DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

GEORGE OTTS SMITH, DIRECTOK

BULLETIN 358

GEOLOGY

OF THE

SEWARD PENINSULA 

TIN DEPOSITS

ALASKA

BY ADOLPH KNOPF

WASHINGTON

GOVERNMENT POINTING OFFICE 

1908

CONTENTS.

Page.

Preface, by Alfred H. Brooks_____________ 5

Introduction __ 7

Production and prospecting_________.. _ _ . S

Geography of the region___________. _ _ 9

General geology _ _____________ _______ 10

Outline______________.________________________ 10

Slates near York_____- _____ _ __ __ 10

Port Clarence limestone-- . 12

Limestone near Palazruk______________ _ 13

Surn'cial deposits_____ = ____ ___ ___ 15

Igneous rocks______ _ 15

Mineralogy of the region___ ____ ______ _ _ 1(5

Economic geology _____________.__..__'___ __ 24

Outline__________________________________ ___ 24

Lodes __________________ _____ ___ 25

Ear Mountain________..__________.-_ __ : 25

Introduction __________________________ 25

General geology_______ ___ ' 25

Granite____________________________..__ 2(;>

Contact metamorphisni ____...._________._.-.._ 28

 Quartz-augite porphyry dikes__..___...__._____ . _ .. 2!)

Mineral occurrences----__.._..___. . .-_.. _.___._.____..... 30

Buck Creek______________._.__. _.__._____.._ ._ 32

Geologic features._____________.__.__..__ ____.... 32

Economic geology ___________.. _ ____...._ _____ 33

Origin of the ores_____________. ___....... .__. ___ 84

Cape Mountain____________..________.._. ____.... 35

General geology_-________________. _..______ __.._ 35

Granite_______________________________ 35

Contact phenomena__________________ 36

Ore deposits _____________________. ______ 38

Developments____________________________ 40

Brooks Mountain______________________________ 41

Lost River______.___________________________ 44

Location ________________________________ 44

General geology _____:________ 44

The rocks____________________________ 44

Orbicular contact metamorphism___________ 45

Cassiterite prospects________________________ 49

Lodes ___________________ _ ___________ 49

Developments^________________________ 52

Seaming of the limestone_________________ 52

Cassiterite and wolframite quartz veins__________ 55

JMetasomatic processes___________________ 56

Wolframite-topaz lode_______ ____ ___ 57

3
4 . CONTENTS.

Economic geology Continued. Page. 

Lodes Continued.

Lost River Continued.

Other mineral deposits______1..______________ 58

Alaska Chief property__________________ 58

Idaho claim____________________________ 59

Origin of the ores_________________________ 60

Placers_______________. __________________ , 61

Buck Creek___________:____________________ 61 

Grouse Creek _____________________ ________ 62

Fairhaven district_____________ ___________..__ 63 

Resume and conclusions ___________________ ________ 63

Practical deductions_i_-______________________ _______ 66

Index________l-i_____________:___ _ 69

ILLUSTRATIONS.

Page.

PLATE I. Topographic map of tin region, showing location of metalliferous prospects ___ 8

II. A, Thin section of paigeite hornfels; B, Port Clarence limestone

near head of Cassiterite Creek_ . ___ 12

III. A, Thin section of uctiuolite-cassiterite rock; B, Thin section of 

stanniferous metamorphosed limestone, from Brooks Mountain_ _ _ _ 84

IV. A, Orbule produced by contact rnetamorphisrn; B, Reverse side 

of orbule shown in A; C, Maximum orbule; D, Irregular 

orbules___ _ ___________ ____ _______ 44

V. Banded vein, supply duct for orbules__________ ______ 46

VI. A, Banded apophysis from garnet-vesnvianite mass on Tin Creek ;

B, Orbules in marble matrix, showing mode of origin____ 46

VII, A, Reticulate seaming of limestone in vicinity of Cassiterite lode; 

B, Surface exposure showing occurrence of fluorite silicate 

rock adjoining veinlets in limestone_l_i____________ 50

VIII. A, Polished surface of seamed limestone, showing intense metasomatism ; B, Thin section of Cassiterite ore__________ 54

IX. A, Polished surface of wall rock adjoining wolframite-quartz 

vein; B, Thin section of wall rock adjoining wolframite-quarts! 

vein _ ___________ _____ ________ 56

FIG. 1. Geologic sketch map of the Seward Peninsula tin region_____ 11

2. Geologic sketch map of Ear Mountain_________________ 27

3. Geologic section through Ear Mountain_______________ 27

4. Diagrammatic section at Eunson's shaft, Ear Mountain_____ 30

5. Geologic sketch map of Cape Mountain_____ _______ 36

6. Geologic section across Lagoon Creek, Cape Mountain________ 37

7. Geologic sketch map of Cassiterite Creek and vicinity______ 45
PREFACE.

By ALFRED H. BROOKS.

Since the discovery of stream tin in the York region by the Geological Survey in 1900 the tin deposits of this district have been discussed in several Survey publications. In all cases, however, the 

statements were based on investigations that were incidental to other 

work, and as this district had attracted much notice as a possible 

source of tin and considerable money had been spent in mining and 

prospecting, it appeared time, in the spring of 1907, to undertake a 

more thorough investigation.

To this task Mr. Knopf was assigned, with instructions to make a 

careful study of the mineral deposits and, so far as time permitted, 

to determine the laws of their occurrence and origin. It was 

thought best to emphasize the more purely scientific phase of the 

investigation, for the commercial phase can best be solved by the 

mining expert and engineer. Therefore, those Avho expect to find 

in this volume a statement of the commercial value of individual ore 

bodies will be disappointed. In the opinion of the writer, however, 

the presentation of the chief facts regarding the mineralogy and 

geology of the tin deposits, together with a careful analysis of these 

data, will have more value to the district as a whole than any attempt to publish a statement of commercial features of .individual 

ore bodies. Moreover, it has become an established policy in the 

Alaskan work not to attempt to sample ore bodies, as such work is 

believed to fall outside of the province of the Federal geologist, and 

obviously without careful sampling the valuation of any given deposit is impossible.

The reader should not infer from the foregoing remarks that this 

paper is regarded as a final statement on the occurrence and genesis 

of the tin deposits. Difficulty of access, limited exposures, and lack 

of time prevented Mr. Knopf from making exhaustive studies. It 

is believed, however, that this report marks a great advance in the 

knowledge of the subject.

5

GEOLOGY OF THE SEWARD PENINSULA TIN 

DEPOSITS, ALASKA."

By ADOLPH KNOPF.

INTRODUCTION.

Stream tin was discovered in the York region of Seward Peninsula 

during the fall of 1900 as a heavy and objectionable constituent which 

accumulated in the sluice boxes of the placer-gold prospectors. Some 

of this material was brought to Washington by A. H. Brooks, of the 

United States Geological Survey, who was engaged in a hasty reconnaissance of the mineral resources of the region, and was identified 

as cassiterite.6 The discovery was soon heralded by the public press.

The true nature and value of the mineral once known, search was 

directed toward finding a wider distribution of the stanniferous 

gravels and toward locating the bed-rock source of the cassiterite. 

Two factors combined to stimulate this search the failure of the gold 

placers of the region and the high market price of metallic tin in 

recent years. Ever since their discovery the Alaskan tin deposits, as 

they are popularly styled, have continued to attract considerable 

attention from the mining public an interest due in large part to 

the fact that there are no producing tin mines in the United States 

proper.

Several reports of a preliminary character have been issued by 

the Geological Survey describing the geology and mineral resources 

of the York region, in which the chief deposits of tin occur. The 

presence of placer tin in Anikovik River and in its tributary, Buhner 

Creek, was first recorded by Brooks.0 On the basis of a reconnaissance of the northwestern part of Seward Peninsula in 1901, Collier d 

was able to offer certain advice as to where search for lode tin might 

profitably be made. In 1903 he assisted a number of prospectors in 

making the original discovery of lode tin in Seward Peninsula and

°A summary of the results given in this paper was published several months ago by 

Brooks, A. I-L, and others, in Mineral resources of Alaska; report on progress of investigations in 1907 : Bull. U. S. Geol. Survey No. 345, 1908, pp. 251-267.

6 Brooks, A. H., A new occurrence of cassiterite in Alaska: Science, new ser., vol. 13, 

No. 328, 1901, p. 593.

c Brooks, A. H., An occurrence of stream tin in the York region, Alaska: Mineral 

resources U. S. for 1900, U. S. Geol. Survey, 1901, p. 270. Also, A reconnaissance of 

the Cape Nome and adjacent gold fields of Seward Peninsula, Alaska, in 1900 (a special 

publication of the U. S. Geol. Survey), 1901, pp. 132-139.

* Collier, A. J., Prof. Paper U. S. Geol. Survey No. 2, 1902, p. 49. 
8 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

was able to publish the first authentic information on the occurrence 

of cassiterite in place." On returning from his examination of the 

Cape Lisburne coal fields in 19.04, he spent a few days in the York 

region and noted the progress of development in opening up the tin 

deposits. 6 Later developments, in the fall of 1905, were fully described by Hess.c

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

popular reports indicated that extensive development work was in 

progress. Owing to the fact that the earlier investigations, which 

were incidental to field work in other parts of Seward Peninsula, 

were hampered by lack of time and by the inadequate development 

of the region, it was deemed advisable to give an entire field season 

to an examination of the tin deposits, including those of Ear Mountain, which had not been previously visited. With this purpose in 

view, the writer was instructed to examine all known occurrences of 

tin ore in Seward Peninsula, giving especial attention to the origin 

of the ores and the commercial importance of the field. Field work 

was commenced at-Tin City on June 23, and ended at Teller on September 6, 1907.

Acknowledgments are due for the many courtesies extended to the 

writer while in the region, and it is an especial pleasure to thank Mr. 

John Vatney, of Ear Mountain, Mr. M. R. Luther, of Tin City, and 

Messrs. William C. Randt and S. Read, of Lost River, for their generous hospitality and substantial aid in furthering the field work. Thanks 

are also due Dr. W. T. Schaller, Dr. E. C. Sullivan, and Prof. E. F. 

Smith for assistance in chemical and mineralogical work.

PRODUCTION AND PROSPECTING.

The total production of tin ore in the entire region to the close of 

1907 was about 160 tons of concentrates, all of which, except a few 

tons from lode deposits, came from the stream tin of Buck Creek.

.The approximate annual value of the tin production of the York 

district since mining began is as follows:

Value of tin produced in York region, Alaska, 1902-1907.

1902 ____________.______________________ $8,000 

1903_______.___________________________ 14/000 

1904 ________.:______________________________________ 8, 000

1905^________.__________________________ 4, 000 

1906__________._.______________________ 38,640 

1907 ____ J______._______________________ 20,000

92, 640

0 Collier, A. J., Tin deposits of the York region, Alaska : Bull. U. S. Geol. Survey No.'225, 

1904. See also Bull. 229.

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

No. 259, 1905, pp. 120-127.

c Hess, F. L., The York tin region : Bull. U. S. Geol. Survey No. 284, 1906, pp. 145-157.
U. S. GEOLOGICAL SURVEY 

GEORGE OTIS SMITH. DIRECTOR BULLETIN NO 358 PL. I

Alfred H. Brooks, Geologist in charge 

Topography by T G. Gerdine, 

D C. Witherspoon, and Alfred H. Brooks 

Surveyed in 1900- 1901

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

S o ale 2BOOOO

4- B 3 miles

20 24 6 a 10 12 14 Mloineters

Contour intei-val 20O feet

1908
GEOGEAPHY OF THE REGTOtt. 9

At present four localities are being prospected for tin. They 

are included in an area of 400 square miles, about 100 miles northwest 

of the city of Nome, the supply point of the region. In geographic 

order from north to south these four localities are Ear Mountain, 

Buck Creek, Cape Mountain, and Lost Kiver. Ear Mountain occupies 

an isolated position 40 miles north of the others, which are grouped 

together in the York region at the west end of the continent.

GEOGRAPHY OF THE REGION.

The region here to be considered comprises the extreme western 

projection of Seward Peninsula, or that portion of the American Continent which approaches nearest to Asia. On the northwest it is 

bounded by the Arctic Ocean, and on the southwest by Bering Sea 

two bodies of water that are frozen over during seven months of the 

year. In 1907 navigation opened to Tin City, which is situated at the 

tip of the continent, on June 22, and a few days later the last of the 

ice had drifted northward through Bering Strait. During the open 

season gasoline schooners maintain a weekly service between Nome 

and points on Bering Sea, and a small passenger steamer, carrying the 

mail, calls every ten days while en route to Kotzebue Sound. A 

nominal ten-day mail service is thus afforded during the summer 

months, but a regular weekly service is obtained in the winter with 

sled and dog team.

The topographic character of' the region is well brought out on the 

map (PL I). Toward the north the Arctic coastal plain forms a 

wide expanse of gently rolling topography with a relief of less than 

50 feet, and so heavily grown over with moss as to be practically impassable to wagons. Numerous lakes dot the landscape, and the 

streams wind across the plain in tortuous courses, emptying into 

broad lagoons impounded behind barrier beaches. In the vicinity of 

Shishmaref Inlet the lower stretches of the streams are affected by the 

tidal ebb and flow.

Toward the south the York Mountains rise abruptly from the 

coastal plain. They are an exceedingly steep and rugged group, with 

an average altitude of 2,500 feet. Broad stream valleys, however, 

penetrate the mountains from both the Bering and Arctic sides and 

render them easily accessible, so that wagons have been taken across 

them at a number of points without difficulty. Where the mountains 

abut upon the Bering coast thejr are broken off by bold sea cliff sf'and 

a magnificent terrace 1 to 4 miles wide with gentle seaward slopeThas 

been carved upon their flanks at an elevation of 600 to 800 fe'et.a 

Westward this feature merges into the York Plateau a level upland 

surface ranging in altitude from 200 to 600 feet. The larger streams

0 Collier, A. J., A reconnaissance of the northwestern portion of Seward Peninsula, 

Alaska : Prof. Paper U. S. Geol. Survey No. 2, 1902, p. 137.
10 TIN DEPOSITS OF REWARD PENINSULA, ALASKA.

have cut wide, shallow valleys in this plateau, and these furnish good 

wagon roadways of easy grade.

The western extremity of the continent is marked by an isolated 

mountain mass, known as Cape Mountain, which rises sheer from the 

water's edge to a height of 2,250 feet. This impressive promontory, 

usually scathed in chill fogs, forms the American portal of Bering 

Strait.

 There are no harbors in the region and consequently landing is 

often impossible on account of fog, surf, or storm. The nearest harbor is that of Port Clarence, 20 miles distant.

GENERAL GEOLOGY.

OUTLINE.

The sedimentary rocks of the tin region comprise .chiefly limestones 

and slates, all probably of Paleozoic age. The oldest rocks are a 

series of impure arenaceous slates of undetermined age. These are 

overlain conformably by a thick formation of thin-bedded Ordovician 

limestones (Port Clarence limestone), which generally show no evidence of metamorphism. Near Cape Prince of Wales there is developed a series of crystalline limestones with subordinate siliceous 

schists and quartzites. These rocks are of " Lower Carboniferous " 

age, and are faulted off against the slates to the east. Greenstones 

are common in the slates and a number of granite stocks have invaded 

the limestones.

The youngest sediments .of the region are the gravels, sands, and 

silts of the Arctic coastal plain. The distribution of the rocks is indicated on the sketch map (fig. 1), in which use has been made of the 

earlier published results. In the succeeding pages the salient features 

of the various formations of the York region are briefly described, but 

no attempt is made to discuss the metamorphic rocks of Ear Mountain 

under the heading of general geology, inasmuch as their stratigraphic 

relations are unknown.

SLATES NEAR YORK.

In the vicinity of York a belt of slates 8 or 10 miles wide trends 

north westward across the west end of Seward Peninsula. The slates 

are prevailingly of an impure arenaceous character and exhibit a 

variable degree of metamorphism throughout the area, though the 

great bulk of the series consists of but slightly metamorphosed rocks. 

Typical black clay slates are locally interstratified as thin beds with 

the more siliceous types. The arenaceous slates are as a rule more 

or less calcareous or dolomitic. Massive members of the series consist of fine-grained dolomitic sandstone. A graphitic siliceous schist 

(the graphite, however, is poorly individualized) represents the
GENERAL GEOLOGY. 11

highest phase of metamorphism in the York region, but this type has 

no great areal distribution. Petrographically the slates consist chiefly 

of comminuted quartz and feldspar, and as an accessory mineral a 

constant though minute . amount of clastic brown tourmaline was 

noted from all parts 'of the area. In the dolomitic member the quartz 

grains are generally well rounded, and the carbonate, whose magFIG. 1. Geologic sketch map of the Seward Peninsula tin region.

nesian character was determined chemically, is commonly in the form 

of small rhombohedra scattered throughout the binding material.

The degree of cleavage of the slates is variable. The rocks range 

from nearly massive rocks little more than shales to thinly fissile 

slates. Cleavage and stratification may coincide locally, but within 

a short distance may stand at right angles. Measurements of strike
12 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA, '

and dip of the cleavage have little value on account of this abrupt 

variation, as the sedimentary banding can rarely be observed. These 

relations are best displayed in the beach section between York and 

Kanauguk Point. The slates are affected by faults, and one was 

noted to have offset a dike 400 feet. They are fractured and veined 

with quartz stringers, some of which carry cassiterite, blue tourmaline, arsehopyrite, and pyrite.

The slates are faulted against a series of crystalline limestones on the 

west, and underlie the Port Clarence limestone conformably on the east.

PORT CLARENCE LIMESTONE.

The Port Clarence limestone was so named by Collier a on account 

of its typical exposure north of Port Clarence, where it occupies an 

area of 1,400 square miles. Here it comprises a thick volume of thinbedded limestones of dense, texture, generally unaffected by metamorphism. Four types of rock can be discriminated an ash-gray variety, a dark lead-gray variety, magnesian and tremolitic phases, and 

an argillaceous banded variety. The first two are the commonest 

types, and occur together in interstratified beds. The dark lead-gray 

limestone forms massive beds up to 6 feet thick, but the ash-gray 

variety, which is fine grained, like lithographic stone, is thin bedded 

and commonly breaks into large, thin slabs whose surfaces are covered 

with fucoid fragments. These limestones were found to be nonmagnesian in the .specimens examined, the ash-gray variety containing 

considerable aluminous material, and the dark lead-gray limestone 

being pure carbonate rock. Some beds of fine-grained dolomite occur 

in the Port Clarence formation, but on account of their close resemblance to the prevailing dense-textured limestones their quantitative 

abundance is not known. Occasionally there occur interbedded with 

the normal Port Clarence limestones strata which show numerous 

small prisms of tremolite in random orientation. This is the highest 

degree of metamorphism displayed by the formation, except for 

purely local manifestations surrounding granitic intrusives.

The basal portion and lower horizons of the Port Clarence formation consist of an impure banded limestone, the banding being 

produced by laminae of argillaceous material. This phase is commonly in a highly contorted condition (PL II, B). Locally, as at 

Cassiterite Creek, tremolite has been noted as an abundant constituent, though the possibility is not excluded that the amphibole 

may have been produced by the action of vein-forming agencies.

In the Lost River region the Port Clarence has a thickness of 

2,000 feet. Collier l has indicated that the thickness may be as great

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

Alaska: Prof. Paper U. S. Geol. Survey No."2, 1902, p. 18.

" Collier, A. .T., Tin deposits of the York region, Alaska: Bull. U. S. Geol. Survey No. 229, 

1904, p. 19.
U. S. GEOLOGICAL SURVEY I.F.TIN NO. 358 PL. II

.-1. THIN SECTION OF PA1GEITE HORNFELS. 

Magnification 60 diameters. Black, opaque fibers of paigeite in tourmaline. Colorless mineral is fluorite.

PORT CLARENCE LIMESTONE NEAR HEAD OF CASSITERITE CREEK. 

Shov/ing crumpled character of the argillaceous banded variety.
GENERAL GEOLOGY. 13

as 5,000 feet, but on account of the prevalence of faults and shatter 

zones in this region it is probable that the smaller figure is more 

nearly correct. The dip of the limestone is usually low about 

20° N. although it may reach as high as 60° along the upper course 

of Kanauguk River.

The Port Clarence limestone overlies the slates of the York region 

conformably. As exposed along the western flank of the York 

Mountains the basal argillaceous schistose limestones of the Port 

Clarence formation merge imperceptibly into members of the slates, 

the transitional zone being several hundred feet thick. The same 

relations are evident on the northwest flank of Brooks Mountain, 

where the transition is more abrupt, but is marked by an intimate 

interlamination of slate and limestone. The transition zone has 

been a zone of weakness, and exhibits more or less severe dynamic 

deformation. As already indicated, the lower horizons of the Port 

Clarence are acutely crumpled, locally passing into a brecciated 

condition. Viewed in the large, however, this phase maintains the 

appearance of undisturbed and simple structure, characteristic of 

the Port Clarence as a whole.

Farther east, in the vicinity of Bay Creek, the Port Clarence 

limestone gives place to the graphitic, chloritic, and calcareous 

schists characteristic of the Nome region. The exact relations are, 

however, obscure. In the hills behind Teller Mission the limestone is 

highly argillaceous and in many places acutely contorted, indicating 

that the basal portion of the Port Clarence is exposed. According to 

this interpretation the Port Clarence is regarded as folded into a 

synclinorium, with one limb exposed on the western flank of the York 

Mountains and the other at Bay Creek. Subordinate folds have 

brought the underlying slates into the zone of erosion in isolated 

areas, as at Brooks Mountain and California River. 

The age of the Port Clarence limestone, according to Collier,* is 

either Ordovician or Silurian. The writer found at the head of 

Cassiterite Creek a few poorly preserved fossils, which, as identified by Kindle, appear to belong to the genera Raphistoma, and 

Liospira; indicating an Ordovician age.

LIMESTONE NEAR PALAZRUK.

Between the granite headland of Cape Prince of Wales and the 

mouth of Baituk Creek is a belt of crystalline limestone, which is 

finely exposed in the cliffs that front Bering Sea. Sericitic siliceous 

schists, phyllites, and thin-bedded white quartzite are present, but are 

of very subordinate importance. The siliceous schists find their main

0 Collier, A. J., The gold placers of a part of Seward Peninsula : Bull. U. S. Geol. Survey 

No. 328, p. 79.
14 TIN DEPOSITS OF SEWAKD PENINSULA, ALASKA.

development on Cape Mountain, where they are apparently 300 feet 

thick, and overlie the limestone capping the summit o'f the granite 

stock.

A striking feature of the limestone is the variable degree of metamorphism which it exhibits in different parts of the area. It varies 

from a dense-textured dark-blue'limestone on the north side of Cape 

Mountain to a faintly schistose snow-white marble in the section displayed along Bering Sea.

The thermal metamorphism produced by the granite invasion is 

strictly local in character. Marmorization extends 200 feet from the 

intrusive at a maximum, producing a coarsely granular aggregate of 

calcite crystals several millimeters in diameter. The calcareous 

quartzites, however, remain unaffected at this distance. In the 'limestone patch overlying the granite the siliceous laminae have been converted into wollastonite bands which exhibit a remarkable degree of 

minute crinkling. Some phases of the limestone show a development 

of metamorphic minerals unconnected with the presence of visible 

intrusives. In the old sea cliff 1 mile east of Tin City a series of interstratified ash-gray and dark-blue beds, each individually a few feet 

or less in thickness, show numerous foils of phlogopite, long prisms 

and radial groups of tremolite, and cubes of pyrite. Phlogopite is 

areally the most persistent mineral, and the phlogopite-bearing limestone can be traced as far west as the head of Cape Creek. A certain original content of magnesia is indicated by the formation of 

phlogopite and tremolite and can be detected chemically in the carbonate of the dark-blue limestone. Whether the formation of these 

minerals is due to thermal metamorphism is ah open question, but 

the field evidence suggests that it has been caused by the same agency 

which produced the crystalline marbles namely, a mild regional 

metamorphism.

The structure within the limestone area is prevailingly simple. 

The beds lie nearly horizontal, with a slight easterly dip. Here and 

there rolls Avith dips up to 20° occur. Locally individual strata are 

acutely crumpled and doubled back upon themselves.. On the basis 

of relative degree of metamorphism this limestone would be regarded 

as the oldest formation in the York region, but paleontologic evidence procured by Collier" has shown it to be of Mississippian 

(" Lower Carboniferous ") age younger than the less highly metamorphosed Port Clarence limestone 8 miles east of it. The limestone 

appears to be faulted against the slates on the east. Both formations 

are lying flat near the contact, though locally the limestone may show 

almost vertical dips. The line of contact, however, does not follow 

the contours as it should if one horizontal formation were resting

a Collier, A. J., The gold placers of a part of Seward Peninsula: Bull. U. S. Geol. 

Survey No. 328, p. 81.
GENERAL GEOLOGY. 15

upon the other, but cuts across them indifferently. Furthermore, various friction breccias are found in the vicinity of the contact, and an 

abundant quartz veination occurs in the slates. These features the 

shattering, crushing, and local dragging of the strata, and the fact 

that the contact is independent of the topography indicate that the 

limestone is cut off to the east by a fault.

SURFICIAL DEPOSITS.

Surficial deposits have a relatively small distribution in the York 

region. They comprise the shallow gravels of the present streams 

and the beach deposits of the narrow coastal shelf bordering Bering 

Sea. Toward the north, however, all bed rock is mantled by the 

silts and gravels beneath the low-rolling arctic tundra that stretches 

between Cape Prince of Wales and Cape Espenberg.0

Some of the stream gravels contain local concentrations of gold

and placer tin, and are therefore of economic interest.

IGNEOUS ROCKS.

Four stocks of granite are intrusive into the limestones of the 

region, all quartzose orthoclase granites containing subordinate 

amounts of sodic oligoclase and biotite, and all prevailingly of a 

coarsely granular porphyritic habit. They appear to represent contemporaneous intrusions; and as the granite at Cape Prince of 'Wales 

is known' to invade limestones of "Lower Carboniferous " (Mississippian) age, it is probable thateall are post-Mississippian. Together 

with related quartz-bearing porphyry dikes, they are the most important igneous rocks of the region, inasmuch as the tin deposits 

are directly associated with them, and their description is therefore 

given in greater detail under the separate localities at which each 

occurs.

Greenstones of diabasic character are common in the slate area 

near York. Where their relations can be determined they are found 

to be present as intrusive sills. The texture of the greenstones ranges 

from aphanitic to coarsely granular, and petrographic examination 

shows that they are composed essentially of chloritized augite and 

altered plagioclase in ophitic arrangement, with abundant accessory 

ilmenite and leucoxene. In many places the alteration is exceedingly 

thorough, but the rocks are unaffected by shearing.

Narrow basalt dikes of rare occurrence are found cutting both 

limestones and granite, and are therefore the youngest igneous rocks 

of the region.

« Collier, A. J., Prof. Paper TJ. S. Geol. Survey No. 2, 1902, p. 25.
16 TIN DEPOSITS OF SEWAED PENINSULA, ALASKA.

MINERALOGY OF THE REGION.

The world's supply of tin is obtained from cassiterite, the dioxide 

of tin (T8.6 per cent Sn). At a very few places stannite, a complex 

sulphide of copper, tin, iron, and zinc, has been found in sufficient 

quantities to be raised as an ore, but apparently never for its tin. 

The stannite formerly found so abundantly in the Carn Brea mines 

in Cornwall was sold simply for the copper it contained.* In recent 

years only one occurrence is known, namely, at the Oonah mine in 

Tasmania,6 where an argentiferous stannite has been mined as a 

copper-silver ore. The tin was rejected as waste product, but arrangements were finally made with the smelter that for ore containing at least 8 per cent of tin $5 per ton should be paid in addition to 

the ordinary returns for copper and silver. The latest reports from 

Tasmania indicate, however, that difficulty has been experienced in 

treating the stannite ore.0 At Borah Creek, New South Wales,'* 

stannite is also found, and occurs associated with chalcopyrite, arsenppyrite, and galena in a silver-quartz ore. With these two exceptions 

stannite is of rare occurrence and is not regarded as an ore of tin.

Cassiterite is the only mineral likely to be of economic importance 

as a source of tin in the Seward Peninsula region. Stannite in association with galena and wolframite is known from one locality 

only, and there the prospective value of the deposit is probably in 

silver' and tungsten, and not in tin.

Placer gold is found with the stream tin of the York region, but 

its paragenesis with relation to the cssssiterite is unknown.

The Seward Peninsula tin deposits are associated with granitic 

intrusives which have invaded various series of limestones. The 

magmas were rich in m i neralizers and produced an intense pneumatolytic contact metamorphism along their margins. Conspicuous 

among the products of this activity are the prevalent boron minerals, 

tourmaline, axinite, a boron A^esuvianite, ludwigite, and two magnesian iron-tin borates new to science, which have been named hulsite 

and paigeite. Fluorite, scapolite, and chondrodite prove the presence 

of the halogens in the magmatic exhalations, and sulphur is indicated 

by the various metallic sulphides that have formed in the contactmetamorphic rocks.

A total of 52 minerals are listed from the region, 16 of which have 

not previously been recorded and two of which are new species.

Gold. Some of the streams of the York region carry placer gold. 

In fact, the prevalence of an undesirable heavy brown mineral in the

0 Trans. Eoyal Geol. Soc. Cornwall, vol. 7, p. 85.

6 Waller, G. A., Zeehan silver-lead mining field, Govt. Geol. Office, Tasmania, 1904. 

c Progress of the mineral industry for the quarter ending 30th September, 1907, 

Govt. Geol. Office, Tasmania, p. 10.

d Min. Res. New South Wales, Geol. Survey, New South Wales, p. 120.
MINERALOGY OF THE REGION. 17

sluice boxes of the placer-gold miners led to the discovery of cassiterite and to the search for its bed-rock source. Placer gold is associated with the cassiterite of Buck Creek, the only locality from 

which there has been an actual production of stream tin, but no 

authentic figures are available as to the amount of gold obtained per 

ton of concentrates. Nuggets up to $20 have been obtained."

 Stibnite (Sb2 S3 ; 71.4 per cent antimony). Some float stibnite, 

associated with purple fluorite, was found in the saddle at the head of 

Tin Creek by H. E. Angstadt, of the Survey party.

Molybdenite. Molybdenite occurs in sparing amount, associated 

with cassiterite, in the Lost River region.

, Galena. On Brooks Mountain, galena, associated with an ironrich zinc blende, occurs intergrown with minerals of contact-metamojphic origin. The Ear Mountain occurrences are of similar nature. 

In the Lost River region it occurs in fracture zones in the Port Clarence limestone. One occurrence in this region is absolutely unique, 

namely, galena associated with stannite and wolframite in a vein 

filling of topaz'-and fluorite.

Sphalerite. Sphalerite (zinc blende) of contact-metamorphic 

origin is common on Brooks Mountain. It possesses a brilliant black 

luster identical with that of wolframite, with which mineral it .has 

been confused. Analysis by W. T. Schaller shows that specimens contain 19 per cent of ferrous iron. It,is distinguished from Avolframite 

by its inferior gravity (sphalerite having a specific gravity of 4 and 

wolframite of 7.3), greater softness, and more complex cleavage. 

Wolframite shows but a single cleavage direction; sphalerite may 

show as many as six. Some thin quartz-tourmaline veinlets carrying 

pyrite and lustrous black sphalerite were found cutting the granite of 

Ear Mountain. In this material blowpipe tests were necessary to distinguish the sphalerite from wolframite. A small amount of sphalerite of somewhat resinous appearance is found associated with the 

tin ore on Cassiterite Creek.

Pyrrhotite. Magnetic iron pyrites, or pyrrhotite, is found, together 

with galena and sphalerite of contact-metamorphic origin, on Brooks 

Mountain. Considerable amounts of it occur in a copper prospect at 

the mouth of Tin Creek. Small amounts are disseminated in the 

pyroxene hornfels at Cape Mountain. Pyrrhotite is commonly regarded as stannite throughout the tin region, but can infallibly be 

distinguished from that mineral by its magnetic character.

Clialcopyrite. Yellow copper pyrites, associated with pyrrhotite, is 

found in a fluorite gangue near the mouth of Tin Creek. The limesilicate hornfels surrounding the granite of Ear Mountain is locally 

.flecked with chalcopyrite.

n Oral communication by F. L. Hess. 

. 54356 Bull. 358 08 2
18 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

Pyrite. Irregular disseminations of pyrite occur in the granite of 

Cape Mountain. It is a common constituent in .the tin ores of the 

region, and occurs in the form of rolled nuggets'with the stream tin 

of Buck Creek.

Arsenopyrite. The silver-white sulphide of iron and arsenic occurs 

in considerable abundance in the tin ore of Cassiterite Creek, and is 

found in the contact-metamorphic deposits of Brooks Mountain, associated with tourmaline, fluorite, sphalerite, etc. It occurs with 

actinolite and cassiterite in the Buck Creek region.

Stannite (tin pyrites; 29.5 per cent copper, 27.5 per cent tin). 

The rare mineral stannite, a sulphide of copper, tin, iron, and usually 

zinc, is known from Lost River only, where it occurs associated with 

galena and wolframite in a gangue of topaz and fluorite. The Alaskan stannite gives a strong qualitative reaction for zinc. It has a 

brown-black color and a metallic luster, and possesses an imperfect 

cleavage. Stannite is a mineral whose identification in any particular 

case must be confirmed by chemical examination.

Fluorite. Fluorite occurs in a variety of. ways throughout the 

region as a contact-metamorphic mineral, as a gangue mineral of tin 

and of copper deposits, and as a metasomatic replacement of limestone 

adjoining stanniferous veinlets. It shows a variety of colors purple, 

green, yellow, rose -and is also colorless, but the purple is the most 

common. Specimens from the Cassiterite lode show numerous cubes 

and, rarely, aggregates of columnar crystals. Fluorite is distinguished from quartz by its relative softness and fine octahedral 

cleavage. A characteristic feature is its power of phosphorescence, 

which becomes highly conspicuous during the drying 'of ore samples.

Quartz. The granites of the region contain abundant quartz, which 

is prevailingly of a smoky character. The quartz porphyry dikes

contain numerous sharply defined crystals of quartz, and these, too, 

are commonly smoky, and consequently have.been mistaken for cassiterite to some extent. Greasy milk-white quartz (" vein quartz ") 

carrying cassiterite forms stringers cutting the slates of the York 

area.

Hematite. Nuggets of red oxide of iron occur with the stream tin 

of Buck Creek. Hematite is used as a pigment by the Eskimos of 

Shishmaref Inlet.

Ilmenite. Ilmenite occurs as an abundant microscopic constituent 

of the greenstones, and is largely converted to leucoxene.

Spinel. Perfect little octahedra of black spinel are found associated with chondrodite in contact-metamorphosed limestone near 

Read's galena prospect on Brooks Mountain.

Magnetite. Nuggets-of magnetite (magnetic iron ore) are fairly 

common in the stream tin of Buck Creek. The ore is found in place 

in visible crystals and clumps associated with calcite, hulsite, and
MINERALOGY OF THE REGION. 19

vesuvianite on Brooks Mountain, and occurs also in narrow bands in 

contact-metamorphic limestone at Tin Creek.

Cassiterite (Sn02 ; 78.6 per cent tin). Cassiterite is a mineral 

which can not be positively identified by the eye alone. The only 

convincing test is the actual production, from specimens in question, 

of beads of metallic tin. On account of the difficulty of recognizing 

Cassiterite the prospectors of the region have mistaken for it a great 

variety of minerals, including garnet, black tourmaline, augite in 

quartz porphyry dikes, pyroxene in contact-metamorphosed limestone, smoky quartz, vesuvianite, and wolframite. In color the 

Alaskan cassiterite varies from black to light yellowish or almost 

colorless. Some from Cape Mountain and Lost River Avas noted to 

show a fair degree of cleavage, which increases its resemblance to 

pyroxene. The specific gravity of cassiterite is about'7 considerably

higher than that of the other minerals mistaken for it, except wolframite. Crushing and panning may therefore serve as a rough test, 

a gray or colorless residue in the pan indicating cassiterite. But 

inasmuch as considerable useless prospecting has been done on minerals mistaken for cassiterite, it seems advisable to test the suspected 

minerals for the only conclusive property of cassiterite its ability 

to yield metallic tin. Pebbles and rolled grains of cassiterite occurring in stream gravels are known as stream tin. That of Buck Creek 

is prevailingly of a brown color, and much of it contains small 

cavities lined with clear, glassy, yellow crystals. Quartz adheres to 

many of the larger nuggets.

Rutile. Rutile occurs as a microscopic constituent in the granites 

of Cape Mountain.

Pyrolusite. Perfect dendrites, which are referred to pyrolusite, 

occur on the joint planes of the limestone near the Cassiterite lode.

Limonite. Limonite occurs very abundantly in the gossan of 

galena bodies in the Lost River region.

Calcite. The limestones in immediate proximity to the granite 

bosses of the region have been converted into aggregates of coarse 

white calc spar in many places. On the Dolcoath lode, near Cassiterite Creek, finely crystallized cassiterite occurs embedded in coarsely 

crystalline calcite associated with danburite, tremolite, and topaz.

Dolomite. Certain strata of the Port Clarence limestone are composed of dolomite. It occurs also in the form of minute rhombohedra in some of the slates of the York area.

Cerusite. The gossan of a galena prospect on Tin Creek was 

noted to contain white crystals of cerusite (lead carbonate).

Azurite. Rock from the wolframite-topaz lode on Lost River is 

slightly incrusted with the blue copper carbonate, azurite, doubtless 

derived from the oxidation of stannite in the ore.
20 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

Feldspar. Large porphyritic crystals of orthoclase and microcline occur in the granites of the region. Albite was detected as a 

constituent of the wolframite-quartz veins on Cassiterite Creek. 

Plagioclase is common in the igneous rocks of the region, and in the 

limestones as a metasomatic mineral.

Pyroxene. Large crystals of augite, up to 2 inches in size, occur as 

a constituent of a quartz porphyry dike on Ear Mountain. They 

are of brown color and rarely show cleavage visible to the eye. They 

have been mistaken for cassiterite and have occasioned considerable 

useless prospecting. Augite is approximately half as heavy as cassiterite, and is readily fusible before the blowpipe. By this simple 

test with the blowpipe, were it available to the prospector, all the 

minerals usually mistaken for cassiterite, except smoky quartz, might 

be rejected from consideration. Pyroxene, probably near hedenbergite in composition, is common in the contact-metamorphosed 

limestone adjoining the granites of the region, particularly that of 

Cape Mountain. It resembles some of the cassiterite very closely, 

and the contact rocks have been prospected for tin. This fallacy 

is encouraged by the fact, that the contact rocks are relatively heavy, 

having a weight corresponding to a 10 per cent quartz-tin ore.

Wollastonite. Wollastonite occurs on Cape Mountain in proximity 

to the granite, locally forming white masses up to 3 feet in thickness.

Amphibole. Tremolite, the colorless variety of amphibole, is 

prevalent in the form of glistening white fibers in the limestone adjoining the cassiterite occurrences on Cassiterite Creek. It also 

occurs as fine radial groups in the limestone east of Tin City. The 

light-green variety, actinolite, constitutes the gangue material of 

some newly discovered tin-bearing rocks on Buck Creek. Hornblende is common in the limestone adjoining cassiterite veinlets,

and gives it a dark-green color.

Garnet. As a product of contact metamorphism garnet is of 

widespread occurrence, the finest specimens coming from Tin Creek, 

on Lost River. It is commonly crystallized in rhombic dodecahedral 

forms, yielding diamond-shaped faces. Where only the apex of the 

dodecahedron is visible garnet bears a deceptive resemblance to the 

four-sided pyramid characteristic of cassiterite. Vesuvianite is 

usilally associated with the garnet.

Olivine. Olivine forms part of the basalt dikes of Cape Mountain, 

and is noted only microscopically.

Scapolite. Chlorine-bearing scapolites are found in specimens of 

lime-silicate hornfels from Ear Mountain and Cape Mountain. Its 

identification is possible only by microscopical and chemical methods. 

In a number of specimens it was found that the birefringence, as 

indicated by the negative uniaxial interference figures, ranged from 

low to comparatively high values in the same thin section.'
MINERALOGY OF THE REGION. . 21

Vesuvianite. Vesuvianite is one of the commonest contact-metamorphic minerals of the region. It is especially prevalent on Brooks 

Mountain and along the headwaters portion of Yankee Creek, where 

fine crystals showing ideal development occur in great abundance 

embedded in a matrix of coarsely crystalline calcite. The color of 

the vesuvianite is some tone of green, ranging from gray-green to 

brown-green. The crystals are usually in the form of square prisms.0 

As radial aggregates and branching forms, vesuvianite associated 

with garnet is abundant near the granite of Tin Creek on Lost 

Kiver. It also forms metasomatically near stanniferous veinlets in 

limestones.

Zircon.- Zircon is noted as a microscopic constituent of the granites of the region.

Danb'urite. The rare borosilicate of lime, danburite, was identified 

as the gangue material of cassiterite in the Dolcoath lode on Cassiterite Creek, where it occurs as replacement both of the dike rock 

and of the contiguous, limestones. (For partial analysis see p. 52.) 

The optical properties of the danburite.in thin section are indecisive; 

it is colorless and biaxial and shows the interference tints of feldspar 

and a relief near that of the associated tourmaline.

Topaz. As a constituent of the tin ore in the Lost River region 

topaz (nuosilicate of aluminum) is exceedingly abundant. It occurs 

in altered quartz porphyry dikes, as vein mineral in veinlets in limestones, and as a metasomatic product in the limestone adjoining such 

veinlets. It is commonly associated with fluorite and zinnwaldite. 

The most remarkable occurrence, however, is that on the Oregon 

claim, where delicately radial topaz with subordinate fluorite forms 

ihe gangue of an argentiferous ore containing wolframite, galena, 

and stannite. (For analysis see p. 58.)

Zoisite. Zoisite occurs in small amount in lime-silicate hornfels 

on Ear Mountain.

Epidofe. Kadial groups of epidote are found on Brooks Mountain 

near the granite contact.

Axinite. Small crystalline aggregates of axinite occur in a tourmaline hornfels from Ear Mountain. It is highly glassy in appearance and of a peculiar brown color. Many of the crystal faces are 

striated and acute edged. Axinite also occurs microscopically in a 

contact-metamorphic deposit on Brooks Mountain, associated with 

tourmaline, nuorite, arsenopj^rite, and sphalerite.

Chondrodite. Small honey-yellow crystals of chonclrodite occur, 

together with numerous minute black octahedra of spinel, in a contact-metamorphosed limestone on Brooks Mountain.

Tourmaline. Tourmaline is exceedingly common throughout the 

entire region and is regarded by many of the local prospectors as an

ft Forms measured crystallographically are given iu Am. Jour. Sci., 4th ser., vol. 25, 

1908, p. 323.
22 TIN DEPOSITS OF SEWABD PENINSULA, ALASKA.

infallible indication of tin ore. Although it is true that tourmaline 

is pretty generally associated with cassiterite, it is far from true that 

cassiterite is always associated with tourmaline. Great deposits of 

tourmaline that carry no tin whatever are known in various parts of 

the world.

In the Seward Peninsula tin region tourmaline occurs in a great 

variety of ways. It is found in granite apophyses on Ear Mountain 

as an original constituent (of the pneumatolytic stage of consolidation). It forms an essential mineral of the lime-silicate rocks surrounding the granite and occurs along seams and stringers in the 

granite itself, and in the quartz porphyry dikes with fluorite, arsenopyrite, and possibly cassiterite. On Cape Mountain it is abundant 

in the granite, and also occurs embedded in coarsely granular calcite. 

In the Buck Creek area the quartz stringers contain small rosettes of 

blue tourmaline. It is common in the Lost River and Brooks Mountain regions. Black, dark blue, and more rarely brown green are 

the prevailing colors. It is commonly crystallized in three- and sixsided prisms and columns, generally arranged in radial groups. As 

a rule the columns are aggregated together, and it is therefore usually 

difficult or impossible to distinguish the geometric form of individual crystals. Tourmaline has a specific gravity of 3, or less than 

half that of cassiterite. This property and its form, where discernible, are its most characteristic differences from cassiterite, with 

which it has frequently been confounded.

Muscovite. White mica occurs at Ear Mountain in granite tongues 

extending from the central mass out into the surrounding sedimentary rocks.

Zinnwaldite. The lithium-iron mica, zinnwaldite, occurs abundantly in the cassiterite veinlets on Cassiterite Creek, and is habitually 

associated with topaz and fluorite. It resembles muscovite, both to 

the eye and under the microscope, but differs from it by possessing a 

smaller axial angle. It fuses readily to a black magnetic globule. 

(For analysis see p. 54.)

Biotite. The granites of the region contain black mica as a subordinate constituent in the form of small lustrous plates. It occurs 

also in a pegmatite on Brooks Mountain, in plates up to one-half inch 

in size.

Phlogopite. Phlogopite, the magnesia mica, is found as small 

flakes associated with vesuvianite in contact-metamorphosed limestone on Brooks Mountain, and with tremolite is abundant in the 

limestone east of Tin City.

Chlorite. Chlorite is common in the gieenstone of the slate area, 

near York.

Kaolin. Kaolin occurs abundantly as an alteration product in the 

Cassiterite lode.
MINERALOGY OF THE REGION. 23

Apatite. Apatite occurs as a microscopic constituent of the 

granites.

Ludwigite- (3MgO.B2 03+FeO.Fe20:! ). A finely fibrous darkgreen mineral, soluble in hydrochloric acid, forms small radial aggregates in a contact-metamorphosed limestone from Brooks Mountain. 

It gives a decided flame reaction for boron, and is therefore tentatively 

identified as ludwigite.

Paigeite. The new boron-tin mineral, paigeite,a was found at two 

localities, near Read's cabin on Brooks Mountain in loose blocks and 

at Ear Mountain in situ. The Brooks Mountain material is intergrown with vesuvianite, calcite, and hedenbergite, with subordinate 

biotite and arsenopyrite in sporadic grains. At Ear Mountain it 

occurs evenly disseminated through a tourmaline-lime-silicate hornfels consisting essentially of calcite, tourmaline, vesuvianite, fluorite, 

and zoisite, with accessory phlogopite, chalcopyrite, and magnetite.

It is a lustrous coal-black foliated mineral, and has a hardness of 

about 3, with a density of 4.71. Analysis of material from Brooks 

Mountain gave the following result, which is recalculated on the 

basis of 100 per cent; it contains probably 15 per cent SnO2 :

Analysis of paigeite from Brooks Mountain.

FeO __________________________________ 51.99 

MgO______________________.___________ 1.68 

Fe2 03 ________________. _______________ 19.54 

H2O__________________..________________ 2.37 

Sn02 _ _.__ __ _______._-_.__-.___.___.______ ___.

100. 00

Hulsite.* On the northwestern flank of Brooks Mountain a prospect cut has been opened on'a showing of contact-metamorphic minerals occurring in a marmorized limestone 10 feet from the granite 

contact. Examination of this deposit indicated that an unknown 

mineral, which subsequent investigation proved to be a new boron-tin 

mineral, was present in considerable abundance. Hulsite, as the mineral was named, is closely associated with magnetite and a boron 

vesuvianite in a matrix of coarse calc spar, in which garnet and fluorite occur in subordinate amounts. The characteristic features of the 

new mineral are its strong submetallic luster, black color, good cleavage after the prism of 57° 38',. and tendency toward a tabular 

development. Its hardness is about 3; density, 4.28. It contains 

probably 20 per cent SnO2 . c

°"Knopf, A., and Schaller, W. T., Two new boron minerals of contact-metarnorphlc 

origin : Am. Jour. Sci., 4th ser., vol. 25, 1908, p. 323. 

6 Op. clt, p. 323.

0 Chemical work Is at present in progress by W. T. Schaller, with a view to determining the formulae of hulsite and paigeite. It appears that the original determinations of 

B203 were defective, so that the formulae proposed are untenable..
24 TIN" DEPOSITS OF SEWABD PENINSULA,, ALASKA.

Wolframite (Fe,MnWO4 ; 76.4 per cent tungsten trioxide). The 

valuable mineral Avolframite occurs associated with cassiterite on 

Cassiterite Creek and with galena and stannite in a topaz-fluorite 

gangue on the Oregon claim on Lost River. Its distinguishing features are its great weight (specific gravity, 7.3), black color, and 

brilliant submetallic luster on fine cleavage surfaces. Its hardness is 

5.5, which means that it can be rather readily scratched with a knife. 

The resemblance of wolframite to the iron-rich zinc blend of Brooks 

Mountain has already been pointed out.

Scheeliie (CaW04 ; 80.6 per cent tungsten trioxide). The mineral 

scheelite has been detected as a microscopic constituent of certain 

lime silicate contact rocks of Cape Mountain. It was also found as 

minute grains in a number of small cassiterite veinlets on Cassiterite 

Creek and in the altered limestone adjoining the veinlets. ,As an alteration product it forms microscopic coatings around Avolframite 

crystals. None of these occurrences are of possible commercial importance. Scheelite is a heavy Avhite mineral of vitreous luster, 

having a specific gravity of 6 and a hardness .ranging from 4.5 to 5.

ECONOMIC GEOLOGY.

OUTLINE.

Tin ore occurs in both lode and placer form, but up to the present 

time practically the only production has been from the placers of 

a single stream Buck Creek. Placer tin is known to be Avidely 

distributed in the streams of Seward Peninsula,** and has been found 

in some of the gold placers near Nome, but in amounts that are commercially unprofitable.

Cassiterite is the only mineral that is. likely to prove of economic 

A*alue as a source of tin. Stannite is also known to occur, but at one 

locality only, where it is associated Avith galena in a remarkable 

argentiferous wolframite-topaz ore. Two neAv tin minerals (magnesian iron-tin borates) have been discovered, but on account of their 

IOAV tin content (approximately 15 per cent SnO2 ) are probably not 

worth exploitation.

The lode-tin deposits are genetically associated Avith the granitic 

intrusives, and on account of the abundance of limestone in the region 

the SeAvard. Peninsula tin occurrences possess a number of unique and 

distinctive features. A Aiiriety of pneumatolytic contact rocks htwe 

been produced around the margins of the granites, and certain of 

these contain the iron-tin borates (hulsite and paigeite) as essential 

constituents. The resemblance of numerous heavy contact-meta*-

" Collier, A. .T., Recent developments of Alaskan tin deposits: Bull. U. S. Geol. Survey 

No. 259, 1905, p. 127.
ECONOMIC GEOLOGY. 25

morphic minerals, such as garnet, to cassiterite, coupled with the fact 

that some of the contact-metamorphic rocks are actually stanniferous, 

has led to much prospecting along the granite-limestone contacts. 

Only one contact rock containing cassiterite in amounts appreciable 

under the microscope was found, however, during the course of the 

present investigation. - , Tin-bearing rock occurs also in tourmalinized granite, in fractured 

quartz porphyry dikes cutting limestone, and in the adjoining limestone itself, intergrown with danburite, tourmaline, tremolite, and 

topaz. Cassiterite is found in quartz stringers in granite, in slate, 

and in limestone, accompanied by an intense metasomatism. Veinlets 

in limestone' may consist wholly of cassiterite, topaz, zinnwaldite, 

and fluorite. In the slate area cassiterite also occurs embedded in 

tabular masses of radial actinolite which appear to be interstratified

with horizontal slates.

Most of the prospects have been but imperfectly opened. Some 

are of promising character, but from the point of view of the conservative mining man their value is yet to be demonstrated.

Each of the four localities at which lode tin is being prospected 

Ear Mountain, Buck Creek, Cape Mountain, and Lost River shows 

certain distinctive features, so it has been found advisable to describe 

them separately in geographic order, as enumerated. To these has 

been added Brooks Mountain, on which some stanniferous contactmetamorphosed limestone has been found, though no cassiterite-bearing rock has yet been discovered there.

LODES. 

EAR MOUNTAIN.

INTRODUCTION.

Ear Mountain is located in the northwestern part of Seward Peninsula, in latitude 66° north and longitude 166° west. It is 50 miles 

north of Teller and 15 miles south of Shishmaref Inlet, a large, shallow embayment from the Arctic Ocean. The region is not readily 

accessible, and with the present means of communication it is practically impossible to bring in supplies during the summer months. 

Steamers of light draft do not venture to approach nearer than 

within 1-| miles of Sarichef Island, at the mouth of Shishmaref Inlet, 

though gasoline schooners make landings directly on the island. 

The inlet itself is not navigable other than by oomiaks and flatbottomed dories. The first steamer of the season of 1907 bound for 

arctic points passed northward through Bering Strait on July 2. 

At this time the last ice floes from the spring break-up were drifting 

out of Shishmaref Inlet.
26 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

Ear Mountain has long, smooth slopes on the north and east sides, 

but on the south and west sides rises abruptly above the tundracovered plateau from an elevation of 1,000 feet to a maximum of 2,308 

feet above sea level. The mountain possesses two flat-topped summits, of which the southern is slightly the higher. Two great granite 

monoliths rest upon the broad northern summit, and are visible, but 

as very diminished objects,'from ships on the Arctic Ocean, 20 miles 

distant. They were noted by the early navigators, and from them the 

mountain received its rather fanciful name.

Cassiterite in the form of stream tin was accidently discovered on 

Eldorado Creek, on the northeast side of the mountain, in 1901. Nuggets several inches in diameter can be picked off the bed-rock riffles 

of this stream, but on account of the small body of gravel the creek 

offers no placer possibilities. The bed-rock source of this cassiterite 

has not yet been discovered. None of the rocks that are being prospected show visible cassiterite, and only a few show small amounts 

microscopically. Attention has lately been directed to some occurrences of chalcopyrite and galena found in contact-metamorphic rock.

GENERAL GEOLOGY.

The rocks flanking Ear Mountain are prevailingly of a calcareous 

character, and comprise contorted limestone and lime-mica schists. 

On the south side of the mountain some black siliceous schists appear. 

Intrusive into these sedimentary rocks and forming the core of Ear 

Mountain is a large mass of coarsely crystalline granite from which 

numerous apophyses of fine-grained white granite extend into the 

surrounding sedimentary rocks. (See figs. 2 and 3.) The limestones 

were highly susceptible to contact metamorphism and succeeded to an 

unusual extent in fixing the magmatic emanations in such minerals as

tourmaline, axinite, paigeite (a magnesian iron-tin borate), fluorite, 

scapolite, galena, and chalcopyrite.

The youngest rocks of the region occur as quartz-augite porphyry 

dikes cutting both limestone and granite. The two most prominent 

of these dikes are 15 feet thick and can be traced for several thousand 

feet, striking N. 30° E. and north and south (magnetically).

Granite. The granite of Ear Mountain consists of feldspar (orthoclase with subordinate amounts of oligoclase-albite and microperthite), quartz, and biotite. Both quartz and feldspar tend to assume 

idiomorphic forms, even in the coarsest grained phases. The quartz 

is smoky, and has not infrequently been mistaken by the prospector 

for cassiterite, especially where the color was more intense. Near the 

contacts the granite is finer grained, and is, in many places, a typical 

granite porphyry. In the central portion of the main granite body 

a small area of this type of rock was found, but whether it represents
ECONOMIC GEOLOGY.

a mere variation in texture or a separate intrusion could not be determined. Black tourmaline is common in the granite of Ear Mountain, but. is always associated Avith seams consisting- of quartz and

Contour interval 200 feet

5 miles

Silts and gravels Limestone and schist Granite Quartz-augite porphyry

Fio. 2. Geologic sketch map of Ear Mountain.

tourmaline, and adjoined by bands of tourmalinized granite 2 or 3 

inches broad. The quartz-tourmaline seams, on account of their resistance to atmospheric attack, Aveather out in relief. Several such

- FEET SW. 

230O -\

1- Schists

2 - Contorted limestone

8-Granite.

4 Quartz-augite porphyry

I BOO

I30OFIG. 3. Geologic section through Ear Mountain.

seams can be seen running up and down the "Ears" the granite 

monoliths from AA'hich the mountain was named.
28 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

The; offshoots from the main granite body are finer grained and 

lighter, colored. They contain, besides quartz and feldspar, white 

mica, and in many places numerous small prisms of tourmaline, evenly 

distributed throughout the body of the rock and unconnected with 

the presence of seams. In addition to the constituents visible to the 

unaided eye, the microscope shows a small amount of topaz. Some 

fluorite is associated with the muscovite, which, like the topaz, occurs 

in skeletal growths. Minute cassiterite prisms surrounded by pleochroic halos occur embedded in the muscovite. The tourmaline, 

which is brown, usually occurs in discrete prisms, but may also form 

skeletal growths. The three minerals muscovite, topaz, and tourmaline were formed after the feldspar and quartz, and apparently by 

similar processes. They are absent from the granite of the main 

mass, and indicate an enrichment of fluorine and boron in the 

apophyses.

Contact metamorphism. An extensive development of lime°-silicate 

hornfels, rich in the so-called pneumatolytic minerals, has been produced around the periphery of the granite mass. The limestones 

that were metamorphosed were originally impure limestones irregularly laminated with thin bands (one-eighth to one-fourth inch 

thick) of argillaceous material. The contact metamorphism has consisted in part in a-recrystallization of the sedimentary material and 

in part in an accession of new material by magmatic emanations.

The simplest case of contact metamorphism is that of the tourmalinized limestone formed on the south side of Eldorado Creek. Here 

the argillaceous lamina have been converted into black tourmaline 

and the calcareous bands have been marmorized, producing a rock 

resembling a gneiss. The tourmaline is pleochroic in tones of blue 

and green. Tremolite has also been developed, with diopside and 

vesuvianite in minor amounts. At other points along the contact 

the original banded character of the limestone is preserved, and even 

emphasized, by the production of trains of vesuvianite crystals. On 

Quartz Creek the limestones have been converted into pyroxenescapolite hornfels. A prospect cut at this locality exposes a darkcolored rock, evenly flecked with a lustrous coal-black lamellar 

mineral, which subsequent investigation has shown to be a new 

boron-tin mineral, and which has been named paigeite.ft

In addition to the paigeite, considerable tourmaline and minor 

amounts of chalcopyrite and magnetite are visible. Under the microscope the rock resolves itself into a confused intergrowth of zonally 

banded tourmaline, calcite, vesuvianite, zoisite, paigeite, fluorite, and 

accessory phlogopite, chalcopyrite, and magnetite. The paigeite is 

embedded in the various other constituents in trichite-like forms,

"Am. .Tour. Sci., 4th ser., vol. 25, 1908, p. 323.
ECONOMIC GEOLOGY. 29

many of which, are of capillary dimensions, and in matted aggregates 

of fibers (PL II, A). Closely associated with this paigeite hornfels is 

a tourmaline-pyroxene-sca polite hornfels, which reacts chemically for 

chlorine. A prospect opening at the granite-limestone contact near 

Tuttle Creek reveals a tourmaline-axinite hornfels. The axinite is 

of highly vitreous appearance and of light-brown color, and is-therefore readily recognizable in the hand specimen. It reacts for boron 

and manganese. In thin section it is colorless with faint blue pleochroisms, but shows no recognizable cleavage, although .agreeing 

otherwise in its optical properties with those cited by Eosenbusch. 

Tourmaline is quantitatively more important than the axinite in 

this hornfels. It is subhedral and in numerous places zonally banded, 

with pleochroism varying from blue to green. In addition to the 

tourmaline and axinite, actinolite, pyroxene, quartz, fluorite, and 

calcite are confusedly intergrown, though the last three minerals 

occur rather in minor amounts. Some cassiterite is found embedded 

in the other constituents in grains large enough to allow its optical 

identification beyond question. This tourmaline-axinite hornfels is 

the only contact-metamorphic rock containing cassiterite found in the 

entire Alaskan tin region, although a large number of such rocks were 

examined on account of the prevalent belief that they are tin bearing.

Quartz-augite porphyry dikes. The quartz-augite porphyry dikes 

consist of a dense-textured rock of dark-blue or green color containing numerous phenocrysts of feldspar and smoky quartz, with lesser 

amounts of augite and biotite. The augites attain unusually large 

dimensions as much as 2 inches or more and are commonly regarded as " tin crystals " by the local prospectors. The presence of 

these augite phenocrysts thus mistaken for cassiterite has led to considerable prospecting of the quartz porphyry dikes. In thin section 

the quartz phenocrysts are seen to be corroded and embayed; the 

plagioclase, which corresponds to bytownite (Ab2r,An7r,)? is also;corroded. Augite and biotite are rare. The groundmass consists of 

plagioclase laths approximating Ab40An60 in composition,, and they 

are disposed in fluxional arrangement. Apatite is unusually abundant. According to this characterization the dikes are augite dacites. 

Calcite and tourmaline are common as secondary minerals. Where 

tourmalinization has been more intense fluorite, arsenopyrite, and 

cassiterite appear.

Two periods of tourmalinization can therefore be distinguished 

one contemporaneous with the contact metamorphism and one following the intrusion of the augite dacite dikes. That a considerable 

lapse of time separated these periods is indicated by the fact that 

the granite now exposed to view had become thoroughly cooled 

during this interval and was able to chill the later intrusives.
30 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

MINERAL OCCURRENCES.

In addition to vesuvianite, tourmaline, axinite, and other contactmetamorphic minerals, the contact rocks at certain localities are 

flecked with galena, chalcopyrite, and a lustrous coal-black iron-tin 

borate, which has been named paigeite. It is this type of rock which 

has raised hopes that copper and lead deposits of economic value 

exist at Ear Mountain. Cassitente-bearing rock also is believed 

by the prospector to occur along the contact at many points.

The practical importance of understanding how the deposits were 

formed arises from the following considerations:

(1) That the size and persistence of the deposits are dependent 

on the mode of origin.

(2) That their probable value, besides being affected by the size 

and persistence of the deposit, is also dependent on the mode of 

origin.

Under the first heading attention may be drawn to certain structural features of the granite boss of Ear Mountain features which 

also occur at Cape Mountain and at Brooks Mountain. It can be

->- __ I_____3 SHAFT

1-Hornfels 2-Limestone 3-Granite

EOOO feet

Fig. 4. Diagrammatic section at Eunson's shaft, Ear Mountain.

shown from various lines of evidence that any such granite mass as 

that of Ear Mountain must have cooled under a considerable blanket 

of protecting rocks. The surface of the granite against these overlying rocks is not necessarily smooth, but may be of gently undulating character, or may even be furrowed by sharp ridges, such as, 

for instance, have been revealed by the extensive mining operations 

in Cornwall. The troughs or depressions between these granite 

ridges will be occupied by the overlying rocks, which will therefore 

appear immersed beneath the general surface of the granite. At 

Ear Mountain erosion has proceeded far enough to strip off much 

of the protective capping of sedimentary rocks which formerly 

arched over the granite, though on the highest peak the south 

peak the granite is still covered by a thickness of 100 feet of schists 

and sheared greenstone. This erosion has revealed the fact that 

the former surface of the granite boss was slightly uneven. The 

limestones and schists resting in the inequalities now occur as isolated patches surrounded on all sides by granite. A section across 

Ear Mountain (fig. 4) illustrates this feature. These rocks may
- ECONOMIC GEOLOGY. 31

show locally indications of ore, but the rock, being of contactmetamorphic origin, will have a small areal extent, will have no 

great depth, and will give out when the underlying surface of the 

granite is reached. The prospector, therefore, should use considerable caution before attempting any exploitation of ore bodies occurring in such patches of rock lying upon the surface of the granite.

Along the periphery of the granite the ore deposits of contactmetamorphic origin are more likely to have permanence in depth. 

What is known of contact-metamorphic deposits in other parts of 

the world, however, is not of a character to encourage extravagant 

hopes for the similar occurrences found at Ear Mountain. They are 

usually of low grade and irregular in form. Such deposits are 

mined in southeastern Alaska for their copper content, but conditions must be exceptionally favorable to make them of commercial 

value.

On the northeastern side of the mountain, near Vatney's cabin, 

where the contact metamorphism has been more pronounced than 

usual, some metamorphosed limestone in proximity to a granite dike 

was being prospected for tin ore. It contains small bunches of a 

reddish-brown mineral showing crystal faces, which, when carefully 

examined, are seen to be diamond shaped. This is typical of garnet. 

When only the apex of the garnet crystal is visible it bears an exceedingly deceptive resemblance to the four-sided pyramid characteristic 

of cassiterite.

Near Tuttle Creek a prospect trench has been opened to uncover 

some contact-metamorphosed limestone at the granite contact. The 

stanniferous tourmaline-axinite hornfels whose microscopic features 

have already been described occurs at this locality, but no cassiterite 

is visible in the rock to the unaided eye. The microscope, however, 

shows the presence of a small fraction of 1 per cent an amount too 

small, in view of the nature of the deposit and the remoteness of the 

region, to give rise to any extensive hopes that commercial bodies of 

cassiterite exist along the contact. 

The quartz-tourmaline seams that cut the granite have already 

been described. At various places on Ear Mountain interlacing networks of such seams occur, and an extensive tourmalinization of the 

adjoining granite has taken place. Masses of quartz-tourmaline rock 

have been produced, little resembling the original granite. Such occurrences have been opened by shallow prospect pits at a number of 

points. It is quite possible that some deposits of this character may 

carry low-grade values in tin, but those opened thus far lend small 

encouragement to this idea.

On the northeast side of Ear Mountain an augite-quartz porphyry 

dike has been opened by a shaft and explored by a drift 112 feet long.
32 TIN DEPOSITS OF SEWAED PENINSULA, ALASKA.

Nothing but hard, barren rock was encountered. Work was suspended, and at the time of visit the sbiaft was flooded with water. 

Farther southwest on the extension of the same dike a number of open 

cuts have been made on account of the prevalence of numerous large 

augite* crystals embedded in the dike rock. Chemical analyses of 

" ore " samples, made in the laboratory of the Survey, show the presence of only traces of tin, amounting to a few hundredths of 1 per 

cent.

A shaft known as Eunson's shaft (fig. 4) was sunk near the point 

where a quartz porphyry dike striking north and south crosses the 

granite-limestone contact. The shaft was reported to be 30 feet deep, 

but at the time of visit was flooded with water and partly caved in. 

On the dump a variety of rock is represented. Contact-metamorphosed limestone, granite, granite porphyry, quartz porphyry, and 

various altered modifications of the quartz porphyry appear. The 

quartz porphyry is partly tourmalinized and contains small patches 

of purple fluorite, abundant arsenopyrite, and microscopic grains of 

cassiterite. The tin ore reported from this locality probably came 

from highly altered portions of the quartz porphyry dike.

BUCK CREEK.

GEOLOGIC FEATURES.

The Buck Creek area is a part of the slate belt previously described. 

The bed rock consists of fine-textured arenaceous slates, usually lying 

flat. Along Buck Creek the rocks do not display a highly metamorphosed aspect, but are in large part shalelike, associated with 

beds of banded yellowish fine-grained sandstone and bluish kaolinic 

sandstone. The- slaty cleavage, however, is developed in an irregular 

and variable degree in different parts of the area. At the head of 

Slitter Creek the arenaceous banded rock is thinly fissile, with the 

cleavage at right angles to the sedimentary banding. Greenstones 

of diabasic character occur with the slates, but can rarely be found 

in place. A prominent exposure, however, forms a low ridge immediately to the north of Buck Creek.

Two quartz porphyry dikes cut the slates at the head of Buck 

Creek, The characteristic feature of these dikes, especially of the 

one extending northward from Potato Mountain, is the abundance 

of large quartz phenocrysts, many of them smoky, embedded in a 

dense light-gray matrix. The feldspars are less conspicuous. The 

margins of the dikes have been strongly chilled, and show a bluishblack rock containing small phenocrysts of quartz and feldspar. On 

the summit of Potato Mountain the dike is but 1 foot thick and consists entirely of this kind of rock. Farther north the thickness
ECONOMIC GEOLOGY. 33

increases to 10 feet. The other quartz porphyry dike, which trends 

N. 34° W. (magnetic), has a maximum thickness of 50 feet and a 

length of several thousand feet. These are the only acidic' igneous 

intrusives known in the region.

ECONOMIC GEOLOGY.

Cassiterite occurs in three ways in the bed rock of the Buck Creek 

area (1) as an impregnation in quartz porphyry dikes, (2) in 

quartz stringers cutting the slates, and (3) intergrown with arsenopyrite in a gangue of radial actinolite. Development, however, has not 

yet proceeded far enough-to demonstrate the commercial importance 

of any of these occurrences.

At numerous points on the ridge of hills at the head of Buck Creek 

open cuts have made made, exposing networks of quartz stringers in 

the slates. The veinlets are usually a few inches thick, and here and 

there contain cassiterite. Some merely sho\v rosettes of blue tourmaline or are barren. Gold is probably present in some of them, as 

placer gold is found in the stream tin of Buck Creek. On the divide, 

at an altitude of 1,140 feet, a prospect pit discloses a fine showing of 

tin quartz: The hole is 10 feet deep and exposes a face of 7 feet of 

quartz carrying a considerable percentage of cassiterite. The developments are inadequate to give either clip or strike of the ore body with 

any degree of certainty, nor is its linear persistence known. The depth 

attained is not great enough to expose solid bed rock, and the walls are 

consequently still in a highly shattered condition due to the heave of 

the frost. The ore is a milk-Avhite quartz of greasy luster, and contains 

cassiterite disseminated through it in crystalline aggregates, intimately 

intergrown Avith arsenopyrite and small needles of blue tourmaline. 

In thin section the tourmaline is found to be light blue in color, the 

pleochroism varying from clear blue to colorless (alkali tourmaline). 

The slate wall rock has a greenish cast, and under the microscope is 

found to consist almost entirely of minute tourmaline prisms in parallel orientation, with a small amount of interstitial feldspar. The 

bright boron flame which the slate yields when treated with fluorite 

mixture abundantly confirms the microscopic diagnosis. About 60 

feet from the prospect pit the outcrops consist of normal arenaceous 

slate lying nearly horizontal.

At the head of Peluk Creek (a small tributary of Buck Creek) a 

shaft,'reported to be 20 feet deep, was sunk on quartz stringers in the 

slate. The material.on the dump shows abundant pyrite and a little 

cassiterite. In the same general vicinity float tin ore has been discovered in which the cassiterite occurs in totally different paragenetic 

association. It is found embedded in a green rock composed of radiating groups of actinolite. Relatively large amounts of arsenopyrite 

54356 Bull. 358 08 3
34 TIN DEPOSITS OF SEWAKD PENINSULA, ALASKA.

are usually intergrown with the cassiterite. A number of open cuts 

have been made in the effort to locate the bed-rock source of this tinbearing rock. At time of visit the owner of the property had 

just succeeded in uncovering the edge of the deposit in place, so that 

few facts are available as to its geologic relations and probable value. 

Where exposed it was highly oxidized, and apparently represented a 

tabular mass intercalated between horizontal slates. In thin section 

the ore rock consists of sheaf-like bundles of actinolite, with which 

some finely granular calcite is associated. A small amount of quartz 

occurs in the immediate vicinity of the cassiterite, and both these 

minerals are transfixed by needles of actinolite (PL III, A). Tourmaline in the form of a few small prisms occurs as an accessory 

mineral. The slate in contact with the stanniferous rock is of a 

dense, poorly fissile variety. Under the microscope it shows no decided evidence of metasomatic alteration. It contains abundant magnetite disseminated through it in minute grains, which may possibly 

be of epigenetic origin.

At a number of points the quartz porphyry dikes are somewhat 

impregnated with pyrite, especially in the vicinity of seams. A 

small amount of tourmaline in radial groups appears, ° and white 

mica replaces the feldspar phenocrysts. Specimens containing a 

small amount of cassiterite have been obtained, but appear to be of 

extremely rare occurrence. The largest quartz porphyry dike has 

been faulted approximately .400 feet in a north and south direction. 

The line of this fault is marked by a great quartz vein 15 feet or more 

in thickness, which can be traced for considerably over a mile. The 

vein contains a vast number of slate fragments, each of which has 

acted as a nucleus around which quartz crystals commenced to grow. 

Failure of «the crystals to coalesce in their outer extremities has produced a vuggy vein lined with innumerable hexagonal pyramids of 

quartz. No mineral except some botryoidal limonite incrusting the 

quartz crystals has been observed in the vein, so that it is without 

economic importance. Whether the formation of this vein was contemporaneous with that of the stanniferous quartz veinlets is of considerable practical interest.' If of the same period of origin there is 

a strong possibility that persistent tin-quartz veins may yet be found 

in the slate area.

. ORIGIN OF THE OKES.

' The developments and exposures, are entirely too inadequate to 

allow a very extended discussion of the origin of the ores. From 

analogy with the Lost Eiver area, however, it is believed that a granite mass underlies the Buck Creek region and that the quartz porphyry 

dikes represent the final expulsive effort of this magma. After the 

advent of the porphyry emanations carrying metallic salts in solution
U. S. GEOLOGICAL SURVEY BULLETIN NO. 358 PL. Ill

A. THIN SECTION OF ACTINOLITE-CASSITERITE ROCK; CROSSED NICOLS. 

Magnification 32 diameters. Shows cassiterite transfixed by actinolite needles.

7.'. THIN SECTION OF STANNIFEROUS METAMORPHOSED LIMESTONE FROM BROOKS

MOUNTAIN. 

Magnification 1 2 diameters. Hulsite (black opaque mineral) intergrown with vesuvianite in a matrix of calcite.
ECONOMIC GEOLOGY. 35

ascended from unknown depths. These solutions moved in part 

along preexisting lines of weakness marked by the quartz porphyry 

dikes and in part along fractures in the slates. They contained the 

elements silicon, oxygen, sulphur, arsenic, boron, iron, aluminum, and 

tin, and probably gold. The metasomatic alterations indicate that 

the solutions were at high temperatures. The actinolite rock was 

probably produced by stanniferous solutions which, as leakages from 

the main channels of circulation, moved laterally through impure 

dolomite beds, such as the petrographic study of the slates has shown 

to occur through the region. The association of cassiterite and arsenopyrite in a gangue of actinolite is unique. Actinolite, together 

with axinite and other borosilicates, however, occurs as a gangue 

material in certain Tasmanian copper deposits.* The explanation 

advocated for these is essentially similar to that given above.

CAPE MOUNTAIN.

Cape Mountain forms the promontory fronting Bering Strait at 

the westernmost extremity of the American Continent. On the eastern flank of the mountain are a few widely scattered houses, which 

form the settlement known as " Tin City." Tin City is 110 miles by 

steamer route from Nome, with which it is connected by telephone.

GENERAL GEOLOGY.

Granite. Cape Mountain consists of a granite mass, which has 

invaded a series of crystalline limestones of Carboniferous age. 

(See fig. 5.) The limestones are lying nearly flat, but with a slight 

easterly dip, and extend eastward nearly to the mouth of Baituk 

Creek, where they are faulted against the slates of the York area. At 

the cape the limestones dip in toward the granite. Locally along the 

contact, as on Village Creek, the limestone is crumpled and turned up 

at high angles phenomena ascribable to the dynamic activity of the 

intrusive. Some fine-grained olivine basalt dikes, vesicular and in 

part filled with calcite amygdules, cut both granite and limestone. 

They are without doubt by far the youngest rocks of the area.

The granite of Cape Mountain is of a coarse-grained gray type, 

containing numerous large porphyritic feldspars, either microcline or 

orthoclase, commonly an inch or so in length. Quartz, acid plagioclase, and biotite comprise the remaining essential constituents. The 

quartz is prevailingly smoky. Along the contact with the limestone 

the feldspars are locally aligned with their longer axes parallel. 

M.ore commonly, however, the granite is fine grained along its margin, 

and fluorite, tourmaline, and white mica appear. Pegmatite blebs

"° Weed, W. H., Copper mines of the world, New York, 1907, p. 171.
36 TIN DEPOSITS OF SEWAKD PENINSULA, ALASKA.

containing tourmaline are common in the body of the granite. Dikes 

and sills of granite or granite porphyry cut the surrounding rocks.

Detailed mapping of the contact between the granite and limestone 

has shown that the underground extension of the granite surface 

dips steeply to the east on the east side of Cape Mountain, but that 

on the north side it slopes gently to the north beneath the limestone 

capping. Erosion has left some thin wedgelike remnants resting 

upon the granite surface. (See fig. 6.) Some limestone masses were 

broken off from the roof which covered the granite at the time of its 

intrusion and were partly submerged in the molten rock. An isolated

Contour interval 200 feet

Limestone with some schist Granite

Fig. 5. Geologic sketch map of Cape Mountain.

limestone patch, which had this mode of origin, rests in the granite 

near the head of Lagoon Creek. Attention is drawn to these foundered blocks of limestone because if any ore occurs along their contacts it can have only a meager distribution. The dikes and sills, 

together with the limestone masses torn off at the time of intrusion, 

tend to produce a highly irregular contact surface between the limestone and the granite. This fact is of practical importance when the 

development rof contact ore bodies is being considered.

Contact phenomena. The principal effect of the intrusion of the 

granite on the surrounding limestone has been to produce a coarse
ECONOMIC GEOLOGY. 37

white marble, extending at a maximum 200 or 300 feet from the contact. Proximity to the granite contact is in many places indicated 

by a peculiar rough appearance of the limestone, due to the development of small patches of lime-silicate minerals, ordinarily invisible 

to the eye. These weather out in relief and give it a characteristic 

" shaggy " appearance near the granite. At other points delicate 

radial groups of tremolite appear on the. weathered surfaces of siliceous phases of the limestone. A faint banding is also evident, due 

to the fact that the pure carbonate laminae have been converted to 

calc spar. The siliceous bands in the limestone overlying the granite 

near the summit of Cape Mountain have been converted into wollastonite. At the contact on Bering Strait beds of almost solid wollastonite 2 or 3 feet thick inclose an alaskite sill 1 foot thick and extend 

30 feet from the main granite mass.

Contact metamorphism involving an addition of material is of relatively rare occurrence and is confined mainly to the limestone adj oining the dikes and sills. On the Canoe claim an open cut exposes a

1-Limestone 2-Granite

o. ipoo zooo feet 

i,___c___j_______i 

FIG. 6. ^Geologic section across Lagoon Creek, Cape Mountain.

sill 8 feet thick, consisting of the normal porphyritic granite of Cape 

Mountain. The upper l-£ feet, however, consist of coarse alaskitepegmatite composed of orthoclase and quartz individuals 4 to 5 

inches in length. Between the'pegmatite and the porphyritic granite a zone of relatively fine-grained granite up to 6 inches thick 

intervenes, but no great regularity can be observed for this feature, 

as all three phases may be confusedly intermingled. The question 

whether the pegmatite did not represent a separate intrusion along 

the contact was considered, but as large quartz individuals with pyramidal terminations were found projecting from the pegmatite into 

the fine-grained granite, and as transitions are everywhere observable 

between the three phases, the conclusion was unavoidable that it was 

practically contemporaneous with the granite. Some tourmaline is 

of later origin and is developed along seams. This occurrence resembles the " stockscheider " a peripheral zone of giant granite enveloping the stanniferous granite bosses of the Saxon Erzgebirge 

and suggests Elie de Beaumont's observation a " that the peculiar

a Cotta,' B., Gangstudien I, Freiberg, 1850, p. 398.
38 TIN DEPOSITS OF SEWAKD PENINSULA, ALASKA.

cause of the coarsely crystalline character of the granite had acted 

mainly upon the marginal portion of the eruptive mass." The limestone overlying the pegmatite selvage is marmorized, and where in 

immediate contact has been converted into a heavy green finely 

granular rock. Under the microscope this proves to be a pyroxenefluorite hornfels composed essentially of hedenbergite and fluorite, 

with accessory calcite and quartz. Scheelite and pyrrhotite occur in 

a few scattered grains. The association of a pyroxene hornfels with 

similar pegmatitic selvages is repeated at various other points on 

Cape Mountain. Near the Percy shaft the hornfels consists of hedenbergite, calcite, and scapolite, with accessory scheelite. The scheelite 

occurs in round, droplike grains embedded in calcite or intercrystallized with the pyroxene. It resembles titanite and cassiterite, but 

may be distinguished from the former by its positive uniaxial character and from the latter by its moderate birefringence (0.016). 

Inasmuch as scheelite is not commonly recorded as a contact-metamorphic mineral, the hornfels was submitted to E. C. Sullivan, of 

the United States Geological Survey, who verified chemically the 

presence of tungsten. The pannings from approximately 10 grams 

of rock were fused with Na,C03 , the fused mass was leached with 

water, the tungsten Avas precipitated with HgCl2 as Hg2W04 , the 

mercury was expelled from the tungstate by heat, and the residue 

was examined by the reduction tests with Zn and Sn. The rock was 

also tested for chlorine by Doctor Sullivan and gave a strong reaction 

for. this element, thus confirming the optical determination of the 

scapolite.

QBE DEPOSITS.

Cassiterite probably occurs in three ways at Cape Mountain (1) 

in tourmalinized peripheral portions of the main granite mass and in 

tourmalinized granite dikes; (2) in contact-metamorphic rock, and 

(3) in veins in granite and as an impregnation of the adjoining wall 

rocks. From experience in developed tin regions it appears that the 

first type of occurrence is apt to be of low grade, and that the second 

is an unlikely source of tin ore. The occurrences on Cape Mountain 

indicate that the Alaskan deposits will prove to be no exceptions to 

this rule. The third is the normal type the world over, and has 

yielded the deepest and most productive mines.

At Cape Mountain no practical distinction is made between the 

first and the second modes of occurrence. The largest amount of 

prospecting has been done along the granite-limestone contacts because some rich pockets of tin ore have been discovered in tourmalinized granite along the contact and because of the distinctive character 

of the contact-metamorphic rocks (pyroxene hornfels). It can not be 

positively asserted that cassiterite occurs in the contact-metamorphic
ECONOMIC GEOLOGY. 39

rock (in fact, all observations have been to the contrary), but it is 

not improbable that some tin ore may occur in the tourmalinized 

limestone adjoining tourmalinized granite.

Some of the dikes cutting the limestone show margins which are 

strongly tourmalinized and considerably impregnated with cassiterite. Large masses of bluish tourmaline occur, and these carry rich 

pockets of tin ore of a light-brownish color. The limestone adjoining the dikes has here and there been converted into coarse white 

spar, in which numerous prisms and columns of tourmaline are embedded. Where tourmalinization has been complete it may be difficult to distinguish tourmalinized limestone from tourmalinized dike 

rock. The tourmalinization, either of granite or limestone, however, 

appears to be purely local and erratic in occurrence. Furthermore, 

the tourmaline rock, although, as already stated, locally rich in cassiterite, is in general quite barren.

Along the periphery of the main granite mass local tourmalinization of the granite has taken place, accompanied by an introduction 

of cassiterite and pyrite. The pyrite has largely been oxidized, and 

in this way heavy red porous masses, consisting chiefly of iron oxide, 

with tourmaline and cassiterite, have been produced.

The contact rocks adjoining the large granite stock and some of the 

granite sills have been prospected at a number of places. A. heavy 

green rock, showing in places finely disseminated pyrrhotite, has been 

regarded as tin ore, but microscopic and chemical analysis fail to 

reveal any tin. The green rock is locally known both as " greenstone " 

and as " tinstone." It is a finely granular rock, consisting of pyroxene, probably hedenbergite, and fluorite in equal proportions. This 

rock is well exposed in a cut on the Canoe claim, where a few feet of 

it directly overlies a granite sill, 8 feet thick. At other points on Cape 

Mountain, notably one-half mile north of the Lucky Queen property, 

the pyroxene is embedded in calcite. The pyroxene, which is unusually well developed for a contact-metamorphic rock, is of brown 

color and does not look unlike cassiterite, especially cassiterite which 

shows a macroscopic, cleavage. It is true that cassiterite has a more 

splendent luster, but the distinction is not one that carries conviction. 

Where the presence of tin is suspected in such rocks, only assays can 

give reliable information.

The statements regarding deposits of contact-metamorphic origin 

made for the Ear Mountain region are equally applicable to Cape 

Mountain. In addition, the highly irregular nature of the granite 

contact makes prospecting for such deposits difficult and would entail 

heavy probably unwarrantable expenses in mining them.

Up to the present time a single narrow quartz vein has been found 

in place, cutting the granite on the north side of Cape Mountain at 

an altitude of 1,850 feet. The vein is accompanied by some alteration
40 TIN DEPOSITS OF .SEWARD. PENINSULA, .ALASKA.

to greisen and partial tourmalinization of the adjoining granite 

changes characteristic of tin-bearing veins. At other points on Cape 

Mountain cassiterite, accompanied by tourmaline, is found as an impregnation of the granite adjoining slips or fault planes. This type 

of occurrence, in the opinion of the writer, holds out greater possibilities for the future of the district as a tin producer than the other 

two. Unfortunately, exploratory work has been mainly confined to 

the contact deposits.

DEVELOPMENTS.

At the time of visit two properties only were being actively prospected at Cape Mountain those of the United States Alaska Tin 

Mining Company and the Bartels Tin Mining Company. The former property is situated near the summit of the mountain, on the 

north side. Developments up to the end of July, 1907, consisted of a 

shaft reported to be 22 feet deep, now filled with water, and a T-foot 

tunnel, 270 feet long. The company has also erected a 10-stamp 

mill near the beach at Tin City. Four men were employed at the 

time of visit. The shaft is sunk on a quartz ledge, 1 foot thick, 

striking N. 45° W. (magnetic) and dipping 80° E. The tunnel, 

whose altitude at the portal is 1,600 feet, according to aneroid measurement, is driven in a direction S. 40° W. through hard, firm granite, and is expected to tap the ledge 250 feet below the collar of the 

shaft.

The principal development work by the Bartels Tin Mining Company has been done on the North Star property. Eight men were 

employed during the summer of 1907. The main tunnel, with its 

drifts and winzes, aggregated 750 feet in length. This tunnel is 

being driven to catch the granite-limestone contact at a depth of 100 

feet below the workings of the Lucky Queen tunnel. About 400 feet 

from the mouth of the tunnel a band of granite, carrying numerous 

visible crystals of cassiterite, associated with some tourmaline, was 

encountered. The width of this band is about 18 inches. The granite 

is soft and iron stained. The succeeding 3 feet consists of hard gray 

granite with pyrite disseminated through it. This is succeeded by 

15 feet of iron-stained granite. At the time of visit the 18-inch belt 

of rich tin ore had not been drifted on to prove its persistence. The 

drifts branching off from the main adit follow the contact of a large 

limestone block, which was evidently torn from the main limestone 

mass during the intrusion of the granite. In following this contact, 

which proved to be of a highly irregular nature, several winzes were 

necessary. A new tunnel, 67 feet below the North Star tunnel, is 

being run from the surface to connect with the lowest drifts reached 

by these winzes. The granite along the limestone contact is charged 

with numerous radiating groups of tourmaline prisms associated
ECONOMIC GEOLOGY. 41

with iron oxide, and locally with pyrite and tin ore. Crushing and 

oxidation have taken place along the contact, and heavily iron 

stained gouge matter,. a foot in thickness, has been produced. At 

points where the red clayey material is absent the granite is fresh 

and the progressive increase of fineness of grain as the contact is 

approached can be noted.

A considerable portion of the energies of the company has been 

expended on assessment work on the numerous claims which it holds 

on Cape Mountain.

On the Carlson & Goodwill property an adit 20 feet long has been 

driven along the contact, of a horizontal limestone and a granitic 

dike. The' dike sends small, irregular tongues into the limestone, 

and the ends of these tongues have been completely converted into 

blue tourmaline rock. The limestone has been rendered coarsely 

crystalline, but evinces no other change. Rock from the dump, however, shows that locally an intense tourmalinization was produced. 

No stanniferous rock was seen in place.

On the northwest side of the mountain, on Village Creek, some 

drill holes have been put down to the granite contact, but the results 

are not known.

BROOKS MOUNTAIN.

Brooks Mountain, the dominant peal? of the York Mountains, lies 

in the watershed of the Bering and Arctic drainages. It is easily 

accessible from the coast by way of Lost River, a distance of 9 miles. 

The mineral deposits are of contact-metamorphic origin, and though 

no cassiterite-bearing rock has been discovered on the mountain, yet 

the character of the mineralization here allies it to that of the tin 

region, of which Brooks Mountain is geographically a part.

An intrusive granite mass, 2 miles long by two-thirds of a mile 

wide, forms the southern flank of the mountain. The granite is 

characterized by the presence of numerous large .porphyritic orthoel ase crystals, commonly an inch in diameter, and large idiomorphic 

crystals of smoky quartz exceeding peas in size. The matrix consists of a coarsely granular assemblage of orthoclase and subordinate 

oligoclase (Ab80An20 ), with quartz and biotite in small amounts. 

The rocks surrounding the granite are chiefly limestones of the Port 

Clarence formation, and are highly crumpled throughout much of 

the region. Along the periphery of the granite they have been marinorized, and an extensive variety of contact-metamorphic minerals 

have been produced in the immediate vicinity of the contact.

At the west end of the granite stock a horizontal pegmatite about 

8 inches thick traverses the white marble. It consists of orthoclase, 

quartz, and biotite, generally segregated in separate masses, the biotite in plates one-half inch in diameter forming selvages 1 to 2 inches
42 TIN DEPOSITS OP SEWARD PENINSULA, ALASKA.

thick. Thin sections cut from equidimensional portions of the pegmatite show, in addition to the three constituents already named, a 

small amount of plagioclase, some faint-colored bluish-green mica, 

accessory corundum, and probably topaz. The structure is allotriomorphic granular. The pegmatite is overlain by 1 foot of finely 

granular, heavy green rock, resembling that adjoining the pegmatite 

selvages on Cape Mountain. This rock is composed of vesuvianite, 

hedenbergite, and fluorite in approximately equal proportions.

Vesuvianite is the commonest mineral in the metamorphic aureole 

of the granite mass. Along the headwaters of Yankee Creek the impure crumpled Port Clarence limestone is brought into contact with 

the intrusive stock, and a thorough recrystallization has ensued. 

Crystallographically perfect vesuvianite in prisms up to three-fourths 

inch in length occurs scattered in great profusion through a matrix 

of coarse white calc spar. The vesuvianite is noteworthy as containing 0.88 per cent B^O;! .a Where this mineral has not individualized 

into large crystals the metamorphosed rock consists of a fine equigranular aggregate of vesuvianite, calcite, and possibly garnet, interspersed with minute plates of phlogopite. The growth of the large 

crystals has produced a clarification, as it were, of the carbonate rock, 

and the boron content of the vesuvianite shows that this process was 

promoted by pneumatolytic agents. In this connection it is significant that an energetic tourmalinization of the granite has taken place 

at a number of points in the vicinity of this portion of the contact.

At the west' end of the Brooks Mountain granite mass a prospect 

trench discloses a body of argentiferous galena ore, occurring 20 feet 

from the granite contact in a coarsely crystalline white limestone. 

The strike of the ore body, as revealed in the open cut, is N. 15° W. 

(magnetic), and the dip 65° toward the granite. A thickness of 

3J feet of solid ore is exposed, consisting of galena strongly admixed 

with a lustrous black zinc blende which, as analysis shows, contains 

19 per cent of ferrous iron. Some pyrrhotite is also present, but this 

mineral is comparatively rare. Where any gangue mineral is visible 

it consists of fluorite. The ore body is frozen to both walls. The 

hanging wall shows a belt of finely granular fluorite several inches 

thick, succeeded by extremely coarse calcite containing scattered crystals of diopside and some galena. The grain of the calcite decreases 

away from the ore body. A thin section of rock taken from a point 

near the hanging wall is composed of calcite, fluorite, diopside, calcic 

plagioclase (Ab2An3 ), an unknown positive uniaxial mineral, and a 

small amount of colorless mica, with sporadic grains of the ore minerals. Assays of the ore made in Nome yielded 34 per cent of lead

a This determination was made by Prof. Edgar F. Smith, of the University of Pennsylvania, according to two new methods for the estimation of B»O3 that he has recently 

discovered.
ECONOMIC GEOLOGY. 43

and 11 ounces of silver per ton. Other assays were reported to give 

ore values ranging from $17 to $44 per ton.

In the- vicinity of the galena prospect there is some contact-metamorphosed limestone containing a fibrous green magnesia-iron boron 

mineral (ludwigite?) intercrystallized with galena. Loose masses of 

paigeite hornfels are also present. Some rock containing abundant 

honey-yelloAv crystals of chondrodite and numerous minute octahedra 

of spinel, embedded in a mesostasis of calcite with accessory magnetite: occurs near b}^.

In the same general locality some contact masses of vesuvianite 

have been prospected for nickel. The vesuvianite is in part finely 

granular and gives the rock a general green color. This feature and 

the unusual weight of the rock (that is, compared to quartz) doubtless caused the nickel prospecting. No indications of nickel are present, and it may be added that such a mode of occurrence is totally 

unknown for nickel. On microscopic examination small amounts of 

calcite, deeply pleochroic biotite, and hederibergite are found associated with the vesuvianite.

In the canyon below the galena claim some assessment work has0 

been done on a showing of contact-metamorphic minerals near the 

granite contact. The deposit is interesting from a scientific standpoint, inasmuch as a hitherto unknown boron-tin mineral has been 

discovered in it, but nothing of great commercial importance has been 

found here. The minerals comprise brown garnet, showing rhombic 

faces, green or yellowish-green vesuvianite, and abundant magnetite 

closely associated with the new mineral a magnesia-iron tin borate 

which has been named hulsite.0 These various minerals are all- included in a matrix of coarse white spar (PL 111,5). On account of 

the low percentage of tin in hulsite- (approximately 10 per cent) the 

deposit can have no economic value.

At the head of the same canyon other contact-metamorphic deposits 

have received attention. Here, at an altitude of 2,000 feet, a small 

prospect hole exposed a mass of metamorphic minerals occurring in 

a white marble a few feet from the granite contact. Tourmaline, 

fluorite, calcite, arsenopyrite, brilliant black sphalerite, pyroxene, and 

axinite occur confusedly intergrown. The ore body is 4 feet thick and 

penetrates the marmorized limestone in irregular tongues a few inches 

thick. Some galena was noted in the ends of these tongues, which 

were examined microscopically and found to consist almost exclusively of monoclinic pyroxene. A few hundred feet north of this occurrence the pure white marble gives way to a highly metamorphosed 

impure limestone, consisting essentially of vesuvianite, calcite, biotite, and accessory tourmaline. Some of this rock contains sphalerite

°Am. Jour. Sci., 4th ser., vol. 25, 1908, p. 323.
44 TIN DEPOSITS OF SEWAED PENINSULA, ALASKA.

and other sulphurets, which on oxidizing give it a gossan appearance. 

High gold assays were claimed from this type of rock. Ledoux & Co., 

of New York, report on an assay sample submitted b}^ the Survey: 

" Gold, trace; silver, 0.23 ounce."

On the north side of Brooks Mountain, at an altitude of 1,850 feet, 

is a small galena prospect. The galena occurs in a gossan, the skeleton 

of which consists of tourmaline. The iron oxide of the gossan contains lead and traces of bismuth, the lead probably as carbonate and 

the bismuth as oxide.

LOST RIVER. 

LOCATION.

Lost River is a small stream rising in the heart of the York Mountains in the western part of Seward Peninsula. It flows, south ward 

into Bering Sea through a comparatively broad and open valley, 

except for a short stretch near its mouth, where it flows in a narrow 

canyon. It has a total length of 9 miles. The region is nearly destitute of vegetation, even Arctic mosses being scarce. The tin prospects 

are on Cassiterite Creek (see fig. 7), a branch of Lost River, 6 miles 

from the coast, and are easily accessible by a good wagon roadway. 

A tungsten-silver prospect, a copper prospect, and some galena 

prospects are also situated in this region.

GENERAL GEOLOGY.

The rocks. The general geologic features of the region are simple. 

The bed rock consists of the Port Clarence limestone, dipping northward at an angle of 20°. Near the head of Cassiterite Creek the 

limestone is intimately banded with argillaceous laminae, and intensely crumpled (PL II, B). Locally the formation is fractured 

and brecciated, and shear zones of white marble have been formed.

On Tin Creek, another tributary of Lost River, a small granite 

boss, a third of a mile in diameter, is intruded into the limestone. 

The granite is a medium-grained aggregate of feldspar, quartz 

(which is partly idiomorphic, smoky, and conspicuous), and scattered foils of biotite. The principal effect of this intrusion has been 

to marmorize the surrounding limestone, though locally some large, 

masses of contact-metamorphic minerals have been formed.

A considerable number of vertical quartz porphyry dikes pierce 

the limestone, but only one has been found extending into the granite 

area. They are fairly persistent, and can be traced for several miles 

across the country. They are not all strictly contemporaneous intrusions, as certain dikes have been found to intersect each other. 

The quartz porphyries are light-colored rocks containing small glassy 

quartz and feldspar crystals embedded in an aphanitic matrix. The
U. S. GEOLOGICAL SURVEY tULLETIN NO. 358 PL. IV

A. ORBULE PRODUCED BY CONTACT METAMORPHISM.

B. REVERSE SIDE OF ORBULE SHOWN IN ,1.

C. MAXIMUM ORBULE: DIAMETER, 8 INCHES.

D. IRREGULAR ORBULES.
ECONOMIC GEOLOGY. 45

main tin prospects of the region occur in a few highly altered dikes 

of this character, but several of the other dikes have received separate 'names and have been more or less prospected, on what encouragement, however, it is difficult to understand. They are usually 

unmiiieralized, except for sporadic cubes of pyrite, ancl there is 

no known reason why they should become tin bearing in depth.

FIG. 7. Geologic sketch map of Cassiterite Creek and vicinity. Topography by Adolph 

Knopf. Elevations determined by aneroid barometer.

Orbicular contact metamorphism. The limestone surrounding the 

granite boss of Tin Creek has been converted into a coarse white 

marble. Near the contact loose blocks showing orbicular forms are 

common (Pis. IV, V, VI. B}. The orbules are compqsed of an alternating succession of concentric black and white bands, commonly a
46 TIN DEPOSITS OF SEWAED PENINSULA, ALASKA.

millimeter or so in breadth. As shown in the photographs, the 

orbicular structure is brought out in detail by the etching action of 

the weather. Many of the sections through the orbules are perfect 

circles, a maximum diameter of 8 inches being noted, but elliptical 

forms, due to the. interference of contiguous orbules,. are common. 

In some places small orbules occur in the outer bands of large orbules 

and cause a wrinkling of the even banding. Where several small 

independent orbules have formed around closely spaced centers highly 

intricate structure resembling that of contorted gneiss has been 

evolved.

The minerals composing the orbules, named in their order of abundance, are fluorite, hornblende, vesuvianite, plagioclase (AbyAn7 ), 

and magnetite. The hornblende is a deep-colored variety, the 

pleochroism ranging from brown to strong greenish blue. The 

vesuvianite has a tendency to form radial groups of short, stout 

columns. Under the microscope the banded structure is not as distinctly apparent as would be expected from inspection of the 

weathered surface of the orbules. The light-colored bands consist 

of mutual intergrowths of fluorite and plagioclase; the dark bands 

consist of fluorite with hornblende or vesuvianite, or with both 

together, and commonly some magnetite. The presence of fluorite 

in both dark and light colored bands causes them to contrast less 

emphatically -under the microscope than they do macroscopically. 

Examined in thin section the central portion of one of the orbules was 

found to consist of calcite anhedra in which are embedded vesuvianite, fluorite, and green amphibole inclosing considerable magnetite. This central area is surrounded by a ring of magnetite. Other 

centers are composed chiefly of fluorite and vesuvianite with intergrown hornblende. An unusual type of orbule is one composed of 

garnet, pyroxene, and fluorite, with narrow black bands of magnetite.

A single exposure outcropping beneath the talus near the granitelimestone contact throws light on the genesis of these remarkable 

forms. Curious veins, symmetrically banded, traverse the marmorized limestone in irregular fashion. The bands are only a fraction 

of a millimeter in thickness and simulate the sinuous flow lines of 

certain acidic volcanic rocks (PL V). Small tongues, a few inches 

in length, project into the marble, and these also are symmetrically 

banded. In irregular expansions of the veins orbicular structures 

have been developed (PL VI, B). The veins are composed of fluorite 

and calcic plagioclase (A^An,), with pyroxene, green mica, hornblende, and accessory arsenopyrite, cassiterite, and scheelite. They 

therefore resemble the orbules in mineralogical constitution to a 

considerable extent.

The orbicular material is cut by quartz porphyry, thus fixing the 

period of its formation , although not very closely. The only quartz 

porphyry dike penetrating the granite shows strong marginal chill-
U. 8. GEOLOGICAL SURVE IULLETIN NO, 353 PL. V

BANDED VEIN; A SUPPLY DUCT FOR THE ORBULES. 

Natural size.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 358 PL. VI

A. BANDED APOPHYSIS FROM GARNET-VESUVIANITE MASS ON TIN CREEK. 

Natural size,

]',. ORBULES IN MARBLE MATRIX, SHOWING MODE OF ORIGIN.
ECONOMIC GEOLOGY. 47

ing, indicating, therefore, a considerable interval between the intrusion of the granite and the injection of the dikes. How nearly contemporaneous were the solidification of the granite and the metamorphism is an open question.

A mass of metamorphic minerals 50 feet wide is exposed on the 

bank of Tin Creek, 1,000 feet from the visible contact of. the granite 

and limestone. It consists of solid masses of radial and arborescent 

vesuvianite and of brown garnet showing dodecahedral faces. The 

microscope reveals in addition small amounts of interstitial fluorite 

and calcite and accessory pyroxene, hornblende, and plagioclase. 

The dark mass of metamorphic minerals has injected apophyses, as it 

were, into the adjoining limestone (PL VI, A). These are arranged 

in black and white bands, a fraction of a millimeter in thickness. The microscope shows that these apophysal veins are composed of fluorite, strongly pleochroic from brown-green to blue-green 

hornblende, vesuvianite, and calcite, with accessory magnetite and 

arsenopyrite. These features ally this occurrence with the orbicular 

contact metamorphism and suggest that the heavy vesuvianite-garnet 

masses were produced by magmatic solutions traveling outward along 

fissures in the limestone.

Forms similar to the orbicular structures have not previously been 

recorded. Triistedt a has recently described some curious occurrences 

(" Erzschlauche," he terms them) from Pitkaranta, in Finland, 

which in their essential aspects resemble the banded veins serving as 

the supply ducts for the Alaskan orbules. Cross sections of the ore 

arteries resemble sections through the orbules and, moreover, show a 

similarity in mineralogical constitution and similar alternating bands 

composed of magnetite and fluorite-vesuvianite with sporadic garnet. 

The explanation advanced for the ore arteries, which anastomose 

through a.limestone, is that they are crustification structures produced 

by juvenile waters subject to rapid changes in composition and temperature. The circular pipes along which the deposition took place 

were produced by the corrosive action of earlier solutions. This 

hypothesis does'not fit the Lost River phenomena. The mode of 

growth of the orbules outward from centers their common interferences, and their position adjoining the banded veins preclude an 

origin by crustification. Their essential similarity to the banded 

veins and their parasitic habit show that the two features are identical 

phenomena, which, as already indicated, are allied to the formation of 

the vesuvianite-garnet masses. The 'explanation which the writer 

would suggest for this related set of phenomena is that it was produced by magmatic emissions traveling outward under,.great pressure 

along fissures in the limestone; that the flow of the solutions was im0 Triistedt, 0., Die Erzlagersttitten von Pitkaranta am Ladoga-See: Bull. Coinm. Geol. 

de Finlande, No. 19, 1907, p. 226,
48 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA. .

peded; that at points of stagnation the wall rock was thoroughly permeated and a local intense metasomatism was produced. The solutions 

contained large amounts of fluorine, aluminum, silicon, iron, and 

sodium. The formation of fluorite (CaF2 ) caused the expulsion of an 

equivalent quantity of CO2 from the limestone and, owing to the absorption of this gas, the capacity of the solutions to dissolve lime was 

increased to a high degree.

It is probable that the channels of circulation were enlarged by this 

process and that the vein filling as now seen consists of minerals 

formed from the lime of the wall rock and fluorine, aluminum, silicon, 

iron, and sodium derived from the magmatic solutions. In such veins 

formed in limestone under conditions of high pressure and temperature it appears to be impossible to discriminate between metasomatically altered wall rock and the filling of the original channels of circulation. The banding was produced either by processes similar to those 

which have given certain igneous dikes their banded character or by 

phenomena similar to those operative in the formation of Liesegang's 

rings,. If a drop of AgNO3 solution be placed upon a gelatin plate 

impregnated with K2Cr2 O7 a series of concentric rings consisting of 

Ag2O2 O7 will be formed, becoming progressively wider spaced with 

increasing distance from the center. The explanation given by Ostwald a is that as the soluble silver salt diffuses outward a periodic 

precipitation of insoluble silver chromate will take place at the loci 

of supersaturation. ' Morse and Pierce 6 have investigated this phenomenon in some detail, using tubes, however, instead of plates, and 

have shown incidentally that gelatin is not essential to the production 

of the rings and bands. Among other substances employed were 

FeCl3 and Na2CO3 , and a recurrent evolution of OCX was noted. 

This reaction proceeds as follows:

2FeCi3 +3H2O+3Na2CO3 =3CO2 +6NaCl+2Fe(OH) 3 .

When the attempt is made to fit this explanation to the natural 

conditions the problem becomes greatly complicated on account of 

the complex character of the solutions, the unknown state of combination of the various elements, and their different rates of diffusion. 

As abundant fluorine was present in the solutions, a reaction analogous to the preceding may be formulated as follows:

2FeF3 -f3H2O+3CaCO3 =3CO2 +3CaF2 +2Fe(OH) 3 . .

The fluorine was thus fixed as fluorite and the iron separated as 

magnetite or .was taken up by the production of amphibole. Enough

0 Lehbruch allegemeinen Chemie, 2d., Band 2, Teil 2, p. 778, 

" Zeitschr. physikal. Chemie, vol. 45, 1903, p. 589.
ECONOMIC GEOLOGY. 49

has been indicated to show that an explanation involving diffusion 

and supersaturation is at least as plausible as one involving crustification and is in closer harmony with the phenomena observed in the 

field.

OASSITEBITE PROSPECTS.

Lodes. At Tin Creek a few thin quartz stringers carrying cassiterite have been found in the granite. Collier tt has shown that some 

pyritiferous granite from the same locality contains 0.3 per cent of 

tin. No cassiterite is visible in this granite to the unaided eye, so on 

account of the prevalence of sulphides he assumed that the tin occurs 

in the form of tin pyrites (stannite). A cut has been opened on this 

occurrence and shows a number of "narrow bands of hard quartzose 

granite containing finely disseminated pyrites, chiefly of iron and 

arsenic. Under the microscope the rock is seen to be composed of 

quartz, topaz, and serialized feldspar, with accessory pyrite, arsenopyrite, and cassiterite in small amount.

The principal tin prospects of the region are located on Cassiterite 

Creek, and occur in the quartz porphyry dike known as the Cassiterite lode. This dike is 6 to 10 feet thick and can be traced from 

the head of Tin Creek in a northwesterly direction to Lost River, a 

distance of 9,000 feet. Near Lost River the dike rock contains a multitude of angular limestone fragments and is really a limestone 

breccia cemented by quartz porphyry. The characteristic feature of 

the Cassiterite lode dike rock, where nonstanniferous, is the abundance of sharply defined quartz phenocrysts embedded in a white 

aphanitic matrix. Thin sections cut from the least-altered portions 

of the dike show numerous phenocrysts of quartz, orthoclase, and 

sodic plagioclase embedded in a cryptocrystalline groundmass. The 

nonlamellated feldspar is opaque from kaolinization, but the plagioclase is unaltered,, Sporadic crystals of clear and limpid plagioclase 

lie inclosed in turbid orthoclase phenocrysts. Fluorite is common 

in the groundmass and patches of topaz occur, also. More highly 

altered phases of the dike merely show quartz phenocrysts lying 

scattered in a matrix of scaly white mica, fluorite, and quartz. Along 

a portion of its course the white quartz porphyry dike has broken 

through an older dike, a gray feldspar porphyry, which is particularly conspicuous on account of the multitude of dull white phenocrysts that it contains.

The tin-bearing portion of the dike is 3,000 feet long, but the whole 

of this length can not be considered ore rock; intermittent barren 

stretches occur, and the ore is probably localized in irregular shoots. 

The limestone in the vicinity of the stanniferous portion, of the dike

0 Collier, A. J., Tin deposits of the York region, Alaska: Bull. U. S. Geol. Survey No. 229, 

1904, p. 22.

54356 Bull. 35S OS 4
50 TIN DEPOSITS OF SEWAKD PENINSULA, ALASKA.

is seamed Avith innumerable veinlets which reticulate the surface of 

the country rock in every conceivable direction (PL VII, A). 

These vary in thickness from a film's breadth to several inches. An 

energetic metasomatic alteratien has accompanied the veinlets, and 

cassiterite is locally observed in them, but nothing in the nature of a 

stanniferous stockwork has been formed.

The tin ore found in the quartz porphyry dike is associated with 

irregular seams and stringers of quartz and lithia mica. Cassiterite 

occurs both in the stringers and as an impregnation of the altered 

dike adjoining the stringers. Where the veinlets are absent the 

quartz porphyry contains no cassiterite, and is hard and barren. 

Wolframite is commonly associated with the cassiterite and, though 

no actual tests have been made, it is probable that the tungsten content of the lode is as valuable as the tin. Pyrite and arsenopyrite 

accompany the tin ore and more rarely sphalerite and galena are 

found. Locally the dike rock contains some molybdenite. The commonest gangue mineral- is fluorite, with zinnwaldite next in order of 

abundance. Thin sections show also the presence of topaz in radial 

aggregates. Where alteration has been most intense large drusy 

masses of cubical fluorite and mica occur, and from such localities 

magnificent specimens of cassiterite in black splendent crystals have 

been obtained. The usual type of ore, however, is a soft kaolinized 

porphyry, stained red with iron oxide, and impregnated with cassiterite, wolframite, and sulphides. The dike is intensely and irregularly slickensided and clay gouge is common. The limestone wall 

rock, however, is firm and hard. It is considerably impregnated with 

fluorite, which glows with a greenish light when struck with the pick. 

Thin sections of wall rock immediately adjacent to the dike show that 

it consists of fluorite and radial topaz, with some colorless mica. 

Cassiterite occurs to a small extent in the wall rock in narrow veinlets (1 inch thick) consisting of divergent columnar topaz. In the 

vicinity of these veinlets the fluoritizecl limestone contains patches of 

coarse fluorspar and rosettes of topaz.

A few hundred feet north of the Cassiterite lode is another quartz 

porphyry dike, known as the Ida Bell lode. It is about 35 feet 

thick on the summit of the hill between Lost River and Cassiterite 

Creek. The rock is dense and fine grained. In the vicinity of Cassiterite Creek quartz stringers an inch or so in thickness, carrying cassiterite with some Avolframite, cut the dike, but alone: its eastern ' " o

extension only sporadic cubes of pyrite can be seen. The alteration 

that is so characteristic a feature of the Cassiterite lode is conspicuously absent from the Ida Bell quartz porphyry dike. The petrographic similarity Avhich a number of other quartz porphyry dikes in 

the Lost River basin bear to this dike has occasioned considerable 

useless prospecting.
U. S. GEOLOGICAL SURVEY BULLETIN NO. 358 PL. VII

A. RETICULATE SEAMING OF LIMESTONE IN VICINITY OF CASSITERITE LODE.

i. SURFACt. LXHOiURt iNu uOCUHKENCE or FLUoRi F L-biLiCA"! E ROCK ADJOINING 

VEINLETS IN LIMESTONE.
ECONOMIC GEOLOGY. 51

One mile north of the Cassiterite lode is another tin-bearing 

porphyry dike named the Dolcoath lode. It is from 2-£ to 3 feet 

thick, strikes N. 50° E. (magnetic) and dips 65° NW. This dike 

differs from the two previously described both in its mineralogy and 

in the mode of occurrence of the tin ore. It is so highly altered and 

mineralized that its original igneous character is not everywhere 

readily apparent. Least-altered phases show a dark-gray finegrained rock containing numerous dull feldspar phenocrysts and a 

few quartz crystals. The feldspars prove to be near labradorite in 

composition, and the groimdrnass is largely obscured by secondary 

minerals, such as tourmaline, quartz, pyrite, mica, chlorite, and others. 

Some movement has taken place along the walls of the dike, especially 

the hanging wall, forming a crushed zone 1 to 6 inches thick. The 

dike rock is heavily charged with arsenical pyrites and tourmaline and 

contains some cassiterite disseminated through it. Locally the dike 

has been converted into large masses of danburite containing radial 

groups of tourmaline and an abundance of arsenopyrite. Cassiterite 

is inclosed in these three constituents, but is visible only under the 

microscope..

The Avail rock of the dike is the dense-textured banded argillaceous 

variety of the Port Clarence limestone. The calcareous portion has 

been converted into coarse white spar containing random prisms of 

tremolite; the argillaceous bands have been converted into matted 

aggregates of tremolite fibers. Finely crystallized cassiterite occurs 

embedded in the coarse calc spar and locally is extremely abundant. 

Topaz in square prisms is found in the limestone to some extent. 

Cassiterite also occurs in the wall rock intimately intergrown with 

pocket-like masses of danburite, which is a calcium borosilicate related to topaz. The danburite is of light pinkish-gray color, with a 

peculiar vitreous greasy luster, and occurs as rude ill-defined columns 

in rough radial arrangement. Examination of the danburite-cassiterite ore in thin section shows that the cassiterite, which is finely idiomorphic, lies embedded in the danburite, and contains innumerable 

microlites of tourmaline. The danburite incloses some small patches 

of calcite, and may, like the cassiterite, inclose great numbers of 

minute tourmaline prisms. These bands of wall rock, which carry 

tin ore and show marmorization with accompanying development of 

danburite, topaz, and tremolite, occur on both foot and hanging walls 

of the dike, though nowhere more than 6 inches thick. Tremolite, 

however, persists to great distances from the dike, though in lesser 

abundance. Arsenopyrite has apparently replaced the wall rock to 

some extent also, as solid lumps of it containing only tremolite fibers 

have been found. The association of danburite and cassiterite is 

unique in the literature of tin deposits. To establish the identity of 

the danburite beyond question the following approximate partial
52 TIN DEPOSITS OF SEWAED PENINSULA, ALASKA.

analysis was made by W. T. Schaller on the purest material that could 

be obtained:

Analysis of danburite from Dolcoath lode, Lost River region. 

SiO2 _ ________________________ _ _________ 47.54

CaO _______________________________ . ..,__ 21. 02 

MgO ___________________ _ ___ _ _______ _____ 1. 57

BsOa ___________ _ 1 ______ ______________ a24. 03

97.59 

' Specific gravity, 2.98.

Developments. At the time of visit five adits had been driven on 

the Cassiterite lode, three of which were open and could be examined. 

Tunnel B, 260 feet above Cassiterite Creek, was 180 feet long. The 

tunnel follows the southern margin of the dike, and is partly in limestone. At 45 feet from the mouth a drift has been run 10 feet to 

the north, crosscutting the lode. Tunnel A, 1,170 feet above the 

creek, was 80 feet long. The dike is soft and can be angered, so that 

an advance of 4 feet a day (single shift) is easily made. Both these 

adits are on the east side of Cassiterite Creek. Tunnel E, driven on 

the Cassiterite lode 10 feet above the creek level on the west side, is 

100 feet long. A crosscut 9 feet long has been driven 50 feet from 

the mouth of the adit. The Ida Bell dike has been explored by an 

adit 55 feet long, at the end of which a winze 69 feet deep was sunk. 

This is now filled with water.

A few hundred feet south of the Cassiterite lode some open cuts 

and adits, now caved in, were opened on a porphyry impregnated 

with cassiterite. No surface croppings of this porphyry body are

visible, for the porphyry is buried under the mantle of slide rock

covering the steep slopes of the region. A shaft 50 feet deep was 

sunk on this occurrence, but was flooded at the time of visit. A 

number of open cuts have partly exposed some thin but rich quartz 

veinlets in the limestone. A shaft reported to be 18 feet deep was 

sunk near such a vein on the Jupiter claim, and a drift 44 feet long 

was run to the ledge, but both were also under water.

The developments on the Dolcoath dike consist of four cuts opened 

at intervals along a length of 3,000 feet. An assay of ore from one 

of the crosscuts was reported to have yielded 1.15 per cent of tin.

Seaming .of the limestone. The most striking feature- in the vicinity of the Cassiterite lode is the vast multitude of veinlets that 

interlace the limestones in every direction (PL VII, A). The area 

thus affected extends for 1,000 feet or more on both sides of the stan0 Two determinations made with concordant results by Messrs. Chapin and Wherry 

xinder the direction of Prof. Edgar C. Smith,
ECONOMIC GEOLOGY. 53

niferous portion of the dike. On the basis of dominant mineralogical 

composition five types of veinlets can be ' discriminated fluoriteamphibole, plagioclase-fluorite, zinnwaldite-topaz, topaz-fluorite, and 

tourmaline-mica. The pure types occur, but innumerable intermediate and transitional forms are found also.

Veinlets (PL VII, Z?), only one-half inch thick, consisting of fluorite and radial amphibole, are paralleled on both sides by altered wall 

rock, 2 inches wide, sharply delimited against marmorized limestone 

containing minute fibers of tremolite. The wall consists of fluorite, amphibole, vesuvianite, and green mica irregularly intergrown. 

Where embedded in fluorite the amphibole is a deep-colored variety, 

pleochroic in tones of brown-green arid blue-green.

Allied to the veinlets just described are dense, white veinlets consisting essentially of fluorite and plagioclase. Locally they contain 

hornblende and scattered grains of pyrite and arsenopyhte. They 

are inclosed by dark-green rock, which is very delicately banded by thin 

white bands running parallel to the central veinlet. The larger bands 

are a millimeter in width, but the most of them are narrower,. and 

some are as narrow as 0.1 millimeter. These features are most strikingly apparent on AA7 eather-etched surfaces. Small seams branching 

from the central veinlet cut across the banded rock. The metasomatically altered wall rock is thus elaborately banded at but few places; 

more commonly it is a structureless green rock. In thin section the 

central veinlet is seen to be composed of an intimate intergrowth of 

fluorite and calcic plagioclase (as calcic at least as Ab1An1 ), with 

scheelite as an accessory mineral. The inclosing rock is formed of 

fluorite, hornblende, and vesuvianite, but green mica, pyroxene, and 

calcic plagioclase also occur, and arsenopyrite and cassiterite are present as rare accessories. The hornblende and vesuvianite are prone to 

form small radial groups. The narrow white bands are composed of 

intergrowths of fluorite and plagioclase. The macroscopic appearance of the banded rock immediately recalls the orbules and banded 

veins of Tin Creek, and this external resemblance is confirmed by the 

similarity of mineralogical constitution as revealed by the microscope. 

The chemical origin of the banding is here more obviously apparent.

Veinlets of zinnwaldite up to 1 inch in thickness are common in 

the limestone. Under the microscope the zinnwaldite forms radiating 

fanlike groups diverging from the vein walls. The mica is dirty 

greenish and somewhat pleochroic at the points of attachment. Elsewhere it resembles muscovite. Some topaz occurs as an interstitial 

filling, and cassiterite and wolframite are rare accessory minerals. In 

the specimen examined optically the wall rock is a cryptocrystalline 

limestone exhibiting only a feeble alteration. Others, however, show 

an energetic fluoritization and notable increase of granularity. An
54 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

analysis of the zinnwaldite has been made by W. T. Schaller a in 

the laboratory of the Survey, and is quoted here to illustrate the 

elaborate composition of this mica.

Analysis of zinmoaltlite in limestone near Cassiterite lode, Lost River region.

SiO2 _ __________ __________ 46. SO

A12O3 ______________...___.._________.___ 24.50

Fe203 ______-____.__._____ __.._ ._..______ . 50

FeO _..______._ ___._.__..__________._ 6.35

MnO_____________________________ T ____ 1.38

CaO ___________ _..______ _ __ ___..___ . 24

Na2O__________________________.__._______ 1. 73

K2O______________________^__..____ ____ 9. 20

Li2O ________..__..____'.____________ _________ 3. 73

H2O ___________________________________ . 88

F ____ __________________-____________ 8. 63

103. 94 

Less O=2F _____________________________:__ 3.63

100.31

Topaz-fmorite veinlets, carrying cassiterite and tabular crystals of 

wolframite, have produced both fluoritization and topazization of the 

adjoining limestone.

On the south side of the Cassiterite lode tourmaline veinlets are 

common in the limestone, though no tourmaline occurs in the tin 

ore of the dike rock. The cause of this peculiarity is not known. 

This type of veinlet is composed of white mica, blue tourmaline, and 

iluorite. Some carry visible cassiterite, but these are rare. Microscopic examination of a stanniferous veinlet (one-half inch thick) 

showed that the cassiterite is intergrown with scheelite, calcite, and 

mica (zinnwaldite?) and embedded in a gangue of mica, tourmaline,

and fluorite. The limestone adjoining the tourmaline stringers has 

been converted into a confused intergrowth of green and indigo-blue 

tourmaline, fluorite, and magnetite, with accessory calcite, vesuvianite, green rnica, and amphibole, forming a rock indistinguishable, 

texturally and mineralogically, from the contact-metamorphosed 

limestone of Ear Mountain.

A feature allied to the seaming of the limestone by the veinlets is 

found in an outcrop on the creek 200 feet below the Cassiterite lode, 

in which a zone of brecciated limestone has been recemented by various dark-colored minerals which here and there form large radial 

groups 3 to 4 inches in diameter. The cement or binding material 

when examined optically is found to consist chiefly of blue-green 

hornblende, vesuvianite, calcic plagioclase (A^An,,), fluorite, minor 

amounts of calcite, and accessory scheelite.

"Am. Jour. Sci., 4th sei1 ., vol. 24, 1907, p. 158.
BULLETIN NO. 358 PL. VIII

A. POLISHED SURFACE OF SEAMED LIMESTONE SHOWING INTENSE 

METASOMATISM ACCOMPANYING THE VEINLETS IN LIMESTONE 

AND NUMEROUS SUBSIDIARY VEINLETS BRANCHING FROM MAIN 

VEINLET.

IS. THIN SECTION OF CASSITERITE ORE. 

Magnification 1 2 diameters. Radial topaz diverging from vein wall.
ECONOMIC GEOLOGY. 55

To sum up briefly: In spite of the diversity of mineral composition of the various veinlets it is found that all are accompanied by 

similar intense alterations of their wall rocks (PL VIII, A). They 

appear to be of practically synchronous origin, except, perhaps, the 

tourmaline veinlets, and the smaller seams represent leakages along 

subsidiary fractures. A more complete interchange of material between wall rock and solution was apparently possible in the minor 

seams, for vesuvianite and garnet appear in them, whereas these minerals occur only in the metasomatically altered wall rock of the 

larger veinlets.

Cassiterite and wolframite quartz veins. It is a notable feature 

that, although the quartz stringers cutting the porphyry dikes contain cassiterite and wolframite together, those cutting the limestone 

contain either cassiterite alone or wolframite alone, and in large proportions. Veinlets of such diverse composition occur not many hundred feet apart. They average only a few inches in thickness and, SO 

far as is now known, 30 or 40 feet in length.

The cassiterite-quartz veins, while rare and of no great persistence, 

are extraordinarily rich in tin. Cassiterite, complexly twinned, occurs in crystals up to an inch in size. The predominant gangue mineral is quartz, with which are associated subordinate amounts of 

fluorite, feldspar, and white mica! The cassiterite is concentrated 

near the sides of the vein, which is frozen to the wall rock. Locally 

a thin band, one-eighth inch thick, of delicate radial topaz intervenes between the quartz gangue and the altered wall rock. Thin 

sections cut from this portion of the ore show groups of topaz prisms 

diverging from the wall of the vein (PL VIII, B}. Closely asso-; 

ciated with them are fluorite, white mica in well-defined plates, 

quartz, and cassiterite. The wall rock consists of a fine-grained aggregate of fluorite, topaz, and mica. Farther away from the vein the 

limestone is marmorized and transfixed with tremolite needles. The 

filling of the veinlets branching off from the cassiterite-quartz vein 

is not quartz, but is chiefly white mica. In the country rock adjoining the cassiterite-quartz vein, veinlets consisting essentially of white 

mica and lamellated plagioclase are common, and some of these are 

metalliferous. In their central portions they carry small amounts of 

galena, chalcopyrite, pyrrhotite, and sphalerite. The wall rock adjoining these .veinlets, which average perhaps one-fourth inch in 

thickness, is altered to fluorite and aggregates of scaly green mica.

Some quartz stringers carrying considerable wolframite have been 

found cutting the limestone. The gangue material is dominantly 

quartz, but fluorite, white mica, and albite also occur. The altered 

wall rock consists of fluorite, green-blue hornblende, vesuvianite, 

green mica, and garnet, with scheelite and cassiterite as rare accessory 

minerals (PL IX, #.). Sphalerite and chalcopyrite occur dissemi-
56 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

nated through the metamorphosed wall rock, though absent from the 

quartz veins themselves. The zone of metasomatism is sharply 

bounded by white saccharoidal limestone containing sporadic prisms 

of tremolite (PL IX, A).

The cause of the segregation of the cassiterite and wolframite into 

separate veinlets is not known. On the hill between Lost River and 

Cassiterite Creek a prospect trench has opened a tin-bearing veinlet 

1 inch to 6 inches thick for a length of 30 feet. At one end the veinlet can be seen to pinch out. Throughout its length it is unusually 

rich in cassiterite and carries some wolframite and arsenopyrite. The 

gangue is. composed essentially of topaz, fluorite, and zinnwaldite, 

arranged in a rudely banded structure. The middle band is the 

most distinctly defined, though only one-half inch broad, and consists chiefly of topaz imperfectly interlocking along a central line. 

The wall rock of this veinlet is highly altered and is impregnated 

to some extent with chalcopyrite, pyrite, and sphalerite. The study 

of this occurrence suggests that the simple quartz veins are due to a 

sort of differentiation from the primary solutions that deposited 

fluosilicates, and that, accompanying this change, a segregation of 

cassiterite from the wolframite may have taken place.

Metasomatic -processes. The Lost River occurrences throw some 

light on the metasomatism produced by stanniferous solutions acting 

on a nearly pure limestone. In the zone of most intense activit}^ 

fluorine has effected a complex'expulsion of the carbon dioxide with 

the production of abundant fluorite. On the assumption that the 

calcium remains constant and is combined as fluorite, this means a 

loss in volume of 34 per cent. This shrinkage appears to have been 

amply compensated for by the production of topaz, hornblende, 

vesuvianite, mica, plagioclase, and garnet, all of which involve an 

introduction of material, chiefly alumina and silica, with some FeO, 

Na2O, K2 O, and Li2O. Beyond the zone of fluoritization the limestone has been marrnorized and some tremolite produced. The 

transition between the two zones is abrupt, and the difference of 

the amphiboles in them is a characteristic feature. The colorless 

tremolite, which is less abundant than the deep blue-green hornblende, 

has doubtless been produced by the recrystallization of impurities in 

the original limestone. Where the stanniferous solutions contained 

boron, alkali tourmaline has commonly been developed as a metasomatic mineral. At the Dolcoath dike the solutions appear to have 

been unusually rich in boron, and extensive danburitization has taken 

place, with accompanying development of cassiterite, tremolite, tourmaline, and topaz. Marmorization, too, has locally been intense,, 

with the production of calcite individuals up to an inch in size.

The metasomatic processes outlined in the preceding paragraph 

are of a synthetic nature and have caused the formation of various
U. S. GEOLOGICAL SURVEY BULLETIN NO. 358 PL. IX

A. POLISHED SURFACE OF WALL ROCK ADJOINING WOLFRAMITE-QUARTZ VEIN. 

Showing marked contrast between the rnarrnorized limestone and the metasomatically altered limestone.

II. THIN SECTION OF WALL ROCK ADJOINING QUARTZ-WOLFRAMITE VEIN. 

Magnification 32 diameters. Showing an intergrowth of hornblende, vesuvianite, and fluorite.
ECONOMIC GEOLOGY. 57

Complex silicates, some of which, like topaz, danburite, vesuvianite, 

and hornblende, have not been described previously as alteration 

products in limestone adjoining fissure veins. Topaz, according to 

Rosenbusch,a is characteristic of the pneumatolytic contact zones of 

many granites. ' Danburite has recently been described as a constituent in some of the numerous contact-metamorphic deposits of 

"Japan.6 Vesuvianite is a typical contact-metamorphic mineral, and 

hornblende is common in the metamorphic aureoles of many granites. 

Tremolite, a common contact-metamorphic mineral, has, however, 

been recorded by Lindgren c from the Clifton-Morenci district as a 

metasomatic product in limestone adjoining fissure veins, and the 

unusual and significant character of this alteration has been pointed 

out. Tremolite, as has been shown, is one of the commonest products 

of the metasomatic activity of stanniferous solutions circulating in

limestone, but only in the zone of least intense activity. To sum up

briefly, the metasomatic alteration accompanying cassiterite veins in 

limestone, as exemplified by these Alaskan occurrences, is closely 

allied to contact metamorphism. The Avail rock immediately adjoining the vein is characterized by the addition of material and the 

formation of fluo-, boro-, and alumino-silicates. This zone passes 

outward into one showing the features 'of simple thermal metamorphism marmorization with the production of sporadic tremolite. 

Wolframite-topaz lode. A unique mineral deposit is exposed opposite the mouth of Tin Creek in an open cut on the ridge between 

Lost River and Left Fork. The surface indications show that the 

mineralization has taken place along a fault, running approximately 

east and west, which has brought two slightly dissimilar limestones 

into juxtaposition. Some brecciation is apparent along this line. 

The open cut shows stringers of ore occurring in a belt 1 foot thick, 

forming a stringer lode. The ore minerals consist of wolframite, 

galena, and stannite, and are embedded in a gangne of radial topaz 

associated with some deep-purple fiuorite. The stannite is usually 

intercrystallized with the galena and is of a brown-black color. It 

reacts for tin, copper, zinc, iron, and sulphur. The topaz forms fine 

spherulitic aggregates, which may in places attain a diameter of half 

an inch, but as a rule are very small and are crystallized in delicate 

radial groups. The high specific gravity of topaz (3.5) gives the 

ore rock an unusually heavy weight. The surface ore is stained black 

by manganese minerals produced by the decomposition of wolframite. 

Some azurite is present also, and is doubtless derived from the copper

0 Mikroskopische Fhysiograplne, vol. 1, pi:. 2, Stuttgart, 1905, p. 189. 

6 Beitriige zur Mineralogie von Japan, No. 3, Tokyo, 1907, p. 102.

" Lindgren, W., Copper deposits of Clifton-Morenci district, Arizona: Prof. Paper U. S. 

Geol. Survey No. 43, 1905, p. 176.
58 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

in the stannite. An assay a of a sample of this ore submitted by the 

Survey gave a return of 22.9 ounces of silver to the ton.

The gangue material was identified optically as topaz from its 

similarity to that associated with the tin ore on Cassiterite Creek. 

This determination was confirmed by an approximate quantitative 

analysis made by W. T. Schaller, with results as follows:

Analysis of topaz from wolframite-topaz lode, near Tin Creek..

Si02 ________________________________ 30.27 

AU):, _________________________________ 54.66 

CaO __________________ ______________ 1.16 

MgO ________________________________- Trace. 

F ____________________________________ 17. 26 

H2O, alkalis___________________________ _ Not det.

103.35 

Less O=2 F_____________________________ 7.27

96. 08

The wall rock of the topaz lode consists of a dense cryptocrystalline limestone which shows no evidence of metasomatic alteration. 

The topaz lode is remarkable in two respects it is the first recorded 

instance of topaz as a fissure-vein filling and as the gangue material 

of Sulphide minerals. Topaz is common in the greisen adjoining cassiterite veins and in certain metasomatically altered quartz porphyry 

dikes, but has not hitherto been noted as a vein-forming mineral. It 

is regarded by Vogt B as distinctive of the cassiterite veins in contrast 

with the ordinary sulphide-ore veins (filcns plombiferes). The absence of metasomatic alteration in the limestone adjoining the topaz 

lode is noteworthy, but it will be recalled that some of the zinnwaldite-topaz veinlets in the vicinity of the Cassiterite lode show a similar

lack of action ©n their wall rocks.

The stannite in the above-described wolframite-topaz lode is the 

only known verified occurrence of this mineral in the Alaskan tin 

region. Stannite is not a valuable tin-ore mineral, both on account 

of its relatively low tin content (30 per cent compared to 78 per cent 

in cassiterite) and its difficult metallurgical treatment. It is a rather 

favorite object of search with the prospector, chiefly because of the 

fascination which its unknown character exercises; and in consequence pyrite and pyrrhotite are commonly mistaken for stannite. 

It is, however, a mineral whose identity can be established only by 

careful chemical examination.

OTHER MINERAL DEPOSITS.

Alaska Chief property. The Alaska Chief claim is situated about 

4£ miles from Bering Sea, on Rapid River, the large western branch

" Made by Ledoux & Co., of New York.

6 Genesis of ore deposits; special publication of Am. Inst. Min. Eng., 1902, p. 666.
ECONOMIC GEOLOGY. 59

of Lost River. The workings are on a small gulch tributary to 

Rapid River. The country rock is a tough, fine-grained limestone 

of the Port Clarence formation, lying nearly horizontal. Locally the 

strata are buckled and show crushing in the crest of the buckles. A 

fault breccia 15 feet wide, consisting of small angular fragments of 

limestone cemented together by white calc spar, is exposed in the 

creek one-third mile west of the mine. Basalt, in the form of a narrow dike 1 foot thick, is the' only other rock known to be in place in 

the near vicinity of the mine. A few thousand feet to the east a 

number of quartz porphyry dikes can be seen cutting the limestone. 

The original shaft was sunk in a heavy body of porous red iron 

oxide containing galena, reported to be 12 feet thick. At a depth of 

35 feet work was suspended. An adit 143 feet long driven 85 feet 

below the collar of the shaft encountered the same ore body 50 feet 

below the bottom of the shaft. The ore was still oxidized. About

7 feet of low-grade galena ore was exposed.

On the east side of the gulch a devious tunnel, about 600 feet in 

length, was driven to catch another body of galena indicated on the 

surface. The tunnel follows a zone of crushed limestone, bounded in 

many places by fine walls marked with striae. The " ledge matter " 

consists of small fragments of limestone bound together by coarse 

calc spar, clay, and red iron oxide. No ore was encountered. The 

tunnel on the west side of the gulch was then commenced, and the 

' ore body already mentioned was struck late in August, 1907.

Idaho claim. A few hundred yards below the mouth of Tin Creek 

a copper prospect has been opened on the edge of the 15-foot bench 

fronting Lost River, and at the time of visit enough work had been 

done to expose the face of ore at this point. The deposit occurs in 

an irregular shattered zone in the limestone, 15 feet wide and including numerous horses of unmineralized limestone. The ore mineral is 

chalcopyrite, associated with abundant pyrrhotite (magnetic iron 

pryites), and occurs in a gangue of calcite, fluorite, and small fragments of slickensided rock. Some of the fluorite is rose-tinted and 

is locally known as rub}' quartz. Stripping has shown that the same 

ore body extends at least 50 feet to the east, where a strong gossan has 

been uncovered. The relatively great width of the deposit, combined with the low chalcopyrite tenor and the abundance of pyrrhotite, reduces the copper percentage to a small figure.

On Tin Creek a galena prospect has "been opened on some gossan 

croppings at an altitude of 1,100 feet, 800 feet above the bed of the 

creek. The deposit occurs in a fracture zone in the limestone, which 

has been coarsely recrystallized in the immediate vicinity, forming 

spar crystals up to an inch in size. The gossan consists of honeycombed masses of iron oxide containing abundant galena and numerous white and colorless crystals of cerusite (lead carbonate). It was
60 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

planned to prove the value of this deposit during the winter of 1907 

and 1908.

A small trench, 650 feet below the galena prospect, has been dug 

in the effort to locate the bed-rock source of some loose bowlders composed of arsenopyrite flecked with a small amount of cupriferous 

pyrite. Assays made in Nome are reported to have yielded $12 to the 

ton in gold. Some stibnite in a gangue of purple fluorite has been 

found in the saddle at the head of Tin Creek.

ORIGIN .OF THE ORES.

The injection of the quartz porphyry dikes represents the final 

intrusive activity of an underlying granite magma, of which a 

portion is now exposed by erosion on Tin Creek. Fracturing of the 

dikes, accompanied by shattering of the adjacent limestone, took 

place after their consolidation, and an energetic mineralization 

ensued. As shown by the vein fillings and metasomatic alterations 

the ore-depositing solutions were characterized by their richness 

in fluorine, aluminum, silicon, calcium, tin, tungsten, iron, manganese, arsenic, sulphur, lithium, potassium, and sodium, but contained also copper, lead, and zinc, and locally boron in abundance. 

The state of combination of the various elements in solution is not 

known. The vein material includes various silicates, such as topaz, 

zinnwaldite, tourmaline, and albite, and proves that conditions 

unusual in the formation of the ordinary types of veins prevailed. 

Albite and tourmaline are commonly regarded as indicative of the 

magmatic derivation of vein-forming waters, and topaz and zinnwaldite are the two most characteristic of the so-called pneumatolytic 

minerals. The presence of topaz in the vein matter itself is somewhat unusual, inasmuch as this mineral, although common in the 

greisen adjoining quartz-cassiterite veinlets in granite as a replacement of feldspar, is comparatively rare as a fissure filling. Of the 

minerals contained in the veinlets as Cassiterite Creek all, with the 

notable exception of quartz, have also developed metasomatically in 

the limestone. It is noteworthy that cassiterite, though present in 

large proportions in many of the veinlets, appears only in insignificant 

amounts in the intensely altered wall rock. In addition hornblende, 

vesuvianite, garnet, and others were formed, and prove that conditions allied to those obtaining during contact metamorphism prevailed. The stanniferous solutions were therefore presumably of 

magmatic origin and at high temperature and pressure.

The abundance of fluorine compounds fluorite, topaz, and zinnwaldite is in harmony with Daubree's generalization 0 that fluorine 

is the active agent in the formation of tin deposits. According to

0 Daubree, A., G6ologie experimentale, p. 38.
ECONOMIC GEOLOGY. 61

his theory cassiterite is produced by the mutual decomposition of 

the vapors of water and stannic chloride reacting as follows:

SnCl4+2H2 0=Sn(X+4:HCl.

He was actually able to synthesize cassiterite in this way. 

According to analogy we Avould expect 

SnF4+2H2O=SnO2H-4HF.

Cassiterite has been produced at red heat by Deville and Caron ° in 

conformity to this equation. Quartz can be synthesized according 

to an analogous reaction. In recent years Vogt & has been a vigorous 

exponent of the pneumatolytic or gas-aqueous origin of cassiterite 

deposits. He conceives that hydrochloric and hydrofluoric acids 

acting on a cooling granite magma effect an acid extraction of tin

and the various elements associated with it. Gaseous conditions are 

therefore considered as dominant during the formation of cassiterite 

bodies, and the final individualization of the minerals is held to be 

due to the .reactions of -Daubree's experiments.

Certain facts in the Lost River area suggest that fluorine is not, 

however, absolutely essential to the formation of cassiterite. On the 

Dolcoath dike some of the richest ore is intergrown with danburite 

(CaB,(Si04 ) 2 ) and the limestone shows no fluorite, although the 

latter is common in the vicinity of the Cassiterite lode. Moreover, 

the cassiterite includes multitudes of tourmaline microlites, so that 

there is obviously a closer association of the tin with boron than with 

fluorine. On the other hand, the wolframite-topaz lode, with its 

content of galena and stannite, proves that tin and abundant fluorine 

may coexist in the same solution and cassiterite not be formed.

PLACERS. 

BUCK CREEK.

Developments subsequent to 1905 have revealed few new facts of 

interest in regard to the placers of Buck Creek. The gravels have a 

length of about 4 miles and are shallow. Work below the mouth of 

Sutter Creek has shown that the gravel in that part of the stream is 

from 120 to 160 feet wide, averaging about 125 feet. The width of 

the pay streak is not known. A pit having a mean depth of 5 feet, 

from which 48 tons of concentrates have been extracted, has demonstrated thatothe gravel may run as high as 25 pounds per cubic yard. 

The gravel is a comparatively fine wash and bowlders are rare, the

"Compt. Rend., vol. 40, 1858, p. 764. 

6 Zeitschr. prakt. Geologie, 1895, p. 4.75.
62 TIN" DEPOSITS OF SEWAKD PENINSULA, ALASKA.

largest noted consisting of greenstone about a foot in diameter. The 

richest gravel rests immediately upon bed rock and is exceedingly 

clayey and toughly bound together. It gives difficulty in washing, 

the clay having a tendency to roll up in balls and carry cassiterite 

nuggets over the sluice boxes. The bed rock is a broken shale or slate, 

very clayey, but contains no cassiterite. On Sutter Creek, the large 

southern branch of Buck Creek, there is a considerable body of gravel, 

and the discovery of stream tin has recently been reported on it. 

The other tributaries, gulches, and " benches " of Buck Creek contain 

little or no gravel, at least in amounts sufficient to warrant an outlay 

for the purpose of placer mining.

The stream tin of Buck Creek is clearly derived from the erosion 

and concentration of the cassiterite occurring in the quartz stringers 

so abundant throughout the area. This source was partly supplemented by the cassiterite occurring in the actinolite rock, and to a 

lesser extent by that contained in the quartz porphyry dikes. As 

these bed-rock sources are known to occur in place on the summit of 

the- hills at the head of Buck Creek, it is probable that some of the 

creeks flowing into Lopp Lagoon carry stream tin. But whether 

cassiterite is present in these streams in payable quantities is purely 

a matter of accurate sampling, and not of opinion or theory an idea 

which prevails in certain quarters to the detriment of the region.

Two companies were in operation on Buck Creek during 1907, but 

on account of a number of adverse circumstances the yield was less 

than Avas expected. Placer mining was confined to a small strip just 

below the mouth of Sutter Creek, and the total output of the year 

was approximately 50 tons of concentrates.

At the beginning of the season the American Tin Mining Company 

was working its ground by means of an automatic scraper and belt

conveyor operated by a 35-horsepower oil-burning engine. Early in 

August, however, extortionate freight rates on the transportation of 

crude oil from Nome to York and the imperfect adaptation of the 

scraper to the character of the gravel necessitated a change in the 

method of working. Shoveling in was then adopted, with results at 

least more satisfactory than those attained with machinery.. The 

other company also employed the shoveling-in method, and the tailings were removed by a horse and scraper.

GROUSE CREEK.

During the summer of 1907 assessment work was done on a number of claims on Grouse Creek and two of its tributaries, Sterling 

and Skookum creeks. The results are not known. Some gold sifted 

out of the stream-tin concentrates from Sterling Creek was flat, coarse, 

and not greatly waterworn, and had quartz still adhering to it.
KESTJME AND CONCLUSIONS. 63 

FAIRHAVEN DISTRICT.

A sample of black-sand concentrates from Humboldt Creek sent 

in to the office for determination proved to be a rich tin ore containing less than $5 per ton in gold. About two-thirds of the sample 

was pyrite. Another sample of concentrates sent in from Kougarok 

River was found to contain considerable cassiterite, but far less than 

the Humboldt Creek sample. It carried, however, 85 ounces of gold 

per ton and contained 66 per cent of pyrite and about 10 per cent of 

magnetite. As the headwaters of Humboldt Creek drain the Hot 

Springs granite area, the tin was probably derived from that region. 

Collier states that samples of tin ore purporting to come from it were 

brought to Nome late in the season of 1902.

RESUME AND CONCLUSIONS.

Four localities in the western part of Seward Peninsula are being 

prospected for lode tin at the present time. From one stream 

Buck Creek placer tin is actually being extracted, and an output of 

approximately 50 tons of concentrates was attained in 1907.

The sedimentary rocks of the York region comprise a series of 

slates of unknown but probably early Paleozoic age, a thick volume 

of thin-bedded limestone of Ordovician age (Port Clarence limestone), and crystalline limestone of Carboniferous age. At Ear 

Mountain contorted limestones and lime-mica schists prevail. These 

rocks are intruded by a number of granite masses, which, though appearing in isolated stocks, show by the many features that they possess in common that they belong to the same irruptive magma. The 

granites are coarse-grained types with large porphyritic feldspars 

and quartz which is commonly of a conspicuously smoky character. 

They were unusually rich in volatile constituents, among which boron, 

fluorine together with chlorine, and iron were the most prevalent, and 

they are therefore characteristically surrounded by pneumatolytic 

contact aureoles. Large amounts of the magmatic emanations were 

retained by the limestones in such minerals as tourmaline, axinite, 

ludwigite, hulsite, paigeite, boron vesuvianite, magnetite, hedenbergite, fluorite, scapolite, and chondrodite.

Complementary contact phenomena occur at Cape Mountain, where 

giant granite selvages are overlain by fluoritic and scapolitic pyroxene hornfels containing accessory scheelite. These phenomena 

are regarded as showing on the one hand the effect of the mineralizers 

on the crystallization of the magma, and on the other hand their effect 

in producing intense metasomatic action on the adjoining limestone. 

Essentially similar metasomatism was produced by a pegmatite intrusive in the marble surrounding the granite stock of Brooks Mountain. 

Along Tin Creek a novel type of contact metamorphism which has
64 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

produced perfect orbicular forms has occurred. The orbules are 

allied in their origin to garnet-vesuvianite masses, which have injected 

small banded apophyses, as it were, into the inclosing limestone. The 

supply ducts for the orbules consist of curious banded veins composed 

of fluorite, calcic plagioclase, and pyroxene, with accessory arsenopyrite, cassiterite, and scheelite.

The tin deposits are genetically associated with the granitic intrusives. Cassiterite occurs in a variety of ways:

(1) In a tourmaline-axinite hornfels.

(2) In beds of actinolite rock which are probably interstratified 

with slates.

(3) In tourmalinized margins of granite masses and granitic dikes.

(4) In mineralized quartz porphyry dikes.

(5) In quartz veins cutting granite and accompanied by impregnation of the adjoining granite.

(6) In quartz stringers cutting slates and limestones.

In addition, tin is found to be present as paigeite (an iron-tin 

borate) in lime-silicate hornfels.

At Ear Mountain cassiterite occurs in a contact-metamorphosed 

limestone consisting essentially of tourmaline, axinite, and actinolite, 

but it is not found in any of the other pneumatolytic contact rocks of 

the region. This is a fact of considerable interest from the standpoint of ore genesis. The stanniferous granite magma was rich in 

halogens and boron, and theoretically the limestones are favorable loci 

for the precipitation of cassiterite. By the action of stannic chloride 

(SnCl4 ) vapor on lime Daubree tt was able to synthesize cassiterite. 

The limestone contacts might therefore be expected to show this mineral. But as a matter of fact the contact rocks, although rich in pneumatolytic minerals, are as a rule barren of tin. The advent of the

cassiterite was postponed to a later stage, and where evidence can be 

obtained as to the relative ages of intrusion and mineralization the 

latter postdates the injection of the quartz porphyry, or rhyolitic 

dikes. .This would appear to indicate that a long period of preliminary concentration was taking place in the cooling magma.

The rarity of cassiterite as a contact-metamorphic mineral the 

world over is anomalous. As far :as the writer is aware the only 

deposit in which it is unequivocally of contact-metamorphic origin is 

that in the Dartmoor Forest, Devonshire, England, described by 

Busz. 6 At this locality hornfels adjoining the granite contact consists 

essentially of light-colored mica, quartz, and tourmaline with innumerable grains and minute crystals of cassiterite scattered throughout 

the rock.

« Compt. Rend. ; vol. 30, 1854, p. 138.

6 Busz, K., Neues Jahrb., Beil. Band 13, 1899, p. 100.
RESUME -AND CONCLUSIONS. 65

At the St. Dizier mine,0 in Tasmania, cassiterite occurs in a magnetite-silicate rock of the Kristiania type, but it is not entirely clear 

whether the cassiterite is contemporaneous with the other constituents. According to Twelvetrees 6 the tin-bearing rock at-the Stony 

Ford mine, Tasmania, is a band of quartz-chlorite rock, charged with 

pink garnets, pyrite, some blende and chalcopyrite, and cassiterite. 

This is regarded as probably resulting from the contact metamorphism of slates and sandstone. At Pitkaranta, Finland, cassiterite occurs lining druses along a definite formation consisting of lime-silicate 

rock (" skarn ") that was produced in early pre-Cambrian time by 

the contact metamorphism of a limestone. The tin ore, however, is 

connected with the intrusion of the Rapikiwi granite of late preCambrian age, and is regarded as of contact-metamorphic origin, 

being due to magmatic solutions flowing along pervious contacts. 

This conception c of the genesis of the deposits would practically extend the term contact metamorphism to all deposits formed by juvenile 

waters of high temperature.

A remarkable similarity, both mineralogic and geologic, exists between these deposits and those of Schwarzenberg in Saxony ,<* where 

cassiterite occurs as a secondary impregnation in a salite-tremolite 

rock which at some distance from the contact encircles a granite mass. 

Dalmer, however, would include these deposits as a phase of contact 

metamorphism, using that term in its largest sense. From this review it would appear that stanniferous contact deposits of the Kristiania type are of extremely rare occurrence. One such deposit of 

commercial importance has yet to be found. This is certainly a surprising fact in view of the commonly accepted theory of the pneumatolytic origin of the majority of tin-ore deposits.

The Alaskan tin deposits exhibit a number of unique features. 

These include the association of cassiterite and arsenopyrite in a 

gangue of actinolite; the intergrowth of cassiterite with the rare calcium borosilicate, danburite; and the occurrence of an argentiferous 

wolframite-topaz lode containing galena and stannite. An opportunity has been afforded by the prevalence of limestones in the region 

to study the metasomatism connected with cassiterite veins in such 

rocks, and the study has shown that it resembles contact metamorphism in its dual aspect that is, metamorphism both with and 

without addition of material.

0 Waller, G. A., Tin-ore deposits of Mount TTeemskirk, .Govt. Geol. Office, Tasmania, 

1902, p. 4G.

6 Twelvetrees, W. 1-1., Tin mines of the Blue Tier, Govt. Geol. Office, Tasmania, 

1901, p. 29.

c Triistedt, 0., Die Erzlagerstatten von Pitkii.rn.nta am Ladoga-Sec : Bull. Coinm. Geol. de 

Finlande, No. 19, 1907, p. 816.

d Beck, R., Erzlagerstiitten, 2d ed., Berlin, 1903, p. 444.

54356 Bull. 358 08 5
66 TIN DEPOSITS OF SEWARD PENINSULA, ALASKA.

.In general, the tin shows the intimate association with fluorine 

and boron observed in most tin deposits the world over, an association that is emphasized by the discovery of the new iron-tin borates 

as essential constituents of lime-silicate contact rocks in the metamorphic aureoles of granitic intrusives.

PRACTICAL DEDUCTIONS.

Developments in this region have been sufficient to demonstrate, 

at least, that the granite-limestone contacts are not favorable places 

to hunt for commercial bodies of cassiterite ore. Although a great 

variety of contact-metamorphic rocks have been produced around the 

peripheries of the granites only a few that are stanniferous were 

found, and in only one was even a small amount of cassiterite detected. 

The bunchy and erratic character of contact-metamorphic ore bodies 

has been repeatedly emphasized in the preceding pages, and attention 

has been drawn to the difficulty of mining such deposits occurring 

along irregular contact surfaces. The same drawbacks pertain also 

to tin ore found in the tourmalinized borders of the granite stocks. 

In view of the widespread belief in Seward Peninsula that contact 

deposits are likely sources of tin ore, it is Avorth while to review here 

what is known of cassiterite contact-metamorphic deposits in other 

parts of the Avorld. There has been an actual production of tin from 

two only Pitkaranta in Finland and Schwarzenberg in Saxony 

and in amounts that are relatively small. The ore at these localities 

contains pyroxene and other minerals common in the Alaskan contactmetamorphic deposits, but the ore formation is confined to certain 

definite strata that were evidently favorable to the precipitation 

of the cassiterite. Some tin ore deposits of probable contact-metamorphic origin have been reported from the Stony Ford mine ft and 

St. Dizier mine 6 in Tasmania, but they have not entered the ranks of 

large producers.

Quartz porphyry dikes, locally known as lodes, or even as quartz 

veins, have been prospected to some extent, owing to the fact that the 

original discovery of lode tin in Alaska was made on a mineralized 

and altered dike of this character. The value of any such dike depends on the number of cassiterite stringers which it contains and the 

closeness with which they are spaced. Of itself, a quartz porphyry 

dike has no value. The unwelcome fact should be speedily realized 

that few of these dikes hold out any inducements whatever as prospective tin producers.

a Twelvetrees, W. H., Tin mines of the Blue Tier, Govt. Geol. Office, Tasmania, 1901, 

p. 29.

* Waller, G. A., Tin ore deposits of Mount Heernskirk, Govt. Geol. Office, Tasmania, 

1902, p. 46.
PKACTICAL DEDUCTIONS. 67

Most of the developments throughout the region are still in the 

prospecting stage, and many of the open cuts have not uncovered 

solid bed rock. No tonnage of tin-bearing rock, except at one place 

on Lost River, has yet been blocked out. Small holes in the ground, 

which give no clew to either dip, strike, or persistence of the ore.rock, 

are held at enormous figures. The great need of the country is less 

desultory prospecting. The slate area deserves more careful examination, as it is possible that valuable quartz veins may exist within its 

confines. The distribution of stream tin in Anikovik River and its 

tributaries proves that the stanniferous mineralization is not limited, 

to the region at the head of Buck Creek, but is more widely spread 

throughout the slate belt.

It is probable that a great granite mass, of which the stocks at 

Brooks Mountain, Tin Creek, and Cape Mountain are protruding 

bosses, underlies the entire York region. As shown by the prospects

of tin, tungsten, copper, lead, and zinc, and probably gold, this 

magma was capable of effecting a varied mineralization. As .the 

region becomes better known and more thoroughly prospected, additional discoveries will probably be made from time to time.

INDEX.

A.

Acknowledgments to those aiding. .......... 8

Actinolite-cassiterite rock, thin section of,

figure showing.. . ................ 34

Alaska Chief property, description of ........ 58-59

American Tin Mining Co., development by. . 62

Amphibole, occurrence and character of .... 20

Anikovik River, tin on. ..................... 67

Apatite, occurrence and character of ......... 23

Apophysis, banded, plate showing. ......... 46

Arsenopyrite, occurrence and character of. ... 18

Axinite, occurrence and character of ......... 21

Azurite, occurrence and character of ......... 19

B.

Banded vein, plate showing ................. 46

Bartels Tin Mining Co. , developments by. ... 40

Basalt dikes ................................ 15

Bay Creek, rocks near ....................... 13

Beaumont, Elie de, on stockscheider. ....... 37-38

Biotite, occurrence and character of ......... 22

Boron minerals, association of, with tin. ..... 16, 66

occurrence of ......................... 16, 41-42

Brooks, A. H. , discovery of tin by. . .......... 7

preface by. . ............................. 5

Brooks Mountain, description of ............. 41

economic geology of ..................... 42-44

geology of ......................... 13, 41-42, 63

limestone from, thin section of, figure

showing......................... 34

Buck Creek, geology at. ..................... 32-33

tin ores at ...................:........... 33-34

origin of ............................. 34-35

tin placers on......................... 61-62,63

C.

Calcite, occurrence and character of. ......... 19

California River, rocks at. ................... 13

Cape Mountain, description of. .............. 35

elevation of ............................. 10

geology at. .............................. 35-38

map of .................................. 36

rocks on ................................. 14

section of, figure showing ................ 37

tin ores at. .............................. 38-40

developments of ..................... 40-41

Carlson & Good win claim, developments on. . 41

Cassiterite, character of. ..................... 16-19

discovery of ........................... 7, 17, 26

occurrence of. ... 13, 16, 19, 24, 33-34, 38-40, 64-65

thin section of, figure showing. ......... 54

See also Tin.

Cassiterite Creek, fossils on .................. 13

geology on.. . ............................ 44

limestone on, view of.................... 12

region of, map of ........................ 45

tin on ................................... 44, 49

Page. 

Cassiterite lode, description of............... 49-50

^ developments on........................ 52

Cassiterite-quartz veins, occurrence and character of.......................... 55

Cerusite, occurrence and character of........ 19

Chalcopyrite, occurrence and character of.... 17

Chlorite, occurrence arid character of........ 22

Chondrodite, occurrence and character of.... 21

Collier, A. J., on Mississippian limestone..... 14

on Port Clarence limestone............... 12,13

on tin ores........................... 7-8,49,63

Contact-metamorphic rocks, tin in........... 29

See also Metamorphism. 

Cornwall, starmite in........................ 16

D.

Danburite. occurrence and character of...... 21

Daubrfe, A., on origin of tin.............. 60-61,64

Deville & Caron, experiments of.............. 61

Dikes, occurrence and character of.. 29/32-33,49-51 

Dolcoath lode, analysis of.................... 52

description of..................... 51-52,56,61

developments on........................ 52

Dolomite, occurrence and character of........ 19

Drainage, data on........................... 9-10

Ear Mountain, description of................ 25-26

geology of......................... 26-29,63,64

map of.................................. 27

minerals of.............................. 30

sections of, figures showing.............. 27,30

tin ores at, occurrence and character of. 30-32,64 

Eldorado Creek, tin on...................... 26

England, Cassiterite in...................... 64

Epidote, occurrence and character of......... 21

Eunson's shaft, section at, figure showing .... 30

F.

Fairhaven district, tin placers on............. 63

Feldspar, occurrence and character of......... 20

Field work, scope of......................... 8

Finland, Cassiterite .in...................:... 65,66

Fluorite, occurrence and character of......... 18

relation of, to tin ..................... 60-61,66

Fluorite-silicate rock, production of, plate

showing......................... 50

G.

Galena, occurrence and character of.......... 42-44

Garnet, occurrence and character of.......... 20

Geography, outline of......................... 9-10

Geology, description of...................... 10-15

Geology, economic, description of............ 24

Gold, occurrence of....................... 16-17,33

69
70 INDEX.

Page. 

Granite, association of, with tin.......... 1 16,24-25

occurrence and character of............ 15,

26-28,35-36,41-42,63,67 

Gravels, character and distribution of........ 15

gold and tin in........................... 15

G reenstones, occurrence and character of..... 15

Grouse Creek, tin placers in.................. 62

H.

Hematite, occurrence and character of. 

Hess, F. L., on tin....................

Hot Springs area, tin of...............

Hulsito, occurrence and character of... 

Humboldt Creek, tin on..............

I.

Ida Bell lode, description of.................. 50

Idaho claim, description of................... 59-60

Igneous rocks, occurrence and character of. 15,63-64 

Ilmenite, occurrence and character of......... 18

J.

Jupiter claim, developments on............. 52

K.

Kanauguk River, rocks on................... 13

Kaolin, occurrence and character of.......... 22

Knopf, Adolph, assignment of............... 5,8

Kougarok River, tin on..................... 63

L.

Lagoon Creek, section of, figure showing..... 37

Ledoux & Co., assay by...................... 44

Limestone, belt of, description of............. 13-15

seaming of, plates showing............... 50,54

thin section of, figure showing............ 34

Limonite, occurrence and character of........ 19

Lindgren, W., on contact metamorphism.... 57

Lost River, description of................... 44

Lost River region, cassiterite veins of 49-52,55-56 

description of........................... 44

geology of......................... 12-13,44-49

limestone of, veins in.................... 52-55

map of.................................. 45

metamorphism in....................... 56-57

tin ores in............................ 49-52,67

developments of..................... 52-56

wolframite veins of................ 55-56,57-58

Ludwigite, occurrence and character of...... 23

M.

Magnetite, occurrence and character of...... 18-19

Map, geologic, of Cape Mountain............ 36

of Ear Mountain........................ 27

of Seward Peninsula.................... 11

Marble, orbules in, plate showing........... 46

Metamorphism, occurrence and character of. 14,16,

24-25,28-29,36-38,45-49,56-57,63-64,65

Minerals, occurrence and character of.. 16-24,29,63

Molybdenite, occurrence and character of.... 17

Muscovite, occurrence and character of...... 22

N.

New South Wales, stannite in............... 16

Nickel, prospects for........................ 43

North Star Claim, developments on......... 40-41

0.

Olivine, occurrence and character of......... 20

Orbicular contact metamorphism, occurrence

and character of............ 45-49,63-64

plate showing........................... 44

Orbules, origin of, plate showing............ 46

plate showing........................... 44

supply duct for, plate showing.......... 4&

P.

Paigeite, analysis of......................... 23

occurrence and character of.............. 23,64

section of, figure showing................ 12-

Palazruk, limestone near, description of..... .13-15

Peluk Creek, tin on......................... 33-34

Phlogopite, occurrence and character of...... 14,22

Port Clarence limestone, character and distribution of................... 12-13,63

view of.................................. 12

Potato Mountain, geology at and near....... 32-33

Production, statistics of..................... 8

Prospecting, caution concerning.'............ 30

status of................................. 9

Pyrite, occurrence and character of.......... 18

Pyrolusite, occurrence and character of..... 19

Pyroxene, occurrence and character of....... 20

Pyrrhotite, occurrence and character of...... 17

QQuartz, occurrence and character of......... 18

Quartz-augiteporphyry, occurrence and character of.......................... 29

Quartz Cree1 :, geology on.................... 28

Quartz porphyry, occurrence and character

of................................ 32-33

tin in..'.........:.................... 33-34,66

R. 

Rapid River, tin on......................... 58-59

Rosenbusch, H., on contact metamorphism. 57 

Rutile, occurrence and character of......'.... 19

 ' S.

Saxony, cassiterite in........................ 65,66

Scapolite, occurrence and character of....... 20

Schaller, W. T., analyses by........... 23,52,54,58

Scheelite, occurrence and character of....... 24

Scope of work............................... 5

Seward Peninsula, geologic map of.......... 11

Skookum Creek, tin on...................... 62

Slates, character and distribution of......... 10-12

Smith, E. F., analyses by.................. 52

Sphalerite, occurrence and character of...... 17

Spinel, occurrence and character of.......... 18

Stannite, occurrence and character of........ 16,

18,24,57-58 

Sterling Creek, tin and gold on.............. 62

Stibnite, occurrence and character of........ 17

Sullivan, E. C., hornfels tested by........... 38

Surficial deposits, character and distribution

of................................ 15

Sutler Creek, geology on.................... 32

tin placers on........................... 61,62

T.

Tasmania, cassiterite in 

stannite in..........
INDEX. 71

Page. 

Teller Mission, rocks at...................... 13

Tin, association of, with granite.......... 16,24-25

contact metarnorphic deposits of....... 29-30,

38-40,56,06 

patchy character of................. 30,66

discovery of............................. 7

lodes of, description of................... 25-61

See also Ear Mountain; Buck Creek;

Cape Mountain; Lost River, 

minerals of.............................. 16

occurrence of, descriptions of............ 24-03

re'sume' and conclusions on.......... 63-60

ores of, origin of......................... 60-61

placers of............................. 24,61-63

production of........................... 8

prospecting for.......................... 9

See also Cassiterite. 

Tin City, geology near...................... 14

Tin Creek, apophysis on, plate showing...... 46

geology on........................... 44,45-40

metainorphism on.................. 45-49,63-64

minerals on............................. 47

tin ores on........................... 49,59-60

Tin region, maps of.......................'. 8,11

Topaz, occurrence and character of.......... 21

Topography, character of.................... 9-10

Tourmaline, occurrence and character of..... 21-22

Trustedt, 0., on orbicular metamorphism... 47 

Tungsten, occurrence of..................... 38

Tuttle Creek, geology on..................... 28

tin on................................... 31

U.

United States Alaska Tin Co., developments 

by..............................

V.

40

Veinlets, occurrence and character of........ 52-55

Vesuvianite, occurrence and character of..... 21,

42-43,47 

\rillage Creek, geology on.................... 35,41

Vogt, J. H. L., on origin of tin............... 61

Wolframite, occurrence and character of..... 24

Wolframite-quartz veins, occurrence and

character of..................... 55-56

wall rock adjoining, plate showing....... 56

Wolframite-topaz lode, occurrence and character of.......................... 57-58

topaz from, analysis of................... 58

Wollastonite, occurrence and character of.... 20

Y.

York, slates near, description of............. 10-12

York Mountains, rocks at............'....... 13

Zinc blende, occurrence and character of..... 42

Zinnwaldite, analysis of..................... 54

occurrence and character of........... 22,53-54

Zircon, occurrence and character of.......... 21

Zoisite, occurrence and character of.......... 21

o