Mineralogical And Petrological Features And Environmental Significance Of Pyrite For The Niutitang Formation Of The Cambrian In The Yangtze Area

In this article,the black shale of Yangtze block Niutitang Formation is taken as the study object.By using the microscopic observation,X-ray powder diffraction(XRD)and pyrite sulfur isotope methods on Niutitang Formation black shale samples,I have analyzed the mineralogy characteristics of the Yangtze block Niutitang Formation black shale,Identified and classified pyrites of different genesis pyrite in Niutitang Formation black shale,analyzed the indicative significance of syngenetic pyrite to sedimentary environment,and have obtained some knowledge.The mineral composition of the Lower Cambrian Niutitang Formation black shale in the Yangtze region is mainly composed of quartz-based siliceous minerals and illite-based clay minerals.The content of quartz based siliceous mineral is about 54.6%,clay mineral content is about 34.8%,TOC is about 0.04%~9.93%,with an average of 2.45%.In this paper,the observation of pyrite by optical microscopy and SEM field scanning electron microscopy revealed that the Lower Cambrian Niututang Formation black shale in the Yangtze area mainly contained syngenetic pyrite and diagenetic pyrite two kind.And only the syngenetic pyrite has the indicating significance of sedimentary environment to reservoir;The framboid pyrite in the black shale of the Niuyutang Formation in the Yangtze area is the syngenetic pyrite.The particle size is concentrated in the range of 4 ~ 6?m,with a median value of 5.15?m and a narrow range of variation,indicates that the Niutitang Formation black shale formed in the stagnation-hypoxia water environment;There are significant differences between syngenetic pyrite and diagenetic pyrite in crystalline forms,occurrences,sulfur isotope values and etc.The average sulfur isotope value(?34SV-CDT)of the syngenetic pyrite is obviously higher than diagenetic pyrite that influenced by fluid.The range of variation is mainly concentrated around 19-25 ‰,an average of 21.60 ‰,and all positive value greater than 17.4 ‰.Indicates the Niutitang Formation black shale formed in a closed marine water environment.