Petrology of Chromite-Bearing 

Rocks from the Lowermost Cyclic 

Units in the Stillwater Complex, 

Montana

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Chapter E

Petrology of Chromite-Bearing 

Rocks from the Lowermost Cyclic 

Units in the Stillwater Complex, 

Montana

By PATRICIA J. LOFERSKI, BRUCE R. LIPIN, and ROGER W. 

COOPER

Petrologic study of a drill core through the lowermost cyclic 

units of the Stillwater's Ultramafic series

U.S. GEOLOGICAL SURVEY BULLETIN 1674

CONTRIBUTIONS ON ORE DEPOSITS IN THE EARLY MAGMATIC ENVIRONMENT
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes 

only and does not imply endorsement by the U.S. Government

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990

For sale by the

Books and Open-File Reports Section,

U.S. Geological Survey,

Federal Center, Box 25425,

Denver, CO 80225

Library of Congress Cataloging in Publication Data

Loferski, Patricia J.

Petrology of chromite-bearing rocks from the lowermost cyclic units in the 

Stillwater Complex, Montana / by Patricia J. Loferski, Bruce R. Lipin, and 

Roger W. Cooper.

p. cm. (Contributions on ore deposits in the early magmatic environment)

(U.S. Geological Survey bulletin ; 1674-E)

Includes bibliographical references (p. )

Supt. of Docs, no.: I 19.3:1674-E

1. Chromite Montana. 2. Petrology Montana. 3. Geology Montana. I. 

Lipin, Bruce R. II. Cooper, Roger W. III. Title. IV. Series. V. Series: U.S. 

Geological Survey bulletin ; 1674-E.

QE75.B9 no. 1674-E

557.3 s-dc20

[553.4'632'09786] 89-600391

CIP
CONTENTS

Abstract El

Introduction and General Geology El

Nomenclature and Definitions E2

Description of the M-16 Drill Core E4

Methods E6

Petrography E6

Silicates E6

Oxides E8

Grain Size E10 

Mineral Chemistry E10

Silicates E10

Chromite E12

Whole-Rock Trace Elements E17 

Discussion E21

Origin of Chromite Seams E21

Infiltration Metasomatism E21

Comparison of Cyclic Units in the Stillwater E23

Calculation of Magma Thickness E24 

Conclusions E27 

References E27

FIGURES

1. Maps showing locations of the Stillwater Complex and the M-16 drill

core E2 

2-5. Diagrams showing:

2. Stratigraphic variation of cumulus minerals in an ideal cyclic unit of the 

Peridotite zone E4

3. Stratigraphy and sample locations in the M-16 drill core E5

4. Data for olivine and orthopyroxene E7

5. Data for chromite E9 

6-8. Photomicrographs taken in reflected light of:

6. Accessory chromite E10

7. Exsolved chromite Ell

8. Ilmenite blebs on chromite Ell

9. Diagram showing data for chromite in and near the chromite seam within the 

M-16 drill core E13

10. Compositional plots of chromite E14 

11-22. Diagrams showing:

11. Compositional fields of segregated chromite from the B, G, and H 

chromite seams E15

12. Microprobe analyses of chromite from the M-16 drill core plotted with 

respect to Cr-Al-Fe3+ E15

13. Compositional fields of accessory chromite in olivine, orthopyroxene, 

and amphibole E17

Contents III
14. Compositional differences between accessory chromite in olivine and 

pyroxene E17

15. Ln KG plotted against Y^r for M-16 chromite-olivine pairs E18

16. Cr2O3 contents in orthopyroxene plotted against distance from an 

enclosed chromite grain E19

17. Whole-rock trace-element contents plotted against stratigraphic 

height E20

18. Whole-rock platinum-group-element contents plotted against stratigraphic 

height E22

19. Whole-rock M-16 drill core samples plotted with respect to PtPd-Rh E23

20. Phase relations in the olivine-chromite-SiO2 system shown in weight 

percent E24

21. Mineral chemistry plotted against stratigraphic height for three cyclic 

units from the Stillwater Complex E25

22. Curves illustrating the variation of Mg/(Mg+Fe2+ ) of a solid such as 

olivine and its parental liquid during fractional crystallization E26

TABLES

1. Correlation coefficients for properties of the silicates in the M-16 drill 

core E12

2. Representative microprobe analyses of accessory chromite in olivine and 

orthopyroxene in sample 289 from the first olivine cumulate E18

3. Samples that represent possible parent liquid compositions for the Stillwater 

Complex, from Helz (1985) E26

Contents IV
Petrology of Chromite-Bearing Rocks From the 

Lowermost Cyclic Units in the Stillwater Complex, 

Montana

By Patricia J. Loferski,1 Bruce R. Lipin,1 and Roger W. Cooper2

Abstract

A detailed petrologic study of a drill core through the 

lowermost cyclic unit of the Ultramafic series of the 

Stillwater Complex, Montana, shows that the mineral 

chemistry of chromite, olivine, and orthopyroxene exhibits complex vertical fluctuations rather than continuous 

iron enrichment as would be expected to result from 

simple fractional crystallization. Intervals of iron enrichment alternate with intervals of magnesium enrichment 

upward in the 42-m-thick olivine cumulate, and erratic 

fluctuations occur in the overlying 8-m-thick bronziteolivine cumulate. Shifts in mineral chemistry, modes, and 

grain sizes occur above and below contacts, even those of 

very thin (several millimeters thick) layers in the core; 

these shifts show that changes in phase equilibria influence both the chemical and physical properties of crystallizing phases.

There is no large difference in Mg/(Mg+Fe2+ ) (XMg) 

between segregated chromite in the 2-m-thick chromite 

seam and accessory chromite above and below the seam. 

However, within the seam itself, accessory chromite in 

olivine-rich layers shows lower XMg than the segregated 

chromite.

Accessory chromite enclosed in olivine is compositionally distinct from chromite in pyroxene or amphibole 

in a given sample. This finding indicates postcumulus and 

(or) subsolidus reequilibration of accessory chromite with 

the surrounding silicate. Olivine-chromite pairs indicate 

equilibration temperatures of about 700 °C.

Abrupt shifts in whole-rock nickel contents and in 

the compositions of chromite and olivine occur 1-3 m 

above changes in lithology, indicating that compositional 

changes that may have been caused by the upward migration of interstitial liquid occurred only over small vertical 

distances. Therefore, infiltration metasomatism was not 

an important process here.

Manuscript approved for publication November 9, 1989.

'U.S. Geological Survey, 954 National Center, Reston, VA 22092.

2Geology Department, Lamar University, Beaumont, TX 77710.

Calculations based on an estimated original liquid 

composition and on olivine compositions over a 24-m 

interval that shows normal fractionation indicate that 

about 1,000 m of magma was necessary to crystallize the 

50-m-thick cyclic unit. Therefore, the magma chamber 

probably reached its total thickness of about 8,000 m early 

in its history.

INTRODUCTION AND GENERAL GEOLOGY

This report describes the petrography, mineral chemistry, and whole-rock trace-element geochemistry of 

chromite-bearing samples from a drill core through the first 

cyclic unit and the base of the second cyclic unit in the 

Ultramafic series of the Stillwater Complex, Montana. This 

study, part of a larger study on the evolution of the 

Stillwater Complex and its ore deposits, was done to 

document textural and chemical changes occurring through 

a cyclic unit in the Ultramafic series, which might provide 

information on the formation of cyclic units and enclosed 

chromite seams.

The Stillwater Complex is a layered mafic-ultramafic 

igneous intrusion that is exposed for about 40 km along the 

northern front of the Beartooth Mountains of south-central 

Montana (fig. LA). The complex strikes northwestsoutheast; the dip ranges from nearly vertical or overturned 

in the eastern part of the complex to about 60° N. in the 

western part (Page, 1977). Numerous isotopic studies, 

outlined by Lambert and others (1985), indicate that the 

Stillwater was probably formed about 2,700 Ma. It intrudes 

older Precambrian metasedimentary rocks and is unconformably overlain by Paleozoic and Mesozoic sediments. 

Page (1977) indicated that there may have been low-grade 

greenschist-facies metamorphism of the Stillwater Complex 

between 1,600 and 1,800 Ma, on the basis of development 

of foliation, the growth of low-grade mineral assemblages, 

and the lead loss or uranium gain in apatite. The Stillwater 

was faulted, tilted, and exposed to erosion before the 

Middle Cambrian, and further deformation and uplift during 

the Laramide orogeny brought it to its present position.

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E1
1KH51

110°05' 110° 109°55' 109°50' 109°45'

MONTANA

... .0 Livmgstonu

Map area

Billings H

45°30'

45°25'

45°20'

EXPLANATION

Sedimentary rocks (Mesozoic and Paleozoic) 

Granite (Precambrian)

Banded series of the Stillwater Complex (Precambrian)

|^ban,^ | Norite, gabbro, and anorthosite

Ultramafic series of the Stillwater Complex (Precambrian)

Bronzitite zone

Peridotite zone Bronzitite and harzburgite 

Metamorphic rocks (Precambrian) Basement complex

FIGURE

 Contact Dashed where approximately located

 Fault Dashed where approximately located

 A_ Thrust fault Sawteeth on upper plate

5 MILES

5 KILOMETERS

Figure 1/4. The Stillwater Complex (modified from Jackson, 1961).

Detailed descriptions of the Stillwater Complex were 

given by Hess (1960), Jones and others (1960), Jackson 

(1961), Page (1977), McCallum and others (1980), Zientek 

(1983), and Raedeke and McCallum (1984). Zientek and 

others (1985) reviewed the stratigraphic nomenclature used 

for units in the Stillwater Complex. Briefly, the Stillwater 

Complex is divided into three main stratigraphic units: (1) 

the thin discontinuous Basal series, as much as 240 m thick, 

which consists mostly of norite and orthopyroxenite; (2) the 

Ultramafic series, which averages about 1,000 m in thickness and is further subdivided into a lower Peridotite zone 

and an upper Bronzitite zone; and (3) the Banded series, 

which is about 4,000 m thick and consists of norites, 

anorthosites, troctolites, and gabbros. It is generally agreed 

that the Stillwater formed by processes involving fractional 

crystallization of basaltic magma. Recent work has raised 

the possibility that the Stillwater Complex may have formed 

from magmas of two distinct compositions, and magma 

mixing and double-diffusive convection may have occurred 

(Lambert, 1982; Raedeke, 1982; Irvine and others, 1983).

The samples studied for this report are from the 

Anaconda Co. drill core M-16, from the Mountain View

area of the complex (fig. IB). The Mountain View block is 

bounded by two thrust faults, the Lake fault to the north and 

the Bluebird thrust to the south. The block has been rotated 

so that the layering strikes approximately north-south, 

which is perpendicular to the strike of the layering in the 

rest of the complex.

NOMENCLATURE AND DEFINITIONS

Before discussing the petrology of the drill core from 

hole M-16, a brief discussion of nomenclature and cyclic 

units is necessary. The rocks in the Stillwater Complex are 

composed of various proportions of cumulus and intercumulus minerals (Jackson, 1961; Irvine, 1980). Cumulus 

minerals fractionate from the magma and are accumulated 

either in situ or by crystal settling, flotation, or current 

action. Intercumulus minerals crystallize from the liquid in 

the pore spaces between the cumulus grains. Texturally, 

cumulus minerals form discrete grains, they are well sorted, 

and they form a framework of crystals that compose about 

65 percent of the layer in which they occur. Cumulus grains

E2 Contributions on Ore Deposits in the Early Magmatic Environment
 -^i/7ut-r ^ ""ir-...

EXPLANATION

Sedimentary rocks and intrusive rocks 

of intermediate composition

Quartz monzonite and aplite

Banded series

Bronzitite zone

Peridotite zone Symbol indicates chromitite

Basal series

Metamorphosed sedimentary rocks

Trace of axial plane of major fold

Anticline 

I Syncline Contact Approximately located; dotted where concealed

Fault Dashed where approximately located; dotted where concealed

Thrust fault Dashed where approximately located; dotted where concealed

Low-angle gravity fault Dashed where approximately located. 

Hachures on downthrown side

Figure 1B. Location of the M-16 drill core (modified from Zientek, 1983).

tend to be euhedral, or nearly so, but may be subhedral or 

anhedral. The intercumulus minerals enclose or are molded 

onto the cumulus grains. In the Stillwater Complex, the 

rocks are named according to their cumulus minerals (for 

example, olivine-bronzite cumulate).

Our samples are from the base of the Peridotite zone 

of the Ultramafic series. The Peridotite zone is composed of 

cyclic units, or repetitive sequences of rocks. An ideal 

Stillwater cyclic unit, as defined by Jackson (1961), and 

shown in figure 2, consists of an olivine cumulate, which 

may or may not contain a chromite seam, overlain by a 

bronzite-olivine cumulate in which the olivine-bronzite 

ratio decreases upward, overlain by a bronzite cumulate. 

Not all cyclic units are ideal, and any one of the layers may 

be missing. Detailed descriptions of the formation of cyclic 

units are given by Jackson (1969, 1970), Raedeke (1982),

and Raedeke and McCallum (1984). Briefly, each cyclic 

unit is formed by fractional crystallization, and the crystallization sequence is repeated for successive cyclic units, 

possibly by the injection and mixing of new batches of 

magma. In the Mountain View area, where the Peridotite 

zone reaches its greatest thickness, Jackson and others 

(1954) recognized 15 cyclic units. Raedeke and McCallum 

(1984) showed 20 cyclic units in the same area.

Chromite seams occur within the olivine cumulates of 

many of the cyclic units. As many as 13 stratigraphically 

superposed chromite seams have been found (Jackson, 

1968). Chromite seams have complex internal stratigraphy; 

most of them consist of alternating layers of chromite 

cumulates that can be as much as 2 m thick and olivinechromite cumulates. The chromite seams are labeled 

A through K, with A the stratigraphically lowest. Two

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E3
CUMULUS MINERALS, IN PERCENT

Olivine > 

50 100

100

-Bronzite and chromite 

50

ROCK TYPE

Bronzite 

cumulate

Bronzite

olivine

cumulate

.

- t Olivine [.&' 

^cumulate3 .

^"" 7- cumulate,

0- °:

 O . 

 cip" ' c 

' '<=> 

, <?\ 

 <^>

o <b>. -.'

Bronzite

Olivine

Figure 2. Stratigraphic variation of cumulus minerals in an 

ideal cyclic unit of the Peridotite zone, modified from 

Jackson (1961).

additional seams, the G' and H', are present in the Mountain View area, where the Ultramafic series reaches its 

greatest thickness. Although some seams are discontinuous, 

the G, which is the thickest seam, and possibly the H, J, and 

K seams can be correlated across most of the length of the 

complex (Jackson, 1968). Only the G and the H seams have 

been mined for chromite.

DESCRIPTION OF THE M-16 DRILL CORE

The stratigraphy of the part of the drill core we 

sampled and the sample locations are shown in figure 3.

Thicknesses discussed here are true thicknesses, which 

were calculated by using the angle of intersection of the drill 

core with cumulate layering. The lowermost part of the core 

is in the bronzite cumulate of the Basal series. Above the 

bronzite cumulate are the cyclic units of the Ultramafic 

series. Using Jackson's (1961) definition of a cyclic unit as 

discussed above, we have three possible ways of subdividing the Ultramafic series part of the core into cyclic units, as 

shown in figure 3. Although all three subdivisions are valid 

according to Jackson's (1961) definition, depending upon 

whether or not very thin layers are considered, we have 

elected to use the subdivision shown in column 1 because 

the thin bronzite seams at around 98 m and the olivine 

cumulate at around 67 m are discontinuous in the Mountain 

View area and therefore do not define traceable cyclic units. 

Also, our subdivision of the drill core is more in line with 

the thickness of the lowermost cyclic units as seen in 

outcrop and other drill core in the area. Therefore, according to our subdivision, the first cyclic unit of the Ultramafic 

series consists of a 42-m-thick olivine cumulate overlain by 

an 8-m-thick bronzite-olivine cumulate. This cyclic unit is 

incomplete because it does not contain a bronzite cumulate 

above the bronzite-olivine cumulate. The olivine cumulate 

encloses two thin bronzite cumulate layers, each only 10-20 

mm thick, and the bronzite-olivine cumulate encloses a 

0.5-m-thick olivine cumulate. The olivine cumulate beginning at about 61m defines the base of cyclic unit 2, and it 

contains a 2-m-thick chromite seam about 2.5 m above the 

base. A large hornfels xenolith occurs at about 50 m and 

upward.

As is typical of the chromite seams in the Stillwater, 

the one in M-16 consists of layers of massive chromitite 

interspersed with layers of olivine-chromite cumulate. The 

massive chromitite layers range from 10 to 120 mm in 

thickness. The chromite seam in M-16 is probably the B 

because it is in the second cyclic unit and it correlates with 

the B chromitite in other nearby drill cores that contain both 

the A and B chromitites. Therefore, the stratigraphically 

lowermost A chromitite is not present in M-16, indicating 

its discontinuous nature even within the relatively small 

area of the Mountain View block. Another indication that 

the stratigraphy within cyclic units varies laterally within a 

small area is the fact that the stratigraphy within M-16 does 

not exactly correlate with other Mountain View stratigraphic columns shown by Jackson (1968) and Raedeke and 

McCallum (1984).

Because of core retrieval problems, only two of the 

contacts were exposed in the drill core. The one at 111 m 

between the Basal bronzite cumulate and the first olivine 

cumulate is sharp, and thin sections were made of samples 

on both sides of the contact within a few centimeters. The 

contact at 69 m between the olivine cumulate and the 

bronzite-olivine cumulate of the first cyclic unit is gradational over about 0.5 m.

E4 Contributions on Ore Deposits in the Early Magmatic Environment
Q.

CD

Q

Columnar 

section

50-

70

80-

90-

110-

-9-W

vbc

Possible cyclic units 

1 2

EXPLANATION 

 ^ Olivine cumulate

Chromite seam 

[ I | I | Bronzite-olivine cumulate 

be Bronzite cumulate

Basal bronzite cumulate zone

Ultramafic series

Basal series

\

Figure 3. Stratigraphy and sample locations (tick marks) in the M-16 drill core and three possible subdivisions into 

cyclic units.

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Still water Complex, Montana E5
METHODS

Fifty-eight samples were collected over a stratigraphic interval of nearly 65 m. The sample numbers 

correspond to the drill-hole footages. The samples collected 

were sections 7-14 cm long from the 5-cm-diameter drill 

core. Most samples were halves and some were quarters of 

the core. Half of the sample was submitted for whole-rock 

analyses, and the other half was used for making polished 

thin sections and for point-counting slabs.

Because of their coarse grain size, the modes of 

olivine and pyroxene were obtained by point-counting 

approximately 500 points on 25-cm2 areas on HF-etched 

slabs, rather than in thin sections, in order to have a more 

representative sample. The objective of point counting was 

to determine the proportions of minerals precipitated from 

the magma. Therefore, the hydrous products of subsolidus 

alteration of olivine and pyroxene were not counted separately. Fortunately, alteration was almost entirely isomorphous, and the original minerals were easily determined. 

Because of the fine grain size of chromite, its modes were 

obtained by point-counting thin sections. Most thin-section 

modes were calculated on the basis of 1,000 points. The 

average grain sizes of chromite and olivine were obtained 

by using a micrometer ocular to measure the maximum 

dimension of either 100 grains or every grain, whichever 

was less, in each thin section.

Mineral chemistry of chromite, olivine, and orthopyroxene was determined by electron microprobe analyses of 

three to seven or more points on a minimum of three grains 

of each mineral species per sample. All minerals were 

analyzed for SiO2 , TiO2 , Cr2O3 , FeO, MgO, MnO, and 

NiO. In addition, chromite was analyzed for A12O3 and 

ZnO; olivine for CaO; and pyroxene for A12O3 , Na2O, and 

CaO. The analytical precision was ±2 percent for major 

elements. Most individual grains were not zoned; the 

compositions of olivine and orthopyroxene were constant 

within individual thin sections except adjacent to enclosed 

chromite grains, where they were zoned. Such zoning is 

discussed separately below, and zoned areas were not 

included in the averages for sample compositions. Chromite 

compositions varied as discussed below. Chromite and 

olivine were analyzed with a focused l-fjim-diameter beam, 

and pyroxene, which contains fine exsolution lamellae, was 

analyzed with a 10-fjLm-diameter beam in order to obtain 

bulk compositions. Data were reduced by using the method 

of Bence and Albee (1968). Calculation of Fe3+ for 

chromite was done assuming ideal spinel stoichiometry.

Whole-rock trace-element analyses of Co, Cr, Ni, V, 

Mn, Ti, and Zn were performed by means of direct-current 

arc spectrography, and Pt, Pd, and Rh were determined by 

means of fire-assay separation and atomic absorption. All 

analyses were performed by analysts at the U.S. Geological 

Survey. Data tables and details of the analytical techniques 

are presented elsewhere (Loferski and others, 1984).

PETROGRAPHY 

Silicates

Samples from the Basal series bronzite cumulate are 

composed of 80-90 percent bronzite that occurs as euhedral 

to anhedral grains. The bronzite grains are enclosed in the 

intercumulus phases, which form 10-20 percent of the 

rocks and which consist mainly of clinopyroxene and 

plagioclase with small amounts of brown biotite.

Samples from the various layers in the Ultramafic 

series show the following textures. In the olivine cumulate 

layers, the morphology of olivine grains varies depending 

on the mineralogy of the intercumulus material, as first 

noted by Jackson (1961). Where it is enclosed in orthopyroxene, olivine is rarely euhedral; it is commonly rounded 

to embayed with irregular resorbed edges because of the 

reaction relationship between olivine and liquid to form 

orthopyroxene. Olivine shows sharp euhedral to subhedral 

boundaries against intercumulus plagioclase and amphibole 

and commonly against clinopyroxene. The predominant 

intercumulus phase in the two olivine cumulate layers is 

oikocrystic orthopyroxene. Clinopyroxene and plagioclase 

each constitute about 5 percent of the rocks; trace phases 

include brown amphibole and brown biotite or phlogopite.

In the bronzite-olivine cumulate, bronzite grain 

boundaries range from sharp and euhedral, against intercumulus clinopyroxene and plagioclase, to anhedral and 

irregular where bronzite grains are interlocking. Olivine 

grains are commonly rounded with irregular grain boundaries, and some are surrounded by a thin rim of orthopyroxene. The most abundant intercumulus phase is oikocrystic clinopyroxene; plagioclase is less abundant. Accessory 

phases are brown amphibole, which is associated with the 

clinopyroxene, and brown biotite or phlogopite.

The amount of late-stage alteration is variable, as 

indicated in figure 4. The degree of serpentinization of the 

olivine and pyroxenes typically is less than 20 percent but 

may be as much as 90 percent in rocks near the contacts, 

probably because of greater ease of fluid migration along 

the contacts. Other alteration products of the pyroxenes 

include talc and patches of brown amphibole, which are 

probably pargasitic (N. Page, written commun., 1984). 

Trace amounts of green chlorite and carbonate minerals also 

occur in some of the rocks.

Figure 4 shows the modes of olivine and both 

cumulus and intercumulus orthopyroxene plotted against 

stratigraphic height. These modal proportions vary 

inversely throughout the drill core, showing the following 

variations. Olivine is absent in the Basal series bronzite 

cumulate. The olivine mode increases sharply to 45 percent 

at the base of the lowermost olivine cumulate, then reaches 

65 percent within about 3 m of the contact, and stays at 

about this proportion through most of the remaining 42- 

m-thick olivine cumulate layer. Orthopyroxene composes

E6 Contributions on Ore Deposits in the Early Magmatic Environment
Mg/(Mg 

.82 .80

Olivine size 

(mm)

o.E 

o> £

OJ OJ

ra9-

X ® D en

Rock 

type

Modal percent ' =

Orthopyroxene Olivine o

0 50 100 50 100

110-

EXPLANATION

['. ^J Olivine cumulate

^^^ Chromite seam

|| | I | Bronzite-olivine cumulate

| be | Bronzite cumulate

I__J Basal bronzite cumulate zone

 Olivine

A Cumulus orthopyroxene

A Intercumulus orthopyroxene

Figure 4. Data for olivine 

and orthopyroxene in the 

M-16 drill core, plotted 

against stratigraphic height. 

Mg/(Mg+Fe2+ ) was calculated from microprobe 

analyses; average grain 

sizes are shown as dots; 

bars indicate one standard 

deviation; degree of serpentinization shown as: S, 

slight, less than 20 percent; 

M, moderate, 20-80 percent; E, extensive, greater 

than 80 percent.

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E7
about 90 percent of the sample at the top of the Basal series 

bronzite cumulate. Its mode drops to 45 percent at the base 

of the olivine cumulate then drops further to a fairly 

constant 25 percent in the rest of the olivine cumulate. 

Intercumulus plagioclase and clinopyroxene form the 

remaining 10 modal percent of the olivine cumulate.

The modes of olivine and bronzite show more variation in the overlying bronzite-olivine cumulate, possibly 

because of modal layering that might only be detected by 

examination of the continuous drill core. The mode of 

olivine shows a sharp increase from about 5 percent at the 

top of the bronzite-olivine cumulate to about 75 percent at 

the base of the olivine cumulate in the second cyclic unit. 

The olivine mode then shows a gradual decrease to about 30 

percent above the chromite seam. Samples within the 

chromite seam will be discussed separately below.

Oxides

Chromite is the most abundant oxide in the M-16 

samples, and it occurs both as a ubiquitous accessory phase 

and in thin massive layers in the chromite seam. Minor 

amounts of magnetite, rutile, and ilmenite also occur in the 

samples.

Chromite composes less than 1 percent of the bronzite 

cumulate in the Basal series (fig. 5). Like the mode of 

olivine, the chromite mode shows a sudden jump (to about 

3 percent) just above the base of the first olivine cumulate 

and then drops back to about 1 percent throughout the rest 

of the core, except in the chromite seam and one sample 

above the chromite seam in which it reaches 3 percent. 

Within the chromite seam, the chromite mode varies from 

less than 5 percent in the layers of olivine cumulate to about 

65 percent in the most chromite-rich layers, and there is a 

general increase in mode toward the center of the seam.

Reflected-light microscopy of polished sections 

shows that most of the chromite occurs as subhedral grains 

with rounded square and hexagonal shapes, some of which 

form short chains or clumps of three or more grains. All 

samples contain a few anhedral grains; euhedral grains are 

rare. The relative proportions of euhedral, subhedral, and 

anhedral grains do not vary systematically with stratigraphic 

height or with rock type. The only sample with an unusual 

chromite texture is the one from the base of the first olivine 

cumulate. That sample contains large anhedral chromite 

grains, some of which have ragged edges.

Samples that contain segregated chromite have 

amphibole as the most common intercumulus silicate; 

pyroxene is less abundant, and olivine is rare. All of these 

silicates are serpentinized to varying degrees. A few of the 

segregated chromite samples also contain rare anhedral 

grains of almandine-grossular garnet.

In the samples that contain chromite as an accessory 

phase, only a few chromite grains are enclosed in the 

cumulus olivine. Most of the chromite occurs between 

olivine grains, either completely enclosed by or at the grain 

boundaries between the intercumulus phases. Regardless of 

where they occur, the chromite grains are commonly 

rimmed by brown amphibole. This amphibole may be a 

late-stage intercumulus phase, or it may be the result of 

post-magmatic reaction between the chromite, the surrounding silicate, and a fluid phase. Intercumulus amphiboles elsewhere in the Stillwater are pargasitic (N. Page, oral 

commun., 1984).

Most of the accessory chromite grains are unaltered 

(fig. 6A), but some grains in serpentinized zones have 

porous, highly reflective rims of ferritchromit±chlorite(?) 

(fig. 6B). Ferritchromit, an Fe-enriched, Mg- and Aldepleted phase, is the common alteration product of 

chromite. Discussions of the occurrence and origin of 

ferritchromit are presented elsewhere (Beeson and Jackson, 

1969; Ulmer, 1974; Bliss and MacLean, 1975; Loferski, 

1986).

Close examination of the chromite at high magnification revealed that six of the samples contain chromite 

grains that show complex exsolution features. These 

chromite grains consist of two Cr-spinel(?) phases, intergrown along the spinel (100) planes, as well as ilmenite 

lamellae of variable size along the chromite (111) planes 

(fig. 7). Exsolution of two Cr-spinel phases is rare and has 

been identified only in three other areas, all of which have 

undergone amphibolite-facies regional metamorphism 

(Muir and Naldrett, 1973; Ghisler, 1976; Steele and others, 

1977; Loferski and Lipin, 1983). We originally thought that 

high-grade metamorphic Fe enrichment was a prerequisite 

for chromite compositions to be driven into the miscibility 

gap between Fe- and Al-rich Cr-spinels (see Loferski and 

Lipin, 1983). The exsolution in the M-16 .chromite indicates that metamorphism is not necessary and that primary 

magmatic chromite may be of appropriate compositions for 

exsolution to occur. The exsolved chromite occurs in 

samples throughout the drill core, as indicated in figure 5.

Magnetite occurs as rims around and veins in 

chromite and serpentinized olivine and as dusty inclusions 

in altered pyroxene and amphibole grains. Ilmenite occurs 

in trace amounts as small discrete grains and on the rims of 

chromite grains as blebs that are in optical continuity (fig. 

8). In the latter occurrence, the ilmenite formed either by 

exsolution from the chromite or by epitaxial overgrowth.

Fine needles, tentatively identified as rutile, occur in 

the silicate phases in a few samples. The rutile is most 

abundant in intercumulus plagioclase in sample 434A, at 

the top of the Basal series bronzite cumulate. The rutile 

occurs along crystallographic directions in the host mineral 

and was probably formed by exsolution during cooling.

E8 Contributions on Ore Deposits in the Early Magmatic Environment
Mg/(Mg + Fe2+) 

0.1 0.2 0.3 0.4

Cr/(Cr + Al) 

0.5 0.6 0.7

Fe 3+/pe total 

0.1 0.2 0.3 0.4

(0 

M

*O!E

CD c 

£ CD

» 2- Rock

type

Mode Grain size (mm)

4060 0.05 0.10 0.15 0.20 I I I T

o .-t: 

> c O 3

E

c

.c

Q.

CD

Q

EXPLANATION 

Olivine cumulate

Chromite seam 

Bronzite-olivine cumulate

V/W 

be Bronzite cumulate

I ^ ^j Basal bronzite cumulate zone 

A Exsolved chromite

Figure 5. Data for chromite in the M-16 drill core, plotted against stratigraphic height. Data in the first three columns 

were calculated from microprobe analyses. Bars indicate one standard deviation. Degree of serpentinization shown as: 

S, slight, less than 20 percent; M, moderate, 20-80 percent; E, extensive, more than 80 percent.

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E9
* X

to the average below 98 m) throughout the rest of the olivine 

cumulate, possibly with slight coarsening upward.

In the overlying bronzite-olivine cumulate, chromite 

shows two intervals of fining upward; the intervals are 

separated by the thin olivine cumulate layer at about 67 m. 

Olivine also becomes finer upward in the bronzite-olivine 

cumulate below the olivine cumulate layer; above the layer 

it was not possible to measure olivine grain sizes because of 

the scarcity of olivine in the top of the bronzite-olivine 

cumulate and extensive alteration in the olivine cumulate of 

the second cyclic unit. Proceeding upward in the drill core, 

the chromite is coarsest in the middle of the chromite seam; 

above the seam, the grain size is more erratic but appears to 

increase upward.

Page and others (1972) described size-graded units in 

the 110-m-thick olivine cumulate from the second cyclic 

unit in the Nye basin area of the Stillwater. A comparison of 

our findings with theirs shows that the grain size changes in 

M-16 are not as pronounced or numerous as those in the 

olivine cumulate from the second cyclic unit. In addition, 

whereas Page and others (1972) found distinct olivine 

size-graded units within a single olivine cumulate layer, in 

M-16 the grain-size breaks occur at lithologic contacts, not 

within homogeneous units. In M-16, the changes in grain 

size appear to occur even at contacts of very thin layers, 

such as the bronzite layers at 98 m and the olivine cumulate 

layer at about 66 m. These lithologic changes also correspond to slight changes in the mineral chemistry of the 

silicates and chromite, as described below.

MINERAL CHEMISTRY

Figure 6. A, Photomicrograph taken in reflected light of 

unaltered accessory chromite. The surrounding silicate is 

orthopyroxene. B, Photomicrograph taken in reflected 

light of accessory chromite altered to ferritchromit 

+chlorite(?). The surrounding silicate is serpentine. F, 

ferritchromit+chlorite(?); C, chromite.

Grain Size

The average grain sizes range from 1 to nearly 3 mm 

for olivine and from 0.04 to 0.17 mm for chromite. Plots of 

average grain sizes against stratigraphic height (figs. 4, 5) 

show little evidence for size-graded units in the drill core, 

although some changes do occur. Both olivine and chromite 

are coarse grained at the base of the Ultramafic series, then 

both minerals show sharp decreases in grain size followed 

by relatively constant size to just below the two thin 

bronzite cumulate seams at about 98 m. Both olivine and 

chromite are slightly finer grained above the two bronzite 

cumulate layers than below the layers; then the grain sizes 

coarsen upward and reach fairly constant values (very close

Silicates

Figure 4 shows Mg/(Mg+Fe2+) (XMg) plotted against 

stratigraphic height for olivine and orthopyroxene in the 

various rock types. The two minerals show similar trends, 

but the XMg of the orthopyroxene is invariably greater than 

that of the coexisting olivine because KD , the Fe-Mg 

distribution coefficient between olivine (ol) and orthopyroxene (opx), is greater than 1, where

ipol/opx_ s-y f-y \°^/fY IV \°PX

*^D v"-FeMg' ' v^Fe' Mg'

2+ 2+ XFe=Fe +/(Fe + +Mg), and

2+ XMg=Mg/(Fe+Mg).

Figure 4 shows that the silicates do not follow simple 

Fe enrichment upward. In the lowermost olivine cumulate, 

olivine compositions show three successive vertical trends. 

First, there is a jump in XMg from Fo77 at the lower contact 

to Fo79 1.5 m above the lower contact, with corresponding 

mode and grain-size changes (fig. 4). The olivine XMg 

continues to increase slightly to just below the two thin

E10 Contributions on Ore Deposits in the Early Magmatic Environment
Figure 7. Photomicrographs taken in reflected light. A, Light-gray two-phase Cr-spinel grain (C) containing blebs and 

lamellae of ilmenite (I). B, Close-up of the grain showing the fine lamellar intergrowth in the Cr-spinel that consists of 

gray Al-rich(?) lamellae along the (100) of the white Fe-rich(?) Cr-spinel host. The ilmenite lamellae and blebs are 

surrounded by medium-gray rims that are probably depleted in Fe and Ti. The dot in A corresponds to the dot in B.

bronzite seams at about 98 m, where the olivine composition is Fo80 . The second trend in olivine composition is a 

general decrease in XMg from Fogo to Fo78 in samples from 

just above the thin bronzite layers to the sample at 80 m. 

This interval is the only one in which olivine compositions 

follow the normal fractionation trend of Fe enrichment 

upward. The third trend is one of Mg enrichment upward 

from Fo78 to Fo81 at the top of the first olivine cumulate, 

from 80 m to 70 m.

In the bronzite-olivine cumulate, olivine compositions fluctuate between Fo77 and Fo80 . Below the thin 

olivine cumulate layer at about 67 m there is a decrease 

followed by an increase upward, and above the layer Fo 

decreases upward.

In the second olivine cumulate, olivine compositions 

show decreasing XMg upward, from Fo81 below the 

chromite seam to Fo79 above it. Within the chromite seam 

olivine is rare; it was found in one sample, and its 

composition was Fo79 . The orthopyroxene, both cumulus 

and intercumulus, follows compositional trends that parallel 

those shown by olivine.

Figure 8. Photomicrograph taken in reflected light of 

ilmenite blebs (I) on the rim of a chromite grain. The 

ilmenite blebs are in optical continuity.

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E11
Table 1. Correlation coefficients for properties of the 

silicates in the M-16 drill core

[A^g =Mg/(Mg+Fe2+) for olivine; X^=Mg/(Mg+Fe2+) for orthopyroxene. Level of significance given as percentage]

Properties being 

correlated

ydl yopx

<g-mode ...................

Olivine:

Correlation 

coefficient

0.82

.51

.04

-.11

Level of 

significance

99.9

95-99

«80

<80

Correlation coefficients for various combinations of 

the properties measured on olivine and orthopyroxene are 

shown in table 1. The closest correlation is a positive one 

between the XMg of olivine and that of coexisting orthopyroxene. A positive correlation also exists between the 

olivine mode and Fo content. This is probably because 

olivine that reacts with the liquid and is partially resorbed 

will be more Fe rich than olivine that does not continuously 

react with liquid, by, for instance, being armored by 

orthopyroxene. No significant correlation was found 

between the grain size of olivine and either its mode or its 

composition.

Chromite

Chromite compositions are shown plotted against 

stratigraphic height in figure 5. The degree of serpentinization for each sample is also indicated in figure 5, in order to 

show that the variation in XMg of chromite is not related to 

the degree of late-stage serpentinization, a process that may 

cause Fe enrichment of chromite. In fact, the sample at the 

top of the olivine cumulate at 70 m has been extensively 

serpentinized, yet chromite in that sample has the highest 

XMg of any sample in the olivine cumulate. The stratigraphic locations of the chromite that shows exsolution 

features are also shown in figure 5. Exsolved chromite 

usually has low XMg and high FFe3+ (Fe3+/(Cr+Al+Fe3+ )). 

However, the exsolution is not related to the amount of 

serpentinization and thus is distinct from the typical 

ferritchromit+chlorite alteration of chromite.

The patterns of variation in chromite compositions 

with stratigraphic height are not clear cut because of the 

large compositional variations within samples. Individual 

chromite grains are generally not zoned; the compositional 

range within samples mainly occurs because chromite 

enclosed in olivine is compositionally distinct from that in 

orthopyroxene, as will be discussed below. Also, the

changes in chromite composition are more complex than 

those of olivine and orthopyroxene (compare figs. 4, 5) in 

part because chromite has five major variables in composition (Mg, Fe2+ , Cr, Al, and Fe3+), whereas olivine and 

orthopyroxene have two (Mg and Fe2+ ). In addition, 

chromite compositions are more sensitive to small /O2 

changes and to subsolidus reequilibration, particularly 

where chromite is an accessory phase.

Figure 5 shows that chromite chemistry changes 

abruptly at the Basal series-Ultramafic series contact. At the 

base of the olivine cumulate, just above the contact, there is 

a sharp increase in XMg , followed by a gradual decrease up 

to the two thin bronzite cumulate layers. Over the same 

interval, both XCr (Cr/(Cr+Al)) and Fe3+/Fetotal drop 

sharply just above the contact, then both ratios gradually 

decrease upward to the bronzite seams. The bronzite layers 

mark an abrupt change in chromite chemistry. XMg starts 

out lower above the bronzite layers than below the layers 

then gradually increases upward to the top of the olivine 

cumulate. Over the same interval, both XCr and Fe3+/Fetotal 

start out higher above the bronzite seams than below the 

seams, and both ratios show a gradual decrease to about 82 

m, followed by relatively constant values upward, and 

finally a sharp drop at the top of the olivine cumulate.

There is another sharp break in chromite chemistry at 

the base of the bronzite-olivine cumulate. From the base of 

the bronzite-olivine cumulate to the thin olivine cumulate 

layer at about 65 m, XMg increases then abruptly decreases, 

and this pattern is repeated above the thin olivine cumulate 

layer. Below the thin olivine cumulate, XCr increases 

steadily upward and Fe3+/Fetotal decreases then increases 

upward. Above the thin olivine cumulate, XCr decreases 

toward the middle of the layer, and Fe3+/Fetotal shows a 

slight decrease upward. The overall decrease in Fe3+/Fetotal 

continues upward through the base of the second cyclic 

unit, through the chromite seam, and above the chromite 

seam. In the second cyclic unit, there is a drop then an 

increase in XMg toward the top of the chromite seam, then 

a sharp decrease above the seam, whereas XCr gradually 

decreases from the base of the chromite seam upward.

In summary, the compositions, grain sizes, and 

modes of olivine, orthopyroxene, and chromite commonly 

show breaks at stratigraphic levels in the drill core that 

correspond to changes in lithology, regardless of the thickness of the new lithologic layer. Therefore, as the cyclic 

units crystallized and new phases precipitated, the result 

was not only new lithologic layers but also changes in both 

the chemical and physical properties of the minerals that 

were precipitating. The breaks between the thick layers are 

not surprising, because these layers represent the start of 

prolonged changes in phase equilibria. However, it is 

surprising that breaks also occur above and below the very 

thin layers, and they indicate a balance between even the

E12 Contributions on Ore Deposits in the Early Magmatic Environment
Mg/(Mg + Fe 2+) 

0.2 0.3 0.4

Cr/(Cr + Al) 

0.50 0.55 0.60 0.65

Fe3+/Fetotal 

0.10 0.12 0.14 0.16

Rock 

type

Mode 

20 40 60

Grain size (mm) = £ 

0.05 0.10 0.15 0.20 oi

62-

EXPLANATION 

\y ^ '] Olivine cumulate 

m| Chromite cumulate 

11 I | || Bronzite-olivine cumulate

Figure 9. Stratigraphic variation in chromite composition, mode, and grain size in and near the chromite seam within the 

M-16 drill core. Dots are averages; bars indicate one standard deviation.

short-lived oscillations in magma chemistry and the various 

properties of the crystallizing phases.

Previous studies have shown that an olivine cumulate 

in the Muskox intrusion (Irvine, 1980) and one in the 

second cyclic unit in the Still water (Page and others, 1972) 

are composed of intervals that show cyclic changes in 

mineral chemistry and corresponding changes in grain size 

within seemingly homogeneous layers. As pointed out by 

Page and others (1972), these intervals are suggestive of 

cyclic processes within individual layers rather than simple 

continuous fractionation. In the M-16 core, the changes in 

grain size and the oscillatory changes in mineral chemistry 

provide evidence of similar cyclic processes. The lesser 

thickness of the M-16 cyclic unit compared with those 

studied previously may be the reason that oscillatory vertical trends were not as well developed in M-16 as in the 

previously studied units.

Thus far we have considered variations in chromite 

chemistry with respect to Stratigraphic height. Another

point of interest is a comparison of the chemistry of 

accessory and segregated chromite. Previous studies on 

both the Stillwater and other layered complexes have noted 

a positive correlation between chromite mode and its XMg 

(Irvine, 1967; Jackson, 1969; Cameron, 1977; Hamlyn and 

Keays, 1979). This correlation has been attributed to postcumulus and (or) subsolidus reaction and reequilibration 

between chromite and silicates. In M-16, large compositional differences between accessory and segregated 

chromite were not found, although the segregated chromite 

samples are on the high end of the range in XMg values for 

all of the samples (fig. 5). Within the chromite seam itself, 

however, there are some differences in chemical composition that correspond to differences in the chromite mode. 

Figure 9 shows the compositions of the samples in the 

chromite seam and of those just above and below the seam. 

Two samples in the chromite seam, at 57.8 m and 58.7 m, 

are olivine cumulates that contain accessory chromite, and 

they have lower XMg values (and smaller average grain size)

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E13
than the segregated chromite samples. When comparison is 

made only among samples that contain segregated 

chromite, however, it can be seen that the mode and XMg do 

not correlate exactly. In the samples from 58.4 to 58 m, the 

average XMg shows a slight increase from 0.32 to 0.35, 

whereas the chromite mode drops from over 60 percent to 

about 15 percent. Therefore, in M-16, the mode and XMg of 

chromite show correlation only when samples with less than 

5 percent chromite are compared to those with more than 15 

percent chromite. There is no correlation between mode and 

XMg for samples with more than 15 percent chromite 

because XMg shows little change.

Accessory and segregated chromite compositions are 

shown in figure 10, which illustrates the point that the 

segregated chromite samples are not chemically distinct 

from the accessory chromite but do fall toward the high end 

of XMg values and the low end of Fe3+ values. The 

Stillwater chromite compositional fields shown in figure 10 

are from Irvine (1967) and are based on 14 analyses of 

segregated chromite from various stratigraphic levels. Segregated chromite analyses from the present study fall 

outside Irvine's (1967) Stillwater field and therefore show 

that the field must be expanded to lower values of XMg and 

XCr to include the M-16 segregated chromite compositions.

The chromite compositions (fig. 10A) show a negative correlation between XMg and XCr, indicating substitution of the components MgAl2O4 and FeCr2O4 in the 

chromite. The correlation coefficient for XMg plotted 

against XCr is 0.61, with a significance of 99.9 percent. 

Figure 10A also shows that the three samples from the Basal 

bronzite cumulate plot in a small, compositionally distinct 

group of high XCr. The seven samples from the bronziteolivine cumulate span a compositional range as wide as that 

of samples from the much thicker olivine cumulate. Figure 

IOB indicates that FFe3+ shows a broad negative correlation 

with XMg ; the correlation coefficient is 0.55, with a 

significance of 99.9 percent.

The chromite compositions in figure 10 fall along 

olivine equipotential surfaces (see Irvine, 1965; Jackson, 

1969), indicating that although the chromite spans a wide 

compositional range, it is in equilibrium with olivine of a 

limited range of Fo contents. Analytical data on M-16 

olivines bear this out (as was shown above).

A comparison of the compositions of segregated 

chromite from the B (in M-16), G, and H chromite seams 

from the Mountain View area is shown in figure 11. We 

calculated the G and H chromite cation ratios from analyses 

published by Jackson (1969). Figure 11 shows the trend, 

which was noted by Jackson (1970), of Mg enrichment 

upward in segregated chromite from the A to the H seam. 

The trend is reversed to one of Fe enrichment in the 

chromite seams above the H seam. Raedeke (1982) found a 

similar trend of Mg enrichment in the lower 400 m of the

0.9 

0.8

= 0.7

o

5 0.6

0.5

0.4 

0.3

CD 

u_

+ 0.2

o

0.1

0.0

1.0 0.8 0.6 

Mg/(Mg

0.4 

Fe2+ )

0.2 0.0

EXPLANATION

 Segregated chromite

Compositional fields:

 Stillwater Complex

 Stratiform intrusions

Accessory chromite

e in chromite seam

o in olivine cumulate

+ in bronzite-olivine cumulate

n in Basal bronzite cumulate

Figure 10. Compositional plots of chromite from the M-16 

drill core from microprobe analyses; each point plotted is 

the average for the sample. A, Cr/(Cr+AI) plotted against 

Mg/(Mg+Fe2+ ); B, Fea!+/(Cr+AI + Fe3+ ) plotted against 

Mg/(Mg+Fe2+ ). Compositional fields from Irvine (1967).

Ultramafic series followed by constant Fe contents in 

olivine and orthopyroxene in the upper part.

Chromite compositions are shown plotted with 

respect to Cr, Al, and Fe3+ in figure 12. As is typical for 

primary magmatic chromite, most compositions, particularly those for segregated chromite, plot near the Cr-Al join. 

The solvus in figure 12 is based on the compositions of 

exsolved Fe- and Al-rich Cr-spinel pairs from Red Lodge, 

Mont., and is probably valid for about 600 °C (Loferski and 

Lipin, 1983). Exsolution in chromite is rare because most 

magmatic chromite compositions fall outside the miscibility

E14 Contributions on Ore Deposits in the Early Magmatic Environment
0.8

0.7

0.5

0.4

EXPLANATION 

 this study

~}from Jackson (1969)

1.0 0.8 0.6 

Mg/(Mg

0.4 

Fe2+ )

0.2 0.0

gap, which is between Al- and Fe-rich compositions. If Fe 

enrichment occurs, however, the chromite compositions 

may be driven into the miscibility gap, and, if kinetics 

permit, exsolution will occur. Chromite from the M-16 drill 

core is unusually Fe rich for the Stillwater; thus some 

compositions fall within the miscibility gap. Of the six 

Stillwater samples that contain exsolved chromite, three fall 

inside and three outside the miscibility gap defined by the 

Red Lodge chromites. Of those outside the miscibility gap, 

two are close in composition to other samples in which

Figure 11. Compositional fields of segregated 

chromite from the B, G, and H chromite seams of the 

Stillwater Complex. G and H chromite compositions 

are from Jackson (1969).

EXPLANATION

Chromite

 Segregated

Accessory

c in chromite seam

o in olivine cumulate

+ in bronzite-olivine cumulate

n in Basal bronzite cumulate

A exsolved

Miscibility Gap 

at ~ 600°C

Figure 12. Microprobe analyses of chromite from the M-16 drill core plotted with respect to Cr-AI-Fe3+ . Position of the 

miscibility gap is for chromite from Red Lodge, Mont., from Loferski and Lipin (1983).

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E15
exsolution was not detected. It is possible that exsolution 

occurs in other samples, but it may be too fine to detect in 

the optical microscope.

Of the minor elements, only TiO2 reaches significant 

quantities in the chromite; it ranges from 0.5 to 2.5 percent 

and is typically about 1 percent. NiO, ZnO, and MnO are 

typically less than 0.5 percent.

As alluded to above, we found that accessory 

chromite grains in or near olivine are compositionally 

distinct from those enclosed in orthopyroxene, and this 

results in the wide compositional ranges, especially 

of Cr/(Cr+Al), for individual samples. Figure 13 shows 

compositional fields for accessory chromite grains enclosed within olivine, orthopyroxene, and amphibole for 

all of the samples analyzed in M-16. Although the 

fields overlap when all of the samples are plotted together, 

figure 13 illustrates that the field for chromite in orthopyroxene extends to higher XCr values and slightly lower 

XMg values than the field for chromite in olivine. Three 

samples from the olivine cumulate were investigated in 

detail, and figure 14 shows a plot of the compositions of 

individual grains for one of those three samples. The other 

two samples show similar compositional relations; there is 

little or no overlap between the compositional fields within 

individual samples. Figure 14 shows that, except for one of 

the chromite grains in olivine, those in intercumulus pyroxene and olivine plot in distinct and non-overlapping compositional fields. Grains in orthopyroxene are higher in XCr 

and Fe3+ than those in olivine. In this sample, XMg of grains 

in olivine does not differ from XMg of those in orthopyroxene, whereas another sample showed slightly lower XMg for 

grains in orthopyroxene.

Table 2 shows the oxide weight percent as determined 

by microprobe analysis of a chromite grain in olivine and 

one in orthopyroxene from sample 289. The most dramatic 

differences are in A12O3 , which is 28 percent for chromite 

in olivine and 15 percent for chromite in orthopyroxene, 

and Cr2O3 , which is 32 percent and 44 percent for chromite 

in olivine and orthopyroxene, respectively. Total Fe is 32 

percent for both, and MgO is about 6.2 percent and 6.5 

percent for chromite in olivine and orthopyroxene, respectively. Of the minor elements, ZnO contents are commonly 

higher for chromite in olivine, whereas TiO2 , NiO, and 

MnO do not show consistent differences in abundance for 

chromite in olivine compared with that in orthopyroxene.

The effect of chromite mode on the compositions, 

especially XMg , of chromite and associated Fe-Mg silicates 

has been well established by other workers as noted above. 

However, differences in accessory chromite composition 

relative to the enclosing silicate are less well known. 

Cameron (1975) noted higher A12O3 but lower XMg in 

accessory chromite within olivine relative to that in postcumulus bronzite in the Bushveld. He concluded that such 

compositional differences were due to postcumulus and (or) 

subsolidus reaction but that changes in composition relative

to chromite mode reflect primary magmatic variations. Two 

other studies, by Hamlyn and Keays (1979) of chromite in 

the Panton Sill, Australia, and by Henderson (1975) of 

chromite from the Rhum intrusion, Scotland, showed 

increases in Cr and Al in chromite in the intercumulus 

minerals outside of olivine grains. Both studies concluded 

that the composition of chromite enclosed in olivine is 

unchanged since the time of its inclusion, whereas the 

chromite outside of olivine grains is enriched in MgAl2O4 

by postcumulus reaction with liquid and cumulus silicates.

The relative compositional changes in the M-16 

chromite are different from those shown in the previous 

studies. Nevertheless, in M-16, as in chromite from the 

previous studies, the fundamental question is whether the 

different compositions of chromite in olivine and orthopyroxene reflect primary magmatic differences, postcumulus 

or subsolidus reequilibration, or late-stage alteration related 

to serpentinization.

Primary magmatic compositions are probably not 

preserved in accessory chromite simply because of the sheer 

volume of silicates relative to chromite available for chemical reaction. Calculated equilibration temperatures for 

chromite-olivine pairs from M-16 support this conclusion. 

Figure 15 shows olivine-spinel equilibration temperatures 

based on the empirical calibration of Evans and Frost (1975) 

and from data by Engi (1978, as reported by Henry and 

Medaris, 1980). M-16 chromite samples that have FFe3+ in 

the range of the diagram (for which FFe3+=0.05) yield 

temperatures of about 600-700 °C, which are well below 

the probable crystallization temperatures of chromite in the 

complex. The 600-700 °C temperatures strongly suggest 

subsolidus reequilibration. Numerous studies on chromiteolivine temperatures using this method invariably obtain 

temperatures of 600 °C or higher (see also Loferski and 

Lipin, 1983), suggesting that 600 °C is the temperature 

below which diffusion becomes too sluggish for any substantial Fe-Mg exchange between olivine and chromite.

Another indication of subsolidus reaction is that both 

olivine and orthopyroxene show zoning of Cr2O3 over 

distances as much as 100 (Jim from chromite grains. Figure 

16 shows a plot of weight percent Cr2O3 in pyroxene against 

distance from an enclosed chromite grain; Cr2O3 decreases 

steadily from about 0.7 percent to less than 0.1 percent with 

increasing distance from the chromite grain. In addition, the 

Fo content of olivine is 1-2 mole percent higher immediately adjacent to an enclosed chromite grain. This evidence 

confirms Irvine's (1965) suggestion that olivine gains Mg 

while chromite gains Fe during subsolidus reequilibration.

The lower Al in chromite within orthopyroxene 

compared to that in olivine may be due to the fact that 

orthopyroxene can accommodate diffusion of Al out of 

chromite during subsolidus reequilibration. Therefore, Fe3+ 

and Cr are relatively increased in the chromite by difference. Chromite in the bronzite cumulate of the Basal series 

is lower in XMg and lower in Al than chromite in the

E16 Contributions on Ore Deposits in the Early Magmatic Environment
0.8

0.6

0.4 -

0.2 

0.4

EXPLANATION

in olivine

 in orthopyroxene

 in amphibole

O.Q

EXPLANATION

in olivine

- in orthopyroxene

 in amphibole

1.0 0.8 0.6

Mg/(Mg

0.4

Fe2+)

0.2 0.0

Figure 13. Compositional fields of accessory chromite in 

olivine, orthopyroxene, and amphibole, for all M-16 drill 

core samples. A, Cr/(Cr+AI) plotted against Mg/ 

(Mg+Fe2+ ); fi, Fe3+/(Cr+AI + Fe3+ ) plotted against Mg/ 

(Mg+Fe2+ ).

olivine-bearing rocks of the overlying olivine cumulates 

(see figs. 5, 10). Because the chromite is affected by 

reaction with the bronzite regardless of whether the bronzite 

is cumulus or intercumulus, a subsolidus reaction is 

implied.

In the M-16 samples, accessory chromite also is 

commonly enclosed in amphibole, and this chromite also 

differs compositionally from chromite in olivine or orthopyroxene. As described above, the amphibole can occur as a 

late-stage intercumulus phase or a reaction rim around 

chromite included in a silicate. Chromite grains with

u./

-0.6

o

5 0.5

0.4 

0.2

<r

u.

*+ 0.1

O

A" 

0 

u_

0.0 

1

I I I 

A

i\ /\

M 0

_ Z5

9)

Chromite

O in olivine CDCD 

A in pyroxene

I I I 

B ^

A

Chromite Qf^®

O in olivine ^^ 

A in pyroxene

i i I

.0 0.8 0.6 0.4 0.2 0

Mg/(Mg + Fe 2+ )

Figure 14. Compositional differences between accessory 

chromite in olivine and in pyroxene in one sample from 

the M-16 drill core. Each point represents one grain.

amphibole rims show a variety of compositional trends. In 

some grains of orthopyroxene, chromite grains with 

amphibole rims have lower XMg and higher XCr and YFe^ 

than chromite without amphibole rims, whereas, in other 

samples, chromite grains with amphibole rims have higher 

XMg and lower XCr and YFe^. Thus, although reaction trends 

are not clear cut, accessory chromite compositions have 

been affected by postcumulus or subsolidus reequilibration 

with the surrounding silicates.

WHOLE-ROCK TRACE ELEMENTS

Trace-element analyses were performed on wholerock samples at the analytical laboratories of the U.S. 

Geological Survey. Details of analytical techniques are 

given elsewhere (Loferski and others, 1984). The samples 

were analyzed for Cr, Ti, V, Zn, and Mn, which should 

reflect variations in the oxide minerals in the rocks, and Ni, 

Co, Pt, Pd, and Rh, which should reflect the amount of 

sulfides. The sulfides identified in the polished sections 

include chalcopyrite, pyrrhotite, and pentlandite. In most 

samples they compose much less than 1 percent of the rock 

and occur mainly as specks in serpentinized areas. In a few 

samples, however, they are more abundant and reach 1 to 2 

modal percent. Samples with high modal sulfide are indicated in figure 17; all four are located at the base of a layer.

The trace-element contents are plotted against stratigraphic height in figure 17. The amount of Cr is fairly

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E17
Table 2. Representative microprobe analyses of accessory 

chromite in olivine and orthopyroxene in sample 289 from 

the first olivine cumulate

[calc=calculated by using calculated Fe3+ ]

4.0

Olivine

Chromite in

Orthopyroxene

Oxides, in weight percent

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

TiO, .................

A1,O3 ................

Cr,O3 ................

FeOtotal ...............

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

NiO .................

ZnO .................

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

Total 

FeO calc. .............

Fe-,O3 calc ............

New total

0.04

.87

27.70

32.08

32.41 

6.16

.07

.62

.34

100.29 

27.07

5.93

100.88

0.04 

1.01 

15.30 

44.15 

32.45 

6.52 

.14 

.05 

.36

100.02 

25.08 

8.19

100.84

Formula proportions1

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

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

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

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

Fe3+ .................

Fe2+ .................

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

Ni ...................

Zn. ..................

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

0.001

.020

1.023

.794

.140

.709

.288

.002

.014

.009

0.001 

.025 

.595 

1.150 

.203 

.691 

.320 

.004 

.001 

.010

3.0

Q 

5* 

c 2.0

1.0

0.0

!!

(0.12) 

(0.12) 

(0.20)'

0.0 0.2 0.4 0.6 0.8 1.0

Figure 15. Ln KD plotted against

olivine pairs.

sp = spinel; ol = olivine;

> for M-16 chromite-

'Cation sum=3.000.

constant at about 2,000 ppm throughout the drill core except 

in the chromite seam, where it is as much as 40,000 ppm. 

Cr is accommodated mainly in chromite. In addition, as 

much as 0.6 percent is in orthopyroxene, as much as 1 

percent in clinopyroxene, and a small amount is present in 

olivine, amphiboles, and micas. The constant amount of 

whole-rock Cr in most of the core is a reflection of the 

constant chromite mode, because even a small change in 

chromite mode would cause a large change in the wholerock Cr content.

Most of the Ti, V, and Zn are in solid solution in the 

chromite; therefore, their concentrations show large 

increases in the chromite seam. Throughout the rest of the 

core, Ti, V, and Zn remain fairly constant at about 1,000, 

100, and 100 ppm respectively. A slight increase in the Zn 

content from 100 to 360 ppm occurs just above the base of 

the second olivine cumulate at 61 m and corresponds to 

increases in Ni, Co, and sulfide mode. The most probable

Y*£= Cr/(Cr+AI + Fe3+ ); and 

V|P3+ = Fe3+/(Cr+AI + Fe3+ ).

Numbers in parentheses next to symbols are Y5^ of the 

chromite. The isotherms are for V^ = 0.05. The dashed 

isotherm is from Evans and Frost (1975); the solid isotherms are from a thesis by Engi (1978; as reported by 

Henry and Medaris, 1980).

explanation is that some of the Zn in that sample is in 

sulfide minerals. There is no corresponding increase in Ti, 

V, or Cr, which would indicate an increase in oxide-mineral 

content, nor is there an increase in the modal amount of 

chromite.

Titanium is also in rutile, which occurs as needles in 

plagioclase grains, and in ilmenite, which occurs as separate grains and as exsolution lamellae in chromite. Petrographic examination of the sample at 97.3 m showed that

E18 Contributions on Ore Deposits in the Early Magmatic Environment
.70

.60

.30

.20

.10

O

O

o

o

o o

o

o

o

5 10 15 20 25 

Distance from chromite grain (|am)

30

Figure 16. Cr2O3 contents in orthopyroxene plotted 

against distance from an enclosed chromite grain. Each 

point represents one microprobe analysis.

the slight increase in Ti was caused by an increase in 

ilmenite lamellae in chromite, and the high value at the top 

of the Basal zone's bronzite cumulate at 111.5 m was 

caused by both ilmenite grains and rutile in plagioclase. A 

small amount of Ti is also present in amphiboles and micas.

The concentration of Mn fluctuates throughout the 

core from about 800 to 2,500 ppm. The Mn contents 

correspond to the Cr contents in the chromite seam but not 

in the rest of the samples, an indication that Mn is entering 

phases other than oxide minerals, probably silicates. One 

such phase is garnet, which contains as much as 4 percent 

MnO as shown by microprobe analyses. The garnet, however, is uncommon and cannot account for all of the Mn. 

Olivine and pyroxenes can also contain small amounts of 

Mn.

Co is concentrated in sulfides; Ni occurs in sulfides 

and in olivine. Increases in both Co and Ni at 111, 67.8, 

and 60.6 m reflect increases in modal sulfides at the base of 

each new rock type in the core. Ni contents are low in the 

bronzite cumulate of the Basal series, about 500-600 ppm. 

A jump to 2,600 ppm occurs at the base of the olivine 

cumulate, then there is an increase toward the middle of the 

olivine cumulate followed by a steady drop toward the top 

of the olivine cumulate. The general trend of decreasing Ni 

continues upward through the bronzite-olivine cumulate 

except for samples at the base and one in the thin olivine

cumulate. Ni contents in the second olivine cumulate are 

high at the base then decrease upward.

The general trend of decreasing Ni content near the 

top of the first olivine cumulate and through the bronziteolivine cumulate is an indication of normal fractionation at 

the top of the first cyclic unit, because as crystallization 

proceeds, Ni is partitioned into the olivine and decreases in 

the melt. The NiO weight percent in olivine also shows a 

decrease at the top of the olivine cumulate, an indication of 

normal fractionation. However, the NiO in olivine does not 

show the general decrease through the bronzite-olivine 

cumulate. Instead, there is a sharp increase at the base of the 

bronzite-olivine cumulate followed by a decrease upward. 

The trace-element indication of normal fractionation at the 

top of the olivine cumulate is contradicted by the mineral 

chemistry of olivine, chromite, and intercumulus orthopyroxene, all of which show increases in XMg upward at the 

top of the first olivine cumulate (see figs. 4, 5).

Within the chromite seam, variations in Mn, Ti, V, 

and Zn correspond to variations in Cr content, indicating 

that those elements are in solid solution in chromite. Ni 

shows a negative correlation to Cr, probably a reflection of 

varying amounts of olivine in the samples. Co, by contrast, 

correlates directly with Cr, an indication of increased 

amounts of sulfides in chromite-rich rocks.

Like Co, and some of the Ni, the platinum-group 

elements (PGE), Pt, Pd, and Rh, are concentrated in 

sulfides, and their abundances are shown plotted against 

stratigraphic height in figure 18. In the samples analyzed, 

the PGE range from below detection limits to as much as 

0.19 ppm Pt, 0.74 ppm Pd, and 0.069 ppm Rh. Because of 

their affinity for sulfides, the PGE show patterns of variation that are similar to those of Ni and Co, such as increases 

in PGE at the base of new rock types, at 111 and 67.8 m, 

and in Pd at 60.6 m and Rh at 56 m.

The highest PGE values occur in the chromite seam, 

most likely because of the increase in sulfide mode. Sulfide 

minerals identified in the samples include chalcopyrite, 

pyrrhotite, and small amounts of pentlandite. The sample at 

Him contains the second highest concentration of PGE 

and a corresponding increase in modal sulfides. The PGE 

contents in the M-16 samples are well below levels of 

economic potential. High concentrations of PGE occur in 

the Stillwater's Lower Banded series in the J-M Reef, 

which is about 400 m above the top of the Ultramafic series. 

Within the J-M Reef, Pt is as much as 15 ppm, and Pd 50 

ppm (Todd and others, 1982), well above the PGE contents 

in M-16. In a study of PGE concentrations in chromite 

seams throughout the Stillwater, Page and others (1969) 

found that most chromite seams have PGE contents similar 

to those in M-16. Parts of the A seam, however, are quite 

high in PGE with maxima of 8 ppm Pt, 11 ppm Pd, and 1.7 

ppm Rh.

Figure 19 shows all of the M-16 samples plotted with 

respect to Pt, Pd, and Rh ratios. The samples are generally

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E19
Cr

110

EXPLANATION

Olivine cumulate 

Chromitc seam 

Bronzite-olivine cumulate

be I Bronzite cumulate

Basal bronzite cumulate zone 

* 1-2 modal percent sulfide

Figure 17. Whole-rock trace-element contents (in parts per million) plotted against stratigraphic height for the M-16 drill 

core.

£20 Contributions on Ore Deposits in the Early Magmatic Environment
high in Pd/(Pd+Pt+Rh). The various rock types do not plot 

in distinct fields with respect to Pt, Pd, and Rh. In figure 

19, the dashed lines enclose the field of ratios found by 

Page and others (1969) for Stillwater rocks, and the M-16 

rocks fall within that field.

DISCUSSION

Origin of Chromite Seams

Various mechanisms have been proposed to explain 

the origin of chromite seams in layered complexes. The 

present study may shed some light on the question of which 

mechanism operated in the Stillwater.

On the basis of empirical evidence from the Muskox 

intrusion and experimental evidence, Irvine (1977) proposed that in the system olivine-chromite-SiO2 , the olivinechromite cotectic is curved as shown in figure 20. He 

proposed that chromite seams are formed by mixing of 

differentiated magma at B with fresh magma at A, which 

places the hybrid magma in the chromite primary-phase 

field. Part of the evidence Irvine used to document the 

curvature of the olivine-chromite cotectic is a steady and 

cyclic drop in accessory chromite mode, from about 4 

percent to 1 percent, with increased fractionation in the 

Muskox. The drop in chromite mode occurs within an 

olivine cumulate over intervals as much as 120 m thick. 

Crystallization along a curved cotectic would cause a drop 

in chromite mode because the point of intersection of the 

tangent to the curve with the olivine-chromite tie line, 

which shows the relative amounts of those phases crystallizing, would change to progressively lower amounts of 

chromite as crystallization proceeds.

In M-16, however, the accessory chromite mode 

remains constant at about 1 percent in the olivine cumulate, 

an indication either that the olivine-chromite cotectic is 

straight or that fractionation did not proceed far enough to 

exhibit what curvature there might be. Therefore, we did 

not find evidence to support magma mixing as a mechanism 

for the origin of the chromite seams in the Stillwater.

Cameron and Desborough (1969) suggested that 

chromite seams are formed by an increase in f02 in the 

magma, but Cameron (1977, 1980) later pointed out difficulties with this hypothesis. The hypothesis was initially 

supported by experimental work, which showed that 

increased fO2 favors the formation of Fe-bearing spinels in 

basaltic rocks. In addition, studies of mineral chemistry in 

chromite seams showed that segregated chromite commonly 

has higher XMg than does accessory chromite, a phenomenon that can be accounted for by higher JD2 . In M-16, XMg 

is higher in segregated than in accessory chromite within the 

seam itself, but when the XMg of chromite in the olivine 

cumulate above is compared with that below the seam, no 

difference in the XMg was found. In addition, change in/O2 

should be reflected in changes in Fe3+/Fetotal in chromite,

but in M-16 the Fe3+/Fetotal is about the same in the 

chromite seam as in accessory chromite in the olivine 

cumulate above and below the seam. Therefore, there is no 

evidence in M-16 to support changes inyO2 as the mechanism of chromite-seam formation.

Another hypothesis, proposed by Cameron (1980), 

involves changes in total pressure, which cause shifts in 

phase boundaries and therefore changes in the nature of the 

phase or phases crystallizing. We find this hypothesis 

appealing for several reasons. One major problem in interpreting the origin of chromite seams is the extreme lateral 

continuity of at least some of the seams over great distances 

for such relatively thin layers. This continuity implies that 

they were formed by a process that was propagated almost 

instantaneously throughout the magma chamber and that 

varied rapidly and repeatedly. Change in total pressure 

would be such a process. In addition, such changes would 

be expected to occur during the crystallization history of a 

magma chamber and could be caused by shifts in the floor 

of the chamber, or by venting of the magma to a volcanic or 

subvolcanic system, or by influxes of new magma into the 

chamber. Experimental evidence has shown that increased 

pressure causes an increase in the size of the spinel phase 

field and would therefore favor its crystallization (Osborn, 

1978). It is not yet clear how to test this hypothesis 

empirically, because the effect on the composition of 

chromite is not well known (Cameron, 1980) and may be 

too small to detect.

Infiltration Metasomatism

In his study of the Muskox intrusion, Irvine (1980) 

found that sharp breaks in cumulus-mineral chemistry and 

whole-rock chemistry occur as much as 20 m upsection 

from modal breaks between cyclic units. Irvine accounted 

for this offset by the process of infiltration metasomatism, 

in which cumulus minerals are changed in composition by 

reaction with intercumulus liquid that moves upward by the 

compaction of underlying layers.

In M-16, the abrupt shifts in whole-rock Ni contents 

and in XMs of chromite and olivine occur less than 1 m 

above changes in lithology. Therefore, infiltration metasomatism, if it occurred at all, did not occur over large 

distances. Another example of the small amounts of infiltration metasomatism in M-16 can be seen in the modal 

break at the contact between the Basal series and the 

Ultramafic series. If liquid were moving upward from the 

Basal bronzite cumulate into the overlying olivine cumulate, we would expect to see a gradual rise in olivine mode 

in the olivine cumulate with increasing distance from the 

contact with bronzite cumulate of the Basal series, because 

the liquid in equilibrium with the bronzite and plagioclase 

would dissolve olivine. In M-16, however, the break in 

modal olivine at the contact is sharp. Olivine is absent in the 

Basal series bronzite cumulate; within several centimeters

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E21
Pt

0.05 0.10 0.15 0.20 0.25

Q. 

0) 

Q

EXPLANATION

I'^.'^j Olivine cumulate 

^H Chromite seam

Bronztte-olivine cumulate

be I Bronzite cumulate

Basal bronzite cumulate zone 

* Below detection limit

E22 Contributions on Ore Deposits in the Early Magmatic Environment
Figure 18. Whole-rock platinum-group-element contents (in parts per million) plotted against stratigraphic 

height for the M-16 drill core.

above the contact, it makes up 43 percent of the rock, and 

within 2.5 m of the contact it climbs to 65 percent, where it 

remains throughout most of the rest of the olivine cumulate. 

Therefore, in M-16, infiltration metasomatism was not an 

important process, and if it did occur, it was only effective 

over short vertical distances of 1-3 m.

Comparison of Cyclic Units in the Stillwater

The Stillwater's Ultramafic series shows an initial 

trend of Mg enrichment upward. This trend was first noted

by Jackson (1970) on the basis of his studies of the 

chemistry of olivine and chromite from the various chromite 

seams. He found that Mg enrichment occurs up to the H 

chromite seam, and above the H there is a reversal to Fe 

enrichment upward. Similarly, in her study of silicate 

mineral chemistry in the Ultramafic series, Raedeke (1982) 

found that Mg enrichment upward occurs both within and 

between cyclic units for the first 400 m of the Ultramafic 

series and that compositions then remain constant throughout the rest of the Ultramafic series. She showed that olivine 

XMg is about 0.79 at the base of the Ultramafic series and 

about 0.84 at about 400 m above the base. Over the same 

interval the XMg of orthopyroxene ranges from about 0.76 to 

about 0.85, and it remains at about 0.85 over the rest of the 

Ultramafic series.

EXPLANATION

-^- Bronzite-olivine cumulate 

A Olivine-chromite cumulate 

O Olivine cumulate

 Chromite cumulate

Figure 19. Whole-rock M-16 drill-core samples plotted with respect to Pt-Pd-Rh. The dashed lines enclose the field of 

Ultramafic-series samples from Page and others (1969).

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E23
Quartz'

Orthopyroxene

Olivine Chromite -

Figure 20. Phase relations in the olivine-chromite-SiO2 

system shown in weight percentage, from Raedeke (1982), 

modified from Irvine (1977). Mixing of an olivine-saturated 

liquid on the olivine-chromite cotectic (A) with a more 

differentiated orthopyroxene-saturated liquid (B) results 

in a hybrid liquid in the chromite field. Ol, olivine; Opx, 

orthopyroxene.

Figure 21 shows the mineral chemistry plotted against 

stratigraphic height for three cyclic units in the Stillwater 

Complex: cyclic unit 1 in Mountain View (this study), the 

olivine cumulate from cyclic unit 2 in Nye Basin (Page and 

others, 1972), and approximately cyclic unit 15 from 

Mountain View (Raedeke, 1982). The olivine compositions 

in the three cyclic units show the trend of Mg enrichment 

upward. The first cyclic unit in M-16 contains olivine that 

ranges from Fo77 to Fo81 , with an average of Fo79 ; the 

second cyclic unit at Nye Basin contains olivine that ranges 

from Fo82 to Fo^ and averages Fo84 ; and the fifteenth cyclic 

unit contains olivine that ranges from Fo85 to Fo86 and 

averages about Fo85 . Similarly, the cumulus orthopyroxene 

in the bronzite-olivine cumulate in M-16 ranges from XMg 

0.80 to 0.82, with an average of about 0.81, whereas in 

cyclic unit 15 cumulus orthopyroxene in the bronziteolivine cumulate and the bronzite cumulate has XMg 0.83 to 

0.86, with an average of about 0.85.

The cyclic units also show Mg enrichment upward 

within individual layers. In M-16, both olivine and 

chromite (see fig. 5) show Mg enrichment toward the top of 

the olivine cumulate. Mg enrichment of olivine also occurs 

from the base to the top of most individual size-graded units 

in the olivine cumulate from cyclic unit 2 (Page and others, 

1972). In her study, Raedeke (1982) found generally no 

change in olivine composition within individual olivine 

cumulates but did find increases in XMg upward in cumulus 

bronzite within bronzite cumulate layers as shown in figure 

21.

Jackson (1970) suggested that the Mg enrichment 

upward was caused by the failure of the Stillwater magma to 

reach equilibrium conditions of crystallization until after

cyclic unit 11. Raedeke (1982) attributed the initial Mg 

enrichment to postcumulus reaction of cumulus minerals 

with progressively decreasing volumes of intercumulus 

liquid until a steady state was reached, after which compositions remained constant. Alternatively, it is possible that 

the Mg enrichment was caused by decreasing amounts of 

contamination of the magma by the country rocks, progressively more Mg-rich primary melt compositions from the 

source, changes in the influx of magma from the source, or 

any combination of these processes. Other layered complexes, such as the Bushveld (Cameron, 1980; Osborn, 

1980) and the Great Dyke (Wilson, 1982), also show initial 

Mg enrichment upward. Therefore, Mg enrichment is a 

common phenomenon in layered intrusions.

Calculation of Magma Thickness

By using the compositional change of olivine in the 

part of the olivine cumulate in M-16 that shows normal 

fractionation, and an estimated original Stillwater magma 

composition, it is possible to calculate the thickness of 

magma necessary to crystallize at least part of the first 

cyclic unit. The olivine compositional change is over the 

24-m thickness, from 104 to 80 m, that exhibits normal 

fractionation from Fo80 to Fo78 . The initial Stillwater 

magma compositions that we used for our calculations are 

two of those suggested by Helz (1985) and shown in table 

3, based on her study of fine-grained mafic rocks in the 

Basal series.

Irvine (1979) outlines a method for calculating the 

amount of magma necessary to produce a given change in 

XMg in crystallizing olivine from a magma of known 

composition. The method is based on the Mg-Fe distribution coefficient between the magma and the crystallizing 

olivine, which is defined as:

(1)

where 2+ XMg =Mg/(Mg+Fe + ) and

XFe^=Fe2+/(Mg+Fe2+) in coexisting liquid (L) 

and olivine (ol).

Using a Rayleigh-type fractionation equation, Irvine 

(1979) showed that the cation fraction of Mg+Fe2+ that is 

transferred from liquid to olivine during fractional crystallization is given by

MgFe

(2)

where the zero superscript indicates an initial magma 

composition, and KD is assumed to be constant.

Figure 21. Mineral chemistry plotted against strati- ^ 

graphic height for three cyclic units from the Stillwater 

Complex. Cyclic unit 1, this study; cyclic unit 2 from 

Page and others (1972); cyclic unit 15 from Raedeke 

(1982).

E24 Contributions on Ore Deposits in the Early Magmatic Environment

Using equations 1 and 2, Irvine (1979) produced 

curves (fig. 22) that show the variation of XMg of a solid 

such as olivine and its parental liquid during fractional 

crystallization. These curves can be used to estimate the 

percent of liquid that has solidified, on the basis of the ratio

(cation percent (Mg+Fe) in initial liquid)/ 

(cation percent (Mg+Fe) in olivine) (3)

For the present purpose, we made two assumptions: (1) that 

the olivine cumulate in M-16 formed by perfect fractional 

crystallization and (2) that the olivine compositions have 

not been substantially modified by postcumulus or subsolidus reequilibration. It should be pointed out that our 

calculations provide an estimate and not an absolute value 

of the amount of liquid needed to crystallize the first cyclic 

unit, because of uncertainties in the composition of the 

original liquid. The liquid compositions that we used in our 

calculations would actually initially crystallize olivine of 

about Fo84 , which is 4 mole percent more forsteritic than 

our first olivine composition. However, the uncertainty 

caused by the compositional discrepancy is probably less 

than the uncertainty caused by our two assumptions above. 

Using the possible initial magma compositions given 

in table 3, we calculated the cation percent Mg+Fe in the 

liquids to be 0.34 and 0.41. The cation percent Mg+Fe in 

olivine is 0.67. Therefore, using equation 3, the ratios are 

0.507 and 0.612. Given a change of 2 percent Fo in the 

M-16 olivine, and the ratio calculated from equation 3, 

Irvine's diagrams show that about 5-6 percent of the liquid

Table 3. Samples that represent possible parent liquid 

compositions for the Stillwater Complex, in weight percent, from Helz (1985) 

[ , not analyzed]

Sample

SiO, ..................

TiO, ..................

A1,O, .................

Fe.O, .................

FeO ..................

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

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

CaO ..................

Na,O .................

K,O ..................

P n

H20+ .................

H,0~ .................

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

Total

NB1 8/378

........ 48.2

........ 1.15

........ 16.2

........ 2.30

........ 11.4

........ .19

........ 9.64

........ 10.82

........ 1.56

........ .25

........ .11

........ .34

........ .07

........ .07

........ .07

100.27

CC2/813

51.2

.30

12.6

12.0

.19

13.0

8.8

1.1

.29

.06

99.5

has solidified. Thus, about 40CM-80 m of magma was 

needed to crystallize 24 m of the olivine cumulate. The 5-6 

percent solidification is probably a maximum because there 

is a reversal to Mg enrichment above the interval of Fe 

enrichment. Thus, the 48-m-thick first cyclic unit crystal90 70 60 50 40 

100 Mg/(Mg + Fe2+ )

30 20

Figure 22. Curves illustrating the variation of Mg/ 

(Mg+Fe2+ ) of a solid such as olivine and its parental liquid 

during fractional crystallization, based on the indicated 

distribution coefficient and equations 1 and 2 in the text. 

The heavy curves describe the composition of the fractionated solid when the first crystals have the compositions given by the intercepts with the horizontal axis. The 

light curves (labeled with negative numbers) indicate the 

compositional difference between the crystals precipitated as fractionation progresses and the first crystals. In 

the case of solids crystallizing from their own melt, the 

vertical scale is equivalent to percent solidification. In 

magmas, however, the liquid generally has a different 

content of Mg+Fe2+ than the minerals that crystallize 

from it (see insert), and more than one mineral may 

precipitate at a time; thus to obtain percent solidification, 

the vertical scale must be multiplied by the ratio:

(cation percent Mg+Fe2+ in the initial liquid)/

(cation percent Mg+Fe2+ in the fractionated solids)

where the denominator is a constant. If the denominator 

changes, for example because of the appearance of a new 

mineral on the liquidus, then it is necessary to start over in 

terms of percent solidification of the remaining liquid, 

using the new value of the ratio above and the fractionation curve appropriate to the solids forming at that stage. 

From Irvine (1979); reprinted with permission.

E26 Contributions on Ore Deposits in the Early Magmatic Environment
lized from about 1,000 m of magma. The total thickness of 

the complex is estimated to have been about 8,000 m (Hess, 

1960). Therefore, the magma chamber probably reached its 

total thickness early in its history.

CONCLUSIONS

1. The lowermost cyclic units studied here show complex 

but modest vertical changes in the mineral chemistry of 

chromite, olivine, and orthopyroxene.

2. Comparison with studies of other cyclic units in the 

Stillwater reveals that simple Fe enrichment upward is not 

typically found. Instead, Mg enrichment upward is found 

both within and between cyclic units. The Mg enrichment 

may be the result of a continuous influx of magma or of 

decreasing amounts of contamination by the country rocks.

3. The composition of accessory chromite has been affected 

by postcumulus and (or) subsolidus reequilibration, as 

shown by different compositions for chromite within olivine, pyroxene, and amphibole from a given sample. In 

addition, some of the chromite shows subsolidus exsolution 

features. Also, olivine and pyroxene are zoned near 

enclosed chromite grains. Finally, calculated olivine-spinel 

equilibration temperatures are about 600-700 °C, which is 

well below magmatic temperatures.

4. No evidence was found to support theories of either 

magma mixing or changes inf02 to explain the origin of the 

chromite seam in M-16. Pressure changes remain a possible 

mechanism.

5. Modal analyses, mineral chemistry, and whole-rock 

trace-element chemistry show that infiltration metasomatism was not an important process in the formation of the 

lowermost cyclic unit and occurred only over a maximum 

vertical distance of 1-3 m.

6. Calculations based on changes in olivine composition in 

the lowermost olivine cumulate and an estimated original 

liquid composition indicate that about 1,000 m of magma 

was needed to crystallize the 48-m-thick first cyclic unit. 

The Stillwater Complex was probably about 8,000 m thick 

initially, which suggests that the magma chamber reached 

its total thickness early in its history.

REFERENCES

Beeson, M.H., and Jackson, E.D., 1969, Chemical composition

of altered chromites from the Stillwater Complex, Montana:

American Mineralogist, v. 54, p. 1084 1100. 

Bence, A.E., and Albee, A.L., 1968, Empirical correction factors

for the electron microanalysis of silicates and oxides: Journal

of Geology, v. 76, p. 382-403. 

Bliss, N.W., and MacLean, W.H., 1975, The paragenesis of

zoned chromite from central Manitoba: Geochimica et Cosmochimica Acta, v. 39, p. 973-990. 

Cameron, E.N., 1975, Postcumulus and subsolidus equilibration

of chromite and coexisting silicates in the Eastern Bushveld

Complex: Geochimica et Cosmochimica Acta, v. 39, 

p. 1021-1034.

 1977, Chromite in the central sector of the Eastern Bushveld complex, South Africa: American Mineralogist, v. 62, 

p. 1082-1096.

 1980, Evolution of the Lower Critical zone, central sector,

Eastern Bushveld Complex, and its chromite deposits: Economic Geology, v. 75, p. 845-871.

Cameron, E.N., and Desborough, G.A., 1969, Occurrence and 

characteristics of chromite deposits Eastern Bushveld Complex: Economic Geology Monograph 4, p. 23-40.

Evans, B.W., and Frost, B.R., 1975, Chrome-spinel in progressive metamorphism A preliminary analysis: Geochimica et 

Cosmochimica Acta, v. 39, p. 959-972.

Ghisler, M., 1976, The geology, mineralogy and geochemistry of 

the pre-orogenic Archean stratiform chromite deposits at 

Fiskenaesset, West Greenland: Monograph Series on Mineral 

Deposits, no. 14, 156 p.

Hamlyn, P.R., and Keays, R.R., 1979, Origin of chromite 

compositional variation in the Panton sill, western Australia: 

Contributions to Mineralogy and Petrology, v. 69, p. 75-82.

Helz, R.T., 1985, Composition of fine-grained mafic rock from 

the Basal zone of the Stillwater complex, Montana, in 

Czamanske, G.K., and Zientek, M.L., eds., The Stillwater 

Complex, Montana, Geology and guide: Montana Bureau of 

Mines and Geology Special Publication 92, p. 97-117.

Henderson, P., 1975, Reaction trends shown by chrome-spinels of 

the Rhum layered intrusion: Geochimica et Cosmochimica 

Acta, v. 39, p. 1035-1044.

Henry, D.J., and Medaris, L.G., Jr., 1980, Application of 

pyroxene and olivine-spinel geothermometers to spinel peridotites in southwestern Oregon: American Journal of Science, v. 280-A, p. 211-231.

Hess, H.H., 1960, Stillwater igneous complex, Montana A 

quantitative mineralogical study: Geological Society of 

America Memoir 80, 230 p.

Irvine, T.N., 1965, Chromian spinel as a petrogenetic indicator, 

Part I Theory: Canadian Journal of Earth Sciences, v. 2, p. 

648-672.

 1967, Chromian spinel as a petrogenetic indicator, Part 

II Petrologic applications: Canadian Journal of Earth Sciences, v. 4, p. 71-103.

 1977, Origin of chromitite layers in the Muskox intrusion 

and other stratiform intrusions: A new interpretation: Geology, v. 5, p. 273-277.

 1979, Rocks whose composition is determined by crystal 

accumulation and sorting, in Yoder, H.S., ed., The evolution 

of the igneous rocks, fiftieth anniversary perspective: Princeton, N.J., Princeton University Press, p. 245-306. 

-1980, Magmatic infiltration metasomatism, doublediffusive fractional crystallization, and adcumulus growth in 

the Muskox intrusion and other layered intrusions, in Hargraves, R.B., ed., Physics of magmatic processes: Princeton, 

N.J., Princeton University Press, p. 325-383. 

Irvine, T.N., Keith, D.W., and Todd, S.G., 1983, The J-M 

platinum-palladium reef of the Stillwater Complex, Montana: 

II, Origin by double-diffusive convective magma mixing and 

implications for the Bushveld Complex: Economic Geology, 

v. 78, p. 1287-1334.

Petrology of Chromite-Bearing Rocks From the Lowermost Cyclic Units in the Stillwater Complex, Montana E27
Jackson, E.D., 1961, Primary textures and mineral associations in 

the Ultramafic series of the Stillwater Complex, Montana: 

U.S. Geological Survey Professional Paper 358, 106 p.

 1968, The chromite deposits of the Stillwater Complex, 

Montana, in Ore deposits of the United States, 1933-1967 

(Graton-Sales volume), v. 2: New York, American Institute 

of Mining, Metallurgical and Petroleum Engineers, 

p. 1495-1510.

 1969, Chemical variation in coexisting chromite and olivine in chromitite zones of the Stillwater Complex: Economic 

Geology Monograph 4, p. 41-71. 

-1970, The cyclic unit in layered intrusions A comparison

of repetitive stratigraphy in the ultramafic parts of the 

Stillwater, Muskox, Great Dyke, and Bushveld complexes: 

Geological Society of South Africa Special Publication 1, 

p. 391-429.

Jackson, E.D., Howland, A.L., Peoples, J.W., and Jones, W.R., 

1954, Geologic maps and sections of the eastern part of the 

Stillwater complex in Stillwater County, Montana: U.S. 

Geological Survey Open-file Report [54-133], 4 sheets.

Jones, W.R., Peoples, J.W., and Howland, A.L., 1960, Igneous 

and tectonic structures of the Stillwater Complex, Montana: 

U.S. Geological Survey Bulletin 1071-H, p. 281-340.

Lambert, D.D., 1982, Geochemical evolution of the Stillwater 

Complex, Montana: Evidence for formation of platinumgroup element deposits in mafic layered intrusion: Golden, 

Colo., Colorado School of Mines, Ph.D. thesis, 274 p.

Lambert, D.D., Unruh, D.M., and Simmons, E.G., 1985, Isotopic investigations of the Stillwater complex: A review, in 

Czamanske, G.K., and Zientek, M.L., eds., The Stillwater 

Complex, Montana, Geology and guide: Montana Bureau of 

Mines and Geology Special Publication 92, p. 46-54.

Loferski, P.J., 1986, Petrology of metamorphosed chromitebearing ultramafic rocks from the Red Lodge District, Montana: U.S. Geological Survey Bulletin 1626-B, p. B1-B34.

Loferski, P.J., and Lipin, B.R., 1983, Exsolution in metamorphosed chromite from the Red Lodge district, Montana: 

American Mineralogist, v. 68, p. 777-789.

Loferski, P.J., Berman, Sol, Smith, Hezekiah, and Lipin, B.R., 

1984, Whole-rock trace element analyses of chromite-bearing 

rocks from the lowermost cyclic unit of the Stillwater 

Complex, Montana: U.S. Geological Survey Open-File 

Report 84-125, 4 p.

McCallum, I.S., Raedeke, L.D., and Mathez, E.A., 1980, 

Investigations of the Stillwater Complex: Part 1. Stratigraphy 

and structure of the Banded zone: American Journal of 

Science, v. 280-A, p. 59-87.

Muir, J.E., and Naldrett, A.J., 1973, A natural occurrence of 

two-phase chromium-bearing spinels: Canadian Mineralogist, v. 11, p. 930-939.

Osborn, E.F., 1978, Changes in phase relations in response to 

change in pressure from 1 atm to 10 kbar for the system 

Mg2SiO4-iron oxide-CaAl2Si2O8-SiO2 : Carnegie Institute of 

Washington Year Book 77, p. 784-790.

 1980, On the cause of the reversals of the normal fractionation trend An addendum to the paper by E.N. Cameron, 

"Evolution of the Lower Critical zone, central sector, Eastern 

Bushveld Complex, and its chromite deposits": Economic 

Geology, v. 75, p. 872-875.

Page, N.J, 1977, Stillwater Complex, Montana: Rock succession, 

metamorphism, and structure of the complex and adjacent 

rocks: U.S. Geological Survey Professional Paper 999, 79 p.

Page, N.J, Riley, L.B., and Haffty, J., 1969, Platinum, palladium, and rhodium analyses of ultramafic and mafic rocks 

from the Stillwater Complex, Montana: U.S. Geological 

Survey Circular 624, 12 p.

Page, N.J, Shimek, Richard, and Huffman, Claude, Jr., 1972, 

Grain-size variations within an olivine cumulate, Stillwater 

Complex, Montana: U.S. Geological Survey Professional 

Paper 800-C, p. C29-C37.

Raedeke, L.D., 1982, Petrogenesis of the Stillwater Complex: 

Seattle, Wash., University of Washington, Ph.D. thesis, 

212 p.

Raedeke, L.D., and McCallum, I.S., 1984, Investigations in the 

Stillwater Complex: Part II. Petrology and petrogenesis of the 

Ultramafic series: Journal of Petrology, v. 25, p. 395^420.

Steele, I.M., Bishop, F.C., Smith, J.V., and Windley, B.F., 

1977, The Fiskenaesset Complex, West Greenland, Part III: 

Chemistry of silicate and oxide minerals from oxide-bearing 

rocks, mostly from Qeqertarssuatsiaq: Gronlands Geologiske 

Undersogelse Bulletin 124, 38 p.

Todd, S.G., Keith, D.W., LeRoy, L.W., Schissel, D.J., Mann, 

E.L., and Irvine, T.N., 1982, The J-M platinum palladium 

reef of the Stillwater Complex, Montana: I. Stratigraphy and 

petrology: Economic Geology, v. 77, p. 1454-1480.

Ulmer, G.C., 1974, Alteration of chromite during serpentinization 

in the Pennsylvania-Maryland district: American Mineralogist, v. 59, p. 1236-1241.

Wilson, A.H., 1982, The geology of the Great "Dyke", Zimbabwe: the ultramafic rocks: Journal of Petrology, v. 23, 

p. 240-292.

Zientek, M.L., 1983, Petrogenesis of the Basal zone of the 

Stillwater Complex, Montana: Stanford, Calif., Stanford 

University, Ph.D. thesis, 246 p. [Also Dissertation Abstracts 

International, v. 45, p. 105-B 106-B]

Zientek, M.L., Czamanske, O.K., and Irvine, T.N., 1985, 

Stratigraphy and nomenclature for the Stillwater Complex, in 

Czamanske, G.K., and Zientek, M.L., eds., The Stillwater 

Complex, Montana, Geology and guide: Montana Bureau of 

Mines and Geology Special Publication 92, p. 21-32.

E28 Contributions on Ore Deposits in the Early Magmatic Environment
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