The feature of Nb-Ta-bearing minerals in the spodumene-bearing pegmatite dykes in Kuqu. eastern Himalaya, and its significance

The Nb-Ta-bearing minerals are important rare-element minerals in granitic pegmatite and record the magma evolution process. Significant progress has been made in exploring lithium deposits in the middle and eastern regions of the Himalayas, China. This study focuses on examining the internal structures and chemical compositions of Nb-Ta-bearing minerals, namely columbite-group minerals (CGM) and microlite, from three spodumene-bearing pegmatite dykes and muscovite granite in the Kuqu pluton. The CGM samples obtained from the spodumene-bearing pegmatite dykes are columbite-(Mn) to tantalite-(Mn) with homogeneous, core-rim, patchy, irregular, oscillatory, and complex structures. These CGM samples can be categorized into three generations: CGM-I, CGM- TJ, and CGM-EI. The transition from CGM- I to CGM-II shows an enrichment of Ta and Fe in columbite-Mn, coupled with a depletion of Y. This evolution suggests that CGM- II crystals were crystallized from a Ta- and Fe-rich fluxed melt or fluid, subsequent to the crystallization of Nb-, Mn-, and Y-bearing minerals. The evolution from CGM-II to CGM-IE docs not exhibit consistent trends in Ta/(Nb + Ta) and Mn/(Fe + Mn) values, reflecting complex interactions between minerals and fluids. Microlite samples from both spodumene-bearing pegmatite dykes and muscovite granite display a variety of internal structures, including homogeneous, corc-rim, and irregular-zoned crystals. These variations suggest the influence of a Ta-Ca-rich fluid activity, a boundary effect or Nb-rich fluid activity, and fluid disturbances in an unstable crystallization environment. The chemical composition of microlite, specifically the Ti02, U02, F contents, as well as Ta/(Nb + Ta) value, may serve as potential indicators for magma evolution. However, late fluid activity that could change the chemical composition of microlite should be considered. The study of Nb-Ta-bearing minerals provides valuable insights into the magma features and evolution processes of the three spodumene-bearing pegmatite dykes in the Kuqu intrusion. The first spodumene-bearing pegmatite magma (No. 1) is relatively enriched in Fc and underwent successive crystallization of Fe-bearing minerals, followed by Nb- and Mn-bearing minerals, and finally Ta- and Mn-bcaring minerals, with a late Ta- and Ca-rich fluid activity that is influenced by the boundary effect. The second spodumene-bearing pegmatite (No. 2) experienced the crystallization of Nb- and Fe-bearing minerals, followed by Mn- and Y-bcaring mineral crystallization, and a buffer of Nb- and Ta-bearing mineral crystallization. In the late stage of its evolution, there was evidence of Nb-rich hydrothermal metasomatism following a Ta- and Ca-rich fluid activity. The third spodumene-bearing pegmatite magma (No. 3) is relatively enriched in fluxes and underwent crystallization of Nb-, Mn-, and Y-bcaring minerals, coupled with complex interactions between minerals and the liquid medium. Late-stage Ta- and Ca-rich fluid activity was also observed. The Kuqu spodumene-bearing pegmatites experienced an evolution from a melt to a rarc-mctal-rich and fluxed melt, and finally to complex fluids, including a Ta-Ca-rich fluid and a late-stage Nb-rich fluid. The occurrence, internal structure, and chemical composition of Nb-Ta-bearing minerals provide valuable insights into the evolution processes, particularly the late-stage fluid activities of the granites and pegmatite dykes in the Kuqu intrusion. © 2024 Geological Society of China. All rights reserved.