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.

Mining method: Underground block cave and sub-level cave.

The sole source of our current tonnage is the C-Cut, a 5.7 Ha area with potential for approximately 34 Mt additional tonnage which is known to include high-value Type II stones, large high quality, white or blue gem-stones.

During the Year, the CC1-East project was approved and the capital spend has commenced. This new area is expected to begin to contribute from FY 2024 and to ramp up to steady state by FY 2026. It extends the mine plan to 2031. Whilst the C-Cut mining block is known for its Type II diamonds, the CC1E mining block has a higher grade than the C-Cut.

The significant opportunities for mine plan extensions beyond 2031 include additional tunnels to the east of the C-Cut, the C-Cut Centre block which has not been mined. The D-Cut and material below CC1-East depends on building a new shaft. This western part of the orebody at deeper levels should continue to produce high value stones.

Petra relies on tunnels to access and extract ore from draw-bells within underground block caves and thus face the inevitable threat of tunnel convergence. While tunnels are designed to withstand the significant structural stresses, geology is neither consistent nor entirely predictable and, where complex structures meet unfavourable geology, there is a possibility of tunnel convergence. Typically, the solution is to plug the affected area of the tunnel with concrete to strengthen it ........

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