Genesis of high-grade hematite orebodies of the Hamersley province, Western Australia

The Hamersley iron ore province of Western Australia contains world-class h igh-purity hematite orebodies hosted within Lower Proterozoic banded iron f ormations at Mount Tom Price, Mount Whaleback, and Paraburdoo-Channar. New evidence indicates that the orebodies are structurally controlled along old fault systems that formed during a period a major uplift and extension in Proterozoic times. Hematite ores are always hosted by the Brockman Iron For mation, and ore formation resulted from a multistage, sequential removal of gangue minerals from the host, giving rise to residual concentration of ir on. The first. hypogene, stage of ore formation removed silica only; leaving a thinned residue enriched in iron oxides, carbonates, magnesium silicates, a nd apatite. with no apparent change in the oxidation state of the iron mine rals, ill this stage of alteration, warm, highly saline bicarbonate-saturat ed fluids from the underlying carbonate-shale Wittenoom Formation leaked up ward along fault zones into the lower;er part of the Brockman Iron Formatio n. During thr second, deep meteoric, stage of ore formation a magnetite-sid erite assemblage oxidized to hematite-ankerite. characteristic microplaty h ematite developed. and magnetite converted to marmite. The fluid responsibl e was moderately warm, of low salinity; and oxidized, and it most likely de rived from the surface. A second stage of gangue removal followed this oxid ation stage and stripped all carbonate from both magnetite and hematite zon es, leaving highly porous anti permeable iron ore bands with a high apatite content interbedded with magnesium-rich shale bands. The final, purely sup ergene, stage of upgrading is indistinguishable from modern weathering but penetrated deep below the present surface. Magnesium silicates were convert ed to a kaolinitic residue, greatly thinning the shale bands, apatite was d estroyed, and both calcium and phosphorus were leached from the ore. The fi nal product is a highly; porous hematite ore of characteristic microplaty t exture interbedded with kaolinitic shale containing a significant amount of aluminum and titanium, which retain their relative proportions throughout the upgrading process.