Quartz-topaz-loellingite rocks near Eldorado, Victoria

Abstract

Two small dykes consisting of a quartz-topaz-loellingite rock type have recently been discovered within the aplitic phase of the Pilot Range granite, near Eldorado, in NE Victoria. Minor biotite, muscovite, chlorite, kaolinite, anatase and pharmacosiderite are associated. Apart from the loellingite, the dykes are similar mineralogically to the ‘topazites’ from New England, NSW. These were considered to be magmatic in origin, based on field relationships and high homogenization temperatures for fluid inclusions in topaz (Eadington & Nashar 1978, Contrib. Mineral. Petrol. 67, 433–438). Although experimental evidence on F-enriched ‘granitic’ systems is inconclusive, the emplacement of the Eldorado topazite dykes most likely involved both magmatic and hydrothermal components operating essentially simultaneously. The topazite melt represented a F-rich residual granitic magma, from which aqueous alkali halide-rich solutions separated during high level intrusion. Separation of these aqueous solutions was responsible for miarolitic cavities into which topaz crystals grew. F-OH equilibration calculations for coexisting topaz-biotite pairs suggest the minerals equilibrated in the presence of hydrothermal solutions of variable composition (in terms of the HF/H2O fugacity ratio), at temperatures around 550°C. Alteration of topaz to muscovite, the precipitation of loellingite and the formation of clay and fluorite in the cavities occurred at progressively lower temperatures. The widespread alluvial topaz in the Beechworth-Eldorado area may be derived from similar quartz-topaz dykes.