Synorogenic hydrothermal origin for giant Hamersley iron oxide ore bodies

Geologic mapping, basin analysis, and calculated fluid compositions indicate that giant orebodies of microplaty hematite, and possibly martite-goethite, in the Hamersley province of Western Australia, were formed by heated fluids driven by early Paleoproterozoic orogenesis. Detrital grains of microplaty hematite in the McGrath trough, a foreland basin in front of the northward-advancing Ophthalmian fold belt constrain the age of the earliest microplaty hematite ore formation to <2.45 Ga and ≥ 2.2 Ga. Relationships between orebody shape and structure show that the orebodies formed during the Ophthalmian deformation, some possibly during orogenic collapse. Oxygen isotopes and fluid inclusions from veins and ore indicate that oxidizing fluids at temperatures >200 °C and locally up to 400 °C were involved. Regional circulation of hydrothermal fluids, including heated surface water, through reduced banded iron formations occurred during or soon after the Ophthalmian orogeny. We speculate that martite-goethite orebodies, previously considered Mesozoic–Cenozoic, could also be related to heated Paleoproterozoic meteoric fluids migrating northward away from the Ophthalmian fold belt.