Two types of hydrothermal veins were found in the Ordovician claystones of the Bohdalec Formation (Barrandian, Prague Basin) during the excavation of tunnel of subway Line D at Prague-Pankrác site. The first type is represented by short hair-thin veinlets of various directions fulfilled by dickite. The second type comprises thicker NNW - SSE trending veins with prevailing quartz, which cut the host rocks across the whole width of the gallery. In addition to quartz, they contain also dickite, chlorite (thuringite-chamosite), carbonates of dolomite-ankerite series (Dol37.5-44.0Ank42.0-46.8Ktn10.9-16.1), calcite, fluorapatite, pyrite (with up to 0.5 wt. % Mn), galena (with ~0.6 wt. % Se) and sphalerite (with ~1 wt. % Fe and up to 0.35 wt. % Sn and 0.36 wt. % Cu). Except for calcite, which forms younger veinlets in older quartz fill, all other mentioned minerals form minute inclusions enclosed in quartz, which are arranged parallel with outer margin of the vein. Based on mineral assemblage and chemical composition of individual minerals, highly variable crystallization temperatures (<100 - 350 °C) can be interpreted in various mineralogically distinct domains of the quartz vein. We assume a polyphase, episodic origin of individual domains of the vein fill, close to the crack-seal mechanism, which was bound to successive evolution of the adjacent fault structure. The maximum formation temperatures exceeding by a value of ca. 100 °C the highest reported temperatures of Variscan thermal overprint of Lower Paleozoic rocks of the Prague Basin are explained by production of friction heat in the fault structure. It is probable that part of parent fluids originated from sedimentary iron ores occurring in the host Ordovician sedimentary sequence.
Technical workings realized near Prackovice nad Labem (České středohoří Mts., Czech Republic) yielded new findings about rocks and mineral veins present in Cenozoic volcanites. The studied xenolith represents a piece of pyrometamorphosed and hydrothermally altered sandstone enclosed in an alkaline basic volcanic rock. The core of the xenolith contains relicts of clasts of quartz, embedded in a matrix composed of laths of quartz (probably pseudomorphs of quartz after tridymite) and symplectitic intergrowths of alkali feldspar (sanidine Or57-81Ab19-41An0-1) and quartz. This core is rimmed by drusy overgrowths of sanidine and crystals of fluorapatite, aegirine-augite and titanite. All silicates are characterized by a significant substitution of Al by Fe3+, which is probably the result of high content of Fe3+ in the sandstone protolith (perhaps in limonite cement). The marginal part of xenolith is formed by zeolites (chabazite-K and phillipsite-K), saponite and calcite. These minerals likely crystallized at very low temperatures (<100 °C) in a vug, leaving after volatiles, which were expelled during pyrometamorphism of the xenolith. In addition, we have studied tiny hydrothermal veinlets hosted by neovolcanites, composed of a mixture of Al-rich phyllosilicates (probably a mineral from the kaolinite group and smectite) and strongly substituted carbonates including siderite (Sid55-91Mag3-38Cal5-31Rdc1) and calcite (Cal58-90 Mag8-41Sid1-6).
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