The Beiya Fe-Au-polymetallic deposit lies in the alkali-rich porphyry polymetallic metallogenic belt of northwest Yunnan Province. There exist the contact metasomatic-hydrothermal alteration, ore types of metasomatite and filling veins in the host rocks, which have formed a mineralization system of porphyry. Within cracks and fracture zones of the host rocks or Fe orebody, there are lots of associated sulfide minerals such as siderites, gelenites, pyrites and chalcopyrites. The siderites at Ⅱ stage is obviously later than magnetites at Ⅰ stage, and they are the ideal minerals. A study of chalcography shows that the siderites can be classified into three types, i.e., magnetite-siderite (Sd1), pyrite-chalcopyrite-magnetite-siderite (Sd2) and gelenite-siderite (Sd3). EPMA and ICP-MS analysis of siderites show that, during the transition from Ⅰ-stage to Ⅱ-stage, the intermediate or weakly alkaline, high temperature and oxidation mineralization was converted to weak acid, low temperature and weak reduction, as marked by the generation of mushketovite. Meanwhile, there were a lot of polymetallic sulfides, with pyrites as the main Au-bearing minerals, and Au deposits were simultaneously developed. With the further decreasing temperature, the ore-forming process completely changed into acidity and strong reduction, with the deposition of gelenites and siderites. So all types of siderites and earlier magnetites had the same trace ore-metal source and ore-forming process, resulting from the evolution of ore-forming fluids at different stages and periods, and mineralization occurred at proper location with different ore-forming processes and decreasing temperature.