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

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