Determination of sulfur isotope of sulfides and hydrogen isotope of fluid inclus ions at the Dajing CuSn polymetallic ore deposit confirms that meteoric water was the dominant fluid at its oreforming stage, but sulfur was mostly derived from a deep magmatic fluid. Magmatic fluid, in fact, plays an important role in the formation of hydrothermal vein deposits. The magmatic fl uid enriched in sulfur and other volatile components may carry many oreforming metallic elements in the mag ma, such as Cu and Sn. The mixture of magmatic fluid with meteoric water is the main reason for successive depositions of oreforming metals such as Sn, C u and Zn.