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Book Chapter

The Nature and Development of the Wyoming Uranium Province

By
W.W. Boberg
W.W. Boberg
Ur-Energy Inc., 10758 West Centennial Road, Suite 200, Littleton, Colorado 80127 E-mail: bill.boberg@ur-energyusa.com
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Published:
January 01, 2010

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.

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Contents

Special Publications of the Society of Economic Geologists

The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries

Richard J. Goldfarb
Richard J. Goldfarb
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Erin E. Marsh
Erin E. Marsh
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Thomas Monecke
Thomas Monecke
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Society of Economic Geologists
Volume
15
ISBN electronic:
9781629490403
Publication date:
January 01, 2010

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