Magmatic-hydrothermal evolution and rare metal enrichment of the Huoshibulake B-rich rare metal granite in the Southern Tianshan: Insights from texture, geochemistry, and Hf-O isotopes of zircon

Deciphering magmatic-hydrothermal evolution and rare metal mineralization mechanisms in granitic systems is always challenging, particularly when elucidating the intricate details of magmatic-hydrothermal transition, the critical stage when the system changes from melt-driven to fluid-driven with associated metal extraction. Here we present a case study of the Huoshibulake Nb-REE-mineralized alkali pluton in the north margin of the Tarim Carton, China. The pluton exhibits abundant tourmaline-quartz orbicules and tourmaline-quartz veins, which are the products of immiscible B-rich melt exsolved during the magmatic-hydrothermal transition. Three types of zircon crystals with different morphology have been recognized in the pluton: antecrystic, autocrystic and hydrothermal zircons, the latter two representing the melt and fluid parts of the system. LA-ICP-MS U-Pb dating of autocrystic zircon from granite and orbicule samples (206Pb/238U ages of 274.8 +/- 1.5 and 273.6 +/- 2.0 Ma, respectively) and cassiterite from the vein sample (lower 206Pb/238U intercept age of 271.4 +/- 4.1 Ma) overlap within error. The similar trace-element patterns and near-identical O-isotopic compositions between hydrothermal and autocrystic zircons indicate that the hydrothermal zircons were crystallized from magmatic fluids. Zircon Hf-O isotopic systematics suggests that the coeval A1-type granites in the Halajun area evolved from a single parental magma chamber which originated from -30% of the upper crustal material mixed with -70% mantle-derived mafic magma. Whole-rock compositional trends reveal that fractional crystallization of felsic minerals contributed to the rare metal enrichment. The rare metal mineralization in the veins and the close association of rare metal mineralization with fluorite indicate the critical role of F-rich hydrosilicate liquids in rare metal mineralization, which is also recorded by the significant increase of rare metal concentrations from the autocrystic to the hydrothermal zircon grains.