Alteration mineralogy of the Xiajiadian Au deposit in southern Qinling, China: application of Short Wavelength Infra-Red (SWIR) spectroscopy for vein-type Au exploration

• Hydrothermal alteration minerals in Xiajiadian vein-type Au deposit are phengite and kaolinite. • Au grades of the mining tunnel sections show positive correlations with SWIR-IC values and negative correlations with Kx values. • SWIR technique applies to the exploration of vein-type Au deposits by identifying hydrothermal sources. To assess the applicability of Short Wavelength Infra-Red (SWIR) spectroscopy for the exploration of vein-type Au deposits, this study conducted SWIR analysis on the Xiajiadian Au deposit in the southern Qinling area, which contains over 50 tonnes of Au with an average grade of 4.98 g/t Au. The SWIR analyses revealed that the Xiajiadian Au deposit consists of a large spectrum of hydrothermal alteration minerals, including dickite, kaolinite, phengite, calcite, dolomite, ankerite, siderite, and gypsum. Among them, phengite and kaolinite are the most significant hydrothermal alteration minerals, accounting for 37.65 % and 21.79 % of the total mineral assemblages, respectively. The hydrothermal fluid-generated phengite is genetically linked to Au mineralization and is characterized by large grains and flat shapes in morphology, distinguished from regional metamorphism-generated sericite with relatively small grains and twisty shapes. The SWIR-IC values of hydrothermal fluid-generated phengite and the Kx values of kaolinite are temperature-dependent, exhibiting positive and negative correlations with Au grade, respectively. Additionally, specific parameters (i.e., SWIR-IC values exceeding 0.85, Kx values ranging from −0.0025 to 0.0025, and wAlOH between 2208 nm and 2214 nm) serve as indicators of Au mineralization at the Xiajiadian deposit. We propose that the hydrothermal/mineralization center at the Xiajiadian deposit is below an elevation of 1160 m. This study demonstrates that detailed SWIR-based analyses of hydrothermal alteration minerals could be an effective approach for the exploration of vein-type hydrothermal Au deposits.