Coal accumulation regularity of Zhiluo Formation and its indication to paleoclimate and uranium metallogenic environment,Ordos Basin

The Jurassic coal measures in Ordos Basin are not limited to Yan’an Formation,the coal accumulation lasted from the end of Fuxian Formation (late Toarchian) to the early and middle Zhiluo Formation (Bathonian stage).The study on the coal accumulation characteristics and evolution law of Zhiluo Formation in the decline and evolution stage of Jurassic coal measures is helpful not only to clarify the stratigraphic structure of Jurassic coal measures in Ordos Basin,but also to reshape the detailed process of paleoclimate change and migration in the Bathonian period,and to analyze the metallogenic environment of sandstone type uranium deposits from the perspective of external reduction medium.Combined with the regional outcrop survey and stratigraphic correlation of large scale borehole data,it is found that the whole basin has a large range of coal accumulation during the sedimentary period of the lower sub member of lower member of Zhiluo Formation,and the coal accumulation area is distributed as a ring belt around the subsidence center of the basin,indicating that the paleoclimate at this time was still in a humid environment.However,the scale of coal accumulation is far less than that of Yan’an Formation.It is characterized by the development of thin coal seams,coal lines and carbonaceous mudstones,which indicates that the Ordos Basin in the Bathonian period was in the paleoclimatic background of transition from humid to arid.It is found that from the upper sub member of the Zhiluo Formation,the coal accumulation in whole basin is not consistent.During this period,the coal accumulation in the southern,central and eastern parts of the basin ended,indicating that the paleoclimate had changed into arid.However,in the north and west of the basin,the coal accumulation continues to develop,and the coal accumulation position gradually rises from east to west.At present,the highest coal accumulation position is found in the middle and upper sub members of the Zhiluo Formation in Erdaoling area of Helan Mountain,and the thickest coal seam can reach 6.60 m.This indicates that the humid paleoclimate,which began to form in the late Toarcian period,evolved through Aalenian Bajocian and Bathonian periods,and finally exited from Erdaoling area of Helan Mountain and was replaced by arid paleoclimate.The migration path of paleoclimate is from southeast to northwest.In recent 10 years,the practice of “coal and uranium exploration” in northern Ordos Basin has confirmed that the weak coal accumulation in Zhiluo Formation is the key factor (external reducing medium) for sandstone type uranium deposit formation.It not only limits the metallogenic horizon of sandstone-type uranium deposits to a certain extent,but also directly participates in the whole process of uranium mineralization.The Coal seam or carbonaceous mudstone,the product of weak coal accumulation in Zhiluo Formation,is considered as one of the important prospecting indicators in Dongsheng uranium ore field.