R. 1. 4361 OCTOBER 1948

UNITED STATES

DEPARTMENT OF THE INTERIOR

J. A. KRUG, SECRETARY

BUREAU OF MINES

JAMES BOYD, DIRECTOR

LIBRARY COPY

U. S. Dfrcaul M'i:1s

Boulder City, Nevada

REPORT OF INVESTIGATIONS

INVESTIGATION OF THE RAINY CREEK MERCURY PROSPECT

BETHEL DISTRICT, KUSKOKWIM REGION

SOUTHWESTERN ALASKA

BY

F. A. RUTLEDGE
R. I. 4361,

October 1948.

REPORT OF INVESTIGATIONS

UNITED STATES DEPARTMENT OF TEE ITERIOR - BUREAU OF MINES

INVESTIGATION OF RAINY CREEK MERCURY PROSPECT, BETIFL DISTRICT,

KUSKOKWIM'REGION, SOU2HWESTERN ALASKA. -

By F. A. Rutledge2/

CONTENTS

Page

Introduction and'summary. .' 1

Acknowledgments . i. 

Location and accessibility .. .- . 2

Physical features and climate . . 2

History and production., .......... , 3

' General geology ............. 3

Descr'iption of the deposits ........... 5

No. I .............................- 5

No. II.. ................ . 6

No. III . . . 6

Additional mercury mineralization . 6

The ore .... ... . 7

- Work by Bureau of Mines ................ 7

INTRODUCTION AND SUMMARY

The Rainy Creek deposits lie at the western base of the Kilbuck Mountains,

about 80 miles southeast of Bethel, in southwestern Alaska.

- The deposits were brought to the attention of the'Bureau of Minesby

placer miners on Rainy Creek after cinnabar concentrates were recovered by

their gold-mining operations.

In August 1947 the Bureau excavated 1,500 feet of-dozer trenches totaling

-2,547 cubic yards. Further hand trenching in the bottoms of the dozer trenches

on the. surface totaled 1,440 linear feet, or 40 cubic yards. Nineteen samples

were taken for analyses.

ACIKNOWIEDGEMENTS

The chemical analyses contained in this report were made. at. the Salt Lake

City Experiment Station of the Bureau of Mines under the general supervision of

S. R. Zimmerley, chief, Sclt Lake City Experiment Station, Metallurgical Division. Analyses were made by Heber E. Peterson, chemist, Metallurgical Division.

1/ The Bureau 'of Mines will welcome reprinting of this paper, provided the

'following footnote acknowledgment is used: "Reprinted from Bureau of

Mines Report of' Investigations 4361."

2/ Mining engineer, Juneau, Alaska.

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R.I. 4361

Field investigations were under the general direction of R. S. Sanford,

acting chief, Alaska Branch, Mining Division. Acknowledgment also is made to

Burr S. Webber, mining engineer of the Bureau of Mines, who made the preliminary examination of the deposit.

Special mention is made of the information and assistance obtained from

Al Jones, owner of the Al Jones Airways, Bethel, Alaska, and operator of the

placer operation on Rainy Creek.

LOCATION AND ACCESSIBILITY

The Rainy Creek deposits lie at. the western base of the Kilbuck Mountains

about 7 miles northwest of Mt. Oratia (figs. 1, 2, and 3) in southwestern

Alaska. The deposits are on Arsenic Creek just above its junction with Rainy

Creek and about 3-1/2 miles above the confluence of Rainy Creek with the North

Fork of the Eek Rivr (f1gg. 3);'and the area is near the headwaters of the Eek

River at longitude 660 'OI 'west;letitide .590 59' north.

Bethel, which is at'the'head of ocean traffic on the Kuskokwim River and

the commercial center of thedistrift, is about N. 450 W. and approximately

78 miles distant airline from thy deposit:

A winter tractor trait etimiated'to be 120 miles long has been established

between Rainy Creek andethe I...Frbightifh of general supplies and placer

equipment from Bethel has avra0ged abut' $40 a ton.

During 1945 and 1. 6,' a'rbsp0tiiig outfit, including two tractors (a D-2

and a D-6), went from Goodnew's ay'to Bethel, Alaska, via Rainy Creek, Canyon

Creek, Cripple Creek, and Aniak. Equipment used during the Bureau program was

moved from Cripple Creek, Alaska, during July 1947 over part of approximately

the same trail. (See fig. 2.)

It would be possible to reach this area during high water by ascending the

Eek River from the Kuskokwim. This would not be practical because of the time

required and the necessity for using a canoe or small poling boat on the upper

reaches of the river. The exact distance has not been determined but is estimated to be not less than 200 miles.

An emergency landing strip in the valley of the North Fork of the Eck River

is about 4 miles from the Rainy Creek camp by tractor trail and'can accommodate

small planes. The landing strip is on the right limit of Eck River approximately

2 miles above the mouth of Rainy Creek. Ordinarily, it is necessary to charter

airplane service to Rainy Creek from Be-thel at a minimum of 2 hours flying time.

Present charter rates range from $60 to $75 an hour for planes of 4 to 6 passenger capacity.

PHYSICAL FEATURES AND CLW.PTE

Rainy Creek and. its. tributaries drain the rolling highland below the foothills flanking the western side of the Kilbuck Mountains. Though peaks in the

vicinity rise to more than 2,000 feet, the area generally is about 400 feet

2541 2-
L3 00 A30

SEWAR Figue I ne apo lsa
d q R , R , ~~~~~~~~~~~MODIFIE 30igureY3A Rie area. - A FROM ID AAF PRELIMINARY

R LAKES ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~ - DZE RI

MANYFiue2 -EkRve ra
0/

LO

'if ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~o

0 I 2

SCALE OF MILES

OJARY 1948

Figure 3.- Rainy Creek area.
R.I. 4361

lower. Drainage lines often coincide with contacts between softer sediments

and sandstones or conglomerates, and the .peaks are composed of the coarse,

more resistant clastics.

The immediate area is not glaciated, though glacial moraine was noted a

few miles eastward.

-Except for numerous thickets of willows along the streams, vegetation consists solely of the usual tundra plants. Spruce suitable for mine use could

be obtained in the Eek and.KwethltLl; River Basins about 30 miles west of Rainy

Creek. These trees do not attain sawlog dimensions.

E~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Climatic records from the Rainy Creek area are not available. The nearest

Weather Bureau station is at Bethel, which. because of its position on the

Kuskokwim River at an altitude of only 22 feet, may be expected to have a milder

climate with a longer open season than Rainy Creek. Over a period of 17 years

prior to 1940, the average annual precipitation at Bethel was 17.81 inches.

The seasonal snowfall of 1939-40 was 12.0 inches. Maximum and minimum temperatures at Bethel in 1040 were 750 F. and -290 F., respectively. The open season

in that year was April 24 to October 23.

TISTaRY AND ERODUCTION

The deposits are believed to have been discovered between 1910-20 by Ed

McCann of Bethel, Alaska, who sampled the outcrop of massive realgar. During

the following decade a little attention was given Deposit I by Neal Corrigal,

of Bethel, Alaska; in connection with his exploration of placer-gold deposits

in the region. Corrigal staked the deposit and made a small cut in the face

of the main showing. Subsequently he allowed the location to lapse, and it

is now a part of the public domain.

Production of quicksilver from the area has been solely from cinnabar concentrates recovered during the course of gold-placer operations along Rainy

Creek. All but a small fraction of the 2,000 pounds of high-grade concentrate

shipped was obtained from that part of Rainy Creek below the mouth of Arsenic

Creek.

GENMRAL GEOLOGY

Sediments ranging from conglomerate through sandstone to black shale constitute the observed country rock in this area. Regional strike is north of

east, and dips, usually high, are toward the southeast. In the vipinity of the

deposit the strike ranged from N. 200 E. to N. 310 E. and the dip averaged 720

southeast.

On the basis of similarity with a succession of clastics in the nearby

Goodnews Bay region, tentatively assigned to the Permain by the Federal Geological Survey3l, a like age is suggested for the sediments in the Rainy Creek area.

3/ Smith, Philip S., Areal Geology of Alaska: U. S. Geol. Surv. Prof. Paper

192, 1939, p. 33.

2541 3
R.I. 4361

Type specimens of each of the vein materials and host rocks were submitted

to the Microscopic Laboratory of the Bureau of Mines at Salt Lake City for

petrographic analyses. Tests were performed by LaMar G. Evans, engineer,

Metallurgical Division, Bureau of Mines, Salt Lake City, with the following

results:

Sandstone

No. 426-A is composed of fine, angular quartz grains and quartzite fragments cemented with chalcedony and calcite. These quartz grains are approximately 0.20 mm. in diameter. The chalcedony and calcite have minute inclusions

of carbonaceous moterial and. iron oxide giving the rock a gray color. A few

sporadic grains of muscovite and microcline were observed in microscopic-grain

studies. Qualitative tests for mercury were negative.

Conglomerate .and Grits

Nos. 426-B and C. These two samples are alike in texture and composition.

Microscopic studies of thin sections show that these samples are composed of

oval igneous rock fragments cemented.in an aggregate mass of chalcedony, altered

feldspars, quartz, oligoclase, muscovite, and iron oxide. The oval igneous

fragments have approximately the same orientation in the rock.

Tests for mercury by the ultraviolet lamp and the Willemite screen were

negative.

Shale

Examination of No. 426-D shows that it is a shale. The shale has calcite

disseminated through it. There-arc also a few quartz and orthoclase fragments

scattered sporadically throughout the sample. The blackness of the sample is

due to minute inclusions of iron oxide and a trace of carbonaceous material.

Qualitative tests for mercury were negative.

Vein Quartz Along Bedding Plane

Sample No. 426-E is composed of a fine-grained gray quartz coated with

limonite. This sample is cut by a vein of white quartz, which contains a few

sporadic red inclusions of realgar and cinnabar. Gray fragments of the host

rock are also scattered throughout the quartz vein.

The realgar is the bright-red inclusion, whereas the cinnabar grains are

a dark, dull red.

Based upon mercury tests with the ultraviolet lamp and Willemite screen,

this sample has a low mercury content.

Specimen of Mineralization from Sheared Zone

Microscopic ,tsts prove sample NO! 426-F is composed of fragments of finegrained gray quartz, vein quartz; realgar, cinnabar, and orpiment. The realgar

is the most abundant sulfide mineral, and the cinnabar occurs as inclusions in

it. The only visual difference is the shade of red between the realgar and

cinnabar.

2541 -4
4~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-.c~~~~~~~~~~~~~~~~~~~~~~~~~~''

X : K C Q E X : : ok~~~~~ILE

A~~~~~~~~~~~~~~~~~~~~fW~~~~~~~~~~~~~~~~~A

1~~~~~~~~~~~~~~~~10

O 00 200 300 400 LEGEND

STRIKE & DIP OEDDING PLANE

SPTC ER 1947 SCPTCMSCR~~~~~~~~~~~~~~~~~~~ -

Yb .04 . STRIKE CINNASAR I DIP MINECRALI FAULT OAT PLANE ION

< HAND TRENCH

Figure 4. - Rainy Creek deposits.
ARSENIC CREEK

TRENCH

a. ~ ~ ~ .

SANDSTONE IRON] \ INTERLAYERED

STAINED AT H.W. OF SHALE AND SANDSTONE

SHEAR

PLAN SHEAR ZONE

4" SANDY SHALE

AND CINNABAR IN SANDSTONE CUT BY

PLANE OF SHEAR STOCKWORK OF REALGAR

AND QLUARTZ VEINLETS

SHEARED SANDSTONE WEAK C I N NABAR

AND SHALE

. ~ ~ o

1413 1211 10 9 a 7

E-W SECT ION

LOOKING NORTH

LBS. MERCURY SAMPLE NO. TRUE WIDTH A TON

7 4.4 3.6

8 4.4 1.6

9 4.4 1.2

10 4.4 0.6

1I 4.0 9.4

12 2.0 1.4

13 0.33 35.4

14 0.7 1. 6

0 10 20 30

SCALE IN FEET

AUGUST 1944

Figure 5. - Deposit No. I, Rainy Creek area.
R.I. 4361

DESCRIPTION OF TEE DEPOSITS

The Rainy Creek. mercury rrospect outcrops in the bluffs along the left

limit of Arsenic Creek, about 1,000 feet above its confluence with Rainy Creek

(fig.. 4). Cinnabar -mineralization is of two types - those deposits associated

with fault planes roughly parallel to the bedding of the sediments, but dipping

to the northeast, and deposits along the bedding planes dipping southeast. The

main occurrences are of the first type.

'4

Deposit No. I

Deposit No. I. is of the first type. It outcrops at creek level and is

exposed through a vertical height of 14 feet. Mineralization has been localized

between two shears striking N. 300 to 350 E. and dipping 700 southwest and

parallel bedding planes dipping 450 southeast. (See figs. 4 and 5.)

Mineralization by realgar, cinnabar, and quartz occurs within and between

the two shear zones separated by 16 feet of thoroughly fractured sandstone.

Mineralization of the enclosed sandstone has a stockwork structure, and the

individual seams of quartz and arsenic sulfide generally conform in attitude to

that of the bedding or of the shearing. No mineralization was observed in the

footwall sandstone, but an 8-inch width of the hanging-*all sandstone adjacent

to the upper shear zone contains 1.6 pounds of quicksilver per ton and 0.2 percent arsenic (sample 14).

Mineralization within the football shear zone and the stockwork is uniformly

weak. Samples 7 to 10, inclusive, represent this 17.6-foot section, which has

an average mercury content of 1.8 pounds per ton and 0.9 percent arsenic. The

lower 4 feet of the upper shear zone contains numerous nods of massive realgar

and is represented by sample 11 containing 9.4 pounds'of mercury per ton and

9.4 percent arsenic. The succeeding 2-foot section of the upper shear zone is

lightly mineralized, containing 1.4 pounds of mercury ton and 0.4 percent

arsenic (sample 12). A 4l-inch seam of sandy and shaly material marks the hanging wall of the upper shear zone. This sample, Nc. 13, contains 35.4 pounds of

mercury per ton and 0.2 percent arsenic. A weighted average of this 6.33-foot

cross section of the upper shear zone represented by samples 11 to 13, inclusive, is 8.22 pounds of mercury per ton and 6.08 percent arsenic.

It

The deposit is a rhombohedral parallelepipe with mineralization confined

to a short segment of a favorable sheared sandstone bed by the two faults. To

the southwest the deposit can be seen plunging under the overlying sediments.

Trenching along the projected strike of the deposit to the northeast failed to

reveal either the faults or associated mineralization.

Seventy feet east of deposit No. I and 25 feet higher in elevation is a

small lens of cinnabar and realgar mineralization associated with a fault plane

striking N. 300 E. and dipping 700 to the northwest. No sample was taken at

thlis location, as the occurrence was too small to warrant sampling.

2541 -5-
R.I. 4361

Deposit No. !I

Approximately 100 feet east of deposit No. I and 25 feet higher in altitude is another occurrence similar to No. I. Here two parallel fault planes

strike N. 45° E.and dip 700 northwest to enclose, with bedding planes, another

parallelepiped deposit. Concentration of mineralization is strongest adjacent

to the four sides of the deposit.

Sample 1 was taken across the concentration along the footwall fault.

This sample, 1.2 feet wide, contains 5.6 pounds of mercury per ton and 18.3

percent arsenic. The 3-foot cross section between the realgar and cinnabar

concentrations along the fault planes is represented by sample 3. Sample 3

contains 1.6 pounds of mercury per ton and 0.2 percent arsenic. The concentration along the hanging-wall fault (sample 2) is 3 inches wide and contains

22.8 pounds of mercury per ton and 9.8 percent arsenic.

Concentration along the hanging-wall bedding plane is represented by sample 4. This sample, 6 inches of silicified shale, contains 44.6 pounds of

mercury per ton and o.6 percent arsenic. The adjacent 3-foot section (sample

5) contains 1.2 pounds of mercury per ton and 0.4 percent arsenic.

This deposit also plunges to the southwest under the overlying sedimnentso

Trenching along the projected strike of the deposit to the northeast failed to

reveal either the faults or associated mineralization.

Deposit No. III

Deposit III is situated 100 feet east of deposit II and is composed of

three small lenses adjacent to a nearly perpendicular fault plane (fig. 4). - 0 Strike of the fault is N. 30 E.

The lower lens is 4 feet above the stream level and is 1 foot long, 6

inches high, and 3 inches thick., The second and largest lens is 16.5 feet

slope distance above the first. It is 4 feet long, 3 feet wide, and 3 inches

thick. The third lens was also 3 inches thick but has been mined out completely.

A composite sample representative of the three lenses (sample 6) contains

45.8 pounds of mercury per ton.

Trenching in both directions along the projected strike of this fault failed

to reveal additional mineraliztion.-

Additional Mlercury Mineralization

Trenching along the projected strike of the three miin occurrences failed

to expose continuations of these deposits, but mercury mineralization was,

revealed in narrow quartz ve ins along bedding planes in the sediments. Though

these veins wero too narrow and of too low grade to 'De economically important,

they were sampled and analyzed to complete evaluation of the prospect. Location of these samples in the dozer trenches is shown on figure 4, and the

analyses follow:

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R.T. 4361

Width, Mercury, Arsenic,

Sample feet onds per ton percent

1 A... 0.75 1.2 0.07

2 A..... 0.80 o.6 0.05

3 A ..... 0.50 0.2 0.05

4 A..... 0.60 3.4 0.07

5 A.....j 1.60 1.8 ! _0.10

In addition to the quartz veins sampled, numerous thin quartz veins were

exposed that were devoid of mercury mineralization. Most of these were along

the bedding planes in the sediments, though several paralleled the faults

dipping to the northwest and cut across the bedding.

Though most of the cinnabar recovered during placer-gold operations on

Rainy Creek was found below the mouth of Arsenic Creek and undoubtedly derived

from the described occurrences, a small amount was recovered above the confluence of the two creeks and came from another place. Attempts to trace this

cinnabar to its source were unsuccessful. However, the probable source is

quartz veins along the bedding planes in the sediments on the left limit of

Rainy Creek about 1-1/4 miles above Arsenic Creek, though no quartz containing

cinnabar was found in place.

THE ORE

Mercury is present as the sulfide cinnabar in sporadic red inclusions in

white vein quartz associated with the fine-grained gray quartz that forms tile

veins along bedding planes. Realgar, arsenic sulfide, also is present.

In the larger deposits mercury occurs in the shear zones enclosed by faults

and bedding planes. Mineralization includes fragments of fine-grained gray

quartz, white-vein quartz, realgar, cinnabar, and orpimenti The realgar is the

most abundant sulfide mineral, and the cinnabar occurs as inclusions in it.

WORK BY BUTEAU OF MINES

The Bureau of Mines' program comprised 1,499 lineal feet of dozer trenching

and 2,547 cubic yards of material excavated. Hand trenching in the bottom of

the dozer trenches and small hand cuts totaled 1,439 feet with 40 cubic yards.

Nineteen samples were taken -For analyses.

2541 7