Provenance of alluvial diamonds in Southern Africa : a morphological and mineral chemistry study of diamonds and related heavy minerals from the Vaalorange system and the West Coast

[摘要] ENGLISH ABSTRACT: The discovery of lucrative diamond deposits along the west coast of Southern Africaabout 1200 kilometres from the Kimberley region during the period 1908 to 1927,gave rise to a number of different theories with respect to their possible provenance.These included the transportation of diamonds from unknown sources in southernNamibia by south-flowing rivers, hidden on- and off-shore kimberlites along the coast,and transportation by west-bound rivers from the hinterland. Subsequent researchhas shown that the latter is the only plausible theory.The discovery of marine and coastal diamond deposits as far south as the OlifantsRiver estuary showed that the Vaal-Orange drainage in its current form could nothave been the only conduit for diamonds to the coast, and the drainage evolutionof southern Africa was interpreted as comprising essentially the following two mainpalaeo-fluvial systems active in the formation of the world's only known diamondmega-placer deposit: The Karoo River with its headwaters similar to those of the modern Orange andVaal Rivers and entering the Atlantic Ocean via the present-day Olifants River; The Kalahari River that drained southern Botswana and followed the route of themodern-day Molopo River, entering the Atlantic Ocean in the vicinity of thepresent Orange River mouth.An important shortcoming of the above model is that it could not account for thefact that diamond distribution along the west coast shows a marked increase ingrade and average stone size at the estuaries of all the major rivers draining from theescarpment to the Atlantic between the Olifants and the Orange Rivers. Thepresence of fluvial diamond deposits along the courses of the Buffels, Swartlintjies,Spoeg, Horees and Groen Rivers confirms that the increased grade and diamondsize at their estuaries is not a function of large bays and rougher bottom topographyassociated with the rivers, although these could have contributed to thisphenomenon. This proves that the catchments of the rivers between the Olifants and Orange Rivers also had access to diamondiferous debris, although they werenot in contact with these two major drainages.A number of researchers proposed that diamonds liberated from pre-Karookimberlites were moved from their primary hosts to the south-western parts of thesubcontinent by Dwyka glacials.From the above it is clear that nearly a century after the discovery of diamondsalong the west coast of southern Africa consensus regarding their origin had notbeen reached. The aim of this study was therefore to establish a model explainingthe most likely sources and distribution history of the more important alluvial diamonddeposits in southern Africa.The methodology comprised a study of 1878 diamonds collected from 25 alluvialand two kimberlitic sources for comparison with known similar data from 12kimberlitic populations in southern Africa. The diamond study was supplemented bya study of sedimentary clasts from bulk gravel samples taken along the Middle andLower Orange River as well as Scanning Electron-microscope (SEM) Analyses ofgarnet grains and zircon geochronology.The evidence from the study does not support the postulated existence of a formerKaroo River. The surface features of diamonds, notably brown spots indicating – inthe context of southern Africa - liberation from pre-Karoo kimberlites, as well as theresults of Fourier Transform Infrared analyses revealed that the populations atKwaggaskop along the Sout River, previously considered an erosion remnant of theLower Karoo River and those occurring south of Brandvlei and Van Wyksvlei in thevalley of the Sak River, previously considered to have been reworked from theMiddle Karoo River, differ profoundly from each other. In addition, the surfacefeature studies and Fourier Transform Infrared Analyses clearly show major distinctionsbetween the diamond populations from the Sout River-Olifants River estuary andthose from the Kimberley kimberlite province which was said to have supplied diamonds in large quantities to the Olifants River estuary via the postulated KarooRiver. Furthermore the idea of a palaeo-Gamoep River playing a significant role inthe transportation of diamonds to the west coast is favoured by the presence ofbrown-spotted diamonds and diamonds with Platelet Preservation Indices revealingsevere platelet destruction that could be traced through Bosluispan in the Koa Rivervalley, the Buffels River valley, the Buffels River estuary and to the shallow marineenvironment north of the Buffels River.Zircon geochronology confirmed the role of the Orange River in the denudation ofthe sub-continent.With respect to the drainage evolution and diamond distribution in southern Africathe results of this study indicate a complex diamond dispersal model that differs insome respects from prevailing theories. It shows that diamonds liberated from pre-Karoo kimberlites in the north-eastern part of the sub-continent were initially moved ina south-westerly direction by pre-Karoo drainages, then by Dwyka glaciers and icesheets. Ultimately, after liberation from exhumed glacial and fluvial deposits andtogether with diamonds subsequently liberated from Jurassic and Cretaceouskimberlites, Cretaceous and younger drainages provided the transport toward theAtlantic Ocean where the diamonds were concentrated along shorelines and inbedrock trap sites. Significant quantities did not reach the coast, but were lockedup in fluvial sediments in erosion remnants like terraces, karstic depressions and othersegments of palaeo-channels along the way.The presence of diamonds with FTIR characteristics reminiscent of those from Orapaand Jwaneng in the Orange River deposits as well as in a raised marine terrace insouthern Namaqualand and in marine deposits north of Concession 12A, alsonegates the possible existence of a palaeo-Kalahari River, unless it was a very youngsystem that did not interrupt the south-bound dispersal of Botswana diamonds duringthe Late Oligocene-Early Miocene.The study also included microscopic examination of a parcel of diamonds from theenigmatic Skeleton Coast deposits, north-western Namibia. These results confirmedthe conclusion based on geological and geomorphic grounds that these diamondscannot be linked to the Oranjemund deposits, while their surface features showedthat pre-Karoo sources comprise the most likely provenance for the Skeleton Coastdiamonds.Thus the combination of FTIR analyses and surface feature studies of diamonds,zircon geochronology and SEM analyses of garnets allowed the formulation of arevised model for the distribution of alluvial diamonds and the drainage history of thesub-continent since the Middle Cretaceous, while the study of sedimentary clastsconfirmed the repeated occurrence of high energy fluvial conditions – especiallyevident in the palaeo-Orange River sediments – that contributed to the highpercentage of gem stones in the surviving alluvial diamond populations due to thedestruction of poor quality diamonds.

[发布机构] Stellenbosch University