Abstract
M.Sc.
The Mesoarchean Witwatersrand and Pongola Supergroups of South Africa are the
oldest, well preserved supracratonic successions worldwide. Various banded iron formation
(BIF) and iron-rich mudstone units occur within the West Rand Group of the
Witwatersrand Supergroup and the Mozaan Group of the Pongola Supergroup. A
granular iron formation (GIF) occurs in a single unit in the Nconga Formation of the
Mozaan Group. The Witwatersrand Supergroup and Mozaan Group have been
lithostratigraphically correlated and are interpreted to have been part of the same
sedimentary basin.
The studied BIF units occur in two associations: shale-associated and diamictiteassociated
BIF. The GIF seem to have been deposited in shallower environments with
greater hydrodynamic activity. The iron-rich mudstone shows a similar stratigraphic
setting to that of the shale-associated BIF. The lithostratigraphic setting of the
Witwatersrand-Mozaan basin BIFs are similar to what is seen for Superior-type ironformations,
with the mudstones and associated BIFs marking marine transgressions.
Various mineralogical facies of BIF were identified, including oxide, carbonate and
silicate facies BIF, as well as mixed facies between these end members. The GIF is a
unique facies and shows abundant petrographic evidence for biological activity. The
iron-rich mudstone has been subdivided into iron-silicate rich, magnetite-bearing,
carbonate-bearing, magnetite-carbonate-bearing and garnet-bearing subtypes. BIF, GIF
and iron-rich mudstone have been subjected to lower greenschist facies metamorphism
with some occurences of localized contact metamorphism. The abundance of magnetite
shows that oxidation played an important part in BIF deposition, whereas the occurrence
of 12C-enriched iron-rich carbonates suggests post depositional reduction of the deposited
oxidized iron-rich minerals by organic matter. Al-bearing minerals are rare in the BIFs
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and abundant in the iron-rich mudstones. Apatite and rare earth element (REE)-
phosphates occur throughout.
The major element geochemistry shows an inverse proportionality for Fe and Si in all the
studied samples. BIFs show slightly higher Fe- and lower Si- and Al-concentrations
compared to iron-rich mudstones which show higher Si- and Al- and lower Feconcentrations.
The studied BIFs show major element geochemical attributes
intermediate to those of Superior- and Algoma-type iron-formations. Provenance studies
on some of the iron-rich mudstones illustrate that they were sourced from a mixture of
mafic and felsic sources. The rare earth element (REE) geochemistry suggests strong
hydrothermal input into the units, and positive correlation with the Fe-concentrations
suggests that the Fe was introduced by high temperature hydrothermal fluids. The
majority of the REEs are hosted by apatite and the REE-phosphates monazite and
xenotime. The REEs were reconcentrated into these phosphates during diagenesis.
A comparison of the studied lithostratigraphically correlatable units between the
Witwatersrand Supergroup and Mozaan Group makes it possible to construct a
depositional model for basin-wide BIF deposition in the Witwatersrand-Mozaan basin.
Shale-associated BIF was deposited during the peak of transgression when reduced Ferich
hydrothermal bottom waters were introduced into shallow ocean water that was
either oxygenated or filled with anoxygenic phototrophic bacteria. Diamictite-associated
BIF, in contrast, was deposited during interglacial periods when the melting of glacial ice
introduced sunlight, nutrients and oxygen to the reduced, hydrothermally influenced Ferich
ocean water. GIF was probably deposited in shallow, above wave base waters cut
off from clastic input, and then washed into deeper depositional environments. Iron-rich
mudstone was deposited in a similar setting as the shale-associated BIF, but in
environments that were not completely cut off from detrital influx.
The study shows that it is impossible to construct a general depositional model for
Precambrian BIFs, since the lithostratigraphic and depositional settings vary between
different examples of BIF.