Resources
The Letšeng Diamond Mine is a unique operational kimberlite Mine with an average grade of 1.8 carats per hundred tonnes which despite the lower grade is commercial, and the highest average dollar per carat kimberlite Mine in the world renowned for producing exceptional high-quality white Type II diamonds.
The Letšeng kimberlite pipes were discovered as a result of pioneering works of the likes of Colonel J Scott and British geologist Peter Nixon in the 1950s. A reported major discovery at Letšeng then was that of the 601 carat Lesotho Brown in 1967 by artisanal miners. The ownership of the mine was taken by Rio Tinto in 1968 and subsequently by De Beers in 1973. The Mine was however, due to its inherently low-grade and the plunging diamond market then, rendered uneconomic by De Beers and closed off in 1982. The mine was reopened in 2004 and was bought by Gem Diamonds (Pty) Ltd in 2006 at 70% and 30% shareholding with the Government of the Kingdom of Lesotho.
Letšeng Diamond Mine exploits two kimberlite pipes (the Main Pipe and the Satellite Pipe) that are adjacent to each other. The mineral inventory is estimated at about 213 million tonnes with 56% in the Indicated resource category and the remainder in the Inferred resource class. The Mine has in the past ten years treated an annual average of 6.2 million tonnes producing an annual average of 110 000 carats at an average price of US$1 900/ct. The Main Pipe has currently been mined to a depth of 252m and Satellite Pipe to 336m below the surface.
The two carrot-shaped kimberlite pipes are hosted exclusively in the basaltic lavas of the Karoo supergroup. The original surface impression of the two pipes was 17 hectares for the Main Pipe, and 6.5 hectares for the Satellite. They are each a multifaceted intrusion comprising a number of different diamondiferous kimberlite phases each with unique lithological imprint, mantle rock content, and diamond population. The pipes are infilled with a variant of Kimberley-type pyroclastic kimberlite (KPK) as well as coherent (CK), hypabyssal (HK), and resedimented volcaniclastic kimberlite (RVK). The current understanding of the geology is that the Main Pipe consists of 13 internal domains – 11 kimberlite domains and two basalt xenolith/basalt breccia domains. The Satellite Pipe comprises six main kimberlite domains (SVK, NVK, GVK, KIMB7, GVK-SVK-Mixed, CCK).
Mining
The two kimberlite pipes are mined using conventional open-pit mining techniques on a split shell design. The split shell concept was adopted to optimise waste stripping and enhance cash flow. Major mining activities, except blasting, are outsourced to contracting companies. Drilling is done with Atlas Copco Rock L8 & L6 machines. The mine employs a mixed loading and hauling fleet, comprising CAT 385C and CAT 390D excavators, together with CAT 777 rigid dump trucks for hauling waste and CAT 745 articulated dump trucks for hauling ore. The mixed fleet results from different ore and waste profiles during different stages of the mine’s development. Approximately 6-7 million tonnes of ore and 20-24 million tonnes of waste are mined per annum.
Treatment
Letšeng Mine operates two main kimberlite treatment plants, which in combination treats 5.1 million tonnes of ore per annum. Plant 1 was commissioned in March 2004, and Plant 2 in March 2008. The processing methodology of the two plants are similar, although some improvements were made to Plant 2 based on learnings from Plant 1. The Plants are fed by primary pre-crushing units that accept run of mine ore and reduces the coarse kimberlite boulders to smaller than 200 millimeters. Each main plant comprises primary, secondary and tertiary crushing, scrubbing, sizing, and DMS (dense media separation) concentration of the different size fractions. The DMS concentrate is fed to a centralized final recovery and sorthouse facility. Coarse tailings and slimes residue from the process is placed in a well-designed valley deposition system from where process water is reclaimed for re-use.
Recovery
Letšeng Recovery is where the final recovery of diamonds is achieved from the concentrated ore produced by Plant 1 and Plant 2. It is the final step in the production process, employing x-ray technology. The Mine continuously explores ways to optimise diamond recovery through the implementation of various new technologies. To this end, a final step has been introduced, which is the re-examination of the recovery tailings to check if any diamonds may have been missed, using an alternate type of x-ray process.