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Orange River Alluvial Diamonds |
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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 Orange River is the principal drainage of southern Africa into the Atlantic Ocean and is the major route for the transport of diamonds to the major deposits on the Namibian coastline north of the Orange mouth and Oranjemund.
Economic accumulations of diamonds were not found along the Orange River until 1966. Subsequently alluvial diamonds have been exploited from terrace gravels on both the South African and Namibian banks of the river over its lower 100 km.
There are two suites of gravels distinguished in the lower Orange River terraces, namely:
1). Older, higher suite, which is generally 50 to 70 m above the present river bed and is of early mid-Miocene in age (19 to 17 Ma), referred to as the Proto-Orange river terraces,
2). Younger, lower suite, normally 30 to 40 m above the current river bed, which may be of Plio-Pleistocene (5 to 2 Ma) age, referred to as the Meso-Orange terraces.
Although these gravels are very low in grade, the diamonds they do yield are large (1 to 2 carat stones) and of >95% gem quality, with the best deposits in the Proto-Orange gravels. Grades are highest near the bedrock contact and in localised trap sites where levels of 10 to 50 carats per hundred tonnes are encountered as stones of 1.5 to 2.8 carats.
Remnants of the Proto-Orange gravels are found over the lower 320 km of the river and thicken from <5 m upstream, to >80 m nearer the mouth. They fine upwards from coarser cobble/small boulder size and locally boulder framework gravels at the base to pebble sized gravels and coarse sands to fine silts and clays reflecting decreasing energy or agradation.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1999.
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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Jacob R J, Bluck B J, Ward J D 1999 - Tertiary-age diamondiferous fluvial deposits of the Lower Orange River Valley, southwestern Africa: in Econ. Geol. v94 pp 749-758
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Moore J M, Moore A E 2004 - The roles of primary kimberlitic and secondary Dwyka glacial sources in the development of alluvial and marine diamond deposits in Southern Africa: in J. of African Earth Sciences v38 pp 115-134
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Spaggiari R I, Bluck B J and Ward J D 2006 - Characteristics of diamondiferous Plio-Pleistocene littoral deposits within the palaeo-Orange River mouth, Namibia : in Ore Geology Reviews v28 pp 475-492
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Van Wyk J P, Pienaar L F 1986 - Diamondiferous gravels of the lower Orange River, Namaqualand: in Anhaeusser C R, Maske S, (Eds.), 1986 Mineral Deposits of South Africa Geol. Soc. South Africa, Johannesburg v2 pp 2309-2321
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Wilson M C G, Henry G and Marshall T R, 2006 - A review of the alluvial diamond industry and the gravels of the North West Province, South Africa : in S. Afr. J. Geol. v109 pp 301-314
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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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