Alkali basalts and associated volcaniclastic rocks as a source of sapphire in eastern Australia

Abstract

Corundum occurs widely in eastern Australia, especially in New England which hosts the world's largest producing sapphire mine at Kings Plains. The alluvial deposits of New England show strong local tribal variations in the nature of the sapphire, suggesting that they are derived by reworking of a source rock that is locally rich in corundum. Studies of corundum textures and mineral inclusions suggest a variety of origins, but the commercial sapphire deposits occur in regions where Tertiary alkali basalts are prominent and governed by episodic revival of volcanism over a prolonged period of time. Corundum is brought to the surface in basaltic eruptions as xenocrysts, but it is found only rarely in basalt lava. Throughout the New South Wales Tertiary volcanic province there are basalt capped remnants of thin units of clay‐altered basaltic volcaniclastic rocks. These rocks display many features found in basaltic diatremes and are interpreted as thin distal aprons produced by base surge flows during an early violent stage of eruption. They carry elevated grades of sapphire, erode quickly and would have been laterally extensive. They are considered to be the major source of alluvial sapphire in New England and in similar provinces elsewhere. Their whole‐rock geochemistry, using residual element ratios, indicates a regional link between the more fractionated volcaniclastic compositions and significant sapphire deposits.