The interpretation of the geologic evolution of the northern part of the Bahariya Depression is more or less controversial. The stratigraphic succession of this area comprises Lower Cenomanian siliciclastic rocks with abundant iron horizons (Bahariya Formation) that are overlain unconformably by Middle Eocene carbonates (Naqb Formation). The latter is characterized by localized occurrence of iron ore bodies at El Gedida, Ghorabi and El Harra areas. They show a spatial relation to the main faults of the area. Two sections were studied at Ghorabi and El Harra areas, where the Bahariya Formation is overlain by the ironstone deposits. A third one was studied at Gabal El-Dist where the Bahariya Formation is overlain by Naqb Formation, which is barren of iron in this location. In this paper, insight provided from the study of clays in the Bahariya Formation is used to ascertain the role played by diagenetic and/or hydrothermal processes in the formation of iron ore deposits in the area. Petrographic studies, scanning electron microscopy, electron microprobe analyses as well as bulk mineralogy (X-ray diffraction) of some selected samples (ironstone crusts) from the Bahariya Formation show that the clay minerals comprise 20 to 30% of the whole rock. XRD study of