Via Geology.com from Science News:

Scientists are decoding the geological secrets of banded iron formations, By Sid Perkins:

…. BIFs, as they’re known to geologists, are enigmatic. All seem to have started out as sediments on ancient seafloors, and by some estimates the oxide mineral accumulated in all known BIFs contains about 20 times as much oxygen as today’s atmosphere does. Yet some of these deposits accumulated long before Earth’s atmosphere became thoroughly oxygenated, so the source of the oxygen stored in these BIFs is baffling….

Discussion of various theories old and new.

(Image courtesy Earth Science World Image Bank, Copyright © Thomas McGuire)