Superimposed mineralization of the Tianbaoshan metallogenic region in Yanbian area(eastern Jilin Province),northeastern China:Indicated by the isotopic dating

The large-scale Tianbaoshan metallogenic region,with over one hundred years of mining history,is located in eastern Yanbian area,northeastern China. Thanks to abundant nonferrous metal resources and diverse deposit types,metallogenic theory and ore-prospecting in the Tianbaoshan region have been paid attention to both at home and abroad for a long time. According to the recent research data,the Yanbian area in NE China belonged to the Paleo-Asian Ocean tectonic region during the Paleozoic Era and to the Paleo-Pacific Ocean tectonic region since the Jurassic period. Evolution and superimposition of the two tectonic regions resulted in multi-cyclic tectonic-magmatic activities and provided a beneficial ore-forming geological condition to the intensive nonferrous and precious metal mineralization in this area. Up to now,at least four types of deposits have been discovered,including skarn lead-zinc deposit( e. g. Lishan deposit and Xuanchanghoushan deposit),crypto-explosive breccia lead-zinc-silver deposit( e. g. Xinxing deposit),sedimentary metamorphic-hydrothermal copper-lead-zinc deposit( e. g. Dongfengnanshan deposit) and lode(-porphyry)molybdenum deposit( e. g. Dondfengbeishan deposit). Among these deposit,the Lishan and Dongfengbeishan are more valuable than others. The Lishan deposit occurs in the contact belt between the Late Paleozoic diorite and the Permian marble,the major ore body is controlled by the contact belt and fractures,metal minerals include pyrite,galena,sphalerite,chalcopyrite,pyrrhotite and minor magnetite. The Dongfengbeishan deposit is characterized by veinlet and stockwork molybdenum mineralization in the Late Paleozoic biotite quartz diorite,there are a small amount of such metal minerals as molybdenite,pyrite,chalcopyrite,pyrrhotite and minor arsenopyrite. Due to few accurate geochronology data,it's still uncertain that the two deposits are derived from either the same or different tectonic-magmatic events. Moreover,their tectonic setting is still in controversy. To determine the geochronology framework of the polymetallic mineralization and associated magmatic events in the this region,isotopic dating has been completed,including LAICP-MS U-Pb dating of zircons from the metallogenic diorite in Lishan skarn deposit and the ore-hosting biotite quartz diorite of lode molybdenum mineralization in Dongfengbeishan deposit,as well as the molybdenite Re-Os dating in Dongfengbeishan deposit. Isotopic ages of fifteen zircon grains from the Lishan diorite change from 277 ± 2Ma to 254 ± 3Ma,with the weight average age of 266. 2 ±3. 9Ma( MSWD = 14). U-Pb isotopic ages of nineteen zircon grains from Dongfengbeishan biotite quartz diorite vary from 283 ± 3Ma to271 ± 3Ma,and the weight average age is 278. 4 ± 1. 8Ma( MSWD = 2. 9). These isotopic dating results indicate that both plutons emplaced in the Late Hercynian period. The REE composition and chondrite-normalized REE patterns of the zircon from the Lishan diorite and Dongfengbeishan biotite quartz diorite,together their similar isotopic age,imply that both plutons were derived from the contemporaneous co-magmatism. The Re-Os isochron age of six molybdenite samples from the Dongfengbeishan deposit is 192 ±3. 1Ma,and model ages change from 190. 3 ± 3. 1Ma to 194. 6 ± 2. 8Ma,which suggests that the molybdenum mineralization took place in the Early Yanshian period. Because the Lishan lead-zinc deposit belongs to the skarn type,the age of the ore-forming diorite,266. 2± 3. 9Ma,can be regarded as the metallogenic age. Although the ore-hosting biotite quartz diorite in the Dongfengbeishan deposit emplaced in the Hercynian period,the molybdenum mineralization took place in Early Yanshanian period,which indicates that there should occur the Early Yanshanian intrusion closely related to molybdenum mineralization. Moreover,it can be concluded that the Tianbaoshan polymetallic metallogenic region underwent at least two large magmatic and metallogenic events,the Late Hercynian and the Early Yanshanian,respectively. The study area is located in the east edge of the Xing'an-Mongolian Orogenic Belt( Central Asian Orogenic Belt),and the northeastern part of the intersection region of the Xilamulun-Changchun and the Dunhua-Mishan faults.According to the popular tectonics views on NE China,this district belonged to the Paleo-Asian Ocean tectonic region in Permian period. There are more and more evidence support that the final closure of the Paleo-Asian Ocean from the latest Permian to the Early Triassic period resulted from the matching between the Xing'an-Mongolian Orogenic Belt and the North China Craton. Since the Jurassic period,intensive tectonic-magmatic actives in the east portion of the Xing 'an-Mongolian Orogenic Belt were probably controlled by the subduction of the Paleo-Pacific plate. Based on previous research and data in this paper,it can be conclude that the Lishan diorite and associated skarn lead-zinc mineralization as well as ore-hosting biotite quartz diorite in Dongfengbeishan deposit were formed under the background of the Paleo-Asian ocean tectonic region,and the molybdenum mineralization in Dongfengbeishan deposit can be classified into the Paleo-Pacific tectonic-magmatic-metallogenic event. In addition,the superimposition of different period of tectonic-magmatic-mineralization may result in such especial characteristics as large-scale,complex types as well as paragenesis of several beneficial elements in Tianbaoshan region.