Geology of Uranium Deposits
- Uranium occurs in a number of different geological environments.
- Most of Australia's uranium resources are in two kinds of orebodies, unconformity-related and iron oxide breccia complex, while sedimentary deposits are less significant than overseas.
- Most Canadian deposits are unconformity-related.
Uranium deposits worldwide can be grouped into 15 major categories of deposit types based on their geological setting. 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. The classification is supported by the International Atomic Energy Agency (IAEA).
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, the main ones are Radium Hill which was mined from 1954-62 (mineralisation was mostly davidite) and the large bodies of low-grade mineralisation at Crocker Well and Mount Victoria in the Olary Province.
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.
Polymetallic iron-oxide breccia complex deposits
Olympic Dam* is one of the world’s largest deposits of uranium, and accounts for the major part of Australia’s uranium resources. 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.
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 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) and Mexico. In Australia, they are minor – Ben Lomond and Maureen in Qld are the most significant.
Metasomatite deposits consist of unevenly disseminated uranium in structurally deformed rocks that were affected by sodium or potassium metasomatism. Major examples of this type include Elkon (Russia), Espinharas deposit (Brazil), Novokonstantinovskoye and those near Zheltye Vody (Ukraine), Valhalla and Skal (Australia), Michelin (Canada) and Lianshanguan (China).
Metamorphic-type uranium deposits occur in metasediments and/or metavolcanics unrelated to granite. Examples include the deposits at Forstau, Austria, Shinkolobwe deposit (DR Congo), Rozna (Czech Rep), Jaduguda (India) and Port Radium (Canada). In Australia the largest of this type was Mary Kathleen uranium/rare earth deposit, near Mount Isa, Qld, 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.
Proterozoic unconformity deposits
Unconformity-related deposits arise from geological changes occurring close to major 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, NT and Rudall River area, WA).
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 over 20% U3O8. 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 are basement-hosted below the unconformity and like their Canadian counterparts Eagle Point, Millennium and Kiggavik, and the Kintyre deposit in the Rudall River area of Western Australia, are generally much lower grade.
Strata-bound structure-controlled unconformity deposits include Chitrial and Lambapur, Cuddapah Basin, India.
Collapse breccia pipe deposits
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. Examples of this type are near the Grand Canyon (USA), notably in the Arizona Strip.
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 mineralised 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).
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-Stráž pod Ralskem (Czech Rep) 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 18% of world uranium resources and 41% of known deposits, and are of major economic importance in Kazakhstan, also in Uzbekistan. Orebodies of this type are commonly low to medium grade (0.05 - 0.4% U3O8) and individual orebodies are small to medium in size (ranging up to a maximum of 50 000 t U3O8) and mined by in situ leach (ISL) methods. 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 situ leach (ISL) mining. 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 South Africa (Karoo Basin).
Palaeo-quartz-pebble conglomerate deposits
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% U3O8. In uranium-dominant deposits such as at Elliot Lake in Canada average grades range as high as 0.15% U3O8.
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 granites 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 the world's largest surficial deposit. 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.
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) and Freital (Germany). Uranium has been adsorbed on to carbonaceous matter and consequently no discrete uranium minerals have formed.
Deposits are hosted in limestone or dolomite, often related to karsts, fractures, faults and folds. Examples include Tummalapalle (India), Mailuu-Suu (Kyrgyzstan) and Bentou-Sanbaqi (China).
Sedimentary phosphorites of marine origin contain low concentrations of uranium in fine-grained apatite. Uranium concentrations are very low – 0.005-0.015% U3O8. 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).
Black shale deposits
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. None have so far proved to be economic.
The major primary ore mineral is uraninite (basically UO2) or pitchblende (U2O5.UO3, better known as U3O8), though a range of other uranium minerals is 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 Ca and other elements replacing U and some Fe, and other elements replacing Ti, 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).
OECD NEA & IAEA, Uranium 2014: Resources, Production and Demand
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
Original edition condensed from
Lambert,I., McKay, A., and Miezitis, Y. (1996) Australia's uranium resources: trends, global comparisons and new developments, Bureau of Resource Sciences, Canberra, with their later paper: Australia's Uranium Resources and Production in a World Context, ANA Conference October 2001
McKay, A., and Miezitis, Y. (2001) Australia’s Uranium Resources, Geology And Development Of Deposits, Geoscience Australia, ISBN 0 642 46716 1
Minerals from: Aust IMM, Field Geologist's Manual, 1989