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

摘要

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