Changes of Ge/Si,REE + Y and SmNd isotopes in alternating Fe- and Si-rich mesobands reveal source heterogeneity of the ~ 2.54 Ga Sijiaying banded iron formation in Eastern Hebei,China

The North China Craton (NCC) is one of the most important regions hosting abundant banded iron formations (BIFs). The ~ 2.54 Ga Sijiaying BIF, the best-preserved and most extensive deposit in Eastern Hebei, is intercalated and closely associated with meta-volcanic rocks of the Luanxian Group. In this context, major and trace element and SmNd isotopic analyses of individual mesobands of a Sijiaying BIF specimen were conducted to characterize the source and depositional environment over a transient period.Low Al₂O₃, TiO₂ and high field strength elements (HFSEs) concentrations show that the BIF is relatively detritus-free. Shale-normalized rare earth and yttrium distributions (REE + Y) of individual BIF mesobands exhibit positive La anomalies, enrichment in HREE relative to LREE and MREE and suprachondritic Y/Ho ratios, which are typical features of marine waters throughout the Archean and Proterozoic. The presence of consistently positive Eu anomalies indicates a significant high-T hydrothermal input, while the absence of true Ce anomalies suggests deposition from an anoxic water column. By comparison with other typical BIFs (e.g., the Isua BIF from Greenland; the Kuruman BIF from South Africa), the Sijiaying BIF is depleted in HREE, and appears to record variations in solute fluxes related to changing intensities of hydrothermal activity. These features, coupled with SmNd isotopic relations and Ge/Si distributional patterns, point to two decoupled sources controlling the depositional environment of the BIF and thus reveal source heterogeneity for silica and iron of the Sijiaying BIF. High Fe fluxes were associated with seafloor-vented hydrothermal fluids, which received their SmNd isotopic signature from alteration of depleted oceanic crust; whereas significant amounts of silica were associated with ambient seawater whose REE signature was controlled by solutes derived from weathering of nearby Mesoarchean continental landmasses. The old (up to ~ 3.0 Ga) Nd (T_DM) model ages of Si-rich mesobands of the BIF support such a scenario.     

Geochemistry and origin of the Neoproterozoic Dahongliutan banded iron formation (BIF) in the Western Kunlun orogenic belt,Xinjiang (NW China)

The Neoproterozoic (593–532 Ma) Dahongliutan banded iron formation (BIF), located in the Tianshuihai terrane (Western Kunlun orogenic belt), is hosted in the Tianshuihai Group, a dominantly submarine siliciclastic and carbonate sedimentary succession that generally has been metamorphosed to greenschist facies. Iron oxide (hematite), carbonate (siderite, ankerite, dolomite and calcite) and silicate (muscovite) facies are all present within the iron-rich layers. There are three distinctive sedimentary facies BIFs, the oxide, silicate–carbonate–oxide and carbonate (being subdivided into ankerite and siderite facies BIFs) in the Dahongliutan BIF. They demonstrate lateral and vertical zonation from south to north and from bottom to top: the carbonate facies BIF through a majority of the oxide facies BIF into the silicate–carbonate–oxide facies BIF and a small proportion of the oxide facies BIF.The positive correlations between Al₂O₃ and TiO₂, Sc, V, Cr, Rb, Cs, Th and ∑REE (total rare earth element) for various facies of BIFs indicate these chemical sediments incorporate terrigenous detrital components. Low contents of Al₂O₃ (<3 wt%), TiO₂ (<0.15 wt%), ∑REE (5.06–39.6 ppm) and incompatible HFSEs (high field strength elements, e.g., Zr, Hf, Th and Sc) (<10 ppm), and high Fe/Ti ratios (254–4115) for a majority of the oxide and carbonate facies BIFs suggest a small clastic input (<20% clastic materials) admixtured with their original chemical precipitates. The higher abundances of Al₂O₃ (>3 wt%), TiO₂, Zr, Th, Cs, Sc, Cr and ∑REE (31.2–62.9 ppm), and low Fe/Ti ratios (95.2–236) of the silicate–carbonate–oxide facies BIF are consistent with incorporation of higher amounts of clastic components (20%–40% clastic materials). The HREE (heavy rare earth element) enrichment pattern in PAAS-normalized REE diagrams exhibited by a majority of the oxide and carbonate facies BIFs shows a modern seawater REE signature overprinted by high-T (temperature) hydrothermal fluids marked by strong positive Eu anomalies (Eu/Eu¹_PAAS = 2.37–5.23). The low Eu/Sm ratios, small positive Eu anomaly (Eu/Eu¹_PAAS = 1.10–1.58) and slightly MREE (middle rare earth element) enrichment relative to HREE in the silicate–carbonate–oxide facies BIF and some oxide and carbonate facies BIFs indicate higher contributions from low-T hydrothermal sources. The absence of negative Ce anomalies and the high Fe³⁺/(Fe³⁺/Fe²⁺) ratios (0.98–1.00) for the oxide and silicate–carbonate–oxide BIFs do not support ocean anoxia. The δ¹³C_V-PDB (−4.0‰ to −6.6‰) and δ¹⁸O_V-PDB (−14.0‰ to −11.5‰) values for siderite and ankerite in the carbonate facies BIF are, on average, ∼6‰ and ∼5‰ lower than those (δ¹³C_V-PDB = −0.8‰ to + 3.1‰ and δ¹⁸O_V-PDB = −8.2‰ to −6.3‰) of Ca–Mg carbonates from the silicate–carbonate–oxide facies BIF. This feature, coupled with the negative correlations between FeO, Eu/Eu¹_PAAS and δ¹³C_V-PDB, imply that a water column stratified with regard to the isotopic omposition of total dissolved CO₂, with the deeper water, from which the carbonate facies BIF formed, depleted in δ¹³C that may have been derive from hydrothermal activity.Integration of petrographic, geochemical, and isotopic data indicates that the silicate–carbonate–oxide facies BIF and part of the oxide facies BIF precipitated in a near-shore, oxic and shallow water environment, whereas a majority of the oxide and carbonate facies BIFs deposited in anoxic but Fe²⁺-rich deeper waters, closer to submarine hydrothermal vents. High-T hydrothermal solutions, with infusions of some low-T hydrothermal fluids, brought Fe and Si onto a shallow marine, variably mixed with detrital components from seawaters and fresh waters carrying continental landmass and finally led to the alternating deposition of the Dahongliutan BIF during regression–transgression cycles.The Dahongliutan BIF is more akin to Superior-type rather than Algoma-type and Rapitan-type BIF, and constitutes an additional line of evidence for the widespread return of BIFs in the Cryogenian and Ediacaran reflecting the recurrence of anoxic ferruginous deep sea and anoxia/reoxygenation cycles in the Neoproterozoic. In combination with previous studies on other Fe deposits in the Tianshuihai terrane, we propose that a Fe²⁺-rich anoxic basin or deep sea probably existed from the Neoproterozoic to the Early Cambrian in this area.     

Geochemistry and Si–O–Fe isotope constraints on the origin of banded iron formations of the Yuanjiacun Formation,Lvliang Group,Shanxi, China

Banded iron formations (BIFs) within the Lvliang region of Shanxi Province, China, are hosted by sediments of the Yuanjiacun Formation, part of the Paleoproterozoic Lvliang Group. These BIFs are located in a zone where sedimentation changed from clastic to chemical deposition, indicating that these are Superior-type BIFs. Here, we present new major, trace, and rare earth element (REE) data, along with Fe, Si, and O isotope data for the BIFs in the Yuanjiacun within the Fe deposits at Yuanjiacun, Jianshan, and Hugushan. When compared with Post Archean Australian Shale (PAAS), these BIFs are dominated by iron oxides and quartz, contain low concentrations of Al₂O₃, TiO₂, trace elements, and the REE, and are light rare earth element (LREE) depleted and heavy rare earth element (HREE) enriched. The BIFs also display positive La, Y, and Eu anomalies, high Y/Ho ratios, and contain ³⁰Si depleted quartz, with high δ¹⁸O values that are similar to quartz within siliceous units formed during hydrothermal activity. These data indicate that the BIFs within the Yuanjiacun Formation were precipitated from submarine hydrothermal fluids, with only negligible detrital contribution. None of the BIF samples analyzed during this study have negative Ce anomalies, although a few have a positive Ce anomaly that may indicate that the BIFs within the Yuanjiacun Formation formed during the Great Oxidation Event (GOE) within a redox stratified ocean. The positive Ce anomalies associated with some of these BIFs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed Ho, LREE, and Ce^4 +; these deposits formed during reductive dissolution at the oxidation–reduction transition zone or in deeper-level reducing seawater. The loss of Ce, LREE, and Ho to seawater and the deposition of these elements with iron hydroxides caused the positive Ce anomalies observed in some of the BIF samples, although the limited oxidizing ability of surface seawater at this time meant that Y/Ho and LREE/HREE ratios were not substantially modified, unlike similar situations within stratified ocean water during the Late Paleoproterozoic. Magnetite and hematite within the BIFs in the study area contain heavy Fe isotopes (⁵⁶Fe values of 0.24–1.27‰) resulting from the partial oxidation and precipitation of Fe^2 + to Fe^3 + in seawater. In addition, mass-independent fractionation of sulfur isotopes within pyrite indicates that these BIFs were deposited within an oxygen-deficient ocean associated with a similarly oxygen-deficient atmosphere, even though the BIFs within the Yuanjiacun Formation formed after initiation of the GOE.     

A mixed seawater and hydrothermal origin of superior-type banded iron formation (BIF)-hosted Kouambo iron deposit,Palaeoproterozoic Nyong series,Southwestern Cameroon: Constraints from petrography and geochemistry

The Kouambo iron deposit contains banded iron formations (BIFs) and is located in the northwestern margin of the Congo craton. The BIFs are hosted in Palaeoproterozoic Nyong series, a dominantly metasedimentary formations, which were metamorphosed into greenschist to granulite facies. The Kouambo BIFs are medium- to coarse-grained banded rocks consisting of alternation of Si-rich and Fe-rich mesobands, and belong to oxide facies iron formations. Geochemistry analyses reveal that these iron formations are composed of > 96 wt% Fe₂O₃ and SiO₂ and have low concentrations of Al₂O₃, TiO₂ and trace HFSE, suggesting chemical precipitates of silica and iron. Moreover, these BIFs have low concentrations of Al₂O₃, TiO₂ and trace HFSEs (high field strength elements, e.g., Zr, Hf, Ta, Pb and Th), suggesting that terrigenous detrital materials contributed insignificantly to the sedimentation. The Post-Archean Australian Shale (PAAS)-normalized REE-Y patterns display seawater-like profile: minor LREE depletion and HREE enrichment, positive Y anomalies. However, they display positive Eu and negative Ce anomalies, and low Y/Ho ratio (average 29), which suggest the influence of the hydrothermal fluids. The weak positive Eu/Eu*(_PAAS) ratio (average 1.5), associated with the low V (17.5 ppm), Co (6.1 ppm) and Ni (27.5 ppm) contents similar to other Superior-type BIFs worldwide, are consistent with the deposition of the Kouambo BIFs in continental marginal sea or back-arc basin environment. In summary, the Kouambo BIFs show a seawater-like REE + Y signature, however, the positive Eu anomalies and reduced Y/Ho ratios relative to seawater indicates a possible mixing with hydrothermal fluids (∼ 0.5%).     

Petrography and geochemistry of the Shilu Fe–Co–Cu ore district,South China: Implications for the origin of a Neoproterozoic BIF system

The Shilu Fe–Co–Cu ore district is situated in the western Hainan Province of south China. This district consists of the upper Fe-rich layers and the lower Co–Cu ores, which are mainly hosted within the Neoproterozoic Shilu Group, a dominantly submarine siliciclastic and carbonate sedimentary succession that generally has been metamorphosed to greenschist facies. Three facies of metamorphosed BIFs, the oxide, the silicate–oxide and the sulfide–carbonate–silicate, have been identified within the Shilu Group. The oxide banded iron formation (BIF) facies (quartz itabirites or Fe-rich ores) consists of alternating hematite-rich and quartz-rich microbands. The silicate–oxide BIF facies (amphibolitic itabirites or Fe-poor ores) comprises alternating millimeter to tens of meter scale, magnetite–hematite-rich bands with calc-silicate-rich macro- to microbands. The sulfide–carbonate–silicate BIF facies (Co–Cu ores) contain alternating cobaltiferous pyrite, cobaltiferous pyrrhotite and chalcopyrite macrobands to microbands mainly with dolomite–calcite, but also with minor sericite–quartz bands. Blasto-oolitic, pelletoidal, colloidal, psammitic, and cryptocrystalline to microcrystalline textures, and blasto-bedding structures, which likely represent primary sedimentation, are often observed in the Shilu BIF facies.The Shilu BIFs and interbedded host rocks are generally characterized by relatively low but variable ∑ REE concentrations, LREE depletion and/or MREE enrichment relative to HREE, and no Ce, Gd and Eu anomalies to strongly positive Ce, Gd and Eu anomalies in the upward-convex PAAS-normalized REY patterns, except for both the banded or impure dolostones with nil Ce anomaly to negative Ce anomalies and negative La anomalies, and the minor sulfide–carbonate–silicate BIF facies with moderately negative Eu anomalies. They also contain relatively low but variable HFSE abundances as Zr, Nb, Hf, Th and Ti, and relatively high but variable abundances of Cu, Co, Ni, Pb, As, Mn and Ba. The consistently negative ε_Nd(t) values range from − 4.8 to − 8.5, with a T_DM age of ca. 2.0 Ga. In line with the covariations between Al₂O₃ and TiO₂, Fe₂O₃ + FeO and SiO₂, Mn and Fe, Zr and Y/Ho and REE, and Sc and LREE, the geochemical and Sm–Nd isotopic features suggest that the precursors to the Shilu BIFs formed from a source dominated by seafloor-derived, high- to low temperature, acidic and reducing hydrothermal fluids but with variable input of detrital components in a seawater environment. Moreover, the involved detrital materials were sourced dominantly from an unknown, Paleoproterozoic or older crust, with lesser involvement from the Paleo- to Mesoproterozoic Baoban Group underlying the Shilu Group.The Shilu BIFs of various facies are interpreted to have formed in a shallow marine, restricted or sheltered basin near the rifted continental margin most likely associated with the break-up of Rodinia as the result of mantle superplume activity in South China. The seafloor-derived, periodically upwelling metalliferous hydrothermal plume/vent fluids under anoxic but sulfidic to anoxic but Fe^2 +-rich conditions were removed from the plume/vent and accumulated in the basin, and then variably mixed with terrigenous detrital components, which finally led to rhythmic deposition of the Shilu BIFs.     

Petrography and geochemical behaviour of trace element,REE and precious metal signatures of sulphidic banded iron formations from the Chikkasiddavanahalli area,Chitradurga schist belt,India

The ～1.2 km long and ～250 m wide Chikkasiddavanahalli (C.S. Halli) hill range consists of mixed sulphidic-oxide banded iron formations (BIFs) and Fe-rich phyllites (±carbonaceous), which overlie carbonated schistose and massive meta volcanics. In stratigraphic succession, the lithologies represent the Ingaldhal Formation and are an integral part of the Chitradurga schist belt in the Western Dharwar Craton. The general strike at C.S. Halli varies from N–S to 340° with vertical to steep dips towards east and west. The sulphides, oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist- and amphibolites- facies metamorphism. The BIFs show mesobands of recrystallised cherts and iron sulphides such as pyrite, arsenopyrite, and silicates such as subordinate grunerite, hornblende, chlorite, muscovite, actinolite and minor carbonates such as ankerite, calcite and magnesian siderite. Chemical data indicate depletion in Ti, Mn, Co, Cu, Cr and Ni in these iron formations. Most chondrite normalized REE patterns of the iron formation show moderate LREE and HREE enrichment coupled with strong positive Eu anomaly; the mineralized portions exhibit characteristic negative Ce and Eu anomalies (Eu/Eu1 0.21 to 3.00). The total REE abundance varies, correlates well with the iron contents of the BIFs, and similar to those exhibited by hydrothermal plumes [Chown, E.H., Dah, E.N., Mueller, W.G., 2000. The relation between iron formation and low temperature alteration in a Archean volcanic environment. Precambrian Research 101, 263–275]. Trace and REE data suggest that primary mantle-derived hydrothermal solutions were added to the pore fluids of sediments of the Chitradurga basin and supplied chemical constituents such as FeO, SiO₂ and REE. Oxidation of FeO to Fe₂O₃ was caused by the photosynthesis of primitive stromatolite-building cyanobacteria. Geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation, quartz-calcite vein activity and sulphidation in the mixed sulphide oxide facies BIF and associated iron phyllites in the C.S. Halli area, Western Dharwar Craton, India.     

The composition and genesis of the Mesoarchean Dagushan banded iron formation (BIF) in the Anshan area of the North China Craton

The Dagushan BIF-hosted iron deposit in the Anshan–Benxi area of the North China Craton (NCC) has two types of iron ore: quartz–magnetite BIF (Fe₂O₃^T < 57 wt.%) and high-grade iron ore (Fe₂O₃^T > 90 wt.%). Chlorite-quartz schist and amphogneiss border the iron orebodies and are locally present as interlayers with BIFs; chlorite-quartz schist and BIFs are enclosed by amphogneiss in some locations. The quartz–magnetite BIFs are enriched in HREEs (heavy rare earth elements) with positive La, Eu and Y anomalies, indicating their precipitation from marine seawater with a high-temperature hydrothermal component. Moreover, these BIFs have low concentrations of Al₂O₃, TiO₂ and HFSEs (high field strength elements, e.g., Zr, Hf and Ta), suggesting that terrigenous detrital materials contributed insignificantly to the chemical precipitation. The high-grade iron ores exhibit similar geochemical signatures to the quartz–magnetite BIFs (e.g., REE patterns and Y/Ho ratios), implying that they have identical sources of iron. However, these ores have different REE (rare earth element) contents and Eu/Eu* values, and the magnetites contained within them exhibit diverse REE contents and trace element concentrations, indicating that the ores underwent differing formation conditions, and the high-grade ores are most likely the reformed product of the original BIFs.The chlorite-quartz schist and amphogneiss are characterized by high SiO₂ and Al₂O₃ contents and exhibit variable abundances of REEs, enrichment in LREEs (light rare earth elements), negative anomalies in HFSEs (e.g., Nb, Ta, P and Ti) and positive anomalies in LILEs (large ion lithophile elements, e.g., Rb, Ba, U and K). A protolith reconstruction indicates that the protoliths of the chlorite-quartz schist are felsic volcanic rocks. SIMS and LA-ICP-MS zircon U–Pb dating indicate that this schist formed at approximately 3110 to 3101 Ma, which could represent the maximum deposition age of the Dagushan BIF. However, two groups of zircons from the amphogneiss are identified: 3104 to 3089 Ma zircons that are most likely derived from the chlorite-quartz schist and 2997 to 2995 Ma zircons, which are interpreted to represent the time of protolith crystallization. Thus, the Dagushan BIF most likely formed before 2997 to 2995 Ma. The ~ 3.1 Ga zircons yield ε_Hf(t) values of − 8.07 to 5.46, whereas the ~ 3.0 Ga zircons yield ε_Hf(t) values of − 3.96 to 2.09. These geochemical features suggest that the primitive magmas were derived from the depleted mantle with significant contributions of ancient crust.     

Petrography and geochemistry of the Mesoarchean Bikoula banded iron formation in the Ntem complex (Congo craton), Southern Cameroon: Implications for its origin

Precambrian banded iron formations (BIFs) represent an important source of mineable iron, as well as an archive recording secular changes in the chemistry of the Earth’s early oceans. Here we report petrographic and geochemical characteristics of unweathered drill core samples from the Bikoula BIF, a virtually uncharacterized oxide facies iron formation, hosted in the Mesoarchean Ntem complex, southern Cameroon. The BIF is cross-cut with syenitic veins. The entire succession is highly deformed and metamorphosed under granulite facies conditions. The BIF is characterized by alternating micro-bands of magnetite, quartz and pyroxene. Sulfides (pyrite, pyrrhotite, and chalcopyrite), oligoclase, ferro-pargasite, biotite and ilmenite occur as minor phases. The presence of pyroxene, ferro-pargasite and oligoclase, relatively high contents of major elements such as Al₂O₃ (0.76–7.52 wt.%), CaO (1.95–4.90 wt.%), MgO (3.78–5.59 wt.%), as well as positive correlations among Al₂O₃, TiO₂, HFSEs, LILEs and transition metals (V, Cr, Ni, Cu and Zn), suggest that the BIF protolith included a significant amount of clastic material. Several samples have preserved seawater-like PAAS-normalized REE-Y patterns, including LREE depletion, and positive La and Y anomalies. Positive Eu anomalies observed in some of the analyzed samples indicate influx of hydrothermal fluids (possibly including Fe and Si) within the basin where the BIF precipitated. However, few samples show unusual negative Eu anomalies that likely result from a large proportion of clastic contamination. The lack of Ce anomalies suggests that the Bikoula BIF was deposited in a basin that was (at least partly) anoxic or suboxic, where it was possible to transport and concentrate dissolved Fe²⁺.     

Extreme positive Ce-anomalies in a 3.0 Ga submarine volcanic sequence,Murchison Province: Oxygenated marine bottom waters

Systematic positive anomalies of Ce, where Ce/Ce* spans 2.1 to 11.4, are present in basalts and rhyolites of a 2.96 Ga submarine volcanic sequence of the Murchison Province, Western Australia. This volcanic sequence is host to a stratabound Cu–Zn deposit that formed on the seafloor from a seawater hydrothermal system. These are true Ce anomalies as Pr/Pr* < 1. In modern oxygenated marine water Ce is sequestered by Mn-oxides and hydroxides, which coprecipitate with Fe-oxides and hydroxides as nodules and crusts on the ocean floor, as well as Fe–Mn chemical sediments from hydrothermal systems at ocean spreading centers. Fe–Mn sediments have positive Ce anomalies and marine water complementary negative anomalies. Such Ce anomalies have not formerly been reported for Archean hydrothermally altered volcanic rocks. These extreme anomalies are attributed to Mn-transport in shallow-circulating oxygenated marine bottom waters peripheral to the deeper, hotter, hydrothermal system from which the Cu–Zn deposit formed, and record an oxygenated marine environment ~ 500 Ma before the so-called great oxidation event at ~ 2.4 Ga. Results for positive Ce anomalies in the Golden Grove volcanic sequence are complementary to negative anomalies in Archean BIF, collectively stemming from particulate scavenging of Ce^+ 3 in an oxic water column.     

Precambrian iron formations from the Cauvery Suture Zone,Southern India: Implications for sub-marine hydrothermal origin in Neoarchean and Neoproterozoic convergent margin settings

Thick horizons of iron formations including Banded Iron Formations (BIFs) and Banded Silicate Formations (BSFs) occur as E–W trending bands in the eastern part of Cauvery Suture Zone (CSZ) in the Sothern Granulite Terrane of India. Some of these occur in close association with the Neoarchean-Neoproterozoic suprasubduction zone complexes, where as some others are associated with metamorphosed accretionary sequences including pyroxene granulites and other high grade rocks. The iron formations are highly deformed and metamorphosed under amphibolite to granulite facies conditions and are composed of quartz–magnetite–hematite–goethite–garnet–pyrite together with grunerite and pyroxene. Here we report the geochemical characteristics of twenty representative samples from the iron formations that reveal a widely varying composition with Fe₂O₃^(t) (22–65 wt.% as total iron) total- Fe₂O₃/TiO₂ (205–6532), MnO/TiO₂ (0.25–12.66) and SiO₂ (33–85 wt.%), broadly representing the two types of iron formations. These formations also show very low Al/(Al + Fe + Mn) ratio (0.001–0.01), Al₂O₃ (0.07–0.76 wt.%), Al₂O₃/TiO₂ ratio (2.7–21), MgO (0.01–4.41 wt.%), CaO (0.1–1.24 wt.%), Na₂O (0.01–0.05 wt.%) and K₂O (0.01 wt.%) together with low total REE (3.38–31.63 ppm). The trace and REE elemental distributions show wide variation with high Ni (274 ppm), and Zn contents (up to 87 ppm) when compared to mafic volcanics of the adjoining areas. Tectonic discrimination plots indicate that the iron formations of the Cauvery Suture Zone are of hydrothermal origin. Their chondrite normalized patterns show slight positive Eu anomaly (Eu/Eu* = up to 1.77) and relatively less fractionation of REE with slight LREE enrichment compared to HREE. However, the PAAS (Post Archean Average of Australian Sediments) normalized REE patterns display significant positive Eu anomaly (Eu/Eu* up to 2.32) with well represented negative Ce anomalies (Ce/Ce* = 0.66–1.28). The above results together with petrological characteristics and available geochronology of the associated lithologies suggest that the iron formations can be correlated to Algoma-type. The Fe and Si were largely supplied by medium to high temperature sub-marine hydrothermal systems in Neoarchean and Neoproterozoic convergent margin settings.     

冀东、五台和吕梁地区条带状铁矿的稀土元素特征及其地质意义

详细报道了冀东、五台和吕梁地区条带状铁矿全岩样品的稀土元素分析结果。结果表明,研究区BIF具有非常相似的特征：稀土总量均较低;经页岩标准化的稀土元素配分模式均呈现轻稀土亏损、重稀土富集的特征;Y/Ho比值较高;具有明显的Eu、Y、La的正异常,且这些特征表明研究区BIF的稀土元素来源于火山热液和海水的混合溶液。虽然BIF均显示Eu正异常,但不同类型、不同沉积年龄BIF的铕异常程度不同：与吕梁地区Superior型铁矿相比,冀东和五台地区的Algoma型铁矿显示了更大的Eu正异常;并且自中太古代-新太古代-古元古代,BIF的铕正异常逐渐减小,这可能反映了随着BIF沉积年龄的减小,进入到该地区海水中的高温热液流体逐渐减少;同时,研究区BIF缺乏明显的Ce负异常,可能暗示在BIF沉积时海水的氧化还原状态为缺氧环境。     

U–Pb zircon chronology,geochemistry and isotopes of the Changyi banded iron formation in the eastern Shandong Province: Constraints on BIF genesis and implications for Paleoproterozoic tectonic evolution of the North China Craton

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. The BIF shows alternating quartz-rich light and magnetite-rich dark bands with magnetite (15–65 vol.%), quartz (25–65 vol.%) and amphibole (15–30 vol.%) constituting the major minerals. Minor garnet, epidote, chlorite, calcite, biotite and pyrite occur locally. The BIF bands are interlayered with amphibolite, hornblende gneiss, biotite quartz schist, garnet biotite schist, biotite gneiss and leptynite, and are intruded by granites. LA-ICP-MS U–Pb dating on zircons separated from the BIF bands and the wallrocks constrains the depositional age as 2240–2193 Ma and metamorphic age as ~ 1864 Ma. The dominant composition of SiO₂ + Fe₂O₃^T (average value of 92.3 wt.%) of the BIF bands suggests their formation mainly through chemical precipitation. However, the widely varying contents of major elements such as Al₂O₃ (0.58–6.99 wt.%), MgO (1.00–3.86 wt.%), CaO (0.22–4.19 wt.%) and trace elements such as Rb (2.06–40.4 ppm), Sr (9.36–42.5 ppm), Zr (0.91–23.6 ppm), Hf (0.04–0.75 ppm), Cr (89.1–341 ppm), Co (2.94–30.4 ppm), and Ni (1.43–52.0 ppm) clearly indicate the incorporation of clastics, especially continental felsic clastics, as also confirmed by the presence of ancient detrital zircons in the BIF bands. When normalized against Post Archean Average Shale (PAAS), the seawater-like signatures of REE distribution patterns, such as LREE depletion, positive La and Y anomalies, and superchondritic Y/Ho ratios (average value of 36.3), support the deposition in seawater. Strong positive Eu anomalies (Eu/Eu*_PAAS = 1.14–2.86) also suggest the participation of hydrothermal fluids. In addition, the sympathetic correlation between Cr, Co and Ni as well as the Co + Ni + Cu vs. ∑ REE and the Al₂O₃ vs. SiO₂ relations further indicates that the iron and silica mainly originated from hydrothermal fluids. Combined with regional geological investigation and protolith restoration of the wallrocks, a continental rift environment is suggested for the Changyi BIF deposition. The appearance of negative Ce_PAAS anomalies might suggest the influence of the Great Oxidation Event at the time of deposition. The Changyi BIF witnessed the major Paleoproterozoic rifting–collision events in the NCC and their unique distribution in the NCC contrasts with other examples elsewhere in the world.     

Structural,textural, geochemical and isotopic signatures of synglaciogenic Neoproterozoic banded iron formations (BIFs) at Bafq mining district (BMD), Central Iran: The possible Ediacaran missing link of BIFs in Tethyan metallogeny

The Neoproterozoic magnetite–apatite–hematite–pyrolusite–jaspilite deposits in the Bafq mining district (BMD) contain more than 1.7 Gt ores with an average grade of 50 wt.% Fe and 0.01 to 7.78 wt.% P and were probably formed between 635 and 547 Ma in a riftogenic felsic submarine exhalative sequence of the Esfordi Formation. The ore zones occur in proximal zone of magnetite-rich albitized rhyolite (keratophyres), interdistal zone of rhyolitic tuff–tuffaceous sediments and distal zone of pyrolusite–jaspilite. These sequences are covered by the diamictites and cap carbonates. The BIFs are linked to the altered rhyolites–rhyodacites, jaspilites and diamictites and contain magnetite, hematite and apatite. The presence of Spriggina, Dickinsonia, Medusites and Persimedusites chahgazensis (Sennewald and Krüger, 1979; Hahn and, Pflug; McCall, 2006) in the Kushk shale member of the Esfordi Formation conforms to the Australian fauna of the Ediacaran period (635–540). This relative age is supported by some reliable Pb isotopic data (635–547 Ma) on galena, monazite and apatite (Huckriede et al., 1962; Torab, 2008; Stosch et al., 2011). The most frequent structures–textures in the ore zones include felsic autobrecciation, massive, colloidal, banded, detrital and glaciogenic. The BIFs are highlighted by high values of LREE fractionation and no significant Eu and Ce anomalies. The ores show high values of Fe₂O₃ (14–60%), and SiO₂ (4–34%), and low contents of Al (3.32%), Cr (21.48 ppm), Co (42.82 ppm), Ni (125 ppm), V (868 ppm), and Ti (0.13%) similar to those of the Ediacaran–Rapitan BIFs. The cap carbonates show depletion in δ¹³C, with a range from − 0.43 to − 6.6 per mil and then return to near excursion of about + 2.97‰ in the Lower Cambrian carbonates. These are followed by δ¹⁸O values, which range from − 6.64 to − 11.86‰. The presence of jaspilites, diamictites, C and O isotopic signatures, REE patterns, and immobile element relationships highlights a glaciogenic BIF. Importantly, the glaciogenic structures–textures and jaspilites do not support the misconception of the previously proposed magmatic–carbonatitic and metasomatic–hydrothermal IOCG–Kiruna ore deposits.     

Precambrian banded iron formations in the North China Craton: Silicon and oxygen isotopes and genetic implications

Banded iron formations (BIFs) are Precambrian chemical marine sedimentary formations that record major transitions of chemical composition, and oxidation–reduction state of oceans at the time of their deposition. In this paper, we report silicon and oxygen isotope compositions of a variety of BIFs from the North China Craton (NCC) in order to deduce the mechanism of their formation. Quartz in the various types of BIFs from the NCC are generally depleted in ³⁰Si, where δ³⁰Si_NBS-28 values range from − 2.0‰ to − 0.3‰ (average, − 0.8‰), similar to δ³⁰Si_NBS-28 values measured from modern submarine black chimneys and sinters. The δ¹⁸O_V-SMOW values of quartz in the BIFs are relatively high (8.1‰–21.5‰; average, 13.1‰), similar to those of siliceous rock formed by hydrothermal activities. The δ³⁰Si_NBS-28 values of quartz in magnetite bands are commonly lower than those of quartz in adjacent siliceous bands within the same sample, whereas δ¹⁸O_V-SMOW values are higher in the magnetite bands. A negative correlation is observed between δ³⁰Si_NBS-28 and δ¹⁸O_V-SMOW values of quartz from siliceous and magnetite bands in BIF from Fuping, Hebei Province. The isotopic compositions of silicon and oxygen of quartz in BIFs provide insights for the formation mechanisms of silicon–iron cyclothems in BIFs. After the silicon- and iron-rich hydrothermal solution was injected onto the seabed, the abrupt temperature drop caused oversaturation of silicic acid, resulting in rapid precipitation of SiO₂ and deposition of siliceous layers. Ferric hydroxide was precipitated later than SiO₂ because of low free-oxygen concentration in the ocean bottom. Progressive mixing of hydrothermal solution with seawater caused a continuous drop in temperature and an increase in Eh values, resulting in gradual oxidation of hydrothermal Fe^2 + and deposition of iron-rich layers. In summary, each silicon–iron cyclothem marks a large-scale submarine hydrothermal exhalation. The periodic nature of these exhalations resulted in the formation of regular silicon–iron cyclothems. The widespread distribution of BIFs indicates that volcanism and submarine hydrothermal exhalation were extensive; the low δ³⁰Si_NBS-28 and high δ¹⁸O _V-SMOW values of the BIFs indicate that the temperature of seawater was relatively high at the time of BIF formation, and that concentrations of Fe^2 + and H₄SiO₄ in seawater were saturated.     

Source characteristics of the ∼2.5 Ga Wangjiazhuang Banded Iron Formation from the Wutai greenstone belt in the North China Craton: Evidence from neodymium isotopes

Here we first present samarium (Sm)–neodymium (Nd) isotopic data for the ∼2.5 Ga Wangjiazhuang BIF and associated lithologies from the Wutai greenstone belt (WGB) in the North China Craton. Previous geochemical data of the BIF indicate that there are three decoupled end members controlling REE compositions: high-T hydrothermal fluids, ambient seawater and terrigenous contaminants. Clastic meta-sediment samples were collected for major and trace elements studies in an attempt to well constrain the nature of detrital components of the BIF. Fractionated light rare earth elements patterns and mild negative Eu anomalies in the majority of these meta-sedimentary samples point toward felsic source rocks. Moreover, the relatively low Th/Sc ratios and positive ε_Nd(t) values are similar to those of the ∼2.5 Ga granitoids, TTG gneisses and felsic volcanics in the WGB, further indicating that they are derived from less differentiated terranes. Low Chemical Index of Weathering (CIW) values and features in the A-CN-K diagrams for these meta-sediments imply a low degree of source weathering. Sm–Nd isotopes of the chemically pure BIF samples are characterized by negative ε_Nd(t) values, whereas Al-rich BIF samples possess consistently positive ε_Nd(t) features. Significantly, the associated supracrustal rocks in the study area have positive ε_Nd(t) values. Taken together, these isotopic data also point to three REE sources controlling the back-arc basin depositional environment of the BIF, the first being seafloor-vented hydrothermal fluids (ε_Nd(t) < −2.5) derived from interaction with the underlying old continental crust, the second being ambient seawater which reached its composition by erosion of parts of the depleted landmass (likely the arc) (ε_Nd(t) > 0), the third being syndepositional detritus that received their features by weathering of a nearby depleted source (likely the arc) (ε_Nd(t) > 0).     

Genesis of the Jinding Zn-Pb deposit,northwest Yunnan Province,China: Constraints from rare earth elements and noble gas isotopes

The giant sediment-hosted Jinding zinc-lead deposit is located in the Lanping Basin, northwestern Yunnan Province, China. The genesis of the deposit has long been debated and the sources of the ore-forming fluids and metals are controversial. This study presents rare earth element (REE) and noble gas isotope data that constrain the origins of the ore fluids and the heat source driving the hydrothermal circulation. The early-stage sulfides are enriched in light REEs and have high ∑REE values (30.8–94.8 ppm) and weakly negative Eu (δEu 0.85–0.89) and Ce anomalies (δCe 0.84–0.95), suggesting that the fluids were likely derived from dissolution of Upper Triassic marine carbonates with input of REEs from aluminosilicate rocks in the basin. In contrast, the late-stage sulfides have irregular REE patterns, generally low ∑REE values (0.24–10.8 ppm) and positive Eu (δEu 1.22–10.9) and weakly negative Ce anomalies (δCe 0.53–0.90), which suggest that the ore-forming fluids interacted with evaporite minerals. The ³He/⁴He (0.01–0.04 R_a) and ⁴⁰Ar/³⁶Ar values (301–340) of the ore-forming fluids indicate crustal and atmospheric origins for these noble gases. These findings are in agreement with the published fluid inclusion microthermometry data and the results of H, O, C, S, Pb and Sr isotope studies. Our data, in combination with published results, support a two-stage hydrothermal mineralization model, involving early-stage basinal brines and late-stage meteoric water that acquired metals and heat from crustal sources.     

The possible synglaciogenic Ediacaran hematitic banded iron salt formation (BISF) at Hormuz Island,southern Iran: Implications for a new style of exhalative hydrothermal iron-salt system

The Ediacaran BISF at Hormuz Island is a newly identified glaciogenic iron-salt deposit in the Tethyan margin of Gondwana. The BISF was formed by synchronous riftogenic A-type submarine felsic volcanism and evaporate deposition. The mineralization occurs in a proximal felsic tuff cone and jaspilitic distal zones and contains 1 million tonne of hematite-rich ore with an average grade of 58% Fe. The ore structure shows cyclicity of macrobandings, mesobandings and microbandings of anhydrite, halite, hematite and chert, which marks a new record in BIFs geohistory. The alteration minerals in the proximal and distal zones are actinolite, ripidolite, epidote, sericite, tourmaline, clinochlore, anhydrite and clay minerals. The occurrence of metamorphosed polygenetic bullet-shape dropstones in BISF attests that there was probably a continuous process of ice melting, episodic submarine volcanism and exhalative hydrothermal banded iron salt formation during the Late Ediacaran time. The non-metamorphosed Neoproterozoic stratigraphy, the presence of genus Collenia, U-Pb dating (558 ± 7 Ma) and the marked negative δ¹³C excursion in cap carbonates are representative of Late Ediacaran glaciation, which has been identified worldwide. The REE+Y display light REE enrichment, unusually strong Tb-Tm anomaly, a weak positive Y anomaly, but no distinguished Eu and Ce anomalies, reflecting the glaciogenic nature of the BISF. The contents of Zr, Hf, Nb, Ta, Th, La, Ce and Y in BISF, dropstones, halite and cap carbonates are similar to those of the Neoproterozoic glaciogenic BIFs. Also, the Ni/Fe, P/Fe ratios and Fe/Ti – Al/Al + Fe + Mn + Ca + Na + K diagram suggest an exhalative hydrothermal Ediacaran-type BISF. The absence of brecciated magnetite in the ore association and the low contents of copper (9–493 ppm) and gold (<5–8 ppb) are not in favor of the IOCG – Kiruna-type iron oxide ores. The co-paragenesis of hematite with several alteration minerals, in particular actinolite, tourmaline and anhydrite, indicates that the exhalative hydrothermal fluids were generated by the interaction of seawater with the felsic rocks and sediments at about 200–500 °C. The interaction of seawater with felsic magma and sediments led to the formation of Mg-rich alteration minerals, leaching Si, Fe, Mn and other elements and forming the potential ore fluids. It is highlighted that the A-type alkaline submarine felsic volcanism could be considered as an exploration target for BISF.     

Petrology and Geochemistry of the Banded Iron Formation of the Kuluketage Block,Xinjiang, NW China: Implication for BIF Depositional Setting

The Kuluketage block, located in the northeast Tarim craton, is one of the largest Precambrian blocks in the Xinjiang province. Recently, many banded iron formation (BIF)‐type (Superior‐type) deposits have been discovered in the western part of the Kuluketage block. These deposits occurred in the Paleoproterozoic Shayiti Formation, Xingditage Group, which has a nearly E–W distribution in the southern Xinger and Xingdi faults. Tremolite biotite schist and quartzite are the main wall rocks. The geochemical characteristics of schist indicate that the BIFs occurred in a passive continental margin environment. The LA–ICP–MS zircon ²⁰⁶Pb/²³⁸U ages of BIF and late syenite are 1945 ± 10 Ma(MSWD = 0.77) (weighted average age) and 1974 ± 27 Ma(MSWD = 1.05) (upper intercept age), respectively, indicating that the BIFs occurred in the Paleoproterozoic. In addition, the approximately 1.9 Ga magmatic and metamorphic events are consistent with the global‐scale 2.1–1.8 Ga collisional orogen events which are associated with the assembly of the Columbia supercontinent. The geochemical characteristics show that magnetite and quartz are dominant components (total content, 91.65–98.22 wt.%), and the Zr(Nb) and TiO₂, Zr(Nb) and Al₂O₃ and Zr and Y/Ho display strongly positive correlations, illustrating the addition of crustal materials into the chemical precipitate of the original BIFs. The higher Zr, Nb and Al₂O₃ contents and a lower Y/Ho ratio of the Kuluketage BIFs indicate a higher terrigenous detrital component contaminant compared to BIFs of North China Craton (NCC). The rare earth and yttrium (REY) distribution patterns show a slight LREE enrichment and weak Eu positive anomaly features, indicating that the source of Fe and Si of the Kuluketage BIFs is mainly from the contribution of low‐temperature hydrothermal alteration of the oceanic crust. In addition, along with the decreasing BIF depositional age, the declining of Eu anomaly values reflects the increasing importance of low‐temperature hydrothermal solutions relative to high‐temperature hydrothermal solutions. Moreover, no Ce anomalies in studied BIFs, NCC and Xinyu BIFs are attributed to relative reducing environmental condition when the original BIFs precipitated.     

REE fractionation,mineral speciation,and supergene enrichment of the Bear Lodge carbonatites,Wyoming, USA

The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing δ¹⁸O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO₄³⁻/CO₃²⁻ and F⁻/CO₃²⁻ ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions.