A tour of Cullinan Diamond Mine, a gem in the Petra portfolio

2022-05-19T09:14:24+00:00 June 1st, 2022|Cover story, Mine Excursion|

By: Dr Nicolaas Steenkamp and Sharyn Macnamara

Robust health across the diamond value chain has been evident since mid-2021. The recent sterling performance of Petra Diamonds Limited (Petra) and the success at the company’s Cullinan Diamond Mine in South Africa, in particular, bears testament to the buoyancy of the diamond market.

Panoramic view of old pit with the collapse caused by subsidence visible on the far side of the pit. Image credit: ©Sharyn Macnamara, African Mining

Panoramic view of old pit with the collapse caused by subsidence visible on the far side of the pit. Image credit: ©Sharyn Macnamara, African Mining

In February 2022 Petra, leading independent diamond mining group and a supplier of gem quality rough diamonds to the international market, announced its unaudited interim results for the six months ended 31 December 2021. The Company’s portfolio incorporates interests in three underground producing mines in South Africa (Finsch, Cullinan and Koffiefontein) and one open pit mine in Tanzania (Williamson).

Richard Duffy, chief executive of the company, summed up the company’s stellar performance, Petra has delivered strong results, growing revenue by 49% and adjusted EBTIDA by 87%, as well as reducing net debt significantly. We have benefitted from the recovery in rough diamond prices, record proceeds from the sale of Exceptional Stones, and the improvements we have made in our operations, resulting in significantly improved safety levels, profitability and cash flow.”

The company highlighted that Petra’s Cullinan Diamond Mine, known as Premier mine, has a history of recovering Exceptional Stones which have contributed an average of USD47-million per annum over the last three years, and USD37-million per annum over the last five years to group revenues.

The living, working monument

Premier mine was bought by Petra Diamonds from De Beers in 2008. The mine is located in the Gauteng province of South Africa 40km east of Pretoria in the quaint Edwardian town of Cullinan. Dr Nicolaas Steenkamp and Sharyn Macnamara visited the mine in March 2022. The guided tour pointed out that the Cullinan Diamond Mine is the only producing diamond mine in the world that allows the public to tour its operation. It is a living, working monument to the men and women who have mined this area for over a century, liberating beautiful gemstones from the fierce grip of the several billion year old volcano that is the heart of the mine, as the guide so aptly put it.

Cullinan is known the world over as a source of some of the largest, high-quality gem diamonds, including Type II stones, as well as being the world’s most important source of blue diamonds. Blue diamonds are formed in the presence of the chemical element moron and are incredibly rare.

The Cullinan mine was made famous in 1905 when the largest gem ever to be found – the Cullinan diamond, weighing an astounding 3106 carats, uncut, was discovered. It was named after Sir Thomas Cullinan, founder of the mine and chairman of the Premier diamond Company Limited at the time, which had begun mining operations two years earlier. The Cullinan Diamond was gifted to Britain’s King Edward VII by the people and government of the Transvaal province in 1907. This gem produced the Great Star of Africa, which is set at the top of the Sovereign’s Sceptre with Cross, and the Lesser Star of Africa, as well as 96 smaller diamonds, amongst others.

In 1986 the mine also produced what became the world’s largest cut and faceted diamond – the Golden Jubilee. It was cut from a 755.5 carat rough diamond into a polished gem of 545.67 carats, weighing 15 carats more than the Great Star Africa.  This was followed by a 26 carat rough blue diamond recovery by Petra in 2008, now called the Star of Josephine. Cut to form a 7.03 carat flawless cushion shape diamond, it was sold for over USD9.4-million in May of 2009, setting a new world record for the highest price achieved by any gem sold at auction at the time.

The geology that produces these magnificent gems

The Cullinan Kimberlite pipe is considered the largest in the Gauteng region and intrudes the Waterberg quartzites and has in turn been cross-cut by several gabbro and diabase pipes. Kimberlite is a type of potassic volcanic rock that erupts explosively from depth and has the highest potential of being diamond-bearing. The Cullinan pipe has three distinct types of kimberlite, namely: brown, grey and black (hyperbyssal). The grey kimberlite forms the bulk of the pipe, while the brown kimberlite is found on the eastern side of the pipe and has a high clay content which swells when exposed to moisture. The black kimberlite is very hard, dark kimberlite found in the western section of the pipe and does not decompose when exposed to moisture.

Headgear of the No 1 vertical shaft. Image credit: ©Sharyn Macnamara, African Mining

Headgear of the No 1 vertical shaft. Image credit: ©Sharyn Macnamara, African Mining

One of the main features of the intrusion is the massive gabbro sill. This intrusion of hard gabbro volcanic rock cuts across the pipe at a depth of 500m below the surface. Above the barren gabbro sill, the kimberlite pipe host rock is composed of felsite, which is a hard competent rock that allowed the sides of the pit to be cut at an almost vertical angle. Below the sill, jointed norite forms the host rock. The surface area of the open pit mine is 32ha. The open cast operation has dimensions of a length of 1,000m and breadth is 400m. At 500m level below the ground, it narrows down to an area of 21ha. The depth of the mine is 190m from the surface. The surface mining operation was able to be advanced until the large gabbro sill was intersected. In recent years the sides of the open pit have started to suffer periodic failure of the sidewalls. This has resulted in the potential of some of the remaining surface infrastructure and the residential area of Cullinan town being affected by subsidence. Monitoring is currently being undertaken, which entails both camera monitoring and the use of passive monitoring measures.

The intersection of the gabbro sill stopped further advance of the open pit and necessitated the sinking of vertical shafts to access the orebody beneath it. Three vertical shafts have been sunk to date, along with ventilation shafts.

Mining Method

The Cullinan mine employs a standard block cave mining method to develop underground resources. This method is suited to a massive orebody, and has the advantage of lower unit costs, since it is not necessary to drill and blast every ton. This is done by establishing a drilling level built-up through which the ore body is cut by drilling and blasting. Once a large area is undercut, caving is initiated. Mining works are also performed at the undercut level. The mining work includes long-hole drilling, charging, blasting, and tramming of required ore. The operational production level is located 15m below the undercut level and tunnels are bored into the orebody at the level that is in production. Draw points are developed on these tunnels and raise-bored. A draw bell is built-up to receive the caved ore, from where it flows into the draw point. Rocks which are still too big to be loaded by the six-ton capacity production machines must be reduced in size. This part of the process is called secondary blasting. The big rocks are drilled and blasted until they are reduced to a manageable size. Load haul dump trucks load the ore and carry it out of the orebody. Currently up to 43% of kimberlite ore is extracted to carry out the mining operation.

The extension of life for the Cullinan diamond mine is referred to as the C-Cut expansion plan. The main elements include the deepening of Shafts 1 and 3, along with the related infrastructure to the shaft and planned level development to the Cullinan ore body. A new block cave will be opened in the C-Cut phase 1 of the Cullinan Kimberlite pipe. The C-Cut phase 1 area is located 200m below the existing operations.

“Cullinan is known the world over as a source of some of the largest, high-quality gem diamonds, including Type II stones, as well as being the world’s most important source of blue diamonds.’”

The 350m-deep existing shaft 1 will be deepened to a depth of 920m below surface. It will hoist the ore and waste from the new C-Cut phase 1 block after the process of deepening the shaft project has been completed. The 60m-deep existing shaft 3 will be deepened to a depth of 904m below surface. The upgraded Shaft 3 will transport workers and material to and from the new C-Cut phase block 1.

Silos. Image credit: © Dr Nicolaas Steenkamp

Silos. Image credit: © Dr Nicolaas Steenkamp

After being tipped by the skip on surface, the ore is either fed onto the surface stockpile, or directly into the plant. The stockpile is indicated to have a capacity of 60 000t. The ore is fed into the plant at an average rate of 550t/h. The ore is washed and screened at the entrance to the plant. The oversized rocks are crushed and, together with the screened product, pass through an X-ray sorter where any large diamonds are extracted. Interparticle crushers and a re-crush section reduce the ore feed to less than six millimetres in size. The ore is then passed through a dense media separation process at various stages. The concentrate is sent to the recovery section where grease belts and X-ray machines are used to recover liberated diamonds before final sorting.

Per Petra, Cullinan contains a world-class gross resource of 149.82 Mcts as at 30 June 2021, which suggests its mine life could be significantly longer than the current mine plan to 2030.

Sources:

Sharyn Macnamara Sources:

  1. Petra Diamonds Limited press release, 22 February 2022: Interim results for the Six Months ended 31 December 2021, Investor update on strategy and operational guidance 
  2. www.petradiamonds.com
  3. Cullinan Mine Tours

Dr Nicolaas Steenkamp Sources:

  1. Cullinan Diamond Mine, South Africa. [https://m.miningweekly.com/article/cullinan-diamond-mine-2003-11-14]
  2. Premier Mine, South Africa. [https://m.miningweekly.com/article/premier-mine-x2013-gauteng-1999-09-10]