Types and Evolution of Columbite-Group Minerals from Pegmatites in the Chinese Altai, NW China: Implications for Regional Petrogenesis and Rare-Element Mineralization

Abstract

Abstract The Chinese Altai orogenic belt is famous for large numbers of pegmatite dikes, various rare-element (REL) mineralization types and its rich REL resources. In REL pegmatites, columbite-group minerals (CGM) display compositional complexity that can be used to decipher magma evolution and REL metallogenesis. Here, we provide compositional data and internal structures for columbite-group minerals from representative Chinese Altai REL pegmatites, including Koktokay No. 3 (Li-Be-Nb-Ta-Cs-Rb-Hf, early Jurassic), Xiaokalasu (Li-Nb-Ta, late Permian), and Dakalasu (Be-Nb-Ta, middle Triassic), in order to elucidate ore-forming processes and identify possible indicators of REL mineralization to enhance exploration success. The CGM were classified into five types based on compositional complexity, each of which provides a window into magmatic evolution and crystallization in the pegmatite. In the Koktokay No. 3 pegmatite, CGM evolution in zone I reveals a silicate melt with fluid at undercooling, while that in zone IV reflects a silicate melt followed by complex Ta-rich boundary-layer melt, and that in zone V suggests an evolution from silicate melt, to Ta-rich boundary-layer melt, and finally to REL-rich and fluxed melt accompanied by fluid activity. In the Xiaokalasu Li-mineralized pegmatite, CGM display a similar evolution trend to that of core-rim CGM in zone V of the Koktokay No. 3 pegmatite. In the Dakalasu pegmatite, CGM and Ti-Nb-Ta phases imply Fe-Mn-Nb-Ta-rich and Ti-Nb-Ta-rich melts, and intergrowths of CGM, Ta/Nb-rich rutile, and microlite result from decomposition of a metastable Ti-Nb-Ta oxide precursor in undercooling conditions. The Nb-Ta and Fe-Mn fractionation and changes in minor/trace element contents in CGM depend on the geochemical features of the elements, chemistry of the pegmatite magma, petrogenetic processes (e.g., fractional crystallization, fluid exsolution, and melt–fluid–mineral interaction), and buffering of these factors. Fractional crystallization prevailed during melt evolution, producing Ta-rich boundary-layer melt and REL-rich and flux-enriched melt. Fluid activity was observed in zones related to magmatic, magmatic–hydrothermal transitional, and hydrothermal stages, leading to CGM chemical redistribution by similar/low-Ta and high-Sb fluid replacement and crystallization of stibiotantalite. Fractional crystallization, host-rock assimilation, rapid undercooling, fluid exsolution, and fluid activity are important for Be, Li, Nb, and Ta mineralization. Combining the potential indicators [i.e., Nb-Ta-oxide phase assemblage, CGM types and evolution, twin-element decoupling (Zr-Hf, Th-U, and Y-REE), trace element content grade, and REE (rare earth element) distribution pattern] could be used to discriminate REL mineralization types and enhance REL exploration success.