Abstract
Rare earth elements (REEs) generate characteristic absorption features in visible to shortwave infrared (VNIR-SWIR) reflectance spectra. Neodymium (Nd) has among the most prominent absorption features of the REEs and thus represents a key pathfinder element for the REEs as a whole. Given that the world’s largest REE deposits are associated with carbonatites, we present spectral, petrographic, and geochemical data from a predominantly carbonatitic suite of rocks that we use to assess the feasibility of imaging REE deposits using remote sensing. Samples were selected to cover a wide range of extents and styles of REE mineralization, and encompass calcio-, ferro- and magnesio-carbonatites. REE ores from the Bayan Obo (China) and Mountain Pass (United States) mines, as well as REE-rich alkaline rocks from the Motzfeldt and Ilimaussaq intrusions in Greenland, were also included in the sample suite. The depth and area of Nd absorption features in spectra collected under laboratory conditions correlate positively with the Nd content of whole-rock samples. The wavelength of Nd absorption features is predominantly independent of sample lithology and mineralogy. Correlations are most reliable for the two absorption features centered at ~744 and ~802 nm that can be observed in samples containing as little as ~1,000 ppm Nd. By convolving laboratory spectra to the spectral response functions of a variety of remote sensing instruments we demonstrate that hyperspectral instruments with capabilities equivalent to the operational Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) and planned Environmental Mapping and Analysis Program (EnMAP) systems have the spectral resolutions necessary to detect Nd absorption features, especially in high-grade samples with economically relevant REE accumulations (Nd > 30,000 ppm). Adding synthetic noise to convolved spectra indicates that correlations between Nd absorption area and whole-rock Nd content only remain robust when spectra have signal-to-noise ratios in excess of ~250:1. Although atmospheric interferences are modest across the wavelength intervals relevant for Nd detection, most REE-rich outcrops are too small to be detectable using satellite-based platforms with >30-m spatial resolutions. However, our results indicate that Nd absorption features should be identifiable in high-quality, airborne, hyperspectral datasets collected at meter-scale spatial resolutions. Future deployment of hyperspectral instruments on unmanned aerial vehicles could enable REE grade to be mapped at the centimeter scale across whole deposits.
Highlights
Rare earth element deposits and reflectance spectroscopyCarbonatites, classified as rocks with >50% primary magmatic carbonate (Le Maître, 2002), and the alkaline igneous rocks with which they are often associated are the primary source of rare earth elements (REEs), and will remain so for the foreseeable future (Wall, 2014)
Nd absorption features, which are the strongest LREE absorption features at VNIR-SWIR wavelengths, have been identified in reflectance spectra collected from a suite of predominantly carbonatitic samples with variable REE contents
Absorption features centered at wavelengths of ~583, ~744, ~802, and ~871 nm were detected by laboratory reflectance spectroscopy in samples with Nd concentrations >1,000 ppm
Summary
Carbonatites, classified as rocks with >50% primary magmatic carbonate (Le Maître, 2002), and the alkaline igneous rocks with which they are often associated are the primary source of rare earth elements (REEs), and will remain so for the foreseeable future (Wall, 2014) With notable exceptions, such as the relatively extensive Bayan Obo and Mountain Pass deposits in China and the United States, respectively (Drew et al, 1990; Castor, 2008; Yang et al, 2011; Smith et al, 2015), carbonatite-hosted REE deposits are often small in areal extent. The high concentration of REEs in carbonatites makes them well suited for developing exploration and mapping strategies that exploit diagnostic features in the reflectance spectra of REE-rich materials (e.g., Rowan et al, 1986) In this contribution, we discuss the feasibility of imaging primarily carbonatite-hosted REE deposits by hyperspectral remote sensing at visible and near- to shortwave infrared.
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