Granitoid-hosted Ta mineralization in the Arabian–Nubian Shield: Ore deposit types, tectono-metallogenetic setting and petrogenetic framework

The Arabian–Nubian Shield (ANS) derived from the accretion of oceanic arc and back arc terranes and forms a large segment of juvenile Neoproterozoic crust within the East African Orogen (900 to 500 Ma). It is presently emerging as a major tantalum mining province with economic deposits associated with 1) peraluminous spodumene–albite pegmatite (Kenticha, Ethiopia), 2) peraluminous muscovite–albite granite (Abu Dabbab, Egypt), and 3) peralkaline arfvedsonite-alkalifeldspar granite (Ghurayyah, Saudi Arabia). In addition, a number of subeconomic tantalum mineralizations occur in the ANS including members of each of the three deposit styles. Formation of the different types of tantalum mineralization is apparently linked to a period of sustained post-accretionary granitoid magmatism, lasting from 610 to 530 Ma. This magmatism occurred in transpressional to extensional settings and was produced from mantle- and crust-derived melts. Ta–Li–Sn mineralized peraluminous granites and pegmatites exclusively belong to the low-phosphorous type of rare-metal granites, indicating that they were generated from I-type or A2-type magmas, most likely by extended fractionation of either crust derived or hybrid crust–mantle derived magmas. Nb–Zr–REE–Ta mineralized peralkaline granites belong to the A1-type clan and were largely derived from fractionation of mafic mantle magmas. Tantalum mineralization occurs more widespread in the ANS than in adjacent remobilized crust of the East African Orogen. It is proposed that this relatively frequent abundance of tantalum mineralization in the ANS is related to post-accretionary ‘cratonization’ of the juvenile crust which produced high volumes of granitoid melts and frequently resulted in highly differentiated, rare-metal specialized plutons. Accretionary orogenic belts assembled from predominantly juvenile material may thus have a generally higher fertility for tantalum mineralization than collisional orogens often comprising multiple reworked crust.