THE EARLY ARCHEAN NONDWENI GREENSTONE-BELT, SOUTHERN KAAPVAAL CRATON,SOUTH-AFRICA .1. STRATIGRAPHY, SEDIMENTOLOGY, MINERALIZATION AND DEPOSITIONAL ENVIRONMENT

The Nondweni greenstone belt is an exceptionally well preserved approx imately 3.4 Ga succession of predominantly volcanic rocks near the sou thern margin of the Kaapvaal Craton. Nondweni Group rocks show both si milarities 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 pyroxen e type. The characteristics of the lavas and magma genesis are discuss ed in Part II of this series of papers. Three Formations are recognize d in the Nondweni Group: the lowermost Magongolozi Formation comprises basaltic andesites, komatiitic andesites and basalts with intercalate d coarser-grained pyroxenites and gabbros. The Toggekry Formation is a felsic unit of quartz-sericite schists with small Cu-Zn massive sulph ide ore bodies of volcanogenic origin. The uppermost Witkop Formation comprises mainly basalts, komatiites and komatiitic basalts intercalat ed with chert layers and capped by a clastic sedimentary succession. T he total exposed thickness of the Nondweni Group is 16,500 m is the re sult of duplication of stratigraphy and tectonic thickening during ear ly D1 thrusting in the bedding plane of much thinner lithological unit s. Tuffaceous sediments comprise a relatively minor component of the s tratigraphy but are indicative of depositional environment. Banded che rt layers are largely derived from accumulations of accretionary lapil li tuffs, although a diverse origin is indicated for some varieties. A t the base of the Witkop Formation a number of undeformed, banded, coa rse-grained graphitic quartzite bodies, entirely devoid of sedimentary structures, are considered to have originated as hydrothermal deposit s resulting from fumarolic activity. Selective brecciation of layers r esulted from explosive release of fluids and gases. The textures of th ese bodies are in marked contrast to the highly rounded pseudo-conglom erates from the clastic sedimentary succession at the top of the Witko p Formation. This approximately 500 m thick succession passes upwards through reworked tuffs and alluvial fan deposits, into a zone of evapo rites and stromatolites. Swallow-tail twin crystals of barite, and an evaluation of crystal interfacial angles, are indicative of replacemen t of primary evaporative gypsum. Most primary features of the Nondweni greenstone belt suggest a shallow-water or subaerial depositional env ironment, and together with characteristics of the magma composition, indicate a tectonic setting in a subduction-related continental margin setting.