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An iron shuttle for deepwater silica in Late Archean and early Paleoproterozoic iron formation

Fischer, Woodward W. and Knoll, Andrew H. (2009) An iron shuttle for deepwater silica in Late Archean and early Paleoproterozoic iron formation. Geological Society of America Bulletin, 121 (1-2). pp. 222-235. ISSN 0016-7606.

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Iron formations are typically thinly bedded or laminated sedimentary rocks containing 15% or more of iron and a large proportion of silica (commonly > 40%). In the ca. 2590-2460 Ma Campbellrand-Kuruman Complex, Transvaal Supergroup, South Africa, iron formation occurs as a sediment-starved deepwater facies distal to carbonates and shales. Iron minerals, primarily siderite, define the lamination. The silica primarily occurs as thin beds and nodules of diagenetic chert (now microcrystalline quartz), filling pore space and replacing iron formation minerals and co-occurring deepwater lithologies. Mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production, anoxygenic photosynthesis, abiotic photochemistry, and chemoautotrophy using Fe(II) as an electron donor. The origin and mechanism of silica precipitation in these deposits have received less attention. Here we present a conceptual model of iron formation that offers insight into the deposition of silica. The model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides. Soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides, which precipitate. Fe(OH)_3 binds silica and sinks from the surface ocean along with organic matter, shuttling silica to basinal waters and sediments. Fe(III) respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters, driving precipitation of siderite from Fe2+ and respiration-influenced CO2. Silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases. This model explains many of the mineralogical, textural, and environmental features of Late Archean and earliest Paleo-proterozoic iron formation.

Item Type:Article
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URLURL TypeDescription
Fischer, Woodward W.0000-0002-8836-3054
Knoll, Andrew H.0000-0003-1308-8585
Additional Information:© 2009 Geological Society of America. Published: JAN-FEB 2009. We thank John Higgins, Paul Hoffman, Ann Pearson, and Ben Kotrc for helpful discussions, and the Agouron Institute for funding. This work benefited by insightful reviews from Bruce Simonson and Bryan Krapež. Half cores of GKP01, GKF01, and BH1-Sacha are currently housed at the Council for Geoscience in Tshwane (formerly Pretoria), Gauteng Province, South Africa. Cores GKP01 and GKF01 were collected as part of the Agouron Drilling Project (
Funding AgencyGrant Number
Agouron InstituteGKP01. GKF01
Subject Keywords:South Africa; iron cycle; banded iron formation; chert; Transvaal; Kuruman
Issue or Number:1-2
Record Number:CaltechAUTHORS:FISgsab09
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13507
Deposited By: Ruth Sustaita
Deposited On:16 Jun 2009 18:02
Last Modified:09 Mar 2020 13:18

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