Origin of the Nchanga copper–cobalt deposits of the Zambian Copperbelt

The Zambian Copperbelt forms the southeastern part of the 900-km-long Neoproterozoic Lufilian Arc and contains one of the world’s largest accumulations of sediment-hosted stratiform copper mineralization. The Nchanga deposit is one of the most significant ore systems in the Zambian Copperbelt and contains two major economic concentrations of copper and cobalt, hosted within the Lower Roan Group of the Katangan Supergroup. A Lower Orebody (copper only) and Upper Orebody (copper and cobalt) occur towards the top of arkosic units and within the base of overlying shales. The sulfide mineralogy includes pyrite, bornite, chalcopyrite, and chalcocite, although in the Lower Orebody, sulfide phases are partially or completely replaced by malachite and copper oxides. Carrollite is the major cobalt-bearing phase and is restricted to fault-propagation fold zones within a feldspathic arenite. Hydrothermal alteration minerals include dolomite, phlogophite, sericite, rutile, quartz, tourmaline, and chlorite. Quartz veins from the mine sequence show halite-saturated fluid inclusions, ranging from ~31 to 38 wt% equivalent NaCl, with homogenisation temperatures (ThTOT) ranging between 140 and 180°C. Diagenetic pyrites in the lower orebody show distinct, relatively low δ 34S, ranging from −1 to −17‰ whereas arenite- and shale-hosted copper and cobalt sulfides reveal distinctly different δ 34S from −1 to +12‰ for the Lower Orebody and +5 to +18‰ for the Upper Orebody. There is also a clear distinction between the δ 34S mean of +12.1±3.3‰ (n=65) for the Upper Orebody compared with +5.2±3.6‰ (n=23) for the Lower Orebody. The δ 13C of dolomites from units above the Upper Orebody give δ 13C values of +1.4 to +2.5‰ consistent with marine carbon. However, dolomite from the shear-zones and the alteration assemblages within the Upper Orebody show more negative δ 13C values: −2.9 to −4.0‰ and −5.6 to −8.3‰, respectively. Similarly, shear zone and Upper Orebody dolomites give a δ 18O of +11.7 to +16.9‰ compared to Lower Roan Dolomites, which show δ 18O of +22.4 to +23.0‰. Two distinct structural regimes are recognized in the Nchanga area: a weakly deformed zone consisting of basement and overlying footwall siliciclastics, and a moderate to tightly folded zone of meta-sediments of the Katangan succession. The fold geometry of the Lower Roan package is controlled by internal thrust fault-propagation folds, which detach at the top of the lowermost arkose or within the base of the overlying stratigraphy and show vergence towards the NE. Faulting and folding are considered to be synchronous, as folding predominantly occurred at the tips of propagating thrust faults, with local thrust breakthrough. The data from Nchanga suggests a strong link between ore formation and the development of structures during basin inversion as part of the Lufilian Orogeny. Sulfides tend to be concentrated within arenites or coarser-grained layers within shale units, suggesting that host-rock porosity and possibly permeability played a role in ore formation. However, sulfides are also commonly orientated along, but not deformed by, a tectonic fabric or hosted within small fractures that suggest a significant role for deformation in the development of the mineralization. The ore mineralogy, hydrothermal alteration, and stable isotope data lend support to models consistent with the thermochemical reduction of a sulfate- (and metal) enriched hydrothermal fluid, at the site of mineralization. There is no evidence at Nchanga for a contribution of bacteriogenic sulfide, produced during sedimentation or early diagenesis, to the ores