Kaolinite genesis and the formation of REE ion adsorption deposits

Ion adsorption deposits (IAD), where the Rare Earth Elements (REE) occur adsorbed onto clay minerals in weathered granitoids, are a major source of the critical REE. In this study we aim to test if there is an influence from the climatic or epigenetic conditions of kaolinite formation on the potential for IAD formation. Samples were collected from logged transects from 3 sites with clay adsorbed REE (Zhaibei, China; Ambohimirahavavy, Madagascar; Monchique, Portugal) and analysed for bulk REE content and ammonium sulphate leachable REE, followed by kaolinite separation, characterisation and stable isotope analysis to determine the conditions of formation. Saprolite weathering zones are enriched in the REE relative to bedrock, with leachable fractions between 10 and 90%. Kaolinite dD and δ18O indicate formation from 86 to ∼20 °C. Higher mean T for Portugal and Madagascar reflects the presence of late epigenetic hydrothermal kaolinite prior to weathering. Once the effects of position in the weathering profile have been isolated using the sign and magnitude of the Ce anomaly, the % leachable REE correlates with T, with high % leachable at lower formation T. Analysis of oriented clay mounts by XRD shows that kaolinite crystallinity is variable between sites and within profiles. The variation in leachable fraction REE correlates to increased adsorption onto finer kaolinite crystallite sizes, with corresponding enhanced surface area, which in turn can be related to kaolinite formation conditions. Kaolinite crystallinity is major factor in formation of economic IAD mineralisation, alongside intensity of weathering and changing pH in soil profiles. Hydrothermal kaolinite is unfavourable for the formation of ion adsorption deposits compared to weathering kaolinite.