Fracture Characterization, Mineral Vein Evolution and the Tectonic Pattern of Igarra Syn-Tectonic Granite, Southwestern Nigeria

Abstract

Extensive outcrops of the pan-African granite were studied in detail with particular interest in the joint pattern and mineral vein characteristics. The attitude of 57 tectonic joints and 60 mineral veins were measured, the length and width of the veins documented. Rose diagrams and graphs representing fractures and veins were plotted and analysed based on their behavior and preferred orientations. Two (2) basic trends of mode I fractures(joints) and veins were observed in the granites, these are the E-W trending joints and veins parallel to the maximum principal stress (σ1) direction and the N-S trending joints and veins parallel to the minimum principal stress (σ3) direction. These granites contain mostly Mode I fractures which require lower stress to initiate and propagate relative to Mode II and Mode III fractures, although other trends (NW-SE and NE-SW) of joints and veins were mapped, they are highly insignificant compared to the major E-W and N-S trending joints and veins within the plutons. Mineral veins in these syn-tectonic granites developed apertures that were sensitive to the orientations of the stresses; the widest veins being those oriented approximately in the E-W direction, parallel to the maximum principal stress (σ1). Mineral veins in N-S orientation are generally thin and non-persistent; their formation indicates fluid pressure-build up in the course of the orogeny. The occurrence of sheet/exfoliation joints lends credence to the occurrence of some sort of near-surface deformation related to cooling and exhumation or unroofing within the granite. The simple pattern of surface joints and veins indicates a fairly constant orientation of the principal stresses σ1 and σ3 in the E-W and N-S directions respectively, through most part of the orogeny in this area. From this study, it would appear that the pan-African stress configuration did not change throughout the period tracked by the evolution of these granites. This work serves to elucidate the joint pattern and mineral vein evolution of the syn-tectonic granite to further discuss the history of Pan-African tectonics of the Igarra region in southwestern Nigeria and it was carried out in Igarra and environs with further analysis done at the Department of Geology, University of Calabar, Nigeria for a period of 18 months.