Chromite Deposits in 

Central Part Stillwater 

Complex, Sweet Grass 

County, Montana

GEOLOGICAL SURVEY BULLETIN 1015-D

Chromite Deposits in 

Central Part Stillwater 

Complex, Sweet Grass 

County, Montana

By A. L. ROWLAND

A CONTRIBUTION TO ECONOMIC GEOLOGY 

GEOLOGICAL SURVEY BULLETIN 101S-D

^4 description of layered deposits of 

chromite in the ultramafic rocks, and 

an estimate of their commercial 

importance

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

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

Washington 25, D. C. - Price $1.50 (paper cover)
CONTENTS

Page

Abstract..-.---________-_______________________._ 99

Introduction._____________________________________________________ 99

Outline of geology----_____-____-____________________________ 101

Pre-Cambrian rocks______________________________________________ 102

Metamorphosed sediments___ __--___-___________________________ 102

Igneous rocks of the Stillwater complex._________________________ 102

Basal zone______________________________________________ 103

Ultramafic zone___________________________________________ 103

Banded zone______________________________________________ 105

Dikes. _____________________ __________________________ 105

Granite______________________________________________________ 105

Cambrian rocks___________________________________________________ 106

.Structural features.._____________________________________________ 106

Primary structure or layering.__________________________________ 106

Tilt -------__________________________________ 106

Faults--.. _____________...____..._________._- 106

 Chromite deposits_________________________________________________ 107

Mineralogy and petrography____________-___-_-_-___-________-__ 108

Description of deposits_-_______--_-___--__--____-_-____--_______-_- 108

East Boulder Plateau area._____________________________________ 110

East Boulder Plateau to East Boulder River______________________ 112

Iron Mountain area__________________________________________ 113

Iron Mountain to West Fork of Stillwater River_________________ 113

West Fork of Stillwater River_________________________ 114

Mining possibilities________________________________________________ 115

Composition of the ore mineral-________-___---_---__________-___ 116

Grade of chromite-bearing rock______--_________-___-_---__--_-__ 116

Thickness and continuity of the chromite-bearing layers-___________ 116

Summary ____ _____________________________________________________ 118

Literature cited___________________________________________________ 119

Index____________________________________________________________ 121

ILLUSTKATIONS

[Plates 8-11 are in pocket]

PLATE 8. Geologic map and sections of the central part of the Stillwater complex.

9. Geologic map and sections of the East Boulder Plateau chromite deposits.

10. Geologic map of the east slope of Iron Mountain.

11. Geologic map and sections of the chromite deposits of the West Fork

of the Stillwater River.

Page

FIGURE 17. Index map of the chromite deposits of the Stillwater complex. _ 101
IV CONTENTS

TABLES

Page 

TABLE 1. Chemical analyses of cleaned chromite__ _____________________ 109

2. Partial chemical analyses of samples of cleaned chromite______ 109

3. Surface extent, thickness, and grade of ore in central part of the

Stiliwater complex____________________________________ 116
A CONTRIBUTION TO ECONOMIC GEOLOGY

CHROMITE DEPOSITS IN THE CENTRAL PART 

OF THE STILLWATER COMPLEX, MONTANA

BY A. L. HOWLAND

ABSTRACT

The chromite deposits of the central part of the Stillwater complex lie in a 

belt 9 miles long between the valleys of Boulder River and the West Fork of the 

Stillwater River in Sweet Grass County, Mont. The chromite occurs as layers 

near the middle part of the ultramafic zone in the lower part of the complex. 

The layers, originally horizontal, have been tilted so that they dip northeastwards 

at angles ranging from nearly horizontal to nearly vertical, and are cut by 

many cross faults, the largest with a horizontal offset of 3,000 feet. Investigations by the United States Geological Survey and the United States Bureau of 

Mines have shown that in this belt there are 5 sections ranging in length from 

850 to 3,800 feet along the strike where the continuity and grade of the chromite 

can be reasonably inferred. In these sections the higher grade part (30 to 40 

percent CtaGs) of the main chromite layer averages about 1 foot in thickness. A 

zone of disseminated chromite (0.5 to 30 percent O2O3 ) adjacent to the higher 

grade layer ranges in thickness from a few inches to 8 feet. Analyses indicate 

that the cleaned chromite contains 43 to 48 percent Cr2O3 and has a chromium 

to iron ratio ranging from 1.46:1 to 2.20:1. The geologic factors are unfavorable 

for obtaining any large tonnage of chromite, and the inaccessibility of the area 

and the rigorous winter climate would make development both expensive and 

difficult.

INTRODUCTION

The occurrence of chromite deposits of possible economic interest in 

Stillwater and Sweet Grass Counties, Mont., has been known since 

the time of the First World War. The deposits occur in the Stillwater complex, a belt of layered noritic and ultramafic rocks 1 to 5 

miles wide, that extends for about 27 miles along the northeastern 

margin of the Beartooth Mountains. Chromite occurs as layers near 

the middle of the ultramafic zone throughout most of the length of the 

belt.

In 1939 the United States Geological Survey, in cooperation with 

the United States Bureau of Mines, began a detailed study of the

99
100 CONTRIBUTIONS TO ECONOMIC GEOLOGY

geology of the chromite deposits. Reports dealing with the deposits 

of the eastern part of the complex and with those of the Boulder Eiver 

area have been published (Peoples and Rowland, 1940; Rowland, 

Garrels, and Jones, 1949). The present report describes the results 

of the geologic work in the area between that described in the Boulder 

River report and the West Fork of the Stillwater River.

Between the valleys of the Boulder River and the West Fork of 

the Stillwater River, which are 9 miles apart, the chromite-bearing 

belt crosses a rolling plateau, in part wooded and in part above timberline (fig. 17 and pi. 8). The chromite-bearing rock reaches an altitude of 10,000 feet on East Boulder Plateau between the Boulder and 

the East Boulder Rivers, descends to 8,700 feet at the East Boulder 

River, rises to nearly 10,000 feet again on Iron Mountain, and slopes off 

on the east to 8,700 feet at the edge of the deep valley of the West Fork 

of the Stillwater River. The highest point on the plateau, Chrome 

Mountain, is more than 10,150 feet high. This part of the complex 

is accessible only over steep pack trails from the road in the Boulder 

River valley and from the United States Forest Service road in the 

valley of the West Fork of the Stillwater River, which connects with 

the Mouat mine road. Both junctions of trail and road are nearly 

45 miles from the nearest railroad. The West Fork road ends at an 

altitude of 6,500 feet, and the altitude of the Boulder River road is 

about 5,000 feet. Good grazing is available in the summer, but snow 

can be expected on the plateau from September to June.

Claims were staked over much of this area during the First World 

War, but none were patented, and many were restaked during the 

Second World War. The claims on East Boulder Plateau were held 

in 1944 by Dewey Whittaker and by the Montana Chrome Corporation, which also held all the claims in the Iron Mountain area. The 

Fry-Taylor claims lie in the valley of the West Fork of the Stillwater 

River and extend westward on the plateau about a mile.

In 1940 the east slope of Iron Mountain was mapped geologically 

by A. L. Rowland and J. W. Peoples, assisted by W. R. Jones and 

M. G. Bennett. This map was revised and extended in 1942 by Howland (pi. 10). In 1941 Rowland, assisted by Jones and R. M. Garrels, 

mapped the East Boulder Plateau (pi. 9), and in 1942 Rowland assisted by J. S. Shelton, D. C. Cox, J. L. Weitz, and George Sendon, 

mapped the western side of the valley of the West Fork of the Stillwater River in detail (pi. 11). During 1942 the U. S. Bureau of Mines 

sampled the deposits. The topography of the area is shown on the 

Mount Douglas and Mount Wood 15-minute quadrangles, but these 

maps were not of sufficiently large scale to meet the requirements of 

the chromite work. In 1943, therefore, a topographic map on the scale
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 101

"Briiey"!

EXPLANATION

Stillwater complex 

showing chromite belt I. Gish 

2. Mouat

 

Mines Mills 

A. Mouat 

B. Benbow

Surfaced road Gravel road

3. Benbow 

10 0

FIGDEE 17, Index map of the chromite deposits of the Stillwater complex, Montana.

of 1:12,000 was compiled from aerial photographs by stereophotogrammetric methods for the purposes of the detailed geologic mapping 

(pl.S).

OUTLINE OF GEOLOGY

The igneous Stillwater complex forms the northern part of the preCambrian crystalline core of the Beartooth Range in this area. To the
102 CONTRIBUTIONS TO ECONOMIC GEOLOGY

south of the complex are metamorphosed sedimentary strata, which 

have been intruded by the complex and by granite that is believed to 

be younger than the complex. To the north are Paleozoic and younger 

sediments which rest on the eroded surface of the complex. The complex and the overlying younger sediments have been tilted and in 

most places dip steeply northward. Patches of Cambrian sedimentary 

rocks lying on the rocks of the Stillwater complex have been preserved 

by downf aulting.

The complex trends about N. 60° W. and is a remnant of a once far 

larger mass. Erosion removed the upper layers before Middle Cambrian time and the maximum thickness of about 16,000 feet is found on 

East Boulder Plateau. The complex has been divided for convenience 

into 3 zones. A thin, basal, noritic zone at the south is succeeded upward by a zone of ultramafic rocks containing layers of chromite; and 

that in turn by a banded zone in which norite, anorthosite, and gabbro 

are the dominant rocks.

PRE-CAMBRIAN ROCKS

METAMORPHOSED SEDIMENTS

Underlying the complex is a series of metamorphosed sediments that 

consist chiefly of hornf els but also contain quartzite and iron-bearing 

beds. On East Boulder Plateau this series extends several miles to the 

south. The hornfels is typically a fine-grained rock, dark-gray on 

fresh surfaces and brown where weathered. It is very similar in hand 

specimen to the norite of the basal zone of the Stillwater complex, but 

it can be distinguished from the norite by its granular texture and by 

the absence of laths of feldspar. The quartzite is commonly pale blue 

and so coarse grained that it resembles vein quartz. The iron-bearing 

member consists of alternating layers of quartz, magnetite, and iron 

silicate, from a fraction of an inch to several inches thick.

IGNEOUS BOCKS OF THE STILLWATEB COMPLEX

The rocks of the Stillwater complex in this area are similar to those 

found elsewhere in its three zones: diabasic norite in the basal zone; 

bronzitite, poikilitic harzburgite, granular harzburgite, dunite, basic 

pegmatite, and chromitite in the ultramafic zone; and gabbro, norite, 

and anorthosite in the banded zone. Most of these occur as well-defined layers. Their general character has been described (Peoples 

and Howland, 1*940) and the present discussion is confined to their 

particular features in the area between East Boulder Plateau and the 

West Fork of the Stillwater River.
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 103 

BASAL ZONE

Norite. Fine-grained generally dibasic norite occurs as a layer 50 

to 200 feet thick nearly everywhere along the southern side or base of 

the complex. It consists of plagioclase, bronzite, and a monoclinic 

pyroxene. It closely resembles in texture and grain size the hornfels 

that forms the floor of the complex, and, as already noted, in many 

places the two are hard to distinguish. The norite grades rather rapidly upward into the ultramafic rocks of the overlying zone.

TJI/TKAMAFIC ZONE

The ultramafic zone averages about 4,000 feet in thickness in this 

area and consists chiefly of layers of bronzitite and harzburgite, with 

minor amounts of dunite, basic pegmatite, and chromitite. The 

chromitite will be described separately under the heading Chromite 

 deposits.

Bronzitite. Layers of bronzitite, a rock that consists essentially 

of the orthorhombic pyroxene bronzite, occur at many horizons in the 

ultramafic zone. Most of the bronzitite is medium grained, with crystals averaging a fourth of an inch in diameter, although in some layers 

grain sizes of from 2 to 4 inches occur. Interstitial feldspar and from 

1- to 2-inch poikilitic crystals of a green monoclinic pyroxene are common, the latter especially in the bronzitite above the main chromite 

horizon. In the upper part of the ultramafic zone, the bronzitite contains 10 to 15 percent feldspar. In the area above timberline on East 

Boulder Plateau and Iron Mountain, the outcrops of the bronzitite 

are distinguished from those of harzburgite by being a somewhat 

lighter brown, and by a more distinct jointing and more blocky 

weathering. On the east slope of Iron Mountain, where the streams 

follow the trend of the layering, glaciers formed valleys in the welljointed bronzitite by plucking. On Chrome Mountain the blocks of 

bronzitite resist weathering, remain angular, and show faintly as 

steps on the southern slopes of the south peak.

The thickest layer of bronzitite lies above the main chromite horizon and immediately beneath the norite of the banded zone. This 

layer, which will be called the upper bronzitite, is from 1,000 to 1,500 

feet thick in the central part of the complex, and reaches a thickness 

of 2,800 feet in the vicinity of the Stillwater Eiver.

Poikilitic harzburgite. Harzburgite is a variety of peridotite that 

consists of olivine and bronzite. In the Stillwater complex it commonly has a poikilitic texture, in which grains of bronzite from 1 to 

5 inches in diameter enclose many small grains of olivine; but there 

is considerable variation in the degree to which this texture is devel-
104 CONTRIBUTIONS TO ECONOMIC GEOLOGY

oped and in the relative proportions of the olivine and bronzite. The 

poikilitic texture is most easily recognized where the bronzite crystals are large. Rocks in which the bronzite crystals, though poikilitic, 

are small and poorly developed grade into granular harzburgite, and 

in a number of places the harzburgite is neither typically granular 

nor typically poikilitic. In many places, particularly near the main 

chromite horizon, olivine-rich poikilitic harzburgite grades into the 

olivine rock dunite. Plagioclase is a common accessory mineral and 

may form as much as 15 percent of the rock by volume. Because of 

their greater resistance to weathering, the bronzite crystals stand out 

on weathered surfaces. Weathered surfaces of olivine-rich varieties 

of harzburgite are smooth and vary in color from red brown to grayish 

yellow, in contrast to the nearly black of fresh surfaces.

Chromite is a common accessory mineral in the poikilitic harzburgite. There are all gradations between harzburgite containing scattered grains of chromite and ore consisting chiefly of chromite.

Granular harzburgite. In the granular harzburgite the olivine and 

bronzite grains average about a fourth of an inch in diameter and 

poikilitic texture is not shown. Weathered surfaces of this rock are 

rougher than those of the bronzitite, owing to the rapid weathering 

of the olivine. Faint regular layers from 1 to 6 inches thick, which 

differ chiefly in olivine content, are visible on some smooth glaciated 

surfaces. With a decrease in olivine content, granular harzburgite 

grades into bronzitite. Chromite occurs locally as an accessory mineral in the granular harzburgite, but is not so abundant as in the 

poikilitic variety.

Dunite. Dunite consists essentially of the mineral olivine. This 

rock is much less abundant in the Stillwater complex than harzburgite or bronzitite. It contains scattered grains of bronzite in most 

occurrences, and it grades into both poikilitic and granular harzburgite. Low, rounded outcrops or fine rubble mark its surface exposures. 

Like the olivine-rich poikilitic harzburgite, it weathers to a smooth 

surface, and its color ranges from grayish yellow through greenish 

tan to dark red-brown.

Pegmatite. Coarse pegmatites composed of olivine and pyroxene, 

of feldspar and pyroxene, or of all three minerals are of widespread 

occurrence but small volume. The pyroxene is chiefly bronzite, 

although green monoclinic pyroxene is also present. Most of the 

pegmatite bodies are irregular in shape, but a coarse bronzite-f eldspar 

pegmatite forms a regular layer from 5 to 10 feet thick in the upper 

part of the ultramafic zone on Iron Mountain. Chromite is a common 

accessory mineral of the pegmatites; it characteristically occurs in 

the feldspar, or in the bronzite if feldspar is not present.

Serpentinization. The harzburgite and dunite of the ultramafic 

zone have been partly serpentinized everywhere in the central part of
CHROMITE IN CENTRAL STILLWATEE COMPLEX, MONTANA 105

the complex; the olivine-rich rocks were serpentinized more completely than those poor in olivine. Locally, near some faults, serpentinization is complete. The differences in color of the weathered 

olivine-rich rocks are probably due to differences in degree of serpentinization. The bronzitite is unaltered except near dikes and faults.

BANDED ZONE

Norite, anorthosite, and gabbro form well-defined layers in the 

banded zone overlying the ultramafic zone, but have not been mapped 

in detail. Only the southern or lower part of this zone is shown on the 

accompanying maps (pis. 8 and 9). The actual contact of the ultramafic and banded zones has been seen only in one place, on East Boulder 

Plateau, where it is sharp and clearly igneous, like the layer contacts 

throughout the complex. It is possible, however, that the boundary 

between the two zones may be a fault in some places.

DIKES

Dikes of gabbroic composition from 1 to 50 feet wide cut the ultramafic zone at a number of places. They are gray and fine grained and 

have several well-developed sets of joints which cause them to break 

into small angular fragments. Their presence can be detected in many 

places by the characteristic alteration of the ultramafic rocks that 

border them. The weathered surfaces of the altered harzburgite become a pinkish white to pinkish brown. Bronzitite, which appears to 

alter much less readily than harzburgite, has a purplish tinge under 

these conditions. A f elsite dike similar to Cretaceous intrusions elsewhere in the Beartooth Mountains occurs a mile east of Iron Mountain 

(pi. 8).

GRANITE

Granite intrudes the metamorphosed sedimentary series on both 

sides of the valley of the West Fork of the Stillwater River, and east of 

the West Fork it crops out near the base of the complex. It is believed 

to be younger than the complex. Where the metamorphic series 

extends for several miles to the south of the complex on East Boulder 

Plateau, the hornf els is clearly related to the base of the complex and 

was produced by the thermal metamorphism associated with that 

intrusion. The intrusion of granite into the metamorphosed sediments, which are several thousand feet from the complex, has formed 

granitized schists. Where there are only small remnants of the old 

sediments between the complex and the granite, they are hornf els 

rather than schist, and it is concluded that the thermal metamorphism 

that altered the sedimentary rock to hornf els preceded the intrusion of 

the granite.
106 CONTRIBUTIONS TO ECONOMIC GEOLOGY 

CAMBRIAN ROCKS

Gray shales and thin-bedded limestones of Cambrian age overlie the 

rocks of the banded zone in a belt 4 miles long between the West Fork 

 of the Stillwater River and Iron Mountain. A small area of Cambrian 

limestone has been mapped at the headwaters of Brownlee Creek 

(Howland, Garrels, and Jones, 1949), 500 feet west of the East Boulder 

Plateau map area.

STRUCTURAL FEATURES

PRIMARY STRUCTURE OR LAYERING

Originally the layers of the rocks of the complex were essentially 

horizontal and later were tilted to their present position. Mapping 

on a scale of 500 feet to the inch, such as has been done on East Boulder 

Plateau (pi. 9), demonstrates that many of the thinner layers of 

foronzitite and harzburgite are not continuous throughout the complex, but are long lenses. The major features of the layering, however, 

can be traced throughout the area. The presence of fine-grained norite 

at most places along the base of the complex has already been mentioned. The upper two-fifths of the ultramafic zone is feldspathic 

bronzitite, showing little or no layering, and is in sharp contact with 

the noritic rocks of the banded zone above. Below the thick upper 

bronzitite are alternating layers, from 5 to 200 feet thick, of granular 

harzburgite, bronzitite, and poikilitic harzburgite. Generally the 

boundaries between layers are clearly defined and regular, although 

there are important exceptions. The dunite has irregular boundaries, 

and a few crosscutting dikelike bodies have been observed. The upper 

contacts of some layers of the poikilitic harzburgite are also irregular 

.and project into the overlying granular harzburgite.

TILT

The layers of the complex have been tilted, and they now dip northward at angles that range from nearly horizontal to nearly vertical. 

The prevailing dips are between 40° and 80° to the north and the general strike of the layers is about N. 60° W. Locally the layers are 

overturned steeply to the south and the strike is irregular. Although 

some of these irregularities are related to drag along cross faults, in 

both the East Boulder map area (pis. 8 and 9) and the Iron Mountain 

map area (pis. 8 and 10) the departures from the regional trends are 

more extensive than can be accounted for by the cross faults with which 

they are associated. In the northeastern part of the Iron Mountain 

map area the chromite and associated layers from an arc, concave to the 

west, and strike nearly at right angles to the boundary between the 

ultramafic and banded zones within 1,000 feet of their contact. Smallscale twists and irregularities occur in the chromite. To the east the
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 107

chromite layers conform to the general northwestward trend and are 

offset southward. These relations could be explained by doming and 

cross faulting in the ultramafic zone and truncation by a strike fault. 

There is no evidence for such a fault contact between the ultramafic 

and banded zones, and as an alternative it may be suggested that the 

complicated structures in the ultramafic zone were developed during 

or immediately after the consolidation of the ultramafic rocks, but 

before the crystallization of the banded zone. The complicated 

structures in the East Boulder map area may have a similar explanation. The low dips and irregular topography in these areas are additional factors in causing the irregular surface pattern of the layers.

FAULTS

The faults of the area are of two kinds: strike faults, roughly 

parallel to the strike of the layering in the complex; and cross faults, 

mostly striking north to northeast. Only a few strike faults are 

known, but there may be others that have not been identified because 

of the difficulty of recognizing them.

The Iron Creek fault is a large strike fault, which has been traced 

for 4 miles from near the mouth of Iron Creek westward to the saddle 

north of Iron Mountain (pi. 8). The Iron Creek fault is a thrust 

dipping northward about 50°, and the rocks of the banded zone have 

been thrust up against Cambrian sediments on the south side of the 

fault. The dip of the layers in the overthrust block is 15° to 20° 

steeper than on the south side of the fault, suggesting rotation during 

the thrusting. The Brownlee Creek fault on the west side of the 

East Boulder Plateau (p. 8) shows a similar relation of fault and 

Cambrian sediments, and is believed to be a continuation of the Iron 

Creek fault, although the faults have not been traced across the valley 

of the East Boulder River.

The Bluebird fault, a southward-dipping thrust, is well exposed on 

the east side of the valley of the West Fork of the Stillwater River, 

where for some distance the ultramafic zone is thrust over the banded 

zone. Its westward extension lies along or south of the West Fork 

valley and will not be discussed in detail in this report.

A strike fault, apparently with downthrow on the north side, extends eastward from the north end of the north peak of Chrome Mountain on the west side of the East Boulder Plateau map area and is 

marked by a strong shear zone (pis. 8 and 9). South of this fault the 

dips are low; north of it they are steep, in places vertical. A diabase 

dike in the saddle of Chrome Mountain marks another fault, which 

terminates against a cross fault along the east side of the mountain.

There are many cross faults, generally of steep dip. Detailed 

mapping on the East Boulder Plateau, on Iron Mountain, and in the 

West Fork valley (pis. 9,10, and 11) has shown the presence of many
108 CONTRIBUTIONS TO ECONOMIC GEOLOGY

small cross faults displacing the chromite layers for distances of a 

few tens to several hundreds of feet. Cross faults on a much larger 

scale are readily recognized by displacements of the base of the complex (pi. 8). Not all seem to displace the contact between the 

ultramafic and banded zones, and the possibility that this contact is a 

fault in some places has been considered. No evidence for such a relation has been found west of the West Fork of the Stillwater Kiver, 

however, and it seems more likely that the cross faults, which are steep, 

have vertical or nearly vertical displacements, producing larger effects 

in the gently to moderately dipping base of the complex than in the 

steeper upper contact of the ultramafic zone. Generally the offset on 

the larger cross faults is to the right, although there are a few 

exceptions.

A major change in the structures of the Stillwater complex is approximately marked by the valley of the West Fork. To the east of 

the valley there are large southward dipping thrusts which have no 

counterpart in the central section of the complex. That part of the 

complex lying east of the West Fork will be discussed in a separate 

report.

CHROMITE DEPOSITS 

MINERALOGY AND PETROGRAPHY

The chromite of this area contains from 42 to 48 percent of Cr2O3 

and has a chromium to iron ratio ranging between 1.46:1 and 2.20:1. 

Analyses of chromite from this part of the complex are shown in tables 

1 and 2. On the basis of the chemical analyses, the formulas of four 

samples have been calculated.

H-354 (Cr57AlS5Fe8 ) 2O3 . (Mg45Fe55 ) O 

H-306a (O58Al36Fe6 ) 2O3 . (Mg51Fe49 ) O 

P-387 (Cr61Al36Fe3 ) 2 03 .(Mg50Fe50 )0 

H-421 (Cr52Al39Fe9 ) 2O3 . (Mg51Fe49 ) O

The composition does not differ greatly from that of the chromite 

elsewhere in the complex, in most of which the O2O3 content is 

between 40 and 49 percent, and the chromium to iron ratio is between 

1.2:1 and 2.0:11. As compared with other analysed specimens of Stillwater chromite these four samples show low to intermediate Fe2O3 , 

high A12O3 , and intermediate to high Cr2O3 content. There is a wide 

range in magnetic susceptibility, the explanation for which is not 

known.

The composition of the pure chromite is important because it shows 

the highest Cr,O3 content attainable by mechanical concentration of 

the ore. Any concentrates produced by mechanical means would be 

low-grade by current metallurgical standards, which require 48 percent Cr2O3 content and a chromium to iron ratio of at least 3:1 to be of 

commercial grade.
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 109 

TABLE 1. Chemical analyses of samples of cleaned chromite

CrsOa-- - - - -- --

AltOt......... .............. .. ...............

fe3Ot.... ...... . ..... .. ............

FeO...... ......... .............................................

MgO - - - - - 

MnO... 

NiO... . 

CaO.__ ... 

Ti02 . VtO9 ........... .......................................... ......

-ptOt....... ....................................................

S .. _ . _____ _

SiOa 

H2O.-K---- 

^tO........... ...................................... ..........

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

Spgr. 

 Cr/Fe 

CnOs in raw sample- _ _ .- ___ __ - _ _ -. percent-.

H-354

43.84

18. 36

16.64

i 20. 46

9.38

.21

.08

.79

.24

.02

100.02

4.35

8.34

30.00

20.55

1.46

32.94 

75

H-306a

45.62

19.16

15.01

i 18. 62

10.75

.18

.02

.44

.20

.06

100.06

4.46

121.94

31.22

17.99

1.74

42.50 

93

P-387

47 79

19.11

12.30

i 18. 86

11.21

.15

.02

.62

.80

.12

OR

100.99

4.43

27.80

32.65

16.27

2.00

42.94 

90

H-421

41.55

20 92

1 7 09

'18.47

11.08

.21

.10

49

06

^one

29

.24

100.50

21.85

28.42

1Q ^Ift

1.47

17.00

11.6

12.7

no

i Calculated from total iron on the assumption that RO:R20a=l:l.

H-354. 2-foot layer, "A" zone, Riverview B claim, southwest slope of Chrome Mountain. Montana 

(south) coordinates 526,750 N., 1,831,900 E. R. E. Stevens and M. K. Carron, analysts.

H-306a. Top of 1-foot layer, "G" zone, Champion no. 3 claim, East Boulder Plateau. Strongly magnetic. 

Montana (south) coordinates 527,000 N., 1,838,900 E. R. E. Stevens and M. K. Carron, analysts.

P-387. 8- to 12-inch layer, "O" zone, Chrome C claim, between Iron Mountain and West Fork of Stillwater River. Montana (south) coordinates 509,900 N., 1,863,800 E. M. K. Carron, analyst.

H-421. 2-inch layer, 350-400 feet stratigraphically above "Q" zone, West Fork of Stillwater River. Montana (south) coordinates 504,550 N., 1,870,900 E. R. E. Stevens, analyst.

TABLE 2. Partial chemical analyses of samples of cleaned chromite

H-306c P-284 P-278 P-281 P-282 P-321

Concentrate

Cr _ - _- . - --....percent-. 

Fe ___ -- _ - ____ ------ _ ..do .

Cr/Fe 

Sp. gr... .... ________________

32.09 

17.22

4.46

55.61

32.23 

16.95

1 90

4.44

31.62 

14.36

2.20

9.53

30.05 

17.63

1.70

4.41

31.48 

19.37

10.33

31.06 

16.75

1.85

45.39

Raw sample

41.73

89 88

07 09

81

41.00

93 80

35.07

77

H-30tiC. Bottom of 1-foot layer, "G" zone, Champion no. 3 claim, East Boulder Plateau. Montana (south)

coordinates 527,000 N., 1,838,900 E. 

P-284. 1-foot layer, "G" zone, Apex 5 claim, Chrome Mountain. Montana (south) coordinates 527,750

N., 1,835,000 E. Magnetic in hand specimen. 

P-278. Near middle of 2.3-iooC layer, "G' zone, discovery pit, Sunshine claim, West Fork of Stillwater

Kiver. Montana (south) coordinates 504,200 N., 1,871,700 E. 

P-281. 1-foot layer, "G" zone, about 120 feet southwest of P-27S. Montana (south) coordinates 504,150

N., 1,871,580 E. 

P-282. 1-foot layer, "G" zone, discovery pit, Ground Hog claim, West Fork of Stillwater River. Montana

(south; coordinates ,'104,500 N., 1,872,200 E. 

P-231. 2-iiich layer, 275 feet siratigraphically above "G" zone, West Fork of Stillwater River. Montana

(south; coordinates 605,200 N., 1,809,700 E. Grains are magnetic.
110 CONTRIBUTIONS TO ECONOMIC GEOLOGY

The chromite-bearing layers have a wide range in chromite content, 

and all gradations from a few percent to 90 percent of chromite 

have been observed. Where the rocks are unaltered, the gangue minerals are olivine and bronzite, and, in places, feldspar. The chromite 

characteristically occurs in the bronzite surrounding the olivine grains,, 

but also in the feldspar where that mineral is present. Rounded 

olivine grains, surrounded by bronzite crowded with chromite grains, 

give a strikingly spotted texture to some of the disseminated ore. 

Alteration of the silicates is widespread, however, and much of the 

matrix of the chromite is serpentine.

The chromite has a granular texture and a grain size ranging from 

0.005 to 0.1 inch (100 to 4 mesh) ; the coarser sizes are rare and 

restricted to small patches in the pegmatites.

The tops of the chromite layers are in most places distinguishable 

by the presence of a disseminated zone above the massive chromite 

layer. The thickness of the disseminated chromite zone may be several times that of the massive layer beneath it. Pegmatite sheets, 

irregular in thickness and extent, occur in many places on the lower 

sides of the chromite layers, and less commonly on the upper sides.

The chromite layers parallel the layered structure of other rocks 

of the ultramafic zone. Although some of the chromite layers are 

only an inch thick and their characteristics are not everywhere the 

same, they can be traced with remarkable continuity in some places 

for distances of several thousand feet, maintaining approximately 

the same position in the ultramafic zone. Thus the thin layers, though 

unimportant in themselves, are useful as markers to determine the 

concealed position of the more important known layers.

Only two chromite layers are of possible economic significance in 

the East Boulder Plateau-Iron Mountain area. One of these, which 

occurs somewhat below the middle of the ultramafic zone, is the more 

persistent and usually the thicker layer. It will be called the "G" 

zone or layer, the name originally applied to the main chromite layer 

of the Stillwater River area, of which this layer is the probable correlative. The other, which is from 300 to 500 feet above the base of 

the zone, will be called the "A" zone. Three or four layers, from 

1 to 3 inches thick, occur between the "main" chromite layer and the 

thick bronzitite layer that makes up the upper part of the ultramafic 

zone, and in the West Fork area a persistent 1-inch layer occurs 

between the "A" and "G" zones.

DESCRIPTION OF DEPOSITS

EAST BOULDER PLATEAU AREA

An area of about 2 square miles on the East Boulder Plateau has 

been mapped on a scale of 500 feet to the inch (pi. 9). Included in
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA HI

the discussion of this area, however, are outcrops of the "A" zone layer 

as much as half a mile west of the map area.

Little is know of the chromite in the timbered country dot ween 

the valley of the Blakely Creek to the west (Rowland, Garrels, and 

Jones, 1949) and the East Boulder Plateau. The westernmost outcrops of the "G" zone on the plateau lie on the western slope of the 

north peak of Chrome Mountain (9,963 feet). The northward dip 

is only 20°, and the chromite outcrop swings to the south around the 

base of the peak. On the southeast side of the peak, the chromite 

layer is broken and interrupted by what is regarded as a deformation 

before complete consolidation. The chromite is believed to persist 

to the cross fault on the east side of the peak, but it probably is discontinuous. The lower part of the "G" zone here consists of massive 

ore averaging 1 foot in thickness, which is overlain by a foot-or-so 

of lean disseminated ore. Analysis of sample P-284 (see table 2), 

taken from the massive ore, shows 88 percent of chromite. The disseminated ore contains from 10 to 15 percent chromite. Two thin 

layers, 1 and 3 inches thick and about a foot apart, occur about 500 

feet stratigraphically above the "G" zone and pass through the 

summit of the north peak, but owing to extensive alteration of the 

containing harzburgite they cannot be traced for more than a few feet. 

South of the strike fault, which is believed to be downthrown on 

the north side (see pi. 9, cross section A-A'}, the "G" zone is shown 

as extending in circuitous outcrop on the western and southern sides 

of the south peak of Chrome Mountain. Its position can be determined within narrow limits from the position of the layer of poikilitic 

harzburgite in which it occurs and from a few fragments of chromite 

float on the west side of the peak, but nothing is known of its thickness 

or grade. The only place where it crops out west of the cross fault 

is on the southern side of the south peak where it consists of three 

1-inch layers in a zone about 6 feet thick, the rock between the layers 

containing a small amount of disseminated chromite.

From the cross fault on the south peak of Chrome Mountain, the 

"G" zone chromite can be followed northeastward with fair continuity 

to the eastward extension of the strike fault described above. In 

the basin at the headwaters of Brownlee Creek the chromite is broken 

by several small faults and has an irregular outcrop because of low 

dips and the presence of a shallow syncline in the basin. On the east 

slope of the south peak the chromite zone consists of two or three 

1-inch layers, but in the bottom of the basin the zone ranges from 3 to 8 

feet in thickness and contains, on the average, about 20 percent of 

chromite. The massive layer at the bottom is 6 inches to a foot thick 

and is overlain by banded disseminated material. Comparison of the 

analyses of the two samples H-306a and H-306c (tables 1 and 2) shows

315410 -55 2
112 CONTRIBUTIONS TO ECONOMIC GEOLOGY

a slight decrease in chromium and an increase in iron in the chromite 

in the upper part of the layer. Sample H-306c, although collected 

at the bottom of the layer, contains 4 percent less chromium than 

sample H-306a, which was taken from the top of the layer. The 

increase of iron in the chromite of the upper part of the layer does 

not seem to be great enough to account for the much stronger magnetic 

property of this part, and information concerning the proportions of 

ferric and ferrous iron is not available.

Between the headwaters of Brownlee Creek and the east edge of 

the East Boulder Plateau (pi. 9) the structure is obscured % overburden and complicated by faults. In the southern part of the 

mapped area east of the trail, the structure appears to be complicated 

by intrusions of dunite too small to be placed on the map. There 

the massive layer of chromite is about 3 inches thick where exposed.

The "A"' zone chromite is not exposed in the area shown in plate 9, 

but crops out and has been prospected west and southwest of the 

limits of the map. On the north side of the valley of Bobcat Creek 

about half a mile west of the mapped area (Howland, Garrels, and 

Jones, 1949, pi. 34), a layer 2 to 3 feet thick is exposed more or less 

continuously for 500 feet along the strike. This layer is also found, 

though poorly exposed, on the talus slope southwest of the south peak 

of Chrome Mountain (pi. 8, 525,800 N., 1,833,500 E.), but there it is 

only from 3 to 4 inches thick. Wherever the "A" zone has been 

observed in the complex, it has proved to be irregular and of uneven 

thickness, but its average thickness is probably 3 or 4 inches. The 

chromite of the "A" zone has also a lower Cr2O3 content and a smaller 

chromium to iron ratio than that of the "G" zone. Analysis of sample 

H-354 (table 1), collected from the "A" zone, indicates a Cr2O3 content 

of 43.84 percent and a chromium to iron ratio of 1.46:1.

EAST BOULDER PLATEAU TO EAST BOULDER RIVER

Between the East Boulder Plateau map area and the East Boulder 

River, the '"G" zone crosses timbered country. Chromite crops out 

at a number of places and has been proved for about 1,000 feet along 

the strike on the Chrome Queen no. 9 claim, about a mile west of the 

East Boulder River (pi. 8, 521,300 N., 1,845,000 E.). (Locations of 

many of the claims in this area are shown by Wimmler, 1948.) It 

is not certain that all the outcrops represent the same layer. The 

thickness of the massive "G" layer ranges from 4 to 15 inches; the 

disseminated zone is not always present, but in places it is as much 

as 5 feet thick and contains from 10 to 20 percent of chromite. The 

"A" zone has been found by prospecting in two places, one close to 

the trail in the flat on the west side of East Boulder River (pi. 8, 

519,000 N., 1,849,000 E.), and the other more than a mile to the west
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 113

(521,850 N"., 1,843,600 E.), north of the Montana Chrome Corp. 

cabin. Though not clearly exposed at the time of examination, the 

chromite zone at these two localities apparently is about 2 feet thick 

and dips from 15° to 20° northward. Disseminated chromite is scattered through dunite over an area of 2,000 square feet or more on the 

Bonanza King claim, which is on the east side of the trail and 1,000 

feet southeast of the East Boulder Plateau map area (pi. 8, 533,900 N., 

1,842,800 E.). This is the approximate horizon of the "A" zone and 

illustrates its irregular character.

IRON MOUNTAIN AREA

Between the East Boulder River and the crest of Iron Mountain no 

chromite has been found. Timber covers the lower part of the valley 

that heads on the west flank of Iron Mountain, and above timberline 

there are few outcrops in the chromite-bearing part of the ultramafic 

zone. A cross fault about 1,600 feet east of the river, marked by a 

strong shear zone and a 3- to 4-foot quartz vein, displaces the base of 

the complex more than o.OOO feet southward on the east side. This is 

the largest displacement along this type of fault that has been noted 

anywhere in the complex.

The first exposures of chromite east of the river are in the basin on 

the east side of Iron Mountain. Good exposures below an altitude of 

9.500 feet show that the "G" zone strikes from north to east and dips 

from 20° to 45° westward and northward. The trend of the outcrops 

forms a large arc in the basin, but the chromite is unknown beyond the 

area of small cross faults in the northeast part of the Iron Mountain 

map area (pi. 10). A major cross fault apparently displaces the 

chromite to the right. The chromite layer on the Chrome F and 

Chrome H claims, on the ridge south of Iron Mountain basin (pi. 8, 

513,000 N., 1,860,150 E.) may be the continuation of the "G" zone 

beyond the fault, but it has the characteristics of the "A" zone, which is 

exposed on the Chrome D claim at triangulation station 9,344 (510,000 

N., 1,862,750 E.), nearly a mile to the east.

The "G" zone in the northeast part of the Iron Mountain map area 

consists of a layer of massive ore 0.6 to 1.2 feet thick, and a 1.5- to 

3-foot zone of disseminated chromite above it. Southwestward, 

however, it becomes a faintly layered, disseminated zone, which has a 

maximum thickness of 120 feet and is estimated to average from 6 to 8 

percent of chromite.

IRON MOUNTAIN TO THE WEST FORK OF STILLWATER RIVER

The "G" zone is exposed at a number of places between Iron Mountain and the edge of the plateau on the north side of the valley of the 

West Fork. On the Chrome group of claims about a mile east of the
114 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Iron Mountain map area (pi. 8, 510,000 N., 1,862,700 E. to 508,700 N., 

1,864,800 E.) prospecting has revealed two segments of the "G" zone 

totaling about 400 feet in a distance of 1,700 feet along the strike. A 

massive layer 0.8 to 1.0 feet thick is overlain by 1.0 to 3.5 feet of disseminated material containing from 5 to 15 percent of chromite. The 

analyzed chromite from the Chrome C claim (P-387, table 1) is 

strongly magnetic. The dip is 65° to 75° N. Several small outcrops 

show the "A" zone to be 1 to 2 inches thick on the east slope of Iron 

Mountain. On the Chrome D claim (pi. 8, 510,000 N., 1,862,700 E.). 

however, it ranges from 0.5 to 1.0 feet in thickness and is almost entirely 

massive, with little disseminated chromite. The chromite layer of 

the Chrome H and F claims on the ridge south of Iron Mountain basin 

(pi. 8, 513,000 N., 1,860,150 E.), 0.8 to 0.9 feet thick, probably is also a 

part of the "A" zone.

On the Fry-Taylor claims on the north side of Crescent Creek valley 

(pi. 8, 507,800 N., 1,865,500 E. to 506,300 N., 1,867,700 E.) the "G" 

/one is intermittently exposed for a distance of 2,700 feet along the 

strike. The massive chromite layer ranges from 0.5 to 1.7 feet, but 

does not average more than 1 foot in thickness. The disseminated 

chromite zone above the massive layer ranges from 0.7 to 2.8 feet in 

thickness and disseminations also occur beneath the massive layer. In 

places the dip of the chromite layer is irregular, but in general it dips 

50° to 60° northward. There is a distance of about 1,500 feet between 

the most easterly exposures on the plateau and the outcrops on the 

north slope of the valley of the West Fork of the Stillwater River.

WEST FORK OF THE STILLWATER RIVER

On the north slope of the West Fork of the Stillwater River, the 

"A" and "G" zones, as well as a number of thin layers at other horizons, 

have been mapped in detail (pi. 11) for a distance of 3,800 feet along 

the strike, and at altitudes ranging from 7,100 feet at the eastern end 

t o 8,600 feet near the edge of the plateau on the west.

Exposures are abundant in this area, and the relationships of the 

chromite and other layers for a stratigraphic distance of about 1,500 

feet are clear. Stratigraphic sections at the position of the cross sections on plate 11 are shown in plate 8. Here, as on the East Boulder 

Plateau, it can be seen that although some of the layers are actually 

lenses, the major features of the layering are remarkably continuous. 

Examples are the occurrence of both the "A" and "G" zones within 

continuous layers of poikilitic harzburgite a relationship true 

throughout the complex and a 1-inch layer of chromite approximately midway between the "A" and "G" zones. Everywhere it has 

been found this midway layer marks the boundary between bronzitite 

and an overlying layer of poikilitic harzburgite. Some variations in
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 115

the sequence of rock from one section to another are less real than 

apparent, for it is difficult to place the boundary between bronzitite 

and granular harzburgite; the relationship is in places a gradational 

one and the determination in the field is not based on accurate quantitative data concerning the relative proportions of olivine and bronzite. The thin layers of chromite both above and below the "G" zone 

make possible the approximate location of the main chromite even 

though it is not continuously exposed.

Cross faults which offset the layers from a few to 200 feet occur 

throughout the West Fork area, the longest section of unfaulted 

chromite being a little more than 500 feet. Cross faulting is, indeed, 

an important feature of every area in the complex that has been 

mapped in detail. The little evidence that is available in the shear 

zones and closely spaced joints near faults, indicates that most of the 

cross faults are steep.

The "A" zone has been followed for 1,900 feet along the strike to 

where it disappears beneath talus at the lower end. The containing 

rock is olivine-rich harzburgite, which usually has poorly developed 

poikilitic texture. The chromite is irregular, ranging from an inch 

to more than 3 feet in thickness; no consistent stratigraphic sequence 

of chromite layers and lenses exists. In one place chromite is disseminated through a stratigraphic thickness of more than 25 feet with 

some layers of massive chromite occurring within the zone. Elsewhere a 1-foot "layer-' is known to be a lens only 6 feet long at the 

surface, and several layers shown on the map are only 1 or 2 inches 

thick.

In contrast, the U G" zone is remarkably uniform throughout the 

mapped area. It consists of from 0.5 to 2 feet of massive chromite 

overlain by from 0.5 to 2 feet of disseminated chromite. In a few 

places the main layer is approximately half silicate and at others 

there is a foot or so of disseminated chromite below the main layer. 

Nevertheless, the massive chromite averages 0.9 foot in thickness 

throughout, and the zone as a whole, including both massive and disseminated chromite, averages 1.8 feet.

The layer between the "A" and "G" zones and those above it average 

only an inch in thickness and have significance only as horizon 

markers.

MINING POSSIBILITIES

The possibilities of mining the chromite deposits of the central part 

of the Still water complex depend on: (1) the composition of the 

chromite mineral, (2) the grade of the chromite-bearing rock. (3) 

and the thickness and continuity of the chromite-bearing la}T ers.
116 CONTRIBUTIONS TO ECONOMIC GEOLOGY

COMPOSITION OF THE QBE MINERAL

The Cr2O8 content of the pure mineral ranges from 43 to 48 percent 

and probably averages close to 4(> percent. The chromium to iron 

ratio ranges from 1.46:1 to 2.00: 1 and averages about 1.75:1. This 

composition is similar to that of the chromite that has been mined in 

other parts of the complex.

GRADE OF CHROMITE-BEARING BOCK

The grade of the chromite-bearing layers is also similar to that in 

the layers at the Gish, Mouat, and Benbow mines. So-called massive 

chromite contains between TO and 94 percent of chromite (approximately 30 to 42 percent of Cr2O3 ), and could possibly be handsorted 

to give a material containing 35 percent Cr2O3 and having a chromium 

to iron ratio of 1:1.2. Disseminated material ranges from 1 to 70 

percent of chromite (0.5 to 30 percent of Cr2O3 ) and would have to be 

concentrated. Channel assays across the chromite, most of them made 

on the "G" zone, have been reported by Wimmler (1948). Experience 

with the milling of the Stillwater chromite ores shows that even where 

the olivine and bronzite gangue minerals are serpentized, as they are in 

this area, and break away easily from the chromite, it is difficult to 

produce concentrates consistently containing more than 40 percent of 

Cr2O3 . A high-grade metallurgical ore cannot be obtained, therefore^ 

by even the best mechanical concentration.

THICKNESS AND CONTINUITY OF THE CHROMITE-BEARING

LAYERS

The value of any particular layer of chromite as ore depends upon 

its thickness as well as upon its grade. The thickness of the massive 

part of the "G" zone does not exceed 2.5 feet and does not average 

more than 1.2 feet even in selected areas. The layer of disseminated 

ore above the massive ore ranges in thickness from a few inches to 8 

feet and contains an average of 10 to 15 percent of chromite. The most 

favorable parts of the area for future mining, as indicated by both 

thickness and grade, are listed in table 3. Because the "A" zone is 

thought to have no commercial value, the "G" zone is alone considered.

TABLE 3. Surface extent, thickness, and grade of ore in the central part of the

Stillwater complex

Locality

West Fork-. . ....... __ _______ _ ___________ ________________

Length 

(feet)

850

2,000

1,700

2,700

3,800

Average 

thickness 

(feet)

3

5

3

2.2

1.8

Percent 

CrzOs

15

15

15-20

15

15
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 117

On the southwestern and southern sides of the north peak of Chrome 

Mountain, chromite crops out for a distance of about 1,000 feet along 

the strike of the "G" zone. Its extension to the west is unknown, 

but the eastern end appears to be interrupted and where it is next 

seen there are only two or three 1-inch layers of chromite. The 

massive layer averages 1 foot in thickness, and the disseminated layer 

above it, only 1 to 2 feet; the grade across the total 3-foot thickness 

is believed to be less than 15 percent of Cr2O3 . The outcrop curves 

around the peak, in part because of the low dip of the layer (20° N.) 

and the topography. Therefore, although the distance along the outcrop is 1,000 feet, a measurement parallel to the strike would be only 

about 850 feet. The depth down the dip presumably is comparable 

to the true strike length of the layer.

In the basin at the headwaters of the south fork of Brownlee Creek, 

east of Chrome Mountain, the "G" zone is exposed for about 1,500 

feet along the strike. The massive layer is from 0.7 to 0.9 foot thick, 

and there is a wide disseminated zone that averages 5 feet in thickness 

and reaches a maximum of 8 feet on the Champion no. 3 claim. To 

the southwest this zone is represented only by two or three 1-inch 

layers. To the north it is interrupted by a strike fault, and only thin 

layers and disseminations are seen in the eastern part of the East 

Boulder Plateau map area. Although at this place the chromite layer 

is thicker than anywhere else in the East Boulder Plateau-Iron Mountain area, an attempt to estimate tonnage would mean little as the 

chromite lies in a shallow syncline and its extension down the dip is 

buried on the downthrown north side of the strike fault (pi. 9, cross 

section C-O').

The part of the "main" layer between East Boulder Plateau and 

East Boulder Eiver has not been mapped in detail, but reconnaissance 

mapping (pi. 8) has indicated that it probably is continuous for several 

thousand feet. At least two important cross faults, marked at the 

surface by long open clearings in the timber, cut the ultramafic zone, 

and several minor cross faults are visible. The few exposures east 

of the eastern fault show only from 3 to 4 inches of massive chromite; 

to the west of the western fault the disseminated layer is either missing or is from 60 to 100 feet above the massive layer. The block 

between the two faults is about 2,000 feet long, and has a thickness of 

0.5 to 1.9 feet of massive chromite and 3 to 5.8 feet of disseminated, 

which together might be equivalent to about 5 feet of ore averaging 

15 percent Cr2O3 . The dip of the chromite ranges from TO 0 southward 

to 65° northward.

The chromite exposure in the basin on the east side of Iron Mountain 

is about 1,200 feet long. The massive layer is 0.6 to 1.2 feet thick,
118 CONTRIBUTIONS TO ECONOMIC GEOLOGY

the disseminated 0.9 to 3.5 feet. To the west the massive layer changes 

to a wide zone of low-grade disseminated chromite (2 to 5 percent 

Cr2O3 ), and to the east it is cut by many small cross faults.

On the Chrome A, B, and C claims, about a mile east of Iron Mountain, chromite is exposed sporadically for a distance of 1,700 feet, but 

the aggregate length of the exposures is only about 400 feet. The 

layer appears to be consistent wherever exposed, however, with an 

average thickness of 0.9 foot of massive ore and a little qver 2 feet 

of disseminated, for an average O2O3 content of 15 to 20 percent. 

The layer dips from 65° to 75° northward. Nothing is known of its 

continuation to the west between the Chrome C claim and the Iron 

Mountain basin, but it apparently continues eastward onto the FryTaylor claims. There is probably a cross fault just west of the 

Chrome A discovery pit (508,800 N., 1,864,800 E.).

The section of the Fry-Taylor claims on the north side of Crescent 

Creek (Vega, Orion, New Deal, Venus, and Jupiter claims) shows 

interrupted exposures of the "G" zone averaging 2.2 feet thick, of 

which a little less than 1 foot is massive chromite, for a distance of 

2,700 feet. The dip of the "G" zone is 50°-60° northward.

The section of the Fry-Taylor claims on the north slope of the valley 

of West Fork of the Still water River contains the longest and best 

known exposure of chromite in the central part of the complex. It is, 

furthermore, the most accessible part of the area. The eastern end of 

the outcrops is low on the slope of the valley, about 350 feet above the 

river. The U. S. Forest Service road along the West Fork reaches 

to within 2 miles of the lower end of the chromite. The "G" zone is 

believed to average about 1.8 feet thick, about 15 percent Cr2O3 , and 

to be continuous for 3,800 feet along the strike between the altitudes 

of 7,100 and 8,600 feet. The many cross faults, which displace the 

chromite in this section, might be expected to cause difficulties and 

substantial losses of ore in mining.

SUMMARY

Although all the central part of the Stillwater complex has not 

been mapped in detail, the mapping of critical areas and careful reconnaissance of the remainder justifies a preliminary statement concerning its chromite reserves. The thin chromite layers are unfavorable 

for the production of a large tonnage of ore. Mining would be expensive and complicated, because of the presence of many faults and 

the occurrence of the chromite in gently to moderately dipping layers, 

and because of the inaccessibility of the plateau areas and the rigorous 

climate. The Fry-Taylor claims in the valley of the West Fork of the 

Stillwater River are the most favorable area.
CHROMITE IN CENTRAL STILLWATER COMPLEX, MONTANA 119

LITERATURE CITED

Rowland, A. L., Garrels, E. [R.] M., and Jones, W. R., 1949, Chroniite deposits of 

Boulder River area, Sweet Grass County, Montana: U. S. Geol. Survey Bull. 

948-C.

Peoples, J. W., and Rowland, A. L., 1940, Chroniite deposits of the eastern part 

of the Stillwater complex, Stillwater County, Montana: U. S. Geol. Survey 

Bull. 922-N.

Thayer, T. P., 1946, Preliminary chemical correlation of chromite with the containing rocks: Econ. Geology, v. 41, p. 202-217.

Wimmler, N. L., 1948, Investigation of chromite deposits of the Stillwater complex, Stillwater and Sweet Grass Counties, Montana: U. S. Bur. Mines Rpt. 

Inv. 436B.

INDEX

Page

A zone-______.--___-___--_-_--_-___ 110,112,114,115

Abstract _-- ----_------ -- 89

Accessibility- -_---_----__--_----___---_-__- 10o

Age rr]ationsbij>s____--.--____-.___________ 10,5

Banded zone ________________________ 105,106,107

Bluebird fault.----.-.--_-______---_-____-_-_- 107

Bronzite .______--_-_---------------__ 103,104,110

Bronzitite- ___-.---_ 103,105,106,114,115 

Brownlee Crook basin _____________________ 117

Brownlee Creek fault___-_________-___________ 107

Cambrian rocks.. ______---._____________ 102,106

Chemical analyses.._____-_._-....._..__.__.__ 109

formulas ______-____-_-__---____.__-__ IDS

Chrome Mountain.__________ 100,107,111,112,117

Chromite, disseminated_-_______ -__.__-..___ 110,

111, 113,114,115,116,117,118 

massive.-,.-- -------------_-_ --_ 110,

111, 112,113,114,115,116.117,118 

mechanical concentration_________________ 116

in pegmatite-... __________________________ 104

occurrence in complex.__________________ 99.110

Chromite assays..._.-.____._____-. 116 

Chromite content__-_______ ___--_ 108,109,110, 111

Chromite grains, size and occurrence_______ 104,110

Chromito layers _._ _. ___ 106,107-108,110.111,115 

displacement by cross faults____________ 108. V*

tops______________________________________ 110

Chromium to iron ratio ____________________ 108,112

Claims ___ _ __ _____ 100.102.112.113-114.118

Comparison of samples, central part of complex._----__ ------_-- ______ 111-112

to those from other parts of the complex. _ 108 

Complicated structures--___________ 106-107

Contacts and boundaries___________ 105,106,108,114

Content, CrjOs --_____-_--__--_ 108.112,116,117.118

Cross faults _______ 106,107-108,111,113,115,117,118

Differences, bronzitite and harzburgite_ ______ 103

hornfels and iiorite_-_-______---__________ 102

Dikes_-___.___-__- _-__-_---___-______--- 105,107

Dips ______________ 106,107,111,11:1,114,117,118

Displacement of base of cotnplex_ __ _. 108,113 

Dunite_ ________________________________ 104,106,113

Earlier reports.. ________-__----_____----_____ 100

East Boulder Plateau _________ 100,102,110-112,117

Factors influencing mining the deposits ______ 115

Feldspar_____________________________ _____ 103,110

Felsite.. _____-_---_____ 105

Fry-Taylor cla'ms _-__,-______.,-______-__ 114,118

G zone___________-_ 109,110, 111, 113-114,115,116,117

Gabbro_____._-__- ____._- _____ 105

Gangup minerals.-- _____ _ _ . , ________ llfi, llfi

Geography. _____ ____________________________ 100

Page 

Granite __ __ _ _ ___._.-.____.. 105

Harzburgite, granular____-. _ ___----___-_____ 104,115

poikilitic.__________________ 103-104,106,114,115

Hornfels ________________________________ 102.105

Igneous rocks ____________________________ 102-117

Iron-bearing member, metamorphosed sedimentary rocks____-_-___-__--.____ 102

Iron content. _-_--_ -_-__-_---__-___-__-__ 112 

Iron Creekfault- __-____-_-___-- _____-__-_ 107

Iron Mountain__--__-____-_____________ 100,104,113

Iron Mountain map area___________________ 106,113

Location _ 100

Magnetic susceptibility_______________ 108,109, 111

Main chromitelayer-----___--__--__---__-_- 104,110

Mapping.-_--__--___._-_--____------_-__-__ 100-101

Metamorphism, thermal_____-__-____________ 105

Metamorphosed sediments -_-_-_-____---__ 102,105

Mining of the chromite, difficulties.___-___-__ 118

Norite __---______---.--__-____-_-_________- 102,103

O]ivine__-_--__---_____________--_---___ 103,104,110

One-inch persistent layer of chromite_______ 110,114

Pegroatite___-__-_______-_____---_-____---_- 104,110

Poikiimccrystals-___--_--_-_--__--_______-- 103,110

Pyro.\ene_--__---------------- ------------ 103,104

Schists, granitized----________________._______ 105

Serpentinization______________-_____ 104-105,110,114

Stillwater complex____.________________ 99,101-102

change in structures.___________..-_-_____ 108

erosion of-----------.--- ---------- 102

layering,..-__--_____----____.------ 106

lenslike nature of thin layers___-_._-- 106 

primary structure-_______- ___-____-_ 106

zoning______-----__---_-_--------__---_-_- 102

maximum thickness..____________________ 102

trend __________ _________________________ 102

Strike_-_____________________-----__-_------_- 106

Thickness, A zone _____ ___________ 112,113,

G zone __ ________ 111, 112,113,115,116.

other.. -_________---_ ___-_--_ 110,111,

Thrust faults _______________ ______________

Topography ___________ ___________________

Ultramaflc zone____________ 103-105,106,107

ujnior part ________ ___________________

Upper bronzitite-.---_-----___------_-_--_-

121

114,115 

117. 118 

114.115 

107, 108 

_ 100

108,117 

_ 106 

103. 110

U S GOVERNMENT PRINTING OFFICE: I9S4