Mineralogical and geochemical features of the coarse saprolite developed on orthogneiss in the SW of Yaoundé, South Cameroon

Abstract

A petrological investigation was performed in the coarse saprolite on orthogneiss in Yaoundé (South Cameroon) using combined whole rock geochemical (XRF, ICP-MS) and mineralogical (XRD, SEM) techniques. The orthogneiss has high contents in SiO2 (61.56wt.%), Ba (916ppm) and REE (209ppm), moderate content in Al2O3 (14.34wt.%) and negative Eu anomaly (Eu/Eu*=0.68). The weathering leads to the formation of three main constituents in the coarse saprolite: (i) the loose materials (∼85vol.%) are basically clayey silty with relic structure. They are composed of kaolinite, quartz and goethite. The loose materials have high contents in SiO2 (56–64.83wt.%) and Al2O3 (21.48–23.96wt.%), and moderate contents in V (163–236ppm), Ba (95–340ppm) and Zr (160–313ppm). The REE content is low (∼49–169ppm) relative to the parent rock with LREE-enrichment (LREE/HREE∼7–17). Positive Ce anomaly (Ce/Ce*∼3.35) is observed in the white veins and slight positive Eu anomalies (Eu/Eu*∼1.2–1.4) are noted in all loose samples. The (La/Yb)N ratios (∼0.8–1.5) indicate high REE-fractionation. The mass balance calculation reveals the depletion of several elements except Al, Ti, Sc, Y, Th, Sb and Hf; (ii) the iron duricrust (∼10vol.%) is located at the bottom and the top of the horizon. The mineral assemblage is dominated by hematite and goethite. The upper iron duricrust has high contents in Fe2O3 (45.60wt.%) and Cr (1641ppm), moderate contents in V (459ppm) and Zn (143ppm), and low REE content (47ppm) with low LREE/HREE ratio (4.28). The upper iron duricrust is more enriched in Fe2O3 (53.26wt.%) than the lower one. Vanadium, Cr and Zr have high contents relative to other trace elements. The REE content is low (39ppm) as well as the LREE/HREE ratio (2.94). The iron duricrust has negative Ce anomalies (Ce/Ce*∼0.66–0.69) and very low (La/Yb)N ratios (0.1–0.3). Several elements reported in the iron duricrust are highly leached except Fe, Cr, Zn, Sc, V, Pb, Zr, Cu and Th; and (iii) the Mn-rich materials (<5vol.%) are made up of birnessite, cryptomelane, and low quantities of quartz, kaolinite and goethite. The SEM investigation reveals that Ba and Pb are linked in Mn-bearing phases and Ce-oxides appear as fine-grained intergrowth between Mn-bearing phases. The Mn-bearing phases are enriched in MnO (33.86wt.%), BaO (4.30wt.%), Co (1716ppm), Pb (1315ppm) and Ce (5202ppm). Positive Ce and Eu anomalies are observed (Ce/Ce*∼15.60 and Eu/Eu*∼2). The mass balance calculations indicate the strong accumulation of Mn, Ni, Co, Zn, Sc, Cu, Ba, Pb, Y, Ga, Zr and REE. The Mn-bearing phases might be derived from the accumulation of silicate residues and Mn within the parent rock.The transition from the parent rock to the coarse saprolite is marked by high leaching of several elements.