The uraninite-pyrite association, a sensitive indicator of changes in paleofluid composition: an example from the Ohře (Eger) graben, Bohemian Massif, Czech Republic
Abstrakt
Trace element variations in uraninite were employed to decipher the chemical age of uraninite and to discuss As-rich nature of fluids during uraninite formation in the Jeníkov area, central part of the Cenozoic Ohře Graben, Bohemian Massif, Czech Republic. Cretaceous sandstones of the graben fill show two distinctive stages of young (post-18 Ma) mineralization, most probably related to topography-driven epithermal fluid flow along the NW boundary fault of the graben (Krušné hory Fault).
The older stage is dominated by pervasive silica precipitation accompanied by pyrite, sphalerite, galena, uraninite and kaolinite, while the younger stage is represented by barite with pyrite inclusions and fluorite. The studied uraninite formed together with microcrystalline quartz (Qtz-4) at the end of the older stage.
Uraninites of subgroup A show high As/Pb ratios (14.6-147.1), higher Ca (1.02-1.99 wt.%), lower Ti (0.27-0.44 wt.%), and other trace elements compatible with uraninite structure.