Hydrothermal mineralization with anatase from Nemojov near Pelhřimov (Czech Republic)Article PDF
Hydrothermal mineralization with anatase from Nemojov near Pelhřimov (Czech Republic)
The quartz vein with anatase cuts discordantly biotite paragneisses and HT/LP cordierite migmatites of the Pelhřimov complex (Moldanubian zone). It has ENE-WSW direction with a steep slope and its average thickness is 10 cm, in the central part up to about 50 cm. The rock adjacent to the vein underwent a lowtemperature alteration where biotite, feldspars and cordierite were replaced by chlorite and fine-grained muscovite (sericite). The zonal texture of the quartz vein shows at least two-phase development. The vein has vugs in the thickest part filled with prismatic quartz crystals (to rock-crystal), up to 4 cm long, with simple growth zonality. They are partially overgrown with smaller quartz-crystal crystals. Locally, anatase forms very abundant tiny dark blue to black crystals of bipyramidal shape, up to 1 mm in size, which overgrowing quartz crystals or rarely forms inclusions in their outer zones. The decomposition of titanium-bearing biotite and/or accessory ilmenite from the adjacent migmatite is considered to be a source of Ti for anatase crystallization. This is confirmed by the chemical homogeneity of anatase without apparent zoning and the low concentration of other elements (Nb < 0.003 apfu; Ta, W and Fe < 0.001 apfu). Quartz formation temperatures are assumed in a wide range 180–340 °C under a pressure of 85–170 MPa. Fluid inclusion investigation in quartz suggests that chemical type of fluids could be defined as H2O – NaCl ± (Na+1, K+1, Mg+2, Fe+2-3) ± CH4 ± N2. The presence of these ions is consistent with the release of components out of rock-forming minerals during their decomposition. Anatase-bearing quartz vein is younger than the nearby quartz veins with base-metal mineralization. The temperature of quartz formation, especially outer zone with anatase inclusion, belongs to rather lower values. It is in line with the assumption that this is the final stage of the hydrothermal processes in the Pelhřimov complex.
Jiří Toman, Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 659 37 Brno, Czech Republic; email@example.com
Vladimír Hrazdil, Department of Mineralogy and Petrography, Moravian Museum, Zelný trh 6, 659 37 Brno, Czech Republic
Marek Slobodník, Department of Geological Sciences, Masaryk University, Kotlářska 2, 611 37 Brno, Czech Republic