THE 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 Archaean greenstone 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.
I
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.
