| Jiangxi Yashan granitic pluton and Dajishan granitic pluton are both typical rare-metal granites in South China, they all contain elements such as W, Sn, Nb and Ta, but they formed different types of ore deposits-Yashan granites developed Nb-Ta mineralization, while Dajishan granites formed tungsten mineralization. To find out the relationship between granite and mineralization, research the mineralization diversity between the two types of rare-metal granites will have important significance.In this paper, the author collected the samples from the different lithology zonings of the two deposits, performed detail petrology, geochemistry, U-Pb isotopic dating of zircon and mineralogy analyses, with aims to find out the Nb-Ta mineralization mechanism and W ore-forming mechanism of the two deposits, then to reveal why magmatic evolution and hydrothermal fluid evolution can generate different ore-forming types. The following conclusions are obtained based on the above studies:(1) Both the Yashan Nb-Ta-bearing granitic pluton and Dajishan W-bearing granitic pluton are sub-alkaline, peraluminous and highly evolved rocks. With relatively high SiO2, Al2O3, lithium and fluorine concentrations, relatively low REE concentration, significant negative Eu anomaly and lanthanide tetrad effect in the REE patterns, both Yashan and Dajishan are typical S-type granite. However, Yashan granites have higher Li, F and P concentrations, exhibiting stronger lanthanide tetrad effect, notable negative Eu anomalies with higher Rb/Sr but lower Zr/Hf and Nb/Ta ratios. Therefore, Yashan granites have the higher evolved nature relative to Dajishan granites.(2) As the magmatic evolution, the concentrations of Li, F, Nb, Ta of the mica in Yashan granites have raised gradually, while Fe, Mn concentrations have decreased continuously. The mica of Yashan granites varies from mafic biotite at early stages to Li- and F-rich lepidolite at last. However, the mica of Dajishan granites relatively have single component. Moreover, relative to Dajishan granites, in the highest evolved lepidolite granite of Yashan pluton, feldspars are closer to end-member component of albite, and contain higher phosphorus. Additionally, there are abundant fluorine-rich minerals(i.e., topaz) in the lepidolite granite of Yashan, as results of extremely evolved magma. All above-mentioned mineral characteristics can manifest that the Yashan granites has been evolved much higher than the Dajishan granites.(3) The diagenesis ages of Yashan protolithionite granite is 158±1.8Ma, and muscovite granite’s diagenesis ages is 156±0.4Ma, indicating Yashan granites belong to Yanshan Period. Based on the geochemistry characteristics, mineralogy characteristics of Yashan granites, and the partition coefficients for Nb and Ta between fluid and melt are extremely low, we can conclude that magmatic evolution could be the most pivotal mechanism for Nb-Ta enrichment in the Yahsan granites. Accumulation of volatile elements such as Li, F and P during the magmatic evolution, can increase the quantity of non-bridging oxygens(NBOs) in the melt, which will make tantalum and niobium gradually enriched in the melt and form deposit eventually. However, the formation of Dajishan tungsten deposit is closely associated with the reaction by fluorine-rich hydrothermal fluid and external fluid, hydrothermal fluid have not only dissolved W, Fe, Mn, but also reduce the oxygen fugacity and elevate pH values of the system through the greisenization, that beneficial to the formation of tungsten deposit. The otherness of magmas evolution and the different metasomatism by hydrothermal fluid are the most important reason of why Yashan granites developed Nb-Ta mineralization, while Dajishan granites formed tungsten mineralization. |
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