Diagenesis And Metallogeny Of Ore-bearing Granites In Tongcun Porphyry Mo-Cu Deposit, Zhejiang

Tongcun medium-sized porphyry Mo-Cu deposit locates in West Zhejiang Depressionand near Jiangnan Orogen, north of Jiangshan-Shaoxing suture zone. It belongs toQinzhou-Hangzhou metallogenic belt, where plenty of ore deposits are distributed such asfamous Dexing, Jinshan, Yinshan deposits. Metallic mineral deposits in Zhejiang arecharacterized by a limited scale and reserves. The discovery and thorough investigation ofTongcun deposit help to better understand the petrogenesis and metallogeny of Mesozoicporphyry deposits in West Zhejiang. Furthermore, based on the research on igneousore-bearing potential, comprehensive contrast on Tongcun and super-sized, neighbourhoodDexing ore deposit has been done to search the essential differences on mineralization typesand scale, and the factors controlling metallogenic potential for granites.On the basis of field investigation, lithology, elemental geochemistry, Rb-Sr、Sm-Ndisotopic geochemistry, SHRIMP zircon U-Pb geochronology, zircon in-situ Hf geochemistryand H、O、S、Pb stable geochemistry are applied in research on characteristics of geology,geochemistry and petrogenisis and metallogeny of Tongcun deposit, evaluation of factorseffecting the granitic metallogenic potential, and considerable comparison with Dexing oredeposit, and the following conclusions are withdrawn:(1) SHRIMP zircon U-Pb geochronology analysis implies that the crystallization ages ofTongcun granites are separately169±2Ma、166±2Ma、157～168Ma, which form as WPG inmedium-late Jurassic lithospheric extension setting. The ore-bearing granites are considered tobe I-type, high K, calc-alkali granites, and rich in large lithophile elements such as Rb、Ba、K、U、Th, depleted in high strength elements as Nb、Ta、Zr、Hf、HREE, with a weak abnormityin Eu and an feature of arc granites;(2) Tongcun granites has low εNd(t)(–5.8～–0.8), low TDM(0.80～1.95), high initial87Sr/86Sr ratio (0.7038～0.7143), broad εHf(t) value (–16.1～15.2) and TDMvalue (0.60～1.79Ga), suggesting that Shuangxiwu Group contributes a lot to the original magma. Siginificantcrust growth of Palaeoproterozoic (2.3～1.8Ga) and Neoproterozoic (～0.8Ga) arediscovered here. Tongcun granites formed in a lithospheric extention setting during medium tolate Jurassic. Neoproterozoic arc and lower crust are heated and melt by underplate hot asthenospheic materials;(3) Fluid inclusions and stable isotopic stydy show that metallogenic fluids are mainlyH2O-NaCl and CO2thriple-faces fluids with low temperature and salinity. Mo originates fromlower crust and Cu is extracted from Neoproterozoic arc and asthenosphere mantle.(4) Tongcun granites have fine potential of Mo-Cu mineralization as the granites arefeatured by more acid, strong oxidation state, fraction and crystallization degree.(5) Comprehensive contrast to Dexing deposit indicates that, Mo-major mineralizationare caused by the feature of granites and more contanmination of crust materials,and thereason of a limited scale is ineffective metallogenic fluids, lower magma temperature andlarger depth of Tongcun ore deposit.Major innovation of the thesis are that:(1) isotopic proof are found in Tongcun deposit ofcrust growth of Palaeoproterozoic (2.3～1.8Ga) and Neoproterozoic (～0.8Ga) time;(2)Main factor controlling the metallic types and scale of deposits are ensured based oncomparison medium-to super-large sized deposits;(3) magma partiality is proposed to betterdescribe the genesis relation between granites and mineralization.Above all,3composit plutons are connected in depth and from episodic intrusion of thesame magma region, forming in a intraplate setting of lithospheric reduction and crustdeepening. The depth of Tongcun deposit is lager and has a better potential for Momineralization relative to Cu. Low temperature geochronology and melt-inclusions areremains to be studied.