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Open File Report 128

The geochemical discrimination of mineralised and barren ironstones from the Selwyn Au-Cu Deposit, NW Queensland

J. E. Wildman

Recent exploration for Au-Cu deposits in the Eastern Succession of the Mt Isa Inlier has concentrated on the type of magnetic ironstones found at Selwyn. Aerial magnetics are used to find the larger occurrences of magnetite but they cannot distinguish between mineralised and barren magnetite bodies. Samples of mineralised and barren ironstones were used to geochemically differentiate the two types of ironstone. Comparisons were also made between the unoxidised sulphide ore and its weathered equivalent found at the surface.

Samples of mineralised and barren ironstones were analysed for 38 elements by XRF and INAA. Photomicrographs and backscattered electron images were used to describe the mineralogy and textures of selected samples. Those elements that distinguish mineralised from barren ironstones have been displayed using several methods of plotting the data.

The mineralised samples are distinguished by Au and W and the barren samples by Ba and Mn. Copper is high in the mineralised samples but there is a large overlap of concentrations with the barren samples.

The SEM investigation shows that Au, W, Sn and Cu can be preserved in hematite derived from the magnetite. This hematite may then be a source of local stream sediment anomalies at surface in the case of Selwyn or at a paleosurface where the ironstones are buried by Mesozoic or recent sediments.


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