Geochemistry of peridotite xenoliths from the Kumba and Nyos areas (southern part of the Cameroon Volcanic Line): Implications for Au-PGE exploration

The Kumba and Nyos areas belong to the southern part of the Cameroon Volcanic Line (CVL), Central Africa. This study reports geochemical data of peridotites and their weathered products for gold and platinum-group elements (Au-PGE) exploration. Peridotite xenoliths are mainly made up of olivine, pyroxene and spinel. MgO contents are high; SiO2/MgO values are close to 1. The contents of Cr (2681 ppm), Ni (2436 ppm) and Co (130 ppm) are also high. Meanwhile, the total REE contents are variable (similar to 2-60 ppm) but with high LREE-enrichment (LREE/HREE values range from 1 to 17). The Au-PGE contents are low (9-21 ppb). PGEs with higher contents are Ru (7 ppb), Pt (6 ppb) and Ir (4 ppb). The Pt/Pd, Pt/Rh and Pt/Ru ratios show that Pd and Rh are more mobile than Pt. However, Pt is more mobile than Ru. The peridotite xenoliths from the southern part of the CVL are more highly enriched in IPGE than PPGE. The chondrite-normalized Au-PGE spectra display positive Ru and negative Pt anomalies due to the melt extraction. The weathered products from both sites possess their specific mineralogical and geochemical features to the weathering degree which decreases from Kumba to Nyos. The Kumba weathered samples are characterized by high SiO2, and moderate Cr, Ni, Ba, Co, Sr and REE contents. The Nyos samples are slightly weathered, marked by the mobilization of major and trace elements. The total REE content is low in the Nyos area (1-10 ppm). The chondrite-normalized REE patterns reveal LREE-enrichment in Kumba and negative Ce anomalies in Nyos. The Ce-anomalies may be due to the oxidation conditions. The Au-PGE contents are variable from Kumba (2-4 ppb) to Nyos (16-24 ppb). Ruthenium and Pt have good positive correlations with Cr, Ni and Co. Rhodium and Ru are more mobile than Pt in Kumba. Ruthenium is more mobile than Pt, while Pt is more mobile than Rh in Nyos. The chondrite-normalized Au-PGE patterns show negative Pt anomalies in both sites as noticed in fresh rocks. The mass balance assessment shows that PGE are leached from the Kumba site whereas they are slightly enriched in the Nyos area. Overall, the geochemical survey has revealed that peridotites and their weathered products have sufficiently low contents in Au-PGE to justify further Au-PGE exploration. (C) 2015 Elsevier B.V. All rights reserved.