Snowball Earth Scenario
Until 1992, it was assumed that the rare, later (younger) banded iron deposits represented unusual conditions where oxygen was depleted locally, and iron-rich waters could form and then come into contact with oxygenated water.
An alternate explanation of these later deposits was undergoing much discussion as part of the Snowball Earth hypothesis. This hypothesis stated that following the breakup of the early equatorial supercontinent (Rodinia), the earth's continents were totally covered in an ice age (implying the whole planet was frozen at the surface to a depth of several kilometers).
If this was the case, Earth's free oxygen may have been nearly or totally depleted during a severe ice age circa 750 to 580 million years ago (mya). Dissolved iron then accumulated in the oxygen-poor oceans (possibly from seafloor hydrothermal vents). Following the thawing of the Earth, the seas became oxygenated once more causing the precipitation of the iron.
Another mechanism for BIF's, also proposed in the context of the Snowball Earth discussion, is by deposition from metal-rich brines in the vicinity of hydrothermally active rift zones. Alternatively, some geochemists suggest that BIFs could form by direct oxidation of iron by microbial anoxygenic phototrophs.
Read more about this topic: Banded Iron Formation
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