In this article we will discuss about:- 1. Introduction to Banded Iron-Formations 2. Regional Geologic Distribution of Banded Iron-Formations 3. Ages of Precambrian Banded Iron-Formations.
Introduction to Banded Iron-Formations:
Banded iron-formations are rocks of mostly Precambrian age that are composed of interlaminated quartz and iron minerals.
They can be subdivided into two varieties:
1. Algoma deposits, which are relatively small with an obvious volcanic association, and
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2. Lake Superior, which are much larger and have a shallow-shelf, orthoquartzite-carbonate association.
The Algoma type is abundant in the Archean, but an Ordovician example is found at Bathurst, New Brunswick and there is a possible analogue associated with the Carboniferous Pb-Zn deposit at Tynagh, Ireland. Lake Superior- type ores, by contrast, are confined to a particular time interval at around 2 billion years before the present.
Oolitic ironstones have a more clastic association than the iron-formations, and are higher in Al. Instead of banding, their most prominent sedimentary structure is ooliths made up of hematite or chamosite. Chert is rare, and they are found in rocks of a variety of ages from Proterozoic to Pliocene.
As of date, these two types constituted more than 90 per cent of the world’s iron production, with Lake Superior ores accounting for about 65 percent. At that time, ironstones made up about 20 per cent of the production, but this proportion has been decreasing: the last iron mines in Great Britain, which are in Jurassic oolitic ironstones.
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The extensive Lake Superior-type deposits of Australia are now coming into production, displacing smaller deposits, so that Lake Superior-type ores probably make up 75 per cent or more of production today. One reason for their predominance is a change in the nature of the feed for blast furnaces.
Formerly, only ores that had been naturally enriched by the leaching of most of their SiO2 (the “direct shipping ores”) could be used. Since the introduction of pelletized feeds in 1955, there has been a shift to artificially concentrated cherty taconites because of lower shipping costs and lower energy costs for blast furnace operation.
Iron is the third most abundant metal, after Si and Al. Therefore, it is a major constituent of most rocks (Table 1.1), and its ores are actually rock types in their own right. The formation of these iron rocks depends on changes in oxidation state: Fe is mobilized under reducing conditions, precipitated under oxidizing.
Thus, it will migrate from areas low in oxygen towards areas high in it, provided a pathway exists. Note, in Table 1.1, that igneous rocks, although varying in total iron, have about the same proportion of Fe3 to Fe2.
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Sedimentary rocks, in contrast, show a wide range of oxidation states, indicative of the presence of environments of differing oxidation potential. Therefore, it is in sediments that the greatest potential for iron enrichment exists, and almost all iron ores are sedimentary.
Iron has been known since antiquity. Iron is ubiquitous in the earth’s lithosphere as either a major element or in trace amounts. In abandons or Clarke Value it ranks fourth behind oxygen, silicon and aluminium. The most important use of iron is in the making of steel, which is essentially an alloy of iron with carbon and other elements depending upon the end use.
India is one of the earliest manufacturers and users of iron and steel in the world. Survey of many documentary evidence such as making of various surgical instruments in the 3rd and 4th century B.C. prove this. Till 18th Century iron and steel making in India was at par with that of Europe in the form of village crafts. This situation changed with introduction of Bessemer process in 1865 and the basic open Hearth Furnace process in 1878. These developments led to significant increase in the world steel production from about 0.5 MT in 1870 to 28 MT in 1900.
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The present annual capacity of primary steel production from the integrated steel plants is placed at about 26 MT from the main producers and about 70 MT in the secondary sector by various processes.
Regional Geologic Distribution of Banded Iron-Formations:
The banded iron-formations are confined to Precambrian shield areas. They are comparable by their significant features on both the Northern Hemisphere and the Southern Hemisphere continents and, in fact, there is no major cratonic area which lacks iron-formations as prominent members of its stratigraphical column.
As most, but not all, iron-formations are metamorphosed and are parts of the regionally metamorphosed and often strongly folded sedimentary sequences that build up the consolidated masses of the Precambrian shields, they occur frequently as resistant erosional remnants of folded ancient sedimentary basins. In many cases, they can be traced as parts of synclinal or anticlinal structures over hundreds or even more than a thousand kilometres in lateral extent, forming prominent iron ranges.
Apparently, oxide-facies iron-formations represent the bulk of all economically significant iron-formations. Hence, most of the geological literature on the subject is based on this type which is the host rock of the largest and best-studied iron ore mines, at present being worked predominantly on the Southern Hemisphere continents – South America, Africa, India and Australia.
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In these areas of sub-tropical climates, many of the originally low-grade iron-formations have been enriched under favourable conditions by supergene weathering processes to high-grade iron ores with 60-69 per cent iron content, which is described in India as float ore.
In Figure 1.1 and Table 1.2 is shown the distribution of the most important occurrences of iron-formations on the Precambrian shield areas of the world, it is obvious that many of them lie close to the borders of the continents or on the margins of the cratonic masses now surrounded by younger fold belts and platform sediments.
As Gross (1973) has pointed out, this distribution may lead to the supposition that the margins of the old continental masses have been favourable areas for the development of depositional basins with sedimentation of iron-formations.
Gross suggested that some of the iron-formations now on the borders of different continents may have formed parts of the same sedimentary basins and fold belts of consistent shield areas as they may have appeared prior to continental drift.
This regional distribution of iron-formations near and along the coastal parts of the continents is clearly seen in South America and Africa as well as in Australia and India.
Where the general geological setting has been studied in more detail, a comparison of iron-formations on opposite continents fits fairly well with the assumption of originally coherent iron-formation belts. This is the case for iron- formations on the Guayana and Liberian shields, as Gross (1973) has shown, comparing the occurrences of the Imataca Series of Venezuela with those of Liberia and Sierra Leone. The type and depositional environment of these iron-formations with their associated rocks are similar, if not identical, and so are their ages and their orogenic and metamorphic histories.
It is most interesting to note that the iron-formations of Venezuela and Western Africa, both deposited 2,500 – 3,000 m.y. ago, were affected by the same younger metamorphic events at 1,800 m.y. and underwent deformation by folding, symmetrical to a central crust zone.
This would mean that the segmentation and continental drift of the Gondwana landmass, supposed to have started in many places during the Early Cretaceous, has followed a latent and much older Precambrian fault and tectonic pattern of worldwide dimensions, which may have controlled the suitable conditions for the depositional environments of iron-formations.
It is a most remarkable fact that the distribution of Precambrian iron-formations in several great cycles coincides fairly well with the ages of profound magmatic and metamorphic events during the evolution of the earth’s crust.
Runcorn (1962) advocated a repeated continental drift and delivered a theory of Precambrian continental drift, caused by changes in the convection mechanism of the earth’s mantle by periodical growth of the core.
Such changes, as Runcorn presumes, occurred in the intervals between 2,750 and 2,450 m.y.; 1,900-1,600 m.y.; and 1,150-900 m.y. ago, thus corresponding in time with the main epochs of deposition of iron-formations many of which lie inside the worldwide fold belts of the “Superior Regime” (2,750-1,950 m.y.) as plotted by Dearnley (1965) on the presumed pre-Gondwana landmass.
Thus, the fundamental reasons for the regional distribution of such huge accumulations of iron and silica seem to be basically connected with certain universal principles in the behaviour of the sialic crust. It is almost certain that any attempt to decipher the final reasons for the deposition of the Precambrian iron-formation has to consider not only detailed studies of single iron-formation environments but relies heavily on more knowledge of the superimposing basic factors of worldwide significance.
Ages of Precambrian Banded Iron-Formations:
Due to the lack of minerals within the iron-formations suitable for radiometric age determinations, a precise dating of these rocks has been possible only in rare cases. However, the progress in radiometric dating techniques and the great number of samples determined in recent years from associated meta-sedimentary and volcanic rocks makes it possible at least to mark the age limits of many prominent iron- formations of all parts of the world. Compilations of age determinations have been given by Lepp and Goldich (1964), Govett (1966), James (1966) and by Goldich (1973).
Though banded iron-formations occur during a period of at least 3,000 m.y. throughout the Precambrian, their prevailing development is confined to a single epoch between 2,600 and 1,800 m.y. ago. As Bayley and James (1973) have shown, this interval was clearly the principal time of deposition in North America, far outweighing by their extent and iron content the more limited older and younger iron-formations.
The same is the case for many major iron-formations on other continents such as those of Russia (Krivoy Rog, Kursk), South Africa, Brazil and Western Australia, all of which seem to have been deposited approximately contemporaneously between 2,600-1,800 m.y. ago. In all these shield areas apparently, there is a progressive decrease in the abundance of iron-formations, in size as well as in quantity, in the post-1,800 m.y. period just as in the pre-2,600 m.y. epoch.
Following the framework classification given by Goldich (1973), the iron- formations should be grouped, according to their ages, in the categories:
(1) Older than 3,000 m.y.;
(2) 3,000 – 2,600 m.y.;
(3) 2,600-1,800 m.y.; and
(4) Younger than 1,800 m.y.
1. With few but notable exceptions, the bulk of the Early Precambrian, Archean iron-formations, though not restricted to the pre-2,600 m.y. period, belong to the Algoma-type as defined by Gross (1965). They are closely related to greenstone belts and form relatively small lenticular bodies.
The oldest known iron-formations, close to or older than 3,000 m.y., are found in the Swaziland System of South Africa (3,000-3,400 m.y.), in the Pilbara and Yilgarn Blocks of Western Australia (2,670-3,000 m.y.), in the Imatacu Complex of Venezuela (3,000-3,400 m.y.) and in the Ukrainian Shield in the Greater Krivoy Rog area (3,100-3,500 m.y.)
2. In the range from 3,000 – 2,600 m.y. numerous occurrences are known on the Canadian Shield, most of them closely associated with greenstone belts. A great number of them were deposited in the remarkably brief interval from 2,750 – 2,700 m.y. ago. In South Africa iron- formations in the Sebakvian, Bulavayan and Shamvayan Systems, stratigraphically associated with volcanic rocks, were dated at, more than 2,700 m.y.
The same ages of >2,700 m.y. are found in some small Algoma-type occurrences in the Rio das Veihas-Series of Brazil, in India in Karnataka and in the “Iron Ore Series” of Jharkhand – Orissa and also in eastern Karelia on the Baltic Shield. Thus, it appears that the span of time between 3,000 and 2,600 m.y., especially around 2,700 m.y., marks a significant epoch, very favourable for the deposition of iron-formations.
3. The most abundant development of iron-formations in the world is found in the interval between 2,600 and 1,800 m.y. On the Canadian shield the ages of the major iron-formations of Labrador and the Lake Superior region range between 2,200 and 1,900 m.y.
The interval of the main deposition was probably much less, since for the iron-formations in Minnesota, thought to be largely contemporaneous with others in North America, a minimum age of 2,000 m.y. has been determined. In Australia the Brockrnan Iron Formation of the Hamersley Group is bracketed between 2,200 and 2,000 m.y.
Similar ages are given for Krivoy Rog/Ukrainia (2,200-1,900 m.y.), for the Brazilian itabirites in Minas Gerais and in the Serra dos Carajas-Par (about 2,000 m.y.) and also for iron-formations of the Svecofennian of the Baltic Shield (minimum age of 1,900 m.y.) and of the Aldan Shield on the southeastern margin of the Siberian platform (2,000-2,750 m.y.).
In South Africa iron-formations in association with clastic rocks are predominantly developed during the interval from 2,300-3,000 m.y., whereas the shelf- carbonate and carbonate-manganese associations belong to the period prior to 2,300 m.y.
4. The post-1800 m.y. banded iron-formations known in the world are quantitatively much less important; compared with the overwhelming concentration in the Lower Proteiozoic and Archean they should rather be considered as exceptions. If a clear definition of “iron-formation” should exist, it most probably would not fit the characteristics of “iron-formation”, principally of the problematic younger iron-formations, described by O’Rourke (1961).
Examples for these are occurrences in Nepal and in Mato Grosso/Brazil. These Late Proterozoic or Early Cambrian cherty iron-formations are also typically banded but, averaging about 56 per cent Fe, they are much richer in iron than the older types. Their origin is still enigmatic. In North America only a few minor occurrences of Late Precambrian age are known.
In the Rapitan Group of the Yukon and Northwest Territories the age of an iron- formation is correlated with that of the Windermere Group (800-600 m.y.). The iron- formation in the Yavapai Series of central Arizona was metamorphosed between 1,820 and 1,760 m.y. ago and may be coeval with those of the Lake Superior region. The iron-formation in the lower part of the Damara Super-group of South West Africa has been deposited between 1,000 and 620 m.y. ago.
It is a most remarkable fact that major iron-formations all over the world can be fairly well grouped in several time intervals of the Precambrian, between 3,400 and 3,000 m.y., about 2,700 m.y. and outstandingly in one period between 2,600 and 1,800 m.y. ago with an accumulation at about 2,000 m.y.
This last statement has been also made for the major iron-formations of the CIS and Russia. Many of them have metamorphic ages of about 2,000 m.y. Younger ages are thought to correspond to the age of the final orogeny in the evolution of the geosyncline, which went across the Russian platform so that they do not reflect the time of deposition.
The iron-formations of the Algoma-type generally are limited in size, but occur over a greater span of time, from the earliest Precambrian through the Phanerozoic, whereas the Superior-type is strictly confined to the Precambrian, the bulk of it most probably to the period prior to about 2,000 m.y. ago.
It must be pointed out that the data for single iron-formations should be used with caution as many of them might represent not the true age of deposition but mixed ages from subsequent metamorphic or orogenic events which have overprinted the old craton areas, for instance, the 500 m.y. “Panafrican Thermal Event” advocated by Kennedy (1964).
Nevertheless, these factors as well as uncertainties in the use of decay constants in the different laboratories should not affect the general statement that the deposition of many major iron-formations of the world was synchronous in certain epochs of the Precambrian.
Figure 1.2 suggests that the development of iron-formations fell into relatively stable epochs precursory to and partly overlapping with the initial stages of major magmatic and metamorphic cycles during the Precambrian.
Unequivocally, as it is also pointed out by Cloud (1973), the epoch between about 2,600 and 2,000 m.y. ago marks a profound change of conditions in the earth’s evolution, terminating the vigorous growth of procaryotic organisms and giving rise to a development under increasingly oxidizing conditions.
Commercial and Specifications:
Iron ore is used mainly for making pig iron, sponge iron and steel. Iron and steel together form the largest manufactured products in the world and each of these centers into every brand of industry and it is a necessary factor in every part of our present day lifestyle. Pure iron use has been relatively few and has been mostly used for specialized uses. Ingot iron is galvanized for roofing, siding and tanks.
In the form of corrugated pipes it is used for end-vents because of its relatively high purity it is used for oxy-acetylene welding, both as material to be welded and as welding rod. It is used in vitreous enameling. Its good ductility makes iron suitable for deep drawing operations as in the manufacturing of appliance parts e.g. washing machine tube.
Relatively low electrical resistance and high magnetic permeability gives to its use in many types of electrical equipment, generator field, and magnetic parts of relays, magnetic brakes and clutches. Iron ore is used also in ferrous alloys; cement foundry, vanaspati and glass factories.
The typical compositions of cast iron are given in the Table 1.3. The different grade of pig iron depends upon their contents of silicon, sulphur, phosphorous and manganese.
Prices are influenced not only by the intrinsic prices of the ore (base price) but also by freight expenses. Freight expenses demonstrate a more relatable behavior to the basic price of the ore. The price of the iron ores of different grade during 1999 – 2004 are given in the Table 1.4.
