Textures and chemistry of columbite-group minerals record magmatic–hydrothermal processes in the Baishitouquan pluton in the Eastern Tianshan, NW China

Abstract

Rare-metal granites result from a combination of magmatic and hydrothermal processes. The mineralogical, textural, and chemical characteristics of columbite-group minerals (CGMs), which are ubiquitous in rare-metal granites and pegmatites, can serve as records of the magmatic–hydrothermal processes responsible for the evolution of these granitic systems. The highly evolved Baishitouquan (BST) granite pluton, located in the Eastern Tianshan of NW China, hosts the large-scale Zhangbaoshan (ZBS) Rb deposit (Rb₂O reserve of 67080 t), making it of economic significance. The BST pluton comprises a series of gradual lithological zones, which, from bottom to top, are leucogranite (Zone-a), amazonite-bearing granite (Zone-b), amazonite granite (Zone-c), topaz-bearing amazonite granite (Zone-d), topaz albite granite (Zone-e), and amazonite pegmatite veins. Based on textural and chemical characteristics, five types of CGMs were identified — CGMs with no zoning, and oscillatory zoned, normally zoned, hydrothermally overprinted, and patchy CGMs. The CGMs that exhibit no zoning, oscillatory zoning, and normal zoning mainly occur in Zones-a to -c, suggesting that the lithological and geochemical variations in these zones formed as a result of the magmatic evolution of the BST magma. Hydrothermally overprinted CGMs and those with a patchy texture mainly occur in Zones-d and -e, and the pegmatite veins, suggesting that the evolution of these zones involved hydrothermal processes. From Zone-a to Zone-c, the Ta# and Mn# of CGMs increase gradually, suggesting a gradual evolution of the BST magma. The CGMs are characterized by REE tetrad effect (TE_1,3) that is consistently greater than 1.1 and increases from Zone-a to the pegmatite veins, indicative of increased melt–fluid interaction during evolution of the BST magma. It is, therefore, suggested that the BST magma evolved not only via high degrees of fractional crystallization, but also by the interaction of the melt with hydrothermal fluids, the latter of which likely originated by exsolution from the evolving melt. Based on the mineralogical, textural, and chemical characteristics of CGMs in the BST pluton, a petrogenetic model is proposed to explain the magmatic–hydrothermal evolution of the BST granitic magma.