Altération supergène des Itabirites du complexe du Ntem : cas de Nkout par Djoum et d’Anyouzok par Akom II dans le sud Cameroun

Abstract

Itabirite and related soils in the humid tropical zone of southern Cameroon have proven economic and scientific interests. However, they have received very little attention. The itabiriteand morphological, mineralogical and geochemical features of two related soil profiles were studied using macroscopic and microscopic observations, conventional X-ray diffraction (XRD), X-ray fluorescence (XRF) spectrometry and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) analyses. The studied sites are respectively Nkout by Djoum and Anyouzok by Akom II in the Ntem Complex, southern Cameroon.The parent rock in both sites is itabirite, dominated by resistant primary minerals including quartz and magnetite with small amount of hematite. Other mineral phases present include amphiboles, pyroxenes, micas, pyrite and ilmenite. The parent rock in Nkout is a banded itabirite (BIF) composed of alternating bluish grey quartz-rich (chert) and dark olive grey to black magnetite/hematite fine to medium bands with granoblastic structure. The Anyouzok itabirite (GIF) displays an equant granular structure with medium to fine-grained minerals.The two weathering profiles exhibit well differentiated sequences with similar sets of horizons, respectively a lower weathering set, a median ferruginous set and a loose upper set. They also show similar mineralogical and geochemical features marked by the prevalence of hematite, goethite and quartz, and weak peaks of kaolinite and gibbsite due to the very low Al2O3 content. Weathering materials are dominated by Fe2O3t (58.90 to 90.76 wt %) and SiO2 (5.63 to 38.45 wt %) which show an opposite trend along the profile (R2 = 0.61 to 0.78) with upward increase of Fe2O3t. The REE behaviour and specifically the positive Ce anomaly (Ce/Ce* : 1.12 – 2.64) suggests a dominance of the oxidation during the weathering process. Oxidizing conditions led to the development of a high-grade hematite/goethite bearing iron ore (Fe: 50.4 – 63.7%), well represented in Nkout by a 30 m thick direct shipping ore (DSO: Fe ~ 57.4 wt %).