Field and petrochemical studies of pegmatites in parts of Lokoja, Central Nigeria

• NE–SW trending mineralized pegmatite in undifferentiated basement complex.
• Post tectonic acidic igneous protolith.
• Selective enrichment of trace elements and REE.
• Muscovite crystals contain higher values of Nb, Rb, Ta, Ga, Cs and Sn than feldspars.
• Gem minerals are products of progressive fractionation from granitic magma.

Field, petrological and geochemical studies of pegmatite bodies in parts of Lokoja, Central Nigeria, have been undertaken with a view to characterizing them and determining the rare earth mineralization potentials. Pegmatite bodies which are zoned or poorly zoned occur as dykes trending N–S, NE–SW and NW–SE consisting dominantly of feldspar–quartz, feldspar–quartz–mica, and feldspar–mica–quartz, respectively. Trace elements concentration in the Na-rich and K-rich feldspars is such that Rb > Ba > Sr > Pb > Ga and the pegmatites contain rare metals with moderately high contents of Nb, Sn, Rb, Li and Cs. The muscovite samples have enrichment pattern of Rb > Nb > Sn > Gn > Ta > Cs: each of the elements has concentration values that are higher than those in either the K-rich or Na-rich feldspars suggesting that muscovite and to a lesser extent feldspars, are the possible carriers of Ta, Sn and REE that are associated with the pegmatite bodies in the Lokoja area. The mineralogy and composition of the pegmatite bodies are indicative of post tectonic anorogenic acidic igneous protolith which underwent alkali metasomatism involving selective enrichment of trace elements and REE, fractionation and rock–fluid interactions.