Abstract

Pitchblendes and secondary uranium minerals from the Krunkelbach vein-type uranium deposit in southwest Germany were investigated by the U-Xe-Kr, U-Pb, and 234 U- 230 Th methods and by microprobe analysis. Both U-Pb and U-Xe-Kr results indicate a late Carboniferous formation age of 295 + or - 7 Ma for all veins of the deposit and an episodic Pb loss during the Tertiary, 50 + or - 8 Ma. 234 U- 230 Th dating yields Quaternary formation ages of 250 to 350 Ka (kiloyears) for five secondary U minerals. U-Pb isotope investigations on three of these samples show a strong component of inherited radiogenic Pb which means that during the Quaternary pitchblende oxidation Pb was not completely separated from U.The three isotopically dated events probably correlate with three generations of fluid inclusions. The Carboniferous hydrothermal veins were formed from low-salinity solutions at temperatures of 100 degrees to 300 degrees C. Uranium probably was derived from leaching of magmatic uraninite in the Brhalde granite by near-surface oxidizing ground waters and later precipitated due to mixing with hot, strongly reducing solutions or by reaction with a previously deposited reduced ore assemblage. A gap of 30 to 40 m.g. between granite emplacement and pitchblende formation indicates that circulation of hydrothermal fluids in the late Carboniferous-Early Permian probably was caused by a high regional heat flow rather than by magmatic heat.During the formation of the Rhinegraben in the early Tertiary, barite, fluorite, and calcite but not ore minerals were deposited from highly saline Na-Ca-Cl brines of 150 degrees to 200 degrees C. These brines leached radiogenic and fissiogenic uranium daughters from the pitchblendes and there are indications of minor uranium remobilization. Eight illite samples from hydrothermally altered wall rocks yield K-Ar ages of 119 to 208 Ma. These values are interpreted as mixing ages of the U-Pb- and U-Xe-Kr-documented Carboniferous and Tertiary events although a Mesozoic event, not recorded in the other isotopic systems, cannot be ruled out with certainty. In the late Tertiary-early Quaternary, the present hydrological conditions were installed with infiltration of oxygen-rich meteoric waters causing uranium remobilization and precipitation of secondary uranyl-minerals (i.e., phosphates, arsenates, silicates, and tungstates) yielding 230 Th- 234 U ages of at least 250 Ka.

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