Mineralogical and geochemical characterisation of the Kgwebe volcanic formation in the Ghanzi-Chobe belt portion of the Kalahari copper belt: insights into copper and silver source (s)

Abstract

A combination of petrography and geochemistry (XRF, ICP-OES and ICP-MS) was utilized in order to investigate the magma source(s), magma evolution and petrogenesis implications of the Kgwebe Volcanic Formation (KVF) rock units in the Ghanzi-Chobe Belt portion of the Kalahari Copper Belt (NW Botswana). Based on their texture, the KVF rock units were discriminated into amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks, porphyritic subvolcanic bodies, quartz-feldspar porphyries and clastic sedimentary rocks (intercalated with amygdaloidal rocks). The amygdaloidal rocks commonly crosscut by calcite veins, are characterized by amygdales-filled vesicles set in hyalopilitic groundmass. The porphyritic mafic flows are essentially made up of plagioclase laths set in a sericitised groundmass. The pyroclastic rocks are made up of both phenocrysts and lithic fragments set in a fine-grained microlithic groundmass. Porphyritic subvolcanic bodies are characterized by high content in subrounded epidote (after plagioclase) set in a microcrystalline groundmass. The quartz feldspar porphyries are characterized by quartz and feldspar phenocrysts in variable proportions set in a fine-grained groundmass of variable colour. Magmatic rocks of the KVF represent two geochemically contrasting series that are not co genetic and derived from different geotectonic settings. The amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies are subalkaline basalt in composition, whereas the quartz-feldspar porphyries span from the rhyolite/dacite (mostly) to the andesite/andesite basalt (accessorily) field compositions. Likewise, while quartz-feldspar porphyries are consistently high-K calc-alkaline bodies, their spatially associated amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and porphyritic subvolcanic bodies counterparts are calc-alkaline. Similarly, quartz-feldspar porphyries are within plate bodies, whereas the more mafic KVF units are arc-related. Furthermore, the ferroan quartz-feldspar porphyries generated from the crust and the magnesian amygdaloidal rocks, porphyritic mafic flows, pyroclastic rocks and 5 porphyritic subvolcanic bodies are mantle-derived, with however, significant crustal contamination fingerprints. Textural evidences (resorbed, embayed and rounded phenocrysts) as well as the inferred high-water content (as supported by the abundance of plagioclase phenocrysts) also suggest a rapid magma ascent and magma mixing occurred during the KVF eruption, which was both effusive and explosive. Concomitant volcanism and sedimentary processes likely occurred as supported by the intercalation of clastic sedimentary rocks within volcanics of the KVF. Furthermore, The KVF rock units' Cu and Ag quantities were calculated to be 1183 million tonnes and 42 million tonnes, respectively. Because the total metal budget of the KCM deposits exceeds 500 million tonnes, the KVF is likely to have accounted for some extent to the total metal budget at the KCM property. But apart from the KVF, additional sources of Cu and Ag such as Ngwako Pan and D’Kar formations are to be considered.