An Alternate Explanation for Jasper Formation Prior to the Great Oxidation Event

Dust to dust: Evidence for the formation of “primary” hematite dust in banded iron formations via oxidation of iron silicate nanoparticles

Authors:

Rasmussen et al

Abstract:

Conventional models for the deposition of banded iron formations (BIFs) envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide/hydroxide particles in surface waters that settled to form laterally extensive layers of iron-rich sediment. A fundamental tenet of this model is that fine-grained hematite (so-called dusty hematite) in least-altered BIFs represents the dehydration product of original oxide/hydroxide precipitates. However, this premise has never been proven. We have investigated the origin of the earliest-formed iron oxides in chert in well-preserved BIFs of the 2.63-2.45 billion-year-old Hamersley Group, Australia. We find that laminated chert in BIFs show progressive stages of in situ alteration from grey-green chert, containing iron-silicate nanoparticles, to red chert with abundant hematite dust. Analysis of textures by transmission electron microscopy of samples from the transition zone between grey-green and red chert reveals that dusty hematite formed after partial dissolution of iron-silicate nanoparticles by the precipitation of iron oxides in resulting cavities. These observations suggest that hematite dust is not a relict of an original seawater precipitate but the end-product of post-depositional oxidation. Our observations are consistent with paleomagnetic results from the Hamersley Group, which record two major phases of magnetic remanence carried by hematite that post-date deposition by more than 200 million years. Our results may provide an alternative explanation for the origin of jasper in BIFs deposited before the start of the Great Oxidation Event about 2.4 billion years ago. If correct, it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early Precambrian seawater. Furthermore, our results suggest that the primary iron precipitate in BIFs was iron-silicate mud that was silicified at or just below the sediment-water interface, a hypothesis that requires neither dissolved oxygen nor photosynthetic life, but was an inorganic, chemical process, reflecting anoxic oceans enriched in iron and silica.

Depositional setting of Algoma-type banded iron formation

Depositional setting of Algoma-type banded iron formation

Authors:

Gourcerol et al

Abstract:

Algoma-type banded iron formations (BIF) are chemical sedimentary rocks characterized by alternating layers of iron-rich minerals and chert that are generally interstratified with bimodal submarine volcanic rocks and/or sedimentary sequences in Archean greenstone belts. However, the geological setting for Algoma-type BIF deposition remains equivocal due to the effects of post-depositional deformation and metamorphism, and absence of modern analogues for comparative studies. It is commonly accepted that the abundance of rare earth element and yttrium (REE + Y) in chert bands may retain a primary geochemical signature and therefore constrain their geological setting. In order to explore the latter, a geochemical study using the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) methodology was done using cherts from four Canadian BIF-hosted gold deposits. These results suggest that chert bands record: (1) interaction of seawater with Fe-oxyhydroxides, as suggested by their heavy REE enrichment coupled with La and Y enrichments; (2) contributions from high-temperature (>250 °C) hydrothermal fluids, as suggested by positive Eu excursions; and (3) detrital contamination, which is suggested by relatively consistent REE concentrations and a chondritic Y/Ho ratio (i.e., Y/Ho ≈ 27). Water-column pH conditions at the time of BIF deposition are evaluated using Ce/Ce∗: a positive Ce/Ce∗ anomaly suggests relatively acidic conditions (i.e., pH ⩽ 5) for most of the chert samples, but more alkaline conditions (i.e., pH ⩾ 5) for samples showing Fe-oxyhydroxide precipitation within chert bands. Finally, in situ using secondary ion mass spectrometry (SIMS) analysis (n = 73) of chert from Meliadine show the δ18O of primary amorphous silica (+27‰) was modified to values of around +8‰ to +20‰ during diagenesis at temperatures >100 °C with a fluid having δ18OH2O=0–5‰δ18OH2O=0–5‰. Thus, whereas there has been O isotopic exchange during diagenesis, the REEs and trace elements are not modified in the chert due to the low concentrations of these elements in the reacting fluid of sea water origin.

There Were two DIFFERENT Sources for the Rhyacian PaleoProterozoic Banded Iron Formations

Decoupled sources of the 2.3-2.2 Ga Yuanjiacun banded iron formation: Implications for the Nd cycle in Earth’s early oceans

Authors:

Wang et al

Abstract:

The recognized worldwide gap in BIF deposition between 2.4-2.0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time. However, the recently dated 2.3-2.2 Ga Yuanjiacun banded iron formation (BIF) in the North China Craton (NCC) offers a possibility to redress these uncertainties. Shale-normalized rare earth element-yttrium (REE+Y) patterns of the BIF and interlayered meta-chert samples show features characteristic of other Archean and Paleoproterozoic BIFs, with HREE enrichment relative to LREE, positive La and Eu anomalies, and superchondritic Y/Ho ratios comparable to modern seawater. Very low Al2O3 (less than 0.5 wt%) and high field strength elements (HFSE) concentrations (less than 10 ppm) indicate an essentially detritus-free depositional setting, while positive Eu anomalies are attributed to an imprint of high-temperature hydrothermal fluids. Sm-Nd isotopic features further point to two periodically interacting water masses controlling the deposition of the BIF. The first is seafloor-vented hydrothermal fluids (εNd(t)∼+3.5) derived from interaction with a depleted mantle source and associated with high Fe fluxes. The second is ambient surface seawater (εNd(t)∼-2.4), which obtained its signature through weathering of the nearby landmasses and associated with high Si fluxes. Our findings suggest that the REE budget of the oceans prior to 2.3 Ga was generally dominated by hydrothermal circulation of seawater through depleted mantle-derived source rocks. However, where evolved local continental crust and/or an enriched mantle source were present, this positive mantle Nd signal became discernible in the BIFs. By comparing the Nd isotopic features of similar aged BIFs and marine carbonates, it is concluded that similar to modern oceans, the early Precambrian ocean was not well-mixed with respect to its Nd isotopic composition.

Evidence of the PaleoProterozoic Oceans From the Joffre Banded Iron Formation in Western Australia

The Joffre Banded Iron Formation, Hamersley Group, Western Australia: Assessing the Palaeoenvironment through detailed Petrology and Chemostratigraphy

Authors:

Haugaard et al

Abstract:

The Joffre Member of the Brockman Iron Formation is by volume the largest single known banded iron formation (BIF) in the world. Here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this ∼2.45 billion year old unit. Oxide BIF and silicate-carbonate-oxide BIF dominate the lithology, with minor amounts of interbedded stilpnomelane mudrock, stilpnomelane-rich tuffaceous mudrock and calcareous mudrock. Beside chert and magnetite, the prominent mineralogy is riebeckite, ankerite, hematite, stilpnomelane and crocidolite. The BIF is characterised by an average of 50 wt.% SiO2 and 44.5 wt.% Fe2O3 and an overall low abundance of Al2O3 (less than 1 wt.%), TiO2 (less than 0.04 wt.%), and trace metals such as Cr (less than 10 ppm), Ni (less than 5 ppm) and Mo (less than 0.5 ppm). It has a high ∑REE (rare earth element) content (up to 41 ppm) and a fractionated shale-normalised (SN) seawater REY (rare earth element + yttrium) pattern having an enrichment of HREE (heavy rare earth elements) relative to LREE (light rare earth elements) with an average (Pr/Yb)SN of 0.24. The REY patterns also show a positive LaSN anomaly, no CeSN anomaly and a weakly developed positive YSN anomaly. Iron isotopes (δ56Fe) with positive δ56Fe values of +0.04‰ to +1.21‰ suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides. No epiclastic grains have been found; rather submarine hydrothermal fluids and fine-grained volcanogenic detritus controlled BIF chemistry. The former source is reflected through a constant positive EuSN anomaly throughout the core (average EuSN anomaly of 1.6 with a peak of 2.1 between 100-155 m depth), while the latter source is best reflected through the stilpnomelane-rich tuffaceous mudrock consisting of volcanic ash-fall tuff with relict shards set in a stilpnomelane matrix. The mudrock is overlain by well-preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re-worked volcanic ash formed either on the seafloor or in the water column prior to deposition. An enriched HREE-to-LREE pattern, a high iron content (∼30 wt%), and a δ56Fe value of +0.59‰ collectively imply that the mudrock facies interacted with the Fe-rich seawater prior to deposition. The TiO2-Zr ratio of the BIF and the associated mudrocks suggest a felsic-only-source related to the same style of volcanics as the slightly younger Woongarra rhyolites. Given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics, we speculate that the fine-grained pelagic ash particles may have sourced bio-available nutrients to the surface water. This would have facilitated enhanced biological productivity, including bacterial Fe(II)-oxidation which is now recorded as the positively fractionated 56Fe iron oxide minerals in the Joffre BIF. Alongside submarine hydrothermal input to the basin, the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways, thereby making the Joffre BIF poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses.

Evidence of PaleoArchean Atmospheric Oxygen

A new study shows that iron-bearing rocks that formed at the ocean floor 3.2 billion years ago carry unmistakable evidence of oxygen. The only logical source for that oxygen is the earliest known example of photosynthesis by living organisms, say University of Wisconsin-Madison geoscientists.

"Rock from 3.4 billion years ago showed that the ocean contained basically no free oxygen," says Clark Johnson, professor of geoscience at UW-Madison and a member of the NASA Astrobiology Institute. "Recent work has shown a small rise in oxygen at 3 billion years. The rocks we studied are 3.23 billion years old, and quite well preserved, and we believe they show definite signs for oxygen in the oceans much earlier than previous discoveries."

The most reasonable candidate for liberating the oxygen found in the iron oxide is cyanobacteria, primitive photosynthetic organisms that lived in the ancient ocean. The earliest evidence for life now dates back 3.5 billion years, so oxygenic photosynthesis could have evolved relatively soon after life itself.

Until recently, the conventional wisdom in geology held that oxygen was rare until the "great oxygenation event," 2.4 to 2.2 billion years ago.

The rocks under study, called jasper, made of iron oxide and quartz, show regular striations caused by composition changes in the sediment that formed them. To detect oxygen, the UW-Madison scientists measured iron isotopes with a sophisticated mass spectrometer, hoping to determine how much oxygen was needed to form the iron oxides.

link.

The Archean Banded Iron Formations of Krivoy Rog, Ukraine

Geochemistry of the Krivoy Rog Banded Iron Formation, Ukraine, and the impact of peak episodes of increased global magmatic activity on the trace element composition of Precambrian seawater

Authors:

Viehmann et al

Abstract:

Pure Superior-type Banded Iron Formation (BIF) samples from the Krivoy Rog Supergroup (Ukraine) are excellent archives of ambient Early Precambrian seawater. They show low concentrations of incompatible elements such as Zr, Hf, and Th, and shale-normalized Rare Earths and Yttrium (REYSN) patterns similar to those of modern seawater, i.e. heavy REYSN enriched patterns with positive LaSN, GdSN and YSN anomalies. Lack of CeSN and presence of positive EuSN anomalies indicate REY contributions to anoxic ferruginous seawater from high-temperature hydrothermal fluids.

The depositional age of the Krivoy Rog BIF is ill-defined, but a Late Archean to Paleoproterozoic age has been suggested based on U–Pb zircon ages for units stratigraphically above and below the BIF. We determined Sm–Nd isotopic compositions of pure and impure samples from the Krivoy Rog BIF, which yield an errorchron with an apparent age of 2406 ± 350 Ma (MSWD 15), that falls within this broad age range. All pure BIF samples show chondrite-normalized (subscript CN) REY patterns with strong positive EuCN anomalies that are typical for Archean but rather rare and much less pronounced in Proterozoic BIFs. Associated schists also show Archean – rather than post-Archean-style REY distributions. The REY geochemistry of both, chemical and epiclastic sediments, therefore, is more consistent with a Late Archean rather than a post-Archean depositional age of the Krivoy Rog Supergroup.

Initial ɛNd values of impure BIFs and of associated schist reveal variable contributions from TTGs less radiogenic in Nd and a more radiogenic component possibly comprised of basement amphibolites or mafic volcanics of the stratigraphically underlying New Krivoy Rog Group. The purest Krivoy Rog BIF, representing local Krivoy Rog seawater, displays an ɛNd2.60 Ga value of −2.3. This value is less radiogenic than impure Krivoy Rog BIFs or other near-contemporaneous Neoarchean pure chemical sediments. To preserve this specific local isotopic fingerprint in anoxic Archean seawater, the Krivoy Rog BIF must have been deposited in an isolated sea basin with limited exchange with ferruginous deep-waters of the open ocean.

A compilation of REY data for high-purity Precambrian BIFs reveals that EuCN/Eu*CN ratios of Precambrian seawater follow a general global evolution curve, that shows specific peaks which reflect times of increased high-temperature hydrothermal REY input into seawater. Following declining EuCN/EuCN ratios from the Eoarchean to the Mesoarchean, the ratios suddenly rise at 2.7 Ga and reach a maximum at 2.6 Ga, indicating an increased flux of high-temperature hydrothermal REY to Neoarchean seawater, which supports the hypothesis that times of widespread BIF deposition coincided with periods of intense submarine hydrothermal activity, probably triggered by major mantle plume events. This association is supported by a strong increase of the ɛNd(t) values of pure seawater archives at 2.7–2.6 Ga, which reflects an increased flux of mantle Nd into seawater. These results suggest that Eu-REY systematics (and potentially ɛNd systematics) are robust tools to indentify episodes of enhanced mantle plume activity.

Kabuno Bay's Iron Oxidizing Bacteria Give Clues to Earth's Archean and Proterozoic Oceans

An isolated, iron-rich bay in the heart of East Africa is offering scientists a rare glimpse back into Earth's primitive marine environment, and supports theories that tiny microbes created some of the world's largest ore deposits billions of years ago.

According to University of British Columbia (UBC) research published this week in Scientific Reports, 30 per cent of the microbes in the Democratic Republic of the Congo's Kabuno Bay grow by a type of photosynthesis that oxidizes (rusts) iron rather than converting water into oxygen like plants and algae.

"Kabuno Bay is a time machine back to the Earth's early history when iron-rich ocean chemistry prevailed," said Marc Llirós of the University of Namur, first author of the paper.

"The bay is giving us real-world insight into how ancient varieties of photosynthesis may have supported Earth's early life prior to the evolution of the oxygen producing photosynthesis that supports life today," said UBC geomicrobiologist Sean Crowe, senior author of the study.

While iron-respiring bacteria were discovered in 1993, the new Scientific Reports study provides evidence that microorganisms could have been directly involved in depositing the Earth's oldest iron formations.

Before 2.3 billion years ago, there was little oxygen in the atmosphere but plenty of dissolved iron and many organisms like bacteria derived energy by metabolizing the metal. Many researchers believe iron-metabolizing microbes might have turned plentiful dissolved iron into minerals, which then settled out of seawater and deposited along the ocean floor.

link.

1/2 of Banded Iron Formations are Microbial in Origin

Think of an object made of iron: An I-beam, a car frame, a nail. Now imagine that half of the iron in that object owes its existence to bacteria living two and a half billion years ago.

That's the upshot of a study published this week in the Proceedings of the National Academy of Sciences (PNAS). The findings have meaning for fields as diverse as mining and the search for life in space.

Clark Johnson, a professor of geoscience at the University of Wisconsin-Madison, and former postdoctoral researcher Weiqiang Li examined samples from the banded iron formation in Western Australia. Banded iron is the iron-rich rock found in ore deposits worldwide, from the proposed iron mine in Northern Wisconsin to the enormous mines of Western Australia.

These ancient deposits, up to 150 meters deep, were begging for explanation, says Johnson.

Scientists thought the iron had entered the ocean from hot, mineral-rich water released at mid-ocean vents that then precipitated to the ocean floor. Now Johnson and Li, who is currently at Nanjing University in China, show that half of the iron in banded iron was metabolized by ancient bacteria living along the continental shelves.

The banding was thought to represent some sort of seasonal changes. The UW-Madison researchers found long-term swings in the composition, but not variations on shorter periods like decades or centuries.

link.

Explaining the Rhyacian PaleoProterozoic Lomagundi Event

The rise of oxygen and siderite oxidation during the Lomagundi Event

Authors:

Bachan et al

Abstract:

The Paleoproterozoic Lomagundi Event is an interval of 130–250 million years, ca. 2.3–2.1 billion years ago, in which extraordinarily 13C enriched (greater than 10‰) limestones and dolostones occur globally. The high levels of organic carbon burial implied by the positive δ13C values suggest the production of vast quantities of O2 as well as an alkalinity imbalance demanding extremely low levels of weathering. The oxidation of sulfides has been proposed as a mechanism capable of ameliorating these imbalances: It is a potent sink for O2 as well as a source of acidity. However, sulfide oxidation consumes more O2 than it can supply CO2, leading to insurmountable imbalances in both carbon and oxygen. In contrast, the oxidation of siderite (FeCO3 proper, as well as other Fe2+-bearing carbonate minerals), produces 4 times more CO2 than it consumes O2 and is a common—although often overlooked—constituent of Archean and Early Proterozoic sedimentary successions. Here we propose that following the initial rise of O2 in the atmosphere, oxidation of siderite provided the necessary carbon for the continued oxidation of sulfides, burial of organic carbon, and, most importantly, accumulation of free O2. The duration and magnitude of the Lomagundi Event were determined by the size of the preexisting Archean siderite reservoir, which was consumed through oxidative weathering. Our proposal helps resolve a long-standing conundrum and advances our understanding of the geologic history of atmospheric O2.

 I think I am going to drop a note to the authors and ask if they have compared the carbon 13 isotopes in the limestones and dolostones to modern ones... or at least to Phanerozoic ones.  Just to potentially rule out this.  ;)

Pre Sturtian Cryogenian NeoProterozoic Deposits Hint at Strange Open PaleoOcean Sea Water Chemistry

Pre-Sturtian (800–730 Ma) depositional age of carbonates in sedimentary sequences hosting stratiform iron ores in the Uppermost Allochthon of the Norwegian Caledonides: A chemostratigraphic approach

Authors:

Melezhik et al

Abstract:

Carbon and strontium isotope chemostratigraphy (52 δ13Ccarb and δ18O, and 50 87Sr/86Sr analyses of carbonate components in whole-rock samples) was applied for constraining an apparent depositional age of the carbonate protolith to amphibolite-grade, calcite marbles occurring in siliciclastic sedimentary sequences hosting iron formations (the Dunderlandsdalen type iron ores) of previously unknown age in the Rödingsfjället Nappe Complex of the Rana region, Nordland, Norway. The least altered 87Sr/86Sr (0.70676) and δ13C (+2.5 to +5.6‰) values of the marbles (Dunderland Marble 2a) in the hanging wall sequence of the Stensundtjern iron formation in the Rana region are consistent with seawater composition in the time interval 800–730 Ma, hence the Middle Cryogenian (pre-Sturtian). The least altered Sr- and C-isotopic values obtained from two other marbles units (Marble 3 and 4) of the Rödningsfjället Nappe Complex are consistent with the Late Cryogenian (c. 660 and 670–700 Ma, respectively). The ages obtained provide the first insight into the depositional time of sediment-hosted iron formations of the Uppermost Allochthon in the North-Central Norwegian Caledonides. The Middle Cryogenian-age of marbles associated with iron formations in the Rana region and those located c. 250 km to the north (the Håfjellet iron ore horizon) share similar 87Sr/86Sr and δ13C ratios and hence similar chemostratigraphic ages. These iron formations were originally accumulated outside of Baltica, on a glacially influenced carbonate-siliciclastic shelf, apparently on a margin of an unknown microcontinent. The Scandinavian Dunderlandsdalen and the Håfjellet iron ores of Middle Cryogenian age (800–730 Ma) were accumulated in an open marine environment distant from volcanic centres, and hence represent an outstanding exception to other reported Neoproterozoic iron formations which were all accumulated in volcanically active continental rift settings.

How Were the Siderian PaleoProterozoic Banded Iron Formations Deposited?

Seafloor silicification and hardground development during deposition of 2.5 Ga banded iron formations

Authors:

Rasmussen et al

Abstract:

Banded iron formations (BIFs) are important archives of the ancient oceans, atmosphere, and biosphere, but fundamental questions remain about their origin. It is widely assumed that BIFs were derived from layers of ferric oxyhydroxides and silica that precipitated directly from a water column that was enriched in dissolved iron and silica. The reported lack of current-generated structures and clastic particles beyond mud grade, and the perceived basin-scale extent of laminae, is regarded as evidence for uninterrupted pelagic settling with no sedimentary reworking. New sedimentological and petrographic results show that laminated cherts in the 2.5 Ga Dales Gorge Member of the Brockman Iron Formation, Western Australia, preserve textures indicative of in situ brecciation immediately below the seafloor and the deposition of intraformational sandstones composed of chert clasts in a chert matrix. Chert intraclasts have two sedimentary components: silt-sized microgranules and submicron-sized particles, indicating that the original sediment comprised iron-rich silicate muds that were cemented on or just below the seafloor by pore-filling silica. Silicified muds were episodically eroded by density currents, and the resulting detritus was transported as sand-sized clasts and locally deposited in a matrix of microgranules and mud. Our results support the hypothesis that high concentrations of silica in early Precambrian seawater favored episodic silica cementation of sediments on the seafloor. We suggest that competition between sediment accumulation and seafloor silica cementation, with subsequent differential compaction, explains primary layering in BIFs between beds of relatively thickly laminated chert and beds of thinly laminated, iron-rich minerals. The thickest laminated chert beds are interpreted to represent intervals when seafloor silicification outpaced deposition of hydrothermal muds, forming the equivalent of Phanerozoic hardgrounds at sequence boundaries.

Fossil Evidence of Iron Oxidizing Bacteria From the Great Oxidation Event

Fossil evidence of iron-oxidizing chemolithotrophy linked to phosphogenesis in the wake of the Great Oxidation Event

Authors:

Crosby et al

Abstract:

The oxygenation of Earth's atmosphere allowed for the diversification of metabolisms to include those that rely on oxygen and its derivatives. For example, chemolithotrophic oxidation of sulfide and iron both require oxygen or nitrate as terminal electron acceptors. A growing number of oxygen-utilizing chemolithotrophs are known to accumulate intracellular polyphosphate as an energy reserve that allows them to adapt to the fluctuating redox conditions in their distinctive-gradient habitats. Polyphosphate is also thought to play an important role in the formation of phosphatic mineral deposits. Here we present fossil evidence of iron-oxidizing bacteria preserved as filamentous iron oxides within phosphatic Paleoproterozoic stromatolites. The filaments include twisted stalks similar to those produced by modern iron-oxidizing bacteria that are known to metabolize polyphosphate and inhabit steep redox gradients. Fossil iron-oxidizing bacteria preserved within some of the oldest known phosphorites serve as indicators of O2-Fe(II) gradients that may have supported microbially mediated phosphogenesis via polyphosphate metabolism and/or an active iron redox pump.

Evidence of Oxygenated Marine Bottom Waters at the NeoArchean/PaleoProterozoic Boundary?!?!

Petrology And Geochemistry Of The Wangjiazhuang Banded Iron Formation And Associated Supracrustal Rocks From The Wutai Greenstone Belt In The North China Craton: Implications For Their Origin And Tectonic Setting

Authors:

Wang et al

Abstract:

The Wutai greenstone belt (WGB) is one of the most extensively studied greenstone belts in China. Together with the Hengshan and Fuping complexes, these three associations compose the central segment of the Trans-North China Orogen (TNCO) in the North China Craton (NCC). The Wangjiazhuang banded iron formation (BIF) is located in the bottom of the Jingangku Formation of the WGB. The associated supracrustal rocks consist of meta-basalts (amphibolites), meta-felsic volcanic rocks (leptynite) and metapelites (mica schist), which have experienced amphibolite-facies metamorphism. Amphibolites are commonly intercalated with the BIF. SIMS zircon U-Pb analyses on amphibolites suggest that the Wangjiazhuang BIF was formed at ca. 2543 ± 4 Ma. Combined with most Neoarchean Algoma-type BIFs in the NCC, these features indicate that a significant tectothermal event of the NCC have occurred at ∼2.5 Ga. Mineral assemblages of the Wangjiazhuang BIF are composed of quartz, magnetite, amphibole, and minor garnet, pyrite and calcite. The precursor deposits of this BIF were likely ferric-oxyhydroxides, fine-grained carbonate oozes, silicate phases rich in Al-Ca-Mg-Fe and amorphous silica. The appearance of garnet and ferro-pargasite, high concentrations of Al2O3, HFSEs, Sc, and positive correlations among Al2O3, TiO2, HFSEs and REE indicate that there was a significant terrigenous input. Even so, the Wangjiazhuang BIF samples display distinctively seawater-like REE + Y profiles, characterized by positive La and Y anomalies and HREE enrichment relative to LREE in PAAS-normalized REE diagrams. Consistently positive Eu anomalies are also observed, which are typically from high-T hydrothermal fluids. In addition, the true negative Ce anomalies recorded in the Wangjiazhuang BIF might indicate the onset of bottom-water oxidation at the Archean-Proterozoic boundary, at least in restricted basins. Amphibolites have geochemical affinity with both MORB- and arc-like components, and the trace element characteristics of leptynites are also consistent with a subduction zone signature. These features suggest that the Wangjiazhuang BIF was deposited in a back-arc basin. Oceanic subduction, coupled with contemporary depleted mantle upwelling related to back-arc basin extension, can account for the typical interaction between these two components.