Lithogenic Concentrations of Useful Elements and Residual Mineralization Indices in the Weathering Products Derived from Mineralized Rocks in Meїganga (Central Cameroon)

Abstract

This paper focuses on the use of morphological, mineralogical and geochemical characterization of weathering profiles from mineralized rocks in Meїganga, a locality in the South-East of Cameroon. It is aimed at assessing weathering processes that induced the lithogenic concentrations of useful elements, and eventually residual mineralization indices in weathered products. Seven weathering profiles derived from four distinct mineralized rocks (micaschists, orthogneiss, granite, and quartzite veins) were studied. The profiles exhibit shallow weathered A/B/C or A/C soil profiles with a moderate thickness (less than 4m). Minerals identified in the weathered products in decreasing contents (%) were: quartz (60.3-93.9), kaolinite (0.8-22.3), phlogopite (0.2-15.3), goethite (1.9-13.0), hematite (0.5-8.5), halloysite (0.2-4.6) and smectite (0.5-4.3). The SiO2 contents generally decrease upward in these weathering profiles, except for those from granitic parent rock. Inversely, Al2O3, Fe2O3 and TiO2 contents increase upward, except for the weathering profiles from granite and orthogneiss. Alkaline and alkaline earths are more or less completely exported during weathering. Chemical weathering parameters have revealed intense rocks weathering in Meїganga, resulting to the important accumulation of quartz in association with 1:1 clay minerals. Trace elements that prevail in these weathered products arranged in decreasing order of abundance include: S, Ba, Sr, Zr, Cr, V, Zn, Rb, Ni, Y, Sb, Cu, Pb, Li, Co, Ga, Nb, Th, Sc, Cs, Hf, Sn, U, Mo, and W. The most significant useful elements identified in these weathered products are arranged in decreasing order of abundance include: Zr, Cr, V, Sb, U, Cu, Nb, Hf, Mo, and W. The weathered products present a CI-chondrite normalized pattern of REEs characterized by the fractionation of HREEs relative to LREEs. The accumulation of trace elements (Ga, W, Y, Sn, Hf, Nb, Cu, Sc, V, Zn, Cr, Sb, Pb, Ni, Co, Li, Mo, Th, Rb, Cs, U, Zr and Ba) and REEs in the weathered products of Meїganga has been attributed to the effects of weathering. The main host minerals are the residual primary minerals (epidote, apatite, pyrite, titanite, zircon, or opaque minerals) and the newly formed secondary minerals (phlogopite, hematite, goethite, kaolinite and smectite). Correlation matrices between useful elements (Cr, Mo, U, V, Nb, Zr, Sb, Hf and W) and major oxides (Fe2O3, TiO2, P2O5 and Al2O3) indicates a strong affinity (>0.80), suggesting the trace elements are noble metals. Therefore, major oxides used as tracers for residual mineralization indices are: Al2O3 (Nb, Mo, U, W and Hf), Fe2O3 (Mo, Cr, V, Zr, Hf and Sb), P2O3 (U, Hf and Sb), and TiO2 (Nb, Zr, V and Cr). In the Meїganga soil profiles, mineralization indices could be assigned to W, V, Hf, Nb, Mo, Cr, Sb and U.