Algoma-type Banded Iron Formation Research Articles

Coexistence of Algoma- and Superior-type BIFs in back-arc systems: Evidence from Xincai, North China Craton

Superior-type and Algoma-type banded iron formations (BIFs) generally developed in distinct tectonic environments and have unique geological characteristics. Superior-type BIFs were deposited on passive continental shelves or intracratonic basins and are laterally extensive, whereas Algoma-type BIFs were formed near volcanic arcs and spreading centers. Here, we present a case where late Archean (∼2.5 Ga) Superior- and Algoma-type BIFs co-developed in a back-arc setting at Xincai, along the southern margin of the North China Craton. The Algoma-type BIFs at Xincai exhibit high Ge/Si ratios (4.84 to 8.84) and a broad range of εNd(t) values (−7.2 to +1.2). These geochemical signatures, along with similar εNd(t) values (+1.2) in contemporaneous amphibolites, indicate a significant influence of hydrothermal fluids associated with submarine volcanism. In contrast, the Superior-type BIFs at Xincai are characterized by positive La anomalies, low Ge/Si ratios (0.21 to 4.60) and narrow range of εNd(t) values (−6.1 to −1.0) suggesting a major contribution from terrigenous sources. The coexistence of Algoma- and Superior-type BIF types at Xincai is related to the interplay of multiple fluid systems in a back-arc environment. The Algoma-type BIFs formed from the confluence of hydrothermal fluids originating from enriched subcontinental lithospheric mantle (high Ge/Si and low εNd(t)) with those emanating from the adjacent back-arc spreading centers (depleted mantle like—high Ge/Si and εNd(t)). On the other hand, the Superior-type BIFs formed near the continental margins of the back-arc basins, where surface seawater carrying terrigenous substances (low Ge/Si and εNd(t)) mixed with hydrothermal fluids from the back-arc spreading centers. The distinct co-existence of Algoma- and Superior-type BIFs at Xincai underscores the dynamic nature of back-arc environments as crucibles for diverse BIF mineralization processes, offering new perspectives on the contributions of submarine volcanic activity and terrigenous inputs to the BIFs.

Genesis of the Neoarchean Algoma-type banded iron formation: constraints from Fe isotope and element geochemistry of the Qian’an iron deposit, eastern North China craton

The Neoarchean Qian’an BIF deposit in eastern Hebei, eastern North China Craton has attracted extensive attention of study in the last decades, but the genesis, e.g., Fe sources, metallogenic mechanism, as well as tectonic attributes of the BIFs remains highly disputed. Based on field investigations, microscopic observations and geochemical analysis, this study tries to unravel the source characteristics of ore-forming materials in the Qian’an deposit. The results show that the chemical compositions of the BIF ore samples are mainly composed of TFe2O3 and SiO2, with minor Al2O3 and TiO2. The total trace element contents of the samples are relatively low. The PAAS-normalized REE distribution patterns of the ores show LREE depletion and HREE enrichment, with robust positive anomalies of Eu, Y and La. These characteristics indicate that the BIFs are attributed to dominant chemical precipitation originated from paleo-ocean with obvious volcanic hydrothermal contributions and minor clastic input. Their positive to no Ce anomalies and positive δ56Femagnetite values unveil that the iron was precipitated at low oxygen fugacity. These results, in collaboration with evidence from previous lithological, structural geology, metamorphic P-T paths, geochemistry, geochronology and numerical modeling studies, support a mantle plume model to explain the complicated tectono-thermal processes, and the sources of BIFs in Eastern Hebei, eastern North China Craton.

Geochemistry of BIF in the Quadrilátero ferrífero, Brazil, as a proxy to neoarchean paleoenvironmental and depositional conditions

Precambrian Algoma-type banded iron formations (BIFs) are biochemical metasedimentary rocks formed in the deep water environment relatively close to centers of volcanic expansion and can be characterized on the basis of their paleodepositional environment and the associated rock types. These rocks may contain geochemical signatures indicative of changes in the redox conditions of the Archean ocean. Such variations in the Meso- and Neoarchean are crucial in understanding the processes that led to the gradual increase of oxygen in the early atmosphere, culminating in the Great Oxidation Event (2.4–2.3 Ga). Oxide- and carbonate-facies BIFs were intercepted by drill cores in the Pilar gold deposit in the NE sector of the Quadrilátero Ferrífero (QF), Brazil, located in the Neoarchean Rio das Velhas greenstone belt. These rocks are interbedded with mafic and ultramafic metavolcanic rocks and carbonaceous phyllites. In light of the potential to reconstruct paleodepositional evidence indicative of the Earth's atmospheric redox history based on this stratigraphy, we have considered specific proxies from geological and geochemical observations. Pilar BIFs geochemistry exhibits positive anomalies of La, Eu, and Y, indicating characteristics inherited from seawater and high-temperature hydrothermal fluids. In addition, a superchondritic Y/Ho ratio further supports the influence of seawater. The signatures of the major elements Al2O3 and other immobile elements typical of detrital sediments corroborate the interpretation that these rocks were deposited in a deep marine environment with low clastic influence. Furthermore, negative Ce/Ce*SN anomalies in some sections of the Pilar BIF indicate limited oxygen availability which, together with Mo, U, V, and Zn trace-element enrichment factors, indicate deposition under suboxic/euxinic conditions. This research, together with other similar BIF data from the NW QF provide new insights into the paleoenvironmental conditions prevailing during the formation of the Archean volcano sedimentary basins that compose the southern São Francisco craton (SFC).

Age and genesis of the Tongshan banded iron formation in the Zhongtiao region, North China Craton

Banded iron formations (BIFs) occur extensively within the late Neoarchean supracrustal successions of the North China Craton (NCC). The Zhongtiao region is located on the southern margin of the NCC and is a terrane that may host BIFs. The region is well known for exposed early Precambrian rocks, such as the Sushui Complex, which can be divided into three lithological units: Neoarchean tonalite-trondhjemite-granodiorite (TTG) rocks, Paleoproterozoic multi-phase granitoids, and supracrustal rocks (i.e., the Sushui Group). Despite previous studies focused on the granites within the Sushui Complex, limited information is available on the supracrustal rocks and the potential for associated BIFs that have undergone amphibolite-facies metamorphism. Here, for the first time, we conduct petrological, geochronological, and geochemical studies on a newly discovered BIF layer (i.e., the Tongshan BIF) and the associated metamorphosed volcanic and sedimentary rocks in the Sushui Group. This study constrains the formation age of the Tongshan BIF, its source characteristics, and the redox state of the depositional environment. SHRIMP zircon U–Pb analyses on biotite plagioclase gneiss (the footwall rock of the BIF) demonstrate that the Tongshan BIF was formed at ca. 2583.1 ± 9.8 Ma. The Tongshan BIF, occurrs in a volcano-sedimentary succession, and may have been deposited in a continental margin arc along the southern segment of the NCC and may best be considered as an Algoma-type BIF. Given a 2.58 Ga depositional age for the Tongshan BIF and the Sushui Group, and the more restricted depositonal nature of Algoma-type BIFs, this work highlights the potential for the identification of additional BIF layers in the Zhongtiao region.Two distinctive sedimentary facies are recognized in the Tongshan BIF based on mineral composition: an oxide facies BIF, composed of magnetite and quartz, and a silicate facies BIF, composed of magnetite, amphibole, and quartz. The Tongshan BIF displays distinct seawater-like, shale-normalized REY characteristics, including super-chondritic Y/Ho ratios, HREE enrichment relative to LREE, and positive La, Gd, and Y anomalies. Consistently positive Eu anomalies are also observed, indicating the influence of a high-T hydrothermal input. No true negative Ce anomalies occur in the Tongshan BIF, suggesting a dominantly anoxic water column contemporaneous to deposition. Further, the lack of true negative Ce anomalies extensively occurring in the Tongshan BIF is consistent with other BIFs observed globally, and confirms that ferruginous (anoxic and Fe-rich) conditions were maintained in deep seawater before to the GOE.

The isotope geochemistry of host rocks of the late Archean Guandi and Banshigou banded iron formations, southern Jilin Province: temporal and tectonic significance

The Guandi and Banshigou iron deposits of southern Jilin Province, China, are Algoma-type banded iron formations (BIFs) located in the Eastern Block of the North China Craton (NCC). LA–ICP–MS U–Pb dating shows the magmatic zircons in plagioclase amphibolite from the Guandi BIF (PA–GD) and plagioclase hornblende gneiss from the Banshigou BIF (PHG–BSG) were crystallised at 2556 ± 17 Ma and 2545 ± 27 Ma, respectively, representing the formation age of the BIFs. Metamorphic overgrowth zircons from both samples gave two U–Pb ages of 2452 ± 23 Ma and 2446 ± 70 Ma, indicating the age of metamorphism. The zircon εHf(t) values of PA–GD (–1.3 to +2.8) and PHG–BSG (–1.5 to +2.7) indicate that the magma source was derived from depleted mantle with contamination of crustal material. According to our study, the PA–GD and PHG–BSG were formed in an island arc setting and support previously proposed subduction zone tectonic models for the northeastern NCC during the Neoarchean.

Geochemistry, U-Pb zircon geochronology, and Fe isotopes of the Shanzhuang banded iron formation in the North China Craton: Implications for genesis and depositional environment

The Shanzhuang banded iron formation (BIF) occurs in the Shancaoyu Formation, Taishan Group, in the western Shandong Province (WSP), eastern North China Craton (NCC). We constrained the Shanzhuang BIF depositional age of ∼2.50 Ga based on the zircon U-Pb dating of the leptynite interlayers and the cross-cutting metagranitoid. After deposition, the Shanzhuang BIF was subsequently metamorphosed into amphibolite facies at 2.50–2.45 Ga. According to different mineral assemblages, the Fe ores can be divided into three types: magnetite + quartz ± hornblende (type-1 ore), magnetite + quartz + hornblende + garnet ± hematite (type-2 ore), and quartz + hematite ± magnetite ± hornblende (type-3 ore). In the Post-Archean Australian Shale (PAAS)-normalized rare earth element and yttrium (REY) patterns, the three ore types show consistent flat REY curves. All ore types have moderate ƩREY concentrations (27.24–143 ppm), (La/Yb)N values of 0.29–2.31, significant positive Eu anomalies (1.05–3.47), slightly negative or positive Ce anomalies (0.87–1.26), slightly positive Y anomalies (1.16–1.48), and no apparent fractionation between light REEs (LREEs) and heavy REEs (HREEs), indicating mixing of hydrothermal fluid and seawater. The similar REY patterns and heavy Fe isotopes (0.47–1.02‰) of the three ore types suggest a certain amount of continental detritus was incorporated during the precipitation of the Fe phases in the Neoarchean-Paleoproterozoic oceans. The post-depositional metamorphism did not significantly affect the original REY geochemical signature of the BIF. The Shanzhuang BIF ores are not highly pure chemical sediments, but the type-1 ore is relatively pure and more representative of the original sedimentary composition. By contrast, the type-3 ore tends to have higher δ56Fe ratios (0.74–1.02‰) but lower δEu (1.27–1.97‰), implying substantial post-depositional hydrothermal alteration. The Shanzhuang Fe deposit is an Algoma-type BIF, and the surrounding rock consists of pyroclastic materials. Our model indicates that the Shanzhuang BIF was probably deposited in a redox stratified paleo-ocean environment.

Fe-Si-C isotope constraints on the genesis of iron ores in Gongchangling iron deposit in northeastern China

The Gongchangling iron deposit is an oxide-facies Algoma-type banded iron formation (BIF) and one of the most representative iron deposits in northeastern China. We present new data from petrological, whole-rock geochemical, and Fe-Si-O isotope compositional data from the Gongchangling BIFs to constrain the origin of ore-forming materials and mineralization processes. The low Al2O3 and TiO2 contents in iron ores indicate that the sources of the BIFs were not contaminated by terrigenous clastic sediments. The Gongchangling BIFs are characterized by depletions in LREEs and 30Si and enrichments in HREEs, with positive Eu and Y anomalies. Given these and the Fe isotope results, we conclude that both high-temperature hydrothermal fluid and seawater contributed to the formation of the BIFs and that a mixture of seawater and ~ 0.1 % hydrothermal fluid, which percolated and dissolved the submarine volcanic rocks, provided the ore-forming materials. Combining the Ce anomalies, enrichments in heavy Fe isotopes and low δ13C compositions, we conclude that the fO2 of Archean seawater was limited. Integrated petrographic and geochemical evidence indicates that the high-grade iron ores from the Gongchangling BIFs experienced incomplete oxidation and low amounts of Fe precipitation in a marine environment and that dissimilatory iron reduction and metamorphic hydrothermal fluid played important roles.

Mesoarchaean banded iron formations of the Fennoscandian Shield: new zirconU-Pb (ID-TIMS and SHRIMP-II) isotope ages of noble metal mineralization and Nd-Sr data on whole rocks

ABSTRACT Banded iron formations (BIFs) are widespread over the world (3.81 Ga to 1.80 Ga) as the Algoma and Superior types. The paper provides new isotope (Nd-Sr, U-Pb) data on associated noble-metal mineralization in Algoma-type BIFs of the Fennoscandian Shield. U-Pb dating of accessory zircon applied to estimate the formation time of noble-metal-bearing skarn and related rocks in the Olenegorsk and Kirovogorsk BIFs, NW Fennoscandian Shield show that they occurred in the interval between the formation of basalt protolith (2.81 Ga amphibolites) and metamorphism in BIFs (2.75 Ga) to the formation of gold-bearing iron-rich skarn with native Au, Ag, Bi and tellurides (2.65 Ga) and low-temperature quartz-zeolite veins with redeposited Au (2.55 Ga). REE distribution in zircons and corresponding rocks indicated an important role of redox reactions in Fe and REE redistribution affected by reducing fluids on haematite-quartz protolith of BIFs.

Origin of the Neoarchean VMS-BIF Metallogenic Association in the Qingyuan Greenstone Belt, North China Craton: Constraints from Geology, Geochemistry, and Iron and Multiple Sulfur (δ33S, δ34S, and δ36S) Isotopes

Abstract The association of volcanogenic massive sulfide (VMS) deposits and Algoma-type banded iron formations (BIFs) in many Precambrian terranes indicates a link between submarine hydrothermal processes, seawater chemistry, and chemical sedimentation. The Neoarchean (~2.55 Ga) Qingyuan greenstone belt VMS-BIF metallogenic association, located on the north margin of the North China craton, is a typical example of such an association. The stratigraphy of the Qingyuan greenstone belt includes three units (from the oldest to youngest): (1) the Shipengzi Formation, composed of tholeiitic-transitional arc basalts with negative Nb anomalies, interlayered normal mid-ocean ridge basalts (N-MORBs) and FI-type dacites, and BIFs; (2) the Hongtoushan Formation, consisting of polycyclic bimodal suites of N-MORB-type basalts and FII-type dacites, as well as VMS mineralization and minor BIFs; and (3) the Nantianmen Formation, composed of schist, quartzite, and marble with minor basalts and BIFs. Positive Fe isotope compositions (δ56Fe of 0.48–0.69‰) for magnetite in the silicate BIF of the Shipengzi Formation indicate partial oxidation of aqueous Fe(II). Using a dispersion-reaction model, the relatively high δ56Fe values (0.72–1.04‰) estimated for primary ferric (oxyhydr)oxides in this BIF constrain local dissolved O2 contents of the Neoarchean surface ocean to 10–4 to 10–3 μmol/L. By comparison, negative δ56Fe values for magnetite (–0.83 to –0.65‰) in silicate BIFs of the Hongtoushan Formation and the Nantianmen Formation suggest deposition from a residual water column that was depleted in 56Fe. Following the formation of the bulk of the VMS deposits in the Hongtoushan Formation, a significant change to positive magnetite δ56Fe values (0.79–1.04‰) occurs in the youngest sulfide-bearing BIF in the Nantianmen Formation. This implies that the VMS-related hydrothermal vents injected a large mass of unfractionated ferrous iron into the ocean. Negative Δ33S anomalies in sedimentary pyrite of bedded VMS ores (avg of –0.08 ± 0.007‰, n = 6) and sulfide-bearing BIFs (avg of –0.06 ± 0.007‰, n = 3) of the Qingyuan greenstone belt, along with mass-independent fractionations (with an average Δ36S/Δ33S ratio of –1.1 ± 0.3), are best explained by incorporation of seawater sulfate of atmospheric photochemical origin during their formation. The systematic differences in whole-rock geochemistry and Δ33S values for different types of VMS ores imply variable seawater sulfate contributions to their mineralization. Our results are consistent with global anoxic conditions during the Neoarchean to Paleoproterozoic transition (i.e., at 2.5 Ga), and confirm that formation of the VMS-BIF metallogenic association took place in dominantly anoxic, ferruginous basins at different depths, with the VMS-related hydrothermal system contributing significant Fe to the deposition of BIFs.

Abiotic anoxic iron oxidation, formation of Archean banded iron formations, and the oxidation of early Earth

Iron in the early anoxic oceans of Archean age (4000-2500 million years ago) is believed to have been oxidized to form banded iron formations (BIF). Previously, it has been proposed that iron was oxidized either by free oxygen, H2O2, microbial oxidation, or photo-oxidation. However, these mechanisms are difficult to reconcile with evidence for the oceans at that time having been largely devoid of dissolved oxygen and oxidants, together with the rarity of microbial remains in BIF and restrictively slow rates of photo-oxidation. Experiments reported here show that ferrous iron readily oxidizes in analogs of Archean anoxic seawater following the precipitation of ferrous hydroxide. Once precipitated, ferrous hydroxide undergoes decomposition to elemental iron that reacts with water at room temperature to form ferric iron and release hydrogen gas. The ferric iron may then be incorporated into green rust, a mixed ferrous-ferric phase that ages into iron minerals commonly found in BIF. Our finding suggests that anoxic iron oxidation may have contributed to the formation of oxide-facies BIF, especially Algoma-type BIF that likely formed in semi-restricted basins where ferrous hydroxide saturation was more easily achieved. Additionally, ferrous hydroxide decomposition would have contributed to early Earth's oxidation, as a result of hydrogen escape to space, thus providing new insights into environmental and biological conditions on early Earth.

Depositional Setting of Archean BIFs from Congo: New Insight into Under-Investigated Occurrences

Archean banded iron formations (BIF) represent a major contributor to better constraining and assessing the paleogeography and evolution of Archean cratons. In this context, we conducted an exhaustive sampling and analysis campaign of BIF units in the Congo Craton, covering several greenstone belts within the Ivindo, Kelle-Mbomo, and Chaillu blocks. The REE + Y patterns suggest: (1) Interaction of seawater with Fe-oxyhydroxides, as illustrated by strong REE enrichment coupled with La and Y enrichment; (2) contributions from high-temperature (>250 °C) hydrothermal fluids, illustrated by positive Eu anomalies; and (3) detrital input as suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio. These observations suggest a typical environment of Algoma-type BIF deposition. Moreover, assessment of the Ce anomalies in a combination of HREE enrichment indicates that some basins in the Chaillu and Ivindo blocks may have known potential oxygen-rich episodes in the early Archean during the deposition of these BIFs.

Review on geology and mineralization of North Korea (I: Precambrian)

AbstractIn this review, we introduce the geology and mineralization of the northern part of the Korean Peninsula based on recently published data in North Korea. Geological characteristics and events are described in order of geological period and the representative mineral deposits are presented according to the geologic period to show the relationship between the geology and mineralization. In particular, we rearrange the stratigraphy of the Precambrian basement and reset the timing of intrusion and mineralization based mainly on the reported zircon U–Pb age data. The Precambrian geotectonic provinces of North Korea contain continental cores and massifs, active continental margins, and rift zones. The Precambrian strata between the eastern North China Craton (NCC) and North Korea are newly correlated considering rearranged stratigraphy in this review. The Precambrian strata of North Korea are characterized by the depletion of the Paleoproterozoic to Mesoproterozoic strata compared to the NCC from 1.8 to 1.5 Ga. It may indicate that the northern part of the Korean Peninsula was the passive continental margin environment during this period. The common occurrences of bimodal igneous suites (mafic‐felsic and alkaline‐subalkaline) are also characteristic of North Korea. The bimodal magmatism is well explained by the progressive tectonic evolution from oceanic crust‐continent subduction (2.0–2.2 Ga) and island arc–continent collision (~1.9 Ga) to post collisional extension (1.82–1.87 Ga) in the NCC including North Korea. The Precambrian geological similarities between Northeastern China and North Korea would have resulted in the same or similar mineralization. For example, Algoma type banded iron formations of the Fe deposits of the Anshan–Benxi area in China and Musan Fe deposit in North Korea were formed during the Neoarchean and Paleoproterozoic. Sedimentary exhalative type Pb–Zn and metasedimentary type magnesite deposits were formed within the Jiao‐Liao‐Ji belt extending from the Liaoning Rift Zone of China (Pb–Zn deposits in the Qingchenzi orefield and magnesite deposits in Dashiqiao magnesite belt) to the Macheollyeong Rift Zone of North Korea (Geomdeok Pb–Zn deposit and Daeheung–Ryongyang magnesite deposits) during Paleoproterozoic. The geotectonic, geological and metallogenic similarities between Northeastern China and North Korea indicate that the Precambrian continental massifs of North Korea might be evolved in accordance with the NCC evolution. This review is expected to be helpful for understanding the geotectonics, geology, and related mineralization of the northern part of Korean Peninsula and correlating with those of the NCC.

Highly Siderophile Elements and Coupled Fe-Os Isotope Signatures in the Temagami Iron Formation, Canada: Possible Signatures of Neoarchean Seawater Chemistry and Earth's Oxygenation History.

Banded iron formations (BIFs) were deposited before and concurrent with the Great Oxidation Event at ∼2.33 Ga. They provide useful archives that document the transformation of the Precambrian hydrosphere from anoxic to progressively oxygenated conditions. Their formation involves removal of oceanic Fe by either inorganic or biologically promoted Fe2+ oxidation, or both. To evaluate depositional settings, elemental sources that affect seawater chemistry, and oxidation pathways, we present the first combined highly siderophile element (HSE) and Fe-Os isotope study for the ∼2.7 Ga Temagami BIF, Abitibi Greenstone Belt, Ontario (Canada). HSE abundances and 187Os/188Os ratios show no systematic variation between alternating magnetite and (meta)chert bands of the Temagami BIF. Whereas HSE concentrations mostly resemble modern crustal values, present-day 187Os/188Os ratios range from ∼0.17 to ∼10.8. Magnetite samples define a regression line corresponding to an age of 2661 ± 126 Ma. A chondrite-like 187Os/188Os initial value is in agreement with earlier studies on Neoarchean marine sediments and is thought to reflect seawater composition, which, unlike modern oceans, is dominated by mantle-like 187Os inventory most likely derived from deep-sea hydrothermal vents. Our δ56Fe data vary from about +0.6‰ to +0.9‰ and define a sawtooth-like pattern between alternating magnetite and (meta)chert layers. Partial oxidation of hydrothermally sourced Fe(II) and a lack of microbially mediated dissimilatory iron reduction provide the most plausible explanation for the positive δ56Fe values. Notably, our δ56Fe data for Temagami are in accord with trends defined by literature results for other Algoma-type BIFs that were deposited throughout the Archean.

Field and zircon U–Pb geochronological evidence for the occurrence of Cambrian banded iron formations in the West Kunlun Orogenic Belt, China

Abundant bedded iron deposits have been discovered recently in the Taxkorgan Terrane, West Kunlun Orogenic Belt, China. Magnetite is the dominant iron mineral, which is interbedded with amphibolite-facies metamorphic silicate minerals, quartz, and calcite. Field observations revealed that the iron orebodies are hosted in the Bulunkuole Complex, which consists mainly of submarine volcanic and sedimentary sequences. The iron orebodies are typically in conformable contact with bimodal volcanic rocks. The nature of iron mineralization in this region is unclear, mainly because it appears to be similar to typical banded iron formations (BIFs), but is of inferred Cambrian age. We undertook detailed field investigations of this area and zircon U–Pb dating of meta-rhyolites from three iron deposits (i.e., the Zankan, Taaxi, and Ziluoyi deposits). Meta-rhyolites from the deposits have ages of 540.2 ± 2.4, 540.2 ± 2.4 to 515.2 ± 2.7, and 526.4 ± 2.5 to 508.8 ± 3.6 Ma, respectively, indicating that the iron orebodies can be subdivided into four groups, based on their formation ages (i.e., 540, 526, 515, and 508 Ma). These ages suggest that the iron orebodies are unique Cambrian BIFs, although the gangue minerals in these orebodies have been mainly converted into amphibolite-facies minerals, owing to the regional high-grade metamorphism. The close relationship between iron orebodies and bimodal volcanic rocks indicates the iron deposits are Algoma-type BIFs. Abundant pyrite in the iron orebodies from the Zankan deposit suggests that the basin in which these rocks precipitated was sulfidic and in a relatively shallow shelf environment. The lack of pyrite in ores from the Taaxi and Ziluoyi deposits indicates they precipitated in ferruginous basins with relatively deep seawater. We propose that intense submarine magmatism and local anoxic basins were the key factors that controlled the formation of these unique Cambrian BIFs in the Taxkorgan Terrane.

Hexagonal magnetite in Algoma-type banded iron formations of the ca. 2.52 Ga Baizhiyan Formation, North China: Evidence for a green rust precursor?

Abstract Banded iron formations (BIFs) are iron-rich marine chemical sedimentary rocks, and their mineralogy and geochemistry can be used to gain insights into ancient ocean chemistry and biospheric evolution. Magnetite is the major iron-bearing mineral in many BIFs (particularly in the Archean) and is variably interpreted to be of primary, early diagenetic, or metamorphic origin. Different genetic interpretations for magnetite lead to divergent pictures of the Precambrian Earth system and its evolutionary models through time. The Baizhiyan Formation of the Neoarchean Wutai Group (Shanxi, North China) features magnetite-bearing, Algoma-type BIFs deposited ca. 2.52 Ga, in the lead-up to a major period of global iron formation deposition in the Paleoproterozoic. Abundant magnetite crystals found in the silica-rich bands of these BIFs show euhedral, hexagonal morphology. We suggest that this hexagonal magnetite likely represents pseudomorphs after green rust, a mixed-valence iron hydroxy-salt formed in the water column. The rare earth element composition of the BIFs shows negligible to slightly positive Ce anomalies (CeSN/CeSN* = 1.03 ± 0.07), which is characteristic of a dominantly anoxic water column. The presence of positive Eu anomalies (EuSN/EuSN* <3.9) suggests a substantial influence from proximal hydrothermal fluids. The co-occurrence of siderite layers associated with the magnetite-bearing strata may indicate iron cycling associated with ferruginous bottom seawater conditions. Geochemical signatures of the Baizhiyan BIFs are consistent with the interpretation that the magnetite was transformed from metastable green rust. This green rust could have formed via several processes, including the partial oxidation of Fe(II) by molecular oxygen/photoferrotrophs, the reaction of settling ferrihydrite with Fe(II)-rich hydrothermal fluids under anoxic conditions, or local dissimilatory iron reduction. In all cases, the contribution of primary green rust to BIF formation requires iron redox cycling, and similar pseudomorphs in the form of hexagonal magnetite may be more common in the geological record. Our findings support the models in which green rust was an important primary constituent of the Precambrian iron cycle, and the potential interactions of green rust with other elements (e.g., phosphorus) should be taken into consideration when reconstructing Precambrian biogeochemical cycles.

Depositional Environment and Genesis of the Nabeba Banded Iron Formation (BIF) in the Ivindo Basement Complex, Republic of the Congo: Perspective from Whole-Rock and Magnetite Geochemistry

The Nabeba high-grade iron deposit (Republic of the Congo) is hosted by banded iron formation (BIF) in the Ivindo Basement Complex, which lies in the northwestern part of the Congo Craton. The Nabeba BIF is intercalated with chlorite-sericite-quartz schist and comprises two facies (oxide and a carbonate-oxide). In this study, whole-rock and LA-ICP-MS magnetite geochemistry of the BIF was reported. Magnetite samples from both BIF facies had fairly similar trace element compositions except for the rare earth element plus yttrium (REE + Y) distribution patterns. The high V, Ni, Cr, and Mg contents of the magnetite in the Nabeba BIF could be ascribed to the involvement of external medium-high temperature hydrothermal fluids during their deposition in relatively reduced environment. The Post-Archean Australian Shale (PAAS)-normalized REY patterns of the Nabeba BIF magnetite were characterized by LREE depletion coupled with varying La and positive Eu anomalies. Processing of the information gathered from the geochemical signatures of magnetite and the whole-rock BIF suggested that the Nabeba BIF was formed by the mixing of predominantly anoxic seawater (99.9%) with 0.1% of high-temperature (>250 °C) hydrothermal vent fluids, similar to the formation mechanism of many Archean Algoma-type BIFs reported elsewhere in the world.

Depositional and Environmental Constraints on the Late Neoarchean Dagushan Deposit (Anshan-Benxi Area, North China Craton): An Algoma-Type Banded Iron Formation

Abstract The late Neoarchean, ~2.53 to 2.51 Ga Dagushan banded iron formation (BIF), is a typical Algoma-type BIF located in the northeast part of the North China craton. Despite having undergone upper greenschist to lower amphibolite facies metamorphism, the Dagushan BIF retains evidence of varied depositional facies, making it an ideal archive to evaluate the paleomarine environment and the paragenesis of the ore minerals. A transition from oxide to silicate to carbonate facies BIF is evident in a northward direction. The mineralogical composition shifts from magnetite and quartz in the south through a magnetite-quartz-cummingtonite/stilpnomelane assemblage in the transition zone to magnetite-siderite in the north. Such a distinct distribution of mineralogical facies correlates well with the depositional environment of the BIF. The carbonate facies BIFs formed in a near-shore, proximal environment, whereas the oxide and silicate facies BIF assemblages formed in deeper waters, distal to the paleoshoreline. The BIF samples display characteristic seawater-like rare earth element + yttrium (REE + Y) profiles with positive La and Y anomalies and heavy REE enrichment relative to the light REEs when normalized to post-Archean Australian shale. Positive Eu anomalies suggest a high-temperature hydrothermal contribution to the BIF. The absence of a negative Ce anomaly in nearly all samples, coupled with positive δ56Fe in magnetite in all mineralogical facies, indicates a dominantly anoxic water column contemporaneous with deposition of the BIF. At ~2.53 Ga in the Anshan area, seawater was mostly anoxic and rich in ferrous iron. Dissolved ferrous iron in upwelling hydrothermal fluids was oxidized and precipitated as Fe(III) oxyhydroxides in the photic zone leading to BIF formation. Proximal to hydrothermal vents, magnetite formed via the reaction of Fe(III) oxyhydroxides and aqueous Fe(II) supplied from the hydrothermal fluids and microbial dissimilatory iron reduction (DIR) coupled to organic carbon oxidation. Proximal to a paleoshoreline, siderite formed through DIR, as evidenced by the depleted δ13C values and the presence of graphite. Silicates, such as stilpnomelane and cummingtonite, are considered to be the metamorphic products of early diagenetic silicates (e.g., nontronite) that formed in the water column from admixtures of Fe(III) oxyhydroxides and amorphous silica.

Tectonic setting of the Bipindi greenstone belt, northwest Congo craton, Cameroon: Implications on BIF deposition

The banded iron formations (BIFs) in the Bipindi greenstone belt (GB) are hosted within metavolcano-sedimentary sequence of the Nyong Group at the northwestern edge of the Congo Craton (Ntem Complex) in southern Cameroon. New data on the BIFs and its associated rocks allow us to discuss the tectonic setting of this greenstone belt in a regional perspective. Metavolcanic rocks consist of epidosites, mafic granulites, and garnet-pyroxene gneisses, whereas the metasedimentary rocks comprise chlorite-epidote schists. These rocks have been metamorphosed under greenschist-to granulite-facies conditions. The geochemical features of the metavolcanic rocks such as the trace element ratios and rare earth elements (REE) patterns suggest that their protolith corresponded to tholeiitic to transitional and calc-alkaline basalts. The associated metasedimentary rocks are characterized by low La/Sc and Th/Sc discrimination ratios, and REE abundances, indicating that they were derived from mafic and intermediate igneous rocks. Similar protoliths were recently described for metamorphic rocks interbedded with BIFs at the Nyong Group, which were formed in a shallow basin near the continent. The Bipindi BIFs are further characterized by both weak (1.31–1.34) and slightly high (2.00–2.51) positive (Eu/Eu)SN ratio, that are typical of Superior- and Algoma-type BIFs respectively. However, the geochemical features of the host rocks (e.g., REE patterns, Th/Nb, La/Yb, and La/Sc ratios) suggest that the Bipindi BIFs were deposited at an extensional geodynamic setting likely related to a back-arc, similar to the depositional environment for many Algoma-type BIFs. Archean-Paleoproterozoic arc setting is widely distributed across the Congo Caton and the West African Craton, suggesting integration of the tectonic evolution of the Bipindi GB into a continental-scale paleogeography.

Age and genesis of the Neoarchean Algoma-type banded iron formations from the Dengfeng greenstone belt, southern North China Craton: Geochronological, geochemical and Sm–Nd isotopic constraints

Banded iron formations (BIFs) are important for understanding the composition and redox condition of paleo-oceans, as well as the evolution of the Precambrian crust. The Neoarchean Dengfeng greenstone belt (ca. 2.54–2.50 Ga) in the southern North China Craton (NCC) consists of metamorphosed igneous and sedimentary assemblages including some BIFs. Here, we present mineralogical, geochronological, geochemical and Sm–Nd isotopic data of the Dengfeng BIFs for the first time to constrain their age, origins and source characteristics, and then discuss their paleo-environmental implications. The BIFs are mainly associated with metabasaltic rocks and quartz-rich schists, and can be classified as Algoma-type BIFs. Zircon U-Pb dating of quartz-rich schists interbedded with the BIFs, coupled with ages of wall-rock metabasites, demonstrate that the Dengfeng BIFs formed at ~2.53–2.51 Ga. Most BIF samples have high contents of SiO2 and Fe2O3T, and extremely low contents of Al2O3, TiO2, and high-field strength elements (e.g., Zr, Hf, Th, U), indicating that they are detrital-free deposits. In the Post-Archean Australia Shale normalized multi-element diagram, the detritus-free BIF samples are characterized by depleted light rare earth elements, positive La and Y anomalies, and suprachondritic Y/Ho ratios, resembling geochemical features of modern seawater. The absence of negative Ce anomaly indicates an anoxic condition. Positive Eu anomalies and relatively high concentrations of transition metal elements suggest that high-temperature hydrothermal fluids contributed to the source of BIFs. Isotopically, all BIF samples have positive εNd(t) values (+0.12 to + 4.53), consistent with the range of those (+0.10 to + 4.31) of ca. 2.66–2.50 Ga Dengfeng metabasites and TTG gneisses, suggesting input of possible juvenile crustal fluxes. The elemental and isotopic characteristics of the Dengfeng BIFs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust-derived fluxes.

Depositional model for banded iron formation host to gold in the Archean Rio das Velhas greenstone belt, Brazil, based on geochemistry and LA-ICP-MS magnetite analyses

Petrographic and geochemical data were obtained for selected carbonaceous and siderite-bearing chert and banded iron formation (BIF) from the Archean Rio das Velhas greenstone belt (RVGB), Quadrilátero Ferrífero, including seven orogenic gold deposits (Cuiabá, Lamego, Raposos, Roça Grande, São Bento, Urubu and Ápis) and two barren BIFs (Sumidouro and Campo Grande). The RVGB has Algoma-type BIFs that differ in terms of lithological association, mineralogical assemblage and geochemistry. Trace element geochemistry of the BIFs is used to investigate their depositional environment, and to evaluate if primary geochemical and petrographic features have implications for gold mineralization. The chert layers of the Lamego and Urubu deposits exhibit significant, positive Eu anomalies (Eu/Eu*PAAS = 1.57–4.01) and low contents of immobile elements (Al2O3 = <0.34 wt%, TiO2 = <0.02 wt%), suggesting a high contribution of hydrothermal fluid in their formation, with relatively low detrital input. The carbonate- and the magnetite-rich BIF of the Cuiabá, Roça Grande, Ápis and Raposos deposits display Eu/Eu*PAAS = 1.34–3.25; Al2O3 = 0.04–1.47 wt%, TiO2 = <0.03 wt%, interpreted to represent an intermediate contribution of hydrothermal fluid, as well as limited detrital input. The São Bento, Campo Grande and Sumidouro BIF show a significant mineralogical variability including magnetite, carbonate and iron-rich silicates. Their Eu/Eu*PAAS = 1.1–2.6; Al2O3 = 0.11–6.82 wt%, TiO2 = 0.03–0.33 wt% suggest high detrital input and low hydrothermal fluid influence. Textural relations and mineral assemblages of magnetite-rich samples indicate three main generations of this mineral: (i) diagenetic (very low grade metamorphic), (ii) early and (iii) main-stage hydrothermal. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses show that Ti, V, Ni and Co contents of magnetite systematically fluctuate. Their concentrations of Ti (50–677 ppm), V (11–207 ppm), Ni (8–166 ppm) and Co (3–14 ppm) in magnetite (i) are the highest comparatively to types (ii) and (iii) (Ti = 7–352 ppm; V = 0.2–59 ppm; Ni = 1–22 ppm; Co = 0.1–7 ppm). Even higher trace element content of the Sumidouro (i) is related to important clastic input to seawater during BIF deposition. The composition of magnetite types (ii) and (iii) is dependent on coexisting minerals (sulfides, carbonates and silicates), and on the type of mechanism of their formation (dissolution-reprecipitation, or re-equilibration). The mineralogical, geochemical and geological diversity of the RVGB BIFs, as well as magnetite composition in BIF, point to a depositional model for the RVGB BIF within the Rio das Velhas basin related to both proximity to the hydrothermal vent and of terrigenous sedimentary input.Deposition of chert layers (e.g., Lamego and Urubu deposits) was favored near the hydrothermal vents on the abyssal plain due to high concentration of silica and organic carbon. Away from the hydrothermal source, chert transitioned to carbonate-rich BIF, typical of Cuiabá, Roça Grande and Ápis, which still formed under the influence of organic matter. Magnetite-rich BIF, i.e. Raposos deposit, was deposited further away from the volcanic center, with limited supply of organic carbon and silica. In a submarine environment with greater clastic input, such as continental shelfs and island arcs, BIF composition was influenced by detrital contamination, and iron-rich silicates (i.e., stilpnomelane and chlorite) are common, besides magnetite and carbonate; this is the case for the São Bento, Campo Grande and Sumidouro BIFs. The BIF types with abundant iron-rich carbonate and-or magnetite host gold mineralization more efficiently since they are especially favorable to sulfides replacement accompanied by gold precipitation.