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Mt Richardson prospect, Western Australia - characteristics of high-grade iron ore

Yilgarn Craton, Western Australia

This six month project examined the physical and chemical properties of high-grade iron ore hosted by BIF. Ore bodies are the product of a combination of early hypogene fluid alteration and late near-surface supergene fluid alteration along reactivated structures.

The Mt Richardson prospect is located in the Illaara greenstone belt in the central-northern part of the Southern Cross Domain of the Youanmi Terrane, Yilgarn Craton. Exploration in the prospect area has identified several BIF macrobands that host high-grade iron ore. In these areas, the high-grade ore is characterised by supergene goethite-martite that overprints an earlier hypogene magnetite-talc and hypogene specularite alteration. Multiple generations of folding have resulted in the fold-repetition and local thickening of BIF macrobands. Ductile shearing along the limbs of NNW-trending upright folds resulted in the circulation of silica-poor hypogene fluids within these structures and the addition of disseminated magnetite or specularite to BIF wallrock. While the resultant altered BIF experienced an upgrade in iron abundance, the fluid was not efficient at removing silica gangue from the BIF, resulting in iron ore with elevated silica (SiO2 > 5%). Subsequent reactivation of existing shear zones parallel to BIF margins resulted in the more recent circulation of oxidised supergene fluids through BIF and the widespread removal or replacement of quartz by goethite. A simplified guide to exploration lies in identifying areas of high permeability in BIF, typically associated with long-lived and reactivated structures that affect the broadest possible zones of thick BIF. De-magnetised zones in structurally-thickened BIF are likely locations for supergene-altered BIF.