Evolution of weathering products over mylonites of the Lom series at Kissi (Bétaré-Oya Gold District, East-Cameroon): Morphological, mineralogical and geochemical mechanisms for trace and rare earth elements enrichment

Abstract

Weathering and lateritization processes and their implication to the fractionation of trace and rare earth elements (REEs) in soil materials developed upon mylonites in Kissi area of the Bétaré-Oya Gold District (Lom series, East Cameroon) were assessed through morphological (position on the slopes), mineralogical (X-ray diffraction) and geochemical (X-ray fluorescence and Inductively Coupled Plasma-Mass Spectrometry) investigations. Three (03) representative soil profiles were selected and described according to the topography (upslope, mid-slope and downslope). The resulted weathering products are rich in kaolinite (16–49%), gibbsite (3–17%), goethite (5–10%) and hematite (2–10%), with quartz (30–49%) and muscovite (4–31%) as inherited minerals. Geochemical proxies and related ternary diagrams signal an intense chemical weathering (CIA: 43.3–100%; PIA: 41.9–100%), but weak lateritization (IOL: 11.3–40.8%) of parent mylonite under warm and wet tropical climate, that induce the formation of kaolinitic laterites. These processes are characterized by a weak leaching of SiO2 (79.7–52.2%), but strong one of labile elements (WIP: 48.7 to 0.8%; TRB: 228.9 to 15.1 mg kg−1). Al2O3 (16.2–22.8%) and Fe2O3 (4.2–14.0%) are the slightly enriched major oxides in the weathered materials. Some trace elements including Sc, V, Cr, Co, Ni, Cu, Ga, As, Li, Y, Mo, Cs, Hf, W, Pb, Th, Tl, U, Zr and REEs are reported as strongly enriched compared to mylonite, but weak mineralized according to their average contents in the earth's crust (Maximum Enrichment factor: 1.1–38.8). This gives rise of low mining potential of the current weathering products. Nevertheless, Ce (37.8–453.6 ppm), Ba (34.2–379.6 ppm), Cr (100.0–270.0 ppm), V (108.9–215.0 ppm), Zn (23.4–192.0 ppm), Ni (29.3–170.0 ppm) and Rb (2.5–140.0 ppm) are the most abundant in the soil profiles. Topographically, trace and REEs are preferentially enriched and mineralized at mid-slope and downslope of the interfluve. But, the overall behavior and enrichment of elements in Kissi are controlled by the dissolution of rock-forming minerals and their release in the residual soil system, their incorporation into the crystalline structure of newly formed secondary minerals and the lateral water runoff along the slope gradient.