A hyperspectral study (V-NIR-SWIR) of the Nechalacho REE-Nb-Zr deposit, Canada

This paper reports the results of a visible, near-infrared and short wave infrared (V-NIR-SWIR) spectroscopic study of the Nechalacho rare earth element (REE)-Nb-Zr deposit (Canada), associated rare metal pegmatites and select samples from other REE deposits. The zoning of minerals produced by hydrothermal alteration is traced using V-NIR-SWIR spectroscopic measurements in combination with geological, geochemical and mineralogical data. The minerals detected using V-NIR-SWIR spectroscopy (analcime, ankerite, biotite, chlorite, illite, REE-fluorocarbonates) display complex spatial distributions that were strongly controlled by the pathways and composition of multiple stages of hydrothermal fluids as well as the precursor lithology. Magmatic-hydrothermal analcime ± amphibole alteration was overprinted by regional biotite + magnetite + illite + ankerite + chlorite + REE-fluorocarbonate replacement. A qualitative mass change analysis indicates that the latter alteration (main-stage) was associated with significant gains in Mg and losses of Na at temperatures of ~300 °C, as indicated by the presence of phengitic to muscovitic illite. Low-temperature ankerite ± chlorite alteration was mainly fluid-controlled and associated with the formation of hydrothermal REE-fluorocarbonate minerals that crystallized as a result of the interaction of aqueous-carbonic hydrothermal fluids with eudialyte syenite protoliths, locally leading to leaching and re-deposition of light REE. The ankerite ± chlorite alteration also affected pegmatites peripheral to the Nechalacho intrusion and surrounding hydrothermally altered zones. Analysis of a variety of REE-mineral specimens and geochemically characterized bulk rock samples showed that the relative depths of the 741 and 864 nm NIR absorption features of Nd can be used as qualitative and quantitative proxies for the detection of the REE. In combination with fast measurement and processing capabilities, this allows field-based V-NIR-SWIR spectroscopy to be deployed as a powerful tool for regional and deposit-scale REE exploration, deposit characterization and mine production grade control.