Genesis Of The Granodiorite Porphyry And Sources Of Metallogenic Materials In The Baoshan Pb-Zn Polymetallic Deposit, Southern Hunan Province

Multiple episodes and extensive granitoids are widespread in South China. Among them, the Mesozoic granitoids are closely related to metal mineralization. Therefore, the genetic mechanism of granitoids in South China has been concerned by many geologists and has long been a hot topic. The Nanling Range is one of the most important ore-concentrated area with many large and super-large deposits. Baoshan Pb-Zn polymetallic deposit is one of the representative hydrothermal deposits in the Nanling Range. According to previous research results, there still exsist some controversies on the metallogenetic mechanism, genesis of the granodiorite porphyry, and their tectonic setting, and the research on ore deposit geochemistry research is weak. In order to resolve the above problems, based on the former studies, we carry out comprehensive researches including in-situ zircon trace elements, U-Pb dating and Hf isotopes, whole-rock geochemistry and Sr-Nd isotopes analyses for the granodiorite porphyry and the recently discoverable mafic microgranular enclave (MME) from Baoshan deposit and the granodiorite porphyry breccia in the dacitic tuff breccia from vicinity. Furthermore, in order to illustrate the source and the evolution of the ore-forming minerals, this thesis presents new S-Pb isotopic data form different sulfides, and C-O isopopic compositions for hydrothermal calcites and their wall rock samples.LA-ICP-MS zircon U-Pb dating yields crystallization ages of156～158Ma,155.2±1.4Ma and156.4±1.4Ma for the granodiorite porphyry, MME and the granodiorite porphyry breccia, respectively, indicating their coeval formation.The Baoshan granodiorite porphyry is metaluminous to weakly peraluminous and high-K calc-alkaline, affinity of I-type granitoids. The granodiorite is enriched in LILEs such as K, Rb and U, and depleted in Nb, Ti and P, and has relative low Nb/Ta ratios with an average Nb/Ta ratio of11.3. It has initial87Sr/86Sr ratios of0.7095～0.7115, εNd(t) values of-7.3to-5.0,t2DM(Nd) age of1.35to1.54Ga, zircon εHf(t) values of-14.0to-9.0. The MME is characterized by microgranular textures and ovoid-like shapes with clear contact to the host rocks. The MME has darkly chilled margins and contains many acicular apatite grains. It has low SiO2(55.46wt.%～57.30wt.%) but high K2O (5.86wt.%｀6.90wt.%), and belongs to metaluminous shoshonitic rock. The MME is enriched in LILEs such as Rb, Ba, Th and U, and depleted in Nb, Ta, Ti, and has relative high Nb/Ta ratio (average of15.3),initial87Sr/86Sr ratios of0.7062～0.7063, εNd(t) values of-2.1～-1.9, zircon εNd(t) values of-12.1～-4.7. The MME has compatible elements such as Fe, Mg, V and Cr much higher than the granodiorite. The granodiorite porphyry breccia is metaluminous peraluminous and high-K calc-alkaline, and enriched in LILEs such as K, Rb and U, and depleted in Nb, Ta and Ti, with an average Nb/Ta ratio of11.3. It has initial87Sr/86Sr ratios of0.7107～0.7109, εNd(t) values of-5.4,t2DM(Nd) age of1.39Ga, zircon εHf(t)values of-11.6to-6.5.Detailed elemental and isotopic data suggest that the Baoshan granodiorite porphyry might have formed from a hybrid magma with mafic and felsic members. The mafic member is hydrous and K-rich mafic magma which might be derived from enriched lithospheric mantle source that had been metasomatized by subducting-sediment-derived melt. The felsic member is generated by partial melting of lower crust induced by the underplating of the hydrous and K-rich mafic magma. The MME might be derived from the mafic magma with minor addition of the felsic magma. The neighborhood granodiorite porphyry breccia was derived from the same period and magma chamber as the granodiorite porphyry in Baoshan deposit. The modeling using Sr-Nd isotopes shows that the mixture of20wt.%～30wt.%mafic magma and70wt.%～80wt.%felsic magma can form the Baoshan granodiorite porphyry. One inherited core and one zircon from the porphyry yield U-Pb ages of892.1±20.1Ma and891.5±15.0Ma, respectively, and the former has the εHf(t) and tDM(Hf) values of+6.0and1.21Ga, respectively, 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 Baoshan granodiorite porphyry, and this arc magma belt and corresponding continent-arc collision belt between the Yangtze and Cathaysia Block might extend southward from Pingxiang to Chenzhou-Linwu Faults. In early Yanshanian (190～150Ma), the Nanling Range has undergone lithosphere extensional and thinning due to the subduction of the Paleo-Pacific Plate. The mantle-derived mafic magma underplated the old lower crust which resulted in partial melting of the crust, then the mixture of crust-derived and mantle-derived magmas generated extensive granitoids.The characteristics of sulfur and lead isotopic compositions in sulfide minerals from Baoshan deposit show that sulfur and lead in ores were mainly derived from magma, and some sulfur and lead in the ore-hosting wall rocks were involved during the formation process of the Baoshan deposit. The carbon and oxygen isotopic compositions for the hydrothermal calcites suggest that carbon and oxygen in the ore-forming fluid were mainly derived from magma, and there might be the interfusion of sedimentary organic matter during the formation process of the magma.