Multistage Evolution of High-Grade Hematite Orebodies From the Hamersley Province, Western Australia

Deformed regions of the Hamersley Province, Western Australia, contain several major iron ore deposits where banded iron-formation (BIF) of the Dales Gorge Member was converted to martite-microplaty hematite. The genesis of these ores remains controversial, in part because no study has systematically documented how stratigraphically equivalent rocks change chemically from undeformed regions into the deposits. Profound chemical and mineralogical differences do occur between rocks in the undeformed north and mineralised south. BIF and shale can be classified into three types: BIF, altered BIF and iron ore, and black shale, altered black shale and red shale. A series of low temperature (20 to 250¦C) chemical reactions can explain the observed differences in chemistry and mineralogy. The inferred reaction paths indicate that no single process could have produced all of the altered rocks at Mt Whaleback. Recently described carbonate-rich, silica-poor rocks at Mt Tom Price (Taylor et al, 2001) suggest this alteration sequence is even more complex. Our favoured explanation for the genesis of hematite ores is one that can explain deposits at Mt Whaleback and Mt Tom Price: following regional metamorphism, hydrothermal fluids locally formed carbonate-rich, silica-poor BIF with magnetite and carbonate-rich altered black shale. Subsequent surface derived fluids oxidised this magnetite, crystallised microplaty hematite (mpl) and eventually dissolved carbonates and silicates leaving high-grade ore. Either during or sometime after mineralisation, acidic, oxygenated fluids converted altered black shale to red shale and unaltered BIF to martite-rich BIF without carbonates across the southeast of the province.