Ages of Selected Intrusive Rocks ana 

Associated Ore Deposits in the 

Colorado Mineral Belt

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Ages of Selected Intrusive Rocks and 

Associated Ore Deposits in the 

Colorado Mineral Belt

By Charles G. Cunningham, Charles W. Naeser, Richard F. Marvin, Robert G. Luedke, 

and Alan R. Wallace

U.S. GEOLOGICAL SURVEY BULLETIN 2109

Sixty-three fission-track and K-Ar age determinations help understand 

the temporal and spatial evolution of the Colorado Mineral Belt

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director

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.

Published in the Eastern Region, Reston, Va. 

Manuscript approved for publication August 2, 1994.

Library of Congress Cataloging in Publication Data

Ages of selected intrusive rocks and associated ore deposits in the Colorado Mineral Belt /

by Charles G. Cunningham ... [et al.]. 

p. cm. (U.S. Geological Survey bulletin ; 2109) 

Includes bibliographical references. 

Supt.ofDocs.no.:! 19.3:2109 

1. Rocks, Igneous Colorado. 2. Intrusions (Geology) Colorado. 3. Radiocarbon

dating Colorado. 4. Ore deposits Colorado. I. Cunningham, Charles G. II. Series. 

QE75.B9 no. 2109 

[QE461] 

557.3 s dc20

[553'.1'09788] 94-32991

CIP
CONTENTS

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

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

Present Study .................................................................................................................. 1

Dated Igneous Centers and Associated Mineral Deposits.............................................. 2

Ute Mountains..............................................................------......-......-....."-----......--... 6

La Plata Mountains................................................................................................. 8

Rico......................................................................................................................... 9

Chicago Basin Stock............................................................................................... 9

Ouray....................................................................................................................... 9

Porphyry Creek and Tomichi Creek Stocks............................................................ 16

Tincup Stock........................................................................................................... 16

Round Mountain Stock........................................................................................... 17

Italian Mountain Intrusive Complex....................................................................... 17

Mount Sopris Stock................................................................................................. 17

Leadville .....................................................'............................................................ 17

West Tennessee Creek Stock................................................................................... 18

West Cross Creek Stock:......................................................................................... 18

East Lake Creek Stock...,....:................................................................................... 18

Fulford Stock.......................................................................................................... 18

Montezuma Stock................................................................................................... 18

Empire Stock........................................................................................................... 19

Apex Stock.!............................................................................................................ 19

Eldora Stock............................................................................................................ 19

Jamestown Stocks................................................................................................... 20

Discussion of the Ages of Intrusive Activity.................................................................. 20

Laramide Igneous Centers ...................................................................................... 20

Middle Tertiary Igneous Centers............................................................................. 21

Late Tertiary Igneous Centers................................................................................. 21

Discussion of the Ages of Mineralization....................................................................... 21

Laramide Ore Deposits........................................................................................... 22

Middle Tertiary Ore Deposits ................................................................................. 23

Late Tertiary Ore Deposits...................................................................................... 23

Conclusions..................................................................................................................... 24

Acknowledgments........................................................................................................... 25

References Cited............................................................................................................. 26

FIGURES

1. Index map of the Colorado Mineral Belt, showing its relationship to the San Juan volcanic field, Rio Grande rift,

and -300-milligal Bouguer gravity anornaly .................................................................................................................. 2

2. Late Cretaceous and early Tertiary (Laramide) intrusions, Colorado............................................................................. 10

3. Middle Tertiary intrusions, Colorado.............................................................................................................................. 12

4. Late Tertiary intrusions, Colorado................................................................................................................................... 14

TABLES

1. Fission-track and K-Ar analytical data............................................................................................................................ 3

2. Sample locations and descriptions .................................................................................................................................. 7

ill

Ages of Selected Intrusive Rocks and Associated Ore Deposits

in the Colorado Mineral Belt

By Charles G. Cunningham, Charles W. Naeser, Richard F. Marvin, Robert G. Luedke,

and Alan R. Wallace

ABSTRACT

The relationships between igneous rocks and temporally and spatially related ore deposits have become better 

understood with improved geochemical and geochronologic 

technologies. Sixty-three age determinations, by fissiontrack and K-Ar techniques, are combined here with the 

results of recent geochemical, isotopic, geochronologic, and 

tectonic studies by many authors to better understand the 

processes that formed the intrusive rocks and related ore 

deposits that constitute the Colorado Mineral Belt (CMB). 

The results of this study have helped to (1) document the 

temporal and spatial evolution of magmatic activity along 

the CMB, (2) recognize multiple igneous and mineralization 

events at individual centers, and (3) understand the tectonicmagmatic processes that formed the igneous centers and 

mineral deposits. These age determinations show that some 

igneous centers, such as the Ute Mountains and some of the 

stocks in the northern Sawatch Range, are Laramide (Late 

Cretaceous and early Tertiary) and have no known associated mineral deposits. Some igneous centers and their associated mineral deposits, such as the La Plata Mountains and 

some stocks in the northeastern part of the CMB, are also 

Laramide. Other igneous centers contain Laramide rocks, 

but spatially associated mineral deposits are of middle Tertiary (Leadville) or late Tertiary (Rico) age and are clearly 

related to younger intrusions (generally rhyolites) at the 

center. Some middle Tertiary stocks dated as part of this 

study (Mount Sopris) have little, or no, associated mineral 

deposits, whereas others of similar age (Italian Mountain 

Intrusive Complex) have genetically related mineral 

deposits.

Other results of this study include (1) recognition of a 

major paleothermal anomaly "bull's-eye" near the silverbase metal deposits of Rico that helped lead to the discovery 

of a major molybdenum deposit, (2) documentation of the 

progression of igneous activity northeastward along the 

CMB at a rate of about 4 cm/yr, (3) documentation of the 

existence of a Laramide northwest-trending group of stocks 

that crosses the CMB, and (4) recognition of 45-Ma igneous 

activity at Jamestown, the northernmost igneous center in

the CMB, that occurred during a generally quiescent period 

between the Laramide and middle Tertiary igneous activity.

INTRODUCTION

The Colorado Mineral Belt (CMB) is defined by a 

group of Late Cretaceous and early Tertiary (Laramide), 

middle Tertiary (approximately Eocene to middle Miocene), 

and late Tertiary intrusive centers and associated hydrothermal ore deposits that trends northeast across Colorado (fig. 

1). The southwestern end is located in the Ute Mountains 

near the Four Corners area, and the northeastern end is 

located in the Front Range near Jamestown, about 50 km 

northwest of Denver. The belt contains most of the major 

mining districts in Colorado and has produced at least $4 

billion in metals, principally molybdenum, gold, silver, copper, lead, and zinc.

Improved fission-track and K-Ar methods have better 

defined the chronology of CMB igneous and hydrothermal 

events. The ages reported here have economic as well as 

tectonic and petrogenetic significance. These data indicate 

that, in some areas, ages of intrusive and hydrothermal 

activity are different than previously assumed, thus changing genetic interpretations.

PRESENT STUDY

This report presents the results of 63 age determinations from intrusive rocks in and near the CMB. These 

determinations began in the 1970's as a part of a study to 

see if there was a pattern in the spatial distribution or chemical evolution of CMB rocks that might correlate with ore 

deposits in the belt. The data herein are a combination of 

information collected for this report as well as data that 

were previously released in U.S. Geological Survey publications (Cunningham and Naeser, 1975; Naeser and Cunningham, 1976; Cunningham and others, 1977).

Zircon fission-track (ZFT), apatite fission-track (AFT), 

sphene fission-track (SFT), and K-Ar dates were acquired 

from the same samples to the extent possible. Fission tracks
2 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT

COLORADO

Glenwood Springs

Boulder

Grand Junction

i

L_

EXPLANATION

Present extent of the San Juan i 

volcanic field. From Steven I 

and Lipman (1976).

Rio Grande rift. 

(1990).

From Bookstro1

Negative Bouguer gravity anomaly; 

edge is -300 milligal contour. 

From Behrendt and Bajwa 

(1974).

Boundary of Colorado Mineral 

Belt as defined by principal ' 

mining districts of Late Cretaceous ' 

and early Tertiary (Laramide) age. 

From Tweto and Sims (1963).

Maximum boundary of Colorado 

Mineral Belt as defined by Tweto 

and Sims (1963).

Figure 1. Index map of the Colorado Mineral Belt, showing its relationship to the San Juan volcanic field, Rio Grande rift, and 

-300 milligal Bouguer gravity anomaly. From Tweto and Sims (1963), Steven and Lipman (1976), Bookstrom (1990), and Behrendt 

and Bajwa (1974).

are stable over different temperature intervals for different 

minerals, and, by dating several different minerals from a 

rock, it is possible to obtain a better understanding of its 

thermal history. This is especially important for deciphering 

settings where multiple hydrothermal events have been 

superimposed upon older rocks. Older fission-track and KAr dates were recalculated by using current constants and 

statistical methods (Dalrymple, 1979; McGee and others, 

1985). The data are presented in two tables. Table 1 gives 

fission-track and K-Ar analytical data for the samples, and 

table 2 gives sample locations and descriptions. Figures 2, 

3, and 4 show the distribution of major Late Cretaceous and 

early Tertiary (Laramide), middle Tertiary, and late Tertiary 

intrusive rocks in Colorado and are keyed to the tables by 

geologic and geographic name and letter symbols.

This report coincides with the publication of a 

1:1,000,000-scale map showing the distribution, composition, and age of early and middle Cenozoic volcanic centers 

in Colorado and Utah (Luedke, 1993). A corresponding 

map of late Tertiary centers has already been published 

(Luedke and Smith, 1978). This report complements the

comprehensive compilation on Luedke's 1993 map by providing analytical data and further interpretation.

There are many compilations of age determinations for 

Colorado igneous rocks. Major compilations include Young 

(1972), Marvin and others (1974), Stein (1985), Mutschler 

and others (1987), Davis and Streufert (1990), Bookstrom 

(1990), and Wallace (in press). Luedke's 1993 map is the 

most recent and comprehensive of the entire CMB.

DATED IGNEOUS CENTERS AND 

ASSOCIATED MINERAL DEPOSITS

The intrusive rocks of the CMB and vicinity have a 

distribution pattern related to age of emplacement that provides insight into the evolution of structural settings and 

magmatic processes. Readers are referred to Tweto's 

(1979a) geologic map of Colorado and Luedke's (1993) 

map compilation of ages and compositions for the ensuing 

discussion. A time boundary between the Late Cretaceous 

and early Tertiary (Laramide) rocks and the middle Tertiary
DATED IGNEOUS CENTERS AND ASSOCIATED MINERAL DEPOSITS

Table 1. Fission-track and K-Ar analytical data.

[Numbers in parentheses are number of tracks counted. Values for constants used in calculating ages are as follows: A.F=7.03xlO~ l7 yr'; 40KA,e=0.581x 

10-iOyr-i; Xp=4.962xlO-|0yr'; ^K/K^l.ieVxlQ-4. , no data. Note: The fission-track ages were all determined prior to 1988. The recommendations of 

Hurford (1990) could not be followed because a zeta calibration was not in place when these ages were determined.]

Sample Lab. ... , i 

r Mineral 'ps 

no. no.

2 pi 3 6X10 15 U K20 4 *4°Ar *4°Ar Age 

v (ppm) (wt. percent) (10 lomol/g) (percent) m.y.±2a

Apex stock, map symbol A

A-l DF-1188 zircon 7.10

(1414)

A-l DF-1189 apatite 0.428 

(99)

A-2 DF-1190 zircon 8.00

(1926)

A-2 DF-1191 apatite 0.285 

(594)

7.20

(717)

0.700 

(162)

8.47

(1020)

0.498 

(1037)

1.05 220 61.7±6.3

1.96 11.4 71.3±18.0

1.04 260 58.515.2

1.96 8.1 66.8±7.5

Chicago Basin stock, map symbol CB

CB-1 DF-908 zircon 1.46

(494)

CB-2 DF-907 zircon 1.87

(480)

9.01

(1522)

13.30

(1688)

1.07 270 10.411.2

1.08 390 9.2±1.0

Eldora stock, map symbol E

E-l DF-934 zircon 7.49

(1560)

E-l DF-1000 zircon 9.31

(1293)

E-l DF-942 apatite 0.168 

(311)

8.50

(885)

10.67

(741)

0.200 

(370)

1.27 210 66.616.4

1.19 290 61.816.3

1.17 5.5 58.619.4

East Lake Creek stock, map symbol EL

EL-1 DF-848 sphene 10.30 

(2564)

EL-1 DF-849 zircon 9.87

(777)

5.50 

(687)

8.77

(345)

0.587 300 65.216.3

0.887 320 59.518.1

Empire stock, map symbol EM

EM-1 DF-932 sphene 11.2 

(1872)

EM-1 DF-941 apatite 0.847 

(1765)

11.4 

(950)

1.568 

(3266)

1.16 315 68.016.2

1.16 43 ' 37.412.8

Fulford stock, map symbol F

F-l DF-1161 zircon 6.21

(si 379)

F-2 D2498B biotite 

6.08

(675)

1.07 180 65.116.8

 8.39 7.635 94 62.211.5

8.38

Italian Mountain Intrusive Complex, map symbol I

1-1 DF-655 zircon 4.31

(694)

8.72

(702)

1.15 240 33.913.9
4 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT 

Table 1. Fission-track and K-Ar analytical data Continued.

Sample Lab. ... . K Mineral 

no. no.

'ps 2 pi 3(1)X10 15 U K20 4 *4°Ar *4°Ar Age 

v (ppm) (wt. percent) (1010mol/g) (percent) m.y.±2o

Jamestown stocks, map symbol JS

JS-1 D2419B biotite

JS-2A 41 FT. zircon

JS-2B 3361 FT. zircon

JS-3 DF-3177 zircon

JS-3 DF-3178 apatite

8.92

(661)

7.32

(351)

4.2

(462)

0.377 

(785)

13.06

(484)

10.56

(253)

5.4 '

(299)

0.477 

(993)

 8.55 7.390 88 58.8±2.0

8.64

1.10 750 44.8±5.7

1.09 400 45.1±7.7

1.05 148 48.4±7.5

1.00 15 , 47.1±5.8

Leadville, map symbol L

L-l D2642S sanidine

L-2 DF-1445 zircon

L-3 DF-4151 zircon

 '-..

13.8

(1343)

4.82

(491)

 

20.2

(984)

8.21

(418)

 11.34 6.360 91 38.5±0.6

1.15 510 . 46.8±4.4

0.993 260 34.8±4.9

La Plata Mountains, map symbol LP

LP-2 DF-993 zircon

LP-3 DF-994 zircon

LP-3 DF-995 apatite

LP-3 D2541B biotite

LP-4 DF-996 zircon .

LP-4 DF-997 apatite

LP-5 DF-998 zircon

LP-5 DF-999 apatite

LP-5 D2542B biotite

12.90

(2508)

8.81

(1916)

1.58 

(1466)

6.52

(1238)

0.139 

(129)

18.70

(1040)

1.06 

(978)

12.2

(1188).

8.57

(932)

1.36 

(1240)

 

7.08

(672)

0.110 

(102)

17.50

(487)

1.31 

(1217)

1.09 360 68.5+5.7

1.09 250 66.7±6.1

1.05 41 73.9±6.6

 8.94 8.915 92 67.8±1.6

8.98

1.09 210 .._...._ _ 59.8+6.3

1.05 3.3 79.0121.0

1.09 510 69.318.2

1.05 40 50.314.9

 8.79 8.682 94 67.511.6

- 8.76

Montezuma stock, map symbol M

M-l DF-674 zircon 7.85

(1235)

16.76

(1319)

1.25 430 35.013.2

Mount Sopris stock, map symbol MS

MS-1 DF-1104 zircon

MS-1 D2552B biotite

3.34

(664)

 .

6.50

(647)

. 1.12 190 34.3+4.1

 8.30 4.108 87 34.210.8

8.25
DATED IGNEOUS CENTERS AND ASSOCIATED MINERAL DEPOSITS 

Table 1. Fission-track and K-Ar analytical data Continued.

Sample Lab. ,.,,. , i 

r Mineral 'ps 

no. no. r

2 pi 3<|>xl0 15 U K20 4 *4°Ar *4°Ar Age 

(ppm) (wt. percent) (10'°mol/g) (percent) m.y.±2o

Ouray, map symbol O

O-l DF-1149 zircon 3.25

(346)

O-l DF-1148 apatite 0.011 

(22)

O-2 DF-1116 zircon 8.06

(1865)

O-2 DF-1117 apatite 0.017 

(35)

5.05 1.06

(269)

0.093 1.14 

(193)

9.81 1.10

(1135)

0.060 0.89 

(125)

150 40.7±6.9

2.6 7.8±3.5

280 54.1 ±4.0

2.1 14.915.7

Porphyry Creek stock, map symbol PC

PC-1 DF-1152 zircon 4.19

(911)

PC-1 D2569B biotite 

7.45 1.05

(810)

 

230 35.3±3.3

 8.92 4.961 84 38.4±0.9

8.86

Rico, map symbol R

R-l DF-1166 zircon 4.78

(1174)

R-l DF-1168 apatite 0.016 

(34)

R-l DF-1167 sphene 8.46 

(1645)

R-l D2573PY pyroxene 

R-2 DF-1146 zircon 9.89

(2151)

R-2 DF-1145 apatite 0.017 

(36)

4.86 1.04

(596)

0.169 1.14 

(353)

12.15 1.89 

(1181)

 

9.72 1.07

(1058)

0.038 1.14 

(121)

150 61.016.7

4.8 6.612.4

210 78.316.9

 0.118 0.1917 68 5 11012 

0.116

290 64.815.6

1.6 20.317.4

Round Mountain stock, map symbol RM

RM-1 DF-1160 zircon 2.16

(489)

RM-1 DF-1147 apatite 0.025 

(52)

RM-1 D2567B biotite 

11.74 1.05

(1332)

0.173 1.14 

(361)

 

360 11.511.3

4.9 9.812.9

 8.66 1.736 62 13.910.3

8.62

Tincup stock, map symbol T

T-l DF-1150 zircon 5.67

(1208)

T-l DF-1151 epidote 0.708 

(177)

10.44 1.06

(1112)

3.91 3.46 

(489)

310 34.413.2

36 . 37.416.8

Tomichi Creek stock, map symbol TC

TC-1 DF-1159 zircon 4.85

(1213)

10.36 1.06

(1295)

310 29.612.7
6 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT 

Table 1. Fission-track and K-Ar analytical data Continued.

Sample 

no.

Lab. ... . Mineral 

no.

'ps 2pi . I « KoO 4 *40Ar *40Ar Age 

4>xlO U(ppm) (wt pe^t) (10'°mol/g) (percent) m.y.±2o

Ute Mountains, map symbol U

U-l

U-2

U-2

U-3

U-3-

DF-1073 zircon

DF-1076 zircon

DF-1075 apatite

DF-1077 zircon

D2548B biotite

10.3

(2377)

6.18

(1258)

1.96

(490)

7.60

(1232)

8.26

(956)

4.49

(456)

7.19

(899)

5.34

(433)

0.867 300 64.2±5.7

0.862 170 70.7±8.3

4.12 50 66.8±8.1

0.852 200 72.1±8.6

 8.41 8.898 76 72.0±1.7

8.41

West Cross Creek stock, map symbol WC

WC-1

WC-1

DF-850 zircon

D2499B biotite

4.36

(1130)

3.89

(504)

0.887 140 59.2±6.9

 8.45 7.668 95 61.8+1.4

8.51

West Tennessee Creek stock, map symbol WT

WT-1

WT-1

DF-1164 apatite

D2568B biotite

0.06

(143)

0.100

(209)

1.14 2.8 46.5±10.3

 8.55 8.319 87 66.4±2.2

8.54

'ps = tracks/cm2 fossil, xlO6. 

2pi = tracks/cm2 induced, xlO6.

3 <|> = neutrons/cm2.

4 *40Ar_ radiogenic argon.

K-Ar age is too old.

rocks is placed at 40 Ma by Tweto and 58 Ma by Luedke; 

the latter, being the Paleocene-Eocene boundary, was 

selected for the boundary between intrusions shown on figures 2 and 3, and 16 Ma was selected for the corresponding 

boundary between figures 3 and 4.

The general northeast trend of igneous rocks and associated ore deposits of the CMB has been described by many, 

especially Burbank and Levering (1933), Lovering and 

Goddard (1938, 1950), Tweto and Sims (1963), and Tweto 

(1968). The Precambrian ancestry of the belt was documented by Tweto and Sims (1963), and Warner (1978) 

showed that the belt was part of a larger northeast-trending 

Precambrian wrench fault system he called the Colorado 

lineament. In the central part of the State, a northwesttrending group of Laramide stocks crosses the CMB. This 

group extends for a distance of about 30 km from the West 

Tennessee Creek stock, 15 km northwest of Leadville, 

across the northern end of the Sawatch Range, and ends at 

the Fulford stock. Wallace and Naeser (1986) noted that 

part of this trend may be related to Laramide uplift.

Some of the igneous centers in the CMB have only one 

age of igneous and hydrothermal activity. Other centers,

however, have superimposed igneous and hydrothermal 

events as recorded by partly reset ages. In this report, the 

centers are discussed geographically to emphasize the effect 

and importance of the overprinting of younger Tertiary 

events on igneous centers that were previously considered 

to be older, mainly Laramide. We will sequentially describe 

the CMB from the southwest to the northeast, including the 

northwest-trending group of stocks at the point it crosses 

the main trend. Figures 2, 3, and 4 present the data in geochronological sequence.

UTE MOUNTAINS

The Ute Mountains (fig. 2) are underlain by stocks, 

laccoliths, and sills emplaced mostly into the Upper Cretaceous Mancos Shale. They range in composition and texture 

from microgabbro to quartz monzonite porphyry and have 

recently been mapped by Condon (1991). All major igneous 

rock units, dated for this study, range from 72.1±8.6 Ma to 

64.215.7 Ma; the geologically reasonable ages, based on 

field relationships and annealing characteristics of the
DATED IGNEOUS CENTERS AND ASSOCIATED MINERAL DEPOSITS

CB

Table 2. Sample locations and descriptions.

Map 

symbol

A Apex stock

A-l: 39°52'03"N., 105°33'18"W.; collected by Robert 

Barker (sample no. R-138) from trench that cuts 

the Indianapolis quartz-Mo vein, Central City 

quadrangle, Gilpin County, Colo. Fine-grained 

hornblende-biotite quartz monzonite that crops out 

over a wide area.

A-2: 39°52'03"N., 105°33'18"W.; same location as A1. Collected by Robert Barker (sample no. R-139). 

Medium-grained hornblende-biotite quartz monzonite present as dikes that intrude A-l in the vicinity 

of the Nye Metals adit.

Chicago Basin stock

CB-1: 37°36'13"N., 107°36'43"W.; collected by Peter 

Holland (sample no. 217) south of Needle Creek, 

La Plata County, Colo. Quartz-sanidine granite 

porphyry that forms a composite stock that is spatially related to base- and precious-metal bearing 

veins.

CB-2: 37°36'09"N., 107°36'30"W; collected by Peter 

Holland (sample no. 137). Sanidine-plagioclasebiotite quartz rhyolite porphyry that intrudes center 

of stock CB-1.

Eldora stock

E-l: 39°57'06"N., 105°35'16"W.; roadcut 0.5 km west 

of the town of Eldora, Boulder County, Colo. Also 

called the Bryan Mountain stock. Biotite-hornblende-pyroxene quartz monzonite.

East Lake Creek stock -. EL-1: 39°28'36"N., 106°33'20"W.; collected by Ogden 

Tweto, 2.8 km east of Gold Dust Peak, Eagle 

County, Colo. Plagioclase-hornblende granodiorite.

EM Empire stock

EM-1: 39°45'48"N., 105°43'20"W.; roadcut along U.S. 

40, 3.4 km west of Empire, Clear Creek County, 

Colo. Hornblende-pyroxene monzonite.

F Fulford stock

F-l: 39°31'36"N., 106°39'24"W.; sec. 14, T. 6 N., R. 83 

W.; Roadcut 0.8 km east of Triangle Reservoir in 

southeast part of stock. Eagle County, Colo. 

Medium-grained, equigranular, quartz monzonite.

F-2: 39°31'40"N., 106°39'28"W.; Collected by Odgen 

Tweto, 0.1 km northwest of F-l. Eagle County, 

Colo. Medium-grained, equigranular quartz 

monzonite.

I Italian Mountain Intrusive Complex

1-1: 38°56'52"N., 106°45'11"W.; 0.4 km north of Italian 

Mountain near the base of the cirque, Gunnison 

County, Colo. Same as sample 1-368 (Cunningham, 1976). Biotite-sanidine-quartz monzonite 

that is the youngest intrusion.

JS Jamestown stocks

JS-1: 40°07'N., 105°24'W.; sec. 25, T. 2 N., R. 72 W., 

Boulder County, Colo. Biotite-fluorite vein cutting 

granodiorite stock.

EL

Map 

symbol

JS-2A: 40°07'12"N., 105°23'15"W.; sec. 19, T. 2 N., R. 71 

W., Boulder County, Colo. Drill core from vertical 

hole in sodic granite Porphyry Mountain stock. 

Sampled 12.5 m below ground surface.

JS-2B: 40°07'12"N., 105°23'15"W.; same location and 

drill core as JS-2A. Sampled 1,024 m below 

ground surface.

JS-3: 40°06'40"N., 105°23'30"W.; collected by William 

Lynch. Jamestown granodiorite stock.

Leadville

L-l:

L-2:

39°15'03"N., 106°14'45"W.; collected by Ogden 

Tweto from dump of the Big Six mine, Eureka 

pipe, Lake County, Colo. Rhyolite porphyry intrusion that cuts the ore.

LP

M

39°15'18"N., 106°14'28"W.; collected by Ogden 

Tweto from White Cloud mine. Quartz monzonite 

considered to be Evans Gulch Porphyry by 

Thompson and Arehart (1990).

L-3: 39°15'00"N., 106°14'28"W.; collected by Ogden 

Tweto (sample no. 90T53) from dump of prospect 

shaft on low spur between Johnson Gulch and 

Evans Gulch. Johnson Gulch Porphyry. Samples 

L-l, L-2, and L-3 are all located about 4 km,east of 

the town of Leadville, Colo.

La Plata Mountains

LP-2: 37°24'15"N., 108°05'26"W.; 0.3 km southwest of 

the Allard mine, Montezuma County, Colo. 

Slightly altered syenite of the Allard stock.

LP-3: 37°23'45"N., 108°04'32"W.; first switchback on 

road up Bedrock Creek on west side of the La Plata 

River, La Plata County, Colo. Fresh biotite-syenite 

of the Allard stock.

LP-4: 37°26'13"N., 108°00'55"W.; 0.4 km southwest of 

Snowstorm Peak, Montezuma County, Colo. 

Monzonite sill.

LP-5: 37°25'28"N., 108°02'38"W.; roadcut at road fork 

between Darby Creek and Basin Creek on west 

side of La Plata River, La Plata County, Colo. 

Fine-grained biotite-granodiorite.

Montezuma stock

M-l: 39°32'02"N., 105°49'33"W.; collected by George 

Neuerburg (sample no. HI200) 0.6 km northeast of 

Webster Pass, Summit County, Colo. Porphyritic 

quartz monzonite from small stock about 5 km 

south of the main body of the Montezuma stock.

MS Mount Sopris stock

MS-1: 39°16'13"N., 107°08'43"W.; sec. 18, T. 9 S., R. 87 

W., 0.3 km southwest of the northernmost Thomas 

Lakes, Pitkin County, Colo. Medium-grained 

biotite quartz monzonite.

O Ouray

O-l: 37°59'05"N., 107°42'38"W.; roadcut in switchback 

toward the south on the road to the Camp Bird 

mine from Ouray, near Senator Gulch, Ouray 

County, Colo. Slightly propylitically altered porphyritic granodiorite sill.
8 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT

Table 2. Sample locations and descriptions Continued.

Map 

symbol

O-2: 37°59'40"N., 107°42'14"W.; west side of the canyon wall at elevation 2,840 m on the Weehawken 

pack trail, 0.4 km west of Thistledown, Ouray 

County, Colo. Slightly propylitically altered porphyritic granodiorite sill.

Porphyry Creek stock

PC-1: 38°28'22"N., 106°24'14"W.; sec. 27, T. 49 N., R. 5 

E., 0.2 km east of where Porphyry Creek passes 

under the highway to Whitepine, Gunnison 

County, Colo. Biotite rhyolite porphyry with betamorphology quartz phenocrysts.

Rico

PC

RM

R-l:

R-2:

37°41'40"N., 108°03'37"W.; 0.4 km northwest of 

Expectation mine, on Expectation Mountain, 

Dolores County, Colo. Medium-grained augite 

monzonite stock.

37°38'23"N., 108°03'22"W.; roadcut 0.4 km eastnortheast of Montelores Bridge, along the east side 

of the road, Rico, Dolores County, Colo. Hornblende latite porphyry sill.

Round Mountain stock

RM-1: 38°46'26"N., 106°51'32"W.; about halfway up 

Round Mountain, 0.8 km southwest of the summit, 

Gunnison County, Colo. Biotite rhyolite.

Tincup stock

T-l: 38°40'39"N., 106°30'34"W.; sec. 15, T. 51 N., R. 4 

E., 0.2 km west of the summit of Green Mountain, 

Gunnison County, Colo. Epidote-bearing quartz 

monzonite porphyry with partly resorbed betamorphology quartz phenocrysts.

Map 

symbol

TC Tomichi Creek stock

TC-1: 38°27'14"N., 106°24'22"W.; sec. 4, T. 48 N., R. 5 

E.; roadcut on road to Whitepine, opposite Cow 

Creek, Gunnison County, Colo. Medium-grained 

biotite-augite rhyodacite.

U Ute Mountains

U-l: 37°14'25"N., 108°46'35"W.; sec. 36, T. 35 N., R. 

18 W.; roadcut alongside Cottonwood Creek, at the 

east end of the Sun Dance Ground, Montezuma 

County, Colo. Hornblende diorite porphyry containing amphibolite fragments.

U-2: 37°16'47"N., 108°46'51"W.; sec. 24, T. 35 N., R. 

18 W.; southwest side of Ute Peak, Montezuma 

County, Colo. Hornblende granodiorite porphyry,

U-3: 37°15'52"N., 108°48'41"W.; sec. 27, T. 35 N., R. 

18 W.; small roadcut at the southwest edge of Ute 

Peak, Montezuma County, Colo. Biotite quartz 

monzonite porphyry.

WC West Cross Creek stock

WC-1: 39°25'40"N., 106°32'15"W.; collected by Ogden 

Tweto in unsurveyed sec. 21, T. 75 N., R. 82 W., 

from the ridge between West Cross Creek and 

Cross Creek, Eagle County, Colo. Biotite monzogranite.

WT West Tennessee Creek stock

WT-1: 39°20'48"N., I06°25'05"W.; 0.5 km southeast of 

the Homestake mine, near the south end of the 

stock, Pitkin County, Colo. Fine-grained biotite 

quartz monzogranite.

various minerals dated, are about 72 Ma (tables 1 and 2). 

The diorite porphyry (sample U-l) is the most abundant 

igneous rock and contains prominent xenoliths of amphibolite, which led to previous, apparently spurious, older dates 

by K-Ar methods (for example, Armstrong, 1969). An Oligocene group of small, minette stocks and dikes is exposed 

about 30 km southeast of the Ute Mountains (Condon, 

1991). No mineral deposits are known to be associated with 

igneous rocks in the Ute Mountains.

LA PLATA MOUNTAINS

The La Plata Mountains (fig. 2) contain a wide variety 

of intrusive rocks, of approximately the same age, that form 

stocks, sills, and dikes. The most abundant rocks form porphyritic monzonitic sills represented by sample LP-4 

(79.0±21.0 Ma, AFT; 59.8+6.3 Ma, ZFT). Nonporphyritic 

intrusions range in composition from syenite to diorite. 

Samples LP-2 (68.5±5.7 Ma, ZFT) and LP-3 (73.9±6.6 Ma, 

AFT; 67.8±1.6 Ma, biotite K-Ar; 66.7±6.1 Ma, ZFT) are 

from the Allard syenite stock, and LP-5 (69.3±8.2 Ma,

ZFT); 67.5±1.6 Ma, biotite K-Ar; 50.3±4.9 Ma, AFT) is 

from one of the two granodiorite stocks. Detailed reports on 

the area include Eckel (1949) and Werle and others (1984).

The relatively close agreement of all but one of the 

dates (table 1) indicates Laramide intrusion of igneous 

rocks at about 67 Ma. Much of the associated mineralization also appears to have taken place in the Laramide. Sample LP-3 is from a syenite stock that has a chalcopyritebearing crackle breccia exposed in Bedrock Creek (Eckel, 

1949), which has been extensively explored by the mining 

industry, and less than 1 km north of this sample location is 

the Copper Hill glory hole. The nonporphyritic syenites and 

diorites crosscut the porphyritic sills (Eckel, 1949). The 

youngest date we obtained (sample LP-5) was from apatite 

in a diorite stock that yields Laramide ages on zircon and 

biotite. This reset apatite date may indicate the local presence of younger magmatic and hydrothermal activity rather 

than regional resetting because apatites in other nearby 

rocks yield Laramide dates.

About $6 million worth of metals was produced from 

mines in this area prior to 1937; gold, the most important 

commodity, was produced from gold- and silver-bearing
DATED IGNEOUS CENTERS AND ASSOCIATED MINERAL DEPOSITS

telluride veins and replacement deposits, but significant 

quantities of silver, lead, and copper also were recovered. 

Mines of the La Plata Mountains have produced some 

unusual ores. In addition to telluride ores, Eckel (1949) 

reported disseminated platinum-bearing chalcopyrite, goldbearing contact ore, veins of base-metal sulfides containing 

silver or native gold, chalcocite veins, and veins of ruby silver. Neubert and others (1991) reported that at least 200 

million tons of 0.4-percent copper, plus minor quantities of 

gold and platinum, are known in deposits at the Allard 

stock. Additional reserves are known at depth. Recent 

(1994) mining has been limited to small-scale production 

along gold-telluride veins in the Bessie G mine.

RICO

The Rico area (fig. 2) contains three principal igneous 

rock types; two are Laramide, and the third is Pliocene. An 

augite monzonite stock (sample R-l; 78.3±6.9 Ma, SFT; 

61.0±6.7 Ma, ZFT; 6.6±2.4 Ma, AFT) was intruded during 

the Laramide along the axis of the west-trending Rico structural dome. At about the same time, many subhorizontal 

hornblende latite sills (sample R-2; 64.8±5.6 Ma, ZFT; 

20.3±7.4 Ma, AFT) were intruded throughout the area. The 

hornblende latite sills principally intrude Lower Permian 

Cutler Formation and are generally located in the vicinity of 

the Rico structural dome. These mapped relations are 

shown in Pratt and others (1969). A third principal igneous 

rock type in the area is rhyolite, which is present as small 

dikes and as a small stock at Calico Peak.

The data in table 1 show that age determinations on 

different minerals from the same sample of Laramide rocks 

give discordant results. The rocks that were intruded during 

the Laramide give Laramide dates on sphene and zircon but 

younger dates on apatite. This difference is due to reheating 

of the Laramide rocks by a major Pliocene thermal event 

that caused partial annealing of fission tracks in the apatite 

(Naeser, 1979; Naeser and others, 1980).

Additional dating of igneous rocks near Rico (Naeser 

and others, 1980) shows that a major Pliocene paleothermal 

anomaly, greater than 11 km diameter, is present. Samples 

collected from Laramide hornblende latite sills give partially reset apatite and zircon dates. When contoured, these 

data form a concentric, younging inward "bull's-eye" of 

apparent ages centered on a paleothermal anomaly, east of 

the town of Rico, that formed at 4 Ma. Naeser and others 

(1980) also obtained 4-Ma dates on mineralized rhyolites 

and predicted the presence and location of a young, unexposed stock having the potential for porphyry-related mineral deposits. Subsequent drilling at the site by Anaconda 

Minerals Company, resulted in discovery of the Silver 

Creek molybdenum deposit 1,200 m below the surface 

(Barrett and others, 1985).

Rico was a major silver and base-metal mining camp 

around the turn of the century; classic reports of the geology 

and ore deposits include Cross and Spencer (1900), Ransome (1901), Cross and Ransome (1905), and McKnight 

(1974). The silver- and base-metal vein and replacement 

deposits form a partial halo surrounding and capping the 

Silver Creek molybdenum deposit and are genetically 

related to it. Sericite, associated with a silver-base metal 

vein along the Blackhawk fault, also gives a K-Ar Pliocene 

age (Naeser and others, 1980).

The Silver Creek molybdenum deposit is large and 

high grade. Approximately 44 million tons of 0.31-percent 

Mo have been indicated by drilling (using a 0.2-percent cutoff), and the deposit has not been totally delineated (Barrett 

and others, 1985). Detailed studies have focused on the porphyry-molybdenum deposit and the changes in the chemistry, fission tracks, isotopes, and mineralogy of the 

hornblende latite sills as a result of the mineralizing event 

(Cunningham and others, 1987; Larson, 1987; Wareham, 

1991; Larson and others, 1994a, 1994b).

CHICAGO BASIN STOCK

The Chicago Basin stock (fig. 4) is a composite rhyolite porphyry stock that intruded Precambrian rocks. It is in 

the center of the Needle Mountains near the southwestern 

edge of the San Juan volcanic field, approximately 40 km 

east of Rico. It was investigated by the U.S. Geological Survey as part of the studies of the San Juan Primitive Area 

(Steven and others, 1969), and studies of the geology and 

ore deposits were reported in Schmitt and Raymond (1977). 

Samples of the older, outer stock (sample CB-1; 10.4±1.2 

Ma, ZFT) and younger, inner stock (CB-2; 9.2±1.0 Ma, 

ZFT) give concordant dates of about 10 Ma.

The Chicago Basin stock is near the center of a radial 

assemblage of veins and a zoned pattern of altered rocks. 

The veins were mined for base and precious metals and 

contain molybdenite in thin quartz veinlets. The stock was 

drilled by Climax Molybdenum Company in the 1970's. If a 

paleothermal anomaly exists, it may have a well-defined 

expression in the surrounding Precambrian rocks that could 

be deciphered by various means (see Cunningham and Barton, 1984).

OURAY

Several groups of igneous rocks are exposed in the 

vicinity of Ouray (fig. 2), the most prominent and important 

of which are granodiorite sills, dikes, and laccoliths. 

Younger volcaniclastic breccias and tuffs and a few dikes 

are related to the Oligocene San Juan volcanic field (Lipman and others, 1973). The granodiorite sills are generally 

concordant and intruded Devonian to Cretaceous sedimentary rocks. The laccoliths were intruded primarily at or near
10

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15
16 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT

the base of the Cretaceous Mancos Shale, and they form 

prominent cliffs on either side of the canyon walls above 

Ouray (Burbank and Luedke, 1964). The sills are Laramide 

in age; zircon from sample O-2 (table 1) gave a 54.1±4.0 

Ma date, which is a minimum age because apatite in the 

same sample shows the rock was reheated. Burbank (1930) 

notes that the sills and laccoliths are overlain by the Eocene 

Telluride Conglomerate, and Luedke and Burbank (1962) 

describe porphyritic granodiorite clasts in the formation. 

Both relations indicate that the igneous rocks are no 

younger than Eocene.

Geochronology reported by Billings (1980) indicates 

the approximate timing of three hydrothermal events in the 

vicinity of Ouray: (1) 27.5 Ma (Lipman and others, 1976) 

following the collapse of the Silverton caldera, (2) approximately 15 Ma, related to the rhyolite at Stoney Mountain, 

and (3) approximately 10 Ma, related to the Camp Bird system. Fission-track ages in both zircon and apatite were reset 

by a large paleothermal anomaly that appears to be related 

to mineralization at the Camp Bird mine. A ZFT date of 

54.1 Ma from sample O-2 may be partly reset, and the zircon date from O-l was even more reset to 40.7±6.9 Ma 

(table 1). Apatites in both of the same samples give discordant, younger dates because of their lower annealing temperatures (Naeser, 1979). The apatite in sample O-2 gives a 

14.9±5.7-Ma date, and apatite in O-l, which is closer than 

O-2 to the Camp Bird mine, was apparently completely 

reset to 7.8±3.5 Ma, which approximates the age of mineralization,of 10.5±0.5 Ma at the Camp Bird mine (Lipman 

and others, 1976).

The Porphyry Creek rhyolite porphyry stock is an 

elongate 1 by 3 km pluton that adjoins the northern edge of 

the Tomichi Creek stock (fig. 3). It was mapped by Raines 

(1971) and Tweto and others (1976). The stock is closely 

associated with and located approximately 6 km south of 

the Mount Princeton batholith, which is the largest of the 

exposed middle Tertiary plutons in the CMB (fig. 3).

Both the Porphyry Creek stock and adjoining Tomichi 

Creek stock have ages similar to those of the Mount Aetna 

cauldron complex. The Mount Aetna complex includes the 

Mount Princeton batholith, which forms a large subcircular 

pluton about 24 by 34 km in diameter and is composed 

mainly of medium-grained quartz monzonite. Principal 

studies of the batholith include those by Crawford (1913), 

Dings and Robinson (1957), Shannon (1988), and Toulmin 

and Hammarstrom (1990). The Mount Aetna volcanic center is located in the southwestern part of the batholith; isotopic dates for this volcanic center indicate igneous activity 

beginning at 40-37 Ma (Toulmin and Hammarstrom, 1990). 

In a detailed study of the Mount Aetna cauldron complex,

Shannon (1988) showed three major magmatic events in the 

south-central Sawatch Range: (1) the emplacement of the 

Mount Princeton pluton at 36.6 Ma; (2) the formation of the 

Mount Aetna cauldron at 34.4 Ma; and (3) chemically 

evolved granites at 29.8 Ma. A date on biotite from the 

Mount Antero Granite, the youngest of seven separate intrusions that make up the Mount Princeton batholith, is 

31.611.1 Ma (Thompson and Pulfrey, 1973). Significant ore 

deposits are not associated with the main batholith intrusion; however, molybdenite and beryllium minerals associated with the Mount Antero Granite have been known for 

many years (Adams, 1953; Sharp, 1976).

Dates on sample PC-1 (tables 1 and 2) from the Porphyry Creek stock are 38.4±0.9 Ma (K-Ar, biotite) and 

35.3±3.3 Ma (ZFT). They are concordant and similar to the 

age of the main quartz monzonite phase of the Mount 

Princeton batholith. The slightly younger date on the zircon 

may reflect partial resetting by the adjacent, younger 

Tomichi Creek stock or the effect of cooling and uplift time.

The Tomichi Creek rhyodacite stock, about 2 km in 

diameter and adjoining the southern margin of the Porphyry 

Creek stock, is approximately 8 km south of the Mount 

Princeton batholith. The zircon date of 29.6±2.7 Ma (sample TC-1, table 1) is similar to the age of the chemically 

evolved granites of 29.8 Ma, the youngest intrusions in the 

Mount Aetna cauldron complex.

TINCUP STOCK

The Tincup stock (fig. 3) is one of several plutons of 

quartz monzonite porphyry mapped by Dings and Robinson 

(1957) in and around the Tincup mining district. Sample T-l 

(tables 1 and 2), collected from a west-trending stock about 

1 km south of Cumberland pass, contains prominent epidote 

crystals. Table 1 shows that epidote and zircon give concordant fission-track dates of 37.4±6.8 Ma and 34.4±3.2 Ma, 

respectively. This indicates that the age of the Tincup stock 

is similar to the Porphyry Creek stock (table 1; sample PC1, 38.4±0.9 Ma, biotite K-Ar; 35.3±3.3 Ma, ZFT) and also 

to the main quartz monzonite phase of the Mount Princeton 

batholith, located about 6 km east of sample T-l. Another 

related, north-trending stock of similar composition, 

mapped as Tincup quartz monzonite porphyry by Dings and 

Robinson (1957), is about 4 km northeast of sample T-l. 

This stock intruded along the Precambrian-Paleozoic contact, and nearby mineral deposits are present in the Mississippian Leadville Limestone.

A series of metal-bearing veins are located about 1 km 

north of the Tincup stock in the vicinity of Cumberland 

pass, and at least one vein cuts the stock (Dings and Robinson, 1957). The Bon Ton mine is located 0.6 km north of the 

stock contact in a vein that cuts Precambrian gneissic granite. The Bon Ton mine produced gold, silver, copper, and 

lead and some molybdenum and tungsten ore (Dings and
DATED IGNEOUS CENTERS AND ASSOCIATED MINERAL DEPOSITS 17

Robinson, 1957). The spatial association and concordant 

ages of the dated minerals suggest that these metal deposits 

are genetically associated with the Tincup stock.

ROUND MOUNTAIN STOCK

The Round Mountain stock (fig. 4) is a fine-grained 

biotite rhyolite stock about 2 km in diameter. It is situated at 

the eastern end of the West Elk volcanic field about 15 km 

southeast of Crested Butte. Cross (1894) noted that it v/as 

different from the quartz monzonite that comprises the West 

Elk laccoliths and that it was probably younger in age. Our 

dating studies support his observations; sample RM-1 

(tables 1 and 2) gives concordant zircon (11.5±1.3 Ma) and 

apatite (9.8±2.9 Ma) dates and a slightly older biotite date 

(13.9±0.3 Ma). This is essentially the same age as the Crystal pluton of the Treasure Mountain dome dated at 12.7 Ma, 

located 35 km to the northwest (Obradovich and others, 

1969). There are no known mineral deposits associated with 

the Round Mountain stock.

ITALIAN MOUNTAIN INTRUSIVE COMPLEX

The Italian Mountain Intrusive Complex (fig. 3) is a 

heterogeneous assemblage of intrusive rocks in the eastern 

Elk Mountains. It includes three intrusive centers and associated plutons of quartz diorite, granodiorite, and quartz 

monzonite composition (Cunningham, 1976). The ZFT age 

published in Cunningham and Naeser (1975) is here revised 

to 33.9±3.9 Ma using a recalibrated thermal neutron dose of 

1.15 x 10 15 and a decay constant of 7.03 x 10~15 yr 1 . The 

Italian Mountain Intrusive Complex is similar in age and 

composition to other stocks in the eastern Elk Mountains, 

including the Whiterock and Snowmass stocks, 34.8±1.0 

Ma and 35.0±1.4 Ma, respectively (Gaskill, 1956; Obradovich and others, 1969), and the Mount Sopris stock (34.3±4.1 

Ma, ZFT; 34.2±0.8 Ma, biotite K-Ar; sample MS-1, 

table 1).

Lead-zinc-silver hydrothermal deposits are present in 

the Leadville Limestone on the east side of the intrusive 

complex. The deposits are zoned outward from the northernmost, youngest, intrusive center, suggesting that they are 

genetically related to that youngest center.

MOUNT SOPRIS STOCK

The circular Mount Sopris stock (fig. 3), about 5 km in 

diameter, is the northernmost of the principal stocks of the 

eastern Elk Mountains. It is composed mostly of biotite 

quartz monzonite but grades locally to granodiorite. 

Aspects of the structure and petrology are described by 

Pilkington (1954) and Poland (1967). The dates of 34.3±4.1 

Ma (ZFT) and 34.2±0.8 Ma (biotite K-Ar) (sample MS-1,

tables 1 and 2) show that the Mount Sopris stock is the same 

age as other small stocks in the Elk Mountains.

Minor lead and silver deposits are associated with the 

Mount Sopris stock, primarily in the contact zone (Pilkington, 1954). Schwartz and Park (1930) describe galena with 

stromeyerite, argentite, covellite, and bornite from these 

deposits.

LEADVILLE

The Leadville district (fig. 3) is one of the world's most 

famous mining districts. Despite many studies, the ages of 

the igneous rocks and ore deposits remain controversial. 

However, new isotopic and analytical methods, coupled 

with the detailed records of early investigators such as 

Emmons (1886), Emmons and others (1927), and Behre 

(1953), have given new understanding in discerning geologic relationships in this district. Much of the recent information on the Leadville district is synthesized in Beaty and 

others (1990).

The Leadville district contains at least six different 

igneous rock units that range in age from Late Cretaceous to 

middle Tertiary. Five of them are pre-ore (Thompson and 

Arehart, 1990): Pando Porphyry (71.8 Ma, biotite K-Ar, 

Pearson and others, 1962); Lincoln Porphyry (66.3 Ma, 

biotite K-Ar, Pearson and others, 1962); Evans Gulch Porphyry (46.8±4.4 Ma, ZFT, sample L-2, this study); Sacramento Porphyry (43.9±4.3 Ma, ZFT, Thompson and 

Arehart, 1990); and Johnson Gulch Porphyry (43.1±4.3 Ma, 

ZFT; 43.1±7.6 Ma, AFT, Thompson and Arehart, 1990). 

This geochronologic order is in disagreement with Beaty 

and others (1987) who reported a ZFT age of 53.6±6 Ma for 

the Sacramento Porphyry and with Tweto (1974) who, on 

the basis of geologic relations, reported that the Sacramento 

Porphyry is post-Pando Porphyry and pre-Lincoln Porphyry. Hydrothermal sericite produced during the principal 

Leadville ore-forming event has been dated by Thompson 

and Arehart (1990) at 39.6±1.7 Ma. The Leadville ore 

deposits are cut by younger rhyolite porphyry dikes and 

plugs and still younger breccias. Sanidine from the postmineralization rhyolite gives a K:Ar age of 38.5±0.6 Ma (sample L-l, tables 1 and 2). Thompson and Arehart (1990), on 

the basis of annealed fission tracks in apatite and zircon, 

show thermal events associated with mineralization and 

alteration extended to 33.8±5.0 Ma. This agrees well with 

our sample L-3, described as Johnson Gulch Porphyry by 

Ogden Tweto (U.S. Geological Survey, written commun., 

1981), which gives a ZFT date of 34.8±4.9 Ma. These 

younger dates may also represent annealing caused by a 

younger intrusion.

The principal ore deposits of the Leadville district are 

massive replacement sulfide bodies in dolostone that are 

zoned around the Breece Hill stock, which is composed of 

Johnson Gulch Porphyry, although the ore is interpreted to
18 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT

be derived from a younger intrusive system centered 

beneath the stock (Thompson and Arehart, 1990). DeVoto 

(1983) and Tschauder and others (1990) suggest that some 

of the silver in the Leadville ores is remobilized from silverrich ores that formed during a late Paleozoic, Mississippi 

Valley-type, mineralization event.

Age relationships of igneous rocks and ore deposits at 

Leadville are similar to those documented at the Oilman 

district, about 32 km to the northwest. At Oilman, the Pando 

Porphyry, dated at 71.8 Ma (Pearson and others, 1962), 

forms large sills in Paleozoic sedimentary rocks, but the 

oldest determined zircon dates are only 60.1±7.8 Ma 

(Naeser and others, 1990), indicating they have been partially reset. The main ore deposits are massive sulfide 

replacement ores in the Mississippian Leadville Limestone. 

The ore is related to an unexposed, middle Tertiary stock 

(Beaty and others, 1990) that formed a large dome-shaped 

paleothermal anomaly in the Pando Porphyry (Naeser and 

others, 1990). The ores at Oilman formed at 34.5±4.4 Ma; 

this age is based on an apatite fission-track determination 

on paragenetically late hydrothermal apatite in the ore 

assemblage. Fission tracks in apatite and zircon in the 

nearby Pando Porphyry are partially reset and give anomalously young ages near the ore. The age of 34.5+4.4 Ma 

from the Oilman ore is similar to the date given by reset zircons of 34.8±4.9 Ma in the Johnson Gulch Porphyry (sample L-3, table 1) from the Leadville district; thus young 

igneous-hydrothermal events of the same age may have 

occurred at both districts.

WEST TENNESSEE CREEK STOCK

The West Tennessee Creek stock (fig. 2) is the southernmost of several small monzogranite to granodiorite 

(Wallace and Blaskowski, 1989) stocks that trend northwest 

across the northern tip of the Sawatch Range. Elongate and 

only about 0.5 km in length, the stock is located immediately north of West Tennessee Lakes and has been mapped 

by Tweto (1974). A date of 66.4+2.2 Ma on biotite (sample 

WT-1, tables 1 and 2) shows that it was emplaced during the 

Laramide, but the younger apatite date of 46.5±10.3 Ma 

(sample WT-1, tables 1 and 2) indicates it was reheated or 

reflects an uplift age. Leadville is about 14 km southeast of 

this locality.

WEST CROSS CREEK STOCK

The West Cross Creek stock (fig. 2), roughly circular 

and about 1.0 km in diameter, is an intrusive complex composed mostly of diorite and granodiorite, which occur 

together with monzogranite and rhyolite porphyry intrusions. It also is referred to as part of the intrusive rocks of 

Middle Mountain (Wallace and Blaskowski, 1989). The

concordant ages of 61.8±1.4 Ma (biotite K-Ar) and 

59.3±6.9 Ma (ZFT) (sample WC-1, tables 1 and 2) indicate 

emplacement during the Laramide. Insufficient apatite was 

recovered to obtain a date to investigate possible younger 

partial annealing as observed in the West Tennessee Creek 

stock. The monzogranite Treasure Vault stock, located 2 km 

to the southeast, gives a K-Ar age of 64.0+.2.3 Ma on biotite 

(Wallace and Blaskowski, 1989) and an AFT date of 52 Ma 

(Wallace, in press).

EAST LAKE CREEK STOCK

The East Lake Creek stock (fig. 2), elongate in plan 

view and about 0.5 by 1.5 km, is composed of leucocratic 

syenite that locally grades to granite (Wallace and 

Blaskowski, 1989). Fission-track dates of 65.2±6.3 Ma on 

sphene and 59.5±8.1 Ma on zircon (sample EL-1, tables 1 

and 2) are concordant and similar to other ages of stocks in 

the northwest trend.

FULFORD STOCK

The Fulford stock (fig. 2), at the northwestern end of 

the trend, ranges from quartz diorite to granite but is mostly 

monzogranite. Two samples (tables 1 and 2) collected close 

to each other give Laramide dates (65.1±6.8 Ma, ZFT, sample F-l;62.2±1.5 Ma, biotite K-Ar, sample F-2). The Fulford stock has small gold-copper-quartz and lead-silver 

veins spatially associated with it. These veins cut the stock 

and surrounding sedimentary rocks in the Fulford mining 

district (Gableman, 1949). Wallace and others (1989) 

showed that many of the gold-bearing veins were related to 

post-Fulford, 62.2- to 61.7-Ma albite syenite intrusions.

MONTEZUMA STOCK

The Montezuma stock (fig. 3) is on the western flank 

of the Front Range and is a 9 by 5 km east-west elongate 

biotite quartz monzonite porphyry stock. It intruded Precambrian rocks, primarily gneisses, as well as Cretaceous 

shales at its western tip (Neuerburg and Botinelly, 1972). 

Smaller, fine-grained plutonic outliers are present south of 

the main stock; sample M-1 came from one of these. The 

stock and its associated ore deposits are described by Lovering and Goddard (1950) and Neuerburg and others 

(1974). Excellent descriptions of the main stock, where it is 

penetrated by the northwest-trending Harold D. Roberts 

water tunnel are given by Wahlstrom and Hornbeck (1962) 

and Warner and Robinson (1967). Simmons and Hedge 

(1978) report a Rb-Sr date on biotite of 39 Ma for the Montezuma stock. Bookstrom and others (1987) report an AFT 

age of 40.2±8.8 Ma and a ZFT age of 39.8±4.2 Ma for the 

relatively fresh Montezuma stock and a ZFT age of
DATED IGNEOUS CENTERS AND ASSOCIATED MINERAL DEPOSITS 19

37.4±3.0 Ma on a sericitized part of the stock. Our ZFT age 

determination of 35.0+3.2 Ma (sample M-l, tables 1 and 2) 

is similar to a K-Ar date on biotite of 39.6±1.2 Ma on 

biotite (McDowell, 1971) and a 37.9±1.4 Ma date on biotite 

(Marvin and others, 1989) and confirms that the stock is 

middle Tertiary in age.

The Montezuma district contains argentiferous leadzinc veins that cut the Montezuma stock (Levering and 

Goddard, 1950). Trace amounts of molybdenite are present 

in the stock (Neuerburg and others, 1974), but much of the 

base metal and molybdenite appear to be related to younger 

intrusive rocks (Bookstrom and others, 1987).

EMPIRE STOCK

The subcircular 1-km diameter Empire stock (fig. 2) is 

within the Front Range and is located about 2 km west of 

the town of Empire. Many intrusive bodies are exposed in 

the area, ranging from dikes to small plutons in size and 

from granodiorite to granite aplite in composition; Braddock (1969) determined relative ages where possible. The 

principal part of the Empire stock consists of hornblendepyroxene monzonite. Our fission-track date on sphene of 

68.016.2 Ma (sample EM-1, tables 1 and 2) and 61.6±2.2 

Ma on biotite (Marvin and others, 1989) agree reasonably 

well with a Rb-Sr date of 65 Ma reported by Simmons and 

Hedge (1978) and reflect the probable age of emplacement; 

a date of 37.4±2.8 Ma on apatite from our sample EM-1 

indicates there has been some thermal annealing of fission 

tracks.

The Empire district has a long history of mining, and 

most of the production in the district was from veins in the 

vicinity of the town of Empire. Braddock (1969) found only 

one vein that actually cuts the Empire stock. Ore deposits in 

the Empire district are mostly gold-pyrite veins, which have 

a strong northeast trend. The veins are clustered in an area 

just north of town. A well-defined geochemical anomaly in 

mull (forest humus layer), which has a central zone of gold, 

copper, and bismuth encircled by silver, lead, zinc, and 

molybdenum, is located in the vicinity of a small, hornblende diorite porphyry stock that is located about 2 km 

north of the town of Empire (Curtin and others, 1971) and 

has not been dated. The age of the ore deposits has not been 

directly determined; however, the biotite quartz monzonite 

porphyritic Mad Creek stock, which cuts the eastern edge of 

the Empire stock, has a peripheral sericitized explosion 

breccia containing molybdenite, pyrite, galena, and sphalerite (A. Bookstrom, U.S. Geological Survey, written commun., 1994). The Mad Creek stock and its explosion breccia 

give concordant ZFT dates of 40.5±6.5 Ma (breccia) and 

39.4±4.3 Ma (stock), which are similar to the age of the 

nearby Montezuma stock (Bookstrom and others, 1987).

APEX STOCK

The next major pluton to the north in the Front Range, 

the Apex stock (fig. 2), cuts Precambrian rocks, is elongate 

north-south, and is about 5 km long by 1 km wide. It is 

composed of hornblende-biotite quartz monzonite and is cut 

by dikes of similar composition. Rice and others (1982) 

report K-Ar hornblende dates of 79.3+1.9 Ma and 76.7±3.1 

Ma. The main body of the stock gives fission-track dates of 

71.3±18.0 Ma (AFT) and 61.7±6.3 Ma (ZFT) (sample A-l, 

tables 1 and 2). Dates from both minerals are concordant at 

about 60±5 Ma. Barker (1979) reports dates of 60.1±5.5 Ma 

and 57.3±4.4 Ma. Sample A-2 (tables 1 and 2), from the 

same location as A-l, is from one of a series of younger, 

crosscutting dikes and yields an AFT date of 66.8±7.5 Ma 

and a ZFT date of 58.5±5.2 Ma. Dates from minerals in A-2 

are slightly younger than dates from corresponding minerals in sample A-l. Apparently there are problems either 

with excess radiogenic argon in the hornblende and the 

stock formed about 60 Ma, or fission tracks in the zircons 

and apatites have been reset to about 60 Ma by a thermal 

event apparently associated with a larger intrusion probably 

related to the dikes sampled by A-2. The apatite ages from 

both samples A-l and A-2 show the same ages as from the 

zircon in the same sample, indicating there has been no 

thermal resetting younger than about 60 Ma, in contrast to 

the thermal history of the nearby Empire stock.

Pyritic gold formed the principal ore in the vicinity of 

the Apex stock. Most of the veins trend northeast and are 

located just east of the stock (Levering and Goddard, 1950); 

however, two veins cut the stock. Low-grade molybdenite 

deposits are present in the stock and in associated veins and 

breccias (Barker, 1979).

ELDORA STOCK

The next pluton north in the Front Range part of the 

CMB is the Eldora stock (fig. 2), which cuts Precambrian 

rocks and is subcircular with a diameter of about 3 km. It is 

composed of biotite-hornblende-pyroxene quartz monzonite. Samples of zircon and apatite from rocks at the same 

locality give fission-track dates (sample E-l, table 1) of 

66.6±6.4 Ma (zircon), 61.8±6.3 Ma (zircon) and 58.6±9.4 

Ma (apatite) that are concordant around 60 Ma, indicating a 

Laramide age for the stock. This agrees with a previous 

40Ar/39Ar date of 63 Ma on biotite (Berger, 1975) and a KAr date of 68.2±4.1 Ma on hornblende (Marvin and others, 

1989) for the stock. The concordant apatite age shows there 

has been little or no thermal resetting. The petrography of 

the stock has been studied in detail by Cree (1948), the contact zone by Hart (1964) and Berger (1975), and the mapped 

relationships are in Lovering and Goddard (1950) and 

Gable (1969).
20 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT

Only a minor amount of ore was produced from the 

Eldora district, mostly from gold-telluride ores, and molybdenite is reported to be moderately abundant (Lovering and 

Goddard, 1950). Some quartz-pyrite veins cut the stock. For 

perspective, the molybdenum mineralization at Central City 

is reported to be 59 Ma (Rice and others, 1985).

JAMESTOWN STOCKS

Jamestown (fig. 3), at the northeastern end of the 

CMB, includes two stocks, an older granodiorite stock and 

a younger sodic granite pluton that intrudes the northern 

end of the granodiorite. The sodic granite is called the Porphyry Mountain stock. Hornblende from the granodiorite 

stock has been dated at 79.6±2.3 Ma and 73.6±2.2 Ma 

(McDowell, 1971). The younger stock was dated by using 

two samples from the same drill core from a hole near the 

summit of Porphyry Mountain. Zircons from the two samples (table 1), one at a depth of 12.5 m (sample JS-2A, 

44.8±5.7 Ma) and the other from near the bottom of the drill 

core at a depth of 1,024.4 m (sample JS-2B, 45.1±7.7 Ma), 

give concordant dates for the stock at about 45 Ma. Zircon 

and apatite,-collected from the older granodiorite stock but 

within 1 km of the contact of the Porphyry Mountain stock, 

give dates of 48.4±7.5 Ma and 47.1±5.8 Ma, respectively; 

these dates are interpreted to be reset, minimum ages. A KAr date on biotite from an unusual vein that contains essentially only biotite and fluorite and cuts the granodiorite was 

58.8±2.0 Ma (sample JS-1, table 1). Igneous and hydrothermal events in nearby Four Mile Canyon have been dated at 

62.06±0.37 Ma, 54.7±1.0 Ma, 48.19±0.29 Ma, and 46±1.0 

Ma (Kane and others, 1988) indicating a complex thermal 

history in the area in which the last significant event 

occurred at about 45 Ma.

The Jamestown district is known for its production of 

gold-telluride ore, fluorspar, pyritic gold ore, and silver-lead 

ore (Goddard, 1946; Lovering and Goddard, 1950; Kelly 

and Goddard, 1969). Most of the fluorite is paragenetically 

older than the base metals, gold, and tellurides (Nash and 

Cunningham, 1973). The igneous-hydrothermal system 

related to the Porphyry Mountain stock and its underlying 

source appears to have been responsible for most of the 

fluorite mineralization in the district because fluorite is a 

primary mineral in the stock as well as in adjacent breccia 

bodies, and fluorite-bearing veins and breccia bodies are cut 

by granitic dikes that appear to be related to the Porphyry 

Mountain stock; the genetic relationship of the pyritic gold 

and gold tellurides to the stock is less certain (Nash and 

Cunningham, 1973).

DISCUSSION OF THE AGES OF 

INTRUSIVE ACTIVITY

Igneous rocks and their associated ore deposits in the 

CMB tend to form three natural groupings by age; they are 

Late Cretaceous and early Tertiary (Laramide), middle Tertiary, and late Tertiary. With the advent of improved technologies to more precisely date rocks, alteration minerals, 

and ore minerals and the addition of new data, especially 

since 1990, this tripartite division has blurred somewhat but 

remains. These natural age groupings have been recognized 

by many investigators, and, with minor modifications, we 

follow suit as shown on figures 2, 3, and 4. The age assignment used is our best interpretation of the age of intrusion 

of stocks or igneous centers and is based on geochronologic 

data, thermal annealing characteristics of minerals, and geologic relationships.

LARAMIDE IGNEOUS CENTERS

The oldest group of intrusions formed mainly between 

70 and 50 Ma, straddling the Cretaceous-Tertiary boundary, 

and were emplaced during the Laramide orogeny (Tweto, 

1975). These intrusions form a well-defined northeastern 

trend that constitutes the innermost zone of the CMB and is 

readily apparent on figure 2. Two northwest-trending zones 

of intrusive rocks are also present. The most prominent is a 

series of small stocks that extends northwest from the West 

Tennessee Creek stock, across the northern Sawatch Range 

to the Fulford stock. Another smaller group of stocks and 

dikes extends northwest in the Front Range from the Denver area.

Along the main northeastern trend, igneous activity 

generally proceeded from the southwest to the northeast; 

this progression is best documented in the southwestern 

segment of the CMB. Igneous activity in the Ute Mountain 

area is well dated at about 72-70 Ma (table 1) by multiple 

dates, including apatite ages, which indicate there has been 

no significant resetting of ages. The La Plata Mountains 

appear to be also well dated, and ages group around 68 Ma. 

The Laramide rocks at Rico are fairly well dated, especially 

the hornblende latite sills; when the data of this study are 

combined with those of Naeser and others (1980), an age of 

about 65 Ma seems most reasonable. The Laramide sills at 

Ouray are probably of similar age, a determination based on 

the proximity, similarity in composition, and style of 

emplacement; the date of 54 Ma (table 1) is a minimum age 

due to apparent reheating. The most comprehensive compilations of ages at Leadville are by Thompson and Arehart 

(1990) and Wallace (in press). Although the ore and some 

of the rocks are younger than Laramide, two major igneous 

units, the Pando Porphyry (72 Ma) and the Lincoln Porphyry (64 Ma), give Laramide ages. These rock units are 

Laramide because the Pando Porphyry appears to have been
DISCUSSION OF THE AGES OF MINERALIZATION 21

emplaced contemporaneously with thrusting related to 

Laramide activity (Thompson and Arehart, 1990). A short 

distance to the north at Oilman, the Pando Porphyry has 

been affected by a middle Tertiary thermal event. The oldest 

zircon ages, the ones that are apparently the least reset and 

closest to the original age of emplacement, range from 58 to 

55 Ma (Naeser and others, 1990).

Northeast of Leadville the age progression picture is 

not as readily apparent. Scattered stocks, such as Apex, 

Empire, Eldora, and Caribou appear to have been emplaced 

about 65-60 Ma. The emplacement age of the older granodiorite stock at Jamestown is not well dated. Ages on hornblende are approximately 80 and 74 Ma (McDowell, 1971) 

and may be too old due to excess argon and to reset dates on 

apatite and zircon of 48 Ma (sample JS-3, table 1). The date 

of 45 Ma on the sodic granite stock is quite different from 

the well-dated Laramide rocks; other dates of approximately 45 Ma have been documented in the nearby Four 

Mile Canyon area (Kane and others, 1988) and for a quartz 

monzonite sill in the central CMB in the Swan Mountain 

area (Simmons and Hedge, 1978; Marvin and others, 1989).

Lovering and Goddard (1950) proposed a general 

northeastward progression of intrusive activity in the Front 

Range; corresponding lithologic types appear earlier in the 

southwestern part than in the northeastern part. If an age of 

about 62 Ma for the Apex and Eldora stocks is accepted as 

the general age of activity near that part of the CMB, a distance of about 400 km from the Ute Mountains, then the 

rate of northeast progression of igneous activity was about 4 

cm/yr. The northeastward progression of igneous activity 

noted at least in the southwestern part of the CMB parallels 

the general direction of Laramide tectonic activity; the 

Laramide orogeny commenced in the southwest before 

marine deposition ended in the northeast (Tweto, 1975).

MIDDLE TERTIARY IGNEOUS CENTERS

Post-Laramide early to middle Tertiary igneous activity, from about 45-25 Ma, was significantly more widespread in Colorado than was Laramide igneous activity (fig. 

3). The middle Tertiary igneous rocks that are relevant to 

our discussion are present today mostly as volcanic rocks in 

the San Juan volcanic field of southern Colorado (Steven, 

1975; Steven and Lipman, 1976) and as epizonal stocks, 

laccoliths, and volcanic breccias in the Elk Mountains of 

central Colorado (Godwin and Gaskill, 1964; Lipman and 

others, 1969; Obradovich and others, 1969). During the 

middle Tertiary, these two igneous centers coalesced into a 

single volcanic field (Steven, 1975). Smaller, local centers 

existed in northern Colorado (fig. 3). Leucocratic stocks 

related to major molybdenum deposits were intruded at Red 

Mountain (Urad-Henderson) and Climax at 33 to 24 Ma 

(Bookstrom and others, 1988) and Mount Emmons at 18 to 

16 Ma (Thomas and Galey, 1982).

The volcanic fields of the San Juan and Elk Mountains 

coincide with two large (> -300 milligal) gravity lows (fig. 

1) that have been interpreted to reflect the location of two 

shallow batholiths (Tweto and Case, 1972; Plouff and 

Pakiser, 1972; Behrendt and Bajwa, 1974). The margins of 

the batholiths have been interpreted to be approximately 

between the -275 mGal and -300 mGal contour lines along 

the trends of the steepest gravity gradients along the borders 

of the gravity lows (Steven, 1975). The central Colorado 

gravity low has a lobe that extends northeastward along the 

trend of the CMB, as defined by Laramide plutons. Middle 

Tertiary stocks coincide with Laramide stocks along this 

part of the CMB all the way to the Porphyry Mountain stock 

at Jamestown, thereby leading to the conclusion that a lobe 

of the northeasternmost of the two batholiths underlying 

central Colorado extends to the northeast (Steven, 1975). 

Gravity modeling (Isaacson and Smithson, 1976) indicates 

that a granitic batholith in central Colorado is present at 

depths of 8 to 25 km.

LATE TERTIARY IGNEOUS CENTERS

Late Tertiary igneous centers are present over a wide 

area of Colorado (fig. 4) including the CMB. Dates suggest, 

that most of the igneous intrusive activity in these centers 

took place between about 15 and about 4 Ma. Several of 

these centers, or stocks, have been dated in the course of 

this study (tables 1 and 2). They include Round Mountain 

(about 14-10 Ma), Chicago Basin (about 10 Ma), and the 

rhyolites associated with the Silver Creek molybdenum 

deposit at Rico (about 4 Ma). Other significant igneous centers include Treasure Mountain dome at 12.7 Ma in the Elk 

Mountains (Obradovich and others, 1969; Mutschler, 1970), 

the Hahns Peak area at about 10 Ma in northern Colorado 

(McDowell, 1971; Segerstrom and Young, 1972), Flat Top 

basalts between about 24 Ma and less than 1 Ma (Marvin 

and others, 1974; Larson and others, 1975), and several centers about 4 Ma in the vicinity of the San Luis valley (Lipman and others, 1970, 1986).

DISCUSSION OF THE AGES OF 

MINERALIZATION

Most ore deposits in the CMB previously were considered to be of Laramide age because of their spatial correspondence to intrusive rocks that also were believed to be 

Laramide (Lovering and Goddard, 1950). Recent studies, 

including this one, have confirmed the Laramide age of 

some deposits but have shown also that the majority of ore 

deposits previously thought to be Laramide are actually of 

middle Tertiary or late Tertiary age, although they may be 

superimposed on or spatially associated with Laramide 

igneous rocks.
22 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT

LARAMIDE ORE DEPOSITS

Many of the Laramide igneous centers of the CMB 

contain no significant mineralization, especially in the 

southwestern part of the belt. These include Laramide rocks 

in the Ute Mountains, sills in the La Plata Mountains, the 

augite monzonite stock and the sills at Rico, the Twin Lakes 

stock in the Sawatch Range (63.8±0.7 Ma 40Ar/39Ar on 

hornblende; Shannon and others, 1987), Leadville, some of 

the small intrusions in the Front Range, the granodiorite 

stock at Jamestown, and many of the stocks in the northwestern trend.

However, some Laramide igneous centers demonstrably generated cogenetic mineral deposits. In the La Plata 

Mountains, the telluride veins and replacement bodies, 

which have yielded more than 95 percent of the district's 

production, are generally in structures peripheral to the central core of igneous rocks in the district; the exceptions are 

those veins related to the diorite stocks (Eckel, 1949). The 

disseminated chalcopyrite deposits in the La Platas are spatially, and apparently genetically, associated with both syenite stocks, and minor gold veins and replacement bodies are 

scattered throughout the district. All of our age determinations in the La Platas, with one exception, have produced 

Laramide dates, including fission-track dates on apatite; the 

major part of the igneous and hydrothermal activity in the 

La Platas took place during the Laramide, and apparently no 

major younger thermal event has been recorded. The one 

exception is an intriguing date of 50.3±4.9 Ma on apatite 

(sample LP-5, tables 1 and 2) from the diorite stock at Diorite Peak.

Near the northwestern margin of the San Juan volcanic 

field, ore was deposited in two or more metallogenetic 

epochs at Ouray; the bulk of the production came from 

deposits genetically associated with the Laramide porphyritic granodiorite intrusions. Deposits, which are zoned 

about a central stock, contain gold, base-metals with gold, 

gold tellurides, and silver, and lead-zinc silver. The deposits 

are present as fissure fillings and tabular bodies in sedimentary rocks adjacent to igneous rocks. These earliest formed 

ore deposits were truncated by early Tertiary erosion and 

overlain by the early Tertiary Telluride Conglomerate and 

middle Tertiary volcanic San Juan Formation. The ages of 

emplacement of the laccoliths and sills at Ouray have not 

been determined; our dates determined on zircon (samples 

O-l and O-2, table 1), as previously stated, must be 

accepted as minimums and those on apatite as reset dates.

Further north along the CMB are other examples of 

Laramide mineralization. The Aspen district has Laramide 

intrusions in which the K-Ar dates cluster about 74-72 Ma 

(Obradovich and others, 1969). Bryant and others (1990) 

have demonstrated with zircon dates that there was a cooling event at 52 Ma and with apatite dates that there was a 

thermal event at 30 Ma, possibly related to igneous activity 

in the Elk Mountains. These dates furnish no definitive evidence for the age of the mineral deposits, but the reconnaissance study does indicate that large middle Tertiary 

paleothermal anomalies, such as those present at the Leadville (Thompson and Arehart, 1990), Oilman (Beaty and 

others, 1990), and Tennessee Pass (Beaty and others, 1987) 

districts, are not present at Aspen (Bryant and others, 1990).

In the northwestern trend, some of the stocks are associated with mineralization, whereas others are barren. The 

Fulford stock is spatially associated with small gold-albitesiderite vein deposits; however, although the stock is about 

65-62 Ma (tables 1 and 2), the spatially associated deposits 

have been shown by Wallace and others (1989) to be related 

to post-Fulford (prior to 61.7 Ma) albite-syenite intrusions. 

The various methods that have been used for age determinations give generally concordant Laramide ages, indicating 

that these stocks have not had superimposed upon them significant igneous or hydrothermal events. A notable exception may be the West Cross Creek stock, also called part of 

the intrusive complex at Middle Mountain by Wallace and 

others (1989). They mapped a rhyolite porphyry in the glaciated valley floor of West Cross Creek, just north of the 

stock, that has textural characters similar to those of Oligocene granite complexes known to host stockwork molybdenum deposits of the Climax type.

The Whitehorn stock (Normand, 1968; Wrucke, 1974; 

Wrucke and Dings, 1979) is a large stock, about 23 by 7 km 

in extent, and is located about 5 km northeast of Salida. The 

stock is slightly off the main northeastern trend of the CMB 

and is approximately on line with a southeastern extension 

of the line of stocks that trend northwest across the Sawatch 

Range. It has been dated at about 70 Ma (Wrucke, 1974), 

and, although it ranges in composition from syenogabbro to 

quartz monzonite, it is mostly granodiorite. Geophysical 

studies (Case and Sikora, 1984) show the stock is marked 

by both positive magnetic and gravity anomalies, and 

detailed modeling suggests that the northern part is sill- or 

laccolithlike. Along the western side of the Whitehorn 

stock, contact metasomatic iron deposits (Behre and others, 

1936) formed where the stock came into contact with limestones. Minor gold, silver, and copper veins of the TurretCalumet district occur in conjunction with the iron deposits 

(Vanderwilt and others, 1947; Marsh and Queen, 1974).

The Central City district, located approximately 50 km 

west of Denver, contains uranium, base-metal, and precious-metal mineral deposits that have been studied and 

dated by Wells (1960), Phair (1979); Rice and others (1982) 

and Wallace (1989). The base- and precious-metal deposits 

are complexly zoned, and although they are directly related 

to the emplacement of a Laramide, alkalic rhyolitic magma, 

there is some uncertainty about the precise age of mineralization (Wallace, 1989). Rice and others (1982) would place 

the base-metal deposits at about 60-58 Ma, but the deposits 

might be as old as 69 Ma (Taylor, 1976; Wallace, 1989). 

Telluride mineralization postdated the base-metal sulfide 

stage (Sims and Barton, 1962). The telluride mineralization
DISCUSSION OF THE AGES OF MINERALIZATION 23.

at Central City has not been directly dated but may be similar to the age of the rest of the mineralization (Rice and 

others, 1982) or significantly younger (Wallace, 1989). 

Mineralization related to the Apex stock is apparently Laramide because apatite and zircon from the stock give Laramide dates that have no suggestion of subsequent thermalmineralization events. The age of mineralization at the 

Empire stock is more questionable, however. The stock is 

clearly Laramide, and the gold-pyrite veins are similar in 

character to those in other Laramide districts, but the anomalously young apatite date (sample EM-1, tables 1 and 2) 

suggests that a younger thermal event may have produced 

the mineralization.

MIDDLE TERTIARY ORE DEPOSITS

Middle Tertiary ore deposits are present mainly in the 

central and northeastern parts of the CMB, especially near 

the middle Tertiary batholiths and the intersection of the 

CMB and the north-trending Rio Grande rift system (fig. 1). 

At Leadville, the water, sulfur, lead, and by inference the 

rest of the metals of the replacement deposits had a magmatic-hydrothermal origin as determined by stable and 

radiogenic isotope analyses (Thompson and Beaty, 1990). 

The age of the replacement ore, 39.6±1.7 Ma, is based on a 

K-Ar analysis of hydrothermal sericite (Thompson and Arehart, 1990). This age agrees with our postmineralization 

rhyolite sanidine date of 38.5±0.6 Ma (sample L-l, tables 1 

and 2). Assuming that the approximately 39-Ma age for 

mineralization is valid, the younger zircon and apatite fission-track dates of 34.8±4.9 Ma (ZFT) (sample L-3, tables 1 

and 2) and 33.415.1 Ma, derived from an average of several 

samples near the ore (Thompson, 1990), must be annealed 

samples. The similarity of the fission-track dates of reset 

apatite and zircon at Leadville to the 34.5±4.4 Ma age on 

the ore at Oilman suggests the possibility that a thermal 

event hot enough to partly reset fission tracks may have 

occurred at Leadville at about the same time as the major 

mineralizing event at Oilman.

Weakly mineralized middle Tertiary stocks are present 

throughout the CMB; they occur north of Rico (fig. 3) and, 

together with unmineralized laccoliths, in the West Elk 

Mountains (Gaskill and Meeves, 1984). Intrusions of similar age in the Elk Mountains are sparsely mineralized (Bryant, 1971) with the exception of the lead-zinc-silver 

replacement and vein deposits associated with the Italian 

Mountain Intrusive Complex dated at 34 Ma (sample 1-1, 

tables 1 and 2). The Montezuma stock, dated at about 40 

Ma, is spatially associated with silver-lead-zinc veins and 

disseminated molybdenum (Neuerburg and others, 1974). A 

fission-track date on zircon of 37.4±3.0 Ma, from a hydrothermally altered part of the Montezuma stock, is similar to 

the ages of nearby small intrusions (39-37 Ma) believed to 

be related to a hidden pluton south of the stock; Bookstrom

and others (1987) suggest this hidden pluton is responsible 

for most of the mineralization and alteration that affected 

the Montezuma stock.

The thermal events associated with the multiple intrusions and large molybdenum deposits at Red Mountain 

(Urad and Henderson) spanned from 29.9±0.3 Ma (possibly 

before 30.38±0.09 Ma) to 26.95±0.08 Ma; the dates were 

based on a 40Arp9Ar thermochronologic study by Geissman 

and others (1992). Corresponding activity at Climax 

occurred between about 33 and 24 Ma (White and others, 

1981; Bookstrom and others, 1988; and a recent molybdenite Re-Os date of 26.6 Ma, Suzuki and others, 1993). Biotite 

from the intrusions at Mount Emmons gives dates of 

17.3±0.7 Ma and 16.4±0.7 Ma (Thomas and Galey, 1982). 

All of these deposits are spatially within the negative 300- 

mGal Bouguer gravity contour in central Colorado. The 

Henderson-Red Mountain and Climax deposits are associated with bimodal leucogranite-lamprophyre magmatic 

suites that are indicative of underplating of the crust by 

basaltic magmas and consequent melting of felsic source 

rocks (Bookstrom and others, 1988). Extensive Pb, Sm-Nd, 

and Rb-Sr isotopic data and rare earth element (REE) data 

(Stein, 1985; Stein and Crock, 1990) indicate that the 

Climax-type granites were probably derived from a felsic to 

intermediate composition lower crustal source.

At Jamestown, fluorite deposits are most probably 

related to the sodic granite Porphyry Mountain stock, dated 

at 45 Ma, because fluorite occurs as a primary mineral in a 

late phase of the stock. The fluorite-biotite veins may reflect 

an earlier event at about 58 Ma or perhaps an anomalously 

old age due to argon from the older rocks perhaps concentrated or scavenged by the fluorine. The genetic relationship 

between pyritic gold, gold telluride veins, and the stock is 

less certain, but the gold is younger than the fluorite (Nash 

and Cunningham, 1973); the telluride and molybdenum 

mineralization most probably was related to the emplacement of the Porphyry Mountain stock (Threlkeld and 

Gonzalez-Urien, 1985). In the nearby Four Mile canyon 

area, 40Ar/39Ar dating of rocks and vein materials has identified a lead-silver vein at 62.06±0.37 Ma, a syenitic stock 

at 54.7±1.0Ma, a bostonitic stock at 48.19±0.29 Ma, and a 

hydrothermal breccia at 46±1.0 Ma, indicating an area of 

complex magmatic-hydrothermal-metalliferous systems 

(Kane and others, 1988).

LATE TERTIARY ORE DEPOSITS

Late Tertiary igneous systems, like Laramide and middle Tertiary systems, are characterized by both barren and 

highly mineralized stocks. At Rico, both lead-zinc-silver 

vein and replacement deposits and the more recently discovered Silver Creek molybdenum deposit are genetically 

related to rhyolitic stocks and dikes dated at 4 Ma. These 

are the youngest known mineralized rhyolites in Colorado.
Sr-Nd-Pb-O isotope and minor element data indicate that 

the rhyolitic stocks and dikes at Rico are isotopically distinct from the igneous rocks associated with Climax-type 

porphyry-molybdenum deposits in Colorado and may have 

contributions from both crust and mantle sources (Wareham, 1991). Chemical analyses of samples from dumps and 

drill core provide additional trace-element data for the Rico 

deposit (Neubert and others, 1991). The systems that 

formed the Chicago Basin molybdenum prospect and the 

Camp Bird deposits imparted anomalously young dates to 

Laramide sills near Ouray, at about 10 Ma. In central Colorado, the Round Mountain stock is slightly older (about 14 

11 Ma) but contains no known, related mineral deposits. 

The nearby granite of similar age at Treasure Mountain contains only small, widely separated quartz-pyrite-sericite-fluorite veins, which locally contain minor amounts of 

molybdenite (Mutschler, 1976). The Hahns Peak (Elkhead 

Mountains) igneous center, about 30 km north of Steamboat 

Springs in northwestern Colorado, is about 12 Ma and contains minor amounts of silver, lead, and zinc veins. The vein 

system and a cone-shaped intrusive breccia containing 

molybdenite are related to a granitic stock (Casaceli, 1983); 

spatially associated alluvial gold deposits may or may not 

be genetically related to the intrusive center (Young and 

Segerstrom, 1973; Casaceli, 1983).

CONCLUSIONS

The Laramide (Late Cretaceous and early Tertiary) and 

middle to late Tertiary stocks, dikes, and sills of the CMB 

tend to be mostly calc-alkaline in composition and range 

from diorite to rhyolite; granodiorites and quartz monzonite 

porphyry predominate. At individual centers, intrusions of 

similar ages, whether Laramide or middle Tertiary, generally increase in silica content and mafic hydrous minerals 

with time (Ute Mountains, Rico, Leadville, Italian Mountain) or toward alkalic compositions (La Plata Mountains, 

Jamestown) or both (Central City). Late Tertiary stocks 

have less compositional range and tend to be rhyolitic.

Rocks of Laramide age in the Front Range are present 

as two petrographically distinct suites: a silica-oversaturated granodiorite-quartz monzonite suite and a silica-saturated, high alkali monzonite suite (Braddock, 1969). 

Simmons and Hedge (1978) examined both Laramide and 

mid-Tertiary plutons in the northeastern part of the CMB. 

They found that the granodiorite suite was present in rocks 

of both ages, whereas the monzonite suite was restricted to 

Laramide rocks. On the basis of Rb-Sr isotopic data, they 

concluded that both suites were derived from Precambrian 

source materials.

In general, monzonitic rocks preceded granodioritic 

rocks during the northeastwardly advance of Laramide 

magmatic activity between 75-45 Ma and most of the 

southwestwardly retreat after 45 Ma (Bookstrom, 1990). At

Central City, granodiorite of Laramide age predated the sulfide mineralization and slightly younger biotite quartz latite 

postdated it (Rice and others, 1982). At Leadville, the rocks 

are generally more silicic, but the same trend persists; 

quartz latites and quartz monzonites predate the ore deposition, and the slightly younger rhyolite postdate it (Thompson and Arehart, 1990). The same overall change from 

intermediate composition to alkalic rhyolites also occurs at 

Rico and Jamestown, although the rocks are of significantly 

different ages.

Gold-telluride mineralization occurred throughout 

Colorado but was generally late in the ore-forming 

sequence at individual centers. In the La Plata Mountains, it 

is associated with Laramide syenites that are among the latest intrusions. Tellurides formed during the latest of the four 

stages of mineralization at Central City (Bastin and Hill, 

1917; Sims and others, 1963); the gold-telluride mineralization has been related genetically to an alkalic rhyolitic stock 

and may be younger than 53 Ma (Wallace, 1989). At 

Jamestown, the telluride mineralization was probably 

related to the sodic granite stock dated at 45 Ma. The most 

extensively mined gold-telluride deposits in Colorado are 

outside of the CMB at Cripple Creek, where they are associated with alkalic rocks (Levering and Goddard, 1950; 

Thompson and others, 1985; Thompson, 1992) that have 

been dated at about 32-28 Ma (Kelley and others, 1993). 

Levering and Goddard (1950) noted that in the Front 

Range, southwest of Georgetown, complex lead-silver-zinc 

ores predominate and are present with diorite, monzonite, 

quartz monzonite, and sodic quartz monzonite; in contrast, 

northwest of Silver Plume, pyritic gold ores are present with 

alkalic syenite and alkalic trachyte porphyry.

Isotopic data can provide insight into the sources of 

magmas that formed stocks in the CMB and presumably the 

sources of metals in ore deposits. Studies of Pb, Rb-Sr, SmNd isotopes, and REE from 52 granitoid intrusive and 

extrusive rocks indicate a predominantly crustal origin for 

igneous rocks of the CMB (Stein and Crock, 1990). Data on 

Climax-type molybdenum systems support magma generation by partial melting of Proterozoic, lower crustal rocks 

followed by differentiation (Stein and Crock, 1990), but 

recent Re-Os studies indicate that a mantle component may 

be part of the process (Stein and others, 1993). These 

deposits are believed to have formed at the time of incipient 

rifting related to the development of the Rio Grande rift system (Stein, 1988). Younger molybdenum systems, such as 

those at Chicago Basin and Rico in southwestern Colorado, 

may also be related to Rio Grande rifting (Bookstrom, 

1981; Stein, 1982).

Documentation of the temporal and spatial evolution 

of igneous rocks and associated ore deposits in the CMB 

provides a unique opportunity to gain insight into subcontinental processes during the past 75 million years. Studies 

of, and references to, the relationship between the igneoushydrothermal systems in the CMB and plate tectonic set-
ACKNOWLEDGMENTS 25

tings include Christiansen and Lipman (1972), Lipman and 

others (1972), Lipman (1980), Bookstrom (1981, 1990), 

Mutschler and others (1987), and Wallace (1990). Among 

the earliest to document the relationship between the Laramide tectonic and magmatic processes and subduction of 

the Farallon plate obliquely beneath the western margin of 

the American plate was Atwater (1970). Lipman (1980) 

presented evidence, based on the chemistry of volcanic 

rocks in a northeast-migrating magmatic arc above the 

plate, that the plate appeared to have an abrupt hinge 

between gently and steeply dipping subduction segments. 

T.A. Steven (U.S. Geological Survey, oral commun., 1976) 

and Barker and Stein (1990) showed that contoured K2OSiO2 values of volcanic rocks were offset along the CMB, 

supporting the idea that Laramide igneous activity was 

related to a leaky transform fault in the subducted slab. 

Bookstrom (1981, 1990) used time-distance plots to document the northeastern migration of igneous activity along 

the CMB between 75-60 Ma. This migration was followed 

successively by tectonism and magmatism in the central 

part of the belt between 60-45 Ma, post-Laramide tectonic 

relaxation and southwestern retreat of magmatism between 

45-25 Ma, bimodal magmatism with incipient rifting 

between 35-28 Ma, and rift-related volcanism associated 

with the Rio Grande rift system after 28 Ma. Alternatively, 

Mutschler and others (1987) suggested that a subducted 

plate explanation was not necessary because the igneous 

activity could be generated by regional compression, crustal 

thickening, and decompression melting of the mantle during 

rebound.

The Rio Grande rift system (fig. 1) has had an important role in forming ore deposits within and near the CMB. 

In Colorado, the north-trending rift is a series of en echelon 

grabens and horsts that is prominent from the south up to 

Leadville. The rift continues northward to the ColoradoWyoming border (Tweto, 1979b), although its surficial 

expression is much compromised. The Climax and Henderson porphyry molybdenum deposits formed in the zone 

where the Rio Grande rift and the central Colorado 

batholith intersect. Bookstrom (1981) and Stein (1982) 

showed that the younger Mount Emmons (17 Ma), Chicago 

Basin (10 Ma), and Rico (4 Ma) molybdenum deposits were 

related to Rio Grande rift-related bimodal volcanism as it 

progressed outward from the rift axis.

Many studies have investigated the genetic relationship between tectonic processes, igneous centers, and oreforming hydrothermal systems. They have shown that the 

CMB formed during processes involving subduction and 

subsequent rifting; both tectonic regimes produced periods 

of complex magmatism. Throughout these magmatic processes, ore deposits formed in the CMB.

These data, combined with the results of many 

authors' studies (especially Tweto and Sims, 1963; Christiansen and Lipman, 1972; Lipman and others, 1972; 

Steven, 1975; Tweto, 1975; Simmons and Hedge, 1978;

Bookstrom, 1981, 1990, 1994; Stein, 1985, 1988; Wallace 

and Naeser, 1986; Mutschler and others, 1987; Bookstrom 

and others, 1988; Stein and Crock, 1990; and Wallace, 

1990) support the following sequence of events. In the late 

Mesozoic, the convergence rate of the American plate and 

Pacific plate increased, resulting in the continued overriding 

of the intervening Farallon plate and a decrease of the 

plate's angle of subduction. The effects of this more shallow 

subduction produced Laramide-style compressional block 

faulting with basement-cored uplifts and associated deformation as far into the craton as the modern Rocky Mountains. Crustal thickening accompanied the subduction. The 

subducted plate descended in a northeast direction; within 

it, several parallel northeast-trending transform faults, being 

breaks in the slab, probably concentrated volatiles and 

localized melting of lower crustal Precambrian rocks. Relatively isolated igneous centers developed above these zones 

during progressive anatexis, and they tended to evolve in 

composition from monzonite to granodiorite to, locally, 

alkalic magmas. Magmatic activity progressed from southwest to northeast; associated ore deposits were composed 

mainly of precious and base metals.

During middle Tertiary time, the subduction rate 

decreased and the plate angle steepened. Asthenospheric 

counterflow over the leading edge of the plate occurred; 

isotherms rose higher into the lower crust, and large volumes of magma were generated. The magma chambers coalesced into two batholiths, one in central Colorado that 

occupies part of the northeastern trend of the CMB and 

another beneath the San Juan volcanic field. Progressively 

more silicic magma was intruded at individual centers. The 

Rio Grande rift began opening about middle Oligocene 

time. Silicic igneous rocks and associated major molybdenum deposits were formed by melting lower crustal Precambrian rocks. As the effects of the rifting spread laterally 

and northward, younger molybdenum-silver-lead deposits 

formed.

ACKNOWLEDGMENTS

The authors appreciate the thoughtful discussions they 

had with many of their colleagues during the course of this 

study and the many suggestions that were made, especially 

by Ogden Tweto and Thomas A. Steven. Excellent, 

thoughtful reviews and suggestions by Arthur A. Bookstrom, Lawrence W. Snee, and Holly J. Stein improved the 

manuscript. Dorothy Manley, Linda Jacobsen, and Jerry M. 

Russell are thanked for their assistance with report 

preparation.
26 AGES OF SELECTED INTRUSIVE ROCKS AND ASSOCIATED ORE DEPOSITS IN THE COLORADO MINERAL BELT

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Sawatch Range, Colorado: The Mountain Geologist, v. 10, 

no. 4, p. 117-122.

Thompson, T.B., Trippel, A.D., and Dwelley, PC., 1985, Mineralized veins and breccias of the Cripple Creek district, Colorado: Economic Geology, v. 80, p. 1669-1688.

Threlkeld, W., and Gonzalez-Urien, Eliseo, 1985, The molybdenum porphyry at Jamestown and some tectonic implications 

of Eocene subduction [abs.]: Geological Society of America 

Abstracts with Programs, v. 17, no. 4, p. 268.

Toulmin, Priestley, III, and Hammarstrom, J.M., 1990, Geology of 

the Mount Aetna volcanic center, Chaffee and Gunnison 

Counties, Colorado: U.S. Geological Survey Bulletin 1864, 

44 p.

Tschauder, R.J., Landis, G.P., and Noyes, R.R., 1990, Late Mississippian karst caves and Ba-Ag-Pb-Zn mineralization in central Colorado Part I. Geologic framework, mineralogy, and 

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U.S. Geological Survey Miscellaneous Investigations Series 

Map 1-830, scale 1:24,000.

 1975, Laramide (Late Cretaceous-Early Tertiary) Orogeny 

in the southern Rocky Mountains, in Curtis, B.J., ed., Cenozoic history of the southern Rocky Mountains: Geological 

Society of America Memoir 144, p. 1^44.

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State Geologic Map, scale 1:500,000. 

-1979b, The Rio Grande rift system in Colorado, in Riecker,

R.E., ed., Rio Grande rift Tectonics and magmatism: Washington D.C., American Geophysical Union, p. 35-56.

Tweto, Ogden, and Case, J.E., 1972, Gravity and magnetic features 

as related to geology in the Leadville 30-minute quadrangle, 

Colorado: U.S. Geological Survey Professional Paper 726-C, 

31 p.

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Colorado Mineral Belt: Geological Society of America Bulletin v. 74, no. 8, p. 991-1014.

Tweto, Ogden, Steven, T.A., Hail, W.J., Jr., and Moench, R.H., 

1976, Preliminary geologic map of the Montrose 1° x 2° 

quadrangle, southwestern Colorado: U.S. Geological Survey 

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Vanderwilt, J.W., 1947, Mineral resources of Colorado: Denver, 

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Brown, S.D., 1989, Mineral resources of the Holy Cross Wilderness Area, Eagle, Pitkin, and Lake Counties, Colorado:
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*U.S. G.P.O.:1994-387-030:41
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