Mineralogical and geochemical evolution of muscovite in the pegmatite group of the Angwan Doka area, Kokoona district: A clue to petrogenesis and tourmaline mineralization potential

Abstract

The pegmatite group in the Angwan Doka area, Kokoona district is a part of the rare-metal pegmatites occurring in a SW–NE trending broad belt extending for about 400km from Ago-Iwoye in SW to Wamba, north-central Nigeria. The pegmatite group consists of discrete dikes of unzoned, simple and mineralized types, which are genetically linked to the late-orogenic Pan-African (600±150Ma) granodioritic host rock and are of the Li–Cs–Ta type. The zoned pegmatite dikes range from simple mineralogy, commonly at the outskirt of the internally zoned to highly mineralized facies in the inner-most zone where extreme fractionation and accumulation of rare lithophile element enrichment have been recorded. Analysis of geochemical composition of representative granitoids and muscovite from the different zones of selected pegmatite dikes reveals that Rb, Cs, Be, Li, F, B, Sn, Zn, Ta and Nb contents in the muscovite increase from the barren, simple pegmatite composed mainly of quartz and mica to the feldspar-rich border–wall zone consisting of muscovite with graphic intergrowth of quartz and schorl to more evolved intermediate spodumene-bearing zone and finally to the Li-mica, spodumene and elbaite-rich lepidolite unit. In general, the Rb, Cs, Be, Li and F contents of the pegmatite increase with decreasing K/Rb and K/Cs ratios. The corresponding increase in Li, F and MnO contents from the border to the core zone is concomitant with the decrease in Fe2O3, MgO, Al2O3 and TiO2 from the border zone toward the core of the pegmatite. These variations are consistent with rare-element enrichment via fractionation processes in which the melt evolved based on the degree of compatibility combined with partitioning of rare elements from the pegmatite melt into minerals and late stage volatiles. Enrichment of Mg and Fe in muscovite from the border and wall zones of the pegmatites reflects an interaction between the host rock and the pegmatite-forming fluids and consequently crystallization of tourmaline of the dravite–schorl solid-solution series. The high activities of F, Li and B coincide with the abundance and quality of tourmaline encountered in the core zone of the pegmatite. Variation in color and intensity of tourmaline ranges from dark blue–black in the border and wall zones to near colorless, pink, green to greenish-yellow, reddish to purplish-pink and dark green varieties in the spodumene to lepidolite (core) zone. The observable sharp contact and lack of metasomatic aureole between the pegmatite and host suggest that the pegmatite dikes crystallized inwards from the wall–rock contact in a closed system with limited alteration along the border to the host granodiorite. Differences in the geochemical evolution trend of muscovite among the pegmatite bodies investigated suggest that they reached variable degrees of fractionation. Transfer of chemical components is probably one way, from the host rock to the pegmatite magma, while infiltration of fluid from the pegmatite to the host is very minimal and almost negligible. The systematic increase and replacement of biotite by muscovite in the fractionated granodiorite of the Angwan Doka area, coupled with the increase in B, Li and F contents of the muscovite toward the lepidolite unit could serve as potential indicators of gem tourmaline mineralization in the pegmatite provinces of Nigeria.