Petrographic and Structural Study of Precambrian Pegmatite Veins in the Igarra Schist Belt, Southwestern Nigeria

Precambrian Pegmatite veins in Igarra were studied and analyzed in order to group the veins based on their structures, orogenic episodes and modal composition. Pegmatite veins are found more in granodiorites, Migmatite-Gneiss and undifferentiated mixed rocks in the study area. The studied veins were predominantly dykes and sills while some were folded into irregular shapes. Exposed studied veins have minimum length of 2.5m and maximum of 10m. A minimum width of 43cm and maximum of 2m were also recorded. Structural elements like strike directions analyzed using rose diagrams suggest that the emplacement of pegmatite veins is related to a dominant structural control orogenic event. Two major deformational episodes were recorded on the pegmatite veins studied. The irregular recumbent folded pegmatite veins represent a ductile phase deformation which resulted in the formation of planar structures in the study area. The second deformation that impacted the pegmatites was a brittle phase deformation that resulted to fractures found in basement rocks many which were filled with pegmatites and quartz veins. Results showed that the dominant strike directions of veins were mainly in the NE-SW and N-S directions. Also recorded were few veins which trend in the NW-SE direction both which were likely the imprint of Pan-African orogeny in the study area. Modal composition of minerals from thin section results on samples shows they are predominantly feldspathic pegmatites with k-feldspar between (32 to 46%) with mean value of 40.9% and plagioclase (18 to 30%) with mean value of 21.5%.The NE-SW veins have more biotite than the NW-SE pegmatite veins. Perthitic structures (mask effect of albitic plagioclase) were recorded in the NE-SW veins. Sericitization of feldspars (hydrothermal alteration) was also seen in the NE-SW pegmatite veins. The mineral Tourmaline is associated with NE-SW veins as seen in thin section petrography while Sphene (Titanite) can be found in the NW-SE pegmatite veins.