Sulfide-Bearing Veinlets Throughout the Stratiform Mineralization of the Central African Copperbelt: Temporal and Genetic Implications

Abstract

The Central African Copperbelt of Zambia and Democratic Republic of Congo, along with its broad time equivalents in Botswana and Namibia (the Kalahari Copperbelt), is the world’s largest sediment-hosted, stratiform copper province. The economically dominant stratiform copper ± cobalt or silver mineralization, accepted by many as at least partly early diagenetic in origin, is ubiquitously accompanied by sulfide-bearing quartz-carbonate veins and veinlets. Traditionally, this veining is considered to be the product of either metamorphic lateral secretion or discrete late-diagenetic to synorogenic mineralization events. This view is challenged because identical sulfide assemblages and textures characterize both the disseminated and accompanying veinlet mineralization throughout the zoned sulfide assemblages that constitute many of the stratiform orebodies, a situation that is taken to imply contemporaneity of the two styles. Consequently, during the mineralization, the siltstone, sandstone, or dolomite host rocks must have been sufficiently competent to undergo widespread brittle fracturing, implying that the disseminated mineralization cannot have been introduced by passive fluid infiltration during early diagenesis. This conclusion is further supported by the local occurrence of minor disseminated and veinlet mineralization hosted by mafic sills and dikes. Both the disseminated and veinlet mineralization styles lack ductile deformation features in many Congolese deposits, but are variably metamorphosed and deformed in some deposits in Zambia, Botswana, and Namibia. In conjunction, these observations suggest that the stratiform copper orebodies were the results of massive saline fluid expulsion by hydraulic fracturing after the mafic magmatism (~765–<715 Ma), most probably spanning peak Damara-Lufilian metamorphism and ductile deformation (~530 Ma) and continuing in places until at least ~500 Ma. Thus, basin inversion, contractional tectonism, and associated uplift and exhumation, and not the earlier extension and rifting, seem more likely to have been the ultimate drivers for the fluid mobilization and expulsion.