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Introduction Iron (Fe) is one of the most abundant rock-forming elements, constituting about 5% of the Earth's crust. It is the fourth most abundant element after oxygen, silicon and aluminium, and after aluminium, the most abundant and widely distributed metal. Iron is indispensable to modern civilisation and people have been skilled in its use for more than 3,000 years. However, its use only became widespread in the 14th century, when smelting furnaces (the forerunner of blast furnaces) began to replace forges.
Iron ores are rocks from which metallic iron can be economically extracted. These rocks are usually found in the form of hematite (Fe2O3) or magnetite (Fe3O4). About 98% of world iron ore production is used to make iron in the form of steel. Iron ore provides the foundation for one of Australia’s major export industries.
Occurrence Most iron ores mined today comprise the iron oxide minerals hematite, Fe2O3 (70% Fe); goethite, Fe2O3s H2O, (63% Fe); limonite, a mixture of hydrated iron oxides (up to 60% Fe); and magnetite, Fe3O4(72% Fe).
Most of the world's important iron ore resources occur in iron-rich sedimentary rocks known as banded iron formations (BIFs) which are almost exclusively of Precambrian age (i.e. greater than 600 million years old). BIFs occur on all continents. In many instances they are mined as iron ores, but most importantly they are the source rocks for most of the large high-grade concentrations of iron ore currently mined throughout the world.
In the Hamersley Province in the Pilbara district of Western Australia there are three main types of deposit: iron oxide enrichments within BIFs; iron oxides deposited along ancient, mainly Tertiary age river channels (palaeochannels); and iron oxide deposits formed from the erosion of existing orebodies (detrital iron ore deposits).
The BIF enrichment deposits comprising hematite and hematite goethite are the most important in regard to resources and production. The iron content of these ores varies widely and until recently most deposits needed to have an average grade of more than 60% Fe for mining to be commercially viable. However, some deposits can now have iron grades between 56-59% Fe and be commercially viable.
The palaeochannel deposits comprised of pisolitic limonite are the next in importance and are prized for their low impurities such as phosphorus. They are not as rich in iron as the BIF enrichment ores. Those mined usually contain 57-59% Fe.
Detrital iron ore deposits, including scree and canga deposits, are found downhill of the BIF enrichment deposits from which they have been eroded. They are usually easily recovered and have a grade of between 40-55% Fe. BIF enrichment deposits also occur elsewhere in Western Australia in the Pilbara (e.g. Yarrie), and the Yilgarn Block (e.g. Koolyanobbing) and in South Australia (e.g. Iron Duke, Middleback Range).
Numerous magnetite deposits of igneous origin or association occur in most States and uneconomic sedimentary sideritic (those containing the mineral siderite, FeCO3) iron ore deposits occur in Queensland and the Northern Territory.
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