Stillwater Complex, Montana 

Structure, Mineralogy, and Petrology 

of the Basal Zone with emphasis on 

the Occurrence of Sulfides

By NORMAN JPAGE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1038

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Page, Norman J

Stillwater Complex, Montana Structure, Mineralogy, and Petrology of the Basal Zone with emphasis on the

Occurrence of Sulfides

(Geological Survey Professional Paper 1038) 

Bibliography: p. 61 

Supt. of Docs. No.: I 19.16:1038

1. Rocks, Igneous. 2. Intrusions (Geology) Montana. 3. Sulphides. 4. Ore-deposits Montana. 

I. Title. II. Series: United States Geological Survey Professional Paper 1038 

QE461.P289 552'.l'09786 77-608351

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

Washington, D.C. 20402 

Stock Number 024-001-03179-5
CONTENTS

Abstract ............................................................................................

Introduction ......................................................................................

Geologic history of the Stillwater Complex and adjacent rocks

Page

1

1

2

Previous work and acknowledgments ............................................... 3

Definition of the Basal zone .................................................................... 4

Distribution and thickness of the Basal zone ................................... 4

Shape of the Basal zone ................................................................... 5

Description of contacts ..................................................................... 6

Contact of the Basal zone with metasedimentary rocks ............ 6

Contact of the Basal zone with the Ultramafic zone ................. 7

Contact of the Basal zone with younger intrusive quartz monzonitic rocks .......................................................................... 7

Contact of the Basal zone with mafic dikes .............................. 7

Contact of the Basal norite with the Basal bronzite cumulate .. 7

Age restrictions on the Basal zone ................................................... 7

Petrology and mineralogy of the Basal zone ........................................... 7

Rock types in the Basal zone ........................................................... 7

Cumulate rocks of the Basal zone .................................................... 9

Cumulate sequences .......................................................................... 16

Size variation of orthopyroxene cumulates ............................... 17

Evidence for current structures ................................................. 18

Modal mineral variation in cumulates ...................................... 19

Lateral and vertical variation of orthopyroxene composition ... 21

Compositional variation of plagioclase ..................................... 21

Discussion of cumulate rocks in the Basal zone ........................ 23

Igneous-textured (noncumulate) rocks of the Basal zone ................. 23

Modal variation of noncumulate rocks ..................................... 23

Chemical composition of the noncumulate igneous-textured rocks 24

Viscosity, density, and settling velocities ................................... 28

Contaminated igneous rocks ............................................................ 31

Types, textures, and mineralogy of the contaminated igneous

rocks ...................................................................................... 31

Relation of mixed rocks to inclusions of metasedimentary rocks 32

Processes for forming mixed or contaminated rocks ................ 33

	Page

Petrology and mineralogy of the Basal zone Continued

Rocks derived by partial melting ...................................................... 34

Occurrence ................................................................................ 34

Petrography and mineralogy ..................................................... 35

Characteristics of hornfelses adjacent to quartz, norite .............. 36

Origin of the quartz norites ....................................................... 36

Other silicate rocks ........................................................................... 38

Ultramafic xenoliths .................................................................. 38

Autohornfelsed Basal zone rocks .............................................. 39

Sulfide-bearing rocks ........................................................................ 39

Occurrence, general terminology, and distribution .................... 39

Development of terminology for sulfide-bearing rocks ............. 41

Textures of sulfide-bearing rocks .............................................. 41

Disseminated and matrix sulfide textures .......................... 42

Textures of massive sulfide rocks ....................................... 44

Textural relations between silicate and

sulfide minerals ............................................................... 44

Mineralogy of the sulfide-bearing rocks .................................... 44

Pyrrhotite ........................................................................... 46

Pentlandite ......................................................................... 47

Chalcopyrite family ............................................................ 48

Magnetite, ilmenite, and hematite ...................................... 48

Other opaque minerals ....................................................... 49

Proportions of sulfide minerals ................................................. 49

Interrelation between sulfide mineralization and

silicate rocks .......................................................................... 50

Interpretation and model of sulfide crystallization .................... 52

Mechanisms for separation of immiscible

sulfide-oxide liquids from basaltic magma ..................... 53

Collection and concentration models ................................. 54

Crystallization of the collected sulfide-oxide liquid ........... 59

Limitations and speculations on origin of Basal zone ............................. 61

References cited ....................................................................................... 61

Index ....................................................................................................... 67

ILLUSTRATIONS

PLATE I. Interpretative map, cross sections, and long sections of the Basal zone ......................................................

2. Vertical variation in orthopyroxene compositions and grain size from 15 cross sections of the Basal zone

Page

In pocket 

In pocket

FIGURH 1. Index map of the Stillwater Complex ................................................................................................................................ 2

2. Geologic map of the Stillwater block ................................................................................................................................. 3

3. Frequency diagram of estimated stratigraphic thickness of Basal zone at 48 localities ...................................................... 5

4. Interpretative sketches of basin structures in Basal zone and Peridotite member .............................................................. 6

5. Sections showing rock types and mineralogy in Basal zone ............................................................................................... 9

6. Histograms of orthopyroxene and plagioclase compositions ............................................................................................. 16

7. Graph of mean length, width, and average grain size of orthopyroxene versus stratigraphic position for part of Basal bron/ite

	cumulate, upper Nye Basin ......................................................................................................................................... '8
IV CONTENTS

Page

FIGURE 8. Length-frequency distributions of orthopyroxene crystals for a segment of Basal bronzite cumulate, upper Nye Creek Basin .. 18

9. Photographs of scour and local unconformities in Basal norite ......................................................................................... 19

10. Plot of modes of cumulates from Basal zone ..................................................................................................................... 20

1 1. Orthopyroxene in Basal zone plotted against stratigraphic position. West Benhow, Golf Course section ......................... 20

12. Diagram of plugioclase composition plotted against stratigraphic position, upper Nye Basin section ............................... 22

13. Plot of coexisting plagioclase and orthopyroxene compositions in Basal /one .................................................................. 22

14. Diagram showing proportions, by volume, of clinopyroxene. plagioclase, and orthopyroxene in noncumulate rocks ...... 24

15. ARM diagram of noncumulate rock compositions compared with cumulate rock compositions ...................................... 27

16. Projections of analyses of noncumulate igneous-textured rocks in the system olivine-clinopyroxene - plagioclase - silica 27

17. Calculated dry density-temperature curves of possible Stillwater magmas ......................................................................... 28

18. Diagram of calculated viscosities of possible parent magmas for the Stillwater complex plotted against reciprocal

	temperature ................................................................................................................................................................. 29

19. Settling \elocitics of plagioclase, orthopyroxene. and olivine in possible Stillwater magma .............................................. 30

20. Photographs of etched slabs of contaminated igneous rocks ............................................................................................. 31

21. Histogram of sizes of metasedimentary inclusions ............................................................................................................. 33

22. Histogram of widths of mixed rocks zones adjacent to inclusions ..................................................................................... 33

23. Diagram of inclusion size and width of mixed rock zone .................................................................................................. 33

24. Diagram showing percentage of hornfelsthat would be consumed to form mixed rocks for various sizes of hornfels inclusions 34

25. Sketches and photographs of wisps, lenses, and discontinuous veins of quartz norite ....................................................... 35

26. Modal compositions of quartz norites and noncumulate igneous rocks ............................................................................ 36

27. Comparison of compositions of metasedimentary rocks, noncumulate igneous rocks, and estimated composition ot quartz

	norite ........................................................................................................................................................................... 37

28. Photomicrographs of ultramafic xenoliths ......................................................................................................................... 38

29. Estimated maximum volume percentage of sulfide minerals in a stratigraphic section of Basal zone ............................... 40

30. Modal volumes of sulfide minerals in 150 selected samples ............................................................................................... 41

31. Photographs and tracings of cumulate rocks showing textures of disseminated and matrix sulfide minerals .................... 43

32. Photographs of slabs of massive sulfide rocks ................................................................................................................... 45

33. Photomicrograph of coexisting monoclinic and hexagonal pyrrhotite ............................................................................... 46

34. Diagram used to estimate bulk composition of pyrrhotite ................................................................................................. 47

35. Photomicrograph of pyrrhotite texture ........................................................................................................................... 47

36. Photomicrographs showing oxide-sulfide mineral relations ............................................................................................... 49

37. Histograms of copper and nickel content ol drill core and mine-tunnel samples ............................................................... 50

38. Diagram correlating copper and nickel content ................................................................................................................. 51

39. Triangular diagram of pyrrhotite, pentlandite. and chalcopyrite modes for sulfide-bearing rocks ................................... 52

40. Settling velocities ot sulfide liquid spheres ......................................................................................................................... 56

41. Ranges of diameters of disseminated sulfide clots for individual samples .......................................................................... 57

42. Diagram showing clot size and grain size distributions of disseminated sulfides and inclusions ........................................ 58

43. Hypothetical size distributions of sulfide spheres in the Stillwater magma ........................................................................ 59

TABLES

Page

1. Generalized geologic history of the Stillwater Complex area, Montana ............................................................................ 4

2. Rocks in the Basal zone of the Stillwater Complex, Montana .......................................................................................... 8

3. Generalized petrographic descriptions of cumulate rock types in the Basal zone .............................................................. 10

4. Chemical, spectrographic, normative, and modal analyses of ophitic, subophitic, and ragged-textured rocks of the Basal

norite member. Basal zone, Stillwater Complex, Montana, with some mineral compositions .................................... 25

5. Comparison of the range and average chemical analyses of noncumulate igneous-textured rocks from the Stillwater Complex

with Hawaiian basalt. Thingmuli basalt, oceanic tholeiitic basalt, and alkali basalt .................................................. 27

6. Summary of types, textures, and mineralogy of contaminated igneous rocks and their relation to igneous and

metasedimentary rocks ................................................................................................................................................ 32

7. Modes of contaminated or mixed rocks and the immediately adjacent hornfelses ............................................................ 32

8. Range and average volume percentage of minerals in quartz norite in lenses, clots, and discontinuous veins .................. 35

9. Compositions of hexagonal pyrrhotite determined by X-ray diffraction ............................................................................ 46

10. Calculations of sulfide compositions based on Roby's (1949) large composite sample of sulfide material ........................ 55
STILLWATER COMPLEX, MONTANA STRUCTURE, MINERALOGY,

AND PETROLOGY OF THE BASAL ZONE WITH EMPHASIS

ON THE OCCURRENCE OF SULFIDES

By NORMAN J PAGE

ABSTRACT

The Basal zone of the Stillwater Complex is divided into two members, a 

Basal norite and an overlying Basal bronzite cumulate. The Basal norite is 

composed of alternating lensoid masses or layers of rock and is extremely 

variable both laterally and vertically in mineralogy, texture, grain size, and 

crystallization sequences, and the Basal bronzite cumulate is composed of 

layers of fine- to coarse-grained granular orthopyroxene cumulates and 

exhibits relatively regular properties laterally.

The contact between the Basal zone and the metasedimentary rocks it 

intrudes does not follow a single stratigraphic horizon but cuts across 

lithologic and structural units in the country rocks. The overall shape of the 

Basal zone is sheetlike with an extremely large length-to-thickness ratio, 

but in detail it consists of a series of basinlike shapes linked by thinner units 

of rock. The Basal zone is composed of a variety of rocks, including: 

Cumulates, principally orthopyroxene cumulates but with minor amounts 

of one- and two-phase cumulates dominated by olivine, orthopyroxene, 

clinopyroxene, inverted pigeonite, plagioclase, or chromite; noncumulate 

igneous rocks with ophitic, subophitic, and ragged textures; contaminated 

and mixed rocks; rocks derived by local partial melting; sulfide-bearing and 

sulfide rocks; and complex breccias of the above types. The rocks and their 

constituents are characterized by fluctuating and repeating patterns of 

chemical and physical properties that, from bottom to top, include: (a) 

Lithologic sequences of ophitic, subophitic, ragged, and cumulus-textured 

rocks; (b) cumulate sequences of orthopyroxene-orthopyroxene+plagioclase, olivine+orthopyroxene-orthopyroxene, and 

plagioclase-plagioclase+ orthopyroxene; (c) sequences of fine- to mediumto coarse-grained cumulate rocks; (d) variation in the volume of cumulus 

phases present; (e) variation of orthopyroxene compositions; (f) variation 

of plagioclase compositions; and (g) variation of the distribution of sulfide 

minerals.

The model for the formation of the Basal zone proposes that immediately 

after or during initial emplacement of magma, several characteristics of the 

basal contact zone developed. Silicate magma and a sulfide-oxide liquid 

were injected into the metasedimentary rocks as tongues, dikes, and lenses, 

accounting for locally complicated geologic relations. Some early silicate 

magma was emplaced in rocks that were cool enough to allow it to 

crystallize as ophitic, subophitic, and ragged-textured noncumulates. 

Subsequent injections of magma cooled more slowly and developed 

textures characteristic of cumulate rocks. The massive sulfide lenses 

crystallized as temperatures decreased.

Noncumulate igneous rocks crystallized by relatively rapid cooling or 

quenching of magma within the main chamber near the contact with 

metasedimentary rocks. Large blocks and inclusions of metasedimentary 

rocks locally having rinds of noncumulate rocks were rafted upward in the 

magma chamber. Locally, cumulate crystalization began, was interrupted 

either by new influxes of magma or by overturning in convection cells, and 

was reestablished. This process was probably the major mechanism

operating during the development of the Basal norite member. The 

approach of the magma to thermal and chemical equilibrium was also 

perturbed by influxes of new magma or convection.

The same cycle of crystallization and perturbation affected the remaining 

magma, probably producing the chemical and mineralogical repetitions 

and inhomogeneities exhibited by the norite member of the Basal zone. 

Perhaps, by the time the upper part of the Basal norite member was 

crystallizing or at least by the time the Basal bronzite cumulate member was 

accumulating, there were no new influxes of magma, and the crystallization 

pattern was established.

INTRODUCTION

The large tabular mass of Precambrian mafic and 

ultramafic rocks that forms the Stillwater Complex crops 

out along a northwest strike on the northern margin of the 

Beartooth Mountains in southwestern Montana (fig. 1). The 

lower part of this mass, called the Basal zone, is discontinuously exposed for about 42 km along strike in the Mt. 

Wood and Mt. Douglas 15-minute quadrangles (Page and 

others, 1973a, b). A considerable mineral potential is known 

for the Basal zone and adjacent rocks (Howland, 1933; 

Roby, 1949; Dayton, 1971; Page and Dohrenwend, 1973). 

Because keys to understanding the early processes involved 

in formation of the stratiform mafic and ultramafic complexes are hidden in the detailed geology of the Basal zone, 

this report concentrates on this zone exclusively.

An informal stratigraphic nomenclature to describe the 

internally conformable layers of the Stillwater Complex has 

developed from the early work of Peoples (1936, p. 358), 

through that of Jones, Peoples, and Howland (1960), Hess 

(1960), Jackson (196la), and Page (1977), to that presented 

here. In addition, an extensive terminology developed by 

Jackson (1967) to describe rocks formed by crystal accumulation in response to gravity is used.

The Stillwater Complex is divided into four main zones: 

Basal, Ultramafic, Banded, and Upper. The Basal zone is 

split into two members; the Basal norite and the overlying 

Basal bronzite cumulate (Page and Nokleberg, 1975; Page, 

1977). Jackson (1961 a) divided the U Itramafic zone into two 

parts, a Peridotite member and a Bronzitite member. Only 

the Peridotite member, which conformably overlies the
STILLWATER COMPLEX, MONTANA

11000' 1,900,000 FEET EAST

Mt. Rae 7.5' 

McLeod Basin 7 

Mt. Cowen 15' 

Mt. Douglas 15' 

Mt. Wood 15' 

Emerald Lake 7.5

EXPLANATION

Tertiary volcanic rocks

Mesozoic and Paleozoic 

stratified rocks

Quartz monzonite

IZ3

Stillwater Complex

Hornfels

Basement 

m

rocks

45 00'

1 Benbow area

2 Nye Basin area

3 Nye Lip area

4 Mountain View area 

Initial area 

Bluebird Peak area 

West Fork of the

Stillwater River area 

Crescent Creek area

9

10

11

12

13

14

15

Iron Mountain area 

Forge Creek area 

Bobcat Creek area 

ChromfrMountain area 

Blakely Creek area 

Gish area 

North side of Falls 

Creek area 10 15 MILES

0 5 10 15 KILOMETERS

FIOURI: I. Index map of the Stillwater Complex.

Basal zone, is pertinent to this report. It consists of cyclic 

units (Jackson, I96la, 1963, 1967, 1968a, 1969, 1970, 1971) 

that are repetitions of the cumulate sequence olivine, 

chromite, olivine+chromite, olivine+bronzite, and bronzite; 

at least 15 cycles are recognized in the Peridotite member, 

and 13 contain chromite and chromite+olivine cumulates 

near the base. The chromite cumulates have been designated 

by letters from A to K sequentially from the lowest to highest 

zone.

GEOLOGIC HISTORY OF THE STILLWATER 

COMPLEX AND ASSOCIATED ROCKS

The geologic history of the Stillwater Complex and 

associated rocks extends back over 3140 m.y. (million years) 

and contains at least two mountain-building events. Page 

(1977) summarized the Precambrian events, and Jones,

Peoples, and Howland (1960) described the post-Middle 

Cambrian events for the Stillwater area. Foose, Wise, and 

Garbarini (1961) and Casella (1969) gave summaries of the 

geology of the encircling Beartooth Mountain region. Figure 

2 is a simplified map of the geology within the Stillwater 

Complex compiled from Page and Nokleberg (1975) and 

Page, Simons, and Dohrenwend (1973a, b) that shows the 

geologic relations of the five major groups of rocks in the area: 

(1) Regionally metamorphased rocks consisting of granitic 

gneisses and associated metasedimentary rocks of Precambrian age; (2) hornfelsed metasedimentary rocks associated 

with the Stillwater Complex; (3) stratiform mafic and 

ultramafic rocks of the Stillwater Complex; (4) an intrusive 

sequence of quartz monzonitic rocks of Precambrian age; 

and (5) sedimentary rocks of Paleozoic and Mesozoic age. 

Table 1 summarizes the geologic history of the area.
INTRODUCTION

PREVIOUS WORK 

AND ACKNOWLEDGMENTS

The Stillwater Complex and adjacent rocks have been 

studied since 1920 as part of several overlapping comprehensive investigations of structure, stratigraphy, geochemistry, 

and petrology of the complex. Mapping and studies of the 

chromite deposits include Westgate (1921), Peoples (1932, 

1933, 1936), Howland (1933, 1955), Vhay (1934), Peoples

and Howland (1940), Wimmler (1948), Howland, Garrels, 

and Jones (1949), Richards (1952,1958), Peoples, Howland, 

Jones, and Flint (1954), Jackson, Howland, Peoples, and 

Jones (1954), Vail (1955), Jones, Peoples, and Howland 

(1960), Jackson (1960, 1961a, 1963, 1967, 1968a, 1969,1970, 

1971), and Page, Shimek, and Huffman(1972). Hess (1938a, 

b; 1939, 1940, 1941, 1949, 1960), and Hess and Phillips (1938, 

1940) concentrated on the Banded and Upper zones. Other 

generalized mapping studies include Page and Nokleberg

110 15' 110 00'

45 30'

45 20' —

EXPLANATION

Sedimentary rocks and intru

Cu

E 

o 

U

Peridotite member with 

chromitite zone

H cph

Coarse-grained quartz 

monzonite

Metamorphosed sedimentary rocks 

cph, orthopyroxenecordierite hornfels 

ach, anthophyttitecordierite hornfels 

bch, biotite-cordierite

hornfels

Regionally metamorphosed rocks

ggn,granitic gneisses

bsh, biotite schist and

amphibolitic gneiss

Contact, approximately located

Fault, approximately located. 

Dotted where concealed

Thrust fault, approximately located. Dotted 

where concealed. Sawteeth on upper plate

-A5 _,_ e^ 

Inclined Vertical Overturned

Strike and dip of beds

.30 ( 80p

Inclined Vertical Overturned 

Strike and dip of cumulus 

layering

FIGURE 2.-Geologic map of the Stillwater Complex after Page and Nokleberg (1974) and Page, Simons, and Dohrenwend (I973a. b).
STILLWATER COMPLEX, MONTANA

(1970b, 1972, 1974), and Page, Simons, and Dohrenwend 

(1973a, b). Studies on sulfide minerals and platinum metals 

include Howland (1933), Howland, Peoples, and Sampson 

(1936), Roby (1949), Page (1971a, b; 1972), Page and 

Jackson (1967), and Page, Riley, and Haffty (1969, 1971, 

1972). Studies of the Basal zone by Howland (1933), Roby 

(1949), and E. D. Jackson (unpub. data) provide the 

foundation for this paper.

E. D. Jackson and A. L. Howland introduced and 

encouraged this study in 1967 and since then have contributed samples, unpublished maps, and recommendations. 

The author is grateful to W. J. Nokleberg and R. A. Koski 

for fieldwork and to John Stuckless, Richard Shimek, and 

John Dohrenwend for laboratory work. AMAX Exploration Inc., Anaconda Co., Cyprus Mines Corp., and 

Johns-Manville and their personnel have cooperated and 

facilitated this project. Richard N. Miller (Anaconda Co.)

TABLE I. Generalized geologic history of the Stillwater Complex,

Montana

[Alter Page (1977). Jones. Peoples, and Howland (I960), and hoose. Wise, and (iarbarim <I96I)J

I ime Fvent

Precambrian W 

(prior to 

3140 m.y.)

Precambrian W 

(2750-3140 m.y., 

probably nearer 

3140 m.y.).

Precambrian W 

(2750+60 m.y.).

Precambrian X 

(1600-1800 m.y.).

Pre-Middle Cambrian. 

Middle Cambrian........

Middle Cambrian

through Early

Cretaceous. 

Late Cretaceous..........

Early Paleocene 

through early 

Eocene.

Middle Eocene

through early

Miocene. 

Miocene through

Pliocene. 

Pleistocene through

Holocene.

(1) Presence of a terrane containing malic, ultramafic, and intermediate composition rocks 

which were being actively eroded.

(2) Deposition of clastic and chemically precipitated magnesium-and iron-enriched sediments 

including iron formation; deposition of a 

diamictite possible local glaciation.

(3) Complex folding of magnesium- and ironenriched sediments, possible regional 

metamorphism.

Strike-slip faulting, fractionation. and accumulation of magmatic sediments from Stillwater 

magma; intrusion of the Stillwater magma and 

contact metamorphism of older rocks.

Intrusion of quart/ monzonitic rocks; contact 

metamorphism of older rocks.

Intrusion of mafic dikes, penetrative deformation, low-grade regional metamorphism, possible thrust and normal faulting.

.Faulting, rotation, tilting, and erosion.

.Subsidence of Stillwater Complex.

Deposition of 2400-3000 m of marine and 

continental sedimentary rocks with breaks 

in the Silurian and Permian.

.Extrusion of volcanic rocks; initiation of 

Laramide deformation.

Thrust, normal, and strike-slip faulting; major 

uplift and deformation; folding; intrusion 

of siliceous and intermediate sills, dikes, and 

stocks; extrusion of volcanic rocks (Laramide 

orogeny).

Erosion.

Erosion and uplift.

Erosion and glaciation, minor faulting.

and Giles E. Walker (AMAX Exploration, Inc.) provided 

access to their diamond drill core for the purposes of logging 

and sampling.

DEFINITION OF THE BASAL ZONE

The Basal zone, as previously defined (Page and 

Nokleberg, 1974; Page, 1977; Page, Simons, and 

Dohrenwend, 1973a, b; Hess, 1960; Jones, Peoples, and 

Howland, 1960; Jackson, 1961a, p. 2; Page, 1971b, p. 1) 

includes all rocks stratigraphically below the lowest cyclic 

unit of the Peridotite member of the Ultramafic zone. It is 

composed of magmatic sediments of orthopyroxene, 

clinopyroxene, plagioclase, olivine, or combinations thereof 

and of noritic and gabbroic rocks that exhibit ophitic or 

subophitic noncumulate igneous textures. Because the 

boundary between the Basal zone and the Ultramafic zone is 

not necessarily marked by the appearance of the first 

cumulus olivine, the definition is referenced to the lowest 

cyclic unit of the Ultramafic zone, which usually begins with 

an olivine cumulate but locally begins with an olivine-bronzite or bronzite cumulate owing to onlapping relations.

The Basal zone is divided into two members. The upper or 

Basal bronzite cumulate member is mainly composed of 

layers of fine- to coarse-grained hypidiomorphic granular 

orthopyroxene cumulates. The lower or Basal norite is 

composed of alternating lensoid masses or layers of rock in 

which are found a variety of crystallization sequences 

composed of olivine, orthopyroxene, clinopyroxene, 

plagioclase, hornblende, biotite, quartz, 

iron-titanium-chromium oxides, and sulfide minerals. The 

sequences in the Basal norite appear to be irregularly 

distributed and most rocks seem to be more enriched in 

feldspar than is the Basal bronzite cumulate. In addition, 

grain size varies from coarse to fine in an irregular manner. 

Within a small area one finds rocks with all gradations of 

texture from magmatic sediment with apposition fabrics to 

current-formed fabrics to ophitic, diabasic, and gabbroic 

textures. Usually the most distinctive texture is one in which 

the pyroxenes show ragged subhedral margins against 

plagioclase. Making the distinction between the two 

members is relatively simple in the field, where exposures are 

large enough to apply the above criteria, but the distinction 

is very difficult to make when logging drill core because of 

the limited surface area and because the contact between the 

two members is in part gradational.

DISTRIBUTION AND THICKNESS 

OF THE BASAL ZONE

The entire Basal zone is found on the southern margin of 

the complex where there are about 3.6 km2 of exposures. This is 

about 9 percent of the area covered by exposures of the 

Stillwater Complex. Although exposed discontinuously 

along strike, the Basal zone is the lowermost, continuous, 

semitabular layer of the complex. Deep glacial valleys
DEFINITION OF THE BASAL ZONE

cutting the complex, valley fill, and complex faulting are 

responsible for the observed discontinuities in exposures. 

For example, exposures are either sparse or nonexistent 

along the trend of the Basal zone in the West Fishtail Creek, 

Stillwater River, West Fork of the Stillwater River, East 

Boulder River, and Boulder River valleys because of 

covering valley fill. The Basal zone is covered by younger 

rocks thrust northward (figs. 1 and 2) from the 

Initial-Cathedral creek to the Upper Forge Creek areas. 

Between Blakely Creek and the Gish area, the surface is 

covered with glacial moraine and talus, but outcrops of 

olivine cumulate and cordierite - orthopyroxene - biotite 

hornfels (Page and Nokleberg, 1975) suggest that the Basal 

zone may in part be absent or very thin in this area. Plate 1 

contains an interpretative geologic map which shows the 

distribution of well-known, inferred and approximate, and 

projected and hypothetical occurrences of the Basal zone of 

the Stillwater Complex. Well-known occurrences include 

those areas marked by abundant surface outcrops, surficial 

float, or detailed drilling where the boundaries of the Basal 

zone are well established, approximate and inferred areas 

include those marked by sparse outcrops, float, and drill 

data, and projected and hypothetical areas are enclosed by 

contacts established by geologic reasoning and are usually 

covered by extensive surficial debris.

Maximum known thicknesses of the Basal zone are on the 

order of 400 m, but most of the zone averages between 60 and 

240 m in thickness. Stratigraphic columns along the strike of 

the complex (pi. 1) show estimated thicknesses of the Basal 

zone at 48 localities. Figure 3 summarizes the estimated 

thickness as a histogram; an arithmetic average of 163.16 m 

was calculated for the set of estimates and agrees well with 

visual estimates from examination of the maps and cross 

sections.

In most of the sections of the Basal zone, 20 to 70 percent is 

composed of the Basal bronzite cumulate. The maximum 

known thickness of the Basal bronzite cumulate exceeds 260 

m and the minimum thickness is less than 18 m. The member 

probably averages about 80 m in thickness; however, the 

Basal bronzite cumulate is twice as thick in the west as in the 

east. On the average, the Basal bronzite cumulate accounts

-

-

m

g

I I I 

(35.3 Average value

I

3 200

I I L

-

IHl li!

400 600 800 1000 1200 1400

08-

STRATIGRAPHIC THICKNESS, IN FEET

FIGURE 3. Estimated stratigraphic thickness of Basal zone at 48 

localities.

for about 50 percent of the Basal zone. The Basal norite has a 

maximum known thickness of about 260 m and averages 

between 60 and 90 m thick.

Maximum topographic relief is between 1300 and 1400m, 

which affords a comparable depth of exposures down dip 

because of the steep tilt of the complex. The largest amounts 

of downdip exposure of the Basal zone are from Chrome 

Mountain to the Boulder River (1372 m) and from Benbow 

to Nye Lip (914 m).

SHAPE OF THE BASAL ZONE

The exposed part of the Basal zone has roughly the shape 

of a tilted, extremely thin, tabular prism about 42 km long, 

about 163 m thick, and with about 1370 m of downdip 

exposure. The eastern part of the Basal zone can be inferred 

to extend down dip below the surficial exposures for a lesser 

distance than the western part, on the basis of the distribution of the aeromagnetic high associated with the Basal and 

Ultramafic zones (U.S. Geol. Survey, 1971) and the 

aeromagnetic low associated with the Upper and Banded 

zones. A preliminary interpretation of the aeromagnetic 

data is that the eastern part of the Basal zone is cut out by 

faulting, intrusive quartz monzonite, or both at depth.

In detail the Basal zone does not, of course, have a 

prismatic shape (pi. 1) but varies in shape along its strike 

length because of changes in thickness. Thickest sections of 

the Basal zone are in the areas of Benbow, Mountain View, 

Crescent Creek, Iron Mountain, Chrome Mountain, and 

Blakely Creek, and west of the Boulder River. If the base of 

the Peridotite member is used as a reference horizon of 

constant elevation, these areas appear as depressions or 

basins in the country rock. If the base of the Bronzitite 

member is used as the reference horizon, the Peridotite 

member also forms several basinlike structures (Jackson, 

1963, p. 48); similarly, basinlike structures can be inferred 

from the columnar sections of Jones, Peoples, and Rowland 

(1960, p. 290). In the basinlike sections both the cyclic units 

and chromitite zones are thicker than on the adjoining shelf 

sections (Jackson, 1963), which suggests that more rapid 

crystal accumulation took place in the basin areas. Figure 4 

compares basins in the Peridotite member with basinlike 

structures in the Basal zone. Slope angles on the sides of the 

basins range from less than 2° to about 20°; most estimated 

slopes are less than 10°. In general, basinlike structures 

occur in the same areas for both rock units, suggesting that 

the structures were present during crystallization of the 

Basal zone and did not begin development at a later time, 

although they may have continued forming during the 

crystallization of the Peridotite member.

Where the most data are available, the basinlike structures 

in the Basal zone are found to be irregular in shape (pi. 1, fig. 

4). In the Benbow-Nye Lip area, the irregularities probably 

are caused in part by the intrusive quartz monzonites and in 

part by faulting. In the Chrome Mountain area and to the 

west, seeming irregularities may result from the wide spacing
STILLWATER COMPLEX, MONTANA

GISH

WEST

CHROME MTN IRON MTN WEST FORK INITIAL 

BASE OF BRONZITITE MEMBER^

MTN VIEW NYE LIP BENBOW

EAST

BASE OF PERIDOTITE MEMBER

METERS FEET 

300 -1-1000

ASE OF PERIDOTITE MEMBER

—-—XL—

BASE OF BASAL ZONE

3 4 MILES

J___|

I I I T 

01234 KILOMETERS

FIGURE 4. Basin structures in Basal zone compared with those in Peridotite member. Peridotite member thicknesses after Jackson (1963, 

p. 48). Dashed lines hypothetical or speculative; vertical scale exaggerated.

of the geologic information used to reconstruct the sections. 

Between the Mountain View and the Chrome Mountain 

areas, the shape of the Basal zone beneath the thrusts has 

been estimated from the few outcrops, drilling data, and the 

shapes of basins in the Peridotite member. Probably the 

thickest parts of this basin structure lie between the Crescent 

Creek and the Iron Mountain areas.

DESCRIPTION OF CONTACTS

Contacts between rock units discussed here are of two 

kinds; those between the Basal zone and younger or older 

rock units which are not part of the complex, and those that 

are internal to the Stillwater Complex, between its various 

units. External contacts include the intrusive contact of the 

Basal zone within the metasedimentary rocks (now pyroxene 

hornfels facies rocks) and those with later intrusive rocks. 

Both the hornfels and Basal zone have been intruded by a 

younger sequence of coarse-grained quartz monzonite and 

aplites (Page and Nokleberg, 1970a, 1972; Page, 1977) and 

all three rock units are intruded by younger mafic dikes. 

Internal contacts include the upper contact of the Basal zone 

with the Peridotite member of the Ultramafic zone and the 

lower contact of the Basal bronzite cumulate member with 

the Basal norite member.

CONTACT OF THE BASAL ZONE WITH 

METASEDIMENTARY ROCKS

The contact between the Basal zone and the metasedimentary rocks has been modified on the east by later intrusive 

quartz monzonites and in the central part of the Stillwater 

Complex its position is concealed by thrust faulting. Surface 

exposures of the contact occur in the Mountain View area,

the Initial area, discontinuously from Chrome Mountain to 

Blakely Creek, and in Falls Creek.

The appearance and characteristics of the rocks and the 

contact between them show variations. Locally, the contact 

may consist of (1) ophitic to subophitic noritic or gabbroic 

rocks in sharp, regular contact with the metasedimentary 

rocks, (2) the same rocks as above in contact but with dikes 

and tongues of gabbroic rock extending into the 

metasedimentary rocks, (3) orthopyroxene cumulates that 

collected on a floor of metasedimentary rocks, or (4) a zone 

of intrusive breccia composed of hornfelsed metasedimentary rock fragments, cumulates, ophitic to subophitic rocks 

and rocks rich in sulfides. Locally, there may be zones of 

hornfels inclusions above the contact or there may be zones 

of noritic dikes and sulfide pods well below the contact for 

example, in the Mountain View area.

Iron formation and blue metaquartzite are exposed near 

the contact from the Boulder River to a point west of 

Chrome Mountain, but from there to the upper part of 

Forge Creek these rock units diverge toward the south from 

the contact. Outcrops of iron formation and metaquartzite 

close to the Basal zone between the upper part of Forge 

Creek and Bluebird Peak owe their position to a thrust fault. 

From Bluebird Peak to the east end of the Basal zone iron 

formation and blue metaquartzite are sparse. Minor 

amounts of these rocks occur as inclusions in the quartz 

monzonite on the ridge between Nye and Flume Creeks. The 

overall relations of rock units below the complex to the Basal 

zone suggests that the Basal zone contact is not parallel to a 

stratigraphic horizon but cuts across stratigraphic units. The 

contact may mark either a fault that was nearly horizontal at 

the time of emplacement of the complex or an unconformity.
DEFINITION OF THE BASAL ZONE

CONTACT OF THE BASAL ZONE WITH 

THE ULTRAMAFIC ZONE

The contact of the Basal zone with the Ultramafic zone is 

located between the top of the Basal bronzite cumulate and 

the first cyclic unit of the Peridotite member of the 

Ultramafic zone. It is marked for 80 percent of the strike 

length by the disappearance of orthopyroxene and the 

appearance of olivine as cumulus minerals in the Peridotite 

member but may also be marked by orthopyroxene being 

joined by olivine as a cumulus crystal. The horizon is sharp 

and distinct and where observed is about one crystal thick. 

Over large areas the layering in the Peridotite member 

appears to be conformable with the layering in the Basal 

bronzite cumulate member, for example in the Benbow area 

(pl- 1).

However, the layering in the Peridotite member is not 

always conformable with the Basal bronzite cumulate 

member. In the Chrome Mountain area the lower olivine 

cumulate, bronzite cumulate, and olivine cumulate of the 

next cycle abut against the Basal bronzite cumulate. This 

relation can be explained as onlap on the margin of a basin, 

similar to onlap in sedimentary rocks. The only other 

example of this type of relation occurs within the Peridotite 

member on the west side of Iron Mountain where bronzite 

and olivine+bronzite cumulates abut against an olivine 

cumulate. Smaller scale examples of onlap have been 

observed by Jones, Peoples, and Howland (1960) in the 

chromitite zones and by W. J. Nokleberg (oral commun., 

1970) in the Banded zone. Other evidence for a basin margin 

is the dramatic thinning of the Basal zone from northeast to 

southwest in the Chrome Mountain area. On the basis of this 

evidence onlap should be expected in the basin structures, 

and its possibility should not be neglected in subsurface 

interpretations.

CONTACT OF THE BASAL ZONE WITH YOUNGER 

INTRUSIVE QUARTZ MONZONITIC ROCKS

Precambrian quartz monzonite, aplite, and hornblende 

quartz diorite intrude the Basal zone of the Stillwater 

Complex' from West Fishtail Creek to slightly west of the 

West Fork of the Stillwater River. Page (1977) and Page and 

Nokleberg (1972) discuss the intrusive, petrologic, and 

structural nature of this sequence of rocks, which is 2750 

m.y. old (Nunes and Tilton, 1971). Dikes of these rocks are 

found in the Basal zone, and inclusions of the Basal zone 

were found in the quartz monzonites. These relations 

demonstrate that the contacts between the Basal zone and 

these rock units are intrusive. From the Benbow area to the 

Stillwater River, the quartz monzonite apparently stoped 

away part of the Basal zone and modified its lower contact. 

Evidence for this suggestion is the discrepancy in thickness 

of the Basal zone across the Big 7 fault. To the east of the Big 

7 fault the Basal zone thickens toward the fault; immediately 

across the fault to the west the Basal zone is only half as 

thick.

CONTACT OF THE BASAL ZONE WITH MAFIC DIKES

Mafic dikes of younger Precambrian age also intrude the 

Stillwater Complex; most of these dikes have basaltic 

compositions and ophitic to subophitic textures (Page, 

1977). The mafic dikes are fairly easily distinguished from 

the Stillwater Complex by their textures and gross form 

except where they intrude the Basal norite member. Within 

the Basal norite,intrusive mafic dikes have chilled margins of 

very fine grained, diabasic-textured rocks or crosscutting 

relations with definable units of the Basal norite. Isotopic 

studies also aid recognition of the dike rocks. Because faults 

tend to follow the mafic dikes, the dike rocks can be highly 

sheared, broken, and altered, a secondary criterion for their 

recognition. Sills or dikes subparallel to the Basal norite unit 

would be extremely difficult to recognize. Mafic dikes have 

been observed in surficial exposures in the Benbow, Initial, 

Chrome Mountain, and Boulder River areas. Where there 

has been extensive drilling, such as the Iron Mountain area, 

dikes have been recognized in the Basal zone in the 

subsurface.

CONTACT OF THE BASAL NORITE WITH 

THE BASAL BRONZITE CUMULATE

The contact between the Basal norite and the Basal 

bronzite cumulate is locally gradational and generally 

marked by a transition from noncumulate, subophitic-, 

ophitic-, diabasic-, and gabbroic-textured rocks of the Basal 

norite to cumulate rocks of the Basal bronzite cumulate. The 

Basal norite generally contains more plagioclase than the 

Basal bronzite cumulate. Locally this contact has been 

placed between apposition fabric cumulates of the Basal 

bronzite member and cumulates with current structures in 

the Basal norite member. Other contacts between lenses and 

layers in the Basal zone are phase contacts, ratio contacts, 

and form contacts. (See Jackson, 1967, for terminology.)

AGE RESTRICTIONS ON THE BASAL ZONE

The Basal zone of the Stillwater Complex is at least 2750 

m.y. old and may be more than 3140 m.y. old (Page and 

Nokleberg, 1972; Page 1977). This conclusion is based on 

uranium-lead study of zircons from the intrusive quartz 

monzonites and from the metasedimentary rocks (Nunes 

and Tilton, 1971).

PETROLOGY AND MINERALOGY 

OF THE BASAL ZONE

ROCK TYPES IN THE BASAL ZONE

In the field, various igneous rocks, concentrations of 

sulfide minerals, hornfelsed metasedimentary rocks, and 

complex breccias were mapped as part of the Basal zone (fig. 

5). The last two rock types are not strictly part of the 

Stillwater Complex but were included in the Basal zone 

where their dimensions are too small to separate at the
STILLWATER COMPLEX, MONTANA

mapping scale and because they are closely related to the 

processes that formed the Basal zone. Table 2 shows 

schematically the interrelations of the six major rock types 

included in the Basal zone. The table shows that contributions from metasedimentary rocks and silicate-sulfide 

magma that formed the Basal zone are clearly interrelated 

and generated two groups of rock with mixed origins the 

partial melts and contaminated rocks.

The Basal zone has been divided into four major textural 

types of igneous rocks: Cumulate rocks, noncumulate 

igneous-textured rocks, contaminated rocks, and rocks 

possibly derived by partial melting. In addition to the four 

major types of igneous rocks there are small areas of 

authornfelsed noncumulate rocks. This division of the Basal 

zone emphasizes the different processes involved in the 

formation of the igneous rocks of the Basal zone. Cumulate 

rocks owe their formation to various sedimentary processes 

in the magma chamber whereas the noncumulate 

igneous-textured rocks were derived by more typical 

crystallization processes. Contaminated rocks formed as a 

result of reaction between the magma and the metasedimentary basement rocks and appear to be distinct from those 

igneous rocks derived by partial melting of the metasedimentary rocks. Autohornfelsed noncumulate rocks are igneous 

rocks that were apparently metamorphosed after solidification by the magma from which they crystallized. The Basal 

norite contains all five rock types, whereas the Basal bronzite 

cumulate consists only of cumulates.

Sulfides are comagmatic with the silicate minerals in the 

Basal zone, and sulfide-bearing rocks are of three types.

Massive sulfide rocks contain more than 60 percent 

pyrrhotite, pentlandite, and chalcopyrite; the balance of the 

rock is plagioclase or pyroxene phenocrysts and hornfels 

inclusions. Sulfides that occur as isolated blebs, aggregates, 

or grains throughout rocks are classed as disseminated 

sulfide. Between these two extremes is matrix sulfide rock in 

which the sulfide material forms a connected and continuous 

network between silicate grains; sulfide content ranges from 

about 10 to 60 percent.

Hornfelsed metasedimentary rocks compose part of the 

Basal zone as inclusions and rafts too small to map. The 

various assemblages in the hornfels have been discussed in 

detail by Page (1977) and Page and Koski(1973). The most 

common assemblage in the metasedimentary rocks is 

cordierite - orthopyroxene - quartz - plagioclase - biotite. A 

commonly occurring material which locally marks the 

contact between the Basal zone and country rocks is a 

breccia consisting of angular hornfels fragments in a matrix 

of igneous rocks and sulfides.

The number of rocks in any one locality or stratigraphic 

section is large and their relations complex. A general idea of 

the abundance, variety, and distribution of the rocks may be 

seen in the individual sections shown in figure 5. Thinner 

cumulate layers have been exaggerated slightly in apparent 

thickness to illustrate the variety of rocks. In general, there is 

a repetitive sequence of noncumulate and cumulate igneous 

rocks associated with metasedimentary inclusions or rafts. 

The sequence is increasingly dominated by cumulates 

upward in the sections. No intrusive relations between the 

repetitions of noncumulates and cumulates have been

TABLE 2.—Rocks in the Basal zone of the Stillwater Complex, Montana

Silicatesulfide 

magma

Metasedimentary 

rocks.

Cumulates .....................

Noncumulates ....... ....

Sulfide rocks ..................

Contaminants ................ 

Partial melts ..................

Contact 

metamorphic 

rocks.

I Pla 

Orthopyroxene cumulates \ yri 

Inverted pigeonite cumulates f 

Plagioclase cumulates 

Clinopyroxene cumulates 

Chromite cumulates 

Orthopyroxene+olivine cumulates 

Orthopyroxene+inverted pigeonite cumulat 

Orthopyroxene+plagioc la se cumulates 

Plagioclase+olivine cumulates 

Plagioclase+inverted pigeonite cumulates 

Clinopyroxene+olivine cumulates

............................................................

i Massive sulfide 

/ Matrix sulfide 

| Disseminated sulfide

I Cordierite replaced by plagioclase 

| Rocks crystallized from contaminated ma 

Orthopyroxene (unexsolved)+plagioclase+c 

minerals 

1 Orthopyroxene-cordierite-biotite-quartz-p 

< Anthophyllite-cordierite-biotite-quartz-pla 

1 Biotite-cordierite-quartz-plagioclase hornf

nar 

ear

es

Ophitic textured

Subophitic textured 

Ragged textured

gma 

quartz+biotite+opaque

agioclase hornfels 

gioclase hornfels 

els

With or 

without 

inverted 

pigeonite, 

phenocrysts 

or olivine 

or plagioclase or 

quartz
CUMULATE ROCKS OF THE BASAL ZONE

observed, and none of the repetitions can be readily 

correlated from area to area. This suggests that repeating 

units have tabular lens shapes with strike dimensions of less 

than 1 km.

MOUNTAIN VIEW

fl-srv*—O-;-.T —'

_4-»_ S. 2-lI'i ——

Inverted . 

pigeonite

ie

co E <0

DQ 2

Plagioclase 

phenocrysts"

Olivine 

phenocrysts^

Inverted _ 

pigeonite

Olivine - phenocrysts 

Inverted ^ 

pigeonite 

Plagioclase/ 

phenocrysts

Inverted 

pigeoniteQuartzCUMULATE ROCKS OF THE BASAL ZONE

Cumulate rocks consist of (1) crystals and other elements 

such as sulfide-oxide liquids that accumulated by settling 

either under the influence of gravity alone or of currents and

UPPER NYE 

BASIN

BENBOW EAST 

GOLF COURSE

Inverted 

pigeonite \ ":,

CO OC <oDQ Z

OT METERS FEET 

W 0-r- 0

15 50

100

EXPLANATION

Olivine and chromite cumulates (Ultramafic zone)

Orthopyroxene cumulate

Inverted pigeonite cumulate

Plagioclase cumulate

Orthopyroxene-plagioclase cumulate

Inverted pigeonite-plagioclase cumulate

Clinopyroxene-olivine cumulate

Orthopyroxene-olivine cumulate

Noncumulate mafic rocks

Intrusive breccia, metasediments

Metasediments

Intrusive quartz monzonite sequence

FIGURK 5. Sections showing rocks and mineralogy in Basal zone, based on detailed logging and extensive thin section examination ot core. 

Horizontal bars on the right of each section indicate locations of thin sections.
TABLE 3. Generalized petrographic descriptions of cumulate rock types in the Basal zone

[From thin sections, polished sections, and etched rock slabs. CPX. clinopyroxene; OPX, orthopyroxene; PX. pyroxene; PIG, pigeonite; OL, olivine; PL, plagioclase; QZ, quartz; KS

hornblende; BIO, biotite; OPAQ, spinels and sulfides; EN, MgxIOO/(Mg+Fe+Ca) by X-ray; AN, CaxlOO/(Ca+Na+K) by X-ray; FO, MgxlOO/(Mg+Fe+Ca) by 

Rock name: Orthopyroxene cumulate

Constituent 

minerals

Orthopyroxene.. 

Clinopyroxene...

Brown 

hornblende.

Biotite...............

Spinels ^

Sulfide \ 

minerals. /

Volume percent Size range

Range

. 44.9-81.8 

0-25.9

. 7.4-40.7

0-3.6 

. 0-1.2

0-6.0

0-21.6 

0-0.2

0-0.7

Average

61.9 <l-35 Well to 

moderate

10.4 <1-30 ..................

239 <1-20

.29 <l-4 ..................

.35 <l-2 ..................

.32 <l-2 ..................

" 1 ^ .............

Tr <1 ..................

.01 <1 ..................

Crystallinity

Euhedralsubhedral.

Anhedralsubhedral.

do.........

do.........

do.........

Anhedral...... 

Subhedraleuhedral.

Euhedral ...... 

Subhedral.

Number of sections examined: 271 Number of mo

Internal features of crystals

Exsolution, twinning Zoning

Cumulus, equidimensional, CPX blebs and None to 

broad, narrow prismatic. lamellae || (100). slight.

Postcumulus, poikilitic, OPX lamellae || (IOO);Present.... 

interstitial, local twinned, 

granulation. 

Postcumulus; pokilitic. Twinned................. Common.

tabular mosaics; 

interstitial. 

Postcumulus, interstitial, 

locally rims PX, intergrown with PX. 

Postcumulus, interstitial, 

locally poikilitic. 

Postcumulus, interstitial.

Fill pores, inclusions in 

OPX.

Inclusions in BIO.

Compo

ran

EN =3

ave

463

Spin

ch

m

m

ilm

gr

Pyrr

pe

ch

Reactions inferred and observation:

1. Orthopyroxene-*clinopyroxene; rounded OPX in CPX.

2. Orthopyroxene-+quartz; irregular OPX locally against 

QZ.

3. Orthopyroxene+magma -> orthopyroxene with overgrowth.

4. Orthopyroxene-»brown hornblende, irregular OPX 

against HBN.

5. Clinopyroxene -»brown hornblende, irregular CPX 

against HBN.

Special features or variants:

1. 0.7 percent olivine, armored 

xenocrysts in one sample.

2. Locally shows bimodal distribu

3. Spinel (dark opaque) euhedral in

4. Local green-brown isotropic (s

parallel to (100) plane of CPX 

Crystallization sequence: OPX, CPX and PL, HBN, BIO, QZ, OPAQ.
CUMULATE ROCKS OF THE BASAL ZONE 11

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TABLE 3. Generalized petrographic descriptions of cumulate rock types in the Basal zone—Continued

Rock name: Orthopyroxene-olivine cumulate Number of sections examined: 7

Constituent Volume percent Si/e range

, (mm) Sorting Crystal Unity 

minerals Range Average

Orthopyroxene... 29.9-50.0 38.2 2-4 Well- Euhedralmoderate. subhedral.

Clinopyroxene 5.0-20.3 14.0 1-1.5 Anhedral 

Olivine............... 3.0-25.5 18.0 .2-1.5 Well- Subhedralmoderate euhedral. 

Plagioclase. ........ 16.7-32.0 22.8 2-5 .................. Anhedralsubhedral. 

Brown 0-5.3 1.5 >.5 .................. do.........

hornblende 

Biotite... ............. 0-1.9 .52 .2-1.0 .................. do. ........

Spinels \ ( >.1-1.0 .................. Euhedral ......

> 0-13.9 6.8 < 

Sulfide 1 j >. 1-2.0 .................. Subhedralminerals. i f anhedral.

Reactions inferred and observation: 

1. Olivine resorbed by magma-anhedral branching grains. 

2. Olivine + magma ->orthopyroxene-subhedral, rounded OL 

against OPX. 

3. Orthopyroxene+magma -»clinopyroxene-subhedral, 

rounded in OPX, 

in CPX.

Crystallization sequence: OL and spinels,

Shape and texture

Cumulus, broad, stubby 

prisms.

Postcumulus, poikilitic 

rims OL, interstitial.

Cumulus, locally anhedral 

branching crystals. 

Postcumulus, poikilitic, 

interstitial, granular 

mosaic. 

Postcumulus, locally 

rims, biotite, rims 

spinels. 

Postcumulus, rims 

opaques, interstitial. 

Interstitial and 

inclusions.

do.............................

OPX, PL and CPX, BIO,

Rock name: Inverted pigeonite cumulate Number of sections examined: 2

Constituent Volume percent Size range

minerals _ A , . Sortin« Crystal Imity 

Range Average (mm)

pigeonite anhedral 

(orthopyroxene). 

Clinopyroxene.... 10.8 .......... ............. ..................... Anhedral

Brown hornblende .9 ............................................ do .........

Biotite................ .1 ............................................ do.........

Spinels and 7.3 

sulfides.

Crystallization sequence: PIG,

Shape and texture

Postcumulus. 

do ............................

do.

do.

CPX, PL, HBN, BIO.

Internal features of crystals Compo

Exsolution, twinning Zoning rang

CPX lamellae and HN=74.3

blebs || (100).

OPX lamellae II (100) 

opaque blades;

twinned. ..,. , ., _ 

1-0=67.7

AN=64.7

Chromitc

........................................................ I'vrrhoti

pentla

chalco

Variants: 

1. Locally olivine occurs in c

CPX. 

2. Olivine grains have reentr

quartz.

HBN.

IntcrnaHcatureso. crystals fompo

Exsolution. twinning /oning ran

... Coarse, discontinuous l :.N=6

lamellae, blebs; fine 

lamellae || (100).

.........................................Common.... AN =7
CUMULATE ROCKS OF THE BASAL ZONE 13

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TABLE 3. Generalized petrographic descriptions of cumulate rock types in the Basal zone—Continued

Rock name: Inverted pigeonite cumulate

Constituent Volume percent

minerals . Range Averaj

pigeonite 

(orthopyroxene).

Quartz................ .3

Brown .1 .....

hornblende. 

Biotite. .............. .3

sulfides.

Size range

ge

original. 

2.0-6.9 

overgrowth. 

.....>1.0 .....................

..... 2.4-5.0 Moderate.....

..... .2:5 .....................

.....>.25 .....................

..... .12-.6 .....................

..... 2.0-5.0 .....................

Number of sections examined: I

Crystal Unity

Subhedral. .......

do. ...........

do............

do............

Reactions inferred: After pigeonite accumulated and inverted had 

overgrowth of orthopyroxene to produce some grains with 

poikilitic texture.

Crystallization sequence: PIG and PL,

Rock name: Plagioclase-olivine cumul:

Constituent Volume percent

minerals Range Averaj

(inverted 

pigeonite). 

Clinopyroxene.... 18.4

hornblende.

sulfides.

ite

Size range

;e (mm)

...... 1-2 ....................

...... 1-2 ....................

...... .5-1.5 Well............

...... 1-1.5 Wellmoderate. 

...... >.5 ....................

...... >.2 ....................

Internal features of crystals r-^m r,,,,

Exsolution, twinning Zoning ^

Cumulus, locally shows Broad, discontinuous EN=60

poikilitic texture. lamellae and blebs.

Postcumulus, OPX lamellae || (100). 

interstitial. 

Cumulus, tabular, Twinned, 

stubby prisms.

do.

do. 

Pore filling........... .................................................................... Pyrrhot

pentla

chalc

Special features: Cordierite xenocryst, one grai

OPX, CPX, HBN, BIO, QZ.

Number of sections examined: 1

Crystallmity

do ...........

euhedral. 

Euhedralsubhedral.

do...........

... . Internal features of crystals Com

Fxsolution. twinning Zoning

Postcumulus, irregular, CPX lamellae || (100) 

interstitial. discontinuous, hN-70

irregular blebs. 

Postcumulus, poikilitic, OPX lamellae || (100). 

interstitial.

Postcumulus, interstitial, 

rims on OPAQ. 

Inclusions and Oxides 

interstitial. chrom

Special features:About one-tenth of OPX is inverted pigeonite. 

Crystallization sequence: OL, PL, OPX, CPX, HBN.
CUMULATE ROCKS OF THE BASAL ZONE 15

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16 STILLWATER COMPLEX, MONTANA

gravity and (2) postcumulus material that crystallized later 

in the pores of the rocks. Cumulate terminology used in this 

report is that of Jackson (1967), who developed it for and 

applied it to other parts of the Stillwater Complex. Other 

cumulate terminologies exist, and cumulate processes in 

other complexes are well described (Wager and Brown, 

1967). Modal or chemical compositions of cumulate rocks 

reflect the combined processes of accumulation and postcumulus deposition and are not equal to the composition of 

the magma from which they formed.

In the Basal zone, orthopyroxene, inverted pigeonite, 

clinopyroxene, plagioclase, olivine, spinel (chromite, 

magnetite), ilmenite, and sulfide phases are the cumulus 

minerals. For a particular rock, any one or as many as three 

of these minerals can be cumulus phases and the other 

minerals, postcumulus phases. Other important postcumulus minerals are brown hornblende, biotite, and 

quartz. As shown in table 3 and figure 5, various cumulates 

occur in the Basal zone. The Basal bronzite cumulate 

member consists of dominantly orthopyroxene cumulates, 

but locally thin lenses or layers of chromite and inverted 

pigeonite cumulates occur.

Table 3 summarizes the petrographic and mineralogic 

descriptions of the cumulate rocks observed in the Basal 

zone. Most of the terminology in the table is that commonly 

used in describing cumulate rocks. The material referred to 

as inverted pigeonite is now orthopyroxene and commonly 

has two sets of exsolution lamellae: (1) Thin lamellae 

(assumed to be clinopyroxene) parallel to the (100) plane of 

orthopyroxene, and (2) coarse discontinuous blebs and 

lamellae oriented on irrational planes in the orthopyroxene. 

The latter set is indicative of primary crystallization of 

pigeonite which, on cooling, exsolved augite and inverted to 

orthopyroxene (Poldervaart and Hess, 1951; Brown, 1957; 

von Gruenewaldt, 1970), which later exsolved clinopyroxene 

parallel to the (100) plane. It is important to distinguish this 

type of orthopyroxene which crystallized from the magma as 

pigeonite from the more abundant orthopyroxene which 

crystallized as orthopyroxene. Sorting is used to describe the 

spread in cumulate grain size in a particular rock (Jackson, 

1961a, p. 28). Compositions of mineral phases were determined by X-ray techniques (Jackson, 1960, 1961b;Hotzand 

Jackson, 1963; Himmelberg and Jackson, 1967).

From figure 5 and table 3 it is apparent that orthopyroxene cumulates are most abundant and that plagioclase and 

plagioclase+orthopyroxene cumulates are next most abundant. The eight other cumulate types form only a small part 

of the rock sequence. Irvine's (1970a) listing of theoretical 

cumulate mineral combinations suggests that other unrecognized combinations of cumulate minerals may be 

present in the Basal zone.

The compositions of orthopyroxene and plagioclase in 

Basal zone rocks are summarized as histograms in figure 6. 

The average composition of orthopyroxene is En&8.9, and the 

plagioclase is An77 8 - Only five olivine compositions were

30

20

10 

n

Averag 

/V=130 

~~ Plagioclase

-

F^iH te: mfe-J"!1

":;•;"::;:::] 

psj ——

8=77.8

,^

IlL: :?.;:',;:•;";•:;:-.-:

l!S>i& !i^: '!fK^ 

";p.;;;ffsi.M.M!:r !!

1

-

m\

m-S : -Ssj •S"i

'52 60 68 76 84 92 

An PERCENTAGE

110

100

90

80

70

60

50

40

30

20

10

1 I I r 

Average=68.9 

/V=529 I

Orthopyroxene

II

30 38 46 54 62 70 78 86 94 

En PERCENTAGE

Kica Ri-6. Orthopyroxene and plagioclase compositions. N, total 

number of samples.

obtained from the Basal zone with an average of Fo 70. 7 and a 

narrow range of Fo6?. 7 to Fo?3.i.

CUMULATE SEQUENCES

The relations of some cumulate rocks are shown in figure 5 

for three sections of the Basal zone; plate 1 shows additional 

sections but in much less detail. In most layered complexes 

(Wager and Brown, 1967; Irvine, 1970a) and in the 

Ultramafic zone of the Stillwater Complex (Jackson, 1970), 

there are repetitive sequences of cumulate minerals and 

distinctive orders of appearance of cumulate rocks which are 

thought to be diagnostic of magma composition and 

conditions of crystallization in those complexes. All 

theoretical cumulate sequences (Irvine, 1970a, p. 468-472) 

begin with a one-phase cumulate rock which is followed by a 

two-phase cumulate rock. The theoretical sequence ends 

with a three- or four-phase cumulate. Such a sequence forms 

a cyclic unit. In the Slillwater Complex, an example of a
CUMULATE SEQUENCES 17

normal complete cyclic unit is the sequence of olivine, 

olivine+bronzite, and bronzite cumulates of the Peridotite 

member (Jackson, 1961a, 1963). Jackson (1970, p. 390-401) 

called cyclic units which lacked one or more members at the 

top of the sequences beheaded. Within this framework there 

are no recognizable complete cyclic units in the Basal zone, 

only parts of units. The partial sequences that are most 

important are (1) orthopy roxene, 

orthopyroxene + plagioclase, (2) olivine + orthopyroxene, 

orthopyroxene and, (3) plagioclase, 

plagioclase+orthopyroxene.

As pointed out by Irvine (1970a, p. 463), such short partial 

sequences of cumulates can represent several crystallization 

orders. In the Stillwater Complex several other pieces of 

evidence limit the choices of crystallization orders. First, the 

presence of olivine and plagioclase phenocrysts in the 

noncumulate igneous rocks indicates that olivine and 

plagioclase were the first minerals to crystallize from the 

magma, even though no olivine cumulates have been 

positively recognized in the Basal zone. Therefore, the 

crystallization order and cumulate sequence in the Basal 

zone must be such that either olivine or plagioclase form the 

first cumulates. Secondly, the order and sequence probably 

should be consistent with the order and sequence in the 

stratigraphically higher zones of the complex. Hess (1960), 

Jackson (196la), and Page (1977) have established the 

crystallization order above the Basal zone as olivine, 

orthopyroxene, plagioclase, clinopyroxene, which produces 

the cumulate sequence olivine, olivine+bronzite, bronzite, 

bronzite + plagioclase, and bronzite + plagioclase + 

clinopyroxene. Two of the beheaded cumulate sequences in 

Basal zone orthopyroxene, orthopyroxene+plagioclase 

and olivine+bronzite, bronzite fall within this general 

scheme and probably account for more than three-fourths of 

the cumulates in the Basal zone.

The sequence plagioclase, plagioclase+orthopyroxene 

seems to represent another crystallization order and 

cumulate sequence. The probable order is plagioclase, 

orthopyroxene, clinopyroxene, and the sequence is 

plagioclase, plagioclase + orthopyroxene, 

plagioclase + clinopyroxene + orthopyroxene. Further 

evidence of this sequence will be presented in later sections of 

the report.

During study of the Basal zone, specific features were 

selected for study in anticipation that they would offer 

means of stratigraphic control. Selection was based on the 

assumption that the feature was controlled by a process 

operating over the whole magma chamber at, or nearly at, 

the same time. Size variation of cumulus phases, possible 

current structures, modal mineral variations, orthopyroxene 

compositional variations, and plagioclase compositions 

were examined. None of these features may be used for 

detailed stratigraphic correlation over distances of 

kilometers either by themselves or in combination with one

another. However, over shorter distances they appear to be 

of some value in correlating stratigraphic horizons.

SIZE VARIATION OF ORTHOPYROXENE CUMULATES

In the Ultramafic zone, Jackson (I96Ia, 1970) 

demonstrated the importance of the variation in grain size of 

cumulus minerals in establishing cyclic units, and Page, 

Shimek, and Huffman (1972) showed that size-graded units 

within individual cumulate layers correlate with other 

physical and chemical parameters of the layer. Therefore, 

grain size was considered an important parameter early in 

the study, and two different approaches were used to record 

its variation in the Basal zone.

During core logging, visual estimates of average grain size 

were classified as fine-grained (less than 1 mm), 

medium-grained (1-5 mm), and coarse-grained (greater than 

5 mm). Locally, terms such as "finer medium" and "coarser 

medium" were used to describe gradational changes in 

size-graded units, and where grain size varied over short 

distances (less than a foot) and apparently in a random 

manner, the grain size was recorded as randomly variable. 

Estimates of grain size were based mainly on orthopyroxene 

size, hence grain-size estimates were infrequent for the 

limited sections of other types of cumulates. Figure 7 

summarizes some of these grain-size data for stratigraphic 

sections representative of the total strike length of the Basal 

zone.

One of the more striking aspects of the plots (fig. 7) is the 

repetition of the ascending sequence fine-, medium-, and 

coarse-grained cumulates, regardless of the stratigraphic 

section examined. The sequence is the same as that recorded 

for the cyclic units of the lower part of the Peridotite 

member (Jackson, 1961a, p. 22-23) and is similar to the 

repetitions found in a single layer of olivine cumulate (Page, 

Shimek, and Huffman, 1972). Attempts to correlate 

grain-size sequences laterally over long distances were not 

successful. Over short distances correlations have met with 

only limited success and suggest that lateral continuity for 

grain-size units is less than 1 km.

The second method of examining grain size was more 

quantitative and was restricted to a selected segment of the 

Basal zone in which no cumulate, composition, or grain-size 

units appeared to form repetitive cycles. The quantified data 

allow a check on the semiquantitative information and allow 

comparisons to be made with previously published 

grain-size studies in the Stillwater Complex (Jackson, 1961a; 

Page, Shimek, and Huffman, 1972). Length and width 

measurements of orthopyroxene were made in thin section 

for six samples spaced over about 56 m of core in the Basal 

bronzite cumulate from the upper Nye Basin section (fig. 7). 

The technique and limitations of this method were discussed 

by Jackson (1961a, p. 21).

The trend from finer to coarser grains expressed as 

arithmetic means, is apparent (fig. 7) and compares well with 

the qualitative data on grain-size changes made during core
18 SIILLWATER COMPLEX, MONTANA 

100

1.00 1.20 1.40 1.60 1.80 

GRAIN DIMENSION, IN MILLIMETERS

2.00

FIGURE 7. Mean length, width, and average grain size of 

orthopyroxene versus stratigraphic position for a segment of 

Basal bron/ite cumulate, upper Nye Basin.

logging. The range of average grain size, 1.22-1.63 mm, is 

mostly within the range of 1.3-2.3 mm reported by Jackson 

(196la, p. 26) for orthopyroxene in the Ultramafic zone but 

is representative of the fact that slightly finer grain sizes are 

observed in the Basal zone. The width to length ratio of 

orthopyroxene remains fairly constant (0.64 to 0.76) 

throughout the section.

Size-frequency distributions of either lengths or widths of 

orthopyroxene for each sample are log-normal and similar 

to size-frequency distributions in other parts of the complex. 

All of the measurements for each sample were combined to 

examine the size-frequency distribution for this segment of 

the Basal zone. Because lengths, widths, and mean diameters 

of the orthopyroxene crystals vary in proportion, only

Sorting coefficients.35 

Median=1.57mflrr 

Mean=1.69 mm

2 1 

LENGTH, IN MILLIMETERS

FiGi'Ri- 8. Length-frequency distributions of orthopyroxene crystals for a 

segment of Basal bronzite cumulate, upper Nye Creek Basin.

lengths were used. Figure 8 illustrates the length-distribution 

both as a histogram and a cumulative frequency curve. The 

orthopyroxene crystals are remarkably well sorted and have 

a low sorting coefficient. This is similar to the sorting of 

olivine, bronzite, chromite, and sulfide grains in the Ultramafic zone (Jackson, 1961a; Page, 1971b; Page, Shimek, 

and Huffman, 1972).

EVIDENCE FOR CURRENT STRUCTURES

Current structures are apparently rare, or at least not easily 

detectable in the Ultramafic zone; Jackson (I961a, p. 29) 

discusses one example of scour and local unconformity in 

olivine and olivine-chromite cumulates and implies that the 

smaller chromite grains were winnowed out in that example. 

Where evidence for magmatic currents is well demonstrated, 

strongly lineated fabrics of the elongate minerals are found 

(for example, the Skaergaard Intrusion, Wager and Brown, 

1967). In the petrofabric studies (Hambleton, 1947; Jackson, 

196la) and fieldwork in the Ultramafic zone lineated rocks 

appear to be rare. The apparent rarity of current structures 

in the Ultramafic zone may in part be due to the poor color 

contrasts of the minerals that compose most of the zone. In 

order to overcome this difficulty, more than 1200 samples of 

core and surface specimens were prepared with flat ground 

surfaces and etched with hydrofluoric acid. Etching attacks 

the plagioclase, giving it a chalky white color that may be 

preserved by spraying the specimen with plastic. Etching 

provides a background that highlights grain-size distributions, shapes, and packing of the orthopyroxene.

Even with this technique no structures that could be 

ascribed to magmatic currents were observed in the Basal
CUMULATE SEQUENCES 19

bronzite cumulate member, although such structures were 

found in the Basal norite member. Most discernible structures consist of scours or cuts and local unconformities; 

many are similar to the ones shown in figure 9. Typically 

these structures are found at the contacts between 

orthopyroxene cumulates and either plagioclase or 

plagioclase+orthopyroxene cumulates. Some show what 

appear to be truncated layers with up to 35° of apparent 

angular discordance (fig. 9A). Others display small basins, 

furrows, and hills in the underlying rock that are filled in by 

the immediately overlying cumulate without the shape being 

reflected in the next cumulate layer (fig. 9B).

In table 2, orthopyroxene cumulates were subdivided into 

two groups, planar and linear. The terms refer to the 

apparent orientation of orthopyroxene when the 

orthopyroxene crystals have a shape other than equidimensional. In planar cumulates, the long dimensions of individual orthopyroxene crystals tend to lie randomly in 

planes parallel to ratio, phase, and form layering. Jackson 

(1967) termed this planar lamination. This fabric has been 

explained (Jackson, 196 la) as due to settling of crystals in a 

magma without currents. In linear cumulates, the long axes 

of orthopyroxene crystals also lie in planes parallel to 

layering but in addition show a linear orientation in that 

plane. Petrofabric studies of the degree of preferred orientation have not yet been made. Linear fabrics are interpreted to 

be the result of magmatic currents.

MODAL MINERAL VARIATION IN Cl Ml'LATES

Variations in the proportion of mineral constituents of the 

cumulates in the Basal zone are given in table 3 as ranges and 

averages. In figure 10, modes of individual samples are 

plotted in terms of clinopyroxene, orthopyroxene, and 

plagioclase. This projection was chosen because the bulk 

modal composition for most samples could be adequately 

represented even though some rocks contain olivine. 

Cumulus and postcumulus minerals are combined in the 

diagram, but the cumulus minerals are indicated in the rock 

name adjacent to the appropriate symbol. The orthopyroxene and plagioclase cumulates cluster in distinctive 

groupings near their respective corners of the diagram (fig. 

10), and the plagioclase+orthopyroxene cumulates fall 

between the single-phase cumulates in a somewhat restricted 

area. Examination of other two-phase cumulates in the 

Stillwater Complex and Great Dyke (Jackson, 1970, p. 394, 

409) suggests that percentages of each cumulate mineral in a 

two-phase cumulate vary from 10 to 45 percent, a fairly 

restricted range.

Mineralogical differences between the Basal and Ultramafic zone cumulates are shown by comparison of the 

averages in table 3 and an average of 132 modes of 

Ultramafic zone rocks (Jackson, 1961a, p. 5). The Basal zone 

contains (1) less olivine and correspondingly more 

orthopyroxene, (2) more plagioclase, generally more than 

the 7.8 percent average for rocks from the Ultramafic zone,

i'Ri- 9. Scour and local unconformities in Basal 

norite. A, Etched sample from Upper Nye Basin. 

B, Etched sample from Benbow area.
20

Clinopyroxene

STILLWATER COMPLEX, MONTANA

50

Plagioclase Orthopyroxene

EXPLANATION

CUMULATE ROCKS

Orthopyroxene cumulates

Plagioclase cumulates

Plagioclase + inverted pigeonite cumulates

Plagioclase + Orthopyroxene cumulates

Plagioclase + olivine cumulates

Clinopyroxene + olivine cumulates

Orthopyroxene + olivine cumulates

FIGURE 10. Modes of cumulates from Basal zone projected on the 

triangular diagram Clinopyroxene, Orthopyroxene, and plagioclase in 

volume percent.

and (3) more Clinopyroxene in all cumulates except for the 

plagioclase cumulates. Brown hornblende, biotite, and 

quartz also appear to be more abundant in the Basal zone 

than in the Ultramafic zone. The relative abundance of 

plagioclase and Clinopyroxene in the Orthopyroxene 

cumulates (fig. 1 1) reflects the tendency for the proportion of 

postcumulus materials to be greater in the Basal zone than in 

the Ultramafic zone. This relation implies that the cumulus 

minerals are less tightly packed and have a smaller amount 

of secondary overgrowth on the average than rocks from 

higher in the complex. On the basis of four samples, Jackson 

(1961a, p. 61) argued that secondary enlargement or 

overgrowth was considerably less in the Basal zone than in 

the Ultramafic zone.

A segment of core from the Benbow West Golf Course 

section was selected for more detailed examination of modal 

variation in the Basal zone. It was chosen because the 

segment contains two breaks in Orthopyroxene compositions and several repetitions of grain-size units. Modes 

based on at least 1000 points were determined on 41 thin 

sections representing samples between core depths 62 and 

280 m. The volume percentage of Orthopyroxene is plotted 

against stratigraphic position for this segment in figure 1 1 .

VOLUME PERCENTAGE 

OF ORTHOPYROXENE

FIGURE 11. Volume percentage of Orthopyroxene in Basal zone plotted 

against stratigraphic position, Benbow West Golf Course section. 

Dotted lines represent Orthopyroxene compositional units.
CUMULATE SEQUENCES 21

The diagram shows that, on the average, the volume 

percentage of orthopyroxene increases from the bottom to 

the top of the section. Within the section there are two or 

three sharp maxima in orthopyroxene content. If the 

maxima reflect breaks in the accumulation process and are 

assumed to represent either the bottom or top of a unit, then 

there are several units in the section marked by a fairly 

regular increase in the volume of orthopyroxene. These 

repeated units could be correlated neither with repeated 

units defined by grain size nor with those defined by 

orthopyroxene compositions. As orthopyroxene is the only 

cumulate mineral, the volume of orthopyroxene in the 

section could reflect relative accumulation rates or varying 

rates of solidification of the postcumulus material.

LATERAL AM) VERTICAL VARIATION OF 

ORTHOPYROXENE COMPOSITION

Orthopyroxene is the dominant and most widespread 

mineral in the Basal zone, and because it usually is a cumulus 

mineral, it has the potential of offering the most information 

regarding the behavior of the parent magma. Himmelberg 

and Jackson (1967) developed an X-ray method for determining the ionic percent of magnesium in orthopyroxene 

based on Stillwater samples. This method yields the value for 

MgXIOO/total octahedral cations in orthopyroxene, which 

will be referred to in this report as the En (enstatite) content 

of orthopyroxene. Determinations of the En content of 

orthopyroxene have been made on 529 core and surface 

specimens from the Basal zones. Fifteen fairly complete 

stratigraphic sections were selected from this data and are 

shown on plate 2. Determinations in other sections are not 

reported because they represented short, incomplete 

segments of the Basal zone. The precision of these 

measurements ranges from +0.05 to ±3.0 percent of the En 

content and averages +0.51 percent, which is well within the 

two-sigma deviation of +1.3 percent assigned to the determinative survey by Himmelberg and Jackson (1967).

The cross sections and data on plate 2 are fairly 

representative of the strike length of the complex; all except 

the Bluebird, Forge Creek, Unnamed Creek, and Boulder 

River sections are based on drill core. Unfortunately, none 

of the sections are complete either because of lack of 

outcrops, limitations in drill core, or inadequate sample 

collection. All section thicknesses represent true thickness 

adjusted for the local attitude of the Basal zone and the 

attitude of the drill hole or traverse section. Schematic rock 

logs are given for each cross section, and grain-size logs are 

included where they were available. The contact between the 

Basal and Ultramafic zones is used as the reference horizon, 

and the location of each section is illustrated on plate 2. Most 

of the En contents for orthopyroxene have been connected 

by lines to emphasize the changes, but the connecting lines 

do not indicate compositions for intervening orthopyroxenes. The presentation of the data in the form of plate 2 

allows lateral and vertical correlations of rock type, grain

size, and orthopyroxene composition to be readily visualized.

Several generalizations can be derived from the data. 

Orthopyroxenes range in En content from 30 to 98, but most 

En contents for the Basal zone fall between 54 and 82, 

comparable with the range 50-90 reported for the whole 

complex (Hess, 1960; Jackson, 1961a). Individual specimens 

from each stratigraphic section (pi. 2) show a similar gradual 

increase in En content from the base to the top of the section. 

The rate of increase in each section is not the same and 

short-range fluctuations in orthopyroxene composition are 

superimposed on the gradual increasing trend of En content. 

Many of the sections show compositional units within the 

individual trends as defined by sharp increases and decreases 

in En content.

Well-defined and inferred or suspected unit boundaries 

are shown on plate 2 as dotted lines. Three to five 

orthopyroxene composition units can be distinguished in 

more detailed stratigraphic sections. In the Benbow West 

Golf Course section the units were numbered sequentially 

from the lowest unit upward. In three of the adjoining 

sections, unit correlations appear possible based on the 

orthopyroxene composition changes, rock types (specifically the presence of plagioclase cumulates), and grain size. For 

the middle and upper Nye Basin and Benbow East Golf 

Course sections, tentative unit numbers were assigned. 

However, the present data do not permit positive correlation 

with other sections.

COMPOSITIONAL VARIATION OF PLAGIOCLASE

Three textural varieties of plagioclase have been recognized in the Basal zone. By far the most abundant is 

postcumulus plagioclase. The next most abundant is the 

cumulus plagioclase of plagioclase and 

plagioclase+orthopyroxene cumulates. The least abundant 

variety is plagioclase phenocrysts in noncumulate rocks and 

phenocrysts as scattered crystals in thin layers but usually 

not in crystal contact with one another as in other cumulate 

rocks (pi. 2). Many estimates of plagioclase compositions 

were made during examination of thin sections, but all 

compositions reported were determined by the X-ray 

method of Jackson (1961b) developed specifically for 

plagioclase from the Stillwater Complex. The uncertainty 

for the determinative curve is+2.1 percent An (anorthite) in 

the range of about An65-80 and otherwise is +2.4 percent An 

(Jackson, 1961b, p. C287). Precision of the measurements 

made in this study is within these limits. The composition of 

all varieties of plagioclase in the Basal zone ranges from An57 

to Angs; most compositions fall between An65 and An80 (fig. 

6). These compositions are comparable to the range of An69 

to An77 reported by Jackson (1967) for postcumulus 

plagioclase in the Ultramafic zone and the range of An63 to 

An86 reported for the whole Stillwater Complex (Wager and 

Brown, 1967, p. 303).
22 STILLWATER COMPLEX. MONTANA

Postcumulus plagioclase compositions were examined in 

detail for the upper Nye Basin section, but for other sections

Base of the Ultramafic zone

METERS 

0-

10

20

30-1

About 50 meters covered

60 70 80 

Art, PERCENTAGE

90

FIGURE 12. Plagioclase composition plotted against stratigraphic position, 

upper Nye Basin section. Dotted lines represent orthopyroxene 

compositional units.

more or less random samples were checked by the X-ray 

method. The trends of plagioclase composition with 

stratigraphic position (fig. 12) are very similar to the trends 

of orthopyroxene compositions (pi. 2). The overall trend of 

plagioclase composition in the Basal zone is one of generally 

increasing An content from the base to the top of the zone. 

Superimposed on this trend are several sharp fluctuations in 

An content (fig. 12). Some of the sharp fluctuations in An 

content coincide with the sharp fluctuations in orthopyroxene compositions in the same section. Postcumulus 

plagioclase compositions apparently form repeating compositional units which may coincide with those based on 

orthopyroxene compositions.

Samples in which both the An content of the plagioclase 

and the En content of orthopyroxene were determined are 

plotted in figure 13. Increasing An content shows weak to 

moderate correlation with increasing En content. The 

correlation was not expected to be strong because the two 

minerals originated in different ways. Orthopyroxene is a 

cumulus mineral and settled under the influence of gravity 

through some of the magma from which it crystallized, 

whereas plagioclase is a postcumulus mineral and crystallized in place from magma that was trapped in the cumulus 

pore spaces. Only if the plagioclase crystallized from trapped 

magma of the same composition as that which precipitated 

the orthopyroxene would there be a strong correlation 

between their compositions. Several processes have been 

suggested that would modify the composition of the trapped 

magma. These include chemical diffusion between trapped 

magma and the main magma (Hess, 1939, 1960), intercumulus convection (Hess, 1972), and 

crystallization-differentiation (Jackson, 1961a, p. 85). Such 

processes are probably responsible for the spread in the

90

80

70

60

O 50

o

40

EXPLANATION 

A Noncumulate igneous rocks 

• Postcumulus plagioclase 

o Cumulus plagioclase

30

FIGUKI- 13.

40 50 60 70 80 

En CONTENT OF ORTHOPYROXENE, IN PERCENT

90

Coexisting plagioclase and orthopyroxene compositions in 

Basal zone.
IGNEOUS-TEXTURED (NONCUMULATE) ROCKS OF THE BASAL ZONE 23

composition plot. In addition, the spread may indicate a 

variation in accumulation rates of orthopyroxene if the 

postcumulus processes are assumed to take place at approximately constant rates.

DlSCl'SSION OF CUMULATE ROCKS IN THE BASAL ZONE

Although consistent mineralogic, petrologic, and 

stratigraphic patterns in cumulate rocks in the Ultramafic 

zone allow correlation over relatively long distances, similar 

patterns in the cumulate rocks in the Basal zone tend to have 

relatively little continuity along strike. Thus, a basic 

distinction between cumulate rocks of the Basal zone and the 

Ultramafic zone concerns the ratios of thickness to breadth 

of the tabular units. The chemical processes that formed 

lithologic and mineral compositional patterns in the Ultramafic zone operated over great lateral distances at the 

same time, in the same batch of magma, whereas the same 

processes active during the accumulation of the Basal zone 

occurred at different times, possibly in different batches of 

magma, and within localized areas. Cumulate sequences and 

possibly compositions of cumulus minerals may tentatively 

be correlated within individual basinlike structures, but 

because such sequences are not the same between the 

basinlike structures (compare fig. 4 and pi. 2), correlation of 

minor units within the Basal zone along the strike of the 

complex does not appear to be possible with the present 

data. For example, plagioclase cumulates are abundant in 

the Nye Basin-Benbow area and appear typical of this 

basinlike structure but are not abundant in the Mountain 

View area, another basinlike structure. This suggests that 

each basinlike structure had its own convection pattern and 

its own chemical and physical controls, at least through its 

early history of accumulation.

The fact that grain-size sequences do not correlate, even 

over short distances and within similar cumulate rock 

sequences or mineral compositional sequences, indicates 

that some type of mechanical differentiation was active. The 

vertical fine- to coarse-grained repetitive sequences are 

regular throughout the Basal zone and must be the results of 

the same process. One possible explanation can be based on 

an analogy to turbidite deposition described by Bouma 

(1962). In general, deposition from a turbidity current results 

in a decrease in thickness and grain size away from its source, 

so that sediments vary laterally in grain size from coarse to 

fine within the same physical unit. If small batches of magma 

begin to crystallize within the basinlike structures, become 

unstable relative to other magma because of the density 

contrast due to cumulus crystals, travel across the basin in a 

manner analogous to turbidity currents, and deposit their 

crystal content, the lateral grain-size variations may be 

plausibly explained. The variation upward from fine to 

coarse may be explained by continued crystallization during 

deposition. Thus, those cumulus crystals at the upper part of 

the postulated magma mass continued to grow relative to 

those crystals already deposited at the base of the mass.

IGNEOUS-TEXTURED (NONCUMULATE) 

ROCKS OF THE BASAL ZONE

Noncumulate igneous-textured rocks of the Basal zone 

include rocks that may or may not contain phenocrysts but 

that crystallized virtually in place. These noncumulate rocks 

have intrusive contacts with the country rocks, and approximate the magma composition from which they formed 

(table 2). Neither crystal settling under the influence of 

gravity nor any of the postcumulate processes is considered 

to have greatly altered the rock compositions or textures.

The noncumulates are subdivided into groups with 

ophitic, subophitic, and ragged textures (fig. 5). The term 

"ophitic" as used in this report refers to rocks that contain 

subhedral to anhedral ortho- or clinopyroxene, which 

totally encloses lath-shaped plagioclase. Ophitic is not used 

to suggest the dominance in amount of either plagioclase or 

pyroxene. Rocks that contain lath-shaped plagioclase partly 

enclosed in either ortho- or clinopyroxene are termed 

"subophitic." Ragged-textured rocks are composed of 

anhedral, very irregularly shaped phenocrysts of pyroxenes, 

olivine, or plagioclase set in poikiolitic, subophitic, or 

mosaic-textured groundmasses. Locally, the ragged 

phenocrysts are closely packed and are surrounded by 

interstitial material. Ragged-textured rocks appear to be 

gradational between ophitic and subophitic rocks at one 

textural extreme and cumulate rocks at the other. These 

three subdivisions may be further divided on the basis of the 

mineralogy of the phenocrysts, by the presence of inverted 

pigeonite or orthopyroxene, and by the presence of absence 

of quartz. Plagioclase phenocrysts range in An content 

between 63.4 and 77.2 and average 71.2; olivine phenocrysts 

have a composition of approximately Fo70 .

The proportions of the noncumulate rocks in the Basal 

norite are difficult to estimate because of the variability in 

texture over relatively short intervals in outcrop and drill 

core. Qualitative estimates made during surface mapping 

suggest that ragged-textured rocks are most abundant, but 

examination of core and thin section study suggests that the 

ophitic and subophitic types may be more abundant.

A generalized sequence of noncumulate rocks has been 

synthesized. This textural sequence from the base upward is 

ophitic texture, subophitic texture, ragged texture, and 

cumulate. However, not all rocks need be present, but if 

more than one type is present, they fit this sequence. As has 

been shown in figure 5, igneous rock sequences may repeat 

several times in any particular section, as do cumulates.

MODAL VARIATION 

OF NONCl'MULATE ROCKS

Changes in the proportions of plagioclase, clinopyroxene, 

and orthopyroxene, representing the major variations in the 

modal composition of the noncumulate rocks, are illustrated 

in figure 14. Modes for some of the specimens upon which 

this diagram is based, those that are chemically analyzed, are
24 STILLWATER COMPLEX, MONTANA

Clinopyroxene

Plagioclase Orthopyroxene

EXPLANATION

OPHITIC TEXTURE

• Inverted pigeonite 

O Orthopyroxene-olivine

• Inverted pigeonite-quartz 

9 Orthopyroxene-quartz 

SUBOPHITIC TEXTURE

• Inverted pigeonite

D Orthopyroxene

a Orthopyroxene-quartz

RAGGED TEXTURE 

A Inverted pigeonite 

A Inverted pigeonite-quartz 

A Orthopyroxene 

A Orthopyroxene-quartz

FIGURE 14. Proportions, by volume, of clinopyroxene, plagioclase, and 

Orthopyroxene in noncumulate rocks.

given in table 4. Each mode was determined by counting 

1,000 points in thin section. Two difficulties in gathering the 

modal data are how to treat alteration products, and how to 

count exsolution lamellae in the ortho- and clinopyroxenes. 

Alteration products were counted as primary minerals if the 

original mineral was readily suggested by shape and texture; 

if not, the material was counted as alteration. Exsolution 

lamellae in either Orthopyroxene or clinopyroxene were 

counted as the host pyroxene phase in which they occurred. 

There is substantial variation in the modal compositions 

of the noncumulate rocks, and textural subdivisions are not 

represented by well-defined clusters in modal composition 

(fig. 14). Comparison of modal data for noncumulate rocks 

with those given in figure 10 for cumulate rocks shows some 

overlap in mineral proportions for most of the ragged-textured rocks and some ophitic-textured rocks. Yet, a large 

part of the ophitic and subophitic rocks have proportions of 

pyroxenes and plagioclase that differ from cumulate rocks 

and form a cluster between the groupings for plagioclase and 

Orthopyroxene cumulates. The overlap in modal proportions of the ragged-textured cumulate rocks reinforces the 

idea that ragged-textured rocks are indeed gradational 

between ophitic and cumulate rocks, whereas the separation

of subophitic and ophitic from cumulate rocks implies that 

they originated by different processes.

CHEMICAL COMPOSITION OF THE 

NONCUMULATE IGNEOUS-TEXTURED ROCKS

In the past, the major interest in the chemical composition 

of the noncumulate igneous-textured rocks has been related 

to attempts to determine the composition of the parental 

magma for the Stillwater Complex. On the basis of field 

occurrence and mineralogical characteristics, Hess (1960) 

and Jackson (1971) selected a limited number of samples 

believed to be possible parents, three of which were analyzed 

and the results were published. These analyses were presented 

as representative of the chilled border of the Stillwater 

Complex, and both authors suggested that the analyses 

represent the original melt or magma composition for the 

complex. Hess (1960) also estimated the composition of the 

parent magma by attempting to calculate the bulk composition of the whole complex and found some agreement 

between this calculation and the composition of border 

rocks.

The approach in this study was to select for analysis 

representative samples of ophitic, subophitic, and 

ragged-textured rocks which in hand specimen and thin 

section showed little alteration or contamination with 

metasedimentary rocks. The purpose was more to determine 

the extent of chemical variation in the igneous-textured 

rocks than to find the original parent magma composition. 

Each ophitic and subophitic rock analyzed is assumed to 

represent a melt or magma composition at the time and place 

the rock crystallized.

Table 4 gives chemical and semiquantitative spectrographic analyses, CIPW norms, modes, mineral compositions, and locations of ophitic, subophitic, and 

ragged-textured rocks. The data demonstrate the variability 

in composition of noncumulate igneous-textured rocks and 

emphasize the difficulty of selecting any particular rock to be 

a parent magma. Analytical data reported by Hess (1960) 

and Jackson (1971) for their chilled border rocks are listed 

for comparison. Hess' (1960) two specimens have the highest 

A1 2O3 content of any analyses presented, and it should be 

noted that Hess called sample EB89 a "contaminated 

hypersthene dolerite or hornfels" (Hess, 196.0, p. 53). 

Jackson (1967, p. 25) criticized Hess' other sample (463G70) 

as not representing the parental magma on the basis of its 

high A12 O3 content. Therefore, neither of Hess' (1960) 

samples appears to be an acceptable representative of the 

parental magma. The analyses in table 4 show a fairly wide 

range of compositions, but all, including Hess' samples, fall 

within the range for basalts. Table 5 summarizes the 

chemical analyses in table 4 and compares them with average 

tholeiitic basalt compositions from other basaltic provinces. 

Jackson's (1971) specimen approaches the average of all the 

analyses in table 4 and can be considered fairly representative of a parental magma. It is immedTiately apparent that
IGNEOUS-TEXTURED (NONCUMULATE) ROCKS OF THE BASAL ZONE

TABLE 4. Chemical, spectrographic, normative, and modal analyses of ophitic, subophitic, and ragged-textured rocks of the Basal norite,

Basal zone, Stillwater Complex, Montana, with some mineral compositions 

[Chemical anahses b\ P. Elmore. J. Glen. H. Smith, and J. Kelsey performed in the Rapid Rock Analysis Laboratory 

under the direction ol I eonard Shapiro. X-ra\ fluorescence analyses o( S and Cl by B. P. Fabbi. Quantitative

25

snectr ^graphic anal\ses of Cr. Cu. and Ni b\ R. E. Mays and Chris Heropoulos. hire-assay spectrochemical 

anah cs ol Pd. Pt. and Rh b\ W. D. Goss. A W. Haubert, and Joseph Hutlty. Semiquantitative six-step 

spectr ^graphic anah ses by Chris Heropoulos and R. E. Ma\s. I ookedlorbut not detected at the limit of detection: 

As. Ai. B. Be. Bi.Cd.l.a. Mo. Nb. Pd. Pt. Sn.Te. 11. V.Zn. Cc. Ht. In, Li. Re. la, 1 h.l 1, Eu.Tr. element present 

but b lo\\ limit of determination: N. not detected; H. interlerence Irom other elements]

Sample number 2NB68 1MV71 MI7( ^7-^ ^387-^ ^ gS74"s| 56BF69 67MV69 ^'^ 38CM7I 32CM7I , N ^ M594^ W-'s MBE69 EB89 463G70 Jackson 

(1971)

Chemical Analyses, Weight Percent

SiO- .............. 

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

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

Ca() ..............

Na-O

KX) ...............

H.O+ .............

H : O- .............

no, ..............

|>,()<;

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

CO, ............

Total ..

F ...................

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

Cl .................

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

Cu .................

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

Pd (pph) .......

Pt (pph) ........

Rh (pph)

....48.7 47.4 48.7 52.4 48.9 53.3 48.8 48.7 48.9 50.3 51.4 48.8 44.5 48.0 46.6 47.34 

.... 15.2 13.5 15.0 14.5 13.8 14.9 13.9 16.6 16.2 13.7 12.1 14.9 10.8 14.2 16.0 17.12 

.... 2.8 1.2 .92 1.3 1.6 .79 3.9 4.0 .50 .73 .36 1.7 1.9 1.7 .57 2.51 

....12.2 14.5 13.8 12.8 14.1 14.1 10.2 11.4 16.0 8.7 10.4 12.8 21.1 13.0 15.3 12.05 

.... 6.7 7.0 6.4 6.9 7.1 3.9 7.6 7.1 6.4 14.9 10.9 7.0 9.4 6.4 7.6 8.23

.... 10.6 10.3 10.6

.... 1.5 .45

.... .24 .89

.... .00 1.7

.... .12 .08

.... 1.4 1.5

.... .17 .22

.... .26 .27

.... <.()5 <.05

1.8

.17

.61

.16

1.4

.19

.26

<.05

8.2 10.5

.77

.49

1.2

.03

.88

.08

.24

.12

.77

.24

1.1

.24

1.2

.08

.23

<.05

6.1 10.6 6.3 7.5 9.7 11.3 11.0

2.2 1.7 2.6 1.3 .65 1.3 1.1

.28 .15 .32 .19 .08 .15 .32

.11 I.I .70 .56 .79 .41 .55

.24 .18 .07 .10 .07 .09 .10

3.3 1.4 1.7 1.4 .13 .45 1.4

.52 .16 .20 .24 .04 .09 .07

.24 .26 .27 .27 .21 .30 .27

<.05 <.05 <.05 <.05 <.05 .27 <.05

9.0 10.0 8.

.88 1

.33 2

.60

.11

.87 1

.12

.23

.08

....100 99 100 100 100 100 100 100 100 100 100 100 100 100

... .01 .01

.... .12 .14

.... .0055 .0136

.... .170 .150

.210 .080

.... .210 .100

.... .8 <4

...<IO <IO

<5 <5

.00

.17

.0067

.190

.200

.085

.00

.44

.0080

.170

.480

.290

Tr

0

.00

1.76

.0042

.210

.700

.950

... <5

.02 .00 .01 .01 .00 .00 .00

.01 .00 .17 .39 .05 .00 .19

.0300 .0022 .0043 .0062 .0141 .0221 .0016

N2 .210 .090 .200 .700 .700 .190

.036 .100 .100 1.40 .170 .070 .380

.015 .100 .140 .260 .250 .240 .100

... <4 <4 ... 32 14 <4

...<10 <10 ... 19 19 <10

... <5 <5 ... <5 <5 <5

.02

4.66

.0085

.067

.120

.220

2

2

76 1

34

6 1

19

28

17

99

00

7

50

42

2

12

6

07

33

0?

00

9.79

.76

.21

.73

.04

.52

.17

50.68

17.64 

.26 

9.88 

7.71

10.47

1.87

.24

.42

.06

.45

.09

.15

99.92

49.41 

15.78 

2.11 

10.25 

7.36

10.88

2.19

.16

.23

.07

1.20

.11

.20

99.95

.17 .61

.0110

.0125

.0145

028

0073

0030

0560

Oil

Semiquantitative six-step spectrographic analyses, parts per million

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

Ag .................

Ba ................

Co ................

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

Cu ................

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

Ph ................

Sc .................

Sr .................

V

Y ..................

Zr .................

Ga ................

Yh ................

.... 1,500 1,500

.... N N

70 150

50 50

150 100

.... 200 100

.... 200 150

.... N N

50 50

150 200

.... 500 500

30 30

30 70

15 20

3 3

1,500

N

70

30

150

200

100

N

30

150

300

30

70

15

3

1,500

N

150

30

150

500

300

20

30

100

200

10

20

15

2

1,500

2

70

50

150

1,000

1,000

15

50

100

500

10

15

15

2

1.500 1,500 1,500 1,500 1,000 1,500 1,500

N N N 3 N N N

150 70 100 70 15 70 70

30 30 50 30 50 50 50

N 200 70 150 700 700 150

20 150 200 3,000 200 100 500

15 150 150 300 300 200 150

15 N N 20 N N N

15 50 30 50 50 50 50

300 150 200 200 100 100 150

150 500 300 300 150 200 500

30 20 20 20 N 20 15

150 15 50 N N 15 15

20 15 20 15 10 10 15

32221 12

1,500 1

N

70

150

1,000

1,500

2,000

10

50

100

200

10

30

10

H

,500 1

N

100

70

150

150

300

N

50

150

300

30

50

15

3

500

N

30

70

30

700

200

N

50

300

500

N

N

15

H

CIPW norms, constituents normalized to 100 percent

Q..................

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

Ah ................

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

Di ................

Di-Wo ..........

Di-En ...........

Di-Fs ...........

Hy ...............

Hy-En .........

Hy-Fs ..........

Ol ...............

OI-Fo .........

OI-Fa .........

Mt ...............

11 ................

Ap ...............

Cc ...............

C .................

.... 2.35 2.58

.... 1.42 5.31

.... 12.71 3.84

.... 34.09 32.51

.... 14.24 14.49

.... 7.16 7.23

.... 3.37 3.07

.... 3.69 4.17

..... 27.93 34.26

..... 13.33 14.53

..... 14.59 19.72

..... 4.06 1.75

..... 2.66 2.87

.40 .52

.11 .11

1.00

15.24

32.37

15.66

7.80

3.23

4.61

29.59

12.19

17.40

.95

.36

.58

1.33

2.66

.45

.11

10.02

2.89

6.52

34.70

3.95

1.98

.87

1.09

36.68

16.32

20.35

1.88

1.67

.19

.27

4.15

1.42

6.53

33.60

14.87

7.44

3.23

4.20

33.39

14.51

18.88

2.32

2.28

.19

.11

... 2.27 1.99 3.46 ... 1.16 2.37

.88 1.89 1.12 0.47 .89 1.89

... 14.41 22.01 11.05 5.50 11.06 9.31

... 29.90 29.64 35.49 34.23 26.89 34.78

... 17.52 ... ... 10.73 22.21 15.82

... 8.94 ... ... 5.56 11.35 7.94

... 4.98 ... ... 3.64 6.44 3.64

... 3.60 ... ... 1.52 4.42 4.23

... 24.07 33.02 43.31 44.84 35.15 29.85

... 13.97 17.69 16.01 31.60 20.85 13.79

... 10.10 15.33 27.29 13.24 14.30 16.06

... 1.91

... 1.30

.60

... 5.66 5.80 .72 1.05 .52 2.46

... 2.66 3.23 2.67 .24 .86 2.66

.38 .47 .57 .09 .21 .16

.11 .11 .11 .11 .61 .11

... 1.11 .91

1.26

1.24

6.46

42.92

4.80

2.47

1.23

1.17

37.81

19 37

18.44

3.66

.99

1.42

15.84

39.08

10.17

5.15

2.63

2.39

31.38

16.43

14.94

.22

.11

.11

.37

.85

.21

0.94

18.55

32.79

16.88

8.57

4.51

3.80

23.52

12.75

10.76

1.46

.75

.70

3.06

2.28

.26
26 STILLWATER COMPLEX, MONTANA

TABLE 4. Chemical, spectrographic, normative, and modal analyses—Continued

Sample number 2NB68 1MV7I 56B 169 67MV69 «CM7I 1JCM7. N JJ,2 3 5

FBXQ

Jackson 

(1971)

Modes, volume percent

Quart/ 6.1 11.4 6.5 5.1 1.1 7.9 1.3 1.6 0.2

Plagioclase ........ 49.2 49.6 51.0 36.3 26.1 56.5 56.8 57.2 41.4 49.3 45.4 48.1 25.9 50.8 47.1

Inverted

28.5 41.4 39.2 ... ... ... ... ... ... 31.2 32.9 25.2 45.7

49.7

Orthopyroxene ..

Clinopvroxene ..

Brown

hornblende ....

Biotite ...............

Opaques ............

Olivines .............

Apatite ..............

Alteration 1 ........

24.7

14.7

1.2

5.7

.1

4.4

27.3

11.4

1.2

.4

4.0

12.3

5.3

.3

2.5

0.1

4.0

3.0

.3

3.6

16.7

4.2

.3

5.7

1.3

28.4 10.8

.5 23.2

.1

.3 Tr

7.8 9.2

1.3

28.5

1.3

11.9

Tr

33.3

2.3

4.3

.9

7.5

2.7

46.2

2.1

2.0

.3

Tr

31.6

18.2

Tr

1.3

2.2

12.6 20.9 18.7 .8

3.1 ... .3

.1 1.3 Tr .1

6.4 16.2 5.2 5.5

Tr

.1 1.3

... 19.9

... 8.6

.6

Tr

Mineral compositions

Normative An ...

Normative En ...

Normative Di-Wo

Normative Di-En

Normative Fo ...

MgxIOO/

2 cations .......

An(X-rav) ........

72.84

47.74

50.35

23.70

44.3

67.2

89.44

42.42

49.97

21.22

67.99

41.20

49.87

20.65

38.30

40.7

60.5

84.18

44.51

50.25

22.08

45.1

79.6

83.73

43.46

50.03

21.72

45.4

80.1

... 67.48

... 58.04

... 51.03

... 28.42

38.3 54.0

55.4 61.9

57.39

53.57

61.1

53.0

76.26

36.97

37.8

72.7

86.16

70.47

51.87

33.96

68.42

68.0

82.5

70.86

59.32

51.10

29.00

78.88 ... ... ... 86.92

46.20 ... ... ... 51.23

50.22 ... ... ... 50.72

23.02 ... ... ... 25.26

43.1 51.0 30.7 64.0

73.8 ... 70.9 82.1

71.16 63.86

52.37 54.23

50.64 50.77

25.86 26.72

50.00 51.72

Sample description and location:

2NB68, ragged-textured gabbro; Nye Basin.

1MV7I, subophitic-textured gabbro; Mountain View area.

M17/ 1091, subophitic-textured gabbro; Mountain View area.

387-339/411, ophitic-textured gabbro; Mountain View area.

387-339/416, subophitic-textured gabbro; Mountain View area.

387-339/481, ophitic-textured gabbro; Mountain View area.

56BE69, ragged-textured gabbro; Benbow area.

67MV69, subophitic-gabbro; Mountain View area.

M I7/1116, ophitic gabbro; Mountain View area.

38CM7I, ragged-textured gabbro or plagioclase-orthopyroxene

cumulate; Iron Mountain area. 

32CM7I, subophitic gabbro; Iron Mountain area.

NBI2/349, ophitic gabbro; Nye Basin area.

M19A/594, subophitic gabbro; Mountain View area.

NBI/386.5, ophitic gabbro; Nye Basin area.

64BE69, ophitic-textured gabbro; Benbow area.

EB89, contaminated hypersthene dolerite or hornfels; analyst, A. H.

Phillips, alkalies by R. B. Ellestad (Hess, 1960, table 12, col. 2, p. 53);

East Boulder Plateau. 

463G70, hypersthene dolerite border facies, collected by Peoples; analyst,

R. B. Ellestad (Hess, 1960, table 12, col. i, p. 53); East Boulder Plateau. 

Jackson (1971), chilled border rock; analyst, Faye H. Neuerburg

(Jackson, 1971, table 1, p. 132), Nye Basin.

'Includes blue-green chnoamphibole. clear amphibole. chlorite. serpentine, talc, and cpidote.

except for Hess' two analyses (1960), the Stillwater noncumulate igneous-textured rocks resemble neither oceanic 

tholeiitic basalts nor alkali basalts. The average compositions of the noncumulate igneous-textured rocks fall 

between the Hawaiian and Thingmuli averages. The 

chemical characteristics of Stillwater noncumulate 

igneous-textured rocks low K2O, Na2O, TiO2 , and high 

FeO are similar to those of chilled border rocks from the 

Bushveld, Muskox, and Great Dyke stratiform intrusions as 

pointed out by Jackson (1971, p. 131).

The variation in composition of noncumulate 

igneous-textured rocks is illustrated by the spread of data 

along the iron-magnesium side of an alkali-iron-magnesium 

diagram (fig. 15). Hess' (1960) calculated liquid trend is 

superimposed upon the diagram as a reference line. Comparison of the spread in compositions with the liquid trend 

line illustrates the difficulties of establishing a single unique 

parent magma composition. Moreover, the analytical data, 

as well as the presence of two crystallization sequences and 

olivine and plagioclase phenocrysts in the Basal zone,

suggest that no single unique parent magma will be 

established by analyzing ophitic and subophitic rocks. At 

best, an average composition and the deviation from it may 

be established by this technique. Because the causes of 

variation from a parent composition, are numerous, a 

definition of the variation may be more important to the 

development of models for the crystallization of the Basal 

zone and the whole complex than the actual parent 

composition. Processes that may cause local variation in 

compositions of parent magma include (1) contamination by 

and reaction with country rocks either within the magma 

chamber or conduits leading to the chamber, (2) fractional 

crystallization and winnowing of crystals in either the 

magma chamber or conduits, (3) initial variation in magma 

compositions due to generation processes or incomplete 

mixing in the magma chamber, (4) alteration by diffusion or 

metamorphic processes at or near magmatic temperatures, 

and (5) lower temperature alteration.

Figure 15 also illustrates that the compositions of 

noncumulate igneous rocks and cumulate rocks from all
IGNEOUS-TEXTURED (NONCUMULATE) ROCKS OF THE BASAL ZONE 27

TABLE 5. Comparison of the range and average chemical analyses of 

noncumulate igneous-textured rocks from the Stillwater Complex with 

Hawaiian basalt, Thingmuli basalt, average oceanic tholeiitic basalt, and 

alkali basalt

Stillwater Complex1

SiO2

AhOi

Fe2O3

FeO

MgO

CaO

Na2O

K2 0

H2O+

H 2OTiO2

P2 05

MnO

C02

Range

44.5-53.3

10.8-17.64

.26-4.0

8.7-21.1

3.9-14.9

6.1-11.3

.45-2.6

.08-2.2

.0-1.2

.04-. 34

.13-1.7

.04-.52

.15-.30

.02-. 17

Average

49.05

14.77

1.60

12.92

7.70

9.53

1.31

.39

.71

.12

1.24

.16

.25

Hawaii2

49.84

14.09

3.06

8.61

8.52

10.41

2.15

.38

2.52

.26

.16

Thingmuli'

48.75

13.12

4.64

9.61

5.46

9.71

2.85

.49

.85

.79

2.85

.48

.25

Oceanic4

49.34

17.04

1.99

6.82

7.19

11.72

2.73

.16

.69

.58

1.49

.16

.17

Alkali

47.41

18.02

4.17

5.80

4.79

8.65

3.99

1.66

.79

.61

2.87

.92

.16

'Range and average of analyses from table 4 of noncumulate igneous-textured rocks trom the 

Stillwater Complex, Mont.

2 Average of 181 tholeiites and olivine tholeiites from the Hawaiian Islands (Macdonald and 

Katsura, 1964, table 9, p. 124).

'Average of 10 analyses olivine tholeiite and tholeiite lavas and dikes from the Thingmuli volcano. Iceland (Carmichael, 1964, table 2, p. 439).

 "Average oceanic tholeiitic basalt (Engel and others, 1965, table 2, p. 721).

s Average alkali basalt from seamounts and islands (Engel and others, 1965. table 2. p. 721).

EXPLANATION

o Noncumulate igneous rocks 

• Cumulate rocks

FIGURE 15. Noncumulate igneous-textured rock compositions compared 

with cumulate rock compositions. A=Na'2O+K,2O, F=FeO+MnO, 

M = MgO. Cumulate rock coordinates calculated from analytical data 

of Bowes, Skinner, and Skinner (1973). Line is Hess' (1960) calculated 

liquid trend.

parts of the complex are different. Cumulate rock compositions plotted in figure 15 were calculated from 61 

chemical analyses of samples collected along three traverses 

across the complex (Bowes and others, 1973). Nine of the 

analyzed samples showed secondary alteration mainly to 

serpentine minerals. For these samples, the Fe 2O3 content 

was converted to FeO and combined with FeO and MnO for

EXPLANATION

• Ophitic textured 

A Subophitic textured

• Ragged textured

Opx+4Q

EXPLANATION

• Ophitic textured 

A Subophitic textured

• Ragged textured

~4Vikb

FIGURE 16. Projections of analyses of noncumulate igneous-textured 

rocks in the system olivine-clinopyroxene-plagioclase-silica. A, 

Plagioclase (Pl)-clinopyroxene (Cpx)-orthopyroxene (Opx)+4SiO 2 

(4Q). B, Olivine (Ol)-plagioclase (pi) SiO 2 (Q). Data are plotted as 

cation equivalents. Numbers refer to identifiable parts of sample 

numbers in table 4. Solid lines represent liquidus boundaries after 

Irvine (1970a) for average basaltic liquids at one atmosphere. Dashed 

lines represent liquidus boundaries inferred after Irvine (1970a)for4'/$ 

kb.

the F coordinate of the diagram. If actual volumetric 

proportions of these analyzed rocks within the complex 

could be determined, it might be possible to determine a 

parent magma composition. Obviously an arithmetic
28 SFILLWATER COMPLEX. MONTANA

average does not yield the parent composition. Unfortunately the Upper and Banded /ones of the complex are too 

poorly known at the present time to justify a volumetric 

calculation.

Interpretation and diagrammatic projections developed 

by Irvine (1970a) for the system olivine-clinopyroxeneplagioclase-silica allow the analytical data from table 4 to be 

examined with respect to possible crystallization paths. 

Ophitic, subophitic. and ragged-textured rock compositions 

have been plotted on two of these projections in figure 16. 

The calculations follow those of Irvine (1970a). In general, 

most of the noncumulate igneous-textured rock compositions would first crystallize small amounts of olivine 

followed by plagioclase or orthopyroxene. or first 

orthopyroxene followed by other phases.

More detailed examination leads to the following observations. Magmas represented by samples projected into the 

olivine volume of figure \6B would first crystallize small 

amounts of olivine at I atmosphere and similarly at about 4 

I 2 kb. Samples such as J would next crystallize plagioclase 

followed by clinopyroxene and then orthopyroxene; in 

others such as 463 the crystallization sequence would be 

olivine, plagioclase, orthopyroxene, clinopyroxene. 

Samples such as 56BE would crystallize in the order olivine, 

orthopyroxene, clinopyroxene, plagioclase; others such as 

2NB in the order olivine, plagioclase, orthopyroxene, 

clinopyroxene. Magmas represented by samples projected in 

the orthopyroxene volume of figure \6B would first 

crystallize orthopyroxene. With this model all of the 

crystallization sequences of cumulates in the Basal zone can 

be accounted for by the various magma compositions except 

for the sequence olivine, olivine+orthopyroxene, 

orthopyroxene.

Irvine (1970a. p. 470-472) proposed a phase-diagram 

model to give this sequence by fractional crystallization. His 

model involves postulating a particular shape and position 

for the olivine-orthopyroxene liquidus boundary to allow 

coprecipitation of olivine and orthopyroxene. If his model is 

accepted, all of the cumulate rocks (see compositions 

projected on fig. 16) within the complex can be accounted for 

by fractional crystallization. Cumulate rocks in the Basal 

zone reflect local variations in magma compositions; the 

rocks in other zones perhaps reflect a more homogeneous 

composition caused by more complete mixing of magma 

over a longer time span.

VISCOSITY, DENSITY, AM) SETTLING VELOCITIES

Estimates of the variation in viscosity and density of the 

Stillwater magma and of the probable settling velocities 

within it are the basis for evaluation of accumulation 

processes in the Basal zone. Both the viscosity and density of 

magmatic liquids are strongly dependent upon chemical 

composition, hence their variation may be more important 

than their absolute values in interpreting features in the 

Basal zone and the processes responsible for them. Estimates 

of these properties were made in the past by Hess (1960) and

Jackson (196la, 1971), but recent studies of the viscosity of 

basaltic rocks (Shaw and others, 1968; Shaw, 1969; Murase 

and McBirney, 1973) and the development of techniques to 

calculate viscosities of magmatic silicate liquids of varying 

composition (Bottinga and Weill, 1972; Shaw, 1972) and 

their densities (Bottinga and Weill, 1970) allow these 

properties to be quantified with more confidence than in the 

past. Viscosity and density ranges have been calculated from 

the theoretical models for the possible liquid compositions 

represented by chemically analyzed noncumulate, ophitic. 

and subophitic rocks.

Bottinga and Weill (1970) calculated and approximated 

partial molar volumes of the major constituents of basaltic 

magmas from binary and ternary silicate melt data and 

presented compositional coefficients and a model from 

which densities of melts in the range of 1,200° to 1.600°C 

may be calculated. Using their method, densities of possible 

dry (without water) magmas represented by compositions of 

the noncumulus ophitic and subophitic rocks in table 4 were 

calculated for a temperature range of 1,200°-1,400°C as 

shown in figure 17. The density calculations were extrapolated below 1,200° C but probably do not represent true 

densities because below 1,200°C crystalline phases would be 

present that would tend to increase the bulk density. Water 

was not included in the calculations because its effect is

2.80

2.76

522.72

2.68

2.64

Average Hprnfels density 'lit

Lower limits of Hornfels densities

MOO

Probable piag pclase dens'ftfes;

1000 1100 1200 

TEMPERATURE, IN °C

1300 1400

iRi 17. Calculated dry density-temperature curves of possible 

Stillwater magmas. Sample Ml 7' 1116 (table 4) represents highest and 

sample 67MV69 represents lowest densities. Densities of samples 

NB-12 349and463G70are near average. Solid lines indicate densities 

calculated after Bottinga and Weill (1970); dashed lines indicate 

extrapolations.
IGNEOUS-TEXTURED (NONCUMULATE) ROCKS OF THE BASAL ZONE 29

sensitive to the total pressure on the system and the pressure 

on the Stillwater magma is unknown. As shown by Bottinga 

and Weill (1970. p. 179), the density difference between dry 

and wet magmas depends upon the amount of water present 

and the total pressure.

In general the calculated densities are slightly higher than 

the densities tor the Stillwater magma estimated in previous 

studies. For example. Jackson (1971, p. 138) and Hess (1972, 

p. 505) used a density of 2.60 g/cm3 . The calculations 

emphasize the variation in magma densities as a function of 

small composition and temperature changes and point out 

the impropriety of assigning one density to the whole 

magma.

The magma density calcuations show that in most of the 

possible Stillwater magma compositions plagioclase (An s3 

to Anv ) could have floated (fig. 17) if it was a primary phase. 

However, stratigraphic and petrologic observations (fig. 5) 

indicate that some plagioclase was a cumulate phase. 

Assuming that the range of plagioclase density(An,,to Anioo) 

at 1,200°C is 2.53 to 2.71 g/cm3 (Bottingaand Weill, 1970, p. 

180) it is possible for plagioclase to have been cumulus in 

some of the lower density compositions of the magma in the 

Basal zone. Previous problems of interpretation concerning 

magma composition and the behavior of plagioclase 

(Jackson, 196la; Bottinga and Weill, 1970) can be 

eliminated by suggesting that plagioclase could sink or float 

depending upon the local composition and physical conditions of the magma.

The appearance of hornfels inclusions in the Basal zone, 

both as isolated fragments and as groups of fragments in 

lenses or layers, raises the question of whether they represent 

pieces of country rock that floated upward from the base, or 

roof rocks that sank to their present position. Measured bulk 

densities range from 2.720 to 2.924 and average 2.813 g/cm3 

(Page, 1977). Thus, hornfels inclusions may have either sunk 

or floated depending on local magma composition, although 

most of the hornfels probably sank.

H. R. Shaw (1972) simplified calculation techniques for 

prediction of viscosities of silicate liquids and extended them 

to treat hydrous compositions. Shaw's (1972, p. 881) data 

indicate that his method yields smaller mean differences 

from measured viscosities of silicate liquids than does the 

more cumbersome technique of Bottinga and Weill (1972). 

Hence Shaw's method of calculation was used to estimate 

viscosities for magmas corresponding to the compositions in 

table 4 over a temperature range from 1,300° to 1,000°C. 

The results of these calculations are shown in figure 18 as a 

plot of In 17, where rj is the viscosity in poises, against 

reciprocal temperature in kelvins. Sample 463G70 (table 4) 

would yield the most viscous magma, similar to those 

measured for a Hawaiian tholeiitic basalt (Shaw, 1969; 

sample 1MV7I (table 4) would yield the least viscous 

magma. Calculated viscosities for samples Ml7/1091, 

67M V69, and 32CM71 are close to the average calculated for 

all the Basal zone rocks over the chosen temperature range.

The mean slopes tor each composition were used to calculate 

the viscosity-temperature curves by the equation

In 77 = S(104 / 7>1.505-6.40,

where Aslope and /^temperature in kelvins (Shaw, 1972, p. 

873). The viscosities in figure 18 were calculated assuming 

that no crystals or gas bubbles are present in the magma. It is 

more likely that the magma contained crystals in the chosen 

temperature range; if so, the viscosities would be higher, 

depending upon the proportion of crystals present (Shaw 

and others, 1968, p. 257). Previous viscosity estimates of 

3X10-' poises (Hess, 1960) and 300 poises (Jackson, 1971) fall 

within the range of calculated magma viscosities. The most 

probable range of viscosity, from 36 to 909 poises, is shown 

in the stippled area of figure 18. It is important to emphasize 

that the slight variations in rock composition represented in 

table 4 lead to variations in calculated viscosities of almost 

an order of magnitude.

Many scientists have calculated Stokes law settling 

velocities of olivine, orthopyroxene, and plagioclase grains 

of different sizes in basaltic magmas under stagnant conditions for various viscosity conditions and particle shapes. 

Jackson (1971, p. 138) showed that in a magma with a

-10,000

5,000

7 —

"26 

2

-2,000

-1,000

-500

5 -

4 -

2 -

1 —

• Viscosity estimate 

(Hess, 1960)

1000°C

200 £

o

QL

1—100 z 

50 £

CO 

O 

o

CO 

20 >

Viscosity estimate 

(Jackson, 1971)

-10 

5

-2

45678 

RECIPROCAL TEMPERATURE (104/K)

10

FIGURH 18. Calculated viscosities (>?) of possible parent magmas for the 

Stillwater Complex plotted against reciprocal temperature. Stippled 

area is the most likely viscosity range for Stillwater magmas. Black dots 

show measured viscosities of Hawaiian basalts (Shaw, 1969). Two 

horizontal lines show viscosity estimates of Hess (I960) (upper line) 

and of Jackson (1971) (lower line).
30 STILLWATER COMPLEX, MONTANA

viscosity of 300 poises, extreme prismatic shapes of 

plagioclase and pyroxenes settled nearly as rapidly as ideal 

spherical shapes. Shaw (1965) demonstrated that if a 

cumulus crystal is assumed to grow continuously while 

settling it will travel only about one-third as far as a crystal of 

fixed size in the same amount of time. It also can be shown 

that the density contrast between crystal and magma is not 

changed enough through the range of compositional solid 

solution of olivine, orthopyroxene, and plagioclase of the 

Basal zone to greatly alter settling velocities of individual 

mineral species. Settling velocities for minerals in the Basal 

zone are shown in figure 19. The conditions correspond to a 

magma with a density of 2.712 g/ cm3 and a viscosity of 59.1 

poises at 1,300°C and a density of 2.726 g/cm3 and a 

viscosity of 149.9 poises at 1,200° C, the average ranges of the 

preceding density and viscosity calculations. For 

orthopyroxene a density of 3.36 g/cm3 representing En80 was 

used at 1,200°C and a density of 3.58(En50 )at 1,300°C. This 

allows a maximum range of settling velocities to be shown. 

Similarly at 1,200°C a density of 3.47 (Fo74 ) and at 1,300°C a 

density of 3.57 (Fo67 ) was used for olivine. A plagioclase 

density of 2.68 corresponding to a composition of about

Ans5 at 1,200° C was used in the calculations. One must 

conclude from figure 19 that settling velocities in the Basal 

/one were from 25 to 61,000 meters per year.

Throughout the lithologic and stratigraphic descriptions 

of the Basal zone, the abrupt variation in grain size along the 

trend of the zone and within individual hand samples has 

been emphasized. One explanation for this phenomenon is 

that grain-size distributions are controlled by magma 

density and viscosity which in turn are dependent upon 

composition. The slight variation in composition of possible 

magmas, represented by ophitic and subophitic rocks in 

table 4, appears to be large enough to change viscosity in the 

temperature range of interest by an order of magnitude and 

to change the density by 4 or 5 percent. These variations 

could locally affect settling velocities by as much as 70 

percent. Therefore, if there were local volumes of magma 

with varying compositions, it is reasonable to expect abrupt 

changes in grain size. This explanation does not consider the 

existence of currents or local convection, which were 

certainly present as evidenced by textures and structures. 

Movement of masses of magma could probably increase the 

observed irregularities without contributing to rapid mixing.

Range of plagioclase grain size

Range of orthopyroxene grain size

-2 —

of olivine grain size

SETTLING VELOCITY, IN METERS PER YEAR 

"100 1000 10,000 100,000

-3

45678 

SETTLING VELOCITY AS In v, IN METERS PER YEAR

10 11 12

FIGURH 19. Settling velocities of plagioclase, orthopyroxene, and olivine in a possible Stillwater magma, r, grain si/c; v, settling velocity.
CONTAMINATED IGNEOUS ROCKS 31

CONTAMINATED IGNEOUS ROCKS

Contaminated igneous rocks or rocks of mixed origin are 

found locally in contact zones between metasedimentary 

rocks and either cumulate or noncumulate igneous-textured 

rocks of the Basal norite. Inclusions of metasedimentary 

rocks are sparse throughout the Basal bronzite cumulate and 

in the lower part of the Peridotite member as well as in the 

Basal norite. In the Basal zone, at least two types of 

contaminated igneous rocks may exist. The first type 

consists of cumulate and noncumulate igneous-textured 

rocks that crystallized from magma whose composition had 

been changed by addition or subtraction of components. 

The second type consists of rocks of mixed origin formed by 

reaction of metasedimentary rocks with magma. The first 

type is recognized by chemical features such as unique trace 

element contents, mineral compositions, or crystallization 

sequences; the second type is recognized on the basis of 

textures, mineralogy, and relation to the metasedimentary 

rock. Recognition of the first type of contaminated igneous 

rocks in the Basal zone is difficult; this discussion is therefore 

restricted to those contaminated rocks that can be unquestionably distinguished on mineralogical, textural, and 

petrologic criteria. They have been observed only in contact 

with metasedimentary rocks that occur either as inclusions 

or at the lower margins of the Basal norite. Mixed rocks are 

present along the strike length of the Basal zone but are 

concentrated in areas where there are abundant inclusions of 

metasedimentary rocks.

TYPES, TEXTURES, AND MINERALOGY OF 

THE CONTAMINATED IGNEOUS ROCKS

The more common variants of the mixed rocks contain 

large (0.2 to 0.5 cm), anhedral, interlocking poikilitic 

crystals. These crystals occur within the igneous rock and 

also extend across the contact and into the metasedimentary 

rock where they enclose relict mineral grains from the 

metasedimentary rock. Table 6 summarizes the textures and 

mineralogy of the mixed rocks. Where mixed rocks occur 

between metasediments and orthopyroxene cumulate, 

plagioclase cumulate, ophitic norite, subophitic norite, or 

ragged norite, they contain relict poikilitic plagioclase and 

varying amounts of relict orthopyroxene, cordierite, biotite, 

and magnetite from the pyroxene hornfels facies 

metasedimentary rocks. Figure 20 shows examples of this 

type of contaminated rock and their contacts with the 

igneous and metasedimentary rocks. Orthopyroxene in 

mixed rocks is distinguished from either euhedral cumulus 

orthopyroxene or ophitic orthopyroxene by its shape, 

texture, and internal characteristics. It occurs as fine (less 

than 1 mm diameter), anhedral, locally branching grains that 

contain neither the fine clinopyroxene exsolution lamellae 

parallel to the (100) plane of orthopyroxene nor the coarser 

discontinuous blebby exsolution bodies typical of inverted 

pigeonite. These characteristics are typical of the 

orthopyroxene in the hornfelses (Page, 1977) and

orthopyroxene of this form and structure is therefore 

considered relict from the hornfelses. Cordierite in the mixed 

rocks retains the characteristics it has in the hornfels but 

locally has rounded or subrounded grain shapes where 

enclosed in plagioclase, instead of the sharp, distinct corners 

it has in the cordierite grain mosaics in the hornfelses. Biotite 

and magnetite have similar shapes in the mixed rock and in 

the hornfels. Green spinel (possibly hercynite) occurs in the 

hornfelses (Page, 1977), but is not common. In the mixed 

rocks, green spinel usually forms fine (<0.x mm diameter) 

inclusions in the center of cordierite crystals or occurs as 

inclusions in poikiloblastic orthopyroxene in positions 

normally occupied by cordierite.

The contacts between the rocks listed in table 6 tend to be 

gradational and irregular rather than sharp, although 

contacts between metasedimentary rocks and mixed rocks 

with poikilitic plagioclase are generally no thicker than the

FIGURE 20. Etched slabs of contaminated igneous rocks. A, Fragments of 

partly reacted metasedimentary rocks (M) in orthopyroxene cumulate 

(be) and in ragged-textured norite (rno). Sample 387-339/240. B, 

Reacted fragment of metasedimentary rock (M), plagioclase phencryst 

(P) at the grain-size boundary and within two orthopyroxene 

cumulates. Sample NB-1/194.5. C, Almost completely reacted fragment (M) in orthopyroxene cumulate. Sample NB-13/490.
32 STILLWATER COMPLEX, MONTANA

I" \HI.K 6 Summarv oj types, textures, and mineralogy of contaminated igneous rocks and their relation to igneous and metasedimcntar\ rocks

Igneous rocks

Ophitic norite .....................................

Subophitic norite J 

Ragged norite I

N

Contaminated or mixed rocks

.................... Large, anhedral poikilitic plagioclase, enclosinganhedral

orthopyroxene (irregular grain shapes of orthopyroxene). 

..................... Large, anhedral, poikilitic plagioclase enclosing anhedral

orthopyroxene, minor cordierite. biotite. magnetite. 

.................... Large anhedral poikilitic Cordierite with green spinel

plagioclase enclosing inclusions, minor orthocordierite and green pyroxene, biotite. magnetite 

spinel. poikilitic plagioclase. 

Large, anhedral poikilitic Large, anhedral poikilitic 

plagioclase enclosing plagioclase enclosing 

green spinel. green spinel and cordierite.

Metasedimentai) rocks

Cordierite+

orthopyro.\ene+ 

biotite+

magnetite 

(+quart/ or

plagioclase).

Orthopyroxene cumulate j 

Ragged clinopyroxenite >

(possible cumulate). ) 

Orthopyroxene cumulate, matrix

of graphically intergrown

plagioclase and quart/., biotite.

and ores. 

Orthopyroxene cumulate, matrix

of micrographic plagioclase

and quart/.

Ophitic gabbro. clinopyroxeneenclosing tabular plagioclase

Graphically intergrown plagioclase and quart/enclosing 

minor cordierite.

Plagioclase forms micrographic intergrowths in quart/ 

grains of metaquart/ite.

Locally layered with 

orthopyroxene or 

cordierite enriched 

bands.

Metaquart/ite 

99 percent quart/, 

minor chlorite. 

magnetite.

crystals of plagioclase. The rnineralogic composition of the 

contaminated rocks is gradational from typical igneous rock 

to metasedimentary rock, which makes it difficult to derive 

average compositions for the mixed rock. Table 7 lists modes 

of two mixed rocks and the adjoining hornfelses. The data 

illustrate the predominance of plagioclase over cordierite in 

the mixed rock and the reverse in the hornfelses. The 

accuracy of the modes may be questioned because of the 

relatively small number of points counted and the 

gradational properties of the rock, but the consistency of 

modes suggests that they represent reasonable approximations.

The lower three types of mixed rocks in table 6 apparently 

occur sparsely but are interesting in that plagioclase is an 

important constituent of two of them. Figures 2GB and C 

illustrate the textures of these two variants. The replacement 

of quartz in the metaquartzite is similar to textures at the 

contact of dikes and quartzite near the Dufek intrusion in

TABLE 7. Modes of contaminated or mixed rocks and the immediately 

adjacent horn/els

[Determined hy point counting in a single thin section per pairxvithagrid I 3mmx I 6mm; variable 

number of points counted because ol limited areas available lor counting)

Rock t\ pe

Sample number... NBI 149.2

Mixed Hornfcls Mixed

Plagioclase .........

Cordierite ...........

Orthopyroxene ... 

Green spinel .......

Opaque minerals 

Biotite ................

Total ...................

Number of points

74.5

17.2

.8

4.7

2.8

71.8

26.5 

........

70.0

22.0

1.2

1.8

5.0

74.6

12.3

6.8

2.5

1.4

2.4

100.0

640

100.0

355

100.0

323

100.0

796

MI7 441: Drill core. Mountain Viev 

NBI 149.2: Drill core. N\e Basin.

Antarctica (A. B. Ford, oral commun., 1969), and textures 

described where the Mashaba igneous complex in Rhodesia 

comes in contact with leucocratic granitic rocks (Wilson, 

1968, p. 76). Ragged clinopyroxenite and ophitic gabbro rich 

in clinopyroxene were observed near hornfels inclusions 

only a few times; the mineral assemblages are strikingly 

different elsewhere in the Basal zone. Although there is no 

textural evidence that these mixed rocks formed by contamination, their unique mineral association suggests this 

origin.

RELATION OF MIXED ROCKS TO 

INCLUSIONS OF METASEDIMENTARY ROCKS

Most of the contaminated igneous rocks observed in this 

study occur around inclusions of metasedimentary rocks, 

therefore it is necessary to digress and describe the inclusions 

at least with respect to their sizes and volumes. Inclusions of 

hornfels range in size from less than a centimeter across to 

approximately 60 by 300 m (exposed on the surface in the 

Benbow area). Larger inclusions possibly occur, especially if 

the east-west-trending outcrop in the Mountain View area 

(pi. 1) is interpreted as an inclusion and not a spine or block 

attached to the basement hornfels. Estimates of the volume 

of included material in the Basal zone are uncertain, but the 

following method offers one approximation. Surfaces of 

drill core amounting to about 6.34 X 106 cm2 of drill core 

were examined and logged and samples with an area of 

about 39140 cm2 were collected. Within the area of the 

samples, 69 inclusions were identified and their sizes, 

including recognizable zones of mixed rock, measured. The 

inclusions range in size, expressed as the average of the 

extreme dimensions, from 0.2 to 6.0 cm. Figure 21 illustrates
CONTAMINATED IGNEOUS ROCKS 33

"0 .4 .8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.0 4.4 4.8 5.2 5.6 6.0

SIZES OF INCLUSIONS, IN CENTIMETERS

FIGURE 21. Sizes of metasedimentary inclusions, expressed as the average 

of maximum length and minimum width. Based on etched core 

samples, maximum core width about 6 cm. N. number ot 

measurements.

the si/e. range, and number of occurrences by size intervals 

of the inclusions in the samples of drill core. Larger 

inclusions than these were probably intersected during 

drilling but. because of the limitations of core widths, are not 

included in this estimate. The average size is 1.68 cm; 

assuming a circle of this diameter as an average for 69 

inclusions yields an area of about 153cm2 . By area or volume 

the Basal zone contains at least 0.4 percent inclusions.

Of the 69 inclusions, 28 showed mixed rock zones less than 

O.I mm wide; the others contained rims of mixed rock 

ranging in width from 0.1 to 3.0 cm; 13 additional zones of 

mixed rock were measured at other contacts. Figure 22 

shows the distribution of the widths of the zones of mixed 

rock. Two different average widths were calculated: One 

average includes those with mixed rock zones less than 0.1 

mm wide, the other, only those with zones wider than 0.1 

mm. It appears that on the average about 0.5 cm of the 

margins of metasedimentary rocks is involved in making 

mixed rocks. Figure 23 illustrates the relation of inclusion 

size and the width of the mixed rock zone.

PROCESSES FOR FORMING MIXED OR 

CONTAMINATED ROCKS

The mineralogic, textural, and petrologic observations 

discussed above demonstrate that the position occupied by 

cordierite in the metasedimentary inclusion is filled by 

plagioclase in the mixed rock or contaminated rock. Because 

the metasedimentary rocks are low in CaO and Na2O(Page, 

1977), these constituents must have been derived from the 

magma from which the igneous rocks crystallized. In turn 

the cordierite in the hornfels must have given up MgO and 

FeO to the magma. All the other hornfels constituents 

appear to remain in the mixed rock. This reaction is similar 

to one suggested by Subramaniam (1956) for the formation 

of cordierite xenoliths in the anorthosites of the Sittampundi 

Complex, India. More recently, Willemse and Viljoen( 1970, 

p. 364-365) proposed calcification of pelitic rock inclusions 

and addition of MgO and FeO to the Bushveld magma on a

cc 

m 4

3 2

Average (/V=82, includes 28 with no zones)=0.33 cm" 

Average (/V=54) = 0.50 cm

/V=82

i

1 at 3.0 ^

ill

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.5 1.6 1.7 1.8 

WIDTH OF MIXED ROCK ZONES, IN CENTIMETERS

FIGURE 22. Widths of mixed rock zones adjacent to inclusions. N, number 

of measurements.

local scale. Such reactions could eventually change the 

composition of the original magma.

To evaluate the magnitude of this process a model was 

developed for the exposed part of the Stillwater Complex. 

Assuming a model volume of magma to form the complex of 

8.2X1.0X42 km3 and using a specific gravity of 2.72, 

calculated from Bottinga and Weill (1970), the weight of 

magma is about 9.4X 10 17 grams. This magma would contain 

about 7.2X 10 16 grams of MgO (see table 5 for average value 

of MgO). If we assume that the total surface area of the base 

of the complex reacted over a thickness of 0.5 cm, and that 

the basal rocks have an average density of 2.81, then 5.9X107 

grams of hornfels would be used to make mixed rock. Of this 

only about 6.4 percent is MgO, and therefore 3.8X106 grams 

of MgO would be added to the magma. This amounts to 

adding about 0.04 parts of MgO per billion to the magma by 

this reaction. The other possible additions and subtractions

" 0 0.5 1.0 1.5 2.0 2.5 

WIDTH OF MIXED ROCK ZONE, IN CENTIMETERS

FIGURE 23. Inclusion si/e (maximum length plus minimum width times 

one-half) plotted against width of mixed rock zone. Plotted line 

represents hornfels totally converted to mixed rock.
34 STILLWATER COMPLEX, MONTANA

to the original magma would be of the same order of 

magnitude. With complete mixing, the production of mixed 

rocks would not produce an easily recognizable change in 

the original magma composition. On the other hand, 

without complete mixing, the basaltic magma would be 

locally enriched with MgO and FeO and depleted in CaO. 

This would force the magma to precipitate more pyroxene, 

iron oxides, or possibly sulfide minerals.

If there is only limited mixing, the surface area of 

metasedimentary rocks exposed to the original magma is an 

important factor because an increase in surface area would 

increase the amount of MgO and CaO available for reaction 

and exchange. Processes associated with the Basal zone that 

increase the surface area of metasedimentary rocks exposed 

to the magma include brecciation, formation of dikes and 

sills, and incorporation of inclusions. Figure 24 shows the 

percentage of hornfels of different sizes that would be 

consumed in reacting with an original magma. For construction of this diagram it has been assumed that: (1) Inclusions 

are spheres, the shape that presents a minimum surface area 

for contact with the magma; and (2) zones of mixed rock

from 0.5 to 3.0 cm thick form from the sphere margins. These 

widths are the average and maximum widths observed in the 

Basal zone. Figure 24 illustrates that as the size of inclusion 

decreases, especially below radii of 20-30 cm, the volume 

percentage of mixed rock derived or contamination per 

inclusion increases rapidly. Although the diagram was 

constructed for the average and maximum widths observed 

in the Basal zone, there is a family of curves of similar shape 

for other widths.

ROCKS DERIVED BY PARTIAL MELTING 

OCCURRENCE

Wisps, clots, lenses, discontinuous veins, and irregularly 

shaped volumes of quartz norite occur within hornfelsed 

metasedimentary rocks immediately below the Basal norite 

member and within the larger inclusions and attached blocks 

of the hornfelses. These quartz norite masses consist of 

orthopyroxene, plagioclase, quartz, biotite, and opaque 

minerals and occur in bodies commonly between 2 and 10 

mm wide, but occasionally as wide as about 5 cm. Most of 

the wisps, lenses, and discontinuous veins have a length five

10 20 30 40 50 60 70 

VOLUME PERCENTAGE OF HORNFELS CONSUMED

80 100

FIGURE 24. Percentage of hornfels that would be consumed to form mixed rocks for various sizes of hornfels inclusions.
ROCKS DERIVED BY PARTIAL MELTING 35

to ten times their width. Many have tabular shapes, but 

others are more like flattened ellipsoids. Figure 25 shows 

several sketches and photographs of lenses, wisps, and veins

EXPLANATION

Quartz norite

Cordicritc-splncl dots 

Homfcls

Massive sulfide

4 cm

D

4 cm i

4 cm

FIGURE 25. Wisps, lenses, and discontinuous veins of quartz norite. A and 

B. Sketches of hand specimen of hornfels showing quart/norite lenses. 

C layered hornfels (hf) with veins of quartz norite (n) and rock 

fragments rich in cordierite (c). D. Hornfels (hf) with irregular patches 

of quart/ norite (n).

taken from etched slabs. The sketches illustrate that a series 

of elongate lenses and discontinuous veins may have a 

semiparallel arrangement but that clots and irregularly 

shaped volumes appear to have no organization. Examples 

of quartz norite veins and lenses that are semiconcordant to 

layering as well as those that crosscut layering have been 

found. Locally the irregular patches, clots, and lenses merge 

and form slightly larger volumes. The quartz norite appears 

to have been mobilized and shows intrusive features in some

Quartz norite veins and lenses were found both in hornfels 

inclusions and in the country rocks in the Benbow, Nye 

Basin, Mountain View, and west of Chrome Mountain areas 

and thus appear to have a widespread areal development in 

practically all the areas in which country rocks and the Basal 

zone are exposed. Within individual sections of hornfels, 

between 5 and 30 percent of the total section contains 

recognizable concentrations of quartz norite veins and 

lenses. Within these segments or areas, the quartz norite 

veins and lenses compose between 1 and 10 percent of the 

country rock. Reconstruction* of the geology suggests that 

the quartz norite veins and lenses in hornfels may extend 

from the nearest Basal norile contact for 120-180 m. As the 

distance from the Basal zone increases, the volume and 

extent of quartz norite decreases.

PETROGRAPHY AM) MINERALOGY

Textures of all the quartz norites are very similar and the 

mineralogy simple. The lenses and veins are composed of 

euhedral to subhedral equidimensional orthopyroxene 

phenocrysts (1-3 mm in diameter) set in a matrix of 

intergrown quartz and plagioclase with scattered biotite 

flakes. Other phases present are pyrrhotite, chalcopyrite, 

pentlandite, and iron oxides. Potassium feldspar occurs in 

the matrix, usually in micrographic intergrowths with 

quartz, but it is not a common constituent in most of the 

lenses and veins. Some of the orthopyroxenes contain 

extremely fine exsolution lamellae parallel to the (001) 

plane. The locally zoned plagioclase occurs in the matrix as 

euhedral to subhedral crystals intergrown with or surrounded by quartz, as graphic, micrographic, or vermicular 

intergrowths with quartz, and as poikilitic crystals that 

enclose orthopyroxene. Quartz may be interstitial or intergrown with plagioclase. Within one lens or patch of 

quartz norite, orthopyroxene, quartz, and plagioclase frequently are uniformly distributed. In a few of the discontinuous veins, plagioclase is the matrix material in one side 

or part of the vein and quartz in another. Sulfide minerals 

generally form clots or discontinuous disseminations at the 

contact of the quartz norite and the hornfels (fig. 25). Apatite 

and clinopyroxene were found in one lens. The textures and 

mineralogy described above indicate that the rock is igneous 

and crystallized from a melt.

M odes of the quartz norites are difficult to obtain because 

of the small areas of quartz norite. Table 8 lists the range and
36 STILLWATER COMPLEX, MONTANA

TABLE 8. Range and average volume percentage of minerals in 

quartz norite in lenses, clots, and discontinuous veins

Mineral

Biotite .........................................

Apatite ........................................

Spinels and sulfide minerals........

Range

29.6-89.4

0-51.0

4.0-19.0

0-15.9

0-3.1

0-1.8

0-.3

0-25. 1

Average 

( 1 1 samples)

65.7

12.6

12.0

2.3

l.C

b.l

average of 11 modes; four based on 400-500 points using a 

one-third millimeter grid and seven on 800-1100 points. The 

average mode of the quartz norites demonstrates dominance 

of orthopyroxene over approximately equal volumes of 

quartz and plagioclase. Figure 26 compares the modes of 

quartz norite rocks with modes of the noncumulate 

igneous-textured rocks. Almost all of the quartz norites have 

different compositions than other noncumulate igneous-textured rocks and are enriched in the orthopyroxene constituents. There are not enough data to determine whether 

the orthopyroxene-rich quartz norites form a gradational 

series with the noncumulate igneous-textured rocks.

Orthopyroxene compositions from six quartz norite veins 

determined by the method of Himmelberg and Jackson 

(1967) average En62 . i and range from En59.4 to En67.6- This is 

comparable with the range of compositions in the hornfels of 

En5()-En7o. Quartz norite plagioclase compositions estimated 

from optical properties and X-ray techniques range from 

An75 to An*,,.

CHARACTERISTICS OF HORNFELSES 

ADJACENT TO QUARTZ NORITE

The most frequently occurring mineral assemblage in 

hornfelsed metasedimentary rocks near the Basal zone is 

cordierite+plagioclase + orthopyroxene + biotite+opaque 

minerals with or without quartz. Near and adjacent to quartz 

norite lenses and discontinuous veins the most frequently 

occurring assemblages are cordierite+orthopyroxene 

+biotite+opaque minerals, cordierite+orthopyroxene 

+opaque minerals, cordierite+orthopyroxene+biotite, and 

cordierite + biotite + opaque minerals. Assemblages containing quartz or plagioclase are extremely sparse. Where 

quartz or plagioclase occurs in the hornfels adjacent to 

quartz norite it is poikilitic and appears to extend outward 

from the quartz norite. However, in one sample of 

cordierite + plagioclase + orthopyroxene + biotite + opaque 

minerals, the plagioclase formed subhedral to anhedral 

grains in a mosaic texture. In addition, near several of the 

quartz norite bodies, there are lenses of the assemblage 

cordierite+green spinel+biotite+opaque minerals. The lack 

of development of the assemblage cordierite+plagioclase 

+orthopyroxene+biotite +opaque minerals with or without 

quartz near the quartz norites and the development of green 

spinel suggest that near the quartz norite lenses and veins,

Plagioclase EXPLANATION

PARTIALLY MELTED ROCKS

A Modes with >500 points 

A Modes with <500 points

•

7~~~ Noncumulate 

/ igneous rocks

Orthopyroxene Quartz

FIGURE 26. Modal compositions in volume percent of quartz norites and 

noncumulate igneous rocks.

the metasedimenta'ry rocks responded differently to 

metamorphic and igneous conditions.

ORIGIN OF THE QUARTZ NORITES

The simple consistent textures and mineralogy of the 

quartz norite and its widespread but sparse occurrence in 

isolated unconnected patches spatially related to the bottom 

of the Basal norite suggest that it was formed by a single 

process. Moreover, the change in mineral assemblages of the 

hornfelses to ones of fewer phases adjacent to the quartz 

norite suggests that the metasedimentary rocks were involved in the process that formed the quartz norite. Two 

reasonable possibilities are presented to explain the origin of 

the quartz norites. The first is that igneous material was 

injected into the hornfelses along fractures; the second is that 

the metasedimentary rocks partially melted to produce 

isolated pockets of liquid, which subsequently crystallized as 

quartz norite.

Injection of basaltic melt, or of magma of quartz norite 

composition produced by differentiation from the Stillwater 

magma, would require a pervasive fracture system within the 

metasedimentary rocks through which the melt could have 

traveled and would require that the fractures healed, 

possibly by annealing, after the melt was injected. The 

restriction of quartz norite volumes to small size, the 

distribution of these volumes as much as 150 to 180 m from 

the base of the complex, and the decrease in abundance of 

quartz norite away from the complex suggest that the 

process was not fracture-controlled. The injection origin 

also requires that either an extreme differentiate of the 

Stillwater magma was formed or that one of the known 

variants of the magma reacted with the metasedimentary 

rocks upon injection to produce magma of the quartz norite
ROCKS DERIVED BY PARTIAL MELTING 37

composition. The fact that no similar compositions are 

known from noncumulate igneous-textured rocks of the 

Basal norite suggests that no differentiated magmas of this 

composition existed. Because of the widespread distribution 

of the quartz norite material, such an event would have 

affected a large volume of the chamber. There is little or no 

evidence that the hornfelses near the quartz norite lenses 

were involved in reactions with a basaltic magma similar to 

those reactions discussed for the mixed rocks. For these 

reasons, an origin by magmatic injection seems unlikely.

Partial melting or fusion of the metasedimentary rocks to 

derive a relatively small amount of quartz norite melt seems 

the only likely means for creating small lenses and pods that 

are not interconnected. The Stillwater magma is a logical 

heat source, although it is difficult to understand the 

distribution of heat outward from the complex in light of 

heat balance studies by Jaeger (1959), Hess (1972), and 

Irvine (1970b) of tabular intrusive bodies and their country 

rocks. The occurrence of the quartz norite in lenses and clots 

is what might be expected if partial fusion occurred where 

the appropriate number of phases were in contact to produce 

a composition that could melt. The distribution and decrease 

in abundance away from the complex also fit the expectation.

The metasedimentary rocks available for melting are 

composed mainly of SiO2 , A1 2O3 , MgO, and FeO with 

minor amount of CaO (Page, 1977). The amounts of K2O 

and Na2O are relatively low. Figure 27 shows the compositions of the metasedimentary rocks projected onto 

various planes in the tetrahedron CaO-MgO-Al 2O3 -SiO2 

and demonstrates the high proportion of SiO2 in the hornfels 

relative to MgO, A1 2O3 , and CaO. The projections also show 

that the quartz norite rocks have a composition distinct from 

the hornfelses and the noncumulate igneous rocks and that 

liquids of quartz norite composition could crystallize to 

form the observed mineral assemblages. The average composition of the quartz norite was calculated from average 

mode (table 8) and determined mineral compositions.

Several low-temperature invariant points exist in the 

systems K2O-MgO-Al 2O3 -SiO2 (Schairer, 1954) and 

CaO-MgO-Al 2O3 -SiO2 (Hytonen and Schairer, 1960; 

Chinner and Schairer, 1962; Schairer and Yoder, 1970). 

Both systems contain phases relevant to the hornfelses and

FIGURE 27. Comparison of compositions of metasedimentary rocks, 

noncumulate igneous rocks, and estimated compositionof quartz 

norite in the tetrahedron CaO-MgO-AhOi-SiCh. A, The 5 percent 

CaO plane. Projection of approximate liquidus surfaces derived from 

data of Osborn, Devries, Gee, and Kraner( 1954) and Prince (1954). B, 

The 10 percent MgO plane. Projection of approximate liquidus 

surfaces after Prince (1954). C, The 15 percent AhOj plane. Projection 

of approximate liquidus surfaces after Osborn, DeVries, Gee, and 

Kraner(1954).

CaO

5% CaO PLANE

EXPLANATION

Metasedimentary rocks 

Noncumulate igneous

rocks 

Average quartz norite

MgO

10% MgO PLANE

MgO

EXPLANATION

METASEDIMENTARY

ROCKS 

o Projected from below

the plane 

Projected from above

the plane 

Average quartz norite

CaO

A1203

Si02 

15%A1203 PLANE /V-MULLITE

EXPLANATION 

METASEDIMENTARY

ROCKS 

o Projected from below

the plene 

• Projected from above

the plane 

Noncumulate igneous

rocks 

& Average quartz norite

CaO
38 STILLWATER COMPLEX, MONTANA

the quartz norite compositions. Consideration of the lower 

temperature invariant points at which liquids would be 

produced in conjunction with the hornfels compositions, 

based on Presnall's (1969) geometrical analysis of fractional 

fusion, shows that only small amounts of melt of uniform 

composition could be produced. This is in agreement with 

the observed amounts of quartz norite. At least three 

invariant points in these systems could correspond to the 

production of partial melts: (1) The eutectic point at 

960°±20° C at which enstatite-tridymite-potassium 

feldspar-cordierite-liquid coexist (Schairer, 1954), (2) the 

invariant point at 1150°±10° C at which enstatite-anorthite 

-diopside-tridymite-liquid coexist (Hytonen and Schairer, 

1960; Schairer and Yoder, 1970), and (3) the invariant point 

at an undetermined temperature at which 

enstatite-anorthite-tridymite-cordierite -liquid coexist. The 

liquid produced at the second invariant point relates most 

closely with the minerals present in the quartz norites. Such 

liquids could eventually crystallize as the assemblage 

plagioclase+quartz+orthopyroxene with diopsidic augite 

exsolution lamellae. As partial fusion continued, one or 

more phases in the adjacent hornfels would be consumed. 

Elsewhere, features ascribed to partial fusion of country 

rocks associated with mafic intrusive rocks have been 

described. Cribble (1966, 1967, 1968, 1970) and Cribble and 

O'Hara (1967) interpreted the cordierite norites in 

northeastern Scotland as having originated by fusion; other 

examples of partial fusion have been reported by Wyllie 

(1959) and Argell (in Chinner and Schairer, 1962, p. 633). 

Cribble and O'Hara (1967) used the invariant point 

enstatite-anorthite-tridymite-cordierite-liquid to explain the 

composition of their rocks from Scotland. Their analytical 

data on the material to be melted, the interpreted partial 

melts, and the residual materials show higher amounts of 

K2O and Na2O than in the Stillwater example, which would 

explain the dissimilarities between the rocks in Scotland and 

the Stillwater example in which quartz norites were produced.

OTHER SILICATE ROCKS

Two other types of rocks composed mainly of silicate 

minerals were recognized in the Basal norite. Discussion of 

their distribution, abundance, texture, and mineralogy is 

limited by the few samples found. One type, which exhibits 

textures and mineralogy resembling ultramafic xenoliths 

and basalts or rocks from alpine ultramafic masses, was 

found only in samples of drill core of the lower part of the 

Basal norite in the Iron Mountain and Chrome Mountain 

areas. The other type, which has compositions similar to the 

Basal norite and exhibits granoblastic textures imprinted 

upon cumulate or ophitic textures, locally occurs intermixed 

with cumulates and noncumulates of the Basal norite from 

the Benbow area to and including the Mountain View area 

and in the area west of the Boulder River. The first type is 

tentatively labeled ultramafic xenoliths and the other type, 

autohornfelsed Basal zone. It is not known whether the

xenoliths have an origin cognate or external to the Stillwater 

Complex.

I'LTRAMAFIC: XENOLITHS

Rocks classified as ultramafic xenoliths occur as discrete 

fragments that are distinguished from the enclosing 

orthopyroxene cumulate matrix by texture and mineralogy. 

Three groups of xenoliths are present: (1) Websterites 

composed of orthopyroxene and clinopyroxene, (2) dunites 

composed of olivine and minor amounts of orthopyroxene, 

and (3) orthopyroxenites composed of orthopyroxene and 

plagioclase. The host rock for all known xenoliths is 

orthopyroxene cumulate composed of euhedral to subhedral 

orthopyroxene with En63 . 5 to En66 .5 surrounded by postcumulate plagioclase, minor clinopyroxene, biotite, and 

opaque minerals. These are similar to all other orthopyroxene cumulates (table 3) in appearance, mineralogy, texture, 

and composition. They also seem to fit into any trends 

established in the section of the Basal norite in which they 

occur.

The websterites (fig. 28/1) typically contain orthopyroxene crystals that are 6 to 11 mm in diameter, anhedral, and 

interlocking, show undulatory extinction in thin section, 

and extinguish in domains that appear to have been rotated 

or moved slightly in relation to one another. Rims of larger 

grains generally lack exsolution lamellae, and exsolution

FiGi'Ri-: 28. Photomicrographs of ultramafic xenoliths. A, Websterite 

showing large strained orthopyroxene with a necklace of smaller 

orthopyroxene. B, Dunite in orthopyroxene cumulate.
SULFIDE-BEAR1NG ROCKS 39

lamellae parallel to the (001) plane of orthopyroxene are 

irregularly developed throughout single crystals. Larger 

orthopyroxenes are partly to totally surrounded by 

necklaces of smaller (0.15 to 0.5 mm diameter) subhedral to 

anhedral orthopyroxene grains without exsolution lamellae. 

The orthopyroxene necklaces are several grains wide and, 

where developed in patches, show a granoblastic or mosaic 

closely packed texture. Clinopyroxene makes up 5-20 

percent of the xenolith and occurs in the necklaces as small 

anhedral grains without exsolution lamellae. Plagioclase 

makes up 1-5 percent of the xenolith, occurs as interstitial 

material, and has altered to epidote where it is near fractures. 

A clear clinoamphibole occurs near fractures where it 

replaces both clino- and orthopyroxene. The opaque 

minerals make up less than 1 percent of the xenolith and are 

dominantly sulfides that occur along grain boundaries or at 

grain junctions.

The texture of the websterites is interpreted as 

metamorphic, with strained and possibly granulated 

orthopyroxene that recrystallized along with clinopyroxene to form necklaces. Such textures are similar to those in 

xenoliths from basalts (White, 1966; Jackson, 1968b) and in 

alpine ultramafic rocks (Loney and others, 1971). Their 

formation within the Stillwater magma chamber would 

require a peculiar sequence of events, and I suggest that the 

websterite xenoliths may have had an external source.

The dunite xenoliths contain 70-90 percent by volume 

subhedral olivine (0.5-0.8 mm in diameter) with a 

granoblastic texture. Most of the olivine is altered to 

serpentine, talc, and iron oxides and thus internal features of 

the olivine grains have been destroyed. Other constituents of 

the xenoliths are 5-20 percent plagioclase, 5-15 percent 

anhedral to subhedral orthopyroxene, less than 1 percent 

biotite, and 3-5 percent opaque minerals. The dominant 

opaque phase is subhedral to euhedral spinel, the thinner 

edges of some grains showing brown or green colors. Inside 

the xenoliths the plagioclase is interstitial to the mafic 

minerals but is most abundant near the margins of the 

xenoliths where it has a poikilitic texture enclosing olivine 

grains. Poikilitic plagioclase crystals near the margins of the 

xenoliths extend into the orthopyroxene cumulate.

Clots of olivine and single crystals of olivine from the 

Basal norite in the eastern sections of the complex show 

undulatory extinction in thin section. Some olivine crystals 

contain parallel bands of slightly different extinction position called kink bands (Raleigh, 1965a, b). Although kink 

bands are not observable in the olivine of the dunite 

xenoliths because of serpentinization, their presence in 

olivine xenocrysts suggests that there was a source of 

deformed olivine. It is possible that the xenoliths are this 

source.

The orthopyroxenite xenoliths have a granoblastic mosaic 

texture and contain 80-90 percent by volume of granular 

anhedral orthopyroxene (average diameter 0.5 mm), interstitial plagioclase, opaque minerals, and a few flakes of

biotite. The orthopyroxene contains no visible exsolution 

lamellae and shows no evidence of having been strained. 

Margins of the xenoliths are similar to those of the dunite 

xenoliths in that they have poikilitic plagioclase extending 

both inward and outward from their contact with the 

orthopyroxene cumulate host.

Two possible origins for the dunite and orthopyroxene 

xenoliths are suggested. First, the xenoliths represent 

ultramafic material from a source external to the Stillwater 

magma that was caught up in the magma and reacted with it 

to form poikilitic plagioclase. Second, the xenoliths are 

cognate and crystallized elsewhere in the magma chamber, 

were mechanically incorporated in the magma crystallizing 

the cumulate host rock, and reacted with it to form the 

poikilitic plagioclase. The granofelsic mosaic texture in 

either type of inclusion only implies some annealing and 

rearrangement of the grain boundaries and is consistent with 

either hypothesis. Although ultramafic xenoliths are rare in 

tholeiitic basalts, they have been reported in the norite near 

the base of the Sudbury Nickel Irruptive in the Strathcona 

ore deposit (Naldrett and Kullerud, 1967).

At TOHORNFELSED RASAL ZONE ROCKS

About a dozen samples of the Basal norite collected from 

drill core and outcrops megascopically appear to be typical 

cumulate or noncumulate rocks but in thin section have 

quite a different aspect, in that the plagioclase forms a 

granoblastic mosaic regardless of whether its texture was 

originally ophitic or poikilitic. In some of the rocks the 

ortho- and clinopyroxenes are granulated along their 

margins and appear to have recrystallized. These 

recrystallized textures are similar to those in the 

metadolerite dikes from the Beartooth terrane (Page, 1977). 

Comparison of the textures in the metadolerite dikes and in 

the hornfelsed metasedimentary rocks with these textures in 

the Basal zone suggests that the granoblastic mosaic feldspar 

and the granulated pyroxene were derived by a metamorphic 

process. It is suggested that these textures result from 

autometamorphism of precrystallized rocks (cognate 

xenoliths) mechanically transported and annealed by the 

Stillwater magma, or local annealing of rocks that crystallized in place.

SULFIDE-BEARING ROCKS

()C:C:t RRENCE, GENERAL TERMINOLOGY, 

AM) DISTRIBUTION

All rocks within the Basal zone contain sulfide minerals. 

By far the majority of Basal zone samples contain less than 1 

percent sulfide, although higher concentrations, even 100 

percent, are not uncommon. To cover this wide range of 

sulfide content, a terminology was developed on the basis of 

natural divisions, resulting in three types of sulfide concentrations: Disseminated sulfide, matrix sulfide, and massive 

sulfide rock.
40 STILLWATER COMPLEX. MONTANA

The occurrence and distribution of sulfide mineralization 

in and adjacent to the Basal zone are difficult if not 

impossible to determine on the basis of surface exposures 

and must await extensive studies of subsurface information. 

However, several generalizations about the distribution of 

mineralization can be made from the extent and development of gossans, sulfide rock exposures, and early prospecting. (1) The mineralization is not homogeneously distributed within nor is it limited to the Basal zone. Locally, 

sulfide concentrations occur within the metasedimentary 

rocks and within the lower part of the Peridotite member of 

the Ultramafic zone. (2) The amount of mineralization varies 

along the strike of the complex and, as shown by surface 

exposures and gossans, is concentrated in the following 

areas (pi. 1): Benbow, Nye Basin, Mountain View, Initial, 

Crescent Creek, between Blakely and the unnamed creek to 

the south, and west of the Boulder River. Other areas with 

fair exposure appear to contain less sulfides, but many areas 

that may contain mineralization are not exposed, owing to 

either thrusting or covering Quaternary deposits. (3) The 

amount of mineralization varies vertically in the Basal zone; 

stratigraphically lower parts of the section, mainly the Basal 

norite, tend to have higher concentrations of sulfide 

minerals. (4) Areas believed to represent depositional basins 

(fig. 4) tend to have higher concentrations of sulfides than 

the interbasinal areas. Moreover, sulfide mineralization 

appears to increase toward the centers of the basin areas. (5) 

Sulfide concentrations in metasedimentary rocks occur as 

pods and lenses, usually associated with rocks of noritic 

mineralogy and composition. (6) No sulfide concentrations 

have been shown to be localized by faulting. (7) Some areas 

in the Basal norite that are characterized by mixed rocks, 

hornfels inclusions, and intrusive breccias tend to contain 

relatively more mineralization.

An example of the vertical distribution of sulfide 

mineralization in the Basal zone is given in figure 29. The 

section shows estimates of maximum sulfide amounts 

present within about a meter of section, a measure easier to 

judge than average sulfide content. Even though this section 

may not be typical of the Basal zone, several generalizations 

which appear valid for the whole zone may be made from 

figure 29. (1) Within the Basal zone, the Basal norite is 

enriched in sulfide minerals relative to the Basal bronzite 

cumulate. (2) Noncumulate, igneous-textured, ophitic and 

subophitic rocks tend to have relatively small amounts of 

sulfide minerals but commonly are followed stratigraphically upward in the section by concentrations of sulfide 

minerals in the cumulate rocks. (3) No particular cumulate 

rock lithology appears to have more sulfide minerals than 

another, suggesting that the accumulation process is not 

closely related to a specific magma chemistry. (4) Increases 

and decreases in maximum sulfide mineral content appear to 

form cycles which may be related to repetitions of noncumulate and cumulate lithologies. (5) Massive sulfide layers 

tend to be relatively thin about a meter thick.

ESTIMATED 

Maximum Volume Percent Sulfide Minerals

-ri ' i °

CMlA «-

lit I

± ±

III!

-20

-30

EXPLANATION

Olivine and chromite cumulates (Ultramafic zone)

Orthopyroxene cumulate

Inverted pigeonite cumulate

Plagioclase cumulate

Orthopyroxene-plagioclase cumulate

Inverted pigeonite-plagioclase cumulate

Clinopyroxene-olivine cumulate

Orthopyroxene-olivine cumulate

Noncumulate mafic rocks

Intrusive breccia, metasediments

Meta sediments

Intrusive quartz monzonite sequence

FIGURE 29. Estimated maximum volume percentage of sulfide minerals in 

a stratigraphic section of the Basal zone in the Mountain View area.
SULFIDE-BEARING ROCKS 41

DEVELOPMENT OF TERMINOLOGY FOR 

St'LFIDE-BEARING ROCKS

Various names and terms such as massive sulfide ore, 

matrix sulfide (Ewers and Hudson, 1972), net-textured ore 

(Naldrett, 1969), and disseminated sulfide have been used by 

other authors to describe the range in proportions of sulfide 

and silicate minerals in mafic and ultramafic rocks. Consideration of the variation in the amount of sulfide minerals 

present and their degree of interconnection suggested that 

there might be a set of natural boundaries upon which to 

base a terminology for sulfide mineralization in the 

Stillwater Complex.

To examine this possibility, modal volume percentages of 

sulfide minerals were determined by point-counting on 

ground rock slabs or polished sections. On the same sections 

or slabs the electrical resistance (conductivity) was measured 

between numerous sulfide grains (Kanehira, 1966) to determine the extent of interconnection among them. In selecting 

specimens for this analysis, relatively few samples were 

chosen whose visual estimates indicated less than 5 percent 

or more than 95 percent sulfide minerals present. Thus, the 

data are most representative in the intermediate concentration ranges and should not be considered to represent the 

natural frequency of sulfide mineral occurrence in the Basal 

zone. As noted, the natural frequency distribution would 

have a very large maximum in the 0 to 5 percent range. 

Figure 30 illustrates that sulfide-bearing rocks can be broken 

into three distinct groups, separated by the minima between 

5 and 10 and 55 and 70 percent sulfide minerals.

Measurements of electrical resistivity between sulfide 

grains indicate that in all the samples that contain more than 

10-15 volume percent sulfides, the sulfide minerals are 

continuously interconnected at least within the volumes

35

30

25

20

15

10

10 20 30 40 50 60 70 80 

VOLUME PERCENT SULFIDE MINERALS

90 10(

FIGURK 30. -Modal volumes of sulfide minerals in ISO selected samples.

represented by the slabs. At sulfide concentrations below 10 

percent, sulfide minerals are interconnected within volumes 

from less than 1 mm to 1-2 cm in diameter (Page, 1971b). 

Ewers and Hudson (1972, p. 1079) reported a somewhat 

higher level for sulfide continuity; rocks with more than 20 

percent sulfide minerals showed electrical continuity. The 

break between continuous sulfide interconnection over 

indeterminate volumes and the much smaller continuous 

areas of 1 -2 cm and less, appears to be a natural break which 

is useful for developing a terminology. Rocks containing less 

than 10 percent sulfide minerals are called by their rock 

name and are said to contain disseminated sulfide minerals.

Rocks containing between 10 and 60 volume percent 

sulfide minerals are called by their rock name and said to 

contain matrix sulfide minerals. In these rocks the sulfide 

minerals occupy interstices in the cumulus silicate phases in 

an exactly analogous texture to that of postcumulate silicate 

phases in normal cumulate rocks. The silicate minerals 

enclosed in the sulfide matrix have the same appearance and 

properties as cumulus phases in typical cumulate rocks. 

Estimates of initial porosity of cumulates range from 20 

percent (Wager and Deer, 1939) to 45 percent (Wager and 

others, 1960). Jackson (196la) reported porosities between 

15 and 50 percent with an average of 35 percent in the 

Stillwater Complex, depending on the cumulate rock type. 

Measurements of beach sand porosities (Eraser, 1935) are in 

accord with the estimates of initial porosities of cumulate 

rocks, and calculations of ideal porosities for various 

packing arrangements of spheres range from 25.9 to 47.6 

percent (Graton and Eraser, 1935). The lack of rocks with 55 

to 70 percent sulfides appears to be correlated with a similar 

lack of normal cumulate rocks with these initial porosities, 

and it appears that the silicate host fixes a natural upper limit 

for the matrix sulfides. The lack of rocks with 20 to 30 

percent sulfides may also be related indirectly to porosity. 

Cumulate rocks containing relatively small amounts of 

interstitial silicate or sulfide minerals frequently show 

extensive overgrowths on the cumulus minerals that tend to 

decrease the initial porosity. Overgrowth on silicate minerals 

requires an adequate supply of silicate magma and abundant 

diffusion paths. Such conditions imply a correspondingly 

small amount of sulfide liquid at the sites of postcumulus 

development, with the implication that conditions in this 

range of porosity of cumulate rocks do not allow concentrations of sulfides.

Rocks containing between 70 and 100 volume percent 

sulfide minerals are termed massive sulfide rocks because the 

framework of these rocks is composed of sulfide minerals. 

Because silicate minerals or rock fragments are not essential 

components in the massive sulfide rocks, their formation 

and sulfide content are not directly related to processes of 

silicate crystallization.

TEXTURES OF SULFIDE-BEARING ROCKS

The large number of apparent textural varieties displayed 

by the sulfide-bearing rocks in the Basal zone and probably
42 ST1LLWATER COMPLEX, MONTANA

throughout the entire Stillwater Complex can be divided 

into two major groups. The first group, composed of 

disseminated and matrix sulfide rocks, has textures controlled by the shapes of interstices between cumulus minerals 

in cumulate rocks or interstices in the ophitic and subophitic 

noncumulate rocks. The second group consists of the 

massive sulfide rocks and has the texture of the sulfide 

framework; the silicate minerals or rock fragments enclosed 

by sulfide are incidental to the texture. This section of the 

report considers only the textures formed by the sulfide 

content regardless of the mineralogy and the silicate 

fraction; textural relations between various sulfide minerals 

are considered in the section entitled, "Mineralogy of the 

sulfide-bearing rocks."

DISSEMINATED AND MATRIX St'LFIDE TEXTURES

Textures of disseminated and matrix sulfide-bearing 

rocks are controlled by the shapes of the interstices between 

cumulus minerals. Interstices take the shape of branching 

concave polygons whose sides are formed by the crystal faces 

of the cumulus minerals. Cross sections of the interstices, as 

viewed in polished sections, approximate the cuspoid shapes 

of the voids resulting from the systematic packing of spheres 

(Graton and Fraser, 1935, p. 810, 815, 852-857). Jackson 

(196la, p. 65 and following pages) discussed the interstitial 

shapes in rocks from the Ultramafic zone, and his discussion 

is applicable to the Basal zone. Jackson demonstrated that 

the shapes of the interstices are altered wherever secondary 

enlargement or overgrowth of cumulus phases occurs or 

when there has been reaction replacement of the cumulus 

phases. No evidence for reaction, replacement, or 

overgrowth textures involving sulfide minerals has been 

observed, however; disseminated or matrix sulfides tend to 

preserve the original polygonal shapes of the interstices, 

unless the whole rock was involved in later alteration. Figure 

31 depicts in photographs and tracings from slabs the typical 

textures exhibited by disseminated and matrix sulfide 

minerals in cumulate rocks. The dashed circles superimposed on figure 3\A show areas within which the disseminated sulfide minerals are continuously interconnected.

Considered within the framework of processes for forming cumulate rocks, the textures exhibited by the disseminated sulfide minerals suggest possible methods of 

deposition. The occurrence of sulfides in polygonal shapes, 

molded onto cumulus minerals, implies that sulfide minerals 

crystallized after the cumulus silicates and that most of the

sulfide crystallization history must be a postcumulus 

process. A similar argument based on common grain 

boundaries with most postcumulus silicates implies that the 

sulfide minerals crystallized after most postcumulus 

silicates. Experimental evidence on crystallization 

temperatures of silicate and sulfide minerals supports these 

inferences.

The sulfide minerals that appear as clots or groups of 

grains in two dimensions but that prove to be continuously 

interconnected in three dimensions suggest concentration of 

sulfide material prior to postcumulus crystallization. At 

least two possible models for concentration can be envisioned: (1) A postcumulus process in which sulfide 

material accumulates by diffusion from the magma above 

the cumulus crystal pile into pore spaces; and (2) a mainly 

cumulus process in which an immiscible sulfide liquid forms 

droplets and coalesces into masses of liquid large enough to 

settle as a cumulus liquid phase which displaces the 

interstitial silicate magma in the crystal pile before the 

postcumulus silicate crystallizes. The postcumulus process 

requires intricate timing of events the sulfide must accumulate before postcumulate silicate crystallization and 

it requires a variable amount of sulfide in solution in the 

magma to account for the volume variation of sulfide 

mineral. The cumulus process requires only a variable 

supply of droplets, possibly controlled by the time taken to 

coalesce into masses that could settle. Evidence for diffusion 

processes has not been found, and the timing of the event 

suggests that such evidence should be available. Therefore 

the hypothesis of a cumulus process is preferred for the 

origin of disseminated sulfides.

Figures 31 D-H show the textures exhibited by cumulate 

rocks with a range of matrix sulfide content from 13 to about 

50 volume percent. All matrix sulfides fill interstices with 

continuity over large volumes. Figures 31£and //illustrate 

the irregular contacts between barren rocks and cumulates 

with matrix sulfides; the slabs in figures 31 F and G show 

comparable pore-filling textures between sulfide and silicate 

material in orthopyroxene and plagioclase cumulates. It is 

thought that the processes responsible for the textures 

exhibited by the matrix sulfides are the same as for the 

disseminated sulfide minerals except that the supply of 

sulfide material was much larger than for the disseminated 

ores. Any model of origin of the matrix sulfides must be 

constrained by the following observations: (1) There is no 

noticeable relation between the volume of sulfide matrix and

FIGURE 31. Textures of disseminated and matrix sulfide minerals in cumulate rock slabs. A, Disseminated sulfide minerals in 

medium-grained orthopyroxene plagioclase cumulate (BB-3/378.5). Sulfide minerals, black; dashed circles enclose continuously 

interconnected sulfide minerals. Traced from ground and etched slab. B, Disseminated minerals (NBI3347). Width of sulfide grain 2 

mm; po.pyrrhotite; pn.pentlandite. C, Part of rock slab with matrix sulfide minerals in orthopyroxene cumulate (387-339/228). Black, 

sulfide minerals. D, Rock slab of orthopyroxene cumulate containing about 13 percent matrix sulfide minerals (368-307A/561). Light 

gray, sulfide minerals. E, Rock slab of orthopyroxene cumulate containing 46 percent matrix sulfide minerals (368-307A/772.5). Light 

gray, sulfide minerals. F, Rock slab of plagioclase cumulate containing matrixsulfideminerals(NB13/295); s,sulfide; o.orthopyroxene; 

p, plagioclase. G, Rock slab of orthopyroxene cumulate containing 48 percent matrix sulfides in right half of the section 

(368-307A/542). Specimen shows irregular contact between matrix sulfide-bearing cumulate and silicate interstitial material. Light 

gray, sulfide minerals.
SULFIDE-BEARING ROCKS 43

2 cm I___2 cm 2 cm
44 STILLWATER COMPLEX, MONTANA

the type of cumulate mineral or minerals forming the 

framework; (2) there is no evidence of reaction between the 

sulfide matrix and postcumulus silicate minerals or at the 

margins of lenses or layers of matrix sulfide; and (3) the 

matrix sulfides are molded tightly to the cumulus silicate 

minerals generally there are no intervening rims of postcumulus silicate phases.

TEXTURES OF MASSIVE SULFIDE ROCKS

A variety of textural features are found in the massive 

sulfide rocks (fig. 32), which generally contain cumulus 

silicate minerals, rock fragments, or both. Orthopyroxene 

and plagioclase occur in the massive sulfide rock as euhedral 

to subhedral phenocrysts that resemble silicate crystals of 

cumulate rocks (figs. 32,4 and B). Rock fragments of 

metasedimentary rocks, orthopyroxene cumulates, or both 

are engulfed and surrounded by massive sulfide material (fig. 

32D). Both types of rock fragments appear to have been 

mechanically incorporated. One process by which fragments 

of hornfelses were incorporated into the massive sulfide rock 

appears to have been brecciation (figs. 32£"and F). Hornfels 

fragments in these samples have margins against sulfide that 

on adjacent hornfels fragments approach puzzle-shaped 

pieces of-one another, and it appears that the original 

metasedimentary rock could be put back together without 

the massive sulfide. In other instances fragments produced 

by such brecciation appear to have been moved or mixed by 

magmatic currents and gravity to their present positions.

As figure 32,4 illustrates, the only difference between 

massive sulfide rocks and cumulates with matrix sulfide 

minerals is in the amount of sulfide present. The contact 

between types of sulfide-bearing rocks may be sharp and 

irregular (fig. 32,4) or gradational (lower part of specimen in 

fig. 32B). The layering of the samples (fig. 32,4, E) is typical 

of many of the massive sulfide rocks. Hornfelsed 

metasedimentary rock fragments engulfed or enclosed by 

massive sulfides may have irregular shapes (upper part of 

specimen in fig. 325), lensoid shapes with tapering tails (fig. 

32C), or angular to subangular blocky shapes (figs. 32Z)and 

F). Many of the metasedimentary fragments are enriched in 

cordierite, but others have mineralogic features like the 

underlying metasedimentary rock basement. Locally sulfide 

material tends to penetrate some of the rock fragments along 

fractures or foliation planes of the metasedimentary rock. 

Fragments of cumulate rocks tend to be angular to 

subangular (fig. 32D).

Any model of the origin of the massive sulfide must be 

constrained by the following observations: (1) Generally no 

other silicate material separates the massive sulfide and the 

phenocrysts or rock fragments, (2) there are no observable 

reactions between silicate minerals and the sulfides at the 

edges of massive sulfide lenses and layers, and (3) massive 

sulfide rocks usually have sharp contacts with matrix sulfide 

cumulates but may have contacts that appear to consist 

merely of a change in sulfide content.

TEXTURAL RELATIONS BETWEEN SILICATE 

AND SULFIDE MINERALS

Little evidence of replacement of primary igneous silicate 

minerals by sulfide minerals has been observed in the three 

sulfide-bearing rocks. The silicate minerals, around which 

the sulfides are molded, retain the characteristic euhedral to 

subhedral crystal margins that they exhibit in unmineralized 

rocks. Sulfide veins rarely cut silicate minerals, and where 

they do the sulfide minerals appear to be fracture fillings 

because the margins of the fractures generally match. The 

regular and smooth margins of sulfide masses against silicate 

minerals, whether the silicate is a cumulus, postcumulus, or 

noncumulate igneous phase, are evidence that replacement 

of primary silicate minerals by sulfide minerals probably did 

not occur.

In mixed rocks where sulfide minerals are present, 

replacement of silicates and oxides is more common. 

Pyrrhotite forms pseudomorphs after magnetite and spinels 

of the metasedimentary part of the mixed rock, and sulfide 

minerals, mainly pyrrhotite and chalcopyrite, penetrate 

silicate minerals along fractures whose margins do not 

match. However, these types of replacements are minor.

MINERALOGY OF THE SULFIDE-BEARING ROCKS

The overall mineralogy of the sulfide-bearing rocks is 

relatively simple, and except for samples from surface 

exposures the sulfides in specimens of sulfide-bearing rock 

are not altered. Locally where silicate minerals are altered to 

amphiboles, serpentine, and other silicate minerals, the 

primary assemblages of sulfide minerals appear to have been 

remobilized, possibly changing the compositions of the 

phases but not altering the primary phase to another species. 

The following table lists the minerals present in the 

sulfide-bearing rocks. Because the sulfide minerals do not 

appear to vary in number or kind, but only in amount and 

proportions, the sulfide minerals from all three texture types 

can be considered together. Pyrrhotite, pentlandite, and

FIGURE 32.- -Rock slabs of massive sulfide rocks. A, Sharp, irregular boundary between massive sulfide rock, lower right side, and 

orthopyroxene cumulate with matrix sulfide. Sulfide minerals have continuity throughout entire sample (389-339/303.5). B. Massive 

sulfide rock, containing hornfels inclusion, and showing layering of massive sulfide and orthopyroxene phenocrysts (387-339/53.5). C, 

Massive sulfide rock enclosing hornfelsed metasedimentary rocks and orthopyroxene phenocrysts (387-339/49). D, Massive sulfide 

rock enclosing fragments of hornfelsed metasedimentary rocks and orthopyroxene cumulates. Adjacent tracing shows distribution of 

larger hornfels and cumulate fragments (M20/ 355.0). E, Massive sulfide rock engulfing fragments of brecciated hornfelsed 

metasedimentary rock (387-339/398.5). F, Massive sulfide rock engulfing fragments of brecciated hornfelsed metasedimentary rock. 

Upper part of massive sulfide is rich in chalcopyrite (368-307A/595).

O
SULFIDE-BEARING ROCKS 45
46 STILLWATER COMPLEX, MONTANA

chalcopyrite are the dominant primary sulfide minerals. 

Local alteration has produced small amounts of bravolite, 

violarite, mackinawite, marcasite, pyrite, cuprite, and native 

copper. Earlier mineralogic studies by Howland (1933) 

established the presence of most of the phases; this report 

documents their occurrence in the Basal zone. Oxide 

minerals are primarily magnetite with exsolved ilmenite, 

ilmenite, chromite, and local goethite.

Sulfides Oxides

Primary

Pyrrhotite 

Monoclinic 

Hexagonal

Pentlandite 

Chalcopyrite 

Gersdorffite

Magnetite Chromite 

Ilmenite 

Magnetite-ilmenite intergrowths 

Ilmenite-hematite intergrowths

Alteration

Bravolite 

Violarite 

Mackinawite

Marcasite 

Pyrite

Magnetite 

Goethite

Cuprite 

Native copper

PYKKHOHTK

Both hexagonal and monoclinic pyrrhotite are present in 

all the specimens of sulfide-bearing rocks from the Basal 

zone. This might have been expected based on Desborough 

and Carpenter's (1965, p. 1437) previous report of both 

phases in one sample from the Mouat mine area, but in the 

context of studies by Cowan (1968) on the Strathcona ore 

deposit and Haynes and Hill (1970) on the Renison Bell 

deposit, the consistent proportions and relations of the 

phases to one another in .the Basal zone are unexpected. The 

hexagonal phase is dominant in all the sections examined, 

but it is everywhere accompanied by the monoclinic phase 

(10-20 percent of total pyrrhotite).

In well-prepared polished sections, both phases can be 

observed without etching. The more reflective lighter hexagonal phase is the host for fine (10-15jum in width) lamellae 

of the less reflective darker monoclinic phase. Immersion in 

a 57-percent solution of hydriodic acid for 15-30 seconds 

increases the contrast between the phases by etching the 

hexagonal phase (Vaughn and others, 1971, p. 1135) and 

makes it possible to distinguish the phases in poorer polished 

sections (fig. 33). Finely ground magnetite applied as a 

suspension adhered to the unetched lamellae, demonstrating 

the ferromagnetic properties of the monoclinic phase. 

Diffractograms over the "d( 102)" reflection (Desborough and 

Carpenter, 1965) for bulk pyrrhotite samples generally 

contain only the reflection of the hexagonal phase, although 

a few samples showed a low-intensity peak for the 

monoclinic phase. This confirms the identification and 

supports the estimates of the monoclinic phase present as 

intergrowths.

Page (1972) reported electron microprobe analyses of 

some pyrrhotites from the Basal zone (C-395'/2, table 1, p. 

815) and obtained values of atomic percent metals between 

46.2 and 47.5. Some of the variation in composition may be 

ascribed to the possibility that the analyses represent

FIGURE 33. Monoclinic and hexagonal pyrrhotite. Etched in 57 percent 

hydriodic acid. M 14/434.5. Lamellae approximately 10)um across.

mixtures of hexagonal and monoclinic phases. The compositions of pyrrhotite from samples showing no monoclinic 

reflections were determined for the hexagonal phase at a 

number of localities using the X-ray diffraction method of 

Arnold (1962, 1967), Arnold and Reichen (1962) and 

Toulmin and Barton (1964). Glass-slide smear mounts were 

oscillated at '/4° per minute through two cycles between the 

"d{ 102)" and the halite (220) reflections (halite annealed for 3 

days at 500°C. a0=5.6400 A) on a diffractometer using Ni 

filtered Cu Ka radiation. The smear mounts were rotated 

180° between cycles. The results of this study are shown in 

table 9. The compositions given are of hexagonal pyrrhotites 

after their inversion to the hexagonal-monoclinic mixture. 

Consideration of compositions of hexagonal pyrrhotite 

given in table 9 and the range of compositions of natural

TABLE 9. Compositions of hexagonal pyrrhotite determined by X-ray 

diffraction

[Atomic percent metals read from Arnold and Reichen's (1962) curve]

Sample 

number

Ml 4/226

Ml 4/253.5

Ml 4/260

Ml 4/262.3

Ml 4/275

M 14/287

Ml 4/292.5

M 14/339.5

M14/391

Ml 4/434.5

Ml 4/463.2

M14/491.5

BB-7/819

BB-//855

BB-7/860

BB-7/863

BB-7/884

BB-7/914

BB-7/937

BB-3/183

(°i0.0005°)

2.0697

2.0695

2.0686

2.0696

2.0705

2.0701

2.0694

2.0705

2.0705

2.0702

2.0701

2.0702

2.0646

2.0672

2.0632

2.0663

2.0655

2.0637

2.0681

2.0659

Atomic percent 

metals (iO.05- 

0.075 percent)

47.61

47.59

47.51

47.60

47.67

47.64

47.58

47.67

47.67

47.66

47.64

47.66

47.18

47.4

47.35

47.3

47.25

47.1

47.48

47.23

Sample 

number

BB-3/233

BB-3/259

BB-3/352

355-1/141.2

355-1 /1 77 A

355-1/177

355-1/286

355-1/389.4

355-1/555.5

355-8/190

355-12/240

355-14/226

368-307 A/ 595

368-307/773.5

368-507/561

387-339/49

387-339/53.5

387-339/303.5

389-339/398.5

Atomic percent 

<K 102') metals (±0.05- 

(°+0.0005°) 0.075 percent)

2.0677 47.5

2.0668 47.38

2.0655 47.25

2.0672 47.4

2.0663 47.25

2.0663 47.25

2.0672 47.4

2.0672 47.4

2.0641 47.15

2.0681 47.48

2.0681 47.48

2.0681 47.48

2.0686 47.5

2.0695 47.6

2.0708 47.7

2.0708 47.7

2.0699 47.62

2.0681 47.48

2.0677 47.5
SULFIDE-BEARING ROCKS 47

pyrrhotite summarized by Misra and Fleet (1973, p. 530) 

allows an estimation of the pyrrhotite composition before 

inversion if the relative amounts of hexagonal and 

monoclinic phases are known. In figure 34, atomic percent of 

metals in pyrrhotite is plotted against percent of the 

hexagonal phase present. All bulk pyrrhotite compositions 

must lie between the sloping lines for the compositions of 

hexagonal pyrrhotite in table 9 and monoclinic pyrrhotite 

(Misra and Fleet, 1973). Estimates of the percentage of the 

hexagonal pyrrhotite range between 10 and 20 percent, and 

therefore the bulk composition of pyrrhotite before exsolution is estimated to fall within the stippled area of figure 34. 

This approach lends support to the supposition that the 

microprobe analyses of Page (1972) were done on mixtures 

of the hexagonal and monoclinic phases.

Pyrrhotite is usually the most abundant sulfide mineral 

present in all three types of sulfide-bearing rocks, and it 

occurs in four distinctive textures. In the most common 

texture, the shapes of the pyrrhotite grains in contact with 

silicate minerals are controlled by the crystal shapes of the 

silicate minerals and may be straight, gently curving, or 

irregular. Grains of pyrrhotite in contact with one another 

(observed under crossed nicols) generally form equigranular 

to inequigranular mosaic textures in which mutual grain 

boundaries form triple points. Grain diameters range from 

0.05 to 40 mm and average about 17 mm. The fine wavy 

lamellae of monoclinic pyrrhotite are the major internal 

features of the grains.

Locally near fault and shear planes deformation twins, 

kink bands, and combinations of the two are present in 

pyrrhotite. Figure 35 shows an example of these features 

which may be compared with similar structures produced 

experimentally by Clark and Kelly (1973, p. 342).

Where the primary silicate minerals of Basal zone rocks 

have been altered and the rock textures changed by the

100

80

Probable bulk pyrrhotite

compositions before

exsolution

46.3 46.5 46.7 46.9 B 47.1 47.3 

ATOMIC PERCENT METALS

47.5 47.7 47.9

FIGURE 34. Diagram used to estimate bulk composition of pyrrhotite. 

Points labeled A are compositions of hexagonal pyrrhotite from table 

9. Points labeled B are the range of compositions for monoclinic 

pyrrhotite after Misra and Fleet (1973). Histogram shows compositions of hexagonal pyrrhotites in Basal zone. Samples also contain 

10 to 20 percent monoclinic pyrrhotite.

FIGURE 35. Photomicrograph of pyrrhotite texture. Crossed nicols. 

Deformation features. Area in photograph is 2 mm across.

development of amphiboles, talc, serpentine, and other 

minerals, the sulfide mineral textures are also changed. 

Pyrrhotite in part retains its original interstitial texture but 

more frequently occurs in veins, stringers, and irregular 

masses that are intimately intergrown with the silicate 

alteration minerals. In some samples the textures grade from 

the primary interstitial shapes in unaltered parts of the 

sample into diffuse, irregular masses of pyrrhotite in the 

altered parts of the sample. The gradation takes place with 

no apparent sharp contacts between pyrrhotite of different 

textures and without any apparent changes in sulfide 

mineralogy. In other samples where veins and stringers are 

more prevalent pyrrhotite shows replacement textures with 

magnetite. Locally pyrrhotite and magnetite may form 

pseudomorphs after each other. The textural changes are 

evidence that pyrrhotite was either mobilized in the solid 

state or recrystallized and locally transported under hydrous 

conditions.

PENTLANDITE

Pentlandite occurs in all three types of sulfide-bearing 

rocks either as blocky masses between pyrrhotite grains or as 

flamelike masses in pyrrhotite. Locally, the blocky pentlandite grains form finer grained necklaces around coarser 

pyrrhotite grains. Grain size of blocky pentlandite ranges 

from 1 to 10 mm and averages 6 mm; the average width of the 

flamelike masses is about 10 to 50 nm. Two types of blocky 

pentlandite have been found: The most common type 

consists of locally fractured pentlandite with magnetite and 

pyrrhotite in the fractures; the other consists of vermicular 

pentlandite intergrowths identical with those described by 

Page (1971b) in the G and H zone chromitites. The identity 

of the fine wormy intergrowth material is unknown.

From analyses of pentlandite and the associated 

assemblages, Graterol and Naldrett (1971), Page (1972) and
48 STILL WATER COMPLEX, MONTANA

Harris and Nickel (1972) concluded that the nickel content 

of pentlandite was a function of the associated sulfide 

mineral assemblage. Misra and Fleet (1973) summarized 

these conclusions, presented new experimental evidence and 

additional compositional data on pentlandite assemblages, 

and showed that the nickel content is indeed a function of 

nickel content of the bulk sulfide mineral assemblage. Page 

(1972, p. 815) reported microprobe analyses of three 

pentlandites from the basal zone ranging from 23.8 to 28.0 

atomic percent Ni. This range agrees with the range 

presented by Misra and Fleet (1973, p. 530) for the 

assemblage pentlandite-hexagonal pyrrhotite-monoclinic 

pyrrhotite. Other microprobe analyses of pentlandite in the 

Basal zone have not yet been made, but it is suspected from 

the homogeneity of mineral assemblages that the results 

would not differ from those presently accepted.

CIIALCOPYRITE FAMILY

Three recently described minerals, mooihoekite, 

haycockite, and talnakhite (Cabri and Hall, 1972; Cabri, 

1967, 1973) have compositional and structural similarities to 

chalcopyrite as well as very similar appearance in polished 

section. According to Cabri and Hall (1972), the close 

similarity of these minerals makes their positive identification extremely difficult and requires detailed electron 

microprobe analyses in conjunction with careful X-ray 

diffraction studies. Because both mooihoekite and 

talnakhite tarnish with prolonged exposure to air and 

chalcopyrite and haycockite do not, the slight variations in 

reflectivity and degree of tarnish are evidence that more than 

one phase may be present.

Sulfide-bearing rocks from the Basal zone contain 

minerals with the appearance of chalcopyrite that show 

small variations in reflectivity, degree of tarnish, and 

anisotrophism. This suggests that more than one phase is 

present. Early in this study an electron microprobe analysis 

of a phase that had not tarnished in air was made using the 

same methods as Page (1972) with the following results in 

weight percent: Cu, 33.0; Fe, 31.1; Ni, 0.06; S, 34.6; for a 

total of 98.7. The calculated metal-to-sulfur ration of 0.998 is 

close to the theoretical ratios of 1.005-1.02 for chalcopyrite. 

Because haycockite, the other phase that does not tarnish, 

has a metal-sulfur ratio of 1.125, the analyzed phase is 

presumed to be chalcopyrite. Because of this early result and 

relatively recent understanding of these complexities in the 

copper-iron-sulfur system, other phases with the appearance 

of chalcopyrite were not analyzed and were assumed to be 

chalcopyrite. Recent reexamination of the polished sections 

has revealed differences in tarnishing; further investigation, 

of the nature and distribution of the chalcopyritelike phases 

must await a later study. In this paper, for convenience, 

"chalcopyrite" is used to include possible occurrence of all 

four minerals, although chalcopyrite is believed to be the 

major phase.

In all three groups of sulfide-bearing rocks chalcopyrite

usually forms mutual grain boundaries with pyrrhotite, 

pentlandite, or both but may form irregular masses apparently not in contact with another sulfide phase. Rare 

cubanite lamellae occur in some samples of chalcopyrite. 

Several textural varieties of chalcopyrite are present, 

perhaps the most common variety consists of blocky, 

granular to ovoid grains at the margins, grain boundaries, or 

triple points between pyrrhotite grains. Chalcopyrite frequently occurs in individual interstices of the silicate phases, 

with one edge or margin against a silicate mineral and its 

other margins against pyrrhotite, pentlandite, or both. 

Irregular masses of chalcopyrite exhibit similar features. In 

some outcrops, hand specimens, polished slabs of massive 

sulfide rocks, and a few sulfide-bearing cumulate rocks, 

chalcopyrite occurs as lenses and layers a few millimeters to 

several centimeters wide approximately concordant with 

other layering or foliation features. Elsewhere masses of 

chalcopyrite form veins or stringers that crosscut foliation or 

layering. On a polished section scale, stringers and veinlets of 

chalcopyrite locally cut across pyrrhotite and pentlandite 

masses.

In general, the blocky to ovoid grains and concordant 

lenses and layers appear to be primary crystallization 

features, whereas some of the irregular masses and all of the 

crosscutting features appear to be later features.

MAGNETITE, ILMENITE, AND HEMATITE

There are at least three distinct generations of magnetite, 

ilmenite, or magnetite-ilmenite-hematite intergrowths in 

sulfide-bearing rocks of the Basal zone. The earliest generation is characterized by euhedral to subhedral magnetite 

crystals that contain abundant lamellae of ilmenite and by 

subhedral to anhedral ilmenite crystals. Locally, composite 

grains of ilmenite with fine lamellae of hematite are mantled 

by magnetite with ilmenite lamellae (fig. 36/1). The shape 

and textures of the oxide grains and their relation to silicate 

and sulfide minerals are evidence that this generation 

crystallized very early in the sequence. The primary oxide 

phases were either titaniferous magnetite or hematitic 

ilmenite that later exsolved ilmenite or hematite, respectively, 

owing to changing physical conditions. Oxide phases of this 

generation occur in pyroxene cumulates with matrix sulfide 

and in massive sulfide rocks probably as cumulates. Included in this early generation of oxides in interstices associated 

with postcumulate silicates are isolated subhedral to 

anhedral grains of ilmenite and magnetite that may be 

postcumulus phases. The presence of ilmenite as lamellae 

and grains in amounts up to 3 volume percent of the rock 

suggests that the early generation of oxides crystallized from 

a silicate rather than a sulfide-oxide melt because magnetite 

that has crystallized from a sulfide-oxide melt generally has a 

low titanium content (Hawley, 1962; Naldrett and Kullerud, 

1967; Naldrett, 1969).

The second generation of oxide minerals consists of 

magnetite included within sulfide minerals in interstices of
SULF1DE-BEARING ROCKS 49

FIGURE 36. Oxide-sulfide mineral relations. A, Ilmenite (il) with hematite 

lamellae with later magnetite-ilmenite(mt-il)in matrix pyrrhotite(po). 

Si, silicate NB1 /310 polished section. B, Magnetite-ilmenite grain with 

magnetite overgrowths etched with hydrofluoric acid.

matrix sulfide rocks and in massive sulfide rocks. The 

magnetite occurs as isolated subhedral grains enclosed in the 

sulfide phase and, because it lacks ilmenite lamellae, 

apparently has a low titanium content. Grains of 

magnetite-ilmenite that are included in massive sulfide 

material occur as bulbous anhedral magnetite intergrown 

with pyrrhotite rimming the magnetite-ilmenite grains (fig. 

36B). Both isolated grains and overgrowth magnetite are 

rare but appear to be more abundant in the massive sulfide 

rocks. According to Naldrett's (1969) experimental studies 

of the Fe-S-O system, the isolated grains may have 

crystallized from a sulfide-oxide melt. The overgrowth 

magnetite could have originated during sulfide crystallization from a sulfide-oxide melt or by reaction of the earlier 

magnetite-ilmenite crystals with the sulfide material.

The third generation of magnetite consists of euhedral to 

anhedral fine-grained magnetite veins and crosscutting 

masses associated with the alteration minerals serpentine, 

amphibole, and chlorite. These formed after the sulfide and 

silicate rocks were crystallized.

OTIIKK OPAQUE MINERALS

Other primary opaque minerals include chromite, 

cubanite, and a member of the cobaltite group. Chromite is a 

cumulus phase and is found locally in trace amounts as 

fine-grained euhedral crystals. Cubanite is rare and has only 

been observed as lamellae in three specimens of chalcopyrite. 

A member of the cobaltite group occurs locally in amounts 

up to 2 percent by volume as euhedral to subhedral crystals 

associated with the other primary sulfide minerals. Page and 

Jackson (1967, p. D125) reported the following range of 

compositions, in weight percent: 20-22 percent Co, 9-11 

percent Ni, 7-8 percent Fe, 16-18 percent S, and 43-49 

percent As. These analyses suggest that the mineral is 

gersdorffite.

Other opaque minerals that are found only in near-surface 

or surface exposures and in faults include violarite, marcasite, pyrite, mackinawite, cuprite, native copper, and 

goethite. Violarite replaces pentlandite locally and 

fine-grained marcasite plus pyrite replaces pyrrhotite near 

fine fractures filled with goethite. Goethite veins and fracture 

fillings appear to be one of the latest features in the rocks. 

Native copper and cuprite were found in surface fractures up 

to a millimeter wide; cuprite appears to replace the native 

copper.

PROPORTIONS OF SI T LFIDE MINERALS

Accurate estimates of the relative amounts of pyrrhotite, 

chalcopyrite, and pentlandite are difficult to make. Part of 

the problem revolves around the requirements of size and 

nature of samples necessary to characterize the three 

different sulfide-bearing rocks. Disseminated sulfide rocks 

probably can be characterized by small samples because 

each relatively small sulfide-bearing area appears to have 

responded to physical and chemical conditions as an entity. 

The other two sulfide-bearing rocks generally require large 

samples to obtain a representative estimate of the amount of 

the minerals present. Two different ways of estimating 

sulfide mineral proportions were used; one is based on 

modes counted on polished sections, the other is examination of published copper and nickel grades for the Basal 

zone.

Perhaps more reliable estimates of the proportions of 

chalcopyrite and pentlandite can be based on the copper and 

nickel grade data given by Roby (1949) and Dayton (1971). 

An average grade of 0.25 percent nickel and 0.25 percent 

copper has been reported for ore reserves of the Basal zone 

(Dayton, 1971). Analyses of core and mine tunnel samples of 

Basal zone rocks from the Mouat area (more specifically the 

Verdigris Creek area) listed by Roby (1949) indicate that the 

nickel content reaches a maximum of 1.35 percent and 

copper content a maximum of 1.3 percent, if three higher 

copper values are ignored (fig. 37). More than 75 percent of 

the analyses for copper and nickel showed concentrations 

less than 0.5 percent, and more than 40 percent of the 

analyses showed concentrations less than 0.2 percent.
50 STILLWATER COMPLEX, MONTANA

0.2 0.4 0.6 0.8 1.0 1.2 1.4 

COPPER GRADE, IN PERCENT

/V=220

10

0.2 0.4 0.6 0.8 1.0 

NICKEL GRADE, IN PERCENT

FIGURE 37. Copper and nickel content of drill core and mine-tunnel 

samples. N, total number of samples. Data |Yom Roby (1949).

\

Copper-to-nickel ratios for Stillwater samples are given in 

figure 38 and range from about 1:0.33 to 1:10, comparable 

with values from the Duluth Complex (Cornwall, 1972), 

Strathcona mine (Naldrett and Kullerud, 1967), Carr Boyd 

Complex (Purvis and others, 1972), and the Lunnon Shoot 

(Ewers and Hudson, 1972).

Relative volumes of sulfide minerals in sulfide-bearing 

rocks have been estimated by point counting over 1000 

points on polished sections that range from 2.5 cm in 

diameter to 4 by 4 cm in area (fig. 39). The modes represent 

wide ranges in proportions of pyrrhotite to chalcopyrite and 

pentlandite, but in general pyrrhotite is the most abundant

sulfide mineral. The pattern of these modal data is similar to 

plots given by Hawley (1962, p. 117) for random samples 

from the Sudbury ores. His data tend to show higher 

concentrations of pentlandite in samples rich in chalcopyrite 

but in general form a broad band parallel to the 

pyrrhotite-chalcopyrite side of the triangular diagram (fig. 

39).

INTERRELATION BETWEEN Sl'LFIDE MINERALIZATION 

AM) SILICATE ROCKS

The relation between sulfide mineralization and the 

enclosing igneous rocks provides evidence on: (1) Correlation of textures, volumes, compositions, and ratios between 

both materials; (2) comagmatic or external sources of the 

sulfide mineralization; (3) the existence of a sulfide-oxide 

liquid immiscible in the silicate magma; and (4) the timing 

and processes of concentration of the sulfide mineralization. 

This evidence imposes limits on models for sulfide 

mineralization in the Stillwater Complex.

Attempts to quantitatively correlate characteristics of the 

sulfide mineralization in the Basal zone quantitatively with 

properties of the silicate rock containing the sulfide minerals 

have been unsuccessful. No consistent correlations of 

volumes, compositions, ratios, and textures of the sulfide 

minerals with lithology, mineralogy, texture, stratigraphic 

position, relative proportions of minerals, and compositions 

of minerals of the silicate rocks have been found. Similar 

comparisons of sulfide and oxide mineral properties were 

unenlightening. Three qualitative generalizations can be 

made on the basis of observations presented earlier: (1) Finer 

grained ophitic and subophitic rocks tend to contain smaller 

volumes of sulfide minerals than cumulate rocks, which may 

locally contain large amounts of sulfide, (2) the rocks lower 

in the Basal zone tend to contain larger volumes of sulfide 

mineralization than those higher in the sequence, and (3) the 

sulfide minerals occupy interstitial spaces that in unmineralized rock would be filled by silicates. Only within the 

framework of these generalizations does the sulfide 

mineralization appear related to the silicate rock which 

contains it.

Most if not all sulfide mineralization is thought to be 

comagmatic with the silicate material, and both are considered to be derived from the same parent magma or 

magmas. Evidence for this opinion includes (1) the overall 

distribution and occurrence of sulfide mineralization, (2) the 

fact that sulfide minerals fill interstices between cumulus 

phases and occur as small masses or molded about the 

silicates with no evidence of replacement of the primary 

silicates, and (3) the occurrence of small, 10-100 pm 

inclusions and clusters of polymineralic inclusions of sulfide 

material within the centers of cumulus plagioclase and 

pyroxene grains. Further evidence stems from incompatibilities between the textures of the sulfide-bearing rocks 

and those expected from other origins such as external 

introduction by hydrothermal processes. Hydrothermal 

deposition of sulfides requires that space be made for the ore
SULF1DE-BEARING ROCKS 51

0.2 0.4 0.6 0.8 1.0 1.2 

Ni PERCENTAGE

1.4 1.6

FiGi'Ri-: 38. Copper and nickel content data from various sources.

by replacement of existing postcumulus silicates. Because of 

the distribution of sulfides in interstices of different types of 

cumulates, such a replacement process must have been 

extremely selective and seems therefore highly unlikely.

If the comagmatic origin of the sulfide and silicate 

minerals is accepted, the next question concerns whether the 

sulfides were part of a sulfide-oxide immiscible liquid or 

were in solution in the silicate magma. Skinner and Peck 

(1969) studied basaltic lava in Hawaii and showed that the 

magma in Alae lava lake was saturated with sulfide at 1065° 

C with a sulfur content of 0.038 weight percent. Other 

measurements of sulfur content at saturation in basaltic 

magmas are scarce, but experimental and theoretical studies 

suggest that the maximum amount of sulfur soluble in a 

basaltic magma varies between 0.05 and 0.2 weight percent 

as the FeO content varies between 5 and 20 weight percent

(Haughton and others, 1974) at 1200° C. For comparison 

several different estimates of the sulfur content of the 

Stillwater magma were made.

Estimates of the sulfur content of the Basal zone or the 

original parental magma are difficult to make with any 

confidence because of sampling problems. This is illustrated 

by the variable sulfur content, from 0 to 4.66 weight percent, 

of the ophitic, subophitic, and ragged noncumulate igneous 

rocks (table 4) that may represent crystallized magmatic 

liquids. One approach in estimating sulfur content of the 

Basal zone is to average many rock analyses. Roby (1949, p. 

7) gave results of sulfur analyses of six composite 

diamond-drill core samples that represented sample lengths 

of 3.0 to 18.9 m in sulfide enriched zones. The sulfur content 

ranges from 4.99 to 14.41 weight percent. The weighted 

average (weighting based on length of sample) is 10.4 weight
52 ST1LLWATER COMPLEX, MONTANA

Chalcopyrite

EXPLANATION

o 0-10% Sulfide 

• 10.1-60% Sulfide 

A 60.1-100% Sulfide

1 Sudbury modes 

random ore samples

Pyrrhotite Pentlandite

Fic;rRE 39. Pyrrhotite, pentlandite, and chalcopyrite modes of sulfide-bearing rocks. Field of modes for Sudbury ore samples from 

Hawley (1962, p. 117.)

percent sulfur for 58.5 m of core sample. Another approach 

is to consider the volume of sulfide minerals estimated 

during core logging. An average volume percent sulfide 

minerals, based on data in figure 30, over a core length of 160 

m, is 18.2 percent. This corresponds to a maximum 

estimated sulfur content of about 8 or 9 weight percent. 

Visual impressions and estimates of sulfide content based on 

field and laboratory studies suggest that these estimates if 

taken for the whole Basal zone are too large and that more 

probable average values are on the order of 1 or 2 percent for 

the Basal zone. Using composite samples Hess (1960) gave 

average values of 0.04 weight percent sulfur for the 

Ultramafic, Banded, and Upper zones suggesting that even 

this estimate may be too high. Comparison of the several 

estimated sulfur contents of the Basal zone with Skinner and 

Peck's (1969) value for a sulfur-saturated basaltic magma 

suggests that the magma of the Basal zone was saturated 

with sulfur and that an immiscible sulfide-oxide liquid was 

present in the Basal zone.

It is appropriate to consider whether the sulfur content of 

the Basal zone may have been derived from the magma 

reservoir that formed the entire Stillwater Complex. A 

calculation of the contribution of the sulfur content of the 

Basal zone distributed throughout the Stillwater magma was 

made using the following assumptions and approximations: 

(1) The maximum exposed thickness for the complex of 5.49 

km approximates the thickness of the magma chamber; (2) 

the magma density was 2.67 g/cm3 , the calculated value for 

Jackson's (1971) analysis of the parent magma (see table 4) 

using Bottinga and Weill's (1970) method; (3) the Basal zone 

is 0.163 km thick, its average thickness; and (4) a rectangular 

prism 1 km square through the chamber is representative of 

the entire Stillwater Complex. For estimates of 1 and 10

weight percent sulfur in the Basal zone, the contribution to 

the entire Stillwater magma is about 0.003 and 0.03 weight 

percent sulfur, respectively. Adding these estimates to the 

average value, Hess (1960) suggests that the total Stillwater 

magma could have contained 0.04-0.07 weight percent 

sulfur. Perhaps a better value for the sulfur content of the 

Stillwater magma would be the value estimated by Moore 

and Fabbi (1971) for the juvenile sulfur content of basaltic 

magma of 0.080±0.015 weight percent based on analyses of 

outer quenched surfaces of submarine basalts. The lower 

calculated value borders on possible sulfur saturation, 

whereas the higher value is definitely saturated, according to 

Skinner and Peck's (1969) value for Hawaiian basaltic 

magmas. From these calculations, estimates, and comparisons of sulfur content of the Basal zone, it is concluded 

that the sulfides were derived from a sulfur-supersaturated 

magma.

Sulfide-oxide liquids were probably present in the 

Stillwater magma at a very early stage in the intrusive history 

of the Basal zone and were continually present throughout 

deposition of most of the lower part of the complex. 

Evidence for the early presence of sulfide-oxide liquids 

includes: (1) Massive sulfide rocks intruded by ophitic to 

ragged-textured noncumulate rocks and showing the reverse 

sequence of intrusion; both groups are intrusive into the 

metasedimentary rocks below the base of the complex; (2) 

intrusive breccias consisting of metasedimentary rock 

fragments and a massive sulfide matrix (fig. 33); and (3) fine 

polymineralic inclusions of sulfide minerals in the centers of 

some cumulus silicate and oxide phases throughout the 

Basal zone. The occurrence of fine-grained ophitic and 

subophitic igneous rocks with low contents of sulfide 

minerals could be interpreted as evidence contrary to early 

separation of sulfide-oxide immiscible liquids, but the 

variable and possibly gradational sulfur content of the 

noncumulate rocks (table 4) suggests that sulfide-oxide 

liquids were present at least locally when these noncumulate 

rocks crystallized. The field and laboratory evidence implies 

that sulfide-oxide liquids developed as an immiscible fraction of the Stillwater magma before or immediately after 

emplacement. Concentration or collection processes of the 

sulfide-oxide melt must have been operative during the early 

formation of the liquids to account for the field relations of 

large masses of sulfide within metasedimentary rocks such as 

those in the Verdigris Creek and part of the Mountain View 

area.

INTERPRETATION AND MODEL OF 

SULFIDE CRYSTALLIZATION

Most modern investigators agree that large amounts of 

basaltic magma are generated by partial melting of an 

ultramafic mantle under physical and chemical conditions 

that are presently poorly defined. Assuming this source for 

the Stillwater basaltic magma, the arguments and deductions of MacLean (1969, p. 878-880) suggest that the magma
SULFIDE-BEAR1NG ROCKS 53

would have been saturated with sulfur. Such a sulfur-saturated magma could contain dispersed, fine-grained, 

immiscible, sulfide-oxide liquid droplets if it contained 

0.080±0.015 weight percent juvenile sulfur, as suggested by 

Moore and Fabbi's (1971) study. The existence of such 

droplets would depend upon the magma's emplacement 

history in the crust. Whether the magma contained such 

droplets before emplacement is unimportant to the interpretation of the sulfide crystallization history.

The postemplacement crystallization history is discussed 

under four topics: (1) Mechanisms for separation of 

immiscible sulfide liquid and evaluation of these 

mechanisms in terms of published experimental data and 

observations of the Basal zone; (2) collection and formation 

of the observed sulfide masses; (3) initial crystallization of 

the sulfic liquids; and (4) later, lower temperature equilibration of the sulfide minerals to the presently observed 

assemblages.

MFCIIAXISMS FOR SKPARATION OF IMMISCIBLK 

Sl'I.FIDF-OXlDF LIQl'lDS FROM BASALTIC MAGMA

Mechanisms that would generate sulfide-oxide liquids 

from basaltic magma either during ascent or after emplacement in the Stillwater magma chamber require changes in 

physical or compositional parameters of part or all of the 

magma system. Such changes must enrich the sulfide-oxide 

liquid in sulfide either by crystallization of other phases, 

specifically those rich in FeO, or by expanding the region of 

liquid immiscibility in the magma system. The activity of 

FeO in the magma is probably the main factor controlling 

sulfide solubility and liquid immiscibility (MacLean, 1969, 

p. 866); the higher the activity of FeO, the higher the sulfide 

solubility, and the less sulfide-oxide liquid generated. The 

experimental studies in compositionally limited sulfide-silicate systems by Fincham and Richardson (1954), 

Naldrett and Richardson (1967), Naldrett(1969), MacLean 

(1969), and Shamazaki and Clark (1973), suggest that the 

following changes in conditions will also produce more 

extensive immiscible sulfide liquid in a basaltic magma: (1) 

Decreasing the total pressure on the system; (2) decreasing 

the temperature; (3) increasing sulfur fugacity; (4) increasing 

oxygen fugacity; and (5) increasing SiO2 Na2O, K2O, MgO, 

and A1 2O3 content (activity) in the magma; or (6) decreasing 

the FeO content.

Reducing the total pressure on the Stillwater magma while 

keeping other parameters equal would probably result in a 

slight release of sulfide to an immiscible sulfide-oxide liquid 

(MacLean, 1969, p. 881-882). There is no direct experimental evidence for this conclusion, but it can be deduced from 

two sets of experiments. Brett and Bell (1969) showed that 

the temperature of the eutectic in the Fe-FeS system is nearly 

the same at 30 kbar as at 1 bar and Naldrett and Richardson 

(1967) demonstrated that 2 kbar fluid pressure has very little 

influence on the melting temperatures of 

pyrrhotite-magnetite liquids. Complete evaluation of this

mechanism must await further experimental studies. All 

mafic magmas generated in the mantle and emplaced in the 

crust undergo a pressure release, but not all such intrusions 

show evidence of immiscible sulfide-oxide liquids or are 

associated with sulfide deposits. By implication, a 

pressure-release mechanism is not the major factor in 

controlling sulfide generation in the Stillwater Complex.

A decrease in the temperature of a basaltic magma can 

result in an increased amount of immiscible sulfide-oxide 

liquid in two ways: (1) By crystallizing and accumulating 

silicate and oxide phases rich in FeO, thus increasing the 

sulfide content of the magma, and (2) by expanding the 

immiscible sulfide-oxide liquid region at lower temperatures 

for certain magma compositions. Interpretations of the 

ophitic and subophitic textures of the fine-grained noncumulate igneous-textured rocks, and the appearance low in 

the Basal zone of more ferrous orthopyroxenes before 

magnesian orthopyroxenes, suggest an early decrease in the 

temperature of the Stillwater magma after or during its 

emplacement. The conventional interpretation of fine-grained ophitic rocks as rapidly cooled or quenched magmas 

implies a temperature decrease, but it is not certain that this 

interpretation can explain these textures in the middle of the 

Basal zone. The appearance of ferrous orthopyroxenes 

before magnesian orthopyroxenes can be interpreted as due 

to supercooling, with higher rocks in the section reflecting 

adjustment of temperatures toward equilibrium conditions. 

Decreasing temperature probably contributed to the 

amount of immiscible sulfide-oxide liquids but, as with the 

pressure-release argument, the consideration that most 

basaltic rocks do not contain sulfide deposits suggests that it 

was not the major mechanism.

An increase in the sulfur fugacity either directly by the 

addition of more sulfur to the magma through contamination or indirectly by a change in other parameters will result 

in increased amounts of immiscible sulfide liquid (Richardson and Fincham, 1954; MacLean, 1969). Sulfurization, the 

process of incorporation of preexisting sulfur into magma, 

has been suggested as a process for forming sulfide 

accumulations similar to those in the Basal zone (Kullerud, 

1963). No positive evidence that such a process occurred in 

the Stillwater Complex has been observed, and while it is 

conceivable that the iron formation below the complex in the 

Initial to Benbow area had a sulfide facies that was 

incorporated into the basaltic magma, none of the iron 

formation elsewhere below the complex is rich in sulfides. 

The other metasedimentary rocks below the complex appear 

to be low in sulfur, and although they have contaminated the 

magma locally, they probably are not a source of major 

additions of sulfur. Thus, the direct addition of sulfur to 

increase the amount of sulfide-oxide immiscible liquid in the 

Stillwater magma appears to be a minor mechanism, and 

increases in sulfur fugacity must have been caused by 

changes in other parameters of the magma or conditions of 

the chamber.
54 STILLWATER COMPLEX, MONTANA

As shown by MacLean (1969) a magma generated in the 

mantle would have an oxygen fugacity close to that fixed by 

the quartz-fayalite-magnetite buffer, and an increase in the 

oxygen fugacity, especially a rapid rise, would cause a large 

proportion of the sulfur originally soluble in the magma to 

separate as a sulfide-oxide immiscible liquid. Rapid oxidation could take place before emplacement in the magma 

chamber and depending upon the magma's rate of upward 

movement, could result in either a large volume of dispersed 

sulfide-oxide immiscible liquid droplets in the magma or in 

larger masses of sulfide liquid (Naldrettand Kullerud, 1967). 

Rapid oxidation could also occur in the magma chamber 

where accumulation of sulfide-oxide liquid would account 

for the sulfide distribution. One obvious cause of rapid 

oxidation might be the incorporation of H2O from the 

country rocks with concomitant loss of H2 to the surface. 

Incorporation of H2O into the magma chamber need not be 

a diffusion-controlled process but might be controlled by 

other mechanisms such as zone refining (D. M. Shaw, 1972; 

Harris, 1957), which would involve circulating magma past 

wet country rocks and scavenging H 2O from them. If such a 

mechanism is applicable to the Stillwater magma, the oxide 

and silicate assemblages should show some evidence for 

high-oxygen fugacities unless assemblages that may have 

crystallized under high-oxygen fugacities were resorbed and 

reequilibrated when the magma system was closed to the 

influx of water and adjusted to lower oxygen fugacity. 

Evidence for high initial oxygen fugacities is sparse in the 

Basal zone, as is evidence for the presence of large volumes of 

water, but it is present. The presence of ilmenite-hematite 

intergrowths mantled by magnetite with ilmenite lamellae, 

found as subhedral inclusions in matrix sulfides and massive 

sulfide rocks, requires that oxygen fugacities were initially 

high, perhaps near those of the magnetite-hematite buffer, 

and later decreased to conditions near the 

quartz-magnetite-fayalite buffer. The close association of 

the ilmenite-hematite intergrowths with the sulfide minerals 

and the absence of the intergrowths in nearby cumulates in 

which the oxide phases are magnetite and ilmenite strongly 

supports the hypothesis of early oxidation. The presence of 

brown hornblende and red-brown biotite (table 3) in most 

rocks from the Basal zone and in the early cumulates of the 

Ultramafic zone (Page, Shimek, and Huffman, 1972) 

requires the presence of some water in the initial stages of 

magmatic accumulation. An oxidation mechanism to 

generate the amount of sulfide-oxide liquid necessary for the 

sulfide mineralization on the Basal zone is strongly supported.

An increase in the SiO2 , Na2O, K2O, MgO, and A1 2O3 

content (activity) in the magma has the same effect as a 

decrease in the FeO content and results in increased amounts 

of immiscible sulfide-oxide liquid. Two processes by which 

the composition of the magma can be changed are contamination from the country rocks and differentiation by 

fractional crystallization. Previous discussion of the

problems of contamination and the mixed rocks 

demonstrated that the MgO, SiO2 , A12 O3 , or CaO content of 

all the magma in the Stillwater magma chamber could not be 

changed appreciably through contamination with 

metasedimentary rocks. Moreover, contamination can only 

change the magma composition locally and is not a likely 

mechanism for the formation of most of the immiscible 

sulfide-oxide liquid. Because the mass of magma from which 

the relatively thin Basal zone probably was derived is large, 

differentiation and fractional crystallization cannot have 

greatly changed the initial composition of the basaltic 

magma in the chamber. In addition such a process of 

differentiation and fractional crystallization does not appear 

capable of producing large amounts of sulfide-oxide immiscible liquid rapidly enough to account for the concentration of sulfide mineralization near and below the base of the 

complex. Fractional crystallization could increase sulfide-oxide liquid by removing phases rich in iron in smaller 

batches of magma or in local environments. Throughout the 

Basal zone, the sulfide content shows repetitive cycles of 

increase and decrease (fig. 29). These cycles appear to be 

roughly correlated with the cycles of changing orthopyroxene compositions and stratigraphy. Higher sulfide contents 

in the Basal zone tend to occur near the zones of more 

ferrous orthopyroxenes and sulfide content tends to 

decrease with increasing magnesium enrichment. Thus, 

fractional crystallization may be the cause of the cycling, 

repetitive nature of the sulfide mineralization on a local 

scale.

In summary, a basaltic magma that may have been 

saturated with sulfide was generated in the mantle and 

emplaced in the crust. Either during emplacement or in the 

magma chamber of the Stillwater Complex it was rapidly 

oxidized by an influx of water. Oxidation caused a rapid 

increase in immiscible sulfide-oxide liquid, which accumulated to varying degrees to form a spectrum of sulfide 

mineralization textures. Soon thereafter, the chamber was 

sealed from the external source of oxygen and reverted to 

more reducing crystallization trends. During deposition and 

accumulation of the silicates and oxides in the Basal zone, 

local fractional crystallization of orthopyroxene appears to 

have depleted the magma of FeO, thus acting as a secondary 

control on the deposition of sulfide-oxide liquid.

COLLECTION AND CONCENTRATION MODELS

Having established that immiscible sulfide-oxide liquids 

were probably generated in the Stillwater magma by one or 

more processes, possible models for the collection and 

concentration of these liquids at the base of the complex 

need to be examined. At least two different situations can be 

visualized: (1) The magma chamber may have filled rapidly 

and remained as a stagnant, nonconvecting mass similar to 

Jackson's (1961 a) model for the deposition of the Ultramafic 

zone; or (2) after filling, the magma chamber may have 

convected in either a single cell or many cells, allowing
SULFIDE-BEARING ROCKS 55

current-controlled processes to become equal in importance 

to or more important than gravity-controlled processes. The 

stagnant magma situation is the simpler one for which to 

develop models and is used here as a basis for discussion; the 

effects of current are considered to be modifications of the 

stagnant process. Models for either situation depend in part 

upon relations between settling velocities, viscosity, and size 

and on diffusion of sulfur in a basaltic magma.

Several assumptions must be made to estimate settling 

velocities of sulfide-oxide immiscible liquids in the Stillwater 

magma. First, the sulfide-oxide liquids are assumed to have 

had spherical or spheroidal shapes, like similar melts 

produced experimentally by MacLean (1969, figs. 3 and 4). 

Second, the spheres are assumed to have behaved as rigid 

spheres in order to apply Stoke's law. Third, a composition 

for the sulfide-oxide liquid must be assumed from which 

their density may be estimated. Finally, assumptions used in 

deriving the viscosity, temperature, and density relations of 

the silicate magma in the previous sections are included in 

these calculations.

A density for the solid sulfide-oxide phases from the bulk 

sulfide composition can be calculated and this value may be 

modified by what is known about the density of molten 

sulfide minerals to yield an estimate of the density of the 

sulfide-oxide liquid. Very few bulk sulfide analyses are 

available for the Stillwater sulfides, but Roby (1949) 

reported the analysis of one large composite sample taken 

for metallurgical testing which is used to calculate the sulfide 

compositions and volume percentages. Comparison of the 

volume percentage of chalcopyrite, pentlandite, and 

pyrrhotite calculated in table 10 with the modal data in 

figure 39 shows that Roby's (1949) sample appears to be 

representative of the modal data. The bulk density 

calculated for the solid massive sulfide is 4.805 g/cm3 , 

assuming that the densities of the mineral components are 

additive proportional to their mole fraction.

Gubanov (1957) reported that the densities of Cu2S, Ag2S, 

Sb2$3, FeS, and CuS decrease by only a few percent 

upon melting. Also, the limited data on thermal expansion 

of sulfides (see Skinner, 1966), suggest only small changes in

TABLE 10. Calculations oj suljide compositions based on Roby's (1945) 

large composite sample of sulfide material

Element

or 

mineral

Weight 

percent

Normalized

mole 

percent

Weight 

percent

Mole 

fraction

Volume 

percent

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

Cu .............

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

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

As .............

Insoluble ...

Chalcopyrite 1 

Pentlandite2 .. 

Pyrrhotite3 ...

0.74

1.28

35.00

20.30

.08

33.40

0.99

1.558

48.480

48.975

6.59

3.92

89.49

0.0339

.0048

.9613

5.64

4.02

90.35

'CuFeS2 . density =4.088 g/cm'. 

2 Ni4 5Fe4. 5Sx, density=4.911 g/cm'. 

'FeS, density=4.830 g/cm'.

the densities of sulfide minerals upon melting. Calculations 

show that a 5 percent range (5.045 to 4.565 g/cm3 ) about the 

estimated solid density, to account for differences between 

solid and melt, causes little difference in calculated settling 

velocities. Figure 40 shows settling velocities in basaltic 

magma at 1,200°C and 1,300°C for various size spheres of 

sulfide (density, 4.805 g/cm3 ) in comparison with settling 

velocities of orthopyroxene. The size ranges of sulfide 

inclusions in cumulus phases and as independent interstitial 

material are indicated (detailed size data given in Page, 

1972). Size ranges of disseminated sulfides that show 

continuous conductivity are also shown in figure 40 and the 

size ranges of disseminated sulfides that have continuous 

electrical conductivity for individual samples are given in 

figure 41. The largest disseminated clot of sulfide observed 

was 4 cm in diameter, but most of the clots are less than 1 cm 

in diameter. Figure 40 shows that sulfide spheres with radii 

greater than about 0.5 mm fall at settling velocities greater 

than 8 m/yr.

Spheroidal droplets of immiscible sulfide-oxide liquid 

could form under stagnant conditions in at least two ways: 

(1) All of the immiscible sulfide spheres may have nucleated 

at about the same time, had the same size, and been equally 

distributed in the magma if the same physical and chemical 

conditions prevailed throughout the magma chamber; or (2) 

sulfide spheres may have nucleated continuously through a 

period of time producing spheres of different sizes in 

different locations and at different levels in the chamber.

In the first case, assuming a constant growth rate of sulfide 

spheres dependent upon the diffusion of sulfide material 

from an adjacent volume of magma, all the immiscible 

sulfide-oxide spheres would have the same settling velocity and 

would not collide and coalesce during settling to form larger 

spheres. The sulfide-oxide liquid spheres must grow large 

enough to allow them to settle at rates equal to or higher than 

the cumulus phases in order to collect as dispersed sulfide 

throughout the magma chamber near the base of the 

complex. Under stagnant conditions sulfide-oxide spheres 

must have a radius of at least 0.04 mm to fall as fast as the 

slowest settling cumulus phases and a radius of at least 0.5 

mm to fall 8 meters per year (fig. 41). Unless nucleated and 

growing sulfide-oxide spheres are able to coalesce, there is 

no process for concentration and simultaneous nucleation in 

a stagnant magma does not appear to be a likely mechanism. 

However, the presence of currents with velocities sufficient 

to cause individual sulfide-oxide spheres to collide and 

coalesce might make the mechanism work. Currents or 

convection of this kind require initial differences in chemical 

and physical properties throughout the magma chamber, 

which in turn would probably not allow simultaneous 

nucleation throughout the chamber. Therefore, the first 

possibliity appears inapplicable to the collection of sulfides 

in the Basal zone of the Stillwater Complex.

Continuous nucleation of sulfide-oxide spheres over a 

period of time, producing spheres of different sizes at
56 STILLWATER COMPLEX, MONTANA

2-

-2-

-4

-6-

-8

1000

100

10

mm

r=10 mm" 

f=5 mm

Sulfide droplets (1200°C) 

r=2.5 mm

Range of orthopyroxene grain size

Sulfide droplets (1300°C)

- 0.001 SETTLING VELOCITY, IN METERS PER YEAR

10

I

100 1000

I

10,000

I

100,000

I

1,000,000 10,000,000

I______I i r r i

6 8 10 12

SETTLING VELOCITY AS In v, IN METERS PER YEAR

14 16

FIGURE 40. Settling velocities of sulfide liquid spheres. Basaltic magma density at 1,200°C=2.726 g/cm3 , viscosity= 149.9 

poises; at 1,300°C, density=2.712 g/cm3 , viscosity=59.1 poises. Sulfide density =4.805 g/cm3 .

different levels in the magma chamber, requires that either 

chemical or physical parameters or both vary within the 

chamber. Given the range in composition of ophitic and 

subophitic rocks of the Basal norite, compositional variation 

within the magma chamber appears to be likely. This 

compositional variation would in turn produce variations in 

liquidus temperatures, viscosity, density, settling velocity, 

and other properties that control the initial distribution of 

sulfide-oxide liquid spheres. The thickness of the magma 

chamber, approximately 8 km, would yield a lithostatic 

pressure differential between top and bottom of over 2,000 

bars, a possible liquidus temperature differential of about 

24° C assuming a gradient of 3°C/km (see Irvine, 1970b), 

and an adiabatic temperature differential of 2.4° C. Such 

variation might affect the nucleation and growth properties 

of immiscible liquids and in part control their distribution. 

Assuming that local inhomogeneities in physical and 

chemical properties can produce a variable size distribution 

of immiscible sulfide-oxide spheres at different levels of a

stagnant magma chamber, there are at least two possible 

processes that would enlarge the spheres to a critical size for 

rapid settling. One is simply growth through diffusion as 

more sulfide is exsolved from the basaltic magma; the other 

is collision and coalescence of spheres during their descent. 

The distribution of sulfide grain sizes offers some clues as to 

how enlargement may have taken place.

In the Stillwater Complex, sulfide minerals occur either as 

inclusions within cumulus phases, as matrix material, or as 

interstitial material to cumulus phases. Page (1971b) examined size distribution of sulfide material in the G and H 

zone chromitites and observed that all of the sulfide grains 

had diameters less than 0.75 mm. Figure 425, constructed 

from Page's (1971b) size data, shows the distribution of 

sulfide grain diameters in terms of the fraction of grains with 

a specific diameter. A large percentage of the sulfide grains 

are less than 0.10 mm in diameter. Figure 40 shows that these 

sulfide grains would have settling velocities in the range of 

meters to tens of meters per year. Similar-sized sulfide in-
IGNEOUS-TEXTURED (NONCUMULATE) ROCKS OF THE BASAL ZONE 57

1-1 M H h4 8 12 16 20 24 28 32 36 

DIAMETERS OF SULFIDE CLOTS, IN MILLIMETERS

40

FIGURE 41. Ranges of diameters of disseminated sulfide clots foi 

individual samples. Each bar represents one sample.

elusions are present in the Basal zone within cumulus phases. 

They represent sulfide materials with relatively slow settling 

velocities and are probably typical of immiscible sulfide-oxide liquids that did not settle far from where they nucleated 

before being enclosed in a cumulus phase. They may also be 

indicative of the size of sulfide spheres that grew only by 

diffusion and not by coalescence with other spheres. Stated 

in another way, the size distribution of sulfide inclusions in 

cumulus phases probably represents the part of sulfide 

trapped or possibly left over from any collection process if 

the process preceded the cumulate process.

The size distribution of disseminated sulfide clots in the 

Basal zone is illustrated in figure 42/4. The distribution was 

calculated from data used in figure 41. Most of this material 

would have settling velocities greater than 1,000 m/yr (fig. 

40). The clots represent the part of sulfide that coalesced and 

was capable of settling at least as rapidly as the other 

cumulus phases.

The idealized hypothetical curves in figure 43 provide a 

frame of reference against which the size distribution curves 

in figure 42 and the process of collision and coalescence may 

be evaluated. Each curve represents the distribution of the 

fraction of sulfide-oxide spheres with a specific diameter at 

different times during the accumulation of the Basal zone. 

The two curves labeled tf represent the final distribution as 

inclusions and disseminated sulfides generalized from the 

distributions in figure 42. The curve /0 represents an 

assumed distribution of the spheres within the magma 

column soon after nucleation and initial growth. If the 

process of collision and coalescence of spheres is active, then 

the curve t- would represent a distribution of sizes at some 

intermediate time.

The process of collision and coalscence envisioned for the 

collection of sulfide-oxide liquids appears to be analogous to 

the process of raindrop growth. Langmuir(1948), Houghton 

(1950), Matveev (1967), Soo (1967), and Mason (1957) 

developed and reviewed the experimental and theoretical 

studies concerning raindrop growth. The size ranges of 

raindrops and their nuclei are similar to those envisioned for 

sulfide spheres; other physical parameters are quite 

different, but such a model appears applicable. The following model for the collision and coalescence of sulfide-oxide 

liquids in the Stillwater magma is based on these studies and 

outlines only one approach to the problem that would give 

averaged or smoothed rates of droplet growth. Telford 

( 1 955), Twomey ( 1 964, 1 966), and Berry ( 1 967) have pointed 

out that coalescence of raindrops is a stepwise process and 

therefore developed stochastic equations for their growth. 

Such extension of the model to sulfide spheres is unnecessary 

here.

First, consider the growth of a single sulfide-oxide liquid 

sphere of radius R caused by collisions with smaller spheres 

of radius r. In a short interval of time dt, a sphere of radius R 

falls under the influence of gravity relative to the smaller 

spheres a distance (v#-vr) dt, where VR and vr are the 

settling velocities of spheres of radii R and r. The change in 

volume of the large sphere dVR , is a function of (1) the 

volume of the small spheres, Fr=4/37rr3; (2) the cross-sectional area of the cylinder inside which spheres of radius r will collide 

with the sphere of radius R, A-Tr(R+r^\ (3) the size 

distribution of the spheres./(r); (4) the number of spheres per 

unit volume n; and (5) the effective collision area or the 

collision efficiency of the sphere with radius R, £ . The 

collision efficiency £ is a function of both R and r and other 

physical parameters. By coalescence of spheres with radii 

from r to r+dr, the volume added to the sphere with radius R 

in time dt is

dVR - £ A(vR -vr)4/3irrinf(r)drdt. 

The total volume increment dVR is

SdYR=4/3TrinS0R t (R+r?ry(r) (vR-vr) dr. 

Since the volume of a large sphere is VR=4/3irR3 , the 

volume increment is dVp=4 irR2dR. Therefore the change of 

sphere radius with time by collision is

Let us examine parts of the above equation in more detail. 

The settling velocities V R and vr probably obey Stoke's law 

up to sphere with radii between 4 and 9 mm in the magma 

under consideration, which has densities of 2.726 and 2.712 

g/cm3 and viscosities of 149.9 and 59.1 poises. This 

conclusion is based on the equation

max

1/3

4gP,(p -P.)

where R is the Reynolds number; 77, the viscosity; g, the 

acceleration of gravity; P , the density of the magma; and ps,
58 STILLWATER COMPLEX, MONTANA

H-Zone inclusions and interstitial 

G-Zone inclusions and interstitial

0.25 0.50 0.75 

DIAMETER OF GRAINS, IN MILLIMETERS

Basal zone disseminated

14 16 18 20 22 24 26 28 

DIAMETER OF CLOTS, IN MILLIMETERS

FIGURE 42. Clot size (A) and grain size (B) distributions of disseminated sulfldes and inclusions.

the density of the spheres (Shaw, 1965). The maximum 

sphere radius rmax for Stoke's law settling is therefore 

dependent upon R, values of which are taken in the range 

0.05 to 0.1 that are the limits for Stoke's settling 

(Christiansen, 1935; McNown and Malaika, 1950). Settling 

of the large spheres (fig. 41) must be described by other 

functions (Rubey, 1933).

Studies of size distributions of cumulus minerals in 

the Stillwater Complex (Jackson, 1961a, p. 28) suggest that 

most size distributions are lognormal. Within sediments 

some size distributions are lognormal but many are not, and 

various equations to describe other size distributions have 

been given (Pettijohn, 1957). For raindrops lognormal size 

distributions have been used, but other distribution laws 

have been suggested (Matveev, 1967). The size distribution 

of the sulfide-oxide spheres during the growth of a sphere

during coalescence /(r), if assumed to be lognormal, may be 

expressed as

r-(lnr-lnr -(lnr-lnr0) 2 ~|

—— — ——— 

|_ 2<r2 J

where In r0 is the arithmetic mean of logarithms of sphere 

radii and<y =Y(m r~ ln >"o)2 -The size distribution f(r) changes 

with time during collisions as does In r0 and <r.

Evaluation of the collision efficiency £ for a sphere of 

radius R involves consideration of the trajectories taken by 

the smaller spheres of radius r as they approach the large 

sphere. Some of the smaller spheres within the cylinder 

swept out by the larger spheres can flow around the large 

spheres by the motion of the magma streamlining around the 

spheres. Besides streamlining, attractive and repulsive forces 

between sulfide-oxide spheres could modify collision ef-
SULFIDE-BEARING ROCKS 59

8 10 12 14 16 18 

DIAMETERS OF SULFIDE SPHERES, IN MILLIMETERS

FIGURE 43.- Hypothetical size distributions of sulfide spheres in the Stillwater magma.

20' '38 40

ficiency. Empirical and theoretical expressions for the 

collision efficiency of raindrops (Langmuir, 1948; Matveev, 

1967) have been developed and the parameter £ has been 

shown to be a function of the Reynolds number, Froude 

number, and drag coefficient, as well as the size of the 

collector and the size of the particles collected. The extension 

of collision efficiencies of raindrops in air to sulfide-oxide 

spheres in basaltic magma does not appear to be possible, 

and solution of the problem must await experimental data. 

In order to complete a model of collision and coalescence 

of sulfide-oxide spheres, values of collision efficiency and 

size distributions of the spheres are needed. Both probably 

could be derived from appropriate experiments. However, 

from the raindrop analogy and analysis presented above, it 

appears that collision and coalscence are adequate to 

concentrate sulfide-oxide liquids from the basaltic magma. 

The present distribution of sulfide material as disseminated clots, matrix sulfide, and massive sulfide is a 

result of supply of sulfide-oxide liquids and their coalescence 

into masses that could settle rapidly enough to collect.

CRYSTALLIZATION OK THE COLLECTED 

SULFIDE-OXIDE LIQUID

Experimental studies of the phase relations in the Fe-Ni-S 

system, recently summarized and extended to lower 

temperatures by Misra and Fleet (1973), imply that the ore 

mineral assemblages of the Basal zone provide only indirect 

evidence on the conditions existing when the sulfides 

collected. Pyrrhotite-pentlandite-chalcopyrite assemblages

probably result from chemical adjustments to changing 

physical conditions down to 230° C and lower temperatures. 

Diffusion of metallic ions within sulfide phases is an 

important process that will alter original high-temperature 

histories (Ewers and Hudson, 1972). Later thermal events 

such as the intrusion of the quartz monzonite sequence or 

low-grade metamorphism and deformation can alter any 

primary high-temperature artifacts. Nevertheless, it is 

worthwhile to try to unravel the crystallization history of the 

sulfide material within this unfavorable framework.

That the sulfide material collected as sulfide-oxide liquids 

is shown by the fact that postcumulus plagioclase locally 

forms euhedral laths where it projects into disseminated, 

matrix, and massive sulfide just as euhedral cumulus 

minerals project into the postcumulus minerals. The sulfide 

fraction must have been liquid and must have completed 

crystallization after the postcumulus phases. Other features 

present before the crystallization of the sulfide liquid include 

the magnetite-ilmenite intergrowths that occur as subhedral 

to euhedral crystals predominantly in the matrix and 

massive sulfide, but locally in disseminated sulfide. These 

oxide intergrowths are interpreted as cumulus crystals that 

were trapped in the sulfide liquid as were the cumulus silicate 

minerals.

The composition of the sulfide-oxide liquid would have 

been controlled by the host basaltic magma. From 

arguments based on his experimental study of the Fe-S-O 

system, Naldrett (1969) suggests that small sulfide-oxide 

droplets interact with the basaltic magma and thus
60 STILLWATER COMPLEX, MONTANA

equilibrate with it, equalizing the oxygen and sulfur 

fugacities between the magma and the sulfide-oxide liquids. 

Such an interaction depends upon rapid exchange or 

diffusion of oxygen and sulfur, or possibly ferrous and ferric 

iron, between the two liquids. If the collision and coalescence 

model developed in the previous section applies to the 

accumulation of the sulfides, then it is important to ask at 

what size of droplet will the processes of diffusion be able to 

maintain equilibrium.

The variable magnetite content of sulfide inclusions and 

interstitial sulfide material suggests that equilibrium was not 

always reached. Naldrett's (1969) analysis predicted that 

there would not be any magnetite crystallized from the 

sulfide-oxide liquid if it were in equilibrium, and therefore 

sulfide inclusions, interstitial sulfide material, and disseminated sulfide material should not contain magnetite. 

One might conclude that sulfide containing no magnetite 

crystallized from sulfide-oxide liquid in equilibrium with the 

adjoining basaltic magma, whereas that containing 

magnetite did not.

Sulfide-oxide liquid that accumulated as disseminated 

sulfides probably was in the form of larger droplets that fell 

onto the accumulating crystal pile and replaced the existing 

interstitial basaltic magma because of the density contrast. 

Matrix sulfide may have been droplets that were supplied 

rapidly to the pile and that coalesced on and in the crystal 

pile as they displaced silicate magma. Conversely, the same 

texture might develop if the cumulus silicates were growing 

slowly. Massive sulfide represents droplets that accumulated 

more rapidly than the silicates and that displaced basaltic 

magma as well as floated cumulus silicates. After collecting 

in their various forms, the sulfides began to crystallize.

Two early crystallization patterns are defined, one based 

on texture, the other on experimental evidence. The 

gersdorffite group of minerals, although sparsely distributed, tends to occur as euhedral to subhedral crystals 

molded by pyrrhotite, pentlandite, and chalcopyrite, and 

probably crystallized early from the sulfide-oxide liquid. 

Similar observations have been made about gersdorffite in 

the sulfide deposits associated with the Sudbury intrusives 

(Hawley, 1962). Experimental studies by Naldrett, Craig, 

and Kullerud (1967), Yund and Kullerud (1966), Craig and 

Kullerud (1969), and Naldrett (1969), in which copper and 

nickel were added to the Fe-S-O system, show that 

sulfide-oxide liquid in the Basal zone with less than 15 weight 

percent nickel and 4 weight percent copper would begin to 

crystallize a nickel-copper pyrrhotite solid solution between 

1150° and 1050°C. A fluid pressure of 2 kb would not alter 

this temperature interval by much (Naldrett, 1969) nor 

would nickel and copper contents below the stated values. 

Over this temperature range nickel-copper pyrrhotite solid 

solution and sulfide liquid would be present. Solidification 

of the sulfide-oxide liquids would continue, and if the sulfide 

mass were isolated from the basaltic magma, crystallization 

of nickel-copper pyrrhotite solid solution would be accompanied by crystallization of magnetite. If the sulfide mass 

were able to exchange with the basaltic magma, the 

crystalline sulfide product would contain no magnetite. In 

the Basal zone sulfide materials, both situations prevailed 

and sulfide-oxide liquids crystallized to nickel-copper 

pyrrhotite solid solutions with variable amounts of 

magnetite.

Coexistence of a solid and liquid phase opens the 

possibility that within larger sulfide masses, the sulfide-oxide liquid could differentiate by gravity separation of the 

nickel-copper pyrrhotite from the liquid, or that the liquid 

could separate from the crystals filter-pressing. Either 

mechanism could produce variable compositions of sulfide-oxide liquid that might have different crystallization 

histories. For some compositions, chalcopyrite solid 

solutions could crystallize at about 970°C and coexist with 

nickel-copper pyrrhotite and copper-enriched sulfide liquids 

(Craig and Kullerud, 1969, p. 356). This would allow 

separation and local concentration of copper-enriched 

sulfide liquid. Whether these processes operated in the Basal 

zone is not yet documented.

Experimental work (Craig and Kullerud, 1969; Naldrett 

and others, 1967; Misra and Fleet, 1973) shows that the 

temperature of exsolution of pentlandite and chalcopyrite 

from the nickel-copper pyrrhotite solid solution depends 

primarily on the nickel, copper, and sulfur contents. Below 

about 600° C pentlandite could exsolve from the solid 

solution, and below 575° C both pentlandite and 

chalcopyrite could exsolve from the pyrrhotite solid solution 

(Craig and Kullerud, 1969). Comparison of 

pyrrhotite-pentlandite-chalcopyrite textures in the sulfide 

masses with other similar types of deposits suggests that the 

sulfide minerals present are products of this type of 

exsolution. Both blocky pentlandite necklaces and flame 

textures could have been formed in this way. According to 

Misra and Fleet (1973), the compositions of the phases 

would continue to adjust as temperature decreases. The low 

nickel content of pyrrhotite (Page, 1972) suggests that 

reequilibration continued below 230° C (see Misra and Fleet, 

1973, p. 531), at which temperature both hexagonal and 

monoclinic pyrrhotite are considered stable phases. The 

combination of experimental evidence and textures suggest 

that the present mineral assemblages and compositions 

reflect only very low temperature attempts at equilibration.

Superimposed on these characteristics of the sulfide 

material are the effects of the quartz monzonite intrusive 

bodies in the eastern part of the complex and deformation 

features near fault zones. In the eastern part of the complex, 

some massive sulfide rocks contain veins enriched in 

chalcopyrite that may have been remobilized. These veins 

appear to be in fractures, possibly related to quartz 

monzonite emplacement. Pyrrhotite deformation features 

have been observed throughout the Basal zone sulfide and 

generally can be related to late faulting.
REFERENCES CITED 61

LIMITATIONS AND SPECULATIONS 

ON ORIGIN OF BASAL ZONE

Models for the origin, emplacement, and crystallization 

history of the Basal zone of the Stillwater Complex that 

include consideration of its shape, stratigraphy, rock type, 

textures, and their relations are limited by the following 

observations and deductions:

1. The contact between the metasedimentary rocks of 

Precambrian age and the Basal zone does not follow a single 

stratigraphic horizon but cuts across stratigraphic and 

lithologic units in the country rocks.

2. The overall shape of the Basal zone approximates a 

tabular prism with an extremely large breadth to thickness 

ratio (about 256 to 1, based on the exposed complex) but in 

detail consists of a series of basinlike shapes linked by 

thinner interbasin areas.

3. A variety of rocks are present: Cumulates, dominated 

in abundance by orthopyroxene cumulates but with minor 

amounts of one- and two-phase cumulates consisting of 

olivine, clinopyroxene, inverted pigeonite, plagioclase, and 

chromite; noncumulate igneous rocks with ophitic, 

subophitic, and ragged textures; contaminated and mixed 

rocks; rocks derived by partial melting of metasedimentary 

rocks; sulfide-bearing and sulfide rocks; and complex 

breccias of these rocks.

4. The rocks and their constituents form fluctuating and 

repeating patterns of chemical and physical properties which 

include: (a) The ascending lithologic sequence ophitic, 

subophitic, ragged, and cumulus-textured rocks; (b) 

cumulate sequences of orthopyroxene, 

orthopyroxene + plagioclase; olivine + orthopyroxene, 

orthopyroxene; and plagioclase, plagioclase 

+orthopyroxene; (c) fine- to medium- to coarse-grained 

cumulate sequences; (d) variation in volume of cumulus 

phases present; (e) variation in orthopyroxene and 

plagioclase compositions; and (f) variation in the 

distribution of sulfide minerals.

5. A lower member, the Basal norite, is extremely 

variable both laterally and vertically in mineralogy, texture, 

grain size, crystallization sequences, and fabric and is 

overlain by an upper member, the Basal bronzite cumulate, 

which exhibits relatively regular properties laterally.

6. Evidence exists for variability in processes and 

products in the Basal zone in contrast to the evidence for 

extreme uniformity in processes and products in the 

overlying Ultramafic zone.

7. Other facets, not covered by this report, such as heat 

transfer and balance during solidification, source(s) of the 

magma(s), original extent and size of the complex, and many 

detailed observations not mentioned specifically in the first 

six limitations have a bearing on the origin of the complex.

Suggestions and interpretations, offered as part of a 

model for the origin of the Basal zone, contain many 

elements included in the above observations but, if applied

stringently on local scales, have their limitations. The 

following interpretation is offered as compatible with the 

essential facts as presently known.

During or immediately after initial emplacement of 

magma, several characteristics of the basal contact zone 

developed. Quantities of silicate magma and sulfide-oxide 

liquid were injected into the metasedimentary rocks, 

forming the tongues, dikes, and lenses that account for the 

complicated intrusive relations in areas such as Verdigris 

Creek in the Mountain View area. Some of the early silicate 

magma was emplaced in rocks that were cool enough to 

cause it to crystallize as ophitic, subophitic, and 

ragged-textured noncumulate rocks; subsequent injections 

of magma cooled more slowly and developed textures 

approaching those of cumulate rocks; and the massive 

sulfide lenses crystallized last. Within the main magma 

chamber near the contact and attached to metasedimentary 

rocks, noncumulate igneous rocks crystallized by relatively 

rapid cooling or possibly quenching. Large blocks and 

inclusions of metasedimentary rocks, locally with rinds of 

noncumulate igneous-textured rocks, were rafted upward 

in the magma chamber. Locally crystallization and 

accumulation of cumulates began, were interrupted either by 

a new influx of magma or by convection, and then began 

anew. This process of approaching a mechanical 

equilibrium, interrupting it, and approaching another was 

probably the major mechanism operating during the 

development of the Basal norite. As the magma underwent 

mechanical perturbation, it tried to reach thermal and 

chemical equilibrium. Most likely the repetitions and 

inhomogeneities exhibited by the Basal norite are a result of 

approaches to both chemical and mechanical equilibrium, 

interrupted by influxes of magma. Perhaps, by the time the 

upper part of the Basal norite was crystallizing or at least by 

the time the Basal bronzite cumulate was accumulating, 

there were no new influxes of magma and the crystallization 

pattern was established without further interruption.

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INDEX

[Page numbers of major references are in italic]

Abstract.................

Acknowledgments ........

Age, Basal zone ..........

Stillwater Complex ... 

Alae lava lake, correlation . 

Alkali basalt .............

Page

............ 1

............ 3

............ 7

............ 2

............ 51

............ 26

Amphiboles ............................... 44, 47, 49

Anorthite (An) content...................... 22, 36

Anorthite-enstatite (An-En) correlation ....... 22

Anorthosites ............................... 33

Apatite .................................... 35

Aplite ..................................... 6, 7

Apposition fabrics.......................... 4

Autohornfelsed rocks ....................... 39

B

Banded zone ............................... 1, 28

aeromagnetic low ....................... 5

Basal bronzite cumulate............ 1, 4, 16, 31, 40, 61

contact ................................ 7

current structures ...................... 18

distribution ............................ 4

exposures .............................. 4

onlap .................................. 7

thickness .............................. 5

Basal norite............... 1, 6, 7, 31, 34, 37, 38, 56, 61

autohornfelsed Basal zone ............... 38

composition ............................ 4

contact ................................ 7

current structures ...................... 18

lensoid masses.......................... 4

mafic dikes............................. 7

noncumulate rocks...................... 23

silicate minerals ........................ 38

sulfide minerals......................... 40

texture ................................ 4, 39

thickness .............................. 5

ultramafic xenoliths..................... 38

Basal zone ........................... 1, 4, 5, 8,16, 61

aeromagnetic high ...................... 5

age .................................... 7

Basal bronzite cumulate.................. 1, 5, 61

Basal norite ............................. 1, 5, 61

Big 7 Fault............................. 7

breccias ................................. 7 8, 34

brown hornblende....................... 54

contacts ............................... 6, 7

contaminated igneous rocks ............. 31

copper-nickel grades .................... 49

crystallization......................... 17, 26, 61

cumulate rocks ......................... 16, 23

cumulate sequences ..................... 16, 17

cumulus minerals ....................... 16

definition .............................. 4

dikes .................................. 34

faulting ................................ 5, 6

grain size ............................. 17, 18, 30

hornfels inclusions ...................... 29

hornsfelsed metasedimentary rocks ..... 7, 8, 36, 39

igneous rocks ........................... 7, 8

inclusions .............................. 34

interstitial shapes....................... 42

Basal zone Continued

lithology ..................

metasedimentary rocks 

mineral constituents .......

mineral potential ..........

mineralogy ................

modal variation............

onlap .....................

oxygen fugacity ...........

parent magma, composition. 

Peridotite member.........

petrology .................

postcumulus minerals......

pyrrhotite deformation .....

quartz norite veins.........

rock types.................

settling velocity ...........

shape .....................

stratigraphy

	Page

....... 61

....... 31

....... 18

....... 1

....... 7,48

....... 20

....... 7

....... 54

....... 28

....... 4

....... 7

....... 16

....... 60

....... 35

....... 7, 8

....... 30

....... 5, 61

....... 61

sulfide ............................... 7, 8, 41, 57

sulfide minerals......................... 39, 50

sulfide spheres.......................... 57

sulfide texture.......................... 41

sulfur content.......................... 51, 52

sulfur saturation........................ 52

thickness .............................. 5

Ultramafic zone ........................ 4

Basaltic magma ............................ 53, 59

partial molar volumes ................... 28

Beartooth Mountains ....................... 1,2

Beartooth terrane, metadolerite dikes ........ 39

Benbow area................ 5, 7, 23, 32, 35, 38,40, 53

Benbow East Golf Course ................... 21

Benbow-Nye Lip area ....................... 5

Benbow West Golf Course ................... 20

Big 7 fault................................. 7

Biotite .................. 4, 8,16, 31, 34, 36, 38, 39, 54

Blakely .................................... 40

Blakely Creek .............................. 5, 6

Bluebird Peak.............................. 6

Bluebird section ............................ 21

Bottinga, Y., analyst ....................... 28

Boulder River....................... 5,6, 7,21, 38,40

BravoKte .................................. 46

Breccia, intrusive................ 6, 7, 8, 34,40, 44, 52

Bronzite .................................... 2, 7, 17

Bronzitite member.......................... 1,5

Bushveld intrusion ......................... 26

Bushveld magma, calcification............... 33

Carr Boyd Complex, correlation.............. 50

Chalcopyrite ............ 8, 35, 44, 46, 48, 50, 55, 59,60

abundance ............................. 50

phases ................................. 48

cubanite lamellae ................... 48

haycockite ......................... 48

mooihoekite ........................ 48

talnakhite .......................... 48

texture ................................ 48

Chlorite.................................... 49

Chrome Mountain...................... 5,6, 7, 35, 38

Chromite ............................... 2, 16, 46, 49

Chromitites ................................ 56

Page 

Clinoamphibole............................. 39

Clinopyroxene ................. 4,16,18, 23,35, 38,39

modal composition...................... 18

Clinopyroxenite ............................ 32

Cobaltite group............................. 49

Contaminated igneous rocks ................. 31

Copper .................................... 60

Cordierite ........................ 8, 31, 32, 33, 36, 44

Cordierite norite, correlation................. 38

Cordierite xenoliths......................... 33

Country rock ........................ 5, 29, 35, 38, 54

Crescent Creek .............................. 5, 6, 40

Crystallization............... 23, 26, 41, 42,53 54, 59

Crystallization history ...................... 59

Crystallization order ................ 17, 28, 42, 52, 53

Cubanite................................... 48, 49

Cumulate sequences ............... 2, 16, 17,18, 36,61

Cumulates ............. 2, 7, 8, 16, 17,18, 23, 31, 36, 58

Cumulus minerals .......................... 16

sulfide phases .......................... 16

Cunnon Shoot, correlation ................... 50

Cuprite .................................... 46, 49

Current-formed fabrics...................... 4

Current structures.......................... 18

D

Deformation twins, pyrrhotite ............... 47

Diabasic texture............................ 4, 7

Disseminated sulfide........... 8, 39, 41, 42, 55, 57, 59

formation .............................. 60

origin .................................. 42

oxide intergrowths...................... 59

sulfide spheres.......................... 55, 57

texture ................................ 42

Dufek intrusion, correlation ................. 32

Duluth Complex............................ 50

Dunite..................................... 38

Dunite xenoliths............................ 39

E, F

East Boulder River ......................... 5

Elmore, P., analyst......................... 25

Enstatite (En) content..................... 21, 22, 36

Exsolved ilmenite .......................... 46

Exsolution lamellae .................... 24, 31, 35, 38

Fabbi, B. P., analyst........................ 25

Falls Creek................................. 6

Faulting .................................... 6, 7, 60

Feldspar ................................... 4

Ferric iron ................................. 60

Ferrous iron................................ 60

Flume Creek ............................... 6

Forge Creek................................. 5, 6, 21

Fractional crystallization.................... 28, 54

Gabbroic rocks ............................. 4, 6

Geologic history ............................ 2

Gersdorffite................................ 49, 60

Gish area .................................. 5

67
68 INDEX

Page

Glen, J., analyst............................ 25

Goethite ................................... 46, 49

Goss, W. D., analyst........................ 25

Gossans ................................... 40

Grain size................................. 17, 18, 30

Granoblastic texture........................ 38

Granofelsic mosaic texture .................. 39

Great Dyke ................................ 19, 26

Green spinel. See spinel.

H

Haffty, Joseph, analyst..................... 25

Haubert, A. W., analyst..................... 25

Hawaiian tholeiitic basalt................... 26, 29

Haycockite ................................ 48

Hematite .................................. 48

Hercynite.................................. 31

Heropoulos, Chris, analyst.................. 25

Hess, H. H., analyst........................ 26

Hornblende .............................. 4, 7, 16, 54

Hornblende quartz diorite ................... 7

Hornfels................... 6, 29, 31, 33, 36, 37, 38, 44

Hornfels inclusions................... 8, 29, 32, 35, 40

Hornsfelsed metasedimentary rocks .... 7, 8, 34, 36, 44

I

Igneous rocks ............................... 7, 8, 23

autohornfelsed noncumulate ............. 8

contaminated........................... 8, 31

sulfide mineralization ................... 50

Igneous-textured rocks ...................... 8, 23, 26

chemical composition ................... 26

Ilmenite ............................... 16, 46, 47, 59

Ilmenite-hematite intergrowths .............. 54

Ilmenite lamellae .......................... 48, 49, 54

Initial area ............................... 6, 7, 40,53

Initial-Cathedral Creek ..................... 5

Initial Creek ............................... 5

Inverted Pigeonite. See orthopyroxene.

Iron formation, metasedimentary rocks....... 6, 60

Iron Mountain ............................ 5, 6, 7, 38

Iron oxide.................................. 35

Iron-titanium-chromium oxides .............. 4

Irvine, T.N., fractional crystallization, phasediagram model.................. 28

K, L

Kelsey, J., analyst.......................... 25

Kink bands ................................ 39, 47

Lamellae..................... 24, 35,38, 46, 48, 49, 54

Linear cumulates ........................... 19

Lithologic sequence......................... 61

M

Mackinawite ............................... 46, 49

Mafic dikes ................................. 6, 7, 38

composition ............................ 7

Mafic rock ................................. 41

Magmatic sediment......................... 4

Magnetite .......... 16, 31, 44, 46, 47, 48, 49, 54, 59, 60

mineralogy ............................. 49

Magnetite-hematite buffer .................. 54

Marcasite .................................. 46, 49

Mashaba igneous complex, correlation........ 32

Massive sulfide ................ 8, 39, 41, 44, 52, 54, 59

formation .............................. 60

oxide minerals .......................... 49

Matrix sulfide........... 8, 39, 41, 42, 44, 49, 54, 59,60

formation .............................. 60

origin .................................. 42

oxide minerals .......................... 49

texture ................................ 42

Mays, R. E., analyst........................ 25

Metadolerite dikes.......................... 39

Page

Metaquartzite .............................. 6, 32

Metasedimentary inclusion .................. 32, 33

Metasedimentary rocks .......... 31, 33, 37, 40, 52, 61

contact ................................ 6

Mineral potential, Basal zone ................ 1

Mineralogy, Basal zone ..................... 7,31

Minerals, amphiboles ...................... 44, 47, 49

apatite................................. 35

aplite .................................. 6, 7

biotite ............... 4, 8,16, 31, 34, 36, 38, 39, 54

bravolite ............................... 46

bronzite ................................. 2, 7,17

chalcopyrite ......... 8, 35, 44, 46, 48, 50, 55, 59,60

chlorite ................................ 49

chromite ............................ 2, 16, 46, 49

chromitites............................. 56

clinoamphibole ......................... 39

clinopyroxene ............. 4, 16, 18, 23, 35, 38, 39

cobaltite group ......................... 49

copper ................................. 60

cordierite ..................... 8, 31, 32, 33, 36, 44

cubanite ............................... 48, 49

cuprite ................................. 46, 49

dunite ................................. 38

exsolved ilmenite ....................... 46

feldspar................................ 4

ferric iron .............................. 60

ferrous iron............................. 60

gersdorffite ............................ 49, 60

goethite................................ 46, 49

green spinel........................ 16, 31,36, 44

haycockite ............................. 48

hematite ............................... 48

hercynite............................... 31

hornblende........................... 4, 7,16, 54

hornfels ................ 6, 29,31, 33, 36,37, 38,44

ilmenite............................ 61, 46, 47, 59

inverted pigeonite. See orthopyroxene.

iron oxide .............................. 35

mackinawite ........................... 46, 49

magnetite....... 16,31, 44, 46, 47, 48, 49, 54, 49, 60

marcasite .............................. 46, 49

metaquartzite .......................... 6, 32

mooihoekite ............................ 48

native copper ........................... 46, 49

nickel .................................. 60

olivine ............ 2, 4, 7, 16, 17, 18, 22, 26, 28, 30,

38, 39, 61 

opaque minerals ........................ 36, 38

orthopyroxene............. 4, 6, 7, 8, 16, 17, 18, 23

24, 28, 30, 31, 34, 36, 38, 39, 44, 54, 61 

oxide minerals......................... 48, 49, 50

pentlandite ...... 8, 35, 44, 47, 48, 49, 50, 55, 59, 60

plagioclase........ 4, 7, 8, 17, 18, 23, 24, 26, 28, 30,

31, 32, 34, 35, 38, 44, 50, 61 

potassium feldspar...................... 35

pyrite .................................. 46, 49

pyroxene............................... 50

pyrrhotite ............. 8, 35, 44, 46, 47, 48, 49, 50,

55, 59, 60 

quartz ............................ 4, 8,16,32,34

quartz norite....................... 34, 36, 37, 38

serpentine ............................ 46, 47, 49

silicate minerals ....................... 41, 44, 47

spinel............................. 16, 31, 36, 44

sulfide minerals....................... 4, 7, 46,49

talc .................................... 47

talnakhite.............................. 48

violarite................................ 46, 49

websterite.............................. 38, 39

zircon.................................. 7

Modal variation..................... 20, 23,24,32,36

Mooihoekite................................ 48

Mosaic-textured groundmasses .............. 23

Mouat area ................................ 49

Mouat mine area ............................ 46

Mt. Douglas ............................... 1

Mt. Wood.................................. 1

Mountain View........... 5,6, 23,32,35, 38,40, 52, 61

Muskox intrusion........................... 26

Page

N

Naldrett, A. J., analyst...................... 60

Native copper .............................. 46, 49

Net-textured ore............................ 41

Nickel ..................................... 60

Noncumulate rocks ........................ 23, 24, 52

chemical composition ................... 24

See also igneous-textured rocks. 

Noritic dikes ............................... 6

Noritic rocks ................................ 4, 6, 31

Nye Basin....................... 17, 21, 22, 23,35, 40

Nye Creek ................................. 6

O

Olivine....... 2, 4, 7, 16, 17,18, 22, 26, 28,30, 38, 39,61

composition ............................ 16

kink bands ............................. 39

Opaque minerals ........................... 36, 38

chromite ............................... 49

cobaltite group ......................... 49

cubanite ............................... 49

Ophitic gabbro ............................. 32

Ophitic norite .............................. 31

Ophitic rocks .................... 23, 24, 26, 31, 50, 52

Orthopyroxene ............. 4, 6,7, 8,16, 17, 18, 23, 24,

28, 30, 31, 34, 36, 38, 39, 44, 54, 61 

abundance ............................. 16

composition ............................ 16, 21

enstatite (En) content................... 21

exsolution lamellae...................... 16, 35

modal composition ...................... 18

settling velocity ........................ 55

Orthopyroxene cumulates, linear............. 19

planar ................................. 19

See also orthopyroxene. 

Orthopyroxenite............................ 38

Oxide minerals ............................ 48, 49, 50

massive sulfide ......................... 49

matrix sulfide .......................... 49

sulfide correlations...................... 50

Oxygen fugacity............................ 54, 60

P

Partial melting ............................ 34, 37,38

Partial melts ............................... 8

Pelitic rock inclusions ....................... 33

Pentlandite .......... 8, 35, 44, 47, 48, 49, 50, 55, 59, 60

abundance ............................. 50

fractured............................... 47

nickel content .......................... 48

vermicular ............................. 47

Peridotite member......................... 1, 4, 6,31

basins ................................. 5

chromite cumulates ..................... 2

cumulate sequence ...................... 2

cyclic units ............................. 2

grain size .............................. 17

orthopyroxene.......................... 7

Petrofabric studies ......................... 19

Petrology .................................. 7

Phenocrysts ............................ 8, 20, 22, 44

Plagioclase ............... 4, 7, 8,17,18, 23, 24, 26, 28,

30, 31, 32, 34, 35, 38, 44, 50, 61 

abundance ............................. 16

anorthite (An) content................... 22

composition ............................ 16, 21

euhedrallaths .......................... 59

modal composition ...................... 18

poikilitic ............................... 31

Planar lamination .......................... 19

Poikilitic crystals ...................... 31, 35, 36, 39

Postcumulus minerals ...................... 16

Potassium feldspar ......................... 35

Pyrite ..................................... 46, 49

Pyroxene .................................. 50

Pyroxene phenocrysts ...................... 8

Pyrrhotite........ 8, 35,44, 46, 47, 48, 49,5ft 55, 59, 60

abundance ............................. 47, 50
INDEX

Pyrrhotite Continued

composition .......

deformation .......

deformation twins .

texture

Page

47

60

47

46

47

Q,R

Quartz ............................... 4, 8, 16, 32, 34

Quartz monzonite, intrusive ............. 5, 6, 7, 59, 60

Quartz norite ....................... 34, 35, 36, 37, 38

An content ......

composition .....

En content ......

lenses ...........

mineralogy ......

modal variation ..

origin ...........

veins ...........

Ragged norite .......

References cited .....

Renison Bell deposit .

36

37

36

37

35

36

36

35

31

61

46

Separation mechanisms ..................... 53

sulfur fugacity.......................... 53

Sequences ........................... 8, 16, 17, 23, 36

Serpentine ................................ 44, 47, 49

Settling velocities...................... 29, 30, 55, 57

Shaw, H.R., settling velocity, Stillwater

magma ......................... 29

Silicate crystallization ...................... 41, 42

Silicate minerals ........................... 41, 44, 47

comagmatic origin ...................... 51

cumulus ............................... 59

Silicate magma ............................ 42, 60, 61

sulfide correlations...................... 50

Sittampundi Complex, correlation............ 33

Smith, H., analyst.......................... 25

Spinel................................. 16, 31, 36, 44

Stillwater Complex ....................... 1, 6, 21, 61

age .................................... 2

Banded zone............................ 1

Basal bronzite cumulate member ......... 16

Basal norite ............................ 38

Basal zone ............................. 1, 16, 61

copper-to-nickel ratios................... 50

crystallization .......................... 17

cumulates .............................. 16, 58

two-phase .......................... 18

cyclic unit.............................. 17

mafic dikes ............................. 7

magma composition..................... 24

metasedimentary inclusion .............. 33

mixing................................. 33

Peridotite member ...................... 17

porosity................................ 41

Precambrian mafic rock ................. 1

sulfide mineralization ................... 41, 42

correlations ........................ 50

sulfide-oxide liquids..................... 52

sulfide spheres.......................... 56, 58

sulfur saturation........................ 52

sulfurization ........................... 53

temperature............................ 53

contamination .................

molar volumes .................

origin .........................

oxygen fugacity ...............

Plagioclase density ............

quartz-fayalite-magnetite buffer 

quartz norite ..................

settling velocity ...............

Page 

Stillwater Complex Continued

ultramafic xenoliths ..................... 38

Ultramafic zone ........................ 1, 16, 61

Upper zone ............................. 1

Stillwater magma ......... 28, 29, 36, 51, 52, 53, 54, 56

... 54

... 28

... 52

... 54

... 29

... 54

... 36

... 29

sulfide-oxide liquids ..................... 52, 53

sulfide spheres .......................... 56

sulfur content .......................... 51

viscosity ............................... 29

Stillwater River, West Fork ................. 5, 7

Strathcona mine, correlation ................. 50

Strathcona ore deposit, correlation ........... 39, 46

Subophitic norite ........................... 31

Subophitic rocks ................. 23, 24, 26, 31, 50, 52

Sudbury intrusive, correlation ............... 60

Sudbury Nickel Irruptive, correlation ........ 39

Sudbury ores, correlation ................... 50

Sulfide-bearing rocks ........................ 8, 39, 41

disseminated sulfide ........ 8, 39, 41, 42, 55, 57, 59

massive sulfide ............ 8, 39, 41, 44, 52, 54, 59

matrix sulfide ....... 8, 39, 41, 42, 44, 49, 54, 59, 60

Sulfide crystallization, concentration models . . 42

separation mechanisms .................. 53

Sulfide crystallization model ................. 52, 53

Sulfide mineralization ....................... 41, 50

Sulfide minerals ............................ 4, 7

Basal norite ............................ 40

bravolite ............................... 46

chalcopyrite ............................ 46

chromite ............................... 46

comagmatic origin ...................... 50, 51

cuprite ................................. 46, 49

exsolved ilmenite ....................... 46

goethite ................................ 46, 49

hydrothermal deposition ................ 50

igneous rocks ........................... 50

ilmenite ................................ 46

mackinawite ........................... 46, 49

magnetite .............................. 46, 49

marcasite .............................. 46, 49

metasedimentary rocks .................. 40

mineralogy ............................. 44

modal proportions ...................... 49

native copper ........................... 46, 49

occurrence ............................. 40

pentlandite ......................... 44, 47, 48, 49

Peridotite member ...................... 40

pyrite .................................. 46, 49

pyrrhotite ...................... 44, 46, 47, 48, 49

resistivity .............................. 41

silicate correlations ..................... 50

Stillwater Complex ..................... 42

terminology ............................ 41

texture ................................ 41

Ultramafic zone ........................ 40

violarite ................................ 46, 49

Sulfide-oxide immiscible liquid .......... 53, 54, 55, 60

collection model ........................ 54, 55

concentration model .................... 54, 55

Page 

Sulfide-oxide immiscible liquid Continued

copper content.......................... 60

nickel content .......................... 60

Sulfide-oxide minerals, chalcopyrite .......... 60

gersdoffite group ....................... 60

pentlandite............................. 60

pyrrhotite.............................. 60

Sulfide pods................................ 6

Sulfide spheres ..................... 55, 56, 57, 58, 60

collision efficiency ...................... 58

collision equations ...................... 57

magnetite content ...................... 60

raindrop analogy ....................... 57

Sulfur fugacity ............................. 53, 60

Talc ....................................... 47

Talnakhite ................................. 48

Texture................. 4, 23, 32, 39,41,42, 44,48,60

diabasic................................ 4, 7

granoblastic ............................ 38, 39

granofelsic mosaic ...................... 39

ophitic ...................... 23, 24, 26, 31, 50, 52

ragged ................................ 23, 31, 52

subophitic................... 23, 24, 26,31, 50, 52

Thingmuli average.......................... 26

Tholeiitic basalt............................ 26, 39

Turbidite deposition ........................ 23

Two-phase cumulates ....................... 18

U, V

Unnamed Creek ............................

Upper Forge Creek. See Forge Creek. 

Ultramafic mantle ..........................

Ultramafic rock ............................

Ultramafic xenoliths........................

Ultramafic zone..................... 1, 4, 6,18,

aeromagnetic high ......................

Bronzitite member......................

contacts ...............................

cumulate rocks .........................

current structures ......................

grain size ..........

interstitial shapes... 

mineral constituents 

Peridotite member .. 

red-brown biotite ...

sequences ........

sulfide minerals.. 

Upper zone...........

aeromagnetic low

21

52

41

38

21,61

5

1

7

23

18

17, 18

42

19

1,7

54

16

40

1, 28

5

Verdigris Creek ........................... 49, 52, 61

Violarite ................................... 46, 49

W 

Websterite ................................. 38, 39

mineralogy ..... 

Weill, D. F., analyst . 

West Fishtail Creek.

X, Z

Xenolith, origin 

Zircon .........

39

28

5, 7

39

7

* U.S. GOVERNMENT PRINTING OFFICE : 1979--O--677-026/41