Petrology of Metamorphosed 

Chromite-Bearing Ultramafic Rocks 

from the Red Lodge District, 

Montana

U.S. GEOLOGICAL SURVEY BULLETIN 1626-B

Chapter B

Petrology of Metamorphosed 

Chromite-Bearing Ultramafic Rocks 

from the Red Lodge District, 

Montana

By PATRICIA J. LOFERSKI

Petrography, mineral chemistry, and 

whole-rock geochemistry of upper 

amphibolite facies chromite-bearing 

ultramafic rocks

U.S. GEOLOGICAL SURVEY BULLETIN 1626 

CONTRIBUTIONS TO THE GEOLOGY OF MINERAL DEPOSITS
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1986

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304

Library of Congress Cataloging in Publication Data

Loferski, Patricia J.

Petrology of metamorphosed chromite-bearing ultramafic rocks 

from the Red Lodge District, Montana.

(Contributions to the geology of mineral deposits) (U.S. Geological Survey bulletin; 1626-B)

Bibliography: p. B32

Supt. of Docs, no.: I 19.3.1626-B

1. Rocks, Ultrabasic. 2. Chromite Montana Red Lodge Region. I. Title. II. Series. III. Series: U.S. Geological Survey 

bulletin; 1626-B.

QE75.B9 no. 1626-B 557.3s [553.4'643] 84-600179

[QE462.U4]
CONTENTS

Abstract Bl

Introduction Bl

Chromite production Bl

Regional setting B2

Metamorphism B3

Field occurrence and sampling B3

Methods B3

Petrography B3

Silicates B4

Oxides B9

Sulfides Bll 

Mineral chemistry Bll

Olivine Bll

Orthopyroxene B12

Clinopyroxene B12

Amphibole B12

Chlorite B12

Phlogopite B14

Serpentine B14

Chromite B14

Segregated chromite B17 

Accessory chromite B18 

Contact metamorphism B20 

Geochemistry B21 

Petrogenesis B23

Formation and post-emplacement history B29 

Economic geology B29 

Conclusions B31 

References B32

FIGURES

1. Map of the Red Lodge chromite district B2

2. Geologic map of the eastern Beartooth Mountains showing the location of the study 

area B2

3. Photomicrograph of hornblende rim around chromite in a diopside porphyroblast B5

4. Photomicrographs of serpentine textures B9

5. Photomicrograph of segregated chromite from assemblage 2 BIO

6. Photomicrograph of recrystallized chromite Bll

7. Photomicrograph of ragged accessory chromite from assemblage 5 Bll

8. Photomicrograph of chromite from assemblage 3 Bll

9. Photomicrograph of ferritchromit-l-chlorite(?) alteration of Al-rich chromite from 

assemblage 1 B12

Contents iii
10. Compositional plot of pyroxene analyses on pyroxene quadrilateral B14

11. Compositional plot of FeO versus MgO weight percent in amphiboles from ultramafic rocks and from amphibolites B14

12. Plot of serpentine compositions with respect to SiO2-MgO-FeO in weight 

percent B16

13. Compositions of segregated and accessory chromite B16

14. Compositions of segregated chromite from the various assemblages B19

15. Compositions of segregated chromite from the various deposits B22

16. Compositions of accessory chromite B23

17. Chromite compositions with respect to Cr-Al-Fe3+ B26

18. Variation diagrams of selected oxides versus SiC>2 in ultramafic rocks and 

amphibolites B27

19. Compositional plots of amphibolites B29

TABLES

1. Assemblages in the Red Lodge ultramafic rocks B4

2. Modal mineralogy of the Red Lodge ultramafic rocks B6

3. Representative microprobe analyses of olivine, orthopyroxene, and 

elinopyroxene B13

4. Representative microprobe analyses of amphibole BIS

5. Representative microprobe analyses of chlorite and phlogopite B17

6. Representative microprobe analyses of serpentine B18

7. Representative mkroprobe analyses of segregated ehromite B2Q

8. Representative microprobe analyses of accessory chromite B24

9. Ferritchromit analyses from other locations B25

10. Chemical analyses of ultramafic rocks near monzonite dike at the HighKne 

claim B25

11. Chemical and normative analyses of Red Lodge rocks, and comparative 

averages B28

12. Trace-element analyses of Red Lodge ultramafic rocks B30

tv Contents
Petrology of Metamorphosed Chromite-Bearing 

Ultramafic Rocks from the Red Lodge District, Montana

By Patricia J. Loferski

Abstract

The chromite deposits near Red Lodge, Mont., occur in 

small ultramafic lenses that are enclosed in remnants of a 

metasedimentary-metavolcanic sequence that in turn are surrounded by Archean granitic gneiss. The stratiform versus podiform origin of the deposits has been debated in the past 

because amphibolite to granulite facies metamorphism at 

about 2,800 m.y. ago has obscured most primary features.

Samples collected from various ultramafic lenses show 

that the most common rocks are (1) serpentinites and (2) those 

composed of hornblende±clinopyroxene. Less common 

assemblages are hornblende+orthopyroxene±olivine, talc + 

carbonate±phlogopite, and backwall rocks.

Chromite occurs as a ubiquitous accessory mineral and 

as local segregated lenses. Optically, much of the chromite is 

homogeneous; some chromite grains have magnetite and rare 

ferritchromit rims. Ferritchromit+chlorite(?) intergrowths occur 

in some accessory aluminum-rich chromite. Compositionally, 

some segregated chromite corresponds to stratiform-type magmatic chromite; other segregated and most accessory chromite 

show metamorphic iron enrichment and magnesium and aluminum depletion, an alteration trend most pronounced in the 

accessory chromite.

Major- and minor-element geochemistry shows continuous variation from mafic to ultramafic compositions. No evidence of metasomatism was found; thus, the various silicate 

assemblages were formed from a variety of rocks, probably 

including dunite, peridotite, feldspathic peridotite, and gabbro. 

The ultramafic bodies are probably tectonically disrupted fragments of a stratiform complex, and this hypothesis is consistent 

with the chromite compositions and textures, the Archean age 

of the rocks, and the geologic environment of the deposits, all 

of which are similar to stratiform deposits.

Most of the known chromite ore in the district has been 

mined. The economic potential of the remaining deposits is 

problematic because of (1) the low concentration (typically 

5-15 percent) of chromite in most rocks; (2) high-iron metamorphic chromite compositions; and (3) ferritchromit-chlorite(?) 

intergrowths formed during chromite alteration that may cause 

milling problems. Contents of cobalt, nickel, and the 

platinum-group elements in the ultramafic rocks are far below 

present economic limits.

If the ultramafic rocks are fragments of a disrupted stratiform complex, other fragments with local chromite concentrations may be buried at depth. However, those undiscovered 

deposits would have been subjected to the same high-grade 

metamorphism, and therefore would have the same characteristics as those exposed at the surface.

INTRODUCTION

Chromite occurs as a ubiquitous accessory mineral 

and as local concentrations in small lenses of ultramafic 

rocks that are scattered along the northern front of the 

Beartooth Mountains of Montana and Wyoming. In the Red 

Lodge district, the ultramafic rocks and the enclosed 

chromite were metamorphosed to upper amphibolite and 

locally to granulite facies about 2,800 m.y. ago (Cast and 

others, 1958; Simons and others, 1979), and to greenschist 

facies about 1,600-1,800 m.y. ago (Nunes and Tilton, 

1971; Rowan and Mueller, 1971). Whether the deposits are 

stratiform or podiform has been debated since the report' by 

James (1946) because primary features have largely been 

obscured by the high-grade metamorphism. This work was 

undertaken to characterize the effects of the metamorphism 

on the ultramafic rocks in general, and on the chromite in 

particular, to determine how the high-grade metamorphism 

has affected the economic potential of the chromite deposits, and to determine their petrogenesis.

CHROMITE PRODUCTION

Chromite was discovered in the Red Lodge district in 

1916 in the Four Chromes deposit on Silver Run Plateau 

(fig. 1). Pods and lenses of chromitite were mined from 

open pits during the early 1940's from deposits on Hellroaring and Line Creek Plateaus, and total production was about 

61,600 metric tons of chromite ore (Simons and others, 

1979). The crude ore averaged about 24-40 percent Cr2O3 ,

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana Bl
^ £' NORTH

^" GALLON STAR

{ JUG #1 & 2 S X 

I

^~X

GALLON JUG #4 ^ To Cooke City i)

3 KILOMETERS

Chromite deposits

Figure 1. Red Lodge chromite district showing deposits on 

Hellroaring, Silver Run, and Line Creek Plateaus, modified 

from Simons and others, 1979.

and Cr/Fe ratios were about 0.59-1.60 (James, 1946). No 

production was recorded from the deposits on Silver Run 

Plateau.

REGIONAL SETTING

The Beartooth Mountains form a northwest-trending 

roughly rectangular block, about 65 by 115 km, that was 

uplifted along high-angle reverse faults during the Laramide 

orogeny (fig. 2). The core of the range consists mainly of 

Archean granitic gneiss variously interpreted to have an 

igneous (James, 1946), metamorphic (Eckelmann and Poldervaart, 1957; Simons and others, 1979), or combined metasomatic, metamorphic, and igneous origin (Butler, 1969). 

Minor migmatites, pegmatites, and orbicular rocks are also 

present. The ultramafic rocks form concordant lenses in 

remnants of an interlayered metasedimentary-metavolcanic 

sequence that in turn occur as isolated roof pendants in the 

granitic gneiss (James, 1946). The interlayered sequence 

consists of interbedded quartzite, amphibolite, biotite 

schist, and banded ironstone (Eckelmann and Poldervaart, 

1957; Casella, 1964; Simons and others, 1979). The Stillwater Complex, a stratiform mafic-ultramafic igneous intruFigure 2. Geologic map of the eastern Beartooth Mountains. 

Location of the study area is outlined (A). The map is modified 

from Page, 1979.

sion, crops out on the northern front of the Beartooths approximately 40 km northwest of the present study area. 

Mafic dikes of several ages and felsic porphyry dikes and 

plugs of Tertiary age cut the older rocks throughout the area 

(Eckelmann and Poldervaart, 1957; Casella, 1969; Simons 

and others, 1979). Paleozoic and Mesozoic sedimentary 

rocks occur along most of the perimeter of the range, and 

Tertiary volcanic rocks occur on the west and southwest 

sides.

The Red Lodge ultramafic rocks are part of a generally northwest trending belt of small ultramafic lenses, 

which approximately parallels the northern edge of the 

Beartooth Mountains (Hess, 1955). The ultramafic lenses in 

the study area range in size from a few tens of meters to 

about 360 m by 45 m; the best exposures are found at edges 

of plateaus at elevations of about 3,000 m. James (1946) 

investigated the chromite deposits in the Red Lodge area, 

with emphasis on their structure and economic geology. His 

detailed geologic maps were used in the present study.

Various theories have been proposed concerning the 

origin and emplacement of the ultramafic rocks. Most workers believe that they were emplaced as solid bodies into the 

plastic country rocks during regional metamorphism 

(James, 1946; Poldervaart, 1958) and that the ultramafic 

rocks possibly were derived by the disruption of a stratiform 

ultramafic sheet (Skinner, 1969). Some of the ultramafic 

lenses appear to have been dikes, which were later pulled 

apart by flowage (James, 1946; Casella, 1969). Amphibolites associated with many of the ultramafic lenses are 

generally considered to have been derived from tholeiitic 

basalt (Armbrustmacher and Simons, 1977; Simons and 

others, 1979).

62 Contributions to the Geology of Mineral Deposits
METAMORPHISM

Peak metamorphic conditions in the eastern 

Beartooths were estimated by previous workers to be from 

upper amphibolite to granulite facies. Eckelmann and Poldervaart (1957) estimated a peak metamorphic temperature of 

500°-600°C on the basis of the mineral assemblages in amphibolites, ironstones, biotite schists, and ultramafic rocks. 

Casella (1969) estimated metamorphic conditions at the upper amphibolite facies with temperatures greater than 550°C 

and low pressure. Skinner (1969), on the basis of the presence of orthopyroxene in the rocks, estimated upper amphibolite to granulite facies metamorphism of ultramafic 

rocks in the nearby Highline Trail Lakes area. However, 

Evans (1977) showed that orthopyroxene may be present in 

the upper amphibolite facies.

Calculations based on the the olivine-spinel geothermometer (Irvine, 1965; Jackson, 1969; Evans and Frost, 

1975) yield temperatures of about 600°-650°C for two samples of the Red Lodge ultramafic rocks (Loferski and Lipin, 

1983). Those temperatures correspond to those of the upper 

amphibolite facies.

FIELD OCCURRENCE AND SAMPLING

Samples were collected from open pits and outcrops 

at the Pick, Shovel, North Star, Drill, and Gallon Jugs 1,2, 

and 4 claims on Hellroaring Plateau; the Highline claim on 

Line Creek Plateau; and the Four Chromes deposit on Silver 

Run Plateau (see fig. 1). Both accessory chromite and 

chromite ore from mine dumps were sampled. Samples collected by H. L. James in 1942 from some of the deposits 

listed above and from the Edsel and Little Nell deposits on 

Silver Run Plateau were also analyzed. The amphibolites 

near the contacts with the ultramafic lenses were sampled at 

the Drill, Highline, and Gallon Jug 4 claims. A 9-m-wide 

monzonite porphyry dike intrudes the ultramafic lens at the 

Highline open pit, and samples were taken at various distances from the dike to check for contact metamorphic 

effects.

Within individual lenses, the rocks are highly sheared 

and .disrupted, and faulting is common. Felsic veins and 

dikes occur throughout the area, especially at the Drill 

claim. What appeared to be igneous layering was observed 

in only one area of the North Star open pit, where chromite 

bands are interlayered with serpentinite. The thickness of 

the layers ranges from several centimeters to about 1 m. The 

layers are parallel to the boundaries of the ultramafic body 

and to the foliation in the country rock. Other layering seen 

in the outcrops and in thin sections is metamorphic in origin.

Contacts with the country rocks are exposed in the 

three largest open pits: Drill, North Star, and Highline. A 

sheared zone, ranging from a meter to a few meters in 

width, was observed at the contact.

Felsic veins and dikes in the ultramafic bodies are 

common, especially at the Drill claim. Metasomatic effects, 

typically showing the formation of hornblende along the 

contacts, can be seen both on the scale of an outcrop and on 

the scale of a thin section.

METHODS

Fifty-seven thin sections, including 5 amphibolites, 

and 12 polished sections were studied. Standard optical 

techniques were used in both reflected and transmitted light. 

Point counts were performed on most of the thin sections 

and polished sections, with at least 1,000 points for each 

sample. In other samples, modal estimates and determinative charts of Terry and Chillingar (1955) were used.

Electron microprobe analyses were done on individual mineral grains, and the method of Bence and Albee 

(1968) was used to reduce data. For all mineral species 

except amphibole and chromite, Fe is reported as total FeO. 

Fe3+ , as well as minimum and maximum values of Fe2O3 , 

was estimated for amphiboles using the method of Papike 

and others (1974). Calculation of Fe3+ in spinels was based 

on ideal spinel stoichiometry.

Selected samples were analyzed by means of X-ray 

diffraction analyses to identify serpentine polymorphs and 

to check for brucite. Whole-rock major- and selected 

trace-element chemical analyses were obtained for many of 

the samples.

PETROGRAPHY

Six different silicate assemblages were recognized in 

the ultramafic rocks, and each individual ultramafic lens is 

dominated by one particular assemblage (table 1). The 

amount of chromite varies between samples within assemblages 1,3,5, and 6, whereas most samples of assemblages 

2 and 4 are chromitites.

Some general characteristics of the various assemblages 

are as follows. Assemblages 1 through 3 preserve the relict 

high-grade silicates amphibole, pyroxene, and olivine all 

of which have undergone various degrees of retrograde alteration. In contrast, the silicates in assemblages 4 and 5 are 

mainly the retrograde minerals serpentine, talc, chlorite, 

phlogopite, and carbonate minerals. By far the most common type of retrograde alteration is serpentinization, and the 

most common assemblage is 5, the serpentinites. Assemblage 6 consists of blackwall rocks, formed by reaction 

between the ultramafic rocks and the country rocks.

Two of the assemblages, 1 and 3, show characteristic 

banded textures in thin sections. The assemblage 1 rocks 

have alternating nearly monomineralic layers, which have a 

maximum thickness of several grains, of hornblende, orthopyroxene, and olivine. Similar types of monomineralic

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B3
Table 1. Assemblages in the Red Lodge ultramafic rocks

[Minerals listed in decreasing order of abundance; characteristic minerals capitalized; retrograde minerals in parentheses. Abbreviations used are hbd, 

hornblende; opx, orthopyroxene; serp, serpentine; chr, chromite; tc, talc; ol, olivine; am, amphibole; chl, chlorite; phi, phlogopite; carb, carbonate minerals; 

cpx, clinopyroxene; pc, plagioclase; ep, epidote, bi, biotite]

Assemblage 

number Assemblage mineralogy

Number of 

samples

Probable 

protolith

1.

2.

3.

HBD + OPX (±serp) + chr (+tc) ±OL

chr + AM (+serp ± tc ± chl ± phi ± carb) ± cpx ±pc±ep

AM + CPX (±serp) + chr (±tc±chl)

7

14

Harzburgite.

Feldspar- or 

clinopyroxenerich chromitite.

Feldspathic 

pyroxenite, or 

gabbro.

4. chr (+serp + TC ±phl) ±hbd (+CARB)

5. (SERP) + chr±ol (±chl±tc±carb±phl)±am±opx

6. Blackwall: am+bi ±pc, and am+chl ±pc

5

24

5

Chromitite.

Dunite.

layering have been attributed by other workers to metamorphic rather than igneous processes (James, 1946; Skinner, 

1969). In the Red Lodge rocks, the layering is obscured by 

shearing and retrograde alteration. Banding is also seen in 

the assemblage 3 rocks, in which layers of clinopyroxene 

alternate with layers of hornblende±chromite. The layers 

are as much as several grains thick, and are metamorphic in 

origin.

Modal analyses of all the rocks are shown in table 2. 

The general characteristics of each mineral species are as 

follows.

Silicates

The amphibole, typically hornblende that is pale 

green to colorless in thin section, is the most widespread of 

the three relict high-grade silicates, and is present in samples 

from all the ultramafic lenses in amounts that range from 

2 percent to about 90 percent of individual samples. Most 

commonly, it occurs as equant grains with 120° triple-grain 

junctions, and as short prisms, some in subparallel orientation. Coarse twins are present in a few grains. Less commonly, the amphibole occurs as porphyroblasts as long as 

2 mm, and, in two assemblage 2 chromitites, it forms 

ragged inclusion-filled laths with sutured grain boundaries.

Zoned crystals, consisting of hornblende cores and 

aluminum-poor tremolite or actinolite rims, occur in three 

assemblage 2 samples from the Little Nell and Edsel claims. 

In thin sections, the zoning is seen as a pronounced difference 

and sharp break in the birefringence between the cores and

rims of grains, and the zonation follows the shape of the 

crystals.

Orthopyroxene was found in nine of the samples. It 

was found primarily in assemblage 1, in amounts as great as 

38 percent, and in trace amounts in two serpentinites. It is 

present in samples from the Gallon Jug 1, Gallon Jug 4, and 

Highline claims. It most commonly forms sheared porphyroblasts as long as 3 mm; alteration to fine-grained talc is 

extensive along rims and cleavages, and patches of green 

hornblende are present in many grains.

Olivine, because it has been altered to serpentine, is 

not very abundant in the Red Lodge rocks. It was identified 

in six samples from assemblage 1 and in six from assemblage 5; the samples came from the North Star, Gallon 

Jug 1, Gallon Jug 4, Highline, and Four Chromes claims. 

Olivine was found in amounts that range from less than 

1 percent to 17 percent, but it composes less than 5 percent 

of most samples. In all samples, the rims and cracks are 

serpentinized to various degrees, and magnetite veins occur 

in the serpentinized areas. Absence of undulatory extinction 

in the olivine grains indicates that they are not strained. A 

few olivine grains from assemblage 1 contain veins of dusty 

magnetite, which probably formed during a serpentinizatibn 

event that preceded the upper amphibolite facies metamorphism. The presence of these veins in olivine indicates that 

the olivine is metamorphic and crystallized from pre-existing 

serpentine. The serpentine presently in the samples is therefore second-generation serpentine.

Clinopyroxene is present is 11 samples in amounts 

that range from less than 1 percent to about 70 percent. It is 

a characteristic mineral in assemblage 3, and it is a rare

B4 Contributions to the Geology of Mineral Deposits
accessory mineral in assemblages 1 and 2. It was found in 

samples from the Drill, Gallon Jug 2, Shovel, and Highline 

claims. It is pale green, and forms anhedral grains, commonly with 120° triple grain junctions, and ragged 

inclusion-filled porphyroblasts that show wavy extinction. 

The grain size is generally about 0.5 mm, and the length of 

porphyroblasts ranges from 4 or 5 mm to 1 cm. The clinopyroxene contains patches and rims of hornblende, and alteration to talc and brown cloudy material along rims and, 

cleavages is common, and in some grains complete.

In assemblage 3, with very few exceptions, the chromite is not in direct contact with the clinopyroxene. In thin 

sections, the layering in the rocks is very conspicuous and 

striking: the chromite is confined to the hornblende layers, 

and the clinopyroxene layers are free of chromite. In addition, small chromite inclusions in clinopyroxene porphyroblasts are rimmed by hornblende (fig. 3). These textures 

suggest a reaction relationship such as:

Cr spinel+diopside+(Na, Al bearing; fluid ^amphibole,

which was proposed by Best (1974) and Frances (1976) to 

account for the presence of amphiboles in ultramafic 

xenoliths.

Of the retrograde minerals, serpentine is by far the 

most common. It was found in nearly every sample in 

amounts that range from trace to about 90 percent. The 

serpentine is variously colorless, light green, brown, or 

reddish brown in thin section. Some serpentinite samples 

contain rare islands of relict high-grade silicates, including 

amphibole, pyroxene, and olivine.

Serpentine minerals are useful petrogenetic indicators 

because they preserve a variety of textures that are characteristic of certain types of alteration. Identification of serpentine minerals based on their textures is as follows. 

Mesh-textured a-serpentine (length fast) is generally composed of lizardite and is a retrograde texture (Cressy, 1979; 

Wicks and Plant, 1979), whereas hourglass-textured serpentine, although also composed of lizardite, is mainly a prograde texture (Wicks and Plant, 1979). Nonpseudomorphic 

 y-serpentine (length slow) is most commonly antigorite 

(Wicks and Plant, 1979), but may also be either lizardite or 

chrysotile. In most places, nonpseudomorphic serpentine 

may form either by shearing during retrograde metamorphism or by recrystallization of mesh-textured serpentine 

during prograde metamorphism (Wicks and Whittaker, 

1977).

Serpentine textures in the Red Lodge rocks vary 

within and between samples. Of the pseudomorphic textures, mesh textures formed at the expense of olivine are the 

most common (fig. 4A), and of 11 samples, the fact that 9 

contain a- and 2 contain ^-serpentine indicates that most are 

composed of lizardite. The mesh rims are variously defined 

by (1) serpentine "veins" with or without central parting; 

(2) magnetite veins; (3) carbonate veins; and (4) brown to 

reddish-brown rims, in which the color is probably caused

Figure 3. Photomicrograph taken in transmitted light of hornblende rim (medium gray), enclosing chromite (black) in a 

diopside porphyroblast (white) from assemblage 3.

by inclusions of fine-grained iron oxides. In some samples, 

the mesh rims are visible only in plane polarized light; under 

crossed nicols the serpentine has a nonpseudomorphic texture which indicates recrystallization. Hourglass textures of 

a-serpentine occur in three of the samples from Gallon Jug 1 

and from the Highline deposits, and, as discussed above, 

these are the result of a prograde event. Other serpentine 

pseudomorphic textures include rare bastites formed from 

pyroxene and amphibole, in which relict cleavages are preserved in some places. Bastite-textured serpentine in thin 

section is colorless and free of dusty magnetite inclusions.

Isotropic serpentine was found in a few samples associated with serpentinized pyroxene, and as mesh cores in 

one sample from the North Star deposit.

Nonpseudomorphic textures range from interlocking 

to interpenetrating (as defined by Wicks and Whittaker, 

1977), usually -y-serpentine. Such textures occur as patches 

in samples that also contain pseudomorphic serpentine, and 

as the only serpentine texture in some samples. In a few 

places, an original mesh texture is obliterated by later 

growth of nonpseudomorphic serpentine. This transition 

from pseudomorphic to nonpseudomorphic textures is exemplified in a sample from North Star in which radiating 

bundles of antigorite (determined on the basis of the texture 

and chemistry) have grown from the cores of brown, nearly 

isotropic mesh serpentine (fig. 4B). Some areas of that thin 

section are composed only of the mesh serpentine, other 

areas only of the interpenetrating (antigorite) fibers plus 

talc, and, in some areas, the two intergrade.

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B5
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B6 Contributions to the Geology of Mineral Deposits
Gallon Jug 

4

Chromite-Bearing Ultramafic Rocks,Red LodgeDistrict, Montana B7

HP-20B -

op -) \

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HP 23 -

HP 1A

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UP OA

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B8 Contributions to the Geology of Mineral Deposits
Selected samples were analyzed by means of X-ray 

diffraction analysis, and methods of Whittaker and Zussman 

(1956) and Aumento (1970) were used for interpretation. 

One sample composed predominantly of mesh-textured 

a-serpentine is mostly lizardite and minor antigorite. Another sample that has a-serpentine mesh and hourglass textures and subordinate nonpseudomorphic ^-serpentine is 

composed of lizardite and antigorite in roughly equal 

amounts, and two samples of nonpseudomorphic 

 y-serpentine are composed of antigorite and subordinate 

lizardite. Chrysotile was not identified on the X-ray diffraction traces, but it is possible that chrysotile composes the 

veinlets in the samples in amounts below the detection limits 

of X-ray diffraction analysis.

Therefore, the textures and mineralogy indicate that 

most of the serpentinites are retrograde in origin, and were 

formed from dunite (now mesh-textured serpentine) and 

peridotite (now mesh-textured serpentine and bastite). 

Later, recrystallization produced the hourglass and nonpseudomorphic textures in some serpentinites.

Brucite was not identified in the serpentinites, which 

may indicate (1) an initially low MgO/SiO2 ratio; (2) introduction of SiO2 ; (3) removal of MgO; or (4) high CO2 

contents, which would react with brucite to form magnesite.

Other retrograde minerals in the Red Lodge rocks 

include talc, chlorite, phlogopite, and carbonate minerals; 

generally they occur in only minor amounts. The talc commonly forms fine-grained patches. The phlogopite forms 

laths, and the cleavage planes typically contain magnetite 

with exsolved hematite. In places, chlorite is associated 

with ferritchromit, and it also occurs as interstitial laths in 

some segregated chromites. The carbonate minerals occur in 

irregular patches, and as mesh rims in one serpentinite.

Epidote was found in two samples. In one assemblage 6 sample, from Highline, epidote replaces dark-green 

hornblende. The other sample, from assemblage 2 at Gallon 

Jug 2, contains rare epidote grains. Partially to completely 

sericitized plagioclase also is present in those samples.

In summary, the silicate minerals record the following 

petrogenetic sequence after formation of the original igneous parent rocks: (1) serpentinization, (2) metamorphism 

to at least upper amphibolite facies conditions, 

(3) retrograde alteration, and (4) slight prograde recry stallization of some serpentine.

A question arises about whether the six assemblages 

were formed at a variety of metamorphic grades. Although 

local differences in the peak metamorphic conditions may 

have existed throughout the area, neither large differences 

between the lenses nor progressive differences across the 

district were detected in the ultramafic rocks. The wide 

geographic distribution of orthopyroxene and the extensive 

occurrence of hornblende indicate conditions of the upper 

amphibolite facies throughout the area. At lower grades, the 

amphibole in metamorphosed ultramafic rocks is tremolite 

(Evans, 1977). Therefore, peak metamorphic grade was

Figure 4. Photomicrographs taken in transmitted light of serpentine textures. A, Pseudomorphic serpentine mesh texture 

formed at the expense of olivine from assemblage 5. 6, Antigorite blades (light gray) radiating from a serpentine mesh core 

(medium gray) with some magnetite rims (black).

apparently fairly homogeneous in the ultramafic rocks 

throughout the area.

Oxides

Magnetite is a secondary mineral in the Red Lodge 

rocks and was formed during both of the serpentinization

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B9
events. It has five modes of occurrence: (1) It is most abundant as veins, dusty stringers, irregular clumps, and angular 

grains in serpentinized samples; (2) thick veins at hornblende rims and cleavages in a few grains in assemblage 1; 

(3) thin ragged rims on some chromite, and veins in 

chromite; (4) along phlogopite cleavage planes; and (5) anhedral grains in some samples of assemblage 1, and, in a 

few places, concentrated at triple-grain junctions of silicates. All these occurrences indicate that the magnetite is a 

late-stage mineral. It commonly contains exsolved needles 

of hematite, which indicates late-stage oxidizing conditions.

Chromite is present in amounts that range from less 

than 1 percent to 83 percent. In the following discussion, 

chromite that composes 25 percent or less of a sample is 

considered accessory and that which composes more than 

25 percent is considered segregated. This terminology is 

modified from Thayer (1970).

Twenty-seven samples contain segregated chromite. 

It was found in all the assemblages except number 3, in 

which the maximum amount of chromite is 23 percent.

Segregated chromite is rarely euhedral, commonly 

subhedral to anhedral, and many grains have ragged boundaries. Most samples contain grains of highly cracked 

chromite, and many contain granulated zones and grains 

that have pull-apart fractures filled with serpentine or magnetite. The grains are typically 0.1-0.5 mm in diameter, and 

rarely as much as 1 mm. They are optically homogeneous 

and many appear to be unaltered, although the chemical 

compositions of some are clearly metamorphic as described 

below. A few grains have magnetite veins and rims, and 

some contain minute silicate inclusions. Relict igneous textures include chromite chains and occluded silicates.

The textures vary between segregated chromite samples. Many have the texture that James (1946) described as 

"closely spaced but not tightly packed grains" (fig. 5). Some 

samples, however, have areas as much as 1 cm across that 

consist of welded clumps of 100 percent chromite, a texture 

that required overgrowth for its formation.

Recrystallization of chromite is present locally; within 

the same ultramafic lens at North Star, one sample shows 

recrystallization along shears (fig. 6), whereas others show 

only angular splintering of chromite grains along shears, 

and still others show very little deformation.

Like the segregated chromite, the accessory chromite 

grains are generally 0.1-0.5 mm in longest dimension, and 

rarely as much as 2 mm. The grains range in shape from 

euhedral to anhedral, and some occur as small welded 

clumps, a texture most commonly seen in the serpentinites. 

Serpentinized samples from assemblages 1 and 5 contain 

some grains that are anhedral, embayed by magnetite rims 

and veins, and have ragged grain boundaries (fig. 7). In 

contrast, chromite in samples showing little or no serpentinization, such as those in assemblage 3, lack magnetite 

rims and veins, although some chromite has ragged grain

boundaries and is flecked with tiny silicate inclusions 

(fig. 8).

Most assemblage 1 samples contain two Cr-spinels: 

Al rich and Fe rich. The green metamorphic Al-rich 

chromite contains minute exsolved blebs of Fe-rich(?) 

chromite, which in turn contain lamellae of Al-rich(?) 

chromite. This exsolution feature as well as others observed 

in the Red Lodge chromites confirm the existence of an 

extensive solvus in the spinel compositional prism between 

Al- and Fe-rich chromite (Loferski, 1980; Loferski and 

Lipin, 1983). Exsoh^d chromites, of various textures, were 

found in all assemblages except the five blackwall samples 

of assemblage 6.

Most accessory chromite grains from Red Lodge are 

optically homogeneous and do not show the extensive zoning found in metamorphosed chromite from other locations. 

Magnetite veins and rims of various thicknesses are seen in 

some grains, particularly those in serpentinized areas, and 

thin oxidized zones are found along rims and cracks in some 

chromites.

Ferritchromit-chlorite intergrowths occur as alteration 

products of accessory chromite from other locations (see for 

example, Pinsent and Hirst, 1977; Lipin, 1980). Such intergrowths are found in a few of the Red Lodge chromites in 

assemblage 5 serpentinites, and in assemblage 1 Al-rich 

chromite where it is in contact with serpentinized zones. 

Such grains are altered to very fine grained crystallographiFigure 5. Photomicrograph taken in reflected light of segregated chromite (white) from assemblage 2, showing subhedral 

to anhedral shapes. The interstitial silicates are hornblende, 

minor chlorite, and talc.

B10 Contributions to the Geology of Mineral Deposits
Figure 6. Photomicrograph taken in reflected light showing 

recrystallized chromite from the North Star open pit. The silicate matrix is serpentine.

cally controlled intergrowths of the ferritchromit+ 

chlorite(?) (fig. 9). In their study, Pinsent and Hirst (1977) 

also found preferential alteration of Al-rich relative to 

Fe-rich chromite The association in the Red Lodge rocks of 

the ferritchromit+chlorite(?) alteration with serpentinized 

zones indicates that it is a late-stage alteration, formed during the last serpentinization, and is not a product of the highgrade metamorphism.

Sulfides

Sulfide minerals, mostly pyrite, chalcopyrite, and 

pentlandite, are present in trace amounts in most samples. 

They occur as minute inclusions in chromite, and as small 

masses with magnetite veins and rims in some serpentinites.

MINERAL CHEMISTRY 

Olivine

Table 3 shows microprobe analyses of olivine in two 

assemblage 5 samples from the North Star and Four

Chromes claims, in which olivine constitutes about 

15 percent of each sample. Individual grains are homogeneous, and there is little variation between grains. The compositions are Fo92 in the sample from North Star and Fo90 

in that from the Four Chromes deposit. The sample from

Four Chromes contains 0.24 percent MnO and 0.14 percent 

NiO. The sample from North Star contains 0.35 percent 

NiO.

Figure 7. Photomicrograph taken in reflected light showing 

ragged accessory chromite from assemblage 5. The white is 

magnetite.

Figure 8. Photomicrograph taken in reflected light showing 

embayed chromite from assemblage 3 that contains tiny silicate inclusions, but no magnetite rims or veins.

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B11
Figure 9. Photomicrograph taken in reflected light of 

ferritchromit+chlorite(?) alteration along the grain boundaries 

of Al-rich chromite from assemblage 1. C indicates chromite; 

F, ferritchromit+chlorite; M, magnetite; and S, serpentine.

Orthopyroxene

Microprobe analyses of orthopyroxene from two of 

the samples from the Highline deposit are given in table 3 

and are plotted in the pyroxene quadrilateral (fig. 10). The 

orthopyroxenes have compositions of En86 and En91 . Calcium contents are low (a maximum of 0.19 percent CaO), 

and aluminum contents are moderate (a maximum of 2.21 

percent A12O3 ). The low CaO contents indicate a metamorphic origin (Springer, 1974). Cr2O3 is less than 0.07 percent, and 0.10 percent NiO was found in one sample.

Clinopyroxene

Clinopyroxene was found at the Drill, Gallon Jug 2, 

Shovel, and Highline deposits. Microprobe analyses of 

three samples, one of which contains both orthopyroxene 

and Clinopyroxene, are shown in table 3, and compositions 

of four samples are plotted in the pyroxene quadrilateral 

shown in figure 10.

Compositionally, three of the clinopyroxenes are 

augite, and one from the Highline deposit plots in the field 

of endiopside. Aluminum contents are less than 2 percent 

A12O3 , except for the sample of endiopside from the Highline deposit which contains about 6.5 percent A12O3 . The 

amount of Cr2O3 is as high as 0.28 percent; NiO is less than

0.2 percent. Titanium was not detected in the augites; the 

endiopside contains 0.10 percent TiO2 .

Amphibole

Magnesio-hornblende is the most common type of 

amphibole in the Red Lodge rocks according to Leake's 

(1978) amphibole classification scheme. Other types include tremolitic-, actinolitic-, edenitic-, ferro-edenitic-, and 

ferroan-pargasitic hornblende. Tremolite occurs in the ultramafic body at the Shovel claim. Microprobe analyses of 

amphiboles are presented in table 4. The method of Papike 

and others (1974) was used in calculating minimum and 

maximum values of Fe2O3 , and cation proportions are given 

in terms of minimum Fe3+ .

All Si ratios in the amphiboles vary widely, both between samples and within some samples. The Edsel and 

Little Nell lenses contain amphiboles that are zoned from 

hornblende cores to tremolite or actinolite rims. The cores 

are slightly enriched in the minor elements Cr, Fe, Na, and 

K, relative to the rims.

The A12O3 in the hornblende ranges from 4.23 percent 

to 11.49 percent, and no correlation was found with the 

whole rock A12O3 content. Most hornblende contains about 

12-13 percent CaO, up to 1.88 percent Na2O, and 1.01 

percent K2O. Cr2O3 is commonly less than 0.5 percent, but 

as much as nearly 3 percent occurs in amphiboles that are 

near chromite grains. A maximum of 0.25 percent NiO and 

1.47 percent TiO2 were detected in the hornblende. Mg/ 

(Mg+Fe2+ ) ratios of the hornblendes are typically greater 

than 0.85, but some are as low as 0.61.

Hornblende from the amphibolites at Highline and 

Gallon Jug 4 (table 4) have higher total FeO (fig. 11), and 

distinctly higher titanium contents than hornblende from 

associated ultramafic rocks. The two types of hornblende 

show no difference in chromium or nickel contents.

Chlorite

Representative microprobe analyses of chlorites from 

three samples are shown in table 5. Most of the chlorites 

analyzed are clinochlore and penninite, according to Hey's 

(1954) classification, and one sample contained Fe-rich 

pyncochlorite. Ratios of Al/Si are variable in chlorites from 

samples with accessory chromite (see analyses 

LC-9(1) and LC-9(2) from table 5); whereas Al/Si is relatively constant in chlorites from samples with segregated 

chromite. The chlorites contain from 0.10 to 0.84 percent 

NiO.

Cr2O3 content is relatively low, from 0.58 to 1.10 

percent in chlorite from samples containing accessory 

chromite, even where it is associated with ferritchromit

B12 Contributions to the Geology of Mineral Deposits
Table 3. Representative microprobe analyses of olivine, orthopyroxene, and clinopyroxene 

[n.d., not determined]

Olivine

HJ 2a-42

5

LJ n 11

North Star 

5

Orthopyroxene

LC 11

Highline 

5

LC-6 

Highline 

1

Clinopyroxene

LC 1

Highline 

Amphibolite

LC 6

Highline 

1

LJ n *i

Drill 

3

Weight Percent

SiO, -

TiO2 - 

Al,0, -- -- -

Cr,03 - 

FeO

MnO -

MgO - 

NiO -

PaO

Na^O

Total

40.90

.00

.00

.00

9.57

.24

48.96

.14

n.d.

n.d.

99.81

40.45 

.00 

.00 

.00 

8.26 

n.d. 

51.44 

.35 

n.d. 

n.d.

100.50

55.42 

.04 

2.21 

.07 

10.21 

.29 

32.37 

n.d. 

.19 

.00

100.80

57.81 

.00 

.71 

.03 

6.50 

n.d. 

35.70 

.10 

.04 

.00

100.89

Number of ions on the basis of 4 oxygens for olivine and 6

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

Ai (]\f\ ......... .

Al tV1\

Ti -_ -__._ .

Cr 

Fe -.

Mg ----- 

Mn 

Ni .

Pa

Na

Total

i nrn

.000

.000

(w\

.000

.196

1 7Q1

.004

nOi

n.d.

n.d.

2.997

0.982 

.000 

.000 

.000 

.000 

.168 

1.861 

n.d. 

.007 

n.d. 

n.d.

3.018

1.932 

.068 

.023 

n.d. 

.002 

.297 

1.682 

n.d. 

n.d. 

.007 

.000

4.011

1.974 

.026 

.003 

n.d. 

.001 

.185 

1.818 

n.d. 

.002 

.002 

.000

4.011

51.73 

.00 

1.22 

.01 

9.72 

n.d. 

13.00 

.07 

22.80 

.38

98.93

oxygens for pyroxene

1.965 

.035 

.020 

n.d. 

.000 

.309 

.736 

n.d. 

.002 

.928 

.029

4.024

52.76 

.10 

6.56 

.19 

4.53 

n.d. 

21.00 

.09 

12.71 

.36

98.30

1.901 

.099 

.181 

n.d. 

.006 

.138 

1.128 

n.d. 

.003 

.491 

.026

3.973

52.91 

.00 

1.21 

.28 

6.24 

n.d. 

14.26 

.13 

24.27 

.38

99.68

1.969 

.031 

.022 

n.d 

.008 

.194 

.791 

n.d. 

.004 

.968 

.028

4.015

Ratio

Mg

Mg + Fe2+ 0.90 0.92 0.89 0.91 0.70 0.89 0.80

grains. In segregated chromite however, the interstitial chlorite contains as much as 3.43 percent Cr2O3 . Chromium 

values are variable within single grains, and show no regular 

pattern relative to the distance of the chlorite from a neighboring chromite grain.

The names kammererite and kotschubeite have been 

used to describe Cr-rich chlorite where Cr3+ is in octahedral 

and tetrahedral coordination respectively. Recent crystal 

structure refinements of Cr-chlorites, however, indicate that

Cr3+ is not found in tetrahedral coordination, and is in fact 

in the interlayer octahedral sites of the chlorite structure 

(Phillips and others, 1980). Those authors suggested that the 

names kammererite and kotschubeite be dropped and that 

Cr-rich chlorite should be named by adding the prefix 

"chromian" to the chlorite species. The Cr-rich chlorites of 

this study, therefore, are chromian-clinochlore, chromianpenninite, and chromian-pycnochlorite, according to the 

classification of Hey (1954).

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B13
Ca Mg Si 2 O6

EXPLANATION

+ Assemblage 1

A Assemblage 3 

 Assemblage 5 

O Amphibolite

HP-7 Fe Si,OB

Mg2 Si 206

LC-5

Fe,Si,O.

Figure 10. Compositional plot of pyroxene analyses on pyroxene quadrilateral.

Phlogopite

Microprobe analyses of phlogopite from two samples 

are shown in table 5. One of the samples, LC-9, contains 

accessory chromite, the other contains segregated chromite. 

The phlogopite in the sample containing segregated 

chromite contains 1.20 percent Cr2O3 , whereas that in the 

sample containing accessory chromite contains 0.49 percent 

Cr2O3 .

20

16

£12

(D

LU

O 8

O)

AT

a n

ni 

^A

A O

EXPLANATION

+ Assemblage 1 

 a Assemblage 2, Core-rim

A Assemblage 3 

~ T Assemblage 6

O Amphibolite

4 8 12 16 20 

FeO TOTAL WEIGHT PERCENT

Figure 11. Compositional plot of weight percent FeO versus 

weight percent MgO in amphiboles from ultramafic rocks and 

from amphibolites. Tie lines connect cores and rims of amphibole grains.

Serpentine

Serpentine minerals occur in all the ultramafic lenses, 

as alteration products of olivine, pyroxene, and amphibole 

minerals. Chemically, antigorite can be distinguished from 

lizardite and chrysotile by higher SiO2 , lower H2O, and 

lower octahedral occupancies (Dungan, 1979). Microprobe 

analyses of serpentine minerals of various textures and signs 

of elongation (a versus y) are given in table 6. Compositions are plotted with respect to SiO2-MgO-FeO (fig. 12), 

and compositional fields of antigorite versus lizardite or 

chrysotile are shown (from Wicks, 1979).

The samples of ^-serpentine plot in or on the boundary of the antigorite field. Three analyses of mesh-textured 

a-serpentine plot in the lizardite-chrysotile field, and two 

plot in the antigorite field. Pairs of analyses are given for 

three samples that contain areas of mesh-textured 

a-serpentine and areas of nonpseudomorphic y-serpentine. 

In all three samples, the nonpseudomorphic 'y-serpentine is 

higher in SiO2 and plots within or on the boundary of the 

antigorite field, whereas the mesh-textured a-serpentine is 

lower in SiO2 , and plots in or close to the lizardite-chrysotile 

field.

Therefore, on the basis of their chemistry and textures, the dominant serpentine minerals are lizardite and 

antigorite, and relative proportions between samples are 

variable.

Chromite

Five samples of exsolved chromite are included in the 

following discussion of the compositions of the single-phase 

chromites. Those five samples contain minute intergrowths 

of host and guest phases, and the microprobe analyses represent bulk compositions for the grains. Detailed descripB14 Contributions to the Geology of Mineral Deposits
Table 4. Representative microprobe analyses of amphibole

HP 2

Drill

3

HP-20B 

Gallon Jug 4 

Amphibolite

HP 27

Shovel 

3

LC-8 

Highline 

6

HJ-4-42 

Core 

Edsel 

2

HJ-4-42 

Rim 

Edsel 

2

Weight percent

SiCh -- .

Tin.

Al.Oi 

Cr2O3 . pen _______

MgO - . 

NiO -- .

PaH

NO n

K PI

Total

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

A] (TV) ____ -

r\l \L V }

Cr(IV) -

Al CVT^

Ti 

pr _____ . ...

Fe3+ - .

Fe2+ - .

Me ... IVlg

Nj ____

Pa

Na (M4)

Mo I A "I

K - _______

Total

45.48

.20

9.74

1.63

11.96

13 23

.25

12 37

1.17

.97

97.00

6.663

1.337

nnn

.407

.022

.220

.152

1.314

2 913

.030

1.942

.000

332

1 01

15.513

43.32 

1.47 

11.49 

.05 

18.91 

8.50 

.11 

11.35 

1.42 

.97

97.59

Number of ions on

6.552 

1.448 

.000 

.604 

.167 

.006 

.000 

2.392 

1.927 

.014 

1.840 

.050 

.366 

.187

15.553

54.94 

.00 

3.35 

.26 

4.32 

21.41 

.14 

12.91 

.51 

.20

98.04

the basis of 23 oxygens for

7.577 

.423 

.000 

.133 

.000 

.031 

.086 

.413 

4.413 

.016 

1.908 

.000 

.136 

.035

15.171

46.65 

.49 

10.36 

.08 

13.39 

14.21 

.07 

11.70 

1.13 

.61

98.69

minimum Fe3+

6.706 

1.294 

.000 

.466 

.053 

.009 

.285 

3.051 

.008 

1.325 

1.802 

.000 

.315 

.112

15.426

53.54 

.07 

4.23 

1.13 

2.42 

21.89 

.11 

13.09 

.89 

.33

97.70

7.385 

.615 

.000 

.100 

.007 

.156 

.048 

.231 

4.510 

.012 

1.935 

.000 

.238 

.058

15.295

57.42 

.01 

.45 

.95 

2.19 

22.89 

.13 

13.73 

.04 

.07

97.88

7.879 

.073 

.048 

.000 

.001 

.055 

.000 

.251 

4.682 

.014 

2.018 

.000 

.010 

.012

15.043

Ratio

Mg

Mg + Fe2+ 0.69 0.45 0.91 0.70 0.95 0.95

Calculated weight percent

Fe 2O3 

Minimum 

Maximum

1.38

3.23

Edenitic 

hornblende

0.00

3.42

Ferroedenitic 

hornblende

0.83 

3.97

Tremolite

2.64 

9.10

Magnesiohornblende

0.46 

2.69

Tremolitic 

hornblende

0.00

.74

Tremolite

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B15
42 44

FeO

Figure 12. Plot of serpentine compositions with respect to 

SiO2 -MgO-FeO in weight percent. Compositional fields for 

antigorite, chrysotile, and lizardite are from Wicks (1979).

tions of those and other exsolved chromites from Red Lodge 

are given elsewhere (Loferski, 1980; Loferski and Lipin, 

1983).

Microprobe analyses of segregated (table 7) and accessory chromite (table 8) indicate that the Cr2O3 content 

ranges from about 14 percent to nearly 55 percent. The 

minor elements titanium and nickel are low in most of the 

chromite. Amounts of TiO2 range from less than 0.1 percent 

up to 0.57 percent; most samples contain about 0.2 percent 

TiO2 . These contents may be contrasted with chromite from 

the Stillwater Complex, which contains about 1 percent 

TiO2 . Nickel contents range from less than 0.1 percent to 

0.42 percent NiO; most samples contain 0.2-0.3 percent 

NiO, and these values are comparable to chromite from the 

Stillwater. No systematic variation exists between nickel 

and iron contents of the Red Lodge chromites.

The major-element composition of chromite may be 

used as a petrogenetic indicator (Thayer, 1970). The composition of much of the Red Lodge segregated chromite and 

nearly all the accessory chromite clearly has been modified. 

This is shown by a plot of the chromite compositions on 

Thayer-type (1970; modified by Thayer, 1980, oral commun.) variation diagrams on which compositional fields of 

primary magmatic chromite from stratiform and alpine-type 

deposits are outlined (fig. 13). Thayer's original stratiform 

field was distorted because Red Lodge and Fiskenaesset 

Complex (in west Greenland) metamorphosed samples were 

included. The field now consists of only primary magmatic 

chromite from the Stillwater Complex, the Bushveld 

Complex in South Africa, and the Great Dyke in Zimbabwe. 

The Red Lodge chromite that falls in a magmatic field has 

high-Fe stratiform-type rather than high-Al alpine-type 

compositions.

The general trend of chromite alteration is toward Feenrichment, and Al- and Mg-depletion (fig. 13), so that 

much of the Red Lodge chromite is actually homogeneous 

ferritchromit. Accessory chromite generally shows a greater

degree of alteration than the segregated chromite, as shown 

in figure 13, by greater departure from the magmatic

CD 50

$50

040

Chromite from 

layered intrusions

Chromite from 

Alpine-type intrusions

CR2 03 WEIGHT PERCENT

Chromite from 

Alpine-type intrusions

Chromite from 

layered intrusions.

B

CR2O3 WEIGHT PERCENT

Figure 13. Compositions of segregated and accessory 

chromite. The fields of chromite from alpine-type and 

stratiform-type intrusions are from Thayer (1970), as modified 

by Thayer (oral commun., 1980). Open symbols, accessory 

chromite; closed symbols, segregated chromite. A, Cr2O3 

weight percent versus total Fe as FeO weight percent. B, Cr2 O3 

weight percent versus AI2O3/(Al 2O3 +Cr2O3 -l-Fe2O3 ) mol percent.

B16 Contributions to the Geology of Mineral Deposits
Table 5. Representative microprobe analyses of chlorite and phlogopite 

[n.d., not determined]

Chlorite Phlogopite

Sample HJ-2a-42 LC-9(1) LC-9(2) HP-31

Location Four Chromes Highline Highline North Star

Assemblage 5 554

	Weight percent

SiQ2 ~ 30.55 29.23 35.64 40.27

TiO2 n.d. .00 .00 .43

A12O3 19.58 19.79 11.75 14.52

Cr2O3 1.10 .86 .58 1.20

FeO 3.17 3.18 2.05 1.24

MgO - 31.25 32.00 35.79 27.09

NiO JO .22 .14 .35

CaO n.d. .00 .00 .00

Na2O n.d. .00 .00 .04

K2O n.d. .00 .00 8.02

Total 85.75 85.28 85.95 93.16 

	Number of ions based on 28 oxygens (chlorite) and 22 oxygens (phlogopite)

Si 5.847 5.642 6.747 5.711

Al (IV) 2.153 2.358 1.253 2.289

Al (VI) 2.263 2.150 1.369 .138

Ti n .d. .000 .000 .046

Cr » .169 .130 .086 .134

Fe .506 .511 .325 .147

Mg 8.912 9.212 10.098 5.726

Ni .011 .035 .033 .040

Ca n.d. .000 .000 .000

Na n.d. .000 .000 .012

K n.d. .000 .000 1.451

LC-9

Highline 

5

38.13

.66

15.84

.49

3.06

27.29

.21

.00

.05

6.42

92.15

Total 19.861 20.038 19.911 15.694

5.469

2.531

.148

.070

.056

.367

5.834

.023

.000

.013

1.175

15.686

Ratio

Mg

Mg + Fe2 0.95 0.95 0.97 0.98 0.94

chromite compositional field. This is to be expected because of the larger amount of silicates available for reaction 

with the accessory chromite.

Segregated Chromite

Segregated chromite shows no significant grain-tograin chemical variation within individual samples, and single grains show virtually no zoning. Compositions of segregated chromite are plotted in figure 14, which also shows 

the compositional fields for chromite from alpine and stratiform complexes in general and for the Stillwater complex 

in particular. The Stillwater field is included to show a 

typical compositional range of chromite from a single stratiform complex.

Figure 14 shows that the Red Lodge segregated 

chromite is more similar to stratiform- than to alpine-type

Chromite-Bearing Uitramafic Rocks, Red Lodge District, Montana B17
Table 6. Representative microprobe analyses of serpentine 

[n.d., not determined]

HP 3(a)

Drill

3

HP-1 3(7) 

North Star 

5

Highline 

5

LC-1K-/) 

Highline 

5

LC-6(a) 

Highline

1

LC-6(-y) 

Highline 

1

SiO2 - 

TiO2 - 

A12O3 

Cr2O3 

FeO - 

MgO 

NiO -

Total

40.79

.10

1.86

.18

4.51

39.49

n.d.

86.93

43.44

.00

.28

.03

3.19

39.09

.19

86.22

43.01

.00

.60

.00

1.33

40.76

.08

85.78

44.62

.00

.24

.00

1.17

39.83

.04

85.90

41.09

.00

1.74

.00

2.76

40.10

.12

85.81

42.39

.00

1.66

.00

2.73

39.80

.07

86.65

Mg

Mg 0.94

Number of ions on the basis of 14 oxygens

Si _. __ _________ 3.857 4.096 4.041

Al (IV) - -- .143 .031 .000

Al (VI) - .065 .000 .066

a nnn nm nnn

Fe .355 .252 .105

Mg ____ _________ 5.567 5.493 5.707 

Ni n.d. 0.014 .006

Total 9.987 9.889 9.952

Ratio

4.163 3.903 3.973

.000 .097 .027

.026 .097 .155

nnn nnn nnn

.091 .221 .214

5.540 5.678 5.558

nn^ ni i nn?

9.823 10.007 9.934

0.96 0.98 0.98 0.96 0.96

chromite. The Red Lodge chromite shows a broader compositional range than unmetamorphosed chromite from a single stratiform complex, such as the Still water, as a result of 

Fe-enrichment during metamorphic alteration.

Most of the segregated chromite occurs in the 

amphibole-rich rocks of assemblage 2 and in the serpentinites of assemblage 5. The chromite from assemblage 2 

shows slightly lower Mg/(Mg+Fe2+ ) ratios, higher Cr/ 

(Cr+Al) ratios, lower Fe3+ /R3+ ratios, and a somewhat 

broader compositional range (fig. 14).

Distinct, although overlapping, compositional fields 

again appear where the segregated chromite is grouped according to the deposit in which it occurs (fig. 15). The 

chromite compositions, even though they are now altered, 

seem to reflect some primary compositional differences between the deposits that cannot be attributed to different host 

rock mineralogy. The most marked compositional difference is in the Mg/(Mg+Fe2+ ) ratio. Segregated chromite

from the Little Nell deposit is the lowest in Mg/(Mg+Fe2+ ) 

and is actually ferritchromit (table 7). Segregated chromite 

from the Highline and North Star deposits has the highest

Mg/(Mg+Fe2+ ) ratios.

Accessory Chromite

Most of the accessory chromite grains are essentially 

homogeneous ferritchromit, and a few have patches of even 

greater iron enrichment at the grain boundaries. Two compositional groups are present in a sample of exsolved 

chromite from the North Star deposit (table 8, sample 

HP-9). Other samples show little or no grain-to-grain 

variation.

The compositions of the accessory chromite according to assemblages are shown in figure 16. Most samples are 

in assemblages 3 and 5, and, like the segregated chromite, 

those from the amphibole-rich rocks of assemblage 3 are

B18 Contributions to the Geology of Mineral Deposits
slightly lower in Mg/(Mg + Fe2+ ) than those from the serpentinites of assemblage 5.

The chromite from the Red Lodge assemblage 5 serpentinites does not fall into the general compositionaLfield 

of chromite from serpentinites as determined by Evans^and 

Frost (1975) (fig. 16). Those authors showed that accessory 

Cr-spinels from alpine-type serpentinites are extremely iron 

rich and become progressively enriched in Cr, Mg, and Al 

as metamorphism increased. The Red Lodge assemblage 5 

samples overlap the higher grade chromite compositional 

fields shown by those authors. This evidence, along with the 

serpentine mineralogy and textures, indicates that the Red 

Lodge serpentinites are retrograde in origin. The chromite 

compositions were established at higher metamorphic 

grade, and were preserved during the retrograde serpentinization, and the subsequent slight prograde event.

The samples from the highest grade orthopyroxenebearing rocks of assemblage 1 all contain green Al-rich and 

opaque Fe-rich chromite as exsolved grains. The formation of 

the metamorphic green spinel indicates a trend toward Al- and 

Mg-enrichment at high metamorphic grades. This finding 

concurs with that of Evans and Frost (1975).

The ferritchromit from the Red Lodge samples, including homogeneous grains and intergrowths with chlorite, 

contains more A12O3 than MgO, which is the reverse of 

ferritchromit compositions reported from most other locations (table 9). The silicate assemblages in many of the Red 

Lodge samples are also high in A1 2O3 as shown by bulk-rock 

chemical analyses. Thus, the bulk-rock chemistry apparently controls the ferritchromit compositions.

Not all ferritchromit in the Red Lodge samples is 

associated with chlorite. Other workers have found ferritchromit without chlorite rims or intergrowths (Ulmer,

0.6

< 

+

CJ

CJ

0.4-

0.2

I T

Stratiform

/ Alpine

0.8 0.6 0.4

Mg/(Mg + Fe2+ )

0.2

0.6

+ 

< 

+

CJ

0.4

0.2

EXPLANATION

D Assemblage 2 

o Assemblage 4 

 Assemblage 5

0.8 0.6 0.4 0.2

Mg/(Mg + Fe2+ )

Figure 14. Compositions of segregated chromite from the various assemblages. The fields of chromite from alpine- and stratiformtype intrusions and from the Stillwater Complex are from Irvine (1967). A, Mg/(Mg+Fe2+ ) versus Cr/(Cr+AI) mol percent B 

Mg/(Mg+Fe2+ ) versus Fe3+ /Cr+Al + Fe3+ ) mol percent. ' '

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B19
Table 7. Representative microprobe analyses of segregated chromite 

[n.d., Not determined; calc., calculated]

Location - -

Sample ---- 

Assemblage -

North Star

HJ-24-42 HP-31 HP-10A 

545

Gallon Jug 2 Gallon Jug 4 Pick

HJ-73-42 HI-26-42 HP-22 HP-30 HJ-68-42 

2 3452

Shovel

HJ-32-42

2

HJ-15-42 

2

Total

Fe 2O3 calc. 

FeO calc.

Weight percent

TiO-. -----

A12O3 ----

Cr2O3 ----

FeO -----

MgO -----

NiO -----

0.24

12.83

52.64

21.71

11.23

n.d.

0.20

13.08

.51.58

24.44

10.32

.28

0.14

10.28

50.14

28.70

8.64

.42

0.20

13.55

49.06

27.23

9.23

.15

0.20

12.32

51.98

25.28

9.91

n.d.

0.37

11.80

46.21

33.32

6.67

.28

0.17

11.36

50.96

26.42

9.57

.25

0.32

11.77

44.18

37.25

5.06

n.d.

ft 90

12.46

47.57

31.97

6.16

.15

0.27

11.19

45.79

38.90

3.35

n.d.

98.65

5.49

16.77

New total 99.20

99.90 98.32

6.79 9.86

18.33 19.83

100.58 99.31

99.42

8.28

19.78

100.25

99.69

6.94

19.03

100.38

98.65 98.73

10.85

23.56

8.37

18.89

99.74 99.57

98.58

12.23

26.25

99.81

98.59

8.35

24.45

99.43

99.50

10.99

29.01

100.60

Formula proportions: cation sum = 3

Al

Cr -------

Fe3+ ----- 

Ti -------

Fe2+ -----

Mg ------ 

Ni -------

0.494

1.360

.135 

f\f\£.

.458 

547

n.d.

0.500

1.324

.166 

nns

.498 

«no

.007

0.408

1.336

.250 

.003

.559

.433

.011

0.521

I

^fLC

.203 

.005

.539

.448

.019

0.474

1.346

.171 

nnc

.521

AQ'l

n.d.

0.470

1 235

.276 

.010

.666 

.336

.007

ft 44 S

1.337

.209

004

.524 

.473

OO8

0.471

1.197

.316 

.008

.754

TCO

n.d.

0.498

i i"7<;

.213

ftfl7

.693 

.310

ftfn

0.454

1.248

.285

flfi7

.836 

.170

n.d.

19.74; Bliss and MacLean, 1975; Hoffman and Walker, 

1978). In this mode of occurrence, ferritchromit may be 

formed by the reaction of chromite with magnetite rims 

during metamorphism of a lizardite+magnetite+chromite 

assemblage to form antigorite+chromite + 

ferritchromit+magnetite (Bliss and MacLean, 1975).

The compositions of the single-phase chromite, both 

accessory and segregated, are shown with respect to Cr-AlFe3+ and the projected solvus defined by the exsolved 

chromite (fig. 17). The solvus extends from the Fe3+ -Al 

join to more Cr-rich compositions than have previously been 

reported in exsolved chromite from other rocks (see Muir 

and Naldrett, 1973). The top of the solvus is where Cr/ 

(Cr+Al+Fe3+ ) = 0.61. The single-phase Red Lodge 

chromites generally plot toward the Cr end of the Cr-Al join 

in figure 17, and a few plot near the Fe3+ end of the Cr-Fe3+ 

join. None of the single-phase chromites plot within the 

two-phase field defined by the solvus.

CONTACT METAMORPHISM

Table 10 shows bulk rock analyses of samples taken 

at distances ranging from 10 cm to 45 m from the monzonite 

dike at the Highline claim. The most pronounced differences are in CaO content, which is much greater near the 

dikes, and in MgO content, which is much less near the 

dike. SiO2 , A12O3 , K2O, Na2O, and MnO contents all are 

greater near the dike. Samples LC-6, 9, and 10 probably 

reflect primary compositional ranges.

Abundant hornblende, with or without feldspar, biotite, and zoisite, was found in samples taken about 

5-10 cm from the dike. Abundant hornblende and talc as 

well as exsolved chromite were found in the samples taken 

about 0.5 m from the dike. Assemblage 1 was found at

distances of 3-10 m and throughout the Highline ultramafic 

body, which indicates that those high-grade rocks were 

probably formed by the regional rather than the contact

B20 Contributions to the Geology of Mineral Deposits
Table 7. Representative microprobe analyses of segregated chromite Continued 

[n.d., Not determined; calc., calculated]

Four Chromes Little Nell Edsel Highline

HJ-33-42 

2

HJ-34a-42

5

HJ-87-42 

2

HJ-84-42 

2

HJ-80-42

2

HJ-10-42

2

HJ-4-42

2

HJ-9-42

2

HJ-70-42 

5

HJ-69-42 

5

LC-13 

4

LC-15

2

99.51

16.10

24.78

101.12

97.90

18.73

24.80

99.78

Weight percent Continued

0.44

13.41

39.90

39.27

6.49

n.d.

0.57

13.30

36.09

41.65

6.09

.20

0.27

12.20

47.02

35.76

2.50

n.d.

0.13

10.13

48.78

37.31

3.21

n.d.

0.02

.42

14.32

79.24

.04

.30

0.22

12.58

53.89

24.56

8.59

n.d.

0.27

10.50

51.61

30.59

5.47

n.d.

0.16

8.97

54.12

31.93

4.91

n.d.

0.23

13.19

53.79

21.84

11.23

n.d.

0.20

10.68

51.60

27.27

9.39

.08

0.12

15.59

50.77

22.27

11.28

.23

0.25

15.23

45.73

27.90

9.45

.30

97.75

6.62

29.81

98.42

99.56

9.35

28.90

100.50

94.34

53.73

30.90

99.73

99.84 98.44 100.09 100.28

3.82

21.12

6.17

25.03

6.31

26.25

5.04

17.31

99.22 100.26 98.86

8.81 5.47 13.16

19.34 17.35 16.06

100.22 99.05 100.72 100.79 100.10 100.81 100.18

Formula proportions: cation sum = 3 Continued

0.526

1.049

.403

.011

.689

.322

n.d.

0.529

.966

.477

.014

.702

.307

.005

0.506

1.307

.175

.006

.876

.130

n.d.

0.413

1.337

.244

.003

.838

.165

n.d.

0.023

.535

1.549

.000

.990

.001

.009

0.489

1.406

.095

.005

.583

.422

n.d.

0.427

1.405

.160

.007

.721

.280

n.d.

0.363

1.457

.163

.003

.753

.251

n.d.

0.500

1.369

.122

.004

.466

.539

n.d.

0.417

1.355

.220

.004

.537

.465

.002

0.585

1.278

.131

.003

.462

.535

.006

0.586

1.179

.323

.006

.438

.460

.008

metamorphism. Contact metamorphic effects, therefore, extend to a distance from 0.5 m and 3 m from the dike.

The bulk composition of the chromite was indirectly 

affected by the metasomatism. Chromite taken 0.5 m from 

the dike contains about 10 percent A12O3 , whereas that 

taken farther from the dike contains about 30-60 percent. 

The high Ca content near the dike produced the Ca-Al silicates, whereas farther from the dike, where the Ca content 

was low, the chromite was enriched in the available Al.

GEOCHEMISTRY

Table 11 shows representative major oxide analyses 

and C.I.P.W. norms of ultramafic samples from the various 

assemblages and of amphibolites from the contacts with 

some of the ultramafic lenses. Average values for ultramafic 

rocks and for continental tholeiitic basalt are included for 

comparison.

The Red Lodge ultramafic rocks show a wide range in 

chemical composition. Silica variation diagrams for the 

major oxides (fig. 18) show that SiO2 values range from 

about 36 percent to more than 52 percent. The other major 

oxides, MgO, FeO, CaO, and A1 2O3 , also have values that 

range between the averages of ultramafic and mafic rocks 

(fig. 18). The plots show smooth continuous trends, and 

samples from each assemblage generally fall in distinct 

'fields of the diagrams. The minor oxides, TiO2 and Na2O, 

are low in the Red Lodge ultramafics and are closer to the 

values of an ultramafic than to those of a mafic rock

(fig. 18).

Analyses of the trace elements, Cr, Ni, Co, and Sc, 

and average values for ultramafic and mafic rocks are shown 

in table 12. Most trace-element contents are similar to values from other ultramafic rocks (Himmelberg and Coleman, 

1968; Muir, 1979; Stosch and Seek, 1980), unlike the major 

oxides, which show wide variations from ultramafic to 

mafic compositions. Only Sc shows a difference among the

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B21
0.8

0.6

<

+

CJ

CJ

0.4

0.2

<0

O o 

EXPLANATION

O North Star

n Pick

® Shovel

 Gallon Jug #4

© Gallon Jug #2

O Highline

 Four Chromes

+ Edsel

 Little Nell

i____i

0.8

0.6

CJ

+

0.4 -

0.2

B

0.8 0.6 0.4

Mg/(Mg + Fe2+)

0.2 0.6 0.4

Mg/(Mg + Fe2+ )

0.2

Figure 15. Compositions of segregated chromite from the various deposits. A, Mg/(Mg+Fe2+ ) versus Cr/(Cr+Al) mol percent.

Mg/(Mg+Fe2+ ) versus Fe3+/(Cr+AI + Fe3+ ) mol percent.

assemblages, and shows low contents in the serpentinites of 

assemblage 5 that correspond to the average of 5 ppm Sc 

in dunites (Goles, 1967). Most of the other rocks are 

between that value and the average for pyroxenites of 40 

ppm Sc, so many of the Red Lodge rocks may have 

originally been peridotites.

Pt, Pd, and Rh contents of 14 of the Red Lodge 

samples, including 5 segregated chromites, are shown in 

table 12. Segregated chromite contains significantly more 

Rh and typically less Pd than accessory chromite, whereas 

Pt contents are variable. High Rh contents have been 

found in segregated chromite from other areas (Page and 

others, 1979). The Pt, Pd, and Rh contents of segregated 

chromite from the unmetamorphosed Stillwater Complex 

(Page and others, 1969) are significantly higher than those 

from the Red Lodge Samples. In contrast, values from the 

Fiskenaesset Complex of Greenland (Page and others, 

1980), an upper-amphibolite facies stratiform intrusion, 

are similar to those from Red Lodge, which may indicate

B22 Contributions to the Geology of Mineral Deposits

mobility of the platinum group metals during high-grade 

metamorphism. The various elemental ratios in the Red 

Lodge rocks are similar to the ratios in both the Stillwater 

and the Fiskenaesset.

The major oxide contents of the Red Lodge 

amphibolites associated with the ultramafic rocks are 

consistently close to the average composition of 

continental tholeiite (table 11 and fig. 19). Plots of 

amphibolite compositions (fig. 19) also indicate tholeiitic 

compositions. This conclusion agrees with that of other 

workers (Armbrustmacher and Simons, 1977; Simons and 

others, 1979).

The Red Lodge rocks, therefore, show a range in 

composition between mafic and ultramafic rocks, with some 

assemblages such as 1 and 7 closer to average ultramafic 

rocks, and other assemblages such as 3 and 4 closer to 

average mafic rocks in terms of the major oxides. A similar 

range in compositions was noted by Skinner (1969) in the
ultramafic rocks from the nearby Highline Trails Lakes 

area.

PETROGENESIS

As stated previously, the origin of the Red Lodge 

ultramafic rocks is problematic because of the high-grade 

metamorphic overprint. The high CaO and A12O3 contents 

of many of the samples relative to average ultramafic rocks 

suggest two possibilities: (1) extensive metasomatism during the metamorphism of some rocks that were all originally 

ultramafic, or (2) essentially isochemical metamorphism of 

diverse rocks.

The first possibility would require the movement of 

large amounts of material. Although there is a source of

calcium and aluminum in the country rocks, the evidence 

against extensive metasomatism includes:

1. In general, blackwall zones, marked by the 

formation of Al-rich phases such as chlorite near the 

margins of ultramafic bodies, are formed by Mg migration out of the ultramafic rocks into the Al-rich country 

rocks. Aluminum does not migrate into ultramafic rocks 

(Chidester, 1962). Even at high metamorphic grade, 

blackwall zones are thin, usually less than 1 m wide,and 

do not extend to the interiors of ultramafic bodies.

2. As shown previously, metasomatic introduction of 

CaO, and to a minor extent A12O3 , occurs near the monzonite porphyry dike in the Highline open pit, but only in 

samples taken within 3 m of the dike. Thus, the calcium 

and aluminum did not migrate over long distances.

0.8

0.6

CJ 

~~c

CJ

0.4

0.2 ' / A

/ Metamorphic

A

0.8 0.6 0.4

Mg/(Mg + Fe2

0.2

0.8

A °-6

CD

u_

+

< 

+

CJ

0.4

0.2

EXPLANATION

n Assemblage 2

A Assemblage 3

 Assemblage 5

T Assemblage 6

n

0.8 0,6 0.4

Mg/(Mg + Fe2+ )

Figure 16. Compositions of accessory chromite. The fields of chromite from alpine- and stratiform-type intrusions are from Irvine 

(1967); the field of metamorphic accessory chromite including chromite from serpentinite (stippled field) is from Evans and Frost 

(1975). A, Mg/(Mg+Fe2+ ) versus Cr/(Cr+Al) mol percent. B, Mg/(Mg+Fe2+ ) versus Fe3+/(Cr+Al + Fe3+ ) mol percent.

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B23
Table 8. Representative microprobe analyses of accessory chromite 

[calc., calculated]

WD ">

 Drill

3

HP-4 A 

Drill

3

l_|n 1 DR

North 

Star 

5

HP 11

North 

Star 

5

HP 13

North 

Star 

5

HP 26

Shovel 

3

CD 1

Four 

Chromes 

6

HP-9

North 

Star 

5

HP-9 

North 

Star 

5

Weight percent

TiO2 

A1 203 -----

Cr203 -----

FeO _____

MgO - 

NiO --.

........ o.39 0.15 0.05 0.48 0.24 0.47 0.14 0.24 0.19

........ 4.37 11.39 15.44 6.02 5.89 6.50 .15 2.85 2.94

........ 37.04 48.40 42.62 43.86 45.57 42.74 21.68 33.72 42.26

..... 52.70 34.32 33.89 41.57 42.60 46.02 71.64 59.71 51.39

..-._ .14 3.23 5.62 4.64 2.70 .28 .22 .65 1.72

 .34 .22 .11 .30 .24 .31 .29 .23 .12

Total

FeO (calc.) 

Fe2O3 (calc.)

New Total

94.98

31.52

23.54

97.34

97.71

28.25

6.74

98.38

97.73

25.29

9.55

98.68

96.87

25.39

17.98

98.67

97.24

27.98

16.25

98.87

96.32

31.92

15.67

97.89

94.12

30.49

45.73

98.70

97.35

31.24

31.65

100.53

98.62

29.94

23.84

101.01

Formula proportions: cation sum = 3

A 1

Cr 

Ti 

Mg - - 

Ni -- 

.......... 0.195

.......... i.ni

£JV)

.......... .oil

i nnn

.......... .001

.......... .010

0.471

1.343

1 78

004

.829

.169

.006

0.616

1.139

243

noi

.715

003

0.253

.483

.013

TCO

.247

.008

0.251

1.302

44?

.007

.846

.146

.006

0 284

1 252

.437

.013

.989

.015

.010

0.007

.660

1.325

.004

.982

.013

ooo

0.124

.984

.879

.006

964

.036

007

0.125

1.212

.653

oos

.908

.093

.004

3. If metasomatism were extensive, then all the ultramafic rocks probably would have been at least somewhat 

affected because the ultramafic rocks all occur in similar 

country rocks. However, there is no evidence of Ca and 

Al metasomatism in many of the rocks, such as the 

serpentinites.

The second possibility, favored here, explains the 

chemical diversity of the Red Lodge rocks by isochemical or 

nearly isochemical metamorphism of diverse original parent 

rock types. On the basis of the chemical and textural evidence, the original rocks included dunites (now assemblage 5 serpentinites), harzurgites (now rocks of assemblage 1), feldspar- or clinopyroxene-rich chromitites (now 

assemblage 2) and pyroxenites, or rocks approaching gabbroic compositions (now assemblage 3). The normative 

analyses (table 11), although by no means considered as 

conclusive proof, indicate mafic assemblages for some of 

the rocks, with as much as 36 percent feldspar. Thus, the 

abundant modal hornblende in the rocks was formed by

metamorphic reaction of original plagioclase with olivine 

and pyroxene.

The deformation and metamorphism have caused debate concerning whether the Red Lodge ultramafic rocks are 

alpine-type (James, 1946) or technically disrupted stratiform-type deposits, possibly fragments of the Stillwater 

Complex (Thayer, 1970; Bowes and Skinner, 1969). The 

characteristics of the two types of complexes have been 

described in the literature (Thayer, 1960, 1967, and 1970; 

Coleman, 1977; Girardi and Ulbrick, 1980), and the three 

possibilities are considered as follows:

1. Alpine type: James (1946) classified the Red 

Lodge ultramafics as alpine type because of their lenticular forms. However, rock types characteristically associated with alpine-type ophiolites (see Coleman, 1977) are 

lacking in the Red Lodge area, and the Archean Red 

Lodge rocks are much older than known ophiolites, 

which are all Phanerozoic in age. The Red Lodge 

chromite compositions do not correspond to those from

B24 Contributions to the Geology of Mineral Deposits
alpine-type deposits (compare fig. 13 and 14). In addition, characteristic textures in alpine-type chromitites, 

such as coarse grain size and chromite nodules (Thayer, 

1960), are not present at Red Lodge. Thus, the Red 

Lodge deposits are probably not alpine type.

2. Fragments of the Stillwater Complex: This idea is 

attractive because of the proximity of the Red Lodge and 

Stillwater areas. However, the Stillwater Complex, except for some serpentinization, is unaltered, whereas the

Table 9. Ferritchromit analyses from other locations 

[n.d. - not determined]

Red Lodge ultramafic rocks were profoundly affected by 

the 2,800m.y. metamorphic event. This makes any 

connection between the Red Lodge and Stillwater ultramafic rocks unlikely because the Red Lodge rocks appear 

to be older.

3. A third possibility is that the Red Lodge ultramafics were derived from a layered intrusion like the 

Stillwater which was tectonically disrupted. A similar 

origin was proposed for small ultramafic bodies in the

TiO2

A1203

Cr2O3

Fe203

FeO

MgO

NiO

MnO

Total

1.

0.6

1.0

13.9

n.d.

76.2

3.0

n.d.

.3

95.0

2.

1.04

3.1

42.0

22.4

30.16

2.1

n.d.

n.d.

100.80

3.

<0.02

.29

38.8

30.4

25.2

3.09

.43

.69

98.92

4.

0.12

1.8

56.0

13.1

14.7

10.1

.49

.47

96.78

5.

0.42

.06

14.87

55.05

25.97

3.15

.47

n.d.

99.99

6.

n.d.

0.3

37.8

n.d.

52.1

3.3

n.d.

n.d.

93.5

7.

0.31

.26

28.10

39.30

27.90

2.29

.39

.75

99.3

1. Springer, 1974, analysis A41B, p. 177.

2. Beeson and Jackson, 1969.

3. Hoffman and Walker, 1978, analysis 6-7-8, p. 705.

4. Onyeagocha, 1974, analysis 2-4A#l.

5. Bliss and MacLean, 1975, analysis 17, p. 977.

6. Ulmer, 1974.

7. Pinsent and Hirst, 1977, analysis 60186(F), p. 589.

Table 10. Chemical analyses of ultramafic rocks near monzonite dike at the Highline claim 

[In weight percent]

SiO2 

A1 203 --------

Co f^

FeO ____________ -

MgO .

(~*nC\

Na2O 

v c\

u f\+

H

r~i~

TiO2 

P2c\ <J5 ----- - - 

MnO 

(~T\

Cr203 ----- 

Total

........ LC-7

 10 cm

AQ C

........ 5.5

........ 6.0

........ 4.0

in 7

H o

........ .71

........ .16

QC

TC

........ .17

........ .03

........ .20

05

98.72

0.5 m

47.0 

6.7 

7.7 

4.2 

18.8 

10.0 

.75 

.11 

.80 

.19 

.22 

.03 

.23 

.03 

2.8

99.56

LC 5

3 m

42.9 

5.4 

8.1 

6.3 

26.8 

4.0 

.20 

.05 

4.1 

.24 

.24 

.03 

.16 

.12

98.64

LC-10 

13 m

45.4 

4.1 

7.3 

6.2 

28.3 

3.0 

.26 

.07 

3.8 

.20 

.22 

.03 

.15 

.14

99.17

LC 9

18 m

39.9 

2.1 

6.9 

1.1 

36.8 

.43 

.00 

.03 

11.3 

.64 

.07 

.02 

.08 

.02

99.39

LC-6 

45 m

42.3 

4.4 

6.8 

3.9 

29.7 

4.7 

.07 

.04 

6.0 

.34 

.15 

.04 

.07 

.13

98.64

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B25
EXPLANATION 

O Exsolved. bulk composition 

O Single phase

Open symbols, accessory chromite 

Closed symbols, segregated chromite

F03+

Figure 17. Chromite compositions with respect to Cr-Al-Fe3+ , showing the solvus as defined by exsolved pairs.

nearby Highline Trail Lakes area (Skinner, 1969). The 

compositions of some of the Red Lodge chromites correspond to those from stratiform intrusions. In addition, the 

segregated chromites in the Red Lodge ultramafics are 

relatively fine grained and many have chain textures that 

are similar to textures of chromite from stratiform complexes (Thayer, 1960; Jackson, 1961). Finally, the age 

and geologic setting of the Red Lodge rocks are similar to 

those of other stratiform deposits.

Thus, tectonic disruption of a stratiform intrusion appears to be the most likely origin of the Red Lodge ultramafic lenses. The diversity of rock types, and the different 

chromite compositions in the various deposits occur because 

the lenses are fragments from various levels in the parent 

stratiform mass. Tectonic emplacement of the ultramafic 

lenses is implied by their sheared contacts against the country rocks. The diverse metamorphic assemblages are the 

result of alteration of feldspathic and nonfeldspathic parent 

rocks, and the chromite, although now altered, probably 

showed primary compositional differences between the various layers, which are still detectable (see fig. 15). Associations of mafic rocks and small bodies of ultramafic rocks in 

high-grade metamorphic terrains similar to that of the Red 

Lodge area have been described by other workers (Bowes 

and others, 1964; Bowes and others, 1966; Girardi and 

Ulbrich, 1980). The origins have variously been interpreted 

as tectonic disruption of a stratiform complex (Bowes and 

others, 1964; Bowes and others, 1970), and as fractional 

crystallization of olivine-tholeiite magma to produce a series 

of ultramafic rocks and a magma residue which formed 

associated gabbroic rocks, all of which underwent tectonic 

disruption (Girardi and Ulbrich, 1980).

B26 Contributions to the Geology of Mineral Deposits
Other workers have recognized two contrasting types 

of metamorphosed layered complexes: anorthositeleucogabbro complexes such as the Fiskenaesset of Greenland (Windley and others, 1981) and ultramafic-gabbro 

complexes such as those in northwest Scotland (Bowes and 

others, 1964) and southern Africa (Robertson, 1973). The 

Red Lodge rocks are similar to the latter type, and in fact the

geologic setting in the Beartooth Mountains appears to be 

like that in the Archean craton of Zimbabwe. The Great 

Dyke (relatively unaltered and intact) has a Rb-Sr age of 

2,510±20m.y. and is flanked by older, altered, 

chromite-bearing ultramafic complexes with a Nd-Sm age 

of 2,640± 140 m.y. (see,for example, Wilson, 1968; Hamilton, 1977; and Hamilton and others, 1977).

LLI

LLI

1.5

1.0

0.5

0

15

10

5

0

20

15

10

LLI

10

5

0

40

35

30

25

20

15

10

5

D + D 

3 A

A'°23

CaO

A

1% .

o

Total Feas FeO

o A^

+0

MgO

38 40 42 44 46 48

Si02 WEIGHT PERCENT

50 52

EXPLANATION

+ Assemblage 1 

D Assemblage 2 

A Assemblage 3 

O Assemblage 4 

 Assemblage 5 

v Assemblage 6 

O Amphibolite

Average ultramafic

rock (Turekian and

Wedepohl, 1961) 

Average continental

tholeiite (Manson,

1967)

Figure 18. Variation diagrams of selected oxides versus SiO2 in ultramafic rocks and amphibolites from the Red Lodge 

district.

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B27
Assemblage 1 Assemblage 2 

HP 16

Gallon Jug 2

Assemblage 3 Assemblage 4 Assemblage 5 

|-|p 2 cp "> |-|D 1 1

Drill Four Chromes North Star

Assemblage 6 

LC 7

Highline

Oxides, weight percent

SiO^ 

A1203 ------

Fe2O3 -- 

FeO ._ . 

MgO 

CaO 

Na^O -

K PI

H

O +

H

c\~

TiO2 

P/"v 2u5 ........ 

MnO -

r~T\

Total

43 5

4.4

6.6

5 9

30.2

3.0

.13

r\(\

5.2

.25

.17

.07

.20

04

99.8

47.3 

10.3 

3.2 

9.1 

13.2 

11.6 

1.1 

.85 

.94 

.16 

.54 

.05 

.21 

.02

98.6

52.4 

6.3 

3.1 

6.1 

12.0 

16.4 

1.2 

.77 

.32 

.25 

.20 

.08 

.38 

.03

99.5

41.1

3.2 

6.5 

5.0 

30.3 

3.6 

.17 

.05 

7.9 

.18 

.09 

.01 

.13 

1.4

99.6

36.9 

.59 

7.7 

2.0 

38.1 

.75 

.01 

.01 

10.1 

.64 

.03 

.02 

.07 

1.6

98.5

49.8 

5.5 

6.0 

4.0 

19.7 

11.2 

.71 

.16 

.95 

.25 

.17 

.03 

.20 

.05

98.7

Normative mineralogy, weight percent

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

c ..... ... ..

Or -

Ah

An

Ne -

Wo -

Fn _______

Fs 

Fo -

Fa

Mt 

Hm - -- 

11 .... 

AAnP 

Cc 

Mg -----

Total

0.53

1.10

H I 0

1 1/i

41.83

3 04

23 53

i on

9 59

32

.17

no

94.53

5.10 

9.45 

20.98

15.45 

20.48 

8.45 

9.04 

4.11 

4.71

1.04 

.12 

.05

98.98

1.01

4.57 

10.20 

9.58

29.84 

30.03 

9.06

4.52

.38 

.19 

.07

99.45

0.30 

1.44 

7.85

.47 

41.40 

2.14 

24.07 

1.37 

9.46

.17 

.02 

3.20

91.89

0.52 

.06 

.09

30.93 

44.18

6.69 

3.21 

.06 

.05 

1.31 

2.00

89.09

6.05 

6.09 

8.96

19.57 

41.64 

2.10 

5.68

.32 

8.82

.33 

.07 

.12

99.75

From Turekian and Wedepohl, 1961. "FromManson, 1967.
LJJ

CJ 4 

CC 

LJJ 

Q_

(D 

LU

O 1

IN 

CC

I

42 44 46 48

Si02 WEIGHT PERCENT

50

52

o

DC

CL 50

X

CD

LU 48

CN

O 

(7) 46

B

_L

1 2 3

FeO TOTAL/MgO

Figure 19. Compositional plots of amphibolites, indicating mostly tholeitiic compositions. A, SiO2 versus (Na2O+K2O) in 

weight percent. B, FeO total/MgO versus SiO2 in weight percent.

FORMATION AND POST-EMPLACEMENT 

HISTORY

The following sequence of events is proposed for the 

genesis of the Red Lodge ultramafic rocks:

1. A layered igneous intrusion is formed and includes 

dunite, periodotite, feldspathic peridotite, pyroxenite,and 

gabbro, with local concentrations of chromite.

2. Some rocks are partially or completely serpentinized, and magnetite rims form on the chromite.

3. The rocks undergo upper amphibolite facies regional metamorphism, tectonic disruptions, and emplacement of various fragments of the layered intrusion. At low 

grades, chromite reacts with magnetite rims to form Ferich, Mg- and Al-depleted homogeneous ferritchromit. At 

high grades, above the break down of chlorite, chromite 

reacts and is enriched in the MgAl2O4 component. Possible reactions are:

5 chlorite=2 talc+7 olivine+5 MgAl2O4 +18 H2O 

chlorite=enstatite+olivine+Mg A12O4+H2O

(Evans and Frost, 1975). Chromite of various compositions undergoes the various types of exsolution between 

Al-rich and Fe-rich end members. Olivine, enstatite, and 

hornblende are formed at the highest grades, with the 

various assemblages controlled by the original 

mineralogy.

4. Some of the rocks undergo retrograde metamorphism, probably controlled by the availability of H2O. 

Serpentinization and limited formation of talc, chlorite, 

phlogopite, carbonate minerals, and magnetite occur. 

Magnetite rims form on some chromite, and Al-rich 

chromite is altered to ferritchromit+chlorite; otherwise 

the chromite is not affected.

5. The rocks are subjected to low-grade metamorphism possibly corresponding to the 1,600-1,800 m.y. 

event (Nunes and Titton, 1971; Rowan and Mueller, 

1971); there is limited recrystallization of serpentine 

(lizardite -» antigorite) and formation of lizardite hourglass textures.

ECONOMIC GEOLOGY

The United States is entirely dependent on foreign 

sources for chromite, which is the only ore mineral of 

chromium. Chromium is an indispensable industrial metal, 

used primarily as a component of stainless steel to increase 

the hardness, strength, and resistance to corrosion, a use for 

which there is presently no known substitute. It is used in 

the chemical industry, in tanning leather, in pigment, and 

for plating, and the mineral chromite is used as a refractory 

in high-temperature metallurgical furnaces (Thayer, 1973; 

Thayer and Lipin, 1978). Most of the U.S. supply of 

chromite is imported from southern Africa, Turkey, the 

Philippines, and the USSR. Together, the Bushveld Complex of South Africa and the Great Dyke of Zimbabwe

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B29
Table 12. Trace-element analyses of Red Lodge ultramafic rocks

[Spectrographic analyses by Norma Rait of the U.S. Geological Survey.; ppm, parts per million; ppb, parts per billion; n.d., not determined]

Cr 

(ppm)

Co

(ppm)

Ni 

(ppm)

Sc 

(ppm)

Pt 

(ppb)

Pd 

(ppb)

Rh 

(ppb)

Rocks containing accessory chromite

Assemblage 1

HP-20A --. 5,000 250 1,200 28 <3 5 1

HP-24 2,400 200 1,800 19 <4 3 <1

LC-5 2,200 210 1,000 30 <2 4 1

Assemblage 2

HP-8 1,000 170 670 51 n.d. n.d. n.d.

Assemblage 3

HP-1-- 3,600 110 910 12 <3 2 <1

HP-27 4,100 200 740 32 10 22 1

Assemblage 4

SR-2 7,100 290 1,200 25 <4 11 <1

Assemblage 5

HP-9 6,400 120 1,400 5 n.d. n.d. n.d.

HP-11 7,100 190 2,300 5 <3 8 1

HP-14 n.d. n.d. n.d. n.d. < 1 < 2 < 1

HP-29 11,000 250 2,100 9 14 48 2

LC-11 3,100 110 1,400 10 n.d. n.d. n.d.

Rocks containing segregated chromite

Assemblage 2 

LC-15 n.d. n.d. n.d. n.d. < 2 < 1 6

Assemblage 4 

HP-31 n.d. n.d. n.d. n.d. 18 6.4 13

LC-13 n.d. n.d. n.d. n.d. <2 <1 4

Assemblage 5

HP-10A n.d. n.d. n.d. n.d. < 2 < 1 7

HP-30 n.d. n.d. n.d. n.d. < 2 < 1 8

1 Goles, 1967 2,400 110 1,500 16

2 Turekian and Wedepohl, 1961 170 48 130 30

'Average for ultramafic rocks from Goles, 1967.

2 Average for basaltic rocks from Turekian and Wedepohl, 1961.

contain an estimated 99 percent of the worldwide chromite 

resources (Thayer and Lipin, 1978).

Within the United States, about 80 percent of the 

chromite resources occur in the Stillwater Complex. Most of 

the remainder is in small podiform deposits scattered in the 

mountain belts on the east and west coasts (Thayer, 1973), 

especially in Oregon and California. All U.S. deposits are 

low grade, and are not currently economic. However, it is

of interest to assess the economic potential of the deposits, 

such as those at Red Lodge, because production of chromite 

from domestic sources may be necessary in the future, in the 

event of a disruption of foreign sources.

Most of the known massive chromite has already been 

mined from the Red Lodge deposits. About 77,000 tons of 

ore, averaging 12.8 percent Cr2O3 , remains in the deposits 

(Simons and others, 1979), and about 100,000 tons of

B30 Contributions to the Geology of Mineral Deposits
undiscovered ore probably remains in the area (James, 

1946). The present study shows that the concentration of 

chromite in the rocks ranges from 1 percent to about 

80 percent, but is mostly between 5 and 15 percent.

The compositions of the chromite have been altered 

by metamorphism, and this will affect their future economic 

potential. Some segregated chromites have compositions 

similar to magmatic chromite from stratiform intrusions. 

Other segregated chromites and most accessory chromites 

have metamorphic compositions, and the general trend of 

alteration is toward Fe-enrichment and Mg- and Aldepletion. Ferritchromit is formed by this chemical alteration and commonly occurs as homogenous grains, less 

commonly as partial rims on chromite, and as fine intergrowths with chlorite(?) as an alteration of Al-rich chromite. 

Accessory chromite is generally more higWy altered 

than segregated chromite.

This study has shown that the composition of the 

chromite is related to the metamorphic silicate assemblage. 

In general, hornblende-rich rocks contain chromite of somewhat higher Fe content than other rocks. Serpentinites contain chromite of slightly lower Fe content, but contain other 

Fe-rich phases such as magnetite and ferritchromit. 

Ferritchromit-chlorite intergrowths that occur as alteration 

products of Al-rich accessory chromite in some of the serpentinites may present sliming problems in milling disseminated ore. The rocks that contain orthopyroxene have two 

coexisting chromites of Fe- and Al-rich compositions, and 

late-stage serpentinization produces the ferritchromitchlorite type of alteration of the Al-rich chromites.

The present study has shown that the Red Lodge ultramafic rocks probably are parts of a disrupted stratiform 

complex. If this is correct, there may be more chromite 

deposits in fragments of the stratiform complex concealed at 

various depths below the surface. Aeromagnetic surveys 

conducted by Simons and others (1979) indicate both small 

and large magnetic anomalies in the present study area and 

in other areas throughout the Beartooth Mountains. Those 

authors concluded that the anomalies were caused by concealed intrusive bodies. The magnetic anomaly caused by 

the ultramafic rocks of the Stillwater Complex is similar in 

magnitude to the other anomalies in the Beartooths. Therefore, it is possible that some of the anomalies are caused by 

buried ultramafic to mafic rocks.

Magnetometer surveys conducted in the Red Lodge 

area by James (1946) showed that the ultramafic rocks produced anomalies, and that chromite ore bodies produced 

anomalies of even greater magnitude. James (1946) concluded that local anomalies near some of the ultramafic 

lenses were produced by other concealed ultramafic rocks.

However, even if concealed deposits are in the area, 

the chromite would probably have compositions similar to 

those in the exposed deposits, and their economic potential 

would thus be hindered by the same factors: (1) the high-Fe 

compositions; (2) the presence of abundant magnetite as

grains, veins, and rims on accessory chromite in serpentinites; (3) ferritchromit-chlorite(?) intergrowths in serpentinized rocks.

Other elements of possible economic interest include 

the platinum group elements (PGE), cobalt, and nickel. This 

study has shown that the concentration of PGE's in the Red 

Lodge ultramafic rocks is one to two orders of magnitude 

lower than that in the nearby Stillwater Complex (Page and 

others, 1969). The economic potential of PGE's from the 

Red Lodge rocks is, therefore, very low.

Likewise, the amounts of cobalt and nickel are about 

an order of magnitude lower than the amounts that are currently economic. In general, only the nickel in sulfides is 

recoverable. The Red Lodge samples contain only small 

amounts of sulfides, including a Ni-Fe sulfide associated 

with serpentinized samples. The bulk of the nickel is in 

chromite and in the silicates, where it is not recoverable by 

present means. It is not known which minerals contain the 

cobalt in the Red Lodge samples because cobalt contents 

were not determined for individual mineral species.

CONCLUSIONS

A petrologic study was made of upper-amphibolite 

facies chromite-bearing ultramafic rocks that occur as small 

lenses in the Red Lodge chromite district, Montana. Results 

indicate:

1. All chemical and mineralogical evidence shows 

that the deposits are not alpine-type. The ultramafic 

lenses are probably fragments of a stratiform complex 

that was tectonically disrupted and emplaced during the 

metamorphism. This interpretation is consistent with the 

stratiform-type compositions of some segregated chromite, the relatively fine grain size of the chromite and 

absence of nodules, the Archean age, and the geologic 

environment.

2. Accessory chromite shows a greater degree of 

chemical alteration than segregated chromite.

3. In the highest grade orthopyroxene-bearing rocks, 

green Al-rich chromite was formed, and it coexists with 

Fe-rich chromite. In other rocks, the general trend of 

chromite alteration is toward the formation of homogeneous ferritchromit through Fe-enrichment, and Mg- and 

Al-depletion. This adversely affects the economic potential of the chromite remaining in the deposits.

4. If it is necessary to produce chromite from the 

deposits in the future, the following should be considered. Most of the massive chromite has already been 

mined from the Red Lodge deposits. Accessory chromite 

is ubiquitous in the ultramafic rocks, but drawbacks to its 

economic potential include the high-Fe metamorphic 

compositions; the presence of magnetite as grains, veins, 

and rims on chromite in serpentinites; ferritchromitchlorite(?) intergrowths as alteration products of Al-rich

Chromite-Bearing Ultramafic Rocks, Red Lodge District, Montana B31
chromite in serpentinized rocks, which may cause problems in milling.

5. Cobalt, nickel, and the Pt-group elements are all 

present in amounts well below current economic limits.

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*U.S. GOVERNMENT PRINTING OFFICE: 1986-^91-405:20118