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In the previous post ,the name of the minerals that contain uranium are listed .
In this post various types of uranium deposits are explained.
The upper 10 km of the Earth’s continental crust have an average abundance of 2.7 g/t U.
The grade of uranium ore deposits ranges from a few hundred g/t U to more than 20 % U. Therefore, an ore-forming process is required which enriches uranium over its global geochemical background by a factor of 100 to 10,000.
Such enrichment is possible by leaching of large rock volumes by oxidized warm water and precipitation of uranium (commonly in the form of uraninite, UO2, also known as pitchblende, due to its black color) in such places where the solubility of uranium changes.
Uranium exists in two oxidation states. Uranium in the 6+ state is highly soluble, while uranium in the 4+ state is highly insoluble.
This can be condensed into the general geochemical formulation:
•U6+ (aqueous) + 2 e- = U4+ ↓ (precipitation)
Or, in a natural system,
•UO2(CO3)2 2- + 2 H+ = UO2 + ½ O2 + 2 CO2 + H2O
The solubility of U4+ at low temperatures is extremely low, similar to thorium as Th4+.
However, thorium, as opposed to uranium, has no oxidized species which is why it
is not enriched in low- temperature hydrothermal deposits.
Only under high-temperature conditions, particularly in silicate melts, uranium and
thorium can become enriched synchroneously due to their common large ionic radius
and high charge and then can form U-Th deposits in pegmatites and alkaline granites.
Uranium ore deposits distribution through geological eras
There is no uniform distribution of the Uranium deposits along the geological eras. No economic deposits exist in the Archaean (Pre-cambric lands aged more than 2500 million years).
The 3 million tons Uranium known until now:
48% are located in Proterozoic land
Less than 7% are located in the Cambrian and Jurassic lands.
45 % of the resources are located in recent lands (less than 230 million years)
Types of Uranium ore deposits:
There are 15 types of uranium deposits of which 3 types are most important .
These three main types of uranium deposits are
unconformity-type deposits,
paleoplacer deposits and
sandstone-type (roll front) deposits.
The various types of the deposits are discussed below:
Unconformity-related deposits:
Unconformity-related deposits arise from geological changes occurring close to major unconformities.
Unconformity-related deposits constitute approximately 33% of the western world's uranium resources and they include some of the largest and richest deposits.
Minerals are uraninite and pitchblende.
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
Sandstone type deposits:
Sandstone uranium deposits occur in medium to coarse-grained sandstones deposited in a continental fluvial or marginal marine sedimentary environment.
Sandstone deposits constitute about 18% of world uranium resources and are of major economic importance in Kazakhstan.
Ore bodies of this type are commonly low to medium grade (0.05 - 0.4% U3O8) and individual ore bodies are small to medium in size (ranging up to a maximum of 50.000t U3O8).
The main primary uranium minerals are uraninite and coffinite.
Quartz-pebble conglomerate (QPC)/paleo placer deposits:
Detrital uranium occurs in some Archaean-early Palaeoproterozoic quartz-pebble conglomerates that unconformable overlie granitic and metamorphic basement.
These deposits make up approximately 13% of the world's uranium resources.
Where uranium is recovered as a by-product of gold mining, the grade may be as low as 0.01% U3O8.
In deposits mined exclusively for uranium, average grades range as high as 0.15%
U3O8. Individual deposits range in size from 6000-170 000 t contained U3O8.
Major examples are the Elliot Lake deposits in Canada and the Witwatersrand gold-uranium deposits in South Africa.
Uraninite is a heavy mineral (density of 9.0 - 9.7 g/cm3) which can be enriched in placers under reducing conditions.
Such conditions do not exist on the present-day Earth with a 20 % oxygen atmosphere.
Oxygen content in the atmosphere prior to about 2.4 Ga was very low (<< 1 % O2), which allowed enrichment of uraninite and pyrite as a clastic heavy mineral in the exogene environment.
There are huge uranium paleo-placers, also known as quartz-pebble meta-conglomerates, in the 3.1-2.7 Ga Witwatersrand basin of South African, and in the 2.45 Ga Blind River/Elliott Lake district in Ontario, Canada.
Vein deposits:
Vein deposits of uranium are those in which uranium minerals fill cavities such as cracks, veins, fissures, pore spaces, breccias and stock works.
The dimensions of the openings have a wide range, from the massive veins of pitchblende to the narrow pitchblende-filled cracks, faults and fissures in some of the ore bodies
Hematite-Granite-Breccia complex deposits/ (IOCG Deposits):
The Olympic Dam deposit is one of the world’s largest deposits of uranium, and accounts for the major part of Australia’s uranium resources.
The deposit occurs in a hematite-rich granite breccias' complex in the Gawler Craton.
Intrusive deposits:
Included in this type are those associated with intrusive rocks including alaskite, granite, pegmatite, and monzonites. Major world deposits include Rossing (Namibia), Ilimaussaq (Greenland) and Palabora (South Africa).
Phosphorite deposits:
Sedimentary phosphorites of marine origin contain low concentrations of uranium in fine-grained apatite. Uranium concentrations are 0.01-0.015% 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.
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 meters in diameter and up to 1000 meters deep. Uranium mineralization is mostly within permeable sandstone breccias within the pipe. The principal uranium mineral is pitchblende. The best-known examples of this type are deposits in the Arizona Strip in Arizona, USA.
Volcanic deposits:
Uranium deposits of this type occur in acid to intermediate volcanic rocks and are related to faults and shear zones within the volcanic. These deposits make up only a small proportion of the world's uranium resources. Significant resources of this type occur in China, Kazakhstan, Russian Federation and Mexico.
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 are the largest of the surficial deposits. Uranium mineralisation is in fine-grained surficial sand and clay, cemented by calcium and magnesium carbonates.
Surficial deposits comprise about 4% of world uranium resources. Calcrete deposits represent 5% of Australia’s total reserves and resources of uranium. They formed where uranium-rich granites were deeply weathered in a semi-arid to arid climate.
The Yeelirrie deposit in WA is by far the world's largest surficial deposit.
Metasomatite deposits:
Metasomatite deposits consist of unevenly disseminated uranium in structurally deformed rocks that were affected by sodium metasomatism. Major examples of this type include Espinharas deposit (Brazil) and the Zheltye Vody deposit (Ukraine).
Metamorphic deposits:
The orebody occurs in a zone of calcium-rich alteration within Proterozoic metamorphic rocks
Lignite:
Uranium mineralization occurs in lignite and in clay and sandstone immediately adjacent to the lignite, in the Serres Basin, Greece, in North and South Dakota, USA and at Mulga Rock, Western Australia.
Black shale deposits:
Black shale-related uranium mineralization consists of marine organic-rich shale or coal-rich pyritic shale, containing synsedimentary disseminated uranium adsorbed onto organic material.
Other types of deposits:
A variety of waste materials may contain elevated concentrations of uranium that might be some times recoverable, for example gold and/or uranium mill tailings, coal ash, and other waste materials.
