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Chemical and Isotopic Studies 

of Granitic Archean Rocks, 

Owl Creek Mountains, Wyoming

Geochemistry and Petrogenesis of an Archean Granite from the 

Owl Creek Mountains, Wyoming

By J. S. STUCKLESS, A. T. MIESCH, and D. B. WENNER

Geochronology of an Archean Granite, Owl Creek Mountains, Wyoming 

By C. E. HEDGE, K. R. SIMMONS, and J. S. STUCKLESS

Uranium-Thorium-Lead Systematics of an Archean Granite from the 

Owl Creek Mountains, Wyoming

By J. S. STUCKLESS, I. T. NKOMO, and K. A. BUTT

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1388-A, B, C

Isotopic and geochemical studies are used

to determine the age and origin of an Archean

granite in the Owl Creek Mountains, Wyoming

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1986
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Main entry under title:

Chemical and isotopic studies of granitic Archean rocks, Owl Creek Mountains, Wyoming.

(Geological Survey Professional Paper ; 1388)

Bibliography: p.

Supt. of Docs, no.: I 19.16:1388A-C

Contents: Geochemistry and petrogenesis of an Archean granite from the Owl Creek Mountains, Wyoming / 

J.S. Stuckless, A. T. Miesch, D. B. Wenner Geochronology of an Archean granite, Owl Creek Mountains, 

Wyoming / C. E. Hedge, K. R. Simmons, J. S. Stuckless Uranium-thorium-lead systematics of an Archean 

granite from the Owl Creek Mountains, Wyoming / J. S. Stuckless, I. T. Nkomo, K. A. Butt.

1. Geology, Stratigraphic Precambrian. 2. Radioactive dating. 3. Geochemistry Wyoming Owl Creek 

Mountains. 4. Geology Wyoming Owl Creek Mountains. I. Stuckless, John S. II. Series.

QE653.C49 1986 551.7'1 85-600130

For sale by the Branch of Distribution

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425 

Denver, CO 80225
CONTENTS

[Letters designate the chapters.]

Page

(A) Geochemistry and Petrogenesis of an Archean Granite from the Owl Creek Mountains,

Wyoming, by J. S. Stuckless, A. T. Miesch, and D. B. Wenner ........ 1

(B) Geochronology of an Archean Granite, Owl Creek Mountains, Wyoming, by C. E. Hedge,

K. R. Simmons, and J. S. Stuckless ......................... 27

(C) Uranium-Thorium-Lead Systematics of an Archean Granite from the Owl Creek Mountains, Wyoming, by J. S. Stuckless, I. T. Nkomo, and K. A. Butt ....... 39

ill

Geochemistry and Petrogenesis of 

an Archean Granite from the 

Owl Creek Mountains, Wyoming

By J. S. STUCKLESS, A. T. MIESCH, and D. B. WENNER

CHEMICAL AND ISOTOPIC STUDIES OF GRANITIC ARCHEAN ROCKS, 

OWL CREEK MOUNTAINS, WYOMING

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1388-A

CONTENTS

Abstract ............................................. 1

Introduction ........................................... 1

Methods ............................................. 2

Analytical procedures .................................... 2

Q-mode factor analysis .......................... 4

A Q-mode model for granites of the Owl Creek Mountains ................ 8

Anomalous oxides and samples .............................. 8

Determination of end members .............................. 9

Evaluation of the model .................................. 13

Protolith considerations ............................ 14

Magma genesis .......................................... 1?

Summary and conclusions .................................... 18

References ............................................ 20

ILLUSTRATIONS

FIGURE 1. Generalized geologic map and sample locations .......................

2. Factor-variance diagram ....................................

3. Stereographic projection of three-factor solution ......................

4. Chondrite-normalized rare-earth-element diagram ......................

5. Ternary diagram of normative quartz, albite, and orthoclase ...............

6. Ternary diagram of normative feldspar compositions ...................

7. Plot of whole-rock 5180 values versus percent of end member La in starting material

2

10

11

12

15

19

20

TABLES

Page

TABLE 1. Original and computed chemical data and normative mineralogy ............................... 3

2. Chemical data and normative mineralogy for samples not used in the factor solution .................. g

3. Miscellaneous chemical data not used in the factor solution ................................. 7

4. Oxygen isotope compositions ................................................... 7

5. Proportions of the variance of 37 oxides in 39 samples accounted for by factor models with 2 through 10 factors . . g

6. Proportions of the variance of 29 oxides in 25 samples accounted for by factor models with 2 through 10 factors . . 9

7. Compositions of the end members for the factor model ................................... 13

8. Mixing proportions for the factor model ............................................ 14

9. Proportional differences between original chemical data and data represented by the factor solution .......... 15

10. Proportional differences between original chemical data and data derived by the factor solution for samples not used to

	develop the factor solution ................................................... ig

VII

CHEMICAL AND ISOTOPIC STUDIES OF GRANITIC ARCHEAN ROCKS, 

OWL CREEK MOUNTAINS, WYOMING

GEOCHEMISTRY AND PETROGENESIS OF AN ARCHEAN GRANITE 

FROM THE OWL CREEK MOUNTAINS, WYOMING

By J. S. STUCKLESS, A. T. MIESCH, and D. B. WENNER

ABSTRACT

Analytical data consisting of 37 oxides in 39 samples of granite 

from the Owl Creek Mountains, Wyo., have been examined by extended Q-mode factor analysis. The results show that 25 samples are 

cogenetic and that 29 oxides in these samples have retained a magmatically controlled distribution. For these samples and oxides, a 

five-factor model can account for most of the variance, and therefore 

the compositional data can be explained as the product of mixing 

(or unmixing) of five end members. The mixing (or unmixing) may 

have resulted entirely from fractional crystallization in two distinct 

stages, or alternatively, the first stage may have been mostly fractional fusion. Either interpretation can be correlated with variations 

in whole-rock values of 818O. These variations reflect inhomogeneities 

in the protolith, which was probably similar to the highly diverse 

suite of metamorphic rocks intruded by the granite. The two-stage 

history indicated by either interpretation of the model is compatible 

with derivation of the granite or initial differentiation at approximately 3 kb and near water-saturated conditions, followed by differentiation at a lower pressure of perhaps 1.0-0.5 kb. Although extended Q-mode factor analysis cannot uniquely identify the exact end 

members that controlled evolution of the granite, it has identified 

the types of processes necessary to explain the chemical complexities 

observed for the pluton, and the model presented here is compatible 

with all available data.

INTRODUCTION

The major uranium deposits of Wyoming are located 

within sedimentary rocks derived from the Archean 

granites of central Wyoming (Seeland, 1976, 1978). 

Isotopic studies have shown that most of these granites 

lost large quantities of uranium (Rosholt and Bartel, 

1969; Rosholt and others, 1973; Nkomo and others, 

1978; Stuckless and Nkomo, 1978; Nkomo and others, 

1979) at about the same time that the uranium deposits 

were formed (Ludwig, 1978, 1979). The genesis of these 

granites is therefore of economic interest. The granite 

of the Owl Creek Mountains (fig. 1) not only is closely 

associated with uranium deposits in overlying Tertiary 

sediments, but locally hosts near-economic deposits of 

uranium (Yellich and others, 1978).

The Owl Creek Mountains consist of a discontinuous 

series of east-west-striking fault blocks that are composed of Archean metamorphic rocks, granites, and 

diabase dikes (Thaden, 1980a, b, and c). The metamorphic rocks are a multiply-folded and petrologically complex assemblage of volcanic and sedimentary precursors 

that can be subdivided into three roughly east-weststriking groups (Hamil, 1971). The southern group is 

dominated by metasediments, with subordinate 

metavolcanic rocks that are preserved as biotite-muscovite schists. The middle group consists of diverse 

lithologies, including schists that contain biotite, garnet, hornblende, cummingtonite, cordierite, sillimanite, 

andalusite, and sericite, as well as quartzite, ironformation, and minor siliceous marble. The northern 

group has the largest metavolcanic component. Rock 

types here include hornblende or biotite quartzplagioclase gneisses and small amounts of crossbedded 

quartzite. Uralatized gabbroic dikes also occur in this 

group (Condie, 1967). The highest metamorphic grade 

observed in the three zones is the sillimanite-cordieritemuscovite-almandine subfacies of the Abukuma type of 

metamorphism (Winkler, 1967). Metabasalts and 

metadacites, which are presumably part of the northern 

group, are exposed in the Wind River Canyon (near 

locality 24, fig. 1). These yield a rubidium-strontium 

whole-rock age of 2,755±100 Ma (Mueller and others, 

1981).

The metamorphic sequence is intruded by granitic 

rocks and associated pegmatites which have reported 

ages of 2,645±60 Ma (lead-lead whole-rock, Nkomo and 

others, 1978) and 2,663 and 2,584 Ma (rubidium-strontium for pegmatitic muscovite and microcline respectively, Giletti and Gast, 1961). The contact between the 

intrusive and the metamorphic rocks commonly forms 

a wide mixed zone (fig. 1). Within the mixed zone the 

metamorphic rocks and xenoliths of these rocks are 

generally migmatitic (Thaden, 1980b). Modal analyses 

(Nkomo and others, 1978) show that the intrusive rock 

is a granite according to the IUGS classification (Streckeisen, 1973).
439 108°10' 

30' I

GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

______05'________ 108°DD'________________W_______

107°50'

43° 

22' 

3D"

POST-PROTEROZOIC COVER 

DIABASE DIKES

GRANITE 

METAMORPHIC ROCKS

SAMPLE LOCALITY 

TDG13

TDG12, DP-1 

DP-8 

DP-2 

GP-5

6 TDG44.TDG45

7 4343

8 DP-3

9 23432

10 58-66, 58-80, 58-98,

58180,58188.58317 

58392, 58398

11 TDG46, TDG47

12 41591,41596

13 30501,30511,29394,

29467

14 32466,32496

15

16

17

18

19

20

21

22

23

24

25

GP-4

TD6

GP-3

DKCH-1

GP-1

GP-2

TD22

TDG21

TDG20

TDG16

TDG24

FIGURE 1. Generalized geologic map of the Owl Creek Mountains (modified from Thaden, 1980a, b, and c) showing sample localities. 

Locality numbers are used as identification numbers on subsequent plots, and where more than one sample were collected at a 

single locality, samples are distinguished by an alphabetic suffix that corresponds to the order shown on figure 1. For example, 

sample TDG12 has a plot identification of 2a.

The granite and metamorphic rocks are cut by 

diabase dikes that trend north to northeasterly. Potassium-argon whole-rock analyses for east-west-trending 

dikes just west of the area shown on figure 1 yield 

ages of 1,900-2,100 Ma (Condie and others, 1969).

The crystalline rocks were uplifted and eroded during 

late Precambrian time. Topographic highs formed on 

this erosional surface protrude 60-100 m into the Paleozoic sedimentary rocks (Nkomo and others, 1978). 

Periods of sedimentation continued through the Paleocene and attained a total thickness of more than 7000 

m of shallow marine and coastal plain sediments. The 

Laramide orogeny, which began in Late Cretaceous 

time, was accompanied by faulting that uplifted the Owl 

Creek Mountains more than 5300 m (Keefer and Love, 

1963) and caused a stratigraphic separation of more 

than 7000 m along the south side of the range (Nkomo 

and others, 1978). The faulting, which may have occurred along zones of weakness in the Precambrian 

basement, was probably responsible for the development of brecciated zones along the southern side of the 

range (Yellich and others, 1978; Bramlett and others, 

1982). Brecciation was particularly intense near the 

north Canning area (sample localities 4-14, fig. 1). This 

brecciated zone was mineralized with uranium during 

the Cenozoic (Nkomo and others, 1978). Most recently, 

the Owl Creek Mountains were partly buried by volcanic detritus during the late Eocene and reexhumed during the Quaternary.

METHODS 

ANALYTICAL PROCEDURES

Major-element concentrations (reported in weight 

percent, tables 1, 2, and 3) were obtained by two 

methods. Most samples were analyzed by the single so-
GEOCHEMISTRY AND PETROGENESIS <

TABLE 1. Original chemical data and CIPW normative mineralogy (0) for 25 samples from the Owl Creek Mountains, Wyo., and

corresponding data represented by the factor solution (R)

Sample No. DP-3 23432 29394 29467 

Plot I.D. 8 9 13c 13d

30501 30511 32466 32496 

13a 13b 14a 14b

OROROROROROROROR

Weight percent

Si02 71.89 72.24 73.92 73.52 70.87 71.21 71.45 71 

A1 20 3 14.88 14.77 14.38 14.52 14.94 14.87 15.01 14 

FeO(t) 2.07 2.07 1.53 1.40 2.92 2.80 2.41 2 

MgO .83 .59 .28 .35 1.01 .91 .78 

CaO 1.90 1.71 1.31 1.53 2.12 2.09 1.91 1 

Na 2 0 4.19 4.07 4.02 4.03 4.34 4.47 4.23 4 

K 20 3.69 4.09 4.22 4.32 3.23 3.04 3.73 3 

Ti02 .30 .25 .16 .14 .33 .36 .27

.61 72.81 72.69 72.29 72.36 73.24 72.94 73.34 73.44 

.97 14.52 15.63 14.54 14.68 14.53 14.72 14.39 14.46 

.31 1.89 1.93 2.24 2.14 1.65 1.58 1.53 1.52 

.70 .68 .56 .67 .65 .30 .41 .25 .38 

.93 1.73 1.72 1.92 1.82 1.60 1.59 1.60 1.49 

.47 4.06 4.15 4.14 4.20 4.11 4.03 4.12 4.01 

.53 3.86 3.89 3.74 3.68 4.21 4.37 4.43 4.36 

.26 .24 .23 .26 .27 .17 .17 .15 .16

Parts per million

MnO 366 360 234 242 537 539 447 439 

Rb 20 205 200 188 195 213 182 217 191 

BaO 1137 931 797 785 693 818 722 802 

SrO 314 267 233 233 281 309 276 296 

Th0 2 43.80 41.15 28.05 26.91 48.55 45.68 29.03 31 

La 2 03 64.87 60.74 22.37 25.81 82.64 79.67 41.53 48 

Ce2 0 3 122.8 103.5 37.66 41.59 133.5 132 67.75 76 

Nd 2 03 36.33 34.96 11.95 14.11 43.56 45.46 24.14 26 

Sm20 3 4.96 4.81 1.71 2.02 5.65 6.32 3.63 3 

Eu 2 03 1.07 .866 .650 .707 .990 .979 .810 

Gd 20 3 3.41 2.60 1.06 1.55 3.42 3.68 3.26 2 

Tb 2 03 .44 .36 .16 .23 .48 .51 .44 

Dy 2 03 2.36 1.89 .93 1.34 2.57 2.80 2.37 2 

Tm203 .140 .099 .070 .083 .150 .148 .120 

Yb 20 3 .81 .52 .42 .46 .80 .76 .60 

Lu 20 3 .110 .066 .060 .062 .100 .095 .080 

Ta 2 0 5 2.04 .78 .44 .53 .69 .80 .59 

Hf02 4.62 4.59 4.15 3.57 4.77 5.73 4.43 4 

Zr0 2 223 203 187 155 263 253 207 224 

Sc 20 3 4.04 3.99 2.71 2.75 5.46 5.83 4.45 4 

CoO 5.56 4.47 2.44 2.70 7.29 6.80 5.39 5

326 352 321 393 214 265 329 273 

209 192 194 186 174 197 212 207 

811 808 788 838 904 885 720 753 

289 259 296 273 238 251 227 224 

.44 39.29 35.49 41.97 40.16 27.48 30.58 27.84 28.85 

.98 51.44 48.36 66.12 60.11 36.41 36.12 32.43 28.86 

.80 81.80 80.25 105.7 101 55.96 59.94 53.56 46.79 

.09 27.76 27.52 35.08 34.53 21.16 19.88 18.75 16.38 

.70 3.86 3.87 4.57 4.75 3.03 2.71 2.52 2.56 

.806 .780 .817 .880 .889 .650 .753 .620 .687 

.87 1.84 2.41 2.33 2.62 1.94 1.71 2.15 2.14 

.43 .29 .34 .36 .36 .27 .25 .32 .33 

.50 1.73 1.90 2.07 1.92 1.35 1.36 1.80 1.91 

.146 .120 .108 .110 .101 .060 .077 .110 .124 

.78 .65 .57 .60 .52 .32 .41 .62 .69 

.100 .090 .074 .070 .066 .040 .055 .090 .094 

.63 .87 .67 .39 .68 .31 .57 .72 .73 

.87 3.96 4.42 4.45 4.70 3.71 3.75 4.15 3.97 

182 192 235 205 169 169 181 169 

.99 4.85 3.89 4.24 4.20 2.95 2.99 2.87 3.29 

.42 4.79 4.22 5.22 4.84 2.56 3.17 2.44 2.95

Norms (weight percent)

Q 26.58 26.88 30.00 28.71 25.02 25.67 25.49 25 

C .53 .55 .82 .43 .45 .43 .54 

Or 21.88 24.20 25.00 25.58 19.12 17.99 22.07 20 

Ab 35.56 34.48 34.09 34.13 36.80 37.92 35.87 37 

An 9.43 8.49 6.50 7.62 10.54 10.38 9.51 9 

Wo .00 .00 .00 .00 .00 .00 .00 

En 2.07 1.47 .70 .87 2.52 2.26 1.94 1 

Fs 3.38 3.46 2.59 2.38 4.92 4.65 4.07 3 

11 .57 .47 .31 .27 .63 .69 .52

lution technique (Shapiro and Brannock, 1962; Suhr and 

Ingamells, 1966). Stated accuracies in terms of percent 

of amount present are ±1 percent for Si02, ±2 percent 

for Al20s, and ±1 to 10 percent for the remaining major 

oxides. Sample numbers prefixed by TD were analyzed 

using high precision X-ray fluorescence spectrometry 

(Taggart and others, 1982), and ferrous/ferric ratios 

were determined by wet chemical methods (Peck, 

1964). Precision for the X-ray fluorescence results is 

±2 percent of the amount present (2a). 

Minor- and trace-element concentrations (those reported in parts per million, tables 1, 2, and 3, except 

for U, Th, Rb, Sr, and Zr) were determined by neutron 

activation analysis using methods modified from those 

of Gordon and others (1968). Estimated accuracies 

range from 5 to 20 percent of the amount present ex-

.21 28.33 27.71 27.16 27.46 28.13 27.59 27.50 28.54 

.29 .52 .46 .20 .50 .30 .47 .00 .44 

.87 22.84 23.00 22.11 21.76 24.91 25.86 26.20 25.78 

.88 34.43 35.22 35.09 35.56 34.82 34.19 34.95 33.99 

.61 8.58 8.55 9.53 9.03 7.97 7.88 7.68 7.41 

.00 .00 .00 .00 .00 .00 .00 .13 .00 

.75 1.70 1.39 1.66 1.61 .75 1.02 .63 .96 

.89 3.13 3.24 3.74 3.57 2.80 2.68 2.63 2.60 

.50 .46 .43 .50 .51 .32 .32 .29 .30

cept for a few samples for which high uranium contents 

produced large interferences for a few rare earth elements (REE). For these samples, the data reported 

in tables 1 and 2 were obtained by extrapolation between the chondrite normalized abundances of the actfacent REE. Chondrite values of Evensen and others 

(1978) are used for normalization throughout this paper. 

Most uranium and thorium concentrations were obtained by isotope dilution and mass spectrometry 

(Stuckless and others, this volume). These values were 

supplemented by delayed neutron determinations for 

uranium (Millard, 1976) and -y-ray spectrometry determinations for thorium (Bunker and Bush, 1966). The 

general accuracy of reported uranium and thorium concentrations is ±2 percent (2cr). 

Most rubidium and strontium concentrations were
4 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

TABLE 1. Original chemical data and CIPW normative mineralogy (0) for 25 samples from the Owl Creek Mountains, Wyo., and corresponding data represented by the factor solution (R) Continued

Sample No. GP-3 GP-5 4343 TDG12 

Plot I.D. 17 5 7 2a

OROROROR

TD-13 TDG21 TD22 TDG47 58180 

1 22 21 lib lOd

OROROROROR

Weight percent

Si02 74.89 74.85 75.55 75.86 73.76 73.72 74.16 74.33 74 

A1 203 13.16 13.39 14.13 13.76 14.03 13.80 13.62 13.78 13 

FeO(t) 1.17 1.39 .30 .61 1.71 2.06 2.11 1.58 1 

MgO .21 .08 .10 .07 .32 .49 .20 .31 

CaO .32 .08 1.39 1.18 1.21 .99 .96 .89 

Na 20 2.49 2.73 3.68 3.67 3.13 3.13 3.03 3.31 3 

K 2 0 7.48 7.26 4.68 4.67 5.35 5.33 5.63 5.46 5 

Ti02 .08 .05 .05 .06 .23 .26 .12 .15

Parts per

MnO 134 229 96 98 351 337 342 271 320 

Rb2 0 429 394 202 193 274 274 303 280 275 

BaO 776 697 556 588 978 940 606 746 762 

SrO 49 5 122 156 147 140 98 121 104 

Th02 50.97 49.91 21.18 23.23 69.00 69.80 35.13 48.48 57 

La2 03 45.61 36.82 6.11 2.78 96.21 94.88 50.57 50.79 66 

Ce2 03 87.95 72.24 11,10 5.51 190.6 175.4 84.92 93.31 128 

Nd2 03 35.13 29.23 3.81 3.09 75.18 62.08 30.93 34.40 43 

Sm2 03 7.23 6.78 .66 .72 11.78 9.54 5.72 6.02 7 

Eu 2 03 .300 .274 .730 .612 1.06 .873 .640 .627 

Gd 2 03 7.36 6.85 .690 .746 4.18 4.93 4.16 4.36 4 

Tb2 03 1.25 1.11 .13 .12 .66 .67 .59 .66 

Dy2 03 7.60 6.54 .79 .69 3.89 3.48 3.18 3.70 3 

Tm203 .575 .472 .070 .058 .250 .209 .200 .251 

Yb2 03 3.34 2.77 .44 .35 1.44 1.16 1.14 1.44 1 

Lu 203 .480 .389 .070 .053 .200 .158 .150 .199 

Ta2 05 3.59 3.09 .96 .51 1.86 2.03 2.28 1.83 2 

Hf02 7.20 6.25 2.68 2.69 7.23 5.91 4.68 5.21 5 

Zr02 293 217 100 92.7 302 215 193 190 205 

Sc 20 3 5.32 5.50 1.16 1.08 5.26 4.48 3.42 4.15 4 

CoO 1.15 .63 .40 .39 3.02 3.40 2.17 2.21 1

.34 74.20 74.50 74.53 74.76 74.98 76.75 76.80 76.19 76.02 

.92 13.76 14.08 13.83 13.72 13.59 12.69 12.83 12.95 13.19 

.55 1.71 1.46 1.35 1.53 1.36 1.50 1.20 .89 1.21 

.30 .35 .31 .21 .20 .24 .30 .35 .27 .26 

.82 .88 .64 .78 .60 .80 1.25 1.23 .91 .98 

.33 3.17 3.27 3.11 3.17 3.04 3.65 3.68 3.19 3.30 

.40 5.53 5.45 5.89 5.75 5.64 3.57 3.62 5.08 4.73 

.14 .19 .11 .13 .10 .15 .16 .19 .36 .16

mi 1 1 i on

281 190 199 163 205 329 279 299 221 

280 279 282 232 268 169 191 231 229 

844 909 879 726 840 263 203 682 545 

123 88 116 85 109 136 107 130 104 

.42 57.86 59.01 51.11 55.40 56.78 39.85 39.88 56.76 48.99 

.71 68.78 57.59 53.23 48.85 62.30 29.66 29.64 47.96 46.07 

.8 127.7 102.6 100.7 95.13 118.4 50.88 53.31 84.30 86.98 

.54 45,93 32.96 36.18 36.55 42.64 22.21 23.01 28.24 33.08 

.37 7.48 5.27 6.04 6.50 6.85 4.45 4.21 4.48 5.52 

.640 .725 .570 .651 .650 .733 .720 .728 .740 .736 

.56 4.53 3.40 3.78 4.38 3.72 3.51 2.96 2.96 3.18 

.60 .65 .50 .55 .55 .52 .51 .44 .46 .45 

.01 3.52 2.74 3.00 2.55 2.70 2.68 2.43 2.49 2.37 

.190 .228 .220 .200 .120 .175 .190 .174 .180 .161 

.06 1.30 1.29 1.16 .61 1.01 1.01 .98 1.08 .92 

.140 .179 .190 .162 .080 .140 .140 .136 .160 .128 

.07 1.94 1.63 1.79 .80 1.76 .93 1.06 2.29 1.40 

.17 5.41 4.73 4.72 4.35 4.78 4.24 4.52 4.27 4.51 

196 109 171 97.9 164 87.4 132 153 141 

.39 4.10 3.00 3.24 2.93 3.01 2.38 3.07 3.46 2.77 

.78 2.47 1.57 1.45 1.51 1.54 1.83 2.02 1.29 1.53

Norms (weight percent)

Q 29.90 29.82 32.90 33.58 30.80 30.81 31.00 30.60 30 

C .39 .90 .48 .53 .89 1.10 .81 .80 1 

Or 44.28 42.94 27.67 27.61 31.67 31.54 33.32 32.32 31 

Ab 21.13 23.12 31.20 31.09 26.53 26.50 25.65 28.10 28 

An 1.59 .42 6.92 5.85 6.02 4.91 4.76 4.42 4 

Wo .00 .00 .00 .00 .00 .00 .00 .00 

En .52 .19 .25 .18 .81 1.22 .50 .77 

Fs 2.04 2.52 .48 1.05 2.84 3.43 3.74 2.70 2 

11 .15 .09 .09 .11 .44 .49 .23 .29

measured by isotope dilution and mass spectrometry 

(Hedge and others, this volume). These determinations 

were supplemented by X-ray fluorescence analyses, 

which have a general accuracy of ± 5 percent. All zirconium concentrations were obtained by X-ray fluorescence for which the estimated precision is ±5 percent 

(2a). 

Oxygen isotopic compositions (table 4) were determined according to the methods of Taylor and Epstein 

(1962). Results are reported relative to SMOW (standard mean ocean water) and are accurate to ±0.1 per 

thousand (2a).

Q-MODE FACTOR ANALYSIS

The mathematical method used to develop the petrogenetic model for the granites of the Owl Creek

.96 30.90 31.71 30.94 31.53 32.63 37.56 37.55 35.42 35.37 

.12 .96 1.65 .93 1.19 1.02 .54 .63 .56 .86 

.94 32.73 32.26 34.86 34.00 33.38 21.13 21.40 30.06 27.99 

.21 26.90 27.68 26.34 26.90 25.79 30.95 31.14 27.01 27.95 

.06 4.38 3.19 3.86 3.00 4.00 6.20 6.10 4.50 4.87 

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 

.75 .88 .76 .52 .51 .60 .76 .87 .67 .65 

.68 2.89 2.54 2.32 2.67 2.29 2.55 1.95 1.11 1.99 

.27 .36 .21 .24 .19 .29 .31 .36 .68 .31

Mountains is the extended form of Q-mode factor analysis (Miesch, 1976a, b, 1981). The main difference between conventional Q-mode factor analysis and the extended form is that the extended method allows conversion of the scaled analyses back to the original metric 

so that the final model can be more easily interpreted 

in petrologic terms.

For extended Q-mode factor analysis, the analytical 

data for each oxide variable are scaled so that each 

variable has the same mean and variance; this assures 

that the major oxides, especially Si02 and A1203, will 

have no greater influence than the minor constituents 

in model development. The scaled analytical data are 

conceptually represented as N vectors in Mdimensional space, where N is the number of samples
GEOCHEMISTRY AND PETROGENESIS

TABLE 1._Original chemical data and CIPW normative mineralogy (0) for 25 samples from the Owl Creek Mountains, 

Wyo., and corresponding data represented by the factor solution (R) Continued

Sample No. 41591 41596 OP-8 DP-2 

Plot I.D. 12a 12b 3 4

0 R 0 R 0 R 0

Weight

TD646 58-66 58-80 58-98 

lla lOa lOb

ROROROROR

percent

Si02 71.36 71.88 74.65 74.50 73.25 73.09 73.83 73.71 75.36 75.19 76.21 76.46 77.29 76.84 76.38 76.44 

A1 2 03 15.00 14.87 14.10 14.32 14.87 14.44 14.43 14.41 13.62 13.73 13.07 13.35 12.95 12.99 13.51 13.29 

FeO(t) 2.26 2.25 1.11 .89 1.23 1.78 1.04 1.38 1.50 1.16 .90 .64 .66 .89 .72 .83 

MgO .70 .68 .16 .14 .37 .46 .30 .32 .30 .28 .10 .05 .25 .19 .14 .19 

CaO 2.01 1.87 1.26 1.28 1.32 1.50 1.39 1.40 1.10 1.28 .87 .93 1.10 1.02 1.19 1.22 

Na20 4.33 4.26 3.57 3.76 4.45 4.16 3.98 3.85 3.73 3.80 3.42 3.43 3.41 3.41 3.77 3.71 

K 2 0 3.82 3.70 4.94 4.88 4.15 4.23 4.68 4.61 4.20 4.29 5.23 4.97 4.12 4.42 4.07 4.09 

Ti02 .28 .27 .05 .07 .18 .17 .17 .14 .06 .13 .08 .06 .10 .13 .12 .11

Parts per million

MnO 465 409 166 127 347 349 209 229 230 228 136 114 111 179 130 176 

Rb 2 0 208 188 205 202 188 226 172 209 193 205 253 222 216 207 181 187 

BaO 897 894 793 800 677 629 937 814 553 515 466 493 414 396 309 377 

SrO 300 288 260 198 203 217 184 214 131 162 112 105 113 97 119 133 

Th02 36.65 39.79 23.72 24.19 28.44 25.79 30.13 31.52 33.25 29.93 33.27 29.95 33.27 40.44 29.71 29.06 

La 203 71.58 62.43 11.76 12.45 21.39 24.35 27.66 31.26 12.28 19.45 7.10 8.11 30.62 27.33 11.73 11.38 

Ce2 03 112.5 104.2 20.29 21.29 38.16 35.63 49.30 53.36 17.24 32.93 14.25 17.35 63.96 52.65 23.71 20.47 

Nd 203 38.33 35.22 7.39 7.10 16.12 13.52 19.38 18.38 8.26 13.44 7.25 8.59 29.03 21.69 10.38 9.93 

Sm2 03 4.83 4.79 1.47 1.06 3.57 2.63 2.67 2.74 1.86 2.50 1.74 2.03 4.61 3.87 2.14 1.99 

Eu 2 03 .990 .894 .610 .637 .650 .599 .740 .704 .630 .648 .570 .564 .730 .695 .580 .658 

Gd 2 03 2.76 2.68 1.16 .854 3.70 3.20 2.02 1.87 1.47 2.19 1.72 1.87 2.70 2.39 1.45 1.62 

Tb 203 .41 .37 .17 .13 .61 .52 .31 .28 .23 .34 .28 .30 .39 .35 .21 .25 

Dy2 03 2.37 1.99 .95 .75 3.60 3.19 1.75 1.54 1.44 1.97 1.68 1.70 2.11 1.83 1.15 1.39 

Tm2 03 .120 .103 .060 .052 .220 .218 .100 .097 .120 .140 .140 .134 .140 .134 .090 .107 

Yb 2 03 .62 .53 .31 .31 1.23 1.23 .58 .54 .70 .80 .82 .80 .82 .77 .52 .62 

Lu2 03 .080 .066 .040 .045 .160 .167 .070 .074 .100 .111 .110 .115 .120 .109 .070 .088 

Ta205 .90 .68 .26 .51 .69 .99 .61 .74 .62 .84 1.08 1.00 .83 1.09 .59 .69 

Hf02 45.11 4.77 2.30 2.83 3.89 4.69 4.55 3.72 5.83 3.87 2.22 3.35 4.00 3.92 3.65 3.43 

Zr02 189 215 107 118 164 198 154 156 131 139 89.4 103 144 112 126 106 

Sc 203 4.06 4.43 1.08 1.52 6.08 4.60 2.97 2.74 2.27 2.76 1.88 1.67 2.06 2.07 2.12 1.88 

CoO 5.63 5.16 1.17 1.12 3.10 3.61 2.40 2.44 1.89 1.88 .72 .15 1.11 .91 .81 1.06

Norms (weight percent)

Q 24.51 26.31 31.21 30.32 27.22 27.46 28.64 29.15 33.59 32.67 33.63 34.97 38.50 37.04 35.59 35.72 

C .08 .47 .61 .51 .67 .30 .29 .53 .94 .51 .20 .64 .87 .75 .72 .55 

Or 22.64 21.88 29.21 28.90 24.55 25.01 27.68 27.30 24.83 25.36 30.93 29.39 24.35 26.13 24.08 24.22 

Ab 36.69 36.07 30.22 31.86 37.72 35.27 33.73 32.65 31.62 32.20 28.96 29.03 28.91 28.84 31.96 31.40 

An 10.00 9.28 6.26 6.37 6.53 7.43 6.92 6.96 5.46 6.36 4.34 4.62 5.48 5.07 5.92 6.05 

Wo .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 

En 1.76 1.69 .40 .36 .93 1.16 .74 .80 .75 .69 .25 .14 .63 .46 .35 .47 

Fs 3.78 3.77 2.00 1.54 2.02 3.06 1.67 2.34 2.70 1.97 1.55 1.11 1.07 1.46 1.14 1.38 

11 .54 .52 .10 .14 .35 .32 .32 .27 .12 .25 .15 .11 .19 .25 .23 .22

and M is the number of oxide variables. Dividing each 

scaled analysis by the square root of the sum of the 

squared values adjusts each vector to unit length. The 

M-reference axes for the vector system (each representing one of the oxide variables) are then replaced 

by the principal-component axes. The first principalcomponent axis represents the average composition of 

all samples and lies at the center of the sample-vector 

cluster. The second principal-component axis lies at a 

right angle to the first and is oriented so that the sample vectors have as high a projection on it as possible. 

Similarly the third principal-component axis lies at 

right angles to the first two, but again is so oriented 

that the sample vectors have as high a projection on 

it as possible. There are M principal-component axes 

in all, the same as the number of oxide variables.

The coordinates (projections) of the sample vectors 

with respect to the principal-component axes constitute 

a new data set that describes the composition of samples in terms of the principal components rather than 

in terms of the M oxides. The advantage here is that 

the successive principal components are of decreasing 

importance in terms of explaining compositional variation in the sample set. Commonly the compositional 

variability represented by the first few principal components (denoted by m) is almost as great as that represented by all M components. Consequently, elimination of all but the first several principal components 

will yield a data set that is not drastically different 

from that represented by all the principal components. 

The simplification thus attained greatly facilitates the 

development of a petrogenetic model that will account
GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING 

TABLE 2. Chemical data and CIPW normative mineralogy for 14 samples not accounted for by the factor solution

Sample No. 

Plot I.D.

58188 

lOe

58317 

lOf

58392 

10g

58398 

lOh

DP-1 

2b

GP-1 GP-2 GP-4 

19 20 15

TD-6 

16

TDG16 

24

TDG20 

23

TDG24 

25

TDG44 

6a

TDG45 

6b

Weight percent

Si0 2

A1 2 03

FeO(t)

MgO

CaO

Na20

K2 0

Ti02

71.06

15.05

2.62

1.32

3.56

3.76

2.13

.30

76.35

13.29

.59

.15

1.20

3.80

4.50

.06

49.90

15.38

11.44

8.05

9.57

2.65

1.32

1.43

52.82

15.30

10.38

8.06

7.75

2.89

1.65

.86

75.22

14.20

.49

.04

.87

4.60

4.50

.02

75.79 70.54 71.35

14.92 18.21 17.33

.51 .36 .89

.04 .04 .20

.23 .14 6.05

5.81 10.31 3.53

2.50 .29 .42

.01 0.00 .09

74.10

14.11

.29

.05

.03

2.32

9.00

.01

76.09

14.00

.64

.05

.91

4.26

4.0D

.01

73.69

15.08

1.20

.05

.32

4.42

4.96

.01

74.58

14.11

1.29

.20

.73

4.13

4.84

.05

75.55

12.96

1.34

.20

.34

2.41

6.94

.07

50.97

13.46

14.68

5.90

10.91

1.83

.84

1.13

Parts per million

MnO

Rb2 0

BaO

SrO

Th02

La203

Ce 2 03

Nd 2 03

Sm2 03

Eu 20 3

Gd203

Tb2°3

Dy 2 03

Tm203

Yb2 03

Lu 20 3

Ta 2 0 5

Hf0 2

Zr02

Sc 2 03

CoO

416

131

801

345

13.30

23.52

36.98

12.65

2.19

.616

1.61

.25

1.35

.104

.65

.097

.56

3.38

189

5.84

7.57

86

227

112

69

30.16

3.95

8.80

5.23

1.36

.471

1.59

.29

1.93

.183

1.14

.173

.81

2.24

65.7

.98

.51

1630

62

356

236

1.75

12.24

25.10

13.87

3.64

1.30

3.44

.59

3.74

.326

2.02

.301

.45

2.73

155

28.7

70.7

1706

86

570

175

2.18

9.44

19.67

11.27

2.95

1.04

3.29

.63

4.33

.472

3.10

.482

.38

2.01

105

64.6

52.8

147

217

40

12

9.30

5.31

10.04

3.59

.72

.059

.807

.14

.92

.081

.50

.075

.32

3.00

76.2

.97

.08

898 940 125

725 22 37

131 6 136

9 8 703

5.94 3.23 44.03

1.71 .53 34.51

4.49 1.41 67.52

3.34 1.04 23.04

1.06 .34 3.63

.042 .037 .445

1.37 .438 2.24

.25 .08 .41

1.69 .54 2.59

.155 .051 .219

.96 .33 1.32

.139 .050 .188

58.2 23.6 1.60

1.46 2.83 4.33

42.7 59.5 183

1.03 .08 4.03

.25 .10 .94

57

598

142

29

2.44

2.48

5.03

2.70

.63

.175

.965

.21

1.43

.127

.88

.138

2.14

.24

27.0

.34

.63

71

148

77

44

6.04

5.18

6.78

2.91

.67

.197

1.23

.30

2.35

.233

1.50

.226

.52

.69

49.7

1.12

.71

2042

403

114

19

8.27

2.22

4.39

2.53

.56

.100

.811

.25

1.67

.155

.98

.147

17.4

4.87

75.6

.40

.96

104

191

296

56

19.01

20.67

35.15

13.72

2.65

.289

2.55

.39

2.36

.150

.83

.108

.87

2.45

17.6

3.23

1.13

841

521

273

52

33.26

9.99

20.04

9.48

2.19

.291

2.43

.49

3.54

.557

4.16

.754

8.58

4.24

28.1

6.91

1.56

2379

26

162

135

2.40

9.30

19.20

11.40

3.12

.909

3.66

.63

4.15

.431

2.88

.450

.30

2.40

47.2

61.2

46.4

Norm (weight percent)

Q

C

Or

Ab

An

Wo

En

Fs

Fo

Fa

11

29.47

.08

12.63

31.88

17.68

.00

3.30

4.39

.00

.00

.58

33.82

.00

26.59

32.15

5.94

.00

.37

1.00

.00

.00

.11

0.00

.00

7.83

22.43

26.19

8.91

6.26

5.92

9.67

10.08

2.71

0.00

.00

9.78

24.52

23.89

6.10

12.14

10.86

5.58

5.50

1.63

28.91

.18

26.61

38.95

4.32

.00

.10

.90

.00

.00

.04

31.42 8.74 35.32

2.24 .69 .08

14.81 1.71 2.50

49.19 87.22 29.88

1.14 .70 30.04

.00 .00 .00

.10 .10 .50

1.09 .84 1.50

.00 .00 .00

.00 .00 .00

.02 .00 .17

25.80

.50

53.24

19.64

.15

.00

.13

.53

.00

.00

.02

33.45

1.00

23.63

36.07

4.53

.00

.13

1.17

.00

.00

.02

27.07

1.85

29.30

37.45

1.60

.00

.13

2.57

.00

.00

.02

29.15

.76

28.61

34.99

3.60

.00

.50

2.30

.00

.00

.10

32.82

.86

41.06

20.42

1.70

.00

.50

2.50

.00

.00

.13

0.00

.00

4.94

15.53

26.02

11.74

13.80

23.96

.63

1.21

2.15

for suitably large proportions of the variations in all 

M oxides.

Retention of only the first m principal components 

projects the sample vectors from an M-dimensional 

space into an m-dimensional space (where M is greater 

than m). Because projection moves vectors, the sample

vectors will represent slightly different compositions 

before and after projection. Methods described in reports cited previously can be used to derive the compositions represented by the vectors after projection, and 

comparison of these recomputed compositions with the 

original compositions is used to judge the degree to
GEOCHEMISTRY AND PETROGENESIS

TABLE 3. Miscellaneous chemical data for 39 samples from the Owl 

Creek Mountains, Wyo.

Sample No. Fe2 03 H 20 P 2 05 C02 U Cs Sb Cr

58-66 0.95 1.06 0.07 0.02 5.29 4.82 0.058 0.89 

58-80 .62 1.46 .05 .01 11.80 3.25 .071 1.76 

58-98 .69 .67 .08 .01 5.37 2.90 .038 1.28 

58180 .42 .65 .10 .02 160.5 2.16 .050 1.50 

58188 .86 2.60 .13 .43 5.77 3.03 .030 11.6

58317 .50 .66 .07 .11 2008 4.73 .090 .50 

58392 2.70 1.61 .16 .06 1.87 .61 .030 181 

58398 2.60 2.65 .15 .02 1.09 1.79 .030 247 

DP-1 .46 .57 .07 .02 2.87 3.00 .048 .71 

DP-2 .83 .76 .11 .01 14.58 2.47 .020 3.21

DP-3 1.50 .91 .15 .01 3.39 6.41 .050 18.8 

6P-1 .30 .94 .16 .02 .99 46.9 .380 1.90 

6P-2 .09 .57 .14 .02 .29 1.62 .950 1.90 

6P-3 .80 .83 .10 .01 9.22 11.4 .020 1.83 

6P-4 .80 1.24 .11 .01 17.00 1.72 .020 .35

6P-5 .11 .56 .06 .01 3.45 12.70 .220 .67 

23432 .47 .37 .09 .04 21.88 1.96 .103 23.9 

29394 .88 .68 .16 .02 11.94 4.54 .100 41.5 

29467 .77 .46 .17 .05 8.42 3.81 .115 34.7 

30501 .40 .50 .12 .04 9.30 2.72 .103 30.0

30511 .24 .53 .13 .04 9.16 2.82 .102 34.3 

32466 .50 .40 .08 .06 6.15 1.77 .020 25.9 

32496 .36 .33 .09 .06 14.44 2.68 .118 24.7 

41591 .61 .49 .12 .04 11.90 3.94 .103 35.7

41596 1.23 .21 .05 .03 12.80 1.68 .098 19.5

4343 .11 .74 .12 .02 16.80 5.74 .082 2.90 

DP-8 .46 .61 .12 .02 4.16 1.83 .020 6.11 

TD-6 .32 .04 .05 .00 .40 34.30 .176 32.1 

TD612 2.32 .24 .05 .00 11.54 5.10 .248 57.4 

TD-13 1.71 .49 .05 .00 3.72 3.21 .106 20.9

TD616 .70 .30 .05 .00 3.77 1.95 .086 23.3 

TD620 1.33 .08 .05 .00 2.42 3.60 .128 39.9 

TD621 1.59 .75 .05 .00 2.29 5.18 .056 15.8 

TD22 1.66 .66 .05 .00 2.44 2.02 .081 27.3 

TD624 1.42 .22 .05 .00 4.68 1.84 .205 45.1

TDG44 1.48 .10 .05 .00 14.08 7.87 .214 5.78 

TD645 16.00 .19 .05 .00 2.78 .42 .300 101 

TD646 1.65 .20 .05 .00 71.79 2.29 .275 14.8 

TDG47 1.64 .02 .05 .00 78.41 1.74 .197 5.47

which the projection distorts the data. The effect of 

projections for different m's is examined on factor-variance diagrams. If large proportions of the variances 

in the oxide variables can be retained only by setting 

ra to some value near M, little reduction in the complexity of the problem is possible; however, if the recomputed data derived for small ra's are nearly the 

same as the original data, the problem may be greatly 

simplified. By convention, the value of ra is referred 

to as the number of factors and is equal to the number 

of end members that must be mixed and (or) unmixed 

in a petrologic mixing model. 

Commonly, ra end members will be sufficient to account for the variation in all but a few of the oxide 

variables or in all but a few of the samples. It is always 

possible to account for the remaining unexplained variance in the oxides or samples by increasing ra, but 

doing so may also obscure a simple underlying compositional structure and greatly complicate the process of 

developing an acceptable mixing model that accounts 

for most of the oxides or samples or both.

TABLE 4. Oxygen isotope compositions for granitic and 

metamorphic rocks from the Owl Creek Mountains, Wyo.

[Values reported in parts per thousand relative to standard mean ocean water]

Sample S I8 0 Sample S I80

DP-3 6.9 4343 7.4 

23432 7.5 TDG13 8.1 

29394 7.7 TDG21 7.5 

29467 8.2 TD22 7.2 

30501 7.6 TDG47 8.1 

30511 7.8 58180 7.6

32466 7.5 58188 5.2 

32496 7.9 58317 8.1 

41591 8.0 58392 7.8 

41596 7.4 58398 4.9 

DP-8 7.8 TD-6 6.5 

DP-2 7.3 DP-1 5.8

TDG46 8.6 GP-2 7.3 

58-66 7.6 DKCH-167 6.7 

58-80 8.0 DKCH-207 5.1 

58-98 8.0 DKCH-263 6.9 

GP-3 7.8 DKCH-274 6.4 

GP-5 7.8 DKCH-279 6.5

The oxide variables not adequately accounted for by 

ra end members are easily identified on the factor-variance diagram. Samples not well accounted for are identified by comparatively large values of a badness-of-fit 

measure which is here defined as equal to the average 

absolute value of (Xcomp-Xobs)/Xobs across the M oxide 

variables, where Xobs is the observed (raw) oxide value 

and Xcomp is the corresponding value after projection 

of the sample vectors into ra-dimensional space.

The badness-of-fit measure has a second application, 

which is used in the current study. Methods described 

in the reports cited previously can be used to project 

sample vectors not included in the original analysis into 

the ra-dimensional vector system. After projection, the 

badness-of-fit measure can be used to test samples of 

doubtful relation to the main body or sequence for compositional compatibility with the main sequence. 

Once the ra-dimensional system of sample vectors has 

been established, the task is then to select ra reference 

vectors that represent ra petrologic phases that are 

thought to have been involved in the processes of mixing and (or) unmixing (precipitation). Methods that can 

be used to find ra petrologically meaningful reference 

axes have been summarized previously (Miesch, 1981), 

but they consist in general of either using compositions 

represented by hypothetical vectors in selected positions within the vector cluster, or using compositions 

of actual sample vectors providing that the compositions are adequately represented in the ra-dimensional 

cluster. One of the reference (end-member) vectors 

used in the current study is an actual sample vector. 

The others were hypothetical, and therefore it is
GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

improbable that they represent the exact compositions 

that controlled evolution of the granite. The hypothetical vectors do, however, represent the types of compositions needed to explain the observed data.

Finally, the derived model consists of m end-member 

compositions, each composition represented by one of 

the selected reference axes, and N sets of m mixing 

proportions. The end-member compositions are in the 

metric of the original data (weight percent), and the 

m mixing proportions for each of the N samples sum 

to unity. Positive mixing proportions indicate addition 

of the respective end member to the compositional system, whereas negative proportions indicate subtraction 

(fractionally crystallized solidus or residuum of fractional fusion). Mathematical mixing of the end-member 

compositions in the quantities indicated by the mixing 

proportions yields an approximation of the original 

data; the closeness of the approximation for each oxide 

and for a given value of m is given by the factor-variance diagram.

A Q-MODE MODEL FOR GRANITES 

OF THE OWL CREEK MOUNTAINS

Q-mode factor analysis was used for two different 

purposes. The first purpose was to identify cogenetic 

samples and oxides in these samples that had been little 

affected by postmagmatic processes. Non-cogenetic 

samples and oxides known to have been affected by 

postmagmatic events were included as a test to insure 

that the method correctly identified misfit data. The 

second purpose was to develop a petrogenetic model. 

It should be pointed out that end members used in the 

model are not unique, but are analogous to describing 

plagioclase compositions in terms of An5 and An95 . Such 

compositions would be as mathematically accurate as 

conventionally used end members and could be used 

if the true end members were unknown.

ANOMALOUS OXIDES AND SAMPLES

The total data set available for the study consisted 

of analyses for 37 oxides in 39 samples. Although it 

was known beforehand that a few of the oxide concentrations were controlled in large part by late-magmatic 

and weathering processes, that some of the samples 

were hydrothermally altered, and that others were of 

doubtful relation to the main bodies of granite, the entire data set was used to derive a factor-variance diagram. The data for the diagram (not shown) are given 

in table 5 and indicate that although most of the oxides 

can be accounted for by five or six factors, there are 

at least nine notable exceptions (Fe203 , FeO, H20, 

P205, C02, U02 , Sb203 , Cr203 and Cs20). H20 and C02 

contents and the oxidation state of iron are known to

TABLE 5. Proportions of the variances of 37 oxides in 39 samples 

accounted for by factor 'models with 2 through 10 factors

Number of factors

Oxide 3 7 10

Si02

A1 203

Fe203

FeO

MgO

CaO

Na 20

K 2 0

H 20

Ti0 2

P 2 °5

C02

MnO

Rb20

BaO

SrO

Th02

U02

La 2 0 3

Ce2 03

Nd 20 3

Sm203

Eu 20 3

6d 203

Tb2 0 3

Dy203

Tm2 0 3

Yb 20 3

Lu 2 03

Cs 2 0

Ta 2 0 5

Hf0 2

Zr0 2

Sb2 03

Sc 2 0 3

Cr20 3

CoO

0.16 

.09 

.08 

.03 

.15 

.10 

.08 

.15 

.01 

.04 

.00 

.00 

.31 

.00 

.58 

.10 

.84 

.01 

.74 

.75 

.72 

.63 

.20 

.31 

.16 

.06 

.01 

.04 

.05 

.09 

.21 

.57 

.57 

.24 

.15 

.12 

.14

0.92 

.07 

.43 

.55 

.93 

.91 

.30 

.35 

.23 

.88 

.08 

.01 

.58 

.32 

.62 

.25 

.85 

.02 

.78 

.79 

.78 

.72 

.73 

.50 

.38 

.31 

.25 

.24 

.22 

.30 

.39 

.58 

.65 

.34 

.89 

.73 

.92

0.93 

.54 

.53 

.60 

.93 

.92 

.64 

.72 

.32 

.88 

.38 

.18 

.60 

.60 

.66 

.57 

.87 

.03 

.79 

.79 

.78 

.77 

.77 

.72 

.71 

.69 

.74 

.74 

.70 

.35 

.39 

.58 

.72 

.35 

.92 

.72 

.91

0.94 

.74 

.60 

.59 

.92 

.93 

.89 

.81 

.38 

.88 

.41 

.38 

.72 

.61 

.66 

.58 

.87 

.18 

.85 

.86 

.86 

.85 

.79 

.80 

.79 

.76 

.77 

.76 

.71 

.35 

.53 

.71 

.73 

.74 

.92 

.71 

.91

0.96 

.75 

.61 

.76 

.96 

.91 

.89 

.84 

.44 

.90 

.63 

.39 

.74 

.90 

.72 

.57 

.88 

.41

.87 

.80 

.81 

.81 

.78 

.78 

.76 

.72 

.78 

.72 

.71 

.77 

.75 

.93 

.80 

.95

0.96 

.82 

.70 

.79 

.97 

.90 

.89 

.87 

.73 

.90 

.76 

.56 

.74 

.91 

.73 

.63 

.88 

.44 

.87 

.88 

.87 

.86 

.86 

.86 

.91 

.94 

.95 

.91 

.85 

.79 

.75 

.73 

.82 

.77 

.93 

.83 

.95

0.95 

.83 

.69 

.86 

.99 

.90 

.94 

.86 

.78 

.92 

.82 

.56 

.73 

.91 

.73 

.81 

.88 

.74 

.89 

.90 

.91 

.92 

.88 

.90 

.94 

.95 

.95 

.91 

.86 

.81 

.79 

.74 

.83 

.80 

.92 

.86 

.98

0.96 

.83 

.80 

.90 

.99 

.98 

.97 

.92 

.78 

.93 

.82 

.60 

.73 

.92 

.78 

.95 

.89 

.82 

.90 

.92 

.93 

.93 

.88 

.90 

.94 

.95 

.95 

.91 

.86 

.94 

.84 

.77 

.83 

.80 

.93 

.87 

.98

0.96 

.88 

.95 

.95 

.99 

.99 

.97 

.95 

.85 

.94 

.83 

.84 

.78 

.92 

.79 

.96 

.89 

.85 

.91 

.92 

.93 

.94 

.88 

.91 

.94 

.95 

.95 

.91 

.86 

.94 

.90 

.78 

.83 

.81 

.94 

.90 

.98

be strongly affected by postmagmatic processes in most 

igneous rocks; therefore, H20 and C02 were eliminated 

as variables, and ferrous and ferric iron were combined 

as total FeO (FeO(t)). Similarly, many of the samples 

used in this study either gained or lost uranium (Stuckless and others, this volume); therefore, U02 was eliminated from the data. Cs20 has been found to be poorly 

explained in at least one other study (Stuckless and 

Miesch, 1981) and was readily removed in leaching experiments of the Proterozoic Sherman Granite 

(Zielinski and others, 1981). It is apparently also lost 

during devitrification of volcanic rocks (Zielinski and 

others, 1977). Cs20 was therefore not used in further 

factor analyses. Also eliminated were P205 , Sb203 , and
GEOCHEMISTRY AND PETROGENESIS

Cr203. Variations in content of these oxides are fairly 

well accounted for by five- and six-factor models; however, many of the determinations were at or slightly 

below the detection limits of the analytical methods.

Of the 39 samples, 14 had badness-of-fit measures 

that were distinctly poorer than the other 25 and were 

not used in derivation of the final model. For geologic 

reasons 12 of the 14 samples were suspected of having 

little or no genetic relationship to the main granite 

body. Samples 58392, 58398, and TDG45 are banded, 

mafic drill-core samples and, on the basis of the Q-mode 

factor analysis, are interpreted to be xenoliths of the 

older metamorphic sequence rather than mafic segregates. Samples GP-2 and GP-4 are albitized and 

silicified-epidotized granite respectively. They have apparently been too severely altered to preserve any recognizable genetic relationship to the granite. Samples 

DP-1, GP-1, and TDG20 are garnetiferous and show 

metamorphic textures; sample TD-6 is a graphic granite; sample TDG16 is a tourmaline-bearing granite; 

sample 58188 is a gneissic granite; and sample 58317 

is a mineralized aplite. Samples TDG24 (the only seemingly normal sample from the Wind River Canyon) and 

TDG44 (a drill-core sample for which the REE pattern 

is similar to that of the garnetiferous samples) were 

eliminated solely on the basis of factor-analysis results. 

The combined adjustments to the original data set resulted in a matrix of 29 oxides by 25 samples. This 

matrix was used to develop the petrogenetic model.

DETERMINATION OF END MEMBERS

Although the factor-variance diagram (fig. 2) shows 

that three factors account for much of the variance in 

the 29 oxides in the 25 samples, five factors are required to account for the variance in some of the minor 

elements. (See also table 6.) The approach used was 

therefore to first derive a plausible three-factor (endmember) model and then refine it into the final five-factor model. Consequently, the sample vectors were first 

projected from the original 29-dimensional space 

(Af=29) into a three-dimensional space. The three-dimensional vector cluster is represented by the stereographic projection in figure 3.

The vector configuration (fig. 3) reveals two separate 

compositional trends, one that extends from the upperright portion of the diagram to the lower-central part 

(trend A) and the other that extends from the lowercentral to the upper-left part (trend B). Judging from 

the differentiation indices of Thornton and Tuttle (1960) 

calculated for the compositions represented by the vectors, magma evolution proceeded along trend A towards the lower-central part of the diagram and then 

towards the upper left along trend B. Therefore, the 

composition of the initial parent material, liquid or

TABLE 6. Proportions of the variances of 29 oxides in 25 samples 

accounted for by factor models with 2 through 10 factors

Number of factors

Oxide 10

SiO,

A1 203

FeO(t)

MgO

CaO

Na 2 0

K 20

Ti02

MnO

Rb 20

BaO

SrO

Th0 2

La203

Ce2 03

Nd 20 3

Sm203

Eu 20 3

Gd 2 0 3

Tb203

Dy 2 03

Tm203

Yb2 03

Lu 20 3

Ta 2 0 5

Hf02

Zr0 2

Sc 203

CoO

0.56

.69

.31

.47

.87

.84

.65

.29

.33

.47

.14

.89

.17

.02

.00

.01

.11

.37

.26

.31

.29

.41

.44

.46

.43

.06

.03

.02

.56

0.81

.70

.79

.81

.92

.88

.74

.63

.76

.68

.30

.88

.79

.92

.90

.88

.92

.62

.78

.73

.65

.63

.63

.63

.66

.60

.54

.65

.91

0.93

.90

.79

.83

.93

.89

.96

.70

.80

.78

.82

.89

.79

.92

.90

.88

.92

.68

.78

.74

.66

.64

.64

.64

.67

.60

.57

.65

.91

0.98

.92

.82

.86

.92

.92

.96

.70

.85

.86

.84

.89

.88

.96

.96

.93

.93

.80

.91

.94

.93

.92

.90

.68

.71

.64

.67

.85

.95

0.99

'.93

.84

.88

.92

.92

.96

.75

.87

.88

.84

.90

.88

.96

.95

.93

.93

.79

.92

.94

.93

.91

.90

.88

.75

.91

.71

.84

.96

0.99

.93

.95

.88

.93

.94

.97

.91

.87

.89

.91

.89

.90

.97

.98

.95

.94

.87

.92

.95

.96

.96

.95

.95

.86

.92

.82

.91

.96

0.99

.95

.98

.89

.95

.97

.97

.91

.87

.96

.91

.90

.90

.97

.98

.96

.95

.88

.94

.96

.96

.96

.96

.95

.93

.95

.91

.95

.96

0.99

.95

.99

.89

.98

.97

.97

.97

.88

.97

.94

.90

.94

.98

.99

.99

.98

.91

.95

.97

.97

.96

.96

.95

.93

.97

.94

.95

.97

0.99

.98

.99

.91

.98

.98

.97

.97

.89

.97

.94

.92

.94

.98

.99

.99

.98

.95

.95

.97

.97

.96

.96

.95

.96

.98

.99

.95

.97

solid, is probably represented best by a vector near 

the upper-right portion of trend A, possibly near the 

vector that represents sample 29394 (13c on fig. 3), 

which has the lowest differentiation index of all 25 samples.

Compositional changes along trend A could have resulted by subtraction of a primary solid (S) from the 

parent material (L). Possible compositions of this solid 

can be represented by vectors near the intersection of 

trend A with the dashed line (fig. 3) that outlines the 

area of entirely positive compositions. If the parent material were a liquid, the material would have had to 

have been subtracted by fractional crystallization and 

settling, but if the parent material were solid crust, 

then the subtraction could have been by partial fusion 

and residual accumulation of more mafic phases. The 

mathematical model provides no means for distinguishing between these two possible processes. In either 

case, differentiation could have proceeded along trend 

A until a liquid evolved with a composition near that 

of a vector at the intersection of trends A and B.

Differentiation along trend B could have been
PROPORTION OF VARIANCE ACCOUNTED FOR

 *t

I
GEOCHEMISTRY AND PETROGENESIS 11

brought about by fractional crystallization of this liquid 

and separation of a material for which the composition 

would be like that of sample GP-5 represented by the 

vector S3 on figure 3. Sample GP-5 has a low REE 

content and a strongly positive europium anomaly (fig. 

4) and may represent a cumulate from the magma.

The three-end-member process just outlined accounts 

for the observed compositional data to the degree indicated in the factor-variance diagram (fig. 2) for a threefactor model. In order to account for some of the minor 

oxide constituents, however, two additional end members must be used. Inasmuch as the three-end-member 

process accounts for the observed data in at least a

general way, we decided to derive the two additional 

end members by simply modifying the three-end-member model. The most realistic modification that could 

be introduced was to allow some variability in the compositions of the initial parent material (L), liquid or 

solid, and in the compositions of the materials separated from it (S). The vector representing the parent 

material (L) was then replaced by vectors L^ and L2 , 

which represent two extremes in a range of parent materials. The vector representing the material subtracted from the parent material (S) was replaced by 

vectors Si and 82, which represent the extremes in a 

range of separated materials. (For illustration purposes

FIGURE 3. Stereographic projection of the three-factor Q-mode vector solution derived from the data for 29 oxides in 25 samples from 

the Owl Creek Mountains. The dashed line outlines the area within which the vectors represent compositions that are entirely positive. 

Also shown are the projected positions for the starting liquids (L! and L.2), the initial crystallizing solids (Si and 82), and the final 

crystallizing solid (83). Samples are identified by Plot I.D. (table 1 and fig. 1).
12 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

500

100

50

10

500

100

50

10

D

La Ce Nd Sm Eu Gd Tb Dy Tm Yb Lu La Ce 

REE ATOMIC NUMBER

Nd Sm Eu Gd Tb Dy Tm Yb Lu

FIGURE 4. Chondrite-normalized, rare-earth-element diagrams for the 25 granite samples used in the factor analysis. Diagrams A 

and B show the patterns of trend A (fig. 2): A, steep patterns with little or no europium anomaly (samples 41591, DP-3, DP-2, 

30511, 30501, and 32466), and B, steep patterns with minor europium anomalies (23432, TDG46, 41596, 32496, 39294, and 39467, 

and DP-8). Diagrams C and D show patterns of trend B (fig. 2): C, strongly positive through weakly negative europium anomalies 

(GP-5, TDG47, 58180, 58-80, 58-98, and 58-66), and D, moderately to strongly negative europium anomalies (4343, TDG13, 

TDG12, TDG21, TD22, and GP-3).
GEOCHEMISTRY AND PETROGENESIS 13

only, vectors Lx , L2 , Si, and S2 were projected back 

into the three-dimensional vector cluster and are shown 

on figure 3 together with sample vectors and composition S3.)

The search for vectors LI, L2, Si, and S2 was made 

using the assumption that a series of liquids had been 

generated by partial melting under fairly constant 

physical conditions and that therefore major-element 

concentrations would be fairly uniform. However, 

minor-element concentrations would be controlled by 

partitioning coefficients of phases existing in an inhomogeneous source and would therefore be more variable. The search for vectors Si and S2 was made using 

similar assumptions. In a fractional-fusion interpretation, as discussed at the end of the next section, the 

compositions represented by vectors LI and L2 would 

be less constrained than in a partial melting model and 

could encompass the full range of compositions observed for the metamorphic assemblage. Similarly, 

compositions Si and 82 could be more variable, but 

without independent constraints, mathematically valid 

end-member compositions are too numerous to 

evaluate.

The compositions represented by the five selected reference vectors the five end-member compositions 

are given in table 7. Combining these compositions 

mathematically, according to the mixing proportions 

given in table 8, produces the recomputed data as given 

in table 1. The proportional differences between the 

original and recomputed data for each oxide in each 

sample are given in tables 9 and 10.

The mixing proportions in table 8 indicate that all 

but 3 of the 25 samples could have been derived from 

a mixture of compositions LI and L2 by subtraction of 

some composition intermediate between compositions 

Si and 82, plus the addition or subtraction of relatively 

small amounts of composition S3. The other three samples require either the subtraction of material outside 

the range of compositions Si and S2 (sample 58-30) or 

negative quantities of the starting material (samples 

58-98 and TDG47).

EVALUATION OF THE MODEL

For most samples, the average agreement between 

the original and recomputed data is within 15 percent 

(table 9). A similar comparison for the 14 rejected samples is given in table 10. The recomputed data for the 

rejected samples were obtained by projecting vectors 

that represented the original compositions into the fivedimensional vector system. All of these recomputed 

compositions are in poor agreement with the original 

compositions. Thus the five-factor model accounts for

TABLE 7. Compositions of the end members for the factor model

End member

Oxides L l L 2 Si S2 S3

Weight percent

Si02

A1 20 3

FeO(t)

MgO

CaO

Na 2 0

K20

Ti02

MnO

Rb 20

BaO

SrO

Th02

La 2 03

Ce2 0 3

Nd 2 03

Sm2 0 3

Eu 2 0 3

Gd 2 0 3

Tb 2 0 3

Dy 2 0 3

Tm2 0 3

Yb 20 3

Lu 2 0 3

Ta20 5

Hf0 2

Zr02

Sc 2 0 3

CoO

66.95

16.26

4.30

1.47

2.87

5.22

2.04

.55

818

165

1161

466

53.46

126.0

205.1

67.94

8.81

1.22

4.84

.66

3.58

.160

.76

.083

.66

7.25

364

8.73

11.4

66.31

16.32

4.71

1.62

3.00

5.41

1.68

.60

Parts per

919

172

1086

478

54.32

132.1

212.9

71.20

9.52

1.22

5.82

.82

4.58

.226

1.12

.131

.83

8.05

399

10.1

12.5

64.39

16.34

6.24

2.22

3.45

5.63

.44

.85

million

1240

167

1114

528

85.38

205.3

341.0

116.2

15.9

1.58

8.61

1.17

6.31

.303

1.48

.171

1.39

10.7

504

13.4

16.8

64.97

16.91

4.99

1.74

3.28

5.65

1.44

.63

956

155

1272

542

53.78

142.0

228.0

74.71

9.49

1.31

5.35

.73

4.03

.174

.80

.085

.54

7.94

416

10.2

13.5

75.86

13.76

.61

.07

1.18

3.67

4.67

.06

98.0

193

588

156

23.23

2.78

5.51

3.09

.72

.612

.746

.12

.69

.058

.35

.053

.51

2.69

92.7

1.08

.39

Norms (weight percent)

Q

C

Or

Ab

An

En

Fs

11

17.27

.25

12.07

44.25

14.29

3.67

7.16

1.05

16.04

.15

9.95

45.93

14.90

4.05

7.85

1.14

14.65

.35

2.60

47.77

17.15

5.55

10.30

1.63

13.24

.09

8.56

47.94

16.30

4.34

8.33

1.20

33.58

.53

27.61

31.09

5.85

.18

1.05

.11
14 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

TABLE 8. Mixing proportions for the factor model

End member

Sample

DP-3

23432

29394

29467

30501

30511

32466

32496

41591

41596

DP-8

DP-2

TDG46

58-66

GP-3

GP-5

4343

TDG12

TD-13

TDG21

TD22

58180

58-80

58-98

TDG47

4.9193

1.9963

.0782

.4517

1.8175

1.6091

8449

9678

4510

3.8435

2.6332

4.6932

.0001

2.8789

27.5097

.0000

17.2184

13.7561

15.9529

16.6719

15.0054

5.3650

.1490

-3.7071

-5.3798

0.8773

.8379

.0645

1.3172

.5829

.0000

.9678

1.8039

.3367

.9832

3.8947

1.5443

1.1258

1.8496

14.6323

-.0000 

4.4041 

6.0112

-0.7261

-.5650 

.2909

-.3913

-.2887

-.0443

-.8382

-.8156

-.2966

-.9080

-1,

-6

,1908

9658

,0000

4870

,1028

-3.6972

-1.5585 

.0000

-.7226

-1.4031

-.9482

-2.8440

-2.9294

-1.6225

-2.9256

-4.0625

-3.9800

-.9262

-3.5200

-28.8249

.4194

.6296

.0000 -.0000

-2.2856 -15.3472

-2.5642 -13.6872

-2.5629 -14.9546

-2.9129 -15.3889

-0.3733 

.2892 

.5663 

.3451 

.2914

.3833

-.1305

-.0267 

.1315 

.0069

-.2747

-.2918 

.8003 

.2786

-6.2143

1.0000

9896

5160

8548

9997

4.1247

1.5566

.1230

-.6311

-.6756

-2.1754

-.2858 

.6220 

.9866

1.6794

-13.5233

-5.4684 

-.9745 

2.4947 

3.1401

-2 4316 

-.1673 

1.0805 

1.8570 

2.2358

most of the variance in all of the 25 samples and discriminates against altered and unrelated samples.

The five-factor model also accounts for more than 64 

percent of the variance in all 29 oxides used and more 

than 80 percent of the variance in all but four oxides 

(table 5 and fig. 2). All four of the oxides that show 

low percentages of variance accounted for are suspected to have large analytical errors relative to 

geologic variation and are therefore poorly explained.

Standard petrologic diagrams and isotopic data can 

be used to evaluate the petrogenetic model. The pronounced break in differentiation trend indicated by Qmode factor analysis suggests a marked change in physical conditions during crystallization. A different crystallization pressure for samples from trends A and B 

is suggested by a plot of the normative data in both 

the Q-Ab-Or (fig. 5) and the An-Ab-Or systems (fig. 

6). If the whole-rock compositions are interpreted as 

equilibrium liquids, both ternary systems indicate crystallization at lower water pressure for trend B.

Three samples (58-80, 58-98, and TDG47) that plot 

near the junction of trends A and B (fig. 3, points lOb, 

lOc, and lla) and that are not explained by the petrologic model as simple differentiates plot as a group 

above the polybaric minimum (fig. 5). These compositions cannot be derived by equilibrium crystal fractionation if a probable starting liquid from trend A is 

assumed as parental to these samples. The position of 

these data points on figure 5 is probably due to a small 

amount of alteration during which potassium was lost, 

or quartz and albite were gained. Because the shift 

from a probable equilibrium liquid is small and may 

only involve a few elements, these samples still fit the 

five-factor model reasonably well (that is, the average 

difference between the original and recomputed data 

is less than 15 percent).

The interpretation of the five-factor model reveals a 

surprising relationship to oxygen isotopic data, even 

though these data were not used in the development 

of the model. Figure 7 shows a plot of whole-rock 8180 

values versus percent of composition LI in the total 

starting material (lOOL^I^+Lg) for each sample. The 

very good correlation between these two variables for 

all but three samples suggests that the starting magma 

is well represented by a mixture of two materials. A 

similar correspondence between oxygen isotopic and 

elemental compositions has been noted for the Carboniferous Elberton Granite, Georgia (Wenner, 1980). 

However, in that granite the trace-element and oxygen 

isotopic data yield a geographic pattern that is interpreted to reflect a similar pattern in the protolith. No 

geographic pattern is noted for the granite of the Owl 

Creek Mountains. However, if the protolith for the 

granite is as lithologically complex as the metamorphic 

sequence intruded by the granite, a simple geographic 

pattern for variables that preserve a protolith imprint 

would be unlikely.

Although the interpretation of the five-factor model 

is reasonable in light of petrologic data, the same end 

members could be interpreted differently. The data 

permit the generation of trend A as a series of fractional melts. In this interpretation, end members Lj 

and L2 would represent a series of protolith compositions and end members Si and S2 would represent a 

series of residuum compositions. In the alternate 

hypothesis, the difference in pressures suggested for 

trends A and B represents derivation at an intermediate pressure followed by differentiation at a 

slightly lower pressure. The generation of trend A by 

fractional fusion would also account for the correlation 

between starting liquid compositions and 8180 values.

PROTOLITH CONSIDERATIONS

The wide range of 8180 values, which appears to be 

a primary magmatic feature; the apparent range in initial ^Sr/^Sr ratios (Hedge and others, this volume) 

and 2°8pb/204Pb ratiog (gtuckless and others, this vol-
GEOCHEMISTRY AND PETROGENESIS 

Q

15

FIGURE 5. Normative data for quartz, albite, and orthoclase for granite samples from the Owl Creek Mountains, Wyo. Trend-A 

samples indicated by circles; trend-B samples indicated by squares; polybaric, water-saturated minimum (Tuttle and Bowen, 

1958; Luth and others, 1964) indicated by solid dots. The expanded portion of the diagram (shown on the inset) is bounded 

by quartz 25 to 60 percent, albite 20 to 55 percent, and orthoclase 20 to 55 percent.

ume); and the correlation of these variables with one 

another all suggest that the magma genesis involved 

some sort of mixing process that did not proceed to 

homogenization. Although the isotopic data could be 

explained by assimilation and contamination of a 

homogeneous magma, chemical data favor derivation 

from a heterogeneous protolith. This conclusion is 

based on the facts that the samples from trend A indicate a higher pressure history than those from trend 

B and that samples from trend A exhibit the entire 

range of 8180 values. Furthermore, the samples from 

trend A are less evolved than those of trend B. In contrast, a contamination-assimilation model would most 

reasonably predict increasing contamination with increasing degree of magma evolution and increasing contamination with decreasing pressure because assimilation is most likely to occur during intrusion or final 

crystallization. Thus the isotopic data seem to reflect 

protolithic characteristics, which became primary features of the initial magma.

Oxygen isotope data provide some constraints on the 

nature of the protolith. Few 8180 values are greater 

than 8.0 per mil (table 4), which by comparison to Paleozoic source regions suggests a low pelitic component 

in the protolith (O'Neil and Chappell, 1977). This suggestion is supported by both the low values for normative corundum (generally less than 1.1 percent, table 

1) and the low pelitic component in the metamorphic 

complex intruded by the granite (Hamil, 1971). The few 

metamorphic rocks analyzed yield low 8180 values 

(table 4, samples with prefix DKCH). The isotopic fraction for oxygen at the temperature required for granite
16 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

TABLE 9. Proportional differences between the original chemical data and the data represented by the factor solution for the 25 samples

used to develop the solution

[Average is the average of the absolute values]

Sample No.

Si0 2

A1 203

FeO(t)

MgO

CaO

Na 20

K20

Ti02

MnO

Rb 20

BaO

SrO

Th02

La 203

Ce2 0 3

Nd 2 03

Sm20 3

Eu 2 03

Gd 2 0 3

Tb 20 3

Dy 20 3

Tm20 3

Yb 20 3

Lu 2 0 3

Ta 2 0 5

Hf02

Zr0 2

Sc 2 0 3

CoO

Average:

DP-3

0.00

.01

.00

.29

.10

.03

-.11

.18

.02

.03

.18

.15

.06

.06

.16

.04

.03

.19

.24

.19

.20

.29

.36

.40

.62

.01

.09

.01

.20

.14

23432

0.01

-.01

.09

-.25

-.17

.00

-.02

.10

-.03

-.04

.02

.00

.04

-.15

-.10

-.18

-.18

-.09

-.46

-.46

-.44

-.18

-.09

-.03

-.21

.14

.17

-.01

-.11

.13

29394

0.00

.00

.04

.10

.02

-.03

.06

-.09

.00

.15

-.18

-.10

.06

.04

.01

-.04

-.12

.01

-.08

-.07

-.09

.01

.05

.05

-.16

-.20

.04

-.07

.07

.07

29467

0.00

.00

.04

.10

-.01

-.06

.05

.04

.02

.12

-.11

-.07

-.08

-.18

-.13

-.08

-.02

.00

.12

.02

-.06

-.22

-.30

-.25

-.08

-.10

-.09

-.12

-.01

.09

30501

0.00

-.01

-.02

.18

.00

-.02

-.01

.06

-.08

.08

.00

.11

.10

.06

.02

.01

.00

-.05

-.31

-.19

-.10

.10

.12

.17

.23

-.12

-.05

.20

.12

.09

30511

0.00

-.01

.04

.03

.05

-.01

.02

-.02

-.22

.04

-.06

.08

.04

.09

.04

.02

-.04

-.01

-.12

.00

.07

.08

.13

.06

-.75

-.06

.13

.01

.07

.08

32466

0.00

-.01

.04

-.35

.01

.02

-.04

.02

-.24

-.13

.02

-.05

-.11

.01

-.07

.06

.11

-.16

.12

.09

.00

-.28

-.30

-.37

-.85

-.01

.00

-.01

-.24

.13

32496

0.00

-.01

.01

-.53

.07

.03

.02

-.03

.17

.02

-.05

.02

-.04

.11

.13

.13

-.02

-.11

.00

-.03

-.06

-.12

-.12

-.04

-.01

.04

.07

-.15

-.21

.08

41591

-0.01

.01

.00

.04

.07

.02

.03

.03

.12

.10

.00

.04

-.09

.13

.07

.08

.01

.10

.03

.10

.16

.14

.14

.17

.25

.07

-.14

-.09

.08

.08

41596

0.00

-.02

.20

.11

-.02

-.05

.01

-.42

.23

.01

-.01

.24

-.02

-.06

-.05

.04

.28

-.04

.26

.22

.21

.13

.00

-.12

-.98

-.23

-.10

-.40

.04

.15

DP-8

0.00

.03

-.45

-.24

-.14

.07

-.02

.07

.00

-.20

.07

-.07

.09

-.14

.07

.16

.26

.08

.13

.14

.12

.01

.00

-.04

-.43

-.20

-.21

.24

-.16

.13

DP-2

0.00

.00

-.32

-.07

.00

.03

.01

.17

-.10

-.22

.13

-.16

-.05

-.13

-.08

.05

-.03

.05

.07

.11

.12

.03

.06

-.06

-.21

.18

-.02

.08

-.02

.09

TDG46

0.00

-.01

.22

.09

-.17

-.02

-.02

-1.18

.01

-.06

.07

-.23

.10

-.58

-.91

-.63

-.34

-.03

-.49

-.48

-.37

-.17

-.14

-.11

-.36

.34

-.05

-.21

.00

.26

melting is expected to be small (O'Neil and others, 

1977), such that the melt fraction should have the same 

or slightly greater 8180 values than the source rock. 

Thus, at least a part of the exposed metamorphic complex could be similar to one end member in the protolith.

Either a fractional-crystallization or fractional-fusion 

interpretation as an explanation for trend A (fig. 3) can 

provide constraints on magma genesis because both interpretations indicate a starting liquid that can be used 

to estimate a minimum pressure for magma genesis.

A unique starting liquid (or series of liquids) cannot 

be defined in a fractional crystallization interpretation, 

but presumably a series of liquids could have been 

found that included the composition of the least evolved 

sample (29394). In a fractional-fusion interpretation, 

this sample and others along trend A suggest a minimum pressure of 3 kb and approximately water-saturated conditions (fig. 5). A pressure of 3 kb and a temperature near that required for granite melting approach conditions indicated by the grade of metamorphism observed for the volcanic-sedimentary sequence.
GEOCHEMISTRY AND PETROGENESIS 17

TABLE 9. Proportional differences between the original chemical data and the data represented by the factor solution for

the 25 samples used to develop the solution Continued

Sample No. 58-66 58-80 58-98 GP-3 GP-5 4343 TDG12 TD-13 TOG21 TD22 TDG47 58180

Si02 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

A1 2 03 -.02 .00 .02 -.02 .03

FeO(t) .29 -.35 -.16 -.19 -1.06 -

MgO .46 .26 -.36 .63 .27

CaO -.06 .08 -.02 .73 .15

Na 2 0 .00 .00 .02 -.09 .00

K 2 0 .05 -.07 -.01 .03 .00

Ti02 .27 -.31 .05 .41 -.13 -

MnO .16 -.61 -.35 -.72 -.03

Rb20 .12 .04 -.03 .08 .05

BaO -.06 .04 -.22 .10 -.06

SrO .06 .14 -.12 .91 -.28

Th02 .10 -.22 .02 .02 -.10 

La 203 -.14 .11 .03 .19 .55

Ce2 03 -.22 .18 .14 .18 .50

Nd 2 03 -.18 .25 .04 .17 .19

Sm2 03 -.17 .16 .07 .06 -.09

Eu 2 03 .01 .05 -.13 .09 .16

Gd 2 03 -.09 .12 -.11 .07 -.08 

Tb 2 0 3 -.06 .11 -.18 .12 .07

Dy 2 03 -.01 .13 -.21 .14 .13

Tm2 03 .04 .05 -.18 .17 .17

Yb 2 03 .03 .06 -.19 .17 .20

Lu 2 0 3 -.04 .09 -.25 .19 .25

Ta2 05 .07 -.31 -.17 .14 .47

Hf02 -.51 .02 .06 .13 .00

Zr02 -.15 .22 .16 .26 .08

Sc2 03 .11 -.01 .11 -.03 .07

CoO .79 .18 -.31 .46 .01

Average: .15 .14 .13 .22 .18

MAGMA GENESIS

The initial liquids from which the granite may have

been derived were probably formed at approximately

3 kb pressure and near water-saturated conditions. (A

somewhat higher pressure would be possible for a fractional-crystallization interpretation because of the unconstrained composition of the initial liquids.) The protolith was heterogeneous in composition, most likely

similar to the observed metamorphic sequence, but at

a greater depth than currently exposed. Metamorphism

.02 -.01 .01 .02 .01 -.01 -.02

-.20 .25 -.10 .07 .11 .20 -.35

-.52 -.53 -.17 .32 -.16 -.14 .03

.18 .07 -.08 -.21 -.33 .02 -.08

.00 -.10 .05 .05 .04 -.01 -.04

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

-.11 -.27 -.32 -.11 -.50 -.17 .54

.04 .21 .12 -.05 -.26 .15 .26

.00 .08 -.02 -.01 -.16 -.13 .01

.04 -.23 -.11 .03 -.16 .23 .20

.05 -.23 -.18 -.31 -.28 .21 .20

-.01 -.38 -.01 .13 -.02 .00 .14

.01 .00 -.03 .08 -.28 .00 .04

.08 -.10 .01 .02 -.25 -.05 -.03

.17 -.11 -.05 -.10 -.17 -.04 -.17

.19 -.05 -.02 -.15 -.05 .05 -.23

.18 .02 -.13 -.14 -.13 -.01 .01

-.18 -.05 .01 -.11 .15 .16 -.07

-.02 -.11 -.09 -.10 .05 .14 .02

.11 -.16 -.17 -.09 -.06 .09 .05

.17 -.25 -.20 .09 -.46 .09 .11

.19 -.26 -.22 .10 -.65 .03 .15

.21 -.33 -.28 .15 -.75 .03 .20

-.09 .20 .07 -.10 -1.20 -.14 .39

.18 -.11 -.05 .00 -.10 -.07 -.06

.29 .01 .04 -.57 -.67 -.51 .08

.15 -.21 .07 -.08 -.03 -.29 .20

-.13 -.02 -.39 .07 -.02 -.10 -.19

.12 .15 .10 .11 .25 .10 .13

apparently predates granite intrusion (Mueller and

others, 1981; Hedge and others, this volume). Therefore at the time of intrusion, the currently exposed

metamorphic rocks were probably under less pressure

than that indicated by maximum metamorphic grade.

Thus for a fractional-crystallization interpretation, the

compositional variations along trend A would be the

result of differentiation prior to intrusion. Following

the generation of the compositions along trend A (by

either fractional crystallization or fractional fusion), the

magma was intruded to its current level of exposure,
18 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

TABLE 10. Proportional differences between the original chemical data and the data represented by the factor solution, for the 14 samples

not used to develop the solution

[Average is the average of the absolute values. Leaders (--) indicate indeterminate value due to division by zero]

Sample No. 58188 58317 58392 58398 DP-1 GP-1 GP-2 GP-4 TD-6 TDG16 TDG20 TDG24 TDG44 TDG45

Si02 0.01 -0.01 0.83 38.16 -0.01 -0.02 -0.06 -0.03 -0.03 -0.01 -0.02 -0.01 -0.01 -2.85 

A1 203 -.03 .02 -1.14 -37.17 .05 .13 .23 .15 .02 .05 .10 .03 -.01 2.35 

FeO(t) .01 .08 -1.56 -80.35 -.28 .53 -1.76 -.88 .79 .12 .02 .18 .31 4.57 

MgO .33 .58 -.48 -40.09 -1.13 5.93 -4.89 -1.73 8.01 -.22 -3.14 .05 .89 4.60 

CaO .30 .17 -.95 -56.93 -.31 -.32 -9.36 .66 -4.83 -.16 -1.96 -.46 -.06 3.53 

Na 20 -.37 .06 -8.17 -197.68 .16 .50 .59 -.33 -.21 .10 .16 .11 -.31 18.72 

K 20 -.09 .02 24.16 517.33 .02 -1.53 -12.58 -5.81 .17 -.09 .00 .01 .12 -75.60 

Ti02 .00 -.01 -1.65 -113.60 -1.00 4.83 -1.19 13.36 -2.07 -6.85 -.73 .78 6.70 

MnO -.26 -.53 -3.06 -111.20 .00 .98 .76 -1.80 2.25 -1.11 .88 -1.01 .77 6.11 

Rb 20 -.06 .10 8.27 79.39 .05 .60 -7.77 -2.79 .46 -.43 .34 -.24 .36 -30.78 

BaO .12 -1.28 .62 19.96 -7.18 -2.70 -63.60 -2.91 -5.02 -1.61 -2.34 -.65 -.21 -17.03 

SrO -.10 -.31 11.28 -414.84 -10.24 -.43 -21.80 .59 -.59 -1.43 -4.66 -1.33 .73 32.63 

Th02 -.27 .19 19.11 994.57 -.50 -4.34 -1.44 .68 -8.84 -1.11 -1.92 -.41 .08 -53.95 

La 2 0 3 -.53 2.21 -38.38 >999.99 4.22 3.19 34.08 .66 6.50 5.51 -.08 .56 1.07 85.39 

Ce20 3 -.23 1.84 -22.43 -685.59 4.35 .71 29.10 .88 4.23 7.59 .75 .58 .95 47.89 

Nd 2 0 3 -.14 .74 -12.66 -386.18 3.43 -.28 12.14 .87 2.61 4.98 -.59 .44 .40 27.39 

Sm20 3 .26 .12 -5.04 -207.82 1.45 -1.15 3.70 .92 -.28 1.67 -2.86 .19 -.50 14.55 

Eu 20 3 -.36 -.13 -1.77 -44.67 -6.64 -7.59 -10.83 -.68 -.27 -1.05 -2.98 -.77 .21 4.27 

Gd 20 3 -.24 -.14 -7.30 -326.26 -.90 -1.14 -2.53 .51 -1.35 -.50 -3.71 -.09 -1.14 19.25 

Tb 20 3 -.27 -.06 -6.74 -283.64 -1.09 -.97 -2.85 .54 -.94 -.18 -1.61 -.18 -.81 18.47 

Dy 20 3 -.48 .05 -6.76 -265.09 -1.08 -.73 -2.88 .53 -.77 .02 -1.41 -.18 -.54 17.94 

Tm20 3 -.07 .15 -3.51 -141.29 -1.00 -.49 -2.32 .65 -.63 .16 -1.02 -.38 .26 10.40 

Yb 20 3 .18 .21 -2.52 -110.61 -.89 -.50 -1.92 .70 -.49 .24 -.79 -.45 .41 8.19 

Lu 2 0 3 .37 .24 -1.61 -87.01 -.68 -.49 -1.69 .74 -.37 .29 -.64 -.53 .54 6.56 

Ta 20 5 1.12 -.07 10.98 73.88 -1.10 .97 .98 1.03 .20 -.50 .91 -.32 .73 -9.18 

Hf02 -.36 -.49 -12.37 -551.06 -.05 -1.44 -.18 .17 -10.81 -3.76 .04 -.65 -.22 29.34 

Zr0 2 -.24 -.39 -13.15 -637.81 -.37 -1.24 -1.22 .10 -2.59 -1.04 -1.20 -7.19 -4.93 85.80 

Sc 2 0 3 .07 -.78 -.82 -33.63 -1.15 -.60 -35.63 .14 -2.16 -1.02 -9.41 .04 .36 3.27 

CoO .09 .83 -.33 -49.25 -5.29 7.81 -17.00 -3.45 4.89 .63 -.58 -.18 .95 4.62 

Average: .24 .41 7.86 271.11 1.90 1.78 9.87 1.11 2.89 1.30 1.75 .62 .64 22.48

where pressure may have been as low as 0.5-1.0 kb 

(fig. 5), and compositions along trend B were generated 

by the separation of the feldspar-dominated solid (S3). 

Compositions along trend A were modified by minor 

addition or subtraction of this solid. Minor modifications 

of some sample compositions may have occurred as a 

result of deuteric or post-crystallization hydrothermal 

alteration. Such alterations may explain the 2-20 percent of the variance in the oxides not accounted for 

by the five-factor solution (table 6) and are supported 

by isotopic data in the rubidium-strontium and thoriumlead systems (Hedge and others, this volume; Stuckless 

and others, this volume).

SUMMARY AND CONCLUSIONS

Q-mode factor analysis has been used to identify 25 

granitic samples from the Owl Creek Mountains, Wyo., 

for which 29 oxide variables can be well represented 

by a five-factor model. The five-factor model serves to 

identify samples of doubtful geologic relation to the 

main granite intrusion, including suspected xenoliths 

and rocks that have been strongly affected by hydrothermal alteration. The model also identifies oxide variables (H20, C02, U02, Cs20, and the oxidation state 

of iron) that have been controlled in large part by postmagmatic effects. The model presented here explains
GEOCHEMISTRY AND PETROGENESIS 19

more than 80 percent of the observed variance for 25 

of the 29 oxides (table 6) and yields excellent agreement between recomputed and original (adjusted to 100 

percent) compositions for all 25 samples.

End members that provide an explanation for the observed compositional variations were identified within 

the five-dimensional space defined by the sample compositions. These end members can be used in either 

a purely fractional-crystallization interpretation or a 

fractional-fusion and fractional-crystallization interpretation. According to either interpretation, the protolith 

is similar to the inhomogeneous metamorphic sequence 

that the granite intrudes, and the final compositions 

reflect a polybaric history. Both interpretations can 

also be reconciled with the wide range of whole-rock 

8180 values (6.9-8.6).

Two of the five end members in the model represent 

either the extremes in a range of starting liquids generated at a pressure equal to or greater than 3 kb, or 

the extremes in a series of protolith compositions. Two 

additional end members may represent either the extremes in a series of early separated solids at a pressure of approximately 3 kb, or the extremes in a series 

of residuum compositions from fractional fusion. The 

fifth end member is a feldspar-dominated solid that is 

either subtracted or added to account for minor variations in the least evolved compositions and major variations along a second trend defined by the most evolved 

compositions. This solid is interpreted to have crystallized at pressures of approximately 1.0-0.5 kb. The fiveend-member model and the selected end members do 

not explain all of the observed variances in the 29

FIGURE 6. Normative An-Ab-Or for granites from the Owl Creek Mountains, Wyo. Samples of trend A are shown by circles; samples 

of trend B shown by squares; 2- and 8-kb eutectic curves from Whitney (1975). The expanded portion of the diagram (shown 

on the inset) is bounded by anorthite 0 to 45 percent, albite 30 to 75 percent, and orthoclase 25 to 70 percent.
20 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

8.8

8.4

8.0

7.6

7.2

6.8

13bo

20 40 60 80 

PERCENT L! IN STARTING LIQUID

100

FIGURE 7. Plot of whole-rock 8180 values versus percent of end 

member LI in the total starting material. The best-fit line showing 

data correlation was determined using data points plotted as solid 

dots.

oxides. Although analytical error may account for some 

of the unexplained variance, hydrothermal or deuteric 

alteration probably redistributed some elements to a 

small extent such that variance within the data set is 

no longer purely magmatic.

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Survey Geologic Quadrangle Map GQ-1537, scale 1:24,000.

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Chemistry of igneous rocks: American Journal of Science, v. 258, 

p. 664-684.

Tuttle, 0. F., and Bowen, N. LM 1958, Origin of granites in the 

light of experimental studies in the system NaAlSiaOg-KAlSiaOgCaAl2Si208-H20: Geological Society of America Memoir 74, 153

PWenner, D. B., 1980, Oxygen isotope variations within the Elberton 

Pluton, in Stormer, J. C., Jr., and Whitney, J. A., eds., 

Guidebook 19, Geological, geochemical, and geophysical studies 

of the Elberton Batholith, eastern Georgia: A guidebook to accompany the 15th annual Georgia Geological Society field trip, 

p. 31^3.

Whitney, J. A., 1975, The effects of pressure, temperature and XH^ 

on phase assemblages in four synthetic rock compositions: Journal of Geology, v. 83, p. 1-27.

Winkler, H. G. F., 1967, Petrogenesis of metamorphic rocks, rev. 

2d ed.: New York, Springer-Verlag, 237 p.

Yellich, J. A., Cramer, R. J., and Kendall, R. G., 1978, Copper 

Mountain, Wyoming, Uranium deposit rediscovered: Wyoming 

Geological Association Guidebook, 30th annual field conference, 

p. 311-327.

ZielinsW, R. A., Lipman, P. W., and Millard, H. T., Jr., 1977, Minorelement abundances in obsidian, perlite, and felsite of calc-alkalic 

rhyolites: American Mineralogist, v. 62, p. 426-437.

Zielinski, R. A., Peterman, Z. E., Stuckless, J. S., Rosholt, J. N., 

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219.

Geochronology of an 

Archean Granite, 

Owl Creek Mountains, 

Wyoming

By C. E. HEDGE, K. R. SIMMONS, and J. S. STUCKLESS

CHEMICAL AND ISOTOPIC STUDIES OF GRANITIC ARCHEAN ROCKS, 

OWL CREEK MOUNTAINS, WYOMING

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1388-B

CONTENTS

Page

Abstract ............................................. 27

Introduction ........................................... 27

Analytical methods ....................................... 27

Data and discussion ....................................... 28

Conclusions ............................................ 32

References ............................................ 33

ILLUSTRATIONS

Page

FIGURE 1. Rubidium-strontium isochron diagram for unaltered granite samples ............................. 28

2. Plot showing correlation of initial ^Sr/^Sr with whole-rock 8180 values .......................... 29

3. Rubidium-strontium isochron diagram for altered granite samples .............................. 29

4. Concordia plot for uranium-lead data from zircons ...................................... 31

5. Concordia plot for uranium-lead data from zircons assuming the added complexity of 600-Ma lead loss ........ 32

TABLES

Page

TABLE 1. Rubidium-strontium data for whole-rock samples ........................................ 28

2. Uranium-lead isotopic data for zircons .............................................. 30

25

CHEMICAL AND ISOTOPIC STUDIES OF GRANITIC ARCHEAN ROCKS, 

OWL CREEK MOUNTAINS, WYOMING

GEOCHRONOLOGY OF AN ARCHEAN GRANITE, 

OWL CREEK MOUNTAINS, WYOMING

By C. E. HEDGE, K. R. SIMMONS, and J. S. STUCKLESS

ABSTRACT

Rubidium-strontium analyses of whole-rock samples of an Archean 

granite from the Owl Creek Mountains, Wyo., indicate an intrusive 

age of 2640± 125 Ma. Muscovite-bearing samples give results suggesting that these samples were altered about 2300 Ma. This event may 

have caused extensive strontium loss from the rocks as potassium 

feldspar was altered to muscovite. Alteration was highly localized 

in nature as evidenced by unaffected rubidium-strontium mineral 

ages in the Owl Creek Mountains area. Furthermore, the event probably involved a small volume of fluid relative to the volume of rock 

because whole-rock 8180 values of altered rocks are not distinct from 

those of unaltered rocks. In contrast to the rubidium-strontium 

whole-rock system, zircons from the granite have been so severely 

affected by the alteration event, and possibly by a late-Precambrian 

uplift event, that the zircon system yields little usable age information.

The average initial ^Sr/^Sr (0.7033 ±0.0042) calculated from the 

isochron intercept varies significantly. Calculated initial ^Sr/^Sr 

ratios for nine apparently unaltered samples yield a range of 0.7025 

to 0.7047. These calculated initial ratios correlate positively with 

whole-rock 8180 values; and, therefore, the granite was probably derived from an isotopically heterogeneous source. The highest initial 

87gr/86gr ra^io js iower than the lowest reported for the metamorphic 

rocks intruded by the granite as it would have existed at 2640 Ma. 

Thus, the metamorphic sequence, at its current level of exposure, 

can represent no more than a part of the protolith for the granite.

INTRODUCTION

The Owl Creek Mountains are located near the center 

of an extensive area of Archean rocks that includes 

most of Wyoming, much of southern Montana, and adjacent parts of Utah and South Dakota. Throughout 

this region, older gneiss and greenstone terranes were 

intruded by granitic rocks between 2500 and 2800 Ma. 

In the Teton Range to the west, Reed and Zartman 

(1973) assigned an age of metamorphism of 2820 Ma 

to the Webb Canyon Gneiss. Several studies of 

metasedimentary rocks (up to granulite grade) in the 

Beartooth Mountains to the northwest have provided 

an age of 3000-3400 Ma (Nunes and Tilton, 1971; Henry

and others, 1981; Wooden and others, 1981). Ages of 

2800-3000 Ma have been reported for the rocks of the 

Big Horn Mountains to the east (Arth and others, 

1980). An amphibolite-grade sedimentary-volcanic sequence in the Granite Mountains to the south has a 

reported rubidium-strontium whole-rock age of 2860 Ma 

(Peterman and Hildreth, 1978); this age probably represents the most recent period of metamorphism, as other 

isotopic studies suggest that at least the original sediments are as old as 3200 Ma (L. B. Fischer and J. 

S. Stacey, unpub. data, 1982). Granites that intrude 

the metamorphic complex of the Granite Mountains 

have been dated by uranium-lead zircon at 2640 and 

2595 Ma (Ludwig and Stuckless, 1978). Two plutons in 

the Wind River Range (southwest of the Owl Creek 

Mountains) yield uranium-lead zircon ages of 2640 and 

2560 Ma (Naylor and others, 1970).

Previous geochronology of the Precambrian rocks of 

the Owl Creek Mountains is rather limited. Giletti and 

Gast (1961) presented rubidium-strontium ages for pegmatitic muscovite and microcline of 2663 Ma and 2584 

Ma, respectively. Lead-lead whole-rock data for the 

granites, reported by Nkomo and others (1978), indicate an age of 2645 ±60 Ma. Metamorphic rocks from 

Wind River Canyon gave a whole-rock rubidium-strontium age of 2755 ±100 Ma (Mueller and others, 1981). 

Diabase dikes are common throughout the Precambrian 

of Wyoming, and Condie and others (1969) reported potassium-argon whole-rock ages of 1900-2100 Ma for six 

dikes in the Owl Creek Mountains.

ANALYTICAL METHODS

Rubidium and strontium concentrations were measured on separate aliquots of whole rock using 87Rb 

and ^Sr tracers. All ^Sr/^Sr ratios (table 1) have been 

normalized to a 86Sr/88Sr of 0.1194, and the mass spectrometer used in this study yielded a value of

27
28 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

0.70802 ±0.00006 (2a) for the Eimer and Amend SrC03 

standard. Replicate analyses of granitic samples indicate an analytical accuracy of ±1.0 percent (2a) for 

concentrations and ±0.02 percent (2<r) for the strontium isotopic composition. These errors were used for 

data regression because they include splitting errors 

that can be significant for granitic materials.

Zircon separates were sized and magnetically separated. Before dissolution, the separates were leached 

first in 8N HC1 and then in 7N HN03. The attack procedure is similar to that given in Krogh (1973), and 

the extraction procedure is that described by Ludwig 

and others (1981). Lead was run on a single rhenium 

filament using H3P04-silica gel. Uranium and thorium 

were run together on a single filament using H3P04 

and aquadag. Analytical uncertainties for zircon ages 

were calculated according to the methods of Ludwig 

(1980). The determination of best-fit lines was made 

using the methods of Ludwig (1979), and the decay constants used are those recommended by the IUGS Subcommission on Geochronology (Steiger and Jager, 

1977).

DATA AND DISCUSSION

The rubidium-strontium analytical data are presented 

in table 1. On the basis of field evidence, petrography, 

and Q-mode analyses of chemical data (Stuckless and 

others, this volume), the first nine samples listed in 

the table are deemed to represent unaltered and 

cogenetic samples of the main granitic intrusion. These 

samples are from the North Canning area. The nine 

unaltered granite samples give a rubidium-strontium 

age of 2640 ±125 Ma and an initial ^Sr/^Sr of 

0.7033 ±0.0042 (all errors 2a). The scatter of data 

points (fig. 1) is greater than would be expected from 

analytical uncertainty alone, and the age and initial 

87Sr/86Sr are calculated from a model-3 fit to the data 

(Ludwig, 1979).

A possible cause for the observed scatter in the 

rubidium-strontium data, as suggested by the model-3 

fit, might be variable initial 87Sr/86Sr ratios. To test 

this hypothesis, we calculated initial 87Sr/86Sr ratios 

(assuming an age of 2640 Ma) and plotted them against 

the 8180 values for those samples for which both types 

of data were available. A positive correlation results 

(fig. 2). The variable initial 87Sr/®6Sr ratios cannot be 

an artifact of assuming the wrong age because the two 

samples with the highest and lowest apparent initial 

ratios (29467 and 41596, respectively) have nearly identical rubidium-strontium ratios. Similar correlative variations between initial 87Sr/86Sr values and whole-rock 

8 180 values have been noted for Paleozoic granites of

the southeastern Piedmont (Wenner, 1980), and the observed variability in these two parameters has been interpreted as reflecting variations within the source region of the granite (Wenner, 1981). By analogy, we conclude that the age of 2640 ±125 Ma is correct (to the

TABLE 1. Rubidium-strontium data for whole-rock samples 

from the Owl Creek Mountains, Wyo.

Sample 

number

Rb 

(ppm)

Sr 

(ppm) Sr 7Sr/ 86Sr

Unaltered granite

41596

29394

23432

41591

29467

30511

23496

30501

32466

185.9

193.2

171.3

188.8

196.6

175.6

193.3

187.5

159.0

218.0

235.3

190.2

252.3

232.1

247.7

190.8

241.2

201.2

2.491

2.396

2.630

2.184

2.474

2.067

2.964

2.267

2.307

0.7982

.7947

.8030

.7871

.7993

.7820

.8166

.7898

.7916

Altered granite

TDG12

TDG47

58-80

58-66

58-98

GP-1

GP-4

292.1

144.9

196.7

230.1

166.5

735.0

34.3

79.5

116.8

95.4

93.6

101.3

8.58

590.1

11.049

3.637

6.087

7.293

4.839

1807.5

.1682

1.1068

.8339

.9162

.9649

.8780

65.04

.7104

Metamorphic rocks

DKCH-207

DKCH-254

DKCH-263

58188

88.0

98.1

94.3

117.6

193.6

99.2

132.5

286.7

1.322

2.894

2.076

1.193

0.7582

.8246

.7932

.7580

0.82

0.80 -

0.78 -

0.76 -

0.74

0.72 -

0.70

Age = 2640 ± 125 Ma 

I = 0.7033 ± 0.0042

FIGURE 1. Rubidium-strontium isochron diagram for unaltered granite samples from the North Canning area, Owl Creek Mountains, 

Wyo. Symbol size corresponds to ±2tr analytical error.
GEOCHRONOLOGY 29

extent that the data fit the mathematical assumptions 

of model 3) and that the scatter of the rubidium-strontium data for the granite of the Owl Creek Mountains 

(fig. 1) is a function of variable 87Sr/86Sr caused by 

melting of an isotopically heterogeneous source and 

crystallization prior to homogenization of the magma. 

Assimilation of metasedimentary wall rock might also 

explain the variable initial 87Sr/86Sr; however, the 8180 

values of the metamorphic rocks are low, and the 87Sr/ 

86Sr ratios of the metamorphic rocks at the time of intrusion were high. Hence, assimilation of these rocks 

would produce a negative trend of 8180 versus initial 

87Sr/86Sr. Also, such a mechanism would be incompatible with the Q-mode analyses of the chemical data 

(Stuckless and others, this volume).

The remaining seven granite samples in table 1 show 

various types of evidence for alteration. Sample GP-4 

is a silicified-epidotized granite, sample GP-1 is an albitized and garnetiferous granite, and the other five 

samples contain abundant (secondary?) muscovite. 

These seven samples do not define a precise rubidiumstrontium isochron (fig. 3) but give an apparent age 

of 2463 Ma. For reasons that are not totally understood, rubidium-rich and strontium-poor samples like 

GP-1 frequently yield low apparent rubidium-strontium 

ages relative to samples of lower Rb/Sr from the same 

rock unit. For geologic (Gilluly, 1933) and isotopic 

reasons (Stuckless and others, 1981), albitized samples 

such as GP-1 are thought to be the product of postmagmatic hydrothermal alteration. Samples similar to GP-4 

have been interpreted to have been altered hydrothermally after crystallization of a granite (Peterman and 

Hildreth, 1978; Stuckless and others, 1981). The remaining five samples, which are all from the North

8.4 

8.2 

8.0 

7.8 

7.6 

7.4 

7.2 

7.0

29467

0.701 0.702 0.703 0.704 

INITIAL 87Sr/86Sr

0.705 0.706

FIGURE 2. Plot showing correlation of initial ^Sr/^Sr with wholerock 8180 values for unaltered granite samples from the Owl Creek 

Mountains, Wyo. Symbol sizes correspond to ±2a.

Canning area, seem to represent an event that is separate from the two types of hydrothermal alteration 

noted for samples GP-1 and GP-4.

The altered samples from the North Canning area 

have distinctly different rubidium-strontium systematics relative to the unaltered samples from the same 

area. The unaltered samples have very limited ranges 

of rubidium and strontium contents (rubidium range 

from 159 to 197, average 183; strontium range from 

190 to 252, average 223). The altered samples from this 

same area have similar rubidium contents (range from 

145 to 230, average 185) but markedly lower strontium 

contents (range from 94 to 116, average 102). The alteration process appears, therefore, to have had little effect on the rubidium, but to have lowered the strontium 

contents to something less than half of the original values. This postulated expulsion of strontium is consistent with the petrographic observation that, during alteration, potassium feldspar was partially transformed 

to muscovite. Potassium feldspar accepts appreciable 

strontium, whereas muscovite does not. Assuming that 

such is indeed the mechanism of strontium loss and that 

the altered samples originally had strontium contents 

like those of the unaltered samples, we can calculate 

that the time of alteration was approximately 2300 Ma. 

According to this interpretation, the "age" of 2463 Ma, 

shown in figure 2, results either from multiple types 

of alteration that may be of different ages or from scatter indicative of a resetting process that was less than 

perfect.

The mechanism and driving force needed to locally 

transform potassium feldspar to muscovite are not un1.1 -

4 6

87Rb/86Sr

10

FIGURE 3. Rubidium-strontium isochron diagram for altered granite 

samples. Data for sample GP-1 (indicated by arrow) plot well 

beyond the boundaries of the diagram, but on the isochron. Symbol 

size corresponds to ±2<r except data for GP-4 which plot as a 

dot in the center of the square.
30 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

derstood, but it is noteworthy that the alteration was 

patchy and that samples separated by less than 100 m 

could exhibit very different rubidium-strontium wholerock systematics. The event was probably not accompanied by regional heating (as documented for granites 

approximately 80 km to the south by Peterman and 

Hildreth, 1978) because rubidium-strontium ages for 

pegmatitic minerals were not reset (Giletti and Gast, 

1961). It also appears that the alteration did not affect 

whole-rock 8180 values because values for four of the 

altered samples from North Canning are within the 

range of values for unaltered samples from the same 

area (Stuckless and others, this volume). This similarity 

in 8180 suggests that the volume of fluid was small relative to the volume of rock. Alternatively, the fluid had 

a fortuitously appropriate isotopic composition relative 

to temperature of alteration, or possibly alteration occurred at too high a temperature to cause isotopic fractionation. The latter seems unlikely in view of the evidence against significant regional heating. Finally, even 

the whole-rock chemistry of samples identified as altered by rubidium-strontium systematics and supported 

by petrographic observations is apparently unaffected 

because at least some of the altered samples are well 

accounted for by Q-mode factor analysis (Stuckless and 

others, this volume). Although the specifics of alteration are not understood, the conclusion that subtle 

changes in mineralogic assemblages can indicate large 

effects on rubidium-strontium systematics of individual, 

closely spaced samples without concomitant evidence 

from major-element chemistry or oxygen isotope data 

has significant implications for rubidium-strontium 

geochronology.

Six zircon fractions from two hydrothermally altered 

rocks and one apparently unaltered rock were analyzed 

for uranium and lead isotopic compositions (table 2). 

The results yield little information about the age of the 

granite. The data fail to define a unique line on a concordia diagram (fig. 4), and all that can be said with 

complete certainty is that the zircons indicate an age 

in excess of 2514 Ma (the oldest observed lead-lead 

age).

If the intrusive age for the granite is 2640 Ma, as 

indicated by the rubidium-strontium data for the unaltered samples, the zircon data points should lie along 

a line whose upper intercept is 2640 Ma and whose 

lower intercept approximates the time of the Tertiary 

uplift (for example, the heavy solid line on fig. 4). By 

assuming that only one Precambrian event has affected 

the samples since crystallization, we can place some 

limits on when that event occurred. It must have happened after 2393 Ma, because this is the youngest leadlead age, but before 1700 Ma, which is the age calculated by passing a line from 2640 Ma through the data

TABLE 2. Uranium-lead isotopic data for zircon separates from 

granites of the Owl Creek Mountains, Wyo.

Concentrations

Atomic ratios Ages

Sample 

number

GP-5-1

GP-5-2

GP-5-3

58-80-1

58-80-2

41596

U 

(ppm)

3124

2701

3965

4128

4562

719

Pb

(ppm)

2161

1321

2776

1468

1733

204

238J

0.4033

.2970

.4264

.3024

.3182

.1482

235J

9.211

6.642

9.3561

6.494

6.766

3.226

206p b

0.1657

.1622

.1591

.1558

.1542

.1579

238J

2184

1676

2290

1703

1789

891

235 0

2359

2065

2375

2045

2081

1463

206y

2514

2479

2446

2410

2393

2433

point for the zircon closest to concordia (GP-5-3). The 

former case requires that the Precambrian event totally 

reset sample 58-80-2, whereas the latter requires that 

no lead has been lost recently from GP-5-3. Neither 

extreme seems particularly appealing, and the second 

case is the more unlikely. We, therefore, conclude that 

the resetting of the zircons occurred between 2393 and 

1700 Ma, and perhaps nearer the upper than the lower 

part of the range. In other words, the zircons may have 

been partly reset at the same time as the rubidiumstrontium systems of the altered rocks (about 2300 Ma), 

and the zircons may also have been disturbed during 

the Tertiary uplift or mineralization event.

In addition to an early and recent lead loss, there 

may have been a lead loss from the zircons during the 

late-Precambrian unroofing. Lead loss at that time 

would not greatly alter the above interpretation, but 

the upper limit for the earlier Proterozoic event would 

be increased to about 2460 Ma. A lead loss during the 

late Precambrian eliminates the paradox of a zircon 

(41596) that was affected by the 1700- to 2300-Ma event 

in a sample for which the rubidium-strontium wholerock system was unaffected (fig. 5).

The mechanism by which most of the zircons were 

reset is not clear, but some of the salient features are 

noteworthy. The alteration event was local and not regional. No Early Proterozoic resetting was encountered 

for zircons in the studies (cited in the introduction) of 

the surrounding Precambrian exposures except for 

small hydrothermally altered outcrops (Ludwig and 

Stuckless, 1978).

The zircon data are unusual in that the common-lead 

and uranium contents of these zircons are exceptionally 

high. Uranium analyses for 963 zircons from granitic 

rocks yield a geometric mean of 636 ppm with a standard deviation of +934 and -379 ppm (Stuckless and 

VanTrump, 1982). All of the uranium contents for zircons from the altered rock are more than two standard 

deviations greater than this mean, and three of the five 

are more than three standard deviations greater than 

this mean. In contrast, the uranium content for the one 

zircon from an apparently unaltered granite is indistinguishable from that of an average granitic zircon. Also
GEOCHRONOLOGY 31

0.5 

207pb/235U

FIGURE 4. Concordia plot for uranium-lead data from zircons. Symbol sizes represent analytical error at the 2a level. The point labeled 

A represents the intrusive age, and B represents the time of alteration as indicated by the rubidium-strontium data. The possible 

limits for B are indicated by B'and B". The line from A to near the origin indicates where the data would have plotted if they 

had not been affected by the event at time B'. According to this interpretation, at time B data points were displaced from A 

towards B and would have plotted along line A-B had no further lead loss occurred. In response to the Tertiary uplift and uncovering, 

the zircons again lost lead such that data points were displaced from line A-B to their current positions along paths indicated by 

the dashed lines.

exceptional is the fact that the frequently observed relationships of increasing degree of discordance with increasing uranium content of the zircons is only partly 

true for the zircons from the altered rock. If all the 

data are considered, no such correlation is seen because 

the zircon with the lowest uranium content is the most 

discordant. Whether these features represent cause or 

effect of the zircon resetting is unknown, but the problem merits further study.

No real effort was made to determine the age of the 

metamorphic complex; however, a few drill-core samples were analyzed in order to test the possibility that 

the granitic magma was derived from the metamorphic 

complex. Mueller and others (1981) reported an initial 

87Sr/86Sr for metadacite of the Owl Creek Mountains

of 0.7037 ±0.0015 at 2755 Ma; they pointed out that although the age was probably applicable to all of the 

metavolcanic rocks, the initial ratio need not be. The 

average metadacite would have been too radiogenic at 

2640 Ma to account for all but the highest calculated 

initial ^Sr/^Sr of the granite. At 2640 Ma, the 87Sr/ 

86Sr ratios calculated for three metagraywackes and

one granitic gneiss analyzed as part of this study (table 

1, the last four samples) range from 0.7077 to 0.7140. 

These analyzed metamorphic rocks were, therefore, too 

radiogenic to have been a source material for a granite 

for which calculated initial 87Sr/86Sr ratios range from 

0.7025 to 0.7047. This conclusion is valid even though 

our samples of the metamorphic sequence do not yield 

an isochronal relationship if it is assumed that these
32 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

12 14

207pb/235(J

FIGURE 5. Concordia plot for uranium-lead data from zircons assuming the added complexity of lead loss at 600 Ma. Symbol sizes represent 

analytical error at the 2o- level. Points A and B represent the time of intrusion and alteration, respectively, and B' and B" represent 

the maximum limits of B. In this alternate interpretation, data for all of the zircons except 41596 would plot along the line A-B at 

time B. All of the zircons would have then responded to the late Precambrian uplift and unroofing such that data points were displaced 

along the dot-dash vectors. Finally, data points were moved along the dash lines in response to the Tertiary uplift event.

samples behaved as a closed system from 2640 Ma ago 

to the present. Thus, the metamorphic sequence as 

presently exposed is not a suitable source for the granitic magma. A suitable protolith would have lower 

rubidium-strontium ratios and consequently lower 87Sr/ 

^Sr values. Such a protolith may have been more mafic 

than the exposed metamorphic sequence. Alternatively, 

the protolith could be similar to the metamorphic sequence if rubidium-strontium ratios were lowered at 

least 300 Ma before generation of the granite.

Earlier depletion of rubidium in deeper parts of the 

metamorphic sequence by granulite fades metamorphism (Heier, 1960; Lambert and Heier, 1968) could 

have created a source for the granite of the Owl Creek 

Mountains that had low and inhomogeneous ^Sr/^Sr 

ratios of 2640 Ma. A source region that had experienced 

granulite facies metamorphism could also account for

the relatively low 8180 whole-rock values for a granite 

as evolved as the granite of the Owl Creek Mountains 

(Hoefs, 1980) and for such low initial ^Pb/^Pb ratios 

which could persist as a result of uranium depletion 

(Gray and Oversby, 1972).

CONCLUSIONS

Rubidium-strontium analyses of whole-rock samples 

of an Archean granite from the Owl Creek Mountains 

indicate an age of 2640 Ma. Results for altered samples 

can be interpreted as indicating an alteration age of 

about 2300 Ma and further suggest extensive strontium 

loss during alteration. Zircons from the granite were 

so severely affected that they give little usable age information. The resetting of the zircons may have occurred about 2300 Ma, but a second resetting in re-
GEOCHRONOLOGY 33

sponse to unroofing at the end of the Precambrian cannot be ruled out. Measured and published strontium 

isotopic data for the intruded metamorphic complex indicate that the granite could not have been derived by 

melting of these rocks at their current level of exposure. However, if the same or similar rocks exist at 

depth and were metamorphosed to granulite facies at 

3000 Ma or earlier, they could account for the low and 

variable initial 87Sr/86Sr ratios observed for the granite.

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Arth, J. B., Barker, Fred, and Stern, T. W., 1980, Geochronology 

of Archean gneisses in the Lake Helen area, southwestern Big 

Horn Mountains, Wyoming: Precambrian Research, v. 11, 

p. 11-22.

Condie, K. C., Leech, A. P., and Baadsgaard, Halfdan, 1969, Potassium-argon ages of Precambrian mafic dikes in Wyoming: Geological Society of America Bulletin, v. 80, p. 899-906.

Giletti, B. J., and Gast, P. W., 1961, Absolute age of Precambrian 

rocks in Wyoming and Montana, in Kulp, J. L., ed., Geochronology of rock systems: Annals of the New York Academy of Sciences, v. 91, p. 454-458.

Gilluly, James, 1933, Replacement origin of albite granite near 

Sparta, Oregon: U.S. Geological Survey Professional Paper 175- 

C, p. 65-81.

Gray, C. M., and Oversby, V. M., 1972, The behavior of lead isotopes 

during granulite facies metamorphism: Geochimica et Cosmochimica Acta, v. 36, p. 939-952.

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metamorphic and igneous rocks on Langoy, northern Norway: 

Norge Geologiska Undersokelse, v. 207, 246 p.

Henry, D. J., Mueller, P. A., and Wooden, J. L., 1981, Early Archean granulite supracrustal assemblages, eastern Beartooth Mountains, Wyoming: Geological Society of America Abstracts with 

Programs, v. 13, p. 471.

Hoefs, J., 1980, Stable isotope geochemistry, 2d ed.: Berlin, 

Springer-Verlag, 218 p.

Krogh, T. E., 1973, A low-contamination method for hydrothermal 

decomposition of zircon and extraction of U and Pb for isotopic 

age determinations: Geochimica et Cosmochimica Acta, v. 37, p. 

485-494.

Lambert, I. B., and Heier, K. S., 1968, Geochemical investigations 

of high grade regional metamorphic and associated rocks in the 

Australian shield: Lithos, v. 1, p. 30-53.

Ludwig, K. R., 1979, A program in Hewlett-Packard BASIC for 

X-Y plotting and line-fitting of isotopic and other data: U.S. 

Geological Survey Open-File Report 79-1641, 33 p.

 1980, Calculations of uncertainties of U-Pb isotope data: Earth 

and Planetary Science Letters, v. 46, p. 212-220.

Ludwig, K. R., Nash, J. T., and Naeser, C. W., 1981, U-Pb isotope 

systematics and age of uranium mineralization, Midnite Mine, 

Washington: Economic Geology, v. 76, p. 89-110.

Ludwig, K. R., and Stuckless, J. S., 1978, Uranium-lead isotope systematics and apparent age of zircons and other minerals in Precambrian granitic rocks, Granite Mountains, Wyoming: Contributions to Mineralogy and Petrology, v. 65, p. 243-254.

Mueller, P. A., Peterman, Z. E., and Granath, J., 1981, An Archean 

bimodal volcanic series, Owl Creek Mountains, Wyoming: 

Geological Society of America Abstracts with Programs, v. 13, 

p. 515-516.

Naylor, R. S., Steiger, R. H., and Wasserburg, G. J., 1970, U-Th-Pb 

and Rb-Sr systematics of 2700 xlO6 year old plutons from the 

southern Wind River Range, Wyoming: Geochimica et Cosmochimica Acta, v. 34, p. 1133-1159.

Nkomo, I. T., Stuckless, J. S., Thaden, R. E., and Rosholt, J. N., 

1978, Petrology and uranium mobility of an early Precambrian 

granite from the Owl Creek Mountains, Wyoming: Wyoming 

Geological Association Guidebook, 30th annual field conference, 

p. 335-348.

Nunes, P. D. and Tilton, G. R., 1971, Uranium-lead ages of minerals 

from the Stillwater igneous complex and associated rocks, Montana: Geological Society of America Bulletin, v. 82, p. 2231-2250.

Peterman, Z. E., and Hildreth, R. A., 1978, Reconnaissance geology 

and geochronology of the Precambrian of the Granite Mountains, 

Wyoming: U.S. Geological Survey Professional Paper 1055, 

22 p.

Reed, J. C., Jr., and Zartman, R. E., 1973, Geochronology of the 

Precambrian rocks of the Teton Range, Wyoming: Geological Society of America Bulletin, v. 84, p. 561-582.

Steiger, R. H. and Jager, E., 1977, Subcommission geochronology: 

Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary Science Letters, v. 36, 

p. 359-362.

Stuckless, J. S., Nkomo, I. T., and Doe, B. R., 1981, U-Th-Pb systematics in hydrothermally altered granites from the Granite 

Mountains, Wyoming: Geochimica et Cosmochimica Acta, v. 45, 

p. 635-645.

Stuckless, J. S. and VanTrump, George, Jr., 1982, An evaluation 

of uranium contents in zircons from igneous rocks of the contiguous United States as an aid to exploration and resource assessment: Proceedings of the IAEA/OECD Symposium on Uranium 

Exploration Methods, Paris, June 1-4, 1982, p. 369-384.

Wenner, D. B., 1980, Oxygen isotope variations within the Elberton 

pluton, in Stormer, J. C., Jr., and Whitney, J. A., eds., Geological, geochemical and geophysical studies of the Elberton 

Batholith, eastern Georgia: A Guidebook to accompany the 15th 

annual Georgia Geological Society field trip, p. 31-43. 

-1981, Oxygen isotopic compositions of the late erogenic granites in the southern Piedmont of the Appalachian Mountains, 

U.S.A., and their relationship to subcrustal structures and 

lithologies: Earth and Planetary Science Letters, v. 54, 

p. 186-199.

Wooden, J. L., Muellers, P. A., and Bowes, D. R., 1981, Archean 

crustal evolution in the Beartooth Mountains, Montana-Wyoming: Geological Society of America Abstracts with Programs, 

v. 13, p. 585.

Uranium-Thorium-Lead 

Systematics of an 

Archean Granite from the 

Owl Creek Mountains, Wyoming

By J. S. STUCKLESS, I. T. NKOMO, and K. A. BUTT

CHEMICAL AND ISOTOPIC STUDIES OF GRANITIC ARCHEAN ROCKS, 

OWL CREEK MOUNTAINS, WYOMING

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1388-C

CONTENTS

Page

Abstract ............................................. 39

Introduction ........................................... 39

Analytical procedures ...................................... 40

Results .............................................. 40

Discussion ............................................ 41

Sample selection ...................................... 41

Age relationship ...................................... 41

Uranium mobility ...................................... 44

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

References ............................................ 48

ILLUSTRATIONS

FIGURE 1. Plot of 2S2Th/2MPb versus ^Pb/^Pb .............................................. 41

2. Plot of whole-rock 8180 values versus initial ^Pb/^Pb ratios ............................... 42

3. Lead-lead isochron plot ...................................................... 42

4. Concordia diagram ......................................................... 43

5. Photomicrograph and fission-track maps ............................................. 44

6. Ternary diagrams for uranium, thorium, and potassium contents .............................. 47

TABLE

TABLE 1. Uranium, thorium, and lead concentrations and the isotopic composition of lead ...................... 40

37

CHEMICAL AND ISOTOPIC STUDIES OF GRANITIC ARCHEAN ROCKS 

OWL CREEK MOUNTAINS, WYOMING

URANIUM-THORIUM-LEAD SYSTEMATICS OF AN ARCHEAN 

GRANITE FROM THE OWL CREEK MOUNTAINS, WYOMING

By J. S. STUCKLESS, I. T. NKOMO, and K. A. BUTT

ABSTRACT

Isotopic analyses of apparently unaltered whole-rock samples of a 

granite from the Owl Creek Mountains, Wyo., yield a lead-lead isochron age of 2,730 ±35 Ma, which is somewhat older than the age 

obtained by the rubidium-strontium whole-rock method. Thoriumlead data for the same samples deviate markedly from an isochronal 

relation; however, calculated initial 208Pb/204Pb ratios correlate with 

whole-rock 8180 values and lead to the conclusion that the 232Th-208Pb 

data are not colinear because of an originally heterogeneous granitic 

magma. This conclusion also has been drawn on the basis of chemical 

and other isotopic data and attributed to inhomogeneities in the protolith. Although the protolith apparently had a wide range in 232Th/ 

^Pb ratios, the range for ^U/^Pb ratios (|x) was small. The (x 

values for the protolith are high relative to an average orogene, and 

thus the source region for the granite was anomalously uraniferous.

Relationships in the mPbP*U-SMPbP*U system show that uranium was mobilized during early Laramide time or shortly before, 

such that most surface and shallow drill-core samples lost 60-80 percent of their uranium, and some fractured, deeper drill-core samples 

gained from 50 to 10,000 percent uranium. Fission-track maps show 

that much uranium is located along edges and cleavages of biotite 

and magnetite where it is readily accessible to oxidizing ground 

water. Furthermore, qualitative comparisons of uranium distribution 

in samples with excess radiogenic lead and in samples with approximately equilibrium amounts of uranium and lead suggest that the 

latter contain more uranium in these readily accessible sites.

Unlike other granites that have uranium distributions and isotopic 

systematics similar to those observed in this study, the granite of 

the Owl Creek Mountains is not associated with economic uranium 

deposits. We attribute this to the small areal extent of the granite, 

the shallow depth of leaching, and the timing of uranium mobilization, 

which apparently predated the deposition of sediments that elsewhere in central Wyoming host uranium ore.

INTRODUCTION

The granite of the Owl Creek Mountains is overlain 

by Tertiary sedimentary rocks that contain uraniummineralized areas, and the granite hosts moderate-sized 

areas of mineralized fractures (Yellich and others, 

1978). The study of uranium-thorium-lead isotope systematics in the Granite Mountains to the south has 

proved useful in demonstrating a probable connection

between Archean granite and Tertiary uranium deposits (Rosholt and others, 1973; Stuckless and Nkomo, 

1978). A preliminary isotopic study of the granite of 

the Owl Creek Mountains has suggested that this granite may also be related to uranium mineralization 

(Nkomo and others, 1978).

Uranium-thorium-lead systematics can yield information on uranium gain or loss from a granite, the timing 

of uranium movement (including multiple events in certain circumstances), the age of the granite, and constraints on the source of the granite (Rosholt and 

others, 1973; Stuckless and Nkomo, 1978). The method 

consists of determining the concentrations of uranium, 

thorium, and lead and the isotopic composition of lead 

for cogenetic samples that were isotopically homogeneous at the time of crystallization. Isochronal relationships are examined in the 232Th-208Pb and 208Pb-207Pb 

systems for linear fits within the limits of analytical 

error and agreement of calculated ages. Internally precise isochrons and agreement between ages calculated 

from the two isochrons indicate that uranogenic lead 

concentrations can be used as a measure of uranium 

gain or loss. If the isotopic composition of lead at the 

time of intrusion can be measured or closely estimated, 

relationships within the 207Pb/235U-206Pb/238U system 

can be used to place limits on the amount and timing 

of uranium mobility.

Results reported in this study build on information 

gained from chemical and other isotopic studies. Oxygen and strontium isotopic studies and uranium-lead 

analyses in zircons (Stuckless and others, this volume; 

Hedge and others, this volume) show that the granite 

of the Owl Creek Mountains was isotopically inhomogeneous at the time of intrusion and that isotopic 

systems in at least some samples have been affected 

by one or more postmagmatic events. Furthermore, not 

all of the spatially associated granitic samples are 

genetically related. These results indicate that many 

samples will not fit a simple interpretation of the ura39
40 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

TABLE 1. Uranium, thorium, and lead concentrations and the isotopic compositions of lead for samples

from the Owl Creek Mountains, Wyo.

Sample

23432

41596

29394

DP-2

32496

30511

DP-3

29467

GP-3

TD22

4343

TDG21

58180

TD-13

TDG46

GP-5

TDG47

TDG12

58-66

58-80

58-98

DP-1

TDG16

TDG20

58317

TDG45

TDG24

TD-6

TDG44

Plot 

I.D.

9

12b

13c

4

14b

13b

8

13d

17

21

7

22

lOd

1

lla

5

lib

2a

lOa

lOb

lOc

2b

24

23

lOf

6b

25

16

6a

U 

(ppm)

21.88

12.80

11.94

14.58

14.44

9.16

3.39

8.42

9.22

2.44

16.08

2.29

160.5

3.72

71.79

3.09

78.41

12.89

5.29

11.80

5.37

2.87

3.77

2.42

1994

2.78

4.68

.40

14.50

Th 

(ppm)

24.51

20.69

42.26

26.61

24.32

36.53

38.55

25.32

44.92

47.54

59.49

50.82

50.09

50.03

28.97

20.22

34.50

34.44

29.08

29.13

26.29

8.17

5.23

7.23

26.35

2.07

16.58

2.13

35.02

Pb 

(ppm)

52.83

45.47

46.09

56.93

50.99

51.20

46.41

38.12

75.61

56.51

61.41

54.92

91.27

67.01

60.66

42.5

75.21

38.26

56.33

44.65

46.41

62.45

41.22

8.15

81.11

10.94

36.03

42.26

70.48

2 oepb/2 .* Pb

27.645

23.573

22.244

26.097

22.748

22.067

22.163

22.126

26.385

22.337

22.723

20.113

59.285

22.980

31.268

22.795

77.352

28.307

26.172

28.465

26.091

17.159

23.261

34.811

54.263

17.230

19.551

15.200

23.199

207pb/20itpb

17.570

16.837

16.568

17.303

16.683

16.533

16.542

16.520

17.350

16.604

16.669

16.158

23.152

16.741

18.102

16.701

26.307

17.616

17.299

17.666

17.299

15.673

16.547

19.191

21.700

15.681

16.070

15.389

16.745

a.»Pb/2 o, Pb

38.245

37.451

42.824

37.698

38.070

40.344

41.229

40.283

39.451

41.184

42.861

42.186

42.011

41.131

39.140

36.001

42.544

41.248

36.310

38.088

36.754

34.338

35.306

39.501

37.491

36.005

37.119

34.470

37.291

mum-thorium-lead systematics. Thus, for isotopic purposes, the data are considered as three subsets: (1) the 

main granite intrusion, which is subdivided into (la) 

samples apparently unaffected by postmagmatic hydrothermal alteration and (Ib) hydrothermally altered 

samples, and (2) samples not genetically related to the 

main intrusion.

ANALYTICAL PROCEDURES

Uranium, thorium, and lead concentrations for 29 

whole-rock samples were determined by isotope dilution and mass spectrometry. Compositions and concentrations were determined on separate dissolutions, and 

therefore a minimum of 1-g splits of 200 mesh powder 

was used for each sample in order to minimize splitting 

errors. Most samples were analyzed in duplicate and 

average results are reported in table 1. Examination 

of duplicate analyses indicates a precision of ±0.5 percent for elemental ratios (2a) and ±0.2 percent for 

isotopic composition ratios (2a). Details of chemical and

mass spectrometric procedures are nearly the same as 

those described by Rosholt and others (1973) except 

that lead was analyzed using the silica gel-H3P04 

method.

Regression analysis of the data used the methods of 

York (1969). Decay and isotopic constants used are 

those recommended by the IUGS Subcommission on 

Geochronology (Steiger and Jager, 1977). Reported errors are at the 95 percent confidence limit.

RESULTS

Data for the granite of the Owl Creek Mountains 

exhibit extremely large ranges in both elemental concentrations and isotopic compositions. Uranium concentrations range from 0.40 to 1,994 ppm. Exclusion of 

four obviously mineralized samples (TDG46, 58180, 

TDG47, and 58314) still leaves an upper limit of 21.88 

ppm (sample 23432) and eight other samples that contain more than 10 ppm uranium. Thus the granite of 

the Owl Creek Mountains is anomalously uraniferous
URANIUM-THORIUM-LEAD SYSTEMATICS 41

relative to the typical range of 0.5-5.0 ppm uranium 

for silicic plutonic rocks (Rogers and Adams, 1969a). 

Thorium contents can be anomalously large and range 

from 2.07 to 59.49 ppm, which compares to a typical 

range of 10-20 ppm (Rogers and Adams, 1969b). Lead 

contents vary from 8.15 to 91.27 ppm and are generally 

greater than 40 ppm. These values contrast with an 

arithmetic mean lead content for granites of 23.6 ppm 

(Wedepohl, 1974) and reflect a large radiogenic lead 

component in the total lead.

The isotopic composition of lead shows a wide range 

of values (^Pb/^Pb, 15.200 to 77.352; and 208Pb/204Pb, 

34.338 to 42.861). Most compositions are generally 

much more radiogenic than that of the average modern 

orogene. Its composition isotopically approximates that 

of the average mantle, upper crust, and lower crust 

(Doe and Zartman, 1979), and is given as 18.70 for

206pb/204pb and 3g gg for 208pb/204pb (Stacey and K^.

ers, 1975). This radiogenic tendency is reasonable in 

view of the Archean age and high uranium and thorium 

contents. However, three samples (DP-1, TDG45, and 

TD-6) are less radiogenic than the average erogenic 

lead.

DISCUSSION 

SAMPLE SELECTION

Results of extended Q-mode factor analyses have 

shown that several of the analyzed samples are not related chemically to the main granite intrusion (Stuckless and others, this volume). Therefore, these samples 

(the last eight on table 1) were not used in the construction of isochron diagrams. Likewise, the rubidiumstrontium system has been largely reset by a postmagmatic, hydrothermal alteration for five samples (Hedge 

and others, this volume). These samples (TDG47 

through 58-98, table 1) were also not used in isochron 

regressions. Ten of the remaining sixteen samples could 

not be evaluated by this criterion because they were 

not analyzed isotopically for rubidium and strontium. 

However, three of these samples (TD-13, TDG46, and 

GP-5) are nearly colinear with the hydrothermally altered samples on a log-log plot of rubidium and strontium concentration data and, therefore, may be reset 

samples. The first 13 samples in table 1 are thought 

to be cogenetic and are apparently unaltered.

AGE RELATIONSHIPS

A plot of the thorium-lead isotopic data (fig. 1) for 

the 21 samples for which a probable genetic relationship 

has been established by extended Q-mode factor analysis reveals a marked deviation from linearity that is 

well in excess of that predicted by analytical error. Fif43

41

| 39

CM

£

837

35

33

2a

20 40 

232Th/204Pb

60 80

FIGURE 1. Plot of 232Th/204Pb versus ^Pb/^Pb for apparently 

cogenetic samples from the granite of the Owl Creek Mountains. 

Reference line corresponds to an age of 2,700 Ma. Samples that 

are suspected to have been affected by postmagmatic alteration 

are shown by x's. Other samples are shown by crosses that correspond to ±2er analytical error.

teen of the samples scatter about a trend with a slope 

that corresponds to an age of 2,700 Ma. Four of five 

hydrothermally altered samples (lOa, lOb, lOc, and 2a, 

fig. 1) as defined by the rubidium-strontium system 

(Hedge and others, this volume) are displaced towards 

higher 282Th/204Pb values and form a second trend. A 

fifth sample (5, fig. 1) lies along this second trend and 

therefore was most likely affected by the postmagmatic 

hydrothermal alteration. Displacements from the first 

to the second trend are probably due to radiogenic lead 

loss as discussed later in this section. One sample (lib, 

fig. 1) that plots with the hydrothermally altered samples in the rubidium-strontium system has an anomalously high 208pb/232Th yalue reiatjve to either Of ^

trends, which suggests that this sample may have 

gained some radiogenic lead long after crystallization 

of the granite.

Analysis of oxygen and strontium isotopic data has 

shown that the granite was isotopically inhomogeneous 

at the time of intrusion and that variations in oxygen 

and initial strontium isotopic compositions are correlative (Hedge and others, this volume). Figure 2 shows 

a similar correlation for 8180 and calculated initial 

2ospb/204pb ratjos Tne good correlation between calculated initial-lead composition and S180 suggests (1) that 

the excess scatter about the reference 2,700-Ma isochron on figure 1 can be explained by isotopic inhomogeneities in the magma at the time of intrusion, 

and (2) that, for most samples, the thorium-lead system 

has remained closed throughout the history of the granite.
42

34.0

33.6 -

33.2 -

< 32.8

32.4 ^

32.0

GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

17.75

818Q

FIGURE 2. Plot of whole-rock 8180 values versus initial 

ratios assuming an intrusive age of 2,700 Ma. Symbol size corresponds to errors of ±1.2 percent for 8180 and ±0.5 percent for 

initial **Pbf**Pb.

The range in calculated initial 208Pb/204Pb ratios is 

very large (32.472 to 33.689) relative to that observed 

for calculated initial 87Sr/86Sr ratios (0.7025 to 0.7047, 

Hedge and others, this volume). However, the range 

in initial lead compositions can be accounted for by a 

range of 11 to 59 for 232Th/204Pb in the source region. 

(The calculations assume an age of 3.2 Ga for the source 

region and an initial ^Pb/^Pb ratio of 32.202 from 

the model of Stacey and Kramers, 1975.) Ranges in 

232Th/204Pb in excess of 200 have been noted for cogenetic suites of granite (for example, Stuckless and 

Nkomo, 1978), and thus, large variations in initial 

208Pb/204Pb are reasonable for a heterogeneous source 

region.

The assumed age of the source region of 3.2 Ga is 

supported by the low initial 208Pb/204Pb calculated for 

sample TD22 (32.472) and a calculated initial 208Pb/ 

204Pb for a feldspar (32.64, Nkomo and others, 1978). 

Such low values require a 232Th/204Pb in the source that 

is significantly lower than that of the average orogene 

of Stacey and Kramers (1975) for at least the time 

period of 3.1-2.7 Ga. Granulite facies rocks of approximately 3.2-Ga age are reported for the Beartooth 

Mountains to the northwest (Henry and others, 1981; 

Wooden and others, 1981), and therefore, 3.2 Ga has 

been used to calculate 28aTh/204Pb ratios in the source 

region.

Data for all but two of the samples that scatter about 

the reference isochron in the thorium-lead system plot 

along a lead-lead isochron within the limits of analytical 

precision (fig. 3). The resulting age is 2,730 ±35 Ma, 

which is slightly older than (but not distinct from) that 

obtained by rubidium-strontium dating (2,640 + 125 Ma, 

Hedge and others, this volume). Not shown on figure

17.35

16.95

S 16.55

16.15

15.75

13c

'22

19 21 23 

206pb/204pb

25 27

FIGURE 3. Lead-lead isochron plot for granite samples from the Owl 

Creek Mountains. Symbol size corresponds to ±2cr errors. Slope 

of the isochron (0.18834 ±0.00380) corresponds to an age of 

2730 ±35 Ma. Intercept is 12.382 ±0.089.

3 are two samples (58180 and TDG46) that plot along 

the trends for thorium-lead isotopic data (fig. 1) and 

the 8 180-initial 208Pb/204Pb data (fig. 2). These samples 

have been mineralized and plot below the lead-lead isochron. This direction of displacement from the leadlead isochron indicates that mineralization occurred 

long after crystallization of the granite, as discussed 

in the next section.

Also excluded from figure 3 are data for the hydrothermally altered samples (TDG47 through 58-98, table 

1). Most of these data points are displaced from the 

lead-lead isochron and plot along a trend for which 

slope corresponds to an age of about 2,600 Ma. This 

trend, if shown, would cross the isochron shown on figure 3 at a shallow angle, and therefore some samples 

(GP-5, 58-98, and 58-66) could be interpreted as belonging to either group.

The excellent linearity of data in the lead-lead system 

is somewhat surprising in view of the large degree of 

scatter in the thorium-lead system. Although the 

source region for the granite had highly variable ^^h/ 

204Pb ratios, the variation in ^U/^Pb ratios must 

have been small. As a result, the initial 206Pb/204Pb and 

207Pb/204Pb ratios were fairly similar throughout the 

granite, and an estimate of the isotopic composition 

from the intersection of the lead-lead isochron and the 

model lead-lead secondary isochron of Stacey and 

Kramers (1975) yield 206Pb/204Pb=14.156 and 207Pb/ 

204Pb=15.044. These correspond to an average JJL (238U/ 

204Pb) of 11.64 for the second stage of the Stacey and 

Kramers (1975) lead evolution. Thus the average jx for 

the source material for the 1 b.y. prior to intrusion 

of the granite was greater than that calculated by
URANIUM-THORIUM-LEAD SYSTEMATICS 43

2.8

2.4

2.0

1.6

1.2

0.8

0.4

0.68

0.64 -

060

0.56

0.52

0.48

20 40 

207pb/235U

60 80

FIGURE 4. Concordia diagram for all granitic samples from the Owl Creek Mountains. Data for genetically related samples (as defined 

by extended Q-mode analysis) are shown by squares. Data for apparently unrelated samples are shown by circles. The regressed 

chord uses data for the first 15 samples in table 1 (those that fit a trend in the thorium-lead system) and intersects concordia 

at 2,695 ±20 Ma and 120 ±40 Ma. The chord is marked to show data positions for Tertiary uranium losses of 40, 60, 70, and 

80 percent. Inset shows detail around the upper intercept.

Stacey and Kramers (1975) for an average orogene 

(9.64). Furthermore, a JJL of 11.64 is a minimum value 

if the metamorphic sequence (which is much less than 

1 b.y. older than the granite) is the protolith for the 

granite. Therefore, the protolith for the granite must 

have been more strongly enriched in uranium than the 

average orogene.

The common-lead composition can be used to calculate the radiogenic component of the lead, and a concordia diagram (fig. 4) can then be constructed. Regression 

of the data for the apparently unaltered samples yields 

intercept ages of 2,695 ±20 and 120 ±40 Ma. Inclusion 

of data for samples that have been affected by the postmagmatic hydrothermal alteration changes the intercept ages to 2,680 ±30 and 120 ±100 Ma. Most of the 

data for the altered samples plots to the left of the

chord for the unaltered samples and indicates at least 

a partial resetting some time after 2600 Ma.

The lower intercept ages calculated with or without 

the hydrothermally altered samples are distinctly older 

than the 20 ±10 Ma for whole-rock samples from the 

Granite Mountains (Stuckless and Nkomo, 1978). This 

difference suggests that the main episode of uranium 

mobility occurred prior to burial by volcanic detritus 

in early Tertiary time and prior to formation of the 

large uranium deposits of the Gas Hills district to the 

south which formed about 35 Ma (Ludwig, 1979). However, a-activity ratios for three drill-core samples 

(Nkomo and others, 1978) show marked disequilibrium 

within the uranium decay chain, which indicates that 

uranium or its daughter products or both have been 

mobile within the last hundred thousand years.
44 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

FIGURE 5 (above and facing page). Photomicrograph (A) and fission-track detector (B) pairs for granite samples from the Owl 

Creek Mountains. Mica detectors, directly below corresponding photomicrographs, were photographed from the back side so 

that correspondence is not mirror image. Mineral abbreviations are M=magnetite, B=biotite, P=plagioclase, I=ilmenorutile, 

A=allanite, E=epidote, and H=hematite. Samples were irradiated with a total neutron dose of approximately 2xl016/cm2. Uranium concentrations in excess of 30 ppm appear as black areas.

URANIUM MOBILITY

Figure 4 shows that most of the samples gained or 

lost some uranium during or just, before earliest 

Laramide time, and that most samples approximated 

closed-system behavior from the time of intrusion until 

the Laramide disturbance. These conclusions are supported both by the fit of the data (for the unaltered 

samples) to a chord within the limits of analytical precision, and by general agreement of the upper intercept 

age with that determined by rubidium-strontium and 

lead-lead methods.

Data for samples that are presumed to have been 

affected by the postulated hydrothermal alteration plot 

to the left of the chord on figure 4. This displacement 

can be caused by uranium gain or lead loss at the time 

of hydrothermal alteration (approximately 2,300 Ma 

ago, Hedge and others, this volume). The position of
URANIUM-THORIUM-LEAD SYSTEMATICS 45

5mm 5mm

these samples relative to the thorium-lead reference 

isochron (fig. 1) can be explained by either lead loss 

or thorium gain. Many studies have shown that thorium 

is generally immobile in granitic rocks and that lead 

can be mobilized under many conditions (Manton, 1973; 

Zielinski and others, 1977; Stuckless and Nkomo, 1978; 

Doe and Delevaux, 1980; Stuckless, Nkomo and Doe, 

1981; Zielinski and others, 1981; Doe and others, 1982). 

Furthermore, potassium feldspar, which forms a major 

site for lead in granite, has been altered in the hydrothermal event (Hedge and others, this volume). Thus, 

all lines of evidence suggest that the altered samples 

lost lead in response to postmagmatic hydrothermal activity.

The uranogenic portion of the lead and measured uranium contents can be used to estimate the amount and 

timing of uranium mobility (Stuckless and Nkomo, 

1978). The apparent immobility of lead in the thoriumlead system for the unaltered samples (as suggested 

by the correlation between calculated initial 208Pb/204Pb 

ratios and 8180 values) indicates that estimates of uranium mobility should be fairly accurate (± 10 percent), 

with most of the error attributed to the uncertainty 

in the initial ^Pb/^Pb and 207Pb/204Pb ratios. Because 

lead loss from the altered rocks probably occurred not 

too long after the time of intrusion (relative to the age 

of the granite), reasonable inferences about uranium 

mobility can be made for these samples; however, esti-
46 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

mates of uranium loss will be slightly low, and estimates of uranium gain will be slightly high. Estimates 

of uranium mobility have also been made for the eight 

samples that do not have a cogenetic relationship to 

the main granite intrusion on the assumptions that the 

common lead compositions and ages for these samples 

are not very different from the parameters used to calculate radiogenic leads for the other samples. Interpretation of uranium mobility for these samples is therefore much less certain.

The chord on figure 4 is marked for percent uranium 

loss. Nine of the data points suggest uranium loss in 

excess of 60 percent during Laramide time or later. 

Seven of these points are for surface samples and two 

are for drill-core samples from a depth of less than 30 

m. In contrast, four drill-core samples from depths in 

excess of 60 m are mineralized and suggest uranium 

enrichments of 50 to nearly 10,000 percent. The fact 

that the data points both above and below concordia 

are colinear indicates that uranium loss from the surface and shallow drill-core samples occurred at the 

same time as uranium mineralization. Thus, the granite 

was the most likely source for, as well as host to, the 

uranium deposit. Samples that lost uranium are confined to the surface and near-surface environment, 

which suggests oxidizing ground water as a likely 

medium for uranium transport.

Both postulated uranium gain and loss for specific 

samples are consistent with the observed uranium distribution within the samples. Figure 5 shows uranium 

sites for samples that apparently gained or lost varying 

amounts of uranium. In general, the same primary and 

secondary uranium sites are present within samples 

that lost little or no uranium (such as DP-2, figs. 5-1 

and 5-4) and those that lost considerable uranium (such 

as DP-3, fig. 5-3). Most of the uranium is located within zircon, apatite, allanite, and ilmenorutile. These minerals probably constitute the primary or magmatic uranium sites. Epidote, which is commonly enclosed within 

biotite or allanite (figs. 5-3 and 5-4), generally contains 

very little uranium. Epidote is paragenetically late, but 

is nonetheless considered to be a primary site for uranium. Secondary sites include uranium intimately associated with hematite and microscopic opaque inclusions that are titanium-rich. Secondary uranium also occurs along edges, cleavage traces, and fractures in magnetite and biotite. Qualitatively, there appears to be 

less uranium in secondary sites in samples that show 

isotopic evidence for more than 50 percent uranium loss 

(figs. 5-3 and 5-4) than in samples that contain near 

equilibrium amounts of uranium and radiogenic lead.

Samples for which there is isotopic evidence for uranium gain reveal the same primary and secondary sites 

for uranium as other samples; but, in addition, secondary uranium occurs along fractures and grain boundaries of felsic minerals (fig. 5-2). There is also a tendency towards replacement of biotite by iron oxides 

that are highly uraniferous. Leaching experiments of 

mineralized granite from the Owl Creek Mountains 

(Maysilles and others, 1977) show that most if not all 

of the secondary uranium is rapidly removed by either 

strong acids or bases.

The primary and secondary sites for uranium noted 

for the granite of the Owl Creek Mountains are similar 

to those noted for granites that are associated with uranium deposits. Uranium associated with biotite and 

magnetite has been postulated as a source for the deposits in central Wyoming (Stuckless and others, 1977) 

and at the Midnite Mine, Wash. (Nash, 1979). A significant difference in uranium distribution for samples from 

the Owl Creek Mountains is that epidote from the granite of the Granite Mountains has been shown to be 

uraniferous and to have lost much uranium in the nearsurface environment (Stuckless and Nkomo, 1980).

Although both the results of Q-mode factor analysis 

and uranium-thorium-lead systematics indicate uranium 

mobility for the granite of the Owl Creek Mountains, 

comparison of radioelement ratios with those of granite 

from the Granite Mountains suggests a generally lesser 

degree of uranium mobility for the former (fig. 6). Both 

granites are enriched in thorium relative to potassium 

if compared to an average of 2,500 granites from the 

contiguous United States (Stuckless and VanTrump, 

1982), but the granite of the Granite Mountains is depleted in uranium relative to thorium whereas the granite of the Owl Creek Mountains is enriched. This reflects, in part, the large proportion of drill-core samples 

for the latter, but Stuckless and Nkomo (1978) reported 

as much as 80 percent uranium loss for samples from 

as deep as 400 m for the Granite Mountains, and the 

deepest sample used in the current study is from 156 

m. Thus, uranium in the granite of the Owl Creek 

Mountains seems to be less labile than that in the granite of the Granite Mountains, both in terms of depth 

of leaching and in amount of leaching from surface samples.

The lesser degree of uranium mobility, shallower 

depth of leaching, and smaller outcrop area may explain 

why the granite of the Owl Creek Mountains is not 

associated with large, economic uranium deposits. In 

addition, uranium mobilization for the granite of the 

Owl Creek Mountains probably predates formation of
URANIUM-THORIUM-LEAD SYSTEMATICS 47

GRANITE U 

MOUNTAINS (ppm)

OWL CREEK 

MOUNTAINS

(ppm) 20 40 60 80

KX10-4 

(ppm) 80

,KX10-4 

(ppm)

FIGURE 6. Ternary diagrams for uranium, thorium, and potassium contents for granites from the Owl Creek and Granite Mountains

(modified from Stuckless, Bunker, and others, 1981).

the Tertiary sedimentary rocks that host the uranium 

ore deposits of central Wyoming. As a result, the granite may provide a reasonable uranium source for only 

those mineralized zones that are located within the fractured portion of the granite.

CONCLUSIONS

Uranium-thorium-lead systematics for granite from 

the Owl Creek Mountains show that samples unaffected 

by hydrothermal alteration yield a lead-lead age of 

2,730 ±35 Ma. Large variations in the initial ^Pb/^Pb 

ratio preclude obtaining an age in the thorium-lead system. These variations, like those in initial 87Sr/86Sr, can 

be correlated with whole-rock 8180 values. Hydrothermally altered samples are displaced from both the leadlead isochron and thorium-lead reference isochron. The 

direction of displacement is consistent with lead loss 

that may have occurred during alteration of potassium 

feldspar to muscovite. A maximum age of 2,600 Ma is 

indicated for alteration. This age, in combination with 

the minimum age of 2,300 obtained by the rubidiumstrontium whole-rock isochron method, indicates the 

existence of a previously unknown Early Proterozoic 

event.

The large variations in initial 208Pb/204Pb ratios indicate a large but reasonable variation in 2saTh/204Pb 

ratios in the source region of the granite. In contrast, 

jx values for the source region were fairly uniform such 

that initial ^Pb/^Pb and ^Pb/^Pb ratios can be calculated. A minimum jx value of 11.64 indicates a uranium-enriched protolith for the granite.

A concordia plot for the unaltered granite samples 

yields intercepts of 2,695 ±20 and 120 ±40 Ma. The 

upper intercept agrees with the age obtained by leadlead and rubidium-strontium whole-rock isochron 

methods, and the data are colinear within the limits 

of analytical precision. Therefore, the uranium-lead system appears to have remained closed from the time of 

intrusion to approximately Laramide time. During 

early Laramide time or shortly before, mineralized 

samples gained 50-10,000 percent uranium, and at the 

same time, most surface and shallow drill-core samples 

lost 60-80 percent of their uranium. Thus the granite 

is interpreted to be both source and host for uranium 

mineralization.

The high degree of uranium mobility in the granite 

of the Owl Creek Mountains is reasonable in view of 

the uranium sites within the granite. Much uranium is 

located along cleavages and grain boundaries of biotite 

and magnetite where it would be readily accessible to 

oxidizing ground water. A similar distribution has been 

noted for uranium in granite associated with large uranium deposits (Stuckless and others, 1977; Nash, 1979). 

Furthermore, leached granite samples appear to have 

less uranium associated with biotite and magnetite than 

samples that contain near-equilibrium amounts of uranium and radiogenic lead.

In spite of the fact that there is much evidence for 

uranium mobility for the granite of the Owl Creek
48 GRANITIC ARCHEAN ROCKS, OWL CREEK MOUNTAINS, WYOMING

Mountains from results of uranium-thorium-lead systematics, Q-mode factor analysis, and petrographic observations, this granite is not associated with large 

economic deposits. The lack of economic association is 

thought to be due to the relatively small area of leached 

granite, the limited depth of leaching, and the fact that 

mobilization of uranium appears to predate formation 

of Tertiary sediments that host most of the uranium 

deposits of central Wyoming.

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