Zn, He-Ar and Sr-Nd isotopic compositions of the Tongkeng Tin-polymetallic ore deposit in south China: Implication for ore genesis.

The study results of Zn, He-Ar and Sr-Nd of the Tongkeng deposit show that Zn source mainly comes from the Cretaceous Longxianggai granite magmatic fluid, classified into the type of post-magmatic hydrothermal filling-metasomatism. He-Ar isotope study shows that the mantle-sourced substances involved in the mineralization are related to that of the Devonian Liujiang Formation exhalative sedimentary sequences. • The Longxianggai granite belongs to the typical remelted S-type granite. • The mantle-sourced substances in ores are from the Devonian exhalative sediments. The Zn most likely derived from the Cretaceous Longxianggai granite. The Tongkeng tin-polymetallic ore deposit in Nandan County, Guangxi Zhuang Autonomous Region, southern China, is the representative deposit of the largest scale in the famous Dachang tin-polymetallic ore field. It consists of the upper tin-polymetallic ores and the lower newly found zinc-copper ores with the Sn metal reserve of 680,000 tons (average grade 1%), 4,500,000 tons of Zn (average grade 4%), and 80,000 tons of Cu (average grade 0.2%). The orebodies occur as layers and veins within the Middle to Late Devonian siliceous rocks and carbonate rocks. In this paper, the isotopic compositions of Zn, He-Ar and Sr-Nd of the Tongkeng deposit have been studied systematically. The results show that the isotopic composition of Zn is quite homogenous with δ66Zn = 0.22 ~ 0.34‰ in sphalerite samples of various colors from brown to black, collected from different orebodies (upper tin-polymetallic and lower zinc-copper ores) in the Tongkeng deposit. It indicates that Zn source mainly came from magmatic fluid, as the calculated δ66Zn (0.42 ~ 0.54‰) of ore-forming fluid is almost same as that of the Longxianggai granite (0.41 ~ 0.55‰), but quite different from that of the marine carbonate (average 0.91‰). He-Ar isotope study of pyrite shows that the exhalative substances might have been involved in the mineralization, with 3He/4He = 1.2 Ra ~ 2.5 Ra in the upper tin-polymetallic ores and 3He/4He = 0.8 Ra ~ 1.9 Ra in the lower zinc-copper ores in the Tongkeng deposit, which is different from that of the Longxianggai granite (3He/4He = 0.7 ~ 0.8 Ra), but close to that of the Devonian Liujiang Formation exhalative sedimentary sequences with 3He/4He = 1.5Ra. The Longxianggai granite has I Sr = 0.719 (in average), εNd(t) = -7.9 ~ -9.6 and Nd model age T 2DM = 1.54 ~ 1.68 Ga. Based on the geochemical characteristics of granite and Sr, Nd and O isotopes, the source rocks of the Longxianggai granite may be mainly derived from the continental crust, which is a typical remelted S-type granite. Based on the results of this study and the previous data in the literature, it is concluded that the mineralization in the Dachang ore field is closely related to the Cretaceous Longxianggai granite in time and space with the main metal substances of Zn most likely derived from the magmatic fluid, classified into the type of post-magmatic hydrothermal filling-metasomatism. The admixing of the early Devonian spouting sediments in the mineralization was evidenced in the study of source trace and has led to the longtime dispute over the ore genesis. [ABSTRACT FROM AUTHOR]