THE ROLE OF MICROBIAL INDUCED OXYGENATION IN THE DEPOSITION OF NEOARCHEAN IRON FORMATION: EXAMPLES FROM WESTERN SUPERIOR PROVINCE CANADA

The deposition of large, Paleoproterozoic iron formations through the mixing of Fe+2 enriched, deep ocean waters with oxygenated waters on shelves is generally accepted. In contrast, many Archean IF's appear to have formed through the venting of hydrothermal fluids. This paper explores a third type of IF: Neoarchean examples formed in volcanically quiescent, shallow marine settings. Three areas in western Superior Province were investigated: Beardmore-Geraldton, the Minnitaki-Savant Lake Groups, and the Eagle Island Group. Magnetite-rich chemical sediments interbedded with siltstones and slates are present in all three areas on flooding surfaces overlying fluvial channel and shore proximal braid delta deposits. Magnetite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces in barforms to ripples. Thicker accumulations of IF (up to 73m) form portions of transgressive systems tracts where chemical sedimentation dominated. Offshore equivalents of these sequences do not contain chemical sediments. Iron formation depositional mechanisms relying on relatively constant iron compound precipitation in the world ocean combined with siliciclastic starvation are not compatible with these data unless dissolution below a chemocline is inferred. The model presented for iron precipitation relies on an elevated nutrient flux in the near shore stimulating microbially induced oxidation of Fe+2. This implies the existence of thriving microbial colonies in climatically suitable, Neoarchean, near shore settings: colonies that were able to oxidize iron, either intra- or extra-cellularly, and generate thick successions of IF.