Open Session - SWISS GEOSCIENCE MEETINGs

clearly enriched in Ag, Sb, Ta and Cd compared to typical Swiss coals and in U, Mo and Ta compared to world-wide coals.
Several other elements such as Hf, Pb, Mo, Cu, As, Ga and Ce show a minor enrichment within the two coals.
XRF measurements together with autoradiographies of polished whole-coal samples indicate that the uranium enrichment
is limited to the coal. In the hand specimen, the uranium is distributed heterogeneously and occurs mostly within black,
shiny coal layers while only minor amounts are contained within (secondary?) pyrite grains. Based on these data, and in
combination with the fact that no uranium-minerals have been detected by XRD analyses, an organic mode of occurrence of
uranium in coal is suggested. However, an inorganic mode of occurrence of U in coal cannot be excluded due to the possible
presence of ultrafine mineral particles intergrown with the organic matter.
The enrichment of uranium occurs in the peat stadium, where it is introduced into the coal swamp as a U-complex in solution.
The binding to the coal takes place either by ion-exchange mechanisms, where an uranyl ion is bond to carboxyl-groups,
or by the precipitation of inorganic U(IV) species in the reducing environment during coalification (von Borstel 1984).
Radon gas measurements in the coal mine galleries revealed activities around 8'000 to 14'000 Bq/m 3 for the Riedhof mine
and around 4'000 to 5'000 Bq/m 3 for the Mühlebach mine.
Water measurements in the Riedhof mine showed increased activities of the radionuclides 238 U, 234 U and 222 Rn together with
a low 226 Ra activity; however, all these values are below the Swiss limits for drinking water.
Figure 1: Photograph and autoradiography of the Mühlebach coal. Magnified areas show cracks within the coal containing radioactive py-
rite grains. The arrow indicates the younging direction of the sample.
REFERENCES
von Borstel, D. 1984: Bindungsformen von Uran in kohligen Ablagerungen von Sedimenten unterschiedlichen
Diagenesegrades. Ein geochemischer Beitrag zur Kenntnis der Bindung von Uran an Stoffgruppen organogener
Substanz. PhD thesis, TU Clausthal.
Hügi, T., Fardy, J.J., Morgan, N.C. & Swaine, D.J. 1993: Trace Elements in Some Swiss Coals. Journal and Proceedings, Royal
Society of New South Wales 126, 27-36.
Swaine, D.J. 1990: Trace Elements in Coal. Butterworth & Co.
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Symposium 2: Mineralogy-Petrology-Geochemistry