Deposition Of Banded Iron Formations Research Articles

A multi-technique evaluation of hydrothermal hematite U[sbnd]Pb isotope systematics: Implications for ore deposit geochronology

Hematite (α-Fe2O3) is a common iron-oxide mineral known to incorporate U into its crystal lattice at up to wt% concentrations and has been previously used to date ore formation within iron-oxide copper gold and banded iron formation deposits. However, there has been no detailed evaluation of the potential challenges this novel mineral geochronometer may present for accurate temporal interpretation. We report a multi-technique UPb geochronological study comprising laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), sensitive high-resolution ion microprobe (SHRIMP) and the first isotope dilution-thermal ionization mass spectrometry (ID-TIMS) procedure and analyses using an in-situ grain micro-sampling technique. We assess intra- and inter-sample data reproducibility, and examine the compatibility of spatial and analytical resolutions to texturally and compositionally diverse samples.Five samples of hydrothermal, primary, oscillatory-zoned hematite were analysed from distinct lithologies throughout the ~1.59 Ga Olympic Dam Cu-U-Au-Ag deposit, with typical U concentrations ranging between ~20 and 1000 ppm. LA-ICP-MS mapping of isotope distributions across mm-sized grains allows pinpointing of U-rich and relatively U/Pb isotopically ‘homogeneous’ domains. Micro-domains were extracted via laser-cut square shaped trenches (100 × 100 μm). UPb datum range in complexity, with concordant age domains measured by ID-TIMS indicating that hematite can retain a near-closed UPb system over ~1.6 Ga, while also demonstrating that younger Pb-loss events may be recorded. Significant reverse discordance is common in the data, likely to be a real feature due to both internal decoupling and mobility of Pb within a single grain and UPb fractionation during microbeam analysis. These effects will obscure the interpretation of real ages for Proterozoic samples where non-zero age Pb-loss events occur, and restricts the evaluation of common Pb mixing components. As a result, microbeam analyses may be biased younger or older in terms of 207Pb/206Pb dates in samples with complex histories. Microbeam datum compare favourably with high precision ID-TIMS ages. Although reproducible ID-TIMS UPb dates were generally not obtained, in the best-case example, a 207Pb/206Pb weighted mean age of 1589.91 ± 0.91 Ma (MSWD = 1.3, n = 5), within 0.2% of the Olympic Dam granite host rock age (1593.87 ± 0.21 Ma) was generated. All five hematite samples dated by LA-ICP-MS and SHRIMP yield weighted mean 207Pb/206Pb dates within 2% of the host granite age after small degrees of data rejection. The results demonstrate that with careful sample petrography, screening and data interpretation, hematite can be considered a very useful UPb mineral geochronometer, with potential application to all U-bearing Fe-oxide rich mineral systems.

Dynamics of oceanic iron prior to the Great Oxygenation Event

We report Fe isotope compositions in banded iron formations (BIF) from three cores from the pre-GOE Transvaal Supergroup, South Africa, and one core from the pre-GOE Joffre Member of the Hamersley Group, Australia. The low abundances of detrital elements such as Al, Ti, Sc, and V suggest that these BIF were deposited in distal positions with respect to Precambrian continents, while the very low P abundances are incompatible with strong biological productivity at these localities. A combination of U–Pb chronology and cobalt accumulation rates is used to establish a high-resolution time scale and deduce chemical fluxes. The e-folding time of δ56Fe variations up stratigraphy is used to determine Fe oceanic residence times and Fe concentrations as well as the dissolved carbonate content of Early Proterozoic seas. Iron oceanic residence times increased from 0.2 to 2.3 Ma during the time interval between 2521 and 2394 Ma covered by the present cores, translating into ocean Fe concentrations increasing from 6.4 to 37 mmolkg−1. Massive BIF precipitation was triggered by release of CO2 into the atmosphere and subsequent surges of alkalinity into the ocean due to the weathering of large subaerial volcanic systems. We argue that a suitable electron acceptor for Fe2+ oxidation to magnetite is the inorganic conversion of CO2 (or dissolved inorganic carbon) to CH4. In the process, H+ is produced, which is reinjected into oceanic hydrothermal systems liberating Fe2+. The couple Fe2+-magnetite may, in the Archean, have played the same buffering role as the couple Ca2+-calcite plays today. Massive injection of methane into the atmosphere would accompany BIF deposition and make the early Earth similar to modern Titan. Therefore, although biological processes may have assisted iron oxidation and precipitation, they are not a prerequisite for BIF deposition.

Mineral prospectivity analysis for BIF iron deposits: A case study in the Anshan-Benxi area, Liaoning province, North-East China

Banded iron formation (BIF) deposits are the most important source of iron in China, and they are dominantly distributed in the North China Craton (NCC). The Anshan-Benxi area is situated in the northeastern part of the NCC and is considered to be the most fertile and productive iron metallogenic province in China. The recent successive discoveries of large BIF deposits in this area, especially the world-class Dataigou BIF deposit, indicates that there is great potential for iron resources in this area. The BIF deposits are well defined by magnetic and gravity anomalies and thereby extensive petrophysical and geophysical studies are critical for BIF prospective mapping. A data-driven weights-of-Evidence (WofE) analysis was used for BIF prospectivity modeling. The results indicated that prospective areas are generally confined to the Precambrian greenstone belt basin and high-prospectivity zones (Class A) are mainly distributed in the middle and northeastern parts of the study area. Moderate-prospectivity zones (Class B) are mainly distributed in southwestern and northeastern parts of the study area. Finally, we employed selective human intervention to redefine priority targets to be more reliable. The results suggest the method is successful as 82 target areas were narrowed to Class A 20 and 11 Class B targets. The total of Class A and Class B targets was just over one-third of the total number of targets, offering a much smaller but effective selection of targets for consideration by mineral explorers.

Iron-oxides constrain BIF evolution in terranes with protracted geological histories: The Iron Count prospect, Middleback Ranges, South Australia

When studied at appropriate scales of observation and using complementary methods, the intrinsic textural and geochemical complexity of iron-oxides can provide unparalleled insights into the evolution of iron ore resources from banded iron formation deposition to ore formation. Iron Count is one of >20 deposits and prospects within the Archean Middleback Ranges iron ore belt, South Australia, in which iron-oxides from BIFs and ores display variable textural and trace element signatures. Integrated petrography, iron-oxide laser-ablation mapping and geochemistry, and in-situ dating of BIF-derived ores was undertaken to assess whether features observed in samples from this representative prospect might be attributed to localized and superimposed overprint processes.The prospect consists of a West and East ridge, each characterized by a sequence of overprints expressed as interconversions between iron-(hydr)oxides and accompanying variation in trace element concentrations. Pseudomorphic replacement of early magnetite by hematite (martite), followed by replacement by iron-hydroxides is recognized in samples from the East Ridge. Further overprints are expressed as veining, brecciation and crystallization of (micro)platy hematite rich in granitophile elements (Sn, Mo, W, U). Martitization of early magnetite in the West Ridge is accompanied by enrichment in Zn, Co and Ni in martite. Subsequent recrystallization of martite/iron-hydroxides by granoblastic hematite is marked by enrichment in Ti, Ta, Nb, REE and granitophile elements, and co-crystallization of REE-minerals. This is followed by crystallization of (micro)platy hematite, similarly rich in these elements. Enrichment in Ti and granitophile elements in granoblastic hematite is interpreted to infer crystallization from highly saline fluids generated by interaction between evaporite-bearing sedimentary rocks and granite-derived fluids. Post-Archean Australian Shale-normalized REY fractionation trends of iron-oxides from the East Ridge are consistent with precipitation of iron-rich minerals from a mixture of anoxic seawater and hydrothermal vent fluids. Analogous trends from the West Ridge lack a seawater signature. Contrasts in REY fractionation trends between the two ridges is interpreted as a combination of the loss of primary seawater signature due to severe overprinting in the West Ridge, and to lateral variation in the marine depositional environment. Results validate the hypothesis that variation in iron-oxide trace element signatures seen throughout the Middleback Ranges iron ore belt results from overprinting due to superimposed tectono-magmatic events that have locally affected each deposit. Despite undergoing different sequences of ore-forming stages, as well as possibly distinct depositional environments, the two ridges share a common overprint, dated here at ~1790 Ma using in-situ UPb geochronology of co-existing hematite, monazite and xenotime. This age is considered to represent the timing of interaction between granite-derived fluids with ore-hosting rock. Such interpretation is supported by the geochemical signatures of the dated iron-oxides showing enrichment in granitophile elements and coincident ages for the Wertigo Granite and Myola Volcanics, which occur proximal to the prospect and were emplaced/erupted during intracontinental rifting. The approach undertaken here, combining petrography, iron-oxide geochemistry and in-situ dating of BIF-derived ores is generically applicable to other iron resources within the Middleback Ranges or analogous terranes elsewhere.

Application of grounded electrical source airborne TEM system in Changyi BIF deposit, Jiaodong Peninsula of eastern China

Semi-airborne transient electromagnetic (S-ATEM) method is a hybrid method which employs a grounded electrical or a loop source and an airborne receiver. It has greater investigation depth compared to the airborne electromagnetic method due to its higher transmitting power and more flexibility and efficiency compared to ground electromagnetic method due to its airborne receiving equipment. A new S-ATEM system based on unmanned helicopter is introduced. Besides some information on the technical specifications of the S-ATEM system and data processing technique, a case study of the investigation of Changyi banded iron formation deposit is illustrated. Within the survey area, the ground magnetic method has found a clear magnetic anomaly, whose location has a good correspondence with a ore-body. In order to investigate the underground electrical characteristics and reveal the relationship of ore-body location, subsurface magnetic field anomaly and underground resistivity distribution, a S-ATEM investigation was conducted in January 2016. During the survey, underground resistivity distribution from ground level to a depth of 400 was reconstructed and it was inferred that a strong gran- ite intrusion in the middle of the survey area influenced the ore-body burial depth and magnetic distribution. Although the effect of in- trusion on magnetic field is not as significant as that of the ore-body, the intrusion uplifted and deformed the ore-bearing strata, reducing the burial depth and changing the tilt angle, which indirectly affect the magnetic field distribution of the survey area.

Petrography, lithostratigraphy and major element geochemistry of Mesoarchean metamorphosed banded iron formation-hosted Nkout iron ore deposit, north western Congo craton, Central West Africa

The Nkout banded iron formation (BIF) or iron deposit is located in the northwestern margin of the Congo Craton (CC) and is hosted within high-grade metamorphic Ntem complex. It is mainly made up of metamorphosed banded iron formations with resource estimation of 1.8 Bt @ 32.6% Fe indicated, and 0.9 Bt @ 30.3% Fe inferred. Nkout iron deposit is subdivided into three prospects namely East, Centre and West deposits. Results from lithostratigraphic studies allow detailed correlations between units in the Nkout succession. The litho-package from the top to the base stratigraphy comprises iron-rich member and the gneissic member. The first member forms the upper and middle units while the second member forms the lower unit and represents the footwall of the deposit. The upper unit is ∼200 m deep and consists of oxidized (hematite) and un-oxidized (fresh metamorphosed BIF) ore types. The middle unit is approximately 150 m thick and consists of fresh metamorphosed BIF with minor intercalation of biotite gneiss. The lower unit is made up of garnet-magnetite micaschist, biotite gneiss and pyroxene-biotite gneiss.The Nkout banded iron formation belongs to oxide facies magnetite-rich type which has suffered greenschist to amphibolite facies metamorphism, leading to recrystallization of chlorite, amphibole and garnet minerals. Its overall major oxide bulk chemistry is very similar to that of most Archaean iron-formations worldwide, with high SiO2 and Fe2O3 contents. However, the garnet-rich BIF has higher contents of Al2O3 (8.73 wt%), TiO2 (0.27 wt%), K2O (2.37 wt%) relative to magnetite-rich BIF, suggesting clastic contamination, thus a depositional environment closer to the continent. Furthermore, the absence of carbonate facies BIF in the entire Nkout deposit suggests deep water distal depositional setting, in an area of little or no organic carbon supplies.The lithostratigraphic correlation of metamorphosed BIF unit reveals both thicknesses and depth variations from West to the East, inferring a folded structure for the Nkout deposit. However the occurrence of BIF units in the entire deposit supports the fact that BIF deposition took place on a basin-wide scale. Considering the extent of the Ntem basin, it is suggested that a huge amount of Fe2+ in sea water was required. Thus, the deposition of Nkout BIF probably involved an iron rich environment of reduced Fe2+ rich hydrothermal fluid in chemically stratified sea water, similar to the depositional model proposed for the Minas BIF Supergroup of Quadrilátero Ferrífero in Brazil.

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.

Meso- and Neoarchean Banded Iron Formations and Genesis of High-Grade Magnetite Ores in the Anshan-Benxi Area, North China Craton

AbstractThe Anshan-Benxi area in the North China craton has numerous occurrences of Algoma-type banded iron formations (BIFs) with subordinate high-grade magnetite ores. These ores provide insight into iron metallogenesis and early evolution of the North China craton. In this paper, we present Sm-Nd-Fe-O isotope, mineralogical, and structural data for four BIF-type iron deposits to place constraints on their depositional ages and formation mechanism.Previous SIMS and LA-ICP-MS zircon U-Pb dating results indicated a Mesoarchean age (ca. 3.10 Ga) for the Dagushan BIF and a Neoarchean age (ca. 2.55 Ga) for other regional BIFs (Dai et al., 2012, 2013, 2014).This is confirmed by Sm-Nd isochron ages of these BIFs, high-grade magnetite ores, and host metavolcanics, which yield two regression lines and match apparent ages of 3149 ± 85 Ma (MSWD = 1.2) for Dagushan, and 2671 ± 120 Ma (MSWD = 3.0) for the other three deposits. Our new chronological data thus suggest Meso- and Neoarchean BIF deposition and potentially significant BIF-type iron deposits at depth.The regional high-grade magnetite ores are all hosted in the BIFs that occur in the same orientation and have transitional boundaries between them. They also show similar Sm-Nd isotope compositions and magnetite rare earth elements + yttrium (REY) profiles, indicating that the Anshan-Benxi BIFs were most likely the source beds. The high-grade magnetite ores contain abundant pyrite and actinolite, with systematically lower δ56Fe values (0.67–0.40‰) when compared to the BIFs (1.88–0.64‰), suggesting a hydrothermal origin. In the field, some high-grade orebodies with schistose textures are adjacent to undeformed granitic plutons. This geologic relationship implies that the high-grade magnetite ores were formed earlier and probably did not result from magmatic hydrothermal fluids. Therefore we suggest that the Anshan-Benxi high-grade magnetite ores were most likely produced by infiltration of metamorphic fluids into primary BIFs, based on the following: (1) magnetite δ18O values within the high-grade magnetite ores (+2.5 to −0.6‰) are significantly lower than those in the BIFs (9.2–2.6‰); (2) magnetite (avg 0.39 ppm) and pyrite (avg 0.098 ppm) in the high-grade magnetite ores have much lower REY abundances than magnetite in the BIFs (avg 14.6 ppm); (3) skeletal quartz in the high-grade magnetite ores shows systematically higher FeOtolal contents (1.36–0.56 wt %) than those in laminated chert bands (0.06–0.00 wt %); and (4) hydrothermal zircons within the Nanfen BIF yield a U-Pb age of 2480 Ma, which is comparable to ca. 2.48 Ga regional metamorphism (Zhu et al., 2015).Furthermore, microstructural textures indicate a maximum regional deformation temperature of up to 500°C, which is lower than the plastic flow temperature (>600°C) of magnetite. Finite strain measurements and electron backscatter diffraction analyses suggest a general flattening deformation and similar crystallographic preferred orientation for all magnetite crystals. These structural features reveal that magnetite in the high-grade magnetite ores never experienced a separate tectonic event. Our microscopic studies also show that microfractures at the interfaces of BIF bands contain fragmented quartz crystals and are filled with abundant metamorphic minerals (e.g., actinolite and chlorite). Considering that the Anshan-Benxi high-grade magnetite ores are commonly adjacent to weak structural planes (e.g., faults), we propose that macro- and microscopic fractures probably provided channels for metamorphic fluids.Recent zircon U-Pb geochronology has indicated widespread BIF formation at ca. 2.55 Ga in the North China craton, corresponding to a pronounced peak in BIF deposition of other Precambrian cratons. It is thus implied that a global geologic event triggered the extensive occurrence of BIFs. We correlate the Neoarchean tectonic evolution of the North China craton with the 2.7 to 2.5 Ga Kenorland supercontinent. Significantly, planar distribution signatures of the North China craton BIFs indicate ca. 2.5 Ga cratonization through the amalgamation of at least seven microblocks that were welded by several Neoarchean greenstone belts. Hf-Nd isotope studies have highlighted the Archean episodic crustal evolution of the North China craton, and the Meso- and Neoarchean BIF deposition could have benefitted from these geologic processes. The Anshan-Benxi high-grade magnetite ores that formed at ca. 2.48 Ga were closely related to important metamorphic events during the North China cratonization process.

Microbial diversity and iron oxidation at Okuoku-hachikurou Onsen, a Japanese hot spring analog of Precambrian iron formations.

Banded iron formations (BIFs) are rock deposits common in the Archean and Paleoproterozoic (and regionally Neoproterozoic) sedimentary successions. Multiple hypotheses for their deposition exist, principally invoking the precipitation of iron via the metabolic activities of oxygenic, photoferrotrophic, and/or aerobic iron-oxidizing bacteria. Some isolated environments support chemistry and mineralogy analogous to processes involved in BIF deposition, and their study can aid in untangling the factors that lead to iron precipitation. One such process analog system occurs at Okuoku-hachikurou (OHK) Onsen in Akita Prefecture, Japan. OHK is an iron- and CO2 -rich, circumneutral hot spring that produces a range of precipitated mineral textures containing fine laminae of aragonite and iron oxides that resemble BIF fabrics. Here, we have performed 16S rRNA gene amplicon sequencing of microbial communities across the range of microenvironments in OHK to describe the microbial diversity present and to gain insight into the cycling of iron, oxygen, and carbon in this ecosystem. These analyses suggest that productivity at OHK is based on aerobic iron-oxidizing Gallionellaceae. In contrast to other BIF analog sites, Cyanobacteria, anoxygenic phototrophs, and iron-reducing micro-organisms are present at only low abundances. These observations support a hypothesis where low growth yields and the high stoichiometry of iron oxidized per carbon fixed by aerobic iron-oxidizing chemoautotrophs like Gallionellaceae result in accumulation of iron oxide phases without stoichiometric buildup of organic matter. This system supports little dissimilatory iron reduction, further setting OHK apart from other process analog sites where iron oxidation is primarily driven by phototrophic organisms. This positions OHK as a study area where the controls on primary productivity in iron-rich environments can be further elucidated. When compared with geological data, the metabolisms and mineralogy at OHK are most similar to specific BIF occurrences deposited after the Great Oxygenation Event, and generally discordant with those that accumulated before it.

Mesoarchean BIF and iron ores of the Badampahar greenstone belt, Iron Ore Group, East Indian Shield

Banded iron formations (BIFs) are chemically precipitated sedimentary rock characterized by alternating Fe-rich and Si-rich bands. The origin of BIF has remained controversial despite years of diligent research. Most models proposed for the BIF origin are based on the observations of well-preserved Neoarchean to Paleoproterozoic BIFs. The present paper is focused on the origin of Mesoarchean BIFs present in the Badampahar greenstone belt (3.3–3.1 Ga), East Indian Shield. Here, BIF is interlayered with metavolcanic rocks, quartzite, phyllite and chert representing a typical greenstone sequence. Geochemical and sedimentological evidence suggest deposition of BIF below the wave base as part of a back-arc basin with insignificant detrital input. Interaction of seawater and volcanogenic high temperature hydrothermal fluids, generated from back-arc spreading centre, supplied metals for BIF deposition. Distinctly negative Ce anomalies in some lower BIF horizons indicate Fe2+ oxidation in an oxygenated hydrosphere and derivation of free oxygen from microbial photosynthesis. Subsequent stages of deformation, metamorphism, hydrothermal and supergene processes after deposition led to the formation of the iron ore bodies at present.

Host rock geochemistry, texture and chemical composition of magnetite in iron ore in the Neoarchaean Nyong unit in southern Cameroon

ABSTRACTA combination of petrography, whole-rock geochemistry, geochronology and compositional variation in magnetite is employed in this study to elucidate the nature and origin of enigmatic magnetite mineralisation hosted within gneissic rocks in the Nyong Unit in southern Cameroon. The mineralisation occurs as magnetite-bearing calc–silicate gneisses. The host rock mineral assemblage comprises quartz–plagioclase–biotite–amphibole–chlorite–clinopyroxene–garnet that provides evidence of medium-grade metamorphism and retrograde alteration. Textural and chemical analyses of the magnetite grains show variable textural and trace element chemical characteristics attributed to metamorphic-hydrothermal overprint and weathering. Magnetite occurs as disseminations and stringers commonly intergrown with amphiboles. It is also observed to show vermiforms wrapped around quartz and clinopyroxenes within a biotite–chlorite- plagioclase groundmass. Massive-granoblastic magnetite is rare and mainly observed within vein-like domains. On backscattered secondary electron images the magnetite grains are anhedral, with minor spinel exsolution lamellae. Electron microprobe analysis on magnetite suggests both a hydrothermal skarn and banded iron formation (BIF) affinity. The lack of negative Ce anomalies excludes a Proterozoic BIF setting, but it is in agreement with Archaean BIF. Sensitive high resolution ion microprobe U-Pb isotope data on zircon in the magnetite gneiss define an identical Wetherill concordia and Tera–Wasserburg Neoarchaean age of 2699 ± 7 Ma (1σ; MSWD (mean square weighted deviation) = 1.3; n = 13), and Pan-African disturbance at about 500 ± 200 Ma. The Neoarchaean age is in accordance with the known onset of BIF deposition at the northern edge of the Congo Craton and therefore constrains the maximum age of formation of the Nyong magnetite gneisses.

Neoarchaean banded iron formation within a ∼2620 Ma turbidite-dominated deep-water basin, Slave craton, NW Canada

Banded iron formations (BIFs) are iron- and silica-rich chemical metasedimentary rocks that span much of the Archaean and early Palaeoproterozoic rock record. Given that BIFs precipitated from ambient seawater, they provide an important archive of Earth’s ancient marine environments. Within the Slave craton occurs one of the largest and well-preserved Achaean turbidite basins in the world. Interstratified within the greywacke-mudstone turbidites is BIF that is commonly associated with ferruginous sediments and locally with a felsic tuff bed. Field investigations were conducted at four BIF locations from west to east across the Slave craton. These BIFs are composed of micro- and mesobands of quartz and magnetite, with various amounts of metamorphic grunerite, ferro-actinolite, ferro-hornblende as well as greenalite and stilpnomelane, the latter two are interpreted as retrograde metamorphic phases. As the BIF was deposited in a deep-water turbidite-dominated basin(s), the chemical component was occasionally diluted by clastic detritus, as illustrated by highly variable Al2O3 content, and Al2O3/TiO2 and Cr/Ni that are similar to the clastic sediments. On a layer-by-layer basis, the iron content in the BIF ranges from as low as 4wt.% and up to ∼75wt.% Fe2O3, whereas the SiO2 content ranges from 20wt.% to as high as ∼94wt.%. This, together with a hyperbolic germanium/silicon distribution, suggests that continentally-sourced silica comprised the background precipitation, which was periodically interrupted by pulses of hydrothermally-sourced iron. High-temperature hydrothermal fluids contributed, with an average shale-normalized positive Eu anomaly of 1.92, which indicates input of reduced Eu2+ to the ambient seawater. The BIF shows characteristic aqueous fluid-derived rare earths+yttrium fractionation (Pr/Yb<1) and positive La and Y anomalies. A negative Ce anomaly in parts of the BIF indicates the presence of an oxidizing agent able to fractionate Ce3+ to Ce4+. The age and trace element signature of the felsic tuff is similar to arc-type volcanic rocks, potentially linking the BIF and turbidite basin(s) to the Defeat Suite plutons (ca. 2630–2620Ma). The slow chemical rainout behind BIF deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump-generated debris flows, sourced and triggered by the tectonically active arc-terrain, episodically delivered clastic sediment.

Greenalite precipitation linked to the deposition of banded iron formations downslope from a late Archean carbonate platform

Banded iron formations (BIFs) were deposited as deep-water facies distal to the late Archean Campbellrand carbonate platform, Transvaal Supergroup, South Africa. They are traditionally interpreted to have formed from iron oxides/hydroxides and silica that precipitated when upwelling hydrothermal water enriched in Fe2+ and silica mixed with cooler, shallow seawater enriched in dissolved oxygen. The ferric oxides/hydroxides were then converted to hematite or, in the presence of organic matter, reduced and incorporated into siderite. New high-resolution microscopy of BIFs from distal facies of the Campbellrand platform questions that interpretation, revealing the presence of abundant greenalite nanoparticles, interpreted to represent primary precipitates from ferruginous seawater, consistent with recent results from equivalent-aged BIFs of the Hamersley Group, Western Australia. The particles locally define primary sedimentary lamination, and are engulfed in diagenetic chert that preserves shrinkage structures, considered to have formed during dehydration and recrystallization of amorphous silica cement. The replacement of greenalite nanoparticles by coarser grained oxide, silicate and carbonate minerals, with destruction of primary textures and structures, indicates that iron-silicate muds were originally much more widespread. Based on our findings we propose a new model for the deposition of the BIFs, involving iron silicate precipitation from seawater followed by diagenetic silica cementation on the seafloor. We suggest that ferrous iron and silica were transported in hydrothermal plumes sourced from acidic vent fluids. Upon mixing with cooler, more alkaline seawater, the solubility of ferrous iron and silica fell to levels that favored rapid nucleation and the precipitation of iron-silicate nanoparticles over vast areas of the seafloor. The close association between iron and silica during the deposition of BIFs may be explained by the precipitation of iron-silicate nanoparticles and possible sorption of silica onto the surfaces of sinking nanoparticles. Furthermore, on the seafloor, iron-silicate nanoparticles presumably acted as nucleation sites for dissolved silica, promoting early diagenetic silica precipitation and cementation of the iron-rich muds. Our results indicate that iron silicates were important primary precipitates and suggest that changes in alkalinity rather than redox state played a key role in the precipitation of the precursor sediments of BIFs.

The role of microaerophilic Fe-oxidizing micro-organisms in producing banded iron formations.

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe-oxidizing micro-organisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder whether similar organisms played a role in producing BIFs. To evaluate this, we review the current knowledge of modern microaerophilic FeOM in the context of BIF paleoenvironmental studies. In modern environments wherever Fe(II) and O2 co-exist, microaerophilic FeOM proliferate. These organisms grow in a variety of environments, including the marine water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O2 concentrations, measured as low as 2μm to date, although lower concentrations have not been tested. While some extant FeOM can tolerate high O2 concentrations, many FeOM appear to prefer and thrive at low O2 concentrations (~3-25μm). These are similar to the estimated dissolved O2 concentrations in the few hundred million years prior to the 'Great Oxidation Event' (GOE). We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O2 and show that microaerophilic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. Based on this synthesis, we propose that microaerophilic FeOM were capable of playing a significant role in depositing the largest, most well-known BIFs associated with the GOE, as well as afterward when global O2 levels increased.

Geochemical and Spectral Footprint of Metamorphosed and Deformed VMS-style Mineralization in the Quinns District, Yilgarn Craton, Western Australia*

Volcanic-hosted massive sulfide (VMS) occurrences in the Quinns district are hosted predominantly by ca. 2814–2800 Ma banded iron formation (BIF) within a sequence of rhyolite, basalt, and minor siltstone. The VMS occurrences and their surrounding rocks are folded, metamorphosed, and deeply weathered and variably covered by transported regolith. This study uses an integrated petrological and geochemical approach to map gradients in synvolcanic mineral and element abundances, with the aim of understanding the effects of postvolcanic processes on Archean VMS ore. Rhyolite-dominant footwall rocks and the BIF record kilometer-scale gradients in alteration mineral patterns and geochemistry. Rhyolite exposed throughout the district and intersected by drill holes records distal alteration assemblages of quartz-white mica ± chlorite. At the Austin deposit in the western part of the district, abundant talc and anthophyllite with minor cummingtonite and hornblende are associated with mineralization in the BIF, indicating intense magnesium metasomatism. Further away from mineralization, chlorite is the dominant alteration mineral. Adjacent rhyolite is altered to chlorite in proximal zones and white micas in more distal areas. Approaching known VMS prospects in the eastern half of the district, the rhyolite grades into a 2 × 1 km zone of schistose rhyolite with generally dispersed, but locally abundant, coarse-grained andalusite ± kyanite ± garnet rhyolitic schist. This broad Al-rich silicate alteration zone envelops two discrete 1 km × 500 m proximal alteration zones in rhyolite, defined by chlorite ± talc, with minor disseminated magnetite, pyrite, and chalcopyrite. The northernmost proximal alteration zone lies stratigraphically below the BIF-hosted Cu-Zn-rich gossan exposed near the Tasman prospect. The proximal zones are interpreted to be the result of the interaction between synvolcanic, Mg-rich fluids and rhyolite footwall at the time of VMS mineralization and BIF deposition. In contrast, outer zones of andalusite ± kyanite ± garnet suggest the removal of silica and alkali elements and residual concentration of aluminium in rhyolite footwall by acidic synvolcanic hydrothermal fluids prior to peak regional metamorphism. The mapped patterns in alteration minerals mirror bulk rock geochemical gradients in rhyolite; proximal alteration zones are enriched in Cu, Zn, Ag, Au, Bi, Fe2O3, In, MgO, Mo, S, Se, and Te, with local enrichments in As, Cd, MnO, Pb, and V. These rocks are depleted in Ba, K2O, Li, Na2O, and Rb relative to least-altered rhyolite. Rhyolite located up to 100 m from proximal alteration zones is enriched in Ag, Fe2O3, In, K2O, MgO, S, and V. Banded iron formation hosting massive sulfides is altered to talc, anthophyllite, chlorite, and magnetite; these rocks are enriched in Cu, Zn, Pb, Ag, and Sn, with local enrichments in Bi, In, and MgO. Positive Eu anomalies are associated with mineralized BIF in the Austin VMS deposit and contrast with the mostly flat REE slopes for the BIF sampled from less mineralized areas in the district. The kilometer-scale gradients in hydrothermal alteration minerals and bulk rock geochemistry in rhyolite and the BIF are interpreted to be mostly controlled by synvolcanic hydrothermal alteration related to the development of VMS-related hydrothermal fluid pathways. Metamorphism of the synvolcanic alteration was isochemical, apart from localized metasomatism related to later shear zones or fault zones. The defined alteration mineral and geochemical gradients serve as useful tools for detecting fluid alteration pathways related to VMS systems in complexly deformed and metamorphosed districts globally.

Depositional setting of Algoma-type banded iron formation

Algoma-type banded iron formations (BIF) are chemical sedimentary rocks characterized by alternating layers of iron-rich minerals and chert that are generally interstratified with bimodal submarine volcanic rocks and/or sedimentary sequences in Archean greenstone belts. However, the geological setting for Algoma-type BIF deposition remains equivocal due to the effects of post-depositional deformation and metamorphism, and absence of modern analogues for comparative studies. It is commonly accepted that the abundance of rare earth element and yttrium (REE+Y) in chert bands may retain a primary geochemical signature and therefore constrain their geological setting. In order to explore the latter, a geochemical study using the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) methodology was done using cherts from four Canadian BIF-hosted gold deposits. These results suggest that chert bands record: (1) interaction of seawater with Fe-oxyhydroxides, as suggested by their heavy REE enrichment coupled with La and Y enrichments; (2) contributions from high-temperature (>250°C) hydrothermal fluids, as suggested by positive Eu excursions; and (3) detrital contamination, which is suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio (i.e., Y/Ho≈27). Water-column pH conditions at the time of BIF deposition are evaluated using Ce/Ce∗: a positive Ce/Ce∗ anomaly suggests relatively acidic conditions (i.e., pH⩽5) for most of the chert samples, but more alkaline conditions (i.e., pH⩾5) for samples showing Fe-oxyhydroxide precipitation within chert bands. Finally, in situ using secondary ion mass spectrometry (SIMS) analysis (n=73) of chert from Meliadine show the δ18O of primary amorphous silica (+27‰) was modified to values of around +8‰ to +20‰ during diagenesis at temperatures >100°C with a fluid having δ18OH2O=0–5‰. Thus, whereas there has been O isotopic exchange during diagenesis, the REEs and trace elements are not modified in the chert due to the low concentrations of these elements in the reacting fluid of sea water origin.

Decoupled sources of the 2.3–2.2 Ga Yuanjiacun banded iron formation: Implications for the Nd cycle in Earth’s early oceans

The recognized worldwide gap in BIF deposition between 2.4 and 2.0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time. However, the recently dated 2.3–2.2Ga Yuanjiacun banded iron formation (BIF) in the North China Craton (NCC) offers a possibility to redress these uncertainties. Shale-normalized rare earth element-yttrium (REE+Y) patterns of the BIF and interlayered meta-chert samples show features characteristic of other Archean and Paleoproterozoic BIFs, with HREE enrichment relative to LREE, positive La and Eu anomalies, and superchondritic Y/Ho ratios comparable to modern seawater. Very low Al2O3 (<0.5wt%) and high field strength elements (HFSE) concentrations (<10ppm) indicate an essentially detritus-free depositional setting, while positive Eu anomalies are attributed to an imprint of high-temperature hydrothermal fluids. Sm–Nd isotopic features further point to two periodically interacting water masses controlling the deposition of the BIF. The first is seafloor-vented hydrothermal fluids (εNd(t)∼+3.5) derived from interaction with a depleted mantle source and associated with high Fe fluxes. The second is ambient surface seawater (εNd(t)∼−2.4), which obtained its signature through weathering of the nearby landmasses and associated with high Si fluxes. Our findings suggest that the REE budget of the oceans prior to 2.3Ga was generally dominated by hydrothermal circulation of seawater through depleted mantle-derived source rocks. However, where evolved local continental crust and/or an enriched mantle source were present, this positive mantle Nd signal became discernible in the BIFs. By comparing the Nd isotopic features of similar aged BIFs and marine carbonates, it is concluded that similar to modern oceans, the early Precambrian ocean was not well-mixed with respect to its Nd isotopic composition.

Cu isotopes in marine black shales record the Great Oxidation Event

The oxygenation of the atmosphere ∼2.45-2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth's redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ(65)CuERM-AE633) in organic carbon-rich shales spanning the period 2.66-2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in (65)Cu, along with the preferential removal of (65)Cu by iron oxides, left seawater and marine biomass depleted in (65)Cu but enriched in (63)Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ(65)Cu values coincides with a shift to negative sedimentary δ(56)Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ(65)Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

Iron isotope and REE+Y composition of the Cauê banded iron formation and related iron ores of the Quadrilátero Ferrífero, Brazil

The Minas Supergroup banded iron formations (BIFs) of the Brazilian Quadrilatero Ferrifero (QF) mineral province experienced multiple deformational events synchronous with hypogene mineralization, which resulted in the metamorphism of BIFs to itabirites and their upgrade to high-grade iron ore. Here, we present rare earth element and yttrium (REE+Y) compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during BIF deposition and the effects of hypogene iron enrichment. The REE+Y characteristics of itabirites (positive Eu anomaly and LREE depletion) indicate hydrothermal iron contribution to the Minas basin. Iron isotope data and Ce anomalies suggest BIFs were precipitated by a combination of anoxic biological-mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen (such as an oxygen oasis), perhaps related to increase in oxygen concentrations before the Great Oxidation Event (GOE). The similarity of the REE+Y composition of the itabirites from the different QF deformational domains, as well as to other Superior-type BIFs, indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original BIFs. However, iron isotope compositions of iron ore vary systematically between deformational domains of the QF, likely reflecting the specific mineralization features in each domain.

Influence of organics and silica on Fe(II) oxidation rates and cell–mineral aggregate formation by the green-sulfur Fe(II)-oxidizing bacterium Chlorobium ferrooxidans KoFox – Implications for Fe(II) oxidation in ancient oceans

Most studies on microbial phototrophic Fe(II) oxidation (photoferrotrophy) have focused on purple bacteria, but recent evidence points to the importance of green-sulfur bacteria (GSB). Their recovery from modern ferruginous environments suggests that these photoferrotrophs can offer insights into how their ancient counterparts grew in Archean oceans at the time of banded iron formation (BIF) deposition. It is unknown, however, how Fe(II) oxidation rates, cell–mineral aggregate formation, and Fe-mineralogy vary under environmental conditions reminiscent of the geological past. To address this, we studied the Fe(II)-oxidizer Chlorobium ferrooxidans KoFox, a GSB living in co-culture with the heterotrophic Geospirillum strain KoFum. We investigated the mineralogy of Fe(III) metabolic products at low/high light intensity, and in the presence of dissolved silica and/or fumarate. Silica and fumarate influenced the crystallinity and particle size of the produced Fe(III) minerals. The presence of silica also enhanced Fe(II) oxidation rates, especially at high light intensities, potentially by lowering Fe(II)-toxicity to the cells. Electron microscopic imaging showed no encrustation of either KoFox or KoFum cells with Fe(III)-minerals, though weak associations were observed suggesting co-sedimentation of Fe(III) with at least some biomass via these aggregates, which could support diagenetic Fe(III)-reduction. Given that GSB are presumably one of the most ancient photosynthetic organisms, and pre-date cyanobacteria, our findings, on the one hand, strengthen arguments for photoferrotrophic activity as a likely mechanism for BIF deposition on a predominantly anoxic early Earth, but, on the other hand, also suggest that preservation of remnants of Fe(II)-oxidizing GSB as microfossils in the rock record is unlikely.