Banded iron formations are iron oxide, not elemental iron. Vast amounts of iron were dissolved in Earths oceans, until oxygen produced by photosynthesis oxidize

Banded iron formations are iron oxide, not elemental iron. Vast amounts of iron were dissolved in Earths oceans, until oxygen produced by photosynthesis oxidized it, to form iron oxide, which precipitated and sank to the bottom. This process went on for 2.5 billion years until all the dissolved iron was consumed.