African Mining | Page 29

EXCURSION •

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 .

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 .

© Sharyn Macnamara , African Mining

Passive subsidence monitoring instrument at the viewing platform on the edge of the open pit .

© Sharyn Macnamara , African Mining

The town with the mine in the distance .

© Dr Nicolaas Steenkamp

Representation of the volume of diamonds produced monthly .