Banded Iron Formation Samples Research Articles

Textures and Rare Earth Elements Composition of Banded Iron Formations (BIF) at Njweng Prospect, Mbalam Iron Ore District, Southern Cameroon

The REE signature of banded iron formations (BIF) and martite-goethite mineralization of the Njweng ridge (South Cameroon) are used here to decipher the nature of the ocean during the period of BIF precipitation. The textures of typical BIF categories are also presented as the only sedimentary feature. Two types of BIF facies are present at Njweng: the oxide and silicate facies. These facies show two stages of phase transformations. The first is the transformation of the original magnetite mineral into martite by oxidation resulting in trellis texture; the second is a simultaneous transformation by hydration into goethite and dehydration into martite. The samples typically show LREE depleted patterns relative to HREE, a characteristic similar to that of modern seawater. Also the strong positive Eu anomalies from the samples indicate the involvement of hydrothermal plumes of volcanic origin such as at mid ocean ridges (MOR). Yttrium (Y) has a strong positive anomaly pointing to the transportation of REEs as aqueous complexes within these ancient seas. The BIF samples have no positive Ce anomalies, suggesting that the oceans at that time were more reducing with no Ce fractionation than in present day oceans. The martite-goethite mineralization rather shows a positive Ce anomaly that derived from the enrichment in REE during the weathering of REE-bearing minerals. These results are integrated into existing literature on REE in BIFs worldwide and provide new insights into these heretofore unstudied BIFs of the northern edge of the Congo Craton.

The rheology of banded iron formation: constraints from axial compression experiments

Samples of Banded Iron Formation (BIF) with different quartz and hematite contents were deformed in axial compression experiments in a Griggs-type apparatus, at T = 900ºC, P =1.5 GPa and strain rates of 10-5 and 10-6 s-1. The aim is to investigate the mineral phase that controls the rheology of multi-phase rocks and to determine the processes that control the observed rock strength. BIF samples from Quadrilátero Ferrífero region-Brazil were cored perpendicular to the foliation, which is defined by parallel bands of quartz and iron oxide. Sample strength decreases with increasing hematite content and decreasing strain rate. At a strain rate of 10-5 s-1, samples with well-developed compositional banding showed higher strength. At the slower strain rate of 10-6 s-1 no difference between samples with or without compositional banding is observed. For comparison, at 10-6 s-1, the highest quartz content sample is as weak as the highest hematite content sample at 10-5 s-1. Strain is localized in hematite-rich layers. One intriguing observation is that quartz grains within the more deformed hematite-rich bands show more evidence for crystal-plastic deformation than grains in quartz-rich bands.

SIMS analyses of silicon and oxygen isotope ratios for quartz from Archean and Paleoproterozoic banded iron formations

Banded iron formations (BIFs) are chemical marine sediments dominantly composed of alternating iron-rich (oxide, carbonate, sulfide) and silicon-rich (chert, jasper) layers. Isotope ratios of iron, carbon, and sulfur in BIF iron-bearing minerals are biosignatures that reflect microbial cycling for these elements in BIFs. While much attention has focused on iron, banded iron formations are equally banded silica formations. Thus, silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in BIFs. BIFs are banded by definition, and microlaminae, or sub-mm banding, are characteristic of many BIFs. In situ microanalysis including secondary ion mass spectrometry is well-suited for analyzing such small features. In this study we used a CAMECA IMS-1280 ion microprobe to obtain highly accurate (±0.3‰) and spatially resolved (∼10 μm spot size) analyses of silicon and oxygen isotope ratios for quartz from several well known BIFs: Isua, southwest Greenland (∼3.8 Ga); Hamersley Group, Western Australia (∼2.5 Ga); Transvaal Group, South Africa (∼2.5 Ga); and Biwabik Iron Formation, Minnesota, USA (∼1.9 Ga). Values of δ 18O range from +7.9‰ to +27.5‰ and include the highest reported δ 18O values for BIF quartz. Values of δ 30Si have a range of ∼5‰ from −3.7‰ to +1.2‰ and extend to the lowest δ 30Si values for Precambrian cherts. Isua BIF samples are homogeneous in δ 18O to ±0.3‰ at mm- to cm-scale, but are heterogeneous in δ 30Si up to 3‰, similar to the range in δ 30Si found in BIFs that have not experienced high temperature metamorphism (up to 300 °C). Values of δ 30Si for quartz are homogeneous to ±0.3‰ in individual sub-mm laminae, but vary by up to 3‰ between multiple laminae over mm-to-cm of vertical banding. The scale of exchange for Si in quartz in BIFs is thus limited to the size of microlaminae, or less than ∼1 mm. We interpret differences in δ 30Si between microlaminae as preserved from primary deposition. Silicon in BIF quartz is mostly of marine hydrothermal origin (δ 30Si < −0.5‰) but silicon from continental weathering (δ 30Si ∼ 1‰) was an important source as early as 3.8 Ga.

Iron isotopes constrain biogeochemical redox cycling of iron and manganese in a Palaeoproterozoic stratified basin

The Hotazel Formation in the uppermost stratigraphic portion of the Neoarchaean–Palaeoproterozoic Transvaal Supergroup of southern Africa is an unusual sedimentary sequence of banded iron-formation (BIF) intercalated with three manganese-rich layers. As such, it is a succession that holds great potential to offer a unique view of one of the most dramatic transitions in early Earth history — the switch to a full oxidative cycle in shallow oceans at ca. 2.3 Ga. We present iron isotope results from BIF and Mn-rich samples collected across the entire Hotazel sequence, with a view to constraining processes of biogeochemical redox cycling for both metals close to the transition from a reducing to an oxidizing ocean–atmosphere system. The recorded de-coupling of Fe- and Mn reduction during anaerobic organic carbon cycling in the Hotazel strata, suggests that manganese became an important electron acceptor in stratified marine environments of the Palaeoproterozoic during periods of increased primary manganese precipitation relative to iron. Very low δ 57Fe values registered across the entire Hotazel sequence and especially in manganese-rich samples (−2.4 to −3.5‰) signify deposition of iron and manganese in a terminal, stratified aqueous reservoir that was depleted in the heavy iron isotopes. These isotopic signatures, in conjunction with the unusual endowment of the Hotazel sequence in manganese, are interpreted to have evolved by Rayleigh distillation processes during protracted deposition of Mn-poor BIFs as preserved in the lower stratigraphic portion of the Transvaal Supergroup (Kuruman and Griquatown BIFs). The unique end-member geochemical and isotopic characteristics of the Hotazel rocks may therefore constitute a potential link between the widespread deposition of BIF during the Neoarchaean and Palaeoproterozoic, and the postulated rise in atmospheric oxygen levels around 2.3 Ga ago.

Brazilian Analog for Ancient Marine Environments on Mars

Banded iron formations (BIFs) are iron oxide‐ and silica‐rich chemical sedimentary rocks that were deposited in great abundance during an approximately 800‐million‐year span from the late Archean to the mid‐Proterozoic eons (circa 2.7–1.9 billion years ago). At that time, the terrestrial atmosphere was changing in composition from early anoxic conditions to a more oxygenated state, due possibly to the increasing activity of photosynthetic organisms. It has been suggested that Mars and Earth were more similar in their early history than they are today, and that analogous geologic processes—including those responsible for BIF deposition—may have operated on both planets. Here we discuss BIF samples collected from drill cores at Carajas, Brazil, that represent a variety of fresh, weathered, and altered rocks. Laboratory and remote sensing measurements of these rocks show unique properties that may assist in the discovery of any similar deposits on Mars.

GEOCHEMISTRY AND MINERALOGY OF NEOPROTEROZOIC BANDED IRON-FORMATIONS AND SOME SELECTED, SILICEOUS MANGANESE FORMATIONS FROM THE URUCUM DISTRICT, MATO GROSSO DO SUL, BRAZIL

This study characterizes the precursor mineralogy and geochemistry of the Neoproterozoic iron ore deposits as well as some associated Na-containing manganese assemblages of the Urucum district, Mato Grosso do Sul, Brazil. It is based on ten mineralogically well characterized samples (six of banded iron-formation (BIF), three of manganese formations, and a sample of a veinlet that crosscuts the manganese assemblage), which were carefully selected so as avoid as much as possible supergene enrichment and secondary alteration (weathering), both of which are pervasive in the Urucum district. The six BIF samples are representative of the extensive Urucum BIF sequence from which the rich iron ores were developed by supergene enrichment and are considered to be precursors to the iron ore. The manganese-rich samples are part of unusual siliceous manganese horizons that contain complex silicate assemblages with braunite, cryptomelane, some pyrolusite, and authigenic aegirine. The Urucum BIF sequence is distinctive because it consists almost entirely of hematite and chert (jasper) with almost all of the iron present as only Fe2O3. This is in sharp contrast to the iron chemistry of much older (Archean and Early Proterozoic) BIF, in which a very large proportion of the iron occurs as ferrous iron in magnetite, carbonates, and silicates. As such, the Urucum BIF are essentially identical to those of the Neoproterozoic Rapitan sequence (755–730 Ma) of the Yukon and Northwest Territories of Canada. The Urucum sequence contains abundant dropstones, whereas the Rapitan sequence is set among diamictites but also contains dropstones. The δ 13C values of carbonates at Urucum are low, ranging from –5.2 to –7.0 per mil, which reflects their deposition in a glaciomarine setting. The REE concentrations of the BIF, as well as three Mn formation samples, are very similar and are almost completely lacking positive Eu anomalies (relative to NASC). This is in sharp contrast to the pronounced positive Eu anomalies of Archean and Early Proterozoic iron-formations. The general trend of the REE profiles (in NASC plots), with some enrichment of the heavy REE, is qualitatively very similar to that of modern seawater. The source of the Fe, Mn, and Si is concluded to be from typical ocean water with some deep-sea hydrothermal component. The reappearance of the Neoproterozoic Urucum sequence with BIF and interlayered manganese formations, together with the Rapitan sequence of similar Neoproterozoic age, after an absence of such sedimentary sequences in the geologic record for about 1.1 billion years, is considered to reflect ocean stagnation (with anoxic conditions), which may have been caused by a near-global ice cover, referred to as snowball Earth.

Apatite in early Archean Isua supracrustal rocks, southern West Greenland: its origin, association with graphite and potential as a biomarker

Rare earth element (REE) abundances in individual apatite crystals in banded iron formations (BIFs), metacherts, metacarbonates and mafic dykes in the Isua supracrustal belt (ISB) have been determined by laser ablation inductively coupled plasma mass spectrometry. The results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential, proposed in earlier studies, for use of the apatite-graphite association as a biomarker. The chondrite-normalized distribution patterns of apatite in metasedimentary BIFs and metacherts fall into three groups. Relatively flat profiles with distinct positive Eu anomaly are interpreted as characterizing sedimentary (diagenetic) apatite that carry the REE signature of the Archean ocean. Secondary apatite in Isua metasdiments with either middle REE enriched profiles or with light REE depleted profiles is interpreted to have crystallized from percolating carbonate-rich metasomatic fluids or from fluids derived from cross-cutting mafic dykes, respectively. The occurrence together of these different genetic types of apatite with distinct REE signatures within cm-scale samples shows the immobility of REE in preexisting apatite during metamorphic episodes. Apatite crystals in Isua rocks of uncontested chemical sedimentary origin (BIF and metachert samples) do not have graphite inclusions or coatings. Graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks. In these rocks graphite is produced by thermal-metamorphic reduction of carbonate ion, derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor, oxidizing to form magnetite. In view of the non-sedimentary, metasomatic origin of Isua metacarbonates and the abiogenic source of graphite, the apatite–graphite assemblage can not be considered as a biomarker and does not provide information on early Archean life in the ISB.

PETROGRAPHY AND GEOCHEMISTRY OF THE LEAST ALTERED BANDEDIRON-FORMATION OF THE ARCHEAN CARAJAS FORMATION, NORTHERN BRAZIL

The Carajas area is in the southeastern part of the state of Para, Brazil, and includes one of the largest iron ore deposits in the world, located about 600 km south-southwest of the city of Belem. The total iron ore resources are estimated at 17.8 billion tons with an average total Fe content of 66.1 percent. The iron ore deposits are hosted in the Carajas Formation, which is a 400-m-thick sequence of banded iron-formation (BIF) and minor dolomitic rocks, intruded by some irregular mafic rocks. Single zircon dates from a mafic sill that cuts the sequence constrain the age to a minimum of 2740 ± 8 Ma. The samples of this study occur in the N4E deposit that lies at the northern limb of a synclinorium. The metamorphic grade is greenschist facies. The BIF occurs as relict, hard jaspilite pods (protore for the deposit) in the hematite open pit mines and is also intersected in places in diamond drill cores. The bulk chemistry of these BIF samples is very simple and consists mainly of SiO2 and Fe2O3. Their REE show an overall trend of relative enrichment in the light REE, a well-defined positive Eu anomaly in three of the samples and a slight hint of a positive Eu anomaly in one more sample. The REE profiles for the Carajas BIF have background trends similar to those reported for some other Archean BIFs such as Isua, Greenland, and the Nova Lima Group of Quadrilatero Ferrifero, Brazil. However, these background trends are quite different from those reported for the Early Proterozoic Transvaal BIFs in South Africa, which show a positive background slope with relative depletion in the light REE and relative enrichment in the heavier REE. The common presence of positive Eu anomalies in the Carajas BIFs is concluded to be the result of input from suboceanic hydrothermal solutions. These solutions are considered to have transported the iron and SiO2 to the BIFs, from deep-sea spreading center sources of the Carajas Rifted basin. Such a deep-sea hydrothermal origin is similar to that postulated for other Archean and Proterozoic BIFs worldwide.

A review of the geology of the dolomite-dominated carbonatite complexes of the Kalahari Craton, southern Africa

Drill core and outcrop samples of pure marine chemical sediments (banded iron formation (BIF), manganese formation (MnF), jaspilites, lutites, and cherts) from the transition of the ~2426 Ma old Ongeluk Formation into the 2413 Ma old Hotazel Formation, Transvaal Supergroup, South Africa, reveal remarkable changes of seawater chemistry in the Transvaal Ocean. Similar to pre-Ongeluk chemical sediments, the shale-normalized rare earths and yttrium (REYSN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive EuSN anomalies, indicating that high-temperature (>250 °C) hydrothermal fluids did not contribute significantly to the REY budget of ambient waters. However, a 10 cm drill core section in the lower Hotazel Formation is characterized by conspicuous positive EuSN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REYSN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REYSN enrichment, positive LaSN, GdSN and YSN anomalies, but no CeSN or EuSN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative CeSN anomalies in some of the lutites, BIFs and MnFs, reflecting Ce depletion in ambient seawater. All Hotazel lutite, BIF, and MnF samples studied show unradiogenic εNd(t) values (−0.5 ± 0.2 to −2.4 ± 0.2), indicating a mostly continental REY source. The REY distribution and Nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved REY to local “Transvaal seawater”. Precipitation of the Hotazel lutites, BIFs and MnFs with negative CeSN anomalies, therefore, suggests that oxic conditions prevailed on the Kaapvaal Craton and in Hotazel seawater already at ~2.413 Ga, i.e. 80 m.y. before the disappearance of mass-independent sulfur isotope fractionation (MIF-S) that defines the Great Oxidation Event at ~2.33 Ga.

Palaeomagnetism and magnetic anisotropy of Proterozoic banded-iron formations and iron ores of the Hamersley Basin, Western Australia

Rock magnetic properties and palaeomagnetism of weakly metamorphosed banded-iron formations (BIFs) of the Palaeoproterozoic Hamersley Group, Western Australia, and Proterozoic BIF-derived iron ores have been investigated. The BIF units sampled here are slightly younger than 2500 Ma. At Paraburdoo, Mount Tom Price and Mount Newman iron ore formation was completed before 1850 Ma. Sampling was mainly from the Mount Tom Price and Paraburdoo mining areas and for the first time a palaeomagnetic fold test on fresh (unweathered and unaltered) BIF samples has allowed the nature of the remanence of the BIFs to be defined. The remanence of the BIFs is carried by late diagenetic/low-grade metamorphic magnetite after primary haematite. This remanence is pre-folding and is unlikely to be greatly affected by the high anisotropy because the palaeofield inclination was demonstrably low. Determination of palaeofield directions from measured remanence directions is complicated by self-demagnetization effects in strongly magnetic, highly anisotropic BIF specimens. We present a method for correcting measured directions for the effects of self-demagnetization and anisotropy. For typical BIFs, the effect of magnetic anisotropy on measured remanence inclinations and inferred palaeolatitudes is minor for low palaeolatitudes, but can lead to large errors in calculated palaeopoles for intermediate to moderately steep palaeolatitudes. Anisotropy also causes cones of confidence to be underestimated, due to compression of the range of inclinations. In principle, deflection of post-folding remanence towards the bedding plane by high magnetic anisotropy can produce an apparent syn-folding signature, with best agreement between directions from different fold limbs after partial unfolding. Thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of errors, but can also mislead interpretation of the time of remanence acquisition. The anisotropy of anhysteretic remanent magnetization (ARM) probably yields an upper limit to the anisotropy of the chemical remanent magnetization (CRM) carried by the BIFs. Therefore, from the anisotropy of ARM, a maximum inclination deflection of 9° is suggested for the sampled BIFs. This corresponds to less than 5° change of palaeolatitude. The palaeomagnetic pole position calculated for BIFs at Paraburdoo is 40.9°S, 225.0°E (dp=2.9°, dm=5.8°) after tilt correction, but without correction for anisotropy. Other pole positions reported include that from flat-lying BIFs from Wittenoom at 36.4°S, 218.9°E (dp=4.6°, dm=9.1°), from Mount Tom Price iron ore at 37.4°S, 220.3°E (dp=5.7°, dm=11.3°) and from Paraburdoo ore at 36.4°S, 209.9°E (dp=4.7°, dm=8.8°). The poles from the BIFs, the Paraburdoo ore and the part of the Tom Price deposit that was sampled in this study are indistinguishable from each other and from the Mount Jope Volcanics overprint pole. The magnetization of the BIFs was probably acquired during burial metamorphism of the Hamersley Group, soon before the main folding and uplift event in the southern part of the Hamersley Province. This tectonic event exposed magnetite-rich BIFs to near-surface oxidizing conditions, producing extensive martite-goethite orebodies and also appears to have produced the syn-folding overprint magnetization recorded by the Mount Jope Volcanics of the underlying Fortescue Group. Ages of magnetization are tentatively interpreted as ∼ 2200±100 Ma for the BIFs, ∼ 2000±100 Ma for the supergene enrichment of BIF to martite-goethite ore, recorded by the Parabudoo and Mount Tom Price orebodies, and ∼ 1950±100 Ma for the metamorphic martite-microplaty haematite ore, recorded as an overprint by the Tom Price orebody and as the only surviving magnetization of the Mount Newman orebody.

Geological, mineralogical and geochemical aspects of Archean Banded Iron-Formation-hosted gold deposits: Some examples from Southern Africa

This paper reports on different styles of gold mineralization observed in Archean gold deposits hosted by Algoma-type Banded Iron-Formations (BIF) in southern Africa. Genetic aspects of various occurrences are discussed in the context of mineralogical as well as geochemical data of BIFs from the greenstone terranes of the Zimbabwe and Kaapvaal cratons. The study revealed that, in spite of their different provenance and age (3.5 to 2.6 Ga), the BIFs are geochemically similar, whereas observed mineralogical differences reflect various degrees of metamorphic overprint. Generally, the BIFs belong to mixed oxide-carbonate-(±sulfide)-facies. REE distribution patterns of the investigated Archean BIF samples exhibit positive Eu-anomalies, which suggest a strongly reducing nature of the solutions which also provided the distinctive element contents now present in the chemical sediments. Irrespective of their formation, gold enrichment in BIF only occurs if the S- and/or As-contents of the BIFs exceed specific threshold values, i.e. gold mineralization is always associated with increased contents of the iron-sulfides pyrite, arsenopyrite and pyrrhotite. The studies indicate that BIF-hosted gold occurrences are not products of a single universal metallogenic process, but may be explained by several different genetic processes such as primary syn-sedimentary formation, diagenetic changes, metamorphic remobilization, and epigenetic hydrothermal emplacement.

The chemical evolution of Precambrian seawater: Evidence from REEs in banded iron formations

We present REE patterns determined by isotope dilution mass spectrometry from 20 Precambrian banded iron formation (BIF) samples, ranging in age from 3.4 to 0.65 Ga. The samples studied represent a variety of depositional settings and sedimentary facies. The oxide facies BIFs typically show LREE depleted patterns relative to shales, with (La/Nd) N ⩾ 1 and (La/Yb) N mostly 0.3–0.8. The NASC-normalized REE patterns from the oxide facies BIFs are similar to modern metalliferous sediments, except for Ce and Eu. Positive Eu anomalies were found in all BIF samples, whereas Ce anomalies were variable. Both the oxide facies BIFs and modern metalliferous sediments typically have La/Yb ratios slightly higher than seawater. The REE data suggest that the Fe and REEs in BIFs coprecipitated from seawater as Fe oxy-hydroxides.The variable Ce anomalies are probably the result of diagenetic processes and redox cycling, similar to modern reducing environments. Comparison of the REE patterns and ϵ Nd data of Jacobsen and Pimentel-Klose (1988a,b) from coeval BIFs and clastic sediments indicates that the source of the REEs in BIFs was distinct from that of the shales. The positive σ Nd values and positive Eu anomalies indicate that submarine hydrothermal sources were important or even dominant for the REEs, and, by extension, the Fe in BIF. We also report new measurements of REEs in hydrothermal waters from the East Pacific Rise 21 °N site. These have very large Eu anomalies (Eu/Eu∗ ∼-70), higher than previously reported. Comparison of mass balance estimates for REEs in the oceans based on Nd isotopic composition and on Eu anomalies results in a large discrepancy, which probably is a result of non-conservative behavior of Eu during mixing of hydrothermal waters with ambient seawater. The REE data are consistent with a model for BIFs in which Fe and REEs derived from submarine hydrothermal systems are upwelled from the deep ocean, mixed with oxidizing surface water (with some continentally derived REEs), and deposited on the continental margins.

Oxygen isotope study of a Precambrian banded iron-formation, Hamersley Range, Western Australia

Oxygen isotope ratios were determined for quartz, magnetite, ankerite, siderite, riebeckite, hematite and talc in samples of banded iron-formation from the Dales Gorge Member of the Brockman Iron Formation and for quartz, dolomite and calcite in samples of the Wittenoom Dolomite and Duck Creek Dolomite Formations, all from the Hamersley Range area of Western Australia. Additionally, in order to interpret the measured isotope ratios, isotopic fractionations for oxygen between quartz, siderite and magnetite and between these minerals and water as a function of temperature were calculated, using a combination of spectroscopic and thermodynamic data and constraints set by experimental determinations of the fractionations. The Dales Gorge Member was found to have undergone isotopic exchange between minerals at a temperature estimated on the basis of the isotopic fractionations to be above 270°C and probably less than 310°C, during burial metamorphism. At these temperatures quartz and the carbonates were almost completely equilibrated with one another, while hematite apparently underwent negligible exchange. Magnetite may have undergone exchange in some samples but not others, as a result of permeability variations, or it may have been as resistant to exchange as hematite. Riebeckite, and probably talc as well, were also subject to exchange, but to a lesser degree or on a smaller scale than quartz and the carbonates. Hematite formed at temperatures of 140°C or below. Magnetite appears to have formed at temperatures above 140°C, and possibly over a range of temperatures between about 180 and 300°C. The Wittenoom Dolomite and Duck Creek Dolomite samples show apparent lack of equilibrium, due to incomplete exchange or to retrograde effects. A chert from the Wittenoom Dolomite, along with two samples from the Marra Mamba Iron Formation, with δ 18 O values of + 24%. can be considered to set a lower limit of about −11%. on the δ 18 O value of the ocean 2.2 × 10 9 yr ago. Internal fractionations in the Wittenoom Dolomite chert sample may be interpreted as yielding an upper limit on this oceanic δ 18 O value of − 3.5%.