Alteration and associated impurity element enrichment in detrital ilmenites from the Murray Basin, southeast Australia: a product of multistage alteration

Alteration and associated impurity element enrichment in detrital ilmenites from the Murray Basin in southeastern Australia was examined using electron-beam imaging and analysis techniques. A database of over 15 000 analyses from 22 deposits (16 coarse grained, six fine grained) showed a broad spectrum of alteration products from primary ilmenite through to pseudorutile, hydroxylian pseudorutile and minor leucoxene. Impurity element levels were strongly affected by the degree of alteration. Ilmenites from fine-grained deposits were less altered than their coarse-grained equivalents. Regional analysis of the data showed an increase in the degree of alteration towards the northern margins of the basin. Two types of ilmenite alteration were observed: a continuous process in which ilmenite alters to pseudorutile and then leucoxene (Type I) plus a second type of alteration which converts ilmenite directly to hydroxylian pseudorutile (Type II). Type I alteration was present throughout all deposits and occurred prior to final emplacement under acidic oxygenated conditions. Type II alteration was restricted to deposits in the more northern sectors of the basin. Conditions for the formation of Type II alteration are unknown but textural evidence indicates alteration was in situ. The Murray Basin ilmenites represent the final products of multi-stage alteration processes.