M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR

Chronology of Late Cretaceous Igneous and

Hydrothermal Events at the Golden Sunlight

Gold-Silver Breccia Pipe, Southwestern Montana

U.S. GEOLOGICAL SURVEY BULLETIN 2155
Chronology of Late Cretaceous Igneous and 

Hydrothermal Events at the Golden Sunlight 

Gold-Silver Breccia Pipe, Southwestern Montana

By Ed DeWitt, Eugene E. Foord, Robert E. Zartman, 

Robert C. Pearson, and Fess Foster

U.S. GEOLOGICAL SURVEY BULLETIN 2155

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996

M A R C H 3, 18 4 9

U.S. DEPARTMENT OF THE INTERIOR
For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

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

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Chronolgy of Late Cretaceous igneous and hydrothermal events at the Golden Sunlight

gold-silver breccia pipe, southwestern Montana / by Ed DeWitt . . . [et al.].

p. cm.—(U.S. Geological Survey bulletin ; 2155)

Includes bibliographical references.

Supt. of Docs. no. : I 19.3 : 2155

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

3. Hydrothermal alteration—Montana. 4. Gold ores—Montana. 5. Breccia 

pipes—Montana. I. DeWitt, Ed. II. Series.

QE75.B9 no. 2155

[QE688]

557.3 s—dc20

[553.4′1′0978667] 96-29319

CIP
III

CONTENTS

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

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

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

Igneous Rock Suites ...................................................................................................... 3

Rhyolite ................................................................................................................. 5

Petrology ........................................................................................................ 5

Chemistry ...................................................................................................... 5

Chronology of Rhyolite Emplacement .......................................................... 6

Breccia Pipe and Adjacent Strata .......................................................................... 10

Chronology of Alteration and Mineralization ............................................... 12

Intermediate to Felsic Plutonic Rocks ................................................................... 16

Petrology and Chemistry ............................................................................... 16

Lamprophyres ........................................................................................................ 16

Petrology of CO2-Poor Lamprophyre ............................................................ 16

Chemistry of CO2-Poor Lamprophyre ........................................................... 17

Geochronology of CO2-Poor Lamprophyre ................................................... 27

Petrology of CO2-Rich Lamprophyre ............................................................ 27

Chemistry of CO2-Rich Lamprophyre ........................................................... 31

Element Gains and Losses During CO2 Metasomatism ........................ 33

Gold Analyses of Lamprophyre and Other Igneous Rocks ........................... 39

Model for CO2 Metasomatism ....................................................................... 39

Correlation of Lamprophyres and Rhyolite with Rocks in Surrounding Areas ............ 42

Conclusions ................................................................................................................... 44

References Cited ............................................................................................................ 47

FIGURES

1. Generalized geologic map of the Golden Sunlight mine area ............................................................................... 2

2. Geologic cross section of the Golden Sunlight breccia pipe ................................................................................. 4

3. Photomicrograph of mineralized rhyolite, Golden Sunlight mine......................................................................... 5

4. Sample locality map of the Golden Sunlight mine area ........................................................................................ 6

5. Major-element geochemistry plots of rhyolite and tonalite from the Golden Sunlight mine area ........................ 11

6. Chondrite-normalized rare-earth-element plot for rhyolite and tonalite in the Golden Sunlight mine area ......... 12

7. Concordia diagram for zircon from rhyolite, Golden Sunlight mine .................................................................... 12

8. Lead evolution diagrams for samples from the Golden Sunlight mine area ......................................................... 15

9. 40Ar-39Ar release spectra for sericite from rhyolite, Golden Sunlight mine......................................................... 17

10–11. Photomicrograph in plane-polarized light of:

10. Olivine and clinopyroxene phenocrysts in lamprophyre.............................................................................. 18

11. Groundmass texture and minerals in lamprophyre....................................................................................... 18

12. Photomicrograph of strongly zoned clinopyroxene phenocryst in lamprophyre................................................... 18

13–16. Photomicrograph in plane-polarized light of:

13. Carbonate mineral(?)- and feldspar-rich inclusions in clinopyroxene phenocryst in lamprophyre ............. 19

14. Clinopyroxene grains in groundmass in lamprophyre.................................................................................. 19

15. Olivine phenocrysts in lamprophyre............................................................................................................. 19

16. Embayed biotite phenocryst in lamprophyre................................................................................................ 20

17. Photomicrographs of biotite phenocryst surrounded by opaque-mineral- and clinopyroxene-poor selvage

of groundmass in lamprophyre .............................................................................................................................. 21
IV CONTENTS

18. Photomicrograph in plane-polarized light of clinopyroxene in groundmass in lamprophyre............................... 21

19. Photomicrographs of intergrown sanidine and biotite in groundmass in lamprophyre......................................... 22

20–21. Photomicrograph in plane-polarized light of:

20. Corona of biotite surrounding opaque mineral in lamprophyre................................................................... 22

21. Ophitic biotite in groundmass in lamprophyre............................................................................................. 23

22. Photomicrographs of fine-grained carbonate mineral interstitial to groundmass sanidine in lamprophyre.......... 23

23. SiO2 versus major-element-oxide plots for all lamprophyre samples, Golden Sunlight mine area...................... 25

24. Major-element plots of CO2-poor lamprophyre, Golden Sunlight mine............................................................... 26

25. Magnesium number versus Co diagram for all lamprophyres, Golden Sunlight mine area ................................. 27

26. Chondrite-normalized rare-earth-element plot for alkalic and sub-alkalic CO2-poor lamprophyre, 

Golden Sunlight mine area.................................................................................................................................... 27

27. P2O5 versus La plot for alkalic and sub-alkalic lamprophyres, Golden Sunlight mine ........................................ 27

28. 40Ar-39Ar release spectra for biotite from lamprophyre, Golden Sunlight mine.................................................. 29

29–33. Photomicrograph in plane-polarized light of:

29. Clinopyroxene phenocrysts replaced by dolomite and chlorite in lamprophyre.......................................... 29

30. Olivine phenocryst replaced by sericite, magnesite, and magnetite in lamprophyre................................... 30

31. Olivine phenocryst replaced by biotite and carbonate minerals in lamprophyre......................................... 30

32. Olivine-rich lamprophyre showing complete replacement of olivine by magnesite.................................... 30

33. Sanidine replaced by micrometer-size quartz and orthoclase(?) in lamprophyre ........................................ 31

34–35. CO2 versus other component plots for:

34. Alkalic and sub-alkalic lamprophyres showing apparent depletion trends.................................................. 32

35. Alkalic and sub-alkalic lamprophyre showing apparent enrichment trends ................................................ 34

36. Plots illustrating relationships between Ni, MgO, and percentage of modal olivine in alkalic and

sub-alkalic lamprophyre........................................................................................................................................ 35

37. Major-element plots of CO2-rich lamprophyre, Golden Sunlight mine................................................................ 36

38. Chondrite-normalized rare-earth-element plot for CO2-rich lamprophyre ........................................................... 37

39. Multielement plots illustrating differentiation and alteration trends in lamprophyre ........................................... 38

40. Location map of southwestern Montana showing localities and rock units discussed in text .............................. 41

41. Major-element plots of Cretaceous-Tertiary mafic rocks in western Montana ..................................................... 43

42. Chondrite-normalized rare-earth-element plot for mafic volcanic rocks in unit 8 of the

Elkhorn Mountains Volcanics................................................................................................................................ 44

43. Major-element plots of Cretaceous-Tertiary felsic rocks in the Elkhorn Mountains Volcanics,

western Montana ................................................................................................................................................... 45

TABLES

1. Major-, minor-, and rare-earth-element geochemistry of igneous rocks in the Golden Sunlight mine area, 

southwestern Montana........................................................................................................................................... 7

2. U-Th-Pb analytical data for zircon from rhyolite, Golden Sunlight mine, southwestern Montana...................... 13

3. Fission-track analytical data for zircon from rhyolite samples, Golden Sunlight mine area,

southwestern Montana........................................................................................................................................... 13

4. Lead isotopic composition of rocks near the Golden Sunlight mine area, southwestern Montana ...................... 14

5. 40Ar-39Ar data for sericite from rhyolite, Golden Sunlight mine area, southwestern Montana ........................... 16

6. Minor- and trace-element geochemistry of phlogopitic biotite phenocryst samples from lamprophyre bodies, 

Golden Sunlight mine, southwestern Montana ..................................................................................................... 20

7. K-Ar analytical data for biotite from lamprophyre, Golden Sunlight mine area, southwestern Montana............ 28

8. 40Ar-39Ar data for biotite from lamprophyre, Golden Sunlight mine area, southwestern Montana..................... 28

9. Recalculated, CO2-free compositions of CO2-metasomatized lamprophyre samples,

Golden Sunlight mine area, southwestern Montana.............................................................................................. 35

10. Gold concentrations of igneous rocks in the Golden Sunlight mine area, southwestern Montana....................... 39
1

CHRONOLOGY OF LATE CRETACEOUS 

IGNEOUS AND HYDROTHERMAL EVENTS AT THE 

GOLDEN SUNLIGHT GOLD-SILVER BRECCIA PIPE, 

SOUTHWESTERN MONTANA

By Ed DeWitt,1 Eugene E. Foord,1 Robert E. Zartman,1

Robert C. Pearson,1 and Fess Foster2

ABSTRACT

Gold mineralization at the Golden Sunlight breccia

pipe, southwestern Montana, is related to emplacement of

Late Cretaceous alkali-calcic rhyolite and subsequent collapse of the Belt Supergroup wallrock and rhyolite in the

pipe. The pipe is inferred to grade downward into an alkalic

porphyry molybdenum system. The pipe is cut by alkalic to

sub-alkalic lamprophyre dikes and sills, which locally contain high-grade gold where emplaced along late shear zones

and vein systems.

Determination of the emplacement age of the rhyolite is

hampered by inherited lead or inherited Late Archean zircon

from the source region of the rhyolite. An emplacement age

of about 80 Ma for the rhyolite can be inferred if a basement

age of 2,600 Ma is assumed. This Late Archean age is in

agreement with basement ages determined in many parts of

southwestern Montana.

A206Pb-238U whole-rock date of 84±18 Ma from altered

and mineralized Belt Supergroup strata and rhyolite in the

breccia pipe indicates hydrothermal alteration related to gold

mineralization in Late Cretaceous time. Although sericite is a

relatively widespread hydrothermal mineral, attempts to date

the very fine grained material by the 40Ar-39Ar method did not

provide a spectra that could be interpreted unambiguously.

A 40Ar-39Ar plateau date of 76.9±0.5 Ma from biotite

phenocrysts in the lamprophyre indciates intrusion of mafic

magma and attendant CO2 metasomatism in the Late

Cretaceous. Fission-track data from zircon in the rhyolite

are permissive of slow uplift of the Belt Supergroup strata,

 1U.S. Geological Survey, Box 25046, Denver Federal Center, Denver,

CO 80225. 2Golden Sunlight Mines, Inc., 453 MT Highway 2 East, Whitehall, MT

59759.

rhyolite, and lamprophyre between 55 and 50 Ma, but the

data are not definitive.

Rhyolitic welded tuff in the informally named units 7, 9,

and 11 of the Elkhorn Mountains Volcanics is most similar in

chemistry and age to the rhyolite at the Golden Sunlight mine.

Trachybasalt in the Adel Mountains Volcanics and andesitic

basalt in the informally named unit 8 of the Elkhorn Mountains Volcanics are the most analogous in chemistry and age

to lamprophyres at the mine. The rhyolitic rocks appear to be

derived from deep crustal sources, but data for the lamprophyres and mafic rocks in the Elkhorn Mountains Volcanics

indicate that they were derived from the mantle. 

INTRODUCTION

The Golden Sunlight gold-silver deposit is hosted by a

breccia pipe that cuts sedimentary rocks of the Middle Proterozoic Belt Supergroup and sills of a Late Cretaceous rhyolite porphyry (Porter and Ripley, 1985; Foster, 1991a, 1991b).

At depth, rhyolite porphyry forms the matrix for fragments of

the pipe. Creation of the pipe appears to be related to emplacement of an underlying hypabyssal stock related to the sills.

Crosscutting the breccia pipe are hydrothermally altered lamprophyre dikes that postdate the gold-silver ore; locally, these

dikes may have created areas of high-grade ore in the breccia

pipe near their margins. The timing of emplacement of various igneous rocks and the hydrothermal alteration related to

mineralization at the deposit is the topic of this paper.

The Golden Sunlight deposit is in the Whitehall mining

district, Jefferson County, Montana, on the faulted west

limb of a north-plunging syncline of Belt Supergroup sedimentary rocks (fig. 1). The 300- to 700-ft-diameter breccia

pipe cuts stratified rocks and rhyolite sills at moderate to

high angles (fig. 2). Emplacement of the pipe caused fragments of sedimentary rock and rhyolite porphyry to be

downdropped relative to their stratigraphic position outside

the pipe (Foster, 1991a, 1991b). Early mining of high-grade
2 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

gold and silver in the region was concentrated along northeast-striking, high-angle faults and shear zones, some of

which cut the breccia pipe and along which lamprophyre

dikes have been emplaced (fig. 2) (Porter and Ripley,

1985). These structures are thought to be part of a regional,

northeast-striking zone of crustal weakness that has been

intermittently active from the Proterozoic to the present

(Foster and Chadwick, 1990; Foster 1991a).

Because some hydrothermally altered and mineralized

lamprophyre dikes are preferentially emplaced along structures that cross-cut the breccia pipe, their relationship to

mineralization of the breccia pipe has been ambiguous. Certainly their emplacement is later than that of the pipe, and the

simplest interpretation is that lamprophyre emplacement

postdates mineralization. But, because the northeast-striking

shear zones, veins, and dikes contain high-grade ore in

places, a mineralizing process was obviously continuing during emplacement of the lamprophyre bodies. Was that process the same one that created the breccia pipe, or was it a

different one? In this paper, we present petrologic and geochronologic data that bear on the timing and genesis of the

breccia pipe and lamprophyre intrusion. 

Boulder Valley

Pzs Ke

Yb

Ylif

Ylmf

Ylof

QTsv

Ylb

K?b

Kr

Kl

Kt

Ylmf

Ylof

Ylof

Ylof

Ylof

QTsv QTsv

Yls

24

32

29

24

22

23

32

18

53

24

34

28

Kt

Kt

16

30

24

20

50

20

20

13

24

St.

Paul

Gulch

QTsv

112 ̊ 2' 30"

42 ̊ 55' Whitetail Valley

Area of fig. 4

Line of section,

figure 2

Golden

Sunlight

breccia

pipe

Bull Mountain

Kr

Kr

Montana 0 2000 ft

Figure 1 (above and following page). Generalized geologic map of the Golden Sunlight mine area.

Modified slightly from Foster (1991a). Line of section for figure 2 and area of figure 4 shown.
IGNEOUS ROCK SUITES 3

ACKNOWLEDGMENTS

Without the extensive logistical, financial, and technical

support of the staff of Golden Sunlight Mines, Inc., this paper

would not have been possible. Generous access to the open

pit and to core and surface samples was made possible by the

mine staff. Many of our samples were collected with the

assistance of Walter Coppinger of Trinity University, who

also supplied location information for the samples of Swanson (1989). We thank David Allerton for preparing mineral

separates used for isotopic age determinations and R.A. Zimmerman for determining the fission-track dates and aiding in

the interpretation of the data. We thank Ross Yeoman and

L.W. Snee for completing the 40Ar-39Ar analyses of mineral

separates from the mine area. Reviews of this manuscript by

W.C. Day, G.B. Sidder, G.A. Desborough, and Walter Coppinger are gratefully acknowledged.

IGNEOUS ROCK SUITES

Our study concentrates on the petrologic, chemical, and

chronologic history of the igneous rocks in the vicinity of the

Golden Sunlight mine because both rhyolite and lamprophyre are spatially associated with ore in the breccia pipe or

in late veins, respectively. Three chemically distinct igneous

rock units intrude clastic and carbonate-bearing sedimentary

rocks of the Belt Supergroup in the area. The oldest unit is

composed of hypabyssal sills and dikes of rhyolite (unit Kr,

figs. 1 and 2) that cut sedimentary rocks. This rhyolite also

forms the matrix of the Golden Sunlight breccia pipe and is

assumed to compose a hypabyssal stock beneath the deposit.

The rhyolite was investigated in detail to determine the

timing of its emplacement. Also studied was the nature of

hydrothermal alteration related to emplacement of the

rhyolite and breccia pipe.

Younger than the rhyolite are numerous lamprophyre

dikes that cut the breccia pipe (unit Kl, figs. 1 and 2). The

petrology and alteration of these bodies was studied in detail

because the dikes have a distinctive and unusual chemistry

and an equally unusual alteration assemblage dominated by

magnesite. Possibly related to the lamprophyre bodies are

dark-gray, fine-grained olivine basalt and basalt porphyries

(unit K?b, fig. 1) exposed on the southern flank of Bull

Mountain, south of the mine, where the basalt is in contact

with strata of the Belt Supergroup and rhyolite sills. These

bodies are typically vesicular, are not altered, and probably

postdate mineralization, although direct evidence is lacking.

The basalt and basalt porphyries are undated but are presumed to be related to the lamprophyre bodies. Geochemical

characteristics of these mafic rocks are summarized here

because we do not deal with them in depth in this report.

Mineralogically, the basalt contains phenocrysts of olivine

altered to chlorite, very pale green clinopyroxene, and minor

opaque minerals. The groundmass consists of intermediatecomposition plagioclase and clinopyroxene intergrown in a

felted texture. Chemically, the one analyzed rock is an alkalicalcic olivine basalt (classification of De LaRoche and others, 1980) that is very Mg rich. The chondrite-normalized

rare-earth-element pattern (REECN) for the sample is lightrare-earth-element enriched (LREE-enriched) and resembles

those for many of the lamprophyre bodies.

Felsic plutonic rocks contained in irregular, small, plutonic bodies (unit Kt, fig. 1) probably are the youngest in the

area, but age relations to the rhyolite are unclear. Because

rocks of this unit are not believed to be related to mineralization, they were investigated only in a cursory fashion.

Plutonic rocks of the Boulder batholith are exposed to the

west of the mine. The two plutonic bodies south of the mine

(fig. 1) may be related to the Boulder batholith.

Explanation

Quaternary

and

Tertiary

Late


Cretaceous

Paleozoic

Middle


Proterozoic

Sedimentary and volcanic rocks

Basalt

Lamprophyre

Rhyolite

Tonalite

Elkhorn Mountains Volcanics

Sedimentary rocks

Bull Mountain shelf and slope

 deposits (Grayson Shale)

LaHood deep-sea fan deposits

 (LaHood Formation)

Submarine canyon facies

Inner submarine fan facies

Middle submarine fan facies

Outer submarine fan facies

Basin plain facies

Fault, bar and ball on

 downthrown side

Mineralized structure

Syncline, showing 

 direction of plunge

Strike and dip of beds

QTsv

K?b

Kl

Kr

Kt

Ke

Pzs

Yb

Yls

Ylif

Ylmf

Ylof

Ylb

20
4 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE                                   





     

    

         



   5500'

5000'

4500'

Yb

Yl

Yl

Yl

Yb

Yb

Yb Yb

Mineral Hill

Yl

Yl

Ybs Ybss

Roper vein

Ohio vein

Range-front fault

Discovery

fault zone

Sunlight

zone

Bacorn

vein

Reese

vein

system

Unconsolidated depositsFault zoneLamprophyre dikes and sillsMineral Hill breccia pipeRhyolite sills and dikesBull Mountain shelf and slope deposits (Grayson Shale) Black shale Pyritic shaleLaHood deep-sea fan deposits (LaHood Formation) 



Marginal stockwork zonePyrite >15% in matrixBarite, magnesite >10% in matrixVein

EXPLANATION

Breccia pipe alteration

Quaternary

Tertiary

Cretaceous

Middle Proterozoic

0 500 FEET

Kl

Tu

Yb

Kr

Ybss

Ybs

Yl

Figure 2. Geologic cross section of the Golden Sunlight breccia pipe. Location of section on figure 1. Heavy dotted lines in pipe show downdropped limit of distinctive stratigraphic

units Yb, near the top of the pipe, and Ybs, lower in the pipe.
IGNEOUS ROCK SUITES 5

RHYOLITE

Numerous sills, dikes, and irregular bodies of rhyolite

porphyry (unit Kr, figs. 1 and 2) cut strata of the Belt Supergroup in the vicinity of the Golden Sunlight mine and are

mineralized near the breccia pipe. At depth in the pipe,

rhyolite porphyry forms the matrix of the breccia.

PETROLOGY

Most rhyolite is porphyritic, fine grained, and hydrothermally altered. Rhyolite in sills and dikes commonly contains fewer phenocrysts than rhyolite in the breccia pipe.

Groundmass-to-phenocryst volumetric ratios greater than 1

to 1 are common in the sills, but ratios much less than 1 to 1

are common in the breccia pipe. Groundmass is composed of

quartz and minor orthoclase. Phenocrysts are plagioclase,

1×2 mm to 3×8 mm. Oscillatory zoning and albite twinning

are strongly developed in the phenocrysts, which are altered

to a fine-grained mixture of sericite and minor epidote-group

minerals. Initial composition of the phenocrysts is difficult

to determine because of hydrothermal sericite. If the extinction angles between albite twins can be used to approximate

the original composition, the phenocrysts were albite to oligoclase. Mafic minerals in the rhyolite are uncommon, but

minor clots and irregularly shaped masses of biotite associated with sulfide minerals are noted. Whether or not any of

this biotite is of primary igneous origin is uncertain. Accessory phases include apatite and zircon.

The rhyolite is commonly hydrothermally altered, with

intensity increasing toward the mineralized breccia pipe.

Alteration, in the form of potassium metasomatism, produced high K2O concentrations (10.3 weight percent in

sample 88GS-L, table 1) and resulted in formation of sericite

from phenocrysts of plagioclase as well as creation of secondary orthoclase, cryptocrystalline quartz, and some calcite. Minor, primary biotite in least altered rhyolite is

commonly replaced by fine-grained, secondary biotite in

altered rhyolite. Sample 88GS-L (table 1), from the open pit

of the mine, has plagioclase phenocrysts and groundmass

replaced by fine-grained orthoclase and contains as much as

3 percent pyrite and 2 to 5 percent sericite. Sulfide minerals,

principally pyrite and lesser molybdenite, and hematite typically occupy and replace the groundmass between feldspar

grains (fig. 3). In strongly altered rhyolite, very fine grained

pyrite replaces the plagioclase phenocrysts.

CHEMISTRY

Although hydrothermally altered in many places, samples of rhyolite were collected from the mine area in order to

define its petrogenesis (fig. 4). Some previous workers suggested that the rhyolite was a latite (Wertz, 1971; Porter and

Ripley, 1985), but our data do not support such a classification. Latite, as used by most petrologists, implies a relatively

high calcium concentration. Rhyolite from the Golden Sunlight mine contain only traces of CaO. Consequently, Swanson (1989), using the alkali-silica diagram of LeMaitre

(1984) and LeBas and others (1986), referred to the felsic

rocks as rhyolite. If the R1R2 classification of De LaRoche

and others (1980) is used, the least hydrothermally altered

samples of this unit (samples 88GS-7 and 191-2025′; table 1)

are rhyolite (fig. 5A). Least altered samples of rhyolite are

alkali-calcic (fig. 5A), mildly peraluminous (fig. 5B), very

sodic (fig. 5C), and Mg rich (fig. 5D). With increasing potassium metasomatism and introduction of pyrite, hematite, and

molybdenite, the rhyolite is altered to an alkalic quartz

syenite (fig. 5A) that is strongly peraluminous (fig. 5B),

potassic to very potassic (fig. 5C), and Fe rich to very Fe rich

(fig. 5D). Thirty-seven samples of the rhyolite from Wertz

(1971) and three samples of the rhyolite from Swanson

(1989) agree with the changes noted for our four samples.

Because much of the linear trend for our samples of rhyolite

(fig. 5A) is defined by altered rocks, and because there is little chemical variability in the fresh rhyolite, we cannot speculate about lines of descent or fractional crystallization paths

of these felsic rocks.

Figure 3. Photomicrograph in plane-polarized

light of mineralized rhyolite (sample 90C6-2300′)

from breccia pipe at the Golden Sunlight mine. Ab,

albite to oligoclase phenocrysts; Otz, quartz; Mo,

molybdenite. Field of view 4.2×3.0 mm.
6 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Yb

Kr

Ylif

Yls

Ylmf

Kt

K?b

Ylmf

QTsv

Ylof

Ylof

Kr

Ylmf QTsv

QTsv

Kr

Ylif

Ylof

88C2-595.5'

189-642'

115-743'

 1537.5'

 1876'

 1882'

151-417'

191-173.5'

 1034'

 2025'

190-270.5'

 1223.5'

88GS-12

88C1-256'

 283'

 442.3'

88C3-292'

 299'

 301.5'

 353.75'

80P074

80P079

88GS-L 719-28

88GS-1

719-29

88GS-2

88GS-4 88GS-8

718-21

718-15

718-6

88GS-5

88GS-3

88GS-6

719-23

718-16

718-9

718-7

718-13

718-1

718-18

718-2

719-27

719-22

88GS-7

718-19

719-25

80P076

0 2000 ft

breccia

pipe

Rare-earth-element (REE) patterns (fig. 6) are LREEenriched and have chondrite-normalized La/Yb ratios (La/

YbCN) ratios of 10 in fresh to 40 in mildly metasomatized

samples. Heavy rare-earth-element (HREE) parts of the

profiles are notably flat. No europium anomalies are

observed. La/YbCN ratios increase to as much as 50 in one

strongly metasomatized sample. Whether or not this apparent LREE enrichment is due to true enrichment of LREE

during mineralization cannot be proven by our data.

CHRONOLOGY OF RHYOLITE EMPLACEMENT

Rhyolite matrix from drill core (sample 90C6-2300′)

that contains fragments of mineralized sedimentary rocks of

the Belt Supergroup was processed to retrieve zircon for UTh-Pb dating. Morphologically, the zircon varied from

rounded, milky white grains to elongate, clear, euhedral crystals. Although the rounded zircon may be detrital grains contributed by the sedimentary rocks, the euhedral variety is

Figure 4. Sample locality map of the Golden Sunlight mine area. Map area shown

on figure 1. Rock-unit abbreviations as in figure 1. Data for all samples listed are in

table 1. Samples not shown on this figure include St. Paul Gulch shown on figure 1

and JC-1 and JC-2 from Jefferson Canyon, south of figure 1.
IGNEOUS ROCK SUITES 7

Table 1. Major-, minor-, and rare-earth-element geochemistry of igneous rocks in the Golden Sunlight mine area, southwestern Montana.

[Major-element chemistry by X-ray fluorescence by Dave Seims and J.E. Taggart. Minor-element chemistry by induction-coupled plasma spectrometry by D. Fey. Rare-earthelement chemistry by induction-coupled plasma spectrometry by A.L. Meier. FeO, H2O, and CO2 analyses by M.G. Kavulak. Leaders (--) indicate "not analyzed." Suite abbreviations: Bas, basalt; Ton, tonalite; Ton N, tonalite normalized; Lam 1, lamprophyre 1 having CO2<5 percent; Lam 2, lamprophyre 2 having CO2 between 5 and 9 percent;

Lam 3, lamprophyre 3 having CO2 between 10 and 14 percent; Lam 4, lamprophyre 4 having CO2 between 15 and 20 percent; Lam N, lamprophyre normalized; Rhy, rhyolite;

Rhy N, rhyolite normalized; R1, calculated values for R1R2 plot; R2, calculated values for R1R2 plot; A/CNK, calculated values for SiO2 versus A/CNK plot; Fe number,

calculated values for (FeO+0.89Fe2O3)/(FeO+0.89 Fe2O3

+MgO) versus SiO2 plot; K number, calculated values for K2O/(K2O+Na2O) versus SiO2 plot; La/YbCN, chondrite normalized ratio of La/Yb; Eu/Eu*, calculated value of europium anomaly (numbers less than 1 indicate negative anomaly, numbers greater than 1 indicate positive anomaly). From ICP asnalyses, 88C3-301.5′ has minimum of 1.95 weight percent SO3, 191-173.5′ has minimum of 0.49 weight percent SO3, 88GS-8 has minimum of 0.18 weight

percent SO3, 88GS-12 has about 2.02 weight percent SO3, and 88GS-L has about 1.35 weight percent SO3]

Field no. 88GS-1 719-29 88GS-2 88GS-4 88GS-? 718-21 718-15 718-6 88C1-256′ 88GS-5 88C3-292′ 190-270.5′ 88GS-3

Lab no. D322303 Swanson D322304 D322306 Swanson Swanson Swanson D322313 D322307 D322316 D322324 D322305

Suite Bas Bas Ton Ton Ton Ton N Ton N Ton N Lam 1 Lam 1 Lam 1 Lam 1 Lam 1

Major elements (weight percent)

SiO2 48.20 49.65 63.10 66.9063.7059.90 68.24 68.91 46.30 46.90 47.30 47.30 50.70

TiO2 1.00 1.01 0.42 0.34 0.55 0.52 0.35 0.25 0.91 0.95 0.94 0.93 0.60

Al2O3 13.00 13.16 15.50 15.90 14.60 14.92 16.36 14.67 12.10 12.30 12.50 12.30 10.40

Fe2O3 3.54 -- 2.59 1.80 1.04 -- -- -- 5.56 5.09 6.41 6.31 3.44

FetO -- 9.34 -- -- -- 5.70 3.24 2.90 -- -- -- -- --

FeO 5.80 -- 1.80 1.30 4.30 -- -- -- 5.70 6.40 5.30 5.30 5.90

MnO 0.16 0.15 0.10 0.04 0.07 0.13 0.04 0.05 0.20 0.19 0.20 0.20 0.16

MgO 11.90 13.56 2.89 1.52 3.48 5.39 1.52 2.18 8.11 9.14 8.30 8.35 13.40

CaO 8.94 9.01 3.42 1.46 2.42 5.54 1.56 2.82 11.40 11.80 11.20 11.20 8.84

Na2O 2.48 2.03 3.82 4.41 4.63 3.56 4.58 4.10 1.43 1.55 1.96 1.31 2.14

K2O 1.55 1.73 3.62 3.77 1.40 3.53 3.76 3.91 4.02 3.60 3.52 4.12 3.15

P2O5 0.35 0.35 0.43 0.32 0.11 0.80 0.34 0.19 0.73 1.10 0.76 0.75 0.61

H2O+ 2.10 -- 1.00 1.40 2.30 -- -- -- 1.20 0.78 1.20 1.70 0.43

H2O– 1.50 -- 0.96 0.55 0.36 -- -- -- 0.83 0.44 0.55 0.51 0.50

CO2 0.04 -- 0.19 0.02 0.73 -- -- -- 1.80 0.03 0.38 0.50 0.06

Total 100.56 99.99 99.84 99.73 99.69 99.99 99.99 99.98 100.29 100.27 100.52 100.78 100.33

R1 1691 1895 1874 1918 2110 1728 1940 2132 1315 1402 1299 1393 1614

R2 1802 1895 813 544 718 1153 563 698 1859 1957 1855 1854 1814

A/CNK 0.59 0.61 0.95 1.14 1.08 0.76 1.14 0.91 0.44 0.44 0.46 0.46 0.45

Fe No. 0.43 0.41 0.59 0.66 0.60 0.51 0.68 0.57 0.57 0.55 0.57 0.57 0.40

K No. 0.38 0.46 0.49 0.46 0.23 0.50 0.45 0.49 0.74 0.70 0.64 0.76 0.60

La/YbCN 13.48 -- 17.03 34.38 4.82 -- -- -- 13.16 18.73 12.87 15.61 10.11

Eu/Eu* 0.92 -- 1.00 0.95 0.81 -- -- -- 0.94 0.87 0.91 0.96 0.92

Minor elements (ppm)

Ba 550 700 1300 1400 330 1142 1270 934 780 870 790 780 780 

Sr 580 717 1100 860 740 989 888 383 1200 1400 1500 1400 870 

Y 19 21 14 10 18 17 10 9 21 20 21 21 14 

Zr -- 114 -- -- -- 155 175 126 -- -- -- -- --

Nb 7 -- 16 13 4 -- -- -- 7 10 13 11 <4

Sc -- 28 -- -- -- 13 4 4 34 35 34 34 28 

Cu 86 79 32 15 49 58 23 9 180 71 160 180 120 

Pb 10 -- 13 19 27 -- -- -- 8 10 9 9 11 

Zn 74 -- 50 27 66 -- -- -- 94 95 97 98 69 

Ni 376 345 51 23 65 227 36 66 80 110 78 80 390 

Cr 800 904 100 30 130 347 193 226 310 430 310 300 940 

Co 56 79 15 9 18 58 23 9 48 51 49 49 57 

Rare earth elements (ppm)

La 28 -- 48 51 15 -- -- -- 41 50 42 44 18 

Ce 58 -- 82 81 30 -- -- -- 78 98 77 85 33 

Pr 6.8 -- 8.1 7.6 3.4 -- -- -- 8.3 10 8.3 8.8 3.7 

Nd 28 -- 30 27 15 -- -- -- 35 45 35 37 16 

Sm 4.4 -- 4.8 4 3 -- -- -- 5.6 7.4 5.9 6.3 2.9 

Eu 1.2 -- 1.4 0.96 0.8 -- -- -- 1.6 1.9 1.6 1.8 0.88 

Gd 3.3 -- 3.5 1.8 3 -- -- -- 4.5 5.6 4.5 4.9 2.9 

Tb 0.56 -- 0.51 0.4 0.45 -- -- -- 0.75 1 0.79 0.8 0.43 

Dy 3.1 -- 3.1 2.1 3.4 -- -- -- 4.1 4.6 4 4.4 2.5 

Ho 0.57 -- 0.58 0.38 0.64 -- -- -- 0.7 0.85 0.84 0.84 0.53 

Er 1.6 -- 1.7 0.88 2 -- -- -- 2.4 2.2 2.3 2.2 1.5 

Tm 0.23 -- 0.31 0.17 0.34 -- -- -- 0.33 0.33 0.28 0.32 0.2 

Yb 1.4 -- 1.9 1 2.1 -- -- -- 2.1 1.8 2.2 1.9 1.2 
8 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Table 1. Major-, minor-, and rare-earth-element geochemistry of igneous rocks in the Golden Sunlight mine area, southwestern Montana—Continued.

Field no. 88GS-6 88GS-8 189-642′ 88C3-299′ 151-417′ 190-1223.5′ 88C2-595.5′88C3-353.75′ 115-1876′ 88C3-301.5′ 115-1537.5′ 115-1882′ 191-1034′

Lab no. D322308 D322310 D322323 D322317 D322329 D322325 D322315 D322319 D322321 D322318 D322320 D322322 D322327

Suite Lam 1 Lam 2 Lam 2 Lam 2 Lam 2 Lam 2 Lam 2 Lam 2 Lam 2 Lam 3 Lam 3 Lam 3 Lam 3

Major elements (weight percent)

SiO2 55.40 45.70 44.00 44.60 45.00 45.10 45.50 45.90 47.90 42.30 42.80 43.20 46.70

TiO2 0.65 0.78 0.85 0.83 0.68 0.73 0.71 0.56 0.76 0.81 0.83 0.47 2.54

Al2O3 12.00 11.70 11.20 11.40 11.10 9.84 10.70 8.97 13.40 11.20 11.20 7.74 10.60

Fe2O3 3.02 0.30 5.64 3.75 3.60 3.62 3.58 2.53 3.34 4.89 3.95 2.21 2.79

FetO -- -- -- -- -- -- -- -- -- -- -- --

FeO 5.80 15.10 11.30 5.40 4.90 5.20 5.20 5.50 5.20 4.30 5.30 5.10 4.60

MnO 0.15 0.26 0.25 0.14 0.15 0.14 0.15 0.14 0.13 0.18 0.15 0.12 0.12

MgO 7.98 6.29 6.35 9.39 7.84 10.10 11.10 15.70 5.34 8.34 8.55 16.60 9.46

CaO 8.61 9.13 7.77 7.77 8.82 9.96 7.08 6.54 6.65 8.30 8.25 4.95 5.82

Na2O 2.40 1.74 1.73 1.46 1.78 1.10 1.64 1.22 1.64 1.59 1.40 1.69 1.89

K2O 3.22 2.08 3.04 3.57 3.45 3.26 3.12 1.98 3.79 3.20 3.66 2.52 3.57

P2O5 0.53 0.92 0.67 0.94 0.76 0.81 0.59 0.32 0.54 0.98 0.98 0.41 0.55

H2O+ 0.38 2.91 1.10 1.50 1.40 2.00 1.60 3.00 2.10 1.80 1.60 1.50 1.10

H2O– 0.49 0.09 0.86 0.73 1.00 0.77 1.20 1.40 0.42 1.00 0.63 0.95 0.72

CO2 0.02 3.17 5.50 8.40 9.29 7.30 8.20 6.60 6.30 10.40 10.30 12.00 10.70

Total 100.65 100.17 100.26 99.88 99.77 99.93 100.37 100.36 97.51 99.29 99.60 99.46 101.16

R1 1831 1491 1128 1352 1314 1597 1466 1930 1474 1242 1230 1478 1343

R2 1552 1518 1366 1521 1550 1760 1518 1654 1239 1521 1526 1505 1300

A/CNK 0.52 0.54 0.55 0.56 0.49 0.42 0.57 0.56 0.71 0.53 0.53 0.53 0.60

Fe no. 0.52 0.71 0.72 0.48 0.51 0.46 0.43 0.33 0.61 0.51 0.51 0.30 0.43

K no. 0.57 0.54 0.64 0.71 0.66 0.75 0.66 0.62 0.70 0.67 0.72 0.60 0.65

La/YbCN 8.33 -- 13.48 14.33 16.48 13.97 8.43 13.48 8.09 12.42 12.14 9.55 9.15

Eu/Eu* 0.91 -- 0.96 0.88 0.92 0.92 1.06 0.94 1.01 0.98 0.83 1.11 0.87

Minor elements (ppm)

Ba 830 -- 690 930 900 620 840 550 840 140 810 580 820 

Sr 580 -- 1200 820 1300 660 730 500 740 860 2200 830 810 

Y 17 -- 18 18 16 16 15 13 16 16 17 10 12 

Zr -- -- -- -- -- -- -- -- -- -- -- -- --

Nb <4 -- 15 6 7 <4 4 <4 5 8 6 <4 <4

Sc 28 -- 33 27 23 34 24 22 23 25 26 18 20 

Cu 95 -- 160 130 110 190 85 74 95 130 140 83 92 

Pb 12 -- 7 13 9 10 10 13 14 37 10 12 10 

Zn 77 -- 220 74 78 71 72 56 84 91 78 50 61 

Ni 97 -- 140 240 120 240 400 650 56 230 230 710 290 

Cr 340 -- 320 520 330 600 500 530 170 470 440 630 500 

Co 41 -- 80 44 40 43 47 59 33 44 45 57 42 

Rare earth elements (ppm)

La 21 -- 38 34 44 29 25 24 24 35 36 17 19 

Ce 40 -- 72 67 86 59 49 49 47 72 73 31 37 

Pr 4.3 -- 7.6 7.3 8.8 6.5 5.3 5.5 4.9 7.9 8.4 3.4 3.9 

Nd 19 -- 32 35 36 29 24 24 22 36 35 14 18 

Sm 3.3 -- 5.2 5.5 6.2 5 4.1 3.9 3.8 5.8 6.1 2.4 2.8 

Eu 0.99 -- 1.6 1.5 1.7 1.4 1.3 1.1 1.2 1.7 1.5 0.85 0.88 

Gd 3.3 -- 4.8 4.7 4.8 4.1 3.2 3.1 3.3 4.5 4.7 2.2 3.4 

Tb 0.57 -- 0.72 0.69 0.73 0.64 0.6 0.53 0.56 0.73 0.72 0.27 0.43 

Dy 3.4 -- 4 3.5 4.2 3.4 3.5 2.7 3.2 3.6 4.1 2 2.7 

Ho 0.6 -- 0.67 0.69 0.69 0.66 0.71 0.56 0.64 0.8 0.67 0.4 0.5 

Er 1.7 -- 2.1 2.1 2 1.6 2.1 1.4 1.8 1.9 2.4 1.2 1.3 

Tm 0.3 -- 0.27 0.29 0.29 0.25 0.28 0.18 0.25 0.28 0.32 0.13 0.19 

Yb 1.7 -- 1.9 1.6 1.8 1.4 2 1.2 2 1.9 2 1.2 1.4 
IGNEOUS ROCK SUITES 9

Table 1. Major-, minor-, and rare-earth-element geochemistry of igneous rocks in the Golden Sunlight mine area, southwestern Montana—Continued.

Field no. 88C1-283′ 191-173.5′ 719-23 718-16 718-9 718-7 718-13 718-1 718-18 718-2 719-27 719-22 88GS-12

Lab no. D322314 D322326 Swanson Swanson Swanson Swanson Swanson Swanson Swanson Swanson Swanson Swanson D322311

Suite Lam 4 Lam 4 Lam N Lam N Lam N Lam N Lam N Lam N Lam N Lam N Lam N Lam N Rhy

Major elements (weight percent)

SiO2 38.80 39.20 -- 47.37 47.76 47.82 47.84 50.57 51.06 51.46 54.45 55.92 66.30

TiO2 0.35 0.44 -- 0.91 0.95 0.97 0.92 0.90 0.59 0.88 0.84 0.65 0.09

Al2O3 6.34 6.69 -- 12.17 12.16 13.60 12.57 13.34 10.57 13.43 13.38 12.51 16.40

Fe2O3 1.59 1.79 -- -- -- -- -- -- -- -- -- -- 2.99

FetO -- -- -- 11.00 11.00 11.60 11.19 10.48 9.12 10.49 9.14 8.61 --

FeO 5.90 5.60 -- -- -- -- -- -- -- -- -- -- <0.01

MnO 0.13 0.12 -- 0.20 0.20 0.22 0.20 0.20 0.16 0.20 0.16 0.16 <0.02

MgO 15.80 17.70 -- 10.11 9.58 8.46 9.29 7.74 13.86 7.58 7.47 7.32 0.14

CaO 5.06 5.03 -- 11.85 12.44 10.72 11.41 9.73 8.64 9.25 8.92 8.39 0.02

Na2O 0.67 0.79 -- 1.73 1.88 2.29 1.50 2.33 2.18 2.44 1.93 2.51 4.34

K2O 3.06 1.74 -- 3.28 2.80 3.30 3.69 3.95 3.22 3.52 2.64 3.31 6.65

P2O5 0.39 0.25 -- 1.37 1.23 0.96 1.39 0.77 0.60 0.75 1.07 0.62 0.07

H2O+ 1.00 2.00 -- -- -- -- -- -- -- -- -- -- 0.68

H2O– 0.92 0.90 -- -- -- -- -- -- -- -- -- -- 0.46

CO2 20.00 15.50 -- -- -- -- -- -- -- -- -- -- <0.01

Total 100.01 97.75 -- 99.99 100.00 99.94 100.00 100.01 100.00 100.00 100.00 100.00 98.17

R1 1418 1711 -- 1444 1528 1253 1456 1303 1604 1423 2048 1803 1243

R2 1449 1547 -- 2008 2045 1833 1928 1687 1819 1629 1587 1506 331

A/CNK 0.47 0.54 -- 0.44 0.42 0.51 0.46 0.52 0.46 0.55 0.60 0.55 1.14

Fe no. 0.32 0.29 -- 0.52 0.53 0.58 0.55 0.58 0.40 0.58 0.55 0.54 0.95

K no. 0.82 0.69 -- 0.65 0.60 0.59 0.71 0.63 0.60 0.59 0.58 0.57 0.61

La/YbCN 4.90 10.37 -- -- -- -- -- -- -- -- -- -- 20.87

Eu/Eu* 0.95 0.87 -- -- -- -- -- -- -- -- -- -- 0.93

Minor elements (ppm)

Ba 970 990 772 832 724 800 865 977 705 732 731 791 1200 

Sr 510 500 505 1333 1166 1432 1282 1278 764 1016 809 521 700 

Y 8 10 18 24 23 24 22 22 15 22 25 19 4 

Zr -- -- 112 132 123 115 130 112 70 110 146 106 --

Nb <4 <4 -- -- -- -- -- -- -- -- -- -- 13 

Sc 17 18 26 33 38 30 32 28 26 28 29 25 --

Cu 71 110 87 175 97 152 51 74 119 125 128 124 49 

Pb 9 140 -- -- -- -- -- -- -- -- -- -- 8 

Zn 46 240 -- -- -- -- -- -- -- -- -- -- 64 

Ni 590 810 192 255 145 107 192 84 397 108 106 109 7 

Cr 680 1000 345 197 400 393 397 282 906 284 290 375 2 

Co 62 66 19 27 31 30 32 26 31 26 16 36 5 

Rare earth elements (ppm)

La 8 20 -- -- -- -- -- -- -- -- -- -- 13 

Ce 17 41 -- -- -- -- -- -- -- -- -- -- 17 

Pr 1.7 4.5 -- -- -- -- -- -- -- -- -- -- 1.6 

Nd 8.5 20 -- -- -- -- -- -- -- -- -- -- 5.7 

Sm 1.4 3.4 -- -- -- -- -- -- -- -- -- -- 0.75 

Eu 0.44 0.88 -- -- -- -- -- -- -- -- -- -- 0.25 

Gd 1.4 2.6 -- -- -- -- -- -- -- -- -- -- 0.9 

Tb 0.27 0.39 -- -- -- -- -- -- -- -- -- -- 0.17 

Dy 1.5 2.1 -- -- -- -- -- -- -- -- -- -- 0.72 

Ho 0.26 0.44 -- -- -- -- -- -- -- -- -- -- 0.12 

Er 0.91 1.1 -- -- -- -- -- -- -- -- -- -- 0.46 

Tm 0.11 0.14 -- -- -- -- -- -- -- -- -- -- 0.05 

Yb 1.1 1.3 -- -- -- -- -- -- -- -- -- -- 0.42 
10 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

interpreted to have crystallized at the time of rhyolite

emplacement and formation of the breccia pipe. A concentrate of the clearest, most elongate zircon crystals—having

length-to-width ratios of greater than 6 to 1—was handpicked for analysis. Unfortunately, even this zircon contains

a large component of Precambrian radiogenic lead (table 2),

probably present in the cores of old zircon upon which the

younger crystals nucleated. Because of this inherited lead

component, a precise age for the emplacement of rhyolite

cannot be determined from this single analysis. However, the

data can be used to approximately date the Precambrian component, which was likely derived from the basement source

rock of the rhyolite magma. Assuming the rhyolite is approximately 80 Ma and making a linear extrapolation on a

concordia diagram (fig. 7) from that age through the analytical point, an upper intercept age of ~2,600 Ma is determined.

Such an age agrees well with other evidence for a Late

Archean crystalline basement in southwestern Montana.

Zircon also was separated from three samples of rhyolite collected in the mine area and dated by fission-track

methods (table 3) in an attempt to date the emplacement of

the sills. Fission-track methods were employed because feldspar and biotite from the sills are too altered for 40Ar-39Ar

techniques. Two outcrop samples (80P076 and 80P079) and

one drill-core sample (80P074) were selected . A zircon fission-track date of 53.5±10.9 Ma was determined for drillcore sample 80P074, and dates of 53.1±7.2 Ma for outcrop

sample 80P076 and 68.9±8.0 for sample 80P079 were determined. The fission-track date of a zircon is reset by temperatures in the range 170°–240°C over geological time scales.

These dates obviously do not reflect the time of emplacement

of the sills because the sills are older than the 77-Ma lamprophyre bodies that crosscut them.

The zircon fission-track dates may, however, record

either localized heating and post-lamprophyre emplacement

cooling or regional cooling during Laramide uplift and denudation. The oldest zircon fission-track date (68.9±8.0

Ma) comes from the topographically highest sample, which

is consistent with simple uplift and cooling of the area starting in Late Cretaceous time. The topographically lowest

samples, less than a kilometer lower, have much younger

dates (53±7–10 Ma). If interpreted as evidence of simple

uplift and cooling, these data would suggest an uplift rate of

about 0.04 m per 1,000 years. This rate is very slow compared to uplift rates of 0.3–3 m per 1,000 years for Laramide uplifts elsewhere in the northern Rocky Mountains

(Perry and others, 1992). Clearly, more fission-track samples are needed, at varying elevations, to fully understand

our reconnaissance data.

BRECCIA PIPE AND ADJACENT STRATA

Within the pipe and the adjacent country rock, gold is

concentrated as electrum in pyrite and within the telluride

minerals calaverite, petzite, sylvanite, and krennerite (Porter

and Ripley, 1985) in both veins and within zones of

Table 1. Major-, minor-, and rare-earth-element geochemistry of igneous rocks in the Golden Sunlight mine area,

southwestern Montana—Continued.

Field no. 191-2025′ 88GS-L 88GS-7 719-28 718-19 719-25

Lab no. D322328 D322312 D322309 Swanson Swanson Swanson

Suite Rhy Rhy Rhy Rhy N Rhy N Rhy N

Major elements (weight percent)

SiO2 66.60 66.70 69.70 69.84 71.86 72.10

TiO2 0.12 0.10 0.12 0.10 0.08 0.13

Al2O3 18.20 16.00 16.20 16.64 16.21 16.48

Fe2O3 2.26 1.99 1.30 -- -- --

FetO -- -- -- 1.13 1.14 1.23

FeO <0.01 <0.01 <0.01 -- -- --

MnO <0.02 <0.02 0.06 -- 0.04 0.01

MgO 0.77 0.15 0.16 0.16 0.08 0.19

CaO 0.10 0.01 0.45 0.01 0.19 0.11

Na2O 3.08 2.09 5.36 3.47 6.33 4.81

K2O 5.15 10.30 4.68 8.55 3.99 4.85

P2O5 0.05 0.02 0.02 0.10 0.08 0.08

H2O+ 1.70 0.58 0.76 -- -- --

H2O– 0.84 0.60 0.58 -- -- --

CO2 0.18 <0.01 0.27 -- -- --

Total 99.11 98.57 99.67 100.00 100.00 99.99

R1 2076 1240 1608 1387 1571 1922

R2 406 322 374 335 342 345

A/CNK 1.68 1.10 1.10 1.11 1.08 1.24

Fe no. 0.73 0.92 0.88 0.88 0.93 0.87

K no. 0.63 0.83 0.47 0.71 0.39 0.50

La/YbCN 14.33 51.74 10.79 -- -- --

Eu/Eu* 1.08 0.62 1.26 -- -- --

Minor elements (ppm)

Ba 240 320 1700 1691 1471 762 

Sr 550 320 590 387 1496 273 

Y 3 <2 6 4 5 5 

Zr -- -- -- 76 118 95 

Nb 7 13 13 -- -- --

Sc -- -- -- 1 -- --

Cu 13 9 1 57 11 10 

Pb 5 4 52 -- -- --

Zn 14 <2 31 -- -- --

Ni 13 4 3 37 20 30 

Cr 6 2 5 195 238 216 

Co 13 3 3 57 11 10 

Rare earth elements (ppm)

La 34 33 16 -- -- --

Ce 48 38 29 -- -- --

Pr 4.1 2.6 2.8 -- -- --

Nd 14 8.5 11 -- -- --

Sm 1.4 0.75 1.7 -- -- --

Eu 0.5 0.14 0.6 -- -- --

Gd 1.4 0.6 1.1 -- -- --

Tb 0.18 0.1 0.2 -- -- --

Dy 1.5 0.42 0.94 -- -- --

Ho 0.35 0.1 0.3 -- -- --

Er 1.1 0.28 0.93 -- -- --

Tm 0.22 0.05 0.07 -- -- --

Yb 1.6 0.43 1 -- -- --
IGNEOUS ROCK SUITES 11 D

C

80

70

60

50

0.5 0.6 0.7 0.8 0.9 1.0

SiO (wt. percent) 2

2 3 2 3 (FeO + 0.89Fe O )/(FeO + 0.89Fe O + MgO)

Mg rich

Very Mg rich

Very Fe rich Average Fe rich

D

B

80

70

60

50

0 0.2 0.4 0.6

SiO (wt. percent) 2

K O/(K O + Na O) 2 2 2

Very sodic

Very potassic Average Potassic

Sodic C

0.90.80.7

1

1.31.21.1

50 60 70

A/CNK

SiO (wt. percent)

Strongly peraluminous

Mildly peraluminous

2

Field of unaltered

igneous rocks Metaluminous

B

A

3000 2500 2000 1500 1000

500

15001000

R = 6000Ca + 2000Mg + 1000Al 2

R = 4000Si - 11,000(Na + K) - 2000(Fe + Ti) 1

mz di

Calcic Calc-alkalic

Alkali-calcic Alkalic

di

ton

gd

rhy

alk rhy

qtz trach

ALTERED

FRESH

ALTERED

FRESH

ALTERED

FRESH

FRESH

ALTERED

EXPLANATION

RhyoliteRhyolite normalizedTonaliteTonalite normalized

Figure 5. Major-element geochemistry plots of rhyolite and tonalite from the Golden Sunlight mine area. FRESH, rhyolite that is as fresh as we could collect in the vicinity of the mine;

ALTERED, hydrothermally altered rhyolite in the vicinity of the mine; normalized, analyses from Swanson (1989), normalized on a volatile-free basis. A, R1 versus R2 plot (De LaRoche

and others, 1980); di, diorite; mz di, monzodiorite; ton, tonalite; gd, granodiorite; rhy, rhyolite (plutonic equivalent is granite); alk rhy, alkali rhyolite (plutonic equivalent is alkali granite);qtz trach, quartz trachyte (plutonic equivalent is quartz syenite). Field of alkalinity modified slightly from DeWitt (1989). B, SiO2 versus A/CNK plot; A/CNK, molar alumina/sum of

molar calcium, sodium, and potassium; field of unaltered igneous rocks from DeWitt (unpub. data, 1994). C, K2O/(K2O+Na2O) versus SiO2 plot; x-axis is “K number” listed in table 1;

field boundaries from DeWitt (unpub. data, 1995). D, (FeO+0.89Fe2O3)/(FeO+0.89 Fe2O3+MgO) versus SiO2 plot; x-axis is “Fe number” listed in table 1; field boundaries modified

from DeWitt (1989).
12 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

1

10

100

1000

CHONDRITE-NORMALIZED RATIO

RARE EARTH ELEMENT

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

Rhyolite

Tonalite

EXPLANATION

hydrothermally altered and silicified rock. Early quartzpyrite-hematite veins (stage 1a of Porter and Ripley, 1985)

are superseded by a period of copper-bismuth veins, some of

which contain electrum (stage 1b). Lead-zinc veins (stage 2),

tellurium-rich veins (stage 3), and late, barren barite-dolomite-magnesite veins (stage 4) complete the mineralizing

process. Within the pipe, quartz, sericite, barite, and potassium feldspar are the common gangue. Magnesite is locally

abundant near late mafic dikes (Alexander, 1955; Lindquist,

1966; Robinson, 1963; Schmidt and others, 1988, 1989; Tilling and others, 1968). Fragments of sedimentary rocks are

typically bleached, silicified, and replaced by pyrite on their

margins or along bedding planes. Secondary sericite is a

common gangue in both the rhyolite and wallrocks. Much of

the pyrite, hematite, chalcopyrite, and sphalerite is concentrated in the matrix, between fragments.

Deep in the pipe, molybdenite is volumetrically significant. Pyrite, hematite, and molybdenite in the deep parts of

the pipe are concentrated in the groundmass of the rhyolite

porphyry, between phenocrysts of oligoclase and albite.

These early sulfide minerals also are cut by veinlets of quartzpyrite-molybdenite-barite, and the groundmass and phenocrysts are silicified and replaced by lesser amounts of pyrite.

The pipe is probably positioned above a hypabyssal stock

related to an alkalic porphyry molybdenum system. Knowledge of the timing of emplacement of the breccia pipe may

aid in refining exploration models for this type of deposit.

Hydrothermal minerals related to gold deposition

proved difficult to date at the Golden Sunlight mine. Sericite

from altered rhyolite was dated by the 40Ar-39Ar technique,

but the results, reported at the end of the following section,

are ambiguous. Therefore, we analyzed the lead isotopic

composition of both unaltered and altered strata of the Belt

Supergroup and altered and mineralized rhyolite in the pipe

in order to limit the age of alteration and mineralization. We

report those results in the following section.

CHRONOLOGY OF ALTERATION

AND MINERALIZATION

An investigation of the lead isotope systematics of

altered and unaltered sedimentary and igneous rock from the

vicinity of the Golden Sunlight mine was undertaken in order

to potentially determine: (1) the extent and nature of hydrothermal alteration, (2) the source of the lead in the rocks and

ores, and (3) the approximate age of mineralization. Ideally,

strata of the Belt Supergroup that shared a common initial

lead isotopic composition and existed undisturbed as closed

systems to U, Th, and Pb would have isotopic ratios that plot

along ~1,400-Ma isochrons on 238U/204Pb versus 206Pb/

204Pb and 232Th/204Pb versus 208Pb/204Pb diagrams. On the

other hand, complete isotopic homogenization of preexisting

lead plus any new lead introduced at the time of hydrothermal activity would serve to reset the chronometers and, if

subsequently remaining closed systems, provide isochron

ages that reflect the mineralization event. Incomplete retention of U, Th, or Pb, or inhomogeneity in initial lead isotopic

composition might prevent calculation of precise ages but

would still give some useful information about the general

timing and conditions of mineralization.

Seven whole-rock and one pyrite sample defined as

altered and three whole-rock samples defined as unaltered

(G.A. Desborough, written commun., 1993) were analyzed

for their U, Th, and Pb concentrations and Pb isotopic composition (table 4). Of the altered whole-rock samples, five

(89C16-586.5′, 169-489′, 115-743′, 6100′ level, and 88C1-

442.3′) are sericitized and (or) potassium-metasomatized

equivalents of the LaHood Formation and contain cross-cutting quartz-pyrite veinlets and (or) disseminated pyrite. The

other two altered whole-rock samples are rhyolite—one

slightly altered with cross-cutting quartz-pyrite veinlets

(90C6-2300′) and the other extensively sericitized and potassium metasomatized and containing abundant disseminated

pyrite (G-010334). Pure pyrite separated from a vein in the

slightly altered rhyolite (90C6-2300′PY) was also analyzed.

The three unaltered whole-rock samples are black shale

(89C10-585.9′ and St. Paul Gulch) and rhythmically banded

0 2 4 6 8 10 12 14

0.00

0.08

0.16

0.24

0.32

0.40

0.48

0.56

2400

2000

1600

1200

Rhyolite 90C6-2300'

Intercepts at 2,600 and 80 Ma

Pb/ U 207 235

Pb/ U 206 238

Figure 6. Chondrite-normalized rare-earth-element plot for

rhyolite and tonalite in the Golden Sunlight mine area.

Figure 7. Concordia diagram for zircon from rhyolite (sample

90C6-2300′), Golden Sunlight mine.
IGNEOUS ROCK SUITES 13

siltite(?) (89C16-860.2′) of the LaHood Formation, all of

which are devoid of secondary silicate minerals or hydrothermally introduced pyrite. These data together with two

previously published analyses of presumably unaltered

LaHood Formation from ~10 km southeast of the Golden

Sunlight mine (Zartman, 1992) are presented in table 4.

The unaltered LaHood Formation samples can be compared to 1,400-Ma reference isochrons on 238U/204Pb versus

206Pb/204Pb, 235U/204Pb versus 207Pb/204Pb, and 232Th/

204Pb versus 208Pb/204Pb diagrams constructed through initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of

16.20, 15.40 and 35.90, respectively (figs 8A, 8B, and 8C).

These initial isotopic ratios were chosen as approximating

the composition of much of the Belt Basin at the beginning

of Belt Supergroup deposition (Zartman and Stacey, 1971).

A close fit of the data to such isochrons might be taken as

evidence for a relatively undisturbed condition of the unaltered rocks since their deposition. However, Zartman (1992)

demonstrated that samples 12 and 13 from the Helena

embayment of the Belt Basin do deviate appreciably from

the above initial ratios, which he found to mainly characterize the more open Belt Basin to the west. Indeed, the scatter

from the isochrons shown by the uncorrected data for these

two samples—particularly manifest in the 235U/204Pb versus

207Pb/204Pb system—largely can be explained by the peculiar initial isotopic composition of the LaHood Formation.

Accordingly, substantial corrections to at least samples 12

and 13 are needed in order to compare their positions to the

1,400-Ma isochrons (figs. 8B, 8C). Whether such corrections should also be applied to the other samples is not

known, but initial lead systematics similar to those identified

by Zartman (1992) would not improve the linearity of the

unaltered array. Sample 11, the only other sample collected

at a distance greater than 1 km from the Golden Sunlight

mine, already plots significantly below the reference isochrons on the 238U/204Pb versus 206Pb/204Pb and 235U/204Pb

versus 207Pb/204Pb diagrams (figs. 8B, 8C), so any downward correction of the point would further displace it from

the reference isochron. Of course, sample 11 was itself collected from a small, abandoned mine adit and may have been

Table 2. U-Th-Pb analytical data for zircon from rhyolite (sample 90C6-2300′), Golden Sunlight mine, southwestern

Montana.

[Uncertainty in analytical data is calculated in the manner suggested by Ludwig (1991a); uncertainty in date (±2 sigma) is an estimated analytical uncertainty calculated according to Ludwig (1991b). Analyses by R.E. Zartman and Loretta Kwak]

Sample U Th Pb Atomic composition of lead* 206Pb/238U 207Pb/235U 207Pb/206Pb 208Pb/232Th

(mesh size) (ppm) (ppm) (ppm) 204Pb 206Pb 207Pb 208Pb [Date (Ma)] [Date (Ma)] [Date (Ma)] [Date (Ma)]

90C6-2300'

(–200+400) 524.8 382.4 57.96 0.0231 78.99 13.07 7.915 0.10098 2.2522 0.16176 0.01215

[620±2] [1198±4] [2474±2] [244±2]

*Laboratory blank of 30 picograms lead with isotopic composition 206Pb/204Pb = 18.7, 207Pb/204Pb = 15.6, 208Pb/204Pb = 38.2 removed. No common lead correction has been applied to these ratios. Common lead correction used for zircon age calculations: 206Pb/

204Pb = 14.31, 207Pb/204Pb = 14.94, 208Pb/204Pb = 33.99.

Table 3. Fission-track analytical data for zircon from rhyolite samples, Golden Sunlight mine area,

southwestern Montana.

[Samples prepared for dating following the procedures of Naeser (1978). Irradiation performed in the U.S. Geological Survey

TRIGA reactor with NBS-SRM 612 glass as a monitor. Ages calculated by the "zeta calibration" procedure of Hurford and Green

(1983) using an experimentally determined zeta factor for zircon of 331.5±7.6 (1-sigma standard deviation). The number of tracks

counted appears in parentheses under the corresponding track density]

Fossil Induced Monitor 95 percent

Sample no. track density track density track density Grains Date confidence

(# counted) (# counted) (106 tr/cm) counted (Ma) level (±)

(# counted)

80PO74 . . . . . . . . . 5.80 . . . . . . . . . .2.75 . . . . . . . . . 0.173 . . . . . . . . . 7 . . . . . . . . . . 53.5 . . . . . . . . . 10.9 *

(850) (403) (1948)

80PO76 . . . . . . . . . 4.22 . . . . . . . . . .2.27 . . . . . . . . . 0.173 . . . . . . . . . 6 . . . . . . . . . . 53.1 . . . . . . . . . . 7.2

(732) (394) (1948)

80PO79 . . . . . . . . . 4.47 . . . . . . . . . .1.85 . . . . . . . . . 0.173 . . . . . . . . . 9 . . . . . . . . . . 68.9 . . . . . . . . . . 8.0

(1283) (531) (1948)

* In this determination, grain-to-grain variance is greater than can be accounted for by Poisson variability.

Date calculated from weighted mean densities (fossil and induced) and the "±" from observed scatter.
14 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

disturbed by hydrothermal alteration related or unrelated to

mineralization at Golden Sunlight mine. Based on this limited data set, we conclude that quite likely all rocks within

and immediately adjacent to the Golden Sunlight mine

underwent at least partial homogenization of their lead isotopes. Whether such effects extended as far as 1 km from the

mine area is uncertain, but our limited evidence would suggest that, beyond that distance, lead isotopes were not disturbed by the mineralization event.

Calculation of best-fit lines for the eight altered samples

on 232Th/204Pb versus 208Pb/204Pb and 238U/204Pb versus

206Pb/204Pb diagrams (figs. 8A and 8C) produces isochrons

of much shallower slope than the 1,400-Ma reference

isochrons. Particularly noteworthy is the model 3 isochron

age of 84±18 Ma obtained for the 238U/204Pb versus 206Pb/

204Pb plot (fig. 8C) using the ISOPLOT program of Ludwig

(1991b). Model 3 ages assign scatter in excess of analytical

uncertainty to variation in initial lead isotopic composition,

an assumption that would be appropriate if complete homogenization had not taken place at the time of mineralization.

The good agreement between this isochron age and other

attempts to date the time of rhyolite intrusion and associated

mineralization suggests to us that the altered samples do

indeed reflect a largely reset chronometer. Considerably

greater scatter of points from a linear array for the 232Th/

204Pb versus 208Pb/204Pb plot (fig. 8A) results in a more

poorly defined model 3 isochron age of 53± 83 Ma.

Table 4. Lead isotopic composition of rocks in the Golden Sunlight mine area, southwestern Montana.

[First part of hyphenated sample number refers to drill-hole number; second part refers to depth from which

sample was taken. St. Paul Gulch, JC-1, and JC-2 are surface samples. Analyses by R.E. Zartman and Loretta

Kwak]

Sample U Th Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

number (ppm) (ppm) (ppm)

89C10-585.9′ 330 17.852 15.608 37.418

89C16-860.2′ 3.42 10.45 13.70 17.711 15.457 37.674

89C12-586.5′ 5.72 8.63 8.06 18.377 15.541 37.581

169-489′ 5.08 7.15 28.65 17.848 15.542 37.763

115-743′ 3.88 9.73 4.66 18.450 15.531 38.330

6100′ level 6.89 21.77 220.20 17.770 15.514 37.751

88C1-442.3′ 3.37 7.04 13.49 18.051 15.547 37.874

90C6-2300′ 1.31 0.58 3.06 18.161 15.538 38.045

90C6-2300′PY 0.46 0.02 76.66 17.786 15.494 37.833

G-010334 1.34 1.40 8.39 18.009 15.561 37.910

St. Paul Gulch 5.86 10.05 14.64 20.143 15.778 38.900

JC-1 0.99 3.38 18.30 17.878 15.878 37.063

JC-2 0.94 2.99 5.44 18.989 15.867 37.925

Sample descriptions

89C10-585.9′ LaHood Formation, unaltered(?) black shale; Pb determined by energy-dispersive X-ray.

89C16-860.2′ LaHood Formation, unaltered rhythmically banded siltite.

89C12-586.5′ LaHood Formation, altered rhythmically banded siltite containing cross-cutting quartz-pyrite veinlets.

169-489′ LaHood Formation, laminated, K-rich, pyritic rock with fine-grained orthoclase as dominant mineral.

115-743′ LaHood Formation, altered with pyrite, sericite, quartz, orthoclase, and albite.

6100′ level LaHood Formation, open pit of mine, bedded authigenic feldspar.

88C1-442.3′ LaHood Formation, altered with abundant pyrite and mineralogy similar to

115-743′.

90C6-2300′ Slightly altered rhyolite containing cross-cutting quartz-pyrite veinlets.

90C6-2300′PY Pyrite from quartz-pyrite veinlets cutting rhyolite of sample 90C6-2300′.

G-010334 Altered rhyolite containing abundant pyrite, quartz, orthoclase, albite, and

sericite.

St. Paul Gulch LaHood Formation, black shale from abandoned mine adit.

JC-1 LaHood Formation, conglomeratic arkose, Jefferson Canyon, 10 km southeast of Golden Sunlight mine.

JC-2 LaHood Formation, arkose, Jefferson Canyon, 10 km southeast of Golden 
IGNEOUS ROCK SUITES 15

In considering the source of the lead associated with the

gold mineralization, it is useful to compare the isotopic composition of average unaltered and average altered LaHood

Formation lead at the time of mineralization. A match

between the lead in both the unaltered and altered rock would

support a hypothesis whereby most of this metal in the

hydrothermally mineralized mine area was scavenged from

the immediately surrounding country rock. Different isotopic compositions, on the other hand, would demand that

either (1) the lead, if scavenged, does not represent an isotopically unbiased sampling of country rock, or (2) another

source of lead, such as lower crustal rock that gave rise to the

magma for the rhyolite, was at least partially involved in the

mineralization process. Unweighted average lead isotope

ratios (and 2σ uncertainties), calculated for the five unaltered

samples and corrected to 80 Ma, yield 206Pb/204Pb =

18.38±1.86, 207Pb/204Pb = 15.70±0.32, 208Pb/204Pb =

37.8±1.24, but the presumably more appropriate weighted

average ratios—dominated by sample 89C10-585.9′—yield

206Pb/204Pb = 17.89±0.47, 207Pb/204Pb = 15.61±0.08, 208Pb/

204Pb = 37.43±0.31. The isochrons fit through the eight

altered samples (figures 1and 2 and an equivalent 206Pb/

204Pb vs. 235U/204Pb diagram) give initial lead-ratio intercepts of 206Pb/204Pb = 17.79±0.06, 207Pb/204Pb =

15.53±0.02, and 206Pb/204Pb = 37.79±0.19. Although the

two sets of ratios are in modest agreement and, therefore,

appear to permit the country-rock-source hypothesis, the

present data are insufficient to accurately constrain the isotopic composition of the LaHood Formation as it may have

existed at the time of rhyolite intrusion and mineralization.

Especially sensitive in making an isotopic match is

207Pb/204Pb, which has a distinctly lower value for altered

samples than unaltered samples. It can be argued, of course,

that this mismatch is a reflection of biased sampling that has

omitted low-U/Pb unaltered rocks. If the LaHood Formation

and other Beltian rocks in the vicinity of the Golden Sunlight

mine behaved essentially as closed systems with a uniform

initial isotopic composition prior to Late Cretaceous alteration, an isochronous relationship would have existed

between 206Pb/204Pb and 207Pb/204Pb. Assuming the initial

ratios at the time of LaHood deposition to be those determined by Zartman and Stacey (1971), permissible “closeapproach” values of 206Pb/204Pb = 17.75 and 207Pb/204Pb =

15.54 do compare very well with the initial lead calculated

for the altered samples.

An attempt was made to date fine-grained, hydrothermal sericite associated with pyrite and gold in the Golden

Sunlight orebody by the 40Ar-39Ar technique. Sericite was

separated from an altered and mineralized rhyolite sill (sample 88GS-11). Because the separation process involved partial digestion in HCl and HNO3, the analyzed mineral (table

5) may have been slightly chemically altered . The resulting

spectra (fig. 9) is disturbed and does not record a simple igneous or cooling history. The largest two gas fractions, with

dates of 93 and 95 Ma, account for 66 percent of the gas from

the sample, but the spectra declines in age up to the highest

temperature gas fractions. We are hesitant to place much

emphasis on the dates from 93 to 95 Ma because of the shape

of the entire spectra.

1

2 3

4

5

6 7

8

9

10

11

12

13

1,400-Ma reference isochron

A

Model 3 isochron age = 52±83 Ma

36

37

38

39

40

0 40 80 120 160

Th/ Pb 232 204

Pb/ Pb 204 208

1

2

3

4

5 6

7 8

9

11

12 13

10

Model 3 isochron age = 44±137 Ma

1,400-Ma reference isochron B

15.2

15.4

15.6

15.8

16

0 0.08 0.16 0.24 0.32 0.4

204

Pb/ Pb 204

U/ Pb 235

207

1

2

3

4

5

6

7 8

9

10

11

12

13

Model 3 isochron age = 84±18 Ma

1,400-Ma reference isochron C

17

18

19

20

21

0 10 20 30 40 50 60

204

Pb/ Pb 204

U/ Pb 238

206

Figure 8. Lead evolution diagrams for samples from the Golden

Sunlight mine area. Filled circles represent altered samples; open

circles represent unaltered samples; 1,400-Ma reference isochron

shown by thick, steeply inclined line in all diagrams. A, 232Th/

204Pb versus 208Pb/204Pb plot. Model 3 isochron constructed for

eight altered samples. B, 235U/204Pb versus 207Pb/204Pb plot.

Model 3 isochron constructed for eight altered samples; arrows indicate direction that samples 12 and 13 should move to be corrected

for lack of equilibration at 1,400 Ma. C, 238U/204Pb versus 206Pb/

204Pb plot. Model 3 isochron constructed for eight altered samples;

arrows indicate direction that samples 12 and 13 should move to be

corrected for lack of equilibration at 1,400 Ma.
16 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

INTERMEDIATE TO FELSIC

PLUTONIC ROCKS

Small stocks and plugs cut the Belt Supergroup rocks in

the mine area. Their contacts with rhyolite sills and lamprophyre are poorly exposed, but the plutonic rocks appear to cut

all other rock units (figs. 1 and 4). In all likelihood, these

intrusive rocks are related to emplacement of either the

Boulder batholith or to intrusive activity associated with the

Elkhorn Mountains Volcanic field.

PETROLOGY AND CHEMISTRY

These rocks do not appear to be genetically related to

the rhyolite sills, as evidenced by their major- and minor-element geochemistry. The intrusive rocks have seriate, hypidiomorphic texture and contain moderate amounts of

granophyre-myrmekite. Primary minerals, in decreasing percentages, are plagioclase, alkali feldspar, quartz, hornblende,

biotite, and traces of clinopyroxene.

Chemically, these plutonic rocks range from monzodiorite to granite, but average tonalite in composition (fig.

5A). All unaltered rocks are alkali-calcic (fig. 5A), metaluminous to mildly peraluminous (fig. 5B), average to sodic (fig.

5C), and Mg rich to very Mg rich (fig. 5D). Where altered,

the tonalitic rocks are strongly peraluminous (fig. 5B).

The suite of rocks have REE patterns (fig. 6) that are

LREE enriched and have La/YbCN ratios of 17–34. HREE

parts of the profiles are slightly concave upward. No

europium anomalies are noted. The patterns plot generally

above those for rhyolite sills in the area.

LAMPROPHYRES

Numerous lamprophyre sills and dikes cut the gold-mineralized breccia pipe at the Golden Sunlight mine and Middle

Proterozoic sedimentary rocks in the surrounding area. Sills

are more numerous than dikes. Lamprophyre bodies range

from several centimeters to several meters thick but normally

do not exceed 3 m. Many sills and dikes are not shown on figure 1 due to their small size. Contact-metamorphic effects

from the lamprophyres are generally minor to nonexistent.

The lamprophyre bodies are described in detail in this report

because of their unusual chemistry, alteration, and possible

tie to late stages of mineralization.

PETROLOGY OF CO2-POOR LAMPROPHYRE

Lamprophyre dikes having low concentrations of CO2

are dark gray where fresh and distinctly porphyritic. Large

phenocrysts, averaging 4–10 mm across, of olivine and clinopyroxene are set in a fine-grained groundmass that also

contains smaller phenocrysts of olivine and clinopyroxene

(fig. 10). In decreasing amounts, the groundmass (fig. 11) is

composed of clinopyroxene, sanidine, albite, biotite, opaque

minerals, and apatite (fig. 11). Swanson (1989) noted that the

lamprophyres average 72 percent fine- and intermediategrained groundmass and 28 percent coarse-grained phenocrysts (average of 10 samples).

Pale-green, euhedral, strongly zoned diopsidic augite is

the main coarse-grained phenocryst (fig. 12). Such large phenocrysts are as much as 1 cm or more in maximum dimension, but phenocrysts as small as 4 mm are present. Many

Table 5. 40Ar-39Ar data for sericite from rhyolite (sample 88GS-11), Golden Sunlight mine

area, southwestern Montana.

[

40Arc, radiogenic 40Ar corrected for mass discrimination and induced 40Ar; 39Arc, 39Ar corrected for mass discrimination and radioactive decay; %40Arrad, percentage of radiogenic 40Ar. Sample weight = 75 mg; J = 0.007283±0.25

percent. 40Ar-39Ar analyses by Ross Yeoman and L.W. Snee]

Temperature Date 2 sigma 

(°C) 40Arc 39Arc F % 40Arrad % 39Artotal (Ma) uncertainty

(Ma)

400 14.59479 2.25239 6.480 92.7 24.7 83.19 0.35

500 21.62990 2.89221 7.479 96.9 31.7 95.68 0.40

600 23.34362 3.18826 7.322 93.2 34.9 93.72 0.39

650 5.13684 0.70804 7.255 83.2 7.8 92.89 0.39

700 0.23069 0.03309 6.972 45.5 0.4 89.36 0.69

750 0.22523 0.04056 5.554 41.1 0.4 71.53 0.39

800 0.00764 0.00309 2.475 3.8 0.01 32.23 9.33

900 0.02506 0.00195 12.821 11.4 0.01 161.04 16.14

1000 0.01111 0.00158 7.032 8.3 0.01 90.10 10.30

1375 0.07121 0.00386 18.463 35.0 0.01 227.59 6.83

Total gas 7.1054 91.62 0.65

No plateau
IGNEOUS ROCK SUITES 17

phenocrysts are strongly resorbed and embayed by groundmass minerals. In some grains, inclusions of carbonate(?)

minerals and feldspar are aligned along zone boundaries that

are parallel to crystal outlines (fig. 13). Diopsidic augite constitutes 20–25 percent of the coarse-grained phenocrysts and

15–20 percent of the smaller phenocrysts in the groundmass

(fig. 14). Augite contains minor amounts of Cr (about 0.5

weight percent in sample 191-939).

Very pale green to colorless, euhedral to subhedral and

broken, primarily unzoned forsteritic olivine (about Fo85–90)

is the second most abundant phenocryst. Most lamprophyres

average 7 percent coarse-grained olivine. Some varieties

contain as much as 5 percent finer grained phenocrysts, principally in the groundmass. Olivine averages several millimeters across but may be as large as 1–2 cm (fig. 15).

Euhedral, brownish-green biotite (much of it Mg rich)

phenocrysts are subordinate to clinopyroxene and olivine

and normally do not exceed 2 percent. Biotite may be several

centimeters across but is usually 5 mm or less. Phenocrysts

are typically embayed by groundmass minerals (fig. 16).

Such phenocrysts contain 1,200–2,400 ppm Cr, 650–1,450

ppm Ni, 340–490 ppm V, and 2,100–2,300 ppm Ba (table 6).

The fine-grained (less than 0.01 mm) groundmass is

composed of subequal amounts of diopsidic augite and sanidine and lesser amounts of biotite, albite, magnetite and

ilmenomagnetite, carbonate minerals, and apatite. In rocks

having an extremely fine grained groundmass, large

phenocrysts are not embayed or altered on their edges. In

rocks having a coarser grained groundmass, diopsidic augite

or biotite are surrounded by thin selvages of biotite- and

opaque-poor, sanidine-rich material (fig. 17). Diopsidic augite in the groundmass is pale green and euhedral to subhedral

(fig. 18). Euhedral to subhedral, lath-shaped sanidine (Carlsbad twinned, low 2V) forms an interstitial mat to surrounding

clinopyroxene (fig. 19). Minor amounts of euhedral, albitetwinned albite are associated with sanidine. The volume percent of magnetite for 10 CO2-poor samples averages 1.5 percent, as determined with a hand-held magnetic susceptibility

meter. Magnetite and ilmenomagnetite are disseminated

throughout the groundmass. Biotite is present in three morphologies, as coronas of magnetite and ilmenomagnetite (fig.

20), as euhedral elongate grains between sanidine and albite

(fig. 19), and as ophitic clots (up to 1 cm across) surrounding

clinopyroxene and sanidine (fig. 21). Fine-grained carbonate

minerals are interstitial to sanidine laths (fig. 22) and appear

to be of primary, magmatic origin. Very fine grained, acicular

apatite needles are disseminated through the groundmass but

are principally within sanidine grains. Sparse sulfide minerals (chiefly pyrite) are noted (samples 88C3-301.5′, 88GS-8,

and 191-173.5′) both in disseminated form and as microveinlets having alteration haloes.

Locally, the lamprophyre dikes are extremely heterogeneous. In one gently dipping dike(?) at the south end of Bull

Mountain, the relative percentages of phenocryst minerals

are highly variable over a thickness of about 6 ft. No regular

pattern of zonation was observed. In certain parts of the

body, no megascopic clinopyroxene or olivine are noted and

biotite is the principal phenocryst. In other parts, just 1–2 cm

away, large crystals (1–2 cm across) of diopsidic augite and

subordinate olivine are the dominant phenocrysts and no

biotite is visible.

CHEMISTRY OF CO2-POOR LAMPROPHYRE

W.L. Coppinger and the Golden Sunlight mine staff

originally noted the alkalic nature of the dikes, their primitive nature, and spatial association of some areas of highgrade gold in the breccia with the dikes. A study of 17 samples of the lamprophyre (Swanson, 1989) revealed many of

the features of the mafic rocks but did not contain analyses

for FeO, CO2, H2O+, H2O–, or REE. Also, Swanson (1989)

leached his samples with dilute HCl prior to analysis and

normalized the resulting oxides to 100 percent. However,

his normalized results can be used to augment our analyses

if care is taken in interpretation of the data. Using the total

alkali-silica (TAS) classification scheme of LeBas and others (1986), Swanson (1989) classified the majority of lamprophyres as trachybasalt and potassic trachybasalt

(because Na2O < K2O). Several samples were basaltic

andesite (or andesibasalt).

For our study, 19 additional samples were collected,

five from the same outcrops as those sampled by Swanson

(1989). Agreement between duplicate samples is very good.

Chemically, the lamprophyres were subdivided into five

88GS-11 sericite

Total gas date = 91.6±0.6 Ma

No plateau

APPARENT AGE (Ma)

0 100

50

90

80

70

60

100

1,000

10,000

Ar/ Ar 39 37

39

K PERCENT Ar RELEASED

Figure 9. 40Ar-39Ar release spectra for sericite from rhyolite

(sample 88GS-11), Golden Sunlight mine.
18 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Figure 10. Photomicrograph in planepolarized light of olivine and clinopyroxene phenocrysts in lamprophyre (sample 88GS-6). Ol,

olivine; Cpx, clinopyroxene. Field of view

4.2×3.0 mm.

Figure 11. Photomicrograph in plane-polarized

light of groundmass texture and minerals in lamprophyre (sample 88GS-3). Cpx, clinopyroxene;

San, sanidine; Bt, biotite; Il/mt, ilmenomagnetite.

Field of view 1.6×1.1 mm.

Figure 12. Photomicrograph of strongly zoned

clinopyroxene phenocryst in lamprophyre (sample 88GS-5). Cpx, clinopyroxene; crossed nicols.

Field of view 4.2×3.0 mm.
IGNEOUS ROCK SUITES 19

Figure 13. Photomicrograph in plane-polarized

light of carbonate mineral(?)- and feldspar-rich inclusions in clinopyroxene phenocryst in lamprophyre (sample 88C3-292′). Cpx, clinopyroxene.

Field of view 1.6×1.1 mm.

Figure 15. Photomicrograph in planepolarized light of olivine phenocrysts in lamprophyre (sample 88GS-3). Ol, olivine; Cpx, clinopyroxene. Field of view 8.5×5.8 mm.

Figure 14. Photomicrograph in planepolarized light of clinopyroxene grains in

groundmass in lamprophyre (sample 88GS-3).

Ol, olivine; Cpx, clinopyroxene; San, sanidine.

Field of view 1.6×1.1 mm.
20 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

informal groups according to their CO2 concentrations.

Group 1 (labeled “lamprophyre 1” on figures) contains less

than 2 weight percent CO2; rocks in this group have not experienced CO2 metasomatism. Group 2 rocks contain 3–9 percent CO2, group 3 rocks contain 10–14 percent CO2, and

group 4 rocks contain 15–20 percent CO2. Analyses from

Swanson (1989) are treated as a separate group (“lamprophyre normalized” on figures).

Plots of SiO2 versus major-element oxides (fig. 23)

show much more scatter than would be expected from one

cogenetic igneous population. Smooth trends of sample

points are not apparent for many major-element oxides. MgO

and K2O versus SiO2 especially show evidence of more than

one population of lamprophyre, as will be discussed in following sections. H2O concentrations of CO2-poor lamprophyres are in the field of “common” basic igneous rocks

(LeMaitre, 1976) or between there and the field of lamprophyres as defined by Rock (1987). Most trace-element concentrations for the CO2-poor lamprophyres are similar to

values reported for “calc-alkaline” kersantites (Streckeisen,

1979; Rock, 1984, 1989; LeMaitre, 1989).

Chemically, lamprophyres with less than 2 weight percent CO2 (group 1) are predominantly alkali gabbro (fig. 24A)

according to the classification of De LaRoche and others

(1980). Two are olivine gabbro and one is a gabbro-diorite.

Five of the samples from Swanson (1989) also are alkali gabbro. Apparently, the bulk composition of the parental magma

was an alkali gabbro to olivine gabbro and was transitional

from truly alkalic to alkali-calcic. The samples that plot as

gabbro-diorite may represent a population that is distinct

from the alkali gabbro, as is shown by their sub-alkalic nature

and by their position on subsequent geochemical plots.

Figure 16. Photomicrograph in planepolarized light of embayed biotite phenocryst in

lamprophyre (sample 115-153.6′). Bt, biotite;

San, sanidine; Carb, carbonate minerals. Field of

view 4.2×3.0 mm.

Table 6. Minor- and trace-element geochemistry of phlogopitic biotite phenocryst samples from lamprophyre bodies, Golden Sunlight Mine, southwestern Montana.

[All values in ppm, except Na2O and MnO, which are reported in weight

percent (wt %). Analyses by F.E. Lichte by incuction-coupled mass spectrometry]

Element 88C3-301.5′ 88GS-8

(average of 3) (average of 2)

Li . . . . . . . . . . . . . . . . . . . . 45 . . . . . . . . . . . . . 45

B . . . . . . . . . . . . . . . . . . . 296 . . . . . . . . . . . . . . 4

Na2O . . . . . . . . . . . . . . . . . . 0.2 wt % . . . . . . . . 0.58 wt %

Sc. . . . . . . . . . . . . . . . . . . . 15 . . . . . . . . . . . . . 11

V . . . . . . . . . . . . . . . . . . . 339 . . . . . . . . . . . . 486

Cr. . . . . . . . . . . . . . . . . . 2413 . . . . . . . . . . . 1255

MnO . . . . . . . . . . . . . . . . . . 0.0 wt % . . . . . . . . 0.13 wt %

Co . . . . . . . . . . . . . . . . . . 103 . . . . . . . . . . . . 113

Ni. . . . . . . . . . . . . . . . . . 1450 . . . . . . . . . . . . 646

Cu . . . . . . . . . . . . . . . . . . . . 2.0 . . . . . . . . . . . . 10.0

Zn . . . . . . . . . . . . . . . . . . 177 . . . . . . . . . . . . 114

Ga . . . . . . . . . . . . . . . . . . . 59 . . . . . . . . . . . . . 25

Ge . . . . . . . . . . . . . . . . . . . . 3.2 . . . . . . . . . . . . . 0.3

Rb . . . . . . . . . . . . . . . . . . 275 . . . . . . . . . . . . 276

Sr . . . . . . . . . . . . . . . . . . . 131 . . . . . . . . . . . . 100

Zr. . . . . . . . . . . . . . . . . . . . . 9.2 . . . . . . . . . . . . 13.8

Cs . . . . . . . . . . . . . . . . . . . . 7.8 . . . . . . . . . . . . . 4.68

Ba . . . . . . . . . . . . . . . . . 2310 . . . . . . . . . . . 2100

Tl . . . . . . . . . . . . . . . . . . . . . 0.5 . . . . . . . . . . . . 58.5

Pb . . . . . . . . . . . . . . . . . . . . 1.7 . . . . . . . . . . . . . 1.9

Bi. . . . . . . . . . . . . . . . . . . . . 0.2 . . . . . . . . . . . . . 0.3

Th . . . . . . . . . . . . . . . . . . . . 0.3 . . . . . . . . . . . . . 0.2

U . . . . . . . . . . . . . . . . . . . . . 0.1 . . . . . . . . . . . . . 2.4
IGNEOUS ROCK SUITES 21

Figure 17. Photomicrographs of biotite phenocryst surrounded by opaque-mineral- and clinopyroxene-poor selvage of groundmass in

lamprophyre (sample 189-194.5′). Selvage is

nearly pure sanidine, and minor carbonate minerals and apatite. Away from selvage, groundmass

contains average concentrations of opaque minerals and clinopyroxene. Bt, biotite; Cpx, clinopyroxene; San, sanidine; Carb, carbonate

minerals; Op, opaque minerals. A, plane-polarized light; B, crossed nicols. Field of view

1.6×1.1 mm.

Figure 18. Photomicrograph in planepolarized light of clinopyroxene in groundmass

in lamprophyre (sample 88GS-6). Cpx, clinopyroxene; Bt, biotite; San, sanidine. Field of view

1.6×1.1 mm.
22 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Figure 19. Photomicrographs of intergrown

sanidine and biotite in groundmass in lamprophyre (sample 189-194.5′). Bt, biotite; San, sanidine. A, plane-polarized light; B, crossed nicols.

Field of view 1.6×1.1 mm.

Figure 20. Photomicrograph in plane-polarized

light of corona of biotite surrounding opaque

mineral in lamprophyre (sample 88GS-3). Op/bt,

opaque minerals surrounded by biotite; Cpx, clinopyroxene. Field of view 1.6×1.1 mm.
IGNEOUS ROCK SUITES 23

Figure 21. Photomicrograph in plane-polarized

light of ophitic biotite in groundmass in lamprophyre (sample 88GS-3). Bt, biotite; Cpx, clinopyroxene; San, sanidine. Field of view 1.6×1.1 mm.

Figure 22. Photomicrographs of fine-grained

carbonate mineral interstitial to groundmass sanidine in lamprophyre (sample 189-194.5′). Textural relations indicate that carbonate mineral is not

alteration product of other mineral, but is of primary, magmatic origin. Bt, biotite; San, sanidine;

Carb, carbonate minerals. A, plane-polarized

light; B, crossed nicols. Field of view 0.4×0.25

mm.
24 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

The CO2-poor alkali gabbro are metaluminous and plot

in the field of unaltered igneous rocks (fig. 24B). Olivine

gabbro and gabbro-diorite plot to the right of the alkali gabbro, outside the field of unaltered igneous rocks. Placement

of the boundary of unaltered igneous rocks at these low SiO2

concentrations is problematic—we do not place much

emphasis on the fact that the olivine gabbro and gabbro-diorite plot outside the field boundary. All the lamprophyre bodies are very potassic (fig. 24C), but the olivine gabbro and

gabbro-diorite are the least enriched in potassium. The alkali

gabbro bodies have no strong iron enrichment (fig. 24D), but

the olivine gabbro and gabbro-diorite are distinctly Mg rich.

The “average” nature of the alkali gabbro is partly a function

of their high concentrations of Fe2O3.

Are the olivine gabbro and gabbro-diorite more primitive than the alkali gabbro? Determination of magnesium

numbers (Mg number) can aid in the classification of lamprophyres, but care must be exercised in interpretation of the

data, as Mg number can be calculated in a number of ways.

If Mg number is calculated as molar Mg/(Mg+Fe2+), lamprophyres of group 1 range from 0.72 to 0.80. Alkali gabbro

have lower Mg number (0.73 average) than gabbro-diorite

(0.75). If Mg number is calculated as molar Mg/

(Mg+0.8(FeOtotal)), group 1 lamprophyres range from 0.63

to 0.77. Again, alkali gabbro have lower Mg number (0.64

average) than gabbro diorite (0.73 average). Frey and others

(1978) suggest that primitive magmas that were in equilibrium with mantle peridotite and underwent little, if any, differentiation prior to solidification should have Mg number of

0.67 to 0.75. Rhodes (1981) favors the range 0.65–0.80. If

Mg number is calculated as Mg/(Mg+Fe2+), the lamprophyres appear primitive; if Mg number is calculated as Mg/

(Mg+0.8(Fetotal)), the lamprophyres do not. We favor the

interpretation that the lamprophyres may be related to primitive magmas but that the alkali gabbro bodies do not represent truly primitive magmas. The gabbro-diorite does,

however, appear to be related to a primitive magma.

Concentrations of the compatible elements, Cr, Ni, Co,

and Sc are also indicators of the primitive nature of magmas. Alkali gabbro of group 1 averages 337 ppm Cr, 87

ppm Ni, 49 ppm Co, and 34 ppm Sc. Gabbro-diorite of

group 1 averages 640 ppm Cr, 243 ppm Ni, 49 ppm Co, and

28 ppm Sc. Rhodes (1981) suggested that primitive magmas

having high Mg number should have concentrations of

200–500 ppm Cr, 90–700 ppm Ni, 25–80 ppm Co, and

15–30 ppm Sc. Gabbro-diorite has the requisite concentrations of all compatible elements, but Ni concentrations for

alkali gabbro to olivine gabbro are slightly low. A plot of

Mg number (calculated in both ways as discussed above)

versus Co reveals that alkali gabbro to olivine gabbro of

group 1 plots either within or slightly outside the field

defined by Rhodes (1981) for primary, primitive magmas

(fig. 25). Gabbro-diorite plots within the primitive field. A

plot of magnesium number versus Sc, although not shown,

is in agreement with the Mg number versus Co plot.

Pearce element-ratio plots (i.e., Ti/K vs. P/K), as used

by Russell and Nicholls (1988), are constructed by many

investigators of mafic igneous rocks; we do not favor their

use for these lamprophyres. A fundamental precept of such

plots is that a conserved constituent (element) must be in the

denominator. That conserved constituent must be one whose

concentration does not change during the crystallization process in order for the system change to be accurately portrayed (Russell and Nicholls, 1988). As an example, in most

calcic to calc-alkalic gabbro norites and tholeiites, K is a

conserved element because biotite or other potassium-bearing phases are not present. However, in alkalic gabbros and

basalts, the presence of primary biotite phenocrysts affects

the concentration of K2O in derivative magmas and indicates

that K is not a conserved element. Because few, if any, major

elements appear to be conserved during the crystallization

history of the alkalic, mafic magma at the Golden Sunlight

mine, Pearce element-ratio plots were not utilized.

Chondrite-normalized rare-earth-element plots for

group 1 lamprophyres show the separation of alkali gabbro

and olivine gabbro versus gabbro-diorite (fig. 26). All rocks

have LREE-enriched patterns having no Eu anomalies.

Alkali gabbro and olivine gabbro plot above and distinctly

separate from gabbro-diorite. Alkali gabbro and olivine gabbro are more LREE enriched than gabbro-diorite. La/YbCN

ratios for the alkali gabbro and olivine gabbro average 15;

the gabbro-diorite averages 9. Lack of any europium anomalies is expected in rocks from which plagioclase did not

crystallize as a phenocryst.

Most of the variability of concentrations of the LREE is

probably related to modal concentrations of apatite because

apatite preferentially concentrates the light and intermediate

REE. In the lamprophyres, the highest concentrations of

REE are probably within apatite and biotite. But, because

biotite normally does not selectively concentrate the LREE

or HREE, it has little effect on fractionating the heavy versus

the light REE. In a plot of La vs. P2O5, the dependence of La

concentrations on modal percentage of apatite is clear (fig.

27). Alkali and olivine gabbro are enriched in La and P2O5

compared to gabbro-diorite and occupy the upper right-hand

corner of the diagram.
IGNEOUS ROCK SUITES 25

5

10

15

Al O2 3

0

0.5

1

1.5

TiO2

2

4

6

8

Fe O2 3

4

6

8

FeO

0.1

0.2

0.3

MnO

4

8

12

16

20

MgO

4

8

12

16

35 40 45 50 55 60

CaO

SiO 2

0

0.5

1

1.5

35 40 45 50 55 60

H O- 2

SiO 2

0

1

2

3

H O+ 2

0

5

10

15

20

CO2

0

0.5

1

1.5

P O2 5

1

2

3

4

5

K O2

1

2

3

Na O2

Figure 23. SiO2 versus major-element-oxide plots (Harker diagrams) for all lamprophyre samples, Golden Sunlight mine area.
26 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE 706050400.4 0.5 0.6 0.7 0.8 0.9 Very Mg rich Very Fe rich Average Fe rich Mg rich

SiO (wt. percent) 2

2 3 2 3 (FeO + 0.89Fe O )/(FeO + 0.89Fe O + MgO)

D

0.2 0.4 0.6

Very sodic

Very potassic Average

K O/(K O + Na O) 2 2 2

Potassic

C

70

60

Sodic

1.0

0.9

0.8

0.7

0.60.50.4

40 50 60

A/CNK

SiO (wt. percent) 2

Field of unaltered

igneous rocks

Metaluminous

B

Alkalic Sub-alkalic

EXPLANATION

Lamprophyre 1Lamprophyre normalized

50

40

0

SiO (wt. percent) 2

A

2500 2000 1500 1000

2500

2000

1500

1000

500

R = 6000Ca + 2000Mg + 1000Al 2

R = 4000Si - 11,000(Na + K) - 2000(Fe + Ti) 1

Gb-No

Gb-Di

Ol Gb

Sy Gb

Mz

Gb

Alk

Gb

Picritic

Thr

Calcic

Calc-alkalic

Alkali-calcic

Alkalic

Figure 24. Major-element plots of CO2-poor lamprophyre, Golden Sunlight mine. Alkalic lamprophyre, filled symbols. Sub-alkalic lamprophyre, open symbols. 

A, R1 versus

R2

plot (De LaRoche and others, 1980); Picritic, picritic composition or ultramafic rocks; Thr, therallite; Alk Gb, alkali gabbro; Ol Gb, olivine gabbro; Gb-No, gabbro-norite; Sy Gb, syenogabbro; Mz Gb, monzogabbro; Gb-Di, gabbro-diorite. Fields of alkalinity modified slightly from DeWitt (1989). 

B, SiO2 versus A/CNK plot; A/CNK, molar alumina/sum of molar

calcium, sodium, and potassium. Field of unaltered igneous rocks from DeWitt (unpub. data, 1994). 

C, K2O/(K2O+Na2O) versus SiO2 plot; x-axis is “K number” listed in table 1.

Field boundaries from DeWitt (unpub. data, 1995). 

D, (FeO+0.89Fe2O3)/(FeO+0.89 Fe2O3+MgO) versus SiO2 plot; x-axis is “Fe number” listed in table 1. Field boundaries modified

from DeWitt (1989).
IGNEOUS ROCK SUITES 27

GEOCHRONOLOGY OF CO2-POOR LAMPROPHYRE

Two samples of biotite-bearing lamprophyre were collected for age determinations. Phenocrysts of phlogopitic

biotite were separated from each for isotopic analyses. One

biotite (sample 81P120 collected by Walter Bauer from drill

core DDH30-6, 644-646′) was from the Ohio adit, which

was destroyed during open-pit mining. The other (88GS-8)

was from the southern end of Bull Mountain (fig. 1). A conventional K-Ar date of 79.8±2.8 Ma (two-sigma uncertainty)

was determined for biotite from 81P120 (table 7). Biotite

from 88GS-8 was analyzed by 40Ar-39Ar techniques (table

8) and yielded a slightly disturbed spectra (fig. 28) having a

40Ar-39Ar total gas date of 76.4±0.7 Ma (two-sigma uncertainty). A plateau date of 76.9±0.4 Ma is defined by 78 percent of the gas. The slightly older conventional K-Ar date of

79.8 Ma for biotite from 81P120 is probably the result of

small impurities in the mineral separate because no excess

argon is apparent in the spectra for biotite from 88GS-8 (fig.

28). We interpret the somewhat disturbed spectra of biotite

from 88GS-8 to indicate cooling below about 375 ̊C for the

phlogopitic biotite phenocrysts and, hence, emplacement of

the dikelike body.

PETROLOGY OF CO2-RICH LAMPROPHYRE

A complete transition, from CO2-poor lamprophyre, to

CO2-rich lamprophyre, exists within the sills and dikes at the

Golden Sunlight mine. Both in drill cores and in outcrop, the

lamprophyres range from CO2 poor (CO2 concentrations of

≤ 0.3 weight percent, and H2Otot concentrations ≤ 0.9 weight

percent) to CO2 rich (CO2 = 20.0 weight percent, H2Otot =

1.9 weight percent for sample 88C1-283′). Petrographic and

X-ray diffraction studies indicate an unusual alteration-mineral assemblage associated with CO2 metasomatism that is

characterized by Mg-rich carbonate minerals. Our findings

expand on the observations of Swanson (1989), who noted

chlorite and opaque minerals replacing olivine but who did

not work extensively with the CO2-rich rocks.

CO2 metasomatism has partially to completely replaced

phenocrysts of light-apple-green diopsidic augite with dolomite and minor magnesite (fig. 29). Both centimeter-size

phenocrysts and groundmass clinopyroxene may be

replaced. In fewer than 10 percent of all samples, clinopyroxene phenocrysts are replaced by dolomite and quartz

(sample 115-1876′).

Lamprophyre 1

Lamprophyre 2

Lamprophyre 3

Lamprophyre 4

Co, IN PARTS PER MILLION

Mg NUMBER

100

80

60

40

20

0

0.4 0.5 0.6 0.7 0.8 0.9 1.0

EXPLANATION

Alkalic lamprophyre 1

 (alkali gabbro and

 olivine gabbro)

Sub-alkalic lamprophyre 1

 (gabbro-diorite)

1

10

100

1000

CHONDRITE-NORMALIZED RATIO

RARE EARTH ELEMENT

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

EXPLANATION

Lamprophyre 1

Lamprophyre 2

Lamprophyre 3

Lamprophyre 4

0

10

20

30

40

50

0 0.3 0.6 0.9 1.2

2 5 P O , IN WEIGHT PERCENT

La, IN PARTS PER MILLION

EXPLANATION

Figure 25. Mg number (magnesium number) versus Co diagram

for all lamprophyres, Golden Sunlight mine area. Lamprophyre 1

contains less than 2 percent CO2. Lamprophyre 2 has 5–9 percent

CO2. Lamprophyre 3 has 10–14 percent CO2. Lamprophyre 4 has

15–20 percent CO2. Large symbols have Mg numbers calculated

on basis of Mg/(Mg + Fe2+). Small symbols have Mg number calculated on basis of Mg/(Mg + 0.8(Fetotal)). Dashed region is limits

of “primitive magmas” from Frey and others (1978) and Rhodes

(1981).

Figure 26. Chondrite-normalized rare-earth-element plot for alkalic and sub-alkalic CO2-poor lamprophyre, Golden Sunlight mine

area.

Figure 27. P2O5 versus La plot for alkalic and sub-alkalic lamprophyres, Golden Sunlight mine. Lamprophyre 1 contains less

than 2 percent CO2. Lamprophyre 2 has 5–9 percent CO2. Lamprophyre 3 has 10–14 percent CO2. Lamprophyre 4 has 15–20

percent CO2.
28 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Forsteritic olivine (about Fo85–90) has a variety of alteration products that change, somewhat, with increasing CO2

concentration. In CO2-poor rocks, olivine is cracked and partially replaced by limonite(?) and cream-white magnesite.

These alteration products are also present in rocks with

higher CO2 concentrations, but are successively replaced by

magnesite, sericite, and minor biotite (figs. 30 and 31). In

extremely CO2-rich rocks, only magnesite remains. Sample

88C1-283′ (the most CO2 rich lamprophyre) originally contained abundant phenocrysts of olivine (fig. 32) and minor

clinopyroxene. That olivine is now replaced by magnesite,

minor dolomite, and a trace of talc or sericite. Cream-colored

replacement products of olivine phenocrysts in sample 191-

173.5′ are chiefly magnesite and minor dolomite, with much

lesser amounts of quartz (X-ray diffraction data). Similarly,

sample 88GS-10, which was collected from a lamprophyre

dike in the mine pit, was rich in olivine phenocrysts. Ironstained, red-brown pseudomorphs are now predominantly

magnesite, minor talc and siderite, and traces of quartz.

Biotite phenocrysts are largely unaffected by CO2 metasomatism and retain their embayed outlines. Minor amounts

of sericite replace biotite in extremely CO2 rich samples.

Chlorite has replaced biotite in some samples but does not

exceed several percent of individual grains.

The groundmass minerals, with the exception of clinopyroxene, are much less replaced than the large phenocrysts. Clinopyroxene in the groundmass is extensively

replaced by dolomite and minor magnesite and limonite(?).

Sanidine, which constitutes the bulk of the groundmass, is

normally fresh but can be replaced by a mat of sericite. In

extremely CO2 rich samples, sanidine is replaced by

micrometer-size orthoclase(?) and quartz (fig. 33). Biotite in

Table 7. K-Ar analytical data for biotite from lamprophyre (sample 81P120), Golden

Sunlight mine area, southwestern Montana.

[Analyzed by R.F. Marvin, H.H. Mehnert, and E.L. Brandt (Branch of Isotope Geology, Denver, Colo., Report

674). Date in this table differs from that in original report because of change in λe. Sample location: lat

45°54′10′′N., long 112°00′40′′W. (sec. 19, T. 2 N., R. 3 W., Black Butte 7Ω-minute quadrangle, Jefferson County, Montana). λb = 4.962×10–10/yr; λe = 0.572×10–10/yr; atomic abundance of 40K = 1.167×10–4]

K2O (wt %) 40Ar (moles/gram) 40Ar (%) 40Ar/40K Date (Ma) 2-sigma 

uncertainty (Ma)

9.395 avg 10.88×10–10 94 0.00467 79.8 2.8

Table 8. 40Ar-39Ar data for biotite from lamprophyre (sample 88GS-8), Golden Sunlight

mine area, southwestern Montana.

[

40Arc, radiogenic 40Ar corrected for mass discrimination and induced 40Ar; 39Arc, 39Ar corrected for mass discrimination and radioactive decay; %40Arrad, percentage of radiogenic 40Ar. Sample weight = 64.8 mg; J =

0.007846±0.25 percent. 40Ar-39Ar analyses by Ross Yeoman and L.W. Snee]

Temperature 40Arc 39Arc F % 40Arrad % 39Artotal Date 2-sigma

(°C) (Ma) uncertainty (Ma)

500 0.06014 0.01960 3.07 31.10 0.30 42.97 12.42

600 0.14907 0.07687 1.94 60.70 1.30 27.26 2.00

650 0.18598 0.04930 3.77 61.90 0.80 52.62 4.52

700 0.31994 0.05558 5.76 50.00 0.90 79.69 1.10

750 0.65532 0.11281 5.81 86.80 1.90 80.40 0.90

800 1.53368 0.26699 5.74 94.40 4.50 79.53 0.86

850 3.93491 0.69505 5.66 96.60 11.70 78.40 0.60

900 5.40134 0.97444 5.54 98.10 16.40 76.80 0.44

950 5.27044 0.93903 5.61 96.80 15.80 77.74 0.42

1000 3.56740 0.63658 5.60 92.90 10.70 77.63 0.60

1050 4.30633 0.77321 5.57 93.40 13.00 77.16 0.54

1150 7.15876 1.30517 5.49 95.30 22.00 76.01 0.42

1300 0.11408 0.02193 5.21 74.80 0.40 72.17 18.12

Total gas 5.51 76.36 0.70

Plateau age 76.94 0.48

Plateau from 900°C to 1,150°C contains 78.1 percent of the gas.
IGNEOUS ROCK SUITES 29

the groundmass remains fresh. The concentration of magnetite in the groundmass, which averages 1.5 percent in CO2-

poor samples, diminishes to less than 0.01 percent in CO2-

rich lamprophyre. Relicts of magnetite and ilmenomagnetite

are replaced by limonite(?). White material filling globular

structures (Rock, 1984) that were originally vesicles or

amygdules is a mixture of quartz and plagioclase feldspar.

Although Porter and Ripley (1985) reported clay minerals in

altered lamprophyres, we observed none in the samples

examined in this study.

The alteration assemblage of the lamprophyres is somewhat unusual: magnesite, dolomite, sericite, and minor

quartz and chlorite. Velde (1968), in a review of metasomatized lamprophyres, noted the common assemblage of

carbonate minerals, chlorite, actinolite, montmorillonite,

quartz, and talc replacing olivine. Our samples lack many of

these minerals and are especially noteworthy because of the

general lack of calcite. Carbonate minerals mentioned by

Rock (1984) in “calc-alkaline” lamprophyres include calcite,

dolomite, ankerite, and siderite, in approximate decreasing

percentages. At the Golden Sunlight mine, carbonate minerals replacing olivine and clinopyroxene are not primary.

Minor, fine-grained carbonate minerals in the groundmass,

however, do appear to be of primary, magmatic origin.

Based on the preceding petrographic observations, the

major, unbalanced replacement reactions in the lamprophyres include:

Olivine + CO2 → 95% magnesite ± 5% sericite 

Mg1.8Fe0.2SiO4 + CO2 → MgCO3±K 2Al6Si6O20(OH)4 (1)

Clinopyroxene + CO2 → 80% dolomite +

15% magnesite + 5% quartz

Ca(Mg0.8Fe0.2)Si2O6 + CO2 → Ca0.4Mg0.6 CO3 +

MgCO3 + SiO2 (2)

Sanidine + H2O → 100% sericite

KAlSi3O8 + H2O → K2Al6Si6O20(OH)4 (3)

Sanidine → quartz(?) + orthoclase(?)KAlSi3O8 →

SiO2 + KAlSi3O8 (4)

Reaction 1, which cannot be balanced using the percentages of reaction products (predominantly magnesite),

indicates that MgO, SiO2 , and FeO are in excess and that

K2O and Al2O3 have been added to the pseudomorphs.

Reaction 2, which also cannot be balanced because of the

88GS-8 biotite

Total gas date = 76.4±0.7 Ma

Plateau date = 76.9±0.5 Ma

APPARENT AGE (Ma)

0 100

50

90

80

70

60

39

K PERCENT Ar RELEASED

100

Figure 28. 40Ar-39Ar release spectra for biotite from lamprophyre (sample 88GS-8), Golden Sunlight mine. Plateau date of

76.9±0.5 Ma.

Figure 29. Photomicrograph in plane-polarized light of clinopyroxene phenocrysts replaced

by dolomite and chlorite in lamprophyre (sample

189-194.5′). Chl, chlorite; Dol, dolomite. Field

of view 1.6×1.1 mm.
30 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Figure 30. Photomicrograph in plane-polarized

light of olivine phenocryst replaced by sericite,

magnesite, and magnetite in lamprophyre (sample

190-1223.5′). Mg, magnesite; Mt, magnetite; Ser,

sericite. Field of view 1.6×1.1 mm.

Figure 31. Photomicrograph in planepolarized light of olivine phenocryst replaced by

biotite and carbonate minerals in lamprophyre

(sample 191-1034′). Ser, sericite; Carb, carbonate minerals, especially magnesite and dolomite.

Field of view 1.6×1.1 mm.

Figure 32. Photomicrograph in planepolarized light of olivine-rich lamprophyre

(sample 88C1-283′) showing complete replacement of olivine by magnesite. Mg, magnesite.

Field of view 8.5×5.8 mm.
IGNEOUS ROCK SUITES 31

predominance of dolomite, reveals that CaO, MgO and SiO2,

and probably FeO, are in excess. Although quartz is a reaction product identified in thin sections, the low percentages

present (normally less than 5 percent) are not enough to balance the reaction. Reaction 3 comes the closest to being balanced because only Al2O3 must be added to the system.

Reaction 4 indicates that SiO2 has been added to the system.

Importantly, point counts of CO2-rich rocks indicate

that the higher the concentration of CO2, the higher the percentage of relict phenocrysts of olivine. Rocks containing

less than 5 percent CO2 average 6 percent olivine. Rocks

having 5–9 percent CO2 have 10–13 percent olivine, and

rocks having 10–14 percent CO2 have 16–20 percent altered

and replaced olivine. Lamprophyres with 15–20 percent

CO2 have 25–28 percent totally replaced olivine phenocrysts. Therefore, the greatest affect of CO2 metasomatism

was replacement of olivine.

Volume change does not appear to have been

significant during CO2 metasomatism. Replaced phenocrysts of both olivine and clinopyroxene retain their aspect

ratios and average sizes. No phenocrysts are flattened, compressed, or expanded. Groundmass plagioclase retains its

characteristic interstitial form and is not decreased or

increased in size. Groundmass biotite and sanidine also do

not deviate from their form and aspect where replaced.

Therefore, volume change, on a significant scale, has not

taken place during metasomatism.

CHEMISTRY OF CO2-RICH LAMPROPHYRE

The addition of CO2 to lamprophyres at the Golden

Sunlight mine has resulted in apparent enrichment of some

elements and apparent depletion of others (figs. 34 and

35). Both alkalic (alkali gabbro and olivine gabbro) and

sub-alkalic (gabbro-diorite) varieties of lamprophyre have

similar apparent enrichment and depletion trends, and

those trends are clearly separable on most multielement

plots. With increasing CO2 concentration, TiO2, Fe number, Al2O3, Sr, K2O, CaO, and P2O5 decrease in apparent

concentration for both types of lamprophyre (fig. 34A–G).

Scatter within some element plots is greater than within

others, but the following trends are clear. On all plots, subalkalic lamprophyre samples plot at lower concentrations

than do alkalic lamprophyre samples. At the highest CO2

concentrations, separation of the two trends is more difficult because the trends tend to converge. With increasing

CO2 concentration, Cr, Ba, Ni, and MgO increase in apparent concentration for both lamprophyres (fig. 35B–E).

Alkalic lamprophyre samples plot at lower concentrations

than sub-alkalic samples, with the exception of Ba. Cobalt

variation is somewhat enigmatic (fig. 35A) because subalkalic lamprophyres show an apparent increase and some

alkalic lamprophyres exhibit an apparent decrease.

These patterns of apparent enrichment and depletion

are deceptive because they appear to suggest addition of

such refractory elements as Co, Cr, and Ni to the lamprophyres during CO2 metasomatism. As noted in the section on

mineralogy, both groups of lamprophyres have higher concentrations of relict olivine phenocrysts as CO2 concentration increases. Therefore, the apparent enrichment trends

may be only a function of original phenocryst concentration.

A test of this possibility lies in the relationships between Ni,

MgO, and percentage of olivine phenocrysts.

Point counts of replaced olivine phenocrysts in CO2-

rich lamprophyres were made in order to assess the apparent

enrichment patterns of Ni, Cr, and MgO. As expected in

rocks containing olivine phenocrysts, Ni concentration in

our samples is highly correlated (r = 0.97) with modal percent olivine (fig. 36A). The regression of figure 36A was

Figure 33. Photomicrograph in planepolarized light of sanidine replaced by micrometer-size quartz and orthoclase(?) in lamprophyre

(sample 88C1-283′). Cpx, clinopyroxene variable replaced by carbonate minerals; Carb, carbonate minerals; Bt, biotite; Qtz + or, quartz(?)

plus orthoclase(?). Field of view 0.4×0.25 mm.
32 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

0.2

0.4

0.6

0.8

1

0 4 8 12 16 20

CO (wt. percent) 2

TiO (wt. percent) 2

0

0.2

0.4

0.6

0.8

0 4 8 12 16 20

Fe number

CO (wt. percent) 2

4

6

8

10

12

14

0 4 8 12 16 20

CO (wt. percent) 2

Al O (wt. percent) 2 3

0

500

1000

1500

2000

2500

0 4 8 12 16 20

Sr (ppm)

CO (wt. percent) 2

0

1

2

3

4

5

0 4 8 12 16 20

CO (wt. percent) 2

2 K O (wt. percent)

3

6

9

12

0 4 8 12 16 20

CO (wt. percent) 2

CaO (wt. percent)

0

0.3

0.6

0.9

1.2

0 4 8 12 16 20

CO (wt. percent) 2

P O (wt. percent) 5 2

A

B

C

D

E

F

G

Alkalic Sub-alkalic

Lamprophyre 1-4

PARTS THROUGH C G

Alkalic Sub-alkalic

Lamprophyre 1

Lamprophyre 2

Lamprophyre 3

Lamprophyre 4

EXPLANATION

PARTS AND A B
IGNEOUS ROCK SUITES 33

constructed using both alkalic and sub-alkalic groups. However, the sub-alkalic group may define a regression of equal

slope but higher y-intercept, as suggested by the limited data.

This finding is substantiated by other element plots (figs. 26,

34, and 35). Ni is also highly correlated (r = 0.96) with MgO

concentration (fig. 36B). These high correlations indicate

that Ni, Cr, and MgO were not added to the lamprophyres

during CO2 metasomatism. Rather, rocks originally rich in

olivine phenocrysts (and, hence, Ni and, to a lesser extent,

Cr) were the most susceptible to replacement by magnesite

and dolomite because of the instability of olivine during CO2

metasomatism. Most of the apparent enrichment and depletion trends (figs. 34 and 35) are therefore mixing lines

between olivine-rich lamprophyre (high MgO, Ni, Cr, Co)

and olivine-poor lamprophyre (low MgO, Ni, Cr, Co).

Compositions of CO2-metasomatized samples can be

compared to fresh samples (fig. 37). The major-element composition of the CO2-rich samples is expressed in the De

LaRoche plot as lowered R2 values, but little change in R1

values (fig. 37A). From originally an alkali gabbro, most

samples of the alkalic lamprophyre change to syenogabbro or

monzogabbro. Sub-alkalic olivine gabbro changes to gabbrodiorite. These changes indicate that some combination of Ca,

Mg, and Al were lost from the system to account for the lowered R2 values. The largely unchanged R1 values indicate

that, although the sum of Si, Na, K, Fe, and Ti remained constant, individual components could have been lost or gained.

CO2 metasomatism resulted in slightly higher A/CNK ratios,

but markedly lower SiO2 concentrations (fig. 37B), approximately constant K2O/K2O+Na2O at decreasing SiO2 (fig.

37C), and lower Fe numbers at decreased SiO2 (fig. 37D).

REE patterns in CO2-rich lamprophyres have LREE

enrichments similar to CO2-poor lamprophyre, but the

curves for CO2-rich rocks plot successively below those for

CO2-poor rocks (fig. 38). The patterns are consistent with

decreasing concentrations of apatite in CO2-rich rocks (fig.

27). No REE mobility is suggested by the data, just lower

concentrations of REE in more olivine-rich rocks.

ELEMENT GAINS AND LOSSES DURING

CO2 METASOMATISM

The apparent enrichment and depletion trends (figs. 34

and 35) are therefore controlled by two factors: (1) compositional variation of the original lamprophyre; and (2) addition

of CO2. Those rocks having low CO2 concentrations represent samples of both groups of lamprophyres, none of which

has been CO2 metasomatized. The CO2-poor rocks are neither the most primitive nor the most evolved; they are merely

those rocks that have totally escaped the metasomatic process. CO2-rich rocks are those olivine-rich lamprophyres

that were most susceptible to replacement because of the

inherent instability of all phenocrysts in general and olivine

phenocrysts in particular. The concentration of CO2 in both

groups of lamprophyres is not a function of the “completeness or intensity” of alteration but rather the percentage of

olivine phenocrysts that existed and that could react with the

CO2-rich phase.

In order to quantify the true element enrichments and

depletions, point counts were made of CO2-rich lamprophyres that contained 15–20 percent CO2. Pre-metasomatism

whole-rock chemical compositions were calculated from the

modes using published mineral chemistries of forsteritic olivine, diopsidic augite, sanidine, biotite, magnetite, ilmenite,

apatite, and albite from alkalic basalt and alkalic gabbro

(Deer, Howie, and Zussman, 1977). Comparison of the resulting, “primary” chemistries (table 9) with CO2-rich analyses

(table 1) allows a semiquantitative estimation of which elements and oxides were depleted during CO2 metasomatism.

Calculation of the percent of each oxide removed or

added during metasomatism is dependent on two factors:

(1) knowledge of the chemistry of the mineral phases, and

(2) accurate point counts of the pre-metasomatism phases.

Because chemistries of the mineral phases were picked

from published analyses of similar rocks, there is a large

potential uncertainty in this factor. We checked this uncertainty by using the published chemistries to recalculate

compositions of CO2-poor samples from the modal mineral

percentages of those samples. Our published analyses and

the recalculated analyses were in excellent agreement.

Therefore, our assumed mineral chemistries do not contribute significantly to uncertainties in the percentages of

oxides removed or added during CO2 metasomatism. Accurate point counts of all minerals in the rocks containing

15–20 percent CO2, however, were more difficult to obtain.

Particularly for trace minerals such as apatite and albite, and

for low-percentage minerals such as ilmenite, the uncertainty of the percentage of TiO2, P2O5 that were lost is high.

Greatest confidence can be placed on the percentages of

SiO2, Al2O3, Fe2O3, FeO, MgO, CaO, Na2O, and K2O that

were lost because accurate point counts were possible for

relict olivine, clinopyroxene, magnetite, and for fresh

biotite, sanidine, and albite.

Figure 34 (previous page). CO2 versus other component plots for

alkalic and sub-alkalic lamprophyres showing apparent depletion

trends. A, CO2 versus TiO2; B, CO2 versus Fe number; C, CO2 versus Al2O3; D, CO2 versus Sr; E, CO2 versus K2O; F, CO2 versus

CaO; G, CO2 versus P2O5. A and B have lamprophyre 1–4 broken

down by different plotting symbols. The remainder of the plots distinguish only between alkalic and sub-alkalic lamprophyres. Individual groups (1–4) can be distinguished on the basis of their CO2

concentration. Lamprophyre 1 contains less than 2 percent CO2.

Lamprophyre 2 has 5–9 percent CO2. Lamprophyre 3 has 10–14

percent CO2. Lamprophyre 4 has 15–20 percent CO2.
34 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

20

40

60

80

100

0 4 8 12 16 20

2 CO (wt. percent)

Co (ppm)

0

200

400

600

800

1000

0 4 8 12 16 20

Cr (ppm)

CO (wt. percent) 2

400

600

800

1000

1200

0 4 8 12 16 20

Ba (ppm)

CO (wt. percent)

0

200

400

600

800

1000

0 4 8 12 16 20

CO (wt. percent) 2

Ni (ppm)

0

4

8

12

16

20

0 4 8 12 16 20

MgO (wt. percent)

CO (wt. percent) 2

A

B

C

D

E

2

Alkalic Sub-alkalic

Lamprophyre 1-4

EXPLANATION

By utilizing the above calculations, we estimate that

about 15 percent SiO2, 20–25 percent Al2O3, 40–50 percent

Fe2O3, 10–15 percent FeO, 40 percent CaO, and 40–50 percent Na2O were lost from the most CO2-rich rocks during

metasomatism. SiO2 was lost from conversion of olivine and

diopsidic augite to carbonate minerals, principally magnesite.

Reaction 1 (above) indicates that SiO2 is lost during the conversion. Petrographic observations reveal very little secondary quartz except in minor clinopyroxene phenocrysts in one

sample. Al2O3 loss is accounted for by breakdown of diopsidic augite, which contains roughly 30 percent of the Al2O3

in the rock. Loss of Fe2O3 is principally from conversion of

magnetite to limonite, and secondarily from clinopyroxene

alteration to carbonate minerals. Magnetite in the lamprophyres accounts for 20–40 percent of the Fe2O3 in the rocks;

total conversion of it to limonite reduces Fe2O3 by 20–40 percent. FeO loss is much smaller than Fe2O3 loss. Because

Figure 35. CO2 versus other component plots for alkalic and subalkalic lamprophyre showing apparent enrichment trends. A, CO2

versus Co; B, CO2 versus Cr; C, CO2 versus Ba; D, CO2 versus Ni;

E, CO2 versus MgO. Plotting symbols distinguish only between alkalic and sub-alkalic lamprophyres. Individual groups (1–4) can be

distinguished on the basis of their CO2 concentration. Lamprophyre

1 contains less than 2 percent CO2. Lamprophyre 2 has 5–9 percent

CO2. Lamprophyre 3 has 10–14 percent CO2. Lamprophyre 4 has

15–20 percent CO2.
IGNEOUS ROCK SUITES 35

clinopyroxene contains most of the FeO in the rock, its breakdown to carbonate minerals liberates most of the FeO. Some

FeO is reincorporated in siderite, but much remains in the

rock in the form of iron oxide stains around other minerals

and as FeO in limonite. CaO loss is chiefly by breakdown of

diopsidic augite because clinopyroxene accounts for more

than 85 percent of the CaO in the rock. Much of the CaO liberated by breakdown of clinopyroxene is retained in the carbonate minerals dolomite and magnesite, but reaction 2

(above) indicates that Ca is in excess. Na2O loss is facilitated

by breakdown of clinopyroxene because diopsidic augite

contains roughly 20 percent of the Na2O in the rock. Breakdown of groundmass albite may aid in the loss, but petrographic observations do not support this possibility.

K2O, although showing some loss in sample 191-173.5′

(table 9), is probably conserved during metasomatism

because biotite phenocrysts are not altered and sanidine in the

groundmass is fresh or only replaced by sericite. Also,

replacement of olivine phenocrysts by sericite and biotite is

evidence that potassium-bearing minerals were being

formed, not destroyed during CO2 metasomatism. Changes

in P2O5 and TiO2 are more difficult to assess because of the

uncertainty of point counts. Petrographic observations do not

indicate a decrease in the amount of apatite needles in the

groundmass, so P2O5 probably is not substantially decreased.

A

B

0

200

400

600

800

1000

4 8 12 16 20

MgO (wt. percent)

Ni (ppm)

y = -364.65 + 61.85x r = 0.96

y = 1.55 + 0.04x r = 0.97

0

5

10

15

20

25

30

0 200 400 600 800 1000

Ni (ppm)

Olivine (modal percent)

Alkalic Sub-alkalic

Lamprophyre 1

Lamprophyre 2

Lamprophyre 3

Lamprophyre 4

Lamprophyre

 normalized

EXPLANATION

Figure 36. Plots illustrating relationships between Ni, MgO, and

percentage of modal olivine in alkalic and sub-alkalic lamprophyre.

A, Ni versus modal olivine; B, MgO versus Ni. Equations are linear

regressions through all data points in each diagram. Lamprophyre 1

contains less than 2 percent CO2. Lamprophyre 2 has 5–9 percent

CO2. Lamprophyre 3 has 10–14 percent CO2. Lamprophyre 4 has

15–20 percent CO2.

Table 9. Recalculated, CO2-free compositions of CO2-

metasomatized lamprophyre samples, Golden Sunlight

mine area, southwestern Montana.

Recalculated CO2-rich Percentage of

composition analysis of CO2-rich

Oxide based on altered oxide in

modal sample recalculated

mineralogy composition

Sample 88C1-283′

SiO2 46.44 38.80 83

TiO2 0.77 0.35 45

Al2O3 8.49 6.34 75

Fe2O3 3.05 1.59 52

FeO 6.62 5.90 89

MgO 16.93 15.80 93

CaO 8.46 5.06 60

Na2O 1.32 0.67 50

K2O 3.08 3.06 99

P2O5 0.42 0.39 93

H2O+ 1.00

H2O– 0.92

CO2 20.00

Sample 191-173.5′

SiO2 46.62 39.20 84

TiO2 0.71 0.44 62

Al2O3 8.12 6.69 82

Fe2O3 2.97 1.79 60

FeO 7.00 5.60 80

MgO 18.64 17.70 95

CaO 7.84 5.03 64

Na2O 1.30 0.79 61

K2O 3.00 1.74 58

P2O5 0.42 0.25 60

H2O+ 2.00

H2O– 0.90

CO2 15.50
36 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE 706050400.4 0.5 0.6 0.7 0.8 0.9 Very Mg rich Very Fe rich Average Fe rich Mg rich

SiO (wt. percent) 2

2 3 2 3 (FeO + 0.89Fe O )/(FeO + 0.89Fe O + MgO)

D

70

60

50

40

0 0.2 0.4 0.6

Very sodic

Very potassic Average

SiO (wt. percent) 2

K O/(K O + Na O) 2 2 2

Potassic

Sodic

C

1.0

0.9

0.8

0.7

0.60.50.4

40 50 60

A/CNK

SiO (wt. percent) 2

Field of unaltered

igneous rocks

Metaluminous

B

A

2500 2000 1500 1000

2500

2000

1500

1000

500

R = 6000Ca + 2000Mg + 1000Al 2

R = 4000Si - 11,000(Na + K) - 2000(Fe + Ti) 1

Gb-No

Gb-Di

Ol Gb

Sy Gb

Alk

Gb

MzGb

Picritic

Thr

Calcic

Calc-alkalic

Alkali-calcic

Alkalic

Alteration

Alteration

Alteration

Alteration

A

SA

SA

A

SA

A

SA

A

Alkalic Sub-alkalic

Lamprophyre 2Lamprophyre 3Lamprophyre 4

EXPLANATION

Figure 37. Major-element plots of CO2-rich lamprophyre, Golden Sunlight mine. Alkalic lamprophyre in filled symbols. Sub-alkalic lamprophyre in open symbols. Fields of CO2-

poor alkalic (A) and sub-alkalic (SA) lamprophyre from figure 24 shown for comparison. Heavy arrows show direction that samples moved during alteration. A, R1 versus

R2 plot (De

LaRoche and others, 1980); Picritic, picritic composition or ultramafic rocks; Thr, therallite; Gb-No, gabbro-norite; Sy Gb, syenogabbro; Mz Gb, monzogabbro; Gb-Di, gabbro-diorite;fields of alkalinity modified slightly from DeWitt (1989). 

B, SiO2 versus A/CNK plot; A/CNK, molar alumina/sum of molar calcium, sodium, and potassium; field of unaltered igneous

rocks from DeWitt (unpub. data, 1994). Samples that plot to the left of the diagram are shown on the left border and have light arrows leading to the left. 

C, K2O/(K2O+Na2O) versus

SiO2 plot; x-axis is “K number” listed in table 1; field boundaries from DeWitt (unpub. data, 1995). Samples that plot below the diagram are shown on the bottom border and have light

arrows leading down. 

D, (FeO+0.89Fe2O3)/(FeO+0.89 Fe2O3+MgO) versus SiO2 plot; x-axis is “Fe number” listed in table 1; field boundaries modified from DeWitt (1989). Samples

that plot to the left of the diagram are shown on the left border and have light arrows leading to the left.
IGNEOUS ROCK SUITES 37

Alkalic lamprophyre 1

 (alkali gabro and

 olivine gabbro)

Lamprophyre 2

Lamprophyre 3

Lamprophyre 4

1

10

100

1000

CHONDRITE-NORMALIZED RATIO

RARE EARTH ELEMENT

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

EXPLANATION

TiO2 may decrease from alteration of ilmenite to limonite,

but some TiO2 that is liberated may be reincorporated in

groundmass biotite or in sericite and biotite replacing olivine.

MgO shows no measurable decrease in rocks with the

highest CO2 concentrations (table 9). Although reactions

1 and 2 support magnesium loss, the mass balance does

not. Perhaps more magnesite is present in pseudomorphs

of clinopyroxene than we estimated through optical and

X-ray techniques.

True losses (and some possible gains in addition to CO2)

of constituent elements and oxides can be shown in multielement plots (fig. 39). In these plots, CO2-poor and CO2-rich

lamprophyres are plotted as well as the recalculated analyses

from the most CO2-rich samples. All plots reveal the distinction between alkalic and sub-alkalic lamprophyres. Most

plots indicate which elements and oxides were lost or gained

during CO2 metasomatism. In the discussion that follows,

most emphasis is placed on the alkalic lamprophyres because

we have more analyses of them. For all plots, we show the

field containing CO2-poor alkalic lamprophyre samples.

Also, a plus indicates recalculated compositions of CO2-rich

alkalic lamprophyres having 6–20 percent CO2. The region

between the field and the plus corresponds to the primary

igneous composition of lamprophyres before CO2 metasomatism. Samples that plot outside that trend have gained or loss

constituents during metasomatism.

In the TiO2 versus K2O plot (fig. 39A), there is little

variation in K2O concentration indicated by data points from

CO2-rich samples outside the trend from CO2-poor, alkalic

lamprophyre to recalculated compositions of CO2-rich alkalic lamprophyres. However, some points, especially lamprophyre with the highest CO2 concentration, plots at TiO2

concentrations much lower than the recalculated point. We

interpret this plot to indicate little loss of K2O during CO2

metasomatism, but probable loss of TiO2. Points

representing sub-alkalic lamprophyres plot at much lower

TiO2 and K2O concentrations than alkalic lamprophyres.

The MgO versus CaO plot (fig. 39B) shows the original control of olivine and clinopyroxene phenocrysts on the

composition of the rock. At highest MgO concentrations,

CaO is low and is explained by the greater percentage of

olivine phenocrysts to olivine plus clinopyroxene phenocrysts. As MgO decreases from more than 16 percent to

about 10 percent, clinopyroxene phenocrysts increase and

CaO increases. From about 10 percent to 8 percent MgO,

both the percentage of clinopyroxene phenocrysts decreases

rapidly and CaO shows a parallel decrease. Notably in this

plot, samples of CO2-rich lamprophyre plot at lower CaO

concentrations than the trend from CO2 poor to recalculated compositions. We interpret this dispersion, in combination with the recalculated chemical compositions, to

show major CaO loss during metasomatism. Minor MgO

gain during alteration may be suggested by the plot, but the

scatter is within our uncertainties of point counts and

recalculated chemistries.

A similar trend that can be explained by the percentage

of clinopyroxene phenocrysts is apparent in the Al2O3 versus CaO plot (fig. 39C). Highest Al2O3 concentrations are

for the most evolved, most sanidine and albite rich rocks.

Highest CaO concentrations correspond to less evolved, clinopyroxene-rich rocks. Lowest CaO and Al2O3 concentrations are for olivine-rich rocks. All samples from CO2-rich

rocks plot below the igneous trend and substantiate CaO

loss. Al2O3 loss is suggested by the shift of CO2-rich samples to lower concentrations.

K2O versus P2O5 (fig. 39D) reveals a similar, crescent

shape for the igneous trend from olivine-rich rocks at low

K2O and low P2O5 concentrations, through clinopyroxenerich rocks at high K2O and intermediate P2O5 concentrations, to olivine- and clinopyroxene-poor rocks at low K2O

and high P2O5 values. A slight loss of P2O5 may be indicated

by the data. Sub-alkalic lamprophyres plot at much lower

concentrations of both oxides than do alkalic lamprophyres.

Some alkalic lamprophyre samples indicate strontium

gain during CO2 metasomatism (fig. 39E), but many retain

their original values despite major loss of CaO. Much of the

strontium must be in albite and clinopyroxene, but a major

component could be in sanidine as well. Because we do not

fully understand the sites in which strontium is residing, we

cannot assess its gain or loss in the system.

The decrease in CaO concentrations is largely independent of P2O5 variation (fig. 39F). This factor is explained by

calcium loss being effected through clinopyroxene, not apatite, replacement. However, this plot (and fig. 39D) does

suggest minor P2O5 loss.

Figure 38. Chondrite-normalized rare-earth-element plot for

CO2-rich lamprophyre. Field for alkalic, CO2-poor lamprophyre

shown for comparison.
38 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

+

Igneous trend

+

+

+ +

E

0

500

1000

1500

2000

2500

4 6 8 10 12 14

Sr (ppm)

CaO (wt. percent)

2

A

0

1

2

3

4

5

0.2 0.4 0.6 0.8 1

TiO (wt. percent)

2

K O (wt. percent)

B

4

8

12

16

0 4 8 12 16 20

MgO (wt. percent)

CaO (wt. percent)

C

4

6

8

10

12

14

4 6 8 10 12 14

Al O (wt. percent) 2 3

CaO (wt. percent)

D

0

0.3

0.6

0.9

1.2

0 1 2 3 4 5

K O (wt. percent) 2

P O (wt. percent)

2 5

F

0

0.3

0.6

0.9

1.2

4 6 8 10 12 14

2 5 P O (wt. percent)

CaO (wt. percent)

+ Recalculated composition of

 CO -rich alkalic lamprophyre

Field of CO -poor alkalic

 lamprophyre

Missing igneous differentiation

 trend

2

2

Alkalic Sub-alkalic

Lamprophyre 1

Lamprophyre 2

Lamprophyre 3

Lamprophyre 4

Lamprophyre

 normalized

EXPLANATION
IGNEOUS ROCK SUITES 39

GOLD ANALYSES OF LAMPROPHYRE

AND OTHER IGNEOUS ROCKS

Although lamprophyre dikes clearly cut the mineralized breccia pipe, their spatial association with late shear

zones and veins that produced high-grade gold during the

period of early mining has led to speculation concerning a

possible genetic relationship between gold deposition and

lamprophyre emplacement. Also, the presence of magnesite

as gangue in the breccia pipe and as replacement products of

olivine and clinopyroxene in the lamprophyres may suggest

that the latest stages of gold mineralization were coincident

with lamprophyre emplacement. In their review of “calcalkaline” lamprophyres, Rock and Groves (1988) suggest

that lamprophyres may be an under-appreciated clan of rocks

related to gold mineralization. As a test of the lamprophyregold model, all the samples of both lamprophyre groups collected at the Golden Sunlight mine were analyzed for gold,

as were the other rock types for which we report major- and

minor-element chemistry.

Of the 21 samples of lamprophyre, all but two have less

than 0.05 ppm Au (table 10). Two samples of pyrite-bearing

(about 2–3 percent pyrite) lamprophyre (88C1-301.5′, and

191-173.5′) have 0.1 and 0.05 ppm Au, respectively. Any

suggestion of a genetic link between lamprophyre emplacement and gold mineralization is difficult to support with our

data. Of three samples of rhyolite, only one, which contains

pyrite (88GS-12), had a detectable concentration of gold

(0.05 ppm Au). This sample, however, came from immediately adjacent to the breccia pipe (fig. 4) and should not be

used to suggest that some of the rhyolite sills contained gold

related to their emplacement. Wertz (1971) also suggested

that rhyolite bodies removed from the breccia pipe were

exceptionally low in base and precious metals.

Porter and Ripley (1985) observed that the lamprophyres they studied lacked sulfide minerals. Two of our core

samples and one outcrop sample contain 2–3 percent pyrite

in veinlets and as disseminations (88C1-301.5′, 191-173.5′,

88GS-8). We speculate that this sulfide mineralization is

related to emplacement of the lamprophyre bodies and is

younger than the breccia pipe mineralization. With our

present data, however, we cannot rule out the possibility that

the latest stages of breccia pipe formation overlapped in time

with lamprophyre emplacement. Another, less likely, possibility is that lamprophyre magma entrained small amounts of

auriferous pyrite from the breccia pipe or remobilized sulfur,

gold, and iron from country rock on its ascent.

MODEL FOR CO2 METASOMATISM

Petrographic and chemical evidence reveal that addition of large amounts of CO2 to lamprophyres in the Golden

Sunlight mine area resulted in profound mineralogic and

chemical changes to the rocks. Phenocrysts of olivine and

clinopyroxene and groundmass clinopyroxene were extensively replaced by magnesite, dolomite, and minor sericite

and biotite. Groundmass sanidine and albite were largely stable during the CO2 metasomatism but were locally converted

to sericite and mixtures of quartz and orthoclase. Shapes and

sizes of relict phenocrysts and the unchanged texture and

size of groundmass minerals indicates that volume change

was insignificant during metasomatism. Mineral chemistries

and modal data for the most CO2-rich lamprophyres indicate

that 40–50 percent of the Fe2O3, CaO, and Na2O; 20–25 percent of the Al2O3; and 10–15 percent of the SiO2 and FeO

were removed during addition of 15–20 weight percent CO2.

Concentrations of MgO, K2O, Cr, Ni, and possibly Co

remained unchanged during metasomatism.

Figure 39 (previous page). Multielement plots illustrating differentiation and alteration trends in lamprophyre. A, TiO2 versus K2O;

B, MgO versus CaO; C, Al2O3 versus CaO; D, K2O versus P2O5; E,

CaO versus Sr; F, CaO versus P2O5. Field of CO2-poor alkalic lamprophyre shown for comparison. Heavy, curved and dashed line

connects CO2-poor alkalic lamprophyre with recalculated composition of original, non-metasomatized alkalic lamprophyre.

Table 10. Gold concentrations of igneous rocks in the

Golden Sunlight mine area, southwestern Montana.

[0.05 ppm = 50 ppb; N, not detected at value shown. Analyses by B.H.

Roushey]

Rock type Sample no. Au (ppm)

Basalt 88GS-1 N0.05

Tonalite 88GS-2 N0.05

Tonalite 88GS-4 N0.05

Rhyolite 88GS-7 N0.05

Rhyolite 88GS-12 0.05

Rhyolite 88GS-L N0.05

Lamprophyre 88GS-3 N0.05

Lamprophyre 88GS-5 N0.05

Lamprophyre 88GS-6 N0.05

Lamprophyre 88GS-8 N0.05

Lamprophyre 88C1-256′ N0.05

Lamprophyre 88C1-283′ N0.05

Lamprophyre 88C2-559.5′ N0.05

Lamprophyre 88C3-292′ N0.05

Lamprophyre 88C3-299′ N0.05

Lamprophyre 88C3-301.5′ 0.10

Lamprophyre 88C3-353.75′ N0.05

Lamprophyre 115-1537.5′ <0.05

Lamprophyre 115-1876′ N0.05

Lamprophyre 115-1882′ N0.05

Lamprophyre 151-417′ <0.05

Lamprophyre 189-642′ N0.05

Lamprophyre 190-642′ N0.05

Lamprophyre 190-1223.5′ <0.05

Lamprophyre 191-173.5′ 0.05

Lamprophyre 191-1034′ N0.05

Lamprophyre 191-2025′ N0.05
40 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

We believe that the most plausible explanation for the

above observations is that a magmatic CO2-rich fluid or gas

enriched in alkali-earth elements attacked olivine and clinopyroxene phenocrysts in the mafic magma as the groundmass minerals were crystallizing during dike emplacement.

The CO2-rich fluid most likely exsolved from the primary

magma upon cooling. Fine-grained, euhedral, interstitial carbonate minerals within the groundmass that are not replacement products indicate that fairly large amounts of CO2 were

dissolved in the magma before solidification. Although we

lack isotopic data that could substantiate a genetic link, we

believe that this CO2-rich fluid or gas was derived from the

original partial melting event in the mantle that generated the

alkalic lamprophyre magma. Except for the possible addition

of minor amounts of Sr to the system, we see no evidence of

the types of element enrichments typically found in carbonatite settings (i.e., LREE, Cu, Na, P, etc.). Therefore, the lamprophyre dikes do not appear to be related to a more

complex, but unexposed, igneous center.

A further indication that CO2 metasomatism was an integral part of the magmatic history, and not of a later, unrelated

alteration, is the paucity of CO2-poor lamprophyre samples

having high MgO and modal olivine. Our sampling of surface

outcrops and drill core indicates that, for the most part, only

CO2-rich samples contain relicts of olivine greater than 5 volume percent. In other words, if the lamprophyre magma contained abundant olivine, that olivine is invariably transformed

into carbonate minerals. The more primary olivine the magma

contained, the greater the present percentage of carbonate

minerals. If CO2 metasomatism had been a later, unrelated

event, or one controlled by structures or fractures, the pattern

of olivine replacement would not be so predictable.

The existence of CO2-poor lamprophyre grading into

CO2-rich lamprophyre in drill-core samples is explained by

original variability in phenocryst percentages. In drill hole

C3, lamprophyre samples at 299′ depth (table 1) contain 8

percent CO2. Lamprophyre sampled at 292′ depth (table 1)

contains 0.4 percent CO2. Petrographically, sample 299 has

10 percent olivine, but sample 292 contains about 5 percent

olivine. More olivine was present in sample 299, so more

was replaced by the CO2-rich fluid or gas phase in the

magma. What at first looks like a possible structural control

to CO2 metasomatism turns out to be a compositional control

imparted by the phenocrysts.

Abundant sanidine in the groundmass of the dikes indicates emplacement temperatures of the lamprophyre dikes of

at least 700°C. This temperature is notably higher than the

maximum temperature of 600°C suggested by Rock (1984)

for typical lamprophyre magmas and suggests that intrusives

at the Golden Sunlight mine may not be “typical” lamprophyres. Based on the phenocryst assemblage (olivine and clinopyroxene and lack of plagioclase and orthopyroxene),

Swanson (1989), using the summary of Edgar (1987), suggested that phenocrysts crystallized at depths between about

24 and 41 km and temperatures of about 1,210°–1,270°C.

Without a critical comparison of the chemistry of the sample

quoted by Edgar (1987) and the lamprophyre bodies at the

Golden Sunlight mine, we cannot evaluate whether the

experimental results are directly applicable.

What became of the elements that were depleted from

the lamprophyres during CO2 metasomatism? No extensive

alteration envelopes related to the mafic dikes are currently

recognized in the vicinity of the Golden Sunlight mine, nor

are there extensive vein networks that could contain the

deposited products of the CO2-rich gas or fluid. If the

depleted elements were transported away from the lamprophyre bodies in a CO2-rich hydrothermal fluid, that fluid,

upon cooling, would precipitate epidote, quartz, calcite, and

trace albite in the approximate ratios 6:2.5:1.5:trace. Because

we do not see this vein or alteration assemblage at the Golden

Sunlight mine, we can infer only that the depleted elements

were carried farther upward or outward from the immediate

area that we sampled.

Hypotheses have been proposed for the formation of

some lamprophyre magma involving direct melting of normal mantle peridotite under high PCO2 conditions with or

without “zone refining” (Velde, 1971; Rock, 1984). Other

models have suggested that lamprophyres result from 1–20

percent partial melting of mantle lherzolite. Regardless of

which model(s) are appropriate, most workers agree that

alkalic rocks of this composition had their ultimate source

in the mantle. Rapidly changing physical and chemical conditions of the lamprophyre magma are indicated by strongly

zoned and partially resorbed clinopyroxene phenocrysts and

strongly embayed biotite phenocrysts. These changes probably took place during rapid ascent of the magma to the surface. The validity of any specific models for the

lamprophyre dikes at the Golden Sunlight mine is uncertain

until additional isotopic studies, especially Rb-Sr, C, and

Sm-Nd, can be undertaken.

The separation on nearly all multielement plots of alkalic lamprophyres (alkalic gabbro and olivine gabbro) from

sub-alkalic lamprophyres (gabbro-diorite) indicates to us that

the two lamprophyres are probably derived from separate,

though possibly related, magmas. Whether or not one group

is older than the other is not known. Both have CO2-metasomatized variants and both have similar apparent and real element enrichment and depletion trends. Were these two

groups formed independently in the mantle, only to arrive at

the same place in the upper crust? Did the two groups evolve

from a common parent magma in the mantle? Only further,

more detailed data will answer these questions.

Another possible, but we think less probable, model for

CO2 metasomatism involves incorporation of CO2 from

reaction with limestone and dolomite in the Proterozoic or

Paleozoic strata in the area. Major thrusting of Precambrian

Belt Supergroup rocks over Paleozoic to Mesozoic sedimentary and volcanic strata during the Late Cretaceous took

place in the vicinity of the Golden Sunlight mine (Schmidt

and others, 1988). Any thrusts are now buried by Cenozoic
IGNEOUS ROCK SUITES 41

deposits south of the Golden Sunlight mine. One fault at the

Mayflower mine in the northern Tobacco Root Mountains

(fig. 40) juxtaposes Belt Supergroup rocks and rocks as

young as Late Cretaceous (Elkhorn Mountains Volcanics),

but it may be a reverse fault. A major transverse (east-weststriking) fault zone has been mapped (Alexander, 1955;

Robinson, 1963; Schmidt and others, 1988) south of Whitehall that could be interpreted as a tear fault between thrust

plates. The aggregate eastward transport of the block occupied by the Golden Sunlight mine was at least 15 km. Possible evidence for major thrusting is provided by a 4,525 m

(14,846 ft) drill hole on the Devils Fence anticline approximately 30 km northeast of the mine (Burton and others,

1991; Ballard and others, 1994). There, a major thrust,

possibly the Lombard, was suggested at a depth of 2,130 m,

which juxtaposed upper plate Belt Supergroup rocks of the

Spokane, Greyson, and Newland Formations with Cretaceous Elkhorn Mountains Volcanics and the Slim Sam and

Blackleaf Formations. The presence of Cretaceous strata was

interpreted, partly, on the basis of Cretaceous dinoflagellates, which W.J. Perry, Jr., M.J. Pawlewicz, T.A. Daws,

T.S. Dyman, and G.A. Desborough (written commun., 1995)

consider to be a possible contaminant from bentonite drilling

mud that contained Cretaceous fossils. Stratigraphy of the

sedimentary rocks in question, the presence of porcellenite

beds, and minor diabasic sills in the sedimentary rocks suggest that the rocks below the possible thrust may be part of

the Belt Supergroup.

Golden Sunlight mine

Yogo dike

Yogo Peak pluton

Adel Mountains Volcanics

Volcanics near Wolf Creek

Boulder batholith

Elkhorn Mountains Volcanics

Kokoruda phase of Boulder batholith

Great Falls

Helena

Bozeman

Butte

Missouri River

Yellowstone River

Jefferson River

Gallatin River

Madison River

47 ̊

46 ̊

45 ̊

113 ̊ 112 ̊ 111 ̊ 110 ̊

Dillon

Lower part

of the 

volcanics of

Cold Spring Creek

Whitehall

Mayflower mine

Devils Fence

anticline

0 50 miles

Tobacco Root

Mountains

Figure 40. Location map of southwestern Montana showing localities and rock units discussed in text. Drill hole on Devils Fence anticline shown by plus sign inside circle.
42 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

CORRELATION OF LAMPROPHYRES 

AND RHYOLITE WITH ROCKS IN

SURROUNDING AREAS

Rocks as mafic or alkalic as the lamprophyres at the

Golden Sunlight mine are not widely known in southwestern

Montana. However, slightly more evolved basaltic to andesitic volcanic rocks in surrounding areas could be related to

the lamprophyres. Felsic volcanic rocks are quite widespread

in southwestern Montana, and some could be related to the

sills of rhyolite that are so numerous in the mine area. We

suggest the following correlation, based on analysis of chemical data (Ruppel, 1963; Schmidt, 1978; Lambe, 1981; Rutland, 1985; Ivy, 1988) from intrusive and extrusive rocks

from south of Great Falls to north of Dillon (fig. 40).

Rocks that are chemically similar to the alkalic lamprophyres are augite trachybasalt found in the Adel Mountains

Volcanics in the Wolf Creek area north of Helena (Schmidt,

1978). Although not as mafic as the lamprophyres (fig. 41A),

the trachybasalt is alkalic to alkali-calcic and ranges in chemical composition from trachybasalt to latite and easily could

be related to the lamprophyres by fractional crystallization.

A relationship through fractional crystallization would also

explain the trends away from the alkalic lamprophyres in alumina saturation (fig. 41B) and iron enrichment (fig. 41D).

The trend toward a more sodic nature (fig. 41C) is not well

explained by fractional crystallization, however. The trachybasalt flows lack olivine phenocrysts, but contain abundant

augite in a groundmass of labradorite, sanidine, magnetite,

and apatite. A Late Cretaceous paleomagnetic pole age

(Gunderson and Sherif, 1991) for the Adel Mountain Volcanics agrees with the 77-Ma date determined by 40Ar-39Ar

analyses for biotite from one lamprophyre in this study. The

best estimate by Harlan and others (1991) for the age of flow

units in the Adel Mountain Volcanics is 75–76 Ma, which is

also in reasonable agreement with the 77-Ma biotite date.

The one picritic (ultramafic) analysis of the Yogo Peak

pluton also is similar in chemistry to the alkalic lamprophyres

(fig. 41A). In terms of alumina saturation (fig. 41B) and alkalinity (fig. 41C), the analysis of the pluton is more similar to

the alkalic lamprophyres than are the analyses of the Adel

Mountains Volcanics. Additional geochemical data would

aid in our interpretation of chemical similarity to the data of

Embry (1987). Existing geochronologic data (Marvin and

others, 1973) suggest an age of 50 Ma age for the Yogo Peak

pluton, so the lamprophyre at the Golden Sunlight mine does

not appear to be time correlative to the Yogo Peak pluton.

Rocks that are chemically most similar to the sub-alkalic lamprophyres are the majority of flows in the informally

named unit 8 of the Elkhorn Mountains Volcanics (Rutland,

1985). These calc-alkalic andesitic basalts to andesites are

slightly more felsic than the gabbro-diorite at the mine (fig.

41A), but have similar alumina saturations (fig. 41B), are

very potassic (fig. 41C), and are very Mg rich (fig. 41D).

Unit 8 of the Elkhorn Mountains Volcanics averages 440

ppm Cr, 113 Ni, and 681 ppm Sr, and has REECN profiles

that are LREE enriched and have no Eu anomalies (Rutland,

1985)—these values are very similar to the sub-alkalic lamprophyres (fig. 42). Estimated ages for the Elkhorn Mountains Volcanics are 77–79 Ma (Tilling and others, 1968),

which are in agreement with the 77-Ma biotite date. Basalt

flows in the Elkhorn Mountains Volcanics just west of the

mine (Prostka, 1966) have a modal mineralogy similar to the

alkalic lamprophyre, but chemical analyses are lacking for

these flows.

Other units (fig. 41) that have chemical similarity to the

sub-alkalic lamprophyres are rocks that intrude the Elkhorn

Mountains Volcanics (Klepper and others, 1971) and some

flow units from the lower part of the volcanics of Cold Spring

Figure 41 (following page). Major-element plots of CretaceousTertiary mafic rocks in western Montana. Fields of alkalic (A) and

sub-alkalic (SA) lamprophyre at the Golden Sunlight mine shown

for comparison. Adel, Adel Mountains Volcanics; data from

Schmidt (1978). Wolf Creek, volcanics near Wolf Creek that are

possibly correlative with Adel Mountains Volcanics; data from

Schmidt (1978). Kokoruda, Kokoruda phase of the northern part of

the Boulder batholith; data from Smedes (1966). Yogo dike, sapphirine-bearing Yogo dike; data from Clabaugh (1952) and Dahy

(1991). Yogo Peak, Yogo Peak pluton; data from Embry (1987).

Boulder batholith (mafic), mafic rocks that predate the main part of

the Boulder batholith; data from Lambe (1981). Elkhorn Volcanics,

unit 8, informal unit 8 of the Elkhorn Mountains Volcanics, which

corresponds to the upper part of the volcanics (Smedes, 1966; Klepper and others, 1971); data from Rutland (1985). Elkhorn intrusives,

intrusive rocks that cut the Elkhorn Mountains Volcanics; data from

Klepper and others (1971). Cold Spring, lower, lower part of the

volcanics of Cold Spring Creek; data from Ivy (1988). A, R1 versus

R2 plot (De LaRoche and others, 1980); Picritic, picritic composition or ultramafic rocks; Thr, therallite; Gb-No, gabbro-norite; T

Bas, trachybasalt; Lat Bas, latibasalt; And-Bas, andesitic basalt

(gabbro-diorite is plutonic equivalent); T And, trachyandesite; Lat,

latite (monzonite is plutonic equivalent); Lat-And, lati-andesite

(monzodiorite is plutonic equivalent); And, andesite (diorite is plutonic equivalent); Q Tr, quartz trachyte; Q Lat, quartz latite; Dac,

dacite (tonalite is plutonic equivalent); R dac, rhyodacite. Fields of

alkalinity modified slightly from DeWitt (1989). Samples that plot

to left of diagram are shown on left margin and have arrows leading

to the left. B, SiO2 versus A/CNK plot; A/CNK, molar alumina/sum

of molar calcium, sodium, and potassium; field of unaltered igneous

rocks from DeWitt (unpub. data, 1994). Samples that plot below the

diagram are shown on the bottom border and have arrows leading

down. C, K2O/(K2O+Na2O) versus SiO2 plot; x-axis is “K number”

listed in table 1; field boundaries from DeWitt (unpub. data, 1995).

Samples that plot below the diagram are shown on the bottom border and have arrows leading down. D, (FeO+0.89Fe2O3)/(FeO+0.89

Fe2O3+MgO) versus SiO2 plot; x-axis is “Fe number” listed in table

1; field boundaries modified from DeWitt (1989). Samples that plot

to the left of the diagram are shown on the left border and have arrows leading to the left. Samples that plot in the very Mg-rich field,

below plot (C), have arrows leading to the left or up. 
CORRELATION OF LAMPROPHYRES AND RHYOLITE WITH ROCKS IN SURROUNDING AREAS 43 706050400.4 0.5 0.6 0.7 0.8 0.9 Very Mg rich Very Fe rich Average Fe rich Mg rich

SiO (wt. percent) 2

2 3 2 3 (FeO + 0.89Fe O )/(FeO + 0.89Fe O + MgO)

D

70

60

50

40

0 0.2 0.4 0.6

Very sodic

Very potassic Average

SiO (wt. percent) 2

K O/(K O + Na O) 2 2 2

Potassic

Sodic

C

1.0

0.9

0.8

0.7

0.60.50.4

40 50 60

A/CNK

SiO (wt. percent) 2

Field of unaltered

igneous rocks

Metaluminous

B

A

2500 2000 1500 1000

2500

2000

15001000

500

R = 6000Ca + 2000Mg + 1000Al 2

R = 4000Si - 11,000(Na + K) - 2000(Fe + Ti) 1

Gb-No

And-Bas

And

Dac

Ol Gb

Lat

T Bas

Lat

Bas

LatAlk Gb

Picritic

R Dac

Q Lat

Q Tr

T

And

Thr

Calcic

Calc-alkalic

Alkali-calcic

Alkalic

And

A

SA

SA

A

SA

A

A

SA

A

Adel volcanicsBoulder batholith (Kokoruda)Wolf CreekYogo dikeYogo Peak plutonBoulder batholith (mafic)Elkhorn Volcanics, unit 8Elkhorn intrusivesCold Spring, lower

EXPLANATION
44 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Creek near Bannack (Ivy, 1988). 40Ar-39Ar dates of hornblende from flows and ash flows in the volcanics of Cold

Spring Creek are 76–78 Ma (Ivy, 1988), in agreement with

the 77-Ma biotite date. Although a direct age-equivalent correlation cannot be proven with existing data between lamprophyre bodies at the Golden Sunlight mine and other mafic

rocks in southwestern Montana, the data do indicate that

mafic, alkalic plutonism and volcanism was taking place in

western Montana in Late Cretaceous time. Further work will

undoubtedly refine the correlations suggested by this work.

Rocks similar in age and chemistry to the rhyolite sills

at the Golden Sunlight mine are present in the informally

named units 7, 9, and 11 of the Elkhorn Mountains Volcanics

(Rutland, 1985). Alkali-calcic lati-andesite to rhyodacite

welded tuff units plot on a trend that would intersect the

freshest felsic sills at the mine (fig. 43). Unit 11 is the most

similar to rhyolite at the mine in terms of alumina saturation

(fig. 40B), alkali enrichment (fig. 43C), and iron enrichment

(fig. 43D). Limited analyses of units 7 and 9 may obscure

their similarity to rhyolite at the mine. The most evolved of

the rhyolites in unit 7 have Sr, Zr, and Fe numbers that are

similar to those of the freshest rhyolite sills at the mine. Samples of rhyolitic welded tuff from the middle part of the

Elkhorn Mountains Volcanics (Smedes, 1966) are also similar in major element composition to the sills at the mine.

If the rhyolite sills at the mine are temporally and genetically related to emplacement of voluminous ash-flow deposits of unit 7, 9, or 11 of the Elkhorn Mountains Volcanics, the

source for both sills and tuffs may have been within the crust,

as suggested by Rutland (1985). If basaltic flows of unit 8 are

temporally and genetically related to emplacement of lamprophyre dikes, their source may have been in the mantle, not

within the crust. Therefore, differing source areas over time

may characterize parts of the Elkhorn Mountains Volcanics.

A complete analysis of this possibility awaits further study.

CONCLUSIONS

The gold-mineralized breccia pipe at the Golden Sunlight mine, southwestern Montana, cuts strata of the Belt

Supergroup and sills of Late Cretaceous rhyolite. Within the

pipe, rhyolite forms the matrix for downdropped fragments

and is mineralized by disseminated, auriferous pyrite,

molybdenite, and hematite. The pipe is inferred to grade

downward into an alkalic, porphyry molybdenum deposit.

Dikes and sills of alkalic and sub-alkalic lamprophyre cut the

pipe, and they possibly locally remobilized minor amounts of

gold along their margins. Late shear zones and veins that

contain high-grade gold also contain mineralized lamprophyre bodies. Determining the petrogenesis and age of the

rhyolite, gold mineralization, and lamprophyres was the

focus of this investigation.

The rhyolite was difficult to characterize chemically

because of its altered nature. Least altered samples are

alkali-calcic, mildly peraluminous, very sodic, and Mg rich.

Alteration near the breccia pipe creates hydrothermal sericite and orthoclase and turns the rhyolite into an alkalic

quartz syenite that is strongly peraluminous, potassic to

very potassic, and Fe rich to very Fe rich. No REE mobility

was verified during alteration.

Golden Sunlight mine,

all lamprophyre bodies

Elkhorn Mountains

Volcanics, unit 8

10

100

CHONRDITE-NORMALIZED RATIO

3

RARE EARTH ELEMENT

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

300

100

EXPLANATION

Figure 42. Chondrite-normalized rare-earth-element plot for mafic volcanic rocks in unit 8 of the Elkhorn Mountains Volcanics.

Data from Rutland (1985). Lamprophyre of group 1 (CO2 less than

2 percent) from Golden Sunlight mine plotted for comparison. Top

four dashed curves are for alkalic lamprophyre; bottom two dashed

curves are for sub-alkalic lamprophyre.

Figure 43 (following page). Major-element plots of CretaceousTertiary felsic rocks in the Elkhorn Mountains Volcanics, western

Montana. Field of “unaltered” rhyolite at the Golden Sunlight mine

shown by stippled area. Elkhorn air-fall tuff, air-fall tuff from the

Elkhorn Mountains Volcanics; data from Rutland (1985). Elkhorn

01 through 13, welded tuff from the Elkhorn Mountains Volcanics

[numbers increase up-section]; data from Rutland (1985). Elkhorn,

lower, welded tuff from lower part of the Elkhorn Mountains Volcanics; data from Ruppel (1963). Elkhorn, middle, middle part of

the Elkhorn Mountains Volcanics; data from Smedes (1966) and

Klepper and others (1971). Elkhorn, upper, upper part of the

Elkhorn Mountains Volcanics; data from Knopf (1913), Ruppel

(1963), and Robertson (1953). A, R1 versus R2 plot (De LaRoche

and others, 1980); T Bas, trachybasalt; Lat Bas, latibasalt; And-Bas,

andesitic basalt (gabbro-diorite is plutonic equivalent); T And, trachyandesite; Lat, latite (monzonite is plutonic equivalent); Lat-And,

lati-andesite; And, andesite (diorite is plutonic equivalent); Tr, trachyte; Q Tr, quartz trachyte; Q Lat, quartz latite; Dac, dacite; R Dac,

rhyodacite; Rhy, rhyolite; Alk Rhy; alkali rhyolite. Fields of alkalinity modified slightly from DeWitt (1989). B, SiO2 versus A/CNK

plot; A/CNK, molar alumina/sum of molar calcium, sodium, and

potassium; field of unaltered igneous rocks from DeWitt (unpub.

data, 1994). C, K2O/(K2O+Na2O) versus SiO2 plot; x-axis is “K

number” listed in table 1; field boundaries from DeWitt (unpub. data, 1995). D, (FeO+0.89Fe2O3)/(FeO+0.89 Fe2O3+MgO) versus

SiO2 plot; x-axis is “Fe number” listed in table 1; field boundaries

modified from DeWitt (1989).
CONCLUSIONS 45

D

C

80

70

60

50

0.5 0.6 0.7 0.8 0.9 1.0

SiO (wt. percent) 2

2 3 2 3 (FeO + 0.89Fe O )/(FeO + 0.89Fe O + MgO)

Mg rich

Very Mg-rich

Very Fe rich Average Fe rich

D

B

80

70

60

50

0 0.2 0.4 0.6

SiO (wt. percent) 2

K O/(K O + Na O) 2 2 2

Very sodic

Very potassic Average Potassic

Sodic

C

0.90.80.7

1

1.31.21.1

50 60 70

A/CNK

SiO (wt. percent)

Strongly peraluminous

Mildly peraluminous

2

Field of unaltered

igneous rocks Metaluminous

B

A

3000 2500 2000 1500 1000

500

1500

1000

R = 4000Si - 11,000(Na + K) - 2000(Fe + Ti) 1

R = 6000Ca + 2000Mg + 1000Al 2Alk Rhy Rhy R Dac Dac

And

And-Bas

Q Tr

Q Lat

Lat

And Lat

Lat

Bas

T

And

Tr

T

Bas

Calcic

Calc-alkalic

Alkali-calcic

Alkalic

"Unaltered" rhyolite

1 3

3

3

4

6

7

7

7

7

7

9

9

0

1

3

3

4

6

7 7

7

7

0 9

9

1

3

3

3

4

6

7

7 7

7 79

9 0

1

33

3

4

6

7

7 7

7 7 9

9 0

80

Elkhorn air-fall tuffElkhorn 01Elkhorn 03Elkhorn 04Elkhorn 06Elkhorn 07Elkhorn 09Elkhorn 10Elkhorn 11Elkhorn 12Elkhorn 13Elkhorn, lowerElkhorn, middleElkhorn, upper

1

3

46

7

90

EXPLANATION
46 LATE CRETACEOUS IGNEOUS AND HYDROTHERMAL EVENTS, GOLDEN SUNLIGHT BRECCIA PIPE

Determination of a precise crystallization age of the

rhyolite from analysis of zircon by the U-Th-Pb method

was made difficult by the presence of inherited lead or

inherited zircon of Late Archean age. If an emplacement

age of about 80 Ma is assumed for the rhyolite, an age of

inheritance of 2,600 Ma can be calculated. That age agrees

with basement ages determined for much of southwestern

Montana. Determination of a crystallization age of the rhyolite by fission-track methods was not possible, but the zircon fission-track data are permissive of indicating slow

uplift during Laramide time. Additional data would be necessary to corroborate this possibility.

Common lead dating of altered Belt Supergroup

strata and rhyolite in and near the breccia pipe resulted in a

206Pb-238U date of 84±18 Ma. We interpret the alteration

to be related to gold deposition in the breccia pipe. Other

Pb-Pb dating techniques corroborate this date, but are less

precise. Attempts to date hydrothermal sericite from the

rhyolite were unsuccessful, possibly in part because of

chemical treatments that had to be employed during the

mineral-separation process.

Lamprophyre dikes and sills that cut the breccia pipe

are of two distinct groups, one alkalic and the other subalkalic. The alkalic group is predominately alkali gabbro

that is metaluminous, very potassic, and has little iron

enrichment. The sub-alkalic group is gabbro-diorite that is

metaluminous very potassic, and very Mg rich. The lamprophyre bodies are heterogeneous, show wide variation in

phenocryst mineral percentages, and range from CO2 poor

to CO2 rich (0.2–20 weight percent CO2). Primary igneous

textures are generally well preserved. The alkalic lamprophyre averages 3.8 percent K2O, 0.9 percent TiO2, 87 ppm

Ni, and 337 ppm Cr. The sub-lamprophyre averages 3.1

percent K2O, 0.64 percent TiO2, 250 ppm Ni, and 640 ppm

Cr. Phenocrysts (in decreasing order of abundance) in both

groups are clinopyroxene (diopsidic augite), olivine (about

Fo85–90), and much lesser amounts of biotite that has elevated concentrations of Cr, Ni, V, and Ba. Magnesium

numbers indicate that the CO2-poor, sub-alkalic lamprophyres represent primitive magmas that are somewhat

enriched in incompatible elements. The two groups are

interpreted to be cogenetic, but they possibly were derived

from different degrees of partial melting or different source

regions in the mantle.

Extensive CO2 metasomatism of both groups resulted in

destruction of olivine phenocrysts and creation of an alteration assemblage dominated by magnesite. CO2-rich lamprophyre has olivine relicts that are replaced by: (1) limonite(?)

and magnesite, (2) magnesite, sericite, and minor biotite, and

(3) magnesite. Skeletal diopsidic augite is altered to dolomite

and minor amounts of magnesite. Quartz is present in one

sample as a breakdown product of augite but is absent in all

other rocks. Biotite phenocrysts are mostly unaltered but are

strongly embayed by groundmass minerals. Much biotite in

the groundmass is unaffected, but some is replaced by sericite. Where carbonate metasomatism is extreme (10–20 percent CO2), clinopyroxene is completely replaced by

dolomite, magnesite, and minor siderite. Sanidine in the

groundmass may or may not be replaced by sericite. Metasomatic alteration of the lamprophyres involved introduction of

CO2, and depletion of CaO, Fe2O3, Na2O, Al2O3, SiO2, and

FeO, in decreasing percentages.

The two groups of CO2-poor lamprophyres have similarly shaped chondrite-normalized rare-earth-element (REE)

patterns, but the alkalic group plots above and distinctly separate from the sub-alkalic group. With increasing concentration of CO2, lamprophyres retain their La/YbCN values, but

possess overall lower concentration of all REE. This apparent decrease may be simply a function of the most olivinerich lamprophyres having originally had the lowest concentrations of REE. True mobility of rare earth elements during

CO2 metasomatism cannot be documented.

The lamprophyres possibly began to crystallize at

depths between 24 and 41 km and temperatures of about

1,210°–1,270°C (Swanson, 1989). Initial crystallization

involved precipitation of olivine and clinopyroxene followed by phlogopite-biotite. Strong zoning of clinopyroxene suggests highly variable physico-chemical conditions

during crystallization, which may have been caused by

rapid ascent of the magma during clinopyroxene crystallization. Crystallization of the fine-grained groundmass took

place during or after dike emplacement because no subsolidus deformation or flow foliation is noted. Textural evidence indicates that CO2 metasomatism during and after(?)

dike emplacement was caused by an alkali-rich, CO2-rich

fluid that was out of equilibrium with early-formed phenocrysts, but in equilibrium with sanidine, biotite, and apatite

in the groundmass. The extent to which lamprophyre bodies

were CO2 metasomatized was controlled by the percentage

of olivine and clinopyroxene phenocrysts that were available to be altered. The CO2-rich fluid is presumed to be of

magmatic origin and related to generation of the lamprophyres from mantle materials. 

Two biotite separates from different lamprophyre bodies have a conventional K-Ar date of 79.8±2.8 Ma and a

40Ar-39Ar plateau date of 76.9±0.5 Ma. We interpret the

76.9-Ma date to indicate lamprophyre emplacement in Late

Cretaceous time. Whether or not the generation of CO2-rich

lamprophyre magma from mantle depths had any impact on

the generation of the Golden Sunlight breccia pipe cannot be

proven without additional isotopic data.

Some volcanic rocks in southwestern Montana bear

close chemical similarity to both rhyolite and lamprophyre at

the Golden Sunlight mine. Rhyolitic ash flow tuffs in the

Elkhorn Mountains Volcanics, especially informally named

units 7, 9, and 11 (Rutland, 1985), are very similar chemically to the least altered rhyolite at the mine. The estimated

age of The Elkhorn Mountains Volcanics of about 75 to 80

Ma is in agreement with the estimated age of the rhyolite at

the mine. The Adel Mountains Volcanics are slightly more
REFERENCES CITED 47

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77-Ma age of biotite from the lamprophyre. Informally

named mafic unit 8 of the Elkhorn Mountains Volcanics is

also very similar to both alkalic and sub-alkalic lamprophyre

at the mine. If the correlation of unit 8 and the lamprophyre

at the mine is valid, unit 8 may prove to be a time line for the

Elkhorn Mountains Volcanics.

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Published in the Central Region, Denver, Colorado

Manuscript approved for publication June 17, 1996

Edited by Richard W. Scott, Jr.

Photocomposition by Carol A. Quesenberry

Graphics by Ed DeWitt

Online composition by Joan Nadeau, Carol Quesenberry,

Jim Hoffman, and Rob Wells