A multistage hydrothermal geological history of Shlema-Alberoda which is the largest uranium deposit in Europe had been already shown by Xe-Xe dating technique (Shukolyukov and Meshik, 1987). It was demonstrated that Xe-Xe plateau ages decreased from the centre of the deposit to its periphery. Obtained ages varied from 140 to 280 Ma, with maxima of age distribution being at 153(I), 175(II), 197(III), 242(IV) and 270(V) ma (Shukolyukov et al., 1992). These temporal and spatial distributions are due to the behaviour of ore forming fluids. The purpose of this work was to apply Xe-Xe dating coupled with mineralogical and X-ray examinations to study secondary events within a single vein rather than to the deposit as a whole. The motivation of the study was our earlier unpublished Xe-Xe and U-Pb data of inside and outside fragments from one compact piece of the Shlema-Alberoda pitchblende. Both inside and outside parts had concordant 206Pb/238U, 207Pb/238U and Xe-Xe ages, with the inside part being 60 Ma younger than the outside one.
Seven 2-4 mg samples drilled from the 2 kg polished section of the pitchblende nest from the Duerre-Henne vein (1620 m deep) were irradiated by thermal neutrons up to the fluence 7.6x1014n/cm2 and then the combination of step-wise annealing and the triple dilution technique (Pravdivtseva, 1995; Pravdivtseva et al., 1995) have been applied prior Xe isotopic analysis to obtain Xe-Xe age spectra of these samples. Relative uncertainties of Xe-Xe age calculation are 5-8% (Reimold et al., 1995). Mineralogical observations prove that the pitchblende is the aggregate of autopseudomorphic transformations developed along primary spherocrystals containing at least four different uranium oxides. For three of them lattice parameters (a0) have been determined.
All seven age spectra have well defined high temperature plateau corresponding to the closure time of U-Xe
system. Having taken into account obtained ages, X-ray examinations and mineralogical observations, three zones have been distinguished in the polished section. At the first approximation the division corresponds to growing zones of the pitchblende. Four samples from the inside and outside zones characterised by a0=5.42 A° yields concordant plateau ages 196, 193, 223 and 208 Ma. These zones consist of rounded blocks separated by metastreaks of latter oxides. Intermediate zone is represented by three drilled samples yielded ages of 130, 123 and 141 Ma and a0=5.42 A°, with some oxides contacting with calcite and diffuse figures having a0=5.41-5.40 A°. There are at least three mineralization stages including ankerite with Bi-Co-Ni phases. In the context we interpret the older ages (av. 205 Ma) as the recrystallisation time of the oxide with Bi-Co-Ni phases, and the younger age (av. 131 Ma) as the time of further diffuse transformation also connected with the Bi-Co-Ni mineralization, as well as the replacement calcite with dolomite.
The combine application of Xe-Xe dating technique,
X-ray examinations and mineralogical study proves that age variations in Shlema-Alberoda deposit manifest themselves even in the cm scale reflecting multistage character of uranium ore redeposition. Two ages 205 and 131 Ma obtained for the Duerre-Henne vein fit to hydrothermal pulses (I) and (III) of five pulses typical for the whole Shlema-Alberoda deposit. The validity of Xe-Xe dating has been checked by independent U-Pb method.
We thank Dr V. L. Barsukov for his help. This work was supported by the INTAS foundation under the grant #93-272.
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