Age Dating of Gem-quality Zircon and Sapphire from the New England Gem Fields, New South Wales, Australia, and its Application to the Geological Origin of the Sapphires.

Transparent, mostly small, pale yellow to brown zircon megacrysts occur with gem corundum in alluvial deposits associated with alkali basalts in the Inverell-Glen Innes District of the New England gem fields, New South Wales, Australia. In a previous study, U-Pb ages of zircon from the Kings Plains, Mary Anne Gully and Swan Brook localities were determined using LA-ICP-MS data. An analytical traverse across one crystal revealed chemical homogeneity, indicating stable crystallisation conditions. The 206Pb/238U ages of the zircons from the three localities fell into two groups: an older set ranging from 216 to 174 million years (Ma) and a younger group of 45–37.7 Ma, indicating two main formational events in the Late Triassic–Early Jurassic and Eocene time periods. New data obtained from zircon inclusions in two gem-quality blue sapphires gave 206Pb/238U concordant average ages of 37 ± 1.7 Ma and 35.1 ± 1.6 Ma for samples from Kings Plains and Mary Anne Gully, respectively. The ages of both inclusions fall within the K-Ar age range of basalt from the Maybole volcanic suite (38–33 Ma) and correlate well with the age range of the younger zircon megacrysts. Combined with the gemmological features and chemical fingerprints of the sapphires, the age data offer valuable insights into their geological origin by providing a genetic link between gem corundum crystallisation in the deep crust or mantle and transport to the Earth’s surface in alkali basaltic magma. [ABSTRACT FROM AUTHOR]