Lateral Geochemical Gradients and Physical Processes Associated with the Genesis of Iron Formations: Examples from the Paleoproterozoic to Mesoarchean of Superior Province

English Abstract:

The presence of Fe, Si, Mn and P in mineral phases with precursors that were quasi-stable precipitates from the overlying water column necessitates that disequilibrium conditions existed during their formation. Commonly a two box model is used to explain iron precipitation in the Precambrian with an oxic to sub-oxic phytoplankton-rich or CO₂-rich surface layer driving iron precipitation in the underlying anoxic, Fe⁺²-rich ocean. But what is the evidence for this model?  On the wide, Paleoproterozoic Gunflint-Mesabi shelf carbonate iron formation (IF) dominated the shallow areas, whereas oxide IF accumulated in more offshore locations. Storm induced geostrophic flows delivered oxygenated inner shelf waters to the offshore instigating formation of the discrete iron to iron+manganese+silica+mud laminae that compose the fine-grained IF. Here frequent storm mixing would have destroyed vertical stratification and Fe⁺² transport across the <150km wide shallow shelf indicates anoxic conditions across the shelf during fair-weather times.  The Neoarchean delta-top IFs in the Lake St Joseph area record extremely rapid accumulation of iron hydroxides during limited sediment flux to portions of distributary mouth bar complexes. IF accumulated on short-lived reactivation surfaces of gravel bars to ripples in water depths of a few meters. No IF accumulated further offshore. Its chemistry is virtually identical to deep-water IF deposits. The IF was probably formed by high nutrient flux to the shallows promoting cyanobacteria.  Carbonate was deposited on the Mesoarchean, stromatolitic, oxygenated Steep Rock platform, while this production caused Fe and Mn precipitation offshore. Onshore water movement, probably driven by storm events, deposited iron-rich layers in the limestone and shifted the offshore from Fe to chert or mud accumulation. In these examples IF deposition was dependent on lateral, not vertical, geochemical differences augmented by storm induced mixing.