The early Archaean Nondweni greenstone belt, southern Kaapvaal Craton, South Africa, Part I. Stratigraphy, sedimentology, mineralization and depositional environment

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The Nondweni greenstone belt is an exceptionally well preserved ∼3.4 Ga succession of predominantly volcanic rocks near the southern margin of the Kaapvaal Craton. Nondweni Group rocks show both similarities and differences to the Barberton Sequence, of similar age, 300 km to the north. In contrast with the latter, none of the Nondweni lavas exceed 23% MgO and spinifex textures are exclusively of pyroxene type. The characteristics of the lavas and magma genesis are discussed in Part II of this series of papers. Three Formations are recognized in the Nondweni Group: the lowermost Mogongolozi Formation comprises basaltic andesites, komatiitic andesites and basalts with intercalated coarser-grained pyroxenites and gabbros. The Toggekry Formation is a felsic unit of quartz-sericite schists with small CuZn massive sulphide ore bodies of volcanogenic origin. The uppermost Witkop Formation comprises mainly basalts, komatiites and komatiitic basalts intercalated with chert layers and capped by a clastic sedimentary succession. The total exposed thickness of the Nondweni Group is 16,500 m is the result of duplication of stratigraphy and tectonic thickening during early D1 thrusting in the bedding plane of much thinner lithological units. Tuffaceous sediments comprise a relatively minor component of the stratigraphy but are indicative of depositional environment. Banded chert layers are largely derived from accumulations of accretionary lapilli tuffs, although a diverse origin is indicated for some varieties. At the base of the Witkop Formation a number of undeformed, banded, coarse-grained graphitic quartzite bodies, entirely devoid of sedimentary structures, are considered to have originated as hydrothermal deposits resulting from fumarolic activity. Selective brecciation of layers resulted from explosive release of fluids and gases. The textures of these bodies are in marked contrast to the highly rounded pseudo-conglomerates from the clastic sedimentary succession at the top of the Witkop Formation. This ∼500 m thick succession passes upwards through reworked tuffs and alluvial fan deposits, into a zone of evaporites and stromatolites. Swallow-tail twin crystals of barite, and an evaluation of crystal interfacial angles, are indicative f replacement of primary evaporative gypsum. Most primary features of the Nondweni greenstone belt suggest a shallow-water or subaerial depositional environment, and together with characteristics of the magma composition, indicate a tectonic setting in a subduction-related continental margin setting.