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Uranium Isanaturallyoccurring,Verydense,Metallic Definition Andcharacteristics Deposits Definition, Mineralogyand Proportion Ofu-235Tobetween 3And5percent

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Uranium March 2010 Definition, mineralogy and Symbol U nt deposits Atomic number 92 opme vel Definition and characteristics Atomic weight 238.03 de l Uranium is a naturally occurring, very dense, metallic 3 ra Density at 298 K 19 050 kg/m UK element with an average abundance in the Earth’s crust ne mi of about 3 ppm (parts per million). It forms large, highly Melting point 1132 °C e bl charged ions and does not easily fit into the crystal struc- Boiling point 3927 °C na ai ture of common silicate minerals such as feldspar or mica. st Accordingly, as an incompatible element, it is amongst the Mineral Hardness 6 Moh’s scale su r last elements to crystallise from cooling magmas and one -8 f o Electrical resistivity 28 x 10 Ohm m re of the first to enter the liquid on melting. nt Table 1 Selected properties of uranium. Ce Minerals Under oxidizing conditions uranium exists in a highly soluble form, U6+ (an ion with a positive charge of 6), and is therefore very mobile. However, under reducing conditions Other physical properties are summarised in Table 1. it converts to an insoluble form, U4+, and is precipitated. It is these characteristics that often result in concentrations Mineralogy of uranium that are sufficient for economic extraction. Uranium is known to occur in over 200 different minerals, but most of these do not occur in deposits of sufficient Uranium is naturally radioactive. It spontaneously decays grade to warrant economic extraction. The most common through a long series of alpha and beta particle emissions, uranium-bearing minerals found in workable deposits are ultimately forming the stable element lead. shown in Table 2. If an atom of uranium is struck by, and manages to absorb, an extra neutron it will undergo nuclear fission. In this process the atom breaks apart forming ‘daughter products’ (typically strontium and xenon) and releasing a large quan- tity of energy, plus more neutrons. If these neutrons collide with further atoms of uranium a chain reaction can occur. The energy released in nuclear fission is used in nuclear power stations to convert water into steam, which is then used to turn a turbine and generate electricity. Uranium occurs as several isotopes, of which the most abundant are uranium-238 (U-238; about 99.3 per cent) and uranium-235 (U-235; about 0.7 per cent). U-235 is required for the operation of nuclear power stations. Most early designs of power station used uranium in its natural state, but all modern plants require enrichment to increase the proportion of U-235 to between 3 and 5 per cent. Unless otherwise stated, copyright of materials contained in this report are vested in NERC. Figure 1 Uranium ore. BGS © NERC 2010. All rights reserved. Courtesy: Mineral Information Institute (www.mii.org) Mineral profile Name Group of minerals Formula Most common depositional environment 1 2 Uraninite Uranium oxide UO2 Magmatic , hydrothermal or sedimentary- hosted3 deposits Pitchblende Uranium oxide (a massive UO2 Magmatic, hydrothermal or sedimentary- variety of uraninite) hosted deposits Coffinite Uranium silicate U(SiO4)0.9(OH)0.4 Hydrothermal or sedimentary-hosted deposits 2+ Brannerite Uranium titanate (UCaCe)(TiFe )O6 Hydrothermal or sedimentary-hosted deposits Carnotite Uranyl vanadate K2(UO2)2(VO4)2·3(H2O) Sandstone-hosted deposits Tyuyamunite Uranyl vanadate Ca(UO2)2(VO4)2·6(H2O) Sandstone-hosted deposits Uranophane Uranyl silicate CaH2(SiO4)2(UO2)·5(H2O) Sandstone-hosted deposits Table 2 The most common uranium minerals found in economic deposits. Deposit type Brief description Typical grade (ppm U) Examples Unconformity-related Associated with unconformities in ancient 5000 to McArthur River, sedimentary basins 200 000 Canada; Ranger, Australia Sandstone-hosted Oxidising-reducing conditions in sandstones 400 to 4000 Beverley, Australia; Inkai, Kazakhstan Hematite breccia Funnel or pipe-shaped deposits of broken 300 to 500 Olympic Dam, Aus- complex rock tralia Vein Cavities such as cracks, fissures, pore 250 to 10 000 Lianshanguan, China spaces or stockworks Quartz-pebble con- Ancient sedimentary deposits buried before 130 to 1000 Hartebeestfontein, glomerates oxidisation took place South Africa Intrusive Associated with the crystallisation or remo- 60 to 500 Rössing, Namibia bilisation of a magma Phosphorite Associated with sedimentary phosphates 60 to 500 Melovoe, Kaza- khstan (closed) Collapse breccia Concentrated in the matrix and fractures 2500 to 10 000 Arizona 1, USA surrounding breccia pipes (closed) Volcanic & caldera Associated with felsic lava, ash fields and 200 to 5000 Xiangshan (Zou- related related sediments (e.g. rhyolite or trachyte) jiashan), China Surficial Unconsolidated near-surface sediments. 500 to 1000 Langer Heinrich, Sometimes cemented with calcium carbon- Namibia; Yeelirre ate deposit Australia Metasomatite Alteration of minerals within a rock, often 500 to 2000 Ingulkii, Ukraine caused by the nearby emplacement of magma Metamorphic Concentration by processes such as partial 500 to 2000 Mary Kathleen, melting. Often remobilised by fluids Australia (closed) Lignite Associated with coalified plant detritus or Less than 1000 Koldjat, Kazakhstan adjacent clay and sandstone (closed) Black shale Rocks of marine origin with high organic Less than 1000 Schaenzel, France content (closed) Table 3 Summary of uranium deposit types. 1Magmatic – related to magma, molten rock and fluid originating deep within or below the Earth’s crust. 2Hydrothermal – hot fluids 3 Uranium Sedimentary-hosted – mineralisation contained within a sedimentary rock. 2 www.MineralsUK.com Deposits overlying the host sandstones. Mineralisation occurs when Uranium deposits are found throughout the world in a oxidising fluids transport the uranium into the sandstone, variety of geological environments. They can be grouped where it is deposited under reducing conditions (caused into 14 major categories based on geological setting by organic matter, sulphides, hydrocarbons or ferromagne- (IAEA, 2009a), but not all of these are actively worked. Key sium minerals such as chlorite). features of these are shown in Table 3. There are four main types of sandstone deposits (NEA/ Major deposit classes OECD, 2006): Unconformity-related deposits These are formed as a result of geological changes close Rollfront — crescent-shaped bodies that crosscut sand- to major unconformities4. Below the unconformity the stone bedding; rocks are usually reduced, deformed, faulted or brecci- Tabular — irregular, elongated lenses within reduced ated, whereas the overlying younger rocks may not be. sediments; Mineralisation is believed to occur where hot, oxidising, Basal channel — elongated or ribbon-like bodies that oc- metal-bearing fluids migrate through overlying porous cur along former watercourses; rocks and encounter reducing conditions below the uncon- Tectonic/lithologic — adjacent to permeable fault zones. formity. Deposits can be found immediately below, across, or immediately above the unconformity, depending on the The host sandstones can be of almost any age and deposit specific sub-type (WNA, 2009; NEA/OECD, 2006). grades are generally in the range 400–4000 ppm U. The oxidised part of the deposit usually contains uraninite or This category of deposit tends to be found in ancient coffinite, but close to the rollfront other minerals occur sedimentary basins where rocks are typically 1600 Ma such as carnotite, tyuyamunite and uranophane. or older. Deposit grades tend to be relatively high, commonly 5000 ppm U, although they can locally reach These are probably the most common type of deposit 200 000 ppm U. Typically, the mineralisation consists but, due to their lower grade, production tends to be less of pitchblende or uraninite, together with coffinite and than unconformity-related deposits. Currently there are other minor uranium oxides. Some deposits, such as mines operating in rollfront type deposits in Uzbekistan, Cigar Lake, Canada, also contain significant quantities of Kazakhstan, the USA and China. Tabular deposits are nickel-cobalt arsenides. worked in Niger, Romania, Czech Republic and the USA, and basal channel deposits are worked in Australia and Canada is the world’s largest producer of uranium, and Russia. both of its currently operating mines are working this type of deposit in the Athabasca Basin, Saskatchewan. Another Hematite breccia8 complex deposits major unconformity-related deposit currently being mined The Olympic Dam deposit in South Australia is one of the is at Ranger in Northern Territory, Australia. world’s largest uranium deposits and is of this type. Brec- cias generally occur within relatively stable continental Sandstone-hosted deposits areas where extensional tectonics have caused rifting and The most significant deposits in this category are con- the formation of grabens9. Mineralisation occurs due to the tained in permeable, medium- to coarse-grained, sand- presence of nearby granitic or volcaniclastic10 sediments stones that are poorly sorted and usually of fluvial5 or and possibly also shallow hydrothermal processes. marginal marine origin. Lacustrine6 or aeolian7 sandstones may also host mineralisation. Mineralisation in these deposits varies widely, from the monometallic ‘Kiruna’ type (mostly iron with some phos- The source of uranium is usually igneous rocks (volcanic phorus) to the polymetallic ‘iron-oxide-copper-gold’ (IOCG) ash or granite plutons) either close by, interbedded with, or type. The Olympic Dam deposit is towards the latter end 4An unconformity is where
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