GEOLOGICAL SURVEY CIRCULAR 734

The Alaskan Mineral Resource Assessment 

Program: Background Information to 

Accompany Folio of Geologic and Mineral 

Resource Maps of the Tanacross Quadrangle, 

Alaska
The Alaskan Mineral Resource Assessment 

Program: Background Information to 

Accompany Folio of Geologic and Mineral 

Resource Maps of the Tanacross Quadrangle, 

Alaska

By H. L. Foster, N. R. D. Albert, D. F. Barnes, G. C. Curtin, 

Andrew Griscom, D. A. Singer, and J. G. Smith

GEOLOGICAL SURVEY CIRCULAR 734

1976
United States Department of the Interior

THOMAS S. KLEPPE, Secretary

Geological Survey

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Foster, Helen Laura, 1919-

The Alaskan mineral resource assessment program.

(Geological Survey Circular 734)

Bibliography: p. 20

1. Geology Alaska Tanacross region. 2. Mines and mineral resources Alaska Tanacross

region. I. Foster, Helen Laura, 1919- II. Series: United States Geological Survey

Circular 734. 

QE75.C5 no. 734[QE84.T27] 557.3'08s[553'.09798] 76-608228

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eads Street, Arlington, VA 22202
CONTENTS

Abstract____________________________________________ 1

Introduction __ ___________ ________ _________ ________________ 1

Purpose and scope __ ____ _________ _________ _______ 1

Geography and access _________________________ 1

Mineral production and exploration __________________________ 2

Acknowledgments _________________________________ 2

Geologic investigations ________________________________ 3

Previous geologic and mineral resource investigations ______ 3

Present study ._____________________________________ 3

Geology of the Tanacross quadrangle ________________ 4

Potassium-argon ages from the Tanacross quadrangle _________________-__ 5

Description of component maps of the Tanacross quadrangle folio _________ -_- __ 7

Geology (Map 1-593) ______________._________________________ 7

Felsic volcanic rocks ____________________ _________ ____ 7

Mafic volcanic rocks _ _____ __ ____ ___ ______ 10

Syenite of Mount Fairplay _____ ___________ _________________ 13

Geophysics _______________________________________________ 13

Aeromagnetic map and interpretation (Map MF-767A, 2 sheets) _ 13

Gravity map (Map MF-767B) ________________________________ 13

Interpretation of Landsat imagery (Map MF-767C, 3 sheets) ______________ 13

Land status (Map MF-767D) _________ .________ _________________ 15

Mineral resources (Map MF-767E) ________________________________ 15

Reconnaissance geochemistry (Maps MF-767F-O) ______ _____________ 17

Collection of samples--__________________________ 17

Description of sample media ___ ___ _ ___ _____ _______ _ ______ 17

Preparation and analysis of samples__ ___ _________ _____________ 18

Results _____________________________________________ 19

References_____________________________________________ 20

ILLUSTRATIONS

FIGURE 1. Index map showing location of Tanacross quadrangle, Alaska _____ _ ________ ______ -_ 2

2. Map showing physiographic provinces of the Tanacross quadrangle and its location in relation to major

faults ____________________________________________________________ 2

3. Map showing potassium-argon ages of rocks from the Tanacross and southern Eagle quadrangles 5

4. Index map showing localities where revisions are made to the Reconnaissance Geologic Map of the Tanacross

quadrangle, Alaska (Foster, 1970) _________________________________________ 8 

5-12. Maps showing:

5. Revisions in the northwest corner of the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) ____________________________________________ 9

6. Revisions in the west-central part of the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) ____________________________________________ 10

7. Revisions in the southwest corner of the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) _______________________-____________________ 11

8. Revisions in the north-central part of the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) _________________________________________-_ 12

m
FIGURES 5-12. Maps showing Continued page 

9. Revisions in the east-central part of the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) _____________________________________________ 12

10. Revisions in the southeastern part of the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) _____________________________________________ 13

11. Revisions in the south-central part of the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) _____________________________________________ 14

12. Revisions in the northeastern corner of the reconnaisance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) _____________________________________________ 15 

13. Explanation for figures showing corrections to the Reconnaissance Geologic Map of the Tanacross quadrangle,

Alaska (Foster, 1970) ___________________________________________________ 16

TABLES

Page

TABLE 1. Component maps of the Tanacross quadrangle mineral resource assessment ______________ ____ 1

2. Potassium-argon ages, Tanacross and southern Eagle quadrangles, Alaska _________________ _ 6

IV
The Alaskan Mineral Resource Assessment Program:

Background Information to Accompany Folio of

Geologic and Mineral Resource Maps of the

Tanacross Quadrangle, Alaska

By H. L. Foster, N. R. D. Albert, D. F. Barnes, G. C. Curtin, 

Andrew Griscom, D. A. Singer, and J. G. Smith

ABSTRACT

The Tanacross quadrangle, consisting of 17,400 km2 (6,700 

mi2) in east-central Alaska, was investigated by an interdisciplinary research team for the purpose of assessing the mineral resource potential of the quadrangle. This report provides 

the background information for a folio of maps on the geology, 

geophysics, reconnaissance geochemistry, Landsat imagery, 

and mineral resource evaluation of the quadrangle. Revisions 

to the previously published geologic map of the Tanacross 

quadrangle, 18 new potassium-argon ages, and a comprehensive bibliography are also included.

INTRODUCTION 

PURPOSE AND SCOPE

This circular, together with a separately available folio of maps of the Tanacross quadrangle, 

Alaska, is one of a series of U.S. Geological Survey reports intended to provide information for 

formulating a sound long-range national minerals policy to aid in Federal, State, and local landuse planning, to provide significant data for minerals explorations, and to increase the geologic 

understanding of the area. The work was carried 

out under the Alaskan Mineral Resource Assessment Program (AMRAP), authorized by Congress 

to begin on July 1, 1974.

The Tanacross mineral resource assessment 

consists of this circular, a previously published 

geologic map, geophysical and geochemical maps, 

interpretations of satellite imagery, and an 

analysis of the mineral endowment (table 1). 

Many of the basic data for this report were collected prior to July 1, 1974. Since then, an interdisciplinary team of scientists has carried on

additional field and laboratory studies. The Tanacross quadrangle is the result of a multidisciplinary study and evaluation in a very poorly 

exposed but highly complex geologic terrane 

which has had only reconnaissance-type geologic 

mapping.

GEOGRAPHY AND ACCESS

The Tanacross quadrangle covers approximately 17,400 km2 (6,700 mi2) in east-central

TABLE 1. Component maps of the Tanacross quadrangle 

mineral resource assessment

Map Subject

U.S. Geological Survey Miscellaneous 

Geological Investigations (It Map

1-593 (Foster, 1970) ____________________ Reconnaissance geology.

U.S. Geological Suri'ev Miscellaneous 

Field Studies (MF) Map

MF-767-A (Griscom, 1976)______________

B (Barnes, 1976) ______ 

C] (Albert and Steele, 1976)

D (Dusel, 1976) ___________________

E (Singer and others, 1976) _______

F (Curtin and others, 1976a)____

G (Curtin and others, 1976b)____. 

H (Curtin and others, 1976c) _______

I (Curtin and others, 1976d)____ 

J (Curtin and others, 1976e)____.

K (Curtin and others, 1976D ___. 

L (Curtin and others, 1976g)___. 

M (Curtin and others, 1976h)___. 

N (Curtin and others, 1976i) ___. 

O (Tripp and others, 1976) _ ____

Aeromagnetic map and interpretation. 

Gravity map. 

Interpretations of satellite

imagery. 

Land status. 

Mineral resources. 

Geochemical distribution and

abundance of copper. 

Geochemical distribution and

abundance of molybdenum. 

Geochemical distribution and

abundance of lead. 

Geochemical distribution and

abundance of zinc. 

Geochemical distribution and

abundance of arsenic and

mercury. 

Geochemical distribution and

abundance of tin. 

Geochemical distribution and

abundance of beryllium. 

Anomalous distribution of

copper and molybdenum. 

Anomalous distribution of

lead and zinc.

Mineralogical and geochemical data for heavy-mineral

concentrates.
FIGURE 1. Location of Tanacross quadrangle, Alaska.

Alaska (fig. 1) between lat 63° and 64° N. and long 

141° and 144° W. Three major physiographic 

provinces are included in the quadrangle: the 

rugged glaciated mountains of the eastern part of 

the Alaska Range, the plains and low rolling hills 

of the Northway-Tanacross Lowland, and the unglaciated, maturely dissected mountains of the 

Yukon-Tanana Upland (fig. 2) (Wahrhaftig, 

1965). Drainage is into the Tanana River, tributaries to the Fortymile River, and the Ladue 

River, all of which are directly or indirectly 

tributary to the Yukon River.

Elevations range from approximately 2,267 m 

(7,438 ft) above sea level in the Alaska Range 

to approximately 466 m (1,530 ft) in the 

Northway-Tanacross Lowland. Elevations in the 

Yukon-Tanana Upland range from about 520 m 

(1,700 ft) to 1,689 m (5,541 ft), Mount Fairplay 

being the highest point in the Yukon-Tanana Upland part of the Tanacross quadrangle.

The Alaska Highway crosses the southern third 

of the quadrangle in a northwesterly direction, 

and the Glenn Highway, from Anchorage, joins it 

at Tok. The Taylor Highway extends northward 

from Tetlin Junction through the east third of the 

quadrangle. The Taylor Highway, an all-weather 

gravel road, is generally closed during the winter 

months.

MINERAL PRODUCTION AND EXPLORATION

There has been no mineral production from the 

Tanacross quadrangle except for the quarrying of 

sand, gravel, fill, and other road building materials, used locally, and a small amount of ornamental building stone (Richter, 1967, p. 19).

A small amount of antimony was mined on

138°

100 150 200 KILOMETRES

EXPLANATION

Yukon - Tirana Upland Northiray 

FIGURE 2. Physiographic provinces of the Tanacross quadrangle and its location in relation to major faults.

Stibnite Creek in 1941 but was never transported 

from the area (Moffit, 1954, p. 207-208). Mining 

was also attempted there more recently without 

any significant production.

There has been considerable exploration for 

porphyry copper deposits since the late 1960's, 

and several localities have been drilled; as yet, 

there has been no development.

In 1975 gold placer claims were staked in the 

southeast corner of the quadrangle on tributaries 

to McArthur Creek after promising placer gold 

discoveries had been made nearby in the Yukon 

Territory. Although the gold potential of this area 

is uncertain, careful prospecting in previous 

years has not produced any significant finds on 

the Alaskan side of the border.

Commercial deposits of oil, gas, and coal are not 

known in the Tanacross quadrangle, and the likelihood of occurrence of these commodities is small.

ACKNOWLEDGMENTS

Many scientists have participated and contributed to the geologic mapping of the Tanacross
quadrangle. Also much assistance was given by 

local Alaskans and by U.S. Army and commercial 

helicopter crews. The assistance of some of these 

is acknowledged on the geologic map (Foster, 

1970) and in Foster (1967, p. B-2).

In the AMRAP field phase of the project special 

thanks are due Gerald Timmons, former Area 

Regional Manager of the Forty Mile Resource 

Area, Bureau of Land Management (BLM), and 

others at BLM Tanacross. Fortymile Air Service 

rendered invaluable assistance on many occasions. Special thanks and commendation are 

given to the late Dale Hefter of Hosking Helicopters Inc. for an exceptionally skillful job as 

helicopter pilot in the summer of 1974. Geologists 

from several mining companies have given assistance in the field and office and offered useful 

information.

The able and cheerful assistance of Alice M. 

Cantelow in the field and office and of Cynthia 

Dusel and Frances Wahl on processing manuscripts is much appreciated. Martha Yount 

helped in preparing the bibliography, using 

computer-assisted methods, and Frederick Wilson assisted in organizing the data on 

potassium-argon ages. Henry Berg, coordinator of 

AMRAP, has given continual assistance and guidance throughout the project.

GEOLOGIC INVESTIGATIONS

PREVIOUS GEOLOGIC AND MINERAL

RESOURCE INVESTIGATIONS

Lieutenant H. T. Alien initiated scientific observations and mapping in the Tanacross quadrangle on his remarkable traverse across the 

eastern Alaska Range and down the Tanana 

River from the mouth of the Tok River to the 

Yukon in 1885 for the U.S. War Department (Alien, 1887). Alien made the first reliable map of 

the Tanana River. The first geologic expedition 

into the Tanacross quadrangle was that of A. H. 

Brooks and W. J. Peters in 1898 when they ascended the White River, portaged to the headwaters of the Tanana River, and descended the 

Tanana to its confluence with the Yukon (Brooks, 

1900b). Also in 1899 Brooks traversed the quadrangle from south to north enroute to Eagle from 

Lynn Canal (Brooks, 1900a).

Systematic geologic study of the YukonTanana Upland was undertaken in 1903 by L. M. 

Prindle with other U.S.G.S. geologists, and in 

1905 Prindle made a lengthy traverse in the

Tanacross quadrangle (Prindle, 1909). J. B. Mertie, Jr., continued the geologic work in the Yukon-Tanana Upland and made a traverse in the 

Tanacross quadrangle in 1928. Mertie (1937) compiled the available geologic information on the 

Yukon-Tanana Upland, and this report has served 

as the basis for all further geologic work and is 

still the principal reference on parts of the upland. 

While Mertie was working in the upland, F. H. 

Moffit was carrying out geologic reconnaissance 

in the Alaska Range. His comprehensive summary report on the eastern part of the Alaska 

Range (Moffit, 1954) has long been the principal 

reference for that part of the Tanacross quadrangle in the Alaska Ranee.

PRESENT STUDY

Field and laboratory work for the present study 

has been carried out intermittently since 1961. 

From 1961 to 1964, some geologic reconnaissance 

work done by the U.S. Geological Survey in cooperation with the Office of the Engineer, U.S. 

Army, and the Defense Intelligence Agency resulted in the publication of a geologic map of the 

Tanacross C-3 and D-3 quadrangles (Foster, 

1967). In 1967 geologic reconnaissance mapping 

and a limited amount of geochemical sampling 

were accomplished. This work, combined with 

that done earlier in the decade, was published on 

the l:250,000-scale reconnaissance geologic map 

of the Tanacross quadrangle (Foster, 1970), which 

is also part of this study and that by Clark, Condon, Foster, and Hoare (1969).

The aeromagnetic map of the Tanacross quadrangle that is used for aeromagnetic interpretation in this study was compiled from a survey 

flown in 1971 for the State of Alaska Division of 

Geological and Geophysical Surveys (1973).

In July and August of 1974 a helicopter-supported geochemical reconnaissance was made of 

the quadrangle, rock samples were collected for 

potassium-argon age determinations, and some 

additional geologic observations were made. More 

than 1,000 samples, including stream sediments, 

rocks, vegetation, and heavy-mineral concentrates were analyzed by six-step semiquantitative 

spectrographic analyses and some by wet-chemical methods during the summer and following 

months. Eighteen new potassium-argon ages 

were determined on rocks from the Tanacross 

quadrangle, and minor revisions were made to 

the geologic map.
GEOLOGY OF THE TANACROSS QUADRANGLE

The Tanacross quadrangle is underlain primarily by a complex group of metamorphic rocks that 

have been intruded by Mesozoic and Tertiary 

granitic rocks and locally covered by volcanic 

rocks. The Denali fault, a major active dextral 

fault that crosses the southwesternmost corner of 

the quadrangle, separates this metamorphic 

terrane from a relatively unmetamorphosed 

Paleozoic and Mesozoic terrane to the south. The 

rocks south of the fault are contiguous with those 

of the Nabesna quadrangle and are described by 

Richter (1975). The metamorphic terrane underlies all three physiographic provinces of the 

Tanacross quadrangle: the Alaska Range, the 

Northway-Tanacross Lowland, and the YukonTanana Upland (fig. 2).

In the Alaska Range, the metamorphic grade 

increases from low greenschist facies in the south 

to amphibolite facies in the north (Foster, 1970). 

Metamorphic rocks in the Yukon-Tanana Upland 

are predominantly amphibolite facies but include 

rocks of greenschist facies. They were derived 

from both sedimentary and igneous parent rocks 

and include phyllite, greenstone, quartz-chlorite 

schist, quartz-mica-chlorite-epidote schist, 

quartz-mica schist, quartz-mica-garnet schist, 

quartzite, quartz-biotite gneiss, quartz-biotitegarnet-staurolite schist, augen gneiss, amphibolite, and recrystallized limestone. Recrystallized 

limestone that contains middle Devonian rugose 

and tabulate corals (Richter, 1975) in the northern part of the Nabesna quadrangle appears to be 

on strike with slightly metamorphosed limestone 

layers in the southern part of the Tanacross 

quadrangle. This relation suggests that at least 

some of the metamorphosed sedimentary rocks in 

the Tanacross quadrangle are Devonian in age, 

but no other age data from fossils are available.

The metamorphic rocks in the Alaska Range 

are intruded by dikes, sills, and small bodies of 

diorite and more mafic rock. The diorite has been 

altered and perhaps slightly metamorphosed. 

Grantitic intrusions are small and sparse in the 

Alaska Range. The largest one occurs near Yerrick Creek and is Late Cretaceous in age (fig. 3 

and table 2, No. 12).

The metamorphic rocks of the Yukon-Tanana 

Upland are extensively intruded by granitic plutons of mostly quartz monzonitic to granodioritic 

composition. The oldest intrusion, probably Late 

Triassic or Early Jurassic in age, is the Taylor 

Mountain batholith in the northern part of the

Tanacross quadrangle and southern part of the 

Eagle quadrangle (fig. 3; table 2, Nos. 2, 3). The 

other large intrusions are probably middle Cretaceous in age. The Mount Fairplay pluton is a 

complex Tertiary intrusion ranging in composition from syenite to granodiorite and quartz monzonite. Shallow felsic porphyritic intrusions, 

mostly hydrothermally altered, are probably of 

Tertiary age.

Volcanic rocks crop out in less than 10 percent 

of the quadrangle, but many may be associated 

with the igneous activity responsible for much of 

the mineralization. Felsic tuff, including welded 

tuff, is abundant, particularly in the area surrounding Mount Fairplay and on Sixtymile 

Butte. Some of the felsic tuff is associated with 

the shallow porphyritic felsic intrusive rocks ana 

in most places so intimately associated that they 

were not mapped separately but were included 

together in the felsic tuff. Most of the tuff is hydrothermally altered, and original textures and 

compositions commonly are difficult to impossible 

to determine. Small areas of more mafic tuff also 

occur. The volcanic rocks are tentatively considered Tertiary in age, but some in the vicinity of 

Mount Fairplay may be of Cretaceous age. There 

are also several areas of basaltic volcanic rocks of 

probable Tertiary and Pleistocene age and, in addition, the very young Holocene volcanic cone, 

Prindle Volcano. Prindle Volcano is of special 

interest because peridotite and granulite inclusions occur in the basaltic lava (Foster and others, 

1966).

The only sedimentary rocks present in the 

Tanacross quadrangle north of the Denali fault 

are nonmarine conglomerate, sandstone, shale, 

tuffaceous shale, and tuff of Late Cretaceous or 

early Tertiary age and Tertiary gravels that lie 

unconformably on metamorphic rocks (Foster, 

1967, p. B6).

The metamorphic rocks of the Tanacross quadrangle are part of a metamorphic complex that 

lies between the Denali fault and the Tintina 

fault (fig. 2). These rocks are complexly folded and 

faulted, but little is known of their structural and 

metamorphic history. The number of metamorphic episodes, for example, is unknown. However, 

it is clear that regional metamorphism occurred 

before the emplacement of the Taylor Mountain 

batholith in Late Triassic or Jurassic time, because well-foliated metamorphic rocks are incorporated in the nonfoliated granitic rock along the 

southern margin of the batholith. Foster and
10 20

i i r

10 20 30 40 KILOMETRES

EXPLANATION

A74ASJWO

Locality and locality number

88.0 ±29

Age in m.y. and mineral dated

(8, biotitt; H, hornblende; t», mutcovitt;*, ««nidint)

141*00*

FIGURE 3. Potassium-argon ages of rocks from the Tanacross and southern Eagle quadrangles.

Keith (1974) have proposed that the entire metamorphic terrane is an allochthonous slice of continental material that has moved northward. 

Tempelman-Kluit, Gordey, and Reed (1976, p. 

103-105) also postulate that the metamorphic 

terrane is allochthonous and suggest that it consists of a metamorphosed Paleozoic eugeosynclinal sequence that overlies an autochthonous 

miogeosynclinal succession.

POTASSIUM-ARGON AGES FROM THE 

TANACROSS QUADRANGLE

Eighteen new potassium-argon age determinations for 12 different rocks were made as a part 

of AMRAP, with emphasis on identifying the 

principal times of mineralization. When added to 

ages determined by others, a total of 26 potassium-argon ages is available on rocks from units 

in the Tanacross quadrangle and from similar
FIGURE 2. Potassium-argon ages, Tanacross and southern Eagle quadrangles, Alaska

[Ages were determined using standard isotope dilution techniques for extraction and measurement of argon and flame photometry for potassium (Dalrymple and Lanphere, 1969). For the new dates, the plus-or-minus value assigned to each is an estimate of the standard deviation of analytical precision (Cox and Dalrymple, 1967), 

together with an estimate of accuracy based on evaluation of the uncertainties in concentration of the 38Ar tracer and potassium measurements. Where analytical 

data and the plus-or-minus value for an age are missing, they were not reported by the original author.]

Sample No.

Location

quadrangle 

and latitude

Dated by longitude Rock type

Mineral

dated

Percent

K2 O

40 AM-ad 

(moles/gm xlO- 10 )

 >°, A"rad

(percent)

Age in

million years

plus or minus 

analytical

uncertainty

Igneous rocks

3

4

5

6.

7

8

9

10

11

1?

13

14

63AFr2C

61AE184

74ASJ113

74ASJ105

74ASJ143

61AE186 

74ASJ144

74ASJ110

(74AFr363) 

74ASJ111

(74AFr364) 

74ASJ140

H. Foster 3

(72128) 

74ASJ139

74ASJ114

74AFr926

1972.

and others, 1963." 

J. G. Smith,

1975.2

J. G. Smith,

1976.2

J. G. Smith,

1976.2

Wasserburg 

and others, 

1963. 1 

J. G. Smith,

1976.2 

J. G. Smith, 1976.2 

J. G. Smith, 1976. 2 

J. G. Smith, 1976. 2 

D. L. Turner, 1975.3 

J. G. Smith,

1975.2

J. G. Smith,

1975.2

J. G. Smith,

1976. 2

64°11'N, 

143°00'W.

64°01'N, 

142°05'W.

63°59'N,

142°09'W.

63°49'N,

63°44'N, 

145°01'W. 

Tanacross C-3; 

63°40'N, 

142°15'W.

63°39'N, 

142°15'W.

63°37'N,

63°29'N, 

141°08'W.

63°24'N, 

143°55'W.

63°20'N, 142°36'W

63°19'N,

143°30'W.

63°10'N,

142°06'W.

63°39'N,

porphyry.

Welded tuff ________

Alkali 

syenite.

monzonite.

quartz 

monzonite. 

Welded tuff ________

gabbro.

Biotite

intrusive.

Biotite ____ _

Amphibole ________ 

Biotite _ __ _

Biotite _ __ _

Biotite _____

do

do

1.465, 1.477

8.00, 7.99

1.011, 1.002

7.38, 7.50

1.138, 1.160

12.43, 12.54

8.64, 8.64

0.510, 0.496 

8.60, 8.60

8.54,- 8.64

9.09, 9.06

0.552, 0.555

7.85, 7.88

7.94, 7.94

8.00

8 03, 8.04

8.05, 8.19

8.20, 8.26

9.02, 8.95

4.035

24.10

2.989

7.2091

1.1647

17.29

17.23 

8.513

4.378 

13.72

7.263

8.859

1.121

10.48

11.70

8.656

95

93

86

80

54

92

87 

81

53 

91

67

74

87.6

94

92

86

177+5

180

194+5

191+5

64.5+3

67.4+3

91.6+3

91.2+3 

66

65.5+2

58±3 

105+3

56.4+2

65.0+2

132.6+4

87.6+3

95.0+3

64.1+2

Metamorphic rocks

15

16

17

18

19

20

89-65A

(72008) 

74ASJ137

69AChlOO

776-65

(72001)

61AE111

D. L. Turner, 1975.3 

J. G. Smith,

1976.2

and others, 

1963. 1 

D. L. Turner,

1975.3

1975. 2

and others, 

1963.1

Eagle A 1;

64°10'N, 

141°20'W. 

Eagle A 1;

141°20'W.

64°07'N, 

141°32'W.

64°05'N, 

141°39'W.

142°43'W.

63°13'N, 

143°03'W.

gneiss.

muscovite 

schist.

gneiss.

muscovite 

quartz schist.

______ do___ 

______ do______ 

Muscovite _____ _

______ do__-______

0.848, 0.857

9.11,9.14

9.192, 9.159

8.86, 8.97

8.99, 9.00

9.21, 9.22 

9.45, 9.48

9.58, 9.74

2.259

24.50

3.314

13.485

13.56

15.02

89.3

91

91

87.6

95

79

171.2+5.1

174+5

177+5

177

96.9+2.9

101+3

105±3

119

'Wasserburg, C. J., Eberlein, G. D., and Lanphere, M. A., 1963. 

2Smith, J. G., unpub. data, 1975. 

TTurner, D. L., oral commun., 1975.

rock units in the southern Eagle quadrangle 

(fig. 3, table 2).

The potassium-argon ages reveal a long history 

of metamorphism, intrusion, and volcanism. The 

available data suggest three periods of intrusion,

the younger two of which coincide with eruptive 

volcanic activity. Most of the mineralization in 

the quadrangle is associated with the youngest 

period of intrusion. 

The oldest intrusive event was the Early Juras6
sic (approximately 175-195 m.y. ago) emplacement of the Taylor Mountain batholith and syenite of Mount Veta. These plutons intruded a 

previously regionally metamorphosed biotite 

gneiss and schist terrane; xenoliths of gneiss are 

present in the Taylor Mountain batholith. Regionally this intrusive event is roughly equivalent in age to that of a diorite complex south of the 

Denali fault in the adjacent Nabesna quadrangle 

(Richter and others, 1975).

The next younger plutonic event took place in 

middle Cretaceous time (85-105 m.y. ago) and is 

not well established in the Tanacross quadrangle. 

The reasons for this uncertainty are that only a 

few plutons of middle Cretaceous age have been 

dated, the ages are scattered, and the outcrops 

sampled lack hornblende; thus ages based on 

coexisting mineral pairs could not be obtained. 

The plutons typically are coarse-grained porphyritic biotite quartz monzonite, locally poor in 

mafic minerals. In texture and petrography, they 

resemble the Gardiner Creek and Tok-Tetlin plutons in the adjacent Nabesna quadrangle (Richter 

and others, 1975) and the Coffee Creek Quartz 

Monzonite in the Yukon Territory (TemplemanKluit and Wanless, 1975). Plutons of similar age, 

composition, and petrography have been dated by 

Frederic H. Wilson (oral commun., 1975) from the 

adjacent Mount Hayes and Big Delta quadrangles. An age of 91 m.y. on sanidine from a welded 

tuff from near Mount Fairplay indicates that at 

least some of the plutons vented to the surface. 

The distribution of plutons of similar age and 

composition over more than 103,600 km2 (40,000 

mi2) indicates a major period of plutonic activity 

in this part of Alaska and adjacent Yukon Territory during middle Cretaceous time.

The youngest plutonic event was marked by intrusion of generally small isolated plutons, including hypabyssal stocks, and by the eruption of 

silicic and mafic volcanic rocks during the 

Paleocene (55-65 m.y. ago). The Mount Fairplay 

pluton and the hypabyssal stock associated with a 

copper prospect near McCord Creek were intruded at this time. Most of the volcanic rocks 

outside of the Mount Fairplay area were probably 

also erupted in the Paleocene. Preliminary data 

suggest that much of the hydrothermal alteration 

in McCord Creek and other areas that contain the 

altered felsic volcanic rocks took place at this 

same time.

The potassium-argon data on the metamorphic

rocks are more difficult to interpret than those on 

the igneous rocks. Most of the metamorphic rock 

samples were dated in an attempt to determine 

the pre-Mesozoic history of the Yukon-Tanana 

Upland. However, the ages show that the 

potassium-argon technique is unable to penetrate 

beyond the Mesozoic metamorphic events. Most of 

the Mesozoic ages on the metamorphic rocks 

probably reflect older ages that have been partly 

or completely reset by younger thermal events.

DESCRIPTION OF COMPONENT MAPS OF 

THE TANACROSS QUADRANGLE FOLIO

GEOLOGY (MAP 1-593)

A multicolor geologic map of the Tanacross 

quadrangle was published by Foster (1970). The 

fieldwork for this map was of a highly reconnaissance nature, and geochemical and geophysical 

data were not available at the time of compilation.

In the summer of 1974 under AMRAP, some 

additional traverses and geologic observations 

were made in parts of the Tanacross quadrangle, 

but the amount of new work is not sufficient to 

justify the publication of a revised geologic map. 

However, some revisions resulting from the 

summer's fieldwork appear on a series of maps in 

this circular (figs. 4-13), each map showing a part 

of the Tanacross quadrangle. Some areas, such as 

the Tanacross A-l, A-2, A-3, and A-4 1:63, 360- 

scale quadrangles, are not included, because no 

new fieldwork was done there.

Much of the quadrangle still has not been covered by "on the ground" observations and traverses. Additional geologic fieldwork would result 

in many more additions and changes to the geologic map. For example, contacts between granitic and metamorphic rocks cannot be detected 

from the air or from aerial photographs and can 

only be determined with confidence when ground 

observations are available. Many contacts are actually contact zones with a considerable admixture of several kinds of rock.

No change in the number or names of map 

units is attempted here, but additional information is given on the felsic and mafic volcanic rocks 

and the syenite of Mount Fairplay.

FELSIC VOLCANIC ROCKS

The felsic volcanic rock unit includes in some 

localities shallow felsic intrusive rocks, many of
64°

144 141'

FIGURE 4. Localities where revisions are made to the Reconnaissance Geologic Map of the Tanacross quadrangle, Alaska

(Foster, 1970). Diagonal lines indicate areas with revisions.

which are porphyritic and hydrothermally altered. The principal occurrences of such hydrothermally altered intrusive rocks are in the 

headwaters of McCord Creek in the C-l quadrangle, 3.2 km (2 mi) southwest of the bench 

mark "Dennis" in the B-4 quadrangle, southeast 

of the bench mark "Mosquito" in D-5 and D-6 

quadrangles, and locally near Mount Fairplay in 

the C-5 quadrangle.

Some of the intrusive rocks included in this 

unit are similar to those included in the granite 

porphyry, except that most of the former are more 

altered.

Rocks mapped as felsic volcanic rocks (Foster, 

1970) near McCord Creek include quartz latite, 

felsic tuff(?), quartz-feldspar and quartz-tourmaline rock, and porphyrytic hypabyssal rocks ranging in composition from quartz diorite to granite.

Some are felsic breccias. Quartz monzonite and 

granodiorite are mapped adjacent to this unit 

and, where highly altered, may be included in it. 

The rocks locally are much silicified, and sericite 

and clay minerals are abundant. Quartz phenocrysts commonly are embayed, and the feldspar 

phenocrysts strongly zoned. Felsic intrusive rocks 

probably cover a larger area than shown on the 

map in the McCord Creek area.

Rocks mapped as felsic volcanic rocks in the 

B-4 quadrangle (Foster, 1970) are mostly altered 

granitic porphyry with abundant highly strained 

quartz, zoned plagioclase, and microcline phenocrysts. Some rocks also have biotite and hornblende phenocrysts. Exsolution features and 

intergrowth are common in the feldspars of some 

rocks. Hydrothermal alteration is indicated by 

abundant epidote, chlorite, clay minerals, and
144°OO 

I I

5 KILOMETRES

FIGURE 5. Revisions in the northwest corner of the Reconnaissance Geologic Map of the Tanacross 

(Foster, 1970).
0 5 MILES

III I I 

T I I I I 

O 5 KILOMETRES

FIGURE 6. Revisions in the west-central part of the Reconnaissance Geologic Map of the Tanacross quadrangle, Alaska

(Foster, 1970).

sericite, and locally the rocks are silicified to various extents. Cataclasis is evident in different degrees in most of the rocks.

Original rock types are difficult to determine 

because of the extensive alteration, but they may 

include lava as well as hypabyssal intrusive 

rocks. Tuff has not been definitely identified but 

may be present.

Felsic volcanic rocks in the "Mosquito" area 

(Foster, 1970) occur mostly as small, isolated 

patches but in a few places cover an area of hundreds of square metres. They are more extensive 

in this area than indicated by the geologic map. 

These rocks are in contact with granitic and metamorphic rocks, but most contacts are obscure 

and are indicated by patches of rubble of mixed 

rock types. The most abundant rock type included 

in the unit in this area is porphyrytic with a 

fine- to medium-grained groundmass. Phenocrysts are euhedral and subhedral quartz, some of 

which is mildly strained, altered feldspar (both 

plagioclase and potassium feldspar), biotite, and 

hornblende. The rocks range from slightly altered 

to so completely altered that their original texture and composition cannot be determined. Sericite, clay minerals, and quartz, and chlorite are 

the most abundant alteration products, but limonite, carbonate, and other minerals also occur.

Tuff has not been definitely identified, although

spherulites and other textural features in some 

rocks indicate that they are extrusive.

Examples of large areas where the unit is composed dominantly of felsic volcanic rocks are Sixtymile Butte and the vicinity of Mount Fairplay 

(Foster, 1970). Sixtymile Butte is a thick pile of 

devitrified and altered welded tuff. The unit 

around Fairplay is composed of several kinds of 

altered tuff, lava, and breccia, including abundant altered welded tuff. A minor amount of shallow intrusive rock, mostly somewhat altered, is 

also included in the unit near Mount Fairplay.

From potassium-argon ages recently determined, it appears that rocks included in the felsic 

volcanic rock unit may be of several ages. For 

example, a single age on tuff (fig. 3; table 2, No. 5) 

north of Mount Fairplay is about 91 m.y., 

whereas welded tuff near the east margin of the 

quadrangle is about 56 m.y. (fig. 3; table 2, No. 

10). However, alteration and mineralization of 

the volcanic and intrusive rocks in the unit may 

have been a single event corresponding to one of 

the Tertiary ages.

MAFIC VOLCANIC ROCKS

The mafic volcanic rocks at the head of Big 

Creek (Foster, 1970) include both mafic lava and 

volcanic sediments. One or more small plugs, 

dikes, or sills and flows are in contact with vol10
FIGURE 7. Revisions in the southwest corner of the Reconnaissance Geologic Map of the Tanacross 

(Foster, 1970).
142°30'

FIGURE 8. Revisions in the north-central part of the Reconnaissance Geologic Map of the Tanacross quadrangle, Alaska

(Foster, 1970).

63°45' 141°30'

FIGURE 9. Revisions in the east-central part of the Reconnaissance Geologic Map of the Tanacross quadrangle, Alaska

(Foster, 1970).

12
0 SMILES

I I I I i I

0 5 KILOMETRES

FIGURE 10. Revisions in the southeastern part of the Reconnaissance Geologic Map of the Tanacross quadrangle, Alaska 

(Foster, 1970).

canic conglomerates and other volcanic sediments. 

Bedding has a northerly dip and northeasterly 

strike, although there are local variations. The 

volcanic sediments are well indurated and locally 

show baking along contacts with sills or flows. 

Elsewhere contact effects are not evident.

SYENITE OF MOUNT FAIRPLAY

The syenite of Mount Fairplay includes 

granodiorite in addition to the other rocks mentioned in the description of the unit (Foster, 

1970). Syenite may not be the dominant rock, and 

the coarsely porphyrytic type of syenite occurs 

only locally, mostly in the west-central part of the 

intrusion.

GEOPHYSICS

AEROMAGNETIC MAP AND INTERPRETATION 

(MAPS MF 767A, 2 SHEETS)

The aeromagnetic map (sheet 1) of the Tanacross quadrangle was flown in 1971 and subsequently released by the State of Alaska as an 

open-file map (Alaska Division of Geological and 

Geophysical Surveys, 1973). The variations in the 

magnetic field on maps such as these provide valuable information concerning the lateral and vertical extent of rocks containing various percentages of magnetic minerals, usually magnetite. 

Aeromagnetic maps thus are a most useful support for a geologic mapping program as well as for 

mineral resource assessment. An interpretive 

map (sheet 2) identifies several kinds of rocks in

the Tanacross quadrangle that possess characteristic magnetic anomalies and enables the interpreter to extrapolate geologic information 

from known areas into covered or inaccessible regions. In addition, the interpretive map portrays 

faults on the basis of linear interruptions in the 

magnetic patterns. By using the magnetic interpretation together with geologic information, 

certain areas are defined that may be favorable 

for gold or porphyry copper mineralization.

GRAVITY MAP (MAP MF-767B)

The gravity map provides a plot of nearly 150 

gravity measurements that have been made in 

the quadrangle and shows some 10-milligal contouring that was initially drawn for a regional 

compilation of the entire State. All of the measurements were made between 1959 and 1963, 

during the early stages of the State reconnaissance, and have been supplemented only by later 

base-station checks. Eighty percent of the measurements were made along the highway network; 

the remaining stations were reached by floatplane and helicopter.

The limited available data suggest that there 

are few large gravitational anomalies within the 

Tanacross quadrangle. The maximum range of 

gravity within the quadrangle is less than 50 milligals, most of which can probably be explained by 

a thinning of the crust below the low topography 

in the Northway-Tanacross Lowland relative to 

the thicker crust beneath the higher mountains of 

the Alaska Range and Yukon-Tanana Upland. 

The data also suggest that some of the plutons 

have gravitational expression and that detailed 

surveys might help to define the shape, size, and 

possibly the mode of emplacement of these bodies. 

Small gravity anomalies are also associated with 

low-density sediments underlying the flatter 

parts of the Tok, Tanana, and Mosquito-Fork 

River valleys.

INTERPRETATION OF LANDSAT IMAGERY 

(MAP MF-767C, 3 SHEETS)

Interpretation of Landsat data of the Tanacross 

quadrangle is based on (1) a black and white 

Landsat photomosaic of the State of Alaska compiled by the U.S. Department of Agriculture and 

(2) computer-enhanced black and white and color 

Landsat imagery processed by the U.S. Geological 

Survey in Flagstaff, Ariz., and by the Jet Propulsion Laboratory in Pasadena, Calif.

13
Tf Qas Tf PzpCb

J__I

142°30'

5 MILES

I I

5 KILOMETRES

FIGURE 11. Revisions in the south-central part of the Reconnaissance Greologic Map of the Tanacross quadrangle, Alaska

(Foster, 1970).

As a geologic mapping tool, Landsat imagery 

is probably most effective for reconnaissance 

studies. Remotely sensed information about 

geomorphology, structural features, and variations in spectral response of surficial materials, 

for example, have use in planning and directing 

geologic mapping and geochemical sampling. 

However, in areas such as the Tanacross quadrangle, where reconnaissance geologic mapping 

and geochemical sampling were completed prior 

to the availability of Landsat data, the Landsat 

imagery can be used to augment geologic and 

geophysical concepts by identifying possible fault 

extensions, identifying linear features not discernible in the field, analyzing joint and smallscale fracture patterns, and identifying structural

14
142°OO' 141°00'

-PfcpCb SMILES

I I I

I I I I I I

0 5 KILOMETRES

FIGURE 12. Revisions in the northeastern corner of the Reconnaissance Geologic Map of the Tanacross quadrangle, Alaska

(Foster, 1970).

features and relations that might have significance for mineral exploration.

Because most of the Tanacross quadrangle has 

a heavy cover of vegetation, variations in spectral 

responses for rock and soil types are difficult to 

assess. In addition, vegetation stresses (extraordinary changes in growth characteristics) possibly caused by anomalous concentrations of minerals are difficult to distinguish on the Landsat 

imagery from variations in vegetation caused by 

the numerous forest fire scars.

Direct applications of Landsat imagery to mineral resource assessment are still developing. As 

a result, interpretation techniques used in the 

Tanacross quadrangle are mostly experimental.

LAND STATUS (MAP MF-767D)

The land status map shows the classifications 

and boundaries of proposed and existing 

categories of land in the Tanacross quadrangle as 

of about December 1975. The map was compiled 

from a much smaller scale map prepared in 

March 1974 by the U.S. Bureau of Land Management. Although changes have now occurred 

primarily because of land selections made under 

terms of the Alaska Native Claims Settlement 

Act (ANCSA) of December 18, 1971, the map is

useful as a general guide until new maps can be 

compiled.

MINERAL RESOURCES (MAP MF-767E)

An assessment of the occurrence of mineral deposits in the Tanacross quadrangle by area and 

by deposit type is presented in the mineral resources map. Both known deposits and areas 

favorable for the occurrence of unfound deposits 

are delineated. The assessment is limited to the 

endowment of resources; whether the unfound 

deposits will at some time be discovered is not 

considered since that will depend upon the effectiveness and intensity of exploration.

Selection of favorable areas was based on information from the known deposits, combined 

with geologic, geochemical, geophysical, and 

earth satellite data presented in other components of the Tanacross folio. A list of the criteria 

used in the selection of each favorable area is provided with the mineral resources map. It should 

be noted that without detailed drilling, it is impossible to insure that all favorable areas are 

shown or that areas selected actually contain deposits; this is particularly true in the Tanacross 

quadrangle where bedrock exposure is poor. Despite this limitation, the resource assessment 

should provide useful information for land clas15
EXPLANATION

Figures 5 to 12 show areas with revisions to "Reconnaissance geologic map of the Tanacross quadrangle, Alaska", Miscellaneous 

Geologic Investigations Map 1-593 (Foster, 1970). For detailed descriptions of units, refer to Map 1-593.

Unconsolidated deposits (Quatemuy)

Qa Alluvium, primarily gravel and sand

Qas Alluvium, primarily silt and sand

Qs Silt and peat deposits

Ql Landslide deposits

Qcg Alluvium and colluvium, primarily gravel, sand, and rubble

Qc Alluvium and colluvium, primarily silt and sand

Or Rock-glacier deposits

Sedimentary rocks (Cretaceous?)

Kr Detrital rocks

Metamorphic rocks

Rza Phyllite and schist unit (Paleozoic?)

Rzu Schist and quartzite unit (Paleozoic?)

PzpCk Quartz-mica schist unit (Precambrian and (or)

Paleozoic) 

PzpCg Quartz-graphite schist unit (Precambrian and

(or) Paleozoic)

Qfl Fluvial and lacustrine deposits

Qls Lake deposits

Qt Terrace deposits

Qm Moraines of the Donnelly Glaciation

Qd Eolian deposits, primarily in dunes

Qof Alluvial fan deposits of the Delta Glaciation

Qdm Moraines of the Delta Glaciation

RzpCb Biotite gneiss and schist unit (Precambrian and 

(or) Paleozoic)

Patterned areas within this unit 

.^ Primarily quartz-feldspar-biotite gneiss 

vv Area intruded by many fine-grained felsic and 

v granite porphyry dikes

Igneous rocks

QTb Basalt (Tertiary or Quaternary)

Tm Mafic volcanic rocks (Tertiary?)

Tf Felsic volcanic rocks (Tertiary?)

Tp Granite porphyry (Tertiary?)

J"Rg Granodiorite of Taylor Mountain batholith (Triassic or Jurassic)

(vfcgb Gabbro and basaltic rocks of Ketchumstuk Mountain (Mesozoic?)

Mts Hornblende syenite porphry (Mesozoic?)

Each triangle indicates approximate locality where porphyritic

syenite rubble was found

Other map symbols 

a Augen gneiss 

s Abundant sulfides 

? Area not visited. Type of rock not definitely known

Meg Granitic rocks (Mesozoic?)

Diagonal dashed lines, area of mostly 

hornblende-biotite granodiorite

IvfcPzu Ultramafic rocks (Paleozoic or Mesozoic)

Rzd Metadiorite (Paleozoic?)

c Conglomerate occurrence 

b Tectonic breccia

Country rock cut by one or more dikes, sills, or other small igneous masses, commonly indicated only by a mixture of rock types in 

rubble. Letter indicates composition.

p, pegmatite

g, granite

k, alaskite

a, aplite

R, rhyolite; definite quartz phenocrysts

v, rhyolite or dacite

d, diorite or quartz diorite

m, mafic, mostly andesite

u, ultramafic

n, greenstone dikes

+++ marble layers

A granite porphyry

Strike and dip symbols and pingo locations are not shown on revised maps. Refer to Map 1-593 for this information.

FIGURE 13. Explanation for revisions to the Reconnaissance Geologic Map of the Tanacross quadrangle, Alaska (Foster, 1970).

sification and developmental planning by State, about mineral commodities that occur or might 

industrial, and native interests. Information occur in the quadrangle will be of use in making a

16
national minerals policy.

Indications of mineralization are widespread in 

the quadrangle, but only two types of mineral deposits are presently known: one consists of porphyry copper and molybdenum deposits; the other 

is an antimony-bearing vein deposit. The porphyry deposits generally are associated with Tertiary felsic volcanic rocks; the vein deposit and 

probably other vein deposits containing a variety 

of metals occur in the metamorphic rocks of the 

Alaska Range and probably less commonly in the 

metamorphic rocks in other parts of the quadrangle. Although skarn copper and stratiform 

copper deposits have not been found, these types 

of deposits are known to exist in similar geologic 

terrane in Canada and by analogy could occur in 

the Tanacross quadrangle.

RECONNAISSANCE GEOCHEMISTRY (MAPS MF-767F-0)

COLLECTION OF SAMPLES

Reconnaissance geochemical studies were 

made in the Tanacross quadrangle during the 

summers of 1967 and 1974 to help determine the 

mineral resource potential of the quadrangle by 

defining areas of anomalous metal content. The 

geochemical data from the 1974 field season are 

presented and discussed in this folio; the 

geochemical data from the 1967 fieldwork are 

presented in other reports (Clark and Foster, 

1969; Clark and others, 1969). The geochemical 

data delineate areas of known mineral occurrences and additional areas possibly containing 

undiscovered mineral resources. The results also 

show that scavenging agents, such as materials 

rich in secondary iron-manganese oxides and decaying organic matter, are the most useful sample media for reconnaissance geochemical studies 

in the maturely dissected areas of the quadrangle. In the more rugged mountainous areas, 

detrital materials such as minus-80-mesh stream 

sediments and heavy-mineral concentrates are 

the most useful types of samples for reconnaissance geochemical exploration. The results 

further indicate that areas of anomalous metal 

content are more completely defined by data from 

a combination of sample media than by data from 

any one sample medium.

During the summer of 1974, geochemical samples were collected at about 600 sites in streams

draining areas ranging from 5 to 25 km2 at an 

average sampling density of one site per 20 km2. 

Samples were also collected at an additional 35 

sites in swampy areas. Comparisons were made of 

the distribution and abundance of copper, molybdenum, lead, zinc, arsenic, mercury, beryllium, 

bismuth, tin, and tungsten in five sample media. 

These media are (1) heavy-mineral concentrates, 

(2) minus-80-mesh stream sediment, (3) oxide residue (oxalic-acid-leachable fraction) of stream 

sediment, (4) aquatic bryophytes (mosses), and (5) 

sireambank sod (mixed organic and inorganic 

material) collected beneath the water level.

Approximately 864 samples of rocks were also 

collected and analyzed by semiquantitative emission spectrography for 30 elements, and by atomic 

absorption for gold. Selected samples were also 

analyzed by atomic absorption for lead, zinc, and 

mercury. The results of these analyses were used 

in the preparation of this folio, and the complete 

analyses are available in an open-file report 

(O'Leary and others, 1976).

DESCRIPTION OF SAMPLE MEDIA

Stream sediments and heavy-mineral concentrates were collected in active channels of 

streams. The material in samples of stream sediment ranges in size from fine sand and silt in 

areas of low relief to coarse sand in areas of high 

relief. The minus-80-mesh stream sediment and 

the heavy-mineral concentrates represent detrital material that has been mechanically introduced into a stream from bedrock and colluvium 

within a particular drainage basin. The minus80-mesh sediment may reflect the presence of 

outcropping mineralized rock upstream. The 

heavy-mineral concentrates are useful for determining the distribution of certain heavy metals 

and resistate minerals such as gold, platinum, 

cassiterite, and scheelite.

The secondary iron and manganese oxides coating stream sediment particles (oxide residue) and 

aquatic bryophytes are considered scavenging 

agents that concentrate elements that have been 

leached from bedrock and colluvium and are migrating as ions in solution. The oxide residue of 

stream sediment contains secondary iron and 

manganese oxides, together with silica and 

alumina as major components along with the 

trace elements scavenged from solution. These

17
components are extracted from the minus-80- 

mesh sediment using a weak oxalic-acid solution 

(Alminas and Mosier, 1976). A leachate residue is 

produced by this extraction process that is a derivative sample of the detrital stream sediment. 

In areas of low relief, where sulfides and other 

unstable ore minerals have been removed by 

chemical weathering, this oxide-rich derivative 

sample has the following advantages over the 

minus-80-mesh stream sediment: (1) The level of 

trace metal content is about an order of magnitude greater in the oxide residue than in the 

detrital sediment and (2) there is a greater contrast of metal values in the oxide residue than in 

the detrital sediment. Weakly anomalous metal 

concentrations that might go unnoticed in the detrital stream sediment are enhanced in the oxide 

residue because the dilution effects of the detrital 

sediment are removed.

Aquatic bryophytes were collected beneath the 

water level from streams and from shallow ponds 

in swampy areas. Generally, Calliergon cordifolium (Hedw.) Kindb., Calliergon giganteum 

(Schimp.) Kindb., or Fontinalis duriaei Schimp. 

were collected at each site. Identification was 

made by Hansford T. Shacklette, of the U.S. 

Geological Survey, who followed the nomenclature of Crum, Steere, and Anderson (1973). The 

bryophytes concentrate elements in solution 

primarily by absorption (Brooks, 1972). The 

bryophytes apparently also concentrate certain 

heavy minerals such as cassiterite (Curtin and 

others, 1976f) by the entrapment of particulate 

matter.

Streambank sod was collected beneath the 

water level from streams in the terrane north of 

the Tanana River but usually was collected above 

the water level from streams in the Alaska 

Range. The streambank sod was not present at 

many of the sites in the Alaska Range and therefore was not a useful sample medium in this terrane. The streambank sod consists of a mixture of 

inorganic material ranging in size from silt to fine 

sand, dead roots, and other pieces of decaying organic material. The element content of the sod 

consists of elements scavenged from solution 

primarily by the organic material and the element content of the detrital material.

Copper, zinc, and molybdenum values showed a 

relatively high positive correlation with the organic content of the sod, which suggests that the 

quantity of organic material noticeably influences the content of these elements in the sod. A 

regression analysis log metal versus organic

content was used to determine the influence of 

organic content on the variation of copper, zinc, 

and molybdenum values in the ash of the sod. 

This type of analysis allows high metal values 

caused by organic concentration of background 

amounts of metals to be distinguished from high 

values derived from a mineralized source. Values 

from the regression analysis, shown as residuals, 

were plotted on the geochemical maps for copper, 

zinc, and molybdenum in the ash of streambank 

sod.

PREPARATION AND ANALYSIS OF SAMPLES

Stream sediment samples were air dried and 

sieved through a minus-80-mesh (0.2 mm) sieve. 

A split of the minus-80-mesh material was 

analyzed for copper, lead, zinc, mercury, and gold 

by atomic absorption (Ward and others, 1969; 

Vaughn and McCarthy, 1964) and for arsenic by a 

colorimetric method (Ward and others, 1963). 

Another split was analyzed for the other elements 

by a semiquantitative spectrographic method 

(Grimes and Marranzino, 1968).

The oxide residue of stream sediment was obtained by leaching a 5-gram split of the minus80-mesh stream sediment with oxalic acid as 

described by Alminas and Mosier (1976). The 

residue was analyzed by the semiquantitative 

spectrographic method of Grimes and Marranzino 

(1968). With the exception of tin, the reproducibility for the elements of interest in the oxide 

residue is within the limits of the analytical 

method used (±30 percent at the 95th percentile). 

The reproducibility for tin shows a variation of as 

much as 100 percent for individual samples. The 

analytical results for tin are considered to be usable, however, because the overall distribution 

pattern of tin did not change noticeably when tin 

results from repeat analyses were plotted and 

compared.

Panned concentrates were sieved through a 

40-mesh (0.4 mm) sieve, and the minus-40-mesh 

fraction was further separated with bromoform 

into two fractions: a light-mineral fraction composed of minerals having a specific gravity of 2.86 

or less and a heavy-mineral fraction composed of 

minerals having a specific gravity of greater than 

2.86. The heavy-mineral fraction was saved and 

separated magnetically. Magnetite and other 

magnetic minerals were removed from the 

heavy-mineral fraction by the use of a hand magnet and a Frantz Isodynamic Magnetic Separator 1

lThe use of trade names is for descriptive purposes only and does not constitute 

endorsement of these products by the U.S. Geological Survey.

18
set at 0.2 amperes. This magnetic fraction was 

saved for analysis. The remaining heavy minerals were again passed through the separator at a 

setting of 0.6 amperes. The nonmagnetic fraction 

at this higher setting was saved for analysis and 

mineralogical examination. This fraction generally contains in order of relative abundance 

muscovite, sphene, zircon, apatite, rutile, 

anatase, and tourmaline. Ore minerals such as 

sulfides and cassiterite also occur in this fraction. 

Both the magnetic and nonmagnetic fractions of 

panned concentrates were analyzed by semiquantitative spectrographic methods.

The streambank sod was air dried and sieved 

through a 2-mm stainless steel sieve to remove 

rock fragments and large pieces of organic litter. 

The minus-2-mm fraction was ashed, and a split 

of the ashed sample was resieved through an 

80-mesh sieve. The minus-80-mesh material was 

analyzed for copper, lead, and zinc by atomic absorption, for arsenic by colorimetry, and for the 

remaining elements by semiquantitative spectrography. A split of the original ashed sample 

was analyzed for gold by atomic absorption.

The aquatic bryophytes (mosses) were rinsed at 

the sample site and dried in the field in a cloth 

bag. The moss was further prepared in the laboratory by hand massaging the dried material to remove remaining sand and silt. The moss samples 

were then pulverized in a blender and ashed in a 

muffle furnace at a peak temperature of 500°C. 

The ashed samples were analyzed by a semiquantitative spectrographic method for plant materials (Mosier, 1972).

RESULTS

The geochemical maps and statistical data 

were generated on a U.S. Geological Survey DEC 

10 computer using the STATPAC statistical 

evaluation library. The geochemical data are 

plotted on base maps showing generalized geology and the drainage pattern. The elements 

shown on the maps were selected for their association with known base-metal occurrences and 

with possible additional undiscovered mineral occurrences.

The results indicate that in the maturely dissected terrane north of the Tanana River, elements apparently migrate in solution from the 

bedrock and colluvium and enter the streams. 

The elements are then scavenged by the secondary iron-manganese oxides in the stream sediment, aquatic bryophytes, and the streambank

sod beneath the water level. The hydromorphic 

anomalies produced by the scavenging processes 

form patterns that outline areas of known mineral occurrences and potential areas of undiscovered resources.

The minus-80-mesh stream sediment is characterized by a lower contrast in element concentration than that observed for panned concentrates 

and the scavenging agents. The relatively weak, 

isolated anomalies defined by the streamsediment data illustrate this low contrast. The 

weak anomalies may be the result of dilution by 

barren material that constitutes the bulk of the 

stream sediment and of the selective removal by 

weathering of sulfides and other unstable ore 

minerals. The effects of weathering and dilution 

are overcome, however, by concentrating the 

heavy-mineral fraction of the stream sediment by 

panning.

The results also demonstrate that, in the terrane north of the Tanana River, mineral occurrences are more completely defined by a combination of sample media than by any single type of 

sample. The results further indicate that oxide 

residue is probably the most useful sample 

medium in identifying base-metal occurrences. 

Heavy-mineral concentrates are most useful for 

detecting occurrences of tin and tungsten 

mineralization and for delineating anomalous 

distributions of other metals such as bismuth. 

Streambank sod and aquatic bryophytes are 

somewhat less useful than the oxide residue, but 

they can provide supporting geochemical data. 

The detrital, minus-80-mesh stream sediment, on 

the other hand, has only limited use for delineating anomalous distributions of copper, lead, zinc, 

and tin and is of very little or no use for reconnaissance exploration for metals such as molybdenum, tungsten, and beryllium.

In the Alaska Range, mechanical weathering is 

the primary destructive process. In this terrane 

relatively fresh fragments from both country rock 

and mineralized occurrences are deposited in the 

streams to produce sediments that reflect the 

mineralogy of the bedrock in the drainage basins. 

Therefore, the minus-80-mesh stream sediments 

and heavy-mineral concentrates adequately 

define areas of mineral potential in this terrane. 

Analysis of bedrock collected at a somewhat lower sampling density than the other media aids 

in defining the chemical composition of the bedrock and in interpretating geochemical results 

of the stream sediment and panned concentrate 

sampling.

19
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20
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Fluorite in the United States, exclusive of Hawaii: 

U.S. Geol. Survey Min. Inv. Res. Map MR-60, 1 sheet, 

scale 1:3,168,000 and 1:12,672,000.