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As part of a study on the origin of the uranium deposits in the Triassic and Jurassic sedimentary rocks of the Colorado Plateau, 21 uraninlte specimens and other primary uranium minerals from 13 deposits were collected for pb2O6/u23Q age determinations. These uraninite specimens are believed to be the best of more than 80 ore samples from the Plateau on which age determinations have been made,, The 21 samples have an average Fb206/!!238 age of approximately 78 million years -when corrected only for common lead* Chemical and mass spectrometric errors change the average pbSoe age by approximately + 6 million years. Uncertainties resulting from the common lead corrections and from the possible presence of old radiogenic lead in the ores id.ll dbecrease the average Pb206/!!238 age by approximately 5 million years. Corrections for the selective loss of uranium iri.ll decrease the average age, whereas selective loss of daughter products will increase the average age by approximately 5 "to 3-0 million years. Holmes gives 127 and 152 million years as the close of the Jurassic and Triassic periods, respectively» If the ages calculated for the uraninite samples are close to the true ages of the ores, then the uranium was probably introduced into the sediments not later than the Late Cretaceous or early Tertiary (55 to 80 million years ago) 0 The average Fb206/^238 age of 78 million years for the 21 primary uranium minerals differs markedly from tbe age -which -would hare to be assumed if the present deposits were formed in the Late Triassic and Late Jurassic sediments of the Colorado Plateau during or soon, after the deposition of the rocks . During the past two years a study of the age of the Colorado Plateau uranium deposits has been, undertaken, by the Geological Surrey on behalf of the Atomic Energy Commission because data on ages would aid in. clarifying some of the problems on the origin of the Plateau uranium deposits and -would aid indirectly in the search for and the economic envelopment of uranium on the Colorado Plateau. These age , determinations have been, made specifically to help decide -whether the uranium deposits in the Plateau -were formed shortly after the enclosing Late Triassic aad Late Jurassic sedimentary rocks -were laid down, or -whether the deposits -were formed at some more recent time, perhaps at the end of the Cretaceous period or during the Tertiary. Ho attempt has been made in this study to set a precise age for the deposits of the Plateau. Instead, our initial objective has been to establish the age of the deposits -within very broad limits, that is, -whether they are of Triassic, Jurassic, Cretaceous, or Tertiary age* In an effort to obtain an acceptable ans-wer to the fundamental question, "When were the deposits formed" , two very general conditions should be observed? first, and .most obviously, it is necessary to collect geologically representative ore samples Secondly, completely reliable age determinations can be made only on ore samples which have not been -H altered in any way since their deposition. These two ideal conditions are seldom if ever completely fulfilled by the Plateau ores which have been studied . In order that the ages calculated for the Plateau ore can "be placed in their proper geologic sequence, it is also necessary to'know accurately the ages of the sedimentary roeks which enclosed the deposits. Holmes (19^6, p 0 IV?) has considered this problem in some detail and 'on the basis of several different methods has assigned an age of 152 million years to the close of the Triassic period, an age of 127 million \ years to the close of the Jurassic period^ and an age of 58 million years to the close of the Cretaceous periodo Of these three ages, only the age of the end of the Cretaceous has been determined from specimens of uraninite by use of the lead-uranium method. The more than 80 samples included in this age study represent erery type of uranium ore that has been found in the Colorado Plateau. These samples have been collected from deposits in the Shinarump conglomerate, the Entrada formation, and the Morrison formation and from all of the major mining districts in the Colorado Plateau. The average age of these 80 samples is approximately 90 million years, and the ages range from 30 to 350 million years. The oxidized ore specimens, -whose ages are impossibly great because they are older than the sediments in -which they are found, reflect, we believe, the selective loss of uranium "with respect to lead due to surface weathering and ground-water leaching. This group of 80 samples also includes 2^ specimens of uraninite and other relatively unoxidized black primary uranium minerals from 13 deposits in both Triassic and Jurassic sediments. These black ores generally are found at depth or in deposits protected from excessive alteration,, They contain uaweathered pyrite and other sulfides and, when secondary uranium minerals have been found in association with these ores, the secondary minerals have been excluded, when possible, from the material prepared for chemical analyses. This paper presents the lead-uranium age determination of 21 of these black ores which are considered to be most reliable for age determinations. The specimens have been selected for presentation because it is generally agreed that the less oxidized ores are less susceptible to leaching by ground waters than are many of the secondary uranium minerals. This conclusion has been documented in considerable detail by Ellsworth (1932, pp. 105, 2^3) in his study of the altered Villeneuve uraninite. The age of the relatively unoxidized core of that specimen was 1100 million years. The apparent age of the alteration products containing no U02 was approximately 1750 million years. Similar studies by Nier (1939, P. 159) on uraninite and its alteration products from the Belgian Congo yielded analogous results. Recently Phair and Levine's work (1953) on the effect of sulfuric acid waters on partially oxidized but black pitchblende shows that, in the course of leaching, U02 , radium, and lead were residually concentrated relative to UOs. The amount of such concentration of both radium and lead was proportional to the amount of U03 leached,

The lead-uranium ages of some uraninite specimens from Triassic and Jurassic sedimentary rocks of the Colorado Plateau