R. I. 4322 AUGUST 1948

UNITED STATES

DEPARTMENT OF THE INTERIOR

J. A. KRUG, SECRETARY

BUREAU OF MINES

JAMES BOYD, DIRECTOR

LIBRARY COPY

U. S. Burcau Mites

Boulder City, Nevada

REPORT OF INVESTIGATIONS

INVESTIGATION OF THE MORELOCK CREEK TIN PLACER

DEPOSITS, FORT GIBBON DISTRICT, ALASKA

DY

BRUCE 1. THOMAS AND W. S. WRIGHT
R.I. 4322,

August 1948.

REPORT OF INVESTIGATIONS

UNITED STATES DEPARTMENT OF THE INTERIOR - BUREAU OF MINES

INVESTIGATION OF THE FORELOCK CREEK TIN P CER

DEPOSITS, FORT GIBBON DISTRICT, ALASKA -

By Bruce I. Thomas / and W. S. Wright /

CONTENTS

Page

Introduction and summary . ......... .. . . 1

Acknowledgments....*... ....... ........ 2

Location and accessibility . ...... ..... , 2

Physical features'and'climate ........... 2

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

Water supply ...... . 3

Occurrence of deposits ........ ... 3

Character of deposits ..... . .... .. &. ...... 4

Exploration by the Bureau of Mines................ 4

Churn drilling ...... ..... .O...*....... .. 4

Test pitting............. ......... t...... 6

Caisson shaft sinking....I........... ........ . 8

Open-cut mining ... ......... 8

INTRODUCTION AND SUMMfIARY

In 1943, the Bureau of Mines investigated the occurrence of cassiterite

in the gravels of Morelock Creek which flows into the Yukon River 32 miles

east of the confluence of the Yukon and Tanana Rivers in central Alaska.

Samples of the gravels were obtained by churn drilling, test pitting,

shaft sinking, and open-cutrmining. The gravels along Morelock Creek are

about 6 feet in depth and covered with about 2-1/2 feet of overburden. The

overburden is frozen, but the underlying gravels are not. Cassiterite, with

gold and other heavy minerals, is usually concentrated in the first. 2 feet of

gravel above bedrock and in some places penetrates into. joint planes in the

bedrock.

The various methods of obtaining samples are discussed, and drill logs

and analyses are tabulated in this report.

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

following footnote acknowledgment is used: Reprinted from Bureau of

Mines Report of Investigation 4322." / Mining engineer, Bureau of Mines, Fairbanks, Alaska.

32 Mining engineer, Bureau of Mines, Juneau, Alaska.

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R.I. 4322

ACKNOWLEDGMENTS

Special acknowledgment'is extended to Edward Vogt, owner-of the Morelock

Creek claims, for supplying the Bureau with various small tools and housing

facilities for the pr6ject crew.

LOCATION AND ACCESSIBILITY

Morelock Creek is at longitude 1510 W. and latitude 650 18' N., about 32

miles east of the confluence 6f the Yukon and Tanana Rivers. The nearest settlement is Tanana, with a population of about 150 people - 50 whites and 100

Indians. Tanana has two general stores, a roadhouse, an air field capable of

accommodating large aircraft, a territorial school, a government-operated

hospital, and a church. Freight is brought in from the coast by railroad to

Nenana, then by river via Tafinia t6 Morelock Creek;- In the winter, travel is

by dog team or airplane. During the Winter months plane6 can land on the

Yukon River at the mouth of Uorelock Creek whenever the ice.,freezes smoothly

enough for skiis.

PHYICAL FEATURES AND CLIMATE

'The area is north of the Yukon-Tanana upland and is characterized by

high, well-rounded hills. Th&e' 'Verage'' elief is about 800 feet.

The climate of the region is'typi'cal of central'Alaska; winters are long

and cold and summers short and rdlatively warm. Temperature ranges from -70

F. in winter to 900 F. or higher in summer, with an annual mean of about

230 F.

A considerable portion of the alluvial-deposits is permanently frozein -

only a few feet at the surface thaw during the summer. A marked exception of

this condition exists on Mor6Io'ck Creek, where circulating ground water has

thawed the ground in places for a considerable distance back from the banks.

The ,annual precipitation recorded for the last 30 years'averages 12,98

inches., The average winter snowfall, based on observations over the same

period, is 53.6 inches. The frozen mantle of upper gravels and moss has a

tendency to restrict ground-water circulation. In summer the moss retards the

run-off and holds much of the precipitation near the surface.

The more common trees growing along the valley floor on the thawed areas

are spruce, cottonwood, and birch, with some dwarf willows and alders. Scrub

spruce and moss grow in the permanent frost areas. Willows and alders, among

spruce and some birch, are the predominant growth on the high.benches and

valley slopes.

In some areas of the valley floor there are fairly large patches of

spruce that attain a diameter of'2 feet or more, which would be suitable for

saw logs.

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s5z* /.f/ ', 50 148 

|TIN AREA a r1

;:~O~

Scale X , o Mi/ Lakes

NoFurktea Aafner P/otaI, Ala

OelgiO I M X /~

/ j °t~~~~~ 50

l l ~~~~~Sca/c /n7 Mile5

Figure 1. -Morelock Creek tin area, central Alaska.
Af~'ic Skeich byJC/luind/ne d\\\f

.T(:j.Pyne-Gea/og/co/ Sur vey I

Figure 2. - Morelock Creek.
R.I. 4322

HISTORY AND PRODUCTION

Placer gold was first discovered on Morelock Creek in the fall of 1901;

as a result, many men stampeded into the area from the Rampart district. All

mining was done by hand, and operations were chiefly confined to the exposed

reefs, where the shallow portions of the deposits were found. Gold was never

found in sufficient quantities to make the area a large producer, and activities soon died down. Since that time various parties have prospected in the

district, but. all have been unsuccessful in finding any large areas of minable

ground.

Vogt has been in the district about 10 years and has been mining virgin

ground on the high reefs. It was through his efforts that attention was given

to Morelock Creek as a possible source of tin.

Ten 20-acre placer claims covering the tin-bearing area are held by Vogt.

All these claims are recorded at the Fort Gibbon.recording office at Tanana,

Alaska.

WATER SUPPLY

There is enough water in Morelock Creek to satisfy the demands of a small

placer-mining operation. During the lowest stages, it is estimated that approximately 500 miners inches of water flow through the tin-bearing area.

After heavy rains such as occurred at various times during the, period of investigation., the flow of water was observed to have increased several times

that of the low stage.

Two small ditches, built several years ago, supply water for hand-mining

operation. These ditches are too small and are not properly located to

furnish water for any mechanized operation of the deposits.

OCCURRENCE OF DEPOSITS

The tin-bearing area on Morelock Creek is about 5-1/2 miles from the

Yukon River. Figure 1 shows the location of the Morelock Creek tin area, and

figure 2 is a more detailed sketch of Morelock Creek.

Tin, in the form of cassiterite, is found in the gravels on Morelock at

the mouth of two small tributaries - Bonanza and Homestake Creeks. It is in

this area that the greatest concentration of cassiterite, with gold, is found,

smaller amounts of both occurring in the gravels that extend downstream for a

distance of 3,000 feet.

The valley floor below Bonanza Creek is about 600 feet in width. -The

hills on the west side of the valley rise rather steeply, but those to the

east slope more gently, and a bench paralleling the creek indicates an old

valley floor. This bench begins at the confluence of Morelock and Bonanza

Creeks and extends the full length of Morelock valley to the Yukon flats.

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R.I. 4322

Bedrock in the tin-bearing area consists mostly of metamorphic rocks.

In the area prospected, quartz-sericite schist, greenstone schist, calcareous

schist, and quartzite schist were observed. Near the lower end of the area,

a bed of limestone outcrops along the east bank of the creek. These rocks

are complexly folde4 and faulted, producing an irregular surface upon which

the gravels have'been deposited. Although it is apparent that the source of

the tin is not local bedrock, the bedrock had a definite influence on the

deposition of tin in the area. The complex folding and faulting, combined

with the erosion of less resistant rocks, has produced, in most places, an

irregular bedrock surface. It is upon these irregularities or "reefs" that

the gold and heavy minerals such as cassiterite, magnetite, pyrite, and limonite, were deposited.

CHARACTER OF DEPOSITS

The gravels in the area prospected are about 6 feet in depth and are

covered with overburden for an average depth of 2-1/2 feet. This veneer of

overburden, composed chiefly of organic matter, is frozen, although the under-'

lying gravel is not. Some of the gravel along the outer rim of the valley was

found to be frozen, but this condition seems to exist only along the margins

of the alluvium. The gravel is fairly coarse and contains many boulders, some

of which are 24 inches in diameter.

The tin mineral and gold, together with other heavy minerals, are usually

concentrated in the first 2 feet of gravel above bedrock, but in places the

minerals penetrate the sediment-filled joint planes in the bedrock.

-EXPLORATION BY TEE BUREAU OF MINES

During the investigation, four methods were used to obtain samples from

the placer deposits - churn drilling, test pitting, caisson shaft sinking,

and open-cut mining.

Churn Drilling

Drill holes were spaced 100 feet apart in two lines across the valley.

The lines crosscut the tin-bearing area, and the purpose of the work was to

determine the possible extent of the deposit up and down the valley as well as

across the width.

Location of drill holes is shown on figure 3. After a hole was drilled,

a permanent stake, on which the line and hole numbers were painted, was placed

in it. These stakes protruded 6 to 7 feet above the ground so that they-could

be found easily. Sections through each line of holes are shown on figures 4

and 5.

A small, portable, 5-inch, airplane-type churn drill was used. In thawed

ground,, a 5-inch casing with a 6-1/4-inch drive shoe and a thin placer bit was

used. The drill crew was composed of a driller, helper, and panner or

sampler.

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---_ -_-__ _ •LLN- e 70

9goi CU7s Nos0.03 5

/ e (Oz Creek.5

a~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1zS\ Jt °aMOoen Cut

Vo9# Cw' NO I Pit/ NoI

//O /20/30 /50/60/70 /80 _- -_ o- -0 -- 0 -0--- LINE 6f5

Legend 0d Oen Cut

o Ori/Il Ho/es es*Pit/7NO. ii 

o' Old Prospc? S st PilIlNO 3 6 Coisson Shoff 9

_ Old_& Old7a0/c Open Cuf F3Sd~onks , t -Tesf Pi7No.4

E Grovel &or.s ,

- 8edrock Reefs

I J ,, //tt~idOpen CuJf a

0 500

5cole in Feeil

Figure 3. - Morelock Creek showing location of Prospect

shafts and drill holes.
Hol/e/ a /20 /30 /40 /50 /60 /70 /80 /90

4C0 :L/NE . 70 .

5 S @2Lis a o zoo 300'

490'ae90

nHowL/NE10. o in F.eto

t,,,. //0 /ZO /30 140 /50 1S0 7 8 9

f Legend 0 /0o 200 300

Mz Over-bUrden +zG<fi

=J Grave/Scale io Fee 0

f3 sedr-ock - -

T Orill ho/e5 Verb Scale n1e7

Figure 4. - Sections through drill holes, Morelock Creek.
560

6.0/

XCreek 4.s.A5° 5

LINE /5.0

530

Legend

IJ Overburden

3 GrOavel

iR Bedrock J DOri/ /H/oles

0 50 /00 /50

Hloriz. Scale in Feet0 /0 20 30

Verf: Scc/e in Feet

Figure 5. - Section through drill holes, Bonanza Creek.
R.I. 4322

The' character of materials drilled and depth pf each change in material

were recorded. When practical, 2-foot samples were taken in barren overburden

and upper gravels, and 1-foot samples were collected in the tin and gold horizons. Each sample was deslimed, measured loose in a volume.bucket, and

panned. Concentrates from the panning of each sample were put in separate

jars, labeled as to depth, number of hole, and line, and brought to a central

point at the close of each shift.

Each individual sample was then panned, the gold extracted by amalgamation and weighed, and the concentrates tested for the presence of tin by the

zinc method, examined with a hand lens, and weighed. The individual samples

from one hole were then combined and labeled to form one sample of tin conPI centrates and one sample of gold.

All holes were drilled into bedrock for at least 2 feet, and some were

drilled deeper, depending on the amount of heavy mineral concentrate found.

Open holes were drilled in the frozen ground. The procedure of sampling

and recording the formations was that used in thawed ground, except that after

completing the hole a volumetric water measurement was made to determine the

size of the hole in the mineral-bearing horizon.

Each deslimed sample was measured to the nearest thousandth of a cubic

foot. The percentage of solids in the mineral-bearing horizon was determined

by using the sum of the total measured volumes loose and the volume of'the

hole as determined by the water measurement.

The first holes drilled were on line 10.. These crosscut the valley

floor and passed through the upper section of a reported tin-bearing area and

approximately 700 feet downstream from another similar area. The results of

analyses of the concentrates in the eight holes that comprised thiis crosscut

revealed only a trace of tin and no gold. Apparently the tin-bearing streak

did not continue downstream to this point. With the establishment of a lower

limit of tin and gold-bearing gravels, drill line 8.5, 1,500 feet upstream

from line 10, was drilled to crosscut the valley floor. Eight holes were

drilled on this line at 100-foot intervals.

In calculating the amount of tin present, 75 percent of the pan concentrate from each hole was estimated to be tin concentrate. The tin concentrate was assumed to be 57 percent tin. By this method, a factor of 0.427 was

determined for tin calculations. Applying this factor, the average weighted

content of tin in line 8.5 was 0.12 pound a cubic yard, or 0.27 pound over a

square yard of bedrock. The weighted value of gold was 0.0037 ounce a cubic

yard, or 0.0085 ounce over a square yard of bedrock. This line crosscut a

supposedly high-grade area.

Two holes were put down on line' 7. No tin or gold was found to be

present.

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R.I. 4322

Five holes were drilled at 50-foot intervals on line 15 on Bonanza Creek,

crosscutting the full width of the valley floor. The average weighted content

of tin in this line,, assuming the concentrates to be 57 percent tin, was

0.0443 pouncd a cubic yard, or 0.1492 pound over a square yard of bedrock, and

the gold content was 0.0097 ounce a cubic yard, or 0.0328 ounce ove'r a square

yard of bedrock.

Table 1 is a summary of drilling results.

Test Pitting

Four small test pits 2 to 4 feet in depth were put down on Morelock

Creek. These were located on the high "reefs" and in virgin ground near old

workings. The material excavated was carefully panned, and the bedrock area

was measured to determine the amount of material extracted.

Pit 1 is above line 8.5, and pits 2 and 3 were put down on a high reef

of bedrock below line 8.5, Pit 4 is in an old cut 700 feet above line 10.

The location of the pits is shown on figure 3,

The indicated tin and gold content in the vicinity of the pits, measured

as total content over each square yard of bedrock, is as follows:

Pounds of tin over each Ounces of gold over each

Pit. square yard of bedrock square yard of bedrock

1 0.0008 0.0026

2 0.0393 0.0022

3 0.0153 0.0051

4 0,0603 0.359

These pits verify the results obtained from the drilling and discount the

existence of a large and continuous tin-bearing area.

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R.I. 4322

TABLE 1. - Summary of drilling results

Dep th - feet Depth of ME- Depth of MvminMining Analysis ing Section x ing Section x

Hole To- Over- Grav- sec- Cu. Yd. Lbs. of Sn a Oz. of Au a

Line No. tal burden el tion*' Lbs. Sn|Oz. Au cu yd. cu. yd.

Morelock Creek

10.01120 15.0 305 8.0 0.0 0.0 0.0

130 15.0 1.5 10.5 .0 .0 .0

140 15.0 3.0 9.0 .0 .0 .0

150 10.0 2.0 4.0 t0 .0 .0

160 7.0 5.0 ,0 .0 .0

170 13.0 10.0 .0 .0 .0

180 14.5 1. 10.0 .0 .Q .0

190 24.0 4.0 13.0 .0 .0 .0

8.5 110 13.0 4.5 4.5 6.5 .0562 .0 0.3653 0.0000

120 12.0 5.0 3.0 5.0 .0300 .0 .1500 .0000

130 8.0 2.5 3.0 5.0 .0556 .0037 .2780, .0185

140 6.5 .5 3.0 5.0 .4562 .0 2.2810 .0000

150 9.0 1.0 5.0 7.0 .1618 .0202 1.1326 .1414

160 9.0 1.0 5.0 7.0 .0362 |.0051 .2534 .0357

170 12.0 1.5 7.5 10.5 .1706 .0010 1.7913 .0105

180 14.0 4.5 6.5 8.5 .0518 Trace .04403 .0000

Total 54.5 6.6919 0.2061

Average 0.1227 0.00378

7.0 180 18.0 1.0 4.5 .0 .0

190 20.0 5.0 5.o 6.0 .0341 .0

Bonanza Creek

15.0 4.0 11.0 0.0 5.0 9.0 0.0045 0.0 0.0405 0.0000

4.5 10.0 2.0 5.5 7.5 .1250 .0127 .9375 .0952

4.5A 14.5 .0 8.0 14.C*' .0675 .0082

5.0 14.01 .0 8.0 10.0 .0687 .0099 .6870 .0990

5.5 17.01 .0 11.0 14.0 .0093 .0143 .1302 .2002

5.5 Hole abandoned because of boulders encountered.

6.0 14.6 0.0 8.0 0.0 0.0 jO.1

Total 40.5 1.7952 944

Average - - 0.0443 10.00973 _

--The mining section includes at least 2 feet of bedrock. No overburden is

included in the mining section.

**Not included in sum.

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R.I. 4322

Caisson Shaft Sinking

One ftcaisson" shaft, numbered 155 on line 8.5, was sunk to bedrock to

verify the drilling. In this work, 1/8-inch steel caissons were used. A

3-1/2-inch gasoline-operated pump was used for unwatering the shaft. A small

sluice box was set up near the hole, and the gravels extracted were sluiced

over pole riifles, using' the water pumped from the hole. This method proved

to be the fastest and simplest means of handling the samples.

Considerable rater was found in the shaft, which salted the material extracted with fines that were washed in below the caisson. All materials removed were run through the sluice box. The sluice was cleaned after the upper

3 feet of gravel was washed to determine whether any concentrates were

present. The last 3 feet of materials were sluiced separately, and the boxes

were again cleaned to determine the content of the bedrock concentration of

tin minerals and gold.

Assuming the concentrates to contain 57 percent tin, the results indicate

a gravel content of 0.0531 pound of tin a cubic yard, or 0.1089 pound over a

square yard of bedrock, and the gold content to be 0.0055 ounce a cubic yard,

or 0.113 ounce over a square yard of bedrock.

Despite the anticipated salting, the recovered amount of gold and tin

concentrate did not vary materially from the results of the drilling!.

Open-Cut Mining

During the period in which the drilling operations were in progress Vogt

mined three small open cuts. At the completion of each cut., the area of the

cleaned bedrock was measured by a representative of the Bureau of Mines, who

was also present during the clean-up. The tin concentrate was turned over to

the Bureau of Mines, and the gold was weighed and recorded in the presence of

the representative.

Cut 1, on a narrow, high. reef of bedrock, produced 6.8 cubic yards of

material, from which 3.94 pounds of tin concentrate and 0.31 ounce of gold

were recovered. The average tin content was 0.329 pound a cubic yard and the

gold 0.0455 ounce a cubic yard.

Cuts 2 and 3 are on high bedrock near the mouth of Bonanza Creek. In

cut 2, 32.9 cubic yards of inaterial were handled, from which 0.563 pound of

tin concentrate and 0.6410 ounce of gold were recovered. The average tin

content was 0.0097 pound a cubic yard, and the gold 0.0195 ounce a cubic yard.'

In cut 3, from which 1.452 pounds of tin concentrate and 1.0 ounce of gold

were recovered, 31.6 cubic yards of material was mined. The average tin

content was 0.0262 pound a cubic yard, and gold 0.0316 ounce a cubic yard.

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