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Abstract

Although the Wyoming uranium province has no individual deposits that can be considered giant deposits, it is nevertheless a major uranium province with occurrences in nearly all rock units. Significant uranium has been mined from geologic units of various ages from Cretaceous to the Oligocene. Approximately 91,000 metric tons (t) of U3O8 have been produced since the early 1950s and approximately 165,000 t U3O8 "forward-cost" reserves are recognized in the region.

The formation of the Wyoming uranium province most likely started during the Archean with the formation of granitic and metamorphic rocks of the Granite Mountains of central Wyoming. This is supported by the observation that the major uranium deposits within Cenozoic sedimentary rocks are generally in clusters that surround the Precambrian core complexes of central Wyoming.

Crustal deformation during the Laramide orogeny initiated formation of the uplifts and basins that characterize present-day Wyoming. Continued development of these structures throughout the early Tertiary resulted in Eocene breaching of the Precambrian cores of the uplifts and the creation of major basins containing significant volumes of Tertiary sediments. Extensive physical and chemical weathering of the Precambrian cores of the uplifts took place during early Tertiary due to the subtropical climate with high rainfall. The central Wyoming Precambrian granitic rocks lost 50 to 75 percent of their uranium content during the Laramide events.

Volcanism in the western United States affected the Wyoming region starting with the Challis-Absaroka volcanism in the middle to late Eocene, followed by extensive periods of volcanism from various centers that continued sporadically through the Pliocene and into the Quaternary. This span of more than 45 m.y. of volcanism resulted in extensive deposition of thick rhyolitic tuff sequences throughout the region. Some of the tuffs incorporated the weathered debris from the Precambrian highlands and this uranium-rich material was included in thick beds of tuffaceous sediments. Such uranium was readily leached by the dissolution of glassy ash and given the huge volumes of ash deposited across the region; additional uranium resources likely remain to be discovered.

Uranium-Pb age measurements demonstrate that formation of many major uranium districts in the province occurred in the late Eocene and throughout the Oligocene. However, in contrast, the southern Powder River basin deposits were formed during the Pliocene. The major deposits in the Wyoming uranium province occur in fluvial sedimentary units of both Paleocene and Eocene age, whereas other economic deposits in the province occur in Cretaceous sedimentary rocks and brecciated rocks of Precambrian age. Regardless of the age or type of host rock, it is likely that many of the deposits have a common genesis.

The ore-forming fluid for the roll-front and related deposits was uranium-enriched surficial water leaching ash fall tuffs and deeply weathered Precambrian rocks. Paleodrainage systems that shifted across the landscape in response to various regional and local tectonic events transported the uranium in surficial waters which, in turn, recharged the ground water below the paleodrainage systems. Permeable rock capable of transmitting significant quantities of the ground water were favorable locations for the deposition of uranium deposits. Precipitation of the uranium in sandstone host rocks was primarily due to reducing conditions caused by organic carbon buried with the original sediments or by the leakage of hydrocarbons into the sediments. Precipitation in karst regions in carbonate rocks was the result of acid neutralization.

Introduction

Throughout the 1970s, Wyoming was second only to New Mexico in United States uranium production. Wyoming is the only state to have had continuous production of uranium, reported to be approximately 91,000 metric tons (t) U3O8, since uranium was discovered in the state in the early 1950s, producing approximately one-third of all uranium produced within the United States. Wyoming has the largest forward-cost uranium reserves in the United States at 164,700 t U3O8 (U.S. Energy Information Administration, 2010a). The production and reserves are entirely from sedimentary rocks, mostly of early Tertiary age.

The Wyoming uranium province (Fig. 1), technically a sub-province of the Rocky Mountain and Intermontane basins uranium province (Finch, 1996), is a major cluster of significant uranium deposits and districts with similar modes of formation within a specific geologic-geographic region. From the point of view of a uranium explorationist, the Wyoming uranium province is distinct, consisting of many uranium occurrences and districts (e.g., Gas Hills, Shirley basin, Crooks Gap, Great Divide basin, Powder River basin, Black Hills, Crawford basin of western Nebraska, and Cheyenne basin of northern Colorado), each of which is made up of a few to numerous deposits of similar origin. No single deposit in the Wyoming uranium province can be considered a "giant" deposit by world standards (~45,000 t recoverable U3O8 as per Cheney, 1981), but taken as a whole it is one of the major uranium provinces of the world.

Fig. 1.

Index map of the western United States, showing the location of the Wyoming uranium province as used in this paper. The outline of the geologic uranium occurrence map (Fig. 4) is shown.

Fig. 1.

Index map of the western United States, showing the location of the Wyoming uranium province as used in this paper. The outline of the geologic uranium occurrence map (Fig. 4) is shown.

An earlier overview of the Wyoming uranium province (Boberg, 1981) provides the foundation on which this new review is developed. As would be expected, work published since the completion of that 1981 overview allows a more complete evaluation of the Wyoming uranium province and, hopefully, a better understanding of its development as globally significant. The 1960s and 1970s were periods of extensive research on uranium geology and uranium deposits—that material formed the basis for the 1981 overview and continues today to be the basis of understanding for the Wyoming uranium province. An updated version of a paper published nearly 30 yrs ago would normally be based on a significant number of new papers published during the subsequent three decades. Unfortunately, for much of those past three decades, the uranium industry was truly functional in only a few areas. Kyser and Cuney (2008) provided a good overview of expenditures on uranium research since 1960 (Fig. 2). They use the relationship between the number of publications on uranium deposits and the spot price to demonstrate the level of expenditure on uranium deposit research. The number of publications increased significantly following the increase in the uranium price in the mid-1970s. The level of exploration and development drilling also followed the increase in uranium price (Fig. 2). Between 1974 and 1980, nearly 75 Mm of drilling were completed just in the United States, averaging 10.7 Mm/yr before dropping off to an average of about 840,000 m/yr for the remainder of the 1980s (U.S. Department of Energy, 2010). However, continued research on uranium geology, which started during the time of high prices, continued after the price began to fall and important research continued to be published for nearly 10 yrs after the drop in price, although there were few uranium explorationists in a position to utilize the new data in exploration. In many respects, these reports were put on the shelves and forgotten because few people were interested in uranium. Once the uranium price started to rise in the early 2000s, nearly all work was on projects originally developed during the 1970s and essentially no original exploration was done for new projects and, surprisingly, little of the research from the 1980s has found its way to current use.

In the Wyoming area, some uranium deposit research continued into the 1980s as demonstrated by a few published papers, but very little, other than review papers, have been completed since the mid-1980s. Papers that postdate the Boberg review (1981) and influence this present paper, generating significant reinterpretation, are primarily those by Zielinski (1982a, b, 1983, 1985), Smith et al (1981, 1982a, b), Santos and Ludwig (1983), Collings and Knode (1984), and Gjeelsteen and Collings (1988). Although studies of the regional geology, structure, and stratigraphy of Wyoming have continued through the past three decades, there has been very little published during that time that changes the basic Wyoming regional geology as reported in the 1981 overview, except for the review of Wyoming topographic evolution by Lillegraven and Ostresh (1988) and Lillegraven (2010) and the reviews of the White River Formation by Larsen and Evanoff (1998) and Terry (1998). An excellent publication on Wyoming regional geology was published by Snoke et al. (1993).

Fig. 2.

Chart demonstrating the relationship between publications on uranium deposits and the spot price of uranium. The number of papers on uranium deposits is taken as a reflection of the funding of research on uranium deposits and there is a clear relationship between the number of publications from the mid-1970s to the early 1990s and the high uranium spot price of the late 1970s. The publication of papers from the mid-1980s to the early 1990s is a reflection of the delay between the initiation of a research project and publication of results. In addition, more research through that period from the mid-1980s was directed toward the high-grade unconformity-style uranium deposits. The peaks in publications through the 1990s reflect results released from the former Soviet Union and additional work from Asian researchers. This chart is modified from Kyser and Cuney (2008), adding the current 2007 US dollar spot price data from UxC consulting (2010) and the 1960 to 2008 United States drilling statistics (U.S. Department of Energy, 2010b).

Fig. 2.

Chart demonstrating the relationship between publications on uranium deposits and the spot price of uranium. The number of papers on uranium deposits is taken as a reflection of the funding of research on uranium deposits and there is a clear relationship between the number of publications from the mid-1970s to the early 1990s and the high uranium spot price of the late 1970s. The publication of papers from the mid-1980s to the early 1990s is a reflection of the delay between the initiation of a research project and publication of results. In addition, more research through that period from the mid-1980s was directed toward the high-grade unconformity-style uranium deposits. The peaks in publications through the 1990s reflect results released from the former Soviet Union and additional work from Asian researchers. This chart is modified from Kyser and Cuney (2008), adding the current 2007 US dollar spot price data from UxC consulting (2010) and the 1960 to 2008 United States drilling statistics (U.S. Department of Energy, 2010b).

Description of the Deposits of the Wyoming Uranium Province

Stratigraphic distribution of uranium deposits

Within the Wyoming uranium province (Fig. 1), rocks from the Archean to the Pliocene (Fig. 3) contain uranium occurrences. Within the Precambrian intrusive and metasedimentary rocks of central Wyoming, small vein-type occurrences and low-grade disseminations are relatively common. Whereas some of these occurrences are likely primary in nature, supergene processes during the early Tertiary resulted in the formation of some occurrences in these older rocks. Various sandstones of Cambrian, Pennsylvanian, Jurassic, Cretaceous, and Oligocene to Pliocene ages contain uranium occurrences as fracture fillings and small roll front-type deposits.

Fig. 3.

Generalized stratigraphic column of the Wyoming uranium province. The black dots within the lithologic symbol for the formations demonstrate known uranium occurrences within that formation or rock unit. The small dots define occurrences, whereas the larger dots demonstrate the units that have produced significant amounts of uranium, the size of the larger dots demonstrating relative production differences.

Fig. 3.

Generalized stratigraphic column of the Wyoming uranium province. The black dots within the lithologic symbol for the formations demonstrate known uranium occurrences within that formation or rock unit. The small dots define occurrences, whereas the larger dots demonstrate the units that have produced significant amounts of uranium, the size of the larger dots demonstrating relative production differences.

The major deposits of the Wyoming uranium province (Fig. 4) occur primarily in correlative Eocene sandstones in the major uranium districts. Additional major deposits in the province in other ages of host rock include the Paleocene Fort Union Formation in the southern Powder River basin, the Lower Cretaceous Inyan Kara Group of the Black Hills, the Upper Cretaceous Lance Formation of the eastern Powder River basin, and the Upper Cretaceous Fox Hills Formation of northeastern Colorado in the Cheyenne basin.

Other significant occurrences of uranium are in south-central Wyoming and northwestern Colorado, where the Miocene Browns Park Formation is mineralized at Baggs and Ketchum Buttes, Wyoming and Maybell, Colorado. Fractures, caverns, and karst breccia (breccia pipes) in the upper part of the Mississippian Madison Formation in the Little Mountain area of the northern Big Horn Mountains and Pryor Mountains, Montana, are also mineralized.

Some black shale members of various Upper Cretaceous shales (Lewis, Pierre, and Cody Shales) are enriched at very low grades of uranium within the province (Harris and King, 1993). In eastern Idaho and western Wyoming, the Permian Phosphoria Formation is enriched in uranium (Finch, 1996). It is not unusual for coal and lignite to contain uranium and a number of the Tertiary coals and lignites in the province do contain uranium. Uraniferous coal and lignite are found primarily in the Great Divide basin of south-central Wyoming and in northwestern South Dakota, southwestern North Dakota, and southeastern Montana (Denson, 1959; Pipiringos, 1961; Masursky, 1962; Denson and Gill, 1965; Murphy, 2007).

Roll-front deposits

The significant production and resource potential of the province primarily occurs as roll-front deposits within Early Cretaceous through early Tertiary sediments. Roll-front deposits have been described in detail by many authors (Gruner,1956; Adler, 1963, 1964; Renfro, 1969; Rubin, 1970; Harshman, 1972; Rackley, 1976; Harshman and Adams, 1981). The overall deposit setting consists of a large pervasive oxidized-altered zone, commonly referred to as the altered tongue, which extends from outcrop to the downdip, or down paleodip, transition to reduced sediments. The uranium deposits occur at or near the transition between oxidized and reduced sedimentary rocks. These redox boundaries can be sharp or diffuse and are laterally extensive with discontinuous uranium concentrations. This redox transition zone is commonly roughly C shaped, with the mineralized part of the sandstone "rolling" across the bedding and thus giving rise to the term roll front (Fig. 5). A number of other terms have been commonly used for this deposit type, including reduction-oxidation or redox front, solution front, or geochemical cell. A characteristic of the larger, better mineralized, roll-front deposits is that the host strata locally have good permeability, which allows for ground-water flow through the deposit. Within the roll front, permeability will be reduced as a result of pore space being filled with precipitated uranium and accessory minerals (Fig. 5E, F).

On the oxidizing or altered side of the roll front, the influx of oxygenated ground water to the front results in continued oxidation of reduced minerals at the front, such as pyrite, uraninite, and coffinite, as well as of organic matter within the fluvial sedimentary rock. As the ground water passes through the reduction-oxidation front, it enters the reduced zone and uranium will be precipitated. This continual process of oxidation and reduction causes the roll front and its contained uranium to slowly migrate downdip, a feature that in some areas continues today. As the mineralizing solution passes through the roll front, pyrite is oxidized to metastable sulfur species (Granger and Warren, 1969). These metastable sulfur species disproportionate to sulfate and hydrogen sulfide. The sulfate, due to sluggish kinetics, does not precipitate other ions in redox reactions, whereas the hydrogen sulfide will reduce the uranium, causing uranium minerals to precipitate.

The oxidized, or altered, side of the roll front is characterized by the destruction of both pyrite and carbonaceous material in the sedimentary rocks (Fig. 5E). In places, larger areas of carbonaceous material, such as those areas of strong reduction that were not fully oxidized, may be left behind, within the oxidized sedimentary rocks, and can be enriched in uranium to concentrations from a few hundred ppm up to several percent. Some of the carbonaceous shale and finegrained sandstone zones above and below an oxidized sandstone may be strongly reducing as well, which can result in upper and lower limbs of the roll front containing several tenths of a percent to several percent uranium in finer grained and more carbonaceous sedimentary rocks. These characteristics of roll-front deposits have been well proven by extensive drill testing over the years and provide a solid and valuable tool to uranium explorationists.

Disequilibrium in roll fronts

Radiogenic disequilibrium is a common consideration in uranium exploration; uranium out of equilibrium with its daughter products makes the measurement of uranium content by gamma-ray spectrometry difficult. It is the result of differing mobilities between uranium and its daughter products and is determined by using natural gamma logs to evaluate a deposit. Natural gamma is an easily measured characteristic of radioactive substances. Pure uranium generates little gamma radiation, but its daughter product bismuth-214 does. As long as uranium is in equilibrium with its daughter products for a long time (i.e., secular equilibrium), a calculation of uranium content based on gamma radiation, which is obtained with a calibrated gamma probe, will be equivalent to the concentration of uranium determined by chemical analysis. However, secular equilibrium is not possible in actively flowing ground-water systems. This is because the various daughter products of uranium have varying solubilities and can be easily separated from the parent uranium by ground-water flow. Figure 5A demonstrates the common disequilibrium found in a roll-front deposit. The oxidized side of a roll front tends to be in negative equilibrium, which means that a calculated amount of uranium based on the gamma log is greater than the actual amount of chemical uranium present. Conversely, the zone within and in front of the roll front, on the reduced side of the front, tends to be in positive equilibrium, which means that more chemical uranium is present than determined by the gamma log (Harshman, 1972; W. Boberg, unpub. data).

Fig. 4.

Geologic and uranium occurrence map of the Wyoming uranium province. Location shown in index map of Figure 1. This map demonstrates the basic basin and range development of the Wyoming uranium province, as well as the general location of major alteration tongues associated with roll-front development in the major districts and a few deposits that are not associated with roll-front development (modified from Boberg, 1981).

Fig. 4.

Geologic and uranium occurrence map of the Wyoming uranium province. Location shown in index map of Figure 1. This map demonstrates the basic basin and range development of the Wyoming uranium province, as well as the general location of major alteration tongues associated with roll-front development in the major districts and a few deposits that are not associated with roll-front development (modified from Boberg, 1981).

Fig. 5.

Generalized diagram of a uranium roll-front deposit that defines changes in mineralogy from the altered and/or oxidized tongue, across the front and into unaltered and/or reduced sand. It also demonstrates the general gamma log character expected from a geophysical probe log run in drill holes that test the front at specific locations across the front. A. Uranium disequilibrium—"—" indicates that chemical uranium will generally be less than that calculated from a gamma log and "+" indicates that chemical uranium will generally be greater than that calculated from a gamma log for drill holes at these locations within the roll front. B. Roll-front geometry—terms commonly used by uranium explorationists to define general position with regard to the roll front. C. Gamma log character—graphs demonstrating actual gamma log character at various locations. D. Visible alteration—a demonstration of the common alteration characteristics, commonly demonstrated by color, of a roll-front system. E. Mineralogy—a general demonstration of the presence and/or absence of common minerals that are subject to alteration and/or oxidation and precipitation by reduction in a roll-front system. F. Metals—a general demonstration of the location of accessory metals commonly associated with a uranium roll front. Modified from Rubin (1970), Harris and King (1993), W. Boberg (unpub. data).

Fig. 5.

Generalized diagram of a uranium roll-front deposit that defines changes in mineralogy from the altered and/or oxidized tongue, across the front and into unaltered and/or reduced sand. It also demonstrates the general gamma log character expected from a geophysical probe log run in drill holes that test the front at specific locations across the front. A. Uranium disequilibrium—"—" indicates that chemical uranium will generally be less than that calculated from a gamma log and "+" indicates that chemical uranium will generally be greater than that calculated from a gamma log for drill holes at these locations within the roll front. B. Roll-front geometry—terms commonly used by uranium explorationists to define general position with regard to the roll front. C. Gamma log character—graphs demonstrating actual gamma log character at various locations. D. Visible alteration—a demonstration of the common alteration characteristics, commonly demonstrated by color, of a roll-front system. E. Mineralogy—a general demonstration of the presence and/or absence of common minerals that are subject to alteration and/or oxidation and precipitation by reduction in a roll-front system. F. Metals—a general demonstration of the location of accessory metals commonly associated with a uranium roll front. Modified from Rubin (1970), Harris and King (1993), W. Boberg (unpub. data).

Age of host rocks

The age of the host rocks can be quite variable within the Wyoming uranium province. Figure 4 not only shows the major uranium districts in the province but also shows the general extent of the altered tongue and a generalized distribution of deposits scattered along the lateral edge of the rollfront systems in the major districts in the basins. The bulk of the past production and forward-cost reserves are hosted by Paleocene and Eocene fluvial sandstones. The deposits in the Gas Hills and Shirley basin districts are roll-front deposits hosted by the Eocene Wind River Formation (Anderson, 1969; Harshman, 1972). The deposits in the Crooks Gap and Great Divide basin districts are roll-front deposits hosted by the Eocene Battle Spring Formation, which is temporally equivalent to the Wind River Formation (Boberg, 1979, 1981; Klingmuller, 1989). In the southern Powder River basin, encompassing the Box Creek, Highland, and Smith Ranch deposits, roll-front deposits are hosted by the Paleocene Fort Union Formation, whereas farther north, the deposits in the Monument Hill and Pumpkin Buttes areas are hosted by the Eocene Wasatch Formation, which is equivalent to the Wind River and Battle Spring Formations (Sharp et al., 1964; Sharp and Gibbons, 1964; Davis, 1969; Rubin, 1970; Seeland, 1976).

In the eastern Powder River basin, in the Moorcroft-Sun-dance area, roll-front deposits are hosted by fluvial sandstones of the Upper Cretaceous Lance Formation. In the Black Hills region of Wyoming, Montana, and South Dakota, the Lower Cretaceous Inyan Kara Formation hosts deposits along roll fronts that surround much of the Black Hills uplift with alteration tongues extending into southeastern Montana. In northwestern Nebraska, the Crow Butte deposits (Collings and Knode, 1984: Gjelsteen and Collings, 1988) are roll fronts hosted in the Lower Chadron sandstone, originally considered to be a basal Oligocene fluvial sandstone, but on the basis of more recent stratigraphic work, it is now considered to be upper Eocene in age and it has been renamed the Chadron Sandstone of the Chamberlain Pass Formation (Terry, 1998). In northeastern Colorado, in the Cheyenne basin, a northern sub-basin of the Denver-Julesburg basin, the Centennial and Grover roll-front deposits are hosted by sandstones of the Upper Cretaceous Fox Hills Formation (Reade, 1976, 1978). The Nine Mile roll-front deposit north of Casper, Wyoming, is hosted by the Upper Cretaceous Teapot Sandstone Member of the Mesaverde Formation. In the southeastern Shirley basin, the Bootheel roll-front deposit is hosted by the Triassic-Jurassic Canyon Springs Sandstone Member of the Sundance Formation (Pool, 2007). Surrounding the flank of the Rock Springs uplift in southwestern Wyoming are many indications of roll-front development but with no economic deposits, which formed in the Eocene Wasatch Formation that crops out surrounding the uplift (W. Boberg, unpub. data). A few kilometers southwest of Rawlins, Wyoming, in the Red Rim area, sandstones of the Paleocene Fort Union Formation are altered and minimally mineralized to the north but host a roll-front deposit to the south (W. Boberg, unpub. data).

Variations in deposits of the Wyoming uranium province

Whereas the roll-front deposit is the most common type of deposit in the Wyoming province, there are local variations. Molybdenum, selenium, and vanadium are commonly associated metals in sandstone uranium deposits, but concentrations can vary significantly between districts (Harshman, 1974). Where present, these metals may be zoned in relationship to the roll front (Fig. 5F). The Gas Hills deposits commonly contain concentrations as high as 1,000 ppm Mo, 500 ppm Se, and 500 ppm V. In the Shirley basin, there is little to no anomalous molybdenum, whereas concentrations of as much as 500 ppm Se and 1,500 ppm V have been measured. At Crooks Gap and in the Great Divide basin, there is little to no anomalous molybdenum, selenium, or vanadium associated with the roll-front deposits (Klingmuller, 1989; W. Boberg, unpub. data) Surficial occurrences of uranium in the Kaycee area of the western Powder River basin contain significant amounts of vanadium, with concentrations of several percent (W. Boberg, unpub. data), which is not common in the adjacent Wyoming deposits in the central Powder River basin. The deposits in the Crooks Gap district have greater amounts of organic carbon associated with uranium within limb ore (Fig. 5b) or pods of ore-grade material within the altered tongue (Klingmuller, 1989). Conversely, whereas carbonaceous material and pyrite are common in the other Wyoming uranium districts, farther downdip from Crooks Gap in the central part of the Great Divide basin, there is little to no carbonaceous material or pyrite in the reduced sandstones such that the altered sandstones often appear to be only slightly oxidized (W. Boberg, unpub. data). In most of the Wyoming province uranium districts, the reductant was entrapped carbonaceous material deposited with the fluvial sediments, but in many parts of the Great Divide basin, the reductant was hydrocarbon that leaked into younger formations from depth along faults. In parts of some districts, deposits are more tabular in form, which appears to reflect a variation of roll-front type. Some of the uranium occurrences in the region display even greater variations than those demonstrated by recognized roll fronts. In the Copper Mountain area of central Wyoming, uranium deposits occur in rollfront and tabular deposits in sedimentary rocks, or as disseminated deposits in fractured xenoliths of amphibolite or serpentinized diabase dikes within Archean granitic rocks, and supergene enrichments within fractured and weathered Archean granitic rocks (Yellich et al., 1978; Shrier and Parry, 1982; Bramlett et al., 1983).

In the Pryor Mountains of southern Montana and the Little Mountain area of the northern Big Horn Mountains of Wyoming, uranium concentrations have been found in karst features, including breccia pipes, in the Mississippian Madison Limestone (McEldowney et al., 1977). Deposition in these carbonate rocks was the result of the neutralization of an acidic ore-forming fluid. In the Maybell district of northern Colorado, uranium has been concentrated within fracture and fault zones in tuffaceous sediments of the Miocene Browns Park Formation. In the southern Great Divide basin of Wyoming and in southeastern Montana and western North and South Dakota, uranium is concentrated in lignites and coal beds (Denson, 1959; Pipiringos, 1961; Masursky, 1962; Denson and Gill, 1965).

Although these deposits demonstrate significant variation in their styles, they are all the product of similar and episodic mineralizing events over tens of millions of years. The source of uranium was the same for all these deposits, with surficial water and ground water carrying uranium to the site of ore deposition. A chemical reaction, in most cases at some type of redox boundary, caused uranium to be deposited in a wide variety of host rocks that include Paleozoic to Pliocene sandstones, lignites and coals, brecciated zones in limestones, Archean metasedimentary rocks, and Archean granitic rocks. The major uranium deposits and districts of the Wyoming uranium province formed in the rocks with the best permeability at the time of ore formation. Obviously, host-rock type, available permeability, and reductant or precipitant are different in these various deposits, but in a broad sense there is a similar process of ore formation.

Geologic History of the Region as it Relates to the Formation of the Uranium Province

Precambrian

Mountain ranges, with cores of Precambrian metasedimentary and igneous rocks, surrounded by sedimentary basins containing thick sequences of Phanerozoic sediments (Fig. 4), characterize the Wyoming uranium province. The Precambrian rocks are Early and Middle Archean gneisses and supracrustal rocks, Late Archean greenstones and metasedimentary rocks derived from continental platform sequences, and Late Archean granitic rocks (Frost and Frost, 1993). In southern Wyoming, there is evidence of an Early Proterozoic accretion of an oceanic-arc terrane, which was followed by Middle Proterozoic emplacement of anorthosites and monzosyenites along the suture zone (Snoke, 1993). Sedimentary rocks in the surrounding basins have generated far greater interest and studies than have the Precambrian core areas, leaving the Precambrian rocks of most of the mountainous core areas known only in fairly simple terms as the result of mainly decades-old reconnaissance studies (Peterman and Hildreth, 1977).

During the 1970s, the Archean granitic rocks were studied primarily for uranium isotope decay systematics and age determinations in attempts to evaluate them as potential source rocks for the uranium deposited in the surrounding sedimentary basins. Stuckless et al. (1976, 1977, 1981) described the radioelement content of the Archean rocks of central Wyoming and stress the apparent relationship of the major uranium districts to the Archean rocks that have anomalous primary uranium content. The general, relatively close grouping of the major uranium districts (e.g., Powder River basin, Shirley basin, Gas Hills, and Crooks Gap-Great Divide basin) surrounding the northern Laramie Range and the Granite Mountains suggests a genetic relationship of the deposits in the major districts to the Archean rocks. However, the Crow Butte deposits in the Crawford basin of western Nebraska are much farther removed from the Archean granitic and meta-morphic rocks than the other districts and do not easily indicate a similar genetic relationship.

Paleozoic and Mesozoic

Middle Proterozoic to early Paleozoic strata are missing in the Wyoming geologic record (Snoke, 1993), a nearly one billion year gap. Precambrian basement rocks were buried in the early Paleozoic and, for most of the Paleozoic and Mesozoic, they were covered by younger sedimentary rocks. Geologic events throughout this vast period of time had no effect on the development of the Wyoming uranium province until the beginning of the Laramide orogeny in the Late Cretaceous. By the end of the Cretaceous, the Laramide orogeny had begun to affect the Wyoming area. The sea, which had covered much of the region during the Middle Cretaceous, had begun to retreat to the east during the Late Cretaceous, and major west-to-east stream drainage (Fig. 6A) developed in the region (Lillegraven and Ostresh, 1988). Laramide deformation, from Late Cretaceous through Eocene, initiated the development of the basins and ranges of the Wyoming province (Brown, 1993).

Tertiary

Deformation, developing the uplifts and basins, began in the west and gradually moved eastward throughout the early Tertiary. Uplift of the ranges resulted in the initial exposure of the Precambrian rocks in the core areas by late Paleocene and the nearly complete exposure of the Precambrian core areas during the Eocene, causing the sediments in the basins to become mainly arkosic. The major drainage systems were controlled by the uplifts and formation of the basins, with streams flowing in the basinal depressions. The uplifts were stripped of tremendous volumes of debris eroded from the Precambrian granitic and metasedimentary rocks. Streams draining the rapidly eroding highlands carried the sediments into adjacent basins where thick accumulations of Tertiary sediments were deposited (Table 1). In the early Tertiary, most of the Wyoming region was still near sea level, with a subtropical, high rainfall climate allowing extensive chemical weathering of the exposed Precambrian highlands. A savanna climate at the time developed a significantly fluctuating water table, as the result of monsoonal seasonal rains, causing highly oxidizing ground waters (Childers, 1974).

During the Late Cretaceous, the Lewis Sea was receding and continental deposits started to form. Lillegraven and Ostresh (1988) and Lillegraven (2010) demonstrated the changes in drainage patterns in the region from the Late Cretaceous to the early Oligocene (Fig. 6). At the end of the Cretaceous, when the Lance Formation was being deposited, the drainage pattern (Fig. 6A) was predominantly to the east. By early Paleocene (Fig. 6B), the major basins and ranges of the region had begun to form and the drainage patterns had shifted to flow through the newly forming basins. Significantly, the ancestral Wind River and/or Powder River formed a major drainage system flowing north through the newly developed Powder River basin.

From the Eocene through the Pliocene, much of the western United States was periodically covered by extensive volcanic ash falls (Fig. 7). Volcanism in the Absaroka-Challis region of northwestern Wyoming and central Idaho began by the middle Eocene. This volcanic activity deposited many extensive ash falls throughout the region from the middle through the late Eocene, forming layers of tuffaceous sedimentary rock, locally mixing with erosional debris from the highlands during deposition. Major volcanic activity in the Great basin area of Nevada and Utah during the early to middle Oligocene led to deposition of huge blankets of ash and ash-fall sediments as demonstrated by the extensive White River Formation covering large areas of Wyoming, Nebraska, Colorado, and parts of both North and South Dakota (Fig. 7). The Marysvale volcanic field in Utah was active periodically from the middle Oligocene into the Pliocene and the Yellowstone hot spot was active at the McDermitt caldera on the Nevada-Oregon border during the late Miocene, with episodic volcanism through to Recent time as it migrated from the McDermitt caldera, through the Snake River plain, and to Yellowstone at the onset of the Quaternary. The fact that volcanic events and ash deposition, from a variety of extrusive centers, has been ongoing for close to 50 m.y. is of great importance to the development of the Wyoming uranium province.

During the early Eocene (Fig. 6C), the Paleocene drainage patterns were controlled by development of ranges in the east. Lake Gosiute, in the Green River basin area, as well as other large lakes in the Wind River and Big Horn basin areas, developed by the middle Eocene (Fig. 6D), probably due to a broad uplift in eastern Wyoming and a subsequent blockage of drainage. The predominant drainage system of the time was still the ancestral Wind River and/or Powder River flowing from the west and turning north through the Powder River basin, with other major drainages flowing internally within the Big Horn basin and within the Green River basin. By the late Eocene (Fig. 6E), the major lakes had been filled with sediment and drainage was continuing through the basins. Continued erosion and gradual burial by volcanic debris caused the mountainous areas to be covered by the early Oligocene.

Table 1.

Thickness of Tertiary Sediments in Wyoming Basins1

Maximum thickness of sediments
BasinPaleocene-Eocene (m)Oligocene-Pliocene (m)
Powder River1,500300
Big Horn2,6002,000
Wind River5,2001,500
Shirley200300
Great Divide3,000600
Green River2,700600
Maximum thickness of sediments
BasinPaleocene-Eocene (m)Oligocene-Pliocene (m)
Powder River1,500300
Big Horn2,6002,000
Wind River5,2001,500
Shirley200300
Great Divide3,000600
Green River2,700600
1

Sources: Welder (1968), Whitcomb and Lowry (1968), Hodson et al. (1973), Lowry et al. (1973, 1976), Boberg (unpub. data)

Fig. 6.

Early Tertiary paleodrainage patterns demonstrating the change in drainage systems over time from Late Cretaceous through early Oligocene. The paleodrainage systems of late Eocene and early Oligocene were likely the pathways along which uranium-enriched surficial and shallow ground waters moved prior to entering ground-water recharge zones of permeable rocks and becoming the oxidizing ground water that formed the major altered and/or oxidized tongues and associated uranium deposits. Each map covers a specific geologic time. A. Late Cretaceous (Lancian). B. Early Paleocene (Puercan). C. Early Eocene (Early Wasatchian). D. Middle Eocene (Late Wasatchian-Bridgerian). E. Late Eocene (Early Uintan). F. Early Oligocene (Early Chadron). Note how the drainage patterns change over time. The development of blocked drainage during middle Eocene resulted in the development of several lakes in the region. Modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

Fig. 6.

Early Tertiary paleodrainage patterns demonstrating the change in drainage systems over time from Late Cretaceous through early Oligocene. The paleodrainage systems of late Eocene and early Oligocene were likely the pathways along which uranium-enriched surficial and shallow ground waters moved prior to entering ground-water recharge zones of permeable rocks and becoming the oxidizing ground water that formed the major altered and/or oxidized tongues and associated uranium deposits. Each map covers a specific geologic time. A. Late Cretaceous (Lancian). B. Early Paleocene (Puercan). C. Early Eocene (Early Wasatchian). D. Middle Eocene (Late Wasatchian-Bridgerian). E. Late Eocene (Early Uintan). F. Early Oligocene (Early Chadron). Note how the drainage patterns change over time. The development of blocked drainage during middle Eocene resulted in the development of several lakes in the region. Modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

Fig. 7.

Western United States Tertiary volcanic activity. The black diamonds in Idaho and Wyoming show the areas of the Absaroka-Challis volcanism that occurred generally in the time frame 52 to 38 Ma, with intense volcanic activity lasting from about 52 to 48 Ma. The black dots define areas of major volcanic activity in the Great Basin area of Nevada and Utah from 37 to 30 Ma and that appear to be the source of the extensive deposition of the Oligocene White River Formation (Larsen and Evanoff, 1998). The outline of the White River Formation tuffaceous sediment depositional area is shown covering much of Wyoming, South Dakota, Nebraska, and adjacent states. The black square in southwestern Utah shows the general location of the Marysvale volcanic field, which was active from 32 to 22 Ma. The black triangles in northern Nevada and northwestern Wyoming connected by the black dashed line demonstrate the position of the Yellowstone hot spot (Yel), which initiated major volcanic activity along the Nevada-Oregon border area about 16.5 Ma. Crustal plate movement caused volcanic activity to track northeasterly, with volcanism occurring near the McDermitt caldera (McD) straddling the Nevada-Oregon border from 16.5 to about 10 Ma, volcanism on the Snake River Plain (SRP) from about 10 to 5 Ma and, more recently, the volcanism in the Yellowstone (Yel) region starting about 2 Ma and continuing to present. The outline of the Powder River basin (PRB) in northeastern Wyoming is the area discussed under "Ash-fall tuffs as a source of the uranium." Map modified from Larsen and Evanoff (1998).

Fig. 7.

Western United States Tertiary volcanic activity. The black diamonds in Idaho and Wyoming show the areas of the Absaroka-Challis volcanism that occurred generally in the time frame 52 to 38 Ma, with intense volcanic activity lasting from about 52 to 48 Ma. The black dots define areas of major volcanic activity in the Great Basin area of Nevada and Utah from 37 to 30 Ma and that appear to be the source of the extensive deposition of the Oligocene White River Formation (Larsen and Evanoff, 1998). The outline of the White River Formation tuffaceous sediment depositional area is shown covering much of Wyoming, South Dakota, Nebraska, and adjacent states. The black square in southwestern Utah shows the general location of the Marysvale volcanic field, which was active from 32 to 22 Ma. The black triangles in northern Nevada and northwestern Wyoming connected by the black dashed line demonstrate the position of the Yellowstone hot spot (Yel), which initiated major volcanic activity along the Nevada-Oregon border area about 16.5 Ma. Crustal plate movement caused volcanic activity to track northeasterly, with volcanism occurring near the McDermitt caldera (McD) straddling the Nevada-Oregon border from 16.5 to about 10 Ma, volcanism on the Snake River Plain (SRP) from about 10 to 5 Ma and, more recently, the volcanism in the Yellowstone (Yel) region starting about 2 Ma and continuing to present. The outline of the Powder River basin (PRB) in northeastern Wyoming is the area discussed under "Ash-fall tuffs as a source of the uranium." Map modified from Larsen and Evanoff (1998).

The major topographic features were more subdued and the strong northerly drainage system through the Powder River basin had been captured and the major drainage systems were again flowing from west to east (Fig. 6F) during the early Oligocene. The climate changed from a warm and humid, subtropical climate to a warm-temperate climate as the overall elevation of the region increased. Deposition of large volumes of volcanic ash, mixed with erosional debris from the remaining highlands, continued to blanket much of the region. During the late Oligocene, extensive deposition ended and regional erosion removed much of the previously deposited Oligocene sediments.

Enormous volumes of tuffaceous sandstones were deposited across much of Wyoming and buried most of the central Wyoming highlands during the Miocene. Subsidence of the central part of the Granite Mountains had started and by the Pliocene, the entire Granite Mountains block was down-dropped along its southern flank. The Wyoming land surface was uplifted to near its present elevation during the late Pliocene, whereas the Granite Mountains block continued its subsidence. Additional large volumes of volcanic ash continued to be deposited over much of the region throughout the Pliocene, forming deposits of tuffaceous sediments. Former drainage patterns were blocked and new drainages established.

Development of the Wyoming Uranium Province

In his 1996 paper describing the uranium provinces of North America, Warren Finch defined the Rocky Mountain and Intermontane basin uranium province: The uranium province is essentially defined by the extent of the Laramide uplifts and basins. Roll front sandstone uranium deposits formed in the basins, and vein uranium deposits formed in fractured rocks of the uplifts. The Laramide fluvial lacustrine basins acquired their individuality in latest Cretaceous to Eocene." (Finch, 1996, p. 7).

The Wyoming uranium province is a subprovince of Finch's (1996) Rocky Mountain and Intermontane basin uranium province, because the subprovince is actually a cluster of significant uranium deposits and districts with similar modes of formation within a specific geologic-geographic region. The Wyoming uranium "province" herein (Figs. 1, 4) encompasses the state of Wyoming and adjacent parts of Colorado, Montana, Nebraska, and North and South Dakota.

Source of the uranium

The source of the uranium has been discussed by many authors and covers a wide range of possibilities, including uranium-rich hydrothermal solutions, weathering of Precambrian uranium-bearing veins, weathering of the uranium-rich granites of the Granite Mountains that includes the leaching of uranium from arkoses derived from those granites, release of uranium through devitrification of volcanic ash, or a combination of the granite-leaching and ash devitrification theories. The concepts that the source of the uranium in the deposits and districts is either hydrothermal fluids from depth or dissolution of preexisting Precambrian vein deposits by surface waters have no supporting evidence and are thus considered unlikely. It is most likely that a combination of a leached Precambrian igneous rock and a Tertiary volcanic-ash source were involved in the formation of the deposits. However, the influence of the volcanic ash is believed to be greater than that of the Precambrian granites.

Central Wyoming Precambrian as source for the uranium

The Precambrian rocks are an obvious possible source for uranium in the Wyoming province. Dahlkamp (1993, p. 35) states, "On a worldwide basis most of the prominent uranium provinces are associated directly or indirectly with Precambrian terrane…"

The major uranium districts of Wyoming are clustered surrounding uraniferous Precambrian rocks of central Wyoming, which is an important fact. It is very likely that the Precambrian of central Wyoming did play a part in the development of the Wyoming uranium province.

Granitic rocks have very low permeability, which significantly limits the ability of surficial and ground waters to take uranium into solution, except along fractures and joints and from weathered erosional debris. It is likely that a significant amount of uranium derived from the granitic rocks of the central Wyoming Precambrian was from deeply weathered granite as it was being eroded and after the debris was deposited in the adjacent basins as flanking arkosic sediments.

The uraniferous granites of the central Wyoming Precambrian appear to have been derived from the partial melting of preexisting metamorphic rocks, which accounts for the initially high uranium concentrations (Nkomo et al., 1978, 1979; Stuckless and Miesch, 1981). Hills and Houston (1979) indicated that the rocks of central Wyoming were buried by over-thrusting during collisional events and partial obduction of an island arc in the vicinity of present-day southern Wyoming between 1730 and 1640 Ma, and this was likely associated with high-grade metamorphism.

Much of the work on the Precambrian within Wyoming has addressed uranium loss and the possibility that the Precambrian rocks were the source for the uranium deposits in Wyoming. The U-Pb systematics presented by Stuckless and Nkomo (1978, 1980) and Rosholt et al. (1973) make a strong case for the Precambrian as a source, because most of the uranium deposits surround these rocks. The average uranium content in the Granite Mountains is 11.5 ppm for biotite granite and 8.6 ppm for leucrocratic granite (Stuckless and Nkomo, 1978). These granites currently contain two to three times as much uranium as average granites (Rogers and Adams, 1969). Studies of U-Pb systematics of uraniferous alkali-rich granites of the Granite Mountains demonstrate that the granite has been subjected to two periods of uranium loss, with a loss of 10 to 45 percent U from 1700 to 1400 Ma and the loss of ≥ 70 percent U when the rocks were brought to the surface during the Laramide orogeny. In the Owl Creek Mountains of central Wyoming, with similar geology to the Granite Mountains, Nkomo et al. (1978) determined that the granitic rocks had lost 50 to 75 percent of their originally contained uranium, very likely during Laramide or post-Laramide and that uranium has been mobilized to depths of at least 122 m within the past 150,000 yrs. Stuckless et al. (1981, p. 32) stated, "In view of the correspondence of high Th/U ratios with large amounts of uranium loss and the correspondence of uraniferous zircons with high whole-rock thorium contents, these radioelement characteristics are considered to be significant indicators of uranium provinces similar to that in central Wyoming."

These studies of the granites of central Wyoming strongly indicate a Precambrian granitic contribution of uranium to the much younger ore deposits. As evidenced by the thicknesses of sediment in the Wyoming basins (Table 1), it is clear that a significant thickness of rock was eroded from the central Wyoming Precambrian province. Given the demonstrated amount of uranium loss from the granites, a significant amount of uranium was likely released into the weathering cycle for potential accumulation into deposits.

Ash-fall tuffs as source for the uranium

Studies of volcanic ash by Smith et al. (1981) and Zielinski (1982a, 1983) make a very convincing argument that devitrification of volcanic ash can release significant amounts of uranium and may be a major source of uranium. Included in these studies are evaluations of the potential for the release of uranium that may be present on fresh ash as adsorbed ions. The uranium would be removed immediately as a result of the initial flushing of the ash by the first rainfall, as well as by more gradual devitrification of the ash.

Uranium released by dissolution of glassy ash as a source of the uranium: This concept has been considered by many authors over the years. Zielinski (1978, 1982a, 1985) and Walton et al. (1981) evaluated glassy ash and the mobility of uranium and other elements during its dissolution. Smith et al. (1981, 1982a, b) studied fresh ash and determined that a significant amount of uranium was released during dissolution of the ash. The largest percentage, about 1 percent of the uranium in the ash, was released during the dissolution of the volcanic glass, suggesting that an ash that shows evidence of extensive dissolution and/or alteration of glass shards should be considered a better source.

Zielinski (1983) evaluated the White River Formation (Fig. 7) a unit as much as 300 m thick of volcanic ash-fall sedimentary rocks that covers large areas of Wyoming, Nebraska, North and South Dakota, and Montana. He estimated that if an area equivalent in size to the Powder River basin, containing 31,337 km2, was covered by 150 m of ash of 50 percent bulk porosity, then a 0.4 ppm U loss by dissolution of volcanic glass would result in 2.38 Mt U. This is more than sufficient to create the deposits of the Wyoming uranium province with production and known reserves of 0.25 Mt U. Given the fact that the area covered by the White River Formation is many times larger than the Powder River basin and that the original thickness of the White River Formation likely exceeded 300 m, it is not unreasonable to assume that the White River Formation by itself could have generated at least one hundred times more uranium than that contained in deposits of the Wyoming uranium province and could easily have been the sole source of the Wyoming uranium province deposits.

Uranium released during initial washing of fresh ash: This concept was used by Boberg (1981), based, in part, on earlier work by Taylor (1969) that described fresh ash collected immediately following eruption of Central American volcanoes before it was affected by rainfall. It was shown that ions are adsorbed on the ash particles during eruption and are then flushed off and taken into solution by the first rainfall or surface water that contacts the ash. Taylor (1969) demonstrated that this first-flush fluid is unique in composition, with extremely varying metal contents and variable but slightly acidic to acidic pH.

A primary consideration of the Boberg (1981) overview was that both the ash and the central Wyoming Precambrian rocks were sources for the uranium. Extensive Tertiary volcanism periodically blanketed the region with ash falls that, when washed with the first rainfall, would develop exotic first-flush solutions of variable compositions, some of which could be quite acidic and reactive. These short-lived first-flush solutions had potential to preferentially leach uranium from the deeply weathered Precambrian rocks of central Wyoming, as well as to enhance devitrification of the ash itself. The fluids would be buffered back to neutrality, although they would already be transporting significant amounts of uranium and other metals leached from both the ash and the Precambrian rocks. Although uranium contained in the Precambrian rocks is liberated and dissolved during the normal weathering and erosional cycles, it may be the first-flush episodes that leach the vast majority of mobile uranium and form the subsequent ore deposits.

In contrast to the above suggestion, Smith et al. (1981, 1982a, b) studied the leaching of fresh ash to determine the amount of soluble material released during initial washing. They concluded that only a small percentage (<0.1%) of total uranium in the ash was released by the initial rinse. This work demonstrates that the initial washing of uranium from fresh ash is probably not a significant contribution as a source of uranium. However, the fact that it does create an ephemeral exotic fluid of extremely varying metal compositions and pH suggests that it could have some local influence on individual deposits.

Probable combined source of uranium

Whereas the Precambrian granitic province of the Granite Mountains has lost from 50 to 75 percent of the uranium calculated to have been present at the beginning of the Laramide, there seems to be little demonstration, other than the general grouping of uranium districts surrounding the Precambrian basement, that it was the source of the uranium. Studies of modern surface and ground water draining the Granite Mountains, including areas of uranium occurrences, seldom contain anomalous levels of uranium (Love, 1970). On the other hand, there are numerous examples of enrichments that are obviously the result of uranium released by the devitrification of ash. The Pliocene Moonstone Formation is roughly centered on the Granite Mountains and is a tuffaceous unit enriched in uranium (Love, 1970). Various units of the Moonstone Formation, including tuffs, tuffaceous sandstones, shales, and lacustrine limestones, are radioactive and contain as much as 0.034 percent U. Water from a well in the Moonstone Formation contained 60 ppb U (Love, 1970). The Moonstone Formation overlies the Split Rock Formation and the uranium in the Moonstone Formation did not come from the underlying Precambrian (Love, 1970).

Both the volcanic ash and the Precambrian granites contributed significant volumes of uranium to the hydrologic system from the Eocene to the Pliocene. With the burial of many of the highlands during Oligocene and Miocene, and the subsidence of the Granite Mountains during Miocene, the volume of Precambrian rock exposed to weathering and erosion during that time was reduced and the potential contribution of the Precambrian as a uranium source may have been reduced. The huge volumes of ash deposited over the Wyoming region for >45 m.y. provided abundant volumes of ash. Initial flushing of the ash falls would have occurred following each eruptive event, possibly generating uranium-rich flow systems at various times and localities.

Studies in other parts of the world have clearly demonstrated that both Precambrian granitic terranes and tuffaceous sedimentary rocks are possible sources for uranium in nearby deposits. The South Texas roll-front uranium deposits are hosted by sedimentary rocks with major tuffaceous content and ore-forming solutions had limited hydrologic access to any Precambrian granitic rocks (Galloway, 1977, 1978; Walton et al., 1981). On the other hand, in Western Australia, there seems to be ample evidence that major calcrete-type paleochannel uranium deposits were derived solely from leaching of weathered granites (Dahlkamp, 1993). The Wyoming uranium province appears to have benefited from the presence of both favorable sources being abundantly available.

Age of Wyoming uranium deposits

The oldest reliable age of a Wyoming uranium deposit is 43 Ma (Ludwig, 1979). Most ages from the Shirley basin, Gas Hills, and Crooks Gap (Fig. 8) are in the range of 35 to 20 Ma (Ludwig, 1978, 1979). Uranium mineral deposition in the Shirley basin started before 35 and continued to 24 Ma. In the Gas Hills and Crooks Gap districts, ore formation also started before 35 and continued to 26 Ma. The 43 Ma age falls well within the Eocene period when the central Wyoming Precambrian rocks were exposed and the Absaroka-Challis volcanism was active. However, most of the Wyoming uranium deposit ages are younger than the Absaroka-Challis volcanism and many of the dates overlap or closely follow deposition of Oligocene White River ash from 35.5 to 30 Ma.

Ages of the younger Highland deposit in the southern Powder River basin range from 11 to 1.8 Ma (Santos and Ludwig, 1983). These southern Powder River basin ages require a mode that thus accounts for a spread of ages of deposit formation covering much of the Tertiary. With ash falls occurring in this region for more than 45 m.y., including through the Pliocene, it is apparent that a volcanic-ash source for the uranium is most consistent with development of the Wyoming uranium province.

Model of mineralizing fluid and formation of roll-front uranium deposits

The mineralizing fluid most likely originated as oxygen-rich surficial water (Hostetler and Garrels, 1962), which was enriched with uranium by dissolution of glassy ash and, to a lesser degree, by leaching Precambrian rocks. An excellent model for uranium deposit formation in the Wyoming basins is that of Granger and Warren (1978), which is summarized in Table 2. They assumed a uranium-enriched, but otherwise nearly normal ground water, entering the sediments shortly after deposition, and thus still very porous and permeable, forming a typical altered tongue and uranium deposit in less than one m.y. They calculated that a typical 10-km-long altered tongue would form in 700,000 yrs and a typical roll-front uranium deposit would form in 50,000 yrs. They determined that it takes the passage of 4,000 volumes of oxidizing ground water to oxidize one volume of sedimentary rocks containing 1.0 percent pyrite.

Fig. 8.

A chart of ages of the Wyoming uranium deposits. Shirley basin dates from Dooley et al. (1974) and Ludwig (1978), Crooks Gap dates from Ludwig (1979), Gas Hills dates from Dooley et al. (1974) and Ludwig (1979), and Powder River basin dates from Santos and Ludwig (1983).

Fig. 8.

A chart of ages of the Wyoming uranium deposits. Shirley basin dates from Dooley et al. (1974) and Ludwig (1978), Crooks Gap dates from Ludwig (1979), Gas Hills dates from Dooley et al. (1974) and Ludwig (1979), and Powder River basin dates from Santos and Ludwig (1983).

The model of Granger and Warren (1978) relates only to the initial formation of a roll-front deposit. A deposit formed in this manner would be subject to continued modification and remobilization by ground-water flow systems. It is suspected that continued movement of a roll front will be at a much slower rate than the rate at which it was originally deposited.

Paleodrainage systems and mineralizing system development

Seeland (1976, 1978) suggested that the distribution of uranium deposits in the Powder River basin was controlled by paleosurface- and paleoground-water systems that were dominant at the time of mineralization. Late Cretaceous to early Oligocene drainage patterns (Lille-graven and Ostresh, 1988; Lillegraven, 2010) evolved with the topography over time (Fig. 6). The late Eocene paleodrainage system has a general coincidence with the distribution of major altered tongues within uranium districts in all of the basins (Fig. 9A), except the Crow Butte deposit in northwestern Nebraska. By early Oligocene (Fig. 9B) the ancestral Wind River and/or Powder River had been captured and was flowing eastward, over the area of the Crow Butte deposit. The Oligocene White River Formation either overlies or is recognized as adjacent to all the districts (Fig. 9) and was likely a major source for the uranium in many of the deposits of the region, probably accounting for the Crow Butte deposits, as well as those in the Shirley basin, Gas Hills, Crooks Gap, and Great Divide basin, the deposits in Eocene sediments in the Powder River basin, and very likely the Owl Creek-Copper Mountain deposits and the Centennial and Grover deposits of northeastern Colorado in the Cheyenne basin.

Table 2.

Summary of a Possible Mineralizing Fluid and the Altered Tongue and Deposit that It could Form (modified from Granger and Warren, 1978)

Parameters of the deposit to be formed
Distance over which ore-stage pyrite redeposition is essentially complete= 60 m
Distance over which uranium deposition is essentially completed= 27 m
Width of ore-grade uranium (>0.1% U3O8)= 10 m
Average grade of uranium deposit over 27-m width= 0.28% U3O8
Maximum content
Uranium in ore= 3.0%
Ore-stage pyrite= 2.0%
Prestage selenium= 0.1%
Distance between visible reduction-oxidation interface and
Maximum pyrite content= 30 cm
Maximum uranium content= 2 m
Parameters of the host rock
Preore pyrite content= 1.0%
Bulk specific gravity= 2.0
Porosity= 20%
Hydraulic conductivity= 40 m/d
Dip of strata= 5.7 m/km
Hydraulic gradient= 3.8 m/km
Parameters of the oxidized tongue
Length of oxidized tongue parallel to ground-water flow= 10 km
Parameters of the ore-forming ground-water solution
These parameters reflect the ore-forming solution as it approaches close to the reduction-oxidation interface
Uranium content of ground water= 50 ppb
Oxygen content of ground water= 5 ppm
Bulk flow rate of ground-water solution= 50 m3/yr through 1 m2
Velocity of ground-water solution= 290 m/yr
Parameters of roll-front movement and deposit formation
Rate of advance of roll front (advance of reduction-oxidation front)= 1.4 cm/yr(1 km/70,000 yrs)
Maximum time required to form above defined deposit
A 10-km-long altered/oxidized tongue formation requires= 700,000 yrs
The uranium deposit forms at the reduction-oxidation front= 50,000 yrs
Parameters of the deposit to be formed
Distance over which ore-stage pyrite redeposition is essentially complete= 60 m
Distance over which uranium deposition is essentially completed= 27 m
Width of ore-grade uranium (>0.1% U3O8)= 10 m
Average grade of uranium deposit over 27-m width= 0.28% U3O8
Maximum content
Uranium in ore= 3.0%
Ore-stage pyrite= 2.0%
Prestage selenium= 0.1%
Distance between visible reduction-oxidation interface and
Maximum pyrite content= 30 cm
Maximum uranium content= 2 m
Parameters of the host rock
Preore pyrite content= 1.0%
Bulk specific gravity= 2.0
Porosity= 20%
Hydraulic conductivity= 40 m/d
Dip of strata= 5.7 m/km
Hydraulic gradient= 3.8 m/km
Parameters of the oxidized tongue
Length of oxidized tongue parallel to ground-water flow= 10 km
Parameters of the ore-forming ground-water solution
These parameters reflect the ore-forming solution as it approaches close to the reduction-oxidation interface
Uranium content of ground water= 50 ppb
Oxygen content of ground water= 5 ppm
Bulk flow rate of ground-water solution= 50 m3/yr through 1 m2
Velocity of ground-water solution= 290 m/yr
Parameters of roll-front movement and deposit formation
Rate of advance of roll front (advance of reduction-oxidation front)= 1.4 cm/yr(1 km/70,000 yrs)
Maximum time required to form above defined deposit
A 10-km-long altered/oxidized tongue formation requires= 700,000 yrs
The uranium deposit forms at the reduction-oxidation front= 50,000 yrs
Fig. 9.

Geologic-uranium occurrence map (Fig. 4) with paleodrainage patterns (blue) superimposed. Geology legend is the same as in Figure 4, but it has been removed from these maps to simplify them. Contour lines overlain in both maps are thicknesses (m) of the tuffaceous Oligocene White River Formation (Larsen and Evanoff, 1998). A. Late Eocene pale-odrainage systems demonstrating the general coincidence of the paleodrainage systems of late Eocene to the major alteration and/or oxidation tongues in the major basins. B. Early Oligocene paleodrainage systems demonstrating how the shifting of drainage systems to the east in early Oligocene provided a means of delivering uranium-enriched surface waters to western Nebraska to enable formation of the Crow Butte deposits. Modified from Boberg (1981), drainage areas modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

Fig. 9.

Geologic-uranium occurrence map (Fig. 4) with paleodrainage patterns (blue) superimposed. Geology legend is the same as in Figure 4, but it has been removed from these maps to simplify them. Contour lines overlain in both maps are thicknesses (m) of the tuffaceous Oligocene White River Formation (Larsen and Evanoff, 1998). A. Late Eocene pale-odrainage systems demonstrating the general coincidence of the paleodrainage systems of late Eocene to the major alteration and/or oxidation tongues in the major basins. B. Early Oligocene paleodrainage systems demonstrating how the shifting of drainage systems to the east in early Oligocene provided a means of delivering uranium-enriched surface waters to western Nebraska to enable formation of the Crow Butte deposits. Modified from Boberg (1981), drainage areas modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

It has been generally accepted that deposits formed in proximity to the Precambrian highlands would be larger and of higher average grade because of the proximity to both the Precambrian rocks and areas of major tuffaceous deposition. However, the Crow Butte deposit of northwestern Nebraska is high grade (~0.2% U3O8) and is not near the Precambrian highlands, whereas the central Great Divide basin deposits, which are of generally lower grade (<0.1% U3O8), are very close to the Granite Mountains. White River tuffaceous sedimentary rocks over the Crow Butte deposits were greater than 200 m thick, whereas the central Great Divide basin deposits appear to lie outside the deposition area of most of the tuffaceous rocks of the White River Formation (Fig. 9). The Gas Hills, Shirley basin, and Powder River basin districts all lie within areas of major White River deposition and they all had a high-grade uranium grade (>0.1-0.2% U3O8). Therefore, the most favorable regions for ore formation must be considered to be along paleodrainage systems that developed within major areas of ash and where permeable rock was widespread. Given that significant volumes of uranium were liberated from the deeply weathered Precambrian highlands, as well as debris eroded from the highlands, the Precambrian also must have contributed some of the uranium to the Wyoming districts. However, the volcanic ash was the major source of uranium in the Wyoming uranium province.

A combination of events during the Tertiary in the Wyoming region was instrumental in the development of the Wyoming uranium province (Fig. 10). The Precambrian core areas of the Wyoming uplifts were breached only a few million years before airborne ash began to be deposited on them. The presence of available uranium in the granites is an important factor. However, the warm, humid climate, combined with large volumes of ash-fall tuffs, would generate the needed volume of fluids as well as greater volumes of uranium released into the system. The climatic conditions at the time would have maintained a steady supply of oxygen to the hydrologic system, thus assuring the uranium and other oxidized metals in solution would remain in solution until the dissolved oxygen was depleted at the redox front (Granger and Warren, 1978).

Fig. 10.

Coincident factors demonstrating the various events during the Tertiary that affected the development of uranium deposits throughout the Wyoming uranium province. The width of a bar generally defines greater or lesser amount or level of activity. The variation in the width of the bars at the top defining the permeability of sediments is for the thicker part to suggest the greater permeability of unconsolidated, early burial, sediments with the thinning of the bar demonstrating the loss of permeability over time as diagenesis and compaction takes place. K = Cretaceous, Pl = Pliocene, Q = Quaternary. Bar defining volcanic ash fall: Absaroka-Challis = Absaroka-Challis volcanism of northwest Wyoming and central Idaho, UT-NV = volcanism of the Great Basin area of Utah and Nevada that appears to be the source of the Oligocene White River Formation, Marysvale = Marsyvale volcanism of Utah, McD = McDermitt caldera, SRP = Snake River Plain, and Yel = Yellowstone, these three defining the general track of the Yellowstone hot spot. Bar defining mineralization (the age range from Fig. 8 is shown: CG = Crooks Gap, GH = Gas Hills, PRB = Powder River basin, SB-Shirley basin. Modified from Boberg (1981).

Fig. 10.

Coincident factors demonstrating the various events during the Tertiary that affected the development of uranium deposits throughout the Wyoming uranium province. The width of a bar generally defines greater or lesser amount or level of activity. The variation in the width of the bars at the top defining the permeability of sediments is for the thicker part to suggest the greater permeability of unconsolidated, early burial, sediments with the thinning of the bar demonstrating the loss of permeability over time as diagenesis and compaction takes place. K = Cretaceous, Pl = Pliocene, Q = Quaternary. Bar defining volcanic ash fall: Absaroka-Challis = Absaroka-Challis volcanism of northwest Wyoming and central Idaho, UT-NV = volcanism of the Great Basin area of Utah and Nevada that appears to be the source of the Oligocene White River Formation, Marysvale = Marsyvale volcanism of Utah, McD = McDermitt caldera, SRP = Snake River Plain, and Yel = Yellowstone, these three defining the general track of the Yellowstone hot spot. Bar defining mineralization (the age range from Fig. 8 is shown: CG = Crooks Gap, GH = Gas Hills, PRB = Powder River basin, SB-Shirley basin. Modified from Boberg (1981).

Favorable rocks available for incursion of mineralizing solution

During the early Tertiary, the ranges were being rapidly uplifted, deeply weathered, and eroded. The debris being carried from the ranges was deposited in the adjacent basins, which were covered with tropical vegetation. Sediments deposited under such conditions, with abundant decaying vegetation incorporated into the rock sequences, were almost immediately reduced (Rackley, 1972). Available sulfur and iron combined to form pyrite, particularly within and near large masses of organic debris.

These young reduced sediments were very permeable, much like present-day alluvium. During diagenesis and early burial, these sediments were exposed at the surface and represented large ground-water recharge areas, such that large volumes of oxidized surface waters could leach uranium-rich ash and Precambrian granitoids. Fluid flow into and through these young sediments and sedimentary rocks could very rapidly oxidize large volumes of rock and deposit uranium and associated metals at the redox boundary. Gradual burial initiated diagenesis and compaction, which gradually reduced permeability. Ash-fall debris incorporated in the sediments also allowed for a high permeability that was lost over time as the glassy ash was devitrified to form clay.

Other permeable rocks, including brecciated Precambrian granites and metasedimentary rocks, cavernous, brecciated limestones, and various Paleozoic and Mesozoic sandstones, were locally exposed at the surface and were also infiltrated by the oxidizing uranium-rich fluids. Less permeable rocks and those with more limited outcrop or subcrop recharge area exposure defined areas with limited ground-water flow and thus were less favorable for ore formation. Rocks with little or no reductant to cause significant precipitation would also provide little opportunity for the development of economic uranium deposits.

Sedimentation and burial were fairly rapid through much of the Paleocene to the Oligocene. As a result, in most areas, the Paleocene Fort Union Formation was probably buried by Eocene sediments by the time that significant volumes of uranium were being leached and moving within the surface-water system. This lack of outcrop exposure during the generation of oxidizing uranium-bearing fluids explains why much of the Paleocene Fort Union Formation has neither roll fronts nor any uranium mineralization. The southern Powder River basin, on the other hand, has a fairly extensive roll-front system that contains many major deposits within rocks of the Paleocene Fort Union Formation (e.g., Highland, Smith Ranch, Reynolds Ranch). These southern Powder River basin deposits have ages (Fig. 8) much younger than the other districts, predominantly Pliocene to Quaternary, with the oldest being middle Miocene. These deposits can be explained by a later ash-fall source, probably the Pliocene Moonstone Formation or equivalent, that was deposited after middle to late Miocene weathering and erosion had removed the overlying Oligocene and Eocene sedimentary rocks and exposed a broad outcrop of the Paleocene Fort Union Formation in the southern Powder River basin. Such exposure, and thus potential for ground-water flow system development, did not exist earlier when the unit was completely buried by younger sedimentary rocks. Some of the uranium in these deposits could also have been sourced from deposits formed during the Oligocene in overlying Eocene sedimentary rocks that were later destroyed during weathering and erosion.

Formation of other deposit types in the Wyoming uranium province

The generation of uranium-enriched fluids during times of extensive volcanic ash fall was responsible for a variety of deposits in the Wyoming province other than roll-front deposits. The karst breccia deposits in the Mississippian Madison Formation of the Pryor Mountain-Little Mountain area, northern Big Horn Mountains, were probably formed from uranium that was leached from ash falls in the area and transported into the karst topography. The intense brecciation of the Precambrian granitoids in the southern Copper Mountain area provided a good trap for the mineralizing fluids derived from volcanic ash and the adjacent uraniferous granites. Tertiary black shales and coals in all the basins commonly have higher than average radioactivity. The peat deposits that eventually formed the coal beds were likely exposed to uranium-enriched surficial or ground water, with the organic matter creating a strong reducing environment that caused precipitation of uranium and thus coal beds with a high content of uranium. For example, within the Great Divide-Green River basin, the more highly uranium enriched coals of the Eocene Wasatch Formation were likely exposed to significant volumes of uranium-enriched fluids (Pipiringos, 1961; Masursky, 1962). In many areas where coal beds or carbonaceous shales are bounded by sandstones that contain oxidation and/or alteration zones, it is common to find as much as a few hundred ppm uranium at the contact with the oxidized sandstone.

At the Lost Creek schroeckingerite deposit and in similar occurrences in the Great Divide basin, the schroeckingerite was deposited by artesian flow of uranium-enriched ground water and its evaporation and crystallization within soils and shallow surficial sediments (Sheridan et al., 1961). In areas of major paleodrainages cutting the central Wyoming Precambrian rocks, older Paleozoic and Mesozoic sedimentary rocks and brecciated Precambrian rocks have zones of alteration and uranium occurrences that have been discovered in many of these rocks. As examples, the Ninemile project contains 4,100 t U3O8 within a roll front in the Cretaceous Teapot Sandstone of the Southwest Powder River basin, just north of Casper, Wyoming (W. Boberg, unpub. data). The Bootheel project in the southeastern Shirley basin contains 2,000 t U3O8 within a roll front in the Jurassic Canyon Springs Sandstone of the Sundance Formation (Underhill and Roscoe, 2009). In the Crooks Gap area, there is a small deposit in a major fault breccia that cuts Precambrian to Tertiary rocks.

Implications for Exploration

As with most deposits, roll-front deposits have several specific exploration characteristics that are related to their formation (Fig. 5). Although not shown in Figure 5, the altered tongue can extend tens of kilometers downdip from the recharge area, where the oxidizing and uranium-enriched surface waters entered the subsurface and became ground water. As a result, roll-front deposits are blind targets for explorationists.

As blind targets, roll-front deposits in Wyoming and elsewhere were subjected, during the uranium boom of the 1970s, to a great deal of research in attempts to locate them utilizing various geochemical and geophysical techniques. Soil gas sampling, both radon and helium, was the primary surface geo-chemical technique that was developed. Various geochemical techniques were attempted for analyses of drill cuttings and drill core, including thermoluminescence studies, the evaluation of uranium in secondary silica, and testing fresh drill cuttings with a redox probe. The surface and drill hole geochemical data generated targets and provided new information to help better understand roll fronts, but seldom, if ever, resulted in the discovery of an economic deposit. Most attempts to use geophysical studies during the uranium boom of the 1970s provided data that had limited usefulness in the actual discovery of a roll-front deposit. Thirty years later, modern developments in digital processing, combined with higher costs of drilling, are resulting in some reevaluation and testing of geophysics as a tool to better define these blind targets.

Exploration for roll-front deposits has the advantage that core drilling is not required. Most programs use rotary mud drilling, which is significantly cheaper than core drilling, because uranium-enriched zones usually can be readily defined by a downhole gamma probe. The low cost of exploration drilling in the Wyoming region during the 1970s boom discouraged the use of geochemical and geophysical approaches, and most uranium explorationists at that time preferred the immediate information from drill data relative to geochemical or geophysical data. Between 1960 and 1981, more than one million holes were drilled in the United States, totaling about 128,000 km of drill hole (U.S. Energy Information Administration, 2010b). Approximately one-third of that drilling was done in Wyoming (W. Boberg, unpub. data). Because almost all present-day work on uranium exploration, during the current resurgence in the Wyoming region, is based on 1970s discoveries and evaluations, there has been little effort to utilize either geochemistry or geophysics to characterize the deposits. Nevertheless, both geochemistry and geophysics should have significant future value considering the greater costs and regulatory requirements for drill programs today compared to the 1970s.

On completion of exploration drilling for roll-front deposits, the hole is probed with a downhole geophysical probe that records natural gamma, spontaneous (self) potential (SP) and resistivity, and the cuttings from the rotary drill hole are described to define the host rock and its alteration. The geophysical log provides the primary means of defining the stratigraphy, using the resistivity and SP curves on the logs, and of determining the uranium content of the drill hole. The calculated content of uranium in the drill hole, using the gamma log, is based on gamma radiation and, as mentioned earlier, is likely somewhat out of equilibrium in comparison to the actual chemical uranium present. There has been little change in the downhole geophysical gamma probe that is used today other than the fact that the data are collected digitally rather than by analog records. One tool that is in relatively common use today, which was not available in the 1970s, is the Prompt Fission Neutron (PFN) tool. This down-hole probe enables direct measurement of chemical uranium by measuring the amount of 235U present. Because 235U is present in natural uranium in a fairly constant ratio to 238U (0.72/99.27%), the content of uranium in a rock unit cut by a borehole can be measured using the PFN tool. The downhole PFN probe generates a pulsed flux of neutrons into the rock surrounding the borehole as the probe is slowly moved through the hole. The generated neutrons cause a fission reaction with any 235U that they encounter and that fission reaction emits characteristic 2MeV prompt fission neutrons which are measured by the PFN probe, enabling a direct determination of uranium.

Although there can be extreme variations in the features of individual roll fronts, the basic roll-front geometry (Fig. 5) is consistently used by uranium exploration geologists to define where a drill hole is located within a roll-front system and to enable planning of additional drill holes to better define the details of the roll front itself. Common gamma log responses at various locations of the roll-front geometry (Fig. 5B, C), as described by geologists, allow experienced roll-front explorationists to determine where a particular drill hole should be located in a roll-front system.

Although there have been no modern paleowind studies, the presence of high mountain ranges during the early Tertiary might have produced a shadow effect on the deposition of airborne volcanic ash, thus resulting in much less ash in some areas than in others, which would have affected the uranium content of surficial fluids. Conversely, the ranges could have acted to build accumulations in certain areas, much like the drifting of snow behind a fence. A suggestion of this can be seen in Figure 9A and B with the isopach of the White River Formation. The thickest parts of the White River are greater than 200 m north and northwest of the Granite Mountains and to the east of the Laramie Range. With the source of the volcanic ash in the Great basin area (Fig. 7) to the southwest, the wind direction had to be from southwest to northeast and a buildup of ash on the leeward side of the mountain ranges could be expected.

The Eocene-Oligocene sedimentation pattern undoubtedly had a major role in controlling the surface-water flow during the Eocene-Oligocene. Any major variation in the pattern of distribution of the ash would have had a direct effect on the ability of a specific area to generate a mineralizing solution. The coarse sediments of the major drainages would accept the greatest surface- to ground-water recharge volumes and any internal impermeable zones or subcrop ridges within a drainage system would redirect flow around them. For that reason, it is important to understand the paleodrainage systems that were prevalent in the region during the period of mineralization because the paleodrainage systems were the pathways for uranium transport. Zielinski (1980) suggested that secondary silica, formed from the dissolution of volcanic ash and thus release of uranium, can provide direct evidence of the migration pattern and history of uranium movement in ancient ground water.

Of major importance to the exploration geologist is the potential extreme variability in the internal stratigraphy and structure of the hosting sandstones and adjacent siltstones and shales. These result in variations in the pattern of the alteration tongue due to changes in internal porosity and permeability that guided the original oxidizing solution and controlled the eventual formation of the roll front. Recognizing and understanding the stratigraphy of a mineralized horizon is vital for the explorationist to properly define the controls on the roll front in a specific stratigraphic unit and any deposits that may occur along the roll front in that stratigraphic unit.

Huge volumes of uranium were released into the interconnected surface- and ground-water systems from both Precambrian granitic highlands and extensive volcanic ash falls during more than 45 m.y. in the Wyoming province. The majority of the uranium carried by surficial waters probably continued out of the region in the rivers. However, a significant volume of uraniferous surface water did recharge ground-water systems, where the ground-water flow would eventually control deposition of the contained uranium. New discoveries should be possible as a more complete understanding of the paleogeohydrology of the province is developed.

Suggestions for Future Research

This overview highlights the fact that much research has been done in the past, which provides a solid basis for understanding the Wyoming uranium province. However, it also suggests a number of areas for future research and these are as follows:

  1. 1.

    Fluid inclusion work, or other thermal chronology work, on uranium ore minerals from the Wyoming province.

  2. 2.

    An evaluation of the provinciality of the source of the accessory metals, vanadium, molybdenum, and selenium, in the various districts in the Wyoming region. Can the source of the accessory minerals be defined and can this assist in future exploration and development work?

  3. 3.

    An evaluation to determine if the nature of the reductant can be defined. This would consider whether or not a deposit formed by reduction due to hydrocarbon leakage could be differentiated from a deposit formed by reduction due to organic carbon and pyrite contained in the sedimentary rock.

  4. 4.

    Paleowind studies of various Tertiary tuffaceous ash-fall sedimentary rocks in an effort to define areas of greater deposition and possible greater potential for source-rock formation.

  5. 5.

    Testing of secondary silica (Zeilinski, 1980, 1982b) as a definition of the pathway of uraniferous solutions that formed uranium deposits.

  6. 6.

    Testing and evaluation of modern geochemical and geophysical techniques for their value in both identifying prospective targets and the potential definition of ore deposits.

Summary

A summary of the geologic history pertinent to the development of the Wyoming uranium province is given in Table 3 and Figure 10. The development of the Wyoming uranium province may have been initiated during the Precambrian with the formation of the uraniferous granites exposed today in the central Wyoming Precambrian. The development of the current basin and range structure during the Laramide orogeny created favorable conditions for formation of the major sedimentary uranium districts of the Wyoming uranium province. During the Eocene through the Oligocene, several coincident factors (Fig. 10) were instrumental in the formation of the major uranium districts of the province. The recently deposited sediments were still unconsolidated and very permeable. Organic matter deposited with the sediments resulted in reducing conditions shortly after deposition. Rainfall was high and the climate was subtropical to savanna, then changing to warm temperate during the Eocene-Oligocene. The Precambrian rocks in the core of the ranges were fully exposed and subsequently deeply weathered and eroded. Volcanism, starting in the Absaroka-Challis region, later shifting to the west in present-day Nevada, Utah, and later Nevada-Oregon and Idaho, deposited extensive layers of airborne ash across the ranges and basins of the region for more than 45 m.y. The volcanic ash, through initial leaching and later devitrification, released significant volumes of uranium to the hydrologic system that transported the uranium-rich surface waters through major drainage systems. These waters entered the ground-water system through permeable zones in outcrop or subcrop, immediately below the major drainage flow paths. High rainfall assured a steady supply of an oxidizing ground water with the capacity to oxidize large volumes of reduced sedimentary rocks, while keeping the uranium in solution until the oxygen was depleted and the uranium was deposited at the maximum extent of the oxidation. This ore-forming process may have taken place multiple times following periods of major ash fall from mid-Eocene through late Pliocene. The young, recently deposited, uncon-solidated sediments of the basins could readily accept large recharge volumes of the ore-forming fluid. Fluid flow through the very permeable young sediments would be relatively fast, thus allowing for the development of large oxidized tongues, as well as scattered uranium deposits at the reduction-oxidation interface, within approximately 1 m.y.

The deposits were formed starting at ca. 40 Ma, with a major period of mineralization between 35 to 24 Ma, but with an additional apparent age of mineralization between 11 to 1.8 Ma. Once the deposits formed, they generally have been well preserved, but some may have undergone minor modification, including downdip movement, if they continued to be located below the water table. It is possible that once the oxidized, altered tongue is formed, subsequent pulses of oxidizing, uranium-rich waters, derived from younger ash falls, could migrate through the altered tongue to the deposit and add new uranium to the preexisting deposit and/or move the deposit farther downdip. In fact, it is entirely possible that this happened multiple times after a deposit was originally formed.

Tuffaceous sedimentary rocks appear to be the primary source for most of the uranium within deposits in the Wyoming uranium province. The major districts within the province all lie within the depositional area of the tuffaceous Oligocene White River Formation and are in locations that appear to be generally defined by the pathways of major paleodrainage systems of the late Eocene and early Oligocene.

Table 3.

Summary of Events in Geologic History which had an Effect in the Development of the Wyoming Uranium Province

Age (m.y. before present)EpochTectonismSurficial activityIgneous activityClimateUranium mobility and deposition
Present–2QuaternaryStable—regional upliftErosionContinued volcanism to westPresent–day aridLimited to oxidation and erosion of existing deposits
2–22.5Pliocene MioceneRelative stability Collapse of central portion of Wyoming (Granite Mountains)Erosion Deposition—major ash-fall depositonContinued volcanism to west, ash–fall depositionTemperateContinued high from ash falls—new deposit creation, some addition to existing deposits; some reworking and destruction by oxidation and erosion
22.5–38OligoceneStable—regional upliftRegional erosion, deposition, and reworking of ash fallsMajor volcanic activity to west, major ash–fall depositionHigher elevation warm temperateContinued high from ash falls and leaching of Precambrian; continued alteration and deposition of uranium
38–55EoceneContinued basin/uplift development. Major thrust and normal faultingContinued erosion of highlands, deposition in basins; Precambrian cover exposed ~55 m.y.Yellowstone–Absoraka volcanism; ash falls over large areasWarm, humid, high rainfall(Savannah?)High from ash falls and leaching of Precambrian granites—development of alteration cells and uranium deposition in porous, permeable rocks
55–65PaleoceneDevelopment of present–day basins and uplifts.Rapid erosion of uplifts, deposition of continental deposits in basinsNoneNear sea level; subtropical, humid, high rainfallLimited
65–1,400CretaceousSeries of regional depressions and upliftsBroad sea coverage and deposition varying with periodic open land and continental depositionNoneVariableLimited
1,400–1,700ProterozoicCollision of plate boundaries to south (Medicine Bow-Sierra Madre)—major structural adjustments and thermal metamorphism in Central Wyoming?Intrusion of granitic rocks along structural zones??Introduction? Remobilization? Improved accessibility
2,500–2,700Archean??Intrusion of large granitic batholiths?Introduction? Remobilization
2,700–3,000Intense deformation??Probable remobilization
>3,000Intense deformationDeposition of sands, shales, and graywackesBasalt, possibly andesite flows??
Age (m.y. before present)EpochTectonismSurficial activityIgneous activityClimateUranium mobility and deposition
Present–2QuaternaryStable—regional upliftErosionContinued volcanism to westPresent–day aridLimited to oxidation and erosion of existing deposits
2–22.5Pliocene MioceneRelative stability Collapse of central portion of Wyoming (Granite Mountains)Erosion Deposition—major ash-fall depositonContinued volcanism to west, ash–fall depositionTemperateContinued high from ash falls—new deposit creation, some addition to existing deposits; some reworking and destruction by oxidation and erosion
22.5–38OligoceneStable—regional upliftRegional erosion, deposition, and reworking of ash fallsMajor volcanic activity to west, major ash–fall depositionHigher elevation warm temperateContinued high from ash falls and leaching of Precambrian; continued alteration and deposition of uranium
38–55EoceneContinued basin/uplift development. Major thrust and normal faultingContinued erosion of highlands, deposition in basins; Precambrian cover exposed ~55 m.y.Yellowstone–Absoraka volcanism; ash falls over large areasWarm, humid, high rainfall(Savannah?)High from ash falls and leaching of Precambrian granites—development of alteration cells and uranium deposition in porous, permeable rocks
55–65PaleoceneDevelopment of present–day basins and uplifts.Rapid erosion of uplifts, deposition of continental deposits in basinsNoneNear sea level; subtropical, humid, high rainfallLimited
65–1,400CretaceousSeries of regional depressions and upliftsBroad sea coverage and deposition varying with periodic open land and continental depositionNoneVariableLimited
1,400–1,700ProterozoicCollision of plate boundaries to south (Medicine Bow-Sierra Madre)—major structural adjustments and thermal metamorphism in Central Wyoming?Intrusion of granitic rocks along structural zones??Introduction? Remobilization? Improved accessibility
2,500–2,700Archean??Intrusion of large granitic batholiths?Introduction? Remobilization
2,700–3,000Intense deformation??Probable remobilization
>3,000Intense deformationDeposition of sands, shales, and graywackesBasalt, possibly andesite flows??

Deposits that have been brought to positions above the water table and into the range of erosion have been severely modified and often totally destroyed. Brecciated zones in any rock type, fractured and karstic limestones, organic shales, peat bogs, and any porous and permeable rock exposed at the surface in the vicinity of a paleodrainage that carried uranium-rich water could host deposits, if they contained sufficient reductant or other chemical capacity to precipitate the uranium. The ability of any of these other environments to serve as areas of anomalous and major recharge, and provide a reducing site, would allow deposits in those environments to be larger and potentially of higher grade. In most cases, these other environments had a limited ability to accept significant fluid volumes and likely accounts for their lack of major deposits.

The formation of the Wyoming uranium province, as presented in this overview, appears to be compatible with currently available data as well as providing explanations for the wide variety of uranium deposits and occurrences in the province. This concept of the formation of the Wyoming uranium province includes:

  1. 1.

    The creation of the ore-forming fluid as surface or near-surface water sourced within widespread tuffaceous ash-fall sedimentary units and/or Precambrian highlands.

  2. 2.

    The transport of the uranium within the pathways of paleodrainage systems, which change over time and interact with zones of significant ground-water recharge.

  3. 3.

    Areas of ground-water recharge, which may be sufficiently permeable to accept large volumes of ore-forming fluid, include recently deposited sedimentary rocks, older permeable strata, or brecciated zones within other rock types.

  4. 4.

    Flow of oxygenated and U-rich ground water forming an oxidized and/or altered tongue within the sedimentary rock and leading to the deposition of the uranium and accessory minerals at the redox front.

  5. 5.

    Repetition of the ore-forming process multiple times and transport of the uranium within newly formed pathways of paleodrainage systems to areas of ground-water recharge, where the ore-forming fluid becomes ground water and is carried to a new redox front or to a preexisting redox front.

This concept can be applied totally, in part, or with modifications to most of the major roll-front uranium provinces of the world to help explain the occurrence and distribution of uranium deposits. The development of this concept accentuates the importance of regional studies and the understanding of the entire geologic framework of a region as related to mineral exploration.

Acknowledgments

The origins of this paper go back to many discussions with many geologists during the 1970s. Among those I would like to single out Dick Fruchey and Ruffin Rackley as those whose advice I sought at that time. Discussions with Fred Groth over the past several years also added to my thoughts concerning the value of paleodrainage as an exploration tool. During my recent research in my attempt to better understand the genesis of the Wyoming uranium province I was aided significantly by correspondence with and discussions with Bob Zielinski of the U.S. Geological Survey. I also need to thank the staff at Ur-Energy for assisting me and providing the support necessary so I had the time to complete this paper. For the figures in this paper I thank Carrie Cisson, Mary Forster, and Mel Lahr of Ur-Energy for creating them. Finally, I would like to thank the SEG editorial team for their patient assistance and encouragement in the completion of this paper. Their comments and suggestions were valuable and aided significantly in making this a better and more useful paper.

References

Adler
,
H.H.
,
1963
,
Concepts of genesis of sandstone-type uranium ore deposits
:
Economic Geology
 , v.
58
, p.
839
852
.
Adler
,
H.H.
,
1964
,
The conceptual uranium ore roll and its significance in uranium exploration
:
Economic Geology
 , v.
59
, p.
46
53
.
Anderson
,
D.C.
,
1969
,
Uranium deposits of the Gas Hills
:
University of Wyoming Contributions to Geology
 , v.
8
, p.
93
103
.
Boberg
,
W.W.
,
1979
,
Applied exploration geology and uranium resources of the Great Divide basin, Wyoming [abs.]
:
American Association of Petroleum Geologists Bulletin
 , v.
63
, no.
5
, p.
822
823
.
Boberg
,
W.W.
,
1981
,
Some speculations on the development of central Wyoming as a uranium province
:
Wyoming Geological Association Annual Field Conference,32nd
 ,
Jackson Hole, Wyoming, September 20-22, 1981, Guidebook
, p.
161
180
.
Bramlett
,
L.B.
Reyer
,
S.L.
Southard
,
G.G.
,
1983
,
Uranium geology and geochemistry, Copper Mountain, Wyoming
:
Society of Mining Engineers of AIME Transactions
  v.
272
, p.
1891
1901
.
Brown
,
W.G.
,
1993
,
Structural style of Laramide basement-cored uplifts and associated folds
:
Geological Survey of Wyoming Memoir 5
 , p.
312
373
.
Cheney
,
E.S.
,
1981
,
The hunt for giant uranium deposits
:
American Scientist
 , v.
69
, p.
37
48
.
Childers
,
M.O.
,
1974
,
Uranium occurrences in Upper Cretaceous and Tertiary strata of Wyoming and northern Colorado
:
Mountain Geologist
 , v.
11
, p.
131
147
.
Collings
,
S.P.
Knode
,
R.H.
,
1984
,
Geology and discovery of the Crow Butte uranium deposit, Dawes County, Nebraska
:
American Institute of Mining Engineers, Practical HydroMet'83, 7h Annual Symposium on Uranium and Precious Metals
 ,
Lakewood, Colorado
,
August 22-24, 1983, Proceedings
, p.
5
14
.
Dahlkamp
,
F.J.
,
1993
,
Uranium ore deposits
 :
Berlin-Heidelberg-New York
,
Springer Verlag
,
460
p.
Davis
,
J.F.
,
1969
,
Uranium deposits in the Powder River basin
:
University of Wyoming Contributions to Geology
 , v.
8
, p.
131
141
.
Denson
,
N.M.
,
1959
,
Uranium in coal in the western United States
:
U.S. Geological Survey Bulletin 1055
 ,
315
p.
Denson
,
N.M.
Gill
,
J.R.
,
1965
,
Uranium bearing lignite and carbonaceous shale in the southwest part of the Williston basin, a regional study
:
U.S. Geological Survey Professional Paper 463
 ,
75
p.
Dooley
,
J.R.
Jr.
Harshman
,
E.N.
Rosholt
,
J.N.
,
1974
,
Uranium-lead ages of the uranium deposits of the Gas Hills and Shirley basin, Wyoming
:
Economic Geology
 , v.
69
, p.
527
581
.
Finch
,
W.I.
,
1996
,
Uranium provinces of North America—their definition, distribution, and models
:
U.S Geological Survey Bulletin 2141
 ,
18
p.
Frost
,
C.D.
Frost
,
B.R.
,
1993
,
The Archean history of the Wyoming province
:
Geological Survey of Wyoming Memoir 5
 , p.
58
77
.
Galloway
,
W.E.
,
1977
,
Catahoula formation of the Texas coastal plain: Depositional systems, composition, structural development, ground water flow history and uranium distribution
:
University of Texas Bureau of Economic Geology Report of Investigation 87
 ,
59
p.
Galloway
,
W.E.
,
1978
,
Uranium mineralization in a coastal plain fluvial system: Catahoula Formation, Texas
:
Economic Geology
 , v.
73
, p.
1655
1673
.
Gjelsteen
,
T.W.
Collings
,
S.P.
,
1988
,
Relationship between ground water flow and uranium mineralization in the Chadron Formation, northwest Nebraska
:
Wyoming Geological Association Annual Field Conference
 ,
39h, Casper, Wyoming, September 9-11, 1988, Guidebook
, p.
271
284
.
Granger
,
H.C.
Warren
,
C.G.
,
1969
,
Unstable sulfur compounds and the origin of roll-type uranium deposits
:
Economic Geology
 , v.
64
, p.
160
171
.
Granger
,
H.C.
Warren
,
C.G.
,
1978
,
Some speculations on the genetic geochemistry and hydrology of roll-type uranium deposits
:
Wyoming Geological Association Annual Field Conference, Guidebook, 30th
 , p.
249
361
.
Gruner
,
J.W.
,
1956
,
Concentration of uranium in sediments by multiple migration-accretion
:
Economic Geology
 , v.
51
, p.
495
520
.
Harris
,
R.E.
King
,
J.K.
,
1993
,
Geologic classification and origin of radioactive mineralization in Wyoming
:
Geological Survey of Wyoming Memoir 5
 , p.
898
916
.
Harshman
,
E.N.
,
1972
,
Geology and uranium deposits, Shirley basin area, Wyoming
:
U.S. Geological Survey Professional Paper 745
 ,
82
p.
Harshman
,
E.N.
,
1974
,
Distribution of elements in some roll-type uranium deposits
:
Formation of uranium deposits
 :
Vienna
,
International Atomic Energy Agency
, p.
169
183
.
Harshman
,
E.N.
Adams
,
S.S.
,
1981
,
Geology and recognition criteria for roll-type uranium deposits in continental sandstones
:
U.S. Department of Energy GJBX-1
  (
81
).
Hills
,
F.A.
Houston
,
R.S.
,
1979
,
Early Proterozoic tectonics of the central Rocky Mountains, North America
:
University of Wyoming Contributions to Geology
 , v.
17
, p.
89
109
.
Hodson
,
W.G.
Pearl
,
R.H.
Druse
,
S.A.
,
1973
,
Water resources of the Powder River basin and adjacent areas, northeastern Wyoming
:
U.S. Geological Survey Hydrologic Investigation Atlas HA-465
 ,
4 sheets
.
Hostetler
,
P.B.
Garrels
,
R.M.
,
1962
,
Transportation and precipitation of uranium and vanadium at low temperatures with special reference to sandstone type uranium deposits
:
Economic Geology
 , v.
57
, p.
157
167
.
Klingmuller
,
L.M.L.
,
1989
,
The Green Mountain uranium district, central Wyoming
:
Type locality of solution front limb deposits, in Uranium resources of North America
 :
Vienna
,
International Atomic Energy Agency
, p.
173
190
.
Kyser
,
K.
Cuney
,
M.
,
2008
,
Recent and not-so-recent developments in uranium deposits and implications for exploration
:
Mineralogical Association of Canada Short Course Series
  Volume
39
,
271
p. (
plus CD
).
Larsen
,
E.E.
Evanoff
,
E
,
1998
,
Tephrostratigraphy and source of the tuffs of the White River Sequence
:
Geological Society of America Special Paper 325
 , p.
1
14
.
Lillegraven
,
J.A.
,
2010
,
Fools rush in—attempting revision of Wyoming's Paleogene topographic evolution
:
Wyoming Geological Association Luncheon Presentation
 
January 8, 2010
. p.
2
, (www.wyogeo.org/docs/jan_10.pdf).
Lillegraven
,
J.A.
Ostresh
,
L.M.
Jr.
,
1988
,
Evolution of Wyoming's early Cenozoic topography and drainage patterns
:
National Geographic Research
 , v.
4
, p.
303
327
.
Lowry
,
M.E.
Rucker
,
S.J.
IV
Wahl
,
K.L.
,
1973
,
Water resources of the Laramie, Shirley, Hanna basins and adjacent areas, southeastern Wyoming
:
U.S. Geological Survey Hydrologic Investigation Atlas HA-471
 ,
4 sheets
.
Lowry
,
M.E.
Lowham
,
H.W.
Lines
,
G.C.
,
1976
,
Water resources of the Bighorn basin, northwestern Wyoming
:
U.S. Geological Survey Hydrologic Investigations Atlas HA-512
 ,
2 sheets
.
Love
,
J.D.
,
1970
,
Cenozoic geology of the Granite Mountains area, central Wyoming
:
U.S. Geological Survey Professional Paper 495-C
 , p.
C1
-
C154
.
Ludwig
,
K.R.
,
1978
,
Uranium daughter migration and U-Pb isotope apparent ages of uranium ores, Shirley basin, Wyoming
:
Economic Geology
 , v.
73
, p.
29
49
.
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
:
Economic Geology
 , v.
74
, p.
1654
1668
.
Masursky
,
H.
,
1962
,
Uranium-bearing coal in the eastern part of the Red Desert area, Wyoming
:
U.S. Geological Survey Bulletin 1099-B
 , p.
B1
-
B152
.
McEldowney
,
R.C.
Abshier
,
J.F.
Lootens
,
D.J.
,
1977
,
Geology of uranium deposits in the Madison Limestone, Little Mountain area, Big Horn County, Wyoming
:
Rocky Mountain Association of Geologists Symposium
 ,
Snowmass, Colorado
,
September 29, 1977, Guidebook
, p.
321
336
.
Murphy
,
E.C.
,
2007
,
Uranium deposits in southwestern North Dakota
:
North Dakota Geological Survey Geologic Investigations 40
 ,
map sheet with text
.
Nkomo
,
I.T.
Stuckless
,
J.R.
Thaden
,
R.E.
Rosholt
,
J.M.
,
1978
,
Petrology and uranium-mobility of a granite of early Precambrian age from the Owl Creek Mountains, Wyoming
:
Wyoming Geological Association Annual Field Conference, 30h
 ,
Casper, Wyoming
,
September, 1978, Guidebook
, p.
335
348
.
Nkomo
,
I.T.
Rosholt
,
J.N.
Dooley
,
J.R.
Jr.
,
1979
,
U-Th-Pb systematics of Precambrian rocks in the Laramie Mountains, Wyoming
:
Earth Science Bulletin
 , v.
12
, p.
1
14
.
Peterman
,
Z.E.
Hildreth
,
R.A.
,
1977
,
Reconnaissance geology of the Precambrian of the Granite Mountains, Wyoming
:
U.S. Geological Survey Open-File Report 77-140
 ,
86
p.
Pipiringos
,
G.N.
,
1961
,
Uranium-bearing coal in the central part of the Great Divide basin
:
U.S. Geological Survey Bulletin 1099-A
 , p.
Al
-
A104
.
Pool
,
T.C.
,
2007
,
Technical report on the Shirley basin uranium properties, Wyoming
:
Canadian National Instrument 43-101 Report prepared for Target Mining and Exploration Corporation
  (http://www.crossairexploration.com/i/pdf/B43-101July82007.pdf).
Rackley
,
R.I.
,
1972
,
Environment of Wyoming Tertiary uranium deposits
:
American Association of Petroleum Geologists Bulletin
 , v.
56
, p.
755
774
.
Rackley
,
R.I.
,
1976
,
Origin of western-state type uranium mineralization
,
in
Wolf
,
K.
, ed.,
Handbook of strata-bound and stratiform ore deposits
 :
Amsterdam
,
Elsevier Publishing
, v.
7
, p.
89
156
.
Reade
,
H.L.
,
1976
,
Grover uranium deposits: A case history of uranium exploration in the Denver basin, Colorado
:
Mountain Geologist
 , v.
13
, p.
21
31
.
Reade
,
H.L.
,
1978
,
Uranium deposits: Northern Denver Julesburg basin, Colorado
:
Rocky Mountain Association of Geologists Field Symposium Guidebook
 , p.
161
171
.
Renfro
,
A.R.
,
1969
,
Uranium deposits in the Lower Cretaceous of the Black Hills
:
University of Wyoming Contributions to Geology
 , v.
8
, p.
87
92
.
Rogers
,
J.J.W.
Adams
,
J.A.S.
,
1969
,
Uranium
,
in
Wedepohl
,
K.H.
, ed.,
Handbook of geochemistry
 :
Berlin
,
Springer Verlag
, v.
2
, p.
92B
-
92O
.
Rosholt
,
J.N.
Zartman
,
R.E.
Nkomo
,
I.T.
,
1973
,
Lead isotope systematics and uranium depletion in the Granite Mountains, Wyoming
:
Geological Society of America Bulletin
 , v.
84
, p.
989
1002
.
Rubin
,
B.
,
1970
,
Uranium roll zonation in the southern Powder River basin, Wyoming
:
Earth Science Bulletin
 , v.
3
, p.
5
12
.
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
:
Economic Geology
 , v.
78
, p.
498
501
.
Seeland
,
D.A.
,
1976
,
Relationships between early Tertiary sedimentation patterns and uranium mineralization in the Powder River basin, Wyoming
:
Wyoming Geological Association Annual Field Conference, 28th, Guidebook
 , p.
53
64
.
Seeland
,
D.A.
,
1978
,
Eocene fluvial drainage patterns and their implications for uranium and hydrocarbon exploration in the Wind River basin, Wyoming
:
U.S. Geological Survey Bulletin 1446
 ,
21
p.
Sharp
,
W.N.
Gibbons
,
A.B.
,
1964
,
Geology and uranium deposits of the southern part of the Powder River basin, Wyoming
:
U.S. Geological Survey Bulletin 1147-D
 , p.
1
60
Sharp
,
W.N.
McKay
,
E.J.
McKeown
,
F.A.
White
,
A.M.
,
1964
,
Geology and uranium deposits of the Pumpkin Buttes area of the Powder River basin, Wyoming
:
U.S. Geological Survey Bulletin 1107-H
 , p.
541
638
.
Sheridan
,
D.M.
Maxwell
,
C.H.
Collier
,
J.T.
,
1961
,
Geology of the Lost Creek schroeckingerite deposits, Sweetwater, County, Wyoming
:
U.S. Geological Survey Bulletin 1087-J
 , p.
391
478
.
Shrier
,
T.
Parry
,
W.T.
,
1982
,
A hydrothermal model for the North Canning uranium deposit, Owl Creek Mountains, Wyoming
:
Economic Geology
 , v.
77
, p.
632
645
.
Smith
,
D.B.
Zielinski
,
R.A.
Rose
,
W.I.
Jr.
,
1981
,
Leachability of uranium and other elements from freshly erupted volcanic ash
:
U.S. Geological Survey Open-File Report 81-118
 ,
95
p.
Smith
,
D.B.
Zielinski
,
R.A.
Rose
,
W.I.
Jr.
,
1982a
,
Leachability of uranium and other elements from freshly erupted volcanic ash
:
Journal of Volcanology and Geothermal Research
 , v.
13
, p.
1
30
.
Smith
,
D.B.
Zielinski
,
R.A.
Rose
,
W.I.
Heubert
,
B.J.
,
1982b
,
Water soluble material on aerosols collected within volcanic eruption clouds
:
Journal of Geophysical Research
 , v.
87
, p.
4963
4972
.
Snoke
,
A.W.
,
1993
,
Geologic history of Wyoming within the tectonic framework of the North American Cordillera
:
Geological Survey of Wyoming Memoir 5
 , p.
2
57
.
Snoke
,
A.W.
Steidtman
,
J.R.
Roberts
,
S.M.
,
1993
,
Geology of Wyoming
:
Geological Survey of Wyoming Memoir 5
 ,
937
p.
Stuckless
,
J.R.
Miesch
,
A.T.
,
1981
,
Petrogenetic modeling of a potential uranium source rock, Granite Mountains, Wyoming
:
U.S. Geological Survey Professional Paper 1225
 ,
27
p.
Stuckless
,
J.R.
Nkomo
,
I.T.
,
1978
,
Uranium-lead isotope systematics in uraniferous alkali-rich granites from the Granite Mountains, Wyoming—implications for uranium source rocks
:
Economic Geology
 , v.
73
, p.
427
441
.
Stuckless
,
J.R.
Nkomo
,
I.T.
,
1980
,
Preliminary investigations of U-Th-Pb systematics in uranium-bearing minerals from two granitic rocks from the Granite Mountains, Wyoming
:
Economic Geology
 , v.
75
, p.
289
295
.
Stuckless
,
J.R.
Van Trump
,
G.
Jr.
Bunker
,
C.M.
Bush
,
C.A.
Hunter
,
W.C.
Lewis
,
N.F.
Jr.
,
1976
,
Radiometric and petrographic results for samples from drill holes GM1 and GM2, Granite Mountains, Wyoming
:
U.S. Geological Survey Open-File Report 76-842
 ,
19
p.
Stuckless
,
J.R.
Bunker
,
C.M.
Bush
,
C.A.
Doering
,
W.P.
Scott
,
J.
,
1977
,
Geochemical and petrologic studies of uraniferous granite from the Granite Mountains, Wyoming
:
U.S. Geological Survey Journal of Research
 , v.
5
, p.
61
81
.
Stuckless
,
J.R.
Bunker
,
C.M.
Bush
,
C.A.
Van Trump
,
G.
Jr.
,
1981
,
Radioelement concentration in Archean granites of central Wyoming
:
U.S. Geological Survey Open-File Report 81-948
 ,
40
p.
Taylor
,
P.S.
,
1969
,
Soluble material on volcanic ash
:
Unpublished M.S. thesis
 ,
Hanover, NH
,
Dartmouth College
,
77
p.
Terry
,
D.O.
,
1998
,
Lithostratigraphic revision and correlation of the lower part of the White River Group: South Dakota to Nebraska
:
Geological Society of America Special Paper 325
 , p.
15
37
.
Underhill
,
D.H.
Roscoe
,
W.E.
,
2009
,
Technical report on the Bootheel uranium property, Shirley basin mining district, Albany County, Wyoming, U.S.A.
:
Scott Wilson Roscoe Postle Associates Inc., available from Crosshair Exploration and Mining Corp
,
Vancouver, BC
,
Canada Report NI 43-101
 ,
128
p. (http://www.crosshairexploration.com/i/pdf/BH43”101_June10_2010.pdf).
U.S. Energy Information Administration
,
2010a
,
Table 1.
U.S. Forward-Cost Uranium Reserves by State
 , Year-End 2008: (http://www.eia.doe.gov/cneaf/nuclear/page/reserves/ures.html)
U.S. Energy Information Administration
,
2010b
,
Table 4.
12 Uranium exploration and development drilling
 , selected years, 1949-2008: (http://www.eia.doe.gov/emeu/aer/pdf/pages/sec4_25.pdf).
UxC Consulting
,
2009
,
UxC historical Ux price charts
 :
UxC Consulting Company
(http://www.uxc.com).
Walton
,
A.W.
Galloway
,
W.E.
Henry
,
C.D.
,
1981
,
Release of uranium from volcanic glass in sedimentary sequences—an analysis of two systems
:
Economic Geology
 , v.
76
, p.
69
88
.
Welder
,
G.E.
,
1968
Ground-water reconnaissance of the Green River basin, southwestern Wyoming
:
U.S. Geological Survey Hydrologic Investigations Atlas HA-290
 ,
2 sheets accompanying text
,
5
p.
Whitcomb
,
H.A.
Lowry
,
M.E.
,
1968
,
Ground-water resources and geology of the Wind River basin area, central Wyoming
:
U.S. Geological Survey Hydrologic Investigations Atlas HA-270
 ,
3 sheets, accompanying text
,
13
p.
Yellich
,
J.A.
Cramer
,
R.T.
Kendall
,
R.G.
,
1978
,
Copper Mountain, Wyoming, uranium deposit rediscovered
:
Wyoming Geological Association Annual Field Conference, 30th
 ,
Casper, Wyoming
,
September 1978, Guidebook
, p.
311
327
.
Zielinski
,
R.A.
,
1980
,
Uranium in secondary silica: A possible exploration guide
:
Economic Geology
 , v.
75
, p.
592
602
.
Zielinski
,
R.A.
,
1982a
,
The mobility of uranium and other elements during alteration of rhyolite ash to montmorillonite: A case study in the Troublesome Formation, Colorado, U.S.A.
:
Chemical Geology
 , v.
35
, p.
185
204
.
Zielinski
,
R.A.
,
1982b
,
Uraniferous opal, Virgin Valley, Nevada: Conditions of formation and implications for uranium exploration
:
Journal of Geochemical Exploration
 , v.
18
, p.
197
216
.
Zielinski
,
R.A.
,
1983
,
Tuffaceous sediments as source rocks for uranium: a case study of the White River Formation, Wyoming
:
Journal of Geochemical Exploration
 , v.
18
, p.
285
306
.
Zielinski
,
R.A.
,
1985
,
Element mobility during alteration of silicic ash to kaolinite a study of tonstein
:
Sedimentology
 , v.
32
, p.
567
579
.

Figures & Tables

Fig. 1.

Index map of the western United States, showing the location of the Wyoming uranium province as used in this paper. The outline of the geologic uranium occurrence map (Fig. 4) is shown.

Fig. 1.

Index map of the western United States, showing the location of the Wyoming uranium province as used in this paper. The outline of the geologic uranium occurrence map (Fig. 4) is shown.

Fig. 2.

Chart demonstrating the relationship between publications on uranium deposits and the spot price of uranium. The number of papers on uranium deposits is taken as a reflection of the funding of research on uranium deposits and there is a clear relationship between the number of publications from the mid-1970s to the early 1990s and the high uranium spot price of the late 1970s. The publication of papers from the mid-1980s to the early 1990s is a reflection of the delay between the initiation of a research project and publication of results. In addition, more research through that period from the mid-1980s was directed toward the high-grade unconformity-style uranium deposits. The peaks in publications through the 1990s reflect results released from the former Soviet Union and additional work from Asian researchers. This chart is modified from Kyser and Cuney (2008), adding the current 2007 US dollar spot price data from UxC consulting (2010) and the 1960 to 2008 United States drilling statistics (U.S. Department of Energy, 2010b).

Fig. 2.

Chart demonstrating the relationship between publications on uranium deposits and the spot price of uranium. The number of papers on uranium deposits is taken as a reflection of the funding of research on uranium deposits and there is a clear relationship between the number of publications from the mid-1970s to the early 1990s and the high uranium spot price of the late 1970s. The publication of papers from the mid-1980s to the early 1990s is a reflection of the delay between the initiation of a research project and publication of results. In addition, more research through that period from the mid-1980s was directed toward the high-grade unconformity-style uranium deposits. The peaks in publications through the 1990s reflect results released from the former Soviet Union and additional work from Asian researchers. This chart is modified from Kyser and Cuney (2008), adding the current 2007 US dollar spot price data from UxC consulting (2010) and the 1960 to 2008 United States drilling statistics (U.S. Department of Energy, 2010b).

Fig. 3.

Generalized stratigraphic column of the Wyoming uranium province. The black dots within the lithologic symbol for the formations demonstrate known uranium occurrences within that formation or rock unit. The small dots define occurrences, whereas the larger dots demonstrate the units that have produced significant amounts of uranium, the size of the larger dots demonstrating relative production differences.

Fig. 3.

Generalized stratigraphic column of the Wyoming uranium province. The black dots within the lithologic symbol for the formations demonstrate known uranium occurrences within that formation or rock unit. The small dots define occurrences, whereas the larger dots demonstrate the units that have produced significant amounts of uranium, the size of the larger dots demonstrating relative production differences.

Fig. 4.

Geologic and uranium occurrence map of the Wyoming uranium province. Location shown in index map of Figure 1. This map demonstrates the basic basin and range development of the Wyoming uranium province, as well as the general location of major alteration tongues associated with roll-front development in the major districts and a few deposits that are not associated with roll-front development (modified from Boberg, 1981).

Fig. 4.

Geologic and uranium occurrence map of the Wyoming uranium province. Location shown in index map of Figure 1. This map demonstrates the basic basin and range development of the Wyoming uranium province, as well as the general location of major alteration tongues associated with roll-front development in the major districts and a few deposits that are not associated with roll-front development (modified from Boberg, 1981).

Fig. 5.

Generalized diagram of a uranium roll-front deposit that defines changes in mineralogy from the altered and/or oxidized tongue, across the front and into unaltered and/or reduced sand. It also demonstrates the general gamma log character expected from a geophysical probe log run in drill holes that test the front at specific locations across the front. A. Uranium disequilibrium—"—" indicates that chemical uranium will generally be less than that calculated from a gamma log and "+" indicates that chemical uranium will generally be greater than that calculated from a gamma log for drill holes at these locations within the roll front. B. Roll-front geometry—terms commonly used by uranium explorationists to define general position with regard to the roll front. C. Gamma log character—graphs demonstrating actual gamma log character at various locations. D. Visible alteration—a demonstration of the common alteration characteristics, commonly demonstrated by color, of a roll-front system. E. Mineralogy—a general demonstration of the presence and/or absence of common minerals that are subject to alteration and/or oxidation and precipitation by reduction in a roll-front system. F. Metals—a general demonstration of the location of accessory metals commonly associated with a uranium roll front. Modified from Rubin (1970), Harris and King (1993), W. Boberg (unpub. data).

Fig. 5.

Generalized diagram of a uranium roll-front deposit that defines changes in mineralogy from the altered and/or oxidized tongue, across the front and into unaltered and/or reduced sand. It also demonstrates the general gamma log character expected from a geophysical probe log run in drill holes that test the front at specific locations across the front. A. Uranium disequilibrium—"—" indicates that chemical uranium will generally be less than that calculated from a gamma log and "+" indicates that chemical uranium will generally be greater than that calculated from a gamma log for drill holes at these locations within the roll front. B. Roll-front geometry—terms commonly used by uranium explorationists to define general position with regard to the roll front. C. Gamma log character—graphs demonstrating actual gamma log character at various locations. D. Visible alteration—a demonstration of the common alteration characteristics, commonly demonstrated by color, of a roll-front system. E. Mineralogy—a general demonstration of the presence and/or absence of common minerals that are subject to alteration and/or oxidation and precipitation by reduction in a roll-front system. F. Metals—a general demonstration of the location of accessory metals commonly associated with a uranium roll front. Modified from Rubin (1970), Harris and King (1993), W. Boberg (unpub. data).

Fig. 6.

Early Tertiary paleodrainage patterns demonstrating the change in drainage systems over time from Late Cretaceous through early Oligocene. The paleodrainage systems of late Eocene and early Oligocene were likely the pathways along which uranium-enriched surficial and shallow ground waters moved prior to entering ground-water recharge zones of permeable rocks and becoming the oxidizing ground water that formed the major altered and/or oxidized tongues and associated uranium deposits. Each map covers a specific geologic time. A. Late Cretaceous (Lancian). B. Early Paleocene (Puercan). C. Early Eocene (Early Wasatchian). D. Middle Eocene (Late Wasatchian-Bridgerian). E. Late Eocene (Early Uintan). F. Early Oligocene (Early Chadron). Note how the drainage patterns change over time. The development of blocked drainage during middle Eocene resulted in the development of several lakes in the region. Modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

Fig. 6.

Early Tertiary paleodrainage patterns demonstrating the change in drainage systems over time from Late Cretaceous through early Oligocene. The paleodrainage systems of late Eocene and early Oligocene were likely the pathways along which uranium-enriched surficial and shallow ground waters moved prior to entering ground-water recharge zones of permeable rocks and becoming the oxidizing ground water that formed the major altered and/or oxidized tongues and associated uranium deposits. Each map covers a specific geologic time. A. Late Cretaceous (Lancian). B. Early Paleocene (Puercan). C. Early Eocene (Early Wasatchian). D. Middle Eocene (Late Wasatchian-Bridgerian). E. Late Eocene (Early Uintan). F. Early Oligocene (Early Chadron). Note how the drainage patterns change over time. The development of blocked drainage during middle Eocene resulted in the development of several lakes in the region. Modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

Fig. 7.

Western United States Tertiary volcanic activity. The black diamonds in Idaho and Wyoming show the areas of the Absaroka-Challis volcanism that occurred generally in the time frame 52 to 38 Ma, with intense volcanic activity lasting from about 52 to 48 Ma. The black dots define areas of major volcanic activity in the Great Basin area of Nevada and Utah from 37 to 30 Ma and that appear to be the source of the extensive deposition of the Oligocene White River Formation (Larsen and Evanoff, 1998). The outline of the White River Formation tuffaceous sediment depositional area is shown covering much of Wyoming, South Dakota, Nebraska, and adjacent states. The black square in southwestern Utah shows the general location of the Marysvale volcanic field, which was active from 32 to 22 Ma. The black triangles in northern Nevada and northwestern Wyoming connected by the black dashed line demonstrate the position of the Yellowstone hot spot (Yel), which initiated major volcanic activity along the Nevada-Oregon border area about 16.5 Ma. Crustal plate movement caused volcanic activity to track northeasterly, with volcanism occurring near the McDermitt caldera (McD) straddling the Nevada-Oregon border from 16.5 to about 10 Ma, volcanism on the Snake River Plain (SRP) from about 10 to 5 Ma and, more recently, the volcanism in the Yellowstone (Yel) region starting about 2 Ma and continuing to present. The outline of the Powder River basin (PRB) in northeastern Wyoming is the area discussed under "Ash-fall tuffs as a source of the uranium." Map modified from Larsen and Evanoff (1998).

Fig. 7.

Western United States Tertiary volcanic activity. The black diamonds in Idaho and Wyoming show the areas of the Absaroka-Challis volcanism that occurred generally in the time frame 52 to 38 Ma, with intense volcanic activity lasting from about 52 to 48 Ma. The black dots define areas of major volcanic activity in the Great Basin area of Nevada and Utah from 37 to 30 Ma and that appear to be the source of the extensive deposition of the Oligocene White River Formation (Larsen and Evanoff, 1998). The outline of the White River Formation tuffaceous sediment depositional area is shown covering much of Wyoming, South Dakota, Nebraska, and adjacent states. The black square in southwestern Utah shows the general location of the Marysvale volcanic field, which was active from 32 to 22 Ma. The black triangles in northern Nevada and northwestern Wyoming connected by the black dashed line demonstrate the position of the Yellowstone hot spot (Yel), which initiated major volcanic activity along the Nevada-Oregon border area about 16.5 Ma. Crustal plate movement caused volcanic activity to track northeasterly, with volcanism occurring near the McDermitt caldera (McD) straddling the Nevada-Oregon border from 16.5 to about 10 Ma, volcanism on the Snake River Plain (SRP) from about 10 to 5 Ma and, more recently, the volcanism in the Yellowstone (Yel) region starting about 2 Ma and continuing to present. The outline of the Powder River basin (PRB) in northeastern Wyoming is the area discussed under "Ash-fall tuffs as a source of the uranium." Map modified from Larsen and Evanoff (1998).

Fig. 8.

A chart of ages of the Wyoming uranium deposits. Shirley basin dates from Dooley et al. (1974) and Ludwig (1978), Crooks Gap dates from Ludwig (1979), Gas Hills dates from Dooley et al. (1974) and Ludwig (1979), and Powder River basin dates from Santos and Ludwig (1983).

Fig. 8.

A chart of ages of the Wyoming uranium deposits. Shirley basin dates from Dooley et al. (1974) and Ludwig (1978), Crooks Gap dates from Ludwig (1979), Gas Hills dates from Dooley et al. (1974) and Ludwig (1979), and Powder River basin dates from Santos and Ludwig (1983).

Fig. 9.

Geologic-uranium occurrence map (Fig. 4) with paleodrainage patterns (blue) superimposed. Geology legend is the same as in Figure 4, but it has been removed from these maps to simplify them. Contour lines overlain in both maps are thicknesses (m) of the tuffaceous Oligocene White River Formation (Larsen and Evanoff, 1998). A. Late Eocene pale-odrainage systems demonstrating the general coincidence of the paleodrainage systems of late Eocene to the major alteration and/or oxidation tongues in the major basins. B. Early Oligocene paleodrainage systems demonstrating how the shifting of drainage systems to the east in early Oligocene provided a means of delivering uranium-enriched surface waters to western Nebraska to enable formation of the Crow Butte deposits. Modified from Boberg (1981), drainage areas modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

Fig. 9.

Geologic-uranium occurrence map (Fig. 4) with paleodrainage patterns (blue) superimposed. Geology legend is the same as in Figure 4, but it has been removed from these maps to simplify them. Contour lines overlain in both maps are thicknesses (m) of the tuffaceous Oligocene White River Formation (Larsen and Evanoff, 1998). A. Late Eocene pale-odrainage systems demonstrating the general coincidence of the paleodrainage systems of late Eocene to the major alteration and/or oxidation tongues in the major basins. B. Early Oligocene paleodrainage systems demonstrating how the shifting of drainage systems to the east in early Oligocene provided a means of delivering uranium-enriched surface waters to western Nebraska to enable formation of the Crow Butte deposits. Modified from Boberg (1981), drainage areas modified from Lillegraven and Ostresh (1988) and Lillegraven (2010).

Fig. 10.

Coincident factors demonstrating the various events during the Tertiary that affected the development of uranium deposits throughout the Wyoming uranium province. The width of a bar generally defines greater or lesser amount or level of activity. The variation in the width of the bars at the top defining the permeability of sediments is for the thicker part to suggest the greater permeability of unconsolidated, early burial, sediments with the thinning of the bar demonstrating the loss of permeability over time as diagenesis and compaction takes place. K = Cretaceous, Pl = Pliocene, Q = Quaternary. Bar defining volcanic ash fall: Absaroka-Challis = Absaroka-Challis volcanism of northwest Wyoming and central Idaho, UT-NV = volcanism of the Great Basin area of Utah and Nevada that appears to be the source of the Oligocene White River Formation, Marysvale = Marsyvale volcanism of Utah, McD = McDermitt caldera, SRP = Snake River Plain, and Yel = Yellowstone, these three defining the general track of the Yellowstone hot spot. Bar defining mineralization (the age range from Fig. 8 is shown: CG = Crooks Gap, GH = Gas Hills, PRB = Powder River basin, SB-Shirley basin. Modified from Boberg (1981).

Fig. 10.

Coincident factors demonstrating the various events during the Tertiary that affected the development of uranium deposits throughout the Wyoming uranium province. The width of a bar generally defines greater or lesser amount or level of activity. The variation in the width of the bars at the top defining the permeability of sediments is for the thicker part to suggest the greater permeability of unconsolidated, early burial, sediments with the thinning of the bar demonstrating the loss of permeability over time as diagenesis and compaction takes place. K = Cretaceous, Pl = Pliocene, Q = Quaternary. Bar defining volcanic ash fall: Absaroka-Challis = Absaroka-Challis volcanism of northwest Wyoming and central Idaho, UT-NV = volcanism of the Great Basin area of Utah and Nevada that appears to be the source of the Oligocene White River Formation, Marysvale = Marsyvale volcanism of Utah, McD = McDermitt caldera, SRP = Snake River Plain, and Yel = Yellowstone, these three defining the general track of the Yellowstone hot spot. Bar defining mineralization (the age range from Fig. 8 is shown: CG = Crooks Gap, GH = Gas Hills, PRB = Powder River basin, SB-Shirley basin. Modified from Boberg (1981).

Table 1.

Thickness of Tertiary Sediments in Wyoming Basins1

Maximum thickness of sediments
BasinPaleocene-Eocene (m)Oligocene-Pliocene (m)
Powder River1,500300
Big Horn2,6002,000
Wind River5,2001,500
Shirley200300
Great Divide3,000600
Green River2,700600
Maximum thickness of sediments
BasinPaleocene-Eocene (m)Oligocene-Pliocene (m)
Powder River1,500300
Big Horn2,6002,000
Wind River5,2001,500
Shirley200300
Great Divide3,000600
Green River2,700600
1

Sources: Welder (1968), Whitcomb and Lowry (1968), Hodson et al. (1973), Lowry et al. (1973, 1976), Boberg (unpub. data)

Table 2.

Summary of a Possible Mineralizing Fluid and the Altered Tongue and Deposit that It could Form (modified from Granger and Warren, 1978)

Parameters of the deposit to be formed
Distance over which ore-stage pyrite redeposition is essentially complete= 60 m
Distance over which uranium deposition is essentially completed= 27 m
Width of ore-grade uranium (>0.1% U3O8)= 10 m
Average grade of uranium deposit over 27-m width= 0.28% U3O8
Maximum content
Uranium in ore= 3.0%
Ore-stage pyrite= 2.0%
Prestage selenium= 0.1%
Distance between visible reduction-oxidation interface and
Maximum pyrite content= 30 cm
Maximum uranium content= 2 m
Parameters of the host rock
Preore pyrite content= 1.0%
Bulk specific gravity= 2.0
Porosity= 20%
Hydraulic conductivity= 40 m/d
Dip of strata= 5.7 m/km
Hydraulic gradient= 3.8 m/km
Parameters of the oxidized tongue
Length of oxidized tongue parallel to ground-water flow= 10 km
Parameters of the ore-forming ground-water solution
These parameters reflect the ore-forming solution as it approaches close to the reduction-oxidation interface
Uranium content of ground water= 50 ppb
Oxygen content of ground water= 5 ppm
Bulk flow rate of ground-water solution= 50 m3/yr through 1 m2
Velocity of ground-water solution= 290 m/yr
Parameters of roll-front movement and deposit formation
Rate of advance of roll front (advance of reduction-oxidation front)= 1.4 cm/yr(1 km/70,000 yrs)
Maximum time required to form above defined deposit
A 10-km-long altered/oxidized tongue formation requires= 700,000 yrs
The uranium deposit forms at the reduction-oxidation front= 50,000 yrs
Parameters of the deposit to be formed
Distance over which ore-stage pyrite redeposition is essentially complete= 60 m
Distance over which uranium deposition is essentially completed= 27 m
Width of ore-grade uranium (>0.1% U3O8)= 10 m
Average grade of uranium deposit over 27-m width= 0.28% U3O8
Maximum content
Uranium in ore= 3.0%
Ore-stage pyrite= 2.0%
Prestage selenium= 0.1%
Distance between visible reduction-oxidation interface and
Maximum pyrite content= 30 cm
Maximum uranium content= 2 m
Parameters of the host rock
Preore pyrite content= 1.0%
Bulk specific gravity= 2.0
Porosity= 20%
Hydraulic conductivity= 40 m/d
Dip of strata= 5.7 m/km
Hydraulic gradient= 3.8 m/km
Parameters of the oxidized tongue
Length of oxidized tongue parallel to ground-water flow= 10 km
Parameters of the ore-forming ground-water solution
These parameters reflect the ore-forming solution as it approaches close to the reduction-oxidation interface
Uranium content of ground water= 50 ppb
Oxygen content of ground water= 5 ppm
Bulk flow rate of ground-water solution= 50 m3/yr through 1 m2
Velocity of ground-water solution= 290 m/yr
Parameters of roll-front movement and deposit formation
Rate of advance of roll front (advance of reduction-oxidation front)= 1.4 cm/yr(1 km/70,000 yrs)
Maximum time required to form above defined deposit
A 10-km-long altered/oxidized tongue formation requires= 700,000 yrs
The uranium deposit forms at the reduction-oxidation front= 50,000 yrs
Table 3.

Summary of Events in Geologic History which had an Effect in the Development of the Wyoming Uranium Province

Age (m.y. before present)EpochTectonismSurficial activityIgneous activityClimateUranium mobility and deposition
Present–2QuaternaryStable—regional upliftErosionContinued volcanism to westPresent–day aridLimited to oxidation and erosion of existing deposits
2–22.5Pliocene MioceneRelative stability Collapse of central portion of Wyoming (Granite Mountains)Erosion Deposition—major ash-fall depositonContinued volcanism to west, ash–fall depositionTemperateContinued high from ash falls—new deposit creation, some addition to existing deposits; some reworking and destruction by oxidation and erosion
22.5–38OligoceneStable—regional upliftRegional erosion, deposition, and reworking of ash fallsMajor volcanic activity to west, major ash–fall depositionHigher elevation warm temperateContinued high from ash falls and leaching of Precambrian; continued alteration and deposition of uranium
38–55EoceneContinued basin/uplift development. Major thrust and normal faultingContinued erosion of highlands, deposition in basins; Precambrian cover exposed ~55 m.y.Yellowstone–Absoraka volcanism; ash falls over large areasWarm, humid, high rainfall(Savannah?)High from ash falls and leaching of Precambrian granites—development of alteration cells and uranium deposition in porous, permeable rocks
55–65PaleoceneDevelopment of present–day basins and uplifts.Rapid erosion of uplifts, deposition of continental deposits in basinsNoneNear sea level; subtropical, humid, high rainfallLimited
65–1,400CretaceousSeries of regional depressions and upliftsBroad sea coverage and deposition varying with periodic open land and continental depositionNoneVariableLimited
1,400–1,700ProterozoicCollision of plate boundaries to south (Medicine Bow-Sierra Madre)—major structural adjustments and thermal metamorphism in Central Wyoming?Intrusion of granitic rocks along structural zones??Introduction? Remobilization? Improved accessibility
2,500–2,700Archean??Intrusion of large granitic batholiths?Introduction? Remobilization
2,700–3,000Intense deformation??Probable remobilization
>3,000Intense deformationDeposition of sands, shales, and graywackesBasalt, possibly andesite flows??
Age (m.y. before present)EpochTectonismSurficial activityIgneous activityClimateUranium mobility and deposition
Present–2QuaternaryStable—regional upliftErosionContinued volcanism to westPresent–day aridLimited to oxidation and erosion of existing deposits
2–22.5Pliocene MioceneRelative stability Collapse of central portion of Wyoming (Granite Mountains)Erosion Deposition—major ash-fall depositonContinued volcanism to west, ash–fall depositionTemperateContinued high from ash falls—new deposit creation, some addition to existing deposits; some reworking and destruction by oxidation and erosion
22.5–38OligoceneStable—regional upliftRegional erosion, deposition, and reworking of ash fallsMajor volcanic activity to west, major ash–fall depositionHigher elevation warm temperateContinued high from ash falls and leaching of Precambrian; continued alteration and deposition of uranium
38–55EoceneContinued basin/uplift development. Major thrust and normal faultingContinued erosion of highlands, deposition in basins; Precambrian cover exposed ~55 m.y.Yellowstone–Absoraka volcanism; ash falls over large areasWarm, humid, high rainfall(Savannah?)High from ash falls and leaching of Precambrian granites—development of alteration cells and uranium deposition in porous, permeable rocks
55–65PaleoceneDevelopment of present–day basins and uplifts.Rapid erosion of uplifts, deposition of continental deposits in basinsNoneNear sea level; subtropical, humid, high rainfallLimited
65–1,400CretaceousSeries of regional depressions and upliftsBroad sea coverage and deposition varying with periodic open land and continental depositionNoneVariableLimited
1,400–1,700ProterozoicCollision of plate boundaries to south (Medicine Bow-Sierra Madre)—major structural adjustments and thermal metamorphism in Central Wyoming?Intrusion of granitic rocks along structural zones??Introduction? Remobilization? Improved accessibility
2,500–2,700Archean??Intrusion of large granitic batholiths?Introduction? Remobilization
2,700–3,000Intense deformation??Probable remobilization
>3,000Intense deformationDeposition of sands, shales, and graywackesBasalt, possibly andesite flows??

Contents

GeoRef

References

References

Adler
,
H.H.
,
1963
,
Concepts of genesis of sandstone-type uranium ore deposits
:
Economic Geology
 , v.
58
, p.
839
852
.
Adler
,
H.H.
,
1964
,
The conceptual uranium ore roll and its significance in uranium exploration
:
Economic Geology
 , v.
59
, p.
46
53
.
Anderson
,
D.C.
,
1969
,
Uranium deposits of the Gas Hills
:
University of Wyoming Contributions to Geology
 , v.
8
, p.
93
103
.
Boberg
,
W.W.
,
1979
,
Applied exploration geology and uranium resources of the Great Divide basin, Wyoming [abs.]
:
American Association of Petroleum Geologists Bulletin
 , v.
63
, no.
5
, p.
822
823
.
Boberg
,
W.W.
,
1981
,
Some speculations on the development of central Wyoming as a uranium province
:
Wyoming Geological Association Annual Field Conference,32nd
 ,
Jackson Hole, Wyoming, September 20-22, 1981, Guidebook
, p.
161
180
.
Bramlett
,
L.B.
Reyer
,
S.L.
Southard
,
G.G.
,
1983
,
Uranium geology and geochemistry, Copper Mountain, Wyoming
:
Society of Mining Engineers of AIME Transactions
  v.
272
, p.
1891
1901
.
Brown
,
W.G.
,
1993
,
Structural style of Laramide basement-cored uplifts and associated folds
:
Geological Survey of Wyoming Memoir 5
 , p.
312
373
.
Cheney
,
E.S.
,
1981
,
The hunt for giant uranium deposits
:
American Scientist
 , v.
69
, p.
37
48
.
Childers
,
M.O.
,
1974
,
Uranium occurrences in Upper Cretaceous and Tertiary strata of Wyoming and northern Colorado
:
Mountain Geologist
 , v.
11
, p.
131
147
.
Collings
,
S.P.
Knode
,
R.H.
,
1984
,
Geology and discovery of the Crow Butte uranium deposit, Dawes County, Nebraska
:
American Institute of Mining Engineers, Practical HydroMet'83, 7h Annual Symposium on Uranium and Precious Metals
 ,
Lakewood, Colorado
,
August 22-24, 1983, Proceedings
, p.
5
14
.
Dahlkamp
,
F.J.
,
1993
,
Uranium ore deposits
 :
Berlin-Heidelberg-New York
,
Springer Verlag
,
460
p.
Davis
,
J.F.
,
1969
,
Uranium deposits in the Powder River basin
:
University of Wyoming Contributions to Geology
 , v.
8
, p.
131
141
.
Denson
,
N.M.
,
1959
,
Uranium in coal in the western United States
:
U.S. Geological Survey Bulletin 1055
 ,
315
p.
Denson
,
N.M.
Gill
,
J.R.
,
1965
,
Uranium bearing lignite and carbonaceous shale in the southwest part of the Williston basin, a regional study
:
U.S. Geological Survey Professional Paper 463
 ,
75
p.
Dooley
,
J.R.
Jr.
Harshman
,
E.N.
Rosholt
,
J.N.
,
1974
,
Uranium-lead ages of the uranium deposits of the Gas Hills and Shirley basin, Wyoming
:
Economic Geology
 , v.
69
, p.
527
581
.
Finch
,
W.I.
,
1996
,
Uranium provinces of North America—their definition, distribution, and models
:
U.S Geological Survey Bulletin 2141
 ,
18
p.
Frost
,
C.D.
Frost
,
B.R.
,
1993
,
The Archean history of the Wyoming province
:
Geological Survey of Wyoming Memoir 5
 , p.
58
77
.
Galloway
,
W.E.
,
1977
,
Catahoula formation of the Texas coastal plain: Depositional systems, composition, structural development, ground water flow history and uranium distribution
:
University of Texas Bureau of Economic Geology Report of Investigation 87
 ,
59
p.
Galloway
,
W.E.
,
1978
,
Uranium mineralization in a coastal plain fluvial system: Catahoula Formation, Texas
:
Economic Geology
 , v.
73
, p.
1655
1673
.
Gjelsteen
,
T.W.
Collings
,
S.P.
,
1988
,
Relationship between ground water flow and uranium mineralization in the Chadron Formation, northwest Nebraska
:
Wyoming Geological Association Annual Field Conference
 ,
39h, Casper, Wyoming, September 9-11, 1988, Guidebook
, p.
271
284
.
Granger
,
H.C.
Warren
,
C.G.
,
1969
,
Unstable sulfur compounds and the origin of roll-type uranium deposits
:
Economic Geology
 , v.
64
, p.
160
171
.
Granger
,
H.C.
Warren
,
C.G.
,
1978
,
Some speculations on the genetic geochemistry and hydrology of roll-type uranium deposits
:
Wyoming Geological Association Annual Field Conference, Guidebook, 30th
 , p.
249
361
.
Gruner
,
J.W.
,
1956
,
Concentration of uranium in sediments by multiple migration-accretion
:
Economic Geology
 , v.
51
, p.
495
520
.
Harris
,
R.E.
King
,
J.K.
,
1993
,
Geologic classification and origin of radioactive mineralization in Wyoming
:
Geological Survey of Wyoming Memoir 5
 , p.
898
916
.
Harshman
,
E.N.
,
1972
,
Geology and uranium deposits, Shirley basin area, Wyoming
:
U.S. Geological Survey Professional Paper 745
 ,
82
p.
Harshman
,
E.N.
,
1974
,
Distribution of elements in some roll-type uranium deposits
:
Formation of uranium deposits
 :
Vienna
,
International Atomic Energy Agency
, p.
169
183
.
Harshman
,
E.N.
Adams
,
S.S.
,
1981
,
Geology and recognition criteria for roll-type uranium deposits in continental sandstones
:
U.S. Department of Energy GJBX-1
  (
81
).
Hills
,
F.A.
Houston
,
R.S.
,
1979
,
Early Proterozoic tectonics of the central Rocky Mountains, North America
:
University of Wyoming Contributions to Geology
 , v.
17
, p.
89
109
.
Hodson
,
W.G.
Pearl
,
R.H.
Druse
,
S.A.
,
1973
,
Water resources of the Powder River basin and adjacent areas, northeastern Wyoming
:
U.S. Geological Survey Hydrologic Investigation Atlas HA-465
 ,
4 sheets
.
Hostetler
,
P.B.
Garrels
,
R.M.
,
1962
,
Transportation and precipitation of uranium and vanadium at low temperatures with special reference to sandstone type uranium deposits
:
Economic Geology
 , v.
57
, p.
157
167
.
Klingmuller
,
L.M.L.
,
1989
,
The Green Mountain uranium district, central Wyoming
:
Type locality of solution front limb deposits, in Uranium resources of North America
 :
Vienna
,
International Atomic Energy Agency
, p.
173
190
.
Kyser
,
K.
Cuney
,
M.
,
2008
,
Recent and not-so-recent developments in uranium deposits and implications for exploration
:
Mineralogical Association of Canada Short Course Series
  Volume
39
,
271
p. (
plus CD
).
Larsen
,
E.E.
Evanoff
,
E
,
1998
,
Tephrostratigraphy and source of the tuffs of the White River Sequence
:
Geological Society of America Special Paper 325
 , p.
1
14
.
Lillegraven
,
J.A.
,
2010
,
Fools rush in—attempting revision of Wyoming's Paleogene topographic evolution
:
Wyoming Geological Association Luncheon Presentation
 
January 8, 2010
. p.
2
, (www.wyogeo.org/docs/jan_10.pdf).
Lillegraven
,
J.A.
Ostresh
,
L.M.
Jr.
,
1988
,
Evolution of Wyoming's early Cenozoic topography and drainage patterns
:
National Geographic Research
 , v.
4
, p.
303
327
.
Lowry
,
M.E.
Rucker
,
S.J.
IV
Wahl
,
K.L.
,
1973
,
Water resources of the Laramie, Shirley, Hanna basins and adjacent areas, southeastern Wyoming
:
U.S. Geological Survey Hydrologic Investigation Atlas HA-471
 ,
4 sheets
.
Lowry
,
M.E.
Lowham
,
H.W.
Lines
,
G.C.
,
1976
,
Water resources of the Bighorn basin, northwestern Wyoming
:
U.S. Geological Survey Hydrologic Investigations Atlas HA-512
 ,
2 sheets
.
Love
,
J.D.
,
1970
,
Cenozoic geology of the Granite Mountains area, central Wyoming
:
U.S. Geological Survey Professional Paper 495-C
 , p.
C1
-
C154
.
Ludwig
,
K.R.
,
1978
,
Uranium daughter migration and U-Pb isotope apparent ages of uranium ores, Shirley basin, Wyoming
:
Economic Geology
 , v.
73
, p.
29
49
.
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
:
Economic Geology
 , v.
74
, p.
1654
1668
.
Masursky
,
H.
,
1962
,
Uranium-bearing coal in the eastern part of the Red Desert area, Wyoming
:
U.S. Geological Survey Bulletin 1099-B
 , p.
B1
-
B152
.
McEldowney
,
R.C.
Abshier
,
J.F.
Lootens
,
D.J.
,
1977
,
Geology of uranium deposits in the Madison Limestone, Little Mountain area, Big Horn County, Wyoming
:
Rocky Mountain Association of Geologists Symposium
 ,
Snowmass, Colorado
,
September 29, 1977, Guidebook
, p.
321
336
.
Murphy
,
E.C.
,
2007
,
Uranium deposits in southwestern North Dakota
:
North Dakota Geological Survey Geologic Investigations 40
 ,
map sheet with text
.
Nkomo
,
I.T.
Stuckless
,
J.R.
Thaden
,
R.E.
Rosholt
,
J.M.
,
1978
,
Petrology and uranium-mobility of a granite of early Precambrian age from the Owl Creek Mountains, Wyoming
:
Wyoming Geological Association Annual Field Conference, 30h
 ,
Casper, Wyoming
,
September, 1978, Guidebook
, p.
335
348
.
Nkomo
,
I.T.
Rosholt
,
J.N.
Dooley
,
J.R.
Jr.
,
1979
,
U-Th-Pb systematics of Precambrian rocks in the Laramie Mountains, Wyoming
:
Earth Science Bulletin
 , v.
12
, p.
1
14
.
Peterman
,
Z.E.
Hildreth
,
R.A.
,
1977
,
Reconnaissance geology of the Precambrian of the Granite Mountains, Wyoming
:
U.S. Geological Survey Open-File Report 77-140
 ,
86
p.
Pipiringos
,
G.N.
,
1961
,
Uranium-bearing coal in the central part of the Great Divide basin
:
U.S. Geological Survey Bulletin 1099-A
 , p.
Al
-
A104
.
Pool
,
T.C.
,
2007
,
Technical report on the Shirley basin uranium properties, Wyoming
:
Canadian National Instrument 43-101 Report prepared for Target Mining and Exploration Corporation
  (http://www.crossairexploration.com/i/pdf/B43-101July82007.pdf).
Rackley
,
R.I.
,
1972
,
Environment of Wyoming Tertiary uranium deposits
:
American Association of Petroleum Geologists Bulletin
 , v.
56
, p.
755
774
.
Rackley
,
R.I.
,
1976
,
Origin of western-state type uranium mineralization
,
in
Wolf
,
K.
, ed.,
Handbook of strata-bound and stratiform ore deposits
 :
Amsterdam
,
Elsevier Publishing
, v.
7
, p.
89
156
.
Reade
,
H.L.
,
1976
,
Grover uranium deposits: A case history of uranium exploration in the Denver basin, Colorado
:
Mountain Geologist
 , v.
13
, p.
21
31
.
Reade
,
H.L.
,
1978
,
Uranium deposits: Northern Denver Julesburg basin, Colorado
:
Rocky Mountain Association of Geologists Field Symposium Guidebook
 , p.
161
171
.
Renfro
,
A.R.
,
1969
,
Uranium deposits in the Lower Cretaceous of the Black Hills
:
University of Wyoming Contributions to Geology
 , v.
8
, p.
87
92
.
Rogers
,
J.J.W.
Adams
,
J.A.S.
,
1969
,
Uranium
,
in
Wedepohl
,
K.H.
, ed.,
Handbook of geochemistry
 :
Berlin
,
Springer Verlag
, v.
2
, p.
92B
-
92O
.
Rosholt
,
J.N.
Zartman
,
R.E.
Nkomo
,
I.T.
,
1973
,
Lead isotope systematics and uranium depletion in the Granite Mountains, Wyoming
:
Geological Society of America Bulletin
 , v.
84
, p.
989
1002
.
Rubin
,
B.
,
1970
,
Uranium roll zonation in the southern Powder River basin, Wyoming
:
Earth Science Bulletin
 , v.
3
, p.
5
12
.
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
:
Economic Geology
 , v.
78
, p.
498
501
.
Seeland
,
D.A.
,
1976
,
Relationships between early Tertiary sedimentation patterns and uranium mineralization in the Powder River basin, Wyoming
:
Wyoming Geological Association Annual Field Conference, 28th, Guidebook
 , p.
53
64
.
Seeland
,
D.A.
,
1978
,
Eocene fluvial drainage patterns and their implications for uranium and hydrocarbon exploration in the Wind River basin, Wyoming
:
U.S. Geological Survey Bulletin 1446
 ,
21
p.
Sharp
,
W.N.
Gibbons
,
A.B.
,
1964
,
Geology and uranium deposits of the southern part of the Powder River basin, Wyoming
:
U.S. Geological Survey Bulletin 1147-D
 , p.
1
60
Sharp
,
W.N.
McKay
,
E.J.
McKeown
,
F.A.
White
,
A.M.
,
1964
,
Geology and uranium deposits of the Pumpkin Buttes area of the Powder River basin, Wyoming
:
U.S. Geological Survey Bulletin 1107-H
 , p.
541
638
.
Sheridan
,
D.M.
Maxwell
,
C.H.
Collier
,
J.T.
,
1961
,
Geology of the Lost Creek schroeckingerite deposits, Sweetwater, County, Wyoming
:
U.S. Geological Survey Bulletin 1087-J
 , p.
391
478
.
Shrier
,
T.
Parry
,
W.T.
,
1982
,
A hydrothermal model for the North Canning uranium deposit, Owl Creek Mountains, Wyoming
:
Economic Geology
 , v.
77
, p.
632
645
.
Smith
,
D.B.
Zielinski
,
R.A.
Rose
,
W.I.
Jr.
,
1981
,
Leachability of uranium and other elements from freshly erupted volcanic ash
:
U.S. Geological Survey Open-File Report 81-118
 ,
95
p.
Smith
,
D.B.
Zielinski
,
R.A.
Rose
,
W.I.
Jr.
,
1982a
,
Leachability of uranium and other elements from freshly erupted volcanic ash
:
Journal of Volcanology and Geothermal Research
 , v.
13
, p.
1
30
.
Smith
,
D.B.
Zielinski
,
R.A.
Rose
,
W.I.
Heubert
,
B.J.
,
1982b
,
Water soluble material on aerosols collected within volcanic eruption clouds
:
Journal of Geophysical Research
 , v.
87
, p.
4963
4972
.
Snoke
,
A.W.
,
1993
,
Geologic history of Wyoming within the tectonic framework of the North American Cordillera
:
Geological Survey of Wyoming Memoir 5
 , p.
2
57
.
Snoke
,
A.W.
Steidtman
,
J.R.
Roberts
,
S.M.
,
1993
,
Geology of Wyoming
:
Geological Survey of Wyoming Memoir 5
 ,
937
p.
Stuckless
,
J.R.
Miesch
,
A.T.
,
1981
,
Petrogenetic modeling of a potential uranium source rock, Granite Mountains, Wyoming
:
U.S. Geological Survey Professional Paper 1225
 ,
27
p.
Stuckless
,
J.R.
Nkomo
,
I.T.
,
1978
,
Uranium-lead isotope systematics in uraniferous alkali-rich granites from the Granite Mountains, Wyoming—implications for uranium source rocks
:
Economic Geology
 , v.
73
, p.
427
441
.
Stuckless
,
J.R.
Nkomo
,
I.T.
,
1980
,
Preliminary investigations of U-Th-Pb systematics in uranium-bearing minerals from two granitic rocks from the Granite Mountains, Wyoming
:
Economic Geology
 , v.
75
, p.
289
295
.
Stuckless
,
J.R.
Van Trump
,
G.
Jr.
Bunker
,
C.M.
Bush
,
C.A.
Hunter
,
W.C.
Lewis
,
N.F.
Jr.
,
1976
,
Radiometric and petrographic results for samples from drill holes GM1 and GM2, Granite Mountains, Wyoming
:
U.S. Geological Survey Open-File Report 76-842
 ,
19
p.
Stuckless
,
J.R.
Bunker
,
C.M.
Bush
,
C.A.
Doering
,
W.P.
Scott
,
J.
,
1977
,
Geochemical and petrologic studies of uraniferous granite from the Granite Mountains, Wyoming
:
U.S. Geological Survey Journal of Research
 , v.
5
, p.
61
81
.
Stuckless
,
J.R.
Bunker
,
C.M.
Bush
,
C.A.
Van Trump
,
G.
Jr.
,
1981
,
Radioelement concentration in Archean granites of central Wyoming
:
U.S. Geological Survey Open-File Report 81-948
 ,
40
p.
Taylor
,
P.S.
,
1969
,
Soluble material on volcanic ash
:
Unpublished M.S. thesis
 ,
Hanover, NH
,
Dartmouth College
,
77
p.
Terry
,
D.O.
,
1998
,
Lithostratigraphic revision and correlation of the lower part of the White River Group: South Dakota to Nebraska
:
Geological Society of America Special Paper 325
 , p.
15
37
.
Underhill
,
D.H.
Roscoe
,
W.E.
,
2009
,
Technical report on the Bootheel uranium property, Shirley basin mining district, Albany County, Wyoming, U.S.A.
:
Scott Wilson Roscoe Postle Associates Inc., available from Crosshair Exploration and Mining Corp
,
Vancouver, BC
,
Canada Report NI 43-101
 ,
128
p. (http://www.crosshairexploration.com/i/pdf/BH43”101_June10_2010.pdf).
U.S. Energy Information Administration
,
2010a
,
Table 1.
U.S. Forward-Cost Uranium Reserves by State
 , Year-End 2008: (http://www.eia.doe.gov/cneaf/nuclear/page/reserves/ures.html)
U.S. Energy Information Administration
,
2010b
,
Table 4.
12 Uranium exploration and development drilling
 , selected years, 1949-2008: (http://www.eia.doe.gov/emeu/aer/pdf/pages/sec4_25.pdf).
UxC Consulting
,
2009
,
UxC historical Ux price charts
 :
UxC Consulting Company
(http://www.uxc.com).
Walton
,
A.W.
Galloway
,
W.E.
Henry
,
C.D.
,
1981
,
Release of uranium from volcanic glass in sedimentary sequences—an analysis of two systems
:
Economic Geology
 , v.
76
, p.
69
88
.
Welder
,
G.E.
,
1968
Ground-water reconnaissance of the Green River basin, southwestern Wyoming
:
U.S. Geological Survey Hydrologic Investigations Atlas HA-290
 ,
2 sheets accompanying text
,
5
p.
Whitcomb
,
H.A.
Lowry
,
M.E.
,
1968
,
Ground-water resources and geology of the Wind River basin area, central Wyoming
:
U.S. Geological Survey Hydrologic Investigations Atlas HA-270
 ,
3 sheets, accompanying text
,
13
p.
Yellich
,
J.A.
Cramer
,
R.T.
Kendall
,
R.G.
,
1978
,
Copper Mountain, Wyoming, uranium deposit rediscovered
:
Wyoming Geological Association Annual Field Conference, 30th
 ,
Casper, Wyoming
,
September 1978, Guidebook
, p.
311
327
.
Zielinski
,
R.A.
,
1980
,
Uranium in secondary silica: A possible exploration guide
:
Economic Geology
 , v.
75
, p.
592
602
.
Zielinski
,
R.A.
,
1982a
,
The mobility of uranium and other elements during alteration of rhyolite ash to montmorillonite: A case study in the Troublesome Formation, Colorado, U.S.A.
:
Chemical Geology
 , v.
35
, p.
185
204
.
Zielinski
,
R.A.
,
1982b
,
Uraniferous opal, Virgin Valley, Nevada: Conditions of formation and implications for uranium exploration
:
Journal of Geochemical Exploration
 , v.
18
, p.
197
216
.
Zielinski
,
R.A.
,
1983
,
Tuffaceous sediments as source rocks for uranium: a case study of the White River Formation, Wyoming
:
Journal of Geochemical Exploration
 , v.
18
, p.
285
306
.
Zielinski
,
R.A.
,
1985
,
Element mobility during alteration of silicic ash to kaolinite a study of tonstein
:
Sedimentology
 , v.
32
, p.
567
579
.

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