science for a changing world

Composition, Age, and Petrogenesis of Late Cretaceous 

Intrusive Rocks in the Central Big Belt Mountains, 

Broadwater and Meagher Counties, Montana

*

:A /••^dfe^

U.S. Geological Survey Professional Paper 1657

U.S. Department of the Interior 

U.S. Geological Survey
Cover. The Needles, about 6 kilometers south-southeast of Gipsy Lake, form spectacular outcrops 

of the quartz monzodiorite of Mount Edith.
Composition, Age, and Petrogenesis of Late Cretaceous 

Intrusive Rocks in the Central Big Belt Mountains, 

Broadwater and Meagher Counties, Montana

ByEA. du Bray andl.\N. Snee

U.S. Geological Survey Professional Paper 1657

U.S. Department of the Interior 

U.S. Geological Survey
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat Director

This publication is also available online at: 

http://geology.cr.usgs.gov/pub/ppapers/p1657/

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Version 1.0 2002

Library of Congress Cataloging-in-Publication Data

Du Bray, E.A.

Composition, age, and petrogenesis of late Cretaceous intrusive 

rocks in the central Big Belt Mountains, Broadwater and Meagher 

counties, Montana / by E.A. du Bray and LW. Snee.

p. cm. — (U.S. Geological Survey professional paper; 1657) 

Includes bibliographical references.

1. Geology, Stratigraphic—Cretaceous. 2. Rocks—Montana— 

Broadwater County—Analysis. 3. Rocks—Montana—Meagher 

County—Analysis. I. Snee, Lawrence W. II. Title. III. Series. 

QE688.D8 2002 

557.86'612—dc21 2002023527

ISBN: 0-607-98231-4
Contents

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

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

Acknowledgments........................................................................................................................ 3

Geology........................................................................................................................................... 3

Petrography................................................................................................................................... 3

Metal Deposits.............................................................................................................................. 4

Geochronology.............................................................................................................................. 5

Whole-Rock Chemistry................................................................................................................ 10

Major Oxide Data............................................................................................................... 10

Trace-Element Data........................................................................................................... 14

Mineral Compositions.................................................................................................................. 19

Discussion...................................................................................................................................... 23

Tectonic Setting................................................................................................................. 23

Origin of Compositional Variation in Granodiorite and Quartz Monzodiorite........... 23

Genesis of Quartz Monzonite in the Outer Zone........................................................... 27

Conclusions................................................................................................................................... 28

References Cited.......................................................................................................................... 29

Figures

1. Geologic map showing distribution of Late Cretaceous intrusive rocks

in central Big Belt Mountains, Montana ........................................................................... 2

2. Quartz-potassium feldspar-plagioclase ternary diagram showing modal

compositions of central Big Belt Mountains intrusions.................................................. 4

3. Diagrams showing 40Ar/39Ar age spectra for separates from intrusions in

central Big Belt Mountains.................................................................................................. 6

4. 40Ar/36Ar versus 39Ar/36Ar isotope correlation diagrams for biotite and

hornblende samples ............................................................................................................. 8

5. Variation diagrams showing abundances of major oxides and zirconium in

intrusions in central Big Belt Mountains........................................................................... 15

6. Total alkali-silica variation diagram for intrusions of central

Big Belt Mountains................................................................................................................ 18

7. Variation diagram showing relative proportions of K20, Na20, and CaO in

intrusions of central Big Belt Mountains........................................................................... 18

8. Chondrite-normalized extended trace-element diagrams showing

compositions of central Big Belt Mountains intrusions ................................................. 20

9. Chondrite-normalized rare-earth element diagrams showing

compositions of central Big Belt Mountains intrusions ................................................ 20

10. Ternary diagram showing molecular end-member proportions of feldspars

in samples of central Big Belt Mountains intrusions ...................................................... 22

III
11. Ternary Mg, R+3 (Al lv+Fe3++Ti), R+2 (Fe2++Mn) diagram showing relative cation

abundances for biotite in samples of central Big Belt Mountains intrusions ............ 22

12. Ternary diagram showing compositions of amphibole and clinopyroxene

in quartz monzodiorite of central Big Belt Mountains intrusions.................................. 22

13. Trace-element tectonic-setting-discrimination variation diagram for central

Big Belt Mountains intrusions............................................................................................. 24

14. Ternary AFM (I\la20+K20-Fe0*-Mg0) diagram showing compositions of

central Big Belt Mountains intrusions............................................................................... 24

15. Th/Sm vs. Th variation diagram showing compositions of central Big Belt

Mountains intrusions............................................................................................................ 25

16. Extended trace-element abundance diagram showing compositions of

central Big Belt Mountains intrusions............................................................................... 26

17. Ternary variation diagram showing relative proportions of Rb, K, and Sr in

granodiorite and quartz monzodiorite in central Big Belt Mountains.......................... 26

18. Rare-earth element diagram showing compositions of central

Big Belt Mountains intrusions............................................................................................. 26

Tables

1. Summary of 40Ar/39Ar age-spectrum results for intrusive rocks from central

Big Belt Mountains, Mont.................................................................................................... 10

2 40^/39^ incremental heating data for samples from central

Big Belt Mountains ............................................................................................................... 11

3. Composition of Cretaceous intrusive rocks from central Big Belt Mountains .......... 16

4. Instrumental neutron activation analysis of selected samples of Cretaceous

intrusive rocks from central Big Belt Mountains ............................................................ 18

5. Major oxide compositions of plagioclase in intrusive rocks from central

Big Belt Mountains ............................................................................................................... 20

6. Compositions of potassium feldspar in intrusive rocks from central

Big Belt Mountains ............................................................................................................... 21

7. Compositions of biotite in intrusive rocks from central Big Belt Mountains............... 21

8. Compositions of hornblende in intrusive rocks from central Big Belt Mountains..... 21

9. Compositions of clinopyroxene in intrusive rocks from central

Big Belt Mountains ............................................................................................................... 22

10. Representative compositions of accessory phases in intrusive rocks from

central Big Belt Mountains.................................................................................................. 23

11. Results of major oxide least-squares crystal fractionation-mixing modeling for

intrusive rocks from central Big Belt Mountains............................................................. 25

IV
Composition, Age, and Petrogenesis of Late Cretaceous 

Intrusive Rocks in the Central Big Belt Mountains, 

Broadwater and Meagher Counties, Montana

ByEA. du Bray andl.\N. Snee

Abstract

The central Big Belt Mountains in western Montana contain two Late Cretaceous intrusions that are satellitic to the 

Boulder batholith. New 40Ar/39Ar data demonstrate that these 

intrusions, their hydrothermally altered margins, and small 

peripheral metal deposits are temporally related, which indicates 

a genetic link among their causative magmato-hydrothermal 

processes. These deposits and associated altered rocks are similar to other recently described, globally dispersed intrusionrelated gold systems. As such, rocks in the central Big Belt 

Mountains help better define the physical and genetic characteristics of these deposits and establish the petrogenesis of their 

host granitoid intrusions.

Introduction

The Big Belt Mountains are in western Montana about 50 

km southeast of Helena (fig. 1). In contrast to igneous rocks in 

nearby areas, especially the Elkhorn Mountains Volcanics (Rutland and others, 1989) and the Boulder batholith (Tilling, 

1973) to the west, very little is known about intrusions exposed 

in the central Big Belt Mountains. A geologic map of the Confederate Gulch area and accompanying tabulation of regional 

geochemical data (Gualtieri, 1975a, b) are the sole published 

geologic information for the northern of the two principal 

intrusions in this area. The geology of the Duck Creek Pass 

quadrangle, immediately south of the area mapped by Gualtieri 

(1975a, b), was described by Nelson (1963), whose map provides geologic data for the extreme west edge of the central 

Big Belt Mountains.

Geologic mapping of the central Big Belt Mountains (du 

Bray, 1995a) documents the distribution of intrusions exposed in 

this area. These intrusions and their associated altered and mineralized rocks are an important research target because they are 

similar to recently described intrusions and associated gold 

deposits located elsewhere in the world (Lang and others, 2000, 

and references contained therein). Characterization of these 

intrusions, including their geochemistry and geochronology, and 

the deposits they host presents an opportunity to put these rocks

in a global context and to establish the tectonic and petrogenetic 

processes that controlled their evolution.

The intrusive suite of Boulder Baldy, composed of quartz 

monzodiorite and granodiorite, constitutes the northern intrusion; the southern intrusion, the quartz monzodiorite of Mount 

Edith, was previously unknown. New Ar/ Ar geochronologic data indicate that magmas represented by these rocks 

solidified sometime before 68.5 Ma, that all are part of a single magmatic event, and that regional cooling to below 150°C 

subsequent to magmatism was complete by about 60 Ma. 

Geochemical and geologic relations suggest that these calcalkaline intrusions are related to subduction and back-arc processes that prevailed along the late Mesozoic margin of western North America. Mixing calculations indicate that 

compositional variation, from mafic to felsic quartz monzodiorite and granodiorite, developed by crystallization and 

removal of varying amounts of plagioclase, clinopyroxene, 

hornblende, titanite, apatite, and magnetite, the principal constituents of these rocks. Solidification of the respective magma 

reservoirs involved crystal nucleation along bounding surfaces; residual silicate liquid at crystallization fronts separated 

from early-formed solids and remixed with undepleted magma 

away from crystallization fronts to produce progressively more 

evolved magma.

Unusual textures, mineralogy, and erratic compositional 

variation in the outer part of the intrusive suite of Boulder 

Baldy reflect extensive hydrothermal, metasomatic alteration 

and recrystallization of quartz monzodiorite to quartz monzonite by metal-bearing fluids. Hornblende was replaced by 

aegirine-augite, which in turn, in the most strongly altered 

rocks, was replaced by andradite. The abundance of fluorite in 

these rocks and the fact that these rocks are hosted by very fine 

grained Newland Formation that lacks significant permeability 

suggest that fluids responsible for the alteration were primarily 

magmatic. Textural features indicate that magma represented 

by rock in the outer part of the intrusive suite was pressure 

quenched, which may have caused rapid exsolution of fluids 

responsible for the observed alteration. The intensity of alteration is correlated with the extent of pressure quenching; 

quartz monzodiorite of Mount Edith, in which pressure 

quenching was limited and restricted to rock near bounding 

contacts, is unaltered.

1
46°35' _

46°30' -

46°25' -

Limit of geologic map

HELENA

° V-MAPAREA

SNOWBANK

MINE MINE

•y

Intrusive suite of 

Boulder Baldy

The Needles 

*+ 885

Quartz monzodiorite 

of Mount Edith

EXPLANATION

Qal Alluvium 

Qls Landslide deposits 

Qaf Alluvial fan deposits 

Qg Glacial deposits 

Kqm Quartz monzodiorite

of Mount Edith

Intrusive suite of Boulder Baldy 

Kbo Outer zone 

Kbi Intermediate zone 

Kbc Core zone 

Yn Ncwland Formation 

V Mine or prospect pit

8 KILOMETERS

n

SMILES

Figure 1. Distribution of I ate Cretaceous intrusive rocks in central Big Belt Mountains, Mont, (du Bray, 1995a). Sample numbers (at localities 

indicated by plus signs) must be prefaced by 202 to obtain complete designations.

Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
Acknowledgments

Field work was facilitated by Beth Ihle of the U.S. Forest 

Service, who helped obtain access to Forest and adjacent lands. 

Local landowners are thanked for providing access across their 

lands. Eugene E. Foord helped identify andradite garnet in some 

of the analyzed samples. Ross Yeoman conducted 40Ar/39Ar 

analyses. Technical reviews by Margaret Hiza and David John 

improved this report.

Geology

The Big Belt Mountains are underlain principally by Middle Proterozoic sedimentary rocks of the Belt Supergroup, 

mostly the Middle Proterozoic Newland Formation; other Belt 

Supergroup formations are also present. Between Confederate 

Gulch and the southern slopes of Mount Edith (fig. 1), the crest 

of the central Big Belt Mountains is composed largely of granitoid intrusions that are satellitic to the nearby Boulder batholith 

(Tilling, 1973). Gualtieri's (1975a) map shows an approximately circular, 10-km diameter intrusion centered between 

Boulder Baldy and Boulder Mountain and a smaller satellitic 

stock 1.5 km to the east. A large pluton, the quartz monzodiorite 

of Mount Edith, crops out north of Mount Edith and east of 

Baldy Mountain (du Bray, 1995a). Nelson (1963) mapped three 

small stocks arrayed along the west flank of the range in the 

north-central part of the Duck Creek Pass quadrangle. Mapping 

by du Bray (1995a) shows that these three stocks are actually 

contiguous apophyses of the quartz monzodiorite of Mount 

Edith. Geochronologic data compiled by Daniel and Berg 

(1981) indicate potassium-argon dates (recalculated using new 

constants of Steiger and Ja'ger, 1977, and tables of Dalrymple, 

1979) of 64.4±2.4 Ma (biotite) and 71.5±3.2 Ma (hornblende) 

for a pluton, several kilometers to the west, that is petrographically similar to quartz monzodiorite intrusions of the central Big 

Belt Mountains. Several factors, including close spatial association and petrographic similarities, suggest that all the plutonic 

rocks exposed along the crest of the central Big Belt Mountains 

are genetically related and part of a single magmatic episode. 

Gravity and aeromagnetic data (A.E. McCafferty, written commun., 1994) further indicate the subsurface continuity of central 

Big Belt Mountains intrusions.

Mapping by Gualtieri (1975a) accurately shows the geologic relations of the somewhat unusual, concentrically zoned, 

and generally poorly exposed intrusive suite centered between 

Boulder Baldy and Boulder Mountain. His map indicates three 

zones: (1) an outer zone composed of a syenite complex, including several textural variants, (2) an intermediate zone composed 

of monzonite porphyry, and (3) a core zone composed of 

monzonite. Mapping by du Bray (1995a) confirms that the Late 

Cretaceous intrusive suite of Boulder Baldy is a 10-km diameter 

intrusion composed of three concentrically arranged, compositionally and texturally distinct zones. Relative age relations 

among the three zones of this intrusion are indeterminate; similarly zoned plutons (Bateman, 1992), are youngest in their central parts, however. The textural and compositional diversity of

the outer 1.5- to 5-km wide zone of the intrusive suite is 

unusual, as is its spatial and apparent temporal and genetic associations with undistinctive calc-alkaline rock that forms the 

inner two zones. The textures and compositions of the outer 

zone rocks are not those of normally solidified granitoid rock; 

rather, they are like those of solidified and subsequently hydrothermally altered granitoid rock. The intermediate and core 

zones are composed of granitoid rock petrographically and compositionally similar to other calc-alkaline Cretaceous intrusions 

(the Boulder batholith, for instance) elsewhere in western Montana, and to calc-alkaline plutons that coalesce to form Mesozoic batholiths throughout the North American Cordillera. The 

annular intermediate zone is 0 to 2.5 km wide, and the elliptical 

core zone is 2.5 km wide and has a northwest-trending axis 

about 5 km long.

A small stock at the southeast edge of Gualtieri's (1975a) 

map is part of a generally elliptical, 5-km long by 2.5-km wide 

north-northwest-trending intrusion whose south end is about 1 

km northwest of Gipsy Lake (fig. 1). The northern two-thirds of 

this stock is composed of rock similar to that which forms the 

outer zone of the intrusive suite of Boulder Baldy. The southern 

third is composed of rock similar to that which forms the quartz 

monzodiorite of Mount Edith and the intermediate zone of the 

intrusive suite.

The area about 1 km northwest of Confederate Gulch contains three small stocks (Kbc, fig. 1) that are mineralogically 

and compositionally similar to rock forming the core zone of the 

intrusive suite of Boulder Baldy (Kbc at Boulder Baldy, fig. 1). 

The largest of these stocks, centered on Miller Mountain (coincident with sample 883, fig. 1), has been extensively prospected 

as a lode-gold source. The stock is cut by a stockwork consisting of numerous high-angle and crosscutting horizontal quartz 

veins. All the stocks exposed in this area are considered to be 

apophyses of the much larger intrusive suite of Boulder Baldy.

The quartz monzodiorite of Mount Edith forms an essentially undocumented pluton that is generally circular and 10 km 

in diameter. It is composed of quartz monzodiorite that is petrographically and compositionally similar to rock that forms the 

intermediate zone of the intrusive suite of Boulder Baldy. The 

close compositional and spatial association between quartz 

monzodiorite in the two plutons of the central Big Belt Mountains implies a possible common genesis. Most of the quartz 

monzodiorite of Mount Edith is covered by a mantle of glacial 

deposits. Only at The Needles (fig. 1), and in cirque walls north 

of Mount Edith and north-northeast of Baldy Mountain, is this 

pluton well exposed.

Petrography

Rock descriptions and classification in this study are a synthesis of petrographic examinations and stained slab modal analyses (du Bray, 1995a). Plutonic rock names are in accord with 

the IUGS system (Streckeisen, 1973). Indicated crystal sizes are 

approximate averages for their maximum dimensions. All mineral colors refer to the primary color noted in thin sections 

viewed in plane polarized light. Specific mineral names are 

based on electron microprobe composition data.

Petrography 3
Granodiorite (inner zone) of the intrusive suite of Boulder 

Baldy (fig. 2) is fine to medium grained and hypidiomorphic 

equigranular. Its color index is about 6; yellowish-brown to 

dark-brown anhedral to subhedral biotite (0.6 mm) is the sole 

mafic silicate mineral. Quartz (1 mm) is anhedral and weakly 

strained. Potassium feldspar (1.2 mm) and plagioclase (1.2 mm) 

are zoned, twinned, and subhedral. Trace constituents are magnetite, titanite, and apatite.

Hornblende quartz monzodiorite (intermediate zone) of the 

intrusive suite of Boulder Baldy is medium grained, hypidiomorphic inequigranular to seriate textured, and it contains distinctive, euhedral, perthitic phenocrysts of potassium feldspar as 

much as 3 cm long. Its color index is about 8, and subhedral yellow-green to dark-green hornblende (1.2 mm) is the sole mafic 

silicate mineral. Quartz (0.6 mm) is anhedral and interstitial. 

Subhedral plagioclase (1.7 mm) is twinned and zoned. Trace 

constituents are titanite, magnetite, apatite, and zircon. In some 

places, the quartz monzodiorite is characterized by a distinctly 

fine grained, anhedral groundmass that is interstitial to otherwise 

coarser grained and better formed crystals. The amount of interstitial groundmass is highly variable between samples.

Aegirine-augite quartz monzonite (outer zone) of the intrusive suite of Boulder Baldy is compositionally and texturally 

diverse alkali-feldspar-rich, syenitoid rock that ranges to monzodiorite, monzonite, syenite, quartz syenite, and alkali-feldspar 

syenite (fig. 2). Anhedral quartz (0.5 mm) is scarce to absent in 

much of this rock. A fine-grained, quenched or recrystallized 

groundmass is a nearly ubiquitous feature of this nearly 

equigranular to coarsely porphyritic rock. In contrast, the outer 

zone in some places is composed almost exclusively of aligned, 

densely packed, euhedral perthite phenocrysts as much as 6 cm 

long. The distribution of this variant is highly irregular; delineation of its extent is not possible because of notably sparse outcrop. In yet other places, the outer zone consists of intermingled 

pegmatitic and aplitic rock. The color index of the quartz 

monzonite is about 3; anhedral, emerald-green aegirine-augite to 

aegirine (0.5 mm) is the sole mafic silicate mineral. This rock 

type contains trace amounts to several modal percent anhedral 

andradite and abundant subhedral titanite. Some andradite is 

associated spatially with small altered grains of clinopyroxene 

that it appears to have replaced. Some samples contain trace 

amounts to several modal percent of widely disseminated purple 

fluorite. Trace constituents are magnetite, apatite, and zircon.

The quartz monzodiorite of Mount Edith is composed of 

perthite phenocryst-bearing hornblende quartz monzodiorite that 

is petrographically and compositionally similar to the intermediate zone of the intrusive suite of Boulder Baldy. Its color index 

is about 6; subhedral yellow-green to dark-green hornblende (1 

mm) is the principal mafic silicate mineral. Quartz (0.6 mm) is 

anhedral and weakly strained. Plagioclase (2 mm) is subhedral, 

twinned, and zoned. Trace constituents are titanite, magnetite, 

apatite, and zircon. Like the quartz monzodiorite of the intrusive 

suite of Boulder Baldy, the quartz monzodiorite of Mount Edith 

locally includes a groundmass that is fine grained and anhedral; 

this type of groundmass is restricted to rock near the contact 

between the intrusion and enclosing Newland Formation rocks, 

however. A mafic border phase, represented by sample No. 

202940 (table 3), is present at the outer edge of this intrusion in

QUARTZ

MONZONITE PLAGIOCLASE

EXPLANATION

Intrusive suite of Boulder Baldy

bo, outer zone

bi, intermediate zone

be, core zone 

Quartz monzodiorite of Mount Edith

Figure 2. Modal compositions of central Big Belt Mountains intrusions. Sample 883 (202883) is from the small stock exposed at Miller 

Mountain (fig. 1). Classification grid and rock names are those of 

Streckeisen(1973).

some places. This phase is characterized by dense accumulations of aligned plagioclase, by elevated hornblende content, 

and by trace amounts of clinopyroxene; these rocks are probably 

cumulates.

Metal Deposits

During the latter half of the 1800's, and sporadically thereafter, the Confederate Gulch area (fig. 1) was a major producer 

of placer gold (Lyden, 1948). Lyden (1948) estimated that 6-18 

million dollars worth of gold was produced from placer deposits 

in Confederate Gulch before 1933. McClernan (1980) showed 

numerous sets of placer workings along drainages whose headwaters are coincident with the intrusive suite of Boulder Baldy. 

The gold in these deposits is attributed commonly to lode 

sources at Miller Mountain (fig. 1) on the north side of Confederate Gulch (Lyden, 1948). Miller Mountain has been the site of 

recent lode-gold exploration activities and was extensively prospected in previous times. A resource of at least 11 million tons 

of ore containing 310,000 oz Au (grade, 0.028 oz/ton) was 

delineated at Miller Mountain by a Pegasus Gold Corporation/ 

FMC Gold Company joint venture drilling program (Pegasus 

Gold Inc., 1992). Widely disseminated gold-mineralized rock is 

hosted by weakly altered, sulfide-poor (pyrite), granitic rock at 

Miller Mountain. Gold grades are highest near major structural

4 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
intersections, particularly along low-angle quartz vein shear 

zones, in granodiorite exposed at Miller Mountain. A close spatial association between mineralized and intrusive rocks in the 

central Big Belt Mountains implies a genetic relationship.

Altered Newland Formation from a prospect pit 5 km north 

northeast of Boulder Baldy (fig. I) contains anomalous but low 

abundances of silver, arsenic, bismuth, cadmium, and lead. Igneous rock exposed 3-4 km west of Gipsy Lake contains several 

shallow prospect pits and trenches, is bleached and locally contains fluorite in veins and disseminations, and contains anomalous abundances of silver, bismuth, and cadmium.

The Snowbank and Bigler mines lie along the contact 

between the south end of the satellitic stock in the Gipsy Lake 

area and enclosing Newland Formation, whereas the nearby Porcupine mine is in potassium feldspar porphyritic quartz monzodiorite (fig. 1). In all three settings the prospected material 

consists of iron-oxide coatings along fractures in the granitoid 

rock and in the Newland Formation near its contact with the 

granitoid rocks. Rock from these areas contains anomalous 

abundances of arsenic, silver, gold, bismuth, cadmium, copper, 

molybdenum, and lead in varying combinations. Rock from a 

small unnamed mine about 1.5 km west of the Porcupine mine, 

again along the granitoid-Newland contact, contains anomalous 

abundances of arsenic, cadmium, and molybdenum. The Little 

Camas Creek drainage along this same contact contains several 

areas that have been extensively placered.

In contrast, mineralized rock appears to be much less abundant around the edges of the quartz monzodiorite of Mount 

Edith. This may be due in part to extremely poor exposure in 

the area underlain by this pluton. Prospects on the south flank of 

Mount Edith (fig. 1) are clearly related to small intrusions satellitic to the quartz monzodiorite of Mount Edith. Gossany iron 

oxide- and hydroxide-rich material from the dump adjacent to 

one of these prospects contains anomalous abundances of silver, 

arsenic, gold, bismuth, copper, and molybdenum. The Newland 

Formation along the edges of these small intrusions is altered to 

calc-silicate hornfels.

Sahinen (1962) described two fluorite occurrences in the 

central Big Belt Mountains. The first of these (prospect symbol, 

fig. 1, near sample site No. 202854), on the northeast ridge of 

Boulder Mountain, consists of a number of small, dark-purple 

fluorite veins associated with a dike intruding the biotite granodiorite phase of the Boulder pluton. The 1^4-cm-wide veins 

form a northwest-trending, steeply dipping zone not more than 

20 cm wide. Sahinen (1962) reported numerous prospect pits in 

this area; assays of fluorite-bearing samples from the prospect 

pits contain trace amounts of gold. The second occurrence 

(prospect symbol, fig. 1, near sample site No. 202850), the Normandy fluorite prospect, is about 5 km along the ridge extending 

east-southeast from Boulder Mountain. It consists of fracturefilled veins of fluorite-quartz-limonite in the outermost phase of 

the Boulder pluton. Veins in this area also strike northwest and 

dip steeply. Neither of these fluorite occurrences is of sufficient 

size or grade to constitute a resource.

Lang and others (2000) have described a class of globally 

dispersed intrusion-related gold systems in which the associated 

magmatic activity is located well-inboard of subduction zones. 

Many of the characteristics of this class of plutonic rocks and

their associated mineral deposits are similar to those of the Big 

Belt Mountains intrusive rocks and their associated metal deposits. In particular, anomalous metal abundances, including combinations of arsenic, silver, gold, bismuth, cadmium, copper, 

molybdenum, and lead, are common to globally dispersed intrusion-related gold systems and to mineralized rock associated 

with quartz monzodiorite intrusions present in the Big Belt 

Mountains. In the compilation of intrusion-related gold systems 

by Lang and others (2000), Miller Mountain is one of the cited 

examples; but very little information concerning either the geology of Miller Mountain or the larger, associated intrusive suite 

immediately to its south is presented. The basic framework data 

are presented here.

Geochronology

Eight mineral separates from five samples of plutons in the 

Big Belt Mountains were analyzed by the 40Ar/39Ar incremental 

heating technique. Three samples. 202879 (potassium feldspar), 

202852 (hornblende and potassium feldspar), and 202853 

(biotite and potassium feldspar) represent the outer, intermediate, and core zones of the intrusive suite of Boulder Baldy; sample 202878 (hornblende and potassium feldspar) represents the 

quartz monzodiorite of Mount Edith; and sample 202883 (potassium feldspar) represents the mineralized stock centered on 

Miller Mountain. Samples were crushed and sieved and mineral 

concentrates were prepared using standard gravimetric (heavy 

liquid) and magnetic techniques. Mineral separates were further 

purified by handpicking. Separates were sealed in silica vials 

and irradiated in the central thimble of the U.S. Geological Survey TRIGA reactor for 30 hours. Neutron fluence during irradiation was monitored using hornblende standard Mmhb-1, which 

has a K-Ar age of 520.4±1.7 Ma (Alexander and others, 1978).

After irradiation, the samples were progressively degassed 

in a double-vacuum resistance furnace in a series of 20-minutelong heating steps to a maximum temperature of 1,450U C. 

Apparent ages were calculated using decay constants of Steiger 

and Ja'ger (1977). The determination of whether the apparent 

ages yielded a "plateau" was made using the critical value test of 

Dairymple and Lanphere (1969) following the plateau determination of Fleck and others (1977). Only one plateau (for hornblende from sample 202852) was achieved; isochron and 

weight-average dates are used as best estimates of cooling ages 

for the other hornblende (202878) and biotite (202853) samples. 

A detailed description of analytical procedures similar to those 

used in this study appears in Snee and others (1988). 40Ar/39Ar 

results are presented as age spectra (fig. 3) and selected isochron 

diagrams (fig. 4). Detailed analytical data are presented in footnotes of table 2. Hornblende and biotite apparent ages are interpreted to represent cooling of each mineral through its closure 

temperature for argon diffusion. For these high-level plutons we 

use 550° and 300°C as argon-closure temperatures for hornblende and biotite, respectively. Potassium feldspar spectra are 

assumed to represent mixing among several cooling domains 

(Lovera and others, 1989). We interpret the high-temperature, 

high-age step of each spectrum to represent the time of argon

Geochronology 5
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55 --- - 202878 Hornblende ~ ^ Preferred age = 66.82±0.25 Ma 202878 Hornblende a _ ' ^ 4 ———— -J rn —— 1 *~-l ———————— i 202852 Hornblende/ ———— " "I • . —————— u" | ———————— ?n?RR^ Rintitfi 4 ———————————————————————————————————— > 202852 Hornblende Plateau age = 68.U0.1 Ma I I I I I I I I I

0 10 20 30 40 50 60 70 80 90 10

Percent 39ArK Released

Figure 3 (above and facing page) . 40Ar/39Ar age spectra for separates from intrusions in central Big Belt Mountains, Mont. Vertical width of each bar represents analytical error of each 

apparent age at ±1o. Individual steps used in calculation of plateau dates are indicated. A, Spectra for biotite and hornblende separates. B, Spectra for potassium feldspar separates.
(Ma) Age Apparent 

o co --j *--j CD CD en 

c CD en CD en CD en 

B

CD

ro

CD

eo

CD

CD 0 -^

O

0

Dh ren GO 

CD CD 

^ >— 

7^

33

CD 0 

CD —— 

0

03

C/)

0

^ ^ 

CD

CO

CD

eo

CD

CD

' T ^r ——— - r 1 U 1 ——— 1 ———— ' ———— ' 

^ — F "JiJ-i 

Pin

^ff L 

1

.

ll - 

—

.

. . • • 

•—

•L

~~

ro

0

ro\

00

CJI

ro — 

/

ro

o ~ 

ro

00 

CJI 

CO

"1 1 ! — 

ro

s° ^°0 -n 1 

^J .' 

00

[- . 

:p

Is s 1 §§ x /3 

W tD 
4000

2000

0

Isochron age = 67.1±0.3 (1o) Ma; 

750°-1250°C; 97.3% released 39Ar

— (40Ar/36Ar)i = 275±10(1o)

11

202853; biotite

0 

A

200 400 600 800

39Ar/36Ar

4000

2000

Isochron age = 67.6±0.1 do) Ma; 

1000°-1250°C steps; 98.5% released 39Ar

(40Ar/36Ar)i = 3oi±l do)

11.

202852; hornblende

200 400 600 800

B

39Ar/36Ar

Figure 4 (above and facing page). 40Ar/36Ar versus 39Ar/36Ar isotope correlation diagrams for A, 202853 biotite, B, 202852 hornblende, and C, 

202878 hornblende. Lines represent linear regressions of data based on method of York (1969). Numbers next to points refer to select 

temperature steps.

8 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
2000

r 1000

0<

Isochron age = 66.5±0.4 (1a) Ma; 

1000°-1400°C steps; 98.1% released 39Ar

(40Ar/36Ar)

10.

12,

11 +

4*"r3

202878; hornblende

0

c

100 200 300

39Ar/36Ar

closure of the highest temperature domain. Analogously, the 

lowest age step or steps represent time of argon closure of the 

lowest temperature domains. We assume the closure temperature for the lowest temperature cooling domain of the potassium 

feldspars to be approximately 150°C.

With these foregoing assumptions, the plateau date of 

68.1 ±0.1 (la) Ma for the hornblende from the intermediate zone 

of the Boulder Baldy pluton represents the time that this part of 

the pluton cooled through 550°C. The apparent age of the 

biotite, 66.5±0.5 Ma, although disturbed, likely represents cooling of the Boulder Baldy pluton's core zone below about 300°C. 

Assuming a coupled system, cooling from 550° to 300°C 

occurred over 1.5 m.y., yielding a cooling rate of approximately 

170°C/m.y. Assuming that the Boulder Baldy plutonic suite was 

emplaced at approximately 750°C and at a depth less than 10 

km, we infer that some time must have passed between pluton 

emplacement and its cooling to 550°C. If the cooling rate from 

time of emplacement to hornblende argon closure was 170°C/ 

m.y. or more, the Boulder Baldy pluton must have been 

emplaced between 70 and 69 Ma. The apparent (isochron date) 

cooling age (66.5±0.4 (la) Ma) for hornblende from the quartz 

monzodiorite of Mount Edith appears to be statistically younger 

than quartz monzodiorite in the intrusive suite of Boulder Baldy. 

If these inferences are accurate, they may reflect a younger 

emplacement age for the quartz monzodiorite of Mount Edith or 

a longer period of cooling subsequent to its emplacement. Consequently, plutonic rocks in the central Big Belt Mountains have 

emplacement and cooling ages similar to the youngest plutons of

the nearby Boulder batholith, which is known to have crystallized between 78 and 68 Ma (Tilling, 1973; Snee and others, 

1999; Martin and others, 1999).

Apparent ages of potassium feldspar separated from intrusive rocks of the central Big Belt Mountains provide valuable 

insight into the temporal extent of these intrusives' postmagmatic cooling. The five analyzed potassium feldspars give minimum argon cooling ages derived from lowest temperature 

extraction steps, of 61.9 to 60.1 Ma. The uniformity of these 

apparent ages, regardless of potassium feldspar compositions, 

indicates that the region had cooled to relatively low temperatures by 60 Ma. We have not experimentally determined closure 

temperatures for low-temperature cooling domains in the potassium feldspars, but 150°C is likely a reasonable estimate. The 

higher temperature maximum cooling ages for the analyzed 

potassium feldspars range from 70.7 to 66.1 Ma; samples giving 

the oldest apparent ages were from near the margin of the intrusive suite of Boulder Baldy and from the small, isolated Miller 

Mountain stock. Again, closure temperatures for the higher temperature cooling domains in the potassium feldspars are 

unknown, but assuming 300°-350°C, which is consistent with 

biotite argon closure, these apparent ages are consistent with the 

general pattern of cooling for the Big Belt Mountains plutonic 

suite.

The relatively limited difference between the hornblende 

and biotite ages of the intermediate and core zones of the intrusive suite of Boulder Baldy suggest that magma represented by 

these samples was part of a single magmatic event. The slightly

Geochronology 9
Table 1. Summary of 40Ar/39Ar age-spectrum results for intrusive rocks from central Big Belt Mountains, Mont.

[--, no data]

Sample

202879

202852

202853

202878

202883

Unit1 Mineral

bo Potassium

feldspar.

bi Hornblende

Potassium

feldspar.

, Biotite

be

Potassium

feldspar.

qm Hornblende

Potassium

feldspar.

MM Potassium

feldspar.

Apparent 

age (Ma)

68.5

60.1

68.1±0.1

66.3

60.1

66.5±0.5

66.1

61.1

66.8±0.2

66.8

61.1

70.7

61.9

Type of Correlation 

apparent age (Ma) age (Ma)

Cooling

maximum.

Cooling

minimum.

Plateau (5 1.7%) 67.6±0.1

Cooling

maximum.

Cooling

minimum.

Weight average 67.1 ±0.3

Cooling

maximum.

Cooling

minimum.

Excess argon 66.5±0.4

saddle-low.

Cooling

maximum.

Cooling

minimum.

Cooling

maximum.

(excess argon?)

Cooling

minimum.

Initial 

40/36

__

-

301±1

—

-

275±10

-

--

322±3

—

-

—

--

1 bo, bi, be are outer, intermediate, and core zones of the intrusive suite of Boulder Baldy, respectively; qm, quartz 

monzodiorite of Mount Edith; MM, mineralized stock at Miller Mountain.

older age for hornblende from the suite's intermediate zone is 

also consistent with the process of margins-inward cooling, 

through conductive heat loss, that is commonly proposed for 

solidification of magma chambers to form plutons. The similarity of ages for the quartz monzodiorite of Mount Edith and for 

the intrusive suite of Boulder Baldy suggests that these masses 

too are part of a single magmatic event. Finally, the similarity 

between the age for potassium feldspar from the small, mineralized intrusion exposed at Miller Mountain and those for the 

intrusive suite of Boulder Baldy and the quartz monzodiorite of 

Mount Edith indicates that the hydrothermal processes responsible for mineralization at Miller Mountain are temporally and 

therefore probably genetically related to igneous activity in this 

area.

analyses were performed (analysts, D.F. Siems and J.S. Mee) 

using X-ray fluorescence techniques (Taggart and others, 1987). 

Fe : total iron as Fe ratios were adjusted to 0.90 and major 

oxide abundances recalculated to 100 percent, volatile free. Rb, 

Sr, Y, Zr, Nb, and Ba abundances were determined (analyst, E.A. 

du Bray) by energy-dispersive X-ray fluorescence spectroscopy 

(Elsass and du Bray, 1982; Yager and Quick, 1992) using 109Cd 

and Am radio-isotope excitation sources. Instrumental neutron activation analyses (table 4) were conducted (analyst, J.N. 

Grossman) using techniques described by Baedecker and 

McKown (1987).

Major Oxide Data

Whole-Rock Chemistry

Compositional variation is documented by analytical data 

for 49 samples (fig. 1; table 3) of the central Big Belt Mountains 

intrusions. A representative subset of 14 samples was selected 

for instrumental neutron activation analysis (table 4). All 

geochemical analyses were performed in analytical laboratories 

of the U.S. Geological Survey in Denver, Colo. Major oxide

Cretaceous intrusive rocks in the central Big Belt Mountains contain between about 60 and almost 75 weight percent 

SiO2 (table 3); compositional variation of quartz monzodiorite 

and granodiorite within this range is fairly smooth and essentially continuous (fig. 5). The composition of the quartz monzodiorite of Mount Edith and that of the intrusive suite of Boulder 

Baldy are overlapping and span most of this composition range. 

Quartz monzonite and quartz monzodiorite compositions overlap, in part. The granodiorite composition, however, is distinct

10 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
Table 2. 40Ar/39Ar incremental heating data for samples 1 from central Big Belt Mountains. 

4°A7R2 3^1 4°ArR/39ArK2 39Ar/37Ar2,3

°C)

Apparent age4'5 

(Ma at ± la)

202879 (Intrusive suite of Boulder Baldy, outer zone) 

Potassium feldspar; 54.3 mg; J-value5=0.007215±0.1% (1o)

500

550

600

650

700

750

800

850

900

950

1000

1050

1100

1150

1200

1250

1300

1400

0.2000

.1800

.2712

.4042

.73494

.90530

.2720

1.8393

2.0147

1.8398

1.8131

1.6475

1.8617

1.7247

2.0056

3.5970

5.9777

7.6344

Total gas date: 65

0.00757

.02291

.05267

.08385

.15517

.19125

.05790

.38231

.41448

.37607

.37083

.33260

.37421

.34124

.39291

.68769

1.1208

1.4239

.75 ±0.13 Ma

26.4

7.856

5.149

4.822

4.736

4.734

4.697

4.811

4.861

4.892

4.889

4.954

4.975

5.054

5.105

5.231

5.333

5.362

223

441

487

240

471

329

341

348

359

536

620

596

562

508

331

836

54015

1506

71.1

74.7

88.3

92.4

94.1

78.1

89.8

96.6

97.2

97.9

97.1

97.1

96.0

96.1

96.0

95.4

95.3

94.3

0.1

.3

.8

1.2

2.3

2.8

.9

5.6

6.1

5.5

5.5

4.9

5.5

5.0

5.8

10.1

16.5

21.0

315. ±7.

99.5 ±0.9

66. ± 1.

61.7 ± .6

60.62± .22

60.58+ .19

60.1 ± .6

61.56± .09

62.18± .10

62.58± .12

62.54± .10

63.35± .11

63.62+ .10

64.61± .17

65.25± .11

66.83+ .10

68.12+ .10

68.47+ .10

No plateau

202852 (Intrusive suite of Boulder Baldy, intermediate zone) 

Hornblende; 234 mg; J-value5=0.007356±0.1% (1o)

650

800

950

1000

1050

1075

1100

1125

1150

1200

1350

0.0894

.0952

.1103

.1235

.2732

.73562

1.5205

2.0200

1.2937

2.9399

3.7046

0.00325

.01327

.02187

.02325

.05122

.14181

.29423

.38940

.24870

.56290

.70760

27.5

7.17

5.04

5.310

5.334

5.187

5.168

5.188

5.202

5.223

5.236

.29

.90

.75

.28

.21

.19

.19

.20

.20

.20

.20

10.4

8.0

31.0

44.8

59.3

83.6

89.4

92.3

89.8

94.0

93.8

0.1

.5

.9

.9

2.1

5.8

12.0

15.8

10.1

22.9

28.8

332. ±8.

93. ±2.

66. ±3.

69.1 ±1.

69.4 + .5

67.56+ .17

67.31+ .14

67.56+ .10

67.74+ .15

68.01+ .10

68.17+ .14

Total gas date: 68.1 ± 0.1 Ma

Plateau date (steps 10-11): 68.1 ± 0.1 Ma; Isochron date (steps 4-11): 67.6±0.1 Ma; (40Ar/36Ar)i=301±l

202852 (Intrusive suite of Boulder Baldy, intermediate zone) 

Potassium feldspar; 65.534 mg; J-value5=0.00727±0.1% (10)

500

550

600

650

700

750

800

850

900

950

1000

1050

1100

1150

1200

1250

1300

1400

0.04333

.20827

.29099

.60455

.75405

1.8674

1.8199

1.5992

2.9771

2.7501

2.1951

2.0348

2.3330

2.0223

2.3583

3.5779

6.9524

5.2371

Total gas date: 63.77

0.00433

.04060

.06131

.12965

.15912

.39011

.37822

.33294

.61436

.56555

.44878

.41506

.46987

.40691

.46612

.70192

1.3589

1.0181

+ 0.11 Ma

10.017

5.130

4.746

4.663

4.739

4.787

4.812

4.803

4.846

4.863

4.891

4.903

4.965

4.970

5.059

5.097

5.116

5.144

8.1

17

11

13

18

17

18

21

29

52

73

67

44

27

20

26

83

92

No plateau

16.1

52.2

64.6

88.6

82.4

97.8

98.2

92.5

95.8

95.7

95.0

95.0

95.6

93.9

94.6

96.0

97.2

96.4

0.1

.5

.8

1.6

2.0

4.9

4.8

4.2

7.7

7.1

5.6

5.2

5.9

5.1

5.9

8.8

17.1

12.8

126. +!

65.7 +

60.87+

59.82+

60.77+

61.38+

61.69+

61.59+

62.12+

62.33±

62.70±

62.84±

63.63±

63.69±

64.81±

65.29±

65.53±

65.87±

B.

.6

.30

.25

.23

.12

.09

.09

.10

.10

.12

.10

.10

.10

.10

.10

.10

.10

Whole-Rock Chemistry 11
Table 2—Continued. 40Ar/39Ar incremental heating data for samples 1 from central Big Belt Mountains.

Temp 40ArR2 

(°C)

550 0.00353

650 .34396

750 2.1995

850 2.7721

900 2.0069

950 1.0112

1000 1.7753

1050 2.8026

1100 2.9671

1150 2.4650

1250 2.7963

Total gas date: 65.74 

Weight-average date:

450 0.00831

500 .03513

550 .05659

650 .20435

700 .38151

750 .64913

800 .72731

850 3.7059

900 1.3853

950 1.5843

1000 2.2355

1050 3.2704

1100 1.5864

1150 1.9413

1200 2.0673

.1250 2.9231

1300 6.1905

1400 4.3768

Total gas date: 63.92

No plateau

650 0.15831

700 .11242

950 .10560

1000 .09226

1050 .16758

1075 .21445

1 100 .56409

1125 1.0039

1150 1.8159

1200 1.6037

1250 1.4797

1400 1.5461

39ArK2 40ArR/39ArK2 39Ar/37Ar2-3 %40ArR

202853 (Intrusive suite of Boulder Baldy, core zone) 

Biotite; 46.2 mg; J-value5=0.007459±0.1% (la)

0.00430 0.820 13 2.0

.11054 3.112 38 43.2

.44783 4.912 172 79.1

.55203 5.022 226 91.8

.39691 5.056 157 89.5

.19899 5.082 106 84.2

.34626 5.127 124 89.6

.55287 5.069 183 93.2

.58904 5.037 68 93.6

.49148 5.016 108 92.9

.55901 5.002 59 93.3

± 0.20 Ma; no plateau 

66.5±0.5 Ma; Isochron date (steps 3-11): 67.1±0.3 Ma

202853 (Intrusive suite of Boulder Baldy, core zone) 

Potassium feldspar; 56.7 mg; J-value5=0.007415±0.1% (la

0.00030 28.0 2.6 41.8

.00520 6.760 7.9 34.7

.01429 3.961 20 61.2

.04742 4.310 15 87.1

.08548 4.463 14 93.2

.13968 4.647 21 85.0

.15608 4.660 20 97.1

.78469 4.723 23 98.9

.29234 4.739 38 98.8

.33287 4.760 43 98.9

.46871 4.770 60 98.6

.68013 4.809 73 98.8

.32858 4.828 54 98.5

.40041 4.849 32 98.3

.42007 4.921 23 98.3

.59010 4.954 31 98.0

1.2362 5.008 96 98.5

.86952 5.034 79 97.8

±0.12 Ma

202878 (Quartz monzodiorite of Mount Edith) 

Hornblende; 195.9 mg; J-value5=0.007492±0.1% (1a)

0.00093 170 .14 48.1

.00862 13 .73 22.0

.02133 4.950 .56 35.0

.01619 5.700 .25 33.2

.02840 5.901 .18 45.8

.03843 5.580 .18 55.9

.11175 5.048 .17 73.4

.20030 5.012 .16 79.0

.35985 5.046 .16 84.3

.31111 5.155 .16 83.7

.28749 5.147 .16 82.3

.29774 5.193 .16 81.2

%39Ar

0.1

2.6

10.5

13.0

9.3

4.7

8.1

13.0

13.9

11.6

13.2

; (40Ar/

)

0.0

.1

.2

.7

1.2

2.0

2.3

11.5

4.3

4.9

6.8

9.9

4.8

5.8

6.1

8.6

18.0

12.7

0.1

.5

1.3

1.0

1.7

2.3

6.6

11.9

21.4

18.5

17.1

17.7

Apparent age 4-5 

(Maat±1a)

11. ±11.

41.39± .40

64.91± .15

66.34± .10

66.79± .10

67.12± .17

67.71± .10

66.95± .10

66.54± .11

66.26± .10

66.09± .26

36Ar)j=301±l

340. ±21.

88. ±3.

55. ±3.

56.8 ± .54

58.73± .42

61.12± .16

61.28± .25

62.09± .10

62.30± .16

62.57± .10

62.70± .10

63.20± .10

63.45± .15

63.72± .10

64.66± .10

65.08± .12

65.77± .10

66.11± .10

1486. ±44.

168. ±5.

66. ±2.

75. ±2.

78.0 ± .8

73.9 ± .9

66.97± .30

66.50± .33

66.95± .13

68.36± .11

68.26± .10

68.85± .11

Total gas date: 69.85 ± 0.24 Ma

No plateau; Isochron date (steps 3-11): 66.5±0.4 Ma; (40Ar/36Ar)i=322±3

12 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
Table 2—Continued. 40Ar/39Ar incremental heating data for samples 1 from central Big Belt Mountains.

Temp 40ArR 2 

(°C)

39ArK2 40ArR/39ArK2 39Ar/37Ar2,3 o/o 40Ar-R o/() 39Ar Apparent age 4'5 

(Maattlo)

202878 (Quartz monzodiorite of Mount Edith) 

Potassium feldspar; 55.9 mg; J-value5=0.00711±0.1% (la)

500

550

600

650

700

750

800

850

900

950

1000

1050

1100

1150

1200

1250

1300

1400

0.08564

.46338

.76503

.67039

1.3384

2.0043

2.5138

2.3266

2.2649

1.9089

.76492

1.9819

1.8723

1.8298

2.6608

3.2391

6.6508

6.5930

Total gas date: 64.52

No plateau

.00678

.09130

.15787

.13734

.27097

.40669

.50728

.46739

.45485

.37996

.15338

.39172

.36998

.35881

.51621

.62232

1.2626

1.2426

±0.13 Ma

12.6

5.075

4.846

4.881

4.939

4.928

4.956

4.978

4.980

5.024

4.987

5.060

5.061

5.100

5.155

5.205

5.267

5.306

75

175

196

175

133

126

134

150

180

251

255

270

256

207

165

251

501

499

56.3

90.3

93.8

80.3

95.0

97.0

97.7

97.7

97.7

97.1

94.5

98.1

97.4

97.4

97.4

97.2

97.6

96.8

0.1

1.2

2.0

1.8

3.5

5.2

6.5

6.0

5.8

4.9

2.0

5.0

4.7

4.6

6.6

8.0

16.2

15.9

155. ±6.

63.95± .

61.11+ .

61.55± .

62.27± .

62.13± .

62.47± .

62.74± .

62.76± .

63.32± .

62.86± .

63.76± .

63.77± .

64.25± .

64.93± .

65.56± .

66.33± .

66.80± .

.24

,19

,19

,14

.14

,10

,10

,16

.10

.23

,15

,17

,17

.13

.10

.10

,10

202883 (Altered quartz monzodiorite at Miller Mountain) 

Potassium feldspar; 71.2 mg; J-value5=0.0075421±0.1% (1o)

450

500

550

600

650

700

750

800

850

900

950

1000

1050

1100

1150

1200

1250

0.38767

.03265

.06844

.21640

.43549

.18095

.55873

2.7939

2.2848

1.9324

1.6311

1.5791

1.5773

2.0466

2.7129

3.7620

5.5118

1300 12.533

1400 6.7157

Total gas date: 67.44

.01533

.00700

.01421

.04487

.09503

.03889

.12069

.60444

.49293

.41651

.34814

.33021

.32587

.42029

.54961

.74787

1.0673

2.3918

1.2672

±0.14 Ma

25.2

4.665

4.818

4.822

4.583

4.653

4.630

4.622

4.635

4.640

4.685

4.782

4.840

4.870

4.936

5.030

5.164

5.240

5.300

21

24

30

28

19

20

24

42

40

41

54

72

75

62

57

59

92

280

338

78.0

51.1

69.7

87.9

90.8

94.2

80.2

93.9

97.1

97.4

97.3

97.7

97.1

96.2

96.5

96.9

97.3

97.5

96.4

0.2

.1

.2

.5

1.0

.4

1.3

6.5

5.3

4.5

3.7

3.6

3.5

4.5

5.9

8.0

11.5

25.7

13.6

315. ±3.

62. ±6.

64. ±3.

64.4 ± .

61.3 ± . 66. ±1.

61.91± .

61.82+ .

61.99+ .

62.05± .

62.65± .

63.92± .

64.68± .

65.07± .

65.94± , 67.18± .

68.93± .

69.92± .

70.70± .

.8

.7

.18

,09

,15

.19

.10

.13

.14

.10

.11

.10

,11

.11

,11

Mineral separates were prepared after crushing, grinding, and sieving by magnetic separator, mica-table, and heavyliquid methods; grains ranged in size between 60 and 120 mesh (250-125 ^mj. Separates were handpicked to greater 

than 99% purity. Individual samples, which ranged in mass from 46 to 234 mg, were cleaned with reagent-grade acetone, alcohol, and deionized water in an ultrasonic bath, air-dried, wrapped in aluminum capsules and sealed in silica 

vials along with monitor minerals before irradiation. Samples were irradiated at the TRIGA reactor facility at the U.S. 

Geological Survey in Denver, Colo., in one irradiation package. For irradiation an aluminum canister was loaded with

Whole-Rock Chemistry 13
Table 2—Continued. 40Ar/39Ar incremental heating data for samples 1 from central Big Belt Mountains.

six quartz vials each of which was loaded with samples and standards. Standards were placed between every two 

unknowns as well as at the top and bottom of each vial. Each standard was degassed to release argon in a single 20- 

minute-long heating step at 1,250°C. Each sample was degassed stepwise in a series of 11 to 18 individual temperature 

steps for 20 minutes each. All analyses were done in the Argon Laboratory, U.S. Geological Survey, Denver, Colo. 

Decay constants are those of Steiger and Jager (1977). The standard for this experiment is hornblende MMhb-1 with 

percent K=1.555, 40ArR=1.624xlO'9 mole/g, and K-Ar age = 520.4 Ma (Samson and Alexander, 1987). 

2 Abundances of "Radiogenic 40Ar" and "K-derived 39Ar" are reported in volts measured on a Mass Analyser Products 

215 rare-gas mass spectrometer using the Faraday cup. Conversion to moles can be made using a sensitivity of 

9.736xlO~13 moles argon per volt of signal. Detection limit at the time of this experiment was 2x10 moles argon. 

Corrections were made for volume, mass fractionation, trap current, radioactive decay of Ar and 39Ar, and interfering Ar isotopes. Mass discrimination in our mass spectrometer is routinely determined by measuring the 4 Ar / Ar 

ratio of atmospheric argon; our measured value is generally 298.9; the accepted value is 295.5. The calculated discrimination is applied to all samples and standards equally. Production ratios measured on pure K2SO4 and CaF2 salts irradiated with the samples were used to correct for irradiation-produced 40Ar (from K) and 39Ar (from Ca). Corrections 

for Cl-derived 36Ar were determined using the method of Roddick (1983). Production ratios determined for samples in 

this experiment are: (40Ar /39Ar)K = 9.35xlO'3 , (38Ar /39Ar)K = 1.304xlO'2, (37Ar /39Ar)K = 3.0xlO~4, (36Ar /37Ar)Ca 

= 2.7x 10'4 (39Ar /37Ar)Ca = 7.2xlO'4, (38Ar /37Ar)Ca = 4.7xlQ-5 . Analytical data for "Radiogenic 40Ar" and 

"K-derived 39Ar" are calculated to five places; "40ArR/39ArK" is calculated to three decimal places. "Radiogenic 

40Ar," "K-derived 39Ar," and "40ArR/39ArK" are rounded to significant figures using associated analytical precisions. 

Apparent ages and associated errors were calculated from unrounded data and then rounded using associated errors. 

To calculate apparent K/Ca ratios, divide Ar/37Ar value by 2.

4 Uncertainties in calculations for the apparent date of individual steps for a sample were calculated using the equations of Dalrymple and others (1981).

5 The reproducibility of split gas fractions from selected monitors was used to calculate imprecision in J values. This 

imprecision is generally 0.1%, 1 sigma. J values for each sample were interpolated from adjacent standards. The 

uncertainty in each apparent age includes the uncertainty in the J value.

and dramatically restricted, as epitomized by SiO2 contents that 

range from 68.3 to only 70.1 (six samples). The granodiorite 

contains more A12O3 and CaO and less Na2O and K2O than other 

central Big Belt Mountains intrusions with similar SiO2 contents. 

As is the case for most subalkaline rocks, abundances of A12C>3, 

TiO2, FeO* (total iron as FeO), MgO, CaO, P2O5 , and MnO in 

the Big Belt intrusions decrease with increasing SiO2 content, 

whereas abundances of Na2O and K2O are relatively constant. 

The mafic border phase of the quartz monzodiorite of Mount 

Edith is characterized by higher contents of TiO2, FeO*, MgO, 

CaO, P2O5 , and MnO and lower contents of SiO2, Na2O, and 

K2O than the remainder of this pluton.

The quartz monzonite is characterized by distinctive compositions, which, because they also vary erratically, blur the compositional variation trends just described (fig. 5). The quartz 

monzonite contains more A12O3 , Na2O, and K2O and less TiO2 , 

FeO*, MgO, CaO, P2O5 , and MnO than other Late Cretaceous 

igneous rocks of the central Big Belt Mountains. The unusual 

and erratically variable composition of these rocks, in addition to 

their mineralogic and textural features, suggests that these are not 

magmatic rocks. Quartz monzonite samples that best retain primary magmatic textures have compositions that overlap those of 

the quartz monzodiorite, whereas quartz monzonite samples 

whose textures indicate the greatest degree of alteration and 

recrystallization have compositions that stray furthest from those 

of the quartz monzodiorite. The quartz monzonite samples 

record a process that caused their compositions to be increasingly perturbed from primary magmatic values.

Most quartz monzodiorite from the central Big Belt Mountains has a total alkali versus silica content in the range between 

the alkaline-subalkaline dividing lines of Irvine and Baragar 

(1971) and Le Bas and others (1986); these rocks are transitional 

from alkaline to subalkaline (fig. 6). In contrast, most of the

quartz monzonite is strongly alkaline, whereas the biotite granodiorite is distinctly subalkaline. Most of the quartz monzodiorite 

compositions plot (fig. 5) in the high-K calc-alkaline field of 

Ewart (1982); most of the quartz monzonite compositions plot in 

the shoshonite field, whereas those of the biotite granodiorite 

and a few of the quartz monzodiorite compositions plot in the 

calc-alkaline field.

Geochronologic data indicate that ages (Martin and others, 

1999; Snee and others, 1999) of plutons in the Boulder 

batholith, which is across the Missouri River valley from the 

Big Belt Mountains, are similar to ages (data from the present 

report) of intrusive rocks of the central Big Belt Mountains. In 

a study of the Boulder batholith, Tilling (1973) defined two distinct granitic rock composition series, main and sodic, for its 

constituent plutons. Sodic series rocks are distinguished from 

those of the main series by slightly higher Na2O and Sr contents 

and lower K2O and Rb contents at a given SiO2 abundance; ternary proportions of Na2O, K2O, and CaO for the two series are 

particularly diagnostic. Compositions of central Big Belt 

Mountains intrusions (fig. 7) are most like those of the sodic 

series plutons of Tilling (1973); however, at a given SiO2 content, the Big Belt intrusions contain higher abundances of A12O3 

and Na2O and" lower abundances of FeO*, MgO, and P2O5 than 

the sodic series plutons.

Trace-Element Data

The extent of trace-element abundance variation in intrusions of the central Big Belt Mountains is moderate (fig. 8). 

Abundances of Sr, Co, Cr, Sc, Nb, Zr, U, and total REE 

decrease with increasing SiO2 , whereas those of Zn, Ba, Th,

14 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
24

22

20

18

16

14

12

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

6

O H 

££

ntffi a

a an

an

• m

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n 

D

D • •+

• • Dtf 

• •

n 

n

to 'o -B-1-

n

cr

li

+ "

DC

LU q£* X

S2

LU

Z

Shoshonitic - 

series

60 65 70 75

z 

o

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C LU 

O Q_

£ C/3 N 1-

DC

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300

200

100

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, , , D 1 , , , , I , , , ,

60

SiO2,IN WEIGHT PERCENT

EXPLANATION

Intrusive suite of Boulder Baldy 

n bo, outer zone 

+ bi, intermediate zone 

x be, core zone 

• Quartz monzodiorite of Mount Edith

65 70 75

Figure 5. Variation diagrams showing abundances of major oxides and zirconium in intrusions in central Big Belt Mountains. Discriminant line 

on K2 0 versus Si02 diagram from Ewart (1982).

Whole-Rock Chemistry 15
Table 3. Composition of Cretaceous intrusive rocks from central Big Belt Mountains.

[Note: bo, bi, be, outer, intermediate, and core zones of intrusive suite of Boulder Baldy, respectively; qm, quartz monzodiorite of Mount Edith. Majo

parts per million. Ferrous iron/total iron as FeO adjusted to 0.9; LOI, loss on ignition; ND, not detected]

o

CD

to

CO

CD

22. 5"

y

o

!•»

fi)

3 

fi)

Unit

Sample

SiO9

A1203

Fe2O3

FeO

MgO

CaO

Na2O

K2O

TiO2

P2O5

MnO

LOI

TOTAL

Rb

Sr

Y

Zr

Nb

Ba

La

Ce

Nd

Unit

Sample

SiO2

A12O3

Fe2O3

FeO

MgO

CaO

Na2O

K2O

TiO2

P2°5

MnO

LOI

TOTAL

bo

202850

65.10

19.20

0.20

0.73

ND

0.68

7.20

5.31

0.10

ND

0.05

0.36

98.94

99

1395

6

94

11

1879

15

18

ND

bo

202953

69.30

17.00

0.38

1.38

0.48

4.00

4.54

1.52

0.22

0.10

0.05

0.34

99.32

bo

202851

70.70

15.50

0.26

0.92

ND

0.48

5.95

5.09

0.08

ND

0.08

0.26

99.32

132

228

13

87

16

766

9

23

2

bo

202955

66.70

17.00

0.43

1.55

0.35

1.81

5.53

5.33

0.25

0.08

0.09

0.30

99.43

bo

202858

62.20

17.90

0.49

1.76

0.78

2.76

5.32

6.23

0.30

0.13

0.08

0.36

98.30

121

2209

21

145

26

2635

33

28

33

bo

202957

68.50

16.90

0.22

0.80

0.14

0.44

6.27

6.31

0.11

ND

0.06

0.15

99.90

bo

202864

64.70

17.60

0.31

1.11

0.12

2.05

8.08

3.98

0.08

ND

0.15

0.34

98.52

97

39

8

335

12

79

ND

4

ND

bi

202852

66.70

16.90

0.53

1.91

0.63

3.40

4.94

3.07

0.31

0.13

0.09

0.27

98.89

bo

202865

62.70

19.20

0.46

1.65

0.14

2.03

6.87

4.53

0.23

ND

0.09

0.21

98.11

86

2659

14

105

48

3075

109

104

43

bi

202934

62.50

18.30

0.73

2.61

0.94

4.21

5.08

3.41

0.43

0.18

0.12

0.26

98.77

bo

202867

70.90

15.50

0.24

0.86

0.14

0.64

6.18

4.40

0.12

ND

0.06

0.16

99.20

99

372

13

171

16

808

6

30

6

bi

202944

64.00

17.70

0.72

2.58

0.93

4.31

5.21

2.50

0.40

0.20

0.13

0.29

98.97

bo

202869

63.90

17.70

0.45

1.62

0.21

1.67

6.28

5.61

0.27

ND

0.10

0.65

98.46

124

1166

15

27

16

2367

36

61

28

bi

202947

69.30

16.50

0.48

1.71

0.50

3.55

4.54

2.18

0.24

0.12

0.09

0.34

99.55

bo

202876

61.20

17.90

0.72

2.59

1.03

3.25

6.60

3.96

0.44

0.24

0.10

0.14

98.17

71

2307

27

146

26

3043

48

79

62

bi

202951

65.80

17.20

0.56

2.03

0.67

3.47

5.02

3.37

0.33

0.15

0.11

0.21

98.92

bo

202879

73.50

13.80

0.22

0.80

0.12

0.66

4.70

5.07

0.12

ND

0.06

0.11

99.16

138

315

5

80

11

662

8

21

6

bi

202954

66.50

17.00

0.62

2.22

0.81

4.00

5.29

2.04

0.35

0.19

0.10

0.23

99.34

bo

202881

60.20

18.30

0.69

2.50

0.17

2.80

4.07

6.54

0.47

0.08

0.12

0.46

96.40

100

3704

39

113

37

6253

88

60

81

bi

202956

64.50

17.60

0.69

2.48

0.82

4.06

5.11

2.81

0.41

0.19

0.13

0.36

99.16

bo

202882

74.00

13.80

0.23

0.81

ND

0.28

6.03

3.71

0.10

ND

0.06

0.07

99.09

92

50

5

147

16

88

42

45

20

be

202853

68.30

16.80

0.49

1.76

0.60

3.95

4.10

1.91

0.24

0.11

0.07

0.50

98.83

bo

202932

60.40

18.90

0.48

1.72

0.27

1.77

4.40

8.26

0.22

0.06

0.09

1.55

98.12

149

2349

10

54

22

4680

73

85

26

be

202854

68.30

16.90

0.48

1.72

0.57

4.11

4.08

1.83

0.24

0.09

0.07

0.49

98.88
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Whole-Rock Chemistry 17
S 10

55

SUBALKALINE

67

SiO2, IN WEIGHT PERCENT

EXPLANATION

Intrusive suite of Boulder Baldy 

n bo, outer zone 

+ bi, intermediate zone 

x be, core zone 

• Quartz monzodiorite of Mount Edith

Na2O CaO

EXPLANATION

Intrusive suite of Boulder Baldy 

D bo, outer zone 

+ bi, intermediate zone 

x be, core zone 

• Quartz monzodiorite of Mount Edith

Figure 6. Total alkali-silica variation diagram for intrusions of central 

Big Belt Mountains. Alkaline-subalkaline dividing lines are those of 

Irvine and Baragar (1971) (dashes) and Le Bas and others (1986) (solid 

line).

Figure 7. Variation diagram showing relative proportions of K20, 

Na 20, and CaO in intrusions of central Big Belt Mountains. Sodic and 

main series fields defined by Tilling (1973) for samples of Boulder 

batholith.

Table 4. Instrumental neutron activation analysis of selected samples of Cretaceous intrusive rocks from central Big Belt Mountains.

[bo, bi, be, outer, intermediate, and core zones of intrusive suite of Boulder Baldy, respectively; qm, quartz monzodiorite of Mount Edith. All data 

in parts per million; ND, not detected]

Unit 

Sample

Co

Cr

Ni

Cs

Hf

Sb

Ta

Th

U

Zn

Sc

La

Ce

Nd

Sm

Eu

Tb

Yb

Lu

bo 

202864

0.330

ND

ND

0.748

8.65

0.053

0.648

125

4.8

78.0

0.143

2.5

7.3

ND

0.51

0.11

0.073

0.67

0.11

bo 

202876

5.54

6.8

ND

0.97

4.52

ND

1.01

7.47

2.3

58.1

4.06

41.8

78.1

31

6.36

1.50

0.614

2.3

0.29

bo 

202882

0.170

1.4

ND

1.04

3.31

0.046

0.463

20.8

2.9

63.9

0.406

22.5

36.1

10

0.92

0.15

0.059

0.29

0.039

bo 

202932

1.43

1.2

ND

4.06

2.19

0.077

0.815

12.3

4.6

51.9

0.40

71.2

91.2

21

2.5

0.548

0.14

0.46

0.053

bo 

202945

3.10

5.8

ND

1.25

6.21

0.061

0.834

25.5

3.9

54.9

3.07

40.0

63.5

24

4.41

1.16

0.457

2.1

0.306

bi 

202934

5.24

3.3

11.2

1.10

3.35

ND

1.13

5.70

1.5

46

4.18

28.4

52.1

22

4.58

1.24

0.482

1.9

0.26

bi 

202947

2.34

1.8

ND

2.82

3.10

0.043

0.757

4.41

1.2

43.5

1.98

21.9

34.6

11

2.14

0.561

0.23

0.99

0.14

be 

202854

2.50

2.6

ND

0.978

2.97

0.030

0.615

2.42

0.78

59.1

1.78

14.8

25.9

10

2.05

0.635

0.214

0.75

0.099

be 

202946

2.08

2.4

ND

1.37

2.94

ND

0.688

2.74

1.1

51.2

1.67

15.6

27

11

2.34

0.646

0.24

0.80

0.10

be 

202954

3.95

2.8

ND

1.0

3.45

ND

0.994

7.59

1.6

41

3.30

26.2

45.4

19

3.54

0.91

0.372

1.5

0.21

qm 

202870

2.52

3.6

7.60

0.60

3.21

0.055

1.06

4.38

1.0

35

2.36

21.3

38

15

2.95

0.772

0.33

1.3

0.18

qm 

202871

1.78

3.8

ND

0.44

2.93

ND

0.728

5.11

1.2

21

1.72

14.9

25.0

10

1.89

0.541

0.21

1.1

0.15

qm 

202937

3.00

2.8

ND

1.39

4.12

ND

1.19

8.34

1.9

23

2.79

37.6

57.3

19

3.71

0.976

0.414

1.9

0.28

qm 

202940

6.03

1.9

ND

0.71

3.69

0.063

0.775

5.62

1.4

45

7.87

35.8

64.3

30

6.73

1.77

0.789

2.5

0.34

18 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
Ta, Hf, Cs, Rb, and Y display no consistent covariation with 

SiO2 content (table 3). With the exception of three of five 

samples of the quartz monzonite of the intrusive suite of Boulder Baldy, chondrite-normalized extended trace-element patterns for intrusive rocks of the central Big Belt Mountains are 

gently negatively sloping and are characterized by variably 

well developed negative Nb-Ta, P, and Ti anomalies and positive Ba, K, and Sr anomalies (fig. 8). In most samples, abundances of Rb and Yb (at opposite ends of chondrite-normalized 

extended trace-element patterns) are between about 155-250 

and 3-10 times chondritic values, respectively. Patterns for 

samples of the central Big Belt Mountains intrusions are essentially parallel and define relatively limited compositional variation. Trace-element contents of the biotite granodiorite are 

systematically lower than those of hornblende quartz monzodiorite. Extended trace-element patterns for the hornblende 

quartz monzodiorites of Mount Edith and the intrusive suite of 

Boulder Baldy are indistinguishable. The slopes and shapes of 

these patterns, as well as trace-element abundances, are similar 

to those of back-arc, subduction-related rocks, including, for 

example, well-characterized middle Tertiary ash-flow tuffs of 

the southern Great Basin (du Bray, 1995b).

Extended trace-element patterns for the two quartz 

monzonite samples that best preserve igneous textures have 

slopes and shapes similar to those of the majority of central Big 

Belt Mountains intrusions but indicate slightly higher traceelement abundances. The three remaining samples of the quartz 

monzonite have extended trace-element patterns that are somewhat more steeply negatively sloping (fig. SB) than those of 

other central Big Belt Mountains intrusions. In particular, two of 

these patterns are distinguished by large negative Ba anomalies 

and large positive Zr anomalies. The central parts of these three 

patterns are considerably irregular and depict broad traceelement abundance ranges.

With the exception of the three unusual samples of the 

quartz monzonite of the intrusive suite of Boulder Baldy, chondrite-normalized REE patterns for the central Big Belt Mountains intrusions are gently negatively sloping in their light REE 

parts, lack Eu anomalies, and are essentially flat in their heavy 

REE parts (fig. 9A). The patterns depict a narrow compositional 

range and are remarkably parallel. (La/Yb)CN values vary from 

about 11 to 16. REE contents are systematically lower in samples with higher SiO2 and lower TiO2 and MgO contents. 

Accordingly, the REE contents of the biotite granodiorite are 

systematically lower than those of hornblende quartz monzodiorite. REE patterns for the hornblende quartz monzodiorites of 

Mount Edith and of the intrusive suite of Boulder Baldy are 

indistinguishable.

Rare-earth element patterns for the two quartz monzonite 

samples that best preserve igneous textures have slopes similar 

to those of the majority of central Big Belt Mountains intrusions 

but indicate slightly higher REE abundances. The three remaining samples of the quartz monzonite have REE patterns that are 

unusual (fig. 9B) relative to those for other central Big Belt 

Mountains intrusions. One of the three unusual patterns indicates distinctly low REE abundances and is essentially flat with 

minor middle REE depletion. The other two unusual patterns 

are distinctly more negatively sloping in their light REE parts; 

the heavy REE parts of these patterns are flat and indicate

abundances significantly lower than is characteristic of other 

central Big Belt Mountains intrusions. The distinctiveness of 

these three patterns, along with their petrographic features, suggests that these rocks were acted upon by processes that did not 

affect the majority of the central Big Belt Mountains intrusions.

Mineral Compositions

Compositions of subhedral to euhedral, non-interstitial 

minerals that may have participated in chemical evolution of the 

central Big Belt Mountains igneous rocks, including plagioclase, 

biotite, hornblende, potassium feldspar, clinopyroxene, titanite, 

apatite, magnetite, and andradite, were determined using a JEOL 

8900 electron microprobe with five wavelength dispersive crystal 

spectrometers. A 5-20 (jm beam, with 15 kV accelerating voltage and 20 nA beam current, was used to make a minimum of 

three 20-s analyses on three grains of each selected mineral in 

the representative sample subset. Most analyzed spots were at 

crystal rims to ensure that determined compositions reflect the 

composition of the interstitial, residual liquid from which the 

analyzed minerals crystallized.

The outer rims of plagioclase crystals (table 5; fig. 10) in 

samples of the central Big Belt Mountains igneous rocks have 

average compositions that range from andesine (An35Ab64Or1 ) 

to albite (An3Ab96Or 1 ); most are composed of calcic oligoclase. 

Compositions of plagioclase in the two quartz monzodiorite 

intrusions are similar. Plagioclase in the granodiorite is distinctly more calcic, whereas sparse plagioclase in the quartz 

monzonite is distinctly less calcic than plagioclase in the quartz 

monzodiorite. Potassium feldspar (table 6; fig. 10) is orthoclase 

and has compositions that range from AnoAb22Or76Cs2 to 

An0Ab95Or5. Compositions of potassium feldspar in the two 

quartz monzodiorites are indistinguishable. Potassium feldspar 

in the granodiorite is distinctly more potassic, whereas that in 

two of three samples of the quartz monzonite is distinctly more 

sodic than that in the quartz monzodiorite. Reconnaissance 

analyses across single feldspar crystals indicate that core-to-rim 

zoning is limited to about 5 mol percent in most grains; however, feldspar grains in a few analyzed samples display as much 

as 15 mol percent compositional variation. Potassium feldspar 

in several analyzed samples contains considerable BaO; orthoclase 202932, in particular, contains an average of 1.09 weight 

percent BaO.

The composition of biotite in the central Big Belt Mountains igneous rocks (table 7; fig. 11) is similar to that of magnesium-rich biotite in granitoids of the Sierra Nevada batholith 

(Dodge and others, 1969). Biotite in the central Big Belt Mountains igneous rocks has slightly elevated BaO contents (0.45- 

0.87 weight percent), relative to Sierra Nevada biotite; 

MgO:FeO* is about 1, and they contain slightly less magnesium 

and distinctly less titanium than biotite in the Big Timber stock 

(du Bray and Harlan, 1996), in the nearby Crazy Mountains. 

Compositional variation of biotite within individual samples is 

quite limited; no zonation was detected.

Amphibole is the principal mafic silicate mineral in the 

quartz monzodiorite intrusions of the central Big Belt Mountains. Iron contents of analyzed hornblendes were partitioned to

Mineral Compositions 19
1000

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

EXPLANATION

Intrusive suite of Boulder Baldy 

D bo, outer zone 

+ bi, intermediate zone 

x be, core zone 

• Quartz monzodiorite of Mount Edith

Figure 8. Chondrite-normalized extended trace-element diagrams 

showing compositions of central Big Belt Mountains intrusions. A, 

Patterns for all analyzed samples except those forthree petrographically and geochemically unusual samples of the quartz monzonite of 

the intrusive suite of Boulder Baldy. B, Patterns for three petrographically and geochemically unusual samples of the quartz monzonite of 

the intrusive suite of Boulder Baldy. All points in B are (bo).

1000

E 100

1000

E 100 202932

tk

202882

202864'

B

\

La Ce Nd Sm Eu Tb Tm Yb Lu

EXPLANATION

Intrusive suite of Boulder Baldy 

n bo, outer zone 

+ bi, intermediate zone 

x be, core zone 

• Quartz monzodiorite of Mount Edith

Figure 9. Chondrite-normalized rare-earth element diagrams showing compositions of central Big Belt Mountains intrusions. A, Patterns 

for all analyzed samples except those forthree petrographically and 

geochemically unusual samples of the quartz monzonite of the intrusive suite of Boulder Baldy. B, Patterns for three petrographically and 

geochemically unusual samples of the quartz monzonite of the intrusive suite of Boulder Baldy. All points in Bare (bo).

Table 5. Major oxide compositions of plagioclase in intrusive rocks from central Big Belt Mountains.

[bo, bi, be, outer, intermediate, and core zones of intrusive suite of Boulder Baldy, respectively; qm, quartz 

monzodiorite of Mount Edith. Data in weight percent; each analysis is the average of nine analyses (three spots on 

each of three grains)]

Unit 

Sample

SiO^

A1203

FeO

BaO

CaO

Na2O

K20

Total

bo 

876PL

66.67

20.82

0.13

0.03

1.56

10.77

0.12

100.11

bo 

932PL

67.83

19.86

0.10

0.02

0.58

11.22

0.13

99.74

bi 

934PL

62.36

23.17

0.14

0.02

4.78

8.53

0.32

99.32

bi 

947 PL

62.01

23.79

0.13

0.03

5.38

8.50

0.36

100.20

be 

854PL

59.89

25.53

0.12

0.02

6.79

7.84

0.19

100.38

be 

946PL

59.38

25.69

0.15

0.02

7.45

7.37

0.16

100.21

qm 

871 PL

63.81

22.35

0.14

0.03

3.62

9.47

0.26

99.67

qm 

937PL

62.74

23.54

0.15

0.02

5.17

8.73

0.25

100.61

qm 

940PL

61.14

25.05

0.14

0.03

6.44

8.12

0.20

101.13

20 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
Table 6. Compositions of potassium feldspar in intrusive rocks from central Big Belt 

Mountains.

[bo, bi, be, outer, intermediate, and core zones of intrusive suite of Boulder Baldy, respectively; qm, 

quartz monzodiorite of Mount Edith. Data in weight percent; each analysis is the average of nine 

analyses (three spots on each of three grains)]

Unit 

Sample

SiO2

A12O3

FeO

BaO

CaO

Na9O

KoO

Total

bo 

876KF

63.83

18.98

0.09

1.49

0.13

2.33

12.76

99.61

bo 

882KF

64.93

17.94

0.36

0.01

0.00

0.60

15.79

99.64

bo 

932KF

64.32

18.14

0.18

1.09

0.02

2.38

12.64

98.77

bi 

934KF

65.17

18.03

0.07

0.46

0.06

1.43

14.09

99.32

bi 

947KF

63.92

18.78

0.05

1.82

0.03

1.32

14.06

99.98

be 

854KF

65.73

18.63

0.05

0.12

0.01

0.98

15.37

100.89

qm 

871 KF

63.42

18.69

0.09

1.12

0.03

1.42

14.28

99.04

qm 

937KF

64.91

18.75

0.09

0.56

0.04

1.53

14.09

99.98

Table 7. Compositions of biotite in intrusive rocks from central Big 

Belt Mountains.

[bi, be, intermediate and core zones of intrusive suite of Boulder Baldy, respectively. Data in weight percent; each analysis is the average of nine analyses 

(three spots on each of three grains); iron contents partitioned assuming Fe~+/ 

(Fe2++Fe3+)=0.85; H2O calculated by stoichiometry; Totala , total adjusted for 

F and Cl]

Unit 

Sample

SiO2

TiO2

A1203

Fe203

FeO

MnO

MgO

BaO

Na2O

F^K2O

Cl

H2O

Total

Totala

bi 

947BI

36.87

2.39

14.51

3.10

15.80

1.40

11.19

0.45

0.08

9.35

0.51

0.01

3.67

99.34

99.13

be 

854BI

36.87

3.26

15.00

3.17

16.18

1.11

10.55

0.87

0.09

9.40

0.54

0.01

3.70

100.74

100.51

be 

946BI

36.17

3.13

14.75

3.24

16.53

1.12

10.10

0.77

0.10

9.36

0.63

0.01

3.60

99.51

99.24

Table 8. Compositions of hornblende in intrusive rocks 

from central Big Belt Mountains.

[bi, qm, intermediate zone of intrusive suite of Boulder Baldy and 

quartz monzodiorite of Mount Edith, respectively. Data in weight 

percent; each analysis is the average of nine analyses (three spots 

on each of three grains); Fe2O3 and H2O calculated by charge balance and stoichiometry, respectively; Totala, total adjusted for F]

Unit 

Sample

SiO2

TiO2

A12O3

Fe203

FeO

MnO

MgO

CaO

Na2O

K2O

H2O

F

Total

Totala

bi 

934HB

44.62

1.11

8.43

3.32

14.35

1.07

10.35

11.38

1.87

1.02

1.80

0.37

99.68

99.52

bi 

947HB

44.28

1.09

9.34

4.92

12.84

1.42

10.15

11.39

1.54

1.07

1.86

0.26

100.16

100.05

qm 

871 HB

48.92

0.54

5.25

4.22

10.25

1.22

13.42

11.40

1.37

0.58

1.79

0.49

99.45

99.24

qm 

937HB

48.10

0.63

6.48

2.93

13.05

1.18

12.16

11.41

1.84

0.85

1.59

0.92

101.15

100.76

qm 

940HB

47.44

0.94

7.08

2.61

13.90

0.94

11.75

11.68

1.73

0.79

1.72

0.65

101.24

100.96

ferrous and ferric iron by charge balancing; H2O contents were 

calculated by stoichiometry (table 8). According to the classification scheme of Leake (1978), the amphibole (table 8; fig. 12), 

with (Ca+Na)B>1.34, NaB<0.5, 0.5>Mg/(Mg+Fe2+)>1.0, and 

7.25<Si<6.5, is magnesio-hornblende. Most of the magnesiohornblende displays limited within-sample compositional variation, and zoning was not detected. As a group, the magnesiohornblendes display a moderate amount of bet ween-sample 

compositional variation with respect to iron and magnesium 

contents. Hornblende from quartz monzodiorite of the intrusive 

suite of Boulder Baldy has a somewhat higher iron-magnesium 

ratio than that in the quartz monzodiorite of Mount Edith (fig. 

12). Hornblende in intrusions of the central Big Belt Mountains 

also has a slightly higher iron/magnesium ratio than is 

characteristic of hornblende in the nearby Big Timber stock (du 

Bray and Harlan, 1996).

The most mafic parts of quartz monzodiorite intrusions in 

the central Big Belt Mountains contain small amounts of

clinopyroxene. The clinopyroxene is salite, whose composition is Wo49En34Fs 1 7 (table 9; fig. 12); the MnO content of the 

salite, 1.2 to 1.5 weight percent, is somewhat distinctive. 

Within- and between-sample clinopyroxene compositional variation is limited, and zoning was not detected; salite in the two 

quartz monzodiorite intrusions is compositionally indistinguishable. Clinopyroxene in the quartz monzodiorites is more 

calcic than that in the nearby Big Timber stock (du Bray and 

Harlan, 1996).

The composition of pyroxene in the quartz monzonite of 

the intrusive suite of Boulder Baldy ranges from aegirine-augite to aegirine (table 9). Pyroxene in sample 202882 contains 

0.7 atoms of Fe3+ per formula unit and 9.8 weight percent 

Na2O; these features and the pleochroism of this pyroxene are 

diagnostic of aegirine. Pyroxene in samples 202876 and 

202932 contains 0.15 and 0.30 atoms of Fe3+ per formula unit 

and 2.0 and 3.8 weight percent Na2O, respectively; these features and the pleochroism of these pyroxenes are diagnostic of

Mineral Compositions 21
ANORTHITE

AB

SANIDINE 

V————V g V .,

OR

EXPLANATION

Intrusive suite of Boulder Baldy 

n bo, outer zone 

+ bi, intermediate zone 

x be, core zone 

• Quartz monzodiorite of Mount Edith

Figure 10. Ternary diagram showing molecular end member proportions 

of feldspars in samples of central Big Belt Mountains intrusions.

x\ 

>\

BIOTITE FROM THE \\ 

SIERRA NEVADA I \

I ) 

I /

V V V V V I V V V V

R +3 3+2

EXPLANATION

Intrusive suite of Boulder Baldy 

bi, intermediate zone 

be, core zone

Ca

Mg Fe

EXPLANATION

Intrusive suite of Boulder Baldy 

+ bi, intermediate zone 

• Quartz monzodiorite of Mount Edith

Figure 12. Ternary diagram showing compositions of amphibole and 

clinopyroxene in quartz monzodiorite of central Big Belt Mountains 

intrusions. Clinopyroxene compositions are those which plot near the 

top of the composition quadrilateral.

aegirine-augite. Sodic pyroxenes such as this are common constituents of alkaline igneous rocks, which accords with the 

compositions of their host rocks.

Representative compositions of accessory minerals in igneous rocks of the central Big Belt Mountains, including titanite, 

apatite, magnetite, and andradite, are summarized in table 10. 

Titanite compositions from four samples are similar; deviations 

from analytical totals of 100 percent probably reflect undetermined components, including H2O, Zr, Nb, Ta, V, and the REE. 

Similarly, apatite compositions from three samples display 

remarkably little variation; these apatites contain notably elevated fluorine abundances. Opaque oxide grains in intrusions of 

the central Big Belt Mountains are composed of magnetite; magnetite compositions in four analyzed samples are indistinguishable. The most unusual mineralogic feature of igneous rocks in 

the central Big Belt Mountains is the presence of garnet in the 

outer zone of the intrusive suite of Boulder Baldy. In terms of 

end-member components, this garnet is primarily andradite (87 

percent) but includes grossularite (10 percent) and spessartine (3 

percent) components as well. The elevated TiO2 content (table

Table 9. Compositions of clinopyroxene in intrusive rocks 

from central Big Belt mountains.

[bo, bi, outer and intermediate zones of intrusive suite of Boulder 

Baldy, respectively; qm, quartz monzodiorite of Mount Edith. Data 

in weight percent; each analysis is the average of nine analyses (three 

spots on each of three grains)]

Figure 11. Ternary Mg, R+3 (Al lv+Fe3++Ti), R+2 (Fe2++Mn) diagram 

showing relative cation abundances for biotite in samples of central 

Big Belt Mountains intrusions.

22 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana

Unit 

Sample

Si02

TiO2

A1203

FeO

MnO

MgO

CaO

Na2O

Total

bo 

876CP

51.85

0.11

1.13

14.04

0.96

9.00

20.62

2.01

99.72

bo 

882CP

52.74

0.18

0.34

24.32

2.28

1.79

7.59

9.84

99.08

bo 

932CP

50.27

0.20

2.20

17.80

1.30

5.78

17.55

3.84

98.93

bi 

934CP

52.68

0.06

0.71

10.41

1.49

11.26

22.61

0.54

99.76

qm 

940CP

53.14

0.12

0.79

9.96

1.21

11.64

23.20

0.56

100.61
Table 10. Representative compositions of accessory phases in intrusive rocks from central Big Belt Mountains.

[bo, bi, be, outer, intermediate, and core zones of intrusive suite of Boulder Baldy, respectively; qm, quartz monzodiorite of Mount Edith. Data in 

weight percent; each analysis is the average of nine analyses (three spots on each of three grains); Fe2O3 in magnetite calculated by charge balance; 

ND, not detected; Totala, total adjusted for F and Cl]

Unit 

Sample

SiO2

TiO2

A1263

Cr2O3

Fe203

FeO

MnO

MgO

BaO

CaO

Na2O

P2O5

F

Cl

Total

Totala

bo 

932SP

30.05

34.60

1.42

ND

ND

2.16

0.05

ND

0.25

27.50

ND

ND

0.41

ND

96.44

96.27

bi be 

947SP 854SP 

Titan ite

30.39

35.69

1.68

ND

ND

1.43

0.23

ND

0.24

27.84

ND

ND

0.29

ND

97.79

97.67

30.79

36.14

1.61

ND

ND

1.27

0.26

ND

0.25

28.17

ND

ND

0.32

ND

98.81

98.68

qm 

937SP

30.36

36.23

1.50

ND

ND

1.42

0.21

ND

0.27

27.98

ND

ND

0.30

ND

98.27

98.14

bi 

947AP

ND

ND

ND

ND

ND

0.26

0.20

ND

ND

55.59

ND

41.81

3.25

0.00

101.11

99.75

be 

854AP 

Apatite

ND

ND

ND

ND

ND

0.27

0.20

ND

ND

56.20

ND

41.85

3.76

0.01

102.29

100.71

qm 

937AP

ND

ND

ND

ND

ND

0.02

0.11

ND

ND

55.71

ND

41.44

4.42

0.02

101.72

99.86

bo 

9320 P

ND

0.07

0.15

0.01

67.53

29.66

0.92

0.01

ND

ND

ND

ND

ND

ND

98.35

bi be 

9470 P 8540 P 

Magnetite

ND

0.07

0.13

0.02

68.13

29.86

0.99

0.01

ND

ND

ND

ND

ND

ND

99.21

ND

0.13

0.10

0.02

67.98

30.53

0.36

0.00

ND

ND

ND

ND

ND

ND

99.12

qm 

9370 P

ND

0.21

0.19

0.03

67.59

29.85

1.03

0.02

ND

ND

ND

ND

ND

ND

98.92

bo 

932GR 

Andradite

35.20

2.48

2.59

ND

26.53

0.32

1.36

0.15

ND

31.64

0.08

ND

ND

ND

100.26

10) of this garnet, 2.5 weight percent, is distinctive and is diagnostic of the variant of andradite known as melanite. Abundances of ferrous and ferric iron in this garnet (table 10) were 

computed by stoichiometry.

Discussion

Tectonic Setting

Granitoid magmas generated in various tectonic settings 

have distinct geochemical signatures (Pearce and others, 1984). 

Trace-element abundances in igneous rocks of the central Big 

Belt Mountains, in particular those of Rb, Y, and Nb, are similar to those of subduction-related arc rocks (fig. 13). Gill 

(1981) suggested that negative Nb-Ta anomalies (fig. 8A) and 

Ba/Ta ratios greater than 450 (tables 3 and 4), both of which 

are characteristic of igneous rocks in the central Big Belt 

Mountains, are diagnostic of magmas generated in a subduction-related arc setting. Major oxide compositions of the Big 

Belt Mountains igneous rocks form a curvilinear array that parallels, but is displaced slightly below, that characteristic of Cascade arc calc-alkaline rocks (fig. 14). Chondrite-normalized 

extended trace-element patterns for igneous rocks of the central 

Big Belt Mountains are similar to those of mid-Tertiary 

volcanic rocks of the southern Great Basin (for example, du 

Bray, 1995b), whose origin is similarly considered to be related 

to diffuse back-arc volcanism that followed a period of shallow 

subduction (Coney and Reynolds, 1977). These compositional 

characteristics are all consistent with a mantle-dominated genesis of central Big Belt Mountains intrusions in a back-arc, subduction-related setting.

A back-arc, subduction-related origin for igneous rocks 

of the central Big Belt Mountains is consistent with the presence of voluminous, Late Cretaceous through early Tertiary 

igneous rocks throughout Idaho (Hyndman and Foster, 1989) 

and western Montana. Late Cretaceous magmatism in western Montana and eastern Idaho, including that in the Idaho 

and Boulder batholiths, can be placed in the context of a magmatic belt that extended southward and eastward during midCretaceous time (Coney and Reynolds, 1977; Lipman, 1992). 

Igneous rocks in the central Big Belt Mountains are among 

the easternmost manifestations of Late Cretaceous magmatism in this region. The Sliderock Mountain stratovolcano, 

another inferred back-arc magmatic center (du Bray and Harlan, 1998), is about 160 km southeast of and about 10 million years older than intrusions of the central Big Belt 

Mountains. Relative age and geographic relations between 

these two centers are consistent with renewed, westwardsweeping back-arc magmatism (following rapid, shallow subduction during the mid-Cretaceous) related to slower, increasingly steep subduction and westward retreat of the subducted 

slab hingeline through time.

Origin of Compositional Variation in Granodiorite 

and Quartz Monzodiorite

Magmatic compositional variation can result from a number of petrogenetic mechanisms, including magma mixing, wallrock stoping and assimilation, varying degrees of partial 

melting, and fractional crystallization. Physical and geochemical evidence was used to evaluate the contribution of these processes to evolution of compositional variation within igneous

Discussion 23
10,000

1000 -

10 100 1000 

Y+Nb, IN PARTS PER MILLION

EXPLANATION

Intrusive suite of Boulder Baldy

bo, outer zone

bi, intermediate zone

be, core zone 

Quartz monzodiorite of Mount Edith

10,000

EXPLANATION

Intrusive suite of Boulder Baldy 

D bo, outer zone 

+ bi, intermediate zone 

x be, core zone 

• Quartz monzodiorite of Mount Edith

Figure 13. Trace-element tectonic-setting-discrimination variation 

diagram for intrusions of central Big Belt Mountains. Tectonic setting-composition boundaries from Pearce and others (1984).

Figure 14. Ternary AFM (Na 2 0+K2 0-FeO*-MgO) diagram showing 

compositions of central Big Belt Mountains intrusions. Cascade calcalkaline trend line from Irvine and Baragar (1971).

rocks of the central Big Belt Mountains. Magma mixing was 

ruled out, in part because the intrusions lack mafic enclaves that 

are common in granitoids intruded by compositionally distinct 

magma (Vernon, 1984; Frost and Mahood, 1987). The igneous 

rocks are also free of wallrock xenoliths that would result from 

the sloping-assimilation process. Chemical data also preclude 

mixing as a source of the stock's compositional variation. In 

contrast to the intrusion compositions that result from two-component mixing, abundance variation patterns for the Big Belt 

Mountains intrusions are not linear.

The possibility that compositional variation within the 

intrusions resulted from varying degrees of partial melting was 

eliminated primarily on the basis of chemical considerations. 

Allegre and Minster (1978) demonstrated that abundance ratios 

of highly and moderately incompatible trace elements can be 

used to help identify processes responsible for compositional 

variation. This observation can be applied to central Big Belt 

Mountains intrusions by consideration of Th and Sm abundance 

variations. Their Th/Sm ratios are essentially constant regardless of Th abundances (fig. 15 and table 4). Open-system behavior, such as varying degrees of partial melting or magma mixing 

involving sequential inputs, is inconsistent with the essentially 

constant Th/Sm ratios characteristic of the Big Belt Mountains 

intrusions. These compositional variations are, however, 

consistent with development of compositional variation by 

closed system fractional crystallization, which causes highly

versus moderately incompatible trace-element abundance ratios 

to be essentially constant as highly incompatible trace-element 

abundances increase.

In order to evaluate crystal fractionation as a process 

responsible for development of magmatic compositional variation in biotite granodiorite and quartz monzodiorite intrusions 

of the central Big Belt Mountains, whole rock and mineral compositions of eight representative samples were used as reactants 

and products in mixing model calculations. Compositions of 

the quartz monzonite of the intrusive suite of Boulder Baldy 

were not modeled because these rocks do not preserve verifiably 

magmatic characteristics. Because of the compositional overlap 

and similarity between quartz monzodiorites of Mount Edith 

and the intrusive suite of Boulder Baldy, their compositions 

were modeled as part of a single, potential fractionation series. 

The composition of the most mafic, noncumulate sample of the 

subset (202934) was presumed to be parental to each successively less mafic (daughter) sample (202937, 202954, 202854, 

202947, 202871, and finally 202946). A least-squares mixing 

calculation (PETMIX) was used to determine the weight fractions of minerals (with compositions like those contained in the 

parent sample) to be subtracted to produce a derivative liquid 

with a composition like that of each daughter sample (table 11). 

The purpose of these calculations is to demonstrate that all parts 

of the stock are plausibly related by fractionation of reasonable 

amounts of their constituent minerals.

24 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
Th, IN PARTS PER MILLION

EXPLANATION

Intrusive suite of Boulder Baldy

bo, outer zone

bi, intermediate zone

be, core zone 

Quartz monzodiorite of Mount Edith

Figure 15. Th/Sm vs. Th variation diagram showing compositions of 

central Big Belt Mountains intrusions.

Major element least-squares crystal fractionation-mixing 

models are viable when the quantity 2r2 (table 11), the sum of 

the squares of each major oxide residual, is less than a value 

ascribed to analytical uncertainty (approximately 0.5 weight percent). Low to very low 2r values for six Big Belt mixing models indicate that all parts of the associated intrusions could have 

been generated by fractionation of constituent minerals. In both 

cases for which the quantity 2r2 is greater than 0.2 (table 11), the 

majority of each 2r2 value results from large Na2O residuals. 

Because Na2O is notoriously mobile, these slightly elevated Zr2

values probably reflect late hydrothermal alteration or weathering processes rather than true misfits between observed and modeled compositions. The composition of the most evolved sample 

(202946) is the silicate liquid whose modeled composition 

results from crystallization and removal of about 68 percent solids from the presumed parental liquid, whose composition is represented by sample 202934 (table 11). The composition of 

cumulate sample 202940 could have resulted from addition (as 

cumulates) of minerals with compositions like those found in 

sample 202940 to a silicate liquid with a composition like that of 

the presumed parent composition (202934).

Trace-element abundances for each representative sample 

were normalized to that of the most primitive, presumed parental 

sample (202934) and plotted on extended trace-element diagrams to evaluate trace-element behavior during the fractionation process just described. These patterns (fig. 16) are 

essentially parallel, cluster relatively tightly, and include minor 

superimposed positive Th and Zr anomalies. Th and Zr appear to 

have been almost equally partitioned between crystals and residual silicate liquid; their abundances in successively more evolved 

rocks remained essentially constant, whereas those of other trace 

elements decreased with differentiation. These patterns show 

that overall trace-element abundances decrease systematically 

with increasing SiO2 contents (differentiation). That none of the 

evaluated trace elements (including Rb, which is usually highly 

incompatible (Hanson, 1978)), display well-developed incompatibility is somewhat remarkable. All these trace elements were 

preferentially partitioned from the residual silicate liquid into 

solid phases, particularly into feldspar and biotite, during 

solidification.

Abundances of Sr, K, and Rb are principally controlled by 

feldspar (Hanson, 1978); abundances of strontium, which typically acts as a strongly compatible element, and rubidium are 

respectively most and least depleted by feldspar fractionation. 

As feldspar fractionation and differentiation proceed, K and to

Table 11. Results of major oxide least-squares crystal fractionation-mixing modeling for intrusive 

rocks from central Big Belt Mountains.

[Numbers under mineral headings (PL, plagioclase; KF, potassium feldspar; HB, magnesio-hornblende; CP, 

clinopyroxene; MG, magnetite; TI, titanite; AP, apatite) are mass percentages of each phase removed in each 

fractionation step]

Weight fractions, in percent, of minerals 

fractionated from parent liquid

Derived 

composition 1

202946

202871

202947

202854

202954

202937

202934

PL

39.02

39.91

37.93

36.68

30.48

23.82

KF

18.89

15.99

17.94

18.53

17.25

9.45

HB

7.34

6.14

7.25

6.90

5.59

3.64

CP MG

1.30

1.38

1.22

1.29

1.00

1.97 0.75

TI

0.73

0.77

0.72

0.71

0.58

0.48

AP

0.23

0.34

0.29

0.20

0.23

0.16

F

32.49

33.88

34.64

35.69

44.87

59.74

2r2 

Residual

0.042

0.239

0.040

0.079

0.091

0.305

CUMULATE:

202940 11.26 1.34 4.80 1.77 0.82 1.34 79.39 0.159

1 For instance, composition 202937 is derived from composition 202934 by fractionation of mineral mass percentages given on the 202934 line. The fractionated minerals have the compositions of those in 202934. The mass 

percentage of the derived liquid (202937) and the sum of the residuals squared are given in the F and 2r~ columns, 

respectively, on the 202934 line.

Discussion 25
10 i—r "i—i—i—r n—i—r K/100

01 i I I I I I I I I I I I I I I I I I 

Rb Ba Th K Nb Ta La Ce Sr Nd P Sm Zr Hf Ti Tb Y Tm Yb

Figure 16. Extended trace-element abundance diagram showing 

compositions of intrusions in central Big Belt Mountains; abundances 

normalized to that of most primitive sample (202934).

an even greater extent, Rb are concentrated in the residual liquid 

relative to Sr. Compositions plotting nearest the Sr apex of the 

Rb-K/100-Sr ternary diagram (fig. 17) represent the least 

evolved magmas. In samples of granodiorite and quartz 

monzodiorite from the central Big Belt Mountains, compositions 

plot along a short array whose origin lies near the Sr apex; they 

define a narrow compositional range. Mean abundance of Sr in 

the eight representative samples is 860±160 ppm. Because plagioclase is the principal residence of Sr, tight clustering of Sr 

abundances indicates that plagioclase fractionation did not 

strongly influence Sr abundances in compositional evolution of 

the central Big Belt Mountains intrusions. The position, relative 

to more evolved rocks (du Bray and others, 1988; du Bray, 

1995b, for example), and the shortness of the array also confirm 

that compositional evolution of central Big Belt Mountains igneous rocks was not strongly influenced by plagioclase fractionation. The shortness of the compositional evolution array on the 

Rb-K/100-Sr ternary diagram (fig. 17) probably results from a 

relatively low distribution coefficient for Sr in plagioclase (DSr) 

but high Sr abundances, relative to those of Rb and K (table 3). 

Fractionation of plagioclase with a somewhat low DSr (for 

instance, less than 1.45 (Hanson, 1978)) from a very Sr-rich 

magma causes the abundance of Sr, relative to those of K and 

Rb, in residual magma to be little affected.

Abundances of REE in granodiorite and quartz monzodiorite from the central Big Belt Mountains, normalized to that of 

the most primitive (presumed parental) sample (202934), yield 

values near or slightly less than 1.0. Resulting patterns (fig. 18) 

are essentially parallel and flat; therefore, the REE were minimally fractionated during differentiation. Consequently, REE 

bulk distribution coefficients for the fractionation assemblage 

must have been slightly more than about one. The absence of 

REE fractionation indicates that variably REE enriched accessory minerals did not play a major role in chemical evolution of 

these igneous rocks. The lack of progressively larger negative 

Eu anomalies with increasing differentiation is enigmatic given 

that plagioclase was a fractionating phase (table 11). Intrusions 

of the central Big Belt Mountains may have solidified under relatively oxidizing conditions, thereby favoring trivalent europium 

over the divalent form that readily substitutes for calcium in

HIGHLY EVOLVED GRANITES

(du Bray and others, 1988) ^

ASH-FLOW TUFFS OF THE

SOUTHERN GREAT BASIN

(duBray, 1995b)

EXPLANATION

Intrusive suite of Boulder Baldy

bi, intermediate zone

be, core zone 

Quartz monzodiorite of Mount Edith

Figure 17. Ternary variation diagram showing the relative proportions 

of Rb, K, and Sr in granodiorite and quartz monzodiorite in central Big 

Belt Mountains.

plagioclase. The presence of magnetite (as the sole iron oxide 

mineral in these intrusions) and abundant titanite (as opposed to 

ilmenite), both of which contain more ferric than ferrous iron, 

further suggests the prevalence of oxidizing conditions. Mineral-melt distribution coefficient patterns for hornblende and 

apatite are characterized by negative Eu anomalies (Hanson, 

1980); their fractionation (table 11) causes residual liquid Eu 

enrichment, which may also have counteracted any tendency 

toward residual-liquid Eu depletion caused by plagioclase fractionation.

0.1

La Ce Nd Sm Eu Tb Tm Yb Lu

Figure 18. Rare-earth element diagram showing compositions of 

central Big Belt Mountains intrusions; abundances normalized to that 

of most primitive sample (202934).

26 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
If crystal fractionation is accepted as a viable process for 

development of compositional variation in intrusions of the central Big Belt Mountains, magma reservoir processes that result 

in crystal fractionation must then be identified. One of the most 

widely accepted models for genesis of concentrically zoned plutons of the Sierra Nevada batholith involves fractional sidewall 

crystallization (Sawka and others, 1990). This solidification 

model is invoked for magma chambers whose cooling is inferred 

to proceed from the margin inward by conductive heat loss to 

host rock forming the chamber walls. As cooling proceeds, 

mafic silicate minerals, calcic plagioclase, and accessory minerals crystallize along the chamber walls.

A variant of the fractional sidewall crystallization model 

may be responsible for development of progressively more 

evolved magmas represented by igneous rocks of the central Big 

Belt Mountains. Sawka and others (1990) described processes 

of interstitial liquid migration toward the top of the chamber 

that, lacking evidence of vertical zonation, seem inapplicable to 

evolution of igneous rocks in the central Big Belt Mountains. 

Instead, melt in the associated reservoir may have become more 

evolved by removal of components necessary for sidewall crystallization of refractory phases. Continuous sidewall crystallization and mixing between refractory-component-depleted 

sidewall melt and undepleted melt inboard from the crystallization front could have produced an inward-crystallizing magma 

reservoir that became more evolved from the walls inward. The 

mafic border phase of the quartz monzodiorite of Mount Edith 

may represent weakly developed cumulates that formed by a 

combination of sidewall crystallization and filter pressing along 

the margins of the magma reservoir.

Genesis of Quartz Monzonite in the Outer Zone

Textural features, erratic compositional variation, unusual 

compositions, and mineralogy of the quartz monzonite that 

forms the outer zone of the intrusive suite of Boulder Baldy 

suggest that these outer zone rocks do not retain primary magmatic characteristics. A single sample (202945) from near the 

western contact of the outer zone with enclosing Newland Formation rocks (fig. 1) consists of quartz monzodiorite identical 

to that exposed about 1.5 km to the east in the intermediate 

zone of the intrusive suite. Rock represented by this sample is 

interpreted as belonging to an isolated domain, with a primary 

magmatic origin, that was minimally affected by alteration that 

variably altered rock forming the remainder of the outer zone. 

All rock in the outer zone is interpreted as the alteration and 

(or) recrystallization product of a quartz monzodiorite protolith. The extent of alteration and recrystallization is highly 

variable in the outer zone and is correlated with compositional 

variation. Specifically, the most and least altered and recrystallized samples are characterized by compositions that plot 

respectively farthest and nearest from trends (fig. 5) defined by 

granodiorite and quartz monzodiorite of the central Big Belt 

Mountains.

The abundance of fluorite throughout the quartz monzonite 

zone, both as disseminated grains and as veinlets, indicates that 

rock in this zone has extensively interacted with a fluorine-rich

fluid. Further evidence of the importance of fluorine-bearing 

solutions in the genesis of outer zone rocks is presented by 

Sahinen (1962), who described fluorite veins in the intrusive 

suite. The abundance of aplite-pegmatite intergrowths and the 

presence of zones of densely packed, euhedral perthite phenocrysts also indicate that these rocks have interacted extensively 

with hydrothermal fluids. The fluids responsible for alteration 

of the quartz monzodiorite caused magmatic hornblende and 

plagioclase to be replaced by sodic pyroxene and alkali feldspar, 

respectively. These reactions are in accord with the absence of 

hornblende, diminished plagioclase abundances, and somewhat 

elevated alkali feldspar abundances in pyroxene-bearing samples of the quartz monzonite. Rocks affected by this initial, 

weak metasomatic alteration retain primary magmatic features, 

including interlocking grain fabrics and seriate textures.

Abundant rock in the outer zone lacks petrographic features indicative of a primary magmatic origin. These rocks lack 

plagioclase, are primarily composed of densely packed coarsegrained to very coarse grained perthite crystals, and contain 

andradite garnet. Many of these rocks contain no mafic silicate 

minerals. However, one group of samples is probably transitional between quartz monzonite that contains sodic pyroxene 

and that which contains andradite; these samples contain unstable and reacted sodic pyroxene. These petrographic features 

reflect intense hydrothermal alteration that resulted in replacement of sodic pyroxene by andradite. Sodic pyroxene, remaining plagioclase, and magnetite reacted with a hydrothermal fluid 

to form andradite, alkali feldspar, and excess MgO and SiO2. 

These reactions are in accord with the absence of aegirineaugite, plagioclase, and magnetite, and significantly elevated 

alkali feldspar abundances in andradite-bearing samples of the 

quartz monzonite.

These reactions yield excess MgO; samples from the outer 

zone are distinctly MgO depleted (fig. 5) relative to unaltered 

granodiorite and quartz monzodiorite of the central Big Belt 

Mountains. The reaction by which andradite was produced also 

resulted in excess SiO2. Fluids responsible for metasomatic 

alteration of rock now exposed in the outer zone probably transported MgO and SiO2 out of the magmatic system into surrounding wallrocks. The presence of numerous large quartz 

veins in Newland Formation rocks peripheral to the intrusive 

suite of Boulder Baldy indicates that SiO2 was expelled from the 

magmatic system, probably in response to the replacement of 

sodic pyroxene by andradite and other desilication reactions. 

Exploration studies at Miller Mountain (fig. 1) indicate that the 

stock exposed there contains a small (at least 11 million tons of 

ore containing 310,000 oz gold), subeconomic gold deposit 

(Greg Whitman, Pegasus Gold Corporation, oral commun., 

1993). Genesis of the gold-bearing quartz stockwork at Miller 

Mountain probably reflects these same processes. The presence 

of numerous smaller gold prospects along contacts between 

central Big Belt Mountains intrusions and enclosing Newland 

Formation rocks (du Bray, 1995a) suggests that the hydrothermal fluid responsible for alteration of quartz monzodiorite to 

quartz monzonite precipitated gold and other metals near bounding contacts, where physico-chemical gradients were steepest.

The source of the fluids responsible for the alteration 

described herein is uncertain. Because rocks of the Newland 

Formation are fine grained and have low porosity, they are an

Discussion 27
unlikely fluid source. However, the Newland Formation rocks 

contain numerous fractures that could have served as fluid conduits. The abundance of fluorine in the hypothesized fluid 

phase, as indicated by accessory fluorite, and the presence of 

aplite-pegmatite intergrowths associated with altered rock indicate a probable magmatic origin. Solidification of the central 

Big Belt Mountains intrusions almost certainly involved latestage fluid-phase exsolution. The presence of a variably developed, fine-grained, anhedral groundmass among many samples 

of the quartz monzonite zone suggests that final solidification of 

magma represented by these rocks involved rapid nucleation, 

possibly in response to pressure quenching events. Pressure 

quenching causes rapid exsolution, which in turn may have initiated variably intense alteration and recrystallization of quartz 

monzodiorite to rock represented by the outer zone of the intrusive suite of Boulder Baldy.

Many features of the altered rocks associated with the intrusions of the central Big Belt Mountains are similar to those of 

granitoid rocks that have been affected by the alteration that 

accompanies late-stage magmatic and hydrothermal processes 

that are characteristic of Climax-type molybdenum deposits 

(White and others, 1981). This observation is consistent with 

the association, identified by Lang and others (2000, fig. 7), 

between moderately reduced intrusion-related gold systems and 

molybdenum and tungsten deposits associated with evolved and 

moderately reduced intrusions emplaced in subduction-related 

continental settings. The geochemical, petrographic, and tectonic characteristics of the central Big Belt Mountains intrusions 

and associated mineralized rocks clearly are similar to those 

identified in intrusions and mineralized rock associated with Climax-type molybdenum deposits as well as those of intrusionrelated gold deposits. Consequently, the central Big Belt Mountains intrusions and associated mineralized rocks may represent 

a genetic transition between the two mineral deposit types. 

Development of the altered, outer zone of the intrusive suite of 

Boulder Baldy appears to have resulted from magmatic exsolution processes common to Climax-type molybdenum deposits. 

The relationship between these hydrothermal processes and genesis of small gold deposits, such as that at Miller Mountain, is 

uncertain but represents a research avenue that is critical to 

determining the genesis of granite-related gold deposits.

Considering the proposed consanguinity of all central Big 

Belt Mountains intrusions, the fact that the quartz monzodiorite 

of Mount Edith lacks post-magmatic hydrothermal alteration 

features is somewhat puzzling. However, most of the quartz 

monzodiorite of Mount Edith lacks a fine-grained, anhedral 

groundmass, which suggests that this intrusion did not experience the extent of pressure quenching and attendant rapid fluid 

exsolution that apparently played a major role in solidification of 

the quartz monzodiorite of the intrusive suite of Boulder Baldy. 

In addition, magma represented by the quartz monzodiorite of 

Mount Edith may have contained lower dissolved water contents 

than similar magma represented by the intrusive suite of Boulder 

Baldy and was therefore less susceptible to significant fluid 

exsolution during cooling. Consequently, the quartz monzodiorite of Mount Edith was not significantly altered and metasomatically recrystallized by the processes that affected quartz 

monzodiorite in the intrusive suite of Boulder Baldy.

The apparently large volume (assuming that surficial exposures extend to some considerable depth) of hydrothermally 

altered rock depicted by the outer zone of the intrusive suite is 

difficult to reconcile with the limited volume of fluid that typically results from solidification and attendant magmatic exsolution of calc-alkaline intrusions. Perhaps the volume of altered 

rock is less than would result by inferring that the altered, outer 

zone extends to considerable depth. In fact, although exposures 

are limited and field relations poorly exposed, altered, outer 

zone rock may only form a thin tabular carapace that overlies 

and marginally encloses otherwise normal intermediate zone 

rock. However, if altered, outer zone rock does extend to considerable depth and an additional source of abundant fluids is 

required, major, fluid-saturated faults may have been intersected 

during magma intrusion. Interaction between early-emplaced, 

outer zone magma and abundant structurally channelized fluid 

could have increased the extent of hydrothermal alteration of 

outer zone rocks. Isotopic analyses of samples from the outer 

zone could help determine whether fluids responsible for alteration and metasomatic recrystallization of rock represented by 

the outer zone were primarily magmatic, meteoric, or a combination of both.

Conclusions

The central Big Belt Mountains of western Montana contain several Late Cretaceous intrusions, including a pluton represented by the quartz monzodiorite of Mount Edith and a 10- 

km diameter intrusive suite centered between Boulder Baldy 

and Boulder Mountain. The intrusive suite consists of three 

compositional and textural zones, of which only the inner two 

are considered to reflect purely magmatic processes. In contrast, with the exception of a weakly developed mafic border 

phase, the quartz monzodiorite of Mount Edith is generally 

homogeneous. 40Ar/39Ar geochronologic data indicate that 

these rocks crystallized before 68 Ma (but probably between 70 

and 69 Ma), that all are part of a single magmatic event, that 

regional cooling related to the magmatism in this area was complete by about 60 Ma, and that metal deposits and plutonic rocks 

of the central Big Belt Mountains are temporally related. 

Geochemical characteristics and geologic relations indicate that 

genesis of these calc-alkaline intrusions was related to subduction and renewed back-arc magmatism along the west edge of 

the late Mesozoic North America plate following a period of 

shallow subduction. Fractionation-mixing calculations indicate 

that compositional variation within granodiorite and quartz 

monzodiorite of the central Big Belt Mountains developed in 

response to varying amounts of plagioclase, clinopyroxene, 

hornblende, titanite, apatite, and magnetite fractionation. Crystals nucleated along the walls and roof of the cooling magma 

chamber while residual silicate liquid continued mixing.

In contrast, unusual textures and erratic compositional variation, characteristic of quartz monzonite in the outer zone of the 

intrusive suite of Boulder Baldy, probably reflect extensive 

metasomatic alteration and recrystallization of quartz monzodiorite by fluorine-rich, gold-bearing fluids. Intrusive rocks and

28 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana
their associated metal deposits bear striking resemblance to 

recently described (Lang and others, 2000) intrusion-related gold 

systems. Additional study, especially isotopic and fluid inclusion 

analyses, of the Miller Mountain deposit and the large, associated intrusion centered on Boulder Baldy could add considerably 

to the understanding of the genesis of this deposit type.

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication October 12,2001 

Graphics by authors and Springfield and Springfield 

Photocomposition by Gayle M. Dumonceaux 

Edited by LM. Carter

30 Late Cretaceous Intrusives, Central Big Belt Mountains, Montana

* U.S. GOVERNMENT PRINTING OFFICE: 2002 — 773-049 ' 30079 Rec'on No. 8