The Zambian Copperbelt is arguably the most significantly mineralized Neoproterozoic basin on earth, preserving a truly spectacular scale of mineralization, with in excess of one billion tonnes of ore at -2.7% copper extracted to date and major cobalt accumulations. The origin of these deposits, of which Nchanga is one of the largest, has been intensely debated for over six decades, yet the driving forces that generated this system are poorly understood, in particular the relationship between tectonics, palaeo-fluid circulation, and ore deposition.The structural evolution of Nchanga involved: (i) early, pre-Katangan extension, represented by highangle structures within the Basement, followed by (ii) inversion on high-angle structures (against stable granite block) and internal thrust-propagation fold development within the basal part of the Katangan sequence (Lower and Upper Roan Groups). This most likely took place during the main phases of the Lufilian Orogeny (550 to 560 Ma), and was strongly controlled by competence contrasts between different lithologies. High-grade mineralization is spatially associated with major structures, and the majority is hosted by shale-capped quartzo-feldspathic units that have undergone recrystallization and potassic hydrothermal alteration, indicated by a mineral assemblage of phlogopite-sericite-quartz (± dolomite, tourmaline and rutile). Ore mineralogy consists of diagenetic/early pyrite (that possibly replaced earlier anhydrite) replaced by disseminated and fracture-related chalcopyrite and bomite, which has subsequently undergone replacement by chalcocite. Carrollite (Co) appears to overprint primary chalcopyrite and bornite where present, but precedes the chalcocite. Supergene oxidation of mineralization is represented by abundances of malachite and chrysocolla.Fluid inclusion data, in combination with alteration mineral assemblages and whole-rock geochemistry, have identified a potential ore-forming fluid of medium temperature (pressure-corrected: 300° to 310°C within faults and 220° to 240°C within ore zones; TH = 230° to 240°C and 160° to 180°C respectively), high salinity (30 to 38 equivalent wt% NaCl), and near-neutral to slightly acidic pH (5-7), that contained significant quantities of Mg and Ca, as well as Cu and Co. Oxygen isotope analysis has revealed S'Ofiuid (SMOW) signatures of +2.1%c to +10.3%« (calculated with corrected temperatures), which are consistent with basinal brines that have undergone significant water-rock interaction. The 8^''S (CDT) of diagenetic pyrites within black shale ranges form -l%c to -17%«, typical of bacterially reduced sea water sulphate. By contrast, the copper sulphides range from -\%c to +18%c, which is more representative of thermochemical reduction of a sulphate-enriched fluid. This is supported by the 5'^C (PDB) of dolomite alteration around ore with values of -5.6%c to -8.3%c, indicative of a component of organic carbon, compared to dolomites of the Upper Roan, which yield 5'^C typical of marine carbon of + 1.5%c to +2.5%c.These...
