The Kalahari Copperbelt in central-eastern Namibia

The continuity of the Kalahari Copperbelt (KCB) beneath the Cenozoic cover of the Kalahari Group in central eastern Namibia has long been assumed, but was only confirmed by exploration drilling in 2010 when Eiseb Prospecting and Mining (EPM) uncovered Ag-bearing Cu sulfide mineralisation comparable to that found elsewhere in the belt. The geology of this region has not been described in any detail in the literature to date. Zircon geochronology suggests that sedimentation of the Eiseb started at ~1170 Ma. An uplifted basement horst of deformed acid volcanics marks the western edge of the Eiseb. There is no eastern border to the Eiseb, which extends into the Ghanzi-Chobe Belt of Botswana. Deformation and folding of the belt occurred during the Pan African Damara Orogen which peaked at ~530 Ma. Cu-Ag mineralisation is disseminated across a range of rock types, from the volcanic basement horst, to sandstones and argillites. Mineralisation also occurs in veins, often discordant to stratigraphic boundaries, and within the coarse laminae of interbedded siltstones. The preservation of delicate sulfide replacement textures of evaporite minerals in micro-folded rocks suggests that the mineralisation is largely epigenetic, favouring pressure shadows and foliation on a local scale, and fold-closures, faults and thrusts on a regional scale. Magnetite is commonly associated with with Cu-Ag mineralisation both textually and spatially, across a range of rock types. Paleomagnetic methods were unable to constrain the timing of magnetite growth. The trace element contents of magnetite, as deduced by laser ablation inductive coupled plasma mass spectrometry (ICP-MS), is able to distinguish between barren and Cu-Ag mineralised host-rocks using element ratios. The V vs. Ni binary plot is effective for the acid volcanic rocks, and the V vs. Co plot distinguishes between mineralised and un-mineralised sedimentary rocks. Magnetite trace element concentrations show that it formed from hot (150-250˚C) metalliferous fluids with an IOCG (iron-oxide-copper-gold) affinity. The most likely mechanism for magnetite formation is by replacement of pyrite, with textures suggesting this occurred during deformation, i.e. during the Damara Orogeny. The syn-deformation, epigenetic Cu-Ag mineralisation recognised in the Eiseb has been reported from numerous other deposits traditionally classified as ‘sediment-hosted stratiform copper’ (SSC). In these deposits mineralisation has been shown to be coeval with regional plate movements and orogenesis, which is fundamentally different to the SSC model where mineralisation is related to diagenetic processes. An alternative mineralisation model is thus proposed, orogenic-sediment-hosted-copper (O-SSC).