Study of Radioactivity in 

Modern Stream Gravels as 

a Method of Prospecting

GEOLOGICAL SURVEY BULLETIN 1030-E

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of the 

Commission

Study of Radioactivity in 

Modern Stream Gravels as 

a Method of Prospecting

By RANDALL T. CHEW, III

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

GEOLOGICAL SURVEY BULLETIN 1030-E

The detection of radioactivity anomalies 

by means of portable instruments of sealer 

type, and application of the method in 

locating deposits of radioactive ores in the 

Colorado Plateau

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and is published with the permission of the 

Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Fred A. Seaton Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 25 cents (paper cover)
CONTENTS

	Page 

Abstract________________________________________________________ 149

Introduction. _____________________________________________________ 149

Previous investigations.____________________________________________ 149

General considerations_____________________________________________ 150

Field methods_______________________________________ 158

Anomalies ________________________________________________________ 160

Areas studied-____________________________________________________ 162

Results, and evaluation of the method, _ _____________________________ 166

Suggestions for future work_________________________________________ 167

Literature cited-2_________________________________________________ 169

ILLUSTKATIONS

PLATE 8. Graph showing radioactivity in stream gravels, Temple Mountain area, San Rafael district, Emery County, Utah___ In pocket

Page 

FIGUKB 33. Index map of part of Colorado Plateau showing areas where

radioactivity of modern stream gravels was studied-______ 153

34. Radioactivity in modern stream gravels range and average 

of four 2-minute readings at each probe setting, Corral 

Canyon, Seven Mile area, Grand County, Utah__________ 154

35. Radioactivity in modern stream gravels range of two 4- 

minute readings at each probe setting, Corral Canyon, Seven 

Mile area, Grand County, Utah________________________ 155

36. Radioactivity in modern stream gravels range of two 

6-minute readings at each probe setting, Corral Canyon, 

Seven Mile area, Grand County, Utah__________________ 156

37. Radioactivity in modern stream gravels one 8-minute 

reading at each probe setting, Corral Canyon, Seven Mile 

area, Grand County, Utah___________-_______-___---_-_ 157

38. Radioactivity of Colorado River gravels near Climax Uranium Company mill, Grand Junction, Colo______________ 161

39. Diagrammatic sketch of the Temple Mountain area, San

Rafael district, Emery County, Utah, for use with plate 8_ 163

40. Graph showing radioactivity in stream gravels and a comparison of radioactivity to grain size of sediments, Corral 

Creek, Seven Mile area, Grand County, Utah____________ 165

41. Radioactivity of gravel in washes draining Happy Jack mine 

and first canyon west, White Canyon area, San Juan 

County, Utah___-_-______.____________________ 168

m

CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

STUDY OF RADIOACTIVITY IN MODERN STREAM 

GRAVELS AS A METHOD OF PROSPECTING

BY RASTDALL T. CHEW, III

ABSTRACT

Traverses along some streams of the Colorado Plateau in areas known to 

contain minable uranium deposits show that anomalous radiation in the stream 

gravels can be detected with a suitable counter downstream from the deposits.

Intensity of radiation is influenced by the- size of the uraninum deposit, the 

size of the drainage area of the stream, the grain size of the sediments, and 

the lithology of the rocks over which the stream flows.

The spacing of the stations where readings are taken is controlled by the size 

of the stream, and special readings are also taken just downstream from important tributaries. An anomaly is empirically defined as a 10 percent rise over 

background.

Radioactive material from large uranium deposits has been detected as much 

as 1 mile downstream. Radioactive material from smaller deposits is detectable 

over shorter distances. The method is slow but appears to be a useful prospecting 

tool under certain conditions.

INTRODUCTION

This report describes investigations of the radioactivity of gravels 

in modern stream channels undertaken by the U. S. Geological Survey 

on behalf of the Division of Raw Materials of the U. S. Atomic Energy 

Commission. The purpose of this study has been to determine 

whether this radioactivity can be used as a guide in locating uranium 

and thorium ore deposits or mineralized areas. Some of the ideas 

presented here are based on assumptions or working hypotheses; 

further work may change these assumptions. However, the author 

feels that the assumptions are basically correct, and that radioactivity of stream gravels can be a useful prospecting tool under some 

conditions.

PREVIOUS INVESTIGATIONS

As early as 1945, the Geological Survey had made investigations of 

the radioactivity of stream gravels in Alaska. R. S. Cannon, tfr. 

(unpublished report, 1953) reported that the results of four radiation

149
150 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

measurements made in alluvium near uranium deposits in South 

Dakota indicated a detectable radioactivity anomaly downstream from 

the uranium deposits.

As a follow-up to Cannon's work, W. E. Bales (personal communication) completed a traverse of about a dozen stations through an 

intensely mineralized area in South Dakota. Bales found that an 

anomaly did exist in the stream gravels downstream from the deposits. 

This anomaly was of about the same magnitude as those described in 

this report.

Shawe * describes a study of stream gravels on the eastern slope 

of the Sierra Nevada where he measured radioactivity of the gravels 

with a Geiger counter equipped with a neon light for the counting 

device and also made a detailed study of the heavy minerals. The 

sources of the stream gravels described by Shawe are granitic and 

metamorphic rocks, and the radioactivity in the gravels appears to 

have monazite as its chief source in some drainages and ilmenite in 

other drainages. The source rock of the various radioactive minerals 

is discussed in some detail. Most of the radioactivity detected comes 

from thorium minerals; no minable uranium deposits have been found 

in the Sierra Nevada area. Some of the concepts and results described 

by Shawe may find application as a possible prospecting tool in future 

study of radioactivity in modern stream gravels.

GENERAL CONSIDERATIONS

The primary purpose of this study was to determine whether uranium ore deposits contribute a measurable amount of radioactive material to the streams that drain them. This material is detected by its 

radioactivity. Other factors which affect the amount of radioactivity 

measured must also be considered. These include fluctuations in 

radiation caused by the diurnal variation in background and differences in the natural radioactivity of various lithologic units. Secondary considerations which influence the validity of the anomalies are: 

selection of the most suitable radiation instrument for the work, 

statistical considerations affecting the adequacy of a reading or series 

of readings, effect of variation in a stream size and drainage area, and 

effect of variation in grain size of the alluvium.

A sealer type instrument must be used in measuring the radioactivity in modern stream gravels because the scaling circuit records 

each gamma ray counted by the Geiger-Muller tube. This feature 

makes small variations in radiation detectable when recorded on a 

counts-per-minute basis over an adequate period of time. A counter

«

1 Shawe, D. R., Heavy detrital minerals in stream sands of the eastern Sierra Nevada 

between Lewining and Independence, Calif. : Calif. State Div. Mines Special Kept, in 

preparation.
RADIOACTIVITY IN MODERN STREAM GRAVELS 151

equipped with a ratemeter showing only radiation detected over a 

relatively short time interval will not show these small variations with 

a sufficient degree of certainty because of the high random variation 

of gamma rays (both as to intensity and direction) at the low counting rate. A portable sealer equipped with a Geiger-Muller tube with 

an effective cathode area of 3 square inches was used in this study. 

At all levels of radioctivity encountered, this tube had sufficiently low 

sensitivity so that all counts recorded by the tube were shown on the 

scaling circuit. The Geiger-Muller tube used is similar to those 

found in most portable counters.

The author made several readings in areas of anomalous and nonanomalous radiation using both a Geiger counter and a scintillation 

counter equipped with ratemeters. Neither of these instruments 

showed any anomalies in areas which are considered anomalous on the 

basis of the sealer readings.

The sealer should make more than 400 counts during the reading 

period in order that the probable error of the reading be safely within 

the limits which constitute an anomaly. The formula for percentage 

probable error mentioned by Faul (1954, p. 22) can be used in computing the number of counts necessary during a single reading period. 

Faul gives the formula £>=0.6745 V-A7 where p equals the probable 

error of a given reading and N equals the number of counts recorded. 

He then states that the "percentage probable error is the ratio of the 

absolute probable error to the value of the quantity measured expressed

0 6745 in percent." Stated algebraicallv, this reduces to E= ' _ XlOO * J' V^

where E equals the percentage probable error and N equals the number of counts recorded as before. Faul shows a graph of this percentage probable error. The probable error of 400 counts is ±3.35 

percent. Experience has shown that if 8-minute readings are made 

using an instrument equipped with the Geiger-Muller tube just described, the relative probable error will be ±3.3 percent or less. This 

relative error is low in comparison with the increase of approximately 

10 percent over the normal radioactivity that is considered anomalous; 

and, if 8-minute readings are made, the statistical error will always 

be low enough that anomalous readings can be considered valid.

At stations which are obviously giving high readings (more than 

100 counts per minute) the reading can be stopped after 500 or 600 

counts have been recorded and the average can be computed. If 625 

counts are recorded, the probable error is ±2.68 percent. If the 

reading is obviously anomalous, as shown by a high counting rate, the 

probable error becomes unimportant.
152 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Variations in radioactivity over a limited geographical area are 

unimportant. The table below shows the results of two traverses

Average reading (counts per minute) and range of three readings taken in closely 

spaced holes at each station, Seven Mile area, Grand County, Utah

Station No. Average Range 

Seven Mile Canyon traverse

I.. ............ ......-.-.... 59

2 . . . 57

3-_...... ..-.........-. -_.-__-. 53

4 .. .. .--- . 53

5. .-- ...-..--.. .-.-..---- 54

58-61

52-63

51-56

49-58

53-55

Corral Canyon traverse

3...----._--._...-.----___._. ...---.- 107 92-116

4_........._....__..._._._............ 84 80-87

5... .....-......................... 99 97-101

made to show variation in readings obtained by moving the probe 

slightly between readings. Three readings were taken at each station, 

but the probe was moved slightly between readings. At six stations 

the readings were taken on a triangular grid with the corners about 

4 feet apart. At two stations the readings were spaced about 4 feet 

apart in a line parallel to the stream flow. The readings show a maximum range of 24 counts per minute (about 50 percent) caused by moving the probe slightly between readings. In general, the range is 

least where the radiation is at the lowest level and greatest where 

radiation is at the highest level. The range is great enough that 

probably no single reading less than 20 percent above average should 

be considered anomalous. Three or more stations with readings of 

more than 10 percent above average can be considered anomalous. 

Apparently a longer low peak is more likely to occur downstream 

from a uranium deposit than a single high one.

In Corral Canyon, Seven Mile area, Utah, (fig. 33), the range in 

a series of reading shown in figures 34-37 illustrates the error made 

possible by readings of insufficient duration. The probe setting numbers at the top of each graph denote the same setting of the probe on 

each graph. Stations 3, 4, and 5 consist of three readings in different 

holes within about 4 feet of each other. An 8-minute reading was 

taken in each hole. While the measurements were being taken, the 

sealer was read at 2-minute intervals to obtain data for the shorter 

readings. Figure 34 shows the average and range of four 2-minute 

readings. The others show the ranges of two readings except figure 

36 which shows only one reading.
RADIOACTIVITY IN MODERN STREAM GRAVELS 153

-4 E M E-^ Y Jj 6 Rl A N D 

Green River

SEVEN MILE 

AREA

AA PLATA 

Cortez / ofourango

COLORADO

FIGUEE 33. Index map of part of Colorado Plateau showing areas where radioactivity 

of modern stream gravels was studied.

The dashed line connects the same readings on each graph. The 

readings connected by the dashed line have been arbitrarily selected 

but might have been obtained in a traverse where only one reading of 

the duration shown on each graph was taken at each station. The 

dashed line shows two reverses in pitch as the result of the theoretically 

improved "sampling." The first change is between stations 1 and 2 

and the other is between stations 3b and 4b.

To some extent, an anomally is influenced by the size of the deposits 

and the size of the stream draining the area. A deposit in the drainage 

area of a small stream will give a larger anomaly than the same size 

deposit in the drainage area of a large stream because the greater 

amount of barren sediments in the large stream tend to mask any 

anomalous radioactivity. On the basis of reserve estimates and pro379303 56 2
154 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

Js £

31HNIW d3d SINOOD
ISO

STATION MUMBER AND PROBE SET

3b 3c 4o 4b 

Line connecting the \ 

same paints as an ' 

figures 34,36,and 37

FIGURE 35. Radioactivity in modern stream gravels range of two 4-minute reading

area, Grand County, Utah.
150

140

130

120

110

100

90

80

70

60

50

3o

STATION NUMBER AND PROBE SET

3b 3c 40 4b 

Line connecting the 

some points as on 

figures 34, 35,and 37

FiatJRB 36. Radioactivity in modern stream gravels -range of two 6-mlnute readings a

Grand County, Utah.
RADIOACTIVITY IN MODERN STREAM GRAVELS 157

310NIW H3d SlNflOD
158 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

duction records, deposits have been considered for this report to be 

either "small" deposits containing less than 1,000 tons of ore, or "large" 

deposits containing more than 1,000 tons. Streams were classed as 

"small," having a width of less than 10 feet where its width is not controlled by the local topography; "intermediate," with a width of from 

10 to 35 feet; and "large," with a width greater than 35 feet. This 

width, reflects, to some extent, the size of the drainage area of the 

stream and the amount of sediment carried by it.

The grain size of the rock probably has some effect upon the radiation through the "mass effect." Sediments of a small median grain 

size show higher radioactivity than sediments of similar composition 

and larger median grain size. Apparently this is caused by close 

packing of the small material around the probe.

It was believed, early in the work, that diurnal variations in the 

cosmic-ray background radiation might affect the readings; therefore, a reading of the background was taken at every station. Headings of the background radiation were also taken in several areas at 

about 1-hour intervals over several days. This was done to obtain 

some record of the diurnal variations in background radiation and also 

to determine the effects of background readings on the radiation recorded from stream gravels. These background readings were about 

the same as or slightly lower than readings in barren gravels and 

seem to have no adverse effect 011 the readings taken in the gravels. 

Readings taken with the probe buried 6 inches in the gravels probably 

record gamma radiation derived from both cosmic rays and radioactive 

material in the sediments. The radiation recorded is the sum from 

these two sources and thus the cosmic-ray background radiation has 

no practical effect on the readings in the gravels.

FIELD METHODS

Iii this study the intensity of gamma radiation was measured at 

stations along streams that cross known deposits and also along 

streams that cross no known deposits. Spacing of the stations was 

determined by the size of the stream, but in many places this spacing was reduced where the amount of radioactivity detected in the 

gravels began to approach the anomalous level.

On small streams, the stations can be spaced at 0.2 mile intervals 

with spacing narrowed to 0.1 mile where an anomaly is suspected. 

Intermediate streams can have the stations spaced at intervals of 

half a mile. These can be narrowed to 0.2 mile intervals where an 

anomaly is suspected. Readings from stations spaced 0.1 mile apart 

on streams of intermediate size will probably not give any additional 

useful information.
RADIOACTIVITY IN MODERN STREAM GRAVELS 159

A somewhat different technique is necessary on large streams, especially perennial ones, because the amount of radiation seems to vary 

significantly on opposite banks of a large stream. In wide streams 

it appears possible to determine from which side of the stream the 

radioactive material is introduced by taking closely spaced readings 

on each side. For ordinary reconnaissance work on large streams, 

stations on one side of the stream are sufficient; stations can be spaced 

1.0 mile apart and narrowed to 0.5 mile where an anomaly is suspected. 

If readings are made on each side of the stream, it appears practical 

to narrow the spacing to 0.2 mile. This may give additional information as to the source of an anomaly. In an entirely unexplored 

region, stations spaced at wider intervals might give some useful 

information as to the location of large areas for more intense exploration. Stations spaced at intervals of 2 to 3 miles might conceivably 

have some significance in an entirely untried area, but more closely 

spaced stations should generally be used. A traverse along the Colorado River from Kifle, Colo., to Moab, Utah, using stations spaced at 

intervals of 5.0 miles, gave readings which have been interpreted as 

too unsystematic to be useful.

The probe was buried 6 inches below the surface in the stream 

gravels and covered as well as possible. Complete burial of the 

probe is important. If it is not well covered, the readings will be 

lower than the true radioactivity of the station. Where the ground is 

frozen or boulders make burial difficult, a small mound of loose material should be piled over the probe to simulate burial.

Because apparently they do not affect the radioactivity of the 

alluvium, moisture content and abundance of organic matter need not 

be noted in prospecting. Topographic features such as constrictions 

in the streams and the location of the station in relation to major 

bends which might affect deposition of gravels should be noted.

A sketch map of the stream is useful in interpreting the results of 

the traverse. The map can be made by using a compass and some 

means of measuring distance either the odometer of a car or pacing 

and stratigraphy, structure, and topography can be plotted in relation 

to the location of the stations while the readings are made. To make 

anomalous areas more easily recognizable the readings are plotted on 

graph paper. The distances between stations and the various geographic check points are plotted on the abcissa and the readings, in 

average counts per minute, are plotted on the ordinate. Readings 

both in gravels and in country rocks, the average size of the smallest 

grains, and points where the contact between distinctive stratigraphic 

units are crossed can be plotted on the graphs.
160 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

ANOMALIES

The number of counts per minute defined in this section as constituting an anomaly is empirical and is based on the field work. These 

figures have proven sufficiently consistent within the limits given here. 

Traverses have been conducted only over barren rock, mill tailings 

piles, and uraninite-type deposits. No traverses have been made over 

vanadium-bearing carnotite-type deposits or oxidized deposits that 

do not contain vanadium. A significant variation from the figures 

cited here may occur in the amount of radioactive material in sediments downstream from these other types of deposits, especially if 

the source of the radiation in the gravels is detrital uranium minerals. 

These minerals have differing solubilities and this may produce various 

amounts of radiation.

Barren gravel shows radioactivity ranging from 55 to 65 counts per 

minute regardless of grain size. However, single readings as high as 

72 counts per minute should not be considered anomalous. A continuous series of readings at several stations averaging less than 55 

counts per minute should be questioned as the sealer may not be 

functioning properly.

Single readings of more than 80 counts per minute, or three or more 

consecutive stations giving readings of more than 72 counts per minute 

can be considered anomalous. An area which gives unequal readings 

of 65 to 75 counts per minute at several stations may be anomalous 

and should be investigated further. This irregularity has been noticed near smaller deposits that do not show definite high counts, and 

is most common in small streams.

In summary, a rise of 10 percent in the average number of counts 

per minute above the average background can be considered anomalous. This figure of 10 percent is a convenient one derived from 

the statements in the preceding paragraphs, that is, multiple readings of 72 counts per minute is considered anomalous over an average 

of 65 counts, a rise of 7 counts or about 10 percent.

The amount of radiation recorded from the gravels tends to rise 

slowly as the deposit is approached from the downstream side, reaches 

a peak where sediments from the deposit are added, and falls off very 

sharply upstream. The best example of this tendency obtained in 

the present study is shown in figure 38, which shows readings taken 

near the Grand Junction, Colo., mill of the Climax Uranium Company. Wet tailings from the mill are emptied into a marsh near 

the river bank. The curve of the actual readings taken is shown on 

the graph, and the projected curve is shown also. The maximum 

number of counts per minute at the mill as shown on the projected
RADIOACTIVITY IN MODERN STREAM GRAVELS 161

Host 

110

IOO

90

80

70

West

50

Direction of flow

EXPLANATION

Projected curve showing 

theoretical radioactivity

Curve showing actual 

measurements

DISTANCE,IN MILES

FIGURE 38. Radioactivity of Colorado River gravels n«ar Climax Uranium Company mill,

Grand Junction, Colo.

curve are, of course, conjectural. Most curves are much more irregular, but they follow this general pattern.

Geomorphic control of radioactivity does not seem important. 

Anomalous radioactivity is apparently not affected by the rate of flow 

or the location of bends in the channel.

The most important outside influences on the variation of the 

amount of radioactivity in stream gravels are the introduction of 

material by side streams and the lithologic variations of the country
162 CONTRIBUTIONS TO THE GEOM)GY-.Off ;

rock. Special stations should be placed immediately below large side 

streams because a high reading below an important side stream would 

indicate that it should be investigated. Any radioactivity of sediments from a side stream is, within a short distance, masked by the 

greater amount of nonradioactive sediments in the main stream; 

therefore, the reading should be taken within a hundred yards below 

the confluence. On a large stream, the reading should be taken on 

the side on which the side stream enters.

The gravels in streams over some stratigraphio or lithologic units 

may show abnormally high radioactivity over wide areas which do 

not contain any minable deposits. The stream gravels on the Mancos 

shale of Late Cretaceous age exhibit this property wherever they have 

been checked during this investigation. At least 30 percent of the 

readings in stream gravels over the Mancos shale have exceeded 70 

counts per minute; the highest reading was 84.

The tributary streams that flow into the Colorado River from the 

north between Palisade, Colo., and Dewey, Utah, cross the Mancos 

shale. These tributaries give high readings and might give the impression of being anomalo.us if this special characteristic of the country 

rock were not taken into account. However, there is no noticeable 

increase in the radioactivity of the Colorado River alluvium in this 

area. Apparently the amount of alluvium from the Mancos contributed to the river gravels is too small in relation to the amount 

of less radioactive gravels already present to affect noticeably the 

radioactivity of the river gravels.

Where the country rock is thought to be abnormally radioactive, a 

few readings should be taken on fresh outcrops. If they are abnormally radioactive, the number of counts per minute considered necessary to constitute an anomaly should be revised upward. A 10-percent 

rise in the number of counts per minute over the higher background 

can then be considered anomalous. Thus, where 85 is the average 

number of counts per minute in the gravels, 93 or more counts could 

be called anomalous.

AREAS STUDIED

The results of two traverses are explained here in detail because 

they demonstrate all the controlling factors discussed in preceding 

sections of this paper.

Traverses in the, Temple Mountain region, Utah, revealed three 

areas that show anomalous radioactivity (fig. 33, pi, 8). One is the 

main Temple Mountain uranium deposit, which is drained by the 

stream here referred to as the north wash (fig. 39). This wash is of intermediate to large size and drains a large area of nonmineralized rock
RADIOACTIVITY EST MODERN STREAM GRAVELS 163

Moenkopi formation 

ond older rocKs

Shinorump conglomerate 

and younger rocks

Cliff face along which 

Shinorump conglomerate is exposed

Adit

1 Mile

Approximate scale

FIGDBB 39. Diagrammatic sketch of the Temple Mountain area, San Rafael district, Emery 

County, Utah, for use with plate 8.

in addition to the main workings. The readings are irregular (pi. 8) 

without being definitely anomalous. The gravels of the side streams 

gave anomalous or nearly anomalous readings. A side stream that 

drains only areas of unaltered Moenkopi gave readings (pi. 8, sta. 37, 

38) well below the average for the north wash as a whole. The irregularity of the graph showing the amount of radioactivity in the
164 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

gravels in the main stream and the general high count of the gravels 

in the side streams is indicative of anomalous radioactivity and, if 

the area were new, would suggest need for further exploration.

South wash (fig. 39, pi. 8) drains an area of mineralized Shinarump 

conglomerate that has been thoroughly prospected, but contains only 

the one minable deposit shown in figure 39 near the confluence of the 

three washes. The amount of Shinarump in the drainage area is 

fairly large, and the radioactivity anomaly is more pronounced than 

that on the north wash. If the area were new, this type of anomaly 

would warrant further exploration.

West wash (pi. 8) has a drainage area of less than 1 square mile and 

is underlain by the Moenkopi formation. About one-half of this area 

is composed of interbedded sandstones and siltstone of the Moenkopi 

showing some alteration under the sandstone beds. This entire unit 

is slightly radioactive. The large amount of slightly radioactive material caused a very sharp anomalous reading. However, the countryrock reading, not shown on the graph, was exactly the same as that in 

the stream, and it was obvious that the anomaly was more apparent 

than real. Upstream from this slightly radioactive area the readings 

are below the anomaly level. If the region were unexplored, further 

investigation would be justified. Careful observation and note-taking 

while traversing the west wash showed the anomaly in the Moenkopi 

to be of small extent and low grade.

The smoothness of the curve (pi. 8) below station 9, which represents the confluence of the three washes, is noteworthy. Temple Wash 

is outside the mineralized area, and any sediments contributed to it 

below station 9 are not significantly radioactive.

The graph also shows, below station 9, that the radioactivity tends 

to decrease to a constant with increasing distance downstream from 

an ore deposit. The constant here averages about 63 counts per minute.

Figure 40 shows the graph of the traverse along Corral Creek in 

the Seven Mile area, Grand County, Utah. The Shinarump ISTo. 3 

mine is located on the east side of Corral Creek Canyon. The stream 

is of intermediate size. This was the first traverse made and 2-minute 

readings were taken at each station. The possible relative error 

ranged from ±7.6 to ±9.2 percent. These figures show that the 

observation time was too short; statistically the chances are about even 

that the average number of counts would show either an anomaly 

where none existed or would miss an anomaly where one did exist. 

Apparently, however, the average number of counts obtained at each 

station was somewhat nearer the number that would have been obtained with a longer counting period than the probable relative error 

formula shows; thus, the graph has some value. This occurred purely
1750 2000 2250

DISTANCE.IN FEET

FIGOEE 40. Graph showing radioactivity in stream gravels and a comparison of radio

Seven Mile area, Grand County, Utah. Grain-size designations are based on wo

sand" includes all sizes smaller than fine sand, "sand" includes all sizes from 

includes granules, and "boulders" includes all sizes larger than pebbles in the We

the smallest class forming an important (> 10 percent) component of the alluvium
166 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

by chance. The curve showing radioactivity is probably much more 

irregular than it would have been had longer readings been taken.

This graph includes a line showing grain size of the stream gravels 

plotted above the line showing the radioactivity. The relation between grain size and the amount of radioactivity is well shown, especially from stations 10 to 17. Especially high readings were obtained 

from stations 15 to 17 where the alluvium consists of fine sand. A low 

reading was obtained from station 14 where the alluvium consists of 

boulders.

The double peak in the graph just below the mine may be significant. 

The Shinarump outcrop is almost 500 feet wide where the stream 

crosses it. Since the traverse was made, miners have stripped the 

gravel from this outcrop and mined some of the underlying Shinarump 

bedrock as ore. This concentration of ore-grade rock immediately 

under the gravels may account for the double peak and the elongate 

shape of the anomaly.

The Moab fault and the rocks of the Morrison formation of Jurassic 

age apparently have no effect on the radioactivity of the gravels.

RESULTS, AND EVALUATION OF THE METHOD

The investigation has shown that a recognizable radioactive anomaly occurs in alluvium downstream from both large and small uranium 

ore deposits. A large deposit in the drainage area of a small stream 

will cause anomalous readings as much as a mile downstream. A large 

deposit in the drainage area of an intermediate stream will cause 

anomalous readings not more than 0.1 to 0.2 mile downstream. A 

small deposit in the drainage area of a small stream will cause anomalies 0.1 to 0.2 mile downstream, but a small deposit in the drainage 

area of a large stream would probably not give an anomalous reading 

unless a station were set by chance exactly upon the point where the 

deposit contributed to the stream gravels.

Measuring the radioactivity of modern stream gravels as a prospecting method is slow compared to other reconnaissance methods such 

as the use of airborne and carborne scintillation counters. Measuring 

the radioactivity of the stream gravels only, it is possible to cover about 

five stations per hour. At 0.1 mile intervals this is somewhat less than 

a half a mile per hour or about 3^ miles per day. Using a 1-mile 

spacing on larger streams, probably only four stations per hour could 

be read because of the time consumed in moving from station to station. About 30 miles per day could be covered under ideals conditions.

The greatest usefulness of the method probably would be in areas of 

poor outcrop and moderate relief that had been erplored by airborne 

scintillometer. Here the sealer could be used in checking anomalous
RADIOACTIVITY IN MODERN STREAM GRAVELS 167

radioactivity detected by the scintillation counter. The method would 

be used to limit the area of search for the deposit and might show the 

presence of a deposit that is covered by soil.

Another somewhat specialized use would be applicable along a dissected cliff face such as is common on the Colorado Plateau. The 

small restricted drainages flowing out onto the fan at the foot of the 

cliff could be checked with one or two stations in each wash. A 

traverse along such a cliff front could be made at the rate of several 

miles per day and might show the presence of a relatively small deposit 

which otherwise would not be found except by the examination of 

many miles of outcrop. This method was tried at the large deposit 

of the Happy Jack mine in the White Canyon area of Utah (fig. 33), 

and the results are shown in figure 41. Two small streams were 

studied. The anomaly shown in the gravel 0.5 mile below the mine is 

very large, over 100 counts per minute. A smaller ore body occurring 

in a similar area would also be noticed though the magnitude of the 

anomaly would naturally be smaller. The drop in radioactivity 

below the point where the two small streams coalesce is very sharp. 

In using this method of prospecting, the prospector must be careful 

to check each individual stream above the point where it coalesces 

with another.

There are probably other uses for this prospecting method that 

have not come to light. Under ideal conditions anomalies have been 

noted in stream gravels as far as 1 mile from the nearest known outcrop of radioactive material in. large deposits. Small deposits will 

cause detectable irregularities in radioactivity that can be traced over 

several hundred feet of stream gravels. Even though the method is 

slow, deposits can apparently be detected farther away than by the 

other means of reconnaissance prospecting; thus the measurement of 

the radioactivity of modern stream gravels would seem to be useful 

in certain instances.

SUGGESTIONS FOR FUTURE WORK

The work reported in this paper shows only that study of the radioactivity in modern stream gravels appears to be a useful tool in prospecting for uranium deposits. Additional work must be done before 

the applicability of the method under all conditions will be completely 

proved.

The traverses described in this paper and from which the conclusions 

stated herein have been drawn were all made over uraninite-type 

deposits that predominantly contain primary uranium minerals. No 

traverses have been made over carnotite-type deposits that predominantly contain oxidized minerals. Additional traverses over other
168 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM

North

100

90

80 -

70 -

60 -

South

Direction of flow

EXPLANATION.

Streom draining Hoppy 

Jack mine area

Stream draining first canyon 

west of Happy Jack mine

* 9

O.I 0.2 0.3 

DISTANCE, IN MILES

0.4 0.5

Happy Jock mine 0.5 mile

FIGDEE 41. Radioactivity of gravel in washes draining Happy Jack mine and first canyon 

west, White Canyon area, San Juan County, Utah.

types of deposits in igneous and metamorphic rocks as well as in sedimentary rocks are necessary before the adequacy of the method under 

all field conditions has been proved.

The source of the radioactivity in the stream gravels is not known. 

It may be detrital uranium or thorium minerals in the gravels, or 

uranium or radium salts coating the sand grains and filling pore 

spaces. The radioactivity may come from both sources or from others.
RADIOACTIVITY IN MODERN STREAM GRAVELS 169

If the source varies with the different types of deposits, gravel downstream from oxidized deposits might produce different amounts of 

radioactivity from those described here.

One method of determining the source of the radioactivity is 

through the use of the nuclear-track plates as described by Stieff and 

Stern (1952). The radioactive particles can be located on the plates, 

but it will probably be necessary to use X-ray photographs and chemical tests to identify the minerals (T. W. Stern, written communication, 1954).

LITERATURE CITED

Faul, Henry (editor), 1954, Nuclear geology. John Wiley and Sons, Inc., New 

York.

Stieff, L. R., and Stern, T. W., 1952, Preparation of nuclear-track plates and stripping films for the study of radioactive minerals: Am. Mineralogist, v. 37, 

p. 184-196.

Wentworth, C. K., 1922, A scale of grade and class terms for clastic sediments: 

Jour. Geology, v. 30, p. 377-392.

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