THE BARBERTON MOUNTAINLAND GEOLOGICAL HERITAGE

The Barberton Mountainland located in eastern Mpumulanga and north western Swaziland, contains some of the oldest and best preserved Archaean rocks on Earth. These ancient rocks (older than 3500 million years, [Ma]) are extremely rare, being remnants of the oldest continental surfaces, and comprise less than half a percent of the world’s land surface. They include volcanic, sedimentary and igneous rocks that uniquely record the history of the earth’s earliest history. Rocks of this age are commonly referred to as greenstone belts

The area consists of a northeast trending, deeply incised hill-and-ridge landscape extending roughly 50 by 100 kilometres. It consists of two main geological components: (1) the central greenstone rock sequences (Barberton Greenstone Belt) which make up the bulk of the mountainland, and (2) the surrounding intrusions of granite.

Stratigraphy

The greenstone belt has been subdivided into three major rock sequences: the Onverwacht, Fig Tree and Moodies groups, all belonging to the Barberton Supergroup.

Onverwacht Group

The Onverwacht Group is the oldest and occurs at the base of the Supergroup. It consists largely of komatiite and basalt lavas formed in oceans with some quartz-rich volcanic rocks. Within this unit numerous chert horizons occur. Cherts are formed by the precipitation of silica on the sea floor and have a banded appearance. Stone Age cultures are identified by their use of these rocks (flint) to make sharp edged hand tools and arrow points. Maximum age of these rocks is of the order of 3472 Ma.

Komatiite rock type

A new igneous rock type called komatiite was discovered in the 1960’s by South African twins Morris and Richard Viljoen. They are located in the Onverwacht rocks of the southern part of the mountainland. Komatiites, which are very rare, were formed under extreme temperatures of around 1600°C and have a high magnesium content.

Pillow lavas

Well preserved pillow lavas can also be seen in the Onverwacht group. Basalt lava tubes flowed into the sea pinching off balls of lava which appear to have rolled down a slope to be deposited on top of each other forming ‘pillows’. The triangular cusp at the base where a pillow of cooling lava sagged between the previous pillows, is used as a way-up indicator, showing in which direction the younger pillows were being formed. This helps geologists establish whether or not rock sequences have been overturned by deformation processes.

Early life

A bacterial microfossil Archaeospheroides barbertonensis has been identified from the chert layers, making it one of the earliest forms of life. Under a scanning electron microscope one can see colonies of fossilised spherical and rod shaped bacteria, some of which can be clearly seen undergoing cell division. Fossilised organic films and wisps are also present in the chert.

Meteorite bombardment

Recent research indicates that intensive bombardment by meteorites occurred on these early land surfaces. The evidence is preserved in spherule layers (shot-sized balls 1 to 2mm diameter) above the chert layers, with chromium isotope values not found on earth. The spherule beds were produced by fallout from the impact dust caused by meteorites slamming into the earth at velocities of 10 to 15 km per second. Four spherule layers have been identified. Based on the thickness of the layer, the meteorites are estimated to have been up to 30km in diameter, three times larger than the meteorite impact which caused the dinosaur extinction at around 65 Ma. These meteorite impacts are estimated to have evaporated up to 3m of sea water globally in a few seconds, followed by giant tsunami waves. No meteorite craters are found in the area.

Fig Tree Group

The overlying Fig Tree Group consists mainly of greywacke (a mud rich sandstone) with subordinate shale, chert and minor lavas and tuffs formed from volcanic ash. The greywacke and shale were deposited from slurries of mud and water flowing down a sea slope in an ocean basin. As the ocean basin filled up with sediment and water depths decreased, sediment derived from surface rocks became the main contributor to the rock record.

Moodies Group

The uppermost Moodies Group consists largely of sandstones composed of feldspar and quartz grains deposited by rivers in shallow marine settings. Minor amounts of siltstone, shale, thin chert and basalt are also present in this group. These sediments were deposited as a response to the uplift and erosion of the Kaap Valley tonalite (a light-coloured granite forming the floor of the Barberton valley) between approximately 3225 and 3080 Ma.

The Moodies rocks contain evidence of one of the oldest tidal environments on earth, in the form of herring-bone cross-bedding. This cross-bedding is formed by the inclined deposition of sediment in two directions, producing a pattern resembling a fish skeleton when viewed in section. The reversal of direction of the bedding suggests tidal change, indicating that the moon was in orbit at this early stage of the Earth’s history.

Deformation

The whole Onverwacht sequence was subjected to large scale squeezing resulting in the development of tight folds and extensive fault zones. Gold mineralisation is reckoned to have formed during the late stages of this process, just before the development of the large granite batholiths (domes) of the Nelspruit area. Gold bearing fluids would have flowed into the faults and shear zones in the rocks. Reaction with the parent rock and changes in pressure and temperature resulted in the concentration and precipitation of quartz, carbonates, free gold and gold bearing sulphides such as pyrite and arsenopyrite in these localities.

Gold mineralisation

The importance of the greenstone belt lies not only in its scientific value but also in its economic potential. Around 330 tons of refined gold have been mined from lodes in the Barberton area since 1883. There is little or no evidence of earlier gold extraction (pre 1880) although the ancient circular dwellings found in the area are estimated to be some 500 years old and have been interpreted as part of a culture which traded gold with eastern (Arabic and Indian) communities in the past.

Although there were more than 300 mines in the area, gold production has stemmed mainly from three centres of extraction, namely the Agnes, New Consort and Sheba-Fairview areas. Other sites of gold mineralisation in the north include the Lily, Barbrook and Albion deposits. In the southeast in Swaziland, significant quantities of gold were encountered at Forbes Reef, Piggs Peak and Hhohho areas. In the southwestern part of the belt only minor gold production was recorded from the Steynsdorp and Komati valleys. Current gold production from the area amounts to around 400 kg per month.

Other minerals

The Barberton Greenstone Belt also contains concentrations of chrysotile asbestos (Msauli and Havelock Mines, now closed), nickel, copper and magnesite from magnesium rich rocks; small mercury deposits; zinc-copper-iron-silver deposits associated with the quartz rich volcanics developed at the top of the Fig Tree Group; baryte deposits within the Fig Tree Group, tungsten in the New Consort area and tin deposits in Swaziland associated with granite rocks. Dimension stone mined in the past includes colourful Fig Tree conglomerates in a light green matrix (pudding stone) and chrome green verdite and chert.

Acknowledgement

Provided by the Barberton Mountainland Geological Society as Adapted from the 1995 Centennial Geocongress Excursion Guide for the Barberton Mountainland, published by the Geological Society of South Africa.

Enquiries: Barberton Mountainland Geological Society, Chris Rippon e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it