Reflectance spectra in synthetic mineral mixtures:an implication for the feasibility of remote detection of rare earth elements in hematite rich carbonatites

Carbonatites are one of the primary geological environments with the potential to contain rare earth element (REE) minerals such as monazite and bastnäsite. Detecting REEs in carbonatite with proximal sensing and remote sensing is challenging due to the presence of iron oxide and iron-bearing minerals, as they mask spectral absorption features of REE ions in the Visible and Near Infrared region (VNIR). This study attempts to understand the influence of varying concentrations of hematite and carbonates on absorption feature parameters of the REEs-bearing minerals using laboratory-synthesised powdered mineral mixtures and to explore the possibility of using REE ion absorption features in VNIR and short-wave infrared (SWIR) regions to detect REEs in carbonatites by examining resampled spectra for various operational airborne and spaceborne sensors.<br/><br/>Monazite was used as the REE mineral for our study. The mineral was synthesised by a precipitation method, with its composition representing the REE abundance in natural monazite. Powdered mineral mixtures with different mineral abundances using synthetic monazite, laboratory standard calcite and hematite powders were prepared in the laboratory, followed by an acquisition of reflectance spectra using an ASD Fieldspec 3 spectroradiometer. The AVIRIS and EnMAP, representative of hyperspectral sensors, and Landsat Next and WorldView3, representative of multispectral sensors, were chosen for resampling laboratory spectra to evaluate remote sensing capabilities.<br/><br/>A linear relationship of the REE-related absorption feature depth with monazite abundance is found in the SWIR region in synthetic mineral mixtures; these features are not masked by the presence of hematite. While most spectroscopic studies so far have focussed on the application of REE-related absorption features in the VNIR region, the current study highlights that REE-related absorption features in the SWIR range are particularly well suited for detecting monazite in mineral mixtures resembling natural hematite containing REE ores. These findings are formalised in a novel praseodymium (Pr) band ratio for detecting the absorption feature in the SWIR region caused by intra-configurational 4f-4f electronic transitions in the Pr3+ ion.<br/><br/>EnMap and AVIRIS resampled laboratory spectra clearly show the capability of detecting Pr in hematite-bearing mixtures. The new Landsat Next platform has several bands in the SWIR range, but this study indicates that the band centred at 1610 nm is too wide (90 nm, FWHM) to detect the Pr feature. However, the multispectral WorldView3 satellite shows potential for detecting REEs in monazite mineral mixtures that have hematite in them: the SWIR 2 band of the WorldView 3 multispectral sensor may be able to detect the Pr feature centred at 1580 nm.