DEPARTMENT OF THE INTERIOR

ALBERT B. FALL, Secretary

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTIS SMITH, Director

Bulletin 733

GEOLOGY OF THE YORK TIN DEPOSITS

ALASKA

BY

EDWARD STEIDTMANN AND S. H. CATHCART

WASHINGTON

GOVERNMENT PRINTING OFFICE

1922
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CONTENTS.

\

Page.

Introduction_____ ___ 1

Scope of report- _ 1

Acknowledgments 1

Geography 2

Location and area _ 2

Topography _____ ___ . 2

j! Drainage________-___ a. ____ _ 3

Climate _-_____ _ _ _ 4

Rock weathering________ ________________ 5

Rock transportation_______________ ________ __ 7

Vegetation and animals____ ____ ____ 9

Accessibility___________ ..__ _ _ 10

Harbor______________,_____________________ 10

Trails_____________________________________ 11

Habitation __________________________________ 12

Industries________________________..____ __ 33

Previous exploration _ _ __ 13

General geology 15

Summary of geologic history _ _ ______ 15

Slate of the York region __________________________ 1C

General characteristics __ ________________ _ 16

Contacfmetamorphisin _____ _____ _ ___ 17

Conditions of deposition _ ____________ ___ 17

Pre-Ordovician basalt and gabbro________ _____ 19

Limestone_______ _ _ ___ _ _ 19

Limestone of the York Mountains _________________ 20

Limestone of Cape Mountain _________________ _ 20

Structural relations of the slate and limestone ____ _ 22

Contact metamorphism ________________________ 22

Fosils and age_ _ _ ______ _ 23

! Conditions of deposition _______________________ 29

Granite, associated dikes, and tin ores __________________ 30

: Amygdaloidal olivine basalt _ 31

Sequence of igneous intrusions _ ______ _ __ 31

Structure______________ __________________ 32

Topographic history ____________________________ 34

Mineralogy ________________.__________________ 36

Lost River area ____________ __________________ 44

Geographic features_____________________________ 44

General geology ___ _____ 45

Summary____________ __ _ ______ ___ 45

Limestone ___ ___ ___ ______ _ 46

v. Quartz porphyry dikes __ ___ _ _________ 48

Granite _____________________________*___ 49

Erosion____________________________________ 49

276046
IV CONTENTS.

Lost River area Continued. Page.

Prospects ____ __ _____________________ 51

Cassiterite Creek tin prospect ____________________ 51

General geologic relations of the tin deposits ________ 51

Limestone __ __________________ 51

Faults _______________________________ 53

Quartz porphyry dikes ______________________ 55

Breccia ___________________________'___ 56 

Effects of the tin-bearing solutions on the quartz porphyry

dikes ______________________________ 58 

Effects of the tin-bearing solutions on the limestone ___ 61 

Developments ___________________________ 63 

Adit No. 3___________________________ 64 

Adit No. 2____________________________ 66 

Adit No. 1___________________________ 66 

Randt extension tunnel___________________ 67 

Workings on Greenstone lode________________ 69 

Ida Bell tunnel________________________ 71 

Probable tonnage of tin-bearing dike above the level of Cassiterite Creek___ ______________________ 72

Tin content of the Cassiterite dike_______________ 73

Probable extent of the tin deposits of the Cassiterite dike

beneath the level of Cassiterite Creek____________ 73

Placer tin on Cassiterite Creek_ _______________ 74

Tin Creek tin prospect________________________ 74

Dolcoath tin-bearing dike_________________ ___ 76

Bessie and Maple antimony and lead claims.__________ 78

Yankee Girl lead and tin prospect__________________ 80

Other prospects _____ __________ 81

Wolframite-topaz lode_______________________ 81

Alaska Chief claim__ ____________________ 81

Idaho copper claim________________________ 81

Brooks Mountain __ 81

Geographic features_ _ _ _ 81

General geology 83

Summary_________ ____________________ 83

Slate__________________________________ 84

Limestone _ _ ___ _______.. 84

Granite_____________-_______________ __ 85

Prospects _____ _ _ _ _ 86

Read lead prospect _ _ _ _ _ 86

Luther lead prospect________ _____ ______ 86

Mineralizing solutions___ - _ _ ___ __ _ 87 

Probability of finding tin and other minerals in commercial quan-'

tities at Brooks Mountain_ _ __ _ _ 88

Potato Mountain _ 88

Geographic features______ _ 88

Rocks________________________________________ 89

Economic geology __ _ 89

Lode developments _ _ 90

Tin placers : 94

Cape Mountain 96

Geographic features_____ -_ ___.. _ Oil
CONTENTS. V

Cape Mountain Continued. Page.

General geology ______________________________ 97

Sumniary__ _______________________ 97

Granite _ _ _____________ _ 98

Granite pegmatite___________________________ 99

Quartz veins ___________________________ 99

Limestone _________ __________________ 100

Alteration of the granite by tin-bearing solutions_________ 101

Summary of occurrence of tin at Cape Mountain____________ 101

Developments____ __________________________ 102

Production of tin ore _ _________ _ _ ___ 102

Placer tin ___________________________________ 102

Ear Mountain ____________________________________ 102

Geographic features _________________________ _ 102

General geology_______________-__.__________ 103

Summary___ __ _ ____ _ 103

Quartz-mica schist and associated basic intrusive rooks 105

Limestone _________________________-_____ 106

Granite______________________________ 106

Quartz-augite porphyry ______________,________ 108

Contact metamorphism_________ ; 108

Limestone ___________________-_______ - 108

Granite________L____ __________________ 109

Quartz porphyry dikes________________________ 109

Tin deposits__________________________________ 110

Developments _________________________________ 110

Black Mountain_________________ _____ __ ._____ 111

Location and general features______ _____________ 113

General geology _. 111

Summary __ _ __ ___ 111

Faults ________-_________________________ 112

Slate ___________________________________ 114

Pre-Ordovician basalt and gabbro ____________________ 114

Limestone____ _ ______ _ _______ __ 115

Granite______,_________..__________________ 116

Alteration of the limestone beds by the granite___________ 116

Quartz veins _ _ _ _ ___ __ 116

Quartz porphyry dikes ______ ,_________ 116

Amygdaloidal olivine basalt-_______ ____________ 117 

Comparison of the tin deposits of the York region with the other tin deposits of the world________'______ __ ___ __ US

Association with granite- _ _____ _______ _ 118

Composition of granite associated with tin deposits _______ 119

Kinds of rocks into which tin-bearing granites are intruded______ 119

1 Location of ores with reference to granite ____ 119

Age of tin deposits _ _____ 120

Classification of tin deposits________________________ 120

Rock alterations associated with tin deposits_____________ 321

Origin of the tin deposits of the York region_________________ 122

Tin deposits associated with granite___________________ 122

Tin minerals deposited from solutions after solidification of granite. 122

Character and source of solutions_______________________ 123

Sequence of the tin-bearing solutions__________________ 124

Suggestions on lode tin prospecting in the York region__________ 124

Index ____ _______ __ ____ __ _ ____________ 127
ILLUSTRATIONS.

Page.

PLATE I. Topographic and geologic reconnaissance map of York tin region, showing location of prospect and localities from which 

fossils have been collected__________ ________ 4

II. Frost cracks in slope wash east of Lost River _ 6

III. Surface features near the mouth of Lost River: A, Canyon 

near the mouth of Lost River; B, The sea terrace near Lost 

River and the York Mountains in the background- ___ 7

IV. A, Contorted Port Clearance limestone and sea cliffs west of

Lost River; B, Sea cliffs and terrace east of Lost River___ 46

V. A, Workings on Cassiterite dike on east side of Cassiterite Creek 

and on the Greenstone lode; B, Lode workings and camp on 

Cassiterite Creek _______________ _ 47

VI. Mineralization of limestone, Cassiterite Creek: A, Polished surface of altered limestone near Cassiterite lode; B, Thin section of cassiterite vein in' limestone near Cassiterite lode__ 62

VII. Mineralization of " reef" limestone, Cassiterite Creek: A, 

Rough surface of " reef " limestone, near Cassiterite lode; B, 

Rough surface of " reef " limestone______-___ _____ 63

VIII. A, Cassiterite and wolframite replacing Cassiterite dike; B,

Cassiterite vein from Ida Bell dike_______________ 72

I.. Concentric bands or orbules near limestone contact at Tin 

Creek: A, Orbule produced by contact metamorpnism; B, Reverse side of orbule shown in A: C, Maximum orbule, diameter 8 inches; D, Irregular orbules__,____________ 73

X. Geologic map and sections of Brooks Mountain_________ 82

XI. A, Arctic coastal flats, Cape Prince of Wales, and Kingegan Mission from Cape Mountain; B, Upland surface (1,000-foot terrace) at Cape Mountain______________________ 96 

XII. Geologic map of Black Mountain__________________ 112 

FIGUEE 1. Outline map of Seward Peninsula showing location of York

region ________________________________ 2

2. Sketch of the coast from Cape York to Cape Prince of Wales,

Alaska______________'_________________ 4

3. Frost-crack pattern _________________________ 8

4. Profile section along west bank of Lost River from mouth 

/ northward _________________________ _ _ 46

5. Geologic sketch map of Cassiterite Creek and vicinity____ 52

6. Idealized sketch showing mineralization of limestone -at Cassiterite Creek tin prospect.___ ___ _ ______ 59

7. Plan of lode workings on Cassiterite Creek and cross section

of workings on Cassiterite dike_________________ 64

8. Geologic sketch map of Randt Extension tunnel_______ 67

9. Plan and sections of workings on Greenstone lode_______ 70

10. Geologic sketch map of Dolcoath tunnel________ ___ 77

11. West wall of Dolcoath shaft___________________ 78

VI
ILLUSTRATIONS. VII

Page.

FIGTJKE 12. Sketch of area around Bessie and Maple claims 78

13. Geologic sketch map of Southern Cross tunnel. 79

14. Geologic sketch map of Potato Mountain area. 91

15. Geologic sketch map of Cape Mountain______ 97

16. Geologic section across Lagoon Creek, Cape Mountain. 98

17. Section through Cape Mountain., _ _ 98

18. Geologic sketch map of Ear Mountain. ___ 104

19. Section through Ear Mountain..... __ 105

20. Sketch of Winfleld workings, Ear Mountain__________ 110

21. Section at head of Deer Branch_____ ___ 112

22. Section near Willow Branch__________________ 112

by slates through faulting and erosion ____ _ 113

23. Block diagram showing a limestone lens became surrounded

GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA,

By EDWARD STEIDTMANN and S. H. CATHCART.

INTRODUCTION.

SCOPE OF REPORT.

Tin is one of the few metals of which the United States needs 

more than can be furnished by its domestic supplies. This country 

is the world's leading consumer of tin but yields less than one-half of 

1 per cent of the world's production. Our normal needs for tin are 

increasing, and the price has been going up during the last 10 years. 

Our normal deficiency in this metal was accentuated by the abnormal 

requirements of the World War.

This report discusses the York tin region of Alaska, the source 

of nearly all our domestic output of tin. It aims to present the, 

facts known regarding the location, character, and history of the 

tin deposits of this region and the rocks associated with them. Emphasis is laid on the study of the origin of the lode deposits, with the 

view of formulating suggestions which may be helpful in the future 

search for tin in this region. Having had the results of Brooks, 

Collier, Hess, and Knopf to build on, the writers hope that they 

have succeeded in making some advances in the study of the structural relations of the tin deposits and of the rocks of the region. 

They have also revised some of the earlier reconnaissance mapping..

ACKNOWLEDGMENTS.

The acknowledgments of the writers are due first to the earlier 

students of this area, especially Messrs. Brooks, Collier, Hess, and 

Knopf, whose publications are listed in this report.

Of the people actively engaged in exploring and operating in the 

York tin region, Mr. J. F. Halpin, of New York City, has been 

especially helpful in extending courtesies. Others associated with 

Mr. Halpin, to whom acknowledgments are due, include Messrs. 

F. C. Fearing, mining engineer, of Philadelphia; W. B. O'Brien, of 

Seattle; and Alex Mclntosh, of New York City. Courtesies and 

valuable information regarding the Potato Mountain country were

1
GEOLOGY OF THE YORK TIN DEPOSITS, ALAEiKA.

received from Messrs. A. S. Graham, George Mahla, and Tom 

Petersen.

Acknowledgments are also due to Messrs. G. C. Martin, Edwin 

"Kirk, and E. S. Larsen, of the United States Geological Survey.

GEOGRAPHY.

LOCATION AND AREA.

The York tin region is the most westerly portion of the North 

American continent, extending within 60 miles of the coast of Asia. 

(See fig. 1.) It lies about 40 miles south of the Arctic circle. The

65-

25____ 50 75 Miles

FIGURE 1. Outline map of Seward Peninsula showing location of York region.

Arctic Ocean bounds it on the north, Bering Sea on the south, and 

Bering Strait on the west. This region is a part of Seward Peninsula, the westerly projection of Alaska.

The area has the form of a triangle whose base extends from Teller 

northward to Ear Mountain and whose apex is formed by Cape Prince 

of Wales. It includes about 600 square miles lying about 100 miles 

northwest of Nome, the largest town and distributing point for all of 

Seward Peninsula.

TOPOGRAPHY.

Three topographic types predominate the mountain type, represented by York, Cape, and Ear mountains; the plateau type, repre-
GEOGRAPHY. 3

sented by the slate area between the York and Cape mountains and 

by a sea terrace fronting Bering Sea between Kanauguk and Don 

rivers; and the coastal-plain type, represented on the Arctic coast 

and on the Bering coast north of Port Clarence. In addition to these 

may be considered the present beaches. (See PL I.)

The York Mountains constitute the greatest upland area of the 

region. They are steep and rugged and rise from the coast to general altitudes of 2,000 to 2,500 feet. The altitude of Brooks Mountain, the highest of the group, is 2,900 feet. The trend of the range 

is east-northeast. On the north the mountains slope gradually seaward and are represented in the broad tundra plain of the Arctic 

coast by rounded hills a few hundred feet above the sea. On the 

south, where they abut upon the Bering coast, they terminate in a 

sea cliff 400 to 600 feet high, capped by a terrace 1 to 4 miles in 

width. To the east the mountains recede from the coast, and cliffs 

and terrace are lost in the valleys of Don and California rivers, 

which, for 6 miles above their mouths, flow across a tundra plain. 

Northeast and east of California Elver the rugged, sharp-crested 

ridges of the main mountain mass gradually give way to more 

rounded hills that have an altitude slightly more than 1,000 feet.

On the west the York Mountains give way abruptly to the York 

Plateau. The plateau area is a rolling tundra-covered plain, with 

an average relief of 200 to 600. feet. Potato Mountain, at its northern 

extremity, is the only notable prominence and has an altitude of 

1,400 feet.

The western extremity of the area is marked by the isolated mass 

of Cape Mountain, which rises abruptly from the water's edge to an 

altitude of 2,250 feet.

Ear Mountain is an isolated butte in the northeast corner of the 

area, separated from the outlying ridges of the York Mountains by 

20 miles of tundra-covered plain.

The coastal marsh and tundra plain bordering the Arctic coast 

ranges from 2 to 10 miles in widtlj, and the tundra extends up some 

of the larger stream valleys 20 miles or more. The marsh is characterized by sluggish meandering streams and numerous lakes. On 

Bering Sea the coastal marsh occurs only within the shelter of Port 

Clarence, where it covers a triangular area of about 70 square miles. 

It is bordered by a shallow lagoon impounded by a barrier beach, 

which extends for 12 miles along the Bering coast.1 (See fig. 2.)

DRAINAGE.

Probably two-thirds of the area is drained northward into Lopp 

Lagoon. The streams are numerous and of no great size. The

1 Collier, A. J., The tin deposits of the York region, Alaska: U. S. Geol. Survey Bull. 

229, pi. 2, 1904.
4 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

largest streams are easily forded, even at their mouths, and the many 

tributaries that dissect the mountains, plateaus, .and terraces are but 

a few feet in width and in depth.

In the higher hills of the York Mountains .the streams flow through 

V-shaped valleys. The smaller stream valleys are almost without 

bottom lands, and the walls are steep talus slopes, which give to the 

valleys canyon-like characteristics. The larger streams, such as Lost 

River, have developed valley bottoms half a mile or more in width, 

and their confining hills are less precipitous and are interrupted at 

short intervals by well-developed tributary valleys.

In the York Plateau, in the lower hills that extend northward and 

eastward from the main mass of the York Mountains, and in isolated 

slate areas throughout the region, the streams flow through valleys 

of mild relief and in general have well-developed lowlands.

Where the streams enter the coastal-plain area and flow through 

tundra marsh, their currents are sluggish and their course meandering.

FIGURE 2. Sketch of the coast from Cape York to Cape Prince of Wales, Alaska.

In the areas not covered by tundra most of the rainfall disappears 

immediately as run-off, owing to the steep slopes, the frozen condition of the ground at no great depth, and the total absence of vegetation. In the areas covered by tundra the run-off is retarded, as if 

held by a sponge, and supplied gradually to the streams.

Gullies and ravines are the sites for the accumulation of ice and 

of much of the winter's snowfall, which is swept from the lowlands 

and ridges by the high winds that prevail throughout the region. 

In the spring most of the snow and ice disappears from the lowlands 

and exposed ridges and supplies water for the usual spring run-off, 

to which the placer miner looks forward.

Isolated masses of ice and snow persist in the sheltered ravines into 

and even throughout the summer. They furnish a constant source 

of supply to the streams, but in dry seasons the miner often finds 

them insufficient.

CLIMATE.

The climate of the region is arctic and is controlled largely by the 

Arctic Ocean and Bering Sea. High winds from the north or south 

are the rule, and fog and rain prevail in the summer. The precipita-
U. S. GEOLOGICAL SURVEY BULLETIN 733 PLATE I

168°00' 166°00'

168 00 40' 20' 167°00' 40

166°00'

Alfred H, Brooks, Chief Alaskan Geologist 

Topography by T. G. Gerdine, D. C. Witherspoon 

and A. H. Brooks.

Surveyed in I900 and I90I

TOPOGRAPHIC AND GEOLOGIC RECONNAISSANCE MAP OF THE YORK TIN REGION, SEWARD PENINSULA, ALASKA

S c ale 250000

4, 6 8 14 miles

Geology by Arthur J. Collier and Frank L. Hess in 1901-1904 

Revised by Edward Steidtmann and S, H, Cathcart in 1918

2 o 2 4 6 8 10 .12 i-i ^kilometers

Contour interval 200 feet 

.Dcttttjn is num,n sea, l&vel

1922

Sea. level

EXPLANATION

Swamp muck and alluvium

Amygdaloidal basalt

Quartz porphyry dike

(probably Mesozoic)

Limestone, in part metamorphosed

(Including Port Clarence limestone, of > > < 

Silurian and Ordovician age; and limestone of Cape Mountain, in part at

least of upper Mississippian age;

metamorphism indicated by darker

Basalt and gabbro 

(Including some slate in areas repre-j 9

sented by lighter color)

Black slate

Dip and strike of stratified rocks

ill

I o:< 

f m o

ct 

o

Strike and vertical dip

Locality where fossils were collected

(M, upper Mississippian; S, Silurian; L. O, Lower 

Ordovician; U. O, Upper Ordovician)

Unclassified prospect

Copper prospect

Trail

SECTION ALONG LINE A-B
GEOGRAPHY. 5

tion is probably not great. St. Michael, at the mouth of the Yukon, 

the nearest recording station, has an annual precipitation of 18 inches 

and a season of 125 days during which rain falls. The precipitation 

of the York region is probably a little more and the season shorter 

24 inches and 100 days might more nearly express it.

The winters are severe and snows often heavy. Temperatures of

 50° F. are frequently recorded.

Bering Sea is frozen from November to June, when the ice pack 

drifts northward through the straits and navigation opens. Inclosed 

waters, creeks, and ditches may be expected to freeze up in October 

and to open in May.

The summers are short. Freezing temperatures are prevalent at 

some hour of the day except for three or four weeks in July and 

August. The days from July to mid-September are not uncomfortably cold to one dressed in woolens. During July and August the 

temperature is above freezing sufficiently to give the creek and pond 

waters a temperature of 40°-50° F.

Snow caps the higher hills until late in July, and many of the 

gullies are never free from snow and ice the year around.

The unconsolidated materials (muck, gravel, etc.) underlying the 

tundra are frozen solid the year around and must be thawed before 

they can be mined. Such frozen masses may include 50 feet or more 

of material, as on the Nome flats, in the southern part of the peninsula. Again, a tundra-covered surface 50 feet from the bedrock may 

be underlain by 40 feet or more of ice with only a few feet of earth. 

Peat is a common constituent of such a section and as a rule immediately underlies the tundra for a thickness of 1 to 3 feet. The stream 

gravels are frozen to a depth of several feet in winter, but are thawed 

by circulating waters in the open season.

BOCK WEATHERING.

Rock weathering in the York region is caused chiefly by solution 

and frost action, of which the latter is by far the more effective. In

 so cold a climate solution is much less vigorous than in warm, moist 

regions. It is checked directly by the low temperatures and to a 

greater extent indirectly by the limitation which the low temperatures in turn place upon the duration and depth of the underground 

water circulation. Low temperature does not decrease the solubility 

of certain substances in water. Calcium carbonate, for example, is 

more soluble in cold carbon dioxide bearing waters than in warm. 

That solution has taken place in this region is evinced by gossans, 

the occurrence of hard stream waters in the limestone areas, the development of residual clays on limestones, and the occasional deposition of limonite in tundra pools.
6 GEOLOGY OF THE YOEK TIN DEPOSITS, ALASKA.

The checking of the underground water flow is most pronounced 

under low tundra-covered areas. In the rugged portions of the slate 

and limestone areas underground circulation penetrates to considerable depth, causing the perennial flow of springs and streams and 

the development of gossans 85 feet or more deep. It is more 

vigorous in limestone areas than in those underlain by slates. Slate 

areas are sufficiently impervious to make the transmission of water 

in open ditches along hillsides a success, but similar undertakings 

in the limestone areas of the York region failed because of the 

permeability of the rock.

But even the rugged areas probably have some deep, frozen ground 

that retards circulation. The formation of ice in summer at the end 

of adit No. 3 on Cassiterite Creek-at a depth of 260 feet below the 

surface and 61 feet above the creek indicates that the temperature 

underground in this hilly limestone area is low enough to cause the 

formation of ice wherever the circulation is not sufficiently vigorous 

to cause thawing. That vigor of circulation is a vital factor in thawing is shown by the practical success of a method of thawing placer 

ground by forcing cold water into it. The existence of deep frozen 

ground in the limestone area may also be a cause for the ra«pid 

swelling of the streams in response to even light showers. A very 

high percentage of run-off is also characteristic of the tundra-covered 

slate country, but here it is to be expected because of the saturation 

of the surface materials. On the other hand, the quick response of 

streams of the limestone area is surprising in view of the porous 

character of the surface deposits.

The underground waters in the slate areas are probably more active 

chemically because of the peat and tundra through which they pass,

but the waters of the limestone areas accomplish more solution because 

of their more vigorous circulation and the greater solubility of the 

limestone. Stream waters of the limestone areas are hard; those of the 

slate areas are generally soft.

Frost action consists mainly in the expansion of freezing water 

held in the pores and cracks of rocks. This expansion tends to reduce 

all outcrops to a mass of broken rock of all sizes down to minute 

grains. Outcrops are therefore rare except where the removal of the 

broken rock is more rapid than its formation, as along cliffs and 

stream channels. The expansion of ice when the temperature rises 

also contributes to the breaking up of rocks, but a rise of 1° in the 

temperature of ice causes an expansion only about 0.006 per cent of that 

due to the change from water to ice.

The conditions most favorable for frost action are southerly exposures, barren surfaces, and a highly fractured rock. It is most 

effective in spring and autumn, because these seasons have the greatest 

frequency of alternate freezing and thaAving. Limestone surfaces are
V. 8. GEOLOGICAL SURVEY BULLETIN '733 PLATE II

A.

B.

FROST CRACKS IN SLOPE WASH EAST OF LOST RIVER. 

A, In foreground polygonal gaping cracks; B, cracks filled with coarse rubble, chiefly limestone.
U. S. GEOLOGICAL SURVEY BULLETIN 733 PLATE III

A. CANYON NEAR THE MOUTH OF LOST RIVER.

B. THE SEA TERRACE NKAR LOST RIVEK. 

York Mountains in the background.

SURFACE FEATURES NEAR THE MOUTH OF LOST RIVER.
GEOGRAPHY. 7

i more barren than slates, but slates, because of their cleavage, aremore subject to frost action, and hence disintegrate more rapidly thart

, limestone.

The reason for the slate being covered by tundra whereas the limestones are nearly all bare is an interesting problem in itself. Thepoorer drainage of the slates is probably a contributing factor, but 

that it is not the only one is shown by the limestones being generallybare, even on low ground. In marked contrast to this the surface of 

^decayed dikes cutting limestones are commonly covered with moss..

i The barreness of the limestone seems to be tied to the composition 

of the limestone and not to any condition of slope or altitude. The 

plants that constitute the tundra, being distinctly of the "acid soil"' 

type, thrive best where there are no neutralizers of acids. The limestones have a strong destructive action on these acids and therefore 

do not offer a favorable environment for these plants. The slates, 

on the other hand, having in themselves neither pronounced acid nor 

acid-destroying properties and being poorly drained, offer favorable 

conditions for the accumulation of the acidity which these plants

- require for their existence.

t BOCK TRANSPORTATION.

The products of weathering in this climate are chiefly broken,.

-1 chemically unaltered rock materials and only to a very minor degree 

leached soil. They are transported by stream action, soil creep or

- flow, landslides, peat and mud flows, and other agencies. The slowr 

unnoticed creep or flow of surface materials down slopes appears tobe very common in arctic regions and is an inviting subject of study. 

The saturation of the thawed surface materials in the summer and 

their consequent mobility is an important factor in this downward 

movement, particularly in tundra-covered areas. More spectacular 

flows occur when lakes on hillsides or hilltops which are held by 

the growth of tundra burst through their organically deposited barriers and send a flood of water, mud, and peat upon'the slopes below. 

An interesting result of combined frost action and the transportation of surface materials is the frost-crack pattern (PL II). In 

the York region this pattern was studied only on barren, clay-covered

r- areas of the limestone country. Leffingwell 2 describes a more gigantic frost-crack pattern developed in the peat bog? bordering the

i Artic Ocean.

i The frost-crack pattern studied in the York region was commonly 

developed on gentle clay-covered slopes. It consists of a network of 

wedge-shaped cracks, at least 3 feet deep in many places and about ]' 

foot wide at the top. Cross sections were not seen. Commonly

3 Leffingwell, E. de K., Ground-ice wedges: Jour. Geology, vol. 23, pp. 635-654, 1915.
GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

large cracks unite into a roughly hexagonal pattern. The blocks 

between the cracks are usually five, four, or three sided or even circular. Not uncommonly they are perfectly hexagonal. The diameter of the large blocks averages about 6 to 8 feet and rarely exceeds 10 

feet (fig. 3). Commonly a miniature frost-crack pattern is shown 

on the blocks between the large cracks. The width of the blocks 

in this minor network is commonly from 4 to 6 inches.

The large pattern presents two extreme phases. In one the cracks 

are empty or nearly empty and gaping. In the other the cracks 

are filled with angular and rounded rocks 1 foot or less in diameter," 

which rise as ridges above the adjacent blocks, in some places to a 

height of 2 feet. The cracks are thus marked by ridges of loose rock.

The rocks in many of the cracks are 

not all limestones but include granites, 

porphyries in fact all the rocks transported over the surface on which they 

occur.

The history of the frost-crack pattern 

is interpreted as follows: The wet clays 

freeze, and as the temperature falls 

linear contraction of the ice tends to 

take place at the rate of about 1/36,000 

for each degree Fahrenheit. A bar of 

ice 8 feet long, for example, would tend 

to contract about 0.2 inch if its temperature was reduced from 32° F. to 50°. 

If the ice is not free to move, tensile 

stresses are set up which, when they 

exceed the tensile strength, cause cracks. 

An interesting unsolved secondary problem is why two frost-crack patterns are 

developed, the large and the small. The 

gaping, wedge-shaped form of the cracks may be due to the fact that 

the clays suffer less reduction in temperature at depth and hence 

contract less, and also to the fact that the surface materials have a 

lower tensile strength. It is probable, too, that the wedge shape is 

accentuated by a later freezing of water in the cracks.

When the spring thaws begin water fills some of the cracks and, 

on freezing, expands and tends to widen and preserve the cracks. 

This is also liable to happen in autumn. Season after season this 

process is repeated and tends to preserve and increase the size of 

the openings.

Over nearly all surfaces of this region rock debris is moving 

downhill. The occasional vigorous rains may cause some of this 

movement, but the large size of some of the particles of the debris

CROSS SECTION, FILLED CRACKS

 CROSS SECTION.OPEN CRACKS 

0______5______ip FEET

FIGDBB 3. Frost-crack pattern.
GEOGBAPHY. 9

 suggests that slow forms of creep are more important. Eains adequate to move materials of the size seen on some of these gentle 

slopes did not fall in 1918. A slow motion of boulders may perhaps 

result from the change in their center of gravity produced by the 

upward heave of freezing water under them, which may cause them 

to move slightly downhill. Boulders in their transit over the 

surface fall into the frost cracks and fill them. The suggestion that 

the boulders may get into the cracks in other ways, such as by 

frost, heaving them up from ledges or boulder beds below, will not

 serve as a complete explanation, for they occur in places where 

underlying boulder beds are nonexistent or highly improbable. 

The material that fills some cracks, comprising several kinds of 

limestone, granite, and porphyry, could not have come from underlying ledges.

The upheaval of the boulders lying in the frost cracks into ridges 

'that rise above the adjacent surface is believed to be due to the 

upward shove of ice that formed in the cracks. Ice expands as 

Tapidly when the temperature rises as it contracts when the temperature falls. Such expansion would be relieved chiefly by upward 

movement, for the opportunity for lateral relief would be very

 slight, and resistance from below would prevent expansion

 downward.

VEGETATION AND ANIMALS.

The only timber available for construction in this region is driftivood deposited on the beach in time of storm. Locally, in some of the

 stream valleys, a scanty growth of scrub willows, ordinarily but a 

few feet high, is available for use as fuel. It is an uncertain and 

unsatisfactory fuel at best and to be recommended only as a last

 resort.

Most of the larger stream valleys and the coastal plains afford 

grass which is suitable for grazing during the short summer season

 of July and August. The great fields of tundra furnish an abundant supply of moss that is well adapted to the needs of reindeer, 

but of little value to other stock.

The higher hills of the York, Ear, Potato, and Cape mountains 

are almost barren of vegetable growth, affording neither timber 

for fuel nor grass for grazing. In general, the York Mountains 

proper and the lower limestone hills extending eastward from them 

are the most destitute of willows and grass. The slate areas usually 

afford a considerable supply of both.

Land animals 0 were once plentiful in the area, ptarmigan, snipe, 

plover, curlew, rabbits, and a few bear furnishing prospectors with 

urn abundance of meat, but within recent years their numbers have 

101987 22 2
10 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

been greatly reduced. Rabbits have practically disappeared, and a 

few small flocks of ptarmigan, with an occasional fox or bear, alone 

remain. Sea fowl, including geese, ducks, sandhill cranes, and 

swans, are plentiful, especially along the coast.

The streams, with the exception of some of those flowing through 

the heart of the York Mountains, which contain scarcely any vege- 1 

table life, are fairly well stocked with grayling and trout.

Geological Survey parties working in the region have found it 

necessary to carry horse feed and advisable to carry oil for fuel.

ACCESSIBILITY. ^

During the open season, from June to November, one steamship 

line maintains a rather erratic schedule between Seattle and the 

ports of Seward Peninsula. Nome is as far north as the passenger t 

service extends, but between July 1 and September 30 freighters 

are occasionally sent through the straits to Arctic ports: The 

freighters touch at Teller and also discharge cargo at points on the ^ 

south coast of the York district, the freight being transferred from 

the boats to shore on lighters sent out from Teller. Except for 

these freighters, communication between Nome and the York region 

is maintained by small coastwise gas schooners of 20 to 50 tons 

burden. This navigation opens about the middle of June and continues until late in October. One schooner licensed to carry mail 

maintains a two-week schedule, as nearly as the weather conditions 

permit, between Nome and Arctic points, carrying passengers and 

freight and stopping at several coast points on signal if the winds 

and fog are favorable. Other schooners also carry freight and passengers but make no pretense at maintaining a schedule. 1

Points on the Arctic coast of the York district are not easily accessible by boat owing to the shallow lagoons impounded behind 

barrier beaches which extend along the shore and which are in turn 

bordered by extensive tundra marshes. Freight landed at such 

points as Shishmaref Inlet must be lightered in shallow-draft boats 

from the coastwise schooners to the shore and up the tortuous stream 

channels of the tundra flats to solid ground, where it can be picked 

up by teams.

HARBOR. v-»

Port Clarence, about 40 miles east of Cape Prince of Wales, is 

the only harbor that affords adequate protection to coastwise schoon- f 

ers plying between Nome and the cape. From Port Clarence west- ^ 

ward the coast is treacherous and subject to frequent storms. Land- r 

ings are not attempted except in favorable weather. Much loss of 

time, hardship, and uncertainty of coast boat service are occasioned
GEOGRAPHY. 11

by the inadequate harbor facilities. The Arctic coast of the area 

has no harbors.

TRAILS.

During the winter communication between Nome and Teller is 

effected by the overland route by means of dog and sledge. A 10- 

day mail service is maintained. The overland trail used in the 

open season follows the coast from Nome westward to Tisuk River, 

up the Tisuk and across the divide to Bluestone River, down the 

Bluestone to Right Fork, and thence by a direct course over tundracovered hills to Teller. The trails are everywhere passable to horse 

and wagon, although they do not afford an easy haul. Pack trains 

experience no serious difficulty. Sinuk River is crossed on a Government ferry; all other streams are fordable. There are roadhouses 

affording accommodations on the trail at Sinuk and Gold Run. A 

shelter cabin has been constructed by the road commission at the 

mouth of the Tisuk. The journey of about 85 miles is readily made 

in four days' travel.

From the sand spit north of the entrance to Grantley Harbor all 

the known mineral deposits of the region can be reached by light 

teams. Where the trails cross the tundra, hoAvever, travel is difficult in the most favorable seasons and almost impossible during wet 

seasons. Pack trains have been found by Geological Survey parties 

to furnish the best means of transportation if it is desired to reach 

all points under adverse climatic conditions.

A trail leads westward along the beach past Teller Mission to the 

mouth of Lost River, a distance of about 27 miles. Don and California Rivers, which are crossed on the way, are easily forded, and 

the trail as a whole is adapted to a team and light loads. Continuing up Lost River for 7 miles is a good.wagon road leading to the 

Cassiterite Creek tin mine. A trail leads from the mouth of Cassiterite Creek across the Lost and Mint rivers .divide into the Brooks 

Mountain country, a distance of 7 miles. Teams can cross by this 

route to reach Potato Mountain from Brooks Mountain, a distance of 

28 miles. An easy trail leads from the Mint River valley into the 

valley of Skookum Creek, down Skookum Creek to Grouse Creek, 

and thence up Grouse Creek to Buck Creek and up Buck Creek to 

Potato Mountain. A good wagon road runs from Buck Creek to 

York, a distance of 17 miles. York may be reached from the mouth 

of Lost River by way of a trail up Rapid River, across the divide 

to the Kanauguk, down the Kanauguk to the point where it turns 

abruptly south, thence along the telephone line across the tundra 

westward into the Anikovik Valley, and down the Anikovik to York, 

a distance of 22 miles. This trail is poor most of the way and 

adapted only to very light loads. Cape Mountain is easily reached
12 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

from York by a trail, which follows the beach the entire distance of 

12 miles. In general, the valleys of the southward-flowing streams 

Don and California rivers, Tozer Creek, Lost and Rapid rivers, Cassiterite Creek, Kanauguk and Anikovik rivers are readily 

traversed. So also are the upper reaches of the northward-flowing 

streams, but as they leave the mountains they flow through wide 

valleys and over flat tundra plains and coastal marshes, which offer 

serious impediments to travel.

Ear Mountain, an isolated mass in the northeastern part of the 

area, is about 50 miles from Teller. It is reached by trail from . 

Teller leading up Bay Creek, across the divide into the Agiapuk ^ 

basin, across the Agiapuk and northward into its upper valley, up 

its headwaters to the Nuluk divide, down the Nuluk to East Branch, 

up East Branch to its head, thence on to a tundra plain which extends to the base of Ear Mountain, 20 miles distant. The last 20 

miles of this trail offers some difficulties in wet seasons but has been 

traveled with a light wagon.

HABITATION.

r

Tin City, at the base of Cape Mountain, and York, at the mouth of 

Anikovik River, have been centers of population in, this region. 

Tin City is now deserted, and its revival is dependent upon the 

renewal of work on the Cape Mountain lode prospects. York, once 

a flourishing tent town, now consists of half a dozen cabins and is 

permanently inhabited by one family of three persons. A representative of the Bureau of Education at the Cape Mission and a 

missionary family at Teller Mission are the only other permanent 

white inhabitants of the region. Winter prospecting is carried on ) 

from year to year and engages possibly an additional 15 white men. 

During the summer mining season probably 50 or 60 men are employed in the placer and lode mines. The Eskimo population, which 

centers at Cape Prince of Wales and Teller missions, numbers probably 500.

Teller, on the south shore of Grantley Harbor, on the sand pit 

separating Grantley Harbor from Port Clarence, is the local post 

office and supply center for the York district. It contains two general stores and road houses and has a population of about 30 whites. j-r 

A lighterage company which transfers freight to points along the 

Bering.coast and to points along the inland waters of Grantley ^ 

Harbor, Tuksuk Channel, and Imuruk Basin as far east as Davidson's landing, maintains a freight and passenger ferry across Grantley Harbor.

A telephone line, which for a time connected Tin City with Nome 

by way of York and Teller, was put out of commission by storms 
PREVIOUS EXPLORATION. 13

in 1913. Kestoration of the line would be a comparatively simple 

matter, as many of the poles and much of the wire are still available.

INDUSTRIES.

Mining, reindeer herding, and fishing are the principal industries 

of the area.

Gold is mined by placer methods in the creeks 'that flow into 

Grantley Harbor just east of the area and was formerly mined in 

the vicinity of York. The tin placers of Buck Creek are the most 

productive mines of the region. Work is in progress on the lode-tin 

deposit at Cassiterite Creek and promises future production.

Several thousand reindeer, some of which are owned by natives, 

but most of which are controlled by outside interests, are grazed

throughout the region.

In the summer some fishing is done along the coast for salmon 

und white whale and in the inland waters for herring.

Walrus, seal, and polar bear are hunted by the natives during 

the winter. Some trapping is done. The total returns from fishing, hunting, and trapping are small compared with those obtained 

in other parts of the peninsula. a

The fuel problem of the area is a serious one. For generating 

power crude oil, distillate, and gasoline are in general use. Coal 

is used for heating, but the consumption is limited by the cost. Coal 

delivered at many localities in this area in 1918 cost $50 or more a 

ton. All fuel is imported, chiefly from Seattle.

PREVIOUS EXPLORATION.

Prior to the discovery of gold at Cape Nome very little was known 

of the York region. -A mission had been established for a number 

of years at Cape Nome, where one of the Government reindeer herds 

was maintained. After the first rush to Nome, prospectors rapidly 

extended their search to all parts of the peninsula, and as early as the 

fall of 1899 some placer gold had been found in the Anikovik basin. 3

In September, 1900, A. H. Brooks, during his investigation of the 

southern part of Seward Peninsula, spent 10 days in the York region 

and made the first topographic and geologic map of it.4 At this time 

there was little gold placer mining in the York region. The miners 

were much disturbed by some heavy minerals which clogged the 

sluice boxes. A part of this heavy concentrate, from Anikovik River 

and Buhner Creek, one of its tributaries, proved to be stream tin. 

Upon his return, Brooks published a brief note 5 calling attention

3 Brooks, A. H., and Schrader, F. C.,, Preliminary report on the Cape Nome gold region, 

Alaska, with maps and illustrations: U. S. Geol. Survey Spec. Pub., pp. 25-26, 1900. 

* Idem. 

6 Science, new ser., vol. 13, p. 593, 1901.
14 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

to this new locality for stream tin. Copies of this note were mailed 

to all the prospectors in the district, so that they might have early 

knowledge of the discovery. The discovery was also given considerable publicity by the daily press, and the first steamers going 

north in the summer of 1901 carried a number of men who were 

bound for York to search for tin.

In 1901 A. J. Collier undertook the geologic mapping of the northwestern part of Seward Peninsula. He accompanied T. G. Gerdine, 

in charge of party, who was engaged in making a topographic survey 

of the same area. Collier 6 made some examinations of the occurrence of tin. His geologic map covered the essential features of the 

region, and has been but little modified by later work. In the later 

part of the season a great many prospectors searched the York region 

for tin, and before winter they had located promising deposits of 

stream tin on Buck Creek, a tributary of Mint River, about 20 miles 

north of York. 7

In 1902 the search was continued and the first real attempt to mine 

tin-bearing gravels was made. The nature of this occurrence and 

the mining conditions which existed there at that time have been 

« described by Rickard.8

In 1903 Collier and Hess were engaged in a study of the geology 

and mineral resources of the southern part of Seward Peninsula.9 

In the course of this work they reached Teller, on Port Clarence, and 

here met Crum, Randt, and O'Brien, three prospectors who ha.d been 

searching for tin. They had about a bushel of samples of supposed 

tin ore. This material was examined by Collier, who found only 

one piece of rock that looked as if it might contain cassiterite. With

an improvised blow pipe and candle Collier determined the mineral as

cassiterite. It came from an angular fragment of float rock. Collier 

decided that the occurrence was important enough to warrant a 

special examination of the locality. He therefore accompanied the 

three prospectors to Lost River, where they pointed out the locality 

from which the specimen had been derived. Collier discovered the 

tin-bearing ledge, which the three prospectors staked under the name 

Cassiterite Lode. Collier was therefore the discoverer of lode tin 

in the York district, though prior to this time some alleged tin ore 

had been found at Cape Mountain, most of which on test proved to be 

a tourmaline granite. Later some- lode tin was found at Cape 

Mountain.

8 Collier, A. J., Reconnaissance of the northwestern portion of Seward Peninsula, 

Alaska: II. S. Geol. Survey Prof. Paper 2, 1902.

'Collier, A. J., Tin deposits of the York region,. Alaska: U. S. Geol. Survey Bull 229, 

p. 11, 1904.

8 Rickard, Edgar, Tin deposits of the York region, Alaska : Eng. and Min. Jour., vol. 

75, pp. 30-31, 1903.

9 Collier, A. J., Recent developments of Alaska tin deposits: U. S. Geol. Survey Bull. 

259, 1905.
l GENERAL GEOLOGY. 15

In 1904 Collier, on returning from the Cape Lisbnrne coal fields, 

spent a few days noting the progress of development in opening the 

tin deposits.10 Hess lx fully described the later developments in 1905. 

No member of the Geological Survey visited the region in 1906, but 

in 1907 Knopf spent an entire season in examining in detail the tin 

/ deposits and general geology of the region. All the localities where 

prospecting was in progress, including Ear Mountain, were visited, 

and the mineralogy and petrology of the region thoroughly investigated.12

In 1910 Kindle 18 studied the stratigraphy of the region and colV lected fossils from numerous localities.

In 1913 Chapin 14 reported on the development of the region.

Eakin 15 visited the region in 1914 and summarized the recent 

developments and the occurrences of tin.

In 1916 Mertie18 and in 1917 Harrington17 made reports on development. 

% GENERAL GEOLOGY.

SUMMAKY OF GEOLOGIC HISTORY.

The principal known events of the geologic history of the York 

tin-bearing region, in the order of their occurence, may be summarized as follows:

1. Deposition in Cambrian or pre-Cambrian time of black muds, 

now represented by black slates.

2. Intrusion (in pre-Ordovician time) of sills, dikes, and stocklike masses of basalt and gabbro into the slates.

3. Probably warping and erosion of the slates. Deposition on 

the slates and greenstones of limestone beds 2,000 feet or more thick, 

ranging in age from Lower Ordovician to upper Mississippian the 

Port Clarence limestone (Ordovician and Silurian) and the limestones of Cape Mountain (in part at least of Missippian age).

4. Compression of the limestone and slate beds into a broad, 

shallow north-south basin with steep sides on the east and west, 

the whole basin tilted 15°-40° N. Minor folds trending roughly 

north and east were superimposed upon the major basin.

10 Collier, A. J., Recent developments of Alaska tin deposits: U. S. Geol. Survey Bull. 

259, pp. 120-127,, 1905. 

r ,_ " Hess, F. L., The York tin region : U. S. Geol. Survey Bull. 284, pp. 145-157, 1906.

"Knopf, Adolph, Geology of the Seward Peninsula tin deposits, Alaska: U. S. Geol. 

Survey Bull. 359, 1908. 

, 13 Kindle, B. M., The formal succession In the Port Clarence limestone, Alaska: Am.

Jour. Sci., 4th ser., vol. 32, pp. 335-349, 1911. / 

»» " Chapin, Theodore, Mining on Seward Peninsula, Alaska: U. S. Geol. Survey Bull. 

592, pp. 385-397, 1913.

16 Eakin, H. M., Tin mining In Alaska : U. S. Geol. Survey Bull. 622, pp. 81-94, 1915. 

18 Mertie, J. B., jr., Lode and placer mining on Seward Peninsula, Alaska : U. S. Geol. 

Survey Bull. 662, pp. 425-451, 1917.

17 Harrington, G. L., Mineral resources of Seward Peninsula, Alaska: U. S. Geol. 

Survey Bull. 692, pp. 353-400. 1919.
16 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

5. Normal faulting of the limestone beds along zones strikinggenerally N. 20° W. and N. 70° E. Some reversed faults striking; 

N. 70° W. may belong to this period.

6. Injection of quartz porphyry dikes into the N. 70° E. fault, 

zones and to a minor extent along those striking N. 20° W. Intrusion (probably in Mesozoic time) of granitic stocks without visibly 

disturbing the structure of the limestones and slate beds.

7. Release of tin-bearing fluorine and boron solutions from thedeep-seated granitic masses, forming tin-bearing veins and replacement deposits in the marginal portions of the granite and in portions of the dikes and limestones.

8. Erosion of the region accompanied by several changes of level r 

(a) Beveling of the Port Clarence limestone to a nearly plane surface, the modified remnants of which are represented by the crests of 

the York Mountains, the so-called Nuluk Plateau.

(&) Elevation of the region and cutting of a marine plain of 

erosion along Bering Sea the York Plateau about 1 ?600 feet lower 

in elevation than the Nuluk Plateau.

(c) Uplift and warping of the region. The marine plain is raised 

400 to 600 feet above Bering Sea, from Cape Mountain to Lost River. 

East of Lost River it was tilted toward the east, thus drowningGrantley Harbor and submerging the Tuksuk Channel and the Imuruk Basin.

(d) Cutting of the present Bering Sea cliff, averaging about 400' 

feet in height.

9. Extrusion of olivine basalt flows. Continued erosion and deposition of stream gravels, some of them tin-bearing. Minor modifications of the Bering Sea coast by processes of cut and fill.

SLATE OF THE YORK REGION. 

GENERAL CHARACTERISTICS.

Erosion of the east and west ends of the structural basin of theYork region has laid bare the slate areas northeast of Teller and 

west of the York Mountains. At Brooks Mountain the slate is exposed by the truncation of an upwarp of the slate and limestone 

beds, complicated by faulting. The slate of Black Mountain has, 

been uncovered by the erosion of fault blocks. (See PI. I.)

In general, the slates are dark-colored to glistening black graphitic 

sediments of exceedingly fine grain without prominent bedding. A 

slaty cleavage is well developed in the most graphitic beds. Alternation of minerals usually produces a faint banding.

The mineral composition and grain are remarkably uniform. Thin 

laminae of fine-grained quartz about one-twentieth of an inch in 

thickness alternate with laminae composed mainly of very thin, wavy 

scales of brown biotite standing obliquely to the banding. Here and 

there the quartzose bands show minute fragments of plagioclase
GENERAL GEOLOGY. 17

feldspar. With the mica are commonly black streaks of amorphous, 

grainless carbon. Carbon also forms distinct bands or is uniformly 

distributed like a fog throughout the rock.

The slate area extending along the seacoast from a point 1-| miles 

east of York to the Kanauguk Valley is different from that described 

above. Here for a distance of 3 miles the slate beds strike N. 15° 

to 20° W. and stand on end or are overturned to the west. They are 

disturbed by several normal faults striking N. 20° W., with the 

downthrow on the west. The beds are also interrupted by intrusive 

greenstones.

The bedding of the slate in this area is distinct. Beds of dark-gray 

sandy slate from 3 to 8 inches thick alternate with beds of dark-green 

to blac^ fine-grained slate of about the same thickness. The quartzose beds are distinctly cross-bedded, and cross beds indicate that the 

west side of the beds is the top of the formation.

CONTACT METAMORPHISM.

Near granite contacts the slate is changed to a dense, hard finegrained brown rock resembling certain rhyolites. In other respects 

it is unchanged except that the carbonaceous matter has been aggregated into tiny brown jagged, rodlike forms.

A characteristic of the slate rarely seen in the limestone is an 

abundance of thin quartz stringers, generally parallel to the bedding. Pyrite cubes are also very abundant in the slate, probably 

because of its high carbonaceous content. Locally, as in the coast 

section between Kanauguk River and York, pyrite constitutes about 

3 per cent of the volume of the slate. Here the pyrite is almost 

invariably embedded in a shell of white vein calcite. The pyrite in 

its growth did not deform the slate materials but has replaced them.

CONDITIONS OF DEPOSITION. °

A hypothesis that would satisfactorily explain the conditions of 

deposition of the slate must take; into account its uniformity, grain, 

mineral composition, bedding, lamination, areal extent, and thickness. Little is known regarding the last two points.

The grain and composition of the slate indicate that originally 

three-fourths of it was composed of grains less than 0.05 millimeter 

in diameter, of a dominantly sandy nature. The remainder was 

clayey and carbonaceous matter. The clayey portion has recrystallized into brown biotite. The sandy material is dominantly quartz, 

of irregular outline, with occasional unaltered plagioclase fragments.

It is evident that these materials represent the smallest particles 

of dissolved rock that can be transported by sluggish streams or 

currents. Winds did not carry them, else the quartz grains would 

have been rounded. The evenness of the lamination or bedding
18 GEOLOGY OF THE YORK TIjST DEPOSITS, ALASKA.

Indicates conditions of quiet water, such as prevailed below the 

wave zone of large seas or lakes, or on the delta flood plains of large 

streams.

The materials were largely the fine-grained end products of rock 

decay, chiefly quartz and clay. The conversion of the clayey portion 

into brown biotite indicates that originally it had a considerable content of iron, magnesium, and potash, characteristic of clays leached 

under conditions of good drainage in climates humid and warm 

enough to permit chemical decomposition. The occasional presence 

of a fresh plagioclase feldspar grain, it is believed, does not exclude 

the probability that the clays came from regions where the rocks 

were decayed. Plagioclase often resists weathering very well, as is 

seen in some thin sections of partly weathered granite and is also 

shown by its presence in some limestones to the exclusion 01 ortho-

 clase.

The carbon of the slate may represent organic, most probably 

vegetable matter, that grew in the waters in which these muds were 

laid down, or it may have been brought down with the streams that 

transported the muds. The carbonaceous matter is exceedingly 

fine grained and amorphous and belongs to an age when tree 

growths probably did not exist. Hence, even if the material was

 carried from the land, coarse fragments of organic matter could 

not be expected. Under present conditions the streams of the Arctic 

region have access to the largest contemporary surface deposits of 

organic matter known, the Arctic tundra, and organic material is 

present in both solid and insoluble forms in all the rivers except 

those of exceedingly barren areas. Available analyses show that

 streamy draining tropical swamps have the highest content of dissolved organic matter, but analyses of Arctic streams are not included in the comparison. It appears that with the facts at hand 

the possibility can°not be excluded that some or all of the organic 

material of the slate in the York region may have been carried with 

the silts.

The thin lamination of the slate caused by alternations of quartzose, micaceous, and organic material presents an interesting problem. It is difficult to believe that such an alternation of layers of 

tvlmost microscopic thickness is due to a rain of fine particles from 

a succession of clouds of these different materials carried horizontally over the area of sedimentation. It is more likely that these 

alternating layers of sandy silt and black clay represent laminae once 

suspended as alternating strata in the waters from which they came.

Barus 18 found that fine tripoli suspended in pure water was stratified and that each stratum sank at a uniformly accelerated rate

18 Barus. Carl, Subsidence of fine solid particles in liquids: U. S. Geol. Survey Bull. 

36, 1886.
GENERAL GEOLOGY. . 19

with respect to the one overlying. This experiment suggests that 

if two or more different fine-grained substances were in suspension 

each would stratify into discrete or possibly overlapping strata, 

thus forming alternating layers of different composition on reaching the bottom. The experimental facts would seem to indicate that 

the conditions most favorable to stratification in suspension are deep, 

quiet, fresh waters. In experiments the flocculating effect of salt 

water has prevented the stratification of suspended materials. It 

is suggested that the fine lamination of the slate in the York region 

may be due to xthe deposition of suspended strata. Furthermore, 

the possibility that certain finely laminated clays of fresh-water 

glacial lakes may be due to the deposition of suspended strata rather 

than the result of seasonal contributions, as is generally believed, 

seems worthy of consideration.

The suggestions made regarding the deposition of the slate from 

materials stratified in suspension does not apply to the slate area 

on the coast between Kanauguk Eiver and York. Here the thickness of the alternating layers and more specifically the cross-bedding 

of the quartzose layers show that they were laid down from materials swept along by currents.

PRE-ORDOVICTAN BASALT AND GABBRO.

The pre-Ordovician basalt, gabbro, and olivine gabbro are fine to 

medium grained compact rocks which were intruded as dikes, sills, 

and stocks into the slate before the limestone was laid down. They 

were folded with the slate and limestone and in general show a 

marked schistosity due to the development of nearly parallel columns 

of secondary hornblende. Intrusions of this kind, rarely large, the 

stocks being as a rule less than half a mile in diameter, are very 

abundant throughout the slate area. They are remarkably alike in 

grain, color, composition, and alteration and undoubtedly came from 

one parent magma.

The topography of the slate area is strongly influenced by the intrusives. The sills of basalt and gabbro form hogback ridges whose 

gentle slopes are parallel to the dip of the sill. The stocks form 

cone-shaped hills resembling an array of'low volcanoes. Through 

these forms the slate area acquires a distinctiveness which does not 

escape even the casual observer.

LIMESTONE.

The limestone underlies two distinct areas separated by slate 

the York Mountains and, in the extreme western part of peninsula, 

vCape Mountain.
20 GEOLOGY OF THE YOBK TIN DEPOSITS, ALASKA. 

LIMESTONE OF THE YOEK MOUNTAINS.

The character and succession of the limestone beds of the York 

Mountain area is poorly known. Numerous faults disturb the beds 

to an unknown degree; hence it is impossible to estimate their thickness. Studies of the limestone beds at the mouth of Lost River, in 

the canyon of California River, 6£ miles above the mouth of this 

stream, at the head of the Mint River valley west of Brooks Mountain, at the northern border of Black Mountain, and on the western 

flank of the York Mountains, show that they comprise at least three 

lithologic members. The lowest, about 100 feet thick, consists of thin 

laminae of black slate alternating with thin beds of gray calcitic 

limestone. The middle member, about 900 feet thick, consists of 

thin-bedded gray to chocolate-colored microgranular limestone with 

gray.to brown shaly partings. The top member includes thickbedded pure dark-gray to light-gray limestone in beds 1 to 3 feet 

thick and in places containing small irregular nodules of black chert. 

Between the middle and top members there is commonly a gradational phase. The thickness of the top member is uncertain, but is 

probably about 1,000 feet. This threefold classification of the limestone beds of the York Mountains is not extended to the entire area 

but only to the localities specified^above.

The limestone of the York Mountains contains very little quartz. 

Some exceedingly fine-grained quartz occurs in the clay beds of the 

middle me(mber. Nearly all the carbonate ia calcite. Dolomite 

debris was found on a branch of Don River, about 400 paces south of 

the fossil locality indicated on Plate I. Knopf also found some 

dolomite at the head of Lost River. Considering the Ordovician ago 

of at least a part of these rocks this scarcity of dolomite is notable in 

view of the emphasis which has sometimes been placed upon the supposed dominance of dolomite among the early Paleozoic and more 

ancient calcareous sediments.

LIMESTONE OF CAPE MOUNTAIN.

Limestone extends from Baituk Creek to Cape Mountain and is 

well exposed in cliffs 400 feet high along Bering Sea. The mapping 

of the limestone and slate contact in this area is by no means complete. A hurried survey up the Baituk showed that the border of 

the limestone area is not straight as mapped but that irregular 

patches of limestone occur in the slate. (See PI. I.) Topographically the limestone area of Cape Mountain, in marked contrast with 

that of the York Mountains, is a feebly undulating plateau about 

400 feet above sea level except in the vicinity of the mountain.

In composition the limestone beds of Cape Mountain are generally 

pure gray to white granular marbles. At Cape Mountain they are
GENERAL GEOLOGY. 21

interstratified with white quartzite beds a few inches thick occurring at intervals of 10 feet or more. On top of Cape Mountain some 

sericite-quartz schists are associated with the limestone. The limestones are dominantly calcitic, but the original presence of some 

magnesium is suggested by the secondary development of phlogopite 

and tremolite.

Fossils found in the limestone beds north of Cape Mountain (see 

PL I) include doubtful forms resembling corals of the genera 

Syringopora and Litliostrotion, like those 'in the upper Mississippian rocks at Cape Lisburne, and probably of the same age. It ia 

not proved that all the limestone beds of this area are upper Mississippian. At the head of Ocean Creek the limestones overlie an exposed section of black slate 100 feet thick, and both formations dip 

about 10° N. The lower 20 feet of the limestone is thin bedded 

and contains black graphitic partings. Above this is 50 feet of thickbedded gray limestone. The position and character of these limestone beds is like that of the lowest beds of the limestone of the 

York Mountains, and they may be of the same age.

It is evident from this that some of the limestone beds of Cape 

Mountain overlie the slate, apparently with concordant bedding. 

Some of them, however, are in fault contact. Fault relations were 

found at the head of the first east branch of Baituk Creek going upstream from the sea. Here the limestone is faulted downward along 

an east-west vertical plane against the greenstone intruded into the 

slate.

The relation of the limestone of York Mountain to that of Cape 

Mountain is not clear. They are separated by about 16 miles of 

black slate, all of which appears to belong to the same formation. 

Both groups of limestone beds overlie the slate apparently with 

concordant bedding. The basal beds of both consist of alternating layers of black slate and limestone. Apparently the only marked 

differences known between the two areas are that the limestone 

of Cape Mountain has so far yielded only Mississippian fossils and 

contains thin beds of white sand. Sand beds of this kind have not 

been found in the limestone of the York Mountains. In the York 

Mountain area Lower and Upper Ordovician marine fossils have 

been found in a number of places,, whereas Mississippian fossils 

have been reported only from a locality north of Brooks Mountain. 

It looks as if the basal beds in the Cape Mountain area are the 

same as those of the York Mountains, and future studies may prove 

that the two areas include the same group of limestone beds, their 

absence in the intervening area having been caused by erosion and 

structural disturbances.
22 GEOLOGY OF THE YOEK TI2ST DEPOSITS, ALASKA.

STRUCTURAL RELATIONS OF THE SLATE AND LIMESTONE.

It has been pointed out that along the slate and limestone contact in both the York Mountain and the Cape Mountain areas beds 

of black slate dip under the limestone, with no apparent difference 

in inclination. Away from the contact the attitude of the slate 

beds is about the same as that of the limestone. The lowest beds 

of the limestone in both areas have the character of gradational 

beds from the slate to, the limestone. These facts suggest that 

the deposition of the slate was followed by that of the limestone 

without an intervening period of erosion.

The conformity of the slate and limestone would appear to fit all 

the known facts except the nature of the basalt and gabbro intrusives. These rocks are very abundant in the slate within a few 

feet of the limestone beds, but none of them enter the limestone. 

Their schistosity and the crenulations of thin sills show that they 

were folded with the limestone and slate beds.

The relations of the slate and limestone may be interpreted in two 

ways. They may be conformable, and before they were folded, the 

basalts and gabbro may have been intruded and reached a level 

within a few feet of the limestones. This is improbable. Basic 

lavas are more mobile than acidic lavas, and acidic intrusions passed 

through the slate and into the -limestone at a later time.

A better interpretation is to assume that the slate was intruded 

by the basic igneous rocks and then without having been folded 

to any marked degree was eroded to a nearly level surface, after 

which it was covered by limestone beds. On this interpretation the 

apparent gradational character of the basaj. limestone is a coincidence rather than a sequel. The deposition of limestone without a 

sandy or gravelly base unconformably on older surfaces has its 

counterpart in other parts of the world.

CONTACT METAMORPHISM.

Where affected by the granite intrusive^ the limestone shows 

changes of two kinds (1) changes that can be attributed to the expulsion of the carbon dioxide by heat and the recrystaUization of theremaining constituents into heavy, complex silicates including wollastonite, tremolite, grossularite, vesuvianite, hedenbergite or augite, 

and phlogopite; (2) changes resulting in the development of minerals all or most of whose constituents have been introduced, including cassiterite, wolframite, scheelite, paigeite, hulsite, tourmaline, 

danburite, topaz, humite, chondrodite, scapolite, fluorite, hornblende, 

green mica, zinnwaldite, muscovite, plagioclase, and the sulphides 

stannite, stibnite, molybdenite, arsenopyrite, galena, sphalerite, and 

chalcopyrite.
GENERAL GEOLOGY. 23

The changes of the first kind are regional, being in some areas 

noticeable for miles from the visible granite contact. The changes 

involving the introduction of materials are local and as a rule are 

pronounced only within 20 feet of the granite contact. To a marked 

degree they probably extend no farther than about 500 feet from the 

contact, as the limestone was very susceptible to reaction with the 

solutions that caused the changes.

FOSSILS AND AGE.

Fossils are rare in the limestone, but in a few localities certain 

species have been found in great abundance. The fossil localities 

found by the writers as well as those found by earlier explorers are 

all shown on Plate I (and the age of the strata is indicated). The 

collections of Carboniferous fossils were examined by George H. 

Girty, and those of early Paleozoic fossils by Edwin Kirk. In view 

of certain changes in paleontologic correlation, Mr. Kirk's classifications are slightly different from those of earlier workers. The fossil 

collections previously assigned to the Upper Cambrian he regards as 

Lower Ordovician, and in this determination C. D. Walcott and E. 0. 

Ulrich concur. Most of the Silurian of earlier correlations he places, 

in the Upper Ordovician, but he identifies one lot of fossils, from the 

Port Clarence limestone in the vicinity of Don River, as " Silurian, 

probably middle to Upper Silurian."

The location and character of the beds from which fossils werecollected by the writers are set forth below.

Lot S. From doloniitic debris on west bank of Don Kiver, elevation about 

200 feet. Debris from outcrops of buff thin, even-bedded, fine-grained chertydolomite, with fine calcite veinlets. Strike east; dip 35° W.

Lot Jj. From debris on north bank of tributary of Don River, elevation 

about 200 feet. Dense, compact, fine-grained rock with conchoidal fracture.. 

Has, many small, irregular calcite spots and small clayey lenses. Does not 

break parallel to the bedding. Slightly cherty and in spots has irregular chert 

masses about 6 inches across.

Lot 5. From ledges on sea cliffs 2 miles west of the mouth of Lost-Riveiv 

Beds dip 30° N. and strike east. Fossils from beds 130 feet above sea. Fossiliferous beds about 15 feet thick, thin bedded. Full of small brachiopods.

Lot 6. From talus on southeast slope of Brooks Mountain, elevation about 

1,000 feet. Thin-bedded, flgggy Port Clarence limestone, showing fucoid markings. Collection made here because Triassic had been reported from this place.-

Lot 7. From Anigovik River, elevation about 300 feet. Limestone and slate 

debris. Slates show a few doubtful brachiopods; the limestones some fucoid 

markings.

Lot 8. From limestone d6bris on Banner Creek.

Lot 9. From dark-gray medium-grained limestone talus north of CapeMountain, elevation about 300 feet.

Lot 10. From chert d6bris from sea cliffs between Ocean Creek and Bailuk. 

Creek, elevation 75 feet. Source uncertain but is near by.
24 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

Lot 11. From d6bris from limestone cliff 2£ miles east of mouth of Lost 

River, elevation about 100 feet.

Lot 12. From rock outcrops on east bank of California River, elevation about 

175 feet. Grayish-blue fine-grained calcitic, dense limestone with uneven conchoidal fracture. Beds 6 inches to 6 feet thick. Difficult to split parallel to' 

the bedding. Strike N. 60° W.; dip 30° N.

Lot 18. From limestone outcrops on ridge extending westward toward the 

Lost River mine, from the highest peak between Tin and Cassiterite creeks, 

elevation about 1,800 feet. Strike of limestone N, 40° E., dip about 30° N. 

Thin, even-bedded limestone with shaly partings, weathering yellow. Contains 

numerous Maclurea.

Mr. Kirk's report on this collection, excluding lots 9 and 10, which 

are from the Carboniferous limestone of Cape Mountain, follows:

I have the honor to submit, the following report on paleontologic collections 

made by Messrs. Steidtmann and Cathcart in the Seward Peninsula, Alaska, 

transmitted under date of December 12, 1918. Two lots of fossils from the 

area brought back by E. M. Kindle not' previously reported on have been included. All previously collected material has been reexamined, and in various 

places in the present report comments will be made in regard to them.

LOWER OBDOVICIAN.

The upper Cambrian faunas as hitherto reported from the Seward Peninsula 

are referable to the Ordovician, as the line is now drawn in North America. 

It will be noted that the Finkelnburffia, sp. determined by Ulrich was stated 

to be " indistinguishable from a species occurring in the Roubidoux formation of 

the Cambrian in Missouri." The Roubidoux is now referred to the Lower 

Ordovician (Beekmantown). The determination of the age of Kindle's collections 7 and 8 rests on brachiopods of a type common in both the Upper 

Cambrian and the Lower Ordovician. The shell structure of the Seward 

Peninsula brachiopods is fibrous, and this character is now held to be characteristic of Ordovician types and wanting in the Cambrian. The evidence, then, is 

for an assignment of the collections to the Ordovician. 

Loc. 5. Sea cliffs 2 miles west of mouth of Lost River. 

Eoorthis cf. E. wimani (Walcott). 

Trilobite fragment.

This collection carries the same brachiopod as Kindle's loc. 9. 

Loc. 12. East bank of California River: 

Maclurea sp. 

Hormotoma sp. 

Endoceras sp. 

Ptychopyge sp. 

Asaphus sp.

Loc. 11. Cliff 2* miles east of the mouth of Lost River: 

Gastropod suggesting Ophileta. 

Fucoids.

The following lot carries two poor gastropods. Their age is certainly Ordovician, and probably Beekmantown. 

Loc. 13. Ridge extending easterly from Lost River mine: 

Maclurea? sp.
GENERAL GEOLOGY. 25

UPPER OBDOVICIAN.

As reported by Ulrich in Kindle, two post-Beekmantown Ordovician faunas 

are indicated in the region, one of Black River or Trenton age and the other 

of Richmond age. Similar faunas are known throughout the Rocky Mountains 

as far south as El Paso, Tex., and a notable development of the beds occurs 

in Manitoba. Evidence accumulated in recent years throughout the Rocky 

Mountain area has tended to disprove the Black River or Trenton correlation of 

the beds formerly given this age assignment. Available evidence points to the 

Fernvale (Richmond) aj?e of these beds, as typified by the lower massive Bighorn 

dolomite of Wyoming. It is conceivable, however, that the fauna can not accurately be correlated with Mississippi Valley formations and may represent a 

gap in the stratigraphic section as known east of the Rocky Mountains. It 

were best, therefore, to call this fauna Upper Ordovician, rather than Richmond. 

I have recast the faunal lists as given by Kindle for his 13a and 13b. It will 

be noted that the changes are chiefly by way of making determinations less 

exact. Some forms have been dropped as irrelevant. In 13a Maclwrina> mamtobensis Whiteaves has definitely been determined and is of high importance. 

It supersedes Maclurea Ugsbyi Hall of Ulrich's list. A detailed study of the 

material has shown that the Seward Peninsula forms are not specifically identical with Middle Ordovician types and in most cases will probably prove new 

species or varieties.

So far as can be determined from the material on hand Kindle's 13a and 13b, 

as well as Steidtmann and Cathcart's lots 3 and 4, represent the same general 

faunal zone. As noted above, they should be listed as Upper Ordovician rather 

than Richmond. 

Kindle's 13a:

Streptelasma sp.

Halysites cf. H. gracilis Hall, several varieties.

Galapoecia cf. canticostiensis Billings.

Columnaria alveolata Goldfuss, two varieties.

Strophomena cf. Strilobata (Owen).

Triplecia cf. T. extans Hall.

Leptaena sp.

Dinorthis sp.

Hebertella sp.

Rhynchotrema sp.

Modiolopsis cf. M. faba Hall.

Maclurina manitobensis (Whiteaves).

Stenotheca? sp.

Lophospira sp.

Hormotoma cf. H. gracilis Hall.

Hormotoma sp.

Trochonema sp. *

Bumastus sp.c 

Kindle's 13b.

Columnaria alveolata Goldfuss, 2 varieties.

Calapoecia cf. C. anticostiensis Billings.

Lichenaria sp.

Halysites gracilis Hall var.

Stromatocerium? sp.

Nicholsonella? sp.

Leptaena richmondensis Foerste.

101987 22 -3
26 GEOLOGY OF THE YOEK TIN DEPOSITS, ALASKA.

Kindle's 13b Continued.

Hebertella sp.

Ilhynchotrema cf. R. capax Conrad.

Ctenodonta cf. C. levata Hall.

Cyrtodonta sp.

Liospira sp.

Lophospira sp.

Trochonema cf. T. umbilieatum Hall. 

Lot 3. West bank of Don River.

Columnaria (Palaeophyllum) cf. C. stokesi Ed-wards and Haime.

Halysites gracilis Hall var.

Dystactospongia? sp.

Hebertella sp.

Maclurina uianitobensis (Whiteaves).

Hormotoma sp.

Endoceras sp. 

Lot 4. North bank of tributary of Don River.

Halysites gracilis Hall.

Streptelasma sp.

Stromatocerium? sp.

Calapoecia anticostiensis.

Columnaria alveolata Goldfuss.

Favosites sp.

Dystactospongia? sp.

Hormotoma sp.

Trochonema sp.

Maclurina sp.

SILURIAN.

As quoted by Kindle, Schuchert-'determined middle Silurian from the Seward 

Peninsula. The locality is given as "near the forks of Rock Creek, a northern 

tributary of Agiapuk River." The -material was collected by Collier.

As redetermined this fauna is clearly of Upper Ordovician age and correlates 

exactly with the faunas on Don River.

INDETERMINABLE.

Lots 6, 7, and 8 carry nothing but fucoids or annelid trails. No age determinations are possible.

Two lots of fossils have been found in the Survey collections that have not 

hitherto been reported on. One is from the north slope of Brooks Mountain 

and the other from Kanauguk River. The latter was collected by Kindle and 

the other given him by a prospector.

Kanauguk River, near junction of south and middle forks: 

Orthoid brachiopods.

Asaphus? sp., apparently same as loc. 12, California River, and of 

Canadian age.

A lot of fossils from Brooks Mountain not previously reported on has been 

found in the Survey collections. The locality is given as "north side of Brooks 

Mountain, Seward Peninsula." The collector was Philip Asth, as written on 

the label by Kindle. The label accompanying the Triassic fossils reported from 

the southeast side of Brooks Mountain gives the collector as Philip Ast, evidently the same man. Kindle in his paper on the Port Clarence limestone gives
GENERAL GEOLOGY. 27

the collector's name as Peter Esch for the latter lot. The collection from the 

north-side of the mountain consists of fragments of some branching cyathophylloid coral. The preservation is poor, but the fossils could not be older than 

late Paleozoic. Considering the Paleozoic section of this general area it would 

appear that this material could not well represent anything other than Mississippian. This would tend to strengthen the claim of Triassic rocks elsewhere 

on the mountain.

The collections of Mississippian fossils from the region were restudied by Mr. Girty, and the results are embodied in the following 

report:

I have reexaniined the collections mentioned in your letter of March 11, 1919, 

and in searching for those collections I came upon several others made by Mr. 

Hess, apparently from the same area. All of these are covered in the following report.

Before giving a list of the faunas collected I must comment on the preservation of the fossils, which is such as to render any inference based upon them 

unusually precarious. The fossils are all' corals, other types, if present, being 

entirely unrecognizable. The corals apparently are pseudomorphs or have been 

so completely altered as to resemble pseudomorphs of crystalline calcite. They 

stand out conspicuously in the black rock which incloses them, but when examined more closely they are found to have lost all trace of the structures 

upon which generic and specific distinctions are largely based. The only evidence of this sort still remaining is to be observed where the specimens show 

the contact of the matrix with the growing ends of the corals into whose calices 

the black mud had penetrated.

The specimens referred to Syringopora show the typical mode of growth of 

that genus, slender corallites connected by supporting stolons. In a few instances the generic reference suggested by this mode of growth is corroborated 

by traces of the funnel-shaped tabulae. The discrimination of the species 

adopted in my lists is based largely on the size of the corallites and partly on 

their mode of growth.

The fossils referred to Zaphi'eiitis are few in number, and the generic reference as against related types, as for instance Triplophyllum, can not be insisted upon.

The forms referred to Lithottrotion exemplify both the bushy or brandling 

mode of growth (species 1 and 2) and the massive (species 3). Some of them 

show the presence of numerous thin septa and of a central columella, such 

being the structures as exhibited in the. calice. The species, as I have discriminated them in my lists, are distinguished largely on the size of the corallites instead of on the number of septa and other structural characters, the 

usual method being here impossible. Obviously nothing was to be gained by 

trying 'to identify species separated in this manner with described species, 

most of the criteria on which the described species were founded being unknown. Not all of the specimens referred to Lithostrotion show the septa and 

columella, the presence of which suggests the generic reference, and it is 

possible that quite diverse forms have been included in the same species. For 

instance, some of the large Syringoporas as identified in the collection may, 

in fact, be small Lithostrotions. This fact does not, however, vitiate the evidence of other specimens whose characters are less imperfectly shown.

Personally I have scarcely a doubt that these fossils represent the same 

coral fauna that occurs in the Lisburne limestone, upper Mississippian, as 

against the Devonian coral faunas or the very deceptive Triassic coral fauna
28 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

found in this region. This belief, however, is not capable of scientific demonstration on the material at hand, and my statement must also be modified 

to this extent, that inasmuch as most of the specimens are from beach gravels 

and as many do not show even the significant structures observable in some 

of them, the collection may contain besides the Lisburne fauna stray specimens that had their origin in other horizons. The following lists will serve 

to show the composition of the several local faunas considered here. 

Lot 2499. Beach near Cape Mountain, Seward Peninsula, Alaska:

Syringopora sp. 4.

Lithostrotion sp. 2.

Lithostrotion sp. 3.

Zaphrentis sp.

Lot 2500. Beach northwest of York. (Six specimens labeled " T. 5 A H 46 

to 51"; one label reads " 6 miles northwest of York.")

Syringopora sp 1. 

. Syringopora sp. 2.

Syringopora sp. 4.

Lithostrotion sp. 1.

Lithostrotion sp. 2.

Lot 2501. Beach east of Tin City, York tin region, Alaska. (Four specimens 

labeled "T. 5 A H 41 to 44 "; one label reads " 1 mile east of Tin City.")

Syringopora sp. 1.

Syringopora sp. 2.

Syringopora sp. 3.

Syringopora sp. 4.

Lot 2502. North side of valley north of North Star mine, Cape Mountain, 

York tin region, Alaska:

Syringopora sp. 4. 

Lot 2503. North side of Brooks Mountain, Seward Peninsula, Alaska:

Lithostrotion sp. 1. 

Lot 7139. Northeast of Kingegan Mission, Cape Prince of Wales, Alaska:

Syringopora sp. 1.

Syringopora sp. 3.

Zaphrentis sp.

Lithostrotion sp. 2.

Mr. Kirk kindly undertook a reclassification of the pre-Carboniferous fossils collected prior to 1918, and his results are given 

below. The notes regarding the location of the fossil beds were 

inserted by the writers and were compiled from the memoranda of 

the collectors. The localities from which the collections restudied 

by Messrs. Girty and Kirk were taken are shown on Plate I by their 

respective permanent collection numbers.

The following lots could not be found: Collier 1900, 3 AC 136, and Kindle 

lot 11. The two latter lots are both probably in Dr. Girty's collections. Lot 

1 AC 77 as found consists merely of an indeterminable thin section on a 

glass slide, with which nothing can be done. 

Collier 1900. Not found.

287. 1 AC 26. (2 miles southwest of forks of Don River.) 

Favosites sp. 

Pentameroid brachiopod.

Of Silurian age, probably middle to upper Silurian, in place of lower 

Silurian.
GENEKAL GEOLOGY. 29

288. 1 AC 28. (Mountain 4 miles north of Rapid River.)

Badly silicifled sponges. This type of sponge strongly suggests Lower 

Ordovician forms, and I think this age assignment would be warranted, in place of middle of lower Silurian.

289. 1 AC 45. (Don River 4 miles north of Tozer Creek.) 

Maclurina manitobensis (Whiteaves). 

Columnaria alveolata Goldfuss. 

Stromatocerium? sp. 

Halysites cf. H. gracilis Hall. 

Upper Ordovician in place of middle of lower Silurian.

290. 1 AC 76. (Boulder from gravel, Nuluk River.) 

Trilobite fragments.

Lower Ordovician, in place of middle of lower Silurian.

291. 1 AC 77. (Bluff above Nuluk River, latitude 65° 4', longitude 166° 20'.) 

Indeterminable thin section.

292. 1 AC 78. (Foot of talus slope, Nuluk River.)

Cephalopod siphuncle, probably referable to Endoceras. 

Lower Ordovician, in place of middle of lower Silurian.

293. 1 AC 185. (Pebble from sand spit 1 mile north of Teller.) 

Ecorthis wimani Walcott.

Lower Ordovician, in place of middle of lower Silurian. 

3 AC 136. (1 mile east of Cape Prince of Wales.) Not found. 

281. 3 AC 146. (Rock Creek, tributary to Agiapuk River.)

Upper Ordovician, in place of Niagaran or the middle third of the 

Silurian.

276. Washburne, 1904. 4 AW 55. (Merril Mountain, 3 miles north-northeast 

of mouth of Lost River.)

Lower Ordovician.

828. Knopf, 1907. (Head of Cassiterite Creek, Lost River.) 

Indeterminable gastropods.

Lower Ordovician. 

Lot 11, Kindle. (North side of Cape Mountain.) Not found.

876. Lot 6. (450 yards southeast of mouth of Lost River, near top of cliffs; 

15 feet of dark-gray thin-bedded limestone.)

877. Lot 7. (Near head of south branch of Tin Creek at its junction with a 

ravine from the east. Hard blue-gray limestone stratum, a few inches in thickness, which lies just above beds with numerous fucoid-like markings.)

878. Lot 8. (A single thin band of limestone. Elevation 2,000 feet, at head 

of north fork of Tin Creek.)

879. Lot 9. (A single bed of limestone on Cape York.) 

1365. Lot 12. Kindle.

The foregoing five lots (6, 7, 8, 9, 12) are lower Ordovician (Beekmantown). These are the lots formerly identified as Cambrian. Ulrich's

generic and specific determinations stand.

235. Lot 13a, Kindle. (Don River, at side of short ravine heading on slope 

of a mountain marked 1242 and joining the Don 9 miles from the coast.)

236. Lot 13b, Kindle. (Immediately north of the mouth of the ravine from 

which 13a was taken.)

Lots I3a and 13b. Upper Ordovician.

CONDITIONS OF DEPOSITION.

The purity of most of the limestone beds, the presence of corals in 

some, and their local interstratification with thin beds of clean white
30 GEOLOGY OF THE YOKK TIN DEPOSITS, ALASKA.

quartz sand indicate that probably most of them were deposited in 

shallow, warm seas. That conditions in these seas were uniform for 

a long time is inferred from the great thickness of the beds. The 

abundance of black carbonaceous matter in the lowest limestone beds 

raises the same questions called forth by the black slates as to 

whether the carbon is detrital or whether it was organically precipitated in place and in either case what were the conditions of its 

deposition.

The thick-bedded purer limestone of the upper part of the Port 

Clarence formation indicates that the seas had become clearer and 

the conditions of deposition more uniform than they were when the 

underlying beds were deposited. The reason for this change is not 

clear. At least four conditions could have given rise to it; (1) the 

area of sedimentation may have become farther removed from land 

masses by a shift in the position of the strand line, (2) the surface 

of the lands from which the detritus came may have been lowered 

either by erosion or by earth movements so as to make stream and 

shore action less vigorous, (3) the streams may have carried less 

detritus because of a climatic change, (4) the ocean currents carrying detritus may have changed their direction.

GRANITE, ASSOCIATED DIKES, AND TIN ORES.

Small stocks of granite have intruded the limestone at five places 

in the region Cape Mountain, Brooks Mountain, Tin Creek, Black 

Mountain, and Ear Mountain. The presence of one stock underneath the tin prospect on Cassiterite Creek and of another under 

Potato Mountain is inferred. These stocks, except the one at Cape 

Mountain, are associated with faults that are older than the granite.

The invasion of the granite seems therefore to have been influenced 

by planes of weakness in the crust.

In form the stocks appear to be steep on all sides except one. The 

Tin Creek stock is not well enough exposed to give any suggestions as 

to its shape. None of them are deeply eroded in fact, all have parts 

of the roof remaining.

The composition of all the stocks is that of a normal granite. Orthoclase is the chief constituent. The plagioclase feldspar is sodic, and 

the ferromagnesian mineral is biotite, except in a 3-foot border zone 

of the granite at Brooks Mountain which contains hornblende in place 

of biotite. Except at their borders the granites at both Brooks Mountain and Cape Mountain are porphyritic, the orthoclase forming 

prominent phenocrysts. Their border phases are even grained. The 

two granites can not be distinguished in hand specimens. The granites of Tin Creek, Black Mountain, and Ear Mountain are even 

grained and resemble one another closely.
GENERAL GEOLOGY. 31

The granites of Tin Creek and Black Mountain are closely associated in time and place with certain quartz porphyry dikes. Those 

of Brooks Mountain and Ear Mountain are similarly associated with 

dacite porphyr}' dikes containing limy plagioclase. The Dolcoath 

dike, between Brooks Mountain and Tin Creek, is also of the latter 

type. ,The granite of Cape Mountain is not accompanied by porphyry dikes but has even-grained granitoid offshoots which are like 

it in composition. It is also closely related in age to certain granite 

pegmatite dikes. A granite pegmatite dike was also found at Brooks 

Mountain.

After the injection of the granites of Cape Mountain, Ear Mountain, Brooks Mountain, and Tin Creek and some of the dikes associated with them, hot tin-bearing solutions containing boron, fluorine, and other elements invaded the border of the granite and the 

adjoining limestones and dikes. They gave rise to tin-bearing replacement and vein deposits in granites, dikes, and limestones. The 

only commercial tin mineral in these deposits is cassiterite, but paigeite, hulsite, and stannite have been found. Tin has not been found 

at Black Mountain. An area on Cassiterite Creek was invaded by 

tin-bearing solutions that formed deposits in the dikes and limestone. Tin-bearing solutions, which are inferred to have come from 

a granite intrusion below, invaded the slates of Potato Mountain.

The granites are believed to have come from a common deep-seated 

source, because of their similarity in chemical and mineral composition and their common association with nearly contemporaneous 

quartz-bearing dikes, including rhyolites and dacites, and because 

both granite and dikes,, with the possible exception of those at Black 

Mountain, were followed by tin-bearing solutions. The strongest 

argument for their consanguinity seems to be their relation to the 

tin-bearing solutions.

AMYGDALOIDAL OLIVINE BASALT.

The amygdaloidal basalt comprises black fine-grained and glassy 

rocks with a scoriaceous structure. The only visible minerals which 

it contains are tiny green grains of olivine. The microscopic texture is diabasic to vitrophyric. The diabasic types contain labradorite and augite. The others have a glassy base containing microscopic rods of labradorite and perfect crystals of olivine.

These rocks occur as dikes at Cape Mountain and as flows at Black 

Mountain and. east of California River. They are the youngest 

igneous rocks of the region. The flows at Black Mountain were 

formed when the topography was essentially the same as at present.

SEQUENCE OF IGNEOUS INTRUSIONS.

The York tin region has had three distinct periods of igneous 

activity. In the first period basalt and gabbro dikes, sills, and
32 GEOLOGY OF. THE YORK TIN DEPOSITS, ALASKA.

stocks were intruded into the slate before the limestone was deposited. The second period came after the deposition of the limestone 

and is divisible into three epochs, during which occurred (1) the 

intrusion of the granite stocks, (2) the injection of rhyolite and 

dacite dikes, (3) the expulsion of hot solutions containing tin, boron, 

and fluorine. In the third period dikes and flows of basic olivine 

basalt were formed, which in composition resemble the products of 

the first period. They are young, having solidified when the topography of the region was about the same as it is <to-day.

STRUCTURE.

The structure of the region is that of a shallow north-south basin 

upturned at the east and west ends, and the whole tilted northward 

on the average about 20°. Superimposed on the major syncline are 

minor cross and longitudinal folds of varying magnitude. Two systems of normal faults disturb the beds of the region. In the main 

one system strikes roughly east, dips steeply to the south, and has 

the downthrow on the south; the other strikes roughly north, dips 

steeply to the west, and has the downthrow on the west. Evidence of 

overthrust faulting has also been found in the Lost River and Black 

Mountain areas. (See pp. 53, 112.)

In the central part of the area the underlying rocks are the beds 

of the Port Clarence limestone, which except in a few places dip 8°- 

45° N. The two systems of faulting alluded to have numerous representatives in the Lost River area. The truncation of a fault block 

has exposed the slate at Black Mountain and in the Agiapuk Valley, 

to the east. At Brooks Mountain slate is exposed by the erosion of a 

fold complicated with faulting. Details are given in the descriptions 

of these areas.

Along the western flank of the York Mountains the black slate 

beds dip to the east, under the limestone. Exposures of the contact 

were not seen. The dip of the slate seems to be conformable with 

that of the limestone. It averages about 30° and reaches a maximum 

of about 60° on Kanauguk River. Minor folds pitching eastward 

are superimposed on the general eastward monocline. Synclinal 

folds of this kind cause certain projections of limestone into the 

slate area, and the anticlines have produced certain embayments of 

slate extending into the limestone.

An exception to this close relation of folds to undulations in the 

limestone border is found in the upper part of the Skookum Creek 

valley. Here the slate forms a deep reentrant in the limestone. 

Along the northern border of the valley the slate dips northward 

at an angle of about 30° under the limestone, but along the southern 

border of the valley it dips eastward at a steep angle which should
GENERAL GEOLOGY. 33

cause its rapid disappearance under the limestone. There is 

evidently a marked discordance in the dip of the slate beds that 

crop out along the two sides of the valley. The extent of the 

slate area is not related to the dip of the limestone and slate beds or 

to the topography. Faulting therefore appears to have disturbed 

the beds along a zone running about N. 70° W., parallel to the 

Skookum Creek valley. Inspection of Plate I shows that this suggested line of faulting is in alinement with a zone of faulting that 

runs nearly due east along the head of the Mint River valley and 

with another zone of faulting that marks the southern border of 

the slate area east of Black Mountain.

Evidence of faulting along a nearly north-south line was noted 

at one place on the western flank of the York Mountains between 

Kanauguk Kiver and Skookum Creek. As shown on Plate I, this 

locality is marked by the close proximity of a basalt intrusive to the 

Port Clarence limestone. The proof of faulting lies in the vertical 

plane of contact between the slate beds and basalt intrusive and the 

limestone beds.

The slate beds in the area west of the York Mountains dip mainly 

to the east and northeast. Along Bering Sea between Kanauguk 

Kiver and York the slate beds are vertical or dip to the east because 

of their having been overturned. In the western part of the slate 

area near the mouth of Baituk and Ocean creeks most of the slate 

beds dip gently to the north. A normal fault was observed near 

the head of the first east branch at the mouth of Baituk Creek. The 

fault has brought the basalt and slate into vertical contact with 

limestone beds. The trend of the fault appears to be east, and the 

displacement was relatively downward on the north side of the fault 

plane. Normal faults striking N. 20° W., with relative downward 

displacement on the west side, were also noted in the section between 

Kanauguk and York rivers..

The limestone beds near the slate contact in the vicinity of Baituk 

Creek show a variable dip, being very steep in some places and nearly 

horizontal in others. West of the mouth of Baituk Creek they dip 

steeply to the west. Some outcrops up Baituk Creek and Ocean Creek 

dip gently toward the north. Knopf speaks of friction breccias in 

the limestone of this vicinity and also alludes to the variable dip 

of the beds as evidence of faulting along a zone parallel to the border 

of the limestone and slate areas.

Along the seacoast, between Baituk Creek and Cape Mountain, the 

limestone beds generally dip about 45° E. and in places are vertical. 

Along the eastern border of the granite of Cape Mountain the limestone beds dip westward into the granite. North of the granite they 

dip northward, away from the granite.
34 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

East of Mountain Creek, to the north of Teller, the Port Clarence 

limestone is bordered by black micaceous schist which is intruded by 

schistose basic igneous rocks. As these intrusions are schistose and 

apparently do not invade the limestone, it is inferred that they are 

older than the limestone. The schistosity of the intrusives precludes 

the possibility that they may have come in after the schist was exposed 

by erosion.

The schist in the vicinity of Mountain and Bay creeks appears to 

belong to the same formation as the slate of the York region. This 

correlation, however, does not include all of the great schist area 

extending eastward from Teller. The proposed correlation of the 

schist east of Mountain Creek and the slate of the York region is 

based on the following grounds:

1. Both schist and slate are stratigraphically below the Port 

Clarence limestone.

2. They are similar lithologically, the only difference consisting 

in the more intense regional metamorphism of the schist.

3. Both are intruded by basic igneous rocks which show about the 

same degree of metamorphism, are similar lithologically, and bear the 

same relations to the limestone.

The hills west and north of Mountain Creek are barren, steep, and 

rugged, being underlain by the Port Clarence limestone, which in this 

locality is generally thin bedded, clayey, buff colored, and contorted. 

Outcrops are scarce. A few frost-shattered exposures indicate that 

the dip is northward. The high hills east of Mountain Creek are less 

rugged and are covered with tundra. Numerous outcrops along Bay 

Creek show that the inclination of the schist is to the north.

At the head of Mountain Creek the surface of the limestone area

lies flush with the surface underlain by schist. It is evident from the 

exposures of schist and limestone in this vicinity that the schist if 

projected downward would meet the limestone beds along a vertical 

or nearly vertical plane, as the limestone beds dip in the same northerly direction as the schist. Mountain Creek therefore seems to mark 

the location of a zone of faulting. As the schist is stratigraphically 

below the limestone, the faulting is inferred to have depressed the 

limestone beds with respect to the schist.

TOPOGRAPHIC HISTORY.

At least three, and possibly four, epochs of erosion due to successive uplifts of the region are suggested by the Nuluk Plateau, the 

York Plateau, and the recently developed lowlands. The Kougarok 

Plateau is less clearly developed within this region, but is said to be 

well defined farther east.
GENERAL GEOLOGY. 35

The Nuluk Plateau 19 is indicated by the even crests of the York 

Mountain ridges, the tilted and faulted strata of which are beveled 

to a markedly uniform elevation of 2,000 to 2,500 feet. The flat tops 

of Cape Mountain and Ear Mountain, which have an elevation of 

2,300 feet, would seem to be remnants of the same plain. This oldest 

plain of erosion of the region has been thoroughly dissected and has 

been deformed by later differential uplift. (See PL III.)

The York Plateau 20 is best developed in the slate area west of the 

York Mountains, at an elevation of about 600 feet. It is further 

shown in the terrace that caps the sea cliff fronting Bering Sea and 

extends eastward to Don River. The terrace is cut on the Port 

Clarence limestone, the highly tilted and faulted strata of which have 

been beveled with marked uniformity. It is from 1 to 4 miles wide 

and occurs at an elevation of 500 to TOO feet. On the north side of the

York Mountains it is present as a system of benches which extend into 

the mountains along the stream channels. Between California and 

Agiapuk rivers it is marked by a plateau surface at an elevation of 

500 feet. This plain extends up the Agiapuk and is correlated with 

the terrace of the upper Nuluk, which in turn is correlated with the 

terraces on the north side of the York Mountains. Evidence of the 

plain is found in the low hills surrounding Ear Mountain at an elevation of 500 feet.

Since its elevation the York Plateau has been well dissected. It 

has a mild relief of 200 to 600 feet, which is in marked contrast with 

the rugged limestone ridges of 2,000 to 2,500 feet forming the York 

Mountains.

At an elevation of about 1,000 feet there is evidence of an old 

erosion surface which was first named the Kugruk Plateau. 21 It is 

marked in this area by benches on Cape and Ear mountains.and is 

said to be well developed in the region adjoining to the east. Collier 10 

speaks of it as " a well-marked plateau surface extending * * * 

across the Kugruk [now Kougarok] River and continuing in a 

system of benches and table-topped hills to Cape Prince of Wales. 

* * * Its relation to the Nuluk Plateau is best seen a few miles 

oast of the Nuluk River, where a prominent bench, at 1,000 feet 

elevation, extends for several miles and encircles one of the mountains on which the Nuluk plain is well marked at 1,600 feet."

The recent lowlands comprise valley and coastal plains. The 

valley plains are covered by unconsolidated Pleistocene deposits.

10 Collier, A. J., A reconnaissance of the northwestern portion of Seward Peninsula, 

Alaska; U. S. Geol. Survey Prof. Taper 2, p. 35, 1902.

20 Brooks, A. I-L, and others, Reconnaissances in the Cape Nome and Norton Bay regions,. Alaska, in 1900, p. 52, U. S. Geol. Survey Spec. Pub., 1901.

21 Idem, p. 54.
36 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

The coastal plains are covered in part by Pleistocene and Recent 

deposits and in part consist of country rock.

Practically all the larger streams have, along their lower channels, 

well-developed gravel terraces, many of which extend well toward 

the headwaters. This feature is most marked on Agiapuk, Lost, 

Kanauguk, Anikovik, Nuluk, and Mint rivers and on Grouse Creek 

and its tributaries. Few of the streams, except the headwaters, flow 

on bedrock. The stream gravels range from a few feet to 20 feet or 

more in depth.

The coastal-plain areas north of Port Clarence and along the 

Arctic Ocean are partly constructional, as is shown by the barrier 

beach, lagoon, and lake features. A considerable part of the fill has 

been in the form of peat. Extending throughout the coastal-plain 

area are low mounds of bedrock which have been degraded almost 

to the plain level. South of Lopp Lagoon the York Plateau merges 

almost imperceptibly into the coastal plain, so that the two features 

are difficult to separate. Along Bering Sea, however, the coastal 

plain is sharply differentiated from the York Plateau and the sea 

terrace.

MINERALOGY. '

The following is a list of minerals known to occur in the York 

region, with a brief outline of their chemical composition and of 

those physical properties which the prospector may use in their identification.

Amphibole. A group of minerals, chiefly silicates of calcium, magnesium, and 

iron, which occur commonly as gangue minerals but which in themselves have 

no economic value. Tremolite, actinolite, and hornblende are members of the 

group observed in the rocks of this region. Tremolite is a colorless variety and 

occurs as glistening slender crystals in the limestone adjacent to the mineralized 

part of the cassiterite dike and as fine radial groups in the limestone of Cape 

Mountain. It is a calcium-magnesium silicate. Actinolite, an iron, magnesium, 

and calcium silicate, is green and occurs as fine, slender prisms forming veins 

in the slate of Potato Mountain. Hornblende is of more complex composition, 

is green to black, and is the most common variety of amphibole. It is found in 

limestone adjoining cassiterite veins and in places colors1 the limestone green. 

As a group, the amphibole minerals may be recognized by the angle which the 

prism faces make with each other 56° or 124°. Similar angles on pyroxenes, 

with which amphibole is easily confused, are 87° or 93°. Amphibole can not 

be scratched with a knife. Its color may vary from white through .brown and 

green to black. Amphibole crystals are usually long and slender as compared 

with crystals of pyroxene, which tend to be short and stout prisms.

Apatite. Essentially calcium phosphate. Widely distributed as a minor cou> 

stituent of granite and some other rocks, but rarely in sufficient quantity to be 

of economic value as a source of phosphate. In this region it occurs as a 

microscopic constituent of the. granite and is of no commercial value.

Arsenopyrite. A compound of iron, arsenic, and sulphur. Silver-white, has 

a metallic luster, and can not be scratched with a knife. Thin splinters fuse 

slowly in a luminous flame, and the arsenic fumes liberated have the odor of
GENERAL GEOLOGY. 37

garlic. The fused mass is magnetic. It is rather abundant in the Cassiterite 

Creek tin ore and also occurs at Brooks and Potato .mountains. Its usual mineral associates are ores of silver, lead, and tin, also pyrite, chalcopyrite, and 

sphalerite.

Aximte. Essentially a calcium, boron, and aluminum silicate. An uncommon mineral occurring with tourmaline at Ear Mountain. The crystals are 

ax-shaped, and its luster is highly glassy. The Ear Mountain material is brown, 

although the mineral may be blue, yellow, or greenish. It can not be scratched 

with a knife. The thinnest of edges fuse slowly in the lamp flame to a black 

mass and may give a greenish color to the flame.

Azurite. Azure-blue copper carbonate. Can be scratched with a knife. In 

places occurs with the green copper carbonate malachite as an incrustation on 

rocks. When treated with nitric acid it effervesces (bubbles), and if to the 

nitric acid solution some ammonia is added a blue solution results. It is 

formed by the alteration of the sulphide ores of copper, and is commonly found 

on or near the surface. It occurs in the wolframite-topaz lode on Lost River, 

where it is probably derived from the alteration of stamnite in the ore.

Biotite. A potassium, magnesium, iron, and aluminum silicate, commonly 

called black mica. Occurs in thin scales which can easily be picked apart with 

a knife point. The flakes are both flexible and elastic. It is a common minor 

constituent of many igneous rocks, in which it occurs as small lustrous flakes. 

Some biotite has been found in a prospect on Village Creek at Cape Mountain, 

where it occurs in a pegmatite vein, but this variety of mica is without economic 

value. Muscovite and phlogopite are white or light-colored micas which may be 

of value where found in pegmatite veins such as that on Village Creek. The 

value of such a deposit depends on the abundance and the quality of the mica, 

and commercial deposits are rare. Biotite occurs in the granite and schist and 

in pegmatite on Brooks Mountain, in plates as much as half an inch in size, and 

at Cape Mountain in plates measuring as much as several inches.

Caloite. Calcium carbonate, the essential constituent of limestone. Locally 

occurs in veins and possibly mistaken for quartz, from which it is easily distinquishable by its softness. Can be scratched with a pin. Gives off gas bubbles 

when treated with cold dilute acid. Is widely distributed and of no commercial 

value in this region.

Cassiterite. Tin oxide, the only commercial source of tin. The pure crystallized mineral contains 78.6 per cent of tin. It is best distinguished by its weight, 

having a specific gravity of 6.8 to 7.1. It can not be scratched by a knife. The 

color may vary from white through gray, yellow, and brown to black. The luster 

of a freshly broken surface is brilliant. It occurs in the York region in quartz 

veins, in marginal portions of granite, and in limestone and porphyry dikes near 

granite intrusions, but its best-known occurrence is that as stream tin on Buck 

Creek. When in placer form its weight serves well to identify it, but frequently 

it is confused with other materials, among which iron oxide is the most common. 

A simply performed and reliable test for Cassiterite is as follows: Using preferably a shallow dish, cover a mineral particle one-quarter to one-half inch in 

size jvith dilute hydrochloric acid, and add a small quantity of granulated zinc. 

Reaction between the zinc and hydrochloric acid will be vigorous, and at the end 

of two or three minutes the mineral, if Cassiterite, will be coated with a white 

incrustation of metallic tin, which may be polished by rubbing the particle on a 

dry cloth.

Cerusite. Lead carbonate. White to gray. Easily scratched with knife. 

High specific gravity. Commonly occurs as groups or bundles of highly lustrous crystals. Effervesces with acid. Fuses easily in the lamp flame. It
38 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

occurs in connection with other lead minerals and is formed from galena, the 

sulphide of lead. Found at the Yankee Girl prospect, southwest of the Tin 

Creek granite boss.

Chalcopyrite. A copper-iron sulphide called copper pyrites. It is easily confused with pyrite, or " white iron," which it resembles in color. Pyrite is light 

brass-yellow; Chalcopyrite is deep brass-yellow. Pyrite can not be scratched 

with a knife; Chalcopyrite can. Pyrite may occur in cubes or octohedrons; 

the crystal form of Chalcopyrite is seldom recognizable. Pyrrhotite, another 

mineral with which Chalcopyrite may be confused, is bronze-yellow and is an 

iron sulphide, is slightly magnetic, and will not react for copper. Chalcopyrite 

was found on Brooks Mountain at Read's prospect. It dissolves in nitric acid, 

and if ammonia is added to the acid solution a blue solution results, indicating 

copper, while the iron forms a brownish precipitate in the solution. The mineral is important as an ore of copper where it occurs in sufficient quantities. 

It is known to occur in this region near the mouth of Tin Creek and in the 

contact limestone of Ear Mountain.

Chlorite. Iron, magnesium, and aluminum silicates with water closely 

related to the micas in many respects; characterized by greenish color and a 

mica-like structure. The thin flakes of chlorite can be separted like mica 

flakes and are flexible but not elastic. They occur in this region in the greenstones of the slate area and are secondary minerals resulting from the the 

alteration of iron-magnesium silicates, such as pyroxene, amphibole, and biotite. 

Chlorite has no commercial value.

Chondrodite. Magnesium silicate with hydroxyl and fluorine. Small honeyyellow crystals of chondrodite occur in the contact-metamorphose limestone of 

Brooks Mountain.

Danburite. Calcium-boron silicate. A rare mineral which occurs as a constituent in the gangue of cassiterite in the Dolcoath lode, on Cassiterite Creek. 

Wine yellowish to yellowish brown. Luster greasy. Can not be scratched with 

knife but will scratch glass. Of no commercial importance.

Dolomite. Calcium-magnesium carbonate. Very similar to calcite in physical properties but a trifle harder. Dolomite will not effervesce in col4 dilute 

acid; calcite will. Certain strata of the Port Clarence limestone are composed

of dolomite. It occurs also in small crystals in the slates of the York area. 

The mineral is of no economic importance.

Epidote. Essentially calcium, iron, and aluminum silicate, differing from 

zoisite only in the iron present. Radial groups of epidote crystals occur at 

Brooks Mountain near the granite contact. It is characterized by its peculiar 

yellowish-green color. Is prismatic in form, and many of the prisms are furrowed or striated. Can not be scratched with a knife. Powder white or grayish. Of no economic importance.

Feldspar. A group of minerals which are very abundant in igneous rocks. 

The more common members of the group are as follows:

Orthoclase_____________.Potassium-aluminum silicate. 

Plagioclase:

Albite_____________.Sodium-aluminum silicate.

Oligoclase_________________

Andesine___________ Sodium, calcium, and aluminum silicate.

Laboradorite_ .

Anorthite____________.Calcium-aluminum silicate.

Orthoclaae feldspar occurs as the principal constituent of the acidic igneous 

rocks, such as granite, quartz porphyry, and syenite; the plagioclase feldspars 

prevail in the more basic rocks, such as greenstones, gabbros, and diorites.
GENERAL GEOLO.GY. 39

Both may be present in the same rock, but to distinguish the one from the other 

by simple methods is not always possible. They are readily distinguished on 

microscopic examination. As a group the feldspars are of economic importance 

only in that they give information concerning the type of magma which produced them, and in a general way they may indicate what valuable minerals 

might be expected to be associated with them. The potash occurring in orthoclase is not of economic value because of the difficulty in separating it from the 

silica with which it is combined. Feldspars have smooth surfaces, with a glasslike luster, and are white or pale yellow to flesh-colored. They can not be 

scratched with a knife. Good examples may be seen in the larger prominent 

crystals of the granite of Cape, Brooks, and Ear mountains or in the granite of 

Tin Creek.

Fluorite. Calcium fluoride occurs in the York region chiefly as a gangue 

mineral with the tin ores of Cassiterite Creek and Ear, Cape, and Potato mountains. Usually it is present as cubic crystals. The color may be white, yellow, 

blue, red, or green. Can be scratched with a knife, but not with a piu. Luster 

like that of glass. Usually breaks easily, giving flat surfaces. Can be distinguished from calcite by not effervescing in boiling acid and by its hardness.

Galena. Lead sulphide. Commonly occurs in cubes, but may be granular. 

Galena in cubes is easily recognized by its form and high specific gravity. Can 

be scratched with a pin. A splinter fuses slowly in a lamp flame. Color of 

freshly broken mineral and of the powder lead-gray. Bright metallic luster of 

freshly broken material becomes dull after long exposure. All galena carries 

more or less silver, but no external characteristic serves to distinguish the 

kinds that carry much silver from those that carry little. The value of lead ore 

in this region depends almost entirely upon its silver content. Galena is one of 

the most widely distributed of the metallic sulphides. It occurs in beds and 

veins in rocks of all sorts. It is in many places associated with pyrite, sphalerite, chalcopyrite, and arsenopyrite in a gangue of quartz, calcite, fluorite, etc. 

Cerusite is locally a surface alteration product of galena. Galena is also common with gold and in veins of silver ores. In this district it occurs at Brooks 

and Ear mountains and on Lost and Rapid rivers.

Garnet. A group of minerals of which grossularite (calcium-aluminum silicate) is the most common in this region and is usually found in the altered 

limestones bordering on igneous masses. Garnets may occur in good crystals 

showing diamond-shaped or irregular four-sided faces. They can not be 

scratched with a knife, are brittle, and have a glassy luster. The commonly 

called " rubies " of sand are red garnets rounded by stream action. Grossularite 

as found in this region is yellow and where only a part of the crystal is exposed 

may be mistaken for cassiterite, but the equidimensional crystals, diamondshaped faces, and lower specific gravity should serve to distinguish. Before accepting a mineral as cassiterite the acid and zinc test described under cassiterite 

should be applied.

Gold. Occurs in places throughout the region.

Hematite. Nuggets of the red oxide of iron occur with the stream tin at 

Buck Creek. Being heavy and dark in color, it is sometimes mistaken for 

cassiterite. If a particle of hematite is crushed, the powder resulting is reddish brown, even though the compact mineral is black. The same test can be 

made by rubbing the mineral on a piece of o unglazecl porcelain, when a red 

streak will result. Hematite is not magnetic. The nodules of magnetic iron 

occurring with stream tin give, when treated as above, a black powder or a 

black streak.

Hulsite. A rare boron-tin mineral known to occur at Brooks Mountain. It 

is black, has a metallic luster, and can be scratched with a pin. As it is rare
40 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

and contains but 20 per cent of tin oxide it will probably have no value as an 

ore of tin.

Ilmenite. Iron-titanium oxide. Occurs as an abundant microscopic constituent of the pre-Ordovician basalts and gabbros of the slate area. It is not 

known to occur in crystals of recognizable size in the York region. Resembles 

hematite, but gives a black powder or streak; not magnetic like magnetite. 

Used in the manufacture of ferrotitanium, but not of great economic importance. Occurs as an objectionable constituent of some magnetic iron ores. No 

commercial deposits known in the York region.

Kaolinite. Aluminum silicate with water. Occurs as an alteration product 

of aluminous minerals, especially the feldspars of the granite and porphry 

dikes. It is abundant In the decomposed portion of the Cassiterite dike and 

elsewhere in the region, although not in the form of a deposit that might be 

of economic value. It has a soapy feeling, and when breathed upon gives a 

strong odor of clay. Can be scratched with a pin.

lAmonite. The yellow-brown hydroxide of iron. Very common as an alteration product of other minerals containing iron, such as pyrite, magnetite, 

pyroxene, amphibole, and mica, when they are exposed to the action of air and 

surface waters charged with carbonic and organic acids. Numerous varieties 

are known, some of which have a nearly black varnish-like exterior. It may 

occur as soft yellow earthy material or may be so hard and compact that a 

knife point will hardly scratch it. In all varieties the powder of the crushed 

mineral or the streak is brown. It occurs commonly on the surface of ore 

bodies, forming the gossan, as in the case of the lead ores of Lost River. Lake 

deposits in the form of bog ore are common. Bodies of commercial importance 

are not known in the York region.

Ludwiffite. Iron magnesium borate. A finely fibrous dark-green mineral 

occurring in the contact limestone of Brooks Mountain. Of no economic value. 

Magnetite. Magnetic oxide of iron. Nuggets are fairly common in the stream 

tin of Buck Creek. Distinguished from associated minerals by its weight, black 

powder, and magnetism occurs in contact-metamorphic limestone of Brooks 

Mountain and Tin Creek. As'found in the region it is of no economic value.

Molybdenite. Molybdenum sulphide. Occurs in small amounts associated 

with the cassiterite ores at Cassiterite Creek. It is lead-gray and has a bright 

metallic luster. When rubbed on paper it leaves a blue streak. Can be 

scratched with the finger nail. Commonly occurs in flakes, scales, or plates, 

which are flexible but not elastic. May be fine granular. Resembles graphite 

in color and form, but graphite leaves a black streak. Molybdenite usually 

occurs distributed through debris of granite, crystalline limestones, porphyry, 

and pegmatite. No occurrence of economic importance is known in the York 

region.

Muscovite. White mica, essentially potassium-aluminum silicate. Its physical properties are the same as those described for biotite except the color. 

Mucovite occurs in certain phases of the granite of Ears Mountain and is the 

chief mica in the quartz porphyry dikes. Muscovite is the most common of the 

micas. It is very common in mica schists and not uncommon in certain granites. 

The largest and best-developed crystals occur in pegmatite dikes associated with 

granite. It is from such dikes that the mica of commerce (isinglass) is usually 

obtained. No deposits of commercial importance are known in this region.

Olivine. Iron-magnesium silicate. Occurs as a microscopic constituent of 

basalt dikes and flows. Can be recognized by its yellow-green color, glassy luster, and granular form. Can not be scratched with a knife. Of tno economic 

importance.
GENERAL GEOLOGY. 41

Paigeite. A rare boron-tin mineral found at Brooks and Ear mountains. 

Lustrous, coal-black, foliated. Can be scratched with a pin. Contains about 1? 

per cent of tin oxide. Not important as an ore of tin.

Phloffopite. A mica very similar to biotite in composition but containing less 

iron. Has all the physical propertes of biotite except color. Phlogopite is 

usually light brown or brownish red, with a copper-like reflection. It is commonly found in crystalline limestone or dolomite. Occurs in the limestone of 

Cape and Brooks mountains.

Pyrite. Iron sulphide, called iron pyrites or white iron. A very common and 

widely distributed mineral in the York region. Easily recognized by its light 

brass-yellow color and its crystal form. Usually occurs as cubes or octohedrons. Can not be scratched with a knife. Distinguished from chalcopyrite 

by its greater hardness, crystal form, and paler color. A common associate of 

the tin and lead ores of the region. Found in the placers and very abundant in 

large crystals in some of the slates. It is of no economic importance as found 

here. Pyrite is not an ore of iron. Great deposits of it are mined and used in 

the manufacture of sulphuric acid.

Pyrolusite. Manganese oxide. Iron-black to bluish black. Powder black. 

Luster dull. Can be scratched with the finger nail or a pin. Soils the fingers. 

Differs from magnetite usually in hardness and in not being magnetic. Occurs 

in the York region as dendrites, or plantlike figures, in the joints of the limestones at Cassiterite Creek. The source of the manganese is probably wolframite, which is iron-manganese tungstate and which on decomposition yields 

manganese to the circulating waters, to be deposited in the limestone fissures.

Pyroxene. A. group of silicates, chiefly of calcium, magnesium, and iron. 

Common constituents of igneous rocks. Of no commercial value. The color 

ranges from white to black, but green and brown pyroxene are most common 

in this region. Powder gray. Can not be scratched with a knife. Prism faces 

on pyroxene crystals are practically at right angles, thus differing from the 

120° angles of the prism faces of amphibole. Pyroxene is sometimes confused 

with cassiterite but is much lighter and will not respond to the acid and zinc 

test outlined under the description of cassiterite. Augite, a brown variety of 

pyroxene, occurs in a quartz porphyry dike at Ear Mountain. Hedenbergite 

occurs in the contact limestones of Cape Mountain associated with tin, with 

which it has been confused.

Pyrrhotite. Magnetic iron sulphide. Resembles pyrite and chalcopyrite but 

can be distinguished from them by its bronze color, by its hardness, and by 

being slightly magnetic. Is often mistaken for stannite.in the tin deposits but 

differs in its magnetic property. Can be scratched with a knife. Luster is 

metallic. Powder is dark grayish black. Uusally granular. It is intimately 

mixed with nickel minerals at Sudbury, Ontario, in the largest nickel deposit 

known. Occurs in the York region at Brooks and Cape mountains and at Tin 

Creek.

Quartz. Silicon dioxide. Very abundant in the region as stringers or veins 

and as a constituent of the granites and porphyry dikes. Clear crystal quartz 

or milky quartz is easily distinguished from any other mineral which it resembles by its hardness and by its lack of cleavage that is, of smooth surfaces 

along which it will break. Can not be scratched with a knife and will scratch 

glass. Calcite is much softer. Feldspar has good cleavage. Quartz crystals 

are six-sided. Smoky quartz is common in the region and has been mistaken 

for cassiterite.

Rutile. Titanium oxide. Occurs as a microscopic constituent of the granite 

at Cape Mountain. May be yellowish, brown, or black. Black rutile might be 

101987 22 4
42 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

confused with cassiterite, but cassiterite is heavier, and the hydrochloric acid 

and zinc test for tin will distinguish them. Used chiefly in manufacture of 

ferrotitanium. No deposits of economic importance known in York region.

Scapolite. Sodium, calcium, and aluminum silicate containing chlorine. Resembles feldspar in appearance and is best distinguished by microscopic 

methods. Occurs at Ear and Cape mountains in contact-metamorphic limestone 

near the granite masses. Of no economic value.

Scheelite. Calcium tungstate, containing 80.6 percent of WO3 . Occurs as a 

microscopic constituent of altered limestone at Cape Mountain and in cassiterite veinlets and their adjoining altered limestone at Cassiterite Creek. The 

occurrence is not of economic importance. Scheelite is usually yellowish to 

brownish white and has a glassy luster. It is heavy, comparing well in weight 

with cassiterite. Powder white. Can be scratched with a knife. A simple chemical test is as follows: Treat the finely powdered mineral with boiling hydrocholiric acid. A yellowish solution results, which on the addition of a little 

granulated zinc or tin and further boiling turns blue.

Sphalerite (zinc blende). Zinc sulphide. Yellow to brown or black. Luster 

resinous. Black sphalerite is known at Brooks and Ear mountains, where it is 

distinguished from wolframite with difficulty. It differs from wolframite 

in being softer. A knife will scratch sphalerite but not wolframite. Sphalerite 

occurs in both igneous and sedimentary rocks, commonly as an associate of 

galena; also with chalcopyrite, pyrite, siderite, fluorite, and other minerals. 

It is an ore of zinc but is not known to occur in commercial quantities in this 

region.

Spinel. Essentially magnesium aluminate. Different varieties contain iron, 

manganese, and chromium. Occurs as small black octahedrons in the contact 

limestone at Head's galena prospect, Brooks Mountain, where it is associated 

with chondrodite. Is not of economic importance.

Starmite ("tin pyrites "). A sulphide containing copper, tin, iron, and usually 

zinc. Contains 29.5 per cent of copper and 27.5 per cent of tin. Color may 

vary from steel-gray to yellow to brownish black. Can be scratched with a 

knife. Powder blackish. Luster metallic. Occurs at Lost River associated 

with galena and wolframite and is brown-black. Stannite is difficult to recognize, and its identity must be determined by chemical methods. It is not

important as a source of tin.

Stilnite. Antimony sulphide, commonly referred to by prospectors as antimony, but contains only 71 per cent of antimony. Color lead-gray, tarnishing 

to black. Can be scratched with finger nail and cut with a knife. Fuses in a 

match flame. Crystals usually elongated and commonly in radiating groups. 

Found at head of Tin Creek in a vein with calcite and on Bessie and Maple 

claims, west of Lost River.

Topaz. Aluminum-flourine silicate. Commonly white or light yellow, but 

may be blue, green, or red. Luster glassy. Can not be scratched with a knife. 

Will scratch quartz. Powder white. Occurs in prismatic crystals. Commonly found in rocks of the granite type in veins and cavities. May be accompanied by fluorite, cassiterite, tourmaline, etc. In this region it is abundant as a constituent of tin ore at Lost River and occurs in veins in limestone 

and as a contact-metamorphic mineral in limestone adjoining granite intrusives. 

It is commonly associated with fluorite and zinnwaldite. Has no economic 

value except as a gem stone. Only minerals having gem qualities can be 

used for that purpose.

TourmaUne. A complex silicate of boron and aluminum, with either magnesium, iron, or the alkali metals prominent. Crystals prismatic, often slender 

and strongly marked by fine lines. The cross section of the prism may be
GENERAL GEOLOGY. 43

three, six, or nine sided. Can not be scratched with a knife. Luster brilliant. 

Color usually black, but may be brown or greenish. Powder white. Best distinguished by triangular shape of its prisms, hardness, and coal-like fracture. 

Tourmaline is very common throughout the York region as an associate of 

tin ore, but its presence does not necessarily indicate tin, as it is found in 

many places where tin ore does not occur. It is abundant in the granite of 

Cape and Ear mountains and in the adjoining limestone. The quartz stringers 

of Potato Mountain carry tourmaline. At Brooks Mountain and Lost River 

it is abundant. Although in places resembling cassiterite, it is less than half 

as heavy. If comparison of the specific gravities of the two minerals is impossible the hydrochloric acid and zinc test for cassiterite should be made. 

Tourmaline has no economic value, except in rare specimens as a gem stone.

Vesuvianite. Essentially calcium-aluminum silicate. One of the commonest 

contact-metamorphic minerals in the region. Crystals are square prisms. Can 

not be scratched with a knife. Color brown to green. Luster glassy to resinous. Powder white. Thin edges and very fine splinters are rounded in the 

lamp flame, which serves to distinguish it from garnet, tourmaline, and cassiterite, for which it might be mistaken. Vesuvianite occurs on Brooks Mountain 

and at Tin Creek. It is usually of some hue of green, ranging from gray-green 

to brownish green. It is of no comraeercial Importance.

Wolframite. Iron-manganese tungstate containing 76.4 per cent of WCk 

Crystals are commonly tabular but may be prismatic. Prism faces are lined. 

Can be scratched by a knife with difficulty, but is easily scratched by quartz.. 

Color usually brownish black. Powder nearly black. It is about as heavy as 

cassiterite. Wolframite is an ore of tungsten and is in many places associated 

with tin ores; it also occurs in quartz, with scheelite, pyrite, galena, sphalerite, 

etc. It is associated Avith cassiterite at Cassiterite Creek. It can be distinguished from cassiterite by its hardness and in this locality by the shape of 

its crystals, most of which are elongated, slender prisms. Wolframite can be 

distinguished from black sphalerite by its greater weight and hardness. It 

is not so easy to prove the presence of WO3 in wolframite as it is in scheelite, 

owing to the difficulty of getting wolframite into solution. The best results 

are obtained by first fusing the mineral with three parts of sodium carbonate 

on charcoal and then treating with acid. However, it is usually possible to 

get some material into solution by boiling with hydrochloric acid. Addition of 

zinc or tin to the clear solution and boiling turns it an intense blue.

Wollastonite. Calcium silicate, a variety of pyroxene. Color white, gray, or 

yellow to brown. Powder white. Can be scratched with a knife. Luster 

vftreous to pearly. Occurs usually in granular limestone at granite contact. 

It occurs on Cape Mountain in proximity to the granite, locally forming white 

masses 3 feet or less in thickness. Of no economic value.

Zinnwaldite. A mica, essentially a lithium, iron, and aluminum silicate, commonly associated with topaz, fluorite, and other minerals. In physical properties resembles muscovite but is yellowish to brown. It is a common constituent of the cassiterite veins of Cassiterite Creek. Of no economic value.

Zircon. Silicate of zirconium. Occurs as a microscopic constituent of the 

granites of the York region. Not distinguishable in hand specimens as known 

to occur here, and of no economic importance. Zircon is commonly found in 

square prisms. Color yellow, grayish, or brownish. Will scratch glass, also 

quartz. Very high luster. Powder white.

Zoisite. Essentially calcium-aluminum silicate. Occurs in small amounts in 

the altered limestones of Ear Mountain. Crystals are prismatic and deeply. 

striated or furrowed. Can not be scratched with a knife. Luster glassy. 

Color gray, yellowish, or brown. Powder white. Of no economic importance.
44 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

LOST RIVER AREA.

GEOGRAPHIC FEATURES.

Lost River rises on the south slope of Brooks Mountain, whence 

it flows southward into Bering Sea, 9 miles distant. Most of it's 

tributaries enter from the west; of these, Rapid River, 9 miles in 

length, is the longest. In the valley of Rapid River is a trail and 

a wrecked telephone line to York and Cape Mountain. The tributaries from the east are short and carry but little water. The area 

of this side of the Lost River drainage basin has been curtailed by 

the growth of many small rival streams flowing southeastward 

into Bering Sea.

Lost River is a swift-flowing perennial stream. Where confined 

to a narrow bed it is about 2£ feet deep and 25 feet wide under 

average conditions, but in the vicinity of Cassiterite and Tin creeks 

it spreads out over flats several hundred feet wide. The water of 

the stream is clear and cold and where stirred by currents has a 

beautiful bluish-green color. It is remarkably free from both vegetable and animal life.

The valley is bordered by steep barren ridges of the York Mountains, about 2,000 feet high, whose slopes range from 25° to 45°. 

Until the middle of August the gulches of these high hills are 

buried in snow and ice and from a distance appear like white 

streaks, gashing the gray of the hills. The distance across the 

valley from ridge to ridge is about 2£ miles, except at the mouth. 

Bordering the stream are low rounded hills having an elevation of 

about 150 feet above Bering Sea. Upstream these hills merge into 

a well-defined bench lying about 30 feet above Lost River at the 

mouth of Cassiterite Creek. Except for scattered patches of mosses 

these low-lying lands are hardly less barren than the high ridges.

Three-eighths of a mile north of Bering Sea the Lost River valley 

is nearly blocked by a ridge half a mile wide, an eroded remnant of 

the great sea terrace referred to on page 35. Through the eastern 

part of this barrier the river has cut a canyon. West of the river 

this ridge merges into a flat a little over-100 feet in elevation and 

about half a mile wide. To the east and west of the lower 3 miles 

of the river valley lies the great sea terrace, about 400 feet high. 

(See PL III.)

South of the ridge that bars the Lost River valley the stream is 

bordered by a low gravelly flat several hundred feet wide, dotted 

with limestone outcrops. This flat is incised into a stream-gravel 

terrace that rises 24 feet above Bering Sea. Nearly 100 feet from 

the strand line the waves have thrown up a gravel bar 15 feet high, 

through which the stream has cut its channel. During great storms 

from the south the waves build a temporary gravel bar across the
LOST EIVEE AREA. 45

mouth of the stream, but the porous debris thus piled up does not 

hold back the water, and shore currents soon destroy the bar. The 

river has no visible delta.

The mouth of Lost River can be reached by trail along the beach 

of Bering Sea, starting on the shore opposite Teller. From Teller 

this beach is reached by means of ferry across a narrow outlet of 

Grantley Harbor. The numerous streams across- which the trail 

passes are not bridged, but under the average summer conditions 

light loads can be taken over this road. Severe northerly storms 

improve the trail by driving the waters away from the shore. From 

the mouth of Lost River the trail continues upstream and branches, 

one branch going westward up Rapid River to York and the cape 

arid the other going northward to Cassiterite and Tin creeks and 

thence across the Lost River and Mint River divide, down the valley 

of the Mint and across a very low divide into the valley of Skookum 

Creek, which leads northwestward into the Potato Mountain country.

During the summer a 40-ton gasoline schooner stops on signal at 

the mouth of Lost River, weather permitting, about once in 14 days 

on its way to and from the Arctic Ocean. Freight is landed,by 

means of small rowboats, the schooner anchoring about a mile from 

land. During times of fog and windy weather the boat commonly 

omits this stop. Larger freight boats from Seattle occasionally 

reach this shore and lighter their cargo by means of barges, which 

come from Teller for this purpose.

GENERAL GEOLOGY. 

SUMMARY.

The Lost River area is underlain by a succession of limestone beds, 

2,000 feet or more in thickness, which generally strike about N. 

70°-80° E. and dip about 25° N. Two systems of faulting disturb 

these beds. The faults of one system strike parallel to the strike of 

the limestone becls, about N. 80° E., and generally dip steeply to the 

south. Those of the other system generally strike parallel to the 

dip of the beds, N. 20° W., and dip steeply to the west. The displacements caused by these faults appear to be mainly of the normal 

type; that is, the overhanging side seems to have moved downward. 

Slickensides on the fault surfaces show, however, that in many places 

the movements have been more complex, having both horizontal and 

vertical components.

After the faulting had occurred the limestone was intruded .by 

stocks of granite and dikes of granite and quartz porphyry.. One 

granite stock on Tin Creek has been exposed by erosion. Another is 

surmised to underlie the locality of the Cassiterite Creek tin prospect.

The dikes were injected mainly along planes having the same attitude as the strike faults, but a few, including one on Camp Creek,
46 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

rose along planes having the same relative position as the dip faults. 

It is probable that all the dikes rose along fault planes, but this inference can not be proved because of the lack of sufficient exposures.

The intrusion of the granite and quartz porphyry was followed by 

the rise of hot solutions carrying tin, fluorine, and boron from the 

granite stocks below. These solutions caused certain alterations of 

the granite, dikes, and limestone, involving both the accession and 

removal of material.

LIMESTONE.

The limestone includes a lower group of thin gray to chocolatecolored layers with grayish-brown and dark clay partings, an upper 

group of pure gray layers from 1 to 3 feet thick, locally with scattered black flint nodules, and a third group of transitional beds between the two. Layers of the lower type are well exposed in the 

sea cliffs 2 miles west of the mouth of Lost River. (See PI. IV, A.) 

The upper beds can be seen in the high ridges between Tin and Camp 

creeks. The transitional beds occur at the Lost River canyon. The

am be

BERING SEA Jo ______-1 I -

-2- &

FIGURE 4. Profile section along west bank of Lost River northward for about 3,600 feet 

from mouth. See text for explanation.

results of faulting in the area being difficult to gage because of the 

lack of distinctive lithologic or fossil markers, it is impossible to 

estimate the thickness of the three limestone groups.

The following notes on a section taken along the west side of Lost 

River from the sea northward about 3,600 feet illustrate the lithology 

and structure of the lower and transitional group of layers. The 

letters refer to positions so marked on the accompanying profile 

(fig. 4).

a. West bank of Lost River at mouth: Pure calcitic microgranular chocolatecolored limestone. Bedding generally obscured by fractures, but locally thin 

bedded. Strike N. 45° E., dip 40° S. Cut by two sets of joints spaced at 

intervals of 1 inch (1) strike N. 10° W., dip 55° W.; (2) strike N. 15° W.t 

dip 60° W. One fault strikes N. 20° W., dips 80° W., and is marked by a 

breccia zone 1 inch wide; the west side has moved upward. Another fault 

strikes N. 20° W., dips 80° W., and shows fine slickenside grooves inclined 

20° S. on fault plane.

6. About 600' feet north of mouth: 10 feet of thin-bedded dark limestone 

with lentils of brown shale. Strike N. 20° W., dip 50° N. Overlain by 10 feet 

of dark pure limestone beds, 6 to 8 inches thick, with rough, hackly surface.

c. About 720 feet north of mouth: 15 feet of dark-brown thin-bedded aphanitic 

limestone, with rough, hackly surface. Strike N. 20° W., dip 15° N. Cut by 

fault that strikes N. 10° E., dips 80° E., and shows slickenside grooves inclined 

5° S.
U. S. GEOLOGICAL SURVEY BULLETIN 733 PLATE IV

A. CONTORTED PORT CLARENCE LIMESTONE AND SKA CUFFS WEST OF LOST

RIVER.

B. SEA CLIFFS AND TERRACE EAST OF LOST RIVER. 

The beach and its welcome driftwood in the forosround.
U. S. GEOLOGICAL SURVEY BULLETIN 733 PLATE V

A. WORKINGS ON CASSITERITE DIKE ON EAST SIDE OF CASSITERITE CREEK (AT 

LEFT) AND ON THE GREENSTONE LODE (AT RIGHT).

B. LODE WORKINGS AND CAMP ON CASSITERITE CREEK.

Looking upstream. The rough surface to the right of the point where the creek disappears, near 

the center of the view, is the "reef rock."
LOST RIVER AREA. 47

d. About 1,520 feet north of mouth, elevation 3 to 60 feet above sea: Dark 

aphanitic brittle limestone in beds 1$ "inches thick, with wavy surfaces and 

brown shale partings a fraction of an inch thick. Strike east, dip 20° N.

e. Overlying d, elevation 60 to 100 feet: Thin-bedded chocolate-colored 

aphanitic limestone like a. Same altitude as d.

f. Elevation 100 to 125 feet: Debris of limestone like a and e.

g. About 1,900 feet north of mouth, elevation 145 to 155 feet: Limestone 

d6bris like f but with thin brown shaly partings.

h. Elevation 155 to 195 feet: Dark-gray granular pure limestone d6bris, with 

vague bedding and rough, hackly surface.

i. About 2,100 feet north of mouth, elevation 195 to 224 feet; Chocolatecolored thin-bedded limestone with hackly fracture.

;. Elevation 224 to 245 feet: Chocolate-colored limestone in thin wavy beds 

with films of light-brown shale.

fc. Elevation 224 to 300 feet: Chocolate-colored limestone with indistinct 

bedding, containing small black irregular flint nodules about 1 inch in diameter. 

Weathered surface of limestone has ash-gray color and is rough from minute 

flint seams.

1. From 2,300 to 2,350 feet north of mouth: No outcrops.

m. Elevation 295 to 300 feet: Dark-colored thin-bedded limestone composed 

of lenses of limestone 2 inches by one-fourth inch, cemented by films of shale.

n. Elevation 300 to 318 feet: Dark-colored pure limestone in beds from 2 to 

8 inches thick, containing black flint nodules which weather white on exposure.

o. Elevation 318 to 370 feet: Alternations of debris of granular dark-gray 

pure limestone and thin-bedded gray to brown shaly limestone. At elevation 

of 270 feet small outcrop of gray shaly limestone, in teds of one-fourth inch thick, 

strike N. 70° E., dip 8° N. To the north a shear zone striking N. 70° E. 

and dipping steeply south.

The character and complexity of the faulting of the Lost River 

area are illustrated by the following tabulated notes of the fault 

observations made near the mouth of Lost River:

Fault observations near the mouth of Lost River.

Locality.

East bank at mouth. . . .

Do.................

West bank at mouth . . . West bank 300 paces

north of mouth.

East bank 28 paces

north of mouth.

East bank 567 paces

north of mouth.

Canyon 932 paces north

of mouth.

Canyon 2,800pacesnorth 

of mouth.

East bank S. 30° W.

from peak farthest

south on Saddle

Mountain.

100 paces south of last 

locality.

Strike.

N.10° W..

N.80°E...

N.20° W..

N.10°E...

East......

N.45° W..

East......

N.80°E...

N.65°E...

N.65°E...

Dip.

60° W. ....

70°-90° S

80° W. . . . .

80° E .....

Vertical . . .

...do......

50° S......

Vertical . . .

35°N.....

80° S......

Remarks.

Several parallel faults. Slickenside grooves on fault

planes dip 25° S. Length, 1} inches. Widest at 

south end. Suggest that movement was upward

toward northwest.

Slickenside grooves on fault planes dip 45° S. Direction

of movement uncertain.

Slickenside grooves inclined 20° S.

Horizontal Slickenside grooves. ^

West side appears to have moved downward.

Several parallel faults; south side hasmoved downward.

South side has moved downward.

Surface of limestone billowy. Billows are 4 to 8 feet

long and 1 foot high. Both ridges and depressions

cut by parallel grooves striking N. 23° \V., 1$ feet

long and about -fa inch deep. Width less than ,» 

inch. Grooves believed to be caused by overthrust

fault.

Down curling of beds on south side of fault indicates 

that south side has moved upward.
48 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA. 

QUARTZ PORPHYRY DIKES.

The quartz porphyry dikes were injected in a molten condition into 

the limestone along planes generally striking N. 60° E. to due east. 

As a rule th'ey are vertical or steeply inclined. The inclined dikes 

dip more commonly to the south than to the north. The planes along 

which the dikes have been injected have the same attitude as those 

along which dislocations are known to have taken place, and it is 

probable that the intrusions followed the planes of dislocation. In 

a few places for example, 400 paces up Camp Creek from its 

mouth thin injections of the dike material took place along a faultplane striking N. 20° W. and dipping steeply west. It seems that 

fault planes afforded the openings through which the molten materials rose toward the surface.

Some of the dikes, such as the Cassiterite dike, have been traced 

continuously for nearly 2 miles; and it is probable that some of them 

are even longer, their, continuity being hidden by slope wash and 

stream deposits. Their width ranges from a fraction of an inch to 

40 feet or more, and averages about 5 feet.

The dike materials appear to have been at a relatively low temperature at the time of the injection, probably less than 800° C., and did 

not release solutions on cooling, for they had no visible effect on the 

limestone at the time of intrusion. Fragments of limestone showing 

no decomposition are found frozen into them. Limestone decomposes 

between 800° and 900° C., at atmospheric pressure and at a much 

lower temperature, about 300° C., in the presence of silica-bearing 

waters. Pressure raises the temperature of decomposition slightly.

The time of intrusion of some of the dikes was later than the solidification of some of the granites, notably that of Tin Creek. A dike 

with chilled margins cuts the granite on Tin Creek, showing that the 

granite had solidified, cooled, and cracked before the dike was injected. That the dikes are not contemporary offshoots of the same 

molten masses as the granite is also suggested by the fact that they 

generally contain muscovite mica, whereas the granite contains biotite.

The color and mineral composition of the dikes are fairly uniform. 

The fresh dike material ranges in color from a light gray to a grayish 

black. The dikes have a fine-grained groundmass, generally not 

identifiable, in which larger grains of glassy colorless or smoky quartz 

having angular or hexagonal forms are embedded. Few of the quartz 

grains exceed 0.2 inch in diameter. Less commonly the dikes contain, 

in addition to the quartz, visible grains of feldspar. Together the 

quartz and feldspar phenocrysts usually constitute only about 15 per 

cent of the total volume of the rock. Colorless micas are an accessory constituent.
LOST RIVER AREA. 49

The dikes form very few outcrops. They can be traced, as a rule, 

by their debris, which mingles with that of the limestone into which 

they were injected. Such fragments rarely exceed a foot in diameter. 

The surface debris marking an unaltered broad dike may rise 1 foot 

or more above the debris of the adjacent limestone. In places decomposed dikes underlie slight depressions a few inches below the bordering limestone. On such dikes is commonly growing a light-gray to 

brownish moss; and, when the surface is dry, they can easily be traced 

by the color contrasts between themselves and the gray limestone 

debris.

GRANITE.

Granite is exposed in two places in the vicinity of Lost River 

north of Tin Creek and on a tributary that enters Lost River from 

the west about 2 miles north of the mouth of Rapid River. At the 

latter place the granite is. on a low hill on the south side of the 

stream about 2 miles above its mouth. It occurs as a group of dikelike intrusions, striking northeast, about 400 paces long and 150 

paces wide. The surface is covered with granite boulders about 1£ 

feet in diameter. Ledges are scarce. The granite is a mediumgrained quartz-feldspar-biotite rock. Several shallow prospect pits 

and trenches have been opened on it, but show no mineralization. 

The limestone adjacent to the granite appears to be unaltered except 

for a slight development of serpentine. A description of the granite 

on Tin Creek is given on pages 74^75, in the section on the Tin Creek 

tin prospect. At the tin prospect on Cassiterite Creek, granite probably underlies the creek bed at a depth estimated at 400 to 600 feet.

EROSION.

After the intrusion of the dikes and the granite and the development of the tin minerals, the Lost River area was eroded by streams 

and by the sea. Several epochs of erosion are recorded.

In spite of the steep dip and notable faulting of the limestone beds 

of the region, the highest ridges rise to one general level, averaging 

about 2,200 feet above the sea. It is believed that the beds were 

eroded to about this general level, either by streams or by marine 

planation. In the next epoch of erosion the sea cut the great sea 

bench now fronting Bering Sea at an elevation of about 400 feet. 

What the Lost River valley was like at that time is not clear, for no 

benches along the stream having this elevation were noted. The 

present epoch of erosion began with the emergence of the sea bench 

to its present level. The sea cliff along Bering Sea was cut by wave 

action. Since then there have been minor changes in the configuration of the coast by slope and shore deposits, which have caused the 

sea to recede from the cliff east of Lost River.
50 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

After the elevation of the sea bench. Lost Kiver lowered its bed 

until it was about 100 feet above the present sea level on the bluffs 

north of Bering Sea. It then flowed through a broad depression about 

1 mile west of the canyon. Benches of this stage about 80 feet above 

the river are found in the lower part of the valley. At the mouth of 

Tin Creek and up Camp and Cassiterite creeks they, are 15 or 20 

feet above stream level. The stream finally found a more direct outlet to Bering Sea when it was tapped by a gully which had cut its 

way back from the sea. This gully is the presnt canyon of the river.

An observer viewing Lost River valley from one of the high peaks 

to the east does not see the present channel. He sees the broad benches 

on each side of the stream and expects to find the river still flowing 

on them. The canyon near the mouth is a surprise, for it does not 

fit the width of the valley farther up.

East of Lost River the raised sea bench has been only slightly 

eroded by streams since its elevation, except by those which head in 

the York Mountains. These have cut narrow and deep trenches 

across the entire width of the bench.

No estimates of the amount of erosion during the first epoch, represented by the plain that bevels the tops of the York Mountains, can 

be made, for this plain cuts across a monoclinal succession of beds 

that are faulted to an unknown degree. Lost River has removed 

from the part of its drainage basin east of the channel about 50 per 

cent of the materials between its present level and the Nuluk Plateau, 

and from the part west of the channel perhaps 20 per cent.

The amount of tin-bearing dike rock eroded from the Cassiterite 

dike is probably somewhat greater than that which now remains 

above the level of Cassiterite Creek. This conclusion is based on the 

assumption that prior to erosion the mineralized part of the dike 

extended from the present creek upward to the level of adit No. 3. 

The end of adit No. 3 practically represents the east end of the 

mineralized part of the dike. It is probable that the mineralization 

of the dike was as extensive in a vertical direction as in a longitudinal 

direction, or perhaps even more extensive. Hence the assumption 

that the mineralization of the dike once extended vertically from 

the present creek level to the level of adit No. 3 seems to be warranted. It is therefore probable that about 150,000 tons of tin-bearing rock has been removed by erosion from the Cassiterite dike.

Prospects of tin, lead, antimony, copper, and tungsten in lodes 

have been located within the Lost River area. The location of the 

lodes is shown on Plate I. A small quantity of placer tin occurs 

on Cassiterite Creek, and the only important lode-tin prospect is on 

the same stream. Lead prospects have been opened on Tin Cr^ek 

;ibout 400 paces west of the granite, on Rapid River about 4 miles
LOST RIYER AREA. 51

above its junction with Lost River, on a tributary of Lost River 

north of Rapid River, and on the west side of the Lost River valley 

opposite the mouth of Tin Creek (the Southern Cross lode). There 

is a lead-tungsten prospect near the Southern Cross lode, and about 

400 paces west of the lode, across a low divide, is a small antimony 

prospect. Copper was found on the east side of the Lost River valley about half a mile south of the mouth of Tin Creek. Of the prospects mentioned, only one, the tin prospect on Cassiterite Creek, has 

been considerably developed.

PROSPECTS.

CASSITERITE CREEK TIN PROSPECT. 

GENERAL GEOLOGIC RELATIONS OF THE TIN DEPOSITS.

' The region in the vicinity of Cassiterite Creek is underlain by 

northward-dipping limestone beds which have been dislocated by a 

series of faults that strike nearly east and dip steeply to the south, 

and by another series that strike roughly N. 20° W. and generally 

dip steeply to the west.

After the faulting of the limestone beds they were invaded by 

molten materials, which probably found their way upward along 

fissures, as most of the quartz porphyry dikes which they formed 

have the same attitude as the easterly faults, and in a few places the 

molten materials followed certain northwesterly faults. One of the 

quartz porphyry dikes thus formed, the Cassiterite dike, is now being prospected for tin ore.

The injection of the dikes was followed by the rise of hot tinbearing solutions from within the earth. These solutions caused 

portions of the dikes to be replaced by fluorite and other minerals, 

including the tin mineral cassiterite. The limestone traversed by 

them was partly replaced by fluorite, serpentine, pyroxene, and similar minerals. They also formed certain fissure veins cutting the 

limestone and to a lesser extent the dikes. The fissure veins contain 

a group of minerals characteristic of tin deposits, including quartz, 

cassiterite, fluorite, topaz, and tourmaline. Present developments indicate that the dikes contain the most promising deposits of tin. 

The veins cutting the limestone show no probability of future production.

Since their introduction the cassiterite and related minerals have 

not been disturbed by any marked faulting. Erosion has exposed 

them and has caused the accumulation of a little placer tin on Cassiterite Creek.

LIMESTONE.

The limestone in the vicinity of Cassiterite Creek includes thin 

layers with light-brownish shaly partings; thick beds of dark-gray
52 GEOLOGY OF THE YOKK TI1ST DEPOSITS, ALASKA.

pure limestone with distinct grain; arid some thin beds with black 

shaly partings.

Outcrops are not common except along the high ridges and in the 

stream beds. Because of numerous faults, of which probably only 

a portion have been recognized, no attempt will be made to present 

a detailed section, but notes on the character and location of limeEXPLA NATION

-<

Tunnel

FIGURE 5. Geologic sketch, map of Cassiterite Creek and vicinity. Topography by 

Adolph Knopf. Elevations determined by aneroid.

stone outcrops are given with the view of setting forth the evidence 

for certain faults.

East of Cassiterite Creek, about 20 paces north of adit No. 3 (see 

fig. 5), a line running east and west marks the boundary between 

thin-bedded shaly limestone on the north, locally called the "reef 

rock," and pure gray limestone on the south. The thin-bedded shaly
LOST EIVER AREA. - 53

limestone extends northward to the area of stream and slope wash 

adjacent to Cassiterite Creek. The beds strike on the average N. 

60° E. and dip 35° N. West of Cassiterite Creek the northern boundary of the thin-bedded shaly limestone is about 25 paces north of the 

Ida Bell tunnel; the southern boundary about 220 paces south of 

adit No. 3. On the west it extends a little beyond the top of the hill 

between Lost River and Cassiterite Creek. These details are given 

because faulting is inferred from them and will be discussed on 

page 54.

Outcrops of dark-gray limestone in thick beds striking N. 70° E. 

and dipping 25° W. extend from the top of Cassiterite Peak to the 

first saddle on the east. Between the saddle and Deer Horn Peak 

dark-gray thick-bedded pure limestone with black flint nodules alternates with thin-bedded shaly limestone.

From the top of Deer Horn Peak eastward to Chimney Peak 

and southward along the ridge between Deer Horn Peak and the 

granite knob of Tin Creek the underlying limestone is of the darkgray pure thick-bedded variety, with scattered nodules of black flint.

At 500 paces down Cassiterite Creek from the junction of Camp 

Creek dark-gray pure limestone cut by closely spaced faults is exposed. The strike of the beds is obscured but is probably about N. 

20° W., and the dip 35° W. ^

About 400 paces upstream from the mouth of Camp Creek darkgray thick-bedded limestone is cut by closely spaced faults that 

strike N. 20° W. and dip steeply West.

At the junction of the second north fork of Camp Creek the darkgray beds show a few dark shaly partings. They are traversed by 

seams of serpentine and contain tremolite. The upper surface of this 

group is covered with rounded hummocks 1-| feet in diameter and 

about 2 inches high. These elevations are encircled by depressions 

about 3 inches deep. The limestone beds in these depressions contrast sharply with the beds underneath, in that they consist of thin 

beds of soft, nonmetamorphosed limestone, alternating with shaly 

beds less than 1 inch in thickness, weathering to a light-brownish 

color. The conditions suggest unconformity. As they were observed in only this one place no conclusions are drawn from them.

FAULTS.

About 1,500 feet downstream from the mouth of Camp Creek 

outcrops of limestone on the south bank of Cassiterite Creek are disturbed by several faults that strike N. 20° W. and dip 80° W. The 

beds on the west side of the fault planes have moved downward 

with respect to those on the east. To judge by the sharp northwestsoutheast boundary between the debris of thin-bedded limestone
54 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

and that of pure gray limestone to the west, on the hill north of the 

creek, it is probable that these faults extend about 2,000 feet or more 

northwestward. Certain dikes north of the west end of the Ida 

Bell dike are in alinement with this zone of faulting.

At 450 paces upstream on Camp Creek from its junction with 

Cassiterite Creek faults having the same strike and dip as those 

just described cut beds of the pure gfcay limestone. At this place 

white fine-grained muscovite-bearing dikes as much as 8 inches in 

width are injected along some of the fault planes.

The southern boundary of the thin-bedded shaly limestone about 

60 feet north of the tunnels, east of Cassiterite Creek (see description on p. 52), is believed to be associated with a series of fault 

planes. These thin beds of limestone, striking on the average N. 60° 

E. and dipping 15°-35° N., should, if undisturbed by faulting, extend considerably south of the tunnels. It is probable that they 

are cut off by a fault striking nearly east and dipping steeply to 

the south. On the west side of Cassiterite Creek 220 paces south of 

adit No. 3 the thin-bedded limestone meets the pure gray limestone 

along a sharp boundary that trends nearly due east. From the hill 

west of adit No. 3 it is clear that the southern boundary of the thinbedded shaly limestone is considerably farther south on the west side 

of the creek than on the east side a relation which can be explained 

by assuming that a fault, having about the same strike as the trend 

of Cassiterite Creek at this place (N. 10° W.)> cuts through the 

limestone in the vicinity of the creek. If such a fault were nearly 

vertical and had displaced the beds in the same direction as the 

faults exposed farther down Cassiterite Creek and up Camp Creek 

that is, if the beds west of the fault plane had been depressed this, 

combined with the existing state of erosion, would bring about the 

present relation of the thin-bedded limestones on the two sides of 

Cassiterite Creek. This inference is confirmed by the fact that some 

of the limestone exposed in the bed of Cassiterite Creek between the 

mine office and the Ida Bell tunnel is broken into angular blocks 

about 2 feet in diameter, which have been cemented by vein materials.

The limestone in the vicinity of the tin prospects on Cassiterite 

Creek is cut by two sets of fissures, one striking northeast to N. 10° 

W. and dipping steeply east, the other striking nearly east and dipping 8°-45° S. Some of them contain tin. Other fissures occur, but 

these are the most prominent.

A rock trench on the east side of Cassiterite Creek 150 paces east 

of the mouth of the Ida Bell tunnel discloses the fact that movement of the limestone has taken place along one of the planes striking N. 85° E. and dipping 35° S., as is shown by the development 

of gouge and minor fractures in the adjacent limestone.
LOST RIVER AREA. 55

That the faults described were developed before the intrusion of 

the quartz porphyry dikes seems to be proved by the fact that they 

do not disturb the dikes, and in at least one place the dike materials 

were injected along a fault plane. Nor do the two sets of joints 

described as cutting the limestone appear to cross the Cassiterite 

dike. Certain tin-bearing fissure veins of the Ida Bell dike, however, have the same attitude as those tin-bearing veins in the limestone which strike east and dip gently south. The only fault displacing dike materials was seen in the workings on the Greenstone 

lode. It strikes N. 10° E. and dips 80° W., and the downthrow 

appears to be toward the west.

QUARTZ PORPHYRY DIKES.

The principal tin-bearing dikes of Cassiterite Creek are the Cassiterite dike and the Ida Bell dike. A massive irregular body of 

intrusive material south of the Cassiterite dike and east of the creek 

will be referred to as the Greenstone lode. The Cassiterite dike 

ranges in width from about 3 to 12 feet. It can be traced from 

Lost River southeastward to the top of the hill west of Cassiterite 

Creek and thence in a general southeastward direction to the west 

side of the north fork of Tin Creek. On the southwest slope of 

Cassiterite Peak it takes a very abrupt turn northeastward. As 

there are no outcrops of the dike, except in one place on Cassiterite 

Peak, it can be traced only by its debris on the surface. It is therefore not certain whether the very abrupt turn of the dike on 

Cassiterite Peak ig an original feature of the dike or whether it is 

due to an offset caused by faulting. It is very probably original. 

From the portal of adit No. 2 a branch of the Cassiterite dike 

extends southwestward within 100 paces of Cassiterite Creek, where 

it appears to end, about midway between adit No. 3 and the tunnel 

of the Greenstone lode. (See fig. 5.)

The undecomposed portion of the Cassiterite dike consists of welldefined quart^e grains, commonly with hexagonal cross section about 

0.1 inch in diameter, which are embedded in a very fine grained lightcolored groundmass. Besides quartz, it contains crystal grains of 

turbid orthoclase and clear oligoclase of about the same size as 

the quartz. Orthoclase incloses many of the oligoclase crystals. 

Foils of white muscovite mica are also common. The visible minerals 

make up about 20 per cent of the total volume of the rock.

The Ida Bell dike has been traced from the Ida Bell tunnel southwestward to Lost River. On top of the hill west of Cassiterite Creel?: 

it crosses the Cassiterite dike. At this place, to judge by the debris, it 

is about 35 feet wide. It differs from the Cassiterite dike in containing fewer feldspar grains. Quartz in sharply defined grains with 

hexagonal outlines is embedded in a very fine grained light-colored
56 GEOLOGY OF THE YORK TI1ST DEPOSITS, ALASKA.

groundmass and constitutes about one-tenth of the volume of the 

rock.

The area south of the Cassiterite dike and the main channel of 

Tin Creek is traversed by a great many quartz-feldspar dikes of 

very dark color. The feldspar grains range from 0.1 to about 0.5 

inch in diameter, and the quartz grains are very fine, dark, and 

glassy. The fine matrix in which these grains are embedded is generally black. The distribution of these dikes is shown on figure 5. 

One of them has been traced from the north branch of Tin Creek 

southwestward to the peak southeast of the mouth of Camp Creek. 

The same group of dikes probably extends under the stream wash 

of the Lost River valley and reappears on the hills south of the 

Southern Cross tunnel, opposite Tin Creek. (See fig. 12, p. 78.)

In the workings of the Greenstone lode a medium-grained granitic 

dike of very irregular shape is exposed. It does not reach the 

surface. In places it is only a few feet thick and dips gently southward between limestone layers, through which it cuts. A more detailed description appears on pages 69-71.

Innumerable felsitic dikes an inch or so in width cut the limestone 

in the vicinity of Cassiterite Creek between adit No. 3 and the Ida 

Bell tunnel.

BRECCIA.

Boulders containing limestone and granite pebbles found in Cassiterite Creek near the tin prospects led to a search for the ledges from 

which they came. It was believed that they might be from a consolidated gravel bed resting on a surface of erosion either interrupting 

the Port Clarence limestone beds or beveling them all. The possibility of finding tin placers associated with such a formation made 

this search of practical interest.

The boulders were traced to dikelike forms, most of which cut the 

limestone beds. The lowlands north of Cassiterite Creek, upstream 

from the tin prospects, show shattered debris and several outcrops of 

this peculiar rock. One of the best exposures is on the west bank of 

Cassiterite Creek at the junction of the third fork upstream from the 

tin prospect. It strikes N. 55° E., stands vertical, and averages 2 

feet in width. The limestone beds at this place strike N. 70° E. and 

dip 25° N. As a rule the limestone walls of the dike are smooth and 

sharply defined, but here and there some of the cementing material 

of the dike invades them along minute cracks and appears to have 

been caught in the act of sloughing off a fragment of the wall. The 

dike was traced eastward for 150 paces to a point where it disappears 

under debris. It does not reappear where it should if it extended 

much farther.
LOST EIVER AREA. 57

The pebbles composing the dike are chiefly round to subangular 

pebbles of limestone, whose maximum diameter is about 5 inches. 

Five different varieties of limestone were noted, differing in color, 

texture, and composition. Generally the longest dimension of the 

larger pebbles is parallel to the walls of the dike. Here and there a 

distinct pebble of medium-grained granite occurs with the limestone 

pebbles. None over 5 inches in diameter were seen.

In places the limestone pebbles are so closely packed that the 

cementing material between them is a mere filmv of fine-grained calcite. 

Where the cement is more abundant it consists of fine-grained calcite 

with numerous secondary quartz grains and in places colorless mica.

The cement, the limestone pebbles, and the limestone bedrock have 

locally undergone severe alteration such as is characteristic of deepseated conditions, which has resulted in the development of abundant 

tremolite in short white prisms. The outer parts of the limestone 

pebbles have been changed into a felt of radial tremolite needles 

standing perpendicularly to the surface.

Where the rock is weathered the limestone and granite pebbles 

stand out in relief, but the cement, because of more rapid solution, 

is marked by depressions. The limestone pebbles with the tremolitic borders are particularly accentuated. In many places the top 

of such a pebble has been broken off, exposing the inner kernel of 

calcite, which then dissolves out much more rapidly than the tremolized shell. In such pebbles solution has been particularly rapid 

along the border of the outer shell and the calcite, thus forming a 

horn-shaped shell tapering downward, with a small concave bump 

on the bottom of the inside, a form very similar to that of an ancient 

horn coral.

At an elevation of about 600 feet on the hill north of the first 

south fork of Cassiterite Creek upstream from the tin prospects a 

dike of this type has been traced with several interruptions for 

about 100 feet. In places it is nearly vertical and cuts across the 

beds, but at its west end it is parallel to the bedding. The dike ends 

abruptly in a rounded contact. The limestone inclosing this dike 

at the west end shows no effects from it.

On the ridge south of Camp Creek and on the crest of the hill 

east of the head of the north fork of Tin Creek there are some small 

local veinlike occurrences of these materials. Because of their being 

more resistant to weathering than the adjacent limestone, they stand 

in slight relief, amidst limestone debris.

A very significant occurrence of this kind is found on the east 

bank of Lost River at an elevation of about 100 feet above the stream, 

L| miles upstream from the junction of Cassiterite Creek. Here a 

101987 22 5
58 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

dike composed of limestone and porphyry fragments grades into a 

quartz porphyry containing angular pebbles of limestone.

That these dikelike materials are not consolidated gravel beds 

which accumulated on an erosion surface cutting both limestone and 

granite, as was first surmised, is evident. They do not lie upon 

limestone beds but generally cut through them. Nor are they sedimentary dikes fissures filled from the surface with rock debris, 

for the exceedingly close packing of the pebbles, the oblique position 

of some of the dikes, the interrupted veinlike character of others, 

and the character of the cement do not fit that explanation. The 

cement of the pebbles where unchanged by deep-seated alteration is 

not a coarse white vein calcite like that deposited by solutions underground but a very fine grained calcite like that of the pebbles.

To assume that these dikes were formed by fracturing and movement along shear planes followed locally by the invasion of molten 

materials is a satisfactory explanation of the larger occurrences, but 

not of those showing lack of continuity, changes in dip, and local 

sporadic appearance. How the granite pebbles got into dikes of 

this type that now crop out on Cassiterite Creek is puzzling. They, 

are rounded and are discrete from the cement. No granite was seen 

in the cement of the etched surface of weathering, nor in several thin 

sections of cement. If the granite pebbles are fragments of granite 

from some larger mass, how did they get into their present position ? 

The only conceivable source is from below. If they had been brought, 

up by shearing, it would be reasonable to look for granite fragments 

resulting from friction all through the cement, and some of the dikes 

should contain granite boulders in greater number, whereas one in. 

several feet is the usual order of abundance. These dikes may hold 

the key to the problem of how dikes of igneous material make way 

for themselves under certain conditions. Laterally some of the 

breccia dikes grade into porphyry with limestone fragments, and 

the porphyry in turn grades laterally into clean igneous dike material. Is it not probable that all the breccia dikes grade downward 

into pure igneous dikes ? It is conceivable that the intrusion of most 

of the true dikes of the region has been accomplished by the molten 

material forcing upward and displacing zones of broken rock, and 

that the breccias represent the materials which were pushed upward 

from below by intrusive forces. On the basis of this inference the 

breccia dikes are tentatively called intrusive breccias.

EFFECTS OF THE TIN-BEARING SOLUTIONS ON THE QUARTZ PORPHYRY DIKES.

The tin-bearing solutions that permeated the rocks in the vicinity. 

of the tin prospect on Cassiterite Creek came up from below, chiefly 

along (1) the contact between the Cassiterite dike and the limestone 

walls, (2) fissures cutting the limestone crisscross in all directions^
LOST EIVEK AREA. 59

(3) nearly vertical fissures that strike roughly north, and (4) 

fissures that strike nearly east and dip gently south. The area most 

intensely affected by these solutions is elliptical, with its longer 

diameter parallel to the Cassiterite dike. The longer axis of this 

altered area extends about 250 feet west and about 1,200 feet east

\Unaltered Port Clarence 

j limestone

Intersecting vein system 

(Set of intersecting veins 

strike N./0°£.,d/p 80°£,and strike 

N. 60°£., dip 8'to 35'S._ Contains 

various comb/nations of the 

following constituents* quartz, 

f/uorite, tourmaline, topaz, zinnwaldite, cassiterite, wolframite, 

and albite)

Ramifying veins 

(Veins, chiefly green hornblende, fluorite, and calcic plagioc/ase, crisscrossing marble 

in all directions and. giving the 

rock a green pattern)

3iLQuartz porphyry dike 

\Zfr (Quartz, feldspar, muscovite,seri- 

' / cite, cass/terite, and wolframite) 

Highly altered limestone

(Adjoining the mineralized 

portion of quartz porphyry dike. 

Chiefly fluorite(SO%) and serpentine with subordinate quantities 

of green hornblende, calcic plagioclase, humite, vesuvianite, ar-. 

senopyrite,pyrite, and 'cassiterite)

Unaltered Port Clarence 

limestone

FIGURE 6. Idealized sketch showing mineralization of limestone at Cassiterite Creek

tin prospect.

of Cassiterite Creek. Its northern limit is in the vicinity of the 

Ida Bell tunnel, about 180 feet north of the Cassiterite dike, and it 

extends about an equal distance southward from the dike. Between 

Casserite Creek and adit No. 3 seems to lie the center of most intense alteration. (See figs. 5 and 6).
60 GEOLOGY OF THE YOKK TIN" DEPOSITS, ALASKA.

Within the area in which the tin-bearing solutions have been 

effective the Cassiterite dike and the Greenstone lode are intensely 

altered. The Ida Bell dike has been but slightly changed. Where 

most altered the Cassiterite dike and Greenstone lode have been 

changed to a gray or brownish soft, porous claylike mass. Intensely 

altered phases of the Cassiterite dike grade into the unaltered variety 

west and east of Cassiterite Creek.

The changes in the dikes effected by the tin-bearing solutions consist chiefly in the alteration of the feldspars and the groundmass to 

a fine-grained sericite. Locally, however, the groundmass and the 

feldspars have been replaced by quartz associated with a small 

amount of fine-grained sericite and cassiterite. The change to sericite is characteristic of all the altered phases, including those which 

show only a feeble change. The most altered portions of the groundmass and to some extent the feldspars have been replaced by fluorite 

of a light yellowish-green color. Fluorite has also invaded quartz 

grains, and the almost complete absence of quartz in some of the 

most altered phases of the dike seems to be due to the removal 

of the quartz. This disappearance of the quartz has given some parts 

the appearance of a feldspar porphyry of which the largest grains 

are feldspars changed to fine-grained sericite or amorphous kaolin 

inclosing a few grains of fluorite.

Cassiterite and wolframite have been introduced in minor quantities in the dike. (See PL VIII.) They are not everywhere closely 

associated. Wolframite occurs in columnar crystals, some of them 

an inch or more in length, and locally in distinct veins associated 

with quartz; more commonly it replaces the dike. Cassiterite generally occurs in minute crystals averaging about 0.1 or 0.2 inch in 

diameter and rarely is seen in distinct fissure veins except in the Ida 

Bell dike. It also replaces the groundmass and the feldspars along 

many minute fissures several inches in length and a fraction of an 

inch wide. Quartz rather than fluorite is the common associate of 

both cassiterite and wolframite.

Zinnwaldite has been introduced in minute flakes and in a few 

tiny veins of very irregular character. In the veins the individual 

plates lie at right angles to the wall.

Other minerals introduced in minute quantities with the cassiterite 

are pyrite in tiny cubes and fine-grained arsenopyrite. Occasionally 

a few small grains of galena, molybdenite, sphalerite, and stibnite are 

found in the altered dikes. Locally the dike material is almost completely replaced by round irregular grains of gray humite 0.05 inch 

in diameter. Topaz in microscopic radial columns is common in the 

silicified portions. Tourmaline needles are rare and were seen only 

in the Cassiterite dike.
LOST RIVER AREA. 61 

EFFECTS OF THE TIN-BEARING SOLUTIONS ON THE LIMESTONE.

The pure gray limestone adjacent to the Greenstone lode and the 

Cassiterite dike in the vicinity of Cassiterite Creek was changed to 

white granular vesuvianite-bearing marble cut by greenish replacement veins crisscrossing in all directions (PI. VI, A). The alteration has been greatest near the dikes. Within 3 feet of the dikes the 

limestone has been changed to a dark-gr^een, finely granular rock 

which becomes reddish brown on weathering. Here the replacing 

materials are about 50 per cent fluorite, and the remainder is chiefly 

serpentine, with some green mica and hornblende, vesuvianite, zinnwaldite, and scattered grains of pyrite, arsenopyrite, and cassiterite. 

Locally humite in small round grains is abundant. Scattered crystals of partly altered plagioclase are occasionally seen. Metamorphose limestone of this type occurs near the entrance of the Greenstone lode tunnel and in many places along the walls of adits on the 

Cassiterite dike.

In the bed of Cassiterite Creek between the mine office and the 

Cassiterite dike the white marble was broken into angular blocks 

ranging in diameter from a few inches to about 2 feet. Three stages 

of mineralization are shown (1) the openings between the blocks 

of limestone are filled with a green mixture of minerals, chiefly fluorite 

and green hornblende, and the adjoining limestone is replaced by 

similar material; (2) this-altered rock in turn is cut by whiteweathering veins several inches in width consisting chiefly of calcic 

plagioclase and fluorite; (3) still later veins, a fraction of an inch 

in thickness, containing quartz, wolframite, cassiterite, and zinnwaldite, cut the limestone along planes striking nearly east and dipping steeply south. Here and there zinnwaldite occurs alone in 

veinlets about half an inch in width. The plates of this mica, have 

grown perpendicular to the walls. Later unmineralized fissures, 

striking approximately east and dipping steeply south, cut through 

the veins.

At 100 paces north of the mine office ;in the bed of Cassiterite 

Creek banded green veins about 6 or 8 inches wide replace the white 

marble along nearly vertical planes crossing roughly from east to 

west and from north to south. The banding consists of alternations 

of green hornblende and fluorite with fluorite and plagioclase. The 

vertical parallel banding grades into a concentric banding, like that 

of the altered limestone adjoining the granites of Tin Creek. Cores 

of white marble about a foot in diameter commonly remain within 

the concentric rings of green materials. Both in mineral composition 

and in structure these veins resemble those bordering the granites 

of Tin Creek. (See p. 75.)
62 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

The thin-bedded shaly limestone or " reef rock " north of the Cassiterite dike on the east side of Cassiterite Creek and that oirthe west 

side of the creek have been very intensely altered by the tin-bearing 

solutions. (See PI. V, B; PI. VII.) The shaly bands have become 

hardened and stand out conspicuously on weathering. They consist 

of fine-grained, colorless mica, embedded in a very fine groundmass of 

indistinguishable minerals.

A very complex network of fissure and replacement veins cuts the 

"reef rocks." (See PI. VI, B.) The replacement veins, like those 

that cut the white marble, usually crisscross in all directions and are 

only a few inches wide. The fissure veins trend in various directions, 

but most of them fall into two groups one striking about N. 10° E. 

and standing vertical or dipping steeply to the east, the other striking 

N. 80° E. and dipping from 8° to about 35° S. Their maximum 

width is about 2 inches, and in the scattered outcrops in which they 

occur they usually can not be traced for more than about 20 feet. 

The following mineral combinations have been observed in the veins, 

of which about a dozen were seen: (1) Quartz, zinnwaldite, wolframite, Cassiterite, with the wolframite more commonly in the walls 

adjacent to the vein; (2) quartz, zinnwaldite, acid plagioclase, tourmaline, topaz; (3) quartz, fluorite, cassiterite, with tiny nests of zinnwaldite; (4) quartz, zinnwaldite, wolframite, topaz; (5) quartz, 

cassiterite, plagioclase, topaz; (6) quartz, cassiterite; (7) quartz, 

zinnwaldite,, plagioclase.

The richest cassiterite vein "Observed in the limestone was in the 

rock trench on the east side of Cassiterite Creek 100 paces from the 

Ida Bell tunnel. Here a cassiterite-zinnwaldite-quartz vein striking 

N. 80° E. and dipping 36° S. fills a fissure along which the limestone 

beds were displaced before the vein was deposited. The cassiterite 

crystals are about an inch in length and show a well-developed foursided prism, whose edges are beveled by a secondary four-sided 

prism. A major and minor pyramid terminates them. Their color 

is light brown and their luster adamantine. This vein is cut by a 

vertical quartz-wolframite vein, 1 inch in width, which strikes north.

On the west side of Cassiterite Creek 20 paces north of the southern limit of the " reef rock " and about 10 feet below the summit of 

the hill two cassiterite veins were found, both striking nearly due 

east. One dips 50° S. and contains quartz, zinnwaldite, topaz, and 

wolframite. Its width is about 1 inch, of which about 2 per cent is 

cassiterite. The other vein dips 70° N. and is about half an inch in 

width. It contains quartz, fluorite, and cassiterite, the last in minute 

scattered grains.

The alteration of the limestone seems to fall into at least two distinct stages, which were probably continuous in time.
U. S. GEOLOGICAL SURVEY BULLETIN 733 PLATE VI

A. 1'OLISHKD SURFACE OF ALTERED LIMESTONE NEA.R CASSITERITE

LODE.

The white areas arc marble; the dark seams are green crisscrossing replacement veins 

consisting chiefly of hornblende and fluorite.

B. THIN SECTION OF CASSITERITE VEIN IN LIMESTONE NEAR CASSITERITE

LODE.

Enlarged 12 diameters. Radial topaz diverging from vein wall. Dark areas are cassiterite.
U. S. GEOLOGICAL SUKVEY HULLETIN 733 PLATE VII

A. ROUGH SURFACE OF "REEF" LIMESTONE NEAR CASSITERITE LODE. 

Ridges represenL hardened shale partings; depressions are underlain by calcite. Note straight

crisscrossing veins.

-B. ROUGH SURFACE OF "REEF" LIMESTONE.

Dark vein consists chiefly of hornblende and iluorite; white veins chiefly of calcic plagioclase and

fluorite.

MINERALIZATION OF "REEF" LIMESTONE, CASSITERITE CREEK.
LOST RIVER AREA. 63

In the first stage the limestone that was most altered was changed 

chiefly into fluorite, hornblende, zinnwaldite, and serpentine, with 

lesser amounts of calcic plagioclase, chondrodite, vesuvianite, pyrite, 

arsenopyrite, and possibly cassiterite. Rock showing this intensive 

alteration grades into crisscrossing veins of fluorite, calcic plagioclase, 

and green hornblende cutting granular marble, with minor amounts 

of the silicates and sulphides characteristic of the more altered portions. The changes involved consisted in the loss of carbon dioxide 

and the local introduction of silica, alumina, iron, magnesium, 

sodium, potassium, lithium, sulphur, arsenic, and tin. It is conceivable that some of these materials, such as silica, alumina, and 

magnesium, were picked up by the solutions in the limestone. Most 

of the other materials must have come from other sources.

In the second stage of alteration the materials formed in the earlier

stage were cut by fissure veins containing various combination of 

quartz, fluorite, tourmaline, zinnwaldite, topaz, acid plagioclase, 

cassiterite, and wolframite. These materials are distinctively of 

deep-seated origin, being derived from the siliceous and the most 

soluble constituents of magmas, such as fluorine, boron, lithium, 

sodium, and potassium, with other substances, notably tin and 

tungsten.

DEVELOPMENTS.

During the season of 1918 the development work on Cassiterite 

Creek was in charge of J. F. Halpin, of New York City. For the 

greater part of the summer 22 men were employed in cleaning out 

rock falls and ice from the drifts and in mine timbering. Most of 

the energies of the crew were spent in rehabilitating the mine and 

preparing it for examination and sampling. At the end of the 

season there were about 1,375 feet of underground drifts on the property, one raise of 100 feet, two winzes, and several crosscuts. The 

mine equipment included a bunkhouse accommodating about 25 men, 

mine office, blacksmith shop, barns, and'two small warehouses on 

the beach at the mouth of Lost River. The mine office on Cassiterite 

Creek is connected by telephone with one of the buildings on the 

beach.

Descriptions of the underground workings on Cassiterite Creek 

follow. Maps showing the location of the underground workings 

and their general character are given in figures 7-9. The principal 

developments are on the Cassiterite dike. East of the creek it is 

opened by three tunnels, referred to as adits or tunnels Nos. 1, 2, 

and 3; the lowest is No. 3. West of the creek the dike is opened 

by the Randt Extension tunnel. Minor tunnels have been opened 

on the Ida Bell dike and Greenstone lode.
64 GEOLOGY OF THE YOBK TIN DEPOSITS, ALASKA.

Adit No. 3 A.t 430 feet east of Cassiterite Creek the Cassiterite 

dike is opened up by adit No. 3, 650 feet in length. The elevation of 

this adit is 61 feet above the creek. From the portal the tunnel trends 

S. 80° E. for.170 feet, east for 130 feet, and S. 85° E. for 350 feet. At 

170 feet from the portal a crosscut extends 5 feet north from the 

main tunnel. A winze 20 feet deep has been sunk on the north side 

of the main tunnel 220 feet from the portal. During the season

Ida Bell tunnel- \ 

end winze

Horizontal and vertical scale

O 100 200 300 400 500 FEET

. Aditf

Greenstone

PLAN

Ad it 3

fiandt extension ^~^ xi47~Determined by aneroid from Bering Sea 

Sea level ___________________, __________

CROSS SECTION

PIGUBB 7. Plan of lode workings on Cassiterite Creek and cross section of workings

on Cassiterite dike.

of 1918 a raise was cut at 325 feet from the portal of the tunnel 

to adit No. 2, 100 feet higher. Small crosscuts have been opened 

up on the south side of the tunnel at 490, 520, and 580 feet from 

the portal.

Adit No. 3 does not expose the full width of the Cassiterite dike 

at any one place but swings from one wall to another in its course. 

At 170 feet from the portal the width of the dike is 11 feet; at 208 

feet from the portal it is only 3 feet. From the narrowest part of 

the dike to the end of the tunnel the width of the dike exposed
LOST RIVER AREA. 65

averages between 11 and 12 feet. The width of the dike in the 

whole length of the tunnel probably averages 9 feet.

The dip of the Cassiterite dike is 67° S. at 130 feet from the 

portal and only 45° S. where the dike is narrowest. At 220 feet 

from the portal the dip is 75° S.

During the season of 1918 the examination of the tunnel was 

hindered by ice, a fraction of an inch to several inches thick, covering the walls. The ice crystals were arranged in aggregates of thin 

hexagonal plates, roughly an inch in diameter. The most favorable 

conditions for examination prevailed in the early part of September, 

when the walls had partly thawed off and sample gouges had been 

cut at intervals of 5 feet.

Nearly all of the Cassiterite dike in adit No. 3 is soft and ranges 

in coherence from that of clay to that of a very firm shale. The 

color varies from a gray to a very light olive-green. The gray portion is firmer than the greenish portion. The gray dike material is 

a porphyry with very distinct phenocrysts of quartz, commonly 

hexagonal in cross section and constituting about 10 per cent of the 

volume of the material. The feldspars are also fine grained and 

include orthoclase and oligoclase. The softer greenish material is 

a porphyry in which what appear to have been feldspar phenocrysts 

about 0.1 inch in diameter constitute about 30 per cent of the 

volume of the rock. These phenocrysts are composed of very finegrained sericite and amorphous kaolin, with tiny granules of fluorite. 

The groundmass is a very fine grained mixture of sericite, amorphous 

koalin, fluorite, and fine granules of quartz. Locally, as at 300 feet 

from the mouth of the tunnel, the feldspars and groundmass have 

been replaced by quartz, with accessory colorless mica, Cassiterite, 

and topaz.

The relations of the gray quartz porphyry to the more thoroughly 

decomposed feldspar porphyry were not entirely clear, because of 

the ice in the tunnel. Two relations are possible: They may represent 

two distinct dikes, or the feldspar porphyry may be an alteration 

product of the quartz porphyry formed by the removal of quartz by 

fluorine solutions. The latter view is probably the right one. No 

fresh feldspar porphyry appears to be exposed on the surface, whereas quartz porphyry is very common. The feldspar porphyry is 

found only in places where the dike has been intensely altered. If 

the.quartz porphyry and the feldspar porphyry were two distinct 

dikes the width of the composite dike would vary considerably, but 

this is not true of the Cassiterite dike.

The alteration of the Cassiterite dike, as observed in adit No. 3, 

consists chiefly in a decomposition of the feldspars and the poorly 

differentiated groundmass into a mixture of fine-grained sericite and
66 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

kaolin. The more decomposed phases of the dike have been partly 

replaced by fluorite, which in the most altered portions composes 

about one-third of the dike. Fluorite is also found here and there 

in very thin veinlets or stringers. Next to the fluorite, the most 

abundant mineral replacing the dike is a lithia mica, which is also 

found to some extent in veins. Thin needle-like aggregates of green 

tourmaline are seen occasionally. Cassiterite occurs in small aggregates of grains about 0.1 to 0.2 inch in diameter, usually replacing 

the groundmass or the feldspars, and is occasionally found adjacent 

to very thin gashes that are mostly no more than 3 or 4 inches in 

length. Locally cassiterite in minute scattered grains is associated 

with quartz. Microscopic radial topaz and colorless mica replace 

the groundmass and feldspars. Wolframite in small columnar 

crystals is seen here and there, either independently or in close association with cassiterite. Pyrite is found in scattered cubical aggregates and is usually associated with arsenopyrite. Copper-bearing 

minerals, either chalcopyrite or stannite, in very fine grains, were 

observed in a few places. Their copper content is indicated by a 

faint green stain.. A few tiny plates of molybdenite were found on 

the dump of adit No. 3.

Adit No. 2. At a distance of 630 feet east of Cassiterite Creek the 

Cassiterite dike is opened by adit No. 2. The elevation of the portal 

is 162 feet above the creek. From the portal the adit trends S. 68° E. 

for 67 feet, S. 70° E. for 54 feet, and S. 60° E. for 24 feet. Its average width is about 8 feet, and its height about 6£ feet. At 125 feet 

from the portal a crosscut 5 feet long extends from its south side. 

The width of the dike at the east end of the adit is about 8 feet. At 

no other place was the full width of the dike observed. The dip is 

steep to the south.

Only the gray quartz porphyry type of dike was observed in adit 

No. 2. The rock has about the firmness of a strong shale. The feldspars and groundmass are altered to amorphous kaolin and sericite 

and show replacement by fluorite. The introduction of secondary 

minerals other than fluorite seems to be slight. In a few places 

scattered grains of cassiterite replace the dike rock. At the east end 

of the tunnel the secondary mineralization appears to have been 

especially feeble.

Adit No. 1. The portal of adit No. 1 on the Cassiterite dike is 

900 feet east of Cassiterite Creek, at an elevation of 100 feet above 

adit No. 2. From the portal the adit runs east for 75 feet, curves 

northward for about 15 feet, and resumes an easterly direction. The 

total length is about 180 feet, the average width about 8 feet, and the 

height 6£ feet.

The full width of the dike was not seen at any one place but 

probably averages between 8 and 11 feet. (See pp. 6<M55.) The dip
LOST RIVER AREA. 67

is steep to the south. The dike is all of the soft gray quartz porphyry type seen in adit No. 2. The visible results of secondary mineralization are inconspicuous, consisting only of scattered tourmaline 

veinlets a fraction of an inch wide and a few thin stringers of cassiterite about 0.2 inch wide and 5 inches long. There are some scattered pyrite crystals, and in one place a feeble malachite stain was 

seen.

Randt Extension tunnel. West of Cassiterite Creek the Cassiterite dike is opened by the Randt Extension tunnel, about 15 feet 

above the creek. The tunnel is 180 feet long. From the portal it 

trends S. 65° W. for 60 feet, S. 60° W. for about 76 feet, and S. 65° 

W. for 44 feet. The west end of the tunnel is about 80 feet below

EXPLANATION 

!uartz porphyry

Feldspar porphyry 

Wolframite vein 

Zinnwaldile vein 

Metamorphosed limestone

Probable limits of dike-

"S, Fractures 

X. Strike and dip

FIGURE 8. Geologic sketch map of Randt Extension tunnel.

the surface of the hill. The average width of the tunnel is about 6 

feet. At 60 feet from the portal a 10-foot crosscut has been opened 

on the south side of the tunnel, and at 155 feet from the portal 

another crosscut, 3 feet in length, has been excavated on the south 

side. (See fig. 8.)

The entire north face of the tunnel is composed of an altered 

silicated limestone, commonly containing crystals of secondary 

pyrite. Oxidation has changed most of the surface of this limestone 

to a brown granular mass. In the first crosscut on the south side of 

the tunnel, 60 feet^from the portal, the Cassiterite dike is 8 feet wide 

and dips 75° N. The first 6 feet of this dike nearest the tunnel is a 

light-gray, dense, fresh-looking quartz porphyry. Its quartz crystals
68 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

are irregular in shape, are about 0.1 inch in diameter, and constitute 

about one-fifth of the rock by volume. The minerals of the groundmass are not recognizable by the eye. The last 2 feet of the dike is 

soft, brown, and kaolinized. Quartz grains appear to be lacking.

At 20 feet west of the first crosscut a soft feldspar porphyry appears on the south wall and widens toward the west. At 10 feet east 

of the second crosscut this rock occupies nearly the full width of the 

tunnel, but a .hard quartz porphyry similar to that of the first crosscut appears on the north wall. The strike of the contact plane between the soft feldspar porphyry and the hard quartz porphyry is 

N. 75° W., and the dip appears to be 65° S. From a point 15 feet 

west of the second crosscut the quartz porphyry extends along the 

entire south wall to the end of the tunnel, where 6 feet of quartz 

porphyry is exposed. In the second crosscut the total width of the 

dike is 8 feet. Here the north or hanging wall is a dark-gray granular limestone, with small grains of fluorite and pyrite.

The soft feldspar porphyry is dark gray to brown and consists of 

completely altered feldspar grains from 0.1 to 0.2 inch in diameter, 

embedded in a fine-grained clayey material showing many specks 

of fluorite. The feldspar grains have changed to fine scales of sericite and amorphous kaolin, of which sericite commonly constitutes 

about 40 per cent by volume. The altered feldspar crystals form 

about 30 per cent of the volume of the rock.

Aside from the sericitization, kaolinization, and fluoritization of a 

considerable part of the dike above described, there is very little visible secondary mineralization of either the dike or .wall rocks. A 

number of small zinnwaldite veins about 1 inch wide appear in the 

limestone of the south wall of the tunnel between the portal and the 

first crosscut. They are also common about 15 feet west^ of the second 

crosscut on the north wall of the tunnel. At the extreme west end 

of the tunnel a quartz-wolframite vein 2 inches wide is exposed. 

The wolframite crystals, which are about 2 inches in length, show 

excellent cleavage and occupy the central part of the vein. Besides 

wolframite, the vein has small scattered patches of zinnwaldite and 

fluorite.

The relation of the decayed feldspar porphyry and the fresh 

quartz porphyry in this dike has not been satisfactorily determined. 

The separation between the two appears to be sharp. If they represent two distinct intrusions the total width of the compound dike 

thus formed would be expected to vary considerably, but this does 

not appear to% be the case, as the dike seems to maintain a uniform 

width of about 8 feet. Nor does it seem probable that the same 

dike material could solidify into quartz porphyry in one place and 

into feldspar porphyry a few feet away. If such differentiation were 

possible it seems that it would be rel.tted to the wall rock, but the
LOST RIVER AREA. 69

distribution of the quartz porphyry and feldspar porphyry shows no 

dependence on the proximity of the wall rock.

One of the outstanding differences between the two rocks is that 

the feldspar porphyry is much softer and is highly kaolinized. This 

difference suggests that the lack of quartz in the feldspar porphyry 

is due to its removal by the fluorine-bearing solutions. The quartz 

grains in the less altered phases are commonly corroded.

Workings on Greenstone lode. On the east side of Cassiterite 

Creek at an elevation of about 55 feet above the creek level, are the 

workings on the Greenstone lode. They are about 300 feet south 

and 50 feet east from the portal of adit No. 3, the longest tunnel 

on the Cassiterite dike. (See figs. 7 and 9.) The workings on the 

Greenstone lode include an abandoned shaft, probably 30 feet deep 

originally, a trench 3 feet deep and about 35 feet long running from

north to south, and a tunnel 60 feet long bearing N. 80° E., with 

a crosscut 40 feet from the portal trending N. 10° E. About 25 feet 

of the crosscut north of the main tunnel was open in the fall of 

1918; the remaining unknown length was closed by ice. To the south 

of the main tunnel, the crosscut was open for about 20 feet, but it 

had once extended farther.

The name " Greenstone lode" was given to the dikelike formations disclosed by these workings because of the green color of the 

altered limestone adjacent to them. The unusual shape of the intrusive materials was a puzzle to the discoverers, and even now their 

probable extent is not disclosed. Nowhere do they crop out.

The north-south trench referred to is about 20 feet west of the 

portal of the tunnel. Near the south end of the trench a sheet of 

whitish granular quartz feldspar about 2 feet thick, dipping 15° S. 

is exposed. In the northern part of the trench it is again exposed, 

but its limits are not clearly defined. Here a part of it occurs as a 

cement of small limestone fragments.

In the tunnel the north wall as far as the crosscut is composed 

entirely of a dark-green massive silicated limestone, which owes its 

color to the secondary minerals. The south wall is composed of the 

same material, except near the portal, where an irregular area of 

the broken limestone is cemented with the white dike material. .East 

of the crosscut the main tunnel is in the white granular quartz-feldspar rock, which is here soft, amorphous, and kaolinized. At the east 

end of the main tunnel the dike is cut off by a fault striking N. 10° 

E. and dipping 80° W. If this fault plane were projected northward, 

it would cut through adit No. 3 about 140 feet from the portal. The 

fault plane is bounded on the east by a white granular marble containing brown garnet. The dike rock and the marble are separated 

by gouge about 10 inches wide. The nature of the breaks along the
70 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

fault plane shows that the materials west of the fault have moved 

downward with reference to those lying to the east. How much

Broken limestone 

cemented with 

dike material

PLAN

SECTION ALONG LINE A-B

, Thickness of dike ^unknown. , , I",,,, .ft/,-.

SECTION ALONG LINE C-D

PLAN SECTION

Granite dike Dark-green dense 

fine-drained 

metamorphosed 

limestone

FIGOBB 9. Plan and sections of workings on Greenstone lode.

they have been" displaced is not clear probably less than 50 feet. 

No evidence of this fault has been found in adit No. 3.
LOST BIVER AREA. 71

In the crosscut northward from the main tunnel the wall on the 

east side is composed of the white, altered dike material. On the 

west wall the lower 6 feet is dark-green silicated limestone, and this 

is overlain by the white dike, whose thickness is unknown but is 

probably less than 10 feet, for otherwise its upper surface would be 

exposed on the surface of the hill. This dike material is nowhere 

exposed on the surface, the surface being mantled by loose limestone 

fragments. The dike is too soft to form coarse debris.

In the crosscut southward from the main tunnel the east wall is 

composed entirely of the white dike, except in one place where a 

horse of the green altered limestone extends across the crosscut. 

The lower 6 feet of the west wall is composed of the green silicated 

limestone, which is overlain by an unknown thickness of the white 

dike. About 15 feet south of the main tunnel the base of the white 

dike dips about 15° S. and descends to the floor of the crosscut.

The white dike, although its dimensions are not completely known, 

is a sheetlike intrusion of unknown thickness and of very irregular 

boundaries. It limits Qan not be predicted. It may widen out to 

great size at a moderate depth, or it may have the opposite character. 

It can, however, be expected to continue to considerable depth. It 

may even be larger than the Cassiterite dike, for its grain is uniform 

and of moderate size, a condition suggesting the slow cooling of a 

large intrusive body.

Embedded in the white dike material are numerous small secondary 

cubes of pyrite, which average about 0.3 inch in diameter, and here 

and there a small plate of molybdenite or a tiny stibnite crystal. 

There is nothing to suggest, however, that the molybdenite and 

stibnite are present in economic quantities. A few nests of purple 

fluorite were seen. Grains of cassiterite were not observed underground,-but pannings from some samples of the decayed dike rock 

on the dump showed black cassiterite crystals about one-eighth of an 

inch in diameter.

Because the extent of the dike is unknown, no estimates of tonnage 

can be made, but it is probably considerably greater than present 

developments show. The dike may be difficult to follow because of 

its shape. Drilling to outline its shape and composition could be 

done advantageously before extensive underground work is undertaken, even for exploration.

Ida Bell tunnel. About 180 feet north of the Randt Extension 

tunnel the Ida Bell dike is opened up by a tunnel 55 feet long, 15 feet 

above the level of Cassiterite Creek. The strike of the dike at this 

place, to judge from the debris on the surface, is N. 60° E. The tunnel strikes N. 45° W. and averages about?feet in height and 6 feet in 

width. At the west end of the tunnel a winze 69 feet deep has been
72 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

sunk. This was filled with water in 1918. The tunnel is timbered 

all the way, and hence visible exposures are few.

The exposures in the tunnel do not indicate the exact width of the 

dike. It is probably more than 10 feet wide. Most of the dike in 

the tunnel is fresh and in this respect differs greatly from the Cassiterite dike, to the south. On the south wall, however, at the extreme end of the tunnel, the dike is decomposed into a soft, claylike 

material.

The freshest dike material consists of quartz crystals with sharp 

hexagonal outline embedded in a fine-grained matrix of angular 

quartz and sericite. Some thin sections show no quartz, but a finegrained groundmass of sericite and fluorite in roughly cubical 

grains about 0.1 inch in diameter, serpentine in irregular masses and 

in roughly rectangular areas, and a few scattered grains of arsenopyrite.

The north wall at the northwest end of the tunnel consists of limestone made up of layers of gray fine-grained calcite about 3 inches 

thick alternating with layers of light-green serpentinized rock. The 

limestone beds strike N. 10° E. and dip 30° E.

On the north wall of the tunnel, 35 feet from the entrance, are 

two quartz-cassiterite veins, each three-quarters of an inch thick 

(PL VIII, B}. The strike N. 40° E. and dip 40° S. In some places 

the entire width of each vein is composed of cassiterite; in others 

it is all quartz. Both cassiterite and quartz occur next to the wall. 

At 40 feet from the portal two quartz-cassiterite veins, each about 

half an inch in width, occur in the north wall. One of them strikes 

northeast and dips 15° S.; and other strikes N. 60° E. and dips 

25° S. They have knife-edge walls, and no cassiterite was seen in 

the wall material. The quartz and cassiterite show the same relation to each other and to the wall as in the veins 35 feet from 

the entrance. At 10 feet from the west end of the tunnel, on the 

south wall, are several quartz-cassiterite veins, about a quarter of 

an inch thick, which strike N. 75° E. and dip 15°-20° S.

If projected southward, the quartz-cassiterite veins of the Ida 

Bell tunnel would intersect the Cassiterite dike at depths ranging 

from 50 to 250 feet below creek level. The higher figure may give 

some intimation as to the depths of the region from which the tin 

solutions emanated.

PROBABLE -TONNAGE OF TIN-BEARING DIKE ABOVE THE LEVEL OF 

CASSITERITE CREEK.

Estimates of the probable tonnage of tin-bearing dike rock 

above the level of Cassiterite Creek justified by the developments 

in 1918 apply only to that portion of the Cassiterite dike extending from Cassiterite Creek westward 200 feet and eastward 1,080
U. S. GEOLOGICAL SURVKY BULLETIN 733 PLATE VIII

A. CASSITEKITE AND WOLFRAMITE REPLACING CASSITKIUTE DIKE.

The rounded black particles are cassiterite. Note two columnar crystals of wolframite on 

the left. The white parts arc fine-grained porphyry. The rounded grains near the bottom, 

are chiefly quartz.

B. CASSITERITE VEIN FROM IDA HELL DIKE. 

The dark part of the vein is cassiterite; the light part is quartz. The wall is line-grained porphyry
11. S. GEOLOGICAL SURVEY BULLETIN 733 1'LATE IX

B.

D.

CONCENTRIC BANDS OR ORBb'LES NEAR LIMESTONE 

CONTACT AT TIN CREEK.

A, Orbule produced by contact metamorphism; B, reverse side of orbule 

shown in A ; C, maximum orbule, diameter 8 inches; D, irregular orbules. 

Dark bands consist chiefly of hornblende, fluorite, and magnetite; 

white bands chiefly of fine-grained fluorite and calcic plagioclase.
LOST RIVER AREA. 73

i'cet. The width of the dike west of the creek averages 8 feet. 

Very little is known about the width of the dike between the creek 

and adit No. 3, a distance of 430 feet, but in the tunnels east of 

the creek it ranges from 3 to 11 feet, and 9 feet is probably a fair 

average. The weight of the dike rock has been estimated as 

1 ton to 12 cubic feet. It is therefore probable that 107,488 tons 

of tin-bearing dike rock is available above the level of Cassiterite 

Creek.

Estimates of the tonnage in the Ida Bell dike and the Greenstone lode can not be made at present. The tin content of the visible portion of the Ida Bell dike appears to be low, and present 

developments do not suggest the extent of the Greenstone lode.

TIN CONTENT OF THE CASSITERITE DIKE,

Mill tests carried through two seasons on dike materials obtained by trenching through the middle of a 2,000-ton dump thrown 

out from adit No. 3 are said to have averaged 4 per cent concentrates. Assays of the concentrates are reported as averaging 62.33 

per cent tin and 11.08 per cent tungsten.

PROBABLE EXTENT OF THE TIN DEPOSITS OF THE CASSITERITE DIKE BENEATH 

THE LEVEL OF CASSITERITE CREEK.

As the cassiterite in the Cassiterite dike was deposited by hot solutions rising from below, it probably extends to considerable depths 

in about the same abundance as near the present surface. The southward-dipping tin veins of the Ida Bell tunnel and the rock trench 

east of the creek from the Ida Bell would, if they extended far 

enough, intersect the Cassiterite dike at a maximum depth of about 

300 feet. This intersection may represent a center of dispersion of 

the tin solutions.

There is some probability, however, that at a depth of 400 to 600 

feet below Cassiterite Creek the limestones and possibly the dikes 

are replaced by a massive intrusion of granite like that of Tin Creek 

or Brooks Mountain. The indications favoring this probability 

are the innumerable small porphyry dikes in the limestone adjacent 

to the creek, the massive intrusions of granitic material in the form 

of the Greenstone lode, and the intense contact alteration of the 

limestone beds. On Tin Creek the development in the limestone 

of the green concentrically banded silicates (" orbules ") is restricted 

to a zone within 10 feet of the border of the granite. (See p. 75.) 

It is probable that the similar orbules in the limestone on Cassiterite 

Creek are also in close proximity to a large intrusion of granite. 

The narrowness of the altered limestone zone adjacent to other 

granites in the region suggests that the bed of Cassiterite Creek at 

101987 22 6
74 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

the tin prospect is not far from large intrusive masses below. At 

Cape, Brooks, and Ear mountains and at other places in the region 

where granite intrusions are exposed the zone adjacent to the granites in which the country rock has been changed into a condition 

similar to that of the altered limestone of Cassiterite Creek is measured at the maximum in the hundreds of feet.

It is difficult to forecast the state of mineralization of the granitic 

masses that undoubtedly underlie Cassiterite Creek at a comparatively shallow depth. They may be deeply sericitized and more or 

less impregnated with tin-bearing minerals like the tin-bearing portion of the Cassiterite dike. If a large tonnage of tin-bearing rock 

is ever to be mined at this locality it must come from masses of this 

character. On the other hand it is equally possible that mineralization may disappear when these granitic intrusives are reached. So 

far as known, this is the case at Brooks Mountain, at Ear Mountain, 

and to a less marked degree at Cape Mountain. It is to be hoped 

that the future developments at Cassiterite Creek will show a departure from the general rule of the region in this regard. Only 

deep exploration will answer the question.

PLACEE TIN ON CASSITERITE CREEK.

Placer tin in the form of Cassiterite in fine grains averaging about 

0.2 inch in diameter lies in the bed of Cassiterite Creek between the 

Ida Bell dike and the junction of Camp Creek, a distance of more 

than 2,000 feet. The width of the stream gravel is about 100 feet, 

and the depth about 4 feet. The placer tin rests on a very irregular surface of limestone having irregularities over 2 feet in height. 

The stream wash associated with the tin is mostly very coarse, including boulders of the "reef" limestone 2 feet in diameter. 

Thorough tests of the quantity of placer tin at this place have not 

been made. The quantity in the creek bed probably does not exceed 

50 tons. In 1918 about 1 ton of placer material was taken out by 

means of sluicing near the junction of Camp Creek.

TIN CREEK TIN PROSPECT.

The granite area of Tin Creek is several hundred feet northwest of 

the junction of the north branch with the main stream. It is roughly 

semicircular and has a diameter of about 1,200 feet. The northern 

border trends northeast and follows a nearly straight line, a peculiarity suggesting that it is marked by a fault. Force is added to this 

suggestion by several faults striking east and dipping steeply to the 

north on the limestone ridges east of the granite and by similar zones 

of fracturing along the north fork of Tin Creek.

The granite crops out chiefly on the southeast side of a domeshaped hill rising about 500 feet above Tin Creek. To the northwest
LOST RIVER AREA. 75

this hill is separated from still higher hills by a low saddle. Small 

exposures of the granite are also found in the bed of the north 

branch of Tin Creek.

Quartz, feldspar (orthoclase and sodic plagioclase) and small scattered scales of brown mica are the chief mineral constituents of the 

granite. The grains of these minerals are nearly all of the same size, 

about 0.1 inch, and show no marked variation from the border toward the interior of the outcrop.

The granite is cut by a quartz porphyry dike about 8 feet wide, 

which extends into the limestone toward the southwest. It can also 

be traced toward the northeast within about 100 feet of the north 

branch of Tin Creek. That this dike was injected when the granite 

was consolidated is shown by its chilled border.

The limestone intruded by the granite is of a light-gray color, 

with a granular ashlike appearance on the weathered surface. It is a 

nearly pure calcite with a few irregular nodules of black chert.

Within 500 feet of the border of the granite the cherts were 

bleached white by the intrusion. The limestone within this zone has 

been changed to a white coarse-grained marble, individual grains of 

which commonly measure half an inch in length. Along the northwestern' border of the granite, where the contact between the limestone and the granite is exposed, the limestone has been replaced by 

a mass of greenish minerals from 3 to 10 feet wide. These green 

minerals extend outward along fissures radiating from the granite 

contact, but they do not invade the granite. Near the fissures the 

green. mineral mass is finely banded parallel to the fissures. Away 

from the fissures this parallel banding grades into a concentric banding which has been brought into relief by weathering.. The concentric bands form circular to elliptical bodies ranging in diameter 

from 1 inch to about 2 feet. Certain bands inclose smaller concentrically banded bodies or nodules. Some of the concentric bodies 

flatten out with reference to one another, as if their growth had 

produced mutual interference (PL IX).

In a few places the core of a concentric band is a circular or elliptical area of white marble, the original limestone. The darkest 

bands consist chiefly of hornblende, with some magnetite and fluorite. 

The lighter-colored bands consist chiefly of fluorite and limy plagioclase. Fluorite is the chief constituent of these bands. Other minerals observed are pyroxene, garnet, and vesuvianite. Cassiterite, 

pyrite, and arsenopyrite have also been found. An excellent description of these concentric bodies is given by Knopf. 22

The restriction of this green mineral mass to the limestone side 

of the .contact, the great difference in composition between it and the

10 Knopf, Adolph, Geology of the Seward Peninsula tin deposits, Alaska: U. S. Geol. 

Survey Bull. 358, pp. 44-45, 1908.
76 GEOLOGY OF THE YORK TIN" DEPOSITS, ALASKA.

limestone, its development along fissures extending into the limestone, and the fact that it incloses circular areas of limestone show 

that it has replaced the limestone. Hot solutions containing fluorine, 

aluminum, silicon, sodium, magnesium, iron and some tin, sulphur, 

and arsenic, appear to have come up along the granite contact and 

entered into reaction with the limestone, thus setting free carbon 

dioxide and with the lime forming a group of lime-bearing silicates. 

The parallel banding along the fissures and the concentric banding 

away from the fissures appears to have resulted from a process of 

rhythmic interaction between the solutions and the limestones.

The introduction of the solutions that altered the limestone preceded the injection of the quartz porphyry dike into the granite, for 

this dike cuts through the green silicate zone. Whether the solutions 

that altered the limestones had any effect upon the granite is uncertain. No effect was noted. Solutions similar to those which altered 

the limestone have affected the granite locally, as shown in a few 

tiny quartz stringers that have been observed to contain a little 

cassiterite. A shallow prospect pit on the top of the granite has exposed slightly altered granite containing sulphides, including pyrite 

and arsenopyrite, and Knopf reports microscopic amounts of cassiterite. The feldspars of the granite in this place are altered to sericite, a change which is characteristic of the effects produced by tinbearing solutions elsewhere.

DOLCOATH TIN-BEARING DIKE.

The Dolcoath tin-bearing dike is about 1 mile north of the Cassiterite Creek tin prospect, at an elevation of 1,200 feet on the south 

slopes of the high ridge northwest of Cassiterite Creek. (See fig. 

5, p. 52.) The present developments have shown no merchantable 

ore, but the mineral associations of the deposit are very interesting.

The Dolcoath dike has an average strike of about N. 50° E. and 

a general northward dip of about 68°. It has been traced on the 

surface by debris for about 3,700 feet but probably does not maintain 

a uniform width for this distance. The width, where the dike has 

been crosscut, averages about 2-| feet. Its color is a 1 dark gray. The 

chief constituents are scattered minute crystals of plagioclase feldspar, approximating labradorite, and of quartz, embedded in a finegrained groundmass.

In most places the original character of the rock is nearly obscured 

by sericite and secondary minerals, chiefly colorless micas, with 

quartz, tourmaline, danburite, chlorite, pyrite, and arsenopyrite in 

small crystals. Cassiterite is not found in visible amounts but occurs 

in microscopic crystals.

The limestone beds adjacent to the dike strike nearly east and dip 

30° N". They are thin bedded, with shaly partings, and show a
LOST RIVER AREA. 77'

regional change, not dependent on the dike, to -granular white marble 

with an abundance of small white tremolite needles. These needles 

are especially prominent along the shaly seams.

Near the dike the limestone contains seams of green minerals, which 

appear to be like those so common in the limestone near the Cassiterite Creek tin prospect. Seams of this sort lying parallel to the 

bedding can be seen near the portal of the tunnel. (See fig. 5.) 

Knopf 23 found danburite, topaz, arsenopyrite, cassiterite, and tourmaline in both limestone walls within 6 inches of the dike.

Movement of the beds since the dike was introduced appears to have 

taken place, but the dike itself apparently was neither displaced nor 

severed. At the portal of the tunnel the beds show slickenside 

grooves on a vertical plane striking N. 60° E. The grooves are gently 

inclined to the west, and indicate an eastward and upward movement 

of the south wall. In the shaft (see figs. 5 and 11) a 6-inch seam 

of gouge lies between 

the dike and the hanging north wall. This

, ,, .,, 

gouge, o'rmo'A together r/irrornoT* with TOirh

certain fractures, ind i c a t e s a downward

movement of the hang- Limestone Dike

ing wall. i 20 3 FEET

accessible por- FIGURE 10. Geologic sketch map of Dolcoatla tunnel.

tion of the Dolcoath

tunnel was about 100 feet long in 1918. For the first 25 feet from 

the portal the tunnel trends westward and is in limestone. Thence 

it runs southwestward for about 10 feet and cuts through two 

branches of the dike separated by a horse of limestone. The first 

dike from the portal is 2 feet wide, the one farther in about 5 feet. 

About 35 feet from the portal these branches unite and continue as 

one dike, 2 feet wide, exposed on the roof of the tunnel for 70 feet or 

more to the west. (See fig. 10.)

The dike as exposed in the tunnel is a soft gray claylike mass, with 

very little visible mineralization. In the wide branch cut by the 

southwest turn of the tunnel a lump of arsenopyrite about 8 inches 

in diameter was found embedded in the dike. Three other lumps of 

this sort were seen in the last 70 feet of the tunnel. Besides arsenopyrite these lumps contain small amounts of danburite, tourmaline, and 

topaz.

The shaft is about 1,300 feet west of the tunnel, and is about 25 feet 

deep. Here the dike is 2J feet wide, strikes' N. 60 p E., and dips 70° 

N. It is soft and of a brownish color from limonite stains. Closely 

spaced strike fractures cut the dike parallel and transverse to the dip.

« Knopf, Adolph, Geology of the Seward Peninsula tin deposits, Alaska : U. S. Geol. 

Survey Bull. 358, p. 51, 1908.
78 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

EXPLANATION 

Limestone

C3ouge

Lines in dike and 

arsenopyrite indicate fractures

FIGURE 11. West wall of Dolcoath 

shaft.

The limestone on the hanging wall at the shaft has been replaced 

by a sheet 6 inches wide, chiefly arsenopyrite and fluorite. This sheet

is sharply separated from the dike 

but invades the limestone along tiny 

fissiures. About 6 inches of darkbrown gouge lies between the arsenopyrite and the limestone hanging wall. 

The sheet of arsenopyrite is cut by 

fractures having the same strike but 

a different dip, as shown in figure 11. 

Both gouge and fractures indicate that 

the hanging wall has moved downward, and that this movement took 

place after the arsenopyrite was deposited. The limestone beds dip 

about 30° N. and are of about the same character as those at the 

tunnel.

BESSIE AND MAPLE ANTIMONY AND LEAD CLAIMS.

Lead and antimony have been found on the Bessie and Maple 

claims, about half a mile west of Lost River, opposite the mouth 

of Tin Creek, on 

a saddle of a ridge 

trending about N. 10° 

W., at an elevation of 

about 900 feet. (See 

fig. 12.) The saddle 

is underlain by thinbedded shaly .limestone, which on the 

eastern slope of the

ridge strikes N. 55° 

W. In an exposure 

on the west slope of 

the saddle the strike 

is N. 10° W., and the 

dip 30° E. The knob 

south of the saddle is 

underlain by beds of 

dark-gray pure limestone that strike east 

and dip 30° N. They 

appear to be separated from the thinbedded limestone to

Dark pure limestone 

500 o

Shaly 

1000

limestone

1500 FEET

Contour interval lOOfeet 

Elevations by aneroid barometer

FIGURE 12. Sketch of area around Bessie and Maple claims.

the north by a zone of faulting which causes the two types of limestone to meet in vertical contact. The southeastern slopes of the
LOST RIVER AREA. 79

saddle and the knob to the south of it are cut by a great many dikes 

of dark-colored quartz porphyry striking N. 50° E.

On the east slope of the saddle, at an elevation of about 750 feet, 

the Southern Cross tunnel, trending N. 55° W., has been driven into 

the hill for a distance of 150 feet (fig. 13). Nearly all of it cuts 

through very fine grained thin-bedded limestones showing two sets 

of fractures, one striking N. 10° W. and dipping steeply west, the 

other striking east and dipping south. Locally these fractures are 

very closely spaced, and movement of the beds has taken place along 

them.

At 130 feet from the portal the tunnel cuts through a soft gray 

dike decayed into a claylike material, 20 feet wide, striking northeast and dipping 65° S. Along the south wall of the tunnel, between 

the dike and the portal, several galena-limonite veins striking N.

60° W. and dipping 

steeply south are exposed. Their maximum width is about 

3 inches. No metal1 i c mineralization 

was noted west of 

the dike.

About 100 paces 

west of the portal of 

the tunnel, at an elevation of 840 feet, a 

small trench exposes 

a soft white kaolinized dike striking 

about N. 70° E. 

Although only 2 feet wide, it is very probably the same dike that is 

seen in the tunnel. Five feet north of it the trench crosscuts a hard 

brecciated gray fine-grained dike for a distance of 2 feet. The total 

width of this dike is said to be 8 feet. The fragments, which are of 

all sizes up to about 2 inches, are cemented with thin seams of galena 

and pyrite, with some chalcopyrite.

At 79 paces N. 70° E. from this trench, the same dike appears to be 

exposed and shows the same sort of mineralization. On platting the 

relative positions of the dike and the tunnel it appears that if the 

tunnel were extended farther it would reach the dike.

About 500 paces t>. 20° W. from these exposures of the brecciated 

lead-bearing dike, at an elevation of about 750 feet on the west slope 

of the saddle, a trench cuts through frost-broken materials of a leadbearing dike of the same character. The ledge is not shown, but

Fracture zones in limestone' 

at mouth of tunnel

FIGURE 13. Geologic sketch map of Southern Cross tunnel.
80 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

the similarity of all these materials and the alinement of this spot 

with the other exposures make it probable that ^tringers and dikes 

of this kind underlie the saddle between the two extreme points at 

which they have been found. They may not be strictly continuous, 

however, nor constitute a considerable body of ore.

Assays of lead-bearing rock from the Bessie and Maple claims 

given to the Geological Survey by Messrs. W. B. O'Brien and Alex. 

Mclntosh are as follows:

Assays of rock from Bessie and Maple claims.

Location.

Gold 

(ounces 

to the 

ton).

Trace.

0.03

Silver 

(ounces 

to the 

ton).

19.8

4.2

25.6

Lead 

(per 

cent).

9.1

.5

4.6

Tungstic 

oxide 

(per 

cent).

 ? 9

0

Tin 

(per 

cent).

1 4<;

.3

.77

On the west slope of the saddle, at an elevation of about 750 feet, 

a tunnel driven eastward has penetrated a white soft kaolinized dike. 

Because of the ice on the walls of the tunnel the width of the dike 

was not determined. The dike is cut by several vertical stibnitebearing veins 3 inches wide trending east, parallel to the dike. The 

dike material has been impregnated with radial green tourmalint 

and fluorite. From a point 100 feet from the portal the tunnel runs 

southeast for 100 feet more, but leaves the dike.

YANKEE GIRL LEAD AND TIN PROSPECT.

A few hundred feet southwest of the Tin Creek boss of granite lead 

was found by Blomquist & Goodwin on the Yankee Girl claim. 

(See fig. 5, p. 52.) A short tunnel on the claim was inaccessible in 

1918, and nothing to throw light on the dimensions and relations of 

the lead deposits could be seen on the surface. About a ton of leadbearing rock lay on the dump, and some had been sacked. Part 

of the mineralized material is a yellow soft granular gossan containing numerous crystals of cerusite half an inch in maximum 

length. Another part is firm and heavy, consisting chiefly of copperbearing pyrite, arsenopyrite, and fluorite replacing calcite. Arsenopyrite was introduced after fluorite. A faint greenish copper carbonate stain covers the surface of the rock in places. In thin section 

the hard material shows a fine-grained indistinguishable groundmass of serpentinous material containing numerous small crystals of 

cassiterite. Arsenopyrite is the chief constituent. The Cow Boy 

tunnel, driven by the same men several hundred feet higher up the 

hill, was in barren limestone.
BROOKS MOUNTAIN. 81 

OTHER PROSPECTS.

No other prospects were being worked in 1919. Descriptions of 

the wolframite-topaz lode on the west side of Lost River, the Alaska 

Chief lead claim on Rapid River, and the Idaho copper claim south 

of the mouth of Tin Creek are given by Knopf,24 from whose report 

the following notes have been condensed.

Wolframite-topaz lode. The wolframite-topaz lode is on the 

ridge west of Lost River, opposite the mouth of Tin Creek. The mineralization has taken place along a fault zone running nearly due 

east. An open cut shows a stringer lode 1 foot wide, in which the 

vein materials are wolframite, galena, stannite, topaz, and fluorite. 

The wall rock is a dense, fine-grained limestone showing no alteration.

Alaska Chief claim. About 4£ miles from Bering Sea on the 

north side of Rapid River is the Alaska Chief claim. The limestones in this locality have been faulted, brecciated, and intruded by 

quartz porphyry. A shaft and tunnel have been driven on a heavy 

porous body of red iron oxide containing galena. When examined 

in 1918 the dump showed very little lead. A 600-foot tunnel near by 

struck no ore.

Idaho copper claim. A copper prospect on the Idaho claim is sev 

eral hundred yards south of the mouth of Tin Creek, on the 15-foot 

bench that fronts Lost River. Stringers of ore have been deposited 

in an irregularly shattered zone in the limestone 15 feet wide, including numerous horses of unmineralized limestone. The copper 

mineral chalcopyrite occurs with abundant pyrrhotite in a gangue 

of calcite, fluorite, and small fragments of slickensided rock. The 

percentage of copper is low.

BROOKS MOUNTAIN.

GEOGRAPHIC FEATURES.

Cassiterite has not been found on Brooks Mountain. Hulsite and 

paigeite, two rare tin and boron minerals, were found in minute 

quantities by Knopf 25 on the southwest slope of the mountain in 

1908. The occurrence of these tin minerals, the intrusion of granite 

into limestone, and the contact alteration of the limestone adjacent 

to the granite, ally this area with certain other tin-bearing localities 

of the York region, particularly those of Cassiterite and Tin creeks 

and Cape and Ear mountains. The facts known regarding Brooks

21 Knopf, Adolph, Geology of the Seward Peninsula tin deposits, Alaska: U. S. Geol. 

Survey Bull. 358, pp. 57-60, 1908. 

28 Idem, p. 43.
82 GEOLOGY OE THE-YORK TIN DEPOSITS, ALASKA.

Mountain do not justify the hope that important tin deposits will be 

found here.

From an east-west ridge that marks the head of the Lost River 

basin Brooks Mountain extends northward for 2£ miles as a narrow, 

precipitous ridge, 2,000 to 2,900 feet high. A short distance north 

of the highest point it turns east and extends 2 miles farther. It 

has the highest peak in the York Mountains. Its western slope is the 

steepest. In places cliffs 400 feet high descend westward from the 

crest. Below them are steep slopes of loose, angular fragments, which 

slide under foot. The eastern slopes, although steep, can be scaled 

anywhere and form a great amphitheater, the site of many gullies 

joining Anderson Creek. The landscape at Brooks Mountain has 

the barren, rugged aspect characteristic of the York Mountains.

The mountain has been carved by the streams of the Arctic 

and Bering Sea drainage systems out of folded slates and limestones 

intruded by granite. Its height as compared with its surroundings 

is due mainly to the hardening which the limestones and slates have 

undergone through the action of the granite. This hardening has 

enabled them to resist their removal by frost and stream action more 

effectively than the rocks around them.

 The only marked contrasts in the surface features of Brooks Mountain itself are those between the granite and the area of hardened, 

altered limestone adjacent to the granite. Although flanked by 

steep slopes, the limestone shows a nearly even crest line, indicating 

a nearly uniform resistance.

The granite surface can be distinguished miles away by its spiny 

protuberances. It is dotted with towers as much as 80 feet high, 

though true spires are lacking. This great irregularity of the granite 

surface is due to the fact that the massive granite is cut by two principal sets of fissures, one striking nearly east and dipping steeply 

south, the other striking nearly north and dipping 70° E. Less 

prominent fissures strike north and dip 20° E. Thus the granite is 

naturally cut into blocks 10 feet or more in length. The disintegration of the granite is accomplished almost entirely by frost, as solution is practically absent. Frost action breaks the granite into a 

coarse rubble, averaging about 1 inch in diameter. Disintegration 

starts at the surface and penetrates downward along the fissures. 

Wind, running water, and gravity remove the waste. Erosion is 

most rapid in the most fractured parts and where freezing and thawing alternate most frequently, as on southerly exposures. By these 

processes of disintegration and removal of the disintegrated materials the least fractured blocks are brought into relief.
U. S. GEOLOGICAL SURVEY BULLETIN 733 PLATE X

EXPLANATION

Map Section

Strike and dip 

of strata

Brooks Mtn.

-Z4OO

-ieoo'

-BOO' 

Sea level

Section along line A~B

Brooks Mtn,

-IffOo'

-aoo' 

Sea level

Section alon^ line C-D

GEOLOGIC MAP AND SECTIONS OF BROOKS MOUNTAIN
BROOKS MOUNTAIN. 83 

GENERAL GEOLOGY. 

SUMMARY.

The rocks at Brooks Mountain include an unknown thickness of 

black slate overlain by at least 1,000 feet of limestone. The contact 

between the slate and limestone is not exposed, hence it is not certain 

whether .the slate was eroded before the limestone was laid down 

upon it. The lower 100 feet of the limestone consists of black slaty 

layers a fraction of an inch in thickness interbedded with thin layers 

of gray fine-grained limestone. From this fact it would appear 

that there is a gradation from slate to limestone. The slates, however, are more intensely folded than the basal limestone beds. In 

position and character they are like the slates of other parts of the 

York region. Outside of the Brooks Mountain area the evidence is 

strong that the slates lie uncomformably below the limestones, hence 

it is probable that they bear the same relation here. (See PL X.)

The slate and limestone beds in the Brooks Mountain area have 

been buckled up into an arch whose axis strikes N. 10° W. and which 

is essentially parallel to the crest of Brooks Mountain. The west 

limb of this arch dips at an angle of about 30°. The east limb is 

much steeper, and it is probable that the west limb has been thrust 

over the eastern part of the arch. This inference is suggested by 

the fact that the slate and limestone contact on the west slope of the 

mountain is about 300 feet below the crest, whereas at the base of 

the east side of the mountain this contact is below the surface. Only 

limestone appears at the eastern foot of the mountain at the head of 

Anderson Creek.

Both slates and limestones have been dislocated by a fault, striking 

N. 70° E. and probably dipping steeply to the south, at the escarpment that forms the south end of the Mint River valley. Faulting 

is shown by the relations of the limestones and slates. The limestones on the escarpment dip northward; the slates in the valley' to 

the north have a similar northward dip but are intensely contorted 

in the vicinity of the limestones. The limestone beds, if projected, 

would occupy regions underlain by the slates to the north. It is inferred that the limestones tying south of the fault plane have been 

depressed with respect to the slates lying north of it.

Along the axis of the slate and limestone arch a large mass of 

granite has been injected. The molten material apparently followed 

the paths of least resistance offered by the arch and may have come 

up along an overthrust fault plane dipping steeply westward.

Dikes are rare in the Brooks Mountain area. One dacite porphyry 

dike was found in the slates on the east side of the valley, descending northwestward from the first saddle north of the highest peak
84 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

of Brooks Mountain. Knopf found a pegmatite dike in the vicinity 

of the limestone and granite contact at the southeast end of Brooks 

Mountain.

Since the intrusion of the granites the area has been carved into 

its present relief by the action of streams. The weak slate west of 

Brooks Mountain was removed more rapidly than the granite and 

the hardened slate and limestone adjacent to the granite. Only a 

small part of the granite mass has been exposed by erosion, and it is 

probable that the surface erosion has descended into the granite at 

most only a few hundred feet.

SLATE.

The slate, where unaffected by the granite intrusion, has an exceedingly fine grain, the individual particles being barely discernible 

even under the microscope. The black color is due to finely divided 

carbonaceous material. Fine-grained quartz and delicate shreds of 

brown mica are the principal constituents. Slate is not exposed in 

contact with the granite, but from several hundred feet to 1,000 feet 

west of the exposed granite contact the slate is altered to a very 

dense, compact brown mass which has lost its original platy character. It is very faintly banded and resembles some fine-grained 

rhyolites. The minerals are slightly coarser than in the unaltered 

slate, and the carbonaceous material has been aggregated into tiny 

rodlike brown masses.

LIMESTONE.

On the west side of the head of the Mint River valley about 1001 

feet of thin-bedded basal limestone with black shaly graphitic partings is exposed in a number of places. Similar basal beds of black 

shaly limestone are exposed in the valley northwest of the highest 

peak of Brooks Mountain. West of the Mint River valley these 

basal limestones are overlain by at least 900 feet of thin-bedded gray 

limestone with brown shaly partings. The gray limestone in turn is. 

covered by an unknown thickness of pure gray granular limestone in 

beds averaging about 3 feet in thickness.

At the south end of Brooks Mountain the limestone in contact with 

the granite is mostly of the pure gray type. About 400 feet west of 

the exposed contact with the granite it has been changed into a very 

coarse white marble traversed by seams of greenish silicates, which 

have replaced the calcite. The minerals replacing the calcite consist 

chiefly of a yellowish-green boron-bearing vesuvianite and a monoclinic pyroxene, usually diopside near the contact and either augite 

or hedenbergite farther away. The minerals known to replace the 

limestone near the granite contact include vesuvianite, diopside,
BROOKS MOUNTAIN. 85

grossularite, augite, hedenbergite, phlogopite, and fluorite. Of 

minor importance are scapolite, tourmaline, hulsite, paigeite, ludwigite, galena, pyrrhotite, black sphalerite, and chondrodite. The occurrences of lead are described in more detail on pages 86-87.

North of the white marble the limestone adjacent to the granite 

is of the thin-bedded type inteiiayered with shale. Along the crest, 

in the vicinity of the highest peak of the mountain and westward 

for about half a mile, this limestone has been converted into an exceedingly fine grained mass resembling chert. Microscopic examination of this material shows that the calcite has nearly all disappeared 

and has been replaced chiefly by thin layers of vesuvianite and colorless pyroxene alternating with grossularite. Farther west the original calcitic layers still remain, but they have become coarser in 

grain and the shaly layers have undergone a recrystallization. 

Weathering emphasizes the shaly layers and gives the rock the same 

appearance as that of the " reef " north of the tin prospect on Cassiterite Creek. 

« GRANITE.

The granite area of Brooks Mountain is elliptical, extending about 

2£ miles from north to south and about 1 mile from east to west. 

Except along the southern part of the mountain, all of the granite 

area lies on the east slope.

The granite contact dips less steeply toward the west than toward 

the east, as is indicated by the great width of the area of contactmetamorphic limestone west of the granite. That the eastern surface 

of the granite is very steep is shown by exposures along the northeast 

side of the granite area and by the narrowness of the zone of altered 

limestone on this side of the mountain.

Away from the border the granite consists mainly of very coarse 

crystals of orthoclase averaging about 1£ inches in length, which 

constitute about three-fourths of the volume of the rock. Between 

the orthoclase crystals is a coarse-grained matrix composed chiefly 

of orthoclase with minor quantities of acidic plagioclase and coarse 

angular grains of glassy quartz. Within 3 feet of the limestone contact the granite has an even grain of medium size. The minerals are 

the same as in the interior, except for the presence of small crystals 

of green hornblende, which takes the place of the biotite. On the 

weathered surface of the granite occur scattered knots or protuberances about 2 inches in diameter. They have a fine, even grain and 

consist chiefly of orthoclase with an abundance of the green hornblende that is characteristic of the border. Two generations of quartz 

are present in these knobs. One with hexagonal outline is older 

than all other minerals; the other lacks crystal outline and was the 

last mineral to crystallize.
86 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

The granite shows no visible evidence of having been affected by 

hot solutions after it had solidified. Samples taken within a few 

inches of the border show the secondary development of fine microscopic needles of green tourmaline, which penetrate the feldspars. 

These needles are probably related in origin to the tourmaline found 

in the altered limestone adjacent to the granite. Chemical decay 

of the granite by weathering is almost entirely absent. As previously noted, the granite disintegrates into a coarse rubble in which 

all the original minerals are present.

PROSPECTS. 

READ LEAD PROSPECT.

On the Read claims, near the contact of the granite and limestone on the southwest slope of Brooks Mountain, at the head of 

the first east branch of Mint Eiver, are several shallow trenches 

and a shaft about 20 feet deep. When" they were examined in 1918 

slope wash had covered the exposures in these openings, and the 

only clue as to the character of the rock and mineralization was 

obtained from samples on the dumps. Knopf, who examined this 

prospect in 1918, says that a trench 20 feet from the granite contact 

disclosed an ore body 3£ feet wide, striking N. 15° W. and dipping 

60° E., into the granite. He reports that assays made of this ore 

in Nome ran 34 per cent of lead and 11 ounces of silves to the ton. 

and that several other assays are said to have shown results ranging 

from $17 to $44 to the ton.

The ore on the dump consists of pyrrhotite intergrown with 

galena in small irregular lenticular masses about 0.4 inch in length. 

In places galena of the same character is embedded in a black opaque 

sphalerite, which has been reported to contain 19 per cent of ferrous iron. The gangue of this ore consists of calcite replaced by 

vesuvianite, diopside, phlogopite, tourmaline, and fluorite. Fluorite 

is intimately associated with the sulphides.

Several hundred feet down the slope from these prospects a small 

trench cuts through a mass of altered limestone in which Knopf discovered the mineral hulsite, a previously unknown compound of tin 

and boron.

LUTHER LEAD PROSPECT.

Across the divide from the first east branch of Mint River, at an 

elevation of about 2,000 feet, there is a lead prospect consisting of one 

trench 30 feet long, 12 feet deep, and 10 feet wide on a claim owned 

by W. B. Luther. It is 20 feet north of the limestone and granite 

contact. Within 4 feet of the granite the limestone has been con-
BROOKS MOUNTAIN. 87

verted to a yellowish-green hard granular mass, consisting chiefly of 

vesuvianite. At 20 feet beyond this point it is changed to a white 

granular marble, traversed by seams 'of green minerals similar to 

those which replace the calcite at the contact. At the north end 

of the trench the limestone has been replaced by the green silicates 

along a fissure striking north and dipping steeply east. The minerals 

replacing the limestone include tourmaline in black columns several 

inches in length and v^esuvianite. Vuglike openings in the mass of 

metamorphic minerals show an occasional cube of galena one-sixteenth 

of an inch in diameter.

Descriptions of several other small prospects in the vicinity of 

the granite contact are given in Knopf's report, already cited.

MINERALIZING SOLUTIONS.

That the alteration of the limestone was effected by gases or solutions or both is shown by the fact that they followed fissures which 

traverse the limestone. From a comparison of the original minerals 

of the limestones with those deposited by the solutions it is concluded 

that the solutions introduced fluorine, boron, chlorine, lead, copper, 

zinc, sulphur, and tin into the limestone. They also carried silica, 

alumina, magnesium, and iron. From the limestone they removed 

carbon dioxide in very large quantities. Pure limestone was almost 

completely replaced by silicates. Impure limestone some distance 

from the granite lost carbon dioxide, and the materials remaining 

underwent a recrystallization, probably with very little introduction 

of material. Such are the hard, finely banded chertlike phases of the 

altered limestone.

It is not proved that these solutions traveled through the granite 

mass now exposed, for the granite so far as known shows no changes 

that such solutions might have effected. It is conceivable that they 

passed through when the granite was unfavorable for interaction 

with the solutions. The border of the granite shows a faint introduction of tourmaline in the form of fine needles penetrating the feldspars. It is also along the contact that the limestone is most altered. 

The alteration is not due to an interaction of the granite with the 

limestone, for the granite contact has the sharpness of a knife-edge, 

but to the intense action of the solutions. To judge by results the 

solutions were most effective in causing deposition and replacement 

along the granite and limestone contact, and this may have been the 

principal path along which they moved.

Some of the constituents which the solutions carried, namely, 

fluorine, chlorine, and the metals, are so foreign, to the content of 

ordinary surface solutions that they are believed to have been released 

in hot solutions from the granite during the process of cooling.
88 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

PROBABILITY OF FINDING TIN AND OTHER MINERALS IN COMMERCIAL QUANTITIES AT BROOKS MOUNTAIN.

Studies made at Brooks Mountain do not warrant the hope that 

tin or other ores will be found in commercial quantity. This conclusion, however, can not be regarded as final. The chances of finding 

ore in the granite are the poorest. Wherever studied the granite 

shows no visible alteration. It would be useless to undertake underground prospecting on the granite unless places were found where a 

considerable body of it shows alterations of a character similar to 

those which have affected the tin-bearing dikes in the vicinity of 

Cassiterite Creek. It is important to bear in mind that such places 

would not be easily recognized. Because of the softness of the 

altered rock it would be eroded rapidly and thus underlie depressions 

that would be easily covered by slope wash of fresh granite.

Mineralized portions in the limestone area therefore bear a relation to the surface generally the reverse of that shown by the mineralized granite. Because of their relative hardness silicates, such as 

vesuvianite, tourmaline, and others characteristic of the metamorphosed limestone, tend to stand out on the weathered surface and are 

therefore easily discovered. It is probable that nearly all mineralized areas of the present limestone surface have been seen. Such 

deposits in the limestone are found only near the granite contact 

or adjacent to fissures that extend near the contact. They have an 

irregular shape, and it is possible that some of those now exposed 

are larger farther down. The proportions of mineralized rock and 

limestone at the present surface, however, are very probably similar 

to those found at greater depths. They are more likely to be large 

where the granite and limestone contact has a gentle dip, as on the 

west side of Brooks Mountain, than where it is steep.

POTATO MOUNTAIN.

GEOGRAPHIC FEATURES.

About two-thirds of the tin produced in North America has come 

from the Potato Mountain area, where, thus far, the Buck Creek 

placers have been the chief source of ore.

The Potato Mountain area is a part of the slate belt, which is 

characterized by a mild relief of 200 to 600 feet and great expanses 

of tundra-covered hills. The topography is in marked contrast with 

the rugged limestone ridges, 2,000 to 2,500 feet high, that form the 

York Mountains, to the east. (See PL I and fig. 4.) As a rule, 

the areas of accentuated relief in the slate belt can be accounted for 

by the presence of pre-Ordovician basalt or gabbro, or locally other 

igneous intrusive rocks. Potato Mountain, which consists of four
POTATO MOUNTAIN. 89

rather well defined knolls from 1,200 to 1,400 feet high, alined in a 

northeasterly direction and extending for a distance of about 2 miles, 

is the highest prominence of the slate belt and rises near its northern 

extremity. Potato Mountain is due partly to granite porphyry dikes 

which intrude the slate, and partly to numerous quartz "veins that 

cut the mountain mass in all directions, thereby increasing its resistance to erosion. The porphyry dike strikes northeast and coincides with the trend of the mountain mass. It is not everywhere 

exposed, being in part concealed by talus on the mountain slope, and 

in part not yet revealed by erosion. The relation of intrusive dike 

overlain by slate is well shown in an excavation on the northeast 

slope of the next most northerly mound, where the slates are exposed 

arching above the dike rock.

ROCKS.

The slate of the Potato Mountain area is fine textured, is sandy to 

calcareous, and ranges in degree of metamorphism from shalelike to 

well-cleaved varieties. It is interbedded with a few strata of yellowish fine-grained sandstone, bluish clayey sandstone, and sandy limestone a few inches in thickness. The beds dip in general at a low 

angle toward the east.

Intruding the slate is a quartz porphyry dike, already referred to 

as having in part caused the mountain mass. Ramifying quartz 

veins which range in width from a few inches to a fraction of an 

inch are to be seen almost everywhere on the area and in some places 

form a stockwork. One quartz ledge is traceable for several miles. 

It cuts Sutter Creek at the mouth and extends up Buck Creek to 

West Fork, where it swings almost due north. No other acidic 

rocks were observed in the district. Pre-Ordovician basalts and 

gabbros do not occur in the immediate vicinity of Potato Mountain.

ECONOMIC GEOLOGY.

The occurrence of cassiterite in the rocks of the Potato Mountain 

area is not commonly observed. A study of the placer concentrates, 

however, shows that it is found in at least two associations, namely, 

with quartz and with porphyry. Knopf 26 also reports it to occur 

" intergrown with arsenopyrite in a gangue of radial actinolite." 

No deposit of lode tin in commercial quantity has yet been discovered 

in the area.

Quartz carrying cassiterite is present chiefly as stringers, a fraction of an inch to several inches in width, in the slate. Some of 

these stringers evidently carry gold, as gold is present in small

28 Knopf, Adolph, Geology of the Seward Peninsula tin deposits, Alaska: U. S. Geol. 

Survey Bull. 358, p. 33, 1908.

101987 22 7
90 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

quantities in the placers of Buck Creek. The only porphyry observed in the area which might be a source of cassiterite is the large 

dike referred to as .cutting the mounds. No cassiterite, however, 

was observed in the specimens examined.

Numerous attempts have been made to locate the source of the tin 

ore. Prospecting has been confined to Potato Mountain and the 

country adjacent to it. Thus far the attempts have met with but 

little success, and no promising lode has been uncovered. There 

is, however, good reason to consider Potato Mountain as the probable source of the placer tin, because (1) cassiterite nuggets as 

much as several pounds in weight have been picked up on the mountain, (2) quartz veins cutting the slate locally show cassiterite 

crystals, (3) the stream placers are limited to those streams that 

head in the mountain, and the coarseness and angularity of the tin 

decrease with distance from the mountain, (4) placer tin from the 

headwaters of the creeks shows attached particles of porphyry and 

quartz, whereas in that from the lower parts of the creeks most of 

the crystals are clean or attached to bits of quartz only.

LODE DEVELOPMENTS.

A quartz ledge, traceable for several miles, crosses Sutter Creek 

near the mouth, occurs in the bed of Buck Creek, and at West Fork 

leaves the stream to continue northward with a strike of N. 10° 

E. The ledge is in places as much as 100 feet wide. It has the 

appearance of felsite rock including numerous shale particles. 

Microscopic examination shows it to consist chiefly of well-developed 

quartz crystals arranged in radial and coxcomb structure about 

nuclei of shale. It represents a filled fissure reopened by repeated 

movement along the fracture zone so that small particles of shale 

from the vein wall adhered to the vein filling and upon further 

introduction of vein material became incorporated in the quartz 

mass.

Numerous prospect pits have been sunk at different points along 

the outcrop of the vein, and assays are said to have been made. No 

tin or other mineral of economic value was observed, and the assays 

are reported to have been unfavorable.

At the head of Buck Creek, on the divide between Peluk and Fox 

creeks, are several shallow prospect trenches on the Eed. Fox claim. 

One trench exposes what appears to be either a vein or decomposed 

dike rock about 6 inches thick dipping 35° W. and striking north 

and east. The beds of slate inclosing it dip at a low angle toward the 

north. The dike rock has the appearance of a limestone, and microscopic examination shows it to consist of calcite with a little quartz 

and to be cut by veinlets of serpentine (chrysotile) inclosing a
POTATO MOUNTAIN. 91

sprinkling of sulphide crystals. The mineralization is scanty, and. 

the total sulphides form but a fraction of 1 per cent of the rockThe rock is sheared and fractured and is seamed with veinlets of 

calcite.

At 150 paces north of the Red Fox claim there is a shaft (figure14) which was filled with water at the time of visit but was reported 

to be 30 to 40 feet deep. The material on the dump is black slatecut by quartz stringers a quarter of an inch in width and a little rustContour interval 200feet 

FIGURE 14. Sketch map of Potato Mountain area, showing tin claims and prospects.

covered quartz showing drusy cavities containing pyrite crystals and 

stannite in quantities of a fraction of 1 per cent of the rock.

On the dump of an east-west trench 20 feet long and 10 feet deep,. 

150 paces northwest of the shaft, some pyritiferous quartz but no> 

other mineralized rock was observed.

Debris of a fine-grained massive dacite showing considerable sulphide mineralization occurs on the mound just southeast of the Fox' 

Creek divide. The rock contains plagioclase feldspar, quartz, micar
92 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

and pyrite in a groundmass consisting chiefly of feldspar and shows 

considerable secondary calcite. On the southeast slope of this mound 

a prospect trench exposes light-gray porphyry debris, with phenocrysts of rectangular feldspar 1 inch in length and quartz phenocrysts a quarter of an inch in diameter. About 80 per cent of the 

rock is dense, compact groundmass. Some pyrite cubes and tourmaline druses were the only evidences of mineralization observed.

At 160 paces farther down the slope to the southeast a pit 15 feet 

deep exposes slate cut by quartz stringers of an average size of 1 inch. 

This is known as the Alan Dale lode. The slate is a massive gray 

rock and is the metamorphic derivative of a sandy shale. It consists of 30 per cent quartz groundmass, in which are embedded 

quartz, mica, numerous small grains of pyrite, and some calcite.

On the northeast slope of the third mound from the north, about 

100 feet from the crest, a pit 18 feet deep exposes an anticline in the 

slate, the core of which is a quartz porphyry dike about 1 foot wide 

at the upper contact and widening to 8 feet at the base of the exposure. The outer margin of the dike is dark gray to brown-black 

and shows pyrite and a brown sulphide, possibly stannite, in minute 

impregnations. The center of the dike is a gray porphyry about 60 

per cent of which is groundmass, containing phenocrysts of feldspar 

measuring half an inch and of quartz one-fourth inch in size and 

an abundant sprinkling of sulphide minerals, pyrite and stannite. 

This is the porphyry dike referred to on page 90 as a possible source 

of the placer tin of Buck Creek. It was not observed to contain 

cassiterite. Microscopic examination shows it to consist of quartz, 

orthoclase, biotite, and secondary sericite. The boron, fluorine, and 

other minerals so commonly associated with tin are absent, and the 

only constituent observed that suggests such an association is the 

sericite, which in the Cassiterite dike of the Lost River area is closely 

related to the stanniferous portion of the dike.

At the top of the third mound from the north two test pits expose 

slate cut by quartz veinlets half an inch wide. The quartz is 

slightly drusy and contains very fine tourmaline needles. No 

cassiterite or stannite was observed. Near the southeast edge of 

the mound six or more pits show slate breccia with fragments 4 to 

8 inches in size cemented by quartz. The quartz is rusty and drusy 

and contains tourmaline needles. No other evidence of mineralization was observed.

In the saddle to the south of the third mound from the north a 

trench 100 feet long exposes a banded slate cut by quartz stringers 

which contain tourmaline and some slight showing of sulphides.

On the top of Little Potato Mountain a shaft 9 feet deep on the 

Eureka claim exposes quartz containing crystals of resinous cas-
POTATO MOUNTAIN. 93

siterite and tourmaline in drusy cavities. About 1 ton of ore is 

reported to have been taken from this shaft to York. The slate 

at this locality is generally cut by quartz stringers several inches in 

width which carry sulphides. Melanterite, a white to greenish 

sulphate of iron, occurs as an incrustation of some rock surfaces in 

this locality. Microscopic examination shows that the quartz and 

tin ore occur in distinct bands associated with tourmaline and arsenopyrite, the tourmaline crystals penetrating both the quartz and the 

cassiterite crystals.

On the Daisy claim, near the east edge of the top of Potato 

Mountain, a shaft 25 feet deep has been sunk. The shaft was rilled 

with water at time of visit. No well-defined ledge is reported to 

have been encountered, but rather a concentration of quartz stringers in the slate. Two types of vein rock were observed on the dump 

one a quartz vein filling, the other a zoisite rock containing 85 per cent 

of zoisite, hedenbergite, fluorite, and calcite.

A north-south tunnel 25 feet long, 100 feet above the left fork 

of Buck Creek on the east side of Little Potato Mountain, cuts 

numerous intersecting quartz-sulphide veinlets half an inch to 1 

inch wide. Some of the veins are accompanied by fault gouge 10 

inches wide.

Too few examples of ore in place are known to permit an extended 

discussion of the origin of the ores at this locality. The occurrence, 

of tin in quartz veins and granitic rock associated with quartz, 

arsenopyrite, tourmaline, and pyrite, as here, is characteristic of 

tin deposits the world over. The source of the tin-bearing solutions 

is almost certainly a granitic mass which underlies the area. That 

such a mass exists is probable also from analogy with the Cape 

Mountain and Lost River areas, where tin-bearing porphyry dikes 

similar to the one found here are closely associated with granite 

masses.

The action of mineralizing solutions in the formation of tin 

deposits is discussed on pages 123-124, and that discussion applies 

to this occurrence so far as known.

Concerning the depth at which the granite mass underlying the 

area occurs figures in feet or hundreds of feet can not be given. 

However, from outcrops of the dike overlain by slate it is certain 

that only the upper parts of the dike have been exposed at this point. 

Further evidence of the marginal position of these exposures with 

respect to the source of mineralization lies in the association of the 

tin minerals with quartz. This position is analogous to that of the 

quartz-cassiterite occurrence in the Cassiterite Creek area, where 

the quartz-cassiterite veins of the Ida Bell dike extend farther from 

the center of mineralization than any other combination of cassiterite-bearing minerals.
94 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

It is certain that the porphyry has been mineralized by tin-bearing 

solutions, as the association of cassiterite and porphyry in pieces of 

float has been observed. It is equally obvious from examination of 

the outcrop that cassiterite is not uniformly distributed throughout 

the dike.

As exposures are few and poor, prospecting can perhaps best be 

directed by considering the occurrence as analogous with other 

better-known occurrences in the York region. At Lost River the 

mineralized porphyry is in the form of an ore shoot and consequently 

fails to persist along the outcrop. In that portion of the dike which 

has been acted upon by tin-bearing solutions the feldspar constituent, especially, has suffered alteration (sericitization), as a result of 

which the roc]?: is softened and made more susceptible to decay when 

exposed to the air.

To apply the same principle to Potato Mountain, the most 

favorable place to prospect would seem to be the saddles between the 

mounds, it being borne in mind, however, that the upper contact 

of a dike is likely to be irregular and the saddles may be due to a low 

point on the dike, as well as to difference in its resistance to erosion 

us just outlined.

The Iron Creek placers show cassiterite associated with porphyry, 

and the creek heads between Big Potato and Little Potato mountains. 

If further work is contemplated in that vicinity the porphyry dike 

.should be sought for and explored in depth.

TIN PLACERS.

Cassiterite was identified in the gravels of Buhner Creek by Alfred 

H. Brooks in 1900. In the course of prospecting for gold on Buck 

Creek some mineral, thought by the miners to be iron, had been 

found. By comparison with material from Buhner Creek it proved 

to be cassiterite. Buck Creek has since that time been the chief 

source of tin on Seward Peninsula. From 1900 to 1911 mining was 

done on the creek by hand and automatic scraper. About 324 tons 

of tin ore averaging about 50 per cent of metallic tin was recovered 

in this way. In 1911 the York Dredging Co., and in 1915 the American Tin Mining Co. installed dredges on Buck Creek. Since 1911 all 

the ore recovered has been won by dredging.

Placer-tin concentrates prodiiced on Buck Greek, 1911-1919, in short tons.

1911 ____ __ __

1912

1913

1914

1915 __ __ ___ .

3916- _

___ _ _ 93

174

__ __ _ 60

______ 160

150

1917

1918

1919 ________ .

146

55

__ _ ___ 56

1,194
POTATO MOUNTAIN. 95

The average assay of dredge concentrates is about 68 per cent of 

metallic tin. Some gold occurs in the concentrates. In 1912 an 

average of $32 gold to the ton was reported. An amalgamation barrel is installed on the American Tin Mining Co.'s dredge in which 

concentrates from the upper 18 feet of riffles are treated.

The accompanying map (fig. 14) shows the distribution of placer 

ground so far as known. Work thus far has been confined to the 

streams southeast of Potato Mountain, and the productive ground 

has not extended much below the mouth of Buck Creek.

The worked placers have shown an average yield of about 8 pounds 

of concentrates, averaging about 68 per cent of metallic tin to the 

cubic yard. Smaller creeks in the area have run as high as 21 pounds 

of concentrates to the cubic yard.

The bedrock is slate. It is very uneven and shows many irregularities in the form of waves or channels parallel to the stream course 

18 to 20 inches deep and 3 to 5 feet apart. It is soft and easily dug, 

and its surface carries most of the tin. The gravels consist of slate 

with a little quartz. More than 60 per cent of the gravel passes a 

2-inch mesh, and nearly all of it is less than a foot in greatest dimension. The average depth of gravel is 5 feet. So little clay is present 

in the upper creek that the American Tin Mining Co. found difficulty 

in constructing a dam to retain water for its dredge. Most of the 

cassiterite occurs as small, slightly rounded crystals, although occasionally nuggets weighing from 1 to 8 pounds are encountered. But 

little foreign material adheres to the placer tin, as is shown by the 

high metallic content of the concentrates. Some magnetite and 

considerable hematite are present in the placers in nodules as much 

as several pounds in weight.

The York Dredging Co. has worked from Sutter Creek, a tributary of Buck Creek, to the mouth of Buck Creek and down Grouse 

Creek to East Fork. The tenor of the gravel at this point did not 

permit further working, so this may be considered the lower limit 

of the pay ground. The American Tin Mining Co. is dredging on 

upper Buck Creek, just below West Fork. The average width of 

gravel dredged on Buck and Grouse creeks is 80 feet, and the 

average depth of gravel 5 feet.

The ground still remaining to be worked and shown to be dredgeable comprises upper Buck Creek, Left Fork, and Peluk creeks 

(headwater tributaries of Buck Creek), several small gulches tributary to Buck Creek, and a strip of ground of undetermined, width 

bordering either side of the dredged area on Buck Creek and extending to the mouth. This border area has been drilled and is known 

to be workable, but it lies below steep tundra-covered banks and the 

overburden which must be removed by groundsluicing is considerable. It is conservatively estimated that an area equal to about one-
96 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

fourth the dredged area of Buck Creek and at least as valuable 

per yard as that area is thus available.

Sutter Creek is reported to be dredgeable above the mouth, and 

Iron Creek, a small tributary of Sutter Creek, shows high-grade 

ground adaptable to shoveling in but not to dredging.

Grouse Creek above Buck Creek and below Left Fork contains no 

tin, or at least too little for mining, so far as known. Skookum 

Creek, Left Fork, and Mint River are barren of tin.

It is thus evident that according to available data the Buck Creek 

placers would seem to be about two-thirds exhausted. It is perhaps 

now time to turn attention to the streams northwest of Potato 

Mountain which head on its flanks and flow into Lopp Lagoon. Most 

of these streams are reported to show tin, and several have recently 

satisfied prospectors as to their dredging possibilities. Plans are 

said to be under way to install a dredge on these waters in the near 

future. They have never been extensively prospected, but the result 

of the work done and their favorable position with reference to 

Potato Mountain would justify further exploration. These streams 

will not be so adaptable to easy working as those southeast of the 

mountain, owing to their low gradient, as they flow for most of their 

length through the wide tundra flats south of the lagoon. Here 

groundsluicing may prove impossible, and the tundra will consequently be a handicap. It is even possible that no great concentration has resulted in these streams, owing to their meandering and 

constantly changing courses in past geologic time.

CAPE MOUNTAIN.

GEOGRAPHIC FEATURES.

Cassiterite occurs in association with granite intrusions at Cape 

Mountain. Although considerable money has been expended here 

in prospecting, no commercial bodies of ore have been discovered. 

In 1918 all workings were abandoned and inaccessible, owing to rock 

falls and the formation of ice. The following notes on the tin ores of 

Cape Mountain are based mainly on surface examinations of the 

ledges and materials on the dumps.

Cape Mountain is at the extreme west end of Seward Peninsula, 

where the shores of the Arctic Ocean and Bering Sea converge and 

form Cape Prince of Wales. It is a dome-shaped peak rising out of 

the sea to an elevation of 2,300 feet from a base nearly 4 miles wide. 

A narrow terrace flanks the mountain at an elevation of 1,000 feet 

(PI. XI, B). Above 1,000 feet the slopes are very steep and in places 

difficult to scale.

The locality can be reached by following the sea beach from York, 

a distance of 12 miles. A light wagon can be taken over this route
U. S. GEOLOGICAL SURVEY BULLETIN 733 PLATE XI

A. ARCTIC COASTAL FLATS, CAPE PRINCE OF WALES, AND KINGEGAN MISSION, 

FROM CAPE MOUNTAIN.

Bering Strait in the background.

B. UPLAND SURFACE (1,000-FOOT TERKACIi ) AT CAPE MOUNTAIN. 

Empire Tin Mining Co.'s mill in the foreground; Bering Sea at the left.
CAPE MOUNTAIN. 97

except during severe south winds. The telephone line that once connected the cape with Nome, 140 miles away, was wrecked by a storm 

in 1913 and has never been repaired. During the open season gasoline schooners sailing between Nome and Kotzebue Sound, in the 

Arctic Ocean, pass the cape at intervals of about two weeks and stop 

on signal. There is no safe landing place near by, and storms and 

fogs are more common than fair weather.

Cape 

Prince of Wales

x Lode

 Unclassified 

prospect

. House

/ UnitedStates-Alaska 

Tin Mining Company

2 Daisy claim, Seward 

Tin Mining Company

3 Empire Tin Mining Company BERING SEA

5 miles

Contour interval 200 feet

Limestone with some schist 

FIGURE 15. Geologic sketch map of Cape Mountain.

'Granite, 

(intrttsive)

GENERAL GEOLOGY. 

SUMMARY.

A large granite intrusion constitutes the core of the mountain. 

Bordering it on the north and east are limestone beds into which the 

granite has been intruded. In places portions of the limestone roof, 

which once covered the granite, still remain. The granite is closely 

associated with granitic dikes, granitic pegmatites, and quartz veins. 

Both the limestone and the granite show contact-metamorphic phases. 

The youngest rock of the area is an intrusion of black olivine porphyry, which in the form of dikes cuts both limestone and granite. 

(See figs. 15-17.)
98 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

GRANITE.

The visible portion of the granite intrusion is a dome-shaped mass 

about 3 miles in diameter and 'over 3,000 feet high. The contact surface with the limestone appears .to be nearly vertical on the east but 

dips more gently to the north. It is irregular, the granite having 

welled upward into the limestone along vertical planes and also between the limestone beds. Occasionally limestone blocks are found 

which foundered into the granite. A boulder of granite on the dump

1-Limestone . 2-Granite

o iboo 2000 feet

FIGURE 16. Geologic section across Lagoon Creek, Cape Mountain.

of the United States Alaska Tin Mining Co.'s tunnel (see fig. 15) 

contains a block of limestone about 6 inches in diameter. Between 

Lagoon and Village creeks an area of several hundred acres is underlain by limestone partly submerged in the granite while it was still 

in a molten condition. The granite on top of the mountain is covered by several feet of limestone overlain by about 200 feet of finegrained quartz-biotite schist.

The main body of the granite and the marginal portion, in most 

places only about 3 feet wide, differ in grain and mineral composition. Texturally and mineralogically the main body of the granite is identical with that of Brooks Mountain, and samples of the 

two can not be distinguished. It is a porphyry consisting of large 

crystals of orthoclase about 1 inch long embedded in a medium-

_____Granite _ ___ Port Clarence limestone

FIGURE 17. East-west section through Cape Mountain. (See fig. 15.)

grained matrix of orthoclase, sodic plagioclase, angular crystals of 

smoky quartz, and minute flakes of brown biotite.

The grain of the marginal portion is uniform and generally 

of medium size. Feldspar is very prominent, and biotite appears to 

be absent. In this respect also the granite of Cape Mountain resembles that of Brooks Mountain.

Dikes and sills of granite several feet in width appear to be very 

common along the margin of the granite and represent offshoots 

from the granite mass. In grain and composition they resemble
CAPE MOUNTAIN. 99

the marginal portion of the granite. As a rule they have minute 

irregular openings lined with druses of quartz and tourmaline and 

locally plates of muscovite mica. On Village Creek, at an elevation of about 600 feet above the sea, below the confluence of the 

small streams that constitute the head of the creek, a vertical granite 

dike 2 feet wide, striking N. 60° W., cuts through the limestone. 

It is full of druses of tourmaline and muscovite. On the road between the sea and the tunnels of the Empire Tin Milling Co, a 

granite dike 15 feet wide, striking S. 70° W., cuts through the limestone at an elevation of about 800 feet. It dips southwest, toward 

the granite. On the Percy claim a trench discloses a granite dike 

about 10 feet wide striking northeast and cutting the limestone 

vertically. It is overlain by limestone beds and is associated with 

tin minerals.

GRANITE PEGMATITE.

Granite pegmatite is scarce at Cape Mountain. *0n the dump 

of the United States Tin Mining Co. boulders of a quartz-feldsparmuscovite pegmatite were seen. At the Percy claim pegmatite veins 

2 inches in width, 'composed of quartz and feldspar, cut through 

limestone boulders on the dump. The largest pegmatite dike seen 

was on Village Creek at an elevation of about 200 feet. It is 

10 feet wide and consists of coarse crystals of quartz, feldspar, and 

biotite plates 5 inches in diameter. It has been opened by a tunnel 

15 feet long.

Knopf 27 describes an occurrence of pegmatite on the Canoe claim. 

He says that a sill of granite 8 feet thick has- a marginal phase of 

fine-grained granite which is overlain by about ! £ inches of very 

coarse pegmatite composed of quartz and feldspar. Observations 

made by Knopf show that the pegmatite was contemporaneous with 

the granite.

QUARTZ VEINS.

Tin-bearing quartz veins, so far as known, occur only in the granite. In both granite and limestone quartz veins are scarce. Pieces 

of quartz-vein material were found on the dump of the Empire Tin 

Mining Co.'s workings. At the tunnel of the United States Alaska 

Tin Mining Co., on the north slope of Cape Mountain, at an elevation of 1,850 feet, a quartz vein bearing N. 45° W. (magnetic) is said 

by Knopf to cut the granite. He reports that secondary quartz, tourmaline, and sericite occur in the granite adjacent to this vein. In 

1918 a boulder of vein quartz containing several crystals of yellowishbrown cassiterite about half an inch in diameter was found on the 

dump of the tunnel, the only cassiterite seen at this place.

" Knopf, Adolph, Geology of the Seward Peninsula tin deposits,, Alaska: U. S. Geol. 

Survey Bull. 358, p. 37, 1908.
100 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

At only one other place was vein quartz observed, namely, on the 

road between the sea and the Empire Tin Mining Co.'s workings. 

Here at an elevation of about 1,000 feet a 2-inch quartz vein cuts 

through the limestone and dips southwest.

LIMESTONE.

The limestone at Cape Mountain is composed chiefly of calcite, with 

some admixture of quartz. In color it is generally white to bluish 

gray. Beds of quartz sand, now changed to quartzite, averaging 

about 5 inches in thickness, occur at intervals of about 15 or 20 feet. 

On top of Cape Mountain the limestone is overlain by a fine-grained 

quartz-biotite schist about 200 feet thick, which appears to be conforma'ble with the limestone.

East of the granite contact the limestone beds dip toward the 

granite at an angle of about 15°. Between Lagoon and Barluk 

creeks they dip0 east. North of the granite intrusion they dip north,, 

away from the granite. It does not appear that the intrusion of 

the granite has produced the present attitude of the limestone beds.

The changes which the limestone has suffered because of the intrusion of the granite fall into two classes those which involve the 

introduction of materials and. those which do not. The changes 

which do not involve the introduction of materials are as follows: 

Near the contact the limestone has been changed into a very coarse 

white marble, in which calcite grains half an inch in diameter are 

common. The quartz sand associated with the limestone has been 

changed into a dense-white quartzite by the recrystallization of the 

quartz grains. Wollastonite was commonly formed by a reaction 

between the calcite and the quartz grains, and locally the quartz beds 

have been changed completely into wollastonite. The change from 

calcite and quartz to wollastonite involves the loss of carbon dioxide 

from the calcite and the combination of the lime and silica.*

Close to the contact the limestone was locally invaded by hot tinbearing solutions which converted it into a heavy medium to coarsegrained rock. The altered zones appear to have a maximum width 

of only a few feet and are found immediately adjacent to the granite 

and along fissures that cut the limestone in the vicinity of the granite. 

This altered limestone was seen in place only at the Percy claim. 

(See fig. 15.) Here narrow seams of green secondary minerals, a 

few inches wide, lie adjacent to a dike and also cut through the 

limestone. The principal minerals of this green rock are green 

pyroxene and green tourmaline. Minor constituents are pyrite, 

pyrrhotite, fluorite, scapolite, accessory sphalerite, quartz, and cassiterite. About 15 tons of tin-bearing altered limestone lies on the 

dump of the Percy claim. The cassiterite is yellowish brown, fine
CAPE MOUNTAIN. 101

grained, and irregularly distributed through the rock. Some of the 

boulders are very rich in cassiterite. It was impossible to enter the 

tunnel from which this rock came, and hence statements as to its 

extent underground can not be made.

ALTERATION OF THE GRANITE BY TIN-BEARING SOLUTIONS.

The tin-bearing solutions that altered the limestone along the 

granite contact have also affected the border portion of the granite 

itself. On one of the dumps of the Empire Tin Mining Co.'s workings samples of granite were found showing a local fracturing along 

which minute seams of cassiterite and tourmaline had been introduced. They formed a cement and also replaced the feldspars. 

Pyrite, in minute scattered grains, accompanied the cassiterite and 

tourmaline. Knopf reports that in tunnel No. 2, called the North 

Star tunnel, a fractured zone of granite 18 inches wide showing 

visible amounts of both cassiterite and tourmaline occurs at 270 feet 

from the portal.

Granite dike materials thrown out on the dump of the Percy 

claim showed tourmaline, sulphides, and sericite. No visible cassiterite was noted.

SUMMARY OF OCCURRENCE OF TIN AT CAPE MOUNTAIN.

Tin, in the form of cassiterite, is known to occur as a constituent of 

quartz veins, as a replacement of limestone near the granite contact, 

and in granite near the limestone contact. The principal associates of 

the tin are tourmaline and pyrite. It is impossible to state whether 

any of these occurences will yield tin in commercial quantities. In 

view of the excellent exposures on Cape Mountain, it would seem 

that if quartz veins of considerable magnitude were present they 

could easily be discovered, because such veins would form strong 

outcrops, and. therefore, because of the scarcity of quartz-vein 

material on the surface it is improbable that large deposits of vein 

quartz occur on Cape Mountain.

Large deposits of tin in the limestone would also tend to form 

conspicuous outcrops on the surface. The cassiterite and the minerals 

associated with it in the contact limestone resist weathering much 

better than the unaltered limestone; hence weathering brings such 

deposits into relief. The lack of such outcrops suggests a lack of 

notable deposits of this kind at the present surface. It is possible 

that in places they may have been covered by slope wash. Such 

replacement deposits are very irregular in form, and the lack of 

them at the surface does not preclude the possibility that valuable deposits of this kind may exist at depths along the granite contact. It 

is more probable that large deposits of this type would occur where
102 GEOLOGY OF THE YOEK TIN DEPOSITS, ALASKA.

the contact plane between the granite and limestone has a gentle 

dip than where it dips steeply. The chances of finding such deposits are, therefore, better underneath the limestone north of the 

granite than along the eastern border.

Altered marginal phases of tin-bearing granite are weak materials, owing to the development of sericite and sulphides, and do 

not tend to form conspicuous exposures. They are easily covered 

up by slope wash and are not readily found. It is impossible to estimate the extent of the deposits of this marginal type.

DEVELOPMENTS.

Facts previously published regarding the developments at Cape 

Mountain indicate that the Empire Tin Mining Co. has three tunnels on the east side of the mountain at an elevation of about 1,200 

feet. Of these tunnels, No. 2 is the longest, its drifts and winzes 

aggregating 750 feet in length. This company also has a 5-stamp mill 

at the beach, a barn, a 7-room house, a blacksmith shop, and several 

minor buildings.

The United States Alaska Tin Mining Co. has a shaft 22 feet 

deep and a 7-foot tunnel trending S. 40° W. for 270 feet on the 

north side of Cape Mountain, at an elevation of 1,650 feet. On the 

beach the company has a 10-stamp mill, which has never been operated.

On the Percy claim is a tunnel probably less than 100 feet long, 

a shaft about 15 feet deep, and several trenches.

PRODUCTION OF TIN OBE.28

Concentrates amounting to 10 tons are reported to have been 

shipped from Cape Mountain. These were produced by the Empire 

Tin Mining Co. (formerly the Bartels Tin Mining Co.) in 1906.

PLACER TIN.

It is reported that a small amount of placer tin was discovered 

on Tin City Creek in 1918. The extent of this deposit is unknown, 

but probably it is unimportant, because of the small size of the 

stream.

EAR MOUNTAIN. 

GEOGRAPHIC FEATURES.

Ear Mountain is 50 miles north of Teller and 15 miles south of 

Shishmaref Inlet. It can be reached from Teller by three or four 

days' travel over trails that are usually passable for pack horses in 

the summer. In dry weather wagons have gone over the trail. For

28 Chapln, Theodore, Lode developments on Seward Peninsula: U. S. Geol. Survey Bull. 

592, p. 407, 1913.
EAE MOUNTAIN. 103

the last 20 miles the trail runs mostly over tundra. Travel from 

Shishmaref Inlet to Ear Mountain is almost as difficult. The inlet 

is very shallow and is navigated only by light dories and Eskimo 

skin boats.

Ear Mountain is isolated, like Cape Mountain. It rises from a 

base about 4 miles in diameter to a height of about 2,300 feet. Above 

.1,000 feet the slopes steepen. The mountain culminates in three 

peaks or mounds of which the one farthest southwest is the highest, 

standing about 2.300 feet above sea level. Most of the mountain is 

underlain by granite, the surface of which is covered with a rubble 

of frost-broken rock and dotted with towers of granite 50 to 100 

feet high. Here, as at Brooks Mountain, the development of these 

towers is controlled, by fissures. From two such towers which rise 

from the south side of the central peak and which from a distance 

have the appearance of two huge ears the mountain receives it name.

The country surrounding Ear Mountain is a low, gently rolling 

plateau from 600 to 800 feet high, cut by numerous stream valleys. 

Nearly all of it is covered with a brown peat bog. Low willows 

grow along the valleys, and in summer horses can be foraged along 

the small streams.

GENERAL GEOLOGY. 

SUMMARY.

The rocks that underlie Ear Mountain include dark quartz-biotite 

schist intruded by gabbro and thin-bedded limestone with shaly 

partings. Both schist and limestone dip northward at angles ranging from 15° to 45° and are intruded by a dome-shaped granite mass, 

steep sided on the south, east, and west but dipping more gently on 

the north. (See figs. 18 and 19.)

At the granite contact certain contact-metamorphic changes have 

affected both the schist and the limestone and locally the granite 

itself. These changes consisted in the expulsion of water and carbon 

dioxide from the limestone and the recrystallization of the materials 

remaining into heavy lime-bearing silicates. Locally this contact 

metamorphism has also involved the introduction of new materials 

by solutions. Some of the constituents of these solutions were derived from the granite intrusions, namely, boron, fluorine, chlorine, 

tin, lead, copper, and sulphur. These materials were introduced 

chiefly into the limestone but locally also into the border portions of 

the granite.

After the intrusion of the granite and the development of the contact alterations, erosion by streams carved this mountain from the 

rocks of the region, thus laying bare the granite and its associated 

materials. The relative resistance to erosion of the granite and of 

the contact-metamorphic rocks bordering it has caused them to be
104 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

brought into relief. Frost action has been the chief process by which 

materials were prepared for steam transportation. The streams

Contour interval 200 feet

Dacite porphyry Strike and dip 

dike of strata.

Fault Unclassified 

^prospect

FIGURE 18. Geologic sketch map of Ear Mountain. A-B, line of section, figure 19.

flowing from the mountain have produced a slight concentration 

of the tin as stream placers.
EAR MOUNTAIN. 105 

QUARTZ-MICA SCHIST AND ASSOCIATED BASIC INTRUSIVE ROCKS.

The quartz-mica schist is a dark graphitic rock, composed chiefly 

of very fine grained quartz of irregular outline and minute shreds of 

brown biotite mica. The mica plates have a parallel arrangement 

and occur in very thin wavy bands. As a rule the bedding is indistinct, but a faint banding due to alternation of mica and quartz 

layers is very common.

On the east mound of Ear Mountain the quartz-mica schist underlies nearly all of the north slope and appears to alternate with limestone on the south slope. The position of the schist beds on the south 

slope is indicated by debris except at one outcrop in which the schist 

beds dip 30° SW., a direction almost opposite to that of the limestone beds near by.

The schist also underlies the top of the highest peak of Ear Mountain. Here it is not in contact with the limestone but has been intruded by the granite.

Alternations of schist and limestone are found in the bed of Quartz 

Creek. The relations of these schist beds to the limestone is not clear

X Port Clarence

v"- - > - v ' - v 7 v'T^rr-Trr -L limestone f-: v Granite-.r-:-"J-\

Sea,lavel,i

FIGURE 19. Section through Ear Mountain along line A-B, figure 18.

in all places. Some of them are overlain by limestone beds having 

the same strike and dip as the schist. In other places the schists dip 

much more steeply than the limestones, which everywhere dip about 

30° N. Near the end of the Quartz Creek canyon, at an elevation of 

900 feet, beds of black graphitic schist striking nearly north are inclined steeply to the west. In dip and strike this outcrop is clearly 

discordant with that of the limestone beds in this vicinity.

The schist on top of the highest peak of Ear Mountain is intruded by a vertical dike of basalt 15 feet wide, trending N. 50° W. 

This dike can be traced to a more massive basic intrusion about 100 

feet wide on the northwest side of the peak. On microscopic examination the rock of the larger mass is found to be a slightly schistose 

altered gabbro. A faint schistosity is also visible in the hand 

specimen. The mineral constituents are limy plagioclase embedded 

in a matrix of hornblende which incloses numerous small grains of 

magnetite. The hornblende shows a tendency toward parallelism 

and probably is an alteration product of augite or a similar pyroxene.

The relation of the quartz-mica schist of Ear Mountain to the black 

slate of the York area is uncertain. Several facts, however, suggest 

101987 22 8
106 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

that the two belong to the same formation. They are alike in their 

composition and in their finely laminated structure. The chief constituents of both are fine-grained quartz, brown mica, and carbon. 

Both the quartz-mica schist and the slate are intruded by basic 

igneous rocks whose schistosity and other secondary structural 

features indicate that they were intruded before the slate and limestone beds were folded. Faults have disturbed the relations of the 

slate and limestone of the York area. The quartz-mica schist and 

limestone of Ear Mountain are similarly disturbed by faulting. 

Black slates like those of the York area apparently underlie all the 

territory between Ear Mountain and Idaho Gulch. The principal 

facts which suggest the identity of the quartz-mica schist of Ear 

Mountain and the slate of the York area are, therefore, their similarity of mineral composition, lamination, instrusions, faulting, and 

association with limestone.

Some of the schist beds at Ear Mountain appear to be conformable 

with the limestone beds; others are not. Their present distribution is 

probably due to faulting, for although they are older than the limestone they seem to overlie it on two of the peaks of Ear Mountain, 

namely, the one farthest east and the one farthest southwest. Faulting is also suggested by some very marked discordances between the 

dip and strike of the limestone beds and those of the schist beds. 

Such discordances were noted on the south slope of the east mound 

and in the gulch of Quartz Creek. In both of these places certain 

alterations of limestone and slate have been interpreted as due to faulting because of discordance in structure between the two. At Black 

Mountain a similar repetition of limestone and schist beds has been 

proved to be due to faulting.

LIMESTONE.

The limestone beds at Ear Mountain consist of alternating layers 

of finely granular calcite, usually about 1 inch In thickness, and thin 

layers of crenulated clayey material. On weathering the clay seams 

stand out in prominent relief. The attitute of the limestone beds is 

fairly uniform. On both the south and the north sides of the mountain they dip 15°-40° N. and generally strike N. 70° E. It is apparent from this uniformity in the position of the limestone beds that 

the granite has invaded them without producing any marked changes 

in their position.

GRANITE.

The exposed portion of the granite intrusion of Ear Mountain is 

dome shaped, and the base is about 2 miles in diameter. All sides of 

this dome appear to be steep except the north side, but this side
EAR MOUNTAIN. 107

also cuts across the limestone beds. The surface of the granite is 

very irregular. Large dikelike masses project upward from the 

main body of granite into the schist and limestone. Most of these 

projecting masses trend east. Offshoots of this type are common on 

the south slopes of the mountain.

Two types of granite are exposed. The main mass is an evengrained rock composed chiefly of orthoclase crystals of square outline 

associated with quartz, sodic plagioclase, and a little brown biotite. 

On the surface small knots 2 or 3 inches in diameter weather out 

in many places and resemble similar forms in the granite of Brooks 

Mountain. They are generally of finer grain than the granite in 

which they are embedded, but their mineral composition, except for 

a larger quantity of brown biotite, is the same.

A minor portion of the granite is a light-colored muscovite-bearing 

rock with prominent quartz crystals. Dikes of this kind a few feet 

in width crop out on the south slope of the mountain east of Crosby 

Creek at an elevation of 1,400 feet. A larger area of unknown form 

is exposed south of the Quartz Creek canyon, and another in Vatney 

Gulch, on the northeast side of the mountain. The crystallization of 

the muscovite granite appears to have been interrupted several times. 

The large quartz and feldspar grains which it contains are generally 

corroded and inclose very fine crystals of feldspar and quartz. The 

matrix consists of granular quartz and feldspar and of the last 

minerals to crystallize, namely, fluorite and muscovite. Muscovite 

is prominent. In the occurrence from Vatney Gulch Knopf found 

minute crystals of cassiterite embedded in the muscovite.

The fissures of the granite were studied with the view of determining their origin and ascertaining whether they were related to the 

mineralization of the granite and adjacent limestone. The directions 

of fissuring are not. uniform. Outcrops near each other may show 

entirely different systems of fracturing. North-south and east-west 

vertical fissures are the most abundant. Fissures trending in intermediate directions, northeast and northwest, are not so common, and 

many of them show gentle dips, either to the north or to the south. 

Some of the north-south and east-west fissures also dip at low angles. 

The spacing of the fissures changes abruptly. As a rule, gently inclined fissures are closely spaced and develop a platy structure. Wide 

spacing no doubt has been an important factor in causing some 

outcrops to persist much longer than others and thus to form pillars 

and towers.

The fissures appear to be independent of the surface of the granite, 

It is difficult to interpret them as shrinkage cracks developed in a 

uniformly cooling magma. If they are shrinkage cracks, they suggest uneven distribution of heat, a condition which may have been
108 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

caused by the movement of vapors or hot solutions frqm within. 

The black tourmaline crystals covering some of the walls of fissures 

show that heated solutions circulated through parts of the granite.

Strike, dip, and spacing of fissures cutting granite of Ear Mountain.

Location.

Near head of Crosby Creek. ..................

Do.......................................

Do.......................................

Do.......................................

Do.......................................

Do.......................................

Kid ge extending northwest from Central Peak

Elevation 

(feet).

1 

' 700

1 800 '

2,000

2,220

2,360

2,200

Strike.

North-south

N.60°W.... 

N.65°E

N.20°E....

N". 40° W....

N.60-E.... 

N.60°E....

N.35" W....

N. 40° W....

N. 80°W....

N.20" W.... 

N. 50° E .... 

N.30°E....

N. 70° W....

North-south 

N.70°E....

N.50° W.... 

N.30" W....

Dip.

8° E..........

80° E. ........

Steep north... 

.....do. .......

15°N. ........

.....do........

15° S.......... 

32°S..........

30° E. ........

80° E. ........

80°N. ........

Vertical.......

85° N. ........

70° W......... 

30° S... ....... 

15° S. .........

.....do........ 

80° S.. ........

Vertical.......

.....do........

35° E. ........

Spacing interval.

Closely spaced. 

Do. 

Do.

Do.

Several feet.

Do.

Closely spaced. 

Platy structure. 

4 to 8 inches.

Closely spaced.

10 feet.

Closely spaced. 

Widely spaced.

Do.

Walls coated with

tourmaline. 

Widely spaced.

Closely spaced.

QUARTZ-AUGITE PORPHYRY.

The youngest known rocks of the Ear Mountain area are quartz 

porphyry dikes that were injected into the granite and limestone 

after the granite had cooled sufficiently to chill the margins 

of the dikes. Two dikes of this .kind, striking north and N. 20° E., 

crop out on the north side of the mountain. They are about 20 

feet wide and have been traced for several hundred feet. They 

have a chocolate-colored dense groundmass, without visible grain, 

in which large crystals of quartz and feldspar and a few of 

augite are embedded. Augite crystals as much as 2 inches in 

length have been found. Both feldspar and quartz show frayed 

and corroded edges. The feldspar is the limy plagioclase bytownite. The groundmass consists of microscopic crystals of plagioclase 

somewhat less limy than the large ones and an abundance of finegrained biotite. The rock may be classed as a dacite.

CONTACT METAMORPHISM. 

LIMESTONE.

The limestone and quartz porphyry adjacent to the granite and 

the marginal portion of the granite itself have undergone cer-
EAR MOUNTAIN. 1.09

tain changes resulting from the intrusion. Some of these changesinvolve the introduction of new materials.

In the limestone these effects extend several hundred feet beyond, 

the visible contact. Far from the contact the changes consist in 

recrystallization and coarsening of the calcite layers and hardening' 

of the shaly seams. Limestone of this type grades into a rock, 

in which nearly all the calcite has disappeared and the original, 

calcite and clayey layers are replaced by alternating bands of grossularite and light-green pyroxene, either diopside or hedenbergite,, 

vesuvianite, and similar minerals. This change has involved the 

loss of carbon dioxide and a recrystallization of the remaining materials into heavy lime-bearing silicates. Within a few feet of the 

contact the introduction of new materials is indicated by the development of tourmaline, axinite, scapolite, paigeite, and occasionally cassiterite, chalcopyrite, pyrite, and pyrrhotite.

A large assortment of contact-metamorphic minerals is found on 

the dump of the Winfield shaft. The minerals that have replaced the 

limestone at this place include green pyroxene, grossularite, vesuvianite, fluorite, and microscopic crystals of scapolite, cassiterite, 

axinite, topaz, danburite, arsenopyrite, pyrite, and chalcopyrite. 

South of Eldorado Creek the contact metamorphism of the limestone 

has developed a banded rock resembling a gneiss. The calcite seams 

of the original limestone persist, but the clayey layers have been replaced by black needles of tourmaline. In the Quartz Creek canyon 

the limestone shows very intense contact-metamorphic changes, and 

it is here that Knopf found the tin mineral paigeite in the form of 

small black particles embedded in the limestone. Associated with the 

paigeite are tourmaline, vesuvianite, zoisite, fluorite, phlogopite, chalcopyrite, and magnetite.

Limestone lying on a prospect dump near the head of Tuttle Creek 

shows changes to a very dense fine-grained mass of tourmaline and 

axinite. Accessory minerals are pyroxene, quartz, fluorite, calcite,. 

and cassiterite in microscopic grains.

GHANITE..

The chief contact alteration of the border portion of the graniteconsists in the development of black tourmaline, which also very 

commonly replaces the granite along certain fissures. Tourmalinized 

granite crops out at the head of Quartz Creek, and pieces of similarrock lie on the dump of the Winfield shaft.

QUARTZ PORPHYHY DIKES.

Where the quartz porphyry dikes cut the granite border, as at theWinfield shaft, they show strong tourmalinization. Commonly the 

tourmaline crystals invade and replace the large feldspars of the-
110 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

dike in preference to other materials. Films of cholcopyrite border 

the tourmaline crystals or fill cracks in them. The chalcopyrite was 

evidently introduced after the tourmaline.

TIN DEPOSITS.

At Ear Mountain tin in the form of cassiterite has been found (1)* 

as very small and microscopic grains embedded in the muscovite of 

certain muscovite granite dikes, (2) in microscopic grains in contactmetamorphic limestone, and (3) in microscopic grains replacing 

quartz porphyry. The muscovite granite has no commercial possibilities. In, neither the contactmetam orphic limestone nor the 

quartz porphyry have visible 

amounts of cassiterite been found, 

but assays of materials of this type 

from the Winfield shaft are reported to have shown as much as 3 

per cent of tin.

DEVELOPMENTS.

In 1918 no work had been done 

at Ear Mountain for several years 

and all underground workings 

were inaccessible. At only two 

places has extensive underground 

work been done, namely, on the 

Winfield property and in Vatney 

Gulch. In Vatney Gulch, on the 

north side of the mountain, a body 

of quartz porphyry was opened by 

a shaft and a drift 112 feet long. : , No visible cassiterite was found, and 

assays showed only traces of tin. 

The Winfield shaft, 29 feet deep, has been sunk on the north side 

of the mountain 10 feet north of the granite contact and 30 feet east 

of a quartz-augite porphyry dike striking north. (See fig. 20.) 

From the bottom of the shaft an incline was sunk 40 feet to the 

northeast. Both shaft and incline are in contact-metamorphic limestone. Some drifting was done westward from the shaft and has 

crosscut the quartz porphyry. The great variety of contact-metamorphic limestone minerals on the dump include conspicuous lenses 

of chalcopyrite, but their position underground could not be determined because the shaft was filled with ice. The following assays 

given to the Geological Survey by Mr. E. A. Winfield, of Teller, show 

both tin and copper:

Granite

30 60 FEET

FIGURE 20. Sketch of Winfield workings, 

Ear Mountain. A, Vertical north-south 

section through shaft; B, plan of 29- 

foot level. From diagrams furnished 

by B. A. Winfield.
BLACK MOUNTAIN. Ill 

Assays of mineralized rock from E. A. Winfteld prospect, Ear Mountain.

Location.

30 feet west of bottom of shaft on first

Do. ...............................

Tin.

3.46

3.7

2.36

18.72

Copper.

Trace.

4

Zinc.

1.04

Impurities.

69.60

66.3

54.20

Iron.

13

Gold.

Trace.

Trace.

Silver.

None.

Trace.

Trace.

Trace.

Trace.

BLACK MOUNTAIN.

LOCATION AND GENERAL FEATURES.

No tin has been found at Black Mountain, but it is described here 

because the area contains a granite stock and quartz porphyry dikes 

intruded into limestone and slate, similar to those of the tin-bearing 

localities of the York Mountains. The contact-metamorphic changes 

of the limestone and slate are the same as those seen at Brooks Mountain and in other intrusive areas except that no introduction of fluorine, boron, or tin was noted. Another reason for studying this area 

lay in the fact that it offered data on the relations of the limestone 

and slate.

Black Mountain comprises a group of nearly isolated hills, of 

which the highest has an elevation of 2,200 feet, about 5 miles north 

of Bering Sea, between Don and California rivers. When seen from 

the sea its dark color contrasts sharply with the dull grays of the 

adjacent limestone region. It can be easily reached from the beach 

by going up Don River and its branch Tozer Creek.

GENERAL GEOLOGY. 

SUMMARY.

The Black Mountain area is underlain by black graphite slate 

intruded by pre-Ordovician basalt and gabbro dikes, stocks, and 

sills. (See PI. XII and figs. 21-22.) The slate and basic intrusive 

rocks were covered by an unknown thickness of the Port Clarence 

limestone. Both slate and limestone were tilted northward and 

broken along fault zones striking in general east. The granite and 

quartz porphyry intrusions followed. The granite caused the development of contact-metamorphic minerals in the adjacent limestone and slate. After the intrusion of the granite and quartz porphyry dikes the area was carved by streams to its present state of 

relief. The granite and its contact-metamorphic border, resisting 

erosion better than the surrounding slate and limestone, have formed 

.Black Mountain. The slate, being weak, has been removed more
112 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

rapidly than the limestone, and hence underlies most of the valleys 

of Tozer and Skull creeks and the low area extending eastward from 

Black Mountain down the Agiapuk \7alley. The greenstone sills 

injected parallel to the slate beds, because of their superior resistance 

to weathering, form elongated ridges parallel to the strike of the

slates, steep on one side and 

gentle on the dip slope. 

Stocklike injections of greenstone into the slate have 

Aveathered into low cones.

FIGURE 21.-Section at head of Deer Branch, c,

After the had been

Metamorphosed limestone ; A, Port Clarence eroded into nearly its present

limestone; e, gabbro and basalt; f, slate. form ^^ ^QT& extmded in

several places. The largest flow of this kind within the area lies east 

of California Eiver. 4

FAULTS.

Several faults striking in general east have dislocated the rocks 

of the Black Mountain area. One of these faults is well exposed. 

The position of the others was inferred from field relations.

Within 500 paces north of the main limestone contact-metamorphic 

area of Black Mountain, there are two isolated lenses of limestone 

inclosed by slate. A fault is exposed at the one farther north. This 

limestone lens, about 800 feet long and less than 100 feet thick, is 

overlain by beds of black slate striking N. 70° W. and dipping 50° N. 

The limestone itself strikes east and dips about 36° N. The fault 

plane separates the limestone from the overlying 

slate along a knife-edge 

contact. At the west end 

of the outcrop the fault 

strikes N. 70° W., but at 

the east end it curves 

southeastward. The dip 

of the fault plane is 45° 

N. From the evidence 

afforded by the fault itself 

it is impossible to say whether the slate moved up or down with reference to the limestone.

The conditions leading to the isolation of this limestone lens between beds of slate are believed to be essentially as follows : Limestone beds and slate beds underlying them were tilted northward and 

later cut by an undulating fault striking roughly east and dipping

1,000 2,000 Feet

FIGURE 22. Section near Willow Branch, a,, Amygdaloid ; 6, porphyry dikes and quartz; c, metamorphosed limestone ; A, Port Clarence limestone; e, 

gabbro and basalt; f, slate.
U. 8. GEOLOGICAL SURVEY BULLETIN 733 PLATE XII

EXPLANATION

GEOLOGIC MAP OF BLACK MOUNTAIN, ALASKA

I _ 0 I________2 3 MILES

Contour interval HOO feet 

Black Mtn. 

r f ^jt , utfffifflJSSSan- - 6

SECTION ALONG LINE A B 

Scale same as map

o, Amygdaloid; 6, porphyry dikes and quartz; C, metamorphosed limestone; d. Port Clarence limestone; «, gabbro and basalt;/, slate.
BLACK MOUNTAIN. 113.

about 45° N., a clip which is steeper than the inclination of the beds 

(fig. 23, A). On a horizontal plane through the beds the trace of the 

fault plane, because of its undulations, divides the horizontal surface 

into two parts, of which the one to the south is composed of slate and 

a lens of limestone (fig. 23, B). Still later the overhanging side was 

upthrust along the fault plane (fig. 23, C) and was thus brought to 

overlie the limestone lens shown in figure 23, A and B. Erosion followed the thrugt faulting and wore the surface down to a nearly horizontal plane. The lens of limestone became isolated and inclosed by

B D

FIGDBH 23. Block diagram showing how a limestone lens became surrounded by Plates 

through faulting and erosion. A, Position of limestone and slate before faulting; B, 

trace of fault on plane o~&-e before displacement; C, hypothetical position of fault 

blocks after displacement; D, condition after erosion has cut block C down to c', flush 

with 0-6. Note isolated patch of limestone in slate.

slates (fig. 23, D). Further erosion would finally cause this lens todisappear.

Within 500 paces south of the fault just described two other faults 

of the same character have been inferred from the relations of slate 

and limestone. In this locality nonmetamorphosed limestone rests 

upon contact-metamorphosed limestone, as if this faulting had taken 

place after the intrusion of the granite.

Faults of the same general type appear to mark the slate and limestone contact of Black Mountain from the basin of California Kiver 

westward to Deer Creek. The limestone beds along this contact, 

dipping uniformly northward, would, if projected northward, ex-
114 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

lend into slate. It seems from this that the slate has been brought 

upward with reference to the limestone. A local condition of this 

sort has also been observed near the head of the South Fork of Willow 

Branch. (See fig. 22.)

The largest fault of the area is inferred to follow the southern 

"border of the slate belt. The limestone beds in the hills between 

Skull Creek and California Kiver dip 15°-50° N. Even at the 

contact with the slate their dip is northward. The slate beds north 

of the limestone generally dip to the north but do not show the uniformity of dip characteristic of the limestone. Locally, as on Skull 

Creek, they dip into the limestone. There are no folds by which the 

«late, although underlying the limestone, is brought to the surface. 

Evidently faulting has brought the slate upward with reference to 

the limestone. There is very probably a series of faults, as suggested by the apparent great thickness of the limestone east of Skull 

dreek and by the presence of two zones of faulting in the canyon of 

'California River 6 miles north of Bering Sea.

SLATE.

The distribution of the slate in the Black Mountain area is shown 

on Plate XII. Along the northern border of the slate the beds dip 

northward under the limestone, at an angle averaging about 20°, 

which is about the same as the dip of the overlying limestone beds. 

"The contact between the limestone and slate was not seen. The 

lowest beds of the limestone, being composed of alternating thin 

layers of black shale and limestone, crumble readily on weathering, 

and their debris obscures the contact. Within the slate area isolated 

patches of limestone stand in fault relations to the slate, and the 

limestone beds along the entire southern border of the slate area are 

inferred to be faulted against the slate.

Throughout the area the slate is remarkably uniform in character 

and of exceedingly fine grain. Microscopic study shows that it is

 composed of very fine grained quartz laminae alternating with thin 

laminae composed of scales of wavy brown mica. Scattered through 

'it are particles of black carbonaceous material, which in the vicinity 

of the granite contact are aggregated into small irregular rodlike 

particles. The granite has also effected a change in the color of the

 slate. Away from the granite contact the slate is black. Near the

 granite it has a brown color like that of maple sugar, 'and in hardness,

 color, and grain resembles some varieties of rhyolite.

, 

PRE-ORDOVICIAN BASALT AND GABBRO.

The basalt and gabbro are green rocks, generally composed of 

nearly parallel columns of green secondary hornblende associated
BLACK MOUNTAIN. 115

with limy plagioclase decomposed into quartz, calcite, and other 

minerals. Many of the intrusions are distinct sills from 3 to 40 

feet thick injected between the slate layers. Intrusions of this type 

are very well exposed west of 'Skull Creek and on the east side of 

California Elver near the Luther prospect. At the latter place the 

sills are repeated at intervals of several hundred feet and form a 

series of east-west hogback ridges with gentle north slopes. East of 

the granite there are several large stocks of greenstone, one of which 

is nearly a mile in diameter.

LIMESTONE.

The limestone in the Black Mountain area shows the threefold 

character noted in other parts of the York region. The lowest member is composed of thin black shaly layers interlaminated with lenses 

of calcitic limestone. In the section on California River these beds 

show chert nodules 2 or 3 inches in diameter. The thickness of these 

beds is unknown but probably does not exceed 200 feet. Good exposures of these basal beds are found in the canyon of California 

River 6| miles north of Bering Sea and between Willow Branch and 

Deer Branch, north of Black Mountain. On California River the section exposed is about 200 paces long and the beds strike N. 65° E. 

and dip steeply south. They lie on the southern flank of an anticline that strikes northeast. To the north the basal beds are cut off by 

a vertical brecciated zone of limestone 20 paces wide, striking northeast. Faults also disturb the black beds along planes that dip 

steeply south. Overlying the dark beds are thin beds of gray to 

chocolate-colored limestone consisting of fine-grained calcite with 

yellowish-brown shaly partings.

In the section between Willow Branch and Deer Branch the thintedded black limestone is covered by several hundred feet of the 

thin-bedded limestone interlaminated with yellowish-brown shale. 

The same relation between these two members of the limestone series 

<exists at other places along the northern border of the slate area.

The uppermost member of the limestone series consists of thickbedded gray, pure limestone, without shaly or sandy constituents. 

The average thickness of the beds is from 1 to 2 feet. These beds 

are well exposed on the hills between Skull Creek and California 

River. In the section east of Skull Creek about 4,000 feet of the 

uppermost thick-bedded limestone seems to overlie at least 2,000 feet 

of the thin shaly layers, but this thickness of limestone is probably 

only apparent. Faults no doubt affect this section and cause a repetition of beds.
116 GEOLOGY OF THE YOEK TIN DEPOSITS, ALASKA.

GRANITE.

The granite intrusion forms the core of the Black Mountain area. 

The portion exposed is irregularly elliptical and is about 1 mile in 

length. The limestone cover has been, stripped off only on the southeast side. The granite surface is very steep on this side, but toward 

the northwest its slope is probably moderate, to judge by the width 

of the contact-metamorphic area in that direction. Like other intrusions of granite in the York region, it appears to be inclined in the 

same direction as the sediments into which it is intruded but more 

steeply.

The granite of Black Mountain is an even-grained rock, composed 

chiefly of orthoclase with small amounts of quartz and sodic plagioclase. Biotite occurs in very small brown plates. In grain and 

mineral content it resembles the granite of Tin Creek and Ear 

Mountain.

ALTERATION OF THE LIMESTONE BEDS BY THE GRANITE.

The thin-bedded shaly and graphitic limestones which lie at the 

granite contact show alterations similar to those of the limestones 

at the contact of other granites of the York Mountain region. Near 

the border the calcite has completely disappeared and the rock has 

been changed into fine to medium grained silicates, chiefly colorless 

garnet, vesuvianite, and pyroxene, probably augite or hedenbergite. 

These minerals occur in thin alternating laminae. Sulphides, including both pyrite and arsenopyrite, have been introduced as 

minute scattered grains and very thin seams. No minerals; of 

economic value have been found associated with them.

QUARTZ VEINS.

Quartz stringers about 1 inch in width are very common in the 

slate, both parallel to and across the slate layers. Valuable minerals have not been found in them- The largest quartz vein was 

found on the saddle north of the head of the South Fork of Willow 

Branch. It is 3 feet wide, strikes about N. 20° E., and was traced 

for several hundred feet. It appears to be composed of a clear 

white unmineralized quartz.

QUARTZ PORPHYRY DIKES.

Two quartz porphyry dikes were found in the Black Mountain 

area. One, 3 feet wide, crops out at an elevation of 1,200 feet on 

a peak west of California River. It strikes north. The other, 

about 5 feet wide, is exposed at an elevation of 250 feet on the
BLACK MOUNTAIN. 117

south slope of the granite. It strikes east and was traced for several hundred feet. These dikes are composed of small scattered 

quartz grains about one-eighth of an inch in diameter and small 

white feldspar phenocrysts embedded in a black dense, fine-grained 

groundmass that forms 95 per cent of the rock. Both dikes were 

found in the slate, but they probably belong to the period of granite 

intrusion and are therefore younger than the limestone.

AMYGDALOIDAL OLIVINE BASALT.

Three bodies of amygdaloidal basalt were found in the Black 

Mountain area. Their identity of character makes it probable that 

they came from the same magmatic reservoir. Except for a finer 

grain they are like the black dike in the tunnel of the Empire Tin 

Mining Co. at Cape Mountain.

The largest of these bodies is on the east side of California River. 

Here several thousand acres is underlain by boulders of this rock aa 

much as 10 feet in diameter, broken by frost action. The rock is a 

very vesicular surface flow poured out on the eroded surface of the 

Port Clarence limestone. The fine groundmass is black. In it are 

scattered grains and segregations of olivine. Microscopic study 

shows that the groundmass consists of fine laths of calcic plagioclase 

in which are embedded larger crystals of augite and olivine that

 crystallized after the feldspar.

About 400 paces west of the limestone contact west of Skull Creek

 stands a small cone 30 feet high composed- of a broken mass of 

vesicular, slaggy black lava. It is full of blow holes several inches 

long. Segregations of olivine grains 1 inch in diameter were seen 

here and there. Under the microscope the rock is seen to consist of 

glass in which tiny needles of calcic plagioclase are embedded. 

Olivine forms scattered crystals. In composition this lava is like 

the flow east of California River. Its recent extrusion is suggested 

by the freshness of the glass.

On the slope south of the south fork of Willow Branch black lava

 of the vesicular type appears at elevations of 900 and 1,200 feet. 

At the higher elevation three beds of ash and bombs of scoriaceous, 

Topy, and vesicular black lava, each about 15 feet thick, are separated 

by beds of massive black lava several feet thick. Small angular inclusions, about 2 inches long, of white dense, grainless rock like certain phases of the metamorphic limestone are embedded in the lava. 

Microscopically this lava is like that in the cone west of Skull Creek. 

The glassy groundmass constitutes about 80 per cent of the volume 

of the rock. The remainder consists of small grains of olivine about 

0.1 inch in diameter.
118 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

COMPARISON OF THE TIN DEPOSITS OF THE YORK 

REGION WITH THE OTHER TIN DEPOSITS

OF THE WORLD.

ASSOCIATION WITH GRANITE.

The data on the characteristics of the tin deposits of the world in 

this discussion have been taken chiefly from a paper by Ferguson 

and Bateman. 29 In their close association with granite batholithic 

intrusions the tin deposits of the York region resemble tin deposits 

throughout the world, which, barring a few apparent exceptions, are 

all in granites or near granite intrusions. All of them are also 

closely related in age to the intrusive granite with which they occur. 

In some of the apparent exceptions to the association with granites 

the tin occurs with rhyolite, quartz porphyry, or andesite. Most of 

these apparent exceptions are in little-known areas, hence the total 

absence of granite may not have been proved for all of them. 

' The granite batholiths with which most tin deposits are associated 

have not been eroded deeply, and as a rule portions of the roof remain. The deposits are generally at or near the border of the 

granite. Where they appear far within an eroded granite area 

there is generally reason for believing that the border was not far 

removed vertically. Tin deposits found several miles from an exposed granite contact have certain characteristic features indicating 

that the granite is at a shallow depth below them. The deposits are 

more likely to occur where the original surface of the granite is 

gently inclined than Where it is steep.

All these characteristics fit the tin deposits of the York region. 

The granitic intrusions represent stocks from a common batholith 

and are only slightly exposed. Portions of the roof remain in all 

of them. Mineralization of the invaded rock is greatest where the 

slope of the granite border is relatively gentle. Thus the limestones 

north of the Ear Mountain and Cape Mountain stocks and west of 

the Brooks Mountain stock show the most intense mineralization. 

Mineralization does not extend far from the contact where the border 

is steep.

Where the granite stock with which the tin minerals are associated 

is exposed the mineralization probably does not extend more than 

200 feet from the contact. None of these tin deposits are directly 

above the tops of the granite stocks. The granite 7 inferred to underlie the Cassiterite Creek tin deposit may be some somewhat deeper 

than 200 feet, because tin solutions may be expected to have traveled 

farther vertically from the top of a stock than from the sides. No 

close estimate of the depth of the granite stock inferred to underlie

29 Ferguson, H. G., and Bateman, A. M., Geologic features of tin deposits: Bcon. Geology, 

vol. 1, pp. 209-262, 1912.
COMPARISON WITH OTHER TIN DEPOSITS. 119

Potato Mountain can be given, though to judge by the condition, 

of the slate it may be about 1,000 feet.

COMPOSITION OF GRANITE ASSOCIATED WITH TIN DEPOSITS.

The granites associated with tin deposits throughout the world, 

so far as known, are alkaline rocks low in magnesia and lime. Almost 

without exception they "contain more potash than soda. Orthoclase 

is generally the most abundant mineral. The plagioclase feldsparsare sodic. Biotite is usually more common than either hornblende 

or muscovite. The acidic dikes associated with the granites usually 

have about the same chemical characteristics as the granites themselves, but silica is commonly a little higher in the dikes.

The granites of the York tin region are also of the normal type, 

orthoclase being their dominant constituent. Plagioclase feldsparsare subordinate and are of the sodic type. In all the granites biotite 

is the chief mica except in certain dikelike forms at Ear Mountain,, 

which contain a conspicuous amount of muscovite as the last product 

of crystallization. The granite at Brooks Mountain contains biotite 

except in a border zone about 3 feet wide which contains a green 

hornblende.

The dikes that are closely associated in time with the granites and 

tin deposits of the York region are generally of the acidic type. 

Muscovite is commonly the predominant primary mica. At Ear 

Mountain an augite dacite porphyry altered by tin-bearing solutions' 

cuts the granite.

KINDS OF ROCKS INTO WHICH TIN-BEARING GRANITES ARE

INTRUDED.

%

In the York region, the tin-bearing granites are intruded into 

slate and limestone. A compilation of the rocks invaded by the tinbearing granites of the world indicates that the nature of the invaded 

rock has no bearing on tin deposition. Ferguson and Bateman mention schist 16 times, slate 11, quartzite 7, sandstone 6, shale 2, limestone 6, gneiss 6, older granite 3, and other igneous rock 4.

LOCATION OF ORES WITH REFERENCE TO GRANITE.

Most of the tin ores of the world are within the granites. Bateman and Ferguson found that in 26 localities the cassiterite occurs 

almost exclusively in the granite, in 8 it is divided between the 

granite and the invaded rock, and only in 4 is it preponderantly in 

the invaded rock.

So far as known the tin in the York region occurs mostly in the 

invaded rocks, chiefly slate, as indicated both by placers and by occurrences in place. The granites themselves have been the direct source
120 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

of very little placer tin. The only commercially important placer 

deposits are derived from slate at Potato Mountain. The invaded 

rocks influence the shape of the tin deposits formed within them. In 

limestone, because of its tendency to react with tin-bearing solutions, 

the tin deposits usually take the form of irregular replacement deposits ; in slate and quartzite the tin deposits are more commonly in 

the form of fissure veins.

AGE OF TIN DEPOSITS.

In age the tin deposits of the world range from Archean to late 

Tertiary. Most of them appear to be late Carboniferous or Mesozoic. 

The granites of the York region are probably Mesozoic. The 

youngest rocks known to be penetrated by them are of Mississippian 

age. The gap between the Mississippian and the Pleistocene is not 

Imown to be represented by sediments, hence the age of the intrusions can not be determined more precisely.

CLASSIFICATION OF TIN DEPOSITS.

Ferguson and Bateman have compiled the frequency of occurrence of certain types of tin deposits of the world as follows: Normal lodes, 27; pegmatite lodes, 7; stockworks, 12; pipes, 2; segregations, 2.

They define normal lodes as including both veins and tabular 

ore bodies formed by a series of parallel, closely spaced fissure veins, 

with the intervening country rock more or less replaced by ore. 

Cornwall, the Zinnwald district, Tasmania, New South Wales, and 

Bolivia have deposits of this type.

Pegmatite lodes are pegmatite veins containing cassiterite. Deposits of this kind are known in North and South Carolina, the 

Black Hills, the Vegetable Creek district, New South Wales, and 

Swaziland.

Stockwork deposits consist of a network of veins and impregnated 

country rock. They are represented by deposits in the Erzgebirge, 

New South Wales, Tasmania, and South Africa.

Pipes are unique cylindrical deposits, composed of a central core 

of altered granite with disseminated cassiterite which is surrounded 

by zones of cassiterite and tourmaline. They have been found in 

Transvaal and New South Wales.

Segregations are interpreted as portions of the original granite 

magma containing unusual amounts of cassiterite. In the Mergui 

district, Bolivia, massive segregations of quartz at the outskirts of 

the granite contain wolframite, chalcopyrite, and cassiterite. A 

segregation of tin-bearing magnetite in .granite gneiss is said to 

occur at Eoss-shire, Scotland.
COMPARISON WITH OTHER TIN" DEPOSITS. 121

Some of the tin deposits of the York region can be classed with 

certain groups established by Ferguson and Bateman, >and some 

represent other types. The classes recognized are fissure veins and 

replacement deposits. The latter are not mentioned as such in the 

above classification. The fissure veins are found at Cassiterite 

Creek in the limestone and in the Ida Bell dike. Distinct veins of 

this type are practically absent from the Cassiterite dike. In these 

veins Cassiterite is associated with quartz, topaz, zinnwaldite, fluorite, 

tourmaline, wolframite, and albite. Quartz-cassiterite fissure veins 

also occur in the granite at Cape Mountain and in the slate at 

Potato Mountain. Quartz veins locally form a stockwork as Potato 

Mountin but generally do not carry cassiterite in amounts visible 

to the eye.

The replacement deposits include limestone contact-metamorphic 

deposits in limestone, deposits formed by the replacement of brecciated border phases of granite, known in the Empire Mining Co.'s 

tunnel at Cape Mountain, and the replacement deposits of the Cassiterite dike and Greenstone lode. In the limestone contact-metamorphic deposits the cassiterite occurs as an irregular replacement of 

marble and is associated with secondary tourmaline, axinite, fluorite, 

paigeite, hulsite, hornblende, pyroxene, vesuvianite, humite, serpentine, danburite, zinnwaldite, phlogopite, green mica, chondrodite, 

grossularite, stannite, wolframite, chalcopyrite, pyrrhotite, arsenopyrite, pyrite, galena, sphalerite, molybdenite, and stibnite.

The brecciated zones in the granite of Cape Mountain show cassiterite-bearing replacement deposits only a few inches wide. The 

cassiterite is associated with secondary quartz, tourmaline, colorless 

mica, and sulphides of iron.

The tin-bearing portion of the Cassiterite dike is almost entirely 

replaced by sericite, fluorite, and zinnwaldite, with scattered c#ssiterite, wolframite, tourmaline, topaz, humite, arsenopyrite, pyrite, 

molybdenite, and stibnite. Similar replacement ^deposits were noted 

in the Greenstone lode. Locally quartz is an abundant replacing 

mineral and is associated with wolframite, sericite, and cassiterite.

BOCK ALTERATIONS ASSOCIATED WITH TIN DEPOSITS.

The contact metamorphism of limestone, slate, and other rocks 

adjacent to tin-bearing granite shows phases due to recrystallization 

which do not differ from contact alterations caused by granite that is 

not tin bearing. It seems that tin has rarely been introduced as a 

contact mineral. In the York region, however, there have been at 

the limestone contacts, except the one at Black Mountain, marked 

accessions of tin, boron, fluorine, and sulphur bearing minerals, such

101987 22 9
122 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

as fluorite, tourmaline, axinite, cassiterite, paigeite, hulsite, clialcopyrite, pyrrhotite, and galena.

The alteration of the granites and dikes effected by tin-bearing solutions is very much like that generally caused by hydrothermal 

solution on acidic igneous rocks, consisting of silicification and sericitization. In the Cassiterite dike sericitization is far more prominent than silicification. In all phases of alteration of the granites 

and dikes in the York region there is a marked introduction of 

fluorite, tourmaline, and sulphides. This is characteristic of the 

alteration of similar rocks by tin-bearing solutions in other parts 

of the world.

ORIGIN OF THE TIN DEPOSITS OF THE YORK REGION.

TIN DEPOSITS ASSOCIATED WITH GRANITE.

Inferences as to the origin of the tin deposits of the York region 

must take cognizance of their constant association with granite 

stocks. The tin minerals occur in the border phases of granite, 

quartz veins, porphyry dikes, and in limestone near granite intrusions.

TIN MINERALS DEPOSITED FROM SOLUTIONS AFTER SOLIDIFICATION OF GRANITE.

With one exception all known occurrences of tin in the granites 

of the York region are in fractured zones. The tin in the form of 

cassiterite replaces with other minerals the feldspars of the granite. 

The secondary minerals that were introduced with the cassiterite 

include quartz, sericite, tourmaline, topaz, fluorite, arsenopyrite, and 

pyrite. These facts imply that the cassiterite did not crystallize 

with the feldspars and other minerals of the granite but was introduced after the granite had solidified. An unimportant exception 

to this statement is seen at Ear Mountain, where a white muscovite 

granite dike was found to contain microscopic crystals of cassiterite 

embedded in muscovite. The muscovite is abundant and with minor 

amounts of topaz and fluorite constitutes the groundmass in which 

an earlier generation of quartz and feldspar phenocrysts are embedded. Several samples of this rock were examined, but only one 

found by Knopf showed cassiterite.

Whether the tin that replaces rock material adjacent to the granite 

was deposited after the granite had solidified can not be stated as 

confidently as in the case of the tin occurring in the granite. The 

cassiterite in the rocks adjacent to the granites has the same mineral 

associates as the cassiterite of the granite. This implies similarity 

in the composition and temperature of the solutions that deposited 

both. The fact that the two occurrences of cassiterite resemble each
ORIGIN. 123

other closely and are near each other undoubtedly means that they 

came from the same source.

CHARACTER AND SOURCE OF SOLUTIONS.

It is apparent from the chemical composition of the tin minerals 

and their occurrence as vein material and as irregular replacement 

deposits, that the solutions which carried and deposited the tin transported a variety of elements silicon, aluminum, iron, magnesium, 

sodium, potassium, lithium, fluorine, chlorine, sulphur, tungsten, and 

other metals. It is improbable that all these elements came from 

the same source, however. The silica, aluminum, iron, magnesium, 

sodium, and potassium may have been derived partly or wholly from 

the limestones and dikes which the solutions traversed. Certain 

other elements which the solutions carried, such as fluorine, lithium, 

chlorine, sulphur, tin, and tungsten, may have been present in minute 

quantities in the original limestones and dikes, but their abundance 

in the solutions can not be accounted for from this source. Their 

close association with igneous intrusions points to their having an 

igneous source.

The solutions that deposited the tin and the secondary minerals 

closely associated with it were hot solutions of deep-seated origin. 

This is inferred from the close association, both as to time and place, 

of such minerals as tourmaline, topaz, danburite. axinite, and scapolite with igneous activity. These minerals are not known to be 

formed under surface conditions. The list of known minerals deposited by the tin-bearing solutions includes cassiterite, quartz, fluorite, tourmaline, topaz, scapolite, danburite, a green mica, muscovite, 

zinnwaldite, phlogopite, wolframite, calcic and sodic plagioclase, 

hornblende, pyroxene, vesuvianite, magnestite, humite, chondrodite, 

serpentine, arsenopyrite, pyrite, stannite, and probably pyrrohotite, 

galena, sphalerite, molybdenite, and stibnite. Some of the minerals 

named were deposited from the solutions in cavities, fissures, and 

other openings. Others replaced previously existing minerals. 

Some of them were probably formed by the interaction of the solutions with rock materials. Quartz, fluorite, tourmaline, topaz, danburite, muscovite, zinnwaldite, wolframite, and plagioclase occur as 

minerals deposited directly from solutions in fissures. This same 

group of minerals and in addition hornblende, magnetite, chondrodite, serpentine, phlogopite, arsenopyrite, pyrite, and stannite are 

known to replace previously existing rock materials. It is probable 

that considerable amounts of the fluorite and hornblende found in 

the tin-bearing limestone were formed by the interaction of the 

solutions with the original constituents of the limestone. The calcium in both the fluorite and the hornblende may have been derived
124 GEOLOGY OF THE YORK TIN DEPOSITS, ALASKA.

from the limestone, and. the aluminum, magnesium, and some of the 

iron in the hornblende may also have come from the limestone. 

Certain dense, compact, alternating laminae of grossularite, vesuvianite, and colorless to light-green pyroxene replacing limestone ad 

jacent to the granite contacts are not regarded as having been caused 

by the introduction of materials in solution, for the laminae are 

parallel to the bedding and show great regularity. They are ascribed 

to the expulsion of the carbon dioxide of the limestone and the recrystallization of the remaining limestone materials. The occurrence of vesuvianite and pyroxene in concentric rings, as at Tin 

Creek, is ascribed to the action of solutions. Tremolite occurs as a 

product of regional metaiftorphism of the limestone. Its distribution parallel to the bedding and in association with fine-grained 

clayey layers indicates its development was influenced by the original 

composition of the limestone.

SEQUENCE OF THE TIN-BEARING SOLUTIONS.

In some tin districts of the world a sequence of deposition of the 

tin and its associates has been inferred. Except at the Cassiterite 

Creek deposits the opportunities for study of the deposits in the York 

region have not been favorable enough to determine a sequence of 

deposition. The limestone that crops out in the creek at the Cassiterite Creek tin prospect shows two stages of mineralization, represented by (1) irregular, crisscrossing replacement veins, consisting 

mainly of fluorite, hornblende, and calcic plagioclase; (2) fissure 

veins containing chiefly quartz, topaz, tourmaline, albite, wolframite, 

cassiterite, and zinnwaldite. No sequence of deposition was noted in 

minerals replacing the dikes of Cassiterite Creek.

SUGGESTIONS ON LODE TIN PROSPECTING IN THE 

YORK REGION.

The following suggestions on tin prospecting in the York region 

apply to deposits in place and not to placers. All the known tin 

deposits of the world are either in granite or closely associated with 

granite. ' A few apparent exceptions are associated with rhyolite or 

similar rocks closely related to granite. Tin prospecting in the York 

region can therefore be limited to granite areas, to the rocks adjacent 

to granite, and to places where granite is inferred to lie at a shallow 

depth below the surface and which can be recognized by the contact 

metamorphism which the surface rocks have suffered.

In the granite tin may be' looked for in quartz veins or in any 

altered portion of the granite. The alterations of the granite associated with tin are either a sericitization or a silicification. Both 

alterations may be accompanied by tourmalinization. The sericitized portions of the granite commonly contain sulphides and
SUGGESTIONS ON LODE TIN PROSPECTING. 125

fluorite. The quartz veins and silicified masses stand out on weathering and usually are easily found. The sericitized granite, being soft, 

does not form prominent outcrops but is generally covered by debris.

It is useless to start underground exploration in fresh, unaltered 

granite. Subsurface work is justified only where there is some 

probability of finding a large tonnage of one of the types of deposits 

enumerated. Tin deposits, in order to pay, must be capable of yielding a considerable tonnage, for they all have a low tin content, 

usually less than 3 per cent.

Tin in the rocks adjacent to the granite occurs as a replacement 

of quartz porphyry dikes and limestone and in quartz veins cutting 

dikes, limestone, and slate. So far, the best-developed deposit in 

this region is a dike in which cassiterite occurs as a disseminated 

replacement mineral in soft sericitized and locally in silicified portions of the dike. The chief alteration is sericitization of the dike, 

which is therefore very soft and is commonly covered at the surface 

by debris.

Dikes of this kind in limestone are not promising unless the limestone adjacent to them has undergone severe contact metamorphism 

resulting in the development of many hard resistant silicates, which 

cause such" deposit to stand out in prominent outcrops that are easily 

distinguished from the soft unaltered limestone. The metamorphosed limestone breaks down into large blocks several feet in diameter, but the unaltered limestone breaks down to small chips.

Slate adjacent to a tin-bearing dike may be expected to be hard, 

brown, and compact and to show very little slaty structure. Tin has 

not been found in place in dikes cutting slate, but large amounts of 

tin could hardly be expected in a dike unless the rocks associated with 

it showed considerable contact metamorphism.

Tin as a replacement of limestone can generally be located without 

difficulty. Such deposits occur in very hard, resistant limestone 

which makes conspicuous outcrops. Siliceous vein deposits, whether 

in limestone, slate, or dikes, are also readily found as outcrops or in 

float.

In a rugged open country nearly free from vegetation and alluvial 

deposits, like the York region, tunneling and shaft sinking ought to 

be resorted to only when surface work has shown the presence of 

deposits. At almost any place in the region it is possible to get down 

to bedrock by means of pits or trenches. Such surface work gives 

just as much information as underground work and is generally 

accomplished in less time and at a lower cost. The nature of tin 

deposits is such that the tin content at the surface is very much like 

that which may be expected below the surface. Hence deep exploration is generally uncalled for in advance of thorough surface 

examination.

101987 22 10

INDEX.

A.

Accessibility of the regionAcknowledgments for aidAd It No. 1 on the Cassiterlte dike,

description of- _ 

Adit No. 2 on the Cassiterite dike,

description of 

Adit No. 3 on the Cassiterite dike,

description of_ 

Alan Dale lode, on Potato Mountain,

description of__- _ 

Alaska Chief claim, description of 

American Tin Mining Co., operations

by - 

Amphibole, description of 

Animals of the region__ ____ 

Apatite, description of __ 

Arctic coastal flats, plate showing_ 

Arsenopyrite, description of 

Axinite, description of__ _ 

Azurite, description of_ _ _

B.

Page.

10-12

1-2

66-67

66

64-66

92

81

94,95

. 36

9-10

36

96

36-37 

87 

37

Basalt, amygdaloidal olivine, nature

and occurrence of _ 31 

pre-Ordovician, features of _ 19

Bessie antimony and lead claim, description of _____ 78-80

Biotite, description of _ __ 37

Black Mountain, basalt, amygdaloidal olivine, in_____ 117

basalt, pre-Ordovician, in _ 114-115 

gabbro, pre-Ordovician, ln___ 114-115 

faulting in____________ 112-114 

geographic features of- _ 111 

geologic map of- _ 112 

geology of_________ 111-117 

granite in _ 116 

limestone in ___ _ 115,116 

quartz in______ _ 116-117

slate in ______________ 114

Blomquist & Goodwin, development

by_________ __ 80

Breccia, dikes formed of __ 56-58

Brooks, A. H., acknowledgment to_ 1 

exploration by _ _ 13-14

Brooks Mountain, action of mineralizing solutions in _ 87 

altitude of __ 3 

chances of finding commercial

deposits in 88 

geographic features of__ __ 81-82 

geologic map and section of___ 82 

geology of _ 83-86

Page.

Brooks Mountain, granite in ___ 85-8(Plimestone in __________ 83, 84-85 

prospects on_ .._________ 86-87 

slate in _ ____________ 83, 84

Buck Creek, production of tin on__ 94-95 

tin placers on___________ Q4-9&

C.

Calcite, description of_________ 37

Camp Creek, faults on_______ 54

Cape Mountain, altitude of_____ 3

developments at- _ _ 102

dikes on_ _____ __ 98-99

geographic features of 96-97

geology of ___ 97-101

granite in____________ 98-99'

limestone of ___ 20-30,100-101

mineralization on_______ 100-101

quartz veins on_________ 99-100'

tin at, occurrence of 101,102

production of____ -- 102

views from I _ 96 

Cape Prince of Wales, plate showing____________ 96. 

Cassiterite, description of______ 37 

Cassiterite Creek, breccia on __ 56-58 

deposits beneath, extent of__ 73-74 

dikes on 55-56 

faults on _______ 51,53-55'

limestone on __ _______ 51-53

mineralizing on__________ 51

placer tin on____________ 74

workings on __ _ __ 63-72

plate showing __ 47

Cassiterite dike, description of___ 55

developments on ______ 63-69

erosion from. ______ 50'

extent of tin deposits of, beneath Cassiterite 

Creek__________ 73-74: 

mineralization of _______ 60

replacement of__ ______ 72"

tin content of_ ______ 73

tonnage in____________ 72-73

Cerusite, description of_______ 37-38 

Chalcopyrite, description of_____ 38 

Chlorite, description of________ 38 

Chondrodite, description of____ 38 

Climate of the region_________ 4-5 

Collier, A. J., acknowledgment to 1 

discovery of lode tin by_____ 14 

Cow Boy tunnel, location of_____ 80 

Creep of surface materials, features

of _____________ 7,8-9'

327
128

D.

Page.

Daisy claim, on Potato Mountain,

description of_____ 93 

Danburite, description of_______ 38 

Deposits of the region, comparison 

of, with other deposits 

of the world_____ 118-122 

of the York region, origin of_ 31,

122-124

of the world, age of_______ 120 

alterations of rocks associated with_______ 121-122 

types of__________ 120-121 

Dikes, association of, with granite_ 31 

Dikes, breccia, occurrence and origin

of _____________ 56-58 

in the Brooks Mountain area_ 83-84 

quartz porphyry, effects of tinbearing solutions on_ 58-60 

nature and source of___ 48-49 

on Cassiterite Creek____ 55-56 

Dip of the strata___________ 32-34 

Dolcoath dike, description of___ 76-77 

development on____ ____ 77-78 

Dolomite, description of_ _ 38 

Drainage of the region________ 3-4

E.

Ear Mountain, contact metamorphism on___ __ 108-110 

deposits in _______ ___ 110

developments on __ __ 110-111 

dikes in_____________ 109-110 

geographic features of ___ 102-103 

geology of____________ 103-110 

granite on_________ 106-108,109 

limestone on_______ 106,108-109 

location of ____ __ 3 

quartz-augite porphyry on___ 108 

quartz mica schist and associated rocks on______ 105-106

Empire Tin Mining Co., development by___ __ 102 

Epidote, description of___-__ 38 

Erosion in the Lost River area_ 49-51 

Eureka claim, on Little Potato 

Mountain, description 

of _____________ 92-93 

Exploration, record of______ 13-15

F.

Fault systems, features of__ _ 32-33, 34 

Faults, occurrence of, at the mouth

of Lost River_____ 47 

Fearing, F. C., acknowledgment to 1 

Feldspar, description of___ ____ 38-39 

Ferguson, H. G., and Bateman, A.

M., cited_______ 118-122 

Fluorite, description of__ __- 39 

Folds, occurrence of___ 32-33 

Frost-crack pattern, development of_ 7-9 

Frost cracks in slope wash, plate

showing____ __ 6

Gabbro, pre-Ordovician features of_ 19 

Galena, description of__ 39

Page.

Garnet, description of___________ 39

Geography of the region_______ 2-13 

Girty, George FT., fossils determined

by _____________ 27-28 

Gold, distribution of__________ 39 

Graham, A. S., acknowledgment to_ 2 

Granite, association of world's tin

deposits with ____ 118-119 

in the Brooks Mountain area_ 85-86 

location of world's tin ores with

reference to _____ 119-120 

occurrence and source of_ 30-31 

tin-bearing, kinds of rock intruded by________ 119 

Gravels, stream, depth and description of__________ 35-36 

Greenstone lode, mineralization of_ 60 

tonnage in______________ 73 

workings on ____________ 69-71 

plate showing_ _ 47

H.

Halpin, J. F., acknowledgment to_ 1 

development by __________ 63-72

Harbor of the region_________ 10-11 

Hematite, description of_______ 39 

Hess, F. L., acknowledgment to__ 1 

History, geologic, of the region 15-16 

topographic, of the region 34-36 

Hulsite, description of_______ 39-40

I.

Ida Bell dike, cassiterite from vein

in, plate showing. 72

description of_______-__ 55-56

mineralization of_________ 60

tonnage in ____________ 73

Ida Bell tunnel, description of _ 71-72

Idaho copper claim, description of_ 81

Ilmenite, description of_ __ 40

Industries of the region _ 13

Intrusions, igneous, sequence of 31-32

Iron Creek, placers on_____ 96

K.

Kaolinite, description of_ 40 

Kingegan Mission, plate showing 96 

Kirk. Edwin, acknowledgment to 2 

fossils determined by__ 24-27,28-29 

Knopf, Adolph, acknowledgment to__ 1 

Kougarok Plateau, remnants of__ 35

L.

Larsen, E. S., acknowledgment to 2 

Limestone, in the Brooks Mountain 

area, nature and occurrence of_______ 83, 84-85 

in the Lost River area_ 46-47 

mineralization of-____ 61-63, 75-76 

mineralized, plates showing_ 62, 63 

of Cape Mountain, age of 21, 23-21) 

contact metamorphism in 22-23 

deposition of__-_________ 29-30 

fossils in____________ 21,23-29 

general features of___ 20-21 

relations of__________ 21-22
INDEX.

Limestone, of the York Mountains, 

general features ofof the York Mountains, relations of ________

" reef," plates showing ____

Limonite, description of-____ __

Location of the region_____ __

Lopp Lagoon, placers on streams

flowing Into_______

Lost River, description of____ __

section on______________

surface features near, plates

Showing____ _

I/ost River area, dikes in_ __

erosion in___ ______ __

geographic features of______

geology of ______ ________

limestone in__ _ _____

minerals in__________________

prospects in.____________

Lowlands, recent, features of_ __ 

Ludwigite, description of_________

Luther load prospect, on Brooks 

Mountain, description 

of__________________

M.

Page.

20

21-22

63

40

2

7

48-49

49-51

44-45

45-51

46-47

50-51

51-81

35-36

40

86-87

Mclntosh, Alex, acknowledgment to_ 1-2 

Magnetite, description of_ 40 

Mahla, George, acknowledgment to__ 2 

Maple antimony and lead claim, description of_______ 78-80 

Martin, G. C., acknowledgment to__ 2 

Mineralization, process and results

of_ 51,58-60,61-63,75-76,87 

Minerals of the region _____._ 36-43 

Molybdenite, description of- 40 

Muscovite, description of_ 40

N.

Nuluk Plateau, remnants of__ _ 35 

0.

O'Brien, W. B., acknowledgment to_ 1

Olivine, description of _ 40

Orbules, occurrence of_ _ __ 73, 75

plates showing________ 73

P.

41

95

Paigelte, description of _______

Peluk Creek, in Potato Mountain

area, placers on____

Percy claim, Cape Mountain, description of_____ 100,101,102 

Petersen, Tom, acknowledgment to__ 2 

Placer tin, occurrence of, on Cassiterite Creek__ 74

Plains, coastal, features of __ 35-36

Population of the region___ 12-13

Potato Mountain, altitude of 3

economic geology of-_ _ 89-90

geographic features of_ 88-89

lode developments on_ 90-94

rocks of-_______________ 89

tin placers near____ _ 94-96

Prospecting, suggestions for 124-125

Pyrite, description of __ 

Pyrolusite, description of_ 

Pyroxene, description of__ 

Pyrrhotite, description ofQuartz, description of_

Page.

41

41

41

41

41

R.

Randt Extension tunnel on the Cassiterite dike, description of ____ 67-69

Read lead prospect, on Brooks 

Mountain, description 

of_________ 86

Red Pox claim, on Potato Mountain,

description of_____ 90-91

" Reef rock." See Limestone.

S.

Scapolite, description of__________

Scheelite, description of___ _ 

Schist, correlation of_____________

Scope of the report ________

Seward Tin Mining Co., claim of 

Slate, contact metamorphism in___

deposition of_________ 17-19

in the Brooks Mountain area, 

nature and occurrence 

of ____________- 83,84 

near Kanauguk River, deposition of_________ 17,19 

of the York region, general

characteristics of__ 16-17 

Southern Cross tunnel, description

of__ ______ 79 

Sphalerite, description of 42 

Spinel, description of _ 42 

Stannite, description of__ 42 

Stibnite, description of____ __ 42 

Structure of the region__ 32-34 

Suggestions on prospecting for tin

lodes__-______ 124-125 

Sutter Creek, possibility of dredging ________ 96

T.

'Tin Creek, prospect on________ 74-76 

Tin deposits of the world, mode of

occurrence of ____ 118-122 

Topaz, description of _ 42 

Topography of the region 2 3 

Tourmaline, description of__ _- 42 43 

Trails and roads of the region_ _ 11-12 

Transportation of debris of rocks_ 7-9 

Transportation to and in the region. 10-

12,45

U.

United States Alaska Tin Mining

Co., development by__ 102

V.

Vegetation of the region___ _ 9-10 

Vesuvianite, description of_ _ 43
130 INDEX.

W.

Page.

Weathering of rocks, agencies of__ 5 7 

Winfield, E. A., operations by__ 110-111 

Wolframite, description of______ 43 

Wolframite-topaz lode, description

of_____________ 81 

Wollastonite, description of_____ 43

T.

Yankee. Girl lead and tin prospect,

description of______ 80

Page.

York Dredging Co., operations by_ 94, 95

York Mountains, description of 3

limestone of _-__________ 20

plate showing ________

York Plateau, remnants of __ 

Yo'rk tin region, map of_____ _

Z.

Zinnwaldite, description of___ 43

Zircon, description of__________ 43

Zoieite, description of_ 43