Airborne hyperspectral mapping and shortwave infrared spectroscopy of white mica from the Baiyanghe uranium district, Northwest China, with applications in exploration

Airborne hyperspectral and shortwave infrared (SWIR) spectroscopy technology has significant advantages and application prospects in the fine identification and mapping of alteration minerals. The white mica spectra contain rich geological information, including the hydrothermal environment in which white mica formed and its relationship with mineralization. How variations in the chemical components, temperature, and pH of hydrothermal fluid constrain the Al–OH wavelengths of white mica in meso-epithermal deposits is still debated. Moreover, the relationships between uranium mineralization and the spectral parameters of white mica in hydrothermal uranium deposits are poorly understood. In this study, we present shortwave infrared airborne spectral imager (SASI) hyperspectral data, SWIR FieldSpec4 hyperspectral data, petrography data, and geochemical data, including those from X-ray diffraction (XRD), electron probe microanalysis (EPMA), and hydrogen and oxygen isotopes, for white mica from the Baiyanghe uranium district. The SASI airborne hyperspectral alteration mineral mapping shows that the distribution of white mica exhibits distinct zoning characteristics. The SWIR and XRD analyses indicate that the alteration minerals are mainly quartz, chlorite, and illite, with Al–OH absorption wavelengths ranging from 2190 nm to 2220 nm. The geochemical compositions indicate an obvious Tschermak substitution in white mica and suggest that variations in the Al–OH wavelengths of white mica are affected mainly by the Fe and Mg contents and Fe/(Fe + Mg) ratios in addition to the wall rock composition and hydrothermal fluid. The presence of Al-rich (Si-poor) white mica with a shorter Al–OH wavelength at approximately 2200 nm indicates a relatively high-temperature and acidic hydrothermal environment. In contrast, the presence of Al-poor (Si-rich) white mica with a longer Al–OH wavelength at approximately 2200 nm suggests a relatively low-temperature and neutral hydrothermal environment. In addition, white micas close to uranium mineralization in the northern contact zone between the Yangzhuang granite porphyry and the Devonian intermediate–acid volcanic rocks of the Baiyanghe uranium district have shorter Al–OH wavelengths, greater illite spectral maturity index (ISM) values, and lower full width at half maximum at approximately 2200 nm (FWHM2200) values than those not associated with uranium mineralization. The spatial distributions and variations in the Al–OH wavelengths, ISM values, and FWHM2200 values for white mica suggest that a hydrothermal mineralization center may be located in the northern contact zone, indicating that white mica can be used as an important indicator of alteration minerals for uranium exploration in the Xuemisitan uranium metallogenic belt or other hydrothermal deposits.