Genesis And Sedimentary Environment Of Se-Rich Black Rock Series Of Lower Cambrian In Ziyang,Shaanxi Province-Characteristics Of Organic Matter And Geochemical Evidence

Ziyang,Shaanxi Province,belongs to the southern Qinling region and is located in the northern foothills of Daba Mountain.Typical black rock series have appeared in the area,and selenium poisoning has occurred.Previous studies have found that the Lower Cambrian Lujiaping Formation not only has black rock series,but the stratum is generally rich in selenium.The predecessors have carried out a lot of research work on the black rock series in the study area in stratigraphy,geochemistry,mineralogy and paleontology.It can be affirmed that the element content and organic matter content of different stratums are obviously different,but what is the background of this difference?What is its formation mechanism?What are the characteristics of organic geochemistry?In this paper,based on field investigation,combined with regional geological background,through the study of petrography,element geochemistry,organic geochemistry and organic matter morphology analysis,the material source,enrichment rules,occurrence state and sedimentary environment of black rock series are discussed,finally establish the formation mechanism of black rock series.The Lower Cambrian black rock series exposed in research area is mainly composed of siliceous rock,carbonate rock,carbon?silicon??shale/slate?rock,stone coal,etc.The main minerals include barite,poisonous barite,Quartz,chlorite,pyrite,calcite,etc.,and volcanic tuff and basic rock are distributed near the black rock series.Calcite veins and quartz veins are common in the field and under the microscope,which all indicating heat liquid activity;a lot of black organic matter is distributed on the rock surface in the wild,and the organic matter is mainly saprophytic body and asphaltene body under the microscope.Massive asphaltene body can be seen in the mineral crevices,and scattered amorphous saprolite can be seen on the mineral surface plastid.The results of element geochemical analysis indicate that the black rock series are enriched with Ba,As,Sb,P,V and other elements,and the element Se content is positively correlated with the element Ba,P,V content.It is speculated that elemental enrichment is related to deep hydrothermal upwelling caused by syn-depositional faults;low total rare earth,light rare earth enrichment and other indicators indicate hydrothermal activity,Si O₂/Al₂O₃>3.6 indicates the mixed cause of hot water and biology,the average value of Mn O/Ti O₂close to or less than 0.5 indicates that the sedimentary water is shallow.The organic carbon content test results indicate that the abundance of organic matter content in the black rock series is high?average 2.95%?,which implies that the biological effect of the diagenesis and mineralization process is greater;the scanning electron microscope analysis results indicate that the organic matter form is mainly saprophytic body and asphaltene body of putrid group,suggesting that the source of organic matter is low-grade marine algae organisms;then analyze the relevant indicators of biomarker compounds(bimodal,insignificant odd-even advantage,low?C_21-/?C₂₂₊value,Pr/Ph<1),indicating that the source of organic matter is mainly low-grade marine algae with the addition of bacteria and terrestrial substances.In the later period,it is simultaneously degraded by heat and microorganisms,and the deposition environment is anoxic reduction environment.It is speculated that the two main influencing factors of black rock series are transgression and syngenetic faults.The black rock series in the research area are distributed near the NW-SE oriented deep and large fault zones and are consistent with the direction of the stratum.Syngenetic faults provided an ascending channel for the deep hydrothermal fluids rich in ore-forming elements,formed anoxic basins and carbonate platforms and changed regional structures.The upwelling caused by the transgression brought a lot of nutrients,and a large number of organisms develop in shallow water.After a large number of organisms died,the bottom water body was anoxic and reducing environment,which was conducive to element enrichment,a large number of ore-forming elements brought by the fault were enriched in the sediments at the bottom of the basin.Therefore,these specific elements are also enriched in organic-rich rocks;The organic matter in the sediment formed on the platform was poorly preserved and there was no large amount of hydrothermal elements to supplement.Eventually,the difference in element content and organic matter content in different lithologies of the black rock series was formed.The black rock series were formed in the effection of hot water and biology,deposited in the shallow-semi-deep water body on the continental margin of anoxic reduction.The sedimentary environments of different lithology are different,but closely related.