Studying On The Electrical Structure Of The Crust In The Nanpanjiang Basin

The Nanpanjiang Basin is located in the Triangle of Yunnan-Guizhou-Guizhou provines in southwestern China,whose geotectonic location is located at the junction of the southwestern margin of the South China Plate and the northeast margin of the Indosinian Plate,and there is a lot of low-temperature hydrothermal metallogenic.Therefore,the Nanpanjiang Basin is important for studying continental dynamics and mineral resources.The use of magnetotelluric methods to detect the deep electrical structure and structural characteristics of the Nanpanjiang basin can not only provide new electrical constraints on the structural deformation mechanism and dynamic process of the Nanpanjiang Basin and its adjacent areas,but also Provide new understanding for studying the metallogenic mechanism and prospecting work in the area.With the support of the 13 th Five-Year Key R & D Project "Deep Detection Technology Demonstration of Sparse Mineral Resource Base",a400-meter-long magnetotelluric sounding profile spanning the Youjiang Basin from west to east(a total of 41 magnetotelluric soundings)was completed along the Funing-Congjiang direction.At the same time,a 6km audio magnetotelluric sounding profile(a total of 9sounding points)was completed on the margin of the Dachang deposit.Through fine processing,analysis and inversion calculation of the measured data,a reliable deep electrical structure of the MT profile and a shallow electrical structure in the middle margin of the Dachang deposit field are obtained.Magnetoelectrical electrical results show that the crust along the profile is a high resistance body.It is speculated that the high-resistance body is the electrical reflection of the ancient basement of the Yangtze plate.There are four low-resistance bands from SW to NE along the survey line.The resistance is between 10-100?·m,combined with geological data,it is inferred that the low-resistance zone is the electrical reflection of the Funing fault,the Baise fault,the Tianlin-Bama hidden fault,and the Ziyun-Yadu fault.Magnetotellurical evidence suggested the Funing fault is an crustal fault,whose depth is near about 15 km,which provides a channel for magmatic activity and the migration of ore-forming fluids.TheYoujiang fault zone is a crustal fault,whose depth is near about 15-20 km.The Tianlin-Bama fault is a hidden fault.According to the electrical result,we can infer that the fault is a deep crust fault,whose depth is near 20 km.The depth of Ziyun-Yadu fault is greater than the crust,magma and ore fluids upwelled along the fault,formed a huge metallogenic area.According to the discussion of the low-resistance bodies in the Ziyun-Yadu fault zone,it is speculated that the low-resistance bodies are mainly local melting of the crust and mantle.Since the late Cretaceous,due to the subduction and retreat of the New Tethys plate,the asthenosphere upwelled in the western South China and induced the partial melting of the lithospheric mantle.The intrusion of the mantle-derived magma caused partial melting of the overlying mature crust.The crust-source felsic magma(included a small amount of mantle material)eventually formed a granite body with cage lid in the shallow part of the crust,and produced mineralization.According to the inversion results of the AMT profile,the granite in the middle section of the Dachang ore field is ridged uplifted and associated with the ore body,which has obvious ore-controlling effect.