Abstract
Reflectance spectroscopy has been used to identify several deposit types. However, applications concerning lithium (Li)-pegmatites are still scarce. Reflectance spectroscopic studies complemented by microscopic and geochemical studies were employed in the Fregeneda–Almendra (Spain–Portugal) pegmatite field to analyze the spectral behavior of Li-minerals and field lithologies. The spectral similarity of the target class (Li-pegmatites) with other elements was also evaluated. Lepidolite was discriminated from other white micas and the remaining Li-minerals. No diagnostic feature of petalite and spodumene was identified, since their spectral curves are dominated by clays. Their presence was corroborated (by complementary techniques) in petalite relics and completely replaced crystals, although the clay-related absorption depths decrease with Li content. This implies that clays can be used as pathfinders only in areas where argillic alteration is not prevalent. All sampled lithologies present similar water and/or hydroxide features. The overall mineral assemblage is very distinct, with lepidolite, cookeite, and orthoclase exclusively identified in Li-pegmatite (being these minerals crucial targets for Li-pegmatite discrimination in real-life applications), while chlorite and biotite can occur in the remaining lithologies. Satellite data can be used to discriminate Li-pegmatites due to distinct reflectance magnitude and mineral assemblages, higher absorptions depths, and distinct Al–OH wavelength position. The potential use of multi- and hyperspectral data was evaluated; the main limitations and advantages were discussed. These new insights on the spectral behavior of Li-minerals and pegmatites may aid in new Li-pegmatite discoveries around the world.
Highlights
Li-bearing minerals distinguishable from each other and other minerals? (ii) How can the alteration products affect the spectral signature of Li-bearing minerals? (iii) Can Lipegmatites be spectrally discriminated from other lithologies in a supervised classification attempt? The results obtained are essential to better direct future remote sensing-based exploration studies
The main mineralogy inferred from the spectra is presented, but a detailed analysis spectrum per spectrum can be found in the Supplementary Materials (Tables S1–S3)
Shape, and symmetry of diagnostic absorption features, as well as the overall reflectance magnitude and absorption depths of a Li-dedicated spectral library, allowed the identification of the spectral mineralogy and characterization of samples of (i) different Li-bearing minerals, (ii) several field lithologies of the study area, and (iii) samples from areas previously misclassified as Li-pegmatite using a machine learning algorithm
Summary
Reflectance spectroscopy studies have shown a relevant economic potential to mineral exploration, either through the direct characterization and identification of the mineralogy of the deposits or through the identification of hydrothermal alteration zonation as a proxy to target mineralizations such as (i) porphyry copper deposits [1], (ii) volcanogenic massive sulfide (VMS) deposits [2,3], (iii) rare-earth element (REE) deposits [4,5], (iv) 4.0/). No. pegmatite (northwestern China), the pegmatite contact wall zone, and from the host rocks, which included several methods, including reflectance spectroscopy. These methods, coupled with mineral chemistry analyses, allowed to better understand the magmatic to hydrothermal evolution of the pegmatite and the fluid interaction with the pegmatite and the country rocks. Carrino et al [16] correlated an electron probe microanalysis (EPMA)
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