Archean metallogeny in southern Africa

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The Archean granite-greenstone terrain of southern Africa possesses a number of unique characteristics. Not only is the shield area one of the smallest in the world but it is also one of the oldest crustal fragments still preserved. Estimates of lithic proportions show that, in the southern African Archean, the granitic rocks outnumber those of the volcanic belts (greenstone belts) by approximately 6:1--in marked contrast to the situation on the Canadian Shield where granite: greenstone ratios have been estimated to be approximately 1:2.In order to establish further details of the lithological proportions, the southern African Archean greenstone belts are described in terms of a three-stage stratigraphic model involving a Lower Ultramafic Unit at the base, followed by a Mafic-to-Felsic Unit and a terminal Sedimentary Unit. Planimeter studies demonstrate that on average the southern African greenstone belts possess a relatively high proportion of ultramafic and mafic volcanic and plutonic rock types and a correspondingly lower proportion of rocks of intermediate to felsic composition. The relative abundances of the various volcanic components of the lower two stratigraphic units are tabulated, and the lithic proportions are compared with similar data available from greenstone belts of the Superior Province in Canada. Geochemical data illustrating the nature of Archean volcanism in southern Africa and elsewhere are made available for purposes of comparison.The distribution patterns of a wide variety of Archean mineralization types, including the deposits associated with the ancient granites and pegmatites, are illustrated by means of mineral distribution maps. A close genetic relationship exists between the various lithological subdivisions of the Archean greenstone belts and the mineralization. Important deposits of chrome, nickel-copper, gold, and chrysotile asbestos are associated with the ultramafic-mafic component of the volcanic pile, whereas antimony, gold, and lesser amounts of mercury, barite, corundum, copper-lead-zinc, and massive sulfide iron-formations occur throughout the greenstone piles and commonly occur as chemical sediments terminating cycles of volcanism. These well-banded rocks, which probably formed by fumarolic-exhalative processes, also contain significant stratiform gold mineralization.The Archean mineralization of southern Africa is briefly compared and contrasted with that of the Canadian Shield and elsewhere. It is concluded that the disparity of lithological types and proportions between the Canadian and southern African shields is, together with differences in age, the underlying cause of the variability in the mineralization thus far encountered in the two regions.