Overberg Geoscientists Group
1200 Mya
Cullinan Kimberlite and
Diamond Mine
Discovered in 1902, the Cullinan kimberlite and diamond mine (previously known as the Premier mine), has yielded many of the Earth’s largest and most desirable and iconic diamonds, and is renowned for very rare and highly valuable Type IIb blue diamonds and high-quality Type IIa white diamonds. This kimberlite is Proterozoic (~1 160 Ma) in age, and the oldest kimberlite-hosted, world-class producing diamond mine.
Cullinan is a large world-class under-ground diamond operation by Petra Diamonds in the town of Cullinan, 40 kilometres east of Pretoria, Gauteng Province, South Africa. Established in 1902, it was renamed the Cullinan Diamond Mine in November 2003 in celebration of its centenary. The mine is a carrot-shaped volcanic pipe. Had an originally surface area of 32ha, and is rapidly approaching a depth of about 1 000 metres as the mining and infrastructure development gets deeper
The mine rose to prominence in 1905, when the Cullinan Diamond – the largest rough diamond of gem quality ever found – was discovered there. Cullinan mine has produced over 750 stones that are greater than 100 carats (20 g) and more than a quarter of all the world's diamonds that are greater than 400 carats (80 g). It is also the only significant source of blue diamonds in the world.
The Cullinan Diamond – 3 016 carats (recovered in 1905)
Today the Cullinan mine is the flagship asset of the Petra Diamonds mining operations and a world-class kimberlite, famous for being home of the largest gem diamond (3,106cts) ever recovered as well as being the world’s most important source of blue diamonds. Cullinan Mine is a world-class resource, with potential extending beyond 2050. Petra Diamonds is a large internationally recognised diamond mining company listed on the London Stock Exchange (LSE).
Cullinan mine – old plant, now replaced with a modern state-of-the art processing plant and final recovery system
Cullinan boasts a rich history of more than 120 years of mining activity. Cullinan became famous with the discovery of the World’s biggest diamond. The Cullinan Diamond was found by Frederick Wells, surface manager of the Premier Diamond Mining Company in Cullinan on 26 January 1905. It was found 5.5 metres below the surface. It was approximately 10.1cms long, 6.35cms wide, 5.9cms deep, and weighed 3,106 carats (621.2 grams). It was three times the size of the Excelsior Diamond (972 carats) found in 1893 at Jagersfontein Mine in the Free State Provins of RSA. Four of its eight surfaces were smooth, indicating that it once had been part of a much larger stone broken up by natural forces. It had a blue-white hue and contained a small pocket of air, which at certain angles produced a rainbow, or Newton's rings.
The stone was named after Sir Thomas Cullinan, the owner who opened the mine in 1902. The second-largest stone cut from the Cullinan diamond is the Cullinan II which is set in the Imperial State Crown, made during the coronation of King George VI in 1937. The 317-carat cushion-shaped diamond is the most valuable stone in the Crown which was mounted on Queen Elizabeth’s coffin as she lay in state at Westminster Hall, prior to her funeral on 19 September, 2022.
As part of the consolidation of the local and international diamond business and diamond production, De Beers which was once considered a monopoly and key custodian of the international gemstone diamond trade, acquired the Cullinan mine in 1905. Following the dismantling of their monopoly and relocation of most of their diamond management, sorting and sales structures to Botswana in the early-2000’s, on 22 November 2007, De Beers, entered into an agreement to sell the Cullinan mine for R1 billion (US$125 million) to Petra Diamonds Cullinan Consortium (PDCC), a consortium consisting of:
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Petra Diamonds – 37% stake, with an option to increase to 60%
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Al Rajhi Holdings W.L.L. – 37% stake
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A Black Economic Empowerment (BEE) foundation – 26% stake (comprising a 12% stake held by an employee share trust and a 14% stake held by Thembinkosi Mining Investments (Pty) Limited)
On 16 July 2008, Petra announced the completion of the acquisition.
Over a century of mining and study of kimberlites, including the Cullinan and Kimberley kimberlites, the diamonds mined from these pipes and the deep mantle-derived rocks they contain, has furthered the understanding of the processes that occurred within the sub-continental lithosphere and in particular the formation of diamonds. The formation of kimberlite-hosted diamond deposits is a long-lived and complex series of processes that first involved the growth of diamonds in the mantle, and later their transport to the earth's surface by kimberlite magmas. Modern high-precision dating of inclusions in diamonds (eg. via Laser ICP Mass Spectrometry) has shown that diamond growth has occurred several times over geological time.
Studies of diamonds and their inclusions from the Cullinan mine and other kimberlites has shown that many diamonds are of Archaean age. However, suites of younger Proterozoic diamonds have also been of recognized at Cullinan and various southern African mines. These younger ages correspond with ages of major tectono-thermal events that are recognized in crustal rocks of the sub-continent. In southern Africa, kimberlite eruptions occurred as discrete events several times during the geological record, including the Early and Middle Proterozoic (~1 700 and 1 200Ma), Cambrian (~650 Ma), Permian (240 Ma), Jurassic (~180 – 140 Ma) and the Cretaceous (~90 – 60Ma)
Apart from the Early Proterozoic (Kuruman, Northern Cape) kimberlites, all of the other intrusive events have produced deposits that have been mined. Notably, only about 1% of all the numerous kimberlites and related-rocks known and discovered (~ 8 500 world-wide) have been evaluated, trial-mined and exploited. They include some of the world's richest mines, such as Jwaneng in Botswana, the extremely large diatremes (including Orapa in Botswana and Mwadui in Tanzania), others that are both small and marginal (eg. Frank Smith, Lace and Jagersfontein in South Africa), Marsfontien which was both one of the smallest (0.50ha) and richest in the world (paid its capital expenditure back in two days), to small dykes or fissures such as those mined at Bellsbank north of Barkly West, Helam near Swartruggens and Star-mine near Theunissen.
These mines are all located on the Archaean Kalahari Craton, and it is apparent that the Craton its associated sub-continental lithospheric-keel played an important role in providing the right environment for diamond growth and the formation of the kimberlite magmas that transported them to the surface, a situation referred to as the famous Clifford-Rule
In southern Africa two main types of kimberlites are recognised by way of Group-I kimberlites in which monticellite is a common groundmass mineral, and Group-II kimberlites which are characterised by abundant mica. These micaceous rocks are also referred to as Olivine-lamproites or Orangeites.
The historical small Town of Cullinan offers many outdoor attractions, including mine tours, walking down Oak Avenue with its Curio Shops, interesting Restaurants, the famous McHardy Museum, and a golf course, all of which have become part of the quaintness of this old Mining Town. The original small mine employee homes have been restored and are used to still portray the true ambience of this little mining town. One of the features of the Cullinan Recreation Club, erected in 1912, are murals painted by Second World War Italian prisoners of war in 1944.
Further Reading:
Field, M., Johann Stiefenhofer, J., Robey, J. and Kurszlaukis, S. (2008). Kimberlite-hosted diamond deposits of southern Africa: A review. Ore Geology Reviews, 34, 33–75.
Macdougall, J. D. and Haggerty, S. E. (1999). Ultradeep xenoliths from South African kimberlites: Sr and Nd isotopic compositions suggest complex history. Earth & Planetary Science Letters, Vol-170, pp 73-82.
Mitchell, R. H. (1995). Kimberlites, Orangeites, and Related Rocks. Springer Link Publishers.
Smit, K. and Shirey, S. B. (2019). How old are diamonds? Are they forever. GEMS & GEMOLOGY, Spring 2019.
Stachel, T. (2011). Diamonds and cratons: does the relationship hold for the Canadian deposits? Gems and Gemology, 47(2), 112-114. http://store.gia.edu/product_p/11su.htm