University of Hull logo

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

Ferrier, Graham; Black, Martin; Broom-Fendley, Sam; Gibson, Sally A.; Neave, David A.; Riley, Teal R.; Wall, Frances

Authors

Graham Ferrier

Martin Black

Sam Broom-Fendley

Sally A. Gibson

David A. Neave

Teal R. Riley

Frances Wall

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 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. 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.

Journal Article Type Article
Publication Date May 1, 2016
Journal Economic geology
Print ISSN 0361-0128
Electronic ISSN 1554-0774
Publisher Society of Economic Geologists
Peer Reviewed Peer Reviewed
Volume 111
Issue 3
Pages 641-665
DOI https://doi.org/10.2113/econgeo.111.3.641
Keywords Rare earth elements, Neodymium, Remote sensing, REE
Publisher URL http://econgeol.geoscienceworld.org/content/111/3/641.abstract
Copyright Statement This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.
Additional Information Copy of article first published in: Economic geology, 2016, v.111, issue 3.

Files



Downloadable Citations