Two types of ooidal ironstone deposits, the chamosite-type deposits and the kaolinite-type deposits are recognized, in the west and southwest Aswan areas respectively. These deposits are distributed within the Nubian sandstone of the Late Cretaceous age, which is divided into three units: the basal Abu Aggag Formation, the Timsah Formation and the uppermost Um Barmil Formation.

        The Timsah Formation ( Coniacian to Santonian), has a thickness varies between 40-50 m and consists of three coarsening-upward cycles, which contain at least six bands of ooidal ironstone graded to worthy iron ore deposits. The ore of these ironstones is either red iron oxides or sandstone impregnated with ferruginous material.

          Based on the microscopic investigation, supported by XRD and XRF analyses, and according the effects of the ferruginization process, the chamositic ironstone as well as the kaolinitic one is distinguished into the following ironstone types; (a) non-altered, (b) slightly altered, (c) moderately altered, (d) redeposited ironstone and (e) strongly altered ironstone.

            The ironstones consist of ooids and groundmass, both of which are composed predominantly of hematite, goethite, chamositic clay and kaolinite. Other constituents quartz, zircon, rutile and pyrite occur as pseudomorphs replaced by hematite.

            The ooids of the ironstone are distinguished into three petrofabric types 1) Tangential fabric ooids, 2) Ooids and/or pisoids with radial fabric and 3) Ooids and/or pesoids with radially- concentric tangential fabric.

          The geochemical studies, supported by XRF, EPMA and ICP-MS analyses; proved that Fe2O3, SiO2, Al2O3 and MgO are the major constitutes of the ironstone, while Sr, V, Co, Y and Zr are the most common trace elements. The chemical composition of the ironstone is greatly affected by the ferruginization process, where the FeO content increases at the expense of the SiO2, Al2O3 and MgO contents. Consequently, the ironstone of both types, chamositic and kaolinitic, are mostly of Fe2O3 composition with the strongly ferruginized ironstone types.

           In conclusion, the genesis, mode of formation and diagenesis of the ooidal ironstone of the west and southwest Aswan areas are explained in the following concepts:

(A) Sedimentation

           The initial materials (protolith) of ooidal ironstones of west and southwest Aswan areas are genetically related to the weathered igneous rock in the hinterland, under lateritic conditions and subsequently transported into the shallow marine basins. Ooids, pisoids and peloids, They were formed by the rolling on the sea floor and accretion of platy kaolinite crystals, iron-bearing kaolinitic material forms the groundmass of the ooids and detritus.

(B) Diagenesis

          Fe-bearing kaolinite are transformed mainly into chamositic clay minerals (under reducing conditions caused by decomposing organic material), by addition of Mg from sea water to the Si, Al and Fe resulted from the decomposition of the kaolinite. Due to oxidation of the organic matter, authogenic pyrite is formed in the form of frambiodal crystals.

(C)Ferruginization process

         The descending meteoric waters cause decomposition of pyrite mineral and thus developing an acidic solution. which penetrated into the underlying sediments, decomposing fresh chamositic clay, give rise to alkaline conditions. These conditions favour the reprecipitation of the dissolved Fe and hence, ferruginization of the rock in a roll front-like manner.