The source material of the Big Pebble Marker and Rosedale reefs in the Welkom Goldfield
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Date
1990-06
Authors
Steenekamp, Johannes Marthinus Andries
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
Description
Keywords
Sedimentary rocks -- South Africa -- Welkom, Petrogenesis, Dissertation (M.Sc. (Geology))--University of the Free State, 1990