Mineralogy and Geochemistry 

of Some Belt Rocks, 

Montana and Idaho

By J. E. HARRISON and D. J. GRIMES

CONTRIBUTIONS TO ECONOMIC GEOLOGY

GEOLOGICAL SURVEY BULLETIN 1312-O

A comparison of rocks from two widely 

separated areas in Belt terrane

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1970
UNITED STATES DEPARTMENT OF THE INTERIOR 

WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

Library of Congress catalog-card No. 75-607766

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

Washington, D.C. 20402 Price 35 cents

4- 

V
CONTENTS

Page

Abstract .................................................................................................................. Ol

Introduction ............................:.................................... .......................................... 2

General geology ...............................................................;.'................................... 3

Methods of investigation ........................ y .............................................................. 7

Sampling procedure ..............................:....................................................... 7

Analytical technique for mineralogy ...............:............................................ 11

Analytical technique and calculations for geochemistry................................ 13

Mineralogy of rock types........................................................................................ 29

Geochemistry .......................................................................................................... 37

Discussion and interpretation ................................................................................ 42

Implications about sedimentation ................................................................ 42

Metamorphic changes .................................................................................... 46

Comparison of Belt rocks with younger rocks............................................... 46

Conclusions ............................................................................................................ 47

References cited ...................................................................................................... 48

ILLUSTRATIONS

Page 

FIGURE 1. Geologic map of northern Idaho and northwestern Montana............ O5

2. Correlation chart for part of the Belt Supergroup,

northern Idaho and western Montana ........................................... 6

3. Geologic sketch map of the Pend Oreille area, showing

sample localities and numbers...............................-......... .............. 9

4. Histograms showing precision of semiquantitative

spectrographic analyses of some Belt rocks................................. 27

5. Diagram showing average modes of rocks of the Belt

Supergroup ...................................................................................... 35

6. Diagram showing ratio among quartz, potassium feldspar,

and plagioclase in siltites .................................................................. 36

7. Diagram showing average mineralogy of clastic Belt rocks.............. 37

8. Diagram showing geometric means of semiquantitative

spectrographic analyses of different Belt rock types.. ................ 40

ra
IV CONTENTS

TABLES

Page 

TABLE 1. Sample distribution by area, formation, rock type,

and metamorphic grade .................................................................... OIO

2. Average modes of Belt rocks from the Mission

Mountains Primitive Area ................................................................ 13

3. Semiquantitative spectrographic analyses of Belt rocks

from the Pend Oreille area.......-......-.---..........------..-.-..------.-....---.-..-.. 14

4. Replicate Semiquantitative spectrographic analyses of

Belt rocks from the Pend Oreille area.....................--.....----.......--.---. 24

5. Replicate analyses of standard sample G-l........................................ 28

6. Mean element content of low-grade Belt rocks from the

Mission Mountains Primitive Area ................................................... 31

7. Mean element content of Belt rocks from the

Pend Oreille area .............................................................................. 32

8. Numbers used to replace nonnumeric codes

(N, L, and > ) prior to geochemical calculations............................ 34

9. Relative concentration of elements in the heavy mineral

fraction of some Belt rocks from the Pend Oreille area.................... 39

10. Average granite and average composition of the

Canadian Shield compared with average Belt rock

from the Pend Oreille area.............-.--......---.....---.....---.......- . 43

11. Average minor element content of pelitic rocks of

different ages and geologic environments........................................ 47
CONTRIBUTIONS TO ECONOMIC GEOLOGY

MINERALOGY AND GEOCHEMISTRY

OF SOME BELT ROCKS,

MONTANA AND IDAHO

By J. E. HARRISON and D. J. GRIMES

ABSTRACT

The mineralogy and geochemistry of about 20,000 feet of Belt rocks in the Mission

Mountains Primitive Area, northwestern Montana, are compared with those of about

A 40,000 feet of Belt rocks in the Fend Oreille area, northern Idaho and northwestern

Montana. Semiquantitative spectrographic analyses of 254 samples from the Mission

* Mountains and 230 samples from the Pend Oreille area form the basis for chemical 

comparison; quantitative X-ray mineralogic analyses of most samples provide the 

mineralogic data. Both analytical techniques are rapid and inexpensive.

, All rocks were classified on the basis of grain size, texture, and carbonate content 

into one of four rock types: argillitic rocks (shale or mudstone equivalents), siltites 

(siltstone equivalents), quartzites (sandstone equivalents), and carbonate rocks. The 

semiquantitative spectrographic method has good precision for all rock types, but the

*~ accuracy is only fair for rocks of moderate to high carbonate content.

Comparison of clastic rocks from both areas shows the mineralogy to be very 

similar even to the ratio among quartz, potassium feldspar, and plagioclase in siltites.

I.. Most rocks are graywacke or like graywacke. Mineralogy is, not surprisingly, directly 

related to grain size; for example, we find that from argillitic rocks to siltites to 

quartzites the amount of quartz increases, the amount of feldspar remains about the 

same, and the illite-sericite-chlorite content decreases.

"** Several elements show distinct patterns of abundance related to the rock types. 

Elements directly related in abundance to amount of illite-sericite-chlorite in clastic 

rocks are B, Co, Cr, Ni, Sc, V, Fe, Mg, Ti, and possibly La and Y. Siltites and 

quartzites contain Cu in sulfide grains and flakes, and Cu is distinctly more abundant 

in those rocks or in fossil algal forms (usually calcareous or dolomitic siltite) in the 

Mission Mountains. Carbonate rocks or rocks containing moderately abundant carbonate minerals show increased amounts of Ca, Mg, Mn, Sr, and Pb. The higher B

"** content of certain Pend Oreille area strata and the higher Co, Cu, and Ni contents 

of Mission Mountains strata seem most likely to reflect differences in source area of 

the original sediments. If so, the Cu abundance and distribution identified in this

i study may aid in the search for stratabound copper deposits in Belt rocks.

The excellent sorting of grain sizes in similar strata through tens of thousands of 

feet of rock in two widely separated areas, the remarkable uniformity of simple miner4 01
O2 CONTRIBUTIONS TO ECONOMIC GEOLOGV ,

alogy, the apparently simple chemistry of depositional environments, and the bulk 

chemistry of the rocks suggest that the weathering and transportation processes of 

Belt time resulted in homogenization of clastic components drawn from large granitic " 

source areas. This homogenization requires a chemical weathering of most mafic 

minerals, a low terrane and low stream gradient, a nearly uniform availability of many 

minor elements for absorption on or inclusion in clay minerals, and a slow isostatic 

adjustment between source areas and basins of deposition during the several hundred 

million years of Belt rock deposition. Sedimentation processes in Belt time were not 

complicated by living plants or animals on the land or by significant life in the sea. 

Organic compounds did not complicate chemical environments of deposition. i~

Regional metamorphism from the chlorite-sericite zone of the greenschist facies 

into the biotite zone in the Pend Oreille area appears to have resulted only in a loss 

of B among the elements studied. Contact metamorphism, also in the biotite zone, v 

resulted in an increase in Ca and Sr, a decrease in Ni, and a possible decrease in 

Co and B.

INTRODUCTION

Much attention has been given to Belt rocks during the past decade. Some 

of the interest has come from general recognition that the vast area of ; 

exposed Belt rocks represents one of the most poorly understood depositional 

basins in the United States; other interest is based on tectonic history yet to 

be deciphered; and more recent impetus for expanded study of Belt rocks ** 

comes from active exploration for stratabound copper deposits. So much 

new stratigraphic, structural, and chemical information has been gained 

recently that a compendium of data representing the state of knowledge 

only a few years ago (Ross, 1963) is rapidly becoming outdated.

This report discusses the mineralogy and minor element content of Belt 

rocks from two widely separated areas in the Idaho-Montana part of Belt 

terrane shown in figure 1. The data, accumulated from 1957 through _^ 

1968, represent samples collected for two purposes: a study of metamorphic 

effects in the Pend Oreille area and an evaluation of mineral potential in * 

the Mission Mountains Primitive Area. The advent of reasonably precise ,j 

and rapid semiquantitative spectrographic analyses, plus the availability of 

electronic computers to handle efficiently large masses of data, has permitted 

the geochemical study described here. -4,

Several colleagues aided in collecting data included in this report. D. A. 

Jobin collected some samples from the Pend Oreille area and made some 

heavy mineral separations. M. W. Reynolds collected about half the samples y 

from the Mission Mountains Primitive Area. Uteana Oda, now deceased, 

analyzed spectrographically the heavy and light mineral fractions. Paul W. 

Schmidt and George M. Fairer made X-ray spectrographic analyses of * 

samples for the quantitative mineralogic studies. We are particularly grateful for the aid and counsel of Albert P. Marranzino on the spectrographic 

parts of the study. The manuscript benefited from reviews and discussions ^ 

with M. R. Mudge and Jon Connor.
BELT ROCKS, MONTANA AND IDAHO O3

GENERAL GEOLOGY

Rocks of the Belt Supergroup crop out over much of western Montana 

and northern Idaho (fig. 1) and northward into Canada. Equivalent Belt 

rocks extend southward from Missoula, Mont., for at least 150 miles, and 

scattered outcrops are known in northeastern Washington near the IdahoWashington State line. The limits of the basin, or basins, of deposition of 

Belt rocks are poorly known, although evidence indicates that the thrust 

zone on the eastern edge of Belt terrane (fig. 1) may mark the depositional 

edge of Belt rocks (Mudge, 1970). The western edge of the Belt terrane 

is so complicated by extensive intrusion and faulting that it has not yet been 

satisfactorily determined whether some high-grade metamorphic rocks in 

this area are Belt or pre-Belt. Between the thrust belt on the east and a 

Cretaceous batholith in the Idaho Panhandle (fig. 1), Belt rocks either are 

nearly unmetamorphosed or are not above the greenschist facies. The grade 

of regional metamorphism progressively increases from east to west and 

with depth in the stratigraphic section, as demonstrated by illite transformations to the 2M polymorph (Maxwell and Hower, 1967).

Belt rocks are generally monotonous in appearance because of fine grain 

size and drab color. The bulk of the supergroup has a grain size of silt or 

clay, and medium sand is the coarsest grain size observed in thousands of 

feet of rock over large areas. Quartzites and relatively pure to impure dolomites and minor amounts of limestones are scattered through the supergroup, although the carbonate rocks are uncommon in the lower part. Some 

carbonate rocks contain fossil algal forms, which are the only megascopic 

evidence of Precambrian life in the rocks. All rocks are dense and hard, 

reflecting the widespread low-grade metamorphism of most of the supergroup. Because of the metamorphism in most rocks of the Pend Oreille area 

and in some rocks of the Mission Mountains area, we use terms such as 

argillite, siltite, and quartzite rather than claystone, shale, siltstone, and 

sandstone to describe the rocks. Mudstone equivalents in the Belt Supergroup include argillite, silty argillite, and laminated argillite and siltite. 

Descriptions of rock types and formations for the Pend Oreille area are 

given by Harrison and Jobin (1963) and for the Mission Mountains Primitive Area by Harrison, Reynolds, Kleinkopf, and Pattee (1969). Most rocks 

contain shallow-water structures, although the abundance of such structures 

decreases noticeably in the lower parts of the section. Disconformities in the 

supergroup are difficult to identify, and angular unconformities are rare.

The stratigraphy of both areas is reasonably well known, and map units 

are traceable for many miles from each area. Correlation between the areas 

is still somewhat unreliable for a few members or formations (see queries 

in fig. 2), although for most it is adequate (fig. 2).

Obradovich and Peterman (1968) stated that the Belt rocks are from 

about 900 m. y. (million years) to more than 1,300 m.y. old. The bulk of the
04 CONTRIBUTIONS TO ECONOMIC GEOLOGY
BELT ROCKS, MONTANA AND IDAHO O5

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O6 CONTRIBUTIONS TO ECONOMIC GEOLOGY

PEND OREILLE 

AREA 

(Harrison and Campbell, 1963)

Top eroded

Libby Fm 

1,900+ ft

(McNamara Fm 

equivalent missing)

:ormation

Striped Peak F

Member 4 

700ft

_ _ _ 7 _ _ _

Members 3 

and 2 

700 ft

Member 1 

600ft

Wallace Fm

St

10,200 ft

. Regis Fm 

1,100ft

Revett Fm

2,000ft

Burke Fm

3,200ft

Prichard Fm

22,000+ ft

Base not exposed

MISSION MOUNTAINS PRIMITIVE 

AREA 

(Harrison and 

others, 1969)

Top eroded

Shields Fm 

1,400+ ft

Shepard Fm 

2,500 ft

Snowslip Fm 

3,800 ft

Helena Fm

9,800 ft

Empire and Spokane 

Fms 2,300+ fy\

Base not exposed

CAMP CREEK, 

SOUTHERN LEWIS 

AND CLARK RANGE 

(McGILL and 

Sommers, 1967)

Top eroded

Garnet Range Fm 

985+ ft

McNamara Fm 

3,070 ft

Bonner Qtz 

1,265 ft

Shields Fm

2,730 ft

Shepard Fm 

1,980 ft

Snowslip Fm 

2,235 ft

Siyeh Fm 

1,480+ ft

\ /\ /\ /\ /

\/ \/\/ \s

Base not exposed

FIGURE 2. Correlation chart for part of the Belt Supergroup, northern Idaho and western Montana.
BELT ROCKS, MONTANA AND IDAHO O7

Belt rocks exposed in the Mission Mountains Primitive Area (Helena Formation and strata above) result from a period of sedimentation about 

1,100 m.y. old. Presumably, the lithostratigraphic equivalents of those rocks 

in the Pend Oreille area are approximately the same age. Rocks approximately 2,000 feet below the Helena and Wallace Formations are dated at 

about 1,300 m.y. at the eastern edge of the Belt terrane. Correlation of these 

dated rocks with those of the Pend Oreille area is not well established, but 

the general 1,800-m.y. age for the metamorphosed basement rocks in western Montana provides a maximum possible age of the Belt Supergroup. 

Geologic differences and similarities between the two study areas are outlined in table 1, which lists samples collected according to area, formation, 

rock type, and metamorphic grade. The Mission Mountains Primitive Area 

contains strata probably laid down closer to shore than those of the Pend 

Oreille area, and subsequent diagenesis and metamorphism of the Mission 

Mountains strata were low grade. Many carbonate rocks in the Mission 

Mountains area are relatively pure, and carbonate cement is common. The 

Pend Oreille area contains strata whose relations to shore are less well 

defined. The stratigraphic units above the Wallace Formation are more 

similar in appearance to their equivalents in the Mission Mountains Primitive Area than are the Wallace and lower formations. Regional metamorphism is evident in the rocks in the Pend Oreille area; those rocks below 

the Revett Formation are clearly in the biotite zone of the greenschist facies. 

Effects of contact metamorphism, also of the biotite zone, are apparent near 

the granodiorite intrusions (fig. 3). Carbonate rocks are rarely pure, and 

carbonate cement or minerals are uncommon in most units.

METHODS OF INVESTIGATION 

SAMPLING PROCEDURE

The sample localities for the Pend Oreille area are shown in figure 3, 

whereas those for the Mission Mountains are in a report by Harrison, 

Reynolds, Kleinkopf, and Pattee (1969, pi. IB). The locality number on the 

maps is the same as the sample number listed in the tables of analytical data. 

The number of samples in table 1 is crudely proportional to the thickness 

and abundance of the rock types in a given formation. For the Mission 

Mountain samples, an exception to this generality is an overemphasis on 

quartzite and siltite in the Empire and Spokane and on quartzite in the 

Helena; stratabound copper minerals have been found in those rock types. 

Exceptions for the Pend Oreille samples are (1) a smaller than proportional 

number of samples of minor rock types in some formations, such as argillite 

in the Revett Formation and in member 4 of the Striped Peak Formation 

and quartzite in the Burke Formation, and (2) a larger than proportional 

number of samples for the Striped Peak because of the different character 

of each of its members.
O8 CONTRIBUTIONS TO ECONOMIC GEOLOGY
BELT ROCKS, MONTANA AND IDAHO O9

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O10 CONTRIBUTIONS TO ECONOMIC GEOLOGY

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of Shields McGill and

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BELT ROCKS, MONTANA AND IDAHO Oil

The field classification of the rocks into five rock types (argillite, laminated argillite and siltite, siltite, quartzite, and carbonate rocks) and into 

three metamorphic grades (simply defined as chlorite-sericite, biotite, and 

gneissic or schistose) was supported by study of thin sections of each sample 

from the Pend Oreille area and about half the samples from the Mission 

Mountains. The final classification for this study grouped most of the laminated rocks with the argillites into a rock group that is equivalent to 

mudstone.

ANALYTICAL TECHNIQUE FOR MINERALOGY

Mineralogical study of Belt rocks is complicated by (1) fine grain size, 

which makes microscopic modal analysis extremely difficult, and (2) 

low-grade metamorphic recrystallization, which effectively prevents simple 

separation into pure mineralogical components. Various staining techniques 

were tried in thin section, but they met with only partial success on the 

coarsest rocks; several standard mineral separation techniques were tried, 

again with only partial success in unmetamorphosed rocks. We resorted to 

X-ray diffraction as a method for mineralogical analysis that would give 

comparable results in all rocks regardless of grain size or degree of 

metamorphism.

Quantitative mineralogic data for most Belt rocks can be obtained by 

X-ray analysis. The simple mineral suite in most Belt rocks led Jobin and 

Harrison to experiment with quantitative X-ray spectrographic analyses in 

1958. The first results of continuing studies were published a few years 

later (Harrison and Campbell, 1963). Studies by others for example, 

Tatlock (1966), who made a thorough investigation of X-ray analysis of 

felsic rocks verify that quantitative mineralogic data of even more complex 

rocks also can be obtained by X-ray analysis.

The analytical technique used for mineralogical determinations was 

specifically designed for Belt rocks and their low-grade metamorphic mineralogy. Replicate X-ray diffraction analyses on various rock types commonly fell within 5 percent of each other and always within 10 percent. 

Such precision is more than adequate for use in our studies because of the 

much greater differences observed among samples. In addition, the mineralogy was checked against partial or total chemical analyses for many 

samples with the result that the X-ray mineralogy appears to be not only 

reasonably precise but also reasonably accurate. All such chemical checks 

were within 10 percent of the X-ray values.

Each sample was crushed, ground for 15 minutes on a power mortar and 

pestle, spooned or dumped (to help avoid mica orientation) into an aluminum slotted powder holder, and run on the X-ray spectrometer from 2° 

to 62° at 2° 20 per minute. Peak heights above background on the spectrometer trace were then measured: the 10 A peak for mica (and illite), the 

7 A peak for chlorite, the 3.35 A peak for quartz, the 3.25 A peak for
O12 CONTRIBUTIONS TO ECONOMIC GEOLOGY

potassium feldspar, the 3.2 A peak for plagioclase, the 3.05 A peak for 

calcite, and the 2.9 A peak for dolomite. The ratio of the 10 A mica peak 

and the 5 A mica peak was also determined.

General formulas for relating peak heights to mineralogy were devised 

by comparing X-ray spectrometer data with some complete chemical analyses, with K and Na analyses on most rocks, and with Ga and Mg analyses 

on carbonate rocks. In addition, estimates of biotite content were made from 

thin sections of all rocks showing visible metamorphic effects. Standard 

chemical formulas for each mineral were used to compute chemical content 

from rock mineralogy; the dark-green chlorite characteristic of the rocks 

was assumed to be iron rich, and the plagioclase (albite-oligoclase) was 

assumed to contain essentially all the Na in the rocks. From these data and 

assumptions, the following basic formulas for translating peak heights into 

quantitative mineralogy of nonbiotitic rocks emerged:

Quarts 3.35A-'/2 (10A)

4

(Subtraction required because of interference between main quartz peak and 

secondary mica peak) ;

3 25 A Potassium feldspar= - 

3

Sericite (plus muscovite plus illite) = - ;

o

Chlorite= 7 A 

Calcite=

2

3.05 A .

2 '

2.9 A Dolomite= '6

Estimates derived from these formulas were then recalculated to 100 

percent. We consider that the simplified formulas given above are justified 

in this study because of the relatively simple mineralogy of the bulk of the 

Belt rocks and the limited intereference between principal and secondary 

peaks of the minerals. The ratio of the 10 A peak to the 5 A peak may be 

used to detect either preferred orientation of mica or presence of biotite in 

the sample. Biotite tends to give a higher 10 A and lower 5 A peak than an 

equal amount of sericite, whereas mica orientation gives an anomalously 

high 10 A peak. A microscopic scan of powdered rock or thin section usually 

distinguished which factor was affecting the normal peak ratio. Mica orientation necessitated a rerun of the sample; the detection of minor biotite 

required a decrease in the amount of the 10 A peak used in the calculation. 

Excessive biotite totally disrupts the mica formula, but the general formulas 

were found satisfactory for proportions of minerals other than mica. Mica 

content was determined from thin sections where biotite is a major constituent, where the metamorphic grade is relatively high, and where the

grain size of the micas is sufficiently large.
BELT ROCKS, MONTANA AND IDAHO

TABLE 2. Average modes, in volume percent, of Belt rocks from 

the Mission Mountains Primitive Area

O13

Number 

of 

Rock type ' samples

PlagioQuartz clase

Potassium Illitefeldspar sericite Chlorite Calcite Dolomite

Shields Formation

Dolomitic laminated 

argillite and siltite. 

Dolomitio siltite. ....

3 

2 

3

41 

45 

73

11 

15 

4

6 

8 

2

20 

12 

10

9 

7 

9

0 

0 

0

13 

13 

2

Shepard Formation

Carbonatic argillite or 

laminated argillite

Carbonatic eiltite. . . 

Carbonatic quartzite . 

Impure limestone. . . .

5 

3 

2 

6

38 

38

44 

18

16 

16 

22 

10

3 

5 

4 

Trace

16 

13 

7 

9

14 

9 

5 

8

8 

7 

9 

53

5 

12 

9 

2

Snowslip Formation

Argillite or laminated 

argillite and siltite. 

Siltite... ...........

6 

5 

5

38

42 

58

21 

22 

19

1 

3 

0

26 

24 

9

11 

6 

6

1 

3 

8

1 

0 

Trace

Helena Formation

Calcareous silty

Carbonatic siltite. . . . Calcareous quartzite. 

Dolomitic limestone .

5

2 

13 

6 

17

33

34 

33 

46 

17

14

13 

14 

19 

6

3

0 

3

4 

Trace

29

16 

17 

7 

8

13

6 

6 

6

4

7

30 

15 

16 

51

1

1 

12 

2 

14

Empire and Spokane Formations

Carbonatic argillite or 

argillite and siltite . 

Calcareous siltite. . . .

3 

2 

7

27 

35 

62

14 

18 

15

0 

0 

0

30 

20 

12

15 

11 

6

3 

12 

3

11 

4 

2

All formations named above

Carbonatic argillite or 

argillite ana siltite . 

Carbonatic siltite .... 

Carbonatic quartzite . 

Carbonate rocks. ....

24 

25 

23 

23

35 

36 

57 

18

16 

16 

16

7

2 

4 

2 

Trace

24 

18 

9 

8

12 

7 

6 

5

6 

10 

8 

51

5 

9 

2 

11

1 Rock types within. stratigraphic units are not necessarily in order of stratigraphic sequence 

or abundance.

Average modes of rocks from the Mission Mountains are given in table 2, 

and average modes for some rocks from the Pend Oreille area have previously been published (Harrison and Campbell, 1963, table 1).

ANALYTICAL TECHNIQUE AND CALCULATIONS FOR GEOCHEMISTRY

Semiquantitative spectrographic analyses presented in this report (table 

3) and similar analyses on rocks from the Mission Mountains (Harrison 

and others, 1969) were done using methods outlined by Grimes and Marranzino (1968). Analytical results were reported as approximate geometric 

midpoints of ranges whose boundaries are 1.2, 0.83, 0.56, 0.38, and 0.18 (or 

multiples of those numbers); results are reported as 1, 0.7, 0.5, 0.3, 0.2, and 

0.15 (or appropriate multiples).
O14 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 3. Semiquantitative spectrographic analyses

[Number in parentheses indicates sensitivity limit for method used. Metamorphic grade: C, 

bedding; Bgn, biotite zone in gneiss or schist. >, undetermined amount of element is 

limit; N, element was looked for but not found. Also looked for but not found in any 

all samples were reported as N(6) for Mo, N(200) for Zn, N(0.5)'for Ag, and N(10) for

(percent)

Sample

071 ]J

072

073 074

075

083

108 ?/

120

121

128

129

116

118

119

082

117

080

081

067

069

097

102

103

110

113

061*

055

068

078

095

098

105

106

107 2/

109

122

125

062

063

070

076

100

114

126

127

21 1

'"

124 J

004

005

008

012

013

(.05)

1.5

3

3

3

2

3

3

2

3

3

3

1

3

5

1.5

1

.7

.7

3

3

3

3

5

3

3

1.5

3

1

1.5

2

1

1 .7

i

3

3

3

1

2

1.5

.7

.7

.5

2

3

1.5

1

2

.7 

2

1.5

3

1.5

M9 

(.01)

0.7

1

1

1

1

3

2

1.5

1.5

.1.5

1

.7

1

1.5

2

.5

.5

.2

.5

.5

.5

.5

.5

.5

.7

1.5

5

3

1.5

5

.3

.7

.7

1.5

3

2

.3

1

2

.15

.1

.5

.7

1

.5

7

10

.5

3

1.5

1.5

2

Ca 

(.05)'

0.05

.1

.07

.05

.05

.2

.7

.05

.07

1.5

.05

L

.1

.07

.15

N

>20

10

.07

.07

.07

.07

.05

.05

.05

.05

.07

10

.1

.07

7

.07

.05

.05

L

.1

.15

.05

.05

1.5

.07

L

L

.07

.1

L

15

20

 L

.15

L

L

L

(.ooi)

Mn 

(10)

B 

(10)

(ppn

Ba Be 

(10) ' (1)

»)

(5)' Cr 

(5)

Libby Formation

0.2

.2

.3

.3

.5

.2

.2

.5

.5

.3

.5

1

.3

.5

.2

.5

.1

.02

.3

.5

.5

1

.2

.5

.3

.3

.7

.2

.2

.2

.2

 .7

.7

1

.3

.7

.5

.3

.5

.3

.2

.2

.05

.15

.5

.15

.1

.15

.2

.2

.3

.2

.3

70

300

 200.

. 200

300

300

300

200

200

. 500

200

15

ISO

300

150

20

>5,000

1,500

Striped Peak

15

30

150

15

15

30 2

15

15 1

15

300

50

50

200

15 1

20 2

'15 2

30

50

' 70 2

30

100

300

30

300

70

100

ISO

50

500 .

5,000

Ual lace Fo

10

30

20

70

50

70

50

100

100

500

100

30

100

100

50

700 '30

50

70

200

30

20

30

-ormation

ISO

500

500

300

100

,000

100

,500

300

ISO

70

50

100

,500

,000

,000

200

200

,000

50

300

150

30 150

15

300

150

30

150

500

rmation

30

70

50

50

30

300 ' L

150 L

500 L

500 1

500 2

200 1

500 1

700 2

700 1.5

300 2

500 1 .5

500 1 .5

500 1 .5

500 2

700 1 .5

500 ' 1.5

150 L

200 L

700 1.5

300 1.5

300 2

700 2

700 1 .5

300 2

500 1 .5

200 . 1

300 1

300 L

300 1.5

200 L

700 L

ISO 2

300 1.5

500 1.5

300 1.5

500 1.5

500 I 

ISO 1.5

200 I .5

ISO 1

100 1

200 L

300 L

ISO 1

200 1 .5

500 L

70 300

500 L

300 L

500 1 .5

300 L

300 1

500 1

L

5

7

7

10

5

5

N

5

N

L

N

10

N

L

N

10

10

L

5

7

5

5

L

5

N

7

L

7

7

5

L

N

N

10

7

N

5

10

7

N

L

L

L

15

5

 L

L

5

7

7

10

10

30

20

50

50

70

20

20

30

30

30

50

30

30

50

15

30

15

5

50

70

70

50

7

'70

70

30

70

30

30

50

: 50

30

30

50 

50

70 '50

20

30

30

15

20

5

20

50

20 1 20

30

20

30

50

50

30

(2)

7

70

7

5 

30

L

50

L

7

50

30

5

10

' 30

L

15

5

15

L

20

30

L

N

107 '

5

5 .

15

L

7

20 ' 7

7

10

5

5

 15

L'

L

L

L

30

5

15

7

15

10

3,000

L

L

7

30

5

La 

(20)

50

20

L

20

50

50

50

50

30

30

50

70

L

N

50

100

70

L

50

50

50

50

70

70

50

50

70

50

3030'

30

-30

50

70

70

59

50

50

50

30

20

30

N

30

50

50

N

30

30

70

70

50

30

See footnotes at end of table.
BELT ROCKS, MONTANA AND IDAHO O15

of Belt rocks from the Pend Oreille area

chlorite-sericite zone including minor secondary biotite; Bb, biotite Zone in rock with prominent 

present above number shown; L, undetermined amount of element is present below sensitivity 

sample were As(200), Au(10), Cd(20), Sb(lOO), and W(50). Except as indicated by footnotes, 

Bi and Sn. D. J. Grimes, analyst]

Sample

(ppm)

Nb 

(10)

Ni 

(2)

Pb 

(10)

Sc

(5)

Sr 

(50)

V 

(10)

Y 

(5)

Zr Me 

(10)

tamorphi 

Grade

C

Sample description

Libby Formation

071

072

073

071*

075

083

108

120

121

128

129

116

118

119

082

117

080

081

067

069

097

102

103

110

113

06<*

065

068

078

095

098

105

106

107

109

122

125

062

063

070

076

100

n't

126

127

211

1 1 1

\2k

oou

005

008

012

013

L

L

L

L

10

L

L

10

10

10

15

15

L

10

L

10

N

N

L

10

10

L

L

10

10

L

10

L

10

L

L

L

10

10

L

10

10

L

L

L

L

L

L

L

10

10

N

L

N

L

N

10

L

10

15

15

20

20

30

15

L

20

20

20

L

20

30

5

L

L

5

10

30

30

7

5

50

15

L

10

7

20

20

15

5

L

N

30

20

7

5

15

15

L

L

L

20

20

10

10

15

5

20

15

20

15

N

15

L

L

L

N

N

N

10

L

N

L

N

N

N

30

30

30

L

L

L

N

N

N

L

L

L

15

N

N

10

N

50

30

N

N

10

N

N

N

N

N

L

N

N

N

20

N

N

N

N

N

N

5

5

5

7

10

7

5

10

10

10

IS

15

7

10

7

10

5

L

10

15

10

15

7

10

7

7

10

5

7

L

5

7

7

10

7

7

15

5

7

5

L

5

N

5

10

5

L

5

7

10

10

10

7

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

200

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

70

70

70

70

100

50

50

70

100

70

100

100

70

70

30

70

30

IS

Striped

70

70

70

70

50

70

70

30

70

30

50

30

50

70

70

70

70

70

70

30

50

70

15

50

20

70

70

50

20

50

Wall

50

50

70

50

50

15

20

15

30

30

30

30

30

30

30

30

50

20

30

30

30

50

30

Peak

30

30

30

50

50

30

30

30

50

IS

30

30

20

30

30

50

30

30

30

20

20

30

15

20

10

20

30

20

10

30

ace Fo

20

30

20

20

IS

100

100

ISO

100

ISO

ISO

150

150

ISO

200

200

200

ISO

ISO

150

150

ISO

15

Fo rma t i on

200

ISO

ISO

200

ISO

200

200

700

1,000

200

ISO

200

ISO

200

200

300

150

700

300

700

700

300

700

200

100

300

1,000

200

20

ISO

rma t ion

ISO

100

100

100

too

C

C

C

C

C

C

C

C

C

C

C

Bb

Bb

Bb

C

Bb

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Bb

C

C

C

C

C

C

C

Black si 1 ty argi 1 1 ite.

Calcareou s si 1 ty argi 1 1 ite.

Black si Ity argi 1 1 ite.

Green si 1 ty argi 1 1 i te.

Black si 1 ty argi 1 1 i te.

Green si 1 ty argi 1 1 i te.

Calcareous green argillite.

Black si 1 ty argi 1 1 ite.

Do.

Green calcareous argillite.

Black si 1 ty argi 1 1 ite.

Do.

Do.

Do.

Green argi 1 1 i tic si 1 tite.

Argillitic siltite.

Stromatol ite.

Cherty ool ite.

Green si 1 ty argi 1 1 ite.

Black si Ity argi 1 1 ite.

Do.

Green si 1 ty argi 1 1 i te.

Maroon argi 1 1 i te.

Black si Ity argi 1 1 ite.

Red si 1 ty argi 1 1 ite.

Olive siltite.

Red siltite with salt casts.

Dolomi tic si It ite.

Dolomitic argillitic siltite.

Red and green si 1 tite.

Dolomitic si 1 tite.

Green si It ite.

Black argi 1 1 i tic si 1 tite.

Do.

Red feldspathic si 1 tite.

Green argi 1 1 i tic s! 1 tite.

Black argillitic siltite.

Red feldspathic quartzite.

Green quartzite.

Calcareous red quartzite.

Red feldspathic quartzite.

Do.

Do.

Green quartzite.

Red feldspathic quartzite.

Green quartzite.

Si Ity dolomite.

Argi 1 1 i tic dolomite.

Black si 1 ty argi 1 1 i te.

Waxy green argi 1 1 i te.

Green si 1 ty argi 1 1 ite.

Black si 1 ty argi 1 1 i te.

Waxy green argillite.
O16 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 3. Semiquantitative spectrographic analyses

Sample

039

051

057

058

059

06 1

09<*

096

151

168

180

186

187

188

197

207

115

175

206

001

052

054

056

143

166

190

192

162

201

204

205

146

003

002

038

053

060

079

104

144

147

165

006

046

152

169

193

195

176

007 , 

084 il

153

(

Fe Mg 

(.05) (.01)

3.0 5.0

2 2

2 .5

2

3

2

3 .5

3 5

3 3

3 5

3 1

2 1

2 1.5

2 .5

2 1.5

3 1.5

3 3

1.5 3

3 .7

1.5 .7

2 1

3 .5

2 1.5

1 3

1.5 3

2 5

5 1

2 3

1.5 .2

2 1.5

5 1

.5 .5

.5 .3

1 3

2 5

5 5

1.5 5

1 >10

.7 >10

1.5 3

3 10

1.5 5

3 2

3 1.5

3 2

3 2

2 .5 .

3 1.5

3 1.5

.7 .5 

3 3

3 .5

aercent)

Ca 

(.05)

7.0

.15

L

L

.05

.07

L

7

.07

.1

.07

.05

.07

.05

.15

.07

.05

5

.05

.05

.05

.1

.1

7

7

7

.07

1.5

.07

.05

.07

10

1.5

20

>20

10

10

>20

20

10

10

20

.07

2

.1

.07

.05

.5

.1

.15 

.1

.2

Ti 

(.001)

Wall

0.15

.2

.5

.5

.5

.2

.3

.3

.2

.3

.3

.3

.2

.2

.3

.2

.5

.2

.3

.15

.2

.3

.2

.2

.2

.2

.2

.2

.3

.2

.5

.15

.15

.1

.1

.3

.2

.07

.05

.2

.1

.15

.2

.2

.5

.5

.2

.3

.5

.15 

.3

.3

(10)

ace Format

500

70

15

100

70

20

30

700

50

50

70

150

100

50

70

30

70

300

150

50

20

100

30

150

70

300

300

150

50

20

200

300

70

500

1,000

3,000

500

1,000

1,500

500

1,000

300

St. Regis

70

1,000

50

30

30

200

300

100 

300

300

B 

(10)

on--Conti

30

150

100

100

70

50

500

150

100

70

50

50

30

20

70

50

200

50

50

30

20

30

30

50

30

70

30

50

30

30

50

15

15

20

30

50

30

50

50

70

70

50

Formation

30

30

70

100

30

70

70

15 

70

30

Ba 

(10)

iued

500

300

500

500

300

500

200

700

500

500

500

300

300

150

700

700

500

700

500

300

300

200

300

700

700

700

150

500

150

500

300

500

500

300

500

200

700

50

200

300

200

700

700

700

500

500

300

700

700

300 

700

500

(ppir

Be 

(1)

1.0

L

1.5

1.5

1

L

L

1.5

3

2

1.5

1.5

1

L

3

2

1.5

3

2

1

N

1.5

L

11.5

1

L

1.5

1

1.5

2

L

L

L

5

L

L

N

L

1.5

1

1

L

1.5

1.5

2

1

2

3

N

1.5

1

)

Co 

(5)

10

10

10

5

7

7

5

10

7

10

7

7

7

5

5

L

L

N

7

10

15

7

L

N

5

5

10

5

L

N

10

L

L

5

7

7

5

N

N

5

L

5

10

7

7

5

7

7

10

L 

10

L

Cr 

(5)

30

50

70

50

50

50

70

50

20

30

20

70

30

30

30

50

70

7

50

30

15

30

30

15

15

50

20

20

30

15

50

15

20

30

30

50

30

10

7

30

15

15

50

50

20

50

50

50

30

15 

50

20

Cu 

(2)

15

5

10

20

5

7

7

20

20

L

20

7

30

30

30

20

20

L

30

5

20

15

7

15

300

L

30

15

10

10

30

7

7

7

20

30

L

L

L

7

N

7

L

L

L

10

10

L

L

L

30

20

La 

(20)

50

20

50

50

50

50

30

50

50

50

100

50

50

20

70

50

50

50

150

50

30

30

20

30

70

50

30

50

70

50

100

30

50

50

30

30

30

N

L

30

50

30

70

50

50

70

50

70

70

20 

50

L

See footnotes at end of table.
BELT ROCKS, MONTANA AND IDAHO 

of Belt rocks from the Pend Oreille area Continued

O17

Samp 1 e

(ppm)

Nb 

(10)

Ni 

(2)

Pb 

(10)

Sc 

(5) (50)

V 

(10)

Wal lace

039

051

057

058

059

061

03^

096

151

168

180

186

187

188

197

207

115

175

206

001

052

05 1*

056

l<0

166

190

192

162

201

20i*

205

]<t6

003

002

038

053

060

079

10*4

l^U

l<*7

165

006

0^6

152

169

193

195

176

007

081*

153

L

N

L

10

L

L

L

L

10

L

10

10

L

L

10

10

10

15

10

N

N

L

L

L

L

L

10

L

L

10

10

L

N

N

L

N

L

N

N

L

L

N

L

10

L

L

1 L

10

15

N

L

10

20

30

30

30

30

30

20

20

20

20

30

30

20

20

20

30

10

5

20

30

30

30

30

15

20

20

30

30

10

20

30

L

L

15

15

20

20

L

L

15

15

15

15

20

20

20

20

30

30

10

20

10

10

20

15

15

N

N

15

N

N

N

L

N

20

N

10

10

L

N

N

L

15

N

N

10

N

N

L

L

20

10

L

10

L

N

N

L

N

15

N

70

N

50

N

L

N

N

N

N

L

N

30

L

7

10

10

10

7

7

10

7

10

10

10

10

10

7

15

10

10

7

10

7

L

10

7

7

7

10

7

7

10

7

15

L

S

7

5

7

5

L

N

7

5

5

7

7

10

10

7

10

10

L

10

7

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

100

N

150

300

N

100

N

N

L

100

150

N

N

N

N

N

N

N

N

N

N

50

50

70

70

70

50

70

70

70

70

70

70

70

70

70

70

70

50

70

50

20

70

50

50

50

70

50

70

50

50

70

20

20

30

30

50

30

30

10

70

50

50

St.

50

70

70

70

70

70

70

20

70

50

Y 

(5)

Format

20

15

30

30

50

30

30

30

30

30

30

30

30

20

30

30

30

50

30

30

15

30

20

20

30

30

30

30

30

20

30

30

10

30

20

30

20

15

10

20

30

20

Reqis

30

30

30

30

30

30

50

10

30

30

Zr Met 

(10)

ion--Continu

150

ISO

200

200

300

150

150

150

150

150

150

150

150

150

150

150

200

150

150

150

100

200

150

150

150

100

150

150

300

150

ISO

100

150

50

30

200

100

30

50

ISO

50

50

Formation

100

150

150

150

150

ISO

200

70

200

200

:amorphic 

Grade Sample description

led

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Bb

Bb

Bb

C

C

C

C

C

C

C

C

Bb

Bb

Bb

Bb

C

Bb

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Bb

C

C

C

Oolomi tic si 1 ty argi 1 1 i te.

Black si 1 ty argi 1 1 i te.

Do.

Do.

Do.

Do.

Do.

Dolomi tic si 1 ty argi 1 1 i te.

Waxy green argi 1 1 i te.

Do.

Black si 1 ty argi 1 1 i te.

Do.

Do.

Do.

Waxy green argi 1 1 i te.

Black si 1 ty argi 1 1 i te.

Do.

Dolomi tic si 1 ty argi 1 1 i te.

Black si 1 ty argi 1 1 i te.

Argi 1 1 i tic si 1 ti te.

White si 1 tite.

Argi 1 1 i tic si 1 1 i te.

Do.

Calcareous si 1 ti te.

Dolomi tic argillitic si 1 tite.

Do.

Argi 1 1 i tic si 1 tite.

Calcareous s i 1 ti te.

Green si 1 tite.

Do.

Do.

Calcareous silty quartzite.

Calcareous quartzite.

Molar tooth limestone.

Do.

Stromatol i te.

Si 1 ty dolomi te.

Do.

Stromatol i te.

Molar tooth limestone.

Argi 1 1 i tic dolomi te.

Molar tooth limestone.

Purple argi 1 1 i te.

Do.

Do.

Do.

Do.

Purple si Ity argi 1 1 i te.

Do.

Purple argillitic siltite.

Green argillitic siltite.

Purple argi 1 1 itic si 1 tite.
O18 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 3. Semiquantitative spectrographic analyses

(percent)

Sample

170

194

196

047

163

184

009

043

048

171

091

174

181

010

042

050

090

203

087

158

173

182

200

086

149

210

150

177

04 1

045

049

089

092

093 %

133

148

155

161

198

199

209

040

131

183

017 2/

020

021

023

026

031

034

Fe 

(.05)

1.5

1.5

2

1.5

5

1.5

1.5

1

1

3

1

2

.7

1 .7

.7

 3

.3

1

.5

.5

.3

.2

3

3

2

2

2

2

1.5

1

3

1

3

2

2

2

1.5

2

1.5

2

3

3

1.5

3

3

2

1.5

2

3

3

Mg 

(.01)

1.5

1.5

1.5

2

1.5

3

1

.15

.3

.3

.3

.7

.3

.07

.05

.07

.07

.1

.3

.07

 '

.15

.05

.7

1.5

.7

.2

.2

.7

.1

.3

.7

.15

1

.7

1

.5

.3

.5

.2

.7

.7

1

.3

.7

1

.7

1

]

1

1.5

Ca 

(.05)

1 .5

2

.07

1

.15

.1

.07

L

L

.07

.05

.05

L

L

N

1

L

L

.05

L

.05

L

N

.15

.05

.05

.05

.07

.1

.07

.05

..07

 '5

.05

1

.15

1.5

.15

L

.07

.05

.05

L

.05

.15

.05

.05

.2

.07

.07

.7

Ti 

(.001)

St.

0.2

.2

.2

.2

.3

.3

.2

.5

.3

.3

.2

.5

.3

.2

.3

.1

.15

.05

.15

.15

.15

.2

.15

.5

.7

.3

.3

.5

.2

.2

.15

.2

.15

.3

.2

.5

.3

.2

.5

.2

.3

.2

.5

.2

.3

.5

.3

.2

.3

.3

.3

(10)

Reqis Form

200

1,000

50

300

50

100

Revett

70

50

20

20

200

15

20

50

100

700

30

15

70

20

50

30

20

Burke

300

100

150

70

200

.300

70

70

500

300

500

700

200

500

500

70

100

150

Prichard

300

300

15

300

300

300

200

200

300

500

B 

(10)

lation Cor

30

30

30

30

50

200

Formation

20

100

30

20

15

30

L

15

30

15

10

L

10

L

10

10

10

Formation

50

30

30

15

20

20

15

15

20

15

30

15

20

15

15

30

20

20

Format i on

30

50

20

15

30

20

10

30

15

15

Ba 

(10)

itinued

300 .

300

300

300

500

150

300

300

300

500

300

700

150

300

150

150

300

200

300

150

200

150

300

700

700

700

200

700

500

200

300

. 300

300

700

150

500

500

700

500

300

500

500

700

500

300

700

500

500

700

700

500

(ppm)

Be 

(1)

1 .0

11.5

1

2

1.5

1

1

1 .5

2

1.5

3

1

L

1.5

L

L

L

1

L

1

L

L

2

2

3

7

1 .5

1

1

L

1

1.5

7

L

1.5

2

1

2

11.5

1.5

10

3

L

1 .5

1

L

1.5

1

1.5

Co 

(5)

L

5

7

7

7

5

10

L

L

L

L

N

N

N

N

L

N

N

7

N

N

N

N

10

15

7

5

5

10

5

5

10

5

10

5

10

7

5

7

5

7

L

N

5

20

15

7

7

7

7

5

Cr 

(5)

15

20

15

30

50

15

30

30

30

20

15

30

7

15

20

7

5

7

7

5

5

5

5

50

50

50

15

15

30

20

20

30

15.

30

15

20

20

10

50

10

30

30

70

7

50

70

30

30

30

50

70

Cu 

(2)

7

10

20

15

5

L

L

L

5

L

7

L

N

5

L

30

L

L

5

5

5

5

15

L

5

15

5

15

7

5

5

10

5

30

7

L

L

15

5

7

10

5

10

20

50

7

20

10

5

L

7

La 

(20)

50

50

50

50

100

50

20

50

50

30

30

50

50

50

20

30

30

L

30

50

20

50

50

70

70

70

30

50

50

30

30

30

50

50

50

50

70

20

50

50

50

30

50

70

30

100

70

30

70

70

70

See footnotes at end of table.

/
BELT ROCKS, MONTANA AND IDAHO 

of Belt rocks from the Pend Oreille area Continued

O19

(ppm)

Sample

170

194

196

047

163

184

009

043

048

171

091

174

181

010

042

050

090

203

087

158

173

182

200

086

I4g

210

ISO

177

04 1

045

04g

089

092

093

133

148

155

161

198

199

209

Nb 

(10)

L

L

10

L

10

L

L

10

L

10

L

10

10

N

L

N

L

N

N

L

L

L

L

10

15

10

L

L

L

L

N

N

L

L

L

15

10

10

10

L

10

Ni 

(2)

10

20

20

15

30

20

15

7

5

20

5

L

L

N

L

L

L

L

15

N

L

L

L

30

30

20

15

10

10

5

10

20

L

30

L

20

7

15

20

20

20

Pb 

(10)

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

L

N

N

N

N

10

N

N

N

N

L

N

N

10

L

30

L

N

N

10

N

L

N

Sc

(S)

5

7

7

7

15

7

7

7

10

7

5

15

7

L

7

L

L

L

L

L

N

L

L

10

10

10

5

7

7

5

5

5

5

10

5

7

10

5

10

5

7

Sr 

(50)

St.

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

L

N

L

N

L

N

N

N

N

V 

(10)

Reqi s

50

50

50

30

100

70

Re

50

50

30

70

20

70

30

10

20

10

10

L

30

30

30

20

15

B

70

70

70

50

70

30

30

20

30

30

70

.50

70

50

50

50

50

50

Y 

(5)

Zr Metamorphic

Formation Continued

30

30

20

30

50

30

vett Format

15

30

50 1,

30

20

30

20 1,

IS

20 >\,

20

10

15

15

30

20

20

20 1,

urke Format

30

50

30

30

30

30

20

20

15

IS

30

30

30

30

30

20

20

30

ISO

ISO

ISO

ISO

200

ISO

ion

100

200

000

300

150

700

000

300

000

300

150

100

150

500

700

200

000

ion

ISO

300

ISO

150

ISO

150

ISO

200 iso

ISO

iso

200

300

200

ISO

ISO

ISO

150

c

c

c

Bb

Bb

Bb

C

C

C

C

Bb

Bb

Bb

C

C

C

C

C

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

C

C

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Dolomitic purple siltite.

Do.

Purple argi 1 1 itic si 1 tite.

Dolomitic purple siltite.

Purple argi 1 1 i tic si 1 ti te,

Waxy green argillitic siltite.

Green argi 1 1 itic si 1 tite.

Do.

Do.

Green si 1 tite.

Do.

Do.

Do.

Purple streaked quartzite.

White quartzite.

Calcareous quartzite.

White quartzite.

Do.

Do.

Do.

Purple streaked quartzite.

White quartzite.

Do.

Gray si Ity argil 1 ite

Do.

Do.

Purple streaked siltite.

Gray argi 1 1 itic si 1 ti te.

Do.

Purple streaked siltite.

Gray argi 1 1 i tic si 1 tite.

Do.

Purple streaked siltite.

Gray-green argillitic siltite.

Gray argi 1 1 itic si 1 ti te.

Do.

Do.

Purple and green siltite.

Purple argillitic siltite.

Do.

Gray-green argillitic siltite.

Pri chard Formation

040

131

183

017

020

021

023

026

031

034

10

10

L

L

IS

L

L

15

L

L

L

L

15

30

20

10

20

15

IS

7

10

30

N

20

L

15

15

15

10

15

7

15

5

7

15

7

7

10

15

10

N

N

N

N

N

N

N

N

N

200

50

100

30

50

100

70

50

50

70

70

20

30

30

IS

30

30

15

30

30

30

ISO

ISO

ISO

100

ISO

100

70 150

100

ISO

C

C

C

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Black and white argillite.

Do.

Do.

Gray pyrrhotitic argillite.

Do.

Black and white argillite.

Pyrrhotitic silty argillite.

Gray si 1 ty argi 1 1 ite.

Spotted silty argi Kite.

Do.
O20 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 3. Semiquantitative spectrographic analyses

(p

Sample Fe Mg 

(.05) (.01)

035 3-0 1.0

037 3 .7

085 3 .7

132 3 1

135 3 .7

136 5 -7

137 3 .7

138 2 .5

139 3 1

I4o 3 .7

I'M 3 1

154 3 .7

157 3 .7

159 3 -7

160 1.5 .7

164 3 1

172 3 1.5

179 3 .7

185 1.5 1

208 3 .7

216 2 .2

218 2 .7

220 3 .7

222 3 .5

228 3 .5

229 2 .5

230 3 .7

233 3 .7

234 3 .3

236 2 .7

215 3 .7

217,, 1.5 .3

219 S/ 3 .7

226 1 .5 .2

227 1 .2

232 3 1

248 3 .5

249 5 1 251 3 .7

252 1 .3

255 3 1.5

256 2 1

257 2 .5

258 1.5 .5

259 3 .7

260 2 .5

261 3 .7

130 3 .7

014 3 .7

015 3 .5

019 1.5 .3

025 3 1 .5

028 2 .7

029 1 .3

036 1.5 .5

142 2 .7

214 .7 .5

225 .5 .2

250 .7 .15

262 .7 .2

ercent)

Ca 

(.05)

0.2

L

.05

.15

.3

.07

.15

.07

.07

.07

.05

.07

.1

.07

.05

.07

.05

.07

.07

.05

.5

.3

.1

.3

L

L

.05

.05

.05

.1

.15

.2

.2

.3

.2

.07

.2

.15

.15

.3

2

.15

.3

I

.7

.7

.05

.3

.05

.07

.07

.5

.05

.07

.1

.1

1.5

.15

.2

.1

Ti 

(.001)

Prichard

0.5

.2

.3

.5

.5

.5

.5

.3

.5

.5

.5

.3

.3

.7

.2

.5.5 '

.7

.3

.5

.2

.3

.3

.3

.3

.2

.3

.5

.3

.5

.3

.2

.3

.2

.15

.3

.5

.7

.3

.2

.7

.5

.2

.3

.3

.3

.5

.2

.3

.3

.2

.2

.2

.15

.15

.2

.2

.1

.2

.2

Mn 

(10)

Format

300

200

300

500

500

300

500

300

300

300

300

300

300

300

150

300

300

300

200

200

200

500

300

300

150

20

300

300

50

300

300

200

300

100

ISO

300

300

500

300

200

700

300

200

200

300

200

300

300

200

150

200

300

150

100

200

200

100

100

200

100

B 

(10)

ion Conti

15

15

20

50

30

30

30

. 30

100

100

70

30

30 ' 50

15

30

20

50

20

30

L

15

70

30

10

30

50

15

10

15

15

15

15

10

10

30

20

20

20

10

L

10

20

L

10

30

30

20

30

L

15

30

L

L

L

30

15

L

20

15

Ba 

(10)

nued

700

700

500

700

700

700

700

500

500

700

500

700

700

700

700

700

700

700

300

700

500

500

500

700

500

700

700

500

500

700

700

300

500

200

300

500

500

700

700

700

300

700

700

500

500

500

500

500

500

500

300

500

300

300

300

200

70

500

300

500

(ppm)

Be Co 

(1) (5)

L 10

1 7

1 10

2 15

3 10

2 5

2 10

2 L

3 10

3 10

2 L

2 7

3 10

2 10

2 5

1.5 7

3 L

2 7

1.5 5

2 10

1.5 5

1.5 L

1 7

2 10

1 L

1 N

1.5 7

3 N

2 N

2 L

1 L

1.5 5

1.5 7

1.5 L

1.5 N

3 L

1.5 5

1.5 7

1.5 N

L 10

1 15

1 L

1.5 L

1.5 L

1.5 L

1.5 N

I .5 5

1 .5 10

L 15

1 10

1.5 7

2 10

L N

L N

L L

1 7

2 N

N N

1 L

1.5 N

Cr 

(5)

70

30

30

70

50

50

30

20

50

70

50

50

50

50

15

50

70

50

30

70

15

50

50

50

50

50

50

70

50

70

50

15

50

10

10

50

50

70

50

30

100

70

78

30

50

30

70

20

30

50

30

30

30

15

20

15

20

5

20

20

Cu 

(2)

L

7

20

10

10

20

5

7

30

20

15

20

15

20

L

20

15

15

15

15

20

20

20

30

15

30

30

20

15

20

30

30

10

15

7

20

7

30

20

20

20

IS

10

10

15

10

30

15

30

L

10

50

5

7

10

20

10

7

7

5

La 

(20)

70

30

50

70

50

30

50

30

50

70

50

50

70

70

20

70

70

30

30

70

50

70

70

70

50

50

70

30

50

20

50

50

70

50

30

N

70

70

70

30

70

50

50

50

30

50

50

50

50

50

50

50

30

L

30

30

30

20

50

30

See footnotes at end of table.
BELT ROCKS, MONTANA AND IDAHO 

of Belt rocks from the Pend Oreille' area Continued

O21

(ppm)

Sample

035

037

085

132

135

136

137

138

139

140

141

154

157

159

160

164

172

179

185

208

216

218

220

222

228

229

230

233

234

236

215

217

219

226

227

232

248

249

251

252

255

256

257

258

259

260

261

130

014

015

019

025

028

029

036

142

214

225

250

262

Nb 

(10)

L

10

L

IS

L

L

10

10

L

L

10

10

10

10

L

L

10

15

10

IS

10

10

10

10

L

L

10

15

10

10

10

10

10

L

L

15

L

10

L

L

10

10

L

10

L

10

10

10

L

L

L

L

L

L

L

L

10

L

L

L

Ni 

(2)

15

7

20

30

20

IS

20

L

30

30

L

20

30

30

15

30

.5

20

20

20

10

L

15

30

7

L

15

L

5

L

7

7

15

L

5

5

L

15

L

20

50

L

L

5

5

L

10

20

30

15

10

30

L

L

L

20

L

L

L

L

Pb 

(10)

10

N

10

30

10

L

10

N

L

L

L

10

L

20

L

L

L

L

50

N

L

10

L

10

L

10

N

L

N

15

L

L

L

N

N

15

L

30

L

L

15

15

10

L

N

L

N

10

L

N

L

20

10

L

L

IS

L

20

L

L

Sc 

(5)

15

7 ' 7

15

 10

15

10

7

IS

15

10

10

15

15

7

15

15

10

10

15

7

10

15

15

10

7

10

10

10

15

15

7

10

5

7

10

10

15

10

7

IS

15

7

7

10

10

IS

7

7

7

5

7

7

5

5

5

5

L

5

7

Sr 

(50)

Pr

L

N

N

N

L

N

N

L

N

N

N

N

N

N

N

N

N

N

N

N

L

L

N

N

N

N

N

N

N

N

N

N

N

N

100

N

L

L

L

L

500

L

100

300

150

100

N

L

N

N

N

N

N

N

N

N

200

N

200

N

V 

00]

ichard

70

50

70

70

100

100

70

70

70

70

70

70

70

100

70

70

100

70

70

70

50

70

70

70

70

70

50

70

70

70

70

50

70

30

30

70

70

70

70

50

70

70

70

50

70

70

70

70

50

50

30

50

50

30

30

50

50

IS

30

50

Y 

(5)

Format!

30

20

30

30

30

20

30

20

30

30

30

30

50

50

20

30

30

30

30

30

30

30

30

30

30

30

30

30

30

30

30

30

30

30

20

30

30

50

30

20

50

30

30

30

30

30

30

50

20

30

20

30

20

15

20

20

30

15

30

30

Zr Metamorphic 

(10) Grade Sample description

on--Cont i

ISO

150

150

ISO

ISO

ISO

200

150

ISO

150

200

150

150

150

ISO

ISO

100

150

150

ISO

ISO

150

ISO

ISO

100

150

150

ISO

ISO

150

150

ISO

ISO

150

ISO

ISO

ISO

150

150

150

200

150

150

ISO

ISO

150

ISO

150

ISO

ISO

150

ISO

150

150

150

100

150

ISO

700 150

nued

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

Bgn

C

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bgn

Bgn

Bgn

Bgn

Argi 1 1 ite with andalusi

Gray pyrrhoti tic argi 1 1

Black and white argilli

Do.

Slaty black argi 1 1 i te.

Gray argi 1 1 ite.

Do.

Slaty black argi 1 1 i te.

Black and white argilli

Do.

Slaty black argi 1 1 ite.

Black and white argilli

Do.

Do.

Spotted black argi 1 1 ite

Black and whi te argi 1 1 i

Spotted black argi 1 1 ite

Black and white argi 1 1 i

Spotted black argi 1 1 ite

Black and white argilli

Do.

Do.

Do.

Do.

Do.

Do.

Gray argi 1 1 ite.

Black and white argilli

Do.

Do.

Schistose argi 1 1 i te.

Gneissic argi 1 1 ite.

Schistose argi 1 1 ite.

Gneissic argi 1 1 ite.

Do.

Do.

Black and white gneiss.

Black and white schist.

Coarse mica schist.

Coarse mica gneiss.

Bioti te gneiss.

Black and whi te schist.

Gneissic argi 1 1 ite.

te.

i te.

te.

te.

te.

te.

te. .

te.

te.

Gneissic si 1 ty argil) ite.

Schistose argi 1 1 ite.

Gneissic argi 1 1 ite.

Schistose argi 1 1 ite.

Gray argi 1 1 i tic si 1 tite

Gray si 1 tite.

Dark gray si 1 1 i te.

Gray argi 1 1 itic si 1 ti te

Gray pyrrhotitic siltlti

Spotted si 1 ti te.

Do.

Salt and pepper si 1 tite

Black and white si 1 tite

Gneissic si 1 tite.

Do.

Do.

Do.

5.
O22 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 3. Sem'iquantitative spectrographic analyses

_________(percent)_______ ____________________(ppm)____________________

Sample Fe Mg Ca Ti Mn B Ba Be Co Cr Cu La

______(.05) (.01) (.05) (.001) (10) (10) (10) (I) (5) (5) (2) (20)

Prichard Formation Continued

263 1.0 0.2 0.15 0.2 100 10 300 1.5 L 15 7 30

016 .7 .2 .15 .15 100 10 200 L 5 20 L 30

018 .7 .1 1 .2 300 L 200 10 L 20 10 70

02 1* 1 .2 .3 .2 300 10 200 1 N 20 7 30

027 I .2 L .15 70 L 200 N 5 20 7 30

030 .7 .2 .5 .2 200 L 200 1 N 20 5 50

032 .7 .1 1.5 .15 300 L 70 L N 10 L 30

221 1 .2 .15 .2 200 10 300 1 N 10 15 50

237 1 .3 .2 .2 200 10 500 1 5 30 20 50

212 .7 .3 1.5 .15 150 L 100 1 N 15 7 50

231 .5 .1 .3 .15 150 L 70 1 N 20 30 30

25** 7 .7 .5 1.5 .1 200 L 70 3 L 20 L 50

223 V 15 .1 20 .03 >5,000 N 20 N 15 5 15 N

253 2/5 .5 >20 .1 >5,000 L L 3 10 20 20 30

\J Reported as L(5) for Mo.

2/ Reported as L(200) for Zn.

3/ Contains 0.7 ppm Ag.

kj Contains 20 ppm Mo, 0.7 ppm Ag, and 10 ppm Bi.
BELT ROCKS, MONTANA AND IDAHO O23 

of Belt rocks from the Pend Oreille area Continued

Sample

263

016

018

024

027

030

032

221

237

212

231

25^

223

253

5/

i/

U

§/

Nb 

(10)

L

N

L

L

N

10

L

L

10

L

L

L

L

10

Contains

Contains

Contains

Contains

Ni Pb 

(2) (10)

15 N

L L

L 10

5 15

10 N

L 10

L N

L L

10 L

L N

L L

7 L

20 N

7 N

5 ppm Mo.

7 ppm Mo.

15 ppm Sn.

200 ppm Zn e

(ppm)

Sc Sr 

(5) (50)

Pri

5 N

L L

5 200

5 L

L N

5 L

L N

5 N

7 L

L N

5 L

L L

L N

7 N

ind 15 ppm Sn.

V 

(10)

i chard

30

20

30

20

20

20

15

30

50

20

30

30

700

70

Y 

(5)

Form;

20

15

30

15

15

30

15

20

30

20

20

20

30

30

Zr 

(10)

3tion--Cont

ISO

150

150

150

100

150

200

150

150

150

500

200

N

150

Metamorphi 

Grade

inued

Bgn

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bb

Bgn

Bgn

Bgn

Bgn

Bgn

ic 

Sample description

Gneissic si 1 1 i te.

White si 1 ty quartz! te.

Gray green quartzite.

Do.

Salt and pepper quartzite.

Gray green quartzite.

Do.

Do.

Do.

Gneissic quartzite.

Biot i tic quartz! te.

Quartz gneiss.

Skarn.

Do.
O24 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 4. Replicate semiquantitative spectrographic

[Number in parentheses indicates sensitivity limit of method used. L, undetermined amount of 

for but not found in any sample were As(200), Au(lO), Cd(20), Sb(lOO), Sn(10), W(50), 

N(10) for Bi, and N(5) for Mo. D. J. Grimes, analyst]

(percent)

Sample Fe Mg 

(.05) (.01)

001 1.5 0.7

Oil 2 1.5

012 3 1.5

022 3 1.5

023 1.5 1

033 3 1.5

03 1* 3 1.5

CM 3 1

OU5 1.5 .1

055 1.5 .1

056 2 1.5

066 2 1 .5

06? 3 1 .5

077 3 2

078 1.5 3

088 2 2

089 3 .7

099 2 .7

09l' 1 .3

101 .7 .2

102 3 1.5

112 5 1.5

113 3 1.5

123 5 3

I2<» I/ 2 10

13 1* ^1.5 7

135 3 .7

1^5 3 1.5

1*»6 .5 .5

156 .5 .3

157 3 .7

16? 3 1

168 3 5

178 3 3

179 3 .7

189 3 .7

181 .7 .3

191 .7 .3

192 5 1

202 5 ' 1

203 .3 .1

213 .3 .07

21<t .7 .5

22<t .5 .3

225 .5 .2

235 .7 .3

Ca 

(.05)

0.05

.05

L

.05

.2

 5

.7

.5

.07

.05

.1

.1

.07

.07

.1

.1

.07

.07

 05

L

.07

.07

.05

.15

20

20

.3

.5

10

15

.1

.1

.1

.07

.07

.07

L

.05

.07

.07

L

L

1.5

.7

' .15

.2

Ti 

(.001)

0.15

.3

.2

.3

.2

.15

.3

.3

.2

.2

.2

.3

.3

.3

.2

.2

.2

.2

.2

.15

1

1

.3

.7

.15

.15

.5

.5

.15

.15

.3

-.3

.3

.5

.7

.3

.3

.2

.2

.2

.05

.05

.2

.15

.1

.2

(10)

50

70

70

100

200

300

500

300

70

50

30

20

15

20

50

50

500

300

200

150

15

15

15

50

5,000

3,000

500

500

300

300

300

300

50

50

300

300

20

15

300

300

15

70

100

100

100

200

B 

(10)

30

70

50

50

10

15

15

15

15

15

30

20

150

300

70

70

20

20

15

10

300

 500

100

200

500

700

30

50

15

10

30

30

70

50

50

30

L

L

30

30

L

10

15

L

L

L

Ba 

(10)

300

500

300

500

500

500

500

300

200

200

300

300

700

700

300

500

300

500

300

200

700

700

500

700

500

300

700

700

500

500

700

700

500

500

700

700

150

150

150

150

200

150

70

50

500

500

Be Cc 

(1) (5)

1 .0 10

1.5 15

1 10

1 .5 10

L 7

L 10

1 .5 5

1 5

1 5

L 5

L L

L N

1.5 L

2 5

1.5 7

1.5 7

1 10

1 10

1.5 L

1 L

2 5

3 5

1.5 5

2 7

L L

L N

3 10

2 10

L L

L L

3 10

2 10

2 10

2 7

2 7

1.5 5

1 N

L L

L 10

1 10

L N

L N

2 N

1.5 N

N N

L N

\J Reported as 0.7 ppm Ag, 10 ppm Bi, and 20 ppm Mo.
BELT ROCKS, MONTANA AND IDAHO O25

analyses of Belt rocks from the Pend Oreille area

element present below sensitivity limit; N, element was looked for but not found. Also looked 

and Zn(200). Except as indicated by footnotes, all samples were reported as N(0.5) for Ag,

Sample

001

on

012

022

023

033

03 1*

OMt

OltS

055

056

066

06?

077

078

088

089

099

091

101

102

112

113

123

12<»

I3<*

135

l<»5

1<*6

156

157

167

168

178

179

189

181

191

192

202

203

213

211*

221*

225

235

Cr 

(51

30

30

50

70

30

30

70

50

20

15

30

20

50

70

30

30

30

30

15

15

50

50

70

100

30

20

50

50

15

15

50 ' 50

30

50

50

30

7

7

20

30

7

5

20

7

5

20

Cu 

(2)

5

7

30

50

10

15

7

5

5

5

7

5

L

L

L

L

10

15

7

L

L

L

7

7

3,000

3,000

10

10

7

5

15

20

L

N

15

10

N

15

30

30

L

7

10

7

7

15

La 

(20)

50

70

50

70

50

30

70

70

30

30

20

20

50

50

30

50

30

30

30

20

50

70

50

70

30

30

50

70

30

50

70

70

50

70

30

20

50

50

30

50

L

30

30

30

20

20

Nb 

(10)

N

N

10

15

L

L

L

10

L

L

L

L

L

10

10

L

N

L

L

L

L

10

10

10

L

L

L

10

L

L

10

10

L

10

15

10

10

L

10

10

N

L

10

L

L

L

Ni 

(2)

30

30

20

30

20

30

7

10

5

5

30

20

10

15

20

20

20

30

5

5

7

30

15

20

15

15

20

30

L

L

30

30

20

20

20

20

L

L

30

30

L

L

L

L

L

L

(ppm!

Pb 

(10)

L

L

N

L

15

15

15

15

N

N

N

N

L

N

N

N

10

15

N

N

N

L

L

N

N

L

10

10

10

L

L

10

N

N

L

L

N

N

L

L

N

N

L

L

20

15

|

Sc 

(5)

7

10

10

10

7

7

10

10

5

5

7

7

10

15

7

5

5

5

5

5

15

15

7

15

5

L

10

10

L

5

15

10

10

10

10

10

7

5

7

10

L

N

5

5

L

L

Sr 

(50)

N

N

N

N

N

N

. 200

ISO

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

L

L

100

L

N

N

N

N

N

N

N

N

N

N

N

N

200

100

N

N

V 

(10)

50

70

50

70

50

50

70

70

30

30

50

50

70

70

50

50

30

50

20

30

70

70

70

100

50

50

100

70

20

20

70

100

70

70

70

70

30

20

50

70

L

15

50

20

15

50

Y 

(5)

30

30

20

30

15

20

30

30

20

20

20

20

30

30

30

30

15

20

20

15

50

50

30

70

30

30

30

30

30

30

50

50

30

30

30

30

20

20

30

30

15

15

30

20

15

20

Zr 

(10)

150

150

100

ISO

70

150

150

100

150

ISO

ISO

ISO

200

200

150

ISO

ISO

150

150

100

200

200

200

300

150

100

ISO

ISO

100

100

ISO

ISO

150

150

150

150

1,000 

700

150

200

100

100

150

ISO

150

150

2j Reported as 0.7 ppm Ag, 10 ppm Bi, and 15 ppm Mo.
O26 CONTRIBUTIONS TO ECONOMIC GEOLOGY

Precision of the sampling and analytical technique was measured by 

analysis of blind replicates of every eleventh sample from the Pend Oreille ., 

area. The replicate samples, which include a range of element concentrations, were taken from a 1-pint crushed control sample representing a split '* 

of the 2-pound or larger sample collected of a given rock type. The precision 

thus includes errors in sampling as well as errors inherent in the preparation 

and analysis of each sample. Analytical results on the 23 pairs of replications 

are given in table 4. >

Analytical precision is shown graphically in figure 4. The solid bars 

show the number of replicate samples having identical analyses for a given 

element; the open bars show the number of replicate samples differing by 

one or more steps in the reported results. Step differences were determined 

by comparing values from the first split with those from the second. Values 

reported as N (no line seen for the elements) and L (element present in an > 

amount less than the sensitivity limit) were arbitrarily counted as two and 

one steps below the limit of sensitivity, respectively, for purposes of comparison. However, a pair of N's was excluded as a valid measure of similarity. 

For elements having a relatively high limit of sensitivity (many reported 

N's or L's), these arbitrary comparisons will tend to result in an artificially 

high precision. Such elements as Pb and Sr are particularly suspect in this 

regard. The precision of Nb and Go and perhaps of Be, Ni, and Cu should 

also be viewed with caution.

The remaining elements exhibit quite satisfactory precisions in that more 

than 80 percent of the comparisons fall within plus or minus one step. Of 

particular interest, however, is the occasional large difference reported for 

replicates of Mn, Gr, Cu, and Ni. To the extent that such disparity is not due 

strictly to analytical inprecision, it may be due to the particulate character of 

small amounts of minerals containing those elements in the sample. Such 

errors then, may reflect sample inhomogeneity which, in turn, may provide 

clues for interpretation of element distributions in these rocks. ^

One factor aids in maintaining high precision for the spectrographic 

method. A split of sample G-l was shot on each plate as a reference standard. If the G 1 values were not sufficiently close to the known values, then > 

that plate was discarded and another made. Values of G 1 for 12 plates 

used for the Pend Oreille samples are given in table 5. If we assume that 

the bottle of G 1 used as the reference standard in this study was an exact 

duplicate of the original G-l, then table 5 can be used by other workers to 

estimate accuracy of the reported values. The table also shows that our 

selected use of N and L in the calculations for table 4 is reasonable, con- ^ 

sidering the "best value" content of elements in G 1 that are below the 

sensitivity limit of the method we used. In other words, L on table 5 is 

reported for elements whose true value is about one reporting step below x 

the sensitivity limit, and N is reported for elements more than one step
15-,

3> 10-

_, uj 5 _

NUMBER Of REPORTING - 

STEPS DIFFERENCE

Percentage of sample pairs:

In O category 

In (-1+0 + 1) category 

Reported as L or N 

Reported only as N

10 -i

5-

Fe

57

96

0

0

0-

NUMBER OF REPORTING 

STEPS DIFFERENCE

lliLi

Mg

35

87

0

0

-2-10 1 23

Ca

48

87

4

0

1l n_n

-4 -101234567

Percentage of sample pairs:

In O category

In (-1+0+1) category

Reported as L or N

Reported only as N

Cu

35

83

17

0

La

43

96

0

0

-2-1012

Ti

52

87

0

0

-101234

Mn

39

91

0

0

-2-1 0 1 2

B

39

83

9

0

-1012 -101 -101234

Nb

45

100

43

4

Ni

61

96

22

0

Pb

44

100

70

30

-101

Sc

57

96

9

0

FIGURE 4. Precision of semiquantitative spectrographic analyses of some Belt rock

Solid bar, number of replicate samples having identical analyses; N, no line seen 

amount less than the sensitivity limit.
[Number in parentheses indicates sensitivity limit of method used. L, an undetermined 

below the sensitivity limit; N, the element was looked for but 

TABLE 5. Replicate analyses of standard samp

Percent

Fe

(0.05)

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.37

Mg

(0.01)

0.2

.2

.2

.2

.2

.2

.2

.2

.2

.2

.2

.2

0.25

Ca Ti

(0.05) (0.001)

0.7 0.15

.7 .15

.5 .15

.7 .15

.5 .15

.7 .15

.7 .15

.7 .15

.7 .15

.7 .15

.7 .15

.7 .15

0.99 0.16

Mn

(10)

150

150

100

150

150

150

150

150

150

150

150

150

230

B Ba

(10) (10)

G-l, '

N 700

N 700

N 700

N 700

N 700

N 700

N 700

N 700

N 700

N 700

N 700

N 700

G-l,

1.5 ? 1,220

Be

(1)

'Best val

1.5

1

1

1.5

1.5

2

1.5

1.5

1

2

1.5

1.5

Co

(5)

Cr

(5)

ue" (Fleischer,

N

N

N

N

N

N

N

N

N

N

N

N

20

20

15

20

20

30

15

20

20

20

20

20

"Beat value" (Fleischer,

3 2.4 22

Parts per millio

Cu

(2)

1965)

20

20

15

20

15

20

15

20

15

30

20

15

1965)

13

La

(20)

Nb

(10)

of original spl

100

100

100

100

100

100

100

100

100

100

100

100

10

10

15

10

10

15

15

15

10

10

10

10

of original split

120 20 
BELT ROCKS, MONTANA AND IDAHO O29

below. Thus L's and N's for the same sample are probably only about one 

reporting step apart, but two N's could reflect any value from about two 

steps below the sensitivity limit down to zero.

Finally, high-calcium samples alter the excitation conditions of the arc 

in the spectrographic method used here. Comparison with mineralogic and 

wet chemical data indicates that values reported as 10 percent and more 

calcium are consistently low. This suggests that in high-calcium samples 

trace elements may also be affected by the altered arcing conditions, which 

could result in a slight positive or negative bias for a given element. This 

then limits the usefulness of the analytical method on carbonate rocks; 

the precision of the method is sufficiently good to permit comparison of 

elements among groups of carbonate rocks, but the potentially poor accuracy 

may not permit comparison with low-calcium rocks. As a consequence, our 

geologic analysis is restricted to the pelitic and psammitic rocks of low 

carbonate content.

Calculations of average contents and dispersions are based on logarithms 

because the analytical data are reported as midpoints of geometric classes 

(Miesch, 1967, p. B4). In this study, the average content is estimated by the 

geometric mean (the antilog of the mean logarithm), and the dispersion or 

scatter is estimated by the geometric deviation (the antilog of the standard 

deviation of the log values). Geometric means and geometric deviations 

for elements in various groups of Belt rocks are given for the Mission 

Mountains Primitive Area in table 6 and for the Pend Oreille area in 

table 7. For purposes of these computations, real, but arbitrary, numbers 

were substituted for the nonnumeric codes N, L, and > (greater than) used 

in reporting the raw data. (See table 3.) Table 8 lists the numbers substituted and the percentage of the substitution needed for each element. The 

particular numbers used for L and > were determined largely on the basis 

of wet chemical or quantitative spectrographic analyses of some samples. 

Numbers substituted for N are simply small positive numbers between the 

limit of sensitivity and zero. Use of logarithms precludes the use of zero. 

Following common practice, results of calculations are given to two significant figures.

MINERALOGY OF ROCK TYPES

Mineral data on most of the clastic rocks of the Mission Mountains 

(table 2) and Pend Oreille (Harrison and Campbell, 1963, table 1) areas 

are summarized in figure 5. Rock groups containing less than 3 samples are 

not shown. The ratios among quartz, potassium feldspar, and plagioclase 

for the siltites shown in figure 5, as well as for the carbonatic siltites from the 

Pend Oreille area, are summarized in figure 6.

The general uniformity of the Belt rocks is apparent in figure 5; most 

rocks are graywacke. The grain size and mineralogy are directly related.
TABLE 6. Mean element content of low-grade Belt rocks from the Miss

[Geometric means (GM) and geometric deviations (GD) calculated from data given b

and Pattee (1969, table 1). _......, all or most samples reported as N (element w

Number of 

samples Formation and rock type

Shields Formation;

Shepard Formation;

Snowslip Formation;

Helena Formation;

Empire and Spokane Formations;

All formations listed above; 

62 Argillite and argillitic rocks ...... 

62 Siltite...................... .. .

Number of 

samples Formation and rock type

Shields Formation;

Shepard Formation;

9 Siltite. ........................

Percent

Fe

GM GD

2.9 1.5 

1.4 1.4

3.9 1.4 

3.4 1.8 

2.6 2.4 

3.0 1.3

3.5 1.5 

2.8 1.8 

1.8 2.2

2.6 1.4 

2.1 1.6 

.84 2.6 

1.9 1.7

3.8 1.6 

2.6 1.4 

1.0 1.9

3.1 1.5 

2.4 1.7 

1.3 2.4 

2.0 1.7 

2.7 1.6

Mg

GM GD

2.5 1.4 

2.1 1.4

2.5 1.6 

1.5 1.8 

1.2 2.1 

2.5 1.8

1.3 1.3 

.93 2.0 

.63 2.9

2.7 1.5 

2.3 1.9 

1.0 2.3 

3.2 1.8

2.5 1.5 

2.5 1.7 

.93 2.0

2.3 1.6 

1.8 2.0 

.91 2.4 

3.1 1.8 

2.0 1.8

Ca

GM GD

2.0 1.5 

.22 3.8

1.7 3.9 

.90 4.3 

3.4 2.8 

12 1.4

.42 2.6 

.34 3.7 

1.5 4.0

1.6 3.8 

1.8 4.1 

2.0 3.2 

13 1.4

.58 4.6 

.89 4.4 

.67 2.3

1.0 4.2 

1.0 4.6 

1.3 3.7 

13 1.4 

1.0 4.4

Ti

GM GD

0.30 0 

.18 1.2

.34 1.2 

.28 1.3 

.07 3.1 

.20 1.6

.39 1.3 

.35 1.4 

.19 1.8

.28 1.4 

.25 1.3 

.15 2.5 

.14 2.0

.33 1.4 

.26 1.2 

.12 2.1

.31 1.4 

.28 1.4 

.14 2.3 

.14 1.9 

.30 1.4

B

GM GD

100 1.4 

21 1.4

73 1.7 

31 2.0 

16.2 3.7 

35 2.3

85 1.7 

53 2.0 

23 2.8

60 1.9 

31 1.8 

'7.2 3.2 

22 2.4

26 1.2 

18 1.5 

'5.3 2.4

64 1.9 

32 2.0 

 9.2 3.3 

24 2.4 

46 2.1

Parts per 

La

GM GD

33 1.4 

"10 0

.32 1.4 

29 1.6 

21 2.8 

41 2.1

Mn

GM GD

500 1.5 

75 2.6

570 3.5 

650 2.6 

1200 2.6 

1800 2.4

Nb

GM GD

 

Ni

GM GD

36 1.5 

18 1.7

41 1.5 

31 1.4 

8.2 2.5 

22 2.2

Pb

GM GD

16.7 4.0

'8.6 2.7 

»6.1 2.3 

15.3 2.8 

33 5.6

V
Snowslip Formation;

14 Siltite.... .....................

Helena Formation;

Empire and Spokane Formations;

9 Siltite.... .....................

All formations listed above;

124 Pelitic rocks. ..................

34 1.5

27 1.6

18 2.6

37 1.5

22 1.8

20 1.9

25 1.6

39 1.5

35 1.5

30 1.7

35 1.4

26 1.7

22 2.2

27 1.7

30 1.6

380 1.9

400 2.2

740 2.4

210 2.1

250 2.2

320 2.0

740 1.6

480 2.1

450 2.7

290 2.1

320 2.4

350 2.4

420 2.7

840 1.9

330 2.4

_ _

_ _

_ _

_ _

_ _

'4.1 2.3

_ _

_ _

_ _

 

44 1.5

21 1.7

14 2.9

22 1.6

19 1.6

8.4 2.4

13 1.8

28 1.4

23 1.6

12 1.9

29 1.6

21 1.6

11 2.4

14 1.9

25 1.7

'8.0 4.7

'4.6 2.6

'4.4 2.9

''9.4 3.1

10 3.6

14 4.4

24 4.2

18 1.5

14 1.5

'8.3 3.7

'8.8 3.2

'8.1 3.0

'7.6 3.7

25 3.3

'8.4 4.1

16 

11 

5

12 

8

'4

5

12 

11 

'4

13 

9

5

5

11 

TABLE 7. Mean element content of Belt rocks from the Pen

[Geometric means (GM) and geometric deviations (GD) calculated from data 

__ , all or most samples reported as N (element was looked for but 

Number Metaof morphic 

samples grade1 Formation and rock type

Libby Formation; 

11 C Argillite and argillitic rocks.. 

3 Bb ....... .do..... ...........

Striped Peak Formation; 

7 C Argillite and argillitic rocks . .

Striped Peak member 4 ;

Striped Peak, members 3 & 2; 

3 C Argillite and argillitic rocks . . 

6 C Siltite.... .................

Striped Peak member 1 ; 

4 C Argillite and argillitic rocks . . 

5 C Siltite. ....................

'Percent

Fe

GM GD

2.6 1.3 

2.5 2.3

3.2 1.2 

1.6 1.7 

1.2 1.9

1.4 1.8

3.0 0 

1.2 1.7

3.4 1.3 

1.9 1.6

Mg

GM GD

1.3 1.5 

1.0 1.5

1.4 1.2 

1.5 2.4 

.49 2.8

.48 3.0

1.3 1.3 

2.0 2.6

1.5 0 

1.4 2.6

Ca

GM GD

0.11 3.3 

.080 1.9

.061 1.2 

.15 6.8 

.080 3.5

.092 3.6

.076 1.2 

.33 13

.060 1.2 

.081 1.2

Ti

GM GD

0.31 1.5 

.53 1.8

.42 1.7 

.40 1.9 

.23 2.1

.28 1.6

.50 0 

.40 2.0

.37 2.0 

.42 2.0

B

GM GD

86 3.3 

43 1.5

301 3.0 

387 4.2 

95 3.0

124 2.4

794 2.2 

751 4.1

146 1.7 

199 3.8

G

40

50

46

32

17

16

30

38

64

27
TABLE 7. Mean element content of Belt rocks from the Pend Orei

Number Metaof morphic 

samples grade l Formation and rook type

Wallace Formation; 

21 C Argillite and argillitic rocks . . 

3 Bb ....... .do ...............

8 C Siltite.... .................

4 Bb ....... .do ...............

St. Regis Formation; 

6 C Argillite and argillitic rocks . .

Revett Formation; 

4 C Siltite...... ...............

3 Bb ........do ...............

5 Bb ....... .do ...............

Burke Formation; 

3 Bb Argillite and argillitic rocks . . 

13 Bb Siltite. ....................

Prichard Formation; : 

3 C Argillite and argillitic rocks . . 

37 Bb ........do ...............

8 . Bb Siltite.... .................

3 Bgn ........ do ...............

All formations listed above; 

48 C Argillite and argillitic rocks . . 

31 C Siltite.....................

79 C Pelitic rocks. ..............

Number Metaof morphic 

samples grade1 Formation and rock type

Libby Formation; 

11 C Argillite and argillitic rocks .. 

3 Bb ........do................

Percent

Fe

GM GD

2.2 1.4 

2.4 1.5 

2.0 1.6 

2.3 1.7 

1.8 1.8

2.8 1.2 

1.8 1.7 

2.2 2.0

1.5 1.7 

1.1 1.7 

.52 . 1.7 

.43 1.8

2.6 1.3 

1.8 1.4

2.4 1.5 

2.7 1.3 

2.2 1.6 

2.0 1.5 

.70 1.3 

.84 1.2 

.63 1.2

2.5 1.4

1.7 1.6 

.85 2.0 

1.4 1.8 

2.2 1.6

Mg

GM GD

2.0 1.9 

1.8 2.3 

1.5 2.2 

.12 3.2 

6.2 1.8

1.4 1.7 

1.2 2.0 

2.1 1.4

.34 2.2 

.40 1.6 

.070 1.3 

.11 2.0

.90 1.6 

.43 2.1

.59 1.9 

.73 1.5 

.56 1.8 

.57 1.7 

.23 1.6 

.18 1.5 

.20 2.3

1.4 1.8 

1.2 2.5 

.24 3.4 

4.1 3.5 

1.3 2.1

Ca

GM GD

.088 4.8 

.23 14 

.40 11 

.14 4.9 

17 1.6

.17 4.3 

.29 4.2 

.25 3.4

2.046 1.6 

2.042 1.3 

3.049 5.8 

2.030 1.9

.072 1.9 

.12 3.2

2.042 1.3 

.087 2.2 

.25 2.5 

.090 2.1 

.23 2.9 

.27 3.4 

.88 2.5

.091 3.7 

.19 6.6 

.094 6.8 

16 1.6 

.12 4.9

Ti

GM GD

.26 1.4 

.31 1.6 

.20 1.2 

.28 1.5 

.12 1.8

.29 1.6 

.22 1.3 

.26 1.3

.31 1.5 

.31 1.6 

.14 2.0 

..16 1.1

.47 1.5 

.24 1.5

.27 1.7 

.36 1.4 

.32 1.6 

.21 1.3 

.17 1.4 

.18 1.2 

.13 1.3

.30 1.5 

.28 1.7 

.18 2.0 

.10 2.0 

.29 1.6

B

GM GD

63 2.0 

79 2.2 

34 1.5 

39 1.3 

43 1.5

49 1.7 

31 1.6 

67 2.7

33 2.1 

13 2.4 

13 1.9 

28.7 1.4

36 1.3 

19 1.3

31 1.6 

25 2.0 

14 1.7 

11 2.4 

12 1.7 

S7.1 1.4

25.0 o

79 2.8 

90 4.7 

41 3.9 

53 2.4 

83 3.5

GM 

400 

560 

360 

330 

270 

540 

380 

280 

340 

320 

210 

210 

70

380 

56

590 

490 

340 

28

21

80 

43

35

20

24

40

Parts per mil

La

GM GD

33 1.7 

J13 4.9

Mn

GM GD

230 1.6 

88 4.8

Nb

GM GD

27.1 1.5 

29.1 1.7

Ni

GM GD

14 2.5 

8.4 6.4

Pb

GM GD

 

GM 

7

10 
Striped Peak Formation; 

7 C ' Argillite and argillitic rocks. . 

12 C Siltite. ....................

Striped Peak member 4;

Striped Peak, members 3 & 2 ; 

3 C Argillite and argillitic rocks . . 

6 C Siltite.... .................

Striped Peak member 1 ; 

4 C Argillite and argillitic rocks . . 

5 C Siltite.... .................

Wallace Formation; 

21 C Argillite and argillitic rocks .. 

3 Bb ........do ...............

8 C Siltite. ....................

4 Bb ........ do ...............

St. Regis Formation; 

6 C Argillite and argillitic rocks . . 

6 C Siltite.... .................

Revett Formation;

3 Bb ....... .do ...............

5 Bb ........ do ...............

Burke Formation; 

3 Bb Argillite and argillitic rocks. . 

13 Bb Siltite. ....................

Prichard Formation; 

3 C Argillite and argillitic rocks . . 

37 Bb ........do ...............

17 Bgn ........ do ...............

8 Bb Siltite.... .................

3 Bgn ........ do ...............

All formations listed above; 

48 C Argillite and argillitic rocks . . 

31 C Siltite.... .................

79 C Politic rocks ...............

55 1.2 

46 1.4

26 2.5

35 1.4

56 1.2 

49 1.3

54 1.2 

39 1.5

46 1.5 

72 1.9

36 1.5

65 1.4

23 2.4

59 1.2 

33 2.0

63 1.5

35 1.6

42 1.3

25 1.8

38 1.5

70 0 

42 1.4

47 1.5 

50 1.5

43 2.1

34 1.7

40 1.4

42 1.3

45 1.5 

39 1.6

26 2.1

20 2.8

43 1.6

25 2.3 

40 2.8

98 2.4

100 2.6

51 2.5 

52 3.8

15 0 

31 1.9

58 2.8 

150 2.1

85 2.7

74 2.9

820 2.0

93 3.9 

210 2.8

110 2.5

34 1.9

39 4.1

69 4.4

OA I Q

160 1.7

110 5.6 

260 1.8

260 1.6

180 1.4

120 1.4

190 1.7

160 1.2

77 3.3 

69 3.1

94 3.0

1100 2.6

74 ^ 9

27.4 1.4 

»7.1 1.4

25.4 1.3

25.5 1.3

10 0 

27.1 1.5

26.0 1.4 

'7.6 1.5

'5.0 2.1 

11 1.3

27.1 1.5

26.3 1.4

26.3 1.5

27.1 1.5

27.9 1.5

_ _

11 1.3 

25.3 2.3

"7.9 1.5 

28.1 1.5

27.7 1.5

'25.0 0

25.7 1.4

25.0 0

26.1 1.8 

25.5 1.9

25.8 1.8

16 2.3 

5.6 5.4

5.1 4.1

6.4 3.9

36 1.3 

2.0 6.4

8.5 1.6 

13 1.9

21 1.5 

10 2.0

25 1.3

21 1.7

8.8 3.4

20 1.2 

14 1.5

21 1.4

10 1.9

1.7 2.5

3 .63 2.8

2 1.1 5.9

26 1.3 

9.5 3.0

2.5 4.8 

10 3.2

4.3 3.5

6.4 4.8

2 1.7 3.4

2.2 3.0

2 1.9 3.1

16 2.3 

11 3.4

O O A Q

7.5 3.3

14 2.8

25.9 3.2

15 2.3

"4.4 2.6

*4.0 2.2

'8.4 1.9

25.9 4.4

_ _

_ _

_ _

»4.2 2.4

28.4 3.9 

27.3 2.2

25.7 2.2

26.6 2.1

25.5 2.3

'5.4 2.1

"4.1 2.7

'7.6 4.2

»4.0 2.4

10 

7

24

5

11 

7

9

6

9

8

6

9

»4

8

6

9

7

8

23

*2

10 

6

8

11 

9

5

24

24

23

8

6

24

24

7

1 C, chlorite-sericite grade rocks that may contain minor secondary biotite ; Bb, bedded rocks containin

composition ;_ Bgn, gneisses and schists of the biotite zone of metamorphism.

2 Value is less than the sensitivity limit of the method and results from substitution of numbers for re

is below sensitivity limit) and (or) N; it indicates that many samples have a content of the element near thre

3 Value is less than the sensitivity limit of the method and results from many substitutions of a low nu

many samples contained no detectable amount of the element.
O34 CONTRIBUTIONS TO ECONOMIC GEOLOGY

TABLE 8. Numbers used to replace nonnumeric codes (N, L, and >,) prior to 

geochemical calculations

[N, element looked for but not found: L, undetermined amount of element present below sensitivity limit; and >, undetermined amount of element present above the number shown. Mg 

and Ca, in percent; other elements, in parts per million]

N L >

Percentage of Percentage of Percentage of

Element

Mg

Ca

Mn

B

Ba

Be

Co

Cr

Cu

La

Nb

Ni

Pb

Sc

Sr

V

Y

Zr

Replacement 

number

0.01

1

.1 1

1

.1

3

1

.1

2

1

10

1

samples

reported 

as code

0.6

2.1

2.9

12.0

.2

.4

2.7

44.4

.6

30.9

2.9

45.6

.2

Replacement 

number

0.03

5

5

.5

3

3

1

10

5

1

5

3

30

5

samples

reported 

as code

5.4

8.5

.2

20.7

12.0

.2

10.2

4.8

30.3

10.8

26.8

.6

17.2

1.0

Replacement 

number

15

30

7,000

10,000

 '

300

1,500

samples

reported 

as code

0.4

1.2

1.0

.4

.2

.2

The mudstones from the Mission Mountains, except for those from the 

Empire and Spokane Formations, probably were deposited nearer an old 

shoreline and are somewhat siltier than those of the Pend Oreille area. 

Siltites and quartzites of the Pend Oreille area tend to be slightly more 

feldspathic than their stratigraphic equivalents in the Mission Mountains. 

The striking contrast between the Belt mudstones and the Pierre Shale, 

which many Belt rocks resemble in outcrop and hand specimen, is brought 

out by figure 5. The position of the Pierre Shale in figure 5 was calculated 

from information given in table 5 of a report on this shale by Tourtelot 

(1962). The Pierre also differs from the Belt in that montmorillonite is a 

principal clay mineral (montmorillonite was included with illite for the 

calculations used to arrive at the Pierre Shale position in figure 5). Ratios 

among quartz, potassium feldspar, and plagioclase for Belt siltites are remarkably uniform (fig. 6), even for rocks containing several percent of 

carbonate minerals, which form the cement.

Mineralogic comparisons that include carbonate minerals in clastic 

Belt rocks from the two study areas are shown in figure 7. The data used 

here are from table 2 of this report for the Mission Mountains rocks, from 

table 1 of Harrison and Campbell (1963) for some Pend Oreille rocks plus 

additional data on low-grade metamorphic rocks in the Pend Oreille area 

accumulated since 1963 and from table 5 of Tourtelot (1962). Here again,
BELT ROCKS, MONTANA AND IDAHO O35

QUARTZ

FELDSPAR 50 ILLITE-SERICITE + 

CHLORITE + BIOTITE

Formation Number of samples

Argillite 

or shale ( ) Siltite ( X ) Quartzite (+)

Pend Oreille area

Libby (1) ...................

Striped Peak (sp).....

Wallace (w) .............

St. Regis (sr) ...........

Revett (r) .................

Burke (b) .................

Prichard (p) .............

................... 4

................... 3

................... 12

................... 6

................... 3

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

4 

11 

16 

4 

5 

14 

7

9 

10

6

Mission Mountains Primitive Area

Shields (ms) .............

Shepard (sh) ...........

Snowslip (sn) ...........

Helena (h) ...............

Empire and Spokane

................... 3

................... 5

................... 6

................... 5

(es) .......... 3

3

5 

13

3

5 

6

7

FIGURE 5. Average modes, in volume percent, of rocks of the Belt Supergroup, 

Pend Oreille area (symbol not circled), and Mission Mountains Primitive 

Area (symbol circled).
036 CONTRIBUTIONS TO ECONOMIC GEOLOGY

QUARTZ

/ . 1 1 1

K-FELDSPAR 50

Formation Number

Pend Oreille area

Libby (1) ..........................................................................

Striped Peak (sp) ...........................................................

Striped Peak; dolomitic (spc). ......................................

Wallace (w) ....................................................................

Wallace; dolomitic and calcareous (we) .....................

St. Regis (sr) ...................................................................

St. Regis; dolomitic (src) ...............................................

Revett (r) .......................................................................

Burke (b) .........................................................................

Prichard (p) ...;................................................................

Mission Mountains Primitive Area

Shepard; dolomitic and calcareous (she).....................

Snowslip (sn) ...................................................................

Helena; calcareous and dolomitic (he). ........................

\

PLAGIOCLASE

of samples

............ 4

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

............ 4

............. 16

............. 6

............. 4

............. 4

............. 5

............. 14

............. 7

............. 3

............. 5

............. 13

FIGURE 6. Ratio among quartz, potassium feldspar, and plagioclase (volume 

percent) in siltite of the Belt Supergroup, Pend Oreille area (symbol not circled) and Mission Mountains Primitive Area (symbol circled), c, siltite contains 

10 percent or more of carbonate minerals.
80

70

60

50

O 40 

oe

30

20

10

QUARTZ

- O

BELT ROCKS, MONTANA AND IDAHO O37

EXPLANATION

FELDSPAR

A

ILLITESERICITE

CHLORITE

+ 

BIOTITE

CALCITE

+ 

DOLOMITE

Belt argillite plus laminated 

argillite and siltite

x 

Belt siltite

+ 

Belt quartzite

O 

Pierre Shale

Mission Mountain rocks (Empire and Spokane through 

Shields Formations)

Pend Oreille area rocks (Prichard through Libby Formations)

FIGURE 7. Average mineralogy of clastic Belt rocks. Number of samples from Mission Mountains: argillite, 24; siltite, 25; quartzite, 23. Number of samples from 

Pend Oreille area: argillite, 120; siltite, 51; quartzite, 20. Also shown for comparison: Pierre Shale, 20 samples.

only slight differences of the Belt rocks from the two widely separated areas 

are evident. A greater carbonate minerals content (also noticeable in the 

field) in the Mission Mountains strata is the most striking difference, 

although the feldspar content of the Pend Oreille rocks is slightly greater 

than that of the Mission Mountain rocks. Sharp contrast between Belt 

"shales" and the Pierre Shale is again apparent. The pronounced decrease 

in the clay mineral-mica-chlorite component with increasing rock grain 

size is a significant factor in the distribution of many minor elements.

GEOCHEMISTRY

The geometric deviations for most elements in a rock type represented
O38 CONTRIBUTIONS TO ECONOMIC GEOLOGY

by five or more samples with real (not coded) values is about the same. 

In contrast, the geometric deviation of most elements tends to increase 

with increasing rock grain size. Thus the quartzites tend to be less homogeneous chemically than the siltites, which in turn tend to be less homogeneous 

than the argillites. The increased chemical variability of the coarser grained 

rocks could reflect small-scale variation (differences between closely spaced 

samples) or large-scale variation (differences from place to place) or both. 

Field evidence supports the conclusion that the coarser rocks are actually 

more variable in composition from place to place.

A comparison of the average element content of the four Belt rock types 

and the metamorphically equivalent (lowest grade) Belt rocks from the two 

study areas is shown in figure 8. The averages (geometric means) are taken 

from tables 6 and 7. The clastic rocks from each area are plotted in order 

of increasing grain size from left to right, and the geometric means for the 

various clastic rock types are connected with a line. Perhaps the most 

obvious feature of the element patterns in figure 8 is a striking similarity of 

the minor element patterns to that of the illite sericite chlorite component 

of these same rocks (fig. 7). These patterns are like that discussed by 

Krauskopf (1955) but are based on a three-unit system (mudstone, siltstone, sandstone) instead of a two-unit system (shale, sandstone).

Elements that are enriched in the illite-sericite-chlorite-bearing clastic 

rocks from both areas apparently include B, Co, Cr, Ni, Sc, V, Fe, Mg, and 

Ti, and possibly La and Y. As these rocks contain no organic compounds, 

which might form organo-metallic complexes, the elements are probably 

contained in or adsorbed on the lattice of the micaceous minerals. Copper is 

more abundant in silty layers of laminated rocks or in siltites and quartzites 

of the Mission Mountains, and zircon appears concentrated in the coarser 

rocks in the Striped Peak Formation of the Pend Oreille area. This suggests 

that Cu and Zr are primarily in grains rather than held in clay minerals 

in those rocks. Mn, Pb, and Sr are more abundant in the rocks of the 

Mission Mountains where the Ca content is higher; such associations are 

common (Mason, 1956, p. 155; Tourtelot, 1962, p. 60) and suggest that 

some aragonite was originally deposited as part of the carbonate minerals. 

The higher Mg of the Mission Mountains reflects, in part, the more dolomitic character of these rocks. Similar rock types of the Mission Mountains 

contain about twice as much Co, Cu, and Ni as their counterparts in the 

Pend Oreille area. The siltites and quartzites of the Pend Oreille area have 

a B content distinctly higher than similar rocks of the Mission Mountains.

Tables 6 and 7 show that the metamorphically low-grade rock types 

within each formation of each area contain similar concentrations of minor 

elements. The unusually high B content of the Striped Peak Formation, 

particularly of members 2 and 3 (table 7), is clearly limited to that formation and has displaced upward the graph for B in Pend Oreille rocks in
BELT ROCKS, MONTANA AND IDAHO O39

figure 8. Low-grade rocks of other formations in the Pend Oreille area have 

the approximate B content of similar rocks in the Mission Mountains.

Comparison of the minor element content of Belt argillitic rocks with 

that of the Pierre Shale is also shown in figure 8; geometric means for the 

Pierre were calculated from data given by Tourtelot (1962, fig. 13). Most 

minor element contents of the Belt mudstones are similar to those of the 

Pierre Shale. The Pierre is relatively high in Cr, Cu, Sr, and V; Sr is correlated with Ca in the Pierre (Tourtelot, 1962, p. 60); and Cr, Cu, and V 

are commonly enriched in black shales (Krauskopf, 1955, table 2; Vine, 

1966, p. E29). Even though much of the Belt argillitic rock is black, the 

organic carbon of the usual black shale is missing, as is normal enrichment 

in black shale suites of trace elements.

Distribution of the elements among constituent minerals was further 

examined by heavy mineral studies of 45 samples of low-grade rocks from 

the Pend Oreille area. Heavy mineral fractions were obtained by a simple 

bromoform separation from ground samples. Splits of the original sample 

and the heavy and light fractions were analyzed spectrographically. Heavy 

mineral fractions ranged from 0.1 to 3 percent of the rocks; most contained 

about 1 percent. The heavy fractions commonly contain compound grains 

that are mostly silicate minerals enclosing tiny black specks (magnetite). 

One exceptional sample, a silty carbonate rock of fossil algal forms, contains 

11 percent heavy minerals. The minor element content of each split of 

the original sample was compared with its content as calculated from its 

fractions and was found to be precise within the limits of the analytical 

method. Average relative concentrations of elements in the heavy mineral

TABLE 9. Relative concentration of elements in the heavy mineral fraction of some 

Belt rocks from the Pend Oreille area

[Number in column is the ratio: average amount in heavy fraction divided by average amount 

in light fraction; 1 indicates equal amounts in the light and heavy fractions. Number in 

parentheses is number of samples of rock type. .... ., the amount of the element in the heavy 

fraction was below the sensitivity limit of the spectrograph]

Element

B

Ba

Be

Co

Cr

Cu

Fe

La

Mg

Mn

Ni

Pb

Sc

Sr

Ti

V

Y

Zr

Argillite 

(17)

1

1

1

5

1

10

2

3

1

5

5

3

y2

i

3

1

5

5

Siltite 

(20)

1

1

1

5

4

20

3

2

1

5

5

5

i

2

1

5

3

Quartzite 

(6)

10

y2

i

5

10

30

5

5

1

20

20

5

1

7

5

7

10

Carbonate rocks 

(3)

%

1

1

i

"s

i

i

i

5

%

y2

i

i

2
O40 CONTRIBUTIONS TO ECONOMIC GEOLOGY

fraction of various rock types are shown in table 9. The ratios range from 

about l/z to 2 times the average shown in the table, except for Mn which 

is about 1/5 to 5 times the average. The relative concentration of B in heavy 

mineral fractions of Wallace argillites and siltites and of Striped Peak siltites 

is about 3 (not shown in table 9).

1500 Ba

FIGURE 8. Geometric means of semiquantitative spectrographic analyses of different 

Belt rock types. Number of samples from Mission Mountains: argillite, 62; siltite, 

62; quartzite, 68; carbonate rocks, 62. Number of samples from Pend Oreille area:
BELT ROCKS, MONTANA AND IDAHO O41

EXPLANATION

Belt argillite plus »^ \^_

laminated argil- Mission Mountains

hte and siltite samples (Empire

x and Spokane

Belt siltite through Shields

Formations) -t1500

1UUU i nnn

700

500

300

200

150

100

Z 70

0

j

-1 en

^ «JV

I

CL.

UJ

Q.

at

£ 20

15

10

7

5 

3

2

1.5

i n

Sr V Y Zr

o *

 X +

A

-

-

_

3h

..*'

-

-

-

O

\JJ

\

r

4-

^

V ±

°*-

-

-

*'Y ~

* 

0

A

A

-

_

_

-

-

Belt quartzite --.. x . - * 

A A Pend Oreille area

Mission Mountains Pend Oreille samples (Revett

Belt carbonate rocks through Libby Formations)

O

Pierre Shale SpeTtr^grTphic

sensitivity limit

lower tfion smallest

volue on graph

where not shown

_ Fe Mg Ca Ti 20 

15

10

7

5

,

3

2

Z

o L5

a

UJ

a. 1.0

0.7

0.5

0.3

0.2

0.15

n i

-

*

A

 ' \ A

*\

 ' \

' I A

i

4-

-

-

-

A

A

-

\

1

\

1 1

i'

i

A

A

4-

/

/

O

*

; '.

 

-

-

_

_

-

\ -

V-ft. t

argillite, 48; siltite, 31; quartzite, 14; carbonate rocks, 13. Also shown for comparison: Pierre Shale, 67 samples.
O42 CONTRIBUTIONS TO ECONOMIC GEOLOGY

The principal heavy minerals are magnetite, ilmenite, chalcopyrite, zircon, 

tourmaline, and iron hydroxides; some bornite, chalcocite, altered biotite, 

and manganese nodules occur in the coarser grained rocks. The copper minerals are largely in irregular or cup-shaped flakes, but some are in subhedral 

or broken grains; zircon, tourmaline, magnetite, and ilmenite are generally in 

broken or abraded crystals; some magnetite, ilmenite, and most pyrite are in 

discrete euhedral crystals. No significant differences in kinds of minerals in 

heavy mineral suites were observed for rocks of this study.

The concentration ratios (table 9) show clearly that many elements are 

about as abundant in the light fraction as in the heavy fraction. Because 

the light fraction forms about 99 percent of the rock, the bulk of most 

elements is associated with the principal rock-forming minerals. This then 

supports the interpretation that those elements shown in figure 8 that have 

a strong correlation in pattern with the pattern for illite-sericite-chlorite in 

figure 7 are adsorbed on or held within the lattice of those minerals.

Heavy mineral fractions of quartzites show relatively large concentrations 

of B, Cr, Cu, Mn, Ni, and Zr. B is in tourmaline; Zr is in zircon; Cu is in 

chalacopyrite, bornite, or chalcocite; Mn is in small blobs of probable 

manganese oxides; and Cr and Ni are probably associated with iron or 

manganese oxides (Krauskopf, 1955, p. 425).

Only copper shows a relatively large concentration in the heavy mineral 

fraction of all clastic rock types. The somewhat more abundant copper in 

rocks of the Mission Mountains (fig. 8) is in minerals that form enough of 

the rock to allow thin section study. Here the copper occurs as chalcopyrite 

grains and chalcocite flakes and blobs in unfractured rocks that suggest a 

detrital sedimentary origin; interstitial bornite blobs, however, suggest that 

diagenetic or other subsequent modification of any original copper is probable. The concentration and occurrence of copper in the heavy mineral 

fraction of Pend Oreille rocks support this conclusion.

DISCUSSION AND INTERPRETATION

IMPLICATIONS ABOUT SEDIMENTATION

The average chemical composition of Belt rocks in the Pend Oreille area 

is between that of an average granite and that of the Canadian Shield 

(table 10). The weighted average for Belt rocks was computed from 23 

chemical analyses of rocks from all formations in the Pend Oreille area 

where the stratigraphic section consists of about 80 percent pelitic rock, 

15 percent quartzite, and 5 percent impure carbonate rock. Other parts of 

Belt terrane have a higher percentage of carbonate rock and perhaps slightly 

more quartzite. Thus the Belt rocks as a whole may be nearer the average 

composition of the Canadian Shield than is indicated in table 10. The 

Canadian Shield is, of course, one likely source terrane for Belt sediments.

Two lines of evidence point to a remarkable uniformity of depositional 

process lasting several hundred million years for Belt rocks. The direct
BELT ROCKS, MONTANA AND IDAHO O43

TABLE 10. Average granite and average composition of the Canadian Shield compared with average Belt rock from the Pend Oreille area

Si02

AhOa...... __ ._..__._.

FeaOa+FeO ... ....

MgO

CaO

NasO

KaO

H2O+ .... .....

COz

Other l

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

Average granite 

(Daly, 1914, 

p. 169)

70.47

...._._.._...._ 14.90

........ __ .... 3.31

.98

. ................. 2.09

..................... 3.31

4.10

...... ............ .84

100.00

Weighted 

average of 

23 Belt rocks

69.32

13.65

3.54

1.93

1.37

1.42

3.74

1.90

1.53

1.16

99.56

Canadian Shield 

(Shaw and others, 

1967, p. 829)

64.93

14.63

4.11

*> 9i

4.12

3.46

3.10

.79

1.28

1.01

99.67

1 Includes H20 , TiOz, P2Oo, MnO, Cl, F , BaO, SrO, C, and acid soluble S as 80s.

relation between mineralogy and rock grain size (fig. 6) implies a consistent 

weathering and transportation system. First, the almost total absence of life 

during deposition of Belt rocks certainly resulted in a simpler sedimentational history than in later geologic time. No plants covered the surface of 

the earth to impede erosion, and no humic acids were formed to accelerate 

leaching or to increase the variety of local weathering products. Little life 

existed in the oceans to produce clastic shell detritus or organic compounds 

which might combine with or alter mineral or chemical products brought 

in from the land. Second, the nearly uniform ratio among quartz, potassium 

feldspar, and plagioclase in widely separated sediments deposited over hundreds of millions of years requires either a nearly uniform source terrane or 

a high degree of integration of clastic components during transportation.

The chemical composition of the Belt seas and atmosphere was probably 

not much different from the composition of today's seas and atmosphere 

(Rubey, 1951, p. 1111). If the atmosphere of the time had been excessively 

high in CC>2, "the rain water would have been so highly carbonated that it 

would leach much Fe and Si from exposed rocks, and the streams would 

transport significant amounts of those elements in solution * * *" (Rubey, 

1951, p. 1124). Because no iron-formations and only insignificant amounts 

of chert occur in the Belt rocks, it seems reasonable to assume that no great 

excess of Fe or Si was transported in solution to or accumulated in 

Belt areas.

The mineralogy and geochemistry of the Belt rocks suggest either that 

rocks of the source area were uniform in composition (near that of granite) 

or that the weathering and transportation processes homogenized the clastic 

assemblage prior to deposition. Homogenization during transportation is 

indicated by the remarkable uniformity of Belt rock mineralogy in a given 

rock type and of the ratio among quartz, potassium feldspar, and plagioclase in siltites. In the few places where the base of the Belt sequence is 

exposed, the rocks lie unconformably on older, crystalline rocks. Conceivably, part of the Belt strata may have been derived by reworking this base-
O44 CONTRIBUTIONS TO ECONOMIC GEOLOGY

ment or older adjacent strata of the thick Belt stratigraphic sequence. 

However, there are no known older sedimentary rocks that could have 

provided the tremendous volume of sediment of which the Belt is composed. 

Moreover, the increase in amount and grain size of feldspar in the upper 

parts of the supergroup, particularly in the feldspathic red quartzites of the 

Striped Peak and equivalent formations, suggests a marked change 

in provenance. The exposed crystalline basement of Montana and the 

Canadian Shield are not uniformly granitic and do contain mafic rocks. 

We are led to the conclusion that the weathering and transportation processes of Belt time resulted in homogenization of clastic components. This 

homogenization requires a chemical weathering of most mafic minerals, a 

low terrane and low-gradient streams carrying only fine-grained material, 

and a slow isostatic adjustment between source areas and basins to provide 

a reasonably stable cycle of erosion and deposition without major tectonic 

surges to cause folding or faulting within the basins. It is of some interest 

that the slow isostatic adjustment of a granitic crust over a long period of 

time resulting in widespread epicontinental seas is a corollary of Rubey's 

(1951, p. 1140-1142) thoughtful analysis of the geologic history of sea water.

The differences in minor element composition between similar rock types 

in various formations and in the two areas may be caused by a variation 

in depositional environment or by a variation in the source areas that 

supplied the Belt detritus. Carbonate is largely a chemical precipitate in 

these rocks; therefore, Ca and elements such as Mg, Mn, Pb, and Sr that 

form carbonate compounds or substitute in mineral lattices of those compounds increase, as expected, in the environment of carbonate deposition.

Other minor elements showing large differences between the two areas 

are B, Co, and Ni, and they are also related to the illite-sericite-chlorite 

fraction of the rocks. The high B content of the Striped Peak Formation, 

particularly members 2 and 3, is in part a reflection of more abundant 

detrital tourmaline in these rocks than in others. The bulk of the B, however, 

in in the fine-grained light fraction and probably represents original B-rich 

clay minerals (Hirst, 1962, p. 1172-1176). Here, then, is an indication of 

a different source area for part of the Belt strata. The siltites of the formation below the Striped Peak are also somewhat enriched in tourmaline, 

which may indicate that the source area for these rocks was similar to that 

of the Striped Peak. If so, the homogenization of clastic components was 

more thorough in the lower (Wallace) strata.

Co and Ni are more abundant in the Mission Mountains rocks than in 

the Pend Oreille rocks. If these elements reflect a difference in crystalline 

source terrane, then we might also expect correspondingly large amounts of 

V, Cr, and Cu from rocks relatively high in Co and Ni (Parker, 1967, table 

19). Cu is present in larger amounts in Mission Mountains rocks. It also is 

primarily in the form of sulfides in the rocks and thus differs from V and Cr,
BELT ROCKS, MONTANA AND IDAHO 045

which are largely with clay minerals. If we can accept the thesis of the existence of Co, Ni, V, and Cr in a simple chemical environment, then the difference in their behavior may be based on ionic potential because Cr +3 >V+3 

» Ni +2 > Co"1" 2, as measured for behavior of their hydroxides (Goldschmidt, 1937; Krauskopf, 1955, table 9). Thus Cr and V are in about 

equal amounts in similar rock types of the two areas because of their 

superior ability to substitute in lattices of the clay minerals which are 

in equal amounts in the comparable rock types. Excess Ni and Co are left 

to be absorbed or precipitated primarily in sulfides. Many simplifying 

assumptions on the chemistry of the environments are required to allow 

such statements to be made. But the hypothesis is reasonable in terms 

of the geologic data and permits one further suggestion. The source area 

for the Mission Mountains strata contained more basic rocks than that 

for equivalent strata of the Pend Oreille area. This has bearing on the 

origin of stratabound copper deposits or occurrences.

Copper in all these rocks is primarily in discrete sulfides that occur 

in greatest abundance in siltites and quartzites of the Mission Mountains. 

The original copper minerals occur primarily as heavy mineral grains and 

interstitial flakes, and some diagenetic recrystallization is probable (Harrison and others, 1969, p. D17). In these rocks, sedimentary copper was 

apparently deposited as heavy mineral grains in the coarser grained rocks 

or with stromatolites (fossil algal forms) or was subsequently concentrated 

in more permeable strata. The living algae formed sticky mats that were 

effective traps for heavy minerals. This is shown by the 11-percent heavy 

mineral fraction of a Pend Oreille algal sample and by the high clastic 

copper mineral content of a persistent zone of stromatolitic rocks in the 

Mission Mountains (Harrison and others, 1969, p. D18). Several Pend 

Oreille stromatolites examined in thin section tend to have high heavy 

mineral content, which is mostly clastic grains of magnetite-ilmenite. 

Apparently, clastic grains of copper minerals were not available in the 

Pend Oreille area environment at the time of deposition of the stromatolitic 

rocks. Bornite flakes do occur as interstitial fillings in some quartzites and 

siltites, but in markedly less amounts than in the Mission Mountains strata.

We did not see a syngenetic copper ore deposit in either study area, 

although copper may occur in concentrations up to 3 percent in some 

1-inch-thick beds. Although part or all of the chalcopyrite and perhaps 

some chalcocite may be syngenetic, the bornite is not; it is either diagenetic 

or epigenetic. Because the Helena and younger Belt rocks probably contain 

more syngenetic copper in the eastern part of the Belt terrane, prospecting 

for copper in these rocks may be more fruitful in the eastern part of the 

old Belt basin. We have no data on distribution or occurrences of copper 

among various areas or formations in the lower (pre-Helena) part of the 

Belt rocks.
O46 CONTRIBUTIONS TO ECONOMIC GEOLOGY .*'

METAMORPHIC CHANGES »

Samples from the Pend Oreille area fall into three subfacies of metamorphism within the greenschist facies. They represent the chlorite-sericite 

zone and the biotite zone of regional metamorphism (G and Bb in tables 

1 and 7), and the biotite zone of contact metamorphism (Bgn in tables ^ 

land 7).

Chemical changes between the chlorite-sericite and biotite zones of * 

regional metamorphism are shown in table 7. Here we can look only for 

similar patterns or trends within different formations because of the limited Y 

numbers of samples in some groups, large values for geometric deviation, 

and limits of the analytical method. We conclude that the trends support * ' 

Shaw's (1956, p. 934) general conclusion that regional metamorphism 

of shales results in little change in minor element content. One trend is sug- . 

gested in our data a loss of boron with increasing metamorphic grade. 

Boron was not included in Shaw's studies, and so a comparison is not ^

possible.

w 

Effects of contact metamorphism can be examined only in the Prichard

Formation. Within this formation, the argillitic rocks give the best measure w 

of metamorphic change because (1) a larger number of samples were ~» 

collected of this rock type. (2) metamorphic equivalents are more easily 

recognized, and (3) among the lower-grade rocks, the Prichard argillitic 

rocks are more nearly uniform in mineralogy from place to place than are 

the siltites and quartzites. The higher grade argillitic rocks show an increase ^ 

in Ca and Sr and a decrease in Ni, B, and possibly Co. Co and Ni are 

commonly associated, as are Ca and Sr. The B loss with increasing meta- ^ 

morphism supports the trend identified in the regional metamorphism. -*- 

Other elements show no detectable change. Further studies of these metamorphic effects are underway.

* * 

COMPARISON OF BELT ROCKS WITH YOUNGER ROCKS

Table 11 compares the element content of the Mission Mountains and y 

Pend Oreille pelitic rocks with that of similar rocks of different ages and 

geologic environments. The Ordovician rocks are highly siliceous eugeo- *^ 

synclinal sedimentary rocks; their geometric means have been calculated 

from analyses given by Ketner (1969, tables 3, 4). The Devonian rocks 

are largely metamorphosed, and the geometric means were calculated from ^ 

analyses given by Shaw (1954, tables 7, 10, 13) by assigning his quantitative -A. 

spectrographic data to the appropriate semiquantitative spectrographic 

classes used to calculate geometric means for the other groups of rock shown 

in table 11. The Cretaceous rocks are the marine Pierre Shale, and the ^^ 

geometric means were calculated from analyses given by Tourtelot (1962,
BELT ROCKS, MONTANA AND IDAHO O47

TABLE 11. Average (geometric means) minor element content of pelitic rocks 

of different ages and geologic environments

[Results in parts per million. . ..., insufficient or no data]

Precambrian

Element

B 

Ba

Co

Cr

Cu

La

Mn

Ni

Pb

So

Sr

V

Y

Zr

Mission

Mountains

Primitive Area: 

124 pelitic

rooks

(this report)

46 

370

14

35

14

30

330

25

8.4

11

42

54

23

260

Pend Oreille

area: 79

pelitic rocks 

(this report)

83 

400

4.8

34

6.8

4374'

14

4.0

7.9

25

58

27

170

Ordovician

Cordilleran

geosyncline: 40

fine-grained 

nonfissile rocks

and shale

(Ketner, 1969)

72 

770

52

47

220

15

61

200

74

Devonian

Littleton

Formation:

59 pelitic rocks 

(Shaw, 1954)

18

110

9

57

18

12

630

110

41

190

Cretaceous

Pierre Shale:

67 shales

(Tourtelot, 

1962)

73 

520

12

70

46

32

220

33

13

15

150

160

21

130

table 14). Data on the Mission Mountains and Pend Oreille rocks are 

given in tables 6 and 7 of this report.

Both the Ordovician and Cretaceous rocks contain abundant black shales, 

and, to choose Krauskopf's (1955, p. 418), careful phrasing, "some elements 

show a tendency to concentrate***" in this particular rock type. The 

elements listed in table 11 that may show this tendency are Cr, Cu, Ni, Pb, 

and V (Krauskopf, 1955, table 2). Most of them are more abundant in the 

Ordovician and Cretaceous rocks than in the Belt rocks. One can cautiously 

infer that the metamorphosed pelitic rocks of the Devonian Littleton 

Formation of the Eastern United States originally included black shales, 

because they are enriched in all but Cu relative to the non-organic Precambrian pelitic rocks. Tourtelot (1962, p. 50-51) noted the close similarity 

in minor element content between the Pierre Shale and the Littleton 

Formation, but he chose not to infer that the close relation might be due 

to similarities among black shales. Perhaps the minor element content 

of Belt rocks more nearly represents that to be expected in pelitic rocks 

containing little or no organic material.

CONCLUSIONS

Relatively inexpensive techniques of quantitative X-ray mineralogic 

determinations and semiquantitative spectrographic chemical analyses have 

provided data adequate for meaningful geologic interpretation of Belt 

rock sedimentation processes. The principal difference in mineralogy of 

three types of Belt clastic rocks involves presence or absence of carbonate 

cement, and this is reflected in the chemistry of those rocks containing
O48 CONTRIBUTIONS TO ECONOMIC GEOLOGY

carbonate minerals by an increase in Ca, Mn, Sr, and Pb. The abundance ^ 

of many elements is directly related to the abundance of the illite-sericitechlorite fraction in the rock. Relative abundances of many elements in this 

fraction reflect ionic potential of the elements. Other differences in minor 

element chemistry are inferred to reflect source terrane. The occurrence 

and distribution of copper are the most significant in terms of economic 

geology. The more abundant copper appears related to source area of the v 

sediments. Within a broad geographic area, certain strata may either con- ; 

tain stratiform deposits or represent source beds for secondary deposits.

The data obtained by using these techniques do not identify any metamorphic changes from the chlorite-sericite zone to the biotite zone of 

regional metamorphism except for a probable loss of boron. Contact 

metamorphism also in the biotite zone but resulting in formation of schists 'fand gneisses was accompanied by an increase in Ca and Sr and a decrease ^ 

in Ni, B, and possibly Co. The minor element content of pelitic Belt rocks 

is probably representative in general of nonorganic pelitic rocks as contrasted with black shales or pelitic rocks containing black shales as studied V 

by others (Shaw, 1954; Tourtelot, 1962; Vine, 1966; Ketner, 1969).

REFERENCES CITED

Anderson, A. L., 1930, Geology and ore deposits of the Clark Fork district, Idaho:

Idaho Bur. Mines and Geology Bull. 12, 132 p. 

Bayley, R. W., and Muehlberger, W. R., compilers, 1968, Basement rock map of the

United States (exclusive of Alaska and Hawaii) : U.S. Geol. Survey map. 

Daly, R. A., 1914, Igneous rocks and their origin: New York, McGraw-Hill Book Co.,

563 p.

Fleischer, Michael, 1965, Summary of new data on rock samples G-l and W-l, 

1962-65: Geochim. et Cosmochim. Acta, v. 29, no. 12, p. 1263-1283.

Goldschmidt, V. M., 1937, Principles of distribution of chemical elements in minerals 

and rocks: Chem. Soc. London Jour., p. 655-673.

Grimes, D. J., and Marranzino, A. P., 1968, Direct-current arc and alternatingcurrent spark emission spectrographic field methods for semiquantitative analysis of geologic materials: U.S. Geol. Survey Circ. 591, 6 p;

Harrison, J. E., and Campbell, A. B., 1963, Correlations and problems in Belt Series 

stratigraphy, northern Idaho and western Montana: Geol. Soc. America Bull., 

v. 74, no. 12, p. 1413-1427.

Harrison, J. E., and Jobin, D. A., 1963, Geology of the Clark Fork quadrangle, IdahoMontana: U.S. Geol. Survey Bull. 1141-K, 38 p.

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