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Summary
In Western Australia, iron ore deposits are divided into three types, Channel Iron Deposits (CID), Detrital Iron Deposits (DID) and Bedded Iron Deposits (BID). The latter are hosted in the banded iron-formation (BIF) of the Marra Mamba and Brockman Iron Formations. Two types of BID are mined, the Martite-Microplaty and the Martite-Goethite (M-G), the latter representing the current bulk of the direct shipping ore. The Upper to Lower Miocene CID have been successfully dated using the (U-Th)/He method. Here we report the first dating of three samples from M-G mineralisation occurring in the Dales Gorge member from the Brockman Iron Formation. In unweathered MG ores, the original texture of the BIF, from the micro- to the macroscale, is fully preserved. The primary hematite is unchanged, but primary magnetite is oxidised to martite and the original gangue minerals such as chert, silicates, and iron-rich carbonates are pseudomorphed by goethite. Three samples were collected from a diamond drill hole at depths of 163m, 116m and 90m from the surface. The samples were characterised using an optical microscope and after microscopic identification of goethite and martite, samples were microdrilled. The average ages of iron oxides were 45 ± 6 Ma or Mid-Eocene for the sample at 163 m; 42 ± 5 Ma or Mid-Eocene for the sample at 116 m and 28 ± 3 Ma or Early Oligocene for the sample at 90 m.