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90 Mya
KIMBERLEY – ITS KIMBERLITES, MANTLE NODULES & DIAMONDS

Background - Following the discovery of diamonds in river deposits in central South Africa in the mid-nineteenth century, it was at Kimberley where the volcanic origin of diamonds was first recognized. These volcanic rocks, named “kimberlite” (after the famous town of Kimberley in the centre of South Africa), were to become the corner stone of the economic and industrial development of southern Africa. Following the discoveries at Kimberley, further valuable kimberlite hosted diamond deposits were discovered in many other parts of South Africa, Botswana, Lesotho, Swaziland and Zimbabwe.

Kimberley Big Hole

A Century of information and Research - The Kimberley kimberlites and mines have provided over a century of study of kimberlites, starting with the exceptional work of father and son Kimberely mine managers Gardiner Alpheaus Williams and Alpheus Williams in the early years (1809, 1932) of the famous Kimberely diamond mines. The kimberlites, mantle xenoliths (non-kimberlitic material included in the host rock), diamonds and crustal xenoliths found in the famous kimberlites, have greatly furthered the understanding of the processes that occurred within the sub-continental lithosphere, and particularly the formation of diamonds.

The formation of occurrence of diamonds in kimberlites and their successful economic mining and exploitation, is a fascinating geological story relating to the long-lived and complex series of processes and transport. The process first involved the growth of diamonds in the mantle, and later their removal and transport to the earth's surface by kimberlite magmas which effectively acted as the passenger trains that stopped at stations in the deep lithospheric mantle to collect their passengers (=diamonds) and transport them to the earth’s surface. If the passengers were late or there were none waiting at a particular station, the kimberlites arrived at surface as empty (barren) pipes with no economic value.

Diamond in Kimberlite

The-classic-South-African-model-of-a-kimberlite-pipe.jpg

Kimberlite Clusters – Kimberlite intrusions are typically found in clusters of pipes, ‘blows’ (the roots of larger pipes), dykes and sills (fissures). The clusters may vary from only a few preserved intrusions (particularly in the case of very old and deeply eroded kimberlites) or up to 50 or more intrusions (e.g. the Kimberley {RSA} and Orapa {Botswana} Clusters. Typically, only a very small number of the kimberlites found in these clusters may be economic with sufficient grade (carats per 100 tonnes) and diamond quality to justify evaluation, development and mining.

A further key aspect of the diamond characteristics of kimberlite pipes, is that the quality of diamonds from any single intrusion varies considerably. Single diamonds from the same pipe may vary in quality from a few Dollars per carat, to many thousands of Dollars per carat for large and exceptional white of coloured diamonds.

Alluvial or secondary diamond deposits found for example in old river systems (e.g. Vaal and Orange Rivers) and on the west Coast of RSA and Namibia typically show more consistent quality of stones and value. This is due to the erosion of old kimberlite pipes or source rocks, milling of the poor quality fractured and boart or industrial diamonds found in the kimberlite pipes. As the diamonds are transported further and further from their source rocks, the overall quality and value of the diamonds in alluvial deposits are considerably enhanced. To the point that the average value of an alluvial diamond parcel may be orders of magnitude greater than the rough diamond parcel in the source original upstream source-rock kimberlite.

Age Characteristics – The immensely well studied and documented Kimberly kimberlites are ~90 million years old (90 Ma), whereas kimberlites found on the Ghaap Plateau north of Kimberley have intrusive ages of approximately 118Ma. The oldest kimberlites found in the greater Kimberley region are the 1 700Ma Kuruman kimberlites in the Northern Cape. Kimberlite pipes found at Cullinan and adjacent areas north of Pretoria (Gauteng Province) are approximately 1 200Ma, and pipes found in the far north of the Limpopo Province, west of Musina (e.g. Venetia, the Oaks) and Beit Bridge and Colossus (Zimbabwe) are approximately 600Ma in age.

Only about 1% of the kimberlites that have been discovered have been successfully exploited. They include some of the world's richest mines, such as Jwaneng in Botswana, down to mines that are both small and marginal, such as the Frank Smith Mine in South Africa. They also include large pipes such as Orapa (approximately 110ha) in Botswana and small kimberlite dykes (or fissures) such as those mined at Bellsbank, Helam (Swartruggens) and Star Mine (eastern Free State). Marsfontein, an immensely rich but small (0,5ha) pipe that was discovered in the mountains east of Makopane in Limpopo Province (RSA).

Jwaneng Botswana

Letseng Lesotho

Cratons and Diamonds - All these diamond-bearing kimberlite mines are located and found on the Archean Kalahari Craton and Kaapvaal Cratons and adhere to the famous predictive model of Prof. Tom Clifford (the ‘Clifford Rule’) from the University of the Witwatersrand. The evolution of ancient cratons and their sub-continental lithosphere and associated peripheral mountain building subduction events played an important role in providing the right environment for diamond growth and preservations in the roots of these Ancient cratons.

Extensive studies of cratons, kimberlites, mantle nodules and diamonds at leading research institutions and Universities in South Africa across the world ever since diamonds were discovered in the Kimberley mines in 1869, has shown that the distribution of diamonds in the mantle is extremely variable and heterogenous.

The Passenger Trains - in which diamonds may be found or mined from, are the passenger-trains or deep plumbing system that may (or may not) transport diamonds from the lithospheric mantle below the earth’s crust in cratonic (continental) settings.

Dating of individual diamonds via radiogenic dating methods applied to diamond inclusions has shown that diamond formation or growth events have occurred several times over geological time in the evolution of the earth since the Archean (>2 5Ga). Diamonds form from graphite or carbon contained in subducted sediments at pressures of +50kilobars (ie. depths greater than 150km below surface) and temperatures of about 1 200 degrees centigrade in a reducing environment. Some diamonds transported to the earth’s surface may show resorption features where the original diamond layers have been sculpted and eaten-away by oxidation processes during transport to the earth’s surface.

Emplacement Models of Kimberlites The geology and emplacement models of kimberlites vary somewhat in respect of their geology, and the great range of mantle xenolith, xenocryst and diamonds they carry from depth to the earth’s surface from the underlying deep lithospheric mantle.

Kimberlites also vary greatly in respect of the diamond tenor (grade), and range from mostly being non or poorly diamondiferous, to rare localities with exceptional diamond grades such as Jwaneng in southern Botswana which is the world’s richest diamond mine. Grades are expressed in carats (one carats is 0,2 grams) per tonne or 100 tonnes. Kimberlite mines vary greatly in size, grade and diamond-value, as well as in the proportions and types of mantle mineral suites that they contain.

Mantle Nodules - Aside from Kimberley being the famous discovery locality at which diamond bearing kimberlite intrusions or pipes were first recognised, the voluminous old Kimberley mine tailings and waste dumps going back to 1869, have provided a unique treasure trove of deep-seated mantle nodules that provide a wonderful look into the structure and composition of the deep earth. These mantle nodules, xenoliths and xenocrysts include periodotites, eclogites, dunites, pyroxenites, megacrysts, diamonds (and their mineral inclusions) and just about every other possible type of deep-seated lower-crust and mantle nodule type that have been documented.

The wonderful preservation, availability, and range of xenoliths was due to the early years of extraction and processing of diamond-bearing ore from the Kimberley mines not being subject to crushing and milling of the kimberlite material and its exceptional sample-collection of xenoliths. There were no large and efficient big jaw-crushers, mills and scrubbers in the early years of mining, and the kimberlite ore was removed from the mines by horse and cart, then stream-engines and carts, and processed by a process of ‘flooring’ being used.

As suitable crushers were simply not available, the chosen method to recover diamonds involved the cutting down of a million and a half trees around Kimberley. Kimberlite removed from the mines was then deposited on these vast expanses of cleared land, which became known as ‘depositing floors’. Here the kimberlite remained, for up to two years. Periodically the kimberlite or “Blueground,” was turned over by steam ploughing engines, to ensure a more even and regular decomposition. De Beers Consolidated Mines Limited had purchased several steam ploughing engines and harrows from John Fowler & Sons, in England, specifically for this purpose.

Diamond Studies and Dating of Individual Diamonds – Research and studies of diamonds, their mineral-inclusions and age of formation has provided a wealth of data, information and insight into the lithosphere and mantle, which underlie the thin layer of earths-crust that we are most familiar with.

Many diamonds are of Archaean age (>2 500 Ga) and many of these are peridotitic in character, but suites of younger Proterozoic diamonds have also been recognized in various southern African mines. These younger ages correspond with ages of major tectono-thermal Mobile-Belt or Mountain forming events that are recognized in crustal rocks of the sub-continent. Most of these diamonds had eclogitic, websteritic or lherzolitic substrate or protoliths. As noted above, in southern Africa, kimberlite eruptions (emplacement) occurred as discrete events several times during the geological record, including the Early and Middle Proterozoic, Cambrian, Permian, Jurassic and the Cretaceous periods.

Summary - Following the discovery of diamonds in river deposits in central South Africa in the mid nineteenth century, it was at Kimberley where the volcanic origin of diamonds was first recognized. These volcanic rocks, named “kimberlite”, became the corner stone of the economic and industrial development of southern Africa. Following the discoveries at Kimberley in the 1860’s, even more valuable deposits were discovered in South Africa and Botswana in particular, and in Lesotho, Swaziland and Zimbabwe.

90 Mya KIMBERLEY – ITS KIMBERLITES, MANTLE NODULES & DIAMONDS

90 Mya
KIMBERLEY – ITS KIMBERLITES, MANTLE NODULES & DIAMONDS