Geodynamic Evolution Of Qilian Orogenic Belt And Metallogenesis Of Endogenous Metals

Qilian orogenic belt was a NW-trending large mountain chain located in the middle of the Qin-Qi-Kun Central Orogenic System and was a part of the northern edge of the Tibetan Plateau. Because of a unique geological background with long evolution history, it became a typical and complex composite orogenic belt and an important metallogenic belt in western China, producing non-ferrous, ferrous and precious metal deposits in different geological periods and environments.Based on the theory of orogenic belt and geological data about this region in the past, the paper further discussed and re-determined the geodynamic processes of the Qilian orogenic belt, established geodynamic evolution model. Supported by modern test technology and application of modern mineralization theory, it studied different genetic types of the region's typical deposits and summarized deposit series in the Qilian orogenic belt in different geological periods, illustrated temporal and spatial distribution regularities and mineralization characteristics of endogenous metal deposits.The results showed that there were three ancient ocean in Qilian orogenic belt (the North Qilian Ocean, Danghenanshan-Lajishan ocean, the North Qaidam ocean) and two blocks within North and South Qaidam blocks as well as inter Alashan block (Middle Qilian Block, South Qilian Block) The determination of the three oceans implied the evidences of the so-called continental rift biased over the past. Meanwhile, it considered in the past that the emergence of the ocean was in late Neoproterozoic, but our studies on volcanic rocks of Zhulongguan group in Mesoproterozoic showed it was part of the oceanic basalt plateau in Mesoproterozoic, controlled by plume activities, formed in mature oceanic plate, therefore, in Mesoproterozoic, the study area already existed mature ocean, which formed earlier than Proterozoic, that the specific time need to be further investigated.This paper identified the control of metamorphic core complex on gold mineralization for the first time, rather than the previous view that the thrust structural control in Qilian orogenic belt, by the study of Tanjianshan gold deposit in Northern Qaidam. Appling LA-ICP-MS U-Pb method, it determined the age of plagioclase granite porphyry at 394.4±6.0Ma which was the composition of metamorphic core complex, production of the continent-continent collision in Early Hercynian; the age of felsic dykes with gold mineralization were 344.9±2.2Ma, production of extension phase in late Hercynian, which was also the age of final emplacement of metamorphic core complex and gold mineralization. All of them represented continuous evolution and mineralization of the same magmatic activity.This paper proposed a systematic summary of exhalative mineralization characteristics and the deposit types in different extensional environment in the region, established the "double rock group" consist of exhalative rock and black rock series, which accompanied by the output in space. As unique in exhalative deposits, they can be used as identification of exhalative deposit.A detailed study on the felsic intrusive-related hydrothermal mineralization types and characteristics was carried out, which formed at different felsic magmatic evolution stages and different parts of their emplacement. Studies on metallogenic ages and fluid inclusions of some deposits showed that the formation of Daheishan tungsten deposit went through two periods and four stages:Fluid temperature of the skarn mineralization at phaseⅠwas 360～480℃, mineralization depth was 4.8-5.6km. temperatures of quartz-sulphide mineralization at phaseⅡconcentrated in the 200～320℃, with mineralization depth of 2～2.4km, considered as middle to high-temperature hydrothermal deposit; U-Pb age of deposit was 422 Ma, formed in the Late Ordovician. Temperature of Shuangkoushan lead-zinc deposit and Lvliangshan copper deposit concentrated in 120～260℃, the depth of mineralization in the 1.4～2.86km, both of them were low-temperature hydrothermal vein-type deposit. Also, the paper determined gold deposit in Lajishan Mountain as skarn type, and proposed ideas to find similar deposits in the high-calcium volcanic areas.Moreover, the paper gave a detailed study on orogenic gold mineralization, established a continuous orogenic gold mineralization model in the region. Fluid inclusion studies showed that the types of most deposits were orogenic gold deposit and shallow type in sub-categories.Based on the characteristics of newly discovered Niubiziliang copper-nickel mineral occurrence in Northern Qaidam and comparative studies on similar deposits in the region and adjacent areas, the paper figured out that the district had a good condition for the formation of magmatic segregation deposits and had great potential for prospecting potential.Finally, based on systematic summary of metallogenic regularity, the results of this study and the new progress made in prospecting area, this paper proposed a number of new prospecting ideas and suggestions on the prospecting orientation, chief prospecting mineral varieties and so on.