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Orapa

Botswana

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The Orapa diamond mine is located about 240 km west of Francistown in north-central Botswana, in southern Africa.

The Orapa kimberlite, along with two other significantly diamondiferous kimberlites in the same area, was discovered in 1967 by De Beers geologists and is the largest in a cluster of over 50 kimberlites. The mine was opened in 1971. The commissioning of the No.2 Plant in December 1999 resulted in a doubling in annual production to around 12 million carats in 2000. Orapa produced just over 16.26 million carats of diamonds in 2003. In 2002 production totalled 14.33 million carats of diamonds from 16.4 Mt of ore treated at a recovered grade of 86.85 cpht (carats per hundred tonnes). The total waste removed in the same year was 8.12 Mt. Reserves + resources in 2000 totalled 653 Mt @ 49 cpht diamonds (=320 million carats).

The 93 Ma Orapa kimberlite intruded granite-gneiss and tonalite of the Palaeoproterozoic Magondi mobile belt, just east of the Archaean Zimbabwe craton, and on the northwestern margin of the subsequent Kaapvaal craton as well as a variety of rocks from the Mesozoic Karoo Supergroup. While it might be regarded as unusual for a highly diamondiferous kimberlite to be emplaced into non-Archean crust, isotopic evidence from peridotite xenoliths at the nearby Letlhakane kimberlite suggests that the underlying mantle lithosphere is Archaean in age and probably a western extension of the Zimbabwe block.

The AK1 kimberlite covers an area of118 ha at surface, although it comprises two individual intrusions that coalesce near the surface. Rocks from all three facies (crater, diatreme and hypabyssal) have been identified, with the former being particularly well preserved. These intrusions have been labelled the "northern" and "southern" lobes respectively. Clear differences exist in the geology of the two pipes. The northern lobe predates the south lobe.

Current mining is restricted to the crater facies units which exhibit a wide variation in type and texture, from primary pyroclastic deposits to epiclastic examples where reworking of the material is obvious. The final post-eruptive phase in the life of the crater was marked by deposition of reworked and resedimented epiclastics which in turn, can be subdivided into the crater lake deposits (lacustrine shales and grits) and mass and grain flow deposits (massive breccias, bedded grain flow deposits, debris flow breccias, and boulder beds).

Unequivocal hypabyssal facies kimberlite has only been intersected at depth within the mine, with two mineralogical varieties being identified, namely: i). a phlogopite-monticellite variety and ii). a monticellite kimberlite.

Diatreme facies kimberlite occurs in both lobes. The "northern lobe" exhibits a gradual transition from the northern pyroclastic kimberlite into a massive, unstructured tuffisitic kimberlite breccia. In contrast, the diatreme facies in the "southern lobe", show a sharp contrast with the overlying southern volcaniclastic kimberlite of the crater facies, and the possibility of a multiple eruption history, particularly with respect to the diatreme facies units, cannot be excluded.

For detail consult the reference(s) listed below.

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:
Burgess R, Kiviets G B and Harris J W  2004 - Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites: in    Lithos   v77 pp 113-124
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
Field M, Gibson J G, Wilkes T A, Gababotse J and Khutjwe P  1997 - The geology of the Orapa A/K1 kimberlite Botswana: Further insight into emplacement of kimberlite pipes: in   Proc. 6th International Kimberlite Conference, Volume 1 - Kimberlites, related rocks and mantle xenoliths Russian Geology and Geophysics, Spec. Issue,    v38, no. 1 pp 24-39
Gautherona C, Cartignyb P, Moreiraa M, Harrisc J W and Allegre C J.  2005 - Evidence for a mantle component shown by rare gases, C and N isotopes in polycrystalline diamonds from Orapa (Botswana): in    Earth and Planetary Science Letters   v240 pp 559-572
McDonald I and Viljoen K S,  2006 - Platinum-group element geochemistry of mantle eclogites: a reconnaissance study of xenoliths from the Orapa kimberlite, Botswana: in    Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc.   v115 pp 81-93
Stachel T, Viljoen KĘS, McDade P,ĘHarris J W  2004 - Diamondiferous lithospheric roots along the western margin of the Kalahari Craton - the peridotitic inclusion suite in diamonds from Orapa and Jwaneng: in    Contrib. to Mineralogy & Petrology   v147 pp 32-47


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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