Geology & Resources
Sierra Leone is situated on the Man Craton of the Southern West African Shield. The Archean granitic shield contains elements of early sedimentary and mafic formations and a group of supracrustal greenstone belts with banded ironstone and detrital sediments. The basement granite and rocks of the younger Kambui Group have been deformed and metamorphosed together with the underlying Leonean gneisses and intruded by late and post orogenic granites.
The granitic rocks are cut by several fracture systems which are widely believed to have controlled the emplacement of kimberlite and dolerite dykes. Foliations and faults in the basement granites are almost parallel, trending roughly north-south to northeast-southwest. Second order fractures developed in northeast orientations. Vertical amphibolite dykes in the Koidu area, formed by the metamorphism of dolerite (diabase) dykes, are sub-concordant with the host gneissic basement. The dolerite dykes and sills of Jurassic or Cretaceous age, generally have strikes of between 110° and 130° (Hall, 1968).
Three dominant brittle structural patterns were observed by Williams (1977) in the Archean gneisses, which are N-S, ENE-WSW and NW-SW, and these were confirmed by Barnett (2009) during a detailed structural assessment of the Koidu Kimberlite Project. Barnett (2009) also notes that both Williams (1979) and Venkatakrishnan and Culver (1989) suggest that the N-S and ENE-WSW brittle structures form as conjugate shears in response to the N-E directed maximum compressional stresses caused by Neoproterozoic orogenesis, specifically the Rokelide orogeny in western Sierra Leone.
Venkatakrishnan and Culver (1989) suggest, based on an oblique rifting model, that the dominant reactivated structures during the Mesozoic rifting would be approximately N-S to NNW-SSE trending normal faults reactivated along ENE-WSW trending sinistral strike-slip corridors.
The Koidu and Tongo kimberlite clusters of pipes and dyke zones are part of a Jurassic age province of kimberlites within the Man Craton, which extends from the 154 Ma Droubja kimberlite in southeast Guinea and curves around to the +145 Ma bodies at Koidu, then extends southwards to the + 140 Ma bodies at Tongo. The distribution of the kimberlites is thought to be related to continental scale movements and tectonic stresses established in the lithosphere rather than to local scale structures (Skinner et al., 2004). Skinner et al. (2004) presented evidence in support of hotspot activity being related to kimberlite genesis and, assuming that kimberlites are generated from a fixed position in the asthenosphere, that it is possible to track the direction and rates of movement of the overriding lithosphere from known kimberlite distributions and ages.