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 (Figure 13). 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.

Both pipes are currently being mined using conventional drill, blast, load and haul open-pit techniques.

Processing at Letšeng is currently carried out through two 350tph DMS plants and an X-ray final recovery unit, whilst Alluvial Ventures processes ore through two pan plants and an associated X-ray final recovery unit.

All ore from Letšeng's operations is transported by truck to the Run of Mine (RoM) feed stockpiles for either Plant 1 or Plant 2. Both these plants have conventional diamond DMS plant designs. A conventional DMS plant, and associated processes used for the treatment of the kimberlite ore and the recovery of diamonds, typically includes:

* primary crushing and conveying;
* wet scrubbing and screening;
* secondary crushing;
* DMS for primary diamond concentration. Diamonds have a density of 3.5g/cm3 and the separation process forces the diamonds to report to the sink component. Particle size distribution of the cyclone feed is one of the key parameters impacting on the separating efficiency of the cyclones. Therefore the -55mm +2mm fractions are screened and separated into coarse (-55mm +27mm) and fine (-27mm +2mm) ractions and fed into separate dense medium circuits. Ferrosilicon is mixed with water to provide a medium with the correct operating density;
* magnetic recovery of the ferrosilicon;
* tertiary crushing for diamond liberation of the coarse DMS discard;
* fines go to slimes dam; and
* coarse tailings to coarse tailings dump.

The ore is fed into the plants and crushed by means of a jaw crusher. While the design capacity of each plant is 350tph, throughput is increased by manipulating the various cut sizes. 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.

The final recovery is carried out by X-ray separation followed by handsorting in gloveboxes.

The coarse tailings (>2mm) are transported via conveyor to the tailings dump located in the Patiseng Valley.

Slimes (<2mm) from the ore treatment plants are disposed of separately in the slimes dam, after passing through the thickener to remove excess water. Submersible pumps extract return supernatant water from the slimes dam to the plant reservoir, from where the process water gravitates to the plant.

Alluvial Ventures treats ore through two pan plants. The first plant comprising one 14 foot and one 16 foot pan was operational from 2003, when it was used to treat alluvial deposits in the valleys adjacent to the pipes. From 2004, Alluvial Ventures started treating the weathered ore from the surface of the Main Pipe.

In 2005, the company increased its original pan plant with the addition of two 16 foot pans into the existing circuitry.

The company was only able to treat weathered kimberlite ore or stockpiled ore from the Main Pipe or the De Beers Stockpile as its pan plant had no crushing facility. Any oversize (+40mm) extracted was delivered to the Alluvial Ventures? Oversize Stockpile.

In January 2011, the company modified its existing plant to include a CJ411 jaw crusher and Sandvik CS420 cone crusher into a crushing circuit and, as a result, is now able to treat hard ore.