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.     

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.     

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.     

Petrology and geochemistry of the banded iron-formations from Ntem complex greenstones belt,Elom area,Southern Cameroon: Implications for the origin and depositional environment

Banded iron-formations (BIFs) form an important part of the Archaean to Proterozoic greenstone belts in the Southern Cameroon. In this study, major, trace and REE chemistry of the banded iron-formation are utilized to explore the source of metals and to constraint the origin and depositional environment of these BIFs. The studied BIF belongs to the oxide facies iron formations composed mainly of iron oxide (mainly magnetite) mesobands alternating with quartz mesobands. The mineralogy of the BIF sample consists of magnetite and quartz with lesser amount of secondary martite, goethite and trace of gibbsite and smectite. The major element chemistry of these iron-formations is remarkably simple with the main constituents being SiO₂ and Fe₂O₃ which constitute 95.6–99.5% of the bulk rock. Low Al₂O₃, TiO₂, and HFSE concentrations show that they are relatively detritus-free chemical sediments. The Pearson’s correlation matrix of major element reveals that there is a strong positive correlation (r = 0.99) of Al with Ti and no to weak negative correlation of Ti with Mn, Ca and weak positive correlation of Si with Ca, suggesting the null to very minor contribution of detrital material to chemical sediment. The trace elements with minor enrichments are transition metals such as Zn, Cr, Sr, V and Pb. This is an indicator of direct volcanogenic hydrothermal input in chemical precipitates. The studied BIF have a low ΣREE content, ranging between 0.41 and 3.22 ppm with an average of 0.87 ppm, similar to that of pure chemical sediments. The shale-normalized patterns show depletion in light REE, slightly enrichment in heavy REE and exhibit weak positive europium anomalies. These geochemical characteristics indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The absence of a large positive Eu anomaly in the studied BIF indicates an important role of low-temperature hydrothermal solutions. The chondrite-normalized REE patterns are characterized by LREE-enriched (Mean La_CN/Yb_CN = 8.01) and HREE depletion (Mean Tb_CN/Yb_CN = 1.61) patterns and show positive Ce anomalies. With the exception of one sample (LBR133), all of the BIF samples analyzed during this study have positive Ce anomalies on both chondrite- and PASS-normalized plots. This may indicate that the BIFs within the Elom area were formed within a redox stratified ocean. The positive Ce anomalies in the studied samples likely suggest that the basin in which Fe formations were deposited was reducing with respect to Ce, probably in the suboxic or anoxic seawaters.     

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.     

Petrology and geochemistry of REE-rich Mafé banded iron formations (Bafia group,Cameroon)

Archaean–Paleoproterozoic foliated amphibole-gneisses and migmatites interstratified with amphibolites, pyroxeno-amphibolites and REE-rich banded-iron formations outcrop at Mafé, Ndikinimeki area. The foliation is nearly vertical due to tight folds. Flat-lying quartz-rich mica schists and quartzites, likely of Pan-African age, partly cover the formations. Among the Mafé BIFs, the oxide BIF facies shows white layers of quartz and black layers of magnetite and accessory hematite, whereas the silicate BIF facies is made up of thin discontinuous quartz layers alternating with larger garnet (almandine–spessartine) + chamosite + ilmenite ± Fe-talc layers. REE-rich oxide BIFs compositions are close to the East Pacific Rise (EPR) hydrothermal deposit; silicate BIFs plot midway between EPR and the associated amphibolite, accounting for a contamination by volcanic materials, in addition to the hydrothermal influence during their oceanic deposition. The association of an oceanic setting with alkaline and tholeiitic magmatism is typical of the Algoma-type BIF deposit. The REE-rich BIFs indices recorded at Mafé are interpreted as resulting from an Archaean–Paleoproterozoic mineralization.     

Heterogeneous bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure

The nucleation of H₂O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 800 °C and either 50 or 125 MPa, and then decompressed by 20–91 MPa at temperatures that ranged between 550 and 700 °C, and held at the lower pressures for 10–720 s before being quenched. Bubbles nucleated in number densities (N_B) that vary between 3 × 10⁷ and 2 × 10⁸ cm^− 3. Blocky shaped magnetite or the ends of needle-shaped hematite acted as sites for nucleation, but only if a minimum super-saturation was exceeded, which increases with increasing melt viscosity. Bubbles did not nucleate along the lengths of hematite needles nor on plagioclase. Both the beginning and ending times of the nucleation event increases with increasing melt viscosity. Using nucleation theory predictions, neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature, water diffusivity, or viscosity. Instead, the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor, resulting from the increasing polymerization of the melt at lower temperatures and water contents. Because only ∼ 10⁸ bubbles cm^− 3 nucleate on magnetite in the rhyolite melt used, almost regardless of experimental conditions, results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions.     

Nature of ore-forming fluid and formation conditions of BIF-hosted gold mineralization in the Archean Amalia Greenstone Belt,South Africa: Constraints from fluid inclusion and stable isotope studies

Orogenic gold mineralization in the Amalia greenstone belt is hosted by oxide facies banded iron-formation (BIF). Hydrothermal alteration of the BIF layers is characterized by chloritization, carbonatization, hematization and pyritization, and quartz-carbonate veins that cut across the layers. The alteration mineral assemblages consist of ankerite-ferroan dolomite minerals, siderite, chlorite, hematite, pyrite and subordinate amounts of arsenopyrite and chalcopyrite. Information on the physico-chemical properties of the ore-forming fluids and ambient conditions that promoted gold mineralization at Amalia were deduced from sulfur, oxygen and carbon isotopic ratios, and fluid inclusions from quartz-carbonate samples associated with the gold mineralization.Microthermometric and laser Raman analyses indicated that the ore-forming fluid was composed of low salinity H₂O-CO₂ composition (~3 wt% NaCl equiv.). The combination of microthermometric data and arsenopyrite-pyrite geothermometry suggest that quartz-carbonate vein formation, gold mineralization and associated alteration of the proximal BIF wall rock occurred at temperature-pressure conditions of 300 ± 30 °C and ∼2 kbar. Thermodynamic calculations at 300 °C suggest an increase in fO₂ (10⁻³²–10⁻³⁰ bars) and corresponding decrease in total sulfur concentration (0.002–0.001 m) that overlapped the pyrite-hematite-magnetite boundary during gold mineralization. Although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site, the calculated low fO₂ values are consistent with previously determined high Fe/Fe + Mg ratios (>0.7) in associated chlorite, absence of sulfates and restricted positive δ³⁴S values in associated pyrite. Based on the fluid composition, metal association and physico-chemical conditions reported in the current study, it is confirmed that gold in the Amalia fluid was transported as reduced bisulfide complexes (e.g., Au(HS)₂⁻). At Amalia, gold deposition was most likely a combined effect of increase in fO₂ corresponding to the magnetite-hematite buffer, and reduction in total sulfur contents due to sulfide precipitation during progressive fluid-rock interaction.The epigenetic features coupled with the isotopic compositions of the ore-forming fluid (δ³⁴S_ΣS = +1.8 to +2.3‰, δ¹⁸O_H2O = +6.6 to +7.9‰, and δ¹³C_ΣC = −6.0 to −7.7‰ at 300–330 °C) are consistent with an externally deep-sourced fluid of igneous signature or/and prograde metamorphism of mantle-derived rocks.     

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.     

Detrital zircon ages and geochronological constraints on the Neoproterozoic Puga diamictites and associated BIFs in the southern Paraguay Belt,Brazil

The Paraguay belt comprises a thick sedimentary succession deposited on the southwestern border of the Amazonian Craton and the Rio Apa Block. The base of the succession in the southern Paraguay belt is marked by a level of glacially derived deposits from the Puga Formation associated with banded iron formations, which has been assumed to be end-Cryogenian in age (635 Ma) by previous authors is spite of the lack of geochronological data. Here we present the first U–Pb SHRIMP ages on detrital zircon grains separated from the matrix of six samples of these diamictites two different localities (Puga Hill and Bodoquena area). U–Pb ages determined from two samples (ca. 130 grains) of Puga Hill show a large variation between 970 Ma and 2100 Ma. Rocks with these ages can be found in the Amazonian Craton suggesting that it is the most probable source of the sediments. Detrital zircons (ca. 230 grains) from the Bodoquena area (about 200 km south of Puga Hill) range from 706 to 1990 Ma. The 1760 Ma source is significantly more important in these samples, comprising more than 70% of analyzed grains, and indicates provenance from the adjacent Rio Apa Block. The youngest zircon was dated at 706 ± 9 Ma, thus constraining the maximum depositional age for the Puga Formation. Possible sources for this younger population could be either the juvenile Mara Rosa magmatic arc in the Brasilia belt, or the rocks from the Laurentian external fold belts located to the west of the sampled area in Neoproterozoic paleogeographic reconstructions. The maximum depositional age of the diamictites (and associated BIFs), together with cap carbonate carbon and strontium isotope data (δ¹³C = ? 5.0 and ⁸⁷Sr/⁸⁶Sr = 0.7077) in Puga Hill, indicate that they were deposited after 700 Ma, suggesting that they may represent the end-Cryogenian event.     

Algoma-type banded iron formation (BIF), Abitibi Greenstone belt,Quebec, Canada

A typical Algoma-type banded iron formation (BIF) occurs in Orvilliers, Montgolfier, and Aloigny townships in the Abitibi Greenstone belt, Quebec, Canada. The BIF is composed of millimeter to decimeter thick beds of alternating fine-grained, dark gray to black, well laminated, magnetite-rich (and/or hematite) beds and quartz–feldspar metasedimentary (graywacke) beds. The BIF is well defined by magnetic anomalies. These BIF layers are commonly associated with decimeter to meter thick horizons of metasedimentary rocks and mafic to intermediate volcanic rocks, which are locally crosscut by dikes of felsic or mafic intrusive rocks and, as well, narrow dikes of lamprophyre. The upper and lower contacts of the BIF are gradational with the adjacent graywacke. All geological units in the area are metamorphosed to the greenschist facies of regional metamorphism. Magnetite is mainly associated with subordinate amounts of hematite, quartz, Na-rich plagioclase, and muscovite. The fine-grained magnetite content is composed of 77% to 89% of the principal iron oxide minerals present. The magnetite occurs as disseminated idiomorphic to sub-idiomorphic small crystals, which average 20 μm ± 5 μm in size. Hematite is the second most abundant iron oxide mineral. Although less abundant, red jasper occurs in cherty horizons with strongly folded fragments and within fault zones. This particular Algoma-type iron formation stratigraphically extends more than 36 km along strike. It dips sub-vertically with a true width from 120 m to 600 m. The origin of the BIF is closely linked to regionally extensive submarine hydrothermal activity associated with the emplacement of volcanic and related subvolcanic rocks in an Archean greenstone belt.     

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⁺.     

Fluid types and their genetic meaning for the BIF-hosted iron ores,Krivoy Rog,Ukraine

This paper contributes to the understanding of the genesis of epigenetic, hypogene BIF-hosted iron deposits situated in the eastern part of Ukrainian Shield. It presents new data from the Krivoy Rog iron mining district (Skelevatske–Magnetitove deposit, Frunze underground mine and Balka Severnaya Krasnaya outcrop) and focuses on the investigation of ore genesis through application of fluid inclusion petrography, microthermometry, Raman spectroscopy and baro-acoustic decrepitation of fluid inclusions. The study investigates inclusions preserved in quartz and magnetite associated with the low-grade iron ores (31–37% Fe) and iron-rich quartzites (38–45% Fe) of the Saksaganskaya Suite, as well as magnetite from the locally named high-grade iron ores (52–56% Fe). These high-grade ores resulted from alteration of iron quartzites in the Saksaganskiy thrust footwall (Saksaganskiy tectonic block) and were a precursor to supergene martite, high-grade ores (60–70% Fe). Based on the new data two stages of iron ore formation (metamorphic and metasomatic) are proposed.The metamorphic stage, resulting in formation of quartz veins within the low-grade iron ore and iron-rich quartzites, involved fluids of four different compositions: CO₂-rich, H₂O, H₂O–CO₂(± N₂–CH₄)–NaCl(± NaHCO₃) and H₂O–CO₂(± N₂–CH₄)–NaCl. The salinities of these fluids were relatively low (up to 7 mass% NaCl equiv.) as these fluids were derived from dehydration and decarbonation of the BIF rocks, however the origin of the nahcolite (NaHCO₃) remains unresolved. The minimum P–T conditions for the formation of these veins, inferred from microthermometry are T_min = 219–246 °C and P_min = 130–158 MPa. The baro-acoustic decrepitation analyses of magnetite bands indicated that the low-grade iron ore from the Skelevatske–Magnetitove deposit was metamorphosed at T = ~ 530 °C.The metasomatic stage post-dated and partially overlapped the metamorphic stage and led to the upgrade of iron quartzites to the high-grade iron ores. The genesis of these ores, which are located in the Saksaganskiy tectonic block (Saksaganskiy ore field), and the factors controlling iron ore-forming processes are highly controversial. According to the study of quartz-hosted fluid inclusions from the thrust zone the metasomatic stage involved at least three different episodes of the fluid flow, simultaneous with thrusting and deformation. During the 1^st episode three types of fluids were introduced: CO₂–CH₄–N₂(± C), CO₂(± N₂–CH₄) and low salinity H₂O–N₂–CH₄–NaCl (6.38–7.1 mass% NaCl equiv.). The 2^nd episode included expulsion of the aqueous fluids H₂O–N₂–CH₄–NaCl(± CO₂, ± C) of moderate salinities (15.22–16.76 mass% NaCl equiv.), whereas the 3^rd event involved high salinity fluids H₂O–NaCl(± C) (20–35 mass% NaCl equiv.). The fluids most probably interacted with country rocks (e.g. schists) supplying them with CH₄ and N₂. The high salinity fluids were most likely either magmatic–hydrothermal fluids derived from the Saksaganskiy igneous body or heated basinal brines, and they may have caused pervasive leaching of Fe from metavolcanic and/or the BIF rocks. The baro-acoustic decrepitation analyses of magnetite comprising the high-grade iron ore showed formation T = ~ 430–500 °C. The fluid inclusion data suggest that the upgrade to high-grade Fe ores might be a result of the Krivoy Rog BIF alteration by multiple flows of structurally controlled, metamorphic and magmatic–hydrothermal fluids or heated basinal brines.     

New paleomagnetic and paleointensity data from Pliocene lava flows from the Lesser Caucasus

A paleomagnetic, rock-magnetic and paleointensity study has been carried out on 14 basaltic lava flows from two Pliocene (K–Ar age between 3.09 ± 0.10 Ma and 4.00 ± 0.15 Ma) sequences (Apnia and Korxi) from the eastern Djhavakheti Highland in southern Georgia (Caucasus).Measurement of strong-field magnetisation versus temperature curves yielded three types of thermomagnetic curves: (i) Reversible curves with magnetite as only remanence carrier (type H); (ii) irreversible curves with magnetite as only carrier of remanence (type H⁻) and (iii) irreversible curves showing a low Curie-temperature phase and magnetite (type L). Analysis of hysteresis curves showed that samples were characterised by a mixture of single-domain and multi-domain grains.Paleomagnetic experiments allowed determining characteristic components for all flows and normal polarities (6 flows), reversed polarities (7 flows) and intermediate polarities (1 flow) were observed.. Paleomagnetic poles were calculated using only those sites unequivocally showing normal or reversed polarities. The paleomagnetic pole obtained from flows of both combined sequences (latitude λ = 77.9°N, longitude ϕ = 152.1°E, n = 13, A₉₅ = 11.8°, k = 13.4) showed a good agreement with the 5 Ma window of the European synthetic apparent polar wander path of Besse and Courtillot (2002). The paleomagnetic direction of the combined Apnia-Korxi flows agrees well with the expected one, showing no significant tectonic rotation. The latter cannot be however, completely excluded in the Korxi section. In that section, analysis of the angular dispersion of virtual geomagnetic poles yields a much higher value than expected.Paleointensity experiments using the Coe method were performed on 31 specimens from 10 flows. After application of specific selection criteria, 19 samples from 8 flows were observed to provide successful determinations, with mean flow values showing a wide scatter. If only flows with more than one successful paleointensity determination are taken into account, virtual dipole moments (VDMs) vary between 3.5 × 10²² A m² and 8.3 × 10²² A m². In intermediate polarity site AP2 no weak transitional paleostrength values were observed.     

Origin of a barite-sulfide ore deposit in the Mykonos intrusion,cyclades: Trace element,isotopic, fluid inclusion and raman spectroscopy evidence

The polymetallic Mykonos vein system in the Cyclades, Greece, consists of 15 tension-gashes filled with barite, quartz, pyrite, sphalerite, chalcopyrite and galena in ca. 13.5 Ma, I-type, Mykonos monzogranite. Zones of silica and chlorite–muscovite alteration are associated with the veins and overprint pervasive silicification, phyllic and argillic alteration that affected large parts of the monzogranite. The mineralization cements breccias and consists of an early barite–silica–pyrite–sphalerite–chalcopyrite assemblage followed by later argentiferous galena. A combination of fluid inclusion and stable isotope data suggests that the barite and associated mineralization were deposited from fluids containing 2 to 17 wt.% NaCl equivalent, at temperatures of ~ 225° to 370 °C, under a hydrostatic pressure of ≤ 100 bars. The mineralizing fluids boiled and were saturated in H₂S and SO₂.Calculated δ¹⁸O_H2O and δD_H2O, initial ⁸⁷Sr/⁸⁶Sr isotope compositions and the trace and REEs elements contents are consistent with a model in which the mineralizing fluids were derived during alteration of the Mykonos intrusion and subsequently mixed with Miocene seawater. Heterogeneities in the calculated δ³⁴S_SO4^− 2 and δ³⁴S_H2S compositions of the ore fluids indicate two distinct sources for sulfur, namely of magmatic and seawater origin, and precipitation due to reduction of the SO₄^− 2 during fluid mixing. The physicochemical conditions of the fluids were pH = 5.0 to 6.2, logf_S2 = − 13.8 to − 12.5, logf_O2 = − 31.9 to − 30.9, logf_H2S(g) = − 1.9 to − 1.7, logf_Te2 = − 7.9 and logα(SO₄^− 2_(aq)/H₂S_(aq)) = + 2.6 to + 5.5. We propose that retrograde mesothermal hydrothermal alteration of the Mykonos monzogranite released barium and silica from the alkali feldspars. Barite was precipitated due to mixing of SO₄^− 2-rich Miocene seawater with the ascending Ba-rich magmatic fluid venting upwards in the pluton.     

Combined paleomagnetic and geochronological study on Cretaceous strata of the Qiangtang terrane,central Tibet

A combined paleomagnetic and geochronological investigation has been performed on Cretaceous rocks in southern Qiangtang terrane (32.5°N, 84.3°E), near Gerze, central Tibetan Plateau. A total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled. Our new U–Pb geochronologic study of zircons dates the volcanic rocks at 103.8 ± 0.46 Ma (Early Cretaceous) while the red beds belong to the Late Cretaceous. Rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers. After removing a low temperature component of viscous magnetic remanence, stable characteristic remanent magnetization (ChRM) was isolated successfully from all the sites by stepwise thermal demagnetization. The tilt-corrected mean direction from the 14 lava sites is D = 348.0°, I = 47.3°, k = 51.0, α₉₅ = 5.6°, corresponding to a paleopole at 79.3°N, 339.8°E, A₉₅ = 5.7° and yielding a paleolatitude of 29.3° ± 5.7°N for the study area. The ChRM directions isolated from the volcanic rocks pass a fold test at 95% confidence, suggesting a primary origin. The volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole (VGP) scatter. The mean inclination from the Late Cretaceous red beds, however, is 13.1° shallower than that of the ~ 100 Ma volcanic rocks. After performing an elongation/inclination analysis on 174 samples of the red beds, a mean inclination of 47.9° with 95% confidence limits between 41.9° and 54.3° is obtained, which is consistent with the mean inclination of the volcanic rocks. The site-mean direction of the Late Cretaceous red beds after tilt-correction and inclination shallowing correction is D = 312.6°, I = 47.7°, k = 109.7, α₉₅ = 3.0°, N = 22 sites, corresponding to a paleopole at 49.2°N, 1.9°E, A₉₅ = 3.2° (yielding a paleolatitude of 28.7° ± 3.2°N for the study area). The ChRM of the red beds also passes a fold test at 99% confidence, indicating a primary origin. Comparing the paleolatitude of the Qiangtang terrane with the stable Asia, there is no significant difference between our sampling location in the southern Qiangtang terrane and the stable Asia during ~ 100 Ma and Late Cretaceous. Our results together with the high quality data previously published suggest that an ~ 550 km N–S convergence between the Qiangtang and Lhasa terranes happened after ~ 100 Ma. Comparison of the mean directions with expected directions from the stable Asia indicates that the Gerze area had experienced a significant counterclockwise rotation after ~ 100 Ma, which is most likely caused by the India–Asia collision.     

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.     

In-situ LA-ICP-MS trace elemental analyses of magnetite: Cu-(Au,Fe) deposits in the Khetri copper belt in Rajasthan Province,NW India

Magnetite is common in many ore deposits and their host rocks, and is useful for petrogenetic studies. In the Khetri copper belt in Rajasthan Province, NW India, there are several Cu-(Au, Fe) deposits associated with extensive Cu ± Fe ± Au ± Ag ± Co ± REE ± U mineralization hosted in phyllites, schists and quartzites of the Paleoproterozoic Delhi Supergroup. Ore bodies of these deposits comprise dominantly disseminated and vein-type Cu-sulfide ores composed of chalcopyrite, pyrite, and pyrrhotite intergrown with minor magnetite. There are also Fe-oxide ores with minor or no Cu-sulfides, which are locally overprinted by the mineral assemblage of the Cu-sulfide ores. In addition to the Fe-oxide and Cu-sulfide ores, the protolith of the Delhi Supergroup includes banded iron formations (BIFs) with original magnetite preserved (i.e. magnetite-quartzites) and their sheared counterparts. In the sheared magnetite-quartzites, their magnetite and quartz are mobilized and redistributed to magnetite and quartz bands. Trace elemental compositions of magnetite from these types of ores/rocks were obtained by LA-ICP-MS. The dataset indicates that different types of magnetite have distinct concentrations of Ti, Al, Mg, Mn, V, Cr, Co, Ni, Zn, Cu, P, Ge and Ga, which are correlated to their forming environments. Magnetite grains in magnetite-quartzites have relatively high Al (800–8000 ppm), Ti (150–900 ppm) and V (300–600 ppm) contents compared to those of BIFs in other regions such as the Yilgarn Craton, Western Australia and Labrador, Canada. The high Al, Ti and V contents can be explained by precipitation of the magnetite from relatively reduced, Al–Ti-rich water possibly involving hotter, seafloor hydrothermal fluids derived from submarine mafic volcanic rocks. Magnetite in sheared magnetite-quartzites is generally irregular and re-crystallized, and has Ni, Mn, Al, Cu and P contents lower than the magnetite from the unsheared counterparts, suggesting that the shearing-related mobilization is able to extract these elements from original magnetite. However, elevated contents of Ti, V, Co, Cr, Ge and Mg of the magnetite in the sheared magnetite-quartzites can be ascribed to involvement of external hydrothermal fluids during the shearing, consistent with occurrence of some hydrothermal minerals in the samples.Compositions of magnetite from the Fe-oxide and Cu-sulfide ores are interpreted to be controlled mainly by fluid compositions and/or oxygen fugacity (fO₂). Other potential controlling factors such as temperature, fluid–rock interaction and co-precipitating minerals have very limited impacts. Magnetite in the Cu-sulfide ores has higher V but lower Ni contents than that of the Fe-oxide ores, likely indicating its precipitation from relatively reduced, evolved fluids. However, it is also indicated that the two types of magnetite do not show large distinctions in terms of concentrations of most elements, suggesting that they may have precipitated from a common, evolving fluid. We propose a combination of Ge versus Ti/Al and Cr versus Co/Ni co-variation plots to discriminate different types of magnetite from the Khetri copper belt. Our work agrees well with previous studies that compositions of magnetite can be potentially useful for provenance studies, but also highlights that discrimination schemes would be more meaningful for deposits in a certain region if fluid/water chemistry and specific formation conditions reflected in compositions of magnetite are clearly understood.     

Rheology of aqueous magnetite suspensions in uniform magnetic fields

Non-magnetized suspensions of magnetite particles with concentrations in excess of 30% by mass and particle size less than 75 μm exhibit Bingham plastic behaviour. When exposed to external magnetic fields of strengths in excess of 41 × 10⁻⁴ T, the rheological behaviour of the suspensions departs from the Bingham model and can be described by a Herschel–Bulkley model of the form τ = τ₀ + kγⁿ. The value of the index n was found to range from 0.38 to 0.9, depending on the magnetic field strength, solids concentration and particle size and correlations are proposed for the apparent viscosity of magnetized suspensions as a function of magnetic field strength and solids concentration.