On the feasibility of imaging carbonatite-hosted rare earth element deposits using remote sensing

• Main Authors: Neave, David A., Black, Martin, Riley, Teal R., Gibson, Sally A., Ferrier, Graham, Wall, Frances, Broom-Fendley, Sam
• Format: Article in Journal/Newspaper
• Language: English
• Published: Littleton, CO : Society of Economic Geologists, Inc 2016
• Subjects: Deposits; Exploratory geochemistry; Heterojunctions; Infrared imaging; Infrared spectrometers; Lithology; Minerals; Neodymium alloys; Photomapping; Rare earth elements; Rare earths; Remote sensing; Rocks; Signal to noise ratio; Thermography (imaging); Airborne visible infrared imaging spectrometer; Characteristic absorption; Environmental mapping; Hyperspectral instrument; Laboratory conditions; Rare earth elements (REEs); Remote sensing instruments; Spectral response functions; Neodymium; absorption; carbonatite; data acquisition; economic geography; feasibility study; igneous geochemistry; igneous intrusion; imaging method; mineral deposit; noise; rare earth element; reflectance; signal-to-noise ratio; spatial resolution; Arctic; Bayan Obo; California; China; Greenland; Mountain Pass; Nei Monggol; United States; ddc:550; Ferro; Obo
• Online Access: http://www.repo.uni-hannover.de/handle/123456789/1046; https://doi.org/10.15488/1022

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> 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. ...