Metallic iron is virtually unknown on the surface of the Earth except as iron-nickel alloys from meteorites and very rare forms of deep mantle xenoliths. Although iron is the fourth most abundant element in the Earth's crust, comprising about 5%, the vast majority is bound in silicate or more rarely carbonate minerals. The thermodynamic barriers to separating pure iron from these minerals are formidable and energy intensive, therefore all sources of iron used by human industry exploit comparatively rarer iron oxide minerals, primarily hematite.
Formation of iron ore
Most deposits of iron ore in the world are found in rocks known as banded iron formations (BIFs). These are sedimentary rocks that have alternating layers of iron-rich minerals and a fine-grained silica rock called chert.
About 3000 million years ago there was no or very little oxygen dissolved in the oceans, because plants that produce oxygen had not yet evolved. However, the oceans did contain a lot of dissolved silica, which came from the weathering of rocks. Every now and again this silica precipitated out from the seawater as layers of silica jelly, which slowly hardened to become the rock we call chert.
Soluble iron oxide was also produced from the weathering of rocks and was washed into the sea by rivers. Some 2500 million years ago oxygen-producing life forms started to evolve and oxygen became part of the Earth's atmosphere. In time some oxygen also dissolved in the seawater where it reacted with the soluble iron oxide to form insoluble iron oxide. This precipitated out of solution and on to the ocean floor as the minerals magnetite (Fe3O4) and hematite (Fe2O3).
Over many millions of years these processes of precipitating silica and iron oxide were repeated over and over again, and resulted in the deposition of alternating layers of chert (which is grey), hematite (which is red) and magnetite (which is black). Thus, the name banded iron formation comes from the characteristic colour banding of these huge deposits, which are well known in 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.
Hematite or Magnetite
Hematite is an iron oxide mineral. It is non-magnetic and has colour variations ranging from steel silver to reddish brown. Pure hematite contains 69.9% Fe. It has been the dominant iron ore mined in Australia since the early 1960s. Approximately 96% of Australia's iron ore exports are high-grade hematite, most of which has been mined from deposits in the Hamersley province of Western Australia (WA). The Brockman Iron Formation in the Hamersley province is the most significant host for high-grade hematite iron ore deposit.
Magnetite is an iron oxide mineral that is generally black in colour and highly magnetic; the latter property aiding in the beneficiation of magnetite ores. Magnetite contains 72.4% iron, which is higher than hematite, but the presence of impurities usually results in magnetite ores having lower ore grade (generally 20-30% Fe) than hematite ores, making it more costly to produce concentrate for steel smelters. Magnetite mining is an emerging industry in Australia with large deposits, including Balmoral's George Palmer deposit, being developed in the Pilbara region of WA.
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