The source material of the Big Pebble Marker and Rosedale reefs in the Welkom Goldfield
Steenekamp, Johannes Marthinus Andries
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English: In this study the clasts of the Big Pebble Marker (BPM) in the Aandenk Formation and the basal conglomerate of the Rosedale Member (ROM) in the Eldorado Formation, in the Welkom Goldfield, were investigated. The aim of this study was to employ a different approach to acquire an insight into the nature of the provenance of the gold-bearing conglomerates in the Welkom Goldfield. To achieve this, the quantitative and qualitative compositions of two coarse and polymictic reefs were studied. Observations were made at six underground IocaIities per reef. At each locality a clean, reasonably flat surface, of three to four square metres, with sufficient conglomerate exposure, was investigated. A modal count of the different types of clasts present was done at one thousand points per exposure with the aid of two grid frames. The long axes of at least two hundred clasts, falling on preselected lines, were measured. The long axes of the ten largest clasts in the selected area were also measured. Representative samples of the various clast types present were taken for laboratory study. Every exposure studied, was photographed. All samples were cleaned meticulously to prevent any contamination with matrix material. A total of 126 samples were analyzed for gold, major and trace elements. A selection of samples were also analyzed by means of XRD and microprobe for mineral composition. Petrographic studies were carried out on thin sections of all the samples. A few samples were investigated by means of electron microscope. Fluid inclusion studies were carried out on four quartz samples. All the field data and analytical results were processed statistically plotted on concentrations on computer. Petrographic information were standard ternary diagrams. Average gold in the different clast types were compared with world wide averages for comparable rock types. Field data indicate different transport mechanisms for the BPM and RDM. Composite or multiple provenances are also indicated for both reefs. Quartz and chert dominate the clast assemblages of both reefs, but more so in the case of the BPM. Results from the clast assemblage plots on the ternary diagrams indicate thrust belt and recycled orogenic provenances for the BPM and RDM. Clasts, derived from greenstone-type source rocks, occur predominantly in both reefs. Silica dominates the chemical composition of the majority of the clasts. On average, chert clasts from the BPM contained the highest gold concentrations (143 ppb) of all the clast types. Based on modal composition, the total c last assemblage of the BPM is approximately 6X more auriferous than that of the RDM (76 ppb vs 12 ppb). Respectively, these figures are approximately 30X and 5X higher than the figure of 2,4 ppb, which is generally accepted for a Barberton-type granite-greenstone crust.