Indicator mineral characteristics of potassic alteration zones in porphyry copper deposits based on infrared spectroscopy technology: A case study of the Qulong porphyry copper deposit, Tibet

• The crystallinity of gypsum(1940 nm/ 1900 nm) plays a crucial role in distinguishing between different types of gypsum. • Type II hydrous gypsum and anhydrite are closely related to mineralization processes in geological systems. • The variation in the wavelength of K-feldspar is positively correlated with Cu and Mo mineralization. • Rb and Sr elements can significantly influence the wavelength shift of K-feldspar in spectral analysis. The Qulong deposit, located in the Gangdese metallogenic belt of Tibet, is a giant porphyry copper deposit formed in a continental collision environment. This study applies shortwave infrared (SWIR) and thermal infrared (TIR) spectroscopy to analyze nine drill cores, including ZK801-1, ZK802, and ZK821, resulting in a refined alteration mineral mapping of the deposit. The key findings are as follows: (1) Gypsum Classification: Using the ratio of the characteristic absorption peak of gypsum at 1940 nm to the peak associated with mineral crystallization water at 1900 nm (1940 nm/ 1900 nm), different types of gypsum were classified. Through spatial cluster analysis, it was determined that: A ratio near 0 indicates anhydrite; Ratios between 0 and 30 % represent Type I hydrous gypsum; Ratios from 30 % to 60 % indicate Type II hydrous gypsum; Ratios ≥ 60 % correspond to Type III hydrous gypsum; There is a spatial correlation between anhydrite, Type II hydrous gypsum, and Cu-Mo mineralization. (2) Feldspar Variations and Mineralization: K-feldspar and oligoclase are primarily found at greater depths, while albite is more abundant at shallower levels. The wavelength variations of different feldspars are closely linked to Cu and Mo mineralization. Rb and Cs elements shift the K-feldspar wavelength towards the long-wave direction, while Sr and Ba shift it towards the short-wave direction. Among these, Rb and Sr have the most significant influence on the wavelength shifts. This study fills the gap in the research on indicator minerals of potassic alteration zones in porphyry systems.