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Chemical and temporal significance of intracontinental magmatism in the central Zambezi belt: implications on craton margin magmatic ore deposits
The evolution of craton margins creates the ideal conditions for Ni–Cu–(PGE) ore formation, though their tectonic setting and location along craton margins also make them important for studying the breakup of continents. The breakup of Rodinia resulted in the development of several long-lived intracontinental rift-basins across the African continent, including the Neoproterozoic Zambezi belt. The onset of extension in the Zambezi belt is constrained by the emplacement of the Kafue Rhyolite Formation at 875.31 ± 0.63 Ma. Age data from mafic–ultramafic intrusions indicate that Zambezi belt developed over a protracted 100 Ma period through several magmatic intervals, up until at least ca. 737 Ma. The chemistry and isotopic signatures of these intrusions depict a complex scenario of chemically and temporally different rocks that can broadly be grouped into: (1) variably-enriched tholeiitic N- to E-MORB compositions emplaced ca. 880–820 Ma; and (2) transitional to OIB-type compositions emplaced 795–737 Ma. The intrusions display a wide range of 143Nd/144Nd (ƐNd -1.8 to +9) and 176Hf/177Hf (ƐHf -15.7 to +19.3) compositions, which for several intrusions, plot above estimates for the depleted mantle for the time of formation. These values indicate a history of mantle depletion that pre-dates the formation of the Zambezi belt. Examples of decoupled Nd and Hf isotope compositions and increased 176Hf/177Hf indicate a complex history of mantle depletion, re-fertilisation, and mixing. These inferences have specific implications for targeting and evaluating Ni–Cu–(PGE) systems in the Zambezi belt. Intrusions containing appreciable sulphide are only recognised between 860–850 Ma. These results indicate that magmas emplaced during rift incipience were most fertile, formed during post-orogenic extensional magmatism after collisional convergence and arc tectonism associated with the Rodinia supercontinent cycle. One of the major challenges in the exploration of Ni–Cu–(PGE) deposits is the lack of a footprint beyond the host intrusions themselves. This new understanding may prove to be an important predictive tool in the Zambezi belt, and more broadly along the margin of the Congo craton, as it provides a way to prioritise segments with enhanced metal fertility. This study highlights that a multidisciplinary approach, combining independent geochronology with chemical and petrogenetic analyses can help to gain a more comprehensive understanding of ore deposit formation.