GEOLOGY AND GEOCHEMISTRY OF GRANITOIDS OF KOKOONA AREA, IN RELATION TO MINERALIZATION, CENTRAL NIGERIA
GEOLOGY AND GEOCHEMISTRY OF GRANITOIDS OF KOKOONA AREA, IN RELATION TO MINERALIZATION, CENTRAL NIGERIA
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Date
2014-05
Authors
AKOH, Juliet Ugbede
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Abstract
The Angwa-Doka area, Kokoona LGA of Nasarawa State is underlain by migmatite, mica schist, granodiorite, topaz-bearing biotite granite and pegmatite. Geological and geochemical studies of the area were undertaken to determine processes of formation and the gemstone-bearing capacity of the pegmatites. Field mapping on a scale of 1:20,000 and petrographic studies were carried out while geochemical analysis of samples of the granitoid and muscovite selected from the different zones of the pegmatites were undertaken using a lithium metaborate/tetraborate fusion ICP-MS for the major, minor and trace elements. In addition, B and Li were analyzed in the muscovite samples using sodium peroxide fusion ICP-MS while fluorine was analyzed by fusion specific ion-electrode method. The results revealed a mineralogical zoning of these rocks from a metaluminious A/CNK ratio (<1) to a more fractionated peraluminous (A/CNK > 1.1) S-type variety. The K/Rb value suggests that biotite granite in Janta area is highly fractionated while chondrite normalized REE distribution patterns display distinct M-type tetrad effect and strong Eu depletion believed to be associated with late stage magmatic processes that brought about associated greisenization in the rock. The pegmatites are commonly zoned and range from barren, simple to highly evolved lepidolite facies in the inner-most zone. Representative muscovite samples selected from different zones of pegmatites were studied to determine whether the pegmatites and the host granodiorite can be related by a common fractionation path. The geochemical data further reveals that Rb, Cs, Be, Li, F, B, Sn, Zn, Ta and Nb contents increase from the host granitoids inwards. 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 lepidolite unit is concomitant with decrease in Fe2O3, MgO, Al2O3 and TiO2 from the border zone towards the core of the pegmatite. Li content of 0.213 wt% was recorded in the intermediate zone, increasing through the spodumene zone to 1.25-2.82 wt% in the core zone. F concentration also ranges from 0.43-0.62 wt% in the intermediate zone to 1.85-4.5wt% in the core zone. These variations are consistent with rare-element enrichment via fractionation processes, based on the degree of compatibility combined with partitioning of rare elements from the pegmatite melt into the minerals and volatile late stages. The pegmatite group in Angwan Doka, Kokoona district can be said to be "heterogeneous" consisting of the different types and subtypes of LCT family, Rare Element Class pegmatites of albite, spodumene, lepidolite to elbaite sub-class. Tourmaline is essentially magmatic and display characteristic that indicate evolution from schorl-dravite series within the outer zones while the high activities of F, Li and B coincide with the abundant and quality of tourmaline (liddicoatite-rossmanite-elbaite solid solution series) encountered in the evolved zones of the pegmatite. Transfer of chemical components is probably one way; from the host rock to the pegmatite magma. The pegmatite dykes crystallized inwards from the wall-rock contact in a closed system. The systematic increase and replacement of biotite by muscovite in the fractionated granodiorite of Angwan Doka area, coupled with the increase in B, Li and F contents of the muscovite towards the lepidolite unit could serve as potential indicators of gem tourmaline mineralization in the pegmatite provinces of Nigeria.
Description
A thesis submitted to the Postgraduate School, Ahmadu Bello University, Zaria, in partial fulfilment of the requirement for the award of the degree of Master of Science in Geology Department of Geology Faculty of Science Ahmadu Bello University Zaria
Keywords
GEOLOGY,, GEOCHEMISTRY,, GRANITOIDS,, KOKOONA AREA,, RELATION,, MINERALIZATION,, CENTRAL NIGERIA,