OCCURRENCE AND GENESIS OF PYRITE IN EL-GEDIDA IRON MINE, EL-BAHARIYA DEPRESSION, WESTERN DESERT, EGYPT A.A. ABDEL-MONEM1 , E.A. KORANY2, A.M. YOUSSSEF3 Nuclear Materials Authority, Cairo, Egypt1 ; Geology Dept Ain Shams Univ., Cairo, Egypt2 ;Iron and Steel Company, Helwan, Cairo, Egypt3 Pyrite is recorded in El-Gedida Mine at the periphery of the iron ore bodies in the form of lenses within zones. It is commonly hosted in mixed facies comprising hydrogeothite/limonite, glauconitic clay, sandy clay, black clays and shales associated with gypsum needles and veinlets. The pyrite bearing zone varies in thickness between 3.2 and 7.2m. It unconformably overlies the clays and sandy clay of the Bahariya Formation and overlain by an intercalation of glauconitic clay and limonitic ore. The XRD analysis showed that the pyrite zone mainly is composed of pyrite associated with small amount of marcasite and few grains of chalcopyrite. In polished sections the pyrite grains exhibit framboidal and colloform textures, whereas the marcasite ones exhibit spherulitic and radial textures. The few grains of chalcopyrite are sometimes altered to covellite and bornite. SEM analyes show that the pyrite grains contain (45-48%) Fe and (52-55%) S indicating that the structural formula is of the ideal FeS2. However, the chemical composition of bulk pyrite samples showed admixtures with clays as well as Fe-oxides. The mineralogy and textures of the studied pyrite grains indicate sedimentary origin where Fe-monosuphides directly precipitate within organic rich clay layers in the presence of S-reducing bacteria (sapropel). Then the Fe-monosuphides were tansformed to framboidal pyrite or marcasite by biotic or abiotic processes. During diagenesis pyrite was oxidised to FeSO4 or FeOOH in the presence of Fe3+ and/or MnO2 Finally, the recorded pyrite may represent relics of old sapropel layers at the top of Bahariya Formation. The pyrite was subjected to intensive diagenetic oxidation.