Koidu, also spelt Koidu Town, is the principal town of Kono District in eastern Sierra Leone and the centre of the country's diamond industry. The town gives its name to the Koidu Kimberlite Project, a hard-rock diamond mine operated by the Octea Mining group on a series of kimberlite pipes and dykes near the town, and to the broader alluvial workings that have characterised the region's diamond production since the original discovery of Sierra Leonean diamonds in 1930.

Geological setting

The Koidu kimberlite cluster comprises two principal pipes, K1 and K2, and several minor dykes intruded into the Archaean basement of the West African Craton. The pipes are of Cretaceous age, comparable to the Kimberley pipes of South Africa and the Mwadui pipe of Tanzania, and were emplaced through explosive volcanism that brought diamonds from the deep mantle to the surface. Erosion through the long subsequent geological history has deposited diamonds from the kimberlites into the surrounding river systems, producing the rich alluvial workings that supported informal mining for decades before the kimberlite pipes themselves were brought into commercial production. The alluvial diamonds of the Koidu region are among the highest-quality alluvial diamonds in the world, with a high proportion of large, well-shaped, gem-quality stones.

History

Diamonds were first discovered in the Sierra Leone river systems in 1930 by Sir John D. Pollett, working for the colonial geological survey. Commercial alluvial mining began in 1935 under the Sierra Leone Selection Trust. The Koidu region became the centre of the country's diamond industry through the colonial period, with the town developing as a centre for the export trade. After Sierra Leonean independence in 1961, the formal mining sector contracted as the country's politics and infrastructure deteriorated, and informal alluvial digging by individual miners came to dominate production. During the Sierra Leonean Civil War of 1991 to 2002, the Koidu region was central to the trade in conflict diamonds, with the Revolutionary United Front controlling much of the production and channelling it through Liberia to the international market in violation of UN sanctions.

Post-conflict and current operations

The Kimberley Process Certification Scheme, established in 2003 in direct response to the Sierra Leonean experience, has materially reduced but not eliminated the trade in undocumented Sierra Leonean diamonds. Formal large-scale mining returned to Koidu in 2003 with the recommencement of work on the K1 and K2 pipes by Koidu Holdings, later operated by Octea Mining as part of the Beny Steinmetz Group. The pipes have produced a continuing flow of large, high-quality stones, including the 706-carat Peace Diamond recovered in 2017 in nearby Kono District and sold at auction in New York for over six million US dollars, with proceeds returned to the local community.

Trade relevance

For the contemporary trade, Koidu is significant as one of the few hard-rock kimberlite producers in West Africa with a continuing output of fine large stones. Sierra Leonean diamonds carry the Kimberley Process Certification, and stones from the Octea Koidu operation are typically routed through Antwerp for sorting and sale. The region's history of conflict diamonds remains a sensitivity in the trade, and dealers handling Sierra Leonean stones are expected to maintain documentation back to the mine of origin to satisfy compliance requirements in the Canadian and US markets.