African Neoproterozoic Mineral Deposits and Pan African Metallogenesis

Abstract

Amalgamation of a number of continental fragments during the Late Neoproterozoic resulted in a united Gondwana continent. The time period in question, at the end of the Precambrian, spans about 250 million years between ∼800 and 550 Ma. Geological activity focused along orogenic belts in Africa during that time period is generally referred to as ‘Pan African’. We identify three age-related classes of tectonic terranes within these orogenic belts, differentiated on the basis of the formation-age of their crust: juvenile (e.g. mantle derived at or near the time of the orogenesis, ∼0.5–0.8 Ga), Paleoproterozoic (∼1.8–2.5 Ga), Archean (>2.5 Ga). We combine African mineral deposits data of these terranes on a new Neoproterozoic tectonic map of Africa. The spatial correlation between geological terranes in the belts and mineral occurrences are determined in order to define the metallogenic character of each terrane, which we refer to as their ‘metallogenic fingerprint’. We use these fingerprints to evaluate the effectiveness of mobilisation (‘recycling’) of mineral deposits within old crustal fragments during Pan African orogenesis. This analysis involves normalization factors derived from the average metallogenic fingerprints of pristine older crust (e.g. Palaeoproterozoic shields and Archean cratons not affected by Pan African orogenesis) and of juvenile Pan African crust (e.g. the Nubian Shield). We find that mineral deposit patterns are distinctly different in older crust that has been remobilized in the Pan African belts compared to those in juvenile crust of Neoproterozoic age, and that the concentration of deposits in remobilised older crust is in all cases significantly depleted relative to that in their pristine age-equivalents. Lower crustal sections (granulite domains) within the Pan African belts are also strongly depleted in mineral deposits relative to the upper crustal sections of juvenile Neoproterozoic terranes. A depletion factor for all terranes in Pan African orogens is derived with which to evaluate the role of mineral deposit recycling during orogenesis. We conclude that recycling of old mineral deposits in younger orogenic belts contributes, on average, to secular decrease of the total mineral endowment of continental crust. This could be of value when formulating exploration strategies.