Geology

The Archaean Period Rock Formation at 14 streams Barberton

The Archaean Period

(3500-2500 million years ago). Greenstone Belts: The Greenstone Belts get their name from the presence of green minerals such as the chromium muscovite “fuchsite”, green chlorite, actinolite, serpentine, epidote and amphiboles. The rocks occur as belts of deformed volcanic and sedimentary strata. They include the areas near to Barberton, with its gold and asbestos deposits. These ancient rocks are among the oldest in South Africa with the Barberton Greenstone belt being almost 3500 million years old. Geology: The Barberton Mountain Land is a rugged tract of country which straddles the border between the Mpumalanga Province and Swaziland. The rocks of the Barberton greenstone belt are assigned to the Barberton sequence and comprise a variety of early Archaean volcanic, igneous and sedimentary lithologies. These are surrounded and intruded by younger granitic plutons of varying compositions and age. This complex and ancient geological terrain has been subject to extensive geological research. In general, the overall stratigraphy consists of the basal Onverwacht group which is an ultramafic and mafic volcanic sequence which is considered to be the source of gold in the region. Komatiites, which are magnesium rich ultramafic and mafic lavas, were discovered and first described from this area. The upper portions of the Onverwacht Group are more felsic and also contain volcano-sedimentary strata. The overlying Fig Tree group consists of fine-grained sedimentary sequences (greywacke, shale, chert, and balanced iron-formation) and minor interbedded lava and tuff. The uppermost Moodies Group is a coarser sequence of conglomerate, quartzite and shale together with minor volcanic horizons, jaspilite and banded iron-formation. This entire sequence has been complexly deformed, producing a number of tight, steeply dipping synclinal folds, in places overturned, and with fold limbs separated by major strike faults. The oldest rocks in the Onverwacht succession are approximately 3490 million years old. Mineralisation is notably less abundant in the central regions of the Mountain Land and this is due to two main reasons: There is a general absence of Onverwacht Group rocks at the surface in this region and these are considered to be the primary source of the gold and sulphide mineralisation and the effect of intrusive granites i.e., thermal mechanical conditions which remobilised the gold, is most prominent along the exposed margins of the greenstone belt.
Information obtained from Minerals of South Africa by Bruce Cairncross & Roger Dixon.

GEOTRAIL

Geotrail panel at the Lebombo Overview stop of the Geotrail, the R40 between Barberton and Bulembu. The panel is one of a series explaining the natural rock formations and types in the area.