Author name: Sean P. Gaynor

Numerous cases have been reported where zircon may have precipitated from a hydrothermal fluid or a fluid-saturated residual melt. Temperatures for hydrothermal zircon formation range from 600°C in late-magmatic systems at the magmatic-to-hydrothermal transition down to 300°C in mesothermal ore-forming systems. Late-magmatic to hydrothermal zircon may precipitate from fluid-saturated magma and possibly from the fluids exsolved from mineralized granites and pegmatites. For example, in the Sn–Wmineralized Mole Granite, New South Wales, Australia, zircon occurs in growth zones in hydrothermal quartz, along with monazite, xenotime and thorite

Natural zircon crystals often show complex secondary textures that cut across primary growth zones. In zircon showing structural damage caused by self-irradiation, such textures are the result of a diffusion– reaction process in which a hydrous species diffuses inwards and “catalyzes” structural recovery. Nanoscale pores develop, solvent elements such as Ca, Al and Fe are gained, and radiogenic Pb is lost. In both aqueous fluids and melts, replacement of zircon with undamaged structure by a coupled dissolution– reprecipitation process can produce similar textures. The reacted domains usually have lower trace element contents and may contain micrometer-sized pores and inclusions of uranium, thorium and/or yttrium phases, originally in solid solution. Both processes have considerable implications for zircon geochronology.