铜陵包村金(铜)矿床中胶状黄铁矿矿物学及成因解析

Mineralogy and genesis of colloform pyrite in Baocun Au(Cu) deposit,Tongling region,and its implication for Mesozoic polymetallic mineralization

黄铁矿是安徽铜陵包村金(铜)矿床中主要硫化物,对热液成因的显晶质黄铁矿已有大量研究,而胶状黄铁矿研究较少且成因存在争议。本文以粉晶X射线衍射、扫描电镜(SEM)、透射电镜(TEM)以及拉曼光谱(RS)为主要研究方法和手段,对包村金(铜)矿床中胶状黄铁矿的矿物组成和微结构进行研究。包村胶状黄铁矿主要由黄铁矿组成,含有白铁矿、菱铁矿、石英、含铁白云石、高岭石和有机质。黄铁矿主要以纳米-亚微米粒径的自形、半自形的立方体为主,少量微米级他形颗粒,显著不同于胶体或非晶态的无定型黄铁矿。由黄铁矿、白铁矿和有机质组成的胶状结构中,白铁矿和有机质富集在暗色环带,黄铁矿相对富集在浅色环带,浅色和暗色交替变化主要由三者含量变化所致,与矿物粒径无关。结合铜陵地区胶状黄铁矿研究成果,我们认为铜陵地区乃至长江中下游成矿带内铜-金-铁多金属硫化物矿床相关研究文献中记载的胶状黄铁矿为铁硫化物、碳酸盐矿物、黏土矿物、石英和有机质组成的矿物集合体,是在陆源物质输入受限的半封闭海盆环境下经生物化学作用直接沉淀的纳米-亚微米黄铁矿为主的矿物集合体。虽然胶状黄铁矿经历沉积成岩作用和中生代岩浆热液叠加改造作用,但是沉积微结构、矿物成因信息仍然被有效保存。

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 in the 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.