The Cycling Redox State of Iron in the Genesis of Banded Iron-Formations and their Associated Enrichment Iron Ores

A simplified eclectic model for the genesis of Banded Iron-Formation (BIF) and associated iron ores in the Hamersley Group of Western Australia can be applied with suitable modifications to comparable situations elsewhere.

BIF was deposited from upwelling ocean currents as a chemical sediment in a platformal environment. Ferrous iron and silica were supplied to seawater by direct input from rivers and sediment-seawater reactions, forming a general background level which was periodically supplemented by exhalative components from major submarine'hot spot' activity. Iron was precipitated largely by photo-oxidation of ferrous iron as it reached the surface rather than by a more commonly advocated biochemical process. During early diagenesis ferrous minerals, such as magnetite, were formed.

Following consolidation, folding and exposure, supergene enrichment occurred in suitable hydrodynamic systems enabling deep ores to form in the BIF while the surface eroded. The enrichment in these small artesian systems was controlled by large electrochemical cells driven by oxygen reactions in the outcrop. This allowed ferrous to ferric reactions to occur at depths well beyond the reach of atmospheric oxygen. Burial and subsequent metamorphism of such goethite-rich deposits, particularly those formed during the Precambrian, resulted in the development of hematite-rich ores. Some of these have been thermally metamorphosed by igneous intrusions to give magnetite-rich deposits. Later re-exposure has further modified the ores by leaching, oxidation, dehydration and surface lateritisation.

Ores formed during later periods are generally unmetamorphosed and retain their goethite-rich character. These have been modified by continuing exposure and are commonly strongly leached.