Banded iron formation

Almost every piece of steel in the world began as banded iron formation. It is a sedimentary rock of striking appearance — millimetre to centimetre layers of dark iron oxide alternating with red or grey chert, repeated through thicknesses of hundreds of metres and traceable across whole basins.

What makes it remarkable is not the ore alone but that it stopped forming. Banded iron formations were deposited overwhelmingly between about 3,000 and 1,800 million years ago and are essentially absent from younger rocks. The world's iron supply comes from a process that ran for a billion years and then ended, and understanding why it ended turns out to be the same question as how the atmosphere came to contain oxygen.

Early oceans held large quantities of dissolved iron, which is possible only in the absence of free oxygen — iron in an oxygenated ocean precipitates almost immediately. When photosynthesising organisms began releasing oxygen into that water, the dissolved iron reacted with it and settled to the sea floor as oxide. The chert layers between record intervals when iron precipitation paused and silica accumulated instead.

The banding is the fingerprint of that alternation, though what drove its rhythm is still argued over: seasonal cycles, blooms of the organisms producing the oxygen, and periodic upwelling of iron-rich deep water have all been proposed.

Deposition ended when the ocean's dissolved iron had been used up and the atmosphere itself began to hold oxygen. The richest ores mined today are not the original rock but weathered enrichments of it, where later groundwater leached the silica away and left the iron behind at grades approaching sixty per cent.

The iron ore trade is the largest bulk commodity trade in the world by volume, and it is overwhelmingly a trade in banded iron formation and its weathered derivatives. The Hamersley Basin of Western Australia, the Carajás deposits of Brazil, the Mesabi Range of Minnesota and the Transvaal Supergroup of South Africa are all the same rock of roughly the same age.

That concentration in time is why iron ore geography is so unequal. A country either has Precambrian shield exposed at the surface or it does not, and no amount of exploration will produce a deposit in rocks that are too young to contain one.