460 



UNITED STATES MINERAL RESOURCES 



vanadate, arsenate, silicate, sulfate, and carbonate 

 compounds, most of which are hydrous and many of 

 which are bright yellow or green. The vanadates, 

 camotite and tyuyamunite, are the most common 

 and abundant. 



TYPES OF DEPOSITS 



Although anomalous uranium concentrations are 

 widespread, concentrations of uranium that make 

 minable deposits, ranging from about 400 to 2,500 

 times crustal abundance, are not common. Most 

 identified resources are in a few kinds of rocks and 

 geologic environments. The principal domestic re- 

 coverable resources occur as peneconcordant masses 

 in continental and marginal marine sandstone and 

 associated rocks. A small part is in coaly rocks and 

 nonmarine limestone. Less than 2 percent of domes- 

 tic resources occur in veins. Identified paramarginal 

 and submarginal resources are chiefly in marine 

 phosphorite and black shale and in igneous rocks. 

 Some major foreign resources are in Precambrian 

 quartz-pebble conglomerate. 



PENECONCORDANT DEPOSITS 



Peneconcordant deposits are widely distributed in 

 the United States; they are especially numerous in 

 some western States (Butler and others, 1962). 

 Typically these deposits occur in sandstone lenses 

 that are interbedded with mudstone. These strata 

 accumulated under fluvial, lacustrine, and near- 

 shore marine conditions in cratonic or marginal era- 

 tonic environments. Most of the host sandstone beds 

 are quartzose and were derived from older sedi- 

 mentary rocks ; some, however, are arkosic and were 

 derived mainly from granitic rocks. Volcanic ash is 

 present in some of the host sandstone beds and is 

 common in overlying and interbedded mudstone. 

 Nearly all of the host sandstone beds contain car- 

 bonized plant matter or organic residues. The host 

 beds range in age from late Paleozoic to Tertiary, 

 but in any given area all or nearly all of the deposits 

 are in one or a few favored stratigraphic units. 



Uraninite and cofiinite are the primary uranium 

 ore minerals ; camotite and other secondary minerals 

 are ore minerals in some deposits. The ore minerals 

 mainly occupy the pore spaces of the sandstone, but 

 in places they replace the sand grains or the car- 

 bonized plant fossils. The average grade of the ore 

 mined ranged from about 0.15 to 0.30 percent UsOs ; 

 0.08 percent UsOs is a common grade cutoff for min- 

 ing and little material below 0.08 percent is mined. 

 Ore bodies range in size from small masses contain- 

 ing a few tons of ore to those containing more than 

 10 million tons. They occur in two distinct geometric 



forms: tabular bodies that are nearly concordant 

 with the gross sedimentary structures of the host 

 sandstone and roll bodies that are crescent-shaped 

 and discordant to bedding in cross section and 

 elongate in plan, with their long axes nearly con- 

 cordant to bedding. The tabular deposits are discrete 

 masses and tend to be clustered in favorable areas a 

 few miles across ; they are enclosed in reduced sand- 

 stone. The roll deposits are scattered along crescent- 

 shaped interfaces between oxidized sandstone on the 

 concave sides and reduced sandstone on the convex 

 sides; in a given mining district an interface may 

 extend for several miles and localize several ore 

 bodies. Large deposits representative of the tabular 

 type occur in the San Juan Basin, N. Mex. (Hilpert, 

 1969) and the Uravan mineral belt, Colorado (Mo- 

 tica, 1968) ; deposits representative of the roll type 

 occur in the Shirley Basin, Wyo. (Harshman, 1968) 

 and the Texas Coastal Plain (Eargle and others, 

 1971). 



Many hypotheses have been proposed for the gene- 

 sis of the deposits (Finch, 1967, table 8). Most 

 geologists now think that the uranium was derived 

 by leaching — either from volcanic glass within or 

 overlying the host rock or from granitic terranes 

 exposed along the margins of the sedimentary ba- 

 sins. Uranium was transported in ground water in 

 the hexavalent state and precipitated under reduc- 

 ing conditions. Carbonaceous matter, HaS gas, and 

 transient sulfite derived from oxidation of preore 

 pyrite are materials that have been cited as reduc- 

 tants that caused precipitation. Roll-type deposits 

 apparently form at a dynamic interface between 

 oxidizing and reducing conditions, that moves down 

 the regional dip leaving altered (oxidized) host rock 

 in its wake. In contrast, tabular deposits are wholly 

 enclosed in reduced rock which in places may be 

 completely surrounded by oxidized rock that is not 

 in contact with ore. 



DEPOSITS IN QUARTZ-PEBBLE CONGLOMERATES 



Uranium-bearing Precambrian quartz-pebble con- 

 glomerates were deposited under deltaic or fluvial 

 conditions in shallow basins in cratonic or marginal 

 cratonic environments more than 2.3 billion years 

 ago. It is postulated that in the nearly oxygen-free 

 reducing atmosphere of early Precambrian time, 

 rounded and polished detrital grains of uraninite and 

 pyrite accumulated with the more typical detrital 

 placer minerals. Although locally the placer forms 

 of some uraninite and pyrite grains were later modi- 

 fied, both minerals are generally coextensive with 

 conglomerate beds or coarse parts of conglomerates. 

 The deposits are very large and extensive. In the 



