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Colorado Plateau Uranium - Uravan, Henry Mountains, Slick Rock, Rifle, Grants District

USA

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The Colorado Plateau uranium province of Utah, Colorado, Arizona and New Mexico has historically produced the most important uranium resources in the USA.

The Colorado Plateau is a generally oval shaped area of some 600 km east-west by 750 km north-south of more stable, thicker cratonic crust, bounded by Late Cretaceous to Tertiary foreland thrust belts, and the younger Rio Grande rift zone to the south-east.

The plateau is largely covered by sequences deposited prior to the Cretaceous to Tertiary Laramide Orogeny. These comprise Palaeozoic and Mesozoic platform sediments deposited within basins on the western margin of the Interior Platform, and intra-cratonic deposits within foreland basins.

The Mesozoic sequence on the Colorado Plateau was deposited in a series of interconnected basins on the margin of the Interior Platform, including the Henry Mountains, Kaiparowits and Black Mesa depocentres. The sequence commences with a lower, widespread early to middle Triassic reddish-brown mudstone-siltstone that thickens westwards from the eastern borders of the two states. To the east it is fluvial, but becomes inter-tidal over much of the Colorado Plateau, becoming more marine with grey calcareous siltstone, fine sandstone, limestone, gypsiferous shale and gypsum members in the west where it reaches 300 to 700 m in thickness in eastern and south-eastern Nevada. It is unconformably overlain by regional upper Triassic fluvial channel fill sandstones and conglomerates at the base of the Chinle Formation. The upper sections of the Chinle Formation are made up of several other members with different colour, grain size and composition which overlie these deposits, ranging from grey to green to purple to red mudstones, siltstones and sandstones, with bentonitic clays and volcanic ash beds. The Chinle Formation ranges from 150 to 300 m in thickness, averaging 250 m (Baars, 1988). These are unconformably overlain by lower Jurassic massive brown aeolian sandstones and lesser lacustrine sediments of the Glen Canyon Group.

Subseqeuent deposition was largely within the series of foreland basins being formed to the east of the emerging contractional Cordilleran Orogen, commencing from the middle Jurassic with 30 to 350 m of mudstone, sandstone, limestone and gypsum of the Lower San Rafael Group, thickest adjacent to the north-south 'Utah-Idaho Trough' to the immediate west of the Sevier-Cordilleran Foreland Thrust Front. Marine limestone and mudstone occur to the west, flanked farther east and south-east by sabkha mudstones and gypsum lenses, with red aeolian sandstones further to the east in the lower sections. This was overlain by the Middle San Rafael Group, a 200 to 250 m sandstone member, comprising flat bedded sabkha facies siltstone and sandstone to the north-west and south to south-east, while aeolian cross-bedded sandstones predominate further east. Above this, following a brief erosional break was the Upper San Rafael Group, composed of 30 to 150 m of sediments, commencing with marine glauconitic greenish-grey sandstone and scarce limestone, followed by red mudstones and siltstone with minor gypsum lenses, grading eastward into grey sandstones. These are in turn unconformably overlain by the late Jurassic Morrison Formation of up to 150 m or more of fluvial sandstone or conglomerate and thin mudstone beds of the basal Tidwell and overlying Salt Wash Members, followed by smectitic mudstone, thin bentonitic clays and some sandstone of the upper Morrison Brushy Creek-Fiftymile-Recapture Members that are equivalents from different sub-basins. Unconformably overlying Lower Cretaceous sediments comprised fluvial sandstone and over-bank mudstones (Baars, et al., 1988).

The basal member of the Morrison Formation, the Tidwell Member is composed of lacustrine mudstone, sandstone and thin limestone lenses with gypsum to the north also. The overlying Salt Wash Member is mainly sandstone ore conglomerate with thin mudstone beds deposited in fluvial, over-bank floodplain and local lacustrine environments, lateral to and transgressively over the Tidwell Member.

During the Late-Cretaceous to Tertiary, sedimentation was within a series of intra-cratonic basins controlled by the monoclinal structures, all of which were confined to the northern and eastern margins of the plateau, namely the Uinta, Henry, Piceance, Kaiparowits, San Juan and Baca Basins containing mainly of Palaeocene to Eocene (approximately 66 to 37 Ma) sediments.

Deposition within the Uinta and Piceance Basins, typical of the plateau cover, has been divided into six stages, namely, i). a pre-basin to 'basin formation' stage within the larger foreland basin on the Interior Platform, associated with the Sevier Orogeny thrust belt to the west. This phase was represented by upper Cretaceous fanglomerates in the west, grading east into finer fluvial sediments, then marginal marine sandstone and marine shale to the east; ii). early basin sediments of Paleocene age, which unconformably followed the Cretaceous, commencing with up to 30 m of transgressive pebble to cobble conglomerate to conglomeratic sandstone, followed by late Palaeocene to early Eocene fluvial sandstone with variegated shale which varies from 20 up to 750 m in thickness. Volcani-clastic conglomerates are common in the latter towards the south; iii). late Paleocene fluvial to paludal (marshy) sediments comprising highly fossiliferous limestone, dolomicrite, mudstone, sandstone and both bedded and nodular gypsum. To the east in the Piceance basin this grades into grey and carbonaceous shale with thin coal beds and fossiliferous limestones; iv). late Paleocene to Eocene alluvial accumulations of variegated mudstone and sandstone; v). early Eocene fresh water lake deposits of fossiliferous sandstone and limestone, grey shale and carbonaceous shale with thin coal beds; vi). saline lake sediments which comprised predominantly oil shales, with marginal fluvial sandstones, carbonate mudstones and fossiliferous limestones, and marginal playa shelves. These were overlain by volcani-clastics derived from the north (Baars, et al., 1988).

Tabular-type uranium deposits are primarily hosted by fluvio-lacustrine sandstones of the Triassic Chinle and the Jurassic Morrison Formations, with the latter having been more significant.  

Within the Chinle Formations, mineralisation is hosted by the Shinarump and Moss Black Members. The deposits within the Shinarump Member are contained within gently dipping channels cut several metres into the underlying strata where the member is normally 10 to 15 m thick. The channel fill comprises interlayered lenses of fluvial sandstone, conglomerate, siltstone and claystone, with uranium localised around accumulations of carbonised wood that are more abundant within the channels. Sediments within the channels have been bleached grey (in contrast to the reddish-brown of the underlying and equivalent strata. Ore occurs as tabular layers at the base of the channels, elongated parallel to the trend of the channel, or as smooth rolls or rods. Individual bodies may be up to 200 m long, 25 m wide and up to 6 m thick. Uraninite and montroseite are the primary ore minerals, accompanied by pyrite, galena, sphalerite and bornite (Shawe, et al., 1991).

The mineralised Moss Black Member is 0 to 40 m thick and comprises mostly fluvial, fine to coarse grained, greyish-green to light-grey arkosic sandstone, interbedded with thinner layers of mudstone and calcarenite-conglomerate. Carbonised and silicified plant trash is locally abundant. The sediments are bleached grey or grey-green in the vicinity of ore. In some areas the sandstone contains substantial petroleum residue. Ore deposits are crudely oval-shaped in plan and are up to 15 m thick locally and up to several hundred metres in length - the largest extending to 1 km and are both massive tabular and roll-front in nature. Uraninite is the principal ore mineral, accompanied by various amounts of coffinite and the vanadium minerals montroseite, paramontroseite, doloresite and vanadium bearing illite-smectite, with minor amounts of sulphides and sulphates, ferroselite, calcite and sericite (Shawe, et al., 1991).

The Morrison Formations hosts orebodies in sandstones of the Salt Wash Member in Colorado, Utah and Arizona and in the overlying Westwater Canyon and Brushy Basin Members in the San Juan Basin of New Mexico.   The Salt Wash Member deposits are rich in vanadium, with V:U ratios of between 2:1 and 20:1. The Salt Wash Member was deposited as a broad, fan-shaped alluvial apron by aggrading streams. Regional structural warping during deposition produced a lesser alluvial fan, the toe of which corresponds to the mineralised zone. The ore bearing sands occur across the toe of the fans a broad, more or less continuous unit of porous, fine-grained sandstone which contains abundant detrital plant trash that was carbonised and silicified by burial. Folding and faulting led to the development of salt anticlines. Most of the ore mined occurs as larger tabular deposits and smaller associated and connected roll-front developments. Ore is commonly enriched along bedding planes, adjacent to carbonised plant debris, against the sharply bounded curving inside edges of rolls, or against either upper ot lower sharply bounded surfaces of the tabular orebodies. The principal primary ore minerals are uraninite, coffinite with montroseite and vanadium clay minerals. Where weathered, a variety of secondary minerals, notably carnotite, are evident (Shawe, et al., 1991).

The main districts and deposits mined and outlined include:

The Uravan Mineral Belt - Colorado and Utah

This group of deposits extends over an arcuate, NW to north-south to SSW (convex to the east) belt over an interval of approximately 180 km from Utah in the north, into Colorado to the south and has historically produced 35 000 t U3O8 and 176 000 t V2O5 at average grades of 0.25% and 1.29% respectively between 1947 and 1968. Mining within the belt supplied about half the world's radium from 1910 to 1922, with vanadium and uranium as by-products. Most mines had closed by 1923, due to the reduction in price from competition elsewhere in the world, but were revived in 1935 when the price of vanadium rose, and then boomed after World War II when the US stockpiled uranium for their nuclear weapons programs. The uranium, vanadium and radium mineralisation is principally concentrated within the Salt Wash member of the Jurassic Morrison Formation, with economic ore minerals being mainly carnotite and tyuyamunite. Significant producers have included the Gateway, Uravan (~8000 t U3O8, ~45 000 t V2O5), Bull Canyon (~7000 t U3O8, ~43 000 t V2O5), Placerville (~6250 t V2O5 at an average grade of 2.5%, from 1910 to 1944), Rifle (~18 450 t V2O5 at an average grade of 1.51%, from 1909 to 1970) and Slick Rock (~9000 t U3O8, ~50 000 t V2O5 to 1978) districts in Utah and the Thompson's, La Sal and Dry Valley districts in Colorado (Shawe et al., 1991).
The current White Mesa operation, also in the Uravan Mineral Belt of Utah, processes ore from the Pandora, Topaz and St. Jude mines and the Sunday mine in neighbouring Colorado. Estimated reserves for the White Mesa operation in 2000 (IAEA 2001) were 4700 t of contained U
3O8.

Temple Mountains - Utah

Significant mineralisation is largely within the Moss Back Member of the Triassic Chinle Formation in the northern Henry Basin of south-eastern Utah. The district produced 2380 t U
3O8 and 3 350 t V2O5 at average grades of 0.25% and 0.28% respectively between 1914 and 1970.

Henry Mountains - Utah

In the NNW-SSE elongated 100 x 30 km Henry Basin, which is around 150 km to the west of the Uravan Mineral Belt, the economic and lateral sub-economic mineralisation of the Chinle and Morrison Formations is hosted by sandstones and is limited above and below by intervals of diagenetic carbonate cement which has occluded porosity.   Carbonate is generally absent from the mineralised interval.   The principal uranium mineral is coffinite occurring as 2 to 5 micron crystals coating detrital quartz grains and plant material and intergrown with pore-lining and pore-filling vanadium chlorite.   Ore deposits, such as the Tony M are elongated and comprise tabular laterally connected and narrowly separated lenses developed over an area of 2 to 3 km in length, widths of 200 to 1000 m and thicknesses of a few metres.   High grade ore may contain up to 10% V, as oxides and vanadium chlorite, though the former will be dominant in the higher grade zones (Shawe et al., 1991)

Lisbon Valley - Utah

Uranium was first discovered in the district in 1913, hosted by sandstones of the Chinle Formation, with some carnotite, being mined on a small scale for vanadium in 1917, 1940 and 1941. In 1948, mining began in sandstone of the Permian Cutler Formation. In 1952, a high grade 20 m thick uraninite orebody was intersected in the Triassic Chinle Formation which represented the main productive unit in the district. Ore is distributed over a 25 km interval on the south-western limb of the Lisbon valley anticline. Significant mineralisation is largely within the Moss Back Member. The district produced 34 250 t of U
3O8 at an average grade of 0.32%, from 1949 to 1979 (Shawe et al., 1991).

White Canyon - Utah

Uranium and associated copper mineralisation was initially identified in 1920, although production did not commence until 1946. The geology is similar to that of the Monument Valley uranium district 65 km to the south in Arizona. Uranium as uraninite and coffinite, is hosted by the Shinarump Sandstone Member of the Triassic Chinle Formation. The district produced 4980 t U
3O8 at an average grade of 0.26%, from 1948 to 1974. The district includes the Happy Jack, Red Canyon, Elk Ridge and Deer Flat mines (Shawe et al., 1991).

Monument Valley - north-eastern Arizona

The first uranium in the district was discovered in 1942, while the first mine in the district opened in 1948. Uranium and uranium-vanadium minerals occur in fluvial channels of the Shinarump Sandstone member of the Triassic Chinle Formation, with ore deposits associated with carbonised wood in the sandstone. Mining ceased in the district in 1969, after producing 4370 tonnes t U
3O8 and 12 400 t V2O5 at grades of 0.32% and 0.94% respectively. Monument Valley has been the largest producer in Arizona to 2006. Significant mines include the Moonlight and Monument N. 2 (Shawe et al., 1991).

Grants Mineral Belt - north-eastern New Mexico

Uranium in New Mexico is almost all in the Grants mineral belt, along the south margin of the San Juan Basin. The NW-SE elongated belt contains the Chuska, Gallup, Ambrosia Lake and Laguna uranium districts. Most of the uranium ore is contained in the Jackpile, Poison Canyon, and Westwater Canyon sandstone members of the Morrison Formation, and in the Todilto limestone, all of Jurassic age. The belt has produced 85 180 t U
3O8, ~3560 t V2O5 at average grades of 0.22% and 0.14% respectively from 1950 to 1970 (Shawe et al., 1991). This mineral belt is the subject of a separate record of the same name.

The most recent source geological information used to prepare this summary was dated: 1990.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Hansley P L, Spirakis C S  1992 - Organic matter diagenesis as the key to a unifying theory for the genesis of tabular Uranium-Vanadium deposits in the Morrison Formation, Colorado Plateau: in    Econ. Geol.   v87 pp 352-365
Northrop H R, Goldhaber M B, Landis G P, Unruh J W, Reynolds R J, Campbell J A, Wanty R B, Grauch R I, Whitney G, Rye R O  1990 - Genesis of the tabular-type Vanadium-Uranium deposits of the Henry Basin, Utah: in    Econ. Geol.   v85 pp 215-269
Sanford R F  1994 - A quantitative model of ground-water flow during formation of tabular sandstone Uranium deposits: in    Econ. Geol.   v 89 pp 341-360
Sanford R F  1992 - A new model for tabular-type Uranium deposits: in    Econ. Geol.   v87 pp 2041-2055
Wanty R B, Goldhaber M B, Northrop H R  1990 - Geochemistry of Vanadium in an epigenetic, sandstone-hosted Vanadium-Uranium deposit, Henry Basin, Utah: in    Econ. Geol.   v85 pp 270-284


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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