Geology of Uranium Deposits
- Uranium occurs in a number of different geological environments.
- Most Kazakh uranium resources are sedimentary.
- Most Canadian resources are unconformity-related.
- Most Australian uranium resources are in unconformity-related and iron oxide breccia complex orebodies.
Uranium deposits worldwide are grouped into 15 major types based on their geological setting. Some categories have several sub-types. The most recent geological classification has been defined by the International Atomic Energy Agency (IAEA) in 2013 and has been adopted in the Red Book since 2014. The deposit types have fundamental characteristics and recognition criteria, and in that respect, while mainly named by host rock, the types are essentially empirical models, based on observable characteristics.
Types of uranium deposit (IAEA, 2013)
1. Intrusive
Included in this type are those associated with a variety of intrusive rocks including alaskite, granite, pegmatite, and monzonites. Major world deposits include Rössing and Husab (Namibia), Kvanefjeld (Greenland), Bancroft area (Canada), and Palabora (South Africa). In South Australia, Radium Hill was mined 1954-62, and large bodies of low-grade mineralisation occur in the Olary Province.
2. Granite-related
Two sub-types: Endogranitic and Perigranitic. Includes many vein deposits in granite, deposits in adjacent meta-sedimentary rocks, and also disseminated mineralisation in granite. The dimensions of the openings have a wide range, from the massive veins of pitchblende at Jachymov deposit (Czech Republic), to the narrow pitchblende-filled cracks, faults and fissures in some of the ore bodies in Europe, Canada and Australia.
3. Polymetallic iron-oxide breccia complex
Olympic Dam* is the world’s largest deposit of uranium, and accounts for the major part of Australia’s uranium resources, though it is only recovered as a by-product with copper. Both it and Carrapateena occur in a hematite-rich granite breccia complex in the Gawler Craton, overlain by approximately 300 metres of flat-lying sedimentary rocks of the Stuart Shelf geological province.
The central core of the complex is barren hematite-quartz breccia, with several localized diatreme structures, flanked to the east and west by zones of intermingled hematite-rich breccias and granitic breccias. These zones are approximately one kilometre wide and extend almost 5 km in a northwest-southeast direction. Virtually all the economic copper-uranium mineralisation is hosted by these hematite-rich breccias. This broad zone is surrounded by granitic breccias extending up to 3 km beyond the outer limits of the hematite-rich breccias.
The deposit contains iron, copper, uranium, gold, silver, rare earth elements (mainly lanthanum and cerium) and fluorine. Only copper, uranium, gold, and silver are recovered. Uranium grades average from 0.07 to 0.035% U, the higher-grade mineralisation being pitchblende. Copper grades average 2.7% for proved reserves, 2.0% for probable reserves, and 1.1% for indicated resources.
Details of the origin of the deposit are still uncertain. However the principal mechanisms which formed the breccia complex are considered to have been hydraulic fracturing, tectonic faulting, chemical corrosion, and gravity collapse. Much of the brecciation occurred in the near surface eruptive environment of a crater complex during eruptions caused by boiling and explosive interaction of water (from lake, sea or groundwater) with magma.
4. Volcanic-related
Uranium deposits of this type occur in and near volcanic calderas, in acid to intermediate volcanic rocks, and are related to faults and shear zones. Uranium occurs in veins or is disseminated and is commonly associated with molybdenum and fluorite. Significant deposits of this type occur in China (Xiangshan), Kazakhstan, Mongolia (Dornod and Gurvanbulag), Russian Federation (Streltovska caldera, the major occurrence), Peru and Mexico. In Australia, they are minor – Ben Lomond and Maureen in Queensland are the most significant.
5. Metasomatite
Metasomatite deposits consist of unevenly disseminated uranium in structurally deformed rocks that were affected by sodium and/or potassium metasomatism. Major examples of this type include Elkon district (Russia), the Lagoa Real-Caetite district (Brazil), Novokonstantinovskoye and those near Zheltye Vody (Ukraine), Valhalla and Skal (Australia), Michelin (Canada) and Lianshanguan (China).
6. Metamorphite
Metamorphic-type uranium deposits occur in metasediments and/or metavolcanics unrelated to granite. Examples include the deposits at Forstau (Austria), Shinkolobwe deposit (Democratic Republic of Congo), Rozna (Czech Republic), Jaduguda (India), Kokshetau District (Kazakhstan) and Port Radium (Canada). In Australia the largest of this type was Mary Kathleen uranium/rare earth deposit near Mount Isa, Queensland, which was mined 1958-63 and 1976-82. That orebody occurs in a zone of calcium-rich alteration within Proterozoic metamorphic rocks. The Itataia deposit in Brazil is marble-hosted phosphate.
7. Proterozoic unconformity
Unconformity-related deposits arise from geological changes occurring close to major Proterozoic unconformities. Below the unconformity, the metasedimentary rocks which host the mineralisation are usually faulted and brecciated. The overlying younger Proterozoic sandstones are usually undeformed. Unconformity-related deposits constitute about one-third of the western world’s uranium resources and they include some of the largest and richest deposits. Minerals are uraninite and pitchblende associated with strong quartz dissolution. The main deposits occur in Canada (the Athabasca Basin, Saskatchewan and Thelon Basin, Northwest Territories); and Australia (the Alligator Rivers region in the Pine Creek Geosyncline, Northern Territories and Rudall River area, Western Australia).
All of Canada’s uranium production in Saskatechewan over the last 40 years is from unconformity-related deposits – Key Lake, Cluff Lake, Rabbit Lake, McClean Lake, McArthur River and Cigar Lake deposits – with some ore around 20% uranium. The deposits in the Athabasca Basin occur below, across and immediately above the unconformity, with the highest grade deposits situated at or just above the unconformity (e.g. Cigar Lake and McArthur River). In the Alligator Rivers region, the known deposits (Ranger, Jabiluka, Koongarra and Nabarlek) are basement-hosted below the unconformity and like their Canadian counterparts Eagle Point, Millennium, Triple R and Kiggavik, and the Kintyre deposit in the Rudall River area of Western Australia, are generally lower grade.
Strata-bound structure-controlled unconformity deposits include the low-grade Chitrial and Lambapur deposits, Cuddapah Basin, India.
8. Collapse breccia pipe
These occur in circular, vertical collapse structures filled with coarse fragments and a fine matrix of the penetrated sediments. The collapse pipes are 30-200 metres in diameter and up to 1000 metres deep. Uranium mineralisation is mostly within permeable sandstone breccias within the pipe. The principal uranium mineral is pitchblende. All examples of this type are near the Grand Canyon (USA), notably in the Arizona Strip.
9. Sandstone
Sandstone uranium deposits occur in medium- to coarse-grained sandstones deposited in a continental fluvial or marginal marine sedimentary environment. Impermeable shale/mudstone units are interbedded in the sedimentary sequence and often occur immediately above and below the mineralized sandstone. Uranium is precipitated under reducing conditions caused by a variety of reducing agents within the sandstone including: carbonaceous material (detrital plant debris, amorphous humate, marine algae), sulphides (pyrite, H2S), hydrocarbons, and interbedded basic volcanic ash with abundant ferro-magnesian minerals (e.g. chlorite).
There are five main sub-types of sandstone deposits, often mixed:
- Basal channel deposits – wide channels filled with permeable sediments. Examples are Dalur and Khiagda (Russia), and Beverley and Honeymoon (South Australia).
- Tabular deposits – irregular, elongate lenticular bodies parallel to the depositional trend, deposits commonly occur in palaeochannels incised into underlying basement rocks. Examples are Akouta, Arlit, and Imouraren (Niger), Hamr-StráΕΎ pod Ralskem (Czech Republic) and those of the Colorado Plateau (USA).
- Roll-front deposits – arcuate bodies of mineralisation that crosscut sandstone bedding, often in palaeochannels. Examples are Budenovskoye, Tortkuduk, Moynkum, Inkai and Mynkuduk (Kazakhstan) and Crow Butte and Smith Ranch (USA).
- Tectonic/lithologic deposits – occur in sandstones adjacent to a permeable fault zone. Examples are in the Lodève District (France) and the Franceville Basin (Gabon).
- Mafic dykes or sills in Proterozoic sandstones – examples at Matoush (Canada) and Westmoreland (Australia).
Sandstone deposits constitute about 28% of world uranium reasonably assured resources and 40% of inferred resources, and are of major economic importance in Kazakhstan, Uzbekistan, USA and Niger. Orebodies of this type are commonly low to medium grade (0.05-0.35%U) and individual orebodies are small to medium in size (ranging up to a maximum of 50,000 tU). Roll-front sub-types are mined by in situ leach (ISL) methods. Imouraren (Niger) and some Kazakh deposits are larger. The main primary U minerals are uraninite and coffinite.
The USA has large resources in sandstone deposits in the Western Cordillera region, and most of its uranium production has been from these deposits, recently by in ISL. The Powder River Basin in Wyoming, the Colorado Plateau and the Gulf Coast Plain in south Texas are major sandstone uranium provinces. Other large sandstone deposits occur in Niger, Gabon (Franceville Basin), and in the eastern part of Africa, in the Karoo Formation (Malawi, Tanzania, Zambia, South Africa).
10. Palaeo-quartz-pebble conglomerate
Detrital uranium occurs in some Archaean-early Palaeoproterozoic quartz-pebble conglomerates that unconformably overlie granitic and metamorphic basement. Quartz-pebble conglomerate uranium deposits occur in conglomerates deposited in the range 3070-2300 million years ago. Fluvial transport of detrital uraninite was possible at the time because of the prevailing anoxic atmosphere. Rare earths and thorium may be present.
Where uranium is recovered as a by-product of gold mining, such as at Witwatersrand in South Africa, the grade may be as low as 0.01%U. In uranium-dominant deposits such as at Elliot Lake in Canada average grades range as high as 0.15%U.
11. Surficial
Surficial uranium deposits are broadly defined as young (Tertiary to Recent) near-surface uranium concentrations in sediments or soils. These deposits usually have secondary cementing minerals including calcite, gypsum, dolomite, ferric oxide, and halite. Uranium deposits in calcrete are the largest of the surficial deposits. Uranium mineralisation is in fine-grained surficial sand and clay, cemented by calcium and magnesium carbonates. The uranium mineralisation is usually carnotite (hydrated potassium uranium vanadium oxide).
They formed where uranium-rich basement formations (granites, sandstones) were deeply weathered in a semi-arid to arid climate and deposited in valley-fill sediments along Tertiary drainage channels, and in playa lake sediments. The Yeelirrie deposit in Western Australia is one of the world's largest surficial deposits. Other significant deposits in WA include Lake Way, Centipede, Thatcher Soak, and Lake Maitland. Calcrete uranium deposits also occur in the Central Namib Desert of Namibia, the largest being the Langer Heinrich deposit, also Trekkopje.
12. Lignite-coal
Uranium occurs in lignite or coal mixed with mineral detritus (silt, clay), and in immediately adjacent carbonaceous mud and silt/sandstone beds. Pyrite content is high. Examples include those in North and South Dakota (USA), Mulga Rock (Western Australia), Springbok Flats (South Africa), Nizhneylyiskoye (Kazakhstan), and Freital (Germany). Uranium has been adsorbed onto carbonaceous matter and consequently no discrete uranium minerals have formed.
There is estimated to be over 7 million tonnes of uranium in these deposits, but most is subeconomic at present.
13. Carbonate
Deposits are hosted in limestone or dolomite, often related to karsts, fractures, faults and folds. Examples include the large strata-bound Tummalapalle (India), Mailuu-Suu (Kyrgyzstan) and Bentou-Sanbaqi (China).
14. Phosphate
Sedimentary phosphorites of marine origin contain low concentrations of uranium in fine-grained apatite. Uranium concentrations are very low – 0.005-0.015%U. Very large phosphorite deposits occur in the USA (Florida and Idaho), Morocco, Jordan and other Middle Eastern countries and these are mined for phosphate. Where phosphoric acid is produced, uranium is sometimes extracted as a by-product, for example, in Florida. Continental phosphate deposits seldom have uranium, an exception is Bakouma (Central African Rep).
Estimates range to 22 million tonnes of uranium in these deposits, but most is subeconomic at present even as by-product of phosphate fertilizer production, unless new processes are deployed.
15. Black shale
Black shale-related uranium mineralisation consists of marine organic-rich shale or coal-rich pyritic shale, containing synsedimentary disseminated uranium adsorbed onto organic material and clays. Examples include the uraniferous Alum shale in Sweden, the Rudnoye and Zapadno-Kokpatasskaya deposits in Uzbekistan, the Chatanooga shale in the USA, deposits in the Guangxi Autonomous Region, China, and the Gera-Ronneburg deposit, Germany.
Estimates range over 50 million tonnes of uranium in these deposits; they are mostly subeconomic at present.
Perspective
A comprehensive listing of world uranium deposits from a purely geological perspective includes much low-grade mineralisation which is subeconomic. There is very little recovery of uranium from the three deposit types with the largest estimated totals – lignite, black shale and phosphate. Three of the next most-abundant comprise most of the 6.1 million tonnes of identified recoverable resources listed in the 2018 Red Book, and contribute much of the world’s supply of uranium today: sandstone, iron-oxide breccia, and Proterozoic unconformity.
Uranium Minerals
The major primary ore mineral is uraninite (basically UO2) or pitchblende (U2O5.UO3, better known as U3O8), though a range of other uranium minerals are found in particular deposits. These include carnotite (uranium potassium vanadate), the davidite-brannerite-absite type uranium titanates, and the euxenite-fergusonite-samarskite group (niobates of uranium and rare earths).
Brannerite (uranium calcium titanium iron oxide, basically uranium titanate – UTi2O6 – with some calcium and other elements replacing uranium and some iron, and other elements replacing titanium) is particularly important since it occurs as up to 30% of the mineralisation at Olympic Dam and also (<10%) at Valhalla near Mount Isa. It does not dissolve readily in sulfuric acid, and so a substantial proportion is not recovered. Research is being undertaken to improve the recovery of this, which has significant implications for the quantum of Australian recoverable low-cost resources of uranium
A large variety of secondary uranium minerals is known, many are brilliantly coloured and fluorescent. The commonest are gummite (a general term like limonite for mixtures of various secondary hydrated uranium oxides with impurities); hydrated uranium phosphates of the phosphuranylite type, including autunite and ningyoite (with calcium), saleeite (magnesian) and torbernite (with copper); and hydrated uranium silicates such as coffinite, uranophane (with calcium) and sklodowskite (magnesian).
Notes & references
General sources
OECD NEA & IAEA, Uranium 2018: Resources, Production and Demand ('Red Book')
P. Bruneton, M. Cuney, F. Dahlkamp, G. Zaluski, IAEA geological classification of uranium deposits, International Symposium on Uranium Raw Material for the Nuclear Fuel Cycle (URAM 2014), June 2014
World Distribution of Uranium Deposits (UDEPO) with Uranium Deposit Classification, IAEA-TECDOC-1629, International Atomic Energy Agency, October 2009
Original edition condensed from: Lambert,I., McKay, A., and Miezitis, Y., Australia's uranium resources: trends, global comparisons and new developments, Bureau of Resource Sciences, Canberra (1996), with their later paper: Australia's Uranium Resources and Production in a World Context, ANA Conference October 2001
McKay, A., and Miezitis, Y., Australia’s Uranium Resources, Geology And Development Of Deposits, Geoscience Australia (2001), ISBN 0642467161
Minerals from: Aust IMM, Field Geologist's Manual, 1989
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