U–Pb age and origin of gem zircon from the New England sapphire fields, New South Wales, Australia

Abstract

Chemical compositions and geochronological data utilising the laser ablation ICP-MS technique are presented for zircon megacrysts found in alluvial gem corundum deposits associated with Upper Cretaceous–Cenozoic alkali basalts in the Inverell district-New England field, New South Wales, eastern Australia. Three localities, Kings Plains, Swan Brook and Mary Anne Gully, produce gem-quality transparent dark brown and yellow zircon megacrysts, mostly under 10 mm in size. Although brown zircon shows relative enrichment in Hf and REE, there are no differences in relative transition metal concentrations between the colours. Chemical homogeneity within a single crystal indicates stable crystallisation conditions. The ²⁰⁶Pb/²³⁸U age of zircon megacrysts from these three localities define older and younger groups of 216–174 Ma and 45–37.7 Ma, respectively. The ϵ_Hf values of zircon megacrysts from Kings Plains show +7.51±0.34 in the older group and +10.72±0.31 in the younger group. Swan Brook zircons give +11.54±0.47 and +8.32±0.58, and Mary Anne Gully zircons are +13.67±0.63 and +8.50±0.48, respectively. These zircons from New England alluvial gem deposits have two main formational events around Upper Triassic–Lower Jurassic and Eocene episodes. Most originated from lithospheric mantle and all were brought-up by later host basaltic magmas.

KEY WORDS:

• Gem zircon
• megacryst
• U–Pb age
• Hf isotopes
• trace elements
• alkali basalt
• mantle
• New England
• NSW
• East Australia

ACKNOWLEDGEMENTS

This is a joint Research Program: GAAJ-Zenhokyo Laboratory–Australian Museum–Gemmological Association of Australia; the field trip study was supported by The Australian Museum and The Gemmological Association of Australia. Ross Pogson and Gayle Webb, Australian Museum, helped with collection access. The authors are very grateful to the corundum mine operator (owner) Mr Jack Wilson in Kings Plains Creek (Wilson Gems and Investments, Australia), Mr Merv Walburn in Mary Ann Gully, Mr Ken (Bill) Dawson in Swan Brook and Frazers Creek in the Inverell district, NSW, Australia, for supporting the field visiting and arranging a donation of zircon samples. This is contribution 191 from the ARC Centre of Excellence for Core to Crust Fluid Systems (<http://www.ccfs.mq.edu.au>) and 834 in the GEMOC Key Centre (<http://www.gemoc.mq.edu.au>). The analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS, industry partners and Macquarie University. The manuscript was read by Ian Graham, School of Biology, Earth and Environmental Sciences, University of New South Wales, Sydney. Andy Gleadow, University of Melbourne, and Larry Barron, Sydney, are thanked for their detailed reviews of the paper.