I 

Geochemistry of Mineralized Quartzite 

Beds in the Spokane Formation 

(Belt Supergroup), Rogers Pass Area, 

Lewis and Clark County, Montana 

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Geochemistry of Mineralized Quartzite 

Beds in the Spokane Formation 

(Belt Supergroup), 

Rogers Pass Area, 

Lewis and Clark County, Montana 

By JON J. CONNOR and JAMES M. MCNEAL 

U.S. GEOLOGICAL SURVEY BULLETIN 1762 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U. S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

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Any use of trade names in this report is for descriptive purposes only and does 

not imply endorsement by the U.S. Geological Survey. 

Library of Congress Cataloging in Publication Data 

Connor, Jon J. 

Geochemistry of mineralized quartzite beds in the Spokane Formation (Belt 

Supergroup), Rogers Pass area, Lewis and Clark County, Montana. 

(U.S. Geological Survey bulletin ; 1762) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3: 1762 

1. Quartzite-Montana-Lewis and Clark County. 

2. Geochemistry-Montana-Lewis and Clark County. 3. Spokane Formation 

(Mont.) I. McNeal, James M. II. Title. Ill. Series. 

QE75.B9 no. 1762 557.3 s 86-600326 

[QE475.Q3] [552'.4] 
CONTENTS 

Abstract 1 

Introduction and acknowledgments 1 

Methods of study 3 

Geologic setting 4 

Mineralized quartzite 6 

Mineralogy and petrography 6 

Regional geochemical variability 6 

Geochemical factor analysis 11 

Mineralization 12 

Alice Creek Mine 14 

Silver resource of Green Mountain 16 

References cited 16 

FIGURES 

1. Index map showing sampling localities 2 

2. Sketch map of the Alice Creek Mine 3 

3. Plan view of some of the mineralized quartzite beds 5 

4. Normative mineralogy of mineralized quartzite 6 

5. Si02

-Al20 3 variation in mineralized quartzite 7 

6. K2

0-Al20 3 variation in mineralized quartzite 7 

7. Oxidized Cu (malachite) in mineralized quartzite 13 

8. Hg in mineralized quartzite 14 

TABLES 

1. Average mode of mineralized quartzite 6 

2. Components of geochemical variance in mineralized quartzite 8 

3. Components of mineralogical variance in mineralized quartzite 9 

4. Locality medians (averages) of minerals, elements, and oxides 9 

5. Summary geochemistry of mineralized quartzite 10 

6. Eigenvalues 10 

7. Rotated and ranked factor loadings 11 

8. Logarithmic correlation coefficients 14 

9. Geochemical comparison of the Alice Creek Mine to typical mineralized 

quartzite 15 

Contents Ill 

Geochemistry of Mineralized Quartzite Beds in the 

Spokane Formation (Belt Supergroup), 

Rogers Pass Area, Lewis and Clark County, Montana 

By Jon J. Connor and James M. McNeal 

Abstract 

Scattered lenses of fine- to very fine grained, vitreous CuAg-bearing quartzite (lithic arkose) in the Spokane Formation 

(Middle Proterozoic Belt Supergroup) crop out in the Rogers 

Pass area of western Montana. Individual beds are thin, lack 

obvious channeling, and are apparently elongated in a northerly 

direction. The lenses are composed largely of quartz, albite, 

and illitic mica, and they apparently represent the feather edge 

of a larger package of quartzite to the west. 

These quartzite lenses throughout the Rogers Pass area 

contain low-grade, stratabound accumulations of Cu, Ag, Hg, 

and Ba. The Cu resides chiefly in malachite but also in 

chalcocite, bornite, and rarely covellite. The Ba is in barite. 

The form of the Ag and Hg is unknown, except at the Alice 

Creek Mine where both acanthite and native silver were observed. These low-grade accumulations are believed to have 

formed during the expulsion of diagenetic fluids. 

Quartzite lenses close to outcrops of Tertiary intrusive 

rocks contain slightly more Hg than those distant from such 

outcrops. They also contain anomalous As, Ge, Sb, Pb, and 

Mo, and such beds are thought to have been weakly overprinted by a later (Tertiary) hydrothermal mineralizing event, 

presumably related to mineralization in the nearby Heddleston 

Mining District. 

The Alice Creek Mine is an exceptionally rich pod of Ag 

(and Hg) in one of these low-grade mineralized beds. The origin 

of the Ag-rich pod is unknown, but its elevated Ag and Hg contents and the presence of acanthite suggest an origin distinct 

from the more typical (low-grade) stratabound Cu-Ag accumulations. The Ag-rich pod may have originated during the Tertiary 

hydrothermal event referred to above. 

INTRODUCTION AND ACKNOWLEDGMENTS 

Stratabound accumulations of Cu- and Ag-bearing 

minerals occur in nearly all the formations of the Belt 

Supergroup of Middle Proterozoic age in western Montana. Harrison and Reynolds (1979, p. 2) classified these 

accumulations into two major kinds of occurrences: one 

whose host is quartzite or vitreous coarse-grained siltite 

and one whose host is green laminated argillite enclosed 

by hematite-rich redbeds. Economic accumulations of Cu 

and Ag are known only from the quartzite type of occurrence and are typified by extensive resources in quartzites of the Revett Formation near Troy, Mont. None of 

the green-bed type of occurrences are known to be 

economic, although in the aggregate they probably exceed the quartzite type in total tonnage (Harrison and 

Reynolds, 1979, p. 3). Lange and Eby (1981, p. 934) 

discuss a third type of anomalous Cu-Ag accumulation, 

much rarer than the other two, in carbonate lenses of the 

Spokane and Helena Formations. 

The Spokane Formation in the Rogers Pass area 

of western Montana contains an abundance of small, 

quartzite-type accumulations (fig. 1), one of which at the 

Alice Creek Mine constituted a small Ag-rich pod containing up to 280 oz/t Ag (fig. 2). The accumulations occur in small, individual quartzite beds scattered through 

the formation. These beds crop out most abundantly on 

Green Mountain, about 6 km north of Rogers Pass, and 

were collectively and informally called the Green Mountain quartzite by Trammell (1975, p. 3). The Spokane in 

this area also contains an abundance of small Cu-Ag 

deposits of the green-bed type, particularly near the contact with the overlying Empire Formation, but the focus 

of this report is on the mineralized quartzite in the area. 

The field work on which this study was based was 

undertaken in the summers of 1977 and 1978. The study 

itself was suggested by R. L. Earhart and J. W. Whipple as 

part of an evaluation of the mineral-resource endowment 

of the Rogers Pass area. We thank J. W. Whipple in particular for his invaluable assistance in locating outcrops 

of Cu-Ag-bearing quartzite, and we thank G. Desborough 

and R. Christian for assistance in polished-section identification and microprobe analysis, respectively. Special 

thanks are due Mr. Leonard Orr of Lincoln, Mont., for 

permission to map and sample the Alice Creek Mine. 

The analytical work for this study was performed 

in the Denver laboratories of the U.S. Geological Survey 

Introduction and Acknowledgments 
47°07'30" 

47°00' 

5 KILOMETERS 

EXPLANATION 

Quaternary and Tertiary sediments, undivided 

Tertiary intrusives-Isolated dikes shown by 

bold-faceT 

Empire and Helena Formations 

Spokane Formation-Some beds of underlying 

Greyson Formation included in area near 

Hoadley thrust 

Zd: mafic sill 

~ Sampling locality (rectangle) with sampling 

sites (dots) 

• Additional outcrop of mineralized quartzite 

....a.__.,..__.,L Thrust fault-Sawteeth on upper plate . Dashed 

where concealed 

----lf--- Plunging syncline 

HMO Heddleston Mining District 

Figure 1. Index map showing sampling localities and distribution of mineralized quartzite beds in the Spokane Formation 

in the Rogers Pass Area, Montana. 

2 Mineralized Quartzite Beds, Spokane Formation, Montana 
A A' 

METERS 

1: L__l_.._____! _______,! 

0 10 20 METERS 

N 

l EXPLANATION 

SAMPLE TYPE 

® Outcrop, with locality number 

t::. Quartzite bed 04 

.& Ag-rich pod in 04 (Alice Creek Mine) 

Figure 2. Sketch map of the Alice Creek Mine and vicinity, Rogers Pass area. Quartzite beds labeled from Ql through QS. 

by J. W. Baker, A. Bartel, C. Bliss, P. Briggs, G. Burrow, M. F. Coughlin, J. G. Crock, C. M. Ellis, K. E. 

Horan, J. McDade, C. McFee, V. Merritt, H. T. Millard, 

Jr., F. Perez, S. Prelipp, G. Riddle, V. Shaw, M. W. Solt, 

J. A. Thomas, M. L. Tuttle, R. B. Vaughn, J. S. 

Wahlberg, and B. Walz. A complete listing of the 

analytical data appears in Connor, McNeal, and Crock 

(1979). 

METHODS OF STUDY 

Mineralized quartzite beds in the Spokane Formation of the Rogers Pass area were sampled according to 

a hierarchical, four-level analysis of variance design. The 

primary purpose of such a design is to test for the possible presence of regional variation in the geochemistry of 

the mineralized lenses. Concurrently, the magnitude of 

smaller scaled (nonregional) variation may also be 

examined, as well as the importance of variation across 

bedding within lenses and the importance of laboratory 

error. Miesch (1976) describes the uses of such designs 

in detail. 

The first, or top, level of the analysis of variance 

design consisted of seven sampling localities (fig. 1) 

spread over the area of Spokane outcrop. A sampling 

locality was defined as an outcrop area approximately 1 

km or less in its largest dimension that contained an exposure or exposures of mineralized quartzite. The second 

level of the design consisted of two sampling sites within 

each sampling locality, except for locality 1 which because 

of its very small size contained only one site. A sampling 

site was defined as a vertical section of exposed mineralized quartzite in the locality. The two sites were selected 

Methods of Study 3 
randomly from all possible vertical sections in the locality. If multiple mineralized lenses were exposed in a section, all were included in the sampling site. The third level 

of the design consisted of two samples within each sampling site (the vertical section). The two samples were 

located randomly in the vertical section, and each sample consisted of chips taken from approximately 15 em 

of section. The fourth, and lowest, level of the design 

consisted of replicated chemical analyses. Twenty-six 

samples were collected from the seven localities; nine were 

split into two parts, giving a total of 35 rock analyses for 

the analysis of variance. 

An exceptionally Ag-rich pod in a quartzite bed in 

sampling locality 2 (Alice Creek Mine) was being mined 

for its Ag at the time of sample collection. Because of 

the atypically high Ag content of this pod, it and the adjacent parts of the lens were not included as part of the 

formal sample design described above. However, a small 

set of samples was collected from the mine workings as 

part of a separate study. Three samples were collected 

from the working face and six others from the same 

quartzite bed (Q4) but outside the Ag-rich pod. Locations 

of these samples are shown on figure 2 by triangles. 

All samples were analyzed for a variety of chemical 

constituents. Si02 , Al20 3, CaO, total Fe (as Fel2

0 3), 

P20 5, K20, total S, and Ti020 were determined using 

X-ray fluorescence (Wahlberg, 1975); total C was done 

by a Leco gasometric device (Huffman and Dinnen, 

1976); total Cu was done by atomic absorption; and soluble Cu was done following an acid extraction (Connor 

and others, 1979). Ag, B, Ba, Be, Ce, Cd, Co, Cr, Dy, 

Er, Eu, Ga, Gd, La, Li, Mg, Mo, Mn, Nb, Nd, Ni, Pb, 

Pr, Sc, Sm, Sr, Tm, V, Y, Yb, Zn, and Zr were determined by an automated spectrographic plate-reading 

device (Sutton, 1976). Ag in samples from the Alice Creek 

Mine was analyzed by conventional fire assay. U and Th 

were determined by neutron activation (Millard, 1975), 

and Hg was analyzed by flameless atomic absorption. Sb, 

As, Ge, Se, and Sn were analyzed by X-ray fluorescence 

following a sulfide extraction (Wahlberg, 1975). 

Semiquantitative estimates of quartz, feldspar, and 

mica were obtained from thin-section, X-ray diffraction, 

and normative analyses. Fifty points were counted in each 

thin section. The thin sections also provided information 

on textures and mineral varieties. A few polished sections 

of sulfide-bearing samples were examined. In the normative analysis, illite was assumed to contain 7 percent 

K20 and 26 percent Al20 3 • 

A persistent problem in geochemical data is the 

presence of left-censored data; that is, data in which some 

samples contain element concentrations below the limits 

of the analytical method. In the analysis of variance and 

the factor analysis used in this report, the following convention was used to circumvent the effects of the missing 

element concentrations. Where less than a third of the 

4 Mineralized Quartzite Beds, Spokane Formation, Montana 

values were censored, a concentration equal to 0.7 of the 

limit of determination was substituted. If censoring was 

greater than one-third, the variable was not analyzed. Except for Si02, typical element concentrations were estimated with the geometric mean. Censored distributions 

were adjusted for the censoring by a procedure of Cohen 

(1959) prior to computation of the geometric mean. 

GEOLOGIC SETTING 

The Spokane Formation was named by Walcott 

(1899, p. 207) for exposures of "siliceous and arenaceous 

shales of a deep red color" in the Spokane Hills of the 

Helena Embayment some 75 km southeast of the Rogers 

Pass area. In the area of the Embayment, the Spokane 

has been subdivided into five informal units, partly on 

the basis of color (Connor, 1982). All five units are 

dominated by argillite, silty argillite, and siltite. Quartzite is rare and occurs mostly in the lower two units as 

laterally discontinuous beds of coarse, calcareous quartzite. The formation is about 1500 m thick where not 

reduced by pre-Flathead erosion. 

The Spokane Formation in the Rogers Pass area 

is anomalously thin-about 500 m-perhaps because of 

structural truncation. The study area lies in an 

allochthonous thrust block between the Hoadley thrust 

below and the Scapegoat thrust above (fig. 1) near the 

western edge of the Montana Disturbed Belt. The formation here, as in the Helena Embayment, is dominated 

by red argillites and siltites, but representatives of only 

the upper two informal units of Connor's divisions (1982) 

were recognized in this thrust plate. 

The mineralized quartzite lenses in the Rogers Pass 

area constitute a minor part of the Spokane section, but 

they contrast strongly in composition with the calcareous 

quartzite beds observed in the lower part of the formation in the Embayment. The lenses in the Rogers Pass 

area are very fine grained, noncalcareous, vitreous, lithic 

arkoses. About 25 km to the west (above the Scapegoat 

thrust), the Spokane Formation contains appreciably 

more of this kind of quartzite, and the lenses in the 

Rogers Pass area probably represent an eastern feather 

edge of a regionally extensive package of quartzite-rich 

beds in the Spokane. These quartzite beds are 

petrographically and texturally similar to much of the 

quartzite in the Revett and adjacent formations of the 

Ravalli Group approximately 150 km to the west. 

The quartzite lenses in the Rogers Pass area are 

small, rarely exceeding 1000 min outcrop length (average 

30 m) or 1 m in thickness (average <0.5 m), and are 

everywhere enclosed, apparently conformably, by red 

argillite or siltite. They tend to be thick-bedded, locally 

ripple-marked and cross bedded, and in places intercalated 

with green argillite or siltite. Their bases appear to lack 
47°07'30" 

Allee Creek \~) .... 

Mine J::" 

112D22'30" 

Rogers 

X Pass 

HeddleBton 

~ Mining District 

5 KILOMETERS 

Figure 3. Geometry of some of the mineralized quartzite beds (stippling) in the Rogers Pass area. 

channeling and, where the lenses are sufficiently exposed 

to be mapped, they are elongated in a northerly direction subparallel to each other (fig. 3). Some outcrops exhibit multiple lenses, stacked upon each other, as at the 

Alice Creek Mine (fig. 2). The origin of the lenses is 

obscure, but the subparallel arrangement, the stacking, 

and the lack of basal scouring are consistent with deposition on or near a strand line. 

In the Rogers Pass area, the Spokane is intruded 

by a thick diorite sill of Late Proterozoic age. Whipple 

and Bregman (1981) noted a maximum thickness of 

185 m for this sill in the Hoadley thrust plate. Locally, 

mineralized quartzite lenses of the Spokane are caught 

up in this sill, but most lenses, including the one at the 

Alice Creek Mine, are not. This intrusion is not likely 

to bear any genetic relation to the events that mineralized the bulk of the quartzite beds. The primary effect 

of this intrusion on the Spokane Formation was to produce a thick zone of dark hornfels on either side of the 

sill. 

The most intensely mineralized locale in the Rogers 

Pass area is the Heddleston Mining District (fig. 1), an 

intrusive complex of Eocene age and a host to Cu-Mo 

porphyry-type mineralization (Miller and others, 1973). 

The stocks and dikes of the district apparently are just 

the upper part of a large system of unexposed porphyritic 

intrusions extending to the northwest. This buried zone 

passes to the south of Green Mountain (Earhart and 

others, 1981). In the Heddleston District, where the 

porphyries intrude the Spokane Formation, they have 

Geologic Setting 5 
produced a characteristic and pervasive yellow-prown 

alteration halo in the Spokane red beds. As no such 

alteration was observed adjacent to any of the mineralized 

quartzites in the area, it is unlikely that the quartzites were 

affected by the Heddleston mineralizing episode in any 

substantial way, with the single (and important) possible 

exception of the Ag-rich pod at the Alice Creek Mine, 

described below. 

MINERALIZED QUARTZITE 

Mineralogy and Petrography 

The quartzite lenses in the Spokane of the Rogers 

Pass area are composed principally of quartz, plagioclase, 

and illitic mica (fig. 4, table 1). Detrital quartz is generally 

clear; some of it is polycrystalline, and rare grains are 

stretched metaquartzite. Quartz is the common cement; 

carbonate is rare but occurs as calcite in patches or veins 

in localities 4, 6, and 7, where it is associated with minor 

epidote and presumably reflects the metamorphic effects 

of the nearby Late Proterozoic sill. Albite (<7 percent 

An) is an important constituent and ranges up to 38 percent in both the norm and the X-ray diffraction analysis. 

Rock fragments are also important and include abundant 

illitic pellets. 

50 

0 

100 

~ 

$' A ~ ~ t) 

~ 25 ~ 

~ 75 66AA6 

v <t ~~ 

0 

6 66 CD ~~ ® 

®6 ®®® 

CD 0® @) ~ 

®®0 ® 

® # ® 

50 25 

ILLITE, IN PERCENT 

EXPLANATION 

SAMPLE TYPE ® Outcrop, with locality number 

Quartzite bed 04 

® 

®@) 

0 

A Ag-rich pod in 04 (Alice Creek Mine) 

~ 

..'() 

~ 

~ 

~ 

50 

0 

Figure 4. Normative mineralogy of mineralized quartzite beds 

in the Spokane Formation of the Rogers Pass area, Montana. 

6 Mineralized Quartzite Beds, Spokane Formation, Montana 

Table 1. Average mode of mineralized quartzite in th~ 

Spokane Formation of the Rogers Pass area 

[Data are in percent and are based on counts of 50 points in each of 

24 thin sections. Locality 1 was excluded because it is anomalous i1 

texture and mineralogy] 

Quartz 

Monocrystalline 

Polycrystalline 

Stretched metaquartzite 

Authigenic 

Vein 

.Feldspar1 

Mica2 

Muscovite 

Chlorite 

Matrix 

Rock fragments 

Other 

30 

7 

1 

~ 

1~ 

3 

1 

12 

11 

.:t 

1

X-ray diffraction showed 25 percent plagioclase (<7 percent 

An); the norm contains 27 percent albite. The underestimate of feldspar 

in the mode probably reflects misassignment of rock fragments. 2

The norm contains 12 percent illite. 

Few samples contain more than 75 percent quartz 

(fig. 5), and those that do are from bed Q4 in the Alic1~ 

Creek Mine. Si02 and Al20 3 are readily accounted fo~ 

in normative quartz, albite, and illite with albite generally 

greater than illite (fig. 6). Thus, the mineralized lense-: 

in the Spokane are in fact lithic arkoses. The term 

"quartzite," however, has been conventionally applierl 

because the lenses are so vitreous, and that term is re· 

tained here. 

Accessory minerals include chlorite, microcline, 

blue-green hornblende, epidote, calcite (as noted above)~ 

and zircon. Epigenetic mineralization was apparentl;r 

restricted largely to chalcocite and bornite, now mostl:r 

oxidized to mc.tlachite. The pervasive development of finegrained illite and chlorite, the extensive albitization of the 

feldspar, and the local ragged overgrowths on the hornblende indicate that these rocks are in the greenschis• 

facies of metamorphism. 

Grain size in the mineralized quartzite beds range:' 

from about 0.01 to 0.1 mm, and most samples are mixed 

coarse silt and fine sand. Authigenic quartz, where 

distinguishable from the detrital core grains, compose~ 

up to 20 percent of the rock, suggesting that, at least locally, original porosity was fair to good. In general, these 

lenses appear to be texturally submature and mineralogically immature. 

Regional Geochemical Variability 

The analysis of variance (table 2) demonstrates that 

strong geochemical differences exist among the sever 

sampled localities, and even stronger differences exist in 
95 I I I I I 

" '.6. 

90 - ').~ -

.6.,.6., 

.... 6 .6. ...... '· !@@f'·"· - 85 - 'ILl. 6 -....::;r~ "3 1 ')( +" 

(I) 

c: i -~ 4 ~6,;& ~ 

. ' ''®.. t ""'< (I) I 3 1 53 ® '0;& c. 80 - • 3 ~ ~~ - .!: 

ON * 7 • 4 2 (?)® '<~_o% 10 9(,~ ' iii " .,.t_ • 

II @~.(?) " 

75 I- ~ q~~ - ~,.& • co 

::;, )(~ 0" ~~,~, 

~&~ 

70 I-- CD v%._ 

65~--~'----~'----~~-----~~--~~--~ 2 4 6 8 10 12 14 

EXPLANATION 

SAMPLE TYPE 

CD Outcrop, with locality number 

Quartzite bed Q4 

• Ag-rich pod in Q4 (Alice Creek Mine) 

Figure 5. Variation in Si02 and Al20 3 in mineralized quartzite beds of the Spokane Formation of the Rogers Pass area. 

Composition based on normative minerals. 

plagioclase and illite (table 3). These differences are quantified in table 4. Clearly, locality 1 is the most unusual 

of the seven; it is distinctly high in K20 (and illite), As, 

Ge, Hg, and V and distinctly low in plagioclase. Locality 1 is also unique in that regional differences for As and 

Ge become nonsignificant if locality 1 is excluded from 

the test. The two samples from locality 1 are strongly 

sheared and sericitically altered, and the high content of 

the volatile elements As, Ge, and Hg suggest that the 

locality may have been altered hydrothermally. This suggestion is examined in more detail below. 

Of the remaining elements showing regiona.I differences, 11 are highest in localities 6 and 7: Al2

0 3

, Co, 

total Fe as Fe20 3, Gd, La, Mg, Mn, Nb, Ni, Sc, and 

TiOr All of these are mica-related elements in sedimentary rocks, and the suggestion here is that these two 

localities are richer in argillaceous rock fragments than 

the other localities. Ba and S are highest in locality 3, 

in part because of elevated barite (about 0.5 percent 

normative barite), and Sc is highest in locality 6. In turn, 

localities 4 and 5 are the lowest in many of these constituents. These differences suggest, but only weakly, that 

quartzite lenses to the west or south tend to be richer in 

transition elements (more lithic) than those to the east 

or north. Expected element concentrations (estimated as 

the geometric mean) based on all seven localities of 

mineralized quartzite are listed in table 5. 

Geochemical and mineralogical differences between 

sites (tables 2 and 3) are nonsignificant except for Eu. 

This finding indicates that the mineralized quartzite 

lenses, although differing strongly among themselves, are 

each rather compositionally homogeneous. The reason 

for anomalous behavior in Eu is unknown, but the large 

imprecision in the analysis (between-replicates component, table 2, is 51 percent of the total variance) makes 

reliable interpretation of this element difficult. Compositional variation among samples within sections is significant for most constituents, which is not unexpected 

inasmuch as the samples are largely mixtures of quartz 

and illitic micas (in either matrix or rock fragments). 

Analytical reproducibility (between-replicates component, table 2) is reasonably good for many elements. 

Error variances for Cr, Eu, Si, and Yb exceed 50 percent, 

+" 

c: 

(I) 

(.) 

a; 

5.0 

2.0 

1.0 

0.05 .5 . 

0.2 

0.1 

Al 20 3, in percent 

EXPLANATION 

SAMPLE TYPE ® Outcrop, with locality number 

L> Quartzite bed Q4 

15 

10 

_.. Ag-rich pod in Q4 (Alice Creek Mine) 

+" 

c: 

(I) 

(.) 

a; 

c. 

.5 

~ 

Figure 6. Variation in Al 20 3 and K20 in mineralized quartzite of the Spokane Formation in the Rogers Pass area, Mon-. 

tana. Composition based on normative minerals. 

Mineralized Quartzite 7 
Table 2. Components of geochemical variance in mineralized quartzite in the Spokane Formation of the Rogers Pass area 

[Components given as percentages of total logarithmic variance (Total Log10S2

); Tot, total element concentration; Sol, acid-soluble fraction: 

*, component significantly different from zero at the 0.05 probability level] 

Total Between Between Between Between 

Constituents Log1052 localities 1 sites samples replicates 

Ag 0.3901 <1 o/o 22% 35% 43% 

AI .0085 30* <1 66* 4 

As .2489 44* 10 <1 45 

Ba .2196 48* 11 36* 6 

C-Tot .1336 11 <1 44 46 

Co .1477 32* 5 58* 4 

Cr .0332 <1 <1 44 56 

Cu-Tot2 .5905 2 <1 95* 2 

Cu-Sol2 .5221 <1 <1 97* 2 

Eu .0549 <1 49* <1 51 

Fe-Tot .1171 40* <1 56* 4 

Gd .1410 36* <1 19 45 

Ge .0614 42* 19 <1 39 

Hg .3848 66* <1 19 15 

K .1458 64* <1 34* 2 

La .0645 55* <1 27 18 

Mg .1556 36* <1 59* 4 

Mn .1333 52* 11 29* 8 

Nb .0319 45* <1 34 21 

Ni .1283 30* <1 61* 9 

Pb .1363 14 <1 83* 3 

s .1115 76* <1 <1 23 

Sc .0708 57 <1 37* 6 

SP 11.05 13 <1 26 60 

Sr .0532 10 <1 64* 26 

Th .0173 7 <1 82* 11 

Ti .0146 34* <1 65* 2 

u .0388 17 <1 80* 3 

v .0615 42* <1 52* 6 

y .0249 16 10 42* 31 

Yb .0130 <1 20 21 58 

Zn .1248 22 <1 73* 5 

Zr .0253 10 18 47* 24 

1

Locality 1 (fig. 1) is fractured and probably affected by hydrothermal activity; exclusion of this locality from the analysis of variance results 

in a nonsignificant component for As and Ge at the between-localities level of the design. 2

Sample JC7Q411 from locality 4 contained only background amounts of Cu (<20 ppm); however, no change in the significance tests were 

noted upon deletion of this sample from the computation. 3

Computed on untransformed data. 

8 Mineralized Quartzite Beds, Spokane Formation, Montana 
Table 3. Components of mineralogical variance in mineralized quartzite in the Spokane Formation of the Rogers Pass area 

[Components given as percentages of total logarithmic variance (Total Log10S2); M, determined in the 

mode; N, determined in the norm; X, determined by X-ray diffraction; *,component significantly different from zero at the 0.05 probability level] 

Total Between 1 Between Between 

Constituents Log10S2 localities sites samples 

Quartz, M 

Monocrystalline 0.0347 18 <1 82 

Polycrystalline .1669 42 <1 58 

Authigenic .2083 23 <1 77 

Matrix, M .1534 38 <1 62 

Plagioclase, X .1456 93* <1 7 

Albite, N .0973 81* 7 12 

Illite, N .1587 61* <1 39 

1

Between-localities variance for quartz, matrix, and grain size is based on only 6 localities; Locality 1 (fig. 1) was too severely fractured to permit distinction of these properties. 

Table4. Locality medians (averages) of minerals, elements, and oxides in mineralized quartzite 

in the Spokane Formation of the Rogers Pass area 

[Data are given in parts per million except where noted as percent; GD, geometric deviation; X, determined by X-ray diffraction; N, determined in the norm. Localities are arranged in decreasing order of 

normative illite. See figure 1 for locations] 

GD Locality No. 

2 7 6 5 4 3 

Minerals: 

Illite, %, N 1.77 29 15 15 14 11 6.3 3.4 

Plagioclase, OJo, X 1.26 1.0 32 30 29 27 20 21 

Albite, OJo, N 1.28 5.9 26 30 21 30 23 24 

Elements and Oxides: 

Al20 3, OJo 1.19 8.5 9.4 10 9.6 6.9 7.1 6.5 

As 2.36 13 .9 1.1 .8 .9 .4 2.2 

Ba 2.18 160 770 390 1300 300 200 3100 

Co 2.07 4.2 3.0 6.7 4.7 2.2 1.3 1.5 

FeT03, 0Jo 1 1.84 1.7 .95 2.5 1.3 .50 .40 .51 

Gd 2.00 10 4.6 10 11 <2.2 <2.2 4.4 

Ge 1.54 4.0 .9 1.5 1.1 1.1 .95 .9 

Hg 2.30 2.6 .16 .04 .02 .16 .06 .41 

K20, OJo 1.69 2.1 1.1 1.1 .98 .80 .45 .25 

La 1.48 19 22 29 35 6.9 22 12 

Mg, OJo 2.07 .33 .29 .63 .54 .10 .14 .19 

Mn 1.79 97 75 310 140 57 66 52 

Nb 1.36 13 12 17 14 7.1 10 7.1 

Ni 1.99 7.1 5.3 15 8.0 3.0 3.4 3.7 

S, OJo 1.46 .1 .15 <.1 <.1 .1 .15 .3 

Sc 1.49 3.4 3.1 5.7 8.7 1.8 2.1 2.5 

Ti02, OJo 1.25 .3 .3 .42 .32 .23 .3 .4 

v 1.54 37 19 20 20 11 8.2 11 

y 1.40 22 31 21 17 17 23 24 

1

Total Fe measured as Fe20 3• 

Mineralized Quartzite 9 
Table 5. Summary geochemistry of mineralized quartzite in the Spokane Formation of the Rogers Pass area 

[Data are in parts per million except where noted as percent (OJo); GM, geometric mean; GD, geometric deviation; GE geometric error; dashes 

(--),no data; Ratio, number of analyses in which constituent was determined to total number of analyses; X, determined by X-ray diffraction; 

N, determined in the norm] 

GM GD GE Ratio GM GD GE Ratio 

Ag 6.2 3.65 2.57 34:35 Mo 1 <1.0 7:33 

Al20 3, OJo 8.1 1.20 1.04 35:35 Mn 88 2.25 1.27 35:35 

As 1.1 2.63 2.16 34:35 Nb 11 1.49 1.21 35:35 

B 8.5 2.37 1.51 20:35 Nd 40 1.50 1.26 11:35 

Ba 510 2.83 1.30 35:35 Ni 5.1 2.13 1.28 35:35 

Be 1.1 1.40 1.28 20:35 P20s,% .059 2.92 2.16 11:35 

C-Tot,% .039 2.28 1.77 35:35 Pb1 6.2 1.74 1.16 33:33 

CaO,% <.10 16:35 Pr <60 1:35 

Ce 38 1.91 1.47 11:35 S-Tot, % .11 1.78 1.45 21:35 

Cd <10 0:35 Sb1 <.20 9:33 

Co 2.7 2.37 1.19 34:35 Sc 3.3 1.72 1.16 35:35 

Cr 17 1.49 1.37 35:35 Se <.20 3:35 

Cu-Sol, % .10 4.10 1.27 34:35 Si02, %2 81 3.2 2.6 35:35 

Cu-Tot,% .12 4.21 1.28 35:35 Sm <46 1:35 

Dy <10 5:35 Sn .58 3.01 3.23 30:35 

Er 5.0 1.24 1.29 21:35 Sr 120 1.54 1.31 35:35 

Eu 1.6 1.63 1.65 26:35 Tb <22 3:35 

FeT03, %3 .88 1.95 1.17 35:35 Th 7.7 1.30 1.11 34:35 

Ga 1.4 2.13 8:35 Ti02,% .31 1.27 1.04 35:35 

Ge 1.1 1.69 1.43 35:35 Tm <2.2 5:35 

Gd 4.7 2.67 2.21 25:35 u 2.1 1.53 1.08 35:35 

Hg .13 3.85 1.74 34:35 v 15 1.67 1.15 35:35 

K20,% .74 2.27 1.13 35:35 y 23 1.43 1.22 35:35 

La 18 1.75 1.28 34:35 Yb 1.5 1.30 1.22 35:35 

Li <100 1:35 Zn 27 2.20 1.20 35:35 

Mg,% .27 2.38 1.20 35:35 Zr 250 1.43 1.20 35:35 

1

Sample JC7Q121 in locality 1 contained anomalously high Pb (25 ppm) and Sb (26 ppm). Sample JC7Q322 in locality 3 contained 

anomalously high Mo (9 ppm) and Pb (85 ppm). 2

Computed on untransformed data. 3

Total Fe measured as Fe20 3• 

and these four elements were dropped from further 

evaluation (except for listing in table 5, the error variance 

Table 6. Eigenvalues and cumulative variance associated with contributes to GE rather than to GM, the expected conthe first 10 principal components of the geochemistry of centration). It is important to remember that each error 

mineralized quartzite variance is given as a percentage of the total observed 

Principal Eigenvalue Cumulative variation for that constituent and that a large percentage 

component variance error may indicate a small total variance rather than an 

(percent) imprecise analysis. 

The analysis of variance tests have, thus, demon1 12.8 36.6 strated that locality 1 is compositionally unique and that 

2 3.9 47.6 the lenses may be slightly more argillaceous to the west 3 3.3 56.9 

4 2.4 63.7 or south. These differences, however, are not paralleled 

5 2.2 70.1 by any observable differences in the primary mineraliz6 2.0 75.8 ing elements, Cu and Ag (if the Ag-rich pod at the Alice 

7 1.4 79.7 Creek Mine in locality 2 is excluded). Variation in Cu and 

8 1.2 83.1 Ag resides chiefly at the between-sample level. The 9 1.1 86.2 

10 1.0 89.1 uniform distribution seen in these two elements at the 

highest level of the design reflects either a uniformity in 

10 Mineralized Quartzite Beds, Spokane Formation, Montana 
the intensity of the original mineralizing processes or of 

the weathering processes. Without an examination of 

unweathered samples, no real basis exists for choosing 

between these two alternatives, although if the weathering effects are of equal intensity over the area of study 

(as well they might be), then the regionally uniform 

distribution of Cu and Ag reflects original distribution. 

Geochemical Factor Analysis 

Element associations were investigated by factor 

analysis (see Davis, 1973, p. 478) of the logarithmic linear 

correlation coefficients among the measured constituents. 

For this examination, the two samples from locality 1 

were excluded because the locality is geochemically 

unique. Also excluded were variables with more than 33 

percent censored values (see ratio, table 5) and variables 

showing more than 50 percent error variance (see betweenreplicates component, table 2). Two mineralogical variables (modal quartz and normative albite) were added to 

the list of chemical elements. Thus, the factor analysis 

was based on 24 samples and 31 variables. Eigenvalues 

of the correlation matrix for this data set are listed in table 

6. A six-factor solution accounted for a respectable 76 percent of the total variability in the system which, considering the average percentage error in table 2, seems reasonable. The rotated and ranked factor loadings for these six 

geochemical factors are listed in table 7. This particular 

rotation is an arbitrary (orthogonal) rotation and was 

chosen because the resulting table of loadings is more easily interpreted geologically than other examined rotations. 

Table 7. Rotated and ranked factor loadings of geochemical and mineralogical properties of mineralized quartzite in the Spokane 

Formation of the Rogers Pass area 

[Tot, total; Sol, soluble. The error variance is the between-replicates component of table 2] 

Factors Variance (percent) 

Logarithmic property 2 3 4 5 6 6-factor model Error 

AI 0.91 0.00 0.00 0.00 0.00 0.00 83 4 

Fe-Tot .90 -.15 -.11 .06 .25 .02 91 4 

Sc .89 -.11 -.21 .10 .09 .24 92 6 

v .88 -.03 -.08 -.14 .35 .04 93 6 

Co .85 -.16 -.18 .24 .28 .15 94 4 

Mg .84 -.19 -.07 .27 .28 .19 93 4 

Ni .84 -.26 -.07 .24 .20 .13 89 9 

Zn .75 -.20 .12 .14 .41 .19 84 5 

Mn .73 -.20 -.18 .30 .18 .39 88 8 

K .72 -.20 -.16 -.50 .07 .01 84 2 

La .69 .19 -.01 .33 -.02 .38 77 18 

Nb .68 .15 -.08 -.15 -.08 .44 71 21 

Ti .61 .16 .43 -.14 .43 .09 80 2 

Gd .58 -.35 .16 -.11 .06 .25 56 45 

Th .50 .40 .02 .10 -.31 -.37 65 11 

Quartz1 -.84 -.14 -.02 -.32 -.22 .00 88 

Zr .00 .85 .00 .00 .00 .00 72 24 

y .04 .85 .29 -.14 -.13 .10 85 31 

Cu-Tot .06 -.21 .87 .00 .00 .00 80 2 

Cu-Sol .07 -.19 .89 .01 .00 -.01 83 2 

Ge .42 .13 -.59 -.05 .23 .21 64 39 

Ag .14 -.09 .56 -.42 .00 .46 73 43 

s -.52 .45 .54 -.05 .19 -.20 84 23 

Ba -.27 .03 .48 -.17 .40 .38 64 6 

Albite -.05 .35 .36 .65 .00 .00 68 

Sr -.23 .15 -.09 .48 .37 .28 53 26 

Hg -.37 .04 .29 -.44 .21 -.36 59 15 

Pb -.04 -.02 -.04 .17 .80 .13 69 3 

As .08 .10 -.12 -.22 .75 .23 69 45 

u .31 .20 -.17 -.23 -.09 .77 82 3 

C-Tot -.02 -.25 -.41 .32 -.29 .46 63 46 

1

Computed on untransformed data. 

Mineralized Quartzite 11 
Geometrically, the six factors may be viewed as 

orthogonal axes in six-dimensional space, and the geochemical constituents may be viewed as a cluster of vectors in that space. The rotation mentioned above was 

performed (conceptually) in the following manner. The 

vector for Log AI was positioned coincident with factor 

1 while holding the angular relations between it and all 

the other vectors constant. Then, the vector for Log Zr 

was positioned as close to factor 2 as possible, consistent with its angular relations to the remaining vectors. 

In this particular case, Log Zr just happened to become 

coincident with factor 2. Then, the vector for Log total 

Cu was positioned as close as possible to factor 3, consistent with its angular relations to the remaining vectors. 

And similarly, the vectors of Log albite and quartz were 

positioned as close as possible to factors 4 and 5, respectively. Following rotation, the constituents were ranked 

within groups in descending order of the strongest 

loadings. 

The loadings in table 7 are the correlations between the geochemical vectors and the new factors, and 

geologic interpretations of each factor are based on that 

particular group of constituents which is most strongly 

associated with the factor. Quartz and a host of illiterelated elements load strongly on factor 1, the quartzmica factor. As noted above, the quartzite beds are to 

a large extent simple mixtures of quartz and mica, and 

factor 1 reflects that property. The negative relation of 

quartz to mica in this factor reflects the mathematical property of closure (Chayes, 1960); that is, as the percentage of mica increases, the percentage of quartz must 

decrease. 

Factor 2 is defined largely by the association of Log 

Zr with Log Y. The independence of these from factor-1 

constituents indicates that they are not being held to any 

great extent by the illitic component; rather, they are in 

their own phase, presumably a heavy-mineral component 

which must include zircon. 

Factor 3 is defined by high positive loadings for the 

Logs of Cu, Ag, S, and Ba and a high negative loading 

for Log Ge. Clearly, factor 3 represents to a large extent 

the mineralizing event which produced the anomalous 

low-grade Cu-Ag stratabound accumulations in the 

quartzite lenses. Element loadings in this factor except 

for Log Cu are somewhat diffuse, perhaps as a result of 

later hydrothermal activity or redistribution during 

weathering. The high negative loading of Log Ge on this 

factor is intriguing, but further work is necessary to determine if this relation actually represents some kind of element depletion during the mineralizing event. 

Factor 4 is also rather poorly defined, and the element association is not a particularly enlightening one. 

The importance of albite indicates that variation in this 

major rock-forming mineral bears little relation to variation in the other two important constituents (quartz, 

12 Mineralized Quartzite Beds, Spokane Formation, Montana 

mica). The positive association of Log Sr and Log Hg 

with this factor, and the negative association with Log 

K, is enigmatic. The Log correlations of albite with both 

Sr and Hg are quite low (0.18 and -0.01, respectively). 

The distribution of Hg, as suggested below, has likely 

been affected by later hydrothermal activity. 

Factor 5 is defined in terms of a pair of common 

ore-forming elements, Pb and As. Pb is anomalous in 

locality 3 (up to 85 parts per million (ppm)), and As is 

anomalous in general in the study area but not in samples 

from the localities used in the factor analysis. Factor 5 

apparently reflects another mineralizing event, separate 

from that of the stratabound one (factor 3), and one 

probably related to the same, later hydrothermal activity mentioned for factors 3 and 4. 

Finally, factor 6 is associated with U and total C. 

This relation is more apparent than real, however, as the 

logarithmic correlation of U and total Cis low, 0.24 (only 

6 percent common variance). The relation is dominated 

by a single sample from locality 7 which contained both 

high U (14.9 ppm) and high calcite (1. 7 percent in the 

norm). Expected concentrations (table 5) are 2.1 ppm for 

U and <0.2 percent for normative calcite (based on <0.1 

percent CaO). If that sample is discarded, the log correlation between U and C becomes -0.10. The variation 

in total C is largely controlled by the distribution of calcite 

which, as noted above, has probably been localized by 

the Late Proterozoic diorite sill. 

U shows a moderate loading on-factor 1, and its 

highest log correlation is with Nb (r=0.56; 31 percent 

common variance). Some part of the variation in U can, 

thus, be attributed to variation in factor 1 (probably in 

the mica distribution), but the larger part of its variability appears to be essentially independent of all other 

elements in the factor analysis. Possibly, this variation 

reflects a postdepositional rearrangement of U, but, as 

no anomalous U concentrations are known in the Rogers 

Pass area, it is unlikely that this represents a mineralizing event. The moderate positive loadings of Log Nb and 

Log Ag on this factor suggest that this rearrangement of 

U may have been accompanied by minor rearrangement 

of Nb and Ag, also. 

In summary, the factor analysis has identified the 

Cu-Ag mineralization as an event independent of hostrock composition, clearly indicated the existence of a 

second mineralizing event (Pb-As), and suggested that the 

U distribution in these rocks results largely from some 

kind of postdepositional process( es) separate from the 

two episodes of mineralization. 

Mineralization 

The typical quartzite lens in the Spokane Formation of the Rogers Pass area is enriched in Cu, Ag, Hg, 
and Ba, when compared to levels in nonmineralized 

sandstone: 

Cu, ppm 

Ag, ppm 

Hg, ppm 

Ba, ppm 

Mineralized quartzite 

(table 5) 

1200 

6.2 

.13 

510 

Nonmineralized sandstone 

(Rossler & Lange, 1972) 

10-40 

o.x 

.03 

xo 

Cu, Ag, and Ba are enriched as much as 100 times 

background, and Hg is enriched four times over 

background. 

Stratabound Cu-Ag mineralization in the typical 

Spokane quartzite in the Rogers Pass area is spotty and 

low-grade and exhibits a simple mineralogy. The main 

sulfide mineral is chalcocite; bornite is rare and always 

occurs as relict cores in chalcocite. The bornite is, 

therefore, prechalcocite but may nevertheless be part of 

the same mineralizing event. Trammell (1975, p. 27) noted 

digenite and native silver in these rocks as well. Locally, 

covellite occurs as needles in or coatings on chalcocite. 

The covellite is postchalcocite and is likely the product 

of weathering. 

The most prominent weathering product is malachite. In general, more than 70 percent of the Cu in these 

:J 

u 

w 

...J 

Ill 

::> 

...J 

0 

en 

~0.15 

w 

u 

ffi 0.1 a.. 

0.03 '------L.______L _ ___,L__-.-J____L_ _ ___L_ _ ___L _ _L__.J. _ ___..L__J 

0.03 0.05 0.07 0.1 0.15 0.2 0.3 0.5 0.7 1.0 1.5 2.0 

PERCENT TOTAL Cu 

EXPLANATION 

SAMPLE TYPE 

G) Outcrop, with locality number 

6 Quartzite bed 04 

• Ag-rich pod in 04 (Alice Creek Mine) 

Figure 7. Oxidized Cu (malachite) in mineralized quartzite 

beds of the Spokane Formation in the Rogers Pass area, Montana. One sample each from localities 4, 5, and 6, and two 

from locality 7 contained less than 0.03 percent total Cu. 

lenses has been oxidized to malachite (fig. 7). Even in the 

relatively fresh working face of the Alice Creek Mine, 

over 50 percent of the Cu is in malachite. The malachite 

occurs as disseminated specks or splotches or in veins and 

cracks and may or may not be associated with relict 

sulfide. 

Most of the Ba in these rocks is in barite, which 

has three modes of occurrence. The most common form 

is anhedral blebs. In samples from locality 3, however, 

the barite occurred as euhedral crystals in large (1-mm 

diameter) clots of intermixed malachite, relict sulfide, and 

low-refringent clay of unknown composition. Locally, 

euhedral crystals of barite were observed in veins. Each 

of these three occurrences might represent a different 

mineralizing episode, although the barite associated with 

the oxidizing sulfide and the dispersed anhedral blebs 

were both likely part of the stratabound Cu-Ag event. 

Hayes (1984b) suggested that barite forms an integral part 

of mineralization in occurrences in the Revett Formation. 

The mineral form of the Ag and the Hg in the 

typical mineralized bed is largely unknown, although 

Trammell (1975, p. 27) noted some disseminated native 

Ag. Logarithmic correlations of Ag and Hg with other 

elements are given in table 8. The correlations are based 

on the same 24 samples used in the factor analysis. The 

highest correlation for Ag is with Cu at r=0.44 (19 percent common variance); the highest correlation for Hg 

is with Sat r=0.64 (41 percent common variance). This 

suggests that at least some of the Ag probably resides in 

the copper minerals and at least some of the Hg may be 

in cinnabar. These low correlations may reflect the effects of dispersion upon weathering or the presence of 

other, unknown phases, or both. 

The timing of the stratabound Cu-Ag mineralization in quartzite beds of the Spokane Formation in the 

Rogers Pass area is unknown. In general, such mineralization in Belt rocks is believed to be diagenetic (Hayes, 

1984a) or at least prelithification (Harrison and Reynolds, 

1979). Trammell (1975) conjectured that at least some occurrences were formed by expulsion of connate fluids. 

The Rogers Pass area displays abundant evidence 

of Tertiary hydrothermal activity, centered on the Heddleston Mining District (fig. 1). As noted previously, 

Earhart and others (1981) suggested that a large area of 

buried porphyritic plutons extends to the northwest from 

the Heddleston District. Part of the evidence for this 

buried system is a regional Hg anomaly above it (Earhart 

and others, 1981). And Hg in the Spokane lenses shows 

a strong regional effect (table 2), at least part of which 

appears to be related to nearness of known Tertiary intrusives (fig. 8). Samples collected 3900 m or more from 

known intrusive outcrops average about 0.2 ppm Hg, 

whereas samples from localities 1 and 3, collected within 

1500 m of known intrusive outcrops, average twice that 

amount and range up to 3 ppm Hg. Samples from 

Mineralized Quartzite 13 
Table 8. Logarithmic correlation coefficients of Ag and Hg 

with other elements in mineralized quartzite in the Spokane 

Formation of the Rogers Pass area 

Log Log Ag Log Hg 

AI -0.12 -0.43 

As .13 .15 

Ba .34 .29 

c -.13 -.32 

Ca .12 -.50 

Co -.06 -.59 

Cr .02 -.26 

Cu .44 .22 

Eu .30 -.04 

Fe -.01 -.43 

Gd .38 .00 

Ge -.14 -.25 

Hg .26 

K .04 -.27 

La -.03 -.52 

Mg .01 -.50 

Mn .08 -.61 

Na -.03 -.53 

Nb .06 -.26 

Ni .03 -.47 

Pb -.05 -.19 

s .29 .64 

Sc -.04 -.59 

Sn -.10 -.19 

Sr -.20 -.27 

Th -.21 -.28 

Ti .21 .03 

u .30 -.44 

v .06 -.30 

y .22 -.02 

Yb .03 -.33 

Zn .21 -.31 

Zr -.06 .04 

localities 1 and 3, however, are not appreciably higher 

in Cu or Ag than samples from other localities (table 2). 

In addition to a possible Hg effect, anomalous concentrations of As, Ge, Sb, Pb, and Mo were also found 

in localities 1 and 3. Locality 1 contained 13 ppm As and 

4.0 ppm Ge on average (table 4) compared to expected 

levels in these rocks of 1.1 ppm for both elements (table 

5). Also, samples from locality 1 contained up to 26 ppm 

Sb (table 5, footnote 1) compared to an expected level 

of <0.2 ppm (table 5). One sample from locality 3 contained 85 ppm Pb and 9 ppm Mo (table 5, footnote 1) 

compared to expected levels of 6.2 and <1.0, respectively (table 5). The factor analysis suggested a mineralizing 

event separate from the Cu-Ag stratabound event based 

on concentrations of Pb and As. These two trace metals, 

along with the Ge, Sb, Mo, and the Hg, appear to constitute a hydrothermally related overprint on the Cu-Agbearing lenses in localities 1 and 3. Both localities lie near 

the trace of the Scapegoat thrust (locality 1 is intensely 

fractured) whose sole conceivably could have acted as a 

conduit for hydrothermal fluids. 

14 Mineralized Quartzite Beds, Spokane Formation, Montana 

Alice Creek Mine 

The Alice Creek Mine consists of a small, highgrade pod of Ag in quartzite bed Q4 of locality 2 (fig. 

2). Inside this pod, samples ranged up to 280 ozlt Ag 

(8400 ppm; see Connor and others, 1979). Outside the 

pod but still in Q4, samples ranged from 0.5 ozlt to 4 oz/t 

Ag (18-130 ppm). Cu concentrations inside the pod are 

no higher than in the richest samples taken from other 

lenses, but the exceptionally high Ag may indicate a 

source of Ag different from that which produced the 

typical stratabound accumulations. Silver minerals in the 

high-Ag pod include acanthite and native silver, but these 

were not unobserved outside the pod. Samples from the 

high-Ag pod are extraordinarily high in Hg also, up to 

427 ppm (Connor and others, 1979). They are also 

anomalously high in Pb (table 9), and one sample from 

the pod contained 55 ppm Ge (table 9, footnote 2). The 

form of the Hg, Ge, and Pb is unknown, but if the 

anomalously high Hg, Ge, and Pb content here represents 

Tertiary hydrothermal activity_ (as it appears to do in 

localities 1 and 3), then most likely so does the anomalously high Ag. 

10 I 

® ~ 1.0 r-- ::::i 

~ ® ffi ® a. 

(J) 

1-

a: 

<( 

a. 

~ ® 

Ol 0.1 f---

:r: 

I 

CD 

CD 

I I I 

~ 

~,@ 

~0 

0®-a 

I I 

-

0.01 '--'-l __ __.l'-----------'-1 _ _,___,11~--'----'----'' 0 0 000 0 0 0 0 000 0 0 0) 10 0)(¥)0) r-- (¥) 

(¥) v v (0 co 

DISTANCE FROM NEAREST OUTCROP OF 

TERTIARY INTRUSIVE ROCKS (METERS) 

EXPLANATION 

SAMPLE TYPE 

® Outcrop, with locality number 

e:. Quartzite bed 04 

Figure 8. Hg in mineralized quartzite beds of the Spokane Formation in the Rogers Pass area, Montana. (Samples from highAg pod not shown.) 
Table 9. Geochemical comparison of the Alice Creek Mine to typical mineralized quartzite 

in the Spokane Formation of the Rogers Pass area 

[Data are in parts per million, except where noted as percent (O!o); GM, Geometric mean; M, Median; 

Tot, Total; Sol, Soluble; dashes (--), no data; number in parentheses is number of analyses on which 

estimate is based] 

Alice Creek Mine (bed Q4) Typical quartzite 

High-Ag Outside pod (from table 5) 

pod 

Constituents M GM GM 

(4) (13) (35) 

Ag 7300.0 53.0 6.2 

Al2

0 3, Ofo 5.4 5.9 8.1 

As <.2 1.2 1.1 

B <5 <5 8.5 

Ba 3400 1700 510 

C-Tot,% .035 .025 .039 

CaO,% <.1 <.1 <.1 

Ce 84 68 38 

Co 1.6 .97 2.7 

Cr 8.3 7.5 17 

Cu-Sol, % .46 .29 .10 

Cu-Tot,% .61 .36 .12 

Er <4.6 <4.6 5 

Eu <2.5 1.9 1.6 

FeO, OJo .15 .15 

FeT03, % 1 .12 .24 .88 

Ga <2.2 <2.2 1.4 

Gd <2.2 5.8 4.7 

Ge 23.1 .68 1.1 

Hg 280 .2 .13 

K20, OJo .48 .64 .74 

La 20 20 18 

Mg, OJo .15 .17 .27 

Mn 110 100 88 

Nb 10 9.2 11 

Nd <46 <46 40 

Ni 2.2 2.6 5.1 

P20s, OJo <.1 <.1 .059 

Pb 65 8.4 6.2 

S-Tot, OJo <.1 <.1 .11 

Sc 2.8 2.7 3.3 

Si02, %3 90 88 81 

Sn <.6 .39 .58 

Sr 160 160 120 

Th 4.8 6.5 7.7 

Ti02, OJo .16 .22 .31 

Tm 3.3 2.8 <2.2 

u 2.2 2.2 2.1 

v 7 6.6 15 

y 14 17 23 

Yb 1.1 1.3 1.5 

Zn 16 17 27 

Zr 140 190 250 

1

Total Fe measured as Fe203" 2Sample JM8BC5 contained 55 ppm Ge. 3

Computed on untransformed data. 

Mineralized Quartzite 15 
The origin of the elevated Ag and Hg in the Alice 

Creek Mine remains obscure. The mine is adjacent to a 

fault, but the fault is small (about 6 m of throw) and unmineralized and appears to be a slip face on a slump block 

rather than a fault of regional extent. The linearity of 

nearby Alice Creek suggests that the creek was developed 

along a fault trace, but no other evidence exists for 

faulting there. The Alice Creek mine does not seem to 

be fault controlled. 

Many of the mineralized lenses on Green Mountain are cut by a series of thin, roughly east-west trending, nearly vertical quartz veins. These veins are 

perpendicular to the probable direction of least stress and 

may have formed during the time of regional thrusting 

in the area. In the Alice Creek mine, these veins contain 

some chalcocite and secondary oxidation products but 

no acanthite or native Ag, and because of the general lack 

of ore mineralization in these veins elsewhere, the origin 

of the high-Ag pod is not likely related to either the vein 

formation or to the thrusting. The Alice Creek Mine sits 

in the axial plane of a gentle syncline of regional extent 

(fig. 1), which also probably formed during thrusting, and 

which may have localized or helped to localize the 

hydrothermal ore-forming fluids postulated as the source 

of the high-Ag pod at the mine. 

SILVER RESOURCE OF GREEN MOUNTAIN 

No other Ag-rich pods have been found in quartzite beds of the Spokane Formation in the Rogers Pass 

area, but a large number of typically low-grade mineralized lenses crop out on Green Mountain (fig. 1}. These 

lenses collectively constitute a moderately large tonnage 

of mineralized rock. 

Almost all of this tonnage lies in two bodies near 

the crest of Green Mountain, labeled GM1 and GM2 on 

figure 3. GM1 (Green Mountain lens 1) is the larger of 

the two and was judged as having a minimum length of 

1200 m, a minimum width of 600 m, and an average 

thickness of 1.5 m. This estimate gives a volume of approximately 1,000,000 cubic meters or about 2.7 million 

metric tons. GM2 (Green Mountain lens 2) was estimated 

to have a minimum length of 1500 m, a minimum width 

of 500 m, and an average thickness of 0.5 m. This 

estimate gives a volume of approximately 375,000 cubic 

meters or a little more than 1.0 million metric tons. 

Of about 20 smaller lenses seen at the surface in 

the area, none exceeded an outcrop length of about 100 

m (a bit larger than some of the lenses in locality 2, fig. 

2), and most were much less than half a meter in 

thickness. Based on an estimated average length of 30m, 

an average width of 15 m, and an average thickness of 

0.5 m, the collective tonnage of 20 bodies is approximately 4500 cubic meters, or at most 12,000 metric tons. The 

16 Mineralized Quartzite Beds, Spokane Formation, Montana 

median Ag content of all lenses is estimated at 6.1 ppm 

(table 5) or 0.2 oz/t. 

The 3. 7 + million metric tons estimated in outcrops 

on Green Mountain thus could contain over 700,000 oz 

Ag. To the extent that the Green Mountain area contains additional Ag-rich pods of the Alice Creek type, 

this estimate would rise by about 10,000 oz for each occurrence. However, as the 700,000-oz estimate is based 

solely on outcrop sampling, it must be viewed very 

cautiously. 

REFERENCES CITED 

Chayes, Felix, 1960, On correlation between variables of constant sum: Journal of Geophysical Research, v. 65, no. 12, 

p. 4185-4193. 

Cohen, A. C., Jr., 1959, Simplified estimators for the normal 

distribution when samples are singly censored or truncated: 

Technometrics, v. 1, no. 3, p. 217-237. 

Connor, J. J., 1982, The Spokane Formation (Belt Supergroup) 

in the Helena Salient, western Montana (abs.): Geological 

Society of America, Abstracts with Programs 1982, v. 14, 

no. 6, p. 307. 

Connor, J. J., McNeal, J. M., and Crock, J. G., 1979, 

Geochemical analyses of copper-silver-bearing rocks in the 

Spokane Formation (Belt Supergroup), Lewis and Clark 

County, Montana: U.S. Geological Survey Open-File 

Report 79-1440, 26 p. 

Davis, J. C., 1973, Statistics and data analysis in geology: John 

Wiley and Sons, Inc., New York, 550 p. 

Earhart, R. L., Mudge, M. R., Whipple, J. W., and Connor, 

J. J., 1981, Mineral resources of the Choteau 1 X 2 degree 

quadrangle, Montana: U.S. Geological Survey Map 

MF-858A. 

Harrison, J. E., and Reynolds, M. W., 1979, Preliminary 

geology of the Blacktail Mountain Drilling Site, Flathead 

County, Montana: U.S. Geological Survey Open-File 

Report 79-983, 30 p. 

Hayes, T. S., 1984a, The relation between stratabound coppersilver ore and Revett Formation sedimentary facies of Spar 

Lake, Montana in The Belt: Montana Bureau of Mines and 

Geology, Special Publication 90, p. 63-64. 

___ 1984b, Zonation and paragenesis of the Spar Lake stratabound copper-silver deposits in the Belt Supergroup, Montana (abs.): Geological Society of America, Abstracts with 

Programs, v. 16, no. 6, p. 534. 

Huffman, Claude, Jr., and Dinnen, J. J., 1976, Analysis of 

rocks and soils by atomic absorption spectrometry and 

other methods in Geochemical Survey of Missouri-Methods 

of sampling, laboratory analysis, and statistical reduction 

of data: U.S. Geological Survey Professional Paper 954-A, 

p. 12-14. 

Lange, I. M., and Eby, D. E., 1981, Stratabound copper-silverbearing oolitic deposits in the Belt Supergroup of Montana: Economic Geology, v. 76, no. 4, p. 933-937. 

Miesch, A. T., 1976, Sampling designs for geochemical 

surveys-syllabus for a short course: U.S. Geological 

Survey Open-File Report 76-772, 128 p. 
Millard, H. T., Jr., 1975, Determinations of uranium and thorium 

in rocks and soils by the delayed neutron technique, in 

Geochemical survey of the western coal regions, July 1975: 

U.S. Geological Survey Open-File Report 75-436, p. 79-81. 

Miller, R. N., Shea, E. P., Goddard, C. C., Potter, C. W., and 

Brox, G. B., 1973, Geology of the Heddleston coppermolybdenum deposit, Lewis and Clark County, Montana: 

American Institute of Mining, Metallurgical, and Petroleum 

Engineers, Inc., Pacific Northwest Metals and Minerals Conference, Coeur d'Alene, Idaho, 1973, Proceedings, p. 1-33. 

Rossler, H. J., and Lange, H., 1972, Geochemical tables: Elsevier 

Publishing Co., New York, 468 p. 

Sutton, A. L., Jr., 1976, Spectrochemical computer analysis, in 

Geochemical survey of the western energy regions, July 1976: 

U.S. Geological Survey Open-File Report 76-729, 

p. 131-132. 

*U.S. GPO: 1988 573-047/66,098 

Trammell, J. W., 1975, Stratabound copper mineralization in 

the Empire Formation and Ravalli Group, Belt Supergroup, 

northwest Montana: Unpublished Ph. D. dissertation, 

University of Washington, Seattle, Washington, 70 p. 

Wahlberg, J. S., 1975, Analysis of rocks and soils by X-ray 

fluorescence, in Geochemical survey of the western coal 

regions, July 1975: U.S. Geological Survey Open-File Report 

75-436, p. 69-70. 

Walcott, C. D., 1899, Pre-cambrian fossiliferous formations: 

Geological Society of America Bulletin, v. 10, p. 207. 

Whipple, J. W., 1979, Preliminary geologic map of the Rogers 

Pass area, Lewis and Clark County, Montana: U.S. Geological Survey Open-File Report 79-719. 

Whipple, J. W., and Bregman, M. L., 1981, Geologic map of 

the Rogers Pass quadrangle, Lewis and Clark County, Montana: U.S. Geological Survey Map MF-1309. 

Reference Cited 17 



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