Koidu vertical pit, Sierra Leone

The Koidu Kimberlite Project is situated in the Kono District of Sierra Leone, approximately 330 km east of the capital city, Freetown. The mining lease area is 4km2 and comprises two kimberlite pipes, four dyke zones and a number of blows off the dykes. In 2003, Koidu Holdings established operations on the property and began dewatering and removing silt from the existing 30m excavation of the No.1 Pipe. While the original planning for the pipes included conventional open pit operations, mitigating factors such as the proximity of the town, the presence of houses close to the perimeter of the pit, the size of the kimberlite pipe and the planned open pit depth, combined with relevant stripping ratios and uncertainties on the grades concerning an underground operation, lent themselves to the vertical pit concept.

The vertical pit was established using an A-frame headgear for the hoisting arrangement, with sidewall support comprising a combination of cable anchors 20m and 40m long, rock bolts, wire mesh and pneumatically applied concrete. The pit reached steady state production in January 2007 and produced some 26,000 tonnes of head feed per month until August 2007, when mining reached a depth of 74m below the pit collar (approximately 126m below surface). Mining was suspended following a sidewall failure that disrupted operations. Now that the grades in No.1 Pipe have been validated, the focus has switched to an underground operation taking account of both No. 1 Pipe and No. 2 Pipe.

While the mining costs for the vertical pit have been higher than for a conventional open pit, the concept did circumvent the social implications, the issue of grade consistency and diamond value at depth; it also provided the time to carry out a full underground mining study.

Peter Terbrugge: pterbrugge@srk.co.za

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