Granular Iron Formation Research Articles

Does water transparency control the banding in shallow water iron formations?

Abstract Iron formations (IFs) are marine chemical sediments that are conined to the Precambrian rock record and provide unique insights into the co-evolution of the atmosphere-hydrosphere and biosphere through almost three billion years of Earth’s history. IFs commonly appear throughout the Archaean until the Palaeoproterozoic ca. 1.8 billion years ago and re-appear during the “Snowball Earth” epoch in the Neoproterozoic. The formation and deposition mechanism(s) of IFs are, however, still incompletely understood, hindering unique interpretations of palaeoenvironments. Many IFs are banded iron formations (BIFs) with layer thickness of alternating Fe- and Si-rich layers ranging over several orders of magnitude from the nanometre to the metre scale. A second textural type, so called granular iron formations (GIFs) that form above storm wave base become widespread in the Palaeoproterozoic. Understanding the fundamental mechanisms that are responsible for the textural types and the periodicity of banding in BIFs is crucial to link these features to the environmental and geochemical evolution of the Earth. We here provide a conceptual model that highlights the role of changing light conditions and water transparency for Iron Formation (IF) precipitation. We show that the model is particularly feasible for IFs deposited in shallow waters but may also be applicable for some IFs deposited in deeper water settings. The model builds on other primary Banded Iron Formation (BIF) precipitation models postulating that Fe(III)-(oxyhydr)oxide production can be dominated by anoxygenic photoautotrophic Fe2+-oxidising bacteria. These so called photoferrotrophs are adapted to very low light levels corresponding to about 1% of the light level required by oxygen-producing phototrophs allowing them to thrive deep down in the water column. The depth of Fe(III)-(oxyhydr)oxide production is mainly controlled by water turbidity which controls how deep photosynthetically available radiation (PAR) penetrates the water column. Eutrophic conditions result in relatively shallow Fe(III)-(oxyhydr)oxide production depth due to turbidity either induced by the biomass itself and/or by particles that are actively or passively produced by microorganisms (e.g., Fe(III) and Mn(IV)-(oxyhydr)oxides, sulphides), triggering the formation of Fe-rich bands. During oligotrophic stages, Fe(III)-(oxyhydr)oxides are only produced relatively deep down in the water column, so that only silica-rich bands form in the Fe(III)-(oxyhydr)oxide free upper water column. Reactive transport modelling adopted from Ozaki et al., (2019) shows that besides upwelling and nutrient supply, alternating Fe(III) production depth is mainly associated with changing light conditions as a result of water transparency. Periodicities reflected by alternating Fe- and Si-rich bands in IFs in our model can thus be associated with: (1) nutrient supply patterns; (2) additional sources of turbidity in the water column such as Fe(III) and Mn(IV) oxide particles, sulphides, and wind-blown silicate particles; or (3) formation and clearing of organic haze in the atmosphere. One or all of these reasons for low light conditions seem to become more important in the Palaeoproterozoic (<2.4 Ga) and could be partly responsible for the more widespread occurrences of shallow marine granular IFs relative to former epochs, which is often assigned to the gradual oxidation of the ocean. Our model shows that variable water transparency should be considered as additional factor for IF deposition especially for shallow marine settings. This model also reasonably explains the prominent layering in BIFs as syn-depositional feature.

Petrology and geochemistry of metamorphosed rocks associated with iron formations of the Toko‐Nlokeng iron deposit, (Southern Cameroon): Implications for geodynamic evolution and mineralization

Detailed petrographic, lithostratigraphic, and geochemical data of metamorphosed orthogneisses and mafic‐ultramafic metavolcanic rocks intruded into the greenstone belt of the Toko‐Nlokeng iron deposit have enabled the reconstruction of the tectonic and geodynamic setting and crustal evolution of the Nyong Complex. Samples were collected from the drill holes TNF11_01 and TNF11_02 from 17.16 m to 335.85 m depth. The lithostratigraphy supported by field and petrographic observations outlines two main lithologies: Iron formations (IFs) and metamorphosed host rocks. The IFs (granular iron formations (GIF) and banded iron formations (BIF)) are intercalated with host rocks consisting of orthogneisses and mafic‐ultramafic rocks. New and published geochemical data of metamorphosed associated IFs from Anyouzok (TNF08 prospect) of the Toko‐Nlokeng iron deposit, Mewengo iron deposit, Bipindi, and Kribi metavolcanic rocks in the Nyong Complex, suggest that these rocks were formed by material from intrusive and extrusive magmatic episodes with mid‐ocean ridge basalts (MORB) contaminated by either subduction or crustal components. CaO/Al2O3 and (Gd/Yb)N ratios >1 and positive Nb anomalies in ultramafic rocks indicate a mantle plume source contaminated by metasomatized subduction mantle lithosphere. These data also show that the mafic‐ultramafic metavolcanic rocks derive from magma of basaltic and basaltic andesite compositions, with a tholeiitic to calc‐alkaline tendency characteristic of the upper mantle. These data reveal that gneisses derive from granite and diorite subalkaline, peraluminous, and ferroan to magnesian compatible with Cordilleran magmas and island arcs with polygenetic crustal signatures. All samples of mafic granulites, garnet‐amphibolite, metabasites, and ultramafic granulites in the Nyong Complex reveal no residual garnet and predominantly show ca. 4% partial melting of an amphibole‐spinel‐peridotite source in the Dy/Yb versus La/Yb. Otherwise, hornblendites of Toko‐Nlokeng display ca. 5% partial melting of an amphibole‐garnet‐peridotite source, average (Gd/Yb)CN ratio of the hornblendites is (Gd/Yb)CN = 7.56 > 2. This result suggests these hornblendites do not have the same magmatic source as other mafic‐ultramafic rocks of the Nyong Complex. Mafic‐ultramafic host rock protoliths are classified as E‐MORB, P‐MORB, and G‐MORB (hornblendites) compositional types and arc, back‐arc ‘B’ in subduction unrelated, rifted margin setting with minor crustal contamination. The tholeiitic to calc‐alkaline and peraluminous affinity of these rocks indicate a mature arc and thickened crust during the Eburnean Trans‐Amazonian orogenic belts of the Congo Craton. This study shows that the host rocks were emplaced in a convergent tectonic setting and affected by a syn‐collisional episode where melts were derived from the partial melting of thick basaltic crust into amphibolite–eclogite facies in the subduction zone. The geochemical signatures of the mafic‐ultramafic rocks support the tectonic accretionary of the Palaeoproterozoic plume arc in Nyong Complex. Furthermore, the resulting hydrothermal alteration process was accompanied by an increase in Fe; particularly under conditions dominated by seawater, the origin and source of iron and silica in the Toko‐Nlokeng deposit.

Study on Iron Source and Original Mineral Assemblage of Iron Formation in Paleoproterozoic Wuzhiling Formation in North China

The Precambrian iron formation is related to the evolution of the earth's environment and is an important record of the evolution of ancient ocean, ancient environment and atmospheric conditions. The Wuzhiling Formation iron formation at the Songshan boundary is one of the newly identified granular iron formations (GIF). Through the study of mineral characteristics and mineral morphology of iron formation in Wuzhiling Formation, it is found that its material source may be hydrothermal fluid; the original sediments may be hematite particles and 'green rust'.

Depositional age and geochemistry of the 2.44–2.32 Ga Granular Iron Formation in the Songshan Group, North China Craton: Tracing the effects of atmospheric oxygenation on continental weathering and seawater environment

The responses of oceanic chemistry and redox state to the oxygenation of atmosphere at the period (2.45–2.10 Ga) of the global Great Oxidation Event remain poorly constrained due to the lacking of coeval marine chemical precipitates, e.g., Iron Formation. Here, we report a Granular Iron Formation (GIF) in the Songshan Group of the North China Craton and its depositional age was constrained at 2.44–2.32 Ga at the first time. In the GIF, hematite is the primary iron oxide and its occurrence indicates that the shallow seawater at 2.44–2.32 Ga was oxygenated, which is in agreement with the positive δ13C excursion of the coeval carbonates up and below the Songshan GIF. No significant correlations between redox-sensitive trace elements (Cr, V, Mo and U) and Al2O3 contents exclude terrigenous clastic origin, suggesting that these elements are enriched during the deposition and have been adsorbed by the Fe-rich chemical precipitates. The high Cr (up to 2000 ppm) and B (up to 300 ppm) contents of the GIF and the authigenic hematites further indicate an increase of chemical weathering and subsequent input into the seawater. Combined with high V (up to 1500 ppm) and P (up to 7000 ppm) contents in the GIF, it can be inferred that continental oxidative weathering was intensive during 2.44–2.32 Ga. The enrichments of Cr contents and Cr/Ti ratios of the Songshan GIF and its authigenic hematites suggest that Cr was transported to the oceans as soluble Cr (VI) species by rivers because of the oxidative Cr(III) in Cr-rich accessory minerals in soil or crust. The enrichment processes of Cr and V in the hematites are likely to be that soluble Cr (VI) and V (V) supplying by the downwelling surface waters transform into Cr (III) and V (III) in the suboxic-anoxic bottom waters and then removed from seawater via authigenic burial of hematite because trivalent cations can substitute in the hematite structure. Based on above, we therefore suggest a new depositional scenario for the Songshan GIF, in which precipitation occurred due to upwelling of deep, anoxic, reduced Cr-V-rich ferruginous waters into an oxygenated, high productivity shallow-water setting.

The Yunmengshan iron formation at the end of the Paleoproterozoic era

Precambrian iron formations (IF) carry rich information on the interplay between the geosphere and the early biospheres. Here, we report mineralogical and petrologic characterizations of the Yunmengshan IF deposited at ~1.7 Ga, the transition between Paleoproterozoic and Mesoproterozoic eras. The IF contains granular hematite with evident oncoidal structures and lacks laminated structures, which are distinct to the typical banded iron formation (BIF) or granular iron formation (GIF) deposited early in the marine environments with high silica content. Based on observations of clastic sedimentary structures around the Yunmengshan IF, including parallel bedding, cross-bedding, asymmetric ripple mark, and mud crack, we deduce that the Yunmengshan IF was mainly deposited in a supratidal to intertidal zone. Electron microscopic observations show two types of hematite euhedral crystals: laminar hematite of 200–500 nm thick and 3-5 μm in diameter and granular hematite of 200–300 nm in diameter randomly distributed in the matrix. Stilpnomelane and minnesotaite, as Fe2+-silicates in the typical Archean- and the early Paleoproterozoic BIF, are identified but with extremely low abundance. The morphology and paragenetic association of these minerals imply that the Yunmengshan IF was probably deposited in an aquatic environment with low Si concentration. These results indicate that the Yunmengshan IF represents a transitional type of iron deposition between BIF/GIF and ironstone in the geological history of the iron cycle on the surface of Earth.

Life on a Mesoarchean marine shelf \u2013 insights from the world\u2019s oldest known granular iron formation

The Nconga Formation of the Mesoarchean (~2.96–2.84 Ga) Mozaan Group of the Pongola Supergroup of southern Africa contains the world’s oldest known granular iron formation. Three dimensional reconstructions of the granules using micro-focus X-ray computed tomography reveal that these granules are microstromatolites coated by magnetite and calcite, and can therefore be classified as oncoids. The reconstructions also show damage to the granule coatings caused by sedimentary transport during formation of the granules and eventual deposition as density currents. The detailed, three dimensional morphology of the granules in conjunction with previously published geochemical and isotope data indicate a biogenic origin for iron precipitation around chert granules on the shallow shelf of one of the oldest supracratonic environments on Earth almost three billion years ago. It broadens our understanding of biologically-mediated iron precipitation during the Archean by illustrating that it took place on the shallow marine shelf coevally with deeper water, below-wave base iron precipitation in micritic iron formations.

Geobiology of the Paleoproterozoic Belcher Group, Nunavut, Canada

The ~2.0-1.8 Ga (billion years old) Belcher Group on the Belcher Islands in Nunavut provide a unique opportunity for studying Paleoproterozoic geobiology. The Belcher Group includes a sequence of low metamorphic grade peritidal carbonate rocks that preserve putative microbiota, as first described by Hofmann and Jackson (1969). Microbial mats, including stromatolites, are abundant in the peritidal carbonate succession. Additionally, morphologies possibly related to blue-green algae were first described in granular iron formation rocks of the Belcher Group by Moore (1918). The Belcher Group microbiota are a group of simple organisms, believed to be prokaryotic in nature. Microbiota morphologies include ellipsoids, spheroids, and filamentous chains of cells interpreted by previous workers to represent blue-green algae and acritarchs. Some microstructures are questionably biogenic and might be abiotic. The most significant field studies on the Belcher Group occurred from the late 1950s to the early 1980s, which provides the geological context for this study. This project aims to build on the previous work of H. Hofmann and others in the ‘60s and bring these microbiota into a modern context, drawing on the analytical advancements of the last 50 years. The main goal of the project is to determine if there is evidence that the microbiota are perhaps eukaryotic organisms. The emergence of eukaryotes is arguably the most significant geobiological event in Earth history, with eukaryotic cells believed to have evolved around 1.6 Ga (Knoll et al. 2006; Javaux and Lepot 2018), but some contentious fossils interpreted to represent eukaryotes have been dated to as early as 2.2 Ga (Retallack et al. 2013). In North America, the oldest discovered eukaryotic remains are around 1.5 Ga (Adam et al. 2017). If eukaryotic fossils were to be discovered in the Belcher Group, this would make them the oldest occurrence in North America. To test the hypothesis, samples from the microbiota-containing units were collected on the Belcher Islands. Both light microscopy and a collection of modern analytical techniques will be used to obtain high resolution images and chemical signatures of the microbiota and their biosignatures. Preliminary data from petrography, Raman Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS) will be presented. Both Raman spectroscopy and XPS have been used as characterization tools in other studies looking at microbiota and organic matter remains (Qu et al. 2018; Arnarson and Keil 2001). Raman collects molecular and structural data from the sample, while XPS collects elemental chemical data. Both techniques are therefore particularly useful for identifying and characterizing organic carbon, which is the base of organic matter.

Hyperspectral imaging of sedimentary iron ores – beyond borders

Banded iron formations (BIFs) and granular iron formations (GIFs) are the two primary types of sediment hosted iron ore deposits and provide the primary source for iron, globally. Although both dep...

Post‐Great Oxidation Event Orosirian–Statherian iron formations on the São Francisco craton: Geotectonic implications

AbstractThe protocratonic core of the São Francisco craton assembled during the 2.1–2.0 Ga Transamazonian orogeny. Orosirian Fe‐rich sequences that extend from the northwestern border of the São Francisco protocraton (Colomi Group) to the southeast under the Espinhaço Belt (the < 1.99 Ga Serra da Serpentina Group) record the opening of an intracratonic basin with the episodically developed ferruginous waters prior to the initiation of the Espinhaço rift at 1.8 Ga. Ferruginous conditions developed again during deposition of the Canjica Iron Formation of the < 1.7 Ga Serra de São José Group in the Espinhaço rift (contemporaneously with felsic magmatism; Conceição do Mato Dentro Rhyolite and Borrachudos Granitic Suite) and extensive sandstones of the < (1666 ±32) Ma Itapanhoacanga and < (1683 ±11) Ma São João da Chapada Formations. In the upper São João da Chapada Formation, banded hematitic phyllite also records input of Fe‐rich fluids. The young age of these iron formations with respect to the conventionally accepted 1.88 Ga age for the youngest shallow‐marine Paleoproterozoic iron formations, the apparent absence of granular facies (granular iron formations), and yet shallow‐water (above fair‐weather base) depositional environment indicate that an unusual setting developed in a large basin after the Great Oxidation Event, in the aftermath of the Transamazonian orogeny. We propose that mantle plumes led to the opening of a previously unrecognized rift system, that could have caused the magmatism, supplied hydrothermal Fe and led to the opening of the Espinhaço, Pirapora, and Paramirim rifts, later obliterated by the Araçuaí orogenic belt during the Neoproterozoic to Early Paleozoic Brasiliano orogeny. The rift system did not develop into an open continental margin but probably evolved into a broad sag basin, stretching across the São Francisco and Congo cratons.

Organic remains in late Palaeoproterozoic granular iron formations and implications for the origin of granules

Toward the end of the Palaeoproterozoic era, over 109 billion tonnes of banded (BIF) and granular (GIF) iron formations were deposited on continental platforms. Granules in iron formations are typically sub-spherical structures 0.2 to 10 mm in size, whereas concretions are >10 mm. Both types of spheroids are preserved throughout the sedimentological record. Their formation has typically been interpreted to originate from reworked Fe-rich sediments in high-energy, wave-agitated, shallow-marine environments. New evidence from six different late Palaeoproterozoic granular iron formations (GIF), however, suggests that some granules are the result of diagenetic reactions, in addition to other features driven by microbial processes and mechanical movements. Characteristic coarse grain interiors and septarian-type cracks inside granules, akin to those features in decimetre- to meter-size concretions, are interpreted as desiccation features from hydrated diagenetic environments where sulphate and/ or ferric iron were reduced while organic matter (OM) was oxidised inside granules. Those granules derived from sulphate reduction preserve diagenetic pyrite rims, whereas those formed via ferric iron reduction preserve diagenetic magnetite along their rims. Other diagenetic minerals including apatite mixed with OM, and various carbonate phases are commonly preserved within granules. Combined with systematically 13C-depleted carbonate, these diagenetic mineral assemblages point to the oxidative decay of OM as a major process involved in the formation of granules. Spheroidal equidistant haematite laminations surround some granules and contain apatite associated with carbonate, OM, and ferric-ferrous silicates, and oxides that further suggest these structures were not shaped by wave-action along sediment-water interfaces, but rather by chemical wave fronts and biomineralisation. Our results demonstrate that the formation mechanisms of GIF also involve microbial activity and chemically-oscillating reactions. As such, granules have excellent potential to be considered as promising biosignatures for studying Precambrian biogeochemistry, as well as astrobiology.

Newly identified 1.89 Ga mafic dyke swarm in the Archean Yilgarn Craton, Western Australia suggests a connection with India

The Archean Yilgarn Craton in Western Australia is intruded by numerous mafic dykes of varying orientations, which are poorly exposed but discernible in aeromagnetic maps. Previous studies have identified two craton-wide dyke swarms, the 2408 Ma Widgiemooltha and the 1210 Ma Marnda Moorn Large Igneous Provinces (LIP), as well as limited occurrences of the 1075 Ma Warakurna LIP in the northern part of the craton. We report here a newly identified NW-trending mafic dyke swarm in southwestern Yilgarn Craton dated at 1888 ± 9 Ma with ID-TIMS U-Pb method on baddeleyite from a single dyke and at 1858 ± 54 Ma, 1881 ± 37 and 1911 ± 42 Ma with in situ SHRIMP U-Pb on baddeleyite from three dykes. Preliminary interpretation of aeromagnetic data indicates that the dykes form a linear swarm several hundred kilometers long, truncated by the Darling Fault in the west. This newly named Boonadgin dyke swarm is synchronous with post-orogenic extension and deposition of granular iron formations in the Earaheedy basin in the Capricorn Orogen and its emplacement may be associated with far field stresses. Emplacement of the dykes may also be related to initial stages of rifting and formation of the intracratonic Barren Basin in the Albany-Fraser Orogen, where the regional extensional setting prevailed for the following 300 million years. Recent studies and new paleomagnetic evidence raise the possibility that the dykes could be part of the coeval 1890 Ma Bastar-Cuddapah LIP in India. Globally, the Boonadgin dyke swarm is synchronous with a major orogenic episode and records of intracratonic mafic magmatism on many other Precambrian cratons.

Oncoidal granular iron formation in the Mesoarchaean Pongola Supergroup, southern Africa: Textural and geochemical evidence for biological activity during iron deposition.

We document the discovery of the first granular iron formation (GIF) of Archaean age and present textural and geochemical results that suggest these formed through microbial iron oxidation. The GIF occurs in the Nconga Formation of the ca. 3.0-2.8Ga Pongola Supergroup in South Africa and Swaziland. It is interbedded with oxide and silicate facies micritic iron formation (MIF). There is a strong textural control on iron mineralization in the GIF not observed in the associated MIF. The GIF is marked by oncoids with chert cores surrounded by magnetite and calcite rims. These rims show laminated domal textures, similar in appearance to microstromatolites. The GIF is enriched in silica and depleted in Fe relative to the interbedded MIF. Very low Al and trace element contents in the GIF indicate that chemically precipitated chert was reworked above wave base into granules in an environment devoid of siliciclastic input. Microbially mediated iron precipitation resulted in the formation of irregular, domal rims around the chert granules. During storm surges, oncoids were transported and deposited in deeper water environments. Textural features, along with positive δ56 Fe values in magnetite, suggest that iron precipitation occurred through incomplete oxidation of hydrothermal Fe2+ by iron-oxidizing bacteria. The initial Fe3+ -oxyhydroxide precipitates were then post-depositionally transformed to magnetite. Comparison of the Fe isotope compositions of the oncoidal GIF with those reported for the interbedded deeper water iron formation (IF) illustrates that the Fe2+ pathways and sources for these units were distinct. It is suggested that the deeper water IF was deposited from the evolved margin of a buoyant Fe2+aq -rich hydrothermal plume distal to its source. In contrast, oncolitic magnetite rims of chert granules were sourced from ambient Fe2+aq -depleted shallow ocean water beyond the plume.

Euxinic conditions recorded in the ca. 1.93 Ga Bravo Lake Formation, Nunavut (Canada): Implications for oceanic redox evolution

The composition of seawater changed dramatically during the initial rise of atmospheric oxygen in the earliest Paleoproterozoic, but the emerging view is that atmosphere–ocean system did not experience an irreversible transition to a well-oxygenated state. Instead, it has been suggested that the oxygen content of the atmosphere–ocean system decreased considerably after ca. 2.06billionyearsago (Ga), which resulted in a crash in marine sulfate concentrations. The end of the deposition of major granular iron formations at ca. 1.85Ga has been linked either to the development of extensive euxinic conditions along continental shelves or a decrease in hydrothermal flux. The record of oceanic redox state is not well constrained for the period between ca. 2.06Ga, the end of the Lomagundi positive carbon isotope excursion, and ca. 1.88Ga when major granular iron formations appeared. We address this gap by presenting new iron-speciation, major and trace element data, as well as sulfur, organic carbon, and molybdenum isotopic data for greenschist facies organic matter-rich mudrocks (ORMs) of the ca. 1.93Ga Bravo Lake Formation, Piling Group, Baffin Island. The iron speciation data suggest deposition of the Bravo Lake Formation under a euxinic (anoxic and sulfidic) water column. Trace metal enrichments and Mo isotope data suggest extensive marine euxinia ca. 90millionyears before the disappearance of large-scale, economic granular iron formations. The addition of new Mo data in this time interval is important, as it contributes to filling in the sparse Proterozoic record. Lastly, this work provides further support for the idea that there was widespread anoxia shortly after the end of the Lomagundi Event.

In-situ LA–ICP-MS trace elemental analyses of magnetite: The late Palaeoproterozoic Sokoman Iron Formation in the Labrador Trough, Canada

The Sokoman Iron Formation in the Labrador Trough, Canada, a typical granular iron formation (GIF), is coeval with the ~1.88Ga Nimish volcanic suites in the same region. It is composed of the Lower, Middle and Upper Iron Formations. In addition to primary and altered magnetite in iron formations of the Hayot Lake, Rainy Lake and Wishart Lake areas, magnetite in volcanic breccia associated with the iron formation is identified for the first time in the stratigraphy. Trace elemental compositions of the most primary, altered and volcanic brecciated magnetite of the Sokoman Iron Formation were obtained by LA–ICP-MS. Commonly detected trace elements of magnetite include Ti, Al, Mg, Mn, V, Cr, Co and Zn. These three types of magnetite have different trace elemental compositions. Primary magnetite in the iron formation has a relatively narrow range of compositions with the depletion of Ti, Pb, Mg and Al. Magnetite from volcanic breccia is rich in Ti, Al, V, Mn, Mg, Zn, Cu and Pb, indicative of crystallization from mantle-derived magmas. Altered magnetite in the iron formation shows a relatively wide range of trace elemental compositions. Mineralizing fluids associated with magmas that generated the ~1.88Ga Nimish volcanic suites circulated through the sedimentary piles to further enrich the iron formations and to form magnetite with variable compositions. The comparisons of different types of primary and altered magnetite in the iron formation in the region show distinct provenance discrimination. Our findings also support the origin of iron formations in association with multiple stages of exhalative volcanic and hydrothermal processes.

Sedimentary and metamorphic lithofacies of the Lower Negaunee Iron Formation, Marquette District, Michigan, USA

The base of the Proterozoic Negaunee Iron Formation is exposed in the open pit at Tilden Mine, Marquette, Michigan. Juxtaposed against the Archean-aged Palmer Gneiss, it is bounded by the regional-scale Southern Shear Zone and cut by two sets of dykes: an older chloritic and schistose set and a younger 1.1 Ga Keweenawan set. Tilden Mine is dominated by a 100 m scale plunging northwest-anticline and is cut by a growth fault locally termed the Tower Hill Fault that intersects the Southern Shear Zone. The base of the exposed iron formation is composed of three lithofacies, including lower clastics that grade into the overlying banded iron formation that in turn grades upward into granular iron formation. This succession is capped by chloritic metadiabases locally termed the Summit Hill Sill and Pillar Intrusive. Petrographic and mineral chemical investigations document primary or early diagenetic hematite, siderite and possibly ferri-hydrite, metamorphic and related hydrothermal magnetite, chlorite, late martite overgrowing earlier magnetite and growth of specularite. All three lithofacies are cut by brittle fractures and late quartz veins. Brittle fractures are coated with chlorite, carbonate minerals, fluor-apatite, and sparse Cu-sulphides. These lithofacies document initial clastic sedimentation of strained detrital quartz into a subsiding fault trough. Over time, as subsidence slowed or sea level fluctuated, clastic deposition competed with quiescent chemical sedimentation, leading to deposition of the banded iron formation facies. As a stable shelf platform emerged, the granular iron formation facies was deposited via wave reworking of hardgrounds. Subsequent diagenesis initiated dissolution of carbonate and chert and promoted diagenetic replacement of primary iron minerals and chert. Regional metamorphism during Penokean orogeny at 1875–1835 Ma produced a suite of secondary metamorphic and related hydrothermal minerals. Metamorphism and hydrothermal flux related to the 1750 Ma development of the Republic Metamorphic Node overprinted the iron formation at Tilden to greenschist facies and infilled brittle fractures with a unique mineral assemblage. This unique mineral assemblage exhibits some striking similarities to Mn, Au, and Cu-sulphides documented at Champion Mine, west of Tilden, and proximal to the core of the Republic Node.

Oxygenation of shallow marine environments and chemical sedimentation in Palaeoproterozoic peritidal settings: Frere Formation, Western Australia

AbstractThe Palaeoproterozoic Frere Formation (ca 1.89 Gyr old) of the Earaheedy Basin, Western Australia, is a ca 600 m thick succession of iron formation and fine‐grained, clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling. Lithofacies stacking patterns suggest that deposition occurred during a marine transgression punctuated by higher frequency relative sea‐level fluctuations that produced five parasequences. Decametre‐scale parasequences are defined by flooding surfaces overlain by either laminated magnetite or magnetite‐bearing, hummocky cross‐stratified sandstone that grades upward into interbedded hematite‐rich mudstone and trough cross‐stratified granular iron formation. Each aggradational cycle is interpreted to record progradation of intertidal and tidal channel sediments over shallow subtidal and storm‐generated deposits of the middle shelf. The presence of aeolian deposits, mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind‐blown sediment input. Iron formation in the Frere Formation, in contrast to most other Palaeoproterozoic examples, was deposited almost exclusively in peritidal environments. These other continental margin iron formations also reflect upwelling of anoxic, Fe‐rich sea water, but accumulated in the full spectrum of shelf environments. Dilution by fine‐grained, windblown terrigenous clastic sediment probably prevented the Frere iron formation from forming in deeper settings. Lithofacies associations and interpreted paragenetic pathways of Fe‐rich lithofacies further suggest precipitation in sea water with a prominent oxygen chemocline. Although essentially unmetamorphosed, the complex diagenetic history of the Frere Formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean‐atmosphere evolution. Unlike studies that focus exclusively on their chemistry, an approach that also considers palaeoenvironment and oceanography, as well the effects of post‐depositional fluid flow and alteration, mitigates the potential for incorrectly interpreting geochemical data.

IRON FORMATION: THE SEDIMENTARY PRODUCT OF A COMPLEX INTERPLAY AMONG MANTLE, TECTONIC, OCEANIC, AND BIOSPHERIC PROCESSES--A DISCUSSION

Sir: Andrey Bekker and co-authors (Bekker et al., 2010) use a holistic approach to the deposition of iron formation (IF) by relating its genesis to a complex interplay of processes within the deep Earth, oceans, and biosphere. They comment (p. 469) that “In contrast to most previous studies, we suggest that no single parameter controlled iron-formation deposi tion.” The word “most” is critical here, since in a Discussion of IF genesis in Economic Geology nearly half a century ago (Trendall, 1965, p. 1069), I noted the need for a multifaceted approach with the words, “It is useless in any considerations of the origin of iron formation to confine attention to the iron formation itself,” and more recently I have discussed, in a paper that has escaped the attention of Bekker et al. (2010), the significance of IF within a wide context of Earth’s evolu tion (Trendall, 2002). Although I disagree with some of their characterization of the Hamersley Group BIFs of Western Australia, my purpose here is only to comment on their use of IF nomenclature and classification. Their adoption of the simple twofold subdivi sion of IF, as a rock type, into banded iron formation (BIF) and granular iron formation (GIF), which I first used in the 2002 paper already cited, is admirable. But I suggest that their parallel use of the terms “Superior-type iron formation” and “Algoma-type iron formation” serves more to obscure than to clarify their message. These names are derived from the classification schemes Gordon Gross developed during the long course of a study, initiated in the 1950s; his focus was the iron deposits of Canada (my italics, here and elsewhere in this paragraph); the contiguous deposits of the United States were also in cluded. Gross (1959, p. 89) initially referred to the “Lake Su perior or Knob Lake type” as one component (of two) within “Group I” (of six) of his classification of iron deposits (i.e., ore deposits); the term “Algoma type” was not used. The criteria for the recognition of the six groups varied widely, but were mainly related to mineralogy and texture. This early scheme underwent substantial transformation, and Gross (1966) later used a fourfold division of iron-formations into Algoma type, Superior type, Clinton type and Minette type. The most ma ture form of Gross’s scheme changed from a classification of Canadian iron deposits into a “classification of iron forma tions based on depositional environments ,” (Gross, 1980), in which the Clinton type and Minette type are grouped as “ironstones,” leaving the “Lake-Superior-type” and the “Al goma-type” as the only two categories of IF. Only generalized criteria were given for the allocation of these labels although, as the title of the paper indicates, the principal criterion was the depositional environment, as deduced from associated rock types. Thus, Gross (1980, p. 215‐216) wrote, “The wellknown Lake-Superior-type iron formations, widely distrib uted in Proterozoic rocks, were deposited in near-shore con tinental environments, and are associated with dolomite, quartzite, black shale, and minor amounts of other volcanic rocks,” while the “Algoma-type iron formations, found in all ages of rock, are consistently associated with greywacke sedi mentary units.” His figure 1, summarizing his scheme in a sin gle octagonal diagram, has the words “Cherty Oolitic” below “SUPERIOR TYPE,” and the single word “Cherty” below “ALGOMA TYPE.” In summary, and as emphasized by the italicized words in the preceding paragraph, Gross’s terminology evolved from a classification of the iron ore deposits of Canada, based largely on mineralogy, to a classification of the IFs of the world, based on their depositional environment. At no stage were di agnostic, as distinct from descriptive, criteria for application of the names “Superior type” and “Algoma type” to specific IF occurrences clearly specified, and it is unsurprising that there was confusion in their practical application: thus Hamersley IFs were Lake Superior type for Gross (1980, Table 1), but Algoma type for Dimroth (1976).

Ion microprobe analyses of δ18O in early quartz cements from 1.9 Ga granular iron formations (GIFs): A pilot study

The low δ18O values of Precambrian cherts have been widely used to infer that temperatures were higher and/or seawater δ18O was lower compared to today's oceans. However, the Precambrian cherts presented as evidence for these temperatures are neomorphosed from amorphous precursors that originally precipitated from ocean water, suggesting diagenetic alteration is an important cause of the widespread low-δ18O values. Many pores between sand-size clasts in granular iron formations (GIFs) are filled with coarsely crystalline quartz cements that are texturally primary. In unmetamorphosed GIF samples from the 1.9Ga Gunflint and Sokoman iron formations, synsedimentary clasts of GIF and high minus-cement porosities (the volume of pore space occupied by recognizable cement, inversely proportional to the degree of compaction) indicate such cements precipitated directly from pore water near the depositional interface. Growth bands identified in quartz cement crystals using cathodoluminescence (CL) imaging further confirm their void-filling and unrecrystallized nature. In order to test the effect of neomorphism on the δ18O of silica in these rocks, unrecrystallized primary quartz cements and neomorphic quartz in adjacent clasts were both analyzed by ion microprobe. Values of δ18O range from 23.5 to 26.4‰ VSMOW in cements and 21.3–26.8‰ in clasts in samples from the Gunflint Iron Formation. Samples from the Sokomon Iron Formation record deeper, more evolved pore waters and in some cases contain lower δ18O cements. In individual pores of Gunflint samples, cement values vary only slightly more than analytical uncertainty from edge to center and no consistent increasing or decreasing trends were observed. Overall, the δ18O of the cements are similar to the most elevated δ18O reported from cherts of similar age, but span a substantially smaller range. This suggests most of the lower values reported previously reflect late-diagenetic conditions at greater burial depths. The shallow, possibly restricted seas above the Gunflint and Sokoman GIFs appear to have been warmer and may have been lower in δ18O than present-day seawater, but they may not have been representative of global seawater.

Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia. Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations.

Black smokers and density currents: A uniformitarian model for the genesis of banded iron-formations

Banded iron-formations (BIFs) were comparatively abundant and widespread marine sedimentary rocks in the Archean and Lower Proterozoic eras, but thereafter they appear to be restricted to the Neoproterozoic and Paleozoic eras, although there are indications of similar rocks forming at present. BIFs are important as the major source of iron ore for industry and have also been used to support hypotheses regarding the evolution of life, oceans, and the atmosphere in the Archean and Proterozoic. They apparently formed in deep water and consisted of a semi-regular alternation of quartz (chert) and iron-rich minerals with little or no terrigenous sediment and low (< 1 wt.%) alumina content, in contrast to Proterozoic to Phanerozoic oolitic ironstones (Clinton–Minette style) that formed in shelf environments, were comparatively small and rare and contain abundant iron aluminosilicates and hydroxides but little or no chert. Although ooliths may be locally abundant in granular iron-formations (GIFs) they differ from the Clinton–Minette style oolitic ironstones that formed in shelf seas during periods of slow sedimentation. A common mode of origin for marine deep-water iron-formations is proposed, in which hot fluids, consisting of marine and connate water leaching iron, silica and other elements from mafic and ultramafic rocks associated with mantle plumes or mid-oceanic ridges and active spreading centres are released into the ocean at underwater hot springs (black smokers). On contact with cold marine water, the least soluble elements are precipitated in the form of colloidal hydrous silicates (clay minerals) and hydroxides close to the hydrothermal vent. The hydrothermal fluids are high in silica and low in alumina causing the precipitation of alumina-poor iron silicates (nontronite) that dissociate into iron hydroxide and amorphous silica during diagenesis. The amorphous silica is typically entrapped by iron oxide laminae to form bands of chert. Breaches of the iron oxide laminae permitted the escape of the gelatinous amorphous silica during compaction and dewatering leaving a chert-free residue as the protore of non-hydrothermal sedimentary high-grade iron ore. The rapid deposition and abundant included water formed unstable mounds and chimneys around the vents. Slumping of the mounds caused by compaction, dewatering, gravity sliding, and seismic events produced turbidity currents forming proximal fans of GIF, but colloidal particles remained suspended in longer-lived density flows to deposit the ultra fine-grained BIF over vast areas of the ocean floor. Episodes of density current deposition were separated by intervals of slow pelagic sedimentation and silicification of the sea floor. The density currents commonly caused minor erosion of the sea floor with rip-out clasts incorporated in the new layers. The iron-rich hydrothermal fluids triggered the precipitation of dissolved ferrous iron accumulated by anoxic weathering to produce the huge deposits of BIF in the Archean and Paleoproterozoic until the rise in atmospheric oxygen stopped the accumulation of ferrous iron in the oceans leaving only the hydrothermal source for later deposits.