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 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 UPb 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.
The Lindeques Drift and Heidelberg Intrtisions and the volcanic Roodekraal Complex lielong to a higlvTi mafic igneous .suite (HITIS) thai was emplaced in the Kaapvaal Craton as km-sized btxiies, 10 the north and north-east of the Vredefort impact Htmcrure, prior to impact. New zircon SHRIMP ages for the Undef [ues Drift Intmsion (2054.8 ± 5.7 Ma) and Roodekraal Complex (2053 ± 9.2 Ma) are indi.slinguishable from that of the Bii.shvetd magmatic event (-2.06 to 2.05 Ga). The dominant rock types in the Lindeques Drift and Heidelberg Intrusions are even-grained and porphyritic spessartites which essentially are clinopyroxenemagnetite (-ilmenite) ± olivine cumulates with variable fractions (estimated 15 to 30^)) of interslitially crystallized magma. The concentration of magmatic amphibole (edenite-magnesiohastingsite), responsible for tlie porphyritic texture of the .spessartite, is directly proportional to the fraaion of trapped magma, which also crystallized interstitial plagitKlase. sphene. sulphide, apatite, micropenhite and bjotite. The Lindeques Drift spe.ssartite is cut by fme-grained dykes and sills(?) of diorite and syenodiorite. Low-siiica diorite in the Lindeques Drift Intmsion was produced by the reaction of the parental magma with the dolomitic country rock of the Chuniespoort Group tlirough desilicificaton and de-alumination. The Roodekraal Complex is composed of multiple flows of predominantly mugearite lava with sulvHow intrusions of diorite. The diorite of Lindeques Drift and the lava and diorite of Roodekraal are chemically comparable in terms of both major and trace elements, suggesting a co-magmatic derivation, The magma parental to the three bcxlies is proposed to be a low-AJ ferrobasalt with alkaline affinities (MgO -4.8%. TiOj =l.fWi, P2O, -0.52%, Zr/(P.Os» 10^) = 0.02, NbA'-1.47, K,OAt) =1.3, TaA'b = 1.1), The latter was probably derived by deep crustal fractionation of amphibole and piagioclase from an alkali basaltC?) precursor in an embryonic continental rift setting. The probability that a gabbro dyke, stratigraphically below and perpendicular to tlie Lindeques Drift Intmsion. is a feeder to the latter is regarded as small.
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