Lesotho is situated on the Kaapvaal Craton, consisting of a very thick crust three billion years old. The diamonds grew from carbon over millions of years within the stable lithospheric mantle. Research shows that most diamonds range in age between one billion and three billion years. Letšeng has two kimberlite pipes and numerous kimberlite dykes. The Main Pipe is 17,2 hectares in size at the surface, and the Satellite Pipe is 6,5 hectares. The kimberlite emplacement event occurred 91 million years ago. The kimberlite magma melted deep in the mantle and ascended through the mantle, collecting mantle rocks along the way. Evidence of these mantle rocks can still be found in the pits of the Main and Satellite Pipes. The diamonds were contained within these mantle rocks and were liberated as the rocks broke apart.
The kimberlite magma then ascended through the earth’s crust, collecting any rocks it passed through on the way. These crustal rocks include basement rocks such as granite gneisses and sandstones, mudstones and basalts derived from the Karoo Supergroup. All these rocks can be found in varying quantities in the two pipes. The rocks derived from the mantle and the continental crust found in the kimberlite pipes are termed “xenoliths”. Kimberlite magma ascends through the crust at 4 to 10 metres per second until it explodes at the earth’s surface. If the magma encounters obstacles such as dolerite sills en route to the surface, pooling beneath that obstacle may occur. When this occurs, the kimberlite magma has time to fractionate (i.e., minerals grow from the kimberlite magma).
A side effect of fractionation is the resorption of the diamonds from octahedron to dodecahedron shapes. Kimberlite magma is very rich in gas, and this forms a highly reactive magma, which has a destructive effect on any xenolithic material (including diamonds) that it may contain. As the kimberlite magma approaches the surface, the gases that were dissolved in the magma exsolve, and the sudden expansion of gas results in the magma breaking through to the earth’s surface. Kimberlite eruptions are explosive because of the large amounts of gas. They are, however, relatively small compared to other types of explosive volcanic eruptions, such as the eruption of Mt St Helens in 1980.
Several small eruptions occur over days or weeks, resulting in excavating the kimberlite pipe shape. These eruptions are called vent-clearing eruptions as they clear the vent of the host rocks and any kimberlite material, resulting in an excavation in the ground. Kimberlite and host rock material are deposited outside the excavated pipe, forming tuff cones. As the eruption wanes, the material falls back into the excavated pipe, forming layers. These layers consist of different-sized grains sorted by gravity. Large basalt host-rock blocks may fall into the pipe from the unstable side walls. This occurred in the Satellite Pipe, where a large basalt raft fills much of the southwestern side of the pipe.
After the explosive phase of the eruption has completely subsided, the force of the gases is still present in the pipe. This results in fluidisation, i.e., the mixing of deposited material through gas that forms a convecting cell. This process is generally confined to the central areas of the pipe, resulting in the preservation of layering along the pipe edges. The more gas circulates through the pipe, the wider the zone of mixing and therefore the less the layering preserved. All the processes discussed above may start again if a renewed magma is injected into the feeder dyke. Sometimes all evidence of previous phases of eruption has been re-ejected out of the vent (e.g., in the case of the Satellite Pipe). If subsequent eruptions are less powerful than previous eruptions, multiple phases of eruption are preserved (e.g., in the case of the Main Pipe).
The pipe may not fill with material to the surface, and an open crater may be left in the ground. Material from the tuff cone as well as blocks of country rock collapse into the open crater until the sediment supply ends or the open crater reaches surface level. In the case of the two Letšeng kimberlites, as soon as the kimberlite eruptions ended, secondary erosion and deposition processes began. Weathering of the kimberlite by rainfall and wind action resulted in the removal of the fine components from the kimberlite material. This effectively concentrated the very coarse material, including the diamonds. This type of deposit is called eluvium and is commonly known as “yellow ground” in mining terms, and the unweathered kimberlite beneath the yellow ground as “blue ground”.
Drainage from the kimberlites because of rainfall run-off resulted in some diamonds (along with other kimberlite-derived material such as garnets) being carried down rivers – the Patiseng River (off the Main Pipe) and the Qaqa River (off the Satellite Pipe) – and deposited as alluvium. The diamonds in the eluvium and the alluvium were mined by artisanal workers in the late sixties. These deposits are less well consolidated than blue ground and therefore diamonds are liberated easily from mined material.