Magmatic-Hydrothermal Evolution And Mineralization Of The Limu Tin-Bearing Granite, Guangxi Province, South China

The Nanling area is a famous rare-metal deposit region,which is closely related to the widespread Mesozoic granitic magmatism.This thesis presents detail petrology,mineralogy and whole-rock major and trace elements for the Limu Sn-Nb-Ta deposits-bearing granite in the middle part of Nanling area in order to investigate the effect of magmatic evolution and hydrothermal processes on Sn mineralization of the rare-metal granite.The limu granite consists of protolithionite granite,lithium muscovite granite and zinnwaldite granite,which are high-silic,alkali-enriched and strongly peraluminous?A/CNK=1.05¹.50?.The studied samples have extremely low whole-rock rare earth elements contents,and show enrichment of Cs,Rb,W,Sn,Nb and Ta but strong depletion of Ba,Ti and REE with strongly negative Eu,Sr and Ba anomalies on the mean Upper Crust-normalized trace elements spidergram,which would be derived from highly evolved granitic melt via significant fractional crystallization of feldspars.In addition,they have strong lanthanide tetrad effect,indicating the remarkable hydrothermal activities during the late magamtic evolution.From protolithionite granite to zinnwaldite granite,whole-rock Zr/Hf,K/Rb and Nb/Ta ratios decrease gradually,consistent with the characteristics of magma evolution.During the magmatic evolution,Rb,Ta,Sn contents of the melt increase gradually,while REE contents decrease.Micas in the Limu tin-bearing granites show compositional variation in consistent with magmatic evolution and hydrothermal process.The dominant micas in the studied granites vary in species from protolithionite to zinnwaldite,and even to lepidolite during the magmatic evolution,or change from the early protolithionite to the late lithium muscovite in the hydrothermal process.The micas in the lithium muscovite granite samples generally show zoning texture with muscovite rim,which should be related to strong hydrothermal activities during the late magmatic evolution.The F,Li,Rb and Cs contents of micas decrease notably from protolithionite to lithium muscovite in protolithionite granite,from core to rim of zoned micas in lithium muscovite granite,and from zinnwaldite to muscovite in zinnwaldite granite,which indicates that the fluids envolved in the hydrothermal processes are mainly exotic.However,F,Li and Rb contents of micas increase notably from zinnwaldite to lepidolite in zinnwaldite granite,indicating that the hydrothermal fluids are mainly post-magmatic fluid exsolved from highly evoled melt.During the magamtic evolution and hydrothermal processes of rare-metal granite,the W,Sn,Nb and Ta show distinct geochemical behaviors,and Sn was concentrated in the melt via high differentiation.In addition,the Fe³⁺/Fe²⁺ratios of the micas decrease from protolithionite granite to zinnwaldite granite,indicating that the oxygen fugacity of magmatic-hydrothermal system decreased gradually with the magmatic evolution.On the other hand,the Fe³⁺/Fe²⁺ratios of micas decrease from core to rim of zoned micas in the lithium muscovite granite,from the early zinnwaldite to the late lepidolite or muscovite in the zinnwaldite,and from the early protolithionite to the late lithium muscovite in the protolithionite granite,which indicates that the hydrothermal activities will result in the increase of oxygen fugacity of magmatic-hydrothermal system.Therefore,the decreasing oxygen fugacity is conducive to Sn migration and enrichment,while the increasing oxygen fugacity in the hydrothermal fluid is crucial to the final precipitation and mineralization of Sn.