Objectives: As a representative stratiform iron sulfide deposit in Tongling metallization cluster in the Middle—Lower Yangtze Metallogenic Belt, the mineralization and mechanism of the Xinqiao iron sulfide deposit remians a topic of debate. The Colloform pyrite ore is an important constituents of the stratabound sulfide orebodies in the Xinqiao deposit, and its formation and genesis are still controversial which on the contrary limits identification of the stratabound sulfide orebody. Methods:The microstructure characteristics of colloform pyrite—siderite ore in Xinqiao deposit were carried out using powder Xray diffraction (XRD), optical microscope, scanning electron microscopy (SEM) and highresolution transmission electron microscopy (HRTEM) to investigate mineral compositions, morphology, further discuss genesis and mechanism of the iron sulfide deposit. Results:Pyrite and siderite (74.3% and 24.4% in the total mass, respectively) are the two dominant minerals with rare quartz and illite (less than 5% in the total mass) in the ore. The grain sizes of pyrite range from several nanometer to hundreds of nanometers and the granular pyrites are dominated by euhedral—subhedral cubic morphology with minor spheroid and irregular microcrystals. While siderite is presented as euhedral rhombohedron with a size of 20 μm ~30 μm. In the ore xenomorphic clasts quartz and illite were discriminated by TEM. Optical microscopic observation indicated that pyrite and siderite formed mosaic texture. It is confirmed that organic matter fragments were also residual in this ore by the analysis of FESEM、HRTEM and EDS. Conclusions: The colloform pyrite—siderite ore in the Xinqiao iron sulfide deposit was direct precipitation through biogeochemistry mechanism in the subclosed marine basin instead of magmatic hydrothermal process. This colloform pyrite layer may also act as geochemical barrier or precipitant for the Mesozoic copper mineralization.