Koffiefontein Mine, South Africa

Koffiefontein is the largest kimberlite diamond mine in the world by average value per carat. Discovered in 1870, the underground mine is situated 110km south-east of Kimberley in the Free State province of South Africa. The mine neighbours some of Kimberley's famous mines, including Kimberley, de Beers, Dutoitspan, Bultfontein and Wesselton mines.

The mine is a joint-ownership between Re-Teng Diamonds (30%) and Petra Diamonds (70%). It is operated solely by Petra Diamonds, who acquired the mine from De Beers in 2007 for R81.9m. Based on its current resources, the mine has an estimated life up to 2013.

Koffiefontein mine reserves

The mine holds 17.66mt of proven and 18.48mt of probable reserves graded at 3.5 carats per hundred tones (cpht) and 8.76cpht respectively.

Indicated and inferred resources at the mine total 102.85mt at a grade of 3.62cpht. Significant resources are expected below a depth of 690m. The mine has been drilled to a depth of 720m.

Koffiefontein mine geology

The deposit is hosted as kimberlite ore body within the Koffiefontein pipe, which extends 11.1ha at surface level. The pipe, together with several other kimberlite pipes and dykes, forms a cluster that intrudes dwyka shales and karoo dolerites. It is characterised by carbonaceous and Karoo age shales besides intercalated dolerite that overlies the granite gneiss basement.

A second pipe, extending 6ha on the surface, lies adjacent to the Koffiefontein pipe. Known as Ebenhaezer pipe, it also hosts kimberlite ore body.

Diamond mine production

The mine produces high-value diamonds, including white diamonds sized between five to 30 carats. In 2009, the mine produced 52,089 carats graded at 6.6cpht, a reduction from the 89,622 carats production achieved in 2008. Delays in producing from new sections on level 49 and 52 resulted in the decline. Production at the mine was 948,000t a year.

Current production levels at the mine are expected to continue from the pipe's east and west extensions, and the recovery segments at level 52. Grades will remain the same.

A gradual increase to 8.7cpht is expected when Block 2 at level 58 comes on stream. Annual production will be higher than one million tons within three years of Block 2's operation. It will reach 1.2mt a year by the fifth year.

By extending to level 68, the mine will produce 1.2mt a year for more than 20 years, delivering 104,000 ROM carats per year.

Koffiefontein mine operations

The mine is a typical underground operation developed using front cave mining methods. Approximately 100m-high ore blocks are extracted between levels 37, 48 and 49.

Access to underground operations is achieved through a roadway that descends down to level 52, approximately 520m below surface and No 2 main rock shaft. Hauling up is carried out via a vertical shaft that declines to a depth of 620m.

Mining is currently taking place at Block 1 at a depth of 490m. Production at Block 2 at level 58 is scheduled to begin by 2012. Preparation and execution of the block has commenced on schedule and the decline development is in progress. Block 2 extends from 490m to a depth of 590m.

Koffiefontein diamond mine history

The Koffiefontein mine was identified as a pipe in 1880, nine years after it was discovered. Mining began in 1887 and by 1911, the mine was acquired by De Beers. It operated as an open-pit mine until 1981.

On completion of the open-cast operations, the mine had a surface area of 44ha with a diameter of 750m and a depth of 270m. Underground mining began in 1981. Due to economic factors, the mine has been closed several times.

The mine produces the highest value diamonds than other kimberlite mines in the world. The largest rough diamond recovered at the mine was in 1994 and weighed 232.34 carats.