 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; June 2003; v. 31; no. 6; p. 497–500; 5 figures. 497 Origin of the copper-cobalt deposits of the Zambian Copperbelt: An epigenetic view from Nchanga Ross R. McGowan Stephen Roberts    School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, Southampton SO14 3ZH, UK Robert P. Foster Exploration Consultants Ltd., Highlands Farm, Greys Road, Henley-on-Thames RG9 4PR, UK Adrian J. Boyce Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, UK Dave Coller ERA Maptec, 36 Dame Street, Dublin 2, Ireland ABSTRACT The Zambian Copperbelt is arguably the most significantly mineralized Neoproterozoic basin on Earth, preserving a truly spectacular scale of mineralization: in excess of 1  10 9 t of ore at 2.7% copper has been extracted to date, and there are also major cobalt accumulations. The origin of these deposits has been hotly debated for more than six decades, yet the driving forces that generated this system are poorly understood, in par- ticular the relationships between tectonics, paleo–fluid circulation, and ore deposition. We present new field and isotopic data for the Nchanga deposits in which the bulk of the mineralization is hosted by shale-capped feldspathic arenites and arkoses that have un- dergone recrystallization and hydrothermal alteration within a host-rock package con- trolled by low-angle thrust faults. By using in situ laser combustion, we show for the first time that the range of  34 S for copper-cobalt ore sulfides (1‰ to 18‰) cannot have the same source as diagenetic pyrite (1‰ to 17‰). We suggest a new epigenetic model for the formation of these spectacular Nchanga orebodies that involves the introduction of metal- and sulfate-bearing hydrothermal fluids into quartzofeldspathic units during basin inversion, with sulfide derived from thermochemical reduction of the sulfate near the site of deposition. Keywords: Zambian Copperbelt, sulfur isotopes, mineralization, thermochemical reduction. INTRODUCTION Debates on the origin of the copper and co- balt of the Zambian Copperbelt are critically focused on the timing of mineralization in re- lation to the development of the host basin. Prevailing models suggest that mineralization occurred either during the early stages of basin evolution and involved the bacterial reduction of seawater sulfate during early diagenesis (Garlick, 1961; Sweeney et al., 1986), or by exhalative hydrothermal fluids that were intro- duced along deep basement structures into variably consolidated cover sequences (An- nels, 1989; Unrug, 1988). Within these mod- els, sulfur isotope data on Copperbelt sulfides have been interpreted to result from the bacterial reduction of penecontemporaneous seawater sulfate, with subsequent homogeni- zation of the  34 S signature during metamor- phism (Dechow and Jensen, 1965). However, we report new field and sulfur isotope data that do not support this interpretation, but in- stead favor an epigenetic model for Nchanga mineralization in which the copper and cobalt were introduced during deformation of the host sequence. This model represents a sig- nificant reinterpretation of the genesis of a ma- jor component of Copperbelt mineralization and has significant implications for our un- derstanding of the processes responsible for the production of such spectacular metal con- centrations in the crust. GEOLOGIC SETTING The copper and cobalt mineralization of the Zambian Copperbelt is hosted by the Lower Roan sedimentary rocks of the late Precam- brian Katangan Supergroup, which are ex- posed on the limb of the Kafue anticline (Fig. 1), a basement culmination forming the south- ern part of the tectonic Lufilian Arc. The cul- mination is the result of major thrust struc- tures cutting up through the basement, with a dominant transport direction and vergence to the northeast (Daly et al., 1984). The maxi- mum age for the Lower Roan rocks is con- strained by the age of the Nchanga Red Gran- ite (877  11 Ma; Armstrong et al., 1999), which is unconformably overlain by the Ka- tangan succession. The main phase of thrust- ing of the Katangan sequence is considered to have taken place between 560 and 550 Ma (Hanson et al., 1993) during the Lufilian orogeny. NCHANGA DEPOSITS The stratigraphic sequence at Nchanga comprises a series of shales, arenites, and do- lomitic horizons of the Lower and Upper Roan Groups unconformably overlying the Nchanga Red Granite and the quartz-mica schists of the Lufubu System (Fig. 2). Katan- gan sediments were deposited on a granite horst block defined by high-angle extensional structures (displacement 150 m) and subse- quently were subjected to lower-greenschist metamorphism. The fold geometry of the Ka- tangan host sequence was controlled by the development of fault-propagation folds (to thrusts). The thrusts detach at a decoupling horizon located adjacent to or within the lower shale unit, which has been used as a preferred plane of slip. The fold-and-thrust structures in the Nchanga deposits, often characterized by phlogopite-rich shear zones, show a transport direction and vergence toward the northeast, although at the northern end of the deposit in the Nchanga open pit, the vergence of the folds is toward the southwest, as a result of backthrusting on a large north-dipping inver- sion fault that controls the development of the Nchanga syncline. Low-grade copper and cobalt occur throughout the stratigraphic sequence, but high-grade ore is economically exploited from two main zones (Fig. 2): (1) a lower (oxide) orebody hosted within both the top of an ar- kose unit (70%) and the base of an overlying black shale unit (30%); and (2) an upper (sul- fide) orebody hosted by a feldspathic arenite (90%) and again at the base of overlying shale (10%). Lower-orebody mineralization (copper only) consists of disseminated, breccia-hosted, fracture-related, and vein-hosted chalcocite and malachite, with some evidence of the pre- cursor primary sulfides bornite and chalco- pyrite, and is continuous along a 5 km section from southwest to northeast across the Nchan- ga orebodies. Not only is the ore associated with the tectonized contact between the arkose and shale, but there is also a strong positive relationship between elevated ore grades in the lower orebody and the presence of the high-angle extensional structures developed in the basement. Alteration of primary sulfides to chalcocite, which has subsequently been re- placed by malachite, indicates the presence of a former sulfide orebody, similar in sulfide mineral assemblage to the upper orebody. Upper-orebody mineralization (copper and co- balt) is only observed where fault-propagation fold structures have developed in the upper part of the Lower Roan sequence. Cobalt min- eralization, in particular, is spatially associated with thrust-propagated fold-hinge zones, whereas copper mineralization is more wide-