ORIGINS OF LARGE HIGH-GRADE IRON ORE DEPOSITS: WILL ONE SIZE EVER FIT ALL?

Large iron ore deposits are associated with iron formations, almost all deposited before 1.8 Ga. Even in unenriched iron formations, different sedimentation styles that evolved through geologic time cause wide variations in composition and abundance of iron minerals. Some magnetite-rich iron formations are mineable (taconite-type ore deposits), but economic high-grade ore bodies only occur where iron formations (typically 35 wt percent Fe) were upgraded by subsequent events to the 60 to 68 wt percent Fe typical of high-grade ore. The two main types of high-grade iron ore deposits, martite-goethite and high-grade hematite, are both diverse in their characteristics and geneses, but the former are less controversial. Martite-goethite deposits are attributed to replacement of gangue minerals by goethite, whereas soft high-grade hematite deposits are attributed to residual concentration of hematite via leaching of silicate/carbonate gangue. High-grade hematite ores account for the majority of high-grade iron ore reserves (>31,000 Mt) and can be further subdivided into hard hematite and microplaty hematite ore types; their origins are more controversial, a consequence of greater diversity. This includes variations in ore textures, presence/absence of carbonate alteration, timing of hypogene mineralization with respect to regional metamorphism, and possibly the types of fluids responsible (e.g., basinal brines vs. meteoric waters vs. magmatic fluids). Proposed origins for high-grade hematite ores include supergene; supergene with subsequent burial metamorphism; hypogene; and supergene-modified hypogene-hydrothermal involving warm basinal brines and ascending or descending heated meteoric waters. Although the supergene-modified hypogene-hydrothermal model has received widespread support, a single model is unlikely to accomodate the wide diversity of deposits around the world. In the final analysis, high-grade hematite deposits will only form where an iron-rich iron formation with the right textures experiences the right sort of fluid flow. The latter usually happens during regional deformation, but broad consensus on specific mechanisms for both the primary variations in iron formations and their subsequent enrichment to form large, high-grade hematite ore deposits have proven elusive.