Abstract

The Koolyanobbing banded iron formation (BIF)-hosted iron ore deposits (total premining resources ~150 million metric tons (Mt), indicated reserves ~32 Mt) are located in the Mesoarchean lower succession BIF of the Koolyanobbing greenstone belt, Younami terrane, Yilgarn craton in Western Australia. In the Koolyanobbing greenstone belt a deformation sequence that broadly correlates with the proposed deformation history of most greenstones belts within the Southern Cross domain includes: D1 structures (mainly small-scale F1a and F1b folds, formed in a north-south to northwest-southeast compressional regime), a ductile to brittle deformation sequence, D2 to D4 (generated during east-west compression) and, a late-stage brittle segmentation of BIF and reactivation of faults, attributed to D5.

The formation of the seven known medium- (45–58 wt % Fe) to high-grade (58–68 wt % Fe) magnetite-, martite-, specularite-, and goethite-bearing orebodies can be subdivided into four Archean stages and one weathering-related upgrade from the Permian and/or Mesozoic to recent times. The Archean ore-forming stages comprise: (1) early Fe-Mg ± Ca metasomatism causing local ferroan carbonate and ferroan talc alteration of the metamorphosed quartz-magnetite BIF protolith; (2) sequential syn-D2a (coaxial) to syn-D4 (transpressional) tight folding-driven removal of carbonate, quartz and minor ferroan talc by solution and mechanical transfer, producing residual enrichment of medium- to high-grade magnetite ore; (3) magnetite mineralization in syn-D2b and syn-D4 breccias and fractures, forming medium-grade ore zones, or overprint magnetite in BIF and first-stage magnetite ore; and (4) mineralization of hydrothermal specularite and locally associated ferroan dolomite-quartz alteration, and local oxidation of magnetite in and near brittle D4 faults, fractures, and reactivated F1 and F2a fold cores.

Modern weathering-related leaching of carbonate (and minor quartz), pseudomorphic goethite replacement of existing iron oxides and gangue, and coeval or subsequent to oxidation in the vadose zone formed goethite-martite ore with local relics of specularite or magnetite and/or kenomagnetite. The intensity and localization of this supergene modification is, in most deposits at Koolyanobbing, controlled by existing hypogene magnetite, specularite-rich medium- to high-grade ore zones and/or carbonate-altered BIF at depth. The existence of high-grade ore below the weathering horizons suggests the possibility of further concealed magnetite- and/or specularite-rich orebodies within the deposits and region.

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