The Letšeng Diamond Mine comprises two kimberlite pipes that contain diamond resources, both indicated and inferred, totalling more than 210 million tonnes at an average grade of 1.90 carats per hundred tons. They were discovered in 1957 by a British geologist Peter Nixon. The surface area of the two pipes was originally 17 hectares and 6.5 hectares, respectively before mining. The pits excavated into these pipes are now approximately 120 metres deep.
The pipes are 3046 metres above sea level and diamond resources have been established at 2570m and 2444m in Main and Satellite Pipes, respectively. Exploration core drilling in 2012 confirmed the continuation of both pipes to greater depths. Geologically, the pipes are similar. Both consist of multiple-phase intrusions of volcaniclastic kimberlite, containing abundant basalt country rock and mantle xenoliths. They are ilmenite-poor but have significant, though variable, garnet content. Three principal kimberlite phases have been identified in the Main Pipe. First intruded was the so-called K Main phase, which has been subdivided for mine planning purposes into K North and K South. This is because empirically, there is a grade difference between the southern and northern portions of the phase. Subsequently, the higher grade K6 intruded the western part of the pipe. Finally, the lower grade and volumetrically very small K4, which has attributes of coherent kimberlite that are under investigation, was intruded adjacent to K6.
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.
Letšeng operates two kimberlite treatment plants, termed Plant 1 and Plant 2, which together treat 5.7 million tonnes of ore per annum. Production through Plant 1 commenced in March 2004 and in March 2008 through Plant 2. Full production commissioning was achieved in record time at the end of the second quarter of 2008. The flowsheets of the two plants are similar, though small improvements were made to Plant 2 based on what was learnt from Plant 1. They are fed by a Primary Crushing Unit (PCA) that crushes raw kimberlite ore to boulders smaller than 200 millimetres. Each comprises primary, secondary and tertiary crushing, scrubbing, sizing and concentration of the different size fractions in DMS (dense media separation) units. The DMS concentrate is fed to the recovery facility.
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.