Abstract
M.Sc.
The Kaapvaal craton is one of few regions on earth with an almost continuous record of wellpreserved
supracrustal rocks ranging in age from ~3.5 Ga to the late Paleoproterozoic at
~1.75 Ga. In this study diagenetic carbonates from the Paleoarchean Buck Reef Chert and
Joe’s Luck Formation of the Swaziland Supergroup, the Mesoarchean Thalu and Promise
Formations of the Mozaan/Witwatersrand Supergroups and the Paleoproterozoic Timeball Hill
and Silverton Formations of the Transvaal Supergroup were sampled and analyzed. The aim
of the study was to determine possible variations in the composition of the carbonates
through time and their significance especially with regards to microbial activity in diagenetic
systems in early Earth history.
Results indicate similar petrographic observations and geochemical signatures in diagenetic
carbonates of iron formations in the Buck Reef Chert, Joe’s Luck and Griquatown Iron
Formation. The carbonates all tend to be siderites with iron derived from hydrothermal input
and all are depleted in 13C relative to Peedee Belemnite standard. It suggested that siderite
formed as a result of microbial respiration. Microbes degrade organic matter and reduce iron
in this process. This resulted in the depletion in 13C and in the precipitation of siderite.
However in order for iron reduction to have occurred the reduced iron first had to be oxidized.
This most probably occurred through iron oxidizing chemolithoautotrophs under
microaerophilic conditions.
Diagenetic carbonate concretions of the Thalu and Promise Formations are manganiferous
and are highly depleted in 13C relative to PDB. There is also strong evidence for hydrothermal
input of manganese and iron into the system because of positive europium anomalies. The
carbonates from both of the formations strongly suggest the presence of some free oxygen.
The reasoning behind this conclusion is as follows: The depletion of 13C in the carbonates
points to microbial decomposition of organic matter and manganese respiration (the
decomposition of organic matter by microbial MnO2 reduction) is shown to be the most
reasonable process that led to the formation of the carbonate concretions. The implication is
that MnO2 must first have been precipitated and that can only be achieved in the presence of
free oxygen with the oxidation reaction often catalyzed by manganese oxidizing
chemolithoautotrophs.
The carbonates of the Timeball Hill and Silverton Formationsare calcites ad contain little no
iron. There is also little or no evidence for hydrothermal input and the basin appears to be a
clastic dominated. It is generally accepted that a major rise in oxygen in the oceans and the
atmosphere occurred at about 2.32 Ga. This rise in oxygen levels is reflected in the
diagenetic calcite concretions of the Silverton Formation. Both iron and manganese reduction
where not very effective because of the depletion in the basin water of these two elements,
organic carbon taken up in the calcite concretions, indicated by negative δ13CPDB carbonate
values, was most probably derived from aerobic and/or nitrate respiration.
The most important conclusion from this study is that sufficient free oxygen and hence
oxygenic photosynthesis were present to oxidize both Fe and Mn at least as far back as the
Paleo-Mesoarchean.