Field School 11 Field Guide - Transvaal Supergroup

1 Previously...

Before we begin with the Transvaal Supergroup, we shall briefly summarise the geology we have seen (will see) thus far: We began with the c. 3.5 Ga Barberton Supergroup. This renowned geological terrane sheds light on the workings of a young Earth and the formation of early continental crust.

Thereafter, we visited the c. 2.9 Ga Pongola Supergroup. The Pongola may be correlated with the Witwatersrand Supergroup. These volcano-sedimentary sequences form some of the earliest sequences of their kind, deposited on a stable continental platform, i.e. the Kaapvaal Craton. Furthermore, here we should also highlight that the Witwatersrand rocks were deposited onto the c. 3.0 Ga Dominion Group lavas. The Dominion lavas, together with the Pongola and Witwatersrand rocks highlights an early possible continental rift environment.

Following the deposition of the Witwatersrand and Pongola was the outpouring of the c. 2.7 Ga Ventersdorp mafic and ultramafic lavas. And finally, deposited on top of the Ventersdorp lavas was the 2.5 Ga Transvaal Supergroup.

2 Introduction

The rocks of the Transvaal Supergroup are comparable in the eastern Transvaal Basin and western Griqualand West Basin. Here, we will be focusing on the Transvaal Basin and especially on the stromatolitic-bearing Malmani Subgroup. The Malmani is important because these represent the earliest and largest carbonate platform deposits on Earth. Moreover, the Transvaal rocks are also important because they mark a major transition from the Archean to the Paleoproterozoic furthering our discussions about the dynamic shift of the early Earth.

The deposition of the Transvaal rocks occurred at a time that saw a rapid accumulation in the growth of continental crust and major, and irreversible shift in chemistry of the Earth's atmosphere and oceans. Furthermore, rocks within this (and the Griqualand West Basin) host important mineral commodities, such as Gold, Iron and precious stones.

3 Stratigraphy

The Transvaal Supergroup is subdivided into the lowermost high-energy siliciclastic Black Reef formation, overlain by the chemical sediments of the Chuniespoort, which is further overlain by the largely siliciclastic Pretoria Group. The stratigraphy of these various rock types may be summarised in the following diagrams:

 Overview of the extent of the Transvaal and Griqualand West Bains (Frauenstein et al., 2009)

The lowermost rocks of the Transvaal Supergroup consists of the Black Reef Formation, which largely consists of conglomerates with interbedded sandstone and mudstone that was deposited in a braided fluvial to shallow marine lagoon/delta environment.

Unconformably overlying the Black Reef is the Chuniespoort Group, which may further be subdivided into seven formations. The lowermost sequence consists of the stromatolite-bearing Malmani Subgroup, which further consists of five formations. This includes from bottom to top, the Oaktree, Monte Christo, Lyttelton, Eccles and Frisco formations. These are distinguished from each other by their chemical, especially Mn-content and occurrence of interlayered chert. The Monte Christo and Eccles are chert rich, while the others are chert poor. The Malmani was deposited as a carbonate platform in a shallow marine setting, with interbedded shale that highlights proximity to the shoreline and terrestrial sediment input.

Overlying the Malmani is the Penge banded iron formation, which was probably deposited in a shallow marine setting, with the Ventersdorp lavas representing a likely Fe-source candidate. The topmost unit of the Chuniespoort is the dolomitic mudstone and interbedded quartzite of the Duitschland Formation.

Overlying the Chuniespoort are the largely volcano-sedimentary Pretoria Group, which is further subdivided into fourteen formations (which I will not write about here, but you may refer to the figures below for an overview).

Stratigraphic logs correlating the Transvaal and Griqualand West Basin (Frauenstein et al., 2009)

Lithostratigraphy correlated with tectonic setting and depositional environments of the Transvaal Basin (Catuneanu and Eriksson, 1999)

4 The Great Oxygenation Event

From our time in Barberton and Pongola, we would have discussed the presence of oxygen in the early Earth atmosphere. However, this concentration was not significant and the atmospheric and ocean chemistry often varied in composition from oxygen-rich to poor. However, the c. 2.33 Ga Great Oxygenation Event signals a time in the history of the Earth that saw an irreversible increase in the atmospheric oxygen concentration (i.e. Luo et al., 2016). This is especially important because this increase of the oxygen concentration heralds a shift and transition of the biological revolution on Earth.

This event occurred after large-scale photosynthesising cyanobacteria organisms consumed large quantities of delta-12 carbon from the largely reducing atmosphere and ocean. Complimentary to this was the release of a vast amount of oxygen that saturated the atmosphere and ocean, changing the chemical make-up completely.

The chemical reaction catalysed by the photosynthesising organism also resulted in an enrichment of delta-13 carbon, i.e. due to the drawdown of delta-12 carbon. This subsequently resulted in elevated d-12 carbon signatures seen in the Pretoria Group:

Location of the Great Oxygenation Event within the Pretoria Group (Luo et al., 2016)

5 Gold in the Malmani 

In keeping with the theme of major mineralisation, the Malmani Subgroup has produced substantial quantities of gold since the early 1800's. Gold mining within the Malmani rocks was focused around the Sabie-Pilgrim's Rest region and largely consists of quartz-vein hosted lode deposits. During the field school, we will plan on visiting some abandoned mines in the Sabie region, however this will depend on safety. Many of these abandoned mines are currently being mined illegally, a risk to surrounding communities and also the miners themselves. 

We will therefore have to discuss the current state of illegal mining in South Africa and how do we protect these miners and what are the possible mechanisms to remedy this situation?

The gold mineralisation in the Malmani is thought to have been sourced and/or remobilised by the Bushveld Complex. The intrusion of the Bushveld resulted in eastward directed thrusting. During the thrusting, remobilised gold-bearing fluids were concentrated along shallow-bedding-parallel detachment zones within the Malamani. Gold mineralisation is largely associated with iron-sulphides and as quartz-inclusions.