Iron formation-hosted iron ores in the Hamersley Province of Western Australia

Iron formation-hosted iron ore deposits account for the majority of current world iron ore production and consist of three classes: unenriched primary iron formation with typically 25 to 45 wt-%Fe; martite–goethite ore formed by supergene processes, with abundant hydrous iron oxides containing 60 to 63 wt-%Fe; high-grade hematite ores thought to be of hypogene or metamorphic origin overprinted by subsequent supergene enrichment with 60 to 68 wt-%Fe. Individual iron ore deposits range from a few millions of tonnes to over two billion tonnes at >64 wt-%Fe, although most are within the range of 200 to 500 Mt. In the Hamersley province of Western Australia, martite–goethite ores are largely developed in the Marra Mamba iron formation, although high (>0·08 %P) phosphorous mineralisation is also well developed in the stratigraphically higher Brockman iron formation. While the vast majority of high-grade microplaty hematite ore is best developed in the Brockman iron formation, the present paper provides the first textural evidence of locally significant microplaty hematite mineralisation in the Nammuldi member of the Marra Mamba iron formation, in the Chichester Ranges at the Christmas Creek, Cloud Break and Mount Nicholas prospects. Petrographic studies have identified a high-grade Fe texture composed of nanometre scale plates of hematite in mineralised sections of primary microplaty hematite deposits in the Pilbara and elsewhere, well below the normal depth of weathering or dehydration. The population of nanometre to micrometre scale hematite plates is interpreted to represent various stages of nucleation, crystallisation and progressive growth of hematite from the primary ore-forming fluid in areas that were once iron-rich carbonates or silicates in the banded iron formation.