Role of F-rich hydrosilicates in Nb mineralization within the Bashisuhong alkaline complex, Chinese southwestern Tianshan: insights from zircon geochemistry

• Zircon U-Pb dating constrains the Bashisuhong Nb mineralization age at ca. 286–283 Ma. • A crystal-mush system is proposed to explain the formation of the Bashisuhong Alkaline Complex. • F-rich hydrosilicates play principal role in Nb-enrichment. The mechanism of Nb mineralization in alkaline–silicate igneous complexes remains debated. Here, we present results from field investigations, petrology, whole–rock geochemistry, zircon U–Pb dating, trace–element analysis, and in–situ Lu–Hf isotopes for representative lithologies from the Nb–Zr–REE mineralized Bashisuhong Alkaline Complex (BAC) in the South Tianshan Orogenic Belt, China, to elucidate the processes responsible for the Nb enrichment of these rocks. The BAC comprises aegirine syenite, ore–barren and ore–bearing granites, and aegirine pegmatite. Based on the morphology and in–situ trace–element characteristics of the zircon grains, the zircons from the aegirine syenite and ore-barren granite are magmatic, while the zircons from the ore-bearing granite and aegirine pegmatite contain three subgroups. The Group–1 zircons (from the ore–bearing granite and aegirine pegmatite) are magmatic, whereas the Group–2 zircons (from the ore–bearing granite) and Group–3 zircons (from the aegirine pegmatite) are magmatic-hydrothermal. The overlapping U–Pb ages of the magmatic and hydrothermal zircons (286–283 Ma) indicate that there was a rapid transition from the magmatic to hydrothermal stages. The similar zircon ε Hf (t) values of all four lithologies suggest that these zircons were derived from a single crystal–mush system. The field observations, micrographic textures, whole–rock geochemistry, and discrepancies between the whole–rock and zircon Eu anomalies indicate that the aegirine syenite represents cumulate material, whereas the ore–barren and ore–bearing granites are fractionated products. The ore–bearing granite and aegirine pegmatite are enriched in Nb, Ta, Zr, rare earth elements (REEs), and other fluid–mobile elements (Rb, Ga, Sn, Hf, Y, U, Th, and Pb). The positive correlations between these elements and Nb, in contrast to the ore–barren granite, demonstrate the critical role of a late–stage, fluid–rich media in mobilizing Nb. The elevated zircon Y/Ho ratios (up to 46.4) of the Group–2 and Group–3 zircons, relative to chondritic values, indicate the presence of a fluorine–rich medium capable of transporting Nb. We propose that fluorine–rich hydrosilicates, rather than a simple hydrothermal fluid, were the principal agents of Nb enrichment in the BAC. Based on the elevated Nb + Ta contents of the Group–3 zircons and the occurrence of pyrochlore and columbite in the ore–bearing granite and aegirine pegmatite, we infer that the dissolution and reprecipitation of Nb from early crystallized Nb–bearing minerals by the fluorine–rich hydrosilicates may be the mechanism that affected the mobilization/fertilization of Nb.