Cherts of the Barberton Greenstone Belt Interpreted as Products of Submarine Exhalative Activity

Abstract

Cherts studied from the southern Barberton greenstone belt are identified as replacement products of various pillow lavas, pyroclastic deposits, and sedimentary deposits. Silica-filled fracture systems within the cherts have been found to be associated with this silicification process. These systems include: (1) silica-filled fractures; (2) silica vein stockworks; (3) silica-cemented breccias; and (4) pseudoconglomerate bodies. Some of the silica-filled fracture systems can be shown to have formed soon after sedimentation. Silica vein stockworks can be seen to develop laterally into silica-cemented breccias, the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate. The cherts are interpreted as silicified hemi-pelagic sediments that overlie pillow lavas, some of which are now also cherts. Cells of convective sea water interacted with oceanic crust to produce silica-rich fluids. These fluids silicified lava and sediments on re-entering the near surface environment and produced silica veins and breccias. Silica deposition was mainly confined to the subsurface, as in Phanerozoic exhalative systems. The relatively large volumes and widespread occurrence of replacement cherts in some greenstone belts might result from differing Archean tectonic conditions or crustal compositions. Features used previously to identify shallow water conditions in some cherts of both Phanerozoic and Precambrian age may be products of convective systems of the type described here. The features described in this paper also have implications for greenstone belt mineralization and for the oxygen isotopic composition of Archean cherts.