ACCRETIONARY LAPILLI ASSOCIATED WITH ARCHEAN BANDED IRON FORMATIONS OF THE KRAAIPAN-GROUP, AMALIA GREENSTONE-BELT, SOUTH-AFRICA

The Kraaipan Group in the Amalia greenstone belt, South Africa, consists of an Archaean volcano-sedimentary assemblage of mafic metavolcanic rocks and banded iron formations, together with subordinate greywacke. slate and carbonate rocks. The sequence is deformed and intruded by a variety of granites and gneisses. Interlayered with the volcano-sedimentary rocks, in close proximity to a small gold prospect, are pyroclastic rocks in which are described well-developed ''coated'' or ''rim-type'' accretionary lapilli of a variety hitherto rarely, if ever, recorded in association with oxide-facies banded iron formations. The accretionary lapilli are discussed in terms of their significance and as palaeoenvironmental indicators. It is concluded that the accretionary lapilli probably developed in a proximal setting with respect to an ancient eruptive centre and that the lapilli themselves do not constitute exceptional indicators of sedimentary environments but are useful mainly as time or stratigraphic markers. Their association with banded iron formations suggests the accretionary lapilli here described were deposited from volcanic centres close to some shallow-marine or barred lagoonal basin, the latter possibly analogous to a playa-lake complex developed in a primitive inter-arc or back-arc basin setting in existence during the evolution of the Kraaipan volcano-sedimentary assemblage found in the Amalia greenstone belt.