U/Pb- und K/Ar-Datierungen des Uranvorkommens Höhenstein/Oberpfalz |
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| Authors: | Dr Claudia Carl Dr Harald Dill Hans Kreuzer Prof Dr Immo Wendt |
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| Institution: | 1. Bundesanstalt für Geowissenschaften und Rohstoffe, Postfach 51 01 53, D-3000, Hannover 51
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| Abstract: | K/Ar dating of micas from rocks of Fichtelgebirge and the northern Oberpfalz (NE-Bavaria) covers the rather wide range from 330 to 300 Ma. In combination with Rb/Sr wholerock isochron ages of post-tektonic granites four Variscian events can be traced: - Regional metamorphism ～ 330 Ma,
- Intrusion of the older post-deformative granite G1 about 320 Ma (Falkenberger Granit) followed by rapid cooling.
- About 300 Ma ago the older mineral ages were more or less reset by the intrusion of younger granites (G2 to G4, Flossenbürger Granit) again followed by a rapid cooling.
- Local influences later than 290 Ma are recorded by some white micas in fault gauges at least up to the middle Jurassic (< 150 Ma).
The different U-black ore types yielded strongly discordant U/Pb dates. Applying the three dimensional U/Pb-discordia plane method the following results have been obtained: the older pitchblende generation with an upper concordia intersection of 336±17 Ma can be correlated with the regional metamorphism as well as with the intrusion of the older post-deformative granites. The younger generation with 298±4 Ma is of equal age as the intrusion of the younger granites. The brannerite mineralisation which according to thin section evaluation must be still younger could be only roughly dated to 288±78 Ma. The low precision is due to low uranium concentration and high common lead contents of the samples. The lower concordia intersection ages are 0 to 7 Ma. They were obtained more precisely by dating of secondary U-minerals: 10 torbernite samples with extremely high238U/206Pb-ratios of 30 000 to 50 000 yielded a positive slope of the discordia which points to an age of (137±12)×103a. The230Th/238U-activity ratio of 0.68 determined byα-spectrometry indicates an age of (130.4±5.0)×103 a. Pyrite and chalcopyrite samples which had U-concentration of some 100 ppm and which according to thin section evaluation are younger than the brannerite showed a positive slope of the207Pb/206Pb vs.238U/206Pb discordia, too. They could not be dated because the207Pb/206Pb-ratios corrected for common lead contribution are lower than 0.046 (radiogenic lead of zero age). This may be explained by an addition of206Pb due to222Rn migration from the pitchblende in only a few cm distance. |
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