The Study Of 3D Magnetotelluric Imaging And Metallogenic Background In The Nanling-Xuancheng Ore Concentration Area

The Nanling-Xuancheng ore concentration area is located in the southeast of the middle and lower Yangtze River metallogenic belt.Large copper and gold deposits have been discovered in the ore concentration area,such as Chating and Magushan.Thus It becomes a newly promising mineralization area.As this area is mostly covered by a wide range of Quaternary sedimentary layers,its deep structure and metallogenic characteristics are still unclear.In recent years,no breakthroughs have been made in prospecting.The Magnetotelluric sounding(MT)can penetrate the Quaternary overburden and obtain the deep three-dimensional electrical structure.It is obviously of great significance to carry out three-dimensional magnetotelluric imaging and metallogenic background research in the Nanling-Xuancheng ore concentration area.This paper uses the observation data of 942 MT sites with a period of 0.001 s to 1000 s in the study area to calculate the impedance tensor,phase tensor,tipper,and other parameters.The magnetotelluric data were qualitatively analyzed to obtain the dimensionality and strike of the study area.The result of the qualitative analysis shows that the deep structure of the study area is three-dimensional.A three-dimensional magnetotelluric inversion must be performed to obtain a reliable three-dimensional resistivity model of the area.In the past years,most of the magnetotelluric surveys in the middle and lower Yangtze River metallogenic belt were profile surveys.Generally,only the non-principal diagonal components Zxy and Zyx of the impedance tensor Z were used as inversion parameters,and the principal diagonal components Zxx,Zyy and tipper vectors were not fully utilized.Compared with impedance tensor data,tipper data is less affected by the near-surface electrical inhomogeneous medium.Therefore,to improve the reliability of the inversion results,the four components of the impedance tensor and the two components of the tipper vector were inverted simultaneously,and the three-dimensional resistivity structure model of the Nanling-Xuancheng mining area was obtained for the first time.The geological structure and the distribution characteristics of mineral deposits in the study area were well reflected in this model.The resistivity 3D model was compared with active source reflection seismic profile and borehole data.The high consistency of the comparison indicating that the magnetotelluric 3D inversion method and inversion results in this paper are stable and reliable.Based on the three-dimensional resistivity model,the structural characteristics of different depths are studied.The large-scale low-resistance distribution near the surface corresponds to the extensive sedimentary overburden in the study area.The extension of faults are is the same as the interface of high and low resistivity.Striped low-resistivity anomalies were found in many areas in the middle and deep crust,showing the low-resistance characteristics of the metal deposit and its surrounding areas.Resistivity slices reveal the thickness of the sedimentary layers in the Nanling Basin and Xuancheng Basin.The different resistivity of the basement of these basins indicates the structural independence of them.The vertical change of the resistivity in the deep part of the Jiangnan fault indicates that after the formation of the fault,the upper part of the fault may have experienced a complex tectonic process.The position of the low-resistance body below the Zhouwang fault moved northwestward relative to the surface in the deep part,reflecting the extension direction of the fault in the deep part.In the plate junction zone in the southern part of the study area,a large-scale low-resistance anomaly was found along the Jiangnan Fault and Zhouwang Fault.This anomaly is a direct reflection of the deep fault,which means that there exist a strong deformation and fractured structure at this location.Through the comparative analysis of the different causes of the deep low-resistance body,we believe that the deep low-resistance body is caused by the broken structure caused by the deep fracture was filled with the water-containing fluid.Besides,the comparison with the seismic activity,gravity field data,ground temperature distribution and other information in the region further supports this conclusion.The deep low-resist:ivity area of the Chating copper-gold deposit is analyzed.Because this deposit is located in the middle and lower Yangtze River metallogenic belt which is highly correlated with the Mesozoic magmatic activity.This low-resistivity area is considered to be the channel for the upwelling of magma during the Mesozoic metallogenic period and the source of surface mineralization.To obtain a more detailed resistivity stru cture model,the MT data of the Chating copper-gold mine area was re-inverted by three-dimensional inversion.Based on the three-dimensional resistivity model and the geological and geochemical research results carried out by predecessors in Chating,combined with the recent research results on the genesis of the middle and lower Yangtze River metallogenic belt,a simplified ore-forming geodynamic model of the Chating copper-gold deposit is presented.The relationship between the distribution characteristics of metal deposits in the study area and the low-resistivity area in the resistivity model is analyzed,and the correlation between the genesis of the deposits and the distribution of magmatic rocks is discussed.Suggestions were made for the prospecting prospects in the area,and several prospecting areas were delineated according to the continuous strip-shaped low-resistivity anomaly areas.