This paper deals with the ironstone rocks encountered in two sedimentary successions (Upper Cretaceous and Lower Tertiary) in the northern Baharyia, Egypt. The occurrence of the two iron-bearing formations in the same area offers an opportunity to provide update on understanding ironstone genesis. The Bahariya ironstones occur either as thin crusts within the Cenomanian clastic rocks (Bahariya Formation) or as big ore bodies, located at three mine areas, associated to the Eocene carbonate units (Naqb, Qazzun and El Hamra formations). Comparison between the two different ironstone types was achieved on basis of field, petrographic, mineralogical and geochemical investigations. The ironstones revealed different fabrics, i.e. conretionary, massive, stromatolitic-like, pisolitic, colloidal, reinifrom aggregates, coating boxwork, oncolitic-like, brecciated, leisegang, geode and brecciated fabrics. The two types of ironstones hosted in the Cenomanian and Eocene rocks show preferential replacement and cementation of carbonates by iron-rich minerals. Goethite and hematite are the main iron minerals in the two types, even though ferroan-dolomite and ankerite were recorded as subordinate amounts in the ironstone crusts of the Bahariya Formation. There are some similarities between the two ironstone types, i.e. both kinds are diagenetic, replacing carbonates, preserving structures, fabrics and thickness of their precursors. Even though, they exhibit differences in origin, mechanism of formation, timing, geometry, morphology and textures.
The ironstone crusts of the Cenomanian Bahariya Formation replaced and/or cemented the dolomite and ankerite after migration of reducing iron-rich fluids mostly through discontinuities and permeable carbonate facies. Such conditions was enhanced mostly by decomposition of organic matter and hydrocarbon migration. The replacement occurred during Turonian-Santonian tectonic activity that led to mixing of reducing and oxidizing fluids. The big ironstone bodies hosted in the Eocene rocks show intensive and local replacement and cementation of the dolomites of the Naqb Formation where silica (quartz) show replacement of the coarse-grained dolomites and the ferromanganese minerals show preferential replacement of the fine grained carbonate crystals. As well, the iron oxides show replacement and cementation of the limestones of the Qazzun and El Hamra formations preserving micritic nature of precursors. The replacement of the Eocene rock units by iron oxides occurred post- Bartonian, mostly during the Priabonian. Occurrence of these deposits are related to structural traps where hydrothermal reducing iron-rich fluids migrated through the major fault systems and mixed with meteoric water.
Nature and composition of the host rocks have good controls on morphology and distribution of ore deposits and circulation of fluids.
