Petrographic and Structural Analysis of Exposed Rocks of Bishini Sheet (Block 2), North Central Nigeria

Abstract

This study is the second of a series of work to be done on Bishini Sheet 165 SW, which is part of the basement complex of the Kushiriki-Minna area. The area lies between latitudes 9°34′ N and 9°40′ N and longitudes 7°00′ E and 7°08′ E. Due to the continuous play of geologic processes on the earth, there is the need to continuously update the geology of the Nigeria basement complex as this has a very important economic implication in areas like mineral and water exploration. Equally, less attention has been given to this sheet unlike other sheets within the zone. The study of the block has revealed the various rock types and their geological characteristics: field relation, macroscopic and microscopic features and structural elements. Results from the field shows that the block consists of migmatites, gneisses, schists and granites which all show a variety of colours, textures and mineralogy. The displacement of distinctive mineralogy, textures, optical and structural characteristics led to the nomenclature of the rocks. The granitic rocks displayed two petrographic varieties based on their textures. These are the fine-medium grained and coarse porphyritic varieties. The granitic rocks have greatly intruded other rocks and this is thought to be responsible for gold mineralization in the area, The NNE-SSW and the NNW-SSE are the major structural trends determined for the joints and faults while the minor ones are the NE-SW and NW-SE. All these trends are consistent with the general trends of the Nigeria basement rocks. A study of thin sections of the rocks under a petrological microscope reveals characteristic optical properties peculiar to each rock type. The type and nature of the minerals the rocks contain together with their geometry reveal that the rocks have undergone more than one deformational event since the time of their formation. The occurrence of numerous fractures (joints and faults) shows that the area has a great potential for water and mineral exploration.