Four Typical Variation Patterns of Mid-Infrared Spectra of the Felsic Mineral Anomalies for Fault Zone Identification

Mid-infrared (IR) spectroscopy has the advantage of being fast and nondestructive for mineral identification and geological analysis. Mineral anomalies in fault zones are geological markers for fault identification. However, fault rock spectra acquired by remote sensing techniques are often a mixture of multiple endmembers. The mid-IR spectral variability of mineral anomaly patterns in fault zones is still unclear, causing difficulties in identifying faults by analyzing mineral anomaly characteristics through mid-IR spectroscopy. From the perspective of remote detection of faults, we constructed 14 groups of binary, ternary, and quaternary mineral assemblages of the felsic anomaly pattern in fault zones, tested the mid-IR spectra, analyzed the influences of mineral component and content changes on spectral characteristics, and summarized four categories of the variation patterns of the mid-IR spectra of mixed minerals: self-stabilization (SS), superimposed interference (SI), camouflage enhancement (CE), and annihilation (AH). Furthermore, we proposed the spectral identification criteria for anomalous mineral assemblages and clarified the identification characteristics and detection limits of endmember minerals in mixed minerals. Besides, we discussed the nonlinear mixing effect and quantitative potential of the mixed spectra. The results show that the variation effect weakens the linear relationship between spectral peak intensity and mineral abundance in the mixed spectrum, making it difficult to calibrate the mineral content by the reflection peak intensity. Our new finding of the spectral reflection peak shift has a strong linear correlation with changes in mineral content, which provides new ideas for the quantitative interpretation of mid-IR spectra of mixed minerals in remote sensing applications.