Geology of SA PART 2 NOTES
ARCHAEAN SEDIMENTARY ROCKS AND IGNEOUS INTRUSIONS
The Kaapvaal Craton had stabilised enough by 3.0 Ga that it could accommodate extensive
deposition. There is compelling evidence that at this time, the planet was dominated by typical
Archaean granitegreenstone assemblages. The following units will be considered:
1. The Pongola Supergroup
2. The Dominion Group
3. The Witwatersrand Supergroup
4. The Transvaal Supergroup
4. PONGOLA SUPERGROUP
This sequence of sediments and volcaniclastics was deposited upon an already stable crust around
3.0 Ga. The rocks are exposed on the south-eastern part of the Kaapvaal Craton, in Mpumalanga,
KwaZulu-Natal and Swaziland. The deposits are developed in two separate basins, the Main Pongola
Basin in the north and the Nkandla Basin in the south. It is thought that sedimentation occurred in a
single basin, and a structural high developed concomitantly with deposition.
The succession consists of:
The Nsuze Group (lower), a sequence of extensive mafic to felsic volcanic rocks with calcareous and
argillaceous sediments – probably the time equivalent of the Wits Dominion Group.
The Mozaan Group (upper), a mainly sedimentary sequence, interpreted as being the time
equivalent to the Wits West Rand Group.
5. THE DOMINION GROUP
The Dominion Group consists of mainly andesitic volcanic rocks with minor sedimentary horizons,
directly overlying the greenstones, and in turn overlain by Witwatersrand conglomerates. There are
few outcrops, mainly in the Klerksdorp area. The Dominion Group is particularly interesting, because
it represents one of the earliest supracrustal deposits on the Kaapvaal craton. It represents an
important milestone in the evolution of the Kaapvaal craton from the granitoid / greenstone
environment to a platform. In addition, it lies immediately below the Witwatersrand Supergroup.
SHRIMP ion microprobe analysis has given the date of 3074 Ma for the Dominion Group. This
provides an important maximum age for the Witwatersrand, which itself cannot be directly dated
due to the absence of suitable dating materials.
6. THE WITWATERSRAND SUPERGROUP
Witwatersrand deposits consist of a series of conglomerates, quartzites and minor shales which
were deposited on the shores of an inland sea. The sediments were worked and re-worked several
times and deposition was extremely slow. The area is of global importance due to the discovery of
,gold on the farm Langlaagte in 1886. Since then, 4.5 billion tonnes of ore with an average grade of 9
g/t from the Witwatersrand goldfield has been treated to obtain gold. This represents 39% of all
gold ever mined in the history of humanity! Due to the economic importance a lot of work has been
carried out on the Witwatersrand geology, and the mines have largely evolved their own
terminology. The first detailed geological mapping was carried out in 1917 by Mellor, and in 1980
the South African Code of Stratigraphic Terminology and Nomenclature (SACS) divided the
Witwatersrand as follows:
West Rand Group (lower)
The quartzite to shale ratio is 1:1. It consists mainly of quartzites and shales. The shales are iron-rich
and are useful markers for magnetic geophysical surveys. One horizon, the Contorted Bed, is a
prominent banded iron formation. The West Rand Group is divided into three Subgroups, however
their characteristics vary considerably from one basin to the next, so a generalised account is
followed here.
Hospital Hill Subgroup
This consists of orthoquartzites, and is thought to represent a marine transgression across the entire
craton.
Government Subgroup
This represents unstable conditions, with fluctuations in sea level, and consists of conglomerates,
BIF, and several diamictites. The diamictites probably represent glacial periods and associated
marine regressions.
Jeppestown Subgroup
This consists mainly of quartzites, most of them very mature, indicating significant re-working of
shallow marine sediments and is evidence of a far more stable environment. The Crown Formation
represents several basaltic to andesitic lava flows.
Central Rand Group (upper)
This group consists of mainly quartzite and conglomerate, representing alluvial plain and fan
deposits and includes most of the gold bearing strata. A large widespread shale unit is used to divide
the Group into the Johannesburg and Turffontein Subgroups.
Johannesburg Subgroup
This consists of quartzites associated with a fluvial braid environment. Due to minor periods of
erosion there are several erosive horizons, and the reworking of these sediments have enriched
, heavy minerals leading to economic deposits known as reefs. These reefs are characterised by
narrow pebble horizons and a carbonaceous layer known as Kerogen. These palaeoplacers are river
braid channels, and these have frequently been eroded and filled with (non-glacial) diamictites
during marine transgressions. Several important conglomerates are worked – the Main Reef Leader,
Carbon Leader, and the Basal Reef and Steyn Reef in Welkom. The top of the unit is marked by shale
resulting from a laterally persistent marine transgression – the Booysens Formation.
Turffontein Subgroup
This includes the Kimberley Reefs, which are conglomerates formed by the re-working of braided
river deposits, often locally eroded and infilled with shale. The group contains important gold
placers.
The overlying Venterspost Formation, part of the Ventersdorp Contact Reef (and therefore not
Witwatersrand in age), is also an important source of gold and is mined in the Carletonville
Goldfield.
The Witwatersrand deposits lie unconformably on the greenstones, and in places, the Dominion
Group. They are unconformably overlain by Ventersdorp lavas. Due to a complex tectonic history,
nine discreet goldfields have been identified, separated either tectonically or by areas of low
payability, called gaps. The main gaps are: Bothaville Gap, Potchefstroom Gap and Witpoortjie Gap.
The goldfields are:
West Rand Goldfield
Central Rand Goldfield
East Rand Goldfield
South Rand Goldfield
Evander Goldfield
Welkom Goldfield
Klerksdorp Goldfield
Venterskroon Goldfield
Carletonville (West Wits) Goldfield
