BARBERTON GREEN-STONE BELTTHE Barberton Greenstone Belt occurs in an area known as the Barberton
Mountain Land - a rugged tract of country in the Lowveld region of Mpumalanga Province and Swaziland. This Archean green-stone belt represents one of the oldest and best pre-served volcano-sedimentary successions not only in South Africa, but in the entire world. It consists of a wide variety of volcanic and sedimentary rock types, collectively referred to as the Swaziland Supergroup.
The volcano-sedimentary Onverwacht Group, at the base of the succession, attains a thickness of approximately 15 km and is subdivided into six formations. The lower three formations (Tjakastad Subgroup) consist mainly of mafic and ultramafic volcanic rocks (komatiites, komatiitic basalts, high-magnesium basalts and tholeiitic basalts) that were erupted approximately 3500 million years ago into an ancient oceanic environment (similar to present-day ocean-floor domains). A persistent sedimentary horizon, termed the Middle Marker, then occurs, followed by the three upper formations (Geluk Subgroup) that are made up principally of repeated cycles of volcanic, volcaniclastict and sedimentary rocks. The volcanic rocks (the modem-day equivalents of which are found in oceanic island arcs) consist mainly of tholeiitic basalts and felsic pyroclastic rocks and agglomerates, together with cherts. Lesser amounts ofkomatiite, basaltic komatiite, carbonaceous shale and chert, and siderite-rich carbonate beds are also present.
Some of the sedimentary rocks represent ocean-floor muds and debris deposited between volcanic eruptions. The carbonaceous shales and chert have been found to contain primitive microfossils indicating that early forms of life were in existence as far back in time as 3500 million years ago. A number of layered ultramafic complexes occur as igneous intrusions in the Onverwacht Group. In many of these, chrysotile asbestos has been mined in the past, and it is still being mined at the Msauli Mine southeast of Barberton and at Havelock Mine in northwest Swaziland. Overlying the Onverwacht Group and occupying the central core of the Barberton Greenstone Belt are two distinctly different groups of sedimentary rocks. The oldest is the Fig Tree Group, consisting of shales, greywackes (impure sandstones), banded iron formations, and cherts, as well as subordinate volcanic rock types. Again, some of the cherts and shales are carbonaceous and contain primitive microfossils.
The Fig Tree rocks were deposited in relatively deep water, and the sediments were derived from the erosion of oceanic islands (island arcs), which developed probably as a result of some form of primitive plate tectonic processes. In contrast, the overlying Moodies Group sediments display evidence of having been deposited by fluvial systems into relatively shallow-water continental environments. The rock succession consists mainly of conglomerates, quartzites, sandstones, shales, banded iron formations, and minor volcanic interlayers. Some sediments appear to have been deposited in areas affected by tidal influences (such as occur in river deltas at the edges of continents). The entire Swaziland Supergroup succession has undergone multiple stages of deformation, which produced large- and small-scale folds and faults, the latter providing ideal channel ways for migrating hydrothermal solutions carrying gold and sulphide minerals. Numerous gold workings exist in the area, gold having been discovered in 1882 (alluvial gold in the Jamestown Schist Belt north of Barberton) and in 1884 in quartz veins in the hills around the town of Barberton. Some of the mines founded over 100 years ago (Sheba, New Consort, Fairview) are still being worked to this day.
The deformation that affected the rocks of the Barberton Greenstone Belt was largely the result of the intrusion of numerous granite bodies. Several stages of granite emplacement have been recognized, beginning with trondhjemite and tonalite (mainly sodium-rich granites) intrusions, which range from 3500 to 3200 million years in age. Later, potash-rich granites were intruded and formed large batholiths, some being over 40 km in diameter. These occur north and south of the Barberton Greenstone Belt (and in Swaziland), and were intruded approximately 3100 million years ago. A final stage of granite emplacement followed and ended with the intrusion of smaller- scale, potassium-rich granite (Mpangeni) and syenite (Boesmanskop) plutons ranging from 3100 to 2700 million years in age. All the granites interacted with the earlier-formed greenstones and produced a variety of metavolcanic and metasedimentary rock types, particularly near the granite-greenstone contacts where heat from the intrusions was greatest. Fluids produced by the heating of the greenstones moved through the rocks and along faults and fractures, and were responsible for the localisation of gold mineralisation.
The entire Barberton Greenstone Belt was later covered by rocks of the Transvaal Supergroup (about 2500 million years ago). This resulted in the Barberton rocks being protected until about 50 million years ago when, follow- ing the break-up of Gondwana, the rivers on the east side of southern Africa began to cut back to where the present-day Drakensberg Escarpment is now situated. The Barberton Mountain Land has provided geologists with a unique opportunity to study the early history of the Earth and the evolution of the primitive crust, as well as of early life on our planet. The sites that have been selected here represent but a small number of illustrations portraying the wide-ranging character of one of the world's best-preserved Archaean granite- greenstone terranes.
