Pyrite Deposits At 

Horseshoe Bay 

Latouche Island

GEOLOGICAL SURVEY BULLETIN 1024-E

Pyrite Deposits At 

Horseshoe Bay 

Latouche Island 

Alaska

By FRANCIS A. STEJER

MINERAL RESOURCES OF ALASKA

GEOLOGICAL SURVEY BULLETIN 1024-E

A description of sulfide-bearing 

deposits in the southwestern part 

of Prince ff^illiam Sound area

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

Fred A. Seaton, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D. C. - Price 45 cents (paper cover)
CONTENTS

Page

Abstract___________________________________ 107

Introduction ___________ ________ _______________ 107

Geology_________________________________________ 110

Structure__________________________________,_ 112

Pyrite deposits._____ ___________ _ _ 113

History and development_____________________ ___________ 113

Description... ________________ __ __. _ _______ 113

Duchess claim__________________________________________ 116

Duke, Iron Mountain No. 6 and Amended Iron Mountain No. 4

claims_______________________________________________ 119

Economic eonsiderations______________________________ 121

Literature eitfed,.____________________________ 122

ILLUSTRATIONS

PLATE 12. Geologic maps and sections of 400 and 500 ft. levels Duchess

claim, Horseshoe Bay, Latouche Island, Alaska.___ ___ In pocket

Page 

FIGURE 11. Index map of Prince William Sound area, Alaska showing

location of Horseshoe Bay____---_-------__-------_---_ 108

12. Topographic map of part of Latouche Island showing location

of surface workings in vicinity of Horseshoe Bay.. _________ 109

13. Sketch map of Prospect adit No. 1, Duke claim.___________ 120

14. Sketch map of Duke shaft, Amended Iron Mountain No. 4

claim_______________________________________________ 121

in

MINERAL RESOURCES OF ALASKA

PYRITE DEPOSITS AT HORSESHOE BAY, LATOUGHE 

ISLAND, ALASKA

By FRANCIS A. STEJER

ABSTRACT

The sulfide-bearing zone at Horseshoe Bay, on Latouche Island in the southwest 

part of Prince William Sound, Alaska, was explored in the early days as a copper 

prospect, and although large bodies of copper-bearing pyritic "ore" were found on 

the Duchess, Duke, and Iron Mountain No. 4 claims, a mine was never brought 

into production. The property has been idle and deserted since November 1916. 

The deposits are currently (1955) of interest as a possible source for sulfur.

The mineral deposits occur on the northwest flank of a northeastward plunging 

anticlinorium in a thick sequence of marine sedimentary rocks which are considered to be part of the Orca Group of late Mesozoic(?) age. Replacement 

lenses of massive and disseminated sulfides, conforming to the stratification of the 

enclosing gray\vacke-type sedimentary rocks, occur in a mineralized zone whose 

maximum extent is not yet known because of limited mine development and 

exploratory work. As exposed in present workings, lenses of massive sulfides, 

ranging up to 60 feet in width and at least 490 feet in length, are apparently 

longer and wider at depth. This feature suggests that the present land surface 

may be near the top of the mineralized zone.

Pyrite is the principal sulfide mineral and at places makes up essentially 100 

percent of the "ore." Other primary sulfide minerals are chalcopyrite, cubanite, 

sphalerite, pyrrhotite, arsenopyrite, and galena. Gold and silver are present in 

small amounts, but the form in which these metals occur is not known. The 

paragenetic sequence is pyrite, arsenopyrite, sphalerite, cubanite, chalcopyrite, 

and galena. Pyrrhotite was not present in the polished specimens examined but 

from field relationships appears to have been deposited at about the same time as 

the sphalerite.

Spectrographic analyses of selected specimens did not reveal economically 

important quantities of minor or trace elements.

INTRODUCTION

Horseshoe Bay (see fig. 11) is in the southwest part of Prince William 

Sound at latitude GO0*)!' N and longitude 147°57' W. It is on the 

west side of Latouche Island about 2 miles southwest of the old mining 

camp of Latouche. Pyrite deposits crop out about one-half mile 

from the head of the bay (see fig. 12) and in the lower part of a broad 

basinlike valley at altitudes between 125 feet and 650 feet. The 

valley, which is a mile wide and 2 miles long, is open to the sea on the

107
108 MINERAL RESOURCES OF ALASKA

148° 147° __ 146°

148° 147° 146° 

FIOUBB 11. Index map of the Prince William Sound area, Alaska, showing location of Horseshoe Bay.

northwest side and is bordered on the northeast, southeast and southwest by a high sharp-crested ridge that reaches a maximum altitude of 

2,255 feet. The area can easily be reached by small boat or float 

plane from Cordova or other towns in this part of Alaska. The 

property at Horseshoe Bay has been idle and deserted since November 

of 1916, and all of the surface improvements are in ruins.

The climate is cool-temperate and wet and a heavy growth of vegetation mantles much of the area. The annual mean temperature is 

about 42° F with the summer highs in the low eighties and the winter 

lows near zero. The annual precipitation is about 184 inches. The 

annual snowfall is about 153 inches. (Climatic Atlas for Alaska, 

Sept. 1943). Muskeg covers much of the lowland area along the coast 

and in the valley bottom. Alder, willow, berry-bushes, and other 

shrubs form dense thickets at many places along the streams and on 

the lower hill slopes. The higher mountain slopes commonly are well 

covered by a thick mat of heather, alpine flowers, grasses, and herbs.
PYRITE DEPOSITS, SORSESHOE BAY, ALASKA 

147° 58'_______ __L47°_57A_ 147° 56'__________147° 55'

109

Duchess Mine 

I 500' level 

2.400' level

3. Prospect odit on Duke claim

4. Creek exposure on Duke claim

5. Adit on Iron Mountain No. 6 

claim

6. Duke shaft on Amended Iron 

Mountain No. 4 claim

Contour interval 100 feet 

Datum is mean sea level

100'

Modified from plate 12 ULS. Geological 

.Survey Bull,4.43 ,i- '"^

FIGUBE 12. Topographic map of part of Latouche Island showing location of surface workings in the

vicinity of Horseshoe Bay.

Spruce and hemlock grow to moderate size along the coast and the 

lower courses of the streams. ** << ***» *i +*

The present investigation was undertaken in 1952 when, because of 

a world shortage of sulfur, it was deemed advisable to learn more about 

deposits of pyrite that could be considered potential sources for sulfur. 

The deposits at Horseshoe Bay were selected for study because 

Latouche Island h known to be well mineralized and its geology has
110 MINERAL RESOURCES OF ALASKA

been more thoroughly studied than any adjacent area. The deposits 

at Horseshoe Bay are fairly large and very similar to the deposits at 

the famous Beatson mine, 2 miles northeast of Horseshoe Bay. The 

Beatson mine was the largest copper producer in Prince William Sound 

area prior to its shutdown in 1934.

The present report is based largely upon field studies made during 

July and August 1952 when the author assisted by K. O. Linscott and 

R. M. Corn, geologic field assistants, studied the geology of Latouche 

Island and made detailed examinations of the accessible mine workings 

at Horseshoe Bay.

During the course of the field work the U. S. Forest Service helped 

the Geological Survey in many ways particularly by loaning equipment and providing transportation. M. F. Butler, president, EADCO 

Incorporated, has been very helpful during the preparation of this 

report by furnishing valuable information about inaccessible parts of 

mine workings.

GEOLOGT

The rocks at Horseshoe Bay are generally considered to be a part 

of the Orca group of late Mesozoic (?) age and in this vicinity consist 

of a thick marine sequence of interbedded graywacke and slate with 

occasional beds of conglomerate, limestone, and chert. The beds 

range in thickness from a fraction of an inch to as much as 40 feet, 

and the color is commonly a shade of medium light to medium dark 

gray. The beds generally are lacking in distinctive features by which 

they may be recognized and are difficult to correlate.

No attempt has been made in this report to distinguish by name the 

many gradational phases that exist among the sedimentary rocks that 

make up this sequence, and the terms such as graywacke and slate 

are used in the broadest sense.

The major part of the sequence consists of graywacke type sedimentary rocks which are typically hard, compact, dull-gray rocks. 

These rocks are composed largely of grains of quartz and feldspar and 

detritus from fine-grained rocks such as chert, quartzite, and argillite 

or slate in a matrix of finely comminuted rock and mineral fragments 

with calcareous or siliceous or sometimes ferruginous cements. The 

rocks have an arenaceous texture in which the grains commonly range 

in size from about 1.0 mm (coarse sand size) to less than 0.062 mm 

(clay size). Some of the graywacke units in the Orca group approach 

quartzite in texture, structure, and composition, and other units 

because of abundant feldspar grains perhaps could be termed arkoses. 

A common type of calcareous graywacke, such as occurs in Sleepy 

Bay at the northeast end of Latouche Island, effervesces freely in 

cold dilute hydrochloric acid, and is seen under the microscope to 

consist of about 25 percent quartz, 10 percent feldspar, 12 percent
PYRITE DEPOSITS, HORSESHOE BAY, ALASKA HI

siltstone or argillite and chert fragments, 3 percent muscovite mica, 

and 50 percent carbonate matrix of which about 30 percent is finely 

comminuted rock and mineral fragments. Several graywacke beds hi 

this vicinity contain zones several inches thick that are essentially 

limestone. A siliceous type of graywacke from the southwest end of 

the island, viewed under the microscope, was seen to consist of about 

40 percent quartz, 10 percent feldspar, 15 percent chert, quartzifce 

and slate fragments, 10 percent muscovite mica, and 25 percent siliceous cement with which there is a small amount of finely comminuted 

rock and mineral detritus and sericite mica. Some of the slate beds 

are calcareous, but typically the slate is hard, compact, and siliceous. 

Beds of light-gray siliceous limestone and a little chert are present in 

the sedimentary sequence at the Duke shaft (Johnson, 1918, p. 207). 

There are two distinctive kinds of conglomerate in the area. One 

consists of well-rounded pebbles and cobbles of sedimentary and 

igneous rocks cemented by a matrix of fine-grained medium-dark-gray 

graywacke. The other consists of scattered angular to subangular 

pieces of slaty rock imbedded in a matrix of graywacke. The fragments are generally about one-half inch in thickness and range in 

length from less than 1 inch to about 6 inches. Many of the graywacke beds contain thin zones of this intraformational conglomerate.

Igneous rocks have not been found in the vicinity of the mineral 

deposits at Horseshoe Bay, and the nearest of the three known occurrences on Lafcouche Island is at the Beatson mine where Bateman 

(1924, p. 346) reported an "unusual lamprophyre dike." Batemau 

does not discuss the petrology of this dike nor does he give its dimensions. The other igneous bodies on the island consist of a small 

mass of serpentinized diallage gabbro on the northeast shore (Grant 

and Higgins, 1910, p. 49-50) and a few small mafic dikes on the 

southeast shore. The gabbro and the mafic dikes appear to be very 

closely related. They have essentially the same composition, and the 

principal difference is that the dike rock is finer grained, is possibly 

more thoroughly serpentinized, and does not contain sulfide minerals 

as does the larger gabbro body. There is no apparent connection 

between any of the known bodies of igneous rocks and the mineralization at Horseshoe Bay.

Unconsolidated deposits of Quaternary age overlie much of the 

bedrock. Glacial deposits mantle large parts of the valley floor, 

alluvium occurs in patches along the streams, and talus forms thick 

blankets along the lower slopes of the high ridge that borders the 

valley.

374649 5 
112 MINERAL RESOURCES OF ALASKA

STRUCTURE

The rocks of the area have been strongly folded and faulted, and the 

dominant structural feature is a large northeastward-plunging anticlinorium. The axial zone of the anticlinorium is located somewhere 

in Montague Strait, and Latouche Island is on the northwest limb of 

the fold.

At Horseshoe Bay the strike of the bedding is commonly about N. 

30° E., and dips range from 55° to 70° NW. This general attitude 

prevails on Latouche Island and the islands to the west. On Montague 

Island the strike of the bedding is similar, but the dips are steep to 

the southeast.

On Latouche Island there are many folds of moderate size (Moffit, 

1954, p. 263-264; Grant and Higgins, 1910, p. 29-30) that reflect the 

major anticlinorial structure. At Horseshoe Bay, however, the beds 

have a monoclinal dip to the northwest, and the local differences in 

strike and dip that have been noted near the pyrite deposits represent 

minor open flexures, and nowhere within the mineralized area has evidence of complete folds been found. In some areas of the slate at 

Horseshoe Bay, especially in the bed that forms the footwall of the ore 

zone, a minor crenulation is conspicuous. The attitude of the axial 

planes of these tiny folds is closely parallel to that of the bedding, and 

they plunge northward at about 15°.

The structural pattern is complicated by many faults which in general are closely parallel to the bedding. A large part of the fault 

movement has taken place along slate beds that separate the stronger 

graywacke units, and the primary faults are reverse faults along which 

the northwest or hanging-wall side has moved upward relative to the 

southeast or footwall side. At a few places on the island vertical 

displacements of from 1 to 10 feet have been measured. Along most 

of the faults, however, the maximum displacement is unknown. It 

is difficult to trace even the strongest of the fault zones between outcrops with any degree of assurance because of the heavy cover of soil 

and vegetation and the general lack of topographic expression of the 

fault zones. There appears, however, to be one persistent zone of 

faulting on the west side of Latouche Island that has been traced by 

Moffit (1954, p. 256-266) from Latouche southward to Pleasant Bay, a 

distance of 9 miles. This is not a simple fault but includes minor 

fractures that may differ in strike and dip from the strike and dip of 

the main fault zone. The amount of displacement along this fault 

has not been determined.

Several sets of joints are well developed throughout the area, some 

of which form conjugate systems that can be related to the major 

structure. This is particularly well illustrated by a very conspicuous 

set that strikes N. 45° W. The joints of this set are vertical and spaced 

at about 100 foot intervals.
PYRITE DEPOSITS, HORSESHOE BAY, ALASKA 113

Much of the slate has a well-developed cleavage. The strike is 

parallel to the major northeasterly structural trend. The dip ranges 

from 65° to 80° NW.

PYRITE DEPOSITS 

HISTORY AND DEVELOPMENT

Initial claim locations are said to have been made at Horseshoe Bay 

in 1898. Soon after the sulfide-bearing bodies were discovered, the 

Reynolds-Alaska Development Company began to explore the deposits; and although large bodies of copper-bearing pyritic ore were found 

on the Duchess, Duke, and Iron Mountain No. 4 claims, a mine was 

never brought into production; and all work ceased in November of 

1916. The property has been idle and deserted since that date, and 

all of the surface improvements are in ruins. In 1940 the ReynoldsAlaska Development Company was reorganized, and its successor, 

RADCO Incorporated, is the present owner of the Iron Mountain 

group of 16 patented claims. (See pi. 12). The early interest in the 

property was in the copper content of the deposits. Today the deposits are of interest as a potential source of sulfur.

During the early period of exploration the mineralized zone was 

examined by geologists of the U. S. Geological Survey in 1905, 1909, 

and 1916, and brief accounts of the geology have been published by 

U. S. Grant (1906, p. 85), U. S. Grant and D. F. Higgins (1910, pp. 

66-67), and B. L. Johnson (1918, pp. 206-208). At the tune of Grant's 

first examination in 1905, there were a 50-foot adit on the Blue Fox 

claim (Iron Mountain No. 6 claim) and a branching adit 300 feet in 

length on the Duchess claim. In 1907 a shaft was put down on the 

property. In 1908 this shaft was reported to be 100 feet deep with a 

crosscut at its bottom to an ore body. An average force of 10 men 

was employed during 1916; and, when Johnson visited Horseshoe 

Bay, the shaft had been pumped out; and the mine workings at the 

shaft and on the Duchess claim had been extended to about their 

present extent. At this tune, mine development consisted of the 

110-foot shaft with about 356 feet of crosscut and drift, about 3,400 

feet of crosscut and drift from 5 adits, and a small amount of surface 

trenching. Small shipments of ore are reported to have been sent to 

the smelter at Tacoma, Wash., but neither RADCO Incorporated 

nor the smelter have any record of them, and the value of the shipments is not known

DESCRIPTION

The sulfide-bearing bodies at Horseshoe Bay are replacement 

deposits and consist of tabular lenses of massive and disseminated 

sulfides in a slate-graywacke country rock. The deposits have been 

formed by the partial to complete replacement of a unit of the sedimentary sequence by sulfide minerals, and the lenses of ore conform to
114 MINERAL RESOURCES OF ALASKA

the stratification of the enclosing rocks. A thick bed of slate forms the 

footwall of the mineralized zone and is believed to have been a controlling factor in localizing the zone of sulfide deposition. The 

mineralogy is relatively simple. The primary sulfides are pyrite, 

chalcopyrite, cubanite, sphalerite, pyrrhotite, arsenopyrite, and 

galena. Assays show that small amounts of silver and gold are also 

present, but the form in which these metals occur has not been determined. The suifide-bearing bodies have been only slightly oxidized 

since the comparatively recent glaciation of the area, and only small 

amounts of secondary minerals are present. The following species 

have been recognized: "Limonite," melanterite (?), chalcanthite (?), 

azurite, malachite, covellite, and native copper. The paragenetic 

sequence as determined from polished specimens is pyrite, arsenopyrite, sphalerite, cubanite, chalcopyrite, and galena. Pyrrhotite was 

not present in the specimens examined, but from field relationships it 

appears to have been deposited at about the same time as the sphalerite. The hydrothermal gangue minerals are calcite and quartz and 

were the last minerals to be deposited.

Pyrite is by far the most abundant mineral, and in many places the 

massive sulfide is essentially pure pyrite. The copper-bearing minerals, cubanite and chalcopyrite, are about equally abundant and 

commonly are very intricately intergrown. Small amounts of 

sphalerite and tiny grains of arsenopyrite and galena are widely 

distributed throughout the suifide-bearing bodies. The mineral 

aggregate commonly is very fine grained. Most of the mineral grains 

are less than 0.10 millimeter in diameter, although, in some lenses, 

the grains of pyrite may be as large as 1.0 millimeter in diameter. 

There is practically no fracturing or brecciation of the mineral grains, 

and all textural and structural relationships indicate continuous and 

uninterrupted deposition of the sulfide minerals.

Sixteen samples taken from the 400-foot level on the Duchess claim 

by engineers of the U. S. Bureau of Mines l were found by assay to 

contain copper, trace to 1.48 percent; zinc, 0.10 to 0.80 percent; lead, 

trace to 0.21 percent; gold, trace to 0.02 ounce per ton; and silver, 

0.40 to 0.70 ounce per ton. Five of these samples were cut from 

massive sulfide zones and eleven from zones of disseminated sulfides.

Spectrographic analyses of samples of the massive sulfides from the 

Duchess claim were made in the Washington, D. C., laboratories of 

the Geological Survey. The results are listed in the accompanying 

table.

> Webber, B. S., and Rutledge, F. A., May 1944, Horseshoe Bay Deposits, Latouche Island, Prince 

William Sound: II. S. Bureau of Mines Unpublished War Minerals Report.
Spectrograpkic analysis of eight ore samples from the Duchess 

[Janet D. Fletcher, analyst. Looked for but not found: Au, Pt, Mo, \V, Qe, Sb, Cd, In, T

Sample

52-2241S____

52-2242S-...

52-2243S..._

52-2244S____

52-2245S __

52-2246S_.._

52-2247S....

52-2248S___.

Percent (X equals a significant fi

Cu

O.X

.OX

.X

.X

.X

.ox

X

.ox

Ag

o.oox

.oox

.oox

.oox

.oox

.oox

.oox

.oox

Sn

o.oox

0

.ox

.oox

.oox

.oox

.oox

0

Bi

O.OOX

0

0

0

0

0

0

0

Pb

O.X

.X

.X

.X

.X

.X

.X

.X

As

O.X

.X

.X

.X

.ox

.X

.X

.ox

Zn

X

.ox

X

X

X

.ox

X

0

Mn

O.OOX

.oox

.ox

.oox

.ox

.oox

.ox

.oox

Co

o.oox

0

.oox

.oox

.ox

0

.oox

.oox

Fe

xo

xo

xo

xo

xo

xo

xo

xo

0

0

0

0

0

0
J16 MINERAL RESOURCES OF ALASKA

Sulfide-bearing bodies have been found on the Duchess, Duke and 

Iron Mountain No. 6 claims, and, although mine development is inadequate to prove either then" size or relationship to one another, 

they appear to outline a mineralized zone that is at least 3,100 feet 

long.

DUCHESS CLAIM

The mineralized zone on the Duchess claim trends N. 30° E., 

parallel to the bedding of the slate-graywacke country rock, and dips 

steeply to the northwest. Mine development consists of two major 

levels whose portals are at altitudes of 380 feet and 470 feet (see pi. 

12) and a 10-foot prospect adit at an altitude of 630 feet. The 

underground workings, which aggregate about 3,000 feet of drift and 

crosscut, explore the mineralized zone for a distance of about 500 feet 

along its strike.

Lenses of massive sulfide occur throughout the mine (see pi. 12) 

and range in width from a few inches to as much as 60 feet and hi 

length from less than one foot to at least 490 feet. However, the 

maximum size of these sulfide-bearing bodies is not known as none of 

the workings reach the limits of the- mineralized zone. Zones of disseminated sulfides also occur throughout the mine. The usual mode 

of occurrence is as halos surrounding lenses of massive sulfide. At a 

few places, however, (see pi. 12 DN 501 and XCW 560) zones of 

disseminated sulfides are present that have no visible connection with 

massive sulfide bodies. The size of these bodies of disseminated sulfide cannot be determined at this time because the boundaries of 

what is commercial ore must be determined by assay and the amount 

of material that will be classed as ore will depend upon the many 

economic factors that prevail at the time of mining.

The country rock is a sequence of interbedded slate and graywacke, 

and the thick bed of slate that forms the footwall of the mineralized 

zone apparently guided the course of the mineralizing solutions by 

effectively damming their migration and dispersion in that direction. 

Because of this channeling effect of the slate, mineralization is strongest 

on the footwall side of the mineralized zone.

The boundaries between bodies of massive and disseminated sulfide 

are almost invariably sharp and distinct, and the contact is commonly 

marked by a paper-thin claylike selvage. On the footwall side of the 

massive sulfide lenses, this selvage marks the boundary between solid 

sulfide and disseminated sulfide. On the hanging wall side, however, 

the transition from solid sulfide to disseminated sulfide is not so 

abrupt, and at many places the outer 3 or 4 inches of the massive 

sulfide lenses contain small residuals of unreplaced country rock.

The boundaries between disseminated sulfides and barren country 

rock are gradational. On the footwall side of the massive sulfide
PYRITE DEPOSITS, HORSESHOE BAY, ALASKA 117

lenses, the zone of disseminated sulfides is narrow, and the transition 

from massive sulfide to barren country rock commonly takes place 

within 2 or 3 feet of the massive sulfide-disseminated sulfide contact. 

On the hanging wall side of the massive sulfide lenses, the mineralizing solutions migrated much farther into the country rock but in a 

very erratic manner; and although geologic control is poor because of 

the lack of mine openings, it appears that the zone in which disseminated sulfide mineralization occurs may be as much as 100 feet wide.

The nature of the rock that has been replaced by the sulfide minerals is obscure, especially in the areas that are now occupied by 

massive sulfides. Because of the texture and structure of the massive 

sulfides, the author believes that the rock was originally thinly bedded 

graywacke-type sandstone and that the course of mineralization was 

guided by the bedding planes, the primary intergranular permeability 

of the sediments, and theii probable original calcareous and feldspathic 

composition. In the disseminated sulfide zones which contain a considerable volume of unreplaced slaty country rock, the author believes 

that the mineralizing solutions preferentially replaced the coarser 

grained "graywackish" sediments first and then the slaty sediments 

as mineralization continued.

For the most part the unreplaced country rock is slate or graywacke, but not without exception for at a few places in the areas of 

disseminated sulfides, notably in drift DN 503 (see pi. 12), there is a 

small amount of a very fine-grained dark-gray chert. A thin section 

of this rock shows it to have been an original member of the sedimentary sequence.

Within and near the zone of mineralization the country rock has 

been altered by the mineralizing solutions and now shows varying 

degrees of silicification, sericitization, and kaolinization. The massive sulfide lenses contain an occasional pod or lens of intensely altered and partially replaced country rock which is soft and friable 

and which consists mainly of sulfide minerals loosely cemented by 

sericite and clay. At many places the massive sulfide bodies are 

bordered by narrow zones of strongly bleached and whitened rock 

that contains large amounts of sericite and clay.

The problem of wall-rock alteration has not been studied thoroughly, 

but it appears that sericite is the principal mineral in the zone of most 

intense alteration within and adjacent to the massive sulfide bodies. 

The zone of sericite alteration is followed, with considerable overlap 

in many places, by an intermediate zone of alteration in which the rock 

is argillized and finally, and again with considerable overlap in many 

places, there is an outer zone in which the rock is silicified. In the 

zones of disseminated sulfides the country rock commonly is strongly 

silicified although in a few areas where alteration was relatively strong
118 MINERAL RESOURCES OF ALASKA

the country rock is strongly bleached and apparently contains a large 

percentage of argilhc alteration products. In other areas of disseminated sulfide mineralization the only apparent alteration is a slight 

silvery whitening of residual slate fragments.

The mineralized zone is relatively free of faults, and none of the 

massive sulfide lenses are cut off or displaced. At many places, massive sulfide bodies are bordered by narrow zones of highly bleached and 

altered rock that at places appear to be somewhat mashed and 

sheared. Some of these zones may represent incipient faults, but the 

amount of movement along them since mineralization is believed to 

have been insignificant. Faults, which appear to be of the reverse 

type, are present in both the hanging wall and footwall of the mineralized zones; but because of the scarcity of exposures, knowledge of them 

is limited. Most of them strike N. 20°-40° E. and dip 50°-60° W. 

A few, however, deviate from this pattern and dip steeply to the east.

A prominent fault zone 70 to 100 feet west of the massive sulfide 

lenses is fairly well defined near the portal of the 400-foot level and 

in crosscut XCW 555 and drifts DN 501 and DN 504 on the 500-foot 

level. It trends parallel to the bedding and dips 60°-80° W. At 

most places the fault zone consists of a number of fault surfaces, but 

in drift DN 501 it is a narrow well-defined break with as much as 1 

foot of gouge and breccia. A considerable volume of water enters 

the workings along this zone, and most of the mine openings along it 

require timber support. It has been suggested that the fault zone at 

the Duchess mine may be a component of the fault zone that Moffit 

traced from Latouche southward to Pleasant Bay. This relationship 

remains to be proved, but if correct the fault may have had considerable influence on the mineralization at Horseshoe Bay, since at 

Latouche the Bonanza fault had an important influence on ore 

deposition and mining operations (Moffit, 1954, p. 298-299) at the 

Beatson mine. At the present stage of development at Horseshoe 

Bay, however, there is no apparent connection between the fault zone 

and the mineralization or between the fault zone at the Duchess and 

the fault zone at Latouche.

Joints are present in both the country rock and the sulfide bodies. 

They are commonly spaced at intervals of several feet, and the joints 

that existed prior to mineralisation apparently have had very little 

influence on ore deposition. At a few places on the hanging wall side 

of the mineralized zone, veinlets of massive sulfide 1 to 2 inches wide 

extend a few feet into the country rock along preore joint surfaces. 

However, most of the joints transect both the country rock and the 

sulfide bodies and are clearly the result of stresses applied subsequent 

to mineralization.

400-foot level. Exploration on this level consists of a 310-foot adit 

crosscut and a 510-foot winding drift along the mineralized zone.
PYRITE DEPOSITS, HORSESHOE BAY, ALASKA 119

(See pi. 12.) Eight crosscuts from the drift total 403 feet in length. 

The mineralized zone is considered tentatively to contain two lenses of 

massive sulfides. One lens is almost continuously exposed along the 

footwall side of the drift for 490 feet and ranges in thickness from 5.8 

feet to 24 feet. The second lens is exposed only in crosscut No. 208. 

This lens, which is 60 feet wide, may be a separate body, or it may be 

an apophysis from the lens exposed in the main drift.

Four samples taken from crosscut No. 203 by U. S. Bureau of Mines 

engineers 2 gave copper assays of 1.44,1.00, 0.59, and 0.68 percent with 

about 0.40 percent of zinc and 0.6 ounce of silver per ton.

500-joot level. Two adits give access to the workings on this level. 

(See pi. 12.) In 1952 the portal of the main entryway was caved, 

and water ponded by the fallen debris had flooded the workings as 

far as crosscut XCW 558. In those parts of the level covered by the 

water, a thick deposit of "iron-mud" had accumulated on the sill and 

the walls.

The sulfide-bearing zone on this level has been explored for a 

distance along its strike of 505 feet by nearly 1,800 feet of drifts and 

crosscuts. Three short inclined raises total about 75 feet and there 

is one small stope.

This zone contains a number of lenses of massive sulfides. The 

largest lens is exposed in drifts DN 505 and DS 506. It is 17 feet 

wide at one point and is 150 feet long. A small tonnage of sulfides 

is said to have been mined from the stope and shipped to the 

Tacoma, Wash., smelter, but neither RADCO Incorporated nor the 

smelter has any record of the shipment.

DUKE, IRON MOUNTAIN NO. 6, AND AMENDED IRON MOUNTAIN 

NO. 4 CLAIMS

The zone of sulfide mineralization on these three claims is explored 

by a 110-foot shaft, two short adits, and surface trenches. The collar of the shaft is at an altitude of 125 feet. The other workings 

are at slightly higher altitudes. Massive sulfide bodies similar to 

those on the -Duchess claim are present at four different ,places, and 

although there is no proved connection between them, they may lie 

along the same mineralized zone. Individual lenses range in thickness from 4 feet to 27 feet, and the maximum proved length along 

the strike is 151 feet.

Prospect adit No. 1. The portal of this short exploratory adit is 

about 450 feet N. 45° E. from corner post no. 4 of the Duke claim and 

is at an altitude of about 150 feet. The portal was caved ,and the 

adit flooded in 1952, but the water was lowered to wading dejjth, and 

a tape and compass survey was made of the workings. (See fig. 13.) 

Where intersected by the workings the mineralized zone is about 4 feet

2 Webber, B. S., and Rutledge, F. A., May 1944, Horseshoe Bay Deposits, Latouche Island, Prince 

William Sound: U. S. Bureau of Mines Unpublished War Minerals Report. 

374649 56 3
120 MINERAL RESOURCES OF ALASKA

EXPLANATION

Interbedded slate and graywacke

Massive sulfide ore

Disseminated ore 

Density of stippling indicates relative grade

40 Feet

Tope and compass survey 

by F.A. Stejer,August 1952

FIGURE 13. Sketch map of prospect adit No. 1, Duke claim.

wide and contains a considerable volume of massive sulfides. The 

country rock is mostly slate. A few thin beds of graywacke are 

present.

Credc ^exposure. A lens of massive sulfides crops out in. - the bed 

of the creek 225 feet N. 83° E. of the Duke shaft. The lens is about 

7 feet wide and strikes N. 25° E. and dips steeply to the west.

Duke shaft. In 1952 the shaft was full of water, so the following 

description has been compiled from earlier reports. (See fig. 14.) 

The 110-foot 2-compartment shaft is on the hanging waE side of the 

niineralized zone. At a depth of 100 feet a crosscut was driven 205 

feet eastward into the footwall. A hundred feet from the shaft a 

lens of massive sulfides 27 feet wide was intersected. On the footwall side of the lens, drifts were driven 91 feet north and 60 feet south 

along the sulfide body. In addition to graywacke and slate, the 

sedimentary sequence also contains some beds of light-gray siliceous 

limestone, and a little chert. The bedding strikes N. 16°-38° E., and 

the dip is mostly to the west, but ranges from 55° W. to 83° E. There 

is some faulting more or less parallel to the bedding. One gouge-filled 

fault, cut by the lower workings, strikes N. 30° E. and dips 50° W.
PYRITE3 DEPOSITS, HORSESHOE BAY, ALASKA 121

Two compartment shaft

60 Feet

Traced from original map 

by Rex Townsend

FIGURE 14. Sketch map of Duke shaft, Amended Iron Mountain No. 4 claim.

Prospect adit No. 2. Near the north end line of the Iron Mountain 

No. 6 claim, an adit has been driven southward along a lens of massive 

sulfides. This adit is caved, but according to Johnson (1918, p. 208) 

it was driven for 60 feet along a 5-foot sulfide lens that strikes N. 20°- 

45° E. and dips 60°-75° W. An outcrop of ore at the caved portal 

indicates% lens of massive sulfides similar to the other sulfide-beafing 

bodies at Horseshoe Bay.

ECONOMIC CONSIDERATIONS

The zone geologically favorable for the occurrence of ore on the 

Iron Mountain group of claims (see pi. 12)~ is at least 3,100 feet long. 

Exploration work fco date has outlined two ore shoots, and although 

the mine workings are not interconnected and no ore has been 

blocked out, it seems reasonable to assume that a considerable 

tonnage of inferred ore is present along the mineralized zone.

The Duchess zone at the north end of the favorable area has a 

proved length along the strike of about 500 feet, and more important, the mine workings terminate in the sulfide body and have not 

delimited the ore shoot either in length or depth.
122 MINERAL RESOURCES OF ALASKA

The Duke zone at the south end of the favorable area has a proved 

length along the strike of about 500 feet. Here, as at the Duchess, 

none of the mine workings reach the limits of the ore shoot.

The Duchess and the Duke zones are separated by a heavily 

mantled and unprospected interval 2,100 feet long. The greatest 

potential for ore reserves on the Iron Mountain group of claims niay 

lie along this unexplored interval.

The lenses of massive sulfides are wider and longer at depth than 

they are at the surface, which suggests that the present land surface 

may be near the top of the ore shoots. If this is true the lenses of 

sulfides may extend a considerable distance below the 400-foot level 

at the Duchess and also a considerable distance below the d*oss{5ut 

level at the Duke.

LITERATURE CITED

Bateman, A. M., 1924, Geology of the Beatson Mine, Alaska: Econ. Geol. v. 19,

p. 338-368. 

Grant, U. S., 1906, Copper and other Mineral Resources of Prince William

Sound: U. S. Geol. Survey Bull. 284. 

Grant, U. S., and Higgins, D. F., 1910, Reconnaissance of the Geology and

Mineral Resources of Prince William Sound: U. S. Geol. Survey Bull. 443. 

Johnson, B. L., 1918, Copper deposits of Latouche and Knight Island districts:

U. S. Geol. Survey Bull. 662. 

Moffit, P. H., 1954, Geology of the Prince William Sound Region, Alaska:

U. S. Geol. Survey Bull. 989-E. 

U. S. Weather Bureau, 1943, Climatic Atlas for Alaska, Rpt. 444.

U. 3. SOVERNMENT PRINTINC OFFICE: IfM