Iron Rich Sediments

8/4/2019 Iron Rich Sediments

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IRON RICH SEDIMENTS

Iron makes up 6% of the earths crust and is available everywhere in the lithosphere either as

a major constituent or in trace amount and is the 4th

most abundant element in the crust

exceeded only by, oxygen, silicon and aluminum. Most sedimentary rocks contain at least

some iron. Like the average iron contain of siliciclastic mud rocks is 4.8% that of sandstone

2.4% and limestone is 0.4%.iron rich sedimentary rocks, when more enriched in iron than

these average sedimentary rocks. The rock contains more than about 15% iron are generally

referred to as iron rich.

Mineralogy of iron-rich sediments

Iron bearing sediments are iron-rich because of the presence or dominance of one or more

iron-bearing minerals in abnormal amounts. On the basis abundance of major kinds of iron-

bearing minerals, James (1966) defines four groups which are oxides, carbonates, silicates

and sulfides. All this minerals falls in this groups are result of primary or diagenetic, although

many are also produced by the metamorphism and weathering.

1) Iron oxides: Goethite, hematite, and magnetite are found as important constituents while

lepidocrocite and maghemite are reported rarely. Hematite is present in both iron

formation and ironstones and it is a dominant mineral in many early Paleozoic ironstone

and important mineral of precambrian.Goethite is the principal constituent of many post-

precambrian, goethite commonly occurs as ooliths occurs in ironstones but absent in

Precambrian iron deposits. Magnetite is abundant in Precambrian iron formation in

which it may be either digenetic or metamorphic in origin.

2) Iron Silicates: The important iron silicates of primary origin are chamosite .glauconite

and greenalite.and other minerals sach as thuringite,minnesotaite and

stilphnomelane.chamosite is the most abundant iron silicate of ironstones other than

those of Precambrian age .it occurs as ooliths of green material in matrix of siderite and

calcite and commonly found associated with ooliths of goethite. Greenalite is a hydrated

ferrous silicate, is a common and abundant primary iron silicate, greenalite is important

only in Precambrian rocks.Glauconite, it appear bright green color granules but also


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known as intergranular fillings and disseminations ,Glauconitic( green sands) is a

constituent of modern sediments. It ranges from near-shore sandy deposits to the

deeper sea.

3) Iron carbonates: This includes siderite and ankerite.The important iron- bearing

carbonates is siderite, which found in abundance in iron- bearing sediments of all ages.

Many iron-formations are dominantly interbedded siderite and chert.Siderite is also

known from clay ironstones, which are concretionary bodies found in some shales,

especially those associated with Paleozoic coal.Ankerite most common in iron

formation.

4) Iron sulfides: pyrite and marcasite are the only sulfides of importance in iron-rich

sedimentary rocks. Pyrite the most common sulfide occurs as scattered isolated crystals

of diagenetic as scattered isolated crystals of diagenetic origin, few cases it forms

layers consisting of pellets, spherules and replacement of shell fragments.Marcasite,is

rare or absent in ironstones. Its most common occurrence is as nodules in the coal

measures.

Classification of iron-rich sediments

I. Detrital chemical iron-rich sediments

A. Cherty iron formation

Composition: iron-irch chert conating hematite,magnetite,siderite,ankeriteor silicates as predominating iron minerals; relatively poor in Al and P.

B. Mineette-type ironstone

Composition: aluminous iron silicates

(chamosite,chlorite,stilpnomelane),iron oxides and carbonates :relatively

rich in Al and P

II. Iron-rich shales

C. Pyritic shales

Bituminous shales containing nodules or laminae of pyrite ; grade into

massive pyrite bodies by coalescence of pyrite laminae and nodules

D. Siderite rich shales with siderite concretions ;grade into massive siderite

bodies by coalescence of concretions

III. Miscellaneous iron-rich deposits


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E. Iron- rich laterites

F. Bog iron ores

G. Manganese nodules and oceanic iron crusts

H. Iron-rich muds precipitated from hydrothermal brines,lahn-dill and

stratiform,volcanogenic sulfide deposits

I. Placers of magnetite,hematite,or ilmenite sand

(Modified slightly from Dimroth.E.1979.Models of physical sedimentation of iron formation. In

.R.G. Walker

The iron-rich sediment are grouped into 3 class as above in table, of these only ironstone

and iron formation ,which are detrital chemical iron-rich sediments are only volumetrically

important. The iron rich sediments explained as under

Iron formations

Iron formations are iron-rich deposits that range in age from early Precambrian to Cambrian

age. They consist of distinctively banded sequence which ranges 50-600 m thick composed of

layers enriched in iron alternating with layers rich in chert.cherty iron formations can grade into

slightly cherty iron-rich sandstone,siltsone and shales.The textures of iron formations resemble

those in limestones.Dimroth recognizes textural types in iron formations equivalent to

micritic,pelleted,intraclastic,oolitic,pisolitic and stromatolitic limestone textures,sedimentay

structures such as ,cross-bedding, graded bedding, load casts, ripple marks, erosion channel,

shrinkage cracks and slump structures. These structures show that many of the constituents of

iron formations have undergone mechanical transport and deposition.

Ironstone

Ironstones are predominantly phanerozoic age sedimentary deposits. They occur mainly in

early Paleozoic and Jurassic rocks, but they range in age from Pliocene to middle

precambrian.They form thin, massive or poorly banded sequence a few meters to tens of

meters thick. Ironstones are commonly interbedded with carbonates, mudrocks and fine-


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grained sandstone of shelf to shallow-marine origin. Sedimentary structures Ironstones include

cross- bedding. Ripple marks. Scour and fill structures,clasts and burrows

Iron-rich shales

Pyritic black shales occur in association with Precambrian iron formation and phanerozoic

ironstone. They commonly occurs in thin beds in which sulfide content may range as high as

75%.pyrite occurs disseminated in these black carbonaceous shales and in some

limestone.Siderite-rich shales occurs primarily in association with other iron-rich deposits,

occurs disseminated in the mudrocks or as flattened nodules and more or less continuous

beds.

Bogs iron ores, iron-rich laterites, manganese crusts and nodules

Bog iron ores are minor accumulation of iron-rich sediments that occurs particularly in small

freshwater lakes of high altitude. These ores range from hard, oolitic, pisolithic and

concretionary forms to soft, earthy types. Iron-rich laterites are residual iron-rich deposits that

form as a product of intense chemical weathering; they are basically highly weathered soils in

which iron is enriched. Manganese crusts and nodules are widely distributed on the modern

seafloors, in areas where sedimentation rates are low and they also reported from ancient

sedimentary deposits in association with such oceanic sediments as red shales,cherts and

pelagic limestone both iron-rich and iron poor varieties of manganese nodules are found

Iron-rich muds (metalliferous sediments)

Metalliferous sediments occur in oceanic setting, particularly near active mid-ocean spreading

ridges. They are believed to form by precipitation from metal-rich hydrothermal fluids that have

become enriched through contact and interaction with hot basaltic rocks

Heavy mineral placers

Placers are sedimentary deposits that form by mechanical concentration of mineral particles

of specific gravity, commonly in beach or alluvial environments. Placers are local

accumulations that occurs mainly in Pleistocene to Holocene age sediments and commonly do

not exceed 1-2 m in thickness


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Origin of iron- rich sediments

The origin of iron-bearing sediments has been one of the debated topics, there is, as yet, no

acceptable model as organized. Although it is believed that iron rich sediments are chemically

precipitated rocks. The two prevailing views on the source which term from below and from

above according to which, iron was derived from a volcanic source, either as exhalations-

vapors and volcanic waters-or by volcanic in fall and submarine eruptions and iron derived

from the surface process were adequate to extract, transfer and concentrate iron derived from

adjacent land areas. Shown in figure (fig: 1) in form flow chart which reveals the origin of iron

rich sediments in simplified manner.

Fig:1

Diagenetic origin of iron-rich sediments

As it difficult to recognize and understand the post depositional changes which the iron-

bearing sediments undergoes which produce change in mineralogy and texture .however

ironstones and iron- formations may be consider secondary deposits, created by iron


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replacement of original carbonates minerals or other minerals during burial and diagensis.it

has long been recognized that some ironstones deposits are at least partly secondary in

origin because of the presence in these deposits of relict calcium carbonates constituents,

such as oolites and fossils, that have been clearly been replaced by iron.Dimorth(1979)

suggested that iron formations as well as ironstone were most likely precipitated initially as

caco3.the calcium carbonate deposits were subsequently replaced during digenesis by silica

and iron to form the cherty iron deposits. The source of the iron needed to bring about such

massive replacement and the mechanisms of selective iron and silica replacement that would

be required to produce the alternating bands of silica- rich and iron-rich sediments. This

postulated replacement mechanism would probably require that ferrous be introduced

uniformly into the carbonates deposits over a wide area and under reducing conditions, with

subsequent intermittent change to oxidizing conditions to allow precipitation of iron to occur.

The formation iron-rich deposits are still mystery, because certain questions are still not

cleared like manner in which the chert in cherty iron formations was deposited and why chert

and iron not deposited together after Precambrian. Why were deposition iron-rich sediments

particularly prevalent only during the Precambrian, early Paleozoic and Jurassic? What role if

any, did organism play in the deposition of iron formations and ironstones? Was the local

production of oxygen by photosynthesizing organisms such as algae important? Did low forms

of life such as bacteria and algae catalyze or initiate precipitation in some manner? If so, how

did they cause precipitation and how important was such biological activity? Answer are still

not cleared origin of iron rich sediments is yet solved

Evidences of diagenesis:-

Through replacement phenomenon siderite replacing fossil, hematite fossil ore and similar

observations prove that iron migrates and readily replaces calcium carbonates. The

importance of these replacements was early recognized by Cayeux and lead to the conclusion

that ironstones were essentially metasomatic replacements.Chamosite ooids were explained

as replacements of crinoidal and other shelly debris. According Cayeux, the original calcite

was altered to siderite, which in turn was replaced by chlorite. Subsequent oxidation led to

conversion to limonite or even to magnetite and hematite. James has noted that diagenetic

reactions are generally in the direction required by a lowerEh, such as siderite replacing


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glauconite.Interstitial waters in which such reactions take place are more reducing than those

above the sediments- water interface. Weathering changes go in reverse direction required by

higherEh. The evidences of replacement cited above are therefore to be considered

evidences of later reorganization-early digenetic in most case and metamorphic in few.

Distribution of iron- bearing sediments

Iron- bearing sediments are widespread in space and time. The iron deposits are distributed in

different regions of the world under varied geological conditions. The thickest and most

impressive deposits are of Precambrian. The well known Precambrian iron- formations occur in

the krivoy-rog area of Russia, in peninsular India, and in Brazil. Good example of phanerozoic

era is North America, ironstones of Cambrian Bliss sandstone of New Mexico. Deposits of

Proterozoic age are Blecher Islands, Richmond gulf in Canada and Hamersley basin of

Western Australia. Cretaceous greensands especially of New Jersey.

Major iron-rich deposits in India distributed in mainly in

Jharkhand,Orissa,Chhatisgarh,Karnataka and Goa small deposits in Maharashtra, Andhra

Pradesh,tamil Nadu,kerala and Rajasthan .Also some deposits are located in

Assam,Meghalaya,Nagaland, West Bengal, Himachal Pradesh, Uttar Pradesh and Jammu-

Kashmir.

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Iron Rich Sediments