Petrology of the

Ultramafic and Gabbroic Rocks of the 

Brady Glacier Nickel-Copper Deposit, 

Fairweather Range, Southeastern Alaska

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1195
Petrology of the

Ultramafic and Gabbroic Rocks of the 

Brady Glacier Nickel-Copper Deposit,

Fairweather Range, Southeastern Alaska

By GLEN R. HIMMELBERG and ROBERT A. LONEY 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1195

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

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Doyle G. Frederick, Acting Director

Library of Congress Cataloging in Publication Data

Himmelberg, Glen R.

Petrology of the ultramafic and gabbroic rocks of the Brady Glacier nickel-copper deposit, Fairweather Range, southeastern 

Alaska

(Geological Survey Professional Paper 1195)

Bibliography: p. 24

Supt. of Docs, no.: I 19.16:1195

1. Rocks, Ultrabasic. 2. Gabbro Alaska Fairweather Range. 3. Ore-deposits Alaska Fairweather Range. 4. PetrologyAlaska Fairweather Range. I. Loney, Robert Ahlberg, 1922- joint author. II. Title. III. Series: United States 

Geological Survey Professional Paper 1195.

QE462.U4H55 552'.1 80-607095

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

Washington, D.C. 20402
CONTENTS

Page

Abstract .................................................................................................. 1

Introduction .......................................................................................... 1

Acknowledgments ................................................................................ l

Extent of the Brady Glacier nickel-copper deposit .................. 1

General description of the La Perouse layered gabbro .......... 4

Rock succession and petrography of the Brady Glacier

cumulates ............................................................................................ 5

Occurrence and mineralogy of sulfide minerals ...................... 9

Rock chemistry .................................................................................... 9

Page

Chemistry of cumulus minerals .........................———-.—...——-.. 10

Olivine ........................................................-.-------—--- 10

Pyroxene .....................................................................................—.. 12

Plagioclase .....................................—.......-—.——.—-...........——.. 12

Chromite .................................-......——.—..—---—--——-—-— 14

Crystallization of the Brady Glacier cumulates ...................... 17

Metamorphic country rocks ...............................—......—-—....—-—. 22

Summary .....................................................................—....——.....——— 23

References cited ...................................—.............................—..—......... 24

ILLUSTRATIONS

Page

FIGURE 1. Index map showing location of major gabbro bodies in the Fairweather Range, Alaska-——-———————...—........ 2

2. Map of La Perouse layered gabbro and surrounding country rocks showing location of Brady Glacier cumulates- 3

3. Map of the Brady Glacier cumulate area showing location of nunataks, drill holes, and cross sections ——..—..... 4 

4-6. Generalized cross sections through the Brady Glacier cumulates:

4. Southwest-northeast cross section ........................................................................................................................ 6

5. Northwest-southeast cross section ....................................................................................................................... 7

6. North-south cross section ...................................................................................................................................... g

7-10. Core sections showing stratigraphic distribution and chemical composition of cumulus minerals, Brady Glacier 

cumulates:

7. Core NUC-5.............................................................................................................................................................. 9

8. CoreNUC-2 ....................................................................................................................................................... 11

9. CoreNUC-7 ............................................................................................................................................................ II

10. CoreNUC-10 ------..-..--------.--.-...-.-.--...-------.-----.-.--.....-..........----................................................................. 12

11-16. Graphs showing:

11. MgO variation of Brady Glacier cumulate rock samples .................................................................................. 13

12. Variation of olivine composition in the olivine cumulate in the lower part

of core NUC-5 .....................................„.................................—...............—..—.—........................................... 15

13. Composition profiles across plagioclase grains ................................................................................................. 16

14. Variation of the cation ratios Mg to (Mg+Fe+2) and Cr to (Cr+Al) chromite in the Brady Glacier cumulates 18

15. Variation in MnO content of chromite in the Brady Glacier cumulates ......................................................... 18

16. Variation of TiO2 content of chromite in the Brady Glacier cumulates .......................................................... 19

TABLES

Page

TABLE 1- Chemical composition of Brady Glacier cumulate samples ............................................................................................................... JQ

2-6. Chemical composition and structural formulas of cumulus phases, Brady Glacier cumulates:

2. Olivine ................................................................................................................^ 14

3. Bronzite ..............................................................................................................................................^^ 18

4. Augite ........................................................................................................................................................................-———.—-——...—. 19

5. Plagioclase ................................................................................................................................................................................——... 20

6. Chromite ............... ........................................................................................................................................................................... 21

in
IV CONTENTS

METRIC-ENGLISH EQUIVALENTS

Metric U.S. Customary equivalent

iMeter(m) = 3.281 feet(ft)

1 kilometer (km) = 0.6214 mile(mi)

Ikilobar(kbar) = 987 atmospheres (atm)

1°C = 5/9(°F-32)
PETROLOGY OF THE ULTRAMAFIC AND GABBROIC ROCKS

OF THE BRADY GLACIER NICKEL-COPPER DEPOSIT,

FAIRWEATHER RANGE, SOUTHEASTERN ALASKA

By GLEN R. HIMMELBERG* an(j ROBERT A. LONEY

ABSTRACT

The basal cumulates of the La Perouse layered gabbro, the 

Brady Glacier cumulates, are the host rocks for the Brady Glacier 

nickel-copper deposit, Fairweather Range, southeastern Alaska. 

Major rock types of the Brady Glacier cumulates are olivinechromite cumulate, plagioclase-augite-olivine cumulate, and olivine-bronzite-chromite cumulate. The cumulates are interlayered 

on a scale of a few centimeters to hundreds of meters; the layers are 

not laterally continuous over large distances but are lenticular 

and wedge out in all directions. The layering seems to be draped 

over the metamorphic basement topography, which has considerable relief. Sulfide minerals are dominantly pyrrhotite, pentlandite, and chalcopyrite. Although disseminated sulfide minerals show no relation to stratigraphic position or rock type, 

massive sulfides tend to occur along boundaries between mafic 

and ultramafic layers. Textures indicate that the sulfide mineralization represents an immiscible sulfide melt.

Rock chemistry indicates that the ultramafic and gabbroic 

cumulates are related by differentiation. Cumulus-mineral compositions are olivine—Fo86 to Fo71; plagioclase—An8i to An63j bronzite—Ca3Mg82Fei5 to CaiMg75Fe2i; augite—Ca4sMg47Fe8 to Ca42 

Mg48Feio.

Spatial distribution of cumulate sequences suggests two orders 

of crystallization: (1) olivine, olivine + bronzite, olivine + bronzite + 

plagioclase, and (2) olivine, olivine + plagioclase, olivine + plagioclase + augite. The differences in crystallization sequence are 

believed to result from separate injections of magma of slightly 

different composition.

INTRODUCTION

Major nickel-copper sulfide deposits are associated 

with ultramafic and mafic rocks. Naldrett and Cabri 

(1976) have demonstrated the relations between 

composition of sulfide ore and the geochemistry of 

the host rocks. Their study showed that the major 

nickel-copper sulfide deposits are associated with 

tholeiitic or komatiitic rock types, the Cu to (Cu+Ni) 

ratio of the sulfide ore increasing with decreasing 

MgO content of the host rocks. Naldrett and Cabri 

pointed up the necessity of determining the nature of 

the igneous host rocks as well as the relations of the

'U.S. Geological Survey and Department of Geology, University of Missouri, Columbia, MO 65211.

ore to the host rock. In this paper, we examine the 

rock succession, chemistry, and petrology of the 

basal rocks of the La Perouse layered gabbro that are 

the host rocks for nickel-copper sulfide deposits. The 

sulfide deposit of the La Perouse body is generally 

known as the Brady Glacier deposit (Brew and 

others, 1978), and the host rock cumulates are herein 

informally referred to as the Brady Glacier cumulates. The La Perouse gabbro intrusion was called 

the Crillon-La Perouse stock by Rossman (1963); for 

convenience in this report, it is called simply the La 

Perouse layered gabbro.

Rock classification and nomenclature used in this 

report is that recommended by the IUGS Subcommission on the Systematics of Igneous Rocks (1973); 

cumulus terminology is that of Wager and others 

(1960) and of Jackson (1967).

ACKNOWLEDGMENTS

We thank Newmont Exploration, Ltd. for generosity in making available to us the diamond-drill 

core and other materials from the company's Brady 

Glacier nickel-copper prospect. We especially acknowledge the cooperation and courtesy of Mr. Bryon 

S. Hardie, of Newmont Exploration, Ltd., Tucson, 

Ariz., and Mr. Earl Craig, of Dawn Mining Co., Ford, 

Wash., where the drill core was stored. Edward M. 

MacKevett, Jr., formerly of the U.S. Geological 

Survey, gave invaluable help in initiating the project.

EXTENT OF THE

BRADY GLACIER

NICKEL-COPPER DEPOSIT

The Brady Glacier nickel-copper deposit lies near the 

southeast margin of the La Perouse layered gabbro 

(figs. 1, 2), where, except for two—sometimes under 

light-snow conditions three—small nunataks, it is 

covered by the Brady Glacier. The deposit was
PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

136°00'

nt fM*vweat hen

. .... 

Woy^t VVilJbttf x\:

Paleozoic strata 

intruded by 

granitic plutons

EXPLANATION

Gabbroic rocks (Tertiary)

Biotite schist and gneiss (Mesozoic?) \Q

*<v

Hornblende schist and gneiss 

(Mesozoic?)

(Granitic rocks not shown)

/Cross 

/Sound

Contact—Dashed where 

approximately located

————— Fault—Dashed where approximately 

located

————— Boundary of Tarr Inlet suture zone

FIGURE 1.—Location of major gabbro bodies in the Fairweather Range, Alaska. Adapted from Brew and others (1978); location

of Tarr Inlet suture zone from Brew and Morrell (1978, fig. 42).

discovered in 1958 by prospectors of the Fremont 

Mining Co.; by late 1959, the company had completed 32 diamond-drill holes (MacKevett and others, 

1971, p. 81-82). In 1960, Newmont Exploration, Ltd. 

became manager of the property through a lease 

agreement with Fremont and during the years 1960 

and 1961 drilled 14 additional diamond-drill holes. 

By the early 1970's, 36 more holes had been drilled, 

bringing the total to 82.

The areal extent of the drilling (fig. 3) was limited 

by glacial ice conditions and by the gabbro-schist

contact rather than by systematic lateral decrease in 

sulfide mineralization (R. D. Ellett, 1975, cited in 

Brew and others, 1978). Further surface drilling to 

the northeast was prevented by thicker ice (greater 

than 320 m); drilling to the west, by higher rates of ice 

flow. In these directions, then, the limits of the 

mineralization are unknown. The gabbro-schist contact south and east of the nunataks probably represents the main outer contact of the La Perouse 

gabbro intrusion; whereas the contact northwest of 

the nunataks may represent an isolated knob of
137°30'

EXTENT OF THE BRADY GLACIER NICKEL-COPPER DEPOSIT

137°00'

EXPLANATION

Snow and Unconsolidated 

glacial ice deposits

Amphibolite Biotite schist 

Fault

Mount Dagelet

Mount 

Perouse

PACIFIC 

OCEAN

FIGURE 2.—La Perouse layered gabbro and surrounding country rocks. Rectangular box (area of fig. 3) shows location of the 

Brady Glacier cumulates. Map based on U.S. Geological Survey Mount Fairweather C-3, C-4, D-3, and D-4 quadrangles, 

1:63,360.

schist basement within the periphery of the intrusion. In general, the stratigraphic units delineated 

by core data tend to parallel the schist contact which 

suggests the draping of layers over the basement 

topography.

In 1972, Newmont Exploration, Ltd., offered the 

U.S. Geological Survey the opportunity to study the 

cores. Key cores were logged and sampled at the 

company warehouse near Ford, Wash., in 1973 and 

1975. During the summers of 1975, 1976, and 1977,
PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

EXPLANATION

Exposed contact

Approximate location of 

gabbro-schist contact beneath ice

Brady Glacier, ice and snow 

Altered gabbro, plagioclaseclinopyroxene±olivine

cumulate

F-r6 Core hole and cross sections 

lines see figures 4, 5, and 6

Peridotite, olivine-(chromite)

cumulate 

Biotite schist T. 38 S. R. 51 E,

500 1000 I 1500

I

2000 FEET

1—— _J

0 500 METERS

FIGURE 3.—Brady Glacier cumulate area showing location of nunataks, drill holes, and cross sections. Data on drill holes 

by courtesy Newmount Exploration, Ltd.; map based on U.S. Geological Survey Mount Fairweather C-3 quadrangle, 1:63,360.

field studies were carried out in the Brady Glacier 

area and in the main outcrop area of the La Perouse 

layered gabbro.

GENERAL DESCRIPTION OF THE 

LA PEROUSE LAYERED GABBRO

The La Perouse layered gabbro is the largest of 

several gabbro bodies exposed in the Fairweather 

Range, southeastern Alaska (fig. 1). Other gabbro 

bodies include the Fairweather body (Plafker and 

MacKevett, 1970) and the Mount Wilbur body (D. A. 

Brew, oral commun., 1977) to the north and the 

Astrolabe-DeLangle body to the south. Both the La 

Perouse and Astrolabe-DeLangle gabbros were 

reported on by Rossman (1963) and studied by

Himmelberg and Loney, but little is known about the 

Fairweather and the Mount Wilbur bodies.

The La Perouse layered gabbro underlies an area 

approximately 12 by 27 km and has an exposed 

thickness of about 6,000 m (fig. 2). The body was 

emplaced in the Tertiary into a sequence of biotite 

schists and amphibolites of probably Mesozoic age 

(Rossman, 1963; MacKevett and others, 1971). The 

dominant rock types are interlayered olivine gabbro 

and norite with lesser amounts of gabbronorite, 

troctolite, and rare anorthosite. Thin layers of ultramafic rocks occur throughout the sequence, but, 

except in the basal cumulates, are extremely rare. 

Layering and textural characteristics indicate that 

the body originated largely by cumulus processes. 

Layers range in thickness from centimeters to hun-
ROCK SUCCESSION AND PETROGRAPHY OF THE BRADY GLACIER CUMULATES

dreds of meters. Although individual layers may be 

well defined by either phase contacts or ratio contacts (see photographs of layering in Rossman, 

1963), there are no distinctive units that can be 

traced throughout the body and none that can be 

used as marker units across the rugged terrain. Most 

layers tend to fade or wedge out over relatively short 

distances. Distribution of cumulus minerals and the 

absence of any systematic variation in mineral 

composition relative to stratigraphic height (Rossman, 1963; Himmelberg, unpub. data, 1979) suggest 

multiple injections of magma rather than large 

volumes of magma crystallizing and differentiating. 

In these aspects, the La Perouse gabbro is similar to 

the Axelgold intrusion in British Columbia (Irvine, 

1975a).

The base of the La Perouse intrusion is not exposed; but basal cumulates, the Brady Glacier cumulates, are exposed in two nunataks in the Brady 

Glacier southeast of the main body (fig. 2) and have 

been penetrated by drilling through the Brady Glacier in the vicinity of the nunataks. These basal 

cumulates are the host rocks for the nickel-copper 

sulfide deposit.

The age of the La Perouse intrusion is uncertain. 

Czamanske and his coworkers (1978) reported the 

following age-related isotopic data: 87Sr/86Sr range 

from 0.7041 to 0.7047 among plagioclase, olivine and 

pyroxene fractions; low rubidium content precludes 

rubidium-strontium dating. Conventional potassium-argon dates for plagioclase, hornblende, and 

clinopyroxene are discordant and range from 32 to 

250 m.y. (million years); 143Nd/144Nd range only 

from 0.51293 to 0.51299, although 147Sm/144Nd range 

from 0.128 to 0.293 for whole-rock, plagioclase, and 

pyroxene fractions. These samarium-neodymium isotopic ratios constrain the rock age to less than 100 

m.y. if it is assumed that the samarium-neodymium 

systematics were not disturbed by nearby granitic 

intrusive activity 26-30 m.y. ago. Dating by the 

40Ar/39Ar technique indicates an age as young as 30 

m.y. (R. J. Fleck, oral commun., 1979), which suggests, along with the samarium-neodymium data, 

that the higher conventional potassium-argon ages 

result from excess argon.

By Naldrett and Cabri's classification of ultramafic and related rocks (1976), the La Perouse 

layered gabbro and sulfide deposit is most similar to 

the "large stratiformly layered complexes." The La 

Perouse gabbro differs from other stratiform bodies 

such as the Bushveld Complex, South Africa (Wager 

and Brown, 1968; Visser and von Gruenewaldt, 

1970), Stillwater Complex, Montana (Hess, 1960; 

Jackson, 1961; Page, 1977), and Muskox intrusion

(Irvine and Smith, 1967) in two significant ways: (1) 

It was not emplaced in a nonorogenic cratonic area 

but rather near a major fault system into country 

rocks similar to an oceanic crust assemblage; (2) It 

shows no systematic relation between mineral chemical variation and stratigraphic height; this suggests 

multiple injection of magma with limited differentiation.

ROCK SUCCESSION AND

PETROGRAPHY OF THE

BRADY GLACIER CUMULATES

Though exposed in only a few small nunataks, the 

extent of the Brady Glacier cumulates under the 

Brady Glacier, and therefore the extent of the sulfide 

deposit, has been relatively well established by diamond-core drilling except to the north and west in 

the direction of the exposed La Perouse gabbro body. 

Ten drill cores were studied and sampled in detail to 

determine the rock succession, petrography, 

chemistry, and crystallization history of the Brady 

Glacier cumulates. The location of structurally 

important drill holes is shown in figure 3. About an 

equal number of drill cores were studied in less detail.

Cross sections constructed from core data, figures 

4, 5, and 6, illustrate the interlayering of major rock 

units. Slightly more layering detail is shown in 

individual core sections of figures 7 through 10. The 

layers are distinguished on the basis of cumulus 

mineralogy only. A cumulate consists of cumulus 

grains that precipitated from the magma and settled 

to the floor of the magma chamber and postcumulus 

material that crystallized later in the places it now 

occupies in the magmatic sediment (Jackson, 1967). 

Identity of layers that result from the accumulation 

of cumulus minerals and in general reflect the 

crystallization sequence of the magma may be 

obscured if whole-rock mineralogy is utilized. In 

rocks of the same cumulus mineralogy, the 

postcumulus material may vary in kind and amount, 

laterally or vertically, within a single cumulate layer 

and from layer to layer. For example, a single olivinechromite cumulate layer may vary from a dunite to 

an olivine gabbro, depending on the amount of 

pos tcumulus plagioclase and clinopyroxene.

Individual layers range in thickness from a few 

centimeters to hundreds of meters; the phase contacts between layers are generally sharp, but some 

contacts between olivine cumulate and overlying 

plagioclase-augite-olivine cumulate appear gradational because of gradational increase in postcumulus plagioclase in the olivine cumulate before plagio-
sw

METERS

1100- 

1000- 

900- 

800- 

700- 

600- 

500- 

400 

300- 

200- 

100- 

SEA LEVEL -

NUC-5

PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

NUC-2

NE

F-32

200 METERS

EXPLANATION 

Ni+Cu>0.5 weight percent

Glacial ice and snow 

Gabbro and peridotite 

intertonguing

Biotite schist

FEET 

-3500

-3000

-2500

-2000

-1500

-1000

-500

L SEA LEVEL

FIGURE 4.—Generalized southwest-northeast cross section through the Brady Glacier cumulates. (See fig. 3 for location.) oc, 

olivine-chromite cumulate; obc, olivine-bronzite-chromite cumulate; poc, plagioclase-olivine-chromite cumulate; po, plagioclaseolivine cumulate; poa, plagioclase-olivine-augite cumulate; pa, plagioclase-augite cumulate.

clase becomes a cumulus phase. The layers are not 

laterally continuous over large distances but are 

lenticular and wedge out in all directions. (See figs.4 

and 5.) Within individual cumulate layers, there is no 

indication of rhythmic layering, that is, alternation 

of dark and light layers generally attributed to 

deposition from convection currents.

The major rock types are olivine cumulate 

(peridotite and feldspathic peridotite), 

plagioclase-augite-olivine cumulate (olivine gabbro), 

olivine-bronzite ± plagioclase cumulate (harzburgite 

and feldspathic harzburgite), and olivine-plagioclase 

cumulate (dominantly olivine gabbro and troctolite, 

rarely, feldspathic dunite and feldspathic 

harzburgite). All these cumulates except the 

plagioclase-augite-olivine cumulate commonly 

contain a minor amount of cumulus chromite. Thin 

layers of plagioclase-augite-cumulate (gabbro) and 

plagioclase cumulate (anorthosite and gabbro) are 

present but not volumetrically abundant. Graphite is 

conspicuous in sulfide ore zones (seams to 30 cm 

thick) and is commonly observed throughout the 

gabbroic rocks. The cumulates are cut by numerous 

dikes that range in composition from hornblende 

diabase to granodiorite.

At most places, an olivine cumulate forms the 

lowermost unit. In the eastern part of the Brady 

Glacier cumulates, olivine cumulate is interlayered

upward with olivine-plagioclase cumulates and 

plagioclase-augite-olivine cumulates. In the western 

part, the gabbroic rocks wedge out, and the olivine 

cumulates are interlayered with olivine-bronzite 

cumulates (fig. 4).

Four drill holes penetrated the contact of the 

cumulates with the country-rock schist. Two of the 

cores, NUC-24 and NUC-1 (not in fig. 3), have 

olivine cumulate in contact with the schist; whereas 

two others, NUC-2 (too thin to show in figs. 3 and 8) 

and NUC-10, have an altered noncumulate gabbro 

in contact with the schist. The contact gabbros are 

approximately 1 m thick and consist of plagioclase 

and amphibole mattes in a diabasic to ophitic texture. The noncumulate gabbro in drill core NUC-2 

grades upward into approximately 12 m of plagioclase-olivine cumulate containing abundant postcumulus clinopyroxene; in drill core NUC-10, the 

gabbro is in contact upward with olivine cumulate. 

Whether such gabbros represent a common contact 

rock, possibly even a chilled marginal gabbro, or are 

only local lenses is indeterminate. The drill-core data 

indicate that the contact with the country rock is 

irregular and has considerable relief. The tendency 

for the layering to parallel basement contacts (figs. 4, 

5, 6) suggests the draping of sediments over an initial 

basement topography. However, the extent to which 

the structure may have been modified by post-
ROCK SUCCESSION AND PETROGRAPHY OF THE BRADY GLACIER CUMULATES

NW

METERS SE

900-

800-

700-

600-

500-

400-

300-

200-

100-

SEA LEVEL J

NUC-20 NUC-7 NUC-3 NUC-10 F-6

FEET

_ r 3000

400 800 FEET

EXPLANATION 

Ni+Cu>0,5 weight percent

Glacial ice and snow 

Gabbro and peridotite

intertonguing 

Biotite schist

I 

200 METERS

-2500

- 2000

-1500

- 1000

- 500

L L SEA LEVEL

FIGURE 5.—Generalized northwest-southeast cross section through the Brady Glacier cumulates, (see fig. 3 for location.) oc, 

olivine-chromite cumulate;poc, plagioclase-olivine-chromite cumulate; poa, plagioclase-olivine-augite cumulate.

intrusion faulting is not determinable with the existing corehole control.

In the olivine cumulates, the olivine habit ranges 

from euhedral in some samples to subhedral and 

rounded in others. Where olivine is enclosed in augite 

or brown hornblende oikocrysts, it is commonly 

embayed, suggesting partial resorption. Cumulus 

chromite generally makes up less than 1 percent of 

the cumulus phases, in a few samples of olivine 

cumulate, 3-5 percent. The chromite is euhedral to 

subhedral and commonly occurs as small grains 

enclosed in olivine, but free grains are present. 

Postcumulus minerals are augite, plagioclase, and 

brown hornblende that form oikocrysts enclosing 

the cumulus olivine; bronzite and biotite are rare. 

Brown hornblende appears to be the latest crystallizing mineral, commonly rimming and partly replacing augite and rimming interstitial sulfide minerals. 

It is not uncommon for the proportion of postcumulus plagioclase to increase stratigraphically upward 

such that the olivine cumulate in total mode is an 

olivine gabbro or troctolite prior to the appearance of 

plagioclase as a cumulus mineral. Most of the olivine 

cumulates are peridotite or feldspathic peridotite; 

some are feldspathic dunite.

Olivine-bronzite cumulates generally have an olivine content greater than orthopyroxene; in some 

samples, the two cumulus minerals are approximately equal. Small euhedral chromite grains are 

present in all samples but make up less than 1 

percent. Cumulus plagioclase, present in the upper 

part of drill core NUC-2, makes up less than 5
PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

S

METERS 

1000-1

900-

800-

700-

600-

500-

400-

300-

200-

100-

SEA LEVEL -

NUC-11

NUC-3 NUC-12 NUC-18

EXPLANATION ^

•o

Ni+Cu>0.5 weight percent:

Glacial ice and snow 

Gabbro and peridotite

intertonguing 

Biotite schist 400

I

800 FEET

_I

200 METERS

FEET

-3000

-2500

-2000

-1500

-1000

- 500

L SEA LEVEL

FIGURE 6.—Generalized north-south cross section through the Brady Glacier cumulates. (See fig. 3 for location.) oc, olivinechromite cumulate; poc, plagioclase-olivine-chromite cumulate; poa, plagioclase-olivine-augite cumulate.

percent. Postcumulus minerals consist of plagioclase, augite, and brown hornblende; the proportion 

of postcumulus minerals is so small that most of the 

olivine-bronzite cumulates are harzburgite or, less 

commonly, feldspathic harzburgite.

The plagioclase-augite-olivine cumulates are olivine gabbro with minor amounts of postcumulus 

bronzite and brown hornblende. The olivine generally occurs as large irregular embayed grains that 

commonly enclose small plagioclase laths; the olivine + plagioclase grains are believed to have settled 

as composite grains. Similar composite grains have 

been described in the Rhum intrusion, Inner Hebrides (Wager and Brown, 1968), where the evidence 

for settling as a composite cluster is clearly demonstrated. Augite is generally anhedral and angular 

and partly replaced by the postcumulus hornblende.

Most of the plagioclase-olivine cumulates have a 

trace of cumulus chromite. Postcumulus minerals 

are generally augite and brown hornblende; some 

samples contain postcumulus orthopyroxene. Most 

samples have plagioclase exceeding olivine in amount and are olivine gabbro or troctolite, depending on the amount of postcumulus augite. Where 

olivine exceeds plagioclase, the samples are most 

commonly feldspathic dunite or feldspathic harzburgite. Olivine may be subhedral but more commonly is irregular and embayed. Where olivine is 

enclosed in augite, it is generally rounded, suggesting a reaction relation.

Secondary alteration of the cumulates is common. 

The peridotite commonly is altered to a mixture of 

chlorite, amphibole, and serpentine; olivine and 

pyroxene of the gabbroic rocks are altered to mattes
ROCK CHEMISTRY

DEPTH

FT M

• 20U

700 H

900-

UNIT

Ice

obc

COMPOSITION OF MINERALS, IN 

MOLECULAR PERCENT

Olivine 

(Fo)

81

obc

obc

Bronzite 

(Ca: Mg: Fe)

3:82:15

Chromite

79

-79

3:81:16

82

FIGURE 7.—Section for core NUC-5 showing major cumulate 

units, stratigraphic distribution of cumulus minerals, and 

composition of analyzed minerals, oc, olivine-chromite cumulate; obc, olivine-bronzite-chromite cumulate.

of amphibole, but the plagioclase generally remains 

unaltered. Coronas on olivine in contact with plagioclase are common.

OCCURRENCE AND MINERALOGY OF 

SULFIDE MINERALS

The Brady Glacier nickel-copper deposit is composed predominantly of disseminated interstitial 

sulfide minerals in the ultramafic and gabbroic 

cumulates; massive sulfide pods as much as 3 m 

thick have been penetrated by drilling. The sulfide 

mineralogy throughout the basal cumulates consists 

predominantly of hexagonal and monoclinic pyrrhotite, pentlandite, and chalcopyrite (Czamanske 

and others, 1978). In some samples, the amount of 

disseminated sulfide minerals is as high as 10 

percent.

Although the occurrence of disseminated sulfide 

minerals shows no relation to stratigraphic position

or rock type, the massive sulfide pods tend to be 

concentrated along the boundaries between mafic 

and ultramafic layers. The distribution of copper + 

nickel greater than 0.5 percent is shown in figures 4, 

5, and 6. Indicated ore reserves are greater than 100 

million tons of approximately 0.5 percent nickel and 

0.30 percent copper (Czamanske and others, 1978). 

The textural characteristics indicate that the sulfide 

minerals formed from an immiscible sulfide melt.

ROCK CHEMISTRY

Bulk chemical compositions were obtained by 

X-ray fluorescence methods for 19 rock samples from 

four drill cores; the analyses are given in table 1 in 

order of increasing stratigraphic height for each 

core. With one exception, all samples are cumulates; 

sample NUC-10-1340 is a noncumulate gabbro in 

contact with the metamorphic rocks. The rock compositions are dependent on bulk modal mineralogy 

and do not reflect the distinction between cumulus 

minerals and the postcumulus material that crystallized from intercumulus liquid. Any interpretation of 

variation in chemistry must consider variation in 

kind and amount of both the cumulus phases and the 

postcumulus material.

The variation in chemistry of the ultramafic and 

gabbroic rocks is best illustrated with MgO variation 

diagrams (fig. 11). For plotting, the analyses were 

normalized to 100 percent dry weight. "FeO" is total 

iron calculated after normalization of the analyses. 

The range of composition of the cumulus phases 

olivine, bronzite, augite, and plagioclase is plotted on 

the variation diagrams to aid in interpreting the 

various mineral controls on the chemical composition of the rocks.

In relation to MgO content, the compositions fall 

into two groups. Those rocks with MgO greater than 

25 percent are, with one exception, olivine cumulates 

and olivine-bronzite cumulates; the exception is an 

olivine-plagioclase cumulate whose modal plagioclase is significantly less than that of the other 

olivine gabbros. Postcumulus minerals in the ultramafic rocks are dominantly augite and plagioclase. 

In these rocks, the range in MgO content is largely 

produced by the variation in amount of plagioclase. The second group has MgO contents less 

than 20 percent. These rocks are plagioclase-olivineaugite cumulates, plagioclase-augite cumulates, plagioclase-olivine cumulates, and the noncumulate 

gabbro. Postcumulus mineralogy is represented by 

augite in those rocks where it is not a cumulus phase 

and commonly by minor brown hornblende.

In the ultramafic rocks (those with MgO greater
10 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA 

TABLE 1.—Chemical composition of basal (Brady Glacier) cumulates of the La Perouse layered gabbro

[ N.d., not detected; *, asterisk, total iron as FeO. Chemical analyses except for FeO and H2O determined by X-ray fluorescence. B. P. Fabbi, analyst; FeO and H2O determined by B. King]

Sample No. and 

depth, in feet

Si02 ..............

AlzOa-- — ----- — -

FezOa .............

FeO. — --. .......

MgO.- .............

CaO

Na2O ..... — .... .

K C\

H

f\-

H2O. ..............

TiO2 ..............

PjOs —— ..........

MnO... ............

S.. ............. ...

Subtotal...

Total ......

NUC-5

1678

39.45 

3.91 

.92 

13.04 

35.53 

2.61 

.44 

.02 

2.52 

.02 

.08 

.01 

.18 

.63

99.36 

.31

99.05

426

42.27 

4.18 

1.23 

12.08 

32.94 

3.10 

.52 

.03 

2.01 

.04 

.16 

.01 

.20 

.14

98.91 

.07

98.84

1770

46.38 

12.47 

.73 

8.38 

18.14 

9.37 

1.81 

.14 

.84 

.02 

.57 

.03 

.15 

.11

99.14 

.05

99.09

NUC-2

1122

38.81 

1.82 

2.60 

11.02 

39.66 

1.29 

.25 

.01 

3.50 

.04 

.13 

.02 

.20 

.36

99.71 

.18

99.53

303

40.98 

4.77 

.43 

15.55 

32.17 

2.89 

.52 

.07 

1.13 

.01 

.13 

.02 

.23 

.30

99.20 

.18

99.05

34A

42.11 

8.28 

1.07 

12.89 

27.79 

5.00 

.74 

.08 

1.01 

.02 

.20 

.02 

.20 

.27

99.68 

.13

99.55

1145

40.58 

2.62 

.52 

11.67 

40.51 

1.92 

.36 

.01 

.17 

.04 

.11 

.02 

.16 

.48

99.17 

.24

98.93

NUC-7

864

46.57 

15.57 

.65 

6.76 

13.22 

13.12 

1.42 

.03 

.57 

.04 

.30 

.00 

.11 

.73

99.09 

.36

98.73

529

51.16 

14.63 

.30 

4.17 

10.67 

16.98 

1.55 

.03 

.35 

.01 

.38 

.00 

.10 

.07

100.40 

.03

100.37

412

45.88 

17.24 

.53 

6.64 

15.21 

10.69 

1.84 

.03 

.48 

.02 

.23 

.00 

.12 

.08

98.99 

.04

98.95

1340

47.96 

18.27 

.81 

5.69 

10.26 

11.52 

2.95 

.20 

1.59 

.01 

.55 

.06 

.10 

.16

100.13 

.08

100.05

1271

43.82 

8.68 

1.15 

9.14 

26.98 

6.20 

1.15 

.04 

1.46 

.08 

.32 

.04 

.14 

.11

99.31 

.05

99.26

879

44.80 

12.18 

1.40 

10.25 

18.55 

8.74 

1.21 

.03 

.69 

.05 

.27 

.02 

.14 

1.09

99.42 

.54

98.88

765

40.98 

16.89 

N.d. 

12.16 

15.34 

8.14 

1.66 

.07 

N.d. 

N.d. 

.27 

.01 

.13 

1.93

98.46 

.96

97.50

NUC-10

505

49.73 

21.08 

.35 

* 3.14 

7.14 

15.52 

2.17 

.03 

.82 

.01 

.26 

.00 

.08 

.05

100.38 

.02

100.36

484

40.66 

8.39 

1.22 

13.50 

26.91 

4.84 

.86 

.02 

1.03 

.01 

.14 

.02 

.18 

.82

98.60 

.41

98.19

458

48.53 

22.56 

.32 

2.65 

7.24 

15.81 

1.72 

.03 

.81 

.01 

.21 

.00 

.06 

.06

100.01 

.03

99.98

424

42.79 

22.50 

2.28 

6.12 

8.39 

12.25 

1.68 

.04 

.72 

.00 

.17 

.00 

.07 

2.50

99.51 

1.25

98.26

381

40.53 

4.24 

1.39 

12.53 

34.50 

3.63 

.59 

.03 

.75 

.11 

.15 

.02 

.17 

1.11

99.75* 

.55

99.20

ROCK TYPE

[ Rocks are classified by cumulus minerals listed in decreasing order of abundance and, in parentheses, by mode of all minerals]

NUC-5-1678 

426

NUC-2-1770 

1122 

303

34A

NUC-7-1145 

864 

529

412

Olivine-chromite cumulate (wehrlite) 

Olivine-bronzite-chromite cumulate (plagioclase

bearing harzburgite)

Plagioclase-olivine cumulate (olivine gabbro) 

Olivine-chromite cumulate (dunite) 

Olivine-bronzite-plagioclase-chromite cumulate

(plagioclase bearing harzburgite)

Plagioclase-olivine-chromite cumulate (olivine gabbro) 

Olivine-chromite cumulate (plagioclase bearing dunite) 

Plagioclase-augite-olivine cumulate (olivine gabbro) 

Plagioclase-augite-olivine cumulate (gabbro) 

Plagioclase-olivine-chromite cumulate (olivine gabbro)

NUC-10-1340 Noncumulate, altered gabbro

1271 Olivine-chromite cumulate (plagioclase bearing periodotite)

879 Plagioclase-augite-olivine cumulate (olivine gabbro)

765 Plagioclase-olvine cumulate (olivine gabbro)

505 Plagioclase-augite-olivine cumulate (gabbro)

484 Olivine-plagioclase cumulate (plagioclase bearing dunite-

	harzburgite)

458 Plagioclase-augite cumulate (gabbro)

424 Plagioclase-olivine cumulate (troctolite)

381 Olivine-chromite cumulate (plagioclase bearing wehrlite)

than 25 percent), the compositions are predominantly controlled by the composition of the cumulus 

olivine. Variation in amounts of major oxides reflects the relative amounts of cumulus olivine and 

post-cumulus minerals, particularly plagioclase. In 

the gabbroic rocks, postcumulus minerals are not so 

abundant and the variation of AhOs, SiO2, and 

Na2O essentially reflects the proportions of cumulus 

olivine and plagioclase. The amounts of CaO, "FeO," 

and MnO are influenced by augite as well as by 

olivine and plagioclase. TiO2 values show a broad 

scatter on the variation diagram and might reflect 

small amounts of ilmenite not identified in thin 

section. The low FezQs content of the rocks indicates 

a reduced state.

In the MgO-oxide diagrams, the data plot on a 

smooth trend, reflecting the fact that the ultramafic 

and gabbroic rocks are related by differentiation of 

the same magma or similar magmas. Further, even 

though the gabbroic rocks are cumulates with a 

small amount of postcumulus material, and therefore not truly representative of successive liquids, the 

trends of the MgO-oxide variations for the gabbroic 

rocks extrapolate to the olivine composition, indicating that the primary mechanism for derivation of the 

gabbro was olivine fractionation.

CHEMISTRY OF CUMULUS MINERALS

Cumulus minerals in 30 specimens from four drill 

cores were analyzed with an ARL EMX-SM electron 

microprobe, using natural and synthetic minerals 

and oxides of known composition as standards. 

Corrections were made for background, mass absorption, secondary fluorescence, and atomic number. Structural formulas for all minerals were calculated by the hydrogen-equivalent method of Jackson 

and others (1967). The analyses are given in tables 2 

through 6 in order of increasing stratigraphic height 

for each drill core. The compositions are plotted 

relative to the cumulate stratigraphy of each core in 

figures 7 through 10.

OLIVINE

Olivine, a cumulus phase in all the Brady Glacier 

cumulates except in rare plagioclase-augite 

cumulates, is the best mineralogical indicator of the 

fractional crystallization of the magma that 

produced the Brady Glacier cumulates. For the 

olivine analyses given in table 2, each analysis is the 

average of a minimum of 5 points per grain. 

Homogeneity of individual grains indicates that
CHEMISTRY OF CUMULUS MINERALS 11

DEPTH

FT. M.

0 -

100-

200 -

300-

400 -

500 -

600 -

700 -

800 -

900- 

1000-

1100 -

1200-

1300 -

1400-

1500-

1600-

1700 - 

1800 -

1900 -

- 0

- 100

-200

-300

- 400

- 500

- 600

—— —— ——

UNIT

Ice

poc

obpc

______

oc

obc

oc

poc 

Schist

COMPOSITION OF MINERALS, IN 

MOLECULAR PERCENT

Plagioclase OIK 

(An) (f

81

--

IE j

nne Bronzite Chromite 

o) (Ca: Mg: Fe) 

F77

-77 -3:79:18

--

76

77 -3:81:16

85

82

84

*

DEPTH FT M.

200 -

300 -

400 - 

500 - 

600 -

700 -

800 -

900 -

1000 -1 

1100 —

1200 -

1300 -

1400 -

1500 -

1600 -

-100

-200

-300

-400

-500

UNIT

Ice

poc

poa

oc

COMPOSITION OF MINERALS, IN 

MOLECULAR PERCENT

Plagioclase Olivine Augite Chromite 

(An) (Fo) (Ca: Mg:Fe)

70 

69

76

--

79 

-76 -r

78 43:47:9

86

FIGURE 9.— Section for core NUC-7 showing major cumulate 

units, stratigraphic distribution of cumulus minerals, and 

composition of analyzed minerals, oc, olivine-chromite cumulate; poa, plagioclase-olivine-augite cumulate; poc, plagioclaseolivine-chromite cumulate.

Jflplrsnn C\ Q7fh and Facre and others (1972^

FIGURE 8.—Section for core NUC-2 showing major cumulate 

units, stratigraphic distribution of cumulus minerals, and 

composition of analyzed minerals, oc, olivine-chromite cumulate; obc, olivine-bronzite-chromite cumulate; poc, plagioclaseolivine-chromite cumulate; obpc,olivine-bronzite-plagioclase-chromite cumulate.

there was no postcumulus lower temperature growth 

of olivine; either the olivine grains are entirely 

cumulus or postcumulus growth was by an 

adcumulus process. There is no significant grainto-grain compositional variation.

The range in composition for all samples is Fose.i 

to Fo7i.2. Olivine in olivine cumulates ranges in 

composition from Fose.i to Fo7a.a; most samples have 

olivine with a forsterite content greater than 80. 

Oliviiie-bronzite cumulates contain olivine of composition Fosi.i to Fo77.2. Although there is some 

compositional overlap, samples that contain cumulus plagioclase generally have olivine that is more 

iron-rich (Fo7g.7 to Fo7i.2) than do those samples without cumulus plagioclase.

documented evidence that in the Stillwater Complex, 

Montana, olivine within a single cumulate layer can 

vary widely in composition. A similar variation in 

olivine composition was shown by G. K. Czamanske 

and L. C. Calk (written commun., 1978) in the lower 

olivine-cumulate layer of core NUC-5, where the 

olivine composition varies from approximately Fosa 

to Fo7g with no relation to stratigraphic height (fig. 

12). Irvine (1965, 1967) and Jackson (1969) 

demonstrated that the composition of olivine with 

cumulus chromite varies sympathetically with both 

chromite composition and abundance. In the Brady 

Glacier cumulates, chromite occurs with many of the 

olivine-bearing cumulates but is never an abundant 

phase (generally less than 1 percent). Because its 

composition varies widely within a single sample, a 

relation between olivine composition and chromite 

composition or abundance has not been established. 

Cumulate layers are repeated stratigraphically 

upward. The data, though not conclusive, suggest 

that olivine in higher olivine cumulate layers, with 

few exceptions (for example, NUC-10), is less
12 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

DEPTH

FT M

200 —

300-

400 -

500 -

600-

700 -

800-

900 -

1000 -

1100 -

1200-1

1300 -

1400 —

1500 -

1600 —

- 100

papo-

- 200

- 300

- 400

- sou

UNIT

Ice

oc

poc

poa

\ ____ N oc

po

poa

pob

poa

oc

/nc

Schist

COMPOSITION OF MINERALS, IN 

MOLECULAR PERCENT

Plagioclase Olivine Bronzite Augite Chromite

(An) (Fo) (Ca: Mg: Fe) (Ca: Mg:Fe)

T T

h84 

'Ijl ± 77 ^ ._ ._ _ 1

-^ '

-68

-73

-74

-75

°7§ T42:48:10

-73 i

-76

-71 1 4:75:21 ::

-75

-76 44:47:9

-82

QO 

OZ

-67

FIGURE 10.—Section for core NTJC-10 showing major cumulate 

units, stratigraphic distribution of cumulus minerals, and composition of analyzed minerals, oc, olivine-chromite cumulate; 

poa, plagioclase-olivine-augite cumulate; pob, plagioclase-olivine-bronzite cumulate; po, plagioclase-olivine cumulate;poc, 

plagioclase-olivine-chromite cumulate; pa, plagioclase-augite 

cumulate; nc, noncumulate gabbro.

forsteritic than olivine in lower olivine cumulates. 

Compositional variation of olivine within a single 

unit and the absence of specific recognizable 

horizons within the units make it difficult to compare 

olivine compositions laterally between cores.

MnO ranges from 0.18 to 0.36 percent and in 

general increases with FeO content. NiO in olivine 

ranges from "not detectable" to 0.28 percent and 

shows no relation to forsterite content or rock type.

PYROXENE

Chemical composition and structural formulas of 

bronzite and augite are given in tables 3 and 4. Both 

the bronzite and augite have narrow, closely spaced 

exsolution lamellae that generally can not be adequately resolved with the electron microprobe; bulk 

analyses of the pyroxenes were obtained by traversing the grains with a broad electron beam (~25 jion 

diameter). Differences in composition from grain to 

grain obtained in this manner are not significant. 

The presence of exsolution lamellae and the method 

of analysis, however, do preclude determination of

any chemical inhomogeneities or zoning that might 

be related to either cumulus or postcumulus growth.

Cumulus bronzite occurs primarily in the western 

part of the Brady Glacier cumulates (cores NUC-5 

and NUC-2, figs. 7 and 8), mainly with cumulus 

olivine, minor amounts of cumulus chromite, and , 

rarely, with cumulus plagioclase. Nowhere in the 

Brady Glacier cumulates does cumulus bronzite 

occur with cumulus augite. The analyzed bronzite 

has a very limited range of composition; the sample 

most enriched in iron occurs in a thin layer (<5 m) of 

plagioclase-olivine-bronzite cumulate (core NUC-10, 

fig. 10).

The cumulus augite occurs in gabbro with cumulus 

plagioclase and generally cumulus olivine; it does 

not occur as a cumulus phase in peridotite or 

harzburgite. All analyzed augite is magnesium rich 

and shows no significant difference in composition 

regardless of stratigraphic height or lateral position. 

Postcumulus augite from two samples of olivine 

gabbro at the base of core NUC-2 (fig. 8) are of the 

same composition as the analyzed cumulus augites 

(table 5).

The major- and minor-element contents of the 

bronzite and augite are similar to values reported for 

pyroxenes in other cumulates from the lower part of 

stratiform complexes (Hess, 1960; Atkins, 1969; 

Cameron, 1978).

PLAGIOCLASE

In those samples where plagioclase is a cumulus 

phase, most grains show compositional zoning. 

Some of this zoning may be related to cumulus 

growth; much of it may be related to postcumulus 

growth about the original primocryst. Maal0e (1976) 

documented the complex compositional zoning of 

plagioclase in the Skaergaard intrusion and 

illustrated criteria used to distinguish primocryst 

from postcumulus overgrowth. In the Brady Glacier 

plagioclase, distinction between primocyrst and 

postcumulus overgrowth is rarely certain. 

Plagioclase analyses given in table 5 are 

representative of average compositions of single 

plagioclase grains that include primocryst and any 

postcumulus overgrowth. Differences in composition 

from grain to grain in the same sample are as great 

as 7 percent of the amount present and probably 

largely reflect different ratios of primocryst to 

postcumulus material and depth of sectioning 

through the compositionally zoned grains. The 

analyzed plagioclase shows a range in average 

composition of An8 i to An6s with the most anorthite-rich plagioclase in the olivine-bronzite-plagio-
CHEMISTRY OF CUMULUS MINERALS 13

u.t

0.3

0.2

0.1

0.0

H 0.6

Z

UJ

£0.5

UJ

CL

£ 0.4

0

UJ

^ 0.3

Z—

I— 0.2

UJ

l°-<

CJ

Q 0 

X

O

0.2

0.1

0

20

15

10

5

i I I I

MnO

/*\

- A**' J

/ * s'

1 /

1 / ^

/ */

/ / o o

- / / + D -

(§/ ° ° 0

o f^\

D I A

o D VAJ DD

n

D

m

TiO2

r\u ° -

W

_ _

D

0

a _

a a

n

a

a a

o + D

o

- o — 

o

rm »\ m

K2O

a -

D

o a n f

oo " nannoD -

O 0

(9 ————— ^ ——— ~^1 + A

"FeO" /^•/

/ * / 

/ •*/ °

— / m / O / • •/ O ~ ' Ux ° ° n

^^^ o 4- an

_ 0 _

a Q a

,— ? D

VAA) n ~~

B

25

20

15

10

5

0

3

2

I I I

CaO

-

6

0 fc

-

D If

D vD D

U D

D

o

— u —

o °

o o i o "*"

^ ^

Na2 0 "

D -

a 1

a D

a D

1

0

30

20

15

10

5

0

50

40

on

o D

-

o

o /^n

O °° (A/ o ^y

o

__ ________ ^S i^ ~l~

/i Ai 2 o3 r

-

D D

a

a

an

a

D

a

cP

©

n -•-

a

/S~~ >

SiO2 ^) ,-

(A/ a

^ a a

B D° ^

o

0 D u a n

0 0 U

i- («W~~^— — * u

X~^^-^-^_*/

0 ——— - ——————— ——————————————————————————————— ou ———————————————————————————————————————— ! i 1 ——————————— 1

50 40 30 20 10 0 50 40 30 20 10 0

MAGNESIUM OXIDE CONTENT, IN WEIGHT PERCENT

FIGURE 11.—MgO variation diagrams of Brady Glacier cumulate rock samples and cumulus minerals. Open circle, peridotite; 

open square, gabbro; solid circle, olivine; solid square, plagioclase; solid triangle, augite; cross, bronzite. Fields of mineral 

composition outlined.
14 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

TABLE 2.—Chemical composition and structural formulas ofoliuinein the basal (Brady

[Nd., not detected.*, asterisk, total iron as

Drill core -

Sample No.

E*«ri*

MgO .........

MnO .........

NiO .........

Total

and depth, in feet 1678

OQ 0

42 9

29

...................... .10

.......... 99.69

NUC-5

1038

38.8 

19.4 

41.3 

.30 

.12

99.92

784

38.9 

19.0 

41.5 

.24 

.28

99.92

426

39.6

17.7 

43.2 

.27 

.06

100.83

1770

39.0 

20.2 

40.3 

.32 

.13

99.95

1746

39.3 

18.8 

41.9 

.26 

.15

100.41

1550

40.0 

14.7 

45.1 

.24 

.13

100.17

1318

38.5 

17.0 

43.0 

.25 

.10

98.85

NUC-2

1122

40.4 

14.4 

45.4 

.22 

.09

100.51

980

38.8 

20.7 

39.9 

.27 

.07

99.74

689

38.6 

21.9 

39.5 

.32 

.06

100.38

303

38.2 

20.7 

39.6 

.30 

.04

98.84

34A

39.1 

20.8 

39.7 

.31 

.06

99.97

1145

40.3 

13.2 

46.5 

.18 

.12

100.30

NUC-7

864

38.6 

19.7 

41.0 

.29 

.09

99.68

529

38.0 

22.0 

39.2 

.30 

N.d.

99.50

412

38.4 

19.3 

40.8 

.33 

.07

98.90

Cations per 4 oxygens

Si .........................

Fe". ................. .... ..

Mg. ...... ..................

Mn ........................

Ni ......................... rvi .——......———

lOOxMg

Mg+Fe+Mn

......... 1.000

......... .364

......... 1.627

......... .006

.--..--.. .002

......... 1.999

ft! ^

0.996 0.997

.417 .407

1.581 1.588

.006 .005

.002 .006

2.006 2.006

7ft Q 7Q 4

0.999

.373

1.622

.006

.001

2.002

fti i

1.004

.434

1.547

.007

.003

1.991

77 ft

1.000 1.001

.400 .308

1.591 1.681

.006 .005

.003 .003

2.000 1.997

7Q 7 fl>l Q

0.989 1.005

.366 .299

1.649 1.684

.005 .005

.002 .002

2.022 1.990

fil £ AA 7

1.003 0.998

.449 .474

1.538 1.523

.006 .007

.002 .001

1.995. 2.005

rtrt c\ rjc f\

0.999 1.008

.452 .449

1.544 1.527

.007 .007

.001 .001

2.004 1.984

1.000

.273

1.721

.004

.002

2.000

86.1

0.996

.425

1.575

.006

.002

2.008

78.5

0.993 0.996

.481 .419

1.526 1.580

.007 .007

N.d. .002

2.014 2.008

7C Q 70 0

ROCK TYPE

[Rocks are classified by cumulus minerals listed in decreasing order of abundance and, in parentheses, by mode of all minerals]

NUC-5-1678

1038

784

426

NUC-2-1770 

1746 

1550 

1318 

1122 

980

Oliv 

Oliv 

Oliv 

Oliv 

ha

ne-chromite cumulate (wehrlite) 

ne-chromite cumulate (peridotite) 

ne-bronzite-chromite cumulate (harzburgite) 

ne-bronzite-chromite cumulate (plagioclase bearing 

zburgite)

Plagioclase-olivine cumulate (olivine gabbro) 

Plagioclase-olivine cumulate (olivine gabbro) 

Olivine-chromite cumulate (olivine gabbro) 

Olivine-chromite cumulate (peridotite) 

Olivine-chromite cumulate (dunite) 

Olivine-bronzite-chromite cumulate (harzburgite)

NUC-2-689 Olivine-chromite cumulate (peridotite)

303 Olivine-bronzite-plagioclase-chromite cumulate

	(plagioclase bearing harzburgite)

34A Plagioclase-olivine-chromite cumulate (olivine gabbro)

NUC-7-1145 Olivine-chromite cumulate (plagioclase bearing dunite)

864 Plagioclase-augite-olivine cumulate (olivine gabbro)

529 Plagioclase-augite-olivine cumulate (gabbro)

412 Plagioclase-olivine-chromite cumulate (olivine gabbro)

clase cumulate. There is no systematic relation 

between plagioclase anorthite content and olivine 

forsterite content, probably largely because postcumulus overgrowth is included in the analyses 

of the plagioclase.

Representative compositional zoning patterns for 

plagioclase, illustrated in figure 13, show a common 

type of zoning wherein the core of the plagioclase 

grain is considerably less anorthite-rich than the 

rest of the grain. These cores generally have abrupt 

boundaries and are strongly embayed, indicating 

resorption. The cores of these grains are probably 

primocrysts that crystallized near the roof or walls of 

the magma chamber, where the magma temperature 

was depressed below the liquidus owing to 

conduction of heat by the enclosing rocks. Either by 

gravity settling or by convection, the primocrysts 

were carried into the hotter interior of the magma 

chamber, where they were partially resorbed, then 

eventually deposited on the floor of the chamber. The 

more anorthitic parts of the grains probably 

represent renewed higher temperature crystallization (relative to the temperature of 

crystallization at the roof) near the floor of the 

magma chamber.

CHROMITE

A total of 73 cumulus chromite grains from 13 

samples were analyzed and the compositions in 

terms of the cation ratios—Cr/(Cr+Al) and Mg/ 

(Mg-f Fe+ 2 )—were plotted in figure 14; selected 

analyses for each sample are given in table 6. 

Individual grains may be chemically inhomogeneous, and there are large compositional 

differences from grain to grain in the same sample. 

The cation ratios of Cr to Cr+Al and of Mg to Mg+Fe+ 2 

range from approximately 0.15 to 0.46 and 0.27 to 

0.57, respectively. MnO ranges from 0.1 to 0.4 percent 

(fig. 15), TiO2 from less than 0.1 percent to 

approximately 1.0 percent (fig. 16). Sample 

NUC-2-1770 contains a red and green chromian 

spinel; the green spinel is aluminum rich (table 6). 

The large range in composition of chromite grains in 

samples where the composition of olivine is 

essentially constant does not fit the theoretically 

determined equilibrium distribution of Mg and Fe+2 

between coexisting olivine and chromite (Irvine, 

1965) and indicates that the olivine and chromite did 

not crystallize together. Enclosure of most of the 

cumulus chromite in olivine grains indicates that it
Glacier ) cumulates of the La Perouse layered gabbro

FeO. All minerals are cumulus]

CHEMISTRY OF CUMULUS MINERALS 

400

15

NUC-10

1271 1079 879 765 711 630 566 505 484 424 381

39.9 39.5 38.6

16.4 16.8 21.6

44.0 43.4 39.6

.27 .28 .34

.11 .06 .12

38.3

22.2

38.2

.31

.10

37.2

25.4

35.7

.33

.10

38.2 37.8 37.9

21.3 23.8 24.4

38.9 37.1 37.0

.36 .32 .28

.07 N.d. N.d.

38.8 38.9 40.1

19.8 20.8 15.3

40.8 40.0 44.4

.29 .33 .22

.06 .06 .04

100.68 100.04 100.26 99.11 98.73 98.83 99.02 99.58 99.75 100.09 100.06

Cations per 4 oxygens— Continued

1.001 1.000 0.998

.343 '.355 .467

1.647 1.637 1.526

.006 .006 .008

.002 .001 .002

1.998 1.999 2.003

82.5 81.9 76.3

1.005

.486

1.495

.007

.002

1.990

75.2

0.997

.570

1.427

.008

.002

2.007

1.002

.466

1.521

.008

.002

1.997

1.000 

.527

1.466 

.007 

N.d.

2.000

0.999 

.539

1.456 

.006 

N.d.

2.001

1.000 1.003

.427 .448

1.566 1.538

.006 .007

.001 .001

2.000 1.994

71.2 76.2 73.3 72.8 78.3 77.2

1.006

.322

1.660

.005

.001

1.988

83.6

NUC-10-1271 Olivine-chromite cumulate (plagioclase bearing peridotite)

1079 Olivine-chromite cumulate (olivine gabbro)

879 Plagioclase-augite-olivine cumulate (olivine gabbro)

765 Plagioclase-olivine cumulate (olivine gabbro)

711 Plagioclase-olivine-bronzite cumulate (olivine gabbro)

630 Plagioclase-olivine cumulate (olivine norite)

566 Olivine-chromite cumulate (peridotite)

505 Plagioclase-augite-olivine cumulate (gabbro)

484 Olivine-plagioclase cumulate (plagioclase bearing dunite-

	harzburgite)

424 Plagioclase-olivine cumulate (troctolite)

381 Olivine-chromite cumulate (plagioclase bearing wehrlite)

crystallized before olivine. This interpretation is 

consistent with the settling rate as predicted by 

Stoke's Law. The olivine crystals, being coarser, will 

settle faster than the finer but denser chromite 

crystals.

The compostions of the Brady Glacier chromite 

generally have Al>Cr and Fe+2 >Mg; these compositions differ significantly from those reported for 

most other magmatic chromites. Continental stratiform intrusions, such as the Stillwater, Bushveld, 

and Muskox, generally have chromite that is chromium-rich (Cr/(Cr+Al), 0.5-0.85) (Irvine, 1967; Jackson, 1969; Cameron, 1978). Concentric-type ultramafic intrusions of southeastern Alaska also have 

chromium-rich chromite (Cr/(Cr+Al), 0.5-0.8) but are 

iron-rich (Mg/(Mg+Fe + 2), 0.35-0.6) (Irvine, 1967). Chromite in some mid-Atlantic Ridge and other midocean ridge basalts are aluminum-rich but are also 

magnesium-rich (Sigurdsson and Schilling, 1976). 

Chromites in ultramafic xenoliths in basalts (Aoki 

and Prinz, 1974; Basu and McGregor, 1975), in 

alpine-type peridotites (Irvine, 1967; Loney and others, 

1971; Himmelberg and Loney, 1973; Dick, 1977), and 

in harzburgite tectonite of ophiolite complexes (Him450

O

x

500

77 78 79 80 81 82 83 

FORSTERITE CONTENT, IN MOLECULAR PERCENT

FIGURE 12.—Variation of olivine composition in the olivine 

cumulate in the lower part of core NUC-5. From G. K. 

Czamanske and L. C. Calk, (written commun., 1978).

melberg and Loney, 1980) may be aluminum-rich but 

are invariably also magnesium-rich and not magmatic. To our knowledge, the only chromites reported 

of composition similar to those in the Brady Glacier 

cumulates occur in the cumulate ultramafic rocks of 

the Canyon Mountain ophiolite, Oregon, (Cr/(Cr+Al), 

0.26-0.42; Mg/(Mg+Fe+2), 0.33-0.59)(Himmelberg 

and Loney, 1980).

In addition to the cumulus chromite in the ultramafic rocks, zoned chromium + iron chromite with 

cores containing as much as 53 percent Cr2O3 occurs 

in the massive sulfide. This chromite is believed to 

have crystallized directly from the immiscible sulfide melt (Czamanske and others, 1976).
16 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

70

60

80

70

LUO <r

70

60

80

70

60

50

400 360 320 280 240 200 160 120 80 40 0 40 80 120 160 200 240 280 320 360 400

DISTANCE, IN MICROMETERS

FIGURE 13(above and facing page).—Composition profiles across plagioclase grains. A, Sample C-2-1746; B, Sample C-7-412; 

C, Sample C-7-529; D, Sample C-10-424; E, Sample C-10-484; F, Sample C-10-1430.
CRYSTALLIZATION OF THE BRADY GLACIER CUMULATES 17

70 -

LLJ 

0

£60

DC

<70

D 

U

Z60

O °50

30 -

J

400 360 320 280 240 200 160 120 80 40 0 40 80 120 

DISTANCE, IN MICROMETERS

160 200 240 280 320 360 400

The composition of chromite should reflect the 

temperature, pressure, and fugacity of oxygen under 

which the chromite crystallized. Unfortunately, 

chromite chemistry as a function of these factors is 

not well known. It has been suggested that 

aluminous chromite reflects crystallization at relatively high pressures (Irvine, 1967; Haggerty, 1976), 

but variation in Cr/Al has also been demonstrated to 

be a function of bulk composition (Irvine, 1967; 

Loney and others, 1971; Sigurdsson and Schilling,

1976). The composition of chromite coexisting with 

olivine is temperature dependent (Irvine, 1965), but 

unfortunately the olivine-chromite geothermometer 

developed by Jackson (1969) generally yields 

erroneous temperature values. (See, for example, 

Meyer and Boyd, 1972; Evans and Frost, 1975; Basu,

1977). Theoretical /Oz isobars for chromian spinel 

coexisting with olivine and orthopyroxene have been

calculated by Irvine (1965). Although the Brady 

Glacier chromites commonly occur only with olivine 

and probably were not in equilibrium with the 

silicate, calculation by Irvine's method suggests that 

they crystallized under a very low /o . A low fo? 

during crystallization is also indicated by the 

common occurrence of graphite throughout the 

Brady Glacier cumulates.

CRYSTALLIZATION OF THE 

BRADY GLACIER CUMULATES

The succession of cumulates underlying the Brady 

Glacier is believed to represent the early stages of 

fractional crystallization of the magma and deposition onto the floor of the magma chamber. Evidence 

for this includes: (1) the cumulus textures; (2) an 

order of crystallization of cumulus minerals similar
18 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

TABLE 3.— Chemical composition and structural formulas o 

bronzite from the basal (Brady Glacier) cumulates of the 

La Perouse layered gabbro

.-' ' t [N.d., not detected. *, asterisk, total iron as FeO. All minerals are cumulus]

Drill core-..------ ....... NUC-5 NUC 2 NUC 10

Sample No. and 

depth, in feet...... ?84 426 ggfl 303 711

SiO2 ..............

AliOa ————

TiO2 ...............

Cr2O3 .............

FeO*

MnO .............

MgONa2O ... --

Total-

...... 53.8 53.9 54.2 54.3 52.8

-- -- 0 t\l 0 QQ 0 7A. O Qi O QA

....... iq 17 on iji 9*

.65 .74 .71 .54 .11

11.0 9.99 10.8 11.7 14.1

On 1 Q OQ f)A OQ

.. .... on A QI n on t» oo n OT n

1.46 1.59 1.56 1.35 2.11 N.d. .03 .04 .06 .05

..... 100.37 100.59 101.98 100.98 99.70

Cations per 6 oxygens

Si

A1HAA

rr ....................

AKVT1

Ti ....................

Cr ....................

FA*2

Mn ..................

Mg ...................

Po

Na

lOOXMg

Mg+Fe+Mn 

Ca ....................

Mg ..................

Fe ....................

...... 1.903 1-894 1.905 1.911 1.911

.079 .106 .095 .089 -089

...... 2.000 2.000 2.000 2.000 2.000

.014 .018 .019 .032 .009

.005 .004 .005 .005 .007

...... .018 .021 .020 .015 .003

.325 .294 .316 .345 -426

.006 .005 .007 .007 -009

...... 1.603 1.626 1.596 1.560 1.495

.055 .060 .059 .051 -082

N.d. .002 .003 .004 .004

-..--. 2.026 2.030 2.025 2.019 2.035

.... 82.9 84.5 83.2 81.6 77.5 

....... 2.79 3.03 2.98 260 4.08

...... 80.84 82.13 80.97 7974 74.63

...... 16.37 14.84 16.05 1766 21.29

ROCK TYPE

[ Rocks are classified by cumulus minerals listed in decreasing order of abundance and, in 

parentheses, by mode of all minerals]

NUC-5-784 Olivine-bronzite-chromite cumulate (harzburgite) 

426 Olivine-bronzite-chromite cumulate (plagioclase bearing 

harzburgite) 

NUC-2-980 Olivine-bronzite-chromite cumulate (harzburgite) 

303 Olivine-bronzite-plagioclasfr-chromite cumulate (plagioclase 

bearing harzburgite) 

NUC- 10-711 Plagioclase-olivine-bronzite cumulate (olivine gabbro)

0-5 ——————————————————————————————————————————————

O 0 

0 0

5 ODD 0 0

S - °o° 80 °

— OO O Q O

O ' ~ ° o coo°o ~

Q ° „ @ o 

S ° ® DC

~Z. ' °

Q «

1— o

o

0.1 -

o ——————————————————————————————— 0.7 0.6 0.5 0.4 0.3 02 0.1 C 

CATION RATIO Mg/(Mg + Fe+2)

FIGURE 14. — Variation of chromite composition of the Brady 

Glacier cumulates in terms of the cation ratios Mg to Mg+ 

Fe+2 and Cr to Cr+Al.

iLU 1 II 1 O 

DC 

LU 

0_

I 0.4 - 0 o

^2 o ° o

^ ° OO 

-_ O O O O

— o°° 

"Z. o o°° oo

LU O

f- 0 0 0

Z o ° ° 

0 0.2 - °

LU

Q 

X

o

" °' 1 ~ o o °°° °°° -

^ 00°

< o —————— ' ————————————— ! —————— I —————— I —————— I ——————

§ 0.7 0.6 05 0.4 0.3 02 0.1 0 

CATION RATIO Mg/TMg + Fe+2

FIGURE 15. — Variation in MnO content of chromite in the Brady 

Glacier cumulates.

to that of other layered gabbros known to have 

originated by fractional crystallization and also 

similar to that expected from experimentally 

determined crystallization sequences of simple 

basaltic systems; and (3) a postcumulus phase that 

commonly becomes increasingly abundant upward 

prior to crystallizing as a cumulus phase. For 

example, postcumulus plagioclase becomes increasingly abundant in olivine cumulates below the 

olivine-plagioclase cumulates or the plagioclaseaugite-olivine cumulates. The crystallization of 

the magma was clearly complex: First, the cumulate 

sequences in the western and eastern parts of the 

area differ markedly (see fig. 4); this indicates that 

the liquid path of crystallization was different for the

two areas. Secondly, not only are cumulate layers 

repeated, but continuous sequences proceeding from 

one-phase cumulates through two-phase cumulates 

to three-phase cumulates are generally absent. 

Instead, one-phase is commonly followed by the 

simultaneous crystallization of two additional 

cumulus phases, resulting in three-phase cumulates, 

and crystallization of one-phase cumulates is 

commonly repeated before a complete sequence is 

developed. Nevertheless, examination of the cumulates present allows interpretation of a general order 

of their crystallization.

The rock succession in the western part of the 

Brady Glacier cumulates is interlayered olivine cumulates, olivine-bronzite cumulates, and olivine-
CRYSTALLIZATION OF THE BRADY GLACIER CUMULATES 19

ROCK TYPE

[ Rocks are classified by cumulus minerals listed in decreasing order of abundance and, in 

parentheses, by mode of all minerals]

NUC-2-1770 Plagioclase-olivine cumulate (olivine gabbro); analyzed augite

is postcumulus 

1746 Plagioclase-olivine cumulate (olivine gabbro); analyzed augite

is postcumulus

NUC-7-864 Plagioclase-augite-olivine cumulate (olivine gabbro) 

NUC-10-879 Plagioclase-augite-olivine cumulate (olivine gabbro) 

505 Plagioclase-augite-olivine cumulate (gabbro) 

458 Plagioclase-augite cumulate (gabbro)

bronzite-plagioclase cumulates, all containing minor amounts of cumulus chromite. Excluding chromite, this succession suggests a general order of 

crystallization of olivine, olivine + bronzite, and 

olivine + bronzite + plagioclase. Since the boundary 

curve between olivine and orthopyroxene based on 

known liquidus phase relations at low pressures is a 

reaction curve (Andersen, 1915; Kushiro, 1972), olivine and orthopyroxene are not expected to crystallize simultaneously by simple fractional crystallization. The association of cumulus olivine and bronzite 

is relatively common in basal ultramafic rocks of 

some layered gabbros, for example, the Stillwater 

Complex, Montana (Jackson, 1961), the Great Dyke

TABLE 4. — Chemical composition and structural formulas of 

augite from the basal (Brady Glacier) cumulates of the La 

Perouse layered gabbro

[ *, asterisk, total iron as FeO ]

Drill core NUC-2 NUC-7 NUC-10

Sample No. and 

d feet-..-. 1770 1746 864 879 505 458

SiO« ................... 50.2 51.0 52.3 52.0 52.7 52.6

AlzOs............. 6.03 5.37 5.25 4.60 3.72 4.13

TiO2 ............. ...... .61 .59 .54 .45 .44 .55

Cr2O3 ................. .98 1.06 .83 .59 .21 .54

FeO*.................. 5.42 4.94 5.72 5.68 5.90 4.50

MnO.................. .16 .10 .14 .16 .17 .14

MgO................... 16.4 16.8 15.9 16.0 16.5 15.6

CaO................... 19.5 21.0 20.4 20.7 20.2 21.3

Na2O.. ................ .59 .43 .47 .45 .40 .69

Total 99.89 101.29 101.55 100.63100.24 100.05

Cations per 6 oxygens

Si ....................... 1.840 1.846 1.883 1-894 1.921 1.918

Al(IV) ................ .160 .154 117 .106 .079 .082

ST 9 ruin 9 ruin o Ann 9 ftnn 9 ftnn 9 non

Al(VI) ............... .101 .075 106 -092 .081 .095

Ti....... ................. .017 .016 015 -012 .012 .015

Cr ....................... .028 .030 024 -017 .006 .016

Fp f2 ICC 1 XQ 1 net 1 7Q 1 Qfi 1 Q7

Mn ..................... .005 .003 005 -005 .005 .004

Mg ....................... .894 .905 .853 -866 .896 .848

Ca ....................... .766 .815 787 .806 .790 .830

Na ...................... .042 .030 033 -032 .029 .049

TM o m Q 9 fl9Q i one 9 fifl3 1 QQQ 1 QQA

iUUAMg QQ Q Q £ £ fl<5 Q Q Q ft JJJJ Q jjj; rj

Mg+Fe+Mn

Ca ....................... 41.94 43.59 43.43 43.69 42.34 45.73 

Mg ..................... 48.96 48.41 47.08 46.93 48.02 46.72

Fe ...................... 9.10 8.00 9.49 9.38 9.64 7.55

1.0 

0.9

0.8

lLU

CC£0.7

1-

o

LU

i 0.6

uj 0.5

•z. 

o (J

LU

90.4

X

Q

§ 0.3

z

g1-

0.2

0.1

0 

0

——————— ... o ,

0 °° 

00

0

o

0 _

Oo°° °

- 00° —

0

0 °

°°

8 o°

o °° ° 0 ° -

00 0 Q

_ o -

° 0° 0

o

a 8 0 80° 0

0 %

0 °0

o

0

o o o

o

oo

I I I I I .... . 7 0.6 0.5 0.4 0.3 0.2 0.1 C

CATION RATIO Mg/(Mg+Fe+2)

FIGURE 16.—Variation in TiO2 content of chromite in the Brady 

Glacier cumulates.

(Jackson, 1970), and the lower zone of the Bushveld 

Complex, South Africa (Cameron, 1978), where the 

crystallization order is olivine, olivine + bronzite, 

bronzite. The occurrence of olivine-bronzite cumulates could be explained by mechanical mixing 

(Irvine, 1970) or different crystallization levels for 

the olivine and bronzite (Jackson, 1961). Evidence 

for mixing is generally absent (Jackson, 1961; Irvine, 

1970), and the similarity of the olivine-bronzite 

cumulates to olivine-augite cumulates of theMuskox 

intrusion led Irvine (1970) to propose that olivinebronzite cumulates are better explained by fractional 

crystallization. It is known that the incongruent 

melting of enstatite does not persist above about 5 

kbar pressure in an anhydrous system (Boyd and 

others, 1964) and that it is eliminated by addition of 

iron to the system (Bowen and Schairer, 1935). Irvine 

(1970) proposed a phase diagram wherein at least a 

part of the olivine-orthopyroxene boundary curve is 

a cotectic and suggested that it might be obtained by 

a combination of pressure and composition effects.
20 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

TABLE 5.—Chemical composition and structural formulas of plagioclase from the basal (Brady Glacier)cumulates of the La Perouse

layered gabbro

[ N.d., not detected. *, asterisk, total iron as FeO ]

Drill core............

Sample No. and 

depth, in feet

SiO2 .-- — — — .

A12O3 ----- -----.

FezOa*- — ----- - —

MgO ..............

CaO

Na 2 O--- ............

K n

.. 1770

51.4 

30.7 

.40 

.21 

13.4 

4.22 

.07

100.40

NUC-2

1746

51.1 

31.9 

.50 

.29 

14.4 

3.59 

.11

101.89

303

47.6 

33.6 

.26 

.44 

16.9 

2.22 

.04

101.06

34A

46.9 

33.5 

.39 

.15 

16.7 

2.30 

.04

99.98

864

49.1 

32.8 

.57 

.23 

15.9 

2.72 

.05

101.37

NUC-7

529

50.6 

31.5 

.24 

.25 

14.9 

3.55 

.09

101.13

412

48.0 

32.7 

.45 

.06 

15.9 

3.65 

.06

100.82

1340

49.2 

31.3 

.30 

.17 

13.8 

3.77 

.04

98.58

879 765

49.0 48.4 

32.6 32 3 

.36 .23 

.41 N.d. 

15.5 15.1 

2.79 2.99 

.05 .04

100.71 99.10

NUC-10

711 630

48.5 50.0 

32.4 31.4 

N.d. .17 

N.d. N.d. 

15.3 14.1 

3.14 3.69 

.04 .06

99.38 99.42

505

49.9 

32.0 

.20 

.16 

14.9 

3.40 

.05

100.61

484

48.8 

32.2 

.12 

.25 

15.3 

2.94 

.04

99.65

458

49.1 

32.4 

N.d. 

.11 

15.2 

3.18 

.04

100.32

424

48.1 

33.0 

.10 

N.d. 

16.1 

2.64 

.05

99.99

Cations per 32 oxygens

si ............ ........

Al

Fe'3.- ..............

Mg-. ----.. ...........

Ca ...................

Na. ..................

K ....................

Or..................

Ab

An ..................

Q ^9Q

. 6.571

.055 

. .057

. 2.596

. 1.485

.016

.40 

36.24 

63.36

9.338 

6.612 

.055 

.057 

2.624 

1.271 

.016 

.41 

32.50 

67.09

8.663 

7.207 

.036 

.120 

3.294 

.784 

.009 

.23 

19.18 

80.59

8.636 

7.266 

.054 

.041 

3.295 

.814 

.009 

.23 

19.76 

80.01

8.882 

6.998 

.078 

.062 

3.077 

.953 

.012 

.29 

23.58 

76.13

9.150 

6.717 

.033 

.067 

2.877 

1.244 

.021 

.50 

30.03 

69.47

8.780 

7.039 

.062 

.016 

3.118 

1.294 

.014 

.32 

29.23 

70.45

9.113 

6.828 

.042 

.047 

2.741 

1.354 

.010 

.23 

32.99 

66.78

8.904 8.933 

6.989 7.034 

.049 .032 

.111 N.d. 

3.026 2.995 

.983 1.070 

.012 .009 

.29 .23 

24.45 26.27 

75.26 73.50

8.929 9.165 

7.036 6.796 

N.d. .024 

N.d. N.d. 

3.024 2.778 

1.120 1.313 

.009 .014 

.23 .34 

26.77 31.98 

72.80 67.68

9.066 

6.847 

.027 

.043 

2.907 

1.199 

.012 

.28 

29.12 

70.60

8.960 

6.965 

.017 

.068 

3.011 

1.046 

.009 

.23 

25.72 

74.05

8.981 

6.972 

N.d. 

.030 

2.982 

1.127 

.009 

.23 

27.36 

72.41

8.817 

7.135 

.014 

N.d. 

3.167 

.938 

.012 

.28 

22.80 

76.92

ROCK TYPE

[Rocks are classified by cumulus minerals listed in decreasing order of abundance and, in parentheses, by mode of all minerals]

NUC-2-1770 

1746 

303

34A

NUC-7-864 

529 

412

Plagioclase-olivine cumulate (olivine gabbro) 

Plagioclase-olivine cumulate (olivine gabbro) 

Olivine-bronzite-plagioclase-chromite cumulate {plagioclase

bearing harzburgite)

Plagioclase-olivine-chromite cumulate (olivine gabbro) 

Plagioclase-augite-olivine cumulate (olivine gabbro) 

Plagioclase-augite-olivine cumulate (gabbro) 

Plagioclase-olivine-chromite cumulate (olivine gabbro)

NUC-10-1340 Noncumulate; altered gabbro

879 Plagioclase-augite-olivine cumulate (olivine gabbro)

765 Plagioclase-olivine cumulate (olivine gabbro)

711 Plagioclase-olivine-bronzite cumulate (olivine gabbro)

630 Plagioclase-olivine cumulate (olivine norite)

505 Plagioclase-augite-olivine cumulate (gabbro)

484 Olivine-plagioclase cumulate (plagioclase bearing dunite-

	harzburgite)

458 Plagioclase-augite cumulate (gabbrol

424 Plagioclase-olivine cumulate (troctolite)

In the Brady Glacier cumulates, the crystallization 

of olivine + bronzite is followed by olivine + bronzite + 

plagioclase rather than bronzite alone; this progression suggests that the entire olivine-orthopyroxene boundary curve was a cotectic. 

The eastern part of the Brady Glacier cumulates 

consists dominantly of olivine-chromite cumulates 

and plagioclase-augite-olivine cumulates. The common occurrence in these cumulates of plagioclaseolivine cumulates with or without chromite suggests 

that the sequence of crystallization was olivine + 

chromite, olivine + plagioclase with or without chromite, and olivine + plagioclase + augite. The crystallization order—olivine, plagioclase, augite—has 

been reported for the Rhum intrusion (Wager and 

Brown, 1968; Durnham and Wadsworth, 1978) and 

for oceanic tholeiites that crystallized at low pressures under anhydrous conditions (Tilley and others, 

1972; Kushiro, 1973; Clague and Bunch, 1976; Bryan 

and others, 1976; Thompson and others, 1976; Bender and others, 1978). The absence of chromite in 

cumulates that contain augite is consistent with the 

reaction relation between chromite and clinopyroxene (Irvine, 1967; Dickey and others, 1971; Hill and 

Roeder, 1974). The order of crystallization as interpreted here assumes that the associated cumulus 

minerals crystallized and accumulated together. Although theoretical and experimental evidence suggests that plagioclase is generally less dense than 

the basaltic liquid from which it crystallized (Bottinga and Weill, 1970; Campbell and others, 1978), 

and therefore should have a tendency to float rather 

than settle with olivine or pyroxene, Irvine (1978) 

has demonstrated that plagioclase may be transported along with denser minerals to the bottom of 

the intrusion and deposited near or on the floor by 

magmatic currents.

The differences in crystallization sequences of the 

eastern and western parts of the Brady Glacier 

cumulates can be discussed using the qualitative 

olivine-clinopyroxene-plagioclase-silica tetrahedral 

phase-diagram model constructed by Irvine (1970). 

Both crystallization sequences originated in the 

olivine primary crystallization volume; the 

crystallization of the western cumulates then 

proceeded to the olivine-orthopyroxene boundary 

surface (presumably a cotectic in the natural 

system), and the crystallization of the eastern cumulates proceeded to the olivine-plagioclase 

cotectic. The two different crystallization sequences
CRYSTALLIZATION OF THE BRADY GLACIER CUMULATES 21

TABLE 6.—Chemical composition and structural formulas of chromite from the basal (Brady Glacier) cumulates of the La Perouse

layered gabbro

[N.d., not detected. *, asterisk, Ferric and ferrous iron calculated from total iron by assuming an RO/R2Os ratio of 1 to 1 and Ti present as Fe2TiO< ]

Sample No. and 

depth, in feetCr2O3- ----- -

M^Ch... ..........

pgjOs*

TiO2 —— ..........

FeO— -• — — - —

MgO- ...........

MnO— ...........

Total.........

. 1678

30.6 

33.7

.58 

.54 

24.7 

8.10 

.35

98.57

NUC-5

1038

29.4 

32.6

2.91 

.61 

25.6 

7.42 

.36

98.90

784

27.3 

36.2 

.63 

.94 

24.7 

8.62 

.29

98.68

426

30.5 

33.8 

.10 

.94 

24.9 

8.29 

.30

98.83

1770r

26.7 

37.7 

.51 

.28 

26.4 

7.43 

.29

99.31

1770g

13.4 

50.0 

1.68 

.16 

22.5 

11.2 

.21

99.15

1746

27.4 

36.9 

.87 

.93 

23.5 

9.62 

31

99.53

NUC-2

1550

27.0 

37.7 

1.36 

.49 

22.9 

9.85 

.35

99.65

1122

34.6 

30.4 

.05 

.91 

22.8 

9.11 

.39

98.26

980

31.3 

32.5 

1.35 

.35 

25.6 

7.45 

.09

98.64

689

32.9 

30.7 

.89 

.40 

27.6 

5.95 

.10

98.54

303

31.0 

32.7 

1.28 

.46 

26.4 

7.05 

.10

98.99

34A

33.2 

32.6 

.52 

.50 

22.6 

9.57 

.08

99.07

NUC-7

1145

27.4 

36.2 

2.48 

.67 

21.2 

10.9 

.30

99.15

NUC-10

1271

28.1 

33.2 

5.00 

.60 

22.3 

9.75 

.33

99.28

1079

25.4 

41.7 

N.d. 

.32 

21.5 

11.3 

.23

100.45

381

30.5 

36.4

N.d. 

.87 

21.0 

11.0 

.28

10005

Cations per 32 oxygens

Cr —....— .....

Al.. ..............

Fe'3—— .—— ..

Ti ...............

Fe-2——— ......

Mg ...............

Mn ...............

XR'3 +Ti —— ....

ZR'2——— ........

ZM—— ..... .....

Cr/ZR-3— .... ..

Al/ZR'3 . ........

Mg/Mg+Fe'2 ...

.. 9.759

. .107

100

.. 5.063

2 964

073

-. 15.901

... 8.100

24 001

... .376

... .617

007

.. .369

5.740

9 492

.542

c Ofi9

9 7^ci

.075

15 902

ft 1 1 9

24.014

Cfl9

034

.340

c 990

10.320

.115

.171

5.000

3111

.060

15.829

Q 1 71

24 000

334

Afi7

.384

9 749

.018

C ftQ9

o fll Q

1 E\ O97

Q 1 70

coq

.372

10.715

.093

.051

15.950

8.050

.320

.674

.006

.334

0 QQQ

13.270

.285

027

4.236

3.750

040

8.026

24.000

.150

.832

.470

5.157

.156

1 G.H

15.834

24 000

.661

.422

10.524

.242

fiQ7

3 479

1 C Q1 0

OQ QQQ

QOA

.665

.015

.434

6.779

8.874

.009

4.722

3.363

.082

15.832

8.167

23.999

.433

.566

.001

.416

6.134

9.483

.252

.065

5.295

2.752

.019

15.934

8.066

24.000

.387

.597

.016

.342

6.560

9.119

.169

.076

5.817

2.237

.021

15.924

8.075

23.999

.414

.575

.011

.278

6.067

9.524

.238

.086

5.466

2.598

.021

15.915

8.085

24.000

.383

.602

.015

.322

6.381

9.340

.095

.092

4.605

3.471

.016

15.908

8.092

24.000

.403

.591

.006

.430

5.155

10.158

.444

.120

4.206

3.859

.060

15.877

8.125

24.002

.327

.645

.028

.478

5.376

9.493

.912

.109

4.522

3.520

.068

15.890

8.110

24.000

.341

.602

.057

.438

4.616

11.285

N.d.

.055

4.131

3.863

.045

15.956

8.039

23.995

.290

.710

.000

.483

5.666

10.100

N.d.

4.139

3.848

.050

15.920

8.043

23.963

.359

.641

.000

.481

ROCK TYPE

[Rocks are classified by cumulus minerals listed in decreasing order of abundance and, in parentheses, by mode of all minerals]

NUC-5-1678

1038

784

426

NUC-2-1770

1746

1550

1122

980

Olivine-chromite cumulate (wehrlite) 

Olivine-chromite cumulate (peridotite) 

Olivine-bronzite-chromite cumulate (harzburgite) 

Olivine-bronzite-chromite cumulate (plagioclase)

bearing harzburgite) 

Plagioclase-olivine cumulate (olivine gabbro)

r, red chromian spinel grain

g, green chromian spinel grain 

Plagioclase-olivine cumulate (olivine gabbro) 

Olivine-chromite cumulate (olivine gabbro) 

Olivine-chromite cumulate (dunite) 

Olivine-bronzite-chromite cumulate (harzburgite)

NUC-2-689 Olivine-chromite cumulate (peridotite)

303 Olivine-bronzite-plagioclase-chromite cumulate (plagioclase

	bearing harzburgite)

34A Plagioclase-olivine-chromite cumulate (olivine gabbro)

NUC-7-1145 Olivine-chromite cumulate (plagioclase bearing dunite)

NUC-10-1271 Olivine-chromite cumulate (plagioclase bearing peridotite)

1079 Olivine-chromite cumulate (olivine gabbro)

381 Olivine-chromite cumulate (plagioclase bearing wehrlite)

imply either a lateral inhomogeneity of magma 

composition relative to the olivine-clinopyroxeneplagioclase-silica tetrahedron or different positions 

of the liquidus boundaries as a result of differences in 

total pressure, water fugacity, oxygen fugacity, compositional variables (such as the An/Ab and FeO/ 

MgO of the magma during fractional crystallization), or contamination by salic material. An increase in pressure (Green and Ringwood, 1976; Osborn, 1976 cited in Cameron, 1978), water fugacity 

(Yoder, 1965), and salic contamination (Irvine, 1975b) 

contracts the plagioclase crystallization volume and 

enhances the crystallization of pyroxene. The plagioclase volume is increased relative to pyroxene by 

an increase in the ratio of An to Ab (Irvine, 1970). 

Considering the close proximity and contiguous 

occurrence of the two cumulate sequences, differences in total pressure as a cause of the two different 

crystallization paths is extremely unlikely, and there 

are no mineralogical data to suggest differences in

water fugacity or existence of contamination. The 

occurrence of graphite throughout the cumulates 

suggests that the oxygen fugacity was on the graphite— (CO+CO2) buffer during crystallization of both 

sequences.

If it is assumed that the conditions of crystallization and the liquidus boundaries for the two 

sequences were essentially the same, then their 

magmas are assumed to differ in composition. These 

differences need not be large if both compositions are 

near the 5-phase point olivine-orthopyroxene-plagioclase-clinopyroxene-liquid.

The repetition of the cumulate layers is similar to, 

but not so well developed as, the cyclic units described for other layered gabbros, such as the Stillwater, 

Bushveld, Great Dyke, Muskox, Rhum, and Jimberlana intrusions (Jackson, 1961, 1970; Wager and 

Brown, 1968; Irvine, 1970; Campbell, 1977). Two 

major models have been proposed to explain the 

origin of cyclic units: (1) periodic influx of undif-
22 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

ferentiated magma (Cooper, 1936; Brown, 1956), and 

(2) bottom crystallization from a stagnant magma 

that is periodically replenished with fresh magma by 

variable depth convection (Jackson, 1961). A third 

model, order of nucleation of cumulus phases from a 

supercooled magma, was proposed for a particular 

type of layering in the Rhum and Bushveld intrusions but was not intended to have general application (Wager, 1959; Wager and Brown, 1968). In 

studying the Jimberlana intrusion, Campbell (1977) 

concluded that marked reversals in mineral chemistry and nickel-chromium whole-rock trends and 

zones of high-sulfide content at the base of cyclic 

units are probably indicative of multiple injection. 

Campbell noted that these features are absent from 

the cyclic units of the Great Dyke, Muskox, and 

Stillwater intrusions and therefore did not propose 

multiple injection as the sole process in forming 

cyclic units.

In the Brady Glacier cumulates, the parameters 

suggested to be indicative of multiple injection are 

not definitive. Sulfide mineralization occurs throughout the layered sequence and shows no correlation 

with rock type or stratigraphic position. Variations 

in mineral chemistry are not consistent; the olivine 

in the olivine-chromite cumulate at the top of core 

NUC-10 (fig. 10) does show a marked reversal in 

forsterite content, but the olivine in the olivinechromite cumulate at the 175-m level of core NUC-10 

does not show a composition reversal. Further, in the 

olivine, olivine-bronzite layer sequence, olivine compositional variation in a single layer is as great as 

between layers. (Compare figs. 7 and 8 with fig. 

14). Nevertheless, the repetition of cumulate layers, 

many of them relatively thin without good development of continuous fractionation sequences and the 

lateral inhomogeneity of the cumulate sequences, 

leads us to tentatively favor multiple injection of 

magma as a major process in producing the repetition of cumulate sequences in the Brady Glacier rock 

succession. This proposal is strengthened by examimation of the exposed La Perouse gabbro, which 

consists dominantly of interlayered olivine gabbro, 

norite, and gabbronorite but also contains thin 

layers of ultramafic rocks (olivine-chromite cumulates) throughout the sequence. Except for repetition 

of cumulate sequences, the La Perouse gabbro shows 

no systematic variation of mineral composition with 

stratigraphic height. This feature is best explained 

by frequent injection of magma, each batch at a 

different stage of fractionation at the time of injection and undergoing a different degree of fractionation before the next injection.

METAMORPHIC COUNTRY ROCKS

Metamorphic country rocks below the cumulates 

were penetrated in four drill holes; maximum penetration, approximately 30 m, was in drill hole NUC2. The metamorphic rocks are fine to medium grained, generally have a well-developed foliation, and are 

dominantly biotite schists, hornblende-biotite schists, 

and cummingtonite-biotite schists. Limiting mineral 

assemblages containing the maximum number of 

coexisting phases are given below. Any combination 

of a smaller number of phases from the limiting 

assemblage is possible. All phases in an assemblage 

are interpreted to have formed in equilibrium.

1. Staurolite-garnet-cordierite-chlorite-biotite-plagioclase

2. Quartz-garnet-cordierite-chlorite-biotite-plagioclase

3. Quartz-garnet-anthophyllite-biotite-plagioclase

4. Sillimanite-cordierite-corundum-staurolite-plagioclase

5. Hornblende-cummingtonite-biotite-plagioclase

6. Quartz-cummingtonite-biotite- chlorite

In addition, all assemblages contain an opaque 

oxide mineral. Assemblages 2 and 3 occur in the 

same thin section, but anthophyllite does not occur 

with cordierite or chlorite.

The number of phases and components suggests 

that all assemblages formed under at least divariant 

conditions if not higher variance. The diagnostic 

association of garnet with cordierite as well as the 

occurrence of sillimanite and anthophyllite indicates that the metamorphic rocks formed under 

conditions of the upper amphibolite facies (Turner, 

1968; Winkler, 1974). The association of cordierite 

with staurolite indicates that the rocks belong to the 

low-pressure, intermediate-type facies series of Miyashiro (1961), which is similar to Read's Buchan type 

(1952).

The common occurrence of cordierite + garnet in 

high-grade metamorphic rocks has prompted numerous experimental and theoretical investigations of 

the cordierite + garnet stability field (Reinhardt, 

1968; Hess, 1969; Currie, 1971; Hensen and Green, 

1971,1972,1973). The stability field as defined by the 

investigations is dependent on the presence of an 

additional phase, such as sillimanite, potassium 

feldspar, or hypersthene, phases not present in the 

schists of this study. Schreyer and Yoder (1964)
SUMMARY 23

investigated the stability of cordierite under hydrous 

conditions in the absence of other phases and found 

a temperature range of approximately 450°-700°C 

for its formation. High-pressure breakdown for anhyrous cordierite has been calculated to be approximately 5.6 kbar at 700°C (Newton and others, 1974); 

hydrous cordierite breaks down at pressures several 

kilobars higher (Newton, 1972).

The stability of staurolite was investigated by 

Richardson (1968) and by Ganguly (1968, 1972). 

Richardson's data require the presence of quartz and 

are not applicable to this study. Ganguly (1972) 

limited the field of staurolite stability without quartz 

between 520° and 790°C at 5-kbar pressure and an 

oxygen fugacity in the middle of the magnetite field. 

The stability of anthophyllite has been investigated 

by Greenwood (1963). At a Pn2O of 1 kbar in the 

presence of quartz, the lower stability limit of anthophyllite is approximately at 675°C, the upper stability limit at 745°C, where it breaks down to enstatite + quartz. At a PH.O of 2 kbar, the field of 

stability of anthophyllite + quartz is 710°-767°C. In 

natural assemblages, the fields of both staurolite 

and anthophyllite would be more restricted.

In assemblage 4, the sillimanite, corundum, staurolite, and plagioclase occur enclosed in large cordierite porphyroblasts, also a part of assemblage 2. This 

occurrence of sillimanite-cordierite-corundum-staurolite-plagioclase is mineralogically and texturally 

similar to the aluminous enclaves discussed by 

Robinson and Jaffe (1969), who explained the origin 

of such enclaves by a series of reactions involving 

anthophyllite (gedrite) + alumino-silicate to yield 

cordierite, corundum, and staurolite. Some similar 

reaction seems plausible for the origin of assemblage 

4, since it occurs in the same thin section with 

assemblages 2 and 3. The presence of sillimanite 

gives a lower temperature limit of approximately 

622°C using the data of Richardson and others (1969) 

and 501 °C using the data of Holdaway (1971).

The temperatures indicated by the various phase 

equilibria for the formation of the metamorphic 

rocks underlying the Brady Glacier cumulates are 

compatible. The occurrence of anthophyllite + quartz 

is probably the best indication of the lower temperature limit and suggests that the temperature of 

metaittorphism was greater than 675°C if PH20--Ptotai.

The regional metamorphic rocks exposed in the 

southern Fairweather Range are in the amphibolite 

facies but are of lower grade than the rocks recovered 

in the drill core. The exposed rocks contain garnetstaurolite assemblages; but garnet-cordierite assemblages are absent, and the aluminosilicate is andalusite rather than sillimanite. Rocks directly in

contact with the La Perouse gabbro, however, are of 

higher grade, having assemblages containing garnet-cordierite, garnet-hypersthene, and sillimanite. 

These high-grade granulite-facies rocks occur as a 

rim or selvage of variable thickness around the 

gabbro and in abrupt contact with the lower grade 

regional metamorphic rocks; the maximum thickness of the higher grade rocks is approximately 200 

m. The metamorphic rocks below the Brady Glacier 

cumulates are probably equivalent to the higher 

grade metamorphic rocks exposed along the contact 

of the La Perouse gabbro.

Abruptness of the contact of the higher grade 

metamorphic rocks with the lower grade regional 

metamorphic rocks suggests that the La Perouse 

gabbro and the rim of high-grade metamorphic rocks 

were tectonically emplaced together into the lower 

grade terrane. It is not known whether the highgrade amphibolite-granulite-facies rocks represent a 

thermal aureole around the gabbro or reflect a 

regional high-grade terrane at a greater depth.

SUMMARY

The Brady Glacier ultramafic and gabbroic cumulates here described are the basal cumulates of the La 

Perouse layered gabbro. The most common rock 

types are olivine-chromite cumulate, olivine-bronzite-chromite cumulate, and plagioclase-augite-olivine cumulate. Individual cumulate layers are lenticular and wedge out in all directions over short 

distances. The contact of the cumulates with the 

underlying country rocks is irregular and has considerable relief.

Whole-rock chemical data indicate that the Brady 

Glacier ultramafic and gabboric rocks are related by 

differentiation and that olivine fractionation was 

the primary mechanism for derivation of the gabbro. 

Spatial distribution of cumulate sequences indicates 

two different orders of crystallization of cumulus 

silicate minerals: (1) olivine, bronzite, plagioclase 

and (2) olivine, plagioclase, augite. The differences in 

crystallization sequence are attributed to their formation by separate injections of magma of slightly 

different composition.

The Brady Glacier cumulates are the host rocks for 

nickel-copper sulfide deposits that consist of hexagonal and monoclinic pyrrhotite, pentlandite, and 

chalcopyrite. Although there is no correlation of 

sulfide mineralogy or abundance of disseminated 

sulfides with stratigraphic height or rock types; 

massive sulfide lenses do tend to occur along boundaries between mafic and ultramafic layers. Textural
24 PETROLOGY, BRADY GLACIER NICKEL-COPPER DEPOSIT, ALASKA

characteristics indicate that the sulfide mineralization represents an immiscible sulfide melt.

By Naldrett and Cabri's (1976) classification of 

ultramafic and related rocks, the La Perouse layered 

gabbro and sulfide deposit is closest to the "large 

stratiformly layered complexes." The La Perouse 

layered gabbro differs from other large stratiform 

intrusions in two ways: (1) The La Perouse body was 

not emplaced in a nonorogenic cratonic area but 

rather near a major fault system into country rocks 

that are similar to an oceanic crust assemblage; (2) 

there is no correlation between mineral chemical 

variation and stratigraphic height; this indicates 

frequent injection of magma batches rather than 

fractional crystallization of large volumes of magma.

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Atkins, F. B., 1969, Pyroxenes of the Bushveld intrusion, South 

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Basu, A. R., and MacGregor, I. D., 1975, Chromite spinels from 

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Research, v. 81, p. 4285-4304.

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