Lithostructural framework and rare metal (Ta-Nb-Sn-Li-Be) mineralisation in lepidolite from the Ijero-Aramoko pegmatite field, south-western Nigeria

Abstract

A lithological framework and petrochemical features relating to Ta-Nb-Sn-Li-Be mineralisation in lepidolite from the Ijero-Aramoko pegmatite field, south-western Nigeria were investigated. Systematic geological mapping revealed that the pegmatite occurs as steeply inclined intrusive bodies in a basement of gneisses and schistose assemblages that are occasionally pulsed with isolated granitic bodies. The lepidolite occurs in the intermediate (coarse-grained microcline-albite-quartz) zone of the inclined complex pegmatite. The lepidolite has a layered structure, tabular pseudo-hexagonal crystals with scaly aggregates of perfectly cleavable masses and an equigranular texture. The major and trace element composition of 30 lepidolite samples was determined using inductively coupled plasma–atomic emission spectrometry with a view to determining compositional features relating to Ta-Nb-Sn-Li-Be mineralisation. The average silica content of the lepidolite ranged from 49.43% to 57.81%. The three sampling localities and their mean SiO₂ contents were Oke-Asa (c. 49.83%), Oke-Igbo Aba (c. 49.99%) and Ijero-Ekiti (c.58.40%). Average Al₂O₃ content was 27.83%, 23.02% and 28.34%, respectively. Similarly, K₂O content was 10.06%, 9.41%, and 5.86%, respectively. The contents of MgO, CaO and TiO₂ collectively contributed less than 0.5% to the minerals bulk composition. Trace element composition revealed enrichment in Li, Be, Cs and Rb, relative to Ta, Nb and Sn. Lepidolite from Oke Asa, Oke Igbo Aba and Ijero-Ekiti contained an average Li content of 1 859 ppm, 1 778 ppm and 1 656 ppm, respectively. Similarly, the average Ta content was 0.36 ppm, 0.6 ppm and 0.03 ppm, respectively, and Nb and Sn contents showed a similar trend. The Li content of lepidolite from Ijero-Aramoko pegmatite fields is marginally less than the 2% threshold value required for its use as an ore of Li. However, the low Li content of the mineral qualifies it for use in pharmaceuticals, glass, alloy, Li storage batteries and smelting of Al ore and other industrial applications requiring a low Li content.

Keywords:

• basement
• Ijero-Aramoko
• lepidolite
• pegmatite
• petrochemical

JEL classification:

• O30
• O31
• O39
• O55

Acknowledgements

The author is highly grateful to all colleagues and students who assisted in the geological mapping exercise. Professor AF Abimbola of the Department of Geology, University of Ibadan assisted in the link for geochemical analysis, Akin Ojutalayo of the Geological Survey Agency of Nigeria, Ibadan office, provided logistic support and Kunle Oshikoya permitted access to the mines in Ijero Ekiti.