Wyoming Basins - Gas Hills, Crooks Gap, Smith Ranch, Highland / Ruby Ranch, Crow Butte, Sweetwater
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Roll front Uranium deposits in the Wyoming Tertiary Basins are distributed across Wyoming and neighbouring Nebraska, USA, including the following main deposits: Gas Hills (Peach), Crooks Gap, Smith Ranch,Pumpkin Buttes,Highland / Ruby Ranch, Crow Butte, North Butte / Brown Ranch, Sweetwater and others, such as Irigaray, Gillette, Cooper Mt District, Casper, Rattlesnake Hills, Granite Mountains,Shirley Basin District, Lost Creek and Cheyenne.
Production from these basins, by both conventional mining and in situ leaching (ISL) has produced more than 90 000 tonnes of U308 to 2005.
A series of elongate, generally NW-SE to NNW-SSE trending Tertiary sedimentary basins cover a large proportion of the state of Wyoming, separated by ranges of Archaean, Proterozoic and Palaeozoic basement. These include the Green River Basin in the south-west connected to the east with the Great Divide Basin in the south-central part of the state. The Wind River Basin is found immediately to the north-east in the centre of Wyoming, with the smaller Shirley River Basin to its south-east and the Big Horn Basin to its north-west in the north-western part of the state. The extensive NNW-SSE elongated Powder River Basin occupies the north-east of the state, extending north into Montana, while the Denver Basin is found to the south-east of the state, extending into neighbouring Nebraska and Colorado.
Uranium was first discovered at Lost Creek in the Crooks Gap area of the Great Divide Basin in 1936, while uraniferous coal and shale beds were recognised within the same basin in 1945. Uranium was found in faulted Cambrian shales at Crooks Gap in 1953 and in the Eocene Battle Springs Formation in 1954. It was also discovered at Pumpkin Buttes in the northern Powder River Basin in 1951; in the Gas Hills on the southern margin of the Wind River Basin in 1953; and in the Shirley River Basin in 1955 (Shawe, et al., 1991).
Within the Great Divide Basin, mineralisation is mainly hosted by the Eocene Battle Springs Formation; in the Powder River Basin the main host is the Eocene Sawatch Formation; while in the Gas Hills of the Wind River Basin and in the Shirley River Basin they are within the Eocene Wind River Formation. These mineralised Eocene beds are widespread, gently dipping to nearly flat, sandstone units which are as much as 60 m thick. They consist of poorly consolidated, coarse grained, arkosic, fluvial sandstone, interbedded with finer clastics, and in places with carbonaceous shales and coal layers. In the Crooks Gap area, carbonaceous mudstone and boulder conglomerate are also mineralised. The mineralised interval in these Eocene sequences is about 100 to 120 m thick, except at Crooks Gap where it is about 450 m (Shawe, et al., 1991).
The mineralisation in the Wyoming Tertiary basins is at the contact between the "altered" and "un-altered" sandstones. Light grey "un-altered" sandstone on the down-dip side of the ore contains pyrite, calcite, Fe-Ti oxides and carbonaceous "trash" derived from detrital woody material. Yellowish to reddish "altered" sandstone contains detrital Fe-Ti oxides, but is devoid of pyrite and carbonaceous "trash" and only contains minor calcite. "Altered" sandstone forms large tongues several kilometres long that penetrate "unaltered" sandstone down-dip. In the Powder River Basin the "unaltered" sandstone averages about 6 ppm U, while the "altered" sandstone contains about 2 ppm U (Shawe, et al., 1991).
Ore typically occurs as large roll-like bodies wrapped around the 'nose' of "altered" sandstone tongues. The rolls connect upper and lower bands that are near conformable and form on either side of the "altered" sandstone tongues near their nose zones. The largest orebodies may be up to 60 m wide from a nose of "altered" sandstone, 10 m thick and a kilometre or more in length, parallel to the edge of the "altered" sandstone. Average grades vary from 1 to 7 kg/t U308, with high grade zones carrying up to 0.5% organic carbon. V205 grades are about 0.27%. Slightly mineralised low grade sandstone with 0.1 to 1 kg/t U308, extends for up to 100 m or more beyond the orebodies into the "un-altered" sandstone (Shawe, et al., 1991).
Gas Hills and Crooks Gap
Typical examples of the deposits above are the Gas Hills and Crooks Gap uranium districts, located in central Wyoming. Gas Hills is located towards the south-eastern tip of the Wind River Basin, while Crooks Gap is around 90 km ti the SW on the north-eastern margin of the Great Divide Basin. These deposits had recoverable reserves plus production estimated at approximately (Ludwig, 1979):
Gas Hills district - 70 000 tonnes U308
Crooks Gap district - 11 000 tonnes U308 (as of 1969).
Proven + probable reserves in 2005 were:
Gas Hills (Peach) district - 3600 tonnes U308 in 6.85 Mt of ore @ 0.14% U308.
Immature arkosic sandstones and conglomerates of the early Eocene Wind River Formation (in the Gas Hills) and Battle Spring Formation (at Crooks Gap) are the host rocks for the economic uranium deposits. These formations are underlain by sediments ranging from Cambrian to Paleocene in age and by Precambrian granitic and metamorphic rocks with radiometric ages of about 2600 and 2900 Ma respectively, which are exposed in the nearby Granite Mountains. Debris from these Precambrian rocks makes up most of the detritus in the Wind River and Battle Spring formations.
In the Gas Hills area, the Wind River Formation dips gently at 1 to 3° SE, with two recognised periods of minor normal faulting in the early Eocene and between post-Miocene and Pleistocene time.
The Crooks Gap district is structurally more complex with the Battle Spring Formation being more strongly folded and faulted, dipping from 10 to 20° SE, influenced by Eocene thrusts and post-middle Eocene to Pliocene age normal faults.
The Wind River and Battle Spring Formation commence with Eocene arkoses, conglomerates and mudstones with volcanic ash from the Absaroka-Yellowstone province to the east being added to the western part of the Wind River basin. Renewed uplift of the Granite Mountains in the late early Eocene was followed by stability in the middle Eocene. Volcanic centres in the nearby Rattlesnake Hills developed in the middle Eocene, with activity continuing through the late Eocene. Uplift in the southern Wind River Range caused extensive erosion of middle Eocene rocks. Accumulation of sediments rich in felsic ash (White River Formation) began in the early Oligocene, on an irregular surface of Eocene and older rocks. This accumulation continued until at least the mid-Oligocene. After an erosional interval, deposition of large volumes of tuffaceous sandstone occurred (Split Rock Formation of Miocene age). Renewed crustal activity began in the early Pliocene and a thick section of tuffaceous sandstone (Moonstone Formation) accumulated. Regional uplift beginning in the late Pliocene-early Pleistocene began the present cycle of erosion.
In both the Gas Hills and Crooks Gap districts, the major uranium occurrences are in 'roll front-type' deposits, where the uranium is concentrated in arcuate zones between relatively oxidised ('altered') and relatively reduced ('unaltered') sandstone. The uranium in such deposits is generally thought to have been transported as U6+ by oxygenated ground water travelling down dip in the host sandstone, and to have precipitated as insoluble U4+ minerals (uraninite and coffinite) along the slowly moving interface between oxidised and reduced ground. Gangue minerals include pyrite, marcasite and calcite, with less common selenides and Mo-sulphides.
The granitic rocks of the nearby Granite Mountains are known to have lost large amounts of uranium within the last few hundred million years in response to uplift and weathering, and thus are a reasonable source material for the uranium. Also, most of the host sandstones of the uranium deposits are made up of detritus from such rocks, so that the host rocks themselves have been suggested as a source. Alternatively, leaching of uranium from relatively U rich felsic ashes has been suggested as an alternative source. Tuffaceous materials in the Pliocene Split Rock Formation, the Miocene Moonstone Formation, and the Oligocene White River Formation and Wagon Bed Formation all possible uranium sources. U-Pb isotope dating suggests the uraninite-coffinite mineralisation was emplaced between 35 and 26 Ma
The Smith Ranch deposit is located in the southern portion of the Powder River Basin near Douglas and Glenrock, Wyoming. The Powder River Basin covers an area of 30 000 sq km in north-eastern Wyoming and southern Montana. It is a structural basin bounded to the south by the Laramie mountain range, the Big Horn mountains to the west, the Black Hills mountains to the east and is open on the north.
Following structural uplift at the end of the Cretaceous, continental deposition commenced during Paleocene time. Most of the basal Paleocene Fort Union Formation rocks was derived from erosion of Cretaceous shales and sandstones and are mostly fine-grained clastic. By the late Paleocene, erosion had cut down into the crystalline core of the Laramie Mountains and intermittent arkosic sediments were deposited in the southern part of the present Powder River Basin. Further subsidence and corresponding uplift of the surrounding mountain blocks in late Paleocene to early Eocene time was accompanied by the development of by large, sluggish streams and associated coal swamps. In the early Eocene, large volumes of coarse clastics derived from the highlands formed large fans and braided stream accumulations. Deposition of the Wasatch Formation was partially cyclic with periods of quiescence followed by periods of uplift and rejuvenation of the coarse clastic cycle, with the Granite Mountains being the main source of clastic material, and minor clastic originating from the ancestral Laramie Mountains and Hartville uplift.
Subsequent to the deposition of the Wasatch Formation, subsidence of the Powder River Basin produced a northerly regional dip of approximately 1.5° of the Eocene and earlier rocks. Erosion from the middle to late Eocene developed a mature topography which was buried by Oligocene deposits. During the Oligocene, Miocene and Pliocene thick deposits of sandstones and tuffaceous sediments filled the Powder River Basin. Incessant vulcanism during this period resulted in thick layers of volcanic ash. A major regional uplift near the close of Pliocene and rejuvenated streams re-commenced erosion of the existing sediments.
Smith Ranch and the other significant uranium deposits in the South Powder River Basin are hosted by arkosic sandstone units of the upper Paleocene Fort Union formation and lower sandstone units of the Eocene Wasatch formation.
Within the deposit area, the Wasatch Formation has a thickness ranging from 60 to 90 m in the north and south to 150 m in the central part of the deposit. The underlying Fort Union Formation is over 300 m thick, although only the upper 180 to 210 m comprises sandstone units with significant uranium mineralisation. The contact between the two formations is defined as the base of the well correlated School Coal Seam.
Uranium resources are principally concentrated in six sandstone units, the thickness of which ranges from 3 to 60 m. The ore occurs in oxidation reduction roll fronts that are generally north-facing, C-shaped features. A sandstone unit, depending upon its thickness, can contain up to 20 mineralised roll fronts with the individual mineral fronts occurring as sub-features of the overall mineral deposit.
Total proven + probable reserves at Smith Ranch in 2005 were (Cameco, 2006): 4.512 Mt @ 0.11% U308 for 5050 tonnes of contained U308.
Sweetwater operation / Green Mountain
The Sweetwater Mill is located some 65 km NW of Rawlins,and is located in the Great Divide Basin in south-central Wyoming. Between 1981 and 1983 the mill 590 t U308 from a total of 2.39 Mt of ore from an adjacent, now depleted orebody which has since been reclaimed, at a reported recovery rate of 90%. The mill has been packaged with adjacent deposits, which includes the Green Mountain properties, the largest of which is the Jackpot Deposit, located some 30 km to the north.
Uranium mineralisation at Jackpot are hosted within the Great Divide Basin, mainly in the Tertiary Battle Spring Formation, comprising thick, fluvial, medium to very coarse-grained, poorly-sorted arkosic sandstones and conglomerates. At Jackpot, uranium mineralisation occurs as dark grey to black interstitial fillings and impregnations, principaly as uraninite and coffinite.
The estimated resource in 1992 has been quoted at 14.8 Mt at 0.195% U308, containing some 26 275 t U308 based on a cut-off criteria of 1.8 m of 0.10% U308. In 2000, remaining resources are reported to have been 19 200 t U308 (IAEA, 2001)
Published reserve and resource figures include:
- Pumpkin Buttes District, which includes deposits such as Christensen Ranch with resources in 2000 of 6000 t U308 (IAEA, 2001).
- Highland / Ruby Ranch 2.832 Mt @ 0.09% U308 for 2500 tonnes of contained U308, remaining reserves, 2005 (Cameco, 2006).
7 300 t U308 - resources in 2000 (IAEA, 2001).
- Crow Butte 1.012 Mt @ 0.35% U308 for 3550 tonnes of contained U308, remaining reserves, 2005 (Cameco, 2006).
14 700 t U308 - resources in 2000 (IAEA, 2001).
- North Butte / Brown Ranch 3.875 Mt @ 0.10% U308 for 3850 tonnes of contained U308, remaining reserves, 2005 (Cameco, 2006).
7 300 t U308 - resources in 2000 (IAEA, 2001).
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 2005.
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.
Ludwig K R, 1979 - Age of Uranium mineralization in the Gas Hills and Crooks Gap Districts, Wyoming, as indicated by U-Pb isotope apparent ages: in Econ. Geol. v74 pp 1654-1668|
Reynolds R L, Goldhaber M B 1983 - Iron Disulfide minerals and the genesis of roll-type Uranium deposits: in Econ. Geol. v78 pp 105-120|
Santos E S, Ludwig K R 1983 - Age of Uranium mineralization at the Highland mine, Powder River Basin, Wyoming, as indicated by U-Pb isotope analyses: in Econ. Geol. v78 pp 498-501|
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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