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Williamson, Mwadui |
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Tanzania |
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Diamonds
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Williamson diamond mine exploits the Mwadui kimberlite which is located in central Tanzania, East Africa.
The Cretaceous Mwadui kimberlite, which was discovered in 1940 by Dr J T Williamson, is a large primary deposit covering a surface area of some 142 ha. This kimberlite has been mined continuously by open pit since its discovery. During 2003, some 4.543 Mt of ore were treated for 0.166 million carats of diamonds at a recovered grade of 3.7 carats per hundred tonnes diamonds. The average value of stones extracted during 2000 was USD 145 per carat.
The Mwadui kimberlite comprises the following five rock types, namely the:
i). Shale Basin - which reflects lacustrine sedimentation towards the end of the in-fill cycle, and forms the upper part of the central depositional crater. Diamond grades are extremely low.
ii). Bouma facies - representing the lower sections of the central depositional crater, distinguished from the Shale Basin by the presence of bouma cycles. Diamond grades are usually low.
iii). Granite Breccias - the lower contact of the central depositional basin is characterised by a substantial zone of brecciated granite, formed after the final eruptive episode, mainly as a result of collapse of fractured and unstable crater walls of basement granite. The breccia units are characterised by a wide variety of intermixed volcaniclastics, resulting in material containing nearly barren, all the way to economic diamond grades.
iv). Reworked Volcaniclastic Kimberlite (RVK) - formed in response to the reworking of primary tuff ring deposits as a result of grain flow and debris/avalanche deposits into the open crater. Dilution by wall rocks is relatively low in comparison with some of the other units, and as a consequence diamond grades are high.
v). Primary Pyroclastic Kimberlite (PK) - which constitutes the material found below the RVK that can only be observed in the deeper sampling tunnels and drill holes.
The kimberlite exploited existing joint and fracture systems during emplacement. The northern and southeastern sections of the crater are terminated by hypabyssal facies dykes. The northern dyke has been classified as a macrocrystic monticellite kimberlite with variable phlogopite and microlitic diopside, while the southern dyke is poorly exposed and highly weathered.
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.
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Selected References:
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Nixon P H and Condliffe E 1989 - Tanzania kimberlites: A preliminary heavy mineral study: in Kimberlites and related rocks, Volume 1, Their origin, Occurrence and emplacement Proc. 4th International Kimberlite Conference, Perth, WA, 1986 Geol. Soc. Aust, Spec. Publ. 14, Blackwell, Melbourne pp 407-418
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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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