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Two important orebodies have been defined by drilling in the Tallahassee Creek uranium district, Fremont County, Colorado. They are the Hansen orebody, which contains about 12 million kg of U3O8, and the Picnic Tree orebody, which contains about 1 million kg of U3O8. Host rock for the Hansen is the upper Eocene Echo Park Alluvium, and host rock for the Picnic Tree is the lower Oligocene Tallahassee Creek Conglomerate. Average ore grade for both deposits is about 0.08% U3O8.
The principal source rock for the uranium in the deposits is the lower Oligocene Wall Mountain Tuff, although a younger volcanic rock, the Oligocene Thirtynine Mile Andesite, and Precambrian granitic rocks probably also contributed some uranium. Leaching and transportation of the uranium occurred in alkaline oxidizing ground water that developed during alteration of the ash in a semiarid Oligocene or early Miocene environment. The uranium was transported to sites where it was deposited in a reducing environment controlled by carbonaceous material and biogenic products such as hydrogen sulfide.
Localization of the ore was controlled by ground-water-flow conditions and by the distribution of organic matter in the host rock. Ground-water flow, which was apparently to the southeast in Echo Park Alluvium that is confined in the Echo Park graben, was impeded by a fault that offsets the southern end of the graben. This fault and attendant displacement prevented efficient discharge into the ancestral Arkansas River drainage, and protected chemically reducing areas from destruction by the influx of excessive amounts of oxidizing ground water. Localization of orebodies in the Echo Park Alluvium may have occurred in areas where overlying rocks of low permeability were breached by erosion during deposition of the fluvial Tallahassee Creek Conglomerate (which overlies the Wall Mountain uff), allowing localized entry of uranium-bearing water. Paleohydrologic control in Tallahassee Creek Conglomerate may have been affected by the alteration of pervious ash beds to impervious clay beds.
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