Field School 11 Field Guide - Pongola Supergroup

1 Introduction

Our time in the Barberton Greenstone Belt would have highlighted many discussion points regarding the functioning of the early Earth. During this time we would have especially focused on how early continental crust would have formed on Earth. In keeping with the topic of early Earth, we will now move onto the next major Supergroup of South African geology, namely the Pongola Supergroup.

The Pongola Supergroup represents one of the best preserved Archean volcano-sedimentary sequences in the world and may be correlated with the immensely gold-bearing Witwatersrand Supergroup. The Pongola is especially important because it also represents one of the earliest examples of a volcano-sedimentary sequence deposited on some kind of stable continental setting, i.e. the Kaapvaal Craton. Here, we are given an ideal opportunity to investigate a major early Earth geodynamic transition from the creation of continental crust, i.e. as seen in Barberton, to the deposition of rocks on that early crust. Deformation field evidence suggests the occurrence of potentially tectonically-relevant processes occurring at a time where much of the Earth had not produced any stabilised continental crust. 

2 Geological Overview

The Pongola Supergroup is generally separated into a relatively undeformed northern domain and a highly deformed southern domain. These rocks may be subdivided into the lower c. 2.9 Ga volcano-sedimentary Nsuze Group, which is overlain by the c. 2.8 Ga mostly siliciclastic Mozaan Group.

The c. 4-9 km thick Nsuze Group consists of basal quartzite and conglomerate and upper sequences of a large variety of mafic and felsic volcanic rocks. The siliciclastic rocks appear interlayered with the volcanic rocks, which further display a wide variety of features, including lapilli tuff beds, volcanic bombs and amygdaloidal lavas. 

Simplified geological overview of the Pongola Supergroup (Ossa et al., 2016)

The c. 5 km thick Mozaan Group predominantly consists of interlayered sandstone, mudstone, stromatolitic carbonates and banded iron formation. There are also lesser interlayered volcanic rocks near the upper part of the sequence. Sedimentary features are common and suggest deposition in a shallow-marine, wave-dominated environment.

The Mozaan Group can be further subdivided into six formations. These include, from bottom to top; the sandstone, shale and banded iron formation of the Sinqeni Formation; the mudstone, ferruginous mudstone, siltstone and sandstone of the Ntombe Formation; the mudstone, ferruginous mudstone, sandstone and banded iron formation of the Thalu Formation; the sandstone, silstone and pebble-conglomerate of the Hlashane Formation; the mudstone, ferruginous mudstone, diamictite and sandstone of the Odwaleni Formation; and the topmost sandsone, mudstone and volcanoclastics of the Nkoneni Formation. 

Detailed stratigraphic log across the Nsuze-Mozaan transition (Ossa et al., 2016)

3 Deposition of the Pongola Supergroup

The Pongola rocks were deposited on the stabilised Kaapvaal Craton in some kind of continental environment. Considering the occurrence of numerous banded iron formation and stromatolitic carbonate assemblages we may further suggest that deposition occurred in shallow marine, highly reducing environment, which saw a major shift in the chemical composition of the atmosphere (e.g.Beukes and Cairncross, 1991), leading up to, and following the formation of stromatolitic carbonates.

Furthermore, structural evidence (e.g. Gold and Von Veh, 1995) suggests the occurrence of at least one major extensional event that reactivated along pre-exisisting discontinuities during the late Archean. Compression is also noted and potentially linked to the intrusion of major granitic plutons. Once more, strain associated with the compressional regime was partitioned along pre-exisisting discontinuities. Considering these discontinuities and their link to apparent sediment transport directions, the Pongola is considered to have potentially been deposited into a continental rift environment.

Cross section across the Pongola Basin (Gold and Von Veh, 1995)

4 Gold in the Pongola Supergroup

We will also investige gold mineralisation within the Pongola Supergroup during our visit to this terrane. Considering the link and possible similarities in the depositional environments of the Pongola and Witwatersrand Basins, extensive gold exploration was performed throughout the Pongola region. However, gold grades and tonnage never met expectations and development remained low.

Gold mineralisation within the Pongola region manifests in two forms; placer, i.e. gold mineralisation concentrated during sedimentary transport, and lode, i.e. remobilisation of gold-bearing hydrothermal fluids usually into brittle structures, deposits. The placer gold deposits are especially notable in conglomerate layers within the Mozaan and Nsuze Groups.

During the field school we will visit the now abandoned Denny Dalton mine, which is formed in the Sinqeni Formation. The Denny Dalton Member is a clast-supported conglomerate layer that hosts an erratic occurrence of gold, iron-sulphides and uranium.