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Fifteen species of the Gondwanan genus Atopobathynella have been described so far from four countries. The position of the genus within the family Parabathynellidae and its species relationships are controversial due to the different characters and species considered by different authors, however most of the phylogenetic reconstructions are based solely on morphological characters. In the past few decades, the arid zones of Western Australia, including the Pilbara region, have been recognized as a hotspot for subterranean fauna. Material is constantly collected to produce Environmental Impact Assessments to protect and manage the subterranean environment. In 2009 the Cleaverville Iron Formation, in the Pilbara region, northern Western Australia, was proposed to be mined, therefore subterranean fauna sampling was conducted as per legislation. Preliminary molecular studies of stygofauna identified two distinct Parabathynellidae lineages from two ridges, however no morphological descriptions were carried out at that time. In this study, we describe two new species from Yarrie and Callawa ridges, respectively: Atopobathynella yarriensis sp. nov. and A. degreyensis sp. nov.. The new species show differences in male antennal organ, ventral tooth of mandible, number of teeth on distal endite of maxillula, absence or presence of epipod of thoracopod I, number of setae on article 4 of thoracopod II, length of distal external seta of exopod of all thoracopods, outer lobe of male thoracopod VIII, number of spines on uropodal sympod and furcal rami, length of the external dorsal plumose seta of the furcal rami. Additionally, we integrated the morphological data with sequences of three genetic loci, Cytochrome Oxidase c Subunit I (COI), 12S rRNA (12S), and nuclear 18S rRNA (18S), calculating p-distances and constructing a multigene molecular phylogeny to support morphology, and explore the monophyletic status of the genus and the relationship amongst its species. The two new species were well supported in our phylogeny, however they appear distantly related. [ABSTRACT FROM AUTHOR]

The demand for iron ore has increased recently and employing geochemical and hyperspectral remote sensing techniques for discovering new ore and mineral resources have primarily been concentrated on the economic phases. The present study aimed to characterize the hyperspectral spectral signatures of iron ores of field samples to map the deposits that occurred in the Tirthamalai hill region, which lies in the parts of Harur Taluk, Dharmapuri district of Tamil Nadu state, India The measurement and study of spectral signatures of the different samples of the deposits showed strong spectral absorptions near 500 nm, 900 nm and 2400 nm wavelength regions and were confirmed with the Fourier Transform Infrared (FTIR) spectroscopy method. The spectral absorption characteristics of the samples were evaluated by the study of the physical, optical, and chemical characteristics of the samples. The study of hyperspectral and FTIR spectral signatures with petrographic and major chemical elements revealed the best absorption characteristics of the iron ore deposits of the study region and can be used elsewhere in the world. This report presents preliminary findings on the use of hyperspectral imaging to characterize iron ore. The mineralogical products produced from hyperspectral images may improve in situ grade control on an iron ore mine face. It will be extremely useful for businesses in measuring large numbers of commodities quickly and objectively. [ABSTRACT FROM AUTHOR]

Most Neoproterozoic iron formations (NIF) are closely associated with global or near‐global "Snowball Earth" glaciations. Increasingly, however, studies indicate that some NIFs show no robust evidence of glacial association. Many aspects of non‐glacial NIF genesis, including the paleo‐environmental setting, Fe(II) source, and oxidation mechanisms, are poorly understood. Here, we present a detailed case study of the Jiapigou NIF, a major non‐glacial NIF within a Neoproterozoic volcano‐sedimentary sequence in North Qilian, northwestern China. New U–Pb geochronological data place the depositional age of the Jiapigou NIF at ~600 Ma. Petrographic and geochemical evidence supports its identification as a primary chemical sediment with significant detrital input. Major and trace element concentrations, REE + Y systematics, and εNd(t) values indicate that iron was sourced from mixed seawater and hydrothermal fluids. Iron isotopic values (δ56Fe = −0.04‰–1.43‰) are indicative of partial oxidation of an Fe(II) reservoir. We infer that the Jiapigou NIF was deposited in a redox stratified water column, where hydrothermally sourced Fe(II)‐rich fluids underwent oxidation under suboxic conditions. Lastly, the Jiapigou NIF has strong phosphorous enrichments, which in other iron formations are typically interpreted as signals for high marine phosphate concentrations. This suggests that oceanic phosphorus concentrations could have been enriched throughout the Neoproterozoic, as opposed to simply during glacial intervals. [ABSTRACT FROM AUTHOR]

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[Display omitted] • Carbon isotope profiles across an Iron Formation sequence show strong inter-site similarities. • Carbon isotope values are independent of carbonate modal abundance or iron content. • Abiotic, low-δ13C carbonate formation in the ambient water column is favoured. The origin of Precambrian iron-formations (IF) remains contentious, particularly with respect to the mineralogy of primary precipitates and the exact processes and conditions leading to their formation. Despite the uncertainties, prevailing hypotheses range from biological precipitation of ferrihydrite to abiotic water-column formation of greenalite. By contrast, iron carbonate minerals (siderite, ankerite) in IF have traditionally been attributed to diagenetic origins based on textural and isotopic relationships. Recent studies on IF from the Neoarchaean-Paleoproterozoic Transvaal Supergroup of South Africa have revealed evidence for apparently primary, low-δ13C, Fe/Mn-bearing Mg calcite as precursor to iron carbonate formation and as a potentially underestimated pathway of isotopically light carbon burial during IF deposition. Here, we present whole-rock δ13C data and carbonate-specific geochemical analyses for samples from five drill cores that capture the entire stratigraphic extent of the Kuruman and Griquatown IF of the Transvaal Supergroup. Our results demonstrate remarkable consistency in stratigraphic profiles among the locations for the trends and magnitudes of bulk δ13C values that are independent of paragenetic association, modal mineralogy, and chemical composition of the bulk carbonate fraction of each sample. We interpret these records as resulting from water-column abiotic carbonate formation that was accompanied by kinetic isotopic effects associated with fluctuating conditions (pH, alkalinity) controlling carbonate supersaturation in ambient seawater. Although our interpretation provides strong support for abiotic, anoxic models for IF genesis prior to the Great Oxidation Event (GOE), it does not entirely preclude additional biological mechanisms of primary ferric oxyhydroxide formation and its possible role in an early biological pump. [ABSTRACT FROM AUTHOR]

• Gunflint Formation petrography, geochemistry, δ13C, δ18O, and clumped isotopes. • Siderite formed from dissimilatory iron reduction at temperatures as low as 40 °C. • Ferrodolomite-ankerite formed from seawater during burial at elevated temperatures. • Seawater T, δ18O constrained to values of 5–35 °C and −1.0 to −6.5‰, respectively. • Outcrop and core differ because of siderite removal during exposure and weathering. Petrographic, geochemical, C-, O-, and clumped isotope measurements of drill core samples from the Paleoproterozoic (1.88 Ga) Gunflint Iron Formation containing siderite, ankerite, Fe-silicate clays (dominantly greenalite) and silica, reveal variable but light carbonate δ13C values (-21.3 to -7.87‰, VPDB), with heavy and less variable carbonate δ18O values (20.6 to 24.9‰, VSMOW). These measurements are consistent with a model in which siderite is formed during early diagenesis as a by-product of the metabolism of dissimilatory iron reducing (DIR) microbes that consume Fe(III)-oxide and organic carbon delivered from the water column to the sediment-water interface. This model indicates that the oxygen isotope compositions of reactant Fe-oxide, organic matter and dissolved inorganic carbon were set in equilibrium with seawater at T = 5–35 °C and δ18O = -1.0 to -6.5‰, prior to being metabolized to form siderite. Clumped isotope measurements on Fe-bearing carbonates yield temperatures (T(Δ 47)) between 40 °C-132 °C, which constrain δ18O fluid to values between -6.22 to 7.32‰. These measurements record the onset temperature of DIR followed by low water-to-rock ratio diagenetic recrystallization at elevated temperature. Combined petrographic, chemical, and isotopic measurements reveal that the major phases delivered to the shallow seafloor were a disequilibrium assemblage of Fe-oxide, greenalite, silica, and organic matter that underwent microbially mediated modification to form an assemblage of siderite, greenalite, silica, and later diagenetic ankerite. We contend that observed differences between carbonate derived from Gunflint core and outcrop may be reconciled by removal of siderite during exposure and weathering, leaving outcrop enriched in late diagenetic ankerite. [ABSTRACT FROM AUTHOR]

• Greenalite was a primary phase in iron formation from the transvaal supergroup. • New methodology to extract REY from greenalite in a chert matrix. • Greenalite formed on the shelf under fully anoxic conditions. • Local pH must have been higher than predicted by some models. • Greenalite could provide a faithful archive of early marine geochemistry. Iron formations (IF) were widely deposited in the Archean Eon and Palaeoproterozoic Era and hold potential as an archive of marine biogeochemistry. However, reconstructions are challenging due to their fine-grained nature and complex mineralogy. Recent work has identified greenalite, an Fe(II)-silicate mineral, as an abundant, primary phase in IF. Several depositional mechanisms have been proposed for greenalite precipitation, spanning hydrothermal vent systems and shelf environments. We report new in situ rare earth element and yttrium (REY) data from greenalite in the Palaeoproterozoic Kuruman Iron Formation from the Transvaal Supergroup, South Africa. We use a new approach, cutting samples parallel to banding to expose concentrated greenalite and developed a methodology to quantify the proportion of greenalite in each ablation crater. Greenalite consistently shows heavy REE enrichment (Yb SN /Nd SN 4–21), a small positive Eu anomaly (∼1.41), super-chondritic Y/Ho ratios (32–43), and low ∑REE (1.4–13.1), consistent with formation in a shelf environment. While greenalite may have been an abundant precipitate in vent settings, at least some of the greenalite preserved in the rock record formed directly on the shelf. This necessitates another trigger for its formation, which may have included small amounts of Fe3+, local increases in pH, or warmer oceans. Our findings suggest that greenalite could provide a valuable archive of local seawater chemistry. [ABSTRACT FROM AUTHOR]

• Whole-rock Fe isotope ratios significantly vary in 3.2 Ga ferruginous rocks. • δ56Fe values correlate with Fe/Al and detrital matrix ratios in Fe-rich siltstones. • Fe isotope trend was consistent between magnetite- and carbonate-rich siltstones. • Small Fe isotope fractionation occurred during Fe2+-silicate precipitation. • Fe isotope signals preserve oxidative iron precipitation in a 3.2 Ga shallow ocean. Iron (Fe) isotopic compositions of Iron formations (IFs) have the potential to constrain the oceanic redox environment and marine biosphere on the early Earth. However, the interpretation of Fe isotope ratios in IFs is controversial and related to various factors, such as Fe sources, mode of primary precipitation, and subsequent mineral transformations. This paper presents whole-rock Fe isotope data for ca. 3.2 Ga unweathered ferruginous siliciclastic sedimentary rocks deposited in a shallow ocean in the lower part (unit MdI1) of the Moodies Group, Barberton Greenstone Belt, South Africa. We also experimentally examined Fe isotope effects during the precipitation of Fe2+-silicates (e.g., greenalite), proposed as primary Fe minerals in IFs. The Fe isotope data show significant variation (δ56Fe = −0.58 ‰ to +0.60 ‰) for different lithologies (i.e., magnetite-rich siltstone, carbonate-rich siltstone, sandy siltstone, and jaspilite). The δ56Fe values (δ56Fe = −0.54 ‰ to +0.60 ‰) of the magnetite-rich siltstones tend to decrease with decreasing Fe 2 O 3(T) /Al 2 O 3 ratios and matrix ratios (the percentage of detrital grains with a size of <30 μm). Carbonate-rich siltstones also fall on the same Fe 2 O 3(T) /Al 2 O 3 – δ56Fe and matrix ratio – δ56Fe trends as magnetite-rich siltstone. The synthetic experiment showed that isotope fractionation during anoxygenic Fe2+-silicate precipitation from dissolved ferrous Fe (Fe2+ (aq)) was much smaller (Δ56Fe Fe2+-silicate–Fe2+(aq) < +0.3 ‰) than that of oxidative precipitation. These results indicate that Fe isotopic variations in Fe-rich siltstones (magnetite- and carbonate-rich siltstones) are only explained by the oxidative precipitation of Fe2+ (aq) supplied from the deep ocean following Rayleigh-type fractionation. Low carbonate-C isotope ratios (δ13C carb = −5.8 ‰ to −3.7 ‰) of the Fe-rich siltstones show that magnetite and ankerite or Mg-siderite formed from a primary Fe3+-bearing mineral by oxidation of organic C after Fe burial. The consistent Fe 2 O 3(T) /Al 2 O 3 – δ56Fe trends between the magnetite- and carbonate-rich siltstones suggest that Fe reduction during diagenetic and/or metamorphic transformation processes of Fe-bearing minerals caused negligible changes in the whole-rock Fe isotope composition, possibly because of limited mobility of Fe2+ in the sediment. Consequently, the Fe isotope compositions predominantly record the primary precipitation process that occurred in the water column of a 3.2 Ga shallow ocean environment. [ABSTRACT FROM AUTHOR]

Post-GOE deep-water Fe-Si precipitates associated with volcanogenic massive sulfide deposits are an important feature of the Proterozoic rock record. Although it is clear that these enigmatic deposits formed in oxygen-deficient and hydrothermally influenced deep-water settings, the oxidation mechanism(s) resulting in their precipitation remain(s) unclear. Whilst existing genetic models typically couple direct and/or bacterially-mediated iron oxidation with abiotic silica precipitation, low temperature diffuse hydrothermal fluids offer a potential mechanistic alternative to explain the observed layering. Herein, via combination of petrographic observations with elemental and isotopic data (Fe, Si, and O), we explore the genesis of the primary mineral phases present within recent Fe-Si precipitates obtained from the Southwest Indian Ridge. Formation of ferrihydrite and opal-A in these precipitates provides insight into the genesis of widely invoked precursor minerals to post-GOE deep-water Fe-Si precipitates. Specifically, we find that the mineralogical layers that typify these Fe-Si precipitates may have originated via biologically mediated ferrihydrite precipitation and abiotic precipitation of opal-A from oversaturated fluids during diffuse flow. By analogy, we propose that diffuse hydrothermal fluid flow played an important role in the formation of post-GOE deep-water Fe-Si precipitates. [ABSTRACT FROM AUTHOR]

• The 1.89 Ga Frere granular Fe formation was deposited on a redox stratified shelf. • Trend in whole-rock and magnetite δ 56Fe data tracks changes in sea level. • The δ 56Fe of iron formations are a robust tracer for Paleoproterozoic ocean circulation. • (Ce/Ce*) PAAS holds a primary seawater signature in spite of terrestrial input. • Primary δ 18O values of minerals were overprinted by diagenesis. The Paleoproterozoic (ca.1.89 Ga) Frere Formation is a ca. 300-m-thick sedimentary succession of five stacked, Fe- and silicate-rich shallowing upwards cycles or parasequences that were deposited during marine transgression. Deposition in an oxygen-stratified water column is supported by the occurrence of magnetite-rich mudstones (anoxic and distal) beneath shallower, hematite- and chert-rich grainstones (suboxic and proximal). Minor (LREE/HREE) PAAS depletion and positive Ce anomalies in grainstones support this interpretation. Sequence stratigraphic relationships suggest that the Frere Formation accumulated on an unrimmed continental shelf associated with coastal upwelling of Fe. Large variations in oxygen isotope ratios (δ 18O) of magnetite (–0.6 ‰ ≤ δ 18O ≤ +2.8 ‰, n = 12) and quartz (+8.8 ‰ ≤ δ 18O ≤ +20.1 ‰, n = 11) from lithofacies composing the Frere Formation indicate that pristine δ 18O of minerals were overprinted by diagenesis. Highly variable δ 18O values calculated at 200 °C in equilibrium with quartz (–3.0 to + 8.3 ‰) and magnetite (+8.0 to + 12.7 ‰) suggest that fluids with heterogeneous δ 18O evolved through oxygen isotope exchange between minerals and porewater in accumulating sediment during diagenesis. The Fe isotope ratios (δ 56Fe) of bulk rock (–0.50 ‰ ≤ δ 56Fe ≤ +1.55 ‰, n = 20) and magnetite separates (–0.09 ‰ ≤ δ 56Fe ≤ +2.06 ‰, n = 11) from deeper anoxic and shallow suboxic paleoenvironments define a decreasing trend through stratigraphically stacked parasequences. The high δ 56Fe values of magnetite (+0.7 ‰ to + 2.06 ‰) characterizing the base of the Frere Formation is interpreted to reflect equilibrium isotopic fractionation between dissolved Fe2+ aq and freshly precipitated Fe-(oxyhydr)oxide (Fe(OH) 3) resulting from sluggish recharge of Fe2+ aq from deeper shelf environments. These high δ 56Fe values of magnetite from shallow paleoenvironments may also reflect microbial dissimilatory Fe reduction at the seafloor. As sea level rose, continued deepening and encroachment of a well-mixed water mass increased the oxidation of Fe2+ aq , which ultimately increased the influence of kinetic isotopic fractionation between Fe2+ aq and Fe(OH) 3. Such a process produced δ 56Fe compositions of magnetite that gradually decrease through the stratigraphic thickness of the Frere Formation (+2‰ to −0.1 ‰). Thus, Fe isotopic data framed in a sequence stratigraphic context provides greater insight into water column processes on Fe-rich, oxygen stratified Paleoproterozoic and Mesoproterozoic shelves, as well as geologically younger restricted basins with a redox gradient. [ABSTRACT FROM AUTHOR]

The Cuité iron ore deposit consists of high-grade schistose lenses (>60 wt% Fe) hosted in a tectonic sliver of iron formation associated to mica schist (Serra da Serpentina Group). It is located at the southeastern border of the São Francisco Craton, enclosed by sheared Sthaterian granitoids (Borrachudos Suite), Neoarchean ortogneiss (Guanhães Complex), quartz and pegmatite veins. The iron formation encloses schistose to massive high-grade ore lenses, with a pervasively, continuous to anastomosed schistosity defined by millimetric elongated hematite plates and quartz grains enclosing hematite granoblastic domains. Kenomagnetite/maghemite and martite pseudomorphs (after magnetite) overgrow the foliation and intergrow with granoblastic hematite. Three high-grade iron ore types are identified, according to the fabric and mineralogy of the main iron oxides: lamellar-granular hematitic (LGO), magnetitic (MO), and granular hematitic ore (GO). Intrusive pegmatite bodies related to the swarms of the Eastern Brazilian Province (late Ediacaran to early Cambrian) and to post-collisional Cambrian vents of the Brasiliano Orogeny caused extensive hydrothermal alteration in the country rocks, encompassing recrystallization of the detrital zircon grains from the associated psammopelitic units, which yielded Cambrian ages, formation of kaolinite-rich lenses, and generation of granoblastic ore bodies. The association of intense stretching along foliation and circulation of hydrothermal fluids from pegmatite intrusion is responsible for the ore formation. • Schistose to massive high-grade ore lenses with hematitic and magnetite granoblastic domains. • Ore formation associated with pegmatite intrusion and shearing during Brasiliano Orogeny. [ABSTRACT FROM AUTHOR]

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations. [ABSTRACT FROM AUTHOR]

There is growing evidence from Mo and Fe stable isotope tracers that shallow-water sediments of the 2.95 Ga Mozaan Group (Pongola Supergroup) in South Africa were deposited under oxidizing conditions, while mineralogical, elemental and S isotopic evidence favors the contemporaneous atmosphere to have remained reducing. In contrast, the first reported Cr-stable isotope data for White Mfolozi riverbed outcrop samples of the Ijzermijn iron formation (IF), Mozaan Group, and drill core samples of the underlying paleosol of the Nsuze Group were interpreted to require oxidative weathering conditions during the time of deposition. More recent Cr-isotope data of at least the Ijzermijn IF samples of the same White Mfolozi riverbed outcrop, however, suggested that modern weathering effects can superimpose oxidative signatures on paleo-redox proxies. This study compares (234U/238U) activity ratios along with stable Mo and Fe isotope data of corresponding drill core and White Mfolozi riverbed outcrop samples in order (i) to test (234U/238U) disequilibria as a potential tool to identify samples affected by modern weathering and (ii) to further investigate weathering effects on outcrop samples, particularly with regard to the stable Mo and Fe isotope systematics. All drill core samples from the Ijzermijn IF have (234U/238U) activity ratios that are within uncertainties of secular equilibrium (i.e. a value of 1), and thus were not significantly affected by modern weathering-related U gain or loss or other sub-recent rock-water interaction. By contrast, the outcrop IF and shale samples from the White Mfolozi riverbed show very strong deviations from the secular equilibrium (234U/238U) activity ratio. Furthermore, these deviations are accompanied by the observations that (i) outcrop samples have on average heavier δ56/54Fe values (−0.96‰) than drill cores (−1.59‰), and (ii) positive correlations in drill core samples between log[Fe/Mn] and δ98/95Mo or δ56/54Fe are disrupted by inclusion of the outcrop samples, as are negative correlations between log[MnO] and δ98/95Mo. However, δ56/54Fe and δ98/95Mo of our outcrop samples do not correlate with the extent of (234U/238U) disequilibria. As such, (234U/238U) activity ratios fail to act as a direct identification tool for modern weathering effects on stable Fe and Mo isotopes in outcrop samples. Overall, this study confirms the adsorption of isotopically light Mo onto MnO 2 particles in oxic surface water conditions in the epicontinental Pongola Sea. Reduction of the Mn-oxides together with microbial Fe reduction in the reduced sediment pile of the Pongola basin led to the formation of Mn- and Fe-carbonates during early diagenesis storing the observed stable Fe and Mo isotopic signatures. We thus extend the so-called MnO 2 -shuttle first proposed for the Neoarchean 2.46 Ga old Koegas IF to late Mesoarchean shallow-marine depositional settings. [ABSTRACT FROM AUTHOR]

Laterally extensive silicate and sulfide iron formation associated with jasper (hematitic chert) beds and volcanogenic massive sulfide (VMS) deposits in Norway provide evidence of early mineral assemblages and redox conditions within coeval early Paleozoic seawater. Calculated detrital-free compositions record mixed hydrothermal (e.g., Fe, Cu) and seawater ± biogenic (e.g., Si, Ni, S, REE, P) components. Rare earth element (REE) patterns are characterized by small to large negative Ce anomalies and insignificant to locally large positive Eu anomalies, reflecting seawater REE carried to the seafloor by Fe–P-rich particles later modified by diagenetic processes. Protoliths of silicate iron formation precipitated in anoxic and intermittently euxinic deep waters by the diagenetic modification of amorphous Si–Fe oxyhydroxides and/or Si–Fe–OOH gels, based on possible modern analogues in the Red Sea. Diagenetic minerals include nontronite, greenalite, stilpnomelane, magnetite, manganosiderite, apatite, and iron sulfides. In sulfide iron formation, a local predominance of pyrrhotite over pyrite records highly reducing conditions caused by organic material. The geochemical data provide evidence for Mn–Fe–P shuttle and redox processes in a stratified basin with oxic or suboxic shallow waters and silica concentrations much higher than those of modern seawater. Hydrothermal plume-derived Fe present within the anoxic layer and near the chemocline formed mixed-valence oxyhydroxides and silicates and, intermittently, sulfides by reaction with aqueous Si and H2S, respectively, the latter derived from bacterial reduction of seawater sulfate at the chemocline. Major sustained fluxes of hydrothermally derived reductants (Fe2+, Mn2+, H2S, H2) produced from large seafloor systems such as Løkken may have changed the redox state of seawater in local, and possibly regional, basins from weakly or moderately oxic to intermittently anoxic or euxinic conditions. [ABSTRACT FROM AUTHOR]

Abstract In the Neoproterozoic Era there was widespread deposition of iron formations in close association with global or near glaciations. These 'Snowball Earth' glaciations likely played a key role in iron formation distribution and deposition. However, the environmental conditions, Fe sources, and formation mechanisms remain debated. Here we present the rare earth element geochemistry and Fe isotope composition of the synglacial iron formation within the Neoproterozoic Fulu Formation, South China. The Fulu iron formation consists of layers of authigenic minerals (mainly hematite) and detrital components (quartz, feldspars, Fe chlorite, and minor biotite). Positive Eu anomalies in one of the Fulu localities indicate a hydrothermal influence, suggesting that Fe was mainly sourced from distal hydrothermal systems. The bulk-rock Fe isotope composition of the Fulu iron formation shows a large range, with δ56Fe from −0.23 to +1.78‰. Correlation between bulk-rock δ56Fe values and Al/Fe ratios demonstrates that δ56Fe variability reflects, in part, varying proportions of authigenic versus detrital components. The Fe isotope composition of authigenic hematite is calculated by a linear regression and shows δ56Fe between +0.83 and +2.21‰, with an average at +1.54 ± 0.50‰ (2σ, n = 41). Using a dispersion-reaction model, the high δ56Fe values of hematite constrain local dissolved O 2 concentrations of the ocean to less than 0.4 nmol/L, even in the shallow part of the water column. This relationship is consistent with highly reducing conditions in the Neoproterozoic oceans favored by isolation from the atmosphere by a sea ice. We attribute the extremely positive values to partial iron oxidation in waters that were cold relative to modern surface oceans. The dominant occurrence of hematite supports an abiotic precipitation pathway, given that biological activity would have introduced organic matter to the sediments and led to partial reduction of Fe(III) oxides and subsequent formation of magnetite and/or siderite, as is observed in Archean and Paleoproterozoic iron formations. Oxic glacial meltwater and/or O 2 vertical transfer from the atmosphere to the upper ocean linked to ice dynamics is likely to have mediated the abiotic oxidation. We propose that vertical transfer of O 2 resulted from the deposition of snow that trapped air bubbles at the top of the glacier coupled to a melting of the bottom of the glacier, which in combination delivered a limited but continuous amount of O 2 to the ocean. [ABSTRACT FROM AUTHOR]

Precambrian iron formations throughout the world, notably in Australia, Brazil, and South Africa, show evidence of hypogene (≥ 110 °C, mostly > 250 °C) oxidation, alteration, and silica dissolution as a result of tectonic or magmatic activity. Although hydrothermal oxidation has been proposed for the prototype Lake Superior-type iron formation, the Biwabik Iron Formation in Minnesota (USA), it has not been documented there. By examining oxidized and unoxidized Biwabik Iron Formation in three mines, including material from high-angle faults that are associated with oxidation, we document an early hypogene oxidation event (~ 175 °C) involving medium-salinity aqueous fluids (8.4 ± 4.9 wt% NaCl equiv) that infiltrated iron formation along high-angle faults. At the Hibbing Taconite Mine, hydrothermal fluids oxidized iron carbonates and silicates near faults, producing goethite ± quartz. In contrast with much of the oxidized iron ores on the Mesabi Range, silica was not removed but rather recrystallized during this event, perhaps lying in a rock-dominated system at low cumulative fluid flux. During the hydrothermal oxidation event in the Hibbing Taconite deposit, quartz-filled microfractures and irregular inclusions commonly formed in coarse variably oxidized magnetite, currently the ore mineral: these inclusions degrade the ore by introducing excess silica in magnetic concentrate. Hydrothermal oxidation at Hibbing Taconite Mine is overprinted by later, relatively minor supergene oxidation both along faults and near the surface, which locally dissolved quartz. At the Fayal Reserve Mine, widespread silicate and carbonate gangue dissolution and iron oxidation was followed by precipitation of pyrite, Mn-siderite, apatite, and other minerals in void spaces, which prevented post-oxidation compaction and significant volume loss in the sampled rocks. Although definitive temperature data for this assemblage are needed, the weight of evidence indicates that this mineralization is hypogene. The association of oxidation with epithermal conditions constrains the oxidation and subsequent mineralization to have taken place during the Precambrian, the only time when these rocks would have experienced the necessary temperatures. The mineralization at Fayal Reserve shows little supergene overprint: pyrite is largely unoxidized. Hydrothermal oxidation in both mines was likely produced by basinal fluids that were expelled during the 1.83-1.87 Ga Penokean Orogeny, and mixing with meteoric fluids along faults, although a 1.1 Ga rift-related fluid flow event is also possible. Later supergene overprinting of the iron formation was minor. [ABSTRACT FROM AUTHOR]

Abstract Continental margin (Superior-type) iron formation of the Cauê Formation, Iron Quadrangle, Brazil, is composed of a laminated, magnetite-rich chemical meta-mudstone that was altered during hydrothermal circulation of basinal fluids, greenschist to amphibolite facies metamorphism, and subsequent supergene enrichment of Fe. The absence of traction current deposits and storm-generated structures suggests that deposition occurred on the distal shelf, possibly as deep as the continental slope. Paragenesis-based lithogeochemical analyses of two drill cores with contrasting alteration styles highlight the importance of hydrothermal, metamorphic, and supergene processes on the rare-earth element plus yttrium (REE + Y) and δ56Fe compositions of iron formation. The first drill core penetrates to a depth of 540 m in the Ouro Fino Syncline (OFS) where lithofacies have experienced intense hydrothermal and supergene alteration. The second core is from the Gandarela Syncline (GS) and contains 450 m of metamorphosed iron formation that has undergone later supergene enrichment of Fe. Petrographically, euhedral to subhedral magnetite in the OFS and GS is fabric destructive, and depending on the style and degree of alteration, can co-occur with dolomite, siderite, goethite, and/or martite. Synsedimentary magnetite and other primary Fe-oxide phases are absent. The effect of supergene enrichment and weathering is best captured along the depth profile of each core by the inverse relationship between SiO 2 and Fe 2 O 3 ; SiO 2 values are lowest and Fe 2 O 3 values highest near the surface. The presence of goethite to 500 m records the maximum depth of these processes in the subsurface. The REE + Y, and Y anomalies from the deepest and least altered lithofacies in both cores are typical of deposition in a redox-stratified water column. The weathered lithofacies show several changes in the REE + Y patterns, Ce anomalies and absence of Y anomalies, caused mainly by fluid percolation. The change in Fe isotopic composition with depth is more complex. In drill core from the OFS, negative δ56Fe values shift from −0.91‰ at depth to less negative values of −0.6‰ near the surface. The most altered samples do not lie on this trend because they are characterized by positive values of 0.30‰ and 1.33‰. The δ56Fe values from the GS increase linearly from −0.11‰ to 1.23‰ towards the surface with the most negative values −0.7‰ and −1.16‰ near the top. The most negative values from the OFS could reflect synsedimentary microbial dissimilatory Fe reduction, and the positive values from the GS may be related to partial Fe2+ oxidation between the hydrothermal Fe source and Fe-(oxyhydr)oxides. However, when interpreted together with mineralogical and REE data, these data likely record the various alteration processes that affected the Cauê Iron Formation. Such findings highlight the importance of coupling sedimentologic analysis with elemental and isotopic chemistry to understand iron formation deposition and Fe ore-forming processes. Highlights • Evaluation and interpretation of the δ56Fe and REE+Y composition of upwelling-related, IF from two drill cores • Stratigraphically lower IF lithofacies were protected from weathering and supergene enrichment to preserve a primary signal • Interpreting geochemical data in paragenetic context suggests that IF chemistry was also affected by hydrothermal alteration • Findings indicate that REE+Y and δ56Fe values must be properly constrained in depositional and post-depositional context [ABSTRACT FROM AUTHOR]

Rare earth element (REE) abundances in iron formations (IFs) provide key evidence to track the redox evolution of the Precambrian oceans. Deriving information about processes and sources from IFs is complicated by the variable roles of diagenetic dissolution and precipitation that modify primary depositional signals in the rocks. Iron formations are typically mineralogically heterogeneous with varying amounts of carbonate (ankerite + siderite), oxide (predominantly magnetite) and silicate (chert and Fe-silicates). To date, little is known about the respective contributions of these fractions to the bulk-rock REE signal, and which fraction most accurately records primary seawater signals. Here we evaluate an optimized sequential extraction scheme for REE analysis in the ca. 2.46 Ga Asbestos Hills IFs from the Griqualand-West Basin in South Africa. These rocks were deposited immediately before the Great Oxidation Event and are potentially an important archive for large-scale redox changes in the (near) surface environment. To fully evaluate our method, we measured lanthanide elements plus ytrrium (REY) in individual minerals, in bulk rock samples and in three separate fractions using acetate, oxalate and total digestion sequential extractions. The extraction techniques were then verified using a mass balance approach, the first time that this has been comprehensively applied to IFs. Similarities between the carbonate fraction data and modern seawater, coupled with strong measurement reproducibility and systematic stratigraphic variability suggests that carbonate most closely tracks the primary seawater composition. The Asbestos Hills IFs show distinctive REY signatures with strongly variable (Yb/Pr) SN ratios, positive La anomalies, positive and constant Y/Ho, weakly positive Eu anomalies and a lack of Ce anomalies in their carbonate fraction. Stratigraphic variations in these parameters suggest deposition in a stratified fully anoxic ocean, which became shallower over time. The silicate fraction was influenced by various amounts of allochthonous material (detrital and volcanic ash), although some minerals including greenalite, minnesotaite and stilpnomelane may have tracked seawater REE variability. The oxide fraction was found to be the most REY-depleted and therefore susceptible to allochthonous influences as well as diagenetic controls. It shows a systematic MREE enrichment that has not been described in IFs before, but has been seen in sub-oxic, ferrous environments. An increase in the ratio of carbonate to oxide minerals ongoing upwards through the Asbestos Hills IF, from deep to shallow water conditions, most likely reflected an increase in dissimilatory iron reduction close to the margin of the anoxic basin. [ABSTRACT FROM AUTHOR]

The origins of iron formations remain somewhat enigmatic despite much progress over the past decades. This is due in part to continuing research demonstrating that these deposits do not have a single origin but rather can be formed under various and variable conditions. This study describes a formation pathway for an Algoma-type banded iron formation (BIF) from the 2.75 billion-year-old Weld Range of Western Australia, based on petrographic and Fe isotope analyses of drill core samples. The BIF is composed of alternating layers of jaspilitic and Fe-poor chert, magnetite layers of varying thickness and abundance, and rare pyrite. Petrographic analysis indicates the minerals formed in the order hematite, magnetite, and then pyrite, with the latter two clearly replacive of “primary” bedded hematite. Hematite δ 56 Fe values from all portions of the drill core are positive and essentially identical (δ 56 Fe = 0.61 ± 0.05‰). Magnetite δ 56 Fe values are positive, as well (δ 56 Fe = 0.51 ± 0.07‰) but vary systematically with total magnetite content of the core samples. Pyrite δ 56 Fe values are positive and higher than both hematite and magnetite (δ 56 Fe = 1.03 ± 0.05‰). Combined, these analyses reveal a multistage formational model for the Weld Range BIF. Initial deposition occurred by the partial oxidation of hydrothermally-sourced aqueous Fe(II) to form a Fe(III)-Si co-precipitate, probably by anoxygenic photosynthetic iron oxidizers. This material would have been converted to hematite under equilibrium conditions with excess aqueous Fe(II). Magnetite formed by a later intrusion of reducing fluids, mostly along bedding planes, but also as cross-cutting veins. This produced large quantities of magnetite layers parallel, and sub-parallel, to the original bedding, but that do not represent a primary depositional feature. Pyrite was produced in some portions of the unit, likely under equilibrium conditions, by later infiltration of reducing fluids containing sulfide. The entirely positive δ 56 Fe values for all Fe phases in the Weld Range BIF stands in stark contrast to the more massive Superior-type BIFs of the Paleoproterozoic Hamersley and Transvaal basins of Western Australia and South Africa, respectively, which have δ 56 Fe values that average close to 0‰ but vary over nearly the entire range of δ 56 Fe values measured in nature (δ 56 Fe = −2.5 to +2.6‰). The Weld Range BIF δ 56 Fe values are similar to those of older Algoma-type BIFs such as the Paleoarchean Isua BIF of Greenland but are less positive on average. This overall trend of decreasing average δ 56 Fe values through time likely records an overall increasing trend of oxygenation of the Archean surface ocean from 3.8 to 2.45 billion years ago. [ABSTRACT FROM AUTHOR]

Toward the end of the Palaeoproterozoic era, over 10 9 billion tonnes of banded (BIF) and granular (GIF) iron formations were deposited on continental platforms. Granules in iron formations are typically sub-spherical structures 0.2 to 10 mm in size, whereas concretions are >10 mm. Both types of spheroids are preserved throughout the sedimentological record. Their formation has typically been interpreted to originate from reworked Fe-rich sediments in high-energy, wave-agitated, shallow-marine environments. New evidence from six different late Palaeoproterozoic granular iron formations (GIF), however, suggests that some granules are the result of diagenetic reactions, in addition to other features driven by microbial processes and mechanical movements. Characteristic coarse grain interiors and septarian-type cracks inside granules, akin to those features in decimetre- to meter-size concretions, are interpreted as desiccation features from hydrated diagenetic environments where sulphate and/ or ferric iron were reduced while organic matter (OM) was oxidised inside granules. Those granules derived from sulphate reduction preserve diagenetic pyrite rims, whereas those formed via ferric iron reduction preserve diagenetic magnetite along their rims. Other diagenetic minerals including apatite mixed with OM, and various carbonate phases are commonly preserved within granules. Combined with systematically 13 C-depleted carbonate, these diagenetic mineral assemblages point to the oxidative decay of OM as a major process involved in the formation of granules. Spheroidal equidistant haematite laminations surround some granules and contain apatite associated with carbonate, OM, and ferric-ferrous silicates, and oxides that further suggest these structures were not shaped by wave-action along sediment-water interfaces, but rather by chemical wave fronts and biomineralisation. Our results demonstrate that the formation mechanisms of GIF also involve microbial activity and chemically-oscillating reactions. As such, granules have excellent potential to be considered as promising biosignatures for studying Precambrian biogeochemistry, as well as astrobiology. [ABSTRACT FROM AUTHOR]

The Kingston Peak Formation records glacial sedimentation during the Cryogenian in Death Valley, California, and contains iron formation horizons. These iron formations are part of a thick sedimentary succession containing glaciogenic diamictites, together with mass flow breccias, conglomerates, sandstones and siltstones. The Kingston Peak iron formations are mineralogically and sedimentologically simple, consisting of finely laminated hematitic siltstones that contain up to 50 wt% Fe. These iron formations have been the subject of controversy as they have previously been described as being either absent, or the product of volcanism, diagenesis, hydrothermalism or weathering. Here we present a detailed case study of the sedimentology, stratigraphy, geochemistry and iron isotope composition of the Kingston Peak iron formations. Unequivocal textural evidence indicates a synsedimentary origin for these iron formations. The iron isotopic signature (0.2 < δ 56 Fe < 1.65) is indicative of partial oxidation of a ferrous iron reservoir. Therefore the Kingston Peak iron formations are interpreted to be primary chemical sediments deposited by the mixing of oxygenated glaciogenic fluids with ferruginous seawater. Pulses of oxidants led to the precipitation of iron oxides, which became enriched under periods of sediment starvation in a basin influenced by episodic mass flows and glacial input. Sedimentological evidence implicates deposition in a range of glaciomarine environments, from ice-distal to relatively ice-proximal, at multiple stages throughout the glacial succession and well before the end of the glacial period. These iron formations have geochemical and sedimentological similarities to other glacially-associated Neoproterozoic iron formations that occur globally and have important implications for our understanding of ocean chemistry and glaciation in the Cryogenian. [ABSTRACT FROM AUTHOR]

Previously reported stable Cr isotopic fractionation in Archaean paleosols and iron formations (IFs) have been interpreted as a signature of oxidative weathering of Cr(III) to Cr(VI) in soils, and delivery of isotopically heavy Cr(VI) into the oceans. One of the oldest reported fingerprints of this process is isotopically heavy Cr preserved in the 2.95 Ga old Ijzermijn IF, Sinqeni Formation of the Mozaan Group (Pongola Supergroup), South Africa and could suggest that atmospheric free oxygen was present ca. 600 million years earlier than the Great Oxidation Event (GOE). However, fractionated stable Cr isotopic signatures have only been found to date in surface outcrop samples of the White Mfolozi Inlier exposed along the White Mfolozi River Gorge. In this study, the latter outcrop was resampled along with two drill cores of the Ijzermijn IF and a drill core of the Scotts Hill IF to represent multiple exposures of Mozaan Group IFs with different states of preservation. A detailed geochemical comparison on bulk samples of different units was undertaken using stable Cr isotopes coupled with trace and major elements. Outcrop iron-rich mudstones (Fe - lutites) show very low LOI [wt] %, and very low Fe(II)/Fe tot ratios, and lower Ca and Mg relative to equivalent facies in drill cores, indicating the effects that oxidative recent surface weathering had on Fe/Mn-rich carbonate minerals of the IF. Overall rare earth element and yttrium (REE + Y) mixing models agree well with previous studies, confirming that they were minimally disturbed by weathering and are consistent with a high magnitude of continental solutes delivered in a near–shore depositional environment, with a minor contribution of hydrothermally derived fluids that upwelled into shallower depositional setting. Importantly, all drill core samples of this study revealed δ 53/52 Cr values within the igneous inventory, despite variable amounts of detrital Cr input that includes nearly detritus-free, chert/jasper-rich units. By contrast, a specific group of Fe-lutite samples near the base of White Mfolozi River outcrop bear fractionated Cr isotopic signatures with δ 53/52 Cr values up to 0.418‰. These outcrop samples also display unusually high U/Th ratios (max. 12.6) as well as enrichments of other elements (W, Tl, As, MREE) that far exceed that observed in correlative drill core units. These observations together with the lack of Cr isotopic fractionation in drill core samples lead us to propose that the heavy δ 53/52 Cr values of Fe-lutites from outcrop Ijzermijn IF samples reported here and in a previous study are the product of modern oxidative weathering rather than an indicator for Mesoarchaean oxidative weathering at ca. 2.95 Ga. [ABSTRACT FROM AUTHOR]

Iron formations (IFs) are important geochemical repositories that provide constraints on atmospheric and ocean chemistry, prior to and during the onset of the Great Oxidation Event. Trace metal abundances and their Mo-Cr-U isotopic ratios have been widely used for investigating ocean redox processes through the Archean and Paleoproterozoic. Mineralogically, IFs consist of three main Fe-bearing fractions: (1) Fe-Ca-Mg-Mn carbonates, (2) magnetite and/or hematite and (3) Fe-silicates. These fractions are typically fine-grained on a sub-μm scale and their co-occurrence in varying amounts means that bulk-rock or microanalytical geochemical and stable isotope data can be influenced by cryptic changes in mineralogy. Fraction specific geochemical analysis has the potential to resolve mineralogical controls and reveal diagenetic versus primary precipitative controls on IF mineralogy. Here we adapt an existing sequential extraction scheme for Fe-phases (Poulton and Canfield, 2005) to the high Fe-content in IF and the specific three-fraction mineralogy. We optimized the scheme for magnetite-dominated Archean IFs using samples from the hematite-poor Asbestos Hills Subgroup IF, Transvaal Supergroup, South Africa. Previously commonly-used hydroxylamine-HCl and dithionite leaches were omitted since ferric oxides are quantitatively insignificant in these IF samples. The acetate leach was tested at variable temperatures, reaction times and under different atmospheres in order to ensure that all micro-crystalline Fe-carbonates were effectively dissolved, resulting in an optimum extraction for 48 h at 50 °C under anoxic conditions. The dissolution of magnetite by NH 4 -oxalate was also tested, resulting in an optimum extraction for 24 h under an ambient atmosphere. Finally, a HF-HClO 4 -HNO 3 leach was used to dissolve the residual silicate fraction which has to date not been considered in detail in IF. Accuracy of the extraction technique was generally excellent, as verified using 1) elemental recoveries, 2) comparison of major and trace element distributions against mineralogy and 3) comparison to results from microanalytical techniques. This study focuses on the distribution of three frequently used geochemical proxies in IF; U, Mo and Cr. Molybdenum abundances in the Kuruman and Griquatown IF are low and show an apparent correlation with mineralogical variability, as determined by the sequential extraction. This suggests that changes in bulk-rock mineralogy, rather than redox chemistry might significantly affect Mo stable isotopes. For Cr, a minor bulk-rock stratigraphic increase can be related to the oxide and silicate fraction. However, a positive relationship with Zr indicates that this was also controlled by detrital or volcanic ash input. Uranium is predominantly bound to the silicate fraction and shows clear correlations with Zr and Sc implying detrital reworking under anoxic conditions. The discrepant behaviour of these three proxies indicate that mineralogy should be taken into account when interpreting heterogeneous bulk-rock samples and that fraction specific techniques will provide new insights into the evolution of atmosphere and ocean chemistry. [ABSTRACT FROM AUTHOR]

The Fulu iron formation (IF) is an iron-rich unit in the Neoproterozoic glacial successions, South China. The major element data suggest that the iron sources of the Fulu IF are derived from binary mixing from hydrothermal and detrital loads. The Fulu IF is characterized by slightly positive Eu anomalies similar to other Neoproterozoic IFs, indicating that a high-temperature hydrothermal input may contribute little to Neoproterozoic IFs. A shift from non-existent to slightly negative Ce anomalies of the Fulu IF indicates that the IF precipitated across an iron chemocline separating a weakly oxic surface ocean from an oxygen-depleted deep ocean. [ABSTRACT FROM PUBLISHER]

• Red Lake Greenstone Belt contains Earth's oldest known, thick, carbonate platform. • Outcrops and multiple cored drill holes provide excellent stratigraphic control. • Interlayering of lithofacies suggests short to long term relative sea level changes. • Precipitation rates of all types of chemical sediments varied temporally. • Archean seawater REE chemistry is similar in shallow and deep areas of the shelf. The > 200 m thick Red Lake carbonate platform of the 2.940-2.925 Ga Ball Assemblage (Red Lake Greenstone Belt, Superior Province, Canada) is the oldest known carbonate platform preserved on Earth. This study examined surface outcrops and multiple industry cored drill holes comprising a diverse assemblage of chemical and siliciclastic sedimentary rocks of the deeper water lithofacies associated with the shallow water carbonates. The offshore lithologies, underlying, interlayered with, and overlying the mainly stromatolitic carbonates, consist of alternating packages of sandstone, siltstone, carbonaceous slate, pyritic carbonaceous slate, chert, and oxide facies iron formation (OFIF). These interlayer with carbonate in transitional slope environments, which consist in places of carbonate with magnetite laminae above slumped carbonate debris in iron formation and/or carbonaceous slate. Chert and OFIF were deposited further offshore in a suboxic environment, with lithology dependent on concentrations of seawater Fe(II) and silicic acid. The iron sulfides were deposited in a reducing, anoxic, and organic-rich bottom water and muddy pore water environment not conducive to Fe hydroxide or chert precipitation. Carbonaceous mud was the background sediment, accumulating during intervals when the precipitation of chert, Fe hydroxides, or massive layers of Fe sulfides was inhibited. The interlayering of differing assemblages of chemical and siliciclastic sedimentary rocks indicates temporal and spatial fluctuations in the chemistry of local paleo-seawater and variable sedimentation rates on the margins of the Red Lake carbonate platform. Post-Archean Australian Shale (PAAS)- normalized rare earth element (REE) systematics of carbonate and OFIF show positive La and Gd anomalies, super-chondritic Y/Ho ratios, and depleted light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), similar to modern seawater, while the positive Eu anomalies in the carbonate and OFIF samples reflect hydrothermally influenced Archean seawater. The chemical sedimentary rocks exhibit enrichment of redox-sensitive elements (i.e., Cr, Mo, V, and U) compared to their siliciclastic associates. This, combined with concentrations of MnO up to 6.3 wt% and positive Ce anomalies in the iron formation directly below the carbonate platform, all indicate the likely presence of some O 2 ca 2.93 Ga. [ABSTRACT FROM AUTHOR]

Based on the studies of the geological evolution, field investigation of the ore-bearing rock series, zircon U-Pb isotope chronology and typical iron ore deposits of Taxkorgan Block in West Kunlun, the characteristics and forming ages of regional ore-bearing formation, ore deposit type and metallogenic regularity were summarized. The results show that the previously defined “Bulunkuole Group” actually contains Paleoproterozoic (2 100-2 500 Ma) and Early Cambrian (510-540 Ma) sedimentary metallogenic events, which are suffered to the intensely metamorphism and deformation in both Middle-Late Proterozoic (800-1 800 Ma) and Hercynian-Indosinian (200-410 Ma). Moreover, the genesis of the local high-grade ores may be related to the later hydrothermal activities. The regional iron deposits can be divided into two types. The first one is the meta-sedimentary type iron deposit that is superimposed and transformed by the later hydrothermal fluids; taking Zankan, Mokaer and Jiertiekegou as examples, they form in weakly oxic to reducing marine sedimentary environment of Early Proterozoic active continental margin and the metallogenetic materials are derived from the leaching of basic volcanic rocks by seafloor hydrothermal fluids and a small amount of terrigenous sediments input. The second type is the Early Cambrian marine volcanic exhalative sedimentary iron deposit, such as Kalaizi, Laobing and Yeleke sulfate-iron deposit; their depositional environment should be bathyal-neritic facies weakly oxic to suboxic faulted basin; and the ore-forming materials come from the submarine hydrothermal sedimentary and terrigenous materials input. According to the regional metallogenic regularity, it is speculated that Kalaben-Kalaizi-Molahe belt is the favorable area for prospecting the iron-polymetallic deposits, which associate with the Early Cambrian marine exhalative metallogenic system. While, Jiertiekegou-Zankanbei area, Mokaer and its southern area, which are the northward and southward extensional zones of Zankan iron ore belt, are both the favorable areas for prospecting the meta-sedimentary type of Paleoproterozoic iron deposits. [ABSTRACT FROM AUTHOR]

Paleogeographic data indicate that the Neoproterozoic Rapitan Group accumulated in two spatially related, silled basins during the first mid-Neoproterozoic glacial episode (ca. 711 Ma), yet the controls on glaciation, reason for basin development, spatio-temporal relationships among stratigraphic units, and constraints on iron formation deposition and distribution in this region and globally at this time have not been adequately explained. The lack of a coherent understanding of the group’s regional stratigraphy has been a significant barrier to deciphering the geologic history of this critically important succession. Based on new stratigraphic data, and incorporating the results of a related geochemical study, the Rapitan Group accumulated in tectonically complex sub-basins of a young rift zone. The spatial and temporal dynamics of these sub-basins controlled the grain size, volume, and distribution of detrital sediment (predominantly turbidites and diamictites) deposited during two glacially influenced episodes with ice cover. During an interglacial episode, the rotated, exposed upper surfaces of newly displaced rift blocks directed drainage away from basin margins, precluding significant clastic sediment delivery, and subaqueous margins of outboard tilted fault blocks acted as sills that isolated local bottom water, allowing the accumulation of an entirely hydrogenous succession (jaspilitic and hematitic iron formation). Two strike-parallel normal faults that strongly controlled basin-margin development are prominent thrust faults in the present-day orogen. Syndepositional activity of strike-normal transfer faults segmented the basins along their length, as reflected in pronounced along-strike variations in the presence/absence, thickness, and composition of the three stratigraphic units (Mt. Berg, Sayunei, and Shezal formations), and in the presence or absence of iron formation (locally >100 m thick; top of Sayunei Formation). Differential subsidence in actively rifting basins was essential for deposition of Rapitan-type iron formation, but glaciation, although coeval with Rapitan Group accumulation, was not a strong control. The timing of the onset of glacially influenced sediment deposition in the closely spatially related Rapitan (∼711 Ma) and Mount Harper groups (∼717 Ma) in northwestern Laurentia differs by ∼6 m.y., arguing for diachronous Sturtian glaciation, both locally and globally. [ABSTRACT FROM AUTHOR]

A high resolution stratigraphically controlled rare earth element and carbon isotope study of the 2.75 Ga old Algoma-type Helen Iron Formation has provided new insights into the chemistry of late Archean seawater. The Helen Iron Formation passes from a sediment dominated by rhyolitic detrital material in the first 20 m of the section into a purely chemical sediment reflecting the chemistry of the seawater from which it precipitated. An overprint of a hydrothermal signature, seen in the development of a positive europium anomaly, increasing in magnitude upsection is in good accordance with field observations indicating a deepening of the Helen basin through time. The lack of a cerium anomaly along with the predominance of Fe 2+ over Fe 3+ indicates that oxygen played a minor role in the deposition of the Helen Iron Formation. Carbon isotope data are unusual for iron formation as they cluster around 0‰ (VPDB) and indicate direct precipitation from seawater. However, highly negative carbon isotope excursions suggest two episodes of dissimilatory iron reduction in sediments rich in organic matter, the negative carbon isotope signatures being inherited from the oxidation of this organic matter. These excursions have high magnitudes of 3 and 10.5‰, respectively. These results indicate that the Helen Iron Formation was deposited in a restricted basin with high input of Fe 2+ from distal hydrothermal sources and high levels of CO 2 , explained by submarine volcanic activity in the Helen Basin. We suggest that CO 2 and Fe oversaturation led to the direct precipitation of siderite and minor ankerite and that the negative carbon isotope excursions observed are the result of the oxidation of organic matter previously produced by bacterial dissimilatory iron reduction. The Helen Iron Formation thus is a direct precipitate from seawater in a basin increasing in water depth from moderately deep to deep water and presents a significant local hiatus in submarine volcanism. [ABSTRACT FROM AUTHOR]

• The 2.9 Ga Ijzermijn Iron Formation Bed is the oldest supracratonic iron formation. • It contains anomalously high manganese concentrations. • Sedimentology and geochemistry support biological manganese oxidation. • The pre-GOE Mozaan Basin likely contained low oxygen concentrations. The Ijzermijn Iron Formation Bed occurs in the Vlakhoek Member of the basal Sinqeni Formation of the Mesoarchean Mozaan Group of the Pongola Supergroup of southern Africa. It represents the oldest known supracratonic Superior-type iron formation that has significant manganese enrichments. Lithostratigraphic correlation between the Mozaan Group and the Witwatersrand Supergroup indicates that there was a larger Witwatersrand-Mozaan depositional basin on the Kaapvaal Craton during the Mesoarchean. Detailed stratigraphy, mineralogy, petrography, carbonate mineral chemistry, major and trace element geochemistry and carbonate oxygen and carbon isotopes are presented for two drill cores intersecting the Ijzermijn Iron Formation Bed. Drill core PMH 24, which represents a more coast-distal depositional setting, is characterized by manganese-rich siderite, higher Fe 2 O 3 and lower MnO and Al 2 O 3 contents, more prominent europium anomalies, lower Gd SN /Yb SN values and less depleted δ13C PDB values. Drill core DDN 1, which represents a more coast-proximal depositional setting, is characterized by iron-rich rhodochrosite, lower Fe 2 O 3 and higher MnO and Al 2 O 3 contents, less prominent europium anomalies, higher Gd SN /Yb SN values and more depleted δ13C PDB values. The presence of early diagenetic magnetite, hematite and 13C-depleted carbonates indicate the iron and manganese, which were sourced from hydrothermal plumes, were deposited by oxidation, with subsequent diagenetic reduction by organic carbon. The most efficient oxidative mechanism would have been biological oxidation. The increase in manganese contents towards the paleocoastline further supports manganese oxidation as it was redox buffered by Fe2+ in the water column and could only be deposited and preserved once Fe2+ had been removed through oxidation further away from the distal hydrothermal plume source. This is supported by previously published iron isotope data, where δ56/54Fe values are more depleted closer to the paleocoastline. Deposition of iron and manganese was restricted to below the photic zone. This implies that microaerophilic chemolithoautotrophs were responsible for iron and manganese oxidation. This requirement of free oxygen in the Ijzermijn Iron Formation Bed has been supported by earlier molybdenum isotope studies. The presented work therefore further supports the idea of pre-Great Oxidation Event marine oxygen oases that did not lead to any appreciable build-up of oxygen in the atmosphere. [ABSTRACT FROM AUTHOR]

The Neoproterozoic Tatonduk (Alaska) and Holowilena (South Australia) iron formations share many characteristics including their broadly coeval (Sturtian) ages, intimate association with glaciogenic sediments, and mineralogy. We show that these shared characteristics extend to their neodymium ( ε Nd ) and iron isotope ( δ 56 Fe) systematics. In both regions δ 56 Fe values display a distinct up-section trend to isotopically heavier values, while ε Nd values are primitive and similar to non-ferruginous mudstones within these successions. The δ 56 Fe profiles are consistent with oxidation of ferruginous waters during marine transgression, and the ε Nd values imply that much of this iron was sourced from the leaching of continental margin sediments largely derived from continental flood basalts. Rare earth element data indicate a secondary hydrothermal source for this iron. [ABSTRACT FROM AUTHOR]

Iron formations, hematitic cherts (jaspers), ‘tuffites’, silica-iron exhalites and other metalliferous chemical sedimentary rocks are important stratigraphic marker horizons in a number of volcanogenic massive sulfide (VMS) districts worldwide, forming during episodes of regional hydrothermal activity. The VMS prospective ca. 484–464 Ma Tyrone Igneous Complex of Northern Ireland represents a structurally dissected arc-ophiolite complex that was accreted to the composite margin of Laurentia during the Grampian orogeny (ca. 475–465 Ma), and a potential broad correlative to the VMS-rich Buchans–Robert's Arm arc system of the Newfoundland Appalachians. Silica-iron-rich rocks occur at several stratigraphic levels in the Tyrone Igneous Complex spatially and temporally associated with rift-related basalts (e.g., Fe–Ti-rich eMORB, IAT, OIB) and zones of locally intense hydrothermal alteration. In the ca. 475–474 Ma lower Tyrone Volcanic Group, these rocks are characterized by massive, 1–5 m thick blood-red jaspers, hematitic siltstones and mudstones, and intensely silica-hematite altered tuffs and flows. Their mineralogy is dominated by quartz–hematite ± magnetite–(chlorite-sericite ± tremolite/actinolite), with Fe concentrations rarely exceeding 10 wt.%. Relict textures (including the presence of coalesced spherules of silica-iron oxides) in rocks exposed at Tanderagee NW, Creggan Lough and Tory's Hole are indicative of seafloor exhalation, whereas replacement of the original volcanic stratigraphy is evident to varying degrees at Tanderagee, Beaghbeg and Bonnety Bush. In the structurally overlying ca. 473–469 Ma upper Tyrone Volcanic Group, chemical sedimentary rocks include recrystallized: (i) thin and laterally-restricted jaspers in thick sequences of graphitic pelite at Boheragh; and (ii) laterally-persistent sulfidic cherts and ironstones dominated by quartz–hematite–magnetite–(chlorite) or quartz–pyrite–(chlorite) in sequences of tuff at Broughderg. Compared to chemical sedimentary rocks associated with VMS deposits worldwide, their geochemical characteristics are most similar to silica-iron exhalites of the Mount Windsor Subprovince (SE Australia) and jaspers of Central Arizona, Bald Mountain (Northern Maine), the Urals, Iberian Pyrite Belt and Løkken ophiolite (Norway). Positive Eu anomalies (at Slieve Gallion and Tanderagee NW), elevated Cu + Pb + Zn, Au, Fe/Ti, Fe/Mn, Sb, Ba/Zr and Fe + Mn/Al, together with low REE, Sc, Zr and Th, are indicative of a greater hydrothermal component and potentially more VMS-proximal signatures. Based on bulk ironstone geochemistry, Bonnety Bush, Tanderagee NW-Creggan Lough, Broughderg and Drummuck (Slieve Gallion) are considered the most VMS prospective areas in the Tyrone Igneous Complex and warrant further exploration. [ABSTRACT FROM AUTHOR]

The Holowilena Ironstone is a Neoproterozoic iron formation in South Australia associated with glacial deposits of the Sturtian glaciation. Through a comprehensive field study coupled with optical and scanning electron microscopy, X-ray fluorescence, and X-ray diffraction, a detailed description of the stratigraphy, sedimentology, mineralogy, and structure of the Holowilena Ironstone was obtained. The Holowilena Ironstone comprises ferruginous shales, siltstones, diamictites, and is largely made up of hematite and jasper, early diagenetic replacement minerals of precursor iron oxyhydroxides, and silica. These chemical precipitates are variably influenced by turbidites and debris flows contributing clastic detritus to the depositional system. Structural and stratigraphic evidence suggests deposition within a synsedimentary half-graben. A model for the Holowilena Ironstone is proposed, in which dense oxic fluids expelled during sea ice formation in the Cryogenian pool in the depression of the half-graben, allowing for long-lived mixing with the ferruginous seawater and the deposition of iron oxides. This combination of glacial dynamics, tectonism, and ocean chemistry may explain the return of iron formations in the Neoproterozoic. [ABSTRACT FROM AUTHOR]

Carbon and strontium isotope chemostratigraphy (52 δ 13 C carb and δ 18 O, and 50 87 Sr/ 86 Sr analyses of carbonate components in whole-rock samples) was applied for constraining an apparent depositional age of the carbonate protolith to amphibolite-grade, calcite marbles occurring in siliciclastic sedimentary sequences hosting iron formations (the Dunderlandsdalen type iron ores) of previously unknown age in the Rödingsfjället Nappe Complex of the Rana region, Nordland, Norway. The least altered 87 Sr/ 86 Sr (0.70676) and δ 13 C (+2.5 to +5.6‰) values of the marbles (Dunderland Marble 2a) in the hanging wall sequence of the Stensundtjern iron formation in the Rana region are consistent with seawater composition in the time interval 800–730 Ma, hence the Middle Cryogenian (pre-Sturtian). The least altered Sr- and C-isotopic values obtained from two other marbles units (Marble 3 and 4) of the Rödningsfjället Nappe Complex are consistent with the Late Cryogenian (c. 660 and 670–700 Ma, respectively). The ages obtained provide the first insight into the depositional time of sediment-hosted iron formations of the Uppermost Allochthon in the North-Central Norwegian Caledonides. The Middle Cryogenian-age of marbles associated with iron formations in the Rana region and those located c. 250 km to the north (the Håfjellet iron ore horizon) share similar 87 Sr/ 86 Sr and δ 13 C ratios and hence similar chemostratigraphic ages. These iron formations were originally accumulated outside of Baltica, on a glacially influenced carbonate-siliciclastic shelf, apparently on a margin of an unknown microcontinent. The Scandinavian Dunderlandsdalen and the Håfjellet iron ores of Middle Cryogenian age (800–730 Ma) were accumulated in an open marine environment distant from volcanic centres, and hence represent an outstanding exception to other reported Neoproterozoic iron formations which were all accumulated in volcanically active continental rift settings. [ABSTRACT FROM AUTHOR]