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Platinum group elements (PGEs) belong to Group 8, periods V and VI of the transition metals and they include platinum (Pt), palladium (Pd), rhodium (Rh), Iridium (Ir), Osmium (Os) and ruthenium (Ru). These six chemically similar elements comprise the light platinum elements (Ru, Rh, Pd) and the heavy platinum elements (Os, Ir and Pt). Together with gold and silver, the PGEs are precious metals and occur rarely in the earth’s crust. The average crustal abundance of Pt and Pd is 5 ppb while Rh, Ru and Ir are even rarer with approximately 1 ppb. Due to their similarity in physical and chemical characteristics, the PGEs often occur naturally together and can be mined together as coproducts with Pt normally providing the reason behind PGE production levels (Polinares Consortium, 2012). These elements are strongly siderophile and chalcophile (e.g. Brain, 2011; Ranvide et al., 2015) and have similar geochemical behaviors during magmatic processes. Traditionally, the PGEs can be subdivided into two groups, the compatible IPGEs which include Os, Ir and Ru and the incompatible PPGEs which are made up of Rh, Pd and Pt (Qing et al., 2012). It has been suggested that the IPGEs are refractory and tend to be retained in the mantle peridotite during partial melting (Brain, 2011; Evans et al., 2012; Barnes et al., 2011). Most economic PGE deposits belong to a continuum of Ni-Cu-PGE sulfide mineralization styles from those that tend to be sulfide-poor and mined mainly for their PGE contents, to those that are sulfide-rich and are mined mainly for their NiCu contents (Keays and Lightfoot, 2002). PGE mineralization in magmatic deposits is formed in three different ways (Holwell and McDonald, 2010). These include (1) fractional separation of PGEs during magma crystallization, (2) PGE concentration in the oxide-ore phase forming segregations, and (3) PGE extraction from magma into immiscible sulfide liquids. Abstract: Meta-ultramafic rocks in the Paleoproterozoic Nyong Series (SE Cameroon) which occur as intrusions within the metasedimentary and meta-igneous units have been studied. The aim was to study the concentration of platinum group elements (PGEs) within the layered sequence and suggests models of their depletion relevant to the rocks. Twenty-seven samples of the ultramafic rocks were collected from the different layers of the sequence and analysed for their Ni, Cu, Au and PGE contents. Ni, Cu, Au, Pt and Pd were analysed by ICP-MS while Ir, Os, Rh and Ru were analysed by INAA. Results of the analyses show that the concentrations of Cu, Au, PGE and ΣPGE are very low throughout the cliff face while Ni concentrations are high and variable. The effect of alteration of the rocks has resulted in the low PGE concentration due to remobilization. It is also suggested that the rocks are PGE-depleted due to low degree of partial melting or depleted mantle source and various models have been used to explain this depletion as related to the Nyong Series.

Geological Models of Platinum Group Elements (PGE) Depletion in Metamorphosed Ultramafic Rocks of the Nyong Series, Southeast Cameroon