The Tumbarumba Basaltic Gem Field, New South Wales: in relation to sapphire-ruby deposits of eastern Australia

Tumbarumba gemfield in the Snowy Mountains basalt province, NSW, yields corundums, zircons and garnet, corroded by magmatic effects and abraded by alluvial transport.Sub-basaltic contours suggest present drainage profiles mimic Miocene sub-basaltic leads.Six types of corundum were identified.Blue, green, yellow (BGY) zoned sapphires (80%) contain ferrocolumbite as a main mineral inclusion and exhibit variable Fe 2 O 3 /TiO 2 and low Cr 2 O 3 /Ga 2 O 3 (<1).Two sub-types differ in colour absorption spectra, one being unusual in lacking the typical Fe 2+ -Fe 3+ charge transfer effects found in such sapphires.Related trapiche-like corundums (5%) show higher Cr 2 O 3 /Ga 2 O 3 , possibly due to Fe-Ti oxide exsolution.Vari-coloured, diffuse-zoned and pale blue sapphires (10%) have higher Cr 2 O 3 /Ga 2 O 3 and colour absorption characteristics intermediate between BGY sapphires and pink to red corundums with elevated Cr 2 O 3 /Ga 2 O 3 .The BGY and trapiche-like sapphires are considered magmatic, the intermediate sapphires magmatic-metasomatic (possibly through interactions with Cr-bearing serpentinite bodies) and the pink to red corundums metamorphic in origin.Zircons include low-to high-U types.The latter show {100}-{110} prism combinations (unusual in eastern Australian zircons) and suggest incompatible element enriched parental melts.The magmatic sapphires and zircons (U-Pb age 23 Ma) crystallised in deep evolved salic melts, before transport in basalt.Magmatic-metasomatic sapphires contain zircon inclusions with both older inherited U-Pb ages (up to 903 Ma) and younger magmatic U-Pb ages .Basalts represent little evolved undersaturated melts (basanites and alkali basalts), and minor near-saturated transitional melts (olivine basalts).Most generated from garnet peridotite sources, but some from spinel peridotite sources.Mantle normalised incompatible multi-element patterns suggest Oceanic Island Basalt (OIB) melts interacted with amphibole (+ apatite) veined mantle.A sapphire and zircon-bearing basalt, also carries kaersutitic amphibole, apatite, alkali feldspar, titanian mica and titanian magnetite xenocrysts from a veined metasomatised source.Olivine micro-dolerite in a plug resembles the Cainozoic basalts in freshness, but its distinct trace element pattern and Early Devonian K-Ar age (400 Ma) indicate an earlier unmetasomatised spinel peridotite source.The Tumbarumba field evolved through explosive gem-bearing basaltic activity between 27-15 Ma and peaked in basalt lava activity.Interactions of basaltic melts with amphibole-rich mantle, serpentinite bodies and metamorphic corundum deposits combined to generate multi-modal gem suites.