Precambrian Banded Iron Formations Research Articles

Precambrian banded iron formations: paleoecological and paleobiological aspects

Precambrian banded iron formations: paleoecological and paleobiological aspects

Y-Ho fractionation during basalt alteration in hydrothermal system: An implication for superchondritic Y/Ho signature recorded in Precambrian banded iron formations

The concentration of rare earth elements (REE) in Precambrian sedimentary rocks is widely used to estimate their depositional environment. Specifically, superchondritic Y/Ho value is a powerful indicator for determining whether a sedimentary rock was deposited in a marine environment. Currently, superchondritic Y/Ho value in seawater is believed to result from the preferential adsorption of Y onto Fe-Mn oxides. In modern oceans, Fe-Mn oxides are considered the sink for subchondritic Y/Ho value. On the contrary, Precambrian banded iron formations (BIFs) display superchondritic Y/Ho value, suggesting that Precambrian seawater also possessed a superchondritic Y/Ho value. However, Precambrian BIFs cannot account for the sink of subchondritic Y/Ho value. Thus, other processes must have been responsible for maintaining the superchondritic Y/Ho value of Precambrian seawater. A hydrothermal experiment involving mid-ocean ridge basalt (MORB) and a Cl-rich fluid was conducted at 300 °C and 500 bars to assess the role of basalt-hosted hydrothermal systems on the Y/Ho value of seawater. Over time, the fluid's Ca concentration increased while Na decreased, suggesting plagioclase albitization occurred. The REE pattern of the reacted fluids exhibited superchondritic Y/Ho values up to 63.2. Based on the presence of positive Eu anomaly and the degree of light REE depletion, the REE in the hydrothermal fluids was mainly sourced from plagioclase during albitization. The superchondritic Y/Ho values in the fluids can be explained by the preferential leaching of these elements due to differences in chloride complexing strength between REE. Subchondritic Y/Ho values observed in Precambrian hydrothermally altered basalts may be interpreted as remnants generating superchondritic Y/Ho values in coexisting Precambrian hydrothermal fluids. Therefore, we believe that the superchondritic Y/Ho value of Precambrian seawater was maintained by basalt-hosted hydrothermal system.

Cyanobacteria-ferrihydrite aggregates, BIF sedimentation and implications for Archaean- Palaeoproterozoic seawater geochemistry

Abstract Precambrian banded iron formations (BIFs) are iron- and silica-rich (bio)chemical sediments that are widely believed to have been precipitated by microbial oxidation of dissolved Fe(II). The by-product of these metabolisms – insoluble ferric iron – would have settled through the water column, often as aggregates with the cell biomass. While the mineralogy, composition and physical properties of cell-iron mineral aggregates formed by anaerobic Fe(II)-oxidising photoferrotrophic bacteria have been extensively studied, there are limited studies that characterise cyanobacteria-iron mineral aggregates that formed during oxygenic photosynthesis. This gap in knowledge is important because it impacts sedimentation velocities and the Fe(III) to organic carbon (Corg) ratios in the marine sediment pile. Here, we used a recently introduced approach to precisely measure the sedimentation velocity of cyanobacteria-ferrihydrite aggregates and the Fe(III):Corg ratios of the cyanobacteria-ferrihydrite aggregates over a wide range of pH and initial Fe(II) concentrations under predicted Palaeoproterozoic atmospheric conditions. Our results indicate that it was highly unlikely BIFs formed at pH <7 via chemical oxidation due to the insufficient sedimentation velocity, even at the maximum predicted Fe(II) concentration of 1800 μM with excess oxygen. Instead, large Banded Iron Formation (BIF) deposits, such as those associated with the ca. 2.47 Ga Kuruman Formation in South Africa, would only had been deposited at minimum Fe(II) concentrations of 500 μM at pH 7 or 250 μM at pH 8. The Fe:Corg ratios in cyanobacteria-ferrihydrite sediments formed during initially anoxic Fe(II) oxidation experiments represent the maximum values under each condition because we specifically extracted samples after all Fe(II) was oxidised. The Fe(III) to organic carbon ratio was consistently below 4, which is also the ratio required for dissimilatory Fe(III) reduction (DIR). This result indicates that biomass in this case was in excess, which contradicts the low organic carbon content seen in most BIFs. Thus, we suggest that biomass was either physically separated from ferrihydrite aggregates during sedimentation under the influence of ocean currents and waves, or it was degraded prior to DIR. The mineralogical and geochemical evidences of both oxide and carbonate facies from the Kuruman Iron Formation (IF) suggest that ferrihydrite was most likely the precursor along with a significant initial organic carbon input, supporting the proposed cyanobacterially-mediated BIF depositional model and experimental results.

Feasibility of Estimating Type and Abundance of Iron Ore Using Field Hyperspectral Radiometry and Spectral Transformation in Precambrian Banded Iron Formations of North Odisha, India

Feasibility of Estimating Type and Abundance of Iron Ore Using Field Hyperspectral Radiometry and Spectral Transformation in Precambrian Banded Iron Formations of North Odisha, India

Using a novel approach to characterize the surface reactivities of silica-rich ferrihydrite and biogenic cyanobacteria-ferrihydrite aggregates and the implications for Archean ocean geochemistry

Precambrian banded iron formations (BIF) are iron- and silica-rich chemical sedimentary rocks that are commonly used as paleo-redox proxies for Archean and Paleoproterozoic seawater geochemistry. At the onset of the Great Oxidation Event (herein GOE) around 2.4 Ga, cyanobacteria flourished with increasing nutrient fluxes due to oxidative weathering on land. In turn, this led to increased primary productivity that facilitated the permanent shift from a reducing Earth atmosphere to an oxidizing one. Interestingly, the duration of GOE also overlapped with one of the most prolific periods of BIF deposition.It is widely accepted that cyanobacteria were likely responsible for BIF formation during the GOE. Oxidation of dissolved Fe(II) by oxygen produced from cyanobacteria forms a metastable and amorphous mineral phase ferrihydrite, Fe(OH)3. As an essential component in both ancient BIF deposits and various modern ecosystems, the surface reactivity of ferrihydrite has been extensively studied under different conditions (i.e., pH and ionic strengths). Not only are the highly reactive surfaces of ferrihydrite particles important shuttles for trace element transport from the water column to the sediment pile, but previous studies have also demonstrated that cyanobacterial cells and ferrihydrite tend to aggregate at seawater pH. This means that ferrihydrite was also a vector for the transport of organic carbon to the seafloor. However, a complicating issue is how co-ions affect the surface reactivity of ferrihydrite, specifically dissolved silica which was abundant in ancient seawater. Although previous studies have demonstrated that silica can passivate the surface reactivity of ferrihydrite, what remains unclear is how silica impacts ferrihydrite-biomass aggregation. To fill this knowledge gap, we formed both silica-spiked ferrihydrite and cyanobacteria-ferrihydrite aggregates in situ and subsequently conducted empirical potentiometric acid-base titrations and Cd adsorption experiments on the fresh aggregate samples at three different ionic strengths (0.56 M, 0.1 M and 0.01 M). We minimized sample processing (i.e., drying and powdering) to a simple washing step, in which the aggregate pellets remained hydrated to avoid any mineral transformation thus altering their true surface reactivity in seawater. Experimental results were then fitted with non-electrostatic model to predict both surface charges and metal-adsorption behavior of ferrihydrite aggregates. Different from previous surface-complexation modelling studies, here we used a novel and more powerful modelling program called Phreefit. It utilizes the global optimization algorithms instead of more commonly used Newton-Raphson method in FITEQL program, which is often too limited for precisely modelling complex systems such as the two samples in this study. Furthermore, we also measured the surface charges of both samples over the pH range from 3 to 9 on a Malvern Zetasizer and characterized the surface functional groups through Fourier-Transform Infrared Spectroscopy to help with our interpretation of the experimental data.Preliminary results show that cyanobacteria-ferrihydrite aggregates formed primarily due to ionic bridging. Cyanobacterial cells likely facilitated the precipitation of dissolved silica. Findings from titration and Cd adsorption experiments indicate that the surface reactivity and capacity of both silica-rich ferrihydrite cyanobacteria-ferrihydrite aggregates to adsorb trace elements differ from their individual components, likely due to site blockage. This distinction is particularly prominent when considering the expected Archean seawater pH from 6 to 8. This disparity implies that the biogenic ferrihydrite aggregates do not exhibit an additive surface reactivity, which is in agreement with similar previous studies. Our combined results are crucial to accurately predict the adsorption of trace elements onto the aggregate surface and, ultimately, comprehend the archive of trace elements in sedimentary rocks used to reconstruct Precambrian ocean chemistry.

Genesis and depositional environment of the Carboniferous Baishanquan iron deposit in Eastern Tianshan, Northwestern China

Submarine volcanic-hosted iron deposits are the most important iron deposit in Northwestern China. However, their origin remains controversial. Here, we present detailed studies of the petrology, geochronology, geochemistry and FeSi isotopes of iron ores from Baishanquan deposit in Eastern Tianshan, Northwestern China. Similar to Precambrian banded iron formations (BIFs), the Baishanquan deposit is hosted in a volcanic-sedimentary succession, the iron ores are characterized by banding with alternate iron-rich and silica-rich layers, and the mineral assemblage is dominant by magnetite, quartz and Fe-silicates. The iron ores show distinct light rare earth elements (REE) depletion [(La/Yb)PAAS = 0.24–0.47], no significant Eu anomalies (Eu/Eu⁎ = 0.88–1.16), relatively high Y/Ho ratios (30–34), and δ30Si values in quartz from the iron ores ranging from −0.80 to 1.67‰. These data indicate mixed seawater and low-temperature submarine hydrothermal source for both the iron and silica. The absence of negative Ce anomalies suggests that the depositional environment was characterized by low oxygen conditions similar to modern oxygen minimum zones. Positive δ56Fe values in most magnetite (−0.45 to +1.02‰) suggest partial Fe(II) oxidation of the iron ores. Interestingly, the iron ores also contain relatively high Al2O3, CaO, MgO, and Zr concentrations, indicating that the sedimentary basin was subject to periodic clastic input. Indeed, zircon LA-ICP-MS UPb dating of interlayered dacite indicates that the Baishanquan volcanic-sedimentary rocks and associated iron ores were deposited at 327.3 ± 2.8 Ma (N = 25, MSWD = 0.52). This is significant because the appearance of these iron deposit provides direct evidence that marine ferruginous conditions – at least locally - may have still been present at Carboniferous period.

Geochemical and mineralogical evaluation of Libale iron- rich manganese deposit, North-central Nigeria

The iron-rich manganese deposits of Libale area is located within the basement complex of North-central Nigeria. Mineralogically, the major mineral phases are: goethite (Fe2O3.H2O), spessartine (Mn32+Al2(SiO4)3), hematite (Fe2O3), and quartz (SiO2), while minor mineral phases are orthoclase (Al2O3.K2O.6 SiO2), illite (KAl2Si3AlO10(OH)2), and manganite (Mn2O3.H2O) suggesting a trend of mineral assemblage attributed to the differential mineralization of the iron-manganese deposit that may have been caused by the geochemical tectonic activity during its formation. Geochemically, on average, the Fe2O3, SiO2, Al2O3, CaO, P2O5 content are 34.59 wt.%; 26.30 wt.%; 6.94 wt%, 2.60 wt%, 2.01 wt% correspondently while K2O and TiO2 are less than 1wt%. Suggesting oxide facies type for the deposit. The trace element composition of V (1040 ppm), Cr (100 ppm), Co (833.33 ppm), Cu (1740 ppm), Nb (1280 ppm), W (240 ppm), Ta (320 ppm) and Zr (302 ppm) were relatively high compared to Maro, Muro and Kakun BIFs in North-central Nigeria. Bivariate plot of Fe2O3 Vs. Al2O3, suggest enrichment of Fe2O3 but depleted in SiO2 and Al2O3, which possibly indicates replacement of precursor silica and kaolinite with goethite which are subsequently dehydrated to hematite. Al2O3-SiO2-Fe2O3 ternary diagram indicates Precambrian Banded Iron Formation (BIF) for the Libale iron- rich manganese deposits, this was confirmed by MnO Vs. FeO. Discriminative plots: FeO. SiO2 - (FeO + MnO) - Fe2O3 and Al2O3+ K2O + Na2O Vs. MnO + FeO confirmed mixed facies as the analyzed data plots between silicate facies and magnetic-silicate facies, this is linked to chemical association between Al2O3 and SiO2 for the iron-rich as it confirms a transitional grade of silicate facies to magnetite-silicate facies. SiO2 Vs. Al2O3 plot revealed hydrothermal and diagenetic processes for its deposition. The iron-rich manganese deposit is of low grade iron (Av. Fe=24.19%) characterized by ferruginous manganese (Av. MnO2=23.82%) based on the generalized percentages of element of major interest in assessing quality of iron and manganese ore, therefore, good for cast iron production.

The Paleoproterozoic Zhaigou banded iron formation in the Fuping Complex, North China Craton: Geochemistry, geochronology and implications for genesis and tectonic setting

Precambrian Banded iron formations (BIFs) provide important insights on the chemical composition of ancient seawater, as well as the tectonic setting of their formation. The Precambrian BIFs deposits in the North China Craton are widely distributed in the Archean greenstone belts and are composed predominantly of Neoarchean Algoma-type including those in the Anshan-Benxi and eastern Hebei areas of the Eastern Block, together with minor Paleoproterozoic Superior-type BIFs in the southern segment of the trans-North China Orogen. The Zhaigou BIF deposit is associated with Paleoproterozoic Wanzi supracrustal sequence and amphibolite in the Fuping Complex in central North China Craton. Here, we present petrological, geochronological and geochemical data of the meta-sedimentary rocks, BIF ores and amphibolites from the Zhaigou BIF deposit in the Fuping Complex for the first time to constrain the formation age, source characteristics and depositional setting, and evaluate the redox states of the Paleoproterozoic seawater. Whole-rock geochemical data show that the BIF ores are enriched in HREE with low (La/Yb)PAAS values (0.037–0.073), and exhibit positive Eu (3.34–6.21) and Pr (1.05–1.10) anomalies. The PAAS-normalized REY patterns resemble 1:100 mixture of high-temperature hydrothermal fluid and seawater, indicating the involvement of the two end members. In combination with whole-rock geochemical data of BIF ores, trace elements of magnetite and U-Pb ages of detrital zircons from the meta-sedimentary rocks, we propose that crustal components from the Fuping TTG gneiss also contributed to the BIFs. The geochemical features suggest that the protolith of amphibolite is basalt formed in island arc setting. The negative Ce anomaly (Ce/Ce* = 0.81 ∼ 0.90) indicates the deposition of the Zhaigou BIF ore in an oxidizing environment. Zircon U-Pb data on the BIF ore yield weighted mean 207Pb/206Pb ages of 2028 ± 33 Ma and 1874 ± 52 Ma, representing the formation age of the Zhaigou BIF deposit and the subsequent metamorphic event, respectively. The Zhaigou BIF deposit belongs to the Algoma-type which shows close association with the Paleoproterozoic sedimentary sequence and basaltic magmatism in oceanic island arc setting. The Paleoproterozoic seawater forming the Zhaigou deposit was oxidized after the Great Oxygenation Event (GOE) during the terrane assembly in the trans-North China Orogen.

Mesoarchean banded iron-formation from the northern Yangtze Craton, South China and its geological and paleoenvironmental implications

Banded iron formations (BIFs) are important sedimentary rocks used for reconstructing the geochemical and environmental evolution of Precambrian Earth surface conditions. Here, we describe the oldest BIFs of Mesoarchean age in the Yemadong Formation, and their implications for paleo-environmental evolution of the Mesoarchean Ocean in the northern Yangtze Craton, South China. U-Pb dating of metamorphic zircons obtained from metavolcanic rocks interbedded within the Yemadong Complex, record two major tecto-thermal events at 2901 ± 8 Ma, in agreement with a Mesoarchean origin of the Dujiagou BIFs, and as recently as 775 Ma. A high SiO2 + total Fe2O3 content of 90.95–94.62 wt%, moderate 0.54–1.42 wt% Al2O3 + TiO2, and broadly elevated levels of incompatible elements, point to a predominantly chemical origin of the BIF deposit, with a minor but influential terrigenous input. Rare Earth Element (REE) plus Yttrium patterns normalized against Post Archean Average Shale composition are characterized by the depletion of light REE relative to heavy REE. However, extremely weak Eu and Ce anomalies, together with low average chondrite Y/Ho ratios compared to average Mesoarchean BIF values, indicate that the primary REE composition of the rocks may have been diluted to some extent by terrigenous input, pointing to proximity or connection of the depositional setting to a continental margin environment. Despite potential terrigenous dilution of the syngenetic seawater-hydrothermal chemical composition, the preservation of an inherited hydrothermal REE signal in some samples, point to submarine hydrothermal provenance of the Dujiagou BIFs, as is the case for most Precambrian BIFs. Positive δ56Fe values, ranging from 0.25 to 0.45 ‰, characteristic of primary Archean BIFs, suggest partial oxidation of reduced Fe(II) to divalent (Fe(II)/Fe(III)) magnetite-rich BIFs in a broadly reducing Archean Ocean lacking a persistent and pervasive redoxcline before the rise of atmospheric oxygen.

Origin of ooids, peloids and micro-oncoids of marine ironstone deposits in Western Siberia (Russia)

This study investigates the authigenically formed Late Cretaceous and Paleogene marine ironstones in Western Siberia (Russia) based on petrographic and spectroscopic investigations. In the Upper Cretaceous Kuznetsovo, Ipatovo, Slavgorod, Gan'kino and Paleogene Lyulinvor Formations of the West Siberian ironstone basin, there are three main ore horizons, known as the Narym, Kolpashevo, Bakchar layers. The marine succession predominantly comprises ooidal or peloidal ironstones, glauconitolites, glauconitic rocks, sandstones, siltstones, claystones and gritstones. The ooidal ironstone exhibits predominantly abiogenic precipitates with subordinate microbial signatures, including micro-oncoids, relicts of lipids, carbohydrates and microfilaments. Iron-rich ooids, peloids and micro-oncoids formed primarily by adsorption of iron and occasionally by microbial iron-oxidising and sulphate-reduction actions. The abiogenic formation of ooid and peloid depends on physico-chemical conditions of environment during ion adsorption. While berthierine-goethite in ooid cortex formed in an oxic or dysoxic environment, siderite laminae therein represent an anoxic condition. Microbial mediation influenced the precipitation of berthierine and goethite, pyrite, siderite, greigite, pyrrhotite, wurtzite, barite, As-Ni-Co-Fe sulphide, and probably monazite. Siderite, pyrite, and to a lesser extent greigite and pyrrhotite formed exclusively within organic remains. Goethite with a high content of phosphate within micro-oncoids probably formed with the mediation of microbial activities. Bacterial microfilaments and Raman peaks of lipids and carbohydrates strongly support a bacterial origin of micro-oncoids. Abundant intraclasts at the base of the Ipatovo Formation that marks the Coniacian-Santonian boundary, and those within the Lyulinvor Formation that marks the Palaeocene-Eocene boundary, correspond to active tectonics, which provided an enhanced supply of metal-rich nutrients. Distribution of trace elements in the main iron-rich minerals supports a hydrothermal source of iron and other metals for iron-rich ooids and peloids in Western Siberia. The study of the Meso-Cenozoic marine ironstones provides insights into the biotic or abiotic processes involved in case of Precambrian banded iron formations (BIFs).

Early Pleistocene banded iron-rich sedimentary rocks at Cape Vani, Milos Island, Greece: A modern analogue of Precambrian banded iron formations?

The genesis of banded iron formations is commonly associated with anoxic and iron-rich (ferruginous) marine conditions throughout the Archean to Palaeoproterozoic, and occasionally in the Neoproterozoic. In recent studies, ~2 million-year-old banded iron-rich sedimentary rocks with alternating Fe- and Si-rich bands occurring in the Cape Vani sedimentary basin on Milos island (Greece) were interpreted to be a modern analogue of Precambrian banded iron formations. This interpretation is mainly due to the discovery of photoferrotrophic-like filamentous fossils in massive iron-rich cherty beddings of another subbasin about 450 m to the south. In this study, we re-examined the stratigraphy and lithology of the Cape Vani basin, carried out detailed petrographic, mineralogical and geochemical investigations of the iron-rich sedimentary rocks, as well as volcaniclastic/volcanic conglomerates and banded hydrothermal veins that are in close spatial association with the iron-rich sedimentary rocks. The Fe2O3 content of the banded iron-rich sedimentary rocks ranges from 0.41 to 11.02 wt% (2.73 wt% on average), far lower than that required per definition for banded iron formations. In addition, the banded iron-rich sedimentary rocks contain substantial clastic materials. Samples with stronger silicification, however, usually contain less clastic materials. The non-clastic components occur as siliceous matrix comprising the same mineral assemblage as the banded hydrothermal veins. These components occur in the interspaces of the clastic materials, within cavities, microfractures and veins crosscutting the layers. Fe2O3, principally reflecting hematite, is intimately linked with the presence of non-clastic materials. Therefore, the non-clastic materials are interpreted as post-depositional products associated with silicification during later stage hydrothermal activities, rather than being syn-depositional as the minerals in Precambrian banded iron formations. Consequently, the Milos banded iron-rich sedimentary rocks cannot be an analogue of Precambrian banded iron formations, seeing their lithological and genetical differences.The geochemical results reveal that both the banded iron-rich sedimentary rocks and the massive iron-rich cherty beddings have the same clastic provenance as the volcaniclastics/volcanic conglomerates, but the massive iron-rich cherty beddings contain more hydrothermal contributions due to more intensive fluid-rock interaction. Thus, the two types of iron-rich sedimentary rocks are considered as clastic sedimentary rocks with varying degrees of hydrothermal overprinting. The massive iron-rich cherty beds were generally subjected to stronger hydrothermal overprinting, and, therefore, contain more siliceous matrix and Fe2O3. The enrichment of Fe is due to post-depositional hydrothermal fluid infiltration to the primary sediment piles. The primary layer planes of the banded iron-rich rocks, their interconnected pores and fractures provided preferential pathways for the hydrothermal fluids, and resulted in denser precipitation of Fe(III) (oxy)hydroxides and SiO2 cement therein. The alternating bands in the banded iron-rich sedimentary rocks, therefore, reflect cyclic permeabilities of the layers (i.e., primary sedimentary planes and porosities), rather than cyclic precipitation of Fe- and Si-minerals. This study also sheds light on an alternative formation mechanism for banded structures in Fe- and Si-rich sedimentary rocks similar to those of Precambrian banded iron formations.

2470 million-year-old banded iron formation reveals a climatic oscillation consistent with the Gleissberg solar cycle

Modern-day solar cycles due to the solar magnetic field oscillation are well recognized. Owing to the response of Earth’s climate to solar activity fluctuation, solar cycles in the Phanerozoic Eon have been recorded by laminites and fossil tree rings. However, the existence of magnetic cycles within the Sun younger than 3100 million-year-old is still unknown. The deposition of Precambrian banded iron formations (BIFs) reflects the primary productivity of the early ferruginous oceans and is coupled to climatic fluctuations. Here we apply synchrotron-radiation-based µ-XRF with a 20 µm interval on a 60 mm long, 2470 million-year-old BIF from the Kuruman Formation, South Africa. Our spectral analyses of multiple elemental concentration series reveal prominent and consistent 80-year cyclicity, which is best explained by the Gleissberg solar cycle. We hypothesize that solar cycles have already modulated the early Paleoproterozoic climate mainly through high energy irradiance coupled solar radiative forcing oscillation and the solar magnetic field controlled galactic cosmic ray fluctuation. Our result provides the record of solar magnetic field oscillations with stable and close-to-present cycle durations at least since the beginning of the Paleoproterozoic Era.

Sulfur isotopic and trace‐element compositions of pyrite from the Zankan iron deposit, West Kunlun Orogenic Belt, China: Possible Early Cambrian banded iron formations

Abundant iron deposits characterized by stratified orebodies have recently been discovered in the Taxkorgan Terrane, West Kunlun Orogenic Belt, China. Of these deposits, the Zankan iron deposit was the first to be discovered and has the largest estimated tonnage. However, the iron mineralization types for this deposit and others in this area are uncertain, mainly because of the apparent conflict between their typical banded architecture and Cambrian ages. In this study, in situ sulfur isotopic and trace‐element compositions of pyrite, including that in orebodies coexisting with magnetite (Py(o)), pyrite veinlets in strata (Py(v)), and rhyolite and magnetite veins cross‐cutting rhyolite (Py(r)), were conducted to ascertain the genesis of the Zankan deposit. Our results reveal that the pyrite in Zankan deposit has marked positive δ34S values (17.20‰–29.86‰). In addition, Py(o) has low Co/Ni ratios, which are distinct to Py(r) and Py(v), meaning that they belong to typical marine chemical precipitates and magmatic–hydrothermal pyrite, respectively. The coexistence of pyrite and magnetite suggests that the Zankan iron deposit was precipitated in a euxinic basin. Together, our results suggest that seawater sulfate concentrations were low, and the scale and sulfur isotopic compositions of this euxinic basin were most likely dominated by changes in sea level eustacy, which controlled the proportions of seawater and riverine inputs. Comparison with typical Precambrian banded iron formations (BIFs) suggests that the Zankan iron deposit belongs to a unique class of Cambrian BIFs that were controlled predominantly by basins in a shelf environment with locally anoxic conditions.

Phosphate remobilization from banded iron formations during metamorphic mineral transformations

Ratios of phosphorous (P) to iron (Fe) in Precambrian banded iron formations (BIFs) have previously been used to estimate dissolved seawater phosphate concentrations in the ancient oceans. Such studies rely on an assumed composition of the primary iron minerals, the concentrations of the major ions in seawater, and empirical partitioning coefficients for phosphate sorption to Fe(III) (oxyhydr)oxides. There is limited data, however, regarding the post-depositional stability of phosphate associated with presumed primary BIF iron minerals, such as ferrihydrite under low-grade metamorphic temperature and pressure conditions. Here we experimentally formed ferrihydrite in the presence of silica, which was abundant in the Precambrian oceans, and then incubated it at 170 °C and 1.2 kbar in the presence or absence of organic carbon (Corg; either glucose or microbial biomass) as a proxy for ancient planktonic biomass. We found that the post-metamorphic mineral assemblage resulting from thermochemical Fe(III) reduction of Si-doped ferrihydrite depended on Corg reactivity: In the presence of highly reactive glucose, siderite, magnetite, and vivianite were formed, with less than 1.2 mol% of phosphate (0.5 M NaCl extractable) being mobilized. In contrast, the reaction of Si-doped ferrihydrite with less reactive microbial biomass resulted in the formation of hematite and siderite, but not vivianite, and approximately 10 mol% of phosphate was remobilized into the pore fluids. Collectively, our data suggest that the fidelity with which BIFs record ancient oceanic phosphate concentrations depends on the mineralogy and diagenetic history of individual BIFs but should be reliable within 10%.

Mineralogical justification for potentiality of producing marketable hematite products


 
 
 Hematite quartzites are a product of weathering of magnetite quartzites, which make up the ferruginous horizons of deposits of the Precambrian banded-iron formation. They occur all over the planet. The largest deposits are found in the iron-producing areas and basins of Central Kazakhstan, the Kursk magnetic anomaly, the Karelian-Kola region, Western Australia, Southeastern India, Brazil, the United States, and Canada. The geological and mineralogical issues of hematite quartzites as raw materials for producing concentrate and sinter ore have been studied most deeply and comprehensively for the deposits of the Kryvyi Rih basin and Central Kazakhstan. However, when developing an effective scheme for producing high-quality metallurgical raw materials, the mineralogical features of hematite ores have been taken into account insufficiently. The aim of the authors of the present work was to study the localization, structure of deposits and mineral composition of hematite quartzites as raw materials for sinter ore and concentrate production. Data from geological observations and mineralogical studies were used as source material. Proven geological, mineralogical, petrochemical methods were used. In accordance with the obtained results, the hematite quartzites are composed of ore-forming (quartz, hematite) and secondary (relict and newly formed) minerals. The total content of the hematite and quartz exceeds 90 mass %. The peculiarity of Ushkatyn III deposit ores is the high content of manganese oxides. The depth of distribution of the weathering crust composed of hematite quartzites varies from 200 to 1000 m. The hematite quartzites’ bodies are characterized by a zonal structure. Their central parts are represented by martite-micaceous hematite, micaceous hematite- martite quartzites; intermediate ones by martite quartzites; peripheral parts – by dispersed hematite-martite, kaolinite-martite-dispersed hematite quartzites. The horizons differ in the quantitative ratio of these varieties. The quantitative ratio of mineral varieties of hematite quartzites, morphology of individuals and aggregates of ore-forming and secondary minerals, their chemical composition and physical properties must be taken into account when developing the optimal technology for the production of high-quality hematite concentrate.
 
 

A novel approach to investigate the deposition of (bio)chemical sediments: The sedimentation velocity of cyanobacteria–ferrihydrite aggregates

ABSTRACT Sedimentation velocities of various chemical sediments are typically calculated using Stokes's law. However, applying it to chemical sediments that form in situ in the water column is not ideal because the particle properties do not fulfill many of the assumptions underpinning the applicability of Stokes' law. As a consequence, it has been difficult to predict the sedimentation rate of ancient chemical sediments, such as Precambrian banded iron formations (BIF), because their primary sediments likely comprised aggregates of ferric hydroxides, such as ferrihydrite [Fe(OH)3], and marine bacterial biomass, including cyanobacteria. In this work we use a new experimental method to address the mechanisms by which primary BIF sediment, formed by the oxidation of dissolved Fe(II) by O2 and simultaneously incubated with cyanobacterium Synechococcus sp., were deposited to the Archean ocean. Specifically, we formed the aggregates in situ over a wide range of initial pH and Fe(II) concentrations, continuously recorded the entire settling processes of aggregates under each condition, and then processed the data in MATLAB according to different settling mechanisms. Our results demonstrate that ferrihydrite–cyanobacteria aggregates settled to the ocean floor either through the formation of uniformly descending concentration fronts or through convective plumes. The sedimentation mechanism depended on both initial Fe(II) concentration and the pH. Correspondingly, two algorithms were developed to characterize the sedimentation velocity. These algorithms tracked the alteration of light intensity from low to high as sediments descended from an initially homogeneous state through a water tank, and as well calculated the average light intensity over time, from which vertical time series were constructed allowing calculation of the sedimentation velocity. Our method not only provides an accurate estimation of the in situ sedimentation velocity of cell–mineral aggregates, but also provides new insights into the physical mechanisms by which the primary sediments composing BIF were deposited.

A brief discussion on the depositional and metamorphic mineralization of Precambrian banded iron formations

条带状铁建造（BIF）是3.5~1.8Ga前陆架和洋盆的常见沉积物。前寒武纪条带状铁建造构成了世界上重要的铁矿资源。虽然它们成矿过程及其演化的许多方面的问题仍未解决，但人们普遍认为，它们沉积方式的长期变化与地球的环境和地球化学演化有关。条带状铁建造记录了前寒武纪古海洋、古环境、大气条件和细菌代谢条件以及铁的来源和沉积过程。大型BIF沉积与大火成岩省有成因联系，其铁的来源与火山物质加入的海底热液体系有关，或有陆缘岩石风化的无机物产物加入，越靠近陆缘，陆源碎屑物质加入的越多。然而，在太古宙到古元古代期间，BIF沉积的深水盆地中陆源物质的加入很少。那时的铁建造沉积在缺氧的海洋中，通过微生物的光合作用、无氧光合氧化和紫外光线辐射氧化等机制对溶解的二价铁进行氧化，从而形成三价铁氢氧化物和氧化物的沉积。大多数BIF大型矿床，自其在沉积环境中形成以来，它们在从太古宙直至中生代的漫长的地质历史演化过程中经历了铁矿的品位由低到高转化的复杂地质过程，一般经历了深部交代变质作用的除硅、除碳酸岩矿物的富集成矿和浅部风化富集成矿过程。许多BIF铁矿经历了从绿片岩相到角闪岩相变质作用，但到达的压力条件都不是很高，这或许与俯冲的高密度BIF铁矿难以折返的动力学机制有关。迄今为止，变质作用、尤其是高级变质作用对成矿过程的影响研究较少，是今后值得关注的领域。

Post-depositional REE mobility in a Paleoarchean banded iron formation revealed by La-Ce geochronology: A cautionary tale for signals of ancient oxygenation

Precambrian banded iron formations (BIF) are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater. Here we investigated trace element signatures in BIF of the 3.22 Ga Moodies Group, Barberton Greenstone Belt (South Africa), which are interbedded with near-shore siliciclastic sedimentary rocks and represent one of the oldest known shallow-water occurrences of BIF. Unusual rare earth element (REE) signatures, notably with pronounced negative Ce anomalies in shale-normalized spectra, have been previously reported for chemical sediments of the Moodies Group, which we confirm here through an expanded dataset for Moodies BIF spanning three different localities. We find negative Ce anomalies as low as 0.2 Ce/Ce⁎ that are associated with unusual enrichment of LREE relative to HREE in the sample set. While total REE abundances and certain REE features appear strongly related to the concentration of detrital indicators (e.g., Zr), and are likely primary, other features, notably LREE enrichment, cannot be explained as a primary feature of the sediment. This is better explained by later addition of REE from a LREE-enriched but Ce-depleted fluid that generated the significant negative Ce anomalies observed in surface samples of Moodies Group BIF. This REE addition event influenced both Sm-Nd and La-Ce isotope systematics, the latter yielding an isochron of 60 ± 32 Ma, thus constraining the timing of emplacement of the negative Ce anomalies to the past 100 Ma, possibly upon surface exposure of the Barberton Greenstone Belt to wetter conditions during the Cenozoic. Our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post-depositional modification than previously thought, and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of Earth surface oxygenation.

Petrographic and geochemical characterization of weathered materials developed on BIF from the Mamelles iron ore deposit in the Nyong unit, South-West Cameroon

Iron ore deposits hosted by Precambrian banded iron formation (BIF) are the most important source of mineable iron. In Cameroon, they are located in the southern part of the country. This study reports the petrological and geochemical data of iron ores collected from a weathering profile in the Mamelles BIF deposit, SW Cameroon. The profile is composed of three levels which are from the bottom to the top: the saprock, the ferruginous horizon, and the loose horizon. Eight representative iron ore samples (rock fragments and loose clayey material) were collected along the profile and were subjected to petrographic and geochemical analyses. Their mineralogy consists of martite, goethite, quartz, and lesser amounts of hematite, magnetite, kaolinite, and halloysite. The presence of minerals such as kaolinite and goethite in the Mamelles iron ores suggests their supergene origin. Geochemically, the saprock is characterized by high iron content (70.25 wt% Fe2O3t), and low silica (26.38 wt% SiO2) and alumina (1.14 wt% Al2O3). The rock fragments collected from the ferruginous horizon display higher Fe2O3t (72–76.40 wt%), Al2O3 (2.80–5.43 wt%), and lower SiO2 (16.70–18.35 wt%) contents, suggesting the leaching of silica during the enrichment process. The loose clayey samples collected from both the ferruginous horizon and the upper loose horizon show lower iron and higher silica contents. When normalized to the underlying BIF saprock, both rock fragments and loose clayey ores display LREE enrichment, suggesting that they formed through supergene processes. Economically, most of the Mamelles iron ores are classified as medium-grade ores and a few display acceptable contents in contaminants. Overall, this petrological and geochemical study of the Mamelles iron ores revealed encouraging results. Given its strategic location near the deep seaport, the deposit should be investigated in more detail for its mining potential.

Geochemical Characteristics of Wuyang Siliceous Rocks in the Southern Margin of North China Craton and its Constraint on the Formation Environment of BIF of Tieshanmiao Formation

Precambrian banded iron formation (BIF) is one of the most important mineral resources in China, mostly abundant in the North China Craton (NCC) with relatively less common in South China. Since the BIF and siliceous rocks both originated from chemical deposition, the syngenetic BIF and Siliceous rocks can help evaluate their environment of formation. We examine here the mineralogy and geochemistry of siliceous rocks associated with the Tieshanmiao Formation BIF, aiming to decipher the conditions of formation of both BIF and Siliceous rocks in the Wuyang area in the NCC. Analysis of the geochemical characteristics of whole rock shows that the SiO2 content of the siliceous rock ranges from 90.11% to 94.85% and is relatively high overall. Trace element contents of Ba and U are also high, the Ba/Sr ratio ranges from 3.89 to 25.28 and the U/Th ratio ranges from 0.09 to 0.20. Finally, the ΣREE value of rare earth elements ranges from 57.03 ppm to 152.59 ppm, and these indexes all indicate that siliceous rock resulted from hydrothermal deposition. Plots of Al2O3‐SiO2, SiO2/(K2O+Na2O)‐MnO2/TiO2 and Mn‐10×(Cu+Co+Ni)‐Fe in discrimination diagrams also verify this interpretation. However, both the MgO content, ranging from 0.16 to 0.32, and the Fe/Ti ratio, ranging from 2.50 to 9.72, suggest that terrigenous material was added during the depositional process. Major and trace element parameters of siliceous rock, such as the Al/(A1+Fe+Mn) ratio (from 0.81 to 0.93), MnO/TiO2 (from 0.00 to 0.17), Al/(Al+Fe) (from 0.82 to 0.93), Sc/Th ratio (from 0.21 to 0.50), U/Th (from 0.09 to 0.20), (La/Yb)N (from 0.83 to 3.04), and the (La/Ce)N (from 0.01 to 0.02) all imply that the siliceous rock formed in a continental margin. In addition, the Sr/Ba ratio from 0.08 to 0.26, the δCe value from 0.31 to 0.90, and the δEu value from 0.14 to 0.58, all indicate that the siliceous rock was formed at a relatively deeper water depth and under weak hydrodynamic conditions. Siliceous rock and BIF formed in the same geological setting, with the SiO2/(K2O+Na2O) ratio of siliceous rock ranging from 28.61 to 47.43, the SiO2/Al2O3 ratio from 16.53 to 32.37, and the SiO2/MgO ratio from 287.28 to 592.81, which are all in agreement with chemical deposition associated with volcanic eruptions. The Al2O3/TiO2 ratio from 37.82 to 50.30 indicates that the magma source of siliceous rock was of slightly intermediate composition. During the Late Archean in the Wuyang area, the high concentration and high purity SiO2 quickly precipitated from hydrothermal fluids to finally result in the accumulation of siliceous rock in a marginal sea, while the input corresponding to iron formation components was deposited to form iron formation layers, and limestone was only the product formed during the deposition intervals of siliceous rock and iron formations. In this study, the synsedimentary siliceous rocks of BIF act as a new way to provide direct evidence to understand the formation environment of BIF due to its high geochemical stability.