The Bangbule skarn lead-zinc(Pb-Zn)deposit(>1 Mt Zn+Pb)is located in the western Nyainqentanglha polymetallic metallogenetic belt,central Tibet.Lenticular orebodies are all hosted in skarn and developed in the cont...The Bangbule skarn lead-zinc(Pb-Zn)deposit(>1 Mt Zn+Pb)is located in the western Nyainqentanglha polymetallic metallogenetic belt,central Tibet.Lenticular orebodies are all hosted in skarn and developed in the contact zone between the quartz porphyry and carbonate strata of the mid Paleozoic Middle to Upper Chaguoluoma Formation as well as in carbonate and sandstone beds of the Upper Paleozoic Laga Formation.As a newly discovered skarn deposit,the geological background and metallogenesis of this deposit remain poorly understood.Detailed petrological,geochemical and geochronological data of the ore-related quartz porphyry,helps constrain the mineralization age and contributes to discussion on the ore genesis of the Bangbule deposit.Both endoskarn and exoskarn are identified in the Bangbule deposit.From quartz porphyry to carbonate formation,the exoskarn is zoned from proximal garnet skarn to distal pyroxene skarn.Zircon U-Pb dating results show that the quartz porphyry formed at 73.9±0.8 Ma.Geochemical analysis results show that the quartz porphyry has high contents of SiO_(2)(71.40–74.94 wt%)and K_(2)O+Na_(2)O(3.76–8.46 wt%)with A/CNK values of 0.69 to 1.06.Besides,the quartz porphyry is enriched in large ion lithophile elements(LILEs)and light rare earth elements(LREEs)and have lowεNd(t)(from-8.25 to-8.19)and high initial(^(87)Sr/^(86)Sr)i values(0.713611–0.714478).Major,trace elements and whole-rock F concentration analysis results from the endoskarn samples show higher TFe_(2)O_(3),MgO,CaO,Pb+Zn,W,Sn,Mo and F etc.,and lower alkalis(K_(2)O,Na_(2)O,Sr and Ba)than those of fresh quartz porphyry,indicating that the early ore-forming fluids were an Ca-Fe-F-enriched fluid.Massive ore in the proximal skarn might be related to the high F content in the magma,which lowered the solidus temperature of the quartz porphyry magma and caused a lower temperature of the ore-forming fluids,as well as facilitating the precipitation of sphalerite and galena.Based on the geochemical characteristics presented in this study,we propose that the ore-related quartz porphyry was formed by partial melting of crust materials with some juvenile crustal component input.The partial melting of the middle-upper crust after the initial enrichment of lead and zinc elements are important for the formation of Pb-Zn deposits.The case study of the Bangbule deposit has proven that there is still a crust-derived magmatic source region in the western segment of the central Lhasa terrane.Therefore,there is still great potential for Pb-Zn mineralization and Pb-Zn exploration.展开更多
High-silica(SiO_(2)>70 wt.%)granites(HSGs)are the main source of W,Sn,and rare metals.However,abundant HSGs,temporally,spatially,and genetically associated with Pb-Zn mineralization,in the Lhasa terrane(LT),provide...High-silica(SiO_(2)>70 wt.%)granites(HSGs)are the main source of W,Sn,and rare metals.However,abundant HSGs,temporally,spatially,and genetically associated with Pb-Zn mineralization,in the Lhasa terrane(LT),provided an ideal opportunity to study the key factors responsible for Pb-Zn enrichment,instead of W-Sn enrichment.Here we contribute to this topic through U-Pb dating of zircon and garnet,and whole-rock and Sr-Nd-Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb-Zn and W deposits in the LT.The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U-Pb ages of 77.3±0.9 Ma and hydrothermal garnet U-Pb ages of 75.7±4.8 Ma,which is 10-15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb-Zn polymetallic mineralization in the LT.The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere,which occurred as a result of the collision of the Indian and Asian plates.These HSGs related to Pb-Zn mineralization,with high totalalkalis and low magnesian contents,are enriched in Ba,Th,and Rb,but depleted in Ti,Eu,Sr,and P.They belong to either the S-type,or I-type granites.The Sr-Nd-Hf isotopic compositions of the Pb-Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input.This was consistent with the postulated source of W enrichment in the LT.The Pb-Zn and W related granites have similar zircon-Ti-saturation temperatures,comparable low whole-rock Fe_(2)O_(3)/FeO ratios,and zircon oxygen fugacity.This indicated that the Pb-Zn-W enrichment in the high-silica magma system could be attributed to a relatively reduced magma.The Pb-Zn related HSGs,abundant quartz and feldspar phenocrysts,and weak fractionation of twin-elements in wholerock analysis,can be used to reconstruct a model of the magma reservoir.We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir,with a loss of magmatic fluids.The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb-Zn related magma;however,different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in wholerock are formed by a magmatic-hydrothermal transition in highly evolved system.As the source and oxygen fugacities of the Pb-Zn and W related magmas are similar,the absence of a giant W-Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation.This implies that the rocks that originated as highly evolved silicaterich parent magmas,with a high degree of magmatic-hydrothermal alteration,would need to be targeted for W-Sn mineral exploration in the LT.In summary,our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb-Zn and W enrichment.This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.展开更多
基金jointly supported by National Key Research and Development Program of China(Grant No.2022YFC2905002)Special Scientific Research Fund of Tibet Bureau of Geology and Mineral Exploration and Development(Grant No.202119)+2 种基金National Natural Science Foundation of China(Grant No.41902101)Research Start-up Fund of Chengdu University of Technology(Grant No.10912-KYQD2020-08395)the Opening Foundation of MNR Key Laboratory of Metallogeny and Mineral Assessment(Grant No.ZS2101)。
文摘The Bangbule skarn lead-zinc(Pb-Zn)deposit(>1 Mt Zn+Pb)is located in the western Nyainqentanglha polymetallic metallogenetic belt,central Tibet.Lenticular orebodies are all hosted in skarn and developed in the contact zone between the quartz porphyry and carbonate strata of the mid Paleozoic Middle to Upper Chaguoluoma Formation as well as in carbonate and sandstone beds of the Upper Paleozoic Laga Formation.As a newly discovered skarn deposit,the geological background and metallogenesis of this deposit remain poorly understood.Detailed petrological,geochemical and geochronological data of the ore-related quartz porphyry,helps constrain the mineralization age and contributes to discussion on the ore genesis of the Bangbule deposit.Both endoskarn and exoskarn are identified in the Bangbule deposit.From quartz porphyry to carbonate formation,the exoskarn is zoned from proximal garnet skarn to distal pyroxene skarn.Zircon U-Pb dating results show that the quartz porphyry formed at 73.9±0.8 Ma.Geochemical analysis results show that the quartz porphyry has high contents of SiO_(2)(71.40–74.94 wt%)and K_(2)O+Na_(2)O(3.76–8.46 wt%)with A/CNK values of 0.69 to 1.06.Besides,the quartz porphyry is enriched in large ion lithophile elements(LILEs)and light rare earth elements(LREEs)and have lowεNd(t)(from-8.25 to-8.19)and high initial(^(87)Sr/^(86)Sr)i values(0.713611–0.714478).Major,trace elements and whole-rock F concentration analysis results from the endoskarn samples show higher TFe_(2)O_(3),MgO,CaO,Pb+Zn,W,Sn,Mo and F etc.,and lower alkalis(K_(2)O,Na_(2)O,Sr and Ba)than those of fresh quartz porphyry,indicating that the early ore-forming fluids were an Ca-Fe-F-enriched fluid.Massive ore in the proximal skarn might be related to the high F content in the magma,which lowered the solidus temperature of the quartz porphyry magma and caused a lower temperature of the ore-forming fluids,as well as facilitating the precipitation of sphalerite and galena.Based on the geochemical characteristics presented in this study,we propose that the ore-related quartz porphyry was formed by partial melting of crust materials with some juvenile crustal component input.The partial melting of the middle-upper crust after the initial enrichment of lead and zinc elements are important for the formation of Pb-Zn deposits.The case study of the Bangbule deposit has proven that there is still a crust-derived magmatic source region in the western segment of the central Lhasa terrane.Therefore,there is still great potential for Pb-Zn mineralization and Pb-Zn exploration.
基金Fundamental Research Funds for the National Foundation of China(42102058)open fund from the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resource,Institute of Geology,Chinese Academy of Geological Sciences (No. J1901-16)Central Universities,and China University of Geosciences (Wuhan)(No.2019132)。
文摘High-silica(SiO_(2)>70 wt.%)granites(HSGs)are the main source of W,Sn,and rare metals.However,abundant HSGs,temporally,spatially,and genetically associated with Pb-Zn mineralization,in the Lhasa terrane(LT),provided an ideal opportunity to study the key factors responsible for Pb-Zn enrichment,instead of W-Sn enrichment.Here we contribute to this topic through U-Pb dating of zircon and garnet,and whole-rock and Sr-Nd-Hf isotopic geochemistry of ore-related quartz porphyries in the Bangbule deposit and compared these results with published data from large and giant Pb-Zn and W deposits in the LT.The magmatism-alteration-mineralization event in the Bangbule deposit was recorded by robust zircon U-Pb ages of 77.3±0.9 Ma and hydrothermal garnet U-Pb ages of 75.7±4.8 Ma,which is 10-15 Ma earlier than the main Paleocene metallogenic event and the first record of late Cretaceous Pb-Zn polymetallic mineralization in the LT.The late Cretaceous-Paleocene magmatism and mineralization events are a response to the subduction of Neotethyan oceanic lithosphere,which occurred as a result of the collision of the Indian and Asian plates.These HSGs related to Pb-Zn mineralization,with high totalalkalis and low magnesian contents,are enriched in Ba,Th,and Rb,but depleted in Ti,Eu,Sr,and P.They belong to either the S-type,or I-type granites.The Sr-Nd-Hf isotopic compositions of the Pb-Zn mineralized granites demonstrate that they were generated by the partial melting of Proterozoic basement with or without mantle-derived melt input.This was consistent with the postulated source of W enrichment in the LT.The Pb-Zn and W related granites have similar zircon-Ti-saturation temperatures,comparable low whole-rock Fe_(2)O_(3)/FeO ratios,and zircon oxygen fugacity.This indicated that the Pb-Zn-W enrichment in the high-silica magma system could be attributed to a relatively reduced magma.The Pb-Zn related HSGs,abundant quartz and feldspar phenocrysts,and weak fractionation of twin-elements in wholerock analysis,can be used to reconstruct a model of the magma reservoir.We postulate that these features could be reproduced by silica-rich crystal accumulation in a magma reservoir,with a loss of magmatic fluids.The magma associated with W mineralization exhibited a higher level of differentiation compared to the Pb-Zn related magma;however,different groups of zircon texture with varying rare earth elements and concomitance of rare earth elements tetrad effect and high fractionation of twin-elements in wholerock are formed by a magmatic-hydrothermal transition in highly evolved system.As the source and oxygen fugacities of the Pb-Zn and W related magmas are similar,the absence of a giant W-Sn deposit in the LT may indicate that parent magmas with a low degree of evolution and magmatic-hydrothermal transition are not conducive to their formation.This implies that the rocks that originated as highly evolved silicaterich parent magmas,with a high degree of magmatic-hydrothermal alteration,would need to be targeted for W-Sn mineral exploration in the LT.In summary,our results emphasize that variations in chemical differentiation and the evolution of high-silica magmatic-hydrothermal systems can lead to differences in Pb-Zn and W enrichment.This has implications for the evaluation of the mineral potential of high-silica granites and hence their attractiveness as targets for mineral exploration.
