Uranium deposits, types sedimentary

Phosphorite Deposits. Sedimentary phosphorites contain low concentrations of uranium in fine-grained apatite. Uranium of this type is considered an unconventional resource. Significant examples of these uranium ore types include the U.S. deposits in Elorida, where uranium is recovered as a by-product, and the large deposits in North African and Middle Eastern countries (16). 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

Unconformity-type uranium deposits from the Athabasca Basin (Saskatchewan, Canada) represent the worid-richest uranium ores with the McArthur River deposit as a prime exampie. They are mainiy iocated ciose to the unconformity between a Paieo- to Meso-Proterozoic sedimentary basin, the Athabasca Basin, and an Archean to Paieoproterozoic metamorphic to piutonic basement. For severai years, a new type of uraniferous mineraiization, entireiy iocated in the basement, has been driiied in the South-Eastern part of the basin. Few data are avaiiabie for this type of deposit, iimiting the comparison of their characteristics and possibie genetic iinks with deposits iocated at the unconformity. 

Uranium occurs in several different igneous, hydrothermal, and sedimentary geological environments. Uranium deposits worldwide have been grouped into 14 major categories of deposit types based on the geological setting of the deposits. The most significant are discussed below. 

Mesozoic and Caenozoic rocks include a variety of supracrustal and plutonic types similar to most Palaeozoic suites. Of particular interest for uranium deposits are the thick intermontane and rift-associated red bed clastic sedimentary sequences of the American Cordillera, Japan and the Middle East, the Laramide intrusions of the Cordillera and the subaerial, felsic volcanic rocks in the western United States. 

Much of the world s currently minable uranium is found in vein-like deposits of uncertain origin. They consist of three general types of unequal importance. Most important are unconformity-related deposits and vein-like deposits in meta-morphic rocks. Least important and probably of different origin are vein-like deposits in sedimentary rocks. The origin of all three types is a matter of much conjecture. Major unsolved genetic questions are the source of the uranium and its mode of transport, the source of the mineralizing solutions, the nature and role o f reductants and the control exerted on uranium deposition by structural and lithologic features of the host rocks. Unconformity-related deposits and vein-like deposits in metamorphics commonly occur in brecciated and foliated metamorphic rocks in stable Precambrian Shield areas. They contain about 24.4% of the Western world s reasonably assured 30 uranium resources. 

Pitchblende is the dominant uranium mineral in all these vein-like deposits in sedimentary rocks. It occurs as small veinlets along fractures within and surrounding the structures and as finely disseminated crystals in porous breccia fragments within the structures. Pitchblende concentrations may be distributed zonally within the ore, and in some deposits may be concentrated in the upper levels of the structure. Associated minerals may include sulphides and sulpharsenides. Calcite and quartz are the most common gangue. Types of alteration include bleaching of red sediments, silicification, carbonatization and argillization. 

Roll-type uranium deposits occur in intracratonic sedimentary basins in the U.S.A. Best known are those in Tertiary strata of Wyoming (Powder River, Shirley and Wind River basins) and the Texas Gulf Coast. Smaller deposits are present in Jurassic rocks of the Colorado Plateau. 

As in the case of igneous processes, the sedimentary processes of rock formation lead to the formation economic mineral deposits. Many valuable mineral deposits of iron, manganese, copper, phosphorus, sulfur, zirconium, the rare Earths, uranium and vanadium owe their origin to sedimentary processes. Some of these constitute special types of sedimentary rocks, while others form important constituents of sedimentary rocks. 

Field measurements in sedimentary fluvial-type calcrete deposits also suggest that present-day groundwater in these areas may also display potential to both dissolve and precipitate uranium in the near surface. Chemical dilatancy and evaporation-driven diffusion that promote de-complexing, diffusion, and reprecipitation mechanisms are seen to play integral parts in the continued chemical reworking and modification of these calcrete-hosted carnotite deposits. 

Brannerite is the third most important reduced uranium mineral in that it occurs in many different types of deposits and is the chief uranium producer in the conglomerates of Blind River-Elliot Lake, Ontario, Canada. Although it has been found in pegmatites, hydrothermal and sedimentary deposits, it is always associated with uraninite and probably forms through reactions with uraninite and titanium phases that are also present. 

One of the major categories of uranium occurrences is that of sedimentary deposits. Such deposits are considered secondary occurrences in that they owe their existence to exogenic processes, but in these types the uranium was deposited at the same... 

Sedimentary marine phosphorites are composed principally of phosphatic minerals, and many contain significant amounts of disseminated uranium. These phosphorites have been classified as either miogeosynclinal or platform types. Four additional types that are low in uranium and of little economic importance are residual phosphorites, phosphatized rock, river pebble deposits and guano. 

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