Abstract :
Only small, low grade occurrences of bauxite are known in Nigeria, associated with ferricretes on the Jos Plateau. Widespread bauxite has been discovered on the Mambilla Plateau. Laterite is developed on gneisses and basalt, while on trachytic parent rocks low-silica bauxite occurs with a thickness of more than 5 m and covering an area of at least 1 km2. This bauxite consists of gibbsite and goethite.
In contrast to the ferricretes of the Jos Plateau, weathering on the Mambilla Plateau led to the formation of stone line profiles. Ferricretes were developed on the Upper Jurassic Gondwana surface under contrasting climate and are preserved as relicts in the southern part of the Mambilla Plateau where weathered Precambrian basement occurs in a highly uplifted position. Dismantling of these ferricretes is attributed to a gradual change of paleoclimate towards more humid conditions. After the extrusion of volcanic rocks in the northern part of the plateau, presumably during Lower to Mid Miocene, these volcanics underwent intense chemical weathering. Bauxite and laterite formed depending on the type of parent rock. Weathering profiles are mostly truncated and overlain by reworked material. Allochthonous components occur both within and also below the stone-line. This is shown by mineralogical indicators such as ilmenite or quartz and by a shift in the Zr/Ti-ratio. Similar profiles are missing further north on the Jos plateau, where ferricretes formed instead. This is attributed to regional differences in paleoclimatic conditions which were more humid at Mambilla while Jos was under the influence of dryer conditions.
The widespread formation of laterite on different parent rocks on the Mambilla Plateau could be related to a global Mid-Miocene climatic optimum, which led to a general intensification of chemical weathering processes.
