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Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

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Zaw, K and Sutherland, FL and Dellapasqua, F and Ryan, CG and Yui, TF and Mernagh, TP and Duncan, D (2006) Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes. Mineralogical Magazine, 70 (6). pp. 669-687.

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Abstract

Corundum xenocrysts from alkaline basalt fields differ in characteristics and hence lithospheric origins. Trace element, fluid/melt inclusion and oxygen isotope studies on two eastern Australian corundum deposits are compared to consider their origins. Sapphires from Weldborough, NE Tasmania, are magmatic (high-Ga, av. 200 ppm) and dominated by Fe (av. 3300 ppm) and variable Ti (av. 400 ppm) as chromophores. They contain Cl, Fe, Ga, Ti and CO2-rich fluid inclusions and give δ18O values (5.1-6.2‰) of mantle range. Geochronology on companion zircons suggests several sources (from 290 Ma to 47 Ma) were disrupted by basaltic melts (47 ± 0.6 Ma). Gem corundums from Barrington, New South Wales, also include magmatic sapphires (Ga av. 170 ppm; δ18O 4.6-5.8‰), but with more Fe (av. 9000 ppm) and less Ti (av. 300 ppm) as chromophores. Zircon dating suggests that gem formation preceded and was overlapped by Cenozoic basaltic melt generation (59-4 Ma). In contrast, a metamorphic sapphire-ruby suite (low-Ga, av. 30 ppm) here incorporates greater Cr into the chromophores (up to 2250 ppm). Fluid inclusions are CO2-poor, but melt inclusions suggest some alkaline melt interaction. The δ18O values (5.1-6.2‰) overlap magmatic sapphire values. Interactions at contact zones (T = 780-940°C) between earlier Permian ultramafic bodies and later alkaline fluid activity may explain the formation of rubies.

Item Type: Article
Keywords: PIXE, Mineral Chemistry, Sapphire, Ruby, Weldborough Gem corundum, trace elements, fluid/melt inclusions, oxygen isotopes, basalts, mantle, eastern Australia.
Journal or Publication Title: Mineralogical Magazine
Page Range: pp. 669-687
Identification Number - DOI: 10.1180/0026461067060356
Date Deposited: 11 Jul 2007
Last Modified: 18 Nov 2014 03:19
URI: http://eprints.utas.edu.au/id/eprint/1351
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