Chemical evolution of micas and Nb-Ta oxides from the Koktokay pegmatites, Altay, NW China: Insights into rare-metal mineralization and genetic relationships

Abstract

This study focuses on the differences in the mineralization potentials and genetic relationships between representative rare-metal pegmatites and the muscovite granite in the Koktokay field in Altay, China, as well as the possible roles that magmatic and/or hydrothermal activities played in enriching the related rare metals. To this end, the major- and trace- element compositions of micas, the major- element compositions of Nb-Ta oxides, and the Li isotopic compositions of Li-rich muscovite, lepidolite, and spodumene were systematically analyzed. The major element compositions of the micas in the No. 1 and No. 2 pegmatites are quite similar. However, the MgO (0.89–2.18 wt%) and F (0.66–2.01 wt%) contents of the micas in the contact zone of the No.3 pegmatite are higher than those of the micas in the No. 1 and No. 2 pegmatites. In addition, the MnO (0.37–0.46 wt%) and MgO (0.39–0.59 wt%) contents of the micas in the muscovite granite are distinctly higher than those of the micas in the studied pegmatites. The trace-element compositions reveal that the primary micas from the small No. 1 and No. 2 pegmatites have significantly lower Li, Be, Ta, and Cs contents and a narrower range of fractionation relative to those in the large No. 3 pegmatite, indicating a significant difference in the rare-metal mineralization potentials of these pegmatites. Moreover, there is a large difference in the δ7Li values of the Li-rich minerals in the muscovite granites (−4.37–0.07‰), No. 3 (4.3–6.81‰) and No. 1 (7.44–14.65‰) pegmatites. These results support the hypothesis that the muscovite granite and pegmatite dikes may have formed independent of each other, and that their formations were a consequence of multi-stage magmatic activities. In addition, the Nb-Ta oxides in the outer zones are mainly columbite-(Mn). The large differences in the Nb/Ta ratios of the columbite-(Mn) (1.83–14.91) and primary micas (2.22–10.41) in the outer zones could be the result of silicate-silicate melt immiscibility. Furthermore, the low Nb/Ta ratios (commonly < 5) of the bright overgrowths of the columbite-(Mn) may have crystallized directly from a hydrous alkali-halogen-rich melt rather than from a hydrothermal fluid. The distinctly higher Ta (39–167 ppm), Cs (275–579 ppm), and B (213–245 ppm) contents; considerable Li contents (400–554 ppm), and lower Nb/Ta ratios (0.5–3.2) of the hydrothermal micas suggest that hydrothermal processes played an important role in the additional enrichment of the related rare metals (e.g., Ta, Cs, Li, and B).