Since acquiring each of our underground mines in South Africa, Petra has been operating in mature caves, where the existing block of ore to be mined has been nearly depleted.
Once the majority of the ore in one block has been removed, waste from the side walls of host rock will start to fall into the cave and subsequently report to the drawpoints, thereby heavily diluting the material being extracted. This means that the overall diamond content per tonne mined (“grade”) is much lower than that of the pure kimberlite ore.
Petra’s expansion programmes are designed to take the next ‘cut’ by deepening and establishing new sub level and block caves in undiluted kimberlite.
As the development plans progress, the grade of each tonne mined is therefore expected to rise significantly, increasing the margin per tonne mined. See our detailed operational guidance sheet for more information on how we expect grades to increase up to FY 2019.
This process is well underway, as evidenced by the rising ROM grades achieved at Finsch and Cullinan of +28% and +29% respectively in FY 2017. Further improvements are expected in FY 2018, which will be the first year in Petra’s history when it will be sourcing the majority of its underground tonnes from undiluted ore in the new mining areas of its operations.