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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.
Major iron compounds
Name | Formula | % Fe |
Hematite | Fe2O3 | 69.9 |
Magnetite | Fe3O4 | 74.2 |
Geothite / 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 |
Uses
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 sulfate 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.
World production and resources
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, though current world production of iron ore is largely by supply from deposits of high-grade ore composed of either rocks of massive hematite or pisolitic goethite.
World iron ore production in 2013 was 2.95 billion tonne, with the major producers being China, Australia, and Brazil. Production in Australia was 530 Mtonne. South Australian total production in 2013 was in the order of 12 Mtonne, including 3.2 Mtonne of magnetite direct shipping ore, and 665,000 tonne of magnetite concentrate.
Ore production in Australia is overwhelmingly from high-grade hematite and pisolitic goethite-limonite deposits, mostly in the Hamersley Basin region of Western Australia. A recent trend has been production of a high-grade magnetite concentrate from beneficiation of low-grade magnetite ore. Resources of this latter material are enormous, in particular in Western Australia, and South Australia, with significant resources also in the Northern Territory, and Queensland.
The history of iron ore prices differs significantly from other metals. For most of its history the price of iron ore has been fixed globally, and for very long periods. Since the beginning of global pricing, which is pre-1900, the price has remained relatively fixed up until 2003. In that ~100 year period the price changed only once, from A$2/tonne to A$9/tonne in 1965. Since 2003 the iron ore price has fluctuated, reaching a peak of ~A$120/t in 2011. Since then the price has fallen in an erratic but steady decline some 40% to a September 2014 price of A$85-90/t.
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.
South Australian iron ore
Information about iron ore deposits in South Australia is available in M20 Iron ore deposits in South Australia (PDF 2.3MB)
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.
Following the spectacular rise in iron ore price in 2003 there has been a surge in exploration activity in South Australia targeting both high grade DSO , and low-grade magnetite-bearing rocks. As a result three major iron ore provinces have been confirmed, the Eyre Peninsula, The Mount Woods inlier to Hawks Nest district, and the Braemar Ironstone of the Nackara Arc.
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 sulfuric 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.
Other significant deposits of massive hematite include the hydrothermal Peculiar Knob deposit in the Mount Woods Inlier, resource size ~20 Mtonne and currently being mined (September 2014). Also the Wilgerup deposit in central Eyre Peninsula of 14 Mtonne at >57%Fe, and other smaller deposits of goethite and goethitic hematite from 10-20 Mtonne east of the Middleback Range.
Iron Baron was closed in 1995 and Iron Monarch was closed in 1998. Both these mines and the Iron Princess (north of the Iron Monarch), and the Iron Cavalier are in the process of being recommissioned.
Current operating mines in the Middleback Ranges are Iron Knight, Iron Duchess, Iron Duke and Iron Magnet, and Iron Chieftain.
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 emerged as a totally independent competitive steelmaker and miner. With OneSteel further rationalising its operations with the emergence of Arrium Mining, a dedicated exporter of iron ore, and supplier of iron ore to OneSteel's integrated steelworks at Whyalla. Arrium are the current major producers of iron ore from massive hematite deposits in the South Middleback Range.
In 2011 Arrium acquired the iron ore assets of WPG Resources at Hawks Nest including the massive hematite deposits at Buzzard and Tui, and the Peculiar Knob deposit of massive specular hematite.
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.
Archean/Proterozoic BIF
There are many short strike ridges of possible late Archaean to Palaeoproterozoic 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 72 Mtonne at 25.9%Fe. Recent drilling in the Mount Christie region has confirmed the presence of significant volumes of magnetite rock, though no resource figures have been released.
Late Archean to Paleoproterozoic 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.
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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. Arrium Mining OneSteel has determined an inferred resource of ~300 Mt @ 36.8%Fe of magnetite-rich rock underlying the Iron Duke deposit, and refer to the deposit as the Iron Magnet.
Arrium also has significant resources of similar BIF at their Hawks Nest project area in the far North. Resources include 220 Mtonne of magnetite-BIF at ~30%Fe at the Kestrel deposit, and 18.4 Mtonne of DSO hematite ore at the Buzzard-Tui deposit, with potential for additional resources.
60 km to the SSE of Hawks Nest is located the Giffen Well deposit. It is owned by Maosen Australia Pty Ltd and has an inferred resource of 689 Mtonne at 31.4%Fe. Coolybring deposit further south near Tarcoola has inferred resources of 700 mtonne at 39% DTR.
Returning to the Eyre Peninsula, there has been considerable resource drilling by several companies throughout the whole of the Eyre Peninsula on rocks of magnetite-bearing BIF. Resource size for ~15 deposits is in the order of 2.5 billion tonne, with head grades from 20-30%Fe, and DTR values from 20-35%. Indeed the Eyre Peninsula region has been confirmed as a major iron ore province in South Australia.
Mesoproterozoic 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. 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. Published resource is 3.69 billion tonne at 16%Fe making it one of the largest JORC-compliant iron resources in Australia.
The Mount Woods Inlier contains considerable strike lengths of linear magnetic anomalies attributed to both BIF and magnetite-rich metasomatite, 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. IMX Resources in 2012-2013 drilled their Tomahawk prospect, and confirmed the source of the magnetic anomaly as a magnetite-bearing BIF. Indicated grades were in the order of 25-30%Fe, no resource figure determined, but likely to be quite significant.
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 remained unexplored for iron ore until Royal Resources began exploration and resource drilling in 2009. Since then several companies have entered into exploration for iron ore in the region (including that part of the Braemar over the border in NSW), with most ground now held under tenure. Considerable exploration and resource drilling has been completed. In September 2014 five companies had identified resources of 7.8 billion tonne of iron ore with a head grade from 15-25%Fe, with Davis Tube recovery (DTR) in the order of 15-25%. There are exploration targets for an additional 3 billion tonne, with potential for significant additional resources. Truly the Braemar Iron Ore province is one of the most significant iron ore resources to emerge in recent times.
Magnetite-rich metasomatite
In the Mount Woods Inlier large accumulations of magnetite-rich metasomatite are evident, and beneath a moderate thickness of cover sediment from a few metres to maximum 100m. Drilling has confirmed significant thicknesses including Manxman, best intersection DD88EN 43 which intersected 402 m at ~34% Fe from 119 to 521 m. IMX Resources have recently closed their mine at Cairn Hill from which they extracted a DSO magnetite ore with copper and gold credits. Resource drilling by them identified a further resource of 569 Mtonne at 27.1%Fe, with significant potential for additional resources. Arrium Mining is currently mining the massive hematite deposit of Peculiar Knob, with a resource of 20 Mtonne at >60%Fe. Indeed the Mount Woods Inlier – Hawks Nest regions have emerged as a major iron ore province in their own right, with potential for considerable addition to defined resources, and with as yet untapped potential in the neighbouring Coober Pedy Ridge region to the west, and also south at Giffen Well and other resources near Tarcoola.
On the northern Yorke Peninsula the significant Hillside IOCG-type deposit of 337 Mtonne at 0.6% Cu, 0.14 g/t Au has recoverable iron as magnetite, with a contained iron of 54 Mtonne. The Agery prospect has 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.
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.
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. In the Stuart Shelf region large deposits exist under a significant thickness of cover rock from 300 to >1500m. 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 on the Stuart Shelf include Acropolis, Emmie Bluff, Oak Dam, Carrapateena, Khamsin, and Freemantle Doctor.
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.
