Superimposed Nb–Ta Mineralization Events in Internally Zoned Pegmatites: Insights from Zoned Muscovite and Coltan

Abstract Pegmatite, a coarse-grained plutonic igneous rock of broadly granitic composition, is a principal source of rare metal ores. Internal zonation is a characteristic feature of extremely fractionated rare-metal pegmatites. Still, the processes that control internal zonation in rare-element pegmatites remain poorly understood, and the mechanisms continue to be debated. In this study, we use major and trace element analysis, alongside laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) mapping of muscovite and coltan (columbite-tantalite), and U–Pb dating of monazite and coltan from internally zoned pegmatites and associated granites, to characterize the geochemical fractionation processes involved in their formation. The Baishawo #3 is an internally zoned pegmatite comprised of four distinct textural zones: the border, wall, intermediate, and core zones, with muscovite and coltan appearing in all zones. Coltan grains range from homogeneous crystals to zoned varieties with growth rims, reflecting equilibrium-disequilibrium reactions driven by chemical changes. These textures were subsequently metasomatized resulting in alteration rims, patches, and irregular zoning patterns. Progressive zoning of coltan in the core zone of the pegmatite yielded two Tera-Wasserburg intercept U–Pb ages of 150.3 ± 5.1 Ma (coltan I) and 139.8 ± 3.3 Ma (coltan II), consistent with a zircon U–Pb age of 147.5 ± 1.6 Ma and a monazite U–Pb age of 139.7 ± 2.4 Ma from the adjacent two-mica granites I and II, respectively. Combined with spatial–temporal relationships, geochemical signatures, mineralogical features, and modeling results, rare-metal mineralization in the border, wall, intermediate, and core zone I of the #3 pegmatite is interpreted to be genetically associated with two-mica granite I, whereas mineralization in core zone II is attributed to a younger magmatic event linked to two-mica granite II. This two episodes of superimposed magmatic–metallogenetic events occurred within the Baishawo #3 rare-metal pegmatite during the Late Jurassic and Early Cretaceous. The geochemical and mineralogical characteristics indicate the border zone of the #3 pegmatite experienced equilibrium with fluids from the two-mica granite I, but both the wall and intermediate zones are of magmatic origin. Backscattered electron (BSE) imaging and geochemical mapping of muscovite and coltan from the core zones I and II reveal distinct major and trace element zoning patterns, reflecting progressive fractionation from magmatic, through magmatic–hydrothermal, and hydrothermal stages. Geochemical modeling of muscovite compositions in the #3 pegmatite reveals spatially variable fractionation across internal zones. Two episodes of superimposed Nb–Ta mineralization are identified, each of three stages: magmatic, magmatic–hydrothermal, and hydrothermal. The comprehensive results of this study establish the petrogenetic links between the two-mica granite and Nb–Ta mineralization while elucidating the mechanisms underlying the evolution of internally zoned pegmatite in the Baishawo region.