Tectonic Setting And Metallogenic Significance Of Late Paleozoic Volcanic Rock Series In The Yanbian Area,NE China

The Yanbian area is located at junction between the southeast margin of the Xing'an–Mongolian Orogenic Belt?SM-XMOB?and the northern margin of the North China Plate?NCP?,and belongs to the central Jilin-Yanbian metallogenic belt.The area had experienced the evolution,superposition and transition of the Paleo-Asian Ocean and Paleo-Pacific Ocean tectonic domains during the Late Paleozoic to the Early Mesozoic.Multi-period tectonic-magmatic activities had resulted in a volume of intermediate-acid plutons and volcanic rock series consisting of volcanic rocks or volcanic-sedimentary formation.The Late Mesozoic continental volcanic rocks in Yanbian area are closely related to the epithermal gold deposits,and the Late Paleozoic marine volcanic rocks are assocoated with VMS-type,hydrothermal vein-type and skarn-type copper polymetallic deposits.Taking the volcanic rock series of Miaoling Formation as an example,this thesis plays major researches on their geological and mineralization alteration characteristics,formation age and tectonic setting,magma source and metallogenic significance.The main study methods include petrography and mineragraphy,LA-ICP-MS zircon U–Pb dating,petro-geochemistry and Sr-Nd-Pb-Hf isotopic analysis.Some conclusions and research developments have been drawn as follows:1.The rock assembles and alteration-mineralization characteristics of the volcanic rock series of Miaoling Formaion have been confirmed.The field geological investigation and petrography and mineragraphy studies indicate that the strata of Miaoling Formation are widely distributed in Hunchun,Kaishantun,Tianbaoshan and Tianqiaoling in the Yanbian area,and belong to a suite of shallow-marine volcanic-pyroclastic-carbonate-terrigenous sedimentary formation.The rocks in the lower member include dark andesitic tuff breccia,tan dacite tuff and a small amount of argillaceous slate;those in middle member are mainly gray-green andesite,dark andesitic tuff,dacite,and tuffaceous slate;and the upper member are composed of gray-green foliated andesite,andesitic tuff,dark-colored tuffaceous sandstone,siltstone,argillaceous siltstone,and partly interbedded with limestone.The VMS-type,hydrothermal vein-type and skarn type copper polymetallic mineralization have been developed in the volcanic rock series of Miaoling Formation with the alteration of silicification,sericitization,carbonation,skarnization,chloriteization and epidotization.2.The formation ages of the volcanic rock series of the Miaoling Formation in Yanbian area have been constrained.The LA-ICP-MS zircon U–Pb dating of the volcanic rocks of the Miaoling Formation in different regions of Yanbian area indicated that the crystallization age of the andesite crystal-lithic tuff in Tianbaoshan district is 272.7±3.6Ma,that of the crystal-lithic tuff collect from Kaishantun district is 272.8±4.4Ma,the crystallization age of the andesitic lithic-crystal tuff in Hunchun is 270.2±3.6Ma,those of the dacite lithic-crystal tuff and andesite brecciferous lithic-crystal tuff in Tianqiaoling are 271.4±3.2Ma,and 268.3±3.2Ma,respectively,which leads to the conclusion that the formation period of the volcanic rocks in the Miaoling Formation is Early Permian.3.The forming environment of the volcanic rock series of the Miaoling Formation has been definitely.The volcanic rocks of the Miaoling Formation are mainly composed of andesite-dacite-rhyolite.According to both previous and this studies,some exhalite usually developing in the nowadays white or black chimney in sea bed have identified in the ore-bearing volcanic rock series of the VMS deposit in the Miaoling Formation.For examples,the exhalite in the Hongtaiping deposit mainly include chlorite quartzite,calcium-rich siliceous rock,and calcium silicate,and there have developed the stratiform fluorite rock,fluorite chlorite rock and garnet zinc-spinel siliceous rock in ore-hosting volcanic rocks of the No.5 ore body in Dongfengnanshan deposit of Tianbaoshan mineralization region.Combined with the paleontologic analysis,the conclusion is that the volcanic rock series were formed in a shallow marine facies environment.4.The tectonic setting of the volcanic rocks of the Miaoling Formation and Late Paleozoic to Mesozoic tectonic evolution of the Yanbian area has been summarized.The volcanic rocks in the Miaoling Formation are similar to the rock assemblage and geochemical characteristics of the Andean-type active continental margin volcanic rocks.These rock samples are enriched in light rare earth elements?LREEs?,large lithophile elements?LILEs?,and depleted high field strength elements?HFSEs?.Combining its sedimentary charactetitics indicate that they were formed in the tectonic evolution environment of the Andean-type active continental margin to the back-arc basin,and imply that the SM-XMOB were in the subduction of the PAO in Early Permian.Based on the temporal and spatial distribution,lithological assemblage,zircon U-Pb chronology and geochemical characteristics of the Late Paleozoic volcanic rocks,combined with regional tectonic magmatic events and previous research results in the area,it is believed that the Yanbian area mainly experienced three major periods from the Late Paleozoic to the Early Mesozoic:the subduction of PAO in Early Permian to Middle Permian;the closure orogenic of the PAO in Late Permian to Early Triassic;the final closure of the PAO in Middle Triassic.The volcanic rocks of Early Permian Miaoling Formation formed in the tectonic setting of the subduction of PAO.5.The geochemical characteristics and the magma source of the volcanic rocks in the Miaoling Formation have been studied.The volcanic rocks of the Miaoling Formation have relatively high contents of Si O₂,K₂O and Al₂O₃,and low contents of Mg O and TFe₂O₃?total Fe?.They are enriched in LREEs and LILEs?Rb,Ba,Th and U?,and depleted in HFSEs?Nb,Ta,Ti and P?,showing a weak negative Eu anomaly??Eu=0.71?1.00?,volcanic rock has a low Mg#value?38?49?,belonging to the weak partial aluminum-peraluminous calc-alkaline series.The average value of Nb/Ta is 14.25,between the average value of mantle?17.5?and crust?11?,the Rb/Sr value is 0.02?0.25,which higher than the average value of mantle?0.034?and less than the average value of crust?0.35?,indicates that the volcanic rocks have the characteristics of mixed crust-mantle sources.The?_Hf?t?values of zircons are all positive values?+1.8?+15.8?,and the two-stage model age(T_DM2)is 348 to 1176Ma.The²⁰⁶Pb/²⁰⁴Pb value,²⁰⁷Pb/²⁰⁴Pb value and ²⁰⁸Pb/²⁰⁴Pb value is 18.368 to 18.631,15.531 to15.625,and 38.261 to 38.549,respectively.The initial value(⁸⁷Sr/⁸⁶Sr)_i is 0.703718 to0.708789,the?_Nd?t?value is+2.5 to+5.1,and the two-stage model age(T_DM2)is between626 to 837Ma.The above studies show that the primary magma of Miaoling Formation volcanic rocks is mainly derived from the partial melting of juvenile lower crustal material in the Middle-Neoproterozoic,with a small amount of mantle mixed in.6.Metallogenic and ore-prospecting significance of volcanic rock series of the Miaoling Formation on the regional polymetallic mineralization has been summarized.Comprehensive analysis indicates that the Late Paleozoic volcanic rock series in the Yanbian area have significant mineralization significance for both regional metallogenies,and ore-prospecting.On the one hand,the volcanic-sedimentary formation of Miaoling Formation is contemporary and homologous with VMS mineralization.On the other hand,precious metal and nonferrous metal in the volcanic-sedimentary formation have high element abundances,implying the volcanic-sedimentary formation can provide metallogenic material for post hydrothermal vein-type and skarn type deposit as the source bed.The result of soil geochemical measurement in the surrounding of Hongtaiping copper polymeallic deposit indicate that there has an abnormal high-background of mineralization elements of Au,Ag,Cu,Pb,Zn,As and Sb in the Miaoling Formation.The result of1/10,000 soil geochemical measurement in the Yantonglazi gold polymeallic deposit indicate that there has an abnormal high-background of mineralization elements of Au,Ag,Cu,Pb,Zn,As,Sb,Bi,W and Mo in the Miaoling Formation.