Geochemistry of pyroxenites, amphibolites and their weathered products in the Nyong unit, SW Cameroon (NW border of Congo craton): Implications for Au–PGE exploration

• Published in: Journal of geochemical exploration 2012-03, Vol.114, p.1-19
• Main Authors: Ebah Abeng, Sandrine A., Ndjigui, Paul-Désiré, Beyanu, Aye A., Teutsong, Tessontsap, Bilong, Paul
• Format: Article
• Language: English
• Subjects: Au–PGE exploration; Geochemistry; Mafic–ultramafic rocks; Nyong unit; SW-Cameroon; Weathering
• ISSN: 0375-6742; 1879-1689
• DOI: 10.1016/j.gexplo.2011.11.003

The Nyong unit is a greenstone belt at the NW border of the Archaean Congo craton. In this study, results of initial geochemical prospection for gold and platinum-group elements (Au–PGE) in pyroxenites, amphibolites, and their weathered products in the Nyong unit are reported from two areas, Lolodorf and Nyabitande. Garnet-bearing pyroxenites and pyroxenites devoid of garnet were investigated in the Lolodorf region. The main minerals in these rocks are clino/orthopyroxene+biotite+amphibole+feldspar±garnet±quartz±opaque minerals corresponding to a granulite facies assemblage. At Nyabitande region, garnet-bearing amphibolites and non-garnetiferous equivalents were identified. These have a granoblastic heterogranular texture, with associations of amphibole+plagioclase+quartz+opaque minerals+garnet±sulphides indicating the amphibolite facies. Both rock types are characterized by: (i) moderate SiO2, Al2O3, Fe2O3, CaO, MgO, Cr, V, Ni, Zn, Cu, Co, and Sc contents; (ii) low lanthanide contents with variable LREE-enrichment; and (iii) positive Ce and negative Eu anomalies. The (La/Yb)N ratios are only high in amphibolites. The Au–PGE contents are low (25.1 to 120.9ppb). Palladium and Pt are the most enriched PGE. These rocks are highly enriched in PPGE than in IPGE. The Pt/Pd ratios indicate that Pt is more mobile than Pd. They have positive Pd anomalies. The Pt/Ir, Pt/Rh, Pt/Ru, and Pt/Au ratios are variable, and indicate that Ir, Rh, Ru, and Au are more mobile than Pt. Weathered pyroxenites and amphibolites are also characterized by moderate SiO2, Al2O3, Fe2O3, and some transition trace metal contents. They are, in addition, marked by moderate REE contents and LREE-enrichment. The (La/Yb)N ratios indicate that weathered pyroxenites have more fractionated signatures than the weathered amphibolites. All the weathered samples have low Au–PGE contents with PPGE-enrichment. The Pt and Pd contents increase from the parent rock to the slightly weathered amphibolites contrary to the weathered pyroxenites. Mass-balance calculations reveal a strong accumulation of Fe, Mn, Ti, Al, some trace and rare-earth elements; and strong leaching of Au–PGE in several weathered samples. Pyroxenites, amphibolites, and their weathered products here, therefore, are not judged to have a favorable potential target for further Au–PGE exploration. ► A petrological investigation was done on mafic–ultramafic rocks in SW Cameroon. ► The studied samples have moderate ferromagnesian element conten