An Unusual Ruby-Sapphire-Sapphirine-Spinel Assemblage from the Tertiary Barrington Volcanic Province, New South Wales, Australia

F. Lin Sutherland and Robert R. Coenraads
Mineralogy and Petrology Section, Australian Museum, 6 College Street, Sydney, N.S.W. 2000, Australia
Gemmological Association of Australia (N.S.W. Division), 24 Wentworth Avenue, Sydney, N.S.W. 2000, Australia

Abstract: Ruby-sapphire-sapphirine-spinel forms small, corroded, crystalline aggregates in corundum bearing alluvials shed from the Tertiary Barrington basalt shield volcano. Sapphirine is near a 7:9:3 (MgO-Al2O3-SiO2) composition and, together with the corundum, shows reaction rims of pleonaste spinel. Spinel in the aggregates has a compositional range Sp 68–73 Hc 27–29 Cm 0–3. The aggregates give new insights into the ruby-sapphire source rocks. Potential origins include metamorphic recrystallization of aluminous material (below 1460°C) or high temperature-high pressure crystallization reactions related to lamprophyric or basaltic magmas (up to 1300°C and 20 kbar). Sapphirine-spinel thermometry suggests final crystallization temperatures for the aggregates around 780 to 940°C and reaction with host magmas at over 1000°C.

The Barrington gemfield includes two distinct corundum suites. One, typical in eastern Australia, is dominated by blue-green, well-crystallized, growth-zoned sapphire, commonly containing rutile silk and Ferich spinel inclusions (Hc 51–73, Mt 18–35, Mf 6–8, Usp 2–6). The other, an unusual suite, is dominated by ruby and pastel coloured sapphires, with little crystal shape or growth zonation and restricted mineral inclusions, mostly chromian pleonaste and pleonaste. The ruby-sapphire-sapphirine-spinel aggregates provoke new thoughts on the origin of rubies and sapphires and their indicator minerals in eastern Australian and southeastern Asian volcanic gemfields.

Keywords: ruby • sapphire • sapphirine • xenoliths • xenocrysts • Barrington volcanic province • Australian gemfields • Thailand gemfields

Mineralogical Magazine; August 1996 v. 60; no. 401; p. 623-638; DOI: 10.1180/minmag.1996.060.401.08
© 1996, The Mineralogical Society
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