Characterisation of the Platinum Group Minerals in the Ombuku North intrusion peripheral to the Kunene Complex: Insights into its PGE potential

Abstract

The Kunene AMCG (Anorthosite-Mangerite-Charnockite-Granite) Complex, located in the southern part of Angola and northern part of Namibia, is known as one of the most extensive Proterozoic anorthosite complexes worldwide, with an extent of the area ≥ 18 000 km2, and positioned along the southwest margins of the Congo craton. The mafic-ultramafic intrusions within the vicinity of the Kunene Complex are mineralised with nickel, copper, and platinum group elements (PGEs). The area has not been extensively explored, and only minor exploration work has been conducted to search for magmatic sulphide deposits, particularly nickel deposits. The PGE potential of the mafic-ultramafic intrusions in the area and their potential mineral system has not previously been extensively studied. The available geochemical assay data of the Ombuku North intrusion in northern Namibia was sourced from African Nickel Limited (ANL) and was used in this study to assess its PGE potential and to better understand its potential mineral system. Petrographic work demonstrated Ombuku North intrusion was commonly associated with magmatic sulphides (pentlandite, pyrrhotite, chalcopyrite), formed from a Mono Sulphide Solution, and magnetite. Samples were selected for further detailed platinum group mineral (PGM) investigations on the basis of their PGE contents. Ombuku North is characterised by slightly anomalous PGE values and the available thin sections from Ombuku North with corresponding bulk rock PGE values of >80 parts per billion (ppb) were selected for the identification and characterisation of the PGMs, and for detailed petrographic analyses. In this study, PGM mineral analyses were done using a Tescan Integrated Mineral Analyser (TIMA) which is a fully automated instrument, whereas previous mineral analyses for PGMs in Ombuku North and some of the other mafic-ultramafic intrusions in this region were collected using Scanning Electron Microscopy Energy Dispersive Xray Spectroscopy (SEM-EDS) method, which is only partially automated. TIMA could identify the PGM in the selected samples from Ombuku North. The identified PGMs included sperrylite, michenerite, stibiopalladinite, and vincentite, all associated with serpentinised rocks. The platinum arsenides (sperrylite) are mainly hosted in magnetite and in pyrrhotite. The palladium bismuthotellurides (michenerite and vincentite) and a platinum antimonide (stibiopalladinite) are hosted in pentlandite. The origin of these PGMs can be associated with both magmatic and hydrothermal processes. Magmatic processes are primarily responsible for the initial formation and crystallisation of silicate minerals within the mafic-ultramafic intrusions, including primary magmatic sulphide assemblage. Late hydrothermal processes are related to the alteration of these rocks, where fluids circulated through the rock leading to the formation of secondary minerals such as serpentine, talc, and the redistribution of PGE. Although PGE data are also provided also for other intrusions, this study mainly focused on Ombuku North intrusion due to the limited availability of assay data. We infer that amongst all the mineralised mafic-ultramafic intrusions related to the Kunene Complex, the most prospective for PGE anomalies are the altered ultramafic lithologies at Ombuku North.