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Koidu Kimberlite Project Location Profit Sharing Geological Setting Project History No. 1 Pipe Background Vertical Pit Development Production Exploration Geological Model No. 2 Pipe Dyke Zone A and B Blows From Pit to Polish

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Geological Setting

Regional Geology

Sierra Leone is situated on the Man Craton of the Southern West African Shield. The Achaean 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 gneisses and intruded by late and post orogenic granites.

Map showing the Koidu, Tongo and Kangama areas with their respective geological formations. View full size | Download PDF

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 are developed in northeast orientations. Vertical amphibolite dykes in the Koidu area, formed by the metamorphism of dolerite dykes, are sub-concordant with the host gneissic basement and the associated minor structures indicate a component of horizontal dextral simple shear in discrete zones up to 20m wide. The dolerite dykes and sills of Jurassic or Cretaceous age generally have strikes of between 110° and 130°.

The diamond fields of Sierra Leone occur within the Eastern Province and the eastern half of the Southern Province, an area of approximately 20,000km², and includes alluvial deposits and primary kimberlite dykes and pipes. The country rock in the area is granodiorite gneiss, containing metamorphic inclusions of amphibolite, ultrabasic schists, quartzites and granulites, among others. The kimberlite dyke systems at Koidu and Tongo have a strike of between 070° and 074° and are restricted to the area between two parallel regional scale 010° trending faults, the Oyie-Shongbo and Yasamba Faults.

Local Geology

The Koidu Kimberlite Project Mining Lease Area contains two kimberlite pipes (No. 1 Pipe and No. 2 Pipe), four dyke zones (Dyke Zones A, B, C and D), the Ring Structure, a small blow on Dyke Zone A and another blow on Dyke Zone B discovered by Koidu Holdings in 2005.

From the 1950s to the 1980s, the drilling, sampling and mining of the two kimberlite pipes and dyke zones was undertaken by The Sierra Leone Selection Trust / National Diamond Mining Company.

Both kimberlite pipes consist of multiple massive volcaniclastic kimberlite (diatreme facies), magmatic kimberlite dykes (hypabyssal facies) pre- and postdating pipe formation, as well as transitional kimberlites. The kimberlite pipes have been eroded down to the root zone, having irregular shapes, particularly in the case of No. 1 Pipe. Three drilling programmes on No. 1 Pipe and No. 2 Pipe were undertaken, the first of which commenced in 2003 and the last having been completed at the end of 2007, the results of which were incorporated into 3D geological models forming the foundation of
the Company's diamond resource estimates.