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Petrogenetic and Compositional Features of Rare Metal PanAfrican Post-Collisional Pegmatites of Southwestern Nigeria; A Status Review

Oyebamiji A., Adewumi A. J. P., Zafar T., Odebunmi A., Falae P., Fadamoro O.

Contemporary Trends in Geoscience

2018

Vol. 7, iss. 2

166--187

This research reviews the geology, petrogenesis, compositional trends and geochronology of the rare-metal pegmatite of southwestern Nigeria. The source of these pegmatites is still presently debated which have been explained as either product of highly fractionated molten material or anatexis of the local crust. However, published works of past authors have been compiled to give a detailed understanding of the formation of the mineral deposits. The basement complex of southwestern Nigeria comprises of Precambrian rocks of amphibolite, the hornblende gneiss and the granite gneisses which were formed as a result of the opening and closing of the ensialic basin with significant, extensive subduction during the Pan-African orogeny. The pegmatites in this region have shown internal zoning and a high degree of evolution from the border zone to the core zone during the crystallization and solidification of the felsic granite to pegmatite melt. The rare-metal pegmatites have distinct chemical compositions and mineralogy, containing quartz, biotite, muscovite, microcline, garnet with localized tourmaline, tantalite and columbite. These pegmatites vary significantly by their bulk-rock and mineral chemistry which indicates a more peraluminous attribute and enrichments of lithophile elements of Rb, Cs, Ta and Ba. Previous K/Ar isotopic ages (502.8±13.0 Ma and 514.5±13.2 Ma) suggest that the pegmatites are related to the post-collisional phase of intensive metasomatism. Adopted from previous studies, a five-stage conceptual model of evolution which is widely accepted have been proposed for the origin of the pegmatites.

Radioactive Mineralization in Pegmatites, of Dara-Um Swassi, Area North Eastern Desert, Egypt

Shalaby M., Osman A., Ali M., Ahamed F., Ali B.

Bulletin of the Polish Academy of Sciences. Earth Sciences

2002

Vol. 50, nr 1

51-66

The Dara-Um Swassi pluton is one of the well-exposed granitic batholiths in the North Desert. It is cut by pegmatites and dykes of different types running in harmony with the prevailing structural trends. The pegmatites are found as pockets and lenses especially in the peripheral zones and at different topographic levels. They usually have a NE-SW trend, which coincides with the main structural trend. The Dara-Um Swassi pegmatites are both, zoned and unzoned. An idealized zonal sequence from outer margin to the center is: (1) plagioclase, quartz, microcline-perthite with, or without muscovite and/or biotite (wall zone); (2) orthoclasc-perthite, quartz and muscovite (intermediate zone); (3) quartz of monophase type (core zone). In some zoned pegmatites, distinct dark alteration zones are found at contact between quartz core and intermediate zone in the radioactive cataclastic pegmatite. Unzoned pegmatites are of coarse-grained granitic composition. The two types of pegmatites contain fluorite, zircon, allanite, bastneasite and yttrian fluorite, while euxenite, curate, carnotite and thorianite minerals are only found in the zoned pegmatites. Whole rock chemical analyses of the host monzogranites and pegmatite wall zones provide strong evidence that pegmatites in a granitic system are attributed to magmatic differentiation of a volatile-enriched granitic magma of calc-alkaline composition. With respect to radioactivity, in the unzoned pegmatites average U content is 150 ppm; and average Th content is 99 ppm, the zoned pegmatites represent the most important radioactive sites. U and Th contents decrease from intermediate zones toward wall zones (av. = 143 ppm U, 105.5 ppm Th for intermediate zones, and 56.5 ppm U, 47 ppm Th for wall zones). The altered zones represent the most radioactive sites (av. = 233 ppm U, and 205 ppm Th); this is due to the alteration processes caused by late-stage hydrothermal solutions (haematitization, kaolinitization, chloritization and fluoritization).