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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 CO2 contents in the francolites range from 3.32% to 7.21% with an average of 5.3%. The δ 13CPDB values range from −4.04‰ to −8.7‰, while the δ 18OPDB 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 CO2 suggest that the CO2 was derived from degradation of organic matter. Low δ 18O 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 δ 13C and δ 18O 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. [Copyright &y& Elsevier]

Abstract: The Duwi phosphorite deposits in Egypt are a part of the Middle East to North African phosphogenic province of Late Cretaceous–Palaeogene age. Their origin is controversial. Most previous authors have considered them to be in situ, authigenic deposits. Phosphatic grains in the Egyptian phosphorites are well-rounded phosphatic mudclasts (45–65%) and phosphatic bioclasts (35–55%). Phosphatic mudclasts are internally structureless but sometimes contain silt-sized detrital grains such as quartz and bone fragments, suggesting a reworked origin from pre-existing phosphatic mudstone. Analysis by Electron Probe Microanalyzer (EPMA) confirmed that the phosphatic mudclasts are chemically homogeneous, without any concentric structure suggestive of authigenic origin. The presence of bone fragments within some of the phosphatic mudclasts, the occurrence of phosphatic mud filling bone cavities, and the similar appearance of the matrix of phosphatic mudclasts and the phosphatic mud in the bone cavities suggest that the bioclasts are also of reworked origin and washed out from the phosphatic mudstone. Scanning electron microscope observations of the fractured surface of the mudclasts have revealed the capsule-like texture that is characteristic of bacterially mediated authigenic phosphorites formed under upwelling conditions. Campanian authigenic phosphorites with similar texture have been reported from Israel, suggesting that the phosphatic grains were reworked from pre-existing authigenic phosphorites equivalent to those in Israel. Sedimentary facies analysis of the Egyptian phosphatic sequence suggests that the major phosphorite beds represent transgressive lag deposits that accumulated in an oxic, bioturbated, wave-dominated shelf environment. It is likely that offshore authigenic phosphorites, formed in an upwelling-dominated environment during an early Campanian highstand, were reworked during a mid Campanian lowstand, transported landward, and concentrated in a near-shore, oxic environment during subsequent late Campanian transgressions. Another transgressive-regressive cycle is recognized to have occurred during the late Campanian–early Maastrichtian, suggesting that formation, reworking and redeposition of the Duwi phosphorites resulted from sea-level changes during the Campanian and Maastrichtian. [Copyright &y& Elsevier]