An introduction to clay-hosted REE deposits in Australia

Abstract

Rare-earth elements (REEs) are on the critical minerals list in multiple countries due to their application in renewable technologies, electromobility, and defence. A key exploration focus is on clay-hosted REE deposits, which are reported to have a higher proportion of heavy REEs (e.g. Tb and Dy) relative to light REEs. Previous studies have focused on ionic clay-hosted REE deposits in South China and Madagascar; however, little is known about the mineralogy and economic viability of clay-hosted REE projects in Australia. This study provides the first overview of current exploration activities in relation to clay-hosted REE projects in Australia and presents microanalytical data to characterise the distribution of REE-bearing minerals and guide metallurgical processing pathways, as well as mineral system criteria to inform exploration targeting. Our review identified 91 clay-hosted REE projects in Australia, most of which are in the Yilgarn Craton, Albany-Fraser Orogen and the Gawler Craton. Sixteen mineral resource estimates have been completed that have an average grade and tonnage of 971 ppm total rare-earth oxide (TREO) and 145 million tonnes (Mt) across all resources. This study identified monazite, bastnäsite, allanite, zircon, and apatite as the main REE sources in granitic-to-mafic protoliths. These are partially broken-down during weathering resulting in the formation of secondary REE minerals including rhabdophane, bastnäsite, aluminium phosphate sulphate (APS) group minerals, and cerianite. The most significant finding is that the REE budget of Australian clay-hosted REE deposits is dominated by the primary and secondary REE minerals accompanied by a minor ionic proportion. However, some deposits in South Australia, Queensland, and Tasmania yield a higher ionic proportion of >40%. Accordingly, the mineral system model for the majority of Australian projects differs from ionic deposits located in South China due to contrasting exogenic (surface processes) and endogenic (geological) factors, particularly within the Yilgarn Craton, and it remains unclear whether emerging projects within Australia host economic and extractable REEs.