Integration of short-wave infrared (SWIR) spectroscopy and magnetic susceptibility in exploration: Implications for the Eastern Au deposit, Songshu’nangou district, North Qilian orogenic belt, China

Accurate characterization of hydrothermal alteration zones is critical for ore targeting within porphyry-epithermal systems. Although short-wave infrared (SWIR) spectroscopy is effective for identifying hydrous alteration minerals, it fails to detect crucial anhydrous phases, such as K-feldspar within potassic zones. Magnetic susceptibility measurements offer a solution to this constraint as magnetite mostly precipitates during potassic alteration but can be destroyed during later alteration (e.g., phyllic, sericite-chlorite, and propylitic). Consequently, this study integrates SWIR spectroscopy with magnetic susceptibility data to reconstruct the alteration architecture of the Eastern Au deposit in the Songshu’nangou district, North Qilian orogenic belt. Chlorite, white mica, and other minerals were identified in ∼48%, ∼44%, and ∼5% of the analyzed samples from the Eastern Au deposit, respectively, where the abundant white mica shows a spatial association with the Au mineralization, reflected by crystallinity (SWIR-IC > 1.7) and wAlOH (Pos2200 > 2200 nm) values. The magnetic susceptibility ( K bulk ) values are highest in potassic alteration zones and lowest in phyllic alteration zones. Combined SWIR and magnetic susceptibility data delineate a previously unrecognized vertical alteration zonation with a deep potassic (average K bulk = 7.679 × 10 −3 SI units) zone transitioning upwards and outwards into propylitic, sericite-chlorite (strong and weak), and local phyllic alteration zones. This zonation, coupled with the relatively high K bulk values, spatially associated with the Au mineralization in the deep potassic alteration zone, provides compelling evidence for the presence of an underlying porphyry Au system beneath the known intermediate-sulfidation (IS) epithermal mineralization, indicating that the Eastern Au deposit likely represents a porphyry-epithermal Au system. Our results demonstrate that combined SWIR-IC and wAlOH values of white micas may serve as potential vectors in epithermal systems with magnetic susceptibility being a reliable vector for targeting deeper porphyry Au mineralization within potassic alteration zones. The identification of potassic alteration at depth reveals the exploration potential of the Eastern Au deposit, and provides useful guidance for exploring similar porphyry-epithermal Au systems in the North Qilian orogenic belt.