On the feasibility of imaging carbonatite-hosted rare earth element deposits using remote sensing
Creators: name: Neave, David A.
name: Black, Martin
name: Riley, Teal R.
name: Gibson, Sally A.
name: Ferrier, Graham
name: Wall, Frances
name: Broom-Fendley, Sam
Publisher: name: Littleton, CO : Society of Economic Geologists, Inc
Year: 2016
Subjects: subject: Deposits
subject: Exploratory geochemistry
subject: Heterojunctions
subject: Infrared imaging
subject: Infrared spectrometers
subject: Lithology
subject: Minerals
subject: Neodymium alloys
subject: Photomapping
subject: Rare earth elements
subject: Rare earths
subject: Remote sensing
subject: Rocks
subject: Signal to noise ratio
subject: Thermography (imaging)
subject: Airborne visible infrared imaging spectrometer
subject: Characteristic absorption
subject: Environmental mapping
subject: Hyperspectral instrument
subject: Laboratory conditions
subject: Rare earth elements (REEs)
subject: Remote sensing instruments
subject: Spectral response functions
subject: Neodymium
subject: absorption
subject: carbonatite
subject: data acquisition
subject: economic geography
subject: feasibility study
subject: igneous geochemistry
subject: igneous intrusion
subject: imaging method
subject: lithology
subject: mineral deposit
subject: neodymium
subject: noise
subject: rare earth element
subject: reflectance
subject: remote sensing
subject: signal-to-noise ratio
subject: spatial resolution
subject: Arctic
subject: Bayan Obo
subject: California
subject: China
subject: Greenland
subject: Mountain Pass
subject: Nei Monggol
subject: United States
subject: Dewey Decimal Classification::500 | Naturwissenschaften::550 | Geowissenschaften

description: Rare earth elements (REEs) generate characteristic absorption features in visible to shortwave infrared (VNIRSWIR) 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 Ilímaussaq 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&gt; 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&gt;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 hyperspec-tral instruments on unmanned aerial vehicles could enable REE grade to be mapped at the centimeter scale across whole deposits.