Chromite mineralization in ultramafic rocks of the Wadi Ghadir area, Eastern Desert, Egypt: mineralogical, microchernical and genetic studies

The chromite deposit of Wadi Ghadir is situated in the Eastern Desert of Egypt and belongs to a Precambrian ophiolitic complex. The mineralization consists of lenses, pod-like bodies or of disseminated grains of chromite (Cr-spinel) and is found mainly in serpentinites and talc-carbonate rocks of the ophiolitic complex. The morphology of the chromitite bodies is deformed, slickensided and sometimes brecciated. The lenses strike NE-SW and so show the same main shear trend of the region. Chromite within the main chromitite ore bodies is generally homogeneous in geochemical composition, in cr-number and in mg-number while the disseminated grains of Cr-spinel show variations in the chemical composition and are often zoned with an inner core of Al-chromite, a transitional zone of Fe-chromite and an outer rim of Cr-magnetite. The cores of the zoned Cr-spinel represent the original composition of the disseminated grains and show less variation in composition than the chromite of the chromitite ore. These compositional differences are attributed to magmatic processes. The compositions of ferritchromit and chromian magnetite are markedly variable in terms of cr- and mg-numbers compared to the cores of the Cr-spinel. Compositional zonation is suggested to be achieved during the major phase of serpentinization. The formation of antigorite and chromiferous chlorite belongs to the same stage. A minimum temperature of 550 degrees C was estimated for antigorite-chlorite-ferritchromit assemblage. On the basis of field observations, textures of minerals and of geochemical composition, the chromite mineralization of Wadi Ghadir area displays features that are characteristics of the Alpine-type chromitite ore deposits. The rock assemblage is interpreted to be derived from harzburgite-dunite parental rocks, is suggested to have been formed at a mid-oceanic ridge and subsequently have been reemplaced by thrusting in a subduction zone setting.