A New Look at the Geology of the Zambian Copperbelt

Abstract

The Zambian Copperbelt accounts for approximately 46 percent of the production and reserves of the Central African Copperbelt, the largest and highest grade sediment-hosted stratiform copper province known on Earth. Deposits in the Zambian Copperbelt are hosted by the Neoproterozoic Katangan Supergroup, a relatively thin (~5 km) basinal succession of predominantly marginal marine and terrestrial metasedimentary rocks that lacks significant volumes of igneous rocks. The stratigraphic architecture of the Katangan Supergroup in the Zambian Copperbelt is comparable to that of Phanerozoic rift systems. The basal portion of the sequence (Lower Roan Group) contains continental sandstones and conglomerates deposited in a series of restricted subbasins controlled by extensional normal faults. These largely terrestrial sediments are abruptly overlain by a regionally extensive, variably organic rich marginal marine siltstone/shale (Copperbelt Orebody Member, or 'Ore Shale') that contains the majority of ore deposits. This horizon is overlain by laterally extensive marine carbonates and finer grained clastic rocks that evolved through time into a platformal sequence of mixed carbonate and clastic (Upper Roan Group) rocks with abundant evaporitic textures, including widespread breccias thought to record the former presence of salt, now dissolved. Rocks of the overlying Mwashia and Kundelungu Groups are dominantly shallow marine in origin. Three significant tectonic events affected the basin. Extension associated with early rifting led to the development of isolated fault-controlled basins and subsequent linkage of these basins along master faults at the time of Copperbelt Orebody Member deposition. A later period of extension occurred during late Mwashia to early Kundelungu time (~765-735 Ma) and is associated with limited mafic magmatism. Basin inversion and later compressive deformation (~595-490 Ma) culminated in upper greenschist-grade metamorphism (~530 Ma) in the Zambian Copperbelt. The majority of ore deposits in the Zambian Copperbelt occur within a 200-m stratigraphic interval centered on the rocks of the Copperbelt Orebody Member. Deposits are broadly stratiform and are grouped into argillite- (70% of ore) and arenite-hosted (30% of ore) types. The distribution, geometry, and size of deposits are fundamentally controlled by early subbasin fault architecture and the availability of both in situ and mobile reductants, the distribution of which is linked to basin structures. Argillite-hosted deposits occur within relatively dark and locally carbonaceous siltstones and shales, suggesting the former presence of an in situ organic reductant. These deposits are laterally extensive with strike lengths up to 17 km. Arenite-hosted deposits occur in both the footwall and hanging wall of the Ore Shale and have maximum strike lengths of 5 km. They occur at sites that were geometrically favorable for mobile hydrocarbon or sour gas accumulation. Both argillite- and arenite-hosted deposits contain so-called barren gaps of weakly to unmineralized strata that are typically associated with the fault-bounded shoulders of early subbasins. Two mineralization assemblages occur in the Zambian Copperbelt. The volumetrically dominant type consists of prefolding disseminated and lesser vein-hosted Cu-Co sulfides. The most typical sulfide assemblage in the deposits is chalcopyrite-bornite with subsidiary chalcocite and pyrite. The Zambian Copperbelt is unusual among sediment-hosted stratiform copper districts in having abundant Co and low Ag, Zn, and Pb. The Cu-Co sulfide carrollite is widespread in the district, although cobalt is present in economic quantities in only some deposits on the western side of the district. The Zambian Copperbelt also contains ubiquitous, but volumetrically minor, Cu-U-Mo-(Au) mineralization in postfolding veins. Cu-Co sulfides display complex textural relationships that are best explained by multistage ore formation. Diagenetic to late diagenetic mineralization is indicated by the typically nonfracture-controlled distribution of both sulfide and gangue phases, replacive textures of Cu-Co sulfides after diagenetic cements and pyrite, and an approximate 815 Ma Re-Os isochron age for sulfide precipitation at the Konkola deposit. Brines capable of mobilizing metals were most likely generated during development of evaporitic environments in units of the Upper Roan Group, and/or subsequent dissolution of these evaporites to form the Upper Roan Group breccias. Late diagenetic to early orogenic mineralization is recorded by prefolding bedding-parallel veinlets and texturally and compositionally comparable disseminated Cu-Co sulfides. An Re-Os isochron age on Cu-Co sulfides from two arenite- and one argillite-hosted deposits of 576 plus or minus 41 Ma is consistent with early orogenic hydrocarbon or sour gas production. The minor Cu-U-Mo-(Au) mineralization event occurred following ostpeak metamorphism, at approximately 500 Ma. The Zambian Copperbelt ore province is characterized by stratigraphically and laterally widespread metasomatism that records a protracted history of basinal brine migration. Although the alteration history is complex, it can be broadly categorized into an early Ca-Mg-SO4, anhydrite- and dolomite-dominant stage involving brine reflux below the level of Upper Roan Group evaporites; a second, K-dominant stage characterized by widespread and commonly intense development of K-feldspar and locally sericite, best developed in rocks of the Lower Roan Group and associated with ore; and a third, Na-dominant stage characterized by development of albite, commonly at the expense of earlier-formed K-feldspar. Albite dominates in Upper Roan Group breccias and Mwashia-Lower Kundelungu strata. It is also locally associated with a late Cu-U-Mo-(Au) vein event. Although none of these alteration types are direct guides to ore, they demonstrate widespread brine circulation within the lower parts of the Katangan Supergroup.