In Relation to Sapphire-Ruby Deposits

ABSTRACT. 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 Fe2O3/TiO2 and low Cr2O3/Ga2O3 (<1). Two sub-types differ in colour absorption spectra, one being unusual in lacking the typical Fe2+-Fe3+ charge transfer effects found in such sapphires. Related trapiche-like corundums (5%) show higher Cr2O3/Ga2O3, possibly due to Fe-Ti oxide exsolution. Vari-coloured, diffuse-zoned and pale blue sapphires (10%) have higher Cr2O3/Ga2O3 and colour absorption characteristics intermediate between BGY sapphires and pink to red corundums with elevated Cr2O3/Ga2O3. 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 sapphire