GEOLOGICAL SURVEY CIRCULAR 300

RECONNAISSANCE FOR 

RADIOACTIVE DEPOSITS 

IN THE DARBY MOUNTAINS 

SEWARD PENINSULA, ALASKA 

1948

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 DEPARTMENT OF THE INTERIOR 

Douglas McKay> Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 300

RECONNAISSANCE FOR RADIOACTIVE DEPOSITS IN THE DARBY MOUNTAINS,

SEWARD PENINSULA, ALASKA, 1948

By Walter S. West

This report concerns work done on 

behalf of the U. S. Atomic Energy 

Commission and is published with 

the permission of the Commission.

Washington, D. C., 1953

Free on application to the Geological Survey, Washington 25, D. C.
RECONNAISSANCE FOR RADIOACTIVE DEPOSITS IN THE DARBY MOUNTAINS,

SEWARD PENINSULA, ALASKA, 1948

By Walter S. West

CONTENTS

Page

Abstract..................................... 1

Introduction. ................................. 1

Geology ..................................... 2

Undivided metamorphic rocks............. 2

Rocks of Paleozoic age. .................. 2

Greenstone and greenstone schist 

Undivided igneous complex ......

Granite........................

Ungalik conglomerate...........

Basalt.........................

Unconsolidated deposits.........

Hot springs....................

Veins .........................

Geology Continued

Mineral deposits. ................

Radioactivity investigations............

Kachauik Creek-Cheenik

Creek area....................

McKinley Creek-Bryan Creek area

Portage Creek-Cape Darby area...

Norton Bay coastal area between 

Cape Darby and Kwiniuk River ..

Kwiniuk River ............. ^.....

Tubutulik River area.............

Summary and conclusions..............

Literature cited ......................

Page

7

ILLUSTRATION

Plate 1. Geologic map of the southern and eastern parts of the Darby Mountains, 

Seward Peninsula, Alaska........................................

Page 

In pocket

TABLES

Table 1. ' Mineralogic composition of selected radioactive heavy-mineral subfractions (specific gravity 

greater than 3. 3) of concentrates from the Portage Creek-Cape Darby area...............

2. Mineralogic composition of iheavy-mineral subfractions (specific gravity greater than 3.3)

of concentrates from the Tubutulik River area ..............:..........................

Page

ABSTRACT

Radioactivity in the southern and eastern parts 

of the Darby Mountains, Seward Peninsula, Alaska, 

appears to be directly related to the occurrence of 

granite. Concentrates from placers derived from areas 

containing granite are more radioactive than concentrates from placers not derived from the granite and, 

generally, contain from 0.01 to 0.05 percent equivalent 

uranium. The radioactivity of these concentrates is 

largely due to radioactive elements in common accessory minerals in granite, such as sphene, allanite, 

zircon, and, locally, monazite. Locally, in the Clear 

Creek-Vulcan Creek area, the headwaters of the 

Kwiniuk River, and on Golovnin Bay near McKinley 

Creek, concentrates from placers derived from granitic 

terrain contain as much as 0.1 percent equivalent 

uranium. The higher radioactivity of the concentrates

from the Clear Creek area and on Golovnin Bay is due 

chiefly to an unidentified uranium-titanium niobate, 

whereas the higher radioactivity at the headwaters of 

the Kwiniuk River is due to thorianite.

INTRODUCTION

The Darby Mountains are situated in the southeastern part of the Seward Peninsula, Alaska, (pi. I). 

The range has a general north-south trend and extends 

from Cape Darby on the south to the southeastern 

portion of the Bendeleben Mountains on the north, a 

distance of approximately 75 miles. The area is 

bounded on the west by Golovnin Lagoon, Golovnin Bay, 

and the Fish River and in general on the east by Norton 

Bay and the Tubutulik River, although the upper half of 

the latter river dissects the eastern flank of the mountains.
The area is accessible from Nome, approximately 

70 miles to the west, by air to two small airstrips, one 

located at Golovin on the western side of the Darby 

Peninsula and the other at White Mountain on the Fish 

River (outside of area shown on pi. 1) and the Moses 

Point Civil Aeronautics Authority airfield near the 

mouth of the Kwiniuk River, 46 miles northeast of 

Golovin. There are no good harbors along the entire 

coast. Transportation of supplies within the area must 

be accomplished by tractor or in part by small boats, 

as there are no roads.

The Darby Mountains are a relatively low range 

with the highest peaks seldom rising to more than 

3, 000 feet above sea level. In the heart of the range, 

glaciation has produced cirques and steep-sided, 

U-shaped valleys. Some streams have cut deep valleys. 

Wave action along the south coast of the Darby Peninsula 

has produced a rugged coast with high cliffs and a beach 

so narrow that it is impossible to walk along the shore 

in many places.

The greater part of the Darby Mountains district is 

made up of well- rounded tundra- or vegetation- covered 

hills and mountains. Rock pinnacles, talus, and rock 

fragments occur on and near the tops of many mountains, but bedrock outcrops are surprisingly scarce.

Previous investigations (Gault, Killeen, West, 

and others, 1953) in the northeastern part of the Seward 

Peninsula to the northeast of the Darby Mountains revealed the presence of uranothorianite and several 

other less radioactive minerals widely distributed, but 

locally concentrated, in areas underlain by felsic 

igneo1 s rocks. The purpose of the 1948 reconnaissance 

in the Darby Mountains was to determine whether similar radioactive minerals also occur in the felsic igneous 

rocks of this district, and, if present, attempt to discover any sites of high concentration and the bedrock 

source of such minerals. The reconnaissance was 

conducted during the summer of 1948 by a field party 

consisting of W. S. West, geologist, and A. E. Nessett, 

J. J. Otoyuk, and I. M. Ikleasik, camp assistants. 

Time permitted only the investigation of the southern 

and eastern parts of the district. The route followed 

is indicated by the location of camps, numbered in 

chronologic order starting at Golovin, on plate 1.

GEOLOGY

The only work of the Geological Survey in the 

Darby Mountains prior to this investigation was done 

by two parties; that of Mendenhall (1901X who in 1900 

mapped both topographically and geologically the Koyuk 

River valley and parts of the Darby Mountains on an 

exploration-reconnaissance scale; and that of Smith 

and Eakin (1911), who in 1909 mapped the area between 

Nulato a.nd Council on a similar scale.

The geologic features of the Darby Mountains 

district as described by Mendenhall (1901) and Smith 

and Eakin (1911), are discussed briefly below.

Undivided metamorphic rocks

The oldest rocks in the district, which occur 

mainly along the western flanks of the Darby Mountains, 

are folded and sheared metamorphic rocks of pre-Silurian

and possibly pre-Cambrian age (pi. 1). These rocks 

consist of quartzose, calcareous, biotitic, chloritic, 

feldspathic, and graphitic schist and thin limestones. 

The schist has been cut by greenstone, granite, diorite, 

and other igneous rocks which have had a share in the 

metamorphism and obliteration of the original chara,cters of the schist.

Rocks of Paleozoic age

Carboniferous, Devonian, and Silurian rocks are 

best represented along the eastern flank of the Darby 

Mountains and are well exposed along the sea coast 

from the mouth of Iron Creek southwest nearly to 

Carson Creek. This group of rocks, composed mainly 

of limestone with subordinate dolomite, black slate, 

and schist, is complexly folded, faulted, metamorphosed, 

and cut by intrusives of greenstone, granite, and diorite.

Greenstone and greenstone schist

The post-Carboniferous greenstone and greenstone 

schist, which occur along the eastern flank of the Darby 

Mountains, were not observed in place, although greenstone cobbles and pebbles were seen on the bars along 

the lower course of the Tubutulik River.

Undivided igneous complex

An undivided igneous complex and the granite 

make up the central core of the Darby Mountains. The 

undivided igneous complex is believed to belong to one 

geologic body of pre-Cretaceous age. It is composed 

mainly of granite, which has undergone varying degrees 

of metamorphism, but also includes some diorite, 

greenstone, and several other types of igneous rocks 

whose characteristics are not sufficiently clear to allow 

their identification and subdivision.

Granite

The granite, also pre-Cretaceous in age, 

ranges from even-grained to porphyritic in texture 

and exhibits considerable variation in mineralogical 

composition. In the area studied the porphyritic 

granite is much more abundant than the evengrained. Outcrops usually take the form of knobs 

a.nd pinnacles on the tops and sides of the mountains. Inclusions of diorite in the porphyritic 

granite noted especially along the east coast of 

the Darby Peninsula indicate that the granite is 

younger than some of the diorites in the undivided 

igneous complex.

Ungalik conglomerate

The Ungalik conglomerate, the oldest formation 

of the Cretaceous sedimentary sequence in west-central 

Alaska, is made up of rounded debris derived from 

older formations including limestone, schist, igneous 

rocks, quartz veins, and sandstone. It forms the first 

main divide east of the Tubutulik River on the extreme 

eastern flank of the Darby Mountains. One outcrop of 

this conglomerate was noted along the bank of the lower 

Tubutulik River.
Basalt

Two isolated outcrops of vesicular basalt of 

Tertiary age were observed in the area. One lies between the headwaters of Grouse Creek and theTubutulik 

River and the other between the third and fourth east 

tributaries to the Right Branch of Kachauik Creek (pi. 1).

Unconsolidated deposits

The youngest deposits shown on plate 1 are marine 

a.nd nonmarine water-laid deposits of Quaternary age. 

No attempt has been made to show the glacial deposits 

or the widespread deposits of unsorted waste rock that 

cover practically all of the area not occupied by marine 

and nonmarine gravels, sands, and silts.

Hot springs

A hot spring, not mentioned in earlier Survey 

literature, was discovered issuing from granite high 

on a mountain slope above Clear Creek, a right tributary to the Tubutulik River. The stream formed by 

this spring constitutes the seventh west tributary to 

Clear Creek above the creek's mouth (pi. 1). There is 

another hot spring on the north side of the Kwiniuk 

River in the vicinity of the mouth of Corral Creek. This 

spring was not visited, but it also probably issues from 

granite.

Veins

Quartz and calcite veins and veinlets are abundant 

in the schistose rocks, and calcite veins are common 

in the limestones. Quartz veins also occur in the granite 

and granitoid rocks, but they are much less common 

than in the schists. Along the beach about 2 miles 

north of Golovin a large quartz lens 30 by 15 by 10 feet 

may be seen, but this lens, like practically all the veins 

and veinlets, contains no sulfides. Half a mile north 

of this lens vein quartz containing chalcopyrite was 

found in float along the beach; and chalcopyrite-quartz 

vein float, stained and incrusted with malachite, was 

observed on the west bank of the Right Branch of 

Kachauik Creek about 75 yards upstream from the 

mouth,of the second east tributary. Iron-stained quartz 

and calcite veins were observed at other localities, and 

some veins have been reported by prospectors to carry 

small quantities of gold.

Mineral deposits

No lode mines were operating in the Darby 

Mountains during 1948. There is evidence that lode 

prospecting, probably for copper and gold, has been 

carried on in the past along the coast of Norton Bay 

between Carson and Walla Walla Creeks, but no deposits 

of economic importance were discovered. The only lode 

prospect of any significance in the entire Darby Mountains is a silver-lead prospect on Omilak Creek, a 

tributary to the Fish River, in the northwestern part 

of the range. This prospect, which lies outside the 

area covered in plate 1, was not visited.

A coal deposit, not previously mentioned in 

Survey literature, was found to crop out on the right

headwater fork of the left branch of Grouse Creek, a 

tributary to the Tubutulik River. This deposit appears 

to have an east-west extent of more than 1 mile, and 

the outcrop from stream level to the top of the bank 

varies from 10 to 15 feet in thickness although the 

depth of the coal below the level of the stream is not 

known. The coal is probably a high-grade lignite and 

may possibly rank as high as subbituminous C coal.

There is no evidence of any recent placer mining 

in the district, and no one was prospecting during the 

summer of 1948. The only old placer-gold operation 

observed was on Grouse Creek. In the past, gold prospecting has been done on Vulcan, Chukajak, and Admiral 

Creeks, all tributaries to the Tubutulik River.

RADIOACTIVITY INVESTIGATIONS

The instrument used for field radiometry in the 

Darby Mountains in 1948 was a standard commercial 

model of a portable survey meter with a 6-inch betagamma probe. Because the greater part of the district 

is covered with vegetation mostly tundra and talus, 

direct radiometric testing of outcrops was of necessity 

limited in scope, and the low sensitivity of the probe 

rendered the instrument ineffectual for traversing. To 

study effectively distribution of radioactive minerals 

in the district the greater part of the field work was 

directed, therefore, to the collection of concentrates 

from slopewash and the gravels of stream and beach 

deposits in a effort to determine whether radioactive 

minerals were being concentrated in placers formed 

by the deposition of material eroded from rocks in the 

various drainage areas. Thus, local concentrations 

of radioactive minerals could be found, the ultimate 

source of which could then be discovered by trenching 

to bedrock at selected localities.

In the course of the season 248 concentrates were 

collected from 215 sites of stream gravels, 20 sites of 

beach gravels, and 13 sites in slopewash. Each concentrate from stream and beach gravels was panned 

from about 50 pounds of sand and gravel, and each 

slopewash concentrate was panned from approximately 

100 pounds of disintegrated rock material. The locations 

of these concentrates are shown on plate 1. Each of 

these concentrates was tested for radioactivity in the 

field to determine whether additional concentrates 

should be taken in any given drainage area. For consistency in expressing the radioactivity of these concentrates, each sample was further concentrated in the 

laboratory by separating the light- and heavy-mineral 

fractions with bromoform (sp gr 2.89). As preliminary 

tests showed that essentially all the radioactive minerals 

occur in the heavy-mineral fraction, the equivalent 

uranium content of each heavier-than-bromoform fraction was then determined by radiometric analysis. To 

concentrate further the radioactive minerals for 

mineralogic study the heavier-than-bromoform fractions 

of selected concentrates were subdivided into light and 

heavy subfractions by using methylene iodide (sp gr 3.3), 

as again preliminary scanning of the samples indicated 

that essentially all the radioactive minerals have a 

relatively high specific gravity.

In addition to the above technique of testing areas 

by studying the heavy-mineral content of its placers, 

the writer collected numerous rock samples, mostly 

float rock from talus slopes and stream gravels.
However, as the radioactivity of none of these samples 

was estimated to exceed 0.003 percent equivalent uranium when tested with the portable survey meter, they 

were not examined in more detail.

The distribution of radioactivity in the southern 

and eastern parts of the Darby Mountains is discussed 

by area in the following pages.

Kachauik Creek-Cheenik Creek area

The rocks in the drainage basins of Kachauik a.nd 

Cheenik Creeks (pi. 1) include two bodies of granite, 

which appear to be the source rocks of the radioactive 

minerals in the concentrates collected from the area. 

The distribution of radioactivity in a total of 46 concentrate samples (nos. 2834-2889, pi. 1) obtained in the 

area is as follows:

Range in equivalent-uranium content 

(percent)

Number of 

samples

< 0.01 

0.01 to 0.02 

> 0.02 (maximum 0.028)-

25 

Ik

1

All the concentrates containing 0. 01 percent or more 

equivalent uranium are from those parts of the two 

main streams or their tributaries which drain the granite bodies. The concentrates that contain more than 

0.02 percent equivalent uranium have an average concentration ratio of about 360 to 1.

The mineral subtraction of sample 2839, greater 

than 3. 3 specific gravity, selected as typical of the 

radioactive concentrates from the Kachauik CreekCheenik Creek area, contains, in decreasing order of 

abundance, hornblende, sphene, garnet, and zircon 

and traces of ma.gnetite, ilmenite, and olivine. Although the radioactive minerals in this sample were 

not specifically identified, it is likely that the radioactive elements occur chiefly in the sphene and zircon.

McKinley Creek-Bryan Creek area

In the McKinley Creek-Bryan Creek area (pi. 1) 

a. small body of granite occurs in the headwaters of 

Bryan Creek and an even smaller body appears in a 

sea cliff on Golovnin Lagoon about midway between the 

mouths of Cheenik and McKinley Creeks. Six concentrates (nos. 2893-2898) were taken in the vicinity of 

the granite at the head of Bryan Creek. The radioactivity of these concentrates, which have concentration 

ratios of 500-1,000 to 1, ranges from 0. 009 to 0.015 percent equivalent uranium. The radioactive minerals in 

these samples were not identified but probably are one 

or more of the common accessory minerals in the 

granite. Sample 2890, a concentrate taken from a 

small unnamed stream, not shown in plate 1, contains 

0.016 percent equivalent uranium. As this stream 

probably heads in the edge of the granite on upper 

Bryan Creek and the radioactivity and concentratidn 

ratio is a.bout the same as the samples from Bryan 

Creek, it is likely that the radioactive minerals are 

the same as those in the samples from Bryan Creek.

The material in the slopewash concentrate 

(sample 2891) taken at the granite on the coast between

Cheenik and McKinley Creeks probably is derived 

mostly from a mineralized zone near the contact between the granite and the older igneous complex 

(pi. 1). The heavier-tha.n-bromoform fraction contains 

0.074 percent equivalent uranium. The minerals of 

specific gravity greater than 3. 3 are chiefly magnetite, 

with lesser amounts of sphene and topaz a.nd minor 

amounts of allanite, hematite, ilmenite, hornblende, 

biotite, scheelite, and an unidentified uranium-titanium 

niobate mineral. The uraniferous niobate is the chief 

radioactive mineral, although radioactive elements in 

the allanite, hematite, and sphene doubtless contribute 

to the total radioactivity of the sample.

Portage Creek-Cape Darby area

Between Portage Creek and Cape Darby 24 concentrate samples (nos. 2899-2922, pi. 1) were collected 

to determine whether radioactive minerals were being 

eroded from the rocks along the numerous short streams 

and sea cliffs in the area. The distribution of radioactivity in these samples follows:

Range in equivalent-uranium content 

(percent)

< 0.01 

0.01 to 0.02-

> 0.02 

Number of 

samples

IT

2

5

Data on the concentrates from this area containing 

0.01 percent or more equivalent uranium are given 

as follows:

Sample 

no.

2899

2900

2905

2909

2910

2918

Location

2921

Portage Creek, near mouth 

Portage Creek, right

tributary. 

Golovnin Lagoon teach near

Mission Creek.

 do 

 do 

Unnamed stream entering

Golovnin Lagoon about

5j miles south of

Mission Creek. 

Golovnin Lagoon sea cliff

about 14J? miles south of

Mission Creek.

Percent

equivalent

uranium

0.022 

.05!*

.015

.015

.030

.035

Concentration ratio

650:1 

550:1

75:1

130:1

30:1

6,1*00:1

1,200:1

The mineralogic composition of selected radioactive samples from the Portage Creek-Cape Darby 

area is given in table 1. As in the Kachauik CreekCheenik Creek area the more radioactive samples are 

concentrates in which the material has been derived 

chiefly from granite.

In this area the radioactivity is due to the presence of radioactive elements in allanite, hematite, 

and monazite as well as sphene and zircon. Of particular 

note is the slopewash concentrate (sample 2918) from 

the small granite mass about 4 miles south of Mission 

Creek which contains chiefly hematite and topaz. The 

material from which this concentrate was obtained 

consisted largely of disintegrated rock from a contact 

zone between the younger granite and the older intrusives, in this case a metamorphosed granitic rock. 

The possibility that the radioactive elements, particularly
Table 1. Mineralogic composition of selected radioactive heavy-mineral subfractions (specific gravity greater than J>.J>] of concentrates from the Portage Creek-Cape Darby area

[X indicates presence of mineral; Tr. indicates presence of only traces of the mineral; zero indicates mineral not present]

Mineral

Hematite 

Rutile - -

<7-t -nn *~\Y\

Sample no.

2900

X

Tr. 

Tr. 

0 

X 

0 

X 

X 

0 

X

2910

0 

X 

X 

X 

X 

X 

0 

X 

Tr. 

X

2918

0 

X 

0 

0 

0 

0 

0 

0 

0 

Tr.

2921

Tr. 

X 

X 

0 

X 

0 

0 

X 

. 0 

X

Mineral

Riotitc -. -. -. 

Sample no.

2900

0 

0 

X 

0 

0 

X 

0 

0 

0

2910

0 

X 

0 

X 

0 

X 

0 

0 

Tr.

2918

X 

0 

0

/o

0 

0 

0 

0 

0

2921

0 

Tr. 

Tr. 

0 

Tr. 

0 

X 

Tr. 

0

uranium, in this area were introduced hydrothermally 

in a late stage of the consolidation of the granite magmas 

is suggested by the occurrence of fluorite and topaz.

Norton Bay coastal area between Cape Darby 

and the Kwiniuk River

Twenty-seven concentrates (samples 2923-2946 and 

2963-2965, pi. 1) were taken in the coastal area along Norton 

Bay between Cape Darby and the mouth of the Kwiniuk River. 

These include three samples from the area of a granite at 

the head of the Kwiniuk River, which extends to within several miles of the coast about 13 miles north of Cape Darby 

(pi. 1). Most of the concentrates were collected in the vicinity of several small granite bodies that occur along the 

coast in the vicinity of Carson Creek. The radioactivity 

data of the 27 concentrates are summarized as follows:

Range in equivalent -uranium content 

(percent)

< 0.01 -

0.01 to 0.02-

>0.02 -

Number of 

samples

15

7

Pertinent data on the heavy-mineral fractions of samples 

containing more than 0. 02 percent equivalent uranium 

follows:

Sample 

no.

2927

2929

Location

2937

2963

2965

Creek between Norton Bay,

2.8 miles south of

Carson Creek. 

Creek between Norton Bay,

0.6 mile south of

Carson Creek.

Carson Creek 

Headwaters Kwiniuk River,

second right tributary

below headwater forks. 

Headwaters of Kwiniuk

River.

Percent

equivalent

uranium

0.021

.02^ 

.029

.039

Concentration ratio

It-10:1 

560:1

380:1 

350:1

Although the radioactive minerals in the concentrates from 

the Norton Bay coastal area were not identified (except for 

one concentrate from the head of the Kwiniuk River), the 

fact that the locations of the more radioactive samples are 

directly related to location of the granites suggests that 

the bulk of the radioactivity is due to common accessory 

minerals of the granite, such as zirconand sphene.

However, because of the proximity of these granites to 

those on the west side of Darby Peninsula in the vicinity of 

Mission Creek, it is also possible that such minerals as 

allanite and monazite might also be found in these concentrates. If present, these minerals would contribute to the 

radioactivity of the samples.

Sample 2965 from the granite area near the head 

of the Kwiniuk River (pi. 1), contains, in decreasing 

order of abundance, hornblende, sphene, zircon, 

garnet, hematite, magnetite) epidote, olivine, and 

thorianite. The thorianite is the chief radioactive 

mineral; however, the zircon and sphene are slightly 

radioactive. Although the bedrock occurrence of the 

thorianite is most likely as an accessory mineral in 

the granite, the proximity of the contact between the 

granite and metamorphosed sedimentary rocks, 

including limestone, suggests that the thorianite may 

well have been derived from altered rocks in the contact zone.

Kwiniuk River

The Kwiniuk River crosses a large belt of granite 

about halfway to its headwaters (pi. 1). Unfortunately 

the investigation of this granite area could not be completed because of the onset of cold weather in the fall. 

However, some 16 concentrates (samples 2947-2962) 

were taken along the Kwiniuk, and a few of its tributaries, between its mouth and the belt of granite. The 

equivalent-uranium content of the heavy-mineral fractions (specific gravity greater than 2.89) of these concentrates are given as follows:

Remarks Sample 

no.

2947

29^8

29^9

2950

2951

2952

2953

295^

2955

2956

2957

2958

2959

2960

2961

2962

Percent 

equivalent 

uranium

0.003

.008

.008

.015

.011

.018

.002

.012

.006

,00k

.015

.011

.013

.015

.017

.017

Concentration ratio

1,080:1

120:1

250:1

180 :1

310:1

210:1

610 :1

2^0:1

330:1

2^0:1

180:1

250:1

210:1

230:1

230:1

180:1

Right-limit tributary.

Left-limit tributary. 

Do.

Left-limit tributary.
As seen in the previous tabulationand on plate 1 the equivalent uranium content of the samples has a fairly general increase in upstream order with the radioactivity 

confined almost entirely to concentrates from the main 

stream. Although the equivalent uranium content of 

the concentrates is not as relatively high when compared 

to other concentrates from the district, the general increase upstream suggests that higher values might be 

obtained in the granite belt, which is believed to be the 

chief source of the radioactive minerals. The hea.vy 

minerals in sample 2961, which was obtained from a 

small tributary draining a small part of the granite 

along its contact with adjacent sedimentary rocks of 

Paleozoic age (pi. 1), are chiefly sphene, hornblende, 

allanite, apatite, hematite, and garnet; also found were 

traces of spinel, scheelite, powellite, magnetite, 

epidote, rutile, pyrite, and chalcopyrite. The radioactivity of the sample is ascribed mostly to thorium in 

the allanite, although it is likely that some of the other 

minerals, such as the sphene and hematite, are also 

radioactive. The presence of traces of scheelite, 

powellite, a.nd chalcopyrite suggests that at least some 

of the minerals in the concentrate were derived from 

some type of lode at or near the contact between the 

granite and the limestone and shale of Paleozoic age. 

It is possible, therefore, tha,t some, if not all, of the 

radioactive minerals might also have been derived from 

a lode source rather than have originated as accessory 

minerals in the granite.

Tubutulik River area

The drainage basin of the Tubutulik River is 

underlain by a large variety of rock types (pi. 1). The 

most significant feature, however, is a wide belt of 

granite along the western side of the area that is a continuation of the granite belt crossing the Kwiniuk River 

to the south. Of a total of 114 concentrates (samples 

2966-2990, 2992-2998, and 3000-3002, 3004, 3081 

(pi. 1), 41 contain between 0. 01 and 0. 02 percent equivalent uranium, a.nd 34 contain more than 0. 02 percent 

equivalent uranium up to a. maximum of about 0.1 percent 

equivalent uranium. The radioactivity of the concentrates from the Tubutulik River and its tributaries is 

confined entirely to samples in which at least part of 

the original material concentrated was derived from 

granitic rocks (pi. 1). In general, the equivalentura.nium content of the concentrates increases as the 

streams a,re ascended to and within the gra.nite. The data 

on the concentrates from the Tubutulik River area tha.t 

contain more than 0. 02 percent equivalent uranium a.re 

given below; these samples are arranged in order of 

location from south to north, rather than in upstream 

order, in an attempt to show more precisely the relationship of the radioactivity of the concentrates to the 

belt of granite.

Sample 

no.

3020

3021

3009

3

Location e

Seventh west tributary

of Clear Creels, near

hot spring.

 do 

Third west tributary of

Clear Creek.

Percent 

equivalent , 

uranium

0.038

.022

.022

/oncentra- 

:ion ratio

375:1

200:1 

1+25:1

Sample 

no.

2983

pQfik

2987

2988

pqftq

2990

2992

2993 

PQQk

2995 

2996

2997 

2982

297^

2975

2979

3029 

3030

3031

3032

3033

^n^k

3035

3052

3072 

3075

3077 

3078 

^nftk

3

Location ec

Second west tributary of 

Clear Creek. 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

 do 

First west tributary of 

Clear Creek.

 do 

 do 

 do 

 do 

 do 

 do 

First south tributary of 

Tubutulik River above 

Rock Creek.

First northeast tributary 

of Tubutulik River above 

Grouse Creek. 

Second northeast tributary 

of Tubutulik River above 

Grouse Creek.

Percent 

luivalent 

uranium

0.086 

.023

.027

.067

n^k

.027

.101+

.079

c\£.c\

nfi6

.065

Qi(.Q

.022

.023

.023

.023

gk8

.027

Qk2

.014.14.

Ok^

067

.023

.025

.036

.0214- 

.023

.oi+6

.055 

nv6

Concentration ratio

1,100:1 

1+00:1

1 7RH 1

2,200:1

1,050:1

k 75:l

1,850:1

i k'SO 1

750:1

i onn i

1 ccn . 1

1,500:1

600:1 

925:1

1*, 000:1

275:1

127:1

825:1

1,850:1

1,1+50:1

1,050:1 

725:1

l+50'l

2,250:1

1,1+50:1 

1,200:1

1,500:1

? 7^0 ?1

From the above tabulation it is apparent that three 

major centers of abnormal radioactivity occur in the 

Tubutulik River area. These centers are on the second 

west tributary of Clear Creek, on Vulcan Creek, and 

in the area between Grouse Creek and the Tubutulik 

River. The mineral composition of representative 

concentrates from these areas are given in table 2. 

For comparison, the mineral composition for radioactive concentrates of the seventh and first west tributaries of Clear Creek, Rock Creek, and Big Creek are 

also given.

Review of the above mineral lists suggests that 

some of the radioactivity in all the concentrates is due 

to radioactive elements in such minerals as hematite, 

allanite, and sphene. It is likely that the zircon also 

is radioactive. Of the more radioactive samples, 

namely those containing more than 0.06 percent equivalent uranium, only one (that from Grouse Creek) is 

not from the area between the second west tributary 

of Clear Creek and Vulcan Creek. The higher radioactivity in the Clear Creek-Vulcan Creek samples is 

attributed primarily to the presence of an as yet unidentified uraniferous titanium niobate mineral or minerals. The heavy-mineral fractions (specific gravity 

greater than 2. 89) of samples 2983, 2996, and 3033 

were estimated to contain as much as b to 10 percent 

of the niobate mineral or minerals. Qualitative specfcrographic analyses of the nioba.tes from these three
Table 2. Mineralogic composition of heavy-mineral siibfractions (specific,-gravity greater than 3-3)

of concentrates from the Tubutulik River area

[X indicates presence of mineral; Tr. indicates presence of only traces of the mineral; zero indicates mineral not present]

Mineral

Pyrrhotite 

P^m i i~ o _ _ _ _ _ _ _ _

Rutile 

A Y) Q t Q C!£i

Garnet 

Olivine 

Zircon 

Tf"M"^fl "7

Kyanite -

Epidote 

Allanite 

Staurolite 

Biotite 

Sphene 

Niobate minerals 1 

Apatite 

Scheelite 

Powellite 

Unlfftn1t.fi- - _

3020

0

Tr.

0

Tr.

0

0

0

0

0

V

0

0

0

0

0

0

0

0

n

29B3

0

0

0

Tr.

0

0

0

X

.

0

0

X

0

0

0

0

V

0

0

n

2992

0

0

0

0

0

0

0

0

0

0

0

0

y

0

0

y

Tr.

0

0

0

n

2993

0

0

Y

0

0

0

0

0

0

0

0

0

0

0

0

y

Tr.

0

0

0

n

2995

0

0

Y

Y

0

0

0

0

0

0

0

0

0

A y

0

0

Tr.

0

0

0

0

n

c

2996

0

0

0

0

Tr.

0

0

0

y

0

0

0

0

0

0

0

n

Jamplf

2974

0

Tr.

0

y

y

0

0

0

0

0

0

0

Tr.

0

Tr.

0

0

0

0

n

3 no.

3028

0

Tr.

y

0

Tr. Tr.'

0

y

0

y

0

0

0

0

0

n

3033

0

0

0

0

0

0

0

0

0

0

Tr.

0

0

0

0

0

n

3052

0

0

y

y

y

0

0

y

0

y

0

0

y

y

y

y

0

0

0

0

0

0

n

3075

Tr.

Tr.

y

0

0

Tr.

Tr,

0

y

0

0

0

0

0

0

0

Tr.

n

3077

0

0

0

V

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

n

3078

0

0

0

0

0

0

0

0

0

0

0

0

y

0

0

0

0

TT»

3064

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

n

Unidentified minerals that contain chiefly niobium, titanium, calcium, and uranium.

samples showed that the chief elements in the mineral 

are niobium, uranium, titanium, and calcium. Traces 

of silicon and iron were also found. In only one of the 

samples was a niobate mineral thorium bearing.

The relatively high radioactivity of the sample 

from Grouse Creek is ascribed to the occurrence of a 

large amount of sphene and the relatively high concentration ratio.

SUMMARY AND CONCLUSIONS

The widespread cover of vegetation, mostly 

tundra a.nd talus, in the Darby Mountains necessitated 

the use of a placer-sampling technique to evaluate 

adequately the potentialities of the district for radioactive materials. Field studies and the radiometric 

analysis and mineralogic study of the heavy-mineral 

fractions of numerous concentrates from placers of 

the district showed that essentially all the radioactive 

minerals in the district occur in, or were derived from, 

bodies of f el sic igneous rocks, chiefly granite.

The most common and widespread radioactive 

minerals are sphene, allanite, hematite, and zircon. 

These minerals, with the possible exception of the 

hematite, probably are all primary accessory minerals 

in the granite. Locally in the Clear Creek-Vulcan 

Creek area and near the mouth of McKinley Creek, 

abnormally high radioactivity of concentrates derived 

from the granite appears to be due to the presence of 

a. uraniferous titanium niobate mineral. Monazite was 

identified in one concentrate from the east shore of

Golovnin Bay between Cape Darby and Portage Creek. 

Thorianite is the chief radioactive mineral in a concentrate from the head of the Kwiniuk River.

Although it is likely that the uranium- and 

thorium-bearing minerals in the southern and eastern 

parts of the Darby Mountains are largely disseminated 

in the granite, local concentrations of the radioactive 

elements may have formed through hydrothermal 

activity in a late stage of consolidation of the granite 

magmas. This is suggested by the common association 

of topaz and, locally, fluorite with the radioactive 

minerals. The association of topaz and traces of 

cassiterite with the uraniferous niobate mineral in the 

Clear Creek-Vulcan Creek area suggests also that 

these minerals may be genetically related to lode tin 

occurrences or pegmatitic phases of the granite.

LITERATURE CITED

Gault, H. R., Killeen, P. L., West, W. S., and

others, 1953, Reconnaissance for radioactive deposits 

in the northeastern part of the Seward Peninsula, 

Alaska, 1945-47 and 1951: U. S. Geol. Survey 

Circ. 250.

Mendenha.il, W. C,, 1901, A reconnaissance in the 

Norton Bay region, Alaska (in Reconnaissance in the 

Cape Nome and Norton Bay regions, Alaska): U. S. 

Geol. Survey Special Publication.

Smith, P. S., and Eakin, H. M., 1911, A geologic 

reconnaissance in southeastern Seward Peninsula and 

the Norton Bay-Nulato region, Alaska: U. S. Geol. 

Survey Bull. 449.

IHT.-DUP. SEC.. WASH.. D.CSoj7t,

UNITCP STATES DEPARTMENT OF THE INTERIOR 

GEOLOGICAL SURVEY 163'

rj»riii y> w> PLATE i

iff

Unconsolidoted deposits 

(Bonds, grovels.and silts of fluvial, glociot,ond marine origin)

w^j^<^^^^ ' ' '"''W^/A^ * * 1\*&<SJIB3& totitiz.m - A >,:-%f^.«5ai^6s

Ungohk conglomerote 

(Conglomerate with some sandstones)

Greenstone ond greenstone schist

Rocks of Paleozoic age

(Limestones,dolomites,and block slates)

Undivided metamorphic rocks 

f Schists.thin limestones.etc., probably some igneous rocks)

less than 0.01 percent equivalent uranium 

contains from 0.01 to 0.02 percent equivalent uranium

Contains more than 0.02 percent equivalent uranium

Concentrates from slope wosh

2645 Q Contains less than 0.01 percent equivalent uranium

3015 n Contains from 001 to 0.02 percent equivalent uranium

2921 Contains more than 0.02 percent equivalent uranium

X Camp 10 

Geological Survey field comps in 1948

Golov n in

Lagoon

Approximate Mean Declination, 1948

Base from U.S.Geol. Survey Bull. 

449, pi. I (modified by W.S. 

West with data from USAAF 

aerial photographs; the GolovrnCandle area mop, by Lamen Com 

mercial Co., Nome, Alaska; and 

field sketches, 1948).

Geology from U.S.Geol. Survey 

Bull 449 pl.6(modified from 

field work by W.S.West, 1948)

INDEX MAP-SHOWING LOCATION OF DARBY MOUNTAINS AREA 

SEWARD PENINSULA, ALASKA

Cape Darby

Norton Sound

162

GEOLOGIC MAP OF THE SOUTHERN AND EASTERN PART OF THE DARBY MOUNTAINS, SEWARD PENINSULA, ALASKA