Abstract:  The genetic mechanism of Chang’an gold deposit in the southern section of  Ailaoshan metallogenic belt in Southwest China remains to be restricted. To understand the source of ore-forming materials and fluids, and to illustrate its genetic type of deposit could provide a basis for the mine prospecting and reserve increasing. In this paper, the source and evolution of metallogenic materials and fluids in Chang’an gold deposit are studied based on detailed geological features, ore H-O-S-Pb isotope analysis and temperature measurement of fluid inclusions, and then the genesis of the deposit is restricted and the metallogenic model is established. The results show that the near-SN-trending F₆ considered by predecessors is not a fault, but a cryptoexplosive breccia tube, which is the main ore hosting structure of Chang’an gold deposit. In the main metallogenic stage, the freezing point temperature of the inclusions is mostly -2.9--0.7 ℃, the corresponding salinity (w (NaCl)) is 1.22%-4.79%, and the homogenization temperature is 162-226 ℃, which belongs to the low temperature and low salinity fluid system. The δ¹⁸O_H2O of quartz in the main metallogenic stage is between 4.4‰ and 5.2‰, and the δD is -93.9‰--85.9‰, which falls between the evolution line of magmatic water and atmospheric water, indicating that the ore-forming fluid is a mixture of magmatic fluid and atmospheric water. The averageδ³⁴S of pyrite in Chang’an gold deposit is 2.1‰ (n = 32), most of which are between 0.0 and 3.6‰, with obvious characteristics of magmatic sulfur suggesting that S comes from magmatic rocks. The lead isotopic composition of pyrite has obvious duality, reflecting the dual contribution of magmatic and upper crustal surrounding rocks. Therefore, it is considered that the ore-forming materials and fluids are mainly derived from magma, and the later atmospheric water and surrounding rocks also contribute to the metallogenic fluids. Chang’an gold deposit, which has similar characteristics to low-sulfidation epithermal gold deposits, is dominated by cryptoexplosive breccias, although its surrounding rocks are not traditional volcanic rocks.

Key words: Ailaoshan metallogenic belt, Chang’an gold deposit, fluid inclusion, H-O-S-Pb isotope, metallogenic model