Genesis of the ores of the Zambian Copperbelt

The theories for the origin of the ores of the Copperbelt of Zambia (and Zaire) are reviewed. Following an account of the historical development of ideas from epigenetic through syngenetic to syngenetic-diagenetic and, ultimately, to "neohydrothermalist" theories, models for the genesis of these deposits are critically assessed in the light of the data currently available. This examination is conducted with particular regard to the questions of the sources of copper and cobalt, the timing of mineralizing events, and the physicochemical conditions of metal sulphide precipitation. Although accepting that the origin of these ores remains controversial, it is concluded that the most likely sources of both copper and cobalt are the rocks of the Basement Complex, that metals were probably transported both in surface and ground waters, and that the main mineralizing event took place during early diagenesis with sulphide precipitation occurring via bacterial reduction of seawater sulphate. The precipitation of sulphides probably occurred at temperatures around 20-60°C and at near-neutral pH, with minor fluctuations in Eh and variations in the solubilities of the metals involved causing mineral zonation. The burial and compaction of these sediments probably resulted in the expulsion of metal-rich fluids and formation of sulphides in the immediate footwall and hanging-wall of the main ore horizon, with continued burial resulting in the formation of sulphide-bearing pods and veins of lateral secretion origin. Tectonic deformation and metamorphism in the Lufilian Orogeny, although resulting in a set of cross-cutting (high-temperature) mineralized veins, appears not to have resulted in significant remobilization of the sulphides.