Kausticky metamorfovaný pískovcový xenolit a doprovodná hydrotermální mineralizace z neovulkanitů od Prackovic nad Labem (České středohoří)

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).