Types Of Iron Formations Research Articles

Precambrian iron formations from the Cauvery Suture Zone, Southern India: Implications for sub-marine hydrothermal origin in Neoarchean and Neoproterozoic convergent margin settings

Thick horizons of iron formations including Banded Iron Formations (BIFs) and Banded Silicate Formations (BSFs) occur as E–W trending bands in the eastern part of Cauvery Suture Zone (CSZ) in the Sothern Granulite Terrane of India. Some of these occur in close association with the Neoarchean-Neoproterozoic suprasubduction zone complexes, where as some others are associated with metamorphosed accretionary sequences including pyroxene granulites and other high grade rocks. The iron formations are highly deformed and metamorphosed under amphibolite to granulite facies conditions and are composed of quartz–magnetite–hematite–goethite–garnet–pyrite together with grunerite and pyroxene. Here we report the geochemical characteristics of twenty representative samples from the iron formations that reveal a widely varying composition with Fe2O3(t) (22–65 wt.% as total iron) total- Fe2O3/TiO2 (205–6532), MnO/TiO2 (0.25–12.66) and SiO2 (33–85 wt.%), broadly representing the two types of iron formations. These formations also show very low Al/(Al+Fe+Mn) ratio (0.001–0.01), Al2O3 (0.07–0.76wt.%), Al2O3/TiO2 ratio (2.7–21), MgO (0.01–4.41wt.%), CaO (0.1–1.24wt.%), Na2O (0.01–0.05wt.%) and K2O (0.01wt.%) together with low total REE (3.38–31.63ppm). The trace and REE elemental distributions show wide variation with high Ni (274ppm), and Zn contents (up to 87ppm) when compared to mafic volcanics of the adjoining areas. Tectonic discrimination plots indicate that the iron formations of the Cauvery Suture Zone are of hydrothermal origin. Their chondrite normalized patterns show slight positive Eu anomaly (Eu/Eu*=up to 1.77) and relatively less fractionation of REE with slight LREE enrichment compared to HREE. However, the PAAS (Post Archean Average of Australian Sediments) normalized REE patterns display significant positive Eu anomaly (Eu/Eu* up to 2.32) with well represented negative Ce anomalies (Ce/Ce*=0.66–1.28). The above results together with petrological characteristics and available geochronology of the associated lithologies suggest that the iron formations can be correlated to Algoma-type. The Fe and Si were largely supplied by medium to high temperature sub-marine hydrothermal systems in Neoarchean and Neoproterozoic convergent margin settings.

Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia. Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations.

Siderite formation and evolution of sedimentary iron ore deposition in the Earth’s history

The role of siderite in Phanerozoic and Precambrian iron formations is discussed. Various types of iron formations are characterized, and their place in the evolution of sedimentary iron ore deposition is outlined. In Precambrian iron ore deposition, siderite is a primary mineral, whereas in Phanerozoic iron formations it becomes a secondary mineral and is commonly related to diagenetic and catagenetic processes.

Structural history of the Manitouwadge greenstone belt and its volcanogenic Cu-Zn massive sulphide deposits, Wawa subprovince, south-central Superior Province

Structural analysis of the Manitouwadge greenstone belt, integrated with detailed mapping and geochronological and petrographic studies, reveals a complex early deformation history that significantly modified the primary distribution of base-metal deposits and alteration zones. The (D3) Manitouwadge synform dominates the map pattern; however, penetrative fabric development and establishment of the tectono-stratigraphy of base-metal deposits mostly predated D3. The D1 Garnet Lake fault, which repeats mineralized horizons within a distinctive lithological sequence, is delineated locally by annealed mylonite. D1 planar fabrics are preserved locally in outcrop and thin section. D2 folding accompanied peak regional metamorphism at upper amphibolite facies. The F2 Agam Lake syncline repeats the volcanic sequence across the southern limb of the Manitouwadge synform. A map-scale F2 sheath fold deforms the Garnet Lake fault. Minor D2 structures include prevalent outcrop-scale folds, locally with sheath geometry, the dominant S2 foliation, and mineral lineations (parallel to fold axes). Northwest-southeast-directed D3 shortening produced the Manitouwadge synform and related regional folds without extensive penetrative fabric development. Flexural slip folding is evident in the inner hinge of the synform where rocks of differing competency are interlayered. Higher strain, stronger fabric development, and a component of simple shear were preferentially partitioned to fold limbs. Relative pre-D3 structural geometries in the inner hinge region of the Manitouwadge synform are not significantly complicated by D3 and younger deformation. Retrodeformation of the mineralized sequence shows systematic stratigraphic patterns in iron formation types, stacked massive sulphide orebodies, and alteration types that can be applied to exploration models.

Iron-formations and epigenetic ironstones in the Palaeoproterozoic Willyama Supergroup, Olary Domain, South Australia

Iron-formations occur as massive to compositionally layered, Fe oxide-rich, concordant bodies in the Palaeoproterozoic Willyama Supergroup of the Olary Domain, South Australia. They have constitutional similarities to those occurring in the neighbouring Broken Hill Domain. The most abundant iron-formations are in the Quartzofeldspathic Suite and comprise magnetite-quartz assemblages (± hematite, barite, actinolite, apatite). Hematite, magnetite, albite, quartz, Ca(Na) amphibole(s), CaNaFe clinopyroxene and andraditic garnet are major constituents of rare calc-silicate iron-formations in the Bimba and Calcsilicate Suites, whereas magnetite, quartz, almandine-spessartine, manganoan fayalite, manganoan grunerite and apatite form manganiferous iron-formations in the Pelite Suite. The pronounced differences in mineralogy of the three iron-formation types are the result of regional metamorphism of diverse hydrothermal precipitates with variable elastic components, together with the local effects of high-temperature metasomatic alteration. Metasomatic fluids were produced as a result of devolatilisation of the evaporite-bearing volcanosedimentary sequence, during and following amphibolite grade metamorphism and deformation, which led to localised and regional-scale hydrothermal alteration. In places, there was extensive metasomatic reconstitution (veining, brecciation, replacement) of iron-formations and associated rocks, caused by high-temperature (350°–650°C), oxidising, saline fluids. The resulting epigenetic ironstones are dominated by magnetite-hematite-quartz with minor sulfides and display enrichment in Fe, Ti, Cu, Au, Sc, U, V, Y, Zn and HREE relative to parental iron-formations.

Debate about ironstone: has solute supply been surficial weathering, hydrothermal convection, or exhalation of deep fluids?

ABSTRACTIronstone is any chemical sedimentary rock with > 15% Fe. An iron formation is a stratigraphic unit which is composed largely of ironstone. The solutes which have precipitated to become ironstone have dissolved from the Earth's surface, from the upper crust, e.g. the basaltic layer of oceanic crust, or from deeper within the Earth. Genetic modellers generally choose between surficial weathering, e.g. soil formation, and hydrothermal fluids which have convected through the upper kilometre of oceanic crust. Most genetic modellers attribute cherty laminated iron formations to hydrothermal convection and noncherty oolitic iron formations to surficial weathering. However, both types of iron formations are attributable to the exhalation of fluids from a source region too deep for convection of seawater. Evidence for a deep source of ferriferous fluids comes from a comparison of ancient ironstone with modern ferriferous sediment in coastal Venezuela. A deep‐source origin for ironstone has wide‐ranging implications for the origins of other chemical sedimentary ores, e.g. phosphorite, manganostone, bedded magnesite, sedimentary uranium ore, various karst‐filling ores, and even petroleum. Preliminary study of a modern oolitic iron deposit described herein suggests that the source of iron and silica to iron formations may have been even deeper than envisioned within most hydrothermal convection models.

Sedimentology and geochemistry of the glaciogenic late Proterozoic Rapitan Iron-Formation in Canada

The Rapitan iron-formation in the Northwest Territories and Yukon of Canada was formed between 755 and 730 Ma. This iron-formation is approximately coeval with several other extensive iron-formations worldwide, all of which are associated with glaciogenic sequences. The mineralogy and major element geochemistry of the Rapitan iron-formation is very simple (consisting mainly of hematite and chert) and is distinctly different from that of most major banded iron-formation types of Archcan and Early Proterozoic age. The rare earth element (REE) chemistry of the Rapitan iron-formation appears to be much less distinctly influenced by hydrothermal input into an ocean system than that of these earlier Archcan or Early Proterozoic iron-formation sequences. This suggests a hydrothermal input that was highly diluted by ocean waters at Rapitan time.The sedimentologic setting of the Rapitan iron-formation, among a thick sequence of glaciogenic materials, the occurrence of dropstones and faceted pebbles in the iron-formation itself, and the very large aerial extent of these sequences strongly suggest an origin as part of glaciomarine conditions. The iron-formation was deposited during a major transgressive event with a rapid rate of sea-level rise during an interglacial period. The deposition of the iron-formation took place under clear water conditions, with the addition of siliciclastic materials as subaqueous gravity flows.The occurrence, during Late Proterozoic time, of several, mineralogically and chemically very similar iron-formations worldwide that are in close association with major glaciogenic sequences, and palcomagnetic data for that same period (Kirschvink, 1992), which suggest widespread continental glaciers within a few degrees of the equator, lead to the concept of a snowball-type Earth: an earth that would have resembled a highly reflective snowball with floating pack ice over most of the ocean surface (Kirschvink, 1992). In such an environment there would have been a buildup of dissolved iron during glacial periods and a deposition of iron during interglacial periods.

Geochemistry and sedimentology of a facies transition from limestone to iron-formation deposition in the early Proterozoic Transvaal Supergroup, South Africa

This study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate-shale to banded iron-formation in the Campbellrand carbonate sequence to the overlying Kuruman Iron Formation of the Transvaal Supergroup in South Africa which is approximately 2.3 Ga old. Four major lithologies are (1) limestone and dolomite, (2) shale and interbedded shale carbonate, (3) siderite-rich banded iron-formation, and (4) iron oxide-rich banded iron-formation. These rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic-diagenetic overprint ranging in temperature from 110 degrees to 170 degrees C and with pressures of not more than 2 kbars.The oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures. Interbedded with the limestones and dolomites are carbonaceous shales, some of which are unusually ferruginous or pyrite rich. These carbonates and shales are overlain by meso- and microbanded siderite-chert iron-formation which grades upward into magnetite-, chert-, and carbonate-rich iron-formation.The averages for major and trace elements and rare earth element (REE) contents of the limestones, dolomites, and shales are distinct from those of the two types of iron-formation. The two most chemically diverse rock types are the shales (enriched in alkalis and most trace elements) and the two iron-formation types (siderite and magnetite rich) which are almost totally depleted in all elements except Si, Fe, Mg, and Ca (and CO 2 ). The high Al 2 O 3 contents of the shales (avg 9.55 wt %) correlate well with their high organic carbon contents (avg 3.91 wt %); the limestones and dolomites have average Al 2 O 3 and organic carbon contents of about 3.0 and 0.7 wt percent, respectively; the two iron-formation types have Al 2 O 3 average values of 0.099 wt percent (siderite rich) and 0.066 wt percent (magnetite rich) and corresponding averages for organic carbon of 0.080 wt percent (siderite rich) and 0.012 wt percent (magnetite rich). The REE are generally concentrated in the shales by a factor of about 10 over the iron-formations. The limestones and dolomites have intermediate values, and the iron-formations are most depleted. The iron-formation REE patterns (in a ratio with North American shale composite (NASC)) have pronounced positive Eu anomalies and slight negative Ce anomalies. Both of these anomalies are absent in the shales, but the limestones and dolomites show slight positive Eu anomalies.The siderite-rich iron-formations consist of chert-siderite-ankerite (or ferroan dolomite) with traces of pyrite and stilpnomelane. The siderite in these commonly microbanded assemblages is very fine grained along well-defined bedding planes. It is concluded, from petrographic study, that siderite is a primary precipitate. The ankerites and ferroan dolomites in these siderite-rich occurrences are commonly euhedral (rhombohedral), much coarser grained, and appear to be of later (diagenetic) origin. The magnetite-rich iron-formations consist of chert-magnetite-ankerite (or ferroan dolomite) + or - siderite + or - hematite + or - stilpnomelane + or - minnesotaite. In these oxide-rich iron-formations the ankerites (and ferroan dolomites) are similarly much coarser grained than the finely banded siderite. On the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate-shale to iron-formation transition, we conclude that the limestone-dolomite-shale lithologies originated in a water column quite distinct from that in which the iron-formations were precipitated. We propose a model with a stratified water column in which the surface waters (during a regressive stage in the depositional basin) were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition; with at somewhat greater depth (below the chemocline) deposition of pyritic carbonaceous shale. Our model depicts the deeper waters (during a transgressive stage of the basin with the Kaapvaal craton more deeply submerged) as the site for iron-formation deposition; these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass, with continued availability of oxygen along the chemocline separating the two water masses. The ultimate source of the iron (and probably the SiO 2 ) in the iron-formations appears to be a very dilute hydrothermal input in the deep ocean waters, as concluded from mixing calculations of REE values for modern Atlantic Ocean water and hydrothermal solutions from the East Pacific Rise.

Distribution and origin of Precambrian banded iron formation

Banded iron formution (BIF) is a useful descriptive term, which characterize a thinly layered sedimentary rock, consisting mainly of silica and iron minerals, of chemical origin and modified by diagenesis and metamorphism. At least four sedimentary facies have been identified (oxide, carbonates, silicate and sulfide), and various types of iron formations were defined (Algoma, Superior, Clinton, Minette) to accomodate the wide range of types based on lithology, geologic setting and supracrustal association. The vast bulk of BIF has a remarkably simple and uniform chemical and mineralogical composition, including chert, hematite and magnetite as dominant components. It is present in all Precambrian cratons of the world and seem to be about equally distributed in the five continents, each one including a very large BIF district: Hamersley range in Australia, Transvaal-Griquatown in Africa, Minas Gerais in South America, Labrador Trough in North America, and Krivoy Rog-KMA in Eurasia. Together, these deposits contain about 90% of Earth's total BIF, estimated at about 6 x 10 14 tones. The problems related to source of Fe and SiO 2 , chemical mechanism and site of deposition of BIF are not complctely solved at present. The favoured current model involves a Precambrian ocean with an upper oxic layer overlying a much larger volume of anoxic water with enormous amounts of Fe anel SiO 2 accumulated in the oceans from varied sources. BIF precipitation only occurred under a conjunction of favourable circunstances, at particular sites, which were repeated periodicaily, but occurred on a uniquc scale only once, in the Lower Proterozoic (2.0-2.5 Ga), when an extraordinary concentration of BIF (92% of total BIF) was formed possibly in shallow marginal basins, during a quiet tectonic period of the Earth's history.

Geochemical distinctions among environmental types of iron formations

Classification of iron formations according to the sedimentary-volcanic environments in which they formed reveals that chemical compositions of major iron-formation types are largely independent of deposit age. No evidence is found in iron formations for atmospheric-hydrospheric compositional evolution. However, chronological variation in relative abundance among major environmental types may indicate an Archean to Phanerozoic tectonic-magmatic evolution from abundant shallow-volcanic platforms through predominantly non-volcanic shallow platforms or flat continental shelves to abundant inland seas. Shallow-volcanic-platform iron formations typically display large positive europium anomalies and vary widely in silica, phosphorus and alkali content, even within individual iron formations. Various minor-element abundances may be substantial, particularly where phosphatic or near sulfide ore bodies. Iron formations which formed on extensive, chemical-sediment-rich continental shelves or non-volcanic oceanic platforms are generally silica-rich, phosphorus-poor, alkali-poor and minor-element-poor. Europium anomalies are normally small and mostly negative. Oolitic-inland-sea iron formations are poor in silica, except where gradational to sandstone, and are typically enriched in phosphorus and several ferrides.