Mineralogical Constraints On Magmatic And Hydrothermal Evolutions Of The Mesozoic Rare-metal Granites In South China

The South China is an important rare-metal deposit region. Most economic raremetal deposits in South China have genetic relationship with the Mesozoic granitoids. Both columbite-tantalite and tungsten mineralizations are commonly associated with magmatic evolution of Li-rich rare-metal granites. However, most rare-metal granites in South China host dominantly W deposits with few or without Ta-Nb mineralization. In rare cases economic W and Nb-Ta deposits could co-exist. It remains poorly understood as to the mechanism of the diversity of W and Nb-Ta mineralization associated with rare-metal granites. As the carriers of major and trace elements in the granite and deposit, the minerals can directly record the processes of magmatic evolution, hydrothermal fluid reaction and related mineralization on their structural and compositional variations. Based on this proposition, the author performed detail mineralogy, petrology, geochemistry and lithium isotope analyses on the typical W and Nb-Ta-bearing granitic plutons in South China, with aims to tackle the genetic relationship between W and Nb-Ta mineralizations and magmatic evolution and/or hydrothermal fluid evolution, then to reveal the regularities of migration and enrichment of such metallogenic elements. These data will offer new constraints on the mineralization mechanism of rare-metal granite in South China.The Mesozoic W- and Nb-Ta-bearing granitic plutons in South China are all granitic complex consisting of multistage-magmatic intrusions. Both the Yashan NbTa-bearing granitic pluton and Xihuashan W-bearing granitic pluton are peraluminous and highly evolved rocks with relatively high Si O2, Al2O3, lithium and fluorine concentrations. The most evolved granites of two plutons have relatively low REE concentration, and show notable lanthanide tetrad effect and significant negative Eu anomaly in the REE patterns. They are also enriched in Cs, Li, Rb and high field strength elements(HFSE) such as Ta and Nb. In comparison with the Xihuashan Wbearing granite, the Yahsan Nb-Ta-bearing granite have higher Li, F and P contents and exhibit stronger lanthanide tetrad effect and more negative Eu anomalies with higher Rb/Sr but lower Nb/Ta and Zr/Hf ratios, suggesting the higher evolved nature of NbTa-bearing granites relative to W-bearing granites.The compositional variations of feldspar, mica and zircon in the Mesozoic W- and Nb-Ta-bearing granites in South China may be as tracers for the extent of magma evolution. During the magmatic evolution, mica varies from mafic biotite or protolithionite at early stages to Li- and F-rich Li-mica or lepidolite at last. In the highest evolved topaz-lepidolite granite of Yashan pluton, feldspars contain higher phosphorus, micas are more enriched in Li, Rb, Cs, Nb, Ta and F, and zircon have higher P2O5 and Hf O2 contents but lower Zr/Hf ratio than those in the Xihuashan W-bearing granite. Additionally, there are abundant Cs-rich(i.e., pollucite and Cs-rich lepidolite) and fluorine-rich minerals(i.e., topaz) in the Yashan topaz-lepidolite granite as results of extremely evolved magma. Above-mentioned mineral characteristics also manifest that the Yashan Ta-Nb-bearing granite has been evolved much higher than the Xihuashan W-bearing granite.During the magmatic evolution, accumulation of volatile elements such as P, F and Li can increase the proportion of non-bridging oxygens(NBOs) in the melt, which will significantly elevate the solubility of tantalum and niobium and make them gradually enriched in the melt. Therefore, the most evolved topaz-lepidolite granite in the Yahsan pluton has the highest bulk-rock Ta and Nb concentration and most Ta-Nb-enriched mica. Additionally, the zoned mica of Yashan topaz-lepidolite granite exhibits increasing F, Rb, Cs, Ta, and Nb but decreasing Fe O and Mn O from core to mantle, consistent with the magmatic evolution trend. However, Rb, Cs, Nb, Ta, F and W contents decrease dramatically in the thin rim of zoned mica in the Yashan topazlepidolite granite, denoting the hydrothermal evolution. Because the partition coefficients for Ta and Nb between fluid and melt are extremely low, magmatic evolution would be the most important mechanism for Ta-Nb enrichment in the Yahsan pluton, while small amount of exotic aqueous fluids during the post-magmatic or hydrothermal evolution cannot be responsible for the Ta-Nb enrichment and mineralization.In contrast to gradually enrichment of niobium and tantalum during the magma evolution, tungsten does not accumulate significantly in the most evolved muscovite granite and greisen relative to less evolved two-mica granite of Xihuashan pluton. But it is noteworthy that there are abundant siderites and zoned micas within the Xihuashan muscovite granite and greisen. The siderite would be product of hydrothermal metasomatism related to CO2-enriched exotic fluid. The primary micas in Xihuashan muscovite granite were also affected by CO2-enriched exotic fluid and formed the zoning texture through the non-equilibrium diffusion of elements, which is characterized by gradually decreasing W, Sn, Fe and Mn from core to rim. Post-magmatic F-rich hydrothermal fluid have not only dissolved W, Sn, Fe, Mn from wall rock or early granite to fluid, but also reduce the oxygen fugacity and elevate p H values of the system through the greisenization, which would be beneficial to the formation of tungsten deposit. However, there are sharp boundary between core and rim of zoned micas in the Xihuashan greisen. Broad but compositional homogeneous rim of the zoned micas in greisen would be results of fast crystallization under sudden changed environment due to the involvement of low-temperature exotic fluid. Therefore, the formation of Xihuashan tungsten deposit is closely associated with the reaction by external fluid and fluorine-rich hydrothermal fluid.Lithium isotopic fractionation is extremely insignificant during the magmatic evolution of Yashan pluton from protolithionite granite to Li-mica granite to topazlepidotite granite, which is similar during the evolution from biotite granite to two-mica granite of Xihuashan pluton. However, both the Xihuashan muscovite granite and greisen exhibit significant lithium isotopic fractionation. δ7Li values increase particularly in the Xihuashan muscovite granite because of non-equilibrium diffusion of lithium isotopes between micas and exotic fluid, in which 6Li would diffuse faster than 7Li from mica to exotic fluid. In contrast, δ7Li value of the Xihuashan greisen decreases due to relatively low δ7Li value of the exotic fluid and/or leaching of heavier lithium relative to lighter lithium from rocks into fluid during the fluid-rock interaction. Very different Li isotopes fractionation between Yashan pluton and Xihuashan pluton further indicates that Ta-Nb enrichment was associated with highly evolved magmas, but W mineralization is closely related to metasomatism by hydrothermal fluid.