Contribution à l’étude de l’altération des pyroxenites du groupe de Yaoundé (site de Mamb) ; Implication pour l’exploration des éléments utiles nickel, cobalt, or et éléments du groupe du platine.

Abstract

The pyroxenites studied in the Yaoundé Group are intrusive rocks in the micaschists and migmatic gneisses. They are usually moderately and highly weathered around their core. In this work, the mineralogical and geochemical characteristics of these rocks in the Mamb, Ngong, and Nkolmbong areas, as well as those of their weathered products were investigated in order to study the behaviour of Ni, Co and Au-PGE during weathering processes. Pyroxenites are essentially made up of clinopyroxene, orthopyroxene and amphibole. The accessory mineral phase includes biotite, plagioclase and opaque minerals. Chlorite, identified in addition to these minerals in pyroxenites in the Ngong site, testifies a hydrothermal alteration undergone by these rocks. They are composed of SiO2 (47.75-50.32 wt. %), Fe2O3 (11.55-13.68 wt. %) and MgO (15.35-19.73 wt. %), and have high contents in Ni (413-583 ppm), Cr (1406- 1778 ppm), Co (89-108 ppm) and Cu (95-172 ppm). These rocks have low to moderate Rare Earth contents (∑REE ranges from 59.46 to 478.10 ppm), and the condrite-nomalized spectra reveal negative Ce and Eu anomalies. In these rocks, contents in Au (1.2-3.5 ppb) and PGE (1.74-7.1 ppb) are low with positive Pd anomaly. The weathered rims of pyroxenites are made up of kaolinite, gibbsite, goethite, hematite, maghemite, anatase, illite, quartz, pyroxene and amphibole. They are characterized by black concentric streaks that allow the peeling off of the rims into fine beds. This peeling off took place systematically during the weathering processes, and the small thickness (≤ 10 cm) of the current weathered rims are probably related to the recent climatic variations acting during their weathering. In the Mamb site, the weathering of these rocks yields a thin soil profile of 2 m deep, which shows a weak lateritization process in the coarse saprolite while the highly weathered rims are in the kaolinitisation stage. The weathering of the studied pyroxenites is essentially subtractive. The contents of Ni (194-58 ppm), Co (137-46 ppm), Au (4.3-1 ppb), EGP (13-3 ppb) decrease during weathering. For all samples, the REE normalized spectra show negative Ce anomalies, except the coarse-grained greenish-black faciès of Ngong, where the hydrothermal alteration (presence of chlorite) preceding weathering (two sources kaolinite) would have favoured a strong positive Ce anomaly due to its adsorption by kaolinite. This cortex also shows a slight increase in Ni and Co contents due to their incorporation into chlorite relics and kaolinite. These transition (Ni and Co) and precious (Au and EGP) metals are leached during weathering. Therefore, the weathered products of the pyroxenites studied cannot be considered as potential sources of mineralization in Ni, Co, Au and EGP. Nevertheless, during the weathering processes, these metals released from the matrix of ferromagnesian minerals and sulphides got mixed with the soils developed on the host rock where they probably mobilized and accumulated at their base, since they could be incorporated into the clay-ferruginous matrix.