| dc.description.abstract | The well-known Archaean gold deposit on the Witwatersrand is the world's principal
source of this precious metal. A tectonically active hinterland comprising Archaean
granite-greenstone terrane and pre-Witwatersrand sediments provided the detritus which
was deposited at major points of fluvial influx along the northern and western periphery
of the Witwatersrand Basin during the sedimentation of the Central Rand Group.
The geochemical characteristics of the environment which prevailed at the earth's
surface at the time of sedimentation can be reconstructed from the response of the detrital
constituents to the changes in surficial conditions during the different stages of the exogenic
cycle and after burial at the final site of deposition. The information is provided by the
following groups of minerals:
(1) those that must have been common in the source rocks but are absent in the reefs,
(2) constituents that were preserved during the exogenic cycle, including those
altered before denudation of the provenance area, and
(3) altered or neo-formed products of post-burial origin.
The absence of carbonates, sulphides other than pyrite, sulphates, apatite, iron oxides,
and primary silicates suggest that the surface conditions during denudation in the
provenance area and en route to the depositional environment must have been acid. The
silicates that survived the pre-depositional processes or formed soon after deposition were
subjected to diagenesis and metamorphism. The metamorphic silicate assemblage:
chloritoid-pyrophyllite-chloriteIIb-muscovite2M1,
indicates that the sedimentary rocks attained the lower stage of low-grade metamorphism
at a temperature below 400°C and a pressure of about 3 kbar.
The thorough elimination of iron oxides and the alteration of ilmenite to leucoxene
pseudomorphs by the removal of iron at, or close to the earth's surface was a common
process during the Archaean. The dissolution of iron requires neutral to acid conditions,
and the required level of acidity increases with an increase in oxidation potential.
Detrital uraninite, pyrite, chlorite, and biotite, which react readily under modern
(oxidizing) surface conditions, were preserved during the exogenic cycle. Low surface
temperatures and rapid erosion and accumulation of large supplies of debris could have
delayed the reaction of these oxygen-consuming minerals during denudation, erosion, and
transportation. When the detritus arrived at the plane of final deposition, part of the
unoxidized uraninite was encapsulated by microbiota, which prevented its reaction, while
the remaining portion of uraninite became a constituent of the heavy-mineral assemblage.
The common presence of relict uraninite in the matrix of the conglomerates indicates that
uranium was dissolved from uraninite which was not shielded by organic matter. The shape
of these partly or entirely leached grains was preserved by cutans which precipitated during
the removal of uranium. The dissolved uranium was recaptured by dissolved titanium, and
the two phases reprecipitated as the uranous titanate species, viz brannerite and uraniferous
leucoxene of variable composition.
In the Dominion Reef, at the base of the sedimentary succession that filled the basin,
and in some of the reefs near the top of the supergroup, part of the uraninite was transformed
in situ to coffinite through the addition of silica, a process which requires an
alkaline-reducing environment. Such conditions were found, for example, in stagnant pools
on the depositional plane onto which a braided channel pattern developed.
Owing to the addition of titania and silica, the neo-formed uranous titanates and coffinite
occupied much more space than the original uraninite, hence, the alteration of uraninite
and, likewise, the formation of cutans took place at a time that the porosity of the sediment
made provision for expansion and for the free circulation of pore solutions, Le. shortly after
burial.
The missing minerals and the behaviour of ilmenite and the iron oxides indicate that
the conditions at the Archaean earth's surface was distinctly acid. It is generally assumed
that the partial pressure of carbon dioxide in the primitive atmosphere was much higher
than it is at present, a condition which must have increased the acidity of the rain water,
groundwater and surface waters.
Of the oxygen-consuming minerals, only uraninite was oxidized, which implies that the
uranium oxide was the most efficient sink for oxygen. The oxidation of uraninite after
deposition at the palaeosurface without that of the detrital minerals pyrite, chlorite, and
biotite, indicates that only limited amounts of oxygen must have been available at the time
of sedimentation. | |
