Abstract

The genesis of Ta-Nb oxides in granitoid environments and, in particular, the role of deuteric fluids in Ta-Nb ore formation remain controversial. Ta-Nb oxides in the Yichun deposit occur disseminated in a topaz lepidolite granite and in the intermediate zone of a zoned marginal pegmatite. Columbite-group minerals and microlite are the principal ore minerals in this topaz lepidolite granite. The principal Ta-Nb oxides in aplite of the marginal pegmatite are columbite-(Mn), tantalite-(Mn), and wodginite-group minerals. Ta-Nb oxides in the topaz lepidolite granite and aplite are mostly prismatic crystals or aggregates associated with lepidolite and albite, whereas in the pegmatitic intermediate zone, they occur in triplite-dominated nodules as aggregates of acicular columbite-(Mn) in association with fibrous molybdenite, zircon, and native bismuth. Most columbite-group minerals in the topaz lepidolite granite and aplite have a columbite-(Mn) core (stage I), overgrown and resorbed by a later stage II columbite-group mineral overgrowth that is distinctly richer in Ta than the stage I crystals. Textural and chemical features of the stage I crystals are consistent with a magmatic origin. Where stage I crystals are enclosed by snowball quartz, there is no overgrowth, and these crystals are richer in Nb, Sc, Fe, Ti, Y, Zr, U, Th, and W, and thus are more primitive in composition than those outside the snowball quartz. Stage II overgrowths have two to five times higher Ta/(Nb + Ta) values than stage I cores, such that is there is a large compositional gap between core and overgrowth, although the Mn/(Mn + Fe) values of the two stages are similar. Subsequent to formation of the stage II overgrowths in aplite, but not in the topaz lepidolite granite, the columbite-group minerals were altered and overgrown by wodginite-group minerals (stage III), with concomitant precipitation of cassiterite, zircon, and uraninite, requiring interaction of the stage II crystals with a liquid that contained high field strength elements and Sn. The textural relationships and the chemical features suggest that lepidolite, albite, and the snowball quartz at Yichun are all metasomatic.

The chemical features of the columbite-group minerals and the low solubility of Ta in aqueous fluids, combined with the evidence for metasomatism at Yichun, suggest that the media from which the stage II crystals precipitated were hydrosilicate liquids, which have high concentrations of Si and Na, and approximately equimolar SiO2 and H2O concentrations. Distinct chemical differences between columbite-group minerals in the topaz lepidolite granite and aplite indicate that a single hydrosilicate liquid did not pervade the entire system; instead, distinct liquids were present in the various units. The triplite-dominant nodules most likely represent residual volatile-rich melts of the marginal pegmatite. The Nb-Ta-Mo-Bi mineral assemblages in these nodules were precipitated during hydrothermal alteration of triplite to fluorapatite from the same aqueous fluid. The chemistry of columbite-group minerals, as well as the published chronological data, suggest that aplite was not derived directly from the topaz lepidolite granite melt; rather, it represents a different magma that originated from the same magma chamber at depth from which the topaz lepidolite granite melt was derived.

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