The C-Cut expansion project at diamond miner Petra Diamonds’ Gauteng-based Cullinan diamond mine is 50% complete in terms of the physical development work and the capital expenditure, the miner tells Mining Weekly.

Petra Diamonds CEO Johan Dippenaar says the new block cave, which is part of the C-Cut project, will deliver its first ore in the 2016 financial year (FY), which will be followed by a production build-up through FY 2017 and FY 2018 to its target of four-million tons a year (Mtpa) by FY 2019.

The Cullinan mine is well known as a source of large, high-quality Type II gem diamonds. The mine first made history in 1905 when the 3 106 ct Cullinan Diamond was discovered at the mine – the largest rough gem diamond ever discovered.

The kimberlite pipe was originally 32 ha in size at the surface, being the largest economic kimberlite yet to be discovered in South Africa. Despite having been in production for over 100 years, Dippenaar states that the Cullinan orebody still holds a major diamond resource of 199.6-million carats and has a potential life-of-mine (LoM) in excess of 50 years.

The Cullinan mine has produced just under 800 stones greater than 100 ct, more than 130 stones weighing more than 200 ct, and more than a quarter of all the world’s diamonds greater than 400 ct.

It is also the world’s only significant source of truly rare and highly valuable blue diamonds.

New Technology
Petra Diamonds is using near-infrared (NIR) technology as part of its waste-sorting process to identify and remove waste from coarse-size fractions of +32 mm.

The NIR equipment has been fully commissioned and is currently operational at the mine.

He explains that the NIR technology is an important initiative at Cullinan, as the dilution of waste rock, compared to diamond- bearing ore – which the mine is currently experiencing in the mature mining blocks – is compounded by increased waste rock from the mine’s development activities.

The waste tonnages have to be processed through the plant along with the mining tonnages, as the mine does not have separate waste-handling facilities. “The host rock at Cullinan consists of granite, which negatively affects the cyclone separation process in the plant,” says Dippenaar, adding that discarding waste will therefore help to free up plant feed capacity and will reduce general wear and tear in the plant.

“The NIR technology supplier chosen by Petra Diamonds is well established in South Africa, which means that backup support is readily available,” he adds.

Dippenaar further highlights that waste reduction increases throughput, which adds to the mine’s capacity to treat fresh underground run-of-mine material. “Higher throughput improves our chances to recover the full spectrum of diamonds from the orebody, including the high-value special diamonds for which Cullinan is known.”

Finsch Mine
Petra Diamonds’ Northern Cape-based Finsch mine also plans to ramp up production from 1.89-million carats a year in 2014 to two-million carats a year by 2017 by establishing new mining areas in Block 5, underneath the current production level, accessing undiluted ore from the sublevel cave.

Petra Diamonds’ initial mine plan at Finsch indicates that it has an LoM of 16 years. However, the actual LoM is expected to be considerably longer owing to residual resources in Block 6 and the adjacent precursor kimberlite.

The Finsch mine has a gross resource of 51.3-million carats.

Petra Diamonds is currently producing from Block 4 in the Finsch orebody, which is situated 630 m below surface. This block is mature and has been extensively mined out, resulting in the orebody being significantly diluted with waste rock.

To provide earlier access to undiluted ore before the main Block 5 cave is established, Petra Diamonds will use a sublevel caving method over four levels in Block 5, from 700 m to 780 m, with the new Block 5 cave being installed at 900 m, and open access to mine the south-west precursor from 630 m to 780 m. The Block 5 sublevel cave will start contributing ore from the second half of Petra Diamonds’ current financial year.

Dippenaar notes that, as with all diamond mines, the economics of the mine are affected by the average value of its diamonds. “Each kimberlite orebody has its own unique fingerprint in terms of the quality and size ranges of its diamond population, taking into account the grade of the orebody and the frequency of diamonds recovered.”

He tells Mining Weekly that the average value of diamonds from the Finsch orebody is about $140/ct, which is higher than the world average.

Dippenaar explains that the Finsch orebody is a relatively high grade, although the mine is currently running at 38 carats per hundred ton (cpht) owing to old areas of the mine being mined which are diluted by waste rock. However, Dippenaar says the grade will improve to about 58 cpht once the mine has access to the new mining areas.

“These factors, combined with the significant scale of the operation, mean that Finsch is set to deliver increasingly healthy returns and free cash flow to the group,” he says.

Underground production at the Finsch mine will be ramping up from 2.9 Mtpa this year to 3.5 Mtpa in 2018. “Going forward, this production profile will be maintained by installing a new main block cave, which is planned to be fully operational from 2023/24,” says Dippenaar.

Taking into account an industry-projected future slump in production, Petra Diamonds will continue to acquire existing mines and add value to them.

“We are now well progressed on our plan to grow production organically from our existing assets to five-million carats a year, and to increase profitability through rising operating margins, enhanced by a focus on greater cost and capital efficiencies,” he concludes.