Structural controls on the Copper-Cobalt-(U) mineralisation of the Chimiwungo Deposit, Mwombezhi Dome, Northwestern Zambia.

Abstract

The Lumwana Cu-(U) deposits located in northwestern Zambia make Lumwana Mine one of Africa’s largest copper mines operating conventional multi-open pits. The Lumwana Project hosts two main deposits, namely, the Malundwe and the Chimiwungo deposits. The current mining operations are exploiting the Chimiwungo deposit, which forms the basis of this study. The deposit occurs within the Mwombezhi Dome, of the Central African Copperbelt’s Domes region, where amphibolite facies metamorphism, thrusting, and recumbent folding have reworked the Paleoproterozoic basement rocks. The deformation events that yielded the Mwombezhi Dome have been associated with the formation of the Lufilian Arc during the Pan African events. The Chimiwungo deposit defines a tabular stratiform shear-zone-hosted mineralised body, which is characterised by two north-south trending ‘ridges’, called Ore Shoots. The deposit strikes E-W with a strike length of ~4 km and a dip of ~15o to the south. The orebody thickness averages around 80 m, with the Ore Shoots defining the thickest (~120 m) portions of the mineralised shear zone. The structurally controlled Cu-sulphide mineralisation dominantly occurs disseminated within the fabric elements of the main shear zone. The Chimiwungo tectonostratigraphy is truncated by north-directed, late, brittle steep normal faults that displace the orebody in the orders of tens to hundreds of metres. The mineralised shear zone is characterised by the presence of barren lithologies that are internal to the shear zone, which predominantly occur as boudins of varying lithological characteristics, sizes, and distribution patterns. In this study, an extensive mapping exercise of the pit walls and creation of a 3D Leapfrog model using a dataset of 1,141 drillholes was combined to understand the nature and distribution of the internal units (amphibolites, basement gneisses, leucosomes, mylonites and pegmatites). The thickest boudins occurring internal to the orebody are amphibolitic in nature and are also responsible for the development and final geometry of the Ore Shoots. These competent lithologies appear to have undergone partial shear deformation, and also played a role during mineralisation and remobilisation of the Cu-sulphide mineralisation. Narrower shear zones that developed between these massive units during D3 deformation, acted as conduits for focusing mineralised fluid flow. High concentrations of Cu-sulphide mineralisation occur in a north-south linear trend, parallel to the shoots. Studies carried out on meso- and macro-structures associated with Cu-sulphide mineralisation show that the final configuration and distribution of the Cu-sulphides was largely driven by boudinage development and the shear fabric. Boudin development facilitated the accumulation of remobilised Cu-sulphides along the interboudin necks and pressure shadows. The presence of S-C and S-C’ planes in the schistose fabric acted as depositional sites of the sulphides. The eastward-directed ductile flow associated with the main shear zone accounted for the presence of a highly developed schistose fabric which is localised along the eastern flanks of both shoots. The D3 deformational phase lasted much longer than the mineralising event and facilitated the remobilisation of Cu-sulphides. Evidence of syn-deformational mineralisation is presented in the form of L3 stretching lineation, meso-scale drag folds in the mineralised schist, late quartz-sulphide veins, and pressure-shadow-hosted mineralisation associated with porphyroblasts. A recrystallisation event facilitated the redistribution of quartz-sulphide assemblages into veins in random fractures within the ore package and partly within the barren internal units.