The Central African Copperbelt<subtitle>Diverse Stratigraphic, Structural, and Temporal Settings in the World's Largest Sedimentary Copper District</subtitle>

Abstract

The Central African Copperbelt, including the Zambian Copperbelt, Congolese Copperbelt, and deposits in the North West Province of Zambia, is the world’s largest and highest-grade sedimentary copper province, with approximately 200 Mt of contained copper and the world’s largest cobalt reserves. It is hosted in Neoproterozoic metasedimentary rocks of the Katangan Supergroup (~880 and ~600 Ma) deposited in a series of intracontinental rift basins with abundant evaporite deposits. Early rift-stage continental rocks were overlain by a sequence of mixed evaporitic carbonate and clastic rocks, followed by a second period of renewed rift-stage clastic and mafic rocks. Widespread glacial and postglacial deposits covered this lower part of the basinal sequence, and mark an uppermost limit to the distribution of major copper deposits. Subsequent depositon of relatively monotonous, nonevaporitic basin fill clastic and lesser carbonate rocks preceded basin inversion during the Pan-African (~590–500 Ma) Lufilian orogeny. The Copperbelt contains copper deposits in a range of rock units at a number of different stratigraphic levels. These deposits display differing styles and textures of mineralization and alteration types. Deposits may contain either or both disseminated, generally fine-grained sulfides and vein-hosted, generally coarse-grained sulfides. Nevertheless, there are shared characteristics among most deposits. Deposits are hosted at stratigraphic or structural redox boundaries. Where deposits occur in the stratigraphically lowermost reduced rocks, overlying reduced or favorable rocks generally were not mineralized. Although redox was a fundamental control for mineralization, the most carbonaceous rocks within an ore horizon are commonly not economically mineralized. Ore sulfide zonation within deposits occurs on multiple scales, with complexity of zoning broadly related to the complexity of the host-rock sequence. Macrostructural controls on deposit position suggest that extensional faults were important in controlling fluid flow, either directly or indirectly through influence on sedimentary and probably diagenetic facies variation. The stratigraphic section within which the deposits are located was affected by regional potassic, magnesian, silicic, and/or sodic alteration controlled partly by lithology and indicative of the passage of basinal brines. Mineralization in the Copperbelt appears to have occurred over a protracted period that spanned diagenesis, basin inversion, and metamorphism. This attests to the longevity of ore-forming brines resident within the Katangan basin and at least the upper part of its basement. The near-surface portions of deposits throughout the Central African Copperbelt have undergone oxidation and supergene enrichment and such enrichment has been important in upgrading the copper tenor of many deposits.