Geochronology of cassiterite in the Nassarawa-Keffi rare metal pegmatite belt, Nigeria: Tectonic linkages to the Gondwana-forming orogeny

Abstract

Pegmatites typically appear in fields, belts, or provinces associated with a tectonomagmatic stage of orogenic evolution, both temporally and spatially. These lithologies can be categorized into multiple types by the presence of rare metals such as Li, Be, and Nb-Ta. However, the tectonic setting responsible for the development of these giant pegmatite belts, as well as their influence on the formation of different rare metal mineralization, remains only partially understood. The Nassarawa-Keffi rare metal pegmatite containing Li, Be, and Nb-Ta mineralization in the 400 km-long Nigeria pegmatite belt presents a unique opportunity to study the timing, melt source, and tectonic setting for the formation of various rare metal mineralization. The petrology, mineralogy, cassiterite geochronology, trace element composition, and Lu-Hf isotopes of cassiterite and zircon from Nb-Ta-, Be-, and Li-rich pegmatites in the Nassarawa-Keffi region of the Nigerian pegmatite belt were systematically analyzed. U-Pb dating of cassiterite yielded ages of 559–548 Ma for Nb-Ta-rich pegmatites, 572–550 Ma for Be-rich pegmatites, and approximately 565–550 Ma for Li-rich pegmatites. These ages confirm that Nb-Ta-Be-Li mineralization in the Nigerian pegmatite belt represents a ca. 20 Myr multi-stage rare metal mineralization period formed during the late Neoproterozoic, corresponding to the prolonged post-collisional extension events in the Brasiliano orogenic belt during the Gondwana-forming orogeny. The cassiterite and zircon Hf isotope compositions from these pegmatites indicate that both the Nb-Ta-Be-rich and Li-rich pegmatites exhibit ε_Hf(t) values ranging between −8.3 and −20.9 and T_DM2 values varying between 2.0 and 2.8 Ga, which suggests that the parent melts of Nb-Ta-Be and Li pegmatites were commonly derived from the remelting of the Paleoproterozoic basement rocks. The lower ε_Hf(t) values observed in Nb-Ta-Be-rich pegmatites, in comparison to Li-rich pegmatites, indicate their origin from the reworking of distinct protoliths. Consequently, the study elucidates that diverse mineralization within these giant pegmatite belts can be derived from various ancient crustal sources. Moreover, the similarities in the tectonic setting between Li-Be-Nb-Ta pegmatites in Nigeria and the Neoproterozoic-Cambrian transition orogenic belts of western Gondwana highlight the importance or rare metal pegmatite exploration within the 400 km-long pegmatite belt. This underscores the boundary of Precambrian cratons’ tectonic zone during the Gondwana assembly as a globally emerging source for Nb-Ta-Be-Li rare metal resources.