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  Lets'eng Diamonds Production

 Resources


The Lets'eng 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 cpht. They were discovered in 1957 by British geologist Peter Nixon. The two pipes, termed Main Pipe and Satellite Pipe, were originally 17 hectares and 6.5 hectares in surface area respectively before mining. The pits excavated into these pipes are now approximately 120 metres deep.

 

Datum is 3046 metres above sea level and diamond resources have been established to 2570m and 2444m in Main and Satellite respectively. Exploration core-drilling in 2012 has established 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 principle kimberlite phases have been identified in Main Pipe. First intruded was the so-called K Main phase, which has been sub-divided for mine planning purposes into K North and K South. This because empirically, there is a grade difference between the southern and northern portions of the phase. Subsequently, the higher grade K6 was intruded into the western part of the pipe and finally the lower grade and volumetrically very small K4, which has attributes of coherent kimberlite that are under investigation, was intruded adjacent to K6.
 

 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 773 rigid dump trucks for hauling waste and CAT 740 articulated dump trucks for hauling ore. The mixed fleet results from different ore and waste profiles during different stages of mine development. Approximately 7 million tonnes of ore and 20 million tonnes of waste are mined per annum.


 

 Treatment


Lets'eng operates two main kimberlite treatment plants, termed Plants No.1 and No.2, which together treat 5.7 million tonnes of ore per annum (approximately 480 000 tonnes a month). An additional 1.3 million tonnes per annum are treated by Alluvial Ventures, one of the contractors, through their two pan plants. Production through Plant No.1 commenced in March 2004 and in March 2008 through Plant No.2 . Full production commissioning was achieved in record time at the end of the second quarter of 2008.

 

The flow-sheets of the two plants are similar, though small improvements were made to Plant No.2 based on what was learnt from Plant No.1. They are fed by a Primary Crushing unit (the 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.
 

 Recovery


Lets'eng Recovery is where final recovery of diamonds is achieved from the concentrated ore produced by Plants 1 and 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 alternate type of x-ray process.