~ Geochemical and Geologic 

Reconnaissance of a Part of

j»»

i the Fortymile Area, Alaska

By HELEN L. FOSTER and SANDRA H. B. CLARK

, CONTRIBUTIONS TO ECONOMIC GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1312-M

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 25 cents (paper cover)
CONTENTS

	Page

Abstract ______________________ - M!

Introduction _____________ 1

Location of area _ _ _ _ 3

Acknowledgments - _ _- - - 3

Previous geologic work ___ 3

Geologic setting ____.._____ .. 4

Mineral deposits _ _ _ -- 5

Geochemical studies __________ __________ 5

Champion, Bear, and Comet Creeks area ________ 10

Alder Creek area __________________ __ 12

North Fork area ______________ ____ - 15

North Peak area ______________________ - 15

Gold Run area _____..____ ____ _ 19

Joseph area ___________________________ . 21

Hutchinson Creek area _________________ 21

My and Our Creeks area ____________________~____ 23

Kechumstuk and Gold Creeks area _____________ ___ 25

Conclusions __________.__________________ _ __ 25

References cited _______________________.._________ 29

ILLUSTRATIONS

FIGURE 1. Index map showing the location of part of the Fortymile

area included in this report ___________________ M2

2. Map showing extent of geochemical sampling in the

Fortymile area __________________________ 6

3. Histograms showing the percentage frequency distribution 

of chromium, copper, nickel, and lead for 448 streamsediment samples from the Fortymile area _________ 8

4. Map showing location of areas discussed in this report ___ 10

5-13. Maps showing the location of stream-sediment and rock 

samples collected from the following areas:

5. Campion, Bear, and Comet Creeks ___________ 11

6. Alder Creek ________________________ 13

in
IV CONTENTS

Page

FIGURES 5-13. Continued

7. North Fork ____..____________________ M14

8. North Peak _________________________ 17

9. Gold Run ___________________________ 18

10. Joseph ____________________________ 20

11. Hutchinson Creek _____________________ 22

12. My and Our Creeks ____________________ 24

13. Kechumstuk and Gold Creeks ______________ 26

TABLES

TABLE 1. Selected statistical data for 448 stream-sediment samples

from the Fortymile area ______..______________ M9

2. Semiquantitative spectrographic analyses and gold analyses

of stream sediments from the North Fork drainage area _ 16

3. Analyses of selected samples from the Mitchell prospect ___ 28
CONTRIBUTIONS TO ECONOMIC GEOLOGY

GEOCHEMICAL AND GEOLOGIC RECONNAISSANCE OF 

A PART OF THE FORTYMILE AREA, ALASKA

BY HELEN L. FOSTER and SANDRA H. B. CLARK

Abstract

Analyses of stream-sediment and rock samples from the Fortymile area, 

Eagle quadrangle, Alaska, indicate that several localities have weak geochemical anomalies. In most places, the source of the anomaly is not known 

and more geological information is needed. Chromium and nickel in amounts 

above normal background in stream sediments probably came in large part 

from ultramafic intrusions and greenstone. Some weak anomalies may be due 

to scattered sulfide minerals in low-grade metamorphic rocks and are not 

necessarily indicative of mineral deposits.

The weak anomaly in the Champion Creek area is one of the most promising for prospecting, because galena has also been found in float. In the 

Alder Creek area, silver was detected in two-thirds of the stream-sediment 

samples. In the Gold Run, Joseph, and Hutchinson Creek areas there is 

evidence of mineralization associated with probable Tertiary igneous activity. 

The abundance of small igneous intrusions^ favorable host rocks, the occurrence of placer gold, and the presence of some bedrock prospects suggest that 

these and several other parts of the Fortymile area warrant further sampling 

and geological exploration.

INTRODUCTION

A geochemical and geologic reconnaissance of parts of the Fortymile River drainage area, Alaska (fig. 1), was conducted in the 

summer of 1968 as a part of the U.S. Geological Survey's Heavy 

Metals program. A helicopter was used for 2 weeks in early June to 

make geologic observations and to collect stream-sediment and rock 

samples for geochemical analysis. Later in the summer, several 

long traverses on foot were made to obtain additional geologic data. 

(Analyses from a few samples which were collected in 1967 are 

also incorporated in this report.) Work in 1968 was concentrated

Ml
M2 CONTRIBUTIONS TO ECONOMIC GEOLOGY

ALASKA

144° 143 142°

EAGLE QUADRANGLE

FIGURE 1. Location of the Yukon-Tanana Upland, 

Eagle quadrangle, and the part of the Fortymile 

area included in this report.

in parts of the Fortymile area not included in recent geologic 

studies by the senior author (Foster and Keith, 1968; Foster, 

1969b) or in geochemical investigations by the Division of Mines 

and Minerals, State of Alaska (Saunders, 1966 and 1967; Burand, 

1968; and Smith, 1968). Several geochemical anomalies were 

located which deserve further exploration and checking, but no 

areas were found which have a large number of strongly anomalous samples.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M3

LOCATION OF AREA

The Fortymile area, known locally as The Fortymile or the 

Fortymile country, is the drainage area of the Fortymile River, 

a tributary to the Yukon River. The area is included in the 

physiographic province known as the Yukon-Tanana Upland 

(Wahrhaftig, 1965), the maturely dissected mountainous terrain 

lying between the Tanana and Yukon Rivers. The Taylor Highway 

extends from Tetlin Junction on the Alaska Highway to Eagle on 

the Yukon River and crosses the eastern part of the YukonTanana Upland. The part of the Fortymile area included in this 

report is in the southern and central parts of the Eagle quadrangle (fig. 1).

ACKNOWLEDGMENTS

Florence R. Weber, geologist, assisted the senior author for 

several days in early June and field assistant Ronald Warbelow 

collected most of the stream-sediment samples. The field party was 

also assisted for a short time by Sharleen McDonald and Katherine 

Nichols.

Mr. and Mrs. Robert McCombe at South Fork Lodge provided 

a comfortable base of operations during the helicopter work and 

the Marvin Warbelow family at Cathedral Bluffs Lodge gave 

assistance and furnished a base of operations during July and 

August. The hospitality of Carl Sorenson, Suzanne Gilbert, and 

Gordon Miller at Joseph is also appreciated.

PREVIOUS GEOLOGIC WORK

Placer gold was discovered on the Fortymile River in 1886 

(Prindle, 1909, p. 9), and within a few years gold was being 

produced from many creeks in the Fortymile drainage area. A 

U.S. Geological Survey party consisting of J. E. Spurr, H. B. 

Goodrich, and F. C. Schrader investigated the placers of the 

Fortymile area in 1896 (Mertie, 1937, p. 5) ; during the fall of 

1899 the area was traversed by the W. J. Peters and A. H. Brooks 

party on their trip from Pyramid Harbor to Eagle (Prindle, 1909, 

p. 8). In 1903 L. M. Prindle visited all of the gold-producing creeks 

and during the course of his work in the Yukon-Tanana region 

from 1904 to 1911 he traversed parts of the Fortymile area 

(Prindle, 1909, p. 8). J. B. Mertie, Jr., carried on geologic studies 

in the Yukon-Tanana region from 1911 to 1931. The work of 

Prindle and Mertie provides the background for our present 

geologic knowledge of the area. More recently, The Division of 

Mines and Minerals of the State of Alaska has carried on geochemical investigations in parts of the Fortymile area (Saunders,
M4 - CONTRIBUTIONS TO ECONOMIC GEOLOGY

1966; Burand, 1968; and Smith, 1968), and the U.S. Geological 

Survey has done reconnaissance geologic mapping (Foster, 1967, 

1969b; and Foster and Keith, 1968). Much of the area in this 

report is not covered by recent geologic maps.

GEOLOGIC SETTING

The Fortymile area is underlain primarily by metamorphic 

rocks which have been intruded by a wide variety of igneous 

rocks. Locally, sedimentary rocks rest unconformably on the metamorphic and igneous rocks and are associated with lava and tuff. 

The Fortymile River and its major tributaries are entrenched, 

and high-level terrace remnants border many of the streams. Rubble covers many ridge tops, and colluvium covers most of the 

slopes. Good exposures of bedrock are scarce.

The metamorphic rocks can be divided into several groups on 

the basis of lithology and metamorphic grade. In the southern 

part of the area quartz-biotite gneiss and schist, amphibolite, 

quartzite, and marble of the amphibolite facies predominate. In 

the northern part of the area, quartz-graphite schist, quartz- muscovite schist, quartzite, marble, greenschist, and greenstone of the 

greenschist facies are most abundant. All of the metamorphic 

rocks are highly deformed and faulted. Some rocks of the greenschist facies are known to be of Paleozoic age; the other metamorphic rocks are of unknown age and could be Precambrian 

and (or) Paleozoic.

The igneous rocks occur in lava flows and in large and small 

instrusive bodies, such as batholiths, stocks, dikes, and sills. The 

rocks range in composition from granite, pegmatite, and rhyolite 

to gabbro and basalt. Most of the large intrusive bodies consist of 

granodiorite, and margins of quartz diorite or diorite occur locally. 

The Taylor Mountain batholith crops out on both sides of Mosquito 

Fork (fig. 4) and is the largest granodiorite intrusive in the area; 

some rocks of the Charley River batholith are found in the northwestern part of the area, and intrusive rocks of the Mount Harper 

batholith extend into the southwestern part of the area. At least 

one syenite intrusion is known in the vicinity of Mount Veta. 

Many small intrusive bodies consisting of quartz monzonite, diorite, or gabbro are found throughout the Eagle quadrangle. In 

several places, particularly south of Joseph, near Gold Run, and 

in the Hutchinson Creek area, granitic hypabyssal-type rocks occur in small masses and dikes. Basalts, some of which were probably formed as flows, crop out north of Bullion Creek. Rhyolitic 

and dacitic lavas have been found, mostly in small patches, in the
RECONNAISSANCE, FORTYMILE AREA, ALASKA M5

central and western parts of the area. Small ultramafic masses 

and dikes, most of which have been serpentized, occur in no recognizable pattern through the Fortymile area. They are most numerous in the eastern part, but this may be partly because mapping 

there is more detailed.

The age of the granodiorite of the Taylor Mountain batholith has 

been determined by isotopic methods to be about 180 million years 

(Wasserburg and others, 1963, p. 258), and most of the other 

granodiorite intrusions are probably about the same age. At least 

some of the other igneous rocks, including some of the basalt and 

probably the hypabyssal rocks, are Tertiary in age, but few data 

are available concerning the age of most of these rocks.

Only a few small widely scattered areas of sedimentary rocks 

are known. The rocks in these areas consist of conglomerate, 

sandstone, shale, and coal; in places they are tuffaceous. Plant 

remains and pollen indicate that most of these deposits are of 

Tertiary age.

MINERAL DEPOSITS

Gold is the only metallic mineral of economic significance presently known in the Fortymile area. Nearly all the gold has been 

derived from placer deposits, and placer gold mining still continues 

on a small scale in a few creeks notably Chicken, Lost Chicken, 

Wade, and Canyon Creeks, and Fortyfive Pup. Lode gold has not 

been found in sufficient quantity to mine, except for small deposits 

in quartz and calcite veins on the ridge between Myers Fork and 

Stonehouse Creek near Chicken. A rich vein and veinlets were 

mined here by Fred and Arthur Purdy, but promising extensions 

of these deposits have not been located. No other metals are presently mined in the area, although there are some silver, copper, 

iron, and antimony prospects (Saunders, 1967, p. 24-28).

At one time a small amount of coal was mined at Chicken for 

local use (Mertie, 1937, p. 262). An occurrence of asbestos has 

been discovered near Gold Run, and the possibility of finding deposits of potentially commercial size and quality does exist (Foster, 1969a).

GEOCHEMICAL STUDIES

The extent of geochemical studies in the Fortymile area on this 

project is shown in figure 2. The 448 stream-sediment samples 

were anlyzed for gold by the atomic absorption method and for 

29 other elements by six-step semiquantitative spectographic anal378-276 O - 70 - 2
M6 CONTRIBUTIONS TO ECONOMIC GEOLOGY

FIGURE 2. Density and general location of analyzed stream-sediment samples 

(dots), and analyzed rock samples (triangles) in the Fortymile area. 

Area included on map is the Eagle quadrangle.

ysis in which the results are reported to the nearest number in 

the series 1, 0.7, 0.5, 0.3, 0.15, 0.1 and so forth. The rock samples, 

199 in number, were similiarly analyzed. The complete analyses 

of these samples with histograms, computed means, and other 

statistical data are available in an open-file report (Foster and 

Clark, 1969). All sample numbers used in this report correspond 

to those in the open-file report.

Most of the stream-sediment sampling was done in June when 

the streams were high. A resampling in some streams later in 

the summer when streamflow was near normal did not show a 

significant difference in the concentration of metals. The majority 

of stream-sediment samples were collected in the active channel 

of small tributaries, some of which are too small to show on the 

map (fig. 2). Where the tributary is not shown the dot or the 

center of the symbol showing the sample location is offset from 

the main stream. Where possible fine sand and silt were collected. 

The samples were dried, sieved, and the minus 80 mesh fractions 

were analyzed.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M7

The upper limits of background for the indicator elements of 

the following discussion are based on the element distribution in 

several hundred stream-sediment samples collected in the Fortymile area over the past few years. The limits are subjective, however, and must be evaluated in terms of the geology of any given 

part of the area. Histograms derived from computer analysis (fig. 

3) are used to infer probable normal values and to help determine 

anomalous values. The upper limit for background is arbitrarily 

chosen as 50 ppm (parts per million) for lead, 100 ppm for copper, 

100 ppm for nickel, and 200 ppm for chromium. The local background values of chromium and nickel are probably higher in 

areas of greenstone, greenschists, and ultramafic rocks. In most 

of the Fortymile area these rocks are only a small part of a wide 

variety of rocks which crop out, and until the geology is known 

in sufficient detail to establish local background limits the figures 

given above are used. In many places, values for nickel and chromium which are near the threshold values (upper limit of background) indicate a local abundance of greenstone or a somewhat 

more distant ultramafic rock, whereas a value of 500 ppm or above 

for chromium may indicate the presence of nearby ultramafic rock.

The limit of determination for silver by the semiquantitative 

spectographic method used is 0.5 ppm. Only 15 of 448 streamsediment samples had 0.5 ppm or more of silver; in 75 samples 

silver was detected but was below the limit of determination (table 

1). Most of the samples in which silver was detected in amounts 

below 0.5 ppm were collected from only a few streams, and silver 

occurred rather consistently in the samples from these streams. 

In contrast, silver was not detected in most of the streams in the 

Fortymile area. Thus, the presence of silver, even in such small 

amounts may have significance, and is shown on the distribution 

maps.

Only 11 samples contained 0.02 ppm or more of gold, so even 

values reported as 0.02 ppm are indicated on the maps. However, 

the accuracy of the instrumental reading for gold at this low value 

is not reliable and values reported as 0.02 ppm should be used 

with caution. The limit of determination for molybdenum is 5 

ppm and only 16 of 448 samples had 5 ppm or more molybdenum. 

Because molybdenum was detected (but below 5 ppm) in only 

50 samples, detected molybdenum is considered sufficiently uncommon to show on some of the distribution maps, particularly where 

it is in association' with other metals which are present in amounts 

above background values.
M8 CONTRIBUTIONS TO ECONOMIC GEOLOGY

25-

20-

1 15-

LU

0.

10-

5- 

n-

-JT

 1 25-

20-

- 15-

LU

o

oc.

LU

 10-

5- 

i i ftu-> o o o o o o o o

-rf ^

PARTS PER MILLION 

Chromium

g § 5 7 10 15 20 30 50 70 100150

PARTS PER MILLION 

Copper

40-

35-

30-

25-

20-

15-

10-

5-

n-_r_r

 

 1

1

7 10 15 20 30 50 70 100 150

PARTS PER MILLION 

Nickel

35-

30-

25-

ISM10 15 20 30 50 70 100 150

PARTS PER MILLION 

Lead

FIGURE 3. Percentage frequency distribution of chromium, copper, 

nickel, and lead for 448 stream-sediment samples from the Fortymile area. Histograms plotted by computer. Percentage frequency 

is rounded to the nearest 1 percent.
M9

TABLE 1. Selected statistical data for 448 stream-sediment samples from

the Fortymile area

Element

Silver

Gold

Bismuth ___

Antimony _

Chromium _ _

Copper

Molybdenum

Nickel _

Lead _

Tungsten

Tin _

Zinc _ _

Value,

Min

20.5

2200

20.02

25

2100

7

5

25

5

10

270

10

2200

in parts per million

Geometric 

Max mean

1.5 __

<200

0.3

5 __

<100

2000 108

150 29

7

200 54

150 22

200

10 _ ..

700

Percentage of

which element 

was found1

20

7

"3

1

1

100

100

15

100

100

2

<1

on

'Includes samples in which value is below limit of determination, except gold. See footnote 8.

aLimit of determination.

3Samples containing 0.02 ppm or more.

Because of the high limit of determination for zinc (200 ppm) 

and arsenic (200 ppm) any detectable zinc and arsenic are considered anomalous. Because of the rarity of detection of tungsten 

at 50 ppm or above, bismuth at 10 ppm or above, tin at 10 ppm or 

above, and antimony at 100 ppm or above, their detection in any 

amount is considered anomalous.

Rock samples which were analyzed include mineralized specimens such as those high in visible sulfides, vein quartz without 

visible mineralization, rock from sheared, oxidized, and hydrothermally altered zones, and specimens of representative rock 

types (to help determine background values). Because of the wide 

variety of rock types analyzed, it is not practicable to set overall 

threshold limits. However, for simplicity on the distribution maps 

the same limits are applied to both rock and stream-sediment 

samples.

No localities with an abundance of strongly anomalous samples 

were discovered in the Fortymile area, but there are several localities with weak anomalies. Nine areas have been selected and 

studied these areas cover most parts of the Fortymile drainage 

area (see fig. 4) and the pertinent geology and analytical results 

are discussed for each area. If more complete numerical informa-
M10 CONTRIBUTIONS TO ECONOMIC GEOLOGY

64° 15

64" 00' L

15 20 MILES

FIGURE 4. Location of areas discussed in text. Outlines of areas correspond 

with maps showing the location of stream-sediment and rock samples 

(figs. 5-13).

tion is desired for any particular sample it can be obtained from 

the complete analysis in the open-file report (Foster and Clark, 

1969).

CHAMPION, BEAR, AND COMET CREEKS AREA

In the drainage area of Champion, Bear, and Comet Creeks, 57 

stream-sediment samples were collected (fig. 5). Values for lead 

in these samples range from 10 to 150 ppm and 16 samples (28 

percent) have 50 ppm or more. Zinc is found in about 14 percent 

of the samples. Chromium and nickel values are fairly high in 

about 22 percent of the samples, but copper and silver occur in 

anomalous amounts in less than 4 percent of the samples.

Eight rock samples collected north of Champion Creek 

(fig. 5) were analyzed. Three samples of diorite (samples 22, 23, 

and 24) containing visible sulfides do not have unusual concentrations of metals, but a limonite-stained zone near the margin of the 

diorite intrusion (sample 21), in which the rock has visible sulfide minerals, has 200 ppm bismuth. Marble rubble containing 

patches of an unidentified dark-gray mineral was present in several prospect pits. Analyses (samples 19 and 20) show high con-
RECONNAISSANCE, FORTYMILE AREA, ALASKA Mil

142° 00' 141° 30'

64° 30'

10U

0(25)

0(40)

EXPLANATION

o

Silver 

(less than 0.5-0.7 ppm)

Chromium 

(200-700 ppm)

D

Lead 

(50-150 ppm)

Sample containing element(s) in anomalous amounts not shown by symbols at 

left. Element(s) contained indicated by 

chemical symbol:

Cu, copper (100-150 ppm)

Mo, molybdenum (5-7 ppm)

Ni, nickel (100-150 ppm)

Zn, zinc (200-300 ppm)

Sample in which gold, chromium, copper, 

nickel, or lead are not found in anomalous amounts and arsenic, silver, molybdenum, antimony, tungsten, and zinc 

are not detected

Rock sample (discussed in text)

FIGURE 5. Location of stream-sediment and rock samples collected from 

the Champion, Bear, and Comet Creeks area. Anomalous amounts of 

selected metals in the stream-sediment samples are shown by symbol. 

Rock sample numbers are in parentheses.
M12 CONTRIBUTIONS TO ECONOMIC GEOLOGY

centrations of lead (>20,000 ppm), zinc (>10,000 ppm), copper 

(150 ppm), iron (>20 percent), and silver (0.7 ppm). A sample 

of serpentinized ultramafic rock (sample 25) had 1,500 ppm of 

both nickel and chromium.

Galena was found as float on a ridge south of Champion Creek 

(fig. 5). The sample (39) contains >20,000 ppm lead, >10,000 ppm 

zinc, "30 ppm silver, 150 ppm cadmium, 700 ppm copper, and >20 

percent iron. Four samples of schist collected south of Champion 

Creek were analyzed. A graphite schist (sample 45) contains 

1 ppm silver, 20 ppm molybdenum, and zinc is detected but is 

<200 ppm. Another graphite schist (sample 47) has 100 ppm 

copper and zinc is detected but is below 200 ppm. In a chlorite 

schist (sample 46), silver and zinc are detected in amounts below 

the limits of determination. A sample of greenstone (sample 41) 

contains more nickel (200 ppm) and chromium (700 ppm) than 

other metamorphic rock types in the area; ultramafic rock (sample 

42) associated with greenstone has 1,000 ppm nickel and 1,500 

ppm chromium. A quartz vein (sample 40) is barren.

Only the general geology of this area is known. Drainage of 

Bear and Comet Creeks is primarily from igneous intrusive rocks 

of granitic to dioritic composition. Narrow zones, a few feet wide, 

of oxidized and possibly hydrothermally alterated rocks occur. 

The contact of the intrusive rocks with the metamorphic country 

rocks is very irregular and complex. Small patches of intrusive 

rock rubble are surrounded by metamorphic rock rubble and small 

patches of metamorphic rocks are in the igneous terrane. Southern tributaries to Champion Creek primarily drain metamorphic 

rocks, including quartz-graphite schist, chlorite schist, quartzmuscovite schist, marble, quartzite, and some greenstone. At the 

locality of sample 39 (fig. 5), south of Champion Creek, galena 

float is believed to be near its place of origin, and this suggests 

the presence of a mineralized vein or veins in the metamorphic 

rocks. Some of the higher metal values found in the stream 

sediment from Champion Creek could be due to such veins, and 

also, to the occurrence of small masses and dikes of ultramafic 

rock.

Further exploration of this area is recommended because it is 

a contact zone of several igneous intrusions, it has favorable host 

rocks such as marble, it has ultramafic rock, and there are indications of some mineralized veins.

ALDER CREEK AREA

Analyses of stream-sediment samples from Alder Creek suggest 

that this area might be considered for further exploration, Silver
RECONNAISSANCE, FORTYMILE AREA, ALASKA M13

is detected in two-thirds of the samples, zinc is detected in more 

than three-fourths of the samples, and a few samples contain 

slightly anomalous amounts of other metals. Alder Creek 'is ex141° 30'

EXPLANATION

. O 

Silver 

(0.5 ppm or less)

O

Zinc 

Oess than 200-700 ppm)

Sample containing element(s) in anomalous amounts not shown by symbols at 

left. Element(s) contained are indicated by chemical symbol: 

As, arsenic (200 ppm or less) 

Cr, chromium (200-1,000 ppm) 

Cu, copper (100 ppm) 

Mp, molybdenum (less than 5-7 ppm) 

Ni, nickel (100-150 ppm) 

Pb, lead (50-150 ppm) 

Sb, antimony detected 

W, tungsten detected

FIGURE 6. Location of stream-sediment and rock samples collected from the 

Alder Creek area. Anomalous amounts of selected metals in the streamsediment samples are shown by symbol. Rock samples (numbers in parentheses) containing one or more elements in possibly anomalous amounts:

43. Graphitic phyllite; lead (50 ppm) and silver and zinc detected.

44. Graphitic schist;-silver and zinc detected.

45. Graphitic schist; silver (1 ppm), molybdenum (20 ppm), and lead (160 ppm).

46. Chlorite schist; silver and zinc detected.

47. Graphitic quartzite; copper (100 ppm) and zinc detected.

48. Garnetiferous greenstone; chromium (300. ppm), nickel (150 ppm), and silver detected.

67. Serpentinized ultramafic; chromium (2,000 ppm), nickel (1,000 ppm), and silver and 

arsenic detected.

68. Quartz-mica schist with sulfides; silver (0.7 ppm) and gold (5 ppm). 

71. Diorite or metadiorite; zinc detected.

73. Sheared quartzite; molybdenum (5 ppm) and silver and zinc detected.

74. Sheared and crushed rock; silver (0.5 ppm).

75. Sheared quartzite: silver, molybdenum, and zinc detected.

76. Gouge; silver, molybdenum, and zinc detected.

77. Sheared rock; zinc (300 ppm).

78. Quartzite; zinc detected.

80. Sheared marble and quartzite; silver (0.7 ppm) and zinc detected.

91. Mafic dike rock; chromium (700 ppm), silver and zinc detected.

378-276 O - 70 - 3
M14 CONTRIBUTIONS TO ECONOMIC GEOLOGY

ceptional in the consistency with which silver is detected in the 

samples. Sample locations and the distribution of anomalous 

amounts of metals are shown in figure 6.

Alder Creek drains an area that consists primarily of metamorphic rocks, of which marble is the most abundant rock type 

exposed along the creek. The south branch of Alder Creek heads 

in metamorphic terrane consisting mostly of marble, quartzite, 

quartz-mica gneiss and schist, and quartz-graphite schist. A few 

granitic dikes cut the metamorphic rocks, and a granitic intrusion 

crops out on the ridge south of the stream for a distance' of about 

3 miles. The tributaries from which samples 217 and 219 were 

taken drain the area of contact between the intrusion and meta64° 35'

2 MILES

EXPLANATION

o

. Silver

Chromium

FIGURE 7. Location of stream-sediment samples collected from North 

Fork showing anomalous amounts of chromium and silver. Some symbols 

are combined to show both metals. Other elements for these samples are 

given in table 2.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M15

morphic rocks. The tributary from which sample 203 was obtained (fig. 6) heads near a small mass of serpentinized ultramafic 

rock which crops out on the ridge above (Foster, 1969a).

Marble is the most abundant rock type between the two main 

branches of Alder Creek. The north branch of Alder Creek heads 

primarily in quartz-graphite schist, quartzite, mica-schist, and 

chlorite schist, and no intrusions are presently known in its drainage area. Further geological and geochemical work is needed to 

learn the source of the anomalous values in the stream sediments 

and to determine whether or not the Alder Creek area has economic mineral protential.

NORTH FORK AREA

Stream-sediment samples from the North Fork drainage area 

all contain amounts of several metals which are slightly above 

threshold values (fig. 7). Twelve out of 19 samples (63 percent) 

have detectable silver, 14 samples (73 percent) have detectable 

zinc, and 13 samples (68 percent) have detectable molybdenum as 

compared with 20, 26, and 17 percent, respectively, for these 

metals in all Fortymile area stream-sediment samples. Gold is 

found in two samples, and chromium, nickel, and lead have values 

slightly above threshold, values in many. Table 2 gives analyses 

for these samples and figure 7 shows the location of the samples.

The North Fork drains an area of greenschist facies metamorphic rocks consisting primarily of quartz-graphite and carbonaceous schists, quartz-muscovite schist, and quartzite. Some 

greenstone and at least one ultramafic mass crop out. No intrusions other than the ultramafic one are known. There are several 

small zones of crushed and possibly hydrothermally altered rock. 

Much of the area is covered by colluvium and vegetation, and the 

geology is not well known. The 2,000 ppm of chromium in sample 

159 may be from ultramafic rock known to crop out about 1 mile 

upstream. Other values for chromium and nickel which are slightly 

above background may be due to greenstones or ultramafic rocks. 

The source of other metals which are in amounts slightly greater 

than background is not known, but it could be from the altered 

rock along shear zones.

NORTH PEAK AREA

Independence Creek and an unnamed stream drain southward 

from the vicinity of North Peak to the North Fork of the Fortymile River (fig. 8). In the sediments of the unnamed stream, silver 

was detected in eight of 13 samples; gold (0.04 ppm) was detected 

in one; molybdenum, in two; zinc, in three; and 11 contained 50
M16 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 2. Semiquantative spectrographic analyses and gold analyses of 

stream-sediment samples from the North Fork drainage area

[Results are given in parts per million. N, element not detected; L, below limit of determination. Limits of determination: Ag, 0.5 ; As, 200 ; Au, 0.02 ; Mo, 5 ; and Zn, 200. Analysts: 

Elizabeth Martinez and K. J. Curry, spectrographic; R. L. Miller and A. L. Meier, gold 

by atomic absorption. Results of analyses of samples (except 977) for 29 elements in 

Foster and Clark (1969)]

Sample 

No.

152 _ _ _

153

154 _ .

155 _

156

157

158

159

160 - -

161

162

163

164

165

166

167

168

169

977

Ag

L

L

_ L

N

N

L

L

.5

 _ N 

L

 _ N

L

. N

L

_ N

N

_ L

_ L

L

As

N

N

N

L

L

N

L

N

 .N

 N

N.

N

N

N

N

L

L

N

L

Au

L

.3

L

L

L

L

L

L

L

L

L

L

L

L

L

L

L

.04

L

Cr

150

200

300

200

300

150

150

2,000

200

200

200

200

200

150

200

200

200

200

150

Cu

50

70

70

50

50

50

70

70

50

70

50

50

30

50

50

70

50

70

100

Mo

N

N

N

L

L

' N

5

L

L

5

L

5

5

N

5

L

L

L

N

Ni

100

100

150

100

70

100

70

100

100

100

70

70

100

70

100

100

70

100

50

Pb

20

15

20

50

50

50

50

70

15

50

30

15

50

50

30

50

150

20

30

Zn

L

N

N

500

L

L

L

200

L

L

N

L

L

L

200

200

N

N

L

ppm or more lead. None of the sediments from Independence 

Creek, which more or less parallels the unnamed stream, contain 

silver, .gold, or molybdenum, but some contain chromium, nickel, 

lead, and zinc in amounts slightly above background values. The 

distribution of anomalous values for some of the metals is shown 

in figure 8.

The streams of this area primarily drain metamorphic rocks 

which include amphibolite, quartz-mica schist, quartzite, quartzgraphite schist, and greenschist. The metamorphic rocks are intruded by granodiorite and diorite and possibly more mafic rocks. 

Locally, the diorite is epidotized. Specks of sulfide minerals are 

abundant locally in the diorite and adjacent metamorphic rocks. 

Felsic rock with euhedral quartz phenocrysts also intrudes the 

metamorphic rocks.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M17

14 2'00'

64° 45'

North Peak

10 MILES

o

Silver 

(0.5 ppm or less)

Chromium 

(200-700 ppm)

D

Lead 

(50-100 ppm)

EXPLANATION

Sample containing element(s) in anomalous amounts not shown by symbols at 

left. Element(s) contained are indicated by chemical symbol:

Au, gold (0.04 ppm)

Cu, copper (100 ppm)

Mo, molybdenum detected

Ni, nickel (100-200 ppm)

Zn, zinc detected

Sample in which gold, chromium, copper, 

nickel, or lead are not found in anomalous amounts and arsenic, silver, molybdenum, antimony, tungsten, and zinc 

are not detected

FIGURE 8. Location of stream-sediment samples collected from the North 

Peak area. Anomalous amounts of selected metals in the samples are 

shown by symbols.
M18

64° 40' -

CONTRIBUTIONS TO ECONOMIC GEOLOGY

143°00' 142° 30'

h

5 MILES

EXPLANATION

o

Silver detected

Nickel 

(100-150 ppm)

D

Lead 

(50-70 ppm)

Sediment sample from spring containing 

gold (0.02 ppm) or zinc detected

Rock sample

Sample containing element(s) in anomalous amounts not shown by symbols at 

left. Element(s) contained are indicated by chemical symbol:

As, arsenic detected

AU, gold (0.02 ppm)

Cr, chromium (300 ppm)

Mo, molybdenun (5 ppm or less)

W, tungsten detected

Zn, zinc detected

Sample in which gold, chromium, copper, 

nickel, or lead are not found in anomalous amounts and arsenic, silver, molybdenum, antimony, tungsten, and zinc 

are not detected

FIGURE 9. Location of stream-sediment, spring-sediment, and rock samples 

collected from the Gold Run area. Anomalous amounts of selected metals 

in the samples are shown by symbol-. No numbers are given for springsediment samples; rock samples (numbers in parentheses) containing one 

or more elements in possibly anomalous amounts:

1, 2. Hypabyssal type granitic rock; molybdenum (5 ppm), lead (50 ppm), and silver 

detected.

3. Hypabyssal type granitic rock; molybdenum (10 ppm) and arsenic detected.

4. Conglomerate; gold (0.02 ppm) and arsenic detected.

5. Basalt; chromium (700 ppm), nickel (150 ppm), and arsenic and zinc detected.

6. Basalt; gold (1.6 ppm), chromium (1,500 ppm), copper (100 ppm), nickel (500 ppm), 

and silver and molybdenum detected.

7. Quartzite; arsenic detected.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M19

These streams drain a considerable area that crosses contacts, 

possibly mineralized, between the various igneous and metamorphic rocks. The nickel and chromium values which are above 

background may be related to greenstone and (or) ultramafic 

rock. Some other anomalous values in other metals may come from 

small amounts of sulfide minerals scattered throughout the metamorphic rocks.

GOLD RUN AREA

The Gold Run area has some weakly anomalous amounts of 

silver, gold, tungsten, and other metals in both stream sediments 

and rocks (fig. 9). The anomalous values are both small and inconsistent, but Gold Run has produced considerable placer gold 

and the geology of the area indicates a possibility of Tertiary (?) 

mineralization. The area is a dominantly metamorphic terrane 

intruded by granitic rocks, but- locally both fine-grained felsic and 

malfic hypabyssal and volcanic rocks occur. Between the head 

of Gold Run and Ruby Creek a small area of basalt is in contact 

with Tertiary (?) conglomerate on both its eastern and western 

sides. A random sample of the fairly fresh basalt (sample 6) collected about 500 feet from its eastern contact contains 1.6 ppm 

gold. Conglomerate (sample 4) adjacent to the basalt on the west 

contains 0.02 ppm gold. Gold is also present (0.02 ppm) in a 

stream-sediment sample taken near the mouth of Ina Gulch (sample 25) and from a hillside spring sediment (0.02 ppm) above 

Slate Creek. Arsenic is detected but is below 200 ppm in eight of 

14 bedrock samples and is 3,000 ppm in sample 13 (fig. 9). Silver 

is detected but in amounts below 0.5 ppm in three bedrock samples 

and in one soil sample, and sample 9 (fig. 9) contains 3 ppm of 

silver.

Stream-sediment samples from Gold Run contain amounts of 

lead, nickel, tungsten, zinc, molybdenum, and arsenic slightly above 

threshold values. Samples from other streams in the vicinity contain fewer metals in anomalous amounts (fig. 9). Slightly anomalous values for chromium (300 ppm) and nickel (150 ppm) in a 

sample (30) collected near the mouth of Happy New Year Creek 

may be due to ultramafic rock known to occur upstream. Further

8. Quartz vein; molybdenum (20 ppm).

9. Quartzite; silver (3 ppm), zinc (300 ppm), and arsenic and molybdenum detected. 

10, 11. Quartz vein; arsenic and tungsten detected.

12. Dike rock with visible sulfides; chromium (700 ppm).

13. Quartz vein; arsenic (3,000 ppm) and silver and molybdenum detected.

14. Quartz vein; copper (100 ppm) and arsenic detected.
M20

64° 30'

CONTRIBUTIONS TO ECONOMIC GEOLOGY

143°00'

5 MILES

EXPLANATION

D O 

Lead (50 ppm) Zinc detected

Sample containing element(s) in anomalous amounts not shown by above symbols. Elements(s) contained are indicated by chemical symbol:

Ag, silver detected

As, arsenic detected

Au, gold (0.02 to 0.08 ppm)

Mo, molybdenum (less than 5-50 ppm)

Sample in which gold, chromium, copper, 

nickel, or lead are not found in anomalous amounts and arsenic, silver, molybdenum, antimony, tungsten, and zinc 

are not detected

FIGURE 10. Location of stream-sediment samples collected from the 

Joseph area. Anomalous amounts of selected metals in the samples are 

shown by symbol.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M21

investigation of the relation between the mineralization and the 

shallow mafic and felsic intrusions and volcanic rocks is needed.

JOSEPH AREA

On the Joseph Creek drainage and on the Middle Fork above 

and below Joseph about two-thirds of the stream-sediment samples 

have one or more metals in amounts slightly above background 

values (fig. 10). Gold is found in five stream-sediment samples, 

and although it occurs in small amounts (0.02 to 0.08 ppm) it is 

found in more samples here than in any other drainage area sampled in the Fortymile area. The significance of the gold is not 

evident.

Joseph Creek primarily drains metamorphic terrane but some 

tributaries to Joseph Creek entering from the west cross contacts 

between Mesozoic(?) granitic rocks and the metamorphic rocks. 

In a single traverse crossing this intrusive contact no favorable 

indications for mineral deposits were observed.

Tributaries entering the Middle Fork from the south near 

Joseph cross contacts between metamorphic rocks and silicic igneous rocks which are shallow intrusive rocks and (or) volcanic 

rocks. They are similar to rocks in the Gold Run area and may 

be of Tertiary age.

HUTCHINSON CREEK AREA

On Hutchinson Creek drainage, including Montana Creek, and 

on Bullion Creek, silver is detected in 16 samples and zinc is detected in 27 samples of 32 stream-sediment samples (fig. 11). 

Although the amounts of silver and zinc found in the samples are 

low, they are detected much more consistently than in sediment samples from most other Fortymile streams. The only other streams 

in which silver is so consistently detected are Alder Creek and 

North Fork. The Hutchinson Creek area has produced placer 

gold in the past.

Hutchinson Creek heads in an area of metamorphic rocks cut 

by a few small Mesozoic(?) granitic intrusive masses and dikes. 

Montana Creek and other tributaries to the north are in an area 

of metamorphic rocks which are intruded by both felsic and mafic 

dikes and other small intrusions. Most of these intrusions are fine 

grained or porphyritic and may be shallow intrusives; some of 

the rocks may be volcanic. Bullion Creek flows across a complex 

of felsic and mafic rocks, probably mostly of volcanic origin. 

They are younger than the granitic intrusions of the type more 

common near the head of Hutchinson Creek. Many of the igneous 

rocks resemble those of the Gold Run area and may be Tertiary
M22 CONTRIBUTIONS TO ECONOMIC GEOLOGY

142°00'

64° 15'

5 MILES

EXPLANATION

o

Silver detected

O 

Zinc detected

Sample containing element(s) in anomalous amounts not shown by symbols at 

left. Element(s) contained are indicated by chemical symbol:

Cr, chromium (200 ppm)

Mo, molybdenum detected

Pb, lead (50 ppm)

Sample in which gold, chromium, copper, W, tungsten (200 ppm)

nickel, or lead are not found in anomalous amounts and arsenic, silver, molybdenum, antimony, tungsten, and zinc Rock sample 

are not detected

FIGURE 11. Location of stream-sediment and rock samples collected from 

the Hutchinson Creek area. Anomalous amounts of selected metals in 

the samples are shown by symbol. Rock samples (numbers in parentheses) containing one or more elements in possibly anomalous amounts:

93. Quartz-biotite schist; zinc detected.

94. Quartz porphyry; arsenic (1,000 ppm), lead (100 ppm), tin (50 ppm), and

molybdenum and silver detected. 

96. Quartz porphyry; lead (150 ppm), molybdenum (7 ppm), tin (30 ppm), and arsenic,

silver, and zinc detected.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M23

in age. The distribution of metals in the sediments suggests that 

the mineralization may be associated with these younger igneous 

rocks. A quartz porphyry in a saddle about 30 feet wide on the 

ridge north of Montana Creek (samples 94 and 95, fig. 11) is unique 

in having 30 to 50 ppm tin; the rock also contains lead (100 to 

150 ppm), detectable arsenic (<200 ppm to 1,000 ppm), silver 

(<0.5 ppm), and molybdenum (<5 to 7 ppm). The quartz porphyry may be intruded along a fault. Although this area was 

rather thoroughly prospected in the early 1900's, more detailed 

stream-sediment sampling coupled with further geologic reconnaissance may be warranted to determine the significance of the 

silver and zinc distribution.

MY AND OUR CREEKS AREA

Several bedrock prospects for silver and one for antimony in 

the vicinity of My and Our Creeks in the Eagle A-5 quadrangle 

have been previously checked by several investigators (Saunders, 

1967, p. 24-25, 57). Because this remote area is largely covered 

by timber and brush and is difficult to prospect, it was hoped that 

geochemical studies of stream sediment might give some indication of the degree and extent of mineralization. Fortyfive streamsediment samples were collected and analyzed, and about half of 

them contain amounts of one or more metals which are slightly 

above background values. The location and metal content of these 

samples are shown in figure 12.

My Creek, Our Creek, and Norvell Creek all head in and drain 

an area of metamorphic rocks consisting primarily of quartzbiotite gneiss and schist, quartz-graphite schist, quartzite, marble, 

amphibolite, greenstone, and greenschist. The metamorphic rocks 

are in contact with a large granitic intrusive along the west side 

of a ridge near the head of these streams. The intrusive is epidotized and has local altered zones near the contact. A few dikes and 

small granitic masses (probably mostly diorite and syenite) and 

quartz veins were noted in the metamorphic country rock west of 

the contact.

The stream-sediment samples do not seem to indicate particularly promising or extensive mineralization of the area. However,

96. Biotite schist; copper (300 ppm) and silver and zinc detected.

97. Felsic hypabyssal rock; lead (70 ppm) and silver detected.

98. Felsite; molybdenum (5 ppm), lead (100 ppm), and tin detected.

99. Gabbro dike; lead (150 ppm) and zinc detected.

100. Diorite (?); zinc detected.

101. Diorite; lead (100 ppm) and zinc detected.

102. Diorite porphyry; lead (160 ppm) and arsenic detected.

103. Quartz vein; arsenic detected.
M24 CONTRIBUTIONS TO ECONOMIC GEOLOGY

the stream-sediment samples were collected in early June when 

water was high and snow and ice still bordered the streams, so 

they may not be as representative as samples collected under better conditions. Nevertheless, there are a few samples containing 

metals in high enough amounts to suggest places where further 

sampling and exploration might be desirable such as the lower part 

of the unnamed creek north of Norvell Creek (sample 382).

EXPLANATION

Chromium 

(200-300 ppm)

o

Zinc 

(less than 200-300 ppm)

Chromium (200-300 ppm) 

and zinc (less than 200-300 ppm)

Sample containing element(s) in anomalous amounts not shown by above symbols. Element(s) contained are indicated by chemical symbol:

Ag, silver (1.5 ppm)

Mo, molybdenum (5 ppm)

Ni, nickel (100-150 ppm)

Pb, lead (50 ppm)

Sample in which gold, chromium, copper, 

nickel, or lead are not found in anomalous amounts and arsenic, silver, molybdenum, antimony, tungsten, and zinc 

are not detected

Rock sample

FIGURE 12. Location of stream-sediment and rock samples collected from 

the My and Our Creeks area. Anomalous amounts of selected metals in the 

samples are shown by symbol. Rock samples (numbers in parentheses) 

containing one or more elements in possibly anomalous amounts:

119. Quartzite breccia; silver (1.5 ppm), copper (100 ppm), lead (1,000 ppm), zinc 

(3,000 ppm), and arsenic detected.

121. Quartz vein; silver (0.7 ppm) and lead (150 ppm).

122. Galena and quartz from vein material of prospect pit; silver (300 ppm), gold 

(0.04 ppm), copper (1,500 ppm), lead{>20,000 ppm), cadmium (>500 ppm), zinc 

010,000 ppm), and arsenic and tungsten detected.

123. Galena and gangue from vein material in prospect pit; arsenic (200 ppm), silver 

(1,000 ppm), gold (0.04 ppm), copper (1,500 ppm), and cadmium (500 ppm).

124. Stibnite from auartz vein in prospect pit; antimony (>10,000 ppm) and silver «0.2

ppm) detected, and gold possibly detected. 

138. Gabbro with specks of sulfide minerals; silver (0.7 ppm), lead (300 ppm), and zinc

(500 ppm). 

140. Greenstone; lead (150 ppm), zine (300 ppm), and arsenic detected.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M25

Mineralization appears to be primarily in quartz veins cutting 

the metamorphic rocks. The presence of minor amounts of gold, 

silver, and copper and considerable lead and zinc is confirmed in 

selected samples from the silver prospects (samples 122, 123; fig.

^ 12). One sample (122) also contains cadmium. One quartz vein 

(sample 121) north of Norvell Creek contains minor amounts of 

silver (0.7 ppm) and lead (150 ppm). Other quartz veins sampled 

in the area are barren.

Some of the metallic values in the stream-sediment samples 

may be coming from scattered specks of sulfide minerals in green-

* schists and greenstone. The chromium and nickel may be from 

greenstone and greenschist possibly from ultramafic dikes, although ultramafic rocks have not yet been observed nearby.

KECHUMSTUK AND GOLD CREEKS AREA

Geochemical studies were undertaken in the Kechumstuk and 

Gold Creeks drainage area because this area is favorably located 

in regard to igneous intrusions, has some placer gold, has favorable host rocks, and there are several prospects in the area 

(Saunders, 1967, p. 25). Of 27 stream-sediment samples collected 

from the Kechumstuk Creek drainage area (fig. 13) only nine have 

metal values slightly above background. Zinc (<200 ppm) is 

detected in seven samples and molybdenum (<5 ppm) is detected

y in three. The content of chromium is "300 ppm in one sample.

Of 20 samples from the Gold Creek drainage area three have 

detectable zinc (<200 ppm) and one has 70 ppm lead. Near the 

head of Gold Creek, silver is detected in samples of quartz monzonite (sample 114), diorite porphyry (sample 111), and greenschist (sample 116). On a ridge near the mouth of Gold Creek,

^ greenstone (sample 147) interlayered with marble and apparently 

intruded by a gabbroic rock has detectable silver and 200 to 700

> ppm copper.

The stream-sediment sampling gives little indication of significant mineralization in the Kechumstuk and Gold Creeks area. 

However, favorable geology and known small mineralized areas

> (Brooks and others, 1912, p. 213; Saunders, 1967, p. 25, 26) including the Mitchell Prospect (Saunders, 1967, p. 25) (see table 

3), suggest that some further checking should be done to determine if stream-sediment sampling truly reflects the mineral potential of the area.

CONCLUSIONS

A study of analyses of 448 stream-sediment samples from the 

( Fortymile area suggests several areas which have not been care-
M26

64° 15'

CONTRIBUTIONS TO ECONOMIC GEOLOGY

143° 00' 142° 30'

<117>o(W4)o<m> 

28

Mount 

Veta

64° 00' L

(138,139)

0(141)

5 MILES

EXPLANATION

A

Chromium (300 ppm)

O 

Zinc detected

Sample containing element(s) in anomalous amounts not shown by above symbols. Element(s) contained are indicated by chemical symbol:

Mo, molybdenum (less than 5-7 ppm)

Pb, lead (50-70 ppm)

Sample in which gold, chromium, copper, 

nickel, or lead are not found in anomalous amounts and arsenic, silver, molybdenum, antimony, tungsten, and zinc 

are not detected

Rock sample

FIGURE 13. Location of stream-sediment and rock samples collected from 

the Kechumstuk and Gold Creeks area. Anomalous amounts of selected 

metals are shown by symbol. Rock samples (numbers in parentheses) 

containing one or more elements in possibly anomalous amounts:

111. Diorite porphyry; silver detected.
RECONNAISSANCE, FORTYMILE AREA, ALASKA M27

fully prospected recently that may be worthy of further sampling 

and geologic exploration; the Champion Creek and Alder Creek 

areas are two of the most promising. The Champion Creek area 

not only has anomalous values in stream-sediment samples, but 

it also has known mineralized bedrock. This preliminary sampling 

suggests that although lead and zinc are present, the minerals 

may not contain much silver.

The large number of samples from Alder Creek that contain 

silver as well as other metals slightly above background suggests 

that this area should receive further consideration. The geology 

of the Alder Creek area is not well known and little recent prospecting has been done there. Because the area is mostly tree and 

brush covered, additional geochemical sampling might be helpful.

The drainage area of North Fork and drainage from North 

Peak have weak unexplained anomalies. They may be due, at least 

in part, to widely scattered specks of sulfide minerals occurring 

in the metamorphic rocks but which are not necessarily associated 

with mineral deposits. The Gold Run, Joseph, and Hutchinson 

Creek areas, formerly mined for placer gold, have minor indications of Tertiary (?) mineralization which have not been thoroughly checked. No new encouraging indications of mineralization were found in the My Creek, Kechumstuk, or Gold Creek 

areas, but geochemical and geologic work and prospecting is still 

not sufficient for a final evaluation of these areas.

The present geochemical studies in the Fortymile area have 

not conclusively indicated either the presence or the absence of 

undiscovered gold placer deposits. Gold can commonly be panned 

from streams in which it is not found in the fine-grained streamsediment samples.

It is likely that the anomalies in some areas are due to sulfide 

minerals in greenschist and greenstones and are not indications

114. Quartz monzonite; silver detected. 

116. Greenschist; silver detected. 

. 117. Greenschist; chromium (500 ppm).

137. Quartz vein; silver (3 ppm), gold (0.06 ppm), molybdenum (50 ppm), and lead 

(150 ppm).

138. Gabbro; silver (0.7 ppm), lead (300 ppm), and zinc (500 ppm).

139.

141.

142.

143.

144.

145.

146.

147.

148.

Greenstone

Greenstone

Greenstone

Greenstone

Greenstone

Greenstone

Greenstone

Greenstone

Greenstone

(silver 0.7), lead (300 ppm), and zinc (500 ppm).

chromium (200 ppm) and copper (100 ppm).

arsenic detected.

; chromium (700 ppm), copper (100 ppm), and nickel

arsenic detected.

molybdenum (7 ppm) and zinc detected.

tin (10 ppm).

copper (200 to 700 ppm) and silver detected.

chromium (500 ppm) and nickel (150 ppm).

(100 ppm).
M28 CONTRIBUTIONS TO ECONOMIC GEOLOGY

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RECONNAISSANCE, FORTYMILE AREA, ALASKA M29

of mineral deposits. Some other metamorphic rocks such as 

quartzites also have locally abundant scattered specks of sulfide 

minerals. The distribution of chromium and nickel in the stream 

sediments may be helpful in locating ultramafic rocks. Although 

these rocks may not contain commercial quantities of nickel or 

other metals they should be checked as soon as possible sources 

of asbestos.

REFERENCES CITED

Brooks, A. H., and others, 1912, Mineral resources of Alaska, Report on progress of investigations in 1911: U.S. Geol. Survey Bull. 520, 360 p.

Burand, W. M., 1968, Geochemical investigations of selected areas in the 

Yukon-Tanana region of Alaska, 1965 and 1966: Alaska Div. Mines and 

Minerals Geochem. Rept. 13, 51 p.

Foster, H. L., 1967, Geology of the Mount Fairplay area, Alaska: U.S. 

Geol. Survey Bull. 1241-B, 18 p.

_____ 1969a, Asbestos occurrence in the Eagle C-4 quadrangle, Alaska: 

U.S. Geol. Survey Circ. 611, 7 p.

1969b, Reconnaissance geology of the Eagle A-l and A-2 quadrangles, Alaska: U.S. Geol. Survey Bull. 1271-G. 

Foster, H. L., and Clark, S. H. B., 1969, Analyses of stream-sediment and 

rock samples from the Fortymile area, Alaska: U.S. Geol. Survey 

open-file rept.

Foster, H. L., and Keith, I. C., 1968, Preliminary geologic map of the Eagle 

B-l and C-l quadrangles, Alaska: U.S. Geol. Survey open-file rept.

Mertie, J. B., Jr., 1937, The Yukon-Tanana region, Alaska: U.S. Geol.

Survey Bull. 872, 267 p. 

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