Geochemical evolution of micas and Sn-, Nb-, Ta- mineralization associated with the rare metal pegmatite in Angwan Doka, central Nigeria Juliet U. Akoh, Paul O. Ogunleye * , Aliyu A. Ibrahim Department of Geology, Ahmadu Bello University, Zaria, Nigeria article info Article history: Received 31 May 2015 Received in revised form 22 August 2015 Accepted 22 August 2015 Available online 1 September 2015 Keywords: Pegmatites Angwan Doka Magmatic Undercooling Columbite-tantalite Metasomatism abstract The pegmatites in Angwan Doka, north central Nigeria are genetically related to the basement granites formed during the Pan-African orogeny, 550e530 Ma ago. They occur as sharply discordant dykes in the granitic and metasedimentary basement rocks. The pegmatite population comprises of mineralogically simple and complexly zoned types that are characterized by LCT (Li, Cs and Ta) geochemical signature. The host granitoids range in composition from hornblende, titanite-bearing to biotiteemuscovite granodiorites. Analysis of geochemical data of whole rock and muscovite from the different zones reveals compositional variations and evolution across the pegmatite body from border zone to the lepidolite- quartz core zone. Fractionation of Rb, Cs, Sr, Li, F, B, Be Sn, Zn, Ta, Nb and Mn which increases from host granitoids, through the border zone to the central core, with decrease in Fe, Mg, Ti, Ba content, is typical and marks the magmatic crystallization trend of the pegmatites. Other distinctive attribute of the pegmatites is occurrence of cassiterite believed to have formed as a consequence of greisenization, albitization and late-stage metasomatism, which led to enrichment in Sn (up to 886 ppm) in the in- termediate zone. Chemical composition of muscovite from the different zones of the pegmatite reveals high concentration of primary magmatic columbite-Fe (ferrocolumbite and ferrotantalite) in the border zone and tantalite-Mn (manganocolumbite and manganotantalite) in the core zone. Ta predominates (352 ppm) in the most evolved lepidolite (Li- and F-rich) zone while Nb was enriched (up to 714 ppm) in the border zone. These geochemical features are ascribed to undercooling of the melt and crystallization in boundary layers accompanied with increased accumulation of incompatible and fluxing components. With increasing fractionation, Nb/Ta and Fe/Mn ratio decreased and is accompanied with increase in Rb, Cs, Li, F and Be typical of crystallization from magmatic process. The sequence of zonation, origin and formation of the different pegmatite zones can be explained by a single path of fractional crystallization. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Rare-element pegmatites are important resources for their economic concentrations of rare elements Sn, Li, Ta, Rb, and Cs, presence of high quality industrial minerals such as muscovite, spodumene, feldspar and mica, and occurrence of gem minerals (e.g. tourmaline, beryl). LCT-type pegmatites are rare-element pegmatites enriched in Li, Cs and Ta, and are often related petro- genetically to fertile, progenitor granites (  Cerný, 1991). Large percentages of pegmatites associated with granite form pods within the granite or coalesce upwards and segregate towards the roof of the granitic plutons. Pegmatites may also intrude in form of dykes into the surrounding rocks and emanate from the carapace of the pluton and are in this case defined as epigenetic pegmatites. The pegmatites in Angwan Doka, north central Nigeria are a part of the large pegmatite belt containing hundreds of pegmatite dykes, which have been reported to be associated with tin and columbite-tantalite mineralizations in a large number of granitic bodies sporadically distributed in the Nigerian Basement (Garba, 2003; Okunlola, 2005). The pegmatites comprise of labyrinth of vertical and low lying horizontally dipping dykes in metasediments and granitoids belonging to the Older Granite suites, which have been dated 600 ± 150 Ma by Matheis and Caen-Vachette (1983). * Corresponding author. Centre for Energy Research and Training, P.M.B. 1014, Ahmadu Bello University, Zaria, Nigeria. E-mail addresses: ugbedeojocool@gmail.com (J.U. Akoh), ogunleyepo@gmail. com (P.O. Ogunleye). Contents lists available at ScienceDirect Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci http://dx.doi.org/10.1016/j.jafrearsci.2015.08.017 1464-343X/© 2015 Elsevier Ltd. All rights reserved. Journal of African Earth Sciences 112 (2015) 24e36