Synorogenic hydrothermal origin for giant Hamersley iron oxide ore bodies

Geologic mapping, basin analysis, and calculated fluid compositions indicat e that giant orebodies of microplaty hematite, and possibly martite-goethit e, in the Hamersley province of Western Australia, were formed by heated fl uids driven by early Paleoproterozoic orogenesis, Detrital grains of microp laty hematite in the McGrath trough, a foreland basin in front of the north ward-advancing Ophthalmian fold belt constrain the age of the earliest micr oplaty hematite ore formation to <2.45 Ga and greater than or equal to 2.2 Ga Relationships between orebody shape and structure show that the orebodie s 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 degrees C and locally up to 400 degrees C were involved. Regional circulation of hydrothermal fluids, inclu ding heated surface water, through reduced banded iron formations occurred during or soon after the Ophthalmian orogeny. We speculate that martite-goe thite orebodies, previously considered Mesozoic-Cenozoic, could also be rel ated to heated Paleoproterozoic meteoric fluids migrating northward away fr om the Ophthalmian fold belt.