Abstract
The Pilbara region of Western Australia represents the world’s largest supply of quality iron ores predominantly sourced from supergene and hypogene enriched Banded Iron Formations. Aluminium is a major contaminant in these iron ores and its mineralogical host impacts the efficiency and productivity of the steel-making process. The aluminium can either be hosted by gangue minerals such as kaolinite (Al2Si2O5(OH)4) or be adsorbed or substituted into the crystal lattices of iron oxides. Gibbsite (Al(OH)3) is widely considered a low abundance gangue mineral in this region, posing a minimal risk to iron ore quality. As a result, investigations into the genesis of gibbsite are scarce and its spatial distribution in Pilbara iron ores is poorly understood.This latest research is prompted by discrepancies between predicted and actual aluminum distribution in world class Pilbara iron ore deposits, and reveals that the current means of prediction does not adequately account for the highly aluminous mineral gibbsite. A micromorphological and geochemical study of iron ore samples from mining benches at various depths found that significant gibbsite forms in some iron ore deposits due to localized alumina remobilization from dolerites. The study documents for the first time the micron-scale transformation of gibbsite to kaolinite in iron ores. Structure, hydrogeology and geomorphology were found to control the formation and transformation of gibbsite and explain why gibbsite is sometimes abundant in iron ore mineralization proximal to dolerite dykes and at other times completely absent.Improved fundamental understanding allows gibbsite distribution to be derived from normative mineralogy calculations, and suggests gibbsite is locally more abundant in Australian BIF-hosted iron ores than previously thought. This finding has important metallurgical implications given that ores are selected for beneficiation based on predicted aluminium deportment and its textural associations with the iron oxides.
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