CHEMICAL EVOLUTION AND LATE-STAGE RE-EQUILIBRATION OF Zr-Hf-U-BEARING COLUMBITE-GROUP MINERALS IN THE KOKTOKAY NO. 1 GRANITIC PEGMATITE, ALTAI, NORTHWESTERN CHINA

The Koktokay No. 1 pegmatite in northwestern China is a highly fractionated Li-Cs-Ta (LCT) - type granitic pegmatite. As main Nb - Ta oxides in this pegmatite, columbite-group minerals (CGMs) not only display variations in Nb/Ta and Fe/Mn, but also are typified by noticeable Zr, Hf, and U contents. Here, we report on the chemical evolution and late-stage re-equilibration of CGMs in the processes of crystallization and evolution of this pegmatite. Columbite-group minerals record large variations in Ta/(Nb + Ta) values (0.06–0.98) and moderate variations in Mn/(Fe + Mn) values (0.58–0.99), which could be a result of fractional crystallization of pegmatite magma. The concentrations of ZrO 2 , HfO 2 , and UO 2 in CGMs from the contact zone inwards to zone II of this pegmatite are uniformly less than 0.35 wt.%. However, primary tantalite-(Mn) with Ta/(Nb + Ta) and Mn/(Fe + Mn) values of 0.63–0.98 and 0.86–0.99, respectively, show relatively high contents of ZrO 2 , HfO 2 , and UO 2 , up to 1.68, 0.73, and 1.25 wt.%, respectively. The contents of ZrO 2 , HfO 2 , and UO 2 are positively correlated with variations in Ta/(Nb + Ta) values within different grains and single grains of primary tantalite-(Mn). Crystal-chemical considerations suggest that Zr, Hf, and U are possibly present in the tantalite-(Mn) structure due to the substitution 3(Zr,Hf,U) 4+ = 2(Nb,Ta) 5+ + (Fe,Mn) 2+ . In some areas within the primary tantalite-(Mn) crystals in zone III, specific tantalite-(Mn) has been identified with sharp but irregular contacts with the host tantalite. Such tantalite-(Mn) is chemically depleted in Zr, Hf, and U, but contains many small inclusions of zircon, the MnZrTa 2 O 8 phase, uraninite, and U-rich member of the microlite group of minerals. In a similar way, the primary Zr-Hf-U-bearing tantalite-(Mn) experienced late-stage local re-equilibration, giving rise to the formation of secondary Zr-Hf-U-depleted tantalite-(Mn) and other Zr- or U-rich mineral inclusions in an open fluid-mediated system. The behaviors of Zr, Hf, and U in CGMs in the Koktokay No. 1 pegmatite are indicative of Zr, Hf, and U enrichment during fractional crystallization of a highly fractionated pegmatite magma on one hand and imply late-stage activities of fluids on the other. Considerable enrichment of U and Hf in CGMs indicates that this type of mineral may act as a target for in situ LA-ICP-MS U-Pb dating and Lu-Hf isotope measurements, which may provide key information for understanding the source of the magma and the evolution mechanism of the Nb-Ta mineralization.