Sedimentary Iron Formation Research Articles

Thermal Decomposition of Siderite and Characterization of the Decomposition Products under O2 and CO2 Atmospheres

Siderite (FeCO3) is an iron-bearing carbonate mineral that is the most abundant sedimentary iron formation on Earth. Mineralogical alteration of four siderite samples annealed at temperatures 200 °C, 300 °C, 400 °C, 500 °C, 750 °C, and 1000 °C in an O2 and a CO2 atmosphere were investigated using such tools as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the X-ray fluorescence (XRF) method, differential scanning calorimetry and thermogravimetric analysis (DSC/TGA), and Mössbauer spectroscopy measurements. The decomposition of three siderite samples with similar iron content in the oxygen atmosphere took place in the temperature range of 340–607 °C. This process begins at approximately ~100 °C higher under a reducing atmosphere, but it is completed just above 600 °C, which is a temperature comparable to decomposition in an oxidizing atmosphere. These processes are shifted toward higher temperatures for the fourth sample with the lowest iron but the highest magnesium content. Magnetite, hematite, and maghemite are products of siderite decomposition after annealing in the oxygen atmosphere in the temperature range 300–500 °C, whereas hematite is the main component of the sample detected after annealing at 750 °C and 1000 °C. Magnetite is the main product of siderite decomposition under the CO2 atmosphere. However, hematite, maghemite, wüstite, and olivine were also present in the samples after annealing above 500 °C in this atmosphere.

Mechanism of formation, mineralogy and geochemistry of the ooidal ironstone of Djebel Had, northeast Algeria

The Djebel Had Ironstone (DHIS), an 8 m thick stratiform sedimentary iron formation, forms part of the important mining district of south Tebessa, in northeastern Algeria. Stratigraphic, lithological, structural and metallogenic similarities, suggest the DHIS may extend further into southwestern Tunisia. We show that mineralization occurs as layers of ooidal ironstones and inter-laminated iron marl within mid-Eocene gypsiferous marls. The more or less rounded 0.1–2.0 mm brownish-blackish ooids, are composed of goethite, limonite, hematite, with traces of magnetite and piemontite. The grains display a smooth outer surface bound by an argilo-ferruginous layer embedded in siliceous-calcite cement. They are unusually friable, crumbling at the slightest shock. A high total iron (FeT) content of 50.12%, is dominated by up to 71.06% iron hydroxide (FeO(OH). Much of the iron is present as goethite, a common feature of iron-rich ooids of North African origin. However, the lack of prominent chlorite minerals suggest the DHIS is not of a detrital origin. Instead, a negligible Ti and Al oxide concentration suggest a chemical provenance for the DHIS. The data suggest that ferruginous conditions developed in a potentially restricted/semi-restricted continental shelf margin where seafloor redox was sensitive to the alternating cycles of sea level change. We propose a new mechanism for the formation of ooidal ironstones, associated with shelf surface water eutrophication, bottom water anoxia promoted by sea level rise and the weathering of iron phosphate-rich rocks. Phosphorus and cerium enrichment, coupled to reconstructed redox depositional conditions and sediment mineralogy, suggest that intense biomass production stimulated the deoxygenation of shelf bottom waters and the deposition of the DHIS beneath a ferruginous water column.

Formation of diagenetic siderite in modern ferruginous sediments

Ferruginous conditions prevailed in the world’s deep oceans during the Archean and Proterozoic Eons. Sedimentary iron formations deposited at that time may provide an important record of environmental conditions, yet linking the chemistry and mineralogy of these sedimentary rocks to depositional conditions remains a challenge due to a dearth of information about the processes by which minerals form in analogous modern environments. We identified siderites in ferruginous Lake Towuti, Indonesia, which we characterized using high-resolution microscopic and spectroscopic imaging combined with microchemical and geochemical analyses. We infer early diagenetic growth of siderite crystals as a response to sedimentary organic carbon degradation and the accumulation of dissolved inorganic carbon in pore waters. We suggest that siderite formation proceeds through syntaxial growth on preexisting siderite crystals, or possibly through aging of precursor carbonate green rust. Crystal growth ultimately leads to spar-sized (>50 μm) mosaic single siderite crystals that form twins, bundles, and spheroidal aggregates during burial. Early-formed carbonate was detectable through microchemical zonation and the possible presence of residual phases trapped in siderite interstices. This suggests that such microchemical zonation and mineral inclusions may be used to infer siderite growth histories in ancient sedimentary rocks including sedimentary iron formations.

The mineralogy, mineral chemistry, and origin of the Wuyang banded iron formations, North China Craton

Several banded iron formations (BIFs) occur in Precambrian strata of the North China Craton (NCC). Two sub-types of BIF occur in the Wuyang area, quartz-magnetite (QMB) and pyroxene-magnetite BIFs (PMB). However, the genesis of pyroxene and the how the BIFs formed are not well understood. Magnetite grains in QMB are relatively large with a straight boundary and sharp corners, and display a pronounced alignment, whereas magnetite in PMB is disseminated and has a smaller grain size. X-ray diffraction (XRD) analyses indicate that the crystal parameters of magnetite in QMB are: a = b = c = 8.396 Å and z = 8, whereas those of the magnetite in PMB are: a = b = c = 8.394 Å and z = 8. These parameters are consistent with magnetite forming in sedimentary iron formation. Based on scanning electron microscopy and energy dispersive spectroscopy (EDS) results, magnetite of both BIFs possesses high stabilities of geochemical property. Compared with magnetite in PMB, magnetite in QMB has higher concentrations of SiO2, Na2O, Al2O3, CaO, K2O, MgO, Sc, Co, Y, Nb, Ta and U. The enrichment of these elements is consistence with their derivation from seawater due to the strong adsorption capability of precursor iron hydroxide. A spider diagram of trace elements reveals that magnetite in QMB is enriched in Ta, whereas magnetite of PMB is depleted in Ce. Based on Raman and electron probe micro-analysis of pyroxene in PMB, synchronous and compositional zoning are found in the degrees of crystallinity (FWHM values of Gaussian fitting for Raman shift next to 1014 cm−1) and the concentration of SiO2, Al2O3, MgO, MnO, CaO and FeO of pyroxene from core to rim. The two types of BIF formed in different sedimentary environments from different precursors. The precursor to the QMB BIF is almost pure chemical sediment, whereas a that for PMB contains a volcanic ash component. The QMB was likely deposited during a period of hydrothermal sedimentation in the Wuyang basin that was associated with Neoarchean magmatism activity, while the PMB was deposited during periods of volcanic paroxysm.

Snowball Earth ocean chemistry driven by extensive ridge volcanism during Rodinia breakup

During Neoproterozoic Snowball Earth glaciations, the oceans gained massive amounts of alkalinity, culminating in the deposition of massive cap carbonates on deglaciation. Changes in terrestrial runoff associated with both breakup of the Rodinia supercontinent and deglaciation can explain some, but not all of the requisite changes in ocean chemistry. Submarine volcanism along shallow ridges formed during supercontinent breakup results in the formation of large volumes of glassy hyaloclastite, which readily alters to palagonite. Here we estimate fluxes of calcium, magnesium, phosphorus, silica and bicarbonate associated with these shallow-ridge processes, and argue that extensive submarine volcanism during the breakup of Rodinia made an important contribution to changes in ocean chemistry during Snowball Earth glaciations. We use Monte Carlo simulations to show that widespread hyaloclastite alteration under near-global sea-ice cover could lead to Ca2+ and Mg2+ supersaturation over the course of the glaciation that is sufficient to explain the volume of cap carbonates deposited. Furthermore, our conservative estimates of phosphorus release are sufficient to explain the observed P:Fe ratios in sedimentary iron formations from this time. This large phosphorus release may have fuelled primary productivity, which in turn would have contributed to atmospheric O2 rises that followed Snowball Earth episodes. The Cryogenian Snowball Earth glaciations were followed by the deposition of massive cap carbonates. Geochemical modelling suggests that shallow-ridge volcanism supplied much of the alkalinity and cations that fuelled this deposition.

Iron isotopic constraints on the genesis of Bayan Obo ore deposit, Inner Mongolia, China

The giant polymetallic Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world. Despite the fact that a great number of works have been done on the deposit, its origin remains controversial. Various genetic models have been proposed, including sedimentary origin, magmatic origin, hydrothermal origin or origin with multiple processes. Here the Fe isotope compositions of different types of rocks from the deposit and related geological formations, such as carbonatites, mafic dykes, and Mesoproterozoic sedimentary iron formation and carbonates, are systematically investigated. For the ore deposit, the average δ56Fe values are −0.03±0.16‰ (2SD, n=14), 0.01‰±0.24‰ (2SD, n=6), −0.07±0.24‰ (2SD, n=19) for bulk samples of fine-grained iron ores, gangue rocks and ore-hosting dolomite marble, respectively; and 0.01±0.14 (2SD, n=14), 0.08±0.18 (2SD, n=3), −0.21 for the magnetite, hematite and dolomite, the main Fe oxide and carbonate minerals in the deposit. The narrow range of the near-zero δ56Fe values of fine-grained iron ores and Fe oxide minerals are consistent with those of magmatic products such as igneous rocks and magmatic iron ores, but different from those of sedimentary or hydrothermal products like Precambrian sedimentary iron formations and hydrothermal iron ores reported previously. The slightly negative Fe isotope values of the ore-hosting dolomite marble are consistent with those of the carbonatite dykes in Bayan Obo area and the typical carbonatites worldwide, but different from those of Mesoproterozoic sedimentary carbonates. The small Fe isotope fractionations between magnetite and dolomite (Δ56Femagnetite-dolomite≈0.23‰), and between hematite and magnetite (Δ56Fehematite-magnetite≈0.07‰), indicate that the ore deposit experienced a very high temperature. Overall, the Fe isotope compositions are inconsistent with either sedimentary or hydrothermal origin, but support a magmatic origin for the Bayan Obo deposit.

Reconstructing Earth's surface oxidation across the Archean-Proterozoic transition

The Archean-Proterozoic transition is characterized by the widespread deposition of organic-rich shale, sedimentary iron formation, glacial diamictite, and marine carbonates recording profound carbon isotope anomalies, but notably lacks bedded evaporites. All deposits refl ect environmental changes in oceanic and atmospheric redox states, in part associated with Earth’s earliest ice ages. Time-series data for multiple sulfur isotopes from carbonateassociated sulfate as well as sulfi des in sediments of the Transvaal Supergroup, South Africa, capture the concomitant buildup of sulfate in the ocean and the loss of atmospheric massindependent sulfur isotope fractionation. In phase with sulfur is the earliest recorded positive carbon isotope anomaly, convincingly linking these environmental perturbations to the Great Oxidation Event (ca. 2.3 Ga).

Isotope stratigraphy of the Lapa Formation, São Francisco Basin, Brazil: Implications for Late Neoproterozoic glacial events in South America

The Lapa Formation is a thick carbonate sequence (∼900m) that constitutes the upper part of the Vazante Group on the São Francisco craton, Brazil. It conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age. The sequence, above the glacial unconformity, consists predominantly of organic-rich shale, subtidal rhythmic dolomicrites and microbialaminites, and intertidal stromatolites. Four boreholes, spanning different depositional settings, were sampled at high-resolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation.The δ13C and δ18O values of well preserved Lapa carbonate microsamples range from −8.2‰ to 3.3‰ (VPDB) and from −13.6‰ to −0.9‰ (VPDB), respectively. Each of the δ13C profiles of the investigated cores reveals two strong negative excursions of up to 8‰, an event in post-glacial dolomicrites immediately above the glaciogenic unit and a 10m interval of organic-rich shale, and a second near the top of the sequence associated with a shale interval. Based on the observation of dropstones and sedimentary iron formation in the underlying diamictite, as well as the distinguishable carbon isotope trends, the Lapa Formation is considered as a cap carbonate lithofacies. The age of the Lapa Formation is presently unknown but the least radiogenic 87Sr/86Sr value (∼0.7068), associated with a negative carbon isotope excursion, matches that from the Rasthof Formation in Namiba on the Congo craton, which is radiometrically constrained to be younger than ca. 750Ma.

The snowball Earth hypothesis: testing the limits of global change

The gradual discovery that late Neoproterozoic ice sheets extended to sea level near the equator poses a palaeoenvironmental conundrum. Was the Earth's orbital obliquity > 60° (making the tropics colder than the poles) for 4.0 billion years following the lunar‐forming impact, or did climate cool globally for some reason to the point at which runaway ice‐albedo feedback created a `snowball' Earth? The high‐obliquity hypothesis does not account for major features of the Neoproterozoic glacial record such as the abrupt onsets and terminations of discrete glacial events, their close association with large (> 10‰) negative δ13C shifts in seawater proxies, the deposition of strange carbonate layers (`cap carbonates') globally during post‐glacial sea‐level rise, and the return of large sedimentary iron formations, after a 1.1 billion year hiatus, exclusively during glacial events. A snowball event, on the other hand, should begin and end abruptly, particularly at lower latitudes. It should last for millions of years, because outgassing must amass an intense greenhouse in order to overcome the ice albedo. A largely ice‐covered ocean should become anoxic and reduced iron should be widely transported in solution and precipitated as iron formation wherever oxygenic photosynthesis occurred, or upon deglaciation. The intense greenhouse ensures a transient post‐glacial regime of enhanced carbonate and silicate weathering, which should drive a flux of alkalinity that could quantitatively account for the world‐wide occurrence of cap carbonates. The resulting high rates of carbonate sedimentation, coupled with the kinetic isotope effect of transferring the CO2 burden to the ocean, should drive down the δ13C of seawater, as is observed. If cap carbonates are the `smoke' of a snowball Earth, what was the `gun'? In proposing the original Neoproterozoic snowball Earth hypothesis, Joe Kirschvink postulated that an unusual preponderance of land masses in the middle and low latitudes, consistent with palaeomagnetic evidence, set the stage for snowball events by raising the planetary albedo. Others had pointed out that silicate weathering would most likely be enhanced if many continents were in the tropics, resulting in lower atmospheric CO2 and a colder climate. Negative δ13C shifts of 10–20‰ precede glaciation in many regions, giving rise to speculation that the climate was destabilized by a growing dependency on greenhouse methane, stemming ultimately from the same unusual continental distribution. Given the existing palaeomagnetic, geochemical and geological evidence for late Neoproterozoic climatic shocks without parallel in the Phanerozoic, it seems inevitable that the history of life was impacted, perhaps profoundly so.

Reactions forming smythite, Fe 9S 11

This study evaluates the processes that form smythite, a mineral associated normally either with siderite in sedimentary iron formations and hydrothermal carbonate veins, or with pyrrhotite in nickel sulfide ore deposits. Smythite was found to form via either of two processes, the replacement of siderite in low temperature aqueous environments as proposed by Rickard (1968) or exsolution from high-temperature, S-rich pyrrhotite during cooling as proposed by Fleet (1982). In solutions with pH > 6 at temperatures below 53°C, surface-bound Fe 2+ reacts with aqueous sulfide on the siderite surface to form smythite. On the other hand, in low pH sulfide solutions with little dissolved CO 2, siderite dissolves rapidly yielding free Fe 2+ and saturates the bulk solution with amorphous Fe(HS) 2. Amorphous Fe(HS) 2 is subsequently replaced by mackinawite and then by pyrite but not smythite. When S-rich (less than ∼46 Fe atomic %) hexagonal pyrrhotite 1C is cooled slowly from 500°C or higher, it re-equilibrates to an assemblage that is stable at low temperature (S-poor pyrrhotite + pyrite). However, if cooling is rapid (several hundred °C/s), in the absence of local pyrite grains, pyrite does not exsolve owing to its slow nucleation. Because smythite is structurally similar to pyrrhotite, its nucleation is possible during such rapid cooling thereby balancing the bulk composition of the system without pyrite exsolution. Smythite exsolution occurs only in a pyrite-free environment because pyrite overgrowths can occur if pre-existing pyrite is present for nucleation. This eliminates the need for metastable exsolution of smythite to achieve a mass balance for sulfur. Pyrrhotite containing some Ni (1.6 ∼ 2.9 atomic %) exsolves smythite during slower cooling because Ni retards the equilibrium exsolution of pyrite.

Metallogenesis of the Abitibi greenstone belt of Canada: a contribution from the analysis of trace lead in sulfide minerals

Pb-isotopic compositions are reported for 12 ore localities within the late Archean Abitibi greenstone belt. The studied samples carry massive or disseminated sulfides hosted in a variety of materials, including mafic–ultramafic igneous rocks, felsic lavas, porphyries, and sedimentary iron formations. Repeated leaching experiments on these sulfides frequently revealed the presence of a radiogenic Pb component, which is attributed to in situ decay of U and Th. The leaching experiments make it possible, in some cases, to separate the radiogenic Pb from the initial Pb included in the minerals. Six Pb–Pb isochrons formed by the analyses on leachates and residues show little evidence of secondary perturbations and yield ages that are, within error, similar to those determined for the supracrustal assemblage. This implies that the ores were concentrated synchronously with the main phases of magmatic activity, close to 2.7 Ga.The initial isotopic compositions of the sulfide specimens point to the existence of two different sources of metals: (i) juvenile, mantle-derived igneous rocks and (ii) older recycled supracrustal series. Ore formation frequently involves mixing of metals from these two sources in variable proportions. The initial isotopic composition of these two reservoirs is best evaluated by examining the composition of sulfides associated with komatiitic lava flows and with sedimentary iron formations, respectively.

Microbial structures in oolitic iron formations

Ferriferous ooids and microoncoids occur in many sedimentary iron formations. These ferriferous-coated grains are found in fine-grained carbonate-rich groundmasses. Iron mineral-encrusted microbiota are observed in both the coated grains and the groundmass. The branching nature and other morphological features of the microorganisms suggest fungal origins for the oolitic iron ores particularly of the Lower Jurassic (Lorraine Minette). The similarity of the microbial structures in coated grains and their groundmasses suggests that both had developed within microbial mats growing under calm environmental conditions. The contribution of stromatolitic marine fungal mats to the fast extraction and immobilization of iron and thus to the genesis of iron ores is demonstrated. Observations on well-preserved freshwater-derived fungal stromatolites of the Tertiary and laboratory experiments with organotrophic fungal mats confirm the findings on the Jurassic and imply a general role of microorganisms in the formation of such deposits.

Fossil iron bacteria may be preserved in Precambrian ferroan carbonate

CONCLUSIVE evidence for microbial participation in the genesis of the massive sedimentary iron formations from the Precambrian is fragmentary. Structures resembling fossilised bacteria have been reported previously, and they tend to resemble modern iron bacteria such as Sphaerotilus, Gallionella and Metallogenium1,2. The atmosphere at that time is believed to have been almost devoid of oxygen3. The concentration of atmospheric oxygen probably ranged from 0.0001 to 0.001% of the present atmospheric level, and may have remained at this level for 2 × 109 yr after the emergence of algal photosynthesis4. The fossils described in this paper were found during a study of the mineralogy of the Lower Proterozoic Brockman Iron Formation (Hamersley Group) of Western Australia (about 2 × 109yr old5). Two specimens (PM9 and PM24) collected from the Whaleback Shale Member, at Wittenoom Gorge, were identified as dolomitic chert by X-ray diffraction. Optical microscopy showed the presence of fibrous stilpnomelane, and the staining technique suggested by Dickson6 distinguished the dolomite as ferroan.

Origin of Conglomerates in Silurian Red Mountain Formation of Central Alabama; Their Paleogeographic and Tectonic Significance

The Lower Silurian Red Mountain Formation in central Alabama consists of interbedded sandstone and siltstone and four prominent intervals of sedimentary iron formation. Conglomerates consisting of discoid limestone cobbles in sedimentary hematite ore matrix commonly are present in one or more beds within the formation. Elongate cobbles have preferred northeast and southeast orientations. Crossbedding within the conglomerate matrix suggests southwestward-directed paleocurrents. Cobbles are deformed by soft-sediment draping. Bioclastic limestone lenses within and directly below the conglomerate intervals were apparently the source material. Although exposures of the Red Mountain Formation are insufficient to prove a beltlike distribution of the conglomerates, or the trend of such a belt, the conglomerates are known to be present along the strike of the southeastern limb of the Birmingham anticlinorium from Bessemer to Gadsden, Alabama, a distance of more than 70 mi. Width of the possible belt may be as much as 2 mi. The Red Mountain Formation is absent in the Helena thrust block southeast of the Birmingham thrust block. Northwest of the Birmingham anticlinorium, the formation grades into calcareous shale and limestone. Clay cobbles similar in size and shape to those in the Red Mountain Formation are forming at a site in the Mississippi Sound on the north coast of the Gulf of Mexico. The clay cobbles originate through the combined effects of desiccation and cracking of clay lenses in the intertidal zone and gentle wave abrasion. It is suggested that during Early Silurian time a slowly rising arch at the site of the Birmingham anticlinorium separated land on the southeast from the open sea on the northwest. The cobbles originated in an intertidal zone on the arch. Southwest-oriented crossbedding foresets evidently resulted from back-bar longshore currents during tidal withdrawal. The arch never was sufficiently emergent to cause erosion during Early Silurian time, and during deposition of the greater part of the Red Mountain Formation the arch remained submerged. The conglomerate zones mark the times of maximum emergence. Basement involvement in the development of the Birmingham anticlinorium is suggested by this evidence of uplift early in the history of Paleozoic sedimentation.

Synthesis of Grunerite, Fe7Si8O22(OH)2

GRUNERITE is a major rock-forming silicate in several metamorphosed sedimentary iron formations and is an important indicator of metamorphic grade in these rocks1. There have therefore been several attempts to determine experimentally the amphibole's thermal stability limits2–6. But these efforts have been frustrated by the inability to synthesize grunerite directly from oxide starting materials. Thus data on the stability range of grunerite must be inferred from field relations and oxygen isotope work7. Flaschen and Osborn2 found that grunerite formed only as a metastable decomposition product of minnesotaite (Fe3Si4O10(OH)2) at temperatures above 480° C and concluded that grunerite did not have a stability range under the conditions investigated. Kopp and Harris8 synthesized grunerite in the system NaOH–SiO2–Fe–H2O and Schurmann5 produced the amphibole by using bulk compositions which contained 2.7 weight per cent CaO. But the chemical composition of the grunerite produced in each case may depart significantly from the ideal Fe7Si8O22(OH)2 formula.

Thermal Metamorphism of the Banded Iron Formation of Orissa, India

Effect of thermal metamorphism on the sedimentary iron formation of Orissa by basic igneous intrusives is described. Two mineral assemblages, quartz-cummingtonite-grunerite-magnetite with amosite and quartz-actinolite-zoisite-magnetite have been developed at the contact of the banded iron formation and the gabbro. The contact rocks show typical hornfelsic texture. The mineralogical assemblages indicate albite-epidote hornfels to hornblende hornfels facies of thermal metamorphism. The metamorphism took place under high water vapor pressure at a temperature of about 800° ± 50° C, which was confirmed by the decomposition temperature of amosite and also by the temperature of formation of cummingtonite. The banded iron formation consists of alternate bands of fine-grained iron oxide and cherty or cryptocrystalline silica. The basic igneous complex has been differentiated into pyroxenites, olivine gabbro, normal gabbro without olivine, anorthositic gabbro, norite, granodiorite and leucogranite. From the chemical analyses of the contact rocks and their mineralogical and textural studies, it has been established that the metamorphism of the iron-formation was accompanied by the introduction of Ca and Mg from gabbro to the iron formation. Cummingtonite was formed by the reaction of quartz and magnetite. Chemical analysis and X-ray powder photograph data of the mineral amosite are also given.

The relation of iron and manganese in sedimentary iron formations

The mean Mn/Fe ratio of 94 samples of Precambrian sedimentary Fe formations is 0.025 which is very close to the crustal average of 0.022 for this ratio. Post Precambrian ironstones, on the other hand, show a mean Mn/Fe ratio of 0.009 indicating a marked geochemical separation of these metals. The Mn/Fe ratios of Precambrian Fe formations exhibit a significant correlation with the degree of oxidation of the samples, whereas this correlation is absent in the younger ironstones. Ores formed on the Precambrian Fe formations in the Lake Superior region generally have abnormally low Mn/Fe ratios (0.009), as do the ores formed from the younger ironstones. The Mn/Fe ratio of a chemical sediment may be influenced by the source rocks, by selective dissolution at the source, by separation during transport, by the conditions of the depositional environment, and by diagenetic modifications. It is suggested that the present data and other chemical characteristics of Fe formations may best be accounted for by assuming a lower O content in the Precambrian atmosphere.