Abstract:Pyrite is the predominant sulfide mineral in Baocun gold (copper) deposit in Tongling region. Much more attention have been paid to the coarse grained crystalline pyrite in hydrothermal origin compared with few investigation of the colloform pyrite with controversial precipitation processes. In order to understand response of colloform pyrite to forming environment. The mineralogy and microstructure of colloform pyrite from the Baocun gold (copper) deposit were studied. X-ray diffraction (XRD) results show that colloform pyrite mainly consists of pyrite, marcasite, siderite, quartz, ferrodolomite, kaolinite and organic matter. Scanning electron microscope (SEM) observations of colloform pyrite indicate pyrite is mainly comprised of cubic euhedral pyrite microcrystals within nanometer-submicron size, and round or irregular nanocrystals, a few micron-sized crystals, whose characteristics are significantly different from colloidal or amorphous state particles. Interestingly primary colloform textures preserved in the colloform pyrite ores mainly consist of pyrite, marcasite and organic matters and have an sequential development of the colloform layers that consists of bright layer and dark layer alternation. The content of organic matter and marcasite in the dark layer of the colloform textures is higher than that in the bright layers, however the size of pyrite and marcasite in the colloform layers is quite uniform, which reveal different solution condition during pyrite precipitation. Based on the mineralogical researches of colloform pyrite from other ore depsotis, it is suggested that greigite occurence in the related documents of stratabound copper-gold polymetallic sulfide deposits inthe Middle-Lower Yangtze River Metallogenic Belt should be colloform pyrite, which is a polycrystalline mineral aggregation consisting of iron sulfide, carbonate, clay minerals, quartz and organic matters. Mineralogy and microstructure characteristics indicate that the colloform pyrite is a direct precipitation induced from biogeochemical processes in the semi-closed stagnant ocean basin in which the input of terrestrial materials is verylimited. Although the ore underwent early diagenesis and superimposition of Mesozoic magmatic hydrothermal processes, the sedimentary microstructures and formation processes of minerals in the ore were still preserved well.