The high-grade iron ore deposits are mostly hosted within banded iron formation (BIF) sequences of the Brockman and Marra Mamba Iron Formations of the Hamersley Group and consist of two types: martite-microplaty hematite containing between 60 and 68 wt. % Fe, and martite-goethite containing between 56 and 63 wt. % Fe. Paleoproterozoic martite-microplaty hematite ore from Mt. Tom Price (North and Southern Ridge deposits) and Paraburdoo (No 4 lens deposit) revealed a three-stage hydrothermal origin. Stage 1 involved the release, from the underlying sedimentary successions, of low (110oC) to high (280oC) temperature, highly saline (20-25.5 eq. wt % NaCl-CaCl2; Ca>Na>K) basinal brines that interacted with the underlying Wittenoom Formation and moved upwards in normal faults, such as the Southern Batter Fault at Mt. Tom Price, and the 4E Fault, at Paraburdoo, into the host BIF. The hypogene fluids migrated laterally within large-scale folds with permeability controlled by shale layers and NW-trending dolerite dike sets. The BIF was laterally and vertically altered into magnetite-siderite-stilpnomelane and hematite-ankerite±magnetite assemblages at Mt. Tom Price, and a hematite-dolomite-chlorite-pyrite assemblage at Paraburdoo. Stage 2 involved deeply circulating, low temperature (<110oC), Na-rich meteoric waters that interacted with evaporites prior to their interaction with the BIF.
The descending meteoric waters interacted with the carbonate-altered BIF to produce a martite-microplaty hematite-apatite assemblage prior to supergene alteration. Stage 3, the supergene stage during the Mesozoic to Tertiary, is the final stage in the transformation of BIF to high-grade ore. Shallow supergene fluids interacted with the martite-microplaty hematite-apatite assemblage to form a highly porous high-grade martite-microplaty hematite ore. Supergene alteration is likely to have occurred for at least 80 Ma and close to the present topographic surface. High-P (>0.10 wt. %) martite-microplaty hematite assemblages can therefore form, and may remain, concealed beneath BIF, below Proterozoic erosion surfaces.