Magnetite Deposits 

at Tuxedni Bay 

Alaska

GEOLOGICAL SURVEY BULLETIN 1024-D

Magnetite Deposits 

at Tuxedni Bay 

Alaska

By ARTHUR GRANTZ

MINERAL RESOURCES OF ALASKA

GEOLOGICAL SURVEY BULLETIN 1024-D

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1956
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C. - Price 15 cents (paper cover)
CONTENTS

	Page 

Abstract_____________________________________________________ 95

Introduetion______________________________________________________ 95

Geology. ________________________________________.______ 96

Bedded rocks__________-__-_-_________-___.__-__-_--__-_______ 98

Intrusive rocks________-__--_-_---___________________-_________ 98

Structure_____________________________________________________ 99

Magnetite deposits _______________________________________________ 99

Geology near the magnetite deposits_-_________-___-___-_-_-_____ 101

Western deposit_______________________________________________ 101

Eastern deposit- ____________________________________________ 104

Origin of the deposits---__--__-____-_-_----_-___--_-_--__--____ 105

Tenor and reserves__-________-_-_____________________-__-___ 106

Literature cited-______________________________________________ 106

ILLUSTRATIONS

Page

FIGURE 7. Index map of southwestern Alaska, showing location of area

studied______________________________ 96

8. Generalized geologic map of the Tuxedni Bay area, showing the

location of magnetite deposits___________________________ 97

9. Geologic map of the area near the magnetite deposits at Tuxedni

Bay___________________________________ 100

10. Generalized cross section of the western magnetite deposit as

exposed along sea eliff__________________________________ 103

TABLES

Page 

TABLE 1. Analyses of samples from the magnetite deposits at Tuxedni

Bay, Alaska___________________________________________ 104

2. Assays of samples from the magnetite deposits at Tuxedni Bay,

Alaska___________________________________ 105

in

364136 56

MINERAL RESOURCES OF ALASKA

MAGNETITE DEPOSITS AT TUXEDNI BAY, ALASKA

By ARTHUR GRANTZ

ABSTRACT

Magnetite deposits on an island in Tuxedni Bay occur in contact-metamorphosed volcanic and sedimentary rocks near their contact with a quartz-diorite 

batholith, which underlies large areas of the adjacent Aleutian Range. The deposits are pyrometasomatic in origin and probably are localized by northeasttrending fractures. The eastern deposit is a low-grade disseminated deposit 

occurring in hornfels. The western deposit occurs as a massive lens of magnetite 

and garnet with a marble hanging wall and a hornfels footwall. The western 

deposit may contain about 75 percent magnetite. The deposit is lenticular and 

can be followed along strike for 55 feet in outcrop. Magnetic effects indicate 

that it extends an additional 55 feet to the northeast, where it is concealed beneath a cover of soil and vegetation. Where it is exposed in a sea cliff, the 

deposit is 30 to 35 feet thick.

INTRODUCTION

Tuxedni Bay is on the west side of Cook Inlet about 120 miles southwest of Anchorage, Alaska. (See fig. 7.) The bay is very shallow 

except for Tuxedni Channel, which separates Chisik Island from the 

mainland. Tuxedni Channel is a secure anchorage. The land adjacent to tidal flats is rugged, and slopes are generally steep. Iliamna 

Volcano (altitude 10,016 feet) and Kedoubt Volcano (altitude 10,197 

feet) are both within 15 miles of Tuxedni Bay. Very heavy brush 

covers slopes to an altitude of about 2,000 feet. Spruce timber may 

be found along the shores of Cook Inlet from Chinitna Bay to Johnson 

River and northward from Tuxedni Bay; however, most of the spruce 

occurs below 750 feet altitude.

The magnetite deposits are located on an island near the head of 

Tuxedni Bay. The island is 91,4 miles N. 60°-65° W. of the northern 

tip of Chisik Island and may be reached by means of small boats or 

small seaplanes. However, trips should ordinarily be attempted only 

at high tide and under favorable weather conditions. Most of Tuxedni Bay is dry at low tide, and even at high tide most of the bay is 

shoal. Swift tidal currents and changeable winds at the head of

95
96 MINERAL RESOURCES OF ALASKA

Tuxedni Bay necessitate caution in landing by seaplane or visiting 

the area by small boat.

160° 150°

REDOUBT VOLCANO*

/ Tuxedni Bay 

ILIAMNA VOLCANO #

100 0 

i i I

200 Miles

FIGURE 7. Index map of southwestern Alaska, showing location of area studied.

The field investigation upon which this report is based was made 

from September 8 through September 10, 1951. Its purpose was to 

determine the nature and possible extent of a reported magnetite 

deposit at Tuxedni Bay. Mapping of the island, upon which the previously reported western deposit is located, revealed the presence of 

the eastern deposit. The interior of the island was not mapped.

The present investigation was made while the writer was assisting 

John K. Hartsock in a study of the petroleum possibilities of the area 

between Iniskin and Tuxedni Bays. The writer was assisted during 

the field work in the vicinity of the magnetite deposits by R. Werner 

Juhle and drew heavily from Juhle's report on Iliamna Volcano and 

its basement* for information on the geology of upper Tuxedni Bay. 

The following geologic description of the Tuxedni Bay area is in 

large part abstracted from Juhle's report, and figure 8 of the present 

report was adapted from part of plate 1 of Juhle's report.

GEOLOGY

The rocks underlying the Tuxedni Bay area are predominantly of 

Mesozoic age. Sedimentary rocks of Late Triassic age are the oldest

1 Juhle, R. W., 1955, Iliamna volcano and its basement: U. S. Geol. Survey open-file 

preliminary report.
MAGNETITE DEPOSITS AT TUXEDNI BAT, ALASKA 97

that have been recognized in the area. They are overlain by volcanic 

rocks of Early Jurassic age and these, in turn, are overlain by marine 

sedimentary rocks of Middle and Late Jurassic age. Continental 

sedimentary rocks of early Tertiary age, and volcanic rocks of later 

Tertiary and probably Quaternary age, overlie the Mesozoic rocks 

both to the north and to the south of Tuxedni Bay. Tertiary rocks 

are not shown in figure 8 because their extent in the map area is small 

and they do not crop out within 5 miles of the magnetite deposits.

Siltstone. sandstone, 

and conglomerate

Volcanic cocks, locally 

altered to hornfels

FIGURE 8. Generalized geologic map of the Tuxedni Bay area, showing the location of

magnetite deposits.

Quartz diorite and granophyric quartz monzonite, probably of Middle Jurassic age, intruded the Lower Jurassic and older rocks. 

Hornfels, marble, and (locally) tactite were formed near the borders 

of the intrusives. With the possible exception of the very earliest 

beds, the Middle Jurassic and younger bedded rocks have not been
98 ' MINERAL RESOURCES OF ALASKA

intruded by granitic rocks. Where tho1, sedimentary rocks of Middle 

Jurassic or younger age have been intruded by mafic dikes, they are 

baked only where they are immediately adjacent to the dike borders.

BEDDED BOCKS

Argillite, limestone, and interbedded andesitic lava flows, probably 

of Late Triassic age, crop out at the head of Tuxedni Bay. The base 

of the Upper Triassic section has not been recognized; however, its 

position may now be occupied by the quartz diorite of the adjacent 

Aleutian Range.

The Upper Triassic rocks grade conformably upward into volcanic 

rocks of probable Early Jurassic age. The contact between the two 

series of rocks has been arbitrarily placed at the base of the first thick 

tuff bed above the argillite beds. The lower part of the Lower Jurassic volcanic sequence consists chiefly of andesitic breccias and flows, 

but it contains some interbedded argillite. The upper part of the 

Lower Jurassic volcanic sequence is composed primarily of water-laid 

rocks of pyroclastic origin and tuffaceous marine sedimentary rocks. 

Fossiliferous marine sedimentary rocks of Middle and Late Jurassic 

age overlie the Lower Jurassic volcanic rocks with apparent conformity. Siltstone predominates, but many thick units of graywacke 

sandstone and arkosic sandstone and some thick beds of conglomerate 

occur.

Continental sedimentary rocks of early Tertiary age overlie the 

Upper Jurassic marine sedimentary rocks with low angular unconformity. These rocks are exposed along the northwest shore of Cook 

Inlet, both north and south of Tuxedni Bay.

Between Chinitna and Tuxedni Bays and north of Tuxedni Bay, 

mafic flows and associated pyroclastic rocks in many places overlie 

the truncated surfaces of the folded and faulted Triassic and Jurassic 

rocks. Both basaltic andesite of middle or late Tertiary age and 

extensive andesitic flows and breccias of late Tertiary or Quaternary 

age were reported by Juhle. The basaltic andesite crops out in limited 

areas immediately west of Squarehead Cove on the north shore of 

Tuxedni Bay and 2% miles west of Fossil Point on the south shore 

of Tuxedni Bay. Both Iliamna Volcano, which lies between Chinitna 

and Tuxedni Bays, and Redoubt Volcano, which lies north of Tuxedni 

Bay, have been active in historic times (Coats, 1952, p. 38 and table 2).

INTRUSIVE BOCKS

The bedded rocks have been intruded by the biotite-hornblendequartz diorite batholith of the Aleutian Range. This intrusive was 

emplaced during Middle or early Late Jurassic time (possibly as early
MAGNETITE DEPOSITS AT TUXEDNI BAY, ALASKA 99

as early Middle Jurassic time). Dikes and stocks of pink granophyric 

quartz monzonite have intruded the quartz diorite and, in places, the 

bedded rocks near the borders of the quartz diorite. The monzonite 

was emplaced after most the quartz diorite had solidified and may 

have been a late differentiate of the quartz diorite magma.

Mafic dikes, sills, and pipes intrude the Upper Jurassic and older 

rocks at many places in the Tuxedni Bay area. With the exception of 

dikes that are related to the diorite and monzonite intrusives, only 

those dikes and pipes that are related to the flows of Iliamna Volcano 

and vicinity can be dated at this time.

STRUCTURE

Most fold axes and major fractures in the bedded rocks strike 

within 10° of N. 35° E. in conformity with the regional strike of the 

eastern border of the quartz diorite batholith. This structural trend 

is also present in the vicinity of the magnetite deposits, both of which 

strike from N. 35° E. to N. 45° E. and are parallel to the predominant 

strike of the bedding, fractures, dikes, and replacement zones in the 

marble and volcanic host rocks.

MAGNETITE DEPOSITS

Two deposits of magnetite are exposed on a small island of metamorphosed volcanic rock at the edge of the tidal marsh at the north 

shore of Tuxedni Bay. (See figs. 8 and 9.) The eastern deposit, magnetite disseminated in hornf els, is exposed almost down to marsh level 

in the hillside on the northern shore of the island. The western deposit, a massive lens of magnetite and garnet with a marble hanging 

wall and a hornfels footwall, is exposed in a sea cliff at the extreme 

western end of the island.

The western deposit was staked and samples were taken as early 

as the summer of 1916. However, the only development of this deposit still apparent in 1951 was a prospect hole about 5 feet wide and 

3 to 5 feet high, which had been driven about 13 feet horizontally 

beyond the end of a sea cave. No development of the eastern deposit 

was noted.

The western deposit of this report is the same one referred to in 

previous reports on magnetite at Tuxedni Bay (Martin, 1920, p. 35; 

Brooks, 1921, p. 42; Moffit, 1927, p. 55-56 and pi. 4). Although the 

location of the magnetite deposit as shown by Moffit (1927, pi. 4) is 

not identical with the location given in this report, the two descriptions refer to the same deposit. The deposit was not visited by Moffit, 

but it was described to him by Roy A. Trachsel, of Anchorage, who 

was the owner of the deposit at that time.
100 MINERAL RESOURCES OF ALASKA

**./ ' /

- " - A Geology by A. Grantz, 1951 

\-Disseminated B Geology » *"""«le, 1952- 

magnetite --fc=_

Massive 

magnetite '/* 

deposit

Area of garnet 

 rock dikes not 

individually mapped

Volcanic rocks altered 

to hornfels, and possibly argillite

Contact, dashed where 

approximately located

Massive magnetite deposit, showing 

dip. Disseminated magnetite 

deposit, horizontal extent not 

known

Inferred or indefinite 

contact. Question marks

Fault showing dip; 

dashed where approximately located Garnet rock dikes, horizontal 

extent not known

FIGUKB 9. Geologic map of the area near the magnetite deposits at Tuxedni Bay.
MAGNETITE DEPOSITS AT TUXEDNI BAT, ALASKA 101 

GEOLOGY NEAR THE MAGNETITE DEPOSITS

Contact metamorphism has altered the rocks in the vicinity of the 

magnetite deposits to hornfels and marble (fig. 9). Garnet rock occurs locally, and small bodies of magnetite or garnet-magnetite rock 

were found at two places. Evidently the metamorphism is related 

to the nearby quartz diorite batholith. The outcrop pattern of the 

quartz diorite indicates that the diorite underlies the magnetite deposits at shallow depth.

Although the original nature of the rocks has been obscured by 

metamorphism, most of the strata on the island are believed to be of 

volcanic origin. However, in a northeast-trending zone which lies 

roughly between the two deposits, some argillite may be interbedded 

with the volcanic rocks. Adjacent to the western deposit is a small 

area of white medium-grained marble that has been completely recrystallized by contact metamorphism. The recrystallized grains 

range from 2 to 6 millimeters in diameter, but streaks of very coarse 

marble contain grains as much as 2.5 centimeters in diameter. Magmatic additions to the marble consist chiefly of a few massive dikes of 

garnet rock and a few small veinlets of magnetite. Streaks of welldisseminated small garnet crystals and of fine-grained magnetite occur in the marble in a few places. Juhle 2 reports that idocrase is 

present in the marble.

WESTERN DEPOSIT

The western deposit is in a skarn zone and consists of a massive lens 

of high-grade magnetite, and massive garnet rock that in places 

contains disseminated magnetite. Exposures are limited to a high sea 

cliff, two sea caves cut into the sea cliff, and a short prospect hole 

driven horizontally beyond the end of the southern sea cave. There 

are no surface exposures inland from the sea cliff in the vicinity of 

the deposit.

The garnet-magnetite body is irregular in shape. It strikes approximately N. 40° E., and the dip averages about 40° SE. The body is 

about 30 to 35 feet thick and is exposed in the sea cliff for about 55 

feet in the dip direction. The deposit may be followed for 55 feet 

along the strike in the sea caves and the prospect hole. A reconnaissance dip-needle traverse indicated that the magnetic anomalies, 

caused by the presence of the magnetite, end about 110 feet back 

from the cliff face. The contact of the deposit with the marble hanging wall is sharply defined. Very little disseminated magnetite or 

garnet occurs in the marble. The contact of the deposit with the hornfels footwall is less well defined because the hornfels near the contact

1 Op. cit.
102 MINERAL RESOURCES OF ALASKA

contains disseminated garnet in places. Contacts between the garnet 

rock and the magnetite, although gradational in places, are locally 

well defined. A dike of garnet cuts the massive magnetite with sharp 

contacts.

The contact between the marble of the hanging wall and the hornfels of the footwall of the western deposit probably was formed prior 

to, or contemporaneously with, the mineralization. Magnetite and 

garnet rock has completely replaced the marble-hornfels contact in 

the exposures at the western deposit, and therefore direct evidence 

concerning the nature of the marble-hornfels contact was not observed. 

The proximity of the marble to the probable metavolcanic hornfels, 

however, suggests the possibility of a fault contact, although no faults 

were observed either in the western deposit or between the western 

deposit and the country rocks. Such a fault could have provided a 

channelway for the mineralizing solutions and could account for the 

localization of the deposit in the contact zone between the marble and 

the hornfels. The attitude of the deposit is similar to the predominant attitude of the bedding and fractures in the area as well as to 

the attitude of the eastern deposit.

Evidence for postmineral movement in the marble is afforded by 

a very thin vein of magnetite that occurs in the marble about 50 feet 

south of the deposit. This vein has been broken, and the pieces have 

been rotated the marble having flowed around the broken pieces.

The marble in the hanging wall of the western deposit apparently 

influenced the composition of the garnet deposited in or adjacent to 

it (see p. 103) and may have influenced the deposition of the magnetite. 

The magnetite in the western deposit is predominantly massive, 

whereas in the eastern deposit, which is entirely in hornfels, the magnetite is disseminated.

Magnetite is the only ore mineral present, although traces of pyrite 

and probably chalcopyrite can be found locally. No other sulfides 

were noted, and chemical analyses of samples taken from the deposit 

show that the sulfur content is low. Copper and gold are absent or are 

present only in traces.

The chief gangue mineral is garnet. Traces of other silicate minerals, including chlorite, clinopyroxene (diopside-hedenbergite), and 

probable idocrase occur in the garnet-magnetite body. The garnet 

occurs as a minor constituent in the massive magnetite lens and as a 

massive dike cutting the lens. Large masses of garnet rock lie adjacent to the areas of massive magnetite (fig. 10), and locally these 

masses contain as much as 50 percent of disseminated magnetite. In 

other places, however, the garnet rock adjacent to the massive magnetite contains only traces of other silicates or magnetite. An irregular 

mass of garnet rock lies in the marble hanging wall immediately
MAGNETITE DEPOSITS AT TUXEDNI BAY, ALASKA 103

adjacent to the lens of massive magnetite. Where associated with 

the magnetite, the garnet is grayish to moderate yellowish green and 

contains between 70 and 90 percent andradite (the calcium-iron garnet) . Where associated with limestone, the garnet tends to be grayish 

or yellowish brown and contains between 60 and 70 percent andradite. 

The remaining percentage of both varieties of garnet is composed of 

grossularite (the calcium-aluminum garnet) and probably several 

percent of almandite (the iron-aluminum garnet). Qualitative 

chemical tests indicated that some of the manganese in the magnetite 

deposits (table 1) occurs in the associated garnet.

Data taken from outcrops within 10 feet-en 

both sides of section line 

->

&I5°E

NI5°W

Data taken from outcrops 

within 30 feet on east 

side of section line

GROUND 

Garnet with

disseminated 

magnetite

Foliation (possibly 

bedding)

Brown-weathering 

marble

Massive magnetite with some 

garnet gangue

Marble with 

disseminated 

magnetite

Massive magnetite 

with some garnet 

\ gangue 

Surface\ of tidal marsh

EXPLANATION

Contact, dashed where 

projected across 

sea cave

Gradational contact

__7___7__

Doubtful contact

Geology by Arthur Grantz 

40 Feet

HORIZONTAL AND VERTICAL SCALE

Note: Section not normal to strike. Strike of section 

approximately N15 W. Strike of deposit approximately N 40 E. Outcrop data projected at right 

angle to plane of section. Location of the deposit is shown on figures 8 and 9

FIGURE 10. Generalized cross section of the western magnetite deposit as exposed along

sea cliff.

Grab samples believed to be representative of high-, medium-, and 

low-grade portions of the massive magnetite lens of the western deposit were collected for assay and chemical analysis during the present 

investigation. (See tables 1 and 2.) Also shown in these tables are 

two analyses and an assay of samples from the deposit that were 

furnished to the Geological Survey by Roy A. Trachsel, of Anchorage, 

and that were published by Moffit in 1927. All the analyzed samples from the western magnetite deposit contain 50 percent or more 

of iron as metal, with no more than a few percent of the iron occurring
104 MINERAL RESOURCES OF ALASKA

TABLE 1. Analyses of samples from the magnetite deposits at Tuxedni Bay,

Alaska

Determinations

Loss on ignition. --_-_-__.-___.... ......

SiOj _

AhOj 

FeO 

FesCU 

MgO ... 

CaO.. _ .. .. ... ...

Na2 0 

KjO 

TiOi......... .............................

PiO.. .. .. ... ...

P.. 

MnO 

Mn 

CrjOj 

&.. .......................................

Fe ... 

Totals ______ _ ___ ....

A

11.4

1.9

17.0

53.2

1.6

11.0

.14

.08

.31

.64

 (.28)

1.2

M.8)

<.02

.98

"(50.4)

99.47

B

7.3

1.8

21.4

58.8

2 2

6.2

.14

.08

.42

.11

 (.06)

1.7

"(1.3)

<.02

.03

 (67.7)

100.2

O

1.4

.72

25.1

67.8

1.4

1.4

.15

.11

.44

.02

 (.009)

1.6

b(1.2)

<.02

.20

"(66.9)

100. 36

D

.11

4.34

1.20

88.89

1.12

1.55

.05

2.06

.59

(64. 27)

99.91

E

2.80

.064

.11

67.82

F

37.4

5.7

9.1

26.7

1.4

19.4

.15

.08

.38

.OS

 (.036)

.40

K.31)

<02

.01

«(25.8)

100.82

 Calculated from P2Os. *> Calculated from MnO. 1 Calculated from FeO and Fc20s.

NOTE. Sample A: Representative sample of low-grade portions of the massive magnetite lens of the 

western magnetite deposit. Collected by Arthur Grantz. Analysis by S. M. Berthold and E. A. Nygaard, 

II. S. Geological Survey.

Sample B: Representative sample of medium-grade portions of the massive magnetite lens of the western magnetite deposit. Collected by Arthur Grantz. Analysis by S. M. Berthold and E. A. Nygaard.

Sample C: Representative sample of high-grade portions of the massive magnetite lens of the western 

magnetite deposit. Collected by Arthur Grantz. Analysis by S. M. Berthold and E. A. Nygaard.

Sample D: Sample probably from the massive magnetite lens of the western magnetite deposit. Described by F. H. Mofflt, II. S. Geol. Survey Bull. 789, p. 55-56, 1927. Analysis by Kansas City Testing 

Laboratory, Kansas City, Mo., October 14,1920.

Sample E: Sample probably from the massive magnetite lens of the western magnetite deposit. Described by F. H. Mofflt, U. S. Oeol. Survey Bull. 789, p. 55-56,1927. Analysis by Abbot A. Hanks, San 

Francisco, Calif., February 25,1921.

Sample F: Sample from the eastern magnetite deposit. Collected by Arthur Grantz. Analysis by 

S. M. Berthold and E. A. Nygaard.

in garnet or possibly other silicate minerals. However, for use as 

iron ore all samples contain an undesirable amount of sulfur, and four 

of the samples contain an undesirable amount of phosphorus. The 

highest phosphorus content and the highest sulfur content are in the 

same sample.

EASTEBN DEPOSIT

This deposit consists of magnetite disseminated in hornfels that 

was probably derived from volcanic rocks. The magnetite occurs in 

two parallel zones. The upper zone is about 30 feet thick and is separated from a lower zone by 20 feet of country rock. The lower zone 

is at least 10 feet thick, but its base is not exposed. The strike length 

of the deposit is not known. The mineralized zones are exposed only 

in a small area near the shore of the island. To the northeast the
MAGNETITE DEPOSITS AT TUXEDNI BAY, ALASKA 105

deposit is covered by tidal marsh, and to the southwest it is covered 

by soil and heavy vegetation. The mineralized zones strike approximately K 35°-45° E. and dip approximately 45° SE. Bedding and 

fractures in the vicinity of the deposit have simliar trends and may 

have controlled the locus of mineralization.

TABLE 2. Assays of samples from the magnetite deposits at Tuxedni Bay, Alaska

Determinations A

0

65

B

0

30

C

0

70

D E

0.01

0.40

F

0

5

Note. Sample A: Representative sample of low-grade portions of the massive magnetic lens of the western 

magnetite deposit. Collected by Arthur Grantz. Analysis by Hy Almond, U. S. Geological Survey.

Sample B: Represeatative sample of medium-grade portions of the massive magnetite lens of the western 

magnetite deposit. Collected by Arthur Qrantz. Analysis by Hy Almond.

Sample C: Representative sample of high-grade portions of the massive magnetite lens of the western 

magnetite deposit. Collected by Arthur Qrantz. Analysis by Hy Almond.

Sample D: Composite of eight samples from the massive magaetite lens of the western magnetite deposit. 

Collected by Arthur Qrantz. Analysis by Dwight L. Skinner, U. 8. Geological Survey.

Sample E: Sample probably from the massive magnetite lens of the western magnetite deposit. Described by F. H. Mofflt, U. S. Qeol. Survey Bull. 789, p. 55-56, 1927. Analysis by Bogardus Testing 

Laboratories, Seattle, Wash., July 25,1916.

Sample F: Sample from the eastern magnetite deposit. Collected by Arthur Qrantz. Analysis by Hy 

Almond.

The magnetite in the eastern deposit is disseminated in a gangue 

in which garnet is the most abundant mineral; epidpte, diopside, chlorite, plagioclase, and other silicates also are present. No sulfides were 

noted. An analysis of the richest sample obtained from the upper 

zone of the deposit contained 25.8 percent of iron and 0.01 percent of 

sulfur. However, about one-fifth of the iron may occur in silicate 

minerals. The exposed portion of the lower zone of the deposit contains even less magnetite; samples from this zone were not analyzed. 

The magnetite content of the deposit as a whole was estimated in the 

field to be about 10 to 20 percent.

ORIGIN OF THE DEPOSITS

The association of the magnetite with andradite-rich garnet, epidote, 

diopside, chlorite, plagioclase, idocrase, and a trace of iron and copper 

sulfides and its occurrence in irregular replacement zones and 

lenses is characteristic of magnetite deposits of pyrometasomatic 

origin. A large quartz diorite batholith crops out within 1,000 feet 

of the deposits and probably underlies the deposits at shallow depth. 

The quartz diorite has an iron content of about 6 percent, and the deposits are probably genetically related to it; the granophyric quartz 

monzonite, which also outcrops in the area, has an iron content of only 

about 0.73 percent.3

3 Op. cit.
106 MINERAL RESOURCES OF ALASKA 

TENOR AND RESERVES

Although, as stated above, the field-estimated grade of the exposed 

parts of the eastern deposit is about 10 to 20 percent of magnetite, a 

selected sample from this deposit contained 25.8 percent of iron. Each 

reported analysis of the western deposit, however, shows more than 50 

percent of iron, with one analysis showing 67.8 percent. Sulfur and 

phosphorus, which were shown by the analyses to be present in all 

the samples from the western deposit, occur in most of the samples 

in amounts that are undesirable in iron ore. Samples from both deposits were assayed for copper, silver, and gold by the U. S. Geological 

Survey, but these metals were present only in traces or were absent. 

No sample of the deposit or of the adjacent country rock produced 

significant radioactivity, as measured with a portable field-type Geiger 

counter. Massive garnet, as found in the western deposit and elsewhere on the island, is not of the quality generally preferred for 

abrasive garnet.

The small outcrop area of marble (the presence of which may have 

controlled deposition of massive magnetite of minable grade), the 

probable presence of quartz diorite at shallow depth, and the tendency 

for such deposits to be irregular and lenticular in shape, would all have 

a bearing on an estimate of the size of the western deposit. The 

depth of mud fill beneath the tidal marsh, which covers the western 

deposit beyond the sea cliff, and the slope of bedrock beneath the mud 

fill might determine how far the deposit extends to the southwest. 

The reserves of magnetite that may be inferred to be present in the 

western deposit above the level of the tidal marsh, and within the 

limits of outcrop data, are only several thousand tons.

A reconnaissance examination of the shore of the peninsula that lies 

immediately east of the island failed to find evidence of mineralization 

other than sparsely disseminated pyrite. The area of argillite and 

limestone of Late Triassic age that occurs west of the island, across 

Tuxedni Bay, was briefly visited in 1951, but no evidence of mineralization was found.

LITERATURE CITED

Brooks, A. H., 1921, The future of Alaska mining: U. S. Geol. Survey Bull. 714r-A. 

Coats, R. R., 1952, Volcanic activity in the Aleutian arc: U. S. Geol. Survey Bull.

974-B. 

Martin, G. C., 1920, The Alaskan mining industry in 1918: U. S. Geol. Survey

Bull. 712-A. 

Moffit, F. H., 1927, The Iniskin-Chinitna Peninsula and the Snug Harbor district,

Alaska: U. S. Geol. Survey Bull. 789.

U. S. GOVERNMENT PRINTING OFFICE: I9SC