From banded iron-formation to iron ore: geochemical and mineralogical constraints from across the Hamersley Province, Western Australia

Abstract

Several major iron ore deposits occur in deformed regions of the Hamersley Province, Western Australia, where banded iron-formation (BIF) of the Dales Gorge Member has been converted to martite and microplaty hematite. The genesis of these high-grade hematite ores remains controversial, in part because no study has systematically documented variations on the chemistry and mineralogy of stratigraphically equivalent rocks from undeformed regions into the deposits. In this study, we examine the powder colour, chemistry and mineralogy of 177 samples of the Dales Gorge Member and surrounding shales from the type section near Wittenoom and the Mt. Whaleback mine near Newman. Profound chemical and mineralogical changes suggest that after early diagenesis, low-grade metamorphism converted clays in black shales to stilpnomelane and talc. Coincident with or following these changes, reduced metamorphic fluids altered phyllosilicates and K-feldspar in these rocks to clinochlore and muscovite around Mt. Whaleback. These metamorphic fluids did not significantly affect BIF. However, subsequent acidic and oxidizing fluids around Mt. Whaleback converted magnetite to martite and dissolved carbonates and silicates from BIF. In black shales, these fluids also dissolved quartz and converted clinochlore and muscovite to hematite and kaolinite, respectively. Late in the paragenetic sequence, BIF or altered BIF was converted to highly porous high-grade hematite ore by the dissolution of Si, a process not requiring iron addition. Our observations in and around Mt. Whaleback suggest that BIF was oxidized prior to silica removal so we infer that the bulk of this oxidation was related to ore formation. However, regionally, this sequence is probably more complicated because rocks previously described at Mt. Tom Price show a stage involving carbonate replacement of silica prior to oxidation that is not evident at Mt. Whaleback. Also, the oxidation stage could (at least in part) have formed by weathering sometime since the Proterozoic. In any case, (1) no single process can produce all of the altered rocks at Mt. Whaleback, (2) oxidation of magnetite to hematite can occur independently of silica removal or replacement and (3) the main mineralization event postdates metamorphism. A model explaining the differences between these two deposits requires either a local carbonate addition step at Mt. Tom Price, or the complete removal (by oxidation) of previous carbonate–magnetite bearing ore at Mt. Whaleback.