Alteration profiles and impurity element distributions in magnetic fractions of weathered ilmenite

Magnetic fractions of a commercial ilmenite concentrate from a near-surface deposit in Western Australia have been analyzed by quantitative X-ray diffraction and automated electron microprobe techniques. Phase distributions, composition histograms and variation of minor element concentrations (aluminium, silicon and manganese) have been determined with respect to the extent of alteration, as measured by the Ti/(Ti + Fe) atomic ratio. The altered grains contain ilmenite/pseudorutile/rutile phase associations from alteration in groundwater, and ferrian ilmenite/rutile/H-239 (high temperature Fe₂Ti₃O₉) from surface alteration mechanisms. Grains from the different alteration environments were differentiated magnetically, with concentration of the products of surface alteration in the most-magnetic fractions.

Within each magnetic fraction the microprobe analyses show decreasing manganese and increasing aluminium and silicon concentrations with increasing Ti/(Ti + Fe) atomic fraction. This confirms that during alteration, manganese is leached together with iron whereas aluminium and silicon are incorporated into the grains from the environment. Negative correlations occur between mean manganese and aluminium-silicon concentrations across the magnetic fractions. These are explained in terms of the different alteration mechanisms operating at depth and at the surface of the deposit.