Implications of mineralogy, textures and landscape context on surface exploration of rare earth elements: A comparison of soil analytical methods near Esperance, Western Australia

With the rising interest in rare earth element (REE) exploration in Australia and beyond, the focus of many greenfields exploration campaigns is on identifying the best methods for recovery of REE from soils to better define exploration targets. In this study, we compared analyses of the ultrafine (<2 μm) fraction of soils with more traditional soil alkaline fusion analysis of the bulk soil fraction at two regolith-hosted REE prospects near Esperance, Western Australia. We complement this data with sequential leaching and scanning electron microscopy on soils to establish where REE are hosted within the samples and to investigate the potential causes for differences in REE abundancies recovered via the two soil analytical methods. At both sites, the majority of REE in mineralised downhole samples were hosted in REE-bearing phosphates. The primary REE sources have been altered within the regolith via oxidation and re-precipitation. Altered and disaggregated primary and secondary REE-minerals are now hosted in clays and altered micas, and <1 % of REE are ionically bound to the clays in soils. Recovery of REE from the ultrafine fraction of soils was generally higher than via alkaline fusion. Correlation between the two methods was only moderate because of the “nugget effect” of particles >5 μm. Soils have been physically transported downslope and REE occur as detrital mineral particles, some of which are encapsulated in other detrital minerals. By excluding the larger REE ‘nuggets’ the ultrafine fraction results return more consistent results of the detrital fraction in transported soils. Our study highlights the importance of understanding landscape context for analysis and interpretation of exploration soil samples.