The petrogenesis of a wolframite-bearing greisen in the Vykmanov granite stock, Western Krušné hory pluton (Czech Republic)


Authors: Štemprok M, Pivec E, Langrová A

Published in: Bulletin of Geosciences, volume 80, issue 3; pages: 163 - 184; Received 1 October 2004; Accepted in revised form 7 June 2005;

Keywords: Krušné hory/Erzgebirge, ore-bearing granite, Variscan magmatism, greisen, feldspathite, protolithionite, ferberite, wall-rock alteration, tungsten mineralization,

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A wolframite-bearing greisen at Vykmanov, near Ostrov (Czech Republic) occurs at the contact of a small granite stock belonging to the Late Variscan Younger Intrusive Complex (YIC) of the Western Krušné hory/Erzgebirge pluton. The stock emerges as an outcrop of 0.8 × 0.3 km size from a hidden granite body in the eastern continuation of the Nejdek-Eibenstock granite massif. The lens-like greisen body consists mainly of quartz, topaz, protolithionite, and muscovite; it also contains wolframite (ferberite) and native bismuth mineralization. It was formed by replacement of a medium-grained, equigranular, slightly porphyritic Li-F granite of the Karlovy Vary pluton characterized by weak postmagmatic albitization and pervasive muscovitization. The greisen is geologically and compositionally transitional between the Li-rich greisens in albite granites (e.g. with zinnwaldite such as at Krásno) and the Li-poor greisens (e.g. phengite greisens at Gottesberg and Přebuz) associated with weakly albitized granites of the Younger Intrusive Complex. The Vykmanov greisen formed in a subsolidus stage of granite evolution by progressive alkali loss and fluorine metasomatism, leading to the formation of Li-mica quartz greisen subsequently replaced by topaz-quartz greisen at the granite/crystalline contact. These greisens were affected by late-stage muscovitization and argillitization (sericitization, the formation of clay minerals). The Li-Fe mica composition of the greisens corresponds to protolithionite (lithian siderophyllite) and is similar to the composition of micas in the enclosing granite. The tungsten-bearing greisenization represents a postmagmatic episode in the development of the Krušné hory/Erzgebirge batholith, and is located at the eastern contact of highly evolved YIC granites of the Western Krušné hory pluton. The greisen formed from CO2 -poor hydrothermal solutions, at about 400 °C, which evolved from highly saline brines as evidenced by fluid inclusion studies. The geological situation suggests that the mineralizing fluids were mostly magmatic and were responsible for the tungsten-bismuth specialization of the greisens, whereas meteoric waters participated in mineralization during later stages.