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

Abstract

Textural, mineralogical and geochemical data on F-rich rhyolite (ongonite) from the Ary-Bulak massif of eastern Transbaikalia help constraining 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 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.

Cathodoluminescence and Trace Element Composition of Scheelite from the Middle‐Lower Yangtze River Metallogenic Belt (MLYB): Implications for Mineralization and Exploration

“…Previous research indicated that the volatile fluorine is conducive to tungsten mineralization (Einaudi et al, 1981;Newberry and Swanson, 1986;Webster, 1990;Newberry, 1998;Hezarkhani et al, 1999;Chen et al, 2008;Bali et al, 2012;Che et al, 2013;Agangi et al, 2014). Theoretical modelling also proposed that fluorine could increase the electronegativity and chemical hardness of the fluid phase, which would promote the solubility of W in such a fluid phase (Vigneresse, 2009).…”

Wolframoixiolite in lithium-fluoric granites of the Arga-Ynnakh-Khaysky massif, Yakutia

“…Долгое время считалось, что вольфрамоиксиолит характерен лишь для редкометалльных пегматитов (Корнетова и др., 1982;Волошин, 1988;Suwimonprecha et al, 1995;Aurisicchio et al, 2002;Černý, Chapman, 2001;Novák et al, 2008;Melcher et al, 2017;и др.). Однако по мере освоения новых редкометалльных провинций количество его находок неуклонно растет, и в последние годы он многократно отмечен как акцессорный минерал ЛФГ и онгонитов в Европе, Африке и Азии (Johan, Johan, 1994;Chalal, Marignac, 1997;Raimbault, Burnol, 1998;Marignac et al, 2001;Kesraoui, Nedjari, 2002;Zhang et al, 2003;Перетяжко и др., 2007;Летников, 2008;Saleh et al, 2008;Agangi et al, 2014;Breiter et al, 2007Breiter et al, , 2017Melcher et al, 2017;Zhao et al, 2017;Broska, Kubis, 2018;René, 2018;Xie et al, 2018), в том числе на Дальнем Востоке России (Руб, Руб, 2006; Алексеев и др., 2010). «Лавина» находок вольфрамоиксиолита косвенно подтверждает его индивидуальность как минерального вида и позволяет считать его типоморфным минералом ЛФГ (Алексеев и др., 2010).…”

Relation between cathodoluminescence and trace-element distribution of magmatic topaz from the Ary-Bulak massif, Russia

“…Topaz phenocrysts contain abundant inclusions of feldspar, aggregates of Nb-Ta-W oxide (W-ixiolite to tantalite-columbite), mica, round grains of quartz, and elongate to irregular grains of cryolithionite [Na 3 Li 3 Al 2 F 12 ]. A detailed sample description is given in Agangi et al (2014).…”