Mineralogical, chemical and isotopic (U-Pb method) studies of Hercynian uraniferous mineralizations (Margnac and Fanay mines, Limousin, France)

Abstract

Mineralogical and chemical studies of pitchblendes, pyrites and coffinites from the Margnac and Fanay mines (Limousin, France) indicate that chemical exchanges between uraniferous solutions and vein wall-rocks are reflected in the chemical compositions of pitchblendes. An age of 276 Ma is given for the primary pitchblende—pyrite association by the U-Pb method. Isotopic analysis of Pb in galena, crystallized during coffinitization of pitchblende, gives an interval of 6 Ma between pitchblende precipitation and its remobilization as coffinite. Analyses by electron microprobe indicate that this hydrothermal transformation affects more or less all the pitchblende spherulites, even if the volume of coffinite is low. This leads to Si-enriched pitchblende and explains the systematic age of 270 Ma obtained by Pb-Pb and concordia diagrams. A mineralogically undetected remobilization stage is also found in some samples at 170–175 Ma. Finally, this work specifies the life time (20–30 Ma) of the geothermal system which is considered to be responsible for the episyenitization of the two-mica granite, the precipitation of pitchblende and its remobilization as coffinite.