Depositional Genesis And Environment Of The Paleoproterozoic Astingbulake Iron Formations From The Quruqtagh Block,Northeast Tarim Craton

Banded iron formations（BIF）is the most important type of iron ore resources in the world.Its formation is closely related to the early geological and environmental evolution,and comprehensively records important information about the unique crust,atmosphere,ocean and biology of the Precambrian.BIF is a key object for understanding the early geological and environmental evolution of the Earth.Tarim is one of the three palaeocontinents in China.At present,there is little research on the early Precambrian BIF of Tarim craton,which restricts the understanding of the early evolution of Tarim.The Astinbulak iron ore deposit is located in the western section of the Quruqtagh block in the Tarim craton,and is one of the representative BIF type iron deposits discovered in Kuruktag in recent years.In this paper,the Astinbulak iron ore deposit is taken as the research object.On the basis of detailed field geological work,systematic sampling of iron ore outcrops,drill cores,host rock series and felsic dikes intruded into ore-body in the ore district is carried out.Through petrographic,geochemical,iron isotope and chronological studies,its formation age is determined,its genesis and formation environment are revealed,and constraints are provided for further understanding of the early Precambrian geology and environmental evolution of the Tarim craton.The Astingbulake iron ore deposit is hosted by the Shayiti Formation of Xingditage Group,and the mineral-hosting rock series is minely quartz schists,tremolite-biotite-quartz schists and quartzite.The protolith of the schists from Xingditage Group may be argillaceous rock and a few sandstones.The Cr/Th of samples the mineral-hosting rock series is 1.66～17.24,La _CN/Yb _CN is 4.80～33.06.Most samples have negative Eu anomalies,and Th/Sc,Th/Co,La/Sc ratios are greater than the average values of post Archean Australian average shale and upper crust,indicating that the source of sedimentary materials is mainly felsic rocks and a small amount of sedimentary rock.Most samples have negative Eu anomalies,and Th/Sc,Th/Co,La/Sc ratios are greater than the Post-Archean Australian shale and the average values of upper crust,indicating that the source of sedimentary materials is mainly felsic rocks and a small amount of sedimentary rock.The CIA value of the sample is 51～73,and the ICV value is 0.92～10.64.There is a clear positive correlation between Th/Sc and Zr/Sc in most schist samples of the Xingditage Group,indicating that they do not undergo significant sorting and sediment recycling.The Xingditage Group is composed of thick sediments and carbonate rocks,combined with the results reflected by its geochemical characteristics,this study suggest that it was deposited along a stable passive continental margin environment.The schist samples from the lower and upper plate of the ore body show a weighted average age of ²⁰⁷Pb/²³⁵U at 1920±6 Ma and 1902±17 Ma,respectively,indicating that its maximum sedimentary age is approximately 1.9Ga.The feldspathic veins invading the iron ore body show a diagenetic age of 1891±4.5Ma and1894±6.9Ma,indicating that the iron ore was formed before 1890 Ma.Therefore,the formation age of the Astinbulak iron ore deposit should be in the Paleoproterozoic（approximately 1.9 Ga）.The lithology of iron ore is magnetite quartzite,its ore mineral is mainly magnetite and gangue mineral is mainly quartz.The TFe₂O₃ content of magnetite quartzite is 15.50%～56.29%,and the Si O₂content is 38.67%～82.72%.All iron ores have a wide range of variations in Al₂O₃ content,as well as higher levels of Zr and Hf,indicating that iron ores may be contaminated by terrigenous debris during their formation process.But the values ofΣREE,Eu/Eu*,Sm/Yb,Pr/Pr*and Ce/Ce*from iron ore have poor correlation with the contents of Al₂O₃ and Ti O₂,indicating that clastic materials have no obvious control over the geochemical characteristics of trace elements.There is a lack of correlation between the values ofΣREE and TFe₂O₃ with Al₂O₃ of iron ore,and there is a positive Eu anomaly with Y/Ho=25.56～34.44.Combined with the results of its geochemical diagram,this study supported that the iron source of the ore is mainly a mixture of continental-sourced iron source with low hydrothermal source.All ore samples lack true negative Ce anomalies,and the variation range ofδ⁵⁶Fe value is-0.33‰to0.54‰,indicating that the iron ore deposit was formed in a low oxygen or reducing environment.Based on the geological and source characteristics of the Astinbulak iron deposit,its formation age,and the tectonic background of the formation of the mineral-hosting rock series,this study supported that the Astingbulake iron ore deposit belong to the Superior-type BIF,which was deposited by sedimentation in low oxygen seawater on the slope of the passive continental margin continental shelf.Based on previous research,this contribution supported that the northern part of the Tarim craton was in a stable passive continental margin environment during the Late Proterozoic.