Deformation and mineralisation in the Basement Complex of Nigeria
|Location||International Geological Congress,oslo 2008|
|Holding Date||06 October 2008|
Economic mineralisation in the basement complex of Nigeria, have been shown in this study to be constrained mostly by late Pan African deformational events which manifests as shears, faults and folds. Three main groups of rocks constitute the basement complex namely: The migmatite gneiss complex, metasedimentary-volcanic series and the Older granite suites.
Field, remotely sensed (SLAR, aerial photographs) data and laboratory studies indicate that the basement complex rocks have responded differently to deformational episodes with the resultant foliations being essentially tectonic and metamorphic in origin. For example, in the migmatite gneiss complex, the dominant foliation defined by alternating dark and light bands and prominent gneissossity trends roughly N-S with variation between NE-SW and NW-SE. E –W foliations are the oldest and mostly preserved as relicts. The schists, phyllites and quartzites of the metasedimentary series have more than four foliations representing four folding phases ranging from tight to isoclinal with steep to vertical hinges, open and smooth, strain slipped cleavages and kink bands. Some of these folds have been overturned and are thus preserved as isolated mega tight folds.
Principal fractures directions are in the N-S, NNE-SSW, NNW-SSW and NW-SE and to a lesser extent E-W, with the N-S fractures marked by considerable shearing and brecciations . The NE-SW and NW-SE conjugate sets are mostly strike slip faults with the north easterly characterised by dextral sense of movement. Some of the important fault systems are the Ifewara , Zungeru ,Anka and kalangai with wide zones of mylonites, cataclastic and silicified rocks. They are interpreted to have resulted from transcurrent movements.
Field evidence suggest that both base metals (Pb-Zn-Cu-S), gold, rare metals,(Ta-Nb-Sn) and uranium hosted in late siliceous veins and pegmatites are constrained in time and space by late Pan African tectonics and associated essentially with the development of transcurrent fault conjugate systems. The rare metal and gold mineralization along anticlinal axis closely associated with the Kalangai, Ifewara, and Anka transcurrent systems in Zuru, Isanlu, Iseyin-Oyan and Kushaka schist belt respectively support this assumption. Uranium mineralization of both primary and secondary autinite ore is confined to N-S and N 130E open structures respectively. The latter trend representing transpression zones which are typified by stratal thinning leading to formation of dilatant structures as tension gashes. Marble deposits are confined to euomiogeosynclinal basins at the eastern margins of the fault bounded schist belts, while extensive barite mineralization is constrained by N-S trending fractures in the central regions.