Abstract The Hamersley province of northwest Western Australia is one of the world's premier iron ore regions. 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 percent Fe, and martite-goethite containing between 56 and 63 wt percent Fe. Examples of martite-microplaty hematite include Mount Whaleback, Mount Tom Price, and Paraburdoo and examples of martite-goethite ore deposits include Mining Area C (Area C), Hope Downs, and the Chichester Range. The high-grade martite-microplaty hematite ores, which formed in the Paleoproterozoic, have a three-stage origin. Stage 1 involved the release, from the underlying sedimentary successions, of low (110°C) to high (280°C) temperature, highly saline (20–25.5 wt % NaCl-CaCl 2 equiv; Ca > Na > K) basinal brines that interacted with the underlying Wittenoom Formation and moved upward in normal faults, such as the Southern Batter fault at Mount Tom Price, the 4E fault at Paraburdoo, and the Central and Eastern Footwall faults at Mount Whaleback, into the host BIF. The hypogene fluids migrated laterally within large-scale folds with permeability controlled by shale layers and northwest-trending dolerite dike sets. The BIF was laterally and vertically altered into magnetite-siderite-stilpnomelane and hematite-ankerite ± magnetite assemblages at Mount Tom Price, a hematite-dolomite-chlorite-pyrite assemblage at Paraburdoo, and formed a dolomite-chlorite assemblage in the Mount McRae Shale at Mount Whaleback. Stage 2 involved deeply circulating, low-temperature (<110°C), 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 (>63 wt % Fe) martite-microplaty hematite ore. Supergene alteration is likely to have occurred for at least 80 m.y. and close to the present topographic surface. High-pressure (>0.10 wt %) martite-microplaty hematite assemblages can therefore form and may remain concealed beneath BIF, below Proterozoic erosion surfaces. The martite-goethite bedded orebodies resulted from late Mesozoic supergene alteration of BIF. During this process magnetite was oxidized to martite, whereas silicates and carbonates were oxidized and hydrated to goethite or leached without replacement. The controls on the localization of supergene martite-goethite deposits, for example, the Hope Downs, Cloud Break, and Area C deposits include preexisting structures, such as faults, thrusts, and folds. These structures acted as fluid conduits that directed descending supergene fluids into the host BIF. Dolerite dikes and shale layers further focused and controlled fluid flow. High iron grades at the Area C and Hope Downs deposits are associated with synclinal structures where increased supergene fluid flow caused multiple phases of goethite leaching, precipitation, and cementation. Microplaty hematite encompasses a variety of sizes, ranging from 20 to 300 μm, and textures, ranging from platy to tabular. Microplaty hematite is commonly associated with supergene-modified hydrothermal deposits but can also form in the hydration zone of supergene deposits. The phosphorus (P) in supergene and supergene-modified hydrothermal deposits was repeatedly remobilized by both hypogene and/or supergene fluids. The P distribution was controlled by several factors, such as fluid flux in fault zones, permeability of shale layers, and synclinal folds, which resulted in locally high concentrations (>0.10 wt %) of P in the deposits. It is unlikely that a single model for the formation of the martite-microplaty hematite ore deposits can explain all the structural, stratigraphic, hypogene alteration, and ore characteristics at the Mount Whaleback, Mount Tom Price, and Paraburdoo deposits. Continued collaborative research directed at elucidation of a single tectonic history of the Pilbara, based on collection of similar structural and geochemical data sets from these deposits, will advance genetic ore models and aid in exploration for concealed orebodies.

Abstract Hydrothermal alteration in structurally controlled, high-grade banded iron formation (BIF)-related iron deposits at Carajás (Brazil), Hamersley (Australia), and Thabazimbi and the Zeekoebaart prospect (South Africa) exhibit significant similarities and differences in geologic setting and hypogene alteration. In Carajás, Paleoproterozoic hematite deposits are hosted in low-metamorphic grade Archean jaspilites that are encased in metabasalts. The Paleoproterozoic BIF-hosted deposits of the Hamersley district, the Thabazimbi deposit, and the Zeekoebaart prospect are surrounded by shales. At Carajás, the hydrothermal alteration of jaspilites is characterized by a distal alteration zone with magnetite-calcite-quartz-pyrite where the primary microcrystalline hematite → magnetite (±kenomagnetite). The intermediate alteration zone consists of martite-microplaty hematite-quartz with magnetite → martite, whereas the proximal alteration zone contains hematite ± carbonate ± quartz with martite → microlamellar hematite → anhedral hematite → euhedral-tabular hematite. The proximal alteration zone represents the high-grade ore (i.e., porous hard to soft and hard ores). Hydrothermal alteration also affected mafic wall rocks with chlorite-quartz-carbonate ± hematite in distal alteration zones, and chlorite-hematite-quartz-albite-mica-carbonate ± titanite ± magnetite ± sulfides and hematite-chlorite-quartz-albite-mica-carbonate ± titanite ± magnetite ± sulfides in intermediate and proximal alteration zones, respectively. At the Mount Tom Price deposit in the Hamersley district, three spatially and compositionally distinct hydrothermal alteration zones are distinguishable: (1) distal magnetite-siderite-iron silicate, where the shape of the magnetite is suggestive of it being pseudomorphous after preexisting minerals, likely siderite; (2) intermediate hematite-ankerite-magnetite, with euhedral and bladed magnetite showing minor replacement by martite along crystal boundaries and replacement of iron-silicates by anhedral and microplaty hematite; and (3) proximal martite-microplaty hematite zones, where carbonate is removed. Martite and anhedral hematite replace magnetite and iron silicates of the intermediate alteration assemblage, respectively. The Thabazimbi deposit and the Zeekoebaart prospect lack unequivocal evidence for the formation of paragenetically early hydrothermal magnetite. Chert in ore zones has been replaced by microplaty hematite or has been leached, giving rise to porosity. Veins contain coarse tabular hematite and coarse crystalline quartz. High-grade hematite-martite orebodies are the result of SiO 2 leaching and associated volume loss that created widespread brecciation of the high-grade hematite ore. In addition to high-grade hematite-martite ores, four mineralogically distinct types of iron ore have been recognized: (1) goethite-rich, (2) low-grade dolomite-hematite, (3) low-grade calcite-hematite, and (4) talc-hematite. The comparison of hydrothermal alteration characteristics in the three case study areas revealed: (1) a similar paragenetic sequence of iron oxides, marked by an abundance of open-space filling and replacement textures; (2) distinct lack of a penetrative fabric in alteration lithologic units and high-grade ores; and (3) the importance of porosity and brecciation to accommodate volume loss. Differences include: (1) the formation of carbonate in different hydrothermal alteration zones of each deposit; (2) the presence of stilpnomelane in BIF that is surrounded by shales and hosted in sedimentary basins but absence in BIF that is bounded by mafic rocks; (3) the presence of significant amount of siderite in distal alteration zone in the Hamersley deposits but absence in the Carajás and Thabazimbi deposits; (4) the presence of significant amount of sulfides in the Carajás deposits but absence in the Hamersley and Thabazimbi deposits; and (5) significant amounts of chlorite, talc, white mica, and albite in basalt-hosted iron ore deposits (e.g., Carajás) or mafic dikes that are spatially and temporally associated with iron mineralization (e.g., in the Hamersley province). The systematic documentation of hydrothermal-alteration minerals and assemblages has significant implications for the exploration of concealed high-grade iron orebodies, because key hydrothermal alteration minerals such as chlorite, talc, carbonates or iron silicates are an expression of the hydrothermal footprint of the BIF iron-ore mineral system and, therefore, can be used as mineral vectors.