The Study On The Genesis And Forming Environment Of Pyrite In Lucaogou Formation In Jimusal Sag,Permian,Xin Jiang

The Permian Lucaogou Formation?P₂l?in Jimusar Sag,Junggar Basin is an important tight oil reservoir.We found that the eastern part of the Junggar Basin is an intracontinental depression-rift formed during the Late Carboniferous to Middle Permian extensional background.A unique exhalative hydrothermal deposition environment is formed within the depression where the deep hydrothermal fluid reached the surface through the fractures and participated in sedimentary process.Pyrite is widely developed in this type of rock,and there are various morphological fabric types.In this paper,a systematic study of pyrite in exhalative hydrothermal sedimentary rocks is conducted through the study of the structure of pyrite,the analysis of macro and trace elements,the characterization of the particle size type,the petrological and mineralogical studies,the analysis of trace elements,and the analysis of carbon,oxygen,and thorium isotopes,etc.We obtained the following conclusions and understandings:?1?The rock formations?exhalative hydrothermal sedimentary rock?where the pyrite were formed were influenced by deep-seated magmatic and hydrothermal activities:are mainly formed with tuffaceous dolomite,and sedimentary tuff.Mainly,this type of rock generally develops laminar horizontal stratification,hot water internal clastic structure,overlying bedding,sutures,and biogenetic structures;the main minerals in this type of rock are affected by the magmatic-geothermal fluid,and volcanic activities.?2?The origin of pyrite in the Lucaogou Formation,Jimusar Sag is classified into exhalative hydrothermal sedimentary type and normal deposition type,and the exhalative hydrothermal sedimentary type pyrite is classified into endogenous hydrothermal type and hot water hydrothermal type.Endogenous hydrothermal pyrite shows layered structure,lumpy structure,brecciated structure,disseminated structure,patchy structure,and reticulate vein structure.The main III structures are column and pseudo-confidential strawberry shape like.The distribution of Co/Ni ratio of pyrite is relatively large,which showed affection by volcanism.The average value of Co/Ni ratio is the largest,and the S/Fe ratio is high,indicating that the characteristics of the mantle-derived hydrothermal fluid are relatively deep.Hot water hydrothermal pyrite shows veined structure,banded structure,which are mainly lath-like and colloidal pyrite.The Co/Ni ratio of this type of pyrite is centered,with little volcanic effect,and S/Fe is high,showing interbedded j endogenous hydrothermal type and normal type.The characteristics between pyrite.Sedimentary pyrite are mainly granular and late-mature strawberry shape.This pyrite has the lowest Co/Ni ratio and is in the range of sedimentary origin;S/Fe is high which shows the shallow degree the hot fluid influence,and it is closely related to the symbiosis of organisms,showing typical sedimentary characteristics.?3?The depositional environment of the Permian Lucaogou Formation in Jimusar Sag is a stagnant-anoxic saline lake with relatively weak water layering.During the deposition,the hydrothermal eruption of lake-derived mantle source is frequent,and it is the lake basin,which bring deep source hydrothermal fluids to make lake basins with high background values of heat flow,and magmatic-hydrothermal fluids will intermittently participate in sedimentary processes,making lake basins turbulent and invading fresh water,with obvious periodic hydrothermal activity effects..