World Distribution of Uranium Deposits


Ajit Kumar AJIT KUMARWISDOM IAS, New Delhi.

The types of Uranium and its distribution may be classified as follows-


1. Unconformity related deposits


Unconformity related deposits comprise massive pods, veins and/or disseminations of uraninite spatially associated with major unconformities that separate Paleoproterozoic metamorphic basement from overlying Paleoproterozoic-Mesoproterozoic siliciclastic basins.The basement has been altered to varying depths by lateritic weathering. Basement and postunconformity rocks show strong alteration effects associated with mineralized zones. The overlying basin sediments are usually flat-lying, un-metamorphosed, fluvial red-bed strata. Uraninite (commonly in the form of pitchblende) is the dominant uranium mineral in the monometallic (or ‘simple’) deposit and the main mineral in the polymetallic (or ‘complex’) deposit that includes variable amounts of Ni, Co, As, Pb and traces of Au, Pt, Cu and other elements. Some deposits include both ore types and transitional types. Monometallic ores are hosted by basement metasediments, while polymetallic ores are generally hosted by sandstones and paleo-weathered basement at the unconformity. Two sub-types of unconformity related deposits have been recognized reflecting both stratigraphic and structural control :
Fracture controlled, dominantly basement hosted (McArthur River, Rabbit Lake, Eagle Point, McClean Lake, Dominique-Peter in Canada; Jabiluka, Ranger, Nabarlek, Koongarra in Australia),
Clay bounded, massive ore developed along and just above or immediately below the unconformity in the overlying cover sandstones (Cigar Lake, Key Lake, Collins Bay A, B and D zones, Midwest, McClean, Cluff Lake D in Canada).


2. Sandstone deposits


Sandstone uranium deposits occur in carbon and/or pyrite-bearing fluvial (less commonly
marine), arkosic, medium to coarse-grained sandstones that contain, are interbedded with, and are bounded by less permeable horizons. Primary uranium minerals are  redominantly pitchblende, coffinite, and to a lesser extent vanadates and phosphates. Uranium is precipitated under reducing conditions caused by a variety of reducing agents within the sandstones (for example, carbonaceous material, sulphides, hydrocarbons and ironmagnesium minerals as chlorite). Major known sandstone deposits range in age from Palaeozoic to Tertiary. There are also small Precambrian sandstone deposits associated with carbonaceous matter of probable algal origin. Sandstone uranium deposits can be divided into four main types, which are listed below along with representative examples.
Roll front type — Moinkum, Inkai and Mynkuduk (Kazakhstan), Crow Butte and Smith Ranch (USA) and Bukinay, Sugraly and Uchkuduk (Uzbekistan).
Tectonic-lithologic type — Mikouloungou (Gabon) and Mas Lavayre, Lodève district (France).
Basal channel type — Dalmatovskoye, Transural region and Khiagdinskoye, Vitim district (Russian Federation) and Beverley (South Australia).
Tabular type — Ambrosia Lake, Grants Mineral Belt, New Mexico (USA): Westmoreland (Australia), Akouta, Imouraren and Arlit (Niger), Coutras (France) and Colorado Plateau (USA).


3. Hematite Breccia Complex deposits


Deposits of this group occur in hematite-rich breccias and contain uranium in association with copper, gold, silver and rare earths. The main representative of this deposit type, Olympic Dam, has been assigned to a broad suite of loosely related iron oxide–copper–gold deposits ranging in age from ~2 570 to 1 000 Ma that include Prominent Hill, Ernest Henry (~1 480 Ma), Starra (~1 500 Ma), Osborne (1 540 Ma) in Australia; Candelaria (~1 100 Ma), Salobo (2 570–1 880 Ma?) and Sossego in South America, Michelin and Sue-Dianne in Canada. Although some of these iron-rich deposits contain uranium in trace to minor amounts, Olympic Dam (formed at ~ 1 596 Ma) is the only known large Proterozoic iron-rich deposit that contains uranium in economic quantities. They are found in a number of different tectonic settings (rift, subduction zones, basin collapse).


4. Quartz-pebble conglomerate deposits


Quartz-pebble conglomerate uranium deposits are restricted to early Proterozoic intracratonic basins (older than 2.3–2.4 Ga) downwarped into Archean basement assemblages that include granites. Host rocks for this deposit type typically consist of trough cross-bedded, oligomictic quartz-pebble conglomerate beds with a pyritic matrix interbedded with quartzite and argillite beds. This suite of lithologies typically occurs as basal units in fluvial to deltaic braided stream systems. Placer uraninite, the principal primary uranium phase, is locally associated with gold, REE and/or other detrital metallic oxide and sulfide minerals. The variable ore mineral assemblages are a function of different geological source provinces. Fluvial transport and accumulation of uraninite depended on the reducing character of the early, oxygen-poor earth atmosphere prior to oxyatmoversion (gradual increase in oxygen content in the earth’s atmosphere). Post-depositional redistribution and mineral crystallization mainly by diagenetic processes led to the formation of modified placers composed of a suite of authigenic ore and associated minerals such as brannerite, rutile, anatase, coffinite, pyrite, and others. Two economic subtypes of quartz-pebble conglomerate uranium deposits have been identified: polymetallic (Au+U, Witwatersrand Basin, South Africa and monometallic (Blind River- Elliot Lake area, Canada).
There are two subtypes: Monometallic (or U-dominant with REE) U Deposits and Polymetallic Au with U Deposits. Example to Monometallic type is Elliot Lake deposit (Canada) Detrital heavy
minerals (dominated by uraninite and REE minerals.


5. Vein type (granite related U deposits)


The deposit type ‘granite related deposits’ or  ‘vein deposits’, a designation which referred to granite related and undifferentiated deposits. The change in terminology was instituted in an effort to better conform to the practice of relating uranium deposits to specific geologic environments and/or lithologies. Uranium-bearing veins occur in a broad range of lithologies and geologic environments including acidic intrusives (granite, etc.) volcanics, metasediments and sedimentary rocks.
Therefore, the term ‘vein deposits’ is not sufficiently specific to convey a sense of the geology of such deposits. Instead, the term conveys more description of mineral occurrence geometry without providing a geological context.
Two broad subtypes of granite related deposits have been recognized based on their spatial relationship with granitic plutons and surrounding (intruded) host rocks: (1) Endo (or intra-)granitic deposits and related contact-granitic deposits (Limousin-Vendée type); and (2) perigranitic deposits in meta-sediments (monometallic Bohemian and polymetallic Erzgebirge types) and in contact-metamorphic rocks (Iberian type).
The Margnac deposit is located in the Limousin region, north-western Massif Central (France, N46°00'–E1°19'). It is located within St. Sylvestre leucogranite in the La Crouzille district.


6.Intrusive deposits


Uranium deposits in intrusive or anatectic rocks consist of disseminated primary, nonrefractory uranium minerals dominantly uraninite, uranothorianite and/or uranothorite These deposits are generally low-grade (20–500 ppm), but may contain substantial resources (more than 100 kt U). Five subtypes of intrusive deposits are based on host rock petrology. Alaskite type: disseminated uranium occurs in medium- to very coarse-grained alaskite bodies (leucocratic, quartz and alkali feldspar-rich granites) that are discordant to concordant with surrounding folded and highly metamorphosed and migmatized sedimentary rocks [96][97][98][99][100]. The alaskite bodies range in size from small lenses and tabular dikes to large stocks and domes several hundred metres across. No alteration is associated with the uranium mineralization. The main example is Rössing, in Namibia. Other examples include Goanikontes, Ida Dome,  alencia/Trekkopje and SJ Claims (Namibia) and Johan Beetz(Canada). A subtype of the alaskite type deposit, which could also be classified as metasomatic deposits are the ‘Generation four’ type (Upper Proterozoic, 700 to 500 m.y. bp), which is restricted to orogenic belts. This subtype is found in the Damara — Katanga Orogen (part of the Pan-African Orogeny) in Shaba, Democratic Republic of the Congo (Zaire), in Namibia, and in the Brazilian mobile belt (Brazil). Granite, monzonite type (Bingham Canyon, Utah, USA): very low-grade uranium disseminations occur in highly differentiated granitic to (cupriferous) quartz-monzonitic (copper porphyries) complexes. Because of their very low U content, uranium is recovered only as a by-product of copper heap leaching. Other examples are Twin Buttes (Arizona, USA) and Yerington (Nevada, USA). Peralkaline syenite type (Kvanefjeld, Greenland): low-grade uranium disseminations occur in peralkaline syenitic domes or stocks. Uranium phases are commonly of a more refractory nature. Other examples are Motzfeld (Greenland), Pilanesberg (South Africa), Lolodorf (Cameroon) and Catalao (Brazil). Carbonatite type (Phalaborwa, South Africa): disseminated uranothorianite occurs in cupriferous carbonatite complexes. Up until 2002, uranium was recovered as a by-product from copper production .Other typical examples are Araxa (Brazil), Sokli (Finland) and Sevathur (India).
Pegmatite type (Bancroft area, Ontario, Canada): uraninite and other uranium-thorium
minerals occur in un-zoned granitic and syenitic pegmatitic dykes (siliceous and mafic
tendency with aegirine and augite) in sedimentary and igneous rocks, metamorphosed to the amphibolite facies. Deformation and metasomatism commonly follow metamorphism. Hematite is a characteristic alteration product .


7. Caldera related volcanic deposits


Caldera related volcanic deposits are located within or in close proximity to volcanic calderas that are filled by complex assemblages of mafic to felsic volcanic rocks and intercalated clastic sediments. Mineralization in volcanic deposits is largely structure bound, occurring in intrusive veins or stockworks in volcanic intrusions, diatremes, and flow or bedded pyroclastic units. Smaller ore accumulations occur in strata bound mineralized zones as disseminations and impregnations in permeable and/or reactive flows, flow breccias, tuffs and intercalated pyroclastic and clastic sediments. Uranium mineralization also extends into underlying and adjoining basement rock, where it is concentrated in fractured granite and metamorphic rocks. The main uranium minerals, pitchblende and coffinite, are commonly associated with molybdenum-sulfides and pyrite. Other metallic minerals/elements include minor traces of As, Bi, Hg, Li, Pb, Sb, Sn and W. Associated gangue minerals include fluorite, quartz, carbonates, barite and jarosite. The uranium deposits of the Streltsovsk district, Russian Federation and the Dornot complex, Mongolia are the most important examples of caldera related volcanic deposits. Other examples include the Gan-Hang volcanic belt, China, Michelin deposit, Labrador, Canada, Nopal/Peña Blanca, Mexico and McDermitt, USA.


8. Metasomatic deposits


Uranium deposits of this type are related to alkaline metasomatites of sodium or potassium series. The metasomatites are developed in ancient shields and median masses, where they form stockworks controlled by long-lived ancient faults. Sodium metasomatites are predominantly albite in composition, usually with minor carbonate and alkaline amphiboles and pyroxenes — albitites and eisites. Potassium metasomatites are essentially potassium feldspar rocks with minor carbonate (elkonites). Some geologists consider them as gumbeites, which is incorrect, because quartz is unstable in the rearmost zone of these metasomatites. The largest uranium deposits in sodium metasomatites occur in the Kirovograd Ore District, Ukraine. Other regions with similar deposits are Beaverlodge (Canada), Itatiaia (Brazil), Jaduguda (India), and Kokchetav Massif (Kazakhstan). Uranium deposits in potassium metasomatites are known in the Elkon Horst, the southern Yakutia (Russian Federation).


9. Surficial deposits


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 (calcium and magnesium carbonates) are the largest of the surficial
deposits. The calcrete bodies are interbedded with Tertiary sand and clay, which are usually cemented by calcium and magnesium carbonates. Calcrete deposits form in regions where uranium-rich granites were deeply weathered in a semi-arid to arid climate . Surficial uranium deposits also occur in peat bogs and karst caverns.
In Western Australia, surficial calcrete related uranium deposits occur in valley-fill sediments along Tertiary drainage channels (e.g. Yeelirrie) and in playa lake sediments (e.g. Lake Maitland). These deposits overlie Archaean granite and greenstone basement of the northern portion of the Yilgarn Craton. The main uranium mineral is carnotite (hydrated potassium uranium vanadium oxide).
Calcrete uranium deposits also occur in the Central Namib Desert of Namibia, the largest
being the Langer Heinrich deposit. Other small deposits are Trekkopje, Tubas and Aussinanis.


10. Collapse breccia pipe deposits


Collapse breccia pipe deposits occur as vertical chimney-like structures that are filled with down-dropped fragments. Uranium mineralization, which was introduced into the pipes by ascending groundwater and was deposited in response to changes in temperature and/or pressure or to changes in chemical environment, occurs in the interstices between breccia fragments and in fractures in the annular ring that separates the breccia-filled column from the surrounding wall rock. Examples include the Hack Canyon deposit complex and the Orphan deposit, northwestern Arizona, USA.


11. Phosphorite deposits


Uraniferous phosphorite deposits consist of syn-sedimentary, stratiform, disseminated uranium in marine phosphate-rich rocks or phosphorite deposits that formed in continental shelf environments (Modified after Dahlkamp [114]). The uranium mineralization is substituted for Ca in cryptocrystalline fluor-carbonate apatite grains. Phosphorite deposits constitute large uranium resources, but are very low grade (25–150 ppm). Phosphate rock is a key raw material for the world’s chemical fertilizer industry. Therefore, uranium can only be recovered as a by-product of phosphoric acid production. Examples of phosphorite deposits include New Wales Florida (land pebble phosphate) (USA), Gantour (Morocco) and Al-Abiad (Jordan). Other types of phosphorite deposits include: 1) Organic phosphate such as the argillaceous marine sediments enriched in phosphatized fish bones with U-REE-Re-Sc mineralization in the Melovoe deposit, Kazakhstan; 2) Continental phosphorite deposits such as the Bakouma deposit in Central African Republic (considered by some to be a ‘paleo karst’ deposit); and 3) Igneous phosphates including the Itataia deposit in Brazil and deposits on the Kola Pennisula, Russian Federation.


12. Black shale deposits


Stratiform black shale hosted uranium mineralization consists of syngenetic, uniformly disseminated uranium adsorbed onto organic and/or clay particles in organic-rich, pyritic marine shale with thin coalified, phosphatic and/or silty intercalations. The organic matter is of a sapropelic-bituminous or humic, coaly nature derived from planktonic marine algae and land plant debris (e.g. wood spores). Limestone, sand/siltstone, and shale strata complete the stratigraphic sequence. Discrete primary uranium minerals are absent. Other metals (Cu, Cr, Mo, Mn, REE, V and P) occur in small quantities.


13. Metamorphic deposits [Example: Mary Kathleen deposit, Queensland, Australia]


Metamorphic uranium deposits result from regional metamorphism of uraniferous sediments or volcanics. Accordingly, they occur in metasediments and/or metavolcanics in which the uranium mineralization resulted directly from metamorphic processes. Examples include the Mary Kathleen deposit, Australia and the Forstau deposit, Austria .


14. Other deposit types


The preceding list includes all deposit types that are known to host uranium deposits of significant commercial interest. There are, however, other deposit types that host uranium, which either because of their low ore-grades or small size, are of limited commercial interest.


                                       World Distribution of Uranium Deposits (UDEPO), 2009




Sunday, 28th Jul 2013, 07:57:01 PM

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