Crustal fracturing, unconformities, and barite deposition, 3.26-3.23 Ga, Barberton Greenstone Belt, South Africa

Small sedimentary barite deposits are widespread in the 3.258-3.225 Ga Fig Tree Group, Barberton Greenstone Belt (BGB), South Africa. This report explores the stratigraphy of these units, the types of barite present, their depositional settings, and the controls on sedimentation in the Barite Valley area. Five morphological types of barite are recognized: masses of irregularly layered bladed barite termed barite plugs, mounded barite, barite splays, flat-layered bladed barite, and barite sand. In all areas studied, barite deposition was closely associated with active faults and fractures in the underlying rock sequences that are inferred to have been conduits by which barium-bearing fluids made their way to the surface. The barite deposits are also associated with unconformities below, within, and/or above the barite layers. Associated sedimentary rocks show evidence of deposition within intertidal, fan delta, and shallow water to subaerial settings. Previous sulfur isotopic studies have shown that the sulfate originated by photolysis in the atmosphere. We infer that the barite was deposited where barium-bearing, shallow subsurface waters vented to the land surface via mostly small, cool springs and interacted directly with sulfate-bearing meteoric waters, probably often rainwater, triggering barite precipitation. Plug barite marks spring vents, mounds developed in pools, and the outflow systems were characterized by layered bladed barite with increasing barite sand toward more distal settings. Associated thin jasper beds are thought to have formed by oxidation of sedimentary pyrite and siderite by meteoric waters carrying photolysis-produced oxidants, probably H2O2 and perhaps O-2. The Barite Valley barite deposits do not appear to have developed in open marine or evaporitic settings and their geochemistry may carry little information on the composition of Archean sea water or nature of the underlying crust.