| Huanggang-Ganzhuermiao metallogenic belt in the South Great Xing'an Range(GXR)is an important silver polymetallic metallogenic belt in northern China.This area is located in the superposition of three tectonic domains:paleo Asian Ocean,Mongolia Okhotsk ocean and paleo Pacific Ocean.It has experienced complex tectonic evolution and multi-stage mineralization since Paleozoic.Shuangjianzishan Ag-Pb-Zn polymetallic deposit,as a super large hydrothermal vein-type deposit in this area,shows complex metallogenic element assemblage,diversity of mineralization and multi-stage superposition of metallogenic process,which has attracted the attention of scholars.Although many scholars have carried out many studies on the mineralization characteristics,age and lithology,the understanding of the genesis of the deposit is not deep enough.On the basis of summarizing previous studies,through the systematic study of the geological characteristics,typical metallogenic mineralogy,geochemistry and fluid inclusions of the deposit,this thesis describes the relatively high-temperature hydrothermal superposition metallogenic process of the deposit in detail from the perspective of micro mineral structure and mineral assemblage.Due to the changes of fluid composition and properties,even on the mineral scale of the same hydrothermal period,the complex symbiotic relationship of sphalerite,pyrite or cassiterite of different generations can be seen(such as superimposed ring characteristics).This study can deepen the understanding of the metallogenic mechanism and mineralization of the deposit,and provide a scientific theoretical basis for looking for similar deposits.Shuangjianzishan deposit is mainly hosted in volcanic sedimentary rocks of Lower Permian Dashizhai formation.The mineralization is closely related to deep concealed granite porphyry.Biotite in the granite porphyry shows different hydrothermal replacements of quartz and calcite.The ore control characteristics of NE and NW faults are obvious,and have superimposed metallogenic characteristics.The ore body is mainly vein,stratoid and lenticular,and filling the fault.The wall rock alteration is mainly medium and low temperature hydrothermal alteration,with silicification in the deep and sericitization,carbonation and chloritization in the shallow.The ore minerals are mainly sphalerite,galena and various silver minerals.According to field and microscopic observation,the metallogenic stages of the deposit are divided into quartz sphalerite arsenopyrite stage,galena silver mineral sphalerite stage,quartz silver mineral pyrrhotite stage and pyrite carbonate stage.The results of in-situ LA-ICP-MS analysis show that the mineralogical characteristics of sphalerite in different generations of the deposit are clearly distinguished.The first generation sphalerite(Sp1)is located in the NW trending vein.The overall content of trace elements is low,relatively rich in Cd,poor in Fe,Mn and other elements.It has an optical rhythmic ring formed by crystallization,indicating that Sp1 is formed in a relatively stable closed anoxic environment and the crystallization rate is slow.The second generation sphalerite(Sp2)mainly exists in NE trending and superimposed veins,strongly dissolves chalcopyrite and pyrrhotite,and significantly enriches Fe,Mn,Ag,Cu and other elements.The high-temperature element combination indicates that sphalerite was formed in relatively high-temperature ore-forming fluid at this stage.The third generation sphalerite(Sp3),characterized by colloidal structure,is often wrapped with pyrite,arsenopyrite and other mineral particles,rich in Cu,Pb and other elements.Sphalerite in this generation mainly wrapped with sedimentary pyrite,showing concentric ring colloidal characteristics,suggesting the contribution of formation to the source of ore-forming materials.Each generation of pyrite and sphalerite shows coupling indication information.The first generation of pyrite(Py1)is mostly distributed in NW trending veins as veinlets,cemented and wrapped by early relatively high-temperature oxides such as magnetite,and contains more silicate mineral inclusions,indicating that it is metasomatized by relatively oxidizing fluids.The second generation pyrite(Py2)mainly shows the mineral combination of pyrite+sphalerite+galena,which is usually metasomatized or proliferated on the early pyrite in the form of large particle aggregate;The third generation pyrite(Py3)is mainly characterized by colloidal structure,which is formed by dissolution and precipitation of pyrite and is not of sedimentary origin.Due to different metasomatic degrees,the author further divides it into partially metasomatic pyrite(Py3a)and colloidal pyrite(Py3b).Py3a is usually in calcite vein as coarse-grained euhedral pyrite,coexisted with marcasite is an obvious distinguishing feature.Py3b has a typical annular colloidal structure,and the sulfur isotope shows that the composition of the core is obviously different from that of the edge,and the edge is colloidal pyrite overgrowth.Py1 and py2 are mainly enriched in As and Pb,in which Py2is more enriched in Cu and Mn than Py1,and Co,Ni and Sb are relatively enriched in Py3.Different from sulfide,cassiterite as oxide can provide different geological information.The first generation cassiterite(Cst-1)is mainly produced in the cement of NW trending vein or breccia type ore,coexisted with quartz and replaced by canfieldite.Se and Ag have high abnormal values,indicating that silver mineral inclusions are wrapped in cassiterite and there is the participation of formation fluid;The second generation cassiterite(Cst-2)is mainly hosted in lead-zinc massive ore.The hand samples show multi-stage superimposed metallogenic characteristics,in which Fe,Mn,W and high field strength elements are relatively enriched,indicating that the metallogenic temperature is relatively high.The content of In is significantly lower than that of Cst-1and Cst-3,suggesting that In tends to migrate together with Sn under oxidation conditions,and the ore-forming fluid is rich in volatiles.The third generation cassiterite(Cst-3)is mainly produced from the ore with relatively deep chalcopyrite and cemented by sulfide such as chalcopyrite and sphalerite.With the evolution of ore-forming fluid,the contents of Fe,Mn and HFSE gradually decrease,the temperature decreases,the oxygen fugacity increases,the p H value increases,and the contents of Co,Ni and copper group elements are higher in the ore-forming system.The total amount of REE in cassiterite in different generations shows an increasing trend,the fractionation of LREE and HREE is not obvious,and Cst-2 has a relatively large negative Eu anomaly.Comparing the REE distribution characteristics of granite porphyry and slate of Dashizhai formation in shuangjianzishan mining area,it indicates that Cst-1 is contaminated by formation fluid,and Cst-2 is mainly dominated by relatively high-temperature hydrothermal fluid from granite porphyry.The results show that the sulfur isotopes of the Shuangjianzishan deposit vary widely(?34S:-37.8‰?2.2‰),the average value is-5.2‰(n=152),in which Py3(?34S:-37.8‰?-5.2‰)is the main reason for the large range of sulfur isotopes.Due to the enrichment of 32S in sedimentary pyrite caused by BSR,special colloidal Py3 is formed by dissolution and recrystallization after later magmatic hydrothermal replacement.The comparison with regional strata and magmatic rocks shows that,the variation range of?34S value is closed to that of sulfur isotopes of typical deposits in the south GXR,and they all tend to enriching 32S,indicating that the ore-forming fluid may be related to the contamination of sedimentary strata.The sulfide in the main metallogenic stage has relatively uniform and characteristic magmatic sulfur characteristics(-4‰?2‰,with an average value of-2.4‰),indicating that the ore-forming materials are mainly from magmatic hydrothermal solution.Based on the above detailed sulfide stage division and mineralogical study,combined with the study of fluid inclusions,the author believes that the formation of Shuangjianzishan deposit has a superimposed mineralization process of medium to high temperature fluids.The ore-forming temperature in the early stage of mineralization is250??280?,and the sulfur fugacity(log?S2)is-11.0?-12.0.Most metals form chloride complex migration.Due to the weak buffer capacity of ore-forming fluid,sulfide such as sphalerite is precipitated in the fractures in the rock mass and near the contact zone of wall rock.At this stage,the fluctuation of physicochemical conditions in the ore-forming fluid is small,and the saturated precipitation rate of Ag is slow.In the main metallogenic stage,the fluid temperature is 300??350?,the salinity is 4.9?6.4 wt.%Na Cleqv,and the sulfur fugacity is log?S2 is approximately-8.0?-10.0.Due to the rise of deep source fluid and strong alteration reaction(mainly silicification)caused by magma pulsating intrusion,it finally led to the multi-stage activation of NW and NE trending faults in Permian slate.In this process,the rapid reduction of fluid pressure led to boiling(H2S and other volatiles in the fluid escaped rapidly),Thus,the oxidation of the fluid,the instability of the complex(such as Ag(HS)2-)and the rapid and massive precipitation of silver into the irregular fine-grained sulfide lattice form a high-grade mesh vein and breccia type ore.The types of silver minerals are different in different metallogenic stages.The silver minerals in quartz sphalerite arsenopyrite stage are mainly naumannite and canfieldite(selenium bearing);The silver minerals in galena silver mineral sphalerite stage are canfieldite,pyrargyrite,and freibergite;In the quartz silver mineral pyrrhotite stage,the silver minerals are mainly pyrargyrite,canfieldite,freibergite.The late fluid has obvious enrichment and mineralization of silver,and the evolution law of metallogenic element association is Sn+Se?Ag+Sb+Pb+Cu?Sb+Sn+Cu+Fe.The results of LA-ICP-MS U-Pb dating of zircon and cassiterite determine that the time limit of diagenesis and mineralization is concentrated at?130Ma,indicating that the shuangjianzi mountain type mineralization is mainly related to the early Cretaceous magmatism.Geochemical characteristics of Shuangjianshan granite porphyry:major elements show that Si,Al,Na,and K are quite enrichment,but Mg,Fe,and Ca obvious depletion;trace elements display that Ba,Sr,P,and Ti are strongly depletion,but Th,U,Zr obvious enrichment;and are characterized by high Rb,Y and low Sr contents;rare earth elements have obvious characteristics of LREE enrichment and a strong negative Eu anomaly.The Sr-Nd-Pb isotopes of the granites show that the rocks belong to the A-type granites formed in the early Cretaceous tectonic atmosphere.The transformation of tectonic system from compression to tension,followed by large-scale thickening of the lower crust and lithospheric mantle delamination.The lithospheric delamination and the subsequent upwelling asthenosphere mantle induced the intrusion of a large number of magmas.These magmas brought a large amount of sulfur and metal from the deep,which provided sufficient material preparation for the later large-scale hydrothermal mineralization.The shuangjianzishan hydrothermal vein mineralization was formed under this tectonic background. |
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