Sulfide Inclusions Within the B Chromitite, 

Stillwater Complex, Montana

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Chapter D

Sulfide Inclusions Within the B Chromitite, 

Stillwater Complex, Montana

By MICHAEL P. FOOSE and SUZANNE W. NICHOLSON

Sulfide inclusions within the B chromitite of the Stillwater 

Complex, Montana, provide evidence for complex 

high-temperature magmatic processes associated with chromite 

precipitation

U.S. GEOLOGICAL SURVEY BULLETIN 1674

CONTRIBUTIONS ON ORE DEPOSITS IN THE EARLY MAGMATIC ENVIRONMENT
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

Foose, M.P.

Sulfide inclusions within the B chromitite, Stillwater Complex, Montana.

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

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

Bibliography: p.

1. Sulfide minerals Montana. 2. Mineralogy Montana. I. Nicholson,

SuzanneW. II. Title. III. Series.

QE75.B9 no. 1674-D 557.3 s [549'.32'09786] 88-600205 

[QE389.2]
CONTENTS

Abstract Dl 

Introduction Dl

Acknowledgments D2 

Previous Studies D3 

Setting of the B Chromitite D3 

Method of Study D4 

Occurrence of Sulfides D5

Inclusion Types D5

Inclusion Mineralogy D6

Inclusion Chemistry D7

Vertical Variations D9

Bulk Sulfide Composition Dll

Comparison with the G and H Chromitites D13 

Discussion D13

Accumulation of Sulfides D13

Sulfide Inclusion Composition D14

Processes of Inclusion Formation D14

Vertical Changes in Composition D18 

Conclusions D19 

References D19

FIGURES

1. Index and generalized geologic map of the Still water Complex, Montana D2

2. Diagram showing correlation of chromitites in the Ultramafic series, Stillwater 

Complex, Montana D3

3. Diagram showing stratigraphy of the B chromitite in the West Fork area, 

Stillwater Complex, Montana D4

4. Photographs showing sulfide inclusion types in the upper part of the B chromitite, 

Stillwater Complex, Montana D6

5. Plot showing the relative abundance of inclusion types in samples from the B 

chromitite, Stillwater Complex, Montana D7

6. Ternary diagrams showing modes of sulfide phases in inclusions from the B chromitite, Stillwater Complex, Montana D9

7. Plot showing the vertical variations in characteristics of sulfide inclusions in the 

upper part of the B chromitite, Stillwater Complex, Montana Dll

8. Portions of ternary diagram showing bulk sulfide compositions of inclusions in the 

upper part of the B chromitite, Stillwater Complex, Montana, plotted in the FeS-(Ni+Co) system D12

9. Portion of ternary diagram showing bulk sulfide compositions of inclusions in the 

upper part of the B chromitite, Stillwater Complex, Montana, plotted in the FeCu-S system at 1,100 °C D13

10. Plot showing the changes in sulfide nickel content with variations in the ratio of 

silicate liquid to sulfide melt for different nickel partition coefficients D17

Contents III
TABLES

1. Average size and abundance of sulfide inclusions in the upper part of the B chromitite, Stillwater Complex, Montana D5

2. The distribution by type of sulfide inclusions in the upper part of the B chromitite, 

Stillwater Complex, Montana D7

3. The distribution of mineral assemblages in sulfide inclusions in the upper part of 

the B chromitite, Stillwater Complex, Montana D8

4. Modes of sulfide phases in inclusions in the upper part of the B chromitite, 

Stillwater Complex, Montana D8

5. Compositions of pentlandite and pyrrhotite from the upper part of the B chromitite, 

Stillwater Complex, Montana DIG

6. Estimated bulk sulfide compositions of inclusions from the upper part of the B 

chromitite, Stillwater Complex, Montana Dll

7. Comparison of compositions of sulfide inclusions from the upper part of the B 

chromitite, Stillwater Complex, Montana, with compositions from diverse magmatic sulfide deposits D14

IV Contents
Sulfide Inclusions Within the B Chromitite, 

Stillwater Complex, Montana

By Michael P. Foose and Suzanne W. Nicholson

Abstract

Variations in the abundance of chromite locally 

define seven layers within the upper part of the B chromitite of the Stillwater Complex, Montana. All chromite-rich 

layers contain sulfide inclusions, which constitute 0.002 to 

0.016 volume percent of the rock. The average diameter of 

inclusions ranges between 11.9 and 18.5 |xm. The sulfide 

inclusions can be divided into three types depending on 

whether they occur (1) alone in chromite grains, (2) in 

chromite grains in association with trapped silicate grains, 

or (3) within olivine and pyroxene. Pentlandite, chalcopyrite, and pyrrhotite are the dominant sulfide phases, 

occurring as both single-phase and polyphase inclusions. 

Electron microprobe analyses show that most pentlandites 

contain 30 to 38 weight percent nickel and 0.4 to 1.3 

weight percent cobalt; most pyrrhotites have atomic metal/ 

sulfur ratios between 0.87 and 0.90. The average bulk 

composition for all sulfide inclusions, estimated from 

mineral modes and microprobe mineral analyses, is 38.9 

weight percent Fe, 17.1 weight percent Ni, 0.45 weight 

percent Co, 8.3 weight percent Cu, and 35.2 weight 

percent S. The estimated average Cu/Ni ratio is 0.83.

In addition to changes in chromite abundance, contacts between some of the layers within the upper part of 

the B chromitite are also defined by abrupt changes in 

composition of included pentlandite and pyrrhotite, volume of sulfide, number of sulfide inclusions, and distribution of sulfide inclusions within chromite grains. Within 

some layers, there are systematic upward changes in 

Co/Ni ratio in pentlandite, the abundance of sulfide, and 

the size and number of sulfide inclusions.

The sulfide inclusions were trapped either as an 

immiscible sulfide melt or as sulfide crystals that separated 

from the silicate liquids from which the B chromitite 

crystallized. The average ratio of silicate liquid to sulfide 

melt (R) was large (minimum range of 6,000 to 50,000), so 

the initial composition of these sulfides was directly controlled by the composition of the silicate liquid with which 

they were in equilibrium. Some fractional crystallization of 

the sulfide melt may have occurred before trapping and 

thereby contributed to the mineralogical diversity 

observed among inclusions. However, local variations in 

partition coefficients and in R were probably more important in creating compositional differences among inclusions. The partitioning of elements between silicate liquid

Manuscript approved for publication, February 18, 1988.

and sulfide melt probably varied as a result of local 

variations in fO2 . Variations in both fO2 and R may have 

resulted from the precipitation and trapping of sulfides in 

a zone of turbulent mixing between two compositionally 

and thermally distinct silicate liquids.

Variations in Co/Ni ratio of pentlandite in inclusions 

reflect compositional variations in the silicate liquid from 

which the sulfides originally separated. The Co/Ni variations also coincide with abrupt changes in chromite 

mode, suggesting that several compositionally distinct 

liquids were involved in forming the B chromitite.

INTRODUCTION

The Stillwater Complex of south-central Montana is 

one of the world's better known layered intrusions. Most 

determinations indicate an age of about 2,700 Ma (De Paolo 

and Wasserburg, 1979), although an older age of 2,896 Ma 

has also been suggested (Coffrant and others, 1980). The 

nearly 6 km of exposed stratigraphic section is commonly 

divided into three major series (fig. 1; Zientek and others, 

1985). The Basal series ranges from 0 to 160 m in thickness 

and consists of two parts. The lower part is composed 

mostly of heterogeneous rocks containing varying amounts 

of bronzite, olivine, plagioclase, and augite. Chromite and 

pigeonite are also locally present. The upper part of the 

Basal series is mostly bronzite cumulates (Page, 1979; 

Zientek, 1983; Zientek and others, 1985). It is overlain by 

the Ultramafic series, which averages about 1,200 m in 

thickness and consists predominantly of olivine, olivine and 

bronzite, and bronzite cumulates. Chromite is also locally 

present and may form seams as much as several meters 

thick. The approximately 4,000-m-thick Banded series 

forms the upper part of the complex, its base being defined 

by the first appearance of cumulus plagioclase above the 

Ultramafic series. It consists predominantly of cumulus 

plagioclase with varying amounts of additional cumulus 

augite, bronzite, and olivine. Paleozoic sedimentary rocks 

unconformably overlie the Banded series so that the true 

thickness of the complex is unknown. Subsequent to consolidation, both Precambrian and Laramide deformations 

have disrupted the intrusion so that it is now exposed only 

over a strike length of 44 km.

Sulfides occur in each of the three series. They are 

most abundant in the Basal series, where large tonnages

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D1
45°30'

45°20'

MONTANA 0

Livingston o Billings

Cody Q 

IDAHO ' WYOMING

Paleozoic sediments 

Footwall rocks

BANDED SERIES

Upper banded series 

Middle banded series 

Lower banded series

BASAL SERIES -'

I Basal bronzite cumulate zone and basal norite zone

- - - Contact-Dashed where 

approximately located

^ N\>,Onrtial area -^

Moun,..,.. _ 0 Q0 ,-,,. 

area area , \/<j

,, .. . .. Idealized cyclic unit

ULTRAMAFIC SERIES

Bronzitite zone

Peridotite zone

Fault

5 MILES

I

I ' ' ' ' I

0 5 KILOMETERS

Bronzite cumulate

Bronzite olivine cumulate

Olivine cumulate 

Chromite olivine cumulate

Olivine cumulate

-Chromite cumulate

Figure 1. Index and generalized geologic map of the Stillwater Complex, Montana. Map shows distribution of Basal, 

Uitramafic, and Banded series rocks and location of the West Fork area. Also shown is an idealized chromite-bearing 

cyclic unit from the Uitramafic series. Modified from Foose (in press).

(>150 million tons) of rock containing about 0.25 weight 

percent copper and 0.25 weight percent nickel have been 

identified (Page and others, 1985). Sulfides are rare in the 

Uitramafic series. Aside from a few small concentrations, 

sulfides in the Uitramafic series are most conspicuous as 

inclusions in chromite. The Banded series is known to 

contain at least 10 sulfide-bearing intervals that range from 

a few centimeters to several tens of meters in thickness 

(McCallum and others, 1980; Zientek and others, 1985). 

The most significant of these is the J-M Reef, a layer that is 

exceptionally rich in palladium and platinum (locally containing 22.3 grams (0.65 oz) per short ton of palladium and 

platinum; Todd and others, 1982). It is currently being 

mined.

Despite their lack of economic significance, the 

disseminated sulfides in the Uitramafic series are of interest 

because they provide a link between the copper- and 

nickel-rich sulfides of the Basal series and the sulfides rich 

in platinum-group elements (PGE) that occur in the Banded 

series. The sulfide inclusions within cumulus chromite

grains may also be important as a record of the hightemperature immiscible sulfide melt that was present during 

the crystallization of the Uitramafic series. Their distribution and composition may, therefore, provide insight into 

the magmatic processes operating during crystallization of 

this part of the Stillwater Complex. Sulfides in the B 

chromite are especially significant because the B chromitite 

is one of the lowest laterally traceable units in the Uitramafic series.

Acknowledgments

This work has benefited from discussions and support 

from many geologists working both with the U.S. Geological Survey and with the former Anaconda Minerals Company. Particular assistance was provided by B.R. Lipin in 

collecting the samples used in this work and in providing a 

detailed stratigraphic description of the B chromitite^ .ty.J 

Page shared insights gained from his extensive knowledge 

of the Stillwater Complex and his similar study of sulfides

D2 Contributions on Ore Deposits in the Early Magmatic Environment
in the G and H chromitites. Additional critical comments 

have been made by G. Czamanske and M. Zientek.

PREVIOUS STUDIES

The geology of the Stillwater Complex has been 

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

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

Most recently, it has been comprehensively reviewed in a 

special volume (Czamanske and Zientek, 1985). Specific 

studies of the sulfide minerals in the complex have focused 

mostly on Basal series rocks (Roby, 1949; Rowland, 1933; 

Page and Simon, 1978; Page, 1979; Humphreys, 1983; 

Zientek and others, 1982; Zientek, 1983; Drew and others, 

1985; Attanasi and Bawiec, 1987) and the recently identified PGE-rich sulfides of the J-M Reef (Conn, 1979; Todd 

and others, 1979, 1982; Wolfgram and Evans, 1980; Bow 

and others, 1982; Irvine and others, 1983; Campbell and 

others, 1983; Barnes and Naldrett, 1985, 1986; Boudreau 

and others, 1986). Sulfides in the Ultramafic series have 

been discussed by Page (1972) and Page and others (1985) 

as part of surveys of sulfide mineral compositions in the 

Stillwater Complex and were treated in detail in a discussion of sulfide inclusions in the G and H chromitites (Page, 

1971).

The B chromitite in the West Fork area was mapped 

by Rowland (1955) who identified it as the "A" chromitite. 

This designation was maintained in subsequent work by 

Page and Jackson (1967) on the platinum-group minerals in 

these chromitites and in studies by Page and others (1969, 

1972, 1976) who determined that platinum-group metals in 

these chromitites show large lateral and vertical variations 

but on average total 4.6 ppm. B.R. Lipin (in Mann and 

others, 1985) restudied rocks in this area and concluded that 

these are part of the B chromitite. This identification has 

been accepted for the purposes of this study.

SETTING OF THE B CHROMITITE

The Ultramafic series, in which the B chromitite 

occurs, is composed predominantly of cumulus olivine and 

bronzite. Following the terminology developed by Wager 

and others (1960) and Jackson (1967), these rocks are 

olivine cumulates (Oc), olivine-bronzite cumulates (OBc), 

and bronzite cumulates (Be). Plagioclase and augite, as well 

as some olivine and bronzite, fill the intercumulus spaces 

between grains.

The Ultramafic series is divided into two zones (fig. 

1). The upper one-third consists of a sequence of bronzite 

cumulates that is known as the Bronzitite zone, while the 

lower two-thirds, known as the Peridotite zone, is made up 

of cyclically layered rocks. Ideally, from bottom to top, 

cycles ijdnsist of layers of Oc, OBc, and Be that reflect 

repetitive crystallization of cumulus olivine, olivine +

EAST WEST

NYE MOUNTAIN WEST IRON CHROME 

LIP VIEW INITIAL FORK MOUNTAIN MOUNTAIN GISH

-BASE OF BRONZITITE ZONE

8 KILOMETERS

Figure 2. Correlation of chromitites A-K in the Ultramafic series, Stillwater Complex, Montana (from Jackson, 1968).

bronzite, and bronzite. Cyclic units are thought to be caused 

by influxes of new magma; often magma influxes disrupted 

this regular crystallization sequence so that these cycles are 

only partly developed (Jackson, 1970). Between 8 and 21 

cyclic units, ranging from 3 to 381 m in thickness, have 

been identified within the Stillwater Complex (Page, 1977).

Chromite is concentrated as a cumulus phase in the 

basal parts of some cycles (fig. 1) but commonly does not 

exceed 2 modal percent. However, the concentration of 

chromite in several olivine cumulates is sufficient to form 

laterally continuous layers. In general, these chromitites 

have sharp basal contacts and grade upward through disseminated chromite into Oc. The most prominent of these 

chromitites have been labeled according to their stratigraphic position, with A at the bottom and K at the top (fig. 

2; Jackson, 1968). Of these, only the G and the H are thick 

enough to have been mined.

Lipin (in Mann and others, 1985) discussed the 

stratigraphic setting of the B chromitite in the West Fork 

area (fig. 3). In this area, the B chromitite ranges from 

about 1.3 to 9 m in thickness. At the specific outcrop 

examined in this study, it is about 3 m thick, and the upper 

1.5 m shows most of the visible internal variation. Twelve 

samples were collected for study across the upper 1.5 m of 

this seam. Over this thickness, seven layers can be defined 

on the basis of abundance of cumulus chromite. From 

bottom to top, these are:

Layer 1. Net-textured to locally massive chromitite 

having a sharp upper contact (4 to 6 cm thick).

Sulfide Inclusions in the B Chromite, Stillwater Complex, Montana D3
METERS 

0

'Top of B chromitite

10

Bottom of B chromitite

Main" B

EXPLANATION

Chromitite (>80% chromite)

Net-textured olivine-chromite 

cumulate (20-60% chromite)

Disseminated chromite (5-20% 

chromite) in olivine-chromite cumulate

Discontinuous veins and stringers 

of chromite

Continuous (within a given outcrop) 

but thin chromite layer less 

than V2 inch thick

Olivine cumulate

Olivine-bronzite cumulate

Contact between olivine cumulate 

and olivine-bronzite cumulate

Layer 7

 CENTIMETERS 

Layers 100

Layer 5

Layer 4

= Layer 3

Layer 2

80

60

40

20

Layer 1

Figure 3. Stratigraphy of the B chromitite in the West Fork area, Stillwater Complex, Montana. A, Local vertical and 

lateral variations; B, Detailed section and sample locations from the main chromite-bearing portion of the B (modified 

from Mann and others, 1985). The section studied is located at coordinates 15,320 E. and 21,480 N. on Rowland's 

1:12,000 map (1955); samples, sample locations, and stratigraphic section provided by B.R. Lipin.

Layer 2. Olivine-chromite cumulate containing 5 to 

10 percent chromite (approximately 80 cm thick).

Layer 3. Net-textured to massive chromite (1 to 2 cm 

thick).

Layer 4. Olivine-chromite cumulate containing 5 to 

10 percent chromite (approximately 46 cm thick).

Layer 5. Chromitite with a sharp lower contact and a 

sharp, but irregular, upper contact (2 to 8 cm thick).

Layer 6. Cross-laminated sequence of olivinechromite cumulate containing 5 to 10 percent chromite 

(about 10 to 15 cm thick).

Layer 7. Chromitite, changing from massive at the 

base to net-textured (12 to 13 cm thick).

METHOD OF STUDY

Polished thin sections from twelve samples collected 

across the chromite-rich layers were examined in reflected 

and transmitted light. All sulfide inclusions that appeared to 

be primary were studied. Inclusions were considered to be 

primary if they occurred within cumulus grains and were

D4 Contributions on Ore Deposits in the Early Magmatic Environment
not associated with fractures. A total thin-section area of 

approximately 440 cm2 was examined, and 1,626 inclusions ranging in size from 1 to more than 100 |xm were 

identified. Each of these inclusions was classified by its 

setting in enclosing minerals.

Scale drawings of each primary inclusion allowed the 

size, area, and shape of the inclusions to be catalogued. 

Drawings were also used to estimate the abundance of each 

phase within the inclusions and the total volume percentage 

of sulfide in each specimen.

Electron microprobe analyses were made for pyrrhotite and pentlandite with an ARL EMX SM microprobe 

using an accelerating voltage of 15 kV and a beam current 

of 0.1 |xA. Synthetic sulfide standards were used, and data 

were reduced with the ZAP computer program provided by 

Tracor Northern. Low nickel values were obtained when 

using this correction package for pentlandite containing 30 

to 40 weight percent nickel. Therefore, correction curves 

determined from analysis of 5 standards containing 9.89 to 

73.3 weight percent nickel were used to obtain nickel values 

for unknowns. Estimated errors based on repeated analyses 

of standards are 0.3 weight percent for iron and sulfur, 0.2 

weight percent for cobalt, and 0.4 weight percent for nickel.

Modal estimates of the abundance of the three major 

sulfide phases and the microprobe analyses of pentlandite 

and pyrrhotite were integrated to obtain an estimate of the 

bulk sulfide composition for each sample. In another study 

of sulfide inclusions, Czamanske and Moore (1977) 

obtained this type of information by using a broad microprobe beam and moving it in a stepwise fashion to cover the 

entire inclusion. This procedure was satisfactory in their 

study because the sulfide melt was quenched rapidly, 

causing exsolution in these sulfides to occur as very fine 

lamellae. In contrast, exsolution in the B chromitite inclusions is relatively coarse, so that averaging the modes from 

individual inclusions and integrating with sulfide mineral 

analyses is the most effective way to estimate a melt 

composition.

OCCURRENCE OF SULFIDES

Sulfide inclusions in the upper part of the B chromitite typically are small and sparsely disseminated. Comparison of the area of sulfide to the total thin-section area 

indicates that primary sulfides average between 0.007 and 

0.008 percent of the rock and range from 0.002 to 0.016 

percent (table 1). Diameters of inclusions vary from 1 to 

more than 100 jxm and average between 11.9 and 22.8 jxm.

Inclusion Types

Primary sulfide inclusions are divisible into three 

groups. Type 1 inclusions occur in isolation within chromite 

grains. Type 2 inclusions are also trapped in chromite but

Table 1. Average size and abundance of sulfide inclusions

in the upper part of the B chromitite, Stillwater Complex,

Montana

[ marks contact between layers defined by chromite mode]

Sample Average area Average diameter Volume percent 

(|xm2) ((xm) of rock

1A

IB

2

3

4A

4B

5B

5A

6B

6A

7A

7B

270

233

206

221

410

322

138

148

112

191

153

198

18.5

17.2

16.2

16.8

22.8

20.2

13.2

13.7

11.9

15.6

13.9

15.9

0.007

.007

.003

.013

.009

.007

.002

.003

.005

.016

.004

.011

are associated with included silicate. Type 3 inclusions are 

trapped within primary silicates (generally olivine and 

pyroxene). Examples of these three inclusion types are 

shown in figure 4.

Type 1 and 2 inclusions occur in cumulus chromite 

grains, and type 3 inclusions occur in both cumulus and 

intercumulus silicate grains. In this part of the B chromitite, 

cumulus olivine is by far the most abundant primary 

silicate, and only a few type 3 inclusions are enclosed in 

intercumulus augite or bronzite. Visual inspection failed to 

show any discernible difference between inclusions in 

olivine and those in pyroxenes, and the relatively few 

inclusions hosted by pyroxene made classifying them as a 

separate group impractical.

The distribution of inclusions by type (table 2, fig. 5) 

shows that most occur as inclusions in chromite. To some 

extent, this association is controlled by chromite mode, but 

type 1 and 2 inclusions predominate in samples containing 

only 5 to 10 percent chromite. It is also apparent (fig. 5) that 

type 1 sulfide inclusions are more common than type 2.

Sulfides may be trapped in adcumulus rims around 

cumulus chromite grains. Although no practical way exists 

to unequivocally separate such inclusions, type 1 inclusions 

were subdivided into those hosted near rims of chromite 

grains and those occurring near grain centers. This subdivision reveals marked variations between samples. Some 

contain abundant inclusions near edges of chromite grains, 

while other samples do not (table 2). A similar subdivision 

was made for type 2 inclusions, but no difference in 

distribution was detected.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D5
Figure 4. Sulfide inclusion types in the upper part of the 

B chromitite, Stillwater Complex, Montana. A, Type 1 

inclusion consisting of pentlandite (Pn) and pyrrhotite 

(Po) (sample 6A); B, Type 1 inclusion containing only 

pentlandite (sample 7B); C, Type 2 inclusions (sample 

7B); D, Type 2 inclusion containing pentlandite, pyrrhotite, and chalcopyrite (Cp) (sample 6A); f, Type 3 inclusion 

containing pyrrhotite and pentlandite in olivine (sample 

2); F, Type 3 inclusion consisting only of pentlandite in 

olivine (sample 2). All photographs in reflected light; field 

of view 0.36 by 0.51 mm.

Included sulfide grains may be euhedral or anhedral. 

In type 1 and 2 inclusions, sulfides typically have euhedral 

to subhedral crystal forms where bounded by chromite and 

are anhedral against included silicates. Grains in type 3 

inclusions are mostly anhedral.

Inclusion Mineralogy

Pyrrhotite, pentlandite, and chalcopyrite are found in 

inclusions. In polished section, sulfide inclusions commonly contain a single sulfide phase (table 3), which, in

D6 Contributions on Ore Deposits in the Early Magmatic Environment
Table 2. The distribution by type of sulfide inclusions in the upper part of the B chromitite, Stillwater Complex, Montana

[Type 1, occurring within chromite grains without accompanying trapped silicates; type 2, occurring within chromite grains with accompanying trapped 

silicates; type 3, occurring within silicate grains. marks contact between layers defined by chromite modes]

Type 1 Type 2 Type 3

sample

1A

IB

2

3

4A

4B

5B

5A

6B

6A

7A

7B

Cores*

76

27

5

78

4

9

14

37

18

63

19

51

Edges**

1

22

0

84

1

0

32

33

19

176

42

106

Percent

62.1

38.9

9.3

71.3

6.8

11.0

68.6

72.9

25.5

76.3

55.5

75.1

Number

47

76

5

65

32

55

11

22

10

45

36

49

Percent

37.9

60.3

9.2

28.6

43.8

67.1

16.4

22.9

6.9

14.4

32.7

23.5

Number

0

1

44

0

36

18

10

4

98

29

13

3

Percent

0.0

.8

81.5

0

49.3

21.9

14.9

4.2

67.6

9.3

11.8

1.4

of inclusions

124

126

54

227

73

82

67

96

145

313

110

209

r <j

chromite in rock

80

85

5

90

5

5

5

10

5

70

10

45

* Number of inclusions within the cores of chromite grains.

** Number of inclusions near edges of chromite grains.

most cases, is pentlandite. Two-phase assemblages, which 

are less common, generally consist of pyrrhotite and pentlandite. Inclusions containing all three of the major sulfide 

phases are relatively rare. Other rare sulfide phases 

observed in inclusions are heazlewoodite, niccolite, and 

sperrylite.

TYPE1

TYPE 3 TYPE 2

Figure 5. Relative abundance of inclusion types in samples 

from the B chromitite, Stillwater Complex, Montana. Type 

1, sulfides trapped in chromite without accompanying 

silicates; type 2, sulfides trapped in chromite with silicates; type 3, sulfides occurring within silicate grains. 

Circled numbers denote modal percentages of chromite 

in samples.

Modal proportions of pyrrhotite, chalcopyrite, and 

pentlandite were determined for all primary inclusions 

(table 4, fig. 6). Differences between these data and those in 

table 3 are largely attributable to small inclusions (<1 |Am), 

which strongly influence data on the abundance of inclusions (table 3) but have little effect on the overall modal 

composition.

Pentlandite is the dominant phase in most samples 

and is followed in abundance by pyrrhotite and chalcopyrite. In some samples, different inclusion types have 

approximately similar modal compositions. In other samples, apparent differences in modal mineralogy between 

inclusion types may be due to the large deviation expected 

in data obtained from small sample populations. However, 

data for samples 3, 5B, and 6B appear to be sufficiently 

abundant to suggest that some real differences in modes 

exist. These differences do not appear to have any pattern.

Inclusion Chemistry

Microprobe analyses of pyrrhotite and pentlandite 

were obtained from some sulfide inclusions. The small size 

of most inclusions, and the polyphase nature of many, made 

accurate analyses of individual phases difficult to obtain. As 

a result, the data presented in table 5 are not statistical 

averages of large numbers of points but instead are considered to be representative of the sample. For pentlandite, the 

analyses used were those showing the highest nickel contents (indicating the least chance of contamination by 

pyrrhotite); for pyrrhotite, analyses showing the lowest 

nickel and cobalt contents were used.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D7
Table 3. The distribution of mineral assemblages in sulfide inclusions in the upper part of the B chromitite, Stillwater 

Complex, Montana

[In percent. Po, pyrrhotite; Pn, pentlandite; Cp, chalcopyrite; Ni, niccolite; Sp, sperrylite; Hz, heazlewoodite; , none found]

Assemblage

Po

Pn

Cp

Po+Pn

Po+Cp

Pn+Cp

Po+Pn+Cp

Ni

Ni+Pn

Hz

Po+Hz

Sp

Sp+Pn

Ni+Pn+Po+Hz

1A

6.2

26.2

10.8

27.6

4.6

18.5

4.6

1.5

...

1B

5.3

34.7

17.1

24.0

5.1

8.0

5.1

.7

...

2

9.2

18.2

20.4

40.9

4.5

6.8

...

3

10.0

20.3

44.7

7.5

8.8

5.6

2.5

.6

...

4A

13.6

47.3

13.5

20.2

2.7

2.7

4B

20.8

26.9

12.7

30.2

3.1

1.6

2.0

2.7

5B

18.0

10.0

56.0

4.0

6.0

6.0

5A

3.0

41.8

1.5

38.8

4.5

4.5

5.9

___

6B

37.1

15.7

14.8

27.8

2.8

.9

...

.9

__.

6A

10.4

24.2

15.4

38.3

3.0

3.1

1.1

.5

3.0

.5

.5

7A

14.5

27.0

12.5

35.8

6.2

1.0

1.0

1.0

1.0

7B

19.6

16.4

8.2

44.0

2.3

3.7

2.2

2.9

.7

Table 4. Modes of sulfide phases in inclusions in the upper part of the B chromitite, Stillwater Complex, Montana

[Type 1, sulfides trapped in chromite without accompanying silicates; type 2, sulfides trapped in chromite with silicates; type 3, sulfides occurring within 

silicate grains. Cp, chalcopyrite; Po, pyrrhotite; Pn, pentlandite. Number of points counted; number in parentheses is percentage]

Sample

1A

IB

2

3

4A

4B

5B

5A

6B

6A

7A

7B

CP

392

(16.2)

1699

(26.5)

55

(61.8)

10688

(69.6)

0

(0.0)

75

(12.1)

1519

(55.1)

580

(12.7)

1983

(43.6)

1718

(9.4)

314

(4.3)

2305

(18.1)

Type 1

Po

698

(28.9)

1006

(15.7)

17

(19.1)

1958

(12.7)

133

(37.5)

344

(55.4)

841

(30.5)

1547

(33.8)

1125

(24.8)

4469

(24.5)

2894

(39.7)

3797

(29.8)

Pn

1323

(54.8)

3700

(57.8)

17

(19.1)

2714

(17.7)

222

(62.5)

202

(32.5)

397

(14.4)

2445

(53.5)

1435

(31.6)

12056

(66.1)

4082

(56)

6627

(52.1)

Cp

3313

(20.6)

2564

(19.3)

44

(13.2)

4619

(26.5)

918

(5.6)

3371

(23.6)

598

(36.7)

505

(14.4)

174

(16.3)

2776

(16.5)

1292

(50.0)

2718

(19.2)

Type 2

Po

5763

(35.8)

1112

(8.4)

112

(33.6)

2357

(13.5)

1360

(8.4)

5677

(39.7)

858

(52.7)

685

(19.6)

120

(11.3)

4159

(24.8)

103

(4.0)

4820

(34.0)

Pn

7008

(43.6)

9618

(72.3)

177

(35.1)

10458

(60.0)

13985

(86.0)

5262

(36.8)

171

(10.5)

2312

(66.0)

771

(72.4)

9852

(58.7)

1184

(45.9)

6644

(46.8)

Cp

0

(0.0)

0

(0.0)

1007

(16.7)

0

(0.0)

920

(22.5)

302

(11.6)

1096

(74.1)

552

(57.9)

1105

(20.8)

1989

(29.5)

21

(1.8)

0

(0.0)

Type 3

Po

0

(0.0)

0

(0.0)

2429

(40.3)

0

(0.0)

1041

(25.4)

762

(29.4)

248

(16.8)

99

(10.4)

2771

(52.1)

2369

(35.1)

563

(49.2)

443

(62.6)

Pn

0

(0.0)

0

(0.0)

2587

(42.9)

0

(0.0)

2130

(52.1)

1532

(59.0)

135

(9.1)

303

(31.8)

1448

(27.2)

2394

(35.5)

561

(49.0)

265

(37.4)

Cp

3705

(20.0)

4263

(2L6)

1106

(17.1)

15307

(46.7)

1838

(8.9)

3748

(21.4)

3213

(54.8)

1637

(18.1)

3262

(29.8)

6483

(15.5)

1627

(14.8)

5023

(18.2)

All

Po

6461

(34.9)

2118

(10.7)

2558

(39.7)

4315

(13.2)

2534

(12.2)

6783

(38.7)

1947

(33.2)

2331

(25.8)

4016

(36.7)

10997

(26.3)

3560

(32.3)

9060

(32.8)

Pn

8331

(45.1)

13318

(67.7)

2781

(43.2)

13172

(40.1)

16337

(78.9)

6996

(39.9)

703

(12.0)

5060

(56.0)

3654

(33.4)

24302

(58.2)

5827

(52.9)

13536

(49.0)

Pentlandite compositions range between 30.3 and 

37.8 weight percent (23.6 and 29.4 atomic percent) nickel 

and 0.45 and 1.3 weight percent (0.23 and 1.0 atomic 

percent) cobalt; Co/Ni ratios fall between 0.0079 and 

0.038. Within previously given estimated analytical errors, 

pentlandites are stoichiometric. Misra and Fleet (1973) 

determined the compositions of natural and synthetic pentlandites in a wide variety of sulfide assemblages. The range 

of compositions determined in this study is nearly identical 

to the range of 24 to 29.3 atomic percent nickel observed by 

them for pentlandites occurring with monoclinic pyrrhotite. 

Most pyrrhotites contain 53 to 53.5 atomic percent 

sulfur, which is consistent with their being monoclinic. 

Pyrrhotites in several samples (7B, 6A, and 4B), however,

D8 Contributions on Ore Deposits in the Early Magmatic Environment
TYPE 2 TYPE 3

Figure 6. Modes of sulfide phases in inclusions from the B chromitite, Stillwater Complex, Montana. Cp, chalcopyrite; 

Po, pyrrhotite; Pn, pentlandite. Type 1, sulfides trapped in chromite without accompanying silicates; type 2, sulfides 

trapped in chromite with silicates; type 3, sulfides occurring within silicate grains.

contain slightly less sulfur, indicating that some hexagonal 

pyrrhotite must also be present. Nickel and cobalt contents 

are 0.08 to 0.54 weight percent and 0.02 to 0.41 weight 

percent, respectively.

Vertical Variations

Compositions of pyrrhotite and pentlandite, volume 

of sulfide, size of inclusions, proportion of type 1 inclusions, and inclusion mode show vertical variations within 

the studied section (fig. 7, table 2). Where data are 

sufficient, variations may be correlated with the variations 

in chromite mode that were used to define the stratigraphy 

of this section. From the bottom up, the major characteristics of sulfide inclusions in these seven layers are as 

follows:

Layers 1 and 2: At the contact between layers 1 and 

2, there is an abrupt increase in (1) amount of sulfide, (2) 

total number of sulfide-bearing inclusions (table 2), (3) 

percentage of both near-center and near-edge type 1 inclusions, (4) nickel content of pentlandite, and (5) metal/sulfur 

ratio (M/S) of pyrrhotite. The cobalt content of pentlandite 

decreases sharply across this contact. There is no discernible change in the relative proportions of the three major 

sulfide phases.

The limited data from layers 1 and 2 suggest that 

inclusion size and abundance of type 1 inclusions decrease 

upward and that abundance of type 2 inclusions increases 

upward. No data exist for the upper part of layer 2.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D9
Table 5. Compositions of pentlandite (Pn) and pyrrhotite (Po) from the upper part of the B chromitite,

Stillwater Complex, Montana

[In weight percent; number in parentheses shows atomic percent]

Sample

1A

IB

2

3

4A

4B

5B

6A

7A

7B

Mineral

Pn

Po

Pn

Po

Pn

Po

Pn

Po

Pn

Pn

Po

Pn

Pn

Po

Pn

Po

Pn

Po

Fe

30.5

(24.95)

59.7

(46.45)

29.1

(23.97)

59.6

(46.45)

28.5

(23.38)

59.2

(46.50)

28.27

(23.11)

60.3

(46.61)

32.6

(26.61)

34.4

(28.18)

60.3

(47.43)

30.6

(25.13)

28.8

(23.45)

61.3

(47.83)

30.7

(25.01)

58.7

(46.27)

33.0

(26.52)

60.1

(47.12)

Ni

34.3

(26.69)

.37

(0.27)

36.5

(28.60)

.22

(.16)

37.5

(29.26)

.23

(.17)

37.77

(29.37)

.22

(.16)

33.1

(25.7)

30.3

(23.61)

.08

(.06)

35.1

(27.41)

37.6

(29.12)

.10

(.07)

34.1

(26.43)

.54

(.40)

33.2

(25.38)

.52

(.38)

Co

0.61

(.47)

.03

(.22)

.45

(.35)

.02

(.02)

.45

(.35)

.41

(.30)

.50

(.39)

.05

(.04)

1.15

(.89)

1.06

(.82)

.05

(.04)

.88

(.69)

.99

(.76)

.05

(.04)

1.30

(1.00)

.06

(.04)

1.19

(.91)

.07

(.05)

S

33.6

(47.88)

39.3

(53.25)

32.8

(47.07)

39.31

(53.37)

32.9

(47.01)

38.75

(53.01)

33.11

(47.14)

39.5

(53.18)

32.9

(46.78)

33.2

(47.38)

38.3

(52.47)

32.7

(46.77)

32.9

(46.66)

38.3

(52.05)

33.5

(47.55)

38.8

(53.27)

33.7

(47.18)

38.4

(52.44)

Total

99.01

99.4

98.85

99.15

99.35

98.59

99.65

100.07

99.75

99.46

98.73

99.28

100.29

99.95

99.6

98.1

101.09

99.09

Atomic metal/sulfur ratio

1.088

.878

1.125

.874

1.127

.886

1.121

.880

1.137

1.111

.906

1.138

1.143

.921

1.103

.877

1.119

.906

Layer 3: No data exist for layer 3.

Layer 4: Abundance of type 1 inclusions and nickel 

content of pentlandite decrease upward, whereas volume of 

sulfides, inclusion size, abundance of type 2 inclusions, and 

cobalt content of pentlandite increase.

Layer 5: In comparison to the upper parts of layer 4, 

there is a marked increase in volume of sulfide, abundance 

of type 1 inclusions, and modal abundance of chalcopyrite, 

whereas there is a sharp decrease in inclusion size. Pentlandite composition changes abruptly to be richer in nickel 

and poorer in cobalt.

Layer 6: Relative to layer 5, volume of sulfide, 

abundance of chalcopyrite, and proportion of type 1 inclusions decrease. The decrease in the number of type 1 

inclusions may be largely attributed to the relative scarcity 

of chromite in this layer.

Layer 7: Sulfide abundance and inclusion size 

increase. Pentlandite becomes poorer in nickel and richer in 

cobalt upward.

In summary, contacts between layers defined by 

variations in chromite mode are also marked by changes in 

the character of the sulfide inclusions. The most conspicuous changes are in volume of sulfide, abundance of type 1 

inclusions, size of inclusions, and composition of pentlandite. Within layers, type 1 inclusions typically decrease in 

abundance upward, whereas the proportion of type 2

D10 Contributions on Ore Deposits in the Early Magmatic Environment
f

Layer 7

Layer6

Layers

Layer 4

Layer3

Layer 2

Layer 1

0.005 0.010 0.015

VOLUME OF

SULFIDE

(PERCENT)

15 20

AVERAGE

GRAIN SIZE

(MICROMETERS)

50 100 

MODE (PERCENT) WEIGHT PERCENT Ni 

IN PENTLANDITE

1.0 2.0 

WEIGHT PERCENT

Co IN 

PENTLANDITE

0.01 0.03

WEIGHT PERCENT

Co/Ni IN 

PENTLANDITE

0.85 0.90

ATOMIC METAL/SULFUR 

RATIO IN PYRRHOTITE

Figure 7. Vertical variations in characteristics of sulfide inclusions in the upper part of the B chromitite, Stillwater 

Complex, Montana. Horizontal lines separate the seven layers defined by variations in chromite abundance; lithologic 

symbols are taken from figure 36.

inclusions increases. Nickel content of pentlandite generally 

decreases upward. Inclusion size may either decrease or 

increase.

Bulk Sulfide Composition

Estimates of bulk sulfide composition for each sample were made by integrating the modal data (table 4) with 

the microprobe analyses of pentlandite and pyrrhotite (table 

5). For these calculations, chalcopyrite was assumed to be 

stoichiometric, and the presence of additional minor phases 

was ignored. The results are given in table 6 and are plotted 

along with some experimentally determined phase boundaries in figures 8 and 9. In addition, data obtained by similar 

methods from the Basal series of the Stillwater Complex are 

plotted (fig. 8).

The Ni/(Ni+Fe) ratio (weight percent) of the estimated bulk sulfide compositions determined in this study 

averages 0.30 (range, 0.09 to 0.44). This value is much 

higher than the values of 0.07 to 0.08 calculated for sulfides 

in the Basal series of the Stillwater Complex (M.P. Foose, 

unpub. data). Cu/Ni ratios (weight percent) for bulk sulfide 

in the B chromitite generally fall between 0.1 and 0.4. Page 

and Dohrenwend (1973) estimated that the overall Cu/Ni

Table 6. Estimated bulk sulfide compositions (weight percent) of inclusions from the upper part of the B chromitite, 

Stillwater Complex, Montana

Sample

1A

IB

2

3

4A**

4B

5B**

5 A***

6B***

6A

7A

7B

Average

Standard

deviation

Fe

41,

33,

41,

33,

35,

43,

40,

38,

41,

37,

40,

41,

38,

3

.0

.0

.5

.6

.1

.9

.3

.6

.7

.5

.0

.0

.9

.3

Ni

15.8

24.9

16.4

15.2

27.6

12.4

4.0

19.6

11.8

21.9

18.4

16.8

17.1

6.0

Co

0.29

.31

.36

.19

.96

.45

.11

.5

.3

.59

.71

.62

.45

.23

Cu

6.

7.

5.

16.

2,

7.

19.

6.

10.

5.

5.

6.

8.

.8

,5

,9

,2

.7

,4

,7

,4

.4

,4

,1

.2

.3

S

36

34

35

34

33

35

.1

.3

.9

.9

.7

.8

36.0

34

35

34

35

35

35

4.7

.8

.7

.6

.7

.3

.2

.7

Cu/Ni*

0.43

.30

.36

1.1

.10

.59

5.0

.32

.88

.24

.28

.37

.83

1.28

Ni/ 

(Ni+Fe)*

0.28

.43

.28

.31

.44

.22

.09

.34

.22

.37

.29

.29

.30

.09

* Weight ratios.

** Calculated by using the pyrrhotite composition of sample 4B.

***Calculated by using the pyrrhotite composition of sample 4B and 

the pentlandite composition of sample 5B.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D11
1,100°C

Fe Ni + Co

^^^^^^^

/ ______ A

Ni + Co

Fe Ni + Co

D12 Contributions on Ore Deposits in the Early Magmatic Environment
50

40

Cu 30 20 10 Fe

Figure 9. Bulk sulfide compositions of inclusions in the 

upper part of the B chromitite, Stillwater Complex, Montana, plotted in a portion of the Fe-Cu-S ternary at 1,100 

°C. Ppss , pyrrhotite solid solution; L, liquid; AVG, average. 

Plot is in weight percent and was made after subtraction 

of amounts of Fe and S needed to make pentlandite. 

Phase boundaries from Kullerud and others (1969).

ratio for sulfide in the Basal series is about 1.0. However, 

Zientek (1983) recognized several subdivisions within the 

Basal series, each having a different Cu/Ni ratio. The Cu/Ni 

ratios in the B chromitite are slightly higher than the range 

of 0.02 to 0.25 observed in sulfides at the top of the Basal 

series and are somewhat lower than both the range found in 

the lower part of the Basal series (0.4 to 1.5) and the range 

determined from the dikes and sills below the base of the 

complex (0.8 to 1.8).

Comparison with the G and H Chromitites

Many of the observations made by Page (1971) on the 

basis of study of sulfide inclusions in the G and H 

chromitites are consistent with results from this study. 

Sulfides in the G and H chromitites have similar abundances 

(0.007 volume percent in the G and 0.01 volume percent in 

the H) and similar average diameters (5.5 jxm in the G and 

11.1 jxm in the H). Single-phase inclusions are also 

common in these two chromitites. The average modal 

pentlandite/(pentlandite + pyrrhotite) ratio in both the G 

and H chromitites is 0.68, as compared with the ratio of 

0.61 observed in the B chromitite. Finally, estimates based 

on pentlandite modes suggest that, as in the B chromitite,

 ^ Figure 8. Bulk sulfide compositions of inclusions in the 

upper part of the B chromitite, Stillwater Complex, 

Montana, plotted in the ternary Fe-S-(Ni+Co). MSS, 

monosulfide solid solution; L, liquid; Pn and Po, fields 

of observed pentlandite and pyrrhotite compositions, 

respectively; AVG, average sulfide composition of all 

inclusions. Plot is in weight percent and was made after 

subtraction of amounts of Fe and S needed to make 

chalcopyrite. Phase boundaries from Kullerud and others (1969). Also plotted are some unpublished data 

from the Basal series.

the Ni/Fe ratio of included sulfides decreases upward in 

both the G and H chromitites. The B chromitite, however, 

differs somewhat from the G and H in containing larger, 

albeit still small, amounts of arsenides. The compatibility of 

these observations strongly indicates that similar processes 

were involved in forming the suliide-bearing inclusions in 

each of these chromitites.

DISCUSSION

Sulfide inclusions provide clear evidence for the 

existence of a high-temperature, immiscible sulfide liquid 

during deposition of this part of the Stillwater Complex. 

Experimental studies at 1,200 °C show that, under most 

conditions, silicate liquids containing 10 to 12 weight 

percent FeO are capable of dissolving about 0.1 weight 

percent sulfur (Haughton and others, 1974; Buchanan and 

Nolan, 1979). This range of iron content is characteristic of 

quench-textured sulfide-free dikes that may represent parental liquids to the Stillwater Complex (Zientek, 1983). An 

increase in fO2 or a decrease in /S2 , activity of FeO, or 

temperature would lower sulfur solubility and cause formation of small amounts of immiscible sulfide melt, as 

observed in the B chromitite.

Sulfides in a number of mafic magmatic bodies are 

thought to have been partly derived from sulfur contained in 

adjacent country rocks. The lack of an identifiable source 

for external sulfur, the absence of sulfide-rich zones that 

might result from assimilation of sulfides, and the presence 

of mantle-type sulfur isotope ratios characteristic of Basal 

series sulfide-bearing zones (Zientek and others, 1982; 

Zientek, 1983) make it unlikely that assimilation caused 

precipitation of the sulfides in the B chromitite.

Accumulation of Sulfides

The accumulation of sulfide melt has been modeled 

by a process whereby, like falling rain drops, sulfide 

globules move downward through less dense silicate liquid. 

Droplets sometimes coalesce to form larger globules that 

then sink with greater speed (Page, 1979). Sulfide melt may 

ultimately accumulate at the floor of the magma chamber to 

form net-textured or massive concentrations. However, 

calculated settling rates for sulfide inclusions of the size 

found in the B chromitite are only about 0.25 cm/yr. This 

calculation is derived from Stokes' Law and is based on an 

estimated viscosity of 59.1 poise for the silicate liquid, a 

sulfide density of 4.804 g/cm3 , and a silicate liquid density 

of 2.712 g/cm3 (Page, 1979). On the other hand, Hess 

(1960) calculated an overall accumulation rate of about 10 

cm/yr for the silicates within the entire intrusion, and 

Jackson (1971) indicated that olivine and chromite in the 

Ultramafic series would settle at rates greater than 100 

m/yr.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D13
These rates indicate that, if sulfides precipitated along 

with oxides and silicates, most should have been trapped 

near their point of formation. If, however, significant 

amounts of sulfide melt had separated before the onset of 

silicate crystallization, the settling rate of sulfide melt 

would still have been sufficient to form a thin sulfide layer 

within 10 to 100 years. Such concentrations are locally 

observed at bottoms of chromitites (Page, 1971) but are not 

found in this part of the B chromitite. Finally, although 

sulfide inclusion size increases downward in some layers, it 

increases upward in others (for example in layer 4) in a 

manner not consistent with a settling process. These observations also suggest that the trapping of sulfide inclusions 

occurred near their point of formation.

Sulfide Inclusion Composition

An unusual aspect of the sulfide inclusions population 

is the large number of single-phase inclusions (table 3). To 

some extent, this may result from incomplete exposure of 

polyphase inclusions in thin section. However, the large 

number of such inclusions suggests that numerous inclusions consist of only one phase (at least within limits of 

optical resolution). Page (1971) noted a similar abundance 

of single-phase inclusions within the G and H chromitites.

When averaged together, data from these B chromitite inclusions yield estimated bulk sulfide compositions 

that differ from those found in most magmatic sulfide 

deposits (table 7). Ni/Fe ratios in the inclusions are generally higher than the range of 0.05 to 0.2 typically observed 

in most deposits, and the range in sulfur content (34 to 37 

weight percent) is lower than the 38 to 40 weight percent 

sulfur typical of most magmatic sulfide deposits (Naldrett, 

1981).

However, sulfide inclusions from fresh Mid-Atlantic 

ridge basalts have bulk compositions similar to those 

estimated for the B chromitite (Czamanske and Moore, 

1977). The average nickel, copper, and sulfur contents in 

samples from Mid-Atlantic ridge basalts (12 to 15 weight 

percent Ni, 8 to 10.5 weight percent Cu, and 33.8 to 34.8 

weight percent S) overlap the values determined for the B 

chromitite, while cobalt values from the Mid-Atlantic ridge 

basalts (average 0.18 weight percent) are somewhat lower. 

In addition, the abundance (0.022 volume percent) and 

diameter (5 to 500 jxm) of the Mid-Atlantic ridge basalthosted inclusions closely match those observed in the B 

chromitite.

Processes of Inclusion Formation

The compositional diversity shown by individual 

inclusions in the B chromitite and the unusual bulk sulfide 

compositions estimated from inclusions within individual 

samples may be attributed to two quite different processes.

Table 7. Comparison of compositions of sulfide inclusions 

from the upper part of the B chromitite, Stillwater Complex, Montana, with compositions from diverse magmatic 

sulfide deposits

[In weight percent. Data from Czamanske and Moore (1977) and Naldrett 

(1981); , no data available]

Deposit 

setting

B chromitite*

Archean

komatiite

Proterozoic

komatiite

Sudbury 

Flood basalts

Tholeiitic

intrusions

Mid-Atlantic

ridge basalts

Nickel

11.1-23.1

10-15

10-16

3-6 

4-9

4.5-9

12.0-15.5**

Copper

3.6-13.0

0.5-1.5

3-4

1-5 

9.7-17

1.3-2.5

8.0-10.5**

Cobalt

0.22-0.68

0.22-0.34

0.21-0.23

0.15-0.20

.2

0.13-0.25

* Range is plus and minus one standard deviation.

**Range of average values.

First, the compositionally diverse sulfide inclusions may be 

substantially unchanged since trapping, so that the bulk 

compositions estimated from them approximate the overall 

composition of sulfide entrained in the silicate melt. Alternatively, the observed diverse inclusion compositions may 

result from subsolidus reequilibration between trapped sulfide and the surrounding chromite and silicate.

This latter possibility may be illustrated by two types 

of subsolidus exchange reactions between sulfide inclusions 

and enclosing chromite grains. The first of these involves 

exchange of Fe for Ni and Co and involves no loss of sulfur:

and

FeS+NiCr2O4-»NiS+FeCr2O4

FeS+CoCr2O4->CoS+FeCr2O4

(1)

(2)

The second type, suggested by Naldrett and Lehman (1987) 

and Naldrett and others (1987), involves the diffusion of S 

through the enclosing chromite:

4/3Fe2O3 (spinel) + l/3FeS-»Fe3O4 ( 1/6S2 (3)

Fe is used to make additional spinel, and the sulfide is 

enriched in residual Ni and Co.

All of these reactions would involve small amounts of 

Fe, Ni, Co, and S. A sulfide inclusion 10 jxm in diameter 

and having the average composition estimated for all 

inclusions in the B chromitite (table 6) will contain only 

1.8X10" 11 moles of Fe, 6.9xlO~ 12 moles of Ni, 

2.2X10" 13 moles of Co, 3.9X10" 12 moles of Cu, and 

2.9xlO~ n moles of sulfur. If this globule had actually 

started with a more Fe- and S-rich composition, typical of

D14 Contributions on Ore Deposits in the Early Magmatic Environment
most magmatic sulfides (table 7), and changed composition 

by reactions (1) and (2), the sulfide would have had to lose 

about l.lx 1(T 12 moles of Fe and acquire about 4.9x 10~ 12 

moles of Ni and 1.5xlO~ 13 moles of Co. If, however, 

compositional changes occurred as a result of reaction (3), 

the initial iron-rich sulfide inclusion would have needed to 

be much larger originally (15 (Jim in diameter) and would 

have had to lose about 70 percent of its Fe (6.1xlO~ u 

moles) and S (6.5X1CT 11 moles) in order to achieve the 

average composition observed for the B chromitite sulfide 

inclusions. The attractive aspect of these reactions is that 

variations in the amount of subsolidus compositional 

change may cause inclusions to vary widely in composition, 

with single-phase inclusions having developed where extensive exchange had occurred.

Several arguments suggest that extensive subsolidus 

exchange of the types in reactions (1) and (2) has not 

occurred. The exchange reactions between chromite and 

sulfide have not been experimentally studied, but those 

between magnetite and sulfide have. Boctor and Mysen 

(1980) and Boctor (1981) found that the partition coefficient 

for nickel between pyrrhotite and magnetite over the range 

of 600 °C to 300 °C decreased from about 25 to a value 

between 21 and 15; between 500 °C and 300 °C, the 

coefficient for Co increased from a value between 2 and 3 

to a value between 6 and 10. Thus, with decreasing 

temperature, sulfide will lose nickel and gain cobalt from 

magnetite.

The results of these studies are considered to be 

applicable to reactions involving sulfide and chromite, 

because the crystal chemistries of magnetite and chromite 

are similar and because Navrotsky (1975) has shown that 

the enthalpies of formation of Co- and Ni-bearing magnetite 

vary over a similar range to the enthalpies of formation of 

Co- and Ni-bearing chromite.

The partitioning data indicate that any change 

induced by subsolidus equilibration would have made these 

inclusions more Fe- and Co-rich. This is inconsistent with 

the relatively pentlandite-rich mineralogy shown by the 

entire inclusion population (table 4) and, in particular, by 

the presence of relatively abundant single-phase inclusions 

consisting of pentlandite (table 3). Similarly, subsolidus 

changes should have increased the Co/Ni ratio of the 

trapped pentlandite. Harris and Nickel (1972), for example, 

pointed out that pentlandite containing more than about 2 

weight percent Co is not typical of magmatic sulfide 

deposits but has been reported from a variety of nonmagmatic deposits. The compositions determined from pentlandite in individual inclusions (table 5) are not rich in Co.

Similarly, the composition of pyrrhotite in the sulfide 

inclusions indicates that there has been little uncoupled loss 

of iron to the surrounding chromite. Such loss of iron would 

cause a decrease in sulfide metal/sulfur ratio and eventually 

result in formation of pyrite. Pyrite is not observed in any of 

the inclusions. Conversely, the addition of iron from

adjacent chromite would cause hexagonal pyrrhotite or 

ultimately troilite to form. However, most of the pyrrhotites 

have the lower iron/sulfur ratios of monoclinic pyrrhotite 

(table 5).

Comparable arguments also suggest that significant 

changes due to reaction (3) have not occurred. Reaction (3) 

requires loss of sulfur. If sulfur were not able to diffuse 

through the chromite structure, the reaction would stop. 

However, if sulfur were lost, the nickel content of the 

sulfide would increase until nickel-rich phases like violarite, millerite, and heazlewoodite were stabilized. In 

many instances, pyrite would be the predicted coexisting 

iron sulfide. Again, pyrite is not observed, nor is violarite 

or millerite. Heazlewoodite is present in some inclusions 

(table 3) but is so rare as to indicate that alteration by 

reaction (3) has not widely affected all inclusions. Further, 

compositional change by reaction (3) should necessitate a 

large volume decrease in the sulfide inclusion, a phenomenon for which no evidence was seen in thin section.

Several additional pieces of evidence argue against 

substantial subsolidus changes in inclusion composition. 

First, abrupt changes in the composition of pyrrhotite and 

pentlandite (fig. 7) are observed to coincide with changes in 

the B chromitite stratigraphy. These variations also coincide 

with changes in other characteristics of the inclusions, such 

as their type, distribution, size, and abundance. If subsolidus changes had altered the composition of inclusions, 

pyrrhotites and pentlandites within a sample should vary 

widely in composition as a result of differing amounts of 

equilibration with their chromite or silicate host. The 

correspondence of compositional and stratigraphic features 

evident in figure 7 would not be expected.

Second, the bulk sulfide compositions estimated from 

these diverse inclusions are similar to the sulfide melt 

composition in Mid-Atlantic ridge basalts. These bulk 

compositions are, therefore, present in magmatic systems, 

even though they are much more nickel rich than compositions reported from most magmatic sulfide deposits.

Third, Lorand (1985, 1987) has documented similar 

compositionally diverse sulfide inclusions within chromites 

in upper mantle peridotites. Many of these inclusions, 

which Lorand showed had escaped chemical alteration, 

consist of a single phase that usually is pentlandite; the bulk 

sulfide compositions estimated by using these inclusions are 

similar to the nickel-rich compositions determined for the B 

chromitite.

Although it is probable that some subsolidus change 

in inclusion composition has occurred, the evidence suggests that any such change was local and relatively minor. 

Thus the overall abundance and composition of these 

inclusions probably closely reflect the original character of 

the sulfides before trapping. Therefore, the compositional 

heterogeneity of inclusions found within single samples 

must largely result from primary processes.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D15
One process that could result in primary differences 

in inclusion mineralogy is the fractional crystallization of 

sulfide liquid. Over the temperature range of 1,100 °C to 

1,000 °C in the Ni-Fe-S system (fig. 8), the monosulfide 

solid solution (MSS) field expands towards the Ni-S join. 

Most of the estimated bulk sulfide compositions plot in the 

divariant (liquid + MSS) field present at temperatures 

between 1,100 °C and 1,000 °C. Therefore, the separation 

of a nickel-rich liquid from an iron-rich solid would be 

possible over this temperature range. Upon further cooling 

to 610 °C, pentlandite would become stable; by 500 °C, 

exsolution of pyrrhotite and pentlandite from MSS would 

have been completed in most of these inclusions.

A similar relation exists in the Cu-Fe-S system (fig. 

9). At 1,100 °C, many of the estimated sulfide compositions 

fall in the divariant (pyrrhotite + liquid) field, so that 

separation of a copper-rich liquid from an iron-rich solid 

would be possible.

The major problem with this argument is that the 

temperature at which most of the chromite crystallized 

appears to have been greater than the temperatures at which 

fractional crystallization of sulfide is believed to be possible. The temperature of chromite precipitation in the Stillwater is not well known. Jackson (1969) proposed that the 

G and H chromitites crystallized between 1,300 °C and 

1,000 °C on the basis of the compositions of olivinechromite pairs. This temperature range is probably due to 

subsolidus equilibration (Irvine, 1967). Estimates based on 

chromite and olivine compositions from monomineralic 

layers, where the effects of subsolidus changes would be 

minimized, indicate temperatures in the middle of Jackson's 

range (Evans and Frost, 1975). Murck (1985) approached 

the question differently by analyzing the mechanisms by 

which Still water chromite may have precipitated. She 

concluded that temperatures of formation were most probably between 1,200 °C and 1,300 °C.

Sharp (1969) and Ryzenkho and Kennedy (1973) 

showed that the liquidus temperature of pyrrhotite increases 

about 7 °C/kbar. Therefore, at pressures between 10 and 20 

kbar, the sulfide liquidus may approach 1,200 °C. However, pressures of only 3-4 kbar are indicated from contact 

metamorphosed rocks at the base of the Stillwater Complex 

(Labotka, 1985). Therefore, the temperature of chromite 

formation appears to have generally been significantly 

higher than temperatures at which the processes shown in 

figures 8 and 9 would have been effective.

Despite trapping temperatures that appear to be too 

high for significant sulfide fractional crystallization, the fact 

remains that some type 1 inclusions have shapes of sulfide 

crystals. This suggests that at least some crystallization of 

the immiscible sulfide liquid occurred before trapping by 

some of the chromite. Page (1971) made similar observations on inclusions in the G and H chromitites and was also 

led to conclude that some crystalline sulfide coexisted with

immiscible sulfide liquid before inclusion in olivine or 

chromite.

Variations in the ratio of silicate liquid equilibrated 

with the sulfide melt (R) may provide a second way to 

explain the compositional diversity among sulfide inclusions. MacLean and Shimazaki (1976) and Rajamani and 

Naldrett (1978) showed that nickel is much more strongly 

partitioned into the sulfide phase than is iron. Therefore, the 

first sulfide to equilibrate with a silicate liquid will be nickel 

rich. Formation of more sulfide (decreasing R) will rapidly 

deplete the nickel in the enclosing silicate liquid with the 

result that the Fe/Ni ratio of subsequent sulfides will 

increase. Because values of R for the sparsely disseminated 

sulfides from the B chromitite probably were much greater 

than those in economic deposits containing vastly larger 

amounts of sulfides, the Ni/Fe ratio in these inclusions may 

have been higher than values typically reported for magmatic sulfides.

In most situations, the relation between silicate liquid 

and sulfide melt can be expressed as (Campbell and 

Naldrett, 1979)

(4)

where

Xsul is the concentration of trace element X in the 

sulfide melt;

Xsil is the concentration of trace element X in the 

silicate liquid;

R is the ratio of silicate liquid to sulfide melt; and

Dx is the partition coefficient for trace element X. 

This expression shows that, if the composition of the 

silicate liquid remains constant, the composition of the 

immiscible sulfide melt will be controlled by R and the 

partition coefficient D.

The effect on sulfide composition of variations in R 

and D is shown in figure 10. This plot is made by assuming 

a silicate liquid containing 350 ppm nickel. This Ni content 

is consistent with data from dikes at the base of the 

Stillwater Complex (Zientek, 1983), but this discussion is 

not critically dependent on it. Nickel partition coefficients 

are generally considered to range from 275 for basaltic 

liquids to about 100 for ultramafic liquids (Naldrett, 1981). 

Estimated values of R in the B chromitite, derived from the 

volume of sulfide observed in rock, range between 6,000 

and 50,000. These are maximum values because individual 

sulfide inclusions may have equilibrated with smaller volumes of silicate liquid. Over this range of R values and 

partition coefficients, figure 10 shows that little change in 

sulfide nickel content can be expected. Czamanske and 

Moore (1977), however, determined nickel partition coefficients for sulfide globules trapped in Mid-Atlantic ridge 

basalts that ranged up to 770. With these larger coefficients, 

variations of up to 5 percent nickel are possible over the

D16 Contributions on Ore Deposits in the Early Magmatic Environment
40.0

35.0-

30.0-

25.0-

20.0-

15.0-

10.0-

LOG ft

Figure 10. Changes in sulfide nickel content with variations in the ratio of silicate liquid to sulfide melt (R) for 

different nickel partition coefficients (D). Calculations 

were made by assuming 350 ppm nickel in the silicate 

liquid, in agreement with data from Zientek (1983). Values 

of D are from Naldrett (1981) and Czamanske and Moore 

(1977).

range in R values given above. Much larger variations in 

nickel content would be possible if individual inclusions 

equilibrated with significantly less silicate liquid (smaller R 

values).

Assuming a partition coefficient of 770 (Czamanske 

and Moore, 1977), the maximum nickel content in sulfide 

equilibrated with a silicate liquid containing 350 ppm nickel 

is about 27 weight percent. This value is close to the 

maximum nickel content estimated for bulk sulfide trapped 

in the B chromitite. Since pentlandite in this chromitite 

averages 34 weight percent nickel (table 5), about 80 

percent of a trapped sulfide melt having this bulk composition would ultimately crystallize as pentlandite. The 

abundance of inclusions apparently consisting of only 

pentlandite may, therefore, largely result from thin-section 

exposures that do not intersect the other phases, which 

make up the remaining 20 percent of the inclusion.

Work by Cowden and Woolrich (1987) and Doyle 

and Naldrett (1987) indicates that variations in jO2 will 

affect the partitioning of metals in sulfide liquids and thus

suggests another mechanism by which primary compositional differences among inclusions might have been generated. Cowden and Woolrich argued that, as/O2 increases, 

the number of 6-fold sites in the silicate liquid is reduced. 

Because Ni2+ has a much stronger preference for 6-fold 

sites than Fe2+ , its fractionation into the abundant 6-fold 

sites present in sulfides is increased. Further, the increase in 

JO2 decreases the Fe2+/Fe3+ ratio of the silicate melt, 

making less Fe2+ available for partitioning into sulfide. In 

effect, variations in JO2 change the nickel partitioning 

coefficient, such that increases in fO2 cause marked 

increases in the Ni content of sulfides. Cowden and Woolrich (1987) concluded that variations in the Fe/Ni ratio in 

sulfides from Kambalda, Australia, are due to variations in 

yO2 and that a change in/O2 from 10~ u to 10~ 10 may cause 

variations in sulfide nickel content from 9 to more than 20 

weight percent. Similarly, in a review of available experimental data, Doyle and Naldrett (1987) demonstrated that 

variations in the ratio of O/(S+O) may markedly change the 

exchange coefficient of nickel between sulfide melt and 

olivine.

Once trapped, sulfides containing different amounts 

of oxygen may undergo further compositional changes. 

During fractional crystallization, sulfides that are oxygen 

rich may crystallize oxides, whereas those that are oxygen 

poor may form only monosulfide solid solution (Naldrett, 

1969). Although magnetite is the most typical oxide to form 

during crystallization of a sulfide melt, in Cr-rich systems, 

such as the B chromitite, a Cr-rich spinel may initially form 

(Czamanske and others, 1976; Ewers and others, 1976; 

Groves and others, 1977). The chromium in the sulfide melt 

is quickly depleted so that magnetite is precipitated as rims 

on the chromite. Therefore, fractional crystallization of 

sulfide inclusions that initially contained different amounts 

of oxygen may result in the formation of different amounts 

of oxides. These oxides most probably would nucleate on 

the walls of the trapping phase and, because of their small 

volume, not be observable in these inclusions. Their crystallization, however, could dramatically influence the 

amount of iron available for incorporation into sulfide and 

thus control the Fe/Ni ratio in sulfide inclusions.

The local variations in JO2 required by this mechanism could occur in at least two ways. Ulmer (1969) 

showed that increases in JO2 will encourage chromite 

precipitation and, conversely, that chromite precipitation 

will tend to lower JO2 in the silicate liquid. Very local and 

relatively short-lived variations in fO2 may therefore be 

expected in the liquid immediately adjacent to a nucleating 

and growing chromite grain. Because trapping of the sulfide 

inclusions is intimately associated with chromite growth, it 

is possible that, just before trapping, these inclusions 

equilibrated with silicate liquids having different values of

Alternatively, Murck (1985) proposed that some of 

the Stillwater chromitites formed as a result of mixing of

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D17
magmas that differed both in composition and in temperature. These differences would also virtually insure a difference in JO2 . For example, two liquids that were both 

equilibrated with the quartz-fayalite-magnetite buffer but 

that differed in temperature by 100 °C could easily have 

differences in JO2 in excess of 1 log unit. Murck (1985) 

suggested that temperature differences of 50 °C to 200 °C 

may exist between the liquids that mixed to form some 

Stillwater chromites. Calculations by Huppert and Sparks 

(1980) indicate that mixing of such liquids would be 

extremely turbulent. The result would be a zone of mixing 

in which chromite precipitated and in which there were 

significant local variations both in liquid composition and in 

P2 -

Variation in Fe3+/(Cr+Al+Fe3+) has been shown to 

be a sensitive indicator of changes in magma JO2 (Ulmer, 

1969). Data from the G and H chromitites (Murck, 1985) 

show local variations in this ratio. In massive chromitites, 

where effects of subsolidus change should be minimized, 

this ratio fluctuates between 500 and 800. Hill and Roeder 

(1974) indicated that this range of compositional variation 

could occur in basaltic liquids at 1,000 °C by changing f02 

about one-half log unit.

To summarize, a turbulent-liquid mixing model for 

formation of the B chromitite allows rapid and localized 

variation of two important parameters that may cause 

inclusions to become compositionally diverse. First, large 

variations in R values could result from turbulent mixing in 

which individual sulfide globules were trapped before they 

had completely equilibrated with the surrounding silicate 

liquid. Second, local variations in fO2 would occur within 

the zone of mixing, causing variations in the partitioning 

coefficients and, perhaps, in the amount of oxide crystallized from the trapped sulfide melt.

Several additional points should be made concerning 

the effect of these processes on sulfide inclusions. First, 

although large local variations in inclusion composition 

may occur as a result of liquid mixing, the entire population 

of sulfide inclusions within a sample may define an average 

bulk sulfide composition that reflects the average composition and./D2 of the silicate liquid forming that part of the B 

chromitite from which the sample was taken. Second, 

although changes in JO2 may dramatically alter the partitioning of nickel into sulfides relative to iron, similar 

relative changes between nickel and cobalt are not to be 

expected. Nickel and cobalt have similar octahedral site 

preferences (Burns, 1970), and, therefore, changes in the 

structure of the silicate melt as a result of changes in fO2 

should affect the partitioning of nickel and cobalt in similar 

ways. Therefore, the Co/Ni ratio of the immiscible sulfide 

liquid should be relatively insensitive to changes in/O2 and 

should be controlled principally by the composition of the 

silicate liquid. Third, any post-trapping crystallization of 

oxides would alter the amount of Fe in the sulfide but would 

not affect the proportions of Ni and Co. Fourth, any

subsolidus change in composition as a result of reaction (3) 

would only involve loss of Fe and S and also would not 

affect the abundance of Ni relative to Co. Thus, variations 

in Co/Ni ratio within sulfide inclusions may directly reflect 

differences in Co/Ni ratio of the silicate liquids from which 

the sulfides initially separated.

Vertical Changes in Composition

Pentlandite is the only sulfide phase in these inclusions that contains significant amounts of cobalt and nickel. 

Because cobalt and nickel both prefer the sulfide melt 

relative to the silicate liquid, variations in the Co/Ni ratio in 

pentlandite should act as an extremely sensitive indicator of 

variations in the composition of the silicate liquid. Duke 

and Naldrett (1978) indicated that the compositions of 

olivine that separates from a liquid from which 50 molecular percent of the olivine has separated and which has 

changed its MgO content from about 30 to about 17 percent 

will range only from Fo94 8 to Fo89 , whereas the nickel 

content of sulfides forming over the same crystallization 

interval will change from more than 17 to less than 2 weight 

percent.

The pentlandite data (table 5, fig. 7) suggest that at 

least three compositionally different batches of liquid were 

involved in the crystallization of the upper part of the B 

chromitite. The changes in Co/Ni that define these batches 

occur between layers 1 and 2 and layers 4 and 5. No data are 

available to determine whether similar changes in liquid 

compositions occurred in layer 3. Relatively unifortn pentlandite compositions in layers 5,6, and the lower part of 7 

suggest that the liquids from which these layers crystallized 

did not have different Co/Ni ratios.

The sharp shift to lower Co/Ni between layers 1 and 

2 and between 4 and 5 is consistent with the introduction of 

a pulse of more primitive liquid. Where sufficient data exist 

to make the determination (layers 1,4, and 7), the Co/Ni 

ratio increases upward, following the expected pattern for 

fractional depletion of nickel and cobalt from a silicate 

liquid.

The two distinct breaks in pentlandite composition 

are also accompanied by changes in the manner in which 

sulfides occur. The most noticeable of these changes is an 

increase, at the base of each section, in the volume 

percentage of sulfide and in the abundance of type 1 sulfide 

inclusions. For reasons outlined earlier, this increase in 

volume percentage of sulfides is not thought to result from 

gravity concentration of sulfides. Instead, it may reflect the 

initial reduction in the solubility of sulfur, which may result 

from mixing two different liquids or from cooling a newly 

injected liquid against the underlying cumulus pile. Local 

sulfide supersaturation may also have induced sulfide separation at a large number of sites in the silicate liquid, thus 

producing the large number of type 1 inclusions found at the 

bottom of these sections.

D18 Contributions on Ore Deposits in the Early Magmatic Environment
CONCLUSIONS

Sulfide inclusions constituting between 0.002 and 

0.016 volume percent of the rock and ranging in diameter 

from 11.9 to 18.5 |xm are present within the B chromitite. 

The inclusions are compositionally diverse. Inclusions having three or more phases are rare; most contain only one or 

two phases. The estimated average bulk sulfide composition 

for the inclusions is 38.9 percent Fe, 17.1 percent Ni, 0.45 

percent Co, 8.3 percent Cu, and 35.2 percent S. The 

Ni/(Ni+Fe) ratio of about 0.3 is unusually high and the 

sulfur content is low when compared to values reported 

from most magmatic sulfides. These compositions, however, are similar to those of sulfide globules trapped in 

Mid-Atlantic ridge basalts.

Several arguments suggest that these compositions 

have not been substantially affected by subsolidus exchange 

with the surrounding hosts. First, data on sulfide exchange 

with magnetite suggest that subsolidus equilibration should 

have decreased the Ni/Fe ratio, a change not supported by 

the abundance of pentlandite in inclusions. Second, the 

exchange data indicate that the Co/Ni ratio in these inclusions should increase as a result of diffusion, whereas 

pentlandites in these inclusions have Co/Ni ratios typical of 

magmatic deposits. Third, the subtraction or addition of Fe 

from adjacent chromite should have formed either pyrite or 

abundant hexagonal pyrrhotite, neither of which is 

observed. Fourth, diffusive loss of sulfur and iron would 

ultimately produce nickel-rich phases that are generally not 

iQbferved. Fifth, changes in inclusion composition occur 

wheref there- are abrupt changes in the internal stratigraphy 

'of the B chromitite. These changes also coincide with 

changes in abundance of inclusions, volume of sulfides, and 

proportion of inclusion types. Post-trapping, diffusioninduced changes should develop locally within the enclosing host mineral so that the observed compositional changes 

should not correlate with the B chromitite stratigraphy.

Several processes may account for the diversity 

among individual inclusions and their unusual bulk compositions. Some fractional crystallization of the sulfide melt 

may have occurred, as indicated by the presence of inclusions having shapes of sulfide crystals. However, this 

process probably was not important because it would have 

occurred at temperatures lower than those at which most 

inclusions were probably trapped. Variations in D (partition 

coefficients) and R (the ratio of silicate liquid to sulfide 

melt) are more likely to have affected the inclusion composition. Extremely local variations in D may have resulted 

from variations in JO2 as a result of chromite precipitation 

and (or) mixing of compositionally and thermally different 

liquids. Sulfide melt trapped under differing JO2 would 

have had different Fe/Ni ratios and, after entrapment, may 

have crystallized different amounts of oxides, thus further 

altering Fe/Ni ratios of inclusions. Variations in R would 

result from entrapment of sulfide melt before complete 

equilibration with the silicate liquid occurred, a process

facilitated by chromite precipitation and sulfide entrapment 

in a zone of turbulent mixing of two different liquids.

The Co/Ni value of the sulfide melt is relatively 

insensitive to changes in magma JO2 and also is not 

substantially changed by post-trapping precipitation of 

oxides or loss of iron and sulfur. Variations in this ratio may 

therefore effectively reflect the composition of the silicate 

liquid from which the sulfide melt separated. Changes in 

Co/Ni content of pentlandites occur where there are abrupt 

changes in the stratigraphy of the B chromitite and indicate 

that at least three separate pulses of silicate liquid were 

involved in the deposition of the upper part of the B 

chromitite. These results support the hypothesis that 

chromite in the upper part of the B chromitite formed as a 

result of the mixing of compositionally different liquids.

REFERENCES

Attanasi, E.D., and Bawiec, W.J., 1987, A resource assessment 

of copper and nickel sulfides within the Mountain View area 

of the Still water Complex, Montana: U.S. Geological Survey 

Bulletin 1674-B, 27 p.

Barnes, S.J., and Naldrett, A.J., 1985, Geochemistry of the J-M 

(Rowland) Reef of the Stillwater Complex, Minneapolis Adit 

area. 1. Sulfide chemistry and sulfide-olivine equilibrium: 

Economic Geology, v. 80, p. 627-645.

 1986, Geochemistry of the J-M Reef of the Stillwater 

Complex, Minneapolis Adit area. 2. Silicate mineral chemistry and petrogenesis: Journal of Petrology, v. 27, p. 

791-825.

Boctor, N.Z., 1981, Partitioning of nickel and cobalt between 

pyrrhotite, pyrite, and magnetite: Carnegie Institution of 

Washington Year Book 80, p. 352-356.

Boctor, N.Z., and Mysen, B.O., 1980, Partitioning of nickel 

between pyrrhotite and magnetite: Carnegie Institution of 

Washington Year Book 79, p. 337-340.

Boudreau, A.E., Mathez, E.A., and McCallum, I.S., 1986, 

Halogen geochemistry of the Stillwater and Bushveld complexes; evidence for transport of the platinum-group elements 

by Cl-rich fluids: Journal of Petrology, v. 27, p. 967-986.

Bow, C., Wolfgram, D., Turner, A., Barnes, S., Evans, J., 

Zdepski, M., and Boudreau, A., 1982, Investigations of the 

Howland Reef of the Stillwater Complex, Minneapolis Adit 

area: Stratigraphy, structure, and mineralization: Economic 

Geology, v. 77, p. 1481-1492.

Buchanan, D.L., and Nolan, J., 1979, Solubility of sulfur and 

sulfide immiscibility in synthetic tholeiitic melts and their 

relevance to Bushveld Complex rocks: Canadian Mineralogist, v. 17, p. 483^94.

Burns, R.G., 1970, Mineralogical applications of crystal field 

theory: Cambridge, England, Cambridge University Press, 

224 p.

Campbell, I.H., and Naldrett, A.J., 1979, The influence of 

silicate:sulfide ratios on the geochemistry of magmatic sulfides: Economic Geology, v. 74, p. 1503-1505.

Campbell, I.H., Naldrett, A.J., and Barnes, S.J., 1983, A model 

for the origin of the platinum-rich sulfide horizons in the 

Bushveld and Stillwater Complexes: Journal of Petrology, v. 

24, p. 133-165.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D19
Coffrant, Denis, Tatsumoto, Mitsunobu, and Obradovich, J.D., 

1980, Sm-Nd age of the Stillwater Complex [abs.]: International Geological Congress, 26th, Paris, 1980, Abstracts, v. 

2, p. 768.

Conn, H.K., 1979, The Johns-Manville platinum-palladium prospect, Stillwater Complex, Montana, U.S.A.: Canadian Mineralogist, v. 17, p. 463-468.

Cowden, A., and Woolrich, P., 1987, Geochemistry of the 

Kambalda iron-nickel sulfides; implications for models for 

sulfide-silicate partitioning: Canadian Mineralogist, v. 25, p. 

21-36.

Czamanske, O.K., Himmelberg, G.R., and Goff, F.E., 1976, 

Zoned Cr, Fe-spinel from the La Perouse layered gabbro, 

Fairweather Range, Alaska: Earth and Planetary Science 

Letters, v. 33, p. 111-118.

Czamanske, G.K., and Moore, J.G., 1977, Composition and 

phase chemistry of sulfide globules in basalt from the 

Mid-Atlantic Ridge rift valley near 37°N lat: Geological 

Society of America Bulletin, v. 88, p. 587-599.

Czamanske, O.K., and Zientek, M.L., eds., 1985, The Stillwater 

Complex, Montana, Geology and guide: Montana Bureau of 

Mines and Geology Special Publication 92, 396 p.

DePaolo, D.J., and Wasserburg, G.J., 1979, Sm-Nd age of the 

Stillwater Complex and the mantle evolution curve of neodymium: Geochimica et Cosmochimica Acta, v. 43, p. 

999-1008.

Doyle, C.D., and Naldrett, A.J., 1987, The oxygen content of 

"sulfide" magma and its effect on the partitioning of nickel 

between coexisting olivine and molten ores: Economic Geology, v. 82, p. 208-211.

Drew, L.J., Bawiec, W.J., Page, N.J, and Schuenemeyer, J.H., 

1985, The copper-nickel concentration log: A tool for stratigraphic interpretation within the ultramafic and basal zones 

of the Stillwater Complex, Montana: Journal of Geochemical 

Exploration, v. 23, p. 117-137.

Duke, J.M., and Naldrett, A.J., 1978, A numerical model of the 

fractionation of olivine and molten sulfide from komatiite 

magma: Earth and Planetary Science Letters, v. 39, p. 

255-266.

Evans, B., and Frost, B., 1975, Chrome-spinel in progressive 

metamorphism A preliminary analysis: Geochimica et Cosmochimica Acta, v. 39, p. 959-972.

Ewers, W.E., Graham, J., Hudson, D.R., and Rolls, J.M., 1976, 

Crystallization of chromite from nickel-iron sulphide melts: 

Contributions to Mineralogy and Petrology, v. 54, p. 61-64.

Foose, M.P., in press, Deposits containing nickel, cobalt, chromium, and platinum-group elements in the United States, in 

DNAG volume on Economic Geology: Boulder, Colo., 

Geological Society of America.

Groves, D.I., Barrett, P.M., Binns, R.A., and McQueen, K.G., 

1977, Spinel phases associated with metamorphosed 

volcanic-type iron-nickel ores from Western Australia: Economic Geology, v. 72, p. 1224-1244.

Harris, D.C., and Nickel, W.H., 1972, Pentlandite compositions 

and associations in some mineral deposits: Canadian Mineralogist, v. 11, p. 861-876.

Haughton, D., Roeder, P., and Skinner, B., 1974, Solubility of 

sulfur in mafic magmas: Economic Geology, v. 69, p. 

451^67.

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

quantitative mineralogical study: Geological Society of 

America Memoir 80, 230 p.

Hill, R., and Roeder, P., 1974, The crystallization of spinels from 

basaltic liquid as a function of oxygen fugacity: Journal of 

Geology, v. 82, p. 709-729.

Rowland, A.L., 1933, Sulphides and metamorphic rocks at the 

base of the Stillwater Complex, Montana: Princeton, N.J., 

Princeton University, Ph.D. thesis, 76 p.

 1955, Chromite deposits in the central part of the Stillwater 

complex, Sweet Grass County, Montana: U.S. Geological 

Survey Bulletin 1015-D, p. 99-121.

Humphreys, R.D., 1983, The characterization of the sulfide 

component in layered igneous and metasedimentary rocks in 

the Mouat Block of the Stillwater Complex: San Jose, Calif., 

San Jose State University, M.Sc. thesis, 44 p.

Huppert, H.E., and Sparks, R.S.J., 1980, The fluid dynamics of 

a basaltic magma chamber replenished by influxes of hot, 

dense ultrabasic magma: Contributions to Mineralogy and 

Petrology, v. 75, p. 279-289.

Irvine, T.N., 1967, Chromian spinel as a petrogenetic indicator: 

Part 2, petrologic applications: Canadian Journal of Earth 

Sciences, v. 4, p. 71-103.

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

platinum palladium reef of the Stillwater Complex, Montana: 

2. Origin by double-diffusive convective magma mixing and 

implications for the Bush veld Complex: Economic Geology, 

v. 78, p. 1287-1334.

Jackson, E.D., 1961, Primary textures and mineral associations in 

the Ultramafic zone of the Stillwater Complex, Montana: 

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

 1963, Stratigraphic and lateral variations of chromite 

compositions from the Stillwater Complex: Mineralogical 

Society of America, Special Paper 1, p. 46-54.

 1967, Ultramafic cumulates in the Stillwater, Great Dyke, 

and Bushveld intrusions, in Wyllie, P.J., ed., Ultramafic and 

related rocks: New York, John Wiley and Sons, p. 20-38.

 1968, The chromite deposits of the Stillwater Complex, 

Montana, in Ridge, J.D., ed., Ore deposits of the United 

States, 1933-1967 (Graton-Sales v. 2): New York, American 

Institute of Mining, Metallurgy, and Petroleurn Engineers, p. 

1495-1510. : ' K

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

H.D.B., ed., Magmatic ore deposits A symposium: 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, 

in Visser, D.J.L., and von Gruenewaldt, G., eds., Symposium on the Bushveld igneous complex and other layered 

intrusions, Pretoria, July 7-14, 1969: Geological Society of 

South Africa Special Publication 1, p. 391-424. 

-1971, The origin of ultramafic rocks by cumulus processes:

Fortschritte der Mineralogie, v. 8, p. 28-174.

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.

Kullerud, G., Yund, R.A., and Moh, G., 1969, Phase relations in 

the Fe-Ni-S, Cu-Fe-S and Cu-Ni-S systems, in Wilson, 

H.D.B., ed., Magmatic ore deposits A symposium: Economic Geology Monograph 4, p. 323-343.

Labotka, T.C., 1985, Petrogenesis of metamorphic rocks beneath 

the Stillwater Complex; assemblages and conditions of metamorphism, 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. 70-76.

Lorand, J.P., 1985, The behavior of the upper mantle sulfide 

component during the incipient alteration of "alpine"-type 

peridotites as illustrated by the Beni Bousera (Northern

D20 Contributions on Ore Deposits in the Early Magmatic Environment
Morocco) and Ronda (Southern Spain) ultramafic bodies: 

Tschermaks Mineralogische und Petrographische Mitteilungen, v. 34, p. 183-209. 

-1987, Cu-Fe-Ni-S mineral assemblages in upper-mantle

peridotites from the Table Mountain and Blow-Me-Down 

Mountain ophiolite massifs (Bay of Islands area, Newfoundland): Their relationships with fluids and silicate melts: 

Lithos, v. 20, p. 59-76.

Lorand, J.P., and Conquere, F., 1983, Contribution ^ 1'dtude des 

sulfures dans les enclaves de Iherzolite a spinelle des basaltes 

alcalins (Massif Central et Languedoc, France) [Contribution 

to the study of sulfide paragenesis in spinel Iherzolite xenoliths (Central Massif and Languedoc, France)]: Bulletin 

Mine"ralogique, v. 106, p. 585-605.

MacLean, W.H., and Shimazaki, H., 1976, The partition of Co, 

Ni, Cu, and Zn between sulfide and silicate liquids: Economic Geology, v. 71, p. 1049-1057.

Mann, E.L., Lipin, B.R., Page, N.J, Foose, M.P., and Loferski, 

P.J., 1985, Guide to the Stillwater Complex exposed in the 

West Fork area, 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. 231-246.

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.

Misra, K.C., and Fleet, M.E., 1973, The chemical composition of 

synthetic and natural pentlandite assemblages: Economic 

Geology, v. 68, p. 518-539.

Murck, B.W., 1985, Factors influencing the formation of 

chromite seams: Part 1. The effects of temperature and 

oxygen fugacity on the behavior of chromium in basic and 

ultrabasic melts; Part 2. The petrology and geochemistry of 

'th'e (j^and H chromite seams in the Mountain View area of the 

1 Stillwa'ter Complex, Montana: Toronto, Ontario, University 

of Toronto, Ph.D. thesis, 142 p.

Naldrett, A.J., 1969, A portion of the system Fe-S-O between 900 

and 1080 °C and its application to sulphide ore magmas: 

Journal of Petrology, v. 10, p. 171-201.

 1981, Nickel sulfide deposits; classification, composition, 

and genesis, in Skinner, B.J., ed., 75th anniversary volume: 

El Paso, Tex., Economic Geology Publishing Co., p. 

628-685.

Naldrett, A.J., and Lehmann, J., 1987, Spinel non-stoichiometry 

as the explanation for Ni-, Cu- and PGE-enriched sulfides in 

chromites, in Paper T10, Abstracts, Geo-Platinum '87 Symposium: Milton Keynes, United Kingdom, Open University, 

p. 1-2.

Naldrett, A.J., Lehmann, J., and Auge, T., 1987, Spinel nonstoichiometry and reactions between chromite and closely 

associated sulphides, in Campbell, Andrew, ed., IGCP 

Project 161, 5th Magmatic Sulphide Field Conference, 

abstracts and field guide, 3-13 August, 1987: Harare, Zimbabwe, Geological Society of Zimbabwe, p. 18-19.

Navrotsky, A., 1975, Thermochemistry of chromium compounds, 

especially oxides at high temperature: Geochimica et Cosmochimica Acta, v. 39, p. 819-832.

Page, N.J, 1971, Sulfide minerals in the G and H chromitite zones 

of the Stillwater Complex, Montana: U.S. Geological Survey 

Professional Paper 694, 20 p.

 1972, Pentlandite and pyrrhotite from the Stillwater Complex, Montana: Iron-nickel ratios as a function of associated 

minerals: Economic Geology, v. 67, p. 814-820.

 1977, Stillwater Complex, Montana: Rock succession, 

metamorphism and structure of the complex and adjacent 

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

-1979, Stillwater Complex, Montana Structure, mineralogy, and petrology of the Basal zone with emphasis on the 

occurrence of sulfides: U.S. Geological Survey Professional 

Paper 1038, 69 p.

Page, N.J, and Dohrenwend, J.C., 1973, Mineral resource potential of the Stillwater Complex and adjacent rocks in the 

northern part of the Mount Wood and Mount Douglas 

quadrangles, southwestern Montana: U.S. Geological Survey 

Circular 684, 9 p.

Page, N.J, and Jackson, E.D., 1967, Preliminary report on sulfide 

and platinum-group minerals in the chromites of the Stillwater Complex, Montana: U.S. Geological Survey Professional 

Paper 575-D, p. D123-D126.

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.

 1972, Vertical and lateral variation of platinum, palladium, 

and rhodium in the Stillwater Complex, Montana: Economic 

Geology, v. 67, p. 915-923.

Page, N.J, Rowe, J.J., and Haffty, J., 1976, Platinum metals in 

the Stillwater Complex, Montana: Economic Geology, v. 71, 

p. 1352-1363.

Page, N.J, and Simon, P.O., 1978, Differentiation of the sulfides 

in the Basal zone of the Stillwater Complex, Montana: U.S. 

Geological Survey Journal of Research, v. 6, p. 473-482.

Page, N.J, Zientek, M.L., Czamanske, G.K., and Foose, M.P., 

1985, Sulfide mineralization in the Stillwater Complex and 

underlying rocks, 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. 93-96.

Rajamani, V., and Naldrett, A.J., 1978, Partitioning of Fe, Co, 

Ni, and Cu between sulfide liquid and basaltic melts and the 

composition of Ni-Cu sulfide deposits: Economic Geology, 

v. 73, p. 82-93.

Roby, R.N., 1949, Investigation of copper-nickel deposits of the 

Stillwater Complex, Stillwater and Sweet Grass Counties, 

Montana: U.S. Bureau of Mines Report of Investigations 

4431, 10 p.

Ryzenkho, B., and Kennedy, G.C., 1973, The effect of pressure 

on the eutectic in the Fe-FeS system: American Journal of 

Science, v. 273, p. 803-810.

Sharp, W.E., 1969, Melting curves of sphalerite, galena, and 

pyrrhotite, and the decomposition curve of pyrite between 30 

and 65 kbar: Journal of Geophysical Research, v. 74, p. 

1645-1652.

Shima, H., and Naldrett, A.J., 1975, Solubility of sulfur in an 

ultramafic melt and the relevance of the system Fe-S-O: 

Economic Geology, v. 70, p. 960-967.

Todd, S.G., Keith, L.W., Le Roy, 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.

Todd, S.G., Schissel, D.J., and Irvine, T.N., 1979, Lithostratigraphic variations associated with the platinum-rich zone of 

the Stillwater Complex: Carnegie Institution of Washington 

Year Book 78, p. 461^68.

Ulmer, G.C., 1969, Experimental investigations of chromitespinel, in Wilson, H.D.B., ed., Magmatic ore deposits: 

Economic Geology Monograph 4, p. 114-131.

Wager, L.R., Brown, G.M., and Wadsworth, W.J., 1960, Types 

of igneous cumulates: Journal of Petrology, v. 1, p. 73-85.

Sulfide Inclusions in the B Chromitite, Stillwater Complex, Montana D21
Wolfgram, D., and Evans, J.C., 1980, Platinum group mineral Zientek, M.L., Czamanske, O.K., and Irvine, T.N., 1985,

and base metal sulfide phase relationships in the "Main Stratigraphy and nomenclature for the Stillwater Complex, in

platiniferous zone" of the Stillwater Complex, Montana Czamanske, O.K., and Zientek, M.L., eds., The Stillwater

[abs.]: Geological Society of America Abstracts with Pro- Complex, Montana, Geology and guide: Montana Bureau of

grams, v. 12, p. 552. Mines and Geology Special Publication 92, p. 21-32.

Zientek, M.L., 1983, Petrogenesis of the Basal zone of the Zientek, M.L., Ripley, E.M., and Cooper, R.W., 1982, Sulfur

Stillwater Complex, Montana: Stanford, Calif., Stanford isotopic studies of the Stillwater Complex, Montana [abs.]:

University, Ph.D. thesis, 229 p. Geological Society of America Abstracts with Programs, v.

	14, p. 652.

D22 Contributions on Ore Deposits in the Early Magmatic Environment


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