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

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 jarositealunite, 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.

Sapphires related to alkali basalts from the Cerová Highlands, Western Carpathians (southern Slovakia): composition and origin

“…Similar models for corundum origin from upper mantle to lower crustal, fractionated syenitic/anorthoclasitic magma are also proposed for other sapphire occurrences (e.g. Coenraads et al 1995;Sutherland et al 1998Sutherland et al , 2009Upton et al 1999Upton et al , 2009Graham et al 2008).…”

The Dusi (Garba Tula) sapphire deposit, Central Kenya––a unique Pan-African corundum-bearing monzonite

“…The association of corundum with zircon, low-Ca plagioclase and alkali feldspar has been noted in several basalt hosted sapphire deposits (Guo et al, 1996). Coenraads et al (1995) noted the association of corundum to zircon and magnetite in zircon-corundum-magnetite xenoliths from Thailand basalt-hosted sapphire fields.…”

Gem‐bearing basaltic volcanism, Barrington, New South Wales: Cenozoic evolution, based on basalt K–Ar ages and zircon fission track and U–Pb isotope dating

“…KPA, Kings Plain, Australia; LPA, Lava Plains, Australia; HNC, Hainan, China; zircon inclusions in sapphire (data from Guo et al 1996b table 3). KWT, Khao Wua, Thailand: zircon from sapphire–magnetite xenolith (data from Coenraads et al 1995; Sutherland et al 1998b). LRS, Loch Roag, Scotland: zircon in sapphire in syenite xenolith (averaged from Hinton & Upton 1991 table 6).…”