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Al-rich diopside in alluvial ruby and corundum-bearing xenoliths, Australian and SE Asian basalt fields

F. L. Sutherland^1,*, R. R. Coenraads², D. Schwarz³, L. R. Raynor^4,, B. J. Barron⁵ and G. B. Webb¹

¹ Mineralogy, Australian Museum, 6 College Street, Sydney, NSW 2010, Australia
² Cluff Resources Pacific NL, Locked Bag 3355, Hornsby, NSW 2077, Australia
³ Gübelin Gem Lab, Maihofstrasse 102, CH 600, Lucerne 9, Switzerland
⁴ Department of Geology and Geophysics, University of Sydney, NSW 2006, Australia
⁵ 7 Fairview Avenue, St Ives, Sydney, NSW 2075, Australia

^* E-mail: lins{at}austmus.gov.au

Alluvial rubies and sapphires are found in palaeodrainage deposits along the Cudgegong-Macquarie River system, central eastern New South Wales, Australia. A pink to red suite contains Cr (up to 0.6 wt.% Cr₂O₃) as the main chromophore, exceeding Fe (up to 0.5 wt.%Fe₂O₃). Corrosive etching suggests a prior xenocrystic Mesozoic-Cenozoic basaltic transport, while Cr₂O₃/Ga₂O₃ to Fe₂O₃/TiO₂ ratios indicate an original metamorphic source. Syngenetic mineral inclusions include Al-rich diopside, meionite and anatase. The Al-rich diopside (‘fassaite’) contains extremely high Al₂O₃ (20–21 wt.%). A blue-green suite contains Fe (up to 0.8 wt.% Fe₂O₃) as a dominant chromophore, while a rare nepheline-anorthoclase composite inclusion supports a magmatic phonolitic origin. The Cudgegong-Macquarie ruby formation is compared with a garnet granulite origin proposed for Thailand rubies and a xenolith of corundum-bearing garnet granulite from Ruby Hill, Bingara, Australia. Clinopyroxene-corundum thermometry suggests the Cudgegong-Macquarie rubies formed at T >1000–1300°C, a high equilibration T for proposed lithospheric granulites. These rubies form a distinctive suite compared to other rubies from Australian and SE Asian basalt fields, but have some similarities with eastern Thailand rubies.

KEYWORDS: corundum, ruby, sapphire, diopside, alluvial, xenocryst, granulite, basalt field, Australia, SE Asia

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