The Hamersley Iron Ore Province of Western Australia is recognized as containing one of the major Precambrian iron formations in the world--the Brockman Iron Formation. The excellent exposure in drill core of the Dales Gorge Member of this formation has provided a unique opportunity for studying iron oxide genesis in this relatively undeformed and only slightly metamorphosed iron formation. Sixteen banded iron formation units alternate with seventeen shale units over a thickness of 400 ft in the sequence studied. The author has established a sequence of mineral development for the iron oxide and associated silicate and carbonate minerals based principally upon textures observed in the banded iron formation macrobands. Chert, hematite, siderite and stilpnomelane are primary minerals and represent the crystallization products of a pre-existing silica-iron oxide-carbonate gel which formed at the bottom of the stable Hamersley Basin. The dominant iron oxide present, magnetite, formed principally during diagenesis by crystallization from hematite and by decomposition of siderite. This type of crystallization was also responsible for the formation of coarse ankerite rhombs. Minnesotaite rosettes, biotite and possibly riebeckite developed during later low-grade metamorphism, which was a response to the deep burial of the rocks.

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