Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Magmatic-hydrothermal evolution of highly evolved granite stock Knöttel near Krupka in Erzgebirge


Tereza Peterková, David Dolejš

Geoscience Research Reports 50, 2017, pages 189–194
Map sheets: Teplice (02-32)

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Published online: 31 October 2017

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Evolved greisenized granites with rare metal mineralization in the eastern part of Erzgebirge provide a unique record of magmatic-hydrothermal transition. We provide new field and petrological observations from an old mining district Knöttel (Krupka, Czech Republic), as basis for subsequent microanalytical study. Biotite or two-mica quartz-feldspar gneisses were intruded by a hidden steep granite stock Knöttel - its deeper part consists of medium-grained topaz-zinnwaldite alkali-feldspar granite, which evolved upwards into a fine-grained variety with local unidirectional solidification textures. The granites were cross-cut by gray-black greisenized zones, with variable proportions of quartz, mica and topaz. Large body of K-feldspar pegmatite with lenses of coarse-grained milky biotite-quartz pegmatite is located in the upper part of the stock. The pegmatite contains veinlets of dark gray fine-grained zinwaldite-quartz greisens and light gray mica-poor quartz greisens. The uppermost part of the Knöttel stock is composed of a coarse-grained quartz rock (quartzite) with accessory molybdenite, fluorite, topaz, mica and clay minerals. The contacts between coarse- and fine-grained quartz-rich rocks or greisens are generally sharp. The coarse-grained quartz stock was intersected by quartz veins with molybdenite mineralization that extend to its exocontact and produced greisenization (zwitterization) of the surrounding gneisses. Younger dykes of granitic aplites have cross-cut the gneisses, K-feldspar pegmatite as well as molybdenite-bearing quartz veins. Spatial zoning of the granite-pegmatite-quartz system is compatible with disequilibrium differentiation promoted by variations in volatile constituents. Crystallization of the volatile-rich melt probably commenced from the intrusion wall with a formation of K-feldspar pegmatite. A boundary melt layer, ahead of the solidification front and enriched in components complementary to feldspar and volatiles, have likely been responsible for solidification of coarse-grained quartz rocks and release of solute-rich aqueous fluids. The sequence of coarse-grained quartz rocks, fine-grained quartz rocks and greisens, and cross-cutting quartz veins may be interpreted as a record of protracted supersaturation in silica-rich media spanning the magmatic-hydrothermal transition.