Zircon of the No. 782 deposit from the Great Xing’an Range in NE China: Implications for Nb-REE-Zr mineralization during magmatic-hydrothermal evolution

Abstract

Niobium, rare earth elements (REE = lanthanides + Y), and zirconium, are widely used in high-tech applications. The Nb-REE-Zr mineralization in the No. 782 deposit of NE China developed in a fine-grained porphyry (porphyry) that occurs as a ring-shaped stock. The ore-forming porphyry is hosted in a coarse-grained granite (granite) that dominates the study area. Zircon is the only Zr-bearing mineral in both the porphyry and the granite. In contrast to the granite that hosts only magmatic zircon, both magmatic (type I) and metasomatic (type II) zircon crystals or aggregates are present in the porphyry. Most type I zircon occurs as individual crystals and are hosted mainly in interstitial quartz, with evident oscillatory zoning. Most type II zircon occurs as aggregates rather than individual crystals and exhibits vague zoning. Distribution of the type II zircon in the porphyry is heterogeneous, with great abundance in places where there are substantial albitization and snowball quartz crystals. Textural and chemical features of the type II zircon and of the associated albite and snowball quartz crystals suggest that all these minerals are metasomatic. Nb is hosted mainly by fergusonite-(Y) and, REE by fergusonite-(Y) and to a lesser extent, by bastnäsite-(Ce) and parisite-(Ce), where parisite-(Ce) occurs as a replacement of bastnäsite-(Ce). The Nb-REE mineralization occurs either as a replacement of the type II zircon or in chlorite-/quartz-dominant veinlets that postdated both the type I and the type II zircon. Most type I zircon crystals in the porphyry were intact from the replacement or alteration by the postdated Nb-REE minerals, possibly because the type I crystals were encased earlier by other magmatic minerals. The chronological and oxygen isotopic data of zircon suggest that the porphyry (453.8 ± 3.1 Ma) is younger than the granite (487.0 ± 4.0 Ma), and that the two granitic phases derived from different magma chambers. The subtle age difference between magmatic type I and metasomatic type II zircon cannot be shown by SHRIMP U-Pb data of zircon. The mineral assemblages associated with type II zircon and the chronological data suggest that the medium from which type II zircon precipitated is likely an internally-derived hydrosilicate liquid (HSL) that lies compositionally in between a silicate melt and a hydrothermal fluid during the magmatic-hydrothermal transition. Textural features of the Nb-REE minerals, combined with the fact that Nb-REE mineralization developed only inside the porphyry, but not in surrounding country rocks (i.e. the granite) suggest that the Nb-REE mineralization originated from an aqueous fluid that was constrained to the porphyry.