One theory likens the early Earth to the wind swept deserts of Mars. It proposes that banded iron formations reflect rhythmic settling of iron oxides and silica from repeatedly stirred up clouds of dust produced by the impact and disintegration of iron rich meteorites. More convincing theories invoke chemical sedimentation in the Earth’s early seas, probably with the assistance of bacteria as the suppliers of oxygen.

“Whatever their origin,” says Mike Harris, RTIO’s managing director for developments in Africa, “banded iron formations can only deliver high grade ore if they have undergone natural enrichment. Something must have happened to them that concentrated iron minerals by removing almost everything else. Otherwise, the best they can deliver is generally iron rich concentrates, made from ores that may only have assayed around 30 per cent iron.”

That’s how it goes with mines working banded iron formations in northern China, mainstays of domestic iron ore production in that steel hungry country. The Donganshan mine is probably typical, with a total output of 212 million tonnes of ore averaging 28.5 per cent iron but yielding magnetite/ hematite concentrates with an iron content of 64 per cent. Much higher primary ore grades are known in other parts of the world – up to 67 per cent iron at Rio Tinto’s Corumbá mine in western Brazil – but still well below the grades you can get with natural enrichment. Proved reserves at mines operated by RTIO have average grades that nowhere fall below 57 per cent iron and go as high as 67.2 per cent.

It used to be thought that the only agent of natural enrichment was weathering in geologically “recent” times (the last 100 million years or so). The original view about the Carajas deposits, for instance, was that weathering was enough to account for the high iron grades. It had obviously been intense, with help from such things as tectonic fracturing (making the rocks very permeable to rainwater), and exposure of the iron formations in uplands with heavy seasonal rainfall.