Shortwave infrared spectral characteristics of white mica in hydrothermal deposits and its application in geological exploration

This paper is the result of mineral exploration engineering.

Objective White mica is widely developed in the hydrothermal deposits, and the shortwave infrared spectrum of white mica contains rich geological information, recording the hydrothermal environment it formed and its relationship with mineralization. Analyzing the variation patterns and geological influencing factors to short wave infrared spectra of white mica in hydrothermal deposits not only provides important theoretical support for the in−depth exploration and application of hyperspectral mineral mapping technology, but also further improves the identification system for exploration of altered minerals.

Methods This paper systematically summarizes the characteristics of the short wave infrared spectral changes of white mica in hydrothermal deposits, and analyzes the relationship between the chemical composition and content of white mica, and geological factors such as hydrothermal fluid temperature, acid−base properties, pressure, and their shortwave infrared spectral characteristics.

Results The absorption characteristics of shortwave infrared spectra in white mica are closely related to the two main hydrogen containing groups Al−OH and H₂O in its structure, and the two main characteristic absorption peaks are located near 2200 nm (main absorption peak) and 2350 nm (secondary absorption peak), respectively. Since the Al−OH characteristic absorption−peak positions of white mica are influenced by geological factors such as hydrothermal fluid temperature, pressure, acidity and alkalinity, white mica mainly undergoes Tschermak substitution^VIAl³⁺ + ^IVAl³⁺ν^IVSi⁴⁺ +^VI (Fe²⁺, Mg²⁺, Mn²⁺), resulting in changes in the content of Si, Al, Fe, Mg, K, and Na plasma, which results in a shift in the Al−OH characteristic absorption−peak position varying between 2180 nm and 2230 nm. Geological factors, such as original rock components, other altered minerals, fluid components, rock permeability, and water/rock ratios, can also affect the changes in the wavelength of the Al−OH characteristic absorption peaks of white mica.

Conclusions The relationship between the Al−OH characteristic absorption peak wavelength of white mica and mineralization may exhibit a positive or negative correlation depending on the geological environment, therefore, when delineating the hydrothermal mineralization center, it is necessary to comprehensively consider the geological characteristics of the deposits. The illite maturity or crystallinity (ISM or SWIR−IC) of white mica is consistent in different hydrothermal deposit systems, that is, the maturity values of illite near the hydrothermal mineralization center are relatively large, while the maturity values of illite far away from the hydrothermal mineralization center are relatively small. In the future, in order to promote the in−depth geological application of satellite and airborne hyperspectral remote sensing mineral mapping technology, it should strengthen the fine and quantitative satellite and airborne hyperspectral remote sensing inversion researches on spectral characteristics of white mica, and establish a demonstration of hyperspectral remote sensing inversion application for mineralization hydrothermal characteristics of typical hydrothermal deposits.