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Mineralogical Magazine; December 2006; v. 70; no. 6; p. 669-687; DOI: 10.1180/0026461067060356
© 2006 Mineralogical Society of Great Britain and Ireland
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Contrasts in gem corundum characteristics, eastern Australian basaltic fields: trace elements, fluid/melt inclusions and oxygen isotopes

Khin Zaw1,*, F. L. Sutherland2, F. Dellapasqua1, C. G. Ryan3, Tzen-Fu Yui4, T. P. Mernagh5 and D. Duncan6

1 CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia
2 Mineralogy, Australian Museum, 6 College Street, Sydney, NSW 2010, Australia
3 CSIRO Exploration and Mining, School of Physics, University of Melbourne, VIC 3010, Australia
4 Institute of Earth Science, Academica Sinica, Taipei, Taiwan
5 Australian Geological Survey Organization, Canberra, ACT 2601, Australia
6 McPherson Duncan & Associates, 18 Old Summerleas Road, Kinston, Tasmania 7050, Australia

* E-mail: Khin.Zaw{at}utas.edu.au

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 {delta}18O 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; {delta}18O 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 {delta}18O 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.

KEYWORDS: Gem corundum, trace elements, fluid/melt inclusions, oxygen isotopes, basalts, mantle, eastern Australia






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