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Ironstone bodies of the Barberton greenstone belt, South Africa: Products of a Cenozoic hydrological system, not Archean hydrothermal vents!

Donald R. Lowe^,1 and Gary R. Byerly²

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA
² Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803-4101, USA

Irregular bodies of goethite and hematite, termed ironstone pods, in the Barberton greenstone belt, South Africa, have been previously interpreted as the Earth's most ancient submarine hydrothermal vent deposits and have yielded putative evidence about Archean hydrothermal systems, ocean composition and temperature, and early life. This report summarizes geologic, sedimentological, and petrographic evidence from three widely separated areas showing that the ironstone was deposited on and directly below the modern ground surface by active groundwater and spring systems, probably during periods of higher rainfall in the Pleistocene. The deposits include a recently active spring terrace and outflow system containing iron oxide–encrusted microbial filaments, shallow subsurface groundwater areas showing ironstone deposited between chert blocks and replacing chert, degraded spring and outflow deposits, and goethite-cemented Pleistocene landslide breccias and regolith. The predominance of goethite, a thermally unstable iron oxide; widely developed slope-parallel stratification; abundant primary open cavities, some with vertical goethite dripstone; and the lack of structural deformation collectively indicate that these bodies formed during relatively recent time. Veins and displaced masses of coarsely crystalline quartz show corrosion and represent relict Archean vein and cavity-fill quartz after iron oxide replacement of surrounding chert. Iron was sourced by Archean sideritic sedimentary units that hold up high ridges on which the ironstone bodies occur. Siderite was dissolved by circulating groundwater, and the iron deposited as oxides within the shallow subsurface groundwater system and around springs where the Fe-rich water flowed onto the surface. These deposits represent a remarkable iron oxide–depositing Quaternary hydrologic system but provide no information about conditions or life on the early Earth.

Key Words: Archean • ironstone • South Africa • Barberton greenstone belt • sedimentology • early life • springs