Most of the major iron deposits worldwide occur in rocks called banded iron formations (or BIFs for short), which are finely layered sedimentary rocks composed of alternating chert (a form of quartz) and iron oxide bands.
The banded iron formations of the Hamersley Province in the Pilbara region of Western Australia, are the thickest and most extensive rocks of this type in the world. It is estimated that after they were originally deposited, about 2470 to 2450 million years ago, they covered some 150 000 km2 and contained about 300 trillion tonnes of iron.
The main minerals that form the layers in banded iron formations include quartz (silicon oxide), hematite (an iron oxide), siderite (an iron carbonate), and stilpnomelane (a potassium, iron, magnesium aluminosilicate).
We do not see any rocks of this type forming in the world today, and this suggests that conditions in the ancient oceans where they formed were quite different than today.
Banded iron formations appear to have been deposited in areas of the ocean where seawater with high contents of dissolved iron and silica came into contact with water containing higher amounts of oxygen, which resulted in the precipitation of hematite and chert (microcrystalline quartz).
Most of the iron and silicon probably came from upwelling iron-rich, deep ocean currents derived from ocean floor volcanic systems.
Because of their great thickness and the enormous areas that they cover, banded iron formations probably accumulated on wide continental shelves at water depths of over 200 metres.
The process of iron deposition in the Proterozoic seas, 2.5 to 1.9 billion years ago, is thought to have involved a fine balance between the chemistry of the ancient atmosphere and oceans at a time when the oxygen content of the atmosphere was beginning to increase. It was the emergence of the earliest forms of life, tiny microbes (cyanobacteria) that produced oxygen through photosynthesis, that probably saw the composition of the early atmosphere begin to change.
Along with banded iron formations, the rocks of the Pilbara region also contain fossilised remnants of marine communities (stromatolites) built up by cyanobacteria at this time.