Application Of AMT 3D Inversion In Xiangshan Uranium Exploration

Xiangshan uranium ore field is one of the seven major uranium ore exploration areas in China.It belongs to the volcanic-intrusive complex of medium-low temperature hydrothermal type uranium deposit.It is located at the junction of Yangtze plate and South China plate.It is about 50 km north of Jiangshao suture zone,25 km east of Yingtan-Anyuan fault zone and 25 km west of Dexing-Suichuan deep fault zone.It has undergone long-term complex geological processes and developed fault structures.The formation in Xiangshan volcanic basin can be divided into two parts: basement and caprock.The main lithology of the basement is Neoproterozoic metamorphic rock,and the volcanic rock series of the upper overburden can be divided into the Daguding Formation and the Ehuling Formation.Because the stratum interface in Xiangshan area is a weak structural zone,with more developed fissures and ore-conducting and ore-hosting functions,the interface between volcanic rock formations(variation site),basement interface and the complex of fault structures in this area have become the most favorable occurrence site of uranium deposits,and also the focus of our research.To further ascertain the distribution of underground uranium deposits,Beijing Research Institute of Uranium Geology has carried out a large area of high-density audio-magnetotelluric sounding in Heyuanbei work area in this area.2222 sounding points have been obtained.The section direction is 132 degrees to the east of the north,the average length of the section is 3.5km,the average line distance is about100 m,and the point distance is 50 km.These data are of good quality,which will lay a foundation for further study of the deep structure and metallogenic regularity of Heyuanbei area and further ascertain the distribution of uranium deposits.In the past,due to the constraints of inversion theory and technology,two-dimensional inversion of magnetotelluric data was mainly carried out.However,the geological structure of Xiangshan area is complex,and the use of three-dimensional inversion can betterdepict the complex underground electrical structure;at the same time,the large area of good quality data obtained in this area also provides the possibility of carrying out3 D inversion.In this paper,the 3D magnetotelluric inversion technique of the NLCG is used to study the lithological distribution,the distribution of fault zones and the interface characteristics in Xiangshan area,and then to study the favorable occurrence sites of uranium deposits.Firstly,all the measured data are filtered and processed.Then,taking the area where the three lines are located as an example,the influence of the main inversion parameters,such as background resistivity,grid size,data variance,data type,frequency number and regularization factor,on the inversion results is discussed.Finally,3D inversion is carried out for all the measured data in the whole area.The main impedance is used for inversion and 2039 data are filtered.The inversion grid is 1/2 of the distance between the measured points.After rotating the measured lines,the inversion section is divided into 289680(136*71*30)inversion units and the initial resistivity is 1000??m.After 81 inversion calculations,the fitting error is 2.64.Detailed analysis of the 3D inversion electrical model in the study area has obtained the following main points:The results show that the 3D electrical model in work area presents three obvious layers of electrical structure: the upper part is high resistivity anomaly,the resistivity is about 3000??m;the middle part is low resistivity anomaly,the resistivity is about500 ? ? m,the lower part is sub-high resistivity anomaly,the resistivity is about2000 ? ? m.According to logging curves and drilling physical property maps,it is proved that the upper high resistivity anomaly corresponds to the formation of Ehuling Formation,the middle low resistivity layer corresponds to the formation of Daguding Formation,and the lower sub high resistivity anomaly corresponds to the Neoproterozoic basement.This consistency shows that the three-dimensional inversion results are reliable.Based on the geological data of the work area and the detailed analysis of the three-dimensional inversion electrical model,we have predicted the lithologicdistribution,fault distribution and favorable locations of uranium deposits in Xiangshan area,and obtained the following results and understandings:(1)Inversion of 3D magnetotelluric data was carried out in this area,and the first three-dimensional electrical structure image of this area was obtained.(2)The analysis of the electrical characteristics of the important surveying lines in work area area shows that the high resistivity of the Ehuling Formation gradually decreases from south to north,and the Daguding Formation is divided into two parts in work area: the East and the middle.(3)The interface between volcanic rock formations is widely distributed in work area.The boundary around work area is cut and broken by faults,and the central interface is well preserved.Therefore,on the interface between volcanic rock formations,the edge of the work area is the most favorable metallogenic area.(4)The basement interface mainly distributes in the central part of the work area,followed by the eastern part.Therefore,the central and eastern part of the work area is the most favorable metallogenic area on the basement interface.