Geology of an Upper Cretaceous 

Copper Deposit in the Andean 

Province, Lassiter Coast, Antarctic 

Peninsula

GEOLOGICAL SURVEY PROFESSIONAL PAPER 984

Work done in cooperation with 

the National Science Foundation

GEOLOGY OF AN

UPPER CRETACEOUS COPPER DEPOSIT

IN THE ANDEAN PROVINCE,

LASSITER COAST, ANTARCTIC PENINSULA
o

s

'V *

Physiographic setting of the Copper Nunataks (circled). View westsouthwest toward the Antarctic Peninsula ice plateau in the 

background. Western part of the Hutton Mountains in the forground; Johnston glacier separates these mountains from the

Guettard Range in the middleground. The narrow RARE Range 

lies beyond the well-defined 6-km-wide Irvine Glacier, and it is 

separated by the Wetmore Glacier from several nunatak clusters 

of the Latady Mountains in the left background.
Geology of an Upper Cretaceous 

Copper Deposit in the Andean 

Province, Lassiter Coast, Antarctic 

Peninsula

By PETER D. ROWLEY, PAUL L. WILLIAMS, 

and DWIGHT L. SCHMIDT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 984

Work done in cooperation with 

the National Science Foundation

A discussion of the southernmost known 

circum-Pacific copper deposit, which has 

affinities to the porphyry copper class 

but has no present economic potential

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1977
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Rowley, Peter D.

Geology of an Upper Cretaceous copper deposit in the Andean Province, Lassiter Coast, Antarctic Peninsula.

(Geological Survey Professional Paper 984)

Bibliography: p.

1. Copper ores Antarctic regions Lassiter Coast. 2. Geology Antarctic regions Lassiter Coast. .3. Geology, Stratigraphic 

Cretaceous. I. Williams, Paul Lincoln, 1932- joint author. II. Schmidt, Dwight Lyman, 1926- joint author.

HI. United States, National Science Foundation. IV. Title: Geology of an Upper Cretaceous copper deposit in the Andean

Province... V. Series: United States Geological Survey Professional Paper 984. 

QE390.2.C6R68__________________ 549'.23 76-45929 __________

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

Washington, D.C. 20402

Stock Number 024-001-02955-3
CONTENTS

Page

Abstract-_____________________________________________ 1

Introduction ________________________________________________ 1

Acknowledgments __________________________________ 2

Igneous rocks ___-------_-_______________________-________ 4

Medium-grained granodiorite pluton ____________________ 4

Aplite and pegmatite dikes ______________________________ 8

Dacite dike ____________________________________________ 8

Copper Nunataks pluton ________________________________ 8

Aplite and pegmatite dikes ______________________________ 14

Granodiorite porphyry dikes ____________________________ 15

Interpretation of the chemical and modal data ____________ 17

Trends of systematic joints ____________________________ 20

Page

Older shear fracturing, hydrothermal alteration, and mineralization____________________________________ - -------- 20

Younger shear fracturing, hydrothermal alteration, and mineralization ______________________________________________ 25

Mineralization__________________________________________ 27

Phyllic and argillic alteration____________________________ 28

Propylitic alteration __________________________________ 29

Epidote alteration ______________________________________ 30

Silicification____________________________________________ 31

Mineralized and altered rock elsewhere in the Lassiter Coast and 

southern Black Coast ________________________________ 31

Conclusions _________________________________________ 35

References cited _______________________________________ 35

ILLUSTRATIONS

Page

FRONTISPIECE. Photograph showing physiographic setting of the Copper Nunataks.

FIGURES 1-4. Maps of:

1. West Antarctica showing location of Lassiter Coast project area and Copper Nunataks___--_-----_-------- 1

2. Lassiter Coast and southern Black Coast showing locations of known mineralized and hydrothermally altered

rock ______________________________________________________________________ 3

3. The Copper Nunataks showing generalized geology ------------------------------------ __________ ___ 4

4. Three of the Copper Nunataks showing the geologic features______________________________________ 5

5. Ternary diagram showing modal distribution of quartz, K-feldspar, and plagioclase from igneous rocks in the Copper

Nunataks ______________________________________________________________________ 6

6. Maps showing sample localities and outcrops in the Copper Nunataks _______________________----- 7

7. Photograph of Nunatak B _______________________________________________________ 8

8. Photomicrograph of granodiorite pluton _____________________________________________________ 14

9. Photograph of mafic inclusion, aplite dikes, and gray shear fractures cutting granodiorite pluton ________________ 14

10. Photomicrograph of dacite dike ________________________________________________________-_-_---_____ 15

11. Photograph showing a typical outcrop of Copper Nunataks pluton, Nunatak A ________________________________ 15

12. Photograph showing a typical outcrop of Copper Nunataks pluton, Nunatak D ______________________________ 16

13. Closeup photograph of outcrop of Copper Nunataks pluton, Nunatak D __________ ___ ___ _______ _ 16

14. Photomicrographs of Copper Nunataks pluton, Nunataks B and E ______________________________ 17

15. Photograph of aplite dikes in a stoped block in the granodiorite pluton, Nunatak A____________ _____________ 19

16. Photomicrograph of chilled margins of granodiorite porphyry dike_____________-__________________ 21

17. Photomicrograph of interior of granodiorite porphyry dike _____________________________________-____---_ 21

18. Variation diagram of common oxides, selected trace elements, and differentiation index plotted against SiCfe in

specimens from igneous rocks in the Copper Nunataks --_-__________________________________________ 22

19. Ternary plots of selected normative minerals, Earth's cations, and oxides in specimens from igneous rocks in the

Copper Nunataks _____________________________________________________________________ 23

20. Rose diagram of strikes of systematic steeply dipping joints in rocks in the Copper Nunataks ___- _____________ 24

21. Photograph of granodiorite pluton and pegmatite dikes offset by several black shear fractures.______________ 24

22. Photomicrograph of black shear fracture-___________________________________----_-__----____-------_______ 24

23. Photographs of sheared phyllic-argillic zones, Nunataks A, B, and C________________________________________ 26

24-29. Photomicrographs of altered rock:

24. Phyllic altered rock of the granodiorite pluton ______________________________________________ 29

25. Phyllic altered rock of the granodiorite porphyry dike ________________________________ 29

26. Sheared and phyllic altered rock of the Copper Nunataks pluton __________________________ _____ 30

27. Argillic altered rock of the Copper Nunataks pluton__________________-----------------_-------_-------- 30

28. Propylitic altered rock of the granodiorite pluton ___________________________________________ 31

29. Silicified rock of the granodiorite pluton ___________________________________________________ 31

V
VI CONTENTS

TABLES

Page

TABLE 1. Modes of fresh igneous rocks in Copper Nunataks, Antarctica.---______________________ _______ _ 10

2. Chemical and modal data of selected fresh rocks from the major rock units from the Copper Nunataks, Antarctica ___ 12

3. Semiquantitative spectrographic analyses of selected fresh, altered, and mineralized rock from the Copper Nunataks,

Antarctica. ___ __________ _____________ ________________________________________________________ 18

4. Results of X-ray determination of sheet silicates from hydrothermally altered rocks of the Copper Nunataks, Antarctica __________________________________________________________________ 20

5. Semiquantitative spectrographic analyses of biotite-hornblende separates from fresh igneous rocks in the Copper

Nunataks, Antarctica _____________________________________________________________________ 28

6. Semiquantitative spectrographic analyses of selected fresh, altered, and mineralized rock from scattered locations

elsewhere in the Lassiter Coast and southern Black Coast __________________________________________ 32
GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT 

IN THE ANDEAN PROVINCE, LASSITER COAST, ANTARCTIC

PENINSULA

By PETER D. ROWLEY, PAUL L. WILLIAMS, and DWIGHT L. SCHMIDT

ABSTRACT

Mineralization with affinities to the porphyry copper class terminated two Upper Cretaceous episodes of similar igneous-tectonic 

events in the Copper Nunataks area of the Lassiter Coast, Antarctica. The first episode, at about 105 m.y. (million years), began with 

intrusion of a medium-grained granodiorite batholith, followed by 

emplacement of thin hornblende dacite dikes, and ended with two 

events of fracturing accompanied by minor hydrothermal alteration 

and pyrite mineralization. The second episode, at about 95 m.y., 

began with intrusion of a coarse-grained and mostly porphyritic, 

concentrically zoned quartz monzonite stock (Copper Nunataks pluton), followed by emplacement of 1- to 20-m-wide granodiorite porphyry dikes, and ended with extensive shearing accompanied by 

major hydrothermal alteration and copper mineralization (Lassiter 

Coast copper deposit). Altered rock associated with the second 

episode occurs along northwest-striking sheared phyllic-argillic 

zones, between which is propylitic-altered rock. Mineralization, in 

the same location as propylitic, argillic, and phyllic rock, produced 

quartz veins, fist-size clots of chalcopyrite and pyrite, and disseminated crystals of chalcopyrite and pyrite. The main ore minerals are 

chalcopyrite, pyrite, and magnetite; malachite, molybdenite, hematite-limonite, and apparently chalococite are less abundant. Average 

mineral grade of exposed rock probably does not exceed 200 ppm Cu, 

100 ppm Pb, and 50 ppm Mo. Thus there is no evidence that the 

deposit is of ore grade even if it were feasible to be mined, but its 

discovery demonstrates that the Antarctic Peninsula may be placed 

within the circum-Pacific copper province. The presence of the occurrence suggests the possibility of undiscovered copper deposits of 

economic grade elsewhere in the peninsula.

INTRODUCTION

The world's southernmost occurrence of significant 

copper minerals was discovered in the Lassiter Coast 

area of Antarctica by D. L. Schmidt and L. E. Brown 

during the 1969-70 austral summer field season. Exposures occur in the Copper Nunataks, 1 a small cluster 

of rock hills about 10 km west of the RARE (Ronne 

Antarctic Research Expedition) Range near the crest of 

the ice plateau (2,000 m elevation) of the Antarctic 

Peninsula about 130 km west of the Weddell Sea (fig. 

1). A more detailed field study was conducted during 5 

days of the 1970-71 season by P. L. Williams (party 

leader), P. D. Rowley, R. L. Reynolds, and A. B. Ford.

JName recommended for initial approval by Advisory Committee on Antarctic Names.

This report presents the results of field studies, detailed petrographic studies, and chemical analyses.

The Antarctic Peninsula is a geologic continuation of 

the Chilean and Patagonian Andes (Adie, 1955, 1964; 

Halpern, 1971; Ford, 1972; Dalziel and Elliot, 1973), 

but the continuation of this Andean orogen south of the 

Antarctic Peninsula is more uncertain. Evidence such 

as a sharp westward bend in the structure at the south 

base of the Antarctic Peninsula (Laudon, 1972; Laudon 

and others, 1969) and some similarities between the 

rocks of the peninsula and those of western Ellsworth

600 MILES

0 200 400 600 KILOMETERS

FIGURE 1. West Antarctica, showing location of Lassiter Coast 

project area (diagonal ruling) and Copper Nunataks (black 

square).
GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

Land and Marie Byrd Land (for example, Wade, 1969) 

suggests that the orogen passes through these parts of 

West Antarctica (Craddock, 1970a, 1972; Halpern, 

1971). Geologic similarities suggest that this belt, 

which is the southern loop of the great circum-Pacific 

mountain chain, may extend through New Zealand 

(Craddock, 1970b; Elliot, 1972; Bradshaw and Andrews, 1973) and into the western Pacific area.

The geology of the Antarctic Peninsula has been 

summarized by Adie (1964, 1969). He and other workers noted that calc-alkaline stocks and batholiths of 

the Cretaceous and Tertiary Andean intrusive suite 

(Adie, 1955) dominate the geology of the Antarctic 

Peninsula. Plutons of this suite intrude (1) a lower 

Paleozoid?) basement complex of igneous and metamorphic rock (Adie, 1954), (2) a thick sequence of black 

sedimentary rock of the Carboniferous(?) to lower 

Mesozoic(?) Trinity Peninsula Series (Adie, 1957) that 

were probably folded in Triassic or Jurassic time, and 

(3) a thick sequence of black Upper Jurassic and Lower 

Cretaceous sedimentary rock and associated volcanic 

rock (Upper Jurassic) that were probably folded in Cretaceous time. Gently folded to horizontal Upper Cretaceous sedimentary rock (Bibby, 1966) and Cenozoic 

sedimentary and volcanic rock unconformably overlie 

the older rocks. Rowley and others (1975) compared the j 

Lassiter Coast copper deposit to other metal occurrences in the Antarctic Peninsula and concluded that 

metallization is associated with Andean plutonic rock 

and that the peninsula belongs to the circum-Pacific 

copper province.

Reconnaissance geologic investigations over about 

30,000 km2 of the Lassiter Coast, northern Orville 

Coast, and southern Black Coast (Williams, 1970; Williams and Rowley, 1971, 1972; Williams and others, 

1972; Rowley, 1973) show that the area is part of the 

Andean orogen. The oldest known rocks consist of 

black fine-grained sedimentary rocks (Latady Formation) of Late Jurassic age (R. W. Imaly and E. G. 

Kauffman, written commun., 1975); these rocks have 

been tightly folded around northeast to northnortheast axes (Kellogg and Rowley, 1974) prior to intrusion of numerous stocks and batholiths. Metamorphic aureoles, containing grades as high as andalusite 

hornfels, surround the plutons (Plummer, 1974). The 

plutons belong to a calc-alkaline suite (Rowley and 

Williams, 1974) consisting largely of quartz diorite, 

granodiorite, and quartz monzonite but they range 

from gabbro to granite. Numerous occurrences of 

minor hydrothermally altered rock, copper sulfides, 

and other ore minerals are associated with the Andean 

plutonic rocks (fig. 2); these occurrences are discussed 

in the section on "Mineralized and altered rock elsewhere in the Lassiter Coast and southern Black 

Coast."

Biotite-hornblende pairs from five plutons in the 

southern and central Lassiter Coast yield K-Ar ages 

ranging between 119 and 99 m.y. (Mehnert and others, 

1975). Potassium-argon ages of biotite and hornblende 

from eight samples of plutonic rocks in the northern 

Lassiter Coast and southern Black Coast range between 114 and 98 m.y. (Rowley and others, 1976). The 

ages agree with three K-Ar ages given below (see pp. 4, 

8, and 15) for rocks from the Copper Nunataks, determined by A. H. Clark, Edward Farrar, and S. L. 

McBride of Queen's University, Ontario, Canada.

The Lassiter Coast copper deposit is by far the highest grade and most extensive of the mineral occurrences on the Lassiter Coast, and it is the only one that 

received any detailed field and laboratory investigation. It has affinities with the porphyry copper class of 

mineral deposits. However, the average copper content 

of samples from the limited exposures is probably no 

more than 200 ppm, well below present economic grade 

even for United States domestic deposits.

The bedrock in the Copper Nunataks (fig. 3) consists 

entirely of igneous rocks of Late Cretaceous age. The 

geologic history of these rocks may be divided into two 

broad episodes consisting of a nearly identical series of 

events; both episodes terminated with mineralization. 

Each episode consisted of intrusion of a pluton followed 

by emplacement of aplite and pegmatite dikes, intrusions of northwest-trending mafic porphyritic dikes, 

and faulting or "crackling" accompanied by mineralization and hydrothermal alteration. The mineralization and alteration during the younger episode were 

more intense and resulted in the Lassiter Coast copper 

deposit.

The well-exposed bedrock (fig. 3) in the Copper 

Nunataks is fresh because it was scoured during 

higher ice stands. Summer air temperatures extend to 

-30°C and rarely exceed 0°C; the humidity is extremely low, so megascopic organisms are limited to 

sparse patches of lichens on the rocks. Rock temperatures, however, have been recorded at above +20°C for 

several hours a week during several weeks at midseason. Under these conditions, chemical weathering 

takes place very slowly (Boyer, 1975); therefore supergene minerals are scarce and most alteration and 

mineralization products discussed below are considered to be hypogene.

ACKNOWLEDGMENTS

This work is part of a U.S. Geological Survey 

geologic study of the Lassiter Coast area done under
INTRODUCTION

»Bo64b

66° 00' 64°00'

73° 00'

73° 30

74° 00'

74° 30

75° 00'

75°30'

60° 00'

EXPLANATION

x Locality exhibiting 

mineralized rock

Locality exhibiting 

hydrothermally 

altered rock

 Other locality men- J 

tioned in table 5 "^

ICE SHELF

60 KILOMETERS

FIGURE 2. Lassiter Coast and southern Black Coast showing locations of known mineralized and hydrothermally altered rock.
GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

74° 20'S

Nunatak D

65° OO'W Fig.4C

Inferred contact 

of Copper Nunataks 

pluton

Nunatak C

Nunatak E

Nunatak A

Nunatak B

a u

Q. eg

Copper Nunataks pluton 

(quartz monzonite)

Granodiorite pluton

Contact Dashed where concealed by snow or ice

Fault Dotted where concealed by snow or ice

0 2 MILES

0 1 2 KILOMETERS 

FIGURE 3. Generalized geology of the Copper Nunataks.

the auspices of, and financed by, the National Science 

Foundation (NSF) (Grant AG-187). The U.S. Antarctic 

Research Program of the National Science Foundation 

and the U.S. Navy Operation Deep Freeze provided 

logistic support in Antarctica. Many persons made the 

study possible. The memory of the late Prof. Duncan 

Stewart VII of Carleton College, a former teacher, colleague, and friend, provided inspiration for this report. 

M. D. Turner, Program Manager of Polar Earth Sciences, Division of Polar Programs of NSF, approved 

the program and provided logistic help. L. E. Brown, 

A. B. Ford, and R. L. Reynolds of the U.S. Geological 

Survey aided the writers with the field mapping of the 

Copper Nunataks. Copper sightings elsewhere in the 

Lassiter Coast area were made by the other geologists 

during the field seasons: C. C. Plummer during the 

1969-70 season, W. H. Nelson during the 1970-71 season, and S. J. Boyer, E. N. Kamenev, K. S. Kellogg,

W. R. Vennum and R. B. Waitt, Jr., during the 1972- 

73 season; all were affiliated with the U.S. Geological 

Survey except Kamenev, who is at the Research Institute of the Geology of the Arctic, Leningrad, USSR. 

Potassium-argon age dating was done by A. H. Clark, 

Edward Farrar, and S. L. McBride of Queen's University, Ontario, Canada. L. G. Schultz of the U.S. Geological Survey aided the senior author in X-ray identification of clay minerals and interpreted the data. Discussions with A. V. Heyl and R. U. King of the U.S. 

Geological Survey helped form conclusions. G. A. Desborough and D. P. Cox reviewed the manuscript.

IGNEOUS ROCKS

Rocks in the Copper Nunataks are discussed in age 

sequence from oldest to youngest; their distribution is 

shown on the geologic sketch map (fig. 4). The modal 

classification (fig. 5) follows Bateman (1961, fig. 2). 

Sample localities and outcrops are shown in figure 6. 

Anorthite content of plagioclase was determined by the 

a-normal method using the low-temperature curves of 

Wahlstrom (1947, fig. 21, curve D).

MEDIUM-GRAINED GRANODIORITE PLUTON

The first event in the earliest igneous episode was 

intrusion of the older of two large plutons in the area. 

This granodiorite body is characterized by its uniform 

composition and texture, light- to medium-gray color 

due to abundant ferromagnesian minerals and dioritic 

inclusions, and at least minor alteration in nearly all 

exposures. The rock is generally nonfoliated, but in 

several places a vague flow foliation of ferromagnesian 

minerals strikes northwest. It is mostly medium 

grained (1-5 mm), but in some places it is gradational 

to fine grained. Studies by A. H. Clark, Edward Farrar, 

and S. L. McBride (written commun., 1974) give a 

K-Ar biotite determination of 104.9± 1.6 m.y. for this 

pluton (specimen M308a).

The granodiorite is exposed at Nunataks A and B 

(figs. 4, 5, 7) as massive resistant outcrops cut by 

medium spaced to finely spaced joints; it underlies at 

least 15 km2 in the area of the five largest nunataks of 

figure 3. No contacts with older rocks are exposed in 

the area, but on the basis of similar petrology (table 1) 

and chemistry (table 2), the granodiorite is probably 

correlative with a batholith (West RARE pluton) in the 

western nunataks of the RARE Range about 8 km to 

the northeast. The West RARE batholith discordantly 

intrudes folded slate and sandstone of the Upper 

Jurassic Latady Formation (Williams and others, 

1972), and contact metamorphism reaches andalusite 

hornfels grade (Plummer, 1974). Pluton contacts are 

commonly chilled and are accompanied by igneous 

apophyses in the metamorphic country rock; as else-
IGNEOUS ROCKS

Nunatak C

Nunatak A

Nunatak B

CO

D 

O

w 

u _

Kgd

Kgd

EXPLANATION

Granodiorite porphyry dike

Copper Nunataks (quartz 

monzonite) pluton

Dacite dike

0.2 0.4 0.6 MILE

0 0.2 0.4 0.6 0.8 KILOMETER

Sheared phyllic-argillic zone 

Dashed where location uncertain; dotted where covered 

by ice of snow

Nunatak ridge crest

Outer edge of snow- and 

ice-covered nunatak

FIGURE 4. Geology of three of the Copper Nunataks (fig. 3).
GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

K feldspar 

(including perthite)

Plagioclase

EXPLANATION

Cretaceous

Younger igneous episode

Aplite of granodiorite porphyry dike

Granodiorite porphyry dike

Aplite of Copper Nunataks pluton

Quartz monzonite of Copper Nunataks pluton

Assimilated rocks Of same age as 

Copper Nunataks pluton

Older igneous episode

A Dacite dike

n Aplite of granodiorite pluton

o Granodiorite pluton

o West RARE pluton Of same age as 

granodiorite pluton

FIGURE 5. Modal distribution of quartz, K-feldspar, and plagioclase 

in fresh rocks from the major igneous units, Copper Nunataks. 

Calculated from data in table 1. Insert shows igneous classification 

used.

where in the Lassiter Coast, evidence supports the conclusion that the plutons originated by magmatic processes of passive to forceful intrusion. If the petrologic 

and age correlations are correct, the medium-grained 

granodiorite is at least 20 km long in a northeast direction and 10 km wide in a northwest direction.

Modal analyses for 10 samples from the mediumgrained granodiorite and their averages are presented 

in table 1. Plagioclase, hornblende, and biotite are 

subhedral in most thin sections; quartz and K-feldspar 

are anhedral (fig. 8). Minerals are equigranular except 

for scattered large inconspicuous light-gray poikilitic 

K-feldspar crystals which are as much as 2 cm long 

and which enclose other minerals. Crystals other than 

K-feldspar average about 1 mm and rarely exceed 

3 mm. K-feldspar is rarely grid-twinned; perthite is 

rare to absent. Most plagioclase is zoned, anorthite 

content in most thin sections ranges from sodic labradorite cores to calcic oligoclase rims. Nonzoned 

crystals have an average composition of intermediate 

andesine. In some places, the boundaries between plagioclase and K-feldspar or plagioclase and quartz contain small areas of myrmekite. Smoky quartz is abundant and has undulatory extinction in irregular 

masses as large as 5 mm across. Biotite is strongly 

pleochroic (X=pale yellow, Z=dark brown to dark reddish brown). Most biotite shows at least some alteration to chlorite (penninite?) along cleavage planes and 

to a much lesser degree to epidote. Hornblende is also 

markedly pleochroic (X=yellowish green, Y=grass 

green, Z=olive green to dark yellowish green). Content 

of biotite is relatively uniform, whereas the content of 

hornblende is quite variable. Magnetite is the most 

abundant opaque mineral; minor amounts of pyrite 

and ilmenite are present. Sphene is rare and occurs as 

small poorly developed crystals. Apatite and zircon are 

present.

A triangular plot of plagioclase, K-feldspar, and 

quartz (fig. 5) shows that most rocks fall in a tight 

cluster in the granodiorite field near the boundary 

with quartz monzonite. Similar data for four samples 

from the West RARE pluton plot in the same area in 

figure 5 add further support to the proposed correlation 

of these two rock bodies.

Three rapid rock chemical analyses and three semiquantitative spectrographic analyses of fresh rock of 

the granodiorite pluton and West RARE pluton are 

presented in tables 2 and 3. The analyses from the 

granodiorite are somewhat lower in potassium and 

sodium and higher in calcium than the average 

hornblende-biotite granodiorite of Nockolds (1954). 

Sample W144a, representative of the West RARE pluton and from 15 km northwest of Nunatak C, is only 

slightly more silicic; it is lower in potassium and 

sodium and higher in calcium than Nockolds' average 

granodiorite. Trace elements of sample W144a have 

concentrations similar to those of the medium-grained 

granodiorite.

The plutonic rock contains many medium- to darkgreenish-gray, ovoid to subspherical inclusions that 

average 5 cm in diameter; some exceed 1 m in diameter (fig. 9). Exposures average three or four inclusions 

per m2 of outcrop face, but in some places inclusions 

constitute more than 5 percent of rock volume. Mafic 

inclusions are remarkably uniform in texture, composition, and size within each pluton; most inclusions are 

fine-grained rocks (average crystal size less than 

1 mm), with or without phenocrysts of plagioclase, 

hornblende, and (or) biotite. Inclusion edges are mostly
IGNEOUS ROCKS

Nunatak A

Nunatak B

M203 

B26(M

Nunatak C EXPLANATION

Outcrops and sample localities 

not shown

M429 at Nunatak D 

M430 at Nunatak E

0 0.2 0.4 0.6 MILE

0 0.2 0.4 0.6 0.8 KILOMETER

FIGURE 6. Sample localities and outcrops (patterned) in the Copper Nunataks.
GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

FIGURE 7. Southwestern face of Nunatak B. Black curved line extending from one end of the nunatak to the other just below the 

crest is the dacite dike.

sharp, but some exhibit narrow gradational contacts 

with the enclosing igneous rock. Two thin sections 

(table 1) are of a hypidiomorphic-granular to allotriomorphic-granular hornblende diorite and hornblende 

syenodiorite; minerals are the same as those of the 

enclosing granodiorite, consisting mostly of abundant 

euhedral hornblende and uncommon biotite with interstitial plagioclase (calcic andesine), K-feldspar, and 

quartz.

APLITE AND PEGMATITE DIKES

Light pink, or less commonly white, aplite and pegmatite dikes are abundant (to 10 percent of rock volume in some areas) end-stage products of the intrusive 

activity that produced the medium-grained granodiorite (fig. 9). Most aplite dikes are 2-15 cm wide, but 

several are more than 10 m wide. Coarse-grained 

(5 mm to 3 cm) to very coarse grained pegmatite dikes 

are intimately associated with, and in some places 

gradational into, aplites; they have the same dimensions and attitudes as the aplite dikes but are significantly less abundant. The dominant strike of aplitepegmatite dikes is northwest; dips range from 40° to 

vertical with no apparent preferred orientation. Two 

thin sections (table 1) have a typical fine-grained 

allotriomorphic-granular texture and consist chiefly of 

K-feldspar (rarely grid-twinned), quartz, and less 

abundant plagioclase; perthite and myrmekite are 

rare. The rocks are in the quartz monzonite and granite fields of the plagioclase, K-feldspar, and quartz 

diagram (fig. 5).

DACITE DIKE

A single thin (1-4 m wide) sub vertical north-northeast- to west-northwest-striking, dark-green to black 

dike cuts the medium-grained granodiorite, associated 

aplites, and pegmatites at Nunatak B (figs. 4, 7). In 

most places it is a hornblende dacite, with finer

grained chilled margins; at several places where it is 

thicker, the interior is a lamprophyre of small (1- 

3 mm) hornblende phenocrysts in a fine-grained 

phaneritic matrix. Small pyrite crystals as much as 

2 mm across are conspicuous in hand specimen.

Contained minerals (3 thin sections, table 1) are the 

same as those in the medium-grained granodiorite. 

The seriate or porphyritic rock consists mainly of 

euhedral green hornblende in a groundmass of mostly 

anhedral crystals of abundant plagioclase and 

hornblende. Minor K-feldspar, quartz, biotite, and 

opaque minerals are present (fig. 10). Ferromagnesian 

minerals are moderately to extensively altered to chlorite and epidote; anorthite content of plagioclase cannot be determined, owing to saussuritization. This alteration is probably deuteric, because the adjacent 

granodiorite wallrock is fresh. The ternary diagram 

(fig. 5) shows that the rocks fall in the dacite field and 

in the lower right part of the rhyodacite field. A single 

chemical analysis of a dacite dike is given in tables 2 

and 3.

COPPER NUNATAKS PLUTON

The dominant rock type in the Copper Nunataks is 

that of the younger stock, which has an average composition of quartz monzonite and underlies an area of 

at least 40 km2 . It is here named the Copper Nunataks 

pluton. It represents the first event in the younger 

igneous episode. Specimens are easily distinguishable 

from the older granodiorite by (1) lack of alteration, 

(2) considerably coarser grain size, and (3) light-gray 

or pinkish-gray color which, owing to the sparseness of 

ferromagnesian minerals and dark inclusions, is significantly lighter in color than the granodiorite pluton 

(fig. 11). In contrast to the older batholith, the Copper 

Nunataks pluton exhibits considerable internal variety in texture and composition. At Nunataks A, B, and 

C (fig. 3), the pluton is mostly granodiorite to quartz 

monzonite, at Nunatak E it is quartz monzonite, and at 

Nunatak D it is granite (figs. 12, 13). Thus the pluton 

appears to be concentrically zoned from a more mafic 

marginal zone to a silicic interior at Nunatak D. A 

K-Ar biotite date of 95.2±1.5 m.y. was determined for 

the stock (specimen M430a) by A. H. Clark, Edward 

Farrar, and S. L. McBride (written commun., 1974).

Sharp intrusive contacts are exposed at Nunataks B 

and C; here the older granodiorite body and the dacite 

dike are sheared and slightly altered in the area from 

the contact to a distance of several centimeters. The 

Copper Nunataks pluton is somewhat finer grained at 

the margin than elsewhere in the body. At one location 

along the contact at Nunatak B, the rock is gneissic for 

a width of several meters. The light-gray, mediumgrained gneissic bands, 5-50 cm wide, vary in quartz 

and feldspar content, grain size, and amount of fer-
IGNEOUS ROCKS 9

romagnesian minerals; the rock in different bands varies from quartz diorite to granodiorite. The foliation is 

considered to be primary, strikes parallel to the contact, and dips east under the older rock.

The Copper Nunataks pluton occurs as resistant, 

massive outcrops that in places have a slightly crumbly weathered surface. Microscopic study of 17 thin 

sections (table 1) shows that different sizes (as much as 

3 cm long) and shapes (anhedral, less commonly subhedral) of pink or gray K-feldspar crystals result in 

different textures that are most commonly porphyritic 

but may be equigranular (hypidiomorphic-granular), 

poikilitic, or seriate (fig. 14); the three latter textures 

appear to predominate in the interior (Nunataks D and 

E). K-feldspar is rarely grid-twinned and perthite is 

scarce to absent. Plagioclase, hornblende, and biotite 

are subhedral and quartz is anhedral; these minerals 

average 2-3 mm in size and few exceed 1 cm. Most plagioclase crystals are zoned and have an anorthite content that is slightly lower than that of the granodiorite 

pluton: most zoned crystals range from calcic andesine 

cores to calcic oligoclase rims; most unzoned crystals 

range from sodic to intermediate andesine. Rare 

amounts of myrmekite occupy small areas near the 

boundaries of plagioclase and K-feldspar or plagioclase 

and quartz. Quartz has undulatory extinction, and in 

some places it exhibits either sutured boundaries or 

mortar texture suggestive of late-stage plutonic 

movement. Green hornblende and brown biotite have 

the same pleochroic formulas as those in the granodiorite pluton. The hornblende content varies considerably 

whereas the content of biotite is more uniform. Most 

biotite is slightly altered to chlorite (penninite?) along 

cleavages and is much less commony altered to epidote. 

Opaque minerals are abundant and most are magnetite; pyrite is virtually absent. Sphene is abundant and 

occurs as large, conspicuous euhedral crystals as much 

as several millimeters long; apatite is relatively abundant, but zircon and other nonopaque accessory minerals are rare.

On the ternary diagram (fig. 5), the rocks of the Copper Nunataks pluton occupy an elongate belt spanning 

the quartz monzonite field extending from just inside 

the granodiorite field to the edge of the granite field. 

Specimens collected close to the contacts are more 

mafic and contain more biotite, hornblende, and 

sphene, and have a higher hornblende-to-biotite ratio 

(table 1) than those collected farther inward from the 

contact. In comparison with rocks of the granodiorite 

batholith, the rocks of the Copper Nunataks pluton are 

characterized by a higher content of K-feldspar, 

opaque minerals, and sphene; a higher hornblende-tobiotite ratio; and a lower content of plagioclase, quartz, 

hornblende, and biotite.

Rapid rock chemical analyses and semiquantitative 

spectrographic analyses of fresh samples collected from 

the Copper Nunataks pluton (tables 2 and 3) show a 

systematic increase inward towards Nunatak D in 

SiO2 , K2 O, Sr, V, Ga, Ce, Y, Cr, Co, Yb and a systematic decrease in AkOs, Fe2 O3 , FeO, MgO, CaO, TiO2 , 

P2 Os, MnO, and perhaps Pb. Proceeding inward from 

the contacts, sample M315a is comparable to Nockolds' 

hornblende-biotite granodiorite; M321i is somewhat 

higher in CaO and lower in K2 O than Nockolds' average granodiorite; M421 is somewhat higher in CaO 

than Nockolds' average hornblende-biotite adamellite; 

M318a and M430 are somewhat higher in CaO than 

Nockolds' average adamellite; and M429c is somewhat 

higher in CaO and lower in Na2 O and KaO than Nockolds' calc-alkali granite. Specimen B25a is from assimilated material near a stope block and is somewhat 

higher in CaO and lower in KaO than Nockolds' average granodiorite.

Dark inclusions in the Copper Nunataks pluton are 

much less abundant (about 1 per m2 of rock area), 

lighter colored, and smaller (average diameter about 

2 cm and maximum length about 10 cm) than those in 

the medium-grained granodiorite. The abundance of 

inclusions increases slightly toward the intrusive contact. The included rock is a syenodiorite (table 1) with 

both porphyritic texture (subhedral to euhedral plagioclase and green hornblende phenocrysts as much as 

5 mm long) and poikilitic texture (sparse K-feldspar as 

much as 3 mm) in a fine-grained (about 0.4 mm) matrix of subhedral plagioclase, hornblende, and opaque 

minerals and of anhedral quartz. Anorthite content of 

twinned, zoned plagioclase phenocrysts ranges from 

calcic oligoclase to calcic andesine; anhedral, slightly 

zoned groundmass plagioclase is mostly sodic to intermediate andesine. The contrast in relative amounts of 

minerals between the Copper Nunataks and granodiorite plutons is paralleled by those in their contained 

inclusions; inclusions in the Copper Nunataks pluton 

contain less hornblende and more K-feldspar, quartz, 

opaque minerals, and sphene than inclusions in the 

granodiorite pluton.

Intrusion of the Copper Nunataks pluton was largely 

passive. Large-scale stoping and partial assimilation of 

the rock of the older granodiorite is apparent at many 

locations at Nunatak A (fig. 15). Some stoped blocks, as 

much as 8 m in diameter, have sharp contacts with 

quartz monzonite of the Copper Nunataks pluton; more 

commonly, however, contacts are gradational over several tens of meters, and the rock in these zones is compositionally and texturally transitional between that 

of the two plutonic rock types. Microscopic study of 

seven specimens of the assimilated stoped blocks (table 

1) indicates that most of those rocks were derived from
TABLE 1. Modes, in volume percent, of fresh igneous rocks in Copper Nunataks, Antarctica

[Five hundred points counted on thin sections and stained rock slabs. Sample localities shown in figure 6. Tr., trace; Tr.?, possible trace; leaders (- - -) indicate no 

granodiorite pluton; Kg (incl), inclusion in granodiorite pluton; Kg (apl), aplite of gran odiorite pluton; Kwr, West RARE pluton; Kdd, dacite dike; Kc, Copper 

Kc (incl), inclusion in Copper Nunataks pluton; Kc (apl), aplite of Copper Nunataks pluton; Kc (assim), assimilated rock of Copper Nunataks pluton; Kgd, granodior

Kgd (incl), inclusion in granodiorite porphyry dike; Kgd (apl), aplite of granodiorite porphyry dike]

Rock unit

Kg 

Kg (incl) 

verage

Kg(apl) -

Average

Kwr 

verage

Kdd 

verage

Kc 

Sample 

No.

B26a 

B26g 

M308a 

M309a 

M321c

M322a 

M323f 3 

M323h 

M326b 

S57h

M308f 

M401b 2

M201a 

M321e

Rol99i 

W142 

W144a 

W183

MlOla 

M307a 

M407b

M300a 

M300b 

M300h 

M301e

M301h 

MSlld 

M315a 

M318a

Plagioclase

34.3 

39.1 

39.2 

34.5 

46.2

40.2 

42.2 

39.7 

35.7 

37.4

38.9

18.5 

59.8

39.2

27.0 

14.0

20.5

34.9 

40.0 

35.4 

45.1

38.9

 

33.8 

34.4 

36.5 

42.0

37.5 

37.3 

38.0 

43.5

Kfeldspar

20.0 

17.4 

11.6 

17.1 

12.1

13.6 

17.6 

16.1 

19.4 

18.4

16.3

4.3 

1.6

3.0

35.1 

45.6

40.4

18.9 

15.2 

11.5 

13.1

14.7

 

 -

30.3 

25.7 

25.7 

23.8

32.8 

28.2 

29.0 

30.5

Quartz

26. 

26. 

30. 

32. 

27.

24. 

24. 

29. 

30. 

28.

28.

0. 

2.

1.

37. 

40.

38.

29. 

28. 

29. 

29.

29.

 

 

24. 

21. 

20. 

25.

24. 

24. 

25. 

20.

7 

7 

2 

0 

5

8 

9 

0 

5 

0

0

8 

6

7

1 

0

6

2 

2 

9 

8

3

3 

3 

0 

8

1 

9 

2 

6

Biotite

10.0 

7.0 

5.8 

8.2 

7.4

9.9 

8.0 

8.6 

7.4 

9.0

8.1

Tr.

Tr.

0.2 

.2

0.2

9.4 

2.4 

14.2 

6.4

8.1

2.0

0.6

0.4 

1.2 

3.6

.4 

.4 

3.2 

1.6

Hornblende

7.4 

7.8 

11.4 

6.6 

6.2

8.5 

1.6 

6.2 

6.4 

6.8

6.9

74.4 

32.8

53.6

Tr. 

Tr.

Tr.

7.0 

7.6 

8.0 

1.8

6.1

25.4 

25.2 

3.8

18.1

9.8 

15.6 

11.6 

5.2

2.4 

7.0 

5.8 

2.8

Opaque 

minerals

Tr. 

0.2 

.2 

.4 

.4

.4 

.8 

.4 

.4

.4

0.4

Tr. 

.6

0.3

0.4 

.2

0.3

Tr. 

Tr. 

Tr. 

Tr.

Tr.

1.4 

.8 

1.4

1.2

0.4 

.8 

1.4 

.8

1.0 

.6 

.6 

.8

Sphene

Tr. 

Tr.

Tr.

0.2

Tr. 

Tr.

Tr.

__

 

 

0.2 

.2

0.1

 

 

1.0 

.6 

.1 

.4

.2 

.4 

Tr. 

Tr.

Chlorite

1.6 

1.8 

1.2 

1.2 

.2

2.4 

3.0 

Tr. 

.2 

Tr.

1.2

1.2 

2.2

1.7

0.2 

Tr.?

0.1

0.6 

6.0 

.8 

3.6

2.8

3.8 

10.8

4.9

Tr. 

.4 

.6 

1.6

.6 

1.2 

.2 

.2

Other 

Epidote alteration 

minerals

Tr.

Tr . 

0.4 

Tr . 

Tr . 

1.2

Tr . 

Tr. 0.1

0.8 

.4

0.6

Tr . 

Tr .

0.4 

.2

Tr . 

0.2

31.2 

6.8 1.8

12.7 0.6

Tr . 

Tr . 

.4 Tr.

1.0

Tr . 

G

Total Plagio- K

clase f

3 

 

__ __ 

 

 

71.2 49.9 

39.0 21.4 

75.4 40.4 

61.9 37.2 

 
Kc(incl) 

Kc(apl) 

Kc (assim) Kgd 

Kgd(incl) Kgd(apl) 

M319a 

M321i 

M321J 

M421

M422b 

M424a 

M428d M429c M430

M300hM201b 

M314b 

M321fM321g M401bj M407b M428d

B25a 

M207a M207cM312a M424b 

M426 

M427M104aM104c

N303a

M303bM303cM310aM311aM311b

M311C! 

M311c 2 M324aM325aM3?5cM325dM404aM404cM404dM311c 2

M310a

45.1 40.8 

42.3 

37.229.4 

28.8 36.7 21.8 29.8"5 £ 0

56.5

23.2 

15.6 23.8

20.8 

19.8 

8.4 

13.7

17.9

40.0 

40.3 

36.841.8 46.1 52.8 

49.1

28.6

31.4

30.530.632.3

40.6

38.0 

29.5 

1.2

3.3

29.235.9 - 4.614.238.6

22.9

39.026

21. 23. 22. 

28.32. 

34. 

23. 39. 34.

9Q

12.

44. 47. 

41.

48. 

48. 

53. 49.

47.19. 

25. 

28.19. 

19. 

12. 

18.20.

0.

Tr.

Tr.

Tr. 

Tr.

/

x - -~

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1.

0.

21.

48

8 

4 6 6

6 

0 7 

9 4

6

62 3 9

4 

9 

8 0

6

4 8 

1

7 

3 4 

44

34

254

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4

3

23.1 

25.0 

22.3 25.628.8 34.0 29.6 35.9 26.625.7

5.9

31.8 

36.1 

34.1

30.4 30.1 

37.2 

37.0O O Q

28.2 24.3 21.1

26.3 

22.6 20.4 

20.7 ___1.3

2.8

2.9

.6.5

3.82.8 4.1 

.8

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2.22.5

2.2

 '- 3.3

2.7

2.0

9.9

25

2.4 

1.4 5.4 

1.2

4.2 1.8 

3.2 

1.2 

4.8

2.1

0.2 

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Tr. Tr.

0.1

5.2 

2.0 

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3.4 3.6 

1.8 

2.62.7 __0.61.6

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1.8

3.43.03.8 

2.6 2.6

2.4

Tr.

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1.2

2.6

1.6

5.21

6.6 

5.8 

6.6 6.03.8 

1.0 5.8 

.6 

3.0

19.4

Tr.

Tr. 

1.20.26.0 

4.8 

9.07.0 

7.0 5.4 

8.06.7

21.0

Tr.

4.89.4

7.87.4

4.8

8.84.8 

8.0 

28.2

31.8

11.4

9.05.2

9.6

18.0

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12 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

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N N 

200 200

10 

300 

50

10 

3 

20 

30

50 

1,000 

150

50 

5 N 

100

10 

700 

5050 

5 

N 

100

10 

700 

50

50 

5 N 

150

7 

700 

50

30 

3 

N 

100

CIPW Norms (weight

Q 

p __________ ____ . ______ Or 

Wo 

Mt 

n ______________ " V

Opaque mineralsOther alteration 

minerals. 

Groundmass:

Plagioclase 

K-f eld spar Ferromagnesian 

minerals.

23.302 24.197

14.191 12.998 27.940 26.22621.770 25.695 

.564 .323 

3.987 3.486 

3.205 3.0562.611 2.029 

.988 .816 .616 .474

99.174 99.300

46.2 39.2 12.1 11.6 

27.5 30.2 

7.4 5.8 

6.2 11.4

.4 .2 

Tr.

.2 1.2 

.4 ____ 27.569 16.037 23.814 21.588 .525 3.755 4.126 .918 .535 .214 99.081 35.4 11.5 29.9 14.2 8.0 Tr..8 .2 13.612 11.261 15.277 26.772 6.585 11.241 4.526 6.253 1.695 .451 97.673 25.2 .8 3.8 31.2 39.0 21.4 2.0 15.6 Tr. 21.234 18.963 27.154 21.540 .184 3.497 2.438 2.763 .895 .689 99.357 38.0 29.0 25.2 3.2 5.8.6 Tr . .2 23.217 18.906 25.380 20.191 .567 3.984 2.593 2.609 .968 .639 99.054 Modes 40.8 23.4 25.0 1.4 5.8.8 .4 2.4 Tr . 21.181 24.252 27.105 17.202 .636 2.992 1.310 2.612 .779 .593 98.662

7 

700 

3030 

3

N 

150

percent)

23.672

23.630 27.069

17.202 

.403 

2.490 

2.3461.594 

.608 .426

99.440

10 

500 

30

30 

3

N 

150

24.05924.821 26.234

16.794 

.631 

2.740 

1.5821.885 

.665 

.379

99.790

7 

300

1530 

3

N 

100

31.938 

.346 

28.602 23.038

11.611

1.281 

.927

1.170 

.383 .143

99.439

10 

1,000 

70

30 

3 

N 

100

23.978

18.938 

24.57620.401 

.478 

3.991 

3.3282.759 

.970 

.474

99.893

10 

700 

50

50 

5 

N 

150

20.199 

.341 

17.149 

26.250

22.992

3.988 

2.585

3.917 

.931 .664

99.016

10 

700 

7050 

5 

N 

200

19.058

21.305 

24.57620.311 

.423 

4.989 2.7833.485 

1.160 

.735

98.825

10 10 

700 700 

50 50

50 50 

5 5 

N N 

150 150

21.950 23.583 

.355 

20.711 18.917 24.573 23.702

20.605 21.496 

.561 

3.741 3.986 1.990 2.0373.339 3.191 

.856 .893 

.688 .663

99.014 98.823

(volume percent)

37.2 28.6 25.6 

1.2 6.0

.8.4 .2

43.5 

30.5 20.6 

1.6 2.8

.8 

Tr. 

.2

29.8 

34.4 

26.6 

4.8 3.01.2 

Tr. 

.2

21.8 

39.9 35.9 

1.2 

.6

.4 

Tr . 

.2

40.0 19.4 

28.2. 

5.2 

6.0

.4 .6

.2

32.3

.5 

3.4 4.8

1.4 

.2

58.6 

28.7 

19.3 

9.4

1.2

35.9

2.5 

Tr. 

9.0

.4 .2 

1.8 1.2

49.0 

9.8 

22.1 

13.2

3.9

38.0 40.6 

Tr. Tr. 

2.8 3.8 

3.8 3.0 

4.8 8.8

1.2 1.2 

.2 .2 

2.4 .6

.4 Tr.

46.4 41.8 

6.0 7.9 

27.0 19.2 

11.1 13.0

2.3 1.7

o

2!

Assimilated rock of Copper Nunataks pluton.

CO
14 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

FIGURE 8. Mostly fresh rock of the granodiorite pluton: small 

anhedral crystals of quartz (Q) and small subhedral crystals of 

plagioclase (pi), hornblende (hb), and biotite (bi) poikilitically enclosed in a large crystal of untwinned K-feldspar (K). Specimen 

M309a. Crossed polars.

a contaminated quartz monzonite melt that was 

perhaps a border phase of the Copper Nunataks pluton. 

The rock is medium-grained hypidiomorphic-granular 

or hypidiomorphic-seriate. Some rocks have a bimodal 

grain size containing small, commonly-altered crystals 

similar to those of the older granodiorite, and larger 

fresh crystals derived from the quartz monzonite melt. 

Partly assimilated stoped blocks exhibit strongly undulose quartz with sutured boundaries or mortar texture; some crystals are bent and broken. One sample is 

allotriomorphic-granular. Most assimilated rocks plot 

between the fields of the granodiorite and Copper 

Nunataks plutons on the triangular diagram (fig. 5).

APLITE AND PEGMATITE DIKES

Gray or pink aplite dikes and associated pegmatite 

dikes are volumetrically abundant end-stage products 

of the Copper Nunataks pluton intrusive episode (figs. 

9, 15); they are especially abundant near the pluton 

contacts where they may make up 10 or 20 percent of

FIGURE 9. Aplite dikes of several ages in granodiorite pluton near 

the Copper Nunataks pluton intrusive contact, outcrop M201, 

Nunatak B. Pocketknife lies on an older aplite dike that is intruded into its parent granodiorite and a mafic inclusion (black) in 

the granodiorite. This aplite dike and the inclusion are offset by a 

thin gray shear fracture. Several younger aplite dikes, of the Copper Nunataks pluton episode, cut the gray shear fracture. Note 

offset of the older aplite dike by the fracture occupied by one of the 

younger aplite dikes, just above and to the right of the knife.

the rock. Pegmatite bodies may occur with aplitic 

material or as separate masses; they are significantly 

more abundant than the pegmatites of the granodiorite 

pluton. Most pegmatite or aplite dikes are a few centimeters to 25 cm wide, but some are more than 10 m 

wide. Crosscutting relations indicate at least two 

events of aplite-pegmatite intrusion. One event produced an older subhorizontal to gently dipping, nonrooted series of pod-form masses and dikes and the second, a younger one, produced a steeply dipping set, 

rooted to deeper parts of the pluton. Crosscutting relations also are exhibited within the younger dike set. 

The steeply dipping dikes have numerous trends. 

Small masses of tourmaline and garnet occur within 

some pegmatites. Seven thin sections show that, with 

the exception of one microgranite, most rocks are typical aplites (table 1). Grid-twinned K-feldspar and 

perthite are rare. Most aplites are classified as granites 

(fig. 5); paralleling the differences between the two 

plutonic rock types, the aplites of the Copper Nunataks 

stock contain less quartz than those of the granodiorite 

batholith.
IGNEOUS ROCKS 15

FIGURE 10. Fresh dacite dike: a lamprophyric texture of subhedral 

to euhedral hornblende crystals of all sizes in a fine-grained 

groundmass of anhedral feldspar and lesser quartz, hornblende, 

biotite, and opaque minerals. Specimen Ml01. Plane light.

GRANODIORITE PORPHYRY DIKES

The youngest igneous rock in the area is granodiorite porphyry exposed in distinctive dikes. Two of these 

are as narrow as 1 m, but most are 5-20 m wide; all are 

subvertical and strike from north-northwest to westnorthwest. The four smallest dikes are at Nunataks A 

and C, but the largest and best exposed are at Nunatak 

B (fig. 4). A K-Ar biotite determination of 

95.6±1.5 m.y. (specimen M311a) by Clark, Farrar, and 

McBride (written commun., 1974) suggests that the 

dikes are related to the igneous activity of the Copper 

Nunataks pluton.

All dikes have medium-gray to black chilled margins that are as much as several meters thick. The 

country rock is sheared and slightly to moderately altered for several centimeters outward from the intrusive contacts, and the chilled margins also commonly 

show similar shearing and deuteric(?) alteration. Thin 

coatings or disseminated crystals of epidote occur locally in the chilled rock and at the contacts in the country rocks. The chilled margins are aphanitic, with rare

FIGURE 11. Typical outcrop of Copper Nunataks pluton, outcrop 

M425, Nunatak A. Snow-filled ravine on left about 3 m wide at 

base of nunatak.

phenocrysts; inward, phenocryst abundance and size of 

both phenocrysts and groundmass increase; the dike 

interiors are light gray. The transition is most commonly gradational, but in several places it is sharp 

across less than a 5-mm width, indicating a younger 

pulse of more crystal-rich rock.

The rock is fresh in most places. However, the 5-8 m 

wide dike at Nunatak A is entirely altered to saussurite and chlorite; altered rock does not continue for 

more than 1 or 2 m into the country rock. This alteration is most probably caused by deuteric or hydrothermal solutions very closely tied in time and origin to 

dike intrusion. Petrography, clay mineralogy (table 4), 

and the results of semiquantitative spectrographic 

analyses (table 3) show that the altered rock is similar 

to hydrothermally altered argillic rock of the copper 

deposit. This similarity, and the presence of disseminated sulfides in some chilled dike margins, adds support to our belief that the main period of copper 

mineralization is related to and shortly followed 

emplacement of granodiorite porphyry dikes.

Modes of 17 thin sections are presented in table 1, 

and four chemical analyses are given in tables 2 and 3. 

Minerals are identical to those of previously described 

rocks, but their size, shape, and relative abundances
16 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

FIGURE 12. Typical outcrop of massive Copper Nunataks pluton. 

Nunatak D. Rock is a fresh coarse-grained granite in presumed 

interior of pluton; note lack of aplite-pegmatite dikes and mafic 

inclusions.

FIGURE 13. Fresh coarse-grained granite in presumed interior of 

Copper Nunataks pluton, Nunatak D. Granite contains 0.5- 

1.5 mm biotite books and hornblende crystals as well as abundant 

smoky quartz.

are different. The most common phenocryst is euhedral 

and subhedral plagioclase; the phenocrysts average 2 

or 3 mm in length with a maximum of more than 

7 mm; less abundant euhedral hornblende crystals average 1 or 2 mm in length with a maximum of 1 cm, 

and euhedral or subhedral biotite grains average 1 mm 

in length with a maximum of 4 mm. Deeply embayed 

"beta" quartz as much as 3 mm across and untwinned 

K-feldspar as much as 1 or 2 mm long, as well as small 

crystals of opaque minerals, are present but uncommon. The groundmass consists of tiny anhedral crystals that are less than 0.1 mm at the chilled margins of 

the dikes (fig. 16) and commonly reach 0.2-0.3 mm in 

the interior; the interiors of the larger dikes contain a 

micrographic groundmass texture (granophyric) and 

the crystals have myrmekitic to cuneiform intergrowths up to 0.6 mm across (fig. 17). The main 

groundmass minerals are K-feldspar (no gridtwinning) and less abundant quartz; plagioclase, 

hornblende, biotite, and opaque minerals are generally 

minor. Most plagioclase phenocrysts are zoned; their 

anorthite content generally ranges between intermediate labradorite cores and sodic andesine rims. 

Hornblende and biotite have the same pleochroic formulas as those in the plutonic rocks. The hornblende 

content has a relatively wide range. Pyrite and magnetite are moderately abundant. Sphene is uncommon as 

small anhedral to subhedral crystals.

Phenocryst and groundmass contents of plagioclase, 

hornblende, and opaque minerals are generally greater in the granodiorite dikes than in the Copper 

Nunataks pluton, but K-feldspar, quartz, biotite, and 

sphene are less abundant (table 1). The granodiorite 

porphyry dike rock occupies a linear belt on the modal 

ternary diagram (fig. 5); this belt extends across the 

quartz monzonite field to a low-quartz position in the 

granodiorite field. The belt is parallel to, and displaced 

below and slightly to the left of, the field for the Copper 

Nunataks pluton. The rather wide compositional scatter within the belt largely reflects the relatively lower 

precision in point counting the fine-grained groundmass; generally the best modal data are those from 

coarser rocks of the interior of the dikes, and these lie 

toward the upper left part of the belt.

Rather inconspicuous, medium-gray fine-grained inclusions as much as 10 cm long rarely are present in 

the granodiorite dikes. A thin section of one specimen 

(table 1) shows a hypidiomorphic-seriate rock with 

grain size averaging 0.4 mm and a maximum of 

1.5 mm. The minerals are the same as those in the 

other igneous rocks and the other inclusions; the 

pleochroic formulas of hornblende and biotite are also 

the same. The larger crystals are plagioclase and 

hornblende. K-feldspar exhibits no grid-twinning. The
IGNEOUS ROCKS 17

FIGURE 14. Rocks of the

A, Fresh rock: Medium- to coarse-grained hypidiomorphic granular 

texture of K-feldspar (K), plagioclase (pi), quartz (Q), hornblende 

(hb), and minor opaque minerals and sphene (sp). Specimen 

MSlld, Nunatak B. Crossed polars.

commonly zoned plagioclase crystals are generally 

carlsbad twinned or untwinned; anorthite content of 

zoned crystals ranges from sodic andesine to sodic labradorite. The rock is modally classified in the low 

quartz and high plagioclase part of the quartz monzonite field, but it is more silicic than inclusions from the 

Copper Nunataks pluton.

INTERPRETATION OF THE CHEMICAL AND MODAL DATA

The major oxides are shown on a silica variation 

diagram (fig. 18). Plotted against increasing silica, the 

oxides of the combined igneous units exhibit a regular 

trend that is typical of the calc-alkaline rock series. 

Average alkali-lime index (Peacock, 1931) is slightly 

greater than 62. Ternary diagrams of selected chemical constituents (fig. 19) also show trends characteristic of the calc-alkaline rock series. The data points of 

the combined igneous rocks define an overall systematic trend on the variation diagram and the ternary 

plots; the smoothness of the overall curve further supCopper Nunataks pluton.

B, Mostly fresh rock: Medium-grained hypidiomorphic-granular texture of untwinned K-feldspar (K), plagioclase (pi), quartz (Q), 

hornblende (hb), chlorite (chl), and biotite (bi). Feldspar is slightly 

sericitized and some biotite is altered to chlorite. Specimen M430, 

Nunatak E. Crossed polars.

ports the proposal that all rocks are genetically related 

to a single magma series.

Each igneous rock unit occupies a distinct and generally separate position on the various modal and 

chemical diagrams. The data points for individual rock 

units may form a tight cluster or a broad belt. Rocks of 

the Copper Nunataks stock occupy the longest belt; 

samples obtained nearer the intrusive contacts of this 

pluton lie progressively closer in all diagrams to the 

mafic end of the overall field, as defined by the 

granodiorite porphyry dike rocks. The consistent 

trends for this pluton are interpreted as formation by 

igneous differentiation of a single magma body. 

Another smaller belt of data points is defined by rocks 

of the medium-grained granodiorite pluton and rocks 

of the presumably equivalent West RARE pluton; 

these data, too, are interpreted to reflect differentiation of a single magma. Assimilated rock occupies a 

position between the fields of the granodiorite pluton 

and Copper Nunataks pluton. Mafic dike rocks are
00

TABLE 3. Semiquantitative spectrographic analyses of selected fresh, altered, and mineralized rock from the Copper^Nunataks, Antarctica

[Sample localities shown in figure 6. Kg, granodiorite pluton; Kc, Copper Nunataks pluton; Kgd, granodiorite porphyry dike; Kgd (apl), aplite of granodiorite porphyry dike; Kc (apl), 

aplite of Copper Nunataks pluton. Analyses of specimens M310a and M405e by J. D. Fletcher. Less than lower limit: As, Au, Cd, Dy, Er, En, Ge, Hf, Ho, In, Ir, Lu, Os, Pd, Pt, Re, 

Rh, Ru, Sb, Sm, Sn, Ta, Tb, Te, Tr,Tl, Tm, U;value of Gd for specimen M310a: 6 ppm; value of Gd for M405e: <3; value of Nd for M310a: 20 ppm; value of Nd for M405e: 26 ppm: 

value of Prfor M310a: 7 ppm; value of Pr for M405e: H. Analyses of specimens B26f, g, h, i, j, k5, LI, L3 and S57e, f, h. i by Jerry Matooka and E. F. Cooley; looked for but not found: 

As, Au, Cd, Sb, Sn. Analyses of remainder of specimens by J. L. Harris; looked for but not found: As, Au, Cd, Pd, Pt, Sb, Sn, Te, U, Ge, Hf, In, Li, Re, Tn, Th, Tl; looked for but not 

found when La or Ce found: Pr, Nd, Sm, Eu. G, greater than 10 percent; H, interference; leaders (- -), not looked for; N, not detected or at limit of detection; L, detected but below limit of determination]

Parts per million

1,000

1,000

300

Rock unit

Kg - Kgd---- -- Kgd (apl) - Kg 

Kg .......Kg 

Kg------ - Kg 

Kg- KgKg - Average-'

Kg Kg KB - Kg 

Kg Kg.......Kg........

Kg - Kg 

Kg - Kg - Kg - Kg - Kg - Kg - Kg - Kg - Kgd------ 

Kgd - Kgd 

AverageSample No. M309a M303a M310a B26g - B26f - M304b M322b M323J M326a S57e- S57f - S57h- S57i - B26h---- M309c M309d - M309g M309h M3091 M323g M323h M3231 - M324d M416a M301e M301h M3011 M301L M3173J-- M317a 2 M303b - M303c - M303e Weight Al Fe Mg 7 3 1.5 73 1.5 53 .7 1 0.2 5 0.7 7 7 7 7 3 1.5 5 3 1 72 .7 --3 .7 5 1.5 5 .7 5 1.5 74 1.7 1 0.1 73 .7 72 .3 3 1.5 .2 T 3 .7 73 .7 10 3 1.5 10 3 1.5 10 3 1.5 73 1 73 1.5 7 2 1.5 7 1.5 .5 7 5 2 72 .3 10 2 .5 10 3 .7 7 3 1.5 73 1.5 73 .3 percent Ca Na K Ti Fresh rock 5 2 2 0.3 5 1.5 2 .3 3 >.3 3 .1 0.7 -- -- 0.07 2 - -- 0.15 7 1 1.5 .3 3 1.5 2 .3 51 .7 .3 1.5 1.5 5 .15 1.5 -- -- .2 2 -- -- .3 1.5 -- -- .2 2 -- -- .3 2 1.5 2 0.2 Sheared phyllic-argillic 0.7 -- -- 0.07 3 1.5 1.5 .3 5 1.5 2 .2 3 .1 1.5 .1 5 1.5 2 .3 5 .72 .3 5 1.5 1.5 .3 5 1.5 2 3 522 3 215 .3 7 1 -- .3 5 1.5 2 .2 3 1.5 2 .15 5 1.5 2 .3 7 72 2 10 3 1.5 .3 722 .3 5 1.5 2 .3 5 1.5 2 .2 5 1.5 2 .3 D 1.5 2 0.3 

Deuteric altered rocl

LNNNN

L

LLL

L

L

N

30

L

L

L

N

NNHNNNNL

NN

NN

20

300

150700150

1,000

300700500500500

50

700

150

200

500300500

1,000

1,000

700500

700

1,000

500500

100

200

1,000

1,000

300

N

1

1

1

N

1

1

L

1

1

N

1

111.5

1.5

1.5

1.51.5

1.5

1111

1

2

2

222

Ce

N 

100 

60

N 

N 

N N

N

150

N N N

N N 

N 

N NN 

N N 

N 

NN N 

100 100 

100

25 

100

Co10 

15 

7

L

10 50 1 

15 

7 

1015 

15 

L5 L5

20

L 

10 

7 

N 

1010 

N 

7 

10 

7

10 10 

N 

15 10

7 

10 10 

10 10

7 

10

Cr

20 10 

10

L

L 

,000 

30 

15 15

1J 20 

15 

20

150

L

15 15 

10 

20

10 15 

15 

15 

15

20 

10 

20 

20 1050 

50 

10 10 

10

15

7

Cu

30 50 

77

10 

7 

50 

100 

70

70 30 30 

150

50

100 

15 

15 

70 

50

50 

150 

70 

100 

100

70 70 30 

150 

70

30 

15 70 20 

3070 

15

15 

15 15

15 15 

10 

15

15

10 10 

10 

1515 

10 

15 

15 15

15 10 10 

15 1015 15 

15 

15 

1515 

15

La

N 

70 30

NN N N 

N NN N N 

NN

N 

100 

50 

N N

N N 

N N NN 

50 

N 

50 

N

50 

50 

70 

70 

70

30 

70

Mn

700 

500 

800

200700 

1,500 

1,500 

500 

300

1,500 

1,500 1,000 

1,000

1,000

700 700 700 

1,000 1,000

1,000 

700 500 500 300

700 

700 

500 

7,000 1,500

700 700 

700 500 300

1,000 

500

Parts 

Mo

N N 

<1

NN N 

7 

30 50

N 

N 

N 

1515 

She

15

N N 

N NN 

50 

30 

10 

3010 

N 

N 

15 

5

10 

10

N N N

10 

N

per 

Nb

3 

5 

5

million 

Ni Pb Sc 

Fresh rock--Conti

7 15 10 

10 15 10 

3 10 7

Sr

700 

700 

300

V

50 

70 

50 <

W

N 

N 

10

N L 10 N 100 70 N 

Black shear- fracture- -Continued

N 7 10 7 300 200 50 

N 150 N 30 300 100 N 

N 15 30 10 700 50 N 

3 10 77 300 70 N 

37 20 7 300 30 N

N 7 50 7 300 100 N N 15 30 15 300 200 N 

N 10 30 7 300 200 N N 15 50 10 300 200 N

1 30 30 10 300 100 5 

tared phyllic-argillic zone Continue

L L 150 L N 70 N 

3 10 30 10 500 50 N 3 7 15 1.0 200 50 N 3 N 150 3 70 30 N 3 10 70 10 300 70 N

3 10 100 10 200 70 N 

37 7 10 500 50 N 

37 7 10 700 50 N 37 7 10 700 50 N 3 N 15 10 300 70 N

37 50 10 300 70 N 

N 5 7 10 1,000 50 N 

N N 7 3 700 30 N 

10 L 100 10 500 100 N 

N N 200 3 200 50 N

37 7 10 500 70 N 

37 7 10 300 70 N 

5 N 15 10 700 70 N 37 15 10 700 70 N 

3 N 30 10 300 70 N35 50 10 500 60 N 

Deuteric altered rock Continued 

3 N 20 10 700 70 N

Y

30 

70 

20

N

15 

30 

30 20 20

15 

20 

15 15

20 

d

N 

30 

20 

N 

30

30 50 

50 50 

2020 30 15 

30 20

50 30 

50 

50 50

30 50

Yb

35 2

3 32 2

3

3 2 

N 

33 

5 

5 5 2

2 3 

1.5

3 2

5 3 5 5 53 

5

N N 

40

N

N N 

N N N

N 

N 

N 

N

N

N N 

N N N

N N 

N N 

NN N 

N N N

N 

N N 

N 

N

N N

Zr

200 

150 

150

30

50 70 

150 

100 

100

20 

70 70 

100

7030 

100 150 

20 

100150 

70 

100 100 

100150 150 

70 

100 

70

100 100 150 100 200

100 150

Sample 

No.

M309a M303a M310a

B26g

B26f 

M304b 

M322b M323J M326a

S57e S57f S57h S57i

B26h 

M309c M309d M309g 

M309h

M309i 

M323g M323h 

M3231 M324dM416a

M301h M3011 M301LM317aj M317a 2 M303b 

M303c M303e

M314a

Rock unit

-Kgd (apl)-----Kg

- -Kg

-- -Kg

- --Kg

----Kg----Kg

----Kg

-----Kg

---Kg

------Kg 

Kgd 

Kgd

- -Kgd

------Kgd

O OO 

TlTl O

0dCO

o 

O

T)G Tl

OO

COd
IGNEOUS ROCKS 19

SSSSS SS SS

FIGURE 15. N. 80°W. aplite dike offset along a fracture occupied by a 

younger N. 20° E. aplite dike, outcrop M424, Nunatak A. Country 

rock is a large stoped block in the granodiorite pluton, not far from 

the contact with the enclosing Copper Nunataks pluton. Affiliation 

of the aplite dikes is unknown.

more tightly grouped on the diagrams although at 

least the spread of modal data on the granodiorite porphyry dikes suggest that magma of these dikes may 

have differentiated internally prior to or during intrusion.

Despite its general smoothness, the overall curve on 

most diagrams (figs. 5, 18, 19) may be separated into 

two parallel belts of data points. The makeup of the 

belts is consistent with the interpretation for two similar Upper Cretaceous igneous-structural episodes in 

the area of the Copper Nunataks. The first belt is defined largely by samples from the granodiorite pluton 

and the presumed correlative West RARE pluton; the 

data points of the slightly younger aplite-pegmatite 

dikes and the dacite dike define the opposite ends of 

this belt. The second belt is defined largely by samples 

from the Copper Nunataks pluton; the data points of 

the slightly younger aplite-pegmatite dikes and 

granodiorite porphyry dikes define the opposite ends of 

it. The axes of the first and second belts are emphasized 

on each diagram by a dashed and a solid line respectively; these lines are referred to as the granodiorite 

pluton trend line and Copper Nunataks pluton trend 

line. Each smooth trend line presumably represents a 

common magma chamber tapped at only slightly different times: the granodiorite pluton and associated 

dikes represent tapping at only slightly different times
20 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

of different levels of one common magma chamber; the 

Copper Nunataks pluton and associated dikes represent tapping at only slightly different times of different 

levels of a second, younger common magma chamber. 

Most dike material intruded along the regional extension joint direction (see section on "Trends of Systematic Joints").

Each of the two magmatic episodes displayed the 

same sequence of plutonism, related late-stage aplitepegmatite dike intrusion, and emplacement of relatively mafic dikes. Both the dacite dike and the 

granodiorite porphyry dikes show similar chemical 

trends away from their respectively older pluton. Both 

dikes are higher than their parent pluton in Fe2 O3 , 

FeO, MgO, CaO, TiO2 , MnO, Co, Ni, Cu, Sc, Sr, V, Y, Ga, 

and Yb, and lower in SiO2 , K2O, Ba, and differentiation 

index (Thornton and Tuttle, 1960). However, the dacite 

dike is more mafic than the granodiorite porphyry dikes, 

and thus its magma doubtless proceeded farther along 

the trend line from the parent pluton than did that of the 

granodiorite porphyry dikes.

The differences between the two trend lines are clear 

but not profound: the younger Copper Nunataks pluton 

trend line reflects less quartz but generally higher alkalies and lower contents of MgO and CaO. The chemical and petrographic differences between the two 

episodes may be explained largely by differentiation of 

a magma. The lO-m.y.-age difference between the 

granodiorite and Copper Nunataks plutons suggests 

the time frame during which differentiation produced 

these changes. The lower position of the Copper 

Nunataks pluton trend line with respect to quartz 

however, may partly reflect the tapping of magma from 

a lower level in a compositionally zoned magma 

chamber as compared with the level of the magma of 

the granodiorite episode.

TRENDS OF SYSTEMATIC JOINTS

Steeply dipping systematic joints cut all rocks in the 

Copper Nunataks. As in rocks of the overall Lassiter 

Coast (Kellogg and Rowley, 1974), joints may be divided into two groups: barren joints and less common 

epidote-coated joints. Most barren joints (fig. 20) strike 

north-northeast and east-west; they are probably shear 

joints. In contrast, most epidote-coated joints strike between north-northwest and west-northwest. They are 

interpreted as extension joints and mark the position 

parallel to the direction of maximum principal compressive stress. This strike direction is paralleled by 

the dacite and granodiorite porphyry dikes and the 

main mode of aplite-pegmatite dikes of the Copper 

Nunataks pluton intrusive event; the parallelism 

suggests intrusion along the extension position and indicates that the stress field direction was long lived. 

The sheared phyllic-argillic zones (p. 25) also have an 

average strike of northwest and may reflect vertical 

movement along the previously established trend of 

open joints. The direction of maximum principal compressive stress interpreted from trends of Upper Cretaceous dikes and Upper Cretaceous and post(?)-Upper 

Cretaceous joints is not far removed from that which 

produced the period of northeast- to north-northeaststriking tight folds sometime between Late Jurassic 

and Late Cretaceous time (Williams and others, 1972; 

Kellogg and Rowley, 1974).

OLDER SHEAR FRACTURING, HYDROTHERMAL 

ALTERATION, AND MINERALIZATION

The medium-grained granodiorite of Nunataks B 

and C was sheared with accompanying minor hydrothermal alteration and mineralization prior to intrusion of the Copper Nunataks pluton. This took place 

during two events, described by their field names as an 

early set of crisscrossing "gray shear fractures" and a 

late set of crisscrossing "black shear fractures;" the 

late set cut across the early set. The black shear fractures cut the dacite dike; presumably the gray shear 

fractures also cut the dacite dike, but this was not verified by the field evidence. The gray shear fractures 

consist of numerous individual fault planes; in most 

places they are 1-5 cm wide but locally are as wide as 

50 cm. The rock has been mylonitized to a light- to

TABLE 4. Results of X-ray determination of sheet silicates from hydrothermally altered rocks of the Copper Nunataks, Antarctica

[Sample localities shown in figure 6. Kgd, granodiorite porphyry dike; Kc, Copper Nunataks pluton; Kg, granodiorite pluton. Interpretation of X-ray data by Leonard G. Schultz. 

Numbers given in estimated percent by volume of total rocks, rounded to nearest 5. Tr., significant trace, probably several percent; Tr.?, possible significant trace; =£, less than or 

equal to; s=, greater than or equal to]

Alteration type

Rock

unit

Sample

No. Sericite Montmorillonite Kaolinite Chlorite

Total sheet 

silicates

Phyllic _

Argillic

Argillic (deuteric)

Propylitic

Ked

Kc 

Kg 

Ke

Ked

________ Kgd

Kc

M303e

M3011 

M309h 

M309c

M314a

M303c

M301h

40

=£35 

30 

25

15

10

=£15

=£5

5 

10

5=5

Tr.

Tr.

Tr.

Tr.?

Tr.? 

5 

Tr.?

Tr.?

Tr.?

Tr.?

Tr.?

5*10

5 

5

=£15

=£10

=£5

45

50 

50 

40

30

20

20
OLDER SHEAR FRACTURING, HYDROTHERMAL ALTERATION, AND MINERALIZATION 21

FIGURE 16. Fresh contact phase (chilled margin) of granodiorite 

porphyry dike: sparse phenocrysts of plagioclase (pi) and embayed 

"beta" quartz (Q) in a groundmass of fine-grained anhedral 

K-feldspar, quartz, and lesser plagioclase and hornblende. Specimen M404c. Crossed polars

medium-gray, fine-grained rock (fig. 9). The black 

shear fractures consist of numerous individual fault 

planes that are generally less than 5 cm wide, but in 

many places are more than 1 m wide. The rock has 

been mylonitized to a black fine-grained rock (fig. 21). 

Typically, the shear planes of both types of fractures 

are spaced at intervals of about one-half to several 

meters. Shear-fracture planes of both the gray and 

black sets dip steeply and strike in many directions; 

relative displacement is difficult to determine in this 

igneous terrane, but where it could be measured, the 

displacement is from several centimeters to over several meters in a predominantly vertical component. 

The relatively widespread extent, minor size, and 

small amount of offset suggest that both shear-fracture 

sets are probably best considered as the results of 

crackling or breaking. These two sets occur in the same 

areas on Nunatak B and are best developed within 

about 100 m of the quartz monzonite intrusive contacts; they also overlap each other over most of

FIGURE 17. Fresh interior phase of a granodiorite porphyry dike: 

abundant phenocrysts of mostly zoned plagioclase (.pi), embayed 

"beta" quartz (Q), hornblende (hb), partly chloritized biotite (bi), 

and accessory magnetite (mag) and sphene (.sp) in a groundmass of 

myrmekitic K-feldspar and quartz and lesser hornblende, plagioclase, and opaque minerals. Specimen MSllci. Crossed polars.

Nunatak C. Similar shear fractures cut the stoped 

blocks of granodiorite within the Copper Nunataks 

pluton at Nunatak A. Minor hydrothermal alteration 

affects both sets although shearing and alteration is 

greater along the black shear fractures and is accompanied by minor pyrite mineralization.

Microscopically the mylonite of the gray shearfracture set consists of very fine grained (mostly 10 or 

20/xm or less) equant anhedral crystals of quartz, less 

abundant feldspar, minor green hornblende, and traces 

of magnetite (fig. 22). The wallrock, from the megascopic edge of the shear fracture outward to about 

5 mm, exhibits partial recrystallization of much of the 

feldspar and quartz creating a fine-grained rock similar to that within the shear fracture. Most relic quartz 

has sutured boundaries, relic green hornblende is fresh 

but ragged and anhedral, and biotite has recrystallized 

to clusters of small (less than 0.3 mm) randomly 

oriented brown biotite. This alteration zone rapidly 

grades into an outer zone 5 cm wide where only clus-
22 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

PERCEN 

20

O

14 

6

0 4 

u. 2 

0 

6

o 4

<i> 

u. 2

0

fc 4 

5 2

0 

10 

8

4

2 

0

10

o 8

V* 6

to 

2 2

0

4 

O

0 

6 

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2 

5

5 n

CD "D (J-Q O (D~ TO TO O

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CO CO CO COCOCOcOrvi *~ CO ^" ^"

5 55 5555m §55 5

i i i i i i i i i

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CALC-ALKALIC , CALCIC

-^-f ^ti. %

-A ^

±_+^j* ~* n

A _rS_- _ -4VP. P. IV 

1200

800 

Ba 

400

0 

80 

Cr 40 

0

80 

La 40

N

40 

Ni 20

N

60 

Y 40 

20 

0

150

Ce 10°

50

IS!

8 

Yb 4 

0

PERC 

16 

14 

12 

D.I. 10 

8 

6 

4

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TO "D O -O U CD

r* <-in T-i-r-co 

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5 555555m -i i uij i i i i A ^ i

ro ra o 

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"""""" -" -~____ +

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 H-

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 $ 

-- 0-

A JN> w 

A^^ 1<I^M^^* .

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ENT

Trend line for rocks

of younger episode \

±^§c ^"^

=* - "

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^^ Trend line for rocks 

^^ of older episode

i i i i i

4 56 58 60 62 64 66 68 70 72 74 54 56 58 60 62 64 66

1 1 1

68 70 72 7i

SiO2

Cretaceous

Younger igneous episode 

A Granodiorite porphyry dike 

 Quartz monzonite of Copper Nunataks pluton 

+ Assimilated rocks Of same age as Copper Nunataks pluton

EXPLANATION

Cretaceous

Older igneous episode 

A Dacite dike 

O Granodiorite pluton 

v West RARE pluton Of same age as granodiorite pluton

FIGURE 18. Variation in common oxides (weight percent), selected trace elements (ppm), and differentiation index plotted against 

SiO2 (percentage content) for specimens from fresh rocks from the major igneous units, Copper Nunataks. From data in table 2.
OLDER SHEAR FRACTURING, HYDROTHERMAL ALTERATION, AND MINERALIZATION 

Q K

23

Ab+An Or Na B

Ca

F=Fef3 +F«?2 +Mn A=Alz 03 -(Na2 0+K2 0)

An

Alk=Na+K M=Mg C=CaO-(3P2 O5 +COz ) F=FeO+MnO+MgO+2Fe2 O3 -TiO2

EXPLANATION

Cretaceous 

Younger igneous episode

A Granodiorite porphyry dike 

 Quartz monzonite of Copper 

Nunataks pluton

+ Assimilated rocks Of same

age as Copper Nunataks pluton

Cretaceous

Older igneous episode

A Dacite dike

o Granodiorite pluton

o West RARE pluton Of same 

age as granodiorite pluton

FIGURE 19. Ternary plots of selected normative minerals, Earth's cations, and oxides from fresh rocks from the major igneous 

units, Copper Nunataks. From data and recalculation of data in table 2. (A) Normative quartz-orthoclase-plagioclase (Ab+An), 

(B) Na-K-Ca (cation percent), (C) Alk-F-M (cation percent), (D) A-C-F (weight percent).
24 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

N

0 5 10 

JOINT READING PER 5° OF COMPASS BEARING

EXPLANATION

Epidote joints Barren joints

FIGURE 20. Rose diagram of strikes of systematic steeply dipping joints in rocks of the Copper Nunataks.

ters of recrystallized biotite distinguish it from fresh 

rock. Hydrothermally altered rock consists of chloritization of biotite and light to moderate saussuritization 

of relic feldspars. It occurs within parts of the shear 

fractures and the adjacent wallrock, but is not necessarily axial to the shear features. A semiquantitative

FIGURE 21. Pegmatite and aplite dikes at outcrop M300, Nunatak 

B. The dikes crosscut their parent granodiorite pluton and are 

offset by a series of parallel small black shear fractures.

FIGURE 22. Edge of a black shear fracture. Sheared and recrystallized fine-grained anhedral crystals of quartz, feldspar, 

hornblende, and, in smaller amounts, magnetite and pyrite in contact with a partly sheared relic quartz crystal. Specimen S57h. 

Partly crossed polars.

spectrographic analysis (table 3, sample B26g) reveals 

that the gray shear-fractures are almost completely 

devoid of element additions. The rock shows a possible 

increase in V and depletions in Fe, Mg, Ca, Ti, Mn, Cr, 

Sc, Sr, Y, Zr, and apparently Ba, Be, Co, Cu, and Ni 

relative to fresh rock.

The black shear fractures have significantly more 

recrystallized and altered rock within their wallrocks 

than do the gray shear fractures. Furthermore, in several places pyrite veins as much as 5 mm wide occur in 

or adjacent to black shear fractures. Microscopic study 

of nine thin sections reveals that the mylonitized rock 

in the core of the shear fracture is in large part recrystallized to equant anhedral crystals of quartz, feldspar, 

hornblende, and opaque minerals that average 10- 

lOO^im in size. Both hornblende and opaque minerals 

are more abundant (as much as 50 percent by volume) 

and commonly occur as larger crystals (as much as 

1 mm) in black shear fractures than in the gray shear
YOUNGER SHEAR FRACTURING, HYDROTHERMAL ALTERATION AND MINERALIZATION 25

fractures. In many rocks pyrite makes up most of the 

opaque minerals, and the abundance of ferromagnesian minerals undoubtedly causes the black color in 

the shear fractures. In some places the shear fractures 

contain sparse to abundant, sheared and recrystallized 

fragments of wallrock and coarse-grained quartz. The 

feldspar and quartz in the wallrock is partially but 

extensively recrystallized to a fine-grained quartzfeldspar rock similar to that within the shear fractures 

for at least 3 cm outward from the shear fracture. Relic 

wallrock in and about 5 mm beyond this zone lacks 

biotite and contains introduced pyrite and possibly 

magnetite. Hornblende occurs as fresh anhedral 

poikilitic crystals, either relic or replacing biotite; plagioclase is at least slightly saussuritized. Rock about 

1 cm beyond this zone is relatively fresh but, like the 

wallrock near the gray shear fractures, original biotite 

has been recrystallized into clusters of randomly 

oriented, small (maximum 0.3-0.4 mm) crystals. This 

rock in turn grades into fresh wallrock 4 or 5 cm beyond the edges of even the smallest shear fractures. 

The shear fractures are commonly hydrothermally altered, but maximum alteration does not necessarily 

coincide with the central part of the shear fracture. 

Alteration within the shear fracture results in extensive saussuritization of feldspar, chloritization of biotite, and occasionally moderate epidotization of plagioclase and biotite. Hydrothermal alteration is presumed 

synchronous with crackling. Semiquantitative spectrographic analyses of nine black shear fractures (table 

3) shows some changes in elements by alteration solutions relative to that of fresh rock. The analyses show 

that Fe, Mg, Mn, Co, Cr, Cu, Mo, Ni, Pb, and V have 

been introduced, and Al, Na, Ca, Ba, Sr, Y, Yb, and Zr 

have been lost.

YOUNGER SHEAR-FRACTURING,

HYDROTHERMAL ALTERATION,

AND MINERALIZATION

Following intrusion of the granodiorite porphyry 

dikes, rocks at scattered places in Nunataks A, B, and 

C were sheared to various degrees with accompanying 

hydrothermal alteration and mineralization. The end 

result of these associated events, which terminated the 

Copper Nunataks pluton intrusive episode, was the 

Lassiter Coast copper deposit.

Shearing took place along narrow linear "sheared 

phyllic-argillic zones" that crosscut all igneous rocks in 

the area (figs. 4, 23A, B, C, D). Mineralization consisted mostly of copper and iron sulfides and oxides 

occurring both as disseminations and as concentrated 

masses (as much as 10 cm across) in veins of coarsegrained quartz and in altered wallrock. Most

mineralized rock occurs at Nunatak C, with a lesser 

amount at Nunatak B, and a minor amount at 

Nunatak A. Mineralized rock is most abundant where 

sheared phyllic-argillic zones are most abundant; however, it rarely occurs within such zones but is usually 

found adjacent to them. Most mineralized rock is in the 

granodiorite batholith within several hundred meters 

of the Copper Nunataks pluton contact. This localization of mineralized rock probably is caused by two 

factors the great abundance here of sheared phyllicargillic zones and the great amount of fracturing (including both gray and black shear-fracturing) that this 

rock has undergone; both factors provide pathways for 

mineralizing solutions. Mineralization and alteration 

are considered to be genetically related to, and roughly 

simultaneous with, each other.

Sheared phyllic-argillic zones are made up of lighttan to medium-brown hydrothermally altered rock. 

They range in width from less than a centimeter to at 

least 20 m, averaging 1-2 m. All sheared phyllicargillic zones are vertical to steeply dipping and most 

strike west-northwest (the dominant direction at 

Nunataks A and B) to north-northwest (at Nunatak C) 

(fig. 23D). Unlike the gray and black shear fractures 

characterized by mylonite and only lesser altered rock, 

sheared phyllic-argillic zones are recognized by intensely altered rock with only minor volumes of observed slickensides or mylonite. Phyllic and argillic 

altered rock (terminology of Lowell and Guilbert, 

1970) and minor silicified rock characterize the zones. 

The contact between zones and less intensely altered 

propylitic rock (Lowell and Guilbert, 1970) is gradational within 10 cm in most places, and thus it is well 

defined. Propylitic altered rock extends for several 

meters to several tens of meters beyond the edges of 

the zones, and there it grades into fresh rock. Sheared 

phyllic-argillic zones are best developed at Nunatak C. 

Here all rock is transected by zones of various widths 

occurring at intervals of from several meters to about 

10 m; thus all rock at Nunatak C is altered to some 

degree by either sheared phyllic-argillic zones or intervening propylitic rock. Phyllic-argillic altered rock 

at Nunataks A and B is restricted to several zones less 

than 3 m wide.

Where offset could be determined for the sheared 

phyllic-argillic zones, it is vertical and commonly less 

than several meters (fig. 23Z>); some aplite-pegmatite 

dikes cut by sheared phyllic-argillic zones are not 

offset. However, two zones at Nunatak B have relatively large measurable displacement. The westnorthwest-striking fault at the northwestern part of 

the nunatak (fig. 4) cuts the subvertical dacite dike, 

subvertical granodiorite dike, and east-dipping quartz 

monzonite contact with right-lateral apparent offset of
26 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

FIGURE 23. Sheared phyllic-argillic zones.

A, Narrow (10 cm) northwest-striking zone cutting lighter colored C, Dark, relatively crumbly rock of a sheared zone near the ice ax, at 

rock of Copper Nunataks pluton, outcrop M301, Nunatak B. outcrop M317, Nunatak A. Lighter, more resistant, fresh to propylitic altered rock of the Copper Nunataks pluton makes up the 

jointed northwest-trending face.

B, Darker and mostly weaker rocks of several bifurcating 1-3 m wide 

zones that transect rock of the Copper Nunataks pluton View is 

southeast along strike of the zone from outcrop M301, Nunatak B. 

The areas of lowest relief and the rock fragments of smallest size 

mark the axial, most intensely altered parts of the zones. The 

freshest rock shown is the lighter colored propylitic altered rock in 

the upper left corner. Ice ax in center of photograph for scale.

D, A series of small parallel sheared phyllic-argillic zones, striking 

to the upper left (northwest) and dipping steeply to the right 

(northeast) at outcrop M326, Nunatak C. Note offset of a subvertical pegmatite dike by several sheared zones, left and above the 

man's head. Most of the outcrop is rock of the granodiorite pluton.
YOUNGER SHEAR FRACTURING, HYDROTHERMAL ALTERATION AND MINERALIZATION 27

about 10 m, 20 m, and 100 m, respectively. The best 

estimate of actual displacement is that the north block 

was upthrown relative to the south block at least 50- 

100 m. The north-northeast-striking fault near the 

eastern end of the nunatak requires one of the following: at least 50-100 m of vertical component displacement, about 20 m of left-lateral strike-slip movement, 

or a combination of both in order to offset the subvertical dacite dike as shown on the map (fig. 4). Elsewhere 

on the Lassiter Coast, significant high-angle faulting 

has not been found. Therefore we consider offset associated with the sheared phyllic-argillic zones to be a 

localized breaking due to late-stage igneous movement; it probably does not represent regional tectonism.

MINERALIZATION

The Lassiter Coast copper deposit consists of 

coarsely disseminated hypogene sulfide and oxide minerals and quartz veins containing hypogene sulfide and 

oxide minerals. Minerals are most abundant in 

nunataks where sheared phyllic-argillic zones are 

most abundant; minerals usually occur in propylitically altered rock and are sometimes found in the 

sheared phyllic-argillic zones themselves. Field evidence on the relative ages of minerals and sheared 

phyllic-argillic zones is unequivocal in four locations at 

Nunatak C. Here quartz veins and minerals appear to 

cut sheared phyllic-argillic zones in some places, but in 

two outcrops sheared phyllic-argillic zones cut quartz 

veins. We conclude that mineralization and shearingalteration probably were nearly concurrent, but 

mineralization locally may have predated shearing 

and alteration slightly.

Mineralized quartz veins do not appear to have alteration envelopes of their own. They have an average 

width of 2 cm and may be more than 10 cm wide, which 

is unusually wide for veins of porphyry deposits. Most 

mineralized quartz veins occur at Nunatak C and the 

western half of Nunatak B, where they are irregularly 

spaced at intervals of from several meters to a dozen 

meters. Thus they are generally widely scattered, 

another feature dissimilar to those of porphyry deposits; more than one quartz vein rarely crops out 

within an area of a square meter. Most veins consist 

largely of quartz, with few or no megascopic sulfide and 

oxide minerals. Some veins, however, consist largely of 

opaque minerals. The principal opaque vein minerals 

are magnetite, pyrite, chalcopyrite, and molybdenite; 

in some places lenticular masses as much as several 

tens of centimeters long consist of fine- to coarsegrained epidote. Minor hornblende may accompany 

magnetite.

Disseminated sulfides and oxides are slightly more 

abundant than veins and occur as either single crystals

or elongated masses as much as 10 cm long. These 

sulfides and oxides are found mostly at Nunatak C and 

in the western half of Nunatak B. They occur as bodies 

of disseminated crystals or clots several cubic meters 

in size or as widely distributed crystals or clots irregularly spaced anywhere from several to 20 m. Finely 

disseminated minerals also occur locally in the chilled 

margins of the granodiorite porphyry dikes. Thus disseminated minerals also are much coarser grained and 

more widely scattered than those in commercial porphyry copper deposits; nowhere did we find large outcrops comprised of finely disseminated sulfides. Chalcopyrite and pyrite are the most common ore minerals, 

whereas molybdenite is uncommon. In some places 

clots are concentrically zoned with chalcopyrite more 

abundant near the center and pyrite and limonite more 

abundant on the margins. Most disseminated crystals 

or clots are surrounded by halos of rusty limonite and 

(or) bright-yellow oxide stains extending 10 cm or 

more beyond the edges of the sulfide minerals.

Supergene minerals are rare in this extremely arid 

region where ground water is absent. Malachite and 

possibly chrysocolla, however, are conspicuous in 

many places adjacent to concentrations of hypogene 

copper minerals and occur as small crusts that rarely 

exceed several square centimeters in size. Limonite 

and hematite occur in cracks or surround magnetite, 

pyrite, and chalcopyrite.

Examination of 7 thin sections and 13 polished sections shows that the major copper mineral is chalcopyrite; malachite, chalcocite, and possibly bornite are 

rare. Pyrite crystals and leafy masses (as much as several centimeters long) of molybdenite are commonly 

intimately associated with the copper minerals. 

Magnetite is one of the most abundant opaque minerals in the veins and occurs either in massive form as 

much as 10 cm in diameter or as small to mediumgrained plates or radiating plates. Magnetite masses 

may occur alone or may be associated with all or some 

of the other minerals. Limonite and hematite are 

mostly secondary and form rims in cavities adjacent to 

magnetite, pyrite, and chalcopyrite.

In many places the wallrock adjacent to ore minerals 

is remarkably fresh with only minor epidotization of 

plagioclase and minor chloritization of biotite. In other 

places feldspar is moderately to highly altered to sericite and clay minerals, biotite is altered to chlorite and 

epidote, hornblende is altered to uralite, magnetite is 

partially altered to hematite, and rutile and limonite 

are present. In some areas biotite is recrystallized to 

clusters of tiny, randomly oriented crystals, and feldspar is partly recrystallized to fine-grained anhedral 

quartz-feldspar masses; quartz is increased in volume 

by addition of rims of fine-grained quartz crystals. Al-
28 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

tered rock generally grades into fresh rock within 1 or 

2 cm from the ore minerals.

Most rocks from mineralized areas (table 3, 19 

semiquantitative spectrographic analyses) show increases in Fe, Ag, Bi, Co, Cu, Mo, Ni, Pb, and Zn, and 

depletions in Al, Mg, Ca, Na, K, Mn, Ti, Ba, Be, Cr, La, 

Sc, Sr, Y, Yb, and Zr. Trace element changes are similar to, but greater than, those due to sheared phyllicargillic zones. Highest values were recorded for the 

quartz veins. A few of these quartz veins locally contain over 10 percent Fe by weight, 3 percent Cu, 0.7 

percent Pb, 0.15 percent Mo, 700 ppm Zn, 700 ppm Bi, 

500 ppm W, and 300 ppm Ag. The grade of the rock at 

Nunatak C, where sulfides are most abundant, probably does not average over 200 ppm Cu or 100 ppm Pb, 

and averages less than 50 ppm Mo and such values 

are well below ore grade for commercial porphyry copper deposits.

In line with recent studies of copper content in biotites of plutons associated with copper deposits (for 

example, Lovering and others, 1970; Banks, 1974), 

semiquantitative spectrographic analyses were determined for a biotite-hornblende separation from three 

samples each of the granodiorite batholith, Copper 

Nunataks pluton, and granodiorite porphyry dikes. All 

samples were fresh rocks collected from three of the 

major nunataks (fig. 6). The results of the nine 

analyses (table 5) show that neither copper nor any 

other minor elements added during mineralization are 

concentrated to any significant degree in the biotite or 

hornblende of the associated fresh igneous units.

PHYLLIC AND ARGILLIC ALTERATION

Most sheared phyllic-argillic zones contain a 

medium-brown, completely sericitized rock and a less 

altered rock in which feldspar is tan or brownish pink 

owing to various stages of alteration and biotite is altered to green chlorite. The former rock type (mostly 

"phyllic alteration" of Lowell and Guilbert (1970); 

"sericitic alteration" of Meyer and Hemley (1967)) 

generally occupies the more central parts of the zones, 

and the latter rock (mostly "argillic alteration") occupies the outer parts; but the two are mutually gradational and were not differentiated in the mapping or in 

this discussion. "Potassic" altered rock may occur locally in some or most sheared phyllic-argillic zones but 

it does not appear to make up significant parts of the 

zones. Phyllic altered rock generally weathers easily to 

medium-brown pebble- or sand-size grus and produces 

long linear "trenches" on the outcrop. The grade of alteration in some small shear fractures is only argillic. 

Eleven thin sections of phyllic and argillic rock (five 

from the granodiorite, four from the Copper Nunataks

TABLE 5. Semiquantitative spectrographic analyses of biotitehornblende separates from fresh igneous rocks in the Copper 

Nunataks, Antarctica

[Sample localities shown in figure 6. Kg, granodiorite pluton; Kc, Copper Nunataks pluton; 

Kgd, granodiorite porphyry dike. Analyses by L. A Bradley. Not detected or at limit of 

detection: Ag, As, Au, B, feCCd, Eu. Ge, Hf, In, Li, P, Pd, Pf, Re, Sb, Sm, Ta, Te, Th, Tl, U, W, 

Zn. Looked for when Y found but not found: Dy, Er, Gd, Ho, Lu, Tb, Tm Not looked for: Pr. 

Leaders ( I, not looked for; N, not detected, at limit of detection, or at value shown; L, 

detected but below limit of determination]

Rock

unit

Kg ________

Kc _ . _ __

Kgd. . ____

Sample

No.

M308a

M321c

S57h

MSOOh

M321j

M422b

_ MSllb

M325cd

M404d

Estimated

volume of

biotite:

hornblende

in sample

80:20

80:20

80:20

50:50

60:40

70:30

50:50

60:40

60:40

Al Fe Mg Ca Na K Ti

Weight percent

7

7

7

7

7

5

5

7

5

10

10

10

10

10

7

7

7

10

5

3

3

7

7

5

5

5

5

5

3

3

7

5

5

7

7

7

0.7

.7

.7

1

.7

1

.7

.7

.7

3

5

5

9

3

3

1.5

3

3

1

1

1

5

.7

.7

.7

1

.7

Ba Be Ce Co Cr Ga La Mn Mo

Parts per million

500

300

300

70

700

500

150

150

700

L

N

N

2

2

3

L

1.5

L

L

L

L

L

L

L

L

200

L

50

50

30

30

50

30

20

30

30

50

50

70

15

15

15

15

50

15

30

7

15

3

3

7

30

30

7

20

20

20

20

20

20

20

20

20

N

50

N

50

70

50

70

100

70

3,000

3,000

3,000

3,000

5,000

3,000

3,000

3,000

3,000

N5

N5

N5

15

N5

N5

N5

N5

N5

Nb Nd Ni Pb Sc Yb

Parts per million

15

20

20

10

15

20

15

20

15

L

100

L

L

L

L

150

150

150

30

30

30

15

15

15

10

15

10

N

N

15

N

10

15

L

10

20

70

70

70

70

100

70

70

70

70

70

70

70

70

30

70

150

300

150

500

300

500

300

300

200

300

300

300

50

70

50

30

30

30

70

70

70

7

7

7

_

5

7

7

7

7

70

100

70

70

150

100

100

100

70

pluton, two from the granodiorite porphyry) were 

studied.

Phyllic rock is mostly intensely altered, and it consists almost entirely of quartz and fine-grained sericite 

and clay (figs. 24, 25, 26). Most quartz forms relic crystals containing some secondary growth on the outside 

of the crystals, and it has undulose extinction and irregular to vaguely sutured boundaries. All feldspars 

have been recrystallized to fine-grained sericite and 

clay. Biotite alters to mostly fine- or medium-grained 

sericite; the sericite commonly is associated with 

minor leucoxene and limonite. Hornblende changes to 

fine-grained sericite and lesser leucoxene, rutile(?), 

limonite, and hematite; the oxide minerals sometimes 

occur as a rim around the sericitized crystals. Hematite blades lace some altered hornblende crystals. Calcite may be present in the rock. Opaque minerals consist of minor magnetite-limonite and pyrite; sphene is 

altered to leucoxene. Tiny sericite veins are locally visible in some thin sections. In less altered rocks, patches 

of fresh K-feldspar show through the sericite and clay.
YOUNGER SHEAR FRACTURING, HYDROTHERMAL ALTERATION AND MINERALIZATION 29

1 T'-'J-V "'f * "! ^

m = >M^-%V^P' /^<l

-*- "r -M- A-:/-- ^

FIGURE 24. Phyllic altered rock of the granodiorite pluton: all 

feldspar (light gray in photo) is converted to fine-grained sericite 

and clay, and relic quartz (white) remains. Hornblende crystal is 

converted to medium-grained sericite spotted with leucoxene and 

is surrounded by a dark limonite-hematite rim. Specimen M309h. 

Plane light.

In argillic rock, plagioclase is moderately to completely converted to sericite, clay minerals, and minor 

calcite, and K-feldspar is slightly altered to sericite 

and clay (fig. 27). Biotite is moderately to totally altered to chlorite (penninite?), minor leucoxene, and 

rare epidote. Hornblende is either fresh or moderately 

altered to chlorite and subordinate leucoxene, hematite, limonite, sericite, and minor epidote. Accessory 

minerals include magnetite, hematite, and pyrite; 

sphene may be totally altered to leucoxene. Relic 

quartz with secondary growth and uralite after 

hornblende develops with more intense alteration.

X-ray studies (table 4) show that total sheet-silicate 

content is greater in the phyllic rocks than in the argillic rocks. Sericite is most abundant in both alteration 

types. Montmorillonite is significantly higher and 

kaolinite may be higher in phyllic rocks than in argilFIGURE 25. Phyllic altered rock of the granodiorite porphyry dike: 

all feldspar, which occurs as phenocrysts and in the groundmass, is 

altered to fine-grained sericite and clay (light gray in photo), 

whereas relic embayed "beta" quartz phenocryst (upper left) and 

quartz in the groundmass remain largely unchanged. The biotite 

phenocryst (lower left) is altered to medium-grained sericite spotted with leucoxene and limonite-hematite. Two large dark 

hornblende phenocrysts (center) are altered to fine-grained 

sericite-leucoxene and are largely rimmed by limonite-hematite. 

Several small crystals of magnetite (mag) are laced with hematite. 

Specimen M303e. Plane light.

lie rocks. Chlorite is less abundant in phyllic rocks 

than in argillic rocks.

Semiquantitative spectrographic analyses of 20 

samples from sheared phyllic-argillic zones (table 3) 

show that Cu, Pb, Mo, and probably Ni, Ag, and Bi 

were introduced, and that Al, Na, Ba, and probably 

Mg, Sr, Y, and Zr were depleted.

PROPYLITIC ALTERATION

Phyllic-argillic altered rock grades sharply into 

propylitic rock over several centimeters or even over 

several millimeters. Study of six thin sections (two of 

the granodiorite, two of the Copper Nunataks pluton, 

two of the granodiorite porphyry) shows that altered
30 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

FIGURE 26. Sheared and phyllic altered rock of the Copper 

Nunataks pluton: feldspar is sheared and almost totally altered to 

fine-grained sericite and clay (ser). Ferromagnesian minerals are 

totally altered to darker sericite, chlorite (chl), leucoxene (leu), 

and epidote (ep). Several of the shear planes are filled with chlorite. Relic quartz (Q) is sheared and broken, and it commonly has 

sutured boundaries. Specimen MSOlc. Plane light.

plagioclase contains tiny crystals of sericite and clay 

minerals and minor epidote and calcite (fig. 28). Biotite 

is slightly to largely altered to chlorite (penninite?) 

and is altered to less abundant epidote and leucoxene; 

altered minerals are concentrated along mineral 

cleavage planes. Hornblende is generally fresh, but it 

may be slightly altered to chlorite or sericite, either of 

which may contain minor leucoxene and hematite. 

Some opaque crystals are rimmed by hematite or limonite. In most rocks K-feldspar is not altered.

X-ray studies (table 4) show that sheet silicates are 

significantly lower in concentration in propylitic rocks 

than in more intensely altered rocks. Sheet silicates 

consist mostly of sericite; montmorillonite and kaolinite are uncommon. Chlorite is somewhat less abundant than in argillic rocks.

FIGURE 27. Argillic altered rock of the Copper Nunataks pluton: 

plagioclase is completely replaced and K-feldspar (K) moderately 

replaced by fine-grained sericite and clay (ser). Hornblende and 

biotite are completely replaced by chlorite (chl) and leucoxene 

spots. Quartz(Q) is unchanged. Magnetite (mag), laced with hematite, and apatite (ap) are present. Specimen M301i2 . Partly crossed 

polars.

EPIDOTE ALTERATION

Epidote occurs as a replacement mineral in the form 

of small irregular masses and coatings along fractures 

not only in the Copper Nunataks but also in plutonic 

rock throughout the Lassiter Coast. Its occurrence is 

almost universally confined to subvertical joints (see 

p. 20) cutting plutonic rocks and striking in the extension direction to the northwest. Crosscutting relations 

show that some epidote-coated joints developed following intrusion of the granodiorite pluton and prior to 

intrusion of the Copper Nunataks pluton, but most appear to postdate the Copper Nunataks pluton. Presumably epidote is related to very late-stage igneous solutions on a regional scale and is not confined to any 

alteration or mineralization event, such as those associated with the copper deposit. However, some epidote masses and coatings are abundant in and adjacent
MINERALIZED AND ALTERED ROCK ELSEWHERE IN THE LASSITER COAST 31

Cr * vi !>;

FIGURE 28. Propylitic altered rock of the granodiorite pluton: plagioclase (pi) is lightly to moderately altered to fine-grained sericite 

and clay, especially along mineral cleavage, but K-feldspar (K) is 

only slightly altered. Biotite is totally altered to chlorite (chl), and 

minor epidote (ep) is spotted with leucoxene, but hornblende (hb) is 

fresh. Quartz (Q) is unchanged. Specimen MSOle. Plane light.

to the sheared phyllic-argillic zones and thus probably 

result from a low-intensity hydrothermal event.

Microscope examination of three epidotized rocks 

shows that most biotite and hornblende in the rock are 

altered to chlorite and fine- to medium-grained epidote. Plagioclase may be altered to sericite and clay 

minerals or it may be fresh with the exception of some 

contained fine epidote grains. Locally, all minerals except quartz are replaced by epidote.

SILICIFICATION

Masses of pale-green silica, as much as several cubic 

meters in volume, were mapped within part of a single 

3-m-wide sheared phyllic-argillic zone at Nunatak B. 

The rock in one thin section (M309g) consists almost 

entirely of introduced fine-grained (mostly less than 

0.4 mm) quartz as anhedral to subhedral prismatic 

crystals (fig. 29). Some sericite, and minor hematite

FIGURE 29. Silicified rock from the interior of a sheared phyllicargillic zone that cuts the granodiorite pluton. Most of the rock 

consists of small prismatic crystals of hydrothermal quartz, grown 

largely along two preferred directions. Two large crystals of relic 

quartz (Q) are visible, and minor sericite and clay minerals have 

replaced small amounts of remaining feldspar and ferromagnesian 

minerals. Specimen M309g. Crossed polars.

and limonite, and some original relic quartz crystals 

constitute the rest of the rock. Semiquantitative spectrographic analysis shows that this same specimen 

(table 3) is similar to other rocks from the sheared 

phyllic-argillic zones.

MINERALIZED AND ALTERED ROCK

ELSEWHERE IN THE LASSITER COAST

AND SOUTHERN BLACK COAST

Other sightings of mineralized and hydrothermally 

altered rock were made in the Lassiter Coast and 

southern Black Coast (figs. 1, 2) by all members of the 

three field parties. No occurrences were found that 

were as rich as or as extensive as the Lassiter Coast 

copper deposit; they were noted during our reconnaissance geologic studies and were not investigated in detail. Many of these were sampled, however, and the
32 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

TABLE 6. Semiquantitative spectrographic analyses of selected fresh, altered, and mineralized

[Sample localities shown in figure 2. Analyses of specimens B21d, B28d, P9b, S6, S3 la, S33 d, S34a, S34g, S38a, S38k, S39b, S41b, S42b, S42f, S45h, S48Ala, S50g, S54c. S59de, S71ca3, 

W39aa, W41d, and W87cs by Jerry Matooka and E. F. Cooley; looked for but not found: As, Au, Cd, Sb, Sn. Analyses of specimens N5, Re37a, Re37b, Rol2a, RolSabc, Ro54a, Ro75bc. 

Ro86a, RolOSb, RollSb, Rol49b, Rol50b, Rol98de, Rol99bc, Rol99g, WllSde, W139d2, W139h, W144b, Wl44c, W150ze by J. D. Fletcher; less than lower limits: As, Au, Cd, Er, Ge,

Hf, In, Ir, Lu, Os, Pd, Pt, Re, Rh, Ru, Sb, Sm, Ta, Tb, Te, Th, Tl, Tm, U; value of As for specimen R054a: 1,400 ppm; value of Ge for spec. R0149b: 4 ppm; value of Ge for spec. Wl44c: 

160 ppm; value of Sm for spec. N5: 5 ppm; value of Sm for spec. Rol50b: 6 ppm; value of Sn for spec.Rol50ze: 5 ppm. Analyses of remainder of specimens by J. L. Harris; less than

Host g -, rock amP e

type no "

PG Rol03b -----

PG S3^a - --

PQM Bo68c- ---

PQM Bo71b - 

DL P9b---- 

PG Rol2a -- --

PG Rol3abc ----

PQD Ro75bc -----

PG Ro86a------

PQD Roll3b - 

PG RolSObPG Rol98de --

PG Rol99bc----

PG Rol99g --

PG Ro215c --

PG Ro225a ---

PG Ro228b -----

PQM Ro313a -----

PQM Ro313b -----

PG S31a ---

PG S34d ---

L S38k ------

DL S71ca 3 - --

PG V16b ------

L W39aa------

PQD W87c 3 --

PG W144b- -----

PG WISOze -

DL Wal7b!- -

PQM Wa37b - 

Average (all)--

PG B21d-- ---

L S33d ------

L S38a -------

DL S39b 

L S42f -- -

L S45h ------

L S50g 

L S59de -- 

Average (all)--

V B28d-- -----

PG Bo44e ------

PQM Bo68b ----- -

PQM Bo71c-- --

PQM Ka316-4 -

L Ke7g 

PD Ke41de -----

PD Ke41fg -

PG N5---- --

PQD Re37a-- - -

L Ro54a --

PD Ro241b -

PG Ro251b 

L Ro256c -----

DL Ro315d -----

L Ro315h 

PQD S6~ - 

PQD S54c ---

PQM V32c-- - 

DL W41d-- -----

PD W113de - -

PQD W139H -- --

PG W144c -

PG Wa9a -- - 

PQM Wa21 - -

Average (all) 

PQM Bo64b ---

PQD Re37b ------

PG Rol49b -- -

PG S41b -------

PG S48Ala -----

PQD W139d2 -----

Average (all)--

Weight percent

Al

3

11

10

5

7

7

6

4

6

6

6

4

11

10

11

11

11

-

10

6

6

10

9

8

3

8

7

3

4

8

9

7

4

.3

9

5

7

11

.2

1

1

.3

5

1

6

5

13

6

6

5

7

Fe

0.3

.1

3

5

3

5

2

6

5

3

4

1

2

2

3

2

2

2

4

5

10

2

5

3

10

5

3

4

4

3

4

2

2

.5

2

10

5

1

7

4

1

5

1

.5

7

1

8

6

4

4

3

5

3

4

5

10

2

2

3

5

10

6

7

2

5

5

5

4

5

.5

2

3

3

Mg

0.01

L

0.6

1.8

1.5

1.8

2.0

4.8

1.0

.9

1.3

.3

1.0

.7

.6

.4

.7

.3

1.3

1.5

1.5

.3

1.5

.7

1.0

.7

1.0

1.3

.8

.4

1.1

0.2

.5

.1

.2

1.5

.1

.02

.3

0.4

0.1

.3

.1

.1

2.1

.6

4.2

3.2

1.0

.9

.01

2.2

.6

1.0

1.8

.03

.3

.03

.5

3.0

6.2

.03

1.7

.3

1.5

1.3

2.2

1.4

.6

.1

1.0

1.0

1.1

Ca

0.3

.1

3

6

2

4

.3

5

3

3

3

1

3

1

4

4

3

3

4

2

2

.1

2

3

2

.1

2

3

2

3

3

15

.2

.3

L

2

10

.1

1

4

0.2

.1

1

1

1

.4

6

8

1

3

<.l

7

1

1

5

.2

2

.2

.1

5

6

.1

4

1

2

2

7

4

8

.2

1

3

4

Na

>0.3

~~

>0.3

>.3

--

>.3

.03

.3

.3

.3

.3

.3

.3

.3

.3

.3

.3

.3

.3

__

--

"

>.3

--

--

>.3

>.3

>.3

>.3

>0.3

--

--

--

__

__

--

--

--

H

3

4

.02

.2

.3

.3

.3

.3

.3

.3

.3

.3

.3

<.01

-_

--

.2

--

.1

.i

>.3

.04

>.3

--

>0.3

>.3

>.3

__

__

>.3

>0.3

K

2

Hy

5

5

--

2

>1

5

1

2

3

4

3

5

5

5

3

5

>1

__

-_

-

5

--

_-

3

3

4

5

4

--

_-

--

__

__

--

--

--

>1

7

6

1

>1

>1

>1

4

2

.1

2

5

>1

3

<.l

_-

--

.1

--

.2

1

3

1

3

--

1

4

2

__

__

1

2

Parts per million 

Be Bi Ce Co

Pegmatite

Cu Dy

0-.01

-

0.1 <10

N L

rothermally altered

0

0

.2 <

.6

.1

.2

.2

1

.1

.1

.4

.2

.1

.1

.2

.1

.1

.2

.2

.5

.3

.2

.2

.2

.2

.3

.1

.4

.2

.1

.2 >

0.1 <10

< 1 <10

N 100

.1 <10

.5 20

<.l <10

< 1 <10

.1 <10

<.l <10

<.l <10

< . 1 <10

< . 1 <10

'.I <10

<.l <10

< . 1 <10

.3 <10

<.l <10

N L

N L

N 15

L L

<.l <10

N 10

N L

<.l <10

.1 <10

< 1 <10

.1 18

0.1 >6

5

100

rock

800

700

100

600

200

100

600

400

500

500

500

300

800

1,200

500

1,000

600

1,000

1,500

300

700

500

50

1,000

400

700

1,000

400

600

2

N

2

4

N

2

1

<1

<1

<1

2

2

<1

1

2

3

4

2

3

2

2

L

L

2

1

L

<1

2

2

2

>1

<1

N

2

<1

N

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

6

<1

N

N

N

N

<1

N

N

<1

<1

<1

4

>0.4

<20

--

50

80

--

50

40

<20

<20

40

100

40

30

60

<20

<20

60

60

<20

__

--

--

--

<20

--

--

50

100

70

40

>40

<1

N

5

11

20

9

3

40

5

8

5

1

8

3

5

1

4

4

7

15

15

5

5

6

10

5

7

6

5

4

8

10

L

10

5

150

10

10

30

5

20

5

7

20

20

7

10

10

10

60

10

10

30

L

10

15

70

20

5

10

7

20

2

5

4

70

150

2

7

90

10

15

>10

7

20

20

10

120

30

70

30

20

15

7

30

20

10

20

10

20

70

100

>40

4

--

<3

<3

--

<3

7

<3

<3

<3

<3

<3

<3

<3

4

4

<3

5

<3

-_

--

--

4

--

<3

<3

<3

<3

>1

<1

--

1

2

--

1

2

<!

<1

<1

2

1

<1

<1

<1

<1

1

<1

<!

__

--

--

1

--

--

<!

2

1

<!

>1

Barren vanadium

0

0

.1

.5

.1

.1

.2

.02

.1

.2

.2

Mineraliz

0 .1

.1

.1

.1

.04

.2

.5

.3

.5

.2

.03

.2

.1

.2

.3

.003

.2

.01

.004

.3

.3

.02

.1

.1

N L

N 30

N L

N L

.5 L

N L

N L

1.5 10

0.3 >5

ed rock

0.5 L

1.3 14

1.2 <10

< 1 <10

<.l <10

<.l 17

< 1 <10

!4 <10

<.l 22

<.l <10

7 <10

< 1 <10

< 1 <10

<!l 36

<.l <10

1.2 <10

2 L

N L

< 1 <10

N L

1.8 <10

< 1 <10

5 <10

.1 33

.1 >10 <10

0 .2 > 1.2 >5

150

700

20

L

L

L

100

300

>160

1,500

100

1,100

400

50

300

400

200

700

400

30

300

400

300

600

<15

100

200

10

L

20

50

300

70

400

>300

2

2

N

N

L

1

1

1

>1

N

1

2

1

2

2

<1

<1

2

<1

<1

<1

1

2

2

<!

2

N

<1

N

<1

<1

<1

<1

<1

>1

N

N

N

N

N

N

N

N

N

N

<1

11

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<1

<]_

N

N

<1

N

<1

<1

<1

<1

1,200

>50

__

--

--

--

__

--

--

--

-

__

60

30

<20

<20

20

<20

<20

100

<20

<20

<20

<20

80

60

<20

--

--

<20

--

<-20

<20

<20

<20

<20

>20

7

L

L

N

10

30

5

15

>8

5

3

5

<1

20

3

30

30

3

20

6

30

7

10

10

60

70

5

10

50

50

2

30

5

30

>20

L

30

L

L

20

N

20

30

>13

10

8

2

7

2

8

20

40

50

30

5

8

7

30

3

5

50

L

1

1,500

10

<1

20

2

6

70

7

20

5

20

70

5

100

20

30

5

200

50

<1

100

300

100

800

>10

5

300

100

200

70

70

100

1,000

100

500

150

800

5

>1,000

70

7,000

>500

_-

--

--

--

_.

--

--

--

--

-_

<3

<3

<3

<3

3

<3

<3

<3

<3

<3

<3

<3

<3

<3

<3

--

--

<3

--

<3

<3

<3

<3

<3

<3

__

--

--

--

__

--

--

-

__

<1

<1

<1

<!

<1

1

1

2

<l

<1

<1

<1

1

1

<1

--

--

<1

--

<1

<1

<1

<1

<l

>0.3

Epidote rock

0

0

.4 <

.2

.1

.02

.2

.1

.2

0.1 <10

< . 1 <10

'.5 <10

N L

N L

<.l <10

0.1 <10

100

500

400

100

500

300

300

2

<1

1

N

N

<1

>1

<1

<1

<1

N

N

<1

<1

80

<20

60

_.

--

40

>40

20

2

1

N

7

7

>7

30

8

3

N

10

8

1

10

5

300

5

30

20

60

7

<3

<3

--

--

<3

>2

2

<1

1

__

--

<!

1
MINERALIZED AND ALTERED ROCK ELSEWHERE IN THE LASSITER COAST 33

rock from scattered locations elsewhere in the Lassiter Coast and southern Black Coast

lower limits: As, Au, Cd, Cs, Er, Ge, Hf, Hg. In, Ir, Li, Lu, Os, Pd, Pt Rb, Re, Rh, Rn, Sb, Sm,Ta,Tb, Te, Th,Tl,Tm, U; value of Cd for spec. Bo44e: 15 ppm; value of Er for spec. Ke7g: 

3 ppm; value of Ge for spec. BoTlb: 2 ppm; value of Li for spec. Ka316-4: 45 ppm; value of Li for spec. V32c: 38 ppm. G, greater than 10 percent; H, interference; leaders ( ), not 

looked for; N, not detected or at limit of detection; L, detected but below limit of determination. Symbols for rock type of host rock: L, sedimentary rocks of Latady Formation; 

V, volcanic rocks; DL, dike cutting Latady Formation; PD, plutonic rocks of diorite composition; PQD, plutonic rock of quartz diorite composition; PG, plutonic rock of granodiorite 

composition; PQM, plutonic rock of quartz monzonite composition]

Ga Gd 

15 <3

18 3 

23 4

17 11

15 7

3 7 

13 4 

9 <3 

18 13

19 6 

9 4 

9 <3 

17 <3

23 <3 

11 <3 

20 4 

21 4

14 <3

9 <3

17 12 

15 <3 

18 <3

15 >4

<2 <3 

13 <3 

14 <3 

5 3

3 <3 

12 5 

14 <3 

20 6 

13 5

<2 5 

13 5 

7 <3 

7 5 

17 5

<2 5 

<2 <3

<2 3 

<2 4 

22 6 

<2 <3 

10 4

>8 >3

27 3 

15 6 

70 <3

12 <3 

31 >2

Ho 

2

<1 

<1

2 

1

1 

<1 

3

<1 

<1 

<1

1 

<1 

2 

2

<1

23 

<1 

<1

>1

<1 

<1

1

<1 

<1

<1

<1 

<1 

<1

<1

La

<15

N

30 

30 

N 

30 

30

<15 

20 

50

30 

15 

40

30 

30 

50

30 

30 

20 

N

N 

50 

20

50 

40

>30

50 

50 

N

N

20 

N 

N 

20

>20

20 

20 

20

50

30 

30

N 

N

20 

20 

>9

30 

15 

40

N 

L 

30

Mn 'to

2,000 <1 

N G2.000

500 3 

3,000 5 

300 N 

1,300 2 

600 50

1,000 2 

1,100 1 

1,000 1 

900 7

400 3 

700 1 

400 <1 

900 2

600 8 

500 2 

600 3 

1,100 3

1,500 N 

1,500 10 

200 N 

700 N

700 2 

1,500 N 

300 N 

700 4

1,000 2 

1,000 10 

1,500 3

1,000 >4

200 N 

300 N 

70 N 

100 N

1,500 N 

2,000 N 

700 N 

1,000 N

700 N

300 N 

8,000 3 

200 3 

300 <1 

2,000 6

300 <1 

2,500 6 

2,000 2 

700 <1 

1,000 2

150 20 

1,500 8 

4,000 2 

2,500 3 

1,500 6

100 300 

500 N 

70 N 

1,000 3 

1,500 N

2,000 4 

30 100 

3,000 3 

300 2 

1,000 4

1,500 >17

2,000 4 

1,000 2 

2,500 4

30 N 

700 N 

700 2

1,000 >2

Nb

16

N

4 

5 

N 

<2 

4

<2 

<2 

2 

9

3 

3

3

5

6

5 

8 

5

10 

10

N 

N

7 

N 

10 

<2

7 

4 

6

>4

N 

15 

N 

N

N 

N

N 

L

>2

N 

8 

9 

14 

6

10 

5 

<2 

4 

<2

<2 

<2 

6 

7 

9

6

N 

N 

3 

N

3 

<2 

<2 

5 

6

>4

9 

<2 

2

N 

N 

<2

>2

Parts

Nd Ni Pb 

P(

<15 2 30

L 50

Hydrothermally

<15 3 40 

30 3 10 

30 N 

<15 3 10 

20 8 30

20 20 3 

30 3 3 

20 5 3 

50 2 10

40 1 10 

50 10 10 

40 7 3 

<15 3 20

<15 2 30 

30 6 30 

20 2 40 

<15 11 30

L 20 

L 30 

L 30 

L 10

<15 3 30 

30 30 

20 10 

20 10 5

70 2 10 

20 5 15 

15 3 20

>24 >7 >17 

Ban

L 20 

5 30

L N 

-- N N

15 N 

70 50 

7 15 

50 50

-- >20 >20

Miner

5 20 

<15 4 2,000 

<15 15 20 

<15 2 5 

<15 5 20

<15 7 20 

<15 15 7 

<15 20 8 

50 5 15 

20 7 3

<15 2 2 

<15 7 10 

<15 5 70 

30 20 15 

30 2 10

<15 3 10 

-- 100 N 

L N 

<15 15 50 

-- 150 30

H 80 7 

<15 1 1 

20 10 40 

<15 9 10 

<15 3 400

>7 >20 >100 

Epi

<15 15 20 

40 4 2 

<15 <1 15

-- N N 

L 20 

<15 5 5

>9 >4 >10

per millio 

Pr Sc 

>gmatite--C

<2 3

N

altered ro

5 5 

8 15 

15 

6 10 

4 7

H 20 

5 7 

6 5 

13 10

5 5 

5 10 

6 7 

4 7

<2 7 

5 10 

6 5 

<2 1

20 

15 

7 

7

<2 10 

7 

15 

4 7

10 15 

7 7 

4 4

>5 10 

 en vanadiu

5 

15 

N 

5

15 

5 

5 

10

-- >10 

 alized roc

N 

3 2 

11 30 

<2 1 

2 2

<2 <1 

<2 30 

<2 20 

8 8 

3 7

<2 <1 

5 20 

3 4 

6 5 

8 10

5 <1 

7 

N 

<2 <1 

20

<2 >70 

<2 <1 

5 5 

<2 1 

3 10

>3 >8 

.dote rock11 30 

5 9 

9 8

N 

10 

4 6

7 >10

n--C 

Sn 

onti

ck--

<3

8

<3 

<3

<3 

<3 

3 

4

<3 

<3 

<3 

<3

3 

<3 

4 

<1

<1

<3

6 

<3

>1 

m--C

k--C

<3 

<3 

<3 

<3

<3 

<3 

<3

<3 

8

<3 

<3 

14 

6 

<3

<3

30

<3 

5 

<3 

<3 

<3

>3 

-Con

5 

10 

17

<3

7

ontinued 

Sr

nued

5 

N

Continued

500 

400 

N 

800 

30

400 

600 

200 

700

200 

300 

200 

500

600 

400 

600 

500

500 

500 

N 

300

400 

200 

200 

200

700 

600 

150

>400 

ontinued

N 

100

N 

N

300 

300

N 

N

>90 

ontinued

N 

40 

700 

150 

15

70 

1,000 

800 

200 

300

30 

500 

150 

150 

600

3 

N 

N 

10 

300

70 > 

7 

200 

70 

100

>200 

tinued

700 

300 

800 >

N 

300 

500

>400 >

V

N

70 

200 

500 

100 

100

100 

70 

50 

150

30 

70 

70 

50

30 

50 

30 

70

150 

150 

100 

150

70 

50 

200 

70

100 

50 

30

100

70 

150 

10 

10

50 

N 

30 

70

>50

20 

20 

200 

2 

70

30 

300 

200 

150 

100

7 

150 

30 

50 

150

3 

50 

15 

10 

200

200 

50 

100 

10 

50

>90

200 

100 

200

30 

100 

100

100

W

<10

N

<10

N

N 

N 

N 

L

N 

N

<10 

<10

N 

N 

N 

N

N 

L 

N 

N

N 

N

<20 

50

L

L 

150

>8

N 

N

Y Yb

170 >15

N

20 2 

50 6 

L 

20 5 

60 4

7 1 

15 2 

10 1 

70 7

20 3 

10 2 

10 1 

15 3

20 4 

30 4 

30 5 

20 3

50 

30 

15 

15

30 6 

20 

30 

10 1

50 7 

20 3

>25 4

15 

30 

L 

30

50 

15 

20 

20

>25

10 

15 1 

50 6 

20 3 

30 2

10 1 

20 3 

10 2 

15 2 

10 2

<3 <.7 

20 3 

20 3 

20 3 

50 6

<3 <.2

15 

N 

7 1 

20

7 2 

<3 <.7

7 1 

5 1 

10 2

>14 >4

50 6 

15 3 

20 4

N 

20 

10 1

>15 3

Sample %£ 

Zn Zr no ' type

<15 30 ----Rol03b PG

30 100 Rol3abc PG 

60 30 - Ro75bc PQD

20 50 ----Roll3b PQD 

50 300 ----RolSOb PG

<15 200 ---Rol98de PG 

20 30 ---Rol99bc PG 

<15 200 -Rol99g PG 

30 70 ----Ro215c PG

30 200 -Ro225a PG 

30 150 ----Ro228b PG 

40 100 ----Ro313a PQM 

40 200 Ro313b PQM

N 70 ----S71ca 3 DL

40 300 W150ze PG 

40 100 - Wal7bj DL

>30 150 --Average (all)

N 100 --Average (all)

100 70 ---Ka316-4 PQM

100 30 ----Ke41de PD 

100 20 ----Ke41fg PD

100 30 Ro241b PD 

100 150 ----Ro251b PG 

50 200 - Ro256c L 

70 200 ----Ro315d DL

20 <3 Ro315h L

150 30 W113de PD 

<15 10 --W139h PQD 

150 30 W144c PG

>250 >80 --Average (all)

100 200 - -Bo64b PQM 

30 70 - Re37b PQD 

50 150 Rol49b PG

N 70 ----S48Ala PG 

30 15 W139d 2 PQD

30 100 --Average (all)
34 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

results of about 70 semiquantitative spectrographic 

analyses of these specimens (table 6) reveals that, in 

overall families of trace elements, the mineralized rock 

is similar to that of the Lassiter Coast copper deposit, 

and that the hydrothermally altered rock is similar to 

that of the sheared phyllic-argillic zones. The main 

element additions consisted of Cu and less abundant 

Fe, Zn, Mo, Pb, and perhaps Ag. Essentially all occurrences were within or immediately adjacent to stocks 

or batholiths that are probably similar in age (Mehnert 

and others, 1975; Rowley and others, 1976) to the plutons at the copper deposit. The number of occurrences 

appears to increase somewhat to the north, paralleling 

the northward increase in the proportion of igneous 

rock to metamorphosed sedimentary and volcanic rock.

Quartz veins are a major locus of mineralized rock 

and are abundant at many outcrops in the Lassiter 

Coast. The results of semiquantitative spectrographic 

analyses of numerous veins reveal two categories of 

quartz veins. Those veins that occur in sedimentary 

rocks far from igneous contacts do not contain sulfide 

minerals and almost invariably do not exhibit any 

high values of minor elements. They undoubtedly are 

veins related to metamorphic and tectonic processes 

during deformation of the sedimentary and volcanic 

rock; many of the veins do not have roots and are metamorphic quartz segregations. Quartz veins in or adjacent to igneous rocks, on the other hand, may contain 

macroscopic sulfide minerals and most exhibit an addition of some minor elements.

Semiquantitative spectrographic analyses of the 

sulfide-bearing quartz veins reveal moderately high 

Cu from the eastern Werner Mountains (specimen 

Ro256c), southern Playfair Mountains (Ke7g), and the 

western RARE Range (S54c). High to moderate values 

of Cu and Zn occur in unnamed mountains of the 

southern Black Coast (Ka316-4), Werner Mountains 

(Ro251b), Ferguson Nunataks (V32c), and chalcopyrite-bearing rock from the northern Latady Mountains (S6). Pyrite-bearing rock (Wa9a) from the eastern Werner Mountains and rock (Ro54a) from the eastern Guettard Range show anomalous As and moderate 

Cu. High Mo and moderate Cu characterize pyritebearing rock in the southern Black Coast (Ro315h), 

and high Mo and W occur in chalcopyrite-molybdenite 

bearing rock from the western RARE Range (W139h). 

High Zn and Pb and moderate Cu values were determined for altered and silicified rock in the western 

Werner Mountains (Bo44e). Sulfides also occur as pods 

or disseminated crystals in localized areas. A zone containing pyrite, chalcopyrite, malachite-chrysocolla(?), 

and azurite, and containing high values of Cu, Pb, Zn, 

Bi, and apparently Ag occurs in plutonic rock (Wa21) 

in the northern Dana Mountains. A possible malachite

stain was noted at Mount Poster (B28d) west of the 

Latady Mountains. Several pyrite zones were found in 

plutonic rock (S53) in the northern Latady Mountains. 

Veins and disseminated crystals of pyrite in silicic porphyry dikes occur at outcrops in the southern Black 

Coast (Ro315d) and Scaife Mountains (W41d); 

malachite-chrysocolla(?) stains occur at the contact of a 

silicic porphyry dike cutting plutonic rocks in the 

northern Dana Mountains (Wa20). Molybdenite was 

observed in pegmatites in the eastern Hutton Mountains (Rol68e) and Latady Mountains (S34a). 

Malachite-chrysocolla(?), occasionally accompanied by 

chalcopyrite and quartz veins, commonly occurs in 

more mafic (mostly pyroxene and hornblende diorite or 

gabbro) plutonic contact phases in the southeastern 

Dana Mountains (Ro303), Werner Mountains (Ke37g, 

Ke41de, Ke41fg, Ro241b), Watson Peaks (V31e), Rivera Peaks (Ke20gk), southwestern Playfair Mountains 

(Fl, V3, V5), and Guettard Range (WllSde); 

semiquantitative spectrographic analyses of samples 

for some of these outcrops reveal high to moderate values of Cu, Zn, and for some rocks Pb.

Hydrothermally altered igneous rock of argillic and 

phyllic facies was sampled in numerous areas in the 

Lassiter Coast and southern Black Coast. Such altered 

rock commonly occurs at igneous contacts, as in the 

southern Dana Mountains (Ro225a), northern Hutton 

Mountains (Rol98de), Guettard Range (Rol2a, 

RolSabc), RARE Range (Ro75bc, Ro86a), and Latady 

Mountains (S34d, S38k, W87c). Some silicic dike rocks 

are intensely hydrothermally altered, as in the southeastern Werner Mountains (Wal7bi), northeastern 

Latady Mountains (P9b), and western Scaife Mountains (S71cas). Poorly defined areas of hydrothermally 

altered rock within plutons were observed in the 

southern Black Coast (Bo71b), southwestern Dana 

Mountains (Ro228b), southern Hutton Mountains 

(RolSOb, WISOze), and Latady Mountains (S31a). In 

some places the altered rock follows narrow linear 

zones as much as several meters in width that cut 

plutonic rocks. These are similar in appearance and 

petrology to the sheared phyllic-argillic zones of the 

copper deposit; this similarity and their linearity indicate that they also were the sites of fracturing and 

perhaps of shearing. Such sheared phyllic-argillic 

zones were mapped in the southern Black Coast 

(Bo68c, Ro313a, Ro313b, Wa37b), Rivera Peaks 

(Ro215c), northern Playfair Mountains (V16b), and 

northwestern RARE Range (RollSb, Rol99bc, Rol99g, 

W144b). Ore minerals occur adjacent to the zones at 

two of these locations; semiquantitative spectrographic 

analyses prove that samples containing pyrite (Bo68b) 

and pyrite and malachite-chrysocolla(?) (W144c) have 

been enriched in Cu and Zn.
REFERENCES CITED 35

CONCLUSIONS

The Lassiter Coast copper deposit evolved during a 

complex series of igneous and structural events. The 

Late Cretaceous history in the immediate vicinity of 

the deposit consists of two essentially identical broad 

episodes, both of which were initiated by batholith intrusion of granodiorite and by stock intrusion of quartz 

monzonite (Copper Nunataks pluton) into a terrane of 

tightly folded Upper Jurassic eugeosynclinal sedimentary and volcanic rocks. Each of these plutonic events 

was followed by intrusion of hypabyssal, mostly porphyritic, more mafic dikes, and each broad episode was 

terminated by shear-fracturing, hydrothermal alteration, and weak mineralization. The major copper 

mineralization occurred during the younger quartz 

monzonite episode. At this time phyllic and argillic 

altered rock was formed along linear sheared zones 

between which is a broad propylitic zone. Mineralized 

rock occurs over a broad area and is not confined to the 

sheared phyllic-argillic zones; however, it is most intensely developed in the wallrock where the zones are 

most abundant.

The Lassiter Coast copper deposit bears many similarities to the typical porphyry copper deposit described by Lowell and Guilbert (1970). The associated 

and slightly older porphyry dikes are similar in 

petrology, chemistry, and age to those that preceded 

mineralization in the typical porphyry deposits of the 

circum-Pacific province. The alteration and mineralization assemblages also have many similarities in petrology and chemistry. Probably the chief departures of 

the Lassiter Coast deposit from the typical commercial 

porphyry deposit are the coarseness and relatively low 

grade of the mineralized rock and the linear pattern of 

altered and mineralized rock instead of the typical concentric zones of altered and mineralized rock.

Using the porphyry model (Lowell and Guilbert, 

1970; Sillitoe, 1973) we can speculate on the location of 

a possible zone of higher grade. The coarseness of the 

quartz veins and disseminated sulfides, the presence of 

magnetite, and the lack of a significant potassic alteration zone suggest a site marginal to a higher grade 

deposit. Reconnaissance geologic mapping of numerous 

plutons elsewhere in the Lassiter Coast area suggests 

that it is unlikely that the roofs of the plutons in the 

area of the copper deposit were more than 1,000 m 

above the present peaks of the nunataks. This may 

suggest that erosion has not removed any significant 

ore deposit. Richer deposits may lie below the present 

erosion surface at the Copper Nunataks site or they 

may occur adjacent to the copper deposit described.

The Lassiter Coast is part of the circum-Pacific 

igneous-tectonic province, which contains most of the

world's economic porphyry deposits. The existence of 

the Lassiter Coast copper deposit and the numerous 

other occurrences of similar metallized and hydrothermally altered rock nearby prove that the 

southern Antarctic Peninsula locally contains anomalous copper concentrations. And from what can be determined by a review of the literature of other types of 

mineralized rock farther north (Rowley and others, 

1975), we conclude that the Antarctic Peninsula is a 

copper province and that at least locally there are 

affinities with the porphyry type of copper deposit in 

this province. A tie between the copper provinces of 

Antarctica and South America has been proposed by 

Sillitoe (1972, p. 193); this proposal is not surprising in 

light of the geologic similarities already mentioned. 

The discovery of the Lassiter Coast copper deposit provides the first geologic data for completing the southern extension of the circum-Pacific copper province. 

While this deposit has very low economic potential in 

the foreseeable future, the presence of the Lassiter 

Coast deposit suggests that economic deposits might be 

present in the Antarctic Peninsula.

REFERENCES CITED

Adie, R. J., 1954, The basement complex; early Paleozoic plutonic 

and volcanic rocks, [pt.] 1 of The petrology of Graham Land: 

Falkland Islands Dependencies Survey Sci. Rept. 11, 22 p.

 1955, The Andean granite-gabbro intrusive suite, [Pt.] 2, of 

The petrology of Graham Land: Falkland Islands Dependencies 

Survey Sci. Rept. 12, 39 p.

 1957, Metamorphic rocks of the Trinity Peninsula Series, [Pt.] 

3 of The petrology of Graham Land: Falkland Islands Dependencies Survey Sci. Rept. 20, 26 p.

 1964, Geologic history, in Priestly, R. E., Adie, R. J., and Robin, G. D., eds., Antarctic research A review of British scientific 

achievements in Antarctica: London, Butterworths, p. 118-162. 

-1969, Geologic map of Antarctica, in Bushnell, V. C., and

Craddock, Campbell, eds., Geologic maps of Antarctica: Am. 

Geog. Soc. Antarctic Map Folio Ser. 12, pis. 1, 2.

Banks, N. G., 1974, Distribution of copper in biotite and biotite alteration products in intrusive rocks near two Arizona porphyry 

copper deposits: U.S. Geol. Survey Jour. Research, v. 2, no. 2, 

p. 195-211.

Bateman, P. C., 1961, Granitic formations in the east-central Sierra 

Nevada near Bishop, California: Geol. Soc. America Bull., v. 72, 

no. 10, p. 1521-1538.

Bibby, J. S., 1966, The stratigraphy of part of northeast Graham 

Land and the James Ross Island Group: British Antarctic Survey Sci. Repts. 53, 37 p.

Boyer, S. J., 1975, Chemical weathering of rocks on the Lassiter 

Coast, Antarctic Peninsula, Antarctica: New Zealand Jour. 

Geology and Geophysics, v. 18, no. 4, p. 623-628.

Bradshaw, J. D., and Andrews, P. B., 1973, Geotectonics and the 

New Zealand geosyncline: Nature; Phys. Sci., v. 241, p. 14-16.

Craddock, Campbell, 1970a, Tectonic map of Antarctica, in 

Bushnell, V. C., and Craddock, Campbell, eds., Geologic maps of 

Antarctica: Am. Geog. Soc. Antarctic Map Folio Ser. 12, pi. 21.

 1970b, Map of Gondwanaland, in Bushnell, V. C., and Craddock, Campbell, eds., Geologic maps of Antarctica: Am. Geog. 

Soc. Antarctic Map Folio Ser. 12, pi. 23.
36 GEOLOGY OF AN UPPER CRETACEOUS COPPER DEPOSIT, ANTARCTIC PENINSULA

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