Anhydrite pseudomorphs and the origin of stratiform Cu-Co ores in the Katangan Copperbelt
El Desouky, H.
Earth and related Environmental sciences
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The stratiform Cu-Co ore deposits in the Katangan Copperbelt consist of dispersed sulphides, sulphides in nodules and lenses, which are often pseudomorphs after evaporites, and in veins. Two types of pseudomorphs can be distinguished in the nodules and lenses. In Type 1 examples, dolomite precipitated first and was subsequently replaced by Cu-Co sulphides and authigenic quartz, whereas in Type 2 examples, authigenic quartz and Cu-Co sulphides precipitated prior to dolomite and are coarse-grained. The sulphur isotopic composition of the copper-cobalt sulphides in the Type 1 pseudomorphs is between -10.3 and 3.1 V-CDT, indicating that the sulphide was derived from bacterial sulphate reduction (BSR). The generation of HCO3- during this process caused the precipitation and replacement of anhydrite by dolomite. A second product of BSR is the generation of H2S. We suggest that metals in the mineralising fluid reacted with the H2S, resulting in the precipitation of Cu-Co sulphides. Initial sulphide precipitation occurred along the rim of the pseudomorphs and continued towards the core. Precipitation of authigenic quartz was most likely induced by a pH decrease during sulphide precipitation. Fluid inclusion data from quartz indicate the presence of a high-salinity (8-18 eq. wt% NaCl) mineralising fluid, possibly derived from evaporated seawater which migrated through the deep subsurface. 87Sr/86Sr ratios of dolomite in Type 1 nodules range between 0.71012 and 0.73576 significantly more radiogenic than the strontium isotopic composition of Neoproterozoic marine carbonates (87Sr/86Sr = 0.7056-0.7087). This suggests intense interaction with siliciclastic sedimentary rocks and/or the granitic basement. The low carbon isotopic composition of the dolomite in the pseudomorphs (-7.02 and -9.93 V-PDB) compared to the host rock dolomites (-4.90 and +1.31 V-PDB) resulted from the oxidation of organic matter during BSR.
CitationMuchez, P.; Vanderhaeghen, P.; El Desouky, H.; Schneider, J.; Boyce, A.; Dewaele, S.; Cailteux, J. (2008). Anhydrite pseudomorphs and the origin of stratiform Cu-Co ores in the Katangan Copperbelt. , Mineralium Deposita, Vol. 43, 575-589, Springer-Verlag,