Abstract
Ruby in diverse geological settings leaves petrogenetic clues, in its zoning, inclusions, trace elements and oxygen isotope values. Rock-hosted and isolated crystals are compared from Myanmar, SE Asia, and New South Wales, East Australia. Myanmar ruby typifies metasomatized and metamorphic settings, while East Australian ruby xenocrysts are derived from basalts that tapped underlying fold belts. The respective suites include homogeneous ruby; bi-colored inner (violet blue) and outer (red) zoned ruby; ruby-sapphirine-spinel composites; pink to red grains and multi-zoned crystals of red-pink-white-violet (core to rim). Ruby ages were determined by using U-Pb isotopes in titanite inclusions (Thurein Taung; 32.4 Ma) and zircon inclusions (Mong Hsu; 23.9 Ma) and basalt dating in NSW, >60–40 Ma. Trace element oxide plots suggest marble sources for Thurein Taung and Mong Hsu ruby and ultramafic-mafic sources for Mong Hsu (dark cores). NSW rubies suggest metasomatic (Barrington Tops), ultramafic to mafic (Macquarie River) and metasomatic-magmatic (New England) sources. A previous study showed that Cr/Ga vs. Fe/(V + Ti) plots separate Mong Hsu ruby from other ruby fields, but did not test Mogok ruby. Thurein Taung ruby, tested here, plotted separately to Mong Hsu ruby. A Fe-Ga/Mg diagram splits ruby suites into various fields (Ga/Mg < 3), except for magmatic input into rare Mogok and Australian ruby (Ga/Mg > 6). The diverse results emphasize ruby’s potential for geographic typing.
Highlights
In this study [22], we present new comparative trace element results and age data from Myanmar ruby fields, which typify metamorphosed and metasomatized carbonate and skarn settings at Mogok and Mong Hsu, and compare them with eastern Australia ruby fields, which typically carry ruby xenocrysts derived from basalt fields and found in placer deposits
Examples of Myanmar ruby samples analyzed for trace elements by LA-inductively coupled plasma mass spectrometer (ICP-MS) and an Minerals 2019, 9, 28for U-Pb age-dating are shown in Figures 6 and 7
Myanmar ruby ages of ~32 Ma at Mogok and at 23 Ma at Mong Hsu reflect the post-Paleozoic collision history and felsic magmatic activity in Myanmar, while older diverse ruby generated within dismembered fold belts in East Australia were delivered to surface placers by post-rift basalts
Summary
Minerals 2019, 9, 28 with sufficient entry of Cr3+ , whereas sapphire develops blue, green, yellow purple, violet, mauve and pink colors due to substitutions of the other chromophore elements in the presence of lesser Cr contents. Geochemical testing, initially leading to finger-printing [12], and further aided by O-isotope studies have become routinely used to enable geographic typing of the corundum suites [13,14,15,16]. This achieves greater control through use of a wider range of minerals included in corundum for providing precise isotopic dating [17,18,19]. Optical mineralogy combined with Raman spectroscopy remains a non-destructive technique for their testing, along with their solid/melt inclusions, for origin determination. [20,21]
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