Magmatic volatiles and seawater contributions to the formation of the Huangtupo VMS deposit, Eastern Tianshan, NW China

• Sulfur in Huangtupo pyrite reflects mixed seawater and magmatic sources. • Magmatic volatiles drove Cu–Zn enrichment during massive ore formation. • Oxidizing seawater fluids dominated late vein-type sulfide mineralization. The Huangtupo Cu-Zn deposit in the Eastern Tianshan, NW China, is a Paleozoic arc-related volcanogenic massive sulfide (VMS) system characterized by well-developed vertical and lateral metal zonation. Trace-element and sulfur isotope (δ 34 S) variations in pyrite record temporal changes in sulfur sources, metal transport, and hydrothermal fluid evolution during formation of the Huangtupo VMS system. Pre-ore massive pyrite (Py1) shows dominantly positive δ 34 S values (+0.75‰ to +4.72‰) and relatively low median trace-metal contents, indicating precipitation from seawater-dominated hydrothermal fluids with minor magmatic overprinting. Syn-ore pyrite from massive and brecciated Cu-Zn ores (Py2a–Py2b) represents the principal ore-forming stage and is enriched in magmatic volatile–related metals (Cu, As, Se, Sb, Au, Tl, and Bi) together with the most negative δ 34 S values (down to −3.49‰), reflecting strong magmatic volatile input and sulfur derived from SO 2 disproportionation, with subordinate seawater mixing. Syn-ore pyrite from silicified and barite-bearing disseminated ores (Py2c–Py2d) displays lower volatile-metal contents and uniformly positive δ 34 S values (+0.53‰ to +1.92‰), indicating waning magmatic influence and increasing control of seawater-derived fluids. Late epigenetic vein-type pyrite (Py3) is metal-poor, has the highest δ 34 S values (+2.62‰ to +4.57‰), and exhibits anomalously high Co/Ni ratios (17.56–165.04) together with very low As contents. These features indicate precipitation from cooler, seawater-dominated and more oxidizing fluids, in which oxygen fugacity exerted a stronger control than temperature on trace-element partitioning, marking a waning and noneconomic stage of the hydrothermal system. Overall, pyrite trace-element and sulfur isotope systematics document a progressive evolution from early seawater-dominated mineralization, through a magmatic volatile–driven main ore stage, to late oxidizing seawater overprinting, highlighting the critical role of magmatic degassing and seawater mixing in the formation of arc-related VMS systems.