Research On End Effect Of Composite Mineralization In The Southeastern Great Xing’an Range Revealed From Integrated Geophysical Data

The research on end effect of mineralization aims to find the conditions,processes and regularities of accumulation of ore-forming materials.It is of great significance for revealing the spatial and temporal evolution and aggregation process of ore-forming fluids,analyzing the control of structures such as faults on the final spatial positioning and ore-forming types of ore bodies,and proposing reasonable metallogenic models.Based on the changes in underground petrophysics and structures caused by the accumulation of ore-forming materials,geophysical methods are used to study the end effect of mineralization from the perspective of determining the distribution characteristics and coupling relationships of ore-controlling elements.They can be employed to analyze the conditions and regularities of ore-forming material accumulation,reveal the genesis of ore deposits and the spatial positioning of ore bodies,and provide scientific basis for mineral exploration.Under the influence of superposition of multiple tectonic systems,five major metallogenetic events occurred in the Great Xing’an Range during the Early Paleozoic to Cretaceous period,which has the characteristics of composite mineralization.In the southeastern Great Xing’an Range,three stages of metallogenetic events occurred during the Mesozoic period.The Shuangjianzishan super-large epithermal Ag-polymetallic deposit and the Baiyinnuoer super-large skarn Pb-Zn deposit showed multi-stage superposition mineralization in the region,which is an ideal area for the study of composite mineralization.The existing geological and geochemical results in the southeastern Great Xing’an Range have effectively determined the controlling effect of ore-forming elements,ore-forming age,type and source of ore-forming materials in different deposits.However,as a composite mineralization region,the southeastern Great Xing’an Range is still a lack of powerful geophysical results to reveal the end effect of composite mineralization,compared with a single genetic metallogenic region,in the research on how mineralization in different periods superimposed in the same space and how different mineralization jointly controlled the accumulation of ore-forming materials in the same period.In this study,based on the National Key R&D Program of China(2017-2021)and the China Geological Survey(2016-2018),the resistivity,density and magnetic susceptibility of 703 rock samples were obtained through sampling and petrophysical properties testing of the main geological units in the southeastern Great Xing’an Range.The geophysical properties of strata and intrusive rock bodies related to mineralization were summarized,and the obvious differences in electrical and magnetic properties were determined.On the basis of the difference in petrophysical properties,this study mainly used integrated geophysical investigation,including gravity method,magnetic method,electric methods(i.e.,magnetotelluric sounding and controlled-source audio frequency magnetotelluric sounding)and shallow seismic reflection method,to carry out multi-scale geophysical observation in two typical ore districts including Shuangjianzishan and Baiyinnuoer in the southeastern Great Xing’an Range.Finally,the gravity and magnetic data of 50 km~2,the data of 11controlled-source audio frequency magnetotelluric lines,and the data of a shallow seismic reflection line in Shuangjianzishan ore district were obtained.53 regionally distributed broadband magnetotelluric data,and the gravity and magnetic data of 290km~2in Baiyinnuoer ore district were obtained.Based on the depth of research on the end effect of mineralization,three-dimensional physical-property inversion was used for gravity and magnetic data to obtain the relative density and magnetic susceptibility characteristics in the two ore districts.For magnetotelluric data and controlled-source audio frequency magnetotelluric sounding data,three-dimensional inversion was used to obtain the electrical structure in two ore districts.In this paper,the shallow seismic reflection data were processed by conventional means to obtain the seismic reflection results of the Shuangjianzishan ore district.The integrated geophysical results in Shuangjianzishan ore district revealed the geophysical properties and distribution characteristics of the main ore-controlling elements of Ag-Pb-Zn mineralization under simultaneous and multiple types of magma emplacement in the end effect of composite mineralization.The depth of Permian Dashizhai group is above 1239 m,and thicker in the southern of Xinglongshan ore block and the eastern of Shuangjianzishan ore block.The ore-controlling fault in the ore district is composed of deep faults and shallow conjugate faults.The magma emplacement in the two main ore blocks in the ore district shows dome-type intrusion,and the horizontal distribution is controlled by faults.Its geophysical characteristics are mainly manifested as a dome-type structure with high resistance and weak reflection.The volcanism is located about 3 km to the southwest of the west ore block,and is characterized by a tree-like structure consisting of a near-conical high resistivity and high magnetic channel and an annular fault system.The NE-trending deep fault system connects these two types of magma emplacement and acts as a channel for magmatic fluid flowing and enrichment.Combined with regional geology,petrophysical properties and drilling information,the paper proposes a composite metallogenic model of Ag-Pb-Zn mineralization in Shuangjianzishan ore district.The volcanism caused ore-bearing magma to migrate to the bottom of the ore block through the fault system,and formed complex with ore-bearing magma in dome-type emplacement below the west ore block.The complex was emplaced into mineralization under the control of the fault system.The integrated geophysical results in Baiyinnuoer ore district revealed the spatial distribution of key ore-forming elements under multiple stages and types of magma emplacement in the end effect of composite mineralization,that is,the Permian strata as ore-bearing surrounding rocks are mainly distributed in the surface exposed range,within a depth of 3000 m.The ore-controlling fault in the ore district is composed of deep faults distributed in a northeast and near east-west direction.Two periods of intrusive rocks are distributed in the ore district,of which the Late Triassic intrusive rocks are mainly distributed under the Permian strata and in the northern part of the ore district,in the form of the vein-type magma emplacement.The Early Cretaceous intrusive rocks are distributed around the Baiyinwula volcanic edifice and in the northwest of the ore district,and are generally characterized by volcanism.Its geophysical and structural characteristics are similar to those of volcanism in the Shuangjianzishan ore district.Both provided ore-forming materials for the ore district.Two parallel deep faults in northeast direction connect these two types of magma emplacement and serve as the channels for the superposition of magmatic fluids.Combined with geological and geochemical studies,this paper puts forward a composite metallogenic model of Sn-Pb-Zn mineralization in Baiyinnuoer ore district.The ore-bearing fluid formed by vein-type magma emplacement in late Triassic was controlled by NE trending faults to form orebodies in Permian strata.The ore-bearing fluid of the Early Cretaceous volcanism migrated along the NE trending fault at a long distance,and superimposed mineralization again in the early mineralization area.Based on the composite metallogenic models of Shuangjianzishan ore district and Baiyinnuoer ore district,this paper reveals the end effect of composite mineralization in the study area by comparing and analyzing the mineralization characteristics of the southeastern Great Xing’an Range and other mineralization areas.Under the multi-stage,volcanic-intrusive composite magmatic emplacement,ore-bearing fluid originated from the magma fluid migrating along the far end of the faults during volcanism,and the ore-bearing magma precipitated from the top of the intrusion.The ore-bearing fluids were enriched or superimposed through deep faults,and form ore bodies in place in the Permian ore-bearing surrounding rocks.The composite mineralization with multiple mineralization periods and multiple magmatic emplacement patterns may be an important condition for the formation of super-large deposits in the area.