The sedimentology and mineralogy of the composite reef on Cooke Section, Randfontein Estates Gold Mine, Witwatersrand, South Africa

Show simple item record Tucker, Rodney Frank 2019-11-11T11:10:00Z 2019-11-11T11:10:00Z 2019
dc.identifier.citation Tucker, R.F. (1980) The sedimentology and mineralogy of the composite reef on Cooke Section, Randfontein Estates Gold Mine, Witwatersrand, South Africa, University of the Witwatersrand, Johannesburg,
dc.description A thesis submitted in fulfilment of the requirement for the degree Master of Science to the Faculty of Science University of the Witwatersrand, Johannesburg, 1980 en_ZA
dc.description.abstract The Composite reef at Cooke Section of Randfontein Estates Gold Mine is a Precambrian gold and uranium paleoplacer, comprising a con-glomeratic quartzite unit of the Turffontein Sup group (formerly Elsburg Series) of the Witwatersrand Supergroup. The reef zone appears to lie on an unconformity, and shows a distinct thickening toward the east, down the paleoslope. Internally, the reef is a multipulse unit, mainly comprising thin layers of gravel, which are interpreted as being longitudinal gravel-bars, and trough cross-bedded sands, which are believed to have been transverse sand-bars. Less frequently, clayey-matrix sands and debris flow type gravels were deposited. Vertical profiles indicate a series of generally fining-upward cycles, which usually start on scour surfaces, and grade upward from a layer of pebbles and heavy minerals, to a medium-grained sand, and rarely to a silt. Lateral profiles reveal the lenticular and channelled nature of the reef. Distinct scour channels may often be traced up to a few hundred metres. The paleocurrent discharge was highly fluctuating, and the dominant dispersal was in a south-easterly direction, away from the Panvlakte fault scarp. The water depth probably varied between 0,1 m and 0,5 m. The depositional environment was a braided alluvial outwash plain, corresponding closely to the Donjek mode1. Two major sub-environments have been recognized. The perennial braided alluvial outwash facies has a high mineralization potential as a result of its high degree of alluvial reworking while the more ephemeral debris-flow food-type environment has dumped clayey-matrix sand and gravel and has almost no economic placer mineralization potential. Gold and uraninite are the two primary ore minerals’ and occur together with a number of other mainly chalcophile minerals’. Four main periods of mineralization are evident. The allogenic detrital phase includes chromite sircon and fine-grained pyrite mineralization. These minerals’ were in hydraulic equilibrium with the sand fraction. During the second period coarse-grained colloform pyrite and magnesia-alumina-rich muds developed and accumulated in temporarily abandoned channels and interfluves areas. Gold and uraninite were concentrated during this period and show a strong association with colloform pyrite. Subsequent fluvial reactivation concentrated this mineralization only a short distance from the site of its origin. Pebble’s and heavy minerals’ became winnowed down to the base of transverse sand bars or else became entrapped in openwork gravels. The third phase of mineralization took place during diagenesis and is represented by local reconstitution of pyrite in the form of diagenetic concretion modules. The final phase resulted from a low-grade metamorphic event which is shown by indicator minerals’ to be lower green schist facies. A partial reconstitution of ore minerals’ and chalcophile elements notably copper, lead and zinc took place during this phase. The mineralogy of the reef was quantitatively investigated by a geochemical analysis of twenty-five elements. An R-mode factor analysis produced five factors~ which have been interpreted as a chalcophile element and colloform-pyrite factor an allogenic detrital oxide factor, a remobilized chalcophile-element factor and a sericite-clay factor. Soda loads onto a fifth factor which is unexplained. A multiple regression analysis of the geochemical data has produced a predictive equation for gold and uranium. This equation relates to the sediment logical and mineralogical observations made and therefore is regarded as realistic. Gold is proportional to colloform pyrite which is estimated by sulphur or iron. This is improved by the inverse proportionality of chrome or zirconium3 which in effect eliminates the effect of the allogenic phase 1 pyrite. The inverse, proportionality of potash reflects the degree of placer reworking3 as it is present as sericite the predominant clay mineral. Grade prediction has been reviewed in the light of sedimentological mineralogical and geostatistical analyses. Optimum prediction can only result from an integration of all these aspects3 in a multi-disciplinary approach. The most pertinent sedimentological aspects in this regard are paleocurrent dispersal and lithofacies analysis. The mineralogical aspect must be quantitative3 and this is best achieved by a multiple regression of geochemical data. An initial geostatistical (kriging) exercise has demonstrated the necessity of carefully considering the sedimentology before dividing the area into sub-areas. The minor (ore-shoot) axis of semivariogram analyses in areas where a unimodal paleocurrent exists corresponds very closely with the mean paleocurrent trend but misleading semivariogram results are obtained when the sub-areas have a divergent paleocurrent pattern. en_ZA
dc.format.extent Online resource (355 leaves)
dc.language.iso en en_ZA
dc.subject.lcsh Sedimentation and deposition
dc.subject.lcsh Heavy metals-- Environmental aspects
dc.subject.lcsh Sediments (Geology)-- South Africa -- Johannesburg
dc.title The sedimentology and mineralogy of the composite reef on Cooke Section, Randfontein Estates Gold Mine, Witwatersrand, South Africa en_ZA
dc.type Thesis en_ZA
dc.description.librarian TL (2019) en_ZA

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