Gold on the Kaapvaal Craton, outside the Witwatersrand Basin, South Africa

Abstract

Gold occurs in a variety of ancient geological environments in South Africa, the most well-known being the unique Witwatersrand Basin in the central part of the Kaapvaal Craton. Outside of the Witwatersrand Basin significant quantities of gold have been recovered from Archaean greenstone belts of the 3500 to 2700 Ma Kaapvaal Craton as well as from various occurrences in the 2600 to 2200 Ma Transvaal Supergroup. Very little gold has been found in younger formations. Archaean greenstone belts in South Africa are characterised by substantial sequences of ultramafic (komatiitic) volcanic rocks generally overlain by mafic volcanic successions which, in turn, are overlain in some cases by greywacke-shale sequences and younger arenaceous lithologies. Highly sheared and carbonate altered ultramafic zones, together with the immediately surrounding country rock, host some of the most important gold deposits on the Kaapvaal Craton. In the northern part of the Barberton Greenstone Belt, shearing tends to be most intense along the contact of the ultramafic schists of the Zwartkoppie Formation and the overlying greywacke-shales, the contact being marked by the presence of a chert “bar” or banded iron formation (BIF) horizon. Gold mineralisation occurs below, above and/or within the more competent and fractured chert/BIF marker horizon. The locus of gold mineralisation is typically controlled by the rheological characteristics of the wallrocks in the proximity of the shear systems, e.g. the ductile greywacke-shale strata are tightly folded with sub-parallel mineralised shear zones, while competent arenaceous rocks have yielded in a brittle manner resulting in high-angle mineralised fractures. Extensive carbonate alteration in ultramafic rocks is often present along, and in the vicinity of, shear zones where potassium alteration results in a vivid green coloration due to the formation of fuchsite (Cr-mica) and where zones of silicification and quartz veining are closely associated with gold, arsenic and antimony mineralisation. Where mafic rocks act as more competent and fractured hosts to mineralisation, carbonate alteration results in the breakdown of ferromagnesian minerals and the liberation of considerable quantities of silica, manifested as silicification and as mineralised quartz veins. The breakdown of mafic minerals results in “bleaching” of the rocks as carbonate minerals replace ferromagnesian minerals, but may also result in the development of green, brown or black schists depending on the composition of the host rocks. One or more of the above alteration rock types and mineralisation styles is present in virtually all of the greenstone-hosted gold deposits of the Kaapvaal Craton. Wherever available, geochemical data is consistent with alteration fluids of relatively uniform composition but the style of the resultant mineralisation is ultimately dependent on the nature of the host rock. Greenstone gold deposits are irregularly distributed, both in terms of their size and distribution. Only about 1% of greenstone deposits can be regarded as successful mines, while the vast majority remain as showings. Geographically, these gold deposits are preferentially located along litho-structural features where both fluid flow and wallrock conditions were suitable for mineralisation. These criteria are restricted to major structural nodes, significant lineaments, or lithological units, with the result that the stronger mineralising systems tend to be concentrated in finite gold “camps” which host the larger deposits and numerous styles of mineralisation, e.g., the Sheba-Fairview Complex on the refolded arcuate Eureka and Ulundi synclines. Gold hosted in BIFs is a widespread association, but rarely economic except where the nature of the mineralisation allows for economic extraction, e.g., in the Kraaipan Greenstone Belt, where the volume of BIF is large enough to contain the mineralised fracture system. Gold in the Paleoproterozoic Transvaal Supergroup occurs in the lowermost Black Reef Formation quartzites as detrital accumulations in conglomerate channels, but more abundantly as epithermal veins in the dolomite of the overlying Malmani Subgroup and shale of the Pretoria Group as vertical cross-cutting veins and sub-horizontal reefs emplaced along parting planes. The quartz-carbonate-sulphide veins differ from Archaean ores in that gold rarely impregnates the wallrocks; they contain significant carbon which impedes gold recovery, and they contain silver, copper, arsenic, bismuth, lead and zinc in addition to gold.