Uranium ore formation

All ores are formed by geological processes. The clarke (Chapter 10) for uranium is about
4 mmol/mol (4 parts per million). Obviously, ore bodies have a higher concentration:
uraninite is 50 to 80% uranium, while davidite is only around 10% uranium. There are
roughly half dozen basic ore formation processes, of which the four most important for
uranium are:
1. sedimentary accumulation,
2. diagenesis,
3. magmatic segregation, and
4. hydrothermal circulation.
Sedimentation can occur in several ways. Limestone is formed by the rain of small marine
organisms onto the ocean bottom, and salt is formed when an interior sea dries up and is
covered by aolian deposits. In the case of gold, silver, uranium, and thorium, for example,
microscopic pieces could be carried off as runoff. In certain areas where the water stilled,
for example, a lake or widening of a stream, the denser pieces could fall to the lakebed or
streambed.
Diagenesis refers to chemical and physical changes that occur after deposition. If oxygen
is available, an element might combine with it to form an oxide. The oxide might replace
another compound, or, alternatively, if oxygen is removed, a sulfur atom could replace an
oxygen atom. Since the uranium-laced sediment might be covered by more sediment, it
could be subject to such chemical changes.
Magma is liquid rock. There could be ores formed by “freezing out” of materials by their
differing melting temperatures. The resulting ore bodies could rise or sink relative to the
Energy, Ch. 19, extension 4 Uranium ore formation 2
liquid depending on its density.
In hydrothermal circulation, hot water flowing can cause chemical changes and cause
migration of materials to help form ore bodies. This mechanism is less important for
uranium than for copper, silver, and gold ore formation.
Magmatic segregation is often involved in formation of uranium-rich rock. The uranium
may or may not be acted on by hot water (hydrothermal circulation) or hot gases during
its formation. In any case, volcanic action and other geological processes can cause regions
of uranium-bearing rock at concentrations higher then the clarke. As Earth’s plates move,
these original low-concentration rocks may be exposed and eroded by weathering. Water
can carry off the small pieces of rock, including uranium.
In the case of the Oklo deposit in Gabon,(29) not atypical of uranium ore formation, the
area that contained the ore that formed the natural reactor was originally a river delta.
When archaeobacteria began making the oxygen atmosphere, the exposed uranium became
oxidized. The oxidized uranium was weathered, traveled to the delta, and became encased
in the bottom ooze, where the uranium exchanged its oxygen for other elements. The
densest metals had accumulated in bottom sediments, and as these became overlain with
more sediments, the pressure eventually made sandstone out of the buried ooze.
The original volcanic rock containing low concentrations of uranium had become
sedimentary rock with higher concentrations. Geological processes compressed the
sandstone further, then uplifted and folded the rock. Water circulated among the broken
pieces of rock and helped the uranium gather in richer pockets. These were again overlain
and compressed, forming the ore body. This ore body was enriched compared to the
sandstone, and therefore greatly enriched compared to the original volcanic rock. It
Energy, Ch. 19, extension 4 Uranium ore formation 3
contained enough uranium (of course with high enough concentration of uranium-235) to
allow the natural Oklo reactor to run (see the box on this unique event in the chapter).
Uranium ore formations elsewhere also involved these four processes. Most ores seem to
be near regions that had experienced magmatic intrusions of volcanism in the distant past.
This is consistent with what happened in Oklo, Gabon.
Herndon argues that Earth contains an operating reactor. As radioactive decay decreases
the amount of uranium-235 relative to uranium-238, the reactor is going to turn off
eventually. He thinks that the georeactor runs Earth’s magnetic field.(30) In this case, if it
turns off sometime within the next several million years, it would have strong impacts on
Earth.