Individual crystals of zircon, from the Koktokay No. 3 granitic pegmatite vein, are variably altered and consist of three types of domains distinguished on the basis of textures and compositions. The pristine domains possess normal chondrite-normalized rare earth element (REE) patterns. Domains affected by metamictization are enriched in U and Th. The presence of abundant fractures, dense pores, and the enrichments of non-formula elements imply that these metamict domains have been altered by a later hydrothermal fluid. Tetrads observed in these altered domains are likely to have been generated by this alteration event.