Channel-iron deposits
Encyclopedia
Channel iron deposits are iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

-rich fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...

 sedimentary deposits of possible Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 age occupying meandering palaeochannel
Palaeochannel
Palaeochannels or paleochannels are deposits of unconsolidated sediments or semi-consolidated sedimentary rocks deposited in ancient, currently inactive river and stream channel systems. The word palaeochannel is formed from the words "palaeo" or 'old', and channel; ie; a palaeochannel is an old...

s in the Early to Mid-Tertiary
Tertiary
The Tertiary is a deprecated term for a geologic period 65 million to 2.6 million years ago. The Tertiary covered the time span between the superseded Secondary period and the Quaternary...

 Hamerlsey palaeosurface of Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

. Examples are also known from Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...

.

The deposits are anomalously high in iron for deterital material, and exclude detrital iron deposits typified by scree
Scree
Scree, also called talus, is a term given to an accumulation of broken rock fragments at the base of crags, mountain cliffs, or valley shoulders. Landforms associated with these materials are sometimes called scree slopes or talus piles...

 of hematitic
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

 banded iron formation
Banded iron formation
Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...

s and accumulations of currently-forming maghemite
Maghemite
Maghemite is a member of the family of iron oxides. It has the same structure as magnetite, that is, it is spinel ferrite and is also ferrimagnetic.Maghemite can be considered as an Fe-deficient magnetite with formula...

 pisolite alluvials. CIDs are a major source of cheap, high grade iron ore exploited primarily in the Pilbara and Murchison regions of Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

.

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 Tertiary ‘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
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...

 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 swamp with thick vegetation, and accumulation of peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...

s or thick detrital vegetation. Most CIDs are underlain by organic-rich clays and/or Miocene aged lignite
Lignite
Lignite, often referred to as brown coal, or Rosebud coal by Northern Pacific Railroad,is a soft brown fuel with characteristics that put it somewhere between coal and peat...

. 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
Pisolite
A pisolite is a sedimentary rock made of pisoids, which are concretionary grains - often of calcium carbonate, but sometimes of rarer minerals - which resemble ooids but are always more than 2 mm in diameter. These grains are approximately spherical and have concentric layers reaching...

 iron gravels”, which are ooids and pisoids of goethite
Goethite
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...

. 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 pisoilitic 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
Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with hematite, it has...

. 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
Goethite , named after the German polymath Johann Wolfgang von Goethe, is an iron bearing oxide mineral found in soil and other low-temperature environments. Goethite has been well known since prehistoric times for its use as a pigment. Evidence has been found of its use in paint pigment samples...

, clays, carbonate minerals (magnesite
Magnesite
Magnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...

, calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 and sometimes siderite
Siderite
Siderite is a mineral composed of iron 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...

), and occasionally silica. This process may form an in-situ concretion
Concretion
A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...

 of pisolites which may be very resistant to erosion - some mesa
Mesa
A mesa or table mountain is an elevated area of land with a flat top and sides that are usually steep cliffs. It takes its name from its characteristic table-top shape....

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
Infrastructure is basic physical and organizational structures needed for the operation of a society or enterprise, or the services and facilities necessary for an economy to function...

 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
Beneficiation
In mining, beneficiation is a variety of processes whereby extracted ore from mining is separated into mineral and gangue, the former suitable for further processing or direct use....

. 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
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 in the rest of the continental land masses, although there are smaller examples in Kazakhstan
Kazakhstan
Kazakhstan , officially the Republic of Kazakhstan, is a transcontinental country in Central Asia and Eastern Europe. Ranked as the ninth largest country in the world, it is also the world's largest landlocked country; its territory of is greater than Western Europe...

.

The Kazakhstan deposits are Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

 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 Lower Tertiary 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 setting of the Pilbara examples.

Grades reported for the Kazahk 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
  • Yilgarn Craton
    Yilgarn craton
    The Yilgarn Craton is a large craton which constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts...

  • Pilbara Craton
    Pilbara craton
    The Pilbara craton , along with the Kaapvaal craton are the only remaining areas of pristine Archaean 3.6-2.7 Ga crust on Earth...

  • Port Hedland
  • Banded iron formation
    Banded iron formation
    Banded iron formations are distinctive units of sedimentary rock that are almost always of Precambrian age. A typical BIF consists of repeated, thin layers of iron oxides, either magnetite or hematite , alternating with bands of iron-poor shale and chert...

  • Palaeochannel
    Palaeochannel
    Palaeochannels or paleochannels are deposits of unconsolidated sediments or semi-consolidated sedimentary rocks deposited in ancient, currently inactive river and stream channel systems. The word palaeochannel is formed from the words "palaeo" or 'old', and channel; ie; a palaeochannel is an old...

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