Petrologie des laterites et des alluvions dans la région de Matomb (plateau Sud-Camerounais) : prospection geologique et geochimique du rutile

Abstract

The study area is located southeast of the Matomb town, 20 km on the Matomb-Hegba section, in the humid tropical zone. The main objective of this study was to conduct a detailed survey of rutile in order to know its distribution in the Matomb area. The study area is subject to a four-season equatorial climate. The relief is marked by the presence of hills with steep slopes. The gneissic substratum is covered by red ferrallitic soils. The vegetation is secondary forest. The hydrographic network is dense and dendritic. To achieve the objective of this work, the methodological approach adopted after bibliographic surveys consisted of systematic prospecting and the collection of samples. The samples were subjected to petrographic, sedimentological, mineralogical and geochemical analyzes. The soils are characterized by the presence of rutile in the relics of quartz veins. Physical analysis of the sediments reveals that the particle size varies between 2 mm and 200 μm. These sediments are rich in rutile for all size fractions. Observations with a binocular magnifying glass indicate that the materials consist on average of 39% quartz, 41% rutile, 6% zircon, 4% kyanite and 3% brookite. The study of heavy minerals shows a mineralogical procession made up of quartz, rutile, zircon, tourmaline, andalusite and kyanite. X-ray diffractions indicate that the studied materials are characterized by a high proportion of quartz and rutile. The samples of rocks, laterites and sediments were subjected to geochemical analyzes. The results show that these samples are rich in SiO2 and to a lesser extent Fe2O3 and Al2O3. There is also a richness in a wide range of trace elements such as Ba, Cr, Ni, Sr, Ti, V, Y Zn and Zr. They are characterized by concentrations of light rare earth elements much higher than those of heavy rare earth elements with strong negative anomalies in europium. TiO2 shows good positive correlations with Fe2O3, SiO2, SiO2, Al2O3, K2O, Cr, V and Ba. The sediment concentrates consist mainly of rutile with TiO2 contents of around 95%. Among the trace elements and rare earth elements, only Cr, V and Nb have significant contents in the concentrates. The low contents of TiO2 in rocks and laterites testify to a heterogeneous distribution of rutile in these materials. The relics of quartz veins in laterites and alluvium are favorable for the exploitation of rutile.