Ore Genesis And Tectonic Setting Of Shanmen Ag Deposit In Siping Area,Jilin Province

The Shanmen Ag deposit is located in Siping area of Jilin Province,and tectonically situated at the intersection of northern margin of the North China Plate and eastern part of the Xing'an Mongolia Orogenic Belt?XMOB?.It occurs in the southern section of Daheishan Horst at the southwest side of the intersection of the Yitong-Yilan and Xilamulun-Changchun faults.This large-scale silver-only deposit was discovered early,however,it has been poorly researched.Based on regional and mining area geology data analysis,such researches have been carried out in this deposit as ore-forming geological and physicochemical conditions,deposit geology and ore fluid characteristics.In addition,comparisions on Shanmen and other representative silver deposits both at home and abroad have been completed.According to the spatial-temporal and genetic relationship between the moderate acid intrusions and the silver mineralization in the area,the metallogenic intrusions have been identified,and the diagenesis and metallogenic age were determined.Based on the geochemical characteristics of the metallogenic intrusions and S-Pb isotope composition of the ore,the diagenetic and ore-forming materials were traced,and combined with the latest data and results of regional tectonic evolution,the tectonic setting of the Shanmen silver deposit was determined.Besides Quaternary,strata in the Shanmen Ag deposit and adjacent area include mainly a set of metamorphic clastic-carbonate rocks of Middle Ordovician Huangyingtun Formation.The Yanshanian monzogranite and quartz diorite are the major magmatic rocks.The Ag orebodies occur in brittle fractures whithin the monzogranite intrusions and in the contact zone between the Yanshanian quartz diorite and marble with metamorphic siltstone of the Huangyingtun Formation.The ore bodies are controlled by the NNE-trending faults and have layered,veined,and lenticular shapes.The ore types of the Shanmen Ag deposit include mainly quartz-vein type?Longwang ore block?and fractured altered-rock type?Wolong ore block?,and the latter is dominant accounting for relatively high Ag grade.The metallic minerals mainly include such sulfides as pyrite,galena,sphalerite,chalcopyrite,and silvery minerals as argentite,native silver,diaphorite and pyrargyrite.The ore structures are dominated by massive,vein,banded and disseminated structures,and the metallic minerals have mainly euhedral-subhedral granular,metasomatic,exsolution and poikilitic textures.Silicification,sericitization,chloritization and carbonation mainly develop in both the ore and wall rocks,and the silicification and sericitization occurred near ore minerals and are closely related to the mineralization.According to the paragenetic association of minerals,ore textures and structures,cross-cutting relationships and wall-rock alterations,the metallogenic process of the Shanmen Ag deposit can be divided into pyrite-quartz stage,quartz-Ag-polymetallic sulfide stage,and quartz-carbonate stage.The researches on the fluid inclusions included in different stages indicate that the early stage?Stage I?mainly contains three types of fluid inclusions:liquid-rich two-phase?L-type?,vapor-rich two-phase?V-type?,and CO₂ aqueous three-phase?C-type?.The fluid belongs to a medium-high temperature and medium-low salinity H₂O-NaCl-CO₂ system and has boiling characteristics.The middle stage?Stage II?is mainly characterized by liquid-rich two-phase?L-type?and vapor-rich two-phase?V-type?inclusions,and the fluid mixing of different nature leads to the escape of CO₂.Only liquid-rich two-phase?L-type?inclusion is identified in the late stage?Stage III?.The fluids of two later stages belong to the medium-low-temperature and low-salinity H₂O-NaCl system.Homogenization temperatures from the early to late stages range from 272.2 to 412.5°C,124.1 to 313.3°C,and 128.6 to 224°C,respectively.Fluid salinities,from the early to late stages,range from1.6 to 12.1,1.4 to 8.9,and 0.4 to 5.8 wt.%NaCl equiv.,respectively.The gradually decreasing trends of homogenization temperatures and salinities,along with the reduction of CO₂ content,indicate that the release of CO₂ and the low-temperature environment are important causes of the precipitation of silver minerals.The?¹⁸O_H2O values of the quartz vein in the different stages range from-3.7‰to+8.1‰,and the?D values range from-112.8‰to-102.8‰,indicating that the original ore-forming fluid was mainly derived from magma and that the amount of meteoric water gradually increased during the mineralization process.The?³⁴S values?ranging from-11.4‰to+1.79‰?and Pb isotope compositions(²⁰⁶Pb/²⁰⁴Pb=18.143¹8.189,²⁰⁷Pb/²⁰⁴Pb=15.543¹5.599,²⁰⁸Pb/²⁰⁴Pb=38.062³8.251)of sulfides suggest that the ore materials have mixed mantle and crustal sources.Therefore,it has been proposed that the release of CO₂ and the low-temperature environment are important conditions for silver minerals precipitation,and the mixing of fluids of different nature is the dominant metallogenetic mechanism.The Shanmen Ag deposit can be classified as the intrusion-related medium-low temperature hydrothermal vein-type deposit.LA-ICP-MS zircon U–Pb dating of the monzogranite,quartz diorite,and diorite-porphyrite in Shanmen deposit were 167.0±1.5 Ma,167.6±1.9 Ma,and 160.1±2.2 Ma,respectively,which indicates that the petro-and ore-forming period of the Shanmen Ag deposit are Middle Jurassic.The metallogenic monzogranite rock samples have high SiO₂?70.43⁷1.74%?,Al₂O₃?13.17¹3.99%?,Na₂O?3.98⁴.03%?,and K₂O?4.56⁴.94%?and low Mg^#?17.6²0?,MgO?0.49⁰.54%?,and CaO?1.32¹.38%?,which indicate high-K calc–alkaline series rocks.The rare earth elements?REEs?are characterized by a relative enrichment of light rare earth elements?LREEs?and a relative depletion of heavy rare earth elements?HREEs?.The monzogranite includes a significant fractionation of both REE types??LREE/?HREE=11.20¹4.09,?La/Yb?N=14.26²1.96?and displays moderately negative Eu anomalies as well as weakly negative Ce anomalies??Eu=0.56⁰.60,?Ce=0.93¹.04?.In addition,the trace elements are generally enriched in large ion lithophile elements?LILEs?such as Ba,Rb,and K and incompatible elements such as Th and U but are depleted in high field strength elements?HFSEs?such as Ta,Nb,P,and Ti.The geochemical characteristics of the monzogranite are consistent with those of igneous rocks in the active continental margin.The Sr-Nd-Pb isotope contents show that the monzogranite has low Sr initial ratios?0.704495⁰.704673?and high Sm/Nd initial ratios?0.512257⁰.512278?,and the?_Nd?t?values and young Nd-model ages are-3.2 to-2.8 and 992 Ma to 1073 Ma,respectively.Moreover,the radioactive content resulted in relatively high ²⁰⁶Pb,²⁰⁷Pb,and ²⁰⁸Pb,whereas the non-radioactive content caused relatively low ²⁰⁴Pb.These data indicate that the original magma was derived from the partial melting of Meso-Neoproterozoic young lower crust enriched in mantle-derived material that was affected by the mixing of ancient crustal material during the intrusion process.According to the geochemical characteristics of the ore-forming rock mass and the regional tectonic evolution analysis,it can be concluded that the monzogranite and related Ag mineralization in the Shanmen area formed in an active continental marginal tectonic setting related to the Early-Middle Jurassic subduction of Paleo-Pacific plate.