GEOLOGICAL SURVEY CIRCULAR 570 

·Suggested Areas for 

Prospecting in the Central 

Koyukuk River Region 

Alaska 

Suggested Areas for 

Prospecting in the Central 

Koyukuk River Region 

Alaska 

By Thomas P. Miller and Oscar J. Ferrians, Jr. 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 570 

Washington l968 
United States Department of the Interior 

STEWART L. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONfENTS 

Page 

Abstract- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Geologic setting- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Mineralized areas - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3 

Introductory statement - - - - - - - - - - - - - - - - - - - - - - - - - - 3 

Indian Mountain- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3 

Sun Mountain - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

Oakli ----------------------------------- 6 

Clear Creek - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 8 

Caribou Mountain- - - - - - - - - - - - - - - - - - - - - - - - - - - - - 9 

Hawk River- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 10 

Purcell Mountain - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 11 

Summary - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 12 

References cited - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 12 

ILLUSTRATIONS 

Page 

Figure 1. Map of central Koyukuk River region showing location of 

areas described in text - - - - - - - - - - - - - - - - - - - - 2 

2. Geologic map of the Indian Mountain area - - - - - - - - - - - 4 

3. Geologic map of the Zane Hills area - - - - - - - - - - - - - - 7 

4. Geologic map of \the Clear Creek area - - - - - - - - - - - - - 9 

5. Geologic map of the Purcell Mountains area - - - - - - - - - - 11 

TABLES 

Page 

Tables 1-3. Analyses of selected grab samples from1. Utopia Creek placer mine and altered fine-grained 

felsic intrusives, Indian Mountain area-- - - - - - - 5 

2. Head of Black Creek, Indian Mountain area - - - - - - 6 

3. Zane Hills area - - - - - - - - - - - - - - - - - - - - - 8 

4. Copper and lead content of stream-sediment samples, 

Clear Creek area - - - - - - - - - - - - - - - - - - - - - 10 

5. Analyses of selected grab samples from the Hawk River 

locality near Purcell Mountain - - - - - - - - - - - - - - 10 

III 

Suggested Areas for Prospecting in the Central 

Koyukuk River Region, Alaska 

By Thomas P .. Miller and Oscar J. Ferrians, Jr. 

Abstract 

Anomalous amounts of copper, lead, zinc, silver, and gold in 

stream-sediment samples and mineralized outcrops define seven 

areas favorable for prospecting in the central Koyukuk River region, west-central Alaska. These areas, listed with their metals 

of interest, are (1) Indian Mountain:silver,lead,copper, and gold, 

(2) Sun Mountain: lead, copper, and silver, (3) Dakli: copper, (4) 

Clear Creek: gold, (5) Caribqu Mountain: uranium and thorium. 

(6) Hawk River: lead and silver, and (7) Purcell Mountain: gold. 

Mineralization at all these localities is closely related to granitic 

plutons of Late Cretaceous age. These plutons are intrusive into 

volcanic rocks ranging in composition from quartz latite to andesite and in age from possibly Late Jurassic to Early Cretaceous. 

INTRODUCTION 

Reconnaissance investigation of the m in era 1 resources of the central Koyukuk River region, westcentral Alaska, for the Heavy Metals program, U.S. 

Geological Survey, suggests seven localities that are 

particularly interesting for mineral prospecting (fig. 

1 ). Although the region currently has a productive 

placer gold deposit, much of the region is relatively 

inaccessible and has received little attention from 

prospectors. 

The central Koyukuk River region includes 6,600 

square miles covering portions of the Hughes, 

Melozitna, and Shungnak 1:250,000-scale quadrangle 

maps. The only town in the district is Hughes, a small 

village on the Koyukuk River (fig. 1 ), which is serviced 

several times a week by scheduled airlines from Fairbanks. 

Areas with known mineral deposits and other areas 

with geologic environments favorable for the occurrence of mineral deposits were investigated in the 

summer of 1967. Over 400 samples were collected and 

analyzed by semiquantitative spectrographic procedures and by atomic-absorption methods. 

The authors were assisted in the field by Vernon 

DeRuyter, geologic field assistant. Semiquantitative 

spec t r o graph i c analyses were done by Arnold 

Farley, Jr., J. M. Motooka, G. W. Sears, Jr., and Chris 

Heropoulos, and atomic-absorption analyses were done 

by W. L. Campbell, R. L. Miller, T. A. Roemer, A. L. 

Meier, and M. S. Rickard. C. W. Maeser assisted on 

the fission-track analysis. 0 ff i cia 1 s of the United 

States Smelting Refining and Mining Co., particularly 

Jack Fisher, manager of the Hogatza placer mine, 

were very helpful in permitting theinvestigationofthe 

mine. 

The first geologic mapping in the region was done 

primarily along the Koyukuk River by Schrader ( 1900, 

1904). Later, mapping of parts of the region and of 

contiguous areas of interest was done by Smith ( 1913), 

Eakin (1916), and Fernald (1964). The most recent 

mapping, by Patton and Miller (1966) and Patton and 

others ( 1968), includes the Hughes, Shungnak, and part 

of the Ambler River quadrangles. 

GEOLOGIC SETTING 

The bedrock of the central Koyukuk River region 

consists chiefly of granitic rocks intruded during two 

late Mesozoic plutonic episodes into marine andesitic 

volcanic rocks, volcanic graywacke, and mudstone of 

older Mesozoic age. Quartz latite exposed near Purcell Mountain (fig. 1) was extruded between plutonic 

episodes, and fine-grained felsic intrusives of Late 

Cretaceous or Early Tertiary age occur near Indian 

Mountain (figs. 1, 2). Mineralization in the region 

appears to have been associated with the granitic intrusions. 

The oldest bedrock unit in the province is a thick 

sequence mainly of andesitic volcanic rocks of Early 

Cretaceous and possibly Late Jurassic age. It consists 

of pyroxene andesite and trachyandesite flows, pyroclastic and volcaniclastic rocks, and sub ordinate 

fossiliferous limestones. The unit crops out over 

large areas of west- central Alaska and has been assigned to the Koyukuk Group by Smith (1913, p. 80). 

Overlying this unit is a thick sequence of interbedded 

volcanic graywacke, mudstone, and quartz conglomerate of late Early Cretaceous and perhaps younger 

age (Patton and Miller, 1966). The graywacke and 

mudstone are composed of angular detritus derived 

primarily from the underlying volcanic rocks and consist chiefly of plagioclase and volcanic rock fragments 
~ 

162• 

0 ~OMILES 

INDEX MAP SHOWING LOCATION OF 

REPORT AREA (SHADED) 

EXPLANATION 

FigtHe 4 

D 

Area shown on detailed map 

AREAS DESCRIBED IN TEXT 

1 

Indian Mountain 

2 

Sun Mountain 

3 

Dakli 

4 

Clear Creek 

5 

Caribou Mountain 

6 

Hawk River 

7 

Purcell Mountain 

Figure 1.--Map of central Koyukuk River region showing location of areas described in text. 
in an argillaceous matrix. Locally, this unit contains 

marine fossils, plant fragments, and coal beds. 

Crystal lithic tuffs and volcaniclastic rocks locally 

occur at the base of this unit. 

Plutons and dikes of medium-grained granodiorite, 

quartz monzonite, and syenite intruded the volcanic 

and sedimentary rocks during two Late Cretaceous 

plutonic episodes. Three intrusive igneous bodies in 

the western part of the region that are tentatively assigned an age of 100 m.y. (million years) based on 

potassium-argon age determinations (M i 11 e r and 

others, 1966) were intruded during the earlier episode. 

These bodies are the Purcell Mountain pluton composed chiefly of porphyritic quartz monzo:tite, the 

Shiniliaok Creek pluton composed of monzonite and 

syenite, and the Hawk River pluton composed chiefly 

of monzonite. Other main igneous bodies of the region 

were intruded during the younger episode and are assigned an age of 80 m.y. (Miller and others, 1966). 

They consist chiefly of granodiorite and quartz monzonite with subordinate alaskite. Late-stage aplite and 

alaskite dikes cut both episodes of plutonic rocks. A 

few quartz veins occur near the borders of the main 

igneous bodies and in the nearby wallrock. A volcanic 

unit composed of quartz latite, latite, and rhyolite separates the two plutonic episodes in the vicinity of Purcell Mountain; it is dated at 86 m. y. (Patton and others, 

1968) and is assigned a Late Cretaceous age. 

The gran it i c rocks of both suites are generally' 

massive and medium grained, and contacts between the 

igneous bodies and country rock are sharp, relatively 

straight, steeply dipping, and everywhere appear to be 

discordant. Rocks of the hornblende-hornfels facies 

form a narrow zone in country rock adjacent to the intrusive and grade into a broader zone of a 1 biteepidote-hornfels facies rocks away from the contacts. 

Elsewhere, the volcanic and sedimentary units are unaltered. The hornfelsic contact rocks are generally 

more resistant than the granitic rocks and form topographic ridges surrounding parts of the igneous bodies. 

The region lies along the Hogatza arch, which is described by Miller and others ( 1959) as a linear structural high extending from the northeast end of the 

Seward Peninsula to the Koyukuk River. The rocks of 

the region have been deformed into broad, gentle to 

moderate folds and are broken by a complex fault sys-· 

tern. The structural grain is roughly east-west for the 

central and western part of the region and northeastsouthwest for the eastern part. 

The lowland areas are underlain by unconsolidated 

deposits in contrast to the mountainous areas and adjacent uplands, which are underlain by bedrock. Most 

of the Koyukuk Flats, in the southwestern part of the 

region, is underlain by fine-grained, water-laid, and 

windbome sediments of Pleistocene and Holocene age. 

The lowlands in the northern part of the region surrounding the Purcell Mountains, Zane Hills, lsahultila 

Mountains, Indian Mountains, and adjacent uplands are 

3 

underlain by glacial drift of Pleistocene age (Coulter 

and others, 1965), which was deposited by large piedmont glaciers that came from the Brooks Range to the 

north. Also, small local glaciers emanated from the 

higher parts of the Purcell Mountains, Zane Hills, and 

Indian Mountains during Pleistocene time and deposited 

glacial drift in some of the larger valleys within these 

mountains. 

Most mineralized areas in the region occur along or 

near the contacts between the late Mesozoic plutons and 

country rock, indicating a close relatioPship between 

the igneous intrusive rocks and the mineralization. 

MINERALIZED AREAS 

Introductory statement 

Mineral occurrences in outcrop or anomalous concentrations of metals in stream-sediment and rock 

samples indicate seven areas that are worthy of further 

investigation and prospecting. These areas are: Indian 

Mountain, Sun Mountain, Dakli, Clear Creek, Caribou 

Mountain, Hawk River, and Purcell Mountain (fig. 1). 

They are described in the following sections of the report and all the areas except Sun Mountain are shown 

on small-scale geologic maps (figs. 2, 3, 5); a more 

detailed map is provided for the Clear Creek area 

(fig. 4). The geologic maps also show the lead and 

copper content of stream-sediment samples. Lead and 

copper values above 50 parts per million are anomalous for streams draining the bedrock of the region. 

Indian Mountain 

Mineralized rock was found in the Indian Mountain 

area (fig. 2) near old placer mines at Utopia and Black 

Creeks and along the east-northeast-trending fa u It 

zone south of Utopia Creek. 

Abundant boulders of massive white barite as much 

as 2 feet in diameter were found in the tailings near 

the head of an abandoned placer gold mine along Utopia 

Creek. Some of the barite boulders contain fine- grained 

tetrahedrite, galena,· and sphalerite. Analyses of two 

samples of barite, one barren of visible metallic minerals and the other containing visible but minor 

amounts of tetrahedrite and other metallic minerals, 

are given in table I (samples lA and 1 B). Both samples contain silver and anomalous amounts of lead, zinc, 

copper, antimony, and gold; the tetrahedrite-bearing 

sample (lB) contains about 21 ounces ofsilverper ton 

and the principal silver-bearing mineral is probably 

tetrahedrite. A stream-sediment sample(table 1, sample 1 C), taken from a tributary on the south side of the 

valley .near the upstream end of the tailings, contains 

the highest values of lead, zinc, and silver found in any 

stream-sediment sample in the region. The lead value 

of 2,000 ppm and the zinc v a 1 u e of 700 ppm are extremely high. Contamination by the placer mining 

operation seems unlikely because the sample was col-
66" 

Alluvium 

I TKi'l I 

Fine-grained 

felsic intrusive 

rocks 

>-

H ::I 

0 

}

(f) 

::I >- 0 a: 

~a:::!: <COlI- a: w w a: 1-

u 

0 2 

~ 

... 

~ 

~vfl"' " APPROXIMATE MEAN 

DECLINATION, 1968 

Tuff, graywacke, 

and mudstone 

Geology by W. W. Patton, Jr., and T. P. Miller, 1966; 

T. P. Miller and 0. J. Ferrians, Jr., 1967 

EXPLANATION 

Granodiorite and 

quartz monzonite 

Graywacke 

and 

mudstone 

Andesitic volcanic 

rock 

Conglomerate 

(f) 

::I 

0 

w 

u 

<( 

1-

w 

a: 

u 

} 

e ~ u 0 -ow 

~zu <(<C<C 

a: 1- ::::l w ..., a: 

u 

Approximate 

contact 

Fault 

~ 

e1o 

Sample of stream sediment 

Copper content above, lead content below, 

in parts per miUicm 

x2 

Location of analyzed rock samples. 

discussed in tables 1 and 2 

~ 

Abandoned mine 

IIJ/fllflllt/ 

'"""""' Tailings 

Figure 2.--Geologic map of the Indian Mountain area. 

4 
Table 1.--~alyses of selected grab samples from Utopia Creek placer mine and altered fine-grained felsic intrusives. Indian Mountain area 

[Results reported in parts per million. Gold analysis by atomic absorption except for sample 2, which is by fire 

assay. All other analyses are semiquantitative spectrographic with results reported to the nearest number in 

the series 1.0, 0.1, 0.5, 0.3, 0.2, 0.15, 0.1. N, not detected; <,present but below limit of determination 

or below values shown. Sample localities are shown in figure 2] 

Sample No. Lab. No. Cu Ag Au 

lA ACN-801 7 3 0.16 

Pb Zn 

1,000 N 

Mo 

7 

Description 

Massive white barite with no visible metallic minerals. 

lB 996 1,500 700 1,500 3,000 5 Massive white barite with minor amounts of 

tetrahedrite, galena, and sphalerite; 

Sb•l,OOO ppm. 

lC 

2 

3 

4A 

4B 

5 

6A 

6B 

6C 

6D 

6E 

6F 

ACC-115 

64Ml705 

ACN-999 

998 

997 

996 

496 

ACC-251 

252 

253 

254 

255 

Limits of determination 

150 

50 

100 

200 

300 

30 

150 

1,000 

20 

100 

700 

20 

5 

3 

7 

3 

N 

N 

N 

2 

15 

7 

.7 

1 

.5 

• 07 t 

.04 

• 1 

N 

N 

N 

.5 

.2 

N 

.1 

6 

N 

.02 

2,000 

300 

100 

70 

70 

30 

3,000 

1,000 

200 

500 

1,000 

20 

10 

lected away from the operation and because of the high 

silver value. Copper, gold, and barium also are anomalously high in the sample. 

The country rock at Utopia Creek is slightly metamorphosed andesite of Late Jurassic(?) and Ear 1 y 

Cretaceous age cut locally by fine-grained, felsic intrusives. The Indian Mountain granodiorite pluton is 

about 2 miles away, and Utopia Creek does not drain 

the granodiorite-andesite contact as do the other 

streams in the region that contain gold-bearing gravel 

deposits. The lack of granitic rocks in the tailings, 

tog e the r with the abundance and size of the barite 

boulders, suggests that the boulders were derived from 

tetrahedrite-sphalerite-galena-bearing veins in the 

andesite volcanic rocks. Veins of this sort may also 

have been the source of the placer gold found here. 

Also, the location of the tailings suggests that the pay 

streak did not lie in the main part of the valley in this 

area but instead continued up the south slope of the 

valley toward the area drained by the tributary stream 

containing sediments rich in lead and zinc. Because 

of inclement weather near the end of the field investigation, little time was available to look for the bedrock 

source of the barite. The drainage basin is not large 

and therefore should be amenable to prospecting. 

An east-northeast-trendingbelt ofpyritiferous, 

silicified, fine-grained intrusive rocks occurs along a 

well-marked fault zo.ae a few miles southofthe Indian 

Mountain pluton and immediately south of the Utopia 

700 3 

7 

Stream sediment • 

N 

N 

N 

N 

N 

N 

.5 

N 

7 

Oxidized pyritiferous fine-grained intrusive. 

Do • 

Do. 

Oxidized pyritiferous andesite. 

Oxidized pyritiferous fine-grained intrusive. 

700 

700 

7 

20 

Oxidized pyritiferous andesite. 

Oxidized and silicified, fine-grained intrusive with disseminated pyrite and 

sphalerite. 

N 

500 

500 

200 

200 

5 

N 

5 

30 

N 

5 

Deeply oxidized pyritiferous fine-grained 

intrusive. 

Oxidized pyritiferous andesite. 

Oxidized pyritiferous fine-grained intrusive. 

Do. 

Creek barite occurrence (fig. 2). The age of these intrusives is unknown, but they maybe as young as early 

Tertiary. Oxidation of disseminated pyrite in the intrusives has resulted in the formation of conspicuous 

orange-red-yellow gossans visible from many miles 

away. Typical intrusive rocks from. these gossans contain small but anomalo:Js amounts of lead, copper, 

silver, and gold (table 1, samples 2-5). 

Highly altered, fine-grained, pyritiferous intrusive 

rocks, similar to the intrusives mentioned above, occur on Indian River at locality 6 (fig. 2). Bo::h the intrusive rocks and the intruded and e site are much 

oxidized and altered; however, some of the fresher 

specimens of intrusive rock contain disseminated pyrite and sphalerite. Both the altered andesite and intrusive show anomalous amounts oflead, zinc, copper, 

molybdenum, silver, and gold (table 1, samples 6A6F). Sample 6E contains about 6 p;>m of gold, equivalent to approximately 0.16 ounce per ton. 

Pyrite and sparse chalcopyrite are disseminated in 

metavolcanic rocks and late-stage aplites near the 

pluton a.~ the head of Black Creek (fig. 2), upstream 

from gold placer operations. The coarse, angular nature of the gold panned here, together with the very 

small drainage-basin area, indicates a local and nearby source. Several bedrock grab samples from this 

area contain disseminated sulfides and show anomalous 

copper values, but only two contain gold (table 2). No 

other large zones of alteration or obvious source rock 
Table 2.--Analyses of selected grab samples from the head of Black Creek. Indian Mountain area 

JResults reported in parts per .million. Gold analysis by atomic absorption; all others are by semiquantitative 

spectrographic analysis with results reported to the nearest number in the series 1, 0.7, 0.5, 0.3, 0.2, 

0.15, 0.1. N, not detected; <, present but below limits of determination or below value shown. Sample 

localities are shown in figure 2] 

Sample No. Lab. No. Cu Pb Ag Au Mo B Description 

7A ACN-492 30 15 N N 7 

7B 493 20 <10 N N N 

7C 494 1,000 20 o.s N 5 

7D 495 50 15 <.5 N N 

7E 862 700 15 N N N 

7F 863 700 15 N N 15 

7G 864 500 30 N 0.04 N 

7H 865 700 10 N N N 

8 ACC-102 1,000 N N • os 7 

Limits of determination 5 10 .5 .02 5 

for the placer gold of the Indian River gold mine were 

found. It is possible that larger zones of mineralization are buried beneath the colluvium and talus of the 

steep-walled canyon. 

The area in the vicinity of the Indian Mountain pluton 

and, in particular, near Utopia Creek warrants further 

investigation. The fine-grained pyritiferous intrusive 

rocks found here are strongly altered and slightly mineralized. Although the rocks are not nearly of commercial grade, they may indicate deeper or hidden mineralization, and it seems p o s sib 1 e that the fa u 1 t 

controlling the main zone of intrusives could be mineralized. The general area is only 15 miles from the 

Koyukuk River, which is navigable by barge during the 

summer months and which provides access to shipping 

points farther south. 

Sun Mountain 

Scattered occurrences of chalcopyrite, malachite, 

and pyrite were observed along the Koyukuk River in 

the southern Isahultila Mountains near Sun Mountain 

(fig. 1). In addition, a 2-foot-long angular float block 

of altered silicified rock, w~1ich contained disseminated 

cerussite, galena, and malachite, was found on a cutbank at the south end ofBatzaSlough(fig. 1). The float 

block contains 5 percent oflead, 0.5percentof copper, 

and 3 ounces of silver per ton. The numerous quartz 

latite porphyry dikes throughout the Sun Mountain area, 

together with a widespreadzoneofthermallymetamorphosed andesite, suggest a buried pluton at s h a 11 ow 

depth. Several massive fossiliferous limestones, as 

much as 65 feet thick in this area, are favorable hosts 

for ore deposits. Prospecting will be handicapped by 

the heavy forest cover and by the thick covering of 

loeas that· mantles most of the area and prevents reliable stream-sediment sampling. 

Dakli 

The Dakli area is just east of the Dakli VABM in the 

northwest part of the Zane Hills (fig. 3) near the 

northern edge of the Zane Hills pluton. The contact between the hornblende-biotite granodiorite pluton and 

6 

15 Pyritiferous hornfels. 

15 Pyritiferous fine-grained felsic intrusive. 

200 Pyritiferous hornfels. 

20 Do. 

1 1 500 Do. 

70 Do. 

N Do. 

N Do. 

N Do • 

10 

andesite wallrock dips gently to the north in contrast 

to the steep contact around the restofthe pluton. This 

contrast in dip is shown by the occurrence of granodiorite in the valleys and of andesite hornfels on the 

ridgetops, as well as by a broad zone (not shown on the 

map) of th·ermally metamorphosed rocks north of the 

pluton. The abundance of hornblende in the northern 

half of the pluton, in contrast to the hornblende-poor 

southern half, suggests contamination of the pluton near 

its roof. Roof areas such as this are known to be favorable zones for mineralization. 

Copper minerals were found in three places (fig. 3, 

locs. 1-3) along the contact between g n e is s i c hornblende-biotite granodiorite and andesite. Anomalous 

copper values in stream-sediment samples from nearby streams draining east and west from the Zane Hills 

divide (fig. 3) indicate an even larger area of mineralization. Massive chalcopyrite occurs in fractured 

quartz gangue in frost-riven blocks of meta-andesite 

near the granodiorite contact at locality 1 (fig. 3). Pyrite and malachite, which are locally abundant, and 

covellite are also constituents of these blocks. The 

chalcopyrite is little oxidized and shows only minor 

replacement by covellite and malachite. Grab samples 

( 5 inches across) of chalcopyrite in quartz from locality 1 (table 3) contain as much as 4.5 oum~es of silver 

per ton ( 150 ppm) and 0. 023 ounce of gold per ton (0. 8 

ppm). Several streams near locality 1 (fig. 3) have 

float of meta-andesite that is cut by chalcopyritepyrite-quartz veins and contains disseminated chalcopyrite and pyrite. 

Malachite and limonitic chalcopyrite occur in fractures in frost-riven blocks of brecciated quartz, andesite, and altered granodiorite along a belt 100 feet 

wide on the north side of the saddle separating the andesite from the gneissic granodiorite at locality 2 (fig. 

3). The mineralized andesite and granodiorite here 

contain as much as 30 ppm of silver, 0.09 ppm of gold, 

700 ppm of zinc, and from 1,000 to 20,000 ppm of copper (table 3). Similar rocks occur at the granodioriteandesite contact at locality 3. 

Nearby, at locality 4 (fig. 3), rusty-colored molybdenite-bearing quartz veins as much as 2 feet thick 
EXPLANATION 

CD 

~ Alluvium. 

1-:::l 

LLIO 

Granodiorite and 

quartz monzonite }

<Ill 

Porphyritic monzonite e5~ and quartz monzonite 

[£1 

Andesitic volcanic rock 

Approximate 

____ contact ,_ Possible fauit 

20 

e20 

Sample of stream sediment 

Copper content above, lead content below, 

in parts per miUion. 

x8 

Location of analyzed rock samples 

discussed in table 3 

h'r,n:r"rq4 

..... _ ... Tail!ngs 

2 

Figure 3.--Geologic map of the Zane Hills area. 

7 

- 24•. 

APPftOKI 

D£CliNATtON, 1968 

Geology by W. W. Patton, Jr., and T. P. Miller, 1966 

Modified slightly by T. P. Miller and 

0. J. Ferrians, Jr., 1967 

0 2 4 6 MILES 
Table 3.--Analyses of selected grab samples from mineralized localities in the Zane Hills area 

[Results reported in parts per million unless otherwise noted. Gold analysis by atomic absorption; all others 

are by semiquantitative spectrographic analysis with results reported to the nearest number in the series 

1, 0.7, 0.5, 0.3, 0.2, 0.15, 0.1. N, not detected; I, interference; -,not looked for; ~ greater than 

value shown; <, present but below limit of determination. Sample localities are shown in figure 3] 

Sample No. Lab. No. Cu Ag Au Pb Zn Mo Description 

lA ACC-103 7,000 15 0.06 15 N 30 Float of chalcopyrite-pyrite-quartz vein let 

in andesite. 

lB 104 500 N N 10 N N Chalcopyrite-bearing quartz veinlet in andesite. 

lC 105 l/>10 150 .3 N 1,000 N Massive chalcopyrite with minor covellite 

l/>10 lD 106 150 • 6 10 

lE 107 l/>10 150 .5 N 

lF 108 10,000 50 .8 15 

2A ACM-184 >5,000 3 .07 20 

2B ACN-852 20,000 N N 200 

2C 487 1,000 30 .09 50 

2D 488 >5,000 20 .05 <10 

3 489 1,000 2 N <10 

4 ACM-188 30 1.5 N <10 

5 ACN-870 5,000 3 N N 

6 871 100 3 .6 150 

7 872 150 1 N 30 

8 ACM-185 100 N N 30 

9 ACN-994 50 N N 100 

10 ACM-189 500 15 .4 300 

11 ACN-840 300 5 2.4 150 

12 867 70 200 .3 500 

13 M102348 5,000 2 0 

Limits of determination 5 .5 .02 10 

1/Results reported in percent. 

cut the andesite, again near the granodiorite contact. 

A grab sample from one vein (table 3, sample 4) contains 0. 2 P·=rcent of molybdenum. 

Sediment in streams draining both east and west 

from the Dakli contact area contains anomalous copper 

values ranging from 100 to 300 ppm over an area of 

about 20 square miles (fig. 3). Hence, copper deposits 

are probably more widespread than indicated by this 

reconnaissance investigation. Pyrite- and chalcopyrite-bearing quartz veins as much as 3 feet thick 

cut meta-andesite north of the Zane Hills pluton at 

localities 5, 6, and 7 (fig. 3). Analyses of grab samples 

are given in table 3 (samples 5, 6, 7). 

Clear Creek 

The Clear Creek gold area is on the east side of the 

Zane Hills pluton (figs. 3, 4) and includes the Hogatza 

placer gold mine. The granitic rocks of the pluton in 

in quartz; chalcopyrite-content estimate, 

50 percent. 

700 N Do • 

N N Do. 

N 150 Chalcopyrite-bearing quartz veinlets in 

andesite. 

N 70 Chalcopyrite and malachite in fractured 

andesite. 

N N Chalcopyrite and malachite in quartz. 

700 I Hematite float. 

700 I Chalcopyrite and malachite in fractured 

andesite. 

N 15 Chalcopyrite and pyrite in quartz. 

N 2,000 Molybdenite-bearing quartz. 

N N Oxidized pyrite and chalcopyrite in 

quartz. 

N N Oxidized pyrite in quartz. 

N 3 Do. 

N N Altered quartz breccia; contains 200 ppm 

of Sb. 

N 200 Molybdenite(?)-bearing vein; contains 

3,000 ppm of Bi. 

N 30 Pyrite in quartz. 

300 10 Do. 

N 15 Pyritiferous silicified fine-grained intrusive; contains 1,000 ppm of Bi and 700 

ppm of W. 

0 10 Chalcopyrite in quart'.; contains 7,000 ppm 

of w. 

200 5 

8 

this area are gneissic monzonite and quartz monzonite 

(kmq, figs. 3, 4) (discussed in greater detail in the 

section on Caribou Mountain) and massive, mediumgrained granodiorite typical of the rest of the pluton 

(kgq, figs. 3, 4). The country rock is andesitic volcanic rock of Early Cretaceous and Late Jurassic age. 

The contact between granitic rock and andesite is in 

colluvium-filled, heavily vegetated saddles, w hi c h 

separate the subdued low hills of andesite from the 

much higher mountains of granitic rock. This abrupt 

scarp-like feature is indicative of a fault contact which 

would be a favorable site for mineralization. 

Sediment samples from streams draining the andesite-monzonite contact on both sides of Clear Creek 

contain anomalous amounts of lead and copper (fig. 4 

and table 4). A sediment sample from a spring near 

the contact high on the south slope of Clear C r e e k 

valley (fig. 4, and table 4, sample 1 lA) contained 1. 74 
Geology by W. W. Patton, Jr., and T. P. Miller, 1966 

--24•__ Modified byT. P. Miller andO. J. Ferrians, Jr., 1967 

A-PPROXIMATE MEAN 

DECLINATION, 1968 

0 2 MILES I I 

EXPLANATlON 

E2J 

Alluvium 

> 

H 0 

8 

u 

c( 

~ 

Porphyritic monzonite t;j 

and quartz monzonite. 5 

Andesitic voleanic rock 

Approximate eontact 

-?--JPoeeible fault 

•'"" Location of streanHtediment samp~ . di.euBBed in table 4 

xa 

Location of analyzed rock sample 

diaeuued in table 3 . 

~ 

lime 

''''' ,,,, ... ~-., """" Tailinp 

Figure 4.~Geologic map of the Clear Creek area. 

ppm of gold-the highest value of gold found in any 

sediment sample from the entire region. Brecciated 

quartz float from the contact zone on the· ridge above 

this spring (fig. 4) contains anomalous am o u n t s of 

antimony (table 3,·sample 8)~ andsoil samples from 

this same area eontain. lead and anomalous amounts of 

copper, as shown below in parts per million: 

Lab. Cu Pb 

No. 

ACN-·873 150 100 

874 300 30 

87!? 70 70 

9 

Mineralized bedrock is found in several places in this 

general area (table 3, locs. 8-11) and contains as much 

as 2.4 ppm of gold (0.07 ozper ton) as well as anomalous amounts of silver, bismuth, copper, and molybdenum. 

The occurrence of these an om a 1 o us values in 

stream-sediment and rock sam:Jles indicates that mineralization may have taken ·pl~ce along and near the 

monzonite-andesite contact in this area. This possibility is further enhanced by the fact that the mining 

operations of the Hogatza gold mine on Bear C reek 

began only a mile downstream. The auriferous alluvial 

gravels in this valley are de r i v e d from o u twas h 

streams emanating from a lobe of the glacier which 

moved down Clear Creek valley in early Pleistocene 

time. This lobe crossed the ridge between Clear and 

Bear Creeks near the monzonite-andesite contact, 

eventually covered the entire ridge, and terminated in 

Bear Creek valley. Glaciers are not efficient sorting 

or mixing agents and rock material incorporated into 

the ice on one side of the glacier would tend to remain 0.:1 that side. Since the bedrock exposed above the 

point where the Clear Creek glacier crossedoverinto 

Bear Creek is unaltered granitic rock, which is not a 

likely host rock for lode gold, the source of the placer 

gold is thought to be in or near the concealed andesitemonzonite contact. The location of the tailings at Bear 

Creek indicates that the pay streak extended to very 

near this contact. · 

The monzonite-andesite contact zone on either side 

of Clear Creek is an attractive site for lode gold prospecting. A small trenching or drilling program would 

give considerable information in e v a I u at in g the eco-

. nomic potential of this zone. 

Caribou Mountain 

Border phases of the Zane Hills pluton in two areas 

alQng the southeastern marginofthepluton(fig. 3, map 

unit kmq) show anomalous radioactivity-five to ten 

times the background radioactivity of the rest of the 

pluton. These border phases are composedofmediumto coarse-grained, trachytoid to gneissic, hornblendebiotite quartz monzonite and monzonite readily distinguishable in the fie I d from the typical m as s i v e, 

granitic-textured granodiorite of the restofthepluton. 

Tourmaline;.. bearing aplite dikes are common in the 

border phase as are dark-colored hybrid ·rocks indicative of contamination of granitic magma by andesitic wallrock. Large potassium-feldsp~rphenocrysts 

(as long as 3 inches) also are common in these border 

phases and locally in the adjacent metamorphosed andesite. 

An !analysis of porphyritic quartz monzonite from this 

border phase shows 20 ppm of uranium. This is five 

to six times more than the published averages for rocks 

of this composition (Smith, 1963, p. 402). Examination 

of thin sections of this rock shows that biotite and hornblende. contain numerous inclusions surrounded by 
Table 4.--Copper and lead content of stream-sediment samples, Clear Creek area 

[Results are from semiquantitative spectrographic analysis and are reported in parts per million to the nearest 

number in the series 1, 0.7, 0.5, 0.4, 0.3, 0.2, 0.15, 0.1. Sample localities are shown in figure 4] 

Sample No. Lab. No. Cu Pb Sample No. Lab. No. Cu Pb Sample No. Lab. No. Cu Pb 

1 ACN-463 30 30 78 ACC-135 100 150 12D ACN-975 70 70 

2 464 30 20 SA 136 70 50 12E 977 50 70 

3 465 5 20 88 137 70 70 12F 978 70 70 

4A ACC-117 50 50 9 139 70 70 12G 980 70 100 

48 119 50 70 10 64M-1.468 10 50 12H 981 50 70 

4C 120 70 50 llA ACN-809 50 100 121 982 70 50 

4D 122 50 70 118 986 30 50 12J 984 70 50 

4E 124 50 100 llC ACN-988 30 so 12K 98S 50 30 

SA 126 30 70 llD 

58 127 so 50 11E 

5C 128 50 50 llF 

SD 130 70 so 12A 

6A 131 1llO 70 128 

68 132 100 150 12C 

7A 134 100 70 

pleochroic halos indicative of radioactivity. Some of 

these halos are obviously around zircon crystals, but 

other much more intense halos are around grains of a 

colorless to faintly yellow, isotropic mineral of high 

relief. A thin section of this porphyritic quartz monzonite was exposed to a thermal neutron beam in a 

reactor in order to cause the fission of u235. The fission events were recorded in a piece of lexan w h i c h 

covered the section. Later, etching of the lexan showed 

the anomalously occurring uranium in the sample to be 

associated with the colorless isotropic mineral. 

Although the uranium-bearing mineral is only a 

minor constituent in the samples studied, it may be 

more abundant elsewhere in the radioactive border 

phase-possibly in amounts large enough to be important economically, or other uranium minerals may 

be present. A panned concentrate collected in 1964 

from Caribou Creek on the southeastern side of the 

Zane Hills contained 200 ppm of thorium, which was 

probably derived from this more radioactive border 

phase of the pluton. 

Other mineralized rock occurrences in this area are: 

(I) a pyritiferous fine-grained intrusive cutting the 

border phase monzonite at locality 12 (fig. 3); a sample 

989 30 50 13 808 30 so 

991 30 50 14 ACM-107 30 so 

992 30 so lS 106 30 so 

971 50 70 16 ACN-846 100 30 

973 so 70 17 848 70 30 

974 50 70 18 849 100 30 

of this material (table 3, sample 12) contains anomalous amounts of silver, gold, lead, bismuth, and tungsten, and (2) a chalcopyrite-bearing quartz vein 1 to 

3 feet wide cutting monzonite at locality 13 (fig. 3); a 

sample of this material (table 3, sample 13) contains 

anomalous amounts of tungsten in addition to copper. 

Hawk River 

Sediment from streams in a large area south of Purcell Mountain, which includes the headwaters of the 

Hawk River, contains anomalous amounts oflead, copper, and silver (fig. 5, table 5). These streams drain 

an area in which quartz veins containing pyrite, and 

locally, argentiferous galena are abundant. 

The bedrock south of Purcell Mountain (fig. 5) consists of andesitic volcanic rocks of Late Jurassic(?) 

and Early Cretaceous age and quartz latite volcanic 

rocks of Late Cretaceous age which are intruded by 

alaskite and quartz monzonite of Late Cretaceous age 

(Patton and others, 1968). The older andesitic volcanic 

rocks are thermally metamorphosed in the vicinity of 

the Wheeler Creek pluton, Purcell Mountain pluton, 

and Hawk River pluton. The Upper Cretaceous quartz 

latite is t h e r m a 11 y metamorphosed only near the 

Table 5.--Analyses of selected grab samples from the Hawk River locality near Purcell Mountain 

(Results reported in parts per million. Gold analyses by atomic absorption; all others by semiquantitative 

spectrographic analysis with the results reported to the nearest number in the series 1, 0.7, o.s, 0.3, 0.2~ 

0.15, 0.1. N, not detected;>, greater than value.sbown. Sample localities are shown in figure 5] 

Sample No. Lab. No, Cu Ag Au Pb Zn Description 

1 ACM-176 10 N 0.02 so N Oxidized pyrite in quartz. 

2A 178 70 3 N 300 N Do. 

28 179 30 100 .03 >S,OOO 1,SOO Galena in quartz; contains 150 ppm of Sb. 

2C ACN-881 700 200 .3 20,000 500 Galena in quartz. 

3 ACM--180 3,000 so .os 1,500 N Chalcopyrite-galena in quartz. 

4 ACN-486 70 3 .05 300 300 Oxidized pyrite in quartz~ 

Limits of determination 5 .5 .02 10 200 

10 
APPROXIMkTE MEAN 

DECLINATION, 1968 

0 2 3 4 MILES 

EXPLANATION 

E:)i~~~/1 Flood-plain deposits 

[E 

Alluvium 

~ Alas kite 

r:-~l:l Quartz latite 

Ande8itie 

~ 

~ voleanie rocks 

.Fnlt 

Qur&-.,.E zone 

~ 2.0 

Sample of stream eediment 

Cof'1WI' ....... eoRtMt 

,.,.,. calaow, ,.,. leG4 .ituo. ~ 

... Lecatiall 

~ 

of...,.... Nelt ....... 

ia tal»>e-5 

)I 

11'1 

;:::) 

0 

ILl 

u 

~ a:: 

u 

Figure 5.--Geoloaic aap of the Purcell Mountains area. 

Jl 

Wheeler Creek pluton. Numerous fine-grainedpyritiferous felsic dikes as much as tens of feet wide cut 

both the andesitic and quartz latite volcanic rocks 

throughout this area. 

Pyritiferous milky-white vuggy quartz veins showing well-developed comb structure cut both volcanic 

units in a northwest-trending belt 6M by IM miles in 

areal extent (fig. 5). This belt is parallel to a zone 

of prominent northwest-trending lineations and faults. 

The veins are mostly narrow, ranging in width from 

1 inch to 2 feet, and they appear to strike northeast 

and dip steeply to the north. They seem to occur as 

clusters of veins 2 to 20 feet apart; the most extensive concentrations of quartz veins are at localities 

2 and 3 (fig. 5). Locally, they contain irregular 

segregations of argentiferous g a I en a with I e s s e r 

amounts of chalcopyrite and malachite. Pyrite occurs 

in cubes up to 1 inch on a side and is almost completely 

oxidized to limonite; galena is fresh and fills vugs and 

fissures along the edge of the veins. Chemical analyses of grab samples representative of these deposits 

are given in table 5. No galena or chalcopyrite was 

found in the andesitic or latitic volcanic rocks; however, the andesitic wallrock locally contains abundant 

dis seminated pyrite. 

The mineralized quartz veins cut the Upper Cretaceous quartz latite, dated at 86 m. y. (Patton and others, 

1968), and may be related to the intrusion of the 

Wheeler Creek pluton dated at 80 m.y. (Miller and 

others, 1966) that is just east of this area (fig. 5). 

Purcell Mountain 

A small "inactive placer mine is on a stream draining to the west from the Purcell Mountain pluton (fig. 

5). The stream crosses the contact between the quartz 

monzonite and andesite. Although no large zones of 

altered rock were observed in the area, abundant 

pebbles and cobbles of black tourmaline as much as 

3 inches in diameter and of albitizedquartzmonzonite 

cut by black tourmaline veinlets were found in the 

tailings a.-,d gravels of this stream and the stream to 

the north. ·The tourmaline is associated with quartz 

and sparse disseminated oxidized sulfide, and it occurs in the veinlets as radiating sunbursts of crystals 

with individual crystal lengths of a few millimeters. 

The quartz monzonite appears to be altered adjacent 

to the veinlets. Analyses of two tourmaline-rich altered quartz monzonite cobbles show 700 and 1,000 

ppm of copper but no detectable gold. The abundance 

of tourmaline, the small drainage basin, and the lack 

of any exposed large alteration zones suggest that the 

source of the placer gold here may be quartztourmaline-sulfide veins which cut and altered the 

quartz monzonite near the andesite contact. If this is 

so, the presence of tourmaline veinlets in place or in 

material in the alluvium may serve as a guide to possible gold mineralization elsewhere in this part of the 

region; similar tourmaline-rich cobbles are found 

within the Bear Creek placer-mine area on the east 
side of the Zane Hills and in a stream ::lraining the 

Dakli copper prospect. 

SUMMARY 

Investigation of the central Koyukuk River region 

located seven areas containing mineral occurrences 

thought to be worthy of further prospecting. Because 

of the size of the district and the short time available 

for detailed investigation, individual areas were not 

stud~ed in detail, and more concentrated surfaceprospectmg and c I o s e r spaced geochemical stream-sediment sampling is needed to determine the extent and 

amount of mineralized rock. These mineralized areas 

are all near granitic plutons, and future prospecting 

elsewhere in the region should be conducted with this 

in mind. 

REFERENCES CITED 

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J. R., 1965, Map showing extent of glaciations in 

Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map 

1-415, scale 1:2,500,000. 

Eakin, H. M., 1916, The Yukon-Koyukuk region, Alaska: 

U.S. Geol. Survey Bull. 631, 88 p. 

Fernald, A. T., 1964, Surficial geology of the central 

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Geol. Survey Bull. 1181 -K, 31 p. 

Miller, D. J., Payne, T. G., and Gryc, George, 1959, 

G~ology of possible petroleum provinces in Alaska, 

with an annotated bibliography by E. H. Cobb: U.S. 

Geol. Survey Bull. I 094, 131 p. 

12 

Miller, T. P., Patton, W. W., Jr., and Lanphere, M. A., 

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U.S. Geol. Survey Prof. Paper 550-D, p. D158-

DI62. 

Patton, W. W., Jr., and Miller, T. P., 1966, Regional 

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U.S. Geol. Survey Misc. Geol. Inv. Mapl-459, scale 

1:250,000. 

Patton, W. W., Jr., and Miller, T. P., 1968, Regional 

geologic map of the Selawik and southeastern Baird 

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Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map 

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Schrader, F. C., 1904, A reconnaissance inn or the r n 

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John, Anaktuvuk, and Colville Rivers and the Arctic 

coast to Cape Lisburne, in 1901: U.S. Geol. Survey 

Prof. Paper 20, 139 p. 

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GPO 944.101