Mineralogical and geochemical evolution of pre-lateritic soil profiles over schist basement of the Lom series (Bétaré-Oya, East Cameroon): Implication to rock weathering and lithologic constraints on trace elements fractionation

Abstract

A petro-geochemical study of two weathering soil profiles developed on Neoproterozoic schist of Bétaré-Oya in eastern Cameroon is conducted in order to assess lithologic constraints of trace elements dynamics through weathering and lateritization. Macroscopic description of soil profiles revealed a saprolitic C horizon, a nodular B horizon and a clayey-sandy B horizon on upslope; while the downslope evidences a saprolitic gleyic C horizon, a pseudo-gleyic B horizon and nodular B horizon. These differentiated weathering soil profiles result from in-situ weathering of schist that implied a mineral assemblage dominated by residual quartz (40–60%), feldspars (5–11%), muscovite (5–9%), chlorite (10%) and secondary kaolinite (22–29%), goethite (5–11%), hematite (5–14%) and halloysite (7–9%). Several geochemical indices i.e. CIA, PIA, TRB, IOL, WIP, RR and correlative ternary plots highlight moderate weathering intensity with kaolinization as the major soil forming process, leading to formation of ferriferous kaolin and kaolin soils. These findings are in contrast with surrounding highly evolved Adamaoua and South Cameroonian plateaus covered by thick lateritic landsurfaces and point out the role of lithology, especially high Si contents on weathering intensity and lateritization in Bétaré-Oya. The dominant kaolinic process is accompanied by enrichment of SiO2 (53.9–80.3%), while Al2O3 (9.1–19.4%), Fe2O3 (3.5–19.9%) and TiO2 (0.8–1.3%) experience complex behavior in soil profiles. Trace elements (LILE, TTE, HFSE, SME, PME and PGE) are mostly retained in nodular horizons and marked by a slight enrichment of U (0.7–4.6 ppm), Pb (4.2–49.1 ppm), Sb (0.2–1 ppm), Mo (0.2–5 ppm), Pt (0.4–2.1 ppm), Sn (0.5–1.3 ppm) and Sc (2.5–8 ppm). Meanwhile, Zr, Th, As, Ni, Ba, Rb, Cs, Au, Pd, Cr, Co, Cu, V, Sr and Zn are leached through the development of residual soil profiles. Secondary Al and Fe rich-minerals, accessory sphene and apatite have been reported as the main potential host minerals of these trace elements, according to Spearman's correlation. REEs contents (67.2–203.4 ppm) and their normalized patterns further show a weak fractionation with preferential enrichment in the lowermost horizons of soil profiles. LREEs are more abundant than HREEs, whereas HREEs are more fractioned than LREEs. Positive Ce anomalies observed in the nodular and surface horizons are attributed to oxidizing conditions, while negative Ce anomalies related to saprolitic and pseudo-gleyic horizons are assigned to reducing conditions. Moreover, positive Eu anomalies are linked to residual feldspars. Spearman's correlation coefficients reveal P-bearing minerals (e.g. apatite and florencite) as the main potential host mineral of REEs. Likewise, accessory minerals (sphene and zircon), newly-formed (kaolinite, hematite and goethite) and resistant primary minerals (feldspars, muscovite and chlorite) play prominent roles in their retention/release in the Bétaré-Oya weathering products. However, downward enrichments of trace elements (including REEs) and their affinity with Fe-enriched horizons could be instrumental for a comprehensive model of their fractionation/enrichment. But, the overall low fractionation rate of trace elements in the study area is correlated to moderate weathering intensity and consequently provides evidence of lithology on distribution and mobilization of trace elements.