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Siliciclastic Associated Banded Iron Formation from the 3.2Ga Moodies Group, Barberton Greenstone Belt, South Africa

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Siliciclastic Associated Banded Iron Formation from the 3.2Ga Moodies Group, Barberton Greenstone Belt, South Africa Precambrian Research 226 (2013) 116–124 Contents lists available at SciVerse ScienceDirect Precambrian Research journa l homepage: www.elsevier.com/locate/precamres Siliciclastic associated banded iron formation from the 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa a,∗ b c Tomaso R.R. Bontognali , Woodward W. Fischer , Karl B. Föllmi a ETH-Zurich, Geological Institute, Zurich, Switzerland b California Institute of Technology, Geological and Planetary Sciences, Pasadena, CA, United States c University of Lausanne, Institute of Earth Sciences, Lausanne, Switzerland a r t i c l e i n f o a b s t r a c t Article history: Most models proposed for banded iron formation (BIF) deposition are based on observations of well- Received 29 June 2012 preserved Late Archean and Paleoproterozoic BIF. Efforts to push the understanding gained from younger Received in revised form successions deeper in time have been hampered by the high metamorphic grades that characterize Early 30 November 2012 Archean BIF. This study focuses on a unique occurrence of well-preserved and contextualized BIF from Accepted 19 December 2012 the Early Archean (∼3.2 Ga) Moodies Group, in the Barberton Greenstone Belt, South Africa. The Moodies Available online xxx BIF occurs thinly interbedded with fine-grained and cross-stratified sandstones, indicating deposition during times of decreased clastic sediment supply. In the Moodies BIF, chert is present as concretions, Keywords: and is never observed in direct contact with the siliciclastic material but is always associated with iron Banded iron formation Chert minerals. This observation suggests that the processes leading to the formation of both chert and iron Iron cycle minerals were coupled. The dominant iron-rich minerals within unweathered Moodies BIF are hematite Early life and magnetite, with less common occurrences of Fe–carbonate phases (mainly ankerite). Petrographic Barberton Greenstone Belt textures reveal that hematite constitutes an early mineral phase, while magnetite and ankerite display textures indicative of a late diagenetic or metamorphic origin. Carbonaceous particles are present in close association with the magnetite crystals. These C-bearing phases may be the preserved organic matter of microbes involved in the production of the ferric iron precursor phases, though it is difficult to rule out an origin from abiotic processes involving thermal decomposition of siderite to magnetite and organic carbon compounds. Nonetheless, the range of textures, mineralogies, and valence states supports the view that diagenetically-stabilized BIF mineralogies reflect the interaction of ferric iron phases with reducing fluids during diagenesis. These patterns are commonly observed in younger Archean and Paleoproterozoic iron formations, and imply a continuity of processes operating in the iron and silica cycles across both a range of paleoenvironments and long intervals of Archean time. © 2013 Elsevier B.V. All rights reserved. 1. Introduction able to link their occurrences to changes in fluid Earth redox chem- istry and geobiology. Banded iron formations (BIF) are chemical sedimentary rocks It is commonly thought that, during times of BIF formation, characterized by alternating layers of Fe-rich minerals and chert ocean basins must have been anoxic and sulfur poor (at least at (microcrystalline quartz) (James, 1954). Despite years of assiduous depth) in order to allow for the transport and accumulation of research, several aspects concerning their genesis remain contro- dissolved Fe(II); and that Fe was subsequently concentrated in versial (Bekker et al., 2010; Beukes and Gutzmer, 2008; Clout and the sediments by oxidation, hydration, and precipitation (Canfield, Simonson, 2005; Klein, 2005; Trendall, 2002). BIF are widespread 1998; Cloud, 1968; Drever, 1974; Holland, 1973; Klein, 2005). Fe(II) in Archean and Paleoproterozoic sedimentary basins, but similar may have been oxidized in the water column forming a hydrous facies is not observed to form in any modern geological setting. ferric oxide phase as a precursor to hematite (Bekker et al., 2010; BIF clearly result from a suite of non-uniform processes. Secular Lepp and Goldich, 1964). Oxidation may have occurred either in changes in their accumulation and sedimentary style continue to the presence of O2 produced by photosynthetic organisms or in the motivate efforts to understand their origins, with the goal of being absence of molecular oxygen, through abiotic photochemical reac- tions (Cairns-Smith, 1978) or through anoxygenic photosynthesis with iron as a primary electron donor (Widdel et al., 1993). Alterna- ∗ tively, direct precipitation from anoxic seawater may have formed Corresponding author. E-mail address: [email protected] (T.R.R. Bontognali). siderite and mixed valence iron–silicate phases. 0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2012.12.003 T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124 117 Although now common, the hypotheses that microbes were suggesting that a global theory explaining all BIF occurrences may involved in the primary oxidation of Fe(II) to Fe(III) (via oxygenic not exist. To answer the question of whether current models can or anoxygenic photosynthesis, or via chemoautotrophy at low oxy- be extrapolated back in time to explain these Early Archean BIF it is gen concentrations (Brown et al., 1995; Cloud, 1973; Harder, 1919; important to identify well-preserved examples of earlier Archean Hartman, 1984; Kappler et al., 2005; Konhauser et al., 2002; Perry BIF, which can be compared in terms of their sedimentary geology, et al., 1973; Posth et al., 2008)) contrast with the general lack of geochemistry, and petrography, with their younger equivalents. microbial biomass (e.g. accumulation of organic carbon, microfos- This study focuses on the BIF from the ∼3.2 Ga Moodies Group sils or biomarkers) within BIF (Beukes and Klein, 1992; Klein and from the Barberton Greenstone Belt, South Africa. These sedimen- Beukes, 1989). A reasonable explanation for this discordance is pro- tary rocks have been noted (Eriksson, 1977, 1983; Heubeck and vided by diagenetic processes that respired much of the organic Lowe, 1999), but not studied in detail because the few outcrops carbon back to dissolved inorganic carbon (DIC) during interactions where they are exposed at the surface are strongly affected by surfi- with ferric oxide or mixed valence phases (Baur et al., 1985; Fischer cial weathering obscuring the original mineralogies. For this study, and Knoll, 2009; Konhauser et al., 2005; Perry et al., 1973; Walker, we were able to collect a suite of samples directly from the under- 1984). This scenario is consistent with the well-documented pres- ground tunnels of an active gold mine. Coupled to observations ence, in many BIF, of diagenetic iron-bearing carbonates (siderite from an outcrop located at the surface, these materials provide a 13 and ankerite) characterized by a C-depleted isotopic composition unique window into the processes responsible for the deposition (Baur et al., 1985; Becker and Clayton, 1972; Beukes et al., 1990; of BIF in Early Archean time. Fischer and Knoll, 2009; Goodwin et al., 1976; Kaufman et al., 1990; Perry et al., 1973). 2. Geological setting The origin of chert – the most abundant phase in BIF –inthese rocks is no less enigmatic than that of iron. In the absence of silicify- The Barberton Greenstone Belt (BGB) is situated in the ing organisms, Precambrian oceans were likely close to saturation central-east part of South Africa, along the border between the with respect to amorphous silica and evaporation may have pro- Mpumalanga Province and Swaziland (Fig. 1). The BGB contains a vided an important driver for the precipitation of chert (Siever, diverse suite of sedimentary strata deposited in one of the oldest 1992; Trendall and Blockley, 1970). However, this interpretation recognized foreland basins; despite their early Archean age, regions does not explain why chert is common in BIF, which are com- of the BGB have remarkably good preservation and provide a unique monly manifest as a deep-water facies. One hypothesis to explain and rich source of insight about sedimentary processes and envi- the transport and precipitation of silica in deep waters, as well ronments on the early Earth (Byerly et al., 1986; Eriksson, 1977; as its close association with iron minerals, has been proposed by Eriksson and Simpson, 2000; Javaux et al., 2010; Noffke et al., 2006; Fischer and Knoll (2009). This mechanism is based on the tendency Simpson et al., 2012). The successions of rocks that comprise the of ferric hydroxides to bind and shuttle silica to basinal waters and BGB were subdivided into three different groups (Hall, 1918; Lowe sediments. Fe(III) respiration taking place within sediments would et al., 1999) (Fig. 2). The Onverwacht Group (3.5–3.3 Ga) is pre- then return the majority of iron to the water column, while silica, dominantly composed of mafic and ultramafic volcanic rocks but it which does not undergo reductive dissolution, remains reactive, also includes some thin cherty units thought to be sedimentary in is concentrated in pore waters, and is ultimately precipitated as origin (Lowe et al., 1999). The overlying Fig Tree Group (3.3–3.2 Ga) diagenetic mineral phases. consists mainly of fine-grained sedimentary rocks including BIF, Finally, not only is the origin of the BIF mineralogy contro- carbonaceous
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