Shallow crust structure and its tectonic implications of granitic rare earth ore based on ambient noise techniques: A case study of Anxi Mining area, Ganzhou, Jiangxi Province

Rare earth resource is national strategic resource, which plays an important role in national economy and national security. At present, researches related to efficient and accurate exploration methods and metallogenic models are needed for the utilization of this resource. In this study, a planar array consisting of about 100 short-period seismographs, and two linear arrays with 40 m length consisting of 25 short-period seismographs, were deployed in a 10 kmx6 km area of Anxi Town rare earth mining area, Ganzhou City, Jiangxi Province. The three-dimensional shear wave velocity structure of the mining area in the shallow crust from the surface to the depth of 2. 2 km was obtained from ambient noise tomography. Taking a typical granite weathering crust as an example, the one-dimensional shear wave velocity structure from the fully weathered surface to similar to 40 m depth was retrieved using the phase-shift imaging method of surface waves. Based on the one-dimensional velocity structure, the HVSR method was used to detect the thickness of loose sediments in the mining area, and combined with the borehole data, it is verified that this method is effective in detecting the basement of granitic weathering crust. Based on the three-dimensional shear wave velocity structure, the distribution and morphology of the ore-forming source rock (granite) in the upper crust of Anxi pluton were delineated. Meanwhile, the result shows that the tectonic background of the hilly basin where Anxi pluton is located provides favorable conditions for the mineralization process of granitic type rare earth deposits. Combined with the Horizontal-to-Vertical Spectral Ratio (HVSR) results from ambient noise, the 1-D shear wave velocity model, and borehole data, the depth and structure of the weathering crust enriched with rare earth minerals from the top of the full regolith to the source rock were detected. The thickness of loose deposition for the study region indicates the difference of sedimentary environment between ore concentration areas and mountain areas. This study can provide a series of ambient noise seismological methods and strategies for the exploration of granite type rare earth resources, and establish a tectonic model for the origin, development, and evolution of the granitic weathering crust, which provides a reference for the study of deep-shallow structural evolution processes and the influence of tectonic activities for such rare earth deposits.