Uranium deposits - geology

This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).

Introduction

caption Uranium ore. Photo: USGS.

Uranium deposits world-wide can be grouped into 14 major categories of deposit types based on the geological setting of the deposits. Australian uranium deposits can be grouped into 6 of these categories, with some mineralization in two others.

Unconformity-related deposits: Unconformity-related deposits arise from geological changes occurring close to major unconformities. Below the unconformity, the metasedimentary rocks that host the mineralization are usually faulted and brecciated. The overlying younger Proterozoic sandstones are usually undeformed.

Unconformity-related deposits constitute approximately 33% of the World Outside Centrally Planned Economies Area (WOCA)'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, Northern Territory, and Rudall River area, Western Australia).

Unconformity-related deposits constitute a major proportion (20%) of Australia's total uranium resources, and much of Australia's total production since 1980 has been mined from two of these deposits - Nabarlek (now mined out) and Ranger 1 & 3. Other major deposits in the Alligator Rivers region are Jabiluka, Koongarra and Ranger 68.

Today, all of Canada's uranium production is from unconformity-related deposits - Key Lake, Cluff Lake, Rabbit Lake (all now depleted), and McClean Lake and McArthur River deposits. Other large, exceptionally high-grade, unconformity-related deposits currently being developed include Cigar Lake (averaging almost 20% U3O8, some zones over 50% 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, known deposits are below the unconformity and, like their Canadian counterparts, are generally much lower-grade.

Uranium exploration in the Alligator Rivers region and Arnhem Land has been restricted since the late 1970s because of political and environmental factors. Much of the Alligator Rivers region and Arnhem Land have only been subjected to first pass exploration designed to detect outcropping deposits and extensions of known deposits. For example, Jabiluka 2 was found by drilling along strike from Jabiluka 1. There has been very little exploration to locate deeply concealed deposits lying above the unconformity similar to those in Canada. It is possible that very high-grade deposits occur in the sandstones above the unconformity in the Alligator Rivers/Arnhem Land area.

The Kintyre deposit in the Rudall River area is similar to the deposits in the Alligator Rivers region. Metallurgical tests have shown that Kintyre ore can be radiometrically sorted and upgraded prior to milling and processing.

Breccia complex deposits: The Olympic Dam deposit is one of the world's largest deposits of uranium, and accounts for about 66% of Australiaís reserves plus resources. The deposit occurs in a hematite-rich granite breccia complex in the Gawler Craton. It is overlain by approximately 300 meters 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 kilometer wide and extend almost 5 km in a northwest-southeast direction. Virtually all the economic copper-uranium mineralization 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 0.08 to 0.04% U3O8, the higher-grade mineralization being pitchblende. Copper grades average 2.7% for proved reserves, 2.0% for probable reserves, and 1.1% for indicated resources. Gold grades vary between 0.3-1.0 g/t.

Details of the origin of the deposit are still uncertain. However, the principal mechanisms that 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.

Sandstone deposits: 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 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 (petroleum), and interbedded basic volcanics with abundant ferro-magnesian minerals (e.g., chlorite).

There are three main types of sandstone deposits:

  • rollfront deposits: arcuate bodies of mineralization that crosscut sandstone bedding;
  • tabular deposits: irregular, elongate lenticular bodies parallel to the depositional trend, deposits commonly occur in palaeochannels incised into underlying basement rocks; and
  • tectonic/lithologic deposits: occur in sandstones adjacent to a permeable fault zone.

Sandstone deposits constitute about 18% of world uranium resources. 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). The primary uranium minerals are uraninite and coffinite. Conventional mining/milling operations of sandstone deposits have been progressively undercut by cheaper in situ leach mining methods.

The United States 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, Kazakhstan, Uzbekistan, Gabon (Franceville Basin), and South Africa (Karoo Basin). Kazakhstan has reported substantial reserves in sandstone deposits with average grades ranging from 0.02 to 0.07% U3O8.

Sandstone deposits represent only about 7% of Australia's total uranium resources. Within the Frome Embayment, six uranium deposits are known, the largest being Beverley, Honeymoon, East Kalkaroo, and Billaroo West-Gould Dam, all amenable to ISL mining methods. Other deposits are Manyingee, Oobagooma, and Mulga Rock in Western Australia and Angela, Northern Territory. At Mulga Rock uranium mineralization is in peat layers interbedded with sand and clay within a buried palaeochannel.

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 mineralization 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 Western Australia is by far the world's largest surficial deposit. Other significant deposits in Western Australia include Lake Way, Centipede, Thatcher Soak, and Lake Maitland.

In Western Australia, the calcrete uranium deposits occur in valley-fill sediments along Tertiary drainage channels, and in playa lake sediments. These deposits overlie Archaean granite and greenstone basement of the northern portion of the Yilgarn Craton. The uranium mineralization is carnotite (hydrated potassium uranium vanadium oxide).

Calcrete uranium deposits also occur in the Central Namib Desert of Namibia.

Volcanic deposits: uranium deposits of this type occur in acid volcanic rocks and are related to faults and shear zones within the volcanics. Uranium is commonly associated with molybdenum and fluorine. 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. In Australia, volcanic deposits are quantitatively very minor – Ben Lomond and Maureen in Queensland are the most significant deposits.

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). In Australia, the main sites are Radium Hill (South Australia), which was mined from 1954-62 (mineralization was mostly davidite), and the large bodies of low-grade mineralization at Crocker Well and Mount Victoria in the Olary Province, South Australia.

Metasomatite deposits: these occur in structurally-deformed rocks that were already altered by metasomatic processes, usually associated with the introduction of sodium, potassium, or calcium into these rocks. Major examples of this type include Espinharas deposit (Brazil) and the Zheltye Vody deposit (Ukraine). Valhalla and Skal near Mount Isa are Australian examples.

Metamorphic deposits: in Australia, the largest of this type was Mary Kathleen uranium/rare earth deposit, 60km east of Mount Isa, Queensland, which was mined 1958-63 and 1976-82. The orebody occurs in a zone of calcium-rich alteration within Proterozoic metamorphic rocks.

Quartz-pebble conglomerate deposits: 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. The mining operations in the Elliot Lake area have closed in recent years because these deposits are uneconomic to recover under current uranium market conditions.

No such economic deposits are known in Australia, although quartz-pebble conglomerate containing low-grade uraninite and gold mineralization exists in several Archaean-Palaeoproterozoic basins in Western Australia. These are similar in lithology and age to the Witwatersrand conglomerates, being formed before there was any oxygen in the atmosphere.

Vein deposits: Vein deposits constitute about 9% of world uranium resources. Major deposits include Jachymov (Czech Republic) and Shinkolobwe (Zaire).

Uranium Minerals

The 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).

A large variety of secondary uranium minerals is known; many are brilliantly colored and fluorescent. The most common 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 (with calcium), saleeite (magnesian), and torbernite (with copper); and hydrated uranium silicates such as coffinite, uranophane (with calcium), and sklodowskite (magnesian).

Further Reading

  • WNA paper on Geology of Uranium deposits
  • 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
  • Minerals from: Aust IMM, Field Geologist's Manual, 1989
Glossary

Citation

Hore-Lacy, I., & Association, W. (2010). Uranium deposits - geology. Retrieved from http://www.eoearth.org/view/article/156798

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