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An unusual sapphire–zircon–magnetite xenolith from the Chanthaburi Gem Province, Thailand

Published online by Cambridge University Press:  05 July 2018

Robert R. Coenraads ,
Pongsak Vichit  and
F. Lin Sutherland
Robert R. Coenraads
Affiliation:
The Gemmological Association of Australia, Gemmology House, 24 Wentworth Ave, Sydney, N.S.W. 2000, Australia
Pongsak Vichit
Affiliation:
Economic Geology Division, Department of Mineral Resources, Rama 6 Road, Bangkok 10400, Thailand
F. Lin Sutherland
Affiliation:
Division of Earth and Environmental Sciences, The Australian Museum, 6-8 College Street, Sydney, N.S.W. 2000, Australia
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Abstract

A sapphire, zircon and magnetite-bearing xenolith from Khao Wua, near Chanthaburi in Thailand, conclusively demonstrates a common origin for the sapphire, zircon and magnetite found in alluvial deposits in the Chanthaburi gem fields. The original aluminium- and titanium-rich octahedral magnetite crystal in the xenolith exsolved into hercynite, magnetite and hematite during cooling. It includes minor anhedral jarosite–alunite, possibly originating as an iron-sulphide-rich immiscible liquid. Uranium-lead isotope dating of zircon in the xenolith gives an age of 1–2 (± <1) million years (Ma). This falls within fission track ages for alluvial zircons (2.57 ± 0.20 Ma) from the Chanthaburi—Trat gem fields and within the potassium-argon ages of 0.44 to 3.0 Ma for the alkali basaltic volcanism in the Chanthaburi Province. These data suggest a common origin for sapphire, zircon and magnetite, and link them with the processes involved in alkali basaltic magma generation. The high iron and zirconium, low magnesium, and the inferred sulphides suggest pegmatite-like crystallization in an incompatible-element enriched, silica-poor magma (partial melt or fractionation product) in the deep crust or upper mantle. Etch features on exposed surfaces of the xenolith indicate that it was transported out of its equilibrium environment by the rise of later magma.

Keywords

sapphirezirconuranium-lead datingxenolithKhao WuaChanthaburiThailand
Type
Mineralogy
Information
Mineralogical Magazine , Volume 59 , Issue 396 , September 1995 , pp. 465 - 479
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1995

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