Open Access Repository

Crystallisation of magmatic topaz and implications for Nb-Ta-W mineralisation in F-rich silicic melts – The Ary-Bulak ongonite massif

Downloads

Downloads per month over past year

Agangi, A and Kamenetsky, VS and Hofmann, A and Przybyłowicz, W and Vladykin, NV (2014) Crystallisation of magmatic topaz and implications for Nb-Ta-W mineralisation in F-rich silicic melts – The Ary-Bulak ongonite massif. Lithos, 202-20. pp. 317-330. ISSN 0024-4937

[img] PDF
Lithos-2014-ong...pdf | Document not available for request/download
Full text restricted
Available under University of Tasmania Standard License.

Abstract

Textural, mineralogical and geochemical data on F-rich rhyolite (ongonite) from the Ary-Bulak massif of eastern Transbaikalia help constrain the formation of magmatic topaz. In these rocks, topaz occurs as phenocrysts, thus providing compelling evidence for crystallisation at the orthomagmatic stage. Cathodoluminescence images of topaz and quartz reveal growth textures with multiple truncation events in single grains, indicative of a dynamic system that shifted from saturated to undersaturated conditions with respect to topaz and quartz. Electron microprobe and Raman analyses of topaz indicate near-pure F composition [Al2SiO4F2], with very limited OH replacement. Laser ablation ICP-MS traverses revealed the presence of a large number of trace elements present at sub-ppm to hundreds of ppm levels. The chemical zoning of topaz records trace element fluctuations in the coexisting melt. Concentrations of some trace elements (Li, Ga, Nb, Ta and W) are correlated with cathodoluminescence intensity, thus suggesting that some of these elements act as CL activators in topaz. The study of melt inclusions indicates that melts with different F contents were trapped at different stages during formation of quartz and topaz phenocrysts, respectively. Electron microprobe analyses of glass in subhedral quartz-hosted melt inclusions indicate F ≤1.2 wt.%, whereas irregular-shaped melt inclusions hosted in both topaz and quartz have F ≤9 wt.%. Cryolithionite [Na3Li3Al2F12] coexists with glass in irregular inclusions, implying high Li contents in the melt. The very high F contents would have increased the solubility of Nb, Ta and W in the melt, thus allowing progressive concentration of these elements during magma evolution. Crystallisation of Nb–Ta–W-oxides (W-ixiolite and tantalite–columbite) may have been triggered by separation of cryolithionite, which would have caused F and Li depletion and consequent drop in the solubility of these elements.

Item Type: Article
Keywords: Topaz Fluorine Cryolithionite Ongonite Cathodoluminescence
Journal or Publication Title: Lithos
Page Range: pp. 317-330
ISSN: 0024-4937
Identification Number - DOI: 10.1016/j.lithos.2014.05.032
Additional Information:

Copyright 2014 Elsevier

Date Deposited: 07 Jul 2014 07:13
Last Modified: 18 Nov 2014 05:02
Item Statistics: View statistics for this item

Actions (login required)

Item Control Page Item Control Page
TOP