Geology and geochemistry of the Kansanshi Cu-(Au) deposit, North-Western Province, Zambia

Abstract

The Kansanshi Cu-(Au) mine exploits a structurally controlled deposit hosted by strongly deformed Neoproterozoic metasediments in the Katangan Basin of southern Africa. It is currently the largest active Cu mine in Africa, with a total endowment of nearly 12 Mt of Cu and 7 Moz of Au. Copper occurs predominately as coarse chalcopyrite in large quartz-carbonate-pyrite vein swarms and as finer-grained disseminations in carbonaceous phyllite and schist wallrocks. The veins and associated disseminated sulfides are concentrated in domes along the crest of the NW-SE trending Kansanshi antiform. The deposit occurs within the Domes region of the Central African Copperbelt, where amphibolite facies metamorphism and recumbent folding have made correlating between local and regional stratigraphy problematic. Between 2013 and 2015, over 100 deep exploration drill holes were completed across the Kansanshi mining lease with the aim of resolving outstanding questions surrounding the stratigraphic position of the deposit, its structural evolution, and controls on the distribution of ore. This study utilizes geological and geochemical data from the drill holes and geologic mapping in the open pits, along with nearly 600 km of prior drill hole data, to develop a revised structural and stratigraphic interpretation for the Kansanshi deposit. This work reveals a number of major, previously unrecognized, structural and stratigraphic features critical for understanding the genesis of this world-class copper deposit. Metasedimentary rocks at Kansanshi can be divided into three tectonostratigraphic domains separated by evaporitic breccia sequences. The Footwall Domain consists of gneiss and schist basement rocks overlain by >1200 m of quartzite and metaconglomerate of the Basal Clastic Sequence. The Footwall Domain is separated from the overlying Kansanshi Mine domain by the Evaporitic Sequence, a >300 m thick anhydrite-bearing sequence containing multiple polylithic breccia horizons interpreted as the residuum of vanished evaporites. The base of the Kansanshi Mine domain is represented by the Dolomitic Sequence, a homogeneous dolostone interpreted as a shallow water equivalent of shales in the Mwashya Subgroup of the Zambian Copperbelt. The Dolomitic Sequence is overlain by the Grand Conglomérat (Mwale Formation), a Cryogenian metadiamictite that marks the Sturtian “Snowball Earth” glacial epoch. Major geochemical variations in Fe, P, Mn, S, and Ti within the metadiamictite were used to map distinct geochemical facies over a >50 km2 area. The lithogeochemical variations are interpreted to indicate a significant and rapid transition from an initially stratified, anoxic, ferruginous, water column in sub-ice conditions, to a mixed, oxidized water column associated with deglaciation. The Grand Conglomérat is overlain by the Kakontwe Formation, a marble dominated post-glacial ‘Cap Carbonate’ sequence. The Kakontwe Formation transitions upward into the Katete Formation, a rhythmically banded, locally anhydrite-bearing, carbonate-clastic sequence, which acted as a local décollement during deformation. The uppermost stratigraphic unit at Kansanshi and the principle ore host is the Monwezi Formation, a mixed clastic sequence dominated by carbonaceous phyllite and garnet schist. All of these units are recumbently folded within the Kansanshi Mine domain resulting in stratigraphic repetition. The Kansanshi Mine domain is overlain, and locally truncated by, the ‘Mafic-breccia Zone,’ which is an albitized dolomitic breccia horizon containing large mafic meta-igneous blocks derived from the Evaporitic Sequence. These meta-igneous blocks are bound above and below by younger strata, suggesting that they were likely emplaced via diapirism during salt tectonics. The ‘Mafic-breccia Zone’ is overlain by the Upper Plate Domain, which is stratigraphically equivalent to the Kansanshi Mine Domain, but structurally inverted, and interpreted as the bottom limb of a large recumbent fold. The complex structural deformation in the Kansanshi area was associated with basin inversion and metamorphism during the Pan African orogeny. The earliest phase of deformation created nearly orthogonal N- and SE-verging recumbent folds, probably through progressive N to SE deformation. Post-orogenic collapse is interpreted to have resulted in detachment along evaporite horizons, resulting in pervasive boudinage and development of the Kansanshi antiform. Interference folding of pre-existing SE-verging recumbent folds created domes along crest of the antiform. Uplift and cooling during this process was associated with a ductile to brittle transition that resulted in rigid siliciclastic horizons forming boudins within ductile carbonate horizons at all scales. Initial veins developed in boudin necks of siliciclastic lithologies followed by later brittle veining and faulting during the final stage of mineralization. Vein patterns suggest that extensional boudinage controlled the initial vein geometries and introduced ore fluids into chemically favorable reduced carbonaceous host rocks during the waning stages of metamorphism.