Oxygen isotope study of Paleoproterozoic banded iron formation, Hamersley basin, Western Australia

A submillimeter-scale variation of δ¹⁸O in quartz was identified in chert and Fe-oxide mesobands of the Brockman Iron Formation, Western Australia, using an in situ CO₂-laser fluorination technique. The total range of variation is 11.2-23.0‰, with >5%‰ variations within a single mesoband of approximately 2cm thickness. These data contradict most previous works, which have suggested that banded iron formations are isotopically homogeneous. The present sample was obtained outside of the iron mining area, and as such is considered to have been less altered by the hydrothermal event recently shown to be responsible for the formation of the iron ore. The data suggest that the largest Paleoproterozoic banded iron formations may have formed not in a stable, quiescent sea, but instead as a result of increased influx of iron- and silica-rich solutions during periods of increased magmatism and submarine hydrothermal activity in a rift basin environment.