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Kimberley, Dutoitspan, Bultfontein, Wesselton, De Beers |
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Northern Cape, South Africa |
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Diamonds
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Super Porphyry Cu and Au
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The Kimberley group of kimberlite pipes are located adjacent to the town of Kimberley in the Northern Cape Province of South Africa.
(#Location: Kimberley - 28° 44' 19"S, 24° 45' 30"E; Dutoitspan - 28° 45' 37"S, 24° 47' 59"E;
Bultfontein - 28° 46' 5"S, 24° 47' 31"E; Wesselton - 28° 46' 2"S, 24° 49' 45"E;
De Beers - 28° 44' 20"S, 24° 46' 30"E).
These deposit are located on the southern extremity of a broad corridor that extends for ~110 km to the NNW of Kimberley and includes the Kareevlei, Klipdam, Holpan, Leicester and Elandslaagte pipes and mines, as well as the Hart's River and Sedinbeng alluvial mines, ~75 km NNW of Kimberley.
The original Kimberley 'Big Hole' pipe was discovered in 1871. From mid-July 1871 to 1914 up to 50 000 miners excavated the hole with picks and shovels, yielding 2.72 tonnes (13.6 million carats) of diamonds. The Big Hole has a surface area of 17 hectares and is 463 m wide. It was excavated to a depth of 240 m.
The De Beers pipe was also discovered in 1871, and to 1985 had produced 25 million carats (5 tonnes) of diamonds from both open pit and underground. The Kimberely and De Beers pipes are two of 5 main pipes in the cluster, namely: Kimberley, Dutoitspan, Bultfontein, Wesselton and De Beers. Other diamondiferous kimberlite intrusions in the Kimberley area include the Kamfersdam, Otto's Kopje and Belgravia pipes. Kamfersdam, situated on the farm Roodepan 70, seven kilometres NNW of Kimberley, was previously mined by De Beers, but closed in 1907. Otto's Kopje, on the Kimberley Townlands, was discovered in 1880, mined from 1891 to 1905 and again from 1911 to 1913. Although the grades from this mine were low, the diamond quality was high, and one exceptional diamond of 336 carats was recovered in 1896.
The pipes of the cluster are dated at between 84 and 87 Ma. They intrude 90 to 120 metres of the Mesozoic Karoo Dwyka Group sediments, overlying porphyritic lavas of the Archaean Ventersdorp Supergroup. It has been estimated that approximately 1400 metres of erosion has taken place since the emplacement of the pipes. At the present day surface, all the pipes were exposed in the diatreme zone, with the gradation into the root zone taking place at between 400 and 800 metres below the surface.
A number of discrete intrusions of kimberlite are recognised, including hypabyssal facies kimberlite and kimberlite breccias with considerable textural and mineralogic variation. These intrusions contain varying quantities of crustal and upper mantle derived xenolithic material, as well as inclusions of earlier generation kimberlites. The root zones are characterised by broad widths of contact breccia up to 50 m across which in part contain no kimberlitic matter at all, while other sections have abundant interstitial kimberlite in the breccia. There is a difference in grade distribution within the various pipes. Some show a decrease in grade with depth, while others do not vary with depth.
There are six discrete intrusive phases (DB1 to 6 inclusive) represened in the main De Beers pipe as well as a series of kimberlitic dykes including precursor, contemporaneous and internal dykes (only found within the pipe). Near surface to the 245 m level, only two intrusive phases are recognised, namely DB3 and DB5, while by 500 m DB2 seperates the two and predominates at the 785 m level. DB6 becomes more significant with depth. The other 2 phases are less extensive.
DB2 is essentially a dark, grey-black, hard, locally brittle, macrocrystic kimberlite with a dense aphanitic groundmass, which in hand specimen is conspicuous for the numerous anhedral olivine macrocrysts (10 to 20%) and scattered xenoliths of country rock as well as macrocrysts of phlogopite, garnet, ilmenite and chrome diopside.
DB3 is a kimberlitic breccia, locally resembling the surrounding DB2, except that it is a paler grey or grey-green, moderately hard macrocrystic rock with a greater abundance of country rock fragments (around 20% by volume). The xenolithic content varies from DB2 in that it has more local country rock, particularly Karoo shales.
DB5 is also a kimberlitic breccia, which is greenish-grey to greenish-black in colour but otherwise resembles DB3. However, it contains far more Karoo shale, which in underground exposures may exceed 1 m in width and be 15 to 25% of the intrusion. It also contains more phlogopite macrocrysts than the other intrusives and has common garnet.
DB6 is a hard, dark, grey-black macrocrystic rock containing scattered xenoliths of mainly basement gneiss, and is similar in appearance to DB2.
The size at surface and production statistics of the 5 main kimberlite pipes is as follows (to 1993; after De Beers Kimberlite Handbook):
Kimberley - Big Hole - 3.7 ha and 32.7 million carats from 1871 to 1914
Dutoitspan - 10.6 ha and 21.3 million carats
Bultfontein - 9.7 ha and 36.2 million carats
Wesselton - 8.7 ha and 33.6 million carats
De Beers - 5.1 ha and 36.4 million carats carats from 1871 to 1990
Mine Dumps - 11.4 million carats
TOTAL for district - 37.8 ha and 171.6 million carats
Production in 2000 was 3.51 Mt @ 16.2 cpht for 0.5686 million carats of diamonds at an average value of USD 76 per carat.
Total reserves + resources in 2000 totalled 284 Mt @ 11.1 cpht containing 31.5 million carats of diamonds.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2004.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Kimberley - Big Hole Dutoitspan Bultfontein Wesselton De Beers
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Selected References:
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Clement C R, Harris J W, Robinson D N and Hawthorne J B 1986 - The De Beers kimberlite pipe - A historic South African diamond mine: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 2193-2214
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Phillips D, Harris J W and Viljoen K S 2004 - Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa: in Lithos v77 pp 155-179
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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