite deposits in Egypt, known as the Duwi Formation, are a part of the Middle East to North Africa phosphogenic province of Late Cretaceous to Paleogene age. Phosphatic grains in these deposites are classified into phosphatic mudclasts and phosphatic bioclasts. Phosphatic bioclasts are subdivided into fish bone fragments and shark tooth fragments. All phosphatic grains are composed of francolite.Chemical mapping of the phosphatic grains using Electron Probe Microanalysis (EPMA) indicated that the phosphatic mudclasts are homogeneous in their chemical composition and no concentric texture nor chemical zoning are observed. Some of the bone fragments show Fe and S zoning. No significant difference in chemical composition is observed between the phosphatic mudclasts and bioclasts. Acid-insoluble residues of the phosphorites show lower values of the Chemical Index of Alteration (CIA) compared to the associated rocks. Structural CO2 contents in the francolites range from 3.32% to 7.21% with an average of 5.3%. The δ13CPDB values range from −4.04‰ to −8.7‰, while the δ18OPDB values range from −4.3‰ to −10.3‰.The compositional homogeneity of the mudclasts, Fe and S zoning in some of the bone fragments and the difference in the Chemical Index of Alteration between the acid-insoluble residues of the phosphorites and the associated rocks suggest that the phosphatic grains in the Duwi Formation are derived from pre-existing authigenic phosphorites, which reworked and concentrated afterward.Negative δ13C values of structural CO2 suggest that the CO2 was derived from degradation of organic matter. Low δ18O values of structural CO2 can be attributed to the influence of meteoric water.Higher CO2, SO3 and F contents compared to the recent authigenic phosphorites and negative δ13C and δ18O values of structural CO2 indicate that diagenesis plays an important role in the modification of the chemical composition of phosphatic grains and that the studied apatite was francolitized during diagenesis.