There are two basic types of uranium ore deposits: magmatic, which
develop due to hydrothermal effects; and sedimentary, which develop by
chemical reduction of uranium in groundwater in subsurface aquifers.
In 1976, scientists George Cowan and Hans Adler analyzed gas mass
spectrometry results of uranium hexafluoride (before artificial
isotopic enrichment processes took place) derived from uranium ores
around the world. This assessment revealed a slight offset in the
distribution of the ratio of U-235 to U-238, with magmatic-type
deposits having on average higher U-235 percentage weight and
sandstone-type deposits having lower.
However, the precision of individual analyses remained approximately 3
per mil (3 parts per thousand) while the average offset between
deposit types was less than this.
With the higher precision now obtainable in the UI geochemistry
laboratory, Bopp and UI geology professor Craig Lundstrom have
observed the same offset between uranium ores from different geologic
settings.
The researchers used a technique called multiple-collector inductively
coupled plasma-mass spectrometry to measure the ratio of U-235 to
U-238 in three sandstone-type and three magmatic-type uranium ores
provided by the Smithsonian Institution.
�Repeated analysis of the ore samples shows the sandstone-type ores to
be consistently depleted in U-235 relative to magmatic-type ores by
approximately 1 per mil, which is a significant amount of variation,�
said Bopp, who presented the findings at
the annual meeting of the Geologic Society of
America.
The observed depletion of U-235 is most likely the result of a nuclear
field shift effect as isotopes partition between the water and the
reduced uranium ore mineral, Bopp said. But what uranium reduction
process � biotic or abiotic � is responsible is not yet clear.
�We can�t parse that apart at this stage,� Bopp said. �We observe a
depletion, and we know there are microbes present in these types of
deposits, but we can�t say for sure who�s doing what without a much
more in-depth study of a single locality.�
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