Hydrothermal Zircon: Characteristics, Genesis and Metallogenic Implications

Zircon is a principal source of zirconium and hafnium, as well as a key mineral both for petrogenetic modeling and age dating. Although zircon occurrences have been reported from hydrothermal mineral systems, detailed studies on zircon that crystallized entirely from saturated hydrothermal fluid are rare. Here we report on hydrothermal zircon crystals dated at about 1911 Ma that crystallized from boiling high-salinity H2O-CO2 hydrothermal fluid with fluid inclusion homogenization temperatures of 295 to 325 degrees C and the corresponding pressure of 71 bar. These zircon crystals were then overprinted by two stages of hydrothermal zircon veins at 1864 Ma and 528 Ma. The zircon veins crystallized from boiling hydrothermal fluids of moderate to low salinities with fluid inclusion homogenization temperatures and the corresponding pressures of 302 to 329 degrees C and 75 bar and 290 to 320 degrees C and 70 bar, respectively. The zircon crystals in our study show sharp and regular cathodoluminescent oscillatory zonation and low and consistent trace element concentrations and Th/U ratios that decrease with fluid salinity. The Th/U ratios of these zircons lie between the magmatic zircon values (>0.5) and metamorphic zircon values (<= 0.1). The rare earth element concentrations and trace element spidergram patterns of the zircon samples do not record their hydrothermal origin. Importantly, the low concentrations of HfO2 (<1%), Y (<400 ppm) and Ti (<4 ppm) can discriminate hydrothermal zircon from igneous and metamorphic zircons. Hydrothermal zircon is considered as an effective tool for the dating of mineral deposits and also represents one of the main sources of critical elements such as zirconium and hafnium.