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.
BIF-hosted deposits occur in Western Australia in the Pilbara region, particularly in the Hamersley Basin (e.g. Mt Tom Price), the Yilgarn (e.g. Koolyanobbing), and in South Australia in the Middleback Ranges.
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