GEOLOGICAL SURVEY CIRCULAR 279

RECONNAISSANCE FOR RADIOACTIVE 

DEPOSITS IN THE RUBY-POORMAN 

AND NIXON FORK DISTRICTS, 

WEST-CENTRAL ALASKA, 1949

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

GEOLOGICAL SURVEY CIRCULAR 279

RECONNAISSANCE FOR RADIOACTIVE DEPOSITS IN THE RUBY-POORMAN 

AND NIXON FORK DISTRICTS, WEST-CENTRAL ALASKA, 1949

By Max G. White and John M. Stevens

This report concerns work done on behalf of the 

U. 8. Atomic Energy Commission and is published 

with the permission of the Commission.

Washington, D. C.,1953

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

U. S. Geological Survey 

Surface Water Branch 

541 Federal Office Btdg. 

San Francisco I, Calif.

CONTENTS

Page Page

Chapter A. Ruby-Poorman district Chapter B. Nixon Fork district

Abstract ............................. 1 Abstract .......................... . 10

Introduction .......................... 1 Introduction ......................... 10

Geology.............................. 1 Geology, mineral deposits, and

Bedrock ......................... 1 radioactivity investigations .......... 10

Alluvium ........... .^........... 1 Hidden Creek area ............... 12

Mineral deposits ..................... 3 Whalen mine................. 12

Radioactivity investigations ........... 3 Ruby Creek area ................. 12

Ruby, area ....................... 3 Nixon Fork mine............... 16

Long area ....................... 4 Crystal shaft ............. 16

Granite on Birch and Other localities examined ......... 18

Straight Creeks ............ 4 Mystery, Puzzle, and

Granite on upper Submarine Creeks........... 18

Flint Creek ............... 4 Eagle Creek ................. 18

Poorman area .................. . 9 Summary and conclusions.............. 18

Summary and conclusions ............. 9 Literature cited ..................... 19

Literature cited ...................... _ 9

ILLUSTRATIONS _ Page

Figure 1. Sketch map of the Ruby-Poorman district, Alaska, showing location of Birch Creek

and upper Flint Creek ............................................................... . 2

2. Geologic sketch map of Birch Creek, Ruby-Poorman district, Alaska ....................... . 5

3. Geologic sketch map of upper Flint Creek, Ruby-Poorman district, Alaska ................... 6

4. Geologic sketch map of the vicinity of the Nixon Fork mines ................................ 11

TABLES

Page

Table 1. Data on samples from the vicinity of the granite on Birch and

Straight Creeks, 1949 .............................................................. . 7

2. Data on samples from the vicinity of the granite on upper Flint and

Monument Creeks, 1949 .............................................................. 8

3. Data on concentrates collected in the Nixon Fork mining district

before 1949 ...........................'............................................... 13

4. Data on placer concentrates collected from Hidden Creek and its

tributaries in 1949 .................................................................... 13

5. Data on samples collected from the Whalen mine and vicinity ............................... 14

6. Data on bedrock samples and concentrates collected in the Ruby Creek

area in 1949 ....................................................................... . 15

7. Mineralogy and radioactivity of three placer concentrates from the

Nixon Fork mining district ........................................................... 16

8. Data on ore, mill, and tailings samples from the Nixon Fork mine, 1949 .................... 17

9. Data on samples of garnet rock from the vicinity of the Crystal shaft,

Nixon Fork mine ................................................................... 17

10. Summary of significant radioactivity data obtained in the

Nixon Fork mining district, 1949 ...................................................... 19

iii

RECONNAISSANCE FOR RADIOACTIVE DEPOSITS IN THE RUBY-POORMAN 

AND NIXON FORK DISTRICTS, WEST-CENTRAL ALASKA, 1949

By Max G. White and John M. Stevens

CHAPTER A. RUBY-PCCRMAN DISTRICT

ABSTRACT

Reconnaissance for radioactive deposits in the 

Ruby-Poorman district, Ruby quadrangle, central 

Alaska during July 1949 showed that two small bodies of 

granite in the Long area, about 30 miles south of Ruby, 

contain an average of 0. 005 percent equivalent uranium. 

This radioactivity is due chiefly to a uraniferous thorium 

silicate, tentatively identified as uranothorite, which is 

disseminated in the granite. Other minerals, such as 

sphene, allanite, and zircon, that contain radioactive 

elements as impurities, however, also contribute to the 

total radioactivity of the granite. The uranothorite(?) 

contains about 57 percent thorium and 8 percent uranium.

Search for the bedrock source of a radioactive 

mineral of the spinel group which occurs in placers on 

upper Solomon Creek in the Poorman area was unsuccessful. Radiometric traversing indicated no anomalous 

radiation at a silver-bearing galena deposit on New York 

Creek in the Ruby area.

Although it is believed that there is little possibility 

of commercial deposits of uranium in the Ruby-Poorman 

district, it should be noted that the heavy cover of vegetation and alluvium prevents complete coverage of the 

district by radiometric surveying.

INTRODUCTION

The Ruby-Poorman district (fig. 1) lies just south 

of the Yukon River mainly along the divide between the 

Innoko and Nowitna Rivers in central Alaska, approximately 

230 miles west of Fairbanks. Ruby, Long, andPoorman 

are the only settlements in the area. Ruby (population 

175) is located on the Yukon River and is the point of entry 

for supplies to the mines in the district. Long is 28 miles 

south of Ruby and is the center of most of the mining activity in the district. Poorman is about 25 miles south of 

Long and is located at the headwaters of the Innoko River 

drainage. Long and Poorman are now virtually abandoned 

and have a combined population of not more than 8 people. 

The Alaska Road Commission maintains a single-lane 

truck road from Ruby to a point near Monument Creek 

(fig. 1). Formerly, this road extended to Poorman, 

but maintenance on this stretch of road was abandoned a 

few years ago and it is now impassable even to tractors.

Study of concentrates in the Geological Survey's 

Alaskan Geology Branch collections showed the presence 

of radioactive minerals in placers on the upper part of

Solomon Creek near Poorman and near two small granite 

intrusives in the vicinity of Long. The occurrence of these 

radioactive minerals, coupled with the facts that tin and 

bismuth are also known in the placers, and a silverbearing galena deposit is located about 12 miles south of 

Ruby indicated that the district might be favorable for the 

occurrence of uraniferous lodes. The party conducting 

the reconnaissance consisted of M. G. White and 

J. M. Stevens, geologists, andEgilSalvesonand 

R. D. Olson, camp assistants. They worked in the area 

during the month of July 1949. This work was done on 

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

Energy Commission.

GEOLOGY

The geology of the Ruby-Poorman district is known 

only on a reconnaissance scale chiefly as a result of investigations by Eakin (1914, pp. 20-27), Mertie and Harrington 

(1924, pp. 12-74), and Mertie (1936, pp. 130-143). The 

mantle of residual soil covering most of the upper slopes 

and tops of all the hills, the depth of alluvium covering all 

the lower slopes and stream valleys, and the blanket of 

moss, low brush, and timber growth covering all but the 

highest hills make any geologic investigation both difficult 

and time-consuming. Outcrops are rare, talus slides are 

uncommon, and float rock is found usually only by digging 

through the moss into the residual soil mantle.

Bedrock

The major part of the country rock in the Ruby-Poorman 

district consists of a complex including schist, phyllite, 

slate, greenstone, quartzite, chert, and limestone of 

Paleozoic age and possibly older. Within this complex it 

has been possible to recognize the following formations: 

a recrystallized limestone of unknown age; Devonian rocks 

consisting mainly of limestone; and a group of rocks that 

include greenstone, tuff, and chert. Granite bodies, 

cropping out in the headwaters of Flint Creek and on 

Birch Creek, are tentatively classified as Mesozoic in 

age. The youngest rocks in the district are believed to 

be soda-granite dikes of possible Eocene age that occur 

in the vicinity of Poorman, and below Ruby on the banks 

of the Yukon River.

Alluvium

Altitudes within the district range from less than 

400 feet above sea level at Ruby to somewhat more than 

1,800 feet on some of the rounded hills south of Ruby. 

The average altitude is probably between 1,300 and
Figure 1. Sketch map of the Ruby-Poorman district, Alaska, showing location of Birch Creek and upper Flint CreeV.
1,400 feet. Alluvial deposits consisting mainly 

of silt of Pleistocene age with some gravel fill 

nearly all the stream valleys almost to their 

heads and in places occur as high as 1,200 feet. 

Faintly outlined terraces are found at altitudes of 

1,000 feet and higher along some of the bedrock 

slopes and on many of the ridges. It is of some 

significance that the only rock outcrops seen 

below that altitude are along stream cuts of 

Recent age. Essentially all of the bedrock float 

found was in frost-heave mounds thrown up 

through the alluvial mantle. Eakin (1916, p. 55) 

in discussing the origin of these alluvial deposits, 

which are widespread in this region of Alaska, 

states:

The character of the silts indicates deposition 

in quiet water; the distribution and form of 

the gravel terraces point to origin by beach 

action. As there are other corroborative 

lines of evidence pointing to the extensive 

inundation of the low-lying parts of the 

region, probably during and following the 

period of maximum glaciation in interior Alaska, 

the silts are interpreted as the deposits made 

by the debris-laden glacial waters and the 

high-lying gravel terraces as largely the product 

of beach action on the shores of lakes that are 

now extinct. There are also more extensive 

high-level gravel deposits that are probably of 

fluvial origin, representing deltas built out into 

the margins of the ancient lakes by glacial and 

other streams.

According to Maddren (1910, p. 68) there 

was an interval following the glaciation when 

downcutting by the present streams was very 

rapid, leaving some of the preglacial gravel 

perched on the sides of the valleys as bench 

gravels. This gravel appears on the west side 

of middle 'Flint Creek, and on lower Birch Creek.

MINERAL DEPOSITS

Placer gold has been the only mineral 

mined profitably in the Ruby-Poorman district. 

Tin is abundant in many of the streams of 

the area, and though attempts have been made 

to ship some of the cassiterite concentrate 

to the American market, none of the attempts 

have been profitable. The lode source of 

both the tin and gold is easily surmised to 

be from the hills and ridges adjacent to the 

placer deposits. However, the residual rock 

and vegetal material on the ridges and the 

alluvial mantle in the stream valleys so effectively mask the bedrock in the district as to 

make any lode prospecting too expensive and

speculative an enterprise. A general description 

of the lode and placer deposits in the district is 

given in a report prepared by Mertie (1936).

Development work was done for a number 

of years on a silver-lead prospect on 

New York Creek at the head of Beaver Creek 

(fig. 1) in the vicinity of Ruby. Attempts to ship the 

ore were unprofitable, and the property was 

abandoned. This deposit (Brown, 1926, pp. 146, 

147; Mertie, 1936, pp. 226,-227) consists of metalliferous lenticular veins as much as several feet 

thick which lie parallel to the schistosity of the 

enclosing micaceous quartz schist. The ore 

consists chiefly of silver-bearing galena, much 

cerussite and limonite, minor amounts of 

rhodochrosite, manganese oxides, calcite, and 

siderite, and traces of gold, quartz, pyrite, and 

ruby silver.

Bismuth occurs at two localities: one in the 

vicinity of Ruby, at the head of Glacier Creek (fig. 1) 

where a sluice-box concentrate from placer-gold 

mining contains 29 percent bismuth by chemical 

analysis; the other on lower Birch Creek in the 

vicinity of Long, where Mertie (1936, p. 157) 

reported finding native bismuth in the placer 

concentrates.

RADIOACTIVITY INVESTIGATIONS

The vegetal and alluvial mantle blanketing a large 

part of the Ruby-Poorman district effectively absorbs 

most of the radiation from radioactive minerals in the 

underlying rocks. Radiometric testing in the district, 

therefore, was efficient only at the few localities where 

bedrock is exposed, in talus, or on the portions of the 

ridges and upper valley slopes where the vegetation 

cover was not too thick to test the underlying residuum 

either directly or with shallow test pits. For example, 

it is estimated that only about 20 percent of the road 

between Ruby and Monument Creeks (fig. 1) was 

satisfactorily tested by traversing with a jeep,because 

of the thick vegetal and alluvial mantle. Two probes, 

one containing six 1- by 14-inch copper-walled gamma 

tubes and the other with four 1-by 18-inch brass-walled 

gamma tubes, were attached to standard commercial 

models of portable survey meters for jeep and backpacking traverses.

Ruby area

Fifteen placer concentrates from the Ruby area were 

in the Survey's Alaskan concentrate collection before the 

present investigation. The creeks (fig. 1) from which 

these samples had been obtained, the number of samples 

available, and the range in equivalent uranium content of the 

samples from each creek are given below.

Creek

Ruby Creek 

R1 ^ rJr^plf ---. _ _-._-.-.-. _ _ _ __ __ - _ __- _ __

Cox Pup (headwater tributary of Big Creek) 

Number of 

concentrates

2

6

2

5

Range in equivalent uranium 

content (percent)

0.000-0.002

.000- .003

.000- .001

.001 .006
Because of the low radioactivity exhibited by these 

concentrates, reconnaissance in the Ruby area 

was limited to the radiometric traversing along 

the road and the testing of the silver-bearing 

galena lode on New York Creek (fig. 1) about 

1..5 miles east of the road. The road traversing 

revealed no significant radioactivity anomalies and 

no significant amount of radioactive material was 

found at the galena prospect. The maximum 

equivalent uranium content of check samples taken 

at this prospect was 0.003 percent.

Long area

A total of 47 placer concentrates from 

the Long area (fig. 1) were available for testing 

prior to the present investigation. The data on 

these concentrates are summarized at the bottom 

of this page. It is apparent from this summary 

that the only concentrates with radioactivity of 

significance are from Birch, Flint, Monument, 

and Greenstone Creeks (fig. 1). These streams 

drain the small areas of granite on Birch Creek 

(fig. 2) and on the divide between Flint Creek 

and Monument and Greenstone Creeks (fig. 3). 

Consequently most of the reconnaissance' in the 

Long area was directed toward determining 

whether the granite bodies were the source of 

the radioactive minerals in the placers, and, if 

so, whether any zone of concentration of radioactive minerals existed within or near the 

granites.

Granite on Birch and-Straight Creeks

Data on the samples collected in the 

vicinity of the granite on Birch and Straight Creeks 

are given in table 1. The source locations of these samples 

are shown on figure 2. The six samples of the 

granite on these creeks (table ! ) range from 

0.003 to 0.006 percent (average 0. 005 percent) in 

equivalent uranium content. The heavy-mineral 

fractions (those greater than 2.8 specific gravity) 

of these samples range from 0.007 to 0.03& percent equivalent uranium, and average 0.027 percent 

equivalent uranium. Similar heavy-mineral fractions 

of panned concentrates from placers and disintegrated granite in the vicinity of Birch Creek

contain from 0.007 to 0.36 percent equivalent 

uranium, obviously higher because of the greater 

degree of concentration. Sample no. 3456, a 

panned placer concentrate, showed the highest 

equivalent uranium content (0. 36 percent) of any of the 

samples collected in the vicinity of Birch Creek. 

The radioactive elements are primarily in zircon 

and allanite, although the sample contains minor 

amounts of hematite and traces of sphene and 

uranothorite(?) (see below) which are also radioactive. Although it is likely that the bulk of 

the radioactivity in this sample is due chiefly 

to thorium, all of the minerals mentioned give 

a positive qualitative sodium-fluoride flux test 

for uranium. Other minerals in the heavy fractions 

of the granite and placer concentrates from the 

vicinity of the granite on Birch Creek are 

anatase, garnet, ilmenite, and a trace of 

malachite.

Granite on upper Flint Creek

Data on the samples collected from the 

vicinity of the granite body on upper Flint Creek 

are given in table 2; the source locations of the samples 

are shown on figure 3. The equivalent uranium 

content of 12 samples from this granite ranges 

from 0.003 to 0.008 percent and averages 

0.005 percent. The range in equivalent uranium 

content of the heavy-mineral fractions (those 

greater than 2.8 specific gravity) of the 12 samples 

is 0.015 to 0.15 percent; the average is 0.037 percent. This average is higher than that of the 

granite on Birch Creek because of one heavy 

fraction, that of sample 3505, which contains 

0. 15 percent equivalent uranium. If this fraction 

is not considered in the range and average, the 

results (range 0.015-0.05 percent equivalent 

uranium; average 0.026 percent equivalent uranium) 

are closely comparable to the range and average 

for the granite on Birch Creek. It is likely that 

this relatively greater radioactivity in the heavy 

fraction of sample 3505 is due to the occurrence 

of fewer nonradioactive minerals in proportion to 

the radioactive minerals rather than a greater overall 

content of the radioactive minerals. This is also 

suggested by the fact that the radioactivity of the unconcentrated granite at this point is the same as the 

average of the granite (0. 005 percent equivalent uranium).

Creek

R1 r»r*h P'ppp'lr- . _______ _ _______ ________ __

T .1 iflrv P"P^^V_ __________ __ _ _ _

Glen Gulch 

Flint Creek 

rPr>Q 11 P Tv^^a V _ _

Monument Creek 

frTv^^TiQ 1~nn^ Pr>^^lr___ _ __ _ __ 

Midnight Creek 

Short Creek 

Fifth of July Creek 

Wi 1 1 nu PTY^plr _- _ -- -__ _ _____ ___

T m~i rf Ofv^f^l/" ^_

Bear Gulch 

Number of 

concentrates

13

1 

1 

1 

1 

1 

J> 

13 

2 

2 

1 

5 

3

Range in equivalent uranium 

content (percent)

0.000-0.013 

.001 

.006 

.43 

.000

.032 

.001- .017

.000- .002 

.000- .001 

.001 

.001 

.000- .001 

.000- .002
Undifferentiated rocks 8

Approximate, contact

 3456

Sample locality

1949

/+ 4 4- L 7*.-1-3473% 4 ' *-*>* 4- ,

/+ 4 + -Hlgi 4 -^13470 f

x x + + 4 + +XV* + 13472 t

/' + 4- 4- + -t VX' + L 

/ 4-4 + 4-1-4 4 V^-^4 -4-/ 

'4 + 4-4-44-14 J>\4 -K

+.-. 4--f4--4-+y4 A^O ^JP/V

4 4 4 4 -* 4 

V ,'44444-4-4 4/4 4-^

sfSS..'^*^^*^ " .-* stjsiatt^rr^^c^ 5 :' + -t-

-34774V 

' ).+ 

+ 

^^^ 

^2W

l**/*..-*""

3449

(

MILES

*\

Figure 2. Geologic sketch map of Birch Creek, Ruby-Poorman district, Alaska.

5
EXPLANATION

V 3494r +

x^ssoej ^

Undifferentiated rocks

Contact, dashed where 

approximately located

* 3645

Sample locality 

1949

Figure 3. Geologic sketch map of upper Flint Creek, Ruby-Poorman district, Alaska.
Table 1. Data on samples from the vicinity of the granite on Birch and Straight Creeks, 1949

Type

Sample no.

of sample

Location 1

Crushed rock

percent eU2

Heavy-mineral fraction 3

Percent eU Concentration ratio

Granite

3471

3472

3473 

3474

3475

3476

Birch Creek 

_ dO 

 do (fine-grained granitic 

dike).

Straight Creek 

. dO 

_ __ do __________ _ _______

0.006

.006

.006 

.003

.006

.005

0.036

.028

.06

.009

.013

.015

45:1

35:1

225:1 

13:1

17:1

30:1

Panned concentrates from disintegrated granite

3469

3470

Birch Creek 

__ _ do _______ ____________

0.30

.22

15,300:1

11,000:1

Placer concentrates

3449 3455

^456 J~JM 

3477

Birch Creek 

 do-T . 

_ ___ Hr>- __ _ - _ ___ ______ - -- _

Straight Creek 

0.007

.024

 36

 15

( 4 )

2,300:1

? 7C1O 1

640:1

1 See also figure 2.

2 eU-equivalent uranium.

That greater than 2.8 specific gravity. 

*Very high.

In addition to sampling the granite itself, 

panned concentrates were taken of stream 

gravels on Flint and Monument Creeks and 

wash from the disintegrated granites at the 

head of Flint Creek. The heavy-mineral 

fractions of these samples show the consistently 

relatively higher equivalent uranium content 

that would be expected with the greater degree 

of concentration (table 2).

The radioactivity of the granite on 

upper Flint Creek and of the placers and wash 

derived from the granite is due chiefly to 

a uraniferous thorium silicate, tentatively 

identified as uranothorite. The heavy fractions 

also contain much sphene, allanite, and zircon, 

which also contribute to the total radioactivity of 

the samples because they contain radioactive 

elements as impurities. Magnetite and ilmenite 

are also found in the heavy-mineral fractions.

The uranothorite(?) is dark green with a 

vitreous to dull luster. It is isotropic and has 

an index of refraction near 1. 86. Following is 

a spectrographic analysis on selected grains from 

sample 3507, the heavy-mineral fraction of which 

contains 1. 53 percent equivalent uranium by

radiometric analyses and 8 percent of the 

uranothorite(?) as estimated by visual inspection.

Over 10 percent

thorium 

silicon

0. 1-l.Opercent

bismuth 

cobalt 

iron 

tin

0.01-0.1 percent

aluminum

cerium

copper

magnesium

molybdenum

potassium

silver

sodium
Table 2. Data on samples from the vicinity of the granite on upper Flint and Monument Creeks, 1949

Sample number Crushed rock 

percent eU 1

Heavy-mineral fraction2

Percent eU L Concentration ratio

Flint Creek (locations plotted on figure 5) 

Granite samples

3484 - --- - - _ - _ _ _ - - _ - __ -

 *JJ.Q^

 Z|IQ|I

"311Q7 __

 z|iQ8 _ ____

TJ4Q9 _ __ __

^ROT

^n-.

3505 

0.005

!oo8 

.005 

.004 

.005 

.004 

.006 

.005 

.005 

.005 

.005

0.015 

.019 

.025 

.024 

.026 

.024 

.045 

.021 

.050

il5 

.015

99:1 

9:1 

6:1 

40:1 

150:1 

65:1 

100: 1 

120:1

15i 1 

2,850:1 

20:1

Panned concentrates from granite wash

 3R04 _ __ ____________________________ 0.75 

.51

5,000:1 

8,200:1

Panned concentrates from creek gravels

 31178 _ __________ ________________

"348l- _ _ _ _ _ _

^uOp

^486

5487 __ _ _ _ _ __ __ _ __ _____ ____

"^ij-QO _ _-.__

 zhqo

^iio^ .

^RO7

^08

5C50Q . 

364^

^687- __________________________

0.11 

.12 

.224 

.70 

.025 

.26 

.62 

.02 

.95 

1.65 

.46 

.64 

.065

.20

1,250:1 

650:1 

1,450:1 

1,000:1 

2,700: 1 

1,100:1 

680:1 

640:1 

1,600:1 

5,800:1 

4,200:1 

5,700:1 

1,250:1 

4,500:1

Monument Creek (locations plotted on figure 5) 

Panned from creek gravels

-z;cn -i _______ _ ____________ _________

5512 

5515 

0.086 

.001 

.002 

.14

290:1 

9,000:1

eU- equivalent uranium. 

2 That greater than 2.8 specific gravity. 

"High.
In addition to the above elements the mineral contains 

8. 2 percent uranium (determined chemically). Phair 

and Shimamoto (1952, p. 662) show the thorium content 

as 56. 6 percent.

Analytical spectrographic analyses on what is 

apparently the same mineral from other samples 

(no. 3496 from Flint Creek, and no. 3477 from 

Birch Creek) show yttrium(?) as a minor constituent. 

The samples from Monument Creek (0. 032 percent 

equivalent uranium) and Flint Creek (0. 43 percent 

equivalent uranium) obtained before 1949 contain a 

green uraniferous thorium-yttrium silicate which 

was tentatively identified prior to this study as yttrlalite. 

It is likely, however, that this mineral is the 

uranothorite(?) discussed on page 7.

The uranothorite(?) and the other radioactive 

minerals are disseminated accessory minerals of 

the granite, which is typically coarse-grained 

and porphyritic. No zones of high concentration were 

found. The uranothorite(?) appears to be less abundant 

in a minor fine-grained facies of the granite than in the 

large mass of coarse-grained rock.

Poorman area

Data on placer concentrates from the Poorman 

area (fig. 1) available for study prior to 1949 are 

summarized below:

Creek

Poorman Creek 

Solomon Creek 

Flat Creek 

M Pi A CIO f^TIOoT^ _ _ _~

Number of 

concentrates

5

1)

1

1

1

1

Range in equivalent uranium 

content (percent)

0.000-0.006

.002- .056

.001

.000

.001

.004

From this summary it appears that the only radioactivity of significance in concentrates from the 

Poorman area is in the samples from Solomon Creek. 

The most radioactive sample came from the upper part 

of the'creek. However, radiometric traversing in the 

upper valley of Solomon Creek and along adjacent 

divides failed to detect any significant radioactivity. 

Mineralogic study of the placer concentrate from 

Solomon Creek having the greatest radioactivity 

(0. 056 percent equivalent uranium) showed that the 

radioactivity there is due to a uraniferous mineral of 

the spinel group. Qualitative spectrographic analysis 

of the mineral shows aluminum, chromium, iron, 

magnesium, titanium, and rare earths as major 

constituents.

SUMMARY AND CONCLUSIONS

Two small bodies of granite in the vicinity of 

Long in the Ruby-Poorman district contain an average 

of 0. 005 percent equivalent uranium. Although the 

radioactivity is due chiefly to a uraniferous thorium 

silicate tentatively identified as uranothorite radioactive allanite, hematite, sphene, and zircon also 

contribute to the total radioactivity.

Radiometric tests in the vicinity of a silver-lead 

lode 12 miles south of Ruby found no anomalous 

radiation at that locality. Search for the bedrock source 

of a radioactive spinel previously found in placers on 

Solomon Creek in the Poorman area was unsuccessful.

It is concluded from this investigation that there 

is little likelihood of finding uranium deposits of 

commercial grade in the Ruby-Poorman district. It 

should be noted, however, that much of the district is 

heavily covered with vegetation and alluvium, which 

prevents the satisfactory use of portable survey meters 

to detect radiation.

LITERATURE CITED

Brown, J. S. , 1926, Silver-lead prospects

near Ruby (Alaska): U. S. Geol. Survey

Bull. 783-D, pp. 145-150. 

Eakin, H. M. , 1914, The Iditarod-Ruby region,

Alaska: U. S. Geol. Survey Bull. 578. 

_________ 1916, The Yukon-Koyukuk region,

Alaska: U. S. Geol. Survey Bull. 631. 

Maddren, A. G., 1910, The Innoko gold placer

district, Alaska: U. S. Geol. Survey Bull. 410. 

Mertie, J. B. , Jr., 1936, Mineral deposits

of the Ruby-Kuskokwim region (Alaska):

U. S. Geol. Survey Bull. 864-C,

pp. 115-255. 

Mertie, J. B. Jr., and Harrington, G. L,, 1924,

The Ruby-Kuskokwim region, Alaska;

U. S. Geol. Survey Bull. 754. 

Phair, George, and Shimamoto, Kiyoko Onoda,

1952, Hydrothermal uranothorite in fluorite

breccias from the Blue Jay mine,

Jamestown, Boulder County, Colo.: Am.

Mineralogist, vol. 37, no. 7,

pp. 659-666.
CHAPTER B. NIXON FORK DISTRICT

ABSTRACT

Reconnaissance for radioactive deposits in 

the Nixon Fork mining district, Medfra quadrangle, central Alaska, in 1949 disclosed the 

occurrence of allanite in samples containing 

as much as 0.05 percent equivalent uranium 

from the dump of the Whalen mine; the presence of radioactive parisite (a rare-earth 

fluocarbonate) in a highly altered limestone 

containing about 0.025 percent equivalent uranium 

near the Whalen shaft, and radioactive idocrase 

in samples of altered garnet rock' with about 

0.025 percent equivalent uranium, from the 

Crystal shaft of the Nixon Fork mine. This 

radioactivity is due mostly to thorium rather 

than uranium. Placer concentrates from Ruby 

and Eagle Creeks contain 0.078 and 0.26 percent 

equivalent uranium respectively, in which the 

radioactivity is due chiefly to uraniferous 

thorianite. The bedrock source of the uraniferous 

thorianite was not located primarily because much of 

the area is overlain by a relatively thick mantle 

of vegetation (mostly moss) which limited the 

effectiveness of radiometric surveying. The 

uraniferous thorianite is believed to occur in 

a restricted zone or zones at or near the 

contact of limestone with monzonite similar 

to the gold-copper ores of the district and the 

deposits of radioactive parisite and garnet rock 

at the Whelan and Crystal shafts respectively.

INTRODUCTION

The Nixon Fork mining district (fig. 4) is 

in a low range of hills of the Kuskokwim Mountains, 

about 12 miles north of Medfra, a small settlement on the Kuskokwim River 95 miles upstream 

from McGrath, in central Alaska. Access to 

the area is by boat or airplane from McGrath 

to Medfra, and from Medfra to the mines 

by truck.

The term "Nixon Fork mines" has long 

been used for both lode and placer mines in 

the district. Most of the lode-mining property 

is held by the Nixon Fork Mining Co., whose 

principal owners are the Mespelt brothers of 

McGrath. Their property is called the Nixon Fork

mine. The only other lode-mining property

of any importance is the Whalen mine, adjacent

to and south of the Nixon Fork mine property.

Some of the radioactive placer concentrates 

from the Nixon Fork mining district available 

in the Geological Survey's Alaskan Geology Branch 

placer-concentrate file before 1949 (table 3) 

contain uraniferous thorianite associated with 

bismuth and copper minerals. This association 

suggested that the district was favorable for 

the occurrence of a uranium-bearing lode deposit. 

Therefore, in 1949, a Geological Survey party 

conducted a reconnaissance in the district to 

determine its uranium possibilities. Approximately 

3 weeks were required for the investigation, 

of which \\ days were spent at a gold-lode 

prospect on Eagle Creek, about 5 to 7 miles 

south of the main Nixon Fork mines (fig. 4). 

The party consisted of M. G. White and 

J. M. Stevens, geologists, and Egil Salveson 

and R. D. Olson, camp assistants. This 

work was done on behalf of the Division of 

Raw Materials of the U. S. Atomic Energy 

Commission.

GEOLOGY, MINERAL DEPOSITS, AND 

RADIOACTIVITY INVESTIGATIONS

The low range of hills on which the 

Nixon Fork mines are located is composed 

of limestone of Paleozoic age and Upper Cretaceous sandstone, shale, and slate. The rocks 

are intruded by quartz monzonite that is probably 

Eocene in age. The lode deposits in the district 

are an enrichment in limestone along contact 

metamorphic zones between the limestone and 

monzonite. Most of the mineral deposits 

are apparently restricted to the valleys of 

the tributaries of Hidden Creek and the slopes 

around the north headwater fork of Ruby Creek 

(fig. 4). No mineral deposits of any note 

have been found along the eastern edge of 

 the monzonite.

Placer gold was found on Ruby Creek 

in the winter 'of 1917, and shortly thereafter 

the lode deposits at the head of Crystal Gulch 

were located. In 1920 a 10-stamp mill was 

installed at the head of Ruby. Creek to process

10
EXPLANATION

SEDIMENTARY ROCKS

Sandstone, shale, and slate

STRAND _ £^^^y^^^^:^^^:^lfll

Contact, dashed where 

approximately located

Geology modified from Plate 6, U.S.6.S. Bull. 783

Figure 4. Geologic sketch map of the vicinity of the Nixon Fork mines.

11
the highly oxidized copper-gold ores from the 

various shafts and prospects in the district. 

Mining and milling has been carried on in the 

district intermittently since 1920.

More detailed descriptions of the geology; 

mineral deposits, and mining of the Nixon Fork 

district are contained in reports by Martin (1922), 

Brown (1926), and Mertie (1936).

As is the case with much of interior 

Alaska, the area around the Nixon Fork mines 

is thickly covered with underbrush of willow, 

alder, spruce, and birch, and by moss, which 

averages about 3 feet in thickness. These 

conditions make the results of radiometric 

traversing at best inconclusive, except in the 

localities where recent mining developments 

have uncovered sufficient bedrock for direct 

examination.

The mineral deposits on the properties of 

the Nixon Fork Mining Co. and at the Whalen mine 

contain both copper and gold, and occur along 

the contact between the limestone and monzonite. 

The highly mineralized part of the contact zone 

is exposed by a large number of shafts and 

prospect pits, and, hence, was examined in 

some detail. Much of the remainder of the 

contact could not be examined because of the 

moss cover.

Hidden Creek area

Some of the samples in7 the Nixon Fork 

district with the greatest concentration of 

thorianite come from Encio Gulch, a tributary 

of Hidden Creek (fig. 4). However, radiometric 

traverses around the head of Hidden Creek and 

its headwater tributaries gave negative results. 

The results are negative, not so much because of the 

lack of any radioactive materials, which may 

be perfectly possible, but probably because the 

valley is covered almost completely by a thick 

shielding cover of moss and low brush. Concentrates from stream gravels collected during 

the 1949 investigations are* listed in table 4. 

They are somewhat lower in radioactivity than 

the pre-1949 samples because the latter were 

obtained when placer operations were active in the valley. 

The occurrence of bismuth in the placers is apparently 

restricted to the valley of Hidden Creek above Dry Gulch, 

and, because it is thought that all of that portion of 

the valley of Hidden Creek is underlain by 

monzonite, it is likely that the bismuth is 

restricted to the monzonite.

Whalen mine

The main shaft of the Whalen mine is 

located at the head of Holmes Gulch very near 

the contact between the limestone and monzonite. 

The limestone forms an island surrounded by 

monzonite (fig. 4). At this shaft boulders of 

limestone on the waste dump are radioactive and 

contain a nigh percentage of allanite. The equivalent 

uranium content of the boulders is 0.05 percent

but fluorometric analysis indicates a uranium 

content of only 0.004 percent (table 5). It is 

assumed, therefore, that most of the radioactivity 

of these rocks is due to thorium. The heavymineral concentrate (that greater than 2.8 specific 

gravity) of the rock consists of 98 percent 

allanite (the radioactive mineral) and 2 percent 

zircon, kyanite, and scheelite. These heavy 

minerals constitute about 25 percent of the 

rock.

A short distance east of the main shaft is 

a large depression about 40 feet deep formed as 

the result of a cave-in on the 40-foot level 

of the mine. Most of the material of the 

cave-in was high-grade gold ore and was mined 

out, leaving a "glory hole." Along the walls 

of the glory hole is exposed highly weathered 

metamorphic rock consisting mainly of quartz 

and recrystallized limestone that has an average 

content of about 0.025 percent equivalent uranium 

(table 5). However, chemical analyses show 

only 0.002 percent uranium in this rock. The 

radioactive mineral is parisite ( a rare-earth 

fluocarbonate ) that makes up about 95 percent 

of the heavy-mineral fraction of the rock.

The main shaft of the Whalen mine is 

200 feet deep and is inclined at an 85-degree 

angle down the contact. This shaft could not be 

examined because it was filled with ice to within 

12 feet of the top.

There are between 150 and 200 small shafts 

and prospect pits along the contact between the 

limestone and monzonite in the vicinity of the 

Whalen mine. Approximately 80 of the prospect 

pits were cleaned out and tested radiometrically 

in an effort to discover any additional concentrations of radioactive minerals. A few of the 

holes tested have a higher-than-normal radioactivity and were sampled for further study 

(table 5). Radiometric analyses of these samples 

indicate an equivalent uranium content ranging 

from 0.001 to 0.006 percent (table 5). The 

radioactive minerals in these samples are 

allanite, hematite, zircon, and sphene.

Ruby Creek area

Samples from Ruby Creek are not plotted 

on figure 4, but their locations are described 

in table 6. The most radioactive sample 

obtained in the Ruby Creek drainage (no. 3570, 

table 6) contains 0.078 percent equivalent uranium. 

The sample is a sluice-box concentrate from 

the Strand placer mine on Ruby Creek at the 

foot of the headwaters gradient of Crystal Gulch. 

Most of the lode deposits on the Nixon Fork 

Mining Co. property are located at the head 

of this gulch. The percentages of the minerals 

in the fraction greater than 2.8 specific gravity 

of this concentrate are given in table 7. Most 

of the radioactivity in this sample is probably 

due to the thorianite. The unknown secondary 

minerals were analysed spectrographically. 

The yellow mineral contains bismuth, lead, copper, 

iron, vanadium, and silicon as major constituents,
Table 3- Data on concentrates collected in the Nixon Pork mining district before 1949

Samples

Pile no. Field no Location and description Percent equivalent 

uranium

.10

11

50

134

900

901

902

903

904

905

906

907

909

910

911

912

913

914

915

916

917

923

926

33AMt 68 Nixon Pork mine; mill concentrates 

70 Birch Gulch, 1/4 mile above mouth; placer concentrate 

73 Hidden Creek, above Dry Gulch; placer concentrate 

 Holmes Gulch(?); probably sluice-box concentrate 

45AW 59 Greer Gulch; placer concentrate 

57 Holmes Gulch; placer concentrate 

57A do - -

51 Whistling Gulch; placer concentrate 

53 Nixon Pork mine; mill concentrates 

44 Ruby Creek; placer concentrate 

56 Birch Gulch; placer concentrate 

54 Hidden Creek, between Whistling and Dry Gulches; placer

	concentrate. 

50 Whistling Gulch; placer concentrate 

58 Puzzle Creek; placer concentrate 

57B Holmes Gulch; placer concentrate - 

47 Encio Gulch; placer concentrate 

48 Hidden Creek, below Encio Gulch; placer concentrate 

60 Mystery Creek; placer concentrate 

45 Ruby Creek; placer concentrate 

46 do -

49 Hidden Creek, above Encio Gulch; placer concentrate «

52 Nixon Pork mine; concentrate from mill tailings 

55 Dry Gulch; placer concentrate 

6l Submarine Creek; placer concentrate---------------------

<0.001 

.014 

.031 

.086 

.003 

.008 

.015 

.006

< .001 

.012 

.007 

.013

.006

.002

.005

.012

.004

.005

.007

.010

.002

.001

.002

Table 4. Data on placer concentrates collected from Hidden Creek and its tributaries in 1949

Samples

Pile no. Field no

Location Concentration 

ratio

Percent equivalent

uranium in 

heavy-mineral fraction 1

3574

3.575

3576

3577

3578

3579

3580

3581

3582

3583

3584

3584

3585

3586

49ASv 108

109

110

111

112

113

114

115

116

117

118

119

120

121

Hidden Creek, above Encio Gulch 

Hidden Creek,.left limit tributary

above Encio Gulch. 

Hidden Creek, extreme head, 1,000 feet

below road. 

 do--- -

Hidden Creek, prospect hole-------

Birch Gulch, 1,000 feet above mouthDry Gulch, 1,500 feet above mouth - 

Hidden Creek, 1/2 mile above Birch 

, Gulch. 

Hidden Creek, 1/2 mil3 above Birch

Gulch; tailings pile. 

Whistling Gulch, near mouth------

Holmes Gulch, left headwaters fork 

Encio Gulch, left headwaters fork 

Encio Gulch, right headwaters fork 

Encio Gulch, near mouth145:1 

400:1

120:1

310:1 

275:1

360: 

630: 

150:

90:1

190:1

270:1

60:1

90:1

80:1

0.009

.006

.006

.007

.006

.012

.003

.005

.005

.007

.005

.004

.005

.006

That greater than 2.8 specific gravity.

13
Table 5« Data on samples collected from the Whalen mine and vicinity

Samples

Pile no. Field no.

Location and description

Crushed rock eU1 

(percent)

U2 

(percent)

Heavy-mineral fraction 3

eU 1 

(percent)

Concentration 

ratio

Samples collected at Whalen mine

5619

5620

5621

3622

362J

3624

3625

3626

3627

3628

49AWe 127

128

129

130

132

133

134

135

150

151

Whalen mine tailings dump; limestone with large amount of

metamorphic minerals.

Same as sample 3619; concentrate

from panning crushed rock.

Whalen mine"glory hole", wash

from west side rim.

"Glory hole"; rock along contact

between highly weathered limestone and highly leached limestone, panned concentrate.

Same as sample 3622; highly

weathered limestone.

Same as 3623;panned concentrate 

Same as 3622; weathered limestone.

"Glory hole"; combination of

the rock types of samples 3623

and 3625.

Slope east of Whalen mine shaft;

decomposed limestone; panned

concentrate.

Same as sample 3627; unconcentrated rock.

0.05

 

 

 

.03

.02

.03

.005

0.004

.002

___

.002

.002

 

.002

0.081

.08

.06

.14

.14

.14

.10

.12

.019

.04

4:1

4:1

900:1

850:1

35:1

160 :1

400:1

30:1

2,400:1

640:1

Samples collected from some of the prospect holes along the contact between the monzonite 

and limestone in the vicinity of the Whalen mine

 *£OQ

~z.f.-z. r\

3631

3632

"^f^T^

^fi^4

-^fczp.

3636

3637

3638

3639 ~*>fik.n

3641

153

154

155

156

157

158

I t~r\ 159

160

161

162

1fv5

164

164

Hole no. 21; limestone 

Hole no. 24; monzonite 

Hole no. 25; monzonite 

Hole no. 32; limestone 

Hole no. 51; limestone 

Hole no. 52; limestone 

Hole no. 54; monzonite 

Hole no. 56; monzonite 

Hole no. 61; limestone 

Hole no. 65; monzonite 

Hole no. 68; limestone 

Hole no. 74; monzonite 

O nnJi

r\n~z

.003 

.000

nno

.006

.002 on"^

om

one

.001

.004

nnc

0.010

m n

.004

nm

.004

r\~\ Q

nnJi

.008

.003

.019

.003

.006

.007

1 TO. 1

90:1

40:1

50:1 

40:1

20:1

20:1

50:1 

25:1

300: 1

30:1

60:1

25:1

l eU- equivalent vranium.

2 U-uranium.

3 That greater than 2.8 specific gravity.

14
Table 6. Data on bedrock samples and concentrates collected in the Ruby Creek area in 1949

Samples

Pile no.

3558

3567

3568

3569

3570

3593

3594

3596

3597

3598

3599

3600

3601

5602

3603

3604

3607

3608

3609

Field no.

49ASv 91

10C

101

102

103

49AWe 89

91

94

95

97

98

99

101

102

104

105

111

112

113

Location and description

Head of Ruby Creek; placer

concentrate above mill and

mill tailings dump.

Ruby Creek; placer concentrate

at foot of headwaters

gradient .

Same as sample 3567 

Crystal Gulch; concentrate from

head of placer workings.

Ruby Creek; sluice -box concentrate from Strand placer

workings .

Along trail on left limit of

Crystal Gulch; monzonite.

Along road from mill to Garnet

Shaft; monzonite.

Bench on right limit of Ruby

Creek above Strand placer

mine; disintegrated monzonite.

Same as sample 3596; undisintegrated rock.

At mouth of Crystal Gulch in

Strand placer mine; shattered

monzonite.

On north side of Strand placer

mine; panned concentrate from

disintegrated granitic dike

in monzonite.

Same as sample 3599; quartz

veins paralleling granitic

dike.

On south side of Strand placer

mine; panned concentrate of

disintegrated granitic dike.

Same as sample 3601; undisintegrated rock.

Strand placer mine; inclusions

in monzonite.

Middle Crystal Gulch; bedrock

from contact zone.

Same as sample 3594; panned

concentrate from disintegrated

monzonite.

Same as sample 3594; panned

concentrate of disintegrated

granitic dike in monzonite.

Same as sample 3594; undisintegrated granitic dike in

monzonite.

Crushed rock eU 1 

(percent)

___

 

 

 

 

0.004

.003

.003

.002

 

.003

.002

.002

 

 

.008

U2 

(percent)

_ »..

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0.004

Heavy-mineral fraction 3

eU 1 

(percent)

0.007

.006

.008

.02

.078

.005

.006

.011

.005

.004

.013

.010

.032

.016

.003

.007

.015

.033

.021

Concentration 

ratio

300:1

130:1

290:1

650:1

( 4 )

15:1

9:1

690:1

10:1

650: 1

2,270:1

220:1

780:1

80:1

5:1

70:1

1,800:1

2,270:1

100:1

1 ell-equivalent uranium. 

2 U-uranium (determined chemically). 

3That greater than 2.8 specific gravity. 

*Very high.

15
Table 7. Mineralogy and radioactivity of three placer concentrates from the Nixon Pork mining

district

Minerals Sample no. 

5570

Sample no. 

5642

Sample no. 

5850

Allan!teAzurite . tr

Cassiterite

Common rock-forming minerals 4

Pluorite tr

GarnetGold a 2

Hematite a*

Ilmenite 70

Malachite tr*

Magnetite

Monazite--

PowellitePyrite -

Scheelite 5

Sphene a*

Thorianite a*

Unknown secondary green mineral tr*

Unknown secondary yellow mineral tr*

Uraninite -

Zircon a*

Equivalent uranium content (percent) 0.078 

Uranium content (percent) n.d.

2

1

5

tr* 

85

tr

5

tr* 

1*

tr

1*

tr 

4

80

tr 

tr 

tr

4

1*

.1* 

tr

0.26 

n.d.

0.18 

.06

1 Estimated volume percent; the asterisk indicates that the mineral is uranium-bearing as determined by a sodium fluoride flux test, 

a indicates that these five minerals total 5 percent of the sample.

and calcium, aluminum, arsenic, and antimony 

as minor constituents. The green mineral 

contains copper, bismuth, silicon, calcium, 

iron, and lead as major constituents, and 

aluminum, vanadium, and phosphorous as minor 

constituents. The source of the thorianite or 

the secondary uranium-bearing minerals was 

not found, though search was extended, to all 

the slopes of Ruby Creek along the contact 

and in the monzonite, where these slopes were 

not thickly covered by moss. It is possible 

that these minerals are derived from a restricted 

zone at or close to the contact, similar to 

the parisite-bearing zone in the Whalen mine 

"glory hole." The radioactivity in all the 

concentrates and rock samples from the head 

of Ruby Creek is probably due mainly to the 

minerals zircon and sphene. Thorianite was 

not found among them.

Nixon Fork mine

Almost all the ore that was processed 

from all the shafts and prospect pits in the 

vicinity of the Nixon Fork mine was processed 

at the stamp mill located at the head of

Ruby Creek (fig. 4). The tailings from the 

milling operation, crushed to fine silt size, 

were dammed up at the rear of the mill 

with the expectation that by further refinement 

of the milling processes additional gold could 

be recovered from these tailings. Radiometric 

tests of these tailings in the field indicated 

.no appreciable radioactivity. However, in order 

to determine whether any radioactive minerals had 

been carried over into the tailings during milling 

operations, the mill-tailings pile was sampled 

both across the surface or top (with holes 3 to 

4 feet deep) and across the base. As the equivalent 

uranium content of the heavy-mineral fractions 

(those greater than 2.8 specific gravity) of these 

samples ranges only from 0.002 to 0.008 percent 

(table 8), it is obvious that very little radioactive 

material occurs in the tailings.

Samples of ore were collected and tested 

from various ore and concentrate bins in 

and near the mill. The maximum equivalent 

uranium content of any of these samples is 

0.019 percent. The radioactivity is apparently 

associated with iron oxides in the ore. Table 8 

lists the data on all the samples collected in 

and near the mill.

16
Table 8. Data on ore, mill, and tailings samples from the Nixon Fork mine, 1949

Samples

Pile no. Field no

Location and description Crushed rock 

ell (percent J

Heavy-mineral fraction 2

eU (percent)1

Concentration 

ratio

3542

3543

3544

3545

3546

3547

3548

3549

3550

3551

3552

3553

3554

3555

3556

3557

3559

3560

3561

3562

3563

3564

3565

3566

3588

3589

3590

3591

3592

3595

3605

3606

49ASv 75 Nixon Fork mill; mill tailings -

76 Small stream along side of Nixon 

Fork mine; mill tailings.

77 Nixon Fork- mine mill; mill tailings78 do 

79 do -

80 do -

81 do -

82 do - 

85 do -

84 do -

85 Nixon Fork mine mill; crushed ore, 

partly concentrated.

86 do -

87 do -

88 do -

89 do -

90 Nixon Fork mine mill; mill tailings92 do - 

95 do -

94 do -

95 do -

96 do -

97 do -

98 do -

99 do 

49AWe 81 Nixon Fork mine; gold ore 

82 Nixon Fork mine; oxidized ore 

85 do -

85 do -

86 do - 

93 Nixon Fork mine; partly milled ore

from Garnet shaft. 

107 Nixon Fork mine; malachite ore from

Crystal shaft.

109 Nixon Fork mine; malachite ore from 

Garnet shaft.

0.000

.007

.009

.006

.019

.002

.002

.006

0.002

.004

.003

.008

.005

.004

.002

.003

.002

.003

.003

.003

.006

.004

.004

.002

.002

.002

.004

.002

.002

.002

.003

.002

.001

.007

.012

.008

.019

.006

1,950:1 

370:1

590:1 

,400:1 

620:1 

120:1 

,360:1 

,360:1 

560:1 

750:1

1,360:1 

500:1

2,700:1 

270:1 

450:1 

210:1 

500:1 

900:1 

380:1

3:1 

2:1

3:1 

3:1 

2:1

1 eU-equivalent uranium.

2 That greater than 2.8 specific gravity.

Table 9- Data on samples of garnet rock from the vicinity of the Crystal shaft, Nixon Fork mine

Samples

File no. Field no

Description

Crushed rock

FT eU 

(percent)

U2 

'percent)

Heavy-mineral fraction 3

eU 1 

(percent)

Concentration 

ratio

3610

3611

3612

3613

3614

3615

3617

3618

49AWe 115

116

117

118' 

119:

120J

122

123

Surface wash on contact between 

limestone and monzonlte 

50 feet northeast of Crystal 

shaft; panned concentrate.

Random fragments of garnet rock 

showing high radioactivity.

Clean, unweathered garnet rock 

from shaft, 

-doSelected specimen of'garnet 

rock showing strongest radioactivity.

Garnet rock showing strong 

radioactivity.

Monzonlte at contact with garnet 

rock.

Same as sample 3617; concentrate 

obtained by panning crushed 

rock.

0.014 270:1

0.018

.000

.000

.025

.017

.003

0.008

.008

.006 .026

.006

.012

10:1 

25:1 

60:1

1eU-equivalent uranium.

2 U-uranium (determined chemically).
Crystal shaft. The Crystal shaft (fig. 4) of the 

Nixon Fork mine is located in a highly metamorphosed zone of rock that probably includes 

both monzonite and recrystallized limestone. 

About 50 feet northeast of the shaft a massive 

garnet rock underlies garnet-bearing, marmorized 

limestone and contains as much as 0.025 percent 

equivalent uranium. Study of samples from this 

locality (table 9) shows that the radioactivity is 

associated with garnet rock that is somewhat impure 

and weathered. The fresh, unweathered garnet is 

nonradioactive. The impurity in the garnet occurs in 

thin beds or irregular masses and is well coated 

with hydrous iron oxides. The composition of 

the garnet rock, exclusive of the hydrous iron oxides, 

is 65 percent garnet, 20 percent idocrase, 12 percent 

common rock-forming minerals, 3 percent sphene, 

and traces of zircon and magnetite. The radioactive 

minerals are idocrase and sphene. The radioactive

material in the garnet appears to be mainly an ironoxide coating and filling in fractures and fissures 

in the rock.

No significant radioactivity was detected at any 

of the numerous other prospect pits and shafts on the 

property of the Nixon Fork mine.

Other localities examined 

Mystery, Puzzle, and Submarine Creeks

Three concentrates were collected from gravels 

in Mystery and Puzzle Creeks to supplement the 

samples from these creeks previously available 

in the Alaskan Geology Branch concentrate file 

(table 3). The radioactivity of the three samples 

is given below:

Samples

Pile no.

3571 

3572 

3573

Field no.

49ASv 104 

105 

106

Location

MTrq-f-ortTr PTVualr

Mystery Creek -

Heavy-mineral fraction

eU (percent)

0.006 

.018

.003

Concentration ratio

125:1 

3,025:1 

130:1

Radiometric traverses were made around the 

heads of these two creeks as well as Submarine Creek, 

particularly along the ridge of which Strand 

and Jumbo Peaks are a part (fig. 4). No radioactivity anomalies were found.

Eagle Creek

Eagle Creek, a tributary of Crooked Creek, 

is located about 5 to 7 miles south of the Nixon Fork 

mines (fig. 4). B. A. Stone of Medfra operates 

a small gold-lode mine on the headwater slopes of 

the creek near the contact between the limestone 

country rock and a small monzonite mass. The 

heavy-mineral fraction (that greater than 2. 8 specific 

gravity) of a sluice-box concentrate (sample no. 3642) 

from a placer operation on Eagle Creek owned by 

Stone contains 0. 26 percent equivalent uranium. 

The mineral composition of the Eagle Creek sluicebox concentrate is given in table 7. The radioactivity appears to be due chiefly to uraniferous 

thorianite, although the allanite, hematite, and 

sphene are also radioactive. Radiometric traverses 

along the headwater slopes of Eagle and Skookum Creeks 

(fig. 4) failed to disclose the bedrock source of 

the radioactive minerals. A sample (no. 3644) of 

the monzonite on Eagle Creek contains only 0. 003 percent equivalent uranium and a sample (no. 3643) 

of garnet rock from the lode mine contains less than 

0. 001 percent equivalent uranium. Again, however, 

the negative radiometric data obtained in traversing 

is not conclusive because of the thick vegetation cover 

over most of the area.

In 1950 a placer concentrate containing 0. 18 percent equivalent uranium and 0. 06 percent uranium was

received from a prospector, who indicated only that 

it came from western Alaska; information obtained 

later suggests that it is probably from a placer mine 

in the Nixon Fork district. The mineralogy of this 

sample (no. 3850) is compared with two other placer 

concentrates from the Nixon Fork district in table 5. 

The marked similarity of this concentrate to that from 

Eagle Creek (no. 3642) suggests the probability that 

it also is from Eagle Creek rather than elsewhere in 

the district. Of note is the occurrence of uraninite 

as well as the uraniferous thorianite.

SUMMARY AND CONCLUSIONS

A summary of significant radioactivity data 

obtained in the Nixon Fork mining district in 1949 

is given in table 10. The only radioactive mineral 

of major importance is uraniferous thorianite, 

although such minerals as allanite, parisite, idocrase; 

sphene, zircon, malachite, and two unidentified 

secondary polymetal minerals are also radioactive. 

Uraninite has been identified in a concentrate that 

may have come from a placer mine in the district. 

The bedrock source of the uraniferous thorianite 

has not been located, but it is believed to occur, at 

least in part, in a restricted zone or zones at or 

near the contact between limestone and monzonite 

similar to the gold-copper ores of the district, 

the radioactive parisite zone at the Whelan mine, and 

the radioactive garnet-rock zone at the Crystal shaft 

of the Nixon Fork mine.

Search for bedrock sources of the radioactive minerals in the district by radiometric surveying is hampered 

considerably by the shielding effect of the heavy moss cover 

so prevalent through much of interior Alaska.

18
Table 10. Summary of significant radioactivity date obtained in the Nixon Pork mining district,

1949

Location and type of material tested Radioactive minerals Maximum radioactivity

Percent eU Percent U

Placers

Hidden Creek and tributaries 

Ruby Creek area:

Sluice-box concentrate from 

Strand placer mine.

Uraniferous thorianiteUraniferous thorianite, hematite, 

zircon, sphene, malachite, and 

two unidentified secondary 

minerals.

Mystery Creek and tributaries 

Eagle Creek:

Sluice-box concentrate from

Stone placer mine. 

Placer concentrate, location unknown but possibly from Eagle 

Creek (submitted by prospector in 

1950).

Not determinedUraniferous thorianite, allanite,

and sphene. 

Qraninite, uraniferous thorianite,

and allanite.

0.012

.078

.018

.26

.18 0.06

Bedrock

Whalen mine: 

Metamorphosed limestone on

waste dump. 

Altered limestone in "glory

hole". 

Limestone and monzonite in 

prospect holes.

Ruby Creek area:

Granitic dike in monzonite 

Nixon Pork mine: 

Mill tailings concentrates.

Ore samples from ore 

bins in mill. 

Altered garnet rock from 

Crystal shaft. 

Fresh garnet rock from

Crystal shaft.

Eagle Creek: 

Monzonite 

nOT'nO'l" "Y*f*\f*\r

Al 1 nni f P-.--_ __ , __ . __ . _ , ___ - __ . _ -

Allanite, hematite, zircon, and 

sphene.

____ c\ n _ _______ _ _____________

Not determined 

Idocraste, sphene, and iron oxides 

Not determined 

 _ ...rln- - ____ . - _ - ________

0.05

.05

.006 

.004

.008

.008

.019

.025 

.000

.005

.001

0.004

.002

.008

It is suggested that geochemical prospecting 

techniques, such as the analysis of samples of 

disintegrated material taken from auger holes through 

the shielding moss cover may prove more successful 

in locating the bedrock source of the uranium- and 

thorium-oxide minerals than surface radiometric 

traversing. Such sampling might well be supplemented 

by the radiometric logging of the auger holes with a 

survey meter such as a portable sealer. Much study 

on the application of geochemical prospecting techniques 

in Arctic and sub-Arctic regions is needed, however, 

before a routine can be developed.

LITERATURE CITED

Brown, J. S. , 1926, The Nixon Fork country 

and silver-lead deposits near Ruby (Alaska): 

U. S. Geol. Survey Bull. 783-D, pp. 97-150.

Martin, G. C., 1922, Gold lodes in the

upper Kuskokwim region (Alaska): U. S. Geol. 

Survey Bull. 722-E, pp. 149-161.

Mertie, J. B., Jr. , 1936, Mineral deposits of 

the Ruby-Kuskokwim region (Alaska): U. fe. Geol. 

Survey Bull. 864-C, pp. 115-255.