Fluid immiscibility in late-Alpine gold-bearing veins,Eastern and Northwestern European Alps |
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| Authors: | D Craw D A H Teagle R Belocky |
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| Institution: | (1) Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand;(2) Department of Earth Sciences, Cambridge University, Downing Street, Cambridge, UK;(3) Institut für Geowissenschaften, Technische Universität Braunschweig, Postfach 3329, W-3300 Braunschweig, FRG |
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| Abstract: | Gold-bearing quartz veins fill late-Alpine brittle structures in Pennine nappes of Austria (in the Tauern window) and in northern Italy. The veins formed in the latter stages of uplift of the Alps. Fluid inclusions in veins sampled from Böckstein, Austria, and Valle Anzasca, Italy have a wide variety of compositions, ranging from aqueous brine (about 5 wt% NaCl equiv.) to about 50 mol% CO2. At room temperature, the inclusions range with increasing CO2 content from two-phase aqueous, through three-phase in which the CO2 homogenizes to vapour, to three-phase with CO2 homogenizing to liquid. This wide range of inclusion compositions is interpreted as evidence for fluid immiscibility, with most inclusions being accidental mixtures of the two end-member immiscible fluids. The homogenization temperatures of the aqueous inclusions, 200–280°C, gives the best estimate of temperature of formation of the veins. Vein formation fluid pressure at Böckstein and Valle Anzasca was about 1 kbar, and Böckstein veins formed at lower pressure than Valle Anzasca veins. Fluid immiscibility may have contributed to deposition of gold at both Valle Anzasca and Böckstein, and possibly many other uplift-related Alpine gold localities. |
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