HOME

TheInfoList



OR:

Channel iron deposits (CID) are
iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
-rich
fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...
sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
deposits of possible
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
age occupying meandering
palaeochannel A palaeochannel, also spelt paleochannel and also known as palaeovalley or palaeoriver, is a geological term describing a remnant of an inactive river or stream channel that has been filled or buried by younger sediment. The sediments that th ...
s in the Early to Mid-
Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...
Hamerlsey palaeosurface of
Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...
. Examples are also known from
Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a transcontinental country located mainly in Central Asia and partly in Eastern Europe. It borders Russia to the north and west, China to the east, Kyrgyzstan to the southeast, Uzbeki ...
. The deposits are anomalously high in iron for deterital material, and exclude detrital iron deposits typified by
scree Scree is a collection of broken rock fragments at the base of a cliff or other steep rocky mass that has accumulated through periodic rockfall. Landforms associated with these materials are often called talus deposits. Talus deposits typically ...
of hematitic
banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...
s and accumulations of currently-forming
maghemite Maghemite (Fe2O3, γ-Fe2O3) is a member of the family of iron oxides. It has the same spinel ferrite structure as magnetite and is also ferrimagnetic. It is sometimes spelled as "maghaemite". ''Maghemite'' can be considered as an Fe(II)-deficie ...
pisolite alluvials. CIDs are a major source of cheap, high grade
iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the ...
exploited primarily in the
Pilbara The Pilbara () is a large, dry, thinly populated region in the north of Western Australia. It is known for its Aboriginal peoples; its ancient landscapes; the red earth; and its vast mineral deposits, in particular iron ore. It is also a g ...
and Murchison regions of
Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to t ...
.


Morphology

Channel iron deposits are typically partly eroded and currently are from between <1m to 100m thick, with preserved channel widths of between 100m and >5 km. Mineralised channels are up to 150 kilometres in length, but not all of the preserved length of the CID is of ore grade. Channel iron systems typically form within a depression on the Cenozoic ‘Hamersley Surface’, and form several pods downstream on the palaeodrainage. The channels show typical fluvial sedimentary morphology, with channel scours truncating or incising the channel iron deposits, and rare examples of graded bedding, and so on. Individual ore deposits are subsets of a larger sub-economic mineralized system, which varies laterally and along the palaeodrainage. The deposits form lensoidal accumulations with interbeds of clays, gravels and siliceous detrital materials.


Age

No clear geochronological data exist for CIDs, as no radioisotope methods are applicable to directly date CID deposits. Palynological data do exist but cannot constrain ages sufficiently beyond centring on the Middle Miocene.


Formation mechanism

The source of iron for the CIDs is believed to be a
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
aged iron-rich soils which developed upon a palaeosurface (since eroded) which developed in the Early Miocene during hot, humid conditions. The erosion of this ferritic palaeosurface in the Mid Miocene transported of the iron-rich soils into the palaeodrainage system, where the iron became consolidated within the existing river courses. The river beds were at the time a rich
humic Humic substances (HS) are organic compounds that are important components of humus, the major organic fraction of soil, peat, and coal (and also a constituent of many upland streams, dystrophic lakes, and ocean water). For a long era in the 19th an ...
swamp with thick vegetation, and accumulation of
peat Peat (), also known as turf (), is an accumulation of partially Decomposition, decayed vegetation or organic matter. It is unique to natural areas called peatlands, bogs, mires, Moorland, moors, or muskegs. The peatland ecosystem covers and ...
s or thick detrital vegetation. Most CIDs are underlain by organic-rich clays and/or Miocene aged
lignite Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat ...
. The iron became fixed in place in the river channels and gradually replaced the existing humic material via replacement with goethite.


Petrology

Channel iron deposits are formed by accumulation of massive deposits of what is generally referred to as “ pisolite iron gravels”, which are
ooid Ooids are small (commonly ≤2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, mo ...
s and
pisoid A pisolite is a sedimentary rock made of pisoids, which are concretionary grains – typically of calcium carbonate which resemble ooids, but are more than 2 mm in diameter. These grains are approximately spherical and have concentric la ...
s of
goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...
. CIDs were initially considered to be analogous to accumulations of pisolite gravels within palaeochannels via sedimentary means. Modern evidence points to an in-situ formation of the classic pisolitic textures. Goethite ooids and pisoids show evidence of being formed by concretion of layers of goethite (cortex) around a core fragment (nucleus) which is typically ferruginised wood fragments, but may be quartz grains, hematite grains or other detrital material. It is considered that the mechanism for enrichment and formation of the goethite cortex is related to near-surface alteration of an existing highly ferruginous material by groundwater action. Ferruginised wood is ubiquitous and a major component of CIDs, existing as porous, friable
limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·H2O, although this is not entirely accurate as the ratio of oxide to hydroxide ...
. Fossilised wood fragments are present but are usually extremely rare and of very small size (<50 mm). The goethite pisolites are cemented via a variety of agents, usually a mixture of
goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...
,
clays Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
,
carbonate mineral Carbonate minerals are those minerals containing the carbonate ion, . Carbonate divisions Anhydrous carbonates *Calcite group: trigonal ** Calcite CaCO3 ** Gaspéite (Ni,Mg,Fe2+)CO3 ** Magnesite MgCO3 **Otavite CdCO3 ** Rhodochrosite MnCO3 ...
s (
magnesite Magnesite is a mineral with the chemical formula ( magnesium carbonate). Iron, manganese, cobalt, and nickel may occur as admixtures, but only in small amounts. Occurrence Magnesite occurs as veins in and an alteration product of ultramafic ...
,
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
and sometimes
siderite Siderite is a mineral composed of iron(II) carbonate (FeCO3). It takes its name from the Greek word σίδηρος ''sideros,'' "iron". It is a valuable iron mineral, since it is 48% iron and contains no sulfur or phosphorus. Zinc, magnesium and ...
), and occasionally
silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
. This process may form an in-situ
concretion A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Concretions are often ovoid or spherical in shape, although irregular ...
of pisolites which may be very resistant to erosion - some
mesa A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a ...
s in the Pilbara, and Yilgarn are in fact old cemented ferruginous pisolite river gravels.


Economic importance

Channel iron deposits are an important source of iron ore, with the deposits at Yandi and Robe River accounting for approximately 47% of iron ore mined from the Hamerley Iron province. Although channel iron deposits are typically low-grade at 53% to 57% Fe in-situ, they are composed of goethite-limonite which are hydrated iron oxide species. Ore typically contains around 8% to 12% water, and <5% SiO2, and <3% Al2O3. The hydrous iron oxides can be calcined, and the CID ore on a volatile-free basis is around 63% Fe or more. The CID deposits relative lack of consolidation and proximity close to the surface in most cases renders them liable to bulk mining with little or no need for drilling and blasting. This then is a significant cost saving to miners, who can offset a lower revenue from Fe percentages in the ore via the ease of extraction. Also, in most cases, beneficiation can increase the in-situ iron grade several percent by washing out the majority of clay, carbonate and hydrous limonite cements. The key economic criteria for channel iron deposits are, firstly tonnage and location relative to
infrastructure Infrastructure is the set of facilities and systems that serve a country, city, or other area, and encompasses the services and facilities necessary for its economy, households and firms to function. Infrastructure is composed of public and priv ...
similar to other bulk commodities. Thereafter, the nature of the cement is important, particularly in the cases of carbonate cements containing magnesite, as magnesium is a problem. Rare channel iron deposits are rendered uneconomic because of a silica cement proving too durable for easy mining and crushing. The water content of channel iron deposits (quoted as Loss on Ignition) is from 7% to 12%, which is the highest of all iron ore types, generally due to the presence of goethite-limonite. Phosphorus, aluminium and sulfur levels are another concern, typically being above normal levels in-situ although if the phosphorus and aluminium are hosted in a weak cement, they can often be washed out during
beneficiation In the mining industry or extractive metallurgy, beneficiation is any process that improves (benefits) the economic value of the ore by removing the gangue minerals, which results in a higher grade product (ore concentrate) and a waste stream ( ta ...
. Most channel irons are upgraded via washing of the pisolite gravels to remove the cements and matrix.


Type examples

The type deposits are those at Pannawonnica and Robe River, in the Pilbara of Western Australia, which are currently mined by Rio Tinto Iron Ore. Channel iron deposits are rarer outside of the West Australian landmass, due to the relative youth of the
regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestr ...
in the rest of the continental land masses, although there are smaller examples in
Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a transcontinental country located mainly in Central Asia and partly in Eastern Europe. It borders Russia to the north and west, China to the east, Kyrgyzstan to the southeast, Uzbeki ...
. The Kazakhstan deposits are
Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...
in age and occur as ooidal ironstone deposits of deltaic or fluvial origin in the north-eastern continental sediments of northern Turgai and Aral'sk Districts. They occur on valleys excavated from uplifted
Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
marine strata during subtropical conditions in the Late Oligocene. There is evidence of densely wooded valley fills and significant input of humic material, similar to the observed
palaeoclimatic Paleoclimatology (British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...
setting of the Pilbara examples. Grades reported for the Kazakh deposits are highly variable, from 29% Fe to 73% Fe, with higher phosphorus (0.5% to 2.5%), calcium and lower silica and aluminium.


See also

*
Iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the ...
*
Yilgarn Craton The Yilgarn Craton is a large craton that constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts. Zircon grains in the Jack Hills, Narryer Terrane have b ...
*
Pilbara Craton The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia. The Pilbara Craton is one of only two pristine Archaean 3.6–2.7 Ga (billion years ago) crusts identified on the ...
*
Port Hedland A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as Ha ...
*
Banded iron formation Banded iron formations (also known as banded ironstone formations or BIFs) are distinctive units of sedimentary rock consisting of alternating layers of iron oxides and iron-poor chert. They can be up to several hundred meters in thickness ...
*
Palaeochannel A palaeochannel, also spelt paleochannel and also known as palaeovalley or palaeoriver, is a geological term describing a remnant of an inactive river or stream channel that has been filled or buried by younger sediment. The sediments that th ...


References

*Ramanaidou E. R., Morris R. C. and Horwitz R. C. (2003). The Channel Iron Deposits (CID) of The Hamersley Province, Western Australia. Australian Journal of Earth Sciences, 50, 669-690. *Morris, R. C., Ramanaidou, E. R. (2007). Genesis of the Channel Iron deposits (CID) of the Pilbara of Western Australia. Australian Journal of Sciences, 54, 735-759. Economic geology Iron ore deposits