Please do not destroy or throw away this publication. If you have no further use 

for it, write to the Geological Survey at Washington and ask for a frank to return it

UNITED STATES DEPARTMENT OF THE INTERIOR 

Harold L. Ickes, Secretary

GEOLOGICAL SURVEY 

W. C. Mendenhall, Director

Bulletin 849 E M,y »v

MINERAL DEPOSITS NEAR THE 

WEST FORK OF THE CHULITNA RIVER

ALASKA

BY

CLYDE P. ROSS

Investigations in Alaska Railroad belt, 1931 

(Pages 289-333)

UNITED STATES

GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1933

For sale by the Superintendent of Documents, Washington, D.C. ------ Price 15 cents

CONTENTS

Foreword, by Philip S. Smith _______________-_-_-___---- .. .._---. v

Abstract..-._.-_------------------------------------------ .-.----- 289

Introduction __ ._______-___-_-__--____----------_--------------- 289

Scope of the work._____________________-----___--------_---_-_ 289

Acknowledgments. ___________-----_--_-------'__-------------_- 290

History...-----.-------------------------------------------- 290

Geography ________-____._-----------------.--_---_.-------------. 291

Topography _-______--__--_ -_---- --_----_--------- ------ 291

Means of access_--.____._._---_-_-.--___--_-.-------__-----_ 292

Climate - - - - -- - ---------.-- - 293

General geology ______-_--_-_-_-------_-_-- ------------------ 293

Stratified rocks._______.._____.___.__----__-___-_---_-_______. 294

Devonian (?) chloritic rocks._______________________________ 294

Carboniferous (Permian ?) rocks_.----_-____-------__----._. 294

General features.____-__---.___.-_.__---------_.-..___ 294

Unit A_____ : 295

Unit B......__ _____ _ __ __ _ 296

Unit C....--------------------------.------------.--- 297

Unit D....--.....--..--.......-----.-...-..._. 297

Unit E_______----___----_--------------------_.---___ 297

Age. ____----_-----_- --------_-_---------____-____ 298

Triassic limestone. ........................................ 298

Triassic argillite. ________-__-__--_----_-_- .__________. 300

Tertiary (Eocene?) rocks___ _._-_-__.__.__..-..__.._.__._. 302

Quaternary deposits._-____________--..______.____..._____. 303

Intrusive rocks.______________________________________________ 303

Biotite-quartz diorite porphyry._____________________ ..______ 304

Quartz diorite.___..__.__-_-_-_----_-_________________.__. 305

Hornblende diorite porphyry.__-____-_--_.._-___.__..__.__. 305

Age__._ ...... -..--- ------.-..--... ........ 305

Structure. _________________ _____ ___ _ _____ 305

Folding- - .-_ . - .. - __._ ...._.. 305

Faulting._--__------_-__----_-_-_-_-_-_-______________.__ 307

Economic geology.-__-------_----_-----------_-_-___----__________ 309

Lodes.____--------_-_---__---------_-------_---.--____._____ 309

. Disseminated deposits.___----_-__--_-----___-_____________ 309

Deposits along bedding_________________'___________________ 310

Tabular or lenticular lodes_____-_-_-__-_____.-_____________ 311

General characteristics of the sulphides ______________________ 312

Oxidation._ ______________________________________________ 314

Origin. _.--._ - _ ___ _ _ _ _____ _ ___ 314

Economic outlook...._____________________________________ 315

Prospects................................................ 317

Ready Cash___ _ ___ _ ______________________________ 318

Copper King.________________________________________ 320

Lindfors. ..........................................^. 321

Golden Zone_________________________________________ 321

ni
IV CONTENTS

Economic geology Continued. 

Lodes Continued.

Prospects Continued. Page 

Mayflower.__________________________________________ 325

Riverside ____________________________________________ 326

Silver King..._.__._.._....____......_.____...__.__.._ 327

Liberty. __________________________________________^__ 328

Lucrata._--.._.------_____._____-_.___._...__________ 328

Eagle.__.....-_.---__......._...._..-...__.._-_..._._ 329

CoaL-___--__--_ ____________________________________________ 331

Stevens.________________________________________________ . 331

Index..___..___._______..__________________________________ 333

ILLUSTRATIONS

Page 

PLATE 25. Geologic sketch map of the area near the West Fork of the

Chulitna River, with structure sections__________________ 296

26. Sketch map of the Ready Cash prospect, Ohio Creek________ 320

27. Sketch map of the Golden Zone and Mayflower prospects__._ 320 

FIGURE 1. Index map showing areas investigated in Alaska Railroad belt,

1931_1_.__--------_-------_------------------------_-- vii

38. Diagrammatic view of the north flank of the valley of Copeland

Creek near the ice-filled gorge.___________-___----____._ 306

39. Diagrammatic sketch of faulting along upper Blind Creek___ 307

40. Sketch map of the lower Ready Cash tunnel ________________ 319

41. Sketch map of the Copper King prospect_________________ 320

42. Tunnel on the Golden Z'one prospect._____________________ 322

43. Sketch map of the Riverside workings.____________________ 326

44. Sketch map of the Eagle prospect....--------------------- 329
INVESTIGATIONS IN ALASKA KAILKOAD BELT, 1931

FOREWORD

By PHILIP S. SMITH

To help the mining industry of Alaska and to assist in the development of the mineral resources of the Territory have been the prime 

motives of the Geological Survey's investigations in Alaska during 

the past 35 years, in which nearly one half of the Territory has been

covered by its reconnaissance and exploratory surveys. It was 

natural, therefore, that the Alaska Railroad, when it undertook intensive consideration of the problem of finding tonnage that would 

increase its revenues, should look to the Geological Survey to supply 

technical information as to the known mineral deposits along its 

route and to indicate what might be done to stimulate a larger 

production of minerals and induce further mining developments and 

prospecting that would utilize its service. Realization of the need for 

this information had long been felt by the officials responsible for the 

operation of the Alaska Railroad, and the need had been partly 

supplied by the Geological Survey, but funds to carry through an 

extensive inquiry of this sort had not been available until 1930, 

when a special committee of the Senate, composed of Senators Howell, 

Kendrick, and Thomas, visited Alaska, studied some of the railroad's 

problems, and successfully urged Congress to grant it $250,000 for 

investigations of this kind.

On the invitation of the Alaska Railroad the Geological Survey 

prepared various plans and estimates for the investigations that 

appeared to be most likely to contribute the desired information as to 

the mineral resources. Selection of the problems to be attacked 

proved difficult, because the choice necessarily was hedged about with 

many practical restrictions. For instance, each project recommended 

must give promise of disclosing valuable deposits a requirement 

that was impossible to satisfy fully in advance, as it involved prophecy 

as to the unknown and undeveloped resources. Then, too, it was 

desirable that the search should be directed mainly toward disclosing 

deposits which if found would attract private enterprises to undertake their development in the near future. Finally, some of the 

deposits that might be worked profitably did not appear likely to 

afford much tonnage to be hauled by the railroad. Under these
VI INVESTIGATIONS IN ALASKA BAILROAD BELT, 193.1

limitations it should be evident that the projects that could be recommended as worth undertaking with the funds available by no means 

exhausted the mineral investigations that otherwise would be well 

justified. In a large sense, all of Alaska may properly be regarded as 

indirectly contributory to the welfare of the railroad, but even in 

that part of Alaska contiguous to its tracks there are large stretches 

of country that are entirely unexplored and large areas that have had 

only the most cursory examination. Although areas of this sort 

might well repay investigation, they were excluded from the list of 

projects recommended because they were not known to contain 

mineral deposits of value, and it therefore seemed better to make the 

selection from other areas that had been proved to hold promise. 

Furthermore, several areas within the railroad zone were excluded 

because their value was believed to He mostly in their prospective 

placers, which would not yield much outgoing tonnage; others because 

their lodes carried mainly base metals, for which development and 

the recovery of their metallic content in a readily salable condition 

were relatively expensive; and still others because their resources 

consisted mainly of granite/ building stone, or some other product 

for which at present there is only a small local demand.

After careful consideration ten projects were selected, and the 

funds required for undertaking them were made available. The 

projects that were selected involved the examination of two areas 

principally valuable for their coal (Anthracite Ridge and Moose 

Creek), five areas likely to be principally valuable for gold (Fairbanks, Willow Creek, Gird wood, Moose Pass, and Valdez Creek), 

, and three areas whose lodes consisted mainly of mixed sulphides (the 

Eureka area in the Kantishna district, Mount Eielson, formerly 

known as Copper Mountain, and the head of West Fork of Chulitna 

River). The general position of these different areas is indicated on 

the accompanying diagram (fig. 1). A general study of the nonmetalliferous resources of the entire region traversed by the railroad 

was included in the projects to be undertaken, but the results obtained 

were not such as to permit adequate determination of their extent at 

this time.

Examinations were made in the field in each of the selected areas, 

all the known prospects and mines being critically examined and 

sampled so far as time and other conditions permitted. The records 

thus obtained, together with all other information bearing on the 

problems, were then subjected to further study in the laboratory and 

office, in the course of which other Geological Survey specialists whose 

knowledge and experience could be of assistance were freely consulted. 

The outcome of all these lines of analysis has been the reports which 

make up this volume. Although each chapter is presented as embodying the latest and most authoritative information available regarding 

the districts and properties described up to the time field work in them
FOEEWOED VII

FiotiBE 1. Index map showing areas investigated in Alaska Railroad belt, 1931. A, Anthracite Ridge; 

B, Fairbanks; C, Willow Creek; D, Mount Eielson; E, West Fork of Chulitna River; F, Eureka and 

vicinity; Q, Glrdwood; H, Valdez Creek; I, Moose Pass and Hope.
VIII INVESTIGATIONS IN ALASKA EAILEOAD BELT, 1931

was finished, the authors make no claim that all the results they have 

presented are to be regarded as final nor as solving all the problems 

that have arisen. Actually none of the mines have been developed 

to such an extent as to furnish all of the evidence desired to solve the 

problems involved. At none of the properties is any considerable 

quantity of ore actually "blocked out" in the engineering sense of 

that term, so that instead of specific measurements as to the quantity 

and grade of ore the different camps will yield, the Survey geologists 

and engineers have necessarily had to make numerous assumptions 

and be content with estimates and generalizations as to the potential 

resources. Furthermore, the work was planned so as not to invade 

the proper field of the private mining engineer in the valuation of 

individual properties, but rather to occupy the open field of considering 

the districts as a whole.

In two of the districts, Anthracite Bidge and Moose Creek, whose 

value lay in their prospective coal resources, the examinations that 

could be made by ordinary geologic means were not adequate to arrive 

at a final judgment of the resources of the area but pointed to the 

desirability of further tests by drilling. As a consequence additional 

exploration of these districts by means of diamond drilling was 

authorized, and this work was undertaken in the season of 1932. The 

results of these tests were not available at the time the manuscripts 

of the other reports were completed, and rather than delay their 

publication until the later reports could be finished and incorporated 

in the volume these reports have been omitted here and will be 

published later elsewhere.

This is not the place to summarize the detailed findings of the 

geologists as to the merits of the different districts, as those findings 

are explained in detail and summarized in the respective chapters. 

Suffice it to say here that on the whole the principal purpose of the 

investigations was carried through satisfactorily and that while the 

studies in some of the districts indicate that they hold little promise 

of extensive mineral development in the near future, others appear to 

encourage development under existing conditions, and still others 

seem to be worth development when some of the existing factors such 

as transportation or price of base metals are improved. That conditions which are now temporarily retarding the development of some 

of the deposits will become more favorable cannot be doubted. The 

entire region is becoming more accessible each year, and as a result 

costs are being lowered and experience is being gained as to the habit 

of the various types of deposits, so that the conclusions expressed in 

this volume as to the resources of the different districts should be 

reviewed from time to time in the light of the then current conditions.
MINERAL DEPOSITS NEAR THE WEST FORK OF THE 

CHULITNA RIVER, ALASKA

By CLYDE P. Boss

ABSTRACT

The area in the vicinity of the West Fork of the Chulitua River, Alaska, one 

of those examined in 1931 in connection with the study of mineral resources 

in districts tributary to the Alaska Railroad, contains numerous prospects 

but, as yet, no productive mines. Its placer deposits are negligible but some of 

its lodes may prove valuable for gold and silver and perhaps also for copper 

and arsenic.

The area is underlain by a steeply inclined succession of Devonian (?), 

Carboniferous (Permian?), and Triass'ic sedimentary and volcanic strata of 

diverse kinds. These beds have been cut by a zone of thrust faulting with 

associated normal faults and have locally been much contorted. They are intruded by small stocks and dikes of quartz diorite, biotite-quartz diorite porphyry, and related rocks. The lodes, which are genetically related to the 

porphyry, are of three interrelated and intergradational kinds (1) disseminated deposits, mainly in the porphyry, (2) replacement deposits along the 

bedding in calcareous rocks, (3) tabular and lenticular lodes with conspicuous 

vein quartz, along fissures and shear zones. The principal metallic minerals are 

arsenopyrite, pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena. The lodes 

are not sufficiently developed for quantitative estimation of grade of ere or of 

available tonnage, but there are considerable quantities, mostly in individually 

small bodies, of ore ranging from $5 to $20 a ton in precious metals and locally 

containing as much as 20 percent of arsenic. A few of the lodes contain 5 to 

10 percent of copper in addition to some gold and silver, but the quantity of 

such material now exposed is very small.

INTRODUCTION

SCOPE OF THE WORK

The mineral deposits in the general vicinity of the West Fork of 

the Chulitna River were examined in 1931 as a part of the investigation of the mineral resources of the region tributary to the Alaska 

Railroad. Those studied include nearly all the prospects in the area 

termed by Capps 1 the upper Chulitna region. The area mapped

S. R., Mineral resources of the upper Chulitna region, Alaska: U.S. Geol. 

Survey Bull. 692, p. 207, 1919.

289
290 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

lies west of the Chulitna Kiver and includes about 84 square miles 

in the drainage basins of Ohio, Copeland, Long, Colorado, and 

Costello Creeks and a section of the West Fork of the Chulitna 

River. (See fig. 1.)

The field examination in this area began July 19 and ended September 5, 1931. It was conducted by a party comprising the writer 

and Marion Peel, packer and rodman. The area was sketched topographically and geologically on a field scale of 1 to 96,000 by means 

of a plane table and explorer's alidade. Control was obtained from 

an enlarged copy of a plane-table survey made by D. C. Witherspoon, 

of the United States Geological Survey, on a scale of 1 to 180,000. 

The location of the principal peaks on this sheet proved satisfactory 

for use as triangulation points for the new map, it being impracticable to measure a base line or otherwise establish independent 

control. Elevations were originally referred to these earlier determined points, but subsequently they have been approximately adjusted so as to conform to the elevation of the Copeland triangulation 

station of the United States Coast and Geodetic Survey, which is on 

one of the peaks in the southern part of the area mapped during 

the present investigation. Because of limitations of time and personnel the map shown as plate 25 is somewhat deficient in both 

horizontal and vertical control but it represents the topographic 

forms with essential correctness. The geologic sketching for plate 

25 was done concomitantly with the topographic sketching and is 

of comparable accuracy. In addition to the study of the general 

geology, 11 prospects were examined, of Avhich 8 are held for gold, 

1 for copper, 1 for silver and lead, and 1 for coal. The major factors 

concerned with ore deposition there were determined, and some 

insight was obtained into the details of the occurrence of individual 

lodes.

ACKNOWLEDGMENTS

The work was greatly facilitated by the cooperation of Henry 

Stevens and Alonzo Wells, owners of the principal prospects, and 

of other residents of the vicinity. The reports of Capps 2 and Moffit 8 

on this and neighboring regions have been freely drawn upon and 

furnish an invaluable setting for .the present study.

HISTORY

Mineral deposits were first discovered in the area in 1907. As 

usual the placer gravel was the first to be found and worked, but the 

returns were so meager as to result in prompt abandonment. The

a Capps, S. R., Mineral resources of the upper Chulitna region, Alaska: U.S. Geol. 

Survey Bull. 692, pp. 207-232, 1919; The eastern portion of Mount McKinley National 

Park, Alaska: U.S. Geol. Survey Bull. 836, pp. 219-300, 1932.

8 Mofflt, F. H., The Broad Pass region, Alaska: U.S. Geol. Survey Bull. 608, 1915.
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 291

presence of lodes was soon recognized, but the remoteness of the region prevented much development at that time. From 1911 to 1915 

much prospecting was done, and nearly all the existing development 

was accomplished during this time. A large part of the territory 

shown in plate 25 was covered with lode claims, and many pits were 

dug. In a few places, notably at the Golden Zone and Ready Cash 

prospects, short tunnels were driven. Since then many of the claims 

have been abandoned, and the annual assessment work on the others 

has been carried out under such handicaps that little has been 

accomplished. In the summer of 1931 outside persons arranged for 

the extension of the Golden Zone tunnel with a view to further 

development if the results were favorable.

GEOGRAPHY

TOPOGRAPHY

The area studied is in the part of the foothills of the Alaska Range 

which is tributary to the upper Chulitna River and its branches. It 

is crossed by several streams that flow southeastward toward the 

Chulitna River and its West Fork. The trunk stream flows southwestward and borders the Alaska Range almost to the southern 

terminus of the range. In the northern part of the area considerable 

parts of the major streams flow S. 65° E., and the average trend of 

their tributaries is N. 30°-35° E. Farther south the main streams 

tend to flow about S. 50° E., and the average trend of the minor 

streams is N. 45° E.

The West Fork of the Chulitna River and Ohio Creek, the two 

largest streams, are braided and carry much silt from the glaciers 

at their heads. The water in some channels of these streams in midsummer exceeds 4 feet in depth. McCallie and Copeland Creeks 

are also silt laden and in part braided. The other streams are clear, 

although several have small glaciers at their heads. The effects of 

recent rejuvenation are visible along most streamways, and the narrow, rock-cut inner gorges of Colorado, Long, and lower Copeland 

Creeks are especially striking. The walls of these gorges are 30 to 

over 50 feet high.

The elevation within the area mapped ranges from less than 2,000 

feet above sea level at the confluence of Ohio and Copeland Creeks 

to nearly 6,000 feet in the peaks at the head of Long Creek. Below 

an elevation of about 4,500 feet the country is generally open, with 

gentle slopes, except on the immediate borders of the streams. Above 

this the topography is rugged, and many slopes are precipitous. In 

the areas of gentle slopes rock outcrops are poor and scanty, but 

in the rugged country exposures are almost continuous, except where 

masked by talus slopes, snow, and ice. Much of the country here
292 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

mapped is below the more rugged mountains, and the locally unsatisfactory exposures and brush cover are the cause of uncertainty as to 

the precise position of geologic boundaries. This uncertainty is indicated on plate 25 by dashed lines.

The Alaska Range rises on the west to elevations of over 10,000 

feet above the sea and is generally rugged. Many of the valleys are 

glacier-filled, and slopes above an elevation of 6,500 feet have extensive banks of perennial snow.

Timber is absent above an elevation of about 2,700 feet and is poor 

and scanty above 2,200 feet. Spruce and cottonwood are the only 

trees of any size and the cottonwood extends somewhat farther up 

the stream valleys. The timber is of potential value mainly for 

local use, as trees over 2 feet in diameter are exceptional and, on the 

average, are neither tall nor straight. For rough mine timbers and 

similar uses, however, there is a considerable supply.

The valley sides up to elevations locally as high as 3,500 feet are 

mantled with a thick growth of brush which hampers travel but, 

on the other hand, is of value for firewood. In most places the country above 2,500 feet is almost devoid of vegetation other than grasses 

and mosses. Grass suitable for forage grows in favorable places up 

to 4,500 feet. Most of it, however, is of inferior quality.

MEANS OF ACCESS

The region is now served by the main line of the Alaska Railroad, 

which in this vicinity follows closely the Chulitna River and after 

crossing its East Fork proceeds up its Middle Fork. A trail extends 

westward from Colorado station, on the railroad, with branches to 

the principal prospects on both sides of the West Fork of the 

Chulitna. Other trails lead from Honolulu station to points in the 

Alaska Range. One of these formerly served the prospects on 

Ohio Creek, but considerable stretches of it are now impassable. 

Practicable routes of travel for pack horses in summer can be found 

through most of the country below an elevation of 4,500 feet. 

Marshes and cliffs cause numerous detours, and it is occasionally 

necessary to cut a way through brush.

Development of the mineral deposits will require the construction of roads 10 to 15 miles long from points on the railroad. 

Colorado is the nearest railroad point to many of the existing prospects and can be reached from them by several alternate routes without special difficulty. It will be necessary to bridge the flood plain 

of the West Fork as well as to throw relatively short bridges across 

a number of minor streams. A small amount of pack-horse traffic 

soon reduces sections of existing trails to quagmires, and it is evident that any road in this region that is to be used for truck trans-
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 293

portation will require care in draining and local reinforcement or 

surfacing. Locally, aerial tramways may supplement the roads, but 

the topography does not readily lend itself to their use.

CLIMATE

Few climatic records are available for this area or its vicinity. 

Snow generally covers the ground from October or early November 

until April or May, and many drifts remain until June - and later. 

Snowfall is frequent and persistent but rarely heavy. Winter temperatures are generally not extreme, although they may occasionally 

range as low as 50° below zero. Summer temperatures may reach 

90° but are in general materially lower, and frosts occur at intervals 

throughout the summer. The annual precipitation may be roughly 

estimated at 40 to 45 inches. Drizzling rain and dense fog are very 

frequent, especially in July and August. High winds are common at 

elevations over 3,000 feet above sea level.

Of the 49 days the party was in the field in the summer of 1931. 

18 were rainy, 20 showery, 6 cloudy, and 5 fair. Heavy downpours 

were exceptional, but the frequent wet and foggy weather interfered 

materially with outdoor work.

GENERAL GEOLOGY

Both stratigraphically and structurally the area has features which 

distinguish it from others in this part of Alaska. The presence 

of strata of Carboniferous (probable Permian) age and of thrust 

faults are among the more striking of these features. Many of the 

rocks are distinctively colored, permitting subdivision into relatively small stratigraphic units. Fossils at several horizons help 

to date these units. Faulting, not completely understood, causes 

minor uncertainties in correlation. In view of the incompleteness 

of present knowledge regarding the stratigraphy of the region in 

general and this area in particular it has not seemed wise to assign 

any formation names. The principal units are designated in accordance with their provisional age assignments, and the minor subdivisions of the Permian (?) rocks are distinguished as unit A, B, etc.

The lowest stratigraphic unit comprises ancient, possibly Devonian, metamorphosed sedimentary rocks in which silicified limestone is prominent. Unconformably above these are tuff, lava, limestone, chert, and argillite of probable Permian age, succeeded, with 

some unconformity, by Triassic limestone, followed by Triassic 

argillite with some limestone, pyroclastic rocks, and lava. There are 

some small dikes and bosses of moderately silicic porphyritic rocks 

of undetermined, presumably Tertiary age, and Tertiary sediments 

of late Eocene or later age.
294 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

STKATIFIED BOCKS 

DEVONIAN (?) CHLORITIC ROCKS

Character. The southeastern part of the area is underlain by 

metamorphosed sedimentary rocks. They trend in general northeast and dip steeply south, with numerous irregularities. The base 

of the formation is not exposed within the area mapped, and as 

there may,be repetitions by folding and perhaps faulting, available 

data afford no estimate as to the aggregate thickness of these beds, 

except that it is probably great.

Most of the rocks are light gray to nearly white, generally with a 

greenish cast, in upland exposures and dark gray to nearly black 

in fresher outcrops, especially in stream gorges. Most of them are 

thin-bedded, but in weathered exposures the banding is inconspicuous. They are calcareous1 sedimentary rocks containing considerable siliceous and argillaceous material. Some, especially beds 

near the upper contact, contain much chlorite. Most beds look 

cherty and liave probably been silicified during metamorphism, but 

many of these retain enough calcite to react readily with dilute 

acid.

Age, The overlying Permian (?) beds rest in angular unconformity on these calcareous and cherty rocks, which are obviously 

the oldest in the area. The contact trends more nearly north than 

the average strike of the beds above. Wherever satisfactory outcrops of the old rocks and the Permian (?) beds are close together, 

differences in the attitude of the beds are evident, but except where 

there has been faulting the divergence in attitude is rarely great.

Further direct evidence as to the age of the older beds is lacking. 

The strata in the general vicinity of Broad Pass, which include

fossiliferous Devonian limestone, have lithologic similarities to the 

old strata here described and are among the nearest known Paleozoic rocks.4 Hence the metamorphosed strata in the vicinity of the 

West Fork of the Chulitna may be of Devonian age.

CARBONIFEROUS (PERMIAN ?) ROCKS

General features. A thick and heterogeneous sequence of rocks 

of probable Permian age stretches through the middle portion of 

the area and includes most of the known mineral deposits. Tuff, 

volcanic breccia, and lava predominate, but argillite, limestone, 

chert, and conglomerate are each locally plentiful. In the southwestern part of the area mapped these rocks are mainly of volcanic 

origin, with the exception of certain limestone beds. Farther north-

* Mofflt, F. H., The Broad Pass region, Alaska: U.S. Geol. Survey Bull. 608, pp. 24-26, 

1915.
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 295

east both the thickness and the heterogeneity increase and the proportion of sedimentary rocks is progressively greater. The aggregate thickness of these rocks in and near Long Creek exceeds 10,000 

feet. These conditions are represented on plate 25 by grouping 

similar or related beds that are colored distinctively into units designated by letters. The beds that are almost exclusively volcanic 

and are characterized by an abundance of red tuff and breccia are 

grouped for purposes of description as unit A, although intercalation of different beds and other causes separate these rocks into 

several areas. So far as the area here mapped is concerned, the 

other units may all be regarded as local members intercalated in 

unit A, which constitutes the major and locally the sole part of the 

formation. In different parts of the area mapped unit A ranges 

from the base to the top of the Permian (?) sequence, In the

vicinity of Long Creek chloritic argillite, conglomerate, and lava, 

with some limestone are grouped as unit B. Stratigraphically above 

unit B and in places separated from it by part of unit A, there are 

beds composed mainly of black argillite, which are designated unit 

C. This unit is in places separated from the underlying portion 

of unit A by a narrow band of fossiliferous limestone. Above unit 

C and in places separated from it by an isolated mass of unit A is 

a thick aggregate of beds characterized by abundant chert and 

cherty limestone but containing also much argillite and some conglomerate and volcanic material. This is called unit D. It is 

succeeded by more of unit A, followed by other beds called unit E, 

in which chert and limestone are also locally prominent but which 

contain more volcanic material and less argillite than unit D.

Unit A. The striking feature of unit A, the principal component 

of the Permian (?) rocks, is the abundance of tuffaceous beds colored 

deep rose and related shades. Some of these are moderately fine, 

even-grained rocks; others are breccias in which either the cement 

or most of the fragments or both are colored red by disseminated 

hematite. All are composed mainly of fragments of lava and of 

igneous minerals, mainly plagioclase. Quartz and locally calcite 

are plentiful in the cement. The lava fragments are mainly aggregates of plagioclase with trachytic texture. Chlorite and other 

alteration products are present, but original ferromagnesian minerals are generally absent. Some of the coarser breccias include 

in addition to the igneous materials fragments of black argillaceous 

rock, quartz, and other materials. The pieces are generally of irregular shape and poorly sorted, but in some beds they are fairly well 

rounded and the rock approaches a conglomerate in appearance. 

Along Long Creek local beds of conglomerate with well-rounded 

pebbles are grouped with unit A in mapping. Interbedded with the
296 INVESTIGATIONS IN" ALASKA RAILROAD BELT, 1931

red beds there are considerable quantities of green and nearly black 

tufts composed of irregular fragments of lava, devitrified glass, and 

other material. The cementing material is mainly quartz and 

calcite. Some of the lava has pisolitic texture. Chlorite is abundant.

Lava flows are widely distributed but are almost everywhere 

subordinate in amount to the tuffaceous beds. The small masses 

mapped as unit A near Costello Creek are, however, mainly lava. 

The lavas are gray, purple, and green andesite, in part amygdaloidal, 

in part porphyritic. They are composed mainly of plagioclase, 

ranging in composition from oligoclase to andesine in different flow's, 

with small amounts of quartz and considerable chlorite and other 

alteration products. Some contain hornblende, but in most of them 

any ferromagnesian minerals which may have originally been present 

are altered beyond recognition. The amygdules are filled with 

calcite.

Nonvolcanic strata are rare in unit A. Approximately midway in 

the sequence, along Ohio Creek and between units A and C south of 

Long Creek, there are persistent beds of ci^stalline limestone which 

south of Long Creek are fossiliferous. The beds in the two localities are probably approximately equivalent stratigraphically. These 

are the only Permian (?) limestones which have been mapped separately. Their light-colored cliffs are prominent features of the landscape. The beds are about 100 feet thick, or locally more. Small 

amounts of limestone, argillite, and conglomerate are included in 

unit A as mapped in several other localities and are particularly 

well exposed on Long Creek.

The Long Creek gorge cuts three separate masses assigned to unit 

A. These have an aggregate thickness of 3,500 feet, of which about

350 feet consists of greenish tuffs and breccias and about 600 feet of

conglomerates with comparatively well rounded pebbles and without conspicuous red color. Along Ohio Creek unit A is fully 3,500 

feet thick and may be materially thicker. In both localities faults 

prevent accurate determination of the thickness.

Unit B. The strata which on Long Creek are locally at the base 

of the Permian (?) sequence constitute unit B. In the inner gorge 

of this creek unit B includes green to black chloritic argillite, conglomerate, and limestone. Some of the chloritic rock may be intensely altered lava, but most of it is of probable sedimentary origin. 

In some exposures it shows almost no trace of bedding, but elsewhere it is banded and much contorted. A few beds are coarse 

grit with a green matrix that is probably tuffaceous. The chloritic 

rocks have an aggregate thickness of about 2,500 feet. The conglomerate contains pebbles, that are better sorted and more waterworn than any seen elsewhere in the area. The pebbles are mostly
U. S. GEOLOGICAL SURVEY BULLETIN 849 PLATE 25

V . EXPLANATION « F".'-i ,

SKETCH MAP AND SECTIONS

VICINITY OF

WEST FORK OF THE GHULITNA RIVER

ALASKA

GEOLOGY BY CLYDE P. ROSS

Scale 125,000

Contour interval 100 feet 

Datum in mean sea, level

Topography by Clyde P. Rose 

1933

Section along line. A-A 

NW.

Section along line B~B

Section along line C-C
MINERAL DEPOSITS NEAE WEST FORK OF CHULITNA RIVER 297

andesitic lava, and the largest are about 5 inches in diameter. The 

thickness of the conglomerate is somewhat less than 200 feet. Limestone is exposed at two places in the Long Creek gorge within the 

limits of unit B and has an aggregate thickness of nearly 150 feet. 

It is somewhat rusty, crystalline, and, at least in part, magnesian. 

It contains fragmentary fossils, but nothing which appeared determinable was found.

On the ridges to the north chloritic material persists, but neither 

conglomerate nor limestone appears at this horizon. There are a 

number of porphyritic and amygdaloidal lava flows. Some are 

dacite, consisting mainly of plagioclase laths of intermediate composition with about 10 percent of interstitial quartz and considerable 

amounts of chlorite, sericite, and epidote. The amygdaloidal flows 

are largely basalt containing roughly 20 percent of augite, TO percent

of plagioclase with the composition of andesine or labradorite, and 

chlorite, calcite, epidote, and other alteration products.

Unit C. Unit C is composed of black to greenish argillite, which 

resembles the Triassic argillite above but is in part more chloritic. 

Most of it is thin-bedded but some is indistinctly laminated. Where 

this unit is cut by the Long Creek gorge it has a thickness of only 

about 200 feet, but on both sides it appears to be materially thicker.

Unit D. Unit D is a comparatively heterogeneous assemblage of 

strata in which chert, cherty limestone, and chloritic argillite predominate. Locally it includes green and gray andesitic lava, tuff 

and tuffaceous breccia, mainly green and black, and some fine conglomerate. Most of the cherty beds are light greenish, but some are 

black and red. The argillite resembles that in unit B. The aggregate thickness of unit D in the Long Creek gorge is about 1,600 

feet, but the unit appears to thicken northeastward. In the vicinity 

of Bryn Mawr Creek the unit includes some coarse tuffaceous breccia 

in which angular fragments of black argillite are abundant. Most 

of the rock on and near Lookout Mountain is sheared, almost schistose 

chloritic argillite with small limestone lenses.

Unit E. Unit E crosses the West Fork of the Chulitna near the 

Riverside prospect. It resembles unit D in the local abundance of 

chert and cherty limestone but contains more tuff and less argillite. 

In the vicinity of the Riverside prospect the principal rock is banded 

light-colored limestone, in part cherty, in part rather coarsely crystalline. There are a few conglomerate beds with rounded pebbles. 

The hills just south of the prospect contain chert, limestone, and 

green tuff, somewhat silicified. West of the prospect there are 

gray and purplish rather fine grained and massive sedimentary 

rocks composed largely of quartz, calcite, and sericitized feldspar.

179268 33 2
298 INVESTIGATIONS IN ALASKA KAILROAD BELT, 1931

In the canyon of Colorado Creek near the Silver King prospect 

there are cherty and chloritic beds with some conglomerate and limestone. At the Silver King and Liberty prospects the prevailing rocks 

are thin-banded cherty limestone. Along Costello Creek the beds 

assigned to unit E include some cherty and calcareous rocks, but 

most of them are chloritic tuff. Some argillite is also present.

In all exposures of unit E the beds are much disturbed, and the 

thin-banded ones commonly are crenulated. The variety of dips 

and strikes recorded on plate 25 illustrates the impracticability of 

determining accurately the thickness of the unit. Presumably it is 

of the order of 2,500 feet.

Age. The strata described as units A to E are of Carboniferous 

age, probably Permian. They are stratigraphically above the old 

Devonian (?) rocks and below Triassic beds. A collection of fossils 

made from limestone beds immediately below unit C near the crest 

of a spur of the ridge southwest of Long Creek has been examined 

by George H. Girty, who states that it is clearly of Carboniferous 

age and probably Permian, "though the facies of the fauna is by 

no means identical with that of the typical Alaskan Permian as we 

know it from Kuiu Island and from the Yukon." The species 

recognized are listed below, all the identifications being more or 

less provisional. The lot is no. 6942 of the permanent collections 

of the United States National Museum.

Dielasma sp.

Spirifer aff. S. rectangulus 

Spirifer aff. S. fasciger 

Martinia? sp. 

Spiriferina? sp. 

Rustedia sp. 

Platyceras sp.

Cyathophyllum sp. 

Chonetes aff. C. geinitzanus 

Procluctus semireticulatus 

Productus n. sp. 

Pustula aff. P. horrida

Pustula aff. P. pustulata

Rhynchopora aff. R. nikitini 

Camarophoria? sp.

TRIASSIC LIMESTONE

Character. Immediately above the Permian (?) rocks are prominent beds of Triassic limestone. A few of these beds, which cross 

upper Long Creek, are somewhat more coarsely crystalline than the 

others and form conspicuous white outcrops. These have been distinguished from the rest in mapping. The fossil collection described 

first in the discussion of the age of the beds (see pp. 299-300) came 

from sandy, rusty limestone just above this unit. The crystalline unit 

appears to be unfossiliferous, whereas fossils are widely distributed 

in the rest of the Triassic limestone, most of which weathers in 

brownish cliffs, is gray to black on fresh fractures, is interbedded 

with argillite, and is somewhat more impure, some beds being sandy 

and others argillaceous. On the south side of Ohio Creek groups
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 299

of such beds separated by argillaceous strata are exposed. The 

whole is much contorted, and the limestone and argillite are intricately interbedded on the south side of Ohio Creek, making it 

impracticable to separate them on a map of the scale of plate 25. 

The aggregate thickness is probably at least 1,500 feet. On Copeland Creek the proportion of argillaceous material is less; the limestone is intricately and closely folded (see fig. 38) and is probably 

fully as thick as on Ohio Creek. On the south side of Long Creek 

the total thickness of the Triassic limestone appears to be materially 

greater than 2,000 feet. On the north side of Long Creek the exposed 

thickness is much diminished by faulting, and fully 250 feet of rusty 

limestone with minor amounts of argillite overlies the crystalline 

unit. There is perceptible angular discordance between the rusty 

limestone and the main body of argillite, but this presumably records

merely local unconformity.

Age. Fossil evidence shows that these limestones are Triassic, 

probably Upper Triassic. Structurally, as is more fully discussed on 

pages 301-302, there is reason to doubt that the hiatus, if there is any 

such hiatus, between the Permian (?) beds and the limestone and 

associated argillite, is sufficiently great to account for the absence of 

representatives of the Lower and Middle Triassic.

Sandy limestone immediately above the prominent crystalline limestone beds mapped separately on Long Creek contains abundant 

fossils. A collection was made from an outcrop on a terrace on the 

north side of the creek at an altitude of 4,125 feet. J. B. Keeside, Jr., 

comments on this collection (U.S.Nat.Mus. no. 15952) as follows:

Several unnamed and interesting species of Spiriferina, make up this lot. 

It has been examined by G. H. Girty, who finds the species unlike any familiar 

to him. They could very well be Triassic forms, and it is suggested that they 

be so assigned until other evidence is available. Rather similar species are 

present in a Triassic fauna from Oregon which contains ammonites.

Another collection of fossils was made from the limestone immediately south of the conspicuous ice-filled gorge on upper Copeland 

Creek at an altitude of 3,250 feet. Reeside reports that this material 

(U.S.Nat.Mus. no. 15954) contains the following species:

Thecosmilia sp. 

Terebratula?.sp. 

Cassianella n. sp.

Halobia? sp. 

Cardinia? sp. 

Borings, undetermined

Reeside says: " This is an unfamiliar assemblage. Only the form 

determined as Cassianella is well preserved and abundant. It should 

be Triassic and more likely Upper Triassic."

Fossils were noted in several places in the Triassic limestone along 

Ohio Creek but were not collected, as Capps 5 had previously made

B Capps, S. R., Mineral resources of the upper Chulltna region, Alaska: U.S. Geol. 

Survey Bull. 692, p. 217, 1919.
300 INVESTIGATIONS IN ALASKA EAILEOAD BELT, 1931

10093. Terebratula sp. 

Lima sp. 

Avicula sp. 

Myophoria? ? sp. 

Cassianella sp. 

Coelostylina ? sp. 

Loxonema (Auoptychia) ? sp. 

Loxonema? sp. 

Triassic.

a collection from limestone talus iy2 miles above the mouth of 

McCallie Creek. T. W. Stanton reported that this collection 

(U.S.Nat.Mus. no. 10094) contains Avicula sp., Cassianella sp.r 

Undularia? sp., and Coelostylina? sp., and is of Triassic age.

Capps also made four collections of fossils from boulders in 

stream bars along Copeland Creek, which were identified by Stanton 

as indicated below, the collection numbers listed being those of the 

United States National Museum. As the Triassic limestone from 

which a collection was obtained during the present study is the 

only limestone exposed along Copeland Creek, there seems little 

doubt that the fossils in the boulders came from this horizon.

10090. Terebratula sp. 

Pecten sp.

Probably Triassic.

10091. Terebratula sp. 

Lima? sp. 

Astarte? sp.

Probably Triassic.

10092. Terebratula sp. 

Pecten sp. 

Aviculopecten ? sp. 

Probably Triassic.

Stanton's comment regarding the material as a whole was that 

" the presence of Cassianella in these collections suggests the Triassic fauna of Gravina Island of southeastern Alaska." The Gravina Island fauna is now regarded as of Upper Triassic age.6

TRIASSIC ARGIL.LITE

Clim^acteT. The mountains along the northwest border of the area

mapped, which extend westward beyond the limits of vision, are

composed mainly of dark-gray to black argillite and argillaceous 

sandstone. These rocks are interbedded at their base with the Triassic limestone above described, with local unconformity in some 

places. At and just above the upper contact of the Triassic limestone there are lenses of tuffaceous material lithologically identical 

with unit A of the Permian (?) rocks. Most of such exposures are in 

zones of intense folding and faulting, but the distinctive reddish 

strata appear to be integral parts of the sedimentary sequence. Locally, especially on Ohio Creek, dark-greenish lava and pyroclastic 

rocks are abundant. As these rocks can be distinguished from the 

coarser beds in the associated argillite only on close inspection, considerable quantities of them may be present in the rugged mountains 

around the head of Ohio Creek. Some are altered andesitic or possibly more calcic lava, of which roughly 50 percent is made up of

8 Martin, G. C., The Mesozoic stratigraphy of Alaska: U.S. Geol. Survey Bull. 776, pp. 

70-71, 1926.
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 301

plagioclase laths nearly 1 millimeter long, which approximate oligo- 

<clase in composition, almost equally abundant bladed blue to brown 

pleochroic hornblende of secondary origin, and some chlorite and 

:?ericite. Other rocks are coarse breccias composed of irregular fragments of this lava and of argillite in a green tuffaceous matrix. 

The fragments are somewhat rounded and range in maximum diameter from a few inches to several feet.

In several places near the eastern border of the argillite, particularly near the headwater branches of Blind Creek and on the hill 

 east of the mouth of this creek, there are lenses of conglomerate containing moderately well rounded pebbles an eighth to half an inch 

or more in maximum diameter. The pebbles include quartz, argillite, 

quartzite, chert, and several fine-grained igneous rocks. The matrix 

is a grit composed mainly of quartz, feldspar, and rock fragments. 

Conglomerate of this kind is locally associated with the tuffaceous 

lenses mentioned above.

The high mountains northwest of Long Creek, beyond the limits 

of the area shown on plate 25, contain several lenses of rusty limestone. The brown outcrops of this rock are most conspicuous in contrast to the surrounding black rocks and are identical in general 

appearance with the main part of the Triassic limestone within the 

area mapped, suggesting repetition of calcareous deposition higher in 

the sequence.

Age. Fossils were not found in the argillite, but its intimate relation to the fossiliferous Triassic limestone on Long and Copeland 

Creeks shows that at least the lower part of the apparently very 

thick sequence is of Triassic age. The presence near its base of beds 

lithologically identical with distinctive tuff of Permian (1) age led to 

the conclusion in the field that there is no great difference in age between these units. The paleontologic evidence (pp. 298-300) tends to 

indicate that there is a gap corresponding to the whole of Lower and 

Middle Triassic time, but as both the Permian (?) and the Triassic 

fossils obtained are unfamiliar assemblages, the data are inconclusive on this point. The possibility is suggested that early Triassic 

sedimentary rocks are present here, though they have not been found 

in other known parts of Alaska.7 The presence of lava and tuff 

strengthens this possibility, as volcanic strata of probable early 

Triassic age exist in many localities such as Chitina Valley, Kenai 

Peninsula, and Kodiak Island.8

Though the argillaceous rocks within the area here considered 

are thus, at least in part, of Triassic age, strata higher in this very 

thick and complexly deformed sequence are evidently materially 

younger, as Capps 9 has found fossils of Upper Jurassic or Lower

7 Martin, G. C., op. cit., pp. 128-130.

8 Idem, p. 6.

8 Capps, S. R., The eastern portion of Mount McKinley National Park, Alaska: U.S. 

Geol. Survey Bull. 836, p. 261, 1932.
302 INVESTIGATIONS IN" ALASKA RAILROAD BELT, 1931

Cretaceous age in an intercalated limestone lens some distance to tlu> 

west. Capps 10 states that the beds associated with this f ossiliferous 

lens are more uniformly shaly and are without the coarser beds 

prominent in the argillaceous rocks associated with the Triassic 

limestone here described. For the purposes of the present report all 

the dark-colored, dominantly argillaceous rocks associated with and 

stratigraphically above the Triassic limestone are grouped on plate 

25 as Triassic argillite, although it is recognized that future work 

may show that the upper part of the group thus mapped is younger.

TERTIARY (EOCENE?) ROCKS

Character. At the northeast end of the area there are extensive 

deposits of more or less thoroughly consolidated gravel, sand, and 

clay, with local coal beds, which are of Tertiary age. In fresh exposures, either natural or artificial, this material is well-cemented 

rock, but the cement, which is largely calcareous, readily disintegrates 

on weathering. In most places these beds are somewhat deformed, 

dips of 10° and more being common.

Conglomerate is the most abundant material in most places. 

Where fresh it is a hard rock, but in cliff faces it is soft enough to 

yield to the pick, and weathering has reduced most of it surficially 

to loose gravel. Interstream areas are mantled with such gravel, 

but the hard material exposed in most gulches proves its origin. 

Here, as in other areas of similar rolling topography in the region, 

there are scattered large erratics and thin, entirely unconsolidated 

beds of gravel and sand, which are doubtless Pleistocene glacial 

outwash. Such material is difficult to distinguish from the weathered 

Tertiary conglomerate, but the amount present everywhere within 

the area mapped appears to be small. The Tertiary conglomerate

was evidently derived from erosion from the nearby mountains, as 

it is composed largely of argillite pebbles and cobbles. Granitic 

cobbles are also plentiful, doubtless derived from such granitic intrusions as those which crop out near Costello Creek both northwest 

and southeast of the area mapped.11 Some of the characteristic 

reddish tuff of unit A, which is not known to crop out in the tributary area, and other representatives of the Permian (?) rocks are 

locally present but are not abundant.

Much of the sandstone and grit associated with the conglomerate 

is composed largely of argillite. On Camp Creek and its tributary 

Coal Creek the most abundant beds are light-colored calcareous 

sandstone. Similar light-colored sandstone is locally interstratified 

with the conglomerate on Costello Creek and in gulches in the in10 Capps, S. R., personal communication.

11 Capps, S. R., op. cit. (Bull. 836), p. 284.
MINERAL DEPOSITS NEAR WEST FORK OP CHULITNA RIVER 303

tervening area. These beds appear to have been derived mainly from 

the erosion of granitic rocks but have abundant calcite cement. In 

the vicinity of Camp Creek light-colored clayey and shaly beds, 

locally darkened by carbonaceous material, are more abundant than 

farther south, and conglomerate less so. The coal exposed on Coal 

Creek is described on pages 331-332. Other exposures of coal are 

stated by Henry Stevens to exist in the area north of the limits of the 

present study.

Age. The character and structural relations of these beds show 

obviously that they are post-Triassic and pre-Pleistocene. Abundant 

and unusually well preserved plant remains in beds of calcareous 

sandstone associated with the coal on Coal Creek have been examined by Koland W. Brown, who regards them as " at least of 

upper Eocene age, if not a little younger." He identified the fol1'owing forms in the collection (U.S.Nat.Mus. no. 8291):

Taxodiuni dubium

Sequoia?

Populus richardsoni

Alnus alaskana 

Carpinus grandis 

Hicoria magnifies

QUATERNARY DEPOSITS

Within the area mapped distinctly glacial deposits are scanty. 

The glaciers at the heads of Ohio, Copeland. and Long Creeks have 

built small moraines, but there is little suggestion of older morainal 

material in the valleys below. The rolling interstream uplands on 

the eastern border of the area have erratic boulders sparsely distributed on their surfaces and locally have thin gravel deposits mantling the bedrock. All the larger streams, especially the West Fork 

of the Chulitna River, are bordered by terraces, more or less obscured 

by weathering, landslides, and frost creep. Some of these still show 

that they were originally covered by gravel and sand, but the deposits 

are nowhere deep.

All the streams are floored to a greater or less extent by alluvium. 

Wherever sufficiently extensive, this has been shown on plate 25. 

Along the West Fork silt and sand predominate, though gravel bars 

are locally present. The deposits along Ohio and McCallie Creeks 

contain less silt and more and somewhat coarser gravel. In most 

places in the other valleys gravel is abundant and silt rare.

INTRUSIVE BOCKS

Small andesite dikes occur in the Permian (?) volcanic rocks south 

of Copeland Creek and are similar to and doubtless of essentially the 

same age as the nearby lava flows. Some of the other porphyries
304 INVESTIGATIONS IN ALASKA EAILEOAD BELT, 1931

included in the Permian (?) rocks may likewise be of intrusive rather 

than effusive origin. None of these were mapped separately.

In addition to these old intrusive rocks there are numerous small 

bodies, generally rendered inconspicuous by alteration, which are 

doubtless much younger. Most of these appear to differ among themselves mainly in details of alteration, and they are grouped as biotitequartz diorite porphyry. One relatively coarse-grained mass between Colorado Creek and the West Fork of the Chulitna is somewhat more calcic and is termed quartz diorite. Dikes on the hill east 

of the mouth of Blind Creek which contain conspicuous hornblende 

may be distinguished as hornblende diorite porphyry.

BIOTITE-QUARTZ DIORITE PORPHYRY

There are numerous bodies of light-colored, fine-grained biotitequartz diorite porphyry which, where not too much altered, are 

characterized by prominent biotite phenocrysts. These range from 

dikes a few feet wide to the stock at the Golden Zone prospect, which, 

as mapped, has a maximum diameter of somewhat over 4,000 feet. 

This stock is probably more irregular in outline than is indicated on 

plate 25 and has apophyses leading from it which are, for the most 

part, too small and too poorly exposed to be mapped. The numerous 

generally small masses of this rock shown on plate 25 indicate somewhat diagrammatically the position of most of the exposures, 

although many of the dikes are far too small to be shown individually 

on a map of this scale. Most of the dikes tend to follow in general 

the trend of the steeply dipping beds they intrude, but in detail they 

cut across the bedding planes irregularly.

This rock is everywhere so much altered hydrothermally as to

prevent accurate determination of its original composition. The

groundmass is a mat of irregular feldspar grains 0.2 to 0.4 millimeter 

long, with interstitial quartz. The phenocrysts are biotite, feldspar, 

and quartz and are 1 to 2 millimeters in maximum dimension.

Zoned and sericitized plagioclase, mainly of the approximate composition of oligoclase, makes up about 80 percent of the rock. A 

small amount of potassium feldspar may be present in some of the 

intrusive bodies. Chloritized biotite and quartz each make up about 

10 percent of the rock. One variety that crops out in a small mass 

north of Long Creek, near the east border of the area, contains about 

68 percent of feldspar, 15 percent of augite, 12 percent of biotite, 

and 5 percent of quartz. These estimates are based on measurements 

in which the attempt was made to get original composition, ignoring 

the abundant alteration products, which include calcite, chlorite, 

quartz, epidote, sericite, and, locally, iron and other sulphides.
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 305 

QUARTZ DIORITE

The quartz diorite is a fairly even grained aggregate with few 

phenocrysts. The feldspar occurs in frayed laths with a maximum 

length of 1 millimeter, and the quartz tends to be interstitial. The 

rock contains about 45 percent of sericitized andesine, 24 percent 

each of biotite and hornblende, 5 percent of quartz, and some hematite, magnetite, sericite, chlorite, and epidote. It is less altered than 

any of the biotite-quartz diorite porphyry.

HORNBLENDE DIORITE PORPHYRY

The hornblende diorite porphyry consists of hornblende blades 

2 millimeters or more long in a fine-grained mat of plagioclase too 

thoroughly obscured by sericitization for more exact determination. 

The rock contains roughly TO percent of plagioclase, 20 percent of

pleochroic brown hornblende, 5 percent each of secondary calcite 

and quartz, and a little epidote.

AGE

The intrusive rocks are later than the Triassic and earlier than 

the Eocene (?) sedimentary strata. The small size of the bodies and 

the prevailing fine-grained and porphyritic texture suggest intrusion 

at no great depth, which in turn suggests that the intrusions are not 

very old geologically. Capps 12 notes that the Cantwell formation, 

of supposed Eocene age, exposed at the head of the West Fork of 

the Chulitna has been intruded by dikes. The sediments within 

the area mapped that contain pebbles of intrusive rock are not older 

than upper Eocene and may be younger. Thus the scanty available 

evidence leads to the inference that intrusion took place near the 

beginning of the Tertiary period.

STRUCTURE 

FOU5ING

In general, the strata strike northeast and dip steeply northwest, 

but there are many local variations due to both folding and faulting. 

The great width of outcrop of the Devonian (?) rocks may perhaps 

result from repetition by folding. The observed dips in the area 

between Ohio and Copeland Creeks (see pi. 25) suggest the presence 

of an anticline in the Devonian (?) rocks, but this has not been 

proved because of the lack of horizon markers and the paucity of

15 Capps, S. R., op. cit. (Bull. 692), pp. 220-221.
306 INVESTIGATIONS IN ALASKA EAILROAD BELT, 1931

good outcrops in much of this 

area. In the gorges of Copeland and Long Creeks the Devonian (?) strata exhibit considerable variations in attitude. 

Some exposures show intricate 

crenulation.

The Permian (?) beds show 

much local variation in attitude, 

and the thin-banded members 

are somewhat crenulated. In 

some places, such as the vicinity 

of the Eiverside prospect, the 

beds tend to strike transverse 

to the general structural trend, 

suggesting cross folding. 

Though regional deformation 

has tilted the Permian (?) 

beds steeply, there is no evidence of any folds within the 

area of these beds here mapped. 

Most of the variation in attitude 

probably results from faulting.

The Triassic rocks include 

many beds comparatively susceptible to folding. In the 

vicinity of the zone of thrusting, the Triassic rocks are

closely and intricately folded. 

The folding is especially well 

displayed on the north flank of 

the upper valley of Copeland 

Creek. As is somewhat diagrammatically shown in figure 

38 the Triassic limestone here 

has been thrown into a number 

of intricate, overturned, and 

recumbent folds. The associated 

argillaceous and sandy beds 

are similarly deformed, but 

the bedding in these is not 

sufficiently distinct to be visible 

in a distant view such as figure 

38 represents.
MINERAL DEPOSITS NEAR WEST FORK OF CHTJLITNA RIVER 307

FAULTING

The major structural feature of the area is a zone of thrusting 

which extends throughout its median portion. This zone trends 

about N. 30° E. and contains two or more related thrust faults, which 

are inclined 30°-40° NW. The result is to cause the Triassic and part 

of the Permian (?) rocks to override Permian (?) and Devonian (?) 

strata. Thrust planes are exposed with diagrammatic clarity in the 

valley of Copeland Creek. (See fig. 38.) On the north and south 

the exposures are less perfect, but crumpling and brecciation, with 

the general distribution of the rocks, make it possible to trace the

-argillaceous and 

. calcareous sandstone.

Breccia vvt 

zones ''.\',: 

cemented. . 

with quartZ;Jy///

FIGURE 39. Diagrammatic sketch of faulting along upper Blind Creek.

position of the thrust faults with essential correctness. As almost 

the entire width of the steeply tilted Permian (?) strata is traversed 

by the thrust zone in the vicinity of Copeland Creek, the horizontal 

component here appears to be fully 2 miles. Near Ohio Creek the 

displacement seems to be less, but more complete knowledge of the 

stratigraphy of the region might show that this is not the case.

Between Long Creek and the West Fork of the Chulitna there are 

steep faults, evidently associated with the thrusting. These trend
308 INVESTIGATIONS IN ALASKA KAILEOAD BELT, 1931

north to N. 30° E. and are vertical or inclined steeply to the east. 

The last movement along these faults was essentially horizontal, as 

shown by well-marked horizontal grooving on fault planes. It 

seems probable that the absence of Triassic limestone near the West 

Fork is to be accounted for by lateral shift along such faults. A 

sketch of an exceptionally well exposed segment of one of these 

steep faults, in the inner gorge of Blind Creek a short distance below 

the point where the headwater branches of this stream come together 

at the border of the mountains is shown in figure 39. Both on Blind 

Creek and in the bank of the West Fork just west of the Riverside 

prospect it is clear that the deposition of vein quartz and the mineralization in the rocks were later than the faulting. On the West Fork 

a vertical fault is exposed. Bending in the contiguous beds indicates downthrow to the east, and immediately east of the fault plane 

there is a horizontal zone of shearing and brecciation.

Most of the steep faults are not so well exposed as those at the 

localities cited and have been mapped mainly on the basis of the 

distribution of the rocks. In most places where faulting is shown 

on plate 25 the evidence is strengthened by brecciation and variation in attitude in nearby rocks. There are undoubtedly more faults 

than are shown, but those omitted for lack of evidence are probably 

comparatively minor.

The tongue of Triassic argillite that is prominent on upper Blind 

Creek and ceases shortly after crossing Long Creek is probably a 

segment of the overthrust mass. In several places near Blind Creek 

small patches of argillite and kindred rocks cap hills composed of 

rock classed as unit A of the Permian (?) strata. These are doubtless outlying remnants of eroded portions of the overthrust rocks. 

It is clear that the steep faults have affected the argillite tongue,

and the most probable explanation of its relatively low position in 

the gorge of Long Creek is that the argillite was dropped along such 

faults after being overthrust. Original irregularities in the thrust 

plane and subsequent folding would aid in bringing about the relations observed. So far as they have been determined, however, the 

contacts between the argillite tongue in the Long Creek gorge and 

the reddish tuff and breccia on both sides are so steep as to suggest 

faulting rather than folding. The actual contacts were zones of 

weakness and hence have been obscured by erosion, but disturbance 

of the beds and minor amounts of mineralization are visible nearby. 

The two largest of the intrusive masses in the area lie on probable faults and as mapped appear to be of somewhat later date. It 

is possible that some of the other intrusions are similarly related to 

zones of movement, but those of the numerous small dikes that are
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 309

well exposed were introduced approximately along bedding planes 

rather than along faults.

ECONOMIC GEOLOGY

The area contains numerous prospects but as yet no productive 

mines. Most of the lodes are valued mainly for their gold and 

silver content, although they contain also notable quantities of arsenic, and several carry copper, lead, and zinc. One is valued solely 

for its copper, another for its silver and lead, and one of the gold 

lodes contains antimony and perhaps also bismuth. Gold occurs 

in the gravel of several of the creeks^ but no valuable deposits have 

yet been found. In the northern part of the area there are coal 

beds that may be of value in connection with the development of 

metal mines.

All the metallic deposits appear to be closely related. The differences between them result largely from differences in the environment in which deposition took place. For purposes of description and in order to emphasize characteristics of some moment in 

planning for the development of the prospects, the lodes are grouped 

into three types. The first type includes deposits in which sulphides 

and related material are disseminated through a broad expanse of 

rock in which there has been a relatively minor amount of fracturing. Though mineralization of this type was common, only locally, 

as at the Golden Zone, has it been so concentrated as to produce a 

possible ore body. The second type, exemplified by the Copper 

King, includes deposits formed by intense local replacement along 

certain calcareous beds which proved especially favorable to this 

process. The third type, to which most of the prospects belong, 

includes veins containing quartz as a conspicuous constituent. Although these veins were deposited within fairly definite zones of 

fracture and shearing, in part by fissure filling, they all show replacement of the wall rocks and hence are closely allied to the deposits of the first two types in which replacement is the outstanding 

characteristic.

LODES 

DISSEMINATED DEPOSITS

Sulphides are disseminated through the rock in numerous localities 

and over wide areas, but in only a few spots are the products of this 

process sufficiently concentrated to be of economic interest. Much of 

the dioritic porphyry, chert, and impure limestone, as well as some of 

the argillite and other rocks, contain enough pyrrhotite (FeSi+) and 

pyrite (FeS2 ) to be visible with a hand lens. Such disseminated 

sulphide is more abundant in the dioritic porphyry than in any of
310 INVESTIGATIONS IN ALASKA EAILEOAD BELT, 1931

the other rocks, and this porphyry commonly shows other evidences 

of hydrothermal alteration, such as a general bleaching, alteration, 

and disintegration of the ferromagnesian constituents, and sericitization of the feldspar. Carbonate is generally present and in the more 

thoroughly altered rock is fairly abundant. Most of this is calcite 

(CaCOs), but locally it is ferruginous and approaches ankerite 

Ca(Mg,Fe)(CO8 ) 2 ) in appearance. The intrusive rock is in places 

partly silicified. Where sufficiently altered to be of economic significance the rock is cut by fracture planes and by breccia and shear 

zones, along which alteration is in places so intense as to obliterate 

its original characteristics. Irregular veinlets and lenses of vein 

quartz and associated minerals occur in these zones. All gradations 

exist between such zones and those in which fissure filling is the 

dominant feature. Although mineralization of the disseminated type 

is generally most intense within the dioritic rock, it is neither uniform throughout the intrusive mass nor limited by its boundaries. 

The Lindfors prospect, for example, is largely in stratified rock, but 

its mineralization i? evidently an extension of that at the Golden 

Zone. Most of the zones of fracturing and shearing within the 

disseminated deposits appear to be individually of small extent. 

Arsenopyrite (FeAsS) and pyrite, with small amounts of other 

sulphides, are concentrated in streaks and bunches in these zones. 

Locally, sulphides compose considerably more than half the lode matter, but generally they make up less than a third; and most of the 

relatively unsheared rock contains less than 10 percent of disseminated sulphide. Arsenopyrite is by far the most abundant, and 

consequently some zones contain enough arsenic to be of possible 

future value. Chalcopyrite (Cu2S.Fe2 S3 ), sphalerite (ZnS), and 

galena (PbS) are locally present but, in the exposures that were 

examined, not in amount to be of economic interest except insofar 

as they may carry gold and silver. Stibnite (Sb2S3 ) occurs in one 

shear zone at the Silver King and may also be present at the Golden 

Zone. Bismuthinite (Bi2S3 ) and marcasite (FeS2 ) have been identified by Charles Milton, of the United States Geological Survey, in 

placer concentrate from Colorado Creek near the Silver King and 

were presumably derived from the lodes of that prospect or others in 

the immediate vicinity. Under present conditions the economic 

value of the disseminated mineral deposits rests solely on their content of gold and silver. The assays quoted in the descriptions of the 

deposits show that locally notable amounts of these metals exist.

DEPOSITS ALONG BEDDING

, The type of lode characterized by replacement along the bedding 

is exemplified by the Copper King and, to a somewhat less extent, by 

the Riverside and Liberty prospects. Limestone, probably originally 

impure and siliceous, has suffered replacement by fine-grained silica
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 311

and the development of silicates, particularly sericite, chlorite, epidote, and pyroxene. .Locally, portions of certain beds have been 

more or less completely replaced by sulphides. The stratification 

has tended to guide replacement even down to microscopic details. 

At the Copper King, so far as can be judged from the scanty development, this is the only structural factor that has influenced the 

mineralization. Along the strike, however, replacement has terminated within short distances and without apparent cause. At the 

Riverside prospect shearing inclined at small angles with the bedding 

has locally controlled deposition somewhat, and at the Liberty pros-, 

pect shearing planes nearly at right angles to the bedding show about 

as much mineralization as those along the bedding.

.The Copper King prospect is unique in that almost the only sulphides present are pyrrhotite and chalcopyrite, and it is the only 

deposit in the area held primarily for its copper content. Here, as

elsewhere in the area, the chalcopyrite consolidated somewhat later 

than the other sulphides, but the relations are so intimate that the. 

interval cannot have been long. The more thoroughly mineralized 

material exposed at this prospect contains approximately 31 percent 

of chalcopyrite, 20 percent of pyrrhotite, and 49 percent of gangue, 

as judged from microscopic measurements.

The other lodes in which stratification played a major part in 

guiding deposition contain mineral assemblages similar to those in 

most of the lodes of the disseminated and lenticular types and are 

clearly more closely allied to them than the Copper King. Arseno-i 

pyrite is relatively plentiful, pyrite and pyrrhotite are common, and 

chalcopyrite, sphalerite, and galena are so rare as to be easily overlooked, although close examination discloses their presence in most 

exposures. Mineralization in the gold lodes persisted farther along 

the strike than at the Copper King, but the sulphides are more 

irregularly distributed. At the Copper King, wherever mineralization has been intense enough to be of any economic interest, a bed 

is completely replaced throughout its width, and except for faintly 

perceptible banding the lode matter has a uniform texture and 

averages about half sulphide. At the other prospects the sulphides 

are much more irregularly distributed in scattered grains and in 

small irregular aggregates that are only locally abundant. It is the 

precious-metal content alone that makes these lodes of possible value, 

although further development may disclose shoots containing enough 

arsenic and perhaps other metals to be of some value as byproducts.

TABULAR OK LENTICULAR LODES

The Ready Cash, Little Lead, Mayflower, Lucrata, and Eagle prospects are on lodes that tend to have a tabular or lenticular form and 

to contain relatively more abundant coarse-grained vein quartz and
312 INVESTIGATIONS IN ALASKA EAILEOAD BELT, 1931

hence are more veinlike than the disseminated deposits or the deposits along bedding. The lodes occur, however, in shear and fracture zones that are irregular in detail and only exceptionally and 

locally possess definite walls. Altered country rock, more or less 

thoroughly impregnated with quartz, carbonate, and sulphides, constitutes a large part of the lode matter. Kock alteration within and 

bordering these lodes has been essentially similar to that characteristic of the disseminated mineral deposits.

In other respects also there is an obviously close relation between 

the tabular or lenticular lodes and the disseminated mineral deposits. 

For example, the so-called Little Lead, on Golden Zone ground, and 

the Mayflower, just north of the Golden Zone end line, are tabular 

lodes, yet both in position and in characteristics they have affinities 

to the mineralized shear zones within the Golden Zone disseminated 

ore body. The tabular lodes tend to be relatively persistent along 

the strike. The Little Lead is the only one that appears to have been 

shown by development to have an uninterrupted length of as much 

as 1,000 feet, and even here there are gaps in the record. It seems 

probable, however, both from the character of the mineralization and 

from the evidence afforded by existing development that some of 

these lodes are persistent both laterally and in depth. Some of 

them, such as the Ready Cash, have been much disturbed by postmineral faulting. In some, such as the Eagle, irregularity in original deposition and later faulting have combined to produce discontinuity and irregularity in trend. In others, such as the Lucrata, 

original deposition was confined to lenses of small dimensions.

The individual masses of vein quartz are nowhere persistent 

throughout the extent of the lodes. They tend to be lenticular and 

to be arranged en echelon, They are rarely more than a few score 

feet long and a few feet wide, and most of the lenses are very much 

smaller. In all the quartz lenses the texture is massive, with little 

suggestion of banding or comb structure, and small open vugs are 

rare. In a few places vein quartz cements breccias composed of 

angular fragments of the country rock.

In the massive quartz lenses much of the quartz consolidated later 

than the sulphides. Some calcite fills crevices in the quartz. In the 

Little Lead there are small irregular chert veinlets, probably of considerably later origin than the other minerals of the lode. Similar 

veinlets are sparsely distributed in the altered porphyry traversed 

by the Golden Zone tunnel nearby.

GENERAL CHARACTERISTICS OF THE SWLPHIDES

Sulphides are irregularly distributed and rarely compose as much 

as 50 percent of the lode matter. Arsenopyrite is generally the most 

abundant sulphide and appears everywhere to have been the first to
MINERAL DEPOSITS NEAR WEST FORK OF CHTJLITNA RIVER 313

form. Pyrite consolidated next and was somewhat shattered before 

being followed by pyrrhotite. Sphalerite, galena, and part of the 

chalcopyrite are intimately associated with one another and are commonly interstitial between and formed later than the more abundant 

grains of the arsenopyrite, pyrite,,and pyrrhotite. Some of the chalcopyrite occurs in veinlets traversing the other sulphides and hence 

must have been of comparatively late origin. This same order of 

deposition occurs in all varieties of lodes in the area.

Nearly all the bands and irregular masses of comparatively solid 

sulphide consist dominantly of coarse, bladed arsenopyrite. As 

some assays show more than 20 percent of arsenic, this metal is a 

possible byproduct of the development of the lodes should conditions of marketing and metallurgy become sufficiently favorable. 

Qualitative chemical tests by Charles Milton and the writer show 

that much of the arsenopyrite has relatively more arsenic as compared to sulphur than typical arsenopyrite and hence approaches 

lollingite (FeAs2 ) in composition. It is possible that a mixture 

of these minerals exists, but microscopic study of the available material gives no evidence of it. It seems probable that there is isomorphous gradation and that part or all of the arsenopyrite of the 

lodes in this area is intermediate in composition between pure arsenopyrite and lollingite.

Galena and sphalerite are rarely abundant but are present in 

almost all the lodes in which any considerable quantity of sulphides is exposed. So far as present development shows, they are 

nowhere sufficiently abundant to constitute lead or zinc ore of commercial value.

The Ready Cash claim contains a larger proportion of these minerals than any of the other prospects, and is reported to be valued 

mainly for its lead and silver content, but the assays listed in the 

description of that prospect are not encouraging.

Chalcopyrite is as widely distributed as sphalerite and galena and 

perhaps a little more abundant, but it is not sufficiently abundant in 

the more tabular lodes, with the possible exception of the lode on 

the Mayflower claim, to offer much hope that copper ore of commercial value will be found.

Argentite has been recognized only as a film coating cleavage 

cracks in galena from the Eeady Cash claim, and there it was visible 

only in etched specimens under the microscope. It is doubtless 

present in galena elsewhere and may also occur in other sulphides. 

Assays commonly shoAV a disproportionately high content of silver 

with reference to the gold, indicating that argentite or some other 

silver mineral is present. As telluride is reported by Henry Stevens 

to have been found in placer concentrates from this area it is possible that some of the silver is present in this form. No telluride

179268 33 3
314 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

was, however, detected in any of the material examined during the 

present investigation.

Assays show that the amount of gold is not proportional to the 

abundance of sulphide in the lode matter, indicating that-part of 

the gold, at least, is free rather than in solid solution in the sulphides. 

Some samples containing abundant sulphide assay lower in gold than 

others in which neither sulphides nor their oxidation products are 

plentiful. The absence of visible gold in the polished sections examined suggests that the sulphides did not act as precipitants for 

gold in solutions of later origin. The lack of recognition of gold 

is by no means conclusive as to its absence, because very finely divided 

gold might well escape detection. Placer prospecting in Bryn Mawr 

Creek below the Golden Zone lode prospect showed that some free 

gold was present, but it was in such small particles that losses are 

reported to have been inordinately high. Placer concentrate from 

Colorado Creek contained a few flakes of rather coarse gold when 

examined by the writer. The apparent scarcity of conditions favorable to the formation of supergene gold leads to the inference that 

much of such native gold as exists is hypogene that is, formed 

by original deposition rather than through the agency of oxidation

and weathering.

OXIDATION

Oxidation is relatively shallow, being far from complete at a 

depth of 5 feet in any of the lodes in which development has penetrated to that depth. Exposures in ciiff faces generally show sulphides with only superficial oxidation. The principal oxidation 

products are limonite (hydrated ferric oxide) and scorodite 

(Fe2 03 .As205.4H2 0), Small amounts of malachite (Cu2 (OH) 2 .-

CO3 ),azurite (Cu3 (OH),(CO3 ) 2 ), chrysocolla (CuSiO3 .2H2O), and 

anglesite (PbS04 ) are present wherever the corresponding sulphides 

occur. Locally a little hematite and manganese oxide exist, but 

both are rare in all the deposits visited. In some of the shear zones 

clay of presumable supergene origin is abundant.

Microscopic study shows that films of chalcocite (Cu2S) and 

covellite (CuS) coat some of the sulphide grains. This is proof 

of enrichment, but the quantity is too meager to be of economic 

significance.

ORIGIN

Evidently all the metallic lodes in the area are closely related in 

origin. Some of them are so closely associated with the larger 

intrusions of biotite-quartz diorite porphyry as to indicate genetic 

connection between mineralization and the intrusion of this rock. 

The texture of the porphyry and the small size of the exposed masses
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 315

suggest consolidation at moderate to fairly shallow depth, possibly 

of the order of thousands of feet rather than of miles below the then 

existing surface. The mineral composition and the structural characteristics of the lodes are such as are generally considered to indicate deposition under moderate temperature and pressure. Evidence 

of mineralization is wide-spread over an area about 2 miles wide 

and 6 miles long, from Costello Creek to Long Creek, and appears 

again along Ohio Creek, nearly 7 miles farther southwest. In the 

area between Costello Creek and Long Creek particularly, the mineralizing solutions permeated large areas, and deposition took place 

in numerous spots. Both this fact and the characteristics of the 

lodes indicate that the solutions were tenuous. Crystallization of 

the different sulphides followed a definite order, which is essentially the same in all the lodes. Although this is evidence that the

mineralization, as usual, occupied a considerable period of time, 

there is little to suggest distinct or widely separated periods of 

deposition.

ECONOMIC OUTLOOK

Insofar as the character of the mineralization indicates that the 

parts of the lodes now exposed are neither the uppermost tips nor 

the eroded roots of a lode system, it argues for persistence in depth 

and uniformity in tenor of individual lodes. The wide-spread extent of the mineralization, particularly the dissemination of sulphides at a distance from the lodes, is unfavorable in that it suggests 

dilution through permeation of the country rocks rather than concentration in well-defined lodes. Though some of the metallic 

constituents of the original solutions have doubtless been scattered 

so thinly that they cannot be profitably recovered, the evidence does 

not preclude the possibility that valuable ore shoots are locally 

present. In many places exposures are poor, and near the larger 

streams the outcrops are concealed by brush. Many of the lodes 

are inconspicuous, as they have neither enough quartz to resist 

erosion greatly nor enough sulphide to stain the rocks deeply through 

oxidation. Consequently, in spite of the systematic surficial prospecting that part of the area has received, future discoveries are 

possible.

The following generalizations as to probable tenor of the ore in 

the different kinds of lodes are offered, though with some hesitation 

because of insufficient data. In the disseminated deposits some of 

the more thoroughly mineralized shear zones contain over $10 a ton 

in precious metals, and locally their content may be more than double 

this amount. It seems probable that some of the relatively unsheared rock may average as much as $5 a ton, but much of this
316 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

material is of extremely low grade. Some of the shear zones contain 

over 20 percent of arsenic.

The precious-metal lodes that follow stratification contain precious 

metals and arsenic in quantities roughly comparable to those of the 

shear zones in the disseminated deposits. Presumably the material that averages as much as $10 a ton in gold and silver will be 

found in shoots, with lower-grade material in the adjoining beds and 

in the intervening parts of the same beds. Present development 

suggests that in the copper replacement lodes the lode matter will 

prove to be either ore containing 5 to 10 percent of copper and about 

$5 to the ton in gold and silver or of a grade too low to be of 

economic value.

In the more tabular and lenticular lodes the tenor is variable. 

Locally it may be as much as $20 a ton in gold and silver, or perhaps 

even more, but much of the lode matter is of materially lower grade. 

Presumably much of the more thoroughly mineralized material in 

these lodes averages $5 a ton in precious metals. Much of that in 

which sulphides are abundant contains 25 percent of arsenic. The 

data obtained from the Ready 'Cash prospect indicate that lodes containing comparatively conspicuous quartz, galena, and sphalerite are 

relatively low in precious metals. With the possible exception of 

arsenic, none of the tabular lodes have been proved to contain 

enough base metals to be of much economic interest, although some 

may locally contain as much as 5 percent of copper.

Development has nowhere been sufficient to block out any ore, 

and therefore any estimate of possible tonnage can be nothing more 

than a first approximation. There are probably 10,000,000 tons of 

mineralized rock in the disseminated deposits that have been prospected, but what proportion of this rock can be mined profitably 

under a given set of conditions remains to be determined.

Present development would permit crediting the precious-metal 

replacement deposits in stratified rock with only a few thousand tons 

of possible ore, but it is likely that much material similar to that 

exposed exists at depth. In the single prospect on a copper replacement lode the exposed ore is of small extent, and the size of possible 

ore shoots at depth is problematical. Oxidized copper ore commonly 

has such striking green, blue, and red colors that outcrops of such 

material are unlikely to have escaped the notice of prospectors.

The total quartz and mineralized rock of the tabular lodes doubtless aggregates tens and possibly hundreds of thousands of tons. 

There is at present no way of determining how much of this material is of relatively high grade or what proportion of the lowergrade ore can be profitably handled.

The metallurgical treatment of ore of the character of most of that 

in this area presents problems requiring careful consideration. It is
probable that much of the gold is in the metallic state but is very 

finely divided. The silver is in part alloyed with the gold and in part 

chemically combined in argentite and similar minerals. Probably 

only a small proportion of either metal is readily recoverable by 

amalgamation. Presumably some combination of fine grinding, flotation, or possibly gravity concentration and cyanidation will prove 

applicable, but such a process would require an elaborate plant.

In other respects conditions affecting mining are good. Most of 

the rock stands well with little or no timbering and is only moderately resistant to blasting. There is little available timber close at 

hand, but considerable amounts suitable for rough construction and 

for a rather poor grade of mine timber can probably be found within 

about 20 miles. Coal can be obtained locally or can be shipped from 

mines in operation within reasonable distances. Once camps are 

built and roads constructed the climate will not seriously interfere

with underground operation, though special provision will need to 

be made for milling in winter if required. Mill sites will have to be 

selected with a view to having a readily available and adequate supply of sufficiently clear water, and at some places, therefore, the mills 

may have to be situated at some distance from the mines. The problem of obtaining sufficient water in winter will require some consideration. The distance from the railroad to most of the prospects is 

little more than 10 miles. Roads can be built without special difficulty and will not have excessive grades. The railroad haul from 

Colorado station to Seward, the ocean terminus, is only 297 miles.

PROSPECTS

The existing prospects are here described in geographic order, 

starting at the southwest end of the area and proceeding northeastward. It is reported that at one time a strip of country seven claims 

wide from Long Creek past the coal prospect near Costello Creek 

was covered with lode claims. Most of the corresponding " discovery " pits were shallow, and in their present slumped condition they 

reveal little. Only prospects that have received more development 

are here considered. Along Ohio Creek there are traces of numerous 

camps, many of which were set up by prospectors about 1915. A few 

of their pits were found, but the Ready Cash is the only prospect 

in this vicinity that has received enough development to warrant 

description in this report.

All the samples collected by the writer were assayed by Paul Hopkins, of the United States Bureau of Mines, Fairbanks, Alaska, 

whose cooperation is gratefully acknowledged. The assay content of 

a number of samples from other sources is listed for comparison. 

The well-known difficulties of sampling gold deposits make it essen-
318 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

tial to consider the weighted mean of many samples to arrive at an 

approximation of the average content of the ore in this area. Even 

samples from the same localities yield divergent results, emphasizing 

the difficulty of getting representative samples of gold ore. Consequently, the few assays available are regarded as merely indicative 

of the character of the ore in the present meager exposures and do 

not constitute an adequate basis for calculation of the value of the 

ore in the ground.

READY CASH

The Keady Cash prospect is on Ohio Creek about 8 miles above 

its junction with Copeland Creek and 2^ miles below the glacier at 

the head of Ohio Creek. The prospect is about 12 miles from Honolulu, the nearest point on the Alaska Railroad, but the trail is now 

in disrepair. The history of the prospect, as gleaned mainly from 

notices on the property and from the patent documents, is as follows: 

The ground was located in May 1915 by Otto Tangel, J. P. Frisly, 

William Murry, and E. Miller; later ownership was acquired by J. H. 

McCallie, who patented the property in 1927; there are nine claims 

covering an area of 132.627 acres, developed by two tunnels and nine 

discovery pits. It appears from the description by Capps 1S that all 

this work had been done prior to his visit in July 1917. The two 

tunnels and their relations to the veins found during the present 

study are shown in plate 26. The rest of the patented ground, was not 

studied in detail, but other mineralized outcrops exist.

The bedrock in the vicinity of the tunnels is uniformly a coarse, 

somewhat chloritic rock without discernible bedding, which microscopic examination shows to be altered andesite or related rock. As 

shown in plate 26, three segments of quartz veins exist. The segment on the west side of Canyon Creek is 3 feet wide, strikes about 

N. 58° W., and stands vertical. There are several minor quartz 

stringers and lenses of oxidized sulphide south of this vein. The 

main vein here contains arsenopyrite, galena, and other sulphides, 

rather sparsely disseminated. A sample from it yielded on assay 0.01 

ounce of gold and 4.20 ounces of silver to the ton and 0.52 percent of 

lead. Obviously, such low assay returns do not indicate ore. Some of 

the rusty lenses south of it were probably high in sulphide before 

oxidation, but none of these are more than a few inches wide and 

apparently none much more than 2 feet long.

Another vein segment is exposed for about 30 feet on the east side 

of Canyon Creek about 80 feet above the tunnel portal. It strikes 

about N. 25° E. and stands vertical. Its walls are curved so that their 

strike ranges from N. 40° E. to N. 20° E. At the north end of the exM.Capps, S. R., op. cit. (Bull. 692), pp. 228-229.
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 319

posure the vein is about 10 feet wide. A sample from this place contained a trace of gold and 0.70 ounce of silver to the ton. Its lead 

content was not determined but is obviously small. About 25 feet 

to the south the vein has a width of only 4 feet. On the east wall 

there are slickensides which dip 55° N. Sulphides are less abundant 

in this vein segment than in that to the west and occur mainly in 

small streaks and bunches. They include arsenopyrite, pyrite, and 

some chalcopyrite. Most of the quartz has been fractured and locally 

is stained and impregnated with the oxidation products of iron, copper, and manganese. The tunnel below trends N. 83° E. for 169 feet 

and hence extends well beyond 'the line of the vein as exposed 80 feet 

above it. As figure 40 shows, ho\vever, it exposes only 'broken ground 

and minor stringers of quartz and calcite with small amounts of sulphides, mainly pyrite.

The third vein segment is exposed on the hillside above and east of 

the segment just described. A tunnel in the face of a steep bluff has

FIGURE 40. Sketch map of the lower Ready Cash tunnel. From compass survey, August

22, 1931, by C. P. Ross.

been driven 60 feet to this vein, and a drift tunnel off from it to the 

southeast follows the vein for a distance of 16 feet. The vein as exposed in this tunnel is 5 feet wide, strikes N. 20° W., and dips about 

60° NE. The face of the lower tunnel is within 120 feet of the point 

where this vein would be if projected northwestward with the same 

strike and dip. A sample across the southeast face of the drift 

yielded on assay a trace of gold and 1.80 ounces of silver to the ton, 

but no lead. This vein contains numerous stringers and bunches of 

sulphides, some as much as 10 inches wide. They include arsenopyrite, pyrrhotite, pyrite, galena, chalcopyrite, and sphalerite, listed 

roughly in the order of decreasing abundance. Most of the bands 

of nearly pure sulphide consist mainly of arsenopyrite. Some of the 

pyrite is in imperfect cubes as much as an inch wide. There are 

small amounts of supergene covellite. The gangue is white, rather 

coarsely crystalline quartz, somewhat fractured.
320 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

The three vein segments described above may be fragments of a 

single lode, displaced by faulting. The shearing and brecciation of 

northerly trend revealed in the lower tunnel were probably related 

to the faulting. This theory, if correct, explains the marked differences in attitude of the three segments and the failure of the vein 

segment on the east side of Canj^on Creek to be exposed in the lower 

tunnel. It implies that extension of the lower tunnel on its present 

course would probably fail to reach the vein disclosed in the upper 

tunnel. In order to find an ore body of commercial value it would 

doubtless be necessary to seek a less shattered portion of the lode.

COPPER KING

The Copper King prospect, formerly the Hector, comprises four 

unpatented claims in a gulch on the north side of Long Creek. They

'Dirt, to be 

deepened

Barren bed// /) \3 

rockatthis//lft. % 

end <3/ \ 

Ift ^/Brown barren -ft 

rock

Sample

sulphide 

at one spot in bottom

7ft.

20 0 80 Feet

FIGURE 41. Sketch map of the Copper King prospect. From pace and compass survey, 

August 27, 1931, by C. P. Ross. Figures refer to depth of trenches.

are about a mile south of the Golden Zone and are generally approached from the Golden Zone. Development is confined to the 

trenches shown in figure 41, several of which were dug in August 1931. 

The mineralized rock comprises lenses and seams of massive pyrrhotite and chalcopyrite in silicified limestone containing chlorite and 

probably other silicates. The larger masses parallel the bedding of 

the cherty rocks and were formed by replacement of them. The 

strike of the beds in this vicinity ranges from N. 20° E. to N. 55° E.,
U.S. GEOLOGICAL SURVEY BULLETIN 849 PLATE 26

Strike of 

vertical lode

Contour interval 25feet;datum 

is approximately 2,475 feet 

above sea level

100 i i i i i i i

200 Feet

SKETCH MAP OF THE READY CASH PROSPECT, OHIO CREEK. 

Surveyed August 22, 1931, by C. P. Rosa and Marion Peel.
U.S. GEOLOGICAL SURVEY BULLETIN 849 PLATE 27

Trench and open cut, 

hachures show dump

3Ooo 4ooo Feet

Contour interval 25 feet

Datum is approx/matefy

3,540feet above sealeveL

SKETCH MAP OF THE GOLDEN ZONE AND MAYFLOWER PROSPECTS.

Surveyed July 28 and August 7, 1931, by C. P. Roes and Marion Peel.
MINERAL DEPOSITS NEAE WEST FORK OF CHULITNA RIVER 321

-with the average closer to the latter. The main trench shows mineralization at intervals throughout its length of about TO feet but 

exposes massive sulphide only over a width of 2 or 3 feet about 15 

feet from its northwest end. A sample of freshly blasted material 

from this place yielded on assay 0.22 ounce of gold and 4.90 ounces 

of silver to the ton and 7.34 percent of copper. Trenches about 45 

feet to the southwest on the line of strike show little mineralization. 

'The continuation of the same beds in bluffs about 1,000 feet to the 

northeast contains no visible copper minerals. A very little copper 

stain and a little pyrite are present in a limestone bed on the approximate line of strike in the gorge of Long Creek about 2,000 feet 

to the southwest. There are several scattered pits in the vicinity, 

outside the area shown in figure 41, but copper was not observed in 

.any of them. 

The assay indicates that the massive sulphide is sufficiently rich to

be mined if found in large enough bodies. The present scanty development has disclosed only small masses, and the paucity of indications of copper in neighboring exposures, both artificial and natural, 

is evidence against the presence of a large body of ore close to the 

surface. Replacement ore bodies of this general type are erratic in 

distribution, size, and shape. It is possible that deeper development 

Avould disclose an ore body of commercial size.

LINDFOKS

The Lindfors prospect lies at the head of Bryn Mawr Creek adjoining the Golden Zone claims. No work had been done here for 

some time, but the prospect was restaked in the summer of 1931. 

The developments comprise a number of trenches and shallow ditches 

with which a very small amount of ground sluicing was done. Most 

of the trenches do not expose bedrock. They are in frost-shattered 

material, largely rusty dioritic porphyry but including representatives 

of the clastic rocks of the vicinity. Capps 14 notes that at the time 

of his visit in 1917 one open cut exposed a vein of massive arsenopyrite 

4 to 20 inches thick on the contact of a decomposed dike and altered 

tuff. Another cut showed disseminated sulphides and veinlets containing sulphides, quartz, and a carbonate resembling ankerite, in 

calcareous strata. He reported that arsenopyrite, pyrite, chalcopyrite, and sphalerite were present there and that gold is reported in 

encouraging amounts.

GOLDEN ZONE

The Golden Zone prospect comprises four claims held by A. 0. 

Wells, at the head of Bryn Mawr Creek, a tributary of the West

"Capps, S. R., op. cit. (Bull. 692), p. 226.
322 INVESTIGATIONS IN ALASKA RAILKOAD BELT, 1931

Fork of the Chulitna River. This prospect is reported to have been 

first staked in 1909. A little placer gold has been produced from 

the gravel of the creek immediately below the prospect, but the 

returns were so meager that the attempt to work the gravel was soon 

abandoned.

The workings consist of one tunnel with a short, partly caved drift 

leading off from it (see fig. 42) and several pits and trenches (see 

pi. 4). A small amount of ground sluicing has been attempted. 

When examined during the present study very little development had 

been accomplished since the prospect was visited by Capps 15 in 1917..

7-inch mineralized slip20 30 40 Feet VerticaldipFIGUKB 42. Tunnel on the Golden Zone prospect. From compass survey, July 28, 1931,.

by C. P. Ross.

In August 1931 work was started with the intention of extending thetunnel an additional 125 feet.

The results of sampling by Harry Townsend (date unknown),, 

J. G. Shepard (July 1925), and the writer (July 1931), all membersof the United States Geological Survey at the time they made their 

examinations, are given in the table below. Townsend's samples aredesignated in the table and on the sketch map (pi. 27) by capitalletters, those of Shepard by small letters, and those of the writer bynumbers.

16 Capps, S. R., op. cit. (Bull. 692), pp. 226-227.
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 323 

Assay of samples from Golden Zone prospect

No. of 

sample

A..... ......

B..~- ...

0.... .

D...........

E...._....._

F...........

a...........

H...._ ..

I............

J.. ...... ...

K...........

L...........

M.... ......

N.. ........

0...........

P..... ......

Q...........

R...._ ...

Width 

sampled 

(feet)

5

5

5

5

10

10

10

10

10

10

10

10

6

5

5

10

10

Dump.

Gold 

(ounces 

to the 

ton)

1 44

1.20

.28

.14

.04

.12

.22

.08

Trace.

.08

.38

.20

.16

.32

.04

Trace.

' 2.58

Silver 

(ounces 

to the 

ton)

2.00

2.80

1.20

.70

.10

.10

8.10

.80

.40

.20

.80

 1.30

.70

2.39

2.39

.40

.30

4.70

Arsenic 

(percent)

26.53

16.69

12.08

9.56

2.01

1.12

4.37

1.05

1.00

.47

4.41

7.34

8.11

2.00

2.00

1.23

1.00

33.22

No. of 

sample

S... ........

b. .-..-.-

d .....

f...... - ....

g..... .. ..

2... ....... ..

3...... ...

4.. ..........

5.. ..... ..

6.. . 

7..... ......

8... ......

9. ........

Width 

sampled 

(feet)

1

7

100

35

6

6

60( )

3

23

.6

.75

3

.6

4

7

Oold 

(ounces 

to the 

ton)

0 4S

2 K9

.24

.10

.10

.16

.08

.86

.59

.06

.37

.03

fU

.16

.99

.36

.03

Silver 

(ounces 

to the 

ton)

2.40

5.10

1.20

3.00

.50

.80

.30

11.60

1.40

.80

1.10

.30

.30

2.40

6.60

2.40

2.70

Arsenic 

(percent)

23.61

22.91

18.10

o Orab sample from Little Lead cuts.

The Golden Zone lies within a small stock of biotite-quartz diorite 

porphyry intruded into argillite and breccias. Almost the entire 

stock contains disseminated pyrite. Some portions are otherwise 

nearly unaltered, so far as can be judged in the hand specimen, but 

the major part of the mass has been colored a rusty red from the 

oxidation of iron sulphides, and this portion has been partly silicified 

and its origin'al igneous minerals have been bleached and altered. In 

several places this altered rock has been fractured and closely sheeted. 

Mineralization has been more intense along these zones. One of 

them, referred to as the main zone, lies on the west side of Rusty 

Hill, which is the most prominent surface feature of the property. 

This zone is 9 feet wide where cut by one of the trenches (samples 

A, B, a, 2). It is exposed in the next trench up the hill but has 

not been reached by any of the others. Natural exposures in the 

small creek on the northwest show no indication of its continuation along the line of strike. In the two trenches it strikes about 

N. 10° W. and dips about 60° W.

The other trenches on this hillside, represented by samples C to J, b, 

and 1, 2, and 3, also uncover mineralized and locally sheared rock. 

Shearing and abundant sulphides do not necessarily indicate high 

precious-metal content, as sample 2 shows. It is noteworthy that 

sample G yielded 8.10 ounces of silver to the ton and comparatively 

small amounts of both gold and arsenic. The location of samples K, 

L, and M could be only approximately identified, and may, therefore, 

not be correctly shown on plate 27. -
324 INVESTIGATIONS IN ALASKA RAILEOAD BELT, 1931

Near the portal of the tunnel (see fig. 42) there is a nearly vertical 

zone of shearing, which strikes about N. 66° E. and stands vertical. 

Samples N, O, and c indicate the average content of the rock in 

this zone, and sample 4 shows that a 7-inch streak selected because of 

apparently more abundant sulphide was of materially lower grade 

than that of the zone as a whole.

The drift leading off from the tunnel follows a pronounced fissure 

with abundant gouge which locally contains abundant fine-grained 

sulphides. It strikes N. 67° E. and dips 65°-90° NW. Samples P, 

Q, d, and e show the content of the rock in the neighborhood of the 

fissure, and sample 5, taken from a 9-inch wide mass of gouge with 

sulphide, shows once more that material of promising appearance is 

not necessarily of high grade. This selected sample proved to be of 

slightly lower grade than sample f, which was taken over a width 

of 60 feet in the inner portion of the main tunnel, where there is 

no evidence of intense mineralization, although the rock throughout 

has been somewhat altered and mineralized and contains minor mineralized seams, mostly striking N. 40°-65° E. Some of these are 

lined with gouge and others with quartz. As plate 27 shows, the 

tunnel had not yet penetrated far enough westward to get below 

the more favorable showings on the hill above it.

A line of pits on the east side of Bryn Mawr Creek opposite the 

tunnel entrance (see pi. 27) traces a shear zone, locally termed the 

Little Lead, over a distance of about 300 feet. Other cuts farther 

down and on the west side of the creek disclose ore which appears 

to be the continuation of this lead. If so, the Little Lead has a 

length of about 1,000 feet. In the closely spaced pits near the tunnel 

the shear zone strikes N. 30°-45° E., dips 50°-60° SE., and is about 

3 feet wide. There are no definite limiting walls, and further development might show a greater width. Samples R, S, g, 6, 7, 8, 

and 9 show the tenor of the ore in this lode. The cuts from which 

samples R and S were taken have not been positively identified. 

Sample 6 was taken across a width of 3 feet of intensely sheared 

and mineralized lode matter in the freshest of the cuts, and sample 

7 came from a 7-inch band of sulphide within this zone. Unlike 

other samples containing sulphide, sample 7 proved to be comparatively rich.

The unoxidized porphyry in the tunnel, away from the shear zones, 

is partly silicified and cut by numerous quartz stringers. Pyrite 

occurs both in the silicified rock and in the stringers. In the stringers 

it is generally in cubical crystals, locally as much as half an inch in 

diameter. Small amounts of chalcopyrite and arsenopyrite are also 

present. In the shear zones, especially that followed by the drift, 

there is considerable gouge, and this contains disseminated sulphides, 

mainly pyrite.
MINERAL DEPOSITS NEAR WEST FORK OP CHULITNA RIVER 325

In all other exposures the character of the original mineralization 

is obscured by oxidation, which, however, has in few places extended 

to a depth of more than 5 feet. Evidently in the more intensely 

mineralized shear zones there are numerous veinlets of quartz with 

some pyrite and other sulphides and somewhat irregular streaks of 

nearly massive sulphide, largely arsenopyrite, rarely more than 6 

inches wide, in a gangue of quartz, with locally some carbonate. In 

places in the Little Lead there are veinlets and irregular bodies of 

brown chert, locally enclosing lenses of sulphide. Capps 10 reports 

that the metallic minerals which have been recognized on this property include arsenopyrite, pyrite, sphalerite, chalcopyrite, galena, 

malachite, and probably stibnite.

MAYFLOWER

The Mayflower prospect comprises two claims adjoining the Golden 

Zone on the northeast, held by Henry Stevens. It is developed only 

by a few pits, shown in plate 27. The rock here is brecciated, ironstained, and soaked with quartz, so that identification is difficult, but 

it appears to be, at least in part, a continuation of the fine-grained 

intrusive mass of the Golden Zone. The lode can be traced with failaccuracy by means of the pits for a length of 350 feet. It trends 

N. 30°-35° E. and dips 70° SE. One of the pits shows mineralization off the line of this lode. The most northerly pit is the largest 

and the only one in which bedrock was satisfactorily exposed at the 

time of visit. Here the well-mineralized part of the zone of shearing 

and brecciation is 32 inches wide. A sample across it yielded on 

assay 0.07 ounce of gold and 2.60 ounces of silver to the ton. Some 

specimens from this pit contain considerable chalcopyrite, but the 

average copper content of the lode as now exposed is low.

Samples taken by Henry Stevens at the Mayflower prospect and 

assayed by Paul Hopkins September 15, 1929, yielded the results 

listed below:

Assay of samples from Mayflower prospect

5076. ..

5077 _ 

5078- ..

5080......

Bureau of Mines sample numbers

Gold 

(ounces to 

the ton)

0. J2

.34

.26

.06

Silver 

(ounces to 

the ton)

3.00

5.80

1.20

15.80

Copper 

(percent)

5.3

The mineralization disclosed on this property was a continuation 

of that on the Gold Zone claims, but it is improbable that the main 

lode here is to be correlated exactly with any of those on the Golden

18 Capps, S. R., op. cit. (Bull. 692), p. 227.
326 INVESTIGATIONS IN ALASKA EAILEOAD BELT, 1931

Zone ground. The available assays indicate that the ore is of low 

grade. The exposures in the principal pits show that brecciation 

and mineralization were so intense as to encourage the hope that 

further development may yield satisfactory ore.

RIVERSIDE

The Riverside prospect comprises five claims held by A. O. Wells, 

on the southwest bank of the West Fork of the Chulitna River. 

There are numerous cuts and pits on the property. Several tunnels 

have been started, but none were carried far. The principal cuts 

now open are shown in figure 43. They disclose two nearly parallel 

lodes about 80 feet apart, both of which are replacement deposits that 

approximately follow the bedding of limestone lenses. The eastern 

lode strikes N. 15° W. and dips about 70° NE. Ore deposition has

Sample 2

Bank of West Fork 

of Chulitna River

Sample

cur Bedd ing N. 1 5 W,70 E. m Samp/e I 

Sheeting N.25°W,55°SW

Dimensions refer to 

width of /ode

2D Feet

FIGURE 43. Sketch map of the Kiverside workings. From compass survey, July 23,

1931, by C. P. Ross.

in part been controlled by a poorly defined sheeting that trends 

N. 25° W. The maximum observed width of mineralization is 5y2 

feet, but most of the ore is contained within a width of 3 feet. This 

lode has been traced for about 60 feet along the strike. The second 

lode is similar, but its average strike is north and it stands nearly 

vertical. It has been traced for fully 80 feet on the strike, and the 

ore averages about 3 feet in width. Small cuts and outcrops in the 

general vicinity show that there are several other lenses of mineralized rock. One of these accords with the bedding of banded limestone which stands vertical and trends N. 6° E., suggesting faulting 

or other structural disturbance.

The lode matter comprises irregular bunches and lenses of vein 

quartz with sulphides, in partly silicified and chloritized limestone. 

The principal sulphide is arsenopyrite, but pyrrhotite, pyrite, and
MINERAL DEPOSITS NEAR WEST FORK OP CHULITSTA RIVEB 327

<;halcopyrite are also abundant, and sphalerite and galena are locally 

present. There is a very small amount of supergene chalcocite. 

Within a few inches of the surface the lode matter is thoroughly

 oxidized and in part clayey.

Information in the files of the United States Geological Survey

 office at Anchorage gives the results of assays of samples from the 

Riverside. The exact source of these samples is not known to the 

writer.

Assays of samples from Riverside prospect 

[Assays made by Paul Hopkins, of the U.S. Bureau of Mines, Sept. 16,1926]

No.

1

2

3

Source

East cut...... .......... ........ ...... ......

West cut.. _ . _______ . ___ ... __ .-. __ - __ ......... .

.....do-...... .-. .. . - -- - 

Gold

(ounces

to the ton)

0.02

.52

.64

Stiver

(ounces

to the ton)

o in

.40

.60

Three samples were taken July 23, 1931, by the writer from this 

prospect and assayed by Mr. Hopkins. Sample 1, from a channel 

across a width of 3 feet 6 inches on the north side of the principal

 cut on the eastern lode, contained 0.14 ounce of gold and 0.60 ounce 

of silver to the ton. Sample 2, across 3 feet 4 inches in the face of

 a tunnel one set long on the second lode, contained 0.02 ounce of 

.gold and 0.30 ounce of silver to the ton. Sample 3, across 2 feet 10 

inches in another cut on the second lode, contained 0.72 ounce of 

.gold and 0.40 ounce of silver to the ton.

The small amount of development on this property renders a tonnage estimate impracticable, but it is clear that there are at least 

two lodes 2 to 5 feet wide and persistent for scores of feet. The

 assays quoted range in value from 50 cents to a little less than $15 

a ton, the average being over $7 a ton. Clearly, the ore so far

 exposed is so low in grade that it can be handled profitably only 

under more advantageous conditions than now exist.

SILVER KING

The Silver King or Stibnite prospect is on a hillock on the northeast side of Colorado Creek nearly 2 miles above its mouth. It is 

developed by trenches and pits but has been abandoned for some 

time. The hillock is composed largely of biotite-quartz diorite 

porphyry, which intrudes sedimentary beds, in part cherty, which 

strike N. 70° E., dip 70° NW., and have major joints trending 

N. 25° W. and standing vertical. Much of the rock is rusty, and 

the excavations show streaks and bunches of sulphide, apparently 

Tery irregular and discontinuous. Pyrrhotite and pyrite are the
328 INVESTIGATIONS IN ALASKA RAILBOAD BELT, 1931

most abundant sulphides, with some chalcopyrite and arsenopyriteand in one pit considerable stibnite. The mineralized rock contains 

calcite. Capps 17 noted in one cut a body of massive stibnite 6 to> 

12 inches thick, which strikes east and dips 23° S. This is a larger 

mass than was found by the present writer, some additional digging' 

having been done in the intervening period. Paul Hopkins assayed 

two samples obtained here by Henry Stevens in 1926 and found 

that one consisting largely of stibnite contained traces of gold and 

silver and another contained 0.04 ounce of gold and 0.20 ounce of 

silver to the ton. The irregularity and sparseness of the mineralization, so far as can be judged from the slight development, are 

not encouraging.

LIBERTY

The Liberty prospect comprises two claims held by Henry Stevens 

and Jack Colvin, nearly 0.4 mile east of the Silver King. The principal development is a pit nearly 10 feet deep in altered cherty sedimentary strata which strike N. 30° E. and clip 70° NW., cut by a shear 

zone that strikes N. 50° W. and dips 75° NE. Within the pit the shear 

zone forks, and part of it swings until it is parallel to the bedding. 

Mineralization has occurred along the shear zone in both directions. 

Oxidation is nearly complete, but there are residual bunches of sulphide, mainly arsenopyrite. A sample across a width of 2^ feet in 

the shear zone parallel to the bedding yielded on assay 0.06 ounce 

of gold and 1.20 ounces of silver to the ton; a sample across 2 feet 

of the major shear zone that cuts across the bedding yielded 0.14 

ounce of gold and 8.60 ounces of silver to the ton. A shallow trench, 

now slumped in, followed the shear zone along the bedding a short 

distance north of the pit. These assays and the appearance of the 

shear zone, however, suggest that if more work is to be done it might 

be well to investigate the shear zone with northwesterly trend.

LTJCRATA

The Lucrata prospect, heretofore described as the Lucrative group' 

(Bull. 692), now abandoned, is on the west side of Costello Creek 

a short distance below the mouth of Camp Creek. Its principal development is a cut in the upper part of the wall of the inner gorge 

of Costello Creek, which reveals a zone of mineralization about 15 

feet long, 3 feet wide, and 7 feet in height, containing cherty quartz 

and bunches of partly oxidized sulphide. At its northwest end this 

lens is cut off by an irregular lamprophyre dike about 2 feet wide, 

which strikes N. 30° E. There has been little displacement along the 

dike, as another small lens of mineralized rock is exposed in the cliff

"Capps, S. R.. op. cit. (Bull. 692), p. 225. .
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 329

face beyond the dike on the line of strike of the main lens. Sulphides are relatively abundant, and a sample taken some years ago 

by Harry Townsend, then of the United States Geological Survey, 

yielded on assay 1.26 ounces of gold and 3.80 ounces of silver to the 

ton and 21.25 percent of arsenic, showing that some of the ore lenses' 

here are of comparative^ high grade. The mineralized mass now 

exposed, however, is much too small to be profitably mined.

ss,

/

\ I ' I 

, \ \ u ;

\ \ \ ^; 

x \ V V \X 

\ \ \ \ \ 

x 

Y^

\

\

\

\

EXPLANATION

Outcrop of 

lode

Strike Strike of 

and dip ve'rtical lode

PIGDRB 44. Sketch map of the Eagle prospect. Surveyed August 29, 1931, by C. P. Boss 

and Henry Stevens. Contours nave sketch value only.

EAGLE

The Eagle prospect, formerly called the Northern Light, comprises two claims on the east side of Costello Creek a short distance 

below' the mouth of Camp Creek. As shown on figure 44, it is 

developed by a tunnel 62 feet long and several small cuts. The

179268 33 -4
330 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

country rock is chloritic and cherty argillite and tuff, with some 

lava in the vicinity. The average trend is northeast, but locally the 

beds trend northwest, probably because of faulting. The average 

trend of the mineralized zones is somewhat east of north, but there 

is much local variation, resulting in part from irregular deposition, 

in part from later faulting. The dip is generally steep to the east. 

If the pits are all on the same lode, it has, as shown in figure 44, a 

known length of about 300 feet, but it is doubtful if underground 

development would show continuous ore throughout this distance. 

The width of the mineralized shear zones varies in different exposures, ranging from 1 to 10 feet. The vertical range between the 

tunnel and the highest cut is roughly 185 feet. The shear zones 

generally contain bands of white, coarsely crystalline quartz from 

a few inches to 2% feet in width. Locally, the quartz contains 

numerous vugs. Most of the sulphides were deposited along fissures, 

and the best samples are reported to have been taken along the 

fissures that traverse the bedding at considerable angles. Most of the 

rock in and near the shear zones contains sparsely disseminated sulphides, mainly pyrite. There are a few lenses of massive arsenopyrite with a little pyrite, which are several inches wide. Small 

amounts of somewhat oxidized chalcopyrite are present at some 

places. Two samples were taken in the cut immediately north of 

the tunnel, which is reported to contain the best ore so far exposed. 

One of these, taken across a quartz lens somewhat more than 2 feet 

wide, yielded 0.20 ounce of gold and 6 ounces of silver to the ton. 

The other, taken across the full 4 feet of exposed lode, yielded 0.18 

ounce of gold and 2.10 ounces of silver to the ton. A sample from 

this vicinity and probably from the quartz lens in this pit, assayed 

by Paul Hopkins, contained 0.56 ounce of gold and 3.20 ounces of

silver to the ton. Capps 1S stated that at the time of his visit in 

1917 the area of heaviest mineralization had been traced along the 

surface for a distance of about 800 feet, considerably farther than 

it can now be traced. He noted the presence of arsenopyrite, pyrite, 

chalcopyrite, sphalerite, and a little stibnite.

The results of the scanty development here are fairly encouraging. 

The mineralized zones have been shown to have considerable extent 

and contain ore which, locally at least, ranges in value from $5 to 

$10 a ton. Apparently the zone marked by the pits, rather than that 

followed by the tunnel, is the principal lode, although it is evidently 

interrupted by various irregularities. The pits suggest that approximately 12,000 tons of mineralized rock exists in this lode above the 

level of Costello Creek. Discontinuity resulting from numerous 

minor faults is likely to prove a handicap in development.

18 Capps, S. R., op. cit. (Bull. 692), p. 224.
MINERAL DEPOSITS NEAR WEST FORK OF CHULITNA RIVER 331

COAL

The northern part of the area contains coal that is probably adequate in quantity and quality for local use in development of the 

metal mines, though it would be unable to compete with the larger 

and more favorably situated deposits elsewhere in Alaska.

8TEVENS

Henry Stevens is developing under permit a coal prospect on Coal 

Creek, a minor gulch that enters Camp Creek from the east just 

above the confluence of Camp and Costello Creeks. At the time of 

visit, September 4, 1931, the principal working was a slope a little 

over 20 feet long, whose portal starts in the face of an open cut 

about 10 feet deep and 7 feet wide and follows a coal bed that strikes 

about N. 30° E., dips 10°-12° SB., and is nearly 7 feet thick. This

bed contains a few small lenses of shaly and sandy material, the 

largest of which is about an inch thick. A bed of sandstone about 

a foot thick at the base of this bed has been cut through, revealing 

more coal beneath, which has the glistening appearance of cannel 

coal. Above the main bed in the slope is about 4 feet of clay with 

considerable carbonaceous material, followed successively by 3 feet 

8 .inches of coal with platy fracture, 1 foot 4 inches of sandstone, 

4 feet of coal with platy fracture containing clay streaks, and finally 

the loose overburden, mainly gravel, on the upper part of the bank. 

The upper coal bed strikes N. 20° E. and clips 7° SE. At all visible 

contacts between coal and sandstone there is distinct evidence of 

erosional unconformity. This unconformity, with the variations in 

attitude and small lateral extent of individual beds, indicates that 

the material forming the coal is of transported rather than residual 

origin.

When Capps 19 visited the prospect in July 1917 he noted three 

coal beds, 6, 5, and 9 feet thick (listed in descending order), which 

dip 14° E., in a vertical section of 24 feet. There was a 15-foot 

tunnel, now caved, in one of these beds, presumably the uppermost. 

It showed a 6-foot face of coal, and neither the top nor the bottom 

of the bed was exposed. The bank below the portal of the present 

slope is largely covered with loose material, but in places here and 

in the creek channel beds of coarse calcareous sandstone are exposed. 

Most of these contain numerous well-preserved plant remains.

The following analysis of coal from this prospect, kindly furnished 

by Henry Stevens, indicates that according to the classification 

adopted by the United States Geological Survey it is of approxi19 Capps, S. R., op. cit. (Bull. 692), pp. 231-232.
332 INVESTIGATIONS IN ALASKA RAILROAD BELT, 1931

mately subbituminous rank. It has been shown to be resistant to 

weathering, although the coal mined close to the surface makes a 

considerable proportion of fines. It has been used in blacksmith 

forges with fair satisfaction. Present exposures show a number of 

clay partings. The fact that no coal is visible either in the west 

bank of Coal Creek or in the walls of a gorge entering this creek 

from the east a few hundred feet south of the prospect proves the 

small lateral extent of the beds. From the present scanty exposures 

it may be assumed that there is a minimum of 10,000 tons of coal in 

the two beds that have been entered, and it is probable that deeper 

development will disclose a materially larger tonnage. Sufficient 

coal probably exists to supply a small power plant, such as might be 

required to develop one of the metalliferous lodes in the vicinity, for 

a considerable period. The small extent and isolated situation of the 

coal beds will prevent their coming into competition with more 

favorably situated and larger deposits for a long time to come.

Analysis of coal from, Stevens prospect

[Sample submitted by Henry Stevens; analyzed Oct. 25, 1927, by A. L. Glover, Inc., 

Seattle, Wash. Air-drying moisture content lost before sample reached laboratory]

Moisture (combined)- __ _ 7.00

Volatile matter_________________:___________ 46. 60 

Fixed carbon______ _ ______'_ _ 40.90

Ash____________ ____ _ _ _ 5.50

100.00

Sulphur ________________________________ . 41 

British thermal units- _______ 11,190
INDEX

Page 

Abstract of report______________ 289

Accessibility of the area___-_ 292-293

Assays of samples of ore__ 318, 319, 321

323, 325, 327, 328, 329, 330

Bedding deposits, features of _ 310-311 

Biotite-quartz diorlte porphyry, character of___ 304 

Brown, R. W., fossils identified by 303

Carboniferous (Permian) rocks, age

of_______:_____ 298

general features of______ 294-295 

subdivisions of________ 295-298 

Climate __________________ 293 

Coal, analysis of, from Stevens

prospect ______ 332 

Coal prospect, development of 331-332 

Copper King prospect ______ 320-321

Devonian (?) chloritic rocks, character and age of___ 294 

Disseminated deposits, features of_ 309-310

Eagle prospect-___________ 329-330 

Economic geology _______ 309-332

Faulting _____________ 307-309

Field work_______________ 289-290 

Folding _____________ 305-306 

Foreword, by Philip S. Smith___ V-VHI

Geologic map of area near West Fork

of Chulitna River__ pi. 25 

Geology, general features of____ 293 

Girty, G. H., fossils identified by__ 298 

Glover, A. L., Inc., analysis of coal

by __________ 332

Golden Zone prospect 321-325, pi. 27

assay of samples of ore from_ 323

Hopkins, Paul, assays of samples of

ore by __ 325,327,328,330

Hornblende diorite porphyry, character of_________ 305

Page

Intrusive rocks, age of___ 305 

occurrence of______ 303-304

Liberty prospect 328 

Lindfors prospect______ _ 321 

Little Lead workings____ 324 

Lode deposits, origin of 314-315 

tabular or lenticular__ _ 311-312 

Lucrata prospect ________ 328-329

Mayflower prospect _____ 325-326, pi. 27

Mining, economic outlook for _ 315 317

history of ___________- 290-291

Oxidation of lodes, occurrence and 

character of________ 314

Quartz diorite, character of 305 

Quaternary deposits, character of 303

Ready Cash prospect 318-320, pi. 26 

Reeside, J. B., Jr., fossils identified

by____________ 299 

Riverside prospect __ 326-327

Silver King prospect________ 327-328 

Stanton, T. W., fossils identified by_ 300 

Stevens coal prospect _ 331-332 

Stibnite prospect __________ 327-328 

Structure _ 305-309 

Sulphides, general characteristics

of___________ 312-314

Tertiary (Eocene) rocks, age of 303 

character of_____ 302-303

Topography___________ _ 291-292

Triassic argillite, age of__ 301-302 

character of_______ 300-301

Triassic limestone, age of_ 299-300 

character of __________ 298-299

West Fork of Chulitna River, geologic map of area near. pi. 25

333

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