Visible/near infrared reflectance (VNIR) spectral features of ion-exchangeable Rare earth elements hosted by clay minerals: Potential use for exploration of regolith-hosted REE deposits

Regolith-hosted ion-exchangeable rare earth element (REE) mineralization in South China dominate the global source of heavy REEs. The lack of studies on spectral features of ion-exchangeable REEs limits the application of visible/near infrared reflectance (VNIR, 350-2500 nm) spectroscopy for rapid identification of regolith-hosted REE deposits in prospective areas. This paper presents a systematic study on VNIR spectral features of ionexchangeable REEs hosted by a series of synthetic REE-adsorbed clay minerals and natural regolith samples. The results show that ion-exchangeable Nd3+, Dy3+, Ho3+, Er3+ and Tm3+ display diagnostic absorption features at the bands of 730-870, 805, 641, 652, and 684 nm. Specifically, the Nd3+ related absorption bands of regolith samples at -800 nm show a shoulder at -802 nm that are distinguishable from unweathered primary REE (-bearing) minerals. The maxima of second derivative curves derived from the Nd3+ absorption at -802 nm, M802_2nd, can be used to quantify variations of ion-exchangeable REEs in regoliths. Nevertheless, the absorption features of REE3+ ions could be subdued due to an overlapping problem caused by the broad absorption bands of Fe-oxides in the range of 700-900 nm. The obtained VNIR spectra of ion-exchangeable REEs provide critical supplement to spectral library for REE-bearing minerals. This study demonstrates that field-based VNIR spectroscopy can be deployed as a powerful tool for regional and deposit-scale exploration of regolith-hosted ionexchangeable REE resources.