The geology and geochemistry of the Lumwana Basement hosted copper-cobalt (uranium) deposits, NW Zambia

摘要

The Lumwana Cu±Co deposits Malundwe and Chimiwungo are examples of pre-Katangan mineralized basement that are located in the Domes Region of the Lufilian Arc, an arcuate North neo-Proterozoic fold belt, which hosts the Zambian and Congolese deposits that make up the Central African Copperbelt. The Lumwana deposits are situated within the Mwombezhi Dome; a Mesoproterozoic basement inlier consisting of highly sheared amphibolite grade schist to gneiss units that host the Cu±Co mineralization. Kinematic indicators such as s-c fabrics and pressure shadows on porphyroblasts suggest a top to the North shear sense. Peak metamorphism of 750ºC ± 25ºC and 13 ± 1 Kb indicated by whiteschist assemblages occurred during the Lufilian Orogeny at ~530Ma, with burial depths of ~50km. A major decollement separates the high pressure mineral assemblages of the basement from the lower pressure mineral assemblages of the overlying Katangan Supergroup. The age range and lithologies of the pre-Katangan basement of the Domes Region is similar to the pre-Katangan basement of the Kafue Anticline, which underlies the neo-Proterozoic Zambian Copperbelt deposits situated 220km to the SW. The origin of the protolith to the mineralization is ambiguous at Lumwana with transitional contacts from unmineralized quartz-feldspar±phlogopite basement gneiss to Cu±Co mineralized quartz-phlogopite-muscovite-kyanite-sulphide Ore Schist. The transitional contacts and structural controls on mineralization has led to the hypothesis that these deposits represent metasomatically altered, mineralized and sheared basement, rather than mineralized neo-Proterozoic sediments with amphibolite grade metamorphism. This hypothesis is supported by petrographic analysis, stable isotope analysis (?34S), whole rock geochemistry, and electron microprobe analysis of ore and host rock assemblages. The transitional contacts observed at Lumwana are due to an alteration event associated with mineralization that removed feldspar from ore horizons resulting in depleted Na and Ca and relatively higher Al components. Sulphides are deformed by the S1 fabric and overprinted by kyanite which formed at peak metamorphism. This indicates that copper was introduced to the basement either syn or pre-peak metamorphism. Post S1 metamorphism with associated quartz-muscovite alteration has remobilized sulphides into low strain zones and pressure shadows around porphyroblasts. ?34SSULPHIDES give values of +2.3 to +18.5‰ that fall within the range of values observed in the Copperbelt of -17 to +23‰. The mechanism of ore formation at Lumwana was dominated by thermochemical sulphate reduction (TSR), indicated by the relatively heavy ?34S values and the absence of the light bacteriogenic ?34S values observed in the Copperbelt. Electron microprobe data of muscovite, phlogopite and chlorite show little variation between early and late mineral phases indicating that metamorphic homogenization of silicate assemblages occurred. The Lumwana deposits are large mineralized shear zones within the pre-Katangan basement. Various styles of basement mineralization are also observed in the Kafue Anticline and the structural controls on mineralization and lithological similarities to the Lumwana deposits suggests that pre-Katangan basement is a viable source for the Cu-Co budget of the Central African Copperbelt and that basement structures had a key role in its formation.