International Geologiical Congress - Oslo 2008

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HPP-01 General contributions to Precambrian geology

 

Mineralization in pegmatites within the late proterozoic granodiorites of the oban massif southeastern Nigeria

 

Barth Ekwueme, University of Calabar (Nigeria)
K. A. Ero, University of Calabar (Nigeria)
 

 

The Oban basement massif of southeastern Nigeria has received considerable attention since the 1980's. Published works in the areas of petrology, geochemistry, geochronology, hydrogeology and geophysics now exist. The Oban massif is one of the giant spurs forming the southeastern Nigerian highlands marking the boundary of Nigeria and the Cameroon Republic. The geology of southeastern Nigeria has recently been shown to be more related to the Central African Fold Belt than the Nigerian-Benin shield. Rocks in the Oban Massif include phyllites schists, gneisses and migmatites intruded dominantly by granodiorites which are termed the "Uwet granodiorites".

These granodiorites yielded a single zircon evaporation age of 617 ± 2Ma and enclosed within the granodiorites are mappable bodies of pegmatites. The pegmatites of southeastern Nigeria unlike those of northcentral and southwestern Nigeria have not been adequately investigated. They have been labelled "barren pegmatites" unlike pegamtites in northcentral and southwestern Nigeria which are described to be productive and Pan-African in age. These pegmatites in northcentral and southwestern Nigeria believed to form the older tin fields of Nigeria. The younger tin fields are those within the Younger Granites of Jurassic age in Jos Plateau of northern Nigeria.
In this contribution, the authors have carried out pioneering field and geochemical studies of pegmatites in parts of the Oban Massif of southeastern Nigeria. Preliminary geochemical study and analysis of these pegmatites which crop out at Uyanga, Akwa Ibami, Iwuru and Igbofia in the Western Oban Massif show that the pegmatiites are highly albitised. This is contrary to earlier postulations that the pegmatites in this part of Nigeria are barren.

Indices of fractionation such as Ba/Rb, K/Rb, Na/K, K/Cs, K/Ba reveal that they are mineralized and discrimination plots show that these pegmatites are enriched in Li, Be, Sn, Ba, Ta, Ni, Cs and Zn. Rare metal indicative elements such as Ta, Nb, Rb, Cs, Sn are enriched in the pegmatites confirming that they are mineralized. An eloquent evidence in support of this finding is the abandoned tin mines in Akwa Ibami which date back to the early 1900's.

 

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