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DEPOSITS RELATED TO CHEMICAL SEDIMENTATION. PART 1: BANDED

IRON FORMATION, AND BOG

1. Details of Module Subject Name

Paper Name & RESOURCES OF

Module Name/Title DEPOSITS RELATED TO CHEMICAL SEDIMENTATION. PART 1: , PHANEROZOIC IRONSTONES AND BOG IRON ORES Module Id GEL-05-148

Pre-requisites Before learning this module, the users should be aware of:  Sedimentary environment  Geology of Iron formations  Palaeoclimatic aspects Objectives To understand:  Iron formation worldwide.  Facies and genetic types of deposits.  Deposition of iron formation. Keywords Iron ores, facies of iron ores, genesis of iron ores,type of iron ores.

2. Structure of the Module-as Outline: Table of Contents only (topics covered with their sub-topics)

1. Introduction 2. Banded Iron Formation 2.1 facies 2.2 facies 2.3 Silicate facies 2.4 facies 2.5 Types of Iron formations: Algoma type, Superior type; Rapian type 2.6 Genetic aspects 2.7 Some examples 3. Phanerozoic ironstones 3.1 Clinton type 3.2 Minette type 4. Bog iron ores

3.0 Development Team: Role Name Affiliation

National Co-ordinator

Subject Co-ordinators Prof. M.S. Sethumadhav Centre for Advanced Studies

(e-mail: Prof. D. Nagaraju Dept of Earth Science [email protected]) Prof. B. Suresh University of Mysore,

Mysore-6

Paper Co-ordinator Prof. M.S. Sethumadhav Centre for Advanced Studies

Dept of Earth Science

University of Mysore,

Mysore-6

Content Writer/Author(CW) Prof. M.S. Sethumadhav Centre for Advanced Studies

Dept of Earth Science

University of Mysore,

Mysore-6

Content Reviewer (CR) Prof. A. Balasubramaian Centre for Advanced Studies

Dept of Earth Science

University of Mysore,

Mysore-6

1. INTRODUCTION

About 95% of the ’s “reserve base” of iron ores, estimated at tons, occur in sedimentary ores [chemical sedimentary (74%) and volcano-sedimentary (19.4%)].

A meagre ~5% belongs to all other categories combined, including magmatic, epigenetic- hydrothermal, metamorphogenic and residual concentration types and iron stones and bog iron ores (Veizer et al., 1989). Five super large iron ore districts, viz., the

Hamersley Basin (), the Transvaal-Griquatown region (S. ), the Minas Gerias

(), the Labrador Trough region (Canada), and the Krivoy-Rog-Kursk Magnetic

Anomaly region (Ukraine) together make up ~90% of the world’s total sedimentary iron ore resources. Other important occurrences are the Damara Belt (Namibia), the Yilgran Block

(Australia), the Northern Basin (W. Australia), the Iron Ore Group and Dharwar Supergroup

(India), the Province (USA), the Belozyorsky-Komsky region (Ukraine), the

Michipicoten (Canada), the Intaka Complex (Venezuela) and the Liberia--

Guinea- Ivory Coast Belt. In India the “reserve base” stands at tons and it ranks sixth in the world’s iron ore resources. Virtually all Indian ore production (57 million tons in

1991) came from Precambrian deposits in Goa, Karnataka, Madhya Pradesh, Bihar and

Orissa states.

Iron ores can be grouped into (a) Banded Iron Formation (BIF), (b) non-cherty ironstones and (c) Bog iron ores. The BIF are known in different continents under terms

Banded , Banded quartzite, itabirite, jaspilite, etc. BIF is classified into three types: Algoma type, Lake Superior type and Rapitan type Non-cherty iron stones are classified into two types: Clinton type and Minette type.

The BIF ores are dominantly Precambrian. In fact, the great bulk of the banded iron formations of the world was laid down in a very short time interval of 2500 – 1900 Ma ago

(James and Trendall 1982). The amount of iron laid down during this period, and still preserved, is enormous- at least tons and possibly tons. Among the BIF subgroups the submarine volcanic-associated “Algoma type” BIF deposits were dominantly Archaean, though younger Algoma type iron ores are also known. The carbonate-orthoquartzite associated shelf/marginal basin deposits of the Lake Superior type BIF were almost exclusively of lower age. Rapitan type continental margin BIF deposits in glaciogenic sequences are essentially of upper Proterozoic age. The non-cherty ironstones of

both Clinton and Minette types are exclusively Phanerozoic. The bog iron ores were formed on land from carboniferous to Recent times.

2. BANDED IRON FORMATION (BIF)

BIF is characterized by its fine layering. The layers are generally 0.5-3 cm thick and inturn they are commonly laminated on a scale of millimetres or fractions of a millimetre. The layering consists of silica layers (in the form of or better crystallized silica) alternating with layers of iron . The simplest and commonest BIF consists of alternate hematite and chert layers. James (1954) identified four important facies of BIF.

2.1 Oxide facies: This is the most important facies and it can be divided into the hematite and magnetite subfacies according to the dominant . There is a complete gradation between the two subfacies. Hematite in least altered BIF takes the form of fine-grained grey or bluish specularite. Oolitic texture is common in some examples, suggesting a shallow water origin, but in others the hematite may have the form of structureless granules.

Carbonates (calcite, and rather than ) may be present. The “chert” varies from fine-grained cryptocrystalline material to mosaics of intergrown quartz grains. In the much less common magnetite subfacies, layers of magnetite alternate with iron silicate or carbonate and cherty layers. Oxide facies BIF typically averages 30 – 35% Fe and these rocks are commercially viable provided they are amenable to beneficiation by magnetic or gravity separation of the iron minerals.

2.2 Carbonate facies: This commonly consist of interbanded chert and siderite in roughly equal proportions. It may grade through magnetite-siderite-quartz rock into the oxide facies, or, by the addition of , may grade into the sulphide facies. The siderite lacks oolitic or granular texture and appears to have accumulated as a fine mud below the level of wave action.

2.3 Silicate facies: Iron silicate minerals are generally associated with magnetite, siderite and chert which form layers alternating with each other. This mineralogy suggests that the silicate facies formed in an environment common to parts of the oxide and facies.

However, of all the facies of BIF, the for silicates is least understood. This is principally because of the number and complexity of the minerals and the fact that primary iron silicates are difficult to distinguish from low ranking metamorphic iron silicates. Probable primary iron silicates include greenalite, chamosite and glauconite, some and probably stilpnomelane. Most of the iron in these minerals is in the ferrous rather than the ferric state, which, like the presence of siderite, suggests a reducing environment. PCO2 may be important, a high value leading to siderite deposition, a lower one leading to iron silicate formation (Gross 1970). Carbonate and silicate facies BIF typically contain 25 – 30% Fe, which is too low to be of economic interest. They also present beneficiation problems.

2.4 Sulfide facies: This consists of pyritic carbonaceous argillites and occur as thinly banded rocks with organic matter plus carbon making up to 7 – 8 %. The main sulfide is pyrite which can be so fine-grained that its presence may be overlooked in hand specimens unless the rock is polished. The normal pyrite content is around 37 %, and the banding results from the concentration of pyrite into certain layers. This facies clearly formed under anaerobic conditions. Its high sulfur content precludes its exploitation as an iron ore; however, its has been mined until recently for its sulfur content at Chvaletice in Czechoslovakia.

2.4 Types of Banded iron formations

Two distinct types, viz., The Algoma and the Superior type BIF, came to be recognised

(Gross 1970) and the identity of the third (the Rapitan type) group as a distinct variety was also established (Trendall 1973; Dorr 1973).

2.4.1 Algoma type: This type is encountered in Archaean high-grade terrains and widespread development of it is in Archaean greenstone belts. It also occurs in younger rocks including the Phanerozoic. It shows a -volcanic association suggesting a geosynclinal environment and the oxide, carbonate and sulfide facies are present, with iron silicates often appearing in the carbonate facies.

Algoma-type BIF generally ranges from a few centimetres to a hundred or so meters in thickness and is rarely more than a few kilometres in strike length. Exceptions to this observation occur in where Late Archaean deposits of economic importance extending to several kms are found. In the Algoma type BIF, oolitic and granular textures are absent or inconspicuous and the typical texture is a streaky lamination. A close relationship in time and space to volcanic rocks hints at a volcanic source of the iron and many investigators regard the deposits of this type as being exhalative in origin, e.g., Fralick et al. (1989). Goodwin (1973) in a study of the Algoma type BIF in the Canadian showed that facies analysis was a powerful tool in elucidating the palaeogeography and could be used to outline a large number of Archaean basins. His section across the Michipicolin

Basin is shown if Fig. 1.

The Algoma type BIF ideally belongs to synsedimentary ores, with discrete hydrothermal sources recognizable through their association with volcanics of Island arc affinity and occasional massive as well as their REE and other geochemical signatures. In the Michipicoten Basin (Fig.1) the mineralogical facies of the BIF closely parallels the lithofacies variation in the associated clastics, pyroclastics and carbonate rock types and this feature clearly indicates a depth-controlled variation of the ore facies. The common occurrence as lenses of few kilometres strike and distribution along several stratigraphic levels strongly suggest that their formation is often a local and recurrent

phenomenon very much like some volcanogenic massive sulfide deposits and is related to periodic pluses of hydrothermal activity.

Additional interest in the Algoma type of BIF stems from their hosting of ores, particularly in the sulfide facies, in association with arsenopyrite (e.g., Kerr-Addison mine, near Larder Lake, ). However, the primary origin of gold in BIF is questioned by many workers who assign epithermal or metamorphogenic derivation of gold from mafic and ultramafic rocks of the greenstone sequence.

Large deposits of Algoma type BIF of middle Archaean age occur in the Guyanan and

Liberian shields (West Africa). Other important occurrences are: Intaca Complex, Venezuela

(~ 3400 Ma), Belzyorsky-Kausky, Ukraine (~3250 Ma), Bihar-Orissa Iron ore Group (~3025

Ma) in India; Michipicoten (~2725 Ma) in Canada; deposits in Yilgarn Block

(~2700 Ma) in Australia and in Canada.

2.4.2 Superior type: These are thinly banded rocks mostly belonging to the oxide, carbonate and silicate facies BIF. They are usually free of clastic material. The rhythmic banding of iron-rich and iron-poor chert layers, which normally range in thickness from a centimetre or so up to a meter, is a prominent feature and this distinctive feature allows correlation of BIF over considerable distance. Individual parts of the main Dales Gorge Member of the

Hamersley Brockman BIF of western Australia can be correlated at the 2.5 cm scale over about 50,000 km2 (Trendall and Blockley 1970), and correlations of within chert bands can be made on a microscopic scale over 300 km (Trendall 1968).

Superior type BIF stratigraphically are closely associated with quartzite and black carbonaceous and usually also with conglomerate, stromatolitic , dolomite, massive chert, chert breccia and argillite. Volcanic rocks are not always directly associated with this BIF, but there are nearly always present in the stratigraphical column; Superior type

BIF may extend for hundreds of kilometres along strike and thicken from a few tens of

meters to several hundred meters. The successions in which these BIF occur usually lie unconformably on high metamorphosed basement rocks with the BIF, as a rule, in the lower part of the succession. In some places they are separated from the basement rocks by only a meter or so of quartzite, grit and shale and in some parts of the Gunflint Range, , they rest directly on the basement rocks.

The development of Superior type BIF reached its acme during the early Proterozoic.

Superior type BIF frequently extended right around early Proterozoic sedimentary basins and

Gross (1964) suggested that Superior type BIF was once present around the entire shoreline of the Ungava craton for a distance of more than 3200 km.

The associated rock sequences and indicate that Superior type

BIF formed in fairly shallow water on continental shelves marginal basins (Goodwin 1982), in evaporitic barred basins (Button 1976), on flat prograding coast lines (Dimroth 1977) or in intracratonic basins (Eriksson and Truswell 1978). Trendall (1973) suggested that the rhythmic microbands of the Hamersley Group so closely resemble evaporitic varves that a common origin is probable. He suggested that the banding originated by the annual accumulation of iron-rich precipitates whose deposition was triggered by evaporation from a partially enclosed basin with an average water depth of about 200m.

Superior type deposits of BIF of large dimensions are: Biwabik and the in Lake Superior Province (USA), Labrador-Cape Smith – Temiscamie marginal basins around the Ungava Craton in the Labrador Trough region, the Krivoy Rog-Kursk Magnetic

Anomaly region (Ukraine), the Transvaal Griquatown region (S.Africa), Minas Gerias

(Brazil) the Hamersley Basin (Australia), etc.

2.4.3 Rapian type BIF: It occurs as lenses of inter-bedded massive hematite and red and is more restricted in extent and thickness than the Clinton and Superior types of BIF. It is usually hosted in diamictite (unsorted, terrigenous sand/conglomerate in clayey matrix) and is

associated with glaciogenic siliciclastics. Rapian type BIF occurs exclusively as oxide facies.

It contains significant amounts of Mn which often grades to Fe-Mn and Mn-Fe ores. Its REE patterns are very similar to modern sea water pattern. It exhibits negative to slightly positive

Eu anomaly. Ores are depleted in LREE which suggests very little influence of hydrothermal inputs. Rapian type BIF are of late Proterozoic age (755 – 730 Ma.) and its formation coincides with the interglacial period of a major transgressive event. Inferred tectonic setting of deposition was in narrow continental rift basins during early stage of cratonic rift under fluvioglacial conditions. The Rapian type BIF is associated essentially with continental sequence with marine incursions and without any volcanics (e.g., Jacadigo Group, Brazil and

Bolivia). Rapian type BIF deposits occur as a basal unit of the Windermere Supergroup sequence which lies in a sinuous rift belt from Alaska to Mexico. Rapian type BIF are also encountered in a few localities in NW Canada.

2.5 Genetic aspects

There is general agreement that BIF are chemically or biochemically precipitated. Blue – green algae and fungi have been identified in the Gunflint Iron Formation of Ontario and some of these resemble present day iron precipitating bacteria which can grow and precipitate ferric under reducing conditions. However, there is no agreement on the source of the iron. One school considers that this was derived by erosion from nearby landmasses; another school believes that it is of volcanic-exhalative origin (e.g., Gross 1986). One major drawback to the terrestrial derivation is that if large amounts of iron silica were transported from the continents, large quantities of aluminous material must either have been left behind

(to form ) or transported and dispersed in the sea with, or not far from, the iron deposits. No such residual or aluminous sediments have been discovered. On the other hand, Miller and O’Nios (1985) gave en estimate for submarine hydrothermal supply of iron to the present oceans of ≤ kg per annum. Yet it is claimed that the BIF of

Hamersley Basin (Western Australia) alone required ≥ kg per annum. They concluded that a major contribution of iron from the continents did occur unless the hydrothermal iron output during the Proterozoic was overwhelmingly greater than the present day one. On the other hand, from their study of the isotopic composition of neodymium in the Hamersley and

Michipicoten (Ontario) BIF, Jacobson and Pimentel-Klose (1988) concluded that much of the iron in BIF was leached by hydrothermal water circulating through Archaean mid-ocean ridge systems.

The mechanism of transport of the iron is also hotly debated. Today, with an - rich atmosphere, very little iron is transported to the oceans in ionic solution and most travels in colloidal solution or as particulate matter and is deposited mainly in muds. There is no true

Phanerozoic analogues of the BIF of early Proterozoic. For those who advocate a CO 2-rich, oxygen-poor early (Archaean) atmosphere the explanation is simple, i.e., iron could then travel as the bicarbonate in ionic solution and, since does not form a bicarbonate, the two would be separated. With the significant development of oxygen in the atmosphere, large scale formation of BIF would cease. Garrels (1988) assumed transport of iron in the

Fe2+ state and developed a quantitative model for the deposition of iron in large basins to which it was carried in stream waters and precipitated by evaporation to produce iron mineral-chert varves like those described from BIF in many parts of the Precambrian, but especially well developed in the Hamersley Group of Western Australia (Trendall 1973).

Conversely, Kimberley (1989), after careful and lengthy appraisal of the genesis of

BIF, concluded that all iron formations are the result of exhalative activity. According to

Kimberley cherty iron formation (the principal facies of Superior type BIF) resulted from low temperature (< 300°C) hydration of newly formed igneous crust by circulating sea water in convection cells. Thus he thinks the marked production of cherty iron formations in the early

Proterozoic and much smaller but significant development in the late Proterozoic to

particularly rapid crustal accumulation in opening rifts, followed by abrupt failure of the rift and hydration of the new crust. Kimberley provided two evidences in support of his views:

(1) Evidence of Phanerozoic deposition of small cherty iron formations with rifts, a possible modern analogue in Red Sea and (2) Evidence of recent exhalative iron deposition in

Venezuela in favour of this theory. Kimberley attributes Non-cherty iron formation to seismic pumping of sea water along strike – slip or transform faults that pass through ophiolite – bearing sedimentary successions along continental margins, where the iron – dissolving waters have become hypersaline because they were pumped through evaporates or cooling plutons. These fluids are believed to have been modified by ascent through argillaceous sediment leading to cooling and loss of silica.

Morris (1993) has proposed a convincing genetic model for the evolution of

Hamersley Group of BIF. The Hamersley Province is distinctive among BIF provinces in being the largest single depository system in the world and yet not having any lateral facies changes within it unlike most other BIF provinces of the Superior type. The Hamersley

Province is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60,000 km2. This geological setting provides opportunity for reasonable estimates for the annual input of components to the depository (basin). Varying supply of materials for the medley of mesobands types, particularly iron and silica in the oxide facies BIF, can be accommodated by the interaction of two major oceanic supply systems: (1) surface currents, and (2) Convective upwelling from mid-oceanic ridge (MOR) or hot-spot activity, both modified by varied output of pyroclastic material. The surface currents were periodically recharged by storm mixing. Precipitation from them gave rise to the banded chert-rich horizons, including the varves whose regular and finely laminated iron/silica distribution resulted from seasonal meteorological influences. Precipitation from convection-driven upwelling of high iron solutions from MOR (Mid Oceanic Ridge) or hot-spot activity

periodically overwhelmed the delicate seasonal patterns of silica-saturated surface currents to produce the iron-dominated mesobands. Intermediate mesoband types resulted where the deepwater supply was modified by varied MOR activity, or by partial blocking of upwelling waters by surface currents (such as by present El Nino). During these periods of oxide- dominated BIF, silica was deposited from saturated solution mainly by evaporative concentration, and iron by oxidation due to photolysis and photo-synthetically produced oxygen.

Superposed on these supply difference was the varying ash from distal volcanic sources, changing the meteorological and depositional conditions. Occasional input of extremely fine ash during BIF precipitation produced mesoband (cm) scale variations involving increased carbonate-silicate precipitation. Sustained volcanic periods resulted in S- macroband deposition (i.e., chert-carbonate-silicate-bearing BIF, with shale), gradually returning to the dominant hematite-magnetite-chert-bearing BIF as the volcanic input waned.

During volcanic periods, the normally high capacity of sunlight to precipitate ferric iron directly by phtolitic oxidization of ferrous iron, and by photosynthetic production of oxygen, was modified by turbidity in the atmosphere (aerosols and dust) and in the water (colloids from reactive ash). Surface-precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone, augmenting the iron content of the zone.

Precursor ferrous carbonates and silicates were precipitated when the iron concentration of this subphotic zone exceeded their respective solubilities. During volcanism, the increased availability of nutrients, particularly phosphorous, to surface water increased the organic contribution despite lower solar light values, leading to an almost total absence of ferric iron in the S-macrobands (i.e., no magnetite or hematite). Cooling of warm, silica-saturated sea-water during these periods of “volcanic winter” increased the ratio of precipitation of silica to iron, which, however, was still controlled by seasonal conditions. Intermediate

concentrations of organic matter, insufficient to totally convert the ferric compounds either during precipitation or resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite, during low-grade regional metamorphism.

The conditions involved in the above genetic model are: (a) a very low oxygen to anoxic atmosphere, (b) a much higher level of MOR activity than at present, (c) the presence of photosynthetic plankton, (d) the absence of silica-secreting organisms, and (e) a deep sea- water temperature higher than 20°C.

Several decades of investigation on the genetic aspects of BIF did not provide universally acceptable views on (a) source of iron and silica, (b) mechanism and conduit of their selective transportation, to the exclusion of other components, (c) site and environment of deposition and (d) mechanism of development of banding.

In spite of the prevailing diversity of views the current models seem to be converging on some common grounds; they are: An anoxic to very low O2 contemporary atmosphere, presence of photosynthetic planktons and absence of silica-secreting organisms are generally agreed upon. Depositional basins in intracratonic, continent margin and isolated platformal settings considered for Superior type deposits in different areas seem to be acceptable.

Preponderant role of the ocean as the immediate reservoir-irrespective of whether iron was derived from terrigenous sources as fine clastics and/or in solution or from submarine hydrothermal discharge along MOR/hot spots-for all deposits, except those in strictly intracratonic basins, seem acceptable to most.

2.6 Some examples

2.6.1 The Lake Superior region, USA: Lake Superior region, located west and south of Lake

Superior was one of the greatest iron ore districts of the world. In the western part of this region BIF is exploited in 4 districts-vermilion, Gunflint, Gogebic and Cuyana mining

districts (Fig.2). The western part of the region can be divided into three major unites; a basement complex (> 2600 Ma old), a thick sequence of weakly to strongly metamorphosed sedimentary and volcanic rocks (the Marquette Range Supergroup) and younger Precambrian

(Keweenawan) volcanics and sediments.

Banded iron formation is mainly developed in the Marquette Range Supergroup, but in the Vermilion mining district it is present in the basement (Fig. 2). In this mining district there is a great thickness of mafic to intermediate volcanic rocks and sediments. Banded iron formation, mainly of oxide facies, occurs at many horizons, generally as thin units rarely more than 10m thick, but one iron formation (the Soudan) is much thicker and has been mined extensively.

The remaining iron ore of this region comes from the Menominee Group of the

Marquette Range Supergroup. All the iron formation of this group in the different mining districts are approximately of the same age. The Marquette Range Supergroup shows a complete transition from a stable craton to deep water conditions. Clastic rocks were first laid down on the bevelled basement but most of those were removed by later erosion and in many places the Menominee Group rests directly on the basement. Iron formation is the principal rock type of this group. Despite the approximate stratigraphical equivalence of the major iron formations, they differ greatly from one mining district to another in thickness, stratigraphical details and facies type. They appear to have been deposited either in separate basins or in isolated parts of the same basin or shelf area. The only evidence of contempora neous volcanism is the occurrence of small lava flows in the Gunflint and Gogebic mining districts.

It has been suggested apropos this region that volcanism appears to have been detrimental rather than conductive to iron concentration.

The BIF of the Mesabi and Gunflint is discontinuous but constitutes a single body

(Fig. 2 ). It is 100-270 m thick and consists of alternate units of dark, non-granular, laminated

rock and cherty, granule-bearing irregularly to thickly bedded rock. The granules are mineralogically complex containing widely different proportions of iron silicates, chert and magnetite; some are rimmed with hematite. The iron formation of the Cuyuna mining district consists principally of two facies, thin bedded and thick bedded, which differ in mineralogy and texture. The first is evidently layered and laminated, the layers carrying varying proportions of chert, siderite, magnetite, stilpnomelane, minnesotaite and chlorite, while the second contains evenly bedded and wavy bedded rock in which chert and iron minerals alternately dominate in layers 2-30m thick. Granules and are present. In the Gogebic district the iron formation is 150-310m thick and consists of an alternation of wavy to irregularly bedded rocks characterized by granule and oolitic textures. The iron in the irregularly bedded rocks is principally in the form of magnetite and iron silicates, and granule textures are common. The evenly bedded iron formation is mineralogically complex, consisting of chert, siderite, iron silicates and magnetite. Each mineral may dominate a given layer and may be accompanied by one or more of the other minerals.

2.6.2 Adams-Sherman Algoma Type BIF Deposits Ontario, Canada: Adams mine is located south of Kirkland Lake region and the Sherman deposit, in the Temagami district; both are part of the Archaean Abitibi belt. The Adams ore body is composed of thinly laminated magnetite-quartz beds of the Boston iron formation a layer that varies in thickness up to several hundred meters and that extends for some 10km along strike (Fig.3 ). The BIF, averaging 27% Fe is magnetically upgraded to provide pellets of 62 to 63% Fe content. The ores at places contain troublesome amounts of pyrite and minor hematite and jasper. The BIF have been mildly contorted, although soft-sediment slump structures are most common.

Minor amounts of “exhalite indicating elements” (Ni, Cr, Mn, Ba and traces of Au) are reported.

The lowest cycle of iron formation at the Adams mine is underlain by massive flows of tholeiite containing pillows; small lenses of lean iron formation are interbedded in these flows. Well-bedded felsic tuffs lie above a sharp contact with the mafic volcanics.

These tuffs may be absent or range up to tens of meters thick; they commonly contain interbedded iron formation. A layer of nodular and fine-grained disseminated sulphides, from several centimeters to 10 meters thick rests on top of the tuffs. Above the sulfide layer, lean, cherty, low-iron sediments are found. Finely laminated chert and magnetite are interbedded, and subsequent metamorphism has destroyed little of the fine layering. Going up in the sequence, the amount of magnetite with respect to chert increases, and magnetite layers thicken. Hematite occurs in magnetite bands, giving them a reddish colouration, and garnet, tremolite, actinolite (a bluish amphibole), pyrite and chlorite are also present. Above the ore unit, lean iron formation with iron content less than 5% reappears. On the BIF, a horizon of graphitic, sulfidic that may be up to 3 meters thick is present; it is overlain by 200 to 300 m thick chert – derived quartzite. Sedimentary deposition ended by fissure volcanism that produced mafic and ultramafic and ultramafic flows unrelated to previous felsic activity. The iron formation units are overlain by komatiites with spinifex texture. The Adams ores are intimately related to proximal submarine volcanism.

Sherman mines ores at Temagami are also magnetite-quartz rich, mainly well banded grey to white magnetite-chert rocks, but they contain more jasper and less pyrite and show only mild greenschist grade metamorphic affects. The iron formation averages about 60 meters in thickness for 2 km of strike length. Maximum thickness is 100 m. Interbedded tuffs a few centimenters thick are composed of chlorite and stilpnomelane. The ore averages 25 to

30 % Fe and permits pellets of 65 % Fe. BIF lies stratigraphically between layered flows and volcanoclastic rocks; the basal units include an andesite unit that has been fractured to the verge of brecciation and silicified by hot spring fluids presumed to have been entering the

basin at the time of deposition. The finely laminar BIF units show soft sediment deformation and are exemplary of Algoma-type.

3. PHANEROZOIC IRONSTONES

These are usually classified into two types, Clinton and Minette, but both are now of very diminished economic importance as they are of low grade and impossible to beneficiate economically on account of their silicate mineralogy. They are moving, as it were, from the category of reserve to that of resource eventhough there are still many megatons in the ground.

3.1 Clinton type: The Clinton type oolitic iron ore is characterized by (a) the absence of , (b) presence of hematite and chamosite oolites, occasional diagenetic siderite and rare

Fe-sulfide, (c) high P2O5, Al2O3 and MgO (compared to BIF) reflected in the presence of high-Al silicates like chamosite and bertheirine, (d) absence of any facies variation; and (e) herringbone cross-stratification implying intertidal deposition.

Clinton type ironstones form lenticular beds usually 2-3 thick and never greater than

13m. The commonest lithostratigraphic package for oolitic ironstone is a shallow water clastic sequence of basinal shale  prodelta siltstone and sandstone  shallow subtidal siltstone  ironstone and this package is indicative of ore formation at the top of a regressive cycle (Asoke Mookherjee,1999). According to Evans (1993) ironstones appears to have formed in shallow water along the margins of continents, on continental shelves or in shallow parts of miogeoclines. Ironstones are common in sedimentary rocks of to Devonian age (e.g., Ordovician Wabana, Newfoundland; Silurian Clinton Formation, New York –

Alabama belt).

Oolitic ore at Wabana occurs as beds that are a few cm to 10m thick and lie within the upper 15m of a sequence of Ordovician sandstones and . Shallow-water to subaerial depositional conditions are evidenced by raindrop imprints, ooids, cross bedding, mud cracks

and the presence of shallow-water marine such as brachiopods and trilobites. The ooids consist of concentric shells of hematite and chamosite, in places embedded in a siderite matrix. The ore averages 51.5% Fe, 11.8% SiO2 and 0.9% and .

Oolitic iron ore in the Clinton Formation crops out intermittently from upper New

York State southward into Alabama. The Clinton Formation is composed of thin – bedded iron-stained sandstone, shale and oolitic hematite. Three types of ore have been mined in

Alabama State: (1) oolitic in which the hematite forms oolites generally in a hematite, calcite or siliceous matrix, (2) flax seed, in which the hematite forms small flat grains, or flattened oolites, and (3) ore, in which hematite has replaced numerous fossils (molluscs, bryozoans). The Clinton Formation exhibits cross-bedding, mud cracks, animal tracks, oolitic structures and shallow water features. The iron ores are of shallow-marine origin.

3.2 Minette type: These are the most common and widespread ironstones. The principal minerals are siderite (FeCO3) and Chamosite [Fe4Si4O10 (OH)8]. The chamosite often occurs as oolites. The ore contains around 30% Fe, while lime runs 5-20% and silica is usually above 20%.

Minette ironstones are particularly widespread and important in the Mesozoic of

Europe, examples being the ironstones of the English Midlands and the Minette ores of

Alsace-Lorraine ( and Luxemburg).

The oolitic ironstones of Alsace-Lorraine, which have fed the steel mills of for decades are in Middle Jurassic shales, sandstones and marls. The ooids consist dominantly of , though siderite, chlorite, chamosite, and hematite are also present. The iron minerals precipitated have chemically and diagenetically replaced original calcite oolites, as they did in the Clinton ironstones.

4. BOG IRON ORES

Bog iron ores are formed in peat bogs, marshes, swamps and lakes in regions of Pleistocene glaciation and in volcanic lakes. These ores constituted a minor source of iron in the early- industrial era and later during emergency (e.g., in Finland some 2 million tons of ore containing 30% Fe and 12% Mn were dredged up to 1947). Lake ores are known from

Ontario and Quebec in Canada, in the eastern USA and Scandinavian countries where the ores formed as oolitic in agitated waters along the lake margins. Volcanic lake ores, well developed in central Honshu, Japan and in the Kurile Islands, are made up of purely

“limonitic” material with up to 50% Fe and owe their origin to thermal springs. Bog iron ores develop best in regions with cool humid climate, ill-developed drainage systems and high water table.

SUMMARY

Five super large Precambrian iron ore districts, viz., Hamersley Basin (Australia), the

Transvaal-Griquatown region (S. Africa), Minas Gerias (Brazil), the Labrador Trough region

(Canada) and Krivoy-Rog-Kursk Magnetic Anomaly region (Ukraine) together make up ~

90% of the world’s sedimentary iron ores.

Iron ores can be grouped into (a) Banded Iron Formation (BIF) (b) Non-cherty Phanerozoic ironstones and (c) Bog iron ores.

Banded Iron Formation (BIF) is classified into three types: Algoma type, Lake Superior type and Rapitan type

Ironstones are classified into two types: Clinton type and Minette type. Both are of

Phanerozoic age. Bog iron ores of little economic significance were formed on land from

Carboniferous to Recent times.

The BIF are dominantly of Precambrian age. Great bulk of the BIFs of the world was laid down in a very short time interval of 2500 – 1900 Ma ago. Among BIF groups, the submarine volcanic-associated Algoma type BIF are dominantly Archaean while the carbonate-

orthoquartzite associated shelf/marginal basin deposits of the Lake Superior type BIF are exclusively of lower Proterozoic age. Rapitan type continental margin BIF deposits in glaciogenic sequences are essentially are of upper Proterozoic age.

The simplest and commonest BIF consists of alternate layer of chert and layer of iron mineral(s). This type of banding/lamination is clearly seen in oxide facies BIF.

Four facies of BIF are known. The oxide-facies is the most important one and provides the bulk of iron ore mined. It can be dvided into hematite and magnetite subfacies according to which iron oxide is dominant. Carbonate-facies commonly consists of interbanded chert and siderite in about equal proportions. In silicate-facies, iron silicate minerals are generally associated with magnetite, siderite and chert which forms layers alternating with each other.

Sulfide-facies BIF is thinly banded rock with pyritic carbonaceous argillites in which pyrite content is around 37% and carbon content makes up to 7-8%.

Superior type BIFs were formed in fairly shallow water on continental shelves, in evaporitic barred basins, on flat prograding coastlines, or intracratonic basins, Dolomite-sandstone-shale are typically associated with Superior type BIFs and this association indicates deposition in stable environment.

Algoma type BIFs are associated with volcanics intercalated with derived from older bimodal tholeiitic and calc-alkaline series similar to those in modern island arcs.

Algoma type BIFs are encountered in early Archaean high-grade terrians and Late Archaean greenstone belts.

Rapital type BIFs were deposited in narrow continental rift basins during early stage of cratonic rift under fluvioglacial conditions and are associated with continental sedimentary sequences with marine incursions, without any volcanics.

Phanerozoic ironstones are usually classified into two types, Clinton and Minette, but both are now of very diminished economic importance owing to their silicate mineralogy.

Clinton type ironstones form 2-3m thick massive beds of oolitic hematite-chamosite-siderite rock and is free from cherts. Minette type ironstones consist of siderite and oolitic chamosite.

Minette ironstones are widespread in the Mesozoic of Europe (e.g., English Midlands and

Luxembourg) Bog iron ores are found in peat bogs, marshes, swamps and lakes in regions of

Pleistocene glaciation and also in volcanic lakes. Lake ores are known from Ontario and

Quebec in Canada and in Scandinavian countries. Volcanic lake ores are found in central

Honshu, Japan. The ores contain essentially limonite.