Archean Banded Iron Formations Research Articles

Depositional age and formation conditions of Archean banded iron formations, Bundelkhand Craton, Central India: Geochemistry, neodymium isotopes and U-Pb zircon geochronology

In this study, we present major and trace element concentrations, U-Pb zircon geochronology as well as neodymium isotopic compositions of the banded iron formations (BIFs) from the Bundelkhand Craton, India. The study aims to constrain the source characteristics, depositional age and evaluate the paleoenvironmental implications of the Meso-Neoarchean seawater from which the BIF precipitated. The presence of a 2898 ± 26 Ma rounded zircon grain in the Girar BIFs indicates their maximum depositional age, which is consistent with the reported age of the metabasalts (2989 ± 190 Ma). Bearing in mind the errors, we consider the age of the studied BIFs to be ca. 2850 Ma. The BIFs from Mauranipur are associated with 2810 Ma dacite and formed at the same time. They have zircon with ages of 2718 ± 22, 2573 ± 66, 2070 ± 29 and 1934 ± 39 Ma. The BIFs from Babina are associated with 2540 Ma felsic volcanics and can be considered of the same age. Our study shows that the sedimentation of BIF in the Girar and Mauranipur greenstone belts took place in the Mesoarchean between ca. 2850 and 2810 Ma, respectively, and in the Babina greenstone belt in the Neoarchean at ca. 2540 Ma.A positive correlation between TiO2 and Zr, Al2O3 and Zr, Zr and Th/U, Y and Zr, Ni and Cr, as well as Hf and Zr, may suggest a detrital component to the BIF derived from terrigenous mafic and felsic sources. The PAAS-normalized REE patterns of the Bundelkhand BIFs are characterized by depleted LREEs, positive La, Eu, Y anomalies and Y/Ho ratios mostly ranging between 18 and 47. The studied BIFs are broadly similar to worldwide BIF occurrences at the time with the exception of the Mauranipur BIFs, which show elevated MnO concentrations (0.92 to 4.82 wt%). The studied BIF samples from the Bundelkhand Craton display a wide range of εNd(t) ranging from −6.57 to + 4.12. The broad range of isotopic values along with the above-noted geochemical signatures suggest that the BIF from the Bundelkhand Craton had significant inputs from submarine hydrothermal sources as well as pre-existing continental crust. The elevated manganese concentrations in BIFs from Mauranipur may imply the availability of free oxygen in seawater on the Bundelkhand Craton during the Mesoarchean.

The Age and Origin of the Rocks of the West Azov Group (Lozuvatka Antiform, the Ukrainian Shield)

Geochronological studies of amphibole-biotite gneisses of the Verkhnii Tokmak Unit and amphibolites and Banded Iron Formation (BIF) of the Kaiinkulak Unit of the West-Azov Group in the Lypovets area of the Lozuvate antiform in the West Azov block of the Ukrainian Shield were carried out. A multi-stage history of the formation of amphibole-biotite gneisses has been established. In terms of chemical composition they correspond to quartz diorites. Rare earth elements are differentiated — (La/YbN = 7.54), with a significant negative europium anomaly — Eu/Eu* = 0.66. The REE distribution indicates that the initial melt for quartz diorites was produced in a crustal magmatic source. In zircon crystals from amphibole-biotite gneisses, relics (cores) of older zircon and mantles that surround them were distinguished. Relict zircon is coarse-zoned, cracked, and its mantles have a thin-zoned structure. A small amount of transparent non-zonal zircon crystals was also found. Based on the results of zircon dating by the LA-ICP-MS method, it was determined that the age of the relict zircon is 3.16 Ga. The thin-zonal mantles have an age of 2.9-2.8 Ga, which probably corresponds to the time of crystallization of quartz diorites. Non-zonal transparent zircon with an age of 2.1 Ga corresponds to the time of dynamometamorphism (collision). BIF of the Lypovets deposit in the West Azov iron ore province, which belongs to the Kaiinkulak Unit of the West Azov Group, has a high Ni/Fe×10–4 = 0.5-2.1 ratio, typical for the Archean BIFs of the Algoma type. This deposit is confined to an Archean greenstone belt in a synclinal structure composed of metamorphosed basaltic komatiites and sedimentary rocks. Thus, the Verkhnii Tokmak Unit comprises Mesoarchean quartz diorites of crustal genesis, while the Kaiinkulak Unit probably represents heavily metamorphosed rocks of an Archean greenstone belt.

New Evidence of the Organic Origin of Carbonaceous Matter in Archean Banded Iron Formation of the Kostomuksha Greenstone Belt of Karelia, Russia

New Evidence of the Organic Origin of Carbonaceous Matter in Archean Banded Iron Formation of the Kostomuksha Greenstone Belt of Karelia, Russia

The Possible Role of Anoxic Alkaline High Subcritical Water in the Formation of Ferric Minerals, Methane and Disordered Graphitic Carbon in a BARB3 Drilled Sample of the 3.4Ga Buck Reef Chert.

The present article reports Raman spectroscopic observations of siderite, hematite, disordered graphitic carbon and possibly greenalite inside the quartz matrix of a banded iron sample from the BARB3 core drilled inside the 3.4Ga Buck Reef Chert of the Barberton Greenstone Belt in South Africa. The article also reports Raman spectroscopic observations of quartz cavities, concluding in the presence of water, methane and sodium hydroxide at high concentration leading to pH ~ 15 inside the inclusion, suggesting an Archean water which was strongly basic. FeIII-greenalite may also be present inside the inclusion. The possible role of anoxic alkaline high subcritical water in the formation of ferric minerals and the CO required for the synthesis of molecules of biological interest has been demonstrated theoretically since 2013 and summarized in the concept of Geobiotropy. The present article experimentally confirms the importance of considering water in its anoxic strongly alkaline high subcritical domain for the formation of quartz, hematite, FeIII-greenalite, methane and disordered graphitic carbon. Methane is proposed to form locally when the carbon dioxide that is dissolved in the Archean anoxic alkaline high subcritical water, interacts with the molecular hydrogen that is emitted during the anoxic alkaline oxidation of ferrous silicates. The carbon matter is proposed to form as deposition from the anoxic methane-rich fluid. A detailed study of carbon matter from diverse origins is presented in a supplementary file. The study shows that the BARB3_23B sample has been submitted to ~ 335°C, a temperature of the high subcritical domain, and that the graphitic structure contains very low amounts of oxygen and no hydroxyl functional groups. The importance of considering the structure of water is applied to the constructions of the Neoproterozoic and Archean banded iron formations. It is proposed that their minerals are produced inside chemical reaction chambers containing ferrous silicates, and ejected from the Earth's oceanic crust or upper mantle, during processes involving subduction events or not.

Hydrothermal vent fluid-seawater mixing and the origins of Archean iron formation

Precambrian iron formation provides valuable windows onto ancient marine environments, but exactly how these Fe- and Si-rich chemical sedimentary rocks formed have remained a matter of debate. Traditional models envisage Fe(III)-oxide formation through chemical and/or biological oxidation of soluble Fe2+, but sedimentological and textural data show that, throughout much of Archean stratigraphy, the finest-grained Fe(III)-oxides, previously interpreted to reflect primary components, were largely derived through the oxidation of the Fe(II)-silicate mineral greenalite. These observations have formed the basis for an alternative model where Fe2+ and SiO2(aq) combined to precipitate greenalite upon the mixing of hydrothermal fluids with ambient seawater. All models for iron formation genesis invoke hydrothermal alteration of seafloor basalt as the principal source of soluble Fe2+, and its subsequent transport in an anoxic deep ocean. However, the processes and products associated with the venting of near-axis hydrothermal fluids into Precambrian seawater, and implications for Archean iron formation, have not been investigated.In order to test models for Archean iron formation genesis, we used reaction path models to investigate the interactions between ancient hydrothermal vent fluids and anoxic seawater. We assembled available thermodynamic data and executed reaction path models to examine: (1) the equilibration between anoxic SO4-free seawater and basalt/gabbro at subseafloor hydrothermal conditions (400oC and 400-500 bar), (2) the cooling and decompression of these fluids during their ascent through the oceanic crust, and (3) their subsequent mixing with anoxic SO4-free seawater. Our results confirm previous suggestions that the effective lack of SO4 in Archean seawater would have allowed fayalite-magnetite-pyrrhotite-quartz equilibria to buffer fluid chemistry, in turn leading to reaction zone fluids characterised by high Fe/H2S ratios. Cooling and decompression of these reducing fluids upon ascent, and further reaction with mafic wall rock, leads to significant Fe-chlorite and quartz precipitation with little to no sulfide precipitation. As these vent fluids mix with cold seawater directly, or with conductively-heated seawater, mineral assemblages are dominated by pyrite, greenalite, and/or siderite. These results were obtained with no constraints imposed by Archean iron formation, aside from the exclusion of minnesotaite, a demonstrably secondary mineral.The quantity of hydrothermally-derived Fe2+ partitioned into Fe(II)-bearing minerals upon mixing (96-99% of total aqueous Fe), the relative proportion of greenalite precipitated, and its further concentration via plume-driven transport (by virtue of its low density), together imply that the mixing between hydrothermal vent fluids and anoxic seawater may have served as a principal mechanism for generating a large proportion of the mineral mass now preserved as Archean iron formation. These results reconcile long-standing sedimentological and geochemical observations indicating that Archean banded iron formations record a strong hydrothermal influence, and suggest that these unique deposits have the potential to constrain the nature of seafloor-hosted hydrothermal systems on the ancient Earth, and their chemical interactions with seawater.

Mesoarchean banded iron-formation from the northern Yangtze Craton, South China and its geological and paleoenvironmental implications

Banded iron formations (BIFs) are important sedimentary rocks used for reconstructing the geochemical and environmental evolution of Precambrian Earth surface conditions. Here, we describe the oldest BIFs of Mesoarchean age in the Yemadong Formation, and their implications for paleo-environmental evolution of the Mesoarchean Ocean in the northern Yangtze Craton, South China. U-Pb dating of metamorphic zircons obtained from metavolcanic rocks interbedded within the Yemadong Complex, record two major tecto-thermal events at 2901 ± 8 Ma, in agreement with a Mesoarchean origin of the Dujiagou BIFs, and as recently as 775 Ma. A high SiO2 + total Fe2O3 content of 90.95–94.62 wt%, moderate 0.54–1.42 wt% Al2O3 + TiO2, and broadly elevated levels of incompatible elements, point to a predominantly chemical origin of the BIF deposit, with a minor but influential terrigenous input. Rare Earth Element (REE) plus Yttrium patterns normalized against Post Archean Average Shale composition are characterized by the depletion of light REE relative to heavy REE. However, extremely weak Eu and Ce anomalies, together with low average chondrite Y/Ho ratios compared to average Mesoarchean BIF values, indicate that the primary REE composition of the rocks may have been diluted to some extent by terrigenous input, pointing to proximity or connection of the depositional setting to a continental margin environment. Despite potential terrigenous dilution of the syngenetic seawater-hydrothermal chemical composition, the preservation of an inherited hydrothermal REE signal in some samples, point to submarine hydrothermal provenance of the Dujiagou BIFs, as is the case for most Precambrian BIFs. Positive δ56Fe values, ranging from 0.25 to 0.45 ‰, characteristic of primary Archean BIFs, suggest partial oxidation of reduced Fe(II) to divalent (Fe(II)/Fe(III)) magnetite-rich BIFs in a broadly reducing Archean Ocean lacking a persistent and pervasive redoxcline before the rise of atmospheric oxygen.

Abiotic anoxic iron oxidation, formation of Archean banded iron formations, and the oxidation of early Earth

Iron in the early anoxic oceans of Archean age (4000-2500 million years ago) is believed to have been oxidized to form banded iron formations (BIF). Previously, it has been proposed that iron was oxidized either by free oxygen, H2O2, microbial oxidation, or photo-oxidation. However, these mechanisms are difficult to reconcile with evidence for the oceans at that time having been largely devoid of dissolved oxygen and oxidants, together with the rarity of microbial remains in BIF and restrictively slow rates of photo-oxidation. Experiments reported here show that ferrous iron readily oxidizes in analogs of Archean anoxic seawater following the precipitation of ferrous hydroxide. Once precipitated, ferrous hydroxide undergoes decomposition to elemental iron that reacts with water at room temperature to form ferric iron and release hydrogen gas. The ferric iron may then be incorporated into green rust, a mixed ferrous-ferric phase that ages into iron minerals commonly found in BIF. Our finding suggests that anoxic iron oxidation may have contributed to the formation of oxide-facies BIF, especially Algoma-type BIF that likely formed in semi-restricted basins where ferrous hydroxide saturation was more easily achieved. Additionally, ferrous hydroxide decomposition would have contributed to early Earth's oxidation, as a result of hydrogen escape to space, thus providing new insights into environmental and biological conditions on early Earth.

Depositional Setting of Archean BIFs from Congo: New Insight into Under-Investigated Occurrences

Archean banded iron formations (BIF) represent a major contributor to better constraining and assessing the paleogeography and evolution of Archean cratons. In this context, we conducted an exhaustive sampling and analysis campaign of BIF units in the Congo Craton, covering several greenstone belts within the Ivindo, Kelle-Mbomo, and Chaillu blocks. The REE + Y patterns suggest: (1) Interaction of seawater with Fe-oxyhydroxides, as illustrated by strong REE enrichment coupled with La and Y enrichment; (2) contributions from high-temperature (>250 °C) hydrothermal fluids, illustrated by positive Eu anomalies; and (3) detrital input as suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio. These observations suggest a typical environment of Algoma-type BIF deposition. Moreover, assessment of the Ce anomalies in a combination of HREE enrichment indicates that some basins in the Chaillu and Ivindo blocks may have known potential oxygen-rich episodes in the early Archean during the deposition of these BIFs.

Three-dimensional structural modeling and deformation analysis of Archean magnetite quartzite from the Anshan–Benxi area, northeastern China

The factors controlling the migration and enrichment of iron in Archean banded iron formations are important for understanding the formation of iron ore resources. Here we present, for the first time, a micro-three-dimensional modeling method that can be applied to hand specimens of iron ores. This method and petrographic observations, electron probe microanalysis, and the structural deformation characteristics of magnetite quartzite are used to assess the mechanisms of iron enrichment in such ores. The Anshan–Benxi area is located in the eastern part of the North China Craton, and is one of the most significant Archean banded iron deposits in the world. Magnetite quartzite is developed on a large scale in this area, which has experienced complex structural deformation, and thus these samples are ideal for our study. As such, we undertook micro-scale, three-dimensional structural modeling of representative hand specimens of the magnetite quartzite. Our methodology involved combined surface and internal modeling. The rock samples were sliced at fixed intervals and detailed scanned images of the slices were obtained, which enabled a three-dimensional model to be constructed of each sample. These three-dimensional models allow the deformational characteristics of the banded iron formation to be quantified on a hand specimen scale. Petrographic analysis showed that the banded magnetite quartzite has experienced brittle deformation, and electron microprobe analysis revealed iron enrichment in the fold axes. Structural deformation, such as folding and ductile shearing, has a controlling effect on iron enrichment, and the microscopic three-dimensional modeling method provides a new approach for studying the deformational characteristics of iron-bearing formations.

Episodic mafic magmatism in the Eyre Peninsula: Defining syn- and post-depositional BIF environments for iron deposits in the Middleback Ranges, South Australia

Banded iron formation (BIF)-derived iron ore deposits of the Middleback Ranges, Eyre Peninsula, South Australia, are hosted within Archean to Paleoproterozoic basement in the southeastern Gawler Craton. Mafic sills and dikes are associated with the orebodies throughout the belt and although described previously, have never been studied in detail. Two main types of mafic rocks, amphibolites and dolerites, are distinguishable from field occurrence and mineralogy. Amphibolites interbedded with BIFs from the southern Iron Magnet deposit are characterized by extensive overprinting. These amphibolites contain metamorphic assemblages of coexisting Mg-Fe- and Ca-amphiboles, garnet, corundum, biotite and relict titaniferous magnetite displaying ilmenite exsolution. Amphibolite petrography and geochemistry in the southern part of the belt indicate formation in a complex BIF depositional environment, with possible detrital input of titaniferous magnetite from igneous rocks. In contrast, NW-trending dolerite dikes occur throughout the belt and contain assemblages consisting of igneous magnesiohornblende and plagioclase. SHRIMP U–Pb zircon ages for amphibolites and dolerites yield two age groups. Amphibolite from Iron Magnet yields a 207Pb/206Pb weighted mean age of 2542 ± 15 Ma. The most robust 206Pb/238U weighted mean age of 782 ± 23 Ma for dolerites comes from the Iron Queen deposit in the central part of the Middleback Ranges. LA-ICP-MS U–Pb geochronology of U-bearing hematite from the Iron Knight South deposit yields ages of ~680 Ma. Taken together, these results suggest mafic magmatism contemporaneous with Archean BIF deposition followed by a high-grade metamorphic event (~2.47–2.41 Ga Sleafordian Orogeny). Dikes from Cape Donington, the southernmost point on the Eyre Peninsula are, instead, associated with the 1.73–1.69 Ga Kimban Orogeny, as evidenced by zircon from a pyroxene-rich dolerite, which yields a 206Pb/238U weighted mean age of 1711 ± 14 Ma. The dolerite ages from the Iron Queen deposit provide the first evidence for the presence of the ~800 Ma craton-scale Gairdner Large Igneous Province in the Middleback Ranges. The young U–Pb hematite age from Iron Knight South and post-emplacement alteration of dikes of Gairdner age from the central part of the Middleback Ranges belt provide additional support for multiple events leading to upgrading of iron ores.

In-site mineral geochemistry and whole-rock Fe isotopes of the quartz-magnetite-pyroxene rocks in the Wuyang area, North China Craton: Constraints on the genesis of the pyroxene-rich BIF

Quartz-magnetite-pyroxene (QMP) rocks with high Fe contents are of greatly economic and petrological importance and generally occur within the oldest and largest exposures of supracrustal rocks preserved on Earth. However, the chemical composition and genesis of QMP rocks remain unclear. To constrain the protolith and genesis, we present new whole-rock geochemistry and Fe isotopic data, mineral EPMA and LA-ICP-MS analysis and X-ray mapping for the QMP rocks in the Wuyang area, North China Craton. The Wuyang QMP rocks have undergone high-temperature metamorphism and have special mineral assemblages with coexisting magnetite, quartz, clinopyroxene and variable orthopyroxene. Pyroxenes of the Wuyang QMP rocks show very distinct major, trace and rare earth element (REE) compositions from those of the nearby pyroxene-marbles and pyroxenite intrusions. Such difference excludes a contact metamorphic origin for pyroxenes of the QMP rocks. X-ray elemental mapping suggests that Fe-poor carbonate minerals (dolomite and calcite) and quartz inclusions are well preserved in these pyroxene grains and no reaction phenomenon can be observed between them, indicating that these pyroxenes are not the product of metamorphic decarbonation reactions of pre-existing sedimentary silica and Fe-rich carbonate phase. Shale-normalized REE + Y patterns of the whole-rock samples and pyroxenes display geochemical features consistent with classic banded iron formations (BIFs), which show light REE depletion, positive LaSN, EuSN and YSN anomalies. This indicates that the geochemical features are compositionally inherited from their original pure chemical sediments. The Wuyang QMP rocks are therefore considered as pyroxene-rich BIF. Neotocite and hisingerite inclusions in pyroxene grains from the QMP rocks could be metamorphic products of some early Fe- and Mn-rich silicates before pyroxene. Moreover, the Wuyang QMP rocks are enriched in light iron isotopes similar to other Fe- and Ca-rich minerals in Archean BIFs. Therefore, we inferred that pyroxene from the QMP rocks could result from dehydration of initial sedimentary hydrous Ca-, Mg- and Mn-containing Fe(II)-silicate gel during diagenesis and re-crystallization in later metamorphism, providing a new interpretation for the origin of pyroxene-rich BIF.

Two billion years of evolution enclosed in hydrothermal rutile: Recycling of the São Francisco Craton Crust and constraints on gold remobilisation processes

The Quadrilátero Ferrífero in the São Francisco Craton, Brazil is known for its world-class gold deposits hosted by the Rio das Velhas greenstone belt (RdV). As in other greenstone belts, gold is also concentrated in late Archean/early Paleoproterozoic supracrustal units; in this case within the Moeda Formation. This unit has been compared to the Witwatersrand basin where there has been a long-term debate over the nature of the gold and whether it is detrital (placer), hydrothermal or hybrid (modified placer). Presently, links between gold in the Moeda Formation and RdV are tenuous. To enlighten this area of ambiguity, a new approach using chemical, isotopic and mineral inclusion data from detrital rutile found in the gold-bearing horizons of the Moeda Formation is presented. Grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile, related to As-Fe-Cu-Sb-Pb-W rich mineralising fluids. Fe, Nb and V indicate that the likely source of the gold-related rutiles is the Archean banded iron formations within the RdV, where most of the lode-gold occurs. Average Nb/Ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and/or competing mineral phases. U-Pb data yield primary crystallization ages older than deposition ages (>2.58 Ga), supporting the detrital nature of these rutiles. Rutiles record either resetting or new growth at 2.1 Ga and at ca. 500 Ma, during which gold was remobilised. Hence, this study supports a modified placer model for the gold-bearing horizons of the Moeda Formation. These fluid-assisted gold remobilisation events can be found in other correlated Paleoproterozoic gold-bearing horizon units that represent intracratonic basins related to the rifting of the Kenorland Supercontinent. This study reinforces the suitability of rutile and mineral inclusions as a powerful tool to elucidate processes and provenance.

Decoupled Hf and Nd isotopes in suspended particles and in the dissolved load of Late Archean seawater

Abstract It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf–Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf–Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled eNd2.60Ga-eHf2.60Ga values of −2.3 and +9.48, respectively, and suggests that decoupled Hf–Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive eNd2.60Ga values, but still reveal decoupled eNd-eHf values relative to an associated schist that plots slightly below the “terrestrial array”. This suggests that mineral sorting between a zircon-bearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the “zircon effect”. Alternatively, the less pronounced decoupling of eHf-eNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial eNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra)mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater.

Updating the Geologic Barcodes for South China: Discovery of Late Archean Banded Iron Formations in the Yangtze Craton

Banded iron formations (BIFs) in Archean cratons provide important “geologic barcodes” for the global correlation of Precambrian sedimentary records. Here we report the first finding of late Archean BIFs from the Yangtze Craton, one of largest Precambrian blocks in East Asia with an evolutionary history of over 3.3 Ga. The Yingshan iron deposit at the northeastern margin of the Yangtze Craton, displays typical features of BIF, including: (i) alternating Si-rich and Fe-rich bands at sub-mm to meter scales; (ii) high SiO2 + Fe2O3total contents (average 90.6 wt.%) and Fe/Ti ratios (average 489); (iii) relative enrichment of heavy rare earth elements and positive Eu anomalies (average 1.42); (iv) and sedimentary Fe isotope compositions (δ56FeIRMM-014 as low as −0.36‰). The depositional age of the BIF is constrained at ~2464 ± 24 Ma based on U-Pb dating of zircon grains from a migmatite sample of a volcanic protolith that conformably overlied the Yingshan BIF. The BIF was intruded by Neoproterozoic (805.9 ± 4.7 Ma) granitoids that are unique in the Yangtze Craton but absent in the North China Craton to the north. The discovery of the Yingshan BIF provides new constraints for the tectonic evolution of the Yangtze Craton and has important implications in the reconstruction of Pre-Nuna/Columbia supercontinent configurations.

Geochemistry, U-Pb zircon geochronology and Sm-Nd isotopes of the Xincai banded iron formation in the southern margin of the North China Craton: Implications on Neoarchean seawater compositions and solute sources

Banded iron formations (BIFs) that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition. Here, we discuss the major and trace elements and Sm-Nd isotopic geochemistry of the poorly-studied Xincai BIF, located west of the Superior-type Huoqiu BIF along a NW-SE trending supracrustal belt in the southern margin of the North China Craton, to understand the compositions and solute sources of seawater during the Xincai BIF precipitation. U-Pb zircon dating of granitic intrusive rock and biotite schist interlayer of the BIF constrains the depositional age of the Xincai BIF to 2.5–2.7 Ga for the first time, indicating that the Xincai BIF could be part of the Huoqiu BIF or consist of contemporaneous sediments formed in a similar tectonic-depositional environment. Shale-normalized rare earth element and yttrium (REY) patterns of the bulk and individual band samples display features that are characteristic of other Archean BIFs with light REE depletion relative to heavy REE, positive LaSN, GdSN and YSN anomalies, indicative of pure chemical sedimentation. The marine origin of these samples is further supported by their super-chondritic Y/Ho ratios (average Y/Ho = 47). The very low Al, Ti and high field strength element contents of the Xincai BIF indicate an essentially detritus-free component. The bulk BIF samples show a narrow range of εNd(t) values from −1.5 to +1.0, and most of the samples have negative εNd(t) values, −0.3 on average, which is different from Archean BIFs with mantle-derived depleted Nd isotopes. Deviations toward negative εNd(t) values of the iron bands (−0.5 to −0.1) and the silica bands (−4.7 to −0.5) further indicate a continental Nd contribution. In addition, positive EuSN anomalies that were found in all of the bulk and individual band samples indicate that a high-T hydrothermal input also contributed to their REY signatures. It is considered that both high-T hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their REY signatures.

Anatomy of the Archean Anshan iron ore belt in the North China Craton: A geophysical approach

The Anshan region of the North China Craton hosts significant iron ore reserves in the form of Archean banded iron formations (BIF). This study focuses on NW–SE trending layered iron ore bodies identified using aeromagnetic data with a 500m point spacing and a new method that combines structural analysis with the 3D inversion of magnetic susceptibility. This approach has identified two prominent parallel magnetic anomalies within the study area, with the bottom boundary of BIF body I estimated to be at a depth of ∼5600m and the bottom boundary of BIF body II at ∼5200m. The two BIF iron ore bodies are separated by granitoids associated with the Archean Anshan micro-continental nucleus. The area close to this micro-continental nucleus has undergone significant tectonic deformation, including ductile shearing. This shearing is the main control on the distribution of BIF iron ore belts in the Anshan area, in contrast to the previous view which folding was thought to control the distribution of BIF. Combining our new data with regional tectonic information yields a model where both folding and ductile shearing controlled the distribution of the main BIF ore bodies in the study area. This model provides insights into the distribution of iron ores in the Anshan area as well as a guide for future deep exploration in this region.

The formation of magnetite in the early Archean oceans

Banded iron formations (BIFs) are iron- and silica-rich chemical sedimentary rocks that were deposited throughout much of the Precambrian. The biological oxidation of dissolved Fe(II) led to the precipitation of a ferric oxyhydroxide phase, such as ferrihydrite, in the marine photic zone. Upon burial, ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological Fe(III) reduction coupled to the oxidation of buried microbial biomass. However, it has always been intriguing as to why the oldest BIFs are characteristically magnetite-rich, while BIFs formed after the Neoarchean are dominated by hematite. Here, we propose that some magnetite in early Archean BIF could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot, Fe(II)-rich hydrothermal fluids that existed in the deeper waters. We conducted experiments that showed the reaction of Fe(II) with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite. Our model further posits that with the progressive cooling and oxidation of the Earth's oceans, the above reaction shuts off, and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism.

Evaluation of siderite and magnetite formation in BIFs by pressure–temperature experiments of Fe(III) minerals and microbial biomass

Anoxygenic phototrophic Fe(II)-oxidizing bacteria potentially contributed to the deposition of Archean banded iron formations (BIFs), before the evolution of cyanobacterially-generated molecular oxygen (O2), by using sunlight to oxidize aqueous Fe(II) and precipitate Fe(III) (oxyhydr)oxides. Once deposited at the seafloor, diagenetic reduction of the Fe(III) (oxyhydr)oxides by heterotrophic bacteria produced secondary Fe(II)-bearing minerals, such as siderite (FeCO3) and magnetite (Fe3O4), via the oxidation of microbial organic carbon (i.e., cellular biomass). During deeper burial at temperatures above the threshold for life, thermochemical Fe(III) reduction has the potential to form BIF-like minerals. However, the role of thermochemical Fe(III) reduction of primary BIF minerals during metamorphism, and its impact on mineralogy and geochemical signatures in BIFs, is poorly understood. Consequently, we simulated the metamorphism of the precursor and diagenetic iron-rich minerals (ferrihydrite, goethite, hematite) at low-grade metamorphic conditions (170 °C, 1.2 kbar) for 14 days by using (1) mixtures of abiotically synthesized Fe(III) minerals and either microbial biomass or glucose as a proxy for biomass, and (2) using biogenic minerals formed by phototrophic Fe(II)-oxidizing bacteria. Mössbauer spectroscopy and μXRD showed that thermochemical magnetite formation was limited to samples containing ferrihydrite and glucose, or goethite and glucose. No magnetite was formed from Fe(III) minerals when microbial biomass was present as the carbon and electron sources for thermochemical Fe(III) reduction. This could be due to biomass-derived organic molecules binding to the mineral surfaces and preventing solid-state conversion to magnetite. Mössbauer spectroscopy revealed siderite contents of up to 17% after only 14 days of incubation at elevated temperature and pressure for all samples with synthetic Fe(III) minerals and biomass, whereas 6% of the initial Fe(III) was reduced to sideritic Fe(II) in biogenic Fe(III) minerals during incubation. Based on our data, magnetite in BIF is unlikely to be formed by thermochemical Fe(III) reduction in sediments of biogenic ferrihydrite, hematite or goethite-dominated sediments with complex microbial biomass present. Instead, our results suggest that diagenetic magnetite in BIF was either formed by microbial Fe(III) reduction during early diagenesis, i.e., below 120 °C, or by thermochemical Fe(III) reduction with simple organic compounds at higher temperatures, whereas siderite was formed by both microbial, diagenetic Fe(III) reduction and thermogenic Fe(III) reduction with complex biomass. Thermochemical Fe(III) reduction coupled to biomass oxidation during metamorphism provides another origin for BIF siderites and could have led to a significant increase in Fe(II) content in BIF after deposition over geological timescales.

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring, Yellowstone National Park.

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron-rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H2 . Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid-phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G.metallireducens-like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat-shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin-cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the firstdetailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe-rich, circumneutral pH geothermal environment.

Protection of phototrophic iron(II)-oxidizing bacteria from UV irradiation by biogenic iron(III) minerals: Implications for early Archean banded iron formation

On the Archean Earth (4.0–2.5 Ga) the lack of a protective ozone layer meant that harmful ultraviolet radiation (UVR) reached the surface almost unattenuated. For survival, primitive photosynthetic bacteria would have required strategies preventing UV-induced damage to their DNA. At that time, a fraction of the planktonic cells were likely anoxygenic photosynthesizers that oxidized dissolved Fe(II) to Fe(III) during their metabolism. The result of their metabolism was most likely the ferric mineral precursors for the deposition of banded iron formations. Although Fe(III) (oxyhydr)oxide minerals absorb UV radiation while still transmitting higher wavelengths, it is unknown whether minerals produced by Fe(II) oxidizers could have acted as an in situ UVR sunscreen. Here we demonstrate that the anoxygenic phototrophic Fe(II) oxidizers Rhodopseudomonas palustris strain TIE-1 and Rhodobacter ferrooxidans strain SW2 form nanometer-sized grains of ferrihydrite that are loosely attached to the cell surfaces. These biogenic Fe(III) minerals were shown to protect the bacteria from UV-C irradiation, while cells grown in the absence of Fe(II) displayed diminished cell viability as a consequence of damage to their DNA. Importantly, this study implies that primitive Fe(II)-oxidizing bacteria would have been able to produce their own UV screen, enabling them to live in the shallow photic zone of ancient oceans.