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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 and Gary R. Byerly
Ironstone bodies of the Barberton greenstone belt, South Africa; products of a Cenozoic hydrological system, not Archean hydrothermal vents!
Geological Society of America Bulletin (January 2007) 119 (1-2): 65-87

Abstract

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.


ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 119
Serial Issue: 1-2
Title: Ironstone bodies of the Barberton greenstone belt, South Africa; products of a Cenozoic hydrological system, not Archean hydrothermal vents!
Affiliation: Stanford University, Department of Geological and Environmental Sciences, Stanford, CA, United States
Pages: 65-87
Published: 200701
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 50
Accession Number: 2007-014826
Categories: Sedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. col., 1 table, geol. sketch maps
S26°00'00" - S25°49'60", E30°45'00" - E31°04'60"
Secondary Affiliation: Louisiana State University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200705
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