Volcanogenic massive sulfide (VMS) deposits within the prolonged felsic dominated volcanism of the Africa's Nubian Shield: A systematic review

The Nubian Shield in Africa, which spans Ethiopia, Eritrea, Sudan and Egypt, is a world-class Neoproterozoic (∼880–550) belt hosting numerous volcanogenic massive sulfide (VMS) deposits. Despite some studies conducted on individual deposits in these countries, no systematic studies across the entire shield exists, and underexplored VMS deposits are still present. In this review, we integrate geological, mineralogical, geochemical data of the host volcanic rocks, along with metallogenic data from representative VMS deposits in each country. Geochemical data suggest that the host volcanic rocks are mainly bimodal–felsic metavolcanic and volcaniclastic sequences (rhyolite, dacite, basaltic andesite, and basaltic rocks) of calc-alkaline affinity, derived from more evolved magmas in a mature arc-related setting (island-arc and back-arc basin), which are favorable for hosting Kuroko-type VMS mineralization. Across the shield, pyrite, sphalerite, chalcopyrite, and galena were the dominant sulfide mineral assemblages. U–Pb zircon ages of host volcanic rocks range from ∼887–693 Ma (∼887–720 Ma: Eritrea, Sudan & Ethiopia; ∼751–693 Ma: Egypt), which are consistent with VMS mineralization ages (∼790–702 Ma: Eritrea & Sudan; ∼624 Ma: Egypt) from Re–Os dating of primary sulfide minerals, suggesting prolonged hydrothermal activity related to primary VMS mineralization. Mineralization ages follow northward–younging in the same fashion as their host volcanic rocks, in which older rocks record in the southern and central of the shield (Eritrea, Ethiopia & Sudan), and younger in the northern (Egypt), thus, implying their coeval nature to their tectonic-magmatic events. A Cu-Zn-Pb-Au-Ag mineralization style, similar to Kuroko-type, formed at about 232–275 °C (minimum mean temperatures for sphalerite formation in the Egyptian and Eritrean deposits) was observed across the shield. Moreover, the δ34SV-CDT values of +5.44 ‰ and +9.08 ‰ indicate that the sulfur in VMS mineralization was derived from leaching of the underlying host volcanic rocks and thermochemical reduction of seawater sulfate. This range further indicates that the VMS deposits formed in back-arc basin settings. Combining all the evidence together, during the tectono-evolution of the shield (∼880–550 Ma), the shield underwent subduction-related arc magmatism and volcanism that provided the conditions for primary VMS formation. Meanwhile, Tonian arc magmatism (∼887–720 Ma) in Sudan, Ethiopia and Eritrea, and late Tonian to middle Cryogenian (∼751–693 Ma) in Egypt, resulted in a sequence of volcanic and volcaniclastic rocks. Simultaneously, VMS mineralization has deposited between ∼790–702 Ma in Eritrea and Sudan, and ∼624 Ma in Egypt, suggesting that VMS mineralization is genetically contemporaneous with Neoproterozoic arc volcanism during the closure of the Mozambique Ocean and assembly of East Gondwana. Subsequent greenschist-to lower amphibolite-facies metamorphism, extensive shearing, and deformation modified, locally remobilized, and overprinted the pre-existing VMS mineralization. Therefore, understanding this shield-scale variation brings a group of separate deposits into a unified metallogenic framework that can help with future research and set priorities for further VMS discoveries in the shield.