BARBERTON GREENSTONE BELT
by C.R Anhaeusser
Economic Geology Research Unit
Deportment of Geology
University of the Witwatersrand, Johannesburg
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.
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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 granitegreenstone 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, following 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.
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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 granitegreenstone terranes.
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