Characteristics Of Natural Bitumen And Metallogenic Relationship In Mayuan Nanmushu Pb-Zn Deposit,Shaanxi

Mayuan Nanmushu area lead-zinc deposit is one of the most important lead-zinc deposit located in the south of the northern margin of the Sichuan Basin dome structure belt.Concentrate formation of lead zinc ore as the Upper Sinian Dengying formation lithology layer third dolomite breccia,in which a large number of natural bitumen and lead-zinc ore are symbiosis/codependents,orebodies are controlled by Dengying formation,and occurs as stratified or stratoid bed,and mineral composition is simple.There're high quality hydrocarbon source rock and source reservoir cap rock assemblage in this study area,which had developed ancient reservoir Micangshan.Zhujia river area is close to Micangshan ancient reservoir in eastern oil aqueous,destroyedby the late tectonic movement damage,the ancient fossil oil reservoir only remained natural bitumen.The field and sample observation found natural bitumen in Dengying brecciform dolomite and crust of weathering,early bitumen is vein and veinlet,advanced bitumen is ball shape,droplet shape,massive,irregular shape.There're no direct contacts between the barite and sphalerite inclusions containing organic origin CH4,indicating that the organic matter and zinc Mineralization is closely related.In the study area,the source rocks of Guojiaba formation and Dengying formation are the saprophy-type organic matter with high organic matter evolution,the average organic carbon TOC is higher than 3.0%in Guojiaba formation,and the average TOC of Dengying formation is 0.19%.Combined with the evaluation criteria of marine source rocks,the light shadows are non-hydrocarbon source rocks.Therefore,the natural bitumen source of Nanmushu lead-zinc deposit is the lower cambrian Guojiaba formation.The reflectivity Rb of different natural bitumen is between 2.87%and 4.73%,the vitrinite reflectance(Ro)is more than 2.0%,and the organic matter is over mature stage.The characteristics of n-alkanes in natural bitumens and source rocks indicate that the organic matter is derived from low marine plankton,bacteria and algae.The sedimentary environment is a reducing sedimentary environment with a certain salinity and undergoes thermal degradation and a certain degree of biodegradation.The characteristics of the terpane compounds show that the organic matter and the source rocks are characterized by a certain salinity environment.The characteristics of the steroid compounds indicate that the organic matter is derived from the mixed sources of the precambrian low phytoplankton(especially blue and green algae and brown algae).The trace elements and rare earth elements of natural bitumen and carbonaceous slate are very similar in the enrichment or loss degree of each element and the trend characteristics of the distribution curve.The average isotope of natural bitumen is-33.4‰,the sulfur isotope average value is 20.85‰,which is similar to that of lead-zinc ores.Based on the comparison of organic geochemistry,family composition,biomarker compounds characteristics,rare earth trace elements and kerosene isotopic characteristics of natural bitumen source rock,the results showed that natural bitumen and lower cambrian Guojiaba formation carbonaceous slate is more similar,and organic matter from the lower cambrian Guojiaba formation carbon slate.In the material source,the source of lead-zinc mineralization and the source of hydrocarbon accumulation are the formation of the lower cambrian Guojiaba formation,and the Guojiaba formation is the "dual source layer".In the time relation,the main ore-forming time of lead-zinc deposit is between the late cambrian and the early ordovician(486.7±3.1Ma),and the lead-zinc mineralization may only be related to the early palaeozoic reservoirs.The Guojiaba formation is a "dual source layer" rich in organic matter and mineralization elements.During the process of continuous burial,the organic matter reaches the hydrocarbon generation threshold to start hydrocarbon generation and hydrocarbon expulsion process.In the initial migration process of oil-gas extraction of a large number of Pb,Zn mineral elements,due to the characteristics of their own secondary migration to the reservoir to form the first phase of ancient reservoirs,ore-containing fluid at the same time or has been moving,followed by a strong rise in activities,the formation temperature,pressure increases,the heat from the ore-containing fluids causes the hydrocarbon to undergo a brief pyrolysis and TSR reaction,reductive H2S precipitates Pb and Zn complexes to form lead and zinc.