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Iron Ore

 

Iron ore and its uses

Elemental Iron (Fe) is ranked fourth in abundance in the earth's crust and is the major constituent of the Earth's core. It rarely occurs in nature as the native metal.

The pure metal is silvery white, very ductile, strongly magnetic and melts at 1528° C.

Iron accounts for approximately 95% of all metals used by modern industrial society.

Metallic iron is most commonly produced from the smelting of iron ore to produce pig iron.

Steel is a processed form of pig iron with impurities such as silicon, phosphorus and sulphur removed and with a reduction in the carbon content. Globally, steel's versatility is unsurpassed. Wrought iron (low carbon) and cast iron (pig iron) also have important markets. One of the most ubiquitous products in Australia is corrugated iron, a structural sheet steel shaped into parallel furrows and ridges. It was invented by Henry Robinson Palmer in 1828 in London and quickly became popular for roofing and farm buildings.

Iron metal may be produced from the smelting of certain iron compounds. Their concentration in economic proportions is referred to as 'iron ore'.

Other well known uses of iron compounds are:

  • iron sulphate used as fungicide, the oxalate of iron in photographic development, limonite, goethite, hematite as pigments and abrasives, magnetite in the production of industrial electrodes and also for washing coal
  • iron chloride and nitrate used as mordents and industrial reagents in the production of several types of inks
  • iron carbonyl as a catalyser of many chemical reactions
  • micaceous hematite as a protective paint on steel superstructures.

 

Major iron compounds

Name Formula %Fe
Hematite Fe2O3 69.9
Magnetite Fe3O4 74.2
Goethite/Limonite HFeO2 ~ 63
Siderite FeCO3 48.2
Chamosite (Mg,Fe,Al)6(Si,Al)414(OH)8 29.61
Pyrite FeS 46.6
Ilmenite FeTiO3 36.81

Types of iron ore

The major rock types mined for the production of metallic iron are massive hematite, pisolitic goethite/limonite, which provide a 'high-grade' ore, and banded metasedimentary ironstone, magnetite-rich metasomatite, to a much lesser degree, rocks rich in siderite, rocks rich in chamosite which provide a 'low-grade' ore.

High-grade ore

Currently most of the iron ore mined in the world comes from large deposits of massive hematite rock formed by the in situ enrichment of a protore already enriched in iron, most commonly a banded iron formation (BIF).

Two of the best known Australian examples of massive hematite deposits are Tom Price and Mount Whaleback in the Hamersley Range, Western Australia. Another type of high-grade deposit is pisolitic limonite/goethite ore formed in ancient river channels, e.g. Yandicoogina, Hamersley Basin, Western Australia.

The consensus model for formation of massive hematite ore is enrichment by the passage of fluids, which remove the non-iron-bearing minerals (dominantly chert), to a much lesser extent add iron minerals. There are several variants of this model with the most accepted being enrichment by supergene processes. Recent models suggest enrichment by mass sideways and upward migration of dominantly superheated meteoric waters perhaps with a minor magmatic component.

High-grade ore generally has a cut off grade of ~>60% Fe. Historically it has provided a direct feed to smelters either as a raw lump or fines, also in a processed form such as sinter or pellets. There are emerging markets for new varieties of feedstock. Examples include sintered iron carbide and 'DRI' ore, which is natural ore with Fe >69% and low levels of specific trace elements suitable as feed to 'direct reduction' smelters.

Low-grade ore

Low-grade ore is a term applied to iron-rich rocks with cut-off grades in the range of 25�30% Fe. It was the main supply of iron ore for many centuries of the World's early history of production of iron. Since the 1950s North America's main supply has been low-grade ore.

The dominant economic iron mineral in low-grade ore is magnetite. The ore may be easily beneficiated by a process know as wet-magnetic separation - this process has been employed for many decades in North America.

BIF with hematite as the dominant iron mineral may also be beneficiated through wet hydrometallurgical processes though it rarely is due to economic constraints.

World production and resources

Current world production of iron ore is dominated by supply from massive hematite deposits.

Ore production in Australia is exclusively from high-grade hematite and pisolitic goethite-limonite deposits, mostly in the Hamersley Basin region of Western Australia.

World resources of crude iron ore are estimated to exceed 800 billion tonnes containing more than 230 billion tonnes of iron. The world's resources are dominated by low-grade ore.

Most significant are resources of BIF preserved in the remnants of Palaeoproterozoic sedimentary basins. The global distribution of Palaeoproterozoic BIF marks a unique period in Earth's geological history. Examples include BIF in the:

  • Hamersley Basin in Western Australia
  • Lake Superior Region in North America
  • Transvaal Region in South Africa
  • Krivoy Rog Region in the Ukraine
  • Minas Gerais Region in Brazil.

Iron oxides of metasomatic origin form a significant resource. The best example is the Kiruna deposit in Sweden which is the world's largest mine developed on a low-grade, magnetite-rich metasomatite rock.

In South Australia iron oxides of metasomatic origin form a potentially significant resource.

South Australian iron ore

Information about iron ore deposits in South Australia is available in:

state_ironore_sm.jpg (20225 bytes)

The major use of iron minerals in South Australia has been for the production of pig iron for the manufacture of steel. Up to 1915 small deposits in the Flinders Ranges and the Olary region were mined for flux for use in lead-zinc smelters. The recorded total production was ~850,000 tonnes from 35 quarries.

There has been minor production of ochre from several mines in the Adelaide Geosyncline. Also minor production of micaceous hematite.

Pyrite (FeS) was mined at Brukunga to make sulphuric acid, which in turn was used, for the manufacture of superphosphate.

Some BIF has been considered for use as an ornamental stone.

Massive hematite rock

Major deposits of this rock type occur in the Middleback Range within a BIF host. The ore was formed by supergene enrichment of host BIF with both structural and mineralogical controls on ore distribution.

Age of ore formation is put at 1800�1650 Ma.

In 1915 the first major iron ore mine in Australia was opened at a massive hematite deposit at Iron Knob by BHP Pty Ltd. Since then some 200 Mt of high-grade ore has been mined from five massive hematite deposits in the Middleback Range. From 1915 to 1965 the Iron Monarch and Iron Baron-Iron Prince mines were the main supply of ore for Australia's iron and steel industry. The favourable logistics of low cost of ore extraction and the nearby portsite at Whyalla, led BHP to establish an integrated steelworks at Whyalla in 1964.

Map showing principal iron ore deposits and infrastructure in the Iron Knob/Middleback Range/Whyalla region

Iron Baron was closed in 1995 and Iron Monarch was closed in 1998.

Current operating mines in the Middleback Ranges are Iron Knight, Iron Duchess, Iron Duke and Iron Magnet.

In 2000 BHP Steel Pty Ltd divested itself of all long products businesses which included the Whyalla operations and its attached iron ore resources. From this announcement OneSteel (external site) emerged as a totally independent competitive, steel maker and producer of long steel products. They are the current major producers of iron ore from massive hematite deposits in the South Middleback Range.

Other small deposits of massive hematite hosted by BIF include the Buzzard and Wilgerup prospects.

Peculiar Knob prospect is a massive specular hematite deposit of hydrothermal origin.

Banded metasedimentary ironstone

Extensive strike lengths of prominent linear magnetic anomalies occur throughout the Southern Gawler Craton, Northern Gawler Craton, Olary Domain of the Curnamona Province, and the Nackara Arc region of the Adelaide Geosyncline. Limited outcrop and drilling has confirmed that the source of the anomalies is a magnetite-rich ironstone, commonly a BIF. These BIFs are described below in order of age.

Archaean BIF
In central Eyre Peninsula there is a prominent east-west linear magnetic anomaly with a length of ~50 km. Drilling at the Warramboo prospect has identified the source as a metasedimentary magnetite-bearing gneiss of granulite facies, possibly originally a BIF. Diagram of a cross-section through the Warramboo deposit (.gif). Magnetite content averaged ~ 25%. Beneficiation testwork by a relatively simple grinding and wet magnetic separation process yielded a grade suitable for use in the production of DRI (direct reduced iron) feedstock (Adelaide Resources, 2000). No resource figures are attempted.

There are many short strike ridges of Archaean BIF in the northern Gawler Craton, particularly in the region of Mount Christie and to the north at Sequoia prospect. Drilling at Sequoia has identified an inferred resource of 22 Mt @ 28.4%Fe.

Palaeoproterozoic BIF
Forms a major low-grade iron ore resource with extensive strike lengths on central and eastern Eyre Peninsula, the Mount Woods Inlier and in a zone from Tarcoola NNE to Hawks Nest.

ironore_wilgena.jpg (12746 bytes)
Wilgena Hill Jaspilite, Middleback Ranges.

BIF of the Middleback Subgroup occurs discontinuously throughout the eastern half of the Eyre Peninsula. It generally has a strong magnetic signature particularly so in Middleback Range, a discontinuous series of strike ridges of BIF extending north-south for 60 km. The source of the magnetic anomaly has been identified as magnetite-rich BIF beneath a cover of haematitic BIF averaging 90m thick. OneSteel has determined an inferred resource of ~300 Mt @ 36.8%Fe underlying the Iron Duke deposit.

SASE Pty Ltd has tenure on significant resources at the Hawks Nest and Giffen Well prospects. During 2000, AuIron Energy carried out exploration programs on behalf of SASE including geophysical surveys and drilling in the Hawks Nest area. They confirmed resources of beneficiable magnetite-rich BIF at Kestrel, one of seven potential targets at Hawks Nest of :

  • Measured resources of 100 Mt @ 37%Fe
  • Indicated resources of 60 Mt @ 36%Fe
  • Inferred resources of 60 Mt @ 36%Fe

Giffen Well has an inferred resources of 290 Mt @ 36.5%Fe to 150m depth.

In late 2000 SASE Pty Ltd was formed and with the aid of a federal grant they constructed a demonstration smelter in Whyalla with an 'Ausmelt' configuration, i.e. a smelter where the injection of feedstock, including iron ore and coal, is via a hollow lance into a molten iron reservoir. This is new technology for iron ore smelters, though well established for other metals. Reported results are encouraging. In March 2001, a parcel of 27 tonnes of pig iron produced by the demonstration plant was sold to a South Australian foundry.

The Mount Woods Inlier contains considerable strike lengths of linear magnetic anomalies attributed BIF, which interpretation has been confirmed by drilling. Much of the region lies beneath a cover of younger sediments whose depth varies from a few tens of metres deepening to >100 m to the south, but generally is in the order of 30�50m. There has been little exploration of these BIFs for iron ore.

The Ooldea prospect lies on a magnetic anomaly associated with the Karari Fault Zone. Drilling has identified a mylonitised quartz-magnetite-feldspar-amphibole-biotite gneiss, with maximum grade reported at 27% Fe. Inferred resources are reported at ~560 Mt. Davis Tube Testwork shows a magnetite concentrate assaying Fe = 68.9 % and SiO2 = 2.4% can be produced. The magnetic signature of the Karari Fault persists discontinuously for 300 km to the northeast.

Neoproterozoic BIF
Braemar ironstone facies occurs as a stratigraphic package of magnetite-rich ironstone associated with diamictite and is located in the Nackara Arc region of the Adelaide Geosyncline. The rock has been described as 'Rapitan'-type BIF (i.e. associated with glacial sequences). Its iron ore potential was assessed in the early 1960s at the Razorback Ridge prospect. The average head grade is ~25% Fe. Much of its strike length of >150 km remains unexplored for iron ore.

Magnetite-rich metasomatite

There is a zone extending for some 600-700 km along the eastern margin of the Gawler Craton, which includes large accumulations of iron oxide generally accepted to be of hydrothermal origin. The most well known example is Olympic Dam, which contains significant volumes of hematite-rich rock. The average grade for the deposit is reported at 26% Fe. The iron-rich rocks are not considered to be an economic resource.

Other large iron-oxide accumulations include Acropolis, Emmie Bluff, Oak Dam all lying beneath several hundred metres of younger cover. In the Mount Woods Inlier large accumulations of magnetite-rich metasomatite include Manxman, best intersection DD88EN 43 which intersected 402 m at ~34% Fe from 119 to 521 m, and in the northern Yorke Peninsula at the Agery prospect where intervals of massive black magnetite were reported below a deeply weathered basement. The polymetallic nature of these rocks, i.e. anomalous Cu, Au, Ag, U, REE may increase their prospectivity for iron ore. Reports on the recently discovered Cu-Au prospect at Prominent Hill discuss a prominent gravity anomaly thought to be sourced by iron-rich rocks.

Iron-rich magmatic rock
These rock types are currently considered to be relatively insignificant as an iron ore resource in South Australia. Iron-bearing igneous rocks are known to occur within the Giles Complex of the Musgrave Block as small, yet rich segregations. Magnetite-ilmenite segregations have been reported in drill holes within the Malbooma Anorthosite Complex. Drilling has confirmed the presence of ultramafic rocks in the western parts of the Gawler Craton including the circular, strongly layered ultramafic complex of Yumbarra Prospect which shows a form comparable to a major ultramafic intrusion, and prospective for a host of metals including iron ore. There are many other reported occurrences of ultramafic rocks from the western portion of the Gawler Craton.

Iron-rich sediments
Their major iron ore potential relates to the economic recovery of ilmenite an Fe-Ti mineral, from mineral sands particularly in the Murray Basin.

Prospectivity

There is potential for discovery of further resources of hematite ore under cover. All outcropping hematite deposits have been discovered. However there are extensive regions of BIF and iron-rich metasomatite which lie beneath a cover of younger sediments and are potential hosts to deposits of massive hematite. Gravity data in these areas is permissive. A recent example of the success of more detailed gravity data is from the Prominent Hill prospect where a large gravity anomaly some 2000m long by 150�400m wide was outlined. Initial drilling has identified an earthy hematite breccia.

South Australia shows outstanding potential for large resources of low-grade deposits of Palaeoproterozoic BIF and magnetite-rich metasomatite. Also there is the potential of the Neooproterozoic, 'Rapitan' type BIF of the Braemar Iron Formation, in the Nackara Arc.