Microplaty hematite—its varied nature and genesis

Secondary microplaty hematite (mplH) occurs in many different environments but importantly, where unmodified by metamorphism, it is the defining minor component of the high-grade BIF-hosted iron ores of the world that are dominated by martite (M, hematite pseudomorphs after magnetite). The term was introduced early during the CSIRO–AMIRA 1976–1994 program on the Hamersley Province iron ores to conveniently distinguish the then main export martite–microplaty hematite (M–mplH) ore type of Mt Whaleback, Mt Tom Price and Paraburdoo (ca2000 Ma) from the newly exploited, prolific supergene martite–goethite (M–G) ores (ca Cretaceous–Paleocene). The latter, with the channel iron ores, are now the major iron-ore exports of the Hamersley Province. The CSIRO-AMIRA model proposed that theM–mplH ores formed during regional metamorphism of Proterozoic M–G ores at low temperatures (∼80–100°C) by mplH growth in the supergene goethite. The conversion of goethite to hematite involves a ∼27% reduction in volume, with suggested mplH growth in the resulting microvoids by iron transfer through water resulting from the process itself, essentially an internal hydrothermal process requiring no introduced hypogene fluids. Alternative models have proposed oxidation of introduced siderite resulted in mplH + ankerite by reaction of heated meteoric fluids with hydrothermally metasomatised BIF ‘protore’ preserved in two locations in the Mt Tom Price deposits. This postulated ‘protore’ had earlier been described during the CSIRO–AMIRA program as local post-ore BIF metasomatic residuals. These new concepts have resulted in varying hydrothermal models that currently dominate the international literature of iron ore. A critical comparison of the two suggested mechanisms of mplH formation includes examples of mplH-related material from a range of milieus including BIF, GIF, iron ores and ferruginous sediments. Conversion of goethite to mplH appears the more likely process.