MINERALOGICAL STUDY OF WEATHERING ON THE BANA COMPLEX, WESTERN PART OF CAMEROON

The Bana complex is composed of various rocks from volcanics to plutonics located in the western part of Cameroon at latitude 5° 08'N and longitude 10° 20'E. The climate of this region is essentially sudanese tropical type characterized by alternation of humid and arid seasons. The annual precipitation in this area amounts to 1500-2500mm. Under these circumstance, the Bana complex is extensively weathered. The basement rock is composed of gneiss (orthogneiss) which is intruded and covered by later plutonic rocks from gabbro to granite and volcanic rocks, from basalt to rhyolite, respectively. Many weathering profiles selected in the granite as well as in volcanic rocks were investigated in detail. Mineralogical characteristics such as chemical composition and micro-morphological variation of the weathering products were examined by means of X-ray diffraction (XRD), microprobe analysis (EPMA), X-ray fluorescent (XRF), scanning and transmission electron microscopy (SEM and TEM).
Halloysite and kaolinite are the most predominant weathering products regardless type of the host rocks. Detailed mineralogical investigation on the selected profiles revealed the transformation sequence of kaolin minerals as weathering proceed. That is, the lower parts are rich in halloysite whereas the upper parts are rich in kaolinite with a small amount of halloysite. The obtained mineralogical sequence was further confirmed by micro-morphological investigation of these minerals, i.e., from spherical halloysite to finally platy kaolinite.
Spherical halloysite associated with short tubular crystal is characteristic in the bottom part whereas irregular platy crystals predominate in the upper parts. These morphological variation together with crystal size reasonably explain the formation of kaolin minerals and crystallographical transformation sequence from spherical halloysite, short tubular halloysite, long tubular halloysite and finally platy kaolinite was established.
Etch pits developed on quartz and K-feldspar grains are commonly observed. However, shape and size of the etch pits are quite complicated suggesting none monotonous micro-weathering condition. Etch pits found in the upper part are much rougher than those of the lower part.
Based on the results obtained, weathering condition prevailed the Bana complex were discussed mainly from the stand point of clay mineralogy.