| The granitoids related W mineralization in the world are mainly S-type,A-type and I-type granitoids.The diagenesis and mineralization of W or Sn deposits associated with highly fractionated reduced S-type or I-type granitoids and A-type granites are deeply studied.The W(MO)deposits closely related to weakly fractionated oxidized I-type granitoids are a new type of tungsten deposit discovered in recent years.And then,the mechanism of diagenesis and mineralization is a scientific problem to be solved urgently.In recent years,a number of W-Mo deposits associated with weakly fractionated oxidized I-type granodiorite(eg.Dongyuan W-Mo deposit,Xiaoyao W deposit,Zhuxiling W-Mo deposit,etc.)have been discovered in the east of Jiangnan tungsten ore belt(hereinafter referred to as JNTB),which is an ideal object for the study of the diagenesis and metallogenic mechanism of these W deposits.In this study,the Zhuxiling W-Mo deposit,one of the newly discovered large-sized skarn W-Mo deposits,locate in the east of JNTB,was selected as a study case using petrological,mineralogical and geochemical methods with the aim to reveal the diagenesis and mineralization mechanism of W deposits related weakly fractionated oxidized I-type granitic rocks,and discuss its dynamic background.Based on this study,the following main outcomes were obtained:(1)The granitoid associated with mineralization in Zhuxiling W-Mo polymetallic deposit is granodiorite,in which Mafic microgranular enclaves(hereinafter referred to as MME)are well developed.Samples selected from the granodiorite are characterized by low Si but high Mg concentrations,low Rb/Sr ratio,high Zr/Hf and Nb/Ta ratios,and depleted in HREE.They belong to K-rich,weakly peraluminous,calcium-alkaline granitoids,and are enriched in large ionic lithophile elements(eg.K,U and so on)and depleted in high field strength elements(eg.Zr,Nb and so on).Calculation results of hornblende and biotite minerals,based on EPMS data,show that the diagenetic temperature of Zhuxiling granodiorite is 690?-841?,and the oxygen fugacity is mainly between the MH buffer and NNO buffer,which belongs to a high-temperature,weakly fractionated,oxidized I-type granodiorite with an emplacement depth of 4.8-7.9 km.(2)LA-ICP-MS zircon U-Pb data show that the MME was formed at 146.9±0.9 Ma,slightly older than the age of granodiorite which yielded an zircon U-Pb age of 144.6±0.8Ma.The weighted Re-Os age of molybdenite is 141.45±0.94 Ma and the Ar-Ar weighted mean plateau age of muscovite is 141.46±1.51 Ma.These ages overlap with the zircon U-Pb age of Zhuxiling granodiorite in uncertainty,indicating the granodiorite is the causative intrusion for skarn W-Mo mineralization at Zhuxiling.(3)Mineralogical and geochemical evidence of granodiorite and MME show that crust-mantle magma mixing is the main genetic mechanism of Zhuxiling granodiorite.The characteristics of major elements and trace elements,Sr-Nd-Hf isotope characteristics and inherited zircon ages indicate that the felsic magma originated from partial melting of the lower crust,and the mafic magma originated from partial melting of the enriched lithospheric mantle.Based on this study,the diagenetic model is:During the Late Jurassic to the Early Cretaceous(161–135 Ma),the Izanagi plate was subducted beneath the Eurasian continent at low angle and the overlying crust was disrupted or broken due to the disharmonious movement of the Yangtze and North China Cratons.Asthenospheric mantle material upwelling after the break-up,which induced the partial melting of the enriched lithospheric mantle to generate water-rich basaltic magma.The water-rich basaltic magma intruded into the crust-mantle boundary,inducing partial melting of the lower crust to generate the felsic magma.Felsic magma mixed with mantle-derived mafic magma in the magma chamber and eventually forming the Zhuxiling granodiorite with abundant MMEs(4)The mineralization of Zhuxiling W-Mo deposit can be divided into five stages:skarn stage,degenerative and alteration stage,hydrothermal garnet stage,quartz-scheelite-sulfide stage and calcite-scheelite-sulfide stage.Detailed mineralogy and mineral chemistry studies on scheelite from Zhuxiling deposits,eight generations of the scheelite were determined,including(1)generations 1-3 scheelite grows during skarn stage;generations 4-5 scheelite grows during degenerative and alteration stage;generations 5-6 scheelite grows during quartz-scheelite-sulfide stage;and generations 7-8 scheelite grows during calcite-scheelite-sulfide stage.In general,from the early to late generations,LREE enrichment gradually changes to HREE enrichment,the Mo content of scheelite decreases,the temperature decreases,the salinity decreases,the f O2decreases constantly,and the atmospheric precipitation mixed with magmatic fluid gradually increased.(5)The spatial and temporal distribution of W deposits closely related to I-type granitoids in the world show that these deposits are mainly distributed in the subduction-related orogenic belt,and the diagenetic and metallogenic time is consistent with the subduction time or syn-collision,post-collision time.The W-Mo deposits which are associated with weakly fractionated,oxidized I-type granitoids,consistence with arc magmatic rocks,which were generated during the subduction stage,possibly related to the upwelling of asthenosphere caused by subduction or plate tear.In contrast,the W deposits,which are related to highly fractionated,reduced I-type granites,were formed in syn-collision or post-collision tectonic setting,relating to the delamination of subducted plates or thicked lithosphere which was possibly promoted by the upwelling of asthenosphere. |
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