Letšeng's pipes are primary vertical kimberlite pipes. Such kimberlite pipes are characteristically carrot shaped in section and comprise volatile-rich, potassic, ultrabasic igneous rock. Kimberlite has an inequigranular texture which results from phenocrysts and xenoliths being set in a fine-grained matrix.

Letšeng consists of two kimberlite pipes, namely the Main Pipe and the Satellite Pipe. The pipes are hosted in unweathered basaltic lavas. Prior to mining, both pipes consisted of the following features, listed from surface to depth:

- a superficial upper organic layer of black cotton-soil, or sponge, with a thickness of less than 1.0m;
- a leached gravel, which either consists of a brown basaltic overwash (<5.0m thick on the Satellite Pipe and poorly developed on the Main Pipe) or a widespread leached white gravel. The white gravel in the immediate vicinity of the pipes carries kimberlitic heavy minerals;
- a soft, clayey, weathered and oxidised yellow-brown kimberlite known as “yellow ground”. It is typically 2.5m thick and contains many heavily weathered basalt xenoliths; and
- a soft blue-green, partially weathered kimberlite commonly known as “blue ground”. It is at least 20m thick on the Main Pipe, with the degree of weathering decreasing with depth. On the Satellite Pipe, the weathered kimberlite is 15m thick in the south, but very thin or absent in the north, with a relatively sharp lower contact with fresh (hard) kimberlite.

Letšeng's Main and Satellite pipes are primary diamond deposits in near vertical volcanic pipes. Kimberlite rock has a typical inequigranular texture which results from phenocrysts and xenoliths being set in a fine-grained matrix. Kimberlites are volatile rich, potassic, ultrabasic igneous rocks of highly variable mineralogical compositions. There are two distinct types of kimberlite, namely:

* Group I (basaltic kimberlites): These are olivine-rich monticellite-serpentinecalcite ilmenite kimberlites; and
* Group II (micaceous kimberlites): These kimberlites consist primarily of rounded olivine macrocrysts in a matrix of phlogopite.

Diamonds in the kimberlite are associated with both eclogite and peridotite sources, with higher diamond grades occurring in the eclogite xenoliths. Diamond grades are determined by several factors:

* the relative abundance and diamond content of the original mantle rocks (preserved as xenoliths) which contributed to the kimberlite magma;
* the degree of contamination (dilution) by barren country rock fragments; and
* the degree of gravity-controlled sorting.

Diamond mineralisation is present within the facies of both pipes. This section discusses the various facies present along with their potential for mineralisation (i.e. their potential diamond carrying capacity).

Letšeng's pipes are primary vertical kimberlite pipes. Such kimberlite pipes comprise volatile-rich, potassic, ultrabasic igneous rock. Kimberlite has an inequigranular texture which results from phenocrysts and xenoliths being set in a fine-grained matrix. The pipes are hosted in unweathered basaltic lavas. Both pipes represent diatreme facies of volcaniclastic kimberlite that has formed as a result of an explosive eruption and contact with the atmosphere. Diamond mineralisation is present within the facies of both pipes.

The Main Pipe is a cone shaped body tapering downward with depth from a surface projection measuring 540m by 365m in extent. The long axis of the pipe trends northeast. The surface expression covers an area of 17ha. The contact with the basalt, as determined by limited historical drilling, dips inwards at 83°. From surface mapping as well as surface and underground sampling, eight different varieties of kimberlite were originally recognised in the Main Pipe, namely the K1 to K8 facies, in the order in which they were mapped. For practical resource and mining purposes the facies were previously grouped into three distinct and easily recognisable facies namely, K1/KOther; K4; and K6. Then, in 2009, the K1/KOther facies within the Pipe was divided into the K North and K South facies. All recent modelling and sampling has been carried out using these facies as the basis.

The Satellite Pipe is an elliptical shaped kimberlite measuring 425m, along its NNE SSW axis, by 130m wide on surface. The surface expression covers an area of 5.2ha, including a large basalt raft. This lenticular basalt raft extends from the central area of the pipe to the southwestern end and is located immediately adjacent to the western sidewall near surface. The basalt raft tapers downward which results in an increase in cross-sectional area of the pipe with depth. Below the base of the basalt raft, the pipe is elliptical in plan, and tapers at 83°, in conformity with the eastern sidewall. The Satellite Pipe is comprised of at least two facies of kimberlite. Within these two major facies, gradual changes in the size and mineral content of the xenoliths are noted. These major facies, namely North Volcaniclastic Kimberlite (NVK) and South Volcaniclastic Kimberlite (SVK), can be identified in-pit and are used for resource classification and mining purposes.

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.

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 mine development.

The ore is fed into the plants and crushed by means of a jaw crusher. The ore is conveyed to a scrubber where it is washed. Ore is then fed into the DMS or undergoes secondary crushing or tertiary crushing in the re-crush circuit, prior to feeding back into the DMS.

Ore is processed through three treatment plants with an annual throughput of 6.5 million to 7.0 million tonnes and carat recoveries of 114 000 carats to 130 000 carats.

Letšeng operates two main kimberlite treatment plants, termed Plants No.1 and No.2, which together treat 5.7 million tonnes of ore per annum. An additional 1.3 million tonnes per annum are treated by Alluvial Ventures, one of the contractors. 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 ........