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The Multiphase origin of the base metal deposits


The Central African Copperbelt hosts world class stratiform Cu(-Co) deposits and veintype base metal deposits in the Neoproterozoic Katangan Supergroup in the Democratic Republic of Congo (DRC) and Zambia. Based on literature review, satellite image interpretations and petrographic, geochemical and fluid inclusion analyses, multiple mineralisation-remobilisation phases that occurred during different stages of the evolution of the basin have been identified for the generation of the mineral deposits.
The first and likely main period of mineralisation occurred after sedimentation in the Katangan part of the Copperbelt, during early to intermediate diagenesis of the lower part of the Roan Group. Early diagenesis started with the precipitation of framboïdal and euhedral pyrite. The main phase of the stratiform mineralisation followed pyrite precipitation and consists of disseminated copper and often cobalt sulphides and sulphides (carrolite, chalcopyrite, bornite, digenite and chalcosite) in nodules and lenses, which are often pseudomorphs after evaporites (Muchez et al. 2008).
A diagenetic origin is supported by the Re-Os dating of chalcopyrite in the Konkola deposit (Zambia) at 816 ± 62 Ma (in Selley et al. 2005) and geochemical data. This mineralisation phase can be related to the early Katangan rifting of the basin during the Roan (Muchez et al. 2007), leading to the formation of a passive continental margin.

A second mineralisation and/or remobilisation phase is characterised by the occurrence of Cu and Co sulphides in dolomite and quartz veins that crosscut the nodules and lenses of the mineralisation phase. A remobilisation is suggested by the identical mineralogy of the sulphides of the first phase and in the veins and geochemical similarities between both phases.
Two periods can tentatively be suggested for formation of these mineralised veins. Firstly during the Late Roan, when volcanic and magmatic rocks were emplaced in the continental rift setting and when an elevated heat flow was present(Muchez et al. 2007). Secondly during the Lufilian orogeny between 592 and 512 (Rainaud et al. 2005) that caused the deformation of the Katangan sediments. Radiometric dating of sulphides at Nkana, Chibuluma and Nchanga (Zambia) revealed mineralisation ages around 583 and 526 Ma (Barra et al. 2004).
During the latter period also mineralisation of tectonic breccias in the Roan, related to the Lufilian orogeny, could have taken place. Vein-type polymetallic and Cu-Ag deposits also occur in the Copperbelt (e.g. Kipushi and Dikulushi). Structural and radiometric evidence indicate these deposits formed during and after the Lufilian orogen.

Superficial weathering of the primary sulphide deposits resulted in the formation of Cu and Co-hydroxides, -oxides, -silicates or -carbonates at the surface. In addition to an extensive set of newly formed minerals, this secondary enrichment also resulted in an upgrade of the Cu and Co content.

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