Genetic Analysis Of Iron Oolitic Deposition In The Middle Devonian Yangmaba Formation In Ganxi Prolife,Beichuan County,Longmenshan Area

The oolitic are diverse in the Middle Devonian Yangmaba Formation in Ganxi Prolife,Beichuan,Longmenshan Area.Among them,iron oolitic is the most representative.It's a typical"Ningxiang"oolitic.Based on the data analysis of carbonate sedimentology and geochemistry theory,this paper studies the sedimentary genesis of iron oolites by selecting the Shigouli section of Ganxi Prolife,Beichuan as the research object.The sedimentary genesis of iron oolitic was studied by thin section identification and geochemical data analysis.It can provide a basis for the development and utilization of this type of iron ore.The result of research shows that the iron is mainly concentrated in bioclastic limestone,oolitic bioclastic limestone and bioclastic oolitic limestone.And they are interbedded with silty mudstone or mudstone in a lamellar or lenticular manner in the Middle Devonian Yangmaba Formation in Ganxi Prolife,Beichuan,Longmenshan Area.There are many types of oolites.According to the composition of the oolites containing iron,they are divided into four types:plaster oolite,oolites of ferrization,iron oolitic and chlorite oolite.According to the analysis of the geochemical data about the samples,Iron oolitic is mainly composed of Fe,Ca and O.The Si O₂/Al₂O₃ratio of all samples was less than10,indicating that the ore-bearing rock series in this area was related to marine deposition.The Ni content of all samples was much higher than crustal abundance that under the microelement partition model.The average content of As was 18.6 ppm and similar to normal seawater.U/Th values were all less than 1,which was judged as normal seawater deposition.After the standardization of North American shale,the average?Eu value was 0.945,presenting a negative anomaly.The average?Ce value was 1.031,presenting a slightly positive anomaly.The average value of La_n/Yb_nwas0.317 and La_n/Ce_nwas 0.85.It shows that it has the characteristics of continental margin environment.The above analysis indicates that the iron in the Iron oolitic in this area is derived from the continental chemical weathering and deposited in the continental marginal environment with the surface water flowing into the offshore area after weathering.According to the comprehensive regional sedimentary environment.Iron oolitic is the non-barrier coast facies.From the shoreface to the foreshore of the coast without barrier,the main oolites formed in sequence are thin-crust chamosite,true chamosite,iron oolitic,oolite of ferrization,and plaster oolite.