Banded Iron Formation Samples 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.

How does weathering influence geochemical proxies in Paleoproterozoic banded iron formations? A case study from outcrop samples of 2.46 Ga banded iron formation, Hamersley Basin, Western Australia

Trace metal and rare earth element (REE) abundances in banded iron formations are critical for assessing the chemical composition of ancient seawater and the long-term evolution of the ocean-atmosphere system. Recent studies, however, have highlighted the potential effects of outcrop weathering, raising concerns about whether banded iron formation samples are suitable proxies for ancient redox conditions or if exposure to surficial weathering regimes may have altered key geochemical signals. Here, we present a detailed, high-resolution study of several banded iron formation outcrop samples from the Hamersley Basin, Western Australia, to investigate microscale differences in composition between banded iron formation and weathered surfaces (i.e., weathered crusts). Elemental mapping and bulk-rock geochemical analyses reveal that weathered crust is more enriched in most elements than the banded iron formation, except for silica, which is significantly depleted. There is also a significant loss of redox-sensitive elements (RSEs) in the weathered surface, which suggests that outcrop samples have been affected by higher degrees of chemical leaching than physical erosion. These results are significant, because we clearly show that the geochemical characteristics of the weathered surface—irrespective of how it formed—are distinct from those of the remainder of the sample. This means that with sufficient screening of samples for obvious signs of alteration, banded iron formation outcrop samples may indeed be used as a reliable proxy for the evolution of Earth’s coupled ocean-atmosphere system. This increases the volume of easily accessible Precambrian sample material, so that researchers no longer solely need to rely on core recovered through costly drilling programs.

Zircon from banded iron formation as a sensitive indicator of its polychronous background: a case study on the Kostomuksha greenstone belt, Karelian Craton, Fennoscandian Shield

ABSTRACT Zircon is not a common accessory mineral of banded iron formation (BIF), but it occurs in all BIF samples from the Kostomuksha Greenstone Belt (KGB) in the Karelian Craton of the Fennoscandian Shield. The Kostomuksha BIF is of Algoma type and is part of three variably old rock associations: BIF-1 is part of a sedimentary sequence in Mesoarchean (2.87–2.84 Ga) basaltic komatiites with dacites, BIF-2 is located in Meso-Neoarchean (2.8–2.79 Ga) felsic volcanics, and BIF-3 is part of a Neoarchean (2.76–2.74 Ga) greywacke unit with felsic volcanics. Analysis of zircon has revealed grains with cores and several generations of rims. The U-Th-Pb systems of zircon indicate that its isotopic age varies considerably from 2.98 to 1.89 Ga, forming up to 4 age clusters in each sample. Most samples contain single inherited grains. The bulk of zircon in BIF is younger than the host rocks and is interpreted as metamorphic. Most of metamorphic zircons display an oscillatory zoning, and their Th/U ratio varies from 0.01 to 4.7, i.e. their origin is hard to interpret reliably, based on the above characteristics. Paleoproterozoic (1.89–1.85 Ga) zircons from BIF-1 and 3, occurring as rims and individual grains, are related to local tectono-thermal processes simultaneous with the formation of the Svecofennian orogen and were described for the central Karelian Craton for the first time. Thus, both inherited and metamorphic zircons have been revealed in Meso-Neoarchean BIF metamorphosed repeatedly in the Neoarchean-Paleoproterozoic under up to amphibolite-facies conditions. All metamorphic generations of zircons from BIF are correlated with the tectono-thermal events in the Karelian Craton, including the effect of the Svecofennian orogeny.

Genesis of the Longwanggou iron deposit in the Yudongzi Complex, South China: Implications for the redox state of seawater at the onset of the Great oxidation Event

Banded iron formations (BIFs) deposited during the onset of the Great Oxidation Event (GOE) are crucial to understanding fundamental changes in the redox conditions of Earth’s surface environments, however, they are poorly understood in South China. Here, we investigate the petrographic, geochronological, and geochemical characteristics of a set of continuous magnetite quartzite samples from a drill core through the Longwanggou iron deposit in the Yudongzi Complex, South China. These magnetite quartzite samples exhibit diagnostic features of BIFs — 1) they comprise alternating magnetite and quartz layers at the centimeter–millimeter scales, 2) they are dominantly composed of SiO2 + Fe2O3T (87.8–95.8 wt%, average = 91.4 wt%), and 3) they are characterized by shale-normalized rare earth elements plus yttrium patterns similar to seawater. The maximum depositional age of the Longwanggou BIF (i.e., magnetite quartzite) is constrained to ca. 2.49 Ga based on U–Pb dating of detrital zircon grains from chlorite schist that is intercalated with the BIF. The metamorphic complex of the Yudongzi Complex that hosts the BIF was intruded by ca. 2.5–2.45 Ga granitoid gneisses, indicating that the BIF must be older than 2.45 Gyr. Accordingly, the depositional age of the Longwanggou BIF is constrained to ca. 2.49–2.45 Ga, coincident with the onset of the GOE. The positive Eu anomalies (1.3–4.2; average = 2.2) that occur throughout BIF samples suggest a continuous flux of submarine, high-temperature hydrothermal fluids to the ocean at the Archean–Proterozoic boundary. Based on elemental mass balance, the proportion of submarine, high-temperature hydrothermal fluids that contributed to the deposition of the BIF was less than 0.1%. Unlike BIFs deposited during the GOE, the Longwanggou BIF samples lack negative Ce anomalies, suggesting that the water column in which the BIF was deposited was at least locally anoxic during the onset of the GOE. Identification of the Longwanggou BIF provides new constraints for the Precambrian evolution of South China and lays the foundation for further study of the environmental changes at the Archean–Proterozoic boundary.

Evidence for abundant organic matter in a Neoarchean banded iron formation

Abstract Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supracrustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic bio-mass, based on its co-occurrence with apatite rosettes and negative bulk rock δ13Corganic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ13Ccarb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity.

Redox-stratified seawater during the GOE: Evidences from rare earth elemental and C-O isotopic compositions of Paleoproterozoic BIF and carbonate rocks from the Taihua Group, North China Craton

The Paleoproterozoic Great Oxidation Event (GOE) is known for the dramatic change in atmospheric composition that led to oxygen becoming a persistent component of global surface processes. Drastic environmental changes greatly affected ocean and atmosphere chemistry. However, the distribution of dissolved oxygen in the ocean during the GOE is still unclear as rare contemporary sediment-records have been found. In order to identify the redox state of both shallow and deep water, and reveal the ocean redox structure during this time, we present new major and trace elemental and C-O isotopic data of bulk samples of the banded iron formation (BIF), dolomitic marble, and graphite-bearing marble of the early Paleoproterozoic upper Taihua Group in the Lushan area, southern margin of the North China Craton (NCC). The BIF samples exhibit typical major and trace element characteristics akin to very pure marine chemical sediments, such as the shale-normalized positive La, Ga, and Y anomalies, indicative of a marine depositional setting. In these BIF samples, both negative Ce anomalies and the lack of Ce anomalies reflect the presence of both oxic and suboxic/anoxic water in the ocean. Samples of the dolomitic marble are characterized by extremely low amounts of crustal material (such as Al, Ti, Zr, Hf) and high Y/Ho ratios (46–66), also implying a pure marine chemical origin. Excluding some contaminated samples, the rest of the graphite-bearing marble samples display similar features of the dolomitic marble. Significant positive Ce anomalies and high light to heavy REE (Pr/Yb(SN)) ratios in both dolomitic marble and graphite-bearing marble are essentially consistent with the 2.2–2.1 Ga carbonates from the Guanmenshan Formation of the Liaohe Group in the NE Sino-Korean Craton. Combined with the Ce anomalies in the BIF samples, we consider that the seawater when the upper Taihua Group deposited was redox-stratified from oxic shallow water to deeper anoxic water. Besides, the graphite-bearing marble record coeval surface water in the column that was enriched in nutritive materials and beneficial to microbiology and high productivity. Moreover, the early Paleoproterozoic IFs in the Songshan Group and the Lüliang Group in the NCC also exhibit significant both positive and negative Ce anomalies and enrichment of redox sensitive elements. These evidences, therefore, suggest that redox-stratified seawater occurred extensively during the GOE.

Major, trace element, and Nd isotopic compositions of banded iron formation and shales from the Sirsi shelf, Dharwar Craton, India: Implications for paleo-seawater chemistry, post-depositional alteration, and provenance

We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb)SN (0.37–0.53) and (Pr/Yb)SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*)SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb)SN (1.73–18.48) and (Pr/Yb)SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*)SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial εNd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(Xelement/XTi)sample/(Xelement/XTi)reference-1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-depositional fluid overprint. Geochemical compositions of the Sirsi clastic rocks suggest a felsic provenance, while Nd isotopic compositions of selected shale samples indicate that these sediments were most likely derived from granitoids in the Goa-Dharwar Sector.

Age and geochemistry of the Naxiguole banded iron formation (BIF), NW China: recurrence of superior-type BIF in the Neoproterozoic

ABSTRACT The Naxiguole Banded Iron Formation (BIF), located in the Eastern Kunlun Orogenic Belt, Northwest China, is hosted by the Jinshuikou Group and has a Fe of 75 Mt at a grade of 15–28.83 wt.% Fe. In this paper, we use integrated petrological, geochemical, and geochronological data to limit the age and origin of the Naxiguole BIF. Country rocks mainly comprise marble, felsic paragneiss, plagioclase amphibolite schist, and biotite plagioclase gneiss. The features of sedimentary structure (blastopsammitic texture and blastobedding structure) and trace elements (e.g. La, Ni, Zr) from the country rock indicate that their protoliths are sedimentary rock. The 141 detrital zircons from the Jinshuikou Group yield a minimum age of 908 Ma, which can be considered the upper limit for the Naxiguole BIF. According to the age (602 Ma) of the mafic rocks intruding into the Jinshuikou Group, it is reasonable to assume that the formation age of BIF can be limited at 908–602 Ma (i.e. Neoproterozoic). The high contents of Al2O3 + TiO2 (3–6 wt.%), ΣREE (36–135 ppm) for BIF samples suggest that terrestrial clastic material contributed to BIF formation. The BIF samples show seawater-like features, such as depletion in LREEs, positive Y anomaly and high Y/Ho ratios (30.7–57.5, average = 42.5), implying that the Naxiguole BIF precipitated from seawater. The positive Eu/Eu*PAAS anomalies (average 1.52) suggest the influence of high-T hydrothermal fluids. Based on positive Eu anomalies and high terrestrial clastic components and no obvious Ce anomalies, the Naxiguole BIF was formed in an anoxic or suboxic sedimentary basin within active continental island arc, and is similar to Superior-type BIFs worldwide.

A multi-methodological approach for mineral exploration and predictive metallurgy: the case of the Pilar gold deposit at the Quadrilátero Ferrífero, Brazil

Gold exploration is generally based on methods such as soil, stream and drill core geochemistry and geophysical techniques, but X-ray diffraction (XRD) and the Rietveld method-based mineral quantification are seldomly used. Here, a multi-methodological approach coupling traditional drill core macroscopic description and multi-element analysis, scanning electron microscopy/energy-dispersive X-ray fluorescence spectrometry (SEM/EDS), and electron probe microanalysis (EPMA) with XRD/Rietveld method and statistical techniques is presented as a useful tool for deposit characterization, mineral exploration and predictive metallurgy purposes. The Pilar gold deposit, situated in the Quadrilátero Ferrífero province, Brazil, was used as a case study for the proposed multi-methodological approach. Carbonate-facies banded iron formations (BIFs) and schist sequences showing an intense quartz and sulfide hydrothermal alteration represent the host rocks of Pilar mineralized intervals, which may be identified by three signatures: Au-As-W-S-quartz-chlorite-pyrrhotite-arsenopyrite-calcite for schists, and Au-As-quartz-chlorite-arsenopyrite-pyrrhotite and Au-Ag-S-Te-quartz-chlorite-pyrrhotite for BIFs. Within the mineralized BIFs, the expected average Au grades and gold textures vary according to the signature. In As-poor BIFs, gold is only observed within the structure of sulfide and telluride phases. On the other hand, native gold is present in As-rich BIF samples, which usually contain relatively higher Au grades, compared to the As-poor BIFs. Native gold is also identified in mineralized schists, whose average Au grades are similar to those of As-poor BIFs. Therefore, the proposed multi-methodological approach is useful in providing the following information: geochemical-mineralogical signatures and the expected average Au grades and mode of occurrence of gold for each one of these signatures. At Pilar, the geochemical-mineralogical behavior of mineralized zones is directly linked to Au grades and textures. In terms of a practical workflow, this study illustrates that, after the multi-methodological approach is applied and the representativeness of the ore-petrography is tested, the general described behavior of gold within each group of ores can be used for mineral exploration and metallurgical purposes without the need of repeated SEM/EDS and EPMA analyses. Instead, the average Au grades and the gold textures may be predicted solely by the signature definition based on the drill core geochemical-mineralogical analyses.

Can Weathering of Banded Iron Formations Generate Natural Hydrogen? Evidence from Australia, Brazil and South Africa

Oxidation of iron-rich rock is known to generate H2 in oceanic as well as in continental domains. Here we tested the possibility of H2 generation as the result of weathering of banded iron formations (BIF). The BIF constitute more than 60% of global iron ore reserves with low Fe3+/Fetot and total Fe ranging from 20 to 40 wt% and are therefore good candidates for H2 production potential. In the vicinity of BIF-hosted iron mines in Australia, Brazil and South Africa, satellite imaging has revealed the presence of sub-circular depressions that usually are the proxy of H2-emitting features. A morphological comparison of the sub-circular depressions with the ones observed in previous studies point to probable H2 seeping in these areas. In parallel, a petrological study conducted on altered and fresh BIF samples from the Hamersley Province in Western Australia also suggests H2 generation during BIF weathering. Indeed, mineral transitions from ferrous silicate (riebeckite and/or minnesotaite) to ferric iron oxi-hydroxides (goethite) or from ferrous and ferric oxides (magnetite) to exclusively ferric oxides (maghemite, hematite, goethite) were observed on the samples. The oxidation of ferrous iron by aqueous fluids circulating through and leaching the BIF is promising for H2 generation. The BIF weathering profile suggests that the limiting factor is the presence of water, and that this reaction is happening at, or near, surface temperature. This challenges the idea that high temperatures are required to generate H2 as it is the case during the serpentinization. The link between BIF and H2 will have however to be further investigated to better constrain the reactions and their kinetics.

Chemostratigraphy of the Carajás banded iron formation, Brazil: A record of Neoarchean Ocean chemistry

One of the most important occurrences of banded iron formation (BIF) worldwide is situated in the Carajás Mineral Province, southeastern Amazonian Craton. The BIFs are jaspilites and are hosted in the Neoarchean (∼2.74 Ga) volcano-sedimentary sequence of the Grão-Pará Group. They are mostly composed of cm-thick intercalations of hematite, jasper, and chert. Their primary textures and structures are still preserved, which make them an ideal archive to evaluate the paleomarine environment. Low abundance of Al2O3 (<1.0 wt%) and HFSE (<1 ppm) for most BIF samples indicate an essentially detritus-free depositional environment. Overall, the rare earth elements and yttrium (REY) patterns show a weak positive lanthanum (La) anomaly, and a pronounced positive europium (Eu) anomaly (Eu/EuPAAS = 1.86 – 5.05), although the presence of true cerium (Ce) anomaly is not evident. Stratigraphic variations in iron isotope compositions, up to 0.80‰ (δ56Fe = +1.10 to +1.90‰) over tens to hundreds of meters of stratigraphic section, point to relative changes in the iron isotope composition of Carajás seawater over periods of a few million years. The jaspilites show heterogeneous distribution of Nd isotopic signature throughout the BIF sequence, and rocks from near the basaltic flows/jaspilite contact (type-II) have negative εNd (t) values (–4.97 to –0.90). In contrast, predominantly positive εNd(t) values (–0.84 to +5.40) are common in the remaining samples (type-I). The strongly positive δ56Fe values indicate a low degree of partial oxidation of Fe(II), which, combined with the pronounced positive Eu anomalies and the absence of Ce anomalies, hint towards that the deposition occurred mainly on a deep-sea environment with intense hydrothermal activity under anoxic and suboxic conditions, distal to continental landmasses. Locally, considerable oxygen was probably present in the ancient ocean's water masses, which led to the precipitation of BIFs.

Origin of the Urucum iron formations (Neoproterozoic, Brazil): Textural and mineralogical evidence (Mato Grosso do Sul – Brazil)

Neoproterozoic banded iron formations (BIF) and ironstones occur in the Urucum mining district (Pantanal, Brazil). Microbially mediated formation as microbialites has been proposed for these deposits. This study aims to provide a high resolution characterization of micro-textural features, mineral assemblage and distribution to gather evidence for two microbial ore forming systems: Fe- and Mn-oxidizing metabolic processes and cyanobacterial activity Optical- and cathodoluminescence rock microscopy, Fourier Transform Infrared (FTIR), and Raman spectroscopy allowed the identification of syngenetic and diagenetic minerals. Evidence from BIF and nodule samples points to low temperature, suboxic, neutral/slightly alkaline formation conditions, but rarely diagenetic acidic and anoxic local conditions occurred. Direct evidence for the action of microorganisms is: (a) microtextural evidence of microbially mediated formation of the ore beds, which occur as microbialites; (b) mineral types of syngenetic origin (ferrihydrite, todorokite, manganite) with various types of organic matter incorporated, (c) light isotope signal (cited negative hematite δ57Fe) and; (d) mineralized sedimentation cycles of Fe, Mn, Ca and Si. Two microbial ore forming systems are proposed as dual systems, characterized by the main Fe- and Mn-oxidizing metabolic processes and intense cyanobacterial activity. More ordered minerals, such as hematite, goethite and anatase, formed during diagenesis of Fe-rich biomat. Calcite-type (calcite, Mn-calcite, rhodochrosite) and dolomite-type carbonate structures (kutnohorite, ankerite, dolomite) occurred. Fine-grained carbonate is the common constituent, with some idiomorphic occurrences and dissemination in the laminae and nodules. The composition of the ankerite and the other carbonates is highly variable. The BIF’s gray laminae consist of hematite-rich part with minor quartz, scarce carbonate and apatite. The red laminae contain high contents of carbonate and hematite, with quartz and further diagenetic minerals (feldspar, apatite). Hematite occurs in considerable amount, but it is enough only for turning jasper reddish in color. Nodules were segregated from laminae by diapirism and/or microbial activity and underwent the same syngenetic and diagenetic changes as the microlayers from which they are segregated. Nodule compositions fit well with the diagenetic trends common in such ore forming systems. Most likely the differences between laminae and nodules are consequences of different initial organic matter contents and fine alternation of the oxygen supply of the system.

Volcano Sedimentary Origin of Archaean Banded Iron Formation of Nellore Schist Belt from Chundi-Malakonda-Ayyavaripalle Area, Prakasam District, Andhra Pradesh and its Petro-chemical Characterization

Banded iron formations (BIFs) are the most significant source of iron in the world. The present study addresses the petrographic and geochemical characterization of the BIF from Chundi-Malakonda-Ayyavaripalle area in the Nellore schist belt of Prakasam district, Andhra Pradesh, their genesis and iron enrichment processes. Six bands of banded magnetite quartzite (BMQ) of Archaean age occur in the area. Field investigations and petrography revealed that the studied BIF samples are hard, compact, weathered and composed of alternate layers of magnetite/martite and subordinate quartz, biotite and muscovite. The BIFs occur in three associations: volcanic (Band 1,3,4,5), sedimentary (Band 6) and volcano-sedimentary (Band 2). The geochemical composition of the whole rock reveals that iron and silica represent 78.48% to 91.74% of the average composition. The total iron (TFe) contents range from 16.25 to 61.80 wt % (average of 38.47 wt %) and is consistent with low-grade siliceous BIF by global standards. All BIFs have high SiO2/Al2O3, Fe/Ti, and low Al/(Al+Fe+Mn), which suggest a mixed hydrogenous and hydrothermal origins. Geochemical results for the BIFs lead to the conclusion that all the BIFs are of Algoma-type with Precambrian affinity and formed in small sloped or terraced silled basins in the back-arc areas with MORB affinity, surrounding an immature island arc. Depending on the degree of replacement and shape of the magnetite crystals, two textural types of magnetite (Magnetite-I and Magnetite-II) are observed. Ore formation occurs in three stages: Magnetite (protore), Martites and Martite-goethites.

Depositional and Environmental Constraints on the Late Neoarchean Dagushan Deposit (Anshan-Benxi Area, North China Craton): An Algoma-Type Banded Iron Formation

Abstract The late Neoarchean, ~2.53 to 2.51 Ga Dagushan banded iron formation (BIF), is a typical Algoma-type BIF located in the northeast part of the North China craton. Despite having undergone upper greenschist to lower amphibolite facies metamorphism, the Dagushan BIF retains evidence of varied depositional facies, making it an ideal archive to evaluate the paleomarine environment and the paragenesis of the ore minerals. A transition from oxide to silicate to carbonate facies BIF is evident in a northward direction. The mineralogical composition shifts from magnetite and quartz in the south through a magnetite-quartz-cummingtonite/stilpnomelane assemblage in the transition zone to magnetite-siderite in the north. Such a distinct distribution of mineralogical facies correlates well with the depositional environment of the BIF. The carbonate facies BIFs formed in a near-shore, proximal environment, whereas the oxide and silicate facies BIF assemblages formed in deeper waters, distal to the paleoshoreline. The BIF samples display characteristic seawater-like rare earth element + yttrium (REE + Y) profiles with positive La and Y anomalies and heavy REE enrichment relative to the light REEs when normalized to post-Archean Australian shale. Positive Eu anomalies suggest a high-temperature hydrothermal contribution to the BIF. The absence of a negative Ce anomaly in nearly all samples, coupled with positive δ56Fe in magnetite in all mineralogical facies, indicates a dominantly anoxic water column contemporaneous with deposition of the BIF. At ~2.53 Ga in the Anshan area, seawater was mostly anoxic and rich in ferrous iron. Dissolved ferrous iron in upwelling hydrothermal fluids was oxidized and precipitated as Fe(III) oxyhydroxides in the photic zone leading to BIF formation. Proximal to hydrothermal vents, magnetite formed via the reaction of Fe(III) oxyhydroxides and aqueous Fe(II) supplied from the hydrothermal fluids and microbial dissimilatory iron reduction (DIR) coupled to organic carbon oxidation. Proximal to a paleoshoreline, siderite formed through DIR, as evidenced by the depleted δ13C values and the presence of graphite. Silicates, such as stilpnomelane and cummingtonite, are considered to be the metamorphic products of early diagenetic silicates (e.g., nontronite) that formed in the water column from admixtures of Fe(III) oxyhydroxides and amorphous silica.

The origin of early-Paleozoic banded iron formations in NW China

There is growing evidence that the earliest Paleozoic ocean experienced fluctuations in ocean redox chemistry, with anoxic ferruginous conditions being a prevalent feature at certain times. However, the general absence of early Paleozoic banded iron formations (BIFs) means that direct supporting evidence for ferruginous conditions inferred from geochemical proxy data is currently limited. Here, we describe an early-Paleozoic BIF (Tiande No.2) from the late Silurian Habahe Group in Altay, NW China. The BIF is hosted in meta-volcano-sedimentary rocks, and is characterized by alternating iron-rich and silica-rich laminae. In addition to abundant magnetite and quartz, Fe-silicates (garnet and amphibole) are dominant minerals in the BIF. Like most earlier Precambrian examples, the BIF horizons are dominated by high Si and Fe contents, but also contain significant Al, Ti and rare earth element concentrations, consistent with a dominant chemical origin, but with significant terrigenous contribution. We observe slightly positive Eu anomalies (1.17 to 1.57) and low εNd(t) values (−4.2 to −0.8) in BIF samples, indicating a mixed seawater and submarine hydrothermal source for the iron. The BIF also generally displays negative Ce anomalies, and homogeneous and slightly negative δ56Fe values. This suggests near-quantitative oxidation of dissolved Fe(II) from the water column, likely following upwelling into oxygenated surface waters as sea level decreased, in a region where the terrigenous input was low relative to the dissolved Fe2+ flux. The Tiande No.2 BIF is conformably bedded within schist, which also has high concentrations of Fe (up to 12.67 wt%), generally elevated Fe/Al ratios (>1.0), and no Ce anomaly, while magnetite in the schist has heavy and variable δ56Fe values. This suggests that the schist precursor, which likely comprised a significant contribution from deposited marine sediment, was also deposited from ferruginous waters, but with only partial oxygenation of dissolved Fe2+ in a dominantly anoxic water column. The occurrence of this late Silurian BIF and associated ferruginous rocks provides new constraints on the nature of ocean redox chemistry at this time, and supports continued dynamic Fe cycling and at least intermittent ferruginous deeper water conditions in this location during the late Silurian.

Preparation and characterization of magnetic photocatalyst from the banded iron formation for effective photodegradation of methylene blue under UV and visible illumination

The present study devoted to the production of an efficient and low-cost magnetic photocatalyst (γ-Fe2O3/ Fe3O4/ SiO2) from the banded iron formation (BIF) via a single-stage heat-treatment process. The X-ray diffraction (XRD) analysis of Argon modified BIF ore showed complete phase transformation of hematite phase into maghemite. The hematite conversion into the maghemite phase was confirmed from the Fe2p and O1s XPS spectra of the Ar-modified BIF ore. SEM-EDX results revealed the existence of carbon sheets around the iron oxide grains due to polyvinyl alcohol (PVA) decomposition. Optical characterization revealed that Ar-modified BIF has the lowest band gap energy of about 4.2 eV and the lowest photoluminescence emission peak suggesting the highest hole-electron recombination rate among all BIF samples. Various affecting parameters on the photocatalytic performance of both original and modified BIF ore samples were consistently optimized for decolorization of methylene blue (MB) as the model reaction under ultraviolet (UV) light including reaction time, pH and photocatalyst dose. The kinetics of MB photocatalysis on the BIF samples showed an excellent fitting to the pseudo-first-order reaction. The Ar-modified BIF samples exhibited the maximum photocatalytic efficiency of about 87.5% under UV and 76.9% under visible light because of the combined effects of structure defects, oxygen vacancies, and carbon sheets. This research opens the way for the significant and multiple uses of natural raw materials as fast collective photocatalyst under magnetic field for wastewater treatment on a large scale.

Contrasting Depositional Environment of Iron Formation at Endengue Area, NW Congo Craton, Southern Cameroon: New Insights from Trace and Rare Earth Elements Geochemistry

The Endengue Banded Iron Formation (BIF) is located in the northwestern edge of the Congo craton in Cameroon. Here, we report geochemical data of trace and rare earth elements (REE) of the Endengue BIF samples from the Archean Ntem complex and investigate their environmental setting. Two types of BIF occur at Endengue area, both containing minimal contamination from terrestrial material. Total REE (ΣREE) contents in the Type 1 BIF are extremely low, ranging from 0.34 to 1.83 ppm, similarly to pure chemical sediments while Type 2 BIF displays ΣREE contents ranging from 2.98 to 24.26 ppm. The PAAS-normalized REE + Y patterns of the two BIF types display LREE enrichment relative to HREE and weak positive Eu anomaly, most likely suggesting that the source of iron and siliceous of the Endengue BIFs is mainly from the contribution of low-temperature hydrothermal alteration of the crust. Type 1 BIF shows very low Nd content (* and positive Ce anomalies which suggests suboxic or anoxic seawater similar to the depositional environment of Elom BIF in Archean Ntem complex. In contrast, Type 2 BIF displays low to moderate Nd contents (1 and 100 ppm with the exception of sample LBR65) with negative correlation between Nd and Ce/Ce* and negative Ce anomalies. These features indicate precipatation of Type 2 BIF from oxic iron-rich solution that changed to oxidized surface by rapid precipitation of the hydrothermal Fe. The Endengue BIFs were deposited in the continental margin ocean in presence of low-temperature hydrothermal fluids mixed with seawater, similar to Paleoproterozoic Kpwa-Atog Boga BIFs within the Nyong group and other Paleoproterozoic Superior-type BIFs worldwide.

Paleobasinal to band-scale REE + Y distribution in iron formations from Carajás, Amazon Craton, Brazil

Banded Iron Formations (BIFs) from the Carajás Mineral Province (CMP), southeastern Amazonian Craton, are the protolith of the world-class iron deposits of the CMP. Besides being one of the largest iron ore deposits in the world, they recorded important paleoenvironmental and regional geodynamic markers across the Neoarchean-Paleoproterozoic transition. We investigate the secular variation of geochemical signature, from several deposit of Carajás Province. The low contents of Al2O3 and of 19 other immobile trace elements (e.g., Zr, Th, Hf, Ti) in 26 high-resolution detritus-free BIF samples indicate their detritus- free nature. Their positive shale-normalized (subscript SN) La and Gd anomalies are similar to those of modern seawater; whereas their Y/Ho ratios are higher. These BIF samples also display large positive chondrite-normalized (subscript CN) Eu anomalies, reflecting the influence of high-temperature hydrothermal vents and bottom solutions. In contrast to modern seawater and to most other Neoarchean/Paleoproterozoic detritus-free BIFs, the Carajás BIFs do not show the typical heavy rare earth element (REE) enrichment relative to light REE; instead they do show variable light to heavy REE fractionation trends (YbSN/PrSN ratios: 0.05–4.91). The Th/U ratios of both bulk BIFs and mesobands are generally lower than the upper-crustal ratio. Although the bulk BIFs show mostly chondritic values, the individual Fe and Si bands exhibit distinctly higher ratios. The general absence of negative CeSN anomalies attests for rather reducing paleoenvironmental conditions. Unlike the great majority of the data which lack negative anomalies of Ce, a few samples do display such anomalies, which may be compatible to the oxidizing conditions shown by the low Th/U ratios of most the Carajás BIFs. Such low Th/U ratios may register terrestrial weathering conditions, which cause preferential U input into the oceans in response to the oxidation of immobile U4+ to more mobile U6+.

Hybrid multivariate typological model for the banded iron formations from the Bonito mine, Northeastern Brazil

Typological ore delineation is a basic procedure that provides various bases in ore geology on which crucial developments rely on, in particular, those supporting mineral resource evaluation. To deal with several complex issues related to typological delineation of heterogeneous ore bodies, an unusual hybrid multivariate approach, based on a skillful combination of geochemical/geological knowledge and correspondence analysis (CA), was established. This approach was tested on banded iron formations (BIF) in the Bonito mine, Northeastern Brazil, enhancing the available geochemical knowledge by revealing grade combinations related to geological processes causing such deposits. Petrographic studies helped identify four main BIF-types: amphibolitic, hematitic, martitic, and magnetitic itabirites. An exhaustive geochemical database comprising 1,384 BIF samples assayed for Fe2O3, SiO2, Al2O3, P, and Mn grades was established and classified for the aforementioned BIF-types. The methodology provides an updated typological model based on a unique aspect of CA that allows a simultaneous projection of samples and their characteristics onto axes of the same abstract space; this helps validate mathematical results with geological information linked to sample attributes. The BIF typology model was established by applying K-means clustering to factorial scores derived through CA. Hence, the BIF typological model was updated, and new geochemical BIF-types helped interpret some of the main geological processes related to them. Geochemically, the factorial axes could be related to: 1) Iron rich BIF rocks, which are clearly represented on the positive side of Axis 1, comprising almost 50% of the total variance; 2) Manganese and alumina, associated with high silica content, by attachment to Axis 2, which describes a possible and critical terrigenous input.

Chemical evolution of seawater in the Transvaal Ocean between 2426 Ma (Ongeluk Large Igneous Province) and 2413 Ma ago (Kalahari Manganese Field)

Drill core and outcrop samples of pure marine chemical sediments (banded iron formation (BIF), manganese formation (MnF), jaspilites, lutites, and cherts) from the transition of the ~2426 Ma old Ongeluk Formation into the 2413 Ma old Hotazel Formation, Transvaal Supergroup, South Africa, reveal remarkable changes of seawater chemistry in the Transvaal Ocean. Similar to pre-Ongeluk chemical sediments, the shale-normalized rare earths and yttrium (REYSN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive EuSN anomalies, indicating that high-temperature (>250 °C) hydrothermal fluids did not contribute significantly to the REY budget of ambient waters. However, a 10 cm drill core section in the lower Hotazel Formation is characterized by conspicuous positive EuSN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REYSN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REYSN enrichment, positive LaSN, GdSN and YSN anomalies, but no CeSN or EuSN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative CeSN anomalies in some of the lutites, BIFs and MnFs, reflecting Ce depletion in ambient seawater. All Hotazel lutite, BIF, and MnF samples studied show unradiogenic εNd(t) values (−0.5 ± 0.2 to −2.4 ± 0.2), indicating a mostly continental REY source. The REY distribution and Nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved REY to local “Transvaal seawater”. Precipitation of the Hotazel lutites, BIFs and MnFs with negative CeSN anomalies, therefore, suggests that oxic conditions prevailed on the Kaapvaal Craton and in Hotazel seawater already at ~2.413 Ga, i.e. 80 m.y. before the disappearance of mass-independent sulfur isotope fractionation (MIF-S) that defines the Great Oxidation Event at ~2.33 Ga.