Lithium Geo-Resources In Pan-African Pegmatites Of Nigeria - An Evaluation Of Their Geological And Geochemical Characteristics And Implications For Mineral Exploration

Abstract

Nigeria has recently witnessed the discovery of several high grade lithium deposits in pegmatites of Late Proterozoic (Pan-African} age. The lithium- bearing rare-metal pegmatites occur as dykes and sill-like bodies of relatively small dimensions (averaging 500m x 20m x 100m) intersecting mostly low to medium-grade metasediments and metavolcanics of Nigeria's Proterozoic “Schist Belts’. These pegmatites are structurally controlled and belong in chemical composition to the Lithium-Cesium-Tantalum (LCT) family of pegmatites and most commonly enriched in the lithophile elements and mineralized in places with Li, Nb-Ta, Sn and Be. Their distribution is confined mainly to a broad WSW-ENE trending zone within the Precambrian basement complex which is now designated as a metallogenic “pegmatite province” regarded as a northeastern extension of the well-known Eastern Brazilian Pegmatite Province (EBPP). Commercial deposits of lithium, first reported in Nigeria in 2018, have been observed in at least six states, namely; Oyo, Kwara, Kogi, Ekiti, Nasarawa, Kaduna and the Federal Capital Territory. Other states with lithium prospects of unknown economic value are Cross River, Bauchi, Niger, Edo and Taraba. Lithium mineralization occurs as disseminated crystals, mineral veins and felted masses or clusters of the main ore minerals, spodumene and lepidolite, with scattered crystals of the minor minerals; polylithionite, amblygonite and petalite. Apparently, there is some sort of regional zoning in the distribution of the lithium minerals in which spodumene is dominant over lepidolite in the southwest region of Oyo and Kwara states (spodumene>>lepidolite); lepidolite is almost exclusive in the west-central region of Kogi and Ekiti States, while lepidolite and polylithionite (both Li micas) exceed or equal spodumene abundance (lepidolite>spodumene) in most parts of the north-central region of Nasarawa and south Kaduna State. The deposits are relatively high grade, ranging from <0.5% Li₂O to up to 13% Li₂O, with the high-end values being attributed mostly to surficial enrichment. The globally accepted cut-off grade for lithium is 0.4% Li₂O. This paper evaluates the geological and geochemical characteristics of the LCT rare-metal pegmatites, and the mode of occurrence of the lithium mineralization and their economic potential. There is an apparent relationship between lithium mineralization and the presence of low to medium-grade metasedimentary host rocks of the Proterozoic schist belts. Most of the lithium pegmatites do not show any spatial, temporal or genetic relationship with the well-known Pan-African granites (aka Older Granites). It is surmised that the lithium pegmatites are mostly of anatectic origin, derived from partial melting and anatexis of Al-rich and Li-rich metapelites during low to medium-grade regional metamorphism, followed by fractionation of the pegmatite melts in the waning and post-tectonic phase of the Pan-African collisional orogeny. The implications of these preliminary findings on mineral exploration strategies and systematics are briefly discussed.