The Dajing giant Cu–Sn polymetallic deposit is located in the Cu–Sn–Ag–Pb–Zn polymetallic belt of the southern Great Xing’an Range, NE China. Research on its ore genesis is of great significance for understanding Sn polymetallic mineralization in this region. In this study, pyrite, arsenopyrite, and sphalerite were analyzed by electron-microprobe analysis (EMPA) and in situ S–Pb isotope analysis. Previously published fluid-inclusion microthermometric and H–O isotope data were also incorporated to constrain fluid evolution and ore genesis. Both in situ S and Pb isotopic compositions fall within short ranges. The δ³⁴S values suggest a sulfur reservoir with possible magmatic contribution, whereas Pb isotopes indicate a mainly crustal Pb signature in an orogenic setting. Arsenopyrite records variations in As, S, Fe, and Co contents from core to rim. The Co-rich core shows Co enrichment accompanied by Fe depletion, consistent with Co-for-Fe isomorphous substitution. These features indicate changes in local fluid chemistry during arsenopyrite growth. Sulfur isotope geothermometry based on coexisting late-stage pyrite–sphalerite pairs yields 118–233 °C, with an average of 159 ± 49 °C, indicating medium- to low-temperature hydrothermal activity during the late sulfide stage. The Dajing deposit is interpreted as a fault-controlled hydrothermal vein-type Cu–Sn polymetallic deposit formed in a Late Jurassic extensional setting. Ore precipitation was likely promoted by cooling during upward fluid migration away from the magmatic heat source, pressure release, meteoric-water mixing, and fluid–rock interaction with granitic rocks and Linxi Formation wall rocks. This study provides mineral-scale constraints on fluid evolution and ore genesis in the Great Xing’an metallogenic belt.

The Dajing Cu-Sn polymetallic deposit, situated within the southern Great Xing’an Range of northern China, is a representative Mesozoic magmatic–hydrothermal system within a major Cu–Sn–Ag–Pb–Zn metallogenic belt. Located at the junction between the Siberian and North China plates, the deposit occurs within a composite tectonic domain overprinted by the Paleo-Asian, Mongol–Okhotsk, and Paleo-Pacific systems. Building on field geological constraints and detailed ore petrography, this study utilizes SEM–CL imaging and in situ LA–ICP–MS trace-element analysis of hydrothermal quartz to reconstruct the multistage physicochemical evolution and fluid dynamics of the ore-forming system. Three quartz generations record successive mineralization stages: early QI forms grain cores associated with Stage I cassiterite–arsenopyrite–quartz mineralization; main-stage QII crystallized during or shortly after Stage II chalcopyrite precipitation and exhibits well-developed oscillatory zoning; and late QIII occurs mainly as rim overgrowths and fracture fillings and postdates Stage III ore-mineral precipitation. Quartz is characteristically Ti-poor (4.7–22.8 ppm), and its trace-element systematics indicate a low- to intermediate-temperature hydrothermal signature and a granitic magmatic–hydrothermal affinity. Al, Li, Na, K, and Ge show coupled behavior consistent with heterovalent substitution and vary markedly among quartz generations, with CL-bright QIIa showing relatively higher Al, Ge, and Na relative to CL-dark QIIb, whereas QIII is generally characterized by lower Al, Li, and Ge. Localized anomalously high Cu and Sn values are mainly attributed to the co-ablation of fine-grained mineral inclusions, metal-rich fluid inclusions, or fracture-filling components. These features indicate a chemically heterogeneous fluid reservoir with inferred relatively acidic conditions during the initial Sn-mineralization stage. The fluid system subsequently experienced recurrent physicochemical fluctuations associated with pulsed fluid input during the principal Cu-mineralization stage. QIII records a trace element-depleted late-fluid system associated with the terminal pyrite–quartz stage. Late fluids migrated along earlier quartz boundaries and fractures and may have undergone further cooling and dilution through fluid mixing and/or water–rock interaction. Overall, the fluid system shows a progressive shift in inferred fluid chemistry, consistent with decreasing acidity during hydrothermal evolution.