GEOLOGICAL SURVEY CIRCULAR 793 

The Alaskan Mineral Resource Assessment 

Program: Background Information to 

Accompany Folio of Geologic and Mineral 

Resource Maps of the Ambler River 

Quadrangle, Alaska 

Prepared in cooperation with the State of Alaska 

Department of Natural Resources, Division of 

Geological and Geophysical Surveys 

THE ALASKAN MINERAL RESOURCE 

ASSESSMENT PROGRAM 
Large boulders of nephrite (jade~ which have weathered from serpentinite at the jade prospect on south side of the Jade Mountains. (Photo by I. L. Tailleur, 1972~ 

Exploratory development at the Arctic copper-zinc-lead-silver deposit on the ridge divide between the Kogoluktuk and Shungnak 

Rivers. (Photo by I. L. Tailleur, 1972~ 
The Alaskan Mineral Resource Assessment 

Program: Background Information to 

Accompany Folio of Geologic and Mineral 

Resource Maps of the Ambler River 

Quadrangle, Alaska 

By C. F. Mayfield, I. L. Tailleur, N. R. D. Albert, 

lnyo Ellersieck, Donald Grybeck, and S. W. Hackett 

GEOLOGICAL SURVEY CIRCULAR 793 

Prepared in cooperation with the State of Alaska 

Department of Natural Resources, Division of 

Geological and Geophysical Surveys 

1983 
United States Department of the Interior 

JAMES G. WATT, Secretary 

Geological Survey 

Dallas L. Peck, Director 

Library of Congress catalog-card No. 83-600555 

Free on application to Branch of Distribution, U.S. Geological Survey, 604 South Pickett Street, Alexandria, VA 22304 
Abstract --------------------------------------

Introduction ----------------------------------

Purpose and scope --------------------------

Geography and access -----------------------

Acknowledgr.nents --------------------------

Summary of mineral exploration and production ------

Geologic investigations --------------------------

Previous geologic and mineral resource investigations ----------------------------------

Present study ------------------------------

Geology of the Ambler River quadrangle ------------

Geologic terranes ---------------------------

~etamorphism -----------------------------

Very low-grade metamorphic rocks ----------

Low-grade metamorphic rocks --------------

Low- to medium-grade contact metamorphic 

rocks-------------------------------

History of metamorphism -----------------

CONTENTS 

Page 

1 

1 

1 

1 

2 

4 

4 

4 

5 

6 

6 

9 

9 

9 

11 

11 

Geology of the Ambler River quadrangle-Continued 

Potassium-argon ages ------------------------

Uranium-lead ages -----------------------

Lead-alpha ages -------------------------

Fossil localities -----------------------------

Component maps of the Ambler River quadrangle 

folio --------------------------------------

Geology report OF 78-120-A ------------------

Reconnaissance geochemistry reports OF 78-120-B 

throughH------------------------------

~ineral occurrences and resources report OF 78-

120-I ---------------------------------

Interpretation of Landsat imagery report OF 78-

120-J ---------------------------------

Aeromagnetic map and interpretation (report OF 

78-120-K) ---------------=-------------

Bibliography ----------------------------------

ILLUSTRATIONS 

FRONTISPIECE. Jade prospect and the Arctic "copper-zinc-lead-silver" deposit in the Ambler River quadrangle 

FIGURE 1. Index map of Alaska showing location of the Ambler River quadrangle and northeastern part of the 

Page 

12 

12 

13 

16 

16 

16 

20 

21 

22 

22 

24 

Page 

Shungnak quadrangle ------------------------------------------------------------------ 3 

2-6. ~aps showing: 

2. Geologic terrances of the Ambler River quadrangle and northeastern part of the Shungnak 

quadrangle--------------------------------------------------------------------------- 7 

3. ~etamorphic boundaries in the Ambler River quadrangle and northeastern part of the Shungnak 

quadrangle--------------------------------------------------------------------------- 10 

4. Location of rocks that have been dated by radiogenic methods in the Ambler River quadrangle and 

northeastern part of the Shungnak quadrangle ---------------------------------------------- 13 

5. Fossil localities in the Ambler River quadrangle and northeastern part of the Shungnak 

quadrangle--------------------------------------------------------------------------- 17 

6. Aeromagnetic terranes of the Ambler River quadrangle and northeastern part of the Shungnak 

quadrangle--------------------------------------------------------------------------- 24 

III 
TABLES 

TABLE 1. Component maps of the Ambler River quadrangle mineral resource assessment -----------------------

2. Potassium-argon ages from the Ambler River quadrangle -----------------------------------------

3. Uranium-lead ages from the Ambler River quadrangle--------------------------------------------

4. Lead-alpha ages from the Ambler River quadrangle----------------------------------------------

5. Fossils collected in the Ambler River quadrangle------------------------------------------------

6. Estimate of number, grade, and tonnage of volcanogenic deposits ----------------------------------

IV 

Page 

2 

14 

16 

16 

18 

21 
The Alaskan Mineral Resource Assessment Program: 

Background Information to Accompany Folio 

of Geologic and Mineral Resource Maps 

of the Ambler River Quadrangle, Alaska 

By C. F. Mayfield, I. L. Tailleur, N. R. D. Albert, lnyo Ellersieck, Donald Grybeck, and S. W. Hackett1 

ABSTRACT 

The Ambler River quadrangle, consisting of 

14,290 km2 (5,520 mP) in northwest Alaska, was 

investigated by an interdisciplinary research team 

for the purpose of assessing the mineral resource 

potential of the quadrangle. This report provides 

background information for a folio of maps on the 

geology, reconnaissance geochemistry, aeromagnetics, Landsat imagery, and mineral resource 

evaluation of the quadrangle. A summary of the 

geologic history, radiometric dates, and fossil localities and a comprehensive bibliography are also 

included. The quadrangle contains jade reserves, 

now being mined, and potentially significant resources of copper, zinc, lead, and silver. 

INTRODUCTION 

PURPOSE AND SCOPE 

This circular and a separately available folio of 

maps constitute a compilation and assessment of 

the mineral resources of the Ambler River quadrangle, Brooks Range, Alaska, prepared under the 

auspices of the Alaskan Mineral Resource Assessment Program (AMRAP). The purpose of AMRAP 

is to provide basic information which will help Federal, State, and local agencies and industry formulate mineral resource policies and provide a basis 

for making local land-use planning decisions. 

AMRAP also intends to update existing mineral 

resource compilations, to help stimulate private resource development, and to increase understanding of the geology of Alaska 

1

Alaska Division of Geological and Geophysical Surveys, Fairbanks, Alaska 99701. 

1 

The 11 maps in the folio of the Ambler River 

quadrangle mineral resource assessment are listed 

in table 1. The folio is an integration of the geology, geochemistry, geophysics, telegeology, and 

mineral occurrences into a series of reports that 

summarize the currently known mineral resource 

potential of the quadrangle. It represents the 

efforts of an interdisciplinary team of geoscientists, each contributing knowledge in his field of 

specialty to the resource assessment. 

This circular is designed to not only draw attention to the folio of mineral assessment maps but 

also to provide background information relevant 

to understanding what is known about the complex geology of the quadrangle. New information 

that has become available since the publication of 

the maps in the folio is also presented. The circular 

includes a brief history of mineral exploration and 

production, a summary of the geology, and description of the regional metamorphism, and a 

compilation of rock ages based on radiometric 

dates and fossil localities. This is followed by a 

short description of each component map in the 

folio. In addition, the circular contains a comprehensive geologic bibliography of the Ambler River 

quadrangle. 

GEOGRAPHY AND ACCESS 

The Ambler River quadrangle covers approximately 14,290 km2 (5,520 mi2

) in northwestern 

Alaska (fig. 1) between 67° and 68° N.latitude and 

156° and 159° W.longitude. An addition of part of 

the Shungnak quadrangle to the southeast comer 

of the Ambler River quadrangle has been included 

in the folio resource assessment maps to show the 
TABLE I.-Component maps of the Ambler River quadrangle 

mineral resource assessment 

U.S. Geological Survey Open-File 

(OF) Reports 

OF 78-120-A (Mayfield and Tailleur, 

Subject 

1978) --------------------------- Geology. 

OF 78-120-B (Ellersieck, 1978f) ------ Stream-sediment 

geochemical 

sample 

locations. 

OF 78-120-C (Ellersieck, 1978a) ------ Analytical results 

for streamsediment 

samples. 

OF 78-120-D (Ellersieck, 1978e) ------ Lead and zinc 

geochemical 

anomalies. 

OF 78-120-E (Ellersieck, 1978d) ------ Copper and 

molybdenum 

geochemical 

anomalies. 

OF 78-120-F (Ellersieck, 1978g) ------ Tin and beryllium 

geochemical 

anomalies. 

OF 78-120-G (Ellersieck, 1978b) ------ Barium, silver, 

and arsenic 

geochemical 

anomalies. 

OF 78-120-H (Ellersieck, 1978c) ------ Chromium, nickel, 

and cobalt 

geochemical 

anomalies. 

OF 78-120-1 (Mayfield and Grybeck, Mineral 

1978) --------------------------- occurrences and 

resources. 

OF 78-120-J (Albert, 1978) ---------- Interpretation of 

satellite imagery. 

OF 78-120-K (Hackett, 1980) -------- Aeromagnetic 

map and 

interpretation. 

entire geology of the Cosmos Hills. The Baird and 

Schwatka Mountains of the southwest Brooks 

/Range trend in a west-northwesterly direction 

through the central part of the area Streams in 

the south half of the quadrangle flow into the 

Kobuk River, and in the north half, they flow into 

2 

the Noatak River. Elevations range from 30 m 

(100ft) along the lower Kobuk River in the southwest to just over 1970 m (6,000 ft) in the Schwatka 

Mountains in the east-central part of the quadrangle. Most of the mountains rise to 1,000-1,300 m 

(3,000-4,000 ft) and intervening valleys from 

about 160 to 650 m (500 to 2,000 ft). The area has 

undergone multiple stages of glaciation in the 

Pleistocene which formed the broad U-shaped 

valleys that contain most of the larger streams in 

the mountains (Fernald, 1964). Remnants of these 

glaciers are visible as small stagnant ice fields in a 

few of the north-facing cirques in the east-central 

part of the quadrangle. 

The Ambler River quadrangle is situated in the 

back country of Alaska There are no roads leading 

into the area, and the most feasible access is by airplane. There are three improved dirt airstrips. One 

is located at the village of Ambler on the Kobuk 

River, and the other two are in the Cosmos Hills at 

the mining camp of Bornite on Ruby Creek and 

near Dahl Creek. As a result, access to most other 

areas is by helicopter or by light plane which must 

land on gravel bars or small lakes. The distance to 

the nearest supply centers are Kotzebue, 230 km; 

Bettles, 260 km; and Fairbanks, 530 km. Access 

can also be gained to the quadrangle by small boat 

on the Kobuk or Noatak Rivers. Larger items can 

be brought into the area by barge from Kotzebue 

during the summer months if the Kobuk River is 

high enough. There is a dirt road between the village of Kobuk on the Kobuk River and the mining 

camp of Bornite in the Cosmos Hills. 

The only permanent town in the quadrangle is 

Ambler, with a population about 200, located at 

the confluence of the Kobuk and Ambler Rivers. 

The mining camps of Bornite, in the Cosmos Hills, 

and Arctic Camp, on a tributary of the Ambler 

River, have a seasonal fluctuation in population 

from caretaker status in the winter up to several 

dozen people in the summer. Except for irregularly 

occupied cabins strung out along the Kobuk River 

and transitory mining exploration camps in the 

southeastern part of the quadrangle, the remainder of the area is infrequently occupied or 

visited. 

Acknowledgments.-The Ambler River AMRAP 

project is a combined effort between geoscientists 

from the Alaska Division of Geological and Geophysical Surveys (ADGGS) and the U.S. Geological Survey. Appreciation is given to G. H. Pessel, 

R. E. Garland, G. R. Eakins, and M. A. Wiltse of 
St Matthew 1% 

0 

ARCTIC 

CHUKCHI 

SEA 

OCEAN 

PACIFIC OCEAN 

t I 

500 KILOMETERS J I 

300 MILES 

FIGURE I.-Index map of Alaska showing the location of the Ambler River quadrangle and the northeastern part of the Shungnak quadrangle. Area of report is shaded. 

the ADGGS and W. P. Brosge of the U.S. 

Geological Survey who helped conduct the early 

phases of investigation but did not take a direct 

role in the compilation of this circular or the folio 

of mineral assessment maps. These geologists 

have graciously made available field notes, rock 

samples, thin sections, and mineral occurrence 

localities as well as provided us with helpful concepts relating to the general geology of the quadrangle. Other scientists, who visited the area and 

helped with geologic mapping, petrography, and 

age dating, are W. B. Hamilton of the U.S. Geological Survey, W. G. Gilbert of the ADGGS, and 

R. B. Forbes, J. R. Carden, and D. L. Turner of the 

University of Alaska Geophysical Institute. 

The work was greatly facilitated by the help of 

local citizens or other workers engaged in various 

aspects of exploring the country. The cooperation 

of many people engaged in minerals exploration, 

3 

particularly H. A. Sichermann, R. H. W. Chadwick, N. L. Lutz, R. C. Babcock Jr., and Richard 

Walters (all formerly with Bear Creek Mining 

Company) is appreciated. They shared their ideas 

and contributed logistical support when needed. 

Tommy Douglas, from Ambler, and John Moyer, 

from Kobuk, provided helpful information on 

mineral occurrences and past mining history. Data 

on some mineral prospects come from the following exploration companies: Anaconda Mining 

Company, Bear Creek Mining Company, Noranda 

Exploration, and WGM Inc. Also Buck Maxson of 

Maxson's Aviation was of great help in giving 

competent and reliable supply by bush plane to a 

remote field camp on the Noatak River. Office and 

laboratory personnel giving much needed assistance include T. C. Trible of the State of Alaska 

Mineral Analysis and Research Laboratory and 

R. M. O'Leary, S. K. McDanal, and C. M. 
McDougal of the U.S. Geological Survey for providing chemical analyses and computer compilations of the geochemical sampling data. 

SUMMARY OF MINERAL EXPLORATION 

AND PRODUCTION 

Gold has been the principal commodity produced from the quadrangle. The first discoveries 

are believed to have been made in the Cosmos Hills 

in 1898 or 1899 which led to a small gold rush 

about 1900. Most gold was produced between 

1900 and 1940 with peak production from 1910 to 

1920 from placers located on Dahl Creek, California Creek, and the Shungnak River. Placer operations continued on a smaller scale to the early 

1960's; in recent years placer claims have been 

largely inactive. The total amount of gold produced is estimated to be between 466 and 1244 kg 

(15,000 and 40,000 oz) almost entirely mined from 

placers in the Cosmos Hills. 

During the gold rush, copper prospects were discovered at Pardners Hill and Ruby Creek in the 

Cosmos Hills, where a small amount of trenching 

and tunneling was done. In 1948, interest in the 

properties was revived when airborne surveys 

showed a radiometric anomaly associated with the 

old copper prospect at Ruby Creek. Rhinehart 

Berg prospected the anomaly by bulldozing, 

trenching, and a small amount of drilling which 

uncovered enough mineralized ground to interest 

Bear Creek Mining Company into conducting a 

systematic drilling program in 1958 and 1959. 

This program resulted in the discovery of significant quantities of high-grade copper ore in carbonate breccia. A more complete history of the 

exploration of the Ruby Creek deposit is reported 

by Fritts (1970). The discovery at Ruby Creek encouraged the Bear Creek Mining Company to conduct geochemical exploration in the mountains 

north of the Cosmos Hills. This exploration, in the 

mid 1960's, resulted in the discovery of several 

volcanogenic deposits of which the Arctic deposit 

has the largest known reserves. In recent years, 

exploration by several mining companies has 

resulted in the discoveries of several additional 

volcanogenic deposits. Mining and development of 

these deposits beyond the initial assessment stage 

have not yet begun. 

Occurrences of asbestos minerals have been 

known from several localities on the south edge of 

4 

the Brooks Range since the gold rush. The only attempt at mining any of these prospects was at the 

Ing-Ihk mine in the Cosmos Hills in 1944-1945 

(Coats, 1944; Heide and others, 1949). The operation was discontinued soon after a 4 7 -ton test shipment of high-grade ore was made. 

Jade from deposits in the Jade Mountains and 

Cosmos Hills had been collected and traded by the 

Eskimos long before white men ventured into the 

Arctic. These are the largest known deposits in 

Alaska. Since World War II, jade mining has supported one- and two-man summer operations in the 

Jade Mountains and Cosmos Hills. Although production estimates have never been reported, 

reserves at these locations are expected to keep 

small operations in business for many years. The 

deposits presently supply most of the commercial 

jade mined in Alaska. 

GEOLOGIC INVESTIGATIONS 

PREVIOUS GEOLOGIC AND 

MINERAL RESOURCE INVESTIGATIONS 

The first exploration of the central Kobuk Valley 

was made in 1884 and 1885 by two separate parties under different auspices. The first was led by 

J. C. Cantwell of the U.S. Revenue Marine Service 

(Cantwell, 1889; McLenegan, 1889), and the second was led by Lt. G. M. Stoney of the U.S. Navy 

(Stoney, 1900). Stoney's party wintered over at a 

camp on the Kobuk River south of the Cosmos 

Hills from which exploration was made into the 

nearby mountains and Noatak valley. The first recorded exploration into the Noatak valley was in 

1885 by S. B. McLenegan of the U.S. Revenue 

Marine Service (McLenegan, 1887). When gold 

was discovered in the Cosmos Hills in 1898 or 

1899, many prospectors ventured into this 

unknown territory. A prospector's account of the 

early conditions in 1898 is given by Grinnell 

(1901). The first expedition into the area by the 

U.S. Geological Survey was led by W. C. Mendenhall, who descended the Kobuk River and mapped 

the geology along the river at a scale of approximately 1:1,250,000 (Mendelhall, 1902). The first 

mineral investigation was conducted by P. S. 

Smith during traverses of the Kobuk and Noatak 

valleys in 1910 and 1911 (Smith, 1913; Smith and 

Eakin, 1911). In 1931, Irving Reed wrote a brief 

description of the placer mining operations in the 
Cosmos Hills (Reed, 1931 ). Many other reports of 

the Alaska mining industry cover the estimated 

yearly gold production from the period 1904 to 

1942. The most notable of these reports, by A. H. 

Brooks and by P. S. Smith, have been published as 

a series of U.S. Geological Survey Bulletins listed 

in the bibliography at the end of this report. A 

systematic compilation of mineral occurrences in 

the northwestern Brooks Range was made by 

Eskil Anderson during World War II (Anderson, 

1945; 1947). At the end of the war, the asbestos 

deposits in the Cosmos Hills and" Jade Mountains 

were evaluated by the U.S. Bureau of Mines 

(Heide and others, 1949) and the U.S. Geological 

Survey (Coats, 1944). 

Post-World War II studies by the U.S. Geological Survey included geologic reconnaissance of 

the Ambler River and adjacent quadrangles by 

I. L. Tailleur in 1955, 1958, and 1965. He compiled 

a large-scale geologic map of the Cosmos Hills and 

smaller scale maps of the mountains to the north; 

these maps are unpublished but have provided a 

basic framework for later geologic maps and studies in the quadrangle. In 1965 and 1966, the southem part of the quadrangle was investigated by 

Tailleur, W. W. Patton, Jr., and T. P. Miller, and 

they compiled a 1:250,000 scale geology map of the 

region (Patton and others, 1968). Additional reconnaissance mapping of the north and central parts 

of the quadrangle was carried out by Tailleur, 

W. P. Brosge, and H. N. Reiser during 1966 

(Brosge and others, 1967). The observed relations 

between Paleozoic clastic and carbonate units contributed significantly to an understanding of the 

stratigraphy and restored depositional settings of 

the Endicott and Baird Groups (Tailleur and 

others, 1967). Detailed aspects of the geology of 

the copper deposit on Ruby Creek are given in a 

Ph.D. dissertation and subsequent publications by 

D. D. Runnells (1963, 1964, 1965, 1966, and 1969). 

Other prospect examinations were made by 

various government agencies and mining companies during the 1950's, 1960's, and 1970's. 

Because most of these reports are unpublished, 

they are not readily available. The reports that 

have been brought to our attention are included in 

the bibliography of this report. 

In 1968 and 1969, C. E. Fritts of the Alaska 

Division of Mines and Geology completed a project of detailed geologic mapping and geochemical 

sampling in the Cosmos Hills (Fritts, 1969a, b; 

1970). In 1971, Fritts began a similar project in the 

5 

southern part of the Schwatka Mountains which 

continued until his untimely death in a canoeing 

accident on the Kogoluktuk River in 1972. G. H. 

Pessel, R. E. Garland, and G. R. Eakins resumed 

the field operations in the same year and requested 

Tailleur' s assistance. This is the year the current 

project began, and it continues with the publication of this circular as a cooperative effort between 

the Alaska Division of Geological and Geophysical 

Surveys and U.S. Geological Survey. 

PRESENT STUDY 

Most of the field and laboratory work closely 

related to this study was begun in 1972 by Pessel, 

Garland, and Eakins of the ADGGS and Tailleur 

of the U.S. Geological Survey. These workers released stream-sediment geochemical analyses 

(Garland and 'Pessel, 1975) and a geologic map of 

the southeastern part of the quadrangle at 

1:63,360 scale (Pessel and others, 1973). In 1973 

and 1974, Pessel and Tailleur were joined by 

Brosge of the U.S. Geological Survey, and the 

project was expanded in the mountains of the 

south-central and southwestern part of the quadrangle. Stream-sediment geochemical analyses 

from this part of the area were released by Garland 

and others (1975) and Pessel (1976). Detailed mapping and study of selected volcanogenic deposits 

were carried out by M. A. Wiltse (1975). Gilbert 

and others (1977) completed a geologic and geophysical cross-section in the Ruby Ridge area of 

the metamorphic terrane which contains the 

volcanogenic deposits. A preliminary geologic 

map at 1:250,000 scale was compiled by Pessel and 

Brosge (1977) from fieldwork of the years 

1955-1966 and 1972-1974). Geologic mapping and 

geochemical sampling in the north half of the 

quadrangle were concluded in 1976 by Tailleur, 

Mayfield, and Ellersieck. An aeromagnetic survey 

was flown in the western and eastern parts of the 

quadrangle in 197 4 and 197 5 under contract to the 

ADGGS. The resulting aeromagnetic contour 

maps were released by S. W. Hackett (1977). Landsat imagery was analyzed in 1978 by N. R. D. 

Albert. Although many of the above-mentioned 

geologists did not take an active role in the mineral 

resource assessment of the Ambler River quadrangle, their published reports as well as rock 

samples, field notes, and informal discussions constitute a valuable contribution to this report and 

to the mineral assessment maps. 
GEOLOGY OF THE 

AMBLER RIVER QUADRANGLE 

GEOLOGIC TERRANES 

Rocks in the Ambler River quadrangle are here 

broadly divided into five terranes on the basis of 

age and lithologic association. These geologic subdivisions are: the Kobuk River terrane, the Jade 

Mountains-Angayucham Mountains terrane, the 

schist belt terrane, the Schwatka Mountains terrane, and the Endicott' Mountains terrane (fig. 2). 

The Kobuk River terrane is located along the 

south edge of the quadrangle where it consists of 

the rocks of the Kobuk valley and southern foothills of the Brooks Range. Most of the area is covered by forest or tundra growing on a veneer of 

unconsolidated alluvial and glacial deposits. 

Bedrock, exposed discontinuously in stream cuts 

or hilltops, consists of folded and faulted Early 

Cretaceous sedimentary rocks which are the 

youngest rocks in the quadrangle. These rocks 

mostly consist of micaceous quartz conglomerate 

and lesser interbedded sandstone and mudstone 

that have a few thin coal seams visible in stream 

cuts along the lower Ambler River. Induration of 

the rocks appears to increase toward the north and 

is greatest in the sedimentary strata flanking the 

Cosmos Hills where, in some places, conglomerate 

clasts appear to be tectonically stretched. Bedrock 

in this terrane generally dips south and unconformably onlaps rocks of the Jade MountainsAngayucham Mountains terrane to the north. 

The Jade Mountains-Angayucham Mountains 

terrane, located along the south flank of the 

Brooks Range, consists of a relatively narrow belt 

of mafic and ultramafic rocks with minor chert, 

fine-grained clastic rocks, and reef carbonate 

rocks. Mafic rocks are mostly diabase and basalt 

with local pillow structures and may be of several 

different ages. In some places, where they were 

most highly recrystallized by regional metamorphism, they have been transformed to fine-grained 

chlorite-albite schist. In the Angayucham Mountains the rocks are not strongly metamorphosed, 

and bedded chert, correlative with Triassic chert 

east of the quadrangle (Plafker and others, 1978), 

is locally found associated with the basalts. Ultramafic rocks consist of discontinuous serpentinites 

in the Cosmos Hills and Angayucham Mountains 

and serpentinized dunite or peridotite in the Jade 

Mountains and near the Hunt River. In the Jade 

Mountains, peridotite or dunite rests upon diabase 

6 

or basalt with a probable fault contact. In many 

places, serpentinite was probably squeezed into 

shear zones(?) in either mafic or carbonate and 

shale sequences. Paleozoic limestone and dolomite 

and intercalated phyllite occur structurally below 

most mafic-ultramafic rocks. However, in the 

breached antiform that is exposed in the Cosmos 

Hills, middle Paleozoic carbonate rocks may have 

been locally intruded by mafic igneous rocks or 

may have been deposited contemporaneously with 

mafic volcanic rocks (Fritts, 1970). Carbonate 

rocks are more than 200 m thick in some places in 

the Cosmos Hills and Jade Mountains and completely absent or thin and discontinuous in other 

places. Except for north-dipping beds along the 

north side of the Cosmos Hills antiform, rocks of 

this terrane dip south away from the Brooks 

Range. In most exposures, ultramafic rocks overlie mafic rocks which in tum overlie carbonate 

rocks and phyllite. This west-trending belt of 

mafic and ultramafic rocks flanking the mountains 

may represent a late Mesozoic plate boundary between what is now the Yukon-Koyukuk Province 

to the south and the Brooks Range to the north 

(Tailleur, 1973; Patton and others, 1978; Roeder 

and Mull, 1978). 

The schist belt terrane is a thick accumulation of 

metamorphosed sedimentary rocks constituting 

the south half of the Baird and Schwatka Mountains. Most rocks are gray phyllite or quartz-mica 

schist thought to have been a thick pile of predominantly pelitic clastic rocks prior to being 

metamorphosed. Greenstone and amphibolite, 

metamorphosed from mafic igneous rocks, are dispersed throughout the terrane but make up only a 

relatively small volume of it. Included within the 

east half of the terrane is the volcanogenic copper 

belt, which, in addition to the rocks described 

above, also contains felsic schist, carbonaceous 

schist, and marble. The volcanogenic stratabound 

copper-zinc-lead-silver deposits are generally 

associated with felsic schist. Metamorphic recrystallization is greatest in the north and central 

parts of the terrane and becomes progressively 

less intense at the south edge of the mountains 

where the rocks are predominantly phyllite. Folding is complex and commonly isoclinal and shows 

more than one period of deformation. The age of 

the schist belt rocks may be both Precambrian and 

Paleozoic. The age of metamorphism of these 

rocks is controversial; it may have occurred at certain stages in the Precambrian, Paleozoic, and 

Mesozoic (Gilbert and others, 1977; Turner and 
25 50 KILOMETERS 

FIGURE 2.-Geologic terranes of the Ambler River quadrangle and the northeastern part of the Shungnak quadrangle. 

others, 1978; Smith and others, 1978; Dillon and 

others, 1980). 

The northern contact of the schist belt terrane is 

a sharp lithologic boundary in the east half of the 

quadrangle. Fritts (1970) originally called this contact the western extension of the Walker Lake 

fault that he first mapped in the Survey Pass 

quadrangle. Later semidetailed maps (Pessel and 

others, 1973; Pessel and Brosge, 1977) show that it 

may instead be a major stratigraphic unconformity. The age of the rocks adjacent to the boundary 

is not known, and the contact is parallel to the 

7 

strike of metasedimentary rocks in most areas. In 

the west half of the Ambler River quadrangle, the 

northern boundary of the schist belt is not so 

sharply defined, possibly because this lithologic 

break has been folded and faulted along the 

contact. 

The north-central part of the Ambler River 

quadrangle is herein called the Schwatka Mountains terrane. This terrane consists of a variety of 

metasedimentary rocks of which the early and 

middle Paleozoic Skajit Limestone is most widely 

exposed. Calcareous schist, quartzite, dolomite, 
phyllite, and felsic volcanic(?) rocks also make up 

large parts of this terrane. Metamorphosed granitic plutons of probable middle Paleozoic age 

have intruded the rocks of the southern part. Recrystallization has obliterated the fossil record in 

most areas, but it is believed that the rocks are 

predominantly Paleozoic. The oldest known rocks 

in this terrane are Ordovician; this age is based on 

stratigraphic correlation with graptolite-bearing 

rocks located west of the quadrangle (U.S. Geological Survey, 1975b). The age of the youngest rocks, 

probably Permian, is based upon stratigraphic correlation with rocks located east of the quadrangle 

(Mull and Tailleur, 1977). The northeast edge of 

the terrane is overlain by progressively less metamorphosed clastic rocks of Late Devonian age. 

The northern part of the Ambler River quadrangle, called the Endicott Mountains terrane (fig. 2), 

is composed of north-dipping sedimentary rocks 

that structurally overlie the Schwatka Mountains 

terrane. It is an allochthonous sequence mostly 

composed of Late Devonian shale, sandstone, and 

conglomerate that was deposited as a thick clastic 

wedge. The rocks were derived from a north-toeast source area (Donovan and Tailleur, 1975; 

Nilsen and others, 1980). These sedimentary rocks 

and the younger Paleozoic and Mesozoic rocks 

that stratigraphically overlie them in the Howard 

Pass quadrangle to the north are distinctly different from the partly coeval sequence of rocks in 

the Schwatka Mountains terrane. The best explanation for these regional structural and 

stratigraphic relations is that the Endicott Mountains terrane in the Ambler River quadrangle 

represents the south edge of the Brooks Range 

allochthon (Martin, 1970; Mayfield and others, 

1978; Ellersieck and others, 1979) and is the lower 

part of a series of coeval allochthonous sequences 

that were thrust northward more than 150 km in 

the Early Cretaceous orogeny to create the 

bedrock that is exposed in the DeLong and Endicott Mountains of the north half of the Brooks 

Range (Mull and others, 1976). If this reconstruction is correct, then the Kanayut ConglomerateHunt Fork Shale clastic wedge was deposited 

south of the schist belt terrane and was derived in 

part from rocks eroded from the Schwatka Mountains terrane, possibly unmetamorphosed parts of 

the schist belt terrane, and other terranes currently in the subsurface north of the quadrangle. 

This hypothesis requires a relatively simple 

structural history during the Brooks Range 

8 

orogeny in which rocks to the south were successively thrust over (or underthrust by) rocks to the 

north (Tailleur and Snelson, 1969; Mull and others, 

1976; Gealey, 1980). Another attraction of this 

scheme is that it requires the Endicott Mountains 

terrane and overlying allochtons to be deposited 

on a continental shelf south of the Schwatka 

Mountains and schist belt terranes which results 

in an orderly succession of the major facies patterns from the Devonian to the Jurassic. Some 

geologists have found this hypothesis unappealing 

because the Endicott Mountains terrane does not 

reappear at the south edge of the Brooks Range. 

For this hypothesis to be correct, it requires that 

the Endicott Mountains terrane was completely 

thrust over the Schwatka Mountains and schist 

belt terranes or that a southward extension of the 

sequence that contains the Kanayut Conglomerate 

and Hunt Fork Shale is buried beneath Cretaceous 

sedimentary rocks south of the Brooks Range. 

Dutro and others (1976) favor an alternative explanation for the similar structural position of the 

Endicott Mountains terrane to the east in the 

Wiseman quadrangle. They suggest that this terrane was first thrust southward over coeval and 

older rocks and later was thrust northward to produce the characteristic northward overturned 

folds and northward directed thrust faults of the 

Endicott Mountains. This hypothesis requires the 

source area for the Kanayut-Hunt Fork clastic 

wedge to be in the subsurface of the North Slope, 

and it requires a more complex structural evolution for the Brooks Range where southward overthrusting, which is not strongly supported by 

structural evidence, was followed by northward 

overthrusting. 

Dutro and others' (1976) hypothesis does not 

take into account the problems that arise in explaining the distinctive allochthons, each composed of Devonian or Mississippian to Cretaceous 

sedimentary rocks, which structurally overlie and 

are coeval with the sequence of rocks that contains 

the Endicott Mountains terrane north and west of 

the Ambler River quadrangle. If the Endicott 

Mountains terrane and the structurally higher 

allochthons were part of the same extensive depositional basin located north of the Schwatka 

Mountains terrane, as was implied by Dutro and 

others' (1976) hypothesis, then still left to be explained are (1) how this greater than 250-km-wide 

Devonian to Jurassic depositional basin became 

stacked up on itself and (2) how the complex depo-
sitional facies patterns were distributed in northem Alaska prior to the Brooks Range orogeny. 

METAMORPHISM 

Most rocks in the central part of the Ambler 

River quadrangle have undergone low-grade regional dynamothermal metamorphism (Winkler, 

1979). The structural grain of both the foliation 

and strike of metasedimentary rocks trends westnorthwest following the higher 'topography of the 

mountain belt. Metamorphic recrystallization is 

highest in the central Baird and Schwatka Mountains where fine-grained clastic rocks have been 

transformed to quartz-mica schist. Flanking the 

mountains along the Kobuk and Noatak valleys, 

fine-grained clastic rocks are best described as 

slate or phyllite. In the southeastern comer of the 

quadrangle, the Cosmos Hills antiform is ringed 

by strongly indurated and partially recrystallized 

metasedimentary rocks, which become progressively more metamorphosed toward the center of 

the arch to phyllite and quartz-mica schist. Rocks 

exposed in the axis of the Cosmos Hills antiform 

resemble rocks in the schist belt both lithologically 

and in metamorphic grade. 

Hundreds of rock samples taken during the 

course of the current and previous studies in the 

area have been examined petrographically. A plot 

of the mineral assemblages was used to construct 

a map that shows areas of different metamorphic 

grade (fig. 3). Each grade has common mineral 

assemblages that reflect differences in temperature and pressure of metamorphism from area to 

area. Metamorphic grade boundaries are drawn using the classification scheme of Winkler (1979). All 

boundaries that are shown in the quadrangle are 

approximate and gradational. 

Metamorphic rocks in the quadrangle are broadly 

grouped into three categories based upon temperatures reached during metamorphism. These categories are very low grade, low grade, and low to 

medium grade. Very low-grade and low-grade 

metamorphic rocks constitute the regionally metamorphosed rocks in the quadrangle and are equivalent to the greenschist facies (Turner, 1968). 

Within areas of low-grade metamorphism, rocks 

that were metamorphosed under relatively highpressure conditions are distinguished by the 

glaucophane zone, and rocks that were metamorphosed under relatively high temperature are 

9 

distinguished by the garnet zone. A third category, low- to medium-grade metamorphic rocks, 

consists of those affected by contact metamorphism near granitic plutons. Rocks shown as contact metamorphic on figure 3 correspond to the 

albite-epidote-hornfels facies or hornblendehornfels facies of Turner (1968). 

VERY LOW-GRADE METAMORPHIC ROCKS 

Metamorphic rocks designated as very low 

grade occur in the foothills of the Baird and 

Schwatka Mountains bordering the Kobuk and 

Noatak River valleys. Most rocks show only partial recrystallization. Fine-grained clastic rocks are 

slate, and mafic rocks are recognizable in hand 

specimen as basalt or diabase. Petrographic study 

of the mafic igneous rocks commonly reveals relict 

and corroded phenocrysts of augite. Common mineral assemblages include quartz, chlorite, calcite, 

kaolinite, and pumpellyite. In the Ambler River 

quadrangle, the mineral most diagnostic of this 

facies is pumpellyite, which occurs only in mafic 

igneous rocks or wacke that was, in part, derived 

from mafic rocks. 

LOW-GRADE METAMORPHIC ROCKS 

Low-grade metamorphic rocks make up most of 

the Baird and Schwatka Mountains and also crop 

out in the core of the Cosmos Hills antiform. 

Rocks that are believed originally to have been 

shale show a range of recrystallization from gray 

phyllite to fine-grained quartz-mica schist. Common mineral assemblages in metasedimentary 

rocks include quartz, albite, muscovite, chlorite, 

calcitl3, tourmaline, and chloritoid. Typical mineral 

assemblages in metamafic rocks include actinolite, 

sphene, albite, stilpnomelane, and clinozoisite. 

Within the broad area of low-grade metamorphic 

rocks, the glaucophane zone, indicative of relatively high pressure, and the garnet zone, indicative of relatively high temperature, are also shown 

on figure 3. They are characterized as follows: 

Glaucophane zone.-The glaucophane zone of 

low-grade metamorphic rocks occurs mainly along 

a discontinuous linear trend in the central and 

eastern part of the schist belt. Common minerals 

are glaucophane or crossite, clinozoisite, stilpnomelane, garnet, paragonite, and rare jadeite 
,;; . 

- Unmetamgrphosed .·t 

. .:/ 

25 50 MILES 

25 50 KILOMETERS 

EXPLANATION 

Very low-grade metamorphic rocks 

Low-grade metamorphic rocks 

Glaucophane zone 

Garnet zone 

Low-to medium-grade contact 

metamorphic rocks 

Metagranite 

Location of single or multiple 

petrographic samples 

F IGURE 3.-Metamorphic boundaries in the Ambler River quadrangle and the northeastern part of the Shungnak quadrangle. 

(Forbes and others, 1974; Gilbert and others, 1977; 

Carden, 1978). These minerals are found discontinuously in both metasedimentary and metamafic 

rocks. Glaucophane almost everywhere shows 

some degree of retrograde alteration to the common minerals of the low-pressure, low-grade meta10 

morphic rocks. It is likely that in many areas, 

glaucophane and jadeite were completely retrograded to lower-pressure mineral assemblages and 

that the area affected by high-pressure metamorphism was more extensive than is shown on figure 

3. The glaucophane zone of metamorphic rocks 
(fig. 3) in the Cosmos Hills is in doubt. Tailleur collected glaucophane from a metamorphic float cobble that he believes came from a nearby outcrop, 

but an attempt by Carden (1978) to recollect 

glaucophane from outcrops in the vicinity was unsuccessful. It is possible the sample was of glacial 

origin which, if true, would nullify the evidence for 

the glaucophane zone in the Cosmos Hills. 

Garnet zone.-The garnet zone, especially 

almandine-rich garnet, is indicative of the higher 

temperature part of low-grade metamorphism. 

This mineral occurs in metasedimentary and metamafic rocks along a northwest trend in the central 

part of the schist belt where regional metamorphism was greatest. This zone mostly overlaps the 

glaucophane zone, but it is more extensive, 

perhaps because garnet is more stable to retrograde metamorphism at lower pressures than is 

glaucophane. There is a gradual transition to lower 

temperature mineral assemblages and smaller 

mineral grain size of micaceous minerals both to 

the north and south. Typical mineral assemblages 

in metasedimentary rocks include quartz, albite, 

muscovite, chlorite, clinozoisite, garnet, tourmaline, chloritoid, and rare biotite. Metamafic 

rocks commonly contain green hornblende, garnet, 

albite, epidote, chlorite, and sphene. In most areas, 

almandine-spessartine garnet is a common and 

conspicuous mineral. It occurs as large poikilitic 

and locally helicitic crystals, many of which are 

partly or completely altered to chlorite or biotite. 

In the Cosmos Hills, garnetiferous metasedimentary rocks are dispersed in a wide band around 

metamorphosed granitic rock cropping out in the 

Kogoluktuk River valley. This relation suggests 

that the granitic rock may have produced a thermal halo during metamorphism. 

LOW- TO MEDIUM-GRADE 

CONTACT METAMORPHIC ROCKS 

Most contact metamorphic rocks in the quadrangle occur as thin discontinuous hornfels or tactite zones at the borders of a linear trend of 

granitic plutons north of the most highly recrystallized regionally metamorphosed rocks. Most 

tactites show some degree of foliation, but in a few 

localized zones, they appear to be unaffected by 

dynamothermal metamorphism. In detail, hornfels 

or tactite zones occur only locally in the areas that 

are shown as contact metamorphic in figure 3. 

11 

Typical mineral assemblages include diopside, 

wollastonite, garnet, andalusite, tremolite, and 

biotite. These minerals indicate relatively low 

pressure conditions and temperatures indicative of 

both low and medium grades of metamorphism 

(Winkler, 1979). 

HISTORY OF METAMORPHISM 

The metamorphic history of the rocks in the 

Ambler River quadrangle is complex because there 

is evidence that metamorphism occurred in the 

middle Paleozoic, Cretaceous, and possibly the late 

Precambrian. Most geologists who have studied 

the metamorphic rocks in the Ambler River quadrangle agree that a regional metamorphic event affected the southern Brooks Range in the early and 

middle Cretaceous, about 130-80 m.y. ago. This 

concept is strongly supported by the stratigraphic 

record of regional tectonism at this time and by 

many potassium-argon dates from metamorphic 

micaceous minerals in the schists and gneisses 

(Turner and others, 1979a). Metamorphism probably accompanied downwarping of the south edge 

of the Alaska Arctic plate as imbricated thrust 

sheets were stacked above the rocks in the central 

part of the quadrangle (Gealey, 1980). Later isostatic rebound and erosion of the crustal welt in 

the southern Brooks Range have exposed these 

metamorphic rocks throughout the Baird and 

Schwatka Mountains. 

Other radiogenic evidence dates the period of 

contact metamorphism that coincided with the intrusion of granitic plutons in the central part of 

the quadrangle during the middle Paleozoic (Dillon 

and Gilbert, 1978; Nelson and others, 1979). These 

data are supported by the stratigraphic record of a 

late Devonian uplift, probably accompanied by 

block faulting, in the Schwatka Mountains terrane 

(fig. 2) where the early Mississippian Kekiktuk 

Conglomerate appears to overlie Devonian and 

older rocks with an angular unconformity (Mull 

and Tailleur, 1977; Mayfield and Tailleur, 1978). 

Although the details of the late Devonian uplift 

are sketchy, there appears to be little evidence 

that the period of contact metamorphism coincided with a regional dynamothermal metamorphic event. 

Scattered potassium-argon metamorphic mineral ages in the Precambrian and Paleozoic suggest to Turner and others (1979a, b) that the 
southern Brooks Range was affected by an earlier 

period of regional high-pressure metamorphism 

that produced the glaucophane mineral assemblages in the late Precambrian. These data are supported by textural evidence from the metamorphic 

minerals which indicates that glaucophane and 

jadeite crystallized at an early stage in the 

metamorphic process and were later retrograded 

to lower pressure mineral assemblages (Forbes and 

others, 1974; Mayfield, 1976; Gilbert and others, 

1977; Carden, 1978). The interpretation of a late 

Precambrian metamorphic event in some rocks of 

the schist belt terrane (fig. 2) is complicated by the 

fact that all the rocks of this area were metamorphosed in the Cretaceous age. Some of the alleged 

Precambrian glaucophane-bearing rocks occur in 

the volcanogenic copper belt (fig. 2) closely 

associated with middle Paleozoic igneous and 

sedimentary rocks. Wiltse and Gilbert (1977) 

believe that the middle Paleozoic rocks were 

deposited unconformably upon a Precambrian 

basement, and during the Cretaceous regional 

metamorphism, the Precambrian and Paleozoic 

rocks became tectonically intercalated (Gilbert 

and others, 1977). Other geologists remain skeptical of these interpretations, which are based on 

mapping and sampling in an area of rocks that is 

recrystallized and isoclinally folded. To the west in 

the Baird Mountains quadrangle where the rocks 

are not so strongly recrystallized, glaucoplane has 

been found in mafic igneous rocks lying between 

homoclinally dipping fossiliferous middle Paleozoic limestones (67°25' latitude, 162°35.5' longitude). Nearby mafic igneous rocks, which appear 

similar to those with glaucophane, have intruded 

the limestones. If these relations are interpreted 

correctly, field observations require the unlikely 

geographic coextension of two separate glaucophane events, one that occurred in the late 

Precambrian and one that occurred after the middle Paleozoic. Reconciling the field observations 

with the Precambrian radiogenic age dates remains an important unresolved problem concerning the metamorphic rocks in the south-central 

and southwestern Brooks Range. 

POTASSIUM-ARGON AGES 

All potassium-argon ages in the quadrangle are 

located on figure 4 and listed in table 2. Most dates 

previously reported in various other publications 

12 

are summarized by Turner and others (1978; 

1979a). Ages from localities 26 and 27 are published here for the first time. The potassium-argon 

date from locality 27 may represent a minimum 

age for detrital muscovite in a feldspathic wacke 

rock unit. Since all the other ages are from rocks 

partly or completely recrystallized by metamorphism, the minerals that were dated either 

formed under metamorphic conditions or had their 

radiogenic constituents affected by a later 

metamorphism. 

Study of table 2 reveals a wide spread of 

potassium-argon ages ranging from Precambrian 

to Cretaceous. Almost all ages obtained from 

muscovite and biotite fall in the Cretaceous between 130 and 88 m.y. These dates are believed to 

reflect the last stage of regional metamorphism in 

the area which coincides with the period of most 

intense tectonic activity associated with the 

Brooks Range orogeny (Tailleur and Brosge, 

1970). Ages obtained from glaucophane and actinolitic hornblende present a much more complex 

and controversial picture. These amphibole ages 

range from 2,255 to 163 m.y. ago. Most preCretaceous ages cannot be taken at face value 

because there are too many instances where similar rocks located in a similar stratigraphic setting 

give widely divergent ages. Glaucophane samples 

yielding ages greater than 1 b.y. are believed to 

have been affected by excess argon so that they 

represent values that are too old by an undetermined amount (Turner and others, 1979a). A group 

of potassium-argon ages in the 760 to 570 m.y. 

range suggests that many of the rocks may have 

been subjected to a late Precambrian period of 

regional metamorphism. Under this hypothesis, 

the various Paleozoic and Mesozoic ages between 

560 and 150 m.y. may represent partial argon loss 

due to the later Cretaceous metamorphic event. A 

more complete discussion of this idea is presented 

by Turner, Forbes, and Mayfield (1978) and 

Turner, Forbes, and Dillon, (1979a, b). 

URANIUM-LEAD AGES 

U-Pb ages from two duplicate samples are reported by Dillon and others (1979); Dillon and 

others (1980) using the mineral zircon separated 

from blastoporphyritic schist (informally called 

the "button schist" by some geologists) and collected at the Arctic deposit in the volcanogenic 
, .. -

I . 

.,.L -~~-----

0 25 50 MILES 

~----~~----~ 25 50 KILOMETERS 

FIGURE 4.-Location of rocks that have been dated by radiogenic methods in the Ambler River quadrangle and the northeastern part of the Shungnak quadrangle. Numbers refer to listings in tables 2, 3, and 4. 

copper belt (table 3 and fig. 4). The ages from these 

samples, when combined with U-Pb ages from 

other similar granitic and rhyolitic rocks east of 

the Ambler River quadrangle, form a chord on a 

zooPb/23sU diagram that intersects concordia at 

about 360 m.y. and 80 m.y. The intersections of 

the chord of the U · Pb dates and the concordia 

curve are interpreted by Dillon to indicate an ap· 

proximate late Devonian crystallization age for 

the zircons in the copper belt volcanic rocks and an 

episodic lead loss from zircon sometime between 

13 

the Devonian and the end of Cretaceous regional 

metamorphism. Smith and others (1978) also report Paleozoic model lead ages, ranging from 350 

to 250 m.y., from the sulfide samples in metavolcanic rocks in the volcanogenic copper belt (fig. 2). 

LEAD-ALPHA AGES 

Two lead-alpha dates have been determined using zircon separated from metagranites, one from 
TABLE 2.-Potassium-argon ages from the Ambler River quadrangle 1 

Age in 

million 

years plus 

Location or minus 

No. on Average K20 40Arrad 40Arrad analytical 

figure 4 Sample no. Source Rock type Mineral dated (weight percent) (mol/g) X 10-ll (percent) uncertainty 

1 ------72E100R Turner and Quartz Muscovite 10.245 197.4 93.1 129 ± 3.9 

others (1978; muscovite 

1979a) schist 

2 ------73RR75-3 do Actinolite Impure .071 3.973 64.1 352±17.7 

schist actinolite 

3 ------74AWM309 do Actinolite Actinolite .075 5.335 49.9 435 ± 30.5 

quartz 

chlorite 

schist 

4 ------73RR52 do Gamet Glaucophane (I) .035 3.705 66.9 621 ± 31 

glaucophane Glaucophane (II) .035 3.979 68.7 659 ± 33 

schist 

5 ------73RR54 do Gamet Glaucophane .037 15.08 81.7 1,694 ± 85 

glaucophane 

schist 

6 ------72E56B do Biotite Muscovite 11.000 307.2 88.1 184 ± 5.5 

muscovite 

schist 

7 ------C-2 do Gamet Glaucophane .044 12.60 86.9 1,338 ± 134 

glaucophane 

schist 

8 ------73RR10-1C do Quartz Muscovite 9.695 190.2 87.5 131 ± 3.9 

muscovite 

schist 

9 ------73RR45C do Actinolite Actinolite .111 2.722 41.8 163 ± 6.6 

schist 

10 ------73RR7G do Glaucophane Glaucophane .038 3.867 52.5 597 ±59 

quartz Muscovite 5.061 96.31 65.1 128 ± 4 

muscovite 

schist 

11 ------73RR6EF do Biotite Biotite (I) 9.530 147.2 88.8 105 ± 3 

feldspar Biotite (II) 9.530 149.2 93.4 106 ± 3 

quartz Muscovite 11.197 190.2 92.8 115±3 

muscovite 

schist 

12 ------73RR18F do Biotite quartz Biotite 9.118 180.0 91.0 132± 4 

muscovite Muscovite 10.837 210.3 84.9 130 ± 4 

schist 

13 ------A-1 do Gamet Glaucophane (I) .012 9.581 45.4 2,535 ± 507 

glaucophane Glaucophane (II) .012 9.530 67.2 2,555 ± 511 

schist Glaucophane (Ill) .012 9.570 63.2 2,535 ± 507 

A-1CNM do Gamet Paragonite .430 15.86 32.2 239 ± 7 

glaucophane 

schist 

14 
TABLE 2.-Potassium-argon ages from the Ambler River quadrangle 1

-Continued 

Age in 

million 

years plus 

Location or minus 

No. on Average K20 40Arrad 40Arrad analytical 

figure 4 Sample no. Source Rock type Mineral dated (weight percent) (mol/g) X 10-11 (percent) uncertainty 

A-2 do Garnet Glaucophane .028 7.501 83.9 1,279 ± 256 

glaucophane Paragonite 5.550 18.37 48.1 218 ± 6 

schist 

A-3 do Garnet Glaucophane .015 4.741 76.7 1,437 ± 359 

glaucophane Paragonite .640 21.75 78.2 222 ± 7 

schist 

14 ------73RR31C do Glaucophane Impure .028 2.681 56.8 2569 ± 68 

schist glaucophane 

15 ------55ATr109 do Tremolite Tremolite .030 1.456 30.3 314± 31 

schist 

16 ------65APa44 Patton and Meta-granite Hornblende .751 13.90 93.0 124 ± 4 

others (1968) 

17 ------75AWM1503.1 Turner and Quartz Muscovite 9.946 160.4 88.9 109 ± 3 

others (1978, muscovite 

1979a) schist 

18 ------75AWM1502 do Actinolite Actinolite .077 4.932 38.0 396 ± 16 

schist 

19 ------74AWM1535.3 do Feldspathic Biotite 8.650 149.4 91.5 116±4 

schist Muscovite 10.539 188.3 94.3 120± 4 

20 ------73ATr105.4 do Biotite Biotite 8.228 127.8 74.9 105 ± 3 

muscovite Muscovite 11.135 190.3 84.9 115±3 

feldspar 

schist 

21 ------75A WM1500 do Feldspathic Muscovite 10.805 266.0 89.2 163 ± 5 

schist 

75AWM1501.1 do Feldspathic Muscovite 10.758 191.7 87.5 120 ± 4 

schist 

22 ------74AWM1745.5 do Skarn Actinolite .262 9.954 81.2 246 ± 15 

23 ------72Pe92 do Meta-granite Biotite (I) 8.435 111.4 85.2 89.5 ± 3 

Biotite (II) 8.435 109.5 80.7 88± 3 

Muscovite 10.983 189.1 95.8 116±3 

24 ------72E222 do Meta-granite Biotite 8.605 122.1 92.7 96 ±3 

Muscovite 10.955 162.6 95.0 100±3 

25 ------60ATr134 Pessel and Meta-granite Muscovite 10.520 157.6 91.0 101 ± 2 

others (1973) 

26 ------75Tr142.2E3 This report Amphibolite Hornblende .773 485.6 91.0 391 ± 20 

27 ------76Tr75B3 This report Wacke Detrital 8.530 602.7 91.0 434 ± 22 

muscovite 

![Previously published dates have been recalculated using new decay constants.) Constants used in calculation of ages: A.e + A.e' = o.5SIX1o-Io yr-1 

AB = 4.963X1o-Io yr-1 

2Minimum age. 

40KfKtotal = 1.167X10-4 Atom/Atom 

3 Analyses by M. S. Robison, Teledyne Isotopes (argonj, and Paul Klock, U.S. Geological Survey (potassiuml. 

15 
TABLE 3.-Uranium-lead ages from the Ambler River quadrangle 

Location No. First Mineral Ages(m.y.) 

on figure 4 Sample no. publication Rock type dated 206Pb*J2asu 207Pb* ;2asu 207Pb*J2ooPb 

20 -------- 77DN76 Dillon and Blastoporphyritic Zircon 329 334 373 

Pessel, 1979 schist 

20 -------- 77Mw76 do do 

the Cosmos Hills and the other from the upper 

Kogoluktuk River near Schishakshinovik Pass 

(table 4 and fig. 4). These ages were determined in 

1960 and 1961 during the early years of lead isotope dating techniques and therefore should not be 

used as specific dates, because it is probable that 

lead and uranium gain or loss has affected the ages 

by an indeterminable amount. However, these 

dates suggest that the metagranites are preCretaceous in age, especially in light of more recent U-Pb dates (Dillon and others, 1979) and RbSr dates (Nelson and others, 1979) from similar 

granitic rocks in the Survey Pass quadrangle. 

FOSSIL LOCALITIES 

Outcrops where fossils have been collected are 

shown on figure 5, and a list of the type of fossil(s), 

age, and geologic rock unit in which they were 

found is given in table 5. Fossil ages range from 

Ordovician to Mississippian. Most fossils come 

from partially recrystallized carbonate rocks 

mapped as the Baird Group, Skajit Limestone, 

Lisburne Group, and various small unnamed limestone beds within clastic rock units. A detailed 

discussion of Paleozoic corals from collections in 

carbonate rocks in the southern part of the quadrangle is reported in Oliver and others (1975). Conodonts identified from the northern part of the 

quadrangle have color alteration indexes, of 5-

5 1/2, indicating postdepositional temperatures of 

do 327 332 364 

300°-400°C. The only fossils which have been 

found in non-carbonate rocks are Silurian graptolites from dark-gray slates in the northwestern 

part of the quadrangle. 

COMPONENT MAPS OF THE AMBLER RIVER 

QUADRANGLE FOLIO 

GEOLOGY REPORT OF 78-120-A 

The first component of the Ambler River folio is 

the geologic map published at 1:250,000 scale, and 

a generalized version appears on the base maps for 

many of the other folio components. Most rocks in 

the quadrangle have been tectonically disturbed 

and metamorphosed to such an extent that fossil 

evidence has been obliterated from many of the 

rock units, thus making it difficult to construct a 

complete stratigraphic column. Without a coherent stratigraphic column, many of the major folds 

and faults in the quadrangle are unrecognized. To 

enable the reader of the map to understand the 

degree of uncertainty attached to the age of 

various map units, sedimentary and metasedimentary rocks are divided into categories of certain 

and uncertain age. Among the rocks of uncertain 

age, some are given queried systemic age assignments usually based on tenuous correlations with 

other rocks located some distance away, commonly outside the quadrangle. 

TABLE 4.-Lead-alpha ages from the Ambler River quadrangle 

[Lead·alpha determinations by T. W. Stem 1960-61] 

Location No. 

on figure 4 Sample no. 

25 -------- 60ATr134 

16 -------- 55ATr149 

First 

publication 

This report 

do 

Rock type 

Metagranite 

do 

16 

Mineral 

dated a/mg-hr Ph (ppm) Age (m.y.) 

Zircon 1,236 105 210 ± 25 

do 2,125 420 490 ± 60 
25 50 KILOMETERS 

FIGURE 5.-Fossillocalities in the Ambler River quadrangle and the northeastern part of the Shungnak quadrangle. Numbers 

refer to listing in table 5. 

Since the publication of the geologic map, radiogenic age dating work on some granitic rocks in 

the Ambler River and adjacent Survey Pass quadrangles shows strong evidence that the time of igneous cooling of both rhyolitic metavolcanic rocks 

in the southeastern part of the quadrangle and the 

granitic metaplutonic rocks stretching across the 

central part of the quadrangle is middle Paleozoic. 

Dillon and Gilbert (1978) have determined that the 

megacrystic felsic schist in the Ambler River 

quadrangle is approximately 360 m.y. old by U-Pb 

17 

dating methods on zircon. Recent dates from granitic plutonic rocks in the central part of the 

Survey Pass quadrangle by Nelson and others 

(1979) using Rb-Sr methods and by Dillon and 

Gilbert (1978) using U-Pb methods also &upport a 

middle Paleozoic age for these rocks. Because the 

granitic plutonic rocks stretching across the central part of the Ambler River quadrangle are mineralogically similar and crop out on the same linear 

belt, they are also probably middle Paleozoic. 

These rocks are shown on the map as Cretaceous, 
TABLE 5.-Fossils collected in the Ambler River quadrangle 

[Field samples listed as Lehner, Babcock, Read, and Bornite are from collections of Bear Creek Mining Co.; sample numbers beginning with 69A and 685 are from collections of Alaskan Division of Mines and Geology (Fritts, 1969a; 1970); and remaining samples are from the collections of the U.S. Geological Survey. The geologic units are 

on the geologic map of Mayfield and Tailleur (1978)] 

Location No. USGS 

on figure 5 Field sample collection No. Fossils Age Geologic map unit Determined by 

1 -------- 66APa99 7983-SD Corals, stromatoporoids Silurian or Devonian Limestone of W. A. Oliver 

Baird Group 

2 -------- 66APa97, 66APa98, 7981-SD, 7982-SD, Corals, fish bones Devonian do W. A. Oliver 

73ATr85 9278-SD D. H. Dunkle 

3 -------- 73ATr82.2, 9276-SD, Corals Middle Devonian do W. A. Oliver 

73ATr82.3 9277-SD 

4 -------- Babcock 119 6668-SD Corals, gastropods, Early(?) and do do 

66ARr26 7984-SD stromatoporoids Middle Devonian 

5 -------- Babcock 117 6669-SD Corals, stromatoporoids Probably Middle or do do 

early Late Devonian 

6 -------- 66ATr18 7985-SD Corals Middle or early do do 

Late Devonian 

7 -------- 69A34B 8742-SD do Late Silurian to do do 

Middle Devonian 

8 -------- Lehner 6462-SD do Probably Devonian do do 

9 -------- 69A12b, c, f 8740-SD Corals, crinoids do do do 

10 -------- 69A15 Corals Paleozoic Undifferentiated C. E. Fritts1 

limestone and 

marble unit 

11 -------- 69A13g 8741-SD do Silurian or Devonian Limestone of W. A. Oliver 

Baird Group 

12 -------- Bornite cores 8741-SD do Early(?) and do do 

Middle Devonian 

13 -------- 65ABe74A 7471-SD do do do do 

14 -------- 55ATr107, 65ABe66, 6460-SD, 7659-SD Corals, brachiopods do do W. A. Oliver 

66ABe104, 69A6 7980-SD, 8738-SD C. W. Merriam 

Read A 6460-SD 

15 -------- Read B 6461-SD do do do W. A. Oliver; 

65B73 J. T. Dutro, Jr. 

16 -------- 69A10B 8739-SD Corals, stromatoporoids Probably Middle or do W. A. Oliver 

early Late Devonian 

17 -------- 69A21d, e, g, h, i Brachiopods, corals Middle Devonian(?) do W. A. Oliver 

gastropods E. L. Yochelson 

18 -------- 68S18 Crinoids Paleozoic Limestone in mafic C. E. Fritts1 

volcanic rocks unit 

19 -------- 68S51p-k Crinoids, conodonts do Limestone in mafic J. W. Huddle 

volcanic rocks unit 

20 -------- 65AMm34 7996-SD Corals, stromatoporoids Probably Middle or Undifferentiated W. A. Oliver 

early Late Devonian limestone and C. W. Merriam 

marble unit 

21 -------- 55ATr129G M1048 Crinoids, ostracods, Paleozoic Kayak Shale A. K. Armstrong 

and bryozoan? 

18 
TABLE 5.-Fossils collected in the Ambler River quadrangle-Continued 

Location No. USGS 

on figure 5 Field sample collection No. Fossils Age Geologic map unit Determined by 

22 -------- 66ABe75 M1004 Corals, sharks teeth, Late Mississippian Lisburne Group A. K. Armstrong 

55ATr132 18631-PC and brachiopods Helen Duncan 

D. H. Dunkle 

23 -------- 72ATr28 M1052 Corals, molluscs, Late and Early Lisburne Group A. K. Armstrong 

76ATr96 25057-PC (M1300) brachiopods, Mississippian and Kayak Shale J. W. Huddle 

76ATr96B 27437-PC gastropods J. E. Repetski 

conodonts, and 

crinoids 

24 -------- 76AMd241 Crinoids Probably Late Sandstone, siltstone, J. T. Dutro, Jr. 

Devonian and slate unit 

25 -------- 76ATr190E 8669-CO Conodonts Middle Ordovician Siltstone and J. E. Repetski 

to Early Silurian slate unit A. G. Harris 

26 -------- 76ATr147C 9692-SD Corals, gastropods, Silurian or Devonian do do 

bryozoan, and 

brachiopods 

27 -------- 76Md27 4 Cephalopods, Probably Silurian do J. T. Dutro, Jr. 

conularids and or Devonian W. A. Oliver 

stromatoporoids 

28 -------- 76ATr148A Corals do Skajit Limestone do 

29 -------- 76ATr149C 9693-SD do Late Silurian to do do 

Middle Devonian 

30 -------- 76Hn29 M1302 Crinoids, corals Ordovician to Silurian Limestone with A. K. Armstrong 

66ARr24A M1048 black chert of 

Lisburne(?) Group 

31 -------- 76ATr85B M1301 Corals Paleozoic Limestone with do 

black chert of 

Lisburne(?) Group 

32 -------- 76ATr86B 9803-SD Conodonts, corals Middle Ordovician Skajit Limestone J. E. Repetski 

to Middle Devonian A. G. Harris 

A. K. Armstrong 

33 -------- 55ATr121 Ostracodes Paleozoic Stream float J. M. Berdan 

34 -------- 76Md235C Gastropods, Paleozoic Skajit Limestone J. T. Dutro, Jr. 

brachiopods 

35 -------- 76ATr118A, C, D, 9691-SD Stromatoporoids Probably Silurian do W. A. Oliver 

or Devonian 

36 -------- 76A Tr3E 9802-SD Conodonts Middle Ordovician Limestone clasts in J. E. Repetski 

to Middle Devonian feldspathic wacke A. G. Harris 

unit 

37 -------- 76ATr5 9598-SD Corals Probably Silurian Skajit Limestone W. A. Oliver 

or Devonian 

38 -------- 76AMd32 Graptolites Middle Silurian Black phyllite and Claire Carter 

siliceous phyllite 

unit 

39 -------- 76ATr50E 8668-CO Conodonts Ordovician, Undifferentiated J. E. Repetski 

probably Early Limestone and A. G. Harris 

or Middle marble unit 

1

Field identification by C. E. Fritts of Alaskan Division of Mines and Geology. 

19 
but for the above-stated reasons are now regarded 

as probably being middle Paleozoic. 

The geologic map of the Ambler River quadrangle shows only bedrock units. Those interested in 

geology of surficial deposits should consult the 

maps of Pessel and Brosge (1977) and Fernald 

(1964). 

RECONNAISSANCE GEOCHEMISTRY 

REPORTS OF 78-120-B THROUGH H 

Geochemical studies in the Ambler River quadrangle were made to define areas of anomalous 

concentrations of selected metallic and nonmetallic elements. A sampling program was undertaken during the summer months of the years 

1972, 1973, 1974, and 1976, in which 1,281 streamsediment samples were collected, averaging about 

one sample per 9 km2 in areas where outcrops are 

exposed. This sampling density completes coverage of the quadrangle on a reconnaissance basis. 

Extensive Quaternary deposits which cover most 

of the Noatak and Kobuk valleys (about 25 percent 

of the quadrangle) were not sampled. 

There are several previously published geochemical studies in the Ambler River quadrangle; most 

are incorporated into this folio. Three reports by 

the Alaska Division of Geological and Geophysical 

Surveys (Garland and Pessel, 1975; Garland and 

others, 1975; Pessel, 1976) present analyses of 657 

stream-sediment samples from the Ambler River 

quadrangle. These samples have been included in 

the present compilation, along with 561 samples 

collected by the U.S. Geological Survey which are 

reported for the first time (OF 78-120-B, C). Two 

reports by Fritts (1969a; 1970) are not included in 

this compilation. They present the analyses of 236 

stream-sediment samples in the Cosmos Hills, including part of the Shungnak quadrangle adjacent 

to the Ambler River quadrangle. The samples were 

collected at 200- to 500-m intervals along a few 

creeks, a more concentrated and less uniform distribution than in the rest of the quadrangle. Some 

elements reported by Fritts have significantly 

higher median values than the values given in 

other reports on geochemistry of the Ambler River 

quadrangle, so these values are excluded from this 

compilation. 

Samples were collected if possible from fine sediment in streams which drain bedrock. If no silt 

was present in the stream bed, samples were taken 

20 

from alluvium on the bank or from around the 

roots of plants near the water's edge. The samples 

were sieved to retain the -80 mesh fraction, which 

was composed of clay and very fine silt. One split 

of each sample was analyzed by the atomic absorption method for copper, lead, and zinc. Samples 

collected in 197 4 and 197 6 were also analyzed for 

gold by the atomic absorption method. Another 

split of each sample was analyzed by the semiquantitative emission spectrographic method for 

30 elements. 

The analyses for the stream-sediment survey are 

listed by Ellersieck (1978a), and the sample locations are plotted on a generalized geologic map by 

Ellersieck (1978f). A threshold limit above which a 

sample is considered to be anomalous has been 

chosen for each element. These levels are determined entirely from data within the Ambler River 

quadrangle; therefore, they may be higher or lower 

than thresholds in other regions of different geology or stream chemistry. Because the data are reported in discrete analytical intervals, the distribution curve of elemental concentrations is not 

smooth, and the number of anomalous samples is 

different for each element. The proportion of 

anomalous samples may be as high as 9.8 percent 

(strontium) or as low as 0. 7 percent (chromium); 

the average is 3.5 percent of the total number of 

samples. 

There are five geochemical anomaly maps in the 

folio (Ellersieck, 1978b, c, d, e, g) which show the 

location of anomalies for the following 12 elements: copper, molybdenum, lead, zinc, barium, 

silver, arsenic, tin, beryllium, chrome, nickel, and 

cobalt. These elements were chosen either because 

they are of possible economic significance or because they appear to be related to the types of mineral deposits that could be found in the quadrangle. 

An estimate of the effectiveness of the sample 

density of this survey can be made by calculating 

the percentage of known mineral occurrences that 

have a geochemical sample site within 5 km downstream and are reflected by anomalously high geochemical sample values for any of the commodities 

reported in the occurrence. Occurrences in the 

Cosmos Hills, nonmetallic mineral occurrences, 

claims, and occurrences that were determined to 

exist on the basis of the stream-sediment survey 

are excluded from consideration. Of the 53 mineral 

occurrences in the area of the geochemical survey, 

39 (74 percent) had at least one stream-sediment 

sample within 5 km downstream. Of these 39 oc-
currences, 25 (64 percent or 4 7 percent of the 53 

total occurrences) showed anomalous concentrations of silver, copper, lead, zinc, or an element reported to be in the occurrence by Mayfield and 

Grybeck (1978). On the basis of these crude assumptions, less than half of the known mineral 

occurrences are reflected in stream-sediment 

samples with anomalously high values. 

There are ·many reasons why sample density and 

sample techniques such as those used are not more 

effective in finding mineral .. occurrences. The 

"halo" of geochemically enriched stream sediments near a mineral deposit is dependent upon a 

number of complex factors. For example, some 

deposits may spread their geochemical signatures 

farther than others, or the principal commodities 

may be effectively sampled only by a panning 

technique (not used in this survey). Therefore, a 

survey of this type cannot be expected to locate 

significant numbers of specific undiscovered mineral occurrences; instead it is most valuable for 

delineating broad areas with favorable resource 

potential and a few specific stream drainages that 

deserve more detailed exploration. 

MINERAL OCCURRENCES AND RESOURCES 

REPORT OF 78-120-I 

The mineral occurrences and resources map of 

the Ambler River quadrangle is intended to show 

three important kinds of mineral information. 

First, it shows the known mineral occurrences in 

the quadrangle. Second, it provides a discussion of 

the important mineral resources. Third, it delineates areas within the quadrangle that have a high 

potential for discovery of certain types of deposits 

which could yield significant mineral resources. 

There are 80 mineral occurrences in the quadrangle that were known to us at the time of publication of this map. The mineral occurrences are 

listed numerically in a table accompanying the 

map. Each numbered occurrence is located on the 

1:250,000-scale map of the Ambler River quadrangle, and the principal mineral commodities of interest are shown at each location. In addition, the 

table provides information on the form and description of the occurrence, the status of development, and the source from which information was 

obtained. 

The map includes a discussion of the known and 

potential mineral resources of the Ambler River 

21 

quadrangle. The most important resources include 

copper, zinc, lead, and silver; these occur in 

volcanogenic and carbonate breccia-type deposits. 

Of the nine known volcanogenic deposits in the 

quadrangle, one has reserves of 30 million tons of 

4 percent Cu, 5.5 percent Zn, 1 percent Pb, and 

54.1 g/ton (15 oz/ton Ag), and the other eight are of 

unknown size and grade. The number of undiscovered deposits at or near the surface is estimated by 

using areas of favorable geology and geochemical 

anomalies. Grade and tonnage of possible undiscovered deposits are estimated using data compiled on Canadian volcanogenic deposits of Phanerozoic age. Table 6 shows a crude assessment of 

the number, grade, and tonnage of undeveloped 

and undiscovered volcanogenic deposits in the 

quadrangle. The volcanogenic deposits of this 

district are at an early stage of development by 

mining companies, and until more public information is available about the known deposits, the 

assessment given above should be regarded only 

as an initial estimate. No attempt is made numerically to assess the base metal resource in carbonate breccia deposits because reserves a the few 

known deposits are not publically available and 

the origin of these deposits is speculative. At the 

largest deposit of this type, Ruby Creek, the 

reserves are significant. The largest known 

deposits of jade in Alaska occur in the quadrangle. 

Though these deposits have a subordinate resource potential, they are expected to provide a 

steady supply of jade for many years. 

The third aim of the mineral occurrences and resources map is to show areas which have high 

potential for future discoveries of deposits that 

could provide significant amounts of mineral 

resources. Four categories of these deposits are 

TABLE 6.-Estimate of number, grade, and tonnage of volcanogenic deposits 

Chance1 

(percent) metric tonnage Grade2 

95 200,000 6-10 percent combined Cu, Pb, 

Zn,andAg 

55 1,000,000 do 

12 10,000,000 do 

2 100,000,000 do 

1

Crude estimate of number of undiscovered deposits 

90 percent chance that there are 7 or more undiscovered deposits 

50 percent chance that there are 10 or more undiscovered deposits 

10 percent chance that there are 12 or more undiscovered deposits 2

Estimate of probable grades of volcanogenic deposits 
delineated on the same map as the known mineral 

occurrences. These categories are (1) volcanogenic 

deposits, (2) carbonate breccia deposits, (3) black 

pyritic phyllite deposits, and (4) jade, podiform 

chromite, and asbestos deposits. An accompanying explanation for favorable mineral resource 

areas provides a list of the reasons for selection of 

the specific areas based on geologic, geochemical, 

geophysical, and telegeologic information available in other segments of the Ambler River folio. 

Without detailed exploration and drilling, it is impossible to ensure that all favorable areas shown 

will actually contain deposits. 

INTERPRETATION OF LANDSAT IMAGERY 

REPORT OF 78-120-J 

The interpretation of Landsat imagery of the 

Ambler River quadrangle was made by using a 

black-and-white Landsat mosaic (band 7) of the 

State of Alaska assembled by the U.S. Department of Agriculture and by using computerenhanced black-and-white and color Landsat imagery processed by the U.S. Geological Survey in 

Flagstaff, Ariz. The mosaic of the Ambler River 

quadrangle is made from parts of three scenes, 

1707-21385 and 1707-21391, taken June 30, 1974, 

and 1758-21201, taken August 20, 1974. Color 

computer-enhanced maps include linearly stretched 

standard false-color, sinusoidally stretched falsecolor, and simulated natural color images. A blackand-white vertical first-derivative image was also 

used. Copies of these products are available at 

nominal cost from the EROS Data Center, Sioux 

Falls, S.Dak. (Albert, 1978). Detailed descriptions 

of the various computer enhancement techniques 

used in these images are given by Albert and 

Steele (1976a, b) and Condit and Chavez (1978). 

Interpretation of Landsat imagery for this folio 

consists of the identification of straight, circular, 

and arcuate lineaments and iron-oxide colored 

areas plotted on 1:250,000-scale maps of the 

Ambler River quadrangle. It is hoped that some of 

these features may prove to be directly or indirectly related to important mineral deposits even 

though a consistent correlation between known 

deposits and sets of lineaments has not yet been 

found in this study. 

A number of straight lineaments correspond to 

mapped faults, and some of these indicate that the 

faults may extend beyond their mapped limits. A 

22 

major east-trending broad lineament zone was 

identified closely corresponding to the Kobuk 

Trench (Grantz, 1966) and partially corresponding 

to a regional magnetic discontinuity (Gilbert and 

others, 1977; Hackett, 1980). 

In previous studies, most circular features have 

been spatially associated with igneous activity, 

whereas arcuate features have been spatially associated with either igneous activity or metamorphism (Albert and Steele, 1976b). A circular 

feature centered on metagranitic plutonic rocks, 

west of the Ambler River in the central part of the 

quadrangle, is the only obvious spatial correspondence between circular features and igneous phenomena observed in the Ambler River quadrangle. 

It is possible that Cretaceous regional metamorphism has overprinted and obscured arcuate lineaments related to granitic plutonism. 

Many of the iron-oxide colored areas identified 

on Landsat imagery were observed in the field 

from aircraft. A substantial number of these occur 

within carbonate rock units, many of which have 

not received adequate follow-up exploration. Many 

of the carbonate rocks observed in the field showed 

some degree of iron-oxide staining, and most of 

these could not be identified on Landsat imagery. 

Several of the iron-oxide areas identified on Landsat imagery, however, correspond to known mineral occurrences and (or) to geochemical anomalies. 

AEROMAGNETIC MAP AND INTERPRETATION 

REPORT OF 78-120-K 

The aeromagnetic interpretation of the Ambler 

River quadrangle is compiled on three sheets: the 

aeromagnetic-contour map, the magnetic-lineament 

and anomaly-trend map, and the geologicinterpretation map. The contoured aeromagnetic 

map appears at 1:250,000 scale on sheet 1. An 

aeromagnetic survey of the quadrangle and adj acent areas was flown by Geometries of Sunnyvale, 

Calif., in 197 4 and 197 5 for the Alaska Department of Natural Resources. The magnetic data 

were collected along north-south flightlines spaced 

1.6 km (1.0 mi) apart (west half) and 1.2 km 

(0.75 mi) apart (east half) and from an altitude of 

300m (1,000 ft) above ground level. The aeromagnetic data were first released at 1:63,360 scale in 

1975-1976 and subsequently compiled as a 

1:250,000-scale aeromagnetic map of the southwestern Brooks Range by the Alaska Division of 
Geological and Geophysical Surveys (Hackett, 

1977). Rugged topography and marginal weather 

conditions within the southwestern Brooks Range 

caused the surveying aircraft to diverge from the 

nominal flight altitude over some areas within the 

Ambler River quadrangle. For this reason, magnetic anomalies that have some appreciable or suspected topographic expression are noted on the 

interpretation maps. 

A magnetic-lineament and anomaly-trend map 

(sheet 2) was prepared to assist in the geologic 

interpretation of the aeromagnetic data. Major 

lineaments exist and indicate that regional discontinuites in the magnetic rocks correlate with the 

gross geologic pattern and structural setting 

within the Ambler River quadrangle. Magnetic 

anomaly patterns coincide in part with many individual geologic terranes and assist in defining 

some of the bedrock geology at depth. 

The geologic interpretation map (sheet 3) of the 

aeromagnetic data identifies major magneticlithologic units, their suspected strike and dip, and 

their relative strength of magnetization. The 

magnetic-lithologic units were assigned to individual anomalies and magnetic gradient changes on 

the basis of correlation with the known geology. 

The units were integrated from a spectrally colored copy of the aeromagnetic map, from the lineament and anomaly-trend map, from detailed and 

generalized geologic maps (Pessel and Brosge, 

1977; Mayfield and Tailleur, 1978), and from 

geologic observations and field notes by Fritts, 

1969-1972, Pessel, 1972-1975, and Hackett, 

197 5-1978. Continuous zones of steepened magnetic gradients and abrupt truncation of magnetic 

anomalies and trends are interpreted as major 

discontinuites (faults, unconformities, or facies 

changes) that may have surface expression. 

In addition to the magnetic-lithologic units, six 

major aeromagnetic terranes (fig. 6) are outlined 

according to characteristic regional anomaly patterns: the Kobuk lowlands, Jade MountainsCosmos Hills-Angayucham Mountains, Ambler 

lowland, southern Brooks Range schist belt, plutonic belt, and thrust belt. The Kobuk lowlands 

terrane is characterized by broad magneticanomaly patterns which indicate considerable and 

varied depth to magnetic basement. Localized 

high-intensity anomalies over the Waring Mountains in the southwest corner of the quadrangle 

may be caused in part by tilted east-trending 

23 

mafic-ultramafic rocks that are overlain by Cretaceous clastic rocks. The Jade Mountains-Cosmos 

Hills-Angayucham Mountains terrane is characterized by sharp, high-amplitude anomalies related 

to south-southwestward-dipping slabs of mafic 

and ultramafic rocks. The Ambler lowlands terrane is characterized by a linear zone of nearly flat 

magnetic gradient which outlines a topographic 

depression. The northern boundary of this terrane 

in the eastern part of the quadrangle is formed by 

the Ambler magnetic lineament, which parallels 

the conjectural Kobuk fault zone (Grantz, 1966; 

Patton, 1973). The southern Brooks Range schist 

belt terrane is characterized by subdued and variable response. Anomalies with variable amplitudes 

of less than 40 gammas occur with irregular 

distribution over the metavolcanic rocks of the 

volcanogenic copper belt. A narrow, composite 

zone of high magnetic relief is associated with the 

Walker Lake lineament, a major fault or unconformity. Convergence of the Walker Lake lineament and the Ambler magnetic lineament may 

reflect a western plunge of the Kalurivik arch, a 

major anticlinal feature in the southeast part of 

the quadrangle. The plutonic belt terrane is characterized by low-amplitude anomalies over carbonate rocks and schist broken by large asymmetric negative anomalies associated with granitic 

plutonic rocks. High-intensity magnetic zones 

near granite contacts may be over contact 

aureoles. The thrust belt terrane is characterized 

by subdued magnetic signature with a few zones of 

large positive magnetic anomalies over possible 

mafic rocks or magnetite-rich chloritic schists in 

the Cutler River area 

The aeromagnetic anomalies, trends, and lineaments in the Ambler River quadrangle are believed to help outline the regional tectonic trends 

and assist in synthesizing the bedrock geology. 

Local magnetic anomalies, caused by bedrock exposures of magnetic rock units, are commonly 

underlain throughout the quadrangle by larger 

magnetic bodies and are appreciably influenced by 

the magnetism of the adjacent terranes. These 

anomalies represent composite sets of juxtaposed 

magnetic boundaries which complicate geologic interpretation. However, the regional aeromagnetic 

data do help to define some of the bedrock geology 

at depth, identify previously unmapped structural 

features, and aid in projecting suspected trends of 

mineral deposits. 
FIGURE 6.- Aeromagnetic terranes of the Ambler River quadrangle and the northeastern part of the Shungnak quadrangle. 

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25 

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26 

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