Sulfide Minerals in the 

G and H Chromitite Zones 

of the Stillwater Complex, 

Montana 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 694 

Sulfide Minerals in the 

G and H Chromitite Zones 

of the Stillwater Complex, 

Montana 

By NORMAN J PAGE 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 694 

The relationship of the amount, 

relative abundance, and size of grains 

of selected sulfide minerals to the 

crystallization of a basaltic magma 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1971 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

Library of CongresR catalog-card No. 70-610589 

For sale by the Superintendent of Documents, U.S. Government Printin&' Otrice 

Washin&'ton, D.C. 20402 - Price 35 cents (paper cover) 
CONTENTS 

Page 

Abstract------------------------------------------------------------------------------------------------------------ 1 

Introduction________________________________________________________________________________________________________ 1 

Acknowledgments--------------------------------------------------------------------------------------------------- 4 

Sulfide occurrences-------------------------------------------------------------------------------------------------- 4 

Sulfide inclusions in cumulus minerals_____________________________________________________________________________ 4 

Fabrtc______________________________________________________________________________________________________ 5 

Phase assemblages__________________________________________________________________________________________ 5 

Sulfide minerals associated with postcumulus materiaL------------------------------------------------------------ 5 

Fabric------------------------------------------------------------------------------------------------------ 7 

Phase assemblages__________________________________________________________________________________________ 7 

Comparison of types of sulfide occurrence _____ ----------------------------------------------------------------------- 10 

Variation of sulfide categories with stratigraphy---------------------------------------------------------------------- 12 

Genetic implications------------------------------------------------------------------------------------------------- 15 

Conclusions--------------------------------------------------------------------------------------------------------- 19 

References cited---------------------------------------------------------------------------------------------------- 19 

ILLUSTRATIONS 

Page 

FIGURE 1. Geologic index map and correlation diagram showing the chromitite zones in the Peridotite member of the 

Ultramafic zone ------------------------------------------------------------------------------------- 2 

2. Geologic map of the Mountain View area showing sample localities ------------------------------------ 3 

3. Sketch of polished sections showing shapes of mineral grains that form inclusions in olivine and chromite___ 6 

4. Sketch of polished sections showing shapes of mineral grains associated with postcumulus minerals -------- 8 

5. •.rriangular diagram showing volume percentages of pyrrhotite, pentlandite, and chalcopyrite in multiphase 

grains ---------------------------------------------------------------------------------------------- 9 

6-9. Graphs showing: 

6. Variation of volume percentage of sulfide minerals correlated with stratigraphic position ---------- 11 

7. The size of sulfide grains and aggregates correlated with stratigraphic position -------------------- 13 

8. Variation of abundance of phases correlated ·with stratigraphic position -------------------------- 14 

9. Volume percentage of sulfide minerals versus Fe-P/ (Fe+2+Fe-P) ratio in chromite --------------- 18 

TABLES 

Page 

TABLE 1. Abundance of mineral assemblages occurring as inclusions ------------------------------------------------- 5 

2. Abundance of all mineral assemblages associated with postcumulus material ------------------------------- 9 

3. Sulfide abundances in the G and H chromitite zones --~------------------------------------------------------ 10 

III 

SULFIDE MINERALS IN THE G AND H CHROMITITE ZONES 

OF THE STILLWATER COMPLEX, MONTANA 

By NORMAN J pAGE 

ABSTRACT 

Pentlandite, pyrrhotite, chalcopyrite, niccolite, and platinumgroup minerals occur in two chromitite zones ( G and H) 

of the Stillwater Complex. Detailed mineralogic studies of 

stratigraphic sections of both zones illustrate that amount, 

relative abundance, and size of sulfide minerals are related 

directly to primary crystallization processes of a fractionating 

basaltic magma. Four textural categories of sulfide grains and 

aggregates are present: inclusions in cumulus chromite and 

olivinP: grains associated with postcumulus phases: fracture 

fillings; and grains associated with alteration minerals. Averaging the sulfide minerals in the two zones, 2.5 percent occurs 

as inclusions, 88.4 percent occurs as grains associated with 

postcumulus phases, and 9.1 percent occurs as fracture fillings 

and in association with alteration minerals. Inclusions probably 

represent both immiscible sulfide droplets and crystalline sulfide minerals trapped by crystallizing cumulus olivine and 

chromite, whereas sulfide grains in postcumulus material represent interactions between sulfide droplets and the trapped 

magma. Temperature, partial pressures of oxygen and sulfur, 

and growth and settling rates of the sulfide and silicate phases 

were important factors in controlling the digtrihution of sulfide 

materials. 

INTRODUCTION 

Sulfide minerals have long been known to occur in 

the Stillwater Complex. southwestern ~fontana (Howland, 1933; Howland and others, 1936; Roby, 1949; 

Jackson and others. 1954; Peoples and others, 1954). 

The Stillwater Complex is a large tabular mass of 

mafic and ultramafic rocks of Precambrian age (Kistler and others, 1969) that crystallized in at least three 

distinct stages. The initial stage consisted of (1) the 

intrusion of basaltic magma into pelitic rocks, quartzites, and iron-formation-now metamorphosed to the 

pyroxene-hornfels facies-and ( 2) the crystallization 

of the Basal zone of the complex before the appearance of the first cumulus olivine (Hess. 1960; Jones 

and others, 1960; Jackson, 1961). The be~inning of 

the second stage, deposition of the Ultramafic zone, is 

marked by the appearance of cumulus olivine .• Jackson (1961) has divided the second stage into two informal stratigraphic units-a lower Peridotite member and an upper Bronzitite member. The Peridotite 

member contains about 15 cyclic units (Jackson, 1961, 

1967, 1968) of which 12 or 13 contain chromitites. The 

third stage began with the appearance of the first 

cumulus plagioclase. During this stage, the Banded 

and Upper zones of the complex crystallized (Hess, 

1960). 

Sulfide minerals are concentrated in the Basal zone 

(Howland, 1933), in the chromitite zones of the Peridotite member of the Ultramafic zone (Page and Jackson, 1967), and in certain horizons of the Banded zone 

(Howland and others, 1936). Sulfide-rich pods and 

lenses, conformable with the igneous layering, occur 

at the base of the G and H chromitite zones (E. D. 

Jackson, oral commun., 1968). 

The chromitite zones in the Stillwater Complex 

originated as direct crystallization products of a fractionating sheet of basaltic magma (Jackson, 1961, 

1963, 1967, 1969), and the sulfide minerals, which are 

intimately associated with these zones, probably are 

comagmatic with the complex. Page and Jackson 

(1967) suggested that the sulfide minerals were localized either by a process of migration in fluids upward 

in the crystal pile, the sulfide minerals being trapped 

owing to the relative impermeability of the finegrained chromitites, or by a mechanism of precipitation of mattes from the individual batches of magma 

that formed the cyclic units. This paper presents the 

available evidence concerning the relations of the sulfide. silicate, and oxide phase assemblages within two 

chromitite zones. These data can be used as limiting 

conditions on the mechanism of sulfide accumulation 

in the chromitites. 

The descriptions of the sulfide mineralogy and petrology are based on samples collected by E. D. Jackson from the G and H chromitite zones (figs. 1, 2) in 

the ~fountain View area of the Stillwater Complex. 

In this area the two chromitite zones are about 12 

feet thick. 

1 
45°30' 

25' 

WEST 

Gish 

G 

20' 

FEET 

110°15' 

Chrome 

Mountain 

!

1000 

800 

600 

400 

200 

0 

Iron 

0 

West 

Fork 

10' 

member 

Mountain Nye 

Initial View Lip 

5 MILES 

5' 

EAST 

Benbow 

I 

H 

G 

,B 

;~,A 

~ "' 

~ 

c3 

!-

~ 

~ 

~ ri) 

110°00' 

"'0 rtJ 

= CIS 0 ~ 

"'0 N 

Q) '"' "'0 Q) 

= p. CIS p. 

~:::> 

Q) 

§ 

N 

<:,) 

<H 

CIS 

55' 

0 2 3 

Sedimentary 

-

rocks 

Principally norite, 

4 

u 

} 

0 0 NON 

ozo 

W<((l) 

...J LIJ 

<( ::E 0.. 

z 

<( 

gabbro, and anorthosite 0:: 

al 

::E 

<( 

u Bronzitite member LIJ 

c::: Principally bronzitite 0.. 

s G chromitit~ CIS zone '"' ,±:! 

:::> Peridotite member 

Principally bronziti~. 

harzburgite, and chromititel 

50' 

5 MILES 

Metamorphic rocks 

be, metasedimentary rocks 

gg, granitic gneiss 

Contact 

Fault 

Thrust fault 

Sawteeth on upper plate 

109°45' 

FIGUBE 1.-Geologic index map and correlation diagram showing the chromitite zones in the Peridotite member of the Ultramafic zone of the Stillwater Complex. After Page and Jackson (1967). 

~ 

~ 

d 

t"' 

~ 

~ 

tj 

~ 

~ 

..... 

~ 

z 

~ 

~ t"' 

'X 

~ 

~ 

~ 

~ ~ 

0 

> z 

..... 

~ 

:I: 

0 

:I: 

~ 

0 

~ ~ 

8 

~ 

8 

~ 

N 

0 

z 

~ 

(f). 

0 

l'%j 

8 

:I: 

~ 

(f). 

8 

~ 

t"' 

t"' 

~ 

> 8 

~ 

~ 

0 

0 

~ 

~ 

t"' 

1.;1 

~ 

~ 

~ 

~ 
INTRODUCTION 3 

35 45 

EXPLANATION 

Landslide debris 

Glacial deposits 

Basaltic dikes 

Quartz monzonite 

>-

0:: 

<( 

z 

0:: 

w 

1-

<( 

:::l 

0 

z 

<( 

0:: Bronzitite member !D 

Principally bronzitite ~ 

<( 

0 

G chomitite W 

~.one ~ 

Peridotite member 

Principally bronzitite, harzburgite, and chromitite 

Contact 

Dashed where approximately located; dotted 

where concealed 

t D 

u 

Fault 

Showing dip and relative 

movement. Dashed where 

approximately located; 

dotted where concealed 

~ ....... ...&.. •••• 

Thrust fault, showing dip 

Dashed U(here approximately located; dotted 

where concealed. Sawteeth on upper plate 

Fault zone 

~ { Principally gabbro and 

o::l norite 

-t-----tAxis of anticline 

60 

-iStrike and dip of layering 

>--

Portal of tunnel or adit 

Metamorphic rocks 

be, metasedimentary rocks 

gg, granitic gneiss 

FIGURE 2.-Geologic map of the Mountain View area showing sample localities. After Jackson (1969). 

Channel sam pies were cut across surface outcrops of 

the chromitite zones by Jackson (1969). Cores or chips, 

about 1 inch in diameter, were cut, mounted in epoxy, 

and polished to form sections suitable for electron 

microprobe investigations. Thirty-seven polished sections were prepared from the G zone, and 20 from the 

H zone. These polished sections were examined in reflected light, and thin sections from the same sample 

were examined in transmitted light. Each sulfide grain 

was located in the polished section, and the following 

characteristics of the grain were cataloged : location, 

size (maximum and minimum outcrop diameter in 

microns) , shape and textural characteristics, orientation with respect to stratigraphy (where possible), 

phases present, and boundary relations of the associated minerals. Photographs were taken for sample 

identification and in preparation for microprobe examination. Corrections were not applied to the measure-
4 SULFIDE MINERALS IN 'l'HE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, MONTANA 

ment of grain diameters in polished sections because 

the results are used for a comparative purpose within 

a single study. (See Jackson, 1961, p. 21, for further 

discussion.) Certain grains were X-rayed in powder 

cameras. 

A large number of grains were studied (both qualitatively and quantitatively) with the electron microprobe, using metals and both synthetic and natural 

minerals as standards. Electron microprobe procedures 

were similar to those described by Page (1967) and 

Page, Calk, and Carr (1968), except that in the present study extensive use was made of X-ray beam scanning techniques and oscilloscope projection of the resultant images for a quick visual and qualitative identification of the elements present. 

In order to obtain the volume percentages of sulfide 

minerals in the rock specimens, the sizes of the sulfide 

grains within a number of fields of view were tabulated, summed, and relative percentages calculated 

with respect to the total area of the fields of view that 

were examined in the section. The diameter of a single 

field of view was 778 microns, and on the average, 

457.1 and 422.6 fields of view were examined per section in the G and H zones respectively. Polaroid photographs and a planimeter were used in estimating 

relative percentages of phases present in polymineralic 

grains. The precision error involved here is on the 

order of ± 20 percent and is based on replicate measurements. 

ACKNOWLEDGMENTS 

E. D. Jackson contributed the samples; he also 

shared knowledge of the silicate and oxide phases in 

the chromitite horizons and read earlier versions of 

this paper. The helpful and penetrating comments on 

earlier versions of this manuscript by A. L. Clark, 

G. K. Czamanske, G. A. Desborough, and P. B. Barton 

are warmly acknowledged. 

SULFIDE OCCURRENCES 

Within the G chromitite zone, a total area of 60.8 

cm2 was examined in which 1,816 sulfide grains were 

observed. In the H chromitite zone, a total area of 

34.1 cm2 was examined; 1,209 sulfide grains were 

counted. The H chromitite zone contains 0.01 percent 

sulfide minerals by volume, and the G contains 0.007 

percent. 

The range in diameter of sulfide grains in the G 

zone is from 1 to 185 microns ( avg 5.5,u), and the 

range in the H zone is from 1 to 135 microns (a vg 

11.1 .u). The average diameter of sulfide grains for the 

G zone is strongly affected by the 1,413 grains of sulfide that have an average diameter of 1.9 microns. 

These grains are associated with alteration materials 

and thus may not represent a true value for the size of 

sulfide grains before alteration. If this size category is 

neglected, an average diameter of 18.1 microns is obtained, instead of 5.5, for the sulfide grains in the 

G zone. 

Four texturaP associations of sulfide mineral grains 

are found in the rocks of the G and H chromitite 

zones. These associations are : ( 1) inclusions of sulfide 

minerals, with or without silicate and oxide minerals, 

in cumulus olivine and chromite crystals; (2) sulfide 

minerals, with or without magnetite, associated with 

postcumulus minerals and interstitial to cumulus chromite and olivine; (3) sulfide minerals that form veins 

and fill fractures in cumulus phases; and ( 4) sulfide 

minerals that occur in veinlets formed by serpentinization and alteration of the primary silicate and oxide 

phases. Over 90 percent of the last two categories consist of pyrrhotite and pentlandite single-phase grains 

with an average diameter of 3.4 microns. Although 

they form 4.8 and 14.7 percent of the total volume of 

sulfide minerals in the G and H zones respectively, 

their occurrence marks them as probable postmagmatic 

features; therefore. they are not discussed further. 

SULFIDE INCLUSIONS IN CUMULUS MINERALS 

Pyrrhotite, pentlandite, chalcopyrite, niccolite, heazlewoodite, molybdenite. and minerals containing platinum-group metals, all with or without magnetite, 

phlogopitic mica. and other silicates, form inclusions 

usually within the centers of cumulus chromite and 

olivine crystals. The inclusions may contain one to 

three sulfide minerals in addition to silicates. These 

are the same types of inclusions discussed by Jackson 

(1961, 1966) and Page and .Jackson (1967). The inclusions contain 1.4 percent and 3.1 percent of all the 

sulfides in the G and H zone chromitites respectively. 

The inclusions may be divided into the following four 

subcategories based on the cumulus host phase and the 

presence or absence of silicate phases within the inclusion: 

1. Sulfide minerals that fill negative crystals, or subhedral sulfide crystals included in chromite .. 

2. Sulfide minerals of similar habit to 1 but included 

in olivine rather than chromite. 

1 "Texture," as I use the term, includes the general fabric of the 

sulfide minerals in the rock and the grain sizes of these phases. The 

term "fabric" refers to the shape of the sulfides, their orientation, and 

the boundary relations between grains. The cumulus terminology ts 

from Jackson ( 1967). 
SULFIDE OCCURRENCES 5 

3. Sulfide minerals with silicate or oxide phases, or 

both, included as irregular negative crystals In 

chromite. 

4. Sulfide minerals of similar habit to 3 but included 

in olivine rather than chromite. 

The great majority of the inclusions (93.5 percent 

for the G zone and 91.0 percent for the H zone) occur 

in olivine. Inclusions in subcategories 3 and 4 have 

varying ratios of sulfide to silicate minerals, and these 

kinds of inclusions possibly represent a gradation between inclusions containing exclusively silicate or sulfide minerals. 

FABRIC 

Figure 3 illustrates the various shapes of minerals, 

types of contacts between the minerals, and kinds of 

sulfide minerals that occur as inclusions in the cumu1 us phases of the G and H chromitite zones. Where 

more than one mineral is present within an inclusion, 

euhedral negative crystals or subhedral forms are 

found, whereas, if a single sulfide mineral is present, 

the shape of the inclusion reflects a crystal form of the 

sulfide mineral. In stratigraphically oriented sections 

from the G chromitite zone, about 58 percent of the 

inclusions of subcategories 3 and 4 shows the sulfide 

minerals at the bottom of the inclusion. 

Other specimens available from the G and H zones 

were oriented with respect to the layering, but not to 

the stratigraphy; for these specimens the stratigraphic 

top was assumed to be perpendicular to the layering. 

If a preferred orientation existed, the sulfide grains 

should be more abundant either toward the assumed 

top or toward the assumed bottom. There was a preferred orientation for about 76 percent of the inclusions in the H chromitite zone but not for the G 

chromitite zone. 

Analyses of grain size data (averaged diameters) 

show that the average diameter of inclusions within 

the G and H zones· is 4.9 and 6.7 microns respectively; 

the range of diameters for the inclusions in the G and 

H zones is :from 1 to 35 and from 1 to 37.5 micr~ns, 

respectively. Subcategories 1 and 2 on the average 

have smaller grain sizes than those of subcategories 

3 and 4. In both the G and H zone chromitites, the 

size distributions are somewhat skewed toward the 

finer size classes and in general show moderate to wellsorted distributions. The sorting coefficients for the 

inclusions in the G and H chromitites are 1.43 and 

1.60 respectively. The sulfide inclusions therefore give 

about the same value of sorting coefficients as ,Jackson 

(1961) found for crystals of cumulus chromite and 

olivine. 

PHASE ASSEMBLAGES 

The assemblages of sulfide minerals present in the 

inclusions are summarized in table 1. The term 

"abundance," as used here, refers to the number of 

TABLE 1.-Abundance, in percent, of mineral assemblages occurring as inclusions 

[Mineral symbols : pn, pentlandite ; po, pyrrhotite; nc, niccolite; Pt, 

platinum-group minerals ; ccp, chalcopyrite ; mo, molybdenite; hz, 

heazlewoodite; mt, magnetite] 

Mineral assemblage 

pn ------------- po ------------- nc ------------- Pt ------------- ccp ------------ mo ------------ pn+po --------- pn+ccp -------- pn+hz --------- pn+nc --------- pn+mt --------- po+ccp --------- pn+phase Q ---- po+mt --------- nc+mt --------- pn+po+Pt ------ pn+po+ccp 

Total ----

Abundance (percent) 

G chromitite 

zone 

H chromitite 

zone 

1 2 3 4 Total 1 2 3 4 Total 

3.3 11.0 4.4 6.6 25.3 5.0 23.3 5.8 13.3 47.5 

3.3 2.2 3.3 6.6 15.4 5.8 .8 9.2 15.8 

1.1 2.2 17.6 1.1 22.0 4.2 3.3 3.3 10.8 

16.5 1.1 17.6 6.7 3.3 .8 --- 10.8 

3.3 3.3 1.1 7.7 .8 1.7 .8 3.3 

.8 .8 

1.1 1.1 .8 .8 

.8 .8 

1.1 1.1 .8 .8 1.7 

1.1 1.1 .8 .8 

.8 1.7 2.5 

5.5 5.5 .8 .8 

1.1 1.1 

1.1 1.1 .8 .8 

1.7 1.7 

.8 .8 

1.1 1.1 

25.3 19.8 29.7 25.3 100.1 13.3 43.2 12.3 30.7 99.7 

1 Sulfide minerals that fill negative cr;rstals, or subhedral sulfide 

crystals included in chromite. 

2 Sulfide minerals of similar habit to 1 but included in olivine rather 

than chromite. 

3 Sulfide minerals with silicate or oxide phases, or both, included as 

irregular negative crystals in chromite. 

4 Sulfide minerals of similar habit to 3 but included in olivine rather 

than cbromite. 

grains of the mineral or mineral assemblage compared 

to the total number of inclusions. The most common 

sulfide minerals present in inclusions in the G and H 

chromitite zones are pentlandite, pyrrhotite, niccolite, 

and the platinum-group minerals. ·Inclusions rarely 

contain more than one sulfide mineral. Platinum-group 

minerals are strongly concentrated as inclusions in 

chromite (subcategory 1), whereas pentlandite is concentrated as inclusions in olivine (subcategory 2). Except for these preferences, the sulfide minerals show 

no direct correlation with the inclusion subcategories. 

SULFIDE MINERALS ASSOCIATED WITH POSTCUMULUS 

MATERIAL 

Pentlandite, pyrrhotite, chalcopyrite, heazlewoodite, 

and complex platinum-group minerals occur in the 

chromitite zones interstitial to cumulus chromite and 
6 SULFIDE MINERALS IN THE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, MONTANA 

:{'{?.#!#,(:~;~ 

~:rr\;.qm:.:}W 

,---., 

20,u 

EXPLANATION 

Chromite 

Olivine 

Biotite 

Other silicate 

mt 

pn 

pn+po 

pn+phase Q 

P9 

ccp 

nc 

hz 

Laurite 

Pt alloy 

Cooperite 

mo 

if~!! • •• •• : • ••• • •• 0 0 •• _ ·~ ••• 

40JL 

FIGURE 3.-Shapes of mineral grains that form inclusions in olivine and chromite. Mineral symbols: pn, pentlandite; 

po, pyrrhotite; nc, niccolite; Pt, platinum-group minerals ; ccp, chalcopyrite; mo, molybdenite; hz, heazlewoodite ; mt, magnetite. 
SULFIDE OCCURRENCES 7 

olivine. Three subcategories were distinguished and 

catalogued for this type of material : 

1. Sulfide grains which have sharp straight contacts 

with postcumulus phases and which are molded 

into all the available space between cumulus 

minerals. 

2. Sulfide grains and grain aggregates that occupy 

interstices but have ragged, embayed contacts 

with the postcumulus phase and retain at least 

one grain boundary with the cumulus phases. 

3. Sulfides with ragged, em bayed, or (rarely) straight 

contacts entirely enclosed by postcumulus minerals. 

Sulfide minerals associated with postcumulus minerals account for 93.8 percent of the total volume of 

sulfide minerals in the G zone and 83.0 percent in the 

H zone. In the G chromitite zone, 69.8, 24.1, and 6.0 

percent of the grains are distributed between subcategories 1, 2, and 3 respectively, whereas in the H zone, 

46.7, 37.4, and 15.8 percent of the grains are distributed 

in subcategories 1, 2, and 3 respectively. 

FABRIC 

Figure 4 illustrates the shapes and general appearance of a selection of grains in postcumulus interstices 

of the G and H chromitite zones. Most of the twodimensional shapes are rectangular, triangular, and 

polyhedral forms which fill, or partially fill, the concave polygons of the interstices bounded by the crystal 

faces of chromite and olivine. The character of the 

interstitial spaces has been discussed by Jackson (1961, 

p. 65). Five field-oriented sections from the G chromitite zone were examined in detail ; three of these a ppear in figure 4. In these sections, about 60 percent of 

the grains rests on the stratigraphic bottom of an 

interstice. In the remainder of the G zone sections, 

for which only the orientation on the layering plane 

is known, about 60 percent of the grains also rests on 

the stratigraphic bottom of an interstice. For the H 

chromitite zone, a maximum of about 59 percent of the 

grains rests on the bottom of the interstice if a stratigraphic top for the layering plane is assumed. No 

systematic distribution with respect to grain size, 

phases present, or subcategory has been found. 

The sizes (outcrop diameter) of sulfide grains associated with postcumulus material range from 1.5 to 

185 microns in the G chromite zone and from 1 to 135 

microns in the H zone; the average size (mean diameter) is 26.5 microns for the G and 19.4 microns for 

the H. Subcategories 1, 2, and 3 have average diameters of 35.0, 32.0, and 13.9 microns respectively in the 

G zone and 18.5, 23.5, and 14.1 microns in the H zone. 

Cumulative size distributions are skewed toward the 

finer grain sizes, and sorting coefficients are 2.25 for 

the G zone and 1.93 for the H; they indicate moderate 

to poor sorting. 

Pentlandite, in single-phase or multiple-phase 

grains, was found in three ways : as subhedral to 

euhedral crystals without extensive fracturing and 

parting, as similar crystals that are highly fractured 

into mosaics of rhomb-shaped pieces, and as unfractured pentlandite grains with wormlike intergrowths 

of either pyrrhotite or phase Q, which is an unidentified yellow isotropic mineral or rarely, both in the 

same grain. 

Pyrrhotite, in multiple-phase grains with pentlandite, was found as a wormlike intergrowth in which the 

worms of pyrrhotite are approximately 1 or 2 microns 

across; it may also be found as euhedral to subhedral 

crystals less than 5 microns wide that form a spongyappearing aggregate. Pyrrhotite also was found as 

subhedral to anhedral irregular grains and as highly 

fractured grains in which the fractures are filled with 

magnetite. Boundaries between either the spongy or 

the subhedral to anhedral pyrrhotite and the pentlandite are usually straight and sharp and show no 

evidence of embayment or veining. 

Chalcopyrite occurring with pentlandite or pyrrhotite has either sharp and straight contacts or forms 

veins and embayments into the other two minerals. 

Contacts between platinum-group minerals and other 

sulfides are usually sharp, and the shape of the contact is determined by the crystal form of the platinumgroup mineral. 

PHASE ASSEMBLAGES 

Sulfide minerals and mineral assemblages filling or 

partially filling interstices between cumulus olivine 

and chromite are given in table 2, and their relative 

abundance can be com pared for the G and H zones. 

In figure 5, volume percentages of pentlandite, pyrrhotite, and chalcopyrite are plotted for a selected 

number of grains containing two- and three-phase associations of sulfides. Also included are some fourphase grains which contain magnetite as the fourth 

phase. Three-phase grains rarely contain more than 

25 percent by volume of chalcopyrite and tend to have 

more pentlandite than pyrrhotite. Examination of the 

three-phase sulfide grains in the H zone suggests a 

weak stratigraphic variation from pentlandite-rich 

grains in the footwall to those rich in pyrrhotite in 

the hanging wall. Such a variation is partly supported 

by data from the G zone. 
8 SULFIDE MINERALS IN THE G AND H CHROl\IITITE ZONES OF THE STILLWATER COMPLEX, MONTANA 

II 1

15JL I 

'II ll 

lOJL 

I 25JL I 

0 

SCALE OF 

POLISHED-SECTION OUTLINES 

EXPLANATION 

Chromite 

Postcumulates 

pn 

po 

pn+po 

pn+mt 

Sulfide minerals 

Chromite cumulate 

FIGURE. 4.-Shapes of mineral grains associated with postcumulus minerals. Mineral symbols: pn, pentlandite; po, 

pyrrhotite, and mt, magnetite. 
SULI1'IDE OCCURRENCES 9 

TABLE 2.-Abttnllance, in percent, of all mineral assemblages associated with posto,u.mulus material 1 

[Mineral symbols : pn, pentlandite ; po, pyrrhotite ; nc, niccolite; Pt, platinum-group mineral>~ ; ccp, chalcopyrite; mo, molybdenite; hz, heazlewoodite ; vi, violarite ; il, ilmenite; and mt, magnetite] 

Abundance (percent) 

Mineral assemblage G chromitite 

zone 

pn -------------------------------

po ------------------------------- ccp 

vi 

DC -------------------------------

Pt 

pn+po ---------------------------

pn+spongy po -----------------

pn+po worms -----------------

pn+spongy po+po worms -------

pn+phase Q ----------------------

pn+hz --------------------------- pn+ccp ---------------------------

pn+nc --------------------------- pn+mt ---------------------------

po+vi ---------------------------- po+ccp ---------------------------

po+Pt ---------------------------

po+hz ---------------------------- po+mt ---------------------------

ccp+mt --------------------------

hz+IDt --------------------------- pn+po+phase Q -------------------

43.8 

21.3 

4.3 

3.0 

10.2 

(3.8) 

(2.6) 

(3.8) 

.4 

1.3 

1.3 

2.1 

1.3 

.4 

1.3 

.4 

H chromitite 

zone 

52.2 

16.4 

.9 

.2 

.3 

.2 

15.5 

(10.0) 

(4.5) 

(1.0) 

1.9 

.9 

.9 

.3 

.2 

.9 

.2 

1.9 

.3 

.9 

1 In the table certain phase asseemblages have been grouped together. For the H zone, the abundance of pyrrhotite, 16.4 percent, includes 0.5 percent grains of pyrrhotitelike phase with an Ni to Fe 

ratio of 1 to 1. In the assemblage po+vi, the po represents a pyrrhotitelike phase with a Ni to Fe ratio of about 1 to 1. Phase Q, an unAbundance (percent) 

Mineral assemblage G chromitite G chromitite 

zone zone 

pn+po+ccp ----------------------- 2.6 2.5 

pn+hz+Pt ------------------------ .4 .2 

pn+po+Pt ------------------------ .4 

pn+ccp+Pt ----------------------- .2 

pn+po+mt ----------------------- 3.4 .9 

pn+phase Q+mt ------------------- .3 

pn+hz+mt ------------------------ .4 

pn+Pt+mt ------------------------ .4 .2 

po+phase R and s ----------------- .3 

pn+po+phase Q+Pt --------------- .2 

pn+po+phase Q+ccp --------------- .4 

pn+po+phase Q+mt --------------- .5 

pn+po+ccp+vi -------------------- .4 

pn+po+ccp+Pt -------------------- .2 

pn+po+nc+mt -------------------- .2 

pn+po+ccp+mt ------------------- 1.3 

pn+po+ccp+Pt + mt --------------- .2 

1 phase grains -------------------- 72.3 70.1 

2 phase grains -------------------- 18.7 23.1 

3 phase grains -------------------- 6.8 5.5 

4 phase grains -------------------- 2.1 1.0 

5 phase grains -------------------- .2 

identified i~otropic mineral with a strong yellow color, forms a wormy 

intergrowth with pentlandite. Phases R and S were found in only two 

grains and in amounts too small to identify, even with the microprobe. 

Pha~e R i~ hhH' grar in color, and phase S is a gold color. 

Pyrrhotite 

EXPLANATION 

Pentlandite 

Multiple-phase grains 

• G zone 

0 

H zone 

(contain magnetite) 

• G zone 

D 

H zone 

Chalcopyrite 

FIGURE 5.-Volum~ percentages of pyrrhotite. pentlandite, and chalcopyrite in 

multiphase grains. 
10 SULI!"'IDE MINERALS IN THE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, MONTANA 

Although pentlandite and pyrrhotite are the major 

sulfide minerals, both minerals are not present in all 

samples as interstitial material. There is no pentlandite in the olivine-chromite cumulate at the base 

of the G zone, and pyrrhotite is absent in the middle 

part of the G zone. In contrast, for the H zone, both 

pentlandite and pyrrhotite are present in every sample 

except those from the base; in the base samples only 

pyrrhotite is present. The distribution of sulfide grains 

in the upper part of the G zone is very similar to that 

of the whole H zone. 

Examination of the sulfide grains and aggregates associated with postcumulus minerals shows that about 

92.7 and 98.1 percent of all grains from the G and H 

zones respectively contain either pentlandite or pyrrhotite or both; about 43.8 and 52.2 percent of all 

grains consist of pentlandite, 21.3 and 16.5 percent of 

pyrrhotite, and 27.6 and 29.4 percent have both minerals. Other minerals, including chalcopyrite, niccolite, and the platinum-group minerals, constitute the 

remaining 7.3 and 1.9 percent of grains for the G and 

H zones, but these figures could be increased by possibly 2 or 3 percent by including the minor amounts 

of these minerals which are found in the complex aggregates of pyrrhotite and pentlandite. The distribution of one-, two-, three-, four-, and five-phase grains 

in the three subcategories is practically the same as the 

distribution of the same phases of grains on the average in both the G and H zones. 

Sulfide grains associated with postcumulus material 

in the G and H zones have only 3.4 and 1.4 percent of 

platinum-group minerals and 0.0 and 0.8 percent of 

niccolite, in contrast to the inclusion type of sulfide 

grains which have 17.6 and 10.8 percent of platinumgroup minerals and 22.0 and 10.8 percent of niccolite. 

Also, the sulfide-phase assemblages are more complex 

and abundant within the postcumulus materials than 

they are within inclusions. 

COMPARISON OF TYPES OF SULFIDE 

OCCURRENCE 

The principal similarity between the sulfide occurrences in the G and H zones is the dominance of pentlandite, pyrrhotite, and chalcopyrite in both zones. 

Less than 10 percent of the sulfide grains and aggregates have neither pentlandite nor pyrrhotite present. 

Table 3 gives the sulfide-phase abundance by assemblages for the G and H zones. Besides the outstanding 

differences between the four textural categories in 

which the sulfides are associated with cumulus, postcumulus, or alteration minerals, there are differences 

in the amounts present in each category and in their 

orientation, grain size, sorting coefficients, and mutual 

TABLE 3.-Sulftde abundances in the G and H chromitite zones 

[Mineral symbols: pn, pentlandite; po, pyrrhotite; nc, niccollte; Pt, 

platinum-group minerals ; ccp, chalcopyrite ; mo, molybdenite ; hz, 

heazlewoodite; vi, vtolarite; tl, ilmenite; mt, magnetite] 

Abundance (percent) 

Mineral assemblage G chromttite 

zone 

pn -------------------------------

po ------------------------------- ccp 

vi 

nc -------------------------------

Pt -------------------------------

mo ------------------------------

hz -------------------------------

pn+po ----------------------------

pn+hz ---------------------------

pn+phase Q ----------------------- pn+ccp ---------------------------

pn+nc ---------------------------

po+vi ---------------------------- po+ccp ---------------------------

po+Pt ---------------------------

po+mt ---------------------------

po+hz ---------------------------- ccp+mt --------------------------

pn+ll ---------------------------- nc+mt ---------------------------

pn+mt --------------------------- hz+mt ---------------------------

pn+po+phase Q -------------------

pn+po+ccp -----------------------

pn+hz+Pt ------------------------

po+ccp+Pt -----------------------

pn+po+Pt ------------------------

pn+po+mt ------------------------

pn+phase Q+mt -------------------

pn+Pt+mt ------------------------

pn+hz+mt -----------------------

pn+po+ccp+mt -------------------

pn+po+phase Q+ccp ---------------

po+phase R+phase S ---------------

pn+po+phase Q+Pt ----------------

pn+po+phase Q+mt ---------------

pn+po+ccp+Pt --------------------

pn+po+nc+mt --------------------

pn+po+ccp+Pt + mt ---------------

pn+po+ccp+vi --------------------

54.2 

25.0 

2.7 

2.7 

3.3 

.4 

3.7 

.3 

.4 

.5 

2.2 

.1 

.3 

.4 

.7 

.1 

1.0 

.1 

1.1 

.1 

.4 

.1 

.1 

H chromitite 

zone 

53.5 

25.1 

.8 

.1 

Ui 

1.3 

.1 

8.8 

.8 

1.0 

.6 

.3 

.1 

.7 

.1 

1.1 

.2 

.1 

.1 

.3 

.5 

1.3 

.1 

_1 

.1 

.5 

.1 

.1 

.2 

.1 

.3 

.1 

.1 

.1 

phase boundaries in multiphase grains. The nearly 

universal presence of pyrrhotite and pentlandite in the 

four categories and two zones suggests a similar ultimate source for the sulfide minerals, whereas the differences imply different mechanisms for the localization of the phases for each category. 

Although there are broad similarities in the phases 

present as inclusions and associated with postcumulus 

minerals, the platinum-group minerals and niccolite 

rarely occur as polymineralic grains and are concentrated as inclusions in cumulus phases, whereas the 

rare platinum-group minerals in sulfides associated 

with postcumulus minerals generally form composite 

grains. Inclusions also contain fewer polymineralic 

grains than are present in the sulfides associated with 

the postcumulus minerals. No four- or five-phase 

grains are present as inclusions. Spongy pyrrhotite 
COMPARISON 0}' 'l'YPES OF SULFIDE OCCURRENCE 11 

and pyrrhotite forming wormy intergrowths in pentlandite have not been observed as inclusions but they 

form an important part of the sulfide minerals occurring with postcumulus phases. 

The study of the orientation of grains relative to 

a stratigraphic top or bottom shows that sulfides in 

inclusions tend to have a greater degree of preferred 

orientation with respect to layering than those that 

occur as interstitial material. The sulfide grains in 

inclusions, on the average, are four to five times smaller than the interstitial sulfide grains and the sulfide 

grains in inclusions show better sorting characteristics 

than sulfide grains in the other textural categories. 

The relatively complex phase assemblages and intricate contacts between sulfide minerals in multiphase 

grains that occur associated with postcumulus minerals 

are in sharp contrast to the lack of these characteristics for the inclusion subcategories. This fact implies 

that if gravitative settling is important in their origin, 

then reactions and changes have occurred since they 

accumulated in the interstices. 

G chromitite zone 

A 

0 0.02 0.04 

A 

0 0.02 0.04 

B 

B c 

0.06 0 0.001 0.002 0.003 0 

VOLUME PERCENTAGE OF SlJLFIDE MINERALS 

H chromitite zone 

.. ' ' ' ' 

c 

0 

I 

• 

0.02 0.03 

VOLUME PERCENTAGE OF SULFIDE MINERALS 

0.02 0.04 

EXPLANATION 

D 

Unsampled interval 

rillillJ 

Olivine-chromite cumulate 

Chromite-olivine • cumulate 

Chromite cumulate 

~ 40 INChlES 

20 

0 

0.06 0.07 

FIGURE 6.-Variation of Yolume percentage of sulfidP minerals correlated with stratigraphic position. A, 'l'otal sulfide 

minerals. B. Sulfide inclusions in cumulus minerals. C. Sulfide minerals associated with postcumulus minerals. 
12 SULFIDE MINERALS IN THE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, MONTANA 

VARIATION OF SULFIDE CATEGORIES WITH 

STRATIGRAPHY 

Figure 6 presents the variation in volume percentage Qf total sulfide minerals (A.) for the G and H 

zones plotted against stratigraphic position. Although 

more sulfide minerals are concentrated in the chromitite zones than elsewhere in the Ultramafic zone, they 

are not particularly numerous in the cumulus chromite 

layers of these zones, at least in the localities where the 

samples were taken. Instead, layers of olivine-chromite 

and chromite-olivine cumulate tend to have relatively 

higher amounts of sulfide minerals. 

The overall pattern of maximum sulfide content for 

both zones is similar, neglecting samples taken from 

the H zone below the footwall and above the hanging 

wall. The pattern consists of a general paucity of sulfide minerals at the bases of the zones, an increase to 

a maximum near the central parts of the zones, and a 

decrease to a minimum in the hanging walls. Here the 

similarity between the two zones ends; the variability 

within the G zone is much larger than the variability 

within the H zone because some layers contain relatively large amounts of sulfide minerals and some 

none. Little gradation between sulfide-rich layers and 

sulfide-poor layers in the G zone is observed. However, in the H zone, each maximum concentration is 

formed by a gradation from sulfide-poor layers to 

sulfide-rich layers. What the sulfide distribution relations are between the upper and lower parts of the 

H zone is unknown. The lower part of the H zone 

begins with a low sulfide concentration which increases 

gradually to a maximum concentration and then decreases in the direction of the hanging wall. The base 

of the upper part of the H zone has a high sulfide 

concentration which diminishes toward the hano-inO' b b 

wall. 

In figure 6, the volume percentages of sulfide inclusions in cumulus mineral (B) and sulfide minerals associated with postcumulus minerals ( 0) are plotted 

against stratigraphy for the G and H zones. The 

broad distribution patterns are similar to total sulfide 

mineral volume percentages, but in detail many differences exist. 

The volume percentages of sulfide inclusions in cumulus minerals (fig. 6B) are similarly distributed in 

the stratigraphic sequence in both the G and H zones. 

In general, sulfide inclusions are fairly abundant near 

the base of each zone, are rather constant in amount 

through the middle of the zone, and increase to a 

maximum volume of inclusions at the hanging walls 

of each zone. Sulfide inclusions in olivine constitute 

the largest volume percent of sulfide inclusions in the 

upper part of each zone. 

Sulfide minerals that are associated with postcumulus minerals (fig. 60) account for the majority of 

the sulfide grains present; therefore, their stratigraphic distribution pattern, in volume percent, corresponds 

closely with the total volume percent of all sulfide 

minerals within both zones. If this textural type of 

sulfide grain were plotted by subcategories, similar 

distribution patterns would be obtained. 

When the average diameter of all sulfide grains 

within an individual sample is plotted versus its stratigraphic position (fig. 7), a pattern remarkably similar to that of the total sulfides (fig. 6A.) is obtained. 

This correlation is more striking in the H zone than 

in the G zone where large numbers of micron-size 

grains affect the weighted average of the diameters. 

The average diameter of sulfide inclusions oscillates 

between large and small within both zones, but the 

trend is from small-sized inclusions at the bases of the 

zones to large-sized inclusions at the tops of the zones. 

However, the average diameter of sulfide grains and 

aggregates associated with postcumulus phases is largest where the volume percentage of sulfide grains is 

large. Regression analyses of the relation of grain size 

of a particular textural category of sulfide versus the 

volume percent present confirm the gene\al impression 

of correlation between grain size of sulfides and volume percent. Correlation coefficients ranged from O.SO 

to 0.95 at levels of significance greater than 99 percent. 

If the variations in percentage of the various minerals present are examined from the bases of the zones 

to their tops, then the patterns are those shown in 

figure S. Inclusion-type sulfide minerals and sulfide 

minerals associated with postcumulus silicate material 

have been plotted separately. Abundance data on sulfide volumes present for the G zone (figs. SA., 0) tend 

to show more scatter and less regularity than those for 

the H zone (fig. SB, D). This probably results from 

lesser statistical significance of fewer sulfide grains. 

More inclusions in the lower part of the H zone (fig. 

SB) contain pentlandite than in the upper part. The 

trends of pentlandite concentration in the G zone (fig. 

SA.) are opposite-pentlandite increases in abundance 

upward across the zone. Pyrrhotite gradually decreases 

in abundance in the upper parts of both zones. Platinum-group minerals and niccolite appear to be more 

irregular in abundance than pyrrhotite and pentlandite. In the H zone, niccolite increases up section as 

platinum-group minerals decrease; in the G zone, platinum minerals and niccolite are present in the lower 

part of the section, increase upward to a maximum. 

then gradually decrease. 

The abundance of pentlandite and pyrrhotite associated with postcumulus phases (fig. SO, D) is simi-
0 5 

0 4 

VARIATION OF SULFIDE CA'.rEGORIES WITH STRATIGRAPHY 

G chromitite zone 

10 15 20 25 30 35 

AVERAGE DIAMETER, IN MICRONS 

H chromitite zone 

---- ---- ----

8 12 16 20 24 28 

AVERAGE DIAMETER, IN MICRONS 

40 

32 

45 

EXPLANATION 

• All sulfide grains 

0 

Sulfide inclusions 

!:::.. 

Sulfide grains associated 

with postcumulates 

INCHES 

t 

20 

10 

0 

36 40 

FIGURE 7.-The size (average diameter) of sulfide grains and aggregates correlated with stratigraphic position. Symbols on the stratigraphic section are the same as in figure 6. 

13 
pn 

• • 

• 

• 

• 

• 

• 

• 

• 

0 50 

~ 40 INCHES 

30 

20 

10 

0 

pn 

I • 

• 

• 

• 

• 

• 

• 

• 

• • • • • 

• 

I • 0 50 

• 

4 f 

• 

100 0 

• 

• 

100 0 

PO 

• I 

• • 

• 

• 

• 

I 

50 

-

• 

• 4 

100 0 

A 

Sulfide inclusions 

nc Pt 

• I I I 

' 

• • • • • • 

• l 

5& 100 (l 

• 

• 

• • 

• 

• 

• 

• 

l 

50 

ABUNDANCE, IN PERCENT 

P,O 

•I 

• • 

• 

• • 

• 

l 

50 

B 

Sulfide inclusions 

I 

, 

• 

• 

• 

nc 

I 

• 

• 

• 

• ' . • 

•l 

100 0 50 

• 

100 0 

ABUNDANCE, IN PERCENT 

• 

• 

• 

• 

• 

Pt 

I 

• 

• 

l 

50 

• 4 

100 

100 

G chromitite zone 

ccp hz Q 

-rrl 

• 

• • 

.l._j__J 

pn • 

Tl• 

• 

• • 

• • 

• 

• 

• I 

• 

•• 

• 

• 

c 

Sulfide minerals associated with postcumulus minerals 

po Composite grains ccp nc hz Q 

• • •1 I l • ~ I 

• 

• 

• • • • • 

r • • 

• 

• 

• • 

• 

• • 

• I __L 

• 

• 

I 

• • 

• • 

• 

• • 

• 

Pt vi 

• 

• 

• 

• 

• 

• 

0 50 100 0 50 100 0 50 100 

H chromitite zone 

ccp mo hz pn 

~~~- • I 

• • • 

• 3 

• • 

• 

• 

• 

• 

• • 

• • • • • • • 

• 

• I 

0 50 

ABUNDANCE, IN PERCENT 

D 

Sulfide minerals associated with postcumulus minerals 

• 

, 

• 

• 

• 

r-

• 

• ' 

• 

100 0 

po Composite grains ccp nc hz Q Pt vi 

~ I• Ill t • I 

• 

• 

• 

50 

• 

It 

• 

• 

100 0 

• 

• • 

• 

• 

• 

• 

• 

• 

• 

• 

50 • 100 

ABUNDANCE. IN PERCENT 

• 

• • 

• • • • • • 

• • • 

• • • • 

• • • 

• •• • • • • • •• • • 

• 

I I I l 

FIGURE 8.-Variation of abundance of phases correlated with stratigraphic position. A, Sulfide inclusions, G zone. B, Sulfide inclusions, H zone. 0, 

Sulfide minerals associated with postcumulus minerals, G zone. D, Sulfide minerals associated with postcumulus minerals, H zone. Symbols 

in the stratigraphic section are the same as in figure 6. 

,....... 

~ 

00. 

0 

~ 

~ ~ 

tj 

t;j 

~ ~ 

z 

t;j 

~ ~ 

00. 

~ 

z 

~ 

ti: 

t:z=j 

0 

> z 

tj 

ti: 

a 

ti: 

~ 

0 

~ ~ 

~ ~ 

~ 

t:z=j 

N 

0 

z 

t:z=j 

00. 

0 

~ 

~ 

ti: 

t:z=j 

00. 

~ ~ 

~ 

~ 

:a 

> 

~ 

t:z=j 

~ 

a 

0 

~ 

"'0 

~ 

t:z=j 

~>< 

~ 

0 

z 

~ 

~ 
GENETIC IMPLICATIONS 15 

lar to the distribution of the total volume percent of 

sulfide minerals within the zones. Pentlandite abundance varies inversely with that of pyrrhotite. 

GENETIC IMPLICATIONS 

One hypothesis for the origin of the sulfide minerals 

in the chromitite zones is that the minerals were produced from the same magma that produced the silicates ; the sulfur either came from sulfur dissolved 

in the magma, in immiscible sulfide liquids, or in crystalline solid solutions. This hypothesis has been offered 

to explain sulfide occurrences within the Bushveld 

Complex (Wagner, 1924), the Insizwa intrusive 

(Scholtz, 1936), the Skaergaard intrusion (Wager 

and others, 1957), the Sudbury intrusive body (Hawley, 1962; Naldrett and Kullerud, 1967), the Muskox 

intrusion (Chamberlain, 1967) , and many other occurences including the modern Hawaiian lavas (Desborough and others, 1968). If this general hypothesis 

is correct, then the observations on the sulfide mineralogy of the G and H zones corroborate it and should 

also contribute to an understanding of how the process 

works. Before examining the observations in detail, a 

few general definitions of concepts and terms are 

needed. 

Three stages in sulfide mineral formation should be 

distinguished : ( 1) the separation and concentration 

of sulfide material from the basaltic magma, ( 2) subsolidus reactions of the sulfide material within itself 

or with oxide or silicate phases, and ( 3) later redistribution of sulfide phases by alteration processes such as 

serpentinization. Only stages 1 and 2 are of importance in this paper. The actual phases observed in the 

G and H chromitite zones are dominantly a result of 

stage 2 processes. 

Three magmas are described. One is the basaltic 

magma which forms the reservoir from which the cumulus minerals of the cyclic units crystallized. Within 

the basaltic magma there is the possibility of a second 

magma, an immiscible sulfide-metal-oxygen liquid, 

which will be referred to as sulfide liquid or immiscible 

sulfide droplets. The third magma is of the same basaltic composition as the reservoir, but this magma was 

trapped in the pore spaces between the cumulus phases 

and soon was separated from the reservoir. It will be 

called trapped magma. It solidified to form part of 

the postcumulus material. 

The basaltic magma that formed the cyclic units of 

the G and H chromitite zones was probably introduced 

into its position above the previous cyclic units by a 

mechanism of variable-depth convection and periodic 

refreshment of the bottom magma (Jackson, 1961, p. 

96-99). The temperature of this magma was at or 

above the crystallization temperature of olivine and 

chromite. Olivine and chromite pairs indicate temperatures during crystallization of perhaps as much as 

1,425° ± 300°C near the base of the chromitite zones 

and as low as 903°-1,100°C near the top of the chromitite zones (Jackson, 1969). The cumulus minerals chromite and olivine and their sulfide mineral inclusions 

record the liquidus history of crystallization of the 

basaltic magma of the reservoir and its subsolidus 

history. Sulfide phases associated with postcumulus 

silicate phases may reflect the liquidus and subsolidus 

history of both the reservoir basaltic magma and the 

trapped basaltic magma as well as the interaction between the two. 

The intimate association of sulfide minerals within 

cumulus phases and with postcumulus phases in the 

Stillwater Complex supports the thesis that sulfide 

material was present at the time and was under the 

conditions of silicate crystallization from the basaltic 

magma. The introduction of small sulfide mineral 

blebs into the centers of cumulus phases at any other 

time than that of crystallization is difficult to accept. 

Furthermore, the presence of different stratigraphically controlled sulfide populations inside cumulus minerals and their repetition in at least two zones leads 

me to believe that the sulfide minerals are directly related to the separate reservoir magmas and. in fact, 

were derived from the magmas. 

Assuming that the sulfide. oxide and silicate minerals in the chromitite zones are comagmatic, the sulfur must have been dissolved in the magma, in an 

immiscible sulfide liquid, in crystalline solid solutions, 

or held in a combination of these. The contribution of 

these sources must be evaulated. 

The average volume percent of sulfide minerals in 

chromitites is on the order of 0.01. This concentration 

in the magma is compatible with the solubility of 0.1 

weight percent of sulfur in a thoeliitic basalt from 

the Alae lava lake (Skinner and Peck, 1969) and 0.06 

weight percent of sulfur estimated for the Skaergaard 

magma (Wager and others, 1957). MacLean (1968) 

has shown experimentally that 3-5 weight percent of 

sulfur can be dissolved at very low oxygen fugacities 

in iron silicate melts. · 

Although this appears to be a sufficient source for 

sulfur, observations from the chromitite zones suggest 

extreme heterogeneity in the sulfur content of the 

magma. First, the proportions of sulfide to silicate 

minerals within individual inclusions in cumulus olivine and chromite range from 0 to 100 percent. If the 

inclusions were trapped by olivine and chromite at the 

basaltic magma liquidus, then the basaltic magma 

must have contained immiscible sulfide droplets or 
16 SULFIDE MINERALS IN THE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, ~-IONTANA 

sulfide crystals. Second, the amount of trapped magma 

varies from about 4-43 percent by volume in the chromitite zones and shows no correlation with the volume 

of sulfide minerals associated with the postcumulus 

minerals. Third, local massive sulfide lenses occur 

conformable to the base of chromitite zones, indicating a larger amount of sulfur in the basaltic reservoir 

magma than the average volume percent of sulfide 

minerals in the chromitite zones would suggest. 

Either immiscible sulfide liquids or crystalline sulfide solid solutions can account for the apparent inhomogeneity in the reservoir basaltic magma as reflected by the observed sulfide mineralogy. Sulfur dissolved in the reservoir magma cannot alone account 

for the inhomogeneity. If immiscible droplets were 

the only source of the single-phase and multiple-phase 

grains with varying proportions of different minerals, 

many different compositions of sulfide liquids would 

be required. Because the distribution of metals in the 

sulfide liquid was most certainly controlled by the 

reservoir basaltic magma, it is impossible to accept the 

theory of many different sulfide liquids. tT ackson's 

(1969) liquidus temperatures of 1,400°0 in the lower 

portions of the chromite zones, based on olivine-chromite pairs, are too high to permit crystalline phases to 

be present if compared with the 1,100°-1,200°0 temperatures at which crystalline nickel-rich pyrrhotite 

solid solutions are present in experimental studies 

(Craig and Kullerud, 1969; Kullerud and others, 

1969). Because immiscible sulfide droplets alone cannot account for the observed sulfide mineral heterogeneity, I am led to conclude that soon after olivine 

and chromite began to crystallize from the magma, 

significant amounts of sulfur must have existed in both 

crystalline solid solutions and immiscible droplets and 

that Jackson's estimates of liquidus temperatures are 

too high. This conclusion is contrary to the inferences 

which may be drawn from Jackson's temperatures. 

In the above discussion, it is implied that the reservoir basaltic magma is the main source of the sulfide 

minerals. Evidence for this is that the amount of postcumulus silicates (representing trapped magma) 

varies from 4 to 43 percent by volume and shows no 

correlation with the· amount of sulfide minerals associated with postcumulus material; hence, contributions 

from the trapped magma were small. The composition 

of the trapped magma could be affected by the reservoir magma as long as there was permeability between 

the two magmas in the crystal pile. The amount of 

permeability was probably small; the distance over 

which the crystal pile was permeable was on the order 

of feet or inches, as evidenced from soft, sedimentary, 

deformation features (Hess, 1960; Jackson, 1961). 

As the pile cooled and crystallized, the trapped magma 

could generate more sulfide liquid, which then could 

react or add to the preexisting sulfide concentrations 

in the pores of the olivine-chromite cumulates. This 

magma, after cooling below the temperature of crystallization of a nickel-rich pyrrhotite solid solution, could 

yield sulfides as crystalline material. The soluble sulfur in the magma must have eventually reacted with 

the concentrations of sulfides already present, but considering the low solubilities of sulfur in basaltic magmas and the amount of trapped magma, any contribution to them must have been small. If this is what happened, it means that the major processes took place in 

the reservoir magma. 

If the above model is valid, then observations on the 

sulfide minerals suggest several possible embellishments. Single phase inclusions of laurite, platinumgroup sulfide minerals and alloys, niccolite, and molybdenite suggest that these were trapped by the cumulus 

minerals after the inclusions existed as crystals. Multiple-phase inclusions may have been trapped as Fe-NiCu-S crystalline solid solutions or as immiscible droplets which, upon cooling, crystallized and reacted to 

form the observed assemblages. After any of these 

appeared in the magma, they must have grown in size, 

because the average diameter of the inclusions increases from the base to the top of the chromitite 

zones. 

This hypothesis cannot account for the single-phase 

grains of pentlandite and pyrrhotite found as inclusions in cumulus phases. Experimental studies indicate 

that pentlandite does not form from a liquid ; only 

Fe-Ni monosulfide solid solutions do (Kullerud and 

others, 1969). If such a solid solution were the phase 

trapped by the cumulus minerals, then it is possible, 

but not proven, that an exchange of Ni or Fe between 

host and inclusion could form pentlandite or pyrrhotite. Assuming that a 10-micron pentlandite inclusion 

with a density of 4.91 gm per cm3 was derived from an 

inclusion of Fe0 •5Ni0 • 5S, only 9.7X10-11 grams of Ni 

and 9.3X10-11 grams of Fe would have to be added to 

form pentlandite with a composition of Fe •. tsNi,.I!Ss. 

Unpublished analyses of the cumulus phases (E. D. 

Jackson, oral commun., 1968) show that there is 

enough Ni and Fe to do this. Because of the small 

amounts of material involved, any zoning that might 

have been produced in the host by this mechanism 

remains undetected. If this process is not accepted, 

then the present phase diagrams are not applicable to 

a differentiating magma, and one must assume that 

a dense phase of pentlandite can exist at magmatic 

temperatures. All evidence from studies of Hawaiian 

lava agrees with the experimental studies. 
GENETIC IMPLICATIONS 17 

More abundant sulfide grains in phases associated 

with postcumulus material must have had at least two 

sources--cumulates from the reservoir basaltic magma 

and minor postcumulus growth from the trapped magma. Determination of the source of each sulfide grain 

is impossible, but the three subcategories of sulfide 

grains associated with postcumulus material suggest 

at least two sources. Also, the poor sorting exhibited 

by grains associated with postcumulus material could 

be interpreted as evidence for multiple origins for the 

grains. Sulfide grains that completely fill the pore 

spaces and that are molded around the cumulus phases 

(subcategory 1) possibly accumulated as immiscible 

droplets or crystalline solid solutions contemporaneously with olivine and chromite. The sulfide grains 

that have sharp contacts with silicates could be interpreted as cumulates because they are properly oriented in the pores and have local growths of postcumulus chromite or olivine around them (subcategory 

2), but they could also represent accumulated droplets. 

Orientation studies of grains in the G and H zones 

show that grains with these characteristics could 

amount to 60 percent of subcategory 2. The rest of 

this subcategory and possibly the third subcategory of 

ragged grains appear to represent sulfide droplets and 

crystalline phases that were formed by the "trapped 

magma" as the pile cooled. Grains with ragged edges 

might result from addition of material from the last 

sulfur dissolved in the magma or from reactions that 

may have taken place. By abundance, about 84 and 64 

percent of the sulfide grains of the G and H zones 

respectively fall in subcategories 1 and 2. 

The number of sulfide grains representing all textural categories correlates closely with the volume of 

sulfide minerals present in a particular horizon. (Note 

that this relation is not necessarily true; there could be 

one grain of 10-micron diameter present in one rock 

and 1,000 grains of 1-micron diameter in the other 

rock, but both would contain the same volume percent 

of grains.) The well-sorted nature of the inclusions 

and their size variations suggest that they were distributed in the magma chamber by some sort of crystal-settling phenomena. For these reasons, sulfide distribution must have the same re.lationship to the rate 

of production and growth of these inclusions as it has 

to the rates of accumulation and growth of cumulus 

olivine and chromite. .T ackson ( 1961) suggests that 

olivine and chromite accumulated on the bottom of the 

magma chamber as rapidly as they were able to grow 

and nucleate. Settling was rapid. Reexamination, 

using viscosities given by Shaw, Wright, Peck, and 

Okamura (1968) for a basaltic magma, shows that 

olivine and chromite crystals of the size range found 

in the G and H zones 2 would settle at rates of thousands to ten thousands of centimeters per year, whereas inclusions with a 5-micron radius would settle at 

rates of tenths to hundredths of a centimeter per year. 

The change in size of inclusions as a function of stratigraphic position must reflect the growth and nucleation rates of sulfide grains and interrelate with the 

amount of time required for them to be concentrated 

at a particular horizon by settling. 

The number of sulfide grains associated with postcumulus phases also correlates well with the amount 

of sulfides. Aggregates and grains with 20-micron 

radii would settle at rates of between 1/10 and 1 centimeter per year. Comparison of these values with rates 

of settling for olivine and chromite implies that nucleation and growth, rather than settling, were also the 

major controls of their concentration. This theory 

assumes that the grains settled as entities and were 

not attached to chromite and olivine or caught up in 

currents or eddies controlled by the settling olivine 

and chromite. Observations of the orientation of sulfide grains suggest that the sulfide grains may have 

been attached to olivine and chromite, but not enough 

evidence is available to evaluate this feature. 

If, instead of the smooth variations in temperature 

and partial pressure of oxygen initially suggested by 

Jackson ( 1969), the actual variations in temperature 

and pressure fluctuated, as indicated by Jackson's 

(1969) detailed data, then it might be possible to explain the distribution of sulfide minerals in another 

manner. The relation between the sulfide volume percentage, the Fe+3 /(Fe+2 +Fe+3 ) ratio in chromite, and 

the temperature of the liquidus from olivine and chromite pairs is shown in figure 9. There is a suggestion 

from these data that higher temperatures at intermediate values of oxidation ratios favor the accumulation of sulfide minerals. Plots of temperature against 

oxidation ratio, contoured with volume percent of sulfides, suggest that this correlation is not strong. Smith 

( 1961) suggested that a low oxidation potential in a 

mafic magma would favor the formation of sulfide 

liquids and that higher temperatures would favor 

sulfur dissolved in the magma. MacLean's (1968} experimental work indicates that the solubility of iron 

sulfide in an iron silicate melt is at a maximum when 

coexisting with iron and that solubility decreases with 

increasing oxygen fugacity. Evaluation of the Stillwater sulfide occurrences neither supports nor refutes 

this evidence, but it does indicate that in order to 

unravel temperature and oxidation effects, effects of 

2 Asl'lumed values: sulfide density, 4.90 g per cm1 ; olivine density, 

3.30 g per cma; chromite denl'litY, 4.55 g per cm3 ; magma density, 

2.65 g per cmz; and magma vlscol'lltiel'l, !lOO and 8,000 poises. 
18 SULFIDE MINERALS IN THE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, MONTANA 

growth, settling, and nucleation rates and the various 

sources of sulfur must first be understood. 

Hess (1960) gives fairly constant values of 0.04 

weight percent of sulfur in the various zones of the 

Stillwater Complex. The Skaergaard intrusion 

(Wager and others, 1957) and the Muskox intrusion 

(Chamberlain, 1967) also show a constant or slightly 

increasing total sulfur content with higher stratigraphic position. A chromitite zone is a small segment 

of a complex and, moreover, is not rich in sulfur; 

therefore, it is suspected that the amount of sulfur in 

the magma remained nearly constant during the 

deposition of a chromitite zone. 

However, the ratios of phases have been shown to 

change within the horizons. Because pyrrhotite and 

pentlandite are the most abundant sulfide minerals 

present, the most significant variation to be considered 

is the distribution of nickel and iron in sulfides from 

the bottom to the top of a zone. The average pentlandite to pyrrhotite-plus-pentlandite ratio for both 

the G and H zones is about 0.68, so that bulk composi-

(/) 

...J 

<( 

ffi 0.008 z 

~ 

(/) 

::> 

...J 

::> 

::;;! 

::> 

u 

I-

(/) 

0 

Q.. 0.006 ::c 

I-

~ 

0 

IJ.I 

I-

<( 

u 

0 

(/) 

~ 

~ 0.004 

0 

i:i: 

...J 

::> 

(/) 

EXPLANATION 

970±68 

G chromitite • zone 

1381±120 

0 

H chromitite zone 

Numbers are temperatures (°C) 

based on olivine-chromite 

pairs 

,/ 

tions and the process of differentiation which yielded 

the sulfides were similar in both zones. The lower 

H zone may be used as an example for interpretation 

although the data may not be representative of the 

whole differentiation and accumulation process. The 

G zone offers less data of statistical significance in the 

distribution of sulfides and the data seem to' be more 

variable, not only with respect to the sulfide amounts, 

but also with respect to compositions of chromite and 

olivine (Jackson, 1969). 

Nickel and iron have complementary distributions 

in the sulfide phases; therefore, only nickel will be 

considered here. The data suggest, in general : ( 1) the 

abundance of pentlandite decreases from the base to 

the top of the lower H zone, ( 2) ·the abundance of 

pentlandite in single-phase grains decreases upwards 

and the number of single-phase grains decreases upwards, and (3) the abundance of pentlandite in multiphase grains decreases upwards but the number of 

these multiphase grains increases upwards. This evidence implies that the weight percent of nickel in all 

/ 

// 

// 

/ 

~ // 1381±120 

/ 

/ 

\:Jr!) 

"'>' / 

o/ 

/1427±148 

/ 

I 

I 

I 

I 

I 

I 

0 

0 

0 

T 

I 

~ I 

r ' 

/ 

I I 

/ h 0 II / .... 0 / I :: I 

// I I I 

_,.,../ I I I 

• 922±97 

LL 

0 

1398± 146 

• .,.,..,.,. 1274±135 I 1 1 IJ.I .,.,-"' • • I I I _,.,..,.,. 0 1068±71 I I I .,.,. .,., 1275±113 

....,................... / I /1 

(.!} 

<( 

1-

z 

~ 0.002 a:: 

IJ.I 

Q.. 

IJ.I 

::;;! 

::> 

...J 

0 

> 

0 

.,.,.1246±98 1273±134 / /// ~-"' o o1231±9o ?/ / _- 1033 

1110±65 0 0 1396±120 / / / 

0 1246±97 // / / • • 970±68 

1260±1~ / / 977-+-101 • 1438::t:160 

0 1274±47 0 / / / -

• 1253±70.........:".,......., , / 978±68 

0 

1024±50 

20 22 24 26 28 30 32 34 36 38 40 42 

(Fe+3/Fe+2+Fe+3) X 100 IN CHROMITE 

FIGURE 9.-Volume percentage of sulfide minerals versus Fe+31 (Fe-P+Fe+3 ) ratio in chromite. 
CONCLUSIONS 19 

sulfide grains decreases from the base to the top of the 

lower H zone and suggests a nickel impoverishment 

in the source liquids. 

CONCLUSIONS 

The sulfide grains found in the G and H chromitite 

zones came from the same magmas that produced the 

respective cyclic units of the zones. Sulfide minerals 

occur as sulfide inclusions in cumulus chromite and 

olivine and as sulfide phases in interstices between 

cumulus phases. Studies of texture, size, phases, abundance, and orientation indicate that sulfides existed in 

the magma batches in three forms : as crystalline 

phases suspended in the basaltic magma; as immiscible 

sulfide liquids; and as sulfide soluble in the basaltic 

magma. During differentiation, nickel and sulfur were 

preferentially concentrated in the sulfide crystals and 

liquids; the nickel depletion of the magma with time 

is shown by the sulfide phases. Platinum-group metals 

were preferentially concentrated in cumulus chromite 

as inclusions of sulfides and alloys, and at the time of 

entrapment they probably existed as crystalline 

phases. 

Although temperature and partial pressures of oxygen and sulfur were the main controls for the :formation o:f sulfide phases, nucleation, growth, and settling 

rates probably determined the present distribution and 

concentrations of sulfide minerals in the G and H 

chromitite zones. 

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20 SULJ!~IDE MINERALS IN THE G AND H CHROMITITE ZONES OF THE STILLWATER COMPLEX, MONTANA 

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'f:l U.S. GOVERNMENT PRINTING OFFICE: 1971 Q-413-044