Tracing hydrothermal fluid evolution of the Tongshankou skarn–porphyry Cu–Mo–W deposit (Eastern China) using integrated infrared spectroscopic characteristics and magnetic susceptibility

The Middle–Lower Yangtze River Metallogenic Belt (MLYRB) is one of the dominant skarn–porphyry Cu–Au metallogenic belts in Eastern China. Within this belt, skarn–porphyry Cu–Au deposits show a close genetic linkage to Late Mesozoic high‑K calc‑alkaline granitoids, and are associated with variable degrees of critical metals mineralization (i.e., Co, Te, Se and Re). Previous studies have generally suggested that these granitoids possess limited W metallogenic potential; nevertheless, several concealed medium‑to‑large W–Cu–Mo deposits have been discovered in their peripheral areas in recent years. At present, the hydrothermal fluid evolution of these W‑rich skarn–porphyry Cu polymetallic deposits remains poorly constrained, and their deep exploration indicators still need further optimization. This study focuses on the Tongshankou skarn–porphyry Cu–Mo–W deposit in the westernmost segment of the MLYRB. The deposit hosts contained metal resources of 0.55 Mt Cu, 0.01 Mt Mo, and 0.012 Mt WO 3 , with ore grades of 0.86 wt%, 0.10 wt%, and 0.19 wt%, respectively. By combining detailed petrographic observations with short-wave infrared (SWIR) and thermal infrared (TIR) spectral alteration mapping, this study pinpoints the precise ore-forming stages and delineates detailed alteration zoning features in the Tongshankou Cu–Mo–W deposit. Building on SWIR and TIR spectral parameters, alongside volumetric bulk magnetic susceptibility ( K bulk ) measurements, we propose four mineral exploration indicators for this deposit: (i) the widespread development of kaolinite, dickite, skarn, and retrograde alteration mineral assemblages; (ii) Fe-rich chlorite, identifiable by its characteristic absorption wavelength with Pos2250 > 2250 nm; (iii) white mica–montmorillonite, distinguished by a diagnostic absorption wavelength with Pos2200 < 2205 nm; and (iv) high magnetic susceptibility anomalies ( K bulk > 1.0 × 10 -3 SI). Notably, the presence of kaolinite and dickite along the intrusive contact zones between ore-forming porphyries and carbonate wall rocks provides direct evidence for acidic fluid activity during the Cu–Mo–W mineralization process. The development of acid fluid conditions in the system is tightly linked to hydrogen ion release during the precipitation of iron oxides and sulfide minerals. Collectively, these results reveal that hydrothermal fluids associated with Cu–Mo–W mineralization in the Tongshankou deposit evolved from alkaline to acidic throughout the mineralization process. Combined with previous research advances, the present findings further suggest that acidic hydrothermal fluids likely represent a ubiquitous component of skarn-hosted Cu–Mo–W mineralization systems. Clay indicator minerals, including kaolinite and dickite, provide robust constraints on the hydrothermal fluid evolution of Tongshankou deposit and serve as newly recognized, effective exploration proxies for analogous skarn–porphyry Cu–Mo–W deposits worldwide. Significantly, this study demonstrates that the integrated application of SWIR and TIR spectroscopic analyses combined with magnetic susceptibility measurements provides a rigorous and reliable approach for identifying concealed skarn–porphyry ore bodies across the MLYRB, Eastern China.