The Yanshanian Minerogenetic Series And Mineralization Of Polymetallic Deposit In The Yangchun Basin Of Yunkai Area, South China

The Yunkai area is located at the intersection of the Yangtze Craton and Cathaysia Block, which is one of the most important polymetallic mineralization area with many large and super-large deposits in China. Especially gold and copper reserves have significant potential, and received more and more attention by many geologists. As a representative volcanic basin in the Yunkai metallogenic belt, the Yangchun basin shows noticeable feature of mineralization and magmatism, which contains Cu polymetallic deposits related to intermediate-acid intrusive rocks and W-Sn polymetallic deposits associated with acidic granite. Howerer, owing to the complex geological conditions, serious surface vegetation cover and other objective conditions, basic geological research is relatively weak and poorly understood. Therefore, many comprehensive research work needs to do, such as the ore genesis, metallogenic regularity and the relationship between magmatism and mineralization. This paper takes the Yangchun basin as the subject, based on the summary of previous studies, detailed geological investigation and microscope observation, combine with geochemistry, geochronology and in-situ laser ablation trace elements analysis methods, aims to achieve a comprehensive understanding of magmatism and mineralization. Detailed study of typical polymetallic deposits include: Potoumian, Shilu, Tiantang Cu polymetallic deposits related to intermediate-acid intrusive rocks and Yingwuling, Xishan W-Sn polymetallic deposits associated with acidic granite. The following achievements and conclusions have been obtained:1) Based on the previous researches, we recognize these deposits in two episodes, i.e. Cu polymetallic deposits minerogenetic series(Ⅰ) related to Middle Juarassic intrusive rocks and Cu-Pb-Zn-W-Sn polymetallic deposits minerogenetic series(Ⅱ) associated with Cretaceous intermediate-acid intrusive rocks. The latter can be detailedly subdivided in two subseries of ore deposits, i.e. Cu-Mo-Pb-Zn polymetallic deposits subseries(Ⅱ-1) related to Early Cretaceous intermediate-acid intrusive rocks and W-Sn polymetallic deposits subseries(Ⅱ-2) associated with Late Cretaceous granite.2)Ⅰminerogenetic series and Ⅱ-1 subseries are distributed along the marginal depression of Yangchun basin and controlled by composite part of NE-NNE trending structures and EW structures. Metallogenic rockbody associated with series are mainly typical shoshonitic Ⅰ rocks, which originated from Pacific plate subduction and generated by partial melting of lower crust induced by the underplating K-rich of the and hydrous mafic magma, and these magmas of high oxygen fugacity were emplaced at a shallow level and formed granodiorite and related porphyry-skarn Cu polymetallic deposits; Metallogenic rockbody related to Ⅱ-1 subseries are mainly high-K calc-alkaline I-type granites, which generated by partial melting of ancient crustal material(compatible with those of the Neoproterozoic arc magmatic rocks) derived from residual oceanic slab remelting even metasomatized subcontinent lithosphere. Enriched mantle or residual plates not only provides a large number of sources of ore-forming elements, such as copper, but also provides abundant fluid. Finally, ore-bearing magma emplaced along the ancient Qin Hang suture, enriched and precipitated gradually on favorable conditions, and formed Ⅱ-1 subseries; Ⅱ-2 subseries are controlled by buried NW-trending structures, and occur in the contact part of uplifting area and hollow zone as well as parts of uplifting areas of the hollow zone. Granitoids associated with mineralization are mainly weak-intermediate peraluminous A-type granites, which formed by the melting of crustal substances, and probably input of a small amount of mantle substances. The magmas occurs strong fractional crystallization, leading to highly enrichment of volatile and rare metals elements in the residual melt, eventually emplaced and formed related W-Sn polymetallic deposits.3) A large number of high-precision dating data shows that:Ⅰminerogenetic series has an age range of 170~160 Ma, Ⅱ-1 and Ⅱ-2 subseries with an age concentrated in 110~98 Ma and 85~76 Ma, respectively. In conclusion, the metallogenetic geodynamical settings ofⅠseries corresponding to the mineralizations above are Pacific plate subduction environment, and Ⅱseries may be related that Chinese mainland including South China Plate was in a sustained stretch phase with the strong crust-mantle interaction, which was caused by changing movement direction of the Pacific Plate after 135 Ma.4) Neoproterozoic inherited zircon are found firstly in Shilu and Tiantang granitoids, which is compatible with those of the Neoproterozoic arc magmatic rocks in the eastern Jiangnan Orogenic Belt. Consequently, the Neoproterozoic arc magma might have been involved in the formation of the Shilu and Tiantang granitoids, and this arc magma belt and corresponding continent-arc collision belt between the Yangtze and Cathaysia Block might extend to Yangchun basin.