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Geochemistry And Geodynamic Backgrounds Of The Tongyu VHMS Copper Deposit And The Songshugou Chromite Deposit In The Northern Qinling Orogenic Belt

Posted on:2011-07-29Degree:MasterType:Thesis
Country:ChinaCandidate:B LiFull Text:PDF
GTID:2120360305959463Subject:Mineralogy, petrology, ore deposits
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The build-up and break-up of the supercontinent Rodinia and evolution of the Paleo-Qinling Ocean during Meso-Neoproterozoic to Early Paleozoic form the middle ocean ridge-trench-arc-basin lithogenic system in the North Qinling. A series of important ore deposits such as the Songshugou chromite deposit, Tongyu, Shangzhuangping, Shuilingdong, and Liushanyan VHMS deposits, Longmiao Zn-Pb deposit, Fanjiashi magnetite deposit, etc. are preserved in this system. Researches of metallogenesises and the corresponding geodynamic backgrounds of these deposits have the following significant implication:finding the spatiotemporal coupled relationship and mechanism between the early evolutional process of the Qinling Orogen and large quantity of metal element precipitation, developing orogenic-metallogenic theories of continental margins, and providing clues for exploration of multimetal resources in the North Qing area. On the basis of exsiting documents of metallogenesis during Meso-Neoproterozoic to Early Paleozoic, we conducted detailed studies on genesis of host rocks, sources of ore-forming fluids and materials, types of depoits and geodynamic backgrounds for the representative Songshugou chromite deposit and Tongyu copper deposit. A preliminary genetic model of ore deposits has been set up based on important geological events occurred in the North Qinling Orogenic Belt (NQOB) during Meso-Neoproterozoic to Early Paleozoic.Our researches reveal that the Songshugou peridotite formed during the period of the supercontinent Rodinia break-up and spreading process of the Grenvillian Songshugou Ocean. It shares the common mantle source with the mafic rocks, which both are segments of the Songshugou ophiolite. The Songshugou chromite deposit is a stratiform chromite deposit hosting in the medium-coarse grained cumulate dunite, which is the product of magmatic crystallization and differentiation during the formation of medium-coarse grained dunite in the thermal boundary layer (TBL). In terms of the lithogenesis, we propose that the fine-grained harzburgites is the residue produced by decompression-near fractional melting of mantle peridotite during ridge spreading, the fine-grained dunite is mainly composed of olivine residue leaving by mantle peridotite melting, olivine produced by the reaction:aCpx+ bOpx+cSpl= dOl+1Melt, and minor harzburgite residue, furthermore, both of the fine-grained dunite and harzburgite have undergone post-melting refertilization; medium-coarse grained dunite and harzburgite are cumulate peridotite, producing by crystallization of porous melts trapped on the path of migration or in decompression-dilatant zone of the thermal boundary layer (TBL) after the following reaction:Melt A= O1+Melt B.The volcanic host rocks of the Tongyu copper deposit, with the weighted average zircon U-Pb age of 462.2±7.0 Ma, belong to calc-alkaline island arc volcanic rocks. Both of the host rocks and ore-forming materials share common sources. During subduction of the Paleo-Qinling oceanic slab in (462.2-472 Ma), high oxygen-fugacity aqueous fluid released by dehydration reaction of the subducting slab rose into the mantle wedge, leached Cu and other metal elements, and triggered partial melting of the mantle wedge. Associated with the ore-forming materials, the arc calc-alkaline magmas erupted and formed the Tongyu VHMS deposit. The volcanic exhalation-sedimentation, which was contemporaneous with the submarine volcanics, produced the primary mineralization of the Tongyu copper deposit. Afterward, the ore bodies and host rocks were reworked and overprinted intensively by structures corresponding with the overall collisional orogenic process of the Qinling Orogenic Belt during the Indosinian period. In contrast to classic VHMS deposits such as the Kuroko deposit in Japan, the Tongyu copper deposit possesses the dual characteristics of island arc volcanic exhalation and sedimentation as well as structural reworking and overprinting. The Meigou granodiorite intruded in the south of the Tongyu mining district. The weighted average zircon U-Pb age of this pluton is 439.3±3 Ma. According to geochemical characteristics, we classify Meigou granodiorite as island arc granite with adakitic geochemical features, which is generated by interaction between the mantle wedge and melt produced by partial melting of the subducted basaltic slab of the Paleo-Qinling Ocean around 440 Ma. The discovery of island arc volcanic rock-adakite association in the Tongyu mining district provides important clues for exploration of island arc volcanic rock-related VHMS deposits and adakite-related Cu(Au) deposits in the Qinling and Qilian orogenic belts.Metallogenesis in the North Qinling during Meso-Neoproterozoic to Early Paleozoic are closely associated with the build-up and break-up of the supercontinent Rodinia and expansion, subduction, and collision processes of the Paleo-Qinling Ocean. Different types of deposits produced in different setting such as the middle ocean ridge, fore-arc basin, arc, back arc basin, etc. are responses to diffenent geological events in this period. Ore deposits formed during Meso-Neoproterozoic to Early Paleozoic are important parts of the present ridge-trench-arc-basin submarine volcanic-sedimental lithogenic and metallogenic system in the NQOB.
Keywords/Search Tags:important geological events, lithogenesis and metallogenesis, geodynamic background, Songshugou chromite deposit, Tongyu Copper deposit, North Qinling Orogenic Belt
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