The source material of the Big Pebble Marker and Rosedale reefs in the Welkom Goldfield

[摘要] English: In this study the clasts of the Big Pebble Marker (BPM) inthe Aandenk Formation and the basal conglomerate of theRosedale Member (ROM) in the Eldorado Formation, in theWelkom Goldfield, were investigated. The aim of this studywas to employ a different approach to acquire an insightinto the nature of the provenance of the gold-bearingconglomerates in the Welkom Goldfield. To achieve this, thequantitative and qualitative compositions of two coarse andpolymictic reefs were studied.Observations were made at six underground IocaIities perreef. At each locality a clean, reasonably flat surface, ofthree to four square metres, with sufficient conglomerateexposure, was investigated. A modal count of the differenttypes of clasts present was done at one thousand points perexposure with the aid of two grid frames. The long axes ofat least two hundred clasts, falling on preselected lines,were measured. The long axes of the ten largest clasts inthe selected area were also measured. Representative samplesof the various clast types present were taken for laboratorystudy. Every exposure studied, was photographed.All samples were cleaned meticulously to prevent anycontamination with matrix material. A total of 126 sampleswere analyzed for gold, major and trace elements. Aselection of samples were also analyzed by means of XRD andmicroprobe for mineral composition. Petrographic studieswere carried out on thin sections of all the samples. A fewsamples were investigated by means of electron microscope.Fluid inclusion studies were carried out on four quartzsamples.All the field data and analytical results were processedstatistically plotted on concentrations on computer. Petrographic information werestandard ternary diagrams. Average goldin the different clast types were comparedwith world wide averages for comparable rock types.Field data indicate different transport mechanisms for theBPM and RDM. Composite or multiple provenances are alsoindicated for both reefs. Quartz and chert dominate theclast assemblages of both reefs, but more so in the case ofthe BPM. Results from the clast assemblage plots on theternary diagrams indicate thrust belt and recycled orogenicprovenances for the BPM and RDM. Clasts, derived fromgreenstone-type source rocks, occur predominantly in bothreefs.Silica dominates the chemical composition of the majority ofthe clasts. On average, chert clasts from the BPM containedthe highest gold concentrations (143 ppb) of all the clasttypes. Based on modal composition, the total c lastassemblage of the BPM is approximately 6X more auriferousthan that of the RDM (76 ppb vs 12 ppb). Respectively, thesefigures are approximately 30X and 5X higher than the figureof 2,4 ppb, which is generally accepted for a Barberton-typegranite-greenstone crust.

[发布机构] University of the Free State