Petrography and geochemistry of weathered pyroxenite boulders from Mamb, Ngong and Nkolmbong (Pan-African Yaoundé group, Cameroon): tracking the Ni, Co, Cr and Cu mineralization processes during the alteration of ultramafic rocks

Abstract

In this study, mineralogical and geochemical characteristics of weathered pyroxenites boulders of Mamb, Ngong and Nkolmbong areas (Pan-African Yaounde group) are determined in order to understand the mineralisation processes of Ni, Co, Cr and Cu during the early stage of weathering of ultramafic rocks. These pyroxenites are intruded in micaschist at Mamb and Ngong or in migmatitic gneiss at Nkolmbong. Fresh samples are made up of clinopyroxene, orthopyroxene, amphibole and accessory biotite, plagioclase, chlorite and sulphides. Their peculiar chemical features include high Ni (58–194 ppm), Co (46–137 ppm), Cr (875–3017 ppm), Zn (37–185 ppm) and Cu (27–159 ppm) contents. REE contents vary in a large range (ΣREE 59–478 ppm) with LREE-enrichment (LREE/HREE 3–6) and negative Ce and Eu anomalies. The thin (≤ 10 cm) weathered rims comprise alternating black streaks and red beds made up of kaolinite, gibbsite, goethite, hematite, halloysite and maghemite. The alteration intensity is marked by a decrease of the rock density and an increase of its porosity and Chemical Index of Alteration (CIA) values. The supergene alteration of fresh pyroxenites boulders yields Ni-, Co-, Cu-poor, but Cr-slightly enriched weathered rims. Conversely, the lateritization of pyroxenite boulders having previously undergone a hydrothermal alteration into chlorite-rich rocks leads to the formation of clay-, Ni-, Co-, and Cu-rich cortices. These cortices are peeled off during weathering and got mixed with the soils formed under and around the boulders. So, the current weathered rims are probably related to the recent climatic variations and the soils are the host of potential concentration in Ni, Co, Cu and Cr.