Application Of CSAMT 3D Inversion In Huaniushan Lead-zinc Deposit Exploration

The Huanniushan lead-zinc field is an important lead-zinc reserve base in northwest China,which belongs to the exhalation-sedimentation lead-zinc deposit,which located in the Beishan Huaniushan region at the border of Gansu province and Xinjiang province,the mine is north of the wujing river concealed fault,the Huaxitan-huanniushan fault in the south,after a long and complex geological process,the fold and fault structures are well developed.The Huanniushan lead-zinc deposit is mainly a deposit area of clastic-carbonate rock,ore bodies are mostly produced near the contact zone between marble and phyllite,interlaminar fracture zone developed,often accompanied by the production of ore bodies,so,prospecting in this area focuses on Sinian strata with lithology dominated by phyllite and marble,this is also the focus of the paper.In the past,the prospecting model of the Huanniushan lead-zinc deposit had always been based on the geological and geochemical,but seldom combined with geophysical methods,there was a great controversy on the understanding of mineralization.At the same time,in recent years,the 1-D and 2-D inversion techniques of controlled source electromagnetic method have become increasingly mature.Due to the complexity of geological structure in the Huanniushan area,the reliability of 1-D and 2-D inversion is poor,so it has become a mainstream method to distinguish the underground complex electrical structures through 3-D inversion.Langfang geophysical and chemical exploration institute carried out a large area of CSAMT exploration in this area,and obtained a total of 312 sounding points,the direction of the survey line was 16 degrees north by east,the length of the survey line was 1.15km,the line distance was 200m,and the point distance was 50m.It was found that the data collected in the field are of good quality,which lays a foundation for further study on the electrical structure of the Huanniushan area and the division of favorable areas for ore formation.In this paper,finite memory quasi-newton method(L-BFGS)is used to study the distribution of rock series and ore-controlling faults in the Huanniushan area,furthermore,the favorable position of lead-zinc mineralization is classified with the support of large area good quality data.Firstly,all the measured data are filtered and processed,and then the work area of L8 line as an example,discuss the background of the resistivity,grid size,data variance,regularization factor number and frequency number of the main influence on inversion results,an optimized inversion parameter table is designed,finally,3-D inversion is carried out for all measured data in the whole area and inversion using the data for the field acquisition of electric field component E_x,after 651 times of inversion iteration,the finally fitting error to reduce two orders of magnitude,the numerical value of 5.7.The results show that the 3-D electrical model of the working area has obvious two-layer electrical structure:the resistivity of the shallow part is lower,and the resistivity of the deep part is higher.Combining with the surface geological data of the working area,the 3-D inversion results are analyzed in detail.We predict the distribution of rock series,ore-controlling fault and the favorable position for the occurrence of lead-zinc deposit in the Huanniushan area and obtain the following results and understandings:(1)inversion of 3-D controlled source electromagnetic data was carried out in the Huaniushan region,and the first 3-D electrical structure map was obtained;(2)the reliability of 3-D inversion is verified by comparing the measurement results of TDIP method in the working area with the 3-D inversion results of controlled source;(3)the 3-D electrical model of the working area is obtained by the inversion of limited memory quasi-newton method(L-BFGS),the model clearly shows the electrical characteristics of the working area,it shows that shallow low resistivity anomaly in the northwest corner of the working area is mainly caused by the carbnaceous layer,the western lithology is dominated by marble,the eastern lithology is dominated by phyllite;(4)delineate the favorable position of the lead-zinc mineralization in the working area,and consider that the ore belt is located near the contact zone of marble and phyllite,and the electrical sign of prospecting is the middle and low resistivity transition zone,meanwhile the Huaheitan-shuangfengshan fault in the working area is the ore controlling structure of the ore belt.