Three-Dimensional Fine Structure And Geodynamics Setting Of Transcrustal Magmatic System Beneath The Xiangshan Volcanic Basin

Xiangshan(XS)volcanic Basin,which locates in Jiangxi province,hosts the largest and richest volcanic-related uranium(U)deposits in China.However,the genetic model for the XS uranium ore-field remains controversial owing to the origin of U and ore-forming fluids has been attributed to various processes by previous studies.In recent years,the Pb-Zn-Ag deposits in deep depth were recognized and the vertical U polymetallic mineralization and alteration was found to reach depths beyond 1000 meters,showing great potentials for prospecting U polymetallic deposits in the deep depth.The formation of endogenic polymetallic deposits is restricted to the deep structure and dynamic process,and that mineralization is the stage or final product of magmatic evolution.Thus,magnetotelluric(MT)survey was conducted to study the deep structure of the XS volcanic Basin,which can visually reveal the spatial distribution and physical state of the transcrustal magmatic system beneath the XS volcanic basin,and investigate the contributions of the magmatic activity for the deep U metallogenic process and enrichment forms.It can provide new insights for the study of magmatic geodynamics and U mineralization in the study area from an electrical perspective,and has important guiding significance for the deep prospecting breakthrough in the XS volcanic Basin.MT original time-series data processing used Fourier transform and a series of denoising algorithms to give high-quality impedance tensor responses.Further phase tensor and induction vector analysis showed that there is no obvious tectonic strike in the shallow and lower crust,but the NEE trending tectonic strike is observed in the middle crust,showing obvious three-dimensional(3-D)property as a whole.In order to obtain the 3-D fine electrical structure beneath the XS volcanic Basin,an algorithm of 3-D MT inversion under topographic relief using a limited-memory quasi-Newton optimization based on the Broyden-Fletcher-Goldfarb-Shanno(BFGS)formula was first implemented.A covariance matrix that keeps the resistivity of air unchanged and to impose the smoothness of the model was established to address the problem of topographic relief,and the cooling method for the regularization factor and a strategy to search for the step length based on the Wolfe condition were adopted in this algorithm to improve the stability of the inversion.The correctness and reliability of this algorithm were verified by comparing the inversion results of the synthetic data with those of the open-source nonlinear conjugate gradient(NLCG)inversion algorithm.Then,this L-BFGS inversion algorithm was used to conduct the 3-D inversion of the XS field observation data.Results from different inversion methods were compared and analyzed,and a large number of inversion trials were conducted by changing the inversion parameters and data components.A stable and reliable 3-D electrical structure model of the XS volcanic Basin was finally obtained.The 3-D electrical structure model of the XS volcanic Basin images a resistive middle and upper crustal column,which corresponds to the pyknomorphic and stable metamorphic basement.The paleo-volcanic conduit near the XS peak is imaged to be conductive owing to the high permeability of the paleo-volcanic conduit.The hydraulic fractures driven by excess pressure of ascending and intruding magma greatly increases the permeability of the magma flowing pathway,which provided favorable conditions for the infiltration of saline crustal fluids and resulted in the electrically conductive paleo-volcanic conduit.This volcanic conduit is spatially inclined downwards to the northwest of the basin and controls the upward eruption and effusion of magma from the deep magma chamber.The intrusive body beneath the northwest side of the basin is an important component of the ancient magma plumbing system in the study area,which is characterized by a crustal conductor and the conductive features are interpreted as electrical reflections of the carbon-rich pluton.The contribution of igneous carbon produced by the multi-stage magmatic activities accompanying the transporting of the red basin organic-bearing material into deep crust by tectonic movement for the generation of graphite and might have been remobilized during the subsequent magmatic thermal events would result in the electrically conductive intrusive body.The emplacement of the intrusive body is controlled by the secondary faults generated by the NE trending regional deep faults,which indicates the regional extensional tectonic environment that may be related to the subduction and retreat of the Palaeo-Pacific plate.On the whole,the ancient magma plumbing system in the study area occurs as a vertically extensive,transcrustal network framework.Combined with the regional geological data,the ore-controlling structures of the XS volcanic Basin were revealed,the regional deep faults controlled the emplacement of magmatic activity in the XS volcanic Basin and established a basic framework for the formation of U deposits,and the deep faults that cutting through the basement provide favorable channels for the upward migration of ore-forming fluids,and the precipitation and enrichment of uranium were prone to occur in the ring faults,interfaces,unconformity surfaces and those geologic contacts intersecting with faults.A genetic model of U deposits in the XS volcanic Basin was further proposed to completely describe the “Source-Transport-Storage” overview of U mineralization in the study area,which reveals that the fault structural system and the ancient transcrustal magmatic system together constitute the Tectono-Magmatic system in the XS volcanic Basin.The former is the favorable place for the hydrothermal convection circulation,while the latter is the main heat source driving the hydrothermal convection circulation and provides abundant U oreforming materials.