Geology and mineralisation in the Meso- to Neoproterozoic Ghanzi-Chobe belt of northwest Botswana

Abstract

The Ghanzi-Chobe Belt defines a linear zone of volcano-sedimentary rocks that form inliers through the Kalahari Desert of western and northern Botswana. Geological investigations along the Ghanzi Ridge have revealed a sequence composed of a basal bimodal volcanic suite with minor intercalated sedimentary rocks, termed the Kgwebe Formation, and an upper siliciclastic sedimentary unit with subordinate carbonates, called the Ghanzi Group. To the northeast in the Chobe District, basal volcanics informally termed the Goha Hills Formation are overlain by carbonate-bearing siliciclastics assigned to the Chinamba Hills Formation. Geochemical analysis and field relations of the Kgwebe Formation volcanics indicate that the sequences were accumulated in a continental rift basin. Rifting was initiated by extensional tectonics associated with a continental collision along the Namaqua-Natal Belt. The Kgwebe Formation volcanics started to accumulate during Mesoproterozoic times about 1 106 Ma ago. The depositional basin developed from fluvial and lacustrine systems during the accumulation of the Kgwebe Formation, into an extended shallow marine basin that accommodated the Kuke, Ngwako Pan, D'kar, and Mamuno Formations of the Ghanzi Group. Stratabound copper-sulfide mineralisation was developed at a redox interface within the Ghanzi Group. Mineralisation is confined to the basal part of the grey-green, argillitic facies of the D'kar Formation and is either disseminated or is found along cleavage planes and fractures. Minor Cu mineralisation also occurs in carbonates of the Chinamba Hills Formation. Tectonic deformation during the Pan African Damaran Orogeny resulted in a fold and thrust belt known as the Ghanzi-Chobe Belt.