The pipe intruded rocks of the Transvaal Supergroup as well as igneous rocks of the Bushveld Igneous Complex. The kimberlite has been radiometrically dated at 1179 ± 36 Ma using Rb–Sr in clinopyroxene and 1202 ± 72 Ma using U–Pb in perovskite. The pipe is itself intruded by a gabbro sill of 1150 Ma.

The internal geology of the pipe was first described by Wagner who noted the presence of a “floating reef” of Waterberg quartzite which occurred in the middle of the pipe and was associated with a considerable rubble zone of rounded quartzite and sandstone boulders. These Waterberg quartzite inclusions indicated the presence of this formation at the time of emplacement, but which has since been eroded from the area. Later geological work identified the presence of a number of discrete kimberlite varieties that included two major types of TKB, named the “Brown” and “Grey” kimberlites. The large floating reef referred to above occurred within the Grey Kimberlite along the boundary with the Brown. In addition, a central complex that consisted of several phases of kimberlite occurred within the Grey. This was referred to as the Black Kimberlite, which has an associated variety called the Piebald Kimberlite. The last phase of magmatism was the intrusion of carbonate-rich dykes.

The new plant utilises gentler processing methods (comminution via attrition) instead of extensive crushing in the form of autogenous milling and high pressure grinding rolls, which are expected to reduce diamond breakage and improve recoveries across the full spectrum of diamonds, including the larger/exceptional stones for which the mine is rewnowned (the top-cut of 75mm will cater for diamonds of +3,000 carats, such as the Cullinan diamond).

In September 2017, Petra completed the construction and commissioning of a modern, fit-for-purpose processing plant at Cullinan, with a throughput capacity of 6 Mtpa.

This has replaced the previous plant at Cullinan, which was originally commissioned in 1947 and which had undergone numerous refurbishments over the years. Due to its age and operational complexity, it had become expensive to maintain, requiring significant stay-in-business capex, and costly to operate, particularly given the large size of its 26 ha footprint. The plant was also based on old crushing technology, which is known to impact large stone recoveries as while diamond is the hardest substance known to man, it cannot take impact and therefore very large stones can be shattered using conventional processing techniques. At the same time, the energy efficiency of the old plant was not aligned with the Company’s vision.

The design of the new plant was therefore planned to address these shortc ........