Iron oxide chemistry supports a multistage hydrothermal genesis of BIF-hosted hematite ore in the Mt. Tom Price and Mt. Whaleback deposits

This study demonstrates the great value of iron oxide chemistry in low-temperature systems. Hematite records elemental signatures of structural control, partial inheritance of pre-existing alteration, and allows chemical fingerprinting of multistage BIF-hosted ore. Mt. Whaleback and Mt. Tom Price are Western Australia's largest banded iron formation (BIF) hosted hematite ore deposits. The mechanisms and timing of iron enrichment from BIF (with ∼20–40 wt% Fe) to high-grade iron ore (with up to 69 wt% Fe) are still controversially debated. In-situ iron oxide chemistry support post-metamorphic, structurally controlled, hydrothermal hematite ore formation via multiple stages of hydrothermally altered BIF. Iron oxides in two hydrothermal alteration zones in BIF at Mt. Tom Price (distal alteration assemblages: magnetite-siderite-stilpnomelane ± pyrite and intermediate alteration assemblages: hematite-magnetite-ankerite-chlorite) show similar trace element patterns that are distinct from metamorphic magnetite in least-altered magnetite-quartz ± hematite ± Fe-carbonate ± Fe-silicates BIF. At Mt. Whaleback, the hydrothermal alteration is completely eradicated by subsequent microplaty-martite hematite ore formation, and solely recorded in hematite composition by the inheritance of various conservative element abundances. Petrographically indistinct zones within a given orebody are chemically distinct and demonstrate formation via contrasting structurally controlled hydrothermal fluid flow: At Mt. Whaleback, specifically in the vicinity of Central Fault and Whaleback Fault, systematic trace element group abundance trends are recorded: major controls are fO₂, pH, and acquired country rock signatures, i.e., evolved (Al, Mn, Mo, Pb, U) at Central Fault, and primitive (Al, Cr, Ti, V, Cu, Ni, Co) at Whaleback fault. Complex zoned growth textures and element distribution in microplaty hematite support precipitation during protracted hydrothermal fluid/rock interaction. In combination, our results falsify the importance of alternative hematite crystallisation models, including goethite dehydration and coalescent nanoparticles.