Tectonic-Magmatic Evolution And Mineralization Of Zetang Orefield In Gangdese,Tibet

From the Tethys subduction to the continental collision, the Gangdese orogenic belt has expericnecd complex evolution and development about tectonism, magmatism and metallization, which forming a gaint metallogenic belt with great varieties of stage,type and mineral.In this thesis,we regard Zetang orefield,located in the southern margin of the Gangdese ore belt and adjacent to the Yarlung Zangbo suture zone,as the research object. We undertake inverstigation and research about the structure feature, magma types,deposit characteristics and metallization based on field geological survey and testing methods,including isotope dating,lithogeochemical testing and Electron Spin Resonance method and so on. Our study was divided into four stages, correspongding to the orogenetic process,namely Tethys subduction stage, main collisional orogenic stage,late-collisional transformation stage and post-collisional extension.According to the research about each stage,summarize the relationship between tectonic evolution and mineralization.It is propitious to strengthen the understanding of ore host rock,ore-controlling structural and metallization,besides,it is also in favor of investigating the metallogenic potential and metallogenic regularities. The achievements are as follows.1.Tethys subduction stage:Plagioclase granite from Zetang terrane has the Zicon U-Pb ages from152.5Ma to156Ma,and show the adakite character. The source of host rock maybe come from the melt plate subduction, associated with the intra-oceanic arc environment. Moreover, the andesite basalt also has the characteristics of magmatic arc, its trace element display IAB type character. All the results suggest that, it was exist a intra-oceanic subduction in Late Jurassic. At the subduction late stage, like-adakite magma had intruded, with94.1Ma age of Zicon U-Pb. The adakite maybe come from subduction plate, and mixed with mantle material. This period adakite intrude wall rock, forming the Sangbujiala deposit. The Re-Os age of molydenite in Sangbujiala deposit is (93.3±4.1) Ma, in Late Cretaceous.2.Main collisional orogenic stage:The age of this period intrusive rock is50-63Ma.and the geochemical characteristics indicate they are island arc magmatic rocks with Ⅰ type, high calc-alkaline,potassium and continental margin arc characters. This kind of magmatic has an advantage of mineralization. Researchers have found this period mineralization nearby Zetang but not in Zetang orefield. Zicon U-Pb ages of the volcano rock interlayer in Dazhuka group are between49.6Ma with52.6Ma. and the ESR ages of the sandstone are from19.8Ma to28.5Ma.We redefine the Dazhuka group age with these results, from early Eocene to Miocene. The age of Dazhuka group illustration the Tethys ocean had closed completely and the mountains caused by collision has been uplifting before52Ma, in Zetang.3.Late-collisional transformation stage:It is the main metallogenic stage of Zetang orefield.The deposits are skarn and the ore bodies are controlled by the contact boundary of intrusive rocks and wall rocks. The era of mother rocks concentrated at29.88-31.64Ma, in accordance with the Re-Os age of molydenite in deposits. The ore-forming mother rock has two types, one with adakitic features and the other with arc magmatic features. Suggesting that the mother rock has diversity sources and resulting in different tectonic environment.4.Post-collisional extension:Gangdese Thrust(GT) and Great Counter Thrust(GCT) thrust upon Dazhuka group in opposite direction, thus the two thrust systems at least later than19.8Ma. Besides, they are become the most important structure destructing the early mineralization. In addition to these faults, Zetang orefield widely developed north-east, north-west and north-south trend joints and cracks, which were filled by hydrothermal, and developmented weak mineralization.