The Mount Gibson Banded Iron Formation-Hosted Magnetite Deposit: Two Distinct Processes for the Origin of High-Grade Iron Ore

      The Mount Gibson banded iron formation lies within the Windanning Formation of the Luke Creek Group, which is found in almost all greenstone belts throughout the Murchison province. The banded iron formation (BIF) typically consists of alternating bands of magnetite and microcrystalline quartz (chert) with rare carbonaceous and iron silicate-rich shale partings and layers and rare, thin fine-grained tuff bands. Owing to repetition by isoclinal folding and attenuation by faulting, the true thickness of the Mount Gibson BIF is unknown but appears to be on the order of 100 m. Although the Mount Gibson BIF is typical of many Algoma-type iron formations, situated within a greenstone belt on an Archean craton, it is also similar to Hamersley-type BIF in petrology, areal extent, and ore genesis.
   High-grade hematite deposits formed within BIF were thought to have formed by the supergene leaching of chert from typical cherty BIF. Recent evidence suggests that at least some of these deposits are formed by hypogene replacement of chert by carbonates with subsequent supergene leaching of the carbonate and accessory minerals and oxidation of magnetite to hematite. Magnetite-carbonate BIF, in which there is clear evidence of hydrothermal replacement of chert by carbonate, forms distinctive magnetite-goethite ore with magnetite locally persisting to the surface. Mount Gibson shows clear evidence of the formation of high-grade ore by this process but also contains high-grade hematite ore and chert-free BIF that show no evidence of the hypogene replacement of chert. High-grade hematite occurrences, up to 1 km in strike length, are found within the weathered zone overlying the magnetite BIF at Mount Gibson and continue into unweathered chert-free BIF at depth that show no evidence of hydrothermal carbonate or supergene enrichment. The cherty BIF shows sharp contacts against chert-free BIF and high-grade ore, even when strongly weathered. This suggests that deep saprolitic in situ high-grade ore may be produced by different processes, including hydrothermal replacement of chert mesobands by carbonates with subsequent supergene leaching of the carbonate and by the oxidation of chert-free BIF, in which chert bands either never developed or were apparently removed during diagenesis. Neither model requires supergene selective leaching of quartz (chert) during deep weathering.

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