The late Variscan (275-278 Ma) Pribram uranium deposit is one of the
largest known accumulations of uraniferous bitumens in hydrothermal
veins. The deposit extends along the northwestern boundary of the
Central Bohemian pluton (345-335 Ma) with low-grade metamorphosed Late
Proterozoic and unmetamorphosed Cambrian rocks. From a net uranium
production of 41,742 metric tons (t), more than 6,000 t were extracted
from 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 veins
gradually decreases toward the contact with the plutonic body and
increases with depth.
Two types of bitumen microtextures are recognized using high-resolution
transmission electron microscopy: amorphous and microporous, the former
being less common in uraniferous samples. A lower Raman peak area ratio
(1,360/1,575 cm(-1)) in mineralized bitumens (0.9) compared with
uranium-free samples (2.0) indicates a lower degree of microtextural
organization in the latter The H/C and O/C atomic ratios in uranium-free
bitumens (0.9-1.1 and 0.09, respectively) are higher than those in
mineralized samples (H/C = 0.3-0.8, O/C = 0.03-0.09). The chloroform
extractable matter yield is Very low in uranium-free bitumens
(0.30-0.35% of the total organic carbon,TOC) and decreases with uranium
content increase. The extracted solid uraniferous bitumen infrared
spectra show depletion in aliphatic CH2 and CH3 groups compared to
uranium-free samples. The concentration of oxygen-bearing functional
groups relative to aromatic bonds in the IR spectra of uranium-free and
mineralized bitumen, however, do not differ significantly. C-13 NMR
confirmed than the aromaticity of a uraniferous sample is higher (F-ar =
0.61) than in the uranium-free bitumen (F-ar = 0.51). Pyrolysates from
uraniferous and nonuraniferous bitumens do not differ significantly,
being predominantly cresol, alkylphenols, alkylbenzenes, and
alkylnaphthalenes. The liquid pyrolysate yield decreases significantly
with increasing uranium content. The delta(13)C Values of bulk
uranium-free bitumens and low-grade uraniferous, asphaltlike bitumens
range from -43.6 to 52.3 per mil. High-grade, cokelike, uraniferous
bitumens are more C-13 depleted (54.5 to -58.4 parts per thousand). In
contrast to the very light isotopic ratios of the high-grade uraniferous
cokelike bitumen bulk carbon, the individual n-alkanes and isoprenoids
(pristane and phytane) extracted from the same sample are significantly
C-13 enriched. The isotopic composition of the C13-24 n-alkanes
extracted from the high-grade uraniferous sample (delta(13)C = -28.0 to
32.6 parts per thousand) are heavier compared with the same compounds in
a 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 the
Upper Proterozoic host rocks that were pyrolyzed during intrusion of the
Central Bohemian pluton. The C-13- depleted pyrolysates were mobilized
from the innermost part of the contact-metamorphic aureole, accumulated
in structural traps in less thermally influenced parts of the
sedimentary complex and were later extracted by hydrothermal fluids.
Bitumens at the Pribram deposit are younger than the main part of the
uranium mineralization and were formed through water-washing and
radiation-induced polymerization of both the gaseous and liquid
pyrolysates. Direct evidence for pyrolysate reduction of uranium in the
hydrothermal system is difficult to obtain as the chemical composition
of the original organic fluid phase was modified during water-washing
and radiolytic alteration. However, indirect evidence-e.g., higher O/C
atomic ratios in uranium-free bitumens (0.1) relative to the Upper
Proterozoic source rocks (0.02-0.05), isotopically very light carbon in
associated whewellite (delta(13)C = 31.7 to -28.4 parts per thousand),
and the striking absence of bitumens in the pre-uranium, hematite stage
of the mineralization-indicates that oxidation of organic fluids may
have contributed to lowering of aO(2) and uraninite precipitation.