Neoproterozoic metasediments and base metal mineralisation in an area north of the Mpande Dome in southern Zambia: an alternative lithostratigraphy classification

King, Heather Leslie
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Prospecting in 2013 to 2015 identified rift-related metasediments in the Mpande Dome area, south of Lusaka, Zambia. The metasediments were thought to be comparable to the Roan Group of the Katanga Supergroup within the Zambian Copperbelt and therefore prospective as a target for sediment-hosted, stratiform copper mineralisation. The metasediments of this area, known as Shantumbu, were last actively explored in the 1930 to 1960s’. Shantumbu is approximately 30km south southeast of Lusaka, Zambia and lies on the northern margin of the Mpande Dome which is south of the Mwembeshi Zone. The Shantumbu area is a highly tectonised region as it is the convergence point of the Zambezi Belt, the Lufilian Arc and Irumide Belt. Simpson et al. (1963), Smith (1964), and Mallick (1966), classified the metasediments as belonging to the Chunga and Cheta groups (Smith, 1964) or the Shamazio, Kafue and Cheta groups (Mallick, 1966), of the Katanga Supergroup. In 2000, Porada and Berhorst proposed correlation of the stratigraphy with the Katanga Supergroup in the Zambian Copperbelt. For many decades, the lack of outcrop and exploration information and the effect of Pan African deformation and high-grade metamorphism, impeded prospecting. The recent exploration activities on Shantumbu, which included logging of cores, petrography, geochemical and mineralogy, highlighted a fresh examination of the metasediments and the copper mineralisation in this area were required. This is the first detailed information in recent years on the metasediments around the Mpande Dome. The result of the research has showed the adoption of the Roan Group stratigraphy of the Katanga Supergroup, as it occurs in the Zambian Copperbelt, is proposed rather than the nomenclature for metasediments forming part of the Zambezi Supracrustal Sequence. An alternative stratigraphic classification to the nomenclature of the Chunga and Cheta formation has been raised, indicating the metasediments north of the Mpande Dome are the southernmost limit of the Roan Group in Zambia. It therefore appears the Proterozoic rift related terrigenous and marine sediments of the Katanga Supergroup are continuous from the Central African Copperbelt southward to the Kafue area and Shantumbu. The metasediments at Shantumbu progress from arenites and siltstones above what is interpreted to be Kafue Rhyolite and Nazingwe metavolcanics. The metavolcanics cap the alkali granites and gneisses of the Basement. The proportion of arenites deposited diminished and gave way to siltstone-dominated units, which in turn gave way to carbonate and lesser siltstone units. A marine transgression surface terminated the detrital units. The siliciclastic units below the transgression surface correlate to the Mindola Formation. The dolomitic carbonate and arenite units correlate with the Copperbelt Orebody Member of the Kitwe Formation, Roan Group. Carbonates, and lesser siltstone and arenite units, deposited in a near-shore marine environment overlie the Copperbelt Orebody Member, and correlated with the Kitwe Formation. The youngest succession identified on Shantumbu comprised calcitic carbonates deposited in a marine basin progressively starved of terrestrial sediments. The carbonates are correlated with the Bancroft Member of the Kirilabombwe Formation. Petrographic examination of the drill core, together with mineralogy studies and geochemical analyses, have resulted in a contemporary account of the rift basin architecture in the Shantumbu area to be presented. New insight into the depositional environments related to rift initiation, climax and cessation, sulphide mineralisation, and oxic and anoxic conditions during deposition, were obtained. The geochemical characteristics and mineralogy indicate the metasediments were subjected to a complex range of alteration and metasomatism. The alteration mineralogy and presence of minerals such as glaucophane, epidote, albite and sphene, indicated the area was subjected to low-temperature (250 - 400oC) and high-pressure metamorphism, that of epidote-amphibolite facies, and retrograde metamorphosed to greenschist facies. Potassic and sodic alteration is characteristic of the detrital lower successions and the upper carbonate successions respectively. Alteration caused by diagenesis-compaction, dewatering of the terrestrial sediments, and later stage remobilisation of potassic and sodium-bearing fluids. Deformation of the area was related to the Pan African Orogeny. Mineralisation on Shantumbu is associated with arenites capped by siltstone near the first occurrences of dolomitic carbonate in the stratigraphy. Most copper deposits in the Zambian Copperbelt are found within 200m of a marine transgression surface, within and adjacent to the Copperbelt Orebody Member. The formation of stratabound base metal sulphides in the lower detrital sequences versus the lack of base metal sulphides in the marine-dominated sequences, suggests the base metals were sourced from the Basement units and transported via basinal brines, rather than introduced from a magmatic origin. The copper sulphides intersected in drill core at Shantumbu, and the copper oxides which crops out, are comparable to the sediment-hosted, stratiform copper deposits of the Zambian Copperbelt, and other sediment-hosted, stratiform copper occurrences globally, such as White Pine in Michigan, the Zechstein Basin in Europe, Spar Lake in Montana, the copper occurrences in Namibia and Botswana on the Kalahari Craton, and the Mangula and Mhangura deposits in Zimbabwe. The continuation of the metasediments from Shantumbu north through the Chongwe Copperbelt and the southerly termination of the Roan Group on the northern margin of the Mwembeshi Zone, strengthens the argument that Shantumbu is contiguous with the Roan Group of the Katanga Basin. The age of the Kafue Rhyolite and Nazingwe formations in the Mpande Dome region, have been age dated at 879 ± 19 Ma (Hanson et al., 1994; Selley et al., 2005; Johnson et al., 2007), which has provided further support that Shantumbu is part of the Neoproterozoic Katanga Basin system, rather than the older rift systems to the south of Zambia, such as the Zambezi Belt or the Magondi Mobile Belt. The examination of the metasediments on Shantumbu has shown that further research would advance the understanding of the rift basin stratigraphy and related sediment-hosted, stratiform copper mineralisation. Such research topics include the investigation of the presence of metavolcanics on the northern margin of the Mpande Dome and the Kafue Rhyolite and Nazingwe Formation hosting copper sulphides, and a comparison and correlation, or otherwise, of the metasediments comprising the Chongwe Copperbelt to the Shantumbu metasediments.
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophyto the Faculty of Science, University of the Witwatersrand, August 2018
King, Heather Leslie (2018) Neoproterozoic metasediments and base metal mineralisation in an area north of the Mpande Dome in southern Zambia: an alternative lithostratigraphy classification, University of the Witwatersrand, Johannesburg,