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

Home , Moodies Group, Onverwacht Group

Precambrian Research 226 (2013) 116–124

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Precambrian Research

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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

ETH-Zurich, Geological Institute, Zurich, Switzerland

California Institute of Technology, Geological and Planetary Sciences, Pasadena, CA, United States

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.

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.

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

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 shales, siltstones, sandstones, and cherts. And ﬁnally

versial, but also the processes resulting in the interlamination

the Moodies Group (3.2–3.1 Ga) includes alluvial to shallow-marine

between the iron-rich and chert-rich beds producing three differ-

sandstones, mudstones, minor conglomerates, and discrete inter-

ent scales of banding – microbands (≤1 mm), mesobands (∼1 mm

calations of BIF (Anhaeusser, 1973; Eriksson, 1977, 1979, 1983;

to 10 cm) and macrobands (≥1 m) (Trendall and Blockley, 1970).

Heubeck and Lowe, 1994, 1999), on which this study focused. The

Proposed explanations include temperature variations (Posth et al.,

metamorphic grade in the Moodies Group in the study locality

2008), microbial blooms (Trendall and Blockley, 1970), episodic

ocean mixing (Hamade et al., 2003), deposition by density currents

(Krapezˇ et al., 2003), and internal dynamics of the geochemical

system (Wang et al., 2009).

The abovementioned concepts, models, and hypotheses were

primarily developed from observations of Late Archean and Paleo-

proterozoic BIF, including the spectacular craton-wide occurrences

in the Hamersley Group of Western Australia, the Transvaal Super-

group in South Africa, and in the Lake Superior Region in the USA Barberton

(Bekker et al., 2010). Many of these successions were only mildly

affected by metamorphism and deformation, and these deposits

offer good sedimentological context into their paleodepositional Study Area

settings and appreciable textural preservation such that informa-

tion regarding their petrogenesis can be obtained. However, the

extent to which models proposed for these younger successions can

be tested and applied for interpreting BIF deposited during Early Moodies Group

Archean time remains unclear. Many of these deposits are thin com- Onverwacht and

ponents of severely deformed and metamorphosed (to greenschist Fig Tree Group

and granulite grade) Greenstone belt successions (Bekker et al., South Africa

2010). Indeed, it is challenging to ascertain whether some of the 30 Km

oldest iron-rich metamorphic rocks even were once BIF, or even if

they had a sedimentary origin (Dauphas et al., 2007; Eiler, 2007).

Fig. 1. Study area of the 3.2 Ga Moodies Group, Barberton Greenstone Belt, South

Moreover, younger and well-studied BIF show local differences in Africa.

sedimentological textures and mineralogies (Bekker et al., 2010), Modiﬁed from Heubeck and Lowe (1994).

118 T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124

(SEM) analyses were performed with a Philips XL-30 FEG equipped

with an EDAX energy dispersive X-ray spectrometer (EDX). Images

and EDX analysis were obtained with a backscatter detector, an

accelerating voltage of 25 kV, and a working distance of 10 mm.

The thin sections were coated with 7 nm of Au prior to analysis to

ensure a proper conducting sample surface.

4. Results

4.1. Stratigraphy and sedimentology

The stratigraphic section through the Moodies Group exposed

in the Moodies Hills Block (Heubeck and Lowe, 1994) in out-

crop along Ameide Road contains a BIF-rich interval within a

succession of shallow marine sandstones (Fig. 3). The section is

62 m thick and is composed of ﬁve different lithofacies, includ-

ing: (1) siliciclastic beds, (2) cm-scale packages of closely packed

sub-mm scale laminations of iron minerals, (3) chert beds and

concretions, (4) non-laminated sandstone, and (5) volcanic tuff.

Fine-grained sandstones, showing common small-scale parallel or

wavy laminations with occasional trough cross-stratiﬁcation dom-

inate the base of the section. Continuing upward, the ﬁrst BIF

iron-rich packages begin to appear, interbedded with cm-scale

wavy-laminated sandstone beds. Initially, these iron-rich pack-

ages are not associated with conspicuous chert. The frequency of

iron packages (relative to sandstone interbeds) gradually increases

relative to the amount of accumulated siliciclastic sediment and

Fig. 2. General stratigraphy of the Barbertone Greenstone Belt within the Moodies the ﬁrst chert concretions appear. Interestingly, chert beds and

Hills Block.

concretions are always preceded and followed by layers of iron

Modiﬁed from Lowe et al. (1999).

minerals and do not occur in direct contact with the siliciclastic

beds. The chert concretions are often characterized by lenticular

shapes and their thickness is laterally heterogeneous – textures

is constrained to lower greenschist facies (Heubeck and Lowe,

◦

seen in cherts of all ages and commonly in younger iron for-

1999), with estimated maximum burial temperatures of ∼220 C

mation deposits (Beukes, 1984; Fischer and Knoll, 2009; Krapezˇ

(De Ronde et al., 1997; Toulkeridis et al., 1998). The minimum age

et al., 2003). The alternation of sandstone and BIF lithologies is

constraints on the Moodies strata are provided by several igneous

abruptly interrupted at 10.5 m by a 0.6 m-thick bed of volcanic

intrusions, and the maximum age by dates of detrital grains within

tuff. Gradually the thickness of the chert and the iron packages

the strata (Kamo and Davis, 1994; Kröner et al., 1991). In spite of the

increases upsection while the frequency of interbedded sandstone

tight structural folding that affected the region, sedimentary struc-

beds diminishes into an interval several meters thick of predom-

tures are preserved unmodiﬁed and are widespread within the

inantly BIF with occasional sandstone beds. Where the banding

Moodies lithologies. These sedimentary structures and associated

typical of BIF is best expressed, the siliciclastic interbeds are absent.

facies reveal a wide range of terrestrial and marine sedimentary

Continuing upward, the thickness and the frequency of iron and

paleoenvironments including aeolian, alluvial fan, braided stream,

chert beds gradually decrease and siliciclastic sediment again dom-

tide-dominated delta, open marine shelf that were distributed

inates the section. These upper sandstones also contain common

across the foreland basin (Anhaeusser, 1973; Eriksson, 1977, 1979,

sedimentary structures, including wavy laminations, trough cross-

1983; Eriksson and Simpson, 2000; Eriksson et al., 2006; Heubeck,

stratiﬁcation, and soft-sediment deformation. In the uppermost

2009; Heubeck and Lowe, 1994, 1999; Simpson et al., 2012).

part of the studied section, two coarsening-upward sequences

were apparent. These intervals consist of massively bedded

3. Locations, sampling, and methods

sandstones.

A detailed stratigraphic section was measured at bed-by-bed

∼

resolution ( 0.5 cm) and samples were collected from an outcrop 4.2. Iron-bearing minerals in Moodies BIF

◦ ◦

located along Ameide Road (S 25 49 56 –E 031 00 50 , in a struc-

tural sub-basin referred to as the Moodies Hill Block; Heubeck In unweathered samples, the iron-rich component of Moodies

and Lowe, 1994) and directly from the underground tunnels of the BIF appears dark gray, shows a typical metallic luster, and is char-

−

Agness Gold Mine (Ramp West, Access South, 600 m). Access to acterized by several scales of banding (Fig. 4). The thickness of the

subsurface materials is particularly important to obtain pristine iron mineral laminations are commonly submillimetric but group

samples and mitigate the ever-present effects of oxidative sur- into rough bundles at centimetric scales (an approximate average

face weathering in South Africa. Samples collected from the mine is 0.5–1.5 cm, the thickest observed interval is about 3.5 cm). In

were not affected by weathering and show good preservation of the nomenclature of Trendall and Blockley (1970), these would be

sedimentary fabrics and textures. These materials were used for described as microbands (millimeter to sub-millimeter units) and

petrographic and geochemical analyses. The total amount of sam- mesobands (centimeter thick units), respectively. The iron-rich

ples collected was approximately 175 kg. Transmitted and reﬂected constituent of the Moodies BIF is always ﬁnely laminated, and con-

␮

light microscopy was performed on 30 m-thick polished thin sec- tains no macroscopic granular, globular or oolitic morphologies like

tions. Bulk mineralogy was determined by X-ray diffraction using those commonly observed in Paleoproterozoic-age granular iron

a Scintag XRD 2000 diffractometer. Scanning electron microscopy formations (Fig. 5). The iron-rich packages are mainly composed

T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124 119

Fig. 3. Stratigraphic section of Moodies Group strata exposed at the Ameide Road outcrop. Iron-rich packages and chert beds are thinly interbedded with siltstones and

sandstones, very ﬁne to ﬁne in grain size. Chert is always associated with iron-rich horizons and does not occur in direct contact with siliciclastic sediment, indicating that

the processes leading to chert accumulation were tightly coupled with those involving the precipitation of the iron minerals.

of hematite and magnetite crystals with a microcrystalline quartz in the middle) were observed. The abundance and petrographical

cement (Fig. 6). The presence of these two iron-bearing phases is relationships of magnetite are closely linked to the accumulation

easily recognized by optical microscopy, but was further conﬁrmed of ﬁne-grained hematite. Some chert lenses include undulated, dis-

by XRD-analysis of bulk rock powders. The laminations are often continuous laminations almost exclusively comprised of hematite

emphasized by magnetite and ﬁne-grained hematite intercalated (i.e. without magnetite overgrowths) (Fig. 6C); the occlusion of

with thin laminae and discontinuous lenses of detrital siliciclastic porosity in these cherty beds may have better preserved hematite

particles (Figs. 5 and 6B). Where iron minerals are predominant by allowing limited permeability of later reducing ﬂuids. Under

with little intervening clastic material, the lamination is often less low magniﬁcation, hematite appears as a gray homogeneous

well expressed. Texturally, magnetite occurs as a diagenetic phase powder. SEM microscopy reveals that the size of single hematite

that primarily replaced or overgrew prior hematite – a feature spheroids is often less than 1 ␮m. Sulﬁde-bearing minerals,

commonly observed in younger iron formations (Beukes et al., including bravoite, pyrite, and chalcopyrite, are also present

1990; Fischer and Knoll, 2009; Han, 1978). Euhedral crystals of within the iron mineral-rich lamina. They are signiﬁcantly less

magnetite overgrowing hematite (with relic hematite still visible abundant relative to hematite and magnetite and they often

120 T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124

Fig. 4. Photograph of a polished slab of Moodies BIF. (A) Chert concretion. (B) Sub-

mm scale laminations of iron-bearing minerals (mainly magnetite and hematite).

laminations due to the variable amount of incorporated iron minerals. Iron-rich

laminations are due to both the ﬁne interlamination with siliciclastic material and

the variable amount of surrounding chert.

Fig. 5. Large-scale photograph of a representative Moodies BIF polished sample. (A)

occur in association with small veins and cracks that cut primary

Contact between two chert beds. The color of the chert varies from dark red to almost

laminations – mineralization related to later metasomatic events. transparent depending on the concentration of microscopic iron minerals (primar-

ily hematite). (B) Iron-rich microbands ﬁnely interlaminated with siliciclastic grains

(mainly ﬁne quartz sand). (C) Siliciclastic bed showing laminations comprised of

4.3. Chert in Moodies BIF

sand draped by iron-bearing minerals. (D) Chert bed including discontinuous lam-

inae comprised of iron minerals. Boxes marked A–D correspond to the microscopy

In unweathered samples, chert concretions appear red (Fig. 4). images presented in Fig. 6.

Microscopy reveals that the intensity of red color is related to

the concentration of ﬁne-grained hematite laminations within the

particles preferentially occur within the iron mineral-rich bands,

chert (Figs. 4 and 5A and C). In the literature, hematitic chert is

often associated with magnetite crystals. Their size is rather regular

sometimes referred to as jasper. The thickness of the chert laminae ∼

( 10 ␮m) and their distribution within the iron-rich layers appears

varies from submillimetric to centimetric, the thickest observed

homogeneous: no clear laminations or morphologies resembling

interval measures about 7 cm. The chert beds often show a nodu-

microfossils were observed.

lar shape absent in iron-rich and clastic-rich layers. In some cases,

small (<1 mm) chert concretions are embedded in packages of

Table 1

iron minerals. In addition to hematite, domains of iron–carbonate Elemental composition of the carbonaceous particles imaged in Fig. 7 from energy

(mainly ankerite) and magnetite crystals are present within the dispersive X-ray spectroscopy.

chert. The growth of the carbonate crystals cut and overprint the

Element Fig. 7A Fig. 7C

chert laminations (Fig. 6C). A later weak overgrowth of chert over wt% wt%

the edges of the same carbonate crystals was also observed.

C 89.51 67.33

O – 12.94

Mg 0.16 0.05

4.4. Carbonaceous particles in Moodies BIF

Al 0.01 0.04

Si 0.55 0.86

SEM observations revealed the presence of carbonaceous parti-

S 0.35 0.31

cles within the Moodies BIF (Fig. 7). Carbon content of the particles Ca 0.06 0.08

Mn 0.08 0.05

was measured by energy dispersive X-ray spectroscopy and it rep-

Fe 4.49 3.02

resents more than the 90% of their total weight (Table 1). These

T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124 121

Fig. 6. Transmitted light photomicrographs of centimeter-scale Moodies BIF mesobands. (A) Chert concretions include ﬁne-grained hematite spheroids (gray dots). The

larger concentration of hematite in the upper band is responsible for the darker color of the chert and its laminated habit. Although less abundant, hematite is also present in

the lower band, which includes lenticular concretions rich in hematite (e.g. black arrow). Magnetite occurs as large opaque crystals with euhedral shapes developed along,

but clearly crosscutting primary laminations (e.g. white arrow). (B) Iron-rich laminations associated with quartz sand. Coarse euhedral magnetite crystals (e.g. black arrow)

overgrowing hematite. At this magniﬁcation, micron-sized hematite crystals appear like a gray powder, barely visible behind the euhedral, larger, and darker magnetite

crystals. Siliciclastic particles are clear (e.g. white arrow) and have diameters in thin section up to 150 ␮m (i.e. ﬁne sand), and were likely transported as a part of bed load

sediment ﬂux. (C) Interbedded sandstone beds show internal laminations, emphasized by ﬁne layers of hematite. (D) Contact between an iron-rich bed (top dark band) and

a chert concretion. Crystals of ankerite are visible within the chert (e.g. white arrow); they preferentially occur at the boundary between iron-rich beds and chert. The chert

also includes discontinuous laminations comprised of iron minerals (e.g. black arrow).

5. Interpretations throughout the iron-rich layers suggests deposition of ﬁne-grained

BIF precursor phases from suspension and not saltation. This dif-

5.1. Depositional setting of the Moodies BIF fers from the sedimentary structures that are common in granular

Paleoproterozoic BIF and that are interpreted as the result of wave

BIF deposition constitutes a non-uniformitarian suite of pro- and current reworking of chemical clasts in shallow waters (Klein,

cesses, but the sedimentary rocks in contact with these lithologies 2005). Parallel or wavy lamination and “soft sediment deforma-

provide substantial insight into the environments of their deposition” are the only sedimentary structures that are observed in the

tion (Fischer and Knoll, 2009; Ojakangas, 1983; Simonson, 1985). clastic material that is intercalated at the cm scale between the

Observations of BIF deposits hosted in siliciclastic sequences high- iron and chert layers. Such structures may have formed in a tidal

light that BIF formed in a large variety of geological settings environment under the inﬂuence of offshore–onshore currents (e.g.

and paleodepths, ranging from abyssal fans to shallow marine Watchorn, 1980), but may also have formed deeper in to the basin,

environments (Bleeker et al., 1999; Eriksson, 1983; Fralick and where the clastic material was transported by turbidity or com-

Pufahl, 2006; Hofmann and Kusky, 2004; Srinivasan and Ojakangas, bined ﬂow currents (Myrow et al., 2002). Independently from these

1986). The siliciclastics beds interbedded with BIF lithologies in two possible paleoenvironmental interpretations, the gradational

the Moodies Group carry sedimentary structures (i.e. trough cross- nature of increasing and decreasing frequency of BIF mesobands

stratiﬁcation, wavy laminations, and graded siliciclastic beds to interbedded with siliciclastics suggests that the accumulation of

coarse sandstone) that were interpreted by previous studies of BIF was greatest during intervals of reduced sand supply, as a

the Moodies Group to characterize subtidal deposition in a shal- sediment-starved or condensed facies.

low marine shelf (Eriksson, 1977, 1979; Heubeck and Lowe, 1994).

Moodies BIF likely represents the most distal facies of this progra- 5.2. Origin of banding

dational shelf sequence (Eriksson, 1983). Though generally shallow,

the precise water depth of deposition is difﬁcult to constrain. The The presence of interbedded siliciclastics with the iron-rich

detrital layers that are interbedded with the iron layers and chert do packages and chert beds in the Moodies BIF provides new insight

not show structures that unambiguously indicate deposition within helpful for understanding the origin of the typical BIF banding. The

the wave action zone. Furthermore, the parallel lamination found chert occurs as beds and nodules with a laterally heterogeneous

122 T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124

Fig. 7. Backscatter scanning electron photomicrographs of polished Moodies BIF thin sections. (A, C and D) Carbonaceous particles with amorphous shape (white arrows) are

closely associated with magnetite crystals. Note the amorphous shape of the particles displaced by the ingrowing magnetite (e.g. Beukes and Klein, 1992). Mag, magnetite;

Dol–Ank, dolomite–ankerite. (B) Enlargement of region boxed in (A).

thickness; these textures are indicative of diagenetic precipitation somewhat rare compared to younger Archean deposits (Fischer

from pore ﬂuids and do not require that the chert originally pre- and Knoll, 2009; Klein, 2005). Though ankerite was likely derived

cipitated from the water column (Beukes, 1984; Fischer and Knoll, from recrystallization of preexisting siderite, the sulﬁde-bearing

2009; Krapezˇ et al., 2003). Furthermore, in stratigraphic context, minerals occur within small veins and cracks, and have a clear

chert is always preceded and followed by iron-rich packages (i.e. metasomatic origin. Euhedral magnetite grains commonly show

does not occur in direct contact with siliciclastic beds) (Fig. 3). crosscutting relationships with regard to hematite. Though this

These observations reveal a tight coupling between the accumu- phase represents a substantial amount of ferric iron in the Moodies

lation of iron-bearing minerals and chert, supporting hypotheses BIF, it has a late diagenetic or metamorphic origin. Similar observa-

that naturally link the cycles of iron and silica. Grenne and Slack tions and interpretations have been made for the younger Early

(2005) proposed that gels protolith of chert were deposited by Archean and Paleoproterozoic BIF (Bekker et al., 2010; Beukes,

fallout from hydrothermal ﬂuids in silica-rich seawater, in which 1984; Ewers and Morris, 1981; Krapezˇ et al., 2003).

plume-derived Fe oxyhydroxide particles promoted ﬂocculation of The observation that hematite is an early phase while mag-

colloidal particles of silica–iron oxyhydroxide. Wang et al. (2009) netite a diagenetic or metamorphic mineral is consistent with the

reﬁned this model, explaining the banding of BIF in terms of self- hypothesis that iron was concentrated in the sediments by oxida-

organized chemical oscillations – ferric hydroxide precipitation tion of dissolved Fe(II) in seawater to form an insoluble hydrous

from Si- and Fe-rich hydrothermal ﬂuids would decrease ambient oxide precipitate, a precursor that can spontaneously transform

pH that subsequently causes silica precipitation. Thus, precipitation into hematite on early diagenetic timescales (Ayres, 1972; Bekker

of chert is the result of a positive feedback linked to the precip- et al., 2010; Trendall and Blockley, 1970). Diagenetic and meta-

itation of iron. A different linkage was suggested by Fischer and morphic transformation of these ferric minerals to mixed valence

Knoll (2009), wherein the silica constituting the chert mesobands and ferrous minerals may have been driven by several processes,

was shuttled to basinal waters and sediments by adsorption on including microbial respiration of organic matter and abiotic ther-

the surfaces of ferric hydroxides. It is important to note that this mal reactions, and are discussed further in the next section.

hypothesis does not imply that BIF deposition was strictly associ-

ated with hydrothermal settings.

5.4. Role of biology and organic carbon in BIF deposition

5.3. The mechanisms of Fe(II) oxidation

In many models for BIF formation, primary oxidation of Fe(II)

In the Moodies BIF, magnetite and hematite are the dominant to Fe(III) is considered either the direct or indirect (via metabolic

Fe-bearing mineral phases. Ferrous iron minerals are present, but intermediates like O2) result of microbial activity (Brown et al.,

T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124 123

1995; Cloud, 1973; Harder, 1919; Hartman, 1984; Konhauser et al., Grenne and Slack, 2005; Wang et al., 2009). The presence of organic

2002; Posth et al., 2008; Walker, 1987). However, bona ﬁde micro- matter within the Moodies BIF is consistent with the hypothesis

fossils from BIF lithologies are very rare and none of the few that BIF formed through the activities of iron-oxidizing and reduc-

documented occurrences (Barghoorn and Schopf, 1966; Klein et al., ing microbes as a part of the iron cycle. However, a metamorphic

1987; Knoll and Simonson, 1981; Planavsky et al., 2009; Walter origin of the observed organic matter through thermal dispro-

et al., 1976) provides evidence for a genetic relationship between portionation processes cannot be excluded. Despite some clear

microfossils and Fe-rich minerals. For these reasons, the effective idiosyncratic differences, the similarities between the Moodies BIF

involvement of microbes has been questioned and alternative abio- and those deposited almost one billion years later support broad

genic precipitation mechanisms have been suggested (Braterman extrapolation of BIF forming processes back into Earth Archean

et al., 1983; Draganic,´ 1991; Franc¸ ois, 1986; Wang et al., 2009). time.

In the Moodies BIF, we observed carbonaceous particles within

the iron-rich packages. The close spatial association with magnetite

Acknowledgments

phases is consistent with the hypothesis that magnetite may have

formed during diagenesis by reduction of hematite (or its hydrous

We would like to thank Jan Kramers for introducing us to the

precursor) with organic matter. This hypothesis, ﬁrst proposed by

Barberton Greenstone Belt, Chris Rippon, Charles Robus and John

Perry et al. (1973) and subsequently reﬁned by Walker (1984), Baur

Robertson for supporting the ﬁeldwork in the Barberton region and

et al. (1985), Konhauser et al. (2005), and Fischer and Knoll (2009)

in the Agness gold mine, Drummond and Jacky Holman for their

implies a two-fold role of microbes in BIF formation. First, microbes

support at Grace Farm, Mariarita Bontognali-Valli for organizing

build biomass in the water column either by directly oxidizing

the shipment of the BIF samples, Alexey Ulianov for his assistance

dissolved Fe(II) through anoxygenic photosynthesis or by others

with optical microscopy and SEM investigations, Thierry Adatte for

oxygenic metabolism that cause passive precipitation of dissolved

his help with the XRD analyses, André Villard for the preparation

iron. Then, the precipitated Fe(OH)3 sinks from the water column

of the thin sections.

along with part of the produced biomass. Within the sediments

the biomass is consumed during a second microbial process as, for

example, dissimilatory iron reduction that couples organic matter

References

oxidation to Fe(III) reduction. This would explain both the min-

eralogy observed in many BIF (i.e. the co-occurrence of minerals Anhaeusser, C.R., 1973. The evolution of the Early Precambrian crust of Southern

with different Fe oxidation state that cannot precipitate simulta- Africa. Philosophical Transactions of the Royal Society of London Series A: Math-

ematical and Physical Sciences 273, 359–388.

neously from the same solution) and the lack of organic material

Ayres, D.E., 1972. Genesis of iron-bearing minerals in banded iron formation

in BIF, although microbes might have played a key role in their for-

mesobands in the Dales Gorge Member, Hamersley Group, Western Australia.

mation. The organic carbon distribution in the Moodies BIF may Economic Geology 67, 1214–1233.

Barghoorn, E.S., Schopf, J.W., 1966. Microorganisms three billion years old from the

record these diagenetic reduction processes, though it is difﬁcult

Precambrian of South Africa. Science 152, 758–763.

to rule out abiotic alternatives. Importantly, simultaneous forma-

Baur, M.E., Hayes, J.M., Studley, S.A., Walter, M.R., 1985. Millimeter-scale variations

tion of magnetite and organic carbon compounds through thermal of stable isotope abundances in carbonates from banded iron-formations in the

Hamersley Group of Western Australia. Economic Geology 80, 270–282.

decomposition of siderite was observed in laboratory experiments

Becker, R.H., Clayton, R.N., 1972. Carbon isotopic evidence for the origin of a banded

that were carried out to evaluate the origin of putative biosigna-

iron-formation in Western Australia. Geochimica et Cosmochimica Acta 36,

tures present in Martian meteorite ALH84001 (McCollom, 2003). 577–595.

Bekker, A., Slack, J.F., Planavsky, N., Krapez,ˇ B., Hofmann, A., Konhauser, K.O., Rouxel,

The experiments were conducted at relatively mild temperatures

◦ O.J., 2010. Iron formation: the sedimentary product of a complex interplay

of 300 C, a temperature not much higher than that experienced by

among mantle, tectonic, oceanic, and biospheric processes. Economic Geology

the Moodies BIF facies during lower-greenschist metamorphism, 105, 467–508.

Beukes, N.J., 1984. Sedimentology of the Kuruman and Griquatown iron-formations,

though for a much shorter time. The thermal decomposition of

Transvaal Supergroup, Griqualand West, South Africa. Precambrian Research 24,

siderite to magnetite and organic carbon, thus, provides a possible

47–48.

alternative explanation for the presence of carbonaceous particles Beukes, N.J., Gutzmer, J., 2008. Origin of banded iron formations. Reviews in Eco-

within the Moodies BIF. nomic Geology 15, 5–47.

Beukes, N.J., Klein, C., 1992. Models for iron-formation deposition. In: Schopf, J.W.,

Klein, C. (Eds.), The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge

University Press, UK, pp. 147–151.

6. Conclusions Beukes, N.J., Klein, C., Kaufman, A.J., Hayes, J.M., 1990. Carbonate petrography,

kerogen distribution, and carbon and oxygen isotope variations in and early

Proterozoic transition from limestone to iron formation deposition, Transvaal

The Moodies BIF share many features with younger Late Archean

Supergroup, South Africa. Economic Geology 85, 663–690.

and Paleoproterozoic iron formations, including a mineralogy that Bleeker, W., Ketchum, J.W.F., Jackson, V.A., Villeneuve, M.E., 1999. The Central Slave

Basement Complex, Part I: its structural topology and autochthonous cover.

reﬂects a ferric iron phase as the earliest sedimentary precursor and

Canadian Journal of Earth Sciences 36, 1083–1109.

the secondary interaction of these ferric iron phases with reducing

Braterman, P.S., Cairns-Smith, A.G., Sloper, R.W., 1983. Photooxidation of

ﬂuids during diagenesis and metamorphism, and a sedimentary hydrated Fe(II): signiﬁcance for banded iron formations. Nature 303,

163–164.

mode that is characterized by alternating iron-rich packages and

Brown, D.A., Gross, G.A., Sawicki, J.A., 1995. A review of the microbial geochemistry

silica-rich beds and concretions. The interbedded siliciclastic mate-

of banded iron-formations. Canadian Mineralogist 33, 1321–1333.

rial indicates deposition during times of muted clastic input, Byerly, G.R., Lower, D.R., Walsh, M.M., 1986. Stromatolites from the 3300–3500-

Myr Swaziland Supergroup, Barberton Mountain Land, South Africa. Nature 319,

perhaps at the base of a progradational shelf sequence. The precise

489–491.

paleodepth is difﬁcult to constrain with any certainty – the Mood-

Cairns-Smith, A.G., 1978. Precambrian solution photochemistry, inverse segrega-

ies BIF may have formed in a shallow, current-swept environment tion, and banded iron formations. Nature 276, 807–808.

or deeper into the basin, during the intervals between successive Canﬁeld, D.E., 1998. A new model for Proterozoic ocean chemistry. Nature 396,

450–453.

turbidite events. The presence of siliciclastic beds provides an addi-

Cloud, P.E., 1968. Atmospheric and hydrospheric evolution on the primitive Earth.

tional insight on the origin of chert. The lack of chert accumulating

Science 160, 729–736.

directly on the siliciclastic material (i.e. the lack of chert not associ- Cloud, P.E., 1973. Paleoecological signiﬁcance of the banded iron-formation. Eco-

nomic Geology 68, 1135–1143.

ated with iron-rich bands) indicates a tight coupling between iron

Clout, J.M.F., Simonson, B.M., 2005. Precambrian iron formations and iron formation-

and silica, and supports models positing that silica precipitation

hosted iron ore deposits. In: Economic Geology 100th Anniversary Volume, pp.

was a direct consequence of iron oxidation (Fischer and Knoll, 2009; 215–250.

124 T.R.R. Bontognali et al. / Precambrian Research 226 (2013) 116–124

Dauphas, N., Cates, N.L., Mojzsis, S.J., Busigny, V., 2007. Identiﬁcation of chemi- Klein, C., Beukes, N.J., 1989. Geochemistry and sedimentology of a facies transition

cal sedimentary protoliths using iron isotopes in the >3750 Ma Nuvvuagittuq from limestone to iron-formation deposition in the early Proterozoic Transvaal

supracrustal belt, Canada. Earth and Planetary Science Letters 254, 358–376. Supergroup, South Africa. Economic Geology 84, 1733–1774.

De Ronde, C.E.J., Channer, D.M.d., Faure, K., Bray, C.J., Spooner, E.T.C., 1997. Klein, C., Beukes, N.J., Schopf, J.W., 1987. Filamentous microfossils in the Early

Fluid chemistry of Archean seaﬂoor hydrothermal vents: Implications for the Proterozoic Transvaal Supergroup: their morphology, signiﬁcance, and paleoen-

composition of circa 3.2 Ga seawater. Geochimica et Cosmochimica Acta 61, vironmental setting. Precambrian Research 36, 81–94.

4025–4042. Knoll, A.H., Simonson, B.M., 1981. Early Proterozoic microfossils and penecontem-

Draganic,´ I.G., 1991. Decomposition of ocean waters by potassium-40 radiation poraneous quartz cementation in the Sokoman iron formation, Canada. Science

3800 Ma ago as a source of oxygen and oxidizing species. Precambrian Research 211, 478–480.

52, 321–336. Konhauser, K.O., Hamade, T., Raiswell, R., Morris, R.C., Ferris, F.G., Southam, G.,

Drever, J.I., 1974. Geochemical model for the origin of Precambrian banded iron Canﬁeld, D.E., 2002. Could bacteria have formed the Precambrian banded iron

formations. Geological Society of America Bulletin 85, 1099–1106. formations? Geology 30, 1079–1082.

Eiler, J.M., 2007. The oldest fossil or just another rock? Science 317, 1046–1047. Konhauser, K.O., Newman, D.K., Kappler, A., 2005. The potential signiﬁcance of

Eriksson, K.A., 1977. Tidal deposits from the Archaean Moodies Group, Barberton microbial Fe(III) reduction during deposition of Precambrian banded iron for-

Mountain Land, South Africa. Sedimentary Geology 18, 257–281. mations. Geobiology 3, 167–177.

Eriksson, K.A., 1979. Marginal marine depositional processes from the Archaean Krapez,ˇ B., Barley, M.E., Pickard, A.L., 2003. Hydrothermal and resedimented ori-

Moodies Group, Barberton Mountain Land, South Africa: evidence and signiﬁ- gins of the precursor sediments to banded iron formation: sedimentological

cance. Precambrian Research 8, 153–182. evidence from the Early Palaeoproterozoic Brockman Supersequence of Western

Eriksson, K.A., 1983. Siliciclastic-hosted iron-formations in the early Archaean Bar- Australia. Sedimentology 50, 979–1011.

berton and Pilbara sequences. Journal of the Geological Society of Australia 30, Kröner, A., Byerly, G., Lowe, D.R., 1991. Chronology of Early Archean granitegreen-

473–482. stone evolution in the Barberton Mountain Land, South Africa, based on precise

Eriksson, K.A., Simpson, E.L., 2000. Quantifying the oldest tidal record: the 3.2 Ga dating by single zircon evaporation. Earth and Planetary Science Letters 103,

Moodies Group, Barberton Greenstone Belt, South Africa. Geology 28, 831–834. 41–54.

Eriksson, K.A., Simpson, E.L., Mueller, W., 2006. An unusual ﬂuvial to tidal transition Lepp, H., Goldich, S.S., 1964. Origin of Precambrian iron formations. Economic Geol-

in the mesoarchean Moodies Group, South Africa: a response to high tidal range ogy 59, 1025–1060.

and active tectonics. Sedimentary Geology 190, 13–24. Lowe, D.R., Byerly, G.R., Heubeck, C., 1999. Structural divisions and development of

Ewers, W.E., Morris, R.C., 1981. Studies of the Dales Gorge Member of the Brockman the west-central part of the Barberton greenstone belt. In: Lowe, D.R., Byerly,

Iron Formation, Western Australia. Economic Geology 76, 1929–1953. G.R. (Eds.), Geologic Evolution of the Barberton Greenstone Belt, South Africa.

Fischer, W.F., Knoll, A.H., 2009. An iron shuttle for deep-water silica in Late Archean Geological Society of America, pp. 37–82, Special Paper 329.

and Early Paleoproterozoic iron formation. Geological Society of America Bul- McCollom, T.M., 2003. Formation of meteorite hydrocarbons from thermal decom-

letin 121, 222–235. position of siderite (FeCO3). Geochimica et Cosmochimica Acta 67, 311–317.

Fralick, P., Pufahl, P.K., 2006. Iron formation in Neoarchean Deltaic Successions and Myrow, P.M., Fischer, W., Goodge, J.W., 2002. Wave-modiﬁed turbidites: combined-

the microbially mediated deposition of transgressive systems tracts. Journal of ﬂow shoreline and shelf deposits, Cambrian, Antarctica. Journal of Sedimentary

Sedimentary Research 76, 1057–1066. Research 72, 641–656.

Franc¸ ois, L.M., 1986. Extensive deposition of banded iron formations was possible Noffke, N., Eriksson, K.A., Hazen, R.E., Simpson, E.L., 2006. A new window into

without photosynthesis. Nature 320, 352–354. early life: microbial mats in Earth’s oldest siliciclastic tidal ﬂats (3.2 Ga Moodies

Goodwin, A.M., Monster, J., Thode, H.G., 1976. Carbon and sulfur isotope abundances Group, South Africa). Geology 34, 253–256.

in Archean iron-formations and early Precambrian life. Economic Geology 71, Ojakangas, R.A., 1983. Tidal deposits in the Early Proterozoic basin of the Lake Supe-

870–891. rior region—the Palms and the Pokegama formations: evidence or subtidal shelf

Grenne, T., Slack, J.F., 2005. Geochemistry of Jasper Beds from the Ordovician Løkken deposition of superior type banded iron formation. Geological Society of America

Ophiolite, Norway: origin of proximal and distal siliceous exhalites. Economic Memoir 160, 49–66.

Geology 100, 1511–1527. Perry, E.C., Tan, F.C., Morey, G.B., 1973. Geology and stable isotope geochemistry

Hall, A.L., 1918. The geology of the Barberton gold mining district. Geological Survey of the Biwabik iron formation, Northern Minnesota. Economic Geology 68,

Republic of South Africa – Memoir 9, 347. 1110–1125.

Hamade, T., Konhauser, K.O., Raiswell, R., Goldsmith, S., Morris, R.C., 2003. Using Planavsky, N., Rouxel, O., Bekker, A., Shapiro, R., Fralick, P., Knudsen, A., 2009. Iron-

Ge/Si ratios to decouple iron and silica ﬂuxes in Precambrian banded iron for- oxidizing microbial ecosystems thrived in late Paleoproterozoic redox-stratiﬁed

mations. Geology 31, 35–38. oceans. Earth and Planetary Science Letters 286, 230–242.

Han, T.M., 1978. Microstructures of magnetite as guides to its origin in some Pre- Posth, N.R., Hegler, F., Konhauser, K.O., Kappler, A., 2008. Alternating Si and Fe

cambrian iron-formations. Fortschritte der Mineralogie 56, 105–142. deposition caused by temperature ﬂuctuations in Precambrian oceans. Nature

Harder, E.C., 1919. Iron-depositing bacteria and their geologic relations. U.S. Geo- Geoscience 1, 703–708.

logical Survey Professional Paper 113, 89 p. Siever, R., 1992. The silica cycle in the Precambrian. Geochimica et Cosmochimica

Hartman, H., 1984. The evolution of photosynthesis and microbial mats: a specula- Acta 56, 3265–3272.

tion on the banded iron formations. In: Cohen, Y., et al. (Eds.), Microbial Mats: Simonson, B.M., 1985. Sedimentological constraints on the origins of Precambrian

Stromatolites. Alan R. Liss, New York, pp. 449–453. iron-formations. Geological Society of America Bulletin 96, 244–252.

Heubeck, C., 2009. An early ecosystem of Archean tidal microbial mats (Moodies Simpson, E.L., Eriksson, K.A., Mueller, W., 2012. 3.2 Ga eolian deposits from the

Group, South Africa, ca. 3.2 Ga). Geology 37, 931–934. Moodies Group, Barberton Greenstone Belt, South Africa: Implications for

Heubeck, C., Lowe, D.R., 1994. Depositional and tectonic setting of the Archean the origin of ﬁrst-cycle quartz sandstones. Precambrian Research 214–215,

Moodies Group, Barberton Greenstone Belt, South Africa. Precambrian Research 185–191.

68, 257–290. Srinivasan, R., Ojakangas, R.W., 1986. Sedimentology of quartz-pebble conglomer-

Heubeck, C., Lowe, D.R., 1999. Sedimentary petrology and provenance of the Archean ates and quartzites of the Archean Bababudan Group, Dharwar Craton, South

Moodies Group, Barberton Greenstone Belt, South Africa. In: Lowe, D.R., Byerly, India: evidence for early crustal stability. Journal of Geology 94, 199–214.

G.R. (Eds.), Geologic Evolution of the Barberton Greenstone Belt, South Africa. Toulkeridis, T., Goldstein, S.L., Clauer, N., Kröner, A., Todt, W., Schidlowski, M., 1998.

Geological Society of America, pp. 259–286, Special Paper 329. Sm–Nd, Rb–Sr and Pb–Pb dating of silicic carbonates from the early Archaean

Hofmann, A., Kusky, T., 2004. The Belingwe Greenstone Belt: ensialic or oceanic? Barberton Greenstone Belt, South Africa: evidence for post-depositional isotopic

In: Timothy, M.K. (Ed.), Developments in Precambrian Geology. Elsevier, pp. resetting at low temperature. Precambrian Research 92, 129–144.

487–538. Trendall, A.F., 2002. The signiﬁcance of iron-formation in the Precambrian strati-

Holland, H.D., 1973. The oceans: a possible source of iron in iron formations. Eco- graphic record. In: Altermann, W., Cocoran, P. (Eds.), Precambrian Sedimentary

nomic Geology 68, 1169–1172. Environments. , pp. 33–66, International Association of Sedimentologists Series.

James, H.L., 1954. Sedimentary facies of iron formations. Economic Geology 49, Trendall, A.F., Blockley, J.G., 1970. The iron formations of the Precambrian Ham-

235–293. mersley Group, Western Australia: with special reference to the associated

Javaux, E.J., Marshall, C.P., Bekker, A., 2010. Organic-walled microfossils in 3.2- crocidolite. Geological Survey of Western Australia Bulletin 119, 366.

billion-year-old shallow-marine siliciclastic deposits. Nature 463, 934–938. Walker, J.C.G., 1984. Suboxic diagenesis in banded iron formations. Nature 309,

Kamo, S.L., Davis, D.W., 1994. Reassessment of Archean crustal development in the 340–342.

Barberton Mountain Land, South Africa, based on U–Pb dating. Tectonics 13, Walker, J.C.G., 1987. Was the Archean biosphere upside down? Nature 329, 710–712.

167–192. Walter, M.R., Goode, A.D.T., Hall, W.D.M., 1976. Microfossils from a newly discov-

Kappler, A., Pasquero, C., Konhauser, K.O., Newman, D.K., 2005. Deposition of banded ered Precambrian stromatolitic iron-formation in Western Australia. Nature

iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria. Geology 261, 221–223.

33, 865–868. Wang, Y., Xu, H., Merino, E., Konishi, H., 2009. Generation of banded iron forma-

Kaufman, A.J., Hayes, J.M., Klein, C., 1990. Primary and diagenetic controls of isotopic tions by internal dynamics and leaching of oceanic crust. Nature Geoscience 2,

compositions of iron-formation carbonates. Geochimica et Cosmochimica Acta 781–784.

54, 3461–3473. Watchorn, M.B., 1980. Fluvial and tidal sedimentation in the 3000 Ma Mozaan basin,

Klein, C., 2005. Some Precambrian banded iron-formations (BIFs) from around the South Africa. Precambrian Research 13, 27–42.

world: their age, geologic setting, mineralogy, metamorphism, geochemistry, Widdel, F., Schnell, S., Heising, S., Ehrenreich, A., Assmus, B., Schink, B., 1993. Ferrous

and origin. American Mineralogist 90, 1473–1499. iron oxidation by anoxygenic phototrophic bacteria. Nature 362, 834–836.