Evolution of Cu–Co mineralizing fluids at Nkana Mine, Central African Copperbelt, Zambia

The Central African Copperbelt hosts numerous world class stratiform Cu–Co deposits in the Neoproterozoic Katanga Supergroup (<880 to ± 500 Ma). These high grade deposits resulted from multiple mineralization and remobilization stages. The Nkana Cu–Co deposit in the Zambian part of the Copperbelt is such a stratiform deposit but the location of the rich ore bodies is structurally controlled, i.e. occurring in the hinge zones of tight to isoclinal folds. Late stage mineralization and/or remobilization caused this enrichment. Three major mineralization/remobilization stages have taken place during the Lufilian orogeny. They are characterized by folded layer parallel veins, highly irregular veins crosscutting the folds, and finally unfolded massive veins. An evolution in the oxygen, carbon and sulphur isotopic composition is present from the layer parallel and irregular to the massive veins. The more negative δ18O values in the carbonates from the massive veins most likely reflect a decrease in the oxygen isotopic composition of the ambient, metamorphic fluids. The δ13C values range between −25‰ and −5‰ V-PDB with a trend towards less negative values in the massive veins, possibly reflecting an ongoing oxidation of organic matter in a relatively closed system. Early framboidal and massive pyrites disseminated in the host rock have distinctly negative δ34S values, i.e. between −16‰ and −9.7‰ V-CDT. The sulphur isotopic composition increases from these early diagenetic pyrites to sulphides in the successive vein generations. The δ34S values of the massive veins are positive and cluster between 1.3‰ and 2.0‰ V-CDT. This enrichment in heavy sulphur is interpreted as a result of the mixing of S remobilized from early sulphides, with S derived from the thermochemical reduction of sulphate. With time, the sulphur derived from TSR became more important. The Sr isotopic composition of the carbonates in all three vein generations shows a wide range between 0.71672 and 0.75407. All values are significantly more radiogenic than the strontium isotopic composition of Neoproterozoic marine carbonates (0.7056–0.7087). The radiogenic values indicate interaction of the mineralizing fluid with the basement or the siliciclastic sediments derived from it. All fluid inclusions measured in the different vein generations have a dominant H2O–NaCl/KCl–MgCl2 composition with the presence of a gaseous component in some inclusions. Fluid inclusions in the layer parallel veins suggest entrapment around 450 °C at a depth of 8.4 km (2100 bars), i.e. during the main period of metamorphism. Secondary fluid inclusions of unknown origin in the layer parallel, irregular and massive veins have a high salinity (18.1 to >23.2 eq. wt.% NaCl) and homogenization temperatures between 100 and 250 °C. These fluids were trapped after formation of the veins, likely during retrograde metamorphism. The study of the veins, which formed between 580 and 520 Ma, nicely demonstrate the complexity of the metallogenesis of the Cu–Co ore deposits in the Copperbelt. Therefore, geochemical, microthermometric and geochronological analyses need to be carried out on individual generations to fully understand the evolution of ore formation through time.status: publishe