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

Abstract

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 CO 2 contents in the francolites range from 3.32% to 7.21% with an average of 5.3%. The δ 13C PDB values range from −4.04‰ to −8.7‰, while the δ 18O PDB 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 δ 13C values of structural CO 2 suggest that the CO 2 was derived from degradation of organic matter. Low δ 18O values of structural CO 2 can be attributed to the influence of meteoric water. Higher CO 2, SO 3 and F contents compared to the recent authigenic phosphorites and negative δ 13C and δ 18O values of structural CO 2 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.