基于背景噪声方法的花岗岩型稀土矿地壳浅部结构特征及构造意义:以江西赣州安西矿区为例

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

稀土资源是国家的战略性资源,在国民经济和国家安全等方面作用重大.目前,针对该资源的开发利用需要进一步加强勘探方法和成矿模式的研究.本研究在江西省赣州市安西镇稀土矿区10 km×6 km的区域内布设了一个约100台短周期地震仪器构成的面状台阵,以及两个48 m长各由25个短周期地震仪构成的线性台阵.通过背景噪声三维成像方法反演获得矿区地表至2.2 km深度的地壳浅部三维横波速度结构.以典型花岗岩风化壳为例,使用面波的相移成像法,反演了从地表全风化层至约40 m深度的一维横波速度结构.以该一维速度结构为参考,使用噪声谱比法(HVSR)探测了矿区内的松散沉积厚度,并结合钻孔数据验证了该方法对花岗岩型风化壳基底面的有效探测.综合上述结果,本研究通过三维横波速度结构圈定了安西岩体中稀土的成矿母岩(花岗岩)在上地壳的分布范围和形态,显示了安西岩体所处的丘陵盆地的构造背景为花岗岩型稀土矿成矿过程提供有利条件;结合HVSR结果、一维横波速度模型和钻孔数据,探测了富集稀土矿的风化壳从全风化层顶部至母岩的深度和结构,松散沉积厚度显示了矿集区和山区的沉积环境差异.本研究可以为花岗岩型稀土资源的勘探提供一系列基于背景噪声地震学的方法和策略,建立了火山岩型风化壳的起源、发育和演化的构造模型,为该类稀土矿的深-浅部结构演化过程和构造活动影响研究提供参考.

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 km×6 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~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.