Abstract

The Yichun Ta-Nb-Li mine is situated within a small, sheetlike body of topaz-lepidolite granite which is the most fractionated phase of the Yashan batholith. Volumetrically, the most important phase in the batholith is a coarse-grained, protolithionite-muscovite granite which has a finer grained, muscovite granite margin. The protolithionite-muscovite granite is intruded by small bodies of muscovite granite which form the locus of a tungsten vein system at the Xin Fang tungsten mine. Two sheetlike bodies, a lower Li mica granite and an upper topaz-lepidolite granite intrude the upper contact between the protolithionite-muscovite granite and the host metasediments. The granites are crosscut by unmineralized porphyry dikes that are exploited as a source of kaolin. The topaz-lepidolite granite is a major, low-grade Ta-Nb-Li deposit which is exploited via open-pit mining. The topaz-lepidolite granite is composed dominantly of albite, lepidolite. and quartz, with topaz. K feldspar, amblygonite, and accessory zircon, monazite, pollucite, columbite-tantalite, microlite, and Ta-rich cassiterite. Phenocrysts of quartz, topaz, and K feldspar contain abundant inclusions of albite laths and occasional lepidolite crystals along growth zones ("snowball" texture), indicating simultaneous crystallization of these minerals from a subsolvus, residual magma. The topaz-lepidolite granite is characterized by high F, Li, P, Al 2 O 3 , and Na 2 O contents (with Na 2 O > K 2 O), and by very low SiO 2 , TiO 2 , MgO, CaO, and K 2 O contents. It is strongly enriched in Rb, Nb, and Ta, and strongly depleted in Sr and Zr compared with the earlier granite phases. Chemical trends are consistent with an origin via fractionation involving especially the removal of plagioclase, K feldspar, quartz, Fe-Mg silicates, and zircon. Most of this fractionation appears to have taken place within a deeper level magma chamber, with successive magma batches emplaced into the present level of exposure. Mineralization at Yichun consists of columbite-tantalite, microlite, and Ta-rich cassiterite which occur throughout the topaz-lepidolite granite closely associated with, and as inclusions, in lepidolite. The ore minerals also occur as interstitial crystals between the major silicate phases. Crystallization of the Nb-Ta-Sn minerals is an integral part of the cystallization of the topaz-lepidolite granite, and the effects of postmagmatic alteration in the formation of mineralization are absent or minor.

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