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Structural controls on roll-front mineralization at the Buss Pit deposit, Gas Hills district, Wyoming
Long, Jena Marie
Long, Jena Marie
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2014
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2014
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2015-06-01
Abstract
A significant portion of the world's uranium resources are hosted by sandstone-hosted roll-front deposits. This type of deposit is an attractive exploration target as it is amenable to in situ mining methods. Roll-front deposits form when oxidized, uranium-bearing groundwater encounters a redox boundary creating a crescent-shaped ore body. Reducing conditions downstream of the ore body are thought to be maintained by the presence of diagenetic pyrite or organic material within the host aquifer. However, some roll-front deposits occur within aquifers that contain only low amounts of in situ reductants. Under such circumstances it is possible that leakage of reduced fluids along faults creates the necessary chemical trap for deposit formation. The Buss Pit is a roll-front deposit in the Gas Hills district of Wyoming hosted by sandstones of the Wind River Formation. The host rocks of this past producer contain low amounts of organic carbon and minor diagenetic pyrite, possibly not providing a sufficient amount of reductant to form a major uranium deposit. The Gas Hills district, however, is known for the surface discharge of hydrocarbons derived from deep in the Wind River Basin. A three-dimensional model of the deposit area has been developed to test whether there is a potential connection between the location of uranium ore at the Buss Pit and faults that may control upflow of mobile reductants. Geophysical logs were used to reconstruct the deposit stratigraphy, shape of ore zones, and location of faults. The stratigraphy of the Wind River Formation was modeled by correlating resistance curves across the study area. The shape of the ore zones and the grade distribution were derived by digitizing gamma ray curves and calculating equivalent percent uranium oxide grades for each log. The interpreted geophysical dataset was combined to create a three-dimensional model that shows resistive and non-resistive lithologies, ore zones, and major faults. The host rock succession is composed primarily of fining-up units of dominantly arkosic conglomerate to sandstone with variably discontinuous siltstone to mudstone interbeds. This sedimentary package has six fining-up units that dip gently to the south. The steeply dipping, normal Buss fault strikes E-SE, bisecting the deposit area and historic open pit. The ore zones were modeled into three grade shells, which generally occur in resistive lithologies in the southwest quadrant of the study area. Grade shells define an amalgamation of intricate planar and crescent-shaped ore bodies that collectively dip to the west. The three-dimensional model highlights the spatial relationship between the ore zones of the Buss Pit deposit and the faults. The complexity of the ore zone morphologies can only be explained by upflow of mobile reductants along the fault. The results of the three-dimensional modeling reveal the importance of large scale structural controls on the formation of at least some roll-front uranium deposits. It is shown that three-dimensional modeling of geologically complex uranium deposits can provide important insights for exploration and in situ extraction.
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