Energem’s Koidu resource figure tightened up

A revised resource estimate done for Energem Resources (ENM-T) gives its Koidu kimberlite complex in northeastern Sierra Leone a smaller but better-defined resource.

Energem engaged South African consulting firm Venmyn Rand to revise the resource estimate based on bulk sampling of the two main pipes and other parts of the Koidu complex during 2004. One result was that the new resource and a reserve figure now quote only the recoverable diamond grade. Another — limiting the extrapolated depth of kimberlite to 30 metres beyond the deepest intersection — significantly decreased tonnage.

Venmyn’s resource estimate shows Pipe 1, the smaller of the two main pipes, with 434,000 tonnes of indicated resources, at a grade of 0.55 carat per tonne; another 1.1 million tonnes are inferred, and grade 0.55 carat per tonne as well. The resource covers a vertical extent of 244 metres, from 95 to 339 metres above sea level.

A 2003 estimate on Pipe 1 put its size at 511,000 tonnes grading 0.67 carat per tonne in the indicated category, and 1.2 million tonnes grading 0.67 carat per tonne in inferred resources.

At Pipe 2, the new indicated resource is 461,000 tonnes at 0.26 carat per tonne and the new inferred resource, 2.5 million tonnes grading 0.26 carat. The estimate takes in a 235-metre vertical extent on the pipe, between elevations of 110 and 345 metres.

The new estimate on Pipe 2 compares with 3.1 million tonnes at 0.28 carat per tonne, all inferred, in the 2003 resource estimate.

Grades appear sharply reduced, but are an apples-to-oranges comparison: the new grades (0.55 carat in Pipe 1 and 0.26 carat in Pipe 2) are grades recoverable by the X-ray sorter in the existing plant, based on the 2004 bulk sampling. They neglect recoveries historically possible on grease tables, which — being largely industrial-quality diamonds valued around US$4 per carat — would have a negligible economic impact on the project anyway.

Two diamondiferous dyke zones, Dyke Zone A and Zone A Enlargement, have lost substantial inferred tonnages in the resource revision. Dyke Zone A, previously interpreted to have an inferred resource of 1.2 million tonnes grading 0.43 carat per tonne, is now listed with an indicated 455,000 tonnes grading 0.54 carat, between elevations of 270 and 391 metres.

The Zone A Enlargement is down to 82,000 tonnes, all indicated, grading 0.75 carat per tonne, from 24,000 tonnes indicated and 158,000 inferred, grading 0.69 carat.

In the 2004 bulk sampling, material mined from Pipe 1 was valued at US$231 per carat, material from Pipe 2 at US$187 per carat, and material from the dyke zones at US$169 per carat. For the resource and reserve calculations, the zones were assigned values of US$230, US$180, and US$160.

Venmyn put the size of the currently minable Koidu reserves at 163,000 tonnes grading 0.55 carat per tonne in Pipe 1 and 461,000 tonnes grading 0.26 carat in Pipe 2.

Energem plans to exploit Pipe 1 with a vertical-wall pit down to the 200-metre elevation (that is, about a 139-metre depth from surface), at a rate of about 20,000 tonnes per month, with the possibility of going underground to mine by sublevel open stoping after that. Pipe 2 would be mined using a conventional open pit at 5,000 tonnes per month, to provide lower-grade feed topping up the plant at its design capacity.

The plan includes about 1 million tonnes of inferred resources, so it cannot be considered to be a feasibility-level mine plan.