Widespread deposition of greenalite to form Banded Iron Formations before the Great Oxidation Event

Journal Pre-proofs Widespread Deposition of Greenalite to Form Banded Iron Formations before the Great Oxidation Event Janet R. Muhling, Birger Rasmussen PII: DOI: Reference:

S0301-9268(19)30536-4 https://doi.org/10.1016/j.precamres.2020.105619 PRECAM 105619

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Precambrian Research

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20 September 2019 7 January 2020 7 January 2020

Please cite this article as: J.R. Muhling, B. Rasmussen, Widespread Deposition of Greenalite to Form Banded Iron Formations before the Great Oxidation Event, Precambrian Research (2020), doi: https://doi.org/10.1016/ j.precamres.2020.105619

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Widespread Deposition of Greenalite to Form Banded Iron Formations before the Great Oxidation Event

Janet R. Muhlinga,*, Birger Rasmussena a

School of Earth Sciences, The University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia *Corresponding author - E-mail: [email protected].

Abstract Banded Iron Formations (BIFs) are Precambrian sedimentary rocks interpreted to have been precipitated from anoxic seawater prior to the first permanent rise in atmospheric oxygen in the Great Oxidation Event at ca. 2.45-2.32 Ga. BIFs hold the key to understanding the chemistry of the oceans and atmosphere, and how these interacted with microbial life, prior to and during the evolution of oxygenic photosynthesis. To unlock this information, it is essential to know how BIFs formed, that is, what minerals were precipitated, by what mechanisms and in which environments. BIFs are found in almost all depositional settings at times when input of clastic detritus was lacking, for example, submarine proximal volcanic environments, basin floor, slope, deep marine shelf and shallow shelf settings. High-resolution electron microscopy of finely laminated BIFs and ferruginous cherts that range in age from 3.45 to ca. 2.4 Ga and that preserve depositional features indicates that the original sediment was a very fine mud composed of nanoparticles of the iron-silicate greenalite. In places, the greenalite mud experienced very early silicification on the sea floor enabling preservation of the mineral and its depositional textures. Very fine grained siderite and hematite in laminated BIFs post-date dehydration of the early

silica cement and are not primary minerals. Greenalite nanoparticles are found in BIFs from all depositional settings indicating a common origin, likely precipitation resulting from mixing of plume water from hydrothermal vents with ambient seawater. The nanoparticles were carried throughout the oceans and were deposited on the seafloor and on continental margins to form the primary sediments of BIFs. Keywords: Banded iron formations; greenalite; silica; hydrothermal plume; ocean chemistry; electron microscopy

1. Introduction Banded Iron Formations (BIFs) are iron- and silica-rich chemical sedimentary rocks formed only in Precambrian times, i.e. there are no modern analogues. They are composed essentially of chert or quartz bands interlayered with bands of a variety of iron-rich minerals, e.g., silicates, carbonates and oxides. BIFs are common in Archean and Paleoproterozoic rocks formed before the Great Oxidation Event (GOE), the first rise in atmospheric oxygen at ca. 2.45-2.32 Ga, e.g. interlayered with volcanic and sedimentary rocks in greenstone belts in granite-greenstone terranes, in remnants of supracrustal sequences in higher metamorphic grade granite-gneiss terranes, and in sedimentary successions in well-preserved Meso- to NeoArchean and Paleoproterozoic sedimentary basins such as the Hamersley basin, Western Australia and the Transvaal basin of South Africa. Major iron formations disappeared from the rock record after the GOE (Bekker et al., 2014), with notable exceptions being the ca. 1.88 Ga granular and banded iron formations of the Lake Superior district and Labrador in North America and the coeval Frere Formation in Western Australia, and the Rapitan-type iron formations deposited in the Neoproterozoic in association with Snowball Earth glaciations.

The importance of BIFs deposited before the GOE lies in their potential to elucidate the chemistry and composition of the oceans and atmosphere, and the evolution of microbial life, before oxygen contents began to increase in the Paleoproterozoic. However, to interpret the information preserved in BIFs we need to know how they formed, i.e., what minerals were precipitated, by what mechanisms and in which environments. The primary mineralogy of BIFs has been modified and overprinted by sediment compaction, early and late diagenesis, and multiple episodes of metamorphism, hydrothermal alteration (including mineralization) and oxidation (including recent weathering) such that the earliest minerals are difficult to identify (e.g., Bekker et al., 2014; Gole, 1980; Haugaard et al., 2016; Klein and Beukes, 1989; Trendall and Blockley, 1970). Even those BIFs that have experienced the lowest grades of metamorphism, ≤300°C, now contain multiple generations of iron oxides (magnetite, hematite, goethite), carbonates (siderite, ankerite, dolomite, calcite), and silicates (greenalite, stilpnomelane, minnesotaite, riebeckite) as well as chert or quartz. Detailed petrographic studies are required to determine which, if any, of these phases is primary. James (1954), in a very influential paper, proposed that depositional environment, most notably redox potential which was interpreted to be associated with water depth, controlled the primary minerals deposited in BIFs of the Lake Superior region. He recognized deposition of oxide (hematite and magnetite subfacies), carbonate, silicate and sulphide facies in BIF with increasing water depth and decreasing oxidation potential. This model has been widely adopted in the study of BIFs, however, Trendall and Blockley (1970) did not identify depositional facies in the Brockman Iron Formation of Western Australia where oxides, carbonates and silicates are distributed throughout the BIF members. In another seminal paper, Cloud (1968) proposed that the primary minerals in BIFs were ferric or ferro-ferric phases precipitated from ferrous-rich

seawater by oxygen produced by photosynthesizing microbes, and that the facies proposed by James were incidental to this process. Although organic carbon and microfossils are extremely rare in pre-GOE BIFs and no textural evidence has been presented to support this proposal, it underpins the preferred “consensus” model (e.g., Konhauser et al. 2017; Robbins et al., 2019). Despite the popularity of this model, recent studies have also proposed magnetite as a primary precipitate in BIFs (e.g., Sun and Li, 2017; Thibon et al., 2019) while others have suggested the Fe-silicate greenalite (Fe3Si2O5(OH)4) as the earliest mineral phase (e.g., Johnson et al., 2018; Rasmussen et al. 2015b, 2017). Experimental studies (Jiang and Tosca, 2019; Tosca et al., 2019) have indicated that while siderite is unlikely to nucleate homogeneously in the pre-GOE water column it will crystallize heterogeneously on the seafloor. Therefore, the primary mineral precipitates in BIFs and how they formed, whether biotically or abiotically, are still the subject of intense debate, and the influence of depositional environment on the minerals that were precipitated and preserved needs to be resolved before the chemical archive preserved in BIFs can be interpreted. 2. Objectives The objectives of this study are to identify the earliest-formed minerals in BIFs from a wide range of depositional environments in order to place constraints on the origin of BIFs and to evaluate the influence of sedimentary environment on the phases that were precipitated and preserved. We have used high-resolution petrographic techniques to examine samples from drill core of ferruginous cherts and BIFs interlayered with volcanic rocks in greenstone belts (Algoma-type BIFs), and those from the best-preserved BIFs in sedimentary rock packages. The samples range in age from 3.46 Ga to 2.45 Ga and are derived from greenstone belts in the Pilbara and Yilgarn cratons in Western Australia, and throughout the Hamersley and Transvaal

basins. They include samples from deep water volcanic environments, from deep water shelf and slope sedimentary environments, and from shallow-marine settings above wave-base. BIFs and granular iron formations (GIFs) from post-GOE successions are not included in this study as they formed under different environmental conditions, namely, the presence of atmospheric oxygen. It has been stated that no original minerals are present in even the best-preserved BIFs (Bekker et al., 2014), these having been destroyed by diagenesis and metamorphism. However, recent nano-scale petrographic examination of finely laminated cherts has revealed particles of the ferrous silicate greenalite that preserve fabrics analogous to those of freshly deposited mud (Rasmussen et al., 2019a), leading to the interpretation that greenalite was a primary precipitate in BIFs of the Brockman Iron Formation, Hamersley Group, Western Australia and the Nauga and Griquatown Formations of the Transvaal Supergroup, South Africa (Johnson et al., 2018; Rasmussen et al., 2015b, 2017, 2019b). These results suggest that nano-scale petrographic investigations can be used to identify the earliest minerals in the best-preserved BIFs and cherts, and thereby provide constraints on the origin of BIFs and the information that they may contain about ocean and atmosphere chemistry prior to the GOE. 3. Analytical Methods Many diamond drill cores from a wide range of low-metamorphic-grade environments were logged to provide geological context for samples collected for studies of BIFs and iron ore. Hundreds of polished thin sections were prepared and examined by petrographic microscope in transmitted and reflected polarized light to document textural relationships and mineral paragenesis. Some samples have layers of chert that have a dusty appearance in plane-polarized light due to inclusions of submicron mineral grains (referred to hereafter as “dusty” chert). The dusty cherts were studied with backscattered electron (BSE) imaging using TESCAN VEGA 3

(LaB6 source) and FEI Verios (field-emission source with better resolution than LaB6 source) scanning electron microscopes (SEMs) located in the Centre for Microscopy, Characterisation and Analysis (CMCA) at the University of Western Australia (UWA). Each SEM was fitted with an Oxford Instruments X-Max energy dispersive X-ray spectrometer (EDS), which was used for quantitative and qualitative chemical analysis of mineral grains using AZtec software from Oxford Instruments. For mineral grains too small for quantitative analysis by EDS (<1-2 m), optical properties and EDS spectra were used for tentative identification of mineral species. For example, submicron grains having EDS spectra with major O, Si and Fe peaks were interpreted to be greenalite [Fe3Si2O5(OH)4], while grains with minor peaks for Mg, Al and K, in addition to O, Si and Fe, were interpreted to be stilpnomelane [K(Fe,Mg)8(Si,Al)12(O,OH)27]. To confirm the identity of the submicron inclusions, lamellae (ca. 10 x 5 m) for transmission electron microscope (TEM) studies were cut from thin sections having areas of dusty chert identified by SEM as enclosing Fe-silicate, Fe-carbonate or Fe-oxide nanoparticles. Focused ion beam (FIB) techniques were used to prepare ca. 100 nm thick TEM lamellae using an FEI Helios NanoLab G3 CX DualBeam instrument located at CMCA, UWA. Areas of polished thin sections selected for TEM analysis were first coated with a strip of Pt ca. 2 m thick to protect the surface, then trenches ca. 7 m deep were milled on either side of the strip using a Ga ion beam with 30 kV voltage and 9 nA current. The foil was then cut away from the sample and welded to a Cu TEM grid. The foils were thinned with the Ga ion beam at 30 kV and 0.79-0.23 nA, before cleaning at 5 kV and 41 pA, and polishing at 2 kV and 23 pA. Transmission electron microscope data were collected at 200 kV using an FEI Titan G2 80– 200 TEM/STEM with ChemiSTEM technology located at CMCA, UWA. High resolution TEM (HRTEM) images and electron diffraction techniques, high angle annular dark-field (HAADF)

scanning TEM (STEM) images, which highlight differences in thickness and mean atomic number, and EDS element distribution maps and spectra were used to identify the nanoparticles. TEM images were processed using TIA (TEM Imaging and Analysis) software from FEI, ImageJ (Fiji) software and Digital Micrograph software from Gatan Incorporated. EDS spectra and maps were collected and processed using Esprit software from Bruker Corporation, which collects a full energy spectrum from each pixel in the map (a hypermap). The drill core samples examined by TEM are detailed in Table 1. 4. Sedimentary Structures If BIFs formed as chemical sediments, then the primary Fe-minerals would likely be similar in size to those precipitated from seawater at mid-ocean ridges today, i.e. <1 m in size (Feely et al., 1992). These grains will not be preserved where the rocks have experienced compaction and recrystallization but may be found where the sediments preserve original sedimentary laminations and other sedimentary structures. The search for primary precipitates has therefore concentrated on finely laminated intervals of dusty chert in ferruginous cherts, jaspilites and BIFs. 4.1 Sedimentary lamination The most obvious sedimentary structures in BIFs are laminations (Fig. 1) defined by differences in composition, i.e. regular alternations of iron-rich and silica-rich laminae on a scale of ca. 0.1-2.0 mm, which reflect deposition of the primary sediment. In dusty cherts, laminations may be defined by variations in the abundance of enclosed nanoparticles (Fig. 1A, B), by laminae of siderite (Fig. 1A, C, E) or by laminae of secondary minerals, e.g. riebeckite (Fig. 1E, F). BIFs are also layered on a larger, less regular scale of ca. 1.0-2.5 cm forming mesobands (Trendall and Blockley, 1970). Mesobands have been derived from primary laminations by differing levels of compaction and post-depositional recrystallization during diagenesis and

metamorphism (Beukes, 1980a; Trendall and Blockley, 1970). Laminations are best preserved in silica-rich mesobands and nodules due to early silicification which prevented compaction (Rasmussen et al., 2015a, 2019a). Examination of laminated cherts by optical microscopy and BSE imaging shows that the primary laminae may be defined by nanoparticles of Fe-silicate, Fecarbonate or hematite enclosed in microquartz. Coarser grained minerals within the laminae, e.g., siderite, ankerite, stilpnomelane, minnesotaite and magnetite are interpreted to have formed during later diagenesis and metamorphism (Rasmussen and Muhling, 2018). 4.2 Microgranules Microgranules are a common feature in Fe-rich BIFs such as the Dales Gorge and Joffre Members of the Brockman Iron Formation (Rasmussen et al., 2013; Trendall and Blockley, 1970). They form in laminae 1-2 mm thick and are composed of spherical bodies ca. 10-30 m in size enclosed in dusty chert (Fig. 2). Mineralogically, they may be composed of stilpnomelane, greenalite or hematite, which form masses of acicular crystals ca. 5-10 m long. Microgranules are interpreted to form by flocculation of mud-sized particles in the water column and deposition on the sea floor, and thus are original sedimentary components of BIF (Rasmussen et al., 2013). The microgranules are commonly surrounded by a moat of clear quartz (Fig. 2B), a texture first described by Spencer and Percival (1952) in weathered surface samples from BIFs of Singhbhum, India, and later termed “Percival texture” by Trendall and Blockley (1970, p. 123) following their recognition in BIFs of the Hamersley Group. The quartz moats were attributed to shrinkage of the spheres and subsequent filling by quartz cement. The microgranules described by Spencer and Percival (1952) were composed of hematite and probably formed from weathering of primary Fe-silicate microgranules. In fresh drill core, the microgranules are composed of greenalite (Fig. 2A, C) or stilpnomelane (Fig. 2B, D), although

they may be partly replaced by hematite or quartz but still preserve the “Percival texture” (Rasmussen et al., 2013). 4.3 Polygonal shrinkage textures Another feature of the best-preserved dusty chert is a network of clear quartz rims that divide it into irregular polygonal shapes ca. 10-30 m in size (Fig. 3A-D). Laminations, due to variations in the abundance of nanoparticles, are present within the chert polygons but are truncated by the clear quartz rims (Fig. 3B). The laminations are interpreted to be primary depositional features and therefore the quartz rims post-date deposition. The polygonal structures were first described in weathered BIFs of the Singhbhum craton, India, by Spencer and Percival (1952) who interpreted them to be shrinkage cracks. It is likely that the original silica cement was an amorphous opal (opal-A) which experienced dehydration and shrinkage on conversion to a more ordered form of opal (opal-CT) or chert. The shrinkage cracks were subsequently filled by a later generation of particle-free quartz cement. 4.4 Mud depositional fabrics Ferruginous cherts, jaspilites and BIFs with the best-preserved sedimentary laminations are composed of a cement of microcrystalline quartz with randomly oriented mineral grains, mostly <1 m, floating in it. These cherts have a dusty appearance in plane-polarized light due to the fine particles (Fig. 1), although these are generally not resolvable by optical microscope. SEM imaging of the chert reveals the presence of abundant, randomly oriented, sub-micron plates of Fe-silicate (Fig. 3) which are confined to irregular polygonal structures separated by rims of inclusion-free quartz. The polygonal structures formed by shrinkage of the original silica cement and the fact that the Fe-silicate particles are confined to the polygons shows that they formed part of the deposit before shrinkage occurred. Grains of siderite and hematite, ca. 1 m in size, may

also be present in the finely laminated chert, although they are generally less abundant than Fesilicate. They occur within the interpolygonal chert cement as well as enclosed in the polygons, suggesting that they were not part of the original sediment. The silicate particles are too small for quantitative EDS analysis, but qualitative analysis indicates that most are greenalite, although stilpnomelane is present in some samples. Analysis by TEM is required for positive identification of the particles. Bright-field and dark-field imaging shows that the particles in the chert are plate-like and range in size from 50 x 2 nm to 1000 x 100 nm (Fig. 4A-H). In some samples they form chains and loose flocs with face-to-face and face-to-edge contacts typical of freshly deposited mud (Bennett et al. 1981; Rasmussen et al., 2019a). Fast Fourier Transforms (diffractograms) of HRTEM images have been used to measure the d-spacing of the (001) plane of the nanoparticles, and most have values of 0.72-0.75 nm, consistent with greenalite (Fig. 4IK). In addition, a 2.1-2.3 nm modulation of the structure, which is characteristic of greenalite, is present in well-preserved crystals (Guggenheim et al., 1982, 1998). Element mapping of TEM lamellae by EDS shows that the greenalite particles may consist solely of Fe, Si and O, although most contain minor amounts of Mg and Al. Although greenalite has been confirmed in all samples, many of the platy crystals show some damage with consequent obliteration of diffraction patterns. Greenalite nanoparticles are the most abundant inclusions in dusty chert and on textural grounds they are interpreted to be primary precipitates: they are <1 micron in size; they define laminations; they preserve the textures of freshly deposited mud; and they predate the formation of shrinkage cracks in the enclosing silica cement. 5. Other “Dusty” Minerals 5.1 Stilpnomelane

Stilpnomelane is the most abundant phyllosilicate in the BIFs and shales of the Brockman Iron Formation. It has replaced glass shards in thin tuff beds and is abundant in shale bands, but also forms mesobands with magnetite and siderite in BIF members, where it is composed of interlocking crystals 10-50 m long, (Trendall and Blockley, 1970). Microgranules of stilpnomelane are common in BIFs of the Dales Gorge Member of the Brockman Iron Formation in Western Australia (Ayers, 1972; LaBerge, 1967; Rasmussen et al., 2013). Stilpnomelane microgranules are more common than those of greenalite, although they may be enclosed in greenalite-bearing dusty chert (Fig. 2D). Nanoparticles of stilpnomelane are much less abundant than those of greenalite but are present in some samples (Fig. 5A). The stilpnomelane nanoparticles have minor K, Mg and Al in addition to Fe, Si and O (Fig. 5B) and can be identified in TEM element distribution maps (Fig. 5C-E). Diffractograms show that the stilpnomelane grains have (001) d-spacing of ca. 1 nm (Fig. 5F-H). Individual grains of stilpnomelane are present in some foils while in others stilpnomelane is interlayered with greenalite (Fig. 5F-H). It is not apparent whether stilpnomelane has replaced greenalite to form microgranules and nanoparticles, or whether it co-precipitated with greenalite. 5.2 Siderite All of the finely laminated ferruginous cherts, jaspilites and BIFs examined in this study have laminae of fine-grained (2-50 m) siderite parallel to or intergrown with greenalite-bearing dusty chert (Fig. 1), as well as scattered coarser (50 m-0.5 mm) rhombic crystals. The size of the siderite crystals (>1 m) suggests that they are not depositional but are likely an early diagenetic phase. Finer crystals of siderite (200-500 nm) occur with greenalite in some dusty chert (Fig. 3A, B, D), but whereas the greenalite nanoparticles predate the development of chert

shrinkage polygons, siderite nanocrystals are found in the clear quartz cement around the polygons (Fig. 3B) indicating that they formed after dehydration of the dusty chert. 5.3 Hematite Although hematite (with goethite) is the dominant Fe-mineral in outcrops of jaspilites and BIFs, it is much less abundant in samples from below the limit of surface weathering (Rasmussen et al., 2016). While some hematite is clearly secondary, having replaced siderite, stilpnomelane or magnetite (Rasmussen et al., 2014a, 2016), the origin of submicron particles of dusty hematite in chert is more controversial. Dusty hematite is present in bands of jaspilite with laminae defined by the abundance of hematite (Fig. 6). The fine grain size and relationship to lamination have suggested that dusty hematite is the dehydration product of primary ferric oxides or hydroxides (e.g., LaBerge, 1964). However, SEM and TEM analysis shows that where dusty hematite is present, greenalite is always present too, and the textural relationships between them indicate that greenalite predates hematite (Rasmussen et al., 2016). Greenalite nanoparticles predate the development of chert shrinkage polygons, but hematite nanocrystals are commonly found in the clear quartz cement around desiccation polygons (Fig. 6G, K, O) indicating that they formed after dehydration of the initial dusty chert. Where hematite nanoparticles are abundant in the chert, they are commonly associated with fine porosity and plates of hematite fill, or partly fill, some pores (Fig. 6G, K, O). Some of the pores contain relicts of Fe-silicate suggesting that silicate nanoparticles have been dissolved and that hematite has grown in some of the resulting voids (Fig. 6L) (Rasmussen et al., 2016). 6. BIFs hosted in Archean Greenstone Belts Banded iron formations and ferruginous cherts are common rock types in Archean greenstone belts, ranging in age from ca. 3.8 Ga (e.g., Isua, Nuvvuagittuq) to ca. 2.7 Ga. While

the tectonic settings of greenstone belts remain open to interpretation, they are dominated by ultramafic to felsic lavas and volcaniclastic sedimentary rocks, implying a largely submarine, volcanic-dominated depositional environment. Most greenstone belts have experienced multiple episodes of deformation, including isoclinal folding and strike-slip faulting, as well as metamorphism at temperatures above 350°C, and for these reasons greenstone-hosted sequences have not featured strongly in investigations into the origin of BIFs. However, less deformed and metamorphosed greenstone belts are preserved in parts of some Archean cratons, and the BIFs in these belts can help constrain the processes by which BIFs formed. 6.1 Marble Bar Chert: 3.46 Ga The Pilbara Craton in Western Australia contains some of the oldest and best-preserved greenstone belts in the world. The craton comprises the East Pilbara Terrane, the stable core, and the West Pilbara Superterrane (Fig. 7) which was accreted to the East Pilbara Terrane along the Tabba Tabba Shear Zone during the Prinsep Orogeny at 3.07 Ga (Van Kranendonk et al., 2007). The Pilbara Supergroup (3.50-3.23 Ga) comprises East Pilbara greenstone belt rocks composed of ultramafic-mafic-felsic volcanic cycles and associated felsic volcaniclastic rocks, chert and BIFs (Hickman, 2004; Smithies et al., 2005; Van Kranendonk et al., 2007) and includes rocks of the Warrawoona, Kelly and Sulphur Springs Groups, as well as the Strelley Pool Formation (Hickman, 2009). It is overlain by the Soanesville Group (Fig. 7), a predominantly sedimentary rock package interpreted to record extension and rifting of the Pilbara Craton at ca. 3.2 Ga (Hickman, 2009; Van Kranendonk et al., 2010). Fragments of the craton that were rifted from it during this event were brought together again in the Prinsep Orogeny, and all were unconformably overlain by rocks of the Gorge Creek Group of the De Grey Supergroup which occur in both the East Pilbara Terrane and the West Pilbara Superterrane (Fig. 7). Ferruginous

cherts, jaspilites and BIFs are components of the Pilbara Supergroup and Soanesville Group, as well as the Gorge Creek Group. The Marble Bar Chert (MBC) is a member of the ca. 3.46 Ga Duffer Formation of the Warrawoona Group (Van Kranendonk et al., 2007). At surface, the MBC comprises white, red and black banded chert or jaspilite and some BIF (>15% Fe). A NASA-funded drill hole, ABDP1, revealed rocks with a complex history of sedimentation, silicification, recrystallization, mineral replacement and deformation in the MBC (Rasmussen et al., 2014b). The sequence in ABDP1 dips steeply to the east and is overturned, but chlorite thermometry (using the calibration of Lanari et al., 2014) indicates a metamorphic temperature of ca. 300°C. The fine lamination and absence of traction structures in the chert, combined with the presence of pillowed basalt above and below it, indicate that the MBC was deposited in relatively deep water in proximity to mafic and felsic volcanoes and may have a dominant hydrothermal or exhalative component. Sample ABDP1 20-62A (SI Fig. 1) is a white to buff, laminated chert (quartz grainsize 2050 m) with scattered euhedra of pyrite (up to 0.25 mm) and a network of fine quartz veins. Finely laminated chert with 0.4-1.0 mm thick laminations due to varying densities of fine mineral inclusions alternates with bands of massive, dust-free chert 5-10 mm thick. Where the dust is most dense, shrinkage polygons are poorly defined but are better developed where the dust is sparse (Fig. 3A, SI Fig. 1C). The dust is mostly composed of greenalite (50-200 x 5-20 nm), with scattered equant grains of siderite ca. 2-5 m in size (SI Fig. 1C-E). Sample ABDP1 19-63B (Fig. 6A-D; SI Fig. 2) is a white, red and buff laminated chert, with pyrite euhedra mostly concentrated in a single layer. The sample is similar to ABDP1 20-62A except that, in the red laminae, chert polygons are dusted with very fine hexagonal plates of hematite (up to 2 m) in addition to greenalite and siderite (Fig. 6C & D; SI Fig. B-E).

Greenalite is confined to shrinkage polygons, but hematite and siderite occur as well in the clear quartz cement between polygons (Fig. 3A). 6.2 Cardinal Formation Soanesville Group: ca. 3200 Ma The Soanesville Group lies disconformably or unconformably on the Sulphur Springs Group (Fig. 7), which is host to volcanogenic Cu-Zn sulphide mineralization in the Kangaroo Caves Formation in the Soanesville greenstone belt (Vearncombe et al., 1995). There is no direct date for the Soanesville Group. Rasmussen et al. (2007) derived a Sensitive High Resolution Ion Microprobe (SHRIMP) U-Pb age of 3.19 Ga for xenotime outgrowths on detrital zircon in sediments near the base of the group, which provides a minimum age, while the underlying Sulphur Springs Group has been dated to ca. 3.23 Ga (Buick et al., 2002). The basal Cardinal Formation comprises grey shale (Van Kranendonk et al., 2006) at surface, while in drill core from the Sulphur Springs volcanic-hosted sulphide deposit the Kangaroo Caves Formation is unconformably overlain by a package of turbiditic sedimentary rocks including sideritic chert and graphitic shale interpreted to have been deposited in a submarine fan or deep marine shelf environment (Vearncombe et al., 1998). Sample 59960B (SI Fig. 3) is a layered sideritic chert of the Cardinal Formation. Part of the sample has been brecciated, with fragments of graphitic shale, siderite and chert, cemented by chert and carbonate. Chlorite in graphitic shale fragments records a metamorphic temperature of 290 ± 50°C using the calibration of Lanari et al. (2014). Pyrite cubes of different sizes (10-100 m) are dispersed throughout. The sample is layered on ca. 1 cm scale, with finer laminations (12 mm) preserved in some layers. Where lamination is preserved there are: massive laminae composed of siderite rhombs (40-50 m); siderite laminae mixed with very fine graphite ± chlorite; chert (ca. 20 m quartz grains) with scattered siderite rhombs; laminae of chert and

siderite rhombs (ca. 20 m) with euhedral pyrite crystals; and thin laminae of chert with very fine dust. The dusty chert laminae do not have shrinkage structures. BSE imaging and TEM analysis show that the dust in the chert laminae comprises siderite rhombs (<1-5 m) and acicular to platey crystals of greenalite up to 2 m long enclosed in chert (SI Fig.3C). The greenalite contains minor Al, while the siderite has minor Mg, Ca and Mn. Some of the siderite in this sample may have been a primary precipitate and is the subject of further investigation. 6.3 Nimingarra Iron Formation: 3104 Ma The Nimingarra Iron Formation, as defined by Sheppard et al. (2017), is confined to the Shay Gap and Goldsworthy belts on the north-eastern margin of the Pilbara craton (Fig. 7). The belts have been folded and extensively faulted, and while metamorphic grade has been described as “very low grade” (Williams, 2003) it has not been quantified. The Nimingarra Iron Formation has been correlated with the Cleaverville Formation (ca. 3.02-3.01 Ga) and included in the Gorge Creek Group (Fig. 7) by Van Kranendonk et al. (2006), but a SHRIMP U-Pb age of ca. 3.1 Ga casts doubt on this correlation (Sheppard et al., 2017). The Nimingarra Iron Formation is bounded by unconformities and comprises three informal members: a lower BIF member, a middle mudstone member and an upper BIF member (Sheppard et al. 2017). The BIF members are composed of monotonous jaspilitic BIFs with rare beds of coarsely laminated chert. The middle mudstone member is made up of turbiditic sandstone, laminated mudrock, pyritic black shale and coarsely laminated chert, indicating a low-energy, deep-water depositional environment. A tuff within the middle mudstone member gave a SHRIMP U-Pb zircon age of 3104 ± 16 Ma (Sheppard et al., 2017). No fresh drill core is available from the Nimingarra Iron Formation, and sample BBSG-446 was collected from Pit 7 of the Shay Gap iron mine (Fig. 7). The sample is layered on 5-10 mm

scale, with bands of laminated chert interlayered with bands of coarse-grained hematite (SI Fig. 4). The chert layers have 1-2 mm laminations, including some of fine-grained microplatey hematite. For the most part, the lamination in the chert is defined by variations in the abundance of dusty inclusions, and some laminae preserve shrinkage structures (SI Fig. 4B). BSE imaging shows that the dustiness of the chert is caused by platey nanoparticles of Fe-silicate (<1-2 m). It is noticeable that there are numerous holes or pores (1-10 m) in the chert in this sample (SI Fig. 4C). In TEM-EDS maps, the silicates are plates from 250 nm up to 1 m and are composed of Fe, Si, O and minor Mg. No diffraction data could be obtained from the silicate nanoparticles as their crystal structure was damaged. Some of the numerous submicron holes in the chert are lined by silicate grains, suggesting that particles have been dissolved and that weathering could have destroyed and modified the silicate nanoparticles in this sample. The presence of greenalite could not be confirmed but this result highlights the necessity of using fresh drill core samples to identify the depositional nanoparticles in BIFs. 6.4 Wilgie Mia Formation: 2752 Ma Greenstone belt supracrustal sequences in the Yilgarn Craton are younger than those of the Pilbara Craton but are generally less well preserved. The Youanmi Terrane is the nucleus of the craton to which the terranes of the Eastern Goldfields Superterrane were accreted (Cassidy et al., 2006). Greenstone belt rocks of the Murchison Domain of the Youanmi Terrane belong to the Murchison Supergroup (Fig. 8) (Van Kranendonk and Ivanic, 2009; Van Kranendonk et al., 2013), which comprises four groups: the Mount Gibson Group (2960-2935 Ma); the Norrie Group (2825-2805 Ma); the Polelle Group (2800-2735 Ma); and the Glen Group (2735-2700 Ma). Each group contains significant thicknesses of BIF interlayered with quartzite and/or felsic volcanic rocks. The Polelle Group has a basal quartzite and BIF formation, the Coodardy

Formation, overlain by the Meekatharra Formation of komatiite, komatiitic basalt and tholeiitic basalt. The andesitic volcanics and felsic volcanics and volcaniclastics of the Greensleeves Formation overlie the mafic volcanics and are in turn overlain by the Wilgie Mia Formation composed of BIFs and interlayered felsic volcanics. In the Weld Range, the Wilgie Mia Formation has been intruded by extensive sills of dolerite and gabbro of the Gnanagooragoo Igneous Complex, leaving screens of BIF interlayered with mafic intrusive rocks (Van Kranendonk and Ivanic, 2009; Van Kranendonk et al., 2013). The mineralogy and petrology of Wilgie Mia Formation BIF from the Weld Range were described by Gole (1980), and as noted by him this is one of very few areas in the Yilgarn Craton where metamorphic grade is sub-greenschist facies. Peak metamorphic temperatures are estimated to be 320±50°C (Gole, 1980). More recently, Czaja et al. (2018) presented Fe-isotope analyses of samples of Wilgie Mia BIF. The drill core examined by Gole (1980) is no longer available, however, a more recent, north-dipping mineral exploration drill hole drilled on the northern side of the range intersected BIFs (from 95-118 m) and felsic volcanics of the Wilgie Mia Formation (Fig. 8). Samples 95WGD2-6 (SI Fig. 5) and 95WGD2-7A (SI Fig. 6) are BIFs and consist of bands of magnetite from ca. 1 mm to 1.5 cm thick, interlayered with bands of minnesotaite, siderite and laminated chert (1-2 cm thick). Zoned rhombs of siderite-ankerite (0.10.5 mm) are distributed throughout. In some intervals, there are fine laminae (ca. 1 mm) of red hematite, but the hematite is secondary having replaced minnesotaite (SI Fig. 6A). The centres of magnetite bands are composed of massive anhedral magnetite, while the edges are made up of octahedral crystals 50-150 m in size. The magnetite bands may be slightly discordant to finer lamination and commonly bifurcate. Siderite bands comprise interlocking rhombic crystals (2050 m) with minor chert and minnesotaite and are mostly interlayered with magnetite bands.

Minnesotaite bands are composed of mats of unaligned plates and needles of minnesotaite, with minor chert, siderite and magnetite. Laminated chert bands comprise equant quartz grains (5-20 m) enclosing discontinuous laminae (ca. 1 mm thick), or scattered rhombs, of siderite (20-50 m) and bowties of minnesotaite. Some chert bands have laminae enclosing dust within polygonal shrinkage structures (SI Figs 5C & 6C). The dustiness of the chert is caused by very fine grains of greenalite, 500-600 nm long and <100 nm wide. There are also tiny rhombs of siderite (1-2 m) in the dusty laminae (SI Figs 5 & 6). 7. Hamersley Group The 2.77 - >2.21 Ga Mount Bruce Supergroup overlies the granite-greenstone terranes of the Pilbara Craton in Western Australia. It comprises the basalt-dominated Fortescue Group (2.772.63 Ga), the BIF- and shale-rich Hamersley Group (2.63-2.45 Ga), and the siliciclasticdominated Turee Creek Group (2.45 - >2.21 Ga). The lack of sediments deposited in high-energy environments and presence of distal turbidites and hemipelagites has led most researchers to propose a deep marine shelf or basin-floor setting for deposition of the Hamersley Group (e.g., Blake and Barley, 1992; Krapež et al., 2003; Morris and Horwitz, 1983; Pickard et al., 2004; Simonson et al., 1993; Trendall and Blockley, 1970). Major iron formations in the Hamersley Group comprise the Marra Mamba, Brockman (Dales Gorge Member), Brockman (Joffre Member), Weeli Wolli and Boolgeeda Iron Formations, with less prominent BIFs and ferruginous chert members in the Wittenoom Formation (Bee Gorge Member), Mount Sylvia Formation (Bruno’s Band) and Mount McRae Shale (Colonial Chert) (Fig. 9). Metamorphic grade, derived from assemblages in basalts of the underlying Fortescue Group (Smith et al., 1982), is low and ranges from ca. 270°C in the vicinity of drill hole ABDP9 (Fig. 9A), through

ca. 325°C (Mitchell 2 and SGP001) to ca. 350°C (DDH44). The intensity of deformation also increases from north to south, towards the craton margin. The Hamersley Group contains the best-preserved pre-GOE BIFs in the world and has been the subject of many detailed petrographic (e.g., Ayers, 1972; Ewers and Morris, 1981; Grubb, 1971; Haugaard et al., 2016; Huberty et al., 2012; Klein and Gole, 1981; Morris, 1980; Pecois et al., 2009; Trendall and Blockley, 1970; Warchola et al., 2018) and geochemical studies (e.g., Alibert, 2016; Baur et al., 1985; Becker and Clayton, 1972; Johnson et al., 2008; Kaufmann et al., 1990; Steinhofel et al., 2010; Swanner et al., 2013; Williford et al., 2011). It has dominated research into the origin of BIFs and the constraints they place on the chemistry of the oceans and atmosphere, and the influence of life in their deposition. 7.1 Marra Mamba Iron Formation: 2597 Ma The Marra Mamba Iron Formation, the basal formation of the Hamersley Group, conformably overlies the Jeerinah Formation of the Fortescue Group and is conformably overlain by shaley dolomite and dolomite of the Wittenoom Formation (Fig. 9A). The formation occurs throughout the Hamersley Basin and ranges in thickness from ca. 180-230 m but is markedly thinner in the north and northeast. Outcrop of the formation is generally poor and it is best known from drill core intersections (Blockley et al., 1993; Davy, 1985; Klein and Gole, 1981). The Marra Mamba Iron Formation has been divided into three members: two comprising predominantly BIF separated by a shaley unit. The lowermost BIF member is the Nammuldi Member whose lowest part has been sampled in drill hole DDH186 (Fig. 9A). The Nammuldi Member is typically about 130 m thick and is characterised by chert pods (Blockley et al., 1993). In DDH186, it consists of alternating bands of greenish minnesotaite, dark brown or black stilpnomelane and light-colored chert with carbonate. The proportion of magnetite in the member

varies widely and it may be entirely absent, particularly at the base (Klein and Gole, 1981). Pyrite is present as scattered crystals, concentrated into layers in some places, and is more abundant towards the base of the member. Hematite is absent. The BIF is interlayered with numerous shale bands which contain minor carbon. Much of the BIF in the Nammuldi Member consists of mats of minnesotaite, but sample DDH186-1C is a chert-rich sample with three bands 1-2 cm thick (SI Fig. 7): a dark brown band composed of clusters of unaligned needles of stilpnomelane (ca. 50 m long) in chert (quartz grain size ca. 20-30 m); a mostly white band composed of chert (quartz grains ca. 20-30 m) with scattered coarse rhombs (0.5-1.0 mm) of zoned carbonate, rosettes and bowties of minnesotaite (ca. 0.25 mm) and fine-grained stilpnomelane needles (ca. 50 m); and a buff to pale green band composed of chert containing Fe-silicate dust, with scattered bowties of minnesotaite (ca. 0.25 mm) and poorly formed rhombs of ankerite with quartz inclusions (ca. 0.5 mm). The chert in the pale green band has variable grain size (ca. 20-100 m) and contains clusters of fine dust in shrinkage polygons (SI Fig. 7B). BSE and TEM imaging confirms the presence of tiny needles (<800 x 200 nm) of greenalite in quartz (SI Fig.7C-E), with larger crystals of minnesotaite and ankerite. 7.2 Wittenoom Formation, Bee Gorge Member: 2565 Ma The Bee Gorge Member of the Wittenoom Formation is composed of shales with lesser amounts of dolomite, chert and BIF. Three marker beds, the Crystal-rich tuff, the Main tuff interval and the Spherule marker bed, are present in the Bee Gorge Member throughout most of the Hamersley Basin (Simonson et al., 1993). Sedimentological analysis indicates that the Bee Gorge Member, and overlying Mt Sylvia Formation and Mount McRae Shale, were deposited in

deep-water, basinal environments. Thin arenite and intraformational conglomerate beds within the shales were deposited from low-density turbidity currents (Simonson et al., 1993). The Bee Gorge Member was intersected in the NASA-funded drill hole ABDP9 between 224.3 and 472.1 m. Sample ABDP9-37C is a thinly laminated, pale-green to buff-colored chert that shows well-developed polygonal shrinkage structures that cut the laminations (Fig. 1A, B; Fig. 3B; SI Fig. 8), and that are separated from each other by clear microquartz. Within the polygons, the laminations are defined by changes in abundance of randomly oriented nanoparticles of greenalite 5-500 nm long (Rasmussen et al., 2015), and discontinuous laminae of very fine siderite rhombs (200-500 nm). The greenalite has minor amounts of Al and Mg and the siderite also has a trace of Mg. In addition to dusty, greenalite-bearing chert, this sample contains bands of inclusion-free chert and fine-grained siderite (5-50 m). Siderite crystals up to 50 m are found in the dusty layers, but where thin layers or lenses of siderite are present, the chert is dust-free. The grain size of quartz is consistent within laminae, but varies between them, from ca. 20 m to 50 m. 7.3 Mt Sylvia Formation: ca. 2505 Ma The Mt Sylvia Formation is approximately 30 m thick and conformably overlies the Wittenoom Formation (Fig. 9). It consists of shale and dolomite with three thin beds of BIF, one of which (Bruno’s Band) forms the top of the formation and displays remarkable lateral continuity across the Hamersley Basin, which makes it a useful stratigraphic marker. Drill hole ABDP9 intersected the Mt Sylvia Formation between 188.6 and 224.3 m depth. Sample ABDP9-56A comprises a mesoband of red chert (ca. 2 cm) between two dark grey mesobands (1-2 cm) composed of siderite, stilpnomelane, magnetite, hematite and chert (Fig. 6E-H; SI Fig. 9). All of the mesobands have finer lamination on <1 mm scale. The dark grey

bands consist of intergrowths of anhedral siderite, stilpnomelane and chert (quartz grain size ca. 30-50 m), with scattered magnetite crystals from 100 m to ca. 1 mm across. There are some laminae (25 m to 0.5 mm) of smaller magnetite crystals (20-50 m) with or without thin bands of pyrite crystals (ca. 5 m). Very fine grains of hematite (ca. 5 m) and pyrite are dispersed through these mesobands. Between the two grey mesobands is a band of dusty red chert with scattered rhombs of zoned siderite-ankerite (30-50 m), euhedral magnetite crystals (up to 150 m) and bowties of minnesotaite (50-100 m) (SI Fig. 9C). The chert is fine-grained (ca. 20 m) with laminations due to the abundance of nanoparticles. The color of the chert band varies from bright red where there are nanoparticles of hematite to buff colored. SEM-BSE imaging shows that even where there are nanoplates of hematite (ca. 1-5 m across) there are also abundant submicron nanoparticles of greenalite (Fig. 6G & H; SI Fig. 9C). The greenalite nanoparticles are confined to polygonal shrinkage structures while hematite is present in both the polygons and the clear chert cement between polygons (Fig. 6G). Where hematite is abundant, the chert enclosing the greenalite nanoparticles is microporous (Fig. 6G & H). STEM HAADF imaging shows abundant loose flocs composed of ca. 1 m greenalite plates, and subhedral crystals of hematite (Fig. 6H; SI Fig. 9D). 7.4 Colonial Chert, Mount McRae Shale: 2504 Ma The Mount McRae Shale conformably overlies the Mount Sylvia Formation, the bottom of the formation being defined by the top of Bruno’s Band and the top by the bottom of the lowest BIF macroband of the Dales Gorge Member of the Brockman Iron Formation (Fig. 9). The Mount McRae Shale comprises black shale, chert, carbonates and cherty BIF. The top 12 m is characterized by interbedded BIF and shale, which have been informally separated out as the

Colonial Chert member. The Colonial Chert was intersected from 418-474 m in drill hole Mitchell 2. Sample Mitchell2-418 has pale-green and black mesobands, 5 mm to 3 cm thick, of laminated chert and magnetite respectively (Fig. 10A; SI Fig. 10). There are abundant stylolites lined with siderite rhombs and plates of greenalite, enclosed in calcite, parallel to the laminations in the green mesobands (Fig. 10B). In some of the pale green bands there are microgranules of greenalite enclosed in dusty chert (Fig. 10C) with scattered rhombs of siderite and bowties of minnesotaite. The dusty chert (quartz grain size ca. 20 m) comprises irregular polygons enclosing abundant nanoparticles of greenalite, surrounded by rims of clear chert (Fig. 10D). TEM imaging shows that the greenalite forms loose flocs ca. 1 m in size, as well as isolated grains 200-500 nm long (SI Fig. 10). Surrounding the polygons and microgranules are loose flocs ca. 5-10 m across of slightly larger greenalite crystals surrounded by dusty chert (Fig. 10D, E). Where microgranules are abundant and have coalesced, greenalite has been partly replaced by zoned siderite, as well as overprinted by minnesotaite (Fig. 10F). Magnetite too has partly replaced greenalite, and forms aggregates of irregularly shaped grains that overprint greenalite-bearing chert (Fig. 10G-I). Larger magnetite crystals have crystallographically aligned inclusions of quartz (Fig. 10I). Within the green chert bands, magnetite forms ragged aggregates or clots (ca. 100 x 500 m) of grains about 20-30 m in size, overprinting greenalite (Fig. 10A, B). Despite its fine grain size, greenalite in this sample can be identified by petrographic microscope and it forms lenses and laminae throughout the green chert (Fig. 10A, B). Zoned siderite forms isolated, subhedral crystals ca. 50 m in size throughout the green chert, as well as laminae that range in width from single grains ca. 50 m wide to bands ca. 0.5 mm wide. Randomly oriented needles and bowties of minnesotaite (50-100 m long) are disseminated

throughout the sample overprinting greenalite and chert. The magnetite bands are composed of aggregates of grains, similar to those in the laminated chert bands, and are surrounded by finegrained rhombs of siderite enclosed in calcite cement. This sample is one of few that contain greenalite that is readily identified by optical microscope. As well as greenalite microgranules and dusty chert polygons, it preserves evidence for greenalite having been replaced by siderite and magnetite, as well as overprinted by minnesotaite. 7.5 Dales Gorge Member, Brockman Iron Formation: 2495-2461 Ma The Brockman Iron Formation is the largest iron formation in Australia, and probably the world, at 610 m thick and covering an area of 77,700 square kilometers. It is certainly one of the best-preserved pre-GOE BIFs and one of the most studied. It comprises alternating units of BIF and shale, with four defined members: the Dales Gorge Member (BIF), Whaleback Shale, Joffre Member (BIF) and Yandicoogina Shale (Fig. 9). The Dales Gorge Member is the best exposed unit, and as host to economic deposits of iron ore, and formerly blue asbestos, it also provides the best access to drill core. The Dales Gorge Member is 180 m thick and consists of 16 macrobands of shale alternating with 16 BIF macrobands (Trendall and Blockley, 1968, 1970). The Joffre Member is much thicker (366 m), one of the thickest continuous sections of BIF in the world, but is not as well exposed. However, drill core of the Joffre Member is available, e.g. from DD98SGP001 from Silvergrass (Fig. 9). The Dales Gorge Member was described in great detail by Trendall and Blockley (1970), although they did not recognize that it contains greenalite. Four samples of Dales Gorge BIF have been examined by TEM in this study: one from Mitchell 2, one from DD98SGP001 from Silvergrass, and two from DDH44 near Paraburdoo (Fig. 9). The samples all have diverse

microbands and mesobands composed of various combinations of chert, siderite, stilpnomelane and magnetite. Sample Mitchell2-405C (SI Fig. 11) consists of laminae (0.5-1.0 mm thick) composed of euhedral magnetite crystals (0.1-0.5 mm), that bifurcate around elongate nodules of calcite (ca. 3 x 1 mm) (see Rasmussen and Muhling, 2018, Fig. 12). The magnetite laminae are interlayered with chert laminae (1-4 mm thick) composed of greenalite microgranules (ca. 15 m) set in chert (quartz grain size 20-30 m) enclosing greenalite nanoparticles (Fig. 2A, C). In places, the greenalite is concentrated into wavy laminae ca. 10 m wide that help define lamination (SI Fig. 11B). Clusters (20-30 m) of brown stilpnomelane needles, and needles and bowties of minnesotaite, are dispersed through the chert cement (SI Fig. 11C). Some of the chert layers have clusters of siderite crystals, and there are a few laminae (ca. 1 mm) of fine-grained siderite-ankerite. The chert matrix is overprinted by coarse (0.5-1.0 mm) rhombs of ankerite and octahedra of magnetite (Fig. 2A; SI Fig. 11B). Sample AGC985D from drill hole DD98SGP001 has centimeter-scale banding, with chertrich bands alternating with siderite bands (SI Fig. 12A, B). The chert bands are finely laminated (0.15-0.30 mm) and are composed of dusty chert (quartz grain size 20-30 m) containing very fine inclusions of greenalite with discontinuous thin laminae (0.05-0.2 mm thick) composed of siderite rhombs ca. 20-30 m in size (SI Fig. 12B, C). The proportion of fine greenalite dust varies between laminae, in part defining them. One lamina of dusty chert (0.2 mm thick) has well-developed shrinkage polygons and continuous laminae of siderite (0.2-0.3 mm) on either side of it (SI Fig. 12B). Rare pyrite cubes are scattered through the laminated chert. STEM HAADF imaging shows that greenalite nanoparticles (up to 1 m long) form face-to-edge aggregates or loose flocs in chert (Fig. 4B; SI Fig. 12D), while TEM-EDS mapping and HRTEM imaging shows that there are nanoparticles of stilpnomelane in addition to greenalite, and that

some nanoparticles are intergrowths of greenalite and stilpnomelane (Fig. 5). Siderite bands are composed of a mass of fine siderite crystals (20-30 m) with scattered coarser ankerite rhombs (SI Fig. 12C) and there are laminae 0.3-0.5 mm thick containing brown stilpnomelane needles (20-30 m), with minor chert and pyrite cubes (20-30 m). Sample DDH44-19 is laminated on 0.5-2.0 mm scale with laminae of clear chert, dusty chert, and chert with abundant siderite rhombs 30-50 m in size (SI Fig. 13A, B). The chert is mostly very fine grained (10-20 m) but is variable, with grains up to 50 m in some laminae. There is a lamina (ca. 5 mm thick) composed of siderite with irregular magnetite grains (30-100 m) surrounded by flakes of hematite (20-50 m) intergrown with the siderite. Magnetite forms poikiloblasts up to 1 mm in some siderite-rich laminae. Pyrite cubes are dispersed through the rock. BSE imaging shows that the dusty chert layers are composed of densely packed greenalite nanoparticles with irregular to spherical patches of clear chert, rather than well-formed polygonal structures (SI Fig. 13C). The spherical structures may be silicate microgranules that have been replaced by quartz (Rasmussen et al. 2013). Zoned siderite rhombs overprint the dusty chert (SI Fig. 13B, C). STEM HAADF images show greenalite nanoparticles from ca. 100 nm to 1 m long, with many of the larger particles forming house-of-cards aggregates or loose flocs (Fig. 4C). Sample DDH44-32A is an iron-rich BIF and is layered on 5 mm to 1 cm scale, with layers of laminated dusty chert alternating with laminated Fe-rich layers composed of siderite and brown stilpnomelane (SI Fig. 14A). The margins of the Fe-rich layers are marked by sinuous laminae containing very fine pyrite crystals. The quartz in the dusty chert layers has variable grain size (ca. 20-50 m) and encloses microgranules of brown stilpnomelane (Rasmussen et al., 2013), wavy laminae of siderite (10 m – 0.15 mm thick), and scattered siderite rhombs ca. 20

m in size (Fig. 2B; SI Fig. 14B). The size of the microgranules varies between different laminae, e.g., ca. 5-10 m in one and 20-25 m in another. The microgranules are surrounded by moats of clear microquartz forming Percival texture (Trendall and Blockley, 1970, p. 123). The stilpnomelane microgranules are enclosed in dusty chert containing abundant nanoparticles (up to 1 m long) of greenalite, which form loose flocs and clots (Fig. 2D; SI Fig. 14C-E) (Rasmussen et al., 2019a). Layers of coarse-grained siderite on the margins of the Fe-rich bands have overprinted and partly replaced adjacent chert which encloses stilpnomelane microgranules. 7.7 Joffre Member, Brockman Iron Formation: 2461-2454 Ma The petrology and chemostratigraphy of the Joffre Member in drill-hole DD98SGP001 have been described by Haugaard et al. (2016). A sample from 313.65 m (Silvergrass 16c B) is composed of a band of laminated chert ca. 1 cm thick between two bands 1-2 cm thick made up predominantly of stilpnomelane and magnetite, with hematite, siderite and minor chert (Fig. 1E, F; Fig. 6I, J; SI Fig. 15A). The laminated chert contains rare laminae (0.25-0.50 mm wide) of fine-grained (ca. 25 m) siderite crystals enclosed in dusty chert and overprinted in places by brown stilpnomelane and fibrous blue riebeckite (Fig. 1E). Lenses of brown stilpnomelane (ca. 100 m wide), composed of unaligned flakes 20-30 m long, have been overprinted by bunches of fibrous blue riebeckite. Lenses of dusty hematite in chert are 10-250 m wide and up to 2 mm long (Fig. 1F; Fig. 6I-K). They consist of tiny plates of hematite (1-5 m) enclosed in chert that has polygonal shrinkage structures (Fig. 6K; SI Fig. 15C). The cores of some dusty hematite lenses have been overprinted by brown stilpnomelane, riebeckite, quartz and rhombs of ankerite up to 250 m across (Fig. 6K). These various laminae and lenses are enclosed in a matrix of laminated dusty chert with polygonal shrinkage structures. Coarse grains (100-300 m) of rhombic ankerite overprint the minerals in the fine laminae and lenses (SI Fig. 15B). BSE

imaging shows that the dusty hematite lenses contain abundant nanopores, some partly occupied by hematite (Fig. 6K; SI Fig. 15C), as well as abundant submicron particles of Fe silicate. Hematite inclusions are present within the cores of shrinkage polygons but also in the clear chert that surrounds the polygons, whereas the fine silicate particles are confined to the polygon cores. The hematite lenses pass into polygonal chert enclosing abundant fine Fe-silicate particles but lacking hematite and nanopores. TEM analysis shows that the Fe-silicate nanoparticles (100 nm to 1 m long) include both greenalite, with minor Al and Mg, and stilpnomelane in this sample (Fig. 5). In the hematite lenses, nanopores have resulted from partial dissolution of silicate nanoparticles and replacement by hematite (Fig. 6L) (Rasmussen et al., 2016). 8. Transvaal Supergroup The Transvaal Supergroup is preserved overlying the Kaapvaal Craton in South Africa in two structural sub-basins: The Transvaal Sub-basin around Pretoria and the Griqualand West Sub-basin on the western margin of the craton. Contact metamorphism associated with the Bushveld Complex has affected the rocks in the Transvaal Sub-basin, but the succession in the Griqualand West Sub-basin has experienced only low-grade metamorphism and the rocks are well preserved in diamond drill cores (Fig. 11). The Ghaap Group in the Griqualand West Subbasin is a time-equivalent of the Hamersley Group in Western Australia and correlations between the two successions have been made based on the ages of interbedded tuffs and impact spherule layers as well as lithology (Beukes and Gutzmer, 2008; Pickard, 2002, 2003). According to some paleomagnetic reconstructions (de Kock et al., 2009) the two successions formed in a single depositional basin, however there are significant differences between their depositional settings. The Campbellrand Subgroup (Fig. 11) comprises a carbonate platform (the Ghaap plateau facies) with abundant stromatolites in intertidal and lagoonal facies, that grades southwards into a

deeper water slope and shelf facies (the Prieska facies). The slope and deep shelf facies are represented by the Nauga and Klein Naute Formations which both contain thin ferruginous cherts and BIFs. A marine transgression resulted in drowning of the Campbellrand carbonate platform followed by deposition of the Asbestos Hills Subgroup comprising the Kuruman BIF, and the overlying, clastic-textured Griquatown Iron Formation (Beukes and Gutzmer, 2008). The Asbestos Hills Subgroup is unconformably overlain in the southern part of the Griqualand West sub-basin by the Koegas Subgroup, comprising interbedded siliciclastics and iron formation. The siliciclastics, immature arkosic greywackes and subarkoses, are interpreted to be regressive units while terrigenous mudstones and iron formations record transgressive systems tracts (Schröder et al., 2011). The Ghaap Group is unconformably overlain by the Postmasburg Group, which comprises diamictite of the Makganyene Formation which interfingers with and underlies mafic to intermediate, submarine lavas of the Ongeluk Formation. The Makganyene diamictite is interpreted to have resulted from a Snowball Earth glaciation event related to the first rise in atmospheric oxygen (Kirschvink et al., 2000). The ca. 2425 Ma Ongeluk lavas (Gumsley et al., 2017) are overlain by the Fe- and Mn-rich chemical sediments of the Hotazel Formation, host to the world’s largest land-based Mn accumulation. The Hotazel Formation is capped by the Mooidraai Formation, a sucession of marine carbonates. 8.1 Upper Nauga Formation: 2521-2480 Ma The Nauga Formation is the deep-water carbonate succession that is equivalent to the Campbellrand carbonate platform. It has been divided into lower and upper members by the Kamden Member, a ferruginous chert and BIF that can be traced from the basin and slope facies onto the carbonate platform (Beukes and Gutzmer, 2008). The lower Nauga Formation contains cycles of microbialites and slope dolostones (Schröder et al., 2006). The upper Nauga Formation

is dominated by muddy and grainy slope dolostones with fewer microbialites. The proportion of chert and siliciclastic mudstones increases towards the top of the formation. The Agouron Drilling Program drilled two deep diamond drill holes through the slope and shelf facies of the Campbellrand carbonate platform: GKF01 on the slope and GKP01 intersecting deeper water basin facies (Fig. 11). Both cores are dominated by the Nauga Formation. Two samples of ferruginous chert from the upper Nauga Formation in drill hole GKF01 were selected for detailed SEM and TEM examination. Sample 100315-79B comprises interlayered green chert and grey carbonate on 1-5 mm scale (SI Fig. 16). The carbonate layers include laminae of fine-grained siderite enclosed in sparry dolomite-ankerite cement, with minor chert and scattered pyrite crystals. The siderite laminae are commonly wavy. The green chert layers show fine laminations (0.1-0.5 mm) due to variations in color caused by differences in the abundance of greenalite dust (Fig. 1C, D). The chert layers are composed of greenalite nanoparticles enclosed in polygonal shrinkage structures ca. 10-20 m in size (Fig. 3D). The chert laminae are graded from almost massive greenalite-bearing polygons with very little clear quartz cement between polygons to isolated polygons in clear quartz cement (Fig. 1C, D). Laminae of fine-grained siderite (5-10 m) occur preferentially in the massive greenalite laminae (SI Fig.16B) and may have partly replaced them. A few coarser grained (0.100.15 mm) ankerite rhombs are present in the siderite-quartz laminae in the green chert (SI Fig. 16B). Smaller (0.5-2 m) siderite crystals are enclosed in the chert polygons with greenalite, although they are coarser grained than the greenalite and occur in the chert cement between polygons as well as within the polygons (Fig. 3D; SI Fig.16C). Sample 100315-72A is a grey, green and red layered chert with layers ca. 1-5 mm thick (Fig. 6M; SI Fig. 17). It comprises predominantly finely laminated (10-100 m) chert enclosing fine

greenalite dust with polygonal shrinkage structures preserved in places. Laminae (50-100 m) composed of zoned dolomite-ankerite with sideritic rims alternate with the chert, and coarser grained rhombs (20-50 m) are scattered through the chert laminae or form discontinuous laminae. The coarser rhombs have abundant submicron inclusions of hematite, mostly in the dolomite-ankerite cores, giving rise to red or black layers where they are concentrated (Fig. 6M; SI Fig. 17A, B). In some laminae, the greenalite chert has abundant nanopores while in others there are inclusions of dusty hematite (Fig. 6N, O). Even sparsely distributed hematite particles give rise to red coloration (Fig. 6P). Greenalite is always present in the red laminae, many of which are discontinuous and pass into green chert lacking hematite (Fig. 6M, N). Minnesotaite is present in most layers of the rock but is particularly abundant in some (SI Fig. 17C). 8.2 Kuruman Iron Formation: 2480-2431 Ma The Kuruman Iron Formation conformably overlies the Campbellrand carbonate platform and is time-equivalent to the Brockman Iron Formation of the Hamersley Group. It is a classical mesobanded and laminated BIF, similar to the Dales Gorge Member of the Brockman Iron Formation, but does not have the macrobands characteristic of the latter (Beukes and Gutzmer, 2008; Trendall, and Blockley, 1970). It is interpreted to be a deep-water deposit resulting from flooding of the Campbellrand carbonate platform, although a persistent granular iron formation band is present at the top of the unit where it grades into the shallow-water Griquatown Iron Formation (Beukes, 1980b; Beukes and Gutzmer, 2008). Samples of the Kuruman Iron Formation were collected from the Gasesa drill hole located in the Kalahari Manganese Field (Fig. 11). In this area, there has been extensive replacement of the BIF by minnesotaite. Sample 030315-37C is composed of dusty chert, with poorly defined layers due to the abundance of dust enclosed in the chert and to irregular laminae of fine-grained

siderite (ca. 25 m), magnetite euhedra (10-200 m) and clusters of brown stilpnomelane crystals (25-30 m long) (Fig. 12A). Magnetite crystals are present throughout the sample and are also concentrated in a few monomineralic layers 1-2 mm thick (Fig. 12A, B). Bowties of fine-grained minnesotaite (up to 0.3 mm long) are present throughout the sample (Fig. 12B-E). The dusty chert (quartz grain size 20-30 m) contains polygonal shrinkage structures with abundant nanoparticles of greenalite (Fig. 12D, E). STEM HAADF imaging shows that the greenalite forms individual flakes and loose flocs composed of flakes up to 1 m in maximum length (Fig. 12F, G). 8.3 Griquatown Iron Formation: 2431 Ma The Kuruman Iron Formation passes upward into the Griquatown Iron Formation which is predominantly a shallow-water deposit characterized by clastic chert granules and chert-pebble conglomerate (Beukes, 1980b; Beukes and Gutzmer, 2008; Rasmussen et al., 2019b). The granules and pebbles have been derived by reworking of pre-existing iron formation deposits. Two samples of Griquatown Iron Formation from the Erin-3 drill hole, located in the Kalahari Manganese Field (Fig. 11), were examined by SEM and TEM. Sample 040315-62A is composed of a band 1 cm thick of massive siderite + stilpnomelane + hematite with scattered octahedra of magnetite (Fig. 13A). On either side of this band are layers ca. 5-7 mm thick composed of ellipsoidal chert-siderite grains ca. 0.5-1.0 mm in size, in microquartz cement (quartz grains 50-100 m) with minor siderite. Many of the grains have irregular overgrowths of hematite, and the hematite-bearing layers grade outwards into layers of chert-siderite grains with chert cement but no hematite (Fig. 13A-C). BSE imaging shows that many of the grains are composed of chert with fine irregular or rhombohedral inclusions of siderite but also with abundant pores. Coarser inclusions of siderite, and rims of siderite that partially or almost

completely replace some chert grains are common (Fig. 13B-D). However, some grains are composed of dusty chert, with or without thin siderite rims, and these grains contain nanoparticles of greenalite (Fig.13D-G). The greenalite particles are up to 500 nm in maximum dimension and contain trace Mg. The greenalite-bearing grains are interpreted to have formed from reworking of silicified greenalite mud (Rasmussen et al., 2019b). Either before or after reworking and redeposition, some greenalite was dissolved forming micropores which were then filled or partly filled by siderite. Continued replacement by siderite rims resulted in conversion of the cherty grains into siderite grains with remnants of chert (Fig. 13B-D; SI Fig. 19B, C). Sample 040315-68F comprises layers from 2-5 mm thick of massive siderite with scattered fine grains of magnetite and stilpnomelane, interlayered with bands ca. 2 mm thick dominated by either magnetite or stilpnomelane (SI Fig. 20A). Within the siderite there is a ca. 2 mm thick layer of ellipsoidal chert grains within chert cement (SI Fig. 20B), overprinted by laminae composed of ankerite rhombs (0.2-0.4 mm). Magnetite octahedra (0.1-0.2 mm) and pyrite crystals are sparsely scattered in the chert cement, and there are discontinuous laminae of fine magnetite grains (5-25 m in size). The chert grains have been flattened (0.3-0.5 x 0.1-0.15 mm) with long axes parallel to layering, and both the grains and the cement enclose nanoparticles of greenalite (SI Fig. 20C-E) (Rasmussen et al., 2019b). The greenalite flakes are up to 500 nm in maximum dimension, and contain a trace of Mg. In this sample, greenalite mud is interpreted to have been disrupted and reworked while the silica cement was partly plastic, before redeposition as intraclasts in younger greenalite mud. 8.4 Hotazel Formation: ca. 2.4 Ga Although not sampled for this study, nanoparticles of possible greenalite have been identified in drill core from the ca. 100–150-metre-thick Hotazel Formation (Lantink et al.,

2018). The Hotazel Formation comprises three Mn-ore layers, interlayered with BIF. Up to several metres of fine-grained cherty iron formation, rich in hematite and Mn-carbonate, occur at the contacts between BIF and Mn-rich bands. The BIF contains a large proportion of laminated jasper layers which consist of very fine grained hematite and Fe-silicate (< 1 μm) enclosed in chert (Lantink et al., 2018). In some BIF layers, the dusty hematite and Fe-silicate are enclosed in possible shrinkage structures. The Fe-silicate could not be identified due to its fine grain size (Lantink et al., 2018) and lack of TEM analysis, but it is strikingly similar to the Fe-silicate nanoparticles identified as greenalite in the BIFs and cherts described above. Paragenetic relationships between the dusty greenalite and hematite particles are not known. 9. Discussion Greenalite is ubiquitous in the best-preserved ferruginous cherts and BIFs deposited before the GOE, those of deep-water volcanic environments in Archean greenstone belts as well as the classical BIFs of the Hamersley and Transvaal basins. Its presence can be predicted in those cherts and BIFs that preserve sedimentary laminations. Although the particles are generally too small to be identified by optical microscope, the presence of dusty chert with a pattern of shrinkage cracks is an almost infallible guide to the presence of greenalite nanoparticles. Its association with sedimentary structures and its fabric preserved in early siliceous cement strongly indicate that greenalite was a primary precipitate in BIFs and cherts in a wide range of depositional environments, from deep-marine to shallow-water shelf. These observations are at odds with suggestions that greenalite formed only in localized environments such as restricted shallow-water settings or sediment pore waters (Konhauser et al., 2017; Robbins et al., 2019), and they have profound implications for how BIFs formed and how we interpret their geochemical signals.

Currently, the most widely accepted model for the deposition of banded iron formations (e.g., Bekker et al., 2014; Konhauser et al., 2017), at least those of Hamersley-Transvaal type, draws on the model proposed by Cloud (1965, 1968, 1972, 1973). In this model, the oceans are enriched in dissolved ferrous ion Fe2+(aq) derived from hydrothermal alteration of basaltic and/or ultramafic crust at oceanic vents. The Archean to Paleoproterozoic oceans are interpreted to be enriched in dissolved silica, SiO2(aq), derived from hydrothermal sources (Chakrabarti et al., 2012; van den Boorn et al., 2007), and close to saturation with respect to opal-A. The Fe2+(aq)and SiO2(aq)-rich ocean waters upwelled onto continental shelves where Fe2+ was oxidized and precipitated as ferric oxyhydroxides. Oxidation is believed to be biologically mediated, although abiotic precipitation by UV light is also possible although unlikely to be sufficiently effective to produce the giant BIF successions of the Hamersley and Transvaal basins (Konhauser et al., 2007). Proposed biological mechanisms for depositing ferric oxyhydroxides include oxidation by oxygen produced by oxygenic photosynthesizers (e.g. Cloud, 1965, 1968, 1972), or by anoxygenic photosynthesizers, e.g. photoferrotrophs (e.g., Konhauser et al., 2002). In this model, the primary sediments of BIF are considered to be ferric oxyhydroxides, organic matter and chert. The primary sediments were converted to siderite and magnetite by dissimilatory iron reduction (DIR) on the seafloor (Konhauser et al., 2017) such that all of the organic matter and most of the ferric oxyhydroxides were consumed. The remaining ferric oxyhydroxides were dehydrated to dusty hematite, considered to be a remnant of the primary sediment. Despite its wide acceptance (Konhauser et al., 2017), there is no petrographic evidence to support this model, and organic matter is exceedingly rare in pre-GOE BIFs. The measured fractionation of Fe isotopes in BIFs has been interpreted to support this model (e.g., Dauphas et al., 2017; Johnson et al., 2008), however, as the fractionation factor of Fe isotopes during precipitation of

Fe silicates in anoxic seawater is not known (Dauphas et al., 2017) alternative models cannot be tested. Until more data is available on the behavior of Fe isotopes during precipitation of Fe silicates and carbonates, during anoxic diagenesis of such sediments, during partial conversion of siderite to magnetite through low-grade metamorphism, and during partial oxidation of these minerals to hematite through repeated episodes of metamorphism and mineralization, then the current interpretation of Fe isotopes in BIFs is not definitive. Iron Formations interlayered with volcanic rocks in greenstone belts (Algoma-type BIFs) are interpreted to have a different origin as proximal volcanic exhalative-hydrothermal deposits (Bekker et al., 2014). Mineralogically they are the same as BIFs from sedimentary successions despite their different origin and depositional setting. All BIFs are Precambrian metamorphic rocks that preserve a wide range of minerals formed during early and late diagenesis, multiple episodes of metamorphism and fluid flow, and oxidation and mineralization. Mineralogically and texturally, they are exceedingly complicated rocks and unravelling their history is challenging. However, we have shown that remnants of the depositional fabric can be found in BIFs and cherts that range in age from 3.45 Ga to ca. 2.4 Ga. They occur in rare preservational windows in ferruginous chemical sediments that experienced very early silicification at the sediment-water interface. These remnants include nanoparticles, loose flocs and microgranules of greenalite and stilpnomelane enclosed in the early-formed chert that enabled the depositional fabric to survive compaction. Compaction, and diagenetic and metamorphic recrystallization, have transformed the bulk of the rock to the dominant mesobanded magnetite, siderite, minnesotaite, stilpnomelane and chert assemblages that characterize low-metamorphic-grade BIF. The same greenalite remnants and depositional fabrics are found in ferruginous cherts, jaspilites and BIFs in volcanic-dominated greenstone belts and in

supracratonic sedimentary basins. Interpreted depositional settings include submarine proximal volcanic environments (Marble Bar Chert, Cardinal Formation, Wilgie Mia Formation), basin floor (Nimingarra Iron Formation, Hamersley Group), slope (Nauga Formation), deep marine shelf (Kuruman Iron Formation) and shallow shelf above wave-base (Griquatown Iron Formation). This indicates a common origin for ferruginous chemical sediments that is independent of depositional environment and suggests that the “depositional facies” of all preGOE BIFs is silicate (greenalite) facies: siderite, magnetite, minnesotaite, stilpnomelane and hematite result from post-depositional diagenesis and recrystallization. The formation of BIFs as predominantly greenalite mud, preserved in places by early silica cement, is supported by recent experiments. Tosca et al. (2016) showed that in solutions simulating anoxic Archean seawater at pH ca. 7.7-8.3 ferrous-silicate, a precursor to greenalite, was readily precipitated abiotically once a critical supersaturation threshold was reached (Tosca et al., 2016, 2019). Jiang and Tosca (2019) investigated the precipitation kinetics of Fe-carbonate from Archean seawater analogues and found that Fe-carbonate (siderite) does not nucleate homogeneously although it is at supersaturation levels, but that the Fe-silicate precursor to greenalite forms instead in solutions containing SiO2(aq). The presence of greenalite nanoparticles in BIFs in proximal volcanic and distal sedimentary settings is consistent with precipitation close to the source of Fe2+(aq) and SiO2(aq), i.e., submarine volcanic vents, and with the dispersal and deposition of nanoparticles throughout the oceans subsequent to mixing between hydrothermal plume water and ambient seawater. Ferrous iron was transported, at least in part, in particulate form rather than dissolved in anoxic seawater and some of the particles grew to form house-of-cards aggregates, loose flocs and microgranules prior to settling on the seafloor to form the precursor sediments to BIFs.

Textural evidence shows that both siderite and hematite formed after the development of shrinkage polygons in the original dusty chert deposit and therefore they are not primary depositional minerals. Organic matter was not observed in any of the well-preserved cherts examined in this study and there is no textural evidence that it was a component of the earliest sediments, consistent with its exceedingly rare presence in pre-GOE BIFs 10. Conclusions Our observations from the rock record show that despite sediment compaction, early and late diagenesis, and multiple episodes of fluid flow and metamorphism (including hematite mineralization), ferruginous cherts, jaspilites and BIFs preserve remnants of primary minerals and sedimentary structures. In all cases, the earliest mineral preserved in chert cement is greenalite. Greenalite nanoparticles are present in samples of ferruginous chemical sediments from a panoply of depositional environments that range in age from 3.45 Ga to ca. 2.4 Ga. They are found in hydrothermal-exhalative sediments deposited in proximal volcanic environments and in distal sedimentary basins formed on stable cratons. They occur in deposits from basin floor, slope, deep marine shelf and shallow-water shelf environments, in short, throughout the oceans. These observations indicate a common mechanism for the formation of all pre-GOE ferruginous chemical sediments, i.e., abiotic precipitation of greenalite from Fe- and Si-rich plumes derived from hydrothermal vents upon mixing with ambient seawater in the vicinity of seafloor volcanoes. The resulting nanoparticles would be distributed throughout the oceans, forming the precursors to BIFs wherever they settled, be it on the seafloor or on continental margins. This process would have been operative from the time of formation of the first oceans

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Turee Creek Group, Western Australia. Geochimica et Cosmochimica Acta 75, 5686– 5705. Figure Captions Figure 1. Plane-polarized light (PPL) images of well-preserved chert samples with laminae of “dusty” chert, clear chert and siderite. A & B: wavy laminae defined by differing densities of fine-grained mineral dust in sample ABDP9-37C (Bee Gorge Member, Wittenoom Formation). C & D: laminae composed of dense mineral dust, patches of mineral dust in clear chert, and siderite in sample 100315-79B (Nauga Formation). E & F: laminae of mineral dust and lenses composed of fine-grained hematite (Hem) in clear chert, overprinted by bunches of fibrous riebeckite (Rbk) in sample Silvergrass 16c B (Joffre Member, Brockman Iron Formation). Sd – siderite. Figure 2. Microgranules in BIFs. A. PPL image of greenalite microgranules in dusty chert, with scattered siderite (Sd) crystals. B. PPL image of stilpnomelane microgranules with rims of clear chert (Percival texture) and scattered siderite grains. C. Backscattered-electron (BSE) image of greenalite (Gre) microgranules, and a grain of apatite (Ap), in a matrix of chert with greenalite nanoparticles. D. BSE image of stilpnomelane (Stp) microgranules in a matrix of chert with greenalite nanoparticles. A & C from sample Mitchell2-405C, Dales Gorge Member, Brockman Iron Formation. B & D from sample DDH44-32A, Dales Gorge Member, Brockman Iron Formation. Figure 3. Polygonal shrinkage structures in chert from BIFs, jaspilites and ferruginous cherts. A. BSE image of greenalite nanoparticles in chert polygons of the Marble Bar Chert (sample MB 20-62A). Slightly coarser grained crystals of siderite (Sd) and hematite (Hem) are present in poorly defined polygons. B. BSE image of greenalite nanoparticles in well-defined polygons in

ferruginous chert of the Bee Gorge Member of the Wittenoom Formation (sample ABDP9-37C). Slightly coarser grained crystals of siderite (Sd) are present within the polygons and in clear chert between polygons (arrows). C. BSE image of chert polygons with greenalite nanoparticles from the Marra Mamba Iron Formation (sample DDH186-1C). Crystals of siderite (Sd) and minnesotaite (Mns) overprint the chert polygons. D. BSE image of chert polygons with abundant greenalite nanoparticles in ferruginous chert of the Nauga Formation (sample 100315-79B). The polygons contain nanopores (black) and grains of siderite (white), with larger rhombs of siderite (Sd) between polygons. Figure 4. A-H. High-angle annular dark field (HAADF) scanning transmission electron micrographs (STEM) of greenalite nanoparticles in chert from BIFs, jaspilites and ferruginous cherts. The nanoparticles form isolated grains, chains of several grains, and loose flocs with faceto-face or face-to-edge aggregates of grains. I-K. High resolution transmission electron micrographs (HRTEM) of greenalite nanoparticles showing 0.72 nm (001) d-spacing and 2.2 nm modulation characteristic of greenalite. A. Jaspilite, Marble Bar Chert (MB 20-62A). B. BIF, Dales Gorge Member, Brockman Iron Formation (AGC985D). C. BIF, Dales Gorge Member, Brockman Iron Formation (DDH44-19). D. Ferruginous chert, Nauga Formation (100315-79B). E & F. BIF, Dales Gorge Member, Brockman Iron Formation (DDH44-32A). G. BIF, Kuruman Iron Formation (030315-37C). H. Griquatown Iron Formation (040315-68F). I. Ferruginous chert, Bee Gorge Member, Wittenoom Formation (ABDP9-37C). J. Ferruginous chert, Nauga Formation (100315-79B). K. BIF, Wilgie Mia Formation (95WGS2-6A). Figure 5. Greenalite and stilpnomelane nanoparticles in chert from the Dales Gorge Member, Brockman Iron Formation (sample AGC985D). A. HAADF STEM of nanoparticles of greenalite and stilpnomelane (outlined by red rectangles) in chert. B. Energy-Dispersive X-ray Spectrum

(EDS) of stilpnomelane nanoparticle showing minor peaks for K, Mg and Al in addition to Fe, Si and O. C-E. EDS maps for the area outlined by the white rectangle in A showing the presence of K in one nanoparticle. F-H. HRTEM of intergrowths of greenalite (Gre) and stilpnomelane (Stp) showing the ca. 0.7 nm (001) d-spacing for greenalite and ca. 1.0 nm (001) d-spacing for stilpnomelane. Figure 6. Hematite in BIFs, jaspilites and ferruginous cherts. A. Scan of thin section of jaspilite from the Marble Bar Chert. B. Crossed polarizers (XP) transmitted light image showing patches of red dusty hematite in chert. C. BSE image showing scattered, rounded grains of hematite (Hem) among abundant elongate grains of greenalite. D. HAADF STEM image of subhedral to rounded grains of hematite (Hem) with isolated flakes and aggregates of greenalite. A-D sample MB 19-63B. E. Scan of thin section of BIF from the Mount Sylvia Formation. F. PPL image of clusters of dusty hematite in chert. G. BSE image showing fine grains of hematite (white) in polygons of greenalite-bearing chert. Nanopores (black) are present among the hematite grains, but hematite and pores are absent in the lower left of the image, where there is only greenalite in polygonal chert. H. BSE image of subhedral plates of hematite (Hem) in chert with flakes of greenalite and nanopores (black).Some hematite grains are growing in the pores (arrows) or in clear chert between polygons. E-H sample ABDP9-56A. I. Scan of thin section of BIF from the Joffre Member, Brockman Iron Formation. J. PPL image of hematite-bearing lens in chert, overprinted by fibrous riebeckite. K. BSE image of hematite-bearing lens in polygonal chert with abundant inclusions of greenalite. Hematite is partly filling nanopores (black). Coarser flakes of stilpnomelane (Stp) overprint polygonal chert. L. BSE image of euhedral hematite (white) with nanopores (black) in greenalite-bearing chert. I-L sample Silvergrass 16c B. M. Scan of thin section of jaspilite from ferruginous chert of the Nauga Formation. N. XP reflected light image

showing discontinuous lenses of hematite-bearing chert from the band at the bottom of the thin section scan. O. BSE image showing fine grains of greenalite and hematite (equant, white) in polygonal chert with nanopores (black). Hematite partly fills some nanopores (arrows). P. HAADF STEM of scattered, rounded grains of hematite (Hem) with flakes of greenalite in chert. M-P sample 100315-72A. Figure 7. Geological map of the Pilbara Craton showing the locations of drill holes ABDP1 and SSD-19, and Pit 7 of the Shay Gap mine. GB – Goldsworthy greenstone belt, MB – Marble Bar greenstone belt, SG – Shay Gap greenstone belt, TTSZ – Tabba Tabba shear zone. After Hickman (2016). Figure 8. Geological map of the north-eastern Murchison Domain, Youanmi Terrane, Yilgarn Craton, showing the location of the Weld Range and drill hole 95WGD2. EGS – Eastern Goldfields Superterrane, MD – Murchison Domain, SCD – Southern Cross Domain, YT – Youanmi Terrane. After Van Kranendonk et al. (2013). Figure 9. A. Geological map of the Hamersley Basin showing the locations of drill holes ABDP9, Mitchell 2, DD98SGP001, DDH186 and DDH44, and the stratigraphic column for the Hamersley Group. B. Drill hole logs for DD98SGP001, DDH44, Mitchell 2 and ABDP9 showing the locations of samples examined in this study. The locations of samples analysed by TEM are indicated as well as those containing greenalite and hematite. Figure 10. Iron-rich BIF of the Colonial Chert member, Mount McRae Shale. A. Scan of thin section of BIF. B. PPL image of pale-green laminated BIF composed of chert, irregular aggregates of fine grains of greenalite (Gre) and clusters of magnetite (black) surrounded by greenalite. Stylolites (Sty) are lined by greenalite and siderite. C. PPL image of greenalite microgranules in dusty chert, with scattered coarser crystals of siderite (Sd) and minnesotaite

(Mns). D. BSE image of polygonal patches of chert enclosing fine greenalite particles, with chains and aggregates of coarser greenalite plates on the margins of the polygons. E. BSE image of greenalite microgranules in chert. F. BSE image of greenalite microgranules in chert with greenalite nanoparticles, overprinted by minnesotaite (Mns) and siderite (Sd). G. BSE image of greenalite microgranules overprinted by magnetite (Mag) and siderite (Sd). H. BSE image of cluster of irregular magnetite grains (Mag) and siderite (Sd) overprinting greenalite microgranules and greenalite-bearing chert. I. BSE image of magnetite (Mag) grain with aligned inclusions of quartz (Qz) overprinting greenalite-bearing chert. A-I sample Mitchell2-418. Figure 11. Geological map of the Griqualand West sub-basin of the Transvaal Basin, South Africa, showing the locations of drill holes Gasesa, Erin-3, GKF01 and GKP01. Figure 12. Greenalite in Kuruman Iron Formation. A. Scan of thin section of BIF. B. PPL image of dusty chert with coarser crystals of magnetite (Mag), siderite and minnesotaite. C. PPL image of area outlined in B highlighting siderite and minnesotaite overprinting dusty chert. D & E. BSE images showing polygonal patches of chert with nanoparticles of greenalite separated by inclusion-free chert, and coarser crystals of siderite and bowties of minnesotaite. F. HAADF STEM of isolated particles and loose flocs of greenalite in chert. G. HRTEM of greenalite particle with 0.7 nm (001) d-spacing and 2.2 nm modulation. A-F sample 030315-37C. Figure 13. Greenalite in Griquatown Iron Formation. A. Scan of thin section of Griquatown Iron Formation showing hematite-rich band with bands of siderite-chert granules with hematite cement on either side, and a band of hematite-free granular siderite chert. B. PPL image of area of dusty chert and siderite granules in clear chert cement outlined in A. C. PPL image of detail of large dusty chert granule with partial rim of siderite in clear chert cement. Smaller granules of chert-siderite and siderite are enclosed in the chert cement. D. BSE image of dusty chert granule

with siderite (Sd) rim shown in C (rotated 90°). E. BSE image of dusty chert in the granule shown in D, showing minute mineral inclusions. F. HAADF STEM of dusty chert showing inclusions of greenalite (Gre) and rare hematite (Hem). G. HRTEM of greenalite flake showing 0.7 nm (001) d-spacing and 2.2 nm modulation. Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Highlights

   

Banded Iron Formations (BIFs) deposited before the Great Oxidation Event preserve remnants of sedimentary structures and minerals. The earliest minerals in the BIFs are nanoparticles of greenalite and silica cement. Depositional environment, from submarine volcanic to shallow-marine shelf, does not dictate the minerals that are precipitated and preserved. Greenalite nanoparticles precipitated after mixing between hydrothermal plume water and seawater near submarine vents.



The nanoparticles were distributed throughout the oceans and deposited to form BIFs.

Table 1. Details of samples of greenalite-bearing chert analyzed by High-Resolution Transmission Electron Microscopy Sample

Description Latitude

Description2 Longitude

Age From

DDH To

Marble Bar 20-62A Duffer Fm 21°12'15”S

Marble Bar Chert Mb 3.46 Ga 119°43'38”E 130.68

ABDP1 130.70

Marble Bar 19-63B Duffer Fm 21°12'15”S

Marble Bar Chert Mb 3.46 Ga 119°43'38”E 124.88

ABDP1 125.00

SS59960B

Cardinal Fm 21°08'48”S

~3.2 Ga 203.76

SSD-19

119°12'23”E

Nimingarra BIF 20°28'09”S

120°04'39”E

3104 Ma na

Shay Gap 7 na

117°33'14”E

2752 Ma 99.15

95WGD2 95.65

BIF in Wilgie Mia Fm 26°58'12”S

117°33'14”E

2752 Ma 109.70

95WGD2 109.75

Marra Mamba IF 22°23'55”S

117°55'13”E

2597 Ma 81.00

DDH186 81.17

ABDP9 37C

BIF band in Wittenoom Fm 21°59'29”S

Bee Gorge Mb 117°25'13”E

2565 Ma 288.23

ABDP9 288.36

ABDP9-56A

BIF band in Mt Sylvia Fm 21°59'29”S

117°25'13”E

~2505 Ma 221.76

ABDP9 221.84

Mitchell2 418

Mount McRae Shale 22°06'45”S

Colonial Chert 116°34'33”E

2504 Ma 418.00

Mitchell2

Mitchell2 405C

Brockman IF 22°06'45”S

Dales Gorge Mb 116°34'33”E

2495-2461 Ma 405.15

Mitchell2

AGC985D

S band in Brockman IF 22°03'30”S

Dales Gorge Mb 116°50'06”E

2495-2461 Ma 445.08

DD98SGP001 446.16

DDH44-19

Brockman IF 23°14'14”S

Dales Gorge Mb 117°35'35”E

2495-2461 Ma 453.16

DDH44

BBSG Nimingarra 95WGD2-6 95WGD2-7A DDH186-1C

BIF in Wilgie Mia Fm 26°58'12”S

DDH44-32A

Brockman IF 23°14'14”S

Dales Gorge Mb 117°35'35”E

2495-2461 Ma 387.09

DDH44

Silvergrass 16c B

BIF band in Brockman IF 22°03'30”S

Joffre Mb 116°50'06”E

2461-2454 Ma 313.65

DD98SGP001

100315-79B

BIF band in Nauga Fm 28°56'06”S

23°15'00”E

2521-2480 Ma 327.11

GKF01 327.32

BIF band in Nauga Fm 28°56'06”S

23°15'00”E

2521-2480 Ma 314.43

GKF01 314.60 Gasesa

23°01'27”E

2480-2431 Ma 503.20

Griquatown IF 27°23'35”S

Erin-3

23°01'27”E

~2431 Ma 407.60

Griquatown IF 27°23'35”S

~2431 Ma 376.30

Erin-3

23°01'27”E

100315-72A 030315-37C 040315-62A 040315-68F

Kuruman IF 27°23'35”S

A

B

Sd

B

1 mm

100 µm

C

D Sd Sd

D

1 mm

100 µm

E

F

Hem

F

Sd

1 mm

Rbk

100 µm

Muhling and Rasmussen Figure 1 Two column

A

B

Sd

Sd 100 µm

100 µm

C

D Stp

Gre Ap

10 µm

Muhling and Rasmussen Figure 2 Two column

15 µm

B

A

Sd

Sd Hem

50 µm

20 µm

C

D

Sd

Sd

Mns

50 µm

Muhling and Rasmussen Figure 3 Two column

20 µm

A

B

C

1 µm

1 µm

0.5 µm

D

E

1 µm

0.5 µm

F

G

H

1 µm

I

1 µm

J

1 µm

K 0.72 nm 2.2 nm

20 nm 10 nm

Muhling and Rasmussen Figure 4 Two column

10 nm

A

B

O Si

Fe Fe

K

Mg Al

1 µm 0

C

2

Fe 4

D

E

G

H

6

keV

600 nm

F

Stp

1.0 nm

Stp

Stp

0.7 nm

Stp

Gre

Gre

Gre

10 nm

Gre

Muhling and Rasmussen Figure 5 Two column

10 nm

5 nm

A

D

C

B

Hem Hem

E

5 µm

50 µm

5 mm

F

Sd

1 µm

G

H Hmt

Hem

50 µm

5 mm

I

5 µm

25 µm

K

J

L Hem

Stp

5 mm

M

100 µm

N

20 µm

O

Gre

5 µm

P

Hem

5 mm

100 µm

10 µm

Muhling and Rasmussen Figure 6 Two column

1 µm

117°

118°

119°

INDIAN OCEAN

120°

20°

Phanerozoic basins

GB

SG SG7

Z T TS

SSD-19

21° MB

20°

Ph ane roz oic

116°

26°

Pilbara Craton Mt Bruce Supergroup

ABDP1

121°

Fortescue and Hamersley Groups

22°

Proterozoic

100 km

Yilgarn Craton

Split Rock Supersuite (2850-2830 Ma) Granite batholiths (>2850 Ma) De Grey Supergroup Gorge Creek Group c. 3066 Ma

TTSZ

Soanesville Group c. 3220 Ma

West Pilbara Superterrane

East Pilbara Terrane

Greenstone belt

Muhling and Rasmussen Figure 7 Two column

Pilbara Supergroup

118° Cue MD SCD YT Perth

EGS

YILGARN CRATON

117°30’



ld R We

27°

e

ang

Cue 27°30’

25 km Archaean granite Glen Group Polelle Group

Norrie Group Unassigned BIF Unassigned greenstone

Muhling and Rasmussen Figure 8 1.5 columns

A

117°E

$!

& '(

119°E

21°S

Western Australia  

  

100 km

350

Depth (m)

250

300

350

500

500

ABDP9 Depth (m) 100

150

200

250

300

TEM sample

400

     !" Greenalite

450

Colonial Chert

450

Dales Gorge member

C Ch

400

Whaleback Shale

450

W Sh

Joffre member

400

  (     + '(

Mitchell 2

Dales Gorge member

Depth (m)

Depth (m)

300

DDH44

W Sh

SGP001

Dales Gorge member

Turee Creek Group

B

 *   (

Hamersley Group Fortescue Group Archean granite Archean greenstone belt

Shingle Creek Group

  !

350

Hematite

Muhling and Rasmussen Figure 9 1.5 column

C Ch

Wyloo Group and cover

  + )

Mt McRae Shale



,-

 + 

.  

1 & !

     

Mt Sylvia

/0 

$ ) '(



  ( 

 

Bee Gorge member

#$%    

23°S

!  *

B

A

C Sd

Sty

Mns Gre

100 µm

500 µm

5 mm

D

E

10 µm

5 µm

F

G

Sd Mns

Mag

Sd

Sd

Sd 25 µm

25 µm

H

I Mag

Mag

Sd

Qz

Sd

10 µm

10 µm

Muhling and Rasmussen Figure 10 Two column

023°E

024°E

Vryburg

27°S South Africa

Hotazel

 

Kuruman

V

BR T

 

V V V

V

28°S

V

V

V V V V

V V

V V V V V V V V V V V

Griquatown

Olifantshoek Group

Transvaal Supergroup

V V

29°S

V V V V

V V V V V V V

Prieska

V V V

 GKP01

50 km

Muhling and Rasmussen Figure 11 1.5 column

Postmasburg Group Koegas Subgrp Asbestos Hills Subgroup Campbellrand Subgroup Schmidtsdrif Subgroup

B

A

C C Mns

Sd

5 mm

100 µm

100 µm

Mag

E

D

Sd

Sd

Mns Mns 20 µm

F

15 µm

G

0.7 nm 2.2 nm

1 µm

10 nm

Muhling and Rasmussen Figure 12 Two column

B

A

C Sd

Hem

chert

B 500 µm

5 mm

100 µm

D

E

Sd

3 µm

150 µm

F

G

0.7 nm 2.2 nm

Hem Gre

1 µm

10 nm

Muhling and Rasmussen Figure 13 Two column