@ Parmnparsley- Banded iron formations were created during the Precambrian Proterozoic and Archean Eons. This is the 4 billion years before hard-bodied organisms began to evolve. More specifically banded iron formations date between the Paleoarchean and Paleoproterozic Periods, some 3.6 to 1.8 billion years ago.
The exact reason for their formation is not entirely known, but scientists concluded that most of the earth’s surface iron comes from meteorites. The meteorites would vaporize in the atmosphere, settling into the earth’s early oceans and continents. Primitive blue green algae would concentrate the dissolved iron in the oceans during photosynthesis. When the algae died, it would sink to the bottom, forming sedimentary layers that would become banded iron ore formations.