Integrating trace element chemistry, and sulfur isotope systematics of alunite in the Zhule (Xizang), central Lhasa terrane: implications for porphyry-high sulfidation epithermal Cu-Au exploration targeting

• Alunite SWIR-geochemistry reveals Zhule’s magmatic-hydrothermal centers. • Sr-Ca-La-δ 34 S vectors delineate Cu-Au targets, As-Pb halos mark central area. • Two-stage mineralization and pyrophyllite confirm potential porphyry. The Zhule high-sulfidation epithermal system (central Lhasa terrane, Xizang) offers critical insights into magmatic-hydrothermal fluid evolution and exploration targeting in potential concealed porphyry-epithermal systems. This study integrates trace element geochemistry, short-wave infrared (SWIR) spectroscopy, and sulfur isotope analyses of alunite to delineate vertical and lateral zonation patterns as vectors toward intrusive centers and Cu-Au mineralization. Two distinct alunite types are recognized: (1) magmatic-hydrothermal alunite (Alu1–Alu4) occurring as nodules and veins with elevated Cu, P, Y, Ca, V, and Eu associated with pyrophyllite-kaolinite alteration; and (2) magmatic-steam alunite (Alu5) in curved veinlets enriched in Rb and coexists with vuggy quartz. SWIR spectral analysis of 80 drill-core samples reveals that alunite absorption peaks shift to longer wavelengths (from ∼ 1,478 to 1,481 nm) toward the hydrothermal center, correlating with higher Na/(Na + K) ratios (0.07 to 0.23). Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) geochemical profiles show that Sr, Ca, and La concentrations increase with depth (e.g., Sr from 2,654 to 11,875 ppm) accompanied by heavier sulfur isotope ratios (δ 34 S from + 16.6 ‰ to + 20.6 ‰), consistent with proximity to a magmatic fluid source. A broad δ 34 S range (−9.8 ‰ to + 20.6 ‰) indicates a dominant magmatic sulfur contribution. Alunite–pyrite isotopic thermometry defines two mineralization stages: Stage I-1 (289–425 °C, pyrite-digenite-enargite-covellite) reflecting high-temperature (>300 °C) SO 2 disproportionation in magmatic vapor; and Stage I-2 (213–293 °C, pyrite-sphalerite-galena) reflecting cooler conditions with meteoric water incursion. The δ 34 S values become progressively heavier from early digenite (–9.8 ‰) to late-stage pyrite (+7.6 ‰), aligning with equilibrium fractionation trends and indicating progressive oxidation of the hydrothermal fluid. Proximal quartz-Na-alunite alteration with Cu-Au-As anomalies grades outward to pyrophyllite-diaspore assemblages with Pb-Zn enrichment, mirroring lithocap architectures at Lepanto (Philippines) and El Indio (Chile). This study advances mineral exploration strategies by integrating SWIR spectroscopy, sulfur isotopes, and geochemical zoning to pinpoint magmatic-hydrothermal centers. The following fact: (1) depth-correlated Sr-Ca-La enrichment in alunite (δ 34 S > +16 ‰), (2) SWIR absorption wavelengths > 1480 nm in alunite, (3) lateral As-Pb geochemical halos, and (4) high-temperature (>350 °C) pyrophyllite-diaspore assemblages indicate a concealed porphyry intrusion at ∼ 1 km depth. The multi-method approach confirms Na-alunite zones and Cu-rich pyrite clusters as reliable mineralization markers, positioning Zhule as a priority target with vertically stacked high-sulfidation epithermal and underlying porphyry Cu-Au mineralization.