Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation

We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of <200, <340, ∼500, <550, and <740°C respectively. δ⁵⁶Fe of bulk layers ranges from −0.8 to +0.8‰; in some samples the layers may differ by >1‰ on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which δ⁵⁶Fe of magnetite > silicate ≥ carbonate. The inter-mineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperature-dependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record. An erratum to this article can be found at     

Micro-scale tracing of Fe and Si isotope signatures in banded iron formation using femtosecond laser ablation

We have detected micrometre-scale differences in Fe and Si stable isotope ratios between coexisting minerals and between layers of banded iron formation (BIF) using an UV femtosecond laser ablation system connected to a MC-ICP-MS. In the magnetite–carbonate–chert BIF from the Archean Old Wanderer Formation in the Shurugwi Greenstone Belt (Zimbabwe), magnetite shows neither intra- nor inter-layer trends giving overall uniform δ⁵⁶Fe values of 0.9‰, but exhibits intra-crystal zonation. Bulk iron carbonates are also relatively uniform at near-zero values, however, their individual δ⁵⁶Fe value is highly composition-dependent: both siderite and ankerite and mixtures between both are present, and δ⁵⁶Fe end member values are 0.4‰ for siderite and −0.7‰ for ankerite. The data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by Fe(III)-reducing bacteria; or more likely an abiotic reaction of organic carbon and Fe(III) during low-grade metamorphism. Si isotope composition of the Old Wanderer BIF also shows significant variations with δ³⁰Si values that range between −1.0‰ and −2.6‰ for bulk layers. These isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal-rich waters. Interestingly, Fe and Si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures. Moreover, the changes of Fe and Si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal-rich water which govern the rates of Fe and Si precipitation and therefore also the development of layering. During periods of low hydrothermal activity, precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate–chert layers. During periods of intensive hydrothermal activity, precipitation rates of ferric oxyhydroxide were high, and subsequent diagenesis triggered only partial reduction, forming magnetite–carbonate–chert layers. We are confident that our micro-analytical technique is able to detect both the solute flux history into the sedimentary BIF precursor, and the BIF’s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors.     

鞍山-本溪地区条带状铁矿的Fe同位素特征及其对成矿机理和地球早期海洋环境的制约

本文报道了鞍山-本溪地区晚太古代条带状铁建造的主量元素、微量元素、稀土元素和Fe同位素的分析结果.结果显示:铁矿主要由Fe2O3T和SiO2组成,具有较低的Al2O3和TiO2含量;微量元素含量和稀土总量均较低;经页岩标准化的稀土元素配分模式呈现轻稀土亏损、重稀土富集;具有明显的La、Eu、Y的正异常;较高的Y/Ho比值.这些特征均表明鞍山-本溪地区条带状铁矿是由极少碎屑物质加入的化学沉积岩,成矿物质主要来源于海底热液的贡献.相对于标准物质IRMM-014,条带状铁矿石的Fe同位素组成最突出的特征是均为正值,这是由于二价铁溶液经部分氧化和沉淀形成富集重铁同位素的三价铁氧化物或氢氧化物的结果,且沉淀程度的不同是控制Fe同位素组成变化的重要因素.条带状铁矿无明显的Ce负异常和Fe同位素组成为正值的特征暗示了铁矿沉淀时的海水为低氧逸度环境.     

Francolite geochemistry—compositional controls during formation,diagenesis, metamorphism and weathering

Francolite in unaltered Tertiary phosphorites is highly substituted with about 1.2% Na, 0.25% Sr, 0.36% Mg and 2.7% SO₄ within the structure in substitution for Ca and PO₄. These substitutions reflect the composition of the solution from which francolite forms and are not influenced by the mechanism, rate or redox conditions of formation. The remarkably constant composition for a wide range of samples reflects the relative constant composition of sea water, during Tertiary times. Francolite in sulphate-reducing sediments forms in an uppermost zone where bioturbation maintains the concentrations of Na, Sr, Mg and SO₄ at sea water concentrations. The structura-CO₂ content of unaltered francolites vary from 1.4% to 6.3%, in response to variations in pH. The maximum level of carbonate substitution is limited by the disruption of the francolite structure at levels above 6.3% CO₂. For the samples considered here F/P₂O₅ ratios vary with X-ray CO₂ contents showing that PO₄^3? is replaced by (CO₃^2? + F?) and not by CO₃^2? alone. In Phosphoria Formation francolite the Na/P₂O₅ ratios and X-ray CO₂ contents show areal trends related to burial depth and thermal metamorphism. These two processes together with weathering and fresh water diageneses explain all observed variation in francolite composition.     

绿片岩-低角闪岩相变质条件下磁铁铁矿与黄铁矿间的Fe同位素分馏

对辽宁省鞍山一本溪地区经历了绿片岩一低角闪岩相变质的新太古代条带状铁建造中磁铁矿和黄铁矿矿物对的Fe同位素分析结果显示:相对于标准IRMM-014,所有样品的磁铁矿和黄铁矿均显示Fe的重同位素富集;且黄铁矿的Fe同位素比值均大于磁铁矿的Fe同位素比值(ε57Fe黄铁矿ε57Fe磁铁矿),两种矿物的Fe同位素比值之差为△57Fe黄铁矿-磁铁矿=2.23～5.13.黄铁矿富集铁的重同位素表明矿物的Fe同位素组成并不代表其原始沉积的特征,而是在区域变质作用过程中Fe同位素发生了交换的结果.由同位素平衡判别图解可知,在绿片岩一低角闪岩相变质作用中,磁铁矿-黄铁矿间的Fe同位素基本达到了平衡,且在平衡条件下黄铁矿比磁铁矿更富集Fe的重同位素,二者之间的Fe同位素平衡分馏系数α黄铁矿-磁铁矿≈1.000 4‰±0.06‰(2σ).这一研究成果是对变质作用过程中Fe同位素的地球化学行为认识的重要进展.     

Microbiological processes in banded iron formation deposition

Banded iron formations have been studied for decades, particularly regarding their potential as archives of the Precambrian environment. In spite of this effort, the mechanism of their deposition and, specifically, the role that microbes played in the precipitation of banded iron formation minerals, remains unresolved. Evidence of an anoxic Earth with only localized oxic areas until the Great Oxidation Event ca 2·45 to 2·32 Ga makes the investigation of O₂‐independent mechanisms for banded iron formation deposition relevant. Recent studies have explored the long‐standing proposition that Archean banded iron formations may have been formed, and diagenetically modified, by anaerobic microbial metabolisms. These efforts encompass a wide array of approaches including isotope, ecophysiological and phylogeny studies, molecular and mineral marker analysis, and sedimentological reconstructions. Herein, the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post‐depositional alteration are described. The main findings of recent years are summarized and compared here, and suggestions are made regarding cross‐disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition.     

Deciphering formation processes of banded iron formations from the Transvaal and the Hamersley successions by combined Si and Fe isotope analysis using UV femtosecond laser ablation

To investigate the genesis of BIFs, we have determined the Fe and Si isotope composition of coexisting mineral phases in samples from the ∼2.5 billion year old Kuruman Iron Formation (Transvaal Supergroup, South Africa) and Dales Gorges Member of the Brockman Iron Formation (Hamersley Group, Australia) by UV femtosecond laser ablation coupled to a MC-ICP-MS. Chert yields a total range of δ³⁰Si between −1.3‰ and −0.8‰, but the Si isotope compositions are uniform in each core section examined. This uniformity suggests that Si precipitated from well-mixed seawater far removed from its sources such as hydrothermal vents or continental drainage. The Fe isotope composition of Fe-bearing mineral phases is much more heterogeneous compared to Si with δ⁵⁶Fe values of −2.2‰ to 0‰. This heterogeneity is likely due to variable degrees of partial Fe(II) oxidation in surface waters, precipitation of different mineral phases and post-depositional Fe redistribution. Magnetite exhibits negative δ⁵⁶Fe values, which can be attributed to a variety of diagenetic pathways: the light Fe isotope composition was inherited from the Fe(III) precursor, heavy Fe(II) was lost by abiotic reduction of the Fe(III) precursor or light Fe(II) was gained from external fluids. Micrometer-scale heterogeneities of δ⁵⁶Fe in Fe oxides are attributed to variable degrees of Fe(II) oxidation or to isotope exchange upon Fe(II) adsorption within the water column and to Fe redistribution during diagenesis. Diagenetic Fe(III) reduction caused by oxidation of organic matter and Fe redistribution is supported by the C isotope composition of a carbonate-rich sample containing primary siderite. These carbonates yield δ¹³C values of ∼−10‰, which hints at a mixed carbon source in the seawater of both organic and inorganic carbon. The ancient seawater composition is estimated to have a minimum range in δ⁵⁶Fe of −0.8‰ to 0‰, assuming that hematite and siderite have preserved their primary Fe isotope signature. The long-term near-zero Fe isotope composition of the Hamersley and Transvaal BIFs is in balance with the assumed composition of the Fe sources. The negative Fe isotope composition of the investigated BIF samples, however, indicates either a perturbation of the steady state, or they have to be balanced spatially by deposition of isotopically heavy Fe. In the case of Si, the negative Si isotope signature of these BIFs stands in marked contrast to the assumed source composition. The deviation from potential source composition requires a complementary sink of isotopically heavy Si in order to maintain steady state in the basin. Perturbing the steady state by extraordinary hydrothermal activity or continental weathering in contrast would have led to precipitation of light Si isotopes from seawater. Combining an explanation for both elements, a likely scenario is a steady state ocean basin with two sinks. When all published Fe isotope records including BIFs, microbial carbonates, shales and sedimentary pyrites, are considered, a complementary sink for heavy Fe isotopes must have existed in Precambrian ocean basins. This Fe sink could have been pelagic sediments, which however are not preserved. For Si, such a complementary sink for heavy Si isotopes might have been provided by other chert deposits within the basin.     

Geochemistry of banded iron formation of Orissa,India

Major and trace element analyses of representative samples of various types of banded iron-formation and its various minerals, associated sediments, iron ores and volcanic tuff from different localities of Orissa, India, are presented in this paper. The Orissa banded iron-formation is classified as Precambrian banded iron formation and is similar to the oxide facies iron formation of Lake Superior type. The Orissa iron formation consists only of iron oxide and silica with total absence of iron silicate, sulfide and carbonate minerals, and is devoid of terrigenous material. The trace element content suggests the source of the underlying quartzite to be a continental igneous rock mass, while the interbedded tuff are of undoubted volcanic origin. The overlying iron formation were chemically precipitated as oxidate sediments in which the principal iron mineral — magnetite — was formed at low temperature in a shallow marine environment. From the overwhelming similarity of major and trace element contents of all the samples from the different localities, it is postulated that these detached outcrops originated in the same continous basin.     

Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from –2.5 to +1.0‰ in ⁵⁶Fe/⁵⁴Fe ratios relative to the bulk Earth. With a current state-of-the-art precision of ±0.05‰ for the ⁵⁶Fe/⁵⁴Fe ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BIFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have δ⁵⁶Fe=0.00±0.05‰, as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BIFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of δ⁵⁶Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)–Fe(II)–hematite–magnetite–Fe carbonate. δ⁵⁶Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high δ⁵⁶Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive δ⁵⁶Fe values; these seem best explained through production of high ⁵⁶Fe/⁵⁴Fe reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low δ⁵⁶Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-δ⁵⁶Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments.     

Preservation of organic matter and alteration of its carbon and nitrogen isotope composition during simulated and in situ early sedimentary diagenesis

The carbon and nitrogen isotope composition of organic matter has been widely used to trace biogeochemical processes in marine and lacustrine environments. In order to reconstruct past environmental changes from sedimentary organic matter, it is crucial to consider potential alteration of the primary isotopic signal by bacterial degradation in the water column and during early diagenesis in the sediments.In a series of oxic and anoxic incubation experiments, we examined the fate of organic matter and the alteration of its carbon and nitrogen isotopic composition during microbial degradation. The decomposition rates determined with a double-exponential decay model show that the more reactive fraction of organic matter degrades at similar rates under oxic and anoxic conditions. However, under oxic conditions the proportion of organic matter resistent to degradation is much lower than under anoxic conditions. Within three months of incubation the δ¹³C of bulk organic matter decreased by 1.6‰ with respect to the initial value. The depletion can be attributed to the selective preservation of ¹³C-depleted organic compounds. During anoxic decay, the δ¹⁵N values continuously decreased to about 3‰ below the initial value. The decrease probably results from bacterial growth adding ¹⁵N-depleted biomass to the residual material. In the oxic experiment, δ¹⁵N values increased by more then 3‰ before decreasing to a value indistinguishable from the initial isotopic composition. The dissimilarity between oxic and anoxic conditions may be attributed to differences in the type, timing and degree of microbial activity and preferential degradation. In agreement with the anoxic incubation experiments, sediments from eutrophic Lake Lugano are, on average, depleted in ¹³C (−1.5‰) and ¹⁵N (−1.2‰) with respect to sinking particulate organic matter collected during a long-term sediment trap study.     

Changes in carbon and oxygen isotope composition during limestone diagenesis

The calcite fossils of the Derbyhaven Beds, Isle of Man, have δ¹³C values (+ 1·8 PDB) similar to modern, shallow-water marine skeletons, but the δ¹⁸O values (?6·1 PDB) are much lighter than modern skeletons. The light oxygen values indicate either re-equilibration with isotopically light water before cementation started, or Carboniferous sea water with δ¹⁸O of ?6‰. Aragonite dissolution was followed by precipitation of zoned calcite cement. In this cement, up to six intracrystalline zones, recognized in stained thin sections, show isotopic variation. Carbon varies from + 3-8 to + 1-2‰. and oxygen from ? 2-6 to ? 12-4‰. with decreasing age of the cement. This trend is attributed to increasing temperature and to isotopic evolution of the pore waters during burial. The zoned calcite is sequentially followed by dolomite and kaolinite cements which continue the trend towards light isotopic values. This trend is continued with younger, fault-controlled dolomite, and is terminated by vein-filling calcite and dolomite. The younger calcite, interpreted as a near-surface precipitate from meteoric waters, is unrelated to the older sequence of carbonates and has distinctly different carbon isotope ratios: δ¹³C ? 6-8‰.     

Carbon isotope fractionation of sapropelic organic matter during early diagenesis

Study of an algal, sapropelic sediment from Mangrove Lake, Bermuda shows that the mass balance of carbon and stable carbon isotopes in the major organic constituents is accounted for by a relatively straightforward model of selective preservation during diagenesis. The loss of ¹³C-enriched carbohydrates is the principal factor controlling the intermolecular mass balance of ¹³C in the sapropel. Results indicate that labile components are decomposed leaving as a residual concentrate in the sediment an insoluble humic substance that may be an original biochemical component of algae and associated bacteria. An overall decrease of up to about 4‰ in the δ ¹³C values of the organic matter is observed as a result of early diagenesis.     

Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis

The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ⁵⁶Fe values for magnetite record a strong inheritance from Fe³⁺ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe³⁺ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ⁵⁶Fe values: (1) addition of marine hydrothermal , (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess that had low δ⁵⁶Fe values and was produced by DIR. Most siderite has slightly negative δ⁵⁶Fe values of ∼ −0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ⁵⁶Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources.Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ⁵⁶Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth’s history.     

Stable isotope and geochemical evidence of formation pore fluid evolution during diagenesis of Tertiary sandstones and mudrocks of the Niger Delta

Petrological data provide evidence that framboidal pyrite, Fe-carbonates and kaolinite are the major diagenetic minerals developed during burial diagenesis in the Tertiary Niger Delta sandstones and associated mudrocks. The pyrite sulphur, carbonate carbon and oxygen and kaolinite oxygen and hydrogen isotope compositions have been determined. These data (pyrite, δ³⁴S = −24.8 to 21.0‰; “siderite”, δ¹³C(PDB) = −14.7 to +5.0‰, δ¹⁸O(PDB) = −19.1 to −0.6‰; Fe-calcite, δ¹³C(PDB) = +17.5 to 17.9‰, δ¹⁸O(PDB) = −8.3 to −8.0‰; kaolinite, δ¹⁸O(SMOW) = +14.7 to 17.5‰, δD (SMOW) = −86 to −43‰) have been used to interpret the isotopic compositions of the precipitating pore fluids and/or the temperatures of mineral formation. The interpretation of these results indicate that in the deltaic depositional setting the syndepositional pore waters had a significant but variable marine influence that favoured the early formation of pyrite. Subsequently the subsurface influence of meteoric waters, showing varying degrees of modification involving organic and/or water-rock reactions, played an increasingly significant role in the development of later diagenetic cements in the sediments when abundant authigenic carbonates and kaolinites were formed.     

Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced ⁵⁶Fe/⁵⁴Fe ratios for Fe(II)_aq that are 2-3‰ lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II)_aq-magnetite fractionation is −1.3‰, and this is interpreted to be the equilibrium fractionation factor at 22°C in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II)_aq-Fe carbonate fractionations were ca. 0.0‰ for siderite (FeCO₃) and ca. +0.9‰ for Ca-substituted siderite (Ca_0.15Fe_0.85CO₃) at 22°C. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have ⁵⁶Fe/⁵⁴Fe ratios that are up to 1‰ lower than Fe(II)_aq. Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II)_aq.The relative order of δ⁵⁶Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite ≈ Fe(II)_aq > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples such as Banded Iron Formations (BIFs). Where magnetite from BIFs has δ⁵⁶Fe >0‰, the calculated δ⁵⁶Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has δ⁵⁶Fe ≤0‰ apparently requires formation from aqueous Fe(II) that had very low δ⁵⁶Fe values. Based on this experimental study, formation of low-δ⁵⁶Fe Fe(II)_aq in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria.     

Native elements in rocks of the banded iron formation,Kola Peninsula

Eleven native minerals and intermetallic alloys were identified in rocks of the banded iron formation (BIF) in the Kola Peninsula: copper, silver, gold, electrum, auricupride, cuproauride, tetraauricupride, bismuth, sulfur, tellurium, and graphite. Graphite is a common mineral of sulfide-bearing BIF and gneiss. Sulfur occurs in supergene-altered sulfide-bearing BIFs together with Fe- and Ca-sulfates. Gold of low fineness (electrum) in association with electrum, acanthite, auricupride, volynskite, hessite, cervelleite, pavonite, petzite, and bismuth is related to the areas of hydrothermally altered skarnoids with greenalite, chamosite, aegirine, and Na-Ca amphibole. Redeposited gold of high fineness associated with auricupride, hessite, silver, electrum, kostovite, cuproauride, tetraauricupride, and sperrylite occurs in low-temperature zonal hydrothermal segregations hosted in aluminous gneiss and which formed under the effect of alkalized, highly siliceous solutions at the regressive stage of BIF metamorphism.     

Kaolinite and dickite formation during shale diagenesis: isotopic data

X-ray diffraction, scanning electron microscopy and O-isotope geochemistry have been used to investigate the origin and possible controls on polymorphic transformation of kaolin minerals filling veins in Cretaceous shales from the Gibraltar Strait area (southern Spain).The mineralogy of the enclosing shales indicates that kaolin minerals formed from smectite dissolution, a process that silmultaneously originated I/S mixed-layers and quartz. Kaolinite and dickite δ¹⁸O values suggest that an increase in the water isotopic composition, from Cretaceous sea water values (−1%) to values of about 3%, occurred parallel to smectite dissolution, the intensity of this process being the main factor controlling the isotopic composition of kaolin minerals. The minimum formation temperature ranges from 62°C for kaolinite to 86–96°C for dickite, indicating that the depth of burial was the main control on polymorph formation. This temperature range agrees with that deduced for illite/smectite ordering. The passage from kaolinite- to dickite-rich veins was accompanied, as deduced from SEM examination, by a morphologic evolution characterized by the division of large vermiculae, dominant in kaolinite samples, and the formation of short stacks and platy crystals, which are predominant in dickite. The mechanism of dickite formation, however, remains uncertain.     

Mineralogy and geochemistry of banded iron formation and iron ores from eastern India with implications on their genesis

The geological complexities of banded iron formation (BIF) and associated iron ores of Jilling-Langalata iron ore deposits, Singhbhum-North Orissa Craton, belonging to Iron Ore Group (IOG) eastern India have been studied in detail along with the geochemical evaluation of different iron ores. The geochemical and mineralogical characterization suggests that the massive, hard laminated, soft laminated ore and blue dust had a genetic lineage from BIFs aided with certain input from hydrothermal activity. The PAAS normalized REE pattern of Jilling BIF striking positive Eu anomaly, resembling those of modern hydrothermal solutions from mid-oceanic ridge (MOR). Major part of the iron could have been added to the bottom sea water by hydrothermal solutions derived from hydrothermally active anoxic marine environments. The ubiquitous presence of intercalated tuffaceous shales indicates the volcanic signature in BIF. Mineralogical studies reveal that magnetite was the principal iron oxide mineral, whose depositional history is preserved in BHJ, where it remains in the form of martite and the platy hematite is mainly the product of martite. The different types of iron ores are intricately related with the BHJ. Removal of silica from BIF and successive precipitation of iron by hydrothermal fluids of possible meteoric origin resulted in the formation of martite-goethite ore. The hard laminated ore has been formed in the second phase of supergene processes, where the deep burial upgrades the hydrous iron oxides to hematite. The massive ore is syngenetic in origin with BHJ. Soft laminated ores and biscuity ores were formed where further precipitation of iron was partial or absent.     

Geologic and metallogenic aspects concerning the Nahuelbuta mountains banded iron formation,Chile

Paleozoic banded-iron-formation (BIF) deposits occur within the Nahuelbuta-Queule Complex (south central Chile) which hosts the following stratigraphic units: Cabo Tirúa (green schists, mica schists, and metacherts), Lleu-Lleu (iron-bearing metacherts, mica schists, and serpentinites), and Colcura (metagraywackes and metapelites). The lithological, structural, and geochemical characteristics of the Lleu-Lleu and Cabo Tirúa units indicate that they were part of a tectonic mélange accreted to the South American paleocontinent during the Paleozoic. BIF ores are restricted to the Lleu-Lleu metacherts and are characterized by oxide-silicate-sulfide BIF facies. The iron-bearing metacherts present mineralogical and geochemical characteristics close to the volcanogenic BIF types and are thought to have been formed by submarine volcanic exhalative activity.