PriscillaC. Pattern

GEOLOGICAL SURVEY CIRCULAR 271

V* ^

3.V

A PRELIMINARY DETERMINATION OF 

THE AGE OF SOME URANIUM ORES 

OF THE COLORADO PLATEAUS BY

THE LEAD-URANIUM METHOD

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 271

A PRELIMINARY DETERMINATION OF THE AGE OF SOME

URANIUM ORES OF THE COLORADO PLATEAUS

BY THE LEAD-URANIUM METHOD

By L. R. Stief£ T. W. Stern, and R. G. Milkey

This report concerns work done on behalf of the

U. 8. Atomic Energy Commission 

and is published with the permission of the Commission

Washington, D. C., 1953

Free on application to the Geological Survey, Washington 25, D. C.

CONTENTS 

Page Page

Abstract ...................................... 1 Calculated ages Continued

Introduction.................................... 1 P^ZOT^JZSS ages o{ 41 uranium ores of

Samples....................................... 2 the Colorado Plateaus.................. 9

Physical constants and formulas................. 2 Pb 206/U ages and Pb 207 /Pb 206 ratios of

Age calculations................................ 4 samples grouped by grade .............. 9

Corrections............................... 4 Pb 206 /U ages and Pb 2(r7 /Pb 206 ratios of

Analytical errors..................... 4 samples grouped by type................ 11

Common lead......................... 4 Pb 206 /U ages and Pb 20 /Pb 206 ratios of

Additions of old radiogenic lead........ 8 samples groupedby stratigraphicposition.. 13

Loss of daughter products ............. 8 Pb 206 /U ages andPb 207 /Pb 206 ratios of

Loss of uranium...................... 8 samples groupedby mineralogic

Calculated ages................................ 8 composition........................... 13

Pb 206 /U ages of 41 uranium ores of the Summary and conclusions...................... 13

Colorado Plateaus....................... 8 Literature cited............................... 16

	Unpublished reports........................... 16

ILLUSTRATIONS

Page

Figure 1. Index map of part of the Colorado Plateaus showing location of samples used

	for age determinations .................................................................... 3

2. The effect of increased Pb 206 on Pb 204 /Pb 208 ................................................ 5

3. Triangular diagram of lead isotopes .......................................................... 6

4. Triangular diagram of common leads ....................................................... . 7

5. Pb 266 /Pb 208 as a function of U/Pb 208 ..'...................................................... 9

6. Pb 207 /Pb 208 as a function of U 235 /Pb 208 ..................................................... 10

7. Histogram of results of age calculations....................................................... 11

TABLES

Page

Table 1. Pb/U and Pb 206 /U ages of 41 uranium ores of the Colorado Plateaus............................. 12

2. Total and corrected Pb 207 /U 235 ages of 41 uranium ores of the Colorado Plateaus................ 12

3. Pb 206 /U ages and Pb 207 /Pb 206 ratios of 41 uranium ore samples of the Colorado Plateaus

grouped by grade of sample ............................................................... 12

4. Pb 206 /U ages and Pb 207 /Pb 206 ratios of 41 uranium ore samples of the Colorado Plateaus

grouped by type of sample ................................................................. 14

5. Pb 206 /U ages and pb 2 °VPb 206 ratios of 41 uranium ore samples of the Colorado Plateaus

grouped by stratigraphic position .......................................................... 14

6. Pb 206/U ages and Pb 207 /Pb 206 ratios of 41 uranium ore samples of the Colorado Plateaus

grouped by mineralogic composition........................................................ 14

7. Uranium ores of the Colorado Plateaus used for dye calculations ............................... 17

8. Lead and uranium content of uranium ores used for age calculations ............................. 18

9. Isotopic composition of lead from uranium ores and lead minerals of the Colorado Plateaus........ 19

iii

A PRELIMINARY DETERMINATION OF THE AGE OF SOME

URANIUM ORES OF THE COLORADO PLATEAUS

BY THE LEAD-URANIUM METHOD

By L. R. Stieff, T. W. Stern, and R. G. Milkey

ABSTRACT

A study of the Pb 206 /U and Pb 207 /U 235 ages 

and the Pb 207 /Pt> 206 ratios of the uranium ores of 

the Colorado Plateaus was undertaken by the Geological Survey on behalf of the U. S. Atomic Energy 

Commission because of its direct bearing on the 

origin of these ores. A preliminary treatment of the 

datafor 41 ore samples containing more thanO. 1 percent uranium gives an average Pb 206 /U age of 

approximately 71 million years, an average Pb 207 /U 235 

age of about 82 million years, and an average Pb 20 7 /Pb 20 6 

ratio equivalent to an age of 425 million years. At 

least part of the large discrepancy between the mean 

lead-uranium ages and the much less reliable 

Pb 207 /Pb 206 ages is due to small systematic mass 

spectrometric errors. The extreme sensitivity of 

the Pb 207 /Pb 206 ratio to small mass spectrometric 

errors invalidates not only the Pb 207 /Pb 206 ages but 

also the corrections for the presence of old radiogenic 

lead and the selective loss of radon which must be 

based in part on precise determinations of the 

Pb 207 /Pb 206 ratio.

Errors in the chemical analyses for lead and 

uranium introduce uncertainties in the mean Pb 206 /U 

age of approximately + 3 million years. The small 

systematic mass spectrometric errors should not 

increase the mean Pb 206 /U age by more than 3 million years. Better corrections for common lead 

and additional corrections for selective loss of 

uranium and the presence of old radiogenic lead 

should lower the mean Pb 206 /U age by approximately 

10 million years. If the entire Pb 207 /Pb 206 ratio 

anomaly is assumed to be due to the selective loss of 

radon, an improbable assumption, the mean Pb 206 /U 

age would increase approximately 10 million years.

The data suggest that the calculated.ages are 

close to the true age of the ores. From these calculated ages it is reasonable to assume that the 

uranium was introduced into the sediments not later 

than the late Cretaceous or early Tertiary (55 to 80 million years ago). This assumption differs markedly 

from the assumption that the present uranium deposits 

were formed in the Late Triassic and Late Jurassic 

sediments of the Colorado Plateaus (152 and 127 million years ago), during or soon after deposition of 

the sediments. Careful study is continuing in order to 

reduce the uncertainties in interpretation of both field 

and laboratory data so that a satisfactory hypothesis 

of origin of these ores may be definitely established. 

To place this laboratory study on a firmer basis, ages 

are being determined by the lead-uranium methods on 

many additional carefully selected samples of 

uraninite and other primary ore minerals.

INTRODUCTION

The immediate objective of the present study of 

the age of the uranium ores of the Colorado Plateaus 

by the lead-uranium methods has been to determine 

whether the uranium deposits in the Sninarump 

conglomerate are Late Triassic (152 million years) 

and those in the Morrison formation Late Jurassic 

(127 million years) in age (Holmes, 1946a, p. 145), 

as some of the field evidence suggests, or whether 

all the deposits are Late Cretaceous to early Tertiary 

(55 to 80 million years in age) as the available leaduranium age determinations indicate. No attempt has 

been made to set more precise age limits on the 

carnotite ores because of the uncertainties that 

accompany age determinations made on minerals that 

are probably secondary in occurrence. The uraninite 

specimens that have been found in some of the Triassic 

and Jurassic rocks, however, are not as susceptible 

as carnotite specimens to solution by ground waters, 

and the ages that have been determined for these 

uraninites are considered more reliable.

This study of the age of the uranium ores of the 

Colorado Plateaus was begun in 1950 by the Geological 

Survey on behalf of the U. S. Atomic Energy Commission. The writers believe that the determination of the 

age of the ores of the plateaus is one of the most promising types of investigation directly related to the 

origin of the ore deposits in the sedimentary rocks of 

the plateaus. The Tertiary ages of Colorado Plateau 

ores published in works by Hess and Foshag (1927, p. 4\ 

and Holmes (1931, p. 351), and the writers'preliminary 

work (Stieff, Girhard, and Stern, 1950, p. 40; Stieff and 

Stern, 1952, p. 707) did not agree with the ages implied 

by the widely accepted theory that the ore was deposited 

shortly after the enclosing Triassic and Jurassic sedimentary rocks were laid down (Hess, 1914, p. 687; 

Webber, 1947; and Fischer, 1950, p. 3).

If even an approximate dating of the deposits can 

be made by laboratory methods, the approach to the 

problems of the source and the chemical and physical 

controls of the mineralizing solutions will be simplified. 

The clarification of these questions on the origin of the 

deposits'has not only considerable scientific interest 

but also will lead to two immediate practical applications: 

(1) greater confidence can be placed in the estimates 

of the ultimate reserves of uranium ore on the Colorado 

Plateaus and in the selection of new areas for exploratory 

drilling; (2) more thorough evaluation can be made of 

the prospecting methods and geologic controls used to 

guide the drilling.

The writers wish to express their appreciation 

not only to the many members of the Survey for their
help but also to the Atomic Energy Commission's Division 

of Research and Division of Raw Materials for the support 

received. Without the isotopic analyses furnished by 

Dr. Roger F. Hibbs and his staff at the Mass Assay Laboratory, Carbide andCarbon Chemicals Co., Oak Ridge Term., 

this work would not have been possible. The mill-pulp 

samples used in this study were obtained through the 

courtesy of the United States Vanadium Co.

SAMPLES

The sampling of the uranium deposits of the Colorado 

Plateaus for age studies is a specialized problem. It requires not only that the ore specimens from each of the 

deposits in the different mineralized stratigraphic units 

must be representative, but also that the specimens must 

be essentially unaltered if the lead-uranium ages of the 

specimens are not to be much greater than the actual age 

of the ore. Obviously, the oxidized and altered near-surf ace 

ores now available on the plateaus offer little opportunity 

to collect specimens that will satisfy both these requirements. The compromises that had to be made in fulfilling 

these conditions, however, have not been considered in the 

arithmetic averages that have been calculated for this preliminary report. For this reason, the averages that are 

presented are probably greater than the true age of the ore.

The geographic and stratigraphic distribution of all but 

2 of the 17 deposits included in this study are shown in figure 1. 

The deposits not plotted are the Rifle district, Garf ield 

County, Colo., and the Tern pie Mountain district, Emery 

County, Utah. A list of the sample localities arranged 

according to the stratigraphic units in which the deposits

occur, together with the type of sample, principle 

uranium-bearing minerals contained, and their Pb 206 /U 

ages and their Pb 207 /Pb 206 ratios corrected only for 

common lead is shown in table 7 at the end of this report.

In collecting samples for this age study, an effort 

wasmadeto obtain as many different types of uranium 

ore as possible. The uraninite specimens were collected 

from the Happy Jack mine, San Juan County, Utah, and 

the Shinarump No. 1 claim, Seven Mile Canyon, Grand 

County, Utah, both in the Shinarump comglomerate of 

Triassic age. The oldest Jurassic ores were of the 

vanadiferous hydromica type ("roscoelite") collected 

from the Entrada sandstone in the vicinity of Placerville, 

San Miguel County, Colo. Also, a new uranium mineral 

(coffinite) now being described from the La Sal No. 2 

mine, Mesa County, Colo., and an asphaltic ore containing uraninite from the Temple Mountain area, Emery 

County, Utah, were collected and analyzed. The remaining samples are typical of the disseminated type 

of carnotite ores and cdl except one (from the Monument 

No. 2 mine, Apache County, Ariz., in the Shinarump 

conglomerate) were collected from deposits in the Salt 

Wash sandstone member of the Morrison formation of 

Jurassic age. Nine mill-pulp samples representing a 

total of about 4,200 tons of carnotite ore from 7 different 

mines in the Uravan mineral belt were also studied.

PHYSICAL CONSTANTS AND FORMULAS

The physical constants and equations used in the 

age calculations on the ores of the plateaus are 

shown below.

Radioactive decay schemes, physical constants, and age formulas.

Decay schemes

 * Pb 206 + 8 He 4

 »-Pb207 + 7 He 4 

Th232 >-Pb208 + 6 He 4

1Pb 204 is not known to be produced by any natural radioactive decay series.

Physical constants

g/aU 

n238 . 0.99286 

Em 222 (Radon) 2.14 x 10" 12 

n235 0.00714 -

Em 219 (Actinon) 1.62x 10" 16 

Equations for the calculation of geologic age (after Keevil, 1939) 

t = 1. 515 x 10" log 10 (1 + 1.156 Pb2° 6 ) [t is in million years] 

t = 2.37x 1C 3

*(

Half-life 

4. 56 x 10 9 years

3.82 days 

7.14 x 10 8 years

3.92 seconds

U238

1.14 Pb

Pb 207 = __ 

Pb 206 139.0

A235 t

,e -1

U 235 

'

X238 t

 e -1,
MONTEZUMA

(LA PLATA

COLO/RADO

ARIZONA MEXICO

Ship Rock

River

N A V A J 0 A P A C H E

oFarmington 

JUAN

EXPLANATION 

Location of samples used for age determinations

 

Morrison formation

A 

Entrada sandstone

 

Shinarump conglomerate

10 0

I i i i > I

40 Miles

Figure 1. -Index map of part of the Colorado Plateaus showing location of samples used for age determinations.
These constants are the same as those used by 

Wickman (1939, p. 5) 1 in the preparation of his nomographs for the calculation of geologic age. These 

nomographs were used in the calculation of some of 

the ages given in this report. The most recent redetermination of the half-life of U236 by Fleming, 

Ghiorso, and Cunningham (1951, p. 967) does not 

differ significantly from the value proposed by Nier 

(1939a, p. 153) which Wickman used.

In this report Pb 206 /U ages instead of 

Pb 206 /U 238 ages were calculated because the correction of total U to U236 is much less than the 

chemical errors in the determination for uranium. 

It has been shown that the isotopic abundance of 

U238 for samples from many different sources does 

not differ significantly from the value 99. 286 percent.

AGE CALCULATIONS 

Corrections

The calculation of the age of the uranium ores 

of the plateaus is subject to corrections for the 

following:

1. Analytical errors.

2. The presence of common lead in the ore.

3. The presence of old radiogenic lead in the ore.

4. The selective loss of certain radioactive 

daughter products such as radon.

5. The loss of uranium and (or) lead due to 

alteration of the ore by ground waters.

Analytical errors. All samples were carefully ground and thoroughly mixed before they were 

analyzed. Samples containing more than 0.1 percent 

uranium were analyzed volumetrically. All samples 

were analyzed for lead colorimetrically (Milkey, 1952, 

p. 9-13) after extraction of the lead with dithizone. 

The results of duplicate volumetric analyses for 

uranium and colorimetric analyses for lead are 

usually reproducible to within2 to 3 percent.

For a sample whose Pb 206 /U age is 65 million 

years, these analytical chemical errors for the 

uranium and lead result in a maximum uncertainty of 

± 3 million years. The results of the quantitative 

analyses of the samples used for age determinations 

are given in table 8, p. 18.

The limits of precision for the lead isotope 

analyses are calculated so that 95 percent of any new 

determinations will fall within the limits given. 

Known systematic mass spectrometric errors 

probably introduce uncertainties in accuracy of less 

than 1 percent in the reported isotopic analyses. 

The isotopic analyses of the lead (as the iodide) extracted from the ore are in table 9, p. 19.

The Pb 204/Pb 208 ratio of many of the radiogenic leads, however, shows considerably greater 

variation than would be expected from the reported 

precision and accuracy (fig. 2). The direction of the 

variation is toward impossibly large amounts of

*See page 16 for literature cited.

Pb 204 relative to the Pb 208 in the samples as the percent abundance of Pb 206 increases. This type of 

impossibly high Pb 204 /Pb 208 ratio clearly indicates 

the presence of small, previously unrecognized systematic mass spectrometric errors.

The additional systematic error in the Pb 204 / 

Pb 208 ratios of the radiogenic leads may be due 

partly to the inability of available mass spectrometers to resolve completely the smaller Pb 204 

and Pb 208 spectrometer peaks from the much larger 

Pb 206 peaks of these radiogenic leads. Similar but 

smaller percentage errors should be expected in 

the determination of Pb 206 /Pb 207 ratios of the radiogenic leads. These errors result in abundances of 

Pb 204 , Pb 207 , and Pb 208 slightly higher than their 

true values, abundances of Pb 20 ^ correspondingly 

lower, and an increase in the mean Pb 206 /U age of 

not more than approximately 3 million years.

The isotopic data on the leads of the Colorado 

Plateaus (fig. 3) are plotted on the Pb 206 , Pb 207 , 

Pb 208 plane of a tetrahedron which has Pb 204 , 

Pb 206 , Pb 207 , and Pb 208 -at its corners. The sum 20 6 20 7

of the isotopic abundances of Pb , Pb , and 

Pb 208 plotted in this way is 100 percent, and 

Pb 204 does not appear. Pb 204 has been deliberately 

omitted from figure 3 because of the large uncertainty involved in determining its abundance. 

Figure 3 shows, as does figure 2, the increasing 

effect of the small mass spectrometric errors as 

Pb 206 becomes more abundant. Many of the samples 

containing more than 90 percent Pb 2 " 6 are to the 

left of the compositional line connecting the average 

radiogenic Pb 207 /Pb 206 ratio, of the ores of the 

plateaus with the common leads of the plateaus 

which fall within the small inscribed triangle. The 

analyses of samples that are plotted to the left of the 

average compositional line have the largest Pb 207 / 

Pb 206 ratio anomalies.

At least a part of the large anomaly in the 

radiogenic Pb 207 /Pb 206 ratio seems to be due to 

systematic mass spectrometric errors. Until the 

amount of these errors can be determined, reliable 

Pb 207 /Pb 206 ages for the uranium ores of the 

Colorado Plateaus cannot be made. For this reason 

Pb 207 /Pb 206 ages are not included in this report. 

Quantitative corrections for the selective loss of 

radon or for the presence of old radiogenic lead depend 

solely on very accurate Pb 207 /Pb 206 determinations. 

Therefore, detailed discussion of these corrections 

is not justified at this time. However, the mean 

Pb 206 /U age would not be expected to increase more 

than 3 million years as a result of these' systematic 

errors. These ages, calculated to the nearest 

5 million years, are presented.

Common lead. Almost all lead-uranium and 

lead-lead age calculations must include a correction 

for the presence of common lead in the samples because not all the Pb 206 and Pb 207 present was produced by the decay of the U 238 and U 23S in the sample. 

Common lead contains, besides relatively small 

amounts of Pb 204 the only isotope of lead which is 

not known to be the product of any radioactive decay 

series varying amounts of Pb 206 ,Pb 207 , and Pb 208 

in roughly the proportions of 2:2:5. It is probable 

that much of the Pb 206 , Pb 207 , and Pb 208 in common 

lead was present at the time of the formation of the 

earth (Holmes, 1946b, p. 680; 1947, p. 27).
.070

30 40 50 60 70 80 90

i r i r

.070

.065 .065

.060 .060

.055 .055

CO

o

.a 

Q.

.050

.045

.050

.045

.a 

Q. .040 .040

.035 .035

.030

.025

O 00

o 

o »

00

e o .030

.025

.020 J________I

30 40 50 60 70 80 

PERCENT Pb 206

90 .020

Figure 2. -The effect of increased Pb 206 on Pb 204 /Pb 208 .

5
Pb206

90

90

Pb 207 10 20 30 40 50 60 70 80 90 Pb208

Figure 3. Triangular diagram of lead isotopes.

The quantitative effects of common lead corrections on the lead-uranium and lead-lead ages are a 

function of the Pb 206 /U, Pb 207 /U235 , and Pb2ov/pb 206 

ratios, the amount of common lead in the ore, and the 

known or assumed isotopic composition of the lead 

originally present (common lead plus variable amounts 

of old radiogenic lead). The lead-uranium ages are 

usually only moderately affected by the choice of the 

isotopic composition of the original lead even when a 

relatively large amount of original lead is present. 

On the other hand, any uncertainties in the isotopic 

composition of the original lead can result in large 

changes in the Pb 207 /Pb 206 age. This is true even 

when only relatively small amounts of original lead 

are present in the sample.

The range in isotopic composition of common 

lead is shown in figure 4, an enlargement of the small 

triangle shown in figure 3. The triangular points 

(fig. 4) represent the isotopic compositions of the 

common leads analyzed by Nier (1938, p. 1573, and 

Nier, Thompson, and Murphey, 1941, p. 115). The 

circles represent the isotopic compositions of the

common leads and a few of the ores of the Colorado 

 Plateaus that contain large amounts of common lead. 

The circle with the inscribed triangle represents the 

isotopic composition of the common lead in the lead 

molybdate, wulfenite, and the chloride-vanadate of 

lead, vanadinite, from the Tucson Mountains, Ariz., 

analyzed by Nier (1938, p. 1573). We do not know 

whether there is any significance to the fact the 

vanadinite from the Tucson Mountains of Miocene age 

is on the composition line of the uranium and vanadium 

ores of the Colorado Plateaus apparently of early 

Tertiary(?) age, although the similarities in isotopic 

composition certainly suggest that all these samples 

are in some way genetically related. In any event, the 

composition of this lead from the Tucson Mountains 

was chosen for many of the age calculations because 

this lead contains less uranogenic lead than any other 

which could legitimately be considered. This lead, 

therefore, will give the greatest Pb 206 /U ages when 

used in the common-lead correction.

In contrast to the variable Pb 204/Pb 208 ratio f6r 

the ores containing large amounts of radiogenic lead,
Pb206

30%

29

22 28

23 27

24 26

30%

25 25

26 24

23

28 22

29 21

51 52 53 54 55 56 57 58 59

Pb207

60% 

Pb208

Figure 4. Triangular diagram of common leads.

the Pb 204 /Pb 208 ratio of the common leads from the 

Colorado Plateaus is almost constant and is approxi-. 

mately 0.02651. The relatively uniform Pb 204 /Pb 208 

ratio of these common leads is due to the absence 

of thorium, even in very small amounts, in the 

ore, and to the relatively low abundance of Pb 208 

in common leads which permits greater accuracy in 

the isotopic analysis of the lead.

Thorium (Th232), the parent of radiogenic Pb 208, 

has not been found within the sensitivity limit of the 

semiquantitative spectrographic determination, which 

is approximately 0.1 percent in the presence of 0.1 

to 10 percent uranium. It is difficult to evaluate 

quantitatively the limits of sensitivity for thorium 

when uranium is above 10 percent; however, the 

thorium limit is known to be higher than 0.1 percent. 

Quantitative chemical analyses of uraninite samples 

from Happy Jack mine, San Juan County, Utah, and 

Shinarump No. 1 claim, Grand County, Utah, showed 

less than 0.01 percent thorium. An isotope dilution 

analysis of some 30-percent uranium ore from the 

La Sal No. 2 mine, Mesa County, Colo., showed that 

the sample contains less than 0.001 percent thorium

(George Tilton, personal communication). The almost 

complete absence of thorium in the uranium ores may 

'be due to the difference in the chemical behavior of 

thorium and uranium. This absence of thorium may 

also be due to the fact that these deposits are possibly 

related to pitchblende veins at depth, which usually 

contain only traces of thorium, rather than to average 

granitic rocks, which contain from 3 to 6 times more 

thorium than uranium.

The customary practice in common-lead corrections is to use the amount of Pb 204 in the sample that 

is being studied as an index of the amount of common 

lead to be deducted. However, in calculating the ages 

of the ores of the Colorado Plateaus, Pb208 was used 

for the three following reasons:

The Pb 208 in the samples is due only to the 

presence of common lead and is not a result of the 

decay of thorium (Th232).

Pb 208 is approximately 40 times as abundant as 

Pb 204 and can be determined with considerably greater 

precision.
The use of the Pb 208 as an index of common 

lead yields ages which are greater than the ages 

obtained when Pb 204 is used as the common lead 

index. This is in line with the general objective of 

calculating a maximum lead-uranium age for the ores 

of the plateaus.

^Additions of old radiogenic lead. The variations *^IgT~17^rl^ngina7^oLaTsTTnThe abundance of 

Pb 206 , Pb 207 , and Pbzos in common lead that were 

analyzed by Nier (1938, p. 1573; Nier, Thompson, 

andMurphey, 1941, p. 115) were conclusively shown 

to be due to the additions of radiogenic lead (Holmes, 

1946b, p. 680; 1947, p. 127).

Although detailed corrections for the presence 

of old radiogenic lead cannot be made until the mass 

spectrometric errors are resolved, the maximum , 

though improbable, reduction in the mean Pb 206 /U 

age would not be more than 5 million years.

Loss of daughter products. An alternative explanation for the distribution of many of the points in 

figure 3 and the apparent presence of old radiogenic 

lead in the ores With a mean Pb 207 /Pb 206 age of 

425 million years is the selective loss or migration of one 

or more of the daughter products in the U 238 decay 

series. Wickman (1942, p. 465) theorized that a preferential loss by diffusion of radon over actinon, the 

radioactive noble gases, might be expected because 

of radon's greater half-life. (See p. 2.) Because of 

the uncertainty of the exact values for the radiogenic 

Pb 207 /Pb 206 ratios, no detailed correction for loss 

of radon was made. A maximum, although improbable, correction for loss of radon would increase the 

mean Pb 206 /U age 10 million years, but the mean 

Pb 207 /U235 age would remain unchanged.

Loss of uranium. It is the belief of the writers 

that, next to the corrections for common lead, the 

largest source of error in lead-uranium age determinations in many of the carnotite ores is the leaching 

by ground water of uranium rather than lead. The 

true age of samples corrected for loss of uranium 

will be younger than the calculated lead-uranium ages. 

In this preliminary treatment of the data, corrections 

were not made for the possible loss of uranium because of the difficulty of determining this factor 

quantitatively. There is abundant evidence on the 

plateaus, however, of the alteration and leaching of 

the ore bodies by surface and ground waters. Therefore, the mean Pb 2D6 /U ages that were calculated 

are probably older than the actual ages of the 

specimens.

Our data confirm the fact discussed by Ellsworth 

(1932, p. 423) and Nier (1939b, p. 159) that often the 

apparent age of the alteration products in the immediate vicinity of the primary uranium minerals is 

greater than that of the primary minerals from which 

they were formed. The ages of the alteration products are apparently greater because under most 

natural environments uranyl salts are more soluble 

than the comparable lead salts. This results in an 

enrichment of lead. Phair andLevine's work(1952, p. 4) 

on the effect of sulfuric acid waters on partly oxidized 

but black pitchblende shows that in the course of leaching, UOg, radium, and lead were residually concentrated relative to UOs, and the amount of such concentration of both radium and lead was proportional to

the amount of uranium leached. Additional evidence 

from the plateaus of the relatively greater mobility 

of uranium when compared to lead may be found in 

several analyses of recent carnotite coatings on 

joints and fractures which contain 40 to 60 percent uranium and less than 0.01 percent lead 

(Stieff, Girhard, and Stern, 1950, p. 30).

Preliminary studies by Phoenix (1951, p. 9) 

of ground waters in the vicinity of some of the 

uranium deposits of the Colorado Plateaus showed 

that these waters are weakly alkaline bicarbonate 

solutions containing a significant amount of uranium. 

A possible explanation of the erratic ages of the 

lower-grade samples and the few samples with less 

than 0.1 percent uranium may be that they have 

lost disproportionately large amounts of uranium 

to percolating ground water. Ores of different 

grade, mineralogic composition, and uranium 

distribution from the plateaus will be studied in 

the laboratory by leaching with weakly alkaline 

solutions.

Many of the events in the geochemical history 

of the uranium deposits of the plateaus, involving 

loss or addition of either uranium or lead, may 

be shown graphically by plotting the ratio of Pb 206 / 

Pb 208 against the ratio of U/Pb 208 (fig. 5). Loss 

of uranium would move the plotted points horizontally to the left, the true age of the sample being 

less than the plotted one. The Pb 206/Pb 2 °6 ratio 

corrected for the addition of old radiogenic lead 

moves the plotted points vertically downward. 

This movement also results in an age which is 

less than the plotted one. Radon loss would shift 

the points vertically upward and increase the age. 

A similar plot of the ratios of Pb 207 /Pb 208 against 

the ratio of u 235 /Pb 206 (fig. 6) shows much the 

same thing except that there can be no radon loss 

in this system.

As more detailed data are accumulated from 

individual deposits, it will be possible by means 

of similar graphs to determine the approximate 

amount of uranium that has been lost and composition of the lead originally present in the ore 

deposit. This information in addition to the data 

now available should show clearly whether radon 

loss is a significant process in the modification 

of the Pb 207 /Pb 206 ages and whether old radiogenic lead was added to the ores at the time of 

their deposition.

Pb

CALCULATED AGES

206 /U ages of 41 uranium ores of the 

Colorado Platea"us

It is true, not only for the uranium ores of the Colorado Plateaus but also for any large group of samples 

usedfor lead-uranium age studies, that all the specimens 

will not give equally reliable ages. The best materials 

are those which are unaltered and which have a high UOg 

content. As previously mentioned, in this preliminary 

determination of the age of the uranium in the ores of the 

plateaus, no direct consideration is given to the quality 

of the samples in the calculation of the arithmetic averages. 

The result of this uncritical grouping of all 41 samples 

studied is that the averages which are given below are 

probably higher than the actual age of the ore.
40

30

CO

o

CSl

20

to 

o

CM

10

500 1000 1500 

U/Pb208

2000 2500

Figure 5. Pb 2 o 6 /Pb 208 as a function of U/Pb 208.

A histogram (fig. 7) of the corrected Pb 206 /Uages 

for the 41 samples of ore containing more than 0.1 percent uranium shows a pronounced maximum in the 70- 

to 80-million-year interval. The first quartile, median, 

and third quartile are also shown because these measures will be used in tables 1 and 2 (see p. 12) to give 

some indication of the distribution of the ages that have 

been determined.

The necessity for isotopic analyses and commonlead corrections is shown (table 1) by the decrease in 

the mean age of the samples from 270 million years for 

total lead to 68 million using the isotopic composition 

of the lead mineral nearest the deposit (best) from which 

the ore sample was taken. The wide range for both total 

lead and total Pb 206 ages is shown by the fact that the 

mean age is greater than the third quartile.

Pb 207 /U235 ages of 41 uranium ores of the Color ado 

Plateaus

Most of the Pb 2 ° 7 /U235 ages (table 2, p. 12) 

are greater than the Pb 206 /U ages because the systematic mass spectrometric errors result in apparent 

Pb 207 abundances which are greater than the actual 

abundances of this isotope. The total pb 207 /U 235 age 

is greater than the total Pb/U age because samples

containing large amounts of common lead contain a 

greater percentage of Pb 207 than do samples of pure 

radiogenic lead. The similarity of the mean (best) lead 

and vanadinite Pb 207 /U23S ages reflects the relatively 

small variations that havebeen found in thePb 2o7 /Pb 208 

ratios of the common leads of the plateaus.

Pb 206 /Uages andPb 207 /Pb 2 .06 ratios of samples grouped 

by grade

The tabulation of the samples according to grade 

(table 3, p. 12) shows a small difference in mean 

Pb 206 /U ages. Generally the lower grade uranium 

ores contain relatively larger amounts of common lead. 

The Pb 206 /U ages of this group, therefore, would be 

lowered the most by a better choice of the common lead 

used for the common lead correction. Also, this group 

would be most affected by a uniform loss of uranium due 

to leaching by ground waters.

The lower Pb 207 /Pb 206 ratio for samples containing 0.1 to 1.0 percent uranium may be due to a 

smaller radon loss, but more probably it reflects both 

the greater common lead correction and the greater 

accuracy of the mass spectrometer measurements. 

The inability of the mass spectrometer completely 

to resolve the Pb 204 , Pb 207', and Pb 208 peaks
1.4

1.3 

1.2 

I.I 

o 1.0

04

-0

Q.

0.9 -

O 

04

o. 0.8 -

0.7 -

0.6 

0.5 -

0.4

0 8 10 12 14

Figure 6. -Pb 207 /Pb 208 as a function of U 235 /Pb 208 . 

10
</)

II

10

9

8

< </)

°5

uj 4 

m

|3

2 

I

0

MEDIAN

MEAN

FIRST QUARTILE

THIRD QUARTILE

10 20 30 40 50 60 70 80 90 100 110 120 130 140 

AGE IN MILLIONS OF YEARS

Figure 7. Histogram of results of age calculations.

from the Pb 206 peak is shown by the larger Pb 207 / 

Pb 206 ratios in those samples containing both the 

largest amounts of Pb 206 and uranium.

Pb 206 AJ ages and Pb 207 /Pb 2 °6 ratios of samples 

" grouped by type~

The largest spread in meanPb 206 /U ages 

is found by grouping the samples into mill pulps 

and small specimens (table 4, p. 14). The millpulp samples were obtained from the ore-sampling 

plants of the U. S. Vanadium Co. Each sample 

is a split from about 235 tons or more of ore from 

a single ore body. The small samples are hand 

specimens especially selected in the mines for 

isotopic lead-uranium age studies. The greater 

mean Pb 206 /U ages of the large mill pulps compared with the small samples correspond to the 

greater ages of the samples which contain less 

than 1.0 percent uranium mentioned in the last 

paragraph. The grade of the mill pulps is between 

0.18 and 0.49 percent uranium. Also, with the

exception of the three Calamity milt pulps, more 

than 40.9 percent of the lead they contain is common 

lead.

The greater age of the ores that lost uranium is 

shown by the inclusion in the mill-pulp group of one 

pulp sample from a surface deposit which was 

mined as an opencut. This pulp gave a Pb 206 /U age 

of 120 million years and the opencut showed abundant 

evidence of the selective loss of uranium by recent 

percolating ground waters. The remaining mill pulps 

might also be expected to show some effect of loss of 

uranium although they come from less exposed der 

posits and thus their ages would not be as great as the 

sample from the opencut.

In addition, the mill pulps have lower Pb 207 / 

Pb 206 ratios than the small samples. This may 

be due in part to the following facts: (1) they 

contain more common lead, (2) the mass spectrometric analyses are better in this range, and 

(3) these samples lost less radon and (or) received 

less old radiogenic lead.

11
Table 1. Pb/U and Pb 206 /[J ages of 4l uranium ores of the Colorado Plateaus

[in millions of years]

Mean 1st quartile Median quartile

Total Pb/Q 

Total Pb 206 /[J - 

Radiogenic Pb 206 /(J (vanadinite) 1 - 

Radiogenic Pb 206 /(J (best) 

270

120

71

77.5

67.5

60

60

110

85

75

75

165

105

90

90

1 The isotopic composition of the lead in vanadinite was used to correct for common lead present. Analysis in Nier, 1938.

Table 2. Total and corrected Pb 207 /U23 5 ages of 41 uranium ores of the Colorado Plateaus

[in millions of years]

Mean 1st quartile Median 3d quartile

Total pb 207/CJ 235.' * 429 119- 168 339

Radiogenic Pb 207 /[J235 (vanadinite) 2 82 66.8 86.9 103.2

Radiogenic Pb207 /U235 (best)- 82 66.5 88.6 103-7

1 Data on 40 samples.

2 The isotopic composition of the lead in vanadinite was used to correct for common lead present. Analysis in Nier, 1938.

Table 3. Pb-^Vu ages and Pb 20 VPb206 ratios of 4l uranium ore samples of the Colorado Plateaus

grouped by grade of sample _________________

Grade of sample Number of Mean *>"!/& Nea« 06

(percent uranium) sample3 &> Jn millions Pb 2 °VPb206

Vi^ ' ________*____ of years 1 ratio_____

>10 16 68 0.0576

1.0-10.0 8 70 .0568

0.1- 1.0 17 75 .0533

The isotopic composition of the lead in vanadinite was used to correct for common lead present. Analysis in Nier, 1938.

12
Pb 206 /U aaes and Pb 207 /Pb 206 ratios of samples 

grouped by stratigraphic position

The mean Pb 206 /U ages of the samples tabulated 

according to the stratigraphic unit in which the deposits 

are found(table 5, p. 14) does not show any correlation 

with the age of the enclosing sediments. In spite of the 

uncertainty concerning the Pb 207 /Pb 206 ratio, it is 

interesting that the mean ratios do not show any evidence of increasing as the age of the sediments increases. The variation of only 0.4 percent in the mean 

Pb 207 /Pb2oe ratio of the ores of the Morrison and 

Shinarump suggests that the ores in both formations 

were deposited at the same time and (or) came from 

similar sources at depth.

The Pb 206/U and Pb 2 ° 7 /Pb 206 ratios ores from 

the Entrada sandstone are not very reliable because 

large common lead corrections were made. A common 

lead with an isotopic composition similar to the lead 

from a galena specimen taken at Placerville, Colo., 

results in much lower Pb 207 /Pb206 ratios for 2 of the 

3 samples.

An unsuspected relation between the deposits of 

argentiferous galena in the Pony Express limestone 

member of the Wanakah formation and the underlying 

uranium-bearing "roscoelite" ores in the Entrada 

sandstone of the Placerville area is very strongly suggested-by the systematic changes in the isotopic composition of the lead from the two different types of ores. 

The composition of the Placerville galena (table 9, 

sample GS 5,' p. 19) is bracketed by the composition of 

the leads from the ores of the Bear Creek mine and 

adjacent Primus claim (table 9, samples GS 22 and 

24, p. 19). This galena contains more radiogenic lead 

than any other galena whose isotopic composition has been 

published. The implication that the Placerville argentiferous galena deposit might also contain workable 

amounts of uranium should be carefully examined.

Pb2Q6/u ages and Pb2° 7 /Pb2Q6 ratios of samples 

grouped by mineralogic composition

A tabulation of the samples according to the major 

uranium-bearing mineral present in the ore is given 

in table 6. (Seep. 14.)

The Pb 206 /U ages of the carnotite ores range 

from 20 to 140 million years. From the field relations of individual samples as well as their chemistry, 

the writers believe that the greater ages reflect selective loss of uranium. The lower ages of the carnotite 

ores, however, may be real and may result wholly or 

partly from actual differences in age of some parts of 

the deposits. Alteration of the early Tertiary primary 

ores and redeposition of some of the secondary carnotite at different times during the Tertiary should be 

considered.

The ages of the vanadiferous hydromica ores 

are doubtful, and it cannot be stated at this time 

if the lower mean age is significant. Additional 

work on these ores is planned. An asphaltic ore 

containing uraninite from Temple Mountains gives 

a Pb 20 ?U age of 80 million years, and a black 

ore containing an undescribed*uranium mineral 

from La Sal No. 2 mine gives a Pb 206 /U age of 

90 million years.

The writers believe that the age determinations 

which were made on the Happy Jack andShinarump No. I 

uraninite specimens, 65 and 75 million years (Stieff 

and Stern, 1952, p. 707), are the most reliable that have 

been made on the uranium ores of the Colorado Plaleaus. 

The mean Pb 206 /U age of 70 million years for the two 

uraninite samples of the Shinarump conglomerate is 

believed to be within 10 million years of the true age 

of the samples. The Happy Jack specimen had the 

highest specific gravity (9.1), was the freshest in 

appearance, and contained the smallest amounts of 

common lead. The anomalous Pb 207 /Pb ratio for 

this sample cannot be readily explained on the basis 

of a required radon loss of 5.5 percent. The measured 

loss of radon from the massive uraninite from the 

Happy Jack mine is less than 1.0 percent under 

atmospheric conditions.

Although adequate pb 206 /u236 ages for the end 

of theTriassic and Jurassic are not available, the beginning of the Tertiary has been dated by Holmes 

(1946a, p. 145) who recalculated the Pb 206 /U23e age 

of two Colorado Front Range pitchblendes as 57.3 and 

59.8 million years. Two additional Front Range pitchblendes which were analyzed in the Geological Survey 

laboratory give Pb 206 /U ages of 60 and 70 million years. 

The average age of the nraninite samples from the 

Shinarump conglomerate is 70 million years, and both 

these samples show slightly high Pb 20 -/Pb 206 ratios. 

If old radiogenic lead is partly responsible for the high 

Pb 207 /Pb 206 ratios, the true ages of the Shinarump 

uraninites would probably be somewhat less than 

65 million years. The similarity in age of the pitchblendes from sedimentary rocks in the Colorado Plateaus 

and in the vein deposits in the Colorado Front Range 

is striking.

SUMMARY AND CONCLUSIONS

The calculation of the age of the uranium ores of 

the Colorado Plateaus is subject to the following 

corrections:

1. Analytical errors.

2. The presence of common lead in the ore.

3. The presence of old radiogenic lead in the ore.

4. The migration of certain radioactive daughter 

products, such as radon.

5. The loss of uranium due to alteration of the 

ore by ground waters.

The analytical chemical errors result in an uncertainty in the mean Pb 206 /U age of approximately 

±.3 million years. However, the mean lead-uranium 

ages presented are given without limits because of the 

inability of the writers at this time to evaluate quantitatively several of the remaining corrections. These 

ages, therefore, were calculated so that any changes in 

them due to more probable common lead corrections, to 

the presence of old radiogenic lead, or to loss of uranium 

will only lower the mean age. The systematic mass spectrometric errors should not increase the mean Pb 206 /U 

age by more than 3 million years. An increase of approximately 10 million years in the mean Pb 206 /U age can be 

expected only if radon loss is demonstrated to have been

13
Table 4. Pb 206 /U ages and Pb 207 /Pb 206 ratios of 41 uranium ore samples of the Colorado Plateaus

grouped by type of sample

Type of sample

Mill-pulp splits 235 or more

 5 pounds or

tons 

less 

Number of 

samples

9 

32

Mean Pb 206 /U 

age in millions 

of years 1

96

64

Mean

Pb 207 /Pb 206 

ratio

0.0523 

.0566

L The isotopic composition of the lead in vanadinite was used to correct for common lead present. Analysis in Nier, 1938.

Table 5. Pb 206 /U ages and Pb 207/Pb 206 ratios of 41 uranium ore samples of the Colorado Plateaus

grouped by stratigraphic position

Formation

Morrison formation i 

Shlnarump conglomerate --- - - 

Age of stratigraphic 

unit in millions 

of years 1

127

i "^n i Uo *

1 CO lOd

Number of 

samples

34

3

4

Mean Pb 206 /U 

age in 

millions of 

years2

72

50

73

Mean 

Pb 207 /Pb 206 

ratio

0.0550

3 .0627

.0548

1 Holmes, 1946 a, p. 145-

2 Isotopic composition of the lead in vanadinite used to correct for common lead present. Analysis in Nier, 1938.

3 This mean Pb 2O7/Pb2OS ratio is appreciably affected by the assumed isotopic composition of the common lead used because of the large percentage 

of common lead in the samples.

Table 6. Pb 206 /U ages and Pb 207 /Pb 2 06 ratios of 4l uranium ore samples of the Colorado Plateaus

grouped by mineralogic composition

Mineralogic composition

Carnotite ______ 

Vanadiferous hydromica 

Undescribed uranium minerals 

Uraninite 

Number of 

samples

34

3

2

2

Mean Pb 206 /U 

age in millions 

of years 1

72 

50Oi- °5

70

Mean

Pb 207 /Pb 206 

ratio

0.0552

2 r^yj

.0568

.0530

1 The isotopic composition of the lead in vanadinite was used to correct for common lead present. Analysis in Nier, 1938.

This mean Pb /Pb ratio is appreciably affected by the assumed isotopic composition of the common lead used because of the large percentage 

of common lead in the samples.

14
a major process on the plateaus and the only cause 

of the Pb 207 /Pb 206 ratio anomaly.

The mean Pb 206 /U age after correction for 

common lead is 71 million years. The isotopic composition of the common lead used in obtaining this 

mean age was determined by Nier. A comparison of 

the common leads of the plateaus with Nier's analysis 

of vanadinite from the Tucson Mountains, Ariz., 

(Miocene age) showed that the latter common lead 

contains less radiogenic Pb 206 and Pb 207 than any 

of the common leads of the plateaus that were 

studied. These corrected ages will therefore be 

maximum ages and any further small improvements 

in common lead corrections will only lower the mean 

Pb 206 /U age of 71 million years.

Although young Pb 207 /Pb 206 ages are not reliable and therefore are not presented in this report, 

the partly corrected but still anomalous Pb 207 /Pb 206 

ratios are included because they may contain valuable 

information on the source of the ore-forming solutions 

and the geologic processes related to the history of the 

ore. These anomalous ratios are given without precision limits because of the difficulty of quantitatively 

evaluating the very significant but small mass spectrometric errors, the actual composition of the common 

lead in the ore, the presence of old radiogenic lead, 

and the migration of daughter products. If part of the 

anomalously high Pb 207 /Pb 206 ratios are shown to be 

evidence for the addition of old radiogenic lead, then 

the source areas of the ore-bearing solutions from which 

the deposits of the plateaus were formed will probably 

be found at depth, and the lead-uranium and lead-lead 

ages will drop to approximately 6 5 million years. If, on 

the other hand, the high Pb 207 /Pb 206 ratios are shown 

to be due to the selective loss of radon, the mean 

Pb 206 /U ages will increase to a maximum of approximately 82 million years.

Of particular interest is the similarity of the mean 

Pbzoe/u ages (?2 and 73 million years) andPb 207 /Pb 206

ratios (0.0550 and 0.0548) of the ores from deposits in 

sedimentary rocks of Late Triassic and Late Jurassic age. 

These rocks are thought to differ in age by about 

30 million years, and the Late Jurassic rocks are believed 

to be about 130 million years old. The absence of any 

correlation between the age of the enclosing sedimentary 

rock and the calculated ages and ratios of the ores they 

contain not only suggests a single period of ore formation 

but also similar or perhaps common sources for both 

the uranium and the lead originally present in the ore.

The age determinations made on the Happy 

Jack and SMnarump No. 1 uraninite specimens, 

65 and 75 million years, are the most reliable that we have 

made on any uranium ore of the plateaus. Of the 2, the 

Happy Jack specimen had the highest specific gravity, 

was the freshest in appearance, and contained the 

smallest amounts of common lead. Its Pb 2o6 /U age 

of 65 million years is probably correct within approximately 5 million years. The age of the enclosing 

Late Triassic rocks, about 160 million years, is more 

than twice as great as the age of the ore these sedimentary 

rocks contain. This discrepancy of almost 100 million 

years is in striking contrast to the almost identical mean 

age of 60. 5 million years of 4 early Tertiary Colorado 

Front Range pitchblende samples, 2 analyzed by Nier 

and 2 determined in the Geological Survey laboratory.

If the true age of the ore is less than the highest mean Pb 207 /U 235 age of 82 million years, andthe 

writers believe that it is, two interpretations of the 

origin and history of the uranium deposits of the Colorado Plateaus can be made:

1. Uranium was deposited either at the same tim e 

or shortly after the enclosing Jurassic andTriassic sedimentary rocks in which it is found were laid down, or the 

uranium was deposited in one of the overlying Cretaceous 

sediments. At the close of the Cretaceous or at the beginning of the Tertiary the uranium andthe radiogenic lead that 

had been formed were taken into solution, and the uranium 

was eventually redepositedin about its present sites. The 

deficiency of radiogenic lead for a syngenetic ore indicates 

that most of the radiogenic lead formed before this Tertiary 

event could not have been redepositedin or near ore. The 

common lead, however, which was originally deposited 

in the sediments with the uranium was redepositedin the 

present sites. The selective removal of the radiogenic 

Pb 206 andPb 207 isotopes, formed before the end of the 

Cretaceous, from the ore solutions poses an extremely 

difficult problem, the solution of which is not immediately apparent. During the period of dissection and 

erosion, the deposits were partly altered, partly 

leached, and partly redistributed by circulating ground 

waters.

2. A simpler and more probable interpretation 

of the lead-isotope data is that at the close of the Cretaceous or the beginning of the Tertiary uranium, common 

lead, a little old radiogenic lead, vanadium, and other 

metals were introduced into the sediments of the plateaus. 

These metals probably had similar sources, or perhaps 

a common source, to be found at depth. The original 

uranium and vanadium ore minerals were probably 

oxides or mixtur.es of oxides. During the period of 

dissection and erosion, the deposits were partly altered, 

partly leached, and partly redistributed by circulating 

gr'ound waters.

The immediate objective of this age study of the 

uranium ores of the Colorado Plateaus was to determine 

whether these ores were deposited during the Triassic 

and Jurassic periods or whether all the deposits were 

formed at the end of the Cretaceous or the beginning of 

the Tertiary period. An unequivocal answer cannot be 

given at this time,.although the writers 1 data strongly 

suggest that the present deposits are of Late Cretaceous 

age or younger. Regardless of the final interpretation 

of the origin and history of these uranium deposits, it 

is certain that the conditions prevailing during the close 

of the Cretaceous and the beginning of the Tertiary must 

have had an important part in localizing the present 

deposits. The partial control of the ore solutions by 

both major and minor sedimentary features is easily 

demonstrated. The relations of the ore to the Late 

Cretaceous and early Tertiary intrusions of igneous 

rocks, movement of the salt, as well as to folding and 

faulting are not so clear, but perhaps the present 

reexamination of these possible relationships may show 

a connection between the regional structure and the ore 

deposits. Also, the possibility of development of the 

ore with depth might be considered, particularly an 

investigation of the Salt Wash sandstone member of the 

Morrison formation below the present "ore zone. " 

If the ore solutions were introduced from below, it is 

possible that the potentialities of the Colorado Plateaus 

as a uraniferous province have merely been touched.

15
LITERATURE CITED

Ellsworth, H. V., 1932, Rare-element minerals of

Canada: Canada Dept. Mines, Geol. Survey, Econ.

Geology, ser. 11, p. 243. 

Fischer, R. P., 1950, Uranium-bearing sandstone

deposits of the Colorado Plateau: Econ. Geology,

v. 45, p. 1-11. 

Fleming, E. H., Jr., Ghiorso, A., and Cunningham,

B. B., 1951, The specific alpha-activity of U23S :

Phys. Rev., v. 82, p. 967. 

Hess, F. L., 1914, A hypothesis for the origin of the

carnotite deposits of Colorado and Utah: Econ.

Geology, v. 9, p. 675-688. 

Hess, F. L., andFoshag, W. F., 1927, Crystalline

carnotite from Utah: U. S. Natl. Mus. Proc.,

v. 72, art. 12, p. 1-6.

Holmes, Arthur, 1931, Age of the earth: Natl. Research Council Bull. 80, p. 124-459. 

_____1946a, The construction of the geological

time-scale: Geol. Soc. GlasgowTrans., v. 21.

pt. 1, p. 117-152.

_1946b, An estimate of the age of the earth:

Nature, v. 157, p. 680.

_1947, A revised estimate on the age of the

earth: Nature, v. 159, p. 127.

Keevil, N. B., 1939, The calculation of geological 

age: Am. Jour. ScL, 5th ser., v. 237, p. 195-214.

Nier, A. 0., 1938, Variations in the relative abundances of the isotopes of common lead from various 

sources: Am. Chem. Soc. Jour., v. 60, p. 1571-1576.

_____1939a, The isotopic constitution of uranium 

and the half-lives of the uranium isotopes I: Phys. 

Rev., v. 55, p. 150-153.

_1939b, The isotopic constitution of radiogenic

leads and the measurement of geologic time II: Phys. 

Rev., v. 55, p. 153-163.

Nier, A. O., Thompson, R. W., and Murphey, B. Fv 

1941, The isotopic constitution of lead and the 

measurement of geologic time HE: Phys. Rev., 

v. 60, p. 112-116

Stieff, L. R., and Stern, T. W., 1952, The identification and lead-uranium age of uraninite from 

the Shinarump conglomerate: Science, v. 115, 

p. 706-708.

Wickman, F. E., 1939, Some graphs on the calculation of geological age: Sveriges geol. 

undersokning, Arsbok 33, p. 1-8.

_____1942, On the emanating power and the 

measurement of geological time: Geol. foren. 

Stockholm Forh., Band 64, p. 465-476.

UNPUBLISHED REPORTS

Milkey, R. G., 1952, Methods of analysis used in 

the treatment of Colorado Plateau carnotite for 

age studies: U. S. Geol. Survey Trace Elements Inv. 

Rept. 245, p. 1-19.

Phair, George and Levine, Harry, 1952, Notes on 

the differentiali leaching of uranium, radium, and 

lead from pitchblende in HzSO4 solutions: U. S. 

Geol. Survey Trace Elements Inv. Rept. 262, p. 1-23.

Phoenix, D. A., 1951, Preliminary statement of the 

analyses of ground water from the Morrison formation, southwestern Colorado and southeastern 

Utah: U. S. Geol. Survey Trace Elements Mem. 

Rept. 137, p. 1-12.

Stieff, L. R., Girhard, M. N., and Stern, T. W., 

1950, A preliminary report on methods of determining 

the age of Colorado Plateau carnotite: U. S. Geol. 

Survey Trace Elements Inv. Rept. 108, p. 24-47.

Webber, B. N., 1947, Geology and ore resources of 

uranium-vanadium deposit!onal province of the Colorado Plateau region. [ In files of Union Mines 

Development Corp.]

16
Table 7. Uranium ores of the Colorado Plateaus used for age calculations 1

Sample 

no. Location Mineralogic 

composition

Pb 206 /U 

age in millions 

of years 2

Corrected

Pfc)207/pfc206

Shinarump conglomerate

3S 63

3S 64

3S 13 

3S 66

Shinarump No. 1 claim, Seven Mile Canyon,

Grand County, Utah. 

Happy Jack mine, San Juan County, Utah 

Monument No. 2 mine, Apache County, Ariz.- 

Camp Bird No. 13 mine, Temple Mountain, 

Emery County, Utah.

Uraninite 

 do 

Carnotite type 

Asphaltic type 

TC75 

65

75 

80

n ORR8

.0501

.0570 

.0560

Entrada sandstone 3

GS 22 

GS 28 

GS 24

Bear Creek mine, San Miguel County, near 

Placerville, Colo. 

Vanadous No. 1 mine, San Miguel County, 

Colo. 

Primus claim, San Miguel County, Colo. 

Vanadiferous 

hydromica. 

 do -

____ rtn ___________

20 

40 

90

.0578 

.0603 

.0692

Morrison formation

GS 20 

GS 65

GS 21 

GS 31 

GS 33 

GS 34 

GS 35 GS 42" 

GS 43 

GS 10 

LRS 28 GS 41" 

GS 40" 

LRS 63 2 

GS 11 

GS 12 GS 48" 

GS 30

GS 47" 

GS 39 3"

GS 18 

GS 15 1 

GS 15 2 

GS 19 1 

GS 19 2 

LRS 64 1 GS 49" 

GS 14 

GS 17 

GS 23 

GS 50"

GS 16

LRS 20 2 

GS 51"

Roc Creek mine, Montrose County, Colo. 

La Sal No. 2 mine, Mesa County, Colo. 

Calamity No. 13 mine, Mesa County, Colo. 

 do 

Calamity No. 21 mine, Mesa County, Colo. 

Calamity No. 27 mine, Mesa County, Colo. 

Club mine, Montrose County, Colo. 

Mill No. 1 mine, Montrose County, Colo. 

Rock Raven No. 3 mine, Montrose County, 

Colo. 

Coloradium mine, Montrose County, Colo. 

Long Park No. 10 mine, Montrose County, 

Colo. 

Bitter Creek mine, Montrose County, Colo.- 

Jo Dandy mine, Montrose County, Colo. 

 do 

Wild Steer mine, Montrose County, Colo. 

 do 

Bob Tail mine, Montrose County, Colo. 

Red Bird mine, Montrose County, Colo. 

Mine D mine, Montrose County, Colo. 

Butterfly mine, Montrose County, Colo. 

Raven mine, Montrose County Colo. 

Charles T No. 2 mine, San Miguel County, 

Colo. 

Radium No. 6 mine, San Miguel County, 

Colo. 

 do 

Radium No. 8 mine, San Miguel County, 

Colo.

Carnotite type 

New uraniumvanadium 

mineral. 

Carnotite type 

 do 

____ rtn ___________

____ rtn ___________

 do 

 do 

 do 

 do 

 do 

 do 

___ _rtn ___________

____ do ___________

____ rtn ___________

 do 

 do 

 do 

 do 

 do 

 do 

____ rtn ___________

__ , _ HO _____ . _____

 do 

 do 

____ rin _ . _________

60 

90

75 

75 

85 

75 

105

100 

140 

70 

75 

90 

95 

75 

60 

70 

85 

15

75 

110

70 

20 

60 

40 

20 

30 

75 

100 

35 

70 

120

60

30 

100

.0537 

.0570

.0523 

.0617 

.0601 

.0599 

.0547 

.0560 

.0608 

.0610 

.0605 

.0541 

.0526 

.0584 

.0593 

.0582 

.0531 

.0437

.0537 

.0467

.0525 

.0653 

.0594 

.0466 

.0464 

.0607 

.0518 

.0543 

.0542 

.0605 

.0521

.0530

.0511 

.0502

See footnotes at die end of die table.

17
Table 7. Uranium ores of the Colorado Plateaus used for age calculations 1 Continued

Sample 

no. Location Mineral Formation

Common lead

GS 1 

GS 2 

GS 7

GS 9 

GS 67 

GS 68

GS 8 

GS 5

GS 6 

GS 70

Rifle mine, Garfield County, Colo. 

Garfield mine, Garfield County Colo. 

Bitter Creek mine, Montrose County, Colo. 

La Sal No. 1 mine, San Juan County, Utah 

Cougar mine, Lower group, San Miguel 

County, Colo. 

Gap Claim, San Miguel County, Colo. 

Philadelphia Claim, Placerville, 

San Miguel County, Colo. 

Argentine mines, San Juan County, Colo. -

Blanding water tunnel, Abajos Mountains, 

San Juan County, Utah.

,_ _r]n __ - __ - __ - Descloizite? 

(?)

 do 

(?)

rj-al p>na -. -. 

 do- 

 do 

Entrada sandstone?. 

Do. 

Salt Wash sandstone member of 

Mt>rrison formation. 

Do. 

Dakota sandstone. 

Salt Wash sandstone member of 

Morrison formation. 

Do. 

Pony Express limestone member 

of the Wanakah formation. 

(?) 

(?)

The isotopic composition of the lead in vanadinite was used to correct for common lead present. Analysis in Nier, 1938. 

2 Age expressed to nearest 5 million years.

9 The Pb2 7/Pbaoe ratio is appreciably affected by the assumed isotopic composition of the common lead present because of the large percentage of 

common lead in the samples. 

4Mill pulp sample.

Table 8. Lead and uranium content of uranium ores used for age calculations

[Analysts, R. G. Milkey, Harry Levine, and Frank Cuttitta, U. S. Geological Surrey]

Sample no.

GS 10

11

12

13

14

15-1

15 2

16

17

18

19 1

19 2

20

21 1

22

25

24

28

50

51

55

g Pb/g sample

0.0053

.0052

.0055

.00185

.00176

.0008

.00166

.00128

.000705

.000052

.0011

.000116

.000060

.000095

.000552

.00062

.00118

.000102

.000025

.000500

.000402

g U/g sample

0.525

.522

.148

.1554

.1168

.511

.1910

.1445

.0740

.0036

.0465

.0086

.0047

.0077

.0064

.0541

.0022

.0051

.0046

.0256

.0514

Sample no.

GS 54

55 1

59 5

40

41

42

45

47

48

49

50

51

65

64

65

66

LRS 20 2

28 1

65-2

64 1

g Pb/g sample

0.000254

.000515

.000155

.000068

.000074

.000052

.0047

.000062

.000089

.00057

.000052

.000152

.0145

.0074

.0014

.00059

.0015

.0058

.0051

.0014

g U/g sample

0.0228

.0209

.0021

.0045

.0049

.0051

.241

.0055

.0046

.0027

.0018

.0054

.559

.7346

.102

.0290

.526

.360

.254

.346

18
Table 9. Isotopic composition of lead from uranium ores and lead minerals of the Colorado Plateaus

[Analyses by Carbide and Carbon Chemical Co., Y-12 Plant, Mass Asssy Laboratory, Oak Ridge, Term.]

Sample Atom percent abundance of lead isotopes

204 206 207 208

GS

GS 

GS 

GS

GS 

GS 

GS 

GS

GS 

GS

GS 

GS 

GS

GS 

GS

GS 

GS

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS 

GS

1-

2 

6 -

7-1- 

7 2-

9-1-

11 -

12 -

13 -

14 - 

GS 15 1- 

GS 15 2- 

<js 16 -

17 -

18 -

19 1-

19 2-

20 -

21 1-

22 -

23 -

24 - 

28 -

30 -

31 -

39 3-

40 -

41 - 

42___- 

43 -

47 -

48 -

49 -

50 -

51 -

63 -

64 

66-

67-

68- 

70-

20 2-

.38 +

.37 + 

.363 + 

.40

.37 

.36 

.39 

.38 

.38

.029 + 

.227 + 

.820 + 

.169 +

.150 + 

.159 + 

.034 + 

.138 + 

.7^6 + 

.506 +

.08 + 

.10 +

.505 +

.184 + 

.36 + 

.696 +

.36 +

.18 +

.768 + 

.107 + 

.082 + 

.113 + 

.057 + 

.09 + 

.245 + 

.223 + 

.275 + 

.042 + 

.614 + 

.588 + 

.30 + 

.608 + 

.900 + 

.839 + 

.15^ + 

.086 + 

.257 + 

.35 + 

.36 + 

.36 +

.01

.03

.008

.01

.02

.02

.05

.01

.01

.003

.004

.013

.012

.003

.009

.003

.003

.008

.026

.02 

.01

.010

.003

.02

.015

.01

.01

.010

.01

.01

.001

.003

.02

.003

.003

.006

.005

.17

.001

.01

.003

.007

.01

.003

.001

.01

.01

.02

.003

13 2- 

LRS 64 1-

Vanadinlte'

.151 + .011

.071 + .007 

.176 + .002

.108 + .003 

1.37

26

26

27.00 + .05

25

25

25

28

26

26

92

83

55

86

91

89

90

88

92

39

84

84

83

93

64

66

28

65

49

53

88

90

82

26

25

25

10 +

07 +

00 +

29 +

73 +

78

02

20

20

92

76

57

87

40

40

23

59

^1

51

85

11

86

26

36

29

78

47 +

36 +

51 +

02 +

93 +

53 +

36 +

20

39

33

10

55

36

66

27

85

54

14

79

85

01

90

33

93.23 +

88

59

71

41

39

71

87

27

60

26

36

.05 

.05

04

.08

.05

+ .03 

.03 

.05 

.03 

.08 

.02 

.03 

.03

.01

.02

.11

.09

.03

.04

.05

.03

03

.06

.02

.05

57.36 + .05

10

.02 

.03 

.02 

.02 

.06 

.05 

.03 

.03 

.08 

.03 

.03 

.06 

.02 

.05 

.03 

.02 

.12 

.03 

.03 

.02

86.91 + .05

92.76 + .04

86.58 + .03

90.25 + .02

25.20

20.

21.

20.

21. 

21. 

20.

20.

21. 

21.

6.

8. 

14.

7.

6.

6.

5.

6. 

13.

10.

17.

17.

10

6. 

20. 

13. 

20. 

18. 

13.

6.

6.

6.

5.

17. 

7.

7-

7

5

12

11

20

11

15

14

21

21

21

97

00

86

48

09

97

45

02

06

05

00

34

16

64

64

87

51

35

46

28

94

43

7776

36

91

72

48

86

65

88

71

88

51

55

90

99

13

68

46

90

46

74

43

28

03

03

05

12

6.79 + 

6.04 + 

7.31 + 

6.62 +

21.27

.02 

.03 

.03 

.03 

.09 

.18 

.04 

.02 

.03 

.07 

.02 

.03 

.04 

.02 

.03 

.02 

.02 

.06

.02 

.05 

.03 

.03

.01 

.03 

.07 

.03 

.04 

.04 

.06 

.03 

.01 

.01

.02 

.02

.03

.02 

.02

.08 

.02 

.02 

.03 

.01 

.04 

.03 

.03 

.05 

.02 

.08 

.02

.03 

.02 

.02 

.02

51- 

51.

50.

51. 

51. 

51. 

50, 

51- 

51.

29 

5 

4, 

1,

26

17

39 

41 

17

5- 

50.

25.66 

50, 

43, 

28,

3

3.

4.

1,

41,

22. 

21, 

49. 

22,

33-

30.88

5-

2,

8.

51-

51.

52.

55 + .05

56 + .08

77 + .05

84 + .04

8l + .02

90 + .07

14 + .05

41 + .04

36 + .04

00 + .02

01 + .02

27 + .10

80 + .02

82 + .02

80 + .01

871 + .013

76 + .02

50 + .09

53 ± .09

80 + .02

86 + .04

21 + .02

78 + .02

52 + .02

	+ .06 

94 + .03 

64 + .05 

39 + .03 

53 ± .07 

25 + .02 

08 + .02 

70 + .01 

67 + .02 

05 + .05 

84 + .01 

49 + .02 

872 + .007 

70 + .08 

37 + .04 

58 + .03 

23 + .05 

79 + .02

	+ .04 

27 + .02 

84 + .03 

86 + .09 

61 + .05 

69 + .03 

19 + .05

6.14 + .04

1.13 ± .03

5.94 + .02

3.02 + .02

52.17

1 The limits of precision are calculated so that 95 percent of any new determination will fall within the limits given.

2 Isotopic composition of lead in wulfenite and vanadinite in Nier, 1938. The probable error in the messurements on this sample is of the order of 

0.5 percent.

19 IMT.-DOP. SEC., HUSH., D.C. t»it5»S-