Mineral inclusions in placer zircon from the Ohře (Eger) Graben: new data on "strontiopyrochlore"

摘要: After qualitative examination of the inclusion inventory 80 zircon megacrysts from alluvial placers Cenozoic alkali basalt area northern Bohemia and south-eastern Saxony, three representative Podsedice deposit were selected for an in-depth study minerals. Primary secondary inclusions distinguished used as monitor zircon-related syn- epigenetic processes. The trace-element characteristics hosts imply silicatic parental rock, probably nepheline syenites or fenites, which occur enclaves in placer-near breccia fillings. genetically most interesting species is “strontiopyrochlore”, its presence within areas oscillatory zonation suggests a magmatic origin. This mode origin contrasts to previous observations indicating formation “strontiopyrochlore” mineral altered carbonatite. Sr concentration varies between 7.4 12 wt.% SrO, corresponding proportion 29–48 at.% total A-site cations. infiltration late- post-magmatic Fe-rich hydrothermal fluids along fractures crosscutting host caused partial substitution Nb5+ by Fe3+ B-site pyrochlore structure breakdown primary fergusonite-like into undefined Nb–Y–REE–Fe oxide phases usually associated with goethite-like Thorite related Y–REE–Th silicate are frequent originated dissolution−re-precipitation. Baddeleyite formed response desilicification that has experienced during contact SiO2-undersaturated melt.