Geochronology of the Nchanga Granite, and constraints on the maximum age of the Katanga Supergroup, Zambian Copperbelt

The age of the Neoproterozoic Katanga Supergroup, which hosts the stratiform Cu-Co deposits of the Central African Copperbelt, has not been well constrained. The Nchanga Granite, the youngest pre-Katangan intrusion in the Copperbelt basement, is nonconformably overlain by sedimentary rocks of the Katanga Supergroup, a relationship that is well exposed in the open pit at Nchanga Mine, Zambia. Previous attempts at dating the Nchanga Granite using the K-Ar and Rb-Sr systems have given only younger Pan-African ages, reflecting resetting during regional metamorphism. Magmatic zircons extracted from this granite have been analysed by ion-microprobe and have yielded an age of 883 ± 10 Ma. Detrital zircons obtained from coarse clastic sediments in the lower part of the Roan Group (at the base of the Katanga Supergroup), immediately above the nonconformity, cluster into 2 age populations, one at ≈880 Ma and the other at between 1800 and 2000 Ma. The Nchanga Granite age provides a reliable maximum estimate of the beginning of deposition of the Katanga Supergroup, or more specifically, of Roan Group deposition. The presence of Nchanga Granite aged zircon detritus in the lower Roan sediments also indicates that sedimentation in the Central African Copperbelt did not commence until the Nchanga Granite had been exhumed and eroded. Our new maximum age for the Katanga Supergroup precludes its previous correlation with the ≈950 Ma Mbuji Mayi Group, and reinforces the likelihood of correlations with similar sequences in the Damara and West Congolian orogenic belts.