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Dismal Swamp 

Placer Deposit 

Elmore County 

Idaho

g GEOLOGICAL SURVEY BULLETIN 1042-K

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CONTRIBUTIONS TO ECONOMIC GEOLOGY/

DISMAL SWAMP PLACER DEPOSIT, ELMORE 

COUNTY, IDAHO

BY FEANK C. AEMSTBONG

The Dismal Swamp placer deposit, Elmore County, Idaho, was explored for 

niobium-, tantalum-, and uranium-bearing minerals by the J. R. Simplot Company under a Defense Minerals Exploration Administration contract in the 

summer of 1953. Sixteen bulldozer trenches, 2 shafts, and 25 churn-drill holes 

were used to explore the deposit. '

The deposit is underlain by granodiorite and related granitic rocks of the 

Idaho batholith. The gravels of the deposit are the product of local stream 

erosion and slope wash. Columbite and samarskite have been identified in the 

deposit and are believed to have been derived, from small pegmatites associated 

with the Idaho batholith. Most of the uranium is believed to occur in uraniumbearing multiple oxide minerals. . Samples from the trenches, shafts, and drill holes were tabled and the heavy 

concentrates were then magnetically separated. Selected magnetic fractions 

were analyzed chemically for niobium, tantalum, and uranium. Most of the 

niobium, tantalum, and uranium is contained in the "weakly magnetic" fraction. 

In the aggregate, the gravel contains between 1.40 and 1.87 pounds of "weakly 

magnetic" material per cubic yard averaging between 14 and 20 percent NbaOj+ 

Ta2 0s and between 0.15 and 0.19 percent UjOg.

INTRODUCTION

The Dismal Swamp placer prospect is in sees. 22, 27, 33, 34, and 

35, T. 5 N., R. 9 E., northeastern Elmore County, Idaho (fig. 44). 

It is not in an organized mining district but is just west .of the Bear 

Creek mining district. The property is on the headwaters of Buck 

Creek, 1% miles by jeep road from a point on the Rocky Bar-Cougar 

Creek road about 6}£ miles northwest of Rocky Bar, Idaho. From 

the property through Rocky Bar toward Mountain Home, a distance 

of about 115 miles, there is a graded dirt road to about 20 miles north 

of Mountain Home; the remaining distance is paved. The main line 

of the Union Pacific Railroad and U. S. Highway 30 pass through 

Mountain Home. Boise, 44 miles northwest of Mountain Home, is 

the nearest source of mining supplies.

383
384 CONTRIBUTIONS TO ECONOMIC GEOLOGY

FIGURE 44. Index map showing location of the Dismal, Swamp ;placer deposit, Elmore County, Idaho. V

The property is at an altitude of 6,900 feet in a mountainous terrain. 

The hills and valleys in the vicinity of the deposit are heavily forested, 

mostly with lodgepole pine. Dismal Swamp is at the headwaters of
DISMAL SWAMP PLACER DEPOSIT, ELMORE COUNTY, IDAHO .385

the westernmost principal fork of Buck Creek and only a few hundred 

yards northwest of the drainage divide between the Middle Fork of 

the Boise, Kiver and the Feather River. The winters are long and 

cold with moderately heavy snowfall and temperatures are frequently 

as low as 30° below zero. The summers are warm and afternoon 

showers are common. The propert}^ is inaccessible 6 to 7 months of 

the year.

EXTENT OF EXPLORATION

In the summer and fall of 1952 four prospect pits and one 10-foot 

shaft were dug on the Dismal Swamp property, and samples' of the 

heavy concentrates from the gravels in the pits and shaft were taken 

for analysis. Analyses of the samples by commercial assay firms 

and the Bureau of Mines indicated that the samples contained niobium, tantalum, and uranium in large enough amounts' to warrant a 

more thorough examination of the property. Subsequently the 

J. R. Simple t Company applied for a Defense Minerals Exploration 

Administration contract. This report is based on exploration done 

by the J. R. Simplet Company under Defense Minerals Exploration 

Administration contract No. Idm-E 545, docket No. DMEA-291, 

in cooperation with the U. S. Geological Survey and the Bureau of 

Mines.,

Exploration under the Government contract was carried out 

between July 22 and the middle of September 1953, and consisted 

of the following:

1. 16 bulldozer trenches, 6 to 12 feet deep.

2. 25, 6-inch churn-drill holes from 6.5 to 18.5 feet deep. The total footage 

drilled was 329.4 feet.

3. Shaft No. 1, 10.5 feet deep and 5 by 5 feet in cross section and untimbered, 

slink to bedrock around drill hole 8.

4. Shaft No. 2, 12.1 feet deep and 5 by 5 feet in cross section and untimbered, 

sunk to bedrock around drill hole 4.

The property explored consists of 10 contiguous, unpatented placer 

mining claims. The principal placer deposits are covered by only 

2 of these claims; therefore, only those 2 claims, the Associated Gem 

groups Nos. 2 and 3 (pi. 18), were explored.

GEOLOGY

The Dismal Swamp prospect is a placer deposit of irregular outline 

lying at the head of a narrow valley at the junction of four minor 

tributaries. It is near the south end of the Idaho batholith, of 

Cretaceous age, about midway between its east and west margins. 

Outcrops are rare; the principal rock types in the vicinity of the 

property are granodiorite and related granitic rocks. The area does 

not appear to have been glaciated in late Wisconsin time, an. impression
386 CONTRIBUTIONS TO ECONOMIC -GEOLOGY - " ;

confirmed by the moderately deep weathering of the granitic bedrock. 

In weathering, the granitic bedrock has broken into individual minerals to form a gruss. The moderately deep weathering has made the 

top foot or two of bedrock comparatively easy to strip with a bulldozer.

The gravels in Dismal Swamp are products of local stream erosion 

and slope wash of weathered granite from the small drainage basin.

In a sample of black sand concentrate from the Dismal Swamp ^ 

placer, submitted by the J. R. Simplot Company, the Bureau of 

Mines laboratory at Albany, Oreg., identified the following minerals 

(S. W. Pressler, written communication): anatase, cassiterite, columbite, cyrtolite, feldspar, garnet, ilmenite, magnetite, monazite, 

quartz, rutile, samarskite, sericite, titaniferous magnetite, topaz, 

xenotime, and zircon.

Another mineral was recognized but not definitely identified. It 

appeared to be a multiple oxide containing niobium, thorium, rare 

earths, and a relatively small amount of uranium. *

Columbite-tantalite and quartz in one sample from drill hole 3 and 

columbite-tantalite, rutile, anatase, and quartz in a sample from drill 

hole 9 were identified by X-ray analysis in the U. S. Geological Survey 

laboratory at Denver, Colo.

Analyses of the "weakly magnetic" (see p. 387) fractions from 12 

drill holes (table 1) show that the approximate ratio of Nb2 05 to 

Ta205 ranges from 15 to 1 to 45 to 1. The high ratio of Nb205 to 

Ta205 indicates that the principal niobium-bearing mineral of 

these samples is very near the columbite end of the columbitetantalite series. Columbite can contain uranium in amounts greater ^ 

than those found in the above samples, 1 thus the columbite in this 

deposit could contain much or all of the uranium shown by these 

analyses. However, the small amount of uranium (table 1) in the 

black sand concentrates that are known to contain some samarskite 

and possibly some other uranium-bearing multiple oxides suggests 

that the uranium occurs in the uranium-bearing multiple oxides and ^ 

not in the columbite.

It has been suggested that most of the niobium-bearing minerals 

occur immediately above bedrock and that a mixture of coarse and ^ 

fine particles contain more niobium than a homogeneous fine-grained 

material. In the mixed gravels, the niobium-bearing minerals are - 

found in the sand-sized particles interstitial to the pebbles. In the ^. 

writer's opinion, the suggested relation between mixed particle size 

and niobium content was never proved, and it is doubtful that this 

relation exists. The results of the drilling suggest that most of the 

niobium-bearing minerals are concentrated near bedrock, as would be 

expected, but this condition was not proved conclusively.

i George, d'Arcy, 1949, Mineralogy of uranium and thorium-bearing minerals: U. S. Atomic Energy <f 

Oomm. RMO-563, p. 69-73, Tech. Inf. Service, Oak Ridge, Tenn.
DISMAL SWAMP PLACER DEPOSIT, ELMORE COUNTY, IDAHO 387

TABLE 1. Analyses of fractions of churn-drill samples from Dismal Swamp 

placer deposit, Elmore County, Idaho l

Drillhole 

No.

6

5 

5

5 

6

7

7 

8

8

8 

8 

9

g

9 

10

10

10 

12

Footage

5-18^

5-18^ 

5-10

10-12 

5-18

5-17

11-15 

5-13

5-13

7-8 

10-12 

5-13 

5-13

5-13 

5-11

5-11

5-9 

5-13

U. S. Geological Survey analyses, in percent

Nb

5

2 

30

4

4

2 

10

13

.06

3 

3

8

15

Ta

0.2

.08 

1

.1 

.1

.08

.5

.1 

.6

.7

eTP

0.68

.35 

.46

.23 

.36

.30 

.13

.35 

.037

,19

.34 

.17

.45

IT 

0.13

.084 

.19

.080 

.10

.080

.15

.10 

.11

.12

J. R.

Simplot 

Co. analyses, 

In percent

Nb20s 

+ 

Ta2 0s

12.3

4.2

1.9

.4

.9

Remarks

May be reject from 

black mineral separate of "weakly 

magnetic" fraction. 

Sludge from above 

sample. 

Black minerals of 

" weakly magnetic" 

fraction. 

"Weakly magnetic" 

fraction. 

Reject from black 

mineral separate of 

" weakly magnetic" 

fraction? 

Reject from black 

mineral separate of 

"weakly magnetic" 

fraction? 

"Weakly magnetic" 

fraction. 

Reject from black 

mineral separate 

(below) of "weakly 

magnetic" fraction? 

Black, minerals of 

" weakly magnetic" 

fraction. 

"Weakly magnetic" 

fraction. 

"Weakly magnetic" 

fraction. 

"Weakly magnetic" 

fraction. 

"Nonmagnetic" fraction. 

"Magnetic" fractionReject from black 

mineral separate 

(below) of "weakly 

magnetic" fraction? 

Black minerals of 

"weakly magnetic" 

fraction? 

"Weakly magnetic" 

fraction. 

"Weakly magnetic" 

fraction.

Thickness of 

dredgeable 

material'

7

9 

10

5

7

4 

6

Approximate 

percent 

of 

NbjOs 3

TaaOs* 

in the 

'weakly 

magnetic 

fraction

12

6 

6

7

19

6 

22

Approximate 

percent 

ofU3 08 » 

in the 

"weakly 

mag- netic" 

fraction

0.15

.09 

.12

.09

.18

.12 

.14

Approximate ratio of 

NbsOs to 

TajOj

25:1

45:1 

45:1

30:1

30:1

25:1 

25:1

See footnotes at end of table.

425539 57 2
388 .-.; -CONTRIBUTIONS TO ECONOMIC GEOLOGYI. Analyses of fractions of churn-drill samples from Dismal Swamp 

placer deposits. Elmore County, Idaho Continued

Drillhole 

,No.

13 

13

13

21

21

21 

22

23 

24 

24 

24

Footage

5-16 

5-16

5-16 

5-12

5-12

5-12 

5^-15

5-14 

0-10 

0-10 

0-10 

Average

U. S. Geological Survey analyses, in percent

Nb

14 

.04

4 

18

.06

5 

8

12 

18 

.1

4

S 7

Ta

0.6

1.0

.7 

.9 

1.0

eU>

0.46 

070

16

.40 

.075

,27

.53 

.45 

.39 

055

.10

U

0.18

.17

.14 

.20 

.15

J. R.

Simplot 

Co. an- 

. alyses, in percent

Nb2 O5 

+ 

Ta2 0j

Remarks

" Weakly magnetic" 

fraction. 

"Nonmagnetic" 

fraction. 

' ' Magnetic' ' fraction. _ 

"Weakly magnetic" 

fraction. 

"Nonmagnetic" 

fraction. 

"Magnetic" fraction.- 

" Weakly magnetic" 

fraction. 

"Weakly magnetic" 

fraction. 

"Weakly magnetic" 

fraction. 

"Nonmagnetic" 

fraction. 

"Magnetic" fractionThickness of dredge- 

 able 

material e

9

6

 8' 

7

9

7.1

Approximate 

percent 

of 

NbzCV 

+ TazOs4 

in the 

"weakly 

magnetic" 

fraction

21

27

12 

18

1 27

14

Approximate 

percent 

of U3 O8 « 

in the 

"weakly 

magnetic" 

fraction

0.21

.20

.16 

.24

.18

.15

Approximate .-ratio of 

Nb2O6 to 

TazOs

25:1

20:1

15:1 

15:1

25:1

1 All samples are of sand-sized material except the one indicated as sludge under "Remarks."

2 Estimate only; insufficient sample for accurate routine analyses.

a The factor for converting Nb to Nb205 is.1.43.

« The factor for converting Ta to TajOs is 1.22. . -

5 The factor for converting U to UsOs is 1.18.

" Taken from J. R. Simplot Co.'s logs of individual holes.

' Excluding drill-hole 24.

The niobium-tantalum minerals and uranium-bearing minerals 

were probably original constituents of the many small pegmatites 

that cut the Idaho batholith. Similar suites of minerals are known to 

be associated with such pegmatites in other parts of central Idaho 

(Fryklund, 1951 ; 2 Mackin and Schmidt, 1956 3).

Evidence that most of the niobium-bearing minerals in the gravels 

were derived from the area west of the deposit consists of these facts: 

(1) the operator, during work done before this exploration, obtained 

his best samples from along the west margin of the swamp, and (2) 

there is a relatively high Nb205 and Ta205 content in drill hole 24 

(table :1. and pi. 18).

2 Fryklund, V. C., 1951, A reconnaissance of some Idaho feldspar deposits: Idaho Bur. Mines and Qeol. 

Pamph. 91, p. 24-25. ' ' ' -' 

s Mackin, J. H., and Schmidt, D. L., 1956, Uranium- and thorium-bearing minerals in placer deposits 

in Idaho: U. S. Geol. Survey Prof. Paper 300, p, 375-380.
DISMAL SWAMP PLACER DEPOSIT, ELMORE COUNTY, IDAHO 389

MINERAL SEPARATION

After completion of the field work, mineral separations were made 

at the J. R. Simplot Company's plant in Boise, Ida,ho, and selected 

separated fractions were analyzed by J. R. Simplot Company and the 

U. S. Geological Survey.

The samples from the churn-drill holes, trenches, and shafts were 

screened to minus 16-mesh in the field. Examination by panning 

showed that the plus 16-mesh material contained few, if any, black 

minerals. Most of the grains of the plus 16-mesh material and a few 

grains of minus 16-mesh material were rock fragments not broken 

down into individual minerals. The minus 16-mesh material was run 

over a half-sized Wilfley table to separate heavy concentrates of 

each sample.

Magnetite was removed from the concentrates with a hand magnet. 

The remaining part of the concentrates was then passed through a 

magnetic separator in which the magnetic intensity was varied. 

Three fractions were made: (1) with the current oft' and only the 

residual magnetism of the separator acting to separate a "magnetic" 

fraction; (2) with the field strength at 1.5 amperes to separate a 

"weakly magnetic" fraction; and (3) the reject from (2) was the 

"nonmagnetic" fraction. The table concentrates from the first eight 

churn-drill holes were also separated on a high voltage electrostatic 

separator into conducting and nonconducting minerals. It was 

found that in this separation a considerable amount of the fine-grained 

part of the sample was lost; therefore, the rest of the samples were 

not separated on the high-voltage separator.

"Weakly magnetic" fractions of concentrate samples from 4 drill 

holes were analyzed for niobium and tantalum by J. R. Simplot Company. Also, certain fractions of concentrate samples from 12 drill 

holes were analyzed at the Geological Survey laboratory, Denver. 

The results of all. these analyses are given in table 1.

The results of the analyses and the testing done by the Bureau of 

Mines Laboratorj^ at Albany, Oreg., before the exploration contract, 

show that most of the niobium-, tantalum-, and uranium-bearing 

minerals in the concentrates are in the "weakly magnetic" fractions. 

The table also shows that the "magnetic" fractions of the samples 

contain a little niobium and uranium.

J. R. Simplot Company's churn-drill-hole-sample data indicate that 

the gravel in the dredgeable area contains about 1.40 pounds of 

"weakly magnetic" material per cubic yard (table 2). A comparison 

of the amount of "weakly magnetic" material in samples from drill 

hole 8 and .from shaft 1 .(fig- 45), which was sunk around the drill hole, 

shows that the drill-hole samples average 26.4 percent less "weakly 

magnetic" material per cubic yard than the shaft samples. A similar
390 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 2. Thickness of dredgeable gravel and pounds of "weakly magnetic" material 

per cubic yard of gravel as indicated by 11 churn-drill holes

Drill hole

5______-_-_-_-___-_-_--__--__-

6________ _ ___. _ ____..._.__

7_. __..___.-.____.___..______.

8____ _______ __ ________ ___ _

9____. ________________________

10____ __ ________ ____ _____

12 __ __________ ___ ___ __ __

13_____-_-_.__. _______________

21___ ___ ____ __ _ __ _ _ ___

22___.________________________

23_____.______________________

Average _________ __ _ ___

Thickness of 

dredgeable 

gravel in drill 

hole (feet)

7.0

9.0

10.0

2 5. 5

7.0

4.0

6.0

9.0

6.0

8. 0

7.0

7. 1

Pounds of 

"weakly magnetic" material 

per cubic yard 

of gravel 1

0. 92

1.10

.54

22. 06

1.74

1.58

1.06

1.24

2.45

2.36

1.01

1.40

i From J. R. Simplot Company's logs of individual drill holes.

' Corrected for depth to bedrock but not for grade as revealed in shaft 1.

comparison made by the operator between drill hole 4 and shaft 2 

(fig. 46), also sunk around the drill hole, indicated the drill-hole 

samples to be about 30 percent lower than the shaft samples. The 

differences in the amount of "weakly magnetic" material contained 

in the drill-hole samples and the samples from the shafts suggest that 

the content of "weakly magnetic" material of all the drill-hole samples 

may be as much as 25 percent low. If 1.40 pounds represents only 

75 percent of the "weakly magnetic" material in the gravel, then the 

gravel contains 1.87 pounds of "weakly magnetic" material per 

cubic yard.

Analyses (table 1) of "weakly magnetic" fractions from the drill-hole 

samples show that the "weakly magnetic" material in the dredgeable

area contains approximately 14 percent combined Nb206 and Ta206 and 

about 0.15 percent U308 . It is possible that the samples in table 1 

most accurately representing "weakly magnetic" material are those 

from holes 9, 12, 13, 21, 22, and 23. Analyses of these samples average about 20 percent combined Nb205 and Ta205 , and 0.19 percent 

U308 . The average niobium-tantalum and uranium contents of 

"weakly magnetic" material from drill holes 5, 6, 7, 8, and 10 were 

more or less arbitrarily determined by using available information.
DISMAL SWAMP PLACER DEPOSIT, ELMORE COUNTY, IDAHO 391

SURFACE OF FILL

WEIGHTED AVERAGE

2.80 Ib per cu yd . ' 3.22 Ib per cu yd

r

-2 FT

a. 0.83 Ib per cu yd

x 1.1| I > per cu yd

1___

 4.7 FT6.1 FT -

10.5 FT

BEDROCK

EXPLANATION

DRILL SAMPLES

-5FT

1.92 Ib per cu yd

-7 FT 

4.42 Ib per cu yd

8 FT --{ 

T 2.06 Ib per cu yd 

1.41 Ib per cu yd

9 FT -j -- 

1.12 Ib per cu yd

-10 FT -) 

1.10 Ib per cu yd

WEIGHTED 

AVERAGE

-12 FT

0.13 Ib per cu yd

 13 FT -iVERTICAL SCALE 

2 0 2 Feet

Logs, roots, and mud Sand and gravel Defense Minerals Exploration 

Administration, contract No. Idm-E 545, 

docket No. DMEA 291

After S.A.Robinson, engineer, J.R. Simplot Co.

45. Comparison of amounts of "weakly magnetic" material in samples from churn-drill hole 8 

, , and shaft I.

Many analyses of samples from these holes are not of the "weakly 

magnetic" fraction, or are of only a part of the total footage of the 

hole or are of only a part of the "weakly magnetic" fraction of the 

sample. Moreover, all the average analyses for these holes are lower 

than for holes' 9, 12, 13, 21, 22, and 23, although holes 5, 6, 7, 8, and 

10 are close to aiid in part surrounded by holes 9, 12, 13, 21, 22, and
392 CONTRIBUTIONS TO ECONOMIC GEOLOGY

SURFACE OF FILL

WEIGH 

0.482

_l

Lul

Z

z 

<

X

o

FED AVE 

>7 Ib per

hU- 

< 

X 

CO

0.0981 Ib pe 

03

UJ , \|

I'

< ;

! 0.2211 Ib pe 

RJAGE. . . 

cu yd '

^ 0.9104 Ib

Q_

5 : 

<

to

r cu yd

: 5.5 FT 

r cu yd

K 8.2 FT

per cu yd

19 1 FT -

.>.;'. .?;:' 

': : : £ '. ' £ ' ' ' V

.'.'. 1.-.V * .'.' '. '. 'A

= ~ x

-Mt

;->--^J

~x-^ir- 

~^^f-Ir

&*££ -ix ^-f

-I^fx^ -j?-jd ws*

r^ci-XV 0 0 o'

00000

00000

0 0 0 00

0 00000

09. 0 0 0

?.". .". f .0.

Vi'o'i*.

00000

00000

0 0 0 00

.°.?.°. .°.?

DRILL SAMPLES 

-3.0 FT rj ' 

0.1768 Ib 

-9.0 FT ^

0.6380 Ib

per cu yd 

WEIGHTED AV

 .-.-; !- l;<|

0.3339 Ib per

per cu yd

BEDROCK

EXPLANATICiN

Deep roots and mud . Sand.and grovel

Defense Minerals. Exploration Administration 

contract No.ldm-E 545, docket No.DMEA 291

-14.0 FT

0.7O82 Ib per cu yd

- 16.2 FT

After S.. A. fiobinsoh, engineer, 

J. R. Simplot Company

FIGURE 4G. Comparison of amounts of "weakly magnetic" material in samples from, churn-drill .hole 4

and shaft 2.

23. Therefore; it is thought that the "weakly magnetic"'fraction of 

the gravel may contain more than 14 percent combined Nl^Os and 

Ta2 05 and 0.15 percent U308 .

U. S. GOVERNMENT PRINTING OFFICE: 1987