The geochemical history of the sedimentary rocks of the Witwatersrand as reflected in the mineralogy of the heavy-mineral assemblage of the uranium-bearing reefs of the Central Rand Group

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.