Uranium 4 - Deposition

          The primary uranium minerals in commercial ores are uraninite (UO2), pitchblende (U3O8), coffinite (U(SiO4), brannerite (UTi2O6), davidite ((REE)(Y,U)(Ti,Fe3)20O38) and thucholite (Uranium-bearing pyrobitumen). There are a number of other common uranium minerals which form hydrated crystals incorporating water molecules.

          The mineralogy of the host minerals, the reduction-oxidation potential of the uranium mineral and the porosity which determines water infiltration are important factors in the formation of uranium deposits. Since uranium is highly soluble, it can be easily moved around by the flow of water underground. This contributed to the variety of places and manners in which uranium may accumulate. The way in which uranium interacts with other elements and compounds in melted rock also influences its distribution.

          Combinations of surface weathering, sedimentation, diagenetic, magmatic and hydropthermal geological processes mentioned in a previous post produce fifteen general types of uranium deposits.

         The richest uranium ore deposits are found near unconformities. An unconformity is a break in between two layers of rock that have been laid down at different times. In the case of uranium deposits, the two layers are a quartz rich sedimentary layer and a metamorphic layer has been altered by heat and pressure. These deposits were formed between two billion five hundred million years ago and five hundred million years ago.

         The second best uranium ore deposits form in sedimentary deposits on continental shelves and freshwater areas such as river deltas, lakes, etc. In an oxygen rich environment, the uranium dissolves and then moves with the water. When it encounters an oxygen poor or reducing environment, it precipitates out of solution.

         Tabular deposits occur parallel to groundwater flow in sandstone. The ores are rich but the deposits are small.

         Roll front uranium deposits form when ground water dissolves the uranium in sandstone and, after flowing underground, collides with some sort of organic matter rich in carbon. The uranium precipitates out at the “front” when the water encounters the organic material.

         Basal channel deposits form from moving ground water like the tabular and roll front deposits, but the deposition occurs along channels of moving surface water such are rivers. When the water evaporates along desert margins or in shallow saline ponds, the uranium is deposited.

         Quartz-pebble conglomerate deposits are created by the separation and movement of particles of uranium in flows of surface water and their deposit in river beds, river deltas and lakes. These deposits generally contain large quantities of low grade ore

          Breccia complex deposits contain uranium along with iron oxide, copper, gold, silver and rare earth elements. Hydrothermal processes enriched the uranium content of  the quartz-hematite breccias.

          Vein deposits are uranium minerals filling in cracks, veins, fractures and breccias in steeply dipping fault systems. Magmatic processes in molten rock create the veins and later hydrothermal activity can concentrate the uranium. Some veins contain a variety of other metals in combination with the uranium.

          Intrusive uranium deposits form when magma is forced into older rocks deep within the Earth’s crust.

          Marine sedimentary deposits of phosphorite (which contain large amounts of phosphorus) sometimes contain uranium.

          Collapsed breccia pipe deposits are created when vertical cylindrical cavities formed by groundwater dissolving limestone are filled with fragments of rock when they collapse. Uranium fills cavities and coats other rocks.

          Volcanic deposits of uranium may be formed by magmatic processes in the molten rock or later mineralization by groundwater and chemical processes. Such deposits are usually small with low grade ore.

          Surface deposits can form in peat bogs, karst caverns and in soil from the weathering of shallow sedimentary deposits of uranium.

          Metasomite deposits are the result of uranium minerals being distributed in rocks that have been subjected to sodium metasomatism which is chemical alteration by hot subsurface solutions of sodium.

          Metamorphic deposits were laid down by sedimentary or magmatic processes and then remained unaltered by any other processes.

          Lignite is a soft brown young coal derived from wood. Some deposits contain significant amounts of uranium minerals.

          Black shale deposits form in oxygen-free submarine sedimentation processes. The uranium is not mineralized by organic materials due to the lack of oxygen. These deposits are considered very low grade ores.

          There are many other types of uranium minerals but these fifteen types constitute the pool from which uranium ores are chosen for extraction.

Sedimentary layers:

Uranium 3 - Minerals

          Uranium is a very common element present in greater quantities than silver. The term “clarke” refers to the average concentration of a particular element in the Earth’s crust.  The clarke of uranium is about 4 parts per million while the clarke of silver is about 1 part per million, the clarke of aluminum is 82,000 parts per million and iron is 63,000 parts per million.

          The term “ore” refers to a mineral deposit which contains a sufficient concentration of a valuable metal to make extraction of that metal profitable. Mineral are formed by geological processes. The four processes that are the most important for formation of uranium minerals are:

  1. The accumulation of uranium ores by tiny flakes pickup up as rain water runs off the land into a body of water when the flakes settle into of sedimentary deposits. When the body of water disappears, the soft sediment eventually become solid rock.
  2. After sedimentary deposit, there are changes in the distribution of particular elements referred to as “diagenesis.” These are low temperature low pressure changes as the sediment is compressed, liquids are squeezed out, chemicals precipitate out of solution. Oxygen in the sediment may combine with uranium forming oxides.
  3. “Magmatic segregation” is a process in which minerals become locally concentrated during the circulation, cooling and crystallization of molten rock. As the molten rock cools, different minerals solidify at different temperatures. Then they may move up or down based on their density.
  4. During “hydrothermal circulation”, water penetrates down rock and then is moved back up out of the rock by a source of heat such as underlying magma. As the water rises it carries minerals out of the rock it is moving through and ultimately deposits those minerals.

          In Gabon, in Africa, a combination of these processes concentrated uranium to the point where a natural “reactor” was formed that generated heat for hundreds of thousands of years. J. Marvin Herndon has a theory that there is a natural nuclear fission “georeator” driven by an accumulation of uranium at the Earth’s core. He believes that this reactor is responsible for the magnetic field of the Earth that permits life to exist.

            Most known uranium deposits formed near areas that experienced volcanism and intrusions of magma. Although uranium is present in many minerals, the quantities are too small to make extraction profitable. Concentration is the most important qualification for a commercial ore. The two most concentrated uranium minerals are pitchblende and uraninite which can contain up to 85% uranium. Carnotite, torbernite, tyuyamunite, autunite, uranophane, and brannerite uranium minerals that may contain up to 60% uranium. In addition to concentration, a good ore must have uranium that is not bound up in a complicated chemical compound that would make it difficult to extract. The distribution of the uranium through the mineral deposit is also important. If it is too sparse in the deposit, then that deposit would not make a good ore.

            Uranium may be present in throughout the crust of the earth and may be found in many minerals but deposits of commercial grade uranium ores are not all that common and are highly sought after.

Picture of pitchblende by Geomartin:

Uranium 2 - Properties

          Uranium is a naturally occurring radioactive chemical element. It was formed in super novae explosions about 6.6 billion years ago. In its pure form it is a silvery colored heavy metal. It is 70% denser than lead but not quite as dense as gold and will burn in a powdered form. A little softer than steel, it is malleable, ductile, paramagnetic, weakly electropositive and a poor conductor of electricity. It will oxidize in air and can be dissolved by acids.

Uranium 1 - History

          A piece of yellow glass made around 79 AD colored with uranium from found near Naples, Italy is the first known use of uranium. The uranium mineral called pitchblende or uraninite was noticed and reported as long ago as 1565 in mines in Saxony in northwest Germany. Uranium was first recovered in 1789 from analysis of mineral samples from the Joachimsal silver mine in the Czech Republic by a German chemist named Martin Heinrich Klaproth.

Downwinders 4 - US Nuclear Test Fallout

          The main location for testing nuclear bombs in the United States between 1951 and 1962 was the Nevada Test Site. Eighty six nuclear bombs were exploded either at ground level or above and fourteen nuclear devices were exploded underground. Radioactive materials were injected into the atmosphere from all the tests. The government told people to sit outside and watch the mushroom clouds caused by the explosions.

Downwinders 2 - Hanford 1

          The United States government began construction at Hanford in south central Washington State in 1943. Three nuclear reactors and two chemical processing plants were built and operated at Hanford during the Manhattan Project to develop nuclear weapons for use in World War II. The U.S. government retained private contractors including DuPont and General Electric to oversee the production of materials for nuclear weapons.

Downwinders 1 - Introduction

          The term “downwinders” refers to people, either individually or in communities, who have been exposed to radioactive materials as a result of nuclear fuel mining, nuclear weapons production and testing, nuclear waste disposal or nuclear accidents. The term derives from the fact that people who are downwind of an event that expels radioactive particles and gases into the atmosphere will be exposed to the fallout when the particles move through the air and fall to the ground.

NGO Radiation Organizations 3 - International Commission on Radiological Protection

          Shortly after the discovery of X rays in 1895, papers began appearing in scientific publications about the negative effects of high levels of exposure to such radiation. In the first year, suggestions about how to protect against ionizing radiation were made including the three main measures that are still emphasized today, limit the exposure to the shortest possible time, maintain as great a distance from the source as possible and employ shielding of some sort.

NGO Radiation Organizations 2 - International Radiation Protection Association

           After the Second World War, weapons research and work on civilian use of nuclear power were producing more and more radioactive materials. Scientists in the Manhattan Project had been assigned to work on what was then called “Health Physics.” A decade after the end of the war, the first conference on Health Physics was held at Ohio State University. One result of the conference was the formation of a professional Health Physics Society (HPS).

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