REE MOBILITY AND TETRAD EFFECTS IN BAUXITES: AN EXAMPLE FROM THE KANISHEETEH DEPOSIT, NW IRAN

The Kanisheeteh bauxite deposit (NW Iran) is located on the Irano-Himalaya karst bauxite belt and was developed as stratiform horizon along the contact of the Permian and Triassic Formations. Based on absolute weathering index (AWI), HREE were leached out, whereas LREE suffered both leaching and fixation during bauxitization. The non-CHARAC characteristic of Y/Ho ratios can be attributed to geochemically known phenomenon as the "tetrad effect". The simultaneous convex and concave curves of the tetrad effect were observed in the chondrite-normalized REE patterns of the bauxite ores. The correlation between the sizes of T-4 and AWI% values revealed two separate populations. The first one is characterized by a wide variation in sizes of T-4 and very narrow AWI% values, and is thought to be related to acidic zone of the profile. In contrast, the second one is marked by a narrow variation in sizes of T-4 and wide variation in AWI% values, and is conceived to be affiliated to alkaline zone of the profile. In this research, the tetrad effect proved to be capable of clarifying the geochemical characteristics of the deposit and can be applied as a good geochemical index to investigate the prevailed paleo-conditions of depositional environments.