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• W2129966128 abstract "The late Variscan (275-278 Ma) Pribram uranium deposit is one of thelargest known accumulations of uraniferous bitumens in hydrothermalveins. The deposit extends along the northwestern boundary of theCentral Bohemian pluton (345-335 Ma) with low-grade metamorphosed LateProterozoic and unmetamorphosed Cambrian rocks. From a net uraniumproduction of 41,742 metric tons (t), more than 6,000 t were extractedfrom bitumen-uraninite ores during 43 years of exploration and mining.Three morphological varieties of solid bitumen are recognized: globular,asphaltlike, and cokelike. While the globular bitumen is uranium free,the other two types are uraniferous. The amount of bitumen in ore veinsgradually decreases toward the contact with the plutonic body andincreases with depth.Two types of bitumen microtextures are recognized using high-resolutiontransmission electron microscopy: amorphous and microporous, the formerbeing less common in uraniferous samples. A lower Raman peak area ratio(1,360/1,575 cm(-1)) in mineralized bitumens (0.9) compared withuranium-free samples (2.0) indicates a lower degree of microtexturalorganization in the latter The H/C and O/C atomic ratios in uranium-freebitumens (0.9-1.1 and 0.09, respectively) are higher than those inmineralized samples (H/C = 0.3-0.8, O/C = 0.03-0.09). The chloroformextractable matter yield is Very low in uranium-free bitumens(0.30-0.35% of the total organic carbon,TOC) and decreases with uraniumcontent increase. The extracted solid uraniferous bitumen infraredspectra show depletion in aliphatic CH2 and CH3 groups compared touranium-free samples. The concentration of oxygen-bearing functionalgroups relative to aromatic bonds in the IR spectra of uranium-free andmineralized bitumen, however, do not differ significantly. C-13 NMRconfirmed than the aromaticity of a uraniferous sample is higher (F-ar =0.61) than in the uranium-free bitumen (F-ar = 0.51). Pyrolysates fromuraniferous and nonuraniferous bitumens do not differ significantly,being predominantly cresol, alkylphenols, alkylbenzenes, andalkylnaphthalenes. The liquid pyrolysate yield decreases significantlywith increasing uranium content. The delta(13)C Values of bulkuranium-free bitumens and low-grade uraniferous, asphaltlike bitumensrange from -43.6 to 52.3 per mil. High-grade, cokelike, uraniferousbitumens are more C-13 depleted (54.5 to -58.4 parts per thousand). Incontrast to the very light isotopic ratios of the high-grade uraniferouscokelike bitumen bulk carbon, the individual n-alkanes and isoprenoids(pristane and phytane) extracted from the same sample are significantlyC-13 enriched. The isotopic composition of the C13-24 n-alkanesextracted from the high-grade uraniferous sample (delta(13)C = -28.0 to32.6 parts per thousand) are heavier compared with the same compounds ina uranium-free sample (delta(13)C = 31.9 to 33.8 parts per thousand).It is proposed that the bitumen source was the isotopically light(delta(13)C = 35.8 to 30.2 parts per thousand) organic matter of theUpper Proterozoic host rocks that were pyrolyzed during intrusion of theCentral Bohemian pluton. The C-13- depleted pyrolysates were mobilizedfrom the innermost part of the contact-metamorphic aureole, accumulatedin structural traps in less thermally influenced parts of thesedimentary complex and were later extracted by hydrothermal fluids.Bitumens at the Pribram deposit are younger than the main part of theuranium mineralization and were formed through water-washing andradiation-induced polymerization of both the gaseous and liquidpyrolysates. Direct evidence for pyrolysate reduction of uranium in thehydrothermal system is difficult to obtain as the chemical compositionof the original organic fluid phase was modified during water-washingand radiolytic alteration. However, indirect evidence-e.g., higher O/Catomic ratios in uranium-free bitumens (0.1) relative to the UpperProterozoic source rocks (0.02-0.05), isotopically very light carbon inassociated whewellite (delta(13)C = 31.7 to -28.4 parts per thousand),and the striking absence of bitumens in the pre-uranium, hematite stageof the mineralization-indicates that oxidation of organic fluids mayhave contributed to lowering of aO(2) and uraninite precipitation." @default.
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• W2129966128 date "1999-11-01" @default.
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• W2129966128 title "Bitumens in the late Variscan hydrothermal vein-type uranium deposit of Pribram, Czech Republic; sources, radiation-induced alteration, and relation to mineralization" @default.
• W2129966128 doi "https://doi.org/10.2113/gsecongeo.94.7.1093" @default.
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