A Resource Assessment of Copper 

and Nickel Sulfides within the 

Mountain View Area of the 

Stillwater Complex, Montana

U.S. GEOLOGICAL SURVEY BULLETIN 1674-B

Chapter B

A Resource Assessment of Copper 

and Nickel Sulfides within the 

Mountain View Area of the 

Stillwater Complex, Montana

By E. D. ATTANASI and W. J. BAWIEC

U.S. GEOLOGICAL SURVEY BULLETIN 1674

CONTRIBUTIONS ON ORE DEPOSITS IN THE EARLY MAGMATIC ENVIRONMENT
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1987

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225

Library of Congress Cataloging in Publication Data

Attanasi, E. D.

A resource assessment of copper and nickel sulfides within

the Mountain View area of the Stillwater Complex,

Montana. 

(Contributions on ore deposits in the early magmatic

environment; B) (Geological Survey bulletin ; 1674-B) 

Bibliography: p.

Supt. of Docs, no.: I 19.3:1674-6 

1. Copper sulphide Montana. 2. Nickel sulphide. I. Bawiec,

W. J. II. Title. III. Series. IV. Series: U.S. Geological

Survey bulletin ; 1674-B. 

QE75.B9 no. 1674-B 557.3. S 86-600027 

[QE390.2.C6] [553.4'3'09786] Rev.

The use of trade names in this publication is for identification only and does not constitute endorsement by the U.S. 

Geological Survey.
CONTENTS

Abstract Bl 

Introduction Bl

Acknowledgments B3 

Geologic setting B4

Distribution of copper and nickel by stratigraphic unit B5 

Distribution of copper and nickel along the Basal series-metasedimentary rock

contact B9 

In situ grade-tonnage relations for fault blocks IV and VIII B12

Methods of appraisal B18

Grade-tonnage relations for block VIII B19

Grade-tonnage relations for block IV B25 

Implications B27 

References cited B27

FIGURES

1. Geologic index map of the Stillwater Complex B2

2. Generalized geologic index map of the Mountain View area B3

3. Base map showing drill-hole locations, faults, and fault-bounded block 

names B4

4. Stratigraphic classification used in this report B5

5. Idealized cross section demonstrating the method used to compute statistics 

for 100-foot horizontal intervals Bll

6. Graphs showing distribution of copper and nickel by 25-foot intervals away 

from the Basal series-metasedimentary rock contact within individual 

fault-bounded blocks B13

7. Contour elevation map showing faults, drill-hole locations, and orientation of 

the base of the Basal series, block VIII B13

8. Three-dimensional perspective view of base of Basal series, block

VIII B16 

9-10. Base maps showing:

9. Spatial distribution of 25-foot intervals above the base of the Basal 

series used in grade-tonnage calculations for block VIII B16

10. Spatial distribution of 25-foot intervals below the base of the Basal 

series used in grade-tonnage calculations for block VIII B17

11. Three-dimensional perspective view of cross sections through block VIII 

showing distribution of 25-foot intervals away from the base of the Basal 

series B17

12. Contour elevation map showing faults, drill-hole locations, and orientation of 

the base of the Basal series, block IV B18

13. Three-dimensional perspective view of the base of the Basal series, block

IV B19 

14-15. Base maps showing:

14. Spatial distribution of 25-foot intervals above the base of the Basal 

series used in grade-tonnage calculations for block IV B20

15. Spatial distribution of 25-foot intervals below the base of the Basal 

series used in grade-tonnage calculations for block IV B21

Contents Bill
16-25. Graphs showing:

16. Cutoff grade (combined copper and nickel) vs. ore tonnage, block 

VIII, for mining blocks 100 X100 X 25 feet B21

17. Average grade (combined copper and nickel) vs. ore tonnage, block 

VIII, for mining blocks 100 X100 X 25 feet B24

18. Cutoff grade (combined copper and nickel) vs. ore tonnage, block 

VIII, for mining blocks 25 X 25 X 25 feet B24

19. Average grade (combined copper and nickel) vs. ore tonnage, block 

VIII, for mining blocks 25 X 25 X 25 feet B24

20. Average copper and average nickel grades vs. ore tonnage, block VIII, 

for mining blocks 25 X 25 X 25 feet B24

21. Cutoff grade (combined copper and nickel) vs. ore tonnage, block IV, 

for mining blocks 100 X100 X 25 feet B25

22. Average grade (combined copper and nickel) vs. ore tonnage, block 

IV, for mining blocks 100 X100X25 feet B25

23. Cutoff grade (combined copper and nickel) vs. ore tonnage, block IV, 

for mining blocks 25 X 25 X 25 feet B26

24. Average grade (combined copper and nickel) vs. ore tonnage, block 

IV, for mining blocks 25 X 25 X 25 feet B26

25. Average copper and average nickel grades vs. ore tonnage, block IV, 

for mining blocks 25 X 25 X 25 feet B27

TABLES

1. Statistics computed from drill-hole data on copper and nickel concentrations in 

the Peridotite zone of the Ultramafic series of the Mountain View study area, 

Stfllwater Complex B8

2. Statistics computed from drill-hole data on copper and nickel concentrations in 

the Basal series of the Mountain View study area, Stillwater Complex B8

3. Statistics computed from drill-hole data on copper and nickel concentrations in 

the cordierite-rich metasedimentary rocks of the Mountain View study area, 

Stillwater Complex B9

4. Statistics computed from drill-hole data on copper and nickel concentrations in 

the quartz-rich metasedimentary rocks of the Mountain View study area, 

Stillwater Complex BIO

5. Statistics computed from drill-hole data on copper and nickel concentrations in 

the sill- and dike-intruded metasedimentary rocks of the Mountain View study 

area, Stillwater Complex BIO

6. Average copper and nickel grades in 25-foot intervals measured away from the 

Basal series-metasedimentary rock contact by fault-bounded block B12

7. Grades and tonnages for in situ ore in fault block VIII, computed by three 

algorithms B22

8. Grades and tonnages for in situ ore in fault block IV, computed by three 

algorithms B23

BIV Contents
A Resource Assessment of Copper and Nickel Sulfides 

within the Mountain View Area of the Stillwater 

Complex, Montana

By E. D. Attanasi and W. J. Bawiec

Abstract

Drill-hole data within the Mountain View area of the 

Stillwater Complex are used to identify specific lithologies 

and locations that could contain commercial-grade copper 

and nickel ore. In addition to outcrops containing sulfide 

mineralization, the Mountain View area has been penetrated 

by over 110 drill holes whose associated cores have been 

recovered and analyzed for concentrations of copper and 

nickel.

The area southwest of the Verdigris fault in the Mountain View area was intensively studied and found to be 

composed of conterminous fault-bounded blocks. Concentration statistics for copper and nickel based on drill-hole data 

were computed for 100-foot horizontal intervals within individual stratigraphic units for the entire area. The resulting 

grade estimates were lower than those for currently operating 

underground copper and nickel mines.

Data on both copper and nickel concentrations were 

analyzed by individual fault-bounded blocks and examined 

in 25-foot intervals both above the base of the Basal series 

and below the Basal series into the metasedimentary rocks. 

Copper and nickel grades tend to decline as distance both 

above and below this stratigraphic contact increases. The 

location of potentially commercial-grade ore was demonstrated.

Only two fault-bounded blocks contained enough data 

for estimating grade-tonnage (in situ tonnage) relationships. 

The fault block immediately west of the Verdigris fault contains 17.1 to 22.3 million short tons of ore having an average 

grade of 0.5 percent combined copper and nickel. In the 

other fault-bounded block studied, which is located farther 

west, estimated tonnage is 2.4 to 5.3 million short tons of ore, 

also having an average grade of 0.5 percent combined 

copper and nickel. These estimates are of in situ material and 

do not allow for losses in mining and metallurgical recovery.

The tonnage estimates are based on relatively sparse 

data and should be regarded as uncertain. In order to reduce 

this uncertainty, additional sampling and verification of estimates are needed. If the additional tonnage of other faultbounded blocks is required for a minimum commercial size 

operation, then future drilling should be focused on establishing the continuity of mineralization within the lithologic 

units lying directly above and below the contact between the

Basal series and metasedimentary rocks in these other 

blocks.

INTRODUCTION

Drill-hole data containing copper and nickel concentrations in the Mountain Mew area of the Stillwater 

Complex, Montana, are examined in order to identify 

specific lithologies and areas that could contain copper 

and nickel ore. The term "ore" as used in this study refers 

to copper- and nickel-enriched rocks and is not restricted 

to rocks that can currently be mined at a profit. According 

to current geologic interpretation, copper and nickel sulfides are concentrated within the lowermost igneous units 

of the Stillwater Complex (Page, 1979) and are also 

frequently associated with sills and dikes intruding the 

adjacent metasedimentary rocks (Zientek, 1983). As 

shown in figure 1, the Mountain Mew area is a pie-shaped 

wedge which has been rotated approximately 90° in a 

counter-clockwise directbn and dipping to the northwest. 

The lowermost igneous unit, the Basal series, lies directly 

on the metasedimentary rocks and cannot be seen at this 

scale (fig. 1).

The rectangular area shown in figure 2 represents 

the drill-hole grid and is the copper and nickel exploration 

target in the Mountain Mew area of the Stillwater Complex. Due to the faulting and structural complexity of the 

region, only that part of this exploration area which is 

southwest of the Verdigris fault could be found to exhibit 

lithologic and structural continuity. This area, hereafter 

called the Mountain Mew study area, contains faultbounded blocks that have been penetrated by enough drill 

holes (fig. 3) to credibly examine the lithologic and structural continuity of the mineralization.

Previously, the Stillwater Complex and adjacent rocks 

have been evaluated with respect to resources of nickel, 

copper, platinum-group metals, chromite, iron, aluminum, 

coal, and industrial rocks and minerals such as limestone, 

sand, and gravel (Page and Dohrenwend, 1973). That 

preliminary study was done at a very large scale and was

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B1
109°45'

45°30' T

45°20' -is

APPROXIMATE MEAN 

DECLINATION, 1985

EXPLANATION

Sedimentary rocks 

Granite

' Benbow area

Peridotite zone Bronzitite and harzburgite

Metamorphic rocks 

Basement complex

- - Contact Dashed where 

approximately located

 Fault Dashed where 

approximately located

Thrust fault T, upper 

plate

Figure 1. Geologic index map of the Stillwater Complex. Stratigraphic classification (Zientek, 1985) modified from Jackson 

(1961).

based on production and reserves of known mines and on 

prospects, mining company exploration, and geologic mapping and information. Our study deals with a much 

smaller part of the Stillwater Complex in greater detail and 

deals only with copper and nickel sulfides. The types of 

information incorporated into the analysis include data on 

copper and nickel concentrations as well as lithologic 

information from drill holes, previously mapped surficial 

geology, and computer-generated structural interpretation 

within the Mountain Mew study area.

The data on copper and nickel concentrations were 

evaluated in several different ways. First, statistics were 

computed using drill-hole concentration data within 100- 

foot horizontal intervals across the inclined stratigraphy of 

the study area southwest of the Verdigris fault. The 

resulting grade estimates were lower than those for currently operating underground copper and nickel mines.

The copper and nickel concentration data were 

analyzed within individual fault-bounded blocks and examined in 25-foot intervals both above and below the Basal

series-metasedimentary rock contact and conformable to 

it. This analysis focused on the relation between the grades 

of copper and nickel and the distance from the Basal 

series-metasedimentary rock contact. We found that copper and nickel grades tend to decline as distance above and 

below this lithologic contact increases. The results also 

demonstrate the locations of rocks of potentially commercial grades in two of the fault-bounded blocks.

Only two fault-bounded blocks contained enough 

data to allow estimation of grade-tonnage relationships of 

in situ material. The fault block immediately west of the 

Verdigris fault (block VIII, fig. 3) contains approximately 

17 to 22 million short tons of ore having an average grade 

of 0.5 percent combined copper and nickel. In the other 

fault-bounded block evaluated (block IV, fig. 3) which is 

located southwest of the first block, estimated tonnage is 

approximately 2 to 5 million short tons of ore, also having 

an average grade of 0.5 percent combined copper and 

nickel. Estimates are of in situ materials and do not allow 

for potential tosses in mining, metallurgical recovery, and

B2 Contributions on Ore Deposits in the Early Magmatic Environment
109°56'

45°23'

ban

Quartz monzonite and aplite

Banded series

Bronzitite zone

Peridotite zone

Chromitite seams In Peridotite zone

EXPLANATION

Metamorphosed sedimentary rocks

 Contact, approximately located Dotted 

where concealed

 ........ Fault Dashed where approximately

located; dotted where concealed

 v vv Thrust fault Dashed where approximately 

located; dotted where concealed. 

Sawteeth on upper plate

Figure 2. Generalized geologic index map of the Mountain View area (modified from Zientek, 1983).

losses due to the design of mines. These tonnages and 

grade estimates are uncertain because they are based on 

relatively sparse data. Additional sampling would be 

required in order to reduce this uncertainty. Future drilling should be focused on defining the continuity of mineralization within fault-bounded blocks in lithologic units 

tying above and below the contact between the Basal series 

and metasedimentary rocks.

This report first presents a general description of 

the geology. The distribution of metals by stratigraphic 

unit within the Mountain Mew study area is described. 

Then, concentration values within fault-bounded blocks in 

25-foot intervals above and below the Basal seriesmetasedimentary rock contact are discussed. After that, 

the definition of potential mining units and the construction of grade-tonnage relationships is described along with 

the results of the analysis of concentration values. The 

concluding section summarizes the implications of the 

results in terms of siting future drill holes.

Acknowledgments

The authors wish to express their deepest gratitude 

to Anaconda Minerals Company for access to their exploration data and to Dr. Roger Cooper and Dr. Allistar

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B3
3000

2500

2000

1500

1000

500

1000

I VIII 

U

1500 2000 2500 3000 3500 4000 4500

FEET

Fault-bounded block numbers

Probable fault Assumed vertical 

U, Upthrown side 

D, Downthrown side

Probable fault Not used in study 

U, Upthrown side 

D, Downthrown side

EXPLANATION

BLOCK ORIENTATIONS

'1" fault Labeled faults referred to in text

Block

IV

V

VI

VII

VIII

Strike 

N. 77°W. 

N. 75°E. 

N. 75°E. 

N. 72°E. 

N. 82°E. 

N. 76°E. 

N. 68°E. 

N. 50°E.

Dip 

35 NE 

74 NW 

74 NW 

49 NW 

35 NW 

54 NW 

61 NW 

65 NW

5000 5500

Ultramafic series 

Basal series

Metasedimentary rocks 

Drill hole 

Inferred contact

Figure 3. Base map showing drill-hole locations, faults, and fault-bounded block names, Mountain View area. Generalized 

geologic map modified from Zientek, 1983. Block orientation modified from Zientek, 1985, unpublished data. Scale is in feet.

Turner, without whose help this study would not have been 

possible. We would like to thank Dr. Norman J Page and 

Dr. Michael L. Zientek of the U.S. Geological Survey for 

their time and patience in familiarizing us with the geology 

of the area and for all the previous work done by them, 

upon which much of this study is based. The authors 

assume all responsibility for the generalized structural 

interpretation of the Mountain View area as presented in 

this study. Technical reviewers were Dr. John H. DeYoung, 

Jr., of the U.S. Geological Survey and Dr. John H. 

Schuenemeyer of the University of Delaware, whose critical comments helped to improve this paper.

GEOLOGIC SETTING

The Stillwater Complex is a large tabular mass of 

layered Late Archean mafic and ultramafic rocks and is 

exposed along the northern border of the Beartooth

Mountains in southwestern Montana (fig. 1). It is approximately 30 miles (48 kilometers) long and has a maximum 

exposed thickness of 18,000 feet (5.5 kilometers)(Jackson, 

1961). The layering in the complex strikes northwest and 

dips northeast. As the Stillwater Complex is only partly 

exposed and its upper units are eroded, its original size 

and shape are unknown.

The downward sequence of stratigraphic units contained within the Stillwater complex is shown in figure 4. 

Because drill holes used in this study are collared no 

higher than the Peridotite zone of the Ultramafic series, 

this is the highest stratigraphic unit included in this study. 

The Peridotite zone of the Ultramafic series, which 

commonly consists of cyclic repetitions of the cumulus 

sequence of olivine, olivine+ bronzite, and bronzite cumulates, is considered to conformably overlie the Basal series 

(Jackson, 1961; Page, 1977; Zientek and others, 1985). 

The Basal series, which has been subdivided into the 

Basal bronzite cumulate zone and the Basal norite zone

B4 Contributions on Ore Deposits in the Early Magmatic Environment
(Page and Nokleberg, 1974), is considered in this study to 

be one continuous cumulate unit consisting of bronzite 

cumulates above and a variety of cumulus minerals (olivine, orthopyroxene, clinopyroxene, plagioclase, and so on) 

plus sulfides near the bottom. Metasedimentary rocks of 

two types are present: cordierite-rich metasedimentary 

rocks, which are found in close proximity to the Basal 

series contact, and quartz-rich metasedimentary rocks, 

which are the dominant metasedimentary rock type and 

are infolded with the cordierite-rich facies (Zientek, 1983). 

An interval of sills and dikes associated with trie Stillwater 

Complex and found only in the cordierite-rich metasedimentary rocks is contained within approximately 650 feet 

(200 meters)of the Basal series and is defined by the 

occurrence of diabase sills and dikes, mafic norite sills and 

dikes, and massive sulfides (Zientek, 1983).

The Mountain Mew area is a triangle-shaped area 

bounded by two high-angle reverse faults, the Lake fault 

and the Bluebird thrust (figs. 1, 2). As a consequence of 

rotation of this fault-bounded area, the stratigraphy strikes 

northeast and dips northwest. The rectangular area (fig. 

2) is defined by a grid over which 110 drill holes are 

located. Eighty-six of these drill holes are vertical and used 

in the structural interpretation as presented here (fig. 3). 

The stratigraphy and fault intersections identified within 

each drill hole (used in conjunction with and constrained 

by the previously mapped surficial geology) resulted in a 

three-dimensional conceptual representation of the Mountain View study area. The structure, as presented here, is 

based on currently available data and is meant to serve as 

a framework from which grade-tonnage calculations can 

be computed. A more detailed geologic and structural 

interpretation of the area describing stratigraphy, morphology, and genesis of the units involved is the subject of 

another study.

The pattern of surface exposure of stratigraphic 

units for the structural interpretation used here is shown 

in figure 3. Shown within this figure are fault-bounded 

blocks, assumed vertical faults, vertical drill holes, and 

generalized surface stratigraphy.

Major northwest-trending faults appear to be the 

youngest and show left-lateral strike-slip movement. As a 

consequence of the faulting and the northwest dip of the 

stratigraphy, the area northeast of the Verdigris fault is 

dominantfy metasedimentary rocks that have no known 

distribution or control on the contained sulfides. Orientations of the igneous rocks south and west of the Verdigris 

fault, however, can be determined from the layering of the 

cumulate rocks. The orientation for each fault-bounded 

block was computed from the lithologic contact between 

the Ultramafic series and the Basal series, which was 

assumed to be horizontal at the time of emplacement.

The distribution of sulfides within the Mountain 

View area is currently attributable to at least two processes: injection of sulfide-enriched mafic norites as dikes

X

LLJ

0_

O 

O

DC 

LJJ

i

p

W

Upper banded

series

Middle banded

series

Lower banded

series

Ultramafic

series

Basal series

Cordierite-rich

metasedimentary

rocks

Bronzitite

zone

Peridotite

zone

Basal bronzite

cumulate zone

Basal norite

zone

Sills and

dikes present

No sills and

dikes present

Quartz-rich

metasedimentary

rocks

Figure 4. Stratigraphic classification of the Banded series, 

Ultramafic series, Basal series, and metasedimentary rocks 

used in this report (modified from Zientek, 1985). Illustration 

is not to scale.

and sills into the cordierite-rich metasedimentary rocks 

(Zientek, 1983), and the coalescing and downward migration of immiscible sulfides through gravitational segregation during crystallization of the cumulate section (Page, 

1979). The occurrence of sulfide-enriched sills and dikes 

with adjacent massive sulfides and the increase in sulfides 

toward the base of the Basal series have been observed and 

documented. Evidence is cited from both Zientek (1983) 

and Page (1979) to support the occurrence of these two 

events. The combination of these processes accounts for 

the concentration of sulfides in the proximity of the contact 

between the base of the Basal series and the metasedimentary rocks.

DISTRIBUTION OF COPPER AND NICKEL BY 

STRATIGRAPHIC UNIT

Tables 1-5 show copper and nickel grades for the 

Peridotite zone of the Ultramafic series, Basal series,

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B5
241 000 N.

The coordinate system of figure 3 and subsequent figures is based on a gridded drilling program (above) developed 

by Anaconda Minerals Company to locate and space drill holes as accurately as possible for the Mouat prospect. The 

long dimension of this grid is oriented parallel to the strike of the stratigraphy and has drill holes located as closely as 

possible to 200-foot centers. The Anaconda grid is superimposed on a 10,000-meter Universal Transverse Mercator 

grid, zone 12 (1927 North American datum).

cordierite-rich metasedimentary rocks, quartz-rich metasedimentary rocks, and the sill- and dike-intruded metasedimentary rocks that have been delineated in the Mountain Mew study area of the Stfllwater Complex (fig. 4). 

Only 46 of the 86 vertical drill holes used in the structural 

interpretation penetrated the Basal series and (or) the 

cordierite-rich metasedimentary rocks within the Mountain "View study area southwest of the Verdigris fault. They 

are used in computing the stratigraphic unit statistics for 

the 100-foot horizontal intervals. All drill holes were 

continuously sampled and analyzed for copper and nickel 

metal concentrations. These values were derived from 

whole-rock analysis with an average sampling interval of 

approximately 6 feet. Because the analysis of copper and

B6 Contributions on Ore Deposits in the Early Magmatic Environment
nickel came from whole rock samples, no distinction was 

made between metals incorporated in sulfide or silicate 

matrix. However, the amount of copper or nickel tied up as 

a silicate is very small whenever sulfide is also present in 

the sample. Although the Peridotite zone of the Ultramafic series and the quartz-rich metasedimentary rocks 

are not considered to contain the primary mining targets, 

grade statistics were computed for those sections that were 

contiguous to the Basal series or the cordierite-rich 

metasedimentary rocks.

All drill holes were calibrated to sea level, and the 

down-hole lithologic contacts were assigned vertical elevations above sea level. Data were examined in 100-foot 

horizontal intervals across the study area southwest of the 

Verdigris fault. An idealized cross section for the 6,700- to 

6,800-foot interval used in tables 1-5 is illustrated in 

figure 5. Within each stratigraphic unit and 100-foot 

horizontal interval above sea level, all well logs were 

searched for the corresponding copper and nickel concentration values. For concentration data from a drill hole to 

be included in the computations for a particular 100-foot 

interval, the drill hole must have penetrated at least half of 

that interval. For each drill hole that met the conditions of 

correct 100-foot interval and stratigraphic unit, the concentration values were tabulated corresponding to the 

individual vertical elevation intervals for each hole, and an 

average grade weighted by length of each sample was 

computed. A drill hole is then represented in each 100- 

foot interval by a point corresponding to the down-hole 

weighted average of concentration measurements. Figure 5 

is an idealized cross section for the 6,750-foot-midpoint 

interval. In reality, the nine holes penetrating the 6,700- to 

6,800-foot interval of the Peridotite zone of the Ultramafic series (table 1), six holes penetrating the 6,700- to 

6,800-foot interval of the Basal series (table 2), and so on, 

are distributed among eight fault-bounded blocks in the 

part of the study area west of the Verdigris fault. Thus, the 

drill holes sample different fault-bounded blocks and 

different stratigraphic elevations above the base of the 

units involved.

The first column in tables 1-5 gives the elevation of 

the midpoints of the 100-foot intervals examined. The 

second column indicates the number of holes that penetrated each 100-foot interval. The next five columns give 

the means and standard deviations of the copper and 

nickel grades and the corresponding correlation coefficients for copper and nickel grades. The columns in the 

remainder of the table provide additional information 

about the distribution of concentration values in the form 

of the 50th percentile value (median), 75th percentfle 

value, and maximum concentration value for a sample 

occurring in each 100-foot interval.

Table 1 shows statistics computed from concentration 

data for 100-foot intervals of drill holes in the Peridotite 

zone of the Ultramafic series. The mean copper grades

are usually less than half the mean nickel grades. Nickel is 

dominantly present as a silicate, occurring within the 

crystal silicate structure of the olivines. However, there is 

also nickel present in the very minor occurrences of sulfide 

minerals pentlandite and pyrrhotite within the Ultramafic 

series. Copper is dominantly present in the sulfide mineral 

chafcopyrite. The results show that the median copper 

grades are consistently smaller than the mean copper 

grade, implying that in each data set the mean is influenced by large copper grade values. The correlation 

coefficient between copper and nickel is always positive, 

but none of the values were significant at the 99-percent 

confidence level.

Similar statistics for the Basal series are shown in 

table 2. Copper grades and nickel grades are substantially 

higher here than in the Peridotite zone. Both copper and 

nickel are present dominantly in sulfide form; copper is 

present in chalcopyrite and nickel in pentlandite and 

pyrrhotite. For each midpoint in table 2, correlations 

between cooper and nickel are considerably higher than 

the correlations observed within the Peridotite zone (table 

1). The mean copper grades are still larger than the 

median grades, indicating a skewed distribution of grades 

within each 100-foot interval. There was only one midpoint, at 5,350 feet, where the combined mean copper and 

nickel grades are greater than 0.5 percent. Other intervals, 

where the sum of the mean copper grade and the mean 

nickel grade exceeds 0.4 percent, are from 6,050 to 5,950 

feet and from 5,550 to 5,050 feet, respectively.

Table 3 shows results for the cordierite-rich metasedimentary rocks, including unintruded areas and areas 

intruded by sills and dikes. Only four of the 100-foot 

intervals have combined mean copper and nickel grades of 

more than 0.4 percent. These midpoints are located in the 

interval from 5,550 to 5,250 feet. Copper and nickel are 

also predominately in their sulfide forms here. Statistics 

for the quartz-rich metasedimentary rocks are presented 

in table 4; both the mean (and even maximum) copper and 

nickel grades are very tow.

The stratigraphy of the cordierite-rich metasedimentary rocks was further refined into intruded metasedimentary rock, representing a sfll and dike association of border 

rocks, and unintruded metasediments (fig. 4). Of the 46 

original drill holes located southwest of the Verdigris fault 

which penetrated either the Basal series or cordierite-rich 

metasedimentary rocks, only 23 contained sills and dikes 

which are presumed to be associated with the Stillwater 

intrusion. Grade statistics for the intruded metasedimentary rocks, table 5, generally show higher copper and nickel 

concentrations than the combined unintruded and intruded 

cordierite-rich metasedimentary rocks (table 3).

Grades presented in tables 2,3, and 5, where copper 

and nickel are predominately in sulfide forms, represent a 

sampling of these units within the Mountain View study 

area. These grades are lower than those for currently

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B7
Table 1. Statistics computed from drill-hole data on copper and nickel concentrations in the Peridotite zone of the Ultramafic series of 

the Mountain View study area, Stillwater Complex

[Grades are in weight percent; , no data]

Midpoint 

of 1 00-ft 

interval

7050----

6950----

6850----

6750----

6650----

6550----

6450- - - -

6350----

6250----

6150----

6050----

5950----

5850----

No. 

of 

holes

-- 4

-- 9

-- 9

-- 9

-- 15

-- 19

-- 23

-- 16

-- 14

-- 12

-- 10

-- 10

-- 3

Mean 

copper 

grade

0.038 

.038 

.051 

.058 

.055 

.042 

.054 

.048 

.046 

.039 

.050 

.038 

.040

Standard 

deviation, 

copper grade

0.021 

.025 

.034 

.046 

.050 

.026 

.031 

.019 

.027 

.020 

.028 

.027 

.010

Mean 

nickel 

grade

0.120 

.086 

.091 

.107 

.098 

.116 

.146 

.146 

.142 

.130 

.174 

.160 

.143

Standard 

deviation, 

nickel grade

0.061 

.062 

.024 

.051 

.052 

.056 

.072 

.050 

.070 

.047 

.040 

.047 

.010

Correlation 

copper vs. 

nickel grade1

0.5073 

.5954 

.2488 

.6585 

.4041 

.1076 

.4823 

.1395 

.1570 

.5179 

.4247 

.7537 

.1555

50 percentile

Copper 

grade

0.023 

.030 

.040 

.035 

.047 

.029 

.048 

.041 

.030 

.030 

.031 

.031

Nickel 

grade

0.097 

.080 

.087 

.099 

.090 

.111 

.155 

.157 

.141 

.126 

.165 

.138

75 percentile

Copper 

grade

0.052 

.040 

.062 

.079 

.063 

.048 

.065 

.052 

.049 

.050 

.068 

.047

Nickel 

grade

0.106 

.096 

.109 

.124 

.125 

.155 

.174 

.177 

.175 

.163 

.189 

.168

Maximum 

values

Copper Nickel 

grade grade

0.060 0.210 

.080 .230 

.120 .120 

.150 .200 

.220 .210 

.110 .220 

.120 .370 

.100 .250 

.100 .210 

.070 .210 

.100 .260 

.090 .280 

.050 .180

'None of the correlations are significant at 99-percent confidence level.

Table 2. Statistics computed from drill-hole data on copper and nickel concentrations in the Basal series of the Mountain View study 

area, Stillwater Complex

[Grades are in weight percent; , no data]

Midpoint No. 

of 1 00-ft of 

interval holes

6850---

6750---

6650---

6550---

6450- - -

6350---

6250---

6150---

6050---

5950---

5850---

5750---

5650---

5550---

5450---

5350--- 

5250---

5150---

5050---

4950- - -

... i

... 6

... 9

... 11

--- 16

---21

... n

... n

--- 10

... 6

... n

--- 10

... n

... 7

--- 5

--- 5 

... 3

... i

... i

... i

Mean 

copper 

grade

0.090 

.142 

.123 

.118 

.134 

.160 

.158 

.140 

.233 

.205 

.125 

.148 

.184 

.220 

.190 

.318 

.222 

.220 

.160 

.130

Standard 

deviation, 

copper grade

0.059 

.105 

.063 

.057 

.091 

.059 

.059 

.099 

.091 

.071 

.068 

.150 

.124 

.107 

.202 

.042

 

Mean 

nickel 

grade

0.130 

.177 

.129 

.142 

.135 

.183 

.186 

.175 

.251 

.200 

.128 

.143 

.175 

.271 

.254 

.362 

.210 

.220 

.250 

.180

Standard 

deviation, 

nickel grade

0.057 

.078 

.104 

.048 

.080 

.074 

.092 

.130 

.109 

.061 

.066 

.113 

.093 

.166 

.341 

.121

 

Correlation 

copper vs. 

nickel grade

0.7560 

.7664 

!.8676 

.4746 

'.7646 

.5731 

! .8199 

.6396 

!.9333 

'.8460 

^8008 

! .9690 

.8141 

.9347 

.8595 

.2774

 

50 percentile

Copper 

grade

0.115 

.077 

.102 

.123 

.139 

.147 

.130 

.218 

.193 

.108 

.134 

.156 

.181 

.142 

.253

 

Nickel 

grade

0.183 

.113 

.120 

.123 

.185 

.196 

.165 

.210 

.147 

.127 

.135 

.166 

.260 

.181 

.222

 

75 percentile

Copper 

grade

0.149 

.185 

.132 

.162 

.227 

.200 

.170 

.280 

.235 

.152 

.200 

.233 

.226 

.254 

.378

 

Nickel 

grade

0.206 

.183 

.170 

.166

.244 

.224 

.213 

.336 

.222 

.166 

.192 

.215 

.325 

.378 

.331

 

Maximum 

values

Copper 

grade

0.240 

.320 

.270 

.270 

.400 

.280 

.260 

.440 

.360 

.260 

.230 

.570 

.460 

.350 

.620 

.270

Nickel 

grade

0.220 

.260 

.410 

.230 

.330 

.360 

.330 

.490 

.390 

.200 

.230 

.460 

.420 

.440 

.960 

.280

 

'Correlations are significant at 99-percent confidence level.

B8 Contributions on Ore Deposits in the Early Magmatic Environment
Table 3. Statistics computed from drill-hole data on copper and nickel concentrations in the cordierite-rich metasedimentary rocks of 

the Mountain View study area, Still water Complex

[Grades are in weight percent; , no data]

Midpoint 

of 100-ft 

interval

6850----

6750----

6650----

6550----

6450- - - -

6350----

6250----

6150----

6050----

5950----

5850----

5750----

5650----

5550----

5450----

5350----

5250----

5150----

5050----

4950----

No. 

of 

holes

-- 1

-- 3

-- 3

-- 5

-- 5

-- 6

-- 9

-- 12

-- 9

-- 10

-- 1

-- 5

-- 3

-- 4

-- 6

-- 3

-- 1

-- 2

-- 1

-- 1

Mean 

copper 

grade

0.110 

.067 

.107 

.156 

.152 

.165 

.131 

.133 

.138 

.163 

.140 

.090 

.113 

.233 

.227 

.263 

.250 

.065 

.010 

.020

Standard 

deviation, 

copper grade

0.025 

.055 

.079 

.071 

.188 

.125 

.074 

.135 

.123

.063 

.078 

.088 

.110 

.206

 

Mean 

nickel 

grade

0.060 

.083 

.103 

.162 

.144 

.123 

.143 

.136 

.081 

.170 

.160 

.116 

.103 

.293 

.178 

.180 

.800 

.105 

.000 

.050

Standard 

deviation, 

nickel grade

0.021 

.061 

.067 

.113 

.114 

.094 

.102 

.048 

.153

.111 

.078 

.214 

.107 

.115

 

Correlation 

copper vs. 

nickel grade

0.7954 

.9707 

'.9763 

.9419 

*.9623 

.7499 

'.8363 

.5524 

'.7729

.9350 

.9990 

.1593 

-0.0261 

.4799

 

50 percent! le

Copper 

grade

 

0.127 

.129 

.063 

.087 

.132 

.069 

.121

.070

.183 

.197

 

Nickel 

grade

 

0.150 

.116 

.092 

.123 

.110 

.076 

.090

.062

.227 

.113

 

75 percent! le

Copper 

grade

 

0.186 

.206 

.180 

.167 

.193 

.150 

.251

.088

.360 

.265

 

Nickel 

grade

 

0.171 

.199 

.123 

.191 

.170 

.111 

.282

.134

.234 

.247

 

Maximum 

values

Copper 

grade

0.090 

.160 

.270 

.230 

.530 

.420 

.289 

.420 

.370

.200 

.200 

.360 

.410 

.480

.120

Nickel 

grade

0.100 

.170 

.270 

.300 

.340 

.310 

.270 

.160 

.390

.300 

.190 

.560 

.330 

.290

.120

'Correlations are significant at 99-percent confidence level.

operating underground mines. As figure 5 shows, the 

concentration data analyzed by 100-foot elevation intervals 

result in a collection of summary statistics that includes 

samples from different elevation levels within each stratigraphic unit. Moreover, figure 3 shows that the area is 

composed of several fault-bounded blocks and that each 

block has a unique orientation. In an effort to identify the 

rock units with high grades and to estimate ore tonnages, 

the drill-hole concentration data will be examined on the 

basis of uniform stratigraphic heights within the lithologic 

unit both above and below the base of the Basal series.

DISTRIBUTION OF COPPER AND NICKEL 

ALONG THE BASAL SERIESMETASEDIMENTARY ROCK CONTACT

As a result of the combination of sulfide-enriched 

mafic norites intruding the cordierite-rich metasedimentary rocks (Zientek, 1983) and of sulfide immiscibility and 

gravitational settling within the Basal series cumulates 

(Page, 1979), the highest concentrations of copper and 

nickel sulfides are expected to be found directly above and 

below the contact between the Basal series and the 

metasedimentary rocks (Drew and others, 1985). Fault

blocks constituting the Mountain Mew study area have 

been rotated, translated, and inclined due to faulting after 

the introduction of the copper and nickel sulfides. Hence, 

the stratigraphy now appears to be discontinuous when 

examined in horizontal intervals.

Another method by which to evaluate the distribution of copper and nickel sulfides is to examine the mean 

grades in 25-foot layers conformable to the Basal series 

and metasedimentary rock contact, both above into the 

Basal series and below into the metasedimentary rocks. 

The thickness of each 25-foot rock layer was measured 

perpendicular to the Ultramafic series-Basal series contact using block orientations as shown in figure 3. All 

drill-hole data, including copper-nickel concentrations and 

lithologies within each fault-bounded block, were recomputed to account for orientation of the block.

Results of these computations are presented in table 

6 and in figure 6. Bl represents the 25-foot interval in the 

Basal series just above the contact, while Cl represents the 

25-foot interval just beneath the contact but within the 

cordierite-rich metasedimentary rocks. B2 and C2 are 

layers above and below Bl and Cl, respectively. Similarly, 

results for 175 to 200 feet above (B8) and 175 to 200 feet 

below (C8) the contact were computed and are displayed.

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B9
Table 4. Statistics computed from drill-hole data on copper and nickel concentrations in the quartz-rich metasedimentary rocks of the 

Mountain View study area, Stillwater Complex

[Grades are in weight percent; , no data]

Midpoint 

of 100-ft 

interval

6750----

6650----

6550----

6450- - - -

6350----

6250----

6150----

6050----

5950----

5850----

5750----

5650----

5550----

5450----

5350----

5250----

5150----

5050----

4950----

4850----

No. 

of 

holes

-- 1

-- 1

-- 2

-- 3

-- 5

-- 7

-- 5

-- 6

-- 1

-- 3

-- 1

-- 2

-- 2

-- 1

-- 1

-- 1

-- 0

-- 0

-- 1

-- 1

Mean 

copper 

grade

0.010 

.020 

.040 

.033 

.054 

.051 

.038 

.032 

.020 

.147 

.120 

.080 

.185 

.070 

.020 

.020

.010 

.010

Standard Mean 

deviation, nickel 

copper grade grade

0.080 

.050 

.050

0.023 .033 

.011 .036 

.041 .024 

.013 .022 

.024 .030

0^0

.193 .033 

020

.015

.240 

.100 

.040 

.060

.050 

.000

Standard Correlation 

deviation, copper vs. 

nickel grade nickel grade1

 

0.015 0.9449 

.023 .4572 

.022 .1028 

.016 -0.6768 

.028 .2278

.006 .9997

 

 

50 percentile 75 percent! le .

Copper Nickel Copper Nickel _ 

j j j j Copper grade grade grade grade ^

0.050

.060 

0.048 0.038 0.059 0.050 .070 

.050 .070 .050 .035 .140 

.037 .016 .046 .029 .050 

.023 .021 .042 .037 .070

 -370

130

.350

num

jes

Nickel 

grade

0.060 

.050 

.060 

.060 

.040 

.080

.040

.020 

.430

'None of the correlations are significant at 99-percent confidence level.

Table 5. Statistics computed from drill-hole data on copper and nickel concentrations in the sill- and dike-intruded metasedimentary 

rocks of the Mountain View study area, Stillwater Complex

[Grades are in weight percent; , no data]

Midpoint 

of 100-ft 

interval

6750-----

6650----

6550-----

fiA^n

6350-----

6250-----

6150----

6050-----

5950----

5850-----

5750-----

5650-----

5550-----

5450-----

5350-----

5250-----

5150-----

No. 

of 

holes

-- 2

-- 3

-- 5

-- 5

-- 3

-- 6

-- 6

-- 7

-- 1

-- 1

-- 1

 - 3

 - 5

-- 3

-- 1

-- 1

Mean 

copper 

grade

0.080 

.110 

.156 

.152 

.283 

.163 

.174 

.187 

.233 

.140 

.200 

.050 

.190 

.212 

.263 

.250 

.120

Standard 

deviation, 

copper grade

0.055 

.079 

.079 

.216 

.138 

.053 

.143 

.134

.026 

.116 

.206

Mean 

nickel 

grade

0.095 

.103 

.162 

.144 

.197 

.167 

.181 

.092 

.230 

.060 

.300 

.040 

.273 

.148 

.180 

.800 

.210

Standard 

deviation, 

nickel grade

0.061 

.067 

.113 

.124 

.101 

.094 

.056 

.142

.026 

.086 

.115

Correlation 

copper vs. 

nickel grade

0.970 

^980 

.940 

.980 

.700 

.680 

.470 

.620

-0.060 

-0.370 

.480

50 percentile

Copper 

grade

0.087 

.127 

.129 

.183 

.141 

.142 

.124 

.230

.178 

.178 

.179

Nickel 

grade

0.078 

.150 

.116 

.130 

.142 

.137 

.104 

.213

.160 

.100 

.137

75 percentile

Copper 

grade

0.129 

.186 

.206 

.300 

.192 

.206 

.316 

.293

.199 

.212

.322

Nickel 

grade

0.118 

.171 

.199 

.190 

.226 

.275 

.154 

.342

 

.373 

.177

.225

Maximum 

values

Copper Nickel 

grade grade

0.090 0.100 

.160 .170 

.270 .270 

.230 .300 

.530 .340 

.420 .310 

.270 .310 

.420 .160 

.430 .410

.220 .560 

.410 .280 

.480 .290

B10 Contributions on Ore Deposits in the Early Magmatic Environment
NORTHWEST SOUTHEAST

Midpoint 

=6.750 ft.

Elevation above 

sea level

6,800 FT

6.700 FT - JFigure 5. Idealized cross section (perpendicular to strike) of the 6,750-foot midpoint interval demonstrating the method used 

to compute statistics for 100-foot horizontal intervals represented in tables 1-5. Stratigraphy dipping approximately 60° to the 

northwest.

Results generally support the hypothesis that within the 

Basal series higher concentrations of copper and nickel 

sulfides are found closer to the Basal seriesmetasedimentary rock contact (Page, 1979). Copper and 

nickel grades tend to decrease with an increase in distance 

both above and below the contact. There may be some 

distortion to the copper and nickel grades assigned to true 

stratigraphic position because of faults which were encountered within these distances from the contact. However, in 

most cases the distortions are not thought to be consistent 

from block to block or drill hole to drill hole and, hence, do 

not constitute a systematic change within the patterns.

Table 6 shows the average copper and nickel grades 

in 25-foot intervals measured away from the Basal seriesmetasedimentary rock contact by fault-bounded block. 

Also shown is the percentage of footage having combined 

copper and nickel grades of at least 0.5 percent for each 

interval. This column, along with the average grades, 

shows whether high grade levels are the result of a few feet 

of anomalously high core measurements or whether they 

are typical of the entire interval of core.

From table 6 it can be shown that the combined 

mean copper and mean nickel grades exceed 0.5 percent in 

only 11 of 111 25-foot intervals across all eight faultbounded blocks for which data are available. Furthermore, 

in only two of the eleven 25-foot intervals having combined 

average grades above 0.5 percent does at least half of the

core assayed exceed that cutoff value (block V, interval 

Bl, and block VIII, interval C6). Even though combined 

mean copper and nickel grades for most of the intervals 

shown do not exceed 0.5 percent, each will generally have 

some core footage, usually greater than 5 percent of the 

total footage assayed, where the combined copper and 

nickel grade values exceed 0.5 percent.

Blocks I, IV, and VI have no 25-foot intervals with 

combined average copper and nickel grades above 0.5 

percent (table 6). Block V has three 25-foot intervals (Bl, 

Cl, and C8) with average combined grades above 0.5 

percent. However, only the Bl and Cl layers in block V, 

which are penetrated by three drill holes each, have 40 

percent or more of their measured core above the 0.5- 

percent combined grade level. In blocks III and VII, the 

intervals having average grades above the 0.5-percent 

combined grade level are penetrated by only one hole; 

therefore, the opportunities for geologic or statistical 

inference are very limited. Block VIII appears, by far, to 

be the most interesting in terms of identifying potentially 

minable sections. It contains three intervals having average grades above the combined 0.5-percent level (Bl, C5, 

and C6), and it contains 16 holes in one of those intervals.

The distribution of copper and nickel in fault blocks 

IV and VIII is examined in more detail in the next section. 

These blocks have the largest number of holes penetrating 

the Basal series-metasedimentary rock contact, where one

Copper and Nickel Sulfides within the Mountain View Area of the Still water Complex, Montana B11
Table 6. Average copper and nickel grades in 25-foot intervals measured away from the Basal series-metasedimentary rock contact 

by fault-bounded block

[Grade is in weight percent; , no data; cutoff = percent of true footage in 25-foot interval where sum of copper and nickel grades is greater or equal 

to 0.50 percent]

25-foot

Interval

B8---

m£>/---

B6---

B5---

B4---

B3---

-DI-) £>/ - --

Bl ---

*

Cl ---

C2--

C3---

C4---

C5 

C6---

C7 ---

- - -

25-foot

Interval

B8---

B7---

B6---

B5---

B4---

B3---

B2---

Bl ---

*. .-

Cl ---

C2---

C3---

C4---

C5---

C6---

C7---

f>0 Lo - - -

No.

of

holes

2

2

2

2

3

3

3

4

4

4

3

1

1

1

No.

of

holes

2

2

2

2

2

2

2

3

3

3

3

3

3

2

2

2

Block 1

Mean Mean

Cu Ni

grade grade

0.16 0.13

.06 .07

.07 .03

.10 .14

.20 .25

.10 .12

.25 .15

.17 .19

.17 .22

.09 .13

.04 .03

.03 .06

.08 .08

.10 .09

Block V

Mean Mean

Cu Ni

grade grade

0.19 0.14

.17 .17

.14 .13

.15 .10

.22 .15

.16 .15

.17 .19

.39 .46

.22 .29

.13 .17

.11 .17

.13 .19

.12 .11

.08 .06

.19 .27

 3O 1 -3 .JO .IJ

Percent

above

cutoff

7

0

5

0

24

0

14

20

26

18

0

0

0

0

Percent

above

cutoff

24

18

2

0

8

0

9

64

41

23

15

25

14

0

39

16

No.

of

holes

 

1

1

1

1

1

2

2

3

3

3

2

3

2

1

1

No.

of

holes

1

2

2

2

2

3

3

3

3

3

3

3

2

1

 

Block II

Mean Mean

Cu Ni

grade grade

 _

0 04. 0 04.

\J . Vy^T \J . V/T"

.07 .04

.11 .09

.14 .14

.22 .22

.12 .15

.12 .13

.22 .29

.09 .10

.10 .07

.13 .13

.14 .14

.14 .02

46 1? . ^\J . i £>

.03 .03

Block VI

Mean Mean

Cu Ni

grade grade

0.13 0.17

.18 .18

.25 .21

.19 .20

.19 .16

.09 .08

.20 .16

.21 .24

.16 .29

.19 .13

.04 .05

.10 .10

.06 .04

.05 .02

 

Percent

above

cutoff

_ _

0

0

0

0

19

15

5

9

7

6

9

9

4

44

0

Percent

above

cutoff

8

12

57

18

16

0

11

29

21

15

1

11

5

0

 

No.

of

holes

 

__

 

 

 

1

1

1

1

1

 

 

No.

of

holes

1

1

1

1

2

2

3

3

4

3

3

3

2

2

2

2

Block III

Mean Mean

Cu Ni

grade grade

 

__ __

 

 

 

0.07 0.07

.14 .12

.25 .25

.22 .23

.06 .04

 

 

Block VII

Mean Mean

Cu Ni

grade grade

0.13 0.17

.15 .20

.24 .31

.25 .29

.14 .16

.14 .21

.13 .16

.17 .24

.08 .15

.11 .12

.09 .14

.08 .11

.04 .05

.05 .05

.04 .06

.06 .13

Percent

above

cutoff

 

__

 

 

 

0

15

46

21

6

 

 

Percent

above

cutoff

8

0

43

49

13

23

9

21

8

11

7

9

0

0

0

10

No.

of

holes

4

5

6

7

7

8

9

9

9

7

6

5

2

2

1

1

No.

of

holes

8

10

12

12

12

13

16

16

10

7

4

3

2

1

1

Block IV

Mean Mean Percent

Cu Ni above

grade grade cutoff

0.06 0.06 6

.06 .04 2

.09 .11 1

.13 .17 6

.14 .17 12

.15 .19 14

.17 .18 15

.21 .23 32

.20 .24 37

.21 .22 25

.20 .14 22

.22 .22 19

.07 .04 5

.20 .05 17

.17 .05 11

.55 O O .Oo f\£. 1 lj^

Block VIII

Mean Mean Percent

Cu Ni above

grade grade cutoff

0.17 0.16 25

.18 .20 20

.16 .19 10

.12 .16 11

.18 .09 22

.18 .25 31

.25 .17 35

.31 .32 49

.20 .27 28

.19 .09 18

.13 .19 27

.12 .13 18

.15 .43 43

.26 .64 85

.16 .27 19

*Dashed lines denote contact between Basal series and cordierite-rich metasedimentary rocks.

would expect to find the highest grades. But even in these 

blocks, the number of holes was insufficient to use anything but the most elementary engineering methods for 

resource evaluation. Substantial additional drilling is necessary in order to further refine the subsurface of these 

blocks or of other fault-bounded blocks.

IN SITU GRADE-TONNAGE RELATIONS FOR 

FAULT BLOCKS IV AND VIII

Previous sections of this paper described coppernickel concentration values in various degrees of detail. 

The concentration values of copper and nickel in horizonB12 Contributions on Ore Deposits in the Early Magmatic Environment
Block 1

Cu Ni

B8

B7

B6

B5

B4

B3

B2

B1

- 

- 

- 

- 

- 

- 

CONTACT L-^IT C1 r ' 5 ' 25

C2

C3

C4

C5

C6

C7

C8

- 

- 

- 

-

-

__l _ I

.25.5

Block II 

Cu Ni

Block III 

Cu Ni

Block IV 

Cu Ni

Block V 

Cu Ni

Block VI 

Cu Ni

Block VII 

Cu Ni

Block VIH 

Cu Ni

.5.25 .25.5 .5.25 ,25.5 .5.2 . 5.5 .5.25 5.5 .5.2 .25.5 .5.25 5.5

Figure 6. Distribution of copper and nickel by 25-foot intervals away from the Basal series-metasedimentary rock contact 

within individual fault-bounded blocks. Grades are in weight percent. Intervals B1 through B8 represent successive 25-foot rock 

layers above the Basal series-metasedimentary rock contact, and intervals C1 through C8 represent 25-foot rock layers below 

this contact.

tal 100-foot intervals by stratigraphic unit in drill holes of 

the Mountain Mew study area were already described. 

Copper-nickel concentration values for 25-foot intervals 

above and below the Basal series-metasedimentary rock 

contact were then examined within individual fault blocks.

In this section, a further refinement of the analysis 

of concentration values is made, and results of the in situ 

ore tonnage calculations of blocks IV and VIII are 

presented. The areal extent of each 25-foot layer was 

delineated according to the structural interpretation of the 

subsurface geology derived from drill-hole information 

and surficial geology. Continuity of 25-foot intervals, both 

above and below the base of the Basal series, was correlated as far as possible. Lateral continuity, however, has 

been affected between blocks by faulting, rotation, and 

translation of the fault-bounded blocks; within blocks by 

erosional truncation and the natural variability of thickness 

resulting in pinching of the beds; and by toss of control due 

to the inclined stratigraphy and the lack of drill-hole 

information.

We assume that mining units within individual faultbounded blocks would be along the lower Basal series 

contact. The mineralization and stratigraphy are assumed 

to be continuous along the contact between the Basal 

series and cordierite-rich metasedimentary rocks where it 

could not be shown otherwise. Intervals of 25 feet, which 

are conformable to the contact and spatially limited by the 

structural interpretation, were examined both upward into 

the cumulate rocks of the Basal series and downward 

from the contact into the metasedimentary rocks. The area 

of the contact for each block (fig. 3) was measured at the 

surface by planimeter and adjusted to account for the 

inclination of the stratigraphy.

Block VIII is penetrated by 14 vertical drill holes 

which are used in the structural interpretation of this block 

(fig. 3). Additionally, two angle holes were incorporated to 

compute the grade-tonnage statistics for this block. Block 

VIII is delineated by (1) an assumed vertical Verdigris 

fault in the northeast; (2) an assumed vertical number "5" 

fault in the southwest; (3) erosional truncation due to 

inclination of the Basal series-metasedimentary rock contact in the southeast; and (4) a vertical boundary positioned by loss of control due to the lack of holes and 

increasing depth of the body to the northwest. Two 

subfaults, which are approximately parallel and supplementary to the Verdigris fault (V2 and V3), are positioned 

as interpreted to be boated at the base of the Basal series 

and assumed to be vertical for calculations. In reality, they 

are interpreted to be dipping at approximately 70° to the 

southwest. At depth, this area becomes 1,475,000 square 

feet due to the 65° angle of inclination.

The orientation of block VIII, based on the Ultramafic series-Basal series contact, is N. 50° E., 65° NW. 

This orientation results from the surficial geology, the 

evidence for minor faulting within this btock, and a combination of multiple three-point problems computed on 

drill holes that penetrate the Ultramafic series-Basal 

series contact. This is the best orientation that can be 

interpreted from currently available information. A computergenerated elevation contour map of the base of the Basal 

series for block VIII is shown in figure 7 which can be 

oriented to figure 3 by means of the horizontal and vertical 

scales. The Basal series-metasedimentary rock contact 

dips northwest at a high angle from 6,300 feet above sea 

level until the number "9" fault is encountered at 5,250 

feet. Here the contour appears flat because the northwest 

side of the number "9" fault has moved the lower Basal 

series contact upward.

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B13
BLOCK VIM

EXPLANATION 

 Drill hole

Figure 7. Faults, drill-hole locations, 

lines of sections, and contour lines 

depicting attitude of the base of the 

Basal series, block VIII. Block Vlll's 

orientation strikes N. 50° E. and dips 

65° NW. Contour interval is 50 feet. 

Scale is in feet.

1000 -

900

3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200

FEET

A three-dimensional perspective of the base of the 

Basal series (fig. 7) is presented in figure 8. In order to 

observe the slope and have all features visible, the viewpoint of this perspective is 235° clockwise from north at an 

inclination of 20° from the horizontal. This faulted surface 

was computed using 14 vertical holes with a spatial distribution as shown in figure 7 and a deviation of the surface 

of less then 0.5 percent of the range of the true elevations. 

Elevations of areas outside of block VIII are to be ignored 

because these areas were not computed. The steep inclination of the base of the Basal series and the resultant 

effect of the minor faults are also shown in figure 8.

The spatial distribution and number of 25-foot 

intervals that vertical drill holes penetrate in the Basal 

series cumulate rocks is shown in a plan view (map) of 

block VIII in figure 9. In displaying thickness and areal 

extent of stratigraphic units, the convention used in figures 

9 and 10 (and later in figs. 14 and 15) is that the tower case 

"b" or "c" denotes map areas above and below the Basal 

series-metasedimentary rock contact, respectively, and the 

number following "b"or "c"denotes thickness in terms of 

the number of 25-foot intervals for that area. In the earlier 

tables and the following discussion, upper case "B" and 

"C" represent 25-foot intervals above and below the Basal 

series-metasedimentary rock contact, respectively, with a

numerical suffix indicating the specific interval away from 

the contact. Figure 9, oriented in the same manner as 

figure 7, shows that 3 of the 14 vertical drill holes (those in 

areas labeled b2) pass through two 25-foot intervals (B2 

and Bl), that one of the holes (in the area labeled b6) 

passes through six of the 25-foot intervals (B6, B5, B4, 

B3, B2, and Bl), and so forth. The holes shown in figure 

9, along with two inclined holes, were used in computing 

the grades presented in table 6 for block VIII.

Due to the inclination of the stratigraphy and exposure at the surface in block VIII, lithologies in the 

southeastern part of the block are truncated by erosion. 

Figure 9 represents the distribution of 25-foot intervals 

between the base of the Basal series and either the surface 

exposure or the base of the Ultramafic series. Erosion and 

inclination of the strata result in a thickness of the Basal 

series that goes from 50 feet (b2) to 200 feet (b8) above 

the base of the contact. As with the metasedimentary 

rocks below the Basal series, the 25-foot intervals are 

spatially limited by the stratigraphic information available 

in drill cores and at the surface. Since most holes are 

collared in the Ultramafic series and terminate in the 

underlying metasedimentary rock, an upward and downward limit to the thickness of the Basal series is defined. 

Surface erosion has eliminated the Ultramafic series and

B14 Contributions on Ore Deposits in the Early Magmatic Environment
truncated the Basal series, affecting the four most easterly drill holes (shown as xfc on fig. 9). For this reason, an 

originally much thicker Basal series in the eastern part of 

block VIII appears to pinch out in an easterly direction.

Figure 10 is a similar map of block VIII that shows 

the number of 25-foot intervals penetrated by drill holes in 

the metasedimentary rocks beneath the Basal series. It 

also shows two important features associated with the 

assigned thickness of cordierite-rich metasedimentary 

rocks below the base of the Basal series. First, it shows the 

spatial distribution of the thickness of the cordierite-rich 

metasedimentary rocks. Areas labeled cl, c2, and c3, 

which indicate the thickness of the cordierite-rich metasedimentary rocks, were used in computing tonnages. Drill 

holes in area c3 pass through three 25-foot intervals (C3, 

C2, and Cl), holes in area c2 pass through two 25-foot 

intervals (C2 and Cl), and holes in area cl pass through 

the Cl interval. All holes start above the base of the Basal 

series, and most holes terminate in the underlying quartzrich metasedimentary rocks. Therefore, the lower extent 

of the rocks of economic interest is known. The remaining 

holes either do not completely penetrate the cordieriterich metasedimentary rocks or, as the two most southeasterly holes, go directly from the Basal series to quartz-rich 

metasedimentary rocks.

The second important feature of the metasedimentary rocks below the base of the Basal series concerns the 

relation between cordierite-rich and quartz-rich metasedimentary rocks. As can be seen in figure 10, cordieriterich metasedimentary rocks appear to pinch out in an 

easterly and southerly direction. This pinching out may be 

a function of an original depositional relationship, folding 

of the metasedimentary rocks, or pre-intrusion erosion of 

a folded surface.

The pinching out of the Basal series due to surface 

erosion is apparent in figure 11, which shows the distribution of 25-foot intervals above and below the base of the 

Basal series along cross sections for block VIII. These 

cross sections correspond to the A-A'', B-B' traces as 

found in figures 7, 9,10 and with the same perspective as 

figure 8. Figure 11 is included to give the reader a true 

composite view of the 25-foot intervals penetrated in 

block VIII, including surface orientations, fault locations, 

and distribution of the volumes of rock used in the 

grade-tonnage calculations.

Block IV is penetrated by eight vertical drill holes as 

shown in figure 12. One vertical drill hole used in the 

structural interpretation (fig. 3) was collared below the 

Basal series-metasedimentary rock contact; therefore, it 

was not used in the grade-tonnage calculations for block 

IV. The area of interest is delineated by the number "1" 

fault in the northwest, the unnamed fault in the northeast, 

the number "2" fault in the west, and exposure of the 

Basal series-metasedimentary rock contact due to erosion

and inclination of the strata in the south. All faults are 

assumed to be vertical. Block IV has an area of approximately 292,300 square feet at the surface and 442,900 

square feet at depth due to the 49° angle of inclination.

The orientation of block IV, based on the Ultramafic series-Basal series contact is N. 72° E., 49° NW. 

This orientation was calculated from the average of four 

three-point problems computed on six drill holes that 

penetrate the Ultramafic series-Basal series contact. A 

contour map of the base of the Basal series in block IV is 

shown in figure 12. The contact dips to the north from a 

surface exposure of approximately 6,450 feet above sea 

level to 6,160 feet before the number "1" fault is intercepted. A three-dimensional perspective view of this surface is shown in figure 13, from the same direction as 

figure 8, 235° clockwise rotation from north and 20° above 

the horizontal plane. This figure shows a perspective 

computed below a horizontal surface, however, in contrast 

to block VIII, which was computed above a horizontal 

surface or plane. These different views give additional 

insight into how the contact appears below the surface. 

Block IV has no apparent internal faults; however, a 

relatively smooth undulating base for the Basal series may 

be indicative of minor movement along faults. The Basal 

series of block IV becomes thinner further to the west, 

which could be indicative of an undulating cumulate floor 

or minor undetected faulting.

The distribution of the 25-foot intervals encountered in the Basal series in block IV is shown in figure 14. 

These thicknesses range from 75 feet (B3, B2, Bl) in the 

east, where erosion has removed the overlying intervals, to 

200 feet (B8 through Bl inclusive) to the west which 

reflects a thickening of the Basal series from east to west. 

The Basal series of block IV again becomes thinner 

further to the west.

The distribution of the cordierite-rich metasedimentary rocks and the number of 25-foot intervals penetrated 

is shown by figure 15. All holes shown in this section start 

above the Basal series-metasedimentary rock contact and 

terminate in the quartz-rich metasedimentary rocks 

below; therefore, complete sections of cordierite-rich 

metasedimentary rocks are identified. A thinning of 

cordierite-rich metasedimentary rocks is present. Where 

the cordierite-rich unit is absent in one drill hole, the 

Basal series rests directly on quartz-rich metasedimentary 

rocks. Cordierite-rich metasedimentary rocks appear to 

be thicker in both an easterly (areas denoted C2a, C3a, 

and C4a) and westerly (areas denoted C2, C3, and C4) 

direction from this point. Tonnages were computed separately for the rock units in the areas in the eastern part of 

block IV that are denoted with the tower case "a" suffix in 

figure 15. Because the southern exposure of the study 

area is the surface exposure of the base of the Basal 

series, erosion does not truncate these metasedimentary 

rocks north of the surface contact.

Copper and Nickel Sulfldes within the Mountain View Area of the Stlllwater Complex, Montana B15
BLOCK VIII

4200

900 FEET

Figure 8. Three-dimensional perspective view of the base of the Basal series, block VIII; inclination is 20°, rotation 235°. 

Perspective is above a horizontal plane. Contour interval is 50 feet.

BLOCK VIM

1900

Possible extension 

of number "9" fault

EXPLANATION

 Drill holes collared in Ultramafic series; therefore, 

entire Basal series is penetrated

x Drill holes collared in Basal 

series; therefore, top of 

series is missing due to 

erosion

® 25 feet of Basal series missing due to faulting; therefore, not used to calculate 

b8 interval

No Basal series present due to 

inclination of the stratigraphy, movement along 

inclined subfaults, and 

erosional truncation

900

3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200

FEET

Figure 9. Spatial distribution of 25-foot intervals above the base of the Basal series used in grade-tonnage calculations for 

block VIII. B8 represents the uppermost 25-foot interval considered conformable to the Basal series-metasedimentary rock 

contact. Drill holes shown in b8 have penetrated intervals B1 through B8. Those shown in b7 have penetrated B1 through B7, 

and so on (b2 is inclusive of b1). All holes penetrate the base of the Basal series. Scale is in feet.

B16 Contributions on Ore Deposits in the Early Magmatic Environment
BLOCK VIII

1900 EXPLANATION

Drill holes which are collared 

above the cordierite-rich 

metasedimentary rock and 

terminate in the quartzrich metasedimentary rock; 

therefore complete unit 

penetrated

Drill holes which are collared 

above the cordierite-rich 

metasedimentary rock and 

terminate in it

Line at which cordierite-rich 

metasedimentary rock 

pinches out, resulting in 

Basal series lying directly 

on quartz-rich metasedimentary rock

No cordierite-rich metasedimentary rock present

900

3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200

FEET

Figure 10. Spatial distribution of 25-foot intervals below the base of the Basal series in the cordierite-rich metasedimentary 

rocks used in grade-tonnage calculations for block VIII. C1 represents the uppermost 25-foot interval conformable to the Basal 

series-metasedimentary rock contact. Drill holes shown in c3 have penetrated intervals C1 through C3. Those shown in c2 have 

penetrated C1 and C2, and those shown in c1 have penetrated only C1. Scale is in feet.

BLOCK VIII

B'

900 FEET

Figure 11. Cross sections A-A' and B-B' through block VIII showing the distribution of 25-foot intervals away from the base 

of the Basal series and used in grade-tonnage calculations.

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B17
BLOCK IV

1400

1300 -

1200 -

1100 -

1000 -

900

800 -

700

2400 2500 3300 3400

Figure 12. Faults, drill-hole locations, lines of sections, and contour lines depicting attitude of the base of the Basal series, 

block IV. Block IV strikes N. 72° E. and dips 49° NW. Contour interval is 20 feet. Scale is in feet.

Methods of Appraisal

Each fault-bounded block examined in detail was 

subdivided into 25-foot intervals which were spatially 

limited as described previously. These 25-foot intervals 

were then subdivided by 25-foot and 100-foot square 

grids. These subdivisions resulted in mining blocks 

25X25X25 feet and 25X100X100 feet for the computations. Concentration data from scattered drill holes within 

each fault-bounded block were used to interpolate grades 

between drill holes for each mining block. Assuming a 

rock density of 0.0952 short tons per cubic foot (3.05 

grams per cubic centimeter) (Kleinkopf, 1985), a tonnage 

was associated with each mining block. An inventory of 

mining blocks and their assigned copper and nickel grades 

was made. For specific combined copper and nickel cutoff 

grades (for each 25-foot layer), the total ore tonnage was 

computed along with the associated average copper and 

nickel grades (tables 7, 8).

Sample grades for each drill hole intersecting each 

25-foot interval were calculated by using a weighted mean

for the copper and nickel values present. Methods used for 

extending sample grades to estimate the mining block 

grades were quite primitive because of the scarcity of data. 

As table 6 indicates, the numbers of observations in any 

individual fault-bounded block and 25-foot interval were 

very limited. Attempts to estimate a well behaved 

variogram for the 100-foot horizontal layers across the 

study area shown in tables 2 and 3 were unsuccessful. 

Consequently, geostatistical methods could not be applied 

to quantify probabilistically the confidence attached to 

grade or tonnage estimates. Methods used for extending 

the sample data to individual mining blocks were (1) the 

inverse distance algorithm, (2) the inverse distance 

squared algorithm, and (3) the algorithm used in the 

commercially available Interactive Surface Modeling Program (Dynamic Graphics, 1984). Each method or procedure weights data in order to compute estimates of the 

copper or nickel grade for points located at the center of 

each mining block. These values are then assumed to be 

the average grade value for the particular block. In

B18 Contributions on Ore Deposits in the Early Magmatic Environment
BLOCK IV

EXPLANATION 

hole

3400

700 FEET

1400

Figure 13. Three-dimensional perspective view of the base of the Basal series, block IV; inclination is 20°, rotation 235°. 

Perspective is below a horizontal plane. Contour interval is 20 feet.

applying these methods, we have assumed continuous 

mineralization between drill holes.

Application of the inverse distance method assumes 

that there are relationships between the grade of a hole 

and the grade of adjoining blocks and that the relationship 

can be expressed as some function of distance between the 

two. Grades from all surrounding holes are used in 

estimating the block grade. Weights of each surrounding 

hole value are determined by distance between the drill 

hole and the block under consideration. Particular weights 

are inversely proportional to this distance;

that is, Z^ =

where

Z^ is the block grade,

Zj is the i-th hole grade, and

d^ is the distance between the i-th hole and center of

the k-th block.

Even more weight can be attached to the samples nearer 

the block by squaring the distance between the k-th block 

and the i-th sample. Thus, the formula for the inverse 

distance squared weighting scheme is

where Z^, Zj and dik are defined as above.

Finally, with some restrictions of extreme values, the 

Interactive Surface Modeling Program algorithm iteratively fits progressively more complicated mathematical 

functions to the data until there is no longer any additional 

improvement in the fit to the sample data. For a technical 

description of the algorithm, see Briggs (1974). This 

algorithm will extrapolate (linear or nonlinear) trends 

beyond the edges of die sample space, whereas the inverse 

distance and inverse distance squared methods extrapolate constant values beyond the area bounded by data 

points.

Grade-Tonnage Relations for Block VIII

Table 6 shows the number of holes in each of the 

25-foot intervals in fault block VIII. Tonnages were 

computed for intervals Bl through B8 (200 feet above the 

base of the Basal series) and Cl through C3 (75 feet 

below the base of the Basal series). In figures 16 and 17,

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B19
BLOCK IV

1400

1300

1200

1100

1000

900

800

700

EXPLANATION

 Holes collared in Ultramafic series; 

therefore, complete Basal series 

present

x Holes collared in Basal series; therefore, top of unit eroded

2300 2400 2500 2600 2700 2800 2900 

FEET

3000 3100 3200 3300 3400

Figure 14. Spatial distribution of 25-foot intervals above the base of the Basal series used in grade-tonnage calculations for 

block IV. B8 represents the uppermost 25-foot interval conformable to the Basal series-metasedimentary rock contact. Drill 

holes shown in b8 have penetrated intervals B1 through B8. Those shown in b7 have penetrated intervals B1 through B7, and 

so on (b3 is inclusive of b1 and b2). All holes penetrate the bottom of the Basal series. Block IV strikes N. 72° E. and dips 49° 

NW. Scale is in feet.

computations were based on mining blocks assumed to be 

100X100 feet and to have a thickness of 25 feet. Results 

in figures 18 and 19 are based on the assumption that the 

mining blocks are cubes 25 feet on each side. The results 

of the inverse distance, inverse distance squared, and 

Interactive Surface Modeling algorithms on the cutoff 

grade-tonnage computations for combined copper and 

nickel are shown in figures 16 and 18. Figures 17 and 19 

show the average grade (combined copper and nickel) at 

each tonnage corresponding to the cutoff grade tonnages 

from the inverse distance, inverse distance squared, and 

Interactive Surface Modeling algorithms shown in figures 

16 and 18. Figure 16 indicates that above and below 

approximately a 0.4-percent cutoff grade, the estimated 

tonnages computed by the three methods diverge. At a 

combined copper and nickel cutoff grade of 0.5 percent, 

the estimated ore tonnages are within 10 percent of each 

other; whereas at a 0.2-percent cutoff grade, the Interactive Surface Modeling algorithm shows tonnages at least 

10 percent below the results computed by the other two 

algorithms. The cutoff grade-tonnage relations computed 

by all three methods were not very sensitive to mining 

block size as seen from a comparison of figures 16 and 18. 

To estimate the grade-tonnage relations displayed in 

figures 16 and 18, the procedure was to first specify

a combined cutoff grade and then calculate the tonnage in 

all those mining blocks having average grades at least as 

high as the cutoff grade. Figures 17 and 19 present the 

average combined copper and nickel grades for the tonnages shown in figures 16 and 18.

Figure 20 shows average copper and nickel grades as 

a function of the tonnages computed on the basis of the 

inverse distance squared method using 25-foot cubic 

mining blocks. Average nickel grades in figure 20 are 

always higher than average copper grades. Average nickel 

grades were also higher than die average copper grades 

when the inverse distance and the Interactive Surface 

Modeling algorithms were used. Metal content of ore 

tonnages associated with each cutoff grade can be calculated using data presented in these figures. Using these 

figures for the inverse distance squared method, at the 

combined average grade of 0.50 percent copper and nickel 

(fig. 19), estimated ore tonnage was 19.7 million short 

tons. According to the data used to generate figure 20, the 

corresponding average copper grade is then 0.235 percent, 

and the nickel grade is 0.265 percent. Consequently, the 

estimated in situ copper metal tonnage is 46,300 short 

tons, and nickel metal tonnage is 52,200 short tons.

Table 7 shows the results for block VIII of the three 

algorithms applied to each 25-foot interval and areas

B20 Contributions on Ore Deposits in the Early Magmatic Environment
BLOCK IV

1400

1300

1200

1100

1000

900

800

700

EXPLANATION 

 Drill hole

2300 2400 2500 2600 2700 2800 2900 

FEET

3000 3100 3200 3300 3400

Figure 15. Spatial distribution of 25-foot intervals below the base of the Basal series used in grade-tonnage calculations for 

block IV. Shaded area represents Basal series which rests directly on quartz-rich hornfels; therefore, no cordierite-rich hornfels 

are present. Block IV strikes N. 72° E. and dips 49° NW. All drill holes penetrate quartz-rich hornfels. C1 represents the 

uppermost 25-foot interval conformable to the Basal series-metasedimentary rock contact. Drill holes shown in c3 have 

penetrated intervals C1 through C3. Those shown in c2 have penetrated through C1 and C2, and those shown in c1 have 

penetrated only C1. For interpretation for areas c2a, c3a, and c4a, see the text. Scale is in feet.

sl

It

1.1

1.0

0.9 A

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1

0 10 15 20 25 

MILLION SHORT TONS OF "ORE"

30

Figure 16. Cutoff grade (combined copper and nickel) vs. 

ore tonnage, computed using the Interactive Surface Modeling algorithm (solid line), the inverse distance squared algorithm (long dashes), and the inverse distance algorithm 

(short dashes), for block VIII. Mining blocks are 100X100X25 

feet.

delineated and displayed in figures 9 and 10. For each 

combined cutoff grade (0.8 percent, 0.6 percent, 0.4 

percent, and 0.2 percent), the corresponding ore tonnage 

and the average copper and nickel grades are computed. In 

general, as one moves further away from the Basal 

series-metasedimentary rock contact, ore tonnage tends to 

decline. This occurs for two reasons. First, as shown by 

table 6, the concentration grades tend to decline as the 

distance from the contact increases, so that a progressively 

larger fraction of tonnage will be classified as waste rock. 

Second, because of irregular stratigraphy, faulting, and 

erosion within block VIII, the plan view surface areas 

corresponding to each 25-foot layer decline as distances 

from the contact increase.

The tonnage of all rock for each 25-foot interval 

can be computed by multiplying the product of the area of 

the interval and the thickness of interval (25 feet) by the 

weight per cubic foot (0.0952 short tons/cubic foot). For 

block VIII, this tonnage is 25.860 million tons of rock.

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B21
Table 7. Grades and tonnages for in situ ore in fault block VIII, computed by three algorithms

[Calculations are based on cubic blocks of 25 feet; - - -, no data. Rows labeled "Composite" represent average grade of the aggregate ore in all intervals and total 

ore tonnage at the specified cutoff grade]

25-foot 

interval

Area 

(ft2 x103 )

0.8% cutoff 

combined grade

Avg. grade

Cu 

(%)

Ni 

(%)

Short 

tons ore 

(xlO6)

0.6% cutoff 

combined grade

Avg. grade

Cu 

(%)

Ni 

(%)

Short 

tons ore 

(xlO6)

0.4% cutoff 

combined grade

Avg. grade

Cu 

(%)

Ni 

(%)

Short 

tons ore 

(xlO6)

0.2% cutoff 

combined grade

Avg. grade

Cu

(%)

Ni 

(%)

Short 

tons ore 

(X106)

Inverse distance algorithm

DO

D7

u/;

DC

B4------

D1

DO

D 1

*

a ------

C2 ------

C3 ------

Composite

o/rn i

971.0

1,022.9

1,092.2

1,127.9

1,165.5 

1,394.4

1,394.4

1,092.6

496.9

9TQ 7

0

-30

.40

.43

.25

.65

.41

0.55

.53

.49

.66

.19

.51

0.003

.054

i fi

023

AA-3

.248

0.21

.29

-30

-3/C

.24

.33

.33

.22

.52

.32

0.41

.40

.24

.35

.42

.36

-3/C

.47

.16

.37

0.003

. V^TVJ

.003

.003

.007

.T^T 1

2.784

.192

.017

3.496

0.19

.22

.21

.17

.22

.20

.27

.33

23

.34

.19

.26

0 25

.27

.22

.27

.22

.27

.27

.34

98

.13

.22

.28

0 335

392

.117

O98

. -JV^T 

2/CC1

3 203

3.301

£***1<J£*

.242

 V^TSJ

13.131

0.17

1 0

.16

.12

1 8

.20

.27

.33

.22

.23

.16

.21

0.16

.20

.19

.16

1 8

o/c

.27

.34

.27

.10

.16

.23

2.015

2 £* . 303 ,-r\J*S

2.426

2

CT/:

2

/C*7C

2 "7 /CO

3.312

3.312

2 £***JS*J 595

1.075

.551

25.608

Inverse distance squared algorithm

DO

P.7

B6------

DC

n/i

D-3

D1

D 1

*

Cl ------

C2 ------

C3 ------

Composite

O/lft ^

971.0

1,022.9 

1,092.2

1,127.9 

1,165.5

1,394.4

1,394.4

1,092.6

496.9

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0.37

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*^^J 25

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0.067

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Oft73,

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3.960

0.20

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98

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440

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223

752

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2 751

3.075

1.936

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13.147

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1 8

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1 8

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1 8

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98

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2

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3 J . 274 <£* / T

2

C*V A .574

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24.343

Interactive surface modeling algorithm

R7

D/C

DC

n/i

D-3

D9

*

a ------

C2 ----- -

C3 ------

Composite

Q/£fl ^t

971.0

1,022.9

1,092.2 

1,127.9

1,165.5 

1,394.4

1,394.4

1

r*r\^ £.

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0.40

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1.072

.573

.152

2.538

0.21

.36

.37

.19

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33

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.38

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. "T*!

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64

.19

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0.081 9-30

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254

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1.065

1.906

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TQ-3

5.535

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.23

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o/c

23

.36

-3/C

32

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.30

0 98

.33

24

.31

.27

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.35

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23

.34

1.019

.734

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1 042

2.035

9 ^-.VFJ*T ftSd

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1 7^8

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1 89

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0.20

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1 8

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33

98

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0 23

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.29

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1 539

1.960

2.270

1 . O*T 1

2.225

2

T/CO

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3.108

9 34fi ^>. ->HU

.673

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22.282

*Dashed lines denote contact between Basal series and cordieriterrich metasedimentary rocks.

B22 Contributions on Ore Deposits In the Early Magmatic Environment
Table 8. Grades and tonnages for in situ ore in fault block IV, computed by three algorithms

[Calculations are based on cubic blocks of 25 feet; -, no data. Rows labeled "Composite" represent average grade of the aggregate ore in all intervals and total

ore tonnage at the specified cutoff grade]

25-foot 

interval

Area 

(ft2 x103 )

0.8% cutoff 

combined grade

Avg. grade

Cu

(%)

Ni 

(%)

Short 

tons ore 

(x106)

0.6% cutoff 

combined grade

Avg. grade

Cu

(%)

Ni 

(%)

Short 

tons ore 

(xlO6)

0.4% cutoff 

combined grade

Avg. grade

Cu

(%)

Ni 

(%)

Short 

tons ore 

(xlO6)

0.2% cutoff 

combined grade

Avg. grade

Cu

(%)

Ni 

(%)

Short 

tons ore 

(xlO6)

Inverse distance algorithm

B6------

DC

B4------

R.-2

R.9

n i

*

Cl ------

C2 ------

C3 ._---.

C4 ------

P2A-----

P^A

C4A-----

Composite

369.3

416.8

435 5

465 HU J . 11

465 HU J . 11

465 1

450.7

-2O-2 Q

i/;/c i

154.3

59.4

35.5

21.5

0.34

.52

.53 

.52

0.49

.34

.54 

.50

0.002

013

.051 

.066

0 98

.39

o/c

.25

.35

.40

.41

.53 

.40

0.40

25

.39

-20

r\f-

.30

.24

.54 

.36

0.027

flfl9

.019

. \j\ff*

flfl9

.110

.034

.051 

.245

0.17

1 8

.20

.24

.22

.22

.19

i/;

-20

.37

.53 

.23

0

^>j£

.24

.27

.21

23

i/:

^ j

.24

.20

.29

. 22^^

.54

.25

0.011

.007

.272

r\f\/i

1.035

QQ1

. £*£**) 223

032

.141

.084

.051 

2.833

0.11

.13

.14

.16

.17

.21

.21

.17

.17

.13

-2Q

.37

.53 

.17

0.13

.17

.17

.20

1 8

23

.24

.21

.13

.15

.29

.22

.54 

.19

0.412

.982

1.032

1.103

1.105

1.105

1.068

764

fjftQ

346

.141

.084

.051 

8.801

Inverse distance squared algorithm

DrfC

DC

B4------

D-2

R.9

p. 1

*

Cl ------

C2 ------

C3 ------

C4 ----- -

P9A

P3A-----

C4A----- 

Composite

-2^n -2

416.8

435 5

465.1

465.1

465 1

450.7

-29-2 o

^>4C4C -3

X*/*T . _*

59.4

35.5

21.5

0

-2Q

.55

.71 

.56

0.56

in

.72 

.47

0.034

.119

.044 

.197

0.32

37

32

98

.25

.35

.55

.47

.71 

.43

0.47

.24

98

.42

.39

r\f35

.27

.72 

.40

O nro

.021

007

.099

.021

nrp

.129

.074

.044 

.489

0.17

1 Q

23

o/c

.23

.24

.20

00

.55

.47

.71 

.26

0.27

25

32

22

23

90

o/;

.21

35

.27

.72 

.27

0.081

.074

.337

.221

817

T1Q

.270

.066

.129

.074

.044 

2.871

0.12

.13

.15

.16

.17

.22

.22

.17

.16

.12

.55

.47

.71 

.18

0.15

1 0

1 8

.21

1 0

22

.25

.22

.13

.14

.35

.27

.72 

.20

0.427

C\f\f

.953

1.053

1 ft8Q

1.089

Q81

.670

460

^>^cc

.129

.074

.044 

8.140

Interactive surface modeling algorithm

D/C

pc

 QA

R.-2

R9

*

ri _._._.

C2 ------

C3 ------

C4 ----- -

C2A-----

P^A

C4A----- 

Composite

-2^n -2

416.8

435.5

465.1

465 l~T\JmS , 1I

465 *T\JJ. 11

450.7

-2O-2 Q

O/C/C 1

154.3

59.4 

35 5

21.5

0

-2Q

.35

.75

.60

.77 

.49

0.55

.49

.76

.37

.78

.52

0.117

.061

Ofll .VA/l

.094

.044 

.317

0.33

-20

.33

.32

.24

.37

.75

.56

.49

.77 

.40

0.48

25

.30

.40

.39

.29

.76

.35

98

.78 

.41

0.216

.025

.074

187

.027

.029

 V/v I

.123 

.074

.044 

.800

0.17

.19

98

25

o/c

98

.20

98

.20

.53 

.49

.70

.27

0.27

25

.39

23

.24

.29

98

25

9<;

.33

98

.70 

.29

0.210

.297

-20Q

293

.723

8^4.

.367

123

.035

.141 

.074

.051 

3.537

0.13

.14

.16

.19

.17

.22

^>^c

1 8

.20

.14

.53 

46

.70 

.20

0.16

.19

.19

.25

1 8

.22

o/c

25

1 8

18

.33 

.26

.70

.22

0.418

8fK

78Q

942

1.105

1.105

QQ8

.572

.322

.174

.141 

.084

.051 

7.506

* Dashed lines denote contact between Basal series and cordierite-rich metasedimentary rocks.

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B23
1.5 

1.4 

1.3 

1.2 

1.1 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3

5 10 15 20 25 

MILLION SHORT TONS OF "ORE"

30

Figure 17. Average grade (combined copper and nickel) 

vs. ore tonnage, computed using the Interactive Surface 

Modeling algorithm (solid line), the inverse distance squared 

(long dashes), and the inverse distance algorithm (short 

dashes), for block VIII. Mining blocks are 100X100X25 feet.

5 10 15 20 

MILLION SHORT TONS OF "ORE"

25 30

Figure 19. Average grade (combined copper and nickel) 

vs. ore tonnage, computed using the Interactive Surface 

Modeling algorithm (solid line), the inverse distance squared 

algorithm (long dashes), and the inverse distance algorithm 

(short dashes), for block VIII. Mining blocks are 25X25X25 

feet.

10 15 20 25 

MILLION SHORT TONS OF "ORE"

30

Figure 18. Cutoff grade (combined copper and nickel) vs. 

ore tonnage, computed using the Interactive Surface Modeling algorithm (solid line), the inverse distance squared algorithm (long dashes), and the inverse distance algorithm 

(short dashes), for block VIII. Mining blocks are 25X25X25 

feet.

s*

< DC DC O 

O QC 5 if Is

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0

V

0 5 10 15 20 25 30 

MILLION SHORT TONS OF "ORE"

Figure 20. Average copper and average nickel grades vs. 

ore tonnage computed with the inverse distance squared 

algorithm for block VIII. Mining blocks are 25X25X25 feet. 

Solid line represents copper. Dashed line represents nickel.

Results computed with the inverse distance, inverse distance squared, and Interactive Surface Modeling algorithms (table 7) show that at the 0.4-percent combined 

copper and nickel cutoff grade, the waste rock (mining 

blocks having average grades below cutoff grades) accounts 

for almost half of the total tonnage delineated within the 

stratigraphic rock layers. At a 0.2-percent combined copper and nickel cutoff grade, only about 6 percent of the 

rock is waste using the inverse distance squared algorithm. 

At both cutoff grades of 0.4-percent and 0.2-percent

combined copper and nickel, the fraction of rock that is 

waste typically increases with the distance away from the 

Basal series-metasedimentary rock contact.

The ore and metal tonnage that can be computed 

from table 7 represents in situ tonnages and does not 

account for losses due to engineering or to economic 

requirements associated with the mining or metallurgical 

recovery of the metal from the ore. Moreover, because 

there are only eight, seven, and four observations for 

intervals B8, C2, and C3 (table 6), respectively, tonnage

B24 Contributions on Ore Deposits in the Early Magmatic Environment
1.1

1.0

iij 0.9

x.

o

0.8

0.7

0.6

0.4

0.3

0.2

0.1

5 10 

MILLION SHORT TONS OF "ORE"

15

Figure 21. Cutoff grade (combined copper and nickel) vs. 

ore tonnage, computed using the Interactive Surface Modeling algorithm (solid line), the inverse distance squared algorithm (long dashes), and the inverse distance algorithm 

(short dashes), for block IV. Mining blocks are 100X100X25 

feet.

estimates for these layers are based on relatively sparse 

data. Because we assumed that all faults are vertical, the 

surface area of each 25-foot interval is invariant with 

respect to depth. This assumption introduces a bias 

because the V2 and V3 faults trend northward toward the 

Verdigris fault with each successive interval away (upward) 

from the base of the Basal series. This trend of the V2 and 

V3 faults is also true below the base of the Basal series in 

the cordierite-rich metasedimentary rocks where these 

faults trend southward. Because no significant volume 

differences would be expected with the 25-foot intervals 

due to trending of these faults, however, this phenomenon 

was considered insignificant.

Grade-Tonnage Relations for Block IV

The surface area of the Basal series-metasedimentary rock contact in block IV (see figs. 12-15) is 

slightly less than one-third the surface area of rock layers 

in block VIII. As is the case for block VIII, the number of 

drill holes penetrating the base of the Basal series contact

5 10 

MILLION SHORT TONS OF "ORE"

15

Figure 22. Average grade (combined copper and nickel) 

vs. ore tonnage, computed using the Interactive Surface 

Modeling algorithm (solid line), the inverse distance squared 

algorithm (long dashes), and the inverse distance algorithm 

(short dashes), for block IV. Mining blocks are 100X100X25 

feet. For clarity in presentation, data points and curves 

representing tonnages smaller than 100,000 short tons are 

not shown.

in block IV is relatively sparse. The 25-foot layers examined for block IV were Bl through B6 and Cl through C3. 

Results of the grade-tonnage calculations using the three 

algorithms described earlier for block VIII are presented 

for block IV in figures 21-25 and in table 8. Figures 21 

and 23 show the relations between the combined cutoff 

grades and their associated ore tonnages, and figures 22 

and 24 show the relations between ore tonnage and 

average copper and nickel grades. These figures were 

drawn to the same scale as figures 16-19 to allow comparison of the grade-tonnage relations between blocks IV 

and VIII.

The results obtained with the inverse distance, 

inverse distance squared, and Interactive Surface Modeling algorithms were quite similar to each other. Also, the 

results show relatively little (10 percent) variation with 

respect to the assumed mining block size when comparing

Copper and Nickel Sulfldes within the Mountain View Area of the Stillwater Complex, Montana B25
5 10 

MILLION SHORT TONS OF "ORE"

15

Figure 23. Cutoff grade (combined copper and nickel) vs. 

ore tonnage, computed using the Interactive Surface Modeling algorithm (solid line), the inverse distance squared algorithm (long dashes), and the inverse distance algorithm 

(short dashes), for block IV. Mining blocks are 25X25X25 

feet.

figures 22 and 24. For example, the 0.5-percent average 

grade (combined copper and nickel) shown in figure 24 for 

the inverse distance squared method corresponds to 3.7 

million short tons. The associated cutoff grade is 0.37 

percent combined copper and nickel (fig. 23). Average 

copper grade is approximately 0.245 percent, and the 

average nickel grade is 0.255 percent. (See fig. 25.) Thus, 

in situ copper metal is about 9,070 short tons, and 

contained nickel metal is 9,440 short tons for these 

intervals.

The relation between average copper and nickel 

grades shown in figure 25 is different than that observed 

for block VIII, where average nickel grades generally 

exceed average copper grades. For each cutoff tonnage 

computed for block IV, the average nickel and copper 

grades are much closer in absolute value, and average 

copper grades actually exceed nickel for some tonnages.

Table 8 presents the tonnage estimates for the 

individual 25-foot intervals in block IV. The interpreted 

boundaries of each interval are presented in figures 

12-15. Entries in table 8 denoted as C2A, C3A, and C4A 

are intervals located in areas c2a, c3a, and c4a shown in

5 10 

MILLION SHORT TONS OF "ORE"

15

Figure 24. Average grade (combined copper and nickel) 

vs. tonnage, computed using the Interactive Surface Modeling algorithm (solid line), the inverse distance squared algorithm (long dashes), and the inverse distance algorithm 

(short dashes), for block IV. Mining blocks are 25X25X25 

feet. For clarity in presentation, data points and curves 

representing tonnages smaller than 100,000 short tons are 

not shown.

the eastern corner of figure 15. For example, tonnage 

indicated in table 8 in C2a includes the rock in the C2 

interval in areas c2a, c3a, and c4a indicated on figure 15. 

These areas are the consequence of one drill hole in the 

eastern part of block IV which intersected a thick 

cordierite-rich metasedimentary rock interval. Even 

though this was a complicating factor, no drill holes were 

omitted because there were so few with which to work. 

Results presented in table 8 show ore tonnage declining as 

distance from the lithotogic contact increases. The combined tonnages associated with layers C2A, C3A, and 

C4A are probably impractical to mine because of the large 

amount of waste rock that would have to be removed.

Even if these sections were minable, results of the 

inverse distanced squared calculations in table 8 show at a 

0.4-percent cutoff grade, almost three-fourths of the rock 

in the delineated layers is waste. At a 0.2-percent combined cutoff grade, 13 percent of the rock is still waste.

B26 Contributions on Ore Deposits in the Early Magmatic Environment
MILLION SHORT TONS OF "ORE"

Figure 25. Average copper and average nickel grades vs. 

ore tonnage computed with the inverse distance squared 

algorithm for block IV. Mining blocks are 25x25X25 feet. 

Solid line represents copper. Dashed line represents nickel.

Again, the fraction of the waste rock to total rock 

increases with distance from the Basal seriesmetasedimentary rock contact.

IMPLICATIONS

A combined estimate of grade and tonnage for 

several areas of the Stillwater Complex has been presented 

in an earlier report (Dayton, 1971). By using computerized detailed copper- and nickel-concentration data from 

within the Mountain Mew area (from relevant drill holes) 

and a current geologic interpretation, we eliminated areas 

where extrapolation of grades was inappropriate or where 

there was insufficient data. The tonnage estimates for the 

blocks that were examined are still tentative due to 

assumptions made concerning continuity of mineralization. These estimates require validation by additional 

drilling in block IV and block VIII. The question of 

whether to focus drilling within the evaluated faultbounded blocks or elsewhere depends on the commercial 

feasibility of mining a small deposit from the evaluated 

blocks. If additional tonnage is required for commercial

development, future drilling should be focused in other 

blocks in rock units immediately above and below the 

lithologic contact between the Basal series and the 

cordierite-rich metasedimentary rock.

REFERENCES CITED

Briggs, lan, 1974, Machine contouring using minimum curvature: Geophysics, v. 39, no.l, p. 39-48.

Dayton, Stanley, 1971, Hot air over Stillwater, profile of a 

hearing on mineral entry: Engineering and Mining Journal, v. 172, no. 10, p. 75-84.

Drew, L. J., Bawiec, W. J., Page, N. J, and Schuenemeyer, J. H., 

1985, The copper-nickel concentration log A tool for 

stratigraphic interpretation within the Ultramafic and 

Basal zones of the Stillwater Complex, Montana: Journal 

of Geochemical Exploration, v. 23, p. 117-137.

Dynamic Graphics, Inc., 1984, Interactive surface modeling: 

Berkeley, Calif.

Jackson, E. D., 1961, Primary textures and mineral associations 

in the Ultramafic zone of the Stillwater Complex, Montana: U.S. Geological Survey Professional Paper 358, 106

PKleinkopf, M. D., 1985, Regional gravity and magnetic anomalies of the Stillwater Complex area, in Czamanske, G. K., 

and Zientek, M. L., eds., Stillwater Complex, Montana; 

Geology and guide: Montana Bureau of Mines and Geology, Special Publication 92.

Page, N. J, 1977, Stillwater Complex, Montana Rock succession, metamorphism and structure of the complex and 

adjacent rocks: U.S. Geological Survey Professional Paper 

999, 79 p.

 1979, Stillwater Complex, Montana Structure, mineralogy, and petrology of the Basal zone with emphasis on 

the occurrence of sulfides: U.S. Geological Survey Professional Paper 1038, 69 p.

Page, N. J, and Dohrenwend, J. C., 1973, Mineral resource 

potential of the Stillwater Complex and adjacent rocks in 

the northern part of the Mount Wood and Mount Douglas 

quadrangles, southeastern Montana: U.S. Geological Survey Circular 684, 9 p.

Page, N. J, and Nokleberg, W. J., 1974, Geologic map of the 

Stillwater Complex, Montana: U.S. Geological Survey 

Miscellaneous Investigation Series Map 1-797, 5 sheets, 

scale 1:12,000.

Zientek, M. L., 1983, Petrogenesis of the Basal zone of the 

Stillwater Complex, Montana: Ph.D. thesis, Stanford University, Palo Alto, California.

Zientek, M. L., Czamanske, G. K., and Irvine, T.N., 1985, 

Stratigraphy and nomenclature for the Stillwater Complex, 

in Czamanske, G. K., and Zientek, M. L., eds., Stillwater 

Complex, Montana; Geology and guide: Montana Bureau 

of Mines and Geology, Special Publication 92.

«U.S GOVERNMENT PRINTING OFFICE 1987 - 181-407 - 226/40113

Copper and Nickel Sulfides within the Mountain View Area of the Stillwater Complex, Montana B27


IM YltW A^tA, SilLLWAihk COMPLEX, MONTANA U