Magnetite In Banded Iron Formations Research Articles

Using Zn and Ni behavior during magnetite precipitation in banded iron formations to determine its biological or abiotic origin

There is longstanding controversy about the genesis of magnetite in banded iron formations (BIFs), particularly concerning whether it is of abiogenic or biogenic origin. The composition of trace elements within magnetite produced by magnetotactic bacteria has been proposed as a promising marker for its biological origin; however further experimental evidence is required to investigate whether this also holds true for iron-metabolizing bacteria. Here, we compared the behavior of zinc (Zn) and nickel (Ni) in magnetite produced either abiogenically (ferrihydrite reacting with dissolved Fe2+) or biogenically (ferrihydrite reacting with dissolved Fe2+ generated via the dissimilatory iron-reducing bacterium - DIRB - Shewanella oneidensis MR-1). These abiotic and biotic incubations were applied for transforming three ferrihydrite (Fh) substrates: (1) Control Fh without added trace elements; (2) Zn-coprecipitated ferrihydrite (ZnFh); and (3) Ni-coprecipitated ferrihydrite (NiFh) in both NaHCO3 and HEPES buffer. X-ray diffraction revealed magnetite as the dominant transformation product for all reaction conditions, while siderite was only detected in experiments containing NaHCO3 as buffer. Based on similar initial dissolved Ni and Zn concentrations, we found opposing behavior of Zn and Ni where Zn showed enrichment in abiogenic magnetite, while Ni was more enriched in biogenic magnetite. Combining supposed Zn and Ni concentrations in Precambrian seawater and magnetite mineral grains from three formations deposited at different ages (i.e. 3.75 Ga Nuvvuagittuq Supracrustal Belt, 2.45 Ga Weeli Wolli Iron Formation, and 1.88 Ga Sokoman Iron Formation), we reconstructed potential Zn and Ni partition coefficients between Precambrian seawater and magnetite minerals. The reconstructed potential Zn and Ni partition coefficients suggested that these magnetite minerals from three formations were abiogenic. These differing behaviors suggest the possibility of using Zn and Ni partition coefficient ratios as a means of determining the presence or absence of DIRB in the formation of ancient magnetite deposits.

Making magnetite late again: Evidence for widespread magnetite growth by thermal decomposition of siderite in Hamersley banded iron formations

Banded iron formations (BIFs) are marine chemical sedimentary rocks that were widely deposited before the start of the Great Oxidation Event (2.45–2.32 Ga). They represent an important archive into the chemistry of the Precambrian oceans and atmosphere, and the evolution of the marine biosphere. It is believed that BIFs were derived from ferric oxides/hydroxides that were deposited by iron-oxidizing bacteria or by reaction with oxygen released by Cyanobacteria. Hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide/hydroxide precipitate, whereas magnetite is thought to have formed from ferric oxides/hydroxides via bacterial iron reduction and oxidation of microbial biomass. However, studies are equivocal about when and how magnetite formed in BIFs. We present the results of a detailed petrographic study of magnetite in BIFs of the Hamersley Group, Western Australia. We find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction. Instead, most of the magnetite formed by replacement of siderite and other ferrous-rich minerals after burial. Our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism. Published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern Pilbara Craton (200–350 °C), the thermal decomposition of siderite could account for much of the magnetite in the Hamersley BIFs (3FeCO3 + H2O → Fe3O4 + 3CO2 + H2).The relationship between magnetite and the primary precipitates, which are preserved as greenalite nanoparticles and microgranules in chert bands, suggests that much of the magnetite formed in siderite-rich, iron-silicate bands that developed by compaction of non-silicified iron-silicate muds. These bands acted as reactive pathways for fluid flow and the formation of magnetite. If most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron. Our results are consistent with models for the deposition of BIFs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial.

Microstructures, crystallography and geochemistry of magnetite in 2500 to 2200 million-year-old banded iron formations from South Africa, Western Australia and North China

As a major mineral in banded iron formation (BIF), magnetite can provide information on depositional environment and mechanism of BIF. It can provide insights into Precambrian biological process. The crystallography and microstructures of magnetite in BIF have been poorly studied due to its simple chemical composition. Here, we report an integrated crystallographic and geochemical study on various magnetites from four BIFs of 2500 to 2200 million-year-old for a better understanding on their geneses and alterations. Euhedral magnetite crystals from the 2480Ma Dales Gorge and 2460Ma Kuruman BIFs all show pure and homogeneous colors in their EBSD phase, Euler and IPF maps, suggesting they are well crystallized and devoid of inclusions or fractures. These crystals are recognized as silician magnetite with appreciable structural Si (ave. 1.22 and 2.05wt% respectively) but rare Al (ave. 0.27 and <0.05wt% respectively). They likely precipitated in a reduced and Si-rich environment and were not subjected to later redox alterations. By contrast, irregularly shaped massive magnetite from the 2500Ma Wutai BIF contains inclusions and/or fractures as revealed by numerous dark areas in its phase map. Such magnetite contains considerable structural Al (ave. 1.51wt%) but little Si (ave. 0.07wt%), suggesting a terrestrial contribution. Magnetite from the 2200Ma Lüliang BIF commonly contains chert inclusions and fractures and has been partially oxidized to hematite/maghemite. Hematite preferentially occurs along fractures or edges of the magnetite crystals, implying a secondary oxidation with possible fluid percolation. It contains Al (ave. 0.19wt%) and Si (ave. 0.505wt%) in a range between that for the Wutai and Kuruman/Dales Gorge magnetites. We suggest primary features of the magnetite from the Wutai and Lüliang BIFs have been obliterated in environments with increased detrital inputs, owing to oxygenation-enhanced continental weathering during or after the Great Oxidation Event. For all BIFs, the magnetite that coexists with carbonates contains much higher trace elements (Al, Mn, Ba, Ti and V) than that coexists with oxides. The former might have been influenced by its surrounding diagenetic to metamorphic carbonates. The implications of these findings suggest only the well crystallized euhedral magnetite coexists with oxides may retain the signatures of BIF’s depositional environments.

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.

The Best Article Award 2013 of the Society of Resource Geology was Presented to K. Adomako‐Ansah, T. Mizuta, N. Q. Hammond, D. Ishiyama, T. Ogata and H. Chiba, for the Following Paper: Gold mineralization in banded iron formation in the Amalia greenstone belt, South Africa: a mineralogical and sulfur isotope study. Resource Geology, 63(2), 119–140

This paper documented the gold mineralization of the Blue Dot deposit, Kaapvaal Craton, South Africa. Gold ore of the deposit is hosted in banded iron formation (BIF). Previous models have suggested that gold is hosted in a quartz-carbonate vein cutting BIF. Based on the detailed observation of gold ore under a microscope, chemical analyses of carbonates and chlorite, application of arsenopyrite geothermometer, and sulfur isotope analyses of pyrite, new model of gold mineralization was established. Gold was accompanied by pyrite, and mineralization of pyrite occurred simultaneously with recrystallization of iron oxides in BIF. In the gold mineralized BIF, the amount of Fe increased since Fe was added by hydrothermal fluid to precipitate pyrite. Primary magnetite in BIF was replaced by hematite and/or chlorite during gold mineralization. An increase of oxygen fugacity made gold-bisulfide complex unstable then gold precipitated. As a conclusion, this paper is appreciated to be valuable and worthy to be awarded as the Best Article Award among papers published in Resource Geology issues in 2013.

Hexagonal platelet-like magnetite as a biosignature of thermophilic iron-reducing bacteria and its applications to the exploration of the modern deep, hot biosphere and the emergence of iron-reducing bacteria in early precambrian oceans.

Dissimilatory iron-reducing bacteria are able to enzymatically reduce ferric iron and couple to the oxidation of organic carbon. This mechanism induces the mineralization of fine magnetite crystals characterized by a wide distribution in size and irregular morphologies that are indistinguishable from authigenic magnetite. Thermoanaerobacter are thermophilic iron-reducing bacteria that predominantly inhabit terrestrial hot springs or deep crusts and have the capacity to transform amorphous ferric iron into magnetite with a size up to 120 nm. In this study, I first characterize the formation of hexagonal platelet-like magnetite of a few hundred nanometers in cultures of Thermoanaerobacter spp. strain TOR39. Biogenic magnetite with such large crystal sizes and unique morphology has never been observed in abiotic or biotic processes and thus can be considered as a potential biosignature for thermophilic iron-reducing bacteria. The unique crystallographic features and strong ferrimagnetic properties of these crystals allow easy and rapid screening for the previous presence of iron-reducing bacteria in deep terrestrial crustal samples that are unsuitable for biological detection methods and, also, the search for biogenic magnetite in banded iron formations that deposited only in the first 2 billion years of Earth with evidence of life.

Rosing, Bird, Sleep &amp; Bjerrum reply

Replying to C. T. Reinhard & N. J. Planavsky , doi:10.1038/nature09959 (2011) ; N. Dauphas & J. F. Kasting doi:10.1038/nature09960 (2011) ; C. Goldblatt & K. Zahnle doi:10.1038/nature09961 (2011) Reinhard and Planavsky 1 and Dauphas and Kasting 2 question whether the formation of Fe2+/Fe3+-rich sediments and the preservation of magnetite in banded iron formations (BIFs) constrains atmospheric . Goldblatt and Zahnle 3 question whether reduced albedo in combination with a slightly increased greenhouse effect could compensate for the reduced luminosity of the faint young Sun. Reinhard and Planavsky1 and Dauphas and Kasting2 suggest that BIF mineralogy is solely controlled by diagenetic and metamorphic processes decoupled from ocean–atmosphere chemistry. We focus on the persistent preservation of magnetite in all Archean BIFs, and of magnetite and siderite in many BIFs4, and estimate the partial pressure of CO2 during the Archean ( ) using the least number of assumptions concerning unconstrained parameters. Our conclusions5 are not undermined by any geochemical data that support high .

Distribution of Precambrian iron and gold deposits on the southwestern East European Platform reflected in underlying transcrustal structure and current river systems

The East European Platform is underlain by Archaean and Proterozoic complexes of the East European Craton. In the southwest these are locally exposed in the Ukrainian Shield and the Voronezh Massif on either side of the ca. 2000 km long ESE-striking late Palaeozoic Pripyat–Dniepr–Donets rift. Evaluation with Landsat imagery of 1 : 1,000,000 scale published maps of the Precambrian complexes [Zaritsky, A.I., Galetsky, L.S. (Eds.), 1992. Geology and Metallogeny of the Southwest of the East-European Platform Map Series, 1 : 1,000,000, Ukrainian State Committee on Geology and Utilization of Mineral Resources, Kiev.] is largely obstructed by a cover of post-Palaeozoic sediments and soils of variable thickness. This obstruction is aggravated by an almost continuous patchwork of farmlands. However, analysis of the current drainage patterns in the Dniepr River basin and surrounding regions reveals a spatial coincidence of numerous stream courses and watersheds with previously inferred steep, transcrustal discontinuities of most probably Precambrian age. Transcrustal dislocations constituted important pathways for heat and fluids as is indicated by the distribution of a large proportion of assumed Early Proterozoic hydrothermal iron and gold deposits along them. This distribution is underpinned by the spatial coincidence of mineralization and elongate areas of highly irregular magnetization attributed to uneven distribution of hydrothermal magnetite in banded iron formation. In view of the extent of these dislocations, both vertically and laterally, the generation of hydrothermal fluid flow, emplacement of mantle-sourced magma and associated mineral potential away from banded iron formation complexes is likely. A second group of gold deposits, of Archaean age, is known to occur in association with still recognizable volcanic edifices in greenstone complexes. It is not known if and to what extent such Archaean gold deposits are related to these major transcrustal discontinuities. The kinematics and dynamics of these dislocations and pathways appear largely unknown and deserve high-priority investigation. The geological longevity of the transcrustal dislocation framework till the present day inferred from the current drainage systems is corroborated, however, by repeated regional topographical levelling surveys.

Geological setting and mineralization model for the Cleo gold deposit, Eastern Goldfields Province, Western Australia

The Cleo gold deposit, containing 4.25 Moz Au (37.1 Mt at 3.6 g/t), is the western part of a continuous orebody divided by a north-south tenement boundary. The eastern part is known as Sunrise, and together the Cleo and Sunrise deposits contain nearly 8 Moz Au in resources and past production. The focus of this study is the Cleo deposit, located 50 km south of Laverton in the Eastern Goldfields Province of the Yilgarn Craton of Western Australia, hosted in an Archean sequence dominated by volcaniclastic rocks. The Sunrise Shear Zone divides the sequence into hanging wall and footwall components. The gently north-west-dipping shear zone controls the orientation of shear-zone parallel ore zones, which characteristically involve pyrite replacement of magnetite in banded iron formation. Steeply-dipping multistage veins in the hanging wall and footwall define the Western Lodes, ore zones that are oriented parallel to adjacent rhyodacite–porphyry dikes. Western Lode veins are developed in all rock types, and commonly contain free gold, as well as pyrite, arsenopyrite, tennantite and chalcopyrite. In the footwall block, the margins of steeply east-dipping rhyodacite porphyry dikes form the main control on localization of the Western Lodes. In the hanging wall, the Western Lodes parallel a porphyry dike, both structures exploiting a favourable orientation in the stratigraphic sequence. Rhyodacite porphyry dikes exhibit strained margins in the Sunrise Shear Zone. Gold-bearing Western Lodes veins cut porphyry dikes and cut the Sunrise Shear Zone with minimal offset. The association of the ~2,675 Ma rhyodacite porphyry dikes with the Western Lodes ore zones is caused by the structurally favourable orientation of the dikes, and not to any direct genetic relationship between rhyodacite porphyry magma and ore fluids, as the latter post-dated the former.

98/04045 Modeling of strongly swirling powdered coal/biomass flames by the finite-element method

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.