Hypogene Alteration Associated with High-Grade Banded Iron Formation-Related Iron Ore
Lydia Maria Lobato, E Silva Figueiredo Rosaline Cristina, Steffen Hagemann, Warren Thorne, Márcia Zucchetti, 2008. "Hypogene Alteration Associated with High-Grade Banded Iron Formation-Related Iron Ore", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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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 SiO2 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.