Comparative geology and metamorphic evolution of the Luswishi Dome, Copperbelt, Zambia: Implications for exploration targeting

Abstract

The Luswishi Dome forms part of the Neoproterozoic Lufilian Arc which hosts copper-cobalt ores of the Central African Copperbelt. We report new data from a study that combines observations from drill core logging, petrographic and chemical analyses, geothermobarometry and phase equilibria modelling. Drill core logging identifies lithostratigraphic units which are partially correlated with the Katanga Supergroup based on the occurrence of regional markers. The Nguba Group constrained by the presence of glacial tillite includes variably deformed, carbonaceous shales, and carbonates. An underlying sequence of quartzites, metapelites, calc-silicates, meta-sandstones, shales, dolomites and gabbros is partially correlated with the Roan Group. Overprinting relationships of structures in drill core provide evidence for possible polyphase deformation. Early deformation generated folds which were overprinted by faults and later by brecciation caused by hydrofracturing during later deformation, and subsequently metasomatism. Petrographic examinations reveal porphyroblast-inclusion-matrix relationships which indicate that the area experienced at least two penetrative ductile deformation events, D1 and D2, which are in turn overprinted by brittle structures at D3 and later phases. Thermobarometry calculations identify peak metamorphic conditions of 580–600 °C with maximum pressures of ~7 ± 1 kbar. Integrating thermobarometry with pseudosection modelling allows reconstruction of clockwise P-T-d paths consistent with consumption of epidote, chlorite, muscovite, quartz and low Ca-plagioclase to form biotite, garnet, high Ca-plagioclase and aluminosilicates. These observations suggest Barrovian intermediate P/T-type metamorphism which can be attributed to a tectonic scenario of burial under geothermal gradients of ~20–25 °C/km prior to uplift and erosion. These data complement previous models of regional metamorphism by crustal thickening during the Lufilian orogeny.