Radioactivity 

Investigations in the 

Cache Creek Area 

Yentna District 

Alaska, 1945

GEOLOGICAL SURVEY BULLETIN 1024-A

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with the 

permission of the Commission

Radioactivity 

Investigations in the 

Cache Creek Area 

Yentna District 

Alaska, 1945

By G. D. ROBINSON, HELMUTH WEDOW, Jr., and J. B. LYONS

MINERAL RESOURCES OF ALASKA

GEOLOGICAL SURVEY BULLETIN 1024-A

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 : 1955
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - Price 65 cents (paper cover)
CONTENTS

Page

Abstract-_________________________________________________________ 1

Introduction ______________________________________________________ 1

Location and geography________________________________________ 2

Geology ______________________________________________________ 5

Field methods.________________________________________________ 7

The sampling problem _________________________________________ 7

The instrument^_______________________________________________ 9

Sampling procedures_______________________________ _____ 9

Field-station method.______________________________________ 10

Rough-screened sample method__--_-_-____--_----_--------_- 10

Field-laboratory sample method.____________________________ 11

Discussion of sampling data----__-_______________-________--___--__- 12

Size-fractions of the gravel types ________________________________ 12

Pebble composition of the gravels________________________________ 14

Radioactivity.___ _______________________________________ 16

Field tests _______________________________________________ 16

Field-laboratory tests. __________________________:__ 16

Pebble effect __ ^ __________________________________ 18

Laboratory studies.________________________________________________ 19

Methods____________________________________________________ 19

Mineralogy ___________________________________________________ 20

Common constituents of concentrates ________________________ 20

Radioactive minerals._____________________________________ 20

Conclusion._ ______________________________________________________ 21

Literature cited ___________________________________________________ 22

Appendix. Subsequent mineralogic analysis, 1950_______-_________-___ 22

Index.___________________________________________________________ 23

ILLUSTRATIONS

[Plates in pocket]

PLATE 1. Topographic map of the Cache Creek area, showing sample localities.

2. Traverses on Cache Creek upstream from Nugget Creek.

3. Traverses on Nugget Creek.

4. Traverses on Cache Creek between Rambler Creek and Nugget Creek.

5. Traverses on Thunder Creek and part of Cache Creek.

6. Maps and traverses, upper Peters Creek and tributaries.

FIGURE 1. Index map of Alaska showing location of the Yentna district-. 3 

2. View looking east showing valley of Thunder Creek and

Haugham's placer workings _____________________________ 4

Late Tertiary gravels in Krummenacher's placer workings. ___ 6 

Low-level Quaternary bench gravels in Morgan's placer

workings. ________ _____________________________________ 8
IV CONTENTS

TABLES

TABLE 1. Data on concentrates collected in Yentna district before 1945__ 2

2. Location and description of traverses_____________________ - 13

3. Percent distribution of several size-fractions in four gravel types, 14

4. Average pebble composition of four gravel types_____________ 15

5. Summary of gamma-count data by types of material tested____ 17

6. Summary of equivalent-uranium data on size-fractions and

gravity concentrates of materials tested____________________ 17

7. Data on sluice-box and similar concentrates obtained from

placer-mining operations in the Yentna district, 1945 _______ 18
MINERAL RESOURCES OF ALASKA

RADIOACTIVITY INVESTIGATIONS IN THE CACHE CREEK 

AREA, YENTNA DISTRICT, ALASKA, 1945

By G. D. EOBINSON, HELMUTH WEDOW, JR., and J. B. LYONS

ABSTRACT

In the summer of 1945 an investigation was made for possible placer deposits 

of radioactive minerals on Cache and upper Peters Creeks in the Yentna district, 

southern Alaska. Five gravel types of different age or origin Eocene, late 

Tertiary, Quaternary glacio-fluvial, Quaternary bench, and the present floodplain deposits (including tailings from placer mining) were examined for their 

content of radioactive minerals. Radioactivity was measured by gamma count, 

as detected by a portable Geiger-Mueller counter, at 455 field stations, and in 

526 rough-screened samples from 8 to 20 per minute; the rough-screened sample 

gamma counts from 8 to 21. Calculations from counts of carefully screened 

samples and gravity concentrates tested in the field laboratory show a maximum 

of 0.009 percent equivalent uranium even where concentration ratios are 500: 1 

or greater. Sluice-box concentrates with an undetermined but extremely high 

concentration ratio obtained in 1945 have a maximum of 0.064 percent equivalent 

uranium. The equivalent-uranium content of the samples may be due either to 

particles of radioactive minerals in the mineral aggregate of pebbles or other 

rock fragments, or to individual mineral grains. Interest in 1945 was focussed 

on mineral grains, the only material recoverable by normal placer-mining methods. Mineralogic study indicates that the radioactivity is due chiefly to uranium 

and thorium in zircon, rnonazite, and uranothorianite.

INTRODUCTION

Study by Harder and Reed (written communication, 1945) of placer 

concentrates from the Yentna district, southern Alaska (fig. 1), collected by J. B. Mertie, Jr., in 191T indicated that several valleys in 

the district might contain placer deposits of radioactive minerals. 

The radioactivity of the concentrates, according to Larsen (written 

communication, 1945, to Harder and Reed) apparently is due chiefly to 

monazite and a black cubic opaque mineral which may be uraninite 

or thorianite, or a solid mixture of the two.1 Data on the concentrates 

collected before 1945 are given in table 1.

1 Mineralogic studies since the preparation of this report show that thisi mineral is 

uranothorianite
MINERAL RESOURCES OF ALASKA

TABLE 1. Data on concentrates collected in Yentna district before 1945

Sample 

no.

482

521 

519

520 

522 

260 

474 

518 

473 

476 

523 

475 

250 

478 

524 

479 

525 

526 

517 

480 

481 

527 

254

eU 

(percent)

0.025

<.001 

.019

.229 

.035 

<.001 

.064 

.029 

.024 

.002 

.030 

<.001 

<.001 

.005 

<.001 

<.001 

.119 

.003 

.237 

.036 

.023 

.190 

.005

II 

(percent)

0.090

.07

.14

.08

ThO2 

(percent)

0.06

.035

.05

Location

Canyon Creek, tributary to Long Creek, tributary to Tokichitna River. 

Do. 

Poorman Creek, tributary to Cottonwood Creek, tributary to 

Peters Creek. 

Do. 

Willow Creek, tributary to Cottonwood Creek. 

Bird Creek, tributary to Peters Creek. 

Peters Creek, below Cottonwood Creek. 

Do. 

Cache Creek, above Gold Creek. 

Nugget Creek, tributary to Cache Creek 

Do. 

Cache Creek. 

Thunder Creek, tributary to Cache Creek. 

Do. 

Do. 

Dollar Creek, tributary to Cache Creek. 

Cache Creek, above Windy Creek. 

Windy Creek. 

Sholan Bar, Kahiltna River, 2 to 3 miles below Cache Creek. 

Round Bend Bar, Kahiltna River. 

Do. 

Do. 

Mill Creek, tributary of Lake Creek.

In the summer of 1945 a field party was organized to search for 

concentrations of radioactive minerals in the placer deposits of the 

Yentna district. G. D. Eobinson and Helmuth Wedow, Jr., geologists, and Fred Freitag and S. H. Dane, camphands, spent from June 

21 to September 20 in the area. After the transfer of Robinson to 

another project, the preparation of the report was carried on by 

Wedow. J. B. Lyons made the alpha-ray and mineralogic studies 

and prepared the section on mineralogy. The investigations were 

part of the Geological Survey's Trace Elements program which was 

being conducted for the Manhattan Engineer district, predecessor of 

the U. S. Atomic Energy Commission.

Grateful acknowledgment is given to the mine operators in the area, 

C. W. Bradley, Martin Carlson, Hans Erickson, F. D. Haugham, 

Joseph Krummenacher, C. P. Morgan, W. C. Stroll, A. J. Taraski, 

Ed Wagner, and G. H. Weatherell, for their aid and cooperation.

Placer gold has been mined in the Yentna district since 1905 ; before 

1916 hydraulic methods were used for all mining. From 1916 to 1926 

a dredge operated on Cache Creek from above Windy Creek to below 

Nugget Creek. After 1926 a dragline was installed in the "bowl" at 

the mouth of the Peters Creek canyon in the Peters Hills and operated 

until 1941. About 10 hydraulic plants were operating in the area in 

the summer of 1945.

LOCATION AND GEOGRAPHY

The Cache Creek-upper Peters Creek area in the Yentna district is 

about 30 miles west-northwest of Talkeetna, a station on the Alaska 

Railroad (fig. 1). Cache and Peters Creeks are southward-flowing 

tributaries of the Kahiltna River, which is a tributary of the Yentna
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945

PACIFIC OCEAN

FIGURE 1. Index map of Alaska showing location of the Yentna district.

Eiver. Cache and upper Peters Creeks have cut their valleys in the 

gently undulating surface of a troughlike basin that trends northeast 

and separates the Dutch Hills on the northwest from the Peters Hills 

on the southeast (fig. 2). The basin surface slopes gently from an 

average altitude of about 2,400 feet at the northeast to an average 

altitude of about 2,000 feet at the southwest. The main streams are 

incised as much as 400 feet below the basin surface. The Dutch Hills 

rise abruptly about 1,800 to 2,500 feet above it. Principal tributaries 

of Cache Creek are the southward-flowing Dollar, Brails, Thunder, 

Rambler, Nugget, and Gold Creeks (named from \vest to east), and 

the westward-flowing Spruce, Windy, and Long Creeks. The main 

tributaries of upper Peters Creek are southeastward-flowing Bird 

Creek and southwestwarcl-flowing Cottonwood Creek.

The usual transportation route into the area during the mining 

season is from Talkeetna where the Susitna River is crossed by boat. 

A truck road, maintained by the Alaska Road Commission, extends 

from the west bank of the Susitna River past Petersville, a small mining camp at the mouth of Peters Creek canyon, to the mouth of Long 

Creek. Tractor trails branch from the road to the different mining 

properties. Ferry service across the Susitna River, and truck and

RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 .5

tractor service into the Cache Creek area are operated by G. H. 

Weatherell of Talkeetna. In 1945 landing strips for small aircraft 

were located and constructed near the month of Bird Creek and at 

Petersville.

Bedrock and gravel deposits are well exposed in the canyon walls 

and in the cut banks of the two major streams and their larger tributaries. The broad upland areas separating the narrow stream valleys 

are largely "iiiggerhead" flats interrupted by swampy "muskegs."

The geography and climate of the area have been described by 

Capps (1913, p. 11-22) and Mertie (1919, p. 234-235).

GEOLOGY

The general geology of the Cache Creek-upper Peters Creek area 

has been described in considerable detail by Capps (1913, p. 22-47) 

and Mertie (1919, p. 236-240).

The bedrock of the area consists of two major units. The older 

unit is a sequence of slate and graywacke, containing minor amounts 

of quartzite and conglomerate, in which the rocks are much fractured 

and in most places tightly folded. Quartz veins and several kinds of 

light-colored dikes cut the rocks of this sequence. Fossils found by 

Mertie (1919, p. 237) indicate that these beds are of Mesozoic age and 

probably are, at least in part, of Cretaceous age. These deformed but 

only mildly metamorphosed sedimentary rocks, called "hard bedrock" by the miners in the area, are the predominant country rocks of 

the Dutch and Peters Hills, and are generally well exposed in the 

upper or canyon parts of the major streams and their larger 

tributaries.

The younger of the two major bedrock units is a sequence of pebble 

gravels, arkosic sands, organic clays, and lignite, wholly or in part of 

Eocene (Kenai) age (Capps, 1913, p. 28-33). These beds are only 

slightly deformed, and dip gently to the southeast with only local 

reversals of dip to the northwest. It is estimated that more than 300 

feet of these sediments are exposed along the valley of Cache Creek 

and in the lower reaches of its tributaries. These slightly consolidated 

rocks are known locally as "soft bedrock," but the sequence includes a 

well-cemented gravel bed, several tens of feet thick, which crops out 

on Cache Creek near the mouth of Gold Creek. Similar cemented 

gravel crops out along lower Nugget Creek and at several places on 

Cache Creek between Nugget and Wind}7 Creeks.

Overlying the bedrock units are many gravel deposits of diverse 

origin and age. The oldest of these is a sequence of clay-matrix 

gravels and boulder-studded clays that is younger than the Eocene 

rocks but older than the Quaternary glacial deposits. For convenience, this sequence is called "late Tertiary" in this report (figs. 2, 3).

326900 55 2
6 MINERAL RESOURCES OF ALASKA

M- r.'.H.*- **£. .f. § * '..f«t« 
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 7

Ancient stream channels filled, with these clay-matrix gravels are exposed by postglacial stream erosion. In places the deposits are more 

than 300 feet thick; they are commonly only a few tens or hundreds of 

feet wide. Away from the channels the gravel deposits are much 

thinner but underlie large areas to thicknesses of a few feet or tens of 

feet. The gravels are of the type deposited by streams in areas undergoing valley? glaciation, and the clays seem to be glacial outwash. 

The late Tertiary gravels and clays are commonly, but not invariably, 

conformable with the Eocene rocks.

The uppermost surface of the basin between the Dutch and Peters 

Hills is thinly covered by till and poorly sorted gravel, apparently 

deposited during the retreat of a great glacier which occupied the basin 

during part of the Quaternary period. Gravels of this sequence of 

deposition are herein designated Quaternary glacio-fluvial gravels.

The streams of the area, in reaching their present courses, cut and 

built a series of terraces or benches below a gently undulating surface 

left by the retreating Quaternary glacier. As many as seven bench 

levels are preserved in a vertical interval as much as 300 feet below the 

postglacial surface. The benches below this surface and above the 

present stream courses are commonly capped with a few feet of streamlaid deposits herein designated Quaternary bench gravels. These 

gravels are locally several tens of feet thick (fig. 4).

The youngest gravel deposits are those of the present flood plains. 

Much of the gravel at stream level has been mined, and large volumes 

of rock have been contributed to the streams by the mining of higher 

benches. As a result, the flood-plain gravels are now slightly sorted 

and redistributed tailings.

FIELD METHODS

THE SAMPLING PROBLEM

To test the gravels throughout the area with any hope of achieving 

representative results would be a project requiring many seasons. On 

the other hand, to confine work merely to localities actively being 

mined or to sands and gravels of the present flood plains might fail 

to give a true evaluation of the distribution of radioactivity and heavy 

minerals in the gravels of the area.

Some gold has been produced from gravels of every age and origin 

in the district. No gravel has yielded spectacular amounts, although 

the district has been intensively prospected and mined. The bulk of 

the production has been from Quaternary gravel and the least production has come from Eocene beds. Well-defined paystreaks in an}7 

of the gravels are reportedly rare, although gold values are consistently highest near bedrock. L/arge yardage of low-grade gravels have 

made moderately profitable hydraulic and dredge operations possible.
8 MINERAL RESOURCES OP ALASKA
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 9

Only about 10 claims were being mined in 1945. Most of the operators 

are sluicing Quaternary bench gravel, but some work is being done 

on glacio-fluvial deposits and in deposits of high-level late Tertiary 

channel gravel (figs. 2, 3,4).

Because most of the gold is coarse and the grade of the gravels low, 

the miners commonly push as much rock through the boxes as possible, 

using all the hydraulic head available. By doing so they tend to blast 

everything out of the riffles except gold and lead shot, and much of 

the other heavy minerals, if present, are scattered in the tailings. In 

the past, few operators saved the heavy minerals that remained in the 

riffles at the cleanup, so that there is little concentrate available for 

testing.

For preliminary reconnaissance purposes, the low-level Quaternary 

bench gravels probably offer the best source of radioactive mineral 

resources. Whatever heavy minerals the older gravels contain should 

be further concentrated in the younger beds largely derived from them; 

this has evidently been the case with the gold. The low-level benches 

have also the obviously practical advantage of being relatively near 

stream level, making it possible to work rapidly the considerable volumes of gravel needed for the sampling procedure (fig. 4). In reaching this conclusion much preliminary testing was done on higher 

level and older gravels.

Although occasional check traverses were made 011 present floodplain deposits and tailings, and on older gravels, systematic testing 

was mainly restricted to (1) sizable areas of Quaternary gravel, 

whether or not currently being mined, and (2) the vicinity of present 

workings, regardless of the age or origin of the gravels being worked.

THE INSTRUMENT

The radioactivity of the gravel and the placer samples was measured 

by a portable Geiger-Mueller gamma counter designed by the Geological Survey. The techniques developed in the use of this instrument 

are discussed below. Because of inclement weather and hard usage, it 

was necessary to protect the instrument in the field by a paraffin-sealed 

wooden carrying case, designed to permit easy operation and removal 

of the instrument. A small canvas sling held the counter tube against 

the side of the wooden box which was then covered with a paraffined 

canvas skirt. Even with these precautions moisture occasionally 

would get inside the instrument and short-circuit some of the batteries.

SAMPLING PROCEDURES

The general plan of sampling was to test selected areas of exposed 

gravels at regular intervals. Samples from sites of relatively high 

radioactivity in the field were then tested in a local field laboratory for 

semiquantitative determination of the equivalent-uranium content.
10 MINERAL RESOURCES OF ALASKA 

FIELD-STATION METHOD

During the first weeks of the season the gravels were tested by taking the instrument to the traverse station. In each area to be tested a 

traverse was laid out and at regularly spaced stations on the traverse 

gamma counts over a 5-minute period, or rarely a 10-minute period, 

were taken with the counter tube laid against the gravel or the underlying bedrock. Air-background readings with the instrument held from 

1 to 4 feet above the ground surface were occasionally interspersed 

with the regular station readings. Samples for testing in the field 

laboratory were taken at stations giving relatively high counts, and 

occasionally, as a check, at stations giving indifferent counts. This 

method of determining the gamma count of the natural gravel or 

bedrock was abandoned when it was decided, after comparison of a 

considerable number of field-station and field-laboratory data, that 

the count was being influenced by "pebble effect" (p. 18) and perhaps 

by mass effect of the surrounding rocks. Essentially, it appeared that 

the effects on the counter of any small but possibly significant amounts 

of heavy radioactive minerals, which could be recovered by hydraulic 

methods, were masked by the effects of volumetrically overwhelming 

proportions of pebbles and larger rock fragments.

ROUGH-SCREENED SAMPLE METHOD

To eliminate part of the pebble effect a screened fraction of the 

gravel from each traverse station was tested. For this purpose the 

rough-screened sample or "can sample" method was devised.

When a gravel bench, exposed for about 1,000 feet on both sides of a 

stream valley, was to be tested, an instrument station was set up on 

the valley bottom midway along the exposure. Sampling locations 

were surveyed by tape-compass methods at about 100-foot intervals 

along both banks, and the section at each location was measured and 

described. As each location was made, a sample was taken at the base 

or lowest exposed part of the gravel bed and at 5-foot vertical intervals if the section was thick enough. A sample was made by passing 

one-half cubic foot of gravel through a 4-mesh screen and collecting a 

1-quart, 14-ounce can full of the screenings. Eight empty cans were 

carried in a 5-galloii gasoline can attached to a packboard. When 

the eight cans were filled and taken to the instrument station, they 

were tested wT hile another group was being collected. The sample was 

poured into a slightly larger can in which the Geiger-Mueller counter 

tube, protected by a waterproofed canvas pouch, had been placed. 

Five-minute readings were taken, interspersed with background readings taken with the tube in the empty can. The "can samples" were 

discarded after testing.

In addition to the elimination of some of the pebble effect and any 

possible mass effect, adoption of the rough-screened sample method
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 11

greatly increased the average number of traverse stations which could 

be tested in a day and greatly decreased the danger to the instrument. 

Removing and handling gravel in the rough-screened sample method 

introduce volume increases which tend to reduce the gamma count per 

unit volume of gravel; this reduction, however, does not affect in any 

significant degree the relative concentrations of radioactive material 

among the different sample localities.

FIELD-LABORATORY SAMPLE METHOD

The method of sampling for tests in the field laboratory varied to 

some extent during the season, but became standardized about the 

same time that the rough-screened sample method was adopted. Early 

in the season, gravel samples one-half cubic foot in volume were 

taken at field stations which had gamma counts of three to four or 

more above the air background. As a check against the reliability of 

field-station tests, similar samples were taken from some field stations regardless of the gamma count at the station. The sample was 

washed through a <±-mesh screen, the coarser material weighed and 

discarded, and the screenings dried. The minus 4-mesh material 

was next screened through 7-mesh, and the resultant fractions tested 

with the counter. The minus 7-mesh material was further screened 

through 12-mesh and again the resultant fractions tested. The tests 

were made over 5-minute intervals and were in the following sequence: 

background; plus 7-mesh fraction; minus 7-mesh fraction; field 

standard; plus 12-mesh fraction; minus 12-mesh fraction; background. 

Any fraction in this sequence which ran more than three counts per 

minute above background was rerun with background and field standard in the following sequence : background; fraction; field standard; 

fraction; background. If the gamma count of any fraction of a 

sample appeared significant, a concentrate of the heavy minerals, 

about 175:1 by volume, was made at the location by running 3 cubic 

feet of gravel through a rocker and panning the cleanup down to 30 

cubic inches.

Later, the minus 4-mesh samples obtained by the rough-screened 

sampling method were reduced to a volume of about 90 cubic inches 

by splitting. This split sample was then dried and screened through 

7-mesh, but not through 12-mesh as was the earlier practice. If the 

material at the field station was sand or clay, rather than gravel, and 

the gamma count was relatively high, a sample of about 90 cubic inches 

was taken directly and dried, and then tested in field laboratory 

in the usual sequence.

Still later, the method of taking the field-laboratory samples of 

gravel was finally standardized as follows: A half-cubic-foot sample 

was taken at each locality which gave a rough-screened sample reading 

of 4 or more counts above air background. If no rough-screened
12 MINERAL RESOURCES OF ALASKA

sample tested 4 or more counts above air background in any traverse 

as much as 1,000 feet long, at least one representative field-laboratory 

sample was taken in the traverse as a check against the rough-screened 

sample data. The sample was washed through a 4-mesh screen. The 

plus 4-mesh material was weighed and discarded; pebble counts were 

made on some. The minus 4-mesh material was weighed and split to 

90 cubic inches. A cubic foot of gravel was put through a rocker at 

each locality and the concentrate was panned down to 30 cubic inches; 

then a 9 cubic-foot sample was similarly concentrated; at a few sites 

a cubic yard of gravel was so treated.

The split from the half-cubic-foot sample was dried and separated 

into two fractions with a 7-mesh sieve. Parts of the fractional split 

and of the rocker concentrate sufficient in volume for quantitative 

comparison with the radioactive standard container were taken. Fiveminute readings on these parts were run in the following sequence: 

Background; plus 7-mesh fraction; minus 7-mesh fraction; standard; 

1-cubic-foot rocker sample; 9-cubic-foot rocker sample; background. 

Any fraction or concentrate having 3 or more counts above background was rerun in the sequence: background; fraction; standard; 

fraction; background.

A traverse involving the collection of 25 to 35 rough-screened 

samples and two or three complete laboratory-sample and rockersample sets, was ordinarily completed in 1 day.

Late in the season it was decided that enough data had been collected on the weight of each size-fraction of the half-cubic-foot 

samples, and these samples were omitted thereafter.

DISCUSSION OF SAMPLING DATA

The sampling data have been plotted on a series of sketch maps 

(pis. 1-6). On these maps the grade of the different samples, as determined from field-laboratory data, has been plotted in percent 

equivalent uranium. The field data have been plotted as total gamma 

counts per minute. For each traverse the stated air background was 

determined by averaging all background readings taken in the traverse 

area. Brief descriptions of the traverses are given in table 2.

SIZE-FRACTIONS OF THE GRAVEL TYPES

The proportions of the different size-fractions in many of the fieldlaboratory samples was determined as a byproduct of the effort to discover whether the radioactivity of the samples was due to grains of 

radioactive minerals or to radioactive minerals in pebbles. The 

average percent of the arbitrary size-fractions in the different gravel 

types is given in table 3.

Because rock fragments of maximum dimension greater than about 

5 inches were avoided in collecting the samples, the Quaternary bench
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 13

TABLE 2. Location and description of traverses

[FS indicates that the gravels were tested by the field-station method; C indicates that gravels were tested 

by the rough-screened sample method]

Method 

of testing

FS 

FS 

FS

FSand 

0 

C

FS 

FS 

FS 

FS 

FS 

FS

FS

FS

FS 

FS

Location and description

Largely Eocene gravel in a cut bank on the right limit of Cache Creek upstream from

Nugget Creek (pi. 2). 

Low-level Quaternary bench gravel and Eocene bedrock on Cache Creek for 4,000 feet

upstream from Gold Creek; some tailings also tested (pi. 2). 

Low-level Quaternary bench gravel and Eocene bedrock on Cache Creek between Gold

and Long Creeks; 'includes Erickson's workings at the mouth of Gold Creek; some

tailings also tested (pi. 2). 

Low-level Quaternary bench gravel and Eocene bedrock on Cache Creek between Long

and Nugget Creeks (pi. 2). 

Quaternary bench gravel at Bradley workings on Cache Creek about 1.85 miles upstream

from Gold Creek (pi. 2). 

Tailings piles and Eocene clav on Nugget Creek about 2.000 feet upstream from its mouth

(pl.3). 

Tailings, Quaternary bench gravel, and Eocene bedrock in a low cut bank on the left

limit of Nugget Creek about 2,500-3,500 feet upstream from its mouth (pi. 3). 

Low-level Quaternary bench gravel on top of late Tertiary gravel in a left-limit cut bank

on Nugget Creek about 4,000 feet upstream from its mo'uth (pi. 3). 

Gravel (largely reworked tailings) on small incipient flood plains in the lower part of the

canyon of Nugget Creek (pi. 3). 

Low-level Quaternary bench gravel, tailings, and Eocene bedrock on Nugget Creek for

about 1,000 feet upstream from its mouth (pi. 3). 

Mesozoic slate and graywacke, late Tertiary gravel, and Quaternary glacio-fluvial gravel

in Morgan's workings on the uppermost bench on the right limit of Nugget Creek

downstream from the canyon (pi. 3). 

Late Tertiary gravel and Quaternary glacio-fluvial gravel in a gully on the left limit of

Nugget Creek downstream from the mouth of the canyon and opposite Morgan's

workings (pi. 3). 

Quaternary glacio-fluvial gravel on the uppermost bench (/n the left limit of Nugget

Creek near the mouth of the valley (pi. 3).

Late Tertiary gravel about 25 feet vertically below traverse 5-1 (pi. 3). 

Quaternary glacio-fluvial gravel and late Tertiary gravel on the uppermost bench on the

left limit of Nugget Creek about 2,500 feet upstream from the mouth of the valley (pi. 3). 

Continuous and largely on low-level Quaternary bench gravel on Cache Creek from

Nugget Creek downstream almost to Trout Creek; some tailings were tested and a

few readings were made on Eocene bedrock; traverse 6-5 includes Traski's workings

(pl.4). 

Do. 

Do. 

Do. 

Do. 

Do. 

Quaternary bench gravel on the left limit of Cache Creek about 1,000 feet upstream from

Rambler Creek; WeatherelFs workings (pi. 4). 

Flood-plain gravel, Quaternary bench gravel, and late Tertiary gravel on Thunder Creek

from its mouth to the mouth of the canyon (pi. 5).

Do. 

Late Tertiary gravel in Haugham's workings on the uppermost bench on the left limit of

Thunder Creek about 2,500 feet upstream from the mouth of the Thunder Creek canyon (pi. 5).

Quaternary bench gravel in Morgan's workings on Cache Creek at Falls Creek (pi. 5). 

Quaternary bench gravel on the right limit of Cache Creek for about 1,500 feet upstream

from Falls Creek (pi. 5).

Late Tertiary gravel in Krummenacher's workings in Cheechako Gulch (pi. 5). 

Late Tertiary gravel in Carlson's workings on Falls Creek about 2}.<j miles upstream

from its mouth (pi. 5).

Flood-plain gravel on Cache Creek at Dollar Creek (pi. 5). 

Flood-plain gravel (mostly reworked tailings) on Bird Creek for a distance of 1,200 feet

upstream from its mouth (pi. 6). 

Late Tertiary gravel in Wagner's workings on high bench on the right limit of Bird

Creek about 2,800 feet upstream from its mouth (pi. 6).

Quaternary glacio-fluvial(?) gravel on the left limit of Peters Creek about 2,200 feet upstream from the mouth of the canyon (pi. 6). 

Flood-plain gravel at Petersville (pi. 6). 

Flood-plain gravel on Willow Creek about 1.6 miles upstream from its mouth (pi. 6).

gravels and the late Tertiary gravels actually average slightly less 

minus 4-mesh material than is indicated in table 3. Large cobbles and 

boulders are not common in Quaternary glacio-fluvial gravels, and 

are rare in Eocene gravels, and therefore the size distribution given 

for them approaches the true proportions.
14 MINERAL RESOURCES OF ALASKA

The data indicate that the gravel types are not readily distinguishable on the basis of size distribution for the mesh-sizes used.

PEBBLE COMPOSITION OF THE GRAVELS

The pebble compositions of the several types of gravel were determined by counts of 100 pebbles selected at random from the coarse 

fraction (plusd-mesh) of each of a number of the half-cubic-foot fieldlaboratory samples.

TABLE 3. Percent distribution of several sise-fractions in four gravel types

Age and type of gravel Number of 

samples

9 

28 

10 

13 

60

Percent by weight

Plus 

4-mesh

71 

67 70

65 

68

Minus 4- 

and plus 

7-mcsh

8 

13 

12 

13 

12

Minus 

7-mesh

21 

20 

16 

22 

20

The types of pebbles recognized were:

Slate and graywacke 

Schistose rock 

Chert and quartzite 

Cemented conglomerate 

Igneous rock, fresh 

Igneous rock, weathered 

Vein quartz 

Clay and lignite

The pebbles of slate and graywacke, characteristically subangular, 

are apparently derived from the Mesozoic bedrock exposed in the 

Dutch and Peters Hills. The schistose pebbles are chloritic and probably are metamorphosed graywacke.

The chert pebbles are usually well-rounded and are commonly light 

gray to black. The quartzite pebbles are also well-rounded and are 

generally made up of well-cemented fine sand grains; fresh fracture 

surfaces are light gray or yellowish white. The cemented conglomerate forms distinctive pebbles consisting of small, well-rounded fragments of chert, quartzite, and vein quartz well-cemented in a matrix 

of argillite or graywacke. The chert, quartzite, and cemented conglomerate may be derived from Paleozoic sedimentary rocks exposed 

in the Alaska Range.

Igneous pebbles are mainly coarse-grained granitic and dioritic 

rocks, but include aphanitic, fine-grained porphyritic and aplitic rocks. 

The pebbles grouped as weathered igneous rocks are well-rounded 

masses of limonitic material and quartz grains, apparently representing fragments of coarse-grained granitic rocks that were subjected to 

extreme weathering after fluvial transportation and deposition.
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 15

The vein quartz pebbles are usually white, and range from wellrounded fragments apparently derived from reworked older gravels 

to subangular fragments from the recent erosion of veins in the 

Mesozoic bedrock.

The clay and lignite fragments are derived from the Eocene bedrock. The clay is in blue-gray to gray balls, and the lignite occurs as 

brown to black fragments, commonly splinter shaped.

The average pebble composition of four of the gravel types are 

shown in table 4.

The tabulated compositions show that Eocene and Quaternary 

bench gravels are readily distinguishable from the late Tertiary and 

the Quaternary glacio-fluvial gravels as well as from one another on 

the basis of pebble composition. There is essentially no difference in 

pebble composition between the late Tertiary and the Quaternary 

glacio-fluvial gravels, thus suggesting a similar origin for them.

The gravels also show differences in other sedimentary characteristics. The pebbles of Eocene gravels are well-sorted and usually wellrounded. Many pebbles are polished and many others superficially 

stained bright yellow-brown.

The pebbles of the late Tertiary gravels are not as well-sorted as 

those of the Eocene gravels and are usually only subrounded. Boulders as much as 2 feet long occur locally, although most of the rock 

fragments are less than 6 inches long. Portions of the late Tertiary 

gravels have been deeply weathered as shown by their yellowish color 

and by the relative abundance of soft friable pebbles.

TABLE 4. Average pebble composition of four yravel types, hi percent

Types of pebbles Eocene

29 

2 

14 

10 

35

9

1

Late Tertiary

87 

4

3^

2^ 

2^ 

^

Quaternary

Glaciofluvial

86 

4 

# 

2J-4 

2^ 

1 

2^ 

3

Bench

62 

4 

5 

5 

13

4

The pebbles in the Quaternary glacio-fluvial gravels are subrounded ; some are striated. These gravels are poorly sorted and in 

many places contain boulders as much as 1 foot or rarely several feet 

in length. The larger boulders are almost invariably granitic or 

dioritic types of igneous rocks.

The Quaternary bench gravels and the gravels of the present flood 

plains are poorly sorted. Some of the pebbles and cobbles are wellrounded, but most are subrounded to subangular.
16 MINERAL RESOURCES OF ALASKA

The large amount of igneous rock material in the Eocene gravels 

indicates that the gravels were largely derived from the core of the 

main Alaska Range. During late Tertiary time the source of the thick 

gravel deposits was closer to the basin of deposition and presumably 

was the Dutch and Peters Hills where large areas of Mesozoic slate 

and graywacke were exposed. The pebble composition of the glaciofluvial gravels points to a local origin for these deposits; they apparently wrere laid down during Quaternary glaciation when local ice 

masses in the Dutch and Peters Hills were eroding the exposed bedrock 

of Mesozoic age. The Quaternary bench and the flood-plain gravels 

have clearly been derived from the erosion of all the older bedrocks 

and gravels.

RADIOACTIVITY 

FIELD TESTS

A total of 981 field tests were made in which the counter was taken 

to the traverse area. Of these, 455 were field-station tests and 526 were 

rough-screen sample tests. The gamma-count data obtained in these 

field tests are summarized in table 5.

FIEUO-LABOKATORY TESTS

More than 350 equivalent-uranium determinations were made on 

different size-fractions and gravity concentrates of placer samples in 

the field laboratory; most of these are showrn on plates 1-6. A total 

of 339 of these determinations was used in the compilation of the 

summary of equivalent-uranium data given in table 6. No significant 

variation in the equivalent-uranium content of the different gravels 

is apparent. The slight increase in equivalent-uranium content as 

the gravels are concentrated by gravity methods (see \veighted averages, table 6) supports the belief that essentially all the radioactivity 

present is in the smaller size heavy minerals of the placers. Plates 

1 and 6 show the number of pounds of gravity concentrate per cubic 

yard recovered in the samples having the highest degree of concentration obtained at each sample location. As the heavy minerals in these 

concentrates are diluted by a considerable volume of lighter minerals 

and rock fragments, the radioactivity is also low, and the highest test 

on a concentrate shows but 0.009 percent equivalent uranium.

Gravity concentration of the moderately heavy minerals (as distinguished from such minerals of high specific gravity as gold) by 

panning and rocking is not efficient. It is probable that from 10 to 

50 percent of the heavy minerals originally present are lost during 

concentration. Such losses, however, are largely inherent in any inexpensive method of working gravels, and determinations from sampling of this sort are thus probably fairly indicative of expectable 

recovery by common placer-mining methods.
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 17

TABLE 5. Summary of gamma-count data by types of material tested

[FS, indicates field-station method; C, indicates rough-screened sample method]

Age and type of material tested

Do. 

Do. 

Do. 

Do.. .........................

Do-.-l... ........... ............

Do...... ........................

Recent flood-plain gravel and tailings. 

Do..............................

Do.. ................. .....

Methods of 

testing

FS

C

FS

C

FS

O

FS

0

FS

0

FS

O

FS 

O

FS

C

Number of

tests

17

47

19

28

1

72

50

4

129

213

84 

217

455

526

Range

11-17

11-18

11-16 8-19

12

10-17

10-20 8-18

10-16

10-20 8-19

9-18 

8-21

8-20

8-21

Weighted 

average 

(gamma 

counts per 

minute)

13.8

14.3

13.4

14.1

12.0

13.4

14.1

13.2

13.0

14.4

13.1

14.3 

13.4

14.0

13.4

Main traverses on which 

materials were tested

3-1

1-5, 1-6, 2-3, 3-1, 4-1, 5-2, 5-3

7-1, 7-3, 9-1, 12-2

3-1, 4-1, 5-1, 5-3

13-1

1-5, 1-6, 1-7

1-7, 1-8, 6-1 to 6-7, 8-1, 8-2

0) 

0)

1 Tested on most traverses on Cache, Peters, and Bird Creeks.

TABLE 6. Summary of equivalent-uranium data on sis e-fr act ions and gravity 

concentrates of materials tested

Material tested

Eocene gravel and sand: 

Size-fraction:

Minus 7-mesh-. . _____-.-

Gravity concentrate: 

>25:1 to <200:1_ ........... ......... ...............

Eocene clay: 

Size-fraction:

Late Tertiary gravels: 

Size fraction:

Gravity concentrates: 

>25:1 to <200:1 

>500:l-_ ... ----- -.-.-..-.-.

Quaternary glacio-fluvial gravels: 

Size fraction:

Gravity concentrates:

>500:1__ ...... . ........ -_ _ __. _.

Quaternary bench gravels: 

Size-fraction:

Gravity concentrate:

>500:1 ... ... 

Recent flood-plain gravels and tailings: 

Size fraction:

Gravity concentrate: 

<25:1 to >200:1 

<500:1 - .--.--.---.

Weighted average for all tests: 

Size fraction:

Gravity fractions:

<500:1 . 

Number 

of 

samples

9

13

9

5

27

27

21

8

11

11

3

2

41

41

24

20

18

19

16

14

106

116

73

44

Equivalenturanium 

range

<0. 001-0. 004

<. 001-. 003

<. 001-. 004

<. 001-. 003

<. 001-. 005

<.001-.004

<. 001-. 004

. 001- 004

<. 001-. 005

. 001- 004

<. 001- 003

. 003-. 004

<. 001- 007

<. 001-. 006

<. 001- 004

<. 001- 007

<. 001-. 003

<. 001-. 004

<. 001-. 004

. 001-. 009

Average (percent)

0. 0012

.0012

.0021

.0012

.0020

.0013

.0017

.0026

.0020

.0022

.0010

.0035

.0017

.0014

.0021

.0029

.0009

.0010

.0019

.0046

.0016

.0014

.0019

.0034
18 MINERAL RESOURCES OF ALASKA

Field-laboratory tests were made on 24 sluice-box concentrates collected from several hydraulic-mining operations in the area. Also 

tested were 17 panned cuttings from drill holes put down by the 

Alaskan Exploration and Mining Company in the vicinity of the 

Bird Creek workings. The data on these concentrates are shown in 

table 7 and the locations of those collected in the Cache Creek watershed are plotted on plate 1. The gravel yardage in the table is 

only approximate, having been derived from rough estimates furnished by the operators. Much of the material of mineral-grain size 

is lacking in the sluice-box concentrates, particularly the very fine, 

heavy "blowings," which probably contain a high proportion of any 

radioactive minerals present.

PEBBLE EFFECT

The pebble effect mentioned earlier in the discussion of sampling 

methods is apparent in the summary of the field gamma-count data 

(table 5). The weighted average gamma count per minute for the 

rough-screened sample tests is 0.6 counts per minute less than that of 

the field-station tests. This average lower counting rate is attributed 

to the screening out of the pebbles, although it may also be due to a 

lessening of the total mass affecting the instrument. The slight difference in the equivalent-uranium content of the minus 7-mesh and 

minus 4-, plus 7-mesh size-fraction (table 6) is also ascribed to the 

removal of coarser material. Thus, it is concluded that a closer approximation of the radioactivity of recoverable minerals in placer 

deposits can be made by the use of the counter on material screened 

down to mineral-grain size.

TABLE 7. Data on sluice-box and similar concentrates obtained from placermining operations in Yentna district, 1945

[Cubic yards of gravel worked unknown unless shown. Age: LT, late Tertiary gravels; M, Mesozoic 

bedrock; Q, Quaternary bench gravels; R, Recent flood-plain gravels]

Sample 

no.

A

Ba

Be

Ca

Cc

Da

DC

E

Fa 

Fc

G

Oa

H

Cubic 

yards of 

gravel 

worked

20, 000 

20,000

2

Age

LT

LT

LT

(?)

(?)

(?)

(?)

M

Q 

Q

Q

Q

Q

Percent 

eU

<0. 001

<.001

.002

<.001

.002

<.001

<.001

.002

.023

.003

.024

.006

Mesh 

size

-7

+7

+7

+7

 7 

+7

0)

Nature of sample

trate pile. 

. do. ...................

 do 

..... do.. -.. ... _-

 ..do-.. ..... ... 

 do. ... .... 

... __do. ... . 

rock. 

Sluice-box concentrate, 

some blowings. 

 .do. .. . ... ... .

Locality

get Creek. 

Do.

Do.

Mouth of Iron Creek.

Do.

North bank of Cache Creek, 1,000

feet upstream from Nugget 

Creek. 

Do.

get Creek. 

Taraski's workings on Cache 

Creek, opposite Lucky Gulch. 

Do.

Weatherell's cut on Cache Creek,

opposite Rambler Creek. 

Do.

Weatherell's cut on Cache Creek,

1,600 feet downstream from 

Taraski's workings.
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 19

TABLE 7. Data on sluice-box and similar concentrates obtained from placermining operations in Yentna district, 1945 Continued

Sample 

no.

Jl 

,T2

J3 

Ka 

Kc

L

M

N

O

P

Q

Ra

S

Tl

T2

Ul

U2

V

W12

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Ifi

17

18

X

Y

Z

Cubic 

yards of 

' gravel 

worked

30, 000 

30, 000

30,000 

1.000 

1,000

5

12, 000 

8,000

........

4,000

4,000

10

Age

LT 

LT

LT 

LT 

LTand

Q 

Q

LT 

LT

Q

LT?

LT?

LT?

LT?

LT?

LT?

LT?

LT?

LT?

 -

R

Q and

R

Q?

Percent 

eU

<.001 

<.001

.003 

.006 

.002

.027

.03 

.023

.011

.001

.002

.001

<.001

.002

.003

.001

.002

<.001

.002

.002

.001

001

.001

.001

.002 

.002

<.001

.001

<. 001

.001

<.001

.001

.003

.001

.064

.032

<.001

Mesh 

size

+4 

-4

-4

 7 

+7

-12 

-12

(?)

(?)-7

(?)

-7, +12

-12

+12 -12

Nature of sample

Tailings from panning of 

sluice-box concentrate, 

do

Concentrate from panning 

of sluice-box concentrate. 

Sluice-box concentrate. ....

do

... .do. ...................

Mostly blowings from 

sluice-box concentrate, 

do

do

 ..do.. ..................

.....do..-.-.--...-. ....

 ..do...... ... ........

___._do__ ..................

.....do....-..... ......

do

. .do.......... ....... ...

Amalgation residue. ______

_ .do....... .. ________

. ....do. ._._._. ............

..... do.. _____.__._._..____

1

Locality

Haugham's upper-bench cut, 

Thunder Creek. 

Do,

Do.

Krummenacher's workings in 

Cheechako Gulch. 

Do.

1,600 feet downstream from 

Taraski's workings. 

Haugham's workings on lower 

Thunder Creek.

Thunder Creek.

opposite Rambler Creek.

of Bird Creek. 

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

H7, L15.

H8, L14.

H8, L15? 38-40 feet.

H8, L15, 65-108 feet (bedrock 114

feet). 

H15, L? (probably H?, L15).

H5, LI 5, 0-9 feet.

H9, L15.

H9, L15. 

H6, L13.

H9, L13.

H5, L15, 91-201 feet (bedrock).

H10, L13.

H9, L14.

H6, L15.

H6, L15.

H5, L15; H8, L15; H6, L13; H8,

L14 samples combined to fill 

container. 

H7, L14; H8, L14, H9, L14

samples combined. 

Peters Creek at Petersville;

one-half from Patricia Bowl 

and one-half from Little Bowl. 

Peters Creek at Petersville;

three-fourths from low-bench 

cut west of canyon mouth and 

one-fourth from Little Bowl. 

Peters Creek at Petersville;

Little Bowl.

1 Fines.

2 Samples W1-W18 are panned drill cuttings from holes put down by the Alaskan Exploration and 

Mining Co.; locality given by hole no. and line no.; samples have been screened through 7-mesh.

LABORATORY STUDIES

METHODS

Most of the field-laboratory samples collected in the Yentna. district 

were taken to Washington, D. C., for additional study, particularly 

for identification of the radioactive minerals. For this purpose sev-
20 MINERAL RESOURCES OF ALASKA

eral concentrates showing relatively high radioactivity were selected 

from each gravel type. From each of these a grain-size portion 

(minus 40-, plus 100-mesh) was selected and five fractions prepared 

using heavy liquids (bromoform, sp gr 2.8; and methylene iodide, 

sp gr 3.3) and a magnetic separator.

Light, specific gravity <2.8. 

Specific gravity >2.8, <3.3 ; magnetic. 

Specific gravity >2.8, <3.3; nonmagnetic. 

Specific gravity >3.3 ; magnetic. 

Specific gravity >3.3 ; nonmagnetic.

Portions of these five fractions of several of the samples were then 

mounted in bakelite on glass slides and exposed by contact to alpharay spectroscopic plates for periods of 4 to 14 days to determine specifically which grains were radioactive.

MINERALOGY 

COMMON CONSTITUENTS OF CONCENTRATES

The concentrates from the several gravel types studied are much 

the same in mineral composition. The proportions of the different 

minerals, however, vary to some extent, and the variation apparently 

is characteristic of certain of the gravel types. The following minerals are the abundant and common constituents of the concentrates: 

zircon, hornblende, hypersthene, augite, epidote, garnet, pyrite, ilmenite, chromite (or a chrome spinel), cassiterite, magnetite, quartz, 

and altered feldspar. The minor and erratic constituents are gold, 

tourmaline, andalusite, biotite, chlorite, iron oxides, allanite (?), arsenopyrite, copper, stibnite (?), apatite, sphene, monazite, gray wacke 

fragments, iddingsite, prehnite, rutile (?), marcasite, galena, and two 

unidentified minerals one of them brown and wedge-shaped, the 

other whitish and anhedral. Platinum has been reported by Mertie 

(1919) in some of the placers of the Yentna district, but was not found 

in the samples collected in 1945.

The Eocene sands and gravels characteristically have heavy-mineral 

suites low in sulfide minerals, and lacking in tourmaline and apatite. 

Heavy-mineral fractions from the late Tertiary gravels are rich in 

pyrite, whereas Quaternary bench gravels have heavy-mineral fractions characteristically high in andalusite and cassiterite. Apatite, 

tourmaline, monazite, allanite (?) and iddingsite (?) are more abundant in the flood-plain gravels than in the other gravel types

RADIOACTIVE MINERALS

The source of the radioactivity in the samples collected in 1945 apparently is due chiefly to monazite which normally contains from a 

few percent to about 11 percent thoria (Frondel and Fleischer, 1952,
RADIOACTIVITY INVESTIGATIONS, YENTNA DISTRICT, 1945 21

p. 7). The monazite occurs as lemon-yellow, euhedral, wedge-shaped 

nionoclinic crystals.

Zircon, which is extraordinarily abundant, may possibly be slightly 

radioactive and responsible in part for the radioactivity measured, but 

direct evidence is lacking. According to Frondel and Fleischer 

(1952, p. 16) zircon may contain several percent uranium and thorium, 

although it is usually low in these elements. The zircon is characteristically in euhedral, tetragonal prisms and pyramids, generally 

colorless although some crystals are tinted purple, blue, yellow, and 

brown.

The only strongly radioactive mineral noted occurs in black, lustrous, anhedral grains and is tentatively identified as uraninite.2 It 

is the only mineral which darkens an alpha-ray spectroscopic plate 

after 2 weeks exposure. Only a few grains of the uraninite( ?) were 

seen. More than likely it is the same mineral identified as uraninite(?) by Larsen (written communication, 1915, to Harder and 

Reed).

CONCLUSION

In conclusion the writers believe that the placer deposits examined 

in the Cache Creek-upper Peters Creek area do not contain sufficient 

amounts of radioactive minerals to be considered as important reserves 

of uranium and thorium. Ho\vever, because the studies made in 1945 

did not investigate the sites of radioactive placers on Canyon and 

Poorman Creeks, and 011 the Kahiltna River (table 1), it cannot be 

positively assumed that all placers in the Yentna district are unfavorable for the occurrence of significant concentrations of radioactive 

minerals. The occurrences on Poorman and Canyon Creeks should be 

considered in specific regard to the relationship of the placer deposits 

to possible mineralized zones in the bedrock (Mertie, 1919, p. 257-262). 

The deposits on the Kahiltna River are at sites where it would be 

possible to work great quantities of gravel by large-scale placer 

methods. The radioactivity of the Kahiltna River placers is also 

due to the occurrence of monazite and uranimte (?). The equivalenturanium content of the samples apparently is due to considerably 

more uranium than thorium (table 1). In addition to the uraninite(?) 

and monazite, Larsen (written communication, to Harder and 

Reed 1945), indicates that the Kahiltna River concentrates contain 

varying amounts of apatite, cassiterite, feldspar, pyroxene, quartz, 

sphene, spinel, tourmaline, and zircon. They also contain garnet, gold, 

hematite, ilmenite, limonite, magnetite, and platinum (Mertie, 1919, 

p. 262, 263). (See also appendix for mineralogic analysis of sample

2 Subsequent mineralogic studies have shown that this mineral is uranothorianite. See 

p. 22.
22 MINERAL RESOURCES OF ALASKA

obtained subsequent to the preparation of this report.) Inasmuch as 

the Kahiltna River placers may offer possibilities for working large 

yardage of gravels it is possible that the total value of recoverable economic minerals may be sufficient to warrant further prospecting of the 

placers in the present flood plain and low-level bench gravels from the 

confluence of the Kahiltna with the Yentna River to the toe of the 

Kahiltna Glacier.

LITERATURE CITED

Capps, S. R., 1913, The Yentna district, Alaska: U. S. Geol. Survey Bull. 534.

Frondel, J. W., and Fleischor, Michael, 1952, A glossary of uranium- and thoriumbearing minerals, [2d eel.] : U. S. Geol. Survey Circ. 194.

Mertie, J. B., 1919, Platinum-bearing gold placers of the Kahiltna valley 

[Alaska] : U. S. Geol. Survey Bull. 692-D, p. 233-264.

APPENDIX 

SUBSEQUENT MINERALOGIC ANALYSIS, 1950

Since the preparation of this report a prospector's sample was received in 1950 which may be a concentrate from one of the holes 

drilled at Red Hill Bar on the Kahiltna River in 1917 (Mertie, 1919, 

p. 263) across the river from Round Bend Bar. The size of the sample 

received was too small to permit an accurate analysis of the radioactivity, but qualitative tests showed that it was extremely radioactive. 

A mineralogic analysis of the sample by M. G. White is given below.

Mineral

Ilmenite

Zircon

Magnetite

Platinum _

Scheelite

Uranothorianite

Garnet _

Estimated 

volume 

(percent)

25

20

15

15

5

5

5

5

Mineral

Rutae_ __--___-_--_-----

Hornblende

Crocoite(?)---____-----~

Gold. ____-.-_ __.-.__....

Estimated 

volume 

(percent)

3

2

0)

0)

0)

0)(')

Trace.
INDEX

Page 

Acknowledgments_______.______________ 2

Airfields______________________________________ 5

Altitude, of area_____ __________________________ 3

Analyses of concentrates___-_______ 2, 21; tab. 1, 7

Areas tested _______________ 9

Bedrock, composition of_-__________ 5

exposures _______________________________ 5

gamma-count data of______________ tab. 5

major units_________________________ 5

"Can sample" method._.__.._._.._-__.._ 10 

Common constituents of concentrates_____ 20 

Composition of pebbles_____ ______ 14, 15; tab. 3

Eocene, clay and lignite, gamma-count data., tab. 5 

gravels, description of- _-_____-_----_-___. 15

equivalent-uranium data.___-____ 16; tab. 6

gamma-count data___ __ _ ________ tab. 5

size distribution___--._________.____ 13,15

thickness______ _ ______________ 5

Gold, production of - --___________ 7

Gravel deposits_ 5

Gravity concentrates, percent equivalent

uranium___________________ 16; tab. 6

Heavy minerals.-.--------_______________ 9,11,16

loss during concentration._.______________ 16

Hydraulic method for mining______________ 2

plants, number of_.____________________ 2

Late Tertiary gravels, description of_ 7,15; tab. 6 

equivalent-uranium data of__.______._. tab. 6

gamma-count data of_____.___.____- tab. 5

size distribution of_____-__-.__ 13,15; tab. 6

thickness- ________________________________ 7

Location of district_____-__-__.__--._____-_- 2

Monazite. .. _ 1,20,21,22

Pebble(s), age ot.............................. 14,15

composition_____-_______-_____-______ 14; tab. 4

description^ __________________________ 14,15

"Pebbleeffect" . 10,18

Placer claims:

Carlson's workings.___-___--____. tab. 2; pi. 5

Erickson's workings_____________ tab. 2; pi. 2

Haugham's placer workings. __ 4; tab. 2, 7; pi. 4 

Krummenacher's placer workings. _ 6; tab. 2, 7;

pi. 5 

Morgan's placer workings. ____ 8; tab. 2, 7; pi. 3

Traski's workings_______.__________ tab. 2; pi. 4

Wagner's workings_________ tab 2; pi. 6

Weatherell's workings.__________ tab. 2, 7; pi. 4

Page 

Portable Geiger-Mueller gamma counter,

precautions against moisture. _ 9 

techniques used in sampling procedure

with-... 9-12

Possibilities of future mining operations ______ 22

Quaternary bench gravels, description of. _... 15 

equivalent-uranium data of_ _________ tab. 6

gamma-count data.________.._... tab. 5

size distribution-.-------.-------..--- 13,15

thickness..-------------------- ---_ 7

glacio-fluvial gravels, description of....... 13,15

equivalent-uranium data. ___-_-_--__. tab. 6 

gamma-count data_._________________ tab. 5

size distribution.-----.-----.--------- 13,15

thickness.-----------____-_-_-_____. 7

Recent flood-plain gravels, equivalent-uranium data.._____-__-____--___ tab. 6 

gamma-count data.--------------.---.--- tab. 5

Size distribution.---..-_-_-____-------- 13

iraction_____.-..-___-----_-_-_. 12, 18; tab. 3, 6 

Sluice-box concentrates, number collected. ___ 18

percent equivalent uranium_____-____ 18; tab. 7

Source of radioactivity in samples............ 20

Standardized sample.-.---_.__.._.__- 11

Tests, field-station..._._-___--_--__-----_- 16; tab. 5

gamma-count data-------------------- 16; tab. 5

number made______-__-____--_-___-_____- 16

rough-screened-_____________-_-------_--_ 16

Transportation routes_ _____ _ ___________ 3, 5

Traverses, description of__-___-___-_-_- 12; tab. 2, 5 

location of____-_-______--_---------- 12; tab. 2,5

Tributaries of Cache Creek:

Dollar Creek---.-.--- --- - 3; tab. 1, 2 

Palls Creek_-______-_-__-_--.____- 3, 8; tab. 2

Gold Creek . - 3, 5; tab. 2

Long Creek-.-..---____.. ____ 3; tab. 1,2 

Nugget Creek..._--_-_____.---- 3, 5; tab. 1, 2, 7

Rambler Creek_-___- - -----------__- 3; tab. 7

Spruce Creek_____-._-______ _ 3

Thunder Creek____________ 3, 4; tab. 2, 7

Windy Creek_______ _______ 3; tab. 1

Uraninite, description of.__.____________ 1,21

mineralogic analysis of_ . ____________ 22

Uranothorianite______________.. . 1,21,22

Weighted average, equivalent-uranium data_ tab. 6 

gamma-count data_-_---_-_-_-_--_- __ tab. 5

Zircon, as a source of uranium and thorium. _. 

description of___- _________. ____

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