Geology of Uranium Deposits in Colorado

Uranium is a widespread and ubiquitous element. It has a crustal abundance of 2.8 parts per million, slightly more than tin. Primary deposits of uranium tend to concentrate in granitic or alkalic volcanic rocks, hydrothermal veins, marine black shales, and early Precambrian age placer deposits. Secondary (or epigenetic) deposits of uranium are formed later than the surrounding rocks that host the mineral deposit. Uranium is soluble in oxidizing aqueous solutions, especially the U+6 valence state, and can be redistributed from primary source rocks into porous sedimentary rocks and structures by groundwater and form secondary (epigenetic) uranium mineral deposits.

Epigenetic deposits of uranium in sedimentary rocks form the bulk of uranium deposits in Colorado. These include the many mines of the Uravan, Cochetopa, Maybe, and Rifle districts, and other scattered places including the Front Range and Denver Basin. Primary uranium deposits in Colorado occur in hydrothermal veins, especially in the Front Range.

Epigenetic Uranium Deposits in Colorado

Epigenetic uranium deposits in the sandstones of the Salt Wash Member of the Jurassic-age Morrison Formation are widespread in the Uravan district of southwestern Colorado. The sandstones of the Salt Wash Member were deposited by meandering streams and were later covered by shales, siltstones, and volcanic ash beds of the Brushy Basin Member of the Morrison Formation. Later, near shore marine sands of the Dakota Formation and marine muds, silts, and sands of the Mancos Shale covered the rocks of the Morrison Formation. The Salt Wash sandstones are porous, permeable, and locally contain abundant fossil plant material. Sometime after the deposition of the sandstones, uranium- and vanadium-bearing waters, probably derived from the overlying volcanic ash beds, flowed through the sandstones. The uranium- and vanadium-bearing water met changing physicochemical conditions, such as a reducing zone occupied by fossil organic material or changes in the acidity of the water, and the uranium precipitated as the minerals uraninite or coffinite and vanadium precipitated with clay minerals. Uranium and vanadium minerals formed irregularly shaped ore deposits, commonly referred to as uranium rolls. Ore deposits in the Uravan district range in size from a few tons to over a million tons. The average uranium grade is about 0.25 percent and the average vanadium grade is about 2 percent.

The Cochetopa uranium district also contains uranium mineralization in the Upper Jurassic Morrison Formation. In the Thornburg Mine, the silicified and brecciated sandstone and mudstone of the Brushy Basin Member of the Morrison Formation contain black, sooty, fine-grained uraninite in veinlets and as finely disseminated grains.

Not all epigenetic uranium deposits in Colorado are located in the much favored Morrison Formation. The Jurassic-age Entrada and Navajo sandstones in the Rifle Creek district, Garfield County host typical vanadium-uranium minerals. The grade of these deposits ranges from 1 to 3 percent V₂O₅ (vanadium pentoxide), and generally less than 0.10 percent U₃O₈ (triuranium octoxide).

Epigenetic uranium deposits also occur in carbonate rocks in the Marshall Pass district, Saguache County. In the 1970s, Homestake Mining Company geologists working on the Pitch Mine recognized that they had discovered a previously unrecognized type of uranium ore deposit in brecciated dolomite of the Mississippian-age Leadville Limestone. Most of the uranium deposits and prospects of the district occur along the north-trending Chester fault. Large uranium deposits were formed where the fault intersected the Leadville Limestone. Ore mined from 1959 to 1963 was probably also from the Leadville Limestone though the host rock was unrecognized.

Uranium ore in the Tallahassee Creek district occurs in two early Oligocene – late Eocene age formations, the Tallahassee Creek Conglomerate and the Echo Park Alluvium. The Echo Park Alluvium consists of sandstone, shale, and conglomerate. The Wall Mountain Tuff, a rhyolite ash flow tuff, overlies the Echo Park Alluvium. The Tallahassee Creek Conglomerate overlies the Wall Mountain Tuff and is mostly composed of boulders derived from the erosion of volcanic rocks and Precambrian igneous and metamorphic rocks. Uranium was dissolved from the Wall Mountain Tuff by groundwater leaching and then deposited as uraninite in favorable zones in the Echo Park Alluvium and Tallahassee Creek Conglomerate (Dickinson, 1981).

Hydrothermal vein deposits

Vein deposits containing uranium minerals are widespread throughout the Precambrian terrain of Colorado. The central Front Range contains numerous uranium occurrences; most of which were not very productive. However, the largest and most productive uranium mine in Colorado was the hydrothermal vein deposit at the Schwartzwalder Mine located in the Ralston Buttes district of Jefferson County.

The hydrothermal veins of the Schwartzwalder Mine are hosted in Precambrian age metamorphic rocks, schists, gneisses, and quartzite. Most of the uranium-bearing veins are located in garnet biotite gneiss and quartzite. The veins fill north- to northwest-trending, mostly steeply dipping, Laramide age (about 70 million years ago) fractures in the garnet biotite gneiss, quartzite, and other rocks. The ore minerals in the veins consist of uraninite (variety pitchblende), some coffinite, copper sulfides, and other base metal sulfides. Quartz and carbonate minerals form the gangue (non-ore) minerals.