Geochemistry and geochronology of the peripheral intrusions to the Kunene Anorthosite Complex in southern Angola and northern Namibia

Abstract

Proterozoic massif-type anorthosites and their associated rocks have become important targets for Ni-Cu-Co(-PGE) exploration since the discovery of the giant Voisey’s Bay Ni-Cu-Co deposit in the Nain Plutonic Suite, with the recent green energy revolution further increasing the demand for Ni and especially Cu. The Mesoproterozoic Kunene Complex is a massif-type anorthosite complex situated in southern Angola and northern Namibia. In the late 1990s, it was discovered that the mafic/ultramafic intrusions along the periphery of the complex host appreciable, although non-economic, Ni-Cu sulphide mineralisation. Civil unrest in Angola impeded research efforts in the past which means that overall knowledge of the complex is still restricted. This study focussed on seven of the Ni-Cu mineralised peripheral intrusions: Ohamaremba, Ombuku North, Ombuku South, Oheuwa, Onyokohe and Ongoro, all in Namibia, and Oncocua in Angola. One of the main unresolved aspects is the temporal relationship between the peripheral mafic/ultramafic intrusions and the main complex. It is crucial to understand this relationship as it would provide analogies to the Voisey’s Bay Intrusion which is coeval and cogenetic with the Nain Plutonic Suite. It is equally important to understand the geochemistry of the intrusions which would help to understand the differences between intrusions and to identify potential vectors to mineralisation. This study produced six new U-Pb zircon and apatite ages on four of the peripheral intrusions. A U-Pb zircon age of ca. 1400 Ma was obtained for one sample from Ohamaremba, whereas another sample from the same intrusion yielded two ages at ca. 1720 and ca. 1750 Ma. A U-Pb zircon age of ca. 1760 Ma was obtained for the Otjijanjasemo intrusion, while in-situ analyses of U-Pb in zircon at Ombuku North produced an age of ca. 1350 Ma and analyses of apatite at Oncocua produced an age of ca. 1400 Ma. Forming part of the investigations was a petrographic study to identify silicate and ore mineral textures. Mineralisation primarily comprises high-temperature magmatic assemblages of pyrrhotite-chalcopyrite-pentlandite with scarce secondary sulphides. Sulphide textures range from disseminated (Ohamaremba) to massive (Ongoro and Oheuwa). A detailed geochemical study was done on over 3000 core assays and various aspects such as major and trace element distribution, chalcophile and siderophile element variation, petrogenetic characteristics and base metal and PGE enrichment processes were investigated. It was determined that the major element distribution in the intrusions define a fractionation trend from ultramafic rocks most prevalent at Ombuku North, Ombuku South and Oncocua, to anorthosite, which is most abundant at Oheuwa. Chalcophile and siderophile elements concentrate primarily in ultramafic lithologies, specifically pyroxenite, harzburgite, dunite and serpentinite, and tend to also concentrate at contacts between these rocks and other less MgO-rich rocks. Metal tenors are highest in Ombuku North, corresponding to blebby and net-textured sulphides, while massive sulphide mineralisation in Ongoro, Oheuwa and Onyokohe shows low-metal tenors. Ombuku North shows PGE enrichment and it is inferred that the intrusion did not experience any sulphide segregation and removal prior to emplacement, which is reinforced by the low Cu/Pd ratios. It is inferred that hydrothermal remobilisation of base and precious metals possibly contributed to the enrichment at Ombuku North based on evidence of intense alteration observed in thin section. The other intrusions do not show PGE-enrichment, which provides evidence that they were either subjected to early sulphide segregation or that the initial magma did not contain significant concentrations of PGEs. Four of the intrusions have a calk-alkaline composition, which provides evidence that they formed in an arc-like setting, in agreement with an Andean-type setting proposed by recent studies. The mafic/ultramafic satellite intrusions to the Kunene Complex, based on the geochemistry and analogies with the Voisey’s Bay Intrusion, represent an excellent target for base and precious metal exploration in Africa.