A REVIEW OF THE ORIGIN, METALLOGENETIC AND TECTONIC IMPLICATIONS OF RARE-METAL ENRICHED A-TYPE GRANITES IN NORTH CENTRAL NIGERIA

Some anorogenic ring complexes in north-central Nigeria harbor significant concentrations of key metals, including Mo, Nb, Pb, REE, Sn, Y, Zn, and Zr. Understanding the primary enrichment processes within these complexes and their geological constraints is crucial for future development and utilization, especially considering that most mined metals in Nigeria are alluvial in nature. Recent studies employing mineralogy, whole rock geochemistry, electron microprobe, U-Pb-Hf isotope analysis, and zircon trace element geochemistry have revealed a systematic decrease in age from north to south (ca 214 to 141 Ma). Certain granites exhibit substantial increases in Nb, REE, Y, and Zr concentrations, though these enrichments are considered transitional when compared to other world-class deposits. The A-type granites, which formed under various redox conditions (log FMQ = -1.1 to +3.0), are highly differentiated hypersolvus granites containing accessory fluorite, topaz, and pyrochlore. Alkaline granite magma and associated mineralizing fluids are sourced from the lower crust with input from the upper mantle (zircon Hf_(t) values range from -10.06 to -4.38). Sn and Mo are predominantly concentrated in peraluminous biotite granites due to sodic-potassic alteration or greisen-related mineralization. Conversely, critical metals such as Nb and REEs are mainly enriched in peralkaline amphibole-bearing granites, wherein Na-rich fluid and fluorine play pivotal roles in complexation and retention of these metals. Late magmatic-stage crystallization leads to the breakdown of F-HFSEs and F-REE complexes, accompanied by the formation of pyrochlore and REE-bearing accessory minerals. Relationships between whole-rock Rb/Sr and zircon Eu/Eu* indicate that the latter can monitor magma fractionation in these systems. Zircon Ce/Ce* and Eu/Eu* may define conditions favorable for the crystallization of rare metal-enriched accessory minerals. Emplacement of these A-type granites is likely associated with the Mesozoic transtension regime, which reactivated deep-seated Pan-African transcurrent faults, allowing pressure release and fluid channeling, resulting in lithospheric mantle partial melting. Mineralization is the outcome of complex magmatic processes involving fractional crystallization, crustal assimilation, and late-stage hydrothermal fluid activity