| Summary: | The Witwatersrand basin is loaded with carbon. The carbon deposits locations
are not site specific. Many localities where deposits occur are related to the
structure and the sedimentology of the particular area. The purpose of this
thesis is to document and describe the distribution of carbon in the
Witwatersrand basin and to establish the mechanisms controlling emplacement.
The approach used was a multidisciplinary one incorporating aspects of
sedimentology and macro to mesoscopic tectonic structures and their
relationships with carbon distribution patterns to establish the controls on carbon
emplacement. One of the major controls on carbon deposition is structural
geology. Bedding parallel fractures that cut pebbles with carbon fill is indicative
of the influence of bigger and more forceful movements within the ore body. The
thesis begins with a back ground study of all the major theories and ends with a
possible explanation for the influence of structure and sedimentology on the
deposition of carbon, and a new catalyst for the dehydration reaction that could
lead to the deposition of carbon.
The various types of carbon have been classified and grouped to specific
sedimentological and tectonic structures. These types are deposited in
lithofacies horizons that are not related in space to one another. They also differ
in texture. Type A is observed in reefs and bedding planes, bedding parallel
fractures and on fault planes. Type B consists of massive carbon and vug type
carbon.
Carbon on the reef contacts in most cases is developed on intersections with
fluid pathways (phylonites or shear zones) which are characterised by the
alignment of the minerals within the pathway. Carbon precipitation is controlled
by the type of footwall and the amount of fluid pathways. The higher the
occurrence of bedding parallel fractures the more consistent the emplacement
of carbon. Phylonites are classified as follows: Type 1 exhibits a low degree of
deformation and the minerals show a low degree of orientation. Type 2 exhibits
a medium degree of deformation and the minerals show a larger degree of orientation. Type 3 phylonite is where all the original sedimentary character of
the original rock has been sheared and the deformation gives rise to a foliated
rock with a distinctive foliation.
It is suggested that the large extensional faults in the Free State Gold Fields and
the Master Bedding plane fault in the West Rand Gold Fields are conduits for
the fluids into the Basin. The in-flow of fluids is from below the reef horizons. It is
further speculated that in the Free State, the fluids had a north-easterly transport
direction.
The SEM analyses showed new mineral associations. The mineral phases are
shown in three dimensions and the order of precipitation can be deduced. The
element tantalum was prominent in one of the high grade samples. The most
prominent mineral in the fluid pathways within the matrix of the various reefs is
pyrophyllite. The carbon is emplaced within the pyrophyllite within a fluid
pathway and this is indicative of the sequence of mineralization. The uranium
replaced the pyrophyllite and the pyrite crystallized in a fracture within the
pyrophyllite. It is concluded that the three main minerals: carbon, uranium and
gold all came in at the same time into the basin.
A hydrothermal origin for carbon and associated minerals is supported by the
study. The proposed hypothesis to explain the timing and origin of carbon and
gold into the Witwatersrand Basin is that of the 2.02 Ga Vredefort Impact event.
Gold and uranium are inferred to have been transported by the carbon plasma
that originates from the mantle during the Vredefort Impact event.
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