Highly differentiated fluorine-rich, alkaline granitic magma linked to rare metal mineralization: A case study from the Boziguo’er rare metal granitic pluton in South Tianshan Terrane, Xinjiang, NW China

Abstract Rare metal-bearing, alkaline granites have attracted extensive interests because of their economic significance. The Early Permian (∼290 Ma) Boziguo’er alkaline granitic pluton, exposed in the middle segment of the South Tianshan Terrane, Xinjiang, NW China, contain Nb-Ta-, Zr-, Th-U- and REE-bearing accessory minerals, and host a large-scale rare metal deposit. The unalterated alkaline granites are mainly composed of alkali feldspars (∼40 vol% orthoclase/microcline and ∼30 vol% albite), quartz (∼20 vol%), arfvedsonite (∼5 vol%) and biotite (∼5 vol%), with accessory minerals at least including zircon, monazite, pyrochlore, Fe-Ti oxides, xenotime, bastnasite and astrophyllite. The existence of two types of alkaline feldspars, which respectively show nearly pure Ab and Or end member compositions, suggest that the Boziguo’er pluton can be classified as “subsolvus granites”. Rare metal elements in the pluton are mainly contained by pyrochlore (Nb-Ta), zircon (Zr-Hf, Th-U and HREE) and monazite (LREEs) grains. Mineral chemistry suggests that the pyrochlore and monazite are of primary igneous mineral. Based on morphology, internal texture and mineral chemistry, four types of zircons have been identified. Type-I zircon may have crystallized from volatile-undersaturated early-stage magmas, while type-IIA zircon crystallized from volatile (fluorine)-saturated magmas. The type-IIB and type-III zircons could have grown in an LREE-depleted silicate melt after the crystallization of the LREE-rich minerals (e.g., monazite) or an early aqueous fluid during the transition from a magmatic to hydrothermal system. The type-IV zircons can be regarded as low-temperature hydrothermal ones. A semiquantitative calculation, based on mineral compositions of biotites, yields 0.51–2.14 wt% for F contents in granitic melts. The high contents of fluorine in the hydrous minerals suggest a fluorine-enriched characteristic for melt, which would not only prolong the duration of the magmatic differentiation but can also enhance the solubility of ore-forming elements in magma. The magmatic differentiation was mainly dominated by fractional crystallization of alkali feldspar and amphibole. Extensive rare metal mineralization in the pluton likely occurred within a limited temperature interval and was related to the crystallization of a highly evolved intergranular melt at the latest magmatic stage.