Ironstone pods in the Archean Barberton greenstone belt, South Africa: Earth's oldest seafloor hydrothermal vents reinterpreted as Quaternary subaerial springs

Abstract

Bodies of iron oxide to 200 m across in the 3.5–3.2 Ga Barberton greenstone belt, South Africa, have been interpreted as deposits of Archean seafloor hydrothermal vents and have provided what are arguably key observations about surface environments on early Earth. These bodies, termed “ironstone pods,” have yielded what are putatively the oldest-known complex organic compounds and have been used to estimate Archean surface temperature, ocean depth and volume, and seawater composition and to deduce relationships between hydrothermal activity and seafloor sedimentation. However, the pods are composed largely of goethite (a thermally unstable hydrated iron oxide mineral), show abundant still-open primary porosity, are undeformed in a terrane of highly deformed rocks, and show bedding and cavity-filling dripstone indicating formation with their present orientation. They are here interpreted to have formed as spring and shallow subsurface deposits of young (Quaternary) groundwater and/or low-temperature hydrothermal systems. Iron was derived by the dissolution of siderite in surrounding Archean sedimentary rocks. Along pod margins, goethite locally replaces Archean chert and silicified ultramafic rock. However, coarse vein and cavity-fill quartz was less susceptible to replacement and remains as disrupted masses and rare remnant veins in the ironstone. This quartz is of Archean age but contains no more information about the Archean surface environment than other late hydrothermal veins throughout the Barberton greenstone belt rocks. The presence of a well-preserved modern iron oxide spring terrace confirms that these are deposits of young subaerial springs and contain no record of Archean life or environments. They do, however, provide a unique view of a remarkable young iron-depositing hydrologic system.