Keywords : Trace Elements
Iraqi National Journal of Earth Sciences,
2013, Volume 13, Issue 1, Pages 13-34
Karst-filling bauxite deposits of early Cretaceous age are widespread in Hussainiyat area to the northeast of Al- Rutba city within the Iraqi Western Desert. Karstified carbonate host rocks represented by the dolostone of the Ubaid Formation (Early Jurassic) contain bauxite lenses preserved inside the karst together with kaolinitic bauxite, kaolinitic clays and sands belonging to the lower clastic unit of the overlying Hussainiyat Formation (Lower-Middle Jurassic). Normative mineralogy reveals that boehmite (up to 47%) and to a lesser extent gibbsite (up to 19%) are the main bauxitic minerals with kaolinite (28%), hematite (2.8%), anatase (up to 4.1%) and calcite (0.4%) as the main accessory minerals. Plots of chemical data against increasing grade of bauxitization show that Al, Ti, Zr, Nb, V, Hf, Th, U, Ga, Cr, Co, Cu, As and W were immobile and highly enriched, whereas SiO2, Fe2O3, CaO, K2O, Na2O, MgO, MnO, Ni, Mo and Gd were mobilized and depleted. Variation diagrams of certain immobile elements indicate that bauxite precursor might be shale derived from acidic granodiorite or its metamorphosed equivalent.