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INTRODUCTION
Numerous cases have been reported where zircon may have precipitatedfrom a
hydrothermal fluid or a fluid-saturated residual melt.Temperatures for
hydrothermal zircon formation range from 600°Cin late-magmatic systems at
the magmatic-to-hydrothermal transitiondown to 300°C in mesothermal
ore-forming systems. Late-magmaticto hydrothermal zircon may precipitate from
fluid-saturatedmagma and possibly from the fluids exsolved from mineralized
granitesand pegmatites. For example, in the Sn-W-mineralized Mole Granite,
NewSouth Wales, Australia, zircon occurs in growth zones in hydrothermal
quartz,along with monazite, xenotime and thorite
(FIG. 1). This
zircon- considered to be hydrothermal - preserves a crystallizationhistory,
from magmatic oscillatory-zoned growth domains producedunder
fluid-undersaturated to fluid-saturated magmatic conditions,to margins
modified by hydrothermal growth and overprinted byreplacement zones unrelated
to crystal surfaces. Hydrothermal,spongy and inclusion-rich zircon
crystallized under similarconditions together with topaz, fluorite, biotite
and monazitein Sn-W-mineralized quartz veins crosscutting the granite.
