Ore Genesis And Tectonic Setting Of Xiaohongshilazi Lead-zinc Deposit In Panshi Area,Jilin Province

The Xiaohongshilazi deposit is located in the eastern section of the north margin of the North China Block,near Xar Moron–Changchun–Yanji suture zone.There occur two types of lead-zinc ore bodies,namely the layered and vein-type.The layered ore bodies are hosted within early Permian marine volcanic rocks of the Daheshen Formation and obviously controlled by strata.The micro sedimentary rhythm in the ore indicate synsedimentary genesis.The main metallic minerals in the ore are sphalerite,galena,pyrrhotite,which display xenomorphic granular,weak metasomatic textures,and banded structure.The principal component characteristics of sphalerite is similar to that of the Hongtoushan zinc-copper deposit(VMS deposit),and different from that of Taolin and Dongpochaishan zinc-copper deposit(Hydrothermal deposit).The ores contain some minerals such as dolomite with the characteristic of thos of Sedex deposit.The wall rock has weak alteration.Therefore,this paper preliminary argue that the genesis of the layerd ore body is the VMS type.The vein-type Pb–Zn ore bodis are distributed along tectonic fissures and granodiorite porphyry dike,which controlled by fracture and granodiorite porphyry dike.The typical low-to medium-temperature hydrothermal mineral assemblage(pyrite sphalerite chalcopyrite galena quartz calcite)is developed in the ores.The fluid inclusion study shows that the ore-forming fluid of the vein-type orebodies belongs to the l low-to medium-temperature magmatic water.The S-Pb isotopic characteristics show that the ore-forming materials are mainly derived from the upper crust.In summary,we argue that the genesis of vein ore bodies in the Xiaohongshilazi deposit was of a low-to medium-temperature hydrothermal vein type,related to magmatism.Comprehensive research shows that the Xiaohongshilazi deposit is the VMS lead-zinc deposit and surperimposed by the hydrothermal vein-type lead-zinc mineralization.The micro sedimentary rhythm of ore bodies and ores indicate its formation period is consistent with volcanic rocks of the Daheshen formation.The diorite-porphyrite vein in the Xiaohongshilazi deposit obviously cut the layered ore bodies,so its formation time is later than layered ore bodies;but there is no cutting or relationship between the diorite-porphyrite vein and the vein Pb–Zn ore body.The granodiorite porphyry also cut through layered orebody,and consistent with the distribution direction of the vein ore body;The extensive development of the vein Pb–Zn ore body or lead-zinc mineralization in the vicinity of the granodiorite porphyry,indicate close relationship between the granodiorite porphyry and vein Pb–Zn ore body.The granodiorite porphyry is younger than the layered ore body and nearly simultaneous to the vein ore body.Zircon U-Pb age of the rhyolite,dacite and andesite in the Daheshen formation is 302-299 Ma.Zircon U-Pb age of the diorite porphyrite is 225.3±5.1Ma.Zircon U-Pb age of the granodiorite-porphyry is 203.6±1.8Ma.Rb-Sr isochron age of the vein ore body is 195±17Ma.In summary,this paper argues that the layered mineralization should be formed in the Early Permian and the vein mineralization should be formed in early Jurassic.The ore-hosting rocks of the layerd ore body in the Xiaohongshilazi deposit are composed of a set of calc-alkaline to alkaline series(basalt,andesite,dacite and rhyolite).Trace elements volcanic rocks enriched in LREE and LILE,depleted in HFSE(Ta,Nb,Ti etc.).The rock assemblage and geochemical characteristics is similar to active continental margin volcano rocks and the igneous magma originated from partial melting of the lower crust.Previous studies have shown that at Early Permian,at the eastern part of the northern margin of the North China plate,the ancient Asian Ocean has not closed,and continue to dive under the North China plate.Therefore,this paper argue that layered mineralization of the Xiaohongshilazi deposit formed in the active continental margin environment during the closure of the paleo Asian Ocean.The granodiorite porphyry,which is closely related to the vein-type mineralization,is a high-k calc alkaline I type granite.It is enriched with Ba,Th,loss of Ta,Nb,Hf,Zr,Sm,Y,Yb,Lu and other trace elements.The fractionation of HREE and LREE is obvious,which obviously enriched in LREE and depleted in HREE.Geochemical characteristics of the granodiorite porphyry are similar to those of the active continental margin arc granite and the magma source is a mixture of partial melting of mantle derived underplating and crustal material.Combined with the latest research data and achievements of regional tectonic evolution,this paper argue that,at northern margin of North China plate,westward subduction of the Paleo Pacific may start at the end of the late Triassic.Vein-type mineralization of the Xiaohongshilazi deposit formed in the active continental margin setting associated with subduction of the Paleo Pacific plate.