Deposition Of Banded Iron Formations Research Articles

Negative Ce Anomaly in the Indian Banded Iron Formations: Evidence for the Emergence of Oxygenated Deep‐Sea at 2.9‐2.7 Ga

Abstract: Major and rare earth element contents are reported for Late Archean banded iron formations (BIFs) in the Bababudan Group of the Dharwar Craton, South India. The BIFs are mostly composed of SiO2 (average1ρ = 54.88.1 wt%) and Fe2O3* (44.38.2 wt%). The Al2O3 and TiO2 contents are remarkably low, suggesting that detrital components were starved during the BIF deposition. The BIFs have a LREE‐enriched pattern with a relatively high (La/Yb)N (6.644.07). Total REE concentrations (RE) vary from 5.2 to 65.3 ppm. The REE patterns are characterized by the presence of a very large negative Ce anomaly (Ce/Ce*: 0.13‐0.83) and a positive Eu anomaly (Eu/Eu*: 0.96‐2.45). The Eu/Eu* decreases and (La/Yb)N increases with a increase of RE. These correlations of REE indices are similar to those of modern hydrothermal iron‐rich sediments near a mid‐ocean ridge (MOR). Greenstones associated with the BIFs have MORB‐like geochemical features. These geochemical and geological lines of evidence indicate that the depositional site of the BIFs was remote from a continent and/or island arc and that the BIFs were in situ hydrothermal sediments near a MOR. A striking negative Ce anomaly in the BIFs indicates that oxygenated deep‐sea environments emerged at 2.9‐2.7 Ga. The existence of contemporaneous Mn deposits in the Dharwar Craton supports this assertion. Our scenario of oxygen in the Earth's surface of the Late Archean is different from long‐held notion that the atmosphere and ocean were persistently anoxic throughout the Archean.

SHRIMP U–Pb zircon ages of tuffaceous mudrocks in the Brockman Iron Formation of the Hamersley Range, Western Australia

Tuffaceous mudrocks are common in the banded iron‐formations (BIF) of the Brockman Iron Formation. These tuffaceous mudrocks are either stilpnomelane‐rich or siliceous. Their compositions reflect bimodal volcanic activity in the vicinity of the Hamersley BIF depositional site. They also contain complex zircon populations that record resedimentation, syndepositional volcanism and post‐depositional isotopic disturbance. The best estimates of depositional age are obtained from siliceous tuffaceous mudrocks in the Joffre Member that contain 2459 ± 3 Ma and 2454 ± 3 Ma zircon populations most likely derived from felsic volcanism coeval with BIF deposition. These dates constrain the sedimentation rates for the ∼370 m‐thick Joffre Member BIF to >15 m per million years. Siliceous tuffaceous mudrocks are not present in the underlying ∼120 m‐thick Dales Gorge Member and it is uncertain whether previously reported ages of ca 2479–2470 Ma for this unit reflect detrital/xenocrystic or syndepositional zircon populations in resedimented stilpnomelane‐rich tuffaceous mudrocks. The increased abundance of tuffaceous mudrocks in the Joffre Member suggests that a pulse of enhanced igneous and hydrothermal activity accompanied deposition of the bulk of the Brockman Iron Formation BIF after ca 2460 Ma. This preceded and culminated in the emplacement of the 2449 ± 3 Ma large igneous province represented by BIF and igneous rocks of the Weeli Wolli Formation and Woongarra Rhyolite.

Plume‐related mafic volcanism and the deposition of banded iron formation

We have compiled a record of the geochronology of mantle plume activity between 3.8 and 1.6 Ga. Over this time period, the ages of komatiites, and those of global plumes, correlate strongly, at the 99% confidence level, with the ages of banded iron formations (BIFs). The ages of continental plumes correlate more weakly, at an overall 85% confidence level. Using the geochronological records of these events, we can define four periods characterized by mantle superplume activity. Three of these periods are also times of enhanced BIF deposition. The fourth mantle plume period may similarly be coeval with increased BIF accumulation, but the BIF chronostratigraphic resolution is not accurate enough to test this rigorously. Mantle superplume volcanism may promote BIF deposition by increasing the Fe flux to the global oceans through continental weathering and/or through submarine hydrothermal processes. It may also be enhanced by increasing the number of paleotectonic environments appropriate for BIF deposition (particularly plume‐induced ocean plateaus, seamounts, and intracratonic rifts) and by promoting global anoxic, Fe‐rich hydrothermal plumes in the shallow to intermediate marine water column.

An overview of the geology of the Transvaal Supergroup dolomites (South Africa)

In the Neoarchaean intracratonic basin of the Kaapvaal craton, between approximately 2640 Ma and 2516 Ma, two successive stromatolitic carbonate platforms developed. Deposition started with the Schmidtsdrif Subgroup, which is probably oldest in the southwestern part of the basin, and which contains stromatolitic carbonates, siliciclastic sediments and minor lava flows. Subsequently, the Nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the Transvaal Supergroup epeiric sea, around 2550 Ma ago. This transgression led to the development of a carbonate platform in the areas of the preserved Transvaal and Griqualand West basins, which persisted for 30–50 Ma. During this time, shales were deposited over the Nauga Formation carbonates in the southwestern portion of the epeiric sea. A subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical, iron-rich silica precipitates of the banded iron formations (BIF) over the entire basin. Carbonate precipitation in the Archaean was largely due to chemical and lesser biogenic processes, with stromatolites and ocean water composition playing an important role. The stromatolitic carbonates in the preserved Griqualand West and Transvaal basins are subdivided into several formations, based on the depositional facies, reflected by stromatolite morphology, and on intraformational unconformities; interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins. Thorough dolomitisation of most formations took place at different post-depositional stages, but mainly during early diagenesis. Partial silicification was the result of diagenetic and weathering processes. Karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater. Such periods occurred locally at the time of carbonate and BIF deposition. Main karstification, however, probably took place during an erosional period between approximately 2430 Ma and 2320 Ma.

Acid Character of Archean Ocean Waters Revealed by 3.3 Ga-old Ferruginous Chert Compositions, Western Australia.

Comparative inspection of Archean ferruginous cherts with Phanerozoic cherts shows that the former precipitated from acid-water hydrothermally supplied during intensive early Precambrian volcanism. The pH of the water is inferred to have been around 4 where Al dissolves, and Ti and Zr do not. The peculiar geochemical environment seems to have been widespread in Archean to early Proterozoic greenstone basins when tremendous volume of banded iron formation (BIF) formed. This suggests that the acid seas were neutralized after the BIF deposition probably during the middle to late Proterozoic; this change of pH in sea water may have promoted the deposition of carbonate rocks.

Aerobic carbon cycling and cerium oxidation: significance for Archean oxygen levels and banded iron-formation deposition

The amounts of organic carbon and iron generally estimated to have been preserved in early Precambrian sediments suggest by implication that only the rapid, oxidative recycling of organic carbon by aerobic heterotrophy could plausibly have kept pace with the oxygen produced annually by net photosynthesis, even with a low global primary productivity of less than 1% of modern global values (Towe, 1990). A global aerobic recycling process would require at least a moderately oxygenated atmosphere to operate. The presence of such an atmosphere as far back in Earth history as 3.8 Ga ago is consistent with, but not proven by, the abundance of organic carbon and the “anomalous” distribution of rare earths in the banded iron-formations of the Isua sediments (Dymek and Klein, 1988). The absence of positive cerium anomalies and the presence of positive europium anomalies in these and other banded iron-formations (Klein and Beukes, 1989; Derry and Jacobsen, 1990) imply that iron oxidation during BIF deposition took place in waters at some depth below the zone of mixing rather than at the surface. A modified Klein-Beukes model using shallow, seasonal downwelling in an otherwise deep-water stratified basin under a moderately oxygenated atmosphere may help explain the origin of some of these enigmatic deposits.

Aerobic carbon cycling and cerium oxidation: significance for Archean oxygen levels and banded iron-formation deposition

The amounts of organic carbon and iron generally estimated to have been preserved in early Precambrian sediments suggest by implication that only the rapid, oxidative recycling of organic carbon by aerobic heterotrophy could plausibly have kept pace with the oxygen produced annually by net photosynthesis, even with a low global primary productivity of less than 1% of modern global values (Towe, 1990). A global aerobic recycling process would require at least a moderately oxygenated atmosphere to operate. The presence of such an atmosphere as far back in Earth history as 3.8 Ga ago is consistent with, but not proven by, the abundance of organic carbon and the “anomalous” distribution of rare earths in the banded iron-formations of the Isua sediments (Dymek and Klein, 1988). The absence of positive cerium anomalies and the presence of positive europium anomalies in these and other banded iron-formations (Klein and Beukes, 1989; Derry and Jacobsen, 1990) imply that iron oxidation during BIF deposition took place in waters at some depth below the zone of mixing rather than at the surface. A modified Klein-Beukes model using shallow, seasonal downwelling in an otherwise deep-water stratified basin under a moderately oxygenated atmosphere may help explain the origin of some of these enigmatic deposits.

Primary and diagenetic controls of isotopic compositions of iron-formation carbonates

Mineralogic, chemical, and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early Proterozoic (2.5 Ga) Dales Gorge Member of the Brockman Iron Formation, Hamersley Basin, Western Australia. Samples were obtained both from banded iron-formation (BIF) macrobands 9–12 at Paraburdoo, on the southern margin of the basin, and from BIF macroband 13 at Wittenoom, 130 km NW. At Paraburdoo, oxygen-isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60–160°C. This range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin (cf. T ~ 300°C at Wittenoom; Becker and Clayton, 1976). Carbon-isotopic compositions of carbonates are near −7%. vs. PDB at Paraburdoo and −10.5%. at Wittenoom, but the greater isotopic depletion at Wittenoom appears related to primary or diagenetic processes, not metamorphism. Contents of organic carbon are consistently low. Isotopic depletion is roughly correlated with iron abundance and, together with petrographie observations and chemical balances, is consistent with the model of BIF deposition introduced by Beukes et al. (1990): primary siderite (δ ~ −5%.) precipitated from an anoxic water column depleted in 13C; additional depletion of 13C is associated with coprecipitation of iron oxides and organic carbon. Oxygen-isotopic abundances of microbanded carbonates are similar to those of under- and overlying massive marine carbonates, ranging from 17.6 to 21.0‰ vs. SMOW (−9.6 to − 12.9‰ vs. PDB). Millimeterscale variations in abundances of 13C and 18O are associated with diagenetic replacement of primary siderite by secondary ankerite and/or magnetite. It is shown that these isotopic variations cannot result from mineral-dependent fractionations, metamorphism, or the influence of large volumes of water in an open system.

Oxygen isotopes in iron (III) oxides: 2. Possible constraints on the depositional environment of a Precambrian quartz-hematite banded iron formation

Quartz (SiO 2) and hematite (Fe 2O 3) possess considerably different gram formula weights and different stoichiometric coefficients for their oxygen. These differences mean that the numerical values of SiO 2 and Fe 2O 3 compositions when reported as weight percent are not the same as the numerical values of X(O), i.e. oxygen in the mineral as a mole percent of the total oxygen in a binary quartz-hematite system. Consequently, end-member quartz and hematite δ 18O-values which are determined from linear extrapolations of δ 18O vs. weight percent Fe 2O 3 data from binary mixtures can be in error by as much as several per mil. Extrapolation to end-member δ 18O-values in binary quartz-hematite systems should always be performed using mole fraction of oxygen as the compositional variable. The hematite-water fractionation curve of Yapp and the quartz-water curve of Knauth and Epstein were used together with the kinetic isotope exchange model of Criss et al. to constrain the depositional environment of the Late Proterozoic quartz-hematite banded iron formation deposits of Urucum, Brazil. The depositional temperatures permitted by the model assumptions employed here range from 0° to ∼ 35°C (or 54°C). The permitted range of depositional water δ 18O-values is from −6.6‰ (or −8.9‰) to 0.0‰. While the range of permitted depositional conditions is rather large, it does encompass the possible cool temperatures and brackish or fresh waters which are indicated by sedimentologic evidence in these deposits.

The paleoecology and significance of the Gunflint-type microbial assemblages from the Frere Formation (Early Proterozoic), Nabberu Basin, Western Australia

Abstract Eight distinct microbial assemblages are preserved in the Frere Formation, which was deposited in the Nabberu Basin, Western Australia. These assemblages contain taxa (Animikiea, Eoastrion, Gunflintia, and Huroniospora) typical of Gunflint-type microbial assemblages, which have been described from Australia, Canada, and South Africa. Four assemblages are interpreted as primary (composed of benthic microorganisms) and four as secondary (containing components that were transported before deposition). These microbial assemblages have similar paleoecologies to other Gunflint-type microbial communities. All Gunflint-type microbial assemblages, including the Frere assemblages, were deposited in a marine subtidal environment. The apparent worldwide distribution of these similar microbial assemblages supports the hypothesis that Gunflint-type microbiotas were characteristic of the biota living in subtidal settings during the Early Proterozoic. However, this hypothesis does not account for the lack of these assemblages in post-Early Proterozoic rocks. This may be explained by the absence of primary inorganic silica precipitation in open-marine settings during the Middle and Late Proterozoic. Primary silica precipitation may be necessary for the preservation of well-preserved microbial assemblages in marine settings of normal salinity. Silica precipitation in Early Proterozoic open-marine settings is commonly associated with banded iron-formation deposition, which is confined mainly to 2500-2000 Ma ago.

Rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations, 2. Precambrian gold‐uranium ore beds and banded iron formations

I previously suggested that heavy runoff during the 12,500–7000 years B.P. tropical pluvial period generated bottom water quiescence and reducing conditions making possible sapropel formation, accumulation of brine originating at the sea bottom, and long‐term preservation of hydrothermal sulfides in the Red Sea Deeps. I hypothesized the same process to account for two Precambrian strata‐bound ores. In the Upper Witwatersrand paleoplacers, South Africa, gold and uranium eroded by running water from upstream greenstones, reached highest concentrations in association with organic matter in carbonaceous seams, and deposited in the distal end of fluvial fans during transgressive phases. I propose that the deep waters over these fans became anoxic when large, sudden floods stratified the water column. The organic matter thus locally preserved concentrated gold and uranium from the partial dissolution of the detrital metallic grains. In the Hamersley basin of Western Australia, banded iron formations (BIF) of the Dales Gorge Member are a major source of iron ore. Regularly fluctuating environmental conditions are shown by repetitious BIF and black shale macrobands. I propose that they were deposited in a confined, silled, deep basin connected with the open ocean. Before initiation of the lowest BIF deposition the active regional thermohaline circulation flushing the deep waters under a rather arid climate diluted and oxidized the Fe2+‐rich hydrothermal effluents exhaling from the basin floor. Then, during a strong wet period, heavy runoff scavenging volcanoclastics from the land surface lowered the basin surface water salinity and prevented deepwater renewal. The hydrothermal effluents began to accumulate and progressively concentrated in the quiescent and progressively oxygen‐depleted deep water. During this wet initial phase, the massive black, organic‐rich pyritic shale at the top of the Mount McRae Shale Member was deposited immediately and conformably underlying the lowest BIF macroband. When aridity returned, the quiescent deep water filling the basin to sill depth had become sufficiently dense to resist its displacement by reactivated vertical circulation. From Fe2+‐rich anoxic brine, Fe3+ was precipitated at the anoxic‐oxic interface near the photic zone base by biogenic photosynthesis or Fe2+ photooxidation and partially reduced to Fe2+ during its settling through the anoxic water mass. Thus the lowest BIF macroband of the Dales Gorge Member was formed. Once high bottom water density became established, black shale macrobands were formed during recurrent wet periods and BIF macrobands during the intervening arid intervals. The macrobands' wet‐arid periodicity is suggestive of control by Milankovitch‐type insolation variation. Mesobands and microbands point to shorter climatic cycles. Termination of the system is achieved only by tectonic movements.

Iron-formation as an end member in carbonate sedimentary cycles in the Transvaal Supergroup, South Africa

The deposition of iron-rich sediments in the dolomite of the Transvaal Supergroup some 2,300 million years ago was not a random process. Iron-formation precursor can be shown to be an end member in a series of well-defined chemical and sedimentary cycles. The stratigraphic column records a repeated chemical conditioning of the depositional environment, which resulted in the deposition of iron-rich carbonates, iron-formation precursor, and of banded iron-formation. Chemical conditioning can usually be related to carbonate depositional structures suggestive of the presence of shoals in the basin. It is envisaged that these banks or shoals of carbonate detritus caused a partial restriction between the iron-formation environment and the open ocean. The abnormal concentrations of iron, silicon, and other elements which developed in these restricted basins resulted in the precipitation of successively more iron-rich sediments and chert and culminated in deposition of banded iron-formation.

Three Great Basins of Precambrian Banded Iron Formation Deposition: A Systematic Comparison

The origin of Precambrian banded iron formations is controversial. One type of evidence is a comparison of different occurrences. Three of the best-preserved basins of Precambrian iron formation deposition are here denned and compared systematically: the Hamersley Basin of Western Australia, the Animikie Basin of North America, and the Transvaal System Basin of South Africa. After a brief summary of the broad structural and stratigraphic evolution of each basin, their differences and similarities are reviewed. They differ mainly in: major stratigraphic sequence, thickness of iron formation, thickness variation, stratigraphic continuity, lithology of iron formation, nature of sedimentary structures, conspicuousness of varves (micro-banding), clastic association, and abundance of diagenetic riebeckite. With some reservations, they resemble each other in: age, basin size, chemistry and mineralogy, broad time-structural evolution of basin, precedence of chert, volcanic association, and in the restricted, medial and transitional stratigraphic status of the main iron formation. It is concluded that the Western Australian and South African iron formations are very closely similar, but that both differ markedly from the Animikie iron formations of the Lake Superior area. However, all three are more closely similar in chemistry than is required by the fact that their definition is chemical. Quantitative geochemical argument indicates a direct volcanic contribution of iron to the Hamersley Basin; it is possible that such vulcanicity is restricted to a limited medial part of basin development in a particular kind of basin-forming process.