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.< ........
