Archaean flow-top alteration zones formed initially in a low-temperature sulphate-rich environment

Abstract

The 3,500–3,400 Myr Onverwacht Group in the southern part of the Barberton Greenstone Belt, South Africa, includes up to 12 km of komatiitic and tholeiitic lavas containing minor sedimentary and felsic volcanic units¹. Komatiitic lavas showing spinifex textures, chrome spinels and high Ni and Cr contents are common throughout the sequence². The rocks have been subject to polyphase Archaean deformation and low-grade metasomatism^2–4. At several positions within the Onverwacht Group (Fig. 1), the tops of komatiitic lavas underlying chert beds are marked regionally by 1–50-m thick zones of silicification, carbonatization and K₂O enrichment (Fig. 2). Because these zones have been variously interpreted to represent the felsic tops of mafic-to-felsic volcanic cycles, weathering horizons and shear zones, their origin is germane to any interpretation of the stratigraphy, structure and evolution of the Barberton belt. Here we present evidence that they formed inlarge part by low-temperature, near-surface alteration of komatiitic lavas during intervals of volcanic quiescence. They do not mark fault zones and cannot be used to subdivide the greenstone belt sequence into a series of fault-bounded structural slices.