Geochemistry and rare-metal bearing potentials of pegmatites of Gbugbu, Lema and Bishewa areas of North Central Nigeria

Abstract

The pegmatites of Gbugbu, Lema and Bishewa areas belong to the pegmatite belt of North-Central Nigeria which has been prospected for minerals since the early 1940's. Detailed geological mapping of the pegmatite bodies and sampling of muscovite extracts were carried out, while major, trace and rare earth elements geochemistry were achieved using ICP-OES and ICP-MS. Variation plots of major and trace elements were used to determine the geochemical characteristics of these pegmatite bodies. Structural features of the study area suggest fracture patterns in the Nigerian Basement rocks controlled the distribution of the pegmatites on a regional NW-SE, N-S and NE-SW and minor E-W trends. The pegmatites are peraluminous with major and trace elements pattern indicating similarities with granitic clan of igneous rocks. Qualitative assessment of the pegmatites using K/Rb vs Cs, K/Cs vs Rb, K/Rb vs Rb/Sr, variation plots suggest the majority of the pegmatites are mineralized with Na/K ratio>1 and belong to the rare metal bearing class with a compositional variation suggesting regional zonation of the pegmatites. Furthermore, these plots suggest the Lema pegmatite field is highly evolved and belong to the Li-BeTa Type (III) and Li-Cs-BeTa Type (IV) while majority of those in Gbugbu and Bishewa fields belong to the muscovite bearing Type (I) and Be bearing Type (II). It is concluded from this study that the pegmatites of Gbugbu, Lema and Bishewa pegmatite fields have the potentials for rare metal mineralization with alkali and rare alkali fractionation playing significant roles in the mineralization processes. Furthermore, field evidence and Rb vs Sr plot, suggests the pegmatites were emplaced at variable depths of between 20-30km indicating that the Gbugbu and Bishewa pegmatite fields have been extensively eroded and are close to the roots of the primary mineralization exposing mainly the interior pegmatites of the Types I-II. On the other hand, the Lema field which suffered lesser degree of erosion has Type III-IV pegmatite still remaining at the center of a regionally zoned pegmatite body. The significance of this study is that the pegmatites of Gbugbu, Lema and Bishewa areas have potentials for economic mineralization of tantalite, wolframite, cassiterite and columbite.