Metal sources for the Nkana and Konkola stratiform Cu-Co deposits (Zambian Copperbelt): Insights from Sr and Nd isotope ratios

The Central African Copperbelt lies within the Lufilian orogenic belt, in the border region between the Democratic Republic of Congo (DRC) and Zambia. A Sr and Nd isotope study was performed on gangue carbonates associated with multiphase mineralisation at the Zambian Konkola and Nkana deposits. Comparison with isotopic signatures of basement rocks provides new insights into the likely metal source(s) for the Cu-Co mineralisation. At least three mineralisation phases can be identified with respect to the Lufilian orogeny. Gangue carbonates of the first, pre- to syn-kinematic mineralisation phase in both deposits have Sr and Nd isotopic compositions that correspond to felsic rocks of the Domes Region, a tectonic zone in the Zambian part of the Lufilian Belt where basement rocks crop out. However, the isotopic signatures from both deposits differ. This can be attributed to local variation in isotopic composition of the basement below the deposits. Radiogenic isotope ratios suggest that subsequent, syn-kinematic mineralisation at both sites occurred due to the remobilisation of precursor ore. Petrographic evidence indicates that the third, late-kinematic mineralisation phase at Nkana resulted from a renewed input of metals with a mafic affinity (e.g. Co, Ni). However, Sr and Nd isotope ratios resemble those of the earlier mineralisation phases and do not reflect a change in source composition. Nonetheless, comparison with the isotopic signatures of the Co-poor Konkola deposit and Co-rich stratiform deposits in the DRC might indicate a mafic component in the Nkana metal source. Calculated mixed isotopic compositions support such a mafic component.