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Letlhakane

Botswana

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The Letlhakane diamond mine is located about 190 km west of Francistown, and 50 km from the Orapa mine in north-central Botswana, in southern Africa.

The 90 Ma Letlhakane D/K1 kimberlite was intruded into units of the Mesozoic Karoo sequence during the Cretaceous period. Wallrocks of this sequence comprise 130 m of Stormberg basalt flows, overlying 120 m of Ntane Sandstone, which in turn overlies grey siltstones, shales and carbonaceous shales. Four to ten metres of Kalahari sand and calcrete masked the Karoo basalt and originally covered the kimberlite. Large xenoliths of basalt are common and in some places make up more than 90 per cent of the volume of the kimberlite.

Three different distinct episodes of intrusion have been recognised in the D/K1 kimberlite.

The first intrusion was responsible for a kimberlitic basalt breccia, which is only present in the south of the pipe. It is probable that some of this breccia is sedimentary, having fallen or been washed back into the original crater. More than 90 per cent of the kimberlitic basalt breccia consists of angular blocks of basalt set in a matrix of kimberlite material and finely ground basalt.

The second intrusion sharply truncated the kimberlitic basalt, and much of the material from the first intrusion would have been ejected from the pipe or incorporated into the second intrusion. The ore formed by this intrusion is termed LM2 (Letlhakane Mine 2). It has a highly variable basalt content, ranging from less than 5 per cent to greater than 80 per cent. It is generally shades of blue/green/grey and is usually a fresh, hard competent rock.

The third intrusion cut through the second, although the contact is not sharp or easy to recognise. This intrusion comprises the northern half of the pipe and its ore is known as LM1(Letlhakane Mine 1). At elevations higher than 950 m above mean sea level (a.s.l.), the ore is a grey colour with a consistently low basalt content (less than 10 percent) and is intensely weathered to a very friable rock with sub-horizontal calcite and talc/chlorite veining. Below 950 m a.s.l. the ore is of blue/green/grey colour, and is fresh, hard and competent.

The Letlhakane mine commenced operation in 1977 and in 2002 produced 1.03 million carats of diamonds from 3.6 Mt of ore at a recovered grade of 25.7 cpht. Some 15.23 Mt of waste was also removed. The average value of stones recovered in 2000 was 191 USD per carat. In 2000 the total reserves + resources were 62.5 Mt @ 26 cpht of diamonds (=16.25 millions carats). The Letlhakane diamonds are the most valuable per carat in Botswana.

The most recent source geological information used to prepare this summary 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.


  References & Additional Information
   Selected References:
Deines P and Harris J W  2004 - New insights into the occurrence of 13C-depleted carbon in the mantle from two closely associated kimberlites: Letlhakane and Orapa, Botswana: in    Lithos   v77 pp 125-142


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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