The Kipushi Cu–Zn deposit (DR Congo) and its host rocks: A petrographical, stable isotope (O, C) and radiogenic isotope (Sr, Nd) study

Abstract

Near the city of Kipushi, located in the southern part of the Central African Copperbelt, a major vein-type Cu–Zn ore deposit occurs. A combination of petrographic techniques and both stable (O, C) and radiogenic (Sr, Nd) isotope analysis is used to investigate the influence of the mineralisation on the Neoproterozoic dolomite host rocks. A quantification of the abundance and size of the different host rock constituents (dolomite types, quartz, phyllosilicates) revealed a lithostratigraphical controlled variation, without trends towards the ore body. The bulk oxygen isotopic composition of the host rock varies between −2.54‰ and −9.64‰ V-PDB, with most values within the range of Neoproterozoic marine dolomite. Samples with more positive δ18O all originate from the same stratigraphic interval and are interpreted as the result of reflux dolomitisation by an evaporated brine. Few samples with depleted δ18O signatures could indicate the influence of a depleted or high temperature fluid, but are not related to the ore deposit. Moreover, the presence of the ore body cannot be traced through the host rock oxygen isotopic composition. δ18O of gangue dolomite is significantly depleted in comparison with the host rocks and ranges between −7.67‰ and −12.46‰ V-PDB. For an estimated mineralisation temperature of 310°C, this implies a δ18Ofluid between 10.7‰ and 15.6‰ V-SMOW. This is a significant enrichment compared to Neoproterozoic seawater, indicating that the mineralising fluid underwent significant fluid–rock interactions. δ13C of both host rock and gangue dolomite are in range of Neoproterozoic marine dolomites. However, a limited stratigraphic interval has clearly more negative δ13C signatures, due to in situ maturation of carbonaceous material. At the time of mineralisation (450Ma), the host rock dolomite has a strontium isotopic composition partly more radiogenic than Neoproterozoic marine carbonates (0.70793<87Sr/86Sr<0.71167). Nevertheless, the signatures show no relation to the ore body. The gangue dolomite is significantly more radiogenic (0.71061<87Sr/86Sr<0.71332) than the host rock. The radiogenic signature may be due to the interaction of formational and mineralising fluids with Neoproterozoic siliciclastic-rich dolomites, e.g. the top the Kakontwe Supérieur which has 87Sr/86Sr values up to 0.72575 at 450Ma. Alternatively, the mineralising fluid could have interacted with basement rocks. According to εNd450 values between −5.4 and −0.9 for gangue dolomites, this basement was mafic in nature. However, mafic rocks also occur in the Roan Group near the Kipushi deposit. Taken into account previous fluid inclusion data, the mineralising fluid most likely derived metals both from mafic and felsic basement units and possibly interacted with Roan rocks.