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Namdeb, Sperrgebiet, SW Namibian On-shore and Marine Placer Diamonds |
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Namibia |
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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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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The placer diamond deposits of the Sperrgebiet in southwestern Namibia are mined within five mining titles that extend northwards for approximately 320 km from the Orange River (the South Africa-Namibia border) to the north of Luderitz and from 5 km offshore into the Atlantic ocean to 20 to 35 km inland into the Namib desert. To the south the area extends further inland along the Namibian (northern) bank of the Orange River.
Well over 100 million carats of +90% gem quality diamonds have been extracted from this regional composite placer deposit since the initial discovery of diamonds in the area in 1908. Most of these diamonds have been mined from a variety of placer types ranging in age from before 40 Ma up to contemporary deposits.
The very high quality of these diamonds is a result of the unique sequence of natural upgrading and concentrating processes active in the area. These processes are the consequence of the sediment transfer from sources within inland southern Africa to their deposition in "sinks" along the Namib Desert coast and in the Atlantic Ocean. Within this train and time span, diamonds have been concentrated in a range of different placer types spanning both terrestrial and marine settings.
Three major, long-standing transport pathways - or sediment conveyors - have been responsible in moving diamondiferous sediment from source localities to sink areas, as follows:
i). Fluvial conveyor - principally the Orange-Vaal River drainage system;
ii). Marine conveyor - comprising a vigorous wave regime along the passive margin Atlantic coast, with a strong northward longshore drift driven by a prevailing southerly wind pattern;
iii). Desert conveyor - resulting from the combination of the prevailing southerly wind pattern and the arid climate of the Namib Desert, facilitating onshore movement of marine-derived sediment back into a land-based sink.
Fluvial Deposits
The primary sources of diamonds in the Namibian placers are interpreted to have been the kimberlite pipes of the KaapVaal craton in the southern African drainage catchments that have fed the Oraange-Vaal drainage system. In addition, diamonds have been derived from secondary accumulation in sediments ranging in age from Archaean (> 2.7 Ga) through to the Cretaceous (70 Ma). For some 100 Ma, the Orange-Vaal drainage system has been the main fluvial conveyor of eroded sediment to the Atlantic coast. From the early Tertiary (about 65 Ma), continental uplift and sea level variations promoted the incision of the Orange valley and the formation of diamondiferous gravel terraces along the lowermost reach of the Orange river. These fluvial placers formed between 17 and 19 Ma, and again at 3 to 5 Ma. The older terrace suite has proved
Onshore Marine Deposits
A wave-dominated delta is developed at the mouth of the Orange River on the Atlantic coast. Variations in sea level during the last 40 million years has produced a number of palaeo-shorelines both onshore (as "raised beaches") and offshore ("submerged beaches") in the Sperrgebiet. In more recent geological time, the wide Orange River mouth facilitated the formation of barrier beach placers, while further north, linear beach placers were developed under the influence of a northbound longshore drift driven by the prevailing southerly wind pattern.
This wind pattern, which has prevailed for the last 40 million years, generates both a powerful northward-directed longshore current but is also, in conjunction with the cold-water upwelling Benguela current, perpetuates the extreme-arid conditions along the south-western African coast. Consequently, sediment transported by the coastal currents, is moved back onto the land and into the desert conveyor, particularly via deposition in south-facing re-entrant bays where ribbon-like pocket beach deposits of diamondiferous gravels marking palaeo-sea level positions are found. The southerly winds drive sediment through aeolian transport corridors and deflation basins northwards into the main Namib sandy desert. Within deflation basins, a combination of ephemeral run-off, salt weathering and aeolian action upgraded placers to a high degree. Further north, aeolian placers were formed within the dune and sand feeder system leading into the main Namib sandy desert.
Offshore Marine Deposits
Drowned placer deposits lie in waters ranging from the surf zone near the coast to depths as great as 150 metres. Evidence to date suggests that the marine environments host the same complex range of beach-barrier, beach, deflation and aeolian systems seen onshore.
The majority of the mining activity in the Sperrgebiet region is conducted by Namdeb, a joint venture between the Namibian Government and de Beers.
Main source: Namdeb web site.
Production & Reserves
During 2001, Namdeb produced 1.385 million carats of diamonds, of which 39% came from offshore marine mining.
The main onshore operation of Mining Area 1 treated 17.8 Mt of materials yielding 0.618 million carats, the Elizabeth Bay Mine treated 1.97 Mt of material for 0.094 million carats of diamonds, beach and marine contractors, many of whom were Namibian empowerment groups recovered 0.099 million carats of diamonds; while the Orange River mines at Daberas treated 2.7 Mt of materials for 0.031 million carats.
The marine mining operations in the Atlantic 1 license area, conducted by De Beers Marine Namibia (Debmarine), recovered 0.537million carats.
At the end of 2000, Namdeb reported remaining resources and reserves (land + marine) as follows:
Probable reserves of 59.4 Mt @ 1.5 carats per hundred metric tons (cpht),
Indicated total resources of 73.6 Mt @ 2.3 cpht, and
Inferred total resources of 301.7 Mt @ 1.5 cpht.
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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Jacob J, Ward J D, Bluck B J, Scholz R A and Frimmel H E, 2006 - Some observations on diamondiferous bedrock gully trapsites on Late Cainozoic, marine-cut platforms of the Sperrgebiet, Namibia: in Ore Geology Reviews v28 pp 493-506
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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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