Geology of an Alpine-Type 

Peridotite in the 

Mount Sorenson Area, 

East-Central Alaska

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1170-A
Geology of an Alpine-Type 

Peridotite in the 

Mount Sorenson Area, 

East-Central Alaska

By TERRY E. C. KEITH, HELEN L. FOSTER, ROBERT L. FOSTER, E. V. POST, and W. L. LEHMBECK

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1170-A

A petrologic and structural study of a 

large ultramafic body in the 

Yukon-Tanana Upland

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1981
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director

Keith, Terry E. C.

Geology of an alpine-type peridotite in the Mount Sorenson 

area, east-central Alaska.

(Shorter contributions to general geology) (Geological 

Survey Professional Paper; 1170-A)

Bibliography: p. A9.

Supt. of Docs, no.: I 19.16:1170-A

1. Peridotite Alaska Sorenson, Mount, region. 

I. Keith, Terry E. C. II. Series. III. Series: Geological 

Survey Professional Paper; 1170-A.

QE462.P45G46 552'.3 81-607942 

___________________________________AACR2____

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

Washington, D.C. 20402
CONTENTS

Page

Abstract _____________________________________________________________________ Al

Introduction __________________________________________________________________ j

Petrology of the peridotite _______________________________________________________ 2

Primary minerals ___________________________________________________________ 3

Secondary minerals _________________________________________________________ 3

Tectonic inclusions __________________________________________________________ 4

Quartz-carbonate veins ______________________________________________________ 4

Peridotite of Flume Creek _______________________________________________________ 4

Minor-element distribution ______________________________________________________ g

Adjacent rocks ________________________________________________________________ 6

Structural relations _____________________________________________________________ 7

Origin and geologic history ______________________________________________________ 3

References cited _______________________________________________________________ 9

ILLUSTRATIONS

Page

FIGURES 1-4. Maps showing:

1. Location of areas which include peridotites of Mount Sorenson and Flume Creek, Alaska ___________________________ Al

2. Geology of the peridotite of Mount Sorenson _______________________________________________________________ 2

3. Geology of the peridotite of Flume Creek __________________________________________________________________ 4

4. Aero magnetic contours of Mount Sorenson area ____________________________________________________________ 7

TABLE

Page

TABLE 1. Semiquantitative spectrographic analyses of ultramafic rocks from the serpentinized peridotites of the Mount Sorenson

and Flume Creek areas ______________________________________________________________________________ A5

III

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGY OF AN ALPINE-TYPE PERIDOTITE

IN THE MOUNT SORENSON AREA,

EAST-CENTRAL ALASKA

By TERRY E. C. KEITH, HELEN L. FOSTER, ROBERT L. FOSTER,' E. V. POST,* and W. L. LEHMBECK3

ABSTRACT

An alpine-type peridotite of the harzburgite subtype, about 47 km 

in area, crops out in the vicinity of Mount Sorenson in the Eagle and 

Charley River quadrangles of east-central Alaska. The peridotite consists mainly of harzburgite, some dunite, and minor clinopyroxenite. 

All of the peridotite has been serpentinized to some extent, most extensively in the eastern part. Lizardite is the dominant serpentine 

mineral. Large tectonic inclusions of diabase have been altered, 

probably by metasomatism during serpentinization. Four small 

quartz-carbonate veins are also included. The peridotite crops out as 

a synform mass trending nearly east-west. Aeromagnetic contours 

closely follow the configuration of the outcrop. The rocks surrounding the peridotite include greenstone, metamorphosed basalt and 

gabbro, quartzite, mica-schist, greenschist, chert, phyllite, and other 

metamorphosed rocks of probable Paleozoic age. The ultramafic 

rocks are believed to have been tectonically emplaced and may be 

part of a dismembered ophiolite. The nearby Tintina fault, a major 

fault system with significant, right-lateral displacement, may have 

been a deep suture along which mantle material was derived and obducted onto a slice of northwestward-moving continental crust.

INTRODUCTION

A large serpentinized peridotite body, about 47 km2 , 

crops out in the northeastern Eagle and the southwestern Charley River 1:250,000 quadrangles in the 

Yukon-Tanana Upland of east-central Alaska (fig. 1). 

Because it includes part of Mount Sorenson, the most 

prominent landmark of the area, the rocks of this 

peridotite body are referred to as the peridotite of 

Mount Sorenson.

The Yukon-Tanana Upland (called the Yukon crystalline terrane by Tempelman-Kluit, 1976, p. 1343 where it 

extends into Canada) is an area of greenschist- to 

amphibolite-facies metamorphic rocks that have been 

intruded by Mesozoic and Tertiary granitic rocks (Foster, 1976). The peridotite body crops out as a synform 

mass trending nearly east-west and having two limbs, a 

northern limb about 30 km long and a southern limb 14

Cache Creek Exploration Company, Reno, Nevada.

Skyline Labs., Inc., Wheat Ridge, Colo.

'Hawley and Hawley Assayers and Chemists (Division Skyline Labs., Inc.), Tucson, Ariz.

Fault—Dotted where 

uncertain

Yukon-Tanana Upland

FIGURE L Location of map areas that include peridotites of Mount 

Sorenson and Flume Creek, Alaska.

km long (fig. 2). About 30 km to the east, in the vicinity of Flume Creek, outcrops of similar ultramafic rock 

lie on strike with the northern limb. Some small, isolated patches of ultramafic rocks also occur to the east 

on strike with the southern limb.

During reconnaissance mapping in the Eagle 

1:250,000 quadrangle, the ultramafic rocks of the 

Mount Sorenson area in the Eagle quadrangle were 

examined by H. L. Foster, T. E. C. Keith, S. H. B. 

Clark, F. R. Weber, and others between 1968 and 1972. 

The peridotite is located more than 60 km from the 

nearest highway and more than 30 km from a navigable river, so access has been primarily by helicopter. 

During September and October of 1968, Cache Creek 

Exploration Co., under the supervision of R. L. Foster, 

contracted Lockwood, Kessler, and Bartlett, Inc. and 

made an aerial magnetometer survey along the trend of 

the Tintina fault in an attempt to delineate ultramafic 

masses that would present ground targets for asbestos 

prospecting. A part of that airborne survey covered the 

Mount Sorenson area, and some of these data are used

Al
A2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

143°

EXPLANATION

Mesozoic gronitic rocks

Ultramafic rocks, mostly serpentmized

Diobase pods

Metamorphosed bosalt and gabbro

Greenstone

Permian)?) metasedimentary rocks

Poleozoicj?) greenschist-facies 

metamorphic rocks

C Quartz-corbonate vein 

A Outcrop of chert

—^_ Fault—Arrows show relative movement ^——

——? Contact—Queried where uncertain 

Strike and dip of foliation 

Bearing and plunge of lineation

35

FIGURE 2. Geologic map of the peridotite of Mount Sorenson.

in this report. In the summers of 1969 and 1970, Cache 

Creek Exploration Co. carried out a reconnaissance 

ground evaluation of several magnetic anomalies and 

in 1971 managed an exploration program for U.S. 

Borax and Chemical Corp., during which time E. V. 

Post, W. L. Lehmbeck, and R. L. Foster prepared a 

geologic map (unpub., 1:63,360 scale) of the Mount 

Sorenson area as part of an asbestos and base-metal 

appraisal project. This work included the parts of the 

ultramafic body that are in the Charley River quadrangle, as well as parts that are in the Eagle quadrangle.

Information on the ultramafic rocks of the Eagle 

quadrangle, including those of Mount Sorenson, has 

been published by Keith and Foster (1973) and Foster 

and Keith (1974). However, because the peridotite of 

Mount Sorenson is one of the largest ultramafic bodies 

in the Yukon-Tanana Upland and has significance in

the geologic history of the area, more detailed information is presented here.

PETROLOGY OF THE PERIDOTITE

The peridotite of Mount Sorenson is composed dominantly of harzburgite with subordinate dunite and 

minor clinopyroxenite. The eastern part of the body is 

highly serpentinized dunite containing as much as 5 

percent orthopyroxene (or bastite); the western part is 

dominantly harzburgite, locally containing as much as 

35 percent orthopyroxene. The general term peridotite 

is used for the mass as a whole in accordance with 

Jackson's (1968, p. 140) classification. The mass is 

classified according to Jackson and Thayer's (1972) 

criteria as an alpine-type ultramafic body of the 

harzburgite subtype.

Nearly all of the peridotite is serpentinized to some 

extent, but original minerals can generally be distin-
GEOLOGY OF AN ALPINE-TYPE PERIDOTITE IN MOUNT SORENSON, EAST CENTRAL ALASKA A3

guished. The peridotite is least serpentinized (10 to 25 

percent) in the central to western part. In the eastern 

part of the main mass and in the two limbs 80 to 100 

percent of the peridotite is serpentinized.

In outcrop, the peridotite is massive and commonly 

broken into huge blocks, 6 to 8 m on a side, bounded 

by joint surfaces. Most of the blocks have been broken 

up by intense frost action. Weathered surfaces are typically orange brown and knobby. The knobs are caused 

by differential weathering of resistant orthopyroxene 

grains, which generally occur in clots or lenses a few 

centimeters long. Knobs in the completely serpentinized areas are bastite, a serpentine polymorph after 

orthopyroxene.

Conspicuous layering of crystals, which is common in 

most alpine-type peridotites (Loney and others, 1971; 

Himmelberg and Loney, 1973; Coleman, 1977), occurs 

in only a few places. Because of the displacement of 

outcropping blocks by frost action and the paucity of 

crystal layering, primary mineral foliation can be 

measured at only a few places.

The unweathered peridotite and serpentinized 

peridotite are dark green to black and massive, but are 

commonly cut by tiny (l to 3 mm thick), irregular, 

pale-green clinochrysotile veinlets. The amount of serpentinization has little effect on the appearance in outcrop.

Shiny, green, slickensided, and blue-green to black 

serpentine is common in many places at the boundaries 

of the mass and in several places within the mass where 

local shearing has taken place. The serpentine is 

sheared or broken up into relatively small pieces with 

curved surfaces. Abundant slip fiber and magnetite 

specks and streaks are conspicuous on some sheared 

surfaces.

The rocks in the least serpentinized areas have a 

coarse- to medium-grained xenoblastic granular texture. The grains appear to have interlocking boundaries, although they are highly fractured and partly 

serpentinized. Average grain size of olivine crystals is 

difficult to determine because of the fracturing of 

grains and serpentinization, but grains generally range 

from 3 to 6 mm in diameter. Unaltered grains of orthopyroxene and bastite are 5 to 8 mm long.

PRIMARY MINERALS

Olivine is forsterite (Fogo-gs) as determined optically. 

Grains are fractured and serpentinized to various degrees, and kink banding is common. In places in the 

southwestern part of the peridotite body, the fresh 

olivine grains are granulated as a result of deformation 

within the peridotite body. The time of granulation relative to the history of the peridotite cannot be determined with present data. However, granulation is more 

likely to have occurred during mantle deformation than 

during emplacement into the crust or subsequent crustal folding and faulting, both of which would result in 

more extensive alteration of the olivine in the granulated area than is observed.

Orthopyroxene is enstatite and generally occurs in 

clusters of several grains in the fresher (western) part 

of the peridotite. The distribution of bastite in the serpentinized peridotite of the eastern part of the area indicates that orthopyroxene was present as solitary orthopyroxene grains and was less abundant than in the 

western part. The grains are commonly bent and 

strained; their shapes are accentuated by abundant 

subparallel exsolution lamellae of a reddish-brown 

opaque mineral, probably ilmenite.

Clinopyroxene is nearly absent in most of the peridotite. In the northwestern area, however, a few layers or 

lenses are very rich in clinopyroxene. One specimen of 

equigranular texture consists of about 90 percent 

clinopyroxene and minor olivine and serpentine minerals. Another specimen from the same vicinity consists 

of equigranular layers (1 to 3 mm thick) of olivine only 

and olivine with clinopyroxene.

Chromite in amounts less than 1 percent is disseminated in the least serpentinized peridotite; it is absent 

in the more serpentinized peridotite. Chromite occurs 

as small anhedral intergranular grains and as large 

poikilitic or embayed grains as much as 1.5 m long. 

Microscopic alinement of tiny chromite grains forms 

short, irregular bands usually not more than one grain 

thick. No megascopic concentrations or layers of chromite were found.

SECONDARY MINERALS

The massive, dark-green to black serpentine occurring throughout the peridotite is dominantly lizardite 

as determined by X-ray diffraction. Bastite was 

X-rayed separately and also is lizardite. Much of the 

sheared serpentine is a mixture of clinochrysotile and 

lizardite, and the slip fiber is clinochrysotile. Late 

crosscutting veinlets (generally about 1 mm thick) of 

pale-green serpentine with cross fiber in various stages 

of development is also clinochrysotile. No antigorite 

was identified from the peridotite.

In thin section, serpentine fills irregular fractures in 

primary mineral grains and replaces olivine along fractures and orthopyroxene along cleavages. Mesh texture 

of serpentine replacing olivine is crudely to well developed in the highly serpentinized parts of the peridotite. These textures are very similar to those described 

in detail for the Burro Mountain alpine-type ultramafic 

body in the California Coast Ranges (Coleman and 

Keith, 1971, p. 314-315).

Magnetite was formed during the serpentinization of 

olivine and pyroxene. Abundant, tiny, anhedral magnetite grains are concentrated in short, crosscutting microfractures filled with serpentine. In a few places, 

large anhedral solitary magnetite grains are found, 

usually in the highly serpentinized areas. Magnetite is 

not found in the clinochrysotile veinlets.

A slightly red, opaque, elongate, bladelike mineral 

has exsolved from orthopyroxene parallel to cleavage 

during serpentization. This mineral has not been positively identified but appears to be ilmenite.
A4 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Brucite is scarce in these rocks but has been identified in a few thin sections and X-ray diffraction 

traces. Brucite formed during serpentinization along 

with the magnetite.

Pennite occurs sporadically throughout the peridotite 

and is moderately abundant in places. In thin section, 

the penninite is colorless and has anomalous blue and 

brown birefringence. This mineral is most commonly 

associated with large anhedral magnetite grains within 

serpentine, but it also occurs as oriented elongate 

plates and randomly oriented bundles within the serpentine. Penninite appears to have formed during regional greenschist-facies metamorphism.

Tremolite (having some iron component) has developed from alteration of pyroxenes and in a few 

places constitutes as much as 10 percent of the peridotite. It is colorless in thin section and occurs as coarse 

to fine, radiating, fibrous crystals. In places, finegrained talc is associated with tremolite as an alteration product from pyroxene and tremolite. Tremolite 

and talc occur in a few places but not throughout the 

peridotite body. These minerals probably developed 

during regional greenschist-facies metamorphism, although textural evidence is not conclusive.

TECTONIC INCLUSIONS

Pods of diabase, irregular in size and distribution, 

are found throughout the peridotite body (fig. 2). The 

diabase pods are small, 3-20 m in longest dimension. 

The diabase is equigranular within the size range of 1 to 

2 mm. Subhedral plagioclase originally made up 40 to 

65 percent of the diabase, and different proportions of 

clinopyroxene and hornblende made up the rest of the 

rock. In thin section, the primary hornblende exhibits 

pale to strong reddish-brown pleochroism, indicating a 

high titanium content.

No contact effects between the diabase and peridotite 

were noted, but the diabase is altered. The diabase is 

least altered in the main mass of the peridotite where 

serpentinization is least developed and totally altered 

in the eastern parts where serpentinization of the 

peridotite is complete. The plagioclase is altered to 

fine-grained minerals; clinopyroxene and hornblende 

are altered to tremolite, penninite, biotite, and magnetite. Skeletal ilmenite crystals surrounded by sphene 

are common. Biotite is secondary, and its reddish 

brown color indicates a high titanium content. Altered 

diabase in the eastern part of the peridotite contains 

microveinlets of prehnite with associated actinolite and 

pale-green chlorite. The diabase pods are probably tectonic inclusions that may originally have been diabase 

dikes. The alteration of the diabase is probably from 

metasomatism during serpentinization (Coleman,

1967). Tectonic inclusions in a similar environment in 

the northwestern part of the Yukon-Tanana Upland are 

described by Foster (1967, p. D120-D122).

QUARTZ- CARBON ATE VEINS

A vertical quartz-carbonate (magnesite and minor 

dolomite) vein with small patches of bright-green stain 

cuts through massive peridotite in the north-central 

part of the mass where the peridotite is little serpentinized. The vein is about 1.1 m wide and 3 m long and 

trends N. 80° W. The vein is close to but not on the 

northern boundary of the peridotite and probably is a 

result of minor hydrothermal activity at some time 

later than serpentinization and emplacement of the 

peridotite (Barnes and others, 1973, p. 388-390). 

Three other similar veins are shown on the map (fig. 

2).

PERIDOTITE OF FLUME CREEK

Thirty kilometers east of Mount Sorenson, a pod of

142°30'

EXPLANATION

Ultramafic rocks, mostly serpentinized

Diabase pods

< > <~| Metamorphosed basalt and gabbro,- local 

* r cumulate gabbro and pillow basalt

Greenstone

^=^%3 Permian(?) metasedimentary rocks

Paleozoic(?) greenschist-facies 

metamorphic rocks

C Quartz-carbonate vein

/\ Outcrop of chert

~^^ Fault—Arrows show relative movement

————— Contact

-+- Strike of vertical foliation

FIGURE 3. Geologic map of the peridotite of Flume Creek.
GEOLOGY OF AN ALPINE-TYPE PERIDOTITE IN MOUNT SORENSON, EAST CENTRAL ALASKA A5

serpentinized peridotite crops out at the mouth of 

Flume Creek and extends north across the Seventymile 

River (fig. 3). Part of this peridotite was described as 

the Flume Creek body (Foster and Keith, 1974, p. 

660). Petrologic similarities and field relations suggest 

that the peridotite of Flume Creek may have been part 

of the peridotite of Mount Sorenson that has been displaced approximtely 11 km to the southeast along a 

northwest-trending strike-slip fault (figs. 2 and 3).

The dominant rock type of the peridotite of Flume 

Creek is partly serpentinized harzburgite with less than 

10 percent orthopyroxene and no clinopyroxene. The 

serpentine is composed mostly of lizardite in the massive rock and of clinochrysotile in the later veinlets. 

Serpentinization of the peridotite to lizardite and 

clinochrysotile is complete in the Flume Creek area 

south of the Seventymile River, although bastite indicates that the original rock was harzburgite.

As in the peridotite of Mount Sorenson, several tectonic inclusions of diabase occur in the main part of the 

peridotite of Flume Creek and a large quartz-carbonate 

(magnesite-dolomite) zone, 12 m wide, is exposed 

along the banks of Flume Creek (Foster and Keith, 

1974, p. 660). There are also a few pods of rodingite, 

which appear from texture to have been gabbro and 

aphanitic volcanic rocks and now consist of hydrogrosTABLE \.-Semiquantitatwe spectrographic analyses of ultramafic rocks from the serpentinized peridotites of the Mount Sorenson and Flume Creek

nrcns

[Analysts: R. L. Miller, A. L. Meier, H. D. King, R. B. Tripp, J. J. Curry, E. Martinez, R. T. Hopkins, J. C. Hamilton, W. W. Vaughn, M. S. 

Rickard, W. L. Campbell, T. A. Roemer, D. G. Murrey, V. Shaw, T. Huffman, L. B. Riley, N. D. Goss, and J. Haffty. Analysts for Pt and Pd: 

R. R. Carlson, E. F. Cooley, and K. J. Curry. Values for Fe, Mg, Ca, and Ti in percent; others in parts per million. Values for Au obtained 

by atomic absorption; values for Pd and Pt obtained by fire assay and spectrographic analysis. N, not detected; L, limit of detection, --, 

not looked for]

Sample No. and 

description Ag

Peridotite of

Mount Sorenson

70AWr31 Approximately N

15 percent

serpentinized

peridotite

70AFr232 Serpentinized N

peridotite

241 Approximately N

20 percent

serpentinized

peridotite

243 Approximately N

50 percent

serpentinized

peridotite

with talc and

actinolite

2378D Serpentinized N

peridotite

2379 Quartz carbon- N

ate rock

71AFr214A Greenstone N

218 Serpentinized N

peridotite

with silica

veinlets

220 Rodingitized N

diabase

222A Greenstone N

71AWr34 Amphibole N

gneiss

36D Serpentinized N

peridotite

126 Serpentinized N

peridotite

44D Diabase N

Peridotite of

Flume Creek

68ACsl81 Serpentine L

183 Fine, dense N

greenstone

with hematite

veinlets

184 Serpentine N

185A Serpentine L

185C Serpentine .5

185E Serpentine L

393B Silicified N

diorite

As

N

N

N

N

N

N

N

N

N

N

n

N

N

N

N

N

N

700

1500

N

N

Au

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

.2

.4

.02

N

Ba

30

20

20

20

20

20

70

L

150

700

200

L

L

70

N

L

300

300

2000

1500

150

Co

300

300

200

200

200

150

70

200

50

50

50

100

100

50

300

70

150

100

70

200

70

Cr

>5000

>5000

>5000

,>5000

>5000

3000

500

3000

100

150

300

3000

2000

150

>5000

20

3000

1500

1500

5000

2000

Cu

50

70

30

70

70

100

150

70

100

150

70

5

50

150

70

200

15

50

15

30

100

Mn

1500

1500

1500

1000

1500

700

2000

1000

2000

2000

1500

700

1000

1500

2000

1500

1500

300

700

500

2000

Ni

5000

5000

5000

5000

5000

2000

150

3000

100

100

150

2000

2000

70

5000

50

3000

1500

1000

>5000

300

Pd

N

L

N

L

L

--

N

.01

N

.005

N

,007

N

--

..

N

L

N

,003

,007

N

Pt

L

N

.01

N

N

--

N

N

N

N

N

N

.005

--

..

N

L

N

N

N

N

Sc

15

15

15

20

15

15

70

30

70

50

50

15

15

50

20

50

15

7

10

15

50

Sr

L

L

L

L

L

L

150

N

1500

300

150

N

N

200

L

150

L

L

300

L

150

V

30

50

50

70

50

50

500

70

300

bOO

300

30

50

300

100

500

50

30

30

30

200

Fe

10

15

15

10

15

7

15

10

10

10

7

7

10

7

20

20

15

7

3

20

15

Mq Ca Ti

>10 1 0.003

>10 1.5 .005

>10 .7 .015

>10 1 .015

>10 1.5 .02

>10 .3 .01

7 7 1

>10 .05 .015

7 5 .7

3 5 .7

5 3 .7

>10 .2 .03

10 L .007

57 .7

>10 .07 .15

5 7 >1

>10 .3 .03

10 1 .02

10 7 .03

>10 .1 .007

>10 20 .3
A6 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

sularite, with subordinate diopside, chlorite, and prehnite. Late crosscutting serpentine veinlets are scarce.

The northern contact of the peridotite of Flume 

Creek is not exposed because of vegetative cover but 

may be a fault against metamorphic rocks consisting of 

greenstone, mica schist, quartzite, phyllite, and some 

felsic volcanic rocks. At the southern boundary the 

metamorphic rocks are in contact with a sequence of 

steeply southward dipping layered cumulate gabbro 

and pillow basalt with sporadic patches of red chert. 

Samples were collected to be examined for radiolarians 

for age determination, but no radiolarians were found 

(D. L. Jones, oral commun., 1979). Most of the cumulate gabbro consists of rounded orthopyroxene grains 

that have been partly serpentinized; one layer consists 

of rounded grains of plagioclase with intergranular 

clinopyroxene. The cumulate gabbros have not developed any metamorphic fabric from the regional 

metamorphism of the area. The pillow basalt sequence 

is probably also dipping to the south, but the pillows 

are right side up and have no metamorphic fabric. A 

patch of very fine grained gray to green tuff and chert 

crops out south of the pillow basalt and probably 

stratigraphically overlies it, but no contact relations 

are exposed. The sequence just described south of the 

peridotite is evidence for an ophiolite, most of which 

has been faulted and dismembered.

MINOR-ELEMENT DISTRIBUTION

Semiquantitative spectrographic analyses were done 

on several samples from the peridotite of Mount Sorenson and the Flume Creek area (table 1). The values obtained are comparable to other alpine-type ultramafic 

rocks (Himmelberg and Loney, 1973, p. 1591; Loney 

and others, 1971, p. 282-283; Faust and Fahey, 1962). 

Chromium is consistently high in the Mount Sorenson 

rocks compared to the Burro Mountain, California 

(Loney and others, 1971) and Vulcan Peak, Oregon 

(Himmelberg and Loney, 1973) peridotites, although 

no concentrations of chromite were found. Only trace 

amounts of platinum-group elements, similar to those 

in the Burro Mountain peridotite, have been detected 

in the peridotites of Mount Sorenson and Flume Creek.

Gold and silver are found in the quartz-carbonate 

rock associated with the southern serpentinized part of 

the peridotite of Flume Creek. Clark and Foster (1971, 

p. 6-10) suggested that placer gold found near the 

mouth of Flume Creek formerly existed in small 

amounts in quartz veins and quartz-carbonate rock associated with the serpentine. Lode gold has been found 

in shear zones at the serpentine contact (Saunders, 

1956). The association of gold in quartz-carbonate 

zones adjacent to ultramafic rocks has been reported

from the Livengood area of the Yukon-Tanana Upland

(Foster, 1968).

ADJACENT ROCKS

The peridotite of Mount Sorenson and the basalt, 

gabbro, and greenstone form an ophiolitic sequence 

that may have been thrust over the metasedimentary 

rocks (fig. 2). Regional deformation, with associated 

regional greenschist metamorphism, then produced 

the present synform and fault system.

The ultramafic rocks are associated with a belt of 

metamorphic rocks that lies between the Tintina fault 

system on the north and a large Mesozoic granitic 

batholith to the south (Foster, 1976). The outcrop pattern of the metamorphic rocks has a northwesterly 

trend, generally parallel to the Tintina fault and its 

splays. The limbs of the peridotite of the Mount Sorenson area also generally parallel this trend. In many 

places the peridotite is in fault contact with the adjacent rocks; in other places the nature of the contact is 

concealed or otherwise uncertain. The peridotite is 

faulted internally in several places.

The rocks bordering the peridotite of Mount Sorenson on the north are poorly known but include much 

massive greenstone of mafic volcanic origin. The rocks 

that border the northern limb on the south are poorly 

exposed metamorphosed mafic igneous rocks, mostly 

basaltic and dioritic. The basaltic rocks are medium to 

fine grained and locally porphyritic and probably had 

an original composition of tholeiitic basalt. Scarce 

pumpellyite and prehnite veinlets are found in places.

Small outcrops of similar metamorphosed basalt 

occur east of the southeast limb of the peridotite and 

near the southern part of the peridotite of Flume 

Creek. A younger basalt outcrop at Alder Creek, a few 

kilometers east of the peridotite of Flume Creek, appears to be fresh and unmetamorphosed and postdates 

the greenschist metamorphic terrane.

Much of the area between the limbs of the peridotite 

consists of a diverse group of greenschist-facies rocks, 

mostly metasedimentary rocks such as gray, tan, and 

pink quartzite, quartz-mica schist, and chloritic-quartz 

schist. Infolded with the quartzite and schist are massive basaltic greenstones that have an outcrop pattern 

similar to that of the peridotite of Mount Sorenson 

(east of the area shown in fig. 2, see Foster, 1976). 

Bordering the southern limb of the peridotite on the 

south is a narrow band of metasedimentary rocks consisting mostly of fine-grained quartzite, phyllite, 

metagraywacke, and meta-argillite. A band of similar, 

but a little less metamorphosed, rocks occurs south of 

the northern limb. Green and gray chert are also included in this band; no radiolarians have been found in 

the cherts. These metasedimentary rocks are consi-
GEOLOGY OF AN ALPINE-TYPE PERIDOTITE IN MOUNT SORENSON, EAST CENTRAL ALASKA A7

dered to be of Permian age because they are on trend 

with a faulted quartzite, 13.3 km southeast of Flume 

Creek, that contains brachiopods of Permian age (Foster, 1976). The quartzite, quartz-mica schists, and 

other rocks between the two limbs cannot be dated 

precisely but are presumed to be of Paleozoic age because of crinoid columnals found in marble layers to 

the east. Relations of these rocks elsewhere in the Eagle 

quadrangle suggest that they are older than the Permian rocks.

The only known occurrence of glaucophane schist in 

the Eagle quadrangle crops out 21 km southeast of 

Flume Creek on trend with the Flume Creek body and 

other small ultramafic masses (Foster, 1976; Foster 

and Keith, 1974). The rock is medium to fine grained; 

foliation in outcrop is not obvious but is well developed 

in thin section. Epidote, glaucophane, and zoisite constitute 70 percent of the rock; albite, garnet, apatite, 

chlorite, and muscovite are minor minerals; and 

sphene, chloritoid, hematite, and calcite are accessory 

minerals. The glaucophane schist has been slightly retrograded, probably during regional greenschist-facies 

metamorphism, and glaucophane appears to have been 

partly altered to a blue-green amphibole on some grain 

boundaries and along some cleavages. The garnets 

have been partly altered to chloritoid and penninite.

The blueschist outcrop and nearby slivers of serpentinite rest on Paleozoic(?) greenschist-facies 

metasedimentary rocks (Foster, 1976). Although poor 

exposures preclude the determination of the relation of 

the blueschist rocks to the peridotite of Mount Sorenson, the implication can be made that both are parts of 

a dismembered ophiolite sequence. Blueschist 

metamorphic rocks are commonly found associated 

with ophiolite sequences that have been obducted onto 

continental margins (Coleman, 1977). The blueschist 

commonly occurs as melange or tectonic blocks as in 

the Franciscan Formation of California.

STRUCTURAL RELATIONS

The Tintina fault system is a major structural break 

extending northwestward more than 960 km from the 

Yukon Territory in Canada (fig. 1) and is probably significant in understanding the emplacement of the ultramafic rocks. The Tintina fault has been postulated to 

have had 400 km of right-lateral strike-slip movement 

(Roddick, 1967, p. 29), but in places vertical displacements have also occurred. In eastern Alaska the Tintina 

has been active in places as recently as Late Cretaceous 

or Tertiary time and separates an unmetamorphosed 

sequence of rocks of the continental margin type on the 

northeast from metamorphic rocks of several origins on

EXPLANATION

Surface outcrop area of peridotite —— -"--200-1-- Magnetic contour—Contour interval of 

500 gammas using arbitrary base. 

Doshed where contour interval is 

less than 500 gammas. Hachured to 

indicate closed magnetic depression

FIGURE 4. Aeromagnetic map of Mount Sorenson area. Aeromagnetic data are derived from Alaska Division of Geological and Geophysical 

Surveys (1972) and from an unpublished aeromagnetic survey for Cache Creek Exploration Co. by Lockwood, Kessler, and Bartlett, Inc. 

(1968).
A8 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

the southwest. The trace of the fault is locally covered 

in Alaska by Late Cretaceous and (or) Tertiary conglomerate locally interbedded with minor sandstone, 

shale, and lignite. In the vicinity of Mount Sorenson, 

there are several northwest-trending faults believed to 

be splays of the Tintina fault, the largest of which is 

along the Seventymile River (Foster, 1976). In the vicinity of Mount Sorenson, foliations in the metamorphic rocks and outcrop patterns also have a northwesterly strike.

The major structure of the peridotite of Mount 

Sorenson and the adjacent rocks that lie between the 

Tintina fault on the north and the granitic rocks on the 

south is that of a synform mass with a west-northwesttrending axis. However, the structure is more complex 

than the reconnaissance mapping shows (Foster, 

1976), and small faults and folds are abundant. The involvement of the peridotite and greenstones in the synform structure indicates that the area was folded subsequent to the emplacement of the peridotite.

The aero magnetic map of the area (fig. 4) shows that 

the surface outcrop pattern of the peridotite of Mount 

Sorenson conforms fairly closely to the configuration of 

a magnetic high. Two magnetic highs are opposite each 

other on the eastern limbs of the peridotite body and 

are of much higher intensity than that over the less 

serpentinized main body of the peridotite. Variations in 

the amount and distribution of secondary magnetite 

generated during the serpentinization process account 

for these magnetic anomaly differentials. Maximum 

aeromagnetic relief on the peridotite of Mount Sorenson is 3,000 gammas (fig. 4); that on the Flume Creek 

body is 1,100 gammas.

ORIGIN AND GEOLOGIC HISTORY

The peridotite of Mount Sorenson as characterized in 

this paper is a typical alpine-type ultramafic body, and 

it can be further classified as a harzburgite subtype according to the classification of Jackson and Thayer 

(1972).

The possible origins of peridotite prior to deformation within the mantle have been discussed by Himmelberg and Loney (1973, p. 1596-1597) and Coleman 

(1977). We assume, in accordance with detailed studies 

by Loney, Himmelberg, and Coleman (1971, p. 303) 

and Coleman (1971; 1977) on other alpine-type peridotites, that the deformational textures were developed in 

the mantle.

The peridotite of Mount Sorenson can be considered 

as part of an ophiolite, probably a dismembered 

ophiolite, on the basis of the following characteristics:

1. Rock type is predominantly serpentinized 

harzburgite and some dunite.

2. Slightly metamorphosed basaltic rocks are proximal to Mount Sorenson.

3. Sequence of peridotite, cumulate gabbro, pillow 

basalt with red chert, and aphanitic volcanic rocks 

occur along the southern part of the peridotite of 

Flume Creek.

4. Green- and gray-banded chert crops out in minor 

amounts in the vicinity of Mount Sorenson and Flume 

Creek.

5. Fault contacts are common.

6. Diabase is present as tectonic inclusions or dikes 

that have been deformed (Foster and Keith, 1974, p. 

665).

The manner of emplacement of the Mount Sorenson 

and other ultramafic bodies in the Eagle quadrangle 

was considered by Foster and Keith (1974, p. 

665-667). The proximity of the peridotite of Mount 

Sorenson and many other ultramafic bodies to the Tintina fault strongly suggests that the proposed mantle 

source of the peridotite could have been derived from a 

deep crustal suture, possibly a forerunner of the Tintina fault (Churkin and others, 1980). The location of 

the Tintina fault may also define the approximate ancient margin of the North American plate, possibly in 

Paleozoic time. Unmetamorphosed rocks to the north 

of the Tintina fault are characteristic of rocks formed 

from continental shelf-platform deposits along continental margins (Churkin and others, 1980). However, 

a large proportion of the metamorphic rocks on the 

south side of the Tintina fault may also have been derived from continental deposits. Thus, we suggest that 

the metamorphic rocks south of the Tintina fault are a 

slice of continental crust or a small plate that has 

moved northwestward over oceanic crust along the 

margin of the North American plate. During this 

movement, postulated to be late Paleozoic time, some 

oceanic crust and mantle (derived from depth along 

the suture) were caught up along the suture and locally 

obducted onto the small continental plate. The occurrence of glaucophane schist in the vicinity of the 

peridotite is tenuous but supporting evidence for the 

obduction process (Coleman, 1971; 1977). Serpentinization of the boundaries of the peridotite took place as 

it was emplaced into the crust and came into contact 

with meteoric waters.

After initial emplacement, the peridotite masses were 

broken up and displaced by faulting (including thrusting), slightly metamorphosed, and infolded with the 

adjacent country rock. This deformation was followed 

by several hundred kilometers of right-lateral movement along the Tintina fault, probably in Late Cretaceous time (Roddick, 1967, p. 30). Extensive granitic 

intrusive activity also took place, probably mostly in 

Late Cretaceous time, as shown by potassium-argon
GEOLOGY OF AN ALPINE-TYPE PERIDOTITE IN MOUNT SORENSON, EAST CENTRAL ALASKA A9

dating on a nearby granitic pluton (Foster, 1976). 

Still later faulting associated with the Tintina fault may 

have caused minor displacements of the peridotite of Mount Sorenson and displaced the peridotite 

of Flume Creek.

Alpine-type ultramafic rocks in the Yukon Territory, 

Canada, which may have a history related to those of 

the Eagle quadrangle, are discussed by TempelmanKluit (1976, p. 1343; 1979). He postulates that some 

small alpine-type ultramafic masses in the Yukon were 

emplaced along steep southwest-dipping faults that 

represented an Early Triassic interval of movement 

along the Tintina fault. Tempelman-Kluit (1976, p. 

1354) explains the associated basalt, peridotite, and 

chert in the metamorphic terrane of the Yukon as 

thrust remnants originating either from the southwest 

or northeast. He also considers the possibility that the 

peridotites are an oceanic assemblage derived from a 

rifted part of the Tintina trench (which was a deep 

crustal fracture) that was thrust southwestward over 

the metamorphic terrane when the trench closed 

(Tempelman-Kluit, 1979, p. 25).

The ophiolitic assemblages of the Yukon, described 

by Tempelman-Kluit (1976), are more obviously thrust 

than those at Mount Sorenson since the basal sections 

are locally exposed. The ophiolitic assemblages of 

Mount Sorenson and Flume Creek are similar to those 

along the Tintina fault zone in the Yukon, and hypotheses of emplacement of the ophiolites are comparable, 

although minor differences in timing and resolution of 

the direction of thrusting have yet to be worked out. 

Ophiolite belts are described in other parts of Alaska 

(Patton and others, 1977) and provide information on 

the tectonic history of Alaska. The relations of the 

ophiolitic complex in the Mount Sorenson area to other 

ophiolite belts in Alaska and Yukon are not clearly understood but may ultimately be significant in unraveling the tectonic history of this part of North America.

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