Garnets in Montana Diatremes: 

A Key to Prospecting for 

Kim berl ites 

I 

U.S. GEOLOGICAL S~RVEY BULLETIN 1604 

I 

Garnets in Montana Diatremes: 

A Key to Prospecting for 

Kimberlites 

By B. C. HEARN, JR .. , and E. S. McGEE 

GEOLOGICAL SURVEY BULLETIN 1604 

Garnets are useful tracer 

minerals for kimberlite prospecting in Montana, and 

garnet color and composition can specifically indicate 

the presence of typical inclusions of upper mantle 

material 
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE 1983 

For sale by the Superintendent of Documents 

U.S. Government Printing Office 

Washington, DC 20402 

Library of Congress Cataloging in Publication Data 

Hearn, B. C. 

Garnets in Montana diatremes. 

(Geological Survey bulletin ; 1604 

Authors: B. C. Hearn, Jr., and E. S. McGee. 

Bibliography: p. 32 

Supt. of Docs. no.: 1 19.3:1604 

1. Kimberlite-Montana. 2. Garnet-Montana. 

I. McGee, E. S. II. Title. Ill. Series. 

QE75.B9 no. 1604 557.3S[549' .6] 83-600109 

[QE462.k5] 
CONTENTS 

Abstract 1 

Introduction 1 

Techniques of color classification 2 

Summary of samples 2 

Geology of the Williams diatremes 8 

Inclusions from deep levels 9 

Garnets from garnet peridotites 9 

Megacryst garnets 9 

Garnets from granulites, mafic granulites, and mafic amphibolites 19 

Garnets from other inclusions in the Williams diatremes 19 

Garnet-bearing inclusions in other diatremes 19 

Garnets from Precambrian rocks in the Little Rocky Mountains 20 

Colluvial garnets from the Williams diatremes 21 

Comparison of Williams colluvial garnets with garnets from known sources 23 

Panned concentrates from other diatremes 31 

Big Slide diatreme 31 

Bullwhacker Coulee diatreme 31 

Conclusions 31 

References 32 

FIGURES 

1. Geologic map of the Williams diatremes and vicinity, north-central 

Montana 10 

2. Diagrams showing Ca-Mg-Fe in colluvial garnets, megacrysts, and garnets 

from inclusions in the Williams 1 and 4 diatremes 14 

3. Histograms showing Cr203 content of color groups of peridotitic and 

colluvial garnets in the Williams diatremes 17 

4-8. Diagrams showing: 

4. Weight percent CaO versus weight percent Cr203 for garnets 

in the Williams 1 and 4 diatremes 18 

5. Chromium cations per 12 oxygens versus Mg/(Mg +Fe) for 

garnets in the Williams 1 and 4 diatremes 20 

6. Chromium versus aluminum cations per 12 oxygens for garnets 

in the Williams 1 and 4 diatremes 22 

7. Ca-Mg-Fe in garnets from localities other than the Williams 

diatremes 29 

8. Chromium cations per 12 oxygens versus Mg/(Mg +Fe) in garnets 

from localities other than the Williams diatremes 30 

Contents Ill 
IV Contents 

TABLES 

1-6. Analyses of: 

1. Colluvial garnets in the Williams 1 and 4 diatremes 3 

2. Garnets from fresh garnet peridotites in the Williams 1 and 4 

diatremes 12 

3. Garnets from altered garnet peridotites in the Williams 4 

diatreme 13 

4. Garnet megacrysts in the Williams 4 diatreme 24 

5. Garnets from other inclusions in the Williams 1 and 4 

diatremes 25 

6A. Garnets from the Big Slide 1 diatreme 26 

6B. Garnets from the Bullwhacker Coulee diatreme 27 

6C. Garnets from other inclusions, Precambrian bedrock, 

terrace deposits, and Paleocene sedimentary rock 28 
Garnets in Montana Diatremes: 

A Key to Prospecting for Kimberlites 

By B. C. Hearn, Jr., and E. S. McGee 

Abstract 

Garnet is a useful tracer mineral for kimberlite prospecting, and garnet color can specifically indicate the presence of 

typical inclusions of upper mantle material. The most distinctive 

indicators are Mg- and Cr-rich purple garnets, predominantly 

derived from upper mantle peridotites. Garnets in peridotite, 

in other inclusions, and in colluvial deposits from the Williams 

kimberlites and other north-central Montana diatremes have 

been classified by color and analyzed by electron microprobe. 

Fifteen peridotites from the Williams diatremes contain purple 

garnets (Cr20 3 content, 4.2-7.8 wt percent), and four contain 

red or red-orange garnets (Cr 20 3 content, 0.7-3-.0 wt percent). 

Twenty of 21 purple colluvial garnets are Cr rich (Cr20 3 content, 2.4-9.9 wt percent), and the Cr-rich purple, red, and 

pink colluvial garnets are compositionally analogous to the 

peridotite garnets. Other colluvial garnets of pink, red, redorange, orange, and light-orange color may have been derived from upper mantle inclusions but are not distinctively 

different in color in comparison with garnets from lower or 

upper crustal rocks. These five color groups have bimodal distribution of Fe/Mg, probably owing to bulk compositional differences between the lower crust and the upper mantle. Garnets having Mg/(Mg+ Fe) greater than 0.67 contain more Cr 

than most of the Fe-rich garnets, an indication that the Mg-rich 

group is derived from upper mantle rocks or megacrysts. 

Many of the Fe-rich garnets are similar in composition to garnets from lower crustal granulites. One of six megacrysts is Fe 

rich; the remaining five are Mg rich, and their Cr20 3 contents 

(0.6-2.3 wt percent) are less than those of most peridotite 

garnets. 

The Cr20 3 content of peridotite garnets increases as CaO 

content increases, a trend that fits garnets coexisting with a 

two-pyroxene assemblage (lherzolite) from localities elsewhere in the world. In that trend, Williams peridotites tend to 

show a gap between bimodal distributions of Cr 20 3 and CaO, 

but that gap is filled by some colluvial garnets that probably 

represent peridotites not yet found as xenoliths. Neither Crand Ca-rich green garnets nor Cr-rich, Ca-poor garnets have 

been found. 

The Big Slide diatreme contains garnets similar to those 

in granulites present in the diatreme and contains purple 

Mg- and Cr-rich garnets of probable upper mantle origin. Limited sampling shows that the Bullwhacker Coulee diatreme 

contains garnets similar to those in granulites and pyroxenites 

that are present as inclusions but has found no Cr-rich purple 

garnets. Garnets from shallow Precambrian rocks are of various 

colors (purple, pink, and orange) and form a small cluster having Fe-rich, Ca- and Cr-poor compositions. 

INTRODUCTION 

Kimberlites typically contain some or all of the 

tracer minerals pyropic garnet, chromian diopside, 

enstatite, and magnesian ilmenite (Dawson, 1980). Examining heavy-mineral concentrates from alluvial and 

colluvial deposits and soils is a well-known technique 

for prospecting for kimberlites all over the world. Heavymineral concentrates are obtained by hand panning, 

sluice-box or rocker-box concentrating, or heavy-liquid 

or hydraulic laboratory techniques. Garnet is particularly 

suitable as a tracer because it is present in most kimberlites, is more abundant than other tracer minerals in 

many kimberlites (Dawson and Stephens, 1975), and is 

resistant to attrition during transport in comparison 

with other common tracer minerals (Leighton and 

McCallum, 1979). 

Garnet in kimberlite is most commonly derived 

from disaggregated upper mantle material. Garnet in 

kimberlite can also be derived from lower or upper 

crustal rocks or from shallow Precambrian rocks. In 

addition, garnets can be derived from shallow sedimentary rocks or from surficial deposits that were disaggregated and incorporated into the kimberlite during its 

eruption. Garnets from any of these sources could be 

distinctive indicators of kimberlite in a drainage basin 

where other garnet-bearing source rocks are not exposed. 

Although many sources can contribute garnet to. kimberlite or to diatremes of other magma types, the garnets 

that are derived from deep upper mantle sources are distinctive indicators of kimberlite. Kimberlitic garnets of 

ultimate upper mantle origin can be derived from garnet 

peridotite, garnet pyroxenite, eclogite, or megacryst 

assemblages. Megacryst assemblages in kimberlite are 

coarse-grained garnet, orthopyroxene, clinopyroxene, 

olivine, and ilmenite that are presumed to coexist at 

Introduction 1 
upper mantle temperatures and pressures. Megacrysts 

commonly occur as monomineralic inclusions, rarely as 

intergrown bimineralic inclusions, and very rarely as 

multimineralic inclusions. The large size of garnet 

( > 1 em) is an obvious clue to the presence .. of kimberlite 

if large garnets are absent in surrounding bedrock. Garnet megacrysts and garnets from most garnet peridotites 

and from many garnet pyroxenites are distinctly rich in 

Cr and thus are indicative of deep source material in the 

kimberlite and of an attendant potential for diamond. 

One type of garnet that is low in Ca and particularly rich 

in Cr is found as inclusions in diamond (Meyer and 

Boyd, 1972) and as a component of diamond-bearing 

harzburgites and dunites (Finnerty and Boyd, 1980) but is 

extremely rare in kimberlite (Gurney and Switzer, 1973). 

Two studies (Dawson and Stephens, 1975, 1976; 

Danchin and Wyatt, 1979) classify kimberlite garnets 

and xenolith garnets into 12 and 52 cluster groups, respectively, on the basis of chemical composition. Their 

garnet populations include separate xenocrysts and 

megacrysts; garnets from lherzolite, harzburgite, dunite, 

pyroxenite, and eclogite xenoliths; and garnet inclusions 

in diamond. Dawson and Stephens (1975, 1976) gave 

limited color data for each group. Both studies exclude 

garnets from granulites, garnet amphibolites, or other 

crustal rocks because such xenoliths are scarce in African kimberlites or because there are no distinctive criteria 

for xenocrysts of crustal origin. 

During field or laboratory examination of heavymineral suites, the color of garnet can be easily and rapidly determined and is a useful criterion for the presence 

of upper mantle garnets. Garnets from peridotites are 

dominantly purple, reddish purple, or deep red, and all 

have a moderate to high Cr content, typically 2.5 to 

12 wt percent Cr203. Garnets from pyroxenites are purple, red, or orange and have low to moderate Cr content. 

Garnets from eclogites are typically red to light orange 

and have low Cr content. Megacryst garnets are generally 

red to red orange, although some are purple or light 

orange. A substantial proportion of purple garnets in a 

kimberlite indicates that garnet peridotite inclusions 

may be present. Substantial proportions of red, orange, 

or light-orange garnets could indicate the presence of 

eclogites, in the absence of xenoliths of more shallow 

derivation such as granulite or amphibolite, which can 

contain garnets of those colors. 

The purplish or deep-red color of peridotite garnets 

is distinctive and can be recognized in the field during 

hand-lens study of garnets in soil, surficial deposits, pan 

concentrates, or the kimberlite itself. Recognition of 

these garnets is enhanced by their tendency to occur as 

clear, unfractured grains or as grains with clean, glassy, 

conchoidal fracture surfaces. Determination of color 

can be improved by viewing in direct sunlight and by 

wetting the grains. For grains that are partially coated 

2 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 

by kelyphitic reaction rims or that are imbedded in kimberlite matrix, determination of color can be aided by 

focusing a bright spot of sunlight onto the grain with a 

hand lens. 

TECHNIQUES OF COLOR CLASS I FICA liON 

The classification of garnets into color groups was 

done megascopically by using a binocular microscope 

with an incandescent unfiltered light source to simulate 

techniques that could easily be used in the field. Such 

classification, however, is subjective. Variables affecting 

color determination are the light source, the size of the 

grains, background color, the color of adjacent grains, 

surface irregularities, the presence of tiny inclusions or 

fractures, and perceptual idiosyncrasies of the observer. 

It is important to compare many grains in the same field 

of view because some color groups might not be represented in a small sample; also, the divisions between the 

groups are usually arbitrary because of complete gradation between some color groups. Surficial abrasion 

makes grains appear lighter in color. Internal fractures 

can decrease the visible volume of a grain and thus 

lighten its color. The color of garnets that are surrounded 

by dark pyroxenes or serpentinized olivine in an inclusion 

can appear to be different from the color that they would 

show as single loose grains; so, for the best comparison, 

the garnets should be picked out of inclusions or matrix. 

Classification was aided by placing white paper underneath the grains to provide a uniform background and 

to make the colors more distinctive. Garnets should be 

approximately the same size because smaller grains or 

thin flakes appear lighter than larger grains. The observer 

can compensate for some of these variables-for example, by placing light-purple, purple, and dark-purple 

grains in the same color group. 

It is likely that, in classifying the same garnet sample, two observers will have slightly different numbers 

of grains in each color group. This difference will be of 

little importance if the number of garnets is large enough 

to give a good statistical base and if the color groups are 

regarded as gradational. Color groups may be chemically 

gradational as well. For example, some red grains are 

chemically similar to some purple grains, and some 

color groups may show extensive overlap in their chemical compositions. 

SUMMARY OF SAMPLES 

This study concentrates on garnets of the Williams 

kimberlite diatremes. Smaller samples of garnets from 

other diatremes and bedrock and detrital sources have 

also been analyzed. Locations of the diatremes are given 

by Hearn (1979) and in the tables. 
!II 

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IN 

Table 1. Analyses of colluvial garnets in the Williams 1 and 4 diatremes, grouped by color 

[Analyses are ordered by color groups: Purple, red, pink, red orange, orange, and light orange. Analyses 114, 115, and 116 follow analysis 18. Analyses 1-16, 

19-36, 38-53, 56-89, 92-112, 114, and 115: Garnets from H68-16N, colluvial material west of southwestern end of Williams 4 dikelike diatreme, southwest of 

autolith-rich breccia; NW1/4SW1/4NE1/4 sec. 7, T. 24 N., R. 24 E. Analyses 17, 18, 37, 54, 55, 90, 91, 113, and 116: Garnets from H68-17C, colluvial lag 

material in topographic saddle between terrace-pediment gravel remnants, south-central part of Williams 1 diatreme; NW1/4SW1/4NW114 sec. 8, T. 24 N., 

R. 24 E.] 

Si02 -------------

Al20, ------------

FeO -------------

MgO ------------

CaO -------------

Ti02 -------------

MnO ------------

Cr 20, ------------

Total --------

Si ---------------

AI---------------

Fe ---------------

Mg --------------

Ca --------------

Ti ---------------

Mn --------------

Cr ---------------

Total --------

Si02 -------------

AI20, ------------

FeO -------------

MgO ------------

CaO -------------

Ti02 -------------

MnO ------------

Cr 20, ------------

Total--------

Si ---------------

AI---------------

Fe ---------------

Mg --------------

Ca --------------

Ti ---------------

Mn --------------

Cr ---------------

Total --------

W-GV-01 1 

40.82 

20.82 

9.42 

18.79 

5.63 

.15 

.46 

3.93 

100.02 

2.956 

1.776 

.569 

2.027 

.436 

.006 

.027 

.223 

8.020 

W-GV-14 

42~2 

20~4 

6~ 

21.02 

5.38 

.43 

27 

4.13 

100~6 

3.004 

1.710 

.381 

2.214 

.406 

.022 

.014 

.230 

7.981 

W-GV-02 

42.55 

20.29 

6.48 

21.04 

5.25 

.49 

.26 

4.12 

100.48 

3.017 

1.696 

.383 

2.224 

.397 

.025 

.014 

.229 

7.985 

W-GV-15 

40.67 

19.31 

6.73 

19.26 

5.99 

.59 

.26 

6.09 

98.90 

2.964 

1.658 

.408 

2.092 

.466 

.032 

.015 

.351 

7.986 

W-GV-03 

41.43 

16.54 

6.34 

19.41 

6.70 

.47 

.32 

9.89 

101.10 

2.992 

1.407 

.381 

2.089 

.517 

.025 

.019 

.563 

7.993 

W-GV-16 

41.44 

20.40 

7.18 

19.90 

5.27 

.71 

.26 

3.95 

99.11 

2.991 

1.734 

.432 

2.141 

.406 

.038 

.015 

.224 

7.981 

W-GV-04 

41.50 

20.77 

7.26 

20.18 

5.42 

.02 

.41 

4.64 

100.20 

2.971 

1.752 

.433 

2.154 

.414 

.000 

.023 

.263 

8.010 

W-GV-17 

41.56 

21.96 

7.74 

20.16 

5.37 

.04 

.38 

3.02 

100.23 

2.963 

1.846 

.460 

2.143 

.410 

.002 

.023 

.169 

8.016 

W-GV-05 

42.16 

20.47 

6.67 

21.05 

5.09 

.61 

.27 

3.68 

100.00 

3.001 

1.718 

.397 

2.236 

.386 

.031 

.014 

.207 

7.990 

W-GV-18 

41.65 

20.45 

7.79 

19.36 

6.05 

.06 

.46 

4.35 

100.17 

2.990 

1.730 

.468 

2.074 

.463 

.002 

.027 

.246 

8.000 

W-GV-06 W-GV-07 

Major oxides, in wt percent 

41.39 

19.67 

6.91 

20.36 

5.39 

.29 

.26 

5.01 

99.28 

40.63 

21.16 

11.35 

21.07 

2.32 

2.79 

.29 

.04 

99.65 

Cations per 12 oxygens 

2.988 

1.674 

.417 

2.191 

.417 

.015 

.015 

.286 

8.003 

W-GV-114 

2.926 

1.793 

.682 

2.260 

.177 

.150 

.017 

.002 

8.007 

W-GV-115 

Major oxides, in wt percent 

41.99 

19.54 

7.02 

19.72 

5.72 

.15 

.64 

5.86 

100.64 

41.61 

19.50 

6.85 

19.60 

5.21 

.12 

.89 

5.81 

99.59 

Cations per 12 oxygens 

3.006 

1.648 

.420 

2.104 

.438 

.007 

.039 

.331 

7.993 

3.007 

1.660 

.413 

2.111 

.403 

.006 

.054 

.331 

7.985 

W-GV-08 

41.55 

19.11 

6.93 

20.34 

5.37 

.61 

.29 

4.86 

99.06 

3.007 

1.629 

.418 

2.194 

.416 

.032 

.017 

.278 

7.991 

W-GV-116 

41.18 

21.48 

7.65 

19.65 

5.52 

.01 

.39 

2.64 

98.52 

2.987 

1.836 

.464 

2.125 

.428 

.000 

.023 

.151 

8.014 

W-GV-09 

41.33 

21.29 

9.89 

18.00 

5.68 

.11 

.69 

2.79 

99.78 

2.995 

1.818 

.600 

1.944 

.440 

.004 

.040 

.159 

8.000 

W-GR-19 

41.38 

22.15 

11.23 

20.73 

2.28 

1.71 

.27 

.06 

99.81 

2.962 

1.867 

.672 

2.212 

.174 

.090 

.015 

.002 

7.994 

W-GV-10 

42.37 

21.20 

7.69 

19.16 

4.48 

.80 

.29 

2.44 

98.43 

3.055 

1.800 

.463 

2.059 

.345 

.042 

.017 

.137 

7.918 

W-GR-20 

42.25 

22.11 

7.98 

20.80 

4.53 

.80 

.27 

1.61 

100.35 

2.990 

1.844 

.471 

2.195 

.342 

.042 

.014 

.089 

7.987 

W-GV-11 

40.80 

19.89 

6.88 

19.41 

5.87 

.00 

.39 

5.68 

98.92 

2.970 

1.707 

.418 

2.106 

.457 

.000 

.023 

.325 

8.006 

W-GR-21 

41.48 

21.98 

10.64 

19.37 

4.10 

1.54 

.30 

.93 

100.34 

2.970 

1.853 

.636 

2.066 

.313 

.082 

.017 

.050 

7.987 

W-GV-12 

41.80 

20.38 

6.76 

20.41 

5.15 

.69 

.30 

3.86 

99.35 

3.001 

1.724 

.404 

2.186 

.396 

.036 

.017 

.217 

7.981 

W-GR-22 

41.16 

21.14 

11.27 

20.22 

3.24 

2.89 

.27 

.08 

100.27 

2.948 

1.784 

.674 

2.158 

.248 

.153 

.015 

.004 

7.984 

W-GV-13 

41.60 

20.86 

7.12 

19.74 

5.68 

.00 

.43 

4.93 

100.36 

2.974 

1.757 

.424 

2.105 

.434 

.000 

.025 

.277 

7.996 

W-GR-23 

42.17 

22.24 

7.16 

21.39 

4.48 

.66 

.28 

2.09 

100.47 

2.975 

1.848 

.422 

2.250 

.337 

.033 

.016 

.ll6 

7.997 
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Table 1. Analyses of colluvial garnets in the Williams 1 and 4 diatremes, grouped by color-Continued 

Si02 -------------

Al20, ------------

FeO -------------

MgO ------------

CaO -------------

Ti02 -------------

MnO ------------

Cr 20, ------------

Total --------

Si ---------------

AI---------------

Fe ---------------

Mg --------------

Ca --------------

Ti ---------------

Mn --------------

Cr ---------------

Total--------

Si02 -------------

Al20, ------------

FeO -------------

MgO ------------

CaO -------------

Ti02 -------------

MnO ------------

Cr 20, ------------

Total--------

Si ---------------

AI---------------

Fe ---------------

Mg --------------

Ca --------------

Ti ---------------

Mn --------------

Cr ---------------

Total --------

W-GR-24 

41.89 

22.86 

8.39 

20.38 

4.36 

.80 

.26 

1.22 

100.16 

2.970 

1.911 

.497 

2.154 

.331 

.042 

.014 

.066 

7.985 

W-GR-37 

38.68 

21.84 

26.84 

6.23 

7.57 

.07 

.48 

.07 

101.78 

2.966 

1.974 

1.721 

.711 

.621 

.002 

.029 

.002 

8.026 

W-GR-25 

41.41 

22.29 

7.42 

21.03 

4.35 

.72 

.20 

1.77 

99.19 

2.960 

1.877 

.442 

2.240 

.331 

.038 

.010 

.098 

7.996 

W-GP-38 

37.86 

21.85 

22.36 

7.48 

7.84 

.01 

1.69 

.04 

99.13 

2.947 

2.003 

1.455 

.868 

.653 

.000 

.109 

.002 

8.037 

W-GR-26 

37.80 

21.30 

27.66 

5.13 

5.98 

.06 

.86 

.01 

98.80 

2.999 

1.990 

1.834 

.605 

.507 

.002 

.056 

.000 

7.993 

W-GP-39 

37.41 

21.57 

32.98 

6.25 

.80 

.00 

.61 

.01 

99.63 

2.967 

2.015 

2.187 

.737 

.067 

.000 

.040 

.000 

8.013 

W-GR-27 

41.86 

22.55 

8.00 

20.52 

4.43 

.50 

.23 

1.09 

99.18 

2.992 

1.899 

.478 

2.187 

.337 

.025 

.012 

.061 

7.991 

W-GP-40 

37.44 

21.21 

33.15 

5.82 

1.14 

.04 

.69 

.06 

99.55 

2.979 

1.989 

2.204 

.688 

.096 

.002 

.044 

.002 

8.004 

W-GR-28 

41.58 

22.10 

10.62 

18.86 

4.35 

1.02 

.33 

.61 

99.47 

2.998 

1.876 

.639 

2.026 

.334 

.055 

.019 

.034 

7.981 

W-GP-41 

36.84 

20.92 

33.02 

4.96 

1.44 

.00 

.86 

.07 

98.11 

2.985 

1.998 

2.238 

.599 

.124 

.000 

.057 

.002 

8.003 

W-GR-29 W-GR-30 

Major oxides in wt percent 

41.48 38.21 

22.30 22.15 

7.50 23.88 

20.59 7.99 

4.23 6.14 

.64 .13 

.28 .48 

1.55 .09 

98.57 99.07 

Cations per 12 oxygens 

2.982 2.962 

1.888 2.022 

.449 1.547 

2.206 .921 

.325 .509 

.034 .006 

.017 .029 

.086 .004 

7.987 8.000 

W-GP-42 W-GP-43 

Major oxides, in wt percent 

37.59 

21.73 

25.04 

7.70 

5.59 

.03 

.50 

.02 

98.20 

39.10 

22.63 

20.71 

10.17 

6.24 

.04 

.37 

.13 

99.39 

Cations per 12 oxygens 

2.957 

2.015 

1.646 

.902 

.471 

.000 

.032 

.000 

3.023 

2.969 

2.026 

1.315 

1.150 

.506 

.002 

.022 

.006 

7.996 

W-GR-31 

38.63 

20.56 

26.22 

4.97 

8.13 

.11 

.52 

.02 

99.16 

3.044 

1.911 

1.728 

.583 

.685 

.004 

.035 

.000 

7.990 

W-GP-44 

36.56 

21.15 

33.88 

3.73 

1.77 

.00 

1.91 

.07 

99.07 

2.963 

2.019 

2.294 

.449 

.152 

.000 

.130 

.002 

8.009 

W-GR-32 

37.96 

21.40 

28.81 

4.40 

6.20 

.07 

.74 

.00 

99.58 

2.998 

1.991 

1.903 

.517 

.524 

.002 

.048 

.000 

7.983 

W-GP-45 

41.17 

21.76 

7.45 

21.50 

4.47 

.63 

.24 

1.93 

99.15 

2.952 

1.839 

.446 

2.297 

.343 

.033 

.012 

.109 

8.031 

W-GR-33 

41.10 

21.89 

7.52 

20.28 

4.77 

.63 

.34 

2.80 

99.33 

2.950 

1.852 

.451 

2.170 

.367 

.034 

.019 

.158 

8.001 

W-GP-46 

42.21 

22.03 

7.31 

22.03 

4.54 

.65 

.28 

1.92 

100.97 

2.968 

1.824 

.429 

2.310 

.340 

.033 

.016 

.105 

8.025 

W-GR-34 

41.28 

20.73 

8.78 

20.20 

4.82 

.91 

.29 

3.31 

100.32 

2.958 

1.749 

.525 

2.157 

.368 

.048 

.017 

.187 

8.009 

W-GP-47 

41.87 

22.14 

7.41 

21.97 

4.64 

.67 

.21 

1.72 

100.63 

2.952 

1.839 

.437 

2.309 

.350 

.035 

.012 

.095 

8.029 

W-GR-35 

37.91 

20.70 

29.17 

4.97 

6.36 

.09 

.76 

.02 

99.98 

2.996 

1.927 

1.927 

.584 

.538 

.004 

.048 

.000 

8.024 

W-GP-48 

42.20 

21.84 

7.54 

21.44 

4.49 

.66 

.24 

1.77 

100.18 

2.987 

1.821 

.446 

2.262 

.338 

.035 

.012 

.097 

7.998 

W-GR-36 

37.55 

21.32 

31.09 

3.75 

6.28 

.03 

.89 

.02 

100.93 

2.965 

1.984 

2.054 

.440 

.531 

.000 

.058 

.000 

8.032 

W-GP-49 

41.78 

21.29 

7.63 

21.39 

4.75 

.65 

.24 

2.35 

100.08 

2.974 

1.785 

.453 

2.270 

.361 

.033 

.012 . 

.131 

8.019 
Table 1. Analyses of colluvial garnets in the Williams 1 and 4 diatremes, grouped by color-Continued 

Si02 -------------

Ab03 ------------

FeO -------------

MgO ------------

CaO -------------

Ti02 -------------

MnO ------------

Cr203 ------------

Total--------

Si ---------------

AI---------------

Fe ---------------

Mg --------------

Ca --------------

Ti ---------------

Mn --------------

Cr ---------------

Total --------

Si02 -------------

A1203 ------------

FeO -------------

MgO ------------

CaO -------------

Ti02 -------------

Mn --------------

Cr203 ------------

Total --------

Si ---------------

AI---------------

Fe ---------------

~ Mg --------------

3 Ca --------------

: Ti ---------------

~ Mn --------------

0 Cr---------------

;;; Total -------- 111 

3 

'1:1 

ti' 

Ill 

VI 

W-GP-50 

41.48 

20.76 

6.73 

20.80 

5.02 

.54 

.28 

3.55 

99.16 

2.981 

1.756 

.402 

2.227 

.386 

.027 

.017 

.200 

7.996 

W-GT-63 

42.18 

22.05 

8.74 

20.59 

4.46 

.81 

.33 

1.52 

100.68 

2.987 

1.839 

.518 

2.174 

.336 

.042 

.019 

.083 

7.998 

W-GP-51 

41.69 

21.66 

6.79 

20.57 

4.83 

.57 

.23 

2.91 

99.25 

2.983 

1.826 

.405 

2.193 

.369 

.029 

.012 

.164 

7.981 

W-GT-64 

39.52 

21.77 

25.39 

8.68 

5.46 

.08 

.70 

.17 

101.77 

2.991 

1.941 

1.605 

.979 

.442 

.004 

.044 

.009 

8.015 

W-GP-52 

42.46 

21.72 

6.59 

21.09 

4.64 

.45 

.23 

2.59 

99.77 

3.010 

1.813 

.389 

2.228 

.351 

.022 

.012 

.143 

7.968 

W-GT-65 

~46 

2~22 

n~ 

10.38 

5.61 

.00 

.~ 

.16 

100~4 

2.974 

1.974 

1.416 

1.166 

.451 

.002 

.028 

.009 

8.020 

W-GP-53 

36.49 

21.06 

33.95 

3.94 

1.20 

.02 

1.76 

.10 

98.52 

2.970 

2.021 

2.311 

.478 

.103 

.000 

.119 

.004 

8.006 

W-GT-66 

42.34 

22.28 

7.21 

21.80 

4.69 

.66 

.23 

1.86 

101.07 

2.967 

1.840 

.421 

2.276 

.352 

.035 

.012 

.103 

8.006 

W-GP-54 

42.55 

23.90 

7.41 

21.64 

4.66 

.09 

.36 

.73 

101.34 

2.966 

1.963 

.431 

2.248 

.348 

.004 

.020 

.039 

8.019 

W-GT-67 

41.49 

22.30 

11.61 

18.89 

4.90 

1.23 

.34 

.05 

100.81 

2.967 

1.878 

.692 

2.014 

.375 

.065 

.019 

.002 

8.012 

W-GP-55 W-GT -56 

Major oxides, in wt percent 

42.59 

23.61 

8.13 

20.56 

4.63 

.10 

.39 

.82 

100.83 

41.33 

22.91 

11.34 

18.95 

4.49 

.65 

.41 

.57 

100.65 

Cations per 12 oxygens 

2.992 

1.955 

.476 

2.153 

.346 

.004 

.022 

.045 

7.993 

W-GT-68 

2.956 

1.931 

.678 

2.021 

.342 

.033 

.023 

.031 

8.015 

W-GT-69 

Major oxides, in wt percent 

38.95 

21.34 

25.96 

6.88 

6.32 

.08 

.77 

.00 

100.30 

41.42 

21.98 

8.86 

20.46 

4.40 

.82 

.29 

.66 

98.89 

Cations per 12 oxygens 

3.011 

1.944 

1.676 

.792 

.521 

.004 

.050 

.000 

7.998 

2.984 

1.865 

.533 

2.199 

.338 

.042 

.017 

.036 

8.014 

W-GT-57 

41.41 

22.65 

8.71 

20.41 

4.61 

.90 

.25 

.88 

99.82 

2.952 

1.902 

.519 

2.169 

.351 

.048 

.014 

.048 

8.003 

W-GT-70 

38.74 

21.38 

25.32 

6.76 

7.46 

.14 

.55 

.06 

100.41 

2.993 

1.945 

1.635 

.777 

.617 

.006 

.034 

.002 

8.009 

W-GT-58 

38.40 

21.99 

27.43 

6.28 

6.03 

.10 

.70 

.04 

100.97 

2.966 

2.002 

1.771 

.723 

.499 

.004 

.045 

.002 

8.012 

W-GT-71 

38.80 

21.46 

24.00 

8.47 

6.13 

.10 

.50 

.04 

99.50 

2.995 

1.951 

1.550 

.974 

.505 

.004 

.031 

.002 

8.012 

W-GT-59 

41.67 

23.14 

10.30 

19.75 

4.34 

.83 

.34 

.47 

100.84 

2.957 

1.934 

.610 

2.088 

.329 

.043 

.019 

.025 

8.005 

W-GT-72 

38.32 

21.25 

25.75 

6.84 

6.53 

.11 

.66 

.06 

99.52 

2.991 

1.953 

1.678 

.794 

.545 

.004 

.043 

.002 

8.010 

W-GT-60 

38.32 

22.34 

25.27 

7.30 

6.50 

.05 

.64 

.04 

100.46 

2.952 

2.027 

1.627 

.836 

.536 

.002 

.041 

.000 

8.021 

W-GT-73 

38.34 

22.11 

25.23 

6.84 

7.45 

.10 

.52 

.07 

100.66 

2.953 

2.008 

1.625 

.784 

.615 

.004 

.034 

.004 

8.027 

W-GT-61 

42.29 

22.92 

8.07 

21.29 

4.14 

.65 

.25 

1.24 

100.85 

2.971 

1.897 

.474 

2.229 

.311 

.033 

.014 

.068 

7.997 

W-G0-74 

41.12 

22.28 

8.52 

20.31 

4.55 

.73 

.27 

1.37 

99.15 

2.957 

1.889 

.510 

2.178 

.350 

.038 

.015 

.076 

8.013 

W-GT -62 

38.60 

22.06 

23.83 

8.52 

6.29 

.05 

.51 

.14 

100.00 

2.961 

1.995 

1.529 

.975 

.516 

.002 

.031 

.006 

8.015 

W-G0-7.5 

41.54 

22.90 

8.95 

19.84 

4.19 

.75 

.27 

.61 

99.05 

2.979 

1.937 

.537 

2.122 

.320 

.040 

.015 

.033 

7.983 
aC"'l 

~ ::I 

~ 

"' :r 

~ 0 

a 

~ 

::I 

~ 

c 

I'TI 

~ 

3 

I'TI 

~ 

> 

~ "' 

0 

"' 3' 

g-

::::!.. 

;: 

"a 

a 

"CC "' 

a 

:r 

OQ 

Table 1. Analyses of colluvial garnets in the Williams 1 and 4 diatremes, grouped by color-Continued 

Si02 -------------

Al203 ------------

FeO--------------

MgO -------------

CaO -------------

Ti02 -------------

MoO-------------

Cr203 ------------

Total --------

Si----------------

Al ---------------

Fe ---------------

Mg --------------

Ca ---------------

Ti ---------------

Mn --------------

Cr ---------------

Total --------

Si02 -------------

Ab03 ------------

F eO--------------

MgO -------------

CaO -------------

Ti02 -------------

MnO-------------

Cr203 ------------

Total --------

Si----------------

Al ---------------

Fe---------------

Mg --------------

Ca ---------------

Ti ---------------

Mn --------------

Cr ---------------

Total --------

W-G0-76 

41.32 

23.37 

10.23 

18.96 

4.54 

.65 

.30 

.32 

100.19 

2.982 

1.963 

.609 

2.016 

.345 

.033 

.017 

.017 

7.982 

W-G0-89 

41.77 

21.85 

8.56 

21.11 

4.18 

.78 

.27 

.74 

99.26 

2.990 

1.842 

.510 

2.252 

.319 

.042 

.015 

.040 

8.010 

W-G0-77 

41.16 

22.98 

9.33 

18.48 

5.41 

.72 

.24 

.37 

98.69 

2.978 

1.960 

.563 

1.992 

.418 

.038 

.013 

.021 

7.983 

W-G0-90 

41.39 

22.20 

12.55 

18.30 

5.14 

1.10 

.38 

.10 

101.16 

2.965 

1.874 

.750 

1.956 

.393 

.059 

.023 

.004 

8.024 

W-G0-78 

39.61 

23.13 

11.05 

10.97 

13.47 

.27 

.25 

.02 

98.77 

2.956 

2.032 

.688 

1.219 

1.075 

.015 

.015 

.000 

8.000 

W-G0-91 

42.02 

22.66 

10.59 

19.82 

4.54 

.73 

.34 

.82 

101.52 

2.971 

1.888 

.624 

2.089 

.342 

.037 

.019 

.045 

8.015 

W-G0-79 

42.03 

22.43 

8.24 

21.19 

4.49 

.68 

.32 

1.04 

100.42 

2.975 

1.870 

.486 

2.235 

.340 

.035 

.019 

.058 

8.018 

W-GL-92 

42.38 

22.17 

8.89 

20.79 

4.42 

.52 

.29 

.72 

100.18 

3.007 

1.854 

.527 

2.199 

.335 

.027 

.017 

.040 

8.006 

W-G0-80 

39.18 

21.15 

27.87 

6.14 

6.31 

.07 

.83 

.05 

101.60 

3.015 

1.917 

1.793 

.704 

.519 

.002 

.052 

.002 

8.004 

W-GL-93 

41.11 

22.70 

11.80 

18.64 

4.17 

.54 

.38 

.28 

99.62 

2.970 

1.931 

.712 

2.007 

.322 

.027 

.023 

.015 

8.007 

W-G0-81 W-G0-82 

Major oxides, in wt percent 

41.22 

22.44 

11.58 

18.82 

4.31 

.90 

.36 

.13 

99.76 

42.33 

22.29 

8.70 

21.29 

4.35 

.79 

.27 

.84 

100.86 

Cations per 12 oxygens 

2.972 

1.907 

.698 

2.023 

.332 

.048 

.021 

.006 

8.007 

W-GL-94 

2.985 

1.853 

.513 

2.240 

.327 

.041 

.014 

.045 

8.018 

W-GL-95 

Major oxides, in wt percent 

41.79 

22.88 

12.01 

18.68 

4.10 

.44 

.37 

.23 

100.50 

41.63 

22.48 

12.62 

18.18 

4.13 

.49 

.46 

.59 

100.58 

Cations per 12 oxygens 

2.992 

1.929 

.718 

1.994 

.313 

.023 

.021 

.012 

8.002 

2.991 

1.905 

.757 

1.947 

.316 

.025 

.027 

.034 

8.002 

W-G0-83 

41.73 

22.06 

8.17 

21.43 

4.02 

.75 

.27 

.80 

99.23 

2.982 

1.858 

.486 

2.284 

.306 

.040 

.015 

.044 

8.015 

W-GL-96 

38.78 

21.42 

22.93 

4.75 

11.29 

.06 

.94 

.09 

100.26 

3.004 

1.956 

1.485 

.548 

.937 

.002 

.061 

.004 

7.997 

W-G0-84 

41.55 

22.18 

9.51 

20.11 

4.46 

.94 

.33 

.75 

99.83 

2.975 

1.870 

.569 

2.145 

.340 

.050 

.019 

.042 

8.010 

W-GL-97 

42.32 

22.73 

8.46 

20.11 

5.65 

.73 

.28 

.13 

100.41 

2.996 

1.897 

.500 

2.124 

.428 

.037 

.017 

.006 

8.005 

W-G0-85 

38.93 

21.64 

22.29 

10.24 

5.32 

.04 

.63 

.20 

99.29 

2.980 

1.953 

1.427 

1.169 

.436 

.002 

.040 

.011 

8.018 

W-GL-98 

42.01 

22.26 

8.62 

20.90 

4.25 

.71 

.30 

.86 

99.91 

2.987 

1.866 

.511 

2.216 

.323 

.037 

.017 

.048 

8.005 

W-G0-86 

41.10 

22.24 

8.73 

20.67 

4.24 

.80 

.28 

.87 

98.93 

2.959 

1.887 

.524 

2.219 

.325 

.042 

.017 

.048 

8.021 

W-GL-99 

39.12 

21.98 

25.40 

10.88 

2.03 

.00 

.75 

.11 

100.27 

2.978 

1.973 

1.617 

1.235 

.165 

.000 

.047 

.004 

8.019 

W-G0-87 

41.56 

21.97 

8.70 

20.74 

4.53 

.84 

.25 

.90 

99.49 

2.975 

1.853 

.519 

2.214 

.347 

.044 

.015 

.050 

8.017 

W-GL-100 

41.05 

22.29 

12.71 

17.83 

4.17 

.47 

.42 

1.11 

100.05 

2.974 

1.902 

.769 

1.926 

.323 

.025 

.025 

.063 

8.007 

W-G0-88 

41.75 

22.28 

8.37 

20.99 

4.43 

.77 

.27 

.77 

99.63 

2;979 

1.873 

.498 

2.232 

.337 

.040 

.015 

.042 

8.016 

W-GL-101 

40.23 

22.38 

13.49 

17.23 

4.25 

.46 

.45 

.22 

98.71 

2.960 

1.941 

.829 

1.889 

.334 

.023 

.028 

.011 

8.015 
tl) 

c 

3 

3 

Dl 

~ 

s. 

: 

3 

"CC 

;&" 

"' 

...... 

Table 1. Analyses of colluvial garnets in the Williams 1 and 4 diatremes, grouped by color-Continued 

W-GL-102 W-GL-103 W-GL-104 W-GL-105 W-GL-106 W-GL-107 

Major oxides, in wt percent 

SiOz ------------- 41.11 41.51 41.45 38.61 41.24 41.58 

AlzOl ------------ 22.49 22.03 22.39 21.05 22.14 21.18 

FeO ------------- 11.65 8.37 11.95 27.26 12.55 10.06 

MgO ------------ 20.04 20.38 18.73 5.51 17.66. 19.30 

CaO ------------- 2.57 4.49 4.38 7.35 4.21 5.20 

TiOz ------------- 1.16 .66 .45 .07 .43 .95 

MnO ------------ .30 .28 .42 .69 .45 .34 

Cr zOl ------------ .26 1.25 .51 .04 .61 1.16 

Total-------- 99.58 98.97 100.28 100.58 99.29 99.77 

Cations per 12 oxygens 

Si --------------- 2.958 2.984 2.981 3.007 3.003 2.997 

AI--------------- 1.906 1.868 1.898 1.931 1.900 1.798 

Fe--------------- .701 .502 .718 1.773 .763 .604 

Mg -------------- 2.149 2.184 2.010 6.37 1.917 2.073 

Ca -------------- .198 .346 .335 .612 .327 .402 

Ti --------------- .061 .034 .023 .002 .021 .050 

Mn -------------- .017 .017 .025 .043 .025 .019 

Cr --------------- .015 .069 .027 .002 .034 .065 

Total-------- 8.005 8.004 8.017 8.007 7.990 8.008 

1 Analyses have been designated according to the following scheme: 

Typical analysis number: BU-G P- 1 0 3 

Spaces: 1, 2 3, 4 5, 6, 7 

Spaces 1, 2-Locality: 

w Williams diatremes 

Space 3-Type of sample analyzed: 

G Grains from panned concentrate, heavy-mineral separate, gravel, or anthill 

Space 4-Color group: 

v Purple 

R Red 

P Pink 

T Red orange 

0 Orange 

L Light orange 

Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality 

W-GL-108 

40.82 

21.82 

11.41 

18.21 

4.55 

.52 

.38 

1.46 

99.17 

2.973 

1.872 

.694 

1.977 

.354 

.027 

.021 

.083 

8.001 

W-GL-109 W-GL-110 W-GL-111 W-GL-112 W-GL-113 

41.52 41.20 38.64 41.04 40.65 

22.37 22.63 21.36 22.51 23.15 

12.88 12.23 27.04 12.59 16.04 

18.43 18.58 5.78 18.56 12.60 

4.19 4.19 6.27 4.16 8.42 

.51 .59 .07 .42 .10 

.46 .34 .77 .39 .40 

.38 .28 .09 .13 .05 

100.74 100.04 100.02 99.80 101.41 

2.982 2.972 3.012 2.972 2.972 

1.892 1.922 1.962 1.922 1.993 

.773 .736 1.761 .761 .980 

1.972 1.999 .670 2.005 1.373 

.322 .322 .523 .323 .659 

.027 .032 .002 .021 .004 

.027 .019 .050 .023 .023 

.021 .015 .004 .006 .002 

8.016 8.017 7.984 8.033 8.006 
Garnets have been classified by color and analyzed 

by using an automated ARL-EMX electron microprobe 

with online data reduction using Bence and Albee's 

(1968) method. Silicate standards were used for all 

elements except Cr, for which a synthetic Cr203 standard was used. A natural pyrope garnet froin Kakanui, 

New Zealand, was always analyzed as a check on the 

standardization. 

Analyzed garnets from the Williams diatremes 

area are from several types of samples: panned concentrates from colluvium adjacent to the Williams 4 diatreme and from a colluvial lag deposit on the Williams 1 

diatreme (table 1), inclusions of fresh and altered garnet 

peridotite (tables 2 and 3), megacrysts (table 4), and 

inclusions of granulite and amphibolite (table 5). Garnets 

in various samples from other diatremes (panned concentrates, granulite, gneiss and schist inclusions, and 

xenocrysts), in Precambrian bedrock, and in detrital 

deposits from presumed Little Rocky Mountains sources 

(table 6) were classified by color and analyzed for comparison with garnets from the Williams diatremes. Published (Hearn and Boyd, 1975) and unpublished (by 

F. R. Boyd and by the authors) analyses of garnet megacrysts and garnets from peridotite inclusions in the Williams diatremes are also given (tables 2-5). 

GEOLOGY OF THE WILLIAMS DIATREMES 

The Williams kimberlites are a group of four 

closely spaced diatremes (fig. 1) in the eastern part of an 

east-northeast-trending swarm of ultramafic alkalic diatremes, dikes, and plugs 46 to 51 m.y. old (middle Eocene) (Marvin and others, 1980) in the Missouri River 

Breaks area of north-central Montana (Hearn, 1968, 

1979). The western end of the swarm is represented by 

the Haystack Butte intrusion (Buie, 1941) on the eastern 

flank of the Highwood Mountains, and the easternmost 

occurrence is the Ricker Butte group of intrusions on 

the southeastern flank of the Little Rocky Mountains. 

The diatremes and intrusions were produced by alkalic 

ultramafic magmas that crystallized to alnoite or monticellite peridotite. Their fine-grained fresh equivalents, 

which would correspond to olivine nephelinite or olivine 

melilitite, are typically altered to secondary assemblages 

in bedded pyroclastic deposits within the diatremes. Such 

alkalic ultramafic magmas occur in the same broad geographic areas as kimberlites in other parts of the world 

(Dawson, 1980) and are likely to have been parental to 

many kimberlites. Although the alkalic ultramafic character of the Missouri Breaks diatremes indicates kimberlitic affinity, only the Williams diatremes can be termed 

true kimberlite on the basis of the presence of the typical 

kimberlite indicator minerals chromian pyrope, chromian diopside, enstatite, and magnesian ilmenite (Dawson, 1980). 

8 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 

Two of the Williams diatremes are major pipes, 

and two are smaller satellitic pipes. Williams 1 is very 

poorly exposed and was mapped by soil types, the presence of indicator minerals in soils and in animal burrows, 

and limited soil auger sampling. Williams 1 is a diatreme 

of rounded triangular shape, about 250 x 350 m in size, 

that has been emplaced against the ring fracture of the 

Siparyann Butte (Thornhill Butte) dome, a dome of 

Cambrian to Cretaceous sedimentary rocks that were 

uplifted at about 61 m.y. above a hornblende syenite 

porphyry intrusion (Brockunier, 1936; Knechtel, 1959; 

Marvin and others, 1980). Wall rock of the western, 

southern, and northern sides of the kimberlite is dominantly shale, interlayered with minor beds of limestone 

and sandstone, of the Colorado Group of Late Cretaceous age. Wall rock to the east is Paleozoic limestone, 

dolomite, and calcareous shale in the dome. The diatreme mainly contains soft clay-rick kimberlite breccia, 

rich in inclusions of Cretaceous and lower Eocene sedimentary rocks. A small intrusion of partially altered 

monticellite peridotite or massive kimberlite is present 

in the southern part. Downfaulted blocks of sandstone 

and claystone of lower Eocene Wasatch Formation, 

probably 5 m to a few tens of meters in size, occur on 

the border of the Williams 1 diatreme, similar to other 

Missouri Breaks diatremes that contain large slices of 

sedimentary rocks that have subsided as much as 1 ,500 m 

from the Wasatch and older formations. Blocks of Cretaceous shale and sandstone 1 m to several meters in size 

are also present. 

The presence of olive-green pyroxenes of slightly 

rounded, prismatic euhedral shape in panned concentrates and of rare pebbles of phonolite indicates that the 

diatreme also incorporated detrital material derived 

from the Bearpaw Mountains volcanic fields in middle 

Eocene time (Hearn, 1979). 

The poorly exposed Williams 2 pipe, about 40 x 

120 m, contains kimberlite breccia that is rich in limestone and dolomite fragments from the surrounding 

Paleozoic rocks. Williams 3, about 30 x 40 m, primarily 

consists of one or more blocks of sandstone and siltstone 

that have subsided a few hundred meters from the Eagle 

Sandstone or from the Telegraph Creek Formation and 

occupy the whole pipe area with the exception of a small 

marginal occurrence of kimberlite breccia. 

Williams 4 is a dikelike diatreme 390 m long and 

up to about 40 m wide. Parts of Williams 4 are well 

exposed. The dike is variable in texture along strike. 

Massive kimberlite forms several areas from 2 m to a 

few tens of meters across; the rest of the dike has fragmental texture or is unexposed. Some massive outcrops 

show cryptic fragmental texture on close examination. 

A portion of the dike near its southwestern end contains 

abundant autoliths 0.5 to 8 em in diameter. Most autoliths contain cores of baked shale, baked sandstone, or 
igneous rock (kimberlite or monticellite peridotite?), the 

texture of which is slightly coarser than that of the peripheral micaceous igneous coating. 

Inclusions from Deep Levels 

Ascended inclusions are from the Precambrian 

basement (2,000 m and deeper), the upper crust (schist, 

gneiss, and amphibolite), the lower crust (granulite, 

mafic granulite, and mafic amphibolite), and the upper 

mantle (spinel peridotite, dunite, garnet peridotite, garnet megacrysts, and xenocrysts of kimberlite indicator 

minerals). Neither eclogites nor diamonds have been 

found. Assignment of lithologies to the upper or lower 

crust is tentative and based on analogies to inclusion 

suites in diatremes elsewhere and on sequences of metamorphic textures and facies elsewhere. Garnets occur in 

rocks from each of the depth zones listed above. Study 

of the entire suite of inclusions is in progress but so far 

has concentrated mainly on the upper mantle garnet 

peridotites. In addition, six megacryst garnets and garnets from three mafic granulites, two mafic amphibolites, and two inclusions of unknown depth of origin 

have been analyzed. 

Garnets from Garnet Peridotites 

Sheared (porphyroclastic and mosaic porphyroclastic) (Harte, 1977) and granular (coarse) (Harte, 

1977) peridotites are present but rather rare in the Williams 1 and 4 diatremes. Some granular-textured peridotites show a necklace texture of thin zones of fine-grained 

recrystallized olivine surrounding large olivine crystals. 

Because of alteration, the texture cannot be determined 

in many peridotites. In altered peridotites, serpentinization and weathering have produced pseudomorphs of all 

of the primary olivine and enstatite, but garnet and, less 

commonly, diopside survive. All eight fresh peridotites 

contain lherzolitic assemblages (olivine + clinopyroxene 

+orthopyroxene), but some contain less than 1 percent 

clinopyroxene. Garnets from 8 fresh peridotites and 

from 11 altered peridotites have been analyzed (tables 2 

and 3). Within each peridotite xenolith, these garnets 

are typically unzoned and of the same composition or 

else show a small range in composition. All the garnets 

are pyrope rich and have Mg/(Mg +Fe) of more than 

0.78 (fig. 2). Nineteen garnets taken from 15 of the 19 

peridotites contain more than 4.0 wt percent Cr20h the 

maximum being 7.83 percent; six garnets from theremaining four peridotites contain 0. 7 to 3.0 wt percent . 

Garnets from one sheared peridotite contain the lowest 

amount of Cr203 (coarse- and fine-grained garnets are 

slightly different in composition), and garnets from the 

other two sheared peridotites plot with the higher Cr203 

group of garnets (figs. 3, 4B, 5). All19 of the analyzed 

garnets containing more than 4.0 wt percent Cr203 

(from 15 peridotites) are purple; the six analyzed garnets 

(from four peridotites) containing less than 4.0 wt percent are red or red orange. Garnets are purple in 15 of 

19 garnet peridotite inclusions. The band of garnet compositions on the CaO-Cr203 plot (fig. 4B) is within or 

slightly more CaO rich than the band of garnet compositions from two-pyroxene assemblages (lherzolites) 

from Russian kimberlites (Sobolev and others, 1973). 

Peridotite garnets show an inverse relationship of Cr and 

AI (fig. 6B) owing to substitution in the six-coordinated 

position. Chromium content is probably related to differences in bulk composition in the upper mantle. 

Garnets from fresh peridotites have a bimodal distribution of CaO content (4.2-4.9 and 5.6-6.9 wt percent) (fig. 4B) and Ca (figs. 2E, 2F). Garnets from 

·altered peridotites fill in part of the Ca gap for compositions richer in Mg, but the Ca gap remains for compositions lower in Mg (figs. 2E, 2F). The bimodal CaO 

distribution is reflected in Cr203 contents of more than 

4.0 wt percent (fig. 4B) for 19 higher CaO garnets (all 

purple) and of less than 3.1 wt percent Cr203 for 6lower 

CaO garnets (red or red orange). A similar bimodal distribution of Cr203 has been noted for kimberlites from 

Somerset Island, Canada, by Mitchell (1979, p. 163) as a 

justification for subdividing Dawson and Stephens' 

(1975, 1976) group 9 garnets ("chrome pyrope") into 

high and low Cr203 subgroups. The bimodal distribution may be present in sparse data shown by McCallum 

and others (1975, p. 174) for garnets from peridotites in 

Colorado-Wyoming kimberlites. 

Megacryst Garnets 

Only six megacryst analyses are available. Colors 

are purple, red, red orange, and orange. Five of the six 

megacrysts ( > 1 em) are pyrope rich and have similar Ca 

contents; these plot in a line across the lower Ca portion 

of the peridotite garnet field (fig. 2E). All five contain 

less than 2.3 wt percent Cr203 and fit the Cr-poor group 

of garnet megacrysts (0.03-4.8 wt percent Cr203) in 

Colorado-Wyoming kimberlites (Eggler and others, 

1979). The three having the lowest Cr203 content (all red 

orange and orange) are close in composition to peridotite garnets having similar low Cr203 contents (figs. 2E, 

2F, 4B). The sixth megacryst is considerably more Fe 

rich and Cr poor and is similar to garnet compositions 

from one of the mafic granulites in the Williams diatremes (figs. 2E, 5B). The Fe-rich megacryst contains 

more Fe than the most Fe rich megacrysts and xenocrysts 

from Kimberley, South Africa, kimberlites and is similar to the most Fe rich garnets from Arizona kimberlites 

(Reid and Hanor, 1970, fig. 1). Additional analyses of 

Geology of the Williams Diatremes 9 
Kcl 

500 1000 FEET Kj '( Ke 

100 200 300 METERS :~ ...___ "-.. 

Figure 1. Geologic map of the Williams diatremes and vicinity, north-central Montana. 

10 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 
EXPLANATION 

> 

tEEl Qal Alluvial and colluvial deposits 

} 

II: --------------· Contact-Dashed where approximately located, <( 

} Hoi""'"' '"d z dotted where concealed II: 

Qls Landslide deposits Pleistocene w 

~ u <( --0----·········· Fault-Dashed where approximately located, 

at Talus deposits ::> dotted where concealed; U, upthrown side; 0 D, downthrown side 

[§] Terrace and pediment } Pleistocene and _2_0 Strike and dip of inclined beds gravel deposits Pliocene? 

> II: 7ri Strike and dip of overturned beds 

ITkmjig } <( 

I Tkm Massive kimberlite Middle i= Tkb II: 

Tkb Kimberlite breccia Eocene w X Prospect pit ~ 

D Wasatch Formation } Lower >- Ad it 

Eocene Arrows indicate direction 

of landslide movement Judith River Formation 

Claggett Shale 

Eagle Sandstone 

Telegraph Creek Formation Upper 

Cretaceous Niobrara Formation and 

Carlile Shale (/) 

Greenhorn Formation and ::> 

0 Mosby Sandstone Member w 

u of Belle Fourche Shale <( 

~ 

Belle Fourche Shale w 

II: 

u 

Mowry Shale, upper member 

Mowry Shale, lower member 

Lower 

Muddy Sandstone Cretaceous 

Skull Creek Shale 

Kootenai Formation u 

~ } Lower } 0 

N 

Paleozoic, undivided Mississippian 0 

w to Cambrian ...J 

<( 

t:L 

Geology of the Williams Diatremes 11 
-

N 

C"l 

~ ::I 

1 

Ill 

:r 

~ 0 

a 

Dol 

::I 

Dol 

0 

i 

~ 

3 

I'D 

~ 

> 

:Ill: 

I'D 

-< 

8' 

:Ill: 

§' 

c:r 

I'D 

a= 

I'D 

.., 

a 

Ill 

~ 

a 

:r 

CIQ 

Table 2. Analyses of garnets from fresh garnet peridotites in the Williams 1 and 4 diatremes 

[Analyses I17-120, I24, and I25 from Hearn and Boyd (1975); analyses 12I and 126 by F. R. Boyd (unpublished data, I973)] 

W-PV-1171,9 W-PV-1182 W-PV-1193 W-PV-1204 W-PV-1215 W-PV-1226 W-PV-1236 

Major oxides, in wt percent 

Si02 ---------- 41.36 41.11 41.27 41.53 40.99 41.99 41.71 

Ab03 --------- 19.09 16.67 17.59 18.23 18.58 18.96 19.03 

FeO ----------- 7.27 6.23 6.22 6.23 5.68 6.13 6.11 

MgO ---------- 19.61 19.92 20.57 21.17 21.43 20.30 19.67 

CaO ---------- 6.32 6.91 6.57 5.96 5.71 6.53 6.41 

Ti02 ---------- .00 .18 .17 .42 .50 .23 .23 

MnO ---------- .50 .30 .30 .29 .28 .51 .60 

Cr203 --------- 5.58 7.83 6.60 5.84 6.22 6.43 6.38 

Total------ 99.73 99.15 99.29 99.67 99.39 101.08 100.14 

Cations per 12 oxygens 

Si------------- 2.997 3.011 3.003 2.997 2.962 2.994 3.001 

AI ------------ 1.630 1.439 1.509 1.551 1.582 1.593 1.613 

Fe ------------ .441 .382 .379 .376 .343 .365 .367 

Mg ----------- 2.118 2.175 2.232 2.278 2.309 2.157 2.110 

Ca ------------ .491 .542 .512 .461 .442 .498 .493 

Ti ------------ .000 .010 .009 .023 .027 .012 .012 

Mn ----------- .031 .019 .018 .018 .017 .030 .036 

Cr ------------ .320 .454 .380 .333 .355 .362 .362 

Total------ 8.028 8.032 8.042 8.037 8.037 8.011 7.994 

1

Garnet lherzolite, granular texture, Williams 4 diatreme; Wl/2NE114 sec. 7, T. 24 N., R. 24 E.; field no. H68-16B. 2Garnet lherzolite, granular with necklace texture, Williams 4 diatreme; field no. H67-28K-l. 3Garnet lherzolite, granular with necklace texture, Williams 4 diatreme; field no. H67-281-2. 4Garnet lherzolite, sheared texture, Williams I diatreme; Wl/2NW1/4 sec. 8, T. 24 N., R. 24 E; field no. H69-I5F. 

'Garnet lherzolite, sheared texture, Williams 1 diatreme; field no. H68-17D-2. 6Garnet lherzolite, granular with necklace texture, Williams 4 diatreme; field mr. H79-6C-l. 

W-PR-1247 

40.88 

20.34 

9.62 

19.59 

4.86 

.48 

.39 

3.03 

99.19 

2.975 

1.745 

.585 

2.126 

.379 

.026 

.024 

.174 

8.034 

7 0rthopyroxene-rich garnet peridotite or olivine-garnet orthopyroxenite, granular texture, Williams 4 diatreme; field no. H67-28K-4. 

'Garnet lherzolite, sheared texture, Williams 4 diatreme; field no. H67-28K-3. 9 Analyses have been designated according to the following scheme: 

Typical analysis number: BU-G P- I 0 3 

Spaces: 1, 2 3, 4 5, 6, 7 

Spaces 1, 2-Locality: 

W Williams diatremes 

Space 3-Type of sample analyzed: 

P Grain picked out of lithic sample 

Space 4-Color group: 

V Purple 

R Red 

T Red orange 

Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality 

W-PT-1258 W-PT-1268 

42.26 41.95 

21.76 21.78 

8.45 8.03 

21.17 21.39 

4.38 4.23 

.70 .57 

.30 .29 

.69 .79 

99.71 99.03 

3.010 3.003 

1.827 1.838 

.503 .481 

2.248 2.283 

.334 .324 

.037 .031 

.018 .018 

.039 .045 

8.016 8.023 
"' I'D 

Q 

0 

OQ 

"< 

Q 

--:::r 

I'D 

~ ~-

3 

Ill 

2 

~ 

;; 

3 

I'D 

Ill 

... 

~ 

Table 3. Analyses of garnets from altered garnet peridotites in the Williams 4 diatreme 

[Analysis number followed by field number: 127, H67-28A-6; 128 and 129, H67-28A-7; 130 and 131, H67-28E-11; 132, H67-28E-14; 133 and 

134, H67-28E-16; 135, H67-28E-17; 136, H67-28E-18; 137, H67-28E-12; 138 and 139, H73-2M; 140, H67-28A-5; 141, H73-2F. Analysis 141 by 

F. R. Boyd (unpublished data, 1973)] 

W-PV-1271 W-PV-128 W-PV-129 W-PV-130 W-PV-131 W-PV-132 W-PV-133 W-PV-134 W-PV-135 W-PV-136 W-PV-137 W-PV-141 

Major oxides, in wt percent 

SiOz--------- 42.62 42.45 41.79 41.63 42.10 42.19 41.65 42.10 41.92 42.45 40.53 40.91 

Ab01-------- 19.09 19.38 19.81 19.05 19.05 20.48 19.81 20.17 18.07 18.88 18.38 18.19 

FeO --------- 6.10 6.93 6.40 7.30 7.84 6.17 6.55 6.95 6.39 6.40 8.19 6.18 

MgO -------- 21.56 19.53 20.29 19.32 19.15 20.04 20.06 20.22 20.01 21.03 18.24 21.21 

CaO--------- 5.66 6.75 5.85 6.73 6.39 6.09 5.86 5.94 6.77 5.74 6.32 5.58 

TiOz -------- .60 .00 .48 .11 .11 .15 .00 .00 .13 .56 .16 .67 

MnO -------- .49 .42 .56 .93 .83 .23 .-54 .36 .27 .35 .34 .29 

Crz01 ------- 5.32 6.31 4.26 6.55 6.30 5.37 6.08 5.65 7.30 5.93 6.55 6.34 

Total---- 101.44 101.77 99.44 101.62 101.77 100.72 100.55 101.39 100.86 101.34 98.71 99.37 

Cations per 12 oxygens 

Si ----------- 3.010 3.012 3.007 2.977 3.004 2.997 2.980 2.986 3.007 3.009 2.990 2.965 

AI ---------- 1.588 1.620 1.680 1.605 1.601 1.714 1.671 1.685 1.527 1.577 1.598 1.554 

Fe ---------- .360 .411 .385 .436 .468 .366 .392 .412 .383 .379 .505 .375 

Mg---------- 2.269 2.065 2.177 2.060 2.037 2.122 2.140 2.138 2.139 2.222 2.006 2.292 

Ca ---------- .428 .512 .451 .515 .488 .465 .449 .451 .520 .435 .499 .433 

Ti ----------- .032 .000 .026 .005 .006 .007 .000 .000 .007 .029 .008 .037 

Mn---------- .029 .025 .034 .056 .050 .013 .032 .021 .016 .020 .021 .018 

Cr ---------- .296 .353 .242 .370 .355 .301 .343 .316 .413 .332 .381 .363 

Total---- 8.012 7.998 8.002 8.024 8.009 7.983 8.007 8.009 8.012 8.003 8.008 8.037 

'Analyses have been designated according to the following scheme: 

Typical analysis number: B U - G P - 1 0 3 

Spaces: 1, 2 3, 4 s. 6, 7 

Spaces 1, 2-Locality: 

W Williams diatremes 

Space 3-Type of sample analyzed: 

P Grain picked out of lithic sample 

Space 4-Color group: 

V Purple 

R Red 

T Red orange 

Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality 

W-PR-138 W-PR-139 W-PT-140 

42.29 42.10 42.08 

21.81 21.44 21.83 

6.89 7.11 6.45 

21.43 21.59 21.87 

4.83 4.80 4.64 

.57 .60 .50 

.50 .28 .13 

1.84 1.99 2.14 

100.16 99.91 99.64 

2.994 2.991 2.985 

1.819 1.795 1.825 

.407 .422 .382 

2.262 2.286 2.313 

.366 .365 .352 

.030 .032 .026 

.029 .017 .007 

.103 .111 .120 

8.010 8.019 8.010 
... 

,flo. 

C') 

~ ::I 

!l 

Ill 

:;-

~ 

0 

::I 

S' 

::I 

AI 

0 

[ 

~ 

3 

tTl 

~ 

> 

~ 

~ 

S' 

~ 

3" 

tTl 

~ "'CC 

a 

Ill 

"C 

a 

:;· 

CJQ 

A EXPLANATION 

\ .--~ ,; 

/'"'\ "-.J •• ~ <> Red orange } ~ \...,_.____) -" .& Orange 

/---~~::---~ V Light orange \' ... ~.. ..._ .. 

30 

r ~, 

\ 

20 

10 c~~/·------. ~ ~---- ................ , 

v 

... 

\j 

q 

\j 

\j 

/;,\ v ./ \ r- • ~ ~\1 I•• ,..--:.~ 

\~/ 

30 

20 

10 

Mg Fe 

8 

\j ... 

\j ... 

<> ~·· d ~ 

~v~ v 

10f- ?- -/,o 

I 

\j 

,/ 

5 5 

Mg 70 Fe 

Figure 2. Ca-Mg-Fe in colluvial garnets, megacrysts, and garnets from inclusions in the Williams 1 and 4 diatremes. A, Redorange, orange, and light-orange garnets. 8, Enlarged plot of red-orange, orange, and light-orange Mg-rich colluvial garnets. 

C, Purple, red, and pink garnets. 0, Enlarged plot of purple, red, and pink Mg-rich colluvial garnets and areas of peridotite 

garnets. f, Megacrysts, garnets from garnet peridotites, mafic granulites, mafic amphibolites, and other inclusions. F, Enlarged 

plot of Mg-rich megacrysts, garnets from garnet peridotites, and a garnet-diopside-phlogopite inclusion. 

I EXPLANATION 

<> Red orange 

.& Orange 

\1 Light orange 

~~--"'----"' 
n 

2 

0 

?J 

a 

-:r 

~ 

;;;;· 

~ 

3 

"' 0 i. 

~ 

3 

~ 

"' 

... 

VI 

c 

!J 

0 

(:~::' 

EXPLANATION 

Area of Cr-rich purple, 

pink, and red garnets 

(Cr203>1.5 wt percent) 

Area of Cr-poor, 

Ti-rich 20 

garnets 

0 Purple 

• Pink 19 

<> Red 

~~/~--~ ~ ~:9 ', \ I .• ·• 

~~ ' ...... -:./· ~~a • • • ':'". ~.,"' •"' 

• 

• 

<> • 

<> 

<> 

o<><> <> 

,.. • • 

._. 

10 

Mg ~ 

D 

., __ 

5 

00 ~ M ~ 20 

Ca 

~~------~----~~-----;~-----;~-----;~-----;20 

50 40 30 

15 

10. 

69boo ...... , <> 

( __ ~~ 

0 0 15 

(l 

<><> 10 

<> 

cS> 5 

MgL ~------~~------~~------~~------~~------~~------7Q~Fe 90 85 

Figure 2. Continued. 

10 

EXPLANATION 

Colluvial garnets: 

0 Purple 

• Pink 

<> Red 

Peridotite garnets: 

c:!> Purple 

c=) Red orange, 

red 
.... 

Cl'l 

~ AI 

"'I 

= ll 

Ill 

:;-

~ Q 

S' = 

AI = 

c 

§: 

ttl 

3 

~ 

> 

;Ill: 

ttl 

"< 

0 

;Ill: 

~r 

g-

::1. 

;: 

""C:C 

a 

Ill 

'0 

a 

:;· 

CJQ 

E EXPLANATION 

D 0 Fresh peridotites 

(·:.:~:_-. Altered peridotites 

'-... ,' 

<] Megacrysts (purple, red, 

red orange, orange) 

,....,.-,, 

(·· ·.· .. ·.• --..._../ 

,..--, 

( I -....__/ 

Mafic granulites 

(red orange) 

Mafic amphibolites 

(orange) 

1Zl Garnet-diopsidephlogopite 

inclusion (pink) 

0 Garnet-biotiteilmenite-rutile 

inclusion (pink) 

10 

30 

20 

Q

············.·, . // 0 .... : 

<lt ... ~ .. ~ () 

<1 

0 

,'"' 

I ' \} 

r"' 

\ \ 

\ I 

,/ 

30 

J\ ;\ 

l ~ 

'i-, 

~ 

20 

10 

Mg ~ 

80 70 60 50 ·- -- --) 10 .. 

F 

L 15 

~ ~~ ~ ~ 0 ~ 

~..,_,. 121 15 

<] .. ~ ~ () 

~ 

10 

5 

Mg 95 90 85 80 75 70 

Figure 2. Continued. 

Ca 

20 

EXPLANATION 

Peridotites: 

Fresh Altered 

()0 Purple ~ 

0 Red ~ 

~ Red orange .. ~G•anula• textu<e 

Sheared texture 

<] Megacrysts (purple, red, 

red orange, orange) 

1Zl Garnet-diopsidephlogopite 

inclusion (pink) 

Fe 

... 

= 

C') 

I» 

"'I 

= 2. 

"" :;· 

~ 0 

a 

I» 

I» = 

0 

a 

"'I 

~ 

3 

~ 

~ 

> 

..,::: 

~ 

-< 

; 

..,::: 

3" 

~ 

~ 

S: 

~ 

-,:, 

a 

"C "" 

~ 

Q. 

:;· 

OQ 

7.0r-------~--------~------~--------~-------.--------.---------r-------~--------~~----~ 

A ~--

1-

z 

w 

u 

a: 

w 

a.. 

~ 

6 

co 

Samples 

excluded 

6.0 I-

~ 

5.0 I-

---- Field of lherzolite garnets ~----- o----_ -----_.-- _.--

(Sobolev and others, 1973) -i --- 0 

\1 

0 

0 

0 

0 

oO 

0 

0 

ooo 

0 

0 oo 

0 

0 

EXPLANATION 

0 Purple 

D Pink 

0 

-------

~---

0 

- ~,_. 

~~ 

-

u 

~~~'Vi>.~~(> 6 <> <> 

\1 6:: <> '\}(:> <> 

0 0 

0 

) 

--- ~-

0 Red 

<> Red orange 

6. Orange 

-- -- ---- \1 Light orange 4.0 I7.0 

1- 6.0 

z 

w 

u 

a: 

w 

a.. 

~ • 5.0 

0 

co 

u 

4.0 

8 

I I 

Field of lherzolite garnets 

(Sobolev and others, 1973) 

<l 

~~ 

• <J<kl 

~ 

1.0 2.0 

.......... 

k-:--- 1 l I l I I 

Cr203, WT PERCENT 

~ -- - --- 0 -------- ------ 0 ~ ------ --~ --- r-.. - -- ., ~~ 

-- -- () 

--

<> 

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

-------

3.0 4.0 5.0 6.0 

Cr203, WT PERCENT 

EXPLANATION 

Fresh 

() 0 Purple 

0 Red 

Altered 

~ -- ~ Red orange + 

L_L Granular texture 

Sheared texture 

<l Megacrysts (purple, red, 

red orange, orange) 

Lines connect garnets from 

same xenolith 

7.0 8.0 9.0 

---

-

10.0 

Figure 4. Weight percent CaO versus weight percent Cr20 3 for garnets in the Williams 1 and 4 diatremes. A, Colluvial garnets with Cr20 3 greater than 0.50 

wt percent. 8, Megacryst garnets and garnets from garnet peridotites. 
megacrysts may fill in part of the gap. However, if 

megacrysts are fully represented in the colluvial garnet 

population (figs. 2A-W), a complete series between Mgand Fe-rich megacrysts is not present. Chromium-rich 

garnet megacrysts (6.3-13.0 wt percent Cr203) have 

been reported from Colorado-Wyoming kimberlites 

(Eggler and others, 1979, p. 215) but have not been 

found in the Williams kimberlites. 

Garnets from Granulites, Mafic Granulites, 

and Mafic Amphibolites 

Granulites, mafic granulites, and mafic amphibolites are rather common in the Williams 1 and 4 diatremes. Granulites have assemblages of garnet+ clinopyroxene+ plagioclase, and mafic granulites have garnet+ clinopyroxene± plagioclase. Both may contain 

small amounts of hypersthene, quartz, K-feldspar, 

rutile, amphibole, apatite, ilmenite, biotite, and carbonate. Mafic amphibolites have assemblages of garnet+ 

amphibole and may contain small amounts of plagioclase, clinopyroxene, apatite, sphene, ilmenite, and 

carbonate. 

Orange and red-orange garnets from three mafic 

granulites and orange garnets from two mafic amphibolites have been analyzed (table 5) and are Fe rich (fig. 

2E), distinctly different from the peridotite garnets and 

most of the megacryst garnets. Granulite garnets show a 

small range of composition within each inclusion and 

have an overall range of 0.27 to 0.45 Mg/(Mg +Fe) and 

5.0 to 8.4 wt percent CaO, the two more Fe rich samples 

being higher in Ca. Their Cr203 contents are less than 

0.2 wt percent, Cr cations per 12 oxygens are less than 

0.02 (fig. 5B), and MnO contents range from 0.4 to 

0. 7 wt percent. Analyses of garnets from other mafic 

granulites may fill in the small compositional gap between mafic granulites and extend their range. However, 

compositions of colluvial garnets (figs. 2A-2D), which 

are probably in part derived from mafic granulites, indicate that garnets of intermediate Mg/(Mg +Fe) (0.45-

0. 70) are rare or absent. 

The two mafic amphibolites contain garnets that 

also show a small range of composition (table 5). These 

garnets contain more Ca and as much or more Fe in 

comparison with the granulite garnets (fig. 2E). Mafic 

amphibolite garnets have Mg/(Mg +Fe) of 0.17 to 0.20 

(fig. 5B) and contain less than 0.15 wt percent Cr203 

and more MnO (2.2-4.5 wt percent) than any other 

xenolith garnets analyzed so far. 

Although mafic granulites that lack plagioclase 

can resemble eclogites in appearance, the granulite 

pyroxenes tend to be dark green rather than the graygreen or pale-green color of eclogitic pyroxenes and are 

low in the jadeite component (NaAlSi206) that is typical 

of eclogitic pyroxenes. The Ca-Mg-Fe contents of granulite garnets do not distinguish them from eclogite garnets. 

The granulite garnets plot within the area of Dawson 

and Stephens' (1975, 1976) group 3 (garnets dominantly 

from eclogites), but Dawson and Stephens did not include granulite garnets in the population used for their 

cluster group classification. 

Garnets from Other Inclusions in the 

Williams Diatremes 

A garnet-diopside-phlogopite inclusion (H67-

28E-3, table 5) is probably related to the garnet peridotite suite because of similar pyrope content in the pink 

garnet. Although the Cr203 content (0.44 wt percent) is 

lower than that of any peridotite garnet or Mg-rich megacryst, the Cr203 content is higher than that of any 

mafic granulite garnet or amphibolite garnet (compare 

Cr contents in fig. 5B). 

The Ca-Mg-Fe composition of pink garnet from a 

coarse-grained garnet-rich garnet-biotite-ilmenite-rutile 

inclusion (H67-50D, table 5) is close to that of garnet 

from one of the mafic granulites (fig. 2E) and contains 

low Cr203 (0.14 wt percent) (compare Cr contents in 

fig. 5B). This inclusion could be from the upper or 

lower crust. 

GARNET-BEARING INCLUSIONS IN 

OTHER DIATREMES 

Two orthopyroxenites from the Bullwhacker 

Coulee diatreme (Hearn, 1979) contain entirely altered 

orthopyroxene; small amounts of pink garnet and phlogopite are present in one, and small amounts of pink 

garnet, diopside, and amphibole(?) are found in the 

other (table 6B). The small ranges of garnet Mg/(Mg 

+Fe) values for each inclusion average 0.53 and 0.63 

and are intermediate between peridotite garnets and 

granulite garnets (figs. 2E, 7A). The Cr203 content is 

0.4 to 0.5 wt percent, higher than the Cr203 contents of 

garnets in mafic granulites (compare Cr contents in 

figs. 5B, SA). Whether the orthopyroxenites are from 

the upper mantle or the lower crust is unknown; the 

Cr20 3 content of the garnets suggests that the inclusions 

originated below upper crustal levels. Granulites and 

mafic granulites are found in the Bullwhacker Coulee 

diatreme, but their garnets have not yet been analyzed. 

Altered spinel peridotites are also present but rare. 

From the Big Slide diatreme (Hearn, 1979), pink 

garnet (table 6A) in a felsic granulitic gneiss that is dominantly composed of K-feldspar, plagioclase, quartz, 

garnet, and kyanite and contains small amounts of 

phlogopite, graphite, and rutile has an intermediate 

Geology of the Williams Diatremes 19 
en 

z 

w 

0.50 

0.40 

~ 0.30 

X 

0 

N 

.... 

c:: 

w 

t:l. 

en 0.20 

z 

0 

~ u 

u 

0.10 

A 

EXPLANATION 

0 Purple 

D Pink 

0 Red 

EXPLANATION 

<> 

t::. 

0.1 0.4 

0 

<> 

<> 

0 

0 

0 

<¥ 

cB 

0 

0 

000 

<> 

a 

so 

<>o 

DO 

Mg/(Mg +Fe) ATOMIC PROPORTIONS 

Figure 5. Chromium cations per 12 oxygens versus Mg/(Mg+ Fe) for garnets in the Williams 1 and 4 diatremes. A, Colluvial 

garnets. B, Garnets from inclusions and megacrysts. 

Mg/(Mg +Fe) value of 0.38 and contains less CaO than 

garnets from granulites, pyroxenites, or peridotites. The 

Cr203 content of the garnet from gneiss is low, less than 

0.2 wt percent. Garnets from granulites that are locally 

abundant in the Big Slide diatreme have not yet been 

analyzed. 

From the Ervin Ridge 1 diatreme (Hearn, 1979), 

pink garnet (table 6C) from an inclusion of garnetbiotite-feldspar-sillimanite schist plots in the cluster of 

garnets from the Precambrian core of the Little Rocky 

Mountains (fig. 7C; see next section). A single xenocryst 

of pink garnet (table 6C) from the Bird Rapids 2 diatreme (Hearn, 1979) is slightly more Mg rich than garnets 

of that cluster (fig. 7C). 

20 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 

GARNETS FROM PRECAMBRIAN ROCKS IN 

THE LITTLE ROCKY MOUNTAINS 

The tight cluster (fig. 7C) of garnets known or 

assumed to be derived from the Precambrian core of the 

Little Rocky Mountains contains data for three types of 

samples: (1) three orange garnets from garnet-kyanitemuscovite-quartz-feldspar schist cropping out in the 

Little Rocky Mountains, (2) two purple garnets that 

were collected from an anthill on the Thorsen dike and 

are probably residual from nearby terrace gravel deposits 

derived from the Little Rocky Mountains, and (3) a separate pink garnet and two pink garnets in pebbles of 

mica schist in conglomeratic sandstones of the Fort 
0.60 r---------.l------,-l-----r-l----.,.--l--------.l-------rl-----r-l----.,.--1-------, 

0.50-

(/) 

a:i 0.40-

(.9 

>-

X 

0 

N 

ffi 0.30 r- Q.. 

(/) 

z 

0 

i= 

~ 

u 

..... 0.20-

u 

0.10~ 

8 

EXPLANATION 

Peridotites: 

Fresh 

() 0 Purple 

() Red 

Altered 

~ 

~ 

<1 

I2J 

D 

Red orange + 

Granular texture 

Sheared texture 

Megacrysts (purple, red, 

red onmge, orange) 

Garnet-diopside-phlogopite 

inclusion (pink) 

Garnet-biotite-ilmenite(?)-

rutile inclusion (pink) 

Mafic granulites (red orange) 

Mafic amphibolites (orange) 

-

0 

-

-

0 

o~--~~--~-~-~~--~---L~-~~~~o~~~~~-~~---~1 0 0.1 0.2 0.3 0.4 0.5 0.6 ____ 0.7 1~--~~---~ 0.8 0.9 

Mg/(Mg +Fe) CATION PROPORTIONS 

Figure 5. Continued. 

Union Formation of Paleocene age that have been downfaulted several hundred meters along the borders of the 

Squaw Creek and Shellenberger Divide diatremes (Hearn, 

1979). Clasts in these conglomeratic sandstones were 

probably derived from the Little Rocky Mountains. In 

spite of their range of colors (purple, pink, and orange), 

garnets of all three types fall within the limited range of 

0.18 to 0.29 Mg/(Mg +Fe) and are consistently low in 

CaO, all containing less than 2.3 wt percent. Cr203 is 

extremely low, less than 0.05 wt percent, although one 

garnet from the Thorsen dike locality contains 0.18 wt 

percent Cr203. The MnO content is variable, ranging 

from 0.55 to 1.48 wt percent. 

COLLUVIAL GARNETS FROM THE 

WILLIAMS DIATREMES 

Loose colluvial material was collected from the 

Williams 1 and 4 diatremes, which are 700 m apart 

(fig. 1), to represent the colluvial garnet population that 

could be found downstream from the Williams diatremes. The samples were panned, sieved, and separated 

with heavy liquids. Approximately 700 garnet grains 

ranging in size from 0.84 to 2.0 mm were classified into 

six color groups on the basis of binocular microscope 

observations. The color groups used were purple, red, 

pink, red orange, orange, and light orange. Eighteen to 

22 grains from each color group were analyzed for Si02, 

Al203, FeO, MgO, CaO, Ti02, MnO, and Cr203 by 

means of electron microprobe. All tabulated analyses 

are averages of two to three analyses of each grain. No 

significant color variation was observed within grains, 

and no zoning was found by probe analysis. The chosen 

size range represents a common grain size of garnets in 

many types of xenoliths but may bias the sample and 

could exclude some xenoliths or megacrysts. 

Chemical data from the six color groups of garnets 

from two samples of panned concentrates from the Williams diatremes were tabulated (table 1) and plotted 

(figs. 2-6) together for comparisons among the color 

groups with garnets of known sources (tables 2, 3; 

figs. 2-6) and with garnets from other diatremes (table 

6A, 6B, 6C; figs. 7, 8). 

The purple garnets are the most distinctive group 

because of their high Cr 203 content and restricted ranges 

of MgO, FeO, and CaO. The range of Al203 contents 

for purple garnets is slightly larger than that for other 

groups as a result of substituting Cr for Al (fig. 6A). 

Twenty of 21 purple garnets (fig. 3) contain more than 

Garnet-Bearing Inclusions in Other Diatremes 21 
0.6.---------~----------.---------~----------.-----------.----------,----------, 

A 

0.5 EXPLANATION 

0 Purple 

0 Pink 

0 Red 

0.4 

(/) 

z 

w 

(.!) 

>- 0 

X 

0 00 0 N 

.-

a: 0.3 w 0 0... 0 0 (/) 0 z 

0 0 i= 

<( 00 0 u 0 

00 u 0.2 8 

cP 00 oO 

oo 

0.1 B~ 00 

0 (/) 

0 Q] 

0 

1.4 1.5 1.6 1.7 2.1 

AI CATIONS PER 12 OXYGENS 

Figure 6. Chromium versus aluminum cations per 12 oxygens for garnets in the Williams 1 and 4 diatremes. A, Purple, pink, 

and red colluvial garnets. 8, Garnets in peridotites and megacrysts. 

2.4 wt percent Cr203, more than any of the analyzed 

red-orange, orange, or light-orange garnets. The Cr20 3 

content of 16 of 21 purple garnets is larger than that of 

any pink or red garnets. One purple garnet (W -G V -07) 

(table 1) appears unique because its CaO content is much 

lower than that of any other purple garnet, it has only 

0.04 wt percent Cr203, and its Ti02 content is much 

higher than that of any other purple garnet. It is similar 

to four red garnets (fig. 2C) and one light-orange garnet 

(W-GL-102) (table 1) that also are very low in Cr203 

and have more than 1.0 wt percent Ti02. 

The red and pink groups both can be divided into 

two subgroups based on MgO and FeO content. The 

MgO-rich subgroup of each color is high in Cr203 but 

low in FeO (figs. 2C, 3). The MgO-rich subgroups are 

close to the purple garnet group in Ca-Mg-Fe composition (fig. 2). All of the MgO-rich pink garnets contain 

22 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 

more than 0.7 wt percent Cr203. The FeO-rich subgroups 

of the pink and red garnets are low in MgO and Cr203. 

Some of the FeO-rich pink garnets have MnO values 

(1.7-1.9 wt percent) higher than those of any other analyzed colluvial garnets. 

Garnets of the three orange groups (red orange, 

orange, and light orange) were the most difficult to distinguish from one another because they are gradational 

in color. The three color groups show considerable overlapping of compositional ranges (figs. 2A, 5A). All 

three orange groups have a small range of AbOh and 

their Cr 203 and Cr range is smaller than that of purple, 

red. and pink groups (figs. 3, 5A). Like the red and pink 

garnets, the three orange groups can be split into two 

subgroups based on relative FeO and MgO contents. 

The Cr20l and Cr content of almost all garnets of the 

MgO-rich subgroup is higher than that of the FeO-rich 
0.6 I I I I I I 

8 

EXPLANATION 

Peridotites: 

0.5 - Fresh Altered - <10 Purple ~ 

0 Red ~ 0 ~ Red orange • ~ Granular texture 

~ Sheared texture 

0.4 1-- <l Megacrysts (purple, red, - (f) 

z 0 ~ red orange, orange) 

()~~ (!) 

w ~ Lines connect garnets 

>- 0 X from same inclusion 

() 0 ~ 

N 0 .... 0 

a: 0.3 1--

~ 0 - w 

a.. 

(f) 

z 

0 

~ u 

u 0.2 - -

0 

0.1 - ~ -

0~------~'------~'--------~~------~~------~l~<l~-----~1----~ 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1 

AI CATIONS PER 12 OXYGENS 

Figure 6. Continued. 

subgroup (figs. 3, SA). The range of CaO of the three 

orange groups is similar to the range of CaO observed in 

the purple, red, and pink garnets, although one orange 

and one light-orange garnet have higher CaO contents 

(compare Ca contents in figs. 2A, 2C). 

COMPARISON OF WILLIAMS COLLUVIAL 

GARNETS WITH GARNETS FROM 

KNOWN SOURCES 

The colluvial purple garnets and the Mg- and Crrich red and pink garnets show a remarkable correspondence with the peridotite and megacryst garnets (figs. 2C, 

2£, 5). In addition, these colluvial garnets, peridotite 

garnets, and megacryst garnets show the same trend of 

increasing Cr203 with increasing CaO (fig. 4); this trend 

is the lherzolitic trend of Sobolev and others (1973). 

However, some of the purple colluvial garnets (fig. 4A) 

fill a gap in the CaO-Cr203 trend between purple and 

red or red-orange garnets in peridotites (fig. 4B). All of 

the purple garnets from peridotites have CaO values 

between 5.5 and 7.0 wt percent and Cr203 values between 

4.0 and 7.9 wt percent, whereas 19 of 21 purple colluvial 

garnets (fig. 3) have generally equivalent or lower values 

of CaO (4.5-6.1 wt percent) and Cr203 (2.4-6.1 wt percent) (fig. 4A). One colluvial garnet is higher in CaO 

(6.7 wt percent) and is exceptionally high in Cr203 (9.9 wt 

percent). Thus, the colluvial purple garnets may, in 

part, represent disaggregated garnet peridotites that 

have not yet been found as xenoliths. 

Cr-rich, Ca-poor garnets (group 10, low-calcium 

chrome-pyrope) (Dawson and Stephens, 1975, 1976) 

have not been found in the Williams colluvial garnet 

suite. Such garnets are present in other kimberlites as 

xenocrysts, inclusions in diamond, and grains in garnet 

harzburgite or garnet dunite xenoliths, commonly with 

Garnet-Bearing Inclusions in Other Diatremes 23 
Table 4. Analyses of garnet megacrysts in the Williams 4 diatreme 

[Analysis number followed by field number: 142, H67-28J-2; 143, H67-28J-4; 144, H67-28B; 145, H67-28J-1; 146, P64-40H-1; 147, 

H67-28J-3. Analyses 143 and 144 from Hearn and Boyd (1975); analyses 142 and 145-147 by F. R. Boyd (unpublished data, 1973)] 

W-XV-1421 W-XR-143 W-XT -144 W-XT-145 W-XT-146 W-X0-147 

Major oxides, in wt percent 

Si02-------- 42.55 42.37 41.90 42.01 39.05 41.86 

Ab03 ------ 21.44 21.47 21.76 22.04 21.05 21.45 

FeO -------- 5.64 7.00 8.57 8.36 22.22 8.66 

MgO ------- 22.44 22.04 20.37 20.91 9.67 20.76 

CaO ------- 4.84 4.93 4.52 4.52 5.98 4.50 

Ti02 ------- .29 .45 .49 .67 .08 .70 

MnO------- .26 .30 .32 .32 .87 .29 

Cr203 ------ 2.13 2.24 .74 .80 .10 .66 

Total--- 99.59 100.80 98.67 99.63 99.02 98.88 

Cations per 12 oxygens 

Si---------- 3.011 2.986 3.019 

AI --------- 1.788 1.783 1.848 

Fe --------- .334 .412 .516 

Mg--------- 2.368 2.315 2.188 

Ca --------- .367 .372 .349 

Ti---------- .015 .024 .027 

Mn -------- .016 .018 .020 

Cr --------- .119 .125 .042 

Total--- 8.018 8.035 8.009 

• Analyses have been designated according to the following scheme: 

Typical analysis number: BU-G P- l 0 3 

Spaces: 1, 2 3, 4 

Spaces 1, 2-Locality: 

w Williams diatremes 

Space 3-Type of sample analyzed: 

X Xenocryst or megacryst 

Space 4-Color group: 

v Purple 

R Red 

T Red orange 

0 Orange 

5, 6, 7 

2.996 3.009 3.012 

1.853 1.912 1.819 

.499 1.432 .521 

2.223 1.111 2.227 

.345 .494 .347 

.036 .005 .038 

.019 .057 .018 

.045 .006 .038 

8.016 8.026 8.020 

Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality 

Cr-rich spinel also present (Finnerty and Boyd, 1980; 

McCallum and Eggler, 1976; Sobolev and others, 1973; 

Gurney and Switzer, 1973). Green garnets of high Ca 

and high Cr content (group 12, knorringitic uvarovitepyrope) (Dawson and Stephens, 1975, 1976) known 

from other localities (summarized by Clarke and Carswell, 1977) have not been found in the Williams 

kimberlite. 

The Mg-rich subgroups of pink, red, red-orange, 

orange, and light-orange colluvial garnets tend to be 

rich in Cr203 (fig. 3) and thus may be fragments of upper 

mantle rocks or fragments of megacrysts. Mg-rich pink 

garnets are richer in Mg and poorer inCa than most of 

the purple garnets and overlap part of the cluster of 

eight red garnets (fig. 2D). Eight of the 10 Mg-rich pink 

garnets contain more than 1.7 wt percent Cr203, and all 

contain more than 0.7 wt percent Cr203. Of the main 

cluster of eight Mg-rich red garnets (fig. 2D), six contain more than 1.5 wt percent Cr203, and all contain 

24 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 

more than 1 wt percent Cr203. A separate group of four 

Mg-rich red garnets and one purple garnet (fig. 2B) and 

one light-orange garnet (fig. 2B) is distinctive in that 

these garnets contain less than 1 wt percent Cr20h 1.0 to 

2.9 wt percent Ti02 (more than any of the Cr-rich red 

garnets), and slightly more Fe and show variable Ca for 

nearly constant Mg/Fe. Three additional Mg- and Tirich, Cr-poor garnets (W-GT-67, W-H0-81, and 

W-G0-90, table 1) may belong to the same group, but 

their slightly higher Ca content places them in the main 

cluster of red-orange and orange Mg-rich garnets (fig. 

2B). If these garnets are indeed related to one another, 

they may represent a series of bulk compositions that 

are related by fractionation of Ca. This group of six to 

nine Mg- and Ti-rich, Cr-poor garnets does not correspond to any analyzed xenoliths or megacrysts. Some of 

these garnets may fit Dawson and Stephens' (1975, 1976) 

cluster group 2 (high-titanium pyrope) (all examples are 

megacrysts or xenocrysts in kimberlite). These garnets 
Table 5. Analyses of garnets from other inclusions in the Williams 1 and 4 diatremes 

[Analyses 148 and 149 by F. R. Boyd (unpublished data, 1973)] 

W-PP-148 1,8 W-PP-1492 W-T0-1503 W-T0-15P W-TQ-1523 W-TI-1534 W-TI-1544 W-TI-1554 W-TI-1565 W-TI-1575 

Major oxides, in wt percent 

Si02 --------- 42.23 38.91 39.08 39.15 39.94 38.93 38.82 38.64 38.40 37.99 

Ab03 -------- 22.28 21.09 22.10 22.46 22.36 21.62 21.48 21.38 21.38 21.80 

FeO --------- 8.32 24.56 24.61 23.97 23.23 26.85 24.88 26.42 25.41 25.18 

MgO--------- 19.62 8.35 9.14 10.25 10.28 6.20 7.21 6.75 5.45 6.77 

CaO --------- 5.28 4.98 5.13 4.97 5.39 6.93 6.81 6.62 8.33 6.85 

Ti02 --------- .13 .04 .09 .06 .09 .07 .08 .21 .03 .07 

MnO -------- .38 .51 .48 .48 .46 .54 .52 .56 .70 .59 

Cr203 -------- .44 .14 .06 .16 .08 .02 .03 .04 .07 .09 

Total---- 98.68 98.58 100.69 101.50 101.83 101.16 99.83 100.62 99.77 99.34 

Cations per 12 oxygens 

Si ----------- 3.039 3.030 2.976 2.947 2.983 2.997 3.002 2.985 3.001 2.964 

Al----------- 1.889 1.935 1.981 1.993 1.968 1.961 1.957 1.945 1.968 2.004 

Fe ----------- .501 1.599 1.566 1.509 1.451 1.729 1.607 1.707 1.661 1.642 

Mg ---------- 2.105 .969 1.037 1.149 1.144 .711 .831 .777 .633 .788 

Ca ----------- .407 .415 .417 .400 .432 .571 .563 .546 .697 .572 

Ti ----------- .007 .002 .004 .002 .004 .002 .003 .011 .000 .002 

Mn ---------- .023 .034 .029 .028 .028 .034 .033 .036 .045 .039 

Cr----------- .025 .009 .002 .009 .004 .000 .001 .002 .002 .004 

Total---- 7.996 7.993 8.012 8.037 8.014 8.005 7.997 8.009 8.007 8.015 

W-TI-1585 W-T0-1597 W-T0-1607 W-T0-16F W-T0-1626 W-T0-1636 W-T0-1646 W-T0-1657 W-T0-1667 W-T0-1677 

Major oxides, in wt percent 

Si02 --------- 38.46 38.11 38.42 38.07 38.13 38.14 38.01 38.46 38.39 37.94 

~ Ab03 -------- 21.18 21.00 21.21 21.03 21.02 21.13 20.90 21.19 21.31 21.13 

ii:i' FeO --------- 26.65 26.16 26.62 25.57 22.53 22.95 22.79 26.28 26.41 25.50 

3 MgO--------- 5.91 3.23 3.37 3.24 2.95 3.07 3.14 3.39 3.54 3.15 <ll CaO --------- 7.41 9.52 9.38 10.20 11.02 10.76 10.42 9.43 9.01 10.14 1'"1 

2. Ti02 --------- .10 .06 .04 .03 .06 .04 .05 .05 .03 .07 

c MnO -------- .58 2.59 2.08 2.21 4.51 4.28 4.50 2.27 2.27 2.16 < ii:i' Cr203 -------- .04 .07 .06 .05 .04 .05 .10 .13 .00 .09 - Total---- 101.33 100.74 101.18 100.40 100.26 100.42 99.91 101.20 100.96 100.18 C'l 

~ Cations per 12 oxygens :I 

!1 

<ll Si ----------- 2.995 2.997 3.002 2.997 3.002 2.997 3.003 3.003 3.002 2.992 Q,l Al----------- 1.943 1.946 1.953 1.951 1.950 1.956 1.945 1.950 1.964 1.963 :I 

Q. Fe ----------- 1.735 1.720 1.739 1.683 1.483 1.508 1.505 1.716 1.726 1.682 

C'l Mg ---------- .686 .378 .392 .380 .346 .360 .370 .394 .412 .369 Q,l 

... 

:I Ca ----------- .617 .801 .785 .860 .929 .906 .882 .788 .755 .856 

!1 Ti ----------- .004 .003 .002 .001 .003 .002 .003 .002 .001 .004 <ll 

- Mn ---------- .036 .172 .137 .147 .300 .284 .301 .149 .150 .144 ... 

0 Cr ----------- .002 .004 .003 .003 .002 .003 .006 .007 .000 .005 3 Total---- 8.018 8.021 8.013 8.022 8.015 8.016 8.015 8.009 8.010 8.015 Q 'Garnet-diopside-phlogopite inclusion, Williams 4 diatreme; field no. H67-28E-3. Spaces 1, 2-Locality: :::r 

II) 'Garnet-biotite-ilmenite(?)-rutile inclusion, garnet rich, Williams 1 diatreme; field no. H67-50D . W Williams diatremes ... 

~ 'Garnet-clinopyroxene-rutile granulite, Williams 4 diatreme; field no. H67-28J-l. Space 3-Type of sample analyzed: 

•Garnet-clinopyroxene-plagioclase-quartz(?)-rutile granulite, Williams 4 diatreme; field no. H67-28H-l. P Grain picked out of lithic sample 5; 'Garnet-clinopyroxene-plagioclase-K feldspar(?)-biotite-carbonate-rutile granulite, Williams 1 diatreme; field no. H67-17A-7A. T Thin section of inclusion t"' 

II) •Garnet-carbonate-apatite-clinopyroxene(?) amphibolite, Williams 4 diatreme; field no. H67-28D-2. Space 4-Color group: <ll 'Garnet-plagioclase-carbonate-apatite amphibolite, Williams 4 diatreme; field no. H67-28D-l. P Pink 

~ • Analyses have been designated according to the following scheme: T Red orange 

1.11 Typical analysis number: 8 U - G P - 1 0 3 0 Orange 

Spaces: 1, 2 3, 4 5, 6, 7 Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality 
N Table 6A. Analyses of garnets from the Big Slide 1 diatreme, SW1/4SE1/4 sec. 22 and NW1/4NE1/4 sec. 27, T. 24 N., R. 19 E. =--

~ [Analyses 1-5 and 9-18: Panned concentrate from coarse alluvium in gully northwest of eastern breccia pipe; field no. H67-17A. Analyses 6-8: Garnete: kyanite-quartz-K feldspar-plagioclase granulitic gneiss with accessory phlogopite, rutile, and graphite; field no. H69-8C-2. Analysis 19: Panned concentrate ::s from surface of western slope of eastern breccia pipe; field no. H69-8B-4] ~ Ill 

:r BS-GV-01 1 BS-GV-02 BS-GV-03 BS-GV-18 BS-GR-04 BS-GR-05 BS-PP-06 BS-PP-07 BS-PP-08 BS-GP-09 

~ Major oxldes, in wt percent 0 

::s 

S' Si02 --------- 40.96 38.68 38.45 41.24 39.06 39.02 39.16 39.30 39.22 40.34 ::s 

Ill Ab03 -------- 17.54 21.71 20.89 20.98 22.01 22.00 21.92 21.72 21.90 22.86 

c FeO --------- 7.89 25.39 28.69 7.62 25.04 23.00 26.69 26.88 26.76 19.11 i' MgO--------- 19.41 5.87 6.13 19.40 7.92 9.11 9.33 9.31 9.43 12.66 ii1 CaO --------- 6.25 7.76 5.23 5.59 6.17 5.24 2.30 2.26 2.18 4.84 3 

I'D Ti02 --------- 1.23 .08 .05 .05 .10 .07 .01 .00 .00 .05 Ill 

.. MnO -------- .39 .50 .56 .43 .52 .56 .30 .27 .42 .49 > 

r;: Cr203 -------- 7.30 .02 .07 3.67 .00 .25 .04 .00 .07 .42 

"< Total---- 100.97 100.01 100.07 98.98 100.82 99.25 99.75 99.74 99.98 100.77 

0 Cations per 12 oxygens :111:: 3' Si ----------- 2.960 3.002 3.009 2.987 2.985 2.996 3.010 3.023 3.009 2.984 

I'D =- AI----------- 1.493 1.986 1.927 1.791 1.982 1.989 1.986 1.968 1.980 1.992 :::!.. ;: Fe ----------- .477 1.646 1.877 .461 1.599 1.475 1.715 1.729 1.717 1.181 

Mg ---------- 2.091 .679 .714 2.095 .901 1.043 1.069 1.068 1.079 1.396 "'CI 

0 

.. Ca----------- .483 .645 .437 .433 .504 .430 .189 .185 .179 .382 Ill 

"C Ti----------- .065 .004 .002 .002 .004 .002 .000 .000 .000 .002 

a Mn ---------- .023 .032 .037 .026 .031 .036 .019 .017 .027 .030 

:r Cr----------- .416 .000 .002 .210 .000 .013 .002 .000 .004 .023 (JQ Total---- 8.008 7.994 8.005 8.005 8.006 7.984 7.990 7.990 7.995 7.990 

BS-GP-10 BS-GP-11 BS-GT -12 BS-GT-13 BS-GT-14 BS-G0-15 BS-G0-16 BS-G0-17 BS-G0-19 

Major oxides, in wt percent 

Si02 --------- 39.04 39.24 38.88 38.18 38.60 39.40 40.14 39.68 39.33 

Al203 -------- 22.26 22.55 21.78 21.49 21.03 21.89 23.00 22.23 21.21 

FeO --------- 27.31 26.54 24.06 23.97 27.78 22.15 20.86 21.95 23.12 

MgO--------- 8.36 9.93 8.21 7.01 5.89 9.43 11.74 9.50 9.48 

CaO --------- 3.43 1.96 6.02 7.23 6.38 7.16 5.29 6.76 5.36 

Ti02 --------- .02 .02 .08 .14 .09 .07 .09 .06 .05 

MnO -------- .20 .22 .39 .38 .52 .35 .49 .55 .43 

Cr203 -------- .14 .01 .02 .04 .13 .22 .23 .09 .00 

Total---- 100.76 100.47 99.44 98.44 100.42 100.67 101.84 100.82 98.98 

Cations per 12 oxygens 

Si ----------- 2.988 2.986 2.998 2.988 3.008 2.981 2.967 2.994 3.029 

AI----------- 2.008 2.023 1.978 1.982 1.930 1.951 2.004 1.975 1.925 

Fe----------- 1.748 1.688 1.550 1.568 1.809 1.401 1.288 1.384 1.489 

Mg ---------- .952 1.125 .943 .817 .684 1.063 1.292 1.067 1.088 

Ca----------- .281 .158 .495 .606 .530 .579 .418 .544 .442 

Ti----------- .000 .000 .004 .007 .004 .002 .004 .002 .002 

Mn ---------- .011 .013 .024 .023 .032 .022 .030 .035 .028 

Cr----------- .006 .000 .000 .002 .007 .011 .013 .004 .000 

Total---- 7.994 7.993 7.992 7.993 8.004 8.010 8.016 8.005 8.003 

'Analyses have been designated according to the following scheme: Space 4-Color group: 

Typical analysis number: 8 U - G P - 1 0 3 V Purple 

Spaces: 1, 2 3, 4 5, 6, 7 R Red Spaces 1, 2-Locality: P Pink BS Big Slide diatreme T Red orange 

Space 3-Type of sample analyzed: 0 Orange 

G Grains from panned concentrate, heavy-mineral separate, gravel, or anthill Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality P Grain picked out of lithic sample 
jij' 

~ 

3 

"' n 

2.. 

c 

< 

e1: 

~ 1:11 

.. 

:::s 

~ 

.... 

"' 1:11 

:::s 

Q. 

C') 

1:11 

.. 

:::s 

a 

"' 

-

a 

3 

Q 

::r 

~ .. 

~ c 

.. 

t"' 

~ 

"' 

~ 

...... 

Table 68. Analyses of garnets from the Bullwhacker Coulee diatreme, NE1/4SE1/4 sec. 33, T. 24 N., R. 21 E. 

[Analyses 1 and 2: Altered garnet-phlogopite orthopyroxenite(?); field no. H68-6A. Analyses 3-5, 9, and 10: Panned concentrate, eastern breccia pipe; field 

no. H71-21G. Analyses 6-8: Altered amphibole-clinopyroxene orthopyroxenite(?); field no. H71-21B] 

BU-PP-01 1 BU-PP-02 BU-GP-03 BU-GP-04 BU-GP-05 BU-PP-06 BU-PP-07 BU-PP-08 

Major oxides, in wt percent 

Si02 --------- 41.06 40.55 40.11 39.44 39.58 40.02 39.68 39.91 

Al203 -------- 22.92 23.06 23.03 22.44 22.69 22.37 22.34 22.14 

FeO --------- 15.84 16.41 21.15 24.22 19.13 18.89 19.02 19.45 

MgO--------- 15.69 14.94 14.15 10.91 12.76 12.36 12.23 11.77 

CaO --------- 4.89 4.84 2.08 3.53 4.71 4.66 4.81 4.64 

Ti02 --------- .00 .00 .04 .02 .09 .01 .00 .02 

MnO -------- .63 .69 .20 .32 .61 .82 .77 .90 

Cr 203 -------- .41 .49 .03 .09 .39 .48 .46 .48 

Total---- 101.44 100.98 100.79 100.97 99.96 99.61 99.31 99.31 

Cations per 12 oxygens 

Si ----------- 2.974 2.962 2.967 2.972 2.960 2.998 2.987 3.009 

AI----------- 1.956 1.985 2.008 1.992 2.000 1.975 1.982 1.967 

Fe----------- .958 1.001 1.308 1.526 1.197 1.183 1.197 1.226 

Mg ---------- 1.694 1.627 1.560 1.225 1.422 1.379 1.372 1.323 

Ca----------- .378 .377 .165 .283 .377 .374 .386 .374 

Ti----------- .000 .000 .002 .000 .005 .000 .000 .000 

Mn ---------- .038 .041 .012 .020 .039 .050 .048 .057 

Cr----------- .023 .028 .002 .004 .023 .028 .026 .028 

Total---- 8.021 8.021 8.024 8.022 8.023 7.987 7.998 7.984 

1 Analyses have been designated according to the following scheme: 

Typical analysis number: B U - G P - 1 0 3 

Spaces: 1, 2 3, 4 5, 6, 7 

Spaces 1, 2-Locality: 

BU Bullwhacker Coulee diatreme 

Space 3-Type of sample analyzed: 

G Grains from panned concentrate, heavy-mineral separate, gravel, or anthill 

p Grain picked out of lithic sample 

Space 4-Color group: 

p Pink 

0 Orange 

L Light orange 

Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality 

BU-G0-09 BU-GL-10 

39.54 40.25 

21.93 22.53 

21.02 15.87 

10.71 13.26 

4.77 5.85 

.09 .13 

.22 .47 

.00 .25 

98.28 98.61 

3.025 3.008 

1.977 1.984 

1.344 .991 

1.222 1.477 

.391 .468 

.004 .007 

.014 .030 

.000 .015 

7.977 7.980 
~ Table 6C. Analyses of garnets from other inclusions, Precambrian bedrock, terrace deposits, and Paleocene sedimentary rock 

~ AI 

.. 

::s 

1 

"' :r 

~ 0 

a 

AI 

::s 

AI 

c 

~: 

~ 

3 

~ 

> 

~ -< 

0 

;Ill:: 

3' 

:r 

::::!.. 

;: 

., 

a 

"C "' 

a 

:r 

QQ 

81-XP-01 1•

7 E-PP-01 2 E-PP-022 E-PP-032 LR-P0-01 3 LR-P0-023 LR-P0-033 SC-PP-01 4 SC-PP-024 SH-PP-01s 

Major oxides, in wt percent 

Si02 --------- 38.71 38.41 37.87 37.60 37.61 37.59 37.83 37.89 38.06 38.15 

Al203 -------- 22.31 21.48 21.36 21.64 21.42 21.70 21.71 21.85 21.72 21.44 

FeO --------- 27.31 33.04 32.60 32.68 33.98 34.38 33.46 33.36 34.10 35.18 

MgO--------- 8.44 5.13 4.77 5.26 4.79 4.56 4.89 4.89 4.43 4.80 

CaO --------- 2.77 2.14 1.83 2.07 1.51 1.63 1.75 2.29 1.71 1.09 

Ti02 --------- .03 .02 .00 .01 .00 .00 .01 .00 .03 .04 

MnO -------- .50 1.48 1.62 1.39 1.39 1.48 1.36 1.32 1.40 .74 

Cr203 -------- .08 .07 .00 .04 .02 .03 .03 .03 .01 .03 

Total---- 100.15 101.77 100.05 100.69 100.72 101.37 101.04 101.63 101.46 101.47 

Cations per 12 oxygens 

Si ----------- 2.981 2.997 3.005 2.963 2.978 2.964 2.978 2.967 2.991 2.997 

AI----------- 2.025 1.976 1.999 2.010 1.999 2.015 2.014 2.016 2.011 1.985 

Fe----------- 1.758 2.154 2.164 2.153 2.250 2.265 2.202 2.183 2.239 2.310 

Mg ---------- .967 .595 .563 .617 .564 .534 .573 .570 .519 .560 

Ca----------- .228 .178 .153 .173 .127 .136 .147 .190 .143 .090 

Ti----------- .000 .000 .000 .000 .000 .000 .000 .000 .000 .002 

Mn ---------- .031 .096 .107 .092 .093 .097 .090 .087 .092 .048 

Cr----------- .004 .002 .000 .002 .000 .000 .000 .000 .000 .000 

Total---- 7.994 7.998 7.991 8.010 8.011 8.011 8.004 8.013 7.995 7.992 

•xenocryst in alnoite, Bird Rapids 2 diatreme, NE1/4SW1/4 sec. 17, T. 23 N., R. 20 E.; field no. H67-20E. 2Garnet-biotite-plagioclase-apatite-rutile sillimanite schist inclusion in Ervin Ridge 1 diatreme, SE114SW114 sec. 26, T. 24 N., R. 20 E.; field no. P64-10B. 3

Garnet-kyanite-muscovite-K feldspar-quartz schist, Precambrian basement, Little Rocky Mountains, 6.5 km north of Zortman, Mont., NE1/4SW114 

sec. 29, T. 26 N., R. 25 E.; field no. P63-11B. 4Garnet from pebbles of mica schist (Precambrian) in conglomeratic sandstone of Fort Union Formation (Paleocene) in large inclusion downfaulted into 

Squaw Creek diatreme, SW1/4NE114 sec. 29, T. 25 N., R. 22 E.; field no. P65-191. 5Garnet in conglomeratic sandstone of Fort Union Formation (Paleocene) in large inclusion downfaulted into Shellenberger Divide diatreme, 

SE1/4SW114 sec. 21, T. 24 N., R. 22 E.; field no. P65-23A. 6Garnets from anthill on Thorsen dike, probably residual grains from erosion of nearby terrace gravel deposits derived from Little Rocky Mountains, 

NW114SE1/4 NW1/4 sec. 28, T. 25 N., R. 23 E.; field no. P64-41A-2. 7 Analyses have been designated according to the following scheme: 

Typical analysis number: B U - G P - 1 0 3 Space 3-Type of sample analyzed: 

Spaces: 1, 2 3, 4 5, 6, 7 G Grains from panned concentrate, heavy-mineral separate, gravel, or anthill 

Spaces 1, 2-Locality: P Grain picked out of lithic sample 

BI Bird Rapids diatreme X Xenocryst or megacryst 

E Ervin Ridge diatreme Space 4-Color group: 

SC Squaw Creek diatreme V Purple 

SH Shellenberger Divide diatreme P Pink 

TH Thorsen dike 0 Orange 

LR Little Rocky Mountains Precambrian Spaces 5, 6, 7-Analysis number, numbered consecutively for data from each locality 

TH-GV-016 TH-GV-026 

37.35 38.42 

21.52 21.83 

32.59 31.40 

5.70 7.21 

1.67 1.19 

.00 .04 

.91 .55 

.04 .18 

99.78 100.82 

2.965 2.981 

2.013 1.997 

2.162 2.038 

.673 .833 

.140 .098 

.000 .002 

.061 .034 

.002 .009 

8.016 7.992 
~ iii" 

3 

(I) 

("") 

2.. 

c 

< i: 

C'l 

e: 

:I 

(I) 

~ 

Dl 

:I 

Q. 

A EXPLANATION 

0 Phlogopite orthopyroxenite(?) (pink) 

,,-... , I \ Amphibole-diopside orthopyroxenite(?) (pink) ,_,,~ 

Pan concentrate: 

~ D Pink 

D. Orange 

'V Light orange 

Mg 

8 

EXPLANATION 

v Garnet-kyanite granulitic gneiss (pink) 

Pan concentrate: 

~ 20 0 Purple 

D Pink 

00 

<> Red 

<> Red orange 10 

D. Orange 

Mg 90 80 

c EXPLANATION 

D Xenocryst, Bird Rapids 2 diatreme (pink) ,.,...-

, __ ... ... , Garnet-biotite-plagioclase-sillimanite schist 

inclusion in Ervin Ridge 1 diatreme (pink) 

0 Garnet-kyanite-muscovite-feldspar-quartz schist, 

Precambrian bedrock, Little Rocky Mountains (orange) 

[SJ Garnet in schist pebbles in Fort Union Formation 

conglomerate, Squaw Creek diatreme (pink) 

,/ov Q~) D. 

-MO D 

I D 

. Fe 

70 60 50 40 30 20 10 

Ca 

30 

0 

~ <> ~20 

<X> <> 

D. /:{) 0 

D 

D 

dJ 

Fe 

70 60 50 40 30 20 

~ 10 C'l 

Dl 

.. 

:I 

(I) 

~ 

0 -

3 

Q 

rzJ Garnet in Fort Union Formation conglomerate, 

Shellenberger Divide diatreme (pink) 

0 Garnet from anthill on Thorsen dike, probably 

residual from Little Rocky Mountains terrace 

gravel (purple) c_) Field of garnets known or assumed to be from I \Fe D ~;~ . 

~-d'~J Mg E ~l ~!.. ~l ~ ~ 

· shallow Precambrian rocks 60 50 40 30 20 ::r 

~ 

~ c 

~ Figure 7. Ca-Mg-Fe in garnets from localities other than the Williams diatremes. A, Garnets, Bullwhacker Coulee diatreme. 8, Alluvial and inclusion garnets, Big Slide 

1 diatreme. C, Garnets from other diatremes and Precambrian bedrock sources. N 

I.C 
Figure 8. Chromium cations per 12 oxygens versus Mg/(Mg+ Fe) in garnets from localities other than the 

Williams diatremes. A, Garnets, Bullwhacker Coulee diatreme. 8, Alluvial and inclusion garnets, Big Slide 1 

diatreme. C, Garnets from other diatremes and Precambrian bedrock sources. 

may be close to Danchin and Wyatt's (1979) cluster 

groups 25 or 50, which contain garnets that are derived, 

in part, from ilmenite-bearing xenoliths. 

Mg-rich red-orange, orange, and light-orange colluvial garnets (fig. 2B) overlap the lower Ca portion of 

the peridotite garnet field (fig. 2E), but none fall into 

the higher Ca portion of that field. The three orange 

groups of Mg-rich garnets overlap part of the megacryst 

trend of constant Ca with varying Mg/Fe and continue 

that trend to higher Fe content. The trend and position 

suggest that garnets of the three orange groups are frag30 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 

ments of megacrysts or are derived from garnet peridotites of generally lower Ca and Cr contents. Separation 

of orange garnets into three color groups (red orange, 

orange, and light orange) has not yet delineated distinctly separate sources for each color group. 

If the colluvial garnets contain a representative 

sample of megacryst fragments, then the megacrysts 

probably are bimodal in Mg/(Mg +Fe) and do not form 

a complete series of compositions between the five Mgrich megacrysts and the one Fe-rich megacryst. Analyses 

of more megacrysts will test this conclusion. 
Some of the Fe-rich colluvial red-orange, orange, 

and light-orange garnets (fig. 2A) are comparable in 

Ca-Mg-Fe composition to the mafic granulite garnets 

(fig. 2E). The low Cr content of Fe-rich colluvial redorange, orange, and light-orange garnets (fig. SA) is 

also similar to that of granulite garnets (fig. SB). The 

three orange groups do not show any generally systematic 

differences in composition, although the red-orange 

group has a higher proportion of Fe-rich, Cr-poor garnets (fig. SA). The Fe-rich, Ca-poor pink garnets (fig. 

2D) that are similar in composition to garnets from Precambrian rocks in the Little Rocky Mountains (fig. 7C) 

are probably derived from Precambrian metamorphic 

rocks adjacent to the kimberlite at relatively shallow 

depths. None of the various colluvial garnets are similar 

to the mafic amphibolite garnets (fig. 2E). None of the 

Williams colluvial garnets are close in composition to 

garnets from orthopyroxenite inclusions in the Bullwhacker Coulee diatreme (fig. 7A) or from the felsic 

garnet-kyanite gneiss in the Big Slide diatreme (fig. 7B). 

The colluvial garnet grains, which are different from 

any of the analyzed garnets from inclusions (peridotites, 

granulites, and amphibolites), may be from other rock 

types or megacrysts, which have not been analyzed or 

found as discrete xenoliths. 

PANNED CONCENTRATES FROM 

OTHER DIATREMES 

Limited numbers of garnets from panned concentrates of alluvium or colluvium from other diatremes 

were classified into color groups and analyzed (table 6; 

figs. 1A, 1B, SA, 8B) to determine whether garnets 

from upper mantle sources were present. Concentrates 

from the Big Slide and Bullwhacker Coulee diatremes 

were selected because both diatremes are known to contain granulite inclusions of probable lower crustal origin; 

the Bullwhacker Coulee diatreme also contains altered 

spinel peridotite of upper mantle origin and garnet orthopyroxenite of possible upper mantle origin. 

Big Slide Diatreme 

Although garnet peridotite inclusions have not yet 

been recognized in the Big Slide diatreme, two analyzed 

garnets are of upper mantle origin. Both are purple and 

distinctly pyrope rich (fig. 7 B) and chromium rich (fig. 

8B) and have Ca-Mg-Fe compositions similar to those 

of garnets from peridotites in the Williams diatremes 

(fig. 2D). These two garnets suggest that the Big Slide 

diatreme probably contains other purple Cr-rich garnets 

and could contain garnet peridotite inclusions. Of the 

other analyzed garnets, one purple and several red, redorange, and orange garnets (fig. 7 B) are close in composition to the garnets from mafic granulites from the Williams diatremes (fig. 2D), and two are similar to orthopyroxenite garnets from the Bullwhacker Coulee diatreme (fig. 7A). Two of the panned garnets are close to 

garnet from the felsic garnet-kyanite gneiss inclusion 

(fig. 7B). 

Bullwhacker Coulee Diatreme 

Only five garnets have been analyzed from a small 

concentrate of heavy minerals from colluvium; none are 

purple, and none are exceptionally rich in Cr203 (table 

6B). One pink garnet is closely similar in Ca-Mg-Fe content (fig. 7 A) to pink garnet from one of the orthopyroxenites and has a similar Cr203 content (0.39 wt percent). The other four garnets have intermediate values 

of Mg/(Mg +Fe) and contain 0.2S wt percent Cr203 or 

less. None of the garnets are compositionally similar to 

garnets from three mafic granulites from the Williams 

diatremes (fig. 2D), although granulites and mafic granulites are the most abundant garnet-bearing inclusions 

in the Bullwhacker Coulee diatreme. This finding suggests that the Mg/(Mg +Fe) value of garnets (not yet 

analyzed) from Bullwhacker Coulee granulites is higher 

than that of garnets from Williams granulites and could 

extend the range of granulite garnets toward more 

Mg rich compositions. 

CONCLUSIONS 

Garnet can be used rapidly and easily in the field 

as a tracer for kimberlite. The presence and color of garnet in surficial deposits and soils can indicate the presence of kimberlite. 

Purple garnets are the most distinctive indicators 

of the presence of kimberlite and its inclusions of upper 

mantle rocks and minerals. Most purple garnets are 

derived from garnet peridotites, are Mg rich, and contain a larger amount of Cr (with Mg/(Mg +Fe) greater 

than 0.77 and Cr203 greater than 1.S wt percent) in comparison with garnets from other rock types. Purple 

colluvial garnets from the Williams diatremes are dominantly (20 of 21) Cr rich. Fifteen of 19 garnet peridotites 

contain purple garnet, and 4 contain red or red-orange 

garnet. 

Pink, red, red-orange, orange, and light-orange 

garnets can also be derived from kimberlite and its mantle inclusions but are not distinctive in color in comparison with garnets from shallower sources. These five 

Colluvial Garnets from the Williams Diatremes 31 
color groups show bimodal distribution of Fe and Mg; 

Mg-rich garnets (with Mg/(Mg +Fe) greater than 0.67) 

contain more Cr than most Fe-rich garnets and are probably fragments of upper mantle rocks or of megacrysts. 

The range of Ca-Mg-Fe compositions of colluvial Cr-rich 

purple, red, and pink garnets is closely similar to the 

Ca-Mg-Fe range of garnets from peridotite inclusions. 

Many of the Fe-rich garnets are compositionally similar 

to the range of garnets from lower crustal mafic granulites and may be derived from them. 

Garnets from garnet peridotites tend to have 

bimodal distribution of Ca and Cr. Similar bimodal distribution has been noted for peridotite garnets from 

kimberlites elsewhere in the world, 

Megacryst garnets are purple, red, red orange, 

and orange. Five of six are Mg rich; one is Fe rich and is 

near to mafic granulite garnets in composition. The 

Cr203 content (0.6-2.3 wt percent) of Mg-rich megacrysts 

is less than that of niost garnets from garnet peridotites. 

Colluvial garnet compositions suggest that there is not a 

complete series of Ca-Mg-Fe compositions between Mgand Fe-rich megacrysts. 

The suite of colluvial garnets from the Williams 

diatremes is bimodal in Mg-Fe composition, the dominant ranges of Mg/(Mg+Fe) being from 0.16 to 0.40 

and from 0.69 to 0.86. These two ranges are interpreted 

as reflecting the bulk compositional differences between 

the lower crust and the upper mantle. 

The Big Slide diatreme contains two Mg- and 

Cr-rich purple colluvial-alluvial garnets of probable 

peridotitic upper mantle derivation. The other 16 analyzed colluvial-alluvial garnets suggest a range of compositions of garnets from granulites and mafic granulites. 

No purple Cr-rich garnets have been found in limited sampling of the Bullwhacker Coulee diatreme. Altered garnet-bearing orthopyroxenite inclusions contain 

garnet of intermediate Mg/(Mg +Fe) values. The few 

alluvial garnets analyzed suggest that garnets from mafic 

granulite inclusions are more Mg rich than those from 

granulites in the Williams diatremes. 

Garnets from shallow Precambrian rocks have a 

distinctive small range of Fe-rich, Ca- and Mg-poor 

compositions in spite of a range of colors (purple, pink, 

and orange). 

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Bence, A. E., and Albee, A. L., 1968, Empirical correction 

factors for the electron microanalysis of silicates and 

oxides: Journal of Geology, v. 76, no. 4, p. 382-403. 

Brockunier, S. R., 1936, Geology of the Little Rocky Mountains, Montana: New Haven, Conn., Yale University, 

unpublished Ph.D. dissertation, 130 p. 

32 Garnets in Montana Diatremes: A Key to Kimberlite Prospecting 

Buie, B. F., 1941, Igneous rocks of the Highwood Mountains, 

Montana, pt. Ill, Dikes and related intrusives: Geological 

Society of America Bulletin, v. 52, no. 11, p. 1753-1808. 

Clarke, D. B., and Carswell, D. A., 1977, Green garnets from 

the Newlands kimberlite, Cape Province, South Africa: 

Earth and Planetary Science Letters, v. 34, no. 1, 

p. 30-38. 

Danchin, R. V., and Wyatt, B. A., 1979, Statistical cluster 

analysis of garnets from kimberlites and their xenoliths: 

Kimberlite Symposium II, Cambridge, England, July 

1979, Abstracts (unpaginated). 

Dawson, J. B., 1980, Kimberlites and their xenoliths: New 

York, Springer-Verlag, 252 p. 

Dawson, J. B., and Stephens, W. E., 1975, Statistical classification of garnets from kimberlite and associated xenoliths: Journal of Geology, v. 83, no. 5, p. 589-607. 

___ 1976, Statistical classification of garnets from 

kimberlite and asociated xenoliths-Addendum: Journal 

of Geology, v. 84, no. 4, p. 495-496. 

Eggler, D. H., McCallum, M. E., and Smith, C. B., 1979, Megacryst assemblages in kimberlite from northern Colorado 

and southern Wyoming-Petrology, geothermometrybarometry, and areal distribution, in Boyd, F. R., and 

Meyer, H. 0. A., eds., The mantle sample-Inclusions 

in kimberlites and other volcanics, International Kimberlite Conference, 2d, Santa Fe 1977, Proceedings, 

v. 2: Washington, D.C., American Geophysical Union, 

p. 213-226. 

Finnerty, A. A., and Boyd, F. R., 1980, Conditions of origin 

of natural diamonds of peridotite affinity: Journal of 

Geophysical Research, v. 85, no. 12, p. 6911-6918. 

Gurney, J. J., and Switzer, G. S., 1973, The discovery of 

garnets closely related to diamonds in the Finsch pipe, 

South Africa: Contributions to Mineralogy and Petrology, v. 39, no. 2, p. 103-116. 

Harte, Ben, 1977, Rock nomenclature with particular relation 

to deformation and recrystallisation textures in olivinebearing xenoliths: Journal of Geology, v. 85, no. 3, 

p. 279-288. 

Hearn, B. C., Jr., 1968, Diatremes with kimberlitic affinities 

in north-central Montana: Science, v. 159, no. 3815, 

p. 622-625. 

___ 1979, Preliminary map of diatremes and alkalic 

ultramafic intrusions in the Missouri River Breaks and 

vicinity, north-central Montana: U.S. Geological Survey 

Open-File Report 79-1128, 1 pl., scale 1:125,000. 

Hearn, B. C., Jr., and Boyd, F. R., 1975, Garnet peridotite 

xenoliths in a Montana, U.S.A., kimberlite, in Ahrens, 

L. H., and others, eds., Physics and chemistry of the 

Earth, v. 9: New York, Pergamon, p. 247-255. 

Knechtel, M. M., 1959, Stratigraphy of the Little Rocky 

Mountains and encircling foothills, Montana: U.S. Geological Survey Bulletin 1072-N, p. N723-N752. 

Leighton, V. L., and McCallum, M. E., 1979, Rapid evaluation 

of heavy minerals in stream sediments of the Prairie 

Divide area of northern Colorado-A tool for kimberlite 

exploration: U.S. Geological Survey Open-File Report 

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