Fluid Evolution of the Dajing Cu-Sn Polymetallic Deposit, Southern Great Xing’an Range: Constraints from Quartz Textures and Trace Elements

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.