Composition of Primary 

Postcumulus Amphibole and Phlogopite 

Within an Olivine Cumulate in the 

Stillwater Complex, Montana

U.S. GEOLOGICAL SURVEY BULLETIN 1674-A

Chapter A

Composition of Primary 

Postcumulus Amphibole and Phlogopite 

Within an Olivine Cumulate in the 

Still water Complex, Montana

By NORMAN J PAGE and MICHAEL L. ZIENTEK

U.S. GEOLOGICAL SURVEY BULLETIN 1674

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

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center, Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Page, Norman J

Composition of primary postcumulus amphibole and phlogopite

within an olivine cumulate in the Stillwater Complex, Montana.

(Contributions on ore deposits in the early magmatic environment; A) 

(U.S. Geological Survey Bulletin 1674-A) 

Bibliography

Supt. of Docs. No.: I 19.3:1674-A

1. Amphiboles Montana. 2. Phlogopite Montana. 3. Chrysolite Montana,

I. Zientek, Michael L. II, Title. III. Series. IV. Series: U.S. 

Geological Survey Bulletin 1674-A.

QE75.B9 No. 1674-A 557.3 s 86-600291 

{QE391.A53] [549'.6]
CONTENTS

Abstract Al 

Introduction Al

Acknowledgments A2 

Distribution, texture, and microscopic relations of amphibole and phlogopite A2

Mineralogy and petrology of the olivine cumulate A2

Variation in amount of amphibole and phlogopite in the olivine cumulate A2

Microscopic textures and associations A2 

Techniques A6

Composition and substitution mechanisms of amphibole A6 

Composition and substitution mechanisms of phlogopite A15 

Assemblage-controlled compositional variations in brown amphibole A18

Compositional variation between assemblage groups within the same sample A18

Compositional variation among the assemblage groups within the same hornblende

crystal A20

Variation in hornblende composition with stratigraphic position A25 

Variation in phlogopite composition with stratigraphic position A28 

Comparison of trace elements information A28 

Interpretations and conclusions A29 

References cited A34

FIGURES

1. Graphs showing combined modal volume of amphibole and phlogopite 

correlated with stratigraphic position in cyclic unit 2 A3

2. Photomicrographs showing textures of primary brown amphibole and

phlogopite in olivine cumulates from cyclic unit 2 of the Peridotite zone 

of the Ultramafic series, Nye Basin area A4 

3-8. Graphs showing:

3. Classification and nomenclature of amphiboles A14

4. Al versus (Na, K)A for the hornblendes in table 2 A17

5. AlIV -(Na, K)A versus A1 VI + 2Ti + Cr for hornblendes in table 2 A17

6. NaB versus A1 VI + 2Ti + Cr for hornblendes in table 2 A17

7. Al versus Ti, Cr, and A1 VI for hornblendes in table 2 A18

8. NaB versus A1 VI + 2Ti + Cr + (Na, K)A for hornblendes in table 2 A18

9. Triangular diagram showing phlogopite compositions from olivine cumulate 

of cyclic unit 2 cast as phlogopite, annite, and siderophyllite end 

members A20

10. Triangular diagram showing phlogopite compositions in terms of Ti,6-OSO 

(octahedral site occupancy), and 6-(Si + A1 VI) in structural formula A20

11. Plots of phlogopite compositions All

12. Sketches showing compositional variation in zoned hornblendes A26 

13-16. Graphs showing:

13. Formula units of Cr and Ti in zoned hornblende and associated 

chromite A29

14. Variation in selected compositional properties of hornblende with 

stratigraphic position A30

15. Variation in selected compositional properties of phlogopite with 

stratigraphic position A32

16. Trace element variation in olivine cumulates with stratigraphic 

position A33

Contents III
TABLES

1. Electron microprobe analyses of standard amphibole and biotite compared 

with reported analyses of the standards A7

2. Electron microprobe analyses and calculated mineral formulas of hornblendes 

from the olivine cumulate of cyclic unit 2, Still water Complex, Montana A8

3. Electron microprobe analyses and calculated mineral formulas of secondary 

amphiboles from the olivine cumulate of cyclic unit 2, Still water 

Complex, Montana A13

4. Means and standard deviations of hornblende analyses and formulas by assemblage 

groups A15

5. Comparison of the differences in means between assemblage groups given 

in table 4 A16

6. Electron microprobe analyses and calculated mineral formulas of phlogopite from 

the olivine cumulate of cyclic unit 2, Stillwater Complex, Montana A19

7. Electron microprobe analyses and calculated mineral formulas of zoned 

amphibole crystals from the olivine cumulate of cyclic unit 2, Stillwater 

Complex, Montana A22

8. Electron microprobe analyses and calculated mineral formulas of chromite from 

the olivine cumulate of cyclic unit 2, Stillwater Complex, Montana A25

9. Electron microprobe analyses and calculated mineral formulas of olivine from 

the olivine cumulate of cyclic unit 2, Stillwater Complex, Montana A25

10. Comparison of selected compositional properties of brown amphiboles 

through a section of olivine cumulate containing a chromite cumulate, 

cyclic unit 2, Stillwater Complex, Montana A32

11. Estimated temperatures of brown amphibole crystallization based on experimental 

studies of the 1921 Kilauea olivine tholeiite A34

The use of trade names hi this report is for descriptive purposes only and does not constitute 

endorsement by the U.S. Geological Survey.

IV Contents
CONTRIBUTIONS ON ORE DEPOSITS IN THE EARLY MAGMATIC ENVIRONMENT

Composition of Primary Postcumulus Amphibole and 

Phlogopite Within an Olivine Cumulate in the Stillwater 

Complex, Montana

By Norman J Page and Michael L. Zientek

Abstract

Postcumulus amphibole and phlogopite from an olivine 

cumulate of cyclic unit 2 in the Archean Stillwater Complex, 

Montana, containing the B-chromitite, were analyzed with an 

electron microprobe. The brown amphiboles are pargasite or 

pargasitic hornblendes containing as much as 4.5 weight percent TiO2 and 1.8 weight percent Cr2O3 , and they are compositionally zoned. Three distinct textural occurrences were noted:

(1) as material rimming chromite but not replacing pyroxene,

(2) as interstitial material replacing augite but not in contact with 

chromite, and (3) as interstitial material in rocks that contain no 

cumulus spinel. Cation-cation plots show that most of the 

coupled substitutions are of tschermakitic and edenitic type, and 

that hornblendes in category (3) are distinct from those in 

categories (1) and (2) in not having substitutions including Na 

in the M4 site and in having lower contents of Ti and Cr. Hornblendes associated with chromite have a larger extent of coupled 

substitution than those associated with augite and may be slightly 

more magnesium rich. However, the large extent of coupled 

substitution is not reflected by systematic increases in any one 

of the elements involved in the coupled substitution (that is, 

Al lv, Ti, orCr). Variations in compositions of zoned crystals in 

contact with chromite, pyroxene, olivine, and plagioclase support this observation, with the crystals having higher cation units 

of Ti, Cr, and Al lv when the hornblende is in contact with 

chromite. Variations in the composition of hornblende with 

stratigraphic position allow the size-graded olivine cumulate 

package of units to be divided into three parts. The lower part 

has complex increasing and decreasing trends of Mg/(Mg + 

Fe2+ ) (with an average ratio of 0.82), increasing upward Ti, 

Al lv, and Cl contents, decreasing upward F content, and constant Cr content. Hornblende from the middle part, which contains the B-chromitite, has slightly higher Mg/(Mg + Fe2 + ), 

about 0.83 to 0.84, with Ti increasing upward. The upper part, 

a portion of which contains no cumulus chromite, has lower 

Mg/(Mg + Fe2 + ), approximately 0.81, with Cr and Ti increasing upward. Nominal crystallization temperatures of hornblendes 

based on their Al lv contents range from 905 to 981 °C and 

average about 930 °C. Single zoned crystals appear to record 

a range of temperatures, with higher temperatures (980 °C) near 

chromite and lower temperatures (930-940 °C) away from 

chromite.

Phlogopite has a narrow range of composition (approximately 70 to 80 percent phlogopite end member) and substitutions that involve about equal amounts of annite and siderophyllite. The phlogopite has TiO2 greater than 3.0 weight 

percent (with two exceptions) and Cr2O3 contents up to 1.5 

weight percent. Variations in composition with stratigraphic position similar to those of the hornblendes are observed. It is not 

yet possible to suggest the conditions under which phlogopite 

crystallized except to conjecture that these conditions must have 

been similar to those prevailing when the hornblendes 

crystallized.

The textural and chemical evidence indicate that the hornblendes crystallized from trapped interstitial liquids at high 

temperatures. The variation in compositions suggests that the 

coexisting crystalline assemblage influenced the hornblende 

composition. Large-scale movement (greater than centimeters) 

of trapped liquid, which would homogenize the final hornblende 

compositions, did not occur or had ceased by the time the hornblende crystallized. The low contents of Cl and F in both 

phlogopites and hornblendes indicate that the magma was not 

noticeably enriched in these elements, nor is there any positive 

evidence for these volatiles streaming up through the olivine 

cumulate to concentrate base and precious metals.

INTRODUCTION

Numerous studies of cyclic units in the Peridotite zone 

of the Ultramafic series 1 of the Archean Stillwater Complex 

have examined the petrologic, mineralogic, and geochemical 

characteristics of the cumulus minerals (Jackson, 1961, 1963, 

1967, 1968, 1969, 1970, 1971; Page and others, 1972; 

Raedeke, 1982; Raedeke and McCallum, 1982a, b, 1984). 

However, the postcumulus (interstitial) minerals have not 

been studied in the same detail. Therefore, this investigation focuses on postcumulus amphibole and phlogopite from 

olivine cumulates containing a chromite seam; this unit has 

been correlated with cyclic unit 2 in the Stillwater Complex, 

which contains the B-chromitite (Jackson, 1963, 1968). The 

unit was selected because it contains an olivine cumulate in 

which the mineralogic, petrologic, and geochemical details 

of the cumulus minerals and their alteration are documented 

in detail (Page and others, 1972; Page, 1976; Page and

'The stratigraphic terminology for the Stillwater Complex follows that 

of Zientek and others (1985).

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A1
others, 1976). The comparative simplicity of this unit should 

reduce the number of assemblages to evaluate, even though 

experimental studies have shown that the compositions of 

amphiboles are not strongly influenced by changes in the 

coexisting crystalline assemblage (Helz, 1982).

Amphibole and phlogopite were chosen for study 

because of the many chemical substitutions permissible within 

these minerals that might help elucidate the last stages of 

crystallization; in addition, these minerals could give information on the possible role of fluids in postcumulus processes 

in layered mafic stratiform intrusions. In particular, the Al^ 

and Ti contents of amphiboles are strongly correlated with 

temperature (Helz, 1973). Furthermore, the Cl and F contents of the amphiboles should give evidence for the presence 

of these volatiles in the original melt (Wones and Gilbert, 

1982).

This report documents the textural and chemical 

characteristics of postcumulus amphibole and phlogopite in 

part of a layered mafic intrusion, compares these results with 

the whole-rock trace-element data on the host rocks, and tests 

hypotheses for late-stage processes, such as (1) the role of 

a fluid phase (Bow and others, 1982) and (2) simple 

crystallization from trapped (interstitial) magma.

Acknowledgments

We are grateful to Anaconda Minerals Company for 

making the drill core available for study and for cooperating 

fully with this project. Discussions with geologists of the 

Anaconda Minerals Company were of great benefit, as were 

the critical reviews by Rosalind Helz and Bruce Lipin. Steven 

W. Novak helped modify the computer programs used in 

calculating the mineral formulas so as to include

DISTRIBUTION, TEXTURE, AND MICROSCOPIC 

RELATIONS OF AMPHIBOLE AND PHLOGOPITE

Mineralogy and Petrology of the Olivine Cumulate

The cyclic unit investigated in this study was intersected 

in a diamond-drill hole in the upper part of Nye Basin, east 

of the Still water River. The stratigraphic position of this section in the lower part of the Ultramafic series and the correlation of the section with measured sections elsewhere in 

the complex are reported by Jackson (1968) and Page and 

others (1972). The samples used in this study are from the 

olivine cumulate at the base of this cyclic unit. Most of the 

mineralogic, petrologic, and geochemical information on the 

cumulus minerals incorporated in this report are from Page 

and others (1972).

Olivine grain-size measurements, olivine compositions, 

and volume percentages of postcumulus material are shown 

in figure 1 as a framework for discussing the distribution

of amphibole and phlogopite. The cumulates can be subdivided into a series of nine subunits based upon the grain 

size of olivine and the abundance of chromite. In the first 

three subunits (fig. 1), the grain size of olivine decreases 

systematically within each subunit. Subunits 4 and 5 contain chromite, olivine-chromite, and olivine cumulates and 

are thus distinct. Subunits 7, 8, and 9 are largely based on 

olivine grain size, which decreases in each subunit; however, 

the lower 80 percent of subunit 9 contains no cumulus spinel 

phase, in contrast to all of the other subunits, which contain 

cumulus chromite in minor amounts. Variations in volume 

percentages of minerals, in mineral compositions, and in 

trace-element contents correlate with size-graded subunits 

of the olivine cumulate (Page and others, 1972).

The olivine cumulate of this cyclic unit is fairly typical 

for the Ultramafic series and on the average contains 70.9 

volume percent cumulus olivine, with an average Mg/(Mg 

+ Fe2+) x 100 = 84.0. In addition, the average total mode 

contains 1.7 percent cumulus chromite and traces of pyrrhotite, pentlandite, and chalcopyrite both as inclusions in 

cumulus grains and as postcumulus grains. Postcumulus 

plagioclase (11.1 percent), orthopyroxene (6.0 percent), 

augite (9.0 percent), hornblende (0.7 percent), and phlogopite 

(0.5 percent) account for the rest of the unserpentinized rock. 

Serpentinization and postcrystallization alteration have produced various amounts of the alteration assemblage lizardite 

+ chrysotile ± magnetite ± thompsonite (or other zeolites) 

± calcite, the details of which are discussed in Page (1976).

Variation in Amount of Amphibole and Phlogopite 

in the Olivine Cumulate

Within the olivine cumulate, the maximum amounts of 

combined amphibole and phlogopite are found in size-graded 

subunits 3, 4, and 5 (fig. ID). These are the subunits immediately below and containing the chromite and olivinechromite cumulate layers. There are no overall systematic 

trends in the abundance of combined amphibole and 

phlogopite with stratigraphic height within the entire section. 

However, the combined abundance of amphibole and 

phlogopite decreases upsection within subunit 3, which is 

the subunit below the first chromite cumulates.

Either phlogopite or amphibole may be the more abundant mineral in any particular sample (fig. ID); however, 

no recognizable patterns to their relative abundance were 

found. Also, there are no stratigraphic patterns to their mode 

of textural occurrence.

Microscopic Textures and Associations

Amphibole and phlogopite are not randomly distributed 

within a thin section but instead show very subtle but consistent textural relations with other postcumulus and cumulus 

minerals. Two generations of amphibole are present: (1)

A2 Contributions on Ore Deposits in the Early Magmatic Environment
brown to red-brown (/3), brown to light-brown (y), and colorless (a) crystals that texturally appear to be part of the 

postcumulus magmatic assemblage; and (2) colorless crystals 

that appear to be the product of a subsolidus alteration event. 

The terms brown amphibole and hornblende are used interchangeably to describe amphiboles of generation (1). 

Phlogopite is the only mica recognized; it forms brownishred (/? and y) and pale-yellow (a) crystals that also appear 

to be part of the postcumulus magmatic assemblage.

Commonly, postcumulus brown amphibole is spatially 

associated with either postcumulus pyroxene, cumulus 

chromite, or both. The association with pyroxene may be 

direct, in which case brown amphibole forms discontinuous 

rims on augite (fig. 2A) or, less commonly, on orthopyroxene (fig. 2B). The textures of these rims indicate that the 

hornblende is replacing the pyroxene. Features suggesting 

replacement include (1) irregular grain boundaries between 

hornblende and pyroxene, (2) optically continuous patches

980

900

800

700 -

600 L

OCCOCI T I I I I I I

EXPLANATION

o Amphibole > phlogopite

 Phlogopite > amphibole

e Amphibole = phlogopite

AVERAGE DIAMETER

OF OLJVINE GRAIN,

IN MILLIMETERS

B

80 85 90

Mg/(Mg + Fe+2) 

X 100 OLiVINE

AMPHIBOLE AND PHLOGOPITE, IN 

VOLUME PERCENT

D

Figure 1. Combined modal volume of amphibole and phlogopite correlated with stratigraphic position in cyclic unit 2. A, 

Columnar section of cyclic unit 2 showing subunits 1 through 

9 (dotted lines), upper part of Nye Basin; oc, olivine cumulate; 

obc,olivine-bronzite cumulate; occ, olivine-chromite cumulate; 

heavy lines, chromite cumulate. Scale is in drilling footage. B,

Size of olivine as average diameter. Hor izontal lines are error 

bars. Dashed lines indicate values extend off scale. Actual value 

is shown by arrow. C, Olivine compositions as determined by 

X-ray diffraction. D, Combined volume percentage of amphiboles 

and phlogopite.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A3
Figure 2. Photomicrographs showing textures of primary brown 

amphibole and phlogopite in olivine cumulates from cyclic unit 

2 of the Periodotite zone of the Ultramafic series, Nye Basin area. 

Long dimension of all photomicrographs is 2.4 mm except as 

noted. All samples were photographed in plane-polarized light 

except as noted. O, olivine; B, orthopyroxene; A, augite; P, 

plagioclase; C, chromite; H, hornblende; ph, phlogopite. A, 

Brown amphibole rim on postcumulus augite. Sample NB9 

691.5. B, Brown amphibole rim on postcumulus orthopyroxene. Sample NB9 665. C, Thin section of entire slide, showing 

brown amphibole adjacent to oikocryst. Long dimension of

photograph is 10 mm. D, Brown amphibole rim on cumulus 

chromite that is continuous with a clinopyroxene rim on olivine. 

Sample NB9 630. £, Brown amphibole rim on cumulus chromite 

that is continuous with a clinopyroxene rim on olivine. Sample 

NB9 715. F, Brown amphibole rim on cumulus chromite. Sample NB9 618. C, Brown amphibole rim on cumulus chromite. 

Sample NB9 830a. H, Subhedral phlogopite associated with 

cumulus chromite and postcumulus plagioclase. Sample NB9 

628. /, Coexisting phlogopite and brown amphibole. Sample 

NB9 715. 1, Coexisting phlogopite and brown amphibole. Sample NB9 755b.

A4 Contributions on Ore Deposits in the Early Magmatic Environment
Figure 2. Continued.

and lenses of hornblende within the cores of pyroxene 

crystals, and (3) examples of twinned hornblende crystals 

that are homoaxial with twinned augite crystals. On a larger 

scale, the distribution of hornblende in a thin section may 

be spatially related to pyroxene even though hornblende does 

not directly form rims on the pyroxene. As an example, 

postcumulus hornblende is concentrated in a zone adjacent 

to an orthopyroxene oikocryst (fig. 2C).

The association between chromite and brown amphibole is much more direct (fig. 2D, E). In these examples, 

thin rims of augite have formed between cumulus olivine and 

interstitial plagioclase. However, where similar rims form 

on cumulus chromite, the rim material is brown amphibole. 

In most cases, however, brown amphibole forms rims adjacent to chromite without any associated augite (fig. 2F, G). 

Rims of hornblende between cumulus olivine grains or between cumulus olivine and interstitial plagioclase are common; however, some of these occurrences may be related 

to the other textural occurrences outside the plane of the thin 

section.

Clear, colorless amphibole is quite rare in these rocks, 

having been observed in only two samples. The clear amphibole may rim and replace brown amphibole, but it also 

replaces pyroxene. In some examples, the hornblende crystals 

are altered to the clear, colorless amphibole through various 

brown color zones. This alteration may be associated with 

serpentinization, as described by Page (1976).

Postcumulus phlogopite forms subhedral to euhedral 

books when it is finer grained, but it may form anhedral irregular grains where it completely fills the interstitial void 

between cumulus grains. Phlogopite may preferentially form 

subhedral crystals growing adjacent to chromite grains (fig. 

2H) or rims on opaques, but it does not tend to be associated 

with postcumulus pyroxene, unlike brown amphibole.

Coexisting brown amphibole and phlogopite was not 

commonly observed, and only three examples of it were 

found in this study. Boundaries between these minerals do 

not yield any information on the relative order of crystallization (fig. 21, J). Instead, the boundaries between brown amphibole and biotite appear to represent minimum free-energy

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A5
grain-boundary configurations for these two minerals (Kretz, 

1966).

The idiomorphic grain boundaries exhibited by phlogopite in contact with plagioclase suggest that the mica 

crystallized prior to or contemporaneously with plagioclase. 

The relative order of plagioclase and brown amphibole 

crystallization is more difficult to gauge. Because brown amphibole does not develop idiomorphic grain boundaries 

against plagioclase, it is assumed to have crystallized contemporaneously with or subsequent to plagioclase.

TECHNIQUES

The methods of handling and gathering data from the 

drill core, in addition to the whole-rock trace-element 

analyses for the rocks studied, are described by Page and 

others (1972). The mineral presented in this report are microprobe analyses collected at facilities at the U.S. Geological 

Survey in Menlo Park, California, and the University of 

California, Berkeley. The amphibole and phlogopite analyses 

reported in the present study are of either single points on 

mineral grains or an average of several points on the same 

crystal. The points on the mineral grains (reported in the 

tables as, for example, 618a-pt 1) were selected to be 

representative of a given textural occurrence and to be free 

of any alteration or grain-boundary interference. Multiple 

analyses were made on the larger grains; commonly, these 

analyses were in the core of the crystal and along its rim. 

If a difference in composition was present, the individual 

analyses are reported. However, if they were similar, the 

average composition of the multiple points is presented.

The analyses of brown amphibole, secondary amphibole, and phlogopite conducted at the U.S. Geological Survey 

at Menlo Park were performed on an automated threechannel ARL-EMX electron microprobe. Point locations 

were recorded on photomicrographs to assist in relocation. 

Each element was determined by wavelength dispersive 

analysis. Standard operating conditions were an accelerating 

potential of 15 kV with a sample current of approximately 

0.03 /^A. For each point analysis, the counts for each element represent the average of six separate counting intervals. The length of the counting interval was approximately 

10 s and was determined by beam-current integration. The 

information was collected, stored on-line, and reduced with 

the program FRAME 64 using the procedure outlined in 

Yakowitz and others (1973).

Analyses of the zoned brown amphiboles, olivines, and 

chromites were performed on an eight-channel ARL-SEMQ 

microprobe at the University of California, Berkeley. The 

elements were determined by wavelength dispersive analysis; 

operating conditions were identical to those for the analyses 

done at Menlo Park. Each point analysis of amphibole represents the average of six 10-s fixed-beam counting intervals. 

The analyses of olivine and chromite represent averages of

analyses of multiple points within a crystal. The information was collected and reduced on-line using the program 

PRMAIN developed by M. Rivers.

The accuracy and precision can be estimated because 

amphibole and biotite standards periodically were analyzed 

as unknown minerals. When compared with the reported 

values of the standard, the average analyses of the standards 

that were analyzed as unknowns provide an estimate of the 

accuracy of the analyses (table 1). Because of the small variations in composition discussed in this report, the data in table 

1 are provided to aid the reader in evaluating the analyses 

presented. The standard deviations demonstrate the level of 

precision and hence the analytical error typical of analyses 

collected on the ARL-EMX microprobe. The precision of 

analyses collected on the ARL-SEMQ microprobe is comparable to or slightly better than that of the ARL-EMX 

microprobe analyses.

All mineral formulas are reported to three decimal 

places and were calculated before rounding the analytical data 

to minimize rounding errors, but they are probably not 

significant to more than two places. Mineral formulas of amphiboles were calculated on the basis of 23 oxygens using 

the program described by Papike and others (1974). The 

Fe3+ values reported for the analyses are the minimum 

values calculated by this program. Mineral formulas for 

phlogopites were calculated using an unpublished program 

developed by S. Ludington. The mineral formulas for 

phlogopites are based on the assumption that the sum of the 

positive charges equals 22. Olivine and chromite mineral formulas were calculated on the basis of 4 oxygens. Estimates 

of Fe2O3 in the chromites are based on stoichiometry.

COMPOSITION AND SUBSTITUTION 

MECHANISMS OF AMPHIBOLE

The chemical compositions and calculated mineral formulas of primary brown hornblendes and secondary colorless 

amphiboles are given in tables 2 and 3, respectively. This 

section outlines the compositional limits of the amphiboles 

analyzed in this study.

The general form of the standard amphibole formula 

is AO_I B2 C^1 TJJV O22 (OH, F, Cl)2 , where T represents 

tetrahedral sites occupied by Si and Al; C represents the MI, 

M2 , and M3 octahedral sites, which are commonly occupied 

by Al, Cr, Ti, Fe, Mg, and Mn; B represents the 6- to 

8-coordinated M4 site occupied by Fe, Mn, Mg, Ca, and 

Na; and A represents the 10- to 12-coordinated A site, which 

may be occupied by Na and K (Leake, 1978).

The brown amphiboles are calcic, with (Ca + Na)e 

^1.34 and NaB < 0.67. In addition, these amphiboles have 

Ti < 0.50 and calculated Fe3 + < A1 VI. These amphiboles 

are classified as pargasite or pargasitic hornblende (fig. 3 A).

The alteration amphiboles are also calcic and are 

classified as pargasite, pargasitic hornblende, edenitic hornblende, and magnesio-hastingsitic hornblende (transitional

A6 Contributions on Ore Deposits in the Early Magmatic Environment
Table 1. Electron microprobe analyses of standard amphibole and biotite compared with reported 

analyses of the standards 

[n.d., not determined]

Si Op 

Alp03 

FeO 

MgO 

TI n 

PaH

Na2 0 

v n

Cr 0 2 nU3 

40

10

26

4

2

10

1

1

.50

.14

.25

.35

.84

.46

.95

.48

.01

.16

.18

0.26

.22

.30

.10

.24

.15

.11

.04

.01

.03

.02

40

10

26

4

2

10

2

1

n

.54

.10

.28

.37

.88

.49

.24

.45

.d.

1 Q

.2?

Si Op 

A1 2 03

Ti02-~

FeO 

MgO 

Cr H

CaO 

NapO 

V n F±:::

ci 

39

13

1

11

19

10

.07

.17

.68

.18

.79

.18

.01

.32

.00

.13

.01

0.80

.22

.04

.18

.10

.07

.02

.04

.04

.01

.00

38

13

1

10

19

10

n

.63

.10

.58

.92

.90

.23

.00

.15

.00

.30

.d.

Total 98.32 98.80 Total- 95.54 94.81

Formula based on 23 oxygens Formula based on sum of positive 

charges=22

Si -

A1 1V

2Tet

Al Vi

Fe -

Ti -

(V _

^Hr»f

 

____ .__

 

Excess OctPo

NaR

VD

Na A

K __

6.

1.

8.

.

3.

1.

*

5.

1.

p

.

.

2A site ---

Mg/(

1.

2.

3.

4.

5.

6.

Mg+Fe )

Average

Standard

Reported

Average

Standard

Reported

.

of

393

607

000

284

465

025

336

001

111

111

770

119

000

478

299

111

23

12 Amp 52

deviations

analysis of

of four Bio

deviations

analysis of

.038

.038

.033

.048

.022

no A

.002

.025

.025

n 1 Q

.027

.031

.009

.029

.01

6.

1.

8.

.

3.

1.

*

5.

1.

2.

.

.

.

382

618

000

256

460

025

341 

000

rtQ o

082

rjf r\

149

000

535

291

826

23

analyses that

on the

Amp 52

average

,

56 analyses

on the

Bio 56

Si 

A1 IV 

A1 IV 

Ti 

Re

Mg 

Cr

20ct 

Po

Na 

F 

___

Xmg 

Xph 

Xan 

Xsid 

were run as

reported in

2.

1.

.

2.

2.

.

.

1.

.

.

.

873

173

018

093

(.Of.

170

006

974

001

046

937 

030

001

969

73

73

20

08

.027

.097

.023

.003

.017

.033

.002

HOC

.001

.005

.010

.004

.000

.004

.01

.01

.01

.01

2.866

1.134

.012

.088

f r»Q

2.201

.007

2.985

.000

.022

.947

.070

n.d.

' n.d.

.74

.74

.19

.07

unknowns .

column 1.

that were run as unknowns.

average

.

reported in column 4.

pale-brown crystals) to tremolite or tremolitic hornblende 

(colorless crystals).

Earlier, we described at least two distinctly different 

textural occurrences of hornblende. Hornblende was found 

both to replace augite (fig. 2A) and to form rims on chromite 

crystals (fig. 2D-G). To assess the role of assemblage on 

hornblende compositions, the analyses were sorted into the 

following categories: (1) hornblende that rims chromite but 

does not replace pyroxene, (2) hornblende that replaces augite 

but is not in contact with chromite, (3) hornblende occurring in the lower portion of subunit 9 in which a cumulus 

spinel phase is not present, and (4) hornblende in ambiguous 

or different textural associations (table 2).

The mean and standard deviation for each textural 

group were calculated, and the differences between the means 

of oxide weight percentage and formula units for the groups 

were tested at a 0.5-percent level by calculating a comparison 

criterion that is based on the pooled standard deviation, on 

Student's t-test, and on the number of samples in each group. 

This value is then compared with the absolute difference between the means of the property between groups that is under 

consideration. Means and standard deviations are presented 

in table 4, and the results of the comparison are given in 

table 5.

If the mean difference is greater than the comparison 

criterion reported in table 5, the means for that property

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A7
Table 2. Electron microprobe analyses and calculated mineral formulas of hornblendes from the olivine cumulate of cyclic 

unit 2, Stillwater Complex, Montana 

[Analyst: M.L. Zientek. n.d., not determined]

618a-pt 1 618b-pt 3 623a-pt 13 623b-pt 14

Microprobe analyses.

A12 03 - 

K26 

Cr 2°3 

42.6 

12.0 

6.0 

16.6 

3.0 

11.5 

2.4 

1.0 

1.1 

.06 

.01 

96.27

42.8 

12.1 

5.7 

16.4 

1.6 

11.6 

2.4 

1.0 

1.6 

.08 

.05 

95.33

44.2 

12.0 

5.7 

15.7 

2.9 

11.6 

2.3 

1.1 

1.4 

.10 

.09 

97.09

Mineral formula

.

A1 IV

if..??*1

2+

3+

Mg -

NaB 

Mg/(Mg+Fe 2+) 

Assemblage 

6.236 

1.764 

8.000 

.304 

.709 

.000 

3.621 

.331 

.128 

5.094 

.094 

1.808 

.098 

2.000 

.572 

.194 

.766 

.84 

Yes

Pargasite 

4

640a-l,2

6.312 

1.688 

8.000 

.417 

.699 

.000 

3.608 

.181 

.190 

5.095 

.095 

1.830 

.074 

2.000 

.603 

.190 

.793 

.84 

Yes 

Yes 

Pargasitic 

hornblende

1

640b-3,4

6.381 

1.619 

8.000 

.423 

.693 

.000 

3.380 

.318 

.164 

4.979 

.000 

1.792 

.208 

2.000 

.437 

.208 

.645 

.83 

No

Pargasitic 

hornblende

1

640c-l,2,3

43.5 

11.6 

6.4 

16.0 

2.7 

11.9 

2.4 

1.0 

1.3 

.12 

.10 

97.02

calculated on

6.340 

1.660 

8.000 

.337 

.781 

.000 

3.461 

.290 

.146 

5.015 

.015 

1.855 

.129 

2.000 

.545 

.180 

.726 

.82 

Yes

Pargasitic 

hornblende

4

640c-4,5

Microprobe analyses.

Cr 2°3

F 

Total 

43.0 

11.9 

6.3 

15.3 

3.0 

11.8 

2.5 

.8 

1.6 

n.d. 

n.d.

96.2

43.0 

12.0 

6.3 

15.5 

2.5 

11.7 

2.4 

1.0 

1.6 

n.d. 

n.d.

96.0

42.9 

12.0 

6.2 

15.4 

2.9 

11.7 

2.4 

1.0 

1.6 

n.d. 

n.d.

96.1

Mineral formula

A1IV

_ 2+

_ 3+

NaB - 

NaA 

2 A site -- 

Mg/ (Mg+Fe"*) 

Name 

6.298 

1.702 

8.000

.767 

.000 

3.343 

.334 

.136 

4.976 

.000 

1.847 

.152 

2.000 

.560 

.142 

.702 

.81 

No 

Yes 

Pargasitic 

hornblende

6.314 

1.686 

8.000 

.388 

.776 

.000 

3.383 

.276 

.185 

5.007 

.007 

1.840 

.152 

2.000 

.528 

.185 

.713 

.81 

Yes

Pargasitic 

hornblende

6.284 

1.716 

8.000 

.364 

.763 

.000 

3.371 

.317 

.183 

4.998 

.000 

1.836 

.163 

2.000 

.515 

.187 

.702 

.82 

Yes

Pargasitic 

hornblende

43.2 

12.0 

6.4 

15.3 

2.8 

11.8 

2.3 

1.0 

1.6 

n.d. 

n.d.

96.4

calculated on

6.316 

1.684 

8.000 

.381 

.784 

.000 

3.337 

.304 

.187 

4.994 

.000 

1.839 

.161 

2.000 

.502 

.179 

.681 

.81 

Yes

Pargasitic 

hornblende

628b-pt 12 628c-pt 10 630b-pt 7 630b-pt 8

weight percent

43.3 

12.0 

6.2 

15.8 

2.5 

11.7 

2.2 

1.1 

1.5 

.11 

.08 

96.49

the basis of

6.330 

1.670 

8.000 

.390 

.762 

.000 

3.445 

.271 

.169 

5.037 

.037 

1.827 

.135 

2.000 

.499 

.205 

.704 

.82 

Yes

Pargasitic 

hornblende

2

43.7 

11.8 

6.3 

15.7 

2.7 

11.9 

2.3 

1.0 

1.5 

.12 

.08 

97.10

23 oxygens

6.340 

1.660 

8.000 

.365 

.766 

.000 

3.405 

.289 

.169 

4.994 

.000 

1.857 

.143 

2.000 

.513 

.191 

.704 

.82 

Yes

Pargasitic 

hornblende

1

645a-l,2 645b-pt 2

43.2 

11.7 

5.5 

15.8 

3.3 

12.1 

2.3 

1.1 

1.4 

.15 

.04 

96.59

6.297 

1.703 

8.000 

.313 

.670 

.000 

3.421 

.365 

.160 

4.929 

.000 

1.895 

.105 

2.000 

.544 

.204 

.749 

.84 

No 

Yes

hornblenc

1

645c-l,2

43.6 

12.2 

5.8 

15.6 

3.2 

11.6 

2.3 

1.1 

1.4 

.03 

.15 

96.98

6.316 

1.684 

8.000 

.402 

.697 

.000 

3.372 

.351 

.158 

4.980 

.000 

1.799 

.200 

2.000 

.452 

.209 

.661 

.83 

No

Je hornblende

1

655b-pt 5 655-pt 6

weight percent

42.6 

12.5 

5.7 

15.2 

3.3 

12.0 

2.3 

1.1 

1.3 

n.d. 

n.d.

96.0

the basis of

6.238 

1.762 

8.000 

.393 

.702 

.000 

3.320 

.367 

.147 

4.929 

.000 

1.880 

.120 

2.000 

.544 

.206 

.750 

.83 

No

Pargasitic 

hornblende

43.1 

11.9 

6.9 

15.6 

2.6 

11.9 

2.5 

.9 

1.3 

n.d. 

n.d. 

96.7

23 oxygens

6.296 

1.704 

8.000 

.343 

.848 

.000 

3.390 

.290 

.150 

5.021 

.021 

1.862 

.116 

2.000 

.581 

.166 

.747 

.80 

Yes

Pargasitic 

hornblende

43.1 

12.0 

6.2 

15.6 

3.0 

11.7 

2.4 

1.1 

1.1 

n.d. 

n.d.

96.2

6.304 

1.696 

8.000 

.374 

.762 

.000 

3.398 

.328 

.128 

4.990 

.000 

1.839 

.161 

2.000 

.520 

.198 

.718 

.82 

Yes

Pargasitic 

hornblende

43.9 

11.3 

6.1 

15.7 

3.3 

11.6 

2.4 

.9 

1.5 

.13 

.08

96.9

6.378 

1.622 

8.000 

.317 

.743 

.000 

3.403 

.356 

.169 

4.988 

.000 

1.801 

.199 

2.000 

.485 

.163 

.648 

.82 

No 

Yes 

Pargasitic 

hornblende

42.9 

11.8 

7.4 

16.5 

3.0 

10.9 

2.2 

.8 

.9 

.14 

.08

96.62

6.273 

1.727 

8.000 

.305 

.903 

.000 

3.587 

.330 

.105 

5.230 

.230 

1.715 

.055 

2.000 

.555 

.157 

.711 

.80 

Yes

Pargasitic 

hornblende

Assemblage 

A8 Contributions on Ore Deposits in the Early Magmatic Environment
Table 2. Electron microprobe analyses and calculated mineral formulas of hornblendes from the olivine cumulate of cyclic 

unit 2, Still water Complex, Montana Continued

655a a-1,2 655a b-pt 2 657a-l,3 657b-l,2,3 660a-pt 2 660a-pt 3 660b-l,2 665a-l,2 665a-pt 3

Microprobe analyses, weight percent

SiO, 42.5 42.2 42.5 42.9 42.9

A1203 12.2 11.8 12.1 11.9 12.1

Fe0 5.8 6.1 6.0 6.1 6.4

MgO 15.6 15.9 15.9 15.9 15.7

Ti02 3.2 3.2 2.9 2.8 3.1

CaO 11.8 11.7 11.6 11.7 11.7

Na-0 2.4 2.6 2.5 2.5 2.6

K26 1.1 .9 1.0 1.0 1.0

Cr 203 1.4 1.4 1.2 1.4 1.5

Cl n.d. n.d. n.d. n.d. n.d.

F n.d. n.d. n.d. n.d. n.d.

Total 96.0 96.5 95.7 96.2 97.0

	e bas

Si - 6.229 6.213 6.257 6.277 6.250

A1 IV 1.771 1.786 1.743 1.723 1.749

2 let 8.000 8.000 8.000 8.000 8.000

All .338 .270 .356 .335 .322

Fe .716 .754 .740 .748 .779

Fe 3+ .000 .000 .000 .000 .000

Mg 3.409 3.487 3.479 3.475 3.404

Ti .351 .352 .316 .307 .334

Cr . 162 .167 .141 .164 .167

S Oct 4.976 5.030 5.033 5.030 5.007

Excess Oct .000 .030 .033 .030 .007

Ca 1.859 1.841 1.823 1.830 1.831

NaB .141 .129 .144 .139 .162

V B 2.000 2.000 2.000 2.000 2.000

Naft .549 .608 .566 .567 .576

K .207 .165 .191 .181 .177

2 A site - .757 .774 .757 .748 .753

Mg/(Mg+Fe 2 ) .83 .82 .82 .82 .81

Accept^ No Yes Yes Yes Yes

Accept Yes

Name Pargasite Pargasite Pargasitic Pargasitic Pargasitic

	hornblende hornblende hornblende

Assemblage3 14441

42.6 

12.0

5.9 

15.7

3.3 

11.7

2.6 

.8

1.5

n.d.

n.d. 

96.1

42.2

12.5 

6.2

15.3 

3.0

11.6 

2.4 

1.1 

1.5 

n.d. 

n.d.

95.8

Mineral formula calculated on the basis of 23 oxygens

6.240

1.760

8.000 

.308 

.718 

.000

3.430 

.365 

.173

4.993 

.000

1.834 

.166

2.000 

.584 

.146 

.730 

.83 

Yes

6.247

1.752

8.000 

.380 

.723 

.000

3.393 

.338 

.172

5.008 

.008

1.823 

.169

2.000 

.513 

.179 

.692 

.82 

Yes

Pargasite Pargasite

6.221

1.779

8.000 

.388 

.763 

.000

3.362 

.330 

.169

5.011 

.011

1.826 

.162

2.000 

.524 

.201 

.726 

.82 

Yes

6.136

1.864

8.000 

.393 

.784 

.000

3.443 

.326 

.157

5.102 

.102

1.813 

.085

2.000 

.562 

.186 

.748 

.81 

Yes

Pargasite Pargasite

665b-l,2 675a-pt 1 675a-2,3 675b-l,2 680a-pt 1 680a-pt 2 690a-l,3,4 690b-l,2 690C-1.2

Microprobe analyses, weight percent

SiO, 

A1263 

FeO 

MgO 

TiO 

CaO 

Na,0 

K,6 

Cl 

F -

42.5 

12.0

6.1 

15.2

3.3 

11.6

2.6 

.8

1.5

n.d.

n.d. 

95.6

42.8

12.5 

5.8

15.6 

2.9

11.9 

2.4 

1.1 

1.4 

n.d. 

n.d.

96.4

43.1

12.0 

6.0

15.6 

3.0

11.6 

2.4 

1.0 

1.3 

n.d. 

n.d.

96.0

43.2

12.2 

5.6

15.7 

2.4

12.1 

2.4 

1.0 

1.2 

n.d. 

n.d.

43.1

11.9 

5.8

15.6 

3.1

11.8 

2.4 

1.0 

1.5 

n.d. 

n.d.

96.1

42.4

12.1 

5.8

15.5 

3.0

11.7 

2.4 

1.0 

1.4 

n.d. 

n.d.

95.3

Mineral formula calculated on the basis of 23 oxygens

2+ "3+

Al

AlV

Fe

FeJ

Mg 

Ti 

Cr 

2 Oct 

Excess Oct 

Ca 

NaB 

2B 

K 

2 A site 

Mg/(Mg+Fe 2 "1') 

Accept, 

Accept 

Name 

Assemblage 

6.267

1.733

8.000 

.351 

.750 

.000

3.335 

.361 

.176

4.974 

.000

1.828 

.172

2.000 

.562 

.145 

.707 

.82

No 

Yes

Pargasitic 

hornblende

6.249

1.751

8.000 

.392 

.707 

.000

3.399 

.323 

.159

4.980 

.000

1.861 

.139

2.000 

.537 

.195 

.732 

.83

No 

Yes 

Pargasite

6.300

1.700

8.000

.375

.732

.000

3.398

.332

.155

4.992

.000

1.825

.175

2.000

.506

.190

.696

.82

Pargasitic 

hornblende

6.259

1.741

8.000 

.338 

.721 

.000

3.363 

.379 

.174

4.976 

.000

1.841 

.159

2.000 

.526 

.153 

.679 

.82

No 

Yes

Pargasitic 

hornblende

6.324 

1.676 

8.000

.438

.687

.000 

3.428

.263

.142 

4.960

.000 

1.895

.105 

2.000

.565

.187

.752

.83 

Ho

Pargasitic 

hornblende

6.301

1.699

8.000 

.352 

.707 

.000

3.400 

.338 

.169

4.966 

.000

1.854 

.146

2.000 

.534 

.183 

.717 

.83

Ho

Pargasitic 

hornblende

6.323 

1.677 

8.000

.428

.768

.000 

3.412

.256

.161 

5.025

.025 

1.849

.126 

2.000

.519

.184

.703

.82 

Yes

Pargasitic 

hornblende

6.320

1.680

8.000 

.372 

.783 

.000

3.392 

.277 

.179

5.003 

.003

1.867 

.130

2.000 

.510 

.194 

.704 

.81 

Yes

6.262

1.738

8.000 

.367 

.713 

.000

3.417 

.331 

.162

4.990 

.000

1.854 

.145

2.000 

.530 

.183 

.712 

.83 

Yes

Pargasitic Pargasitic 

hornblende hornblende

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A9
Table 2. Electron microprobe analyses and calculated mineral formulas of hornblendes from the olivine cumulate of cyclic 

unit 2, Stillwater Complex, Montana Continued

690c-pt 3 691.5a-3,4 691.5b-l,2 697.5a-pt 6 697.5b-4,5 701b-pt 7 715a-8,9 715b-pt 10 715b-pt 11 

	Microprobe analyses, weight percent

SiO, 42.8 43.4 44.1 42.9 42.6 42.7 42.4 42.2 42.7

A1203 11.8 11.1 11.3 11.9 11.4 11.6 11.9 11.4 12.1

FeO 6.2 6.4 6.3 6.3 6.0 6.1 6.1 5.7 6.3

MgO 15.4 15.8 15.9 16.5 16.4 16.3 16.0 16.1 16.5

TiO. 2.9 3.6 3.4 2.8 3.5 2.9 3.1 3.6 2.4

CaO 12.0 11.7 12.0 11.2 11.3 11.3 11.2 10.3 11.3

Na-0 2.4 2.4 2.5 2.5 2.5 2.6 2.4 2.3 2.3

K20 .9 .8 .7 .8 .6 .6 1.0 1.2 1.1

Cr 20 1.6 1.4 1.5 1.2 1.5 1.5 1.6 1.6 1.4

cl n . d . >17 . 15 . 18 . n .22 .30 .10 .12

F n.d. .07 .09 .06 .06 .03 .05 .02 .06

Total 96.0 96.84 97.94 96.34 95.97 95.85 96.5 94.52 96.28

	Mineral formula calculated on the basis of 23 oxygens

Si - 6.303 6.335 6.357 6.280 6.254 6.279 6.239 6.281 6.259

A11V 1.697 1.665 1.643 1.720 1.746 1.721 1.761 1.718 1.741

2 Tet 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

AlY1 .355 .244 .274 .334 .221 .283 .298 .283 .347

Fe 2 .767 .776 .760 .765 .735 .747 .745 .714 .774

Fe .000 .000 .000 .000 .000 .000 .000 .000 .000

Mg 3.391 3.436 3.411 3.590 3.583 3.569 3.516 3.568 3.607

Ti . 318 .390 .agg .308 .384 .319 .346 .400 .267

Cr . 181 . 156 . 165 . 142 . 175 , 174 . 186 .185 .162

2 Oct 5.012 5.002 4.976 5.140 5.099 5.092 5.092 5.150 5.157

Excess Oct .012 .002 .000 .140 .099 .092 .092 .150 .157

Ca 1.830 1.825 1.848 1.752 1.778 1.788 1.768 1.636 1.766

NaB .158 .173 .152 .108 .122 .119 .141 .213 .077

2B 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000

NaA .513 .511 .541 .588 .587 .629 .533 .444 .573

K . 169 >147 . 12 9 .148 .116 .116 .191 .220 .204

2 A site .683 .658 .670 .736 .703 .745 .724 .665 .777

Mg/(Mg+Fe2+ ) .82 .82 .82 .82 .83 .83 .83 .83 .82

Accept^ Yes Yes No Yes Yes Yes Yes Yes Yes

Accept Yes

Name Pargasitic Pargasitic Pargasitic Pargasitic Pargasitic Pargasitic Pargasite Pargasitic Pargasitic

hornblende hornblende hornblende hornblende hornblende hornblende hornblende hornblende

Assemblage 3 12222241 2

730a-pt 1 730a-pt 2 730a-pt 3 730c-l,2 747b-pt 1 747b-2,3,4,5 747c-l,2 747c-pt 4 755a-12,13,14

	Microprobe analyses, weight percent

SiO. 42.3 42.6 42.6 42.7 43.2 42.5 42.4 43.2 42.2

A1203 12.1 12.1 11.5 12.3 12.5 12.4 12.4 12.3 11.1

Fe0 5.9 5.8 7 . 5 5 . 8 5.9 5 . 6 6 . 0 5.9 6.2

MgO 15.4 15.2 16.1 15.0 15.9 15.6 16.4 18.7 16.3

Ti02 2.7 3.4 2.9 3.3 2.6 3.5 3.9 3.2 4.5

CaO 11.6 11.9 11.7 11.8 12.0 12.0 11.9 11.9 10.9

Na.O 2.5 2.3 2.4 2.3 2.4 2.6 2.4 2.3 2.4

K20 1.0 1.1 .8 1.0 .9 1.0 1.1 .7 1.1

Cr 2°3 L - 4 1 - 4 !-2 1.0 .9 1.4 1.2 1.5 .7

Cl n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. .16

F n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. .08

Total 94.9 95.8 96.7 95.2 96.3 96.6 97.7 99.7 95.64

Mineral formula calculated on the basis of 23 oxygens

Si - 6.276 6.265 6.235 6.291 6.302 6.199 6.127 6.058 6.231

Al 1<724 -L.735 1-765 1>709 Lggs ^301 1.873 1.942 1.769

2 Tet 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Al. .383 .356 .216 .432 .443 .328 .240 .101 .160

Fe* .737 .710 .924 .714 .720 .684 .723 .453 .761

Fe .000 .000 .000 .000 .000 .000 .000 .243 .000

Mg 3. 40 8 3.330 3.524 3.300 3.452 3.393 3.523 3.905 3.580

Ti .301 . 372 .318 .361 .285 .381 .422 .337 .500

Cr . 165 . 165 -142 . 118 .099 .i 57 . 139 . 16 7 .086

2 Oct 4.994 4.934 5.125 4.925 4.999 4.943 5.047 5.205 5.087

Excess Oct .000 .000 .125 .000 .000 .000 .047 .205 .087

Ca 1.843 1.872 1.838 1.870 1.874 1.878 1.841 1.795 1.729

NaB .157 .128 .036 .130 .126 .122 .112 .000 .183

2B 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000

Naft .548 .523 .657 .531 .544 .603 .563 .626 .507

K . 195 -205 . 149 . 184 . 169 . 188 . 19g . 131 . 2 oo

2 A site .743 .728 .806 .715 .714 .791 .762 .757 .706

Mg/(Mg+Fe 2+) ' 82 ' 82 - 79 - 82 « 83 - 83 - 83 - 90 - 82

Accept Yes No Yes No Yes No Yes Yes Yes 

Accept2 

Name Pargasitic Pargasitic Pargasite Pargasitic Pargasitic Pargasite Pargasite Magnesio- Pargasite 

	hornblende hornblende hornblende hornblende hastingsite

Assemblage 3 41144111 2

A10 Contributions on Ore Deposits in the Early Magmatic Environment
Table 2. Electron microprobe analyses and calculated mineral formulas of hornblendes from the olivine cumulate of cyclic 

unit 2, Stillwater Complex, Montana Continued

755ba-l,2,3,4 755-bc 755ca-l,2,3 755ca-pt 4 755cb-pt 1 755cb-2,3 767a-l,3 780a-pt 2 780a-3,4

Microprobe analyses, weight percent

SiO, 43.0 43.1 43.6 43.4 43.6 43.5 43.4 43.6

A1 263 11.5 12.0 11.8 11.6 11.5 11.6 12.4 11.8

Fe0 6.5 6.5 5.8 5.8 6.0 5.8 5.1 6.1

MgO 16.0 16.0 16.2 16.0 16.2 16.1 16.6 17.2

Ti02 2.6 2.9 3.2 4.1 3.2 3.4 2.7 2.4

CaO 11.8 11.6 11.8 11.7 11.7 11.8 12.0 11.7

Na-0 2.8 2.6 2.6 2.7 2.6 2.7 2.3 2.3

K26 .5 .8 .7 .7 .8 .7 .9 .6

Cr 2°3 1>2 2 ' 9 1 ' 4 1-2 1>4 1 ' 6 1-4 1>2

Cl n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

n.d. n.d. n.d. n.d. n.d. n.d.

Total 95.9 98.4 97.1 97.2 97.0 97.2 96.8 96.9 96.9

Mineral formula calculated on the basis of 23 oxygens

S i 6.317 6.205 6.305 6.279 6.317 6.293 6.276 6.308 6.307

A1 IV 1.683 1.795 1.695 1.721 1.683 1.707 1.724 1.692 1.693

2 Tet 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

Ai; 1 .309 .234 .324 .251 .279 .269 .392 .320 .486

Fe .800 .787 .703 .700 .727 .698 .618 .734 .649

Fe .000 .000 .000 .000 .000 .000 .000 .000 .000

Mg 3.503 3.426 3.490 3.445 3.506 3.469 3.567 3.706 3.588

Ti .285 .311 .343 .441 .349 .370 .294 .261 .224

Cr .139 .327 .154 .133 .150 .178 .160 .137 .136

S Oct 5.037 5.085 5.014 4.976 5.021 4.984 5.031 5.158 5.082

Excess Oct .037 .085 .014 .000 .021 .000 .031 .158 .082

Ca 1.858 1.790 1.823 1.811 1.811 1.835 1.855 1.815 1.854

NaB .105 .125 .163 .189 .167 .165 .114 .027 .064

SB 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000

NaA .678 .590 .558 .562 .574 .581 .525 .624 .573

K .092 .147 .137 .124 .139 .135 .171 .116 .114

S A site - .769 .737 .694 .686 .713 .167 .696 .740 .687

Mg/(Mg|Fe 2+ ) .8] .81 .83 .83 .83 .83 .85 .83 .85

Accept^ Yes Yes Yes No Yes No Yes Yes Yes

Accept Yes Yes

Name Pargasitic Pargasite Pargasitic Pargasitic Pargasitic Pargasitic Pargasitic Pargasitic Pargasitic 

	hornblende hornblende hornblende hornblende hornblende hornblende hornblende hornblende

Assemblage 3 44114114 1

787c-pt 9 787a-pt 10 787a-pt 11 830a-12,13,14 845a-l,2,3 845b-l,3 861a-l,2,3 890-b pt 2

	Microprobe analyses, weight percent

SiO, 42.3 42.5 42.0 42.1 43.0 43.0 42.6 42.8

A12&3 11.7 11.6 12.2 11.9 12.4 12.1 12.3 12.3

FeO 5.7 5.5 5.3 5.9 6.1 5.9 5.6 7.4

MgO 17.3 17.2 17.0 17.2 16.3 16.3 15.8 17.1

Ti02 3.2 3.4 3.6 3.1 2.5 2.8 3.6 2.0

CaO 11.5 11.4 11.5 11.4 11.9 12.0 11.9 11.2

Na20 2.4 2.5 2.5 2.4 2.4 2.4 2.3 2.3

K 20 1.0 .9 1.0 1.0 1.1 1.0 1.1 .8

Cr ° 1 ' 7 1 ' 8 1 ' 6 1 ' 6 1 ' 2 1 ' 5 1 ' 3 ' 9

.08 .09 .08 .11 n.d. n.d. n.d. .07

.06 .08 .09 .03 n.d. n.d. n.d. .01

96.94 96.97 96.87 96.74 96.9 97.0 96.5 96.88

Mineral formula calculated on the basis of 23 oxygens

Si - 6.164 6.185 6.109 6.150 6.249 6.249 6.211 6.225

A1IV 1.836 1.815 1.891 1.850 1.751 1.751 1.789 1.775

S Tet 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000

All* .169 .168 .205 .198 .374 .321 .322 .337

Fe* .689 .666 .650 .714 .744 .722 .680 .815

Fe3+ .000 .000 .000 .000 .000 .000 .000 .084

Mg 3.755 3.717 3 . 684 3 . 743 3.527 3.518 3.430 3.697

Ti .3 46 .375 . 394 . 335 . 275 .305 .391 .223

Cr .201 .203 .203 .188 .141 .170 .152 .105

v oct 5.159 5.131 5.120 5.178 5.062 5.035 4.975 5.261

Excess Oct .159 .131 .120 .178 .062 .035 .000 .261

Ca 1.793 1.778 1.793 1.787 1.847 1.860 1.859 1.739

NaB .048 .091 .087 .035 .091 .105 .141 .000

2B 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000

NaA .643 .619 .615 .647 .573 .574 .511 .651

K .180 .165 .182 .182 .204 .182 .212 .152

2 A site - .823 .784 .797 .830 .777 .755 .723 .803

Mg/(Mg+Fe Z+) .84 .85 .85 .84 .83 .83 .83 .82

Accept^ Yes Yes Yes Yes Yes Yes No Yes

Accept Yes

Name Pargasite Pargasite Pargasite Pargasite Pargasite Pargasite Pargasite Pargasite

Assemblage3 211 1 4113

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A11
Table 2. Electron microprobe analyses and calculated mineral formulas of hornblendes from the olivine cumulate of cyclic unit 2, Stillwater Complex, 

Montana Continued

890a-4,5,6 894a-l,2,3 905a-l,2 930a-l,2,3

1. Sum tetrahedral sites=8 ± 0.02

2. Sum octahedral sites >4.98

3. Excess octahedral site occupancy+Ca

4. Sum B site=2 ±0.02

5. Sum A site si.02

6. Residual charge <0.02 

Accepted, if:

1. Sum tetrahedral sites = 8 ±0.02

2. Sum octahedral sites >4.97

3. Excess octahedral site occupancy+Ca

4. Sum B site=2 ±0.02

Assemblage

1. With chromite

2. With augite; no chromite

3. No spinel in rock

4. Others; combination; ambiguous

£2.02

966-7,8

Microprobe analyses, weight percent

A1 263 

Cr 2 03 

F 

Total 

. Si - -----

A1 vl __ _ .

P_2+

p 3+

NaB

NaA

Mg/(Mg+Fe 2+ )

Name 

Assemblage

 17.0

 .05

 97.64

Mineral formula

 1.788

 .81

 Pargasitic 

hornblende

43.5 

12.6 

6.7 

16.5 

1.8 

12.0 

2.4 

1.0 

.7 

n.d. 

n.d.

97.22

calculated

6.310 

1.690 

8.000 

.455 

.810 

.000 

3.554 

.201 

.080 

5.100 

.100 

1.856 

.043 

2.000 

.620 

.178 

.797 

.81 

Yes

Pargasitic 

hornblende

3

43.3 

12.5 

6.9 

16.6 

1.7 

11.9 

2.4 

.9 

.8 

n.d. 

n.d.

97.0

on the basis of

6.294 

1.706 

8.000 

.431 

.837 

.000 

3.593 

.183 

.095 

5.140 

.140 

1.851 

.009 

2.000 

.654 

.169 

.823 

.81 

Yes

Pargasitic 

hornblende

3

42.8 

12.1 

6.7 

16.1 

2.5 

12.1 

2.4 

1.1 

1.2 

n.d. 

n.d.

97.0

23 oxygens

6.241 

1.759 

8.000 

.321 

.820 

.000 

3.500 

.273 

.139 

5.053 

.053 

1.887 

.059 

2.000 

.605 

.208 

.813 

.81 

Yes

Pargasite 

3

44.0 

12.0 

6.3 

16.4 

3.4 

11.5 

2.5 

1.0 

1.4 

.13 

.03

98.5

6.285 

1.715 

8.000 

.303 

.755 

.000 

3.494 

.365 

.161 

5.079 

.079 

1.752 

.169 

2.000 

.515 

.175 

.690 

.82 

Yes

Pargasitic 

hornblende

1

Accented . if:

between the two groups being compared are statistically different for the confidence level being used (in this case 99.5 

percent). The results of this test show that the hornblende 

compositions in rocks from the lower part of subunit 9 (group 

3) are statistically higher in Fe, Na (in the A site), and total 

(Na, K)A and lower in Ti, Cr, and Na (in the M). site) than 

are the compositions of hornblende that rims chromite (group 

1) or that replaces augite (group 2). Groups 1, 2, and 4 have 

nearly identical compositions.

On an expanded portion of the Mg/(Mg + Fe2+) 

versus Si diagram of Leake (fig. 35), the three groups of 

hornblende analyses form overlapping clusters of data. The 

hornblendes associated with chromite seem to be the most 

variable in composition. The composition of hornblende 

associated with augite decreases in Mg/(Mg + Fe2+) as Si

increases. The hornblendes from the lower portion of subunit 

9 have a restricted range of composition and low Mg/(Mg 

+ Fe2 +).

The compositional diversity of amphiboles can be most 

easily represented on cation-cation plots that emphasize some 

of the possible substitutions that take place within the amphibole structure. Two broad types of substitutions are possible: (1) exchange reactions, and (2) coupled substitutions. 

Exchange reactions include, for example, Fe and Mg exchange in the smaller M sites, Ca and Fe exchange in the 

M4 site, or Na and K exchange in the A site. These exchanges do not require additional charge compensation. 

Coupled substitutions, however, require exchange of components in at least two structural sites in the amphibole to 

maintain charge balance. Examples of coupled substitutions

A12 Contributions on Ore Deposits in the Early Magmatic Environment
Table 3. Electron microprobe analyses and calculated mineral formulas of secondary amphiboles from the olivine cumulate of 

cyclic: unit 2, Stillwater Complex, Montana 

[Analyst: M.L. Zientek]

Microprobe analyses, weight percent

6l8a-pt2 774a-pt1-core

Si Op 51.5 

A120 3 2.2

FpO - U P

MgO 23.8 

Ti02 .00

faO 11 "3

NapO .12

el _£ ___ on

p _ _____ 00

Total 93TT4

Si - 7.446 

A1 IV .368 

2 Tet 7.814 

A1VI .000 

Fe2+ .170 

Fe3+ .334

Ti .000 

Cr .000

ZQct 5.252 

Exess oct - .252

To 1 VflQ

Na R .000

D

ZB 2 000

Naft .034 

K .000

2 A site 0.34 

Mg/(Mg+Fe2+ ) .97 

Accept 1 No 

Accept 

Name Tremolitic 

hornblende

42.9 

10.9 

5.7 

18.3 

1.5 

11.4 

2.2 

.7 

1.6 

.31 

.00 

95.51

Mineral

6.290 

1.710 

8.000 

0.180 

.428 

.270 

3.990 

.160 

.183 

5.211 

.211 

1.789 

.000 

2.000 

.634 

.123 

.757 

.90 

Yes

Magnesiohastingsitic 

hornblende

774a-pt2-rim

45.2 

9.0 

5.3 

20.4 

.9 

10.9 

1.8 

.19 

1.0 

.28 

.01 

94.99

774b-3,4

43.1 

11.4 

5.7 

18.1 

1.7 

11.5 

2.3 

.7 

1.7 

.30 

.00 

96.5

formula calculated on the basis

6.531 

1.469 

8.000 

.060 

.097 

.545 

4.392 

.100 

.112 

5.306 

.306 

1.694 

.000 

2.000 

.516 

.035 

.551 

.98 

Yes

Edenitic 

hornblende

6.260 

1.740 

8.000 

.213 

.510 

.184 

3.916 

.183 

.198 

5.204 

.204 

1.796 

.000 

2.000 

.643 

.137 

.780 

.88 

Yes

Pargasitic 

hornblende

774c-pt5

42.1 

11.5 

5.8 

17.5 

2.4 

11.2 

2.3 

.9 

1.6 

.31 

.03 

95.64

of 23 oxygens

6.211 

1.789 

8.000 

.201 

.686 

.030 

3.854 

.271 

.191 

5.232 

.232 

1.768 

.000 

2.000 

.652 

.175 

.827 

.85 

Yes

Pargasite

774d-pt7

54.4 

1.4 

2.7 

24.0 

.2 

12.4 

.44 

.00 

.32 

.00 

.06 

95.86

7.648 

.225 

7.873 

.000 

.278 

.038 

5.025 

.016 

.035 

5.139 

.139 

1.861 

.000 

2.000 

.120 

.000 

.120 

.95 

No 

No 

Tremolite

890b-pt1

42.7 

11.8 

6.8 

18.2 

1.7 

11.4 

2.4 

.8 

.78 

.06 

.01 

96.67

6.187 

1.813 

8.000 

.199 

.487 

.331 

3.939 

.190 

.089 

5.232 

.232 

1.768 

.000 

2.000 

.663 

.153 

.816 

.89 

Yes

Magnesiohastingsite

Accepted, if: 1. Sum tetrahedral sites=8±0.02

2. Sum octahedral sites >4.98 

3. Excess octahedral site occupancy+Ca <2.02 

4. Sum B site=2 ±0.02 

5. Sum A site <1.02 

6. Residual charge <0.02

^Accepted, if:

1. Sum tetrahedral sites = 8 ±0.02

2. Sum octahedral sites >4.97

3. Excess octahedral site occupancy+Ca <2.02 

H. Sum B site=2 ±0.02 

^Assemblage

1. With chromite

2. With augite; no chromite

3. No spinel in rock

1. Others; combination; ambiguous

listed by the name previously assigned to them (Cameron 

and Papike, 1979) include the following:

2. MgVI + Si = A1 VI + Al Tschermakite

3. [ ]A + CaB = NaA + NaB Richterite

1. [ ]A + Si = NaA + Al Edenite 4. Mg + CaB = A1VI + NaB Glaucophane

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A13
In the above equations, [ ] represents a vacancy in a site. 

Other substitutions are possible, such as tschermakite-type 

or glaucophane-type substitutions using Cr3+ , Fe3 + , or Ti4+ 

instead of A1 VI or NasThe variations of hornblende compositions in cyclic unit 

2 are displayed as cation-cation diagrams in figures 4,5, and 

6 and show the important coupled substitutions. The compositions of hornblendes in the assemblage groups are also 

compared on these plots and indicate that the hornblende

(Na,K)A > 0.5 ; TK0.5 ; Fe< AI V '

Ferroan

pargasitic

hornblende

6.4 6£

0.86

0.84

0.82

0.80

0.78

B

6.0 6.1 Q2. 6.3 

Si CONTENT, IN FORMULA UNITS

6.4

Figure 3. Classification and nomenclature of amphiboles. A, 

Calcic amphibole classification showing hornblende compositions (table 2; dots) and alteration amphiboles (table 3; squares). 

Some dots represent more than one analysis. B, Enlarged section of calcic amphibole classification diagram showing individual analyses for which textural assemblage could be determined (table 2). Hornblendes associated with chromite (dots) 

outlined in solid lines, with augite (circles) outlined by dashdot line, and with no chromite (crosses) outlined by dashed lines.

compositions in the lower part of unit 9 (where spinel is not 

present) are distinct from those in augite- or chromite-related 

assemblages. On these plots, the assemblages of hornblende 

plus chromite are compared with the assemblages of hornblende plus augite and hornblende without any spinel in the 

rock (lower portion of subunit 9 in the cyclic unit).

Edenitic and tschermakitic coupled substitutions predominate (fig. 4). The end-member pargasite composition 

results from a linear combination of the tschermakite and 

edenite substitutions. It is obvious that charge imbalance 

created by the substitution of Na and K into the A site could 

be totally compensated by substitution of A1 IV into the 

tetrahedral sites. AI , in excess of that required to offset 

Na and K substitution in the A site, almost compensates the 

charge imbalance created when trivalent and quadravalent 

cations substitute into the octahedral sites (fig. 5). This 

residual could be compensated by substitution of Na into the 

M4 site (fig. 6). Glaucophane-like or riebiekite substitutions 

are most likely, although some richterite substitutions may 

be possible for those hornblendes in the lower portion of 

subunit 9, in which chromite is absent.

Hornblendes associated with chromite overlap the 

ranges of A1 IV and (Na, K)A (fig. 4) for the other textural 

assemblages, but they appear to have a broader range of 

A1 IV and (Na, K)A substitution than those hornblendes 

associated with augite or with no spinel (lower part of subunit 

9). Hornblendes in the lower part of subunit 9 show distinct 

compositions when plotted on figures 4 and 5, an indication 

that there are lesser amounts of A1 VI + Ti + Cr in this 

group than in the other assemblages.

The variation in amounts of substitution in hornblende 

for Ti, Cr, and A1 VI in the different textural assemblages 

is illustrated on figure 7, where these cations are plotted versus Al^. The fields of hornblende compositions for the different textural assemblages overlap; however, hornblendes 

in rocks with no chromite from the lower portion of subunit 

9 tend to have less Ti and Cr and more A1VI than hornblendes in the other assemblages. Similarly, hornblendes 

from the lower portion of subunit 9 without chromite have 

lesser amounts of NaB than hornblendes associated with 

augite or chromite (fig. 8).

In summary, the substitution mechanisms in hornblendes from cyclic unit 2 are dominated by tschermakiticand edenitic-type substitutions that produce pargasite, with 

slightly more tschermakitic substitutions than edenitic 

substitutions. The substitution mechanisms are similar for 

hornblendes in each of the different textural assemblages, 

but hornblendes in the lower part of subunit 9 without 

chromite appear to be differentiated from the other assemblages by some richterite substitution and lesser amounts of 

Ti and Cr. The mean ratios of weight percent Cl to F in hornblendes associated with chromite and augite are 1.4 and 2.0, 

respectively, whereas the ratio in rocks from the lower portion of subunit 9 with no chromite is 6.0. Chlorine-to-fluorine 

ratios can be used to discriminate the hornblendes of the

A14 Contributions on Ore Deposits in the Early Magmatic Environment
Table 4. Means and standard deviations of hornblende analyses and formulas by assemblage groups

Group 1 Group 2 Group 3 Group 4

Number of 

analyzed 

samples Mean

Standard 

deviation

Number of 

analyzed 

samples Mean

Standard 

deviation

Number of 

analyzed 

samples Mean

Microprobe analyses, weight

Si02 

A1 2 0, 

FeO-- 

MgO 

TiOp 

CaO 

Na 20 

K26 

S£=

£*=_

y*±= 

Fe2+____

Fe 3+____

Mg _______

Tl 

Cr

S Oct 

Excess Oct 

Ca

Na B 

2 B 

Na A 

2 A site 

XHg _______

32 

32 

32 

32 

32 

32 

32 

32 

32 

10 

10

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32 

32

12.9 

12.1 

5.9 

16.0 

3.13 

11.7 

2.4 

.96 

1.4 

.10 

.07

6.252 

1.748 

8.000 

.322 

 707 

.008 

3.482 

.342 

.166 

5.026 

.041 

1.829 

.130 

2.00 

.550 

.179 

.729 

.832

0.57 

.35 

.42 

.72 

.47 

.33 

.11 

.15 

.15 

.03 

.04

0.069 

.069 

.000 

.082 

.070 

.043 

.135 

.051 

.016 

.074 

.061 

.048 

.050 

.000 

.053 

.028 

.046 

.017

15 

15 

15 

15 

15 

15 

15 

15 

15 

9 

9

15 

15 

15 

15 

15

15 

15 

15 

15 

15 

15 

15 

15 

15 

15 

15 

15

42.9 

11.7 

6.2 

16.0 

2.99 

11.6 

2.4 

.90 

1.4 

.14 

.07

6.287 

1.713 

8.000 

.315 

.753

3.483 

.330 

.165 

5.046 

.053 

1.814 

.132 

2.000 

.550 

.167 

.718 

.821

0.48 

.40 

.25 

.55 

.57 

.32 

.10 

.17 

.24 

.04 

.02

Mineral

0.050 

.050 

.000 

.085 

.031

.122 

.063 

.027 

.069 

.063 

.046 

.037 

.00 

.047 

.031 

.043 

.009

5 43 

5 12 

5 6 

5 16 

5 2 

5 11 

5 2 

5 

5 

2 

2

formula calculated

5 6 

5 1 

5 8 

5 

5 

5 

5 3 

5 

5 

5 

5 

5 1 

5 

5 2 

5 

5 

5 

5

.3 

.3 

.9 

.6 

.04 

.7 

.4 

.94 

.86 

.18 

.03

on the

.277 

.723 

.000 

.381 

.824 

.017 

.604 

.223 

.098 

.147 

.147 

.824 

.028 

.00 

.636 

.175 

.810 

.812

Standard 

deviation

Number of 

analyzed 

samples Mean

Standard 

deviation

percent oxides

0.47 

.25 

.29 

.39 

.31 

.38 

.04 

.11 

.23 

.16 

.03

basis of 23

0.041 

.041 

.000 

.059 

.013 

.038 

.083 

.034 

.026 

.079 

.079 

.060 

.024 

.000 

.022 

.021 

.010 

.004

23 

23 

23 

23 

23 

23 

23 

23 

23 

5 

5

oxygens

23 

23 

23 

23 

23

23 

23 

23 

23 

23 

23 

23 

23 

23 

23 

23 

23

42.9 

12.0 

6.2 

15.9 

2.89 

11.7 

2.4 

0.90 

1.4 

.15 

.06

6.276 

1.724 

8.000 

.342 

.759

3.464 

.318 

.156 

5.039 

.045 

1.826 

.129 

2.000 

.562 

.169 

.731 

.820

0.47 

.37 

.37 

.46 

.30 

.22 

.14 

.15 

.38 

.09 

.04

0.051 

.051 

.000 

.053 

.045

.090 

.033 

.043 

.066 

.059 

.035 

.040 

.000 

.041 

.028 

.030 

.010

lower part of subunit 9 in the olivine cumulates. A similar 

analysis of the compositions of the alteration amphiboles 

shows that the end product of the alteration process is a 

tremolitic hornblende or tremolite. Intermediate compositions are developed in single crystals in which the amphibole 

changes in composition from pargasite to tremolite. The 

major compositional changes are decreases in A12O3, TiO2, 

K2O, 0*203, and perhaps Cl and F, and an increase in MgO. 

Alteration amphiboles are relatively rare in the samples 

studied.

COMPOSITION AND SUBSTITUTION 

MECHANISMS OF PHLOGOPITE

Phlogopite is used to refer to a trioctahedral mica in 

which most of the octahedral positions are occupied by Mg 

(Foster, 1960). For the analyses in table 6, 70 to 80 percent 

of the octahedral positions are occupied by Mg. Normally, 

phlogopites have less than 0.10 cation positions occupied by 

Ti (Foster, 1960); however, the calculated formulas for 

phlogopites from the olivine cumulates generally contain 

more than 0.2 formula units Ti, with a range from 0.095 

to 0.347. With two exceptions, all analyses in table 6 contain greater than 3.0 weight percent TiO2; in addition, they 

all have A1 VI that is less than A1 IV (with the A1 VI also accompanied by a Si deficiency) and a K-site deficiency of less

than 0.2. Because it was not possible to determine 

the analyses when cast as end members plot as combinations 

of phlogopite, annite, and siderophyllite (fig. 9). The phlogopite compositions form a relatively tight cluster between 

72 and 80 percent phlogopite end member, with about equal 

proportions of annite and siderophyllite.

Various substitution mechanisms have been suggested 

for phlogopites, including the phlogopite-annite series 

(K2Mg6 [Si6Al202ol (OH)4 to K2Fe6 [Si^C^ol (OH)4) and 

the phlogopite-siderophyllite series (K^Mgg [SieA^C^ol 

(OH)4 to K2Fe5AlVI [Si5Al3VO2o] (OH)4); both of these 

substitution mechanisms seem to operate in the phlogopites 

of the olivine cumulates (see fig. 9). Arima and Edgar (1981), 

among others, have examined the possible substitution 

mechanisms involving Ti in phlogopite, and the discussion 

that follows is based on their calculation and plotting 

methodology.

In figure 10, all the phlogopite analyses have A1 VI less 

than Al^ and enough Al to compensate for the Si deficiency 

so that no Fe +3 or Ti^ is needed. They cluster on a line 

between two end members, K2Mg5TiAl4Si4O2o(OH)4 and 

K2Mg4 [ ] TiAl2Si6O2o(OH)2 , indicating that Ti is likely to 

occupy the octahedral site. The phlogopites have trends of 

increasing Ti with both octahedral site occupancy and Si + 

A1VI, as shown in figure 11. This relation suggests that the 

two substitution mechanisms, 2MgVI ^ Ti^ and Mg^Si 

, are operative in the Still water phlogopites.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A15
Table 5. Comparison of the differences in means between assemblage groups given in table 4 

[Comparison criteria are calculated for 99.5-percent confidence level]

Si - 

A1 IV -

A1 VI- -

Fe 2+ 

Fe 3+ -.

Mg .

Excess

lM3g

Na A -

2 A sil 

XM

Groi

Mean

- 0.035

- .035

 .013

Oct- .011 

 .014

 .012

te .011 

 .011

>P 2

Comparison

.129

.154 

.051

0.053

.110

.017

.052 

.040

.038 

.013

Mean

0.345

.570

0.025

.123

.119

.106

.102

.081

Group 1

Group 3

Comparison

.537

.437

.139

0.087

.105

.170

.083

.063

.057

Mean

.157

n £Q

.051

0.024

.004

.002 

.011

Group 4

Comparisoi

Micropr

.197

Mineral formula

0.045

.044 

.031

.035

.029

i Mean

obe analyses,

0.329

.675

.145

.575

calculated on

0.010

.017 

.121

.094

.104

.092

Croup 2

Group 3

Comparison

weight percent

.231

.148

the basis of 23 oxygens

0.071 0

.099

.051

.056

Group

Mean

.077

.011

.027

.008 

.012

.013

4

Com

Comparison criteria = t (s (1/nj + l/n2) ) where:

t is the value of a test statistic derived from Students' t-distribution for a level of significance of 99.5 percent, 

ni and no are the sample sizes of the two populations, 

s is the pooled standard deviation and is equal to

, where s, and Sy are the standard deviations of the two populations.
CO

1.5

Figure 4,

Symbols

0.7 0.8 0.9 

(Na, K)A CONTENT, IN FORMULA UNITS

1.0

1.0

{Na, K)A CONTENT, 

IN FORMULA UNITS

Plot of AI IV versus (Na, K)A for hornblendes in table 2, divided into textural assemblages and an end-member diagram, 

are same as in figure 3.

1.3

03 1.0 1.1 12 13 

AI VI + 2Ti + Cr CONTENT, IN FORMULA UNITS

Figure 5. AI IV - (Na, K)A versus AI VI + 2Ti + Cr for hornblendes in table 2, divided into textural assemblages. Symbols 

are same as in figure 3.

0.2

0.1

0.0

0.8 0.9 1.0 11 12 

AI VI + 2Ti + Cr CONTENT, IN FORMULA UNITS

Figure 6. NaB versus AI VI + 2Ti + Cr for hornblendes in table 

2, divided into textural assemblages. Symbols are same as in 

figure 3.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A17
2.0

1.9

1.7

1.6

0.1 0.2 0.3 0.4

TITANIUM CONTENT, IN FORMULA UNITS

0.5

2,0

Z 1.9

1.8

1.7

1.6

B

0.0 0.06 0.12 0.18

CHROMIUM CONTENT, IN FORMULA UNITS

024

2,0

1.9

1.8

1.7

1.6

0.1 02. 03 0.4 

AI VI CONTENT, IN FORMULA UNITS

0.5

Figure 7. AI IV versus Ti, Cr, and Al vl for hornblendes in table 

2, divided into textural assemblages. Symbols are same as in 

figure 3. A, Al lv versus Ti. B, Al lv versusCr. C, Al IV versus Al Vl .

ASSEMBLAGE-CONTROLLED COMPOSITIONAL 

VARIATIONS IN BROWN AMPHIBOLE

Two facets of the composition information warrant further discussion. First, is there any compositional difference 

between the various distinct textural assemblages of brown 

amphibole within single samples? Second, is there any 

systematic variation in the composition of single hornblende 

crystals related to the coexisting assemblage?

Compositional Variation Between Assemblage 

Groups Within the Same Sample

Although the analysis presented in the previous section indicates there was little difference in composition 

between hornblendes in different assemblages taken as a 

whole, the compositional range may have obscured any 

systematic variation that might have existed within individual 

samples, here defined as a single thin section. The recognition of hornblende assemblages with different compositions 

within individual samples is important in any interpretation 

of hornblende compositions as a function of stratigraphic 

position.

Comparison of hornblende analyses for different textural assemblages in a single sample show that there is a 

systematic difference based on assemblage when examined 

on cation-cation plots. Hornblende associated with chromite

0.30

0.24

0.18

0.12

0.06

0.0

1.60 1.68 1.76 1.84 1.92 2.00 

AI VI + 2Ti + Cr + (Na, K) A CONTENT, IN FORMULA UNITS

Figure 8. NaB versus AI VI + 2Ti + Cr + (Na, K)A for hornblendes in table 2, divided into textural assemblages. Symbols 

are same as in figure 3.

A18 Contributions on Ore Deposits in the Early Magmatic Environment
Table 6. Electron microprobe analyses and calculated mineral formulas of phlogopite from the olivine cumulate of cyclic unit

2, Stillwater Complex, Montana

[Analyst: M.L. Zientek, n.d., not determined; ph, phlogopite; an, annite; sid, siderophyllite]

Microprobe analyses

)Q 5 ________

Ti02 -

NagO 

d __________ 

Total -_ 

623c

38.7 

15.4 

5.0 

5.6 

20.1 

1.1 

.00 

.5 

9-3 

.01 

.07

95.78

628a

36.9 

13.2 

4.6 

4.7 

20.5 

1.0 

.30 

.5 

9.9 

.00 

.04

91.64

630c

37.6 

15.4 

5.2 

6.0 

19.6 

1.2 

.00 

.8 

9.6 

.01 

.18

95.59

630a

37.8 

15.5 

5.6 

6.2 

19.3 

1.2 

.31 

.8 

9.1 

.00 

.17

95.97

685a-bt1

38.2 

15.8 

3.5 

6.0 

20.3 

1.4 

.00 

.9 

9.3 

.07 

.31

95.78

685a-bt2

38.0 

15.9 

3.6 

5.9 

20.2 

1.5 

.00 

.9 

9.1 

.08 

.33

95.51

Formula based on sum of

A!" ________

A1VI ___

Ti 

Fe 2+ ________

Mg 

K _._ ___ _ ~ 

F _ .______ 

Cl 

OH 

F/F+OH 

C1/C1+OH 

log (F/OH) 

log (Cl/OH) -

2.766 

1.234 

.065 

.269 

.336 

2.139 

.032 

2.841 

.00 

.069 

.845 

.002 

.009 

1.989 

.75 

.001 

.004 

-2.998 

-2.344 

.75 

.12 

.12

2.774 

1.226 

.000 

.261 

.293 

2.300 

.030 

2.884 

.024 

.073 

.948 

.00 

.005 

1.995 

.81 

.00 

.003 

.00 

-2.592 

.81 

.12 

.07

2.717 

1.283 

.033 

.285 

.362 

2.114 

.034 

2.829 

.00 

.105 

.881 

.002 

.022 

1.976 

.75 

.001 

.011 

-2.936 

-1.952 

.75 

.12 

.14

2.718 

1.282 

.033 

.304 

.371 

2.063 

.035 

2.806 

.024 

.113 

.833 

.00 

.021 

1.979 

.74 

.00 

.010 

.00 

-1.981 

.74 

.12 

.14

2.756 

1.244 

.095 

.192 

.362 

2.182 

.041 

2.872 

.00 

.123 

.857 

.016 

.038 

1.946 

.76 

.008 

.019 

-2.086 

-1.711 

.76 

.11 

.13

2.747 

1.253 

.102 

.195 

.359 

2.182 

.042 

2.879 

.00 

.121 

.843 

.018 

.040 

1.941 

.76 

.009 

.020 

-2.026 

-1.681 

.76 

.11 

.13

, weight

690d

38.0 

16.1 

3.3 

5.8 

21.1 

1.0 

.00 

.5 

9.6 

n.d. 

n.d.

95.4

positive

2.736 

1.264 

.104 

.177 

.347 

2.260 

.029 

2.917 

.00 

.066 

.878 

n.d. 

n.d. 

n.d. 

.77 

n.d. 

n.d. 

n.d. 

n.d. 

.77 

.10 

.13

Microprobe analyses, weight

SiOg 

Ai 2 53 _______

Ti02 

Cr 2 0o 

Na 20 

if r\ P2^:::::::::

Si A1 iv _____

A1VI ______

Ti 

Fe 2+ ________

ZOct 

Cl

Xmg 

F/F+OH 

C1/C1+OH 

log (F/OH) 

log (Cl/OH) _ 

Xph _- 

730b

38.0 

15.8 

4.4 

5.2 

20.9 

1.0 

.00 

.5 

9.4 

n.d. 

n.d.

95.2

2.730 

1.270 

.071 

.236 

.315 

2.236 

.028 

2.886 

.00 

.063 

.863 

n.d. 

n.d. 

n.d. 

.77 

n.d. 

n.d. 

n.d. 

n.d. 

.77 

.10 

.12

747a

37.8 

15.3 

5.1 

5.3 

20.6 

1.0 

.00 

.5 

9.4 

n.d. 

n.d.

95.0

2.724 

1.295 

.023 

.277 

.322 

2.216 

.029 

2.867 

.00 

.074 

.861 

n.d. 

n.d. 

n.d. 

.77 

n.d. 

n.d. 

n.d. 

n.d. 

.7 

.11 

.12

755b

38.5 

15.6 

3.4 

5.5 

20.7 

.7 

.00 

1.0 

9.0 

.11 

.28

94.79

2.781 

1.270 

.111 

.187 

.334 

2.222 

.020 

2.873 

.00 

.141 

.827 

.025 

.034 

1.941 

.77 

.013 

.017 

-1.888 

-1.753 

.77 

.11 

.12

755b b

38.0 

15.3 

3.7 

6.4 

20.4 

1.0 

.00 

.7 

8.8 

n.d. 

n.d.

94.3

2.761 

1.276 

.073 

.204 

.389 

2.209 

.029 

2.904 

.00 

.102 

.819 

n.d. 

n.d. 

n.d. 

.76 

n.d. 

n.d. 

n.d. 

n.d. 

.76 

.12 

.12

755bc

38.6 

15.3 

4.5 

5.9 

20.7 

.8 

.00 

.9 

8.9 

n.d. 

n.d.

95.6

Formula

2.758 

1.219 

.049 

.243 

.352 

2.208 

.021 

2.874 

.00 

.125 

.813 

n.d. 

n.d. 

n.d. 

.77 

n.d. 

n.d. 

n.d. 

n.d. 

.77 

.12 

.11

755ca

37.3 

15.4 

6.4 

5.6 

19.7 

1 .1 

.00 

.7 

9.0 

n.d. 

n.d.

95.2

based on sura

2.690 

1.239 

.000 

.347 

.337 

2.116 

.032 

2.832 

.00 

.094 

.832 

n.d. 

n.d. 

n.d. 

.75 

n.d. 

n.d. 

n.d. 

n.d. 

.75 

.12 

.14

percent

691 .5c-btl

38.5 

16.2 

1.8 

5.5 

21.7 

.8 

.00 

1.3 

8.7 

.02 

.41

94.93

charges=22

2.777 

1.223 

.155 

.095 

.334 

2.332 

.022 

2.937 

.00 

.182 

.800 

.005 

.050 

1.945 

.79 

.002 

.025 

-2.630 

-1.589 

.79 

.90 

.12

percent

767b 787b-bt1

38.1 

15.7 

3.5 

4.7 

21.8 

1.0 

.00 

.7 

9.0 

n.d. 

n.d.

94.5

positive

2.744 

1.242 

.079 

.187 

.284 

2.345 

.029 

2.923 

.00 

.096 

.831 

n.d. 

n.d. 

n.d. 

.80 

n.d. 

n.d. 

n.d. 

n.d. 

.80 

.09 

.11

39.0 

16.2 

4.2 

4.8 

20.4 

1.2 

.00 

.8 

9.4 

.02 

.05

96.07

charges=22

2.773 

1.310 

.130 

.223 

.286 

2.154 

.035 

2.829 

.00 

.109 

.848 

.004 

.006 

1.989 

.76 

.002 

.003 

-2.646 

-2.519 

.76 

.11 

.13

691.5c-bt2 701a-bt1

38.4 37.2 

16.3 15.5 

3.6 4.7 

5.9 6.0 

20.3 19.3 

1.1 1.4 

.01 .02 

1.1 1.1 

8.9 8.8 

.05 .08 

.34 .26

96.00 94.36

2.747 2.729 

1.253 1.271 

.126 .065 

.193 .256 

.352 .367 

2.171 2.105 

.032 .041 

2.873 2.835 

.001 .002 

.146 .156 

.817 .824 

.011 .019 

.041 .032 

1.947 1.949 

.76 .74 

.006 .009 

.021 .016 

-2.236 -2.022 

-1.674 -1.781 

.76 .74 

.10 .12 

.14 .14

787b-bt2 815a 830ab

38.5 37.7 37.8 

15.9 16.1 15.8 

5.2 2.9 5.2 

4.5 5.2 5.2 

20.5 21.0 19.7 

1.3 1.5 1.3 

.02 .00 .03 

.7 .6 .7 

9.5 9.5 9.6 

.02 n.d. .01 

.06 n.d. .07

96.20 94.5 95.41

2.738 2.744 2.724 

1.256 1.227 1.262 

.070 .121 .064 

.276 .157 .283 

.268 .317 .315 

2.174 2.277 2.115 

.036 .042 .037 

2.823 2.912 2.815 

.002 .00 .002 

.094 .078 .094 

.862 .879 .879 

.005 n.d. .002 

.007 n.d. .009 

1.988 n.d. 1.989 

.77 .78 .75 

.002 n.d. .001 

.004 n.d. .004 

-2.645 n.d. -2.940 

-2.439 n.d. -2.366 

.77 .78 .75 

.11 .90 .11 

.12 .13 .14

701a-bt2

38.2 

15.7 

4.7 

6.0 

19.2 

1.4 

.00 

1.1 

8.8 

.11 

.26

95.47

2.758 

1.242 

.094 

.253 

.363 

2.072 

.039 

2.820 

.00 

.154 

.809 

.025 

.032 

1.943 

.73 

.013 

.016 

-1.888 

-1.786 

.73 

.12 

.14

861b

38.0 

15.7 

5.2 

5.2 

20.0 

1.2 

.00 

.5 

9.5 

n.d. 

n.d.

95.3

2.730 

1.256 

.063 

.281 

.310 

2.148 

.034 

2.834 

.00 

.070 

.873 

n.d. 

n.d. 

n.d. 

.76 

n.d. 

n.d. 

n.d. 

n.d. 

.76 

.11 

.13

715a

38.1 

15.7 

4.7 

6.0 

19.5 

1.2 

.00 

.6 

9.5 

.02 

^35

95.67

2.749 

1.251 

.081 

.257 

.362 

2.098 

.035 

2.834 

.00 

.081 

.872 

.005 

.043 

1.953 

.74 

.002 

.021 

-2.632 

-1.659 

.74 

.12 

.14

874a

37.6 

15.4 

5.4 

4.0 

21.3 

1.1 

.00

.r

9.3 

n.d. 

n.d.

94.7

2.705 

1.276 

.013 

.294 

.240 

2.283 

.030 

2.860 

.00 

.092 

.851 

n.d. 

n.d. 

n.d. 

.80 

n.d. 

n.d. 

n.d. 

n.d. 

.80 

.09 

.11

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A19
PHLOGOPITE

50 50

shows a larger extent of coupled substitution and may be 

slightly more magnesium rich. The compositional variation 

between different assemblages within single samples is 

similar to the variations found in the whole group of samples, 

and the same substitution mechanisms predominate.

Compositional Variation Among the Assemblage 

Groups Within the Same Hornblende Crystal

In order to understand variations in hornblende compositions in the chromite-associated assemblage and the extent of coupled substitutions involving individual elements, 

three samples were selected to study composition zoning in 

single crystals. In each sample, the hornblende crystal was 

large enough so that it was in contact with a variety of silicate 

and (or) oxide crystals. The compositions of the hornblende 

in contact with each adjoining phase were then determined,

Figure 9. Phlogopite compositions, in percent, from olivine 

cumulate of cyclic unit 2 cast as phlogopite, annite, and 

siderophyllite end members.

Ti

75 25

6-(Si + AI VI )

K2 Mg 4 [ ] Ti Al 2 Si e O 20 (OH) 2 

50

K 2 Mg 5 Ti AI 9 Si4 020 (OH)4

75 50 25

6-OSO

Figure 10. Phlogopite compositions, in percent, in terms of Ti, 6-OSO (octahedral site occupancy), and 6-(Si + AI VI) in structural 

formula.

A20 Contributions on Ore Deposits in the Early Magmatic Environment
and the results are presented in table 7. In addition, the compositions of the coexisting chromite and olivine were analyzed; these analyses are given in tables 8 and 9, respectively. Sketches of the areas examined, with the locations of the 

points analyzed, are shown in figure 12. The results show 

that the crystals are zoned and that the composition of the 

hornblende is related to the composition of the chromite.

Although the samples come from different stratigraphic 

positions in the olivine cumulate of cyclic unit 2, Mg/(Mg 

+ Fe2+) of the olivine crystals is the same (0.85). This ratio 

reflects the composition of olivine in the core of crystals; 

no large-scale zoning was recognized. The spinels are 

chromites with Mg/(Mg + Fe2+) between 0.329 and 0.412 

and with Cr/(Cr + Al) between 0.546 and 0.596. TiO2 

varies from 0.76 to 2.2 weight percent. In one sample (747b), 

the rims tend to be slightly Mg enriched.

Two approaches to examining the hornblende data in 

table 7 were used. One was to look at cation contents plotted 

spatially with respect to the adjacent chromite and silicate 

minerals, and the other was to examine cation-cation plots 

to evaluate the amounts and types of substitution.

The distribution of cation and selected cation ratios is 

shown for the three samples containing zoned hornblendes 

(fig. 12). Mg/(Mg + Fe2+) in hornblende does not vary by 

more than 0.03 units in any of the three samples nor does 

its variation appear to be related systematically to the adjoining chromites and silicates. However, variations in the 

cation proportions of Cr and Ti (fig. 125, C) are more 

strongly influenced by the adjoining minerals to the amphibole. The best example of this relation is the distribution 

of Ti in sample 747b (fig. 12C). In this sample, the Ti content (formula units) of amphibole next to olivine and 

plagioclase is 0.29 to 0.32, whereas the Ti contents of amphibole next to chromite and plagioclase and of amphibole 

next to chromite and olivine range from 0.36 to 0.38 and 

0.35 to 0.38, respectively. Similar, although not as strong, 

correlations are found in samples 830a and 755c. Cation units 

of Cr are the highest next to chromite plus plagioclase or 

chromite plus olivine and are the lowest next to plagioclase 

plus olivine (fig. 125). These observations strongly suggest 

that Cr and Ti zoning in hornblende is influenced by the 

nearest neighbor minerals to the hornblende. The distribution of Al cations (fig. 12£>) in hornblende also appears 

to be influenced by the adjoining minerals. Hornblende in 

contact with chromite and plagioclase has higher amounts 

of Al than hornblende in contact with other combinations 

of minerals. The distribution of A1VI in hornblendes appears 

to be lower in hornblende in contact with chromite (fig. VIE). 

Examination of the distribution of (Na, K)A and NaB cations 

in the hornblendes suggests that their distribution is not influenced by the adjoining minerals, except that there is a suggestion that NaB cations are lowest in hornblende that is in 

contact with plagioclase and olivine. In summary, both 

chromite and silicate minerals appear to be related to changes 

in composition of the zoned brown amphiboles, and the

presence or absence of chromite seems to affect the composition more than the presence or absence of silicates.

Changes in the composition of the zoned hornblendes 

depending on assemblage were also examined on cationcation plots similar to those in figures 3-8. Based on this 

examination, it was concluded that the substitution 

mechanisms for these zoned crystals are the same as those 

discussed earlier for all hornblendes. As might be expected, 

hornblende adjacent to chromite-bearing assemblages has a 

greater extent of coupled substitution of edenite and tschermakite and higher Ti and NaB contents. Cr content is

to

0.8

0.6

0.4

CO

t 0.2

tr 

o o.o

5.00 5.24 5.48 5.72 5.96

OCTAHEDRAL SITE OCCUPANCY (6-OSO), 

IN FORMULA UNITS

5.00 5.24 5.48 5.72 

Si + AI VI, IN FORMULA UNITS

5.96

Figure 11. Phlogopite compositions. A, Ti versus 6-OSO. B, Ti 

versus Si + AI VI .

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A21
Table 7. Electron microprobe analyses and calculated mineral formulas of zoned amphibolecrystalsfrom the olivine cumulate of cyclic un

[Analyst: M.L. Zientek]

Microprobe analyses, weight percent

747b-ptl 747b-pt7 747b-pt8 747b-pt2 747b-pt9 747b-pt3 747b-ptlO 747b-ptll 747b-ptl2 7

42.7

12.4

5.7

16.4

.08

3.5

12.2

2.6

.95

1.6

98.13

42.1

12.5

5.7

16.4

.05

3.5

12.2

2.6

.91

1.6

97.56

42.0

12.7

6.2

16.2

.35

3.3

11.7

2.6

.96

1.6

97.61

=r Si02 43.2 42.9 43.7 42.3 43.3

^ A1 2 03 12.3 12.2 11.9 12.2 12.2

S1 FeO 5.9 6.3 6.1 5.8 5.8

^- Mg o 16.5 17.1 16.6 16.2 16.3

 £ MnO .08 .11 .09 .07 .08

£ Ti02 2.7 2.7 3.0 3.4 3.5

3 CaO 12.3 11.9 12.1 12.1 12.2

Si Na 2 0 2.3 2.3 2.7 2.6 2.6

«* K 2 0 .89 .87 .83 .96 .88

S1 Cr 2 03 .98 .94 1.2 1.5 1.6 <_ 

| Total 97.10 97.32 98.22 97.13 98.45

I ____________________________________________________________________________________________________

 Mineral formula calculated on the basis of 23 oxygens

Si 6 . 252 6.215 6.263 6.149 6.205

A1 IV 1.748 1.785 1.737 1.851 1.795

STet 8.000 8.000 8.000 8.000 8.000

A1 VI .349 .299 .282 .241 .265

Fe2+ >711 >76? >731 >70Q >694

Mg 3.555 3.684 3.544 3.508 3.475

Mn .010 .013 .011 .009 .010

Ti .294 .289 .325 .372 .376

Cr .112 .108 .133 .177 .175

SOct 5.031 5.160 5.025 5.007 4.996

Excess Oct .031 .160 .025 .007 .00

Ca 1.909 1.840 1.866 1.878 1.866

Na B .060 .000 .109 .115 .134

SB 2.000 2.000 2.000 2.000 2.000

NaA .594 .640 .631 .626 .583

K .164 .161 .152 .178 .161

SA site .759 .801 .783 .803 .743

Mg/(Mg+Fe 2+) .83 .83 .83 .83 .83

Accept yes yes yes yes yes

Name Pargasitic Pargasite Pargasitic Pargasite Pargasite

hornblende hornblende

42.3

12.4

5.6

16.5

.07

3.5

12.1

2.6

.91

1.6

97.58

6.140

1.860

8.000 

.241 

.682

3.518 

.010 

.382 

.179

5.011 

.011

1.875 

.113

2.000 

.615 

.174 

.790 

.84 

yes

6.095

1.905

8.000 

.228 

.687

3.532 

.006 

.385 

.180

5.018 

.018

1.898 

.084

2.000 

.643 

.168 

.811 

.84 

yes

Pargasite Pargasite

6.086

1.914

8.000 

.265 

.757

3.507 

.043 

.361 

.178

5.111 

.111

1.813 

.076

2.000 

.647 

.178 

.825 

.82 

yes 

Pargasite

6.113

1.887

8.000 

.299 

.675

3.556 

.009 

.380 

.185

5.034 

.034

1.880 

.086

2.000 

.631 

.168 

.799 

.84 

yes 

Pargasite

P
Table 7. Electron microprobe analyses and calculated mineral formulas of zoned amphibole crystals from the olivine cumulate of 

Montana Continued

o

s,

 o1'

3

 Q

51 

O5"

SiQ2 

A1 2 03 

FeO 

T-T r\

Na 2 0 

Kf\

Cr 2 03 - 

Total 

Si - 

A1 IV _____

A1 VI ____

9 4- Fe 2 + _____

Ti 

Cr -

SOct 

Excess Oct 

Ca 

Na B 

yn

NaA -

SA site 

Mg/(Mg+Fe 2+) 

Accept 

755ca-ptl

43.0 

11.9 

6.1 

16.3 

.08 

3.3 

11.8 

2.8 

.69 

1.4

97.37

6.221 

1.779 

8.000 

.253 

.735 

3.514 

.010 

.356 

.160 

5.029 

.029 

1.836 

.135 

2.000 

.660 

.127 

.787 

.83 

yes 

Pargasite

747ca-pt5

43.1 

11.7 

6.6 

16.4 

.10 

3.3 

11.8 

2.9 

.66 

1.5

98.06

6.225 

1.775 

8.000 

.210 

.791 

3.529 

.012 

.353 

.167 

5.062 

.062 

1.823 

.114 

2.000 

.684 

.122 

.806 

.82 

yes 

Pargasite

755ca-pt6

42.7 

12.1 

6.1 

16.3 

.08 

3.3 

12.0 

2.7 

.54 

1.4

97.22

6.185 

1.815 

8.000 

.257 

.744 

3.516 

.010 

.358 

.164 

5.048 

.048 

1.856 

.096 

2.000 

.674 

.100 

.773 

.83 

yes 

Pargasite

755ca-pt7

42.6 

11.6 

5.8 

16.4 

.07 

4.2 

12.0 

2.9 

.62 

1.4

97.59

6.158 

1.842 

8.000 

.138 

.705 

3.524 

.009 

.456 

.162 

4.994 

.000 

1.860 

.140 

2.000 

.664 

.114 

.779 

.83 

yes 

Pargasite

755ca-pt8

42.2 

12.5 

6.3 

15.9 

.10 

4.2 

11.9 

2.7 

.84 

1.3

97.94

Mineral

6.094 

1.906 

8.000 

.225 

.763 

3.417 

.012 

.450 

.151 

5.019 

.019 

1.837 

.144 

2.000 

.619 

.155 

.774 

.82 

yes 

Pargasite

Microprobe

755ca-pt4

43.0 

11.6 

6.0 

16.0 

.07 

4.3 

11.8 

2.8 

.62 

1.4

97.59

analyses, weight percent

755ca-pt9

43.3 

11.5 

5.8 

16.3 

.08 

4.3 

12.0 

2.4 

.52 

1.4

97.60

formula calculated on the

6.211 

1.789 

8.000 

.184 

.720 

3.451 

.009 

.463 

.155 

4.983 

.000 

1.829 

.171 

2.000 

.614 

.114 

.728 

.83 

no 

Pargasite

6.232 

1.768 

8.000 

.178 

.692 

3.507 

.010 

.467 

.164 

5.017 

.017 

1.852 

.131 

2.000 

.528 

.095 

.623 

.84 

yes 

Pargasite

755ca-ptlO

43.2 

11.6 

5.9 

16.4 

.08 

4.4 

11.9 

2.9 

.61 

1.4

98.39

basis of 23

6.189 

1.811 

8.000 

.148 

.705 

3.497 

.010 

.475 

.161 

4.996 

.000 

1.821 

.179 

2.000 

.628 

.112 

.739 

.83 

yes 

Pargasite

755ca-pt2

543.4 

11.9 

6.0 

16.4 

.06 

3.1 

11.8 

2.6 

.77 

1.4

97.43

oxygens

6.261 

1.739 

8.000 

.285 

.729 

3.526 

.007 

.338 

.163 

5.048 

.048 

1.827 

.125 

2.000 

.597 

.142 

.739 

.83 

yes 

Pargasitic 

hornblende
Table 7. Electron microprobe analyses and calculated mineral formulas of zoned amphibole crystals from the olivine cumulate 

Complex, Montana Continued

A1 2 03 

FeO 

Cr 2°3 

Total 

Si 

A1 IV_ _

A1 VI ____

Mn 

Ti 

Excess Oct

a B 

y B

Na A 

SA site 

Mg/(Mg+Fe 2 ' 

Accept 1 

755ca-pt3

43.2 

11.5 

6.0 

16.3 

.07 

3.4 

11.9 

2.8 

.72 

1.6

97.49

6.243 

1.756 

8.000 

.206 

.722 

3.519 

.009 

.374 

.181 

5.010 

.010 

1.845 

.145 

2.000 

.634 

.133 

.767 

k ) .83 

yes 

Pargasite

755ca-ptl4

43.6 

11.9 

6.2 

16.4 

.07 

3.3 

11.8 

2.8 

.72 

1.5

98.29

6.255 

1.745 

8.000 

.264 

.737 

3.497 

.009 

.354 

.168 

5.029 

.029 

1.819 

.152 

2.000 

.626 

.132 

.758 

.83 

yes 

Pargasit ic 

hornblende

830a-ptl

42.9 

11.8 

6.1 

16.5 

.07 

3.0 

11.9 

2.6 

.97 

1.4

97.24

6.226 

1.774 

8.000 

.253 

.739 

3.566 

.009 

.323 

.164 

5.054 

.054 

1.851 

.096 

2.000 

.625 

.180 

.805 

.82 

yes 

Pargasite

Accepted, if: 

1. Sum of tetrahedral sites = 8 ±0.02 

2. Sum of octahedral sites >4.98 

3. Excess octahedral site occupancy +Ca <2 

4. Sum B site=2 ±0.0.2 

5. Sum A site 51.02 

6. Residual charge <0.02

830a-pt2

42.9 

11.9 

6.3 

16.7 

.08 

3.1 

11.6 

2.6 

.89 

1.3

97.37

6.215 

1.785 

8.000 

.254 

.765 

3.600 

.010 

.334 

.143 

5.107 

.107 

1.807 

.086 

2.000 

.642 

.165 

.806 

.84 

yes 

Pargasite

.02

830a-pt3

43.0 

12.4 

5.6 

16.5 

.07 

3.5 

12.2 

2.5 

.96 

1.7

98.43

Mineral

6.154 

1.846 

8.000 

.249 

.672 

3.525 

.008 

.374 

.195 

5.023 

.023 

1.870 

.108 

2.000 

.587 

.175 

.762 

.84 

yes 

Pargasite

830a-ptlO

Microprobe

42.6 

12.5 

5.7 

16.4 

.05 

3.0 

12.2 

2.6 

.97 

1.4

97.42

830a-ptl5 830a-pt6 830a-pt

analyses, weight percent

42.7 

12.2 

5.7 

16.4 

.06 

3.1 

12.0 

2.5 

1.0 

1.5

97.16

formula calculated on the

6.170 

1.830 

8.000 

.295 

.685 

3.543 

.006 

.328 

.165 

5.022 

.022 

1.885 

.093 

2.000 

.628 

.179 

.807 

.84 

yes 

Pargasite

6.187 

1.813 

8.000 

.277 

.695 

3.541 

.007 

.338 

.176 

5.035 

.035 

1.860 

.105 

2.000 

.600 

.189 

.789 

.84 

yes 

Pargasite

42.7 

12.3 

6.1 

16.4 

.07 

2.8 

12.1 

2.6 

.93 

1.6

97.60

basis of 23

6.188 

1.812 

8.000 

.293 

.733 

3.534 

.009 

.301 

.179 

5.048 

.048 

1.878 

.074 

2.000 

.642 

.172 

.814 

.83 

yes 

Pargasite

43.0 

12.0 

6.0 

16.5 

.09 

2.6 

11.9 

2.7 

.85 

1.5

97.14

oxygens

6.23

1.76

8.00

.29

.72

3.56

.01

.28

.17

5.05

.05

1.85

.09

2.00

.66

.15

.81

.83 

yes 

Pargasi
Table 8. Electron microprobe analyses and calculated mineral formulas of chromite from the olivine cumulate of cyclic unit 2, 

Still water Complex, Montana 

[Analyst: M.L. Zientek]

Microprobe analyses, weight percent

747b core 747b rim 747b rim 4 755 sp 1 core 755 sp 1 rim 830a sp 1 830a sp 2 830a sp 3 830a sp 4

Fe 0

FeO

Total ______

Si _________

Al -_ ____

Fe3+ 

Ti ____________

Fe2+ ________

Mn 

Me 

Mg/(Mg+Fe 2+ ) ~-

Fe 2V(Fe 2++Fe 3+ )

Cr/(Cr+Al) - 

Fe3+/(Cr+Al+Fe3+ )

1 Q c

37.1

.37 

8.4

99.70

0.001-N

.731

.254 f2.000

.040

.974J

1.088

.387

.722

.571 

.130

36.9

.38

99.09

0.029^

.720 }

.255 >2.000

.038

.958-1

1.068

.._.

.405

.711

.571 

.132

1.3

1 Q 1

10.7 

1.6

36.2

.39

99.43

0.042>

.705

.269

.040

.944J

.621>

.010 

.435

.002-

.412

.698

.572

.140

16.6

11.6

36.6

25.2

.44

7.0

99.64

Mineral formula

, 0.000>,

I .665 I

>2.000 .297 >2.000

.056

' .982-'

p. 068 '°^2 H.093

> .oooJ

.331

.709

.596 

.153

11.3

.40

98.89

based on

0.000-,

.669

.295

.053

.983^

.725>

.012 

.356 1

.002 J

.329

.711

.595 

.151

8.6

.76

37.3

23.1 

.33

98.89

4 oxygens

0.793>,

>2.000 .215 U.OOO

.019

.973-^ L ' 61t3 1

__, I

.394-J

.750

.551 

.108

19.9

9.2

.94

37.2

23.5

.34 8-3

99.38

_

0.775^

231 U.OOO

.023 I

.971^

.656-j

.008 | 

.413>1-077

__. 1

.386-^

.740

.556 

.117

20.5

9.1

QO

36.9

23.0 

.34

99.26

_

0.796^

226 U.ooo

.020 1

.958J

.640^

.010 

.427 ^1-077

_._ I

.400 J

.739

.546 

.114

9.9

36.5

23.7

.33

8

-3

99.55

_

0.777-^

21t8 U.OOO

.020 f

.K5-J

.663}

.010 I 

J"U15 13]

? ! .086

.384-^

.728

.551 

.125

distinctly higher in the chromite-bearing assemblage in sample 747b than for sample 755c and slightly higher for the 

chromite-bearing assemblage in sample 830a than for sample 755c. Similar variability is encountered for the Al^, 

A1 VI, and (Na, K)A contents in the hornblendes.

The variation between samples may in part reflect the 

composition of the phases coexisting with the brown hornblendes. Within the three samples studied, the amount of Ti 

in the hornblende formulas appears to correlate with the 

amount of Ti in the chromite formulas; however, the amount 

of Cr in the hornblendes does not correlate with the formula 

amounts of Cr in the chromites (fig. 13).

In summary, assemblage-controlled compositional 

variations in hornblende account for some of the variability 

seen in the hornblende analyses, such as with hornblendes 

in the chromite-absent interval in subunit 9 and with the Ti 

contents of hornblendes associated with chromite. However, 

variations in other compositional components do not appear 

to be systematically related to the crystalline assemblage 

coexisting with the hornblendes.

VARIATION IN HORNBLENDE COMPOSITION 

WITH STRATIGRAPHIC POSITION

A true average composition of hornblende at any particular stratigraphic position is difficult to obtain because, 

as has been shown previously, the hornblende composition 

is variable on a thin-section scale and in part depends upon 

the minerals with which it is in contact.

Nevertheless, arithmetical averages of selected properties of the hornblende analyses are plotted as a function of 

stratigraphic position (fig. 14), along with arithmetical 

averages of selected properties of hornblende analyses for 

different associated assemblage groups. Examining the 

overall patterns, especially for Mg/(Mg + Fe2+), we see 

that the olivine cumulate package of cyclic unit 2 can be 

divided into three parts.

The lower part contains grain-size subunits 1, 2, 3, and 

4 and is characterized by an average value of Mg/(Mg + 

Fe2+) of approximately 0.82. Although the average Mg/(Mg

Table 9. Electron microprobe analyses and calculated mineral 

formulas of olivine from the olivine cumulate of cyclic unit 2, 

Still water Complex, Montana 

[Analyst: M.L. Zientek. A12O3 , TiO2 , Cr2O3 , and CaO were not detected]

Microprobe analyses, weight percent

Si02 

FeO 

MnO 

MgO 

Total 

Fe 

Mn -

Mg -

747b

39.5 

14.7 

0.20

45.7

100.10

Mineral

0.990 

.308^1 

.004 i- 2

.85

755c

39.3 

14.5 

0.23

45.5

99.53

formula based on '

0.990 

.306^1 

.020 .004 I- 2. 

1.710J

.85

830a

39.4 

14.4 

0.21

45.8

99.81

t oxygens

0.990

.son

,010 .004 ) 2.015 

1.714J

.85

Composition of Primary Postcumulus Amphibole and Phlogopite, Still water Complex, Montana A25
Sample 830a

-3

OL

OL

OL

OL

EXPLANATION

Olivine

Chromite

Plagioclase

Hornblende

High number of cations 

Moderate number of cations 

Low number of cations 

No information

 12 Analyses location

Figure 12. Sketches of compositional variation in zoned hornblendes of samples 830a, 747b, and 755c. A, Locations of points 

analyzed. B, Distribution of Cr cations per 23 oxygens. C, Distribution of Ti cations per 23 oxygens. D, Distribution of Al lv cations 

per 23 oxygens. £, Distribution of AI VI per 23 oxygens.

A26 Contributions on Ore Deposits in the Early Magmatic Environment
Sample 747b

OL

14

OL

OL 

0.16

0.17

0.11-

OL

0.29 

OL 0.32

0.35- 

0.37- '

OL

OL

1.73- 

1.76

OL

B

D

OL

OL \

Figure 12. Continued.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A27
+ Fe2+) (fig. 145) trends are quite complex and show increasing and decreasing trends with one discontinuity, the 

trend of Mg/(Mg + Fe2+) for hornblendes associated with 

chromite and plagioclase (fig. 14C) is much simpler. The 

Mg/(Mg + Fe2+) trend of hornblende associated with 

chromite and plagioclase is nearly constant; the compositional 

discontinuity between grain-size subunits 2 and 3 seen in 

figure 14B is not present. The slight iron-enrichment trends 

at the base of grain-size subunit 1 and the tops of subunits 

are still apparent. The compositional trend of Mg/(Mg + 

Fe2+) for hornblende associated with augite (fig. 14D) is 

complex; a compositional discontinuity is present between 

grain-size subunits 2 and 3, and the base of subunit 3 is 

characterized by a magnesium-enrichment trend in the hornblende compositions. These results indicate that many of the 

small-scale fluctuations in the overall pattern result from 

averaging ratios of Mg/(Mg + Fe2+) that vary between different assemblages; the significance of the small-scale fluctuations can only be assessed when this information on 

assemblages is available. Overall, this lower part is characterized by increasing Ti and Cl contents with stratigraphic 

position (fig. 14E, G). The F contents (fig. 14G) are lower 

than the Cl contents and have an inverse relation to the 

distribution of Cl. A1 VI is quite variable within samples; the 

trend of the Al^ contents is erratic but shows an overall increase in the lower unit (fig. 14F). Cr is nearly constant (fig.

The middle part of the olivine cumulate package of 

cyclic unit 2 contains grain-size subunits 5,6,7, and 8; the 

chromitites appear in subunits 5 and 6. This middle part has 

a slightly higher range of Mg/(Mg + Fe2+), 0.83 to 0.84, 

than the lower part. The lack of samples prohibits any conclusive generalizations about trends within the subunits. The 

Cr and Ti values are erratic (in part as a result of local alteration to colorless amphibole); in general, the Ti contents increase upward in the middle unit, with a compositional 

discontinuity between the lower and the middle parts. A1 IV 

values are erratic but slightly higher than the lower part. Cl 

contents are lower than those in the lower grain-size subunits.

The upper part contains grain-size subunit 9; the lower 

portion of this subunit contains no cumulus chromite. The 

values of Mg/(Mg + Fe2+) in hornblende are lower than 

in the other grain-size subunits and average 0.81. Both Cr 

and Ti increase upward in this part. A sharp compositional 

discontinuity in Mg/(Mg + Fe2+ ), Cr, and Ti is present 

between grain-size subunits 8 and 9. A1 IV is erratic and 

slightly less than the middle part but similar to the values 

seen in grain-size subunit 3 (lower part). Cl values are quite 

high.

One sample (755c), contains the boundary between two 

grain-size subunits (4 and 5). This sample represents about 

7 cm of pegmatitic olivine cumulate, overlain by 0.3 cm of 

chromite cumulate, and followed by 4.1 cm of olivinechromite cumulate. The pegmatitic olivine cumulate contains 

cumulus olivine and postcumulus plagioclase, orthopyroxene, phlogopite, and hornblende. The chromite cumulate occurs in an olivine-chromite cumulate with postcumulus 

plagioclase, augite, orthopyroxene, and hornblende. It also 

contains accessory amounts of pyrrhotite, pentlandite, 

magnetite, and chalcopyrite. The overlying olivine-chromite 

cumulate contains postcumulus hornblende, plagioclase, and 

minor amounts of magnetite, pyrrhotite, and pentlandite. 

Selected properties of the hornblendes through this section 

are compared in table 10 based on analyses given in table 

2. The largest changes in the hornblende composition 

cumulate and involve decreases in formula units of Al VI and 

Cr and increases in the amount of A1 IV and formula units 

of Ti and K.

VARIATION IN PHLOGOPITE COMPOSITION 

WITH STRATIGRAPHIC POSITION

Selected compositional parameters of phlogopite are 

shown (fig. 15) as a function of stratigraphic position relative 

to the size-graded subunits of the olivine cumulate. Average 

and single-grain analyses have been used to construct the tentative trend lines. The overall pattern is erratic. Each sizegraded subunit (fig. 15/1), where the data are available, 

appears to have its own patterns of increasing and (or) 

decreasing Mg/(Mg + Fe2+), Ti, and Cr with stratigraphic 

height. The overall average trend of Mg/(Mg + Fe2+) X 

100 in phlogopite for the olivine cumulate of cyclic unit 2 

appears to increase slightly with stratigraphic height (fig. 

155). No overall pattern is suggested by the Ti and Cr data 

(fig. 15C, D). Cl is more abundant than F in the phlogopites, 

which is the same relation observed in the hornblendes. 

Phlogopites in grain-size subunit 3 and the base of grainsize subunit 5 contain higher concentrations of both Cl and 

F (fig. 15£).

The most striking feature about the compositional variations shown in figure 15B-D is the sympathetic variation of 

Cr with Ti and the antithetic variation of Mg/(Mg + Fe2+) 

X 100 with both Cr and Ti. The coupled substitution involving Mg and Ti in the octahedral sites in the phlogopites 

discussed earlier would predict an antithetic relation between 

Mg/(Mg + Fe2+) x 100 and Mg and Ti, and this suggests 

that a coupled substitution involving Cr and Mg in the octahedral sites of these phlogopites may also be operative. Such 

a substitution could take the forms Mg ^ Cr or Mg + Si 

^ Cr + Al, analogous to the Ti substitution schemes.

COMPARISON OF TRACE ELEMENTS 

INFORMATION

The variation of whole-rock Cr concentration with 

stratigraphic position in the olivine cumulates is mainly a 

function of the amount of chromite (Page and others, 1972). 

The concentration of whole-rock Ti with the olivine

A28 Contributions on Ore Deposits in the Early Magmatic Environment
cumulates has a more irregular distribution but can be 

generally correlated with the concentration of Cr. When the 

variation in the concentrations of Cr and Ti are compared 

in detail, there is a significant noncorrespondence of Cr and 

Ti abundances in grain-size subunit 3 (fig. 16). The concentration of Ti does closely correspond to the abundance of 

amphibole plus phlogopite in this unit. This suggests that the 

whole-rock Ti contents of these cumulates reflect the abundances of both chromite and hornblende plus biotite. In addition, these results suggest that the Ti contents of cumulates 

cannot necessarily be used to approximate the volume of 

trapped, interstitial material (for example, Irvine, 1980, 

fig. 15).

INTERPRETATIONS AND CONCLUSIONS

Experimental studies have shown that few compositional parameters of hornblende are affected by changes in 

the coexisting assemblage at constant temperature and 

pressure as long as the bulk composition is constant (Helz, 

1973, 1982). The results of the present study generally support this finding. As noted previously, there are only very 

subtle changes in the composition of hornblende in different 

textural associations. In the olivine cumulates of subunits 1 

through 8, the overall compositions of hornblendes that 

replace augite cannot be statistically discriminated from the 

compositions of hornblendes that rim chromite (table 5). All 

analyses of brown amphibole define a very narrow compositional range (fig. 3).

However, the presence of chromite does have a very 

slight effect on the composition of hornblende. The hornblendes in the lower part of subunit 9 are characterized by 

less extensive coupled substitution and lower amounts of Cr 

and Ti; cumulus chromite is absent from the rocks in this 

part of the subunit. In single thin sections, hornblendes 

associated with chromite exhibit a higher degree of coupled 

substitution. Within individual crystals, the proportions of 

Ti and to a lesser extent of A1 IV and Cr increase in abundance adjacent to chromite crystals (fig. 12). The Ti contents of these hornblendes are related to the Ti contents of 

the associated chromite (fig. 13). Mg/(Mg + Fe2+) of the 

hornblendes is not significantly affected by the presence or 

absence of chromite.

^ i

TITANIUK IN FOR

0.05

0.04 

0.03

0.02 

0.01 

nn

I I T """ I ' I ' " " I I I

^^^""^ m m.'^"

^^^ /

S i 

X /

' *

- / *

1 1 1 1 1 1 1

EXPLANATION

 830a 

 747b 

A 755c

i

-

-

030 0.32 0.34 0.36 0.38 0.40 0.42 

TITANIUM IN HORNBLENDE, IN FORMULA UNITS

0.44 0.46 0.48

0.98

5 K. 0.96 

a: z

0.0 0.01 0.02

CHROMIUM IN HORNBLENDE, 

IN FORMULA UNITS

Figure 13. Formula units of Cr and Ti in zoned hornblende and associated chromite for samples 830a, 747b, and 755c. Dashed 

lines bound observed data.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A29
These observations are also in accord with experimental 

observations. Helz (1973, 1982) has noted that the TiO2 contents of hornblende are strongly affected by changes in the 

oxide assemblage. The compositions of the hornblendes in 

cyclic unit 2 are influenced by the local environment of 

crystallization.

Experimental studies by Helz (1973) demonstrated that 

the Al^ and Ti contents of hornblende were strongly correlated with temperature. If the results from the QFM-buffered, 

1921 Kilauea olivine tholeiite experiments are used to model 

the Stillwater magma, crystallization temperatures or (more 

likely) the blocking temperature can be estimated for the 

brown hornblendes. The estimated temperatures are summarized in table 11. Temperatures based upon the Al^ contents of the hornblendes generally fall within a narrow range, 

905 to 981 °C. The average temperature appears to be

930 °C. Difference in temperatures between different 

assemblages in the same thin section are less than the 

estimated difference based on analytical error. A range of 

temperatures within single hornblende crystals is recorded 

in samples 747b, 755c, and 830a. The temperatures are 

highest adjacent to chromite grains (approximately 980 °C) 

and are approximately 930 to 940 °C away from chromite. 

Temperatures based on the Ti contents of the hornblendes 

are much less systematic. Values range from 748 to 999 °C. 

The high and low temperatures given in these ranges do not 

correspond to the same analyzed points that give the maximum and minimum recorded temperatures based upon the 

A1 IV contents of the hornblendes. The Ti contents of the 

hornblendes in these rocks do not accurately reflect the 

temperatures at which they crystallized, because they did not 

crystallize in equilibrium with ilmenite.

980

900

800

700

600 L

4--

obc

obc

<

EXPLANATION

 Chromium 

o Titanium

 O---O----O

..

80 82 84 86 88 90 

Mg/(Mg + Fe 2+ )x100

B

80 82 84 86 88 90 

Mg/(Mg + Fe 2+) x 100

80 82 84 86 88 90 

Mg/(Mg + Fe 2+)x100

D

I___I___I___I___I___I

0 0.1 0.2 03 0.4 0.5

CHROMIUM OR

TITANIUM, IN

FORMULA UNITS

Figure 14. Variation in selected compositional properties of 

hornblende with stratigraphic position. A, Columnar section of 

olivine cumulate of cyclic unit 2, Still water Complex, Montana, 

with size-graded units (dotted lines) numbered; occ, olivinechromite cumulate; oc, olivine cumulate; obc, olivine-bronzite 

cumulate; heavy lines, chromite cumulate. B, Arithmetical 

average of Mg/(Mg + Fe2+ ) x 100 in hornblende; analyses

from table 2. C, Average Mg/(Mg + Fe2+ ) x 100 in hornblende 

associated with chromite and plagioclase. D, Average Mg/(Mg 

+ Fe2+ ) x 100 of hornblende associated with augite. E, 

Average formula units of Cr and Ti in brown amphiboles from 

table 2. F, AI IV in brown amphiboles. C, Average weight percent Cl and F in brown amphiboles.

A30 Contributions on Ore Deposits in the Early Magmatic Environment
The values of Mg/(Mg + Fe2+ ) for the hornblendes 

vary only slightly but change systematically as a function 

of stratigraphic position. As noted previously, this change 

in hornblende composition can be correlated with changes 

in the mode and grain size of the olivine cumulates. In 

essence, the rocks below the chromite seams, the rocks 

associated with and slightly above the chromite seams, and 

the rocks within the chromite-free lower part of subunit 9 

all have distinct amphibole compositions. The relative depletion of Cr in hornblendes above the chromite seams suggests 

that the processes responsible for chromite seam formation 

within the cumulates perhaps also influenced the composition of hornblendes that formed later.

The textural and chemical evidence indicate that the 

brown hornblendes crystallized from trapped interstitial 

liquid at high temperatures (in excess of 900 °C). Local

variations in hornblende composition suggest that the local 

environment influenced the composition. The hornblendes 

that rim chromite may have formed earlier than the hornblendes that replace augite, because the highest recorded 

temperatures are from hornblendes immediately adjacent to 

chromite crystals. On average, about 96 percent of the interstitial space is occupied by plagioclase, augite, and orthopyroxene, all of which crystallized before or slightly overlapping the crystallization of hornblende and phlogopite. 

Although the quantitative relation between porosity and 

permeability is variable in both sandstones and cumulates, 

in general the higher the porosity, the greater is the permeability (Levorsen, 1958). Porosities of less than 5 percent 

in sandstones are associated with extremely low permeabilities (less than 0.1 millidarcy). Thus, the small volume 

of pore space left at the time of hornblende and phlogopite

EXPLANATION

 Average analysis

 Grain analysis

\ 1 \^>

EXPLANATION

 Fluorine 

o Chlorine

o o

1 I I I I I I I L I I I I I I 1 I I I

1.60 1.70 1.80 1.90

AI IV CONTENT, IN FORMULA UNITS

0.10 020

CHLORINE OR FLUORINE CONTENT, 

IN WEIGHT PERCENT

Figure 14. Continued.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A31
Table 10. Comparison of selected compositional properties of 

brown amphiboles through a section of olivine cumulate containing a chromite cumulate, cyclic unit 2, Stillwater Complex, 

Montana 

[oc, olivine cumulate; occ, olivine-chromite cumulate; cc, chromite cumulate]

Sample 755 755 755b

Rook . - A1 iv _

Ai VI _____

MS ATi '

Cr

K

a 4

1 1

OQ

T£ .7o

QO

Q7

.16

.13

1 n

1 Q

.16

.70

QO

.50

.09

.20

1 P

1.7

.27

. 70( U

Q 1

.30

.24

.12

.11

crystallization suggests that the permeability of the olivine 

cumulate was small. Apparently, large-scale movement of 

trapped residual liquid, which would completely homogenize 

the final compositions of hornblende, did not occur or had 

ceased by the time hornblende finally crystallized. This 

discussion does not confirm or deny the movement of residual 

liquids prior to the crystallization of hornblende. However, 

if the residual liquids were derived from the underlying cyclic 

unit, they would have been in equilibrium with both cumulus 

olivine and orthopyroxene before migration and probably 

should have made the compositions of the interstitial minerals 

in subunit 1 different from those in the succeeding two 

subunits, both in which the initial interstitial liquid would 

have been in equilibrium with olivine alone. There is little 

difference between subunit 1 and the next two overlying 

cyclic units, which suggests that residual liquids did not 

migrate very far even at the time when the porosity and 

permeability would have been relatively high.

980 r

tO Oo

CD Oo

--J Oo

600 L

4--

obc

obc

EXPLANATION

 Fluorine 

o Chlorine

\

A

0.70 0.72 0.74 0.76 0.78 0.80 0.82 0.0 

Mg/(Mg + Fe^) x 100

B

0.2 0.4 0.0 0.02 0.04 0.0 0.1 02 0.3 0.4

TITANIUM, 

IN FORMULA UNITS

CHROMIUM, 

IN FORMULA UNITS

D

FLUORINE AND CHLORINE CONTENT, 

IN WEIGHT PERCENT

Figure 15. Variation in selected compositional properties of 

phlogopite with stratigraphic position. A, Columnar section of 

the olivine cumulate of cyclic unit 2, Stillwater Complex, 

Montana, with size-graded units (dotted lines) numbered, occ, 

olivine-chromite cumulate; oc, olivine cumulate; obc, olivinebronzite cumulate; heavy lines, chromite cumulate. B, Mg/(Mg 

+ Fe2+ ) x 100 in phlogopite. C, Formula amounts of Ti in 

phlogopite. D, Formula amounts of Cr in phlogopite. f, Weight 

percent Cl and F in phlogopite.

A32 Contributions on Ore Deposits in the Early Magmatic Environment
The colorless tremolitic amphiboles formed later than 

the hornblendes and are generally associated with alteration 

that postdates final consolidation of the cumulates.

The phlogopites are compositionally homogeneous, 

varying only in the extent of Ti and Cr substitution in the 

octahedral sites. Experimental studies indicate that the Ti 

solubility in phlogopites is dependent on pressure, temperature, oxygen fugacity, and the bulk composition of the 

magma from which they crystallize (Arima and Edgar, 

1981). Experimental studies that approximate the conditions 

at which the Stillwater Complex magma would have 

crystallized have not been done, nor has the extent of Cr 

solubility in phlogopite as a function of temperature, 

pressure, and bulk composition been determined. Therefore, 

at this time, it is not possible to estimate the changes in any 

of these parameters that would be necessary to produce the 

limited variations noted in this report. It can be stated, 

however, that the compositions of phlogopites from the 

olivine cumulate of cyclic unit 2 are closer to those of 

phlogopites from high-pressure experimental runs, from 

potassium mantle-derived rocks, and from potassium-rich

rocks that crystallized at depth than to those of other rocks 

that crystallized at shallow depths (compare with Arima and 

Edgar, 1981). Nevertheless, the phlogopites certainly did not 

crystallize at high pressure and must have crystallized under 

conditions similar to those prevailing when hornblendes 

crystallized.

Finally, the Cl and F contents of both hornblende and 

phlogopite indicate that the magma from which they 

crystallized was not notably enriched in these elements. Many 

recent studies have suggested that volatiles in mafic magmas 

may play an important role in the concentration of platinumgroup elements (Bow and others, 1982; Kinloch, 1982). 

Studies of platiniferous hortonolitic dunite pipes in the 

Bushveld Complex have shown that amphiboles are enriched 

in chlorine (0.41 to 0.83 weight percent; Schiffries, 1982) 

and phlogopites are enriched in fluorine (Wagner, 1929). The 

B-chromitite of cyclic unit 2 is enriched in platinum-group 

elements (Page and others, 1976); however, there is no indication that the Cl or F contents of the magma played a 

significant role in the concentration of platinum-group 

elements in this part of the Ultramafic series.

980

900

800

700

600 L

4-:

o Amphibole > phlogopite

 Phlogopite > amphibole

e Amphibole = phlogopite}

1000 1200

AMPHIBOLE AND PHLOGOPITE, 

IN VOLUME PERCENT

B

TITANIUM CONTENT, 

IN PARTS PER MILLION

2000 4000 6000 8000 10POO

CHROMIUM CONTENT, 

IN PARTS PER MILLION

D

Figure 16. Trace-element variation in the olivine cumulates with 

stratigraphic position. A, Columnar section of the olivine 

cumulate of cyclic unit 2, Stillwater Complex, Montana, with 

size-graded units (dotted lines) numbered, occ, olivine-chromite

cumulate; oc, olivine cumulate; obc, olivine-bronzite cumulate; 

heavy lines, chromite cumulate. B, Distribution of amphibole 

and biotite. C, Ti content. D, Cr content. Dashed lines represent values going off the scale.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A33
Table 11. Estimated temperatures of brown amphibole crystallization based on experimental 

studies of the 1921 Kilauea olivine tholeiite (Helz, 1973)

[Estimated errors in temperature based on analytical methods are ± 14 °C for the A1 IV estimates and ±35 °C 

for the Ti estimates. These results are based on one standard deviation determined for Al and Ti for the standard (table 4)]

Temperatures based 

on the Al contents 

of the amphibole

Temperatures based

on the Ti contents

of the amphibole

Average composition of 32 

amphiboles associated with

Average composition of 15 

amphiboles that replace

Average composition of 5 

amphiboles from unit 9 

in which chromite is not

Range of assemblages

Range of assemblages

Range of assemblages

Range of assemblages

Range of assemblages

Range of assemblages 

in sample 787 

Crystal zoning in 

sample 747 

Crystal zoning in

Crystal zoning in 

sample 830 

938

923

930

911-911

QDR QP'R

938-941

905-916

930-941

954-976 

932-981

936-981

941-966

932

918

784

711 Q n 1 Q

Q/* o r\ 1 Q

918-930

All O Q 1 A

QC 1 07 Q

930-976 

872-974

908-999

870-971

REFERENCES CITED

Arima, M., and Edgar, A.D., 1981, Substitution mechanisms and 

solubility of titanium in phlogopites from rocks of probable 

mantle origin: Contributions to Mineralogy and Petrology, v. 

77, p. 288-295.

Bow, C., Wolfrom, D., Turner, A., Barnes, S., Evans, J., Zdepski, 

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

Reef of the Stillwater Complex, Minneapolis Adit Area: 

Stratigraphy, structure and mineralization: Economic Geology, 

v. 77, p. 1481-1492.

Cameron, M., and Papike, J.J., 1979, Amphibole crystal chemistry: 

a review: Fortschritte Mineralogie, v. 57, p. 28-67.

Foster, M.D., 1960, Interpretation of the composition of trioctahedral micas: U.S. Geological Survey Professional Paper 

354-B, p. 11-49.

Helz, R.T., 1973, Phase relations of basalts in their melting range 

at PH2o = 5 kb as a function of oxygen fugacity, Pt. 1, Mafic 

phases: Journal of Petrology, v. 14, p. 249-302.

_____ 1982, Phase relations and compositions of amphiboles 

produced in studies of melting behavior of rocks, in Veblen,

D.R., and Ribbe, P.H., eds., Amphiboles: Petrology and experimental phase relations: Mineralogical Society of America, 

Reviews in Mineralogy, v. 9B, p. 279-346.

Irvine, T.N., 1980, Magmatic infiltration metasomatism, doublediffusive fractional crystallization, and all cumulus growth in 

the Muskox intrusion and other layered intrusions, in 

Hargroves, R.B., ed., Physics of magmatic processes: 

Princeton, N. J., Princeton University Press, p. 327-383.

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

in the ultramafic zone of the Stillwater Complex, Montana: 

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

_____ 1963, Stratigraphic and lateral variation of chromite 

composition in the Stillwater Complex: Mineralogical Society 

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

_____ 1967, Ultramafic cumulates in the Stillwater, Great

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

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

1968, The chromite deposits of the Stillwater Complex,

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

(Graton-Sales Volume): New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, v. 2, p. 

1495-1510.

A34 Contributions on Ore Deposits in the Early Magmatic Environment
___ 1969, Chemical variation in coexisting chromite and 

olivine in chromitite zones of the Stillwater Complex (with 

discussion): Economic Geology, Monograph 4, p. 41-75. 

1970, The cyclic unit in layered intrusions, a comparison

of repetitive stratigraphy in the ultramafic parts of the Stillwater, 

Muskox, Great Dyke, and Bushveld complexes (with discussion), in Bushveld igneous complex and other layered intrusions: Symposium, Geological Society of South Africa, Special 

Publication 1, p. 391-424.

1971, The origin of ultramafic rocks by cumulus

processes: Fortschritte Mineralogie, v. 48, p. 128-174. 

Kinloch, E.D., 1982, Regional trends in the platinum-group

mineralogy of the Critical zone of the Bushveld Complex, South

Africa: Economic Geology, v. 77, p. 1328-1347. 

Kretz, R., 1966, Interpretation of the shape of mineral grains in

metamorphic rocks: Journal of Petrology, v. 7, p. 68-94. 

Leake, B.E., 1978, Nomenclature of amphiboles: American

Mineralogist, v. 63, p. 1023-1052. 

Levorsen, A.I., 1958, Geology of petroleum: San Francisco, W. H.

Freeman, 703 p. 

Page, N.J, 1976, Serpentinization and alteration in an olivine

cumulate from the Stillwater Complex, southwestern Montana:

Contributions to Mineralogy and Petrology, v. 54, no. 2, p.

127-137. 

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

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

no. 7, p. 1352-1363. 

Page, N.J, Shimek, R., and Huffman, C., Jr., 1972, Grain-size

variations within an olivine cumulate, Stillwater Complex,

Montana: U.S. Geological Survey Professional Paper 800-C,

p. C29-C37. 

Papike, J. J., Cameron, K L., and Baldwin, K., 1974, Amphiboles

and pyroxenes: Characterization of other than quadrilateral

components and estimates of ferric iron from microprobe data

[abs.]: Geological Society of America Abstracts with Programs,

v. 6, p. 1053-1054.

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

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

212 p.

Raedeke, L.D., and McCallum, I.S., 1982a, Modal and chemical 

variations in the ultramafic zone of the Stillwater Complex, 

in Walker, D., and McCallum, I.S., eds., Workshop on 

magmatic processes of early planetary crusts: Magma oceans 

and stratiform layered intrusions: Houston, Tex., Lunar and 

Planetary Institute, LPI Technical Report 82-01, p. 135-137.

_____ 1982b, Field guide to the Stillwater Complex, in Walker, 

D., and McCallum, I.S., eds., Workshop on magmatic 

processes of early planetary crusts: Magma oceans and 

stratiform layered intrusions: Houston, Tex., Lunar and 

Planetary Institute, LPI Technical Report 82-01 p. 169-194. 

1984, Investigations in the Stillwater Complex: Part II.

Petrology and petrogenesis of the Ultramafic series: Journal 

of Petrology, v. 25, p. 395-420.

Schiffries, C.M., 1982, The petrogenesis of a platiniferous dunite 

pipe in the Bushveld Complex: Infiltration metasomatism by 

a chloride solution: Economic Geology, v. 77, p. 1439-1453.

Wagner, P.A., 1929, The platinum deposits and mines of South 

Africa: Edinburgh, Scotland, Oliver and Boyd, 321 p.

Wones, D.R., and Gilbert, M.C., 1982, Amphiboles in the igneous 

environment, in Veblen, D.R., and Ribbe, P.H., eds,, 

Amphiboles: Petrology and experimental phase relations: 

Mineralogical Society of America, Reviews in Mineralogy, v. 

9B, p. 355-390.

Yakowitz, H., Myklebust, R.L., and Heinrich, K.F.J., 1973, 

FRAME: an on-line correction procedure for quantitative electron microprobe analyses: National Bureau of Standards 

Technical Note 796, 46 p.

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

Stratigraphy and nomenclature for the Stillwater Complex: 

Montana Bureau of Mines Special Publication 92, p. 21-32.

Composition of Primary Postcumulus Amphibole and Phlogopite, Stillwater Complex, Montana A35