Ore Genesis And Geodynamic Background Of Lead-zinc Polymetallic Deposits In Erguna Area,Inner Mongolia

The study region is located in the northeastern part of the Derbugan metallogenic belt,the northeastern part of Great Xing'an Range.Geotectonic location belongs to the eastern part of the Central Asian Orogenic Belt?CAOB?,the eastern part of the Erguna Massif and the intersection zone of the Xing'an Block,and the western part of the middle part of the Derbugan Fault.And the Derbugan metallogenic belt is one of the important lead-zinc?silver?polymetallic metallogenic belts in China.Along the southwest to northeast direction of Derbugan deep fault,a number of important lead-zinc polymetallic deposits have been explored successively,such as Dongjun lead?small?,Xiahulin?medium?,Erdaohezi?large?,Derbur?large?,Biliya?large?,etc.In order to study the genesis and metallogenic geodynamic background of the lead-zinc polymetallic deposits in this region,based on the work and scientific research of predecessors,we selected Erdaohezi,Derbur and Biliya large-scale lead-zinc polymetallic deposits that newly discovered in recent years as typical deposit research objects,and carried out systematic research on deposit geology,ore-forming fluids,diagenetic and metallogenic chronology,element geochemistry and isotope geochemistry of the deposits.On the basis of discussing the genesis,diagenetic ore-forming epoch,and diagenetic and metallogenic dynamic background of the deposits,this paper systematically summarizes the metallogenic geological characteristics,metallogenic geodynamic mechanism and metallogenic geological process,and establishes the"model of diagenesis and metallogenic dynamics"and"model of metallogenesis"of the lead-zinc polymetallic deposits in the study region,and the following phases of achievements and progress have been achieved:1.By studying the geological characteristics of three typical deposits,we believe that Erdaohezi lead-zinc polymetallic deposit occurs in felsic volcanic rocks of Manketou Obo Formation of Middle Jurassic,intermediate basic volcanic rocks of Tamulangou Formation of Middle Jurassic and quartz porphyry of Late Jurassic,and near the contact zone between andesite porphyry of Early Cretaceous and quartz porphyry of Late Jurassic.Ore bodies are mainly vein-like,followed by lenticular and breccia-like,with characteristics of expansion and contraction,branching and compound and lateral reappearance;The host rocks of the Derbur lead-zinc polymetallic deposit are intermediate-basic volcanic rocks of Tamulangou Formation of Middle Jurassic,felsic volcanic rocks of the Manketou Obo Formation,andesite of the Manitu Formation and quartz porphyry of the Late Jurassic.The ore bodies are mainly vein-like,followed by lentil-like and breccia-like,with obvious characteristics of branching and lateral reappearance;Biliya lead-zinc polymetallic ore body mainly occurs in acidic volcanic rocks of the Manketou Obo Formation.The ore bodies mainly occur in vein form and have the characteristics of branching,compounding and lateral reappearance.The ore bodies of the three lead-zinc polymetallic deposits occur in the secondary NW-trending tensional-torsional fault system at the intersection of the NE-trending Derbugan deep fault and the NNE-trending Jierbugan deep fault.Quartz porphyry and andesite porphyry are closely related to lead-zinc polymetallic mineralization in study region.In addition,the monzonite porphyry cuts through the lead-zinc orebody,which is an intrusive body in the later stage of mineralization.2.In the aspect of wall rock alteration and mineralization,the wall rock alteration of Erdaohezi deposit mainly develops silicification,sericitization,argillization,fluoritization and propylitization,and a small amount of adularia,opal and calcite can be seen;Compared with Erdaohezi deposit,the low-temperature alteration of fluoritization,opalization and propylitization in Derbur deposit is particularly obvious.And the wall rock alterations of sericitization,fluoritization,opalization and propylitization are relatively developed in the Biliya deposit;The wall rock alteration closely related to lead-zinc polymetallic mineralization are silicification and sericitization;Ore types are mainly lead-zinc ore,followed by silver-lead-zinc ore and copper-lead-zinc ore;Ore structure is mainly vein-like,followed by massive,veinlet and breccia-like;Major textures of ores are euhedral-subhedral granular texture,metasomatic texture,solid solution separation structure,etc;Metallic minerals are galena,sphalerite,pyrite,chalcopyrite and tetrahedrite.Supergene oxidized minerals are limonite,covellite,etc.Silver-bearing mineral is only argentite.Gangue minerals are mainly quartz,calcite,fluorite,opal,chlorite,etc;The metallogenic process can be divided into supergene period and hydrothermal period,in which hydrothermal stage is the main lead-zinc polymetallic metallogenic stage,and the corresponding mineralization stage is divided into three main metallogenic stages and seven sub-stages.In conclusion,the three Pb-Zn polymetallic deposits in the study area have the geological characteristics of epithermal and low-sulfidation type.3.The LA-ICP-MS single-grain zircon U-Pb dating of volcanic rocks?wall rocks?,subvolcanic rocks or porphyries related to mineralization and late intrusive rocks in the three lead-zinc polymetallic ore areas and the Rb-Sr isotopic dating of sphalerite,pyrite and galena in the main stage of hydrothermal period reveal that:?1?The diagenetic ages of quartz porphyry,andesite porphyry and metal sulphide Rb-Sr isochron in Erdaohezi mining area are 160.3±1.4 Ma,133.9±0.9 Ma and 130.5±3.6Ma,respectively;?2?The diagenetic age of rhyolitic tuff of Manketou Obo Formation is 164.0±1.6 Ma,basic-intermediate volcanic rocks of Tamulangou Formation is167.0±2.0 Ma,andesite of Manitou Formation is 140.2±2.6 Ma.The diagenetic age of monzonite porphyry?cutting ore body?is 125.2±1.1 Ma;?3?The diagenetic age of rhyolite in Manketou Obo Formation is 163.7±1.1 Ma and the Rb-Sr isochron age of metal sulfide in the main stage of hydrothermal mineralization is 131.3±2.4 Ma;?4?The intermediate-basic magma of Tamulangou Formation and felsic magma of Manketou Obo Formation in the study area occurred at 167-164 Ma,and their magmatism time was approximately the same.The lead-zinc polymetallic mineralization time is limited between the late Jurassic?160 Ma?and the early Cretaceous?125 Ma?.The precise metallogenic age should occur in the early Cretaceous?130-131 Ma?,which is closely related to the andesite magmatism of the early Cretaceous.4.Geology,petrography,geochemistry and Sr-Nd-Pb-Hf isotope analysis of volcanic rocks and subvolcanic or porphyries in the study area reveal that:?1?The intermediate-basic volcanic rocks?wall rocks?of the Tamulangou Formation are high in aluminium and alkali,and are obviously enriched in large ion lithophile elements?LILE?and light rare earth elements?LREE?,and depleted in high field strength elements?HFSE?.They have low(⁸⁷Sr/⁸⁶Sr)_i?0.705007-0.705240?,?Nd?t??+0.6⁺1.7?and older Nd model age?699-883 Ma?.Based on the lead isotope data of the whole rock,we conclude that the diagenetic magma has a mixed source area of lower crust and depleted mantle or orogenic belt,which is the Neoproterozoic mantle-derived basalt magma undermining the lower crust and formed by partial melting of the accretive lower crust of the Mesoproterozoic;The rhyolitic volcanic rocks of Manketou Obo Formation show weak negative Eu anomaly?average value of?Eu is 0.64?,and obvious Sr element loss,¹⁷⁶Hf/¹⁷⁷Hf ranges from 0.282721 to0.282870,and the corresponding?Hf?t?values range from 1.7 to 6.8?both greater than 0?.The corresponding zircon two-stage model age of T_DM2 is 693-985 Ma,indicating that the magma should be the product of partial melting of the lower crustal material of the Mesoproterozoic.The felsic magmatism in the study area is accompanied by the process of gradual weakening of intermediate-basic volcanism in the Tamulangou Formation,which constitutes a"bimodal volcanism"in the region;?2?Late Jurassic quartz porphyry is felsic,strongly peraluminous and high-K calc-alkaline rock series.They are enriched in large ion lithophile elements?LILE?Rb,Th,U,K and LREE,depleted in high field strength elements?HFSE?Nb,Ta,Zr,Hf and HREE,and obviously depleted in Sr,Ti,Ba,etc.Their diagenetic magma has the properties of volcanic arc or active continental margin.The values of?Hf?t?are 5.7-8.0and 3.1-5.8,respectively,and the two-stage model ages of T_DM2 are 920-1130 Ma and1106-1343 Ma,respectively.The values of ¹⁷⁶Hf/¹⁷⁷Hf are between the depleted mantle and the lower crust,indicating that the magma originated mainly from the mantle material with depleted mantle property partially melted the young lower crust newly accreting from the depleted mantle in the Mesoproterozoic?partial melting in different degrees during the evolution of diagenetic magma?,and the magma was contaminated by crustal materials during the process of magma upwelling or diagenesis.5.On the basis of petrogeochemical studies,the elements of Cr,Ni,Co,Cu,Pb and Zn in volcanics,subvolcanics or porphyries in the region generally hold similar element contents.Compared with the primitive mantle-normalized elements,they generally deplete in siderophile elements or compatible elements,such as Cr,Ni and Co,strongly enriched sulfophile element or large ion lithophile element?such as Pb?.Furthermore,dual attribute elements?siderophile and sulfophile,Cu?and sulfophile element?such as Zn?are close or the same as those of the primitive mantle.They generally hold more than 100 orders of magnitude of Pb elements?compared with Cu and Zn?,and the values of Cu and Zn elements match those of the primitive mantle.This may indicate that the magma-hydrothermal they hold during diagenesis are likely to provide minerals such as Pb,Zn and Cu,or are influenced by hydrothermal processes of Pb,Zn and Cu in later diagenesis,these analysis results provide a favorable information for mineralization.6.The comprehensive analysis of mineral features,fluid inclusions,Pb-Rb-Sr isotopes in metallic sulfides and H-O isotopes in transparent minerals at different metallogenic stages of hydrothermal period shows that:?1?Inclusions in lead-zinc polymetallic deposits in the study area are mainly gas-liquid two-phase?W-type?and contain a few of CO₂ three-phase.Initial ore-bearing fluids are characterized by coexistence of medium-low temperature,high-low salinity and medium-low density.They belong to H₂O-NaCl with a few CO₂(rich in Fe²⁺,Zn²⁺,S^2-,etc.)neutral reduction multiphase fluid system;The ore-bearing fluids of the main stage are low-temperature,high-low salinity coexistence,low-density and H₂O-NaCl±CH₄with few CO₂(rich in Fe²⁺,Zn²⁺,Pb²⁺,etc.),belong to the fluid system of neutral reduction;The residual ore-bearing fluids in the late stage are mainly H₂O-NaCl(rich in Ca²⁺,Cl^-,Cl^-,etc.)liquid-rich or pure liquid phase which is dominated by meteoric water;?2?Preliminary studies show that weak boiling or local boiling of ore-bearing fluids and isothermal mixing of different source fluids or fluid immiscibility are important metallogenic mechanisms of?silver?lead-zinc polymetallic deposits in the region;?3?The strontium,neodymium,lead,hafnium isotopes and elemental geochemical evidence show that the ore-forming material of?Ag?Pb-Zn polymetallic deposits are mainly derived from the neogenic lower crust of the Mesoproterozoic,and a small amount of depleted mantle source material with mid-ocean ridge mantle?depleted mantle?property.On the whole,it has the characteristics of crust-mantle mixing source,which has the characteristics of crust-mantle mixing source.7.Based on the above analysis and analogy with other?silver?lead-zinc polymetallic deposits in the region,we preliminarily believe that three lead-zinc polymetallic deposits in the study area are large epithermal metal deposits of low-sulfidation type related to continental intermediate-acid volcanic magmatism;The magma related to the diagenesis of bimodal volcanic rocks in the region may provide some mineralizing materials for the lead-zinc polymetallic mineralization,and also lay a foundation for the large-scale silver-lead-zinc mineralization in the region;The large-scale activity and shallow achievement of felsic magma occurred in an extensional environment after the subduction of the Pacific plate in the early Late Jurassic?160 Ma?,and the diagenetic magma originated from the newly accretive basaltic lower crust partially melted by depleted mantle.The metallogenic dynamic background is that on the basis of the subduction of the Pacific plate and the partial melting of the lower crust of the Neoproterozoic basalt to produce rhyolitic magma?160.3±1.4 Ma?,and subsequent transfer to the back arc extensional environment under the subduction and compression background of the paleo-Pacific plate?Izanagi?,which resulted in the delamination of the residual neogenic lower crust,the upwelling of asthenospheric mantle material interacts with the residual the accreted lower crust of Mesoproterozoic,forming a magma-hydrothermal rich in lead-zinc polymetallic materials,it may be the origin of the formation of epithermal lead-zinc polymetallic deposits in this area.Finally,based on the results of this study,in order to provide a theoretical basis for the deepening of metallogenic theory and deep prospecting in this field,we established the"model of diagenesis and metallogenic dynamics"and"model of metallogenesis"for lead-zinc polymetallic deposits in study area.