Gold Distribution in Relation to Depositional Processes in the Proterozoic Carbon Leader Placer, Witwatersrand, South Africa

Nami, Mustapha

doi:10.1002/9781444303773.ch44

Cited by 5 publications

(2 citation statements)

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“…Tungsten was chosen as one of the heavy minerals in this study because it has nearly the same density as gold, which is the mineral of economic interest in many fluvial placers like the Witwatersrand deposits in South Africa. Gold grain sizes in the Witwatersrand range from 0.025 mm to 0.3 mm, with a median size of 0.07 mm (Nami 1983). Only the coarsest end of this size range overlaps the size range of our tungsten.…”

Section: Figmentioning

confidence: 82%

Flume Experiments on the Transport of Heavy Minerals in Gravel-Bed Streams

Kuhnle¹,

Southard²

1990

SEPM JSR

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AaSTRACr: Heavy-mineral transport and deposition by shallow unidirectional flows in a gravel-bed channel were studied in a water-recirculating sediment-feed flume 6 m long and 0.15 m wide. The sediment was poorly sorted gravel with mean size 3 mm and 3% by weight of finer magnetite, lead, and tungsten.The heavy minerals became concentrated into a thin layer (the heavy infralayer) composed of nearly 100% heavy minerals lying beneath a thin surficial layer of low-density sediment (the light supralayer). Heavy minerals were not transported past a given location on the bed until the heavy infralayer was fully developed there. Heavy minerals were transported on the upper surface of the heavy infralayer only when local and temporary erosion of the light supralayer, caused by large clasts, clast jams, or downstream-moving dunelike bedforms, exposed the heavy infralayer.In two runs, one with sediment feed and one without, overall degradation of the bed was forced by slow lowering of a tailgate at the downstream end of the channel. In the sediment-feed run a downstream-prograding heavy infralayer formed as in the nondegrading runs. In the run without feed, the surficial heavy-mineral concentration increased gradually at all points along the channel to the end of the run. In light of these results, we hypothesize that ultimately the surficial heavy-mineral concentration would reach a steady state and the bed slope would change until the flow can transport all of the heavy-mineral sediment supplied by local exhumation.Although no runs were designed for aggradation, a too-small initial slope caused overall bed aggradation in the early parts of two runs. Heavy minerals in the feed sediment were transported only a short distance downstream to form a zone of higher but downstream-decreasing heavy mineral concentration extending vertically through the deposit. We hypothesize that the downstream length of this zone would increase as the heavy-mineral feed rate increases relative to the bed aggradation rate.

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Section: Figmentioning

confidence: 82%

Flume Experiments on the Transport of Heavy Minerals in Gravel-Bed Streams

Kuhnle¹,

Southard²

1990

SEPM JSR

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“…The granular heavy metals added to this sieved sediment included well-sorted round lead shot, subangular tungsten and round stainless steel (Table 1). These specific metals were chosen to be representative of typical placer mineral densities (see Minter, 1978;Nami, 1983;Kuhnle, 1986). The dry mass proportion of heavy minerals in these mixtures (5-10%) is much greater than typically found in placer deposits (normally 0.01-1.0%; D,,,,,, average maximum grain size present.…”

Section: Experimental Equipment and Proceduresmentioning

confidence: 99%

An experimental study of flow, bedload transport and bed topography under conditions of erosion and deposition and comparison with theoretical models

Bennett

Bridge

1995

Sedimentology

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The nature of flow, sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size-density sediment under equilibrium and non-equilibrium (aggradational, degradational) conditions and compared with theoretical models. During each experiment, water depth, bed and water surface elevation, flow velocity, bed shear stress, bedload transport and bed state were continuously monitored. Equilibrium, uniform flow was established with a discharge of about 0.05 m3 s ~ ', a flow depth of about 0-01 m, a flow velocity of about 0-81-0.88 m s *Present address: Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK. 1 1 7 118 S. J. Bennett and J. S. Bridgeand non-equilibrium flow conditions, total and fractional bedload transport rates and changes in bed topography and texture very well.

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Nature and origin of ‘carbon’ in the Archaean Witwatersrand Basin, South Africa

Gray

Lawrence

Kenyon

et al. 1998

JGS

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The nature and origin of the carbonaceous matter ('carbon') in the gold-bearing reefs of the Archaean Witwatersrand basin of South Africa has long been a subject of controversy and debate. The importance of the carbon results from its direct association with the gold grade. The studies reported here were directed at resolving the nature and origin of carbon. Techniques used include stope-scale (macroscopic) mapping, mesoscopic (hand specimen) examination, and microscopic studies incorporating the use of optical reflectance measurement. Macroscopic mapping has demonstrated the braided habit of carbon bodies over tens of square metres, with a close parallel to sedimentology. Mesoscopic observations, however, show the vein-like carbon seams are often cross-cutting sedimentary features, with the carbon occupying an anastomosing set of bed-parallel fractures. Microscopic observations show a globular form for the carbon and inclusions of uraninite particles. In seam carbon the globules are generally modified into elongated 'spindles' oriented perpendicular to bedding. Optical properties are dominated by generally high reflectance and anisotropy with a 'swirling' liquid crystal structure, classifying seam carbon as 'carbonaceous mesophase'. It is this fine mesophase texture that has previously been wrongly attributed to plant (algal) morphology. Highly significantly, the optical anisotropy is bed perpendicular (rather than bed parallel as in normal sedimentary burial) indicating mesophase formation during fracture opening.Both reflectance and bireflectance of the seam mesophase show a range of values at all scales. No stratigraphic or depth trends were apparent, though geographic variation points to localization of the maximum temperature event(s). Given high anisotropies, maximum reflectance values in the 4-8% R o range and minima in the 1-2% R o range give arithmetic mean reflectance values of about 4.0% R o . Kinetic modelling equates these values with short term (50 000 years) temperature events of c. 400 C or burial temperatures of c. 280 C. Lower reflectance values of c. 2.0% R o obtained from the shales suggests thermal detachment of permeable and impermeable beds, a characteristic of pulsed hydrothermal flux. The included uraninite grains influence the orientation of the surrounding anisotropic mesophase demonstrating that uraninite predated the mesophase formation event. Regional palaeo-stress 'frozen' into the mesophase anisotropy indicates a NW-SE deviatoric stress.The combination of hydrocarbon, radiation and hydrothermal fluid flow marks Witwatersrand Basin pyrobitumens out as being different from conventional oil. Putative source rocks for the oil are identified in the intra-basinal Witwatersrand shale units. Lithological control in carbon seams is explained by the poro-perm and mechanical properties of the unconformity sequence of footwall, reef conglomerates and hanging wall quartzites favouring fracture along the unconformity surfaces. An overpressured oxidizing fluid created the fracture network that h...

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Gold Distribution in Relation to Depositional Processes in the Proterozoic Carbon Leader Placer, Witwatersrand, South Africa

Cited by 5 publications

References 0 publications

Flume Experiments on the Transport of Heavy Minerals in Gravel-Bed Streams

Flume Experiments on the Transport of Heavy Minerals in Gravel-Bed Streams

An experimental study of flow, bedload transport and bed topography under conditions of erosion and deposition and comparison with theoretical models

Nature and origin of ‘carbon’ in the Archaean Witwatersrand Basin, South Africa

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