Field School 11 Field Guide - Barberton Supergroup

1 Introduction

The c. 3.57-3.22 Ga Barberton Greenstone Belt is one of the most important geological terranes in the world. This is primarily because it is one of very few locations where a pristine fragment of the early Earth is preserved. The rocks found here hold abundant information about the geodynamic functionality and evolution of the early Earth, the formation of continental crust and even clues about the origins of life as we know it. The Council for Geoscience Field School will provide the participants an opportunity of visiting this famous geological terrane. Here we will investigate how fundamental geological field observations throughout the Barberton Greenstone Belt, together with advanced analytical techniques have shed light about the early Earth. We will also spend a few days mapping the lower part of the Onverwacht Group and look for evidence of early Archean geodynamic processes.

2 Geological Overview

The rocks of the Barberton Greenstone Belt comprises the oldest Supergroup in South Africa, namely, the Barberton Supergroup. This forms the eastern portion of the Kaapvaal Craton and appears as a NE trending belt of highly folded volcano-sedimentary rocks that has undergone greenschist facies metamorphism. The Barberton Greenstone Belt is further defined by several granitic and granite gneiss TTG (tonalite, trondhjemite and granodiorite) plutons that have intruded the belt at various locations and times throughout its evolution. The Barberton Greenstone Belt can be separated into a two geochemically distinctive terranes, i.e. a Northern and Southern terrane, which are separated by the Saddleback-Inyoka Fault.

Geological overview of the Barberton Greenstone Belt, with simplified stratigraphic log (Hofmann, 2005)

3 Stratigraphy

The Barberton Supergroup is subdivided into three groups. This includes the lowermost c. 3.57-3.30 Ga Onverwacht Group of mafic and ultramafic volcanic rocks. Overlying the Onverwacht are the c. 3.55-3.25 Ga Fig Tree Group of chemical sedimentary rocks interlayered with mafic and ultramafic lavas. The topmost are the c. 3.23-3.22 Ga Moodies Group of mostly siliciclastic rocks.

3.1 The Onverwacht Group

The Onverwacht Group is a 8-10 km thick sequence of mafic/ultramafic mostly komatiitic basaltic rocks, interlayered with felsic tuff, chert and carbonate rocks. These rocks were probably formed in either a volcanic hot spot, or in an island arc setting. The Onverwacht is further subdivided into six formations. This includes, from bottom to top; the Sandspruit, Theespruit, Komati, Hooggenoeg, Kromberg and Mendon formations.

Generalised geology of the Onverwacht Group (De Wit et al., 2011)

3.2 The Fig Tree Group

The Fig Tree Group attains a thickness of c. 3 km and predominantly consists of turbidite deposits, banded iron formation, stratiform barite and interlayered volcanoclastic rocks. The rocks of the Fig Tree Group can be further subdivided into two facies-type deposits separated across the Saddleback-Inyoka Fault. This includes a shallow-water and deep-water depositional facies, south and north of the Saddleback-Inyoka Fault, respectively.

The southern facies can be subdivided into a lower Mapepe Formation and an overlying Auber Villiers Formation. The Mapepe Formation consists of ferruginous shales, banded iron formation, sandstone and barite-bearing conglomerate. The Auber Villiers Formation consists of felsic volcanics interlayered with turbidites and chert-bearing conglomerate.

The northern facies can be subdivided into, from bottom to top; Sheba, Belvue Road uppermost Schoongezicht formations. The Sheba Formation consists of banded iron formation with interbedded turbidites. The Belvue Road Formation consists of shale, banded iron formation and interlayered felsic volcanic rocks. The Schoongezicht Formation comprises felsic volcanics rocks with interlayered turbidites and conglomerate.

Stratigraphy of the Fig Tree and Moodies Groups north in the northern region of the Barberton Greenstone Belt (Anhaeusser, 1974)

3.3 The Moodies Group

The Moodies Group attains a thickness of c. 3.7 km and consists of sandstone, siltstone and conglomerate that was probably deposited in a shallow marine setting, such as a braided fluvial-tidal system.

Generalised stratigraphy of the Moodies Group north of the Saddleback-Inyoka Fault (Heubeck et al., 2013 )

3.4 TTG rocks

The various TTG plutons that surround the Barberton Greenstone Belt can be summarised into five distinctive age groups:

3510-3502 Ma - e.g. Steynsdorp Pluton

3469-3437 Ma - e.g. Stolzburg, Theespruit, Doornhoek Plutons

3229 Ma - e.g. Kaap Valley Tonalite

3216 Ma - e.g. (undeformed) Dalmein Pluton

3105 Ma - e.g. (undeformed) Mpuluzi Batholith

4 Deformation

The structural evolution of the Barberton Greenstone Belt is contentious. On the basis of the existence of plate tectonics during the early Earth, a polyphase structural evolution can be summarised as follows (after De Ronde and De Wit, 1994):

D0 (3490-3450 Ma) - extensional tectonics and alteration processes near volcanic spreading centers.

D1 (3450-3416 Ma) - subduction-like processes with the emplacement of a possible ophiolite complex in an intraoceanic setting and intruded by TTG plutons.

D2 (3260-3225 Ma) - a second phase of subduction-related accretion of the northern and southern terranes and the formation the Saddleback-Inyoka Fault.

D3 (c. 3100 Ma) - transtensional and transpressional tectonics along this terrane boundary, which also resulted in the NE-trending expression of the Barberton Greenstone Belt and the formation of strike-parallel faults.

D4 (< 3100 Ma) - NW-directed extension and the formation of gold mineralisation-controlling normal faulting development.

Another deformation model considers little to no plate tectonics at play, and rather the effect of rising and falling of mantle plumes and convective overturn (i.e. Van Kranendonk et al., 2009). These features are especially notable when considering the overall pattern across the Barberton Greenstone Belt, which highlights a dome and keel structure:

Dome and keel morphology of the Barberton Greenstone Belt, surrounding TTG plutons (Lana et al., 2010)

5 Gold mineralisation in Barberton

The Barberton Greenstone Belt is an important gold producing terrane in South Africa, with more than 350 operational gold mines. Gold mineralisation in Barberton is structurally controlled and is associated to the later deformational event, i.e. D4. Gold mineralisation was likely formed during a major protracted hydrothermal event as remobilised gold-bearing fluids flowed through permeable brittle fractures, i.e. extensional features. Two kinds of gold mineralisation is noted, namely, mineralisation associated to iron-sulphides and gold within quartz.

Overview of large gold mineralisation locations throughout the Barberton Greenstone Belt (Altigani et al., 2016)

Overview of structural features around the Sheba region (Altigani et al., 2016)

6 A model for the formation of early continental crust

A model describing the formation of Archean continental crust is provided by De Wit, 1998, and can by summarised as followed, starting with some important facts about Archean crust:

1. Archean Cratons, such as the Kaapvaal Craton have very deep subcontinental mantle keels, which reach depths of almost 400 km and were formed by c. 3.5-3.1 Ga. Trace element analysis on eclogite inclusions suggest that these keels may represent hydrothermally altered Archean oceanic crust (i.e. basaltic crust).

2. Deep seismic analysis suggest the presence of mantle discontinuities, which may represent crustal-scale shear zones. These shear zones apparently separate different crustal domains that were amalgamated together.

3. The Archean crust highlights low mantle heat flow patterns. This is probably due to low heat production and insulating effects.

Combining these factors suggests that the early Archean crust formed by tectonic stacking of hydrothermally altered oceanic crust. The model proposed by De Wit, 1998 suggests that this process formed as follows:

During Hadean times, there was dry recycling of ocean crust, where seawater levels remained low and the oceanic spreading centers were above sea level. Continued mantle degassing from c. 4.5 Ga onward, together with meteorite bombardment, would have seen the sea level rise enough to drown these oceanic spreading centers c. 4.0 Ga,

Thereafter, subduction of oceanic crust would have undergone adequate hydrothermal alteration, which allowed for the formation of more buoyant oceanic-lithosphere. This lighter and more buoyant material would resist subduction, thus forming duplex structures, i.e. stacks of oceanic crust. As this stacking process continued and the lower parts of the hydrothermally altered mafic rocks were buried deeper, these rocks underwent dehydration melting, with the products of melting representing the early TTG rocks. The TTG plutonic emplacement would have been facilitated along extensional shear zones. 

Over time, the gradual burial and partial melting of TTG rocks would have resulted in the formation of younger GGM rocks. During this evolution, the slab-pull effect became more prevalent and plate tectonic processes began to function more rapidly. This would eventually result in these early Archean fragments colliding and amalgamating to form larger continental masses.

Geodynamic model for the creation of early Archean crust (Polat, 2012)

7 Conditions on the early Earth

A model for the conditions of the early Earth is provided by Lowe and Tice, 2007, and can be summarised as followed:

The Earth was covered by large oceans and had few continental masses. Meteorite impacts were common and there would have been lots of tsunamis. There was a high concentration of volcanic activity and therefore a large outpouring of mantle gas. Due to a high concentration of these greenhouse gasses (i.e. carbon dioxide and methane), the temperature on Earth was substantially hotter than today, probably c. 70 degrees Celsius. In these conditions cyanobacteria would have struggled to survive and thus the Earth would likely have been dominated by thermophile bacteria. These conditions were however suitable for the formation of early Algoma-type banded iron formation.

Early Earth might have looked something like this (without the humans and drama)

As continental crust continued to grow, rates of weathering increased. This resulted in a drawdown of the carbon dioxide and eventually methane. This eventually resulted in global cooling and even glaciation c. 3.0-2.9 Ga.

The global cooling and relatively stable conditions on the earth Earth, in addition to the changes to the chemical atmospheric conditions allowed for the growth of stromatolitic-producing cyanobacteria and oxygentation of the Archean atmosphere. 

This process of an increase in greenhouse gas, growth of continental crust and eventual drawdown of greenhouse gas seen c. 3.5-2.9 Ga appears cyclic throughout the Earth history and appears again c. 2.75-2.2 Ga and 1.0-0.5 Ga.