TECTONIC SETTING AND CHARACTERS OF RAS SHAIT PODIFORM CHROMITE, EASTERN DESERT, EGYPT. Nadi A. Saad, Department of Geology, Faculty of Science, University of Alexandria, Egypt

The chromite deposits of Ras Shait area occur in an ophiolite complex as isolated lenses, pods, nodules or as disseminated grains mainly enclosed in serpentinites and talc carbonates. Incomplete sequence of the ophiolite complex is recorded in the studied area. Metamorphic peridotites and the overlying foliated gabbro are the main units, while sheeted dykes and massive basalts are encountered as limited exposures. The peridotite and associated chromite mineralization display features characteristic of the Alpine-type. The Ni/Co and MgO/SiO2 ratios of the serpentinites and talc carbonates, beside their petrographic characteristics suggest that the parental rocks were harzburgite-lherzolite. The main opaque constituents in the studied area is chromite and its alteration products (ferritchromite and chromian magnetite). Ilmenite, titanomagnetite and rutile are present in subordinate amounts. Pyrite, chalcopyrite and pyrrhotite are rare. The associated gangues are antigorite, lizardite, talc, magnesite, calcite, fuchsite and kaemmererite. The microprobe analysis of the zoned chromite grains shows chemical compositional variations from the core to the rim and reveal that the alteration zones of chromite crystals are enriched in Fe and Ni and depleted in Al, Mg and Mn. This alteration is concomitant with the serpentinization process which affected the ultramafic masses. A plate tectonic model is proposed to explain the genesis of Ras Shait chromites. The ophiolite complex, with its associated chromite, are considered to be a mass of oceanic lithosphere obducted over continental crust. During obduction, the original chromite layers were subjected to intense tectonic disturbances with the result that the chromite deposits were dismembered and variously rolled giving the present pod form.