science for a changing world

Geochronology and Geology of 

Late Oligocene Through Miocene 

Volcanism and Mineralization in the 

Western San Juan Mountains, Colorado

U.S. Geological Survey Professional Paper 1642

U.S. Department of the Interior 

U.S. Geological Survey
Cover. View of outflow Fish Canyon Tuff (lower cliffs), near Lake City, Colo.
Geochronology and Geology of Late Oligocene 

Through Miocene Volcanism and Mineralization in 

the Western San Juan Mountains, Colorado

By Dana J. Bove, Ken Hon, Karin E. Budding, John F. Slack, 

Lawrence W. Snee, and Boss A. Yeoman

U.S. Geological Survey Professional Paper 1642

U.S. Department of the Interior 

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

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Version 1.0

First printing 2001

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center 

Denver, CO 80225

This publication is also available online at: 

http://geology.cr.usgs.gov/pub/ppapers/p1642

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

is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Geochronology and geology of Late Oligocene through Miocene volcanism and mineralization in the western San Juan Mountains, Colorado / by Dana J. Bove ... [et al.].

p. cm. - (U.S. Geological Survey professional paper; 1642) 

Includes bibliographical references.

1. Geology, Stratigraphic—Oligocene. 2. Geology, Stratigraphic—Miocene. 3. 

Geology—San Juan Mountains (Colo. and N.M.). 4. Geology—Colorado. 5. Volcanism—San Juan Mountains (Colo. and N.M.). 

I. Bove, Dana. II. Series.

QE693.G46 2001

551.7'85'097883—dc21 2001018752

ISBN= 0-607-97486-9
Contents

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

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

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

Early Caldera Cycles............................................................................................................................ 2

Geology and Previous Age-Related Studies .......................................................................... 2

New 40Ar/39Ar Ages and Interpretations.................................................................................. 2

Regional 26-25 Ma Calc-Alkaline Intrusions................................................................................... 5

Geology and Previous Age-Related Studies.......................................................................... 5

New ^Ar/^Ar Ages and Interpretations ................................................................................. 5

Veins and Porphyry-Style Alteration Related to 26-25 Ma Intrusions........................................ 9

Geology and Previous Age-Related Studies .......................................................................... 9

New ^Ar/^Ar Ages and Interpretations ................................................................................. 12

Evolution of the Lake City Caldera..................................................................................................... 12

Geology and Previous Age-Related Studies........................................................................... 12

New ^Ar/^Ar Ages and Interpretations ................................................................................. 14

Hydrothermal Activity and Mineralization Related to the Lake City Caldera............................. 14

Geology and Previous Age-Related Studies .......................................................................... 14

New 40Ar/39Ar Ages and Interpretations ................................................................................. 15

19-17 Ma Rhyolite Intrusions North and South of the Lake City Caldera and

Associated Mineralization......................................................................................................... 16

Discussion and Summary................................................................................................................... 16

Acknowledgments............................................................................................................................... 17

References Cited.................................................................................................................................. 17

Appendix................................................................................................................................................ 20

Figures

1. Map of the San Juan volcanic field, Colorado, showing location of

Oligocene calderas.............................................................................................................. 3

2. Generalized geologic maps of western San Juan Mountains areas,

showing localities of dated samples................................................................................ 6

3. Diagrams showing 40Ar/39 Ar age-spectra for dated samples...................................... 8

4. Generalized geologic map of Lake City caldera............................................................ 13

Tables

1. ^Ar/^Ar age-spectrum data for rocks and mineralized material from

western San Juan Mountains........................................................................................... 4

2. Summary of new geochronologic data for western San Juan Mountains and

previous K-Arages............................................................................................................ 11

3. Sample descriptions and localities.................................................................................. 20

4. 40Ar/39Ar data for mineral separates from rocks and mineralized material from

western San Juan Mountains........................................................................................... 21

5. K-Ar analytical data for rhyolitic rocks in Lake City area............................................. 30

III
Geochronology and Geology of Late Oligocene Through 

Miocene Volcanism and Mineralization in the Western 

San Juan Mountains, Colorado

By Dana J. Bove, Ken Hon, 1 Karin E. Budding,2 John F. Slack, Lawrence W. Snee, 

and Ross A. Yeoman

Abstract

Twenty-five new 40Ar/39Ar ages from volcanic rocks and 

veins in the western San Juan Mountains clarify relationships 

between volcanism and mineralization in this classic area. Five 

calc-alkaline ash-flow sheets erupted from caldera sources (Ute 

Ridge, Blue Mesa, Dillon Mesa, Sapinero Mesa, and Crystal 

Lake Tuffs) from 28.6 to 27.6 Ma. This is a much more 

restricted time interval than previously thought and indicates that 

the underlying batholith rose and evolved very rapidly beneath 

the western San Juan Mountains. The new ages and geologic 

relations constrain the timing of joint resurgence of the 

Uncompahgre and San Juan calderas to between 28.2 and 27.6 

Ma. The collapse of the Silverton caldera produced a set of 

strong ring fractures that intersected with graben faults on the 

earlier resurgent dome to produce the complex set of structures 

that localized the mid-Miocene epithermal gold veins.

Later calc-alkaline monzonitic to quartz monzonitic plutons 

solidified at 26.5-26.0 Ma as the underlying batholith rose 

through its volcanic cover. A new age from lavas near 

Uncompahgre Peak supports earlier interpretations that these 

lavas were fed by nearby 26 Ma monzonite intrusions. Nearly 

all of these intrusions are associated with subeconomic molybdenum and copper mineralization and associated alteration, and 

new ages of 26.40 and 25.29 Ma from the Ute-Ulay and Lilly 

veins in the Lake City region show that some of the most important silver and base-metal veins were temporally and possibly 

genetically connected to these plutons. In addition, the Golden 

Fleece telluride vein cuts all of the post-Uncompahgre caldera 

volcanics in the area and is probably temporally related to this 

cycle, though its age of 27.5±0.3 Ma was determined by less precise U/Pb methods.

The 22.9 Ma Lake City caldera collapsed within the older 

Uncompahgre caldera structure but is petrologically unrelated 

to the older calc-alkaline activity. The distinctive suite of highsilica rhyolite tuff and alkaline resurgent intrusions indicates 

that it is closely related to the early stages of bimodal high-silica rhyolite-alkali basalt volcanism that accompanied the onset 

of extensional tectonism in the region. Both 40Ar/39Ar ages and 

paleomagnetic data confirm that the entire caldera sequence 

formed in less than 330,000 years. Only weak quartz vein mineralization occurred in the center of the caldera, and it appears 

to be related to leaching of metals from the intracaldera tuffs 

above the resurgent intrusion. Massive alunitization and weak

University of Hawaii, Hilo, HI 96720. 

2 150 Green Rock Drive, Boulder, CO 80302.

molybdenum and copper mineralization along the eastern ring 

fracture are associated with calc-alkaline lavas and stocks 

related to late stages of the caldera cycle. These calc-alkaline 

stocks also appear to be genetically and temporally linked to a 

radial pattern of barite-precious-metal veins on the northeastern margin of the Lake City caldera.

Introduction

The rich epithermal vein and breccia pipe deposits of the 

western San Juan Mountains have been used as a model of the 

relationship between epithermal mineralization and volcanic 

activity for the past century (Ransome, 1901; Cross and Larsen, 

1935; Varnes, 1963). During the past two decades, great 

advances have been made in understanding the style of calderarelated volcanism and its relationship to ore deposits in this classic area (Steven and Lipman, 1976). Though almost all of the 

mineralization is spatially related to the calderas that formed 

from 29 to 27 Ma, Lipman and others (1976) demonstrated that 

the large epithermal vein deposits are 10-15 Ma younger. The 

caldera-forming eruptions produced fractures and faults suitable 

for later mineralization but appear to have depleted the associated magmatic systems of any ore fluids capable of producing 

mineralization during the caldera cycle.

More recent studies by Slack (1980) identified base- and 

precious-metal veins that are mineralogically zoned and spatially distributed around the outside of the 23 Ma Lake City 

caldera. Although Slack proposed that these veins formed contemporaneously with the Lake City caldera, a U-Pb-isochron 

(27.5±0.3 Ma) date on one of the richest of these veins indicates 

formation near the end of the earlier caldera cycle rather than 

during the Lake City caldera cycle (Hon and others, 1985). This 

in turn suggested the possibility of some mineralization in the 

western San Juan Mountains associated with the period of major 

caldera formation or the episode of calc-alkaline intrusive activity immediately following. However, massive alunitization and 

porphyry-style mineralization of postcollapse lavas within the 

Lake City caldera are undoubtedly contemporaneous with Lake 

City caldera formation and with similar acid-sulfate alteration 

and mineralization at Summitville and Red Mountain Pass 

(Bove and others, 1990; Gray and Coolbaugh, 1994; Gilzean, 

1984).

Our goal was to obtain more precise ages from a variety of 

fresh and mineralized rocks to clarify key relationships between 

caldera formation and mineralization in the western San Juan 

Mountains. We have obtained a series of new 40Ar/39Ar ages 

from the main calc-alkaline ash-flow sheets and related younger 

calc-alkaline plutons, the ash-flows and lavas of the Lake City

1
caldera cycle, and veins and other altered rocks in the Lake City 

region. Although these ages do not alter the general geological 

story of the western San Juan Mountains, they provide a much 

more precise chronological framework for interpreting durations 

of events and their relationship to mineralization. These ages are 

of similar quality to 40Ar/39Ar ages from the eastern and central 

San Juan Mountains (Lipman and others, 1997; Lanphere, 

1988), making chronological interpretations between the regions 

more comparable. The detailed ages from the Lake City caldera 

and surrounding veins call for a further reevaluation of the timing of mineralization in this area and also allow testing of the 

paleomagnetic constraints on duration of the caldera cycle proposed by Reynolds and others (1986).

Methods

The ages of 25 mineral separates (see Appendix) were 

determined using the 40Ar/39Ar step-heating technique. With the 

exception of two sericite samples that were hand separated, samples were crushed and sieved to recover the 125 to 250 um fraction. The concentrates were washed in 10 percent HC1 in an 

ultrasonic bath to remove carbonate and were hand picked to 

remove grains that showed evidence of incipient alteration. 

Samples were irradiated for about 30 hours at 1 megawatt in the 

U.S. Geological Survey TRIG A reactor in Denver. Corrections 

for radiation-produced interfering isotopes of argon were made 

by applying measured production ratios for the interfering isotopes of argon produced in pure CaF2 and K2SO4 irradiated 

along with the samples (see Appendix). Neutron flux during 

irradiation was monitored using hornblende standard MMhb-1, 

which has a K-Ar age of 520.4±1.7 Ma (Samson and Alexander, 

1987). Errors of 0.1 percent in the calculated J-value were 

experimentally assessed by calculating the reproducibility of 

multiple monitors. Samples were heated in a low-background, 

resistively heated, double vacuum Ta-furnace in a series of eight 

to nineteen 20-minute-long steps. Isotopic abundances of argon 

were measured using a mass spectrometer operated in the static 

mode. All data were reduced using the decay constants and isotopic abundances suggested by Steiger and Ja'ger (1977). Age 

plateau criteria were based on Fleck and others (1977), whereby 

a plateau is defined by two or more contiguous gas fractions 

whose apparent ages are statistically indistinguishable at the 95 

percent confidence level (Dalrymple and Lanphere, 1969) and 

which together total greater than 50 percent of the total 39ArK 

released. Plateau ages represent a weighted mean, where 

weighting is by the inverse of the analytical variance (Taylor, 

1982). The error was determined using equations of Dalrymple 

and Lanphere (1981). For further details on this analytical 

method, see Snee and others (1988). All ages listed in this publication are reported at the la confidence level; previous K-Ar 

ages have been recalculated using the 1977 IUGS constants 

(Hon and Mehnert, 1983).

Early Caldera Cycles

Geology and Previous Age-Related Studies

The Ute Ridge Tuff erupted from the Ute Creek caldera, the 

first of six calderas to form in the western San Juan caldera 

cluster (fig. 1) (Lipman and others, 1973). This initial eruption

2 Western San Juan Mountains, Colo., Lake City Caldera Area

of mafic dacite was followed by eruption of the Blue Mesa Tuff, 

Dillon Mesa Tuff, Sapinero Mesa Tuff, and Crystal Lake Tuff, a 

series of nearly indistinguishable low-silica rhyolites. Each of 

these ash-flow eruptions produced calderas, though the Dillon 

Mesa and Sapinero Mesa Tuffs appear to have both erupted from 

the coalesced San Juan-Uncompahgre caldera structure (Lipman 

and others, 1973).

Earlier reported K-Ar dates for the Ute Ridge Tuff (29.1 

Ma) and Fish Canyon Tuff (27.8 Ma; central San Juan caldera 

cluster), which overlie the Sapinero Mesa Tuff, suggested that 

the four early calderas of the western San Juan complex 

formed in rapid succession (Lipman and others, 1970). However, the age of the Ute Ridge Tuff was actually an average of 

two discordant K-Ar ages (30.1±0.6 Ma and 28.1±0.5 Ma) on 

coexisting biotite and sanidine. These age discrepancies underscore some of the inherent uncertainties in the K-Ar technique, 

which make it unsuitable for more rigorous age comparative 

studies.

We have redated the Ute Ridge Tuff and provided the first 

published dates from the Blue Mesa, Dillon Mesa, and Sapinero 

Mesa Tuffs to further refine the duration of the caldera-forming 

intervals and the timing of individual eruptions. Although the 

age of the Crystal Lake Tuff is bracketed by the underlying 27.8 

Ma Fish Canyon Tuff (Kunk and others, 1985) and the overlying 

27.6 Ma Carpenter Ridge Tuff (Lanphere, 1988) from the central 

San Juan caldera cluster, we also dated it to test the concordancy 

of our ages. The timing of caldera-related events for both the 

eastern and central caldera clusters has recently been refined 

using precise 40Ar/39Ar dating (Lipman and others, 1997; Lanphere, 1988), and here we present similar 40Ar/39Ar data for the 

western San Juans, so that the timing of events can be more easily compared.

New 40Ar/ 39Ar Ages and Interpretations

Ten new 40Ar/ 39Ar age determinations were made on outflow pyroclastic units from early calderas (>23 Ma) of the western San Juan volcanic field (fig. 2A, B; table 1). Sanidine 

sample UR-1 from outcrops of Ute Ridge Tuff southwest of Finger Mesa (fig. 2A, loc. 1) has a slightly U-shaped spectrum (fig. 

3A) suggesting the presence of minor excess 40Ar, but a 

weighted mean based on steps 4-9, containing 57 percent of the 

39ArK released, gives a weighted mean age 28.63 Ma that we 

consider to best represent the age of this sample. This is confirmed by an 40Ar/36Ar vs. 39Ar/36Ar isochron plot (steps 2-12) 

with a slightly elevated atmospheric intercept [(40Ar/36Ar)i] of 

316±5, which is greater than the atmospheric value of 295.5.

Sanidine (BM-2) from the Blue Mesa Tuff was obtained 

from a sequence of ash-flow tuffs on the north edge of Alpine 

Plateau (fig. 2B, loc. 2). This sample has a relatively simple age 

spectrum (fig. 35) with a plateau age of 28.40±0.07 Ma. In contrast, the age spectrum for sanidine sample BM-3 from Blue 

Mesa Tuff east of Telluride (Potosi Peak; fig. 2A, loc. 3) is somewhat discordant, yielding a plateau age of 28.30±0.05 Ma; 

apparent ages within the spectrum show a gradual rise in the 

middle to higher temperature steps (fig. 3C). An isochron plot 

using all the steps gives an age of 28.59 ± 0.2 Ma with an atmospheric intercept [(40Ar/36Ar)i] of 231±15, indicating apparent 

argon loss. Although the isochron date is our best estimated age
107 C

38C

COLORADO

SAN JUAN VOLCANIC FIELD

Figure 1. San Juan volcanic field, Colorado, showing location of Oligocene calderas 

(hachured lines). Calderas of western San Juan Mountains include U, Ute Creek; L, Lost 

Lake; UN, Uncompahgre; SJ, San Juan; S, Silverton; LC, Lake City. Calderas of central 

and eastern San Juan Mountains include LG, La Garita; B, Bachelor; C, Creede; SR, 

South River; MH, Mount Hope; CP, Cochetopa Park; SU, Sumrrvtville; P, Platoro; BN, 

Bonanza. Modified from Lipman and others (1997) and Lipman and Steven (1976).

for this sample, we use this age only to confirm the simple plateau age from sample BM-3 (28.40±0.07 Ma), our preferred age 

for the Blue Mesa Tuff.

Sanidine from a single pumice (DM-3) collected from outcrops of Dillon Mesa Tuff on the south edge of Alpine Plateau 

(fig. 2B, loc. 4) gives an age spectrum that is slightly discordant 

(fig. 3D), but a weighted mean age using steps 3-9 gives a preferred age mean of 28.38±0.05 Ma. Sanidine from Dillon Mesa 

Tuff at Potosi Peak (DM-4, loc. 5) has a simple age spectrum 

with a plateau age of 28.42±0.06 Ma (fig. 3£). Our preferred 

age from sample DM-3 (28.38±0.05 Ma) and the plateau age 

from sample DM-4 (28.42±0.06 Ma) are analytically indistinguishable at the 95 percent confidence level and yield an arithmetic mean age of 28.40±0.04 Ma.

Three sanidine samples were analyzed from outflow of the 

Sapinero Mesa Tuff. The 40Ar/ 39Ar age spectra for all three 

samples are remarkably simple and easy to interpret. Sample

SM-4 (fig. 3F), collected from fiamme in vitrophyre near Gateway north of Lake City (fig. 2B, loc. 6), gave a plateau age of 

28.22±0.07 Ma. Sample SM-5, which was collected from outcrops of the Sapinero Mesa Tuff southeast of Sapinero (fig. 2B, 

loc. 7), yielded a plateau age of 28.17±0.06 Ma (fig. 3G). 

Finally, sample SM-6 from an ash-flow sequence near Potosi 

Peak (fig. 2A, loc. 8) gave a plateau date of 28.18±0.05 Ma (fig. 

3H). Data for all three samples overlap statistically at the 95 

percent confidence level, yielding a weighted mean age of 

28.19±0.03 Ma.

Sanidine sample CL-6 from a vitrophyre at the base of the 

Crystal Lake Tuff near Uncompahgre Peak (fig. 2A, loc. 9) has a 

simple age spectrum with a plateau age of 27.62±0.07 Ma (fig. 

37). The age spectrum for sanidine sample CL-7 (fig. 37) collected from Crystal Lake Tuff in South Clear Creek, near Hermit 

Lakes (fig. 2A, loc. 10), is somewhat discordant, although the 

intermediate steps of the age spectrum give a weighted mean age

Early Caldera Cycles 3
Western SanJuan IV

o e

1

3

O

o~B> 

CD OoQJ_

n.

CDB)

CDQJ

Table 1. 40 Ar/39Ar age-spectrum data for rocks and mineralized material from the western San Juan Mountains. Sample localities shown in fig. 2A and B.

Map 

Figure 

No.

1

23

45

6

7 

8

9 

10

Sample 

No.

UR-1BM-2

BM-3

DM-3DM-4

SM-4 

SM-5 SM-6

CL-6 CL-7

Rock type

Early caldera cycles

Ute Ridge Tuff

Blue Mesa Tuff

Blue Mesa Tuff

Dillon Mesa TuffDillon Mesa TuffSapinero Mesa Tuff Sapinero Mesa Tuff Sapinero Mesa Tuff

Crystal Lake Tuff Crystal Lake Tuff

Spectrum Locality _ disturbed?

Rio Grande Res.

Alpine PlateauPotosi Peak Yes?

Alpine Plateau

Potosi Peak

Gateview Ten Mile Spring 

Potosi PeakNellie Creek 

S. Clear Creek Yes

Mineral 

dated 1

sasa

sasa

sa

sa sa sa

sa sa

Average Plateau age weighted

(Ma + 1 s.d.) mean age 

(Ma ± 1 s.d.)

28.57 + 0.08 28.63 ± 0.05

28.40 + 0.07

28.30 + 0.0528.38 ±0.05

28.42 + 0.0628.22 + 0.07 

28.17 + 0.06 

28.18 + 0.0527.62 + 0.07 

27.73 ± 0.05

Reset age Isochron age Preferred age 

(Ma ± 1 s.d.) (Ma ± 1 s.d.) (Ma + 1 s.d.)

28.72 ± 0.03 28.63 ± 0.05

28.40 + 0.07

28.59 ±0.2 28.59 ±0.2

28.45 ± 0.04 28.38 + 0.05

28.42 ± 0.0628.22 + 0.07 28. 17 ±0.06 28.18 + 0.05

27.64 ± 0.02 27.62 ± 0.07 

27.81 ±0.06 27.73 + 0.05

Regional 26-25 Ma calc-alkaline intrusions and related mineralization

1112 1314

1516

SM-1SM-2 

MP-8

UPL-1

UHT-10

LV-9

Sultan Mountain stockSultan Mountain stock 

Monzonite intrusionVolcanics of Uncompahgre Peak

Ute-Hidden Treasure vein

Lilly vein

Sultan Mountain YesSultan Mountain Yes 

Capitol City

Uncompahgre Pk.

East Henson ( 'k. Yes

Capitol City

b

b b

sa

scsc

26.56 ± 0.04

26.66 ± 0.06 

26.39 ±0.125.63 ± 0.04

26.40 ± 0.04

25.29 ± 0.05

26.56 ± 0.04

26.73±0.06 26.66 + 0.06 

26.39 ±0.1

25.64 ± 0.03 25.63 + 0.04

22.89 ±0.1 26.40 ±0.04

25.29 + 0.05

Lake City caldera cycle and temporally associated mineralization

17

18

19

2021

22

2324

25

SP-11SP-12

DGM-13DRM-14

DRM-15

DRM-16

DRM-17

CA-18

BWV-19

Sunshine Peak Tuff (silicic pumice)Sunshine Peak Tuff (mafic pumice)

Dacite of Grassy Mountain

Dacite of Red Mountain (dike)

Dacite of Red Mountain (dike)

Dacite of Red Mountainpost-alunite dikeDacite of Red Mountainpost-alunite dike

Coarse-grained vein alunite

Post-Lake City caldera mineralization

Black Wonder vein

middle membermiddle member

Grassy Mtn.

S. flank Red Mtn.

N. flank Red Mtn.Red MountainRed Mountain YesRed Mountain Yes

Sherman

sasasasa

sasa

b

al

ad

22.94 ±0.1

22.94 ± 0.05

22.91 ±0.05

23.01 + 0.04

22.87 + 0.07

22.77 ± 0.06

23.24 ± 0.0423.11 ±0.04

16.52 ±0.03

22.94 ±0.122.94 + 0.05

23.01 ±0.03 22.91 ±0.05

22.96 ± 0.04 22.96 ± 0.05

22.87 ± 0.07

22.56 ± 0.3 22.77 + 0.06

16.47 + 0.02 16.47 + 0.02

1 ad= adularia, al= alunite, b= biotite, sa= sanidine, sc= sericite.
of 27.73±0.05 Ma that probably represents a reasonable estimate 

of the age of this sample. However, given the complicated 

nature of this sample, we consider the date from the Nellie Creek 

sample CL-6 (27.62±0.07 Ma) our preferred age of the Crystal 

Lake Tuff.

A summary of our new 40Ar/ 39Ar dates for the pyroclastic 

units of the western San Juan volcanic complex is presented in 

table 2. From these new data we can constrain early calderarelated volcanism in the western San Juan volcanic field to 

between 28.63 Ma (Ute Ridge Tuff) and 27.62 Ma (Crystal Lake 

Tuff) within a variance of 0.88 to 1.13 million years at the lo 

confidence level. In addition, 40Ar/39Ar ages for the intervening 

Blue Mesa (28.40±0.07 Ma), Dillon Mesa (28.40±0.04) and 

Sapinero Mesa (28.19±0.03 Ma) Tuffs record a very brief eruptive time interval ranging from 0.11 to 0.31 million years (lo). 

The weighted mean or preferred ages of these units (table 2), 

with the exception of the Blue Mesa and Dillon Mesa Tuffs, are 

analytically distinguishable at the 95 percent confidence level 

using the Critical Value Test of Dalrymple and Lanphere (1969).

Regional 26-25 Ma Calc-Alkaline Intrusions 

Geology and Previous Age-Related Studies

A large swarm of calc-alkaline intrusions was emplaced 

between 26 and 25 Ma over a broad region of the western San 

Juan Mountains. These intrusions range from large stocks at Mt. 

Wilson west of Ophir and Sultan Mountain near Silverton to 

smaller plugs intruded near Capitol City in the Lake City area 

(fig. 2A). Although the overall size of the exposed plutons 

decreases from west to east, this may be an artifact of erosion 

rather than a reflection of their actual size. These plutons vary 

from simple to complexly zoned and in many places are associated with veins and and disseminated and stockwork Mo-Cu 

mineralized rocks (Ringrose and others, 1986; Slack, 1976; Caskey, 1979; Pyle, 1980). Many of these intrusions are completely 

crystalline and cut through the entire ash-flow sequence, suggesting that they may represent cores of now-eroded stratovolcanoes that fed thick sequences of lavas nearby (Lipman and 

others, 1973). Alternatively, the widespread distribution of 

intrusions also suggests that they represent the uppermost portion of the underlying batholith rising into and consuming the 

earlier volcanic pile—similar to the emplacement of the Boulder 

batholith into the Elkhorn Mountains Volcanics in Montana 

(Lipman, 1984).

Previous K-Ar determinations on biotite from the monzonitic Ophir and Sultan Mountain stocks (fig. 2A) gave concordant dates with a weighted mean age of 25.9±0.6 Ma (Jackson 

and others, 1980). A weighted mean age of 25.2±0.6 Ma (Jackson and others, 1980) was also determined from two concordant 

K-Ar dates on biotite from slightly more evolved monzonite porphyry phases of the Sultan Mountain stock in the Middle ForkOphir Pass area near Silverton (fig. 2A) (Ringrose and others, 

1986, fig. 1, p. 358). A K-Ar date of 25.7±0.6 Ma was determined by H.H. Mehnert (in Slack, 1980) on biotite from one of

several small monzonite to monzogranite porphyry stocks in the 

Capitol City area (fig. 2A). Like the nearby Capitol City stocks, 

small monzonite to dacite porphyry intrusions in the Iron Beds 

and Matterhorn Peak areas (fig. 2A) are also associated with 

zoned alteration and weak mineralization (Slack, 1976; Caskey, 

1979; Pyle, 1980) and have K-Ar ages of 26.6±0.4 (hornblende) 

and 25.2±2 Ma (plagioclase), respectively (F. McDowell, written 

commun., 1994).

New 40Ar/ 39Ar Ages and Interpretations

Three biotite separates from samples related to the 26-25 

Ma period of intrusion and mineralization were analyzed for this 

study (table 1). In addition, sanidine from the top of a thick 

sequence of lavas exposed near Uncompahgre Peak (fig. 2A) 

(volcanics of Uncompahgre Peak; Lipman, 1976) was dated to 

test the hypothesis of Lipman and others (1973) that these lavas 

were extrusive equivalents of the monzonite intrusions.

Two samples (SM-1 and SM-2) from the Sultan Mountain 

stock, near Silverton (fig. 2A, Iocs. 11, 12) overlap at the 95 percent confidence level and have a weighted mean age of 

26.60±0.04 Ma. Both samples have slight humps in the middle 

spectrum region that probably reflect the presence of incipiently 

chloritized biotite grains. Sample SM-1 has a weighted mean 

age of 26.56±0.04 Ma (fig. 3K), whereas SM-2 has a weighted 

mean age of 26.66±0.06 Ma (fig. 3L). Although both biotites 

gave disturbed age spectra, we consider a weighted mean date of 

26.60±0.04 Ma to be our best estimate of the cooling age of 

these samples.

Biotite MP-8 is from a monzonite intrusion near the Capitol 

City townsite, about 30 km northeast of Silverton (fig. 2A, loc. 

13). This sample yields a simple age spectrum (fig. 3M) with a 

plateau age of 26.39±0.1 Ma, which represents the cooling age of 

this intrusion. Sanidine sample UPL-1 from dacitic lavas of the 

volcanics of Uncompahgre Peak (fig. 2A, loc. 14) yielded a simple age spectrum (fig. 3AO with a plateau age of 25.63±0.04 Ma.

The weighted mean age of 26.60±0.04 Ma from Sultan 

Mountain stock biotite samples (SM-1, SM-2) is statistically 

indistinguishable from the plateau age (26.39±0.1 Ma) of sample 

MP-8 from an intrusion in the Capitol City area (table 2). 

Although these ages compare well to previous K-Ar determinations, late-phases of these intrusions such as documented in the 

Middle Fork-Ophir Pass area (25 Ma; Ringrose and others, 

1986) are not represented in the present study. Still, these new 

ages show a minimum time break of about 1 million years 

between the end of early caldera-related magmatism in this area 

(27.6 Ma) and emplacement of these 26-25 Ma mineralized 

intrusions. The age of sanidine sample UPL-1 (25.63±0.04 Ma) 

collected from the uppermost flows of the volcanics of

Figure 2 (following two pages). Generalized geologic maps of western 

San Juan Mountains area, showing localities of dated samples. Generalized caldera boundaries shown in figure 1. A, Geology modified from 

Lipman and others (1976) and Hon (1987b). B, North of the Lake City 

caldera. Geology modified from Tweto and others (1976).

Regional 26-25 Ma Calc-Alkaline Intrusions
I

CO

fi)

r+

Oi

o_ 

o5T?r

(D O

Lake Fork 

center

A

20 KILOMETERS

Peak stock Uncompahgre CV Tof p) *\

Cow Creek center Iron Beds 

intrusions Ute-Ulay mine

^1&££

Carson 

center

Middle 

Ophir phir stock stock f\ Sultan Mtn ,y stock 

Sllverton

/'Hermit Lake's ^ /./,''/•'//>', ','/s«j>.
107°30' 107°15' 107°00'

30 

CD C2. 5'09a

ITSw5'

EXPLANATION 

Late to middle Tertiary

volcanic rocks 

Mafic lava (16-21 m.y.)

Rhyolite-dacite intrusions

(10-23 Ma) 

Lake City caldera fill (22.9

Ma) 

Post-caldera intrusions

(25-26 Ma) 

Uncompahgre-San Juan

and Silverton caldera

complex (27.6-28.4 Ma) 

Regional ash-flow sheets

and minor lavas (27-30

Ma) 

Lavas, breccias, and

related volcaniclastic

rocks

Prevolcanic rocks 

Mesozoic and Paleozoic

sedimentary rocks 

Proterozoic rocks (1.4-1.8

38° 30'38° 

15'

ContactFault—Bar and ball on 

downthrown side

//, //\ + Alpine + 

Plateau
Via) Apprent Ag(Ma)

10 cco ro-> 

01 co 

Apparent Age(Ma) 

co ro-> ro_> 

O U1OO1O O1OO1O OUl

Apparent Ag -> ro 01 o 01

UR-1 Sanidine 

Ute Ridge Tuff 

Isochron age = 28.72+0.03 Ma (10) 

(40Ar/36Ar): = 316+5

- 1

•< ————————————————— >•

Plateau age = 28.6310.05 Ma (10)

A

BM-2 Sanidine 

Blue Mesa Tuff

* —————————————————————————————— »

Plateau age = 28.40+0.07 Ma (10)

B

BM-3 Sanidine 

Blue Mesa Tuff 

Isochron age = 28.5910.2 Ma (10) 

(40Ar/36Ar)i = 231+15

Plateau age = 28.30+0.05 Ma

C

U

DM-3 Sanidine 

Oil Ion Mesa Tuff 

Isochron age = 28.45+0.04 Ma (10) 

(40Ar/36Ar)j = 293+4

r, — 

Weighted mean age = 28.3810.05 

_^ 

Ma (10) ——

D

DM-4 Sanidine 

Oil Ion Mesa Tuff

« ——————————————————————————————— J

Plateau age = 28.42+0.06 Ma

£

i i i i i i i i i

>

35

30

- 25

20

15

30

25

20

15

30

en 

< 25

20

15

„ 30

10 Ul

20

15

H

SM-4 Sanidine 

Sapinero Mesa Tuff

Plateau age = 28.22±0.07 Ma (la)

SM-5 Sanidine 

Sapinero Mesa Tuff

Plateau age = 28.17±0.06 Ma (10)

SM-6 Sanidine 

Sapinero Mesa Tuff

Plateau age = 28.18+0.05 Ma

CL-6 Sanidine

Crystal Lake Tuff

Isochron age = 27.64+0.02 Ma (10)

Plateau age = 27.62+0.07 Ma (10)

10 20 30 40 50 60 70 80 90 100

Percent 39Ar^ Released

40A../39 Figure 3. 4UAr/JbAr age-spectra for dated samples.

10 20 30 40 50 60 70 

Percent 39Ar^ Released

80 90 100

8 Western San Juan Mountains, Colo., Lake City Caldera Area
35

Agt 

N) 01

20 -

15

30

ro 

< 25

20

15

30

< 25

20

15

35

1 30

20

15

10

K

M

CL-7 Sanidine

Crystal Lake Tuff

Isochron age = 27.81±0.06 Ma (1o)

(40Ar/36Ar): = 300±6

Weighted mean age = 27.73±0.05 Ma do)

SM-1 Biotite

Sultan Mountain stock

Weighted mean age = 26.56±0.04 Ma

SM-2 Biotite

Sultan Mountain Stock

Isochron age = 26.73±0.06 Ma do)

(40Ar/36Ar). = 293+4

Weighted mean age = 26.66±0.04 Ma

MP-8 Biotite 

Monzonite intrusion

Plateau age = 26.39+0.1 Ma do)

0 10 20 30 40 50 60 70 

Percent 39Ar|< Released

90 100

Figure 3—Continued. 40Ar/39Ar age-spectra for dated samples.

Uncompahgre Peak represents the minimum age of this thick 

sequence of dacitic to andesitic lavas. As this sample represents 

a minimum age for this thick sequence of flows, we consider it 

to be close enough in proximity to the 26.4 Ma date on the 

monzonite intrusion in Capitol City (table 2) to corroborate the 

assumption of Lipman and others (1973) that the Capitol City 

intrusions may have been the primary feeders for the volcanics 

of Uncompahgre Peak.

35

30

UPL-1 Sanidine

Volcanics of Uncompahgre Peak 

Isochron age = 25.64±0.03 Ma do) 

(40Ar/36Ar) = 304±0.2

03 I O) yi_. i 25 r

(D I

20

35

30h

Q. 20

15

10

- 30

(D

CD 

CD

~ 25

CD

CD 

Q. 

Q.

20

15

35

"(D

^ 30

CD 

O)

c

CD

a 20

15

10

Plateau age = 25.63±0.04 Ma

N

UHT-10 Sericite (1M polytype) 

Ute-Hidden Treasure vein

700°C

400°C I 

22.89±0.1 Ma (1o)

o

26.4±0.04 Ma do)

LV-9 Sericite (1M/2M1 polytype) 

Lilly Vein

Plateau age = 25.29±0.05 Ma do) 1

SP-11 Sanidine

Middle member, Sunshine Peak Tuff

Plateau age = 22.94±0.1 Ma do)

Q

0 10 20 30 40 50 60 70 80 90 100 

Percent 39Ar|< Released

Veins and Porphyry-Style Alteration Related to 

26-25 Ma Intrusions

Geology and Previous Age-Related Studies

Previous studies of the Middle Fork-Ophir Pass, Capitol 

City, Iron Beds, and Matterhorn Peak areas (fig. 2A) document a

Veins and Porphyry-Style Alteration Related to 26-25 Ma Intrusions
35

"(0

1 30

as

O)

1*1 

1

£ 20 -

15

10

SP-12 Sanidine

Middle member. Sunshine Peak Tuff

35

"(0

1 30

0)

c 25

ID

(D 

I" 2°

15

10

"CD 

1 30

R

Plateau age = 22.94±0.05 Ma (la)

DGM-13Sanidine

Dacite of Grassy Mountain

Isochron age = 23.01+0.03 Ma (1o)

Plateau age = 22.91±0.05 Ma (la)

DRM-14Sanidine 

Dacite of Red Mountain 

Isochron age = 22.96±0.04 Ma (1o) 

(40Ar/36Ar): = 302±3

a. 20k Weighted mean age = 23.0110.04 Ma (la)

10

35

"(0

^ 30

<u

D)

1 25

a)

(D

S 20

15

10

DRM-15 Sanidine 

Dacite of Red Mountain

Plateau age = 22.87±0.07 Ma (a)

u

0 10 20 30 40 50 60 70 80 90 100 

Percent 39Ar|< Released

Figure 3—Continued. ^Ar/^Ar age-spectra for dated samples.

35

IS 

1 30

a>

D)

< 

- 25

0)

(D

| 20 

15 

10

35

"to 

^ 30

a>

O)

< 

£ 25

a>

(D

a 20

15

10

35

"CD

S 30

(U 

D) 

<

£ 25

<u

(D

a 20

15

10

18

"CD

| 16

a> 

w 14

Q.

a. 

<

12

10

DRM-16 Sanidine

Post-alunite dacite of Red Mountain 

Isochron age = 22.5610.3 Ma 

(40Ar/36Ar)j = 319±4

Weighted mean = 22.77±0.06 Ma (la)

V

DRM-17 Biotite

Post-alunite dacite of Red Mountain

Plateau age = 23.24±0.04 Ma do)

w

CA-18Alunite 

Late-stage vein alunite

Plateau age = 23.11±0.04 Ma (la)

X

BWV-19Adularia

Black Wonder Mine

Isochron age = 16.4710.02 Ma (la)

(40Ar/36Ar); = 30512

Plateau age = 16.52±0.03 Ma (1o)

0 10 20 30 40 50 60 70 80 90 100 

Percent 39Arx Released

close spatial association between porphyry Mo-Cu mineralization, base-metal sulfide veins, and 26-25 Ma monzonite intrusions (Ringrose and others, 1986; Slack, 1980; Caskey, 1979; 

Pyle, 1980). A weakly mineralized Mo-Cu porphyry system 

present in the Middle Fork-Ophir Pass area is temporally and 

genetically related to a late quartz monzonitic phase (25 Ma) of 

the Sultan Mountain stock (Ringrose and others, 1986). At the 

Middle Fork-Ophir Pass locality, quartz-molybdenite stockwork 

veins associated with intense phyllic alteration are cut by

10 Western San Juan Mountains, Colo., Lake City Caldera Area

molybdenite-bearing, base-metal veins present mostly on the 

margins of the porphyry system (R.T. McCusker, written commun. to Amax Exploration, Inc., 1982). Widespread porphyrystyle alteration in the Capitol City area (fig. 2A) is also spatially 

associated with several small 26 Ma monzonite to monzogranite 

porphyry stocks. Veins in this area, which largely correspond to 

the early quartz-base-metal assemblage of Slack (1980), commonly cut the margins of these intrusions and extend for short 

distances into surrounding volcanic country rock.
Table 2. Summary of new geochronologic data for western San Juan mountains and previous K-Ar ages.

[ad, adularia; b, biotite; sa, sanidine; sc, sericite; al, alunite; m, mean age from various minerals]

Unit Source

Mineralization 

stage

40Ar/39Ar age 

Mat la

New 

K-Ar age 

Ma ±1a

Previous K-Ar 

age Ma ± 1a

Early caldera cycle

Ute Ridge Tuff Ute Creek caldera n.a.n 28.63 ± 0.05 Ma b 30.1 ±0.6 Ma1 

sa 28.1 ±0.5 Ma1

Blue Mesa Tuff 

Dillon Mesa Tuff 

Sapinero Mesa Tuff

Lost Lake caldera 

San Juan caldera?

San JuanUncompahgre

calderas

n.a.

n.a.

n.a.

28.40 ± 0.07 Ma 

28.40 ± 0.04 Ma 

28.19 ±0.03 Ma

Crystal Lake Tuff Silverton caldera n.a. 27.62 ±0.07 Ma

Regional 26-25 Ma calc-alkaline intrusions and related vein mineralization

Sultan Mountain stock near n.a. n.a. 26.60 ± 0.03 Ma 

Silverton

b 25.9 ± 0.6 Ma2 

b 25.2 ± 0.6 Ma3

Monzonite intrusion at 

Capitol City

n.a.

Volcanics of Uncompahgre 26-25 Ma 

Peak monzonite

intrusions

n.a.

26.39 ±0.1 Ma

25.63 ± 0.04 Ma

b 25.7 ± 0.6 Ma4

Sericite from Ute-Hidden n.a. 

Treasure vein (Lake City)

Sericite from Lilly vein n.a. 

near Capitol City

Early quartz-basemetal assemblage

Early quartz-basemetal assemblage

26.40 ± 0.04 Ma

25.29 ± 0.05 Ma

sc 20.8 ± 0.4 Ma •

Lake City caldera and related mineralization

Sunshine Peak Tuff and 

post-resurgence lavas and 

intrusions

Lake City caldera 

and associated 

magmatism

Coarse-grained vein alunite Hydrothermal 

at Red Mountain system at Red

Mountain

n.a.

n.a.

22.93 ± 0.02 Ma

23.11±0.04Mat

m 23.1+0.5 Ma 6

a 23.3 ± 0.6 Ma

Sericite from Ute-Hidden 

Treasure vein (Lake City)

n.a. Late-bariteprecious-metal

assemblage

22.9 ±0.1 Ma |

19-17 Ma rhyolite intrusions and associated mineralization

Rhyolite intrusions (north n.a. n.a.

margin of Uncompahgre

caldera)

Cuba Gulch rhyolite dike n.a. n.a.

18.8 ±0.4 Ma S a 19.1 ±0.3 Ma 5 

sa 19.0 ±0.2 Ma 5

17.1 ±0.3 Ma

Adularia from Black 

Wonder vein

n.a. 16.47 ±0.02 Ma

^ Sample contains excess argon and slightly older than 22.93 Ma host rocks. 

* Apparent age of resetting related to nearby barite-precious-metal vein shoots.

n Not applicable; Age references: !Lipman and others, 1970; 2Jackson, 1980; 3Jackson, 1980 (young phase, Sultan Mountain Stock); 4Slack, 1980; 

5Lipman and others, 1976; 6Hon 1987b; Mehnert and others, 1978; Lipman and others, 1970; Lipman and others, 1976; 7Mehnert and others, 1979.

Veins and Porphyry-Style Alteration Related to 26-25 Ma Intrusions 11
The same early quartz-base-metal assemblage is also 

present in the Ute-Hidden Treasure vein of the Ute-Ulay mine 

(fig. 2A), one of the most productive vein systems in the Lake 

City area (Slack, 1980). Veins of the quartz-base-metal assemblage are mostly present along Henson Creek west of Lake City 

(Slack, 1980), where most trend northeast, transverse to the ring 

fault of the Lake City c alder a. However, unlike the early quartzbase-metal veins of the Capitol City area, veins of the Ute-Ulay 

mine are not spatially associated with exposed and altered 26 Ma 

monzonitic intrusions. Lacking this relationship, earlier studies 

by Slack (1976, 1980) postulated a genetic relationship between 

the quartz-base-metal veins and the 23 Ma Lake City caldera.

Previous K-Ar determinations from coarse-grained sericite 

related to porphyry-style phyllic alteration in the Middle ForkOphir Pass area gave ages of 24.9±0.6 Ma and 25.0±0.6 Ma 

(Ringrose and others, 1986); these ages overlap statistically with 

their quartz monzonite host intrusions that had a weighted mean 

K-Ar date of 25.2±0.6 Ma (Jackson and others, 1980). Base 

metal vein mineralization and alteration associated with monzonitic intrusions in the Capitol City, Iron Beds, and Matterhorn 

Peak areas were not previously dated. However, Lipman and 

others (1976) described haloes of intensely altered and pyritized 

rock surrounding the Capitol City intrusions that are truncated 

abruptly at the topographic wall of the Lake City caldera. Their 

findings clearly demonstrated that intrusion-related alteration at 

this locality preceded caldera collapse at 23 Ma.

New 40Ar/ 39Ar Ages and Interpretations

Coarse-grained sericite (sample UHT-10) from the early 

quartz-base-metal assemblage in the Ute-Hidden Treasure vein 

(fig. 2A, loc. 15) was previously dated by H.H. Mehnert (in 

Slack, 1980) using the K-Ar method. However, the validity of 

this sample's apparent age (20.1 ±0.6 Ma) has been questioned 

due to geologic inconsistencies and relatively large errors associated with the analytical technique (Slack, 1980; Hon and others, 

1985). Because establishing accurate age constraints on this significant vein assemblage is important, we redated sericite from 

sample UHT-10 (Slack, 1980) by the 40Ar/39Ar method. In addition, we obtained an 40Ar/39Ar date on sericite from another 

quartz-base-metal vein (Lilly mine; Irving and Bancroft, 1911) 

near Capitol City to further constrain vein and intrusive relationships observed there.

40Ar/39Ar step-heating experiments suggest that 1M sericite 

(UHT-10) from the early quartz-base-metal assemblage in the 

Ute-Hidden Treasure vein originally formed at about 26.4 Ma. 

The step-up in apparent age with increasing temperature as 

observed in the age spectrum (fig. 36>) may indicate an original 

age of sericite formation as well as an apparent age of resetting. 

We consider the apparent age from step 5 (26.40±0.04 Ma), one 

of the statistically best steps with highest radiogenic yields, to be 

the best estimate of the maximum age of the sericite. The apparent resetting at about 22.9 Ma probably records a younger thermal event, which will be discussed in a later section of this 

report. A sample of green, 1M/2MJ sericite from the Lilly vein 

(LV-9) about 1 km north of Capitol City (fig. 2A, loc. 16) rendered an 40Ar/39Ar plateau age of 25.29±0.05 Ma (table 1).

An original age of 26.40±0.04 Ma for the Ute-Hidden 

Treasure vein sericite (UHT-10) is analytically indistinguishable from that of Capitol City monzonite sample MP-8 

(26.39±0.1 Ma). Sericite from the Lilly vein (LV-9; 25.29±0.05 

Ma) is significantly younger than sericite from the Ute-Hidden 

Treasure vein, indicating that they formed during two distinct 

episodes of mineralization. However, these new ages clearly 

depict the close association between early quartz-base-metal 

assemblage veins in the Lake City area and 26-25 Ma monzonite intrusions (table 2).

Evolution of the Lake City Caldera

Geology and Previous Age-Related Studies

Collapse of the Lake City caldera postdated all other 

caldera-related eruptions in the western San Juan Mountains by 

more than 4 million years. The Lake City caldera is thought to 

be associated with bimodal magmatism related to the onset of 

extensional tectonism in this general region at about 25 Ma (Lipman and others, 1978). It is surrounded by the older Uncompahgre, San Juan, and Silverton calderas (fig. 1), which formed from 

28.4 to 27.5 Ma during an earlier period of volcano-tectonism 

(Steven and Lipman, 1976).

Initial pyroclastic eruptions from the Lake City caldera 

were accompanied by large-scale collapse along an arcuate ring 

fault zone, which also served to control later eruptive vents. The 

Sunshine Peak Tuff comprises three compositionally distinct 

ash-flow members (fig. 4) that record eruption from a magma 

chamber that was zoned downward from high-silica rhyolite to 

quartz trachyte (Hon and Lipman, 1989; Hon, 1987b). The lower 

member of the Sunshine Peak Tuff, a high-silica rhyolite (76 

percent SiO2 ), marks the onset of pyroclastic activity; it spread 

widely from the source region and accumulated to a thickness of 

more than 900 m within the contemporaneously subsiding 

caldera (Hon and Lipman, 1989; Hon, 1987b). Subsequent 

pyroclastic eruptions resulted in recurrent caldera collapse; these 

eruptions first deposited a 300-m-thick rhyolite unit (74 percent 

SiO2), which forms the middle member of the Sunshine Peak 

Tuff. The middle member has two distinct pumice types: a highsilica rhyolite identical to that in the lower member, and a mafic 

trachyte (65 percent SiO2). The upper member is quartz trachyte 

(67-69 percent SiO2) and marked the final pyroclastic eruptions 

from the zoned magma chamber. No erosional breaks occur 

within this entire ash-flow sequence, indicating that all three 

members of the Sunshine Peak Tuff erupted in rapid succession 

(Hon and Lipman, 1989).

A large quartz syenite pluton was intruded into the intracaldera fill causing resurgence of the Lake City caldera. Intrusion of quartz syenite was accommodated by asymmetric doming and uplift of more than 1 km along the north-central ring 

fault. The mineralogic, petrologic, and isotopic characteristics 

of the resurgent intrusions (Hon and Lipman, 1989; Hon, 

1987b) indicate that they were derived from the same alkalic 

magma chamber that produced the caldera-forming ash-flow 

eruptions.

12 Western San Juan Mountains, Colo., Lake City Caldera Area
107°26' 107°20'

38°00' —

37°57' —

5 KILOMETERS

EXPLANATION

Late calc-alkaline rocks 

Dacite porphyry dikes

Quartz monzonite and dacite

porphyry intrusions 

Postcollapse dacite lavas

Contact

Fault—Bar and ball on downthrown side

Resurgent intrusions 

High-silica rhyolite

Quartz syenite and rhyolite porphyry

Sunshine Peak Tuff 

Upper member

Middle member 

Lower member

Figure 4. Generalized geologic map of Lake City caldera (modified from Hon, 1987b).

Continued volcanic activity produced a thick sequence of 

post-caldera-collapse lavas collectively referred to as the dacite 

of Grassy Mountain (63-66 percent SiO2) (fig. 4). These 

calc-alkaline lavas and domal masses, which both predate and 

postdate caldera resurgence, accumulated within the large 

topographic scallop outside the ring fault on the eastern margin 

of the caldera.

The last phase of igneous activity within the Lake City 

caldera was marked by intrusion of the dacite of Red Mountain 

(63-66 percent SiO2) (Bove and Hon, 1992; Bove and others, 

1990) and the quartz monzonite of Alpine Gulch (67-68 percent 

SiO2) along the east and northeastern sections of the ring fault, 

respectively (fig. 4). These calc-alkaline intrusions are similar in 

major-element chemistry, mineralogy, and phenocryst textures,

Evolution of the Lake City Caldera 13
and have nearly identical isotopic compositions, suggesting derivation from a common magmatic source. Geophysical data 

(Grauch, 1987) indicate that a large intrusive body underlies the 

eastern portion of the caldera, and studies by Hon (1987b) suggest that this body represents the crystallized calc-alkaline 

magma chamber that fed these intrusions. Geochemical and isotopic data indicate that these intrusions—as well as the dacite 

lavas of Grassy Mountain—were derived from magma that 

evolved separately from the main alkaline magma chamber 

related to caldera eruptions and resurgent intrusion (Hon and 

Lipman, 1989).

Previous K-Ar determinations were made on 10 mineral 

separates from rocks—ash-flow tuffs, resurgent intrusions, and 

preresurgence lavas—representing all stages of development of 

the Lake City caldera (Lipman and others, 1970; Hon, 1987b; 

Lipman and others, 1976). The most reliable of these ages 

range between 22.6±0.5 Ma and 23.7±0.4 Ma; however, all of 

these ages within this apparent 1.1 million year time interval are 

statistically indistinguishable at the 95 percent confidence level. 

Based on these ages and associated errors, the absolute time 

span for caldera formation could vary from a maximum of 2 Ma 

(22.1 to 24.1 Ma) to a minimum of 0.2 Ma (23.1 to 23.3 Ma) at 

the lo confidence level. Further restrictions on the duration of 

the Lake City caldera cycle have been inferred from paleomagnetic studies (Reynolds and others, 1986), which record 

emplacement of the ash-flow tuffs and the postcollapse lavas, 

and initial cooling of the resurgent intrusions during a reversed 

polarity chron. Based on seafloor records of reversed magnetic 

periods within the Earth's field from 24 to 22 Ma, Reynolds and 

others (1986) have estimated that the time span from eruption 

to resurgence of the caldera was probably less than 300,000 

years.

New ^Ar/^Ar Ages and Interpretations

Seven new 40Ar/39Ar dates from mineral separates reported 

here (table 1; fig. 2A) span all stages of development of the Lake 

City caldera. This study was undertaken to ascertain whether 

individual phases of caldera activity could be distinguished 

using the higher analytical precision of the 40Ar/39Ar step-heating method and to test the time-span estimate of Reynolds and 

others (1986).

The ages of sanidine samples from silicic (high-silica rhyolite; SP-1 1) and mafic (trachyte; SP-12) pumice from vitrophyre 

of the middle member of the Sunshine Peak Tuff (fig. 2A, Iocs. 

17, 18) precisely constrain the age of emplacement of this unit. 

The age spectra for both samples are simple (fig. 3Q, R), and 

give plateau ages of 22.94±0.1 and 22.94±0.05 Ma, respectively. 

Sanidine from sample DGM-13 from postresurgence dacite of 

Grassy Mountain lavas (fig. 2A, loc. 19) gives a statistically 

identical plateau age of 22.91±0.05 Ma (fig. 35).

Sanidine from two dikes of the dacite of Red Mountain that 

crop out southeast (DRM-14) and northeast (DRM-15) of Red 

Mountain (figs. 2A and 4, Iocs. 20, 21) were also analyzed. The 

age spectrum for sample DRM-14 (fig. 37) displays some excess 

argon and has a slightly elevated atmospheric intercept [(40Ar/ 

36Ar)J of 302±3 derived from an isochron plot of 40Ar/36Ar versus 39Ar/36Ar. This sample has an isochron date of 22.96 ± 0.04

Ma after eliminating steps 13-14, which exhibit poor analytical 

reliability. The isochron date is our preferred age and is in close 

agreement with a statistically valid date of 23.01 ±0.04 Ma 

defined by eight temperature steps from 700° to 1,050°C that 

contain 41 percent of the released 39ArK. The age spectrum for 

sanidine sample DRM-15 (fig. 3t7) is relatively uncomplicated 

and has a plateau age of 22.87±0.07 Ma that is represented by 

four steps containing 86 percent of the released 39ArK.

Sanidine from a late-stage dike of the dacite of Red Mountain (DRM-16) (fig. 2A, loc. 22, and fig. 4) exhibits a relatively 

uncomplicated spectrum (fig. 3V) with a plateau age of 

22.77±0.06 Ma. The age spectrum from coexisting biotite (sample DRM-17 (fig. 2A, loc. 23, and fig. 3W)) is complicated, 

clearly showing the effects of excess argon in the low and high 

temperature steps. A plateau age of 23.24±0.04 Ma from the 

intermediate temperature steps is strikingly discordant with our 

preferred age from the coexisting sanidine (22.77±0.06 Ma). 

Such discordance is common in volcanic biotites, which in some 

cases have 40Ar/39Ar ages that are older than those of coexisting 

sanidine by 0.1-0.2 Ma (Harlan and others, 1998; Kellogg and 

others, 1994). However, we consider this age unreliable as it is 

statistically older than all other Lake City 40Ar/39Ar ages that are 

consistent with geologic data. We consider the 22.77 Ma age of 

the sanidine to be the best estimate for the age of the late phase 

of the dacite of Red Mountain intrusions.

Excluding biotite sample DRM-17, all of the previously 

discussed ages related to Lake City caldera volcanic events 

are statistically indistinguishable at the 95 percent confidence 

level and yield a weighted mean age of 22.93±0.02 Ma. The 

maximum age difference within this set of samples at the lo 

level is 330,000 years. Our maximum determined age variance for the lifespan of the caldera is in close agreement with 

the 300,000-year time span estimated by Reynolds and others (1986), emphasizing the benefit of using these techniques 

collaboratively.

Hydrothermal Activity and Mineralization 

Related to the Lake City Caldera

Geology and Previous Age-Related Studies

Hydrothermal systems were established both within the 

core of the Lake City caldera over the cooling resurgent plutons, and along and adjacent to the eastern margin of the 

caldera contemporaneous with emplacement of late calc-alkaline intrusions. Convective circulation of large volumes of 

meteoric water around the resurgent plutons and along the western and northern margins of the caldera was related to the formation of veins and associated alteration along extensional 

faults in the resurgent dome (Hon, 1987a). Veins that formed 

above the resurgent intrusions appear to have scavenged metals 

largely from the intracaldera Sunshine Peak Tuff, and have 

lower base-metal contents than similar veins peripheral to the 

intrusions (Hon, 1987b). This interpretation is confirmed by 

lead isotope studies (Sanford, 1992) showing that veins above 

the intrusions have Pb-isotopic ratios compatible with a lead 

source from the Sunshine Peak Tuff. Veins on the western

14 Western San Juan Mountains, Colo., Lake City Caldera Area
margin of the caldera, however, have Pb-isotopic ratios indicating that hydrothermal fluids interacted with less radiogenic, precaldera andesites (Lipman and others, 1978), whereas veins on 

the east side of the caldera have significantly more radiogenic 

Pb-isotopic ratios (Sanford, 1992), suggesting that vein-related 

hydrothermal fluids that circulated deeply along the margins of 

the resurgent pluton interacted with underlying radiogenic Precambrian crystalline rocks.

Massive alunitization (roughly 70 million metric tons of 

alunite) of postcollapse lavas and intrusions on Red Mountain 

formed by degassing of sulfur-rich dacitic magmas emplaced 

along the eastern ring fracture zone (fig. 4) (Bove and others, 

1990). Alteration is zoned vertically from alunite downward 

into phyllic and finally into a deeper potassic assemblage related 

to emplacement of one or more dacite porphyry intrusions. Subeconomic Mo and Cu mineralization also increases with depth 

and proximity to the deeper potassic alteration assemblage 

(Bove and Hon, 1992). Replacement alunite, related to the 

ascent of buoyant magmatic vapor plumes, makes up the largest 

proportion of alunite at Red Mountain (Bove and others, 1990); 

however, late-stage, coarse-grained vein alunite is present 

locally and has been interpreted to have formed by decompression of magmatic steam in open fractures (Bove and others, 

1990; Rye and others, 1992). A weakly mineralized quartz 

monzonite intrusion in nearby Alpine Gulch (fig. 4) is genetically associated with the dacite of Red Mountain intrusions 

(Hon, 1987b). The Alpine Gulch intrusion has features associated with Cu and (or) Mo mineralization including fragmental 

dikes, tourmaline breccias, magnetite veinlets, and quartz veins 

containing pyrite, hematite, sphalerite, galena, and chalcopyrite 

(Hon, 1987a). However, alteration haloes are conspicuously 

absent around this intrusion.

Slack (1980) noted that veins on the east and north sides of 

the Lake City caldera fill fractures roughly perpendicular to the 

ring fault that appear to form a radial pattern around the Red 

Mountain area. This pattern of radial veins that contain the late 

barite-precious-metal assemblage described by Slack (1980) 

was postulated to be related to emplacement of the 23 Ma dacite 

of Red Mountain intrusions (Slack, 1980). Thin veinlets of this 

late barite-precious-metal assemblage are present in the UteHidden Treasure vein of the Ute-Ulay mine (fig. 2A) and are 

shown by Slack (1980, fig. 4) to cut late ore shoots of the quartzbase-metal assemblage. Mixing of meteoric water off the resurgent dome of the caldera, peripheral to the sulfate-rich magmatic 

plume, may have caused flushing of sulfate from the hydrothermal system into the surrounding radial vein systems (Hon, 

1987b).

Previous K-Ar determinations were made on replacement 

and vein alunite from the Red Mountain complex on the east 

side of the 23 Ma Lake City caldera by Mehnert and others 

(1979). Replacement alunite yielded an age of 22.9±0.8 Ma, 

whereas an age of 23.3±0.6 Ma was determined on paragenetically later vein alunite. Studies by Hon and others (1985) disproved earlier assertions by Slack (1980), who claimed that the 

historically productive Golden Fleece vein (Irving and Bancroft, 1911)—geographically situated among barite-preciousmetal veins on the east side of the Lake City caldera (Slack, 

1976, 1980)—was contemporaneous with the 23 Ma Red 

Mountain hydrothermal system. Although originally thought to

be part of the barite-precious-metal assemblage (Slack, 1980), 

late-stage ore minerals from the Golden Fleece vein are mineralogically distinct, containing abundant gold and silver tellurides intergrown locally with minor colloform pitchblende 

(Hon and others, 1985); pitchblende is unknown elsewhere in 

the Lake City area (Slack, 1976; Hon and others, 1985). A UPb isochron age of 27.5±0.3 Ma and well-documented geologic relations (Hon and others, 1985) clearly indicate that formation of the Golden Fleece vein preceded collapse of the 

Lake City caldera.

New ^Ar/^Ar Ages and Interpretations

Coarse-grained vein alunite from Red Mountain (sample 

CA-18) (fig. 2A, loc. 24, and fig. 4) exhibits a relatively simple 

spectrum (fig. 3X). Two steps yield a plateau age of 23.11 ±0.04 

Ma, but this age is statistically older than 40Ar/39Ar dates for 

unaltered host rocks. The reason for this discrepancy is 

unknown, but may be due to the unrecognized presence of uniformly distributed excess 40Ar. Because the alunite plateau age 

is greater than that of the host rocks, we consider this age unreliable. Overall, we consider the weighted mean 40Ar/39Ar age 

from all Lake City caldera samples (22.93 Ma) as the preferred 

age of contemporaneous alunite formation. Similar incongruously old 40Ar/39Ar dates have also been recorded for replacement alunite from the 23 Ma Summitville deposit in the eastern 

San Juan Mountains (S. Harlan, oral commun., 1995). As at Red 

Mountain, these dates postdate the 23.0 Ma dacite porphyry host 

rock by 100,000 to several hundred thousand years. Alunite 

from the Summitville mine shows evidence of excess argon in 

the age spectra, which, combined with the old apparent ages 

from both areas, may indicate an inherent problem in the isotopic dating of some alunites.

40Ar/39Ar studies of 1M sericite (UHT-10) from the UteHidden Treasure vein, located just outside the northern ring fault 

of the Lake City caldera (fig. 2A), may indicate both an original 

age of sericite formation and an apparent age of resetting (fig. 

30). As discussed previously (see section on 26-25 Ma veins), 

an apparent age of 26.40±0.04 Ma is our best estimate of the 

maximum age of the sericite. However, this sample appears to 

have experienced partial 40Ar loss at about 22.9 Ma (steps 1 and 

2) and may record the effects of a younger thermal event. This 

event may be related to hydrothermal fluids associated with the 

formation of late-stage, barite-precious-metal veins <40 m 

away. Scanning electron microscope studies of this sample 

reveal two discrete populations of sericite. We interpret the 

shape of the spectrum to reflect the combined effects of argon 

released from both mica populations. The earlier formed, 

coarser sericite degassed primarily at higher temperatures, 

whereas the younger and and finer mica fraction degassed at 

lower temperatures.

Although fission track data for zircon from the Capitol City 

area (Johnston and Naeser, 1976) probably record a younger 

thermal event related to the 22.9 Ma Lake City caldera cycle, 

thermal effects of this event were apparently insufficient to completely reset the 40Ar/39Ar system of sericite in the nearby Lilly 

vein (sample LV-9; table 1; fig. 2A). Thus by analogy, thermal 

effects from the Lake City caldera likely were also insufficient

Hydrothermal Activity and Mineralization Related to the Lake City Caldera 15
and therefore did not reset the 40Ar/39Ar systematics of sericite 

within the Ute-Hidden Treasure vein. In addition, the absence 

of Lake City caldera-age intrusions north of the ring fracture 

precludes resetting due to local intrusive activity. Thus by inference we suggest that the late barite-precious-metal assemblage 

in the Ute-Hidden Treasure vein system formed at 22.9 Ma, and 

that this age most probably relates the radial barite-preciousmetal veins on the east side of the caldera to intrusive and hydrothermal activity at Red Mountain, as proposed earlier by Slack 

(1980).

19-17 Ma Rhyolite Intrusions North and 

South of the Lake City Caldera and 

Associated Mineralization

A series of silicic alkalic rhyolite intrusions and one extrusive dome was emplaced along the northern margin of the 

Uncompahgre caldera about 18.8 Ma (fig. 2A). These rhyolites 

are highly enriched in U, Li, Th, Be, Mo, and F and are similar 

petrogenetically to the F-rich, topaz rhyolites described by Burt 

and Sheridan (1980). The rhyolite bodies are characterized by 

erratic concentrations of uranium along their fractured margins 

(Slack, 1976; Steven and others, 1977) and contain late fluorite 

and topaz as vapor-phase vug-fillings. The volatiles H2O and 

HF were probably depleted during the cooling and crystallization of these magmas and lost as volcanic emanations from 

fumaroles (Zielinski and others, 1977); outside of local fumarolic alteration and devitrification, these intrusions and the surrounding host rocks are mostly unaltered.

Dikes of high-silica rhyolite associated with mineralized 

pebble dikes, uranium-mineralized fractures, fluorite, and anomalous molybdenum and tungsten are also present about 2-3 km 

south of the Lake City caldera, just southwest of the Sherman 

townsite (fig. 2A). In contrast to the 18.8 Ma high-silica rhyolites to the north, these dikes are strongly hydrothermally 

altered, as is the surrounding granite, and are cut by thin veinlets 

of microaplitic quartz, pyrite, and sericite. As these rocks have 

many of the characteristic signatures of Climax-type Mo-porphyry deposits (White and others, 1981), they were the focus of 

considerable exploration activity in the late 1970's through early 

1980's.

South of the Lake City caldera and north of the former 

townsite of Sherman, a major mineralized structure is hosted 

almost entirely within a 30-m-thick diabase dike that cuts Precambrian granite (Sanford and others, 1991). The mineralized 

vein structure, known as the Black Wonder vein (fig. 2A), is 

along a highly sheared zone that contains breccia fragments of 

the diabase dike and the granite. Cockscomb quartz is the dominant vein material and is accompanied by later fluorite and calcite, plus minor adularia, pyrite, chalcopyrite, galena, and barite. 

Chalcopyrite and pyrite are also disseminated throughout the 

brecciated wall rock.

Earlier K-Ar determinations from an alkali rhyolite intrusion on the northern margin of the Uncompahgre caldera yielded 

an age of 19.1±0.2 Ma (Lipman and others, 1976). This age is 

confirmed by six nearly identical K-Ar ages (H.H. Mehnert and 

K. Hon, unpub. data, 1985) from as many of these rhyolites that

16 Western San Juan Mountains, Colo., Lake City Caldera Area

have a weighted mean age of 18.8±0.4 Ma (table 5). Previously 

unpublished K-Ar determinations on sanidine from one of the 

high-silica rhyolite dikes in the Cuba Gulch area yielded a statistically younger age of 17.11±0.3 Ma (table 5).

The age spectrum (fig. 3Y) for adularia separated from the 

Black Wonder vein (sample BWV-19) is slightly U-shaped, suggesting the presence of excess 40Ar. This sample yielded a plateau age of 16.52±0.03 Ma over steps 6-14 with 50 percent of 

the released 39ArK. An 40Ar/36Ar vs. 39Ar/36Ar isochron plot 

using all the steps gives an age of 16.47±0.02 Ma (table 1) with a 

slightly elevated atmospheric intercept [(^Ar/^Ar)^] of 305±2, 

which is only slightly higher than the atmospheric value of 

295.5. We consider the isochron date (16.47±0.02 Ma) the preferred age for this sample due to the presence of minor excess 

40Ar.

The 16.5 Ma date for the Black Wonder vein is the first age 

to document post-23 Ma vein mineralization in the Lake City 

area. The similarity in age between the Black Wonder vein and 

the mineralized rhyolite in Cuba Gulch (17.1 Ma) suggests the 

likelihood of a genetic association between mineralized veins 

and intrusion of high-silica rhyolite in this area. The rhyolites to 

the north of the caldera may be high-level and volatile-poor precursors to the deeper unexposed rhyolite intrusions beneath the 

Cuba Gulch area.

Discussion and Summary

Our new 40Ar/39Ar isotopic dates allow establishment of a 

significantly more refined chronology of caldera formation and 

mineralization in the western San Juan Mountains. From these 

data we document the geochronology of the following events: 

(1) formation of the Ute Creek (Ute Ridge Tuff), Lost Lake 

(Blue Mesa Tuff), San Juan-Uncompahgre (Dillon Mesa and 

Sapinero Mesa Tuffs), and Silverton calderas between 28.6 and 

27.6 Ma; (2) emplacement of 26.5-25 Ma intermediate-composition intrusions and related subeconomic Mo-Cu porphyry 

and associated vein mineralization; (3) eruption and resurgent 

doming of the 22.9 Ma Lake City caldera and temporally associated mineralization in the Lake City area; and (4) emplacement of 18-17 Ma high-silica rhyolite intrusions on the 

margins of the Lake City caldera and related 16.5 Ma vein 

mineralization.

Previous K-Ar dates for the Ute Ridge Tuff (30.1 and 28.1 

Ma) and Fish Canyon Tuff (27.8 Ma) loosely documented the 

ages of pre-Lake City-age calderas of the western San Juan 

Mountains (Lipman and others, 1976). New 40Ar/39Ar dates, 

however, precisely constrain the time interval (0.8 to 1.1 million 

years at the la level) between emplacement of the Ute Ridge 

(28.63 Ma) and Crystal Lake Tuffs (27.62 Ma) and show the 

brief intervening eruptive chronology of the Blue Mesa (28.40 

Ma), Dillon Mesa (28.40 Ma), and Sapinero Mesa (28.19 Ma) 

Tuffs. Integrating our new dates with previously established 

geologic relationships, we can now determine the time interval 

for joint resurgence of the San Juan-Uncompahgre calderas. 

Studies by Luedke and Burbank (1968) showed that resurgent 

doming began shortly after collapse of the Sapinero Mesa Tuff, 

before early accumulation of caldera-filling lavas. However, evidence of late stages of resurgence has been recognized in key
exposures of the Crystal Lake Tuff (Lipman and others, 1973; 

P.W. Lipman, written commun., 1998), indicating that resurgent 

doming took place from about 28.2 to 27.6 Ma. Our new 40Ar/ 

39 Ar ages are of similar quality to those of Lipman and others 

(1997) and Lanphere (1988), now enabling correlation of 

regional ash-flow stratigraphy across the entire San Juan volcanic field.

A regional magmatic event beginning about 26.5 Ma 

resulted in intrusion of monzonite and quartz monzonite from 

Wilson Mountain northeast to Capitol City (fig. 2A). This event 

was probably related to intrusion of a high-level batholith into its 

own volcanic cover. Ages of sericite from quartz-base-metal 

veins near Capitol City (25.3 Ma) and Lake City (26.4 Ma) confirm relationships observed in the Middle Fork-Ophir Pass area 

indicating that these veins formed coevally with various phases 

of the monzonite intrusions. These new ages indicate a minimum hiatus of about 1 million years between mineralization 

related to the 26-25 Ma intrusions and pyroclastic eruptions 

from the 27.6 Ma Silverton caldera.

The Lake City caldera is the youngest of more than 15 collapse calderas that have been recognized within the San Juan 

volcanic field (Steven and Lipman, 1976; Lipman and others, 

1997). Six new 40Ar/39Ar dates are indistinguishable at the 95 

percent confidence level and have a minimum and maximum 

apparent age variance of 80,000 to 330,000 years at the lo confidence level. A weighted mean of these determinations yields an 

age of 22.93±0.02 Ma. A maximum time restriction of 330,000 

years for the evolution of the Lake City caldera is consistent with 

paleomagnetic studies of Reynolds and others (1986); however, 

our minimum age variance of 80,000 years is more similar to 

durations known from some other well-dated Pleistocene 

calderas (Gansecki and others, 1998; Gazis and others, 1995; 

Hildreth and others, 1984).

Intrusion of the calc-alkaline dacite of Red Mountain late in 

the Lake City caldera cycle is clearly linked to massive alunitization and related subeconomic Mo-Cu porphyry mineralization at 

depth. Radial veins of the late barite-precious-metal assemblage (Slack, 1980) located 2-3 km north and east of Red Mountain (fig. 4) are also probably genetically related to the dacite of 

Red Mountain intrusions. We interpret the sericite from the 

Ute-Hidden Treasure vein, which originally crystallized at 

26.40±0.04 Ma, to have been partially reset at 22.9±0.1 Ma, perhaps by hydrothermal fluids related to nearby veins of the latestage barite-precious-metal assemblage. Based on these data, 

we concur with the conclusion of Slack (1980) that the late barite-precious-metal veins distributed radially around the east side 

of the Lake City caldera were associated with 23 Ma hydrothermal activity at Red Mountain. Recharge of meteoric water from 

the top of the resurgent dome of the caldera may have resulted in 

the flushing of sulfate and metals from the Red Mountain hydrothermal system into the surrounding moat zone where these 

veins formed.

Intrusion of compositionally and petrographically similar 

dacite porphyry took place regionally throughout the San Juan 

volcanic field at about 23 Ma. Calc-alkaline dacite porphyry 

intrusions at Red Mountain Pass north of Silverton, Summitville 

(Gray and Coolbaugh, 1994), and those at Red Mountain near 

Lake City—aside from being related to significant mineralization—were all rich in sulfur and genetically associated with

large, acid-sulfate alteration systems (Gilzean, 1984; Lipman 

and others, 1976; Gray and Coolbaugh, 1994; Bove and others, 

1990). Mass-balance studies of hydrothermally altered rocks at 

Red Mountain (Lake City) document that nearly 14 million metric tons of sulfur was derived from the underlying dacitic magmas (Bove, 1988). Although only speculative, one possible 

mechanism for sulfur enrichment in these 23 Ma dacitic magmas 

could have been upward migration of a fluid phase derived from 

an underplating basaltic magma similar to processes related to 

the 1991 high-SO2 eruptions at Mount Pinatubo (Pallister and 

others, 1992). As extrapolated to the 23 Ma setting in the San 

Juan volcanic field, regional extension and crustal thinning may 

have promoted the ascent of basaltic magma to higher crustal 

levels, which then gave rise to the sulfur-rich dacitic magmas.

Economically important post-23 Ma vein mineralization in 

the western San Juan Mountains appears to be closely associated 

with intrusion of high-silica alkali rhyolite (Lipman and others, 

1973). The distinctive early Miocene (23-16 Ma) quartz-bearing 

rhyolites in the San Juan Mountains are part of a bimodal igneous suite associated with extensional tectonism that began about 

25 Ma. These rhyolite intrusions, such as the dikes in Cuba 

Gulch, south of Lake City (fig. 2A), and those around Red Mountain Pass north of Silverton (fig. 2A), are petrologically very similar to Climax-type high-silica rhyolites (White and others, 1981; 

Lipman and others, 1976) that are associated with large molybdenum porphyry systems in the Western United States.

Acknowledgments

The authors would like to express their gratitude to 

Stephen Harlan, who went far beyond his editorial responsibilities in the various stages of this manuscript. Early informal 

reviews and discussions with Peter Lipman also greatly 

improved the manuscript. Editorial reviews by Thomas Steven 

and Philip Bethke are also appreciated.

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geochemistry of acid sulfate alteration: Economic Geology, v. 87, p. 

225-262.

Samson, S.D., and Alexander, C.E., 1987, Calibration of the interlaboratory 

""Ar/^Ar dating standard, MMhb-1: Isotope Geoscience, v. 66, p. 27- 

34.

Sanford, R.F, 1992, Lead isotopic compositions and paleohydrology of 

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991.

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economic mineral deposits; Dating the stages of mineralization 

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354.

Steiger, R.H., and Ja'ger, E., 1977, Subcommission on geochronology, 

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321-329.

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Paper 958,35 p.

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E,126p.

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Manuscript approved for publication December 20,2000 

Published in the Central Region, Denver, Colorado 

Graphics by the author and Alessandro J. Donatich 

Photocomposition by Norma J. Maes and LM. Carter 

Edited by LM. Carter

References Cited 19
Appendix

Table 3. Sample descriptions and localities (note localities also shown 

in fig. 2A and B).

16.

1. 37°46'19" 107°21'46" (UR-1) Ute Ridge Tuff from outcrops near 

the Rio Grande Reservoir, southwest of Finger Mesa at elevation 

of 2,910 m, Finger Mesa quadrangle.

2. 38°21'47" 107°22'09" (BM-2) Blue Mesa Tuff collected from 

pyroclastic sequence about 1 km south of U.S. Highway 50, on 

the road to Arrowhead Lodge at elevation of 2,741 m, Poison *' • 

Draw quadrangle.

3. 37°58'18" 107°43'43" (BM-3) Blue Mesa Tuff from vitrophyre

near Potosi Peak, northeast of Telluride. Unit Tpg 3 of Gilpin 18. 

Peak sequence of Burbank and Luedke (1969). Elevation at 3,868 

m, Telluride quadrangle.

4. 38°10'48" 107°20'35" (DM-3) Single pumice of Dillon Mesa Tuff 19- 

collected from outcrops at the south edge of Alpine Plateau, just 

south of the southern mapped extent of Hinsdale basalt flows. 

Elevation at 3,222 m, Alpine Plateau quadrangle. 20.

5. 37°58'23" 107°43'43" (DM-4) Basal vitrophyre of Dillon Mesa 

Tuff near Potosi Peak, northeast of Telluride. Unit Tpg5 of 

Gilpin Peak sequence of Burbank and Luedke (1969). Elevation 

at 4,006 m, Telluride quadrangle. 21.

6. 38°17'38" 107°12'02" (SM-4) Fiamme from vitrophyre of

Sapinero Mesa Tuff in Dutch Gulch, near Gateview. Elevation at 

2,522 m, Gateview quadrangle. 22.

7. 38°23'36" 107°1 1'43" (SM-5) Vitrophyre of the Sapinero Mesa 

Tuff collected near Ten Mile Spring on Sapinero Mesa, about 10 

km southeast of Sapinero. Elevation at 2,477 m, Carpenter Ridge 

quadrangle.

8. 37°59'18" 107°44'56" (SM-6) Basal vitrophyre of Sapinero near

Potosi Peak, northeast of Telluride. Unit Tpg6 of Gilpin Peak 24. 

sequence of Burbank and Luedke (1969). Elevation at 4,052 m, 

Ironton quadrangle.

9. 38°04'07" 107°25'45" (CL-6) Vitrophyre of Crystal Lake Tuff in

upper Nellie Creek near Uncompahgre Peak, Uncompahgre Peak 25. 

quadrangle. Elevation at 3,536 m.

10. 37°49'08" 107°12'47" (CL-7) Crystal Lake Tuff near Hermit 

Lakes, in the South Clear Creek drainage at elevation 3,050 m. 

Finger Mesa quadrangle.

11. 37°48'56" 107°43'15" (SM-1) Quartz monzonite phase of 

Sultan Mountain stock collected at 2,945 m, Silverton quadrangle.

12. 37°49'05" 107°39'52" (SM-2) Quartz monzonite phase of 

Sultan Mountain stock collected at 2,990 m, Silverton quadrangle. 26.

13. 38°00'44" 107°27'55" (MP-8) Fresh monzonite porphyry

collected from a small mine dump on the North Fork of Henson 

Creek, near the former townsite of Capitol City. Although 27. 

monzonite porphyry is exposed in adit, freshest sample was 

found on mine dump. Elevation at about 2,882 m, Uncompahgre Peak quadrangle.

14. 38°04'18" 107°27'43" (UPL-1) Fresh dacite porphyry flows in the 28. 

upper part of the volcanics of Uncompahgre Peak. Collected 

at 4,333 m near the top of Uncompahgre Peak in the Uncompahgre Peak quadrangle. 29

15. 38°02'05" 107°22'05" (UHT-10) Vein sericite from the UteHidden Treasure vein of the Ute-Ulay mine. Sample collected 

about 30 m before the end of the main 5th level adit (Sample 30. 

UHT-555 of Slack, 1980, fig. 8). The sericite is pale green and 

is present as discontinuous lenses (<1 cm thick and up to several 

centimeters long) in massive fine-grained rhodochrosite 31. 

within the paragenetically later parts of stage Ic (part of quartzbase-metal assemblage). Sample collected <40 m from

20 Western San Juan Mountains, Colo., Lake City Caldera Area

younger barite-precious-metal vein assemblage (Slack, 1980).

Elevation about 2,795 m, Lake City quadrangle.

38°01'47" 107°28'00" (LV-9) Sample collected from an outcrop of

the Lilly vein on the north side of Yellowstone Gulch about 1 km

north of Capitol City. Sericite is directly in contact with largely

barren, massive to crudely banded white quartz that is capped by

late crystals of small euhedral quartz. Sericite is relatively coarse

and is present along the contact between the outer edge of the vein

and altered wall rock. Elevation at 3,156 m, Uncompahgre Peak

quadrangle.

37°56'44" 107°23'50" (SP-12) Black vitrophyric pumice from the

middle member of the Sunshine Peak Tuff. Head of the Middle

Fork of Alpine Gulch, Redcloud Peak quadrangle at 3,975 m.

37°56'46" 107°24'45" (SP-11) Black vitrophyric pumice from

middle member of Sunshine Peak Tuff. Collected from ridge

dividing Silver Creek and Bent Creek, Redcloud Peak quadrangle.

37°59'21" 107°19'20" (DGM-13) Black vitrophyric lava of the

dacite of Grassy Mountain just south of the top of Round

Mountain. Elevation at 3,521 m, Lake San Cristobal quadrangle.

37°58'27" 107°20'13" (DRM-14) Fresh dike of the dacite

porphyry of Red Mountain located about 1.5 km south of Red

Mountain at the head of Red Mountain Gulch. Elevation at 3,465

m, Lake San Cristobal quadrangle.

37°58'55" 107°19'21" (DRM-15) Fresh dike, dacite of Red

Mountain about 1 km northeast of Red Mountain. Lake San

Cristobal quadrangle at 3,506 m.

and 23. (DRM-16; DRM-17) Porphyro-aphanitic dacite porphyry

dike. Latest intrusive phase of the dacite porphyry of Red

Mountain; only phase of dacite of Red Mountain that postdates

alunite alteration. Sample from 202 m depth in drill hole LC-1

collared on southwest side of Red Mountain (Bove and Hon,

1992), in Lake San Cristobal quadrangle.

(CA-18) Late-stage, coarse-grained vein alunite (Stage 4 alunite;

Bove and others, 1990) collected at 29 m depth from drill hole

LC-3 (Bove and Hon, 1992) collared about 1.3 km southwest of

Red Mountain. Lake San Cristobal quadrangle.

37°54'15" 107°25'25" (BWV-19) Adularia from vein material

from the Black Wonder mine, hosted almost entirely within a 30-

m-thick Ordovician-Cambrian diabase dike. The mineralized vein

structure is along a highly sheared zone that contains breccia

fragments of the diabase dike and granite from the surrounding

wallrock. Cockscomb quartz is the dominant vein material, and

it is accompanied by later fluorite and calcite, plus minor adularia,

pyrite, chalcopyrite, galena, and barite. Mine located at an

elevation of 3,109 m, less than 1 km north of the Sherman

townsite in the Redcloud Peak quadrangle.

38°00'30" 107°31'55" (80L-106A) Rhyolite vitrophyre from

extrusive dome at Dolly Varden Mountain. Elevation 3,884 m,

Handles Peak quadrangle.

37°59'30" 107°31'58" (1-6-ID) Rhyolite vitrophyre from

intrusion near the junction of the North Fork of Henson Creek

and Mary Alice Creek. Elevation at 3,024 m, Weterhorn Peak

quadrangle.

38°02'28" 107°28'00" (SOL-103) Rhyolite vitrophyre collected

from the Broken Hill sill just east of Broken Hill at 3,797 m.

Uncompahgre Peak quadrangle.

38°03'42" 107°24'37" (80L-111 A) Intrusion of rhyolite from

prominent peak just north of the upper fork of Nellie Creek.

Uncompahgre Peak quadrangle at 3,767 m.

38°04'15" 107°20'27" (80L-110) Rhyolite felsite from intrusion in

the upper reaches of Larson Creek. Elevation at 3,024 m, Lake

City quadrangle.

37°53'05" 107°29'30" (RC-5-2A) Dike of high-silica rhyolite near

the intersection of Cuba Gulch and Snare Creek. Elevation at 3,327

m, Redcloud Peak quadrangle.
Table 4. 40 Ar/39Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic" ""Ar K-derived 

(°C) (V) 39Ar(v)

Sample: UR-1 Ute Ridge Tuff

Measured 40Ar/36Ar = 298.9

700 0.00731 0.0107

800 0.34276 0.1606

900 0.77042 0.3617

1000 1.53864 0.7283

1050 1.54720 0.7308

1100 2.04625 0.9684

1150 2.47059 1.1640

1200 2.42818 1.1398

1250 2.71143 1.2779

1300 4.27081 1.9979

1350 3.36476 1.5678

1450 0.47836 0.2275

Total - Gas

Plateau date (1000°-1250° C)

Sample: BM-2 Blue Mesa Tuff

Measured 40Ar/36Ar = 297.0

700 0.01288 0.00337

800 0.18312 0.08499

900 0.53721 0.26176

1000 1.31802 0.62876

1050 0.99207 0.47174

1100 1.09531 0.52430

1150 1.65374 0.78793

1200 1.46323 0.69605

1250 1.66555 0.65975

1300 2.78365 1.32758

1350 1.75271 0.83351

1450 0.34991 0.16343

Total - Gas

Plateau date (1000°-1350° C)

Sample: BM-3 Blue Mesa Tuff

Measured 40Ar/36Ar = 298.9

900 0.08649 0.05499

1000 0.76485 0.36371

1100 1.02447 0.48661

1200 1.24364 0.58990

1250 1.46346 0.68821

1300 1.78006 0.83672

1350 2.04508 0.95664

1400 2.37909 1.11449

1450 3.57580 1.67640

1500 5.64558 2.63270

1550 2.50126 1.15848

1650 0.74579 0.35358

Total - Gas

Plateau date (1350M5000 C)

40Ar/3SAr 

ratio

0.684

2.134

2.130

2.113

2.117

2.113

2.123

2.130

2.122

2.138

2.146

2.103

2.126

3.816

2.155

2.052

2.096

2.103

2.089

2.099

2.102

2.101

2.097

2.103

2.141

2.100

39Ar/37Ar 

ratio

7.84

10.95

8.33

8.56

9.79

11.51

14.16

16.11

21.45

33.36

37.50

13.47

33.11

59.80

59.59

59.27

58.43

60.91

61.85

62.07

63.32

64.33

64.79

63.44

Radiogenic 

yield (%)

6.8

42.9

89.2

94.4

95.4

96.0

97.0

97.5

97.1

97.5

97.4

90.5

9.5

38.0

90.4

95.6

96.8

97.2

98.0

98.3

98.5

98.4

97.1

96.4

%39Ar Apparent age and error* 

(Ma at ICT)

Sanidine; 59.0 mg

J- value = . 007551 ±.

0.1 9.3 ± 3.27

1.6 28.84 ± .08

3.5 28.79 ± .09

7.0 28.55 ± .09

7.1 28.61+ .09

9.4 28.56 ± .04

11.3 28.68 ± .06

11.0 28.79+ .04

12.4 28.67+ .06

19.3 28.89 ± .04

15.2 29.00+ .04

2.2 28.42 ± .13

28.74 ± .07

28.57 ± .08

Sanidine; 60.1 mg

J- value =. 007559 ±.

0.1 51.30± 6.29

1.3 29.14± .70

3.9 27.77 ± .08

9.5 28.36 ± .05

7.1 28.45 ± .09

7.9 28.26 ± .04

11.9 28.40 ± .09

10.5 28.44 ± .08

22.3 28.40 ± .06

20.0 28.37 ± .07

12.5 28.45 ± .04

2.5 28.96 ± .15

28.41 ± .08

28.40 ± .07

Sanidine; 57.9 mg

1 (Icr)

1% (la)

J- value = .007394 ± .1% (la)

1.573

2.103

2.105

2.108

2.126

2.127

2.138

2.135

2.133

2.144

2.159

2.109

2.131

25.64

24.59

34.09

41,14

46.91

47.67

46.15

46.34

49.61

54.64

49.58

35.23

46.3

59.2

93.1

94.5

95.4

96.0

96.5

95.8

94.7

93.7

94.3

88.8

0.5 20.86 ± .19

3.3 27.83 ± .15

4.5 27.87 ± .04

5.4 27.90 ± .06

6.3 28.14± .07

7.7 28.16± .04

8.8 28.29 ± .05

10.2 28.25 ± .08

15.4 28.23 ± .04

24.1 28.38+ .04

10.6 28.57 ± .04

3.2 27.92 ± .04

28.20 ± .06

28.30 ± .05

Appendix 21
Table 4—Continued. 40 Ar/39 Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic* 40Ar 

(°C) (V)

K-derived 

39Ar(v)

Sample: DM-3 Dillon Mesa Tuff 

Measured ^Ar/^Ar = 297.0 

700 0.14750 0.07410

800 0.23375

900 0.65997

1000 1.11994

1050 1.04354

1100 1.37191

1150 1.59227

1200 1.56668

1250 1.27557

1300 1.61371

1400 1.24104

Total - Gas

0.10940

0.31496

0.53273

0.49681

0.65749

0.75557

0.74524

0.60681

0.76352

0.58807

40Ar/39Ar 

ratio

1.991

2.137

2.095

2.102

2.100

2.087

2.107

2.102

2.102

2.114

2.110

2.102

39Ar/37Ar 

ratio

15.38

11.93

13.68

16.25

16.35

24.24

30.25

37.82

44.28

46.27

43.76

Radiogenic 

yield (%)

24.7

48.3

91.1

95.3

96.1

97.4

98.8

98.7

98.4

98.7

97.4

Weighted mean date (900°- 1250° C)

Sample: DM-4 Dillon Mesa Tuff 

Measured ^Ar/^Ar = 298.9 

900 0.04824 0.03323

1000 0.52159

1100 1.00843

1200 1.84679

1250 1.56614

1300 1.81071

1350 2.53846

1400 2.92788

1450 3.44036

1500 3.31880

1550 2.27900

1650 0.67719

Total - Gas

Plateau date (1200°- 1500° C)

0.24566

0.47672

0.86914

0.73752

0.85462

1.19353

1.37896

1.61844

1.55588

1.06500

0.32556

Sample: SM-4 Sapinero Mesa Tuff 

Measured 40Ar/36Ar= 297.0 

700 0.06400 0.02733

800 0.37331

900 0.81236

1000 1.45020

1050 1.46160

1100 1.77050

1150 2.37449

1200 1.82216

1250 1.53077

1300 1.31648

1400 0.42560

Total - Gas

Plateau date (900°- 1250° C)

0.15749

0.35900

0.63742

0.64483

0.77903

1.04554

0.79933

0.67279

0.57403

0.18169

1.289

2.126

2.115

2.125

2.124

2.119

2.127

2,123

2,126

2.133

2.140

2.080

2.123

2.341

2.370

2.263

2.275

2.267

2.273

2.271

2.280

2.275

2.293

2.342

2.280

19.54

9.54

8.21

12.30

17.24

22.65

25.85

24.87

21.68

24.05

21.40

11.88

18.22

27.62

40.98

51.55

56.52

59.53

63.75

64.50

65.64

67.38

59.15

33.5

51.4

91.2

95.9

96.5

96.8

97.1

96.7

97.1

97.4

97.0

92.2

22.9

54.7

93.9

96.5

97.3

98.5

98.6

98.7

98.4

98.5

98.7

%39Ar Apparent age and errorb 

(Ma at la)

Sanidine; 54. 1 mg 

J- value = .007550 ± .1% (la) 

1.3 26.91 ± .47

1.9 28.87 ±

4.6 28.32 ±

9.4 28.41 ±

8.8 28.38 ±

11.6 28.20 ±

13.4 28.48 ±

13.2 28.41 ±

10.8 28.41 ±

13.5 28.56 ±

9.4 28.52 ±

28.41 ±

28.38 ±

.25

.05

.05

.04

.05

.05

.05

.09

.05

.06

.07

.05

Sanidine; 58.5 mg 

J- value = . 0074701.1 (la) 

0.3 17.29± 2.65

2.4 28.43 ±

4.6 28.28 ±

9.8 28.41 ±

7.1 28.39 ±

8.3 28.33 ±

11.5 28,44 ±

13.3 28.391

15.6 28.42 ±

15.0 28.52 ±

10.3 28.61 ±

3.1 27.82 ±

28.38 ±

28.42 ±

.14

.04

.05

.13

.09

.04

.04

.04

.04

.04

.21

.07

.06

Sanidine; 61.2mg 

J- value = . 006938 ±.1 (la) 

0.5 29.07 ± .50

2.7 29.43 ±

6.1 28.10 +

10.8 28.25 ±

11.0 28.15 ±

13.3 28.22 ±

17.8 28.20 ±

13.6 28.3 1±

11.4 28.26±

9.8 28.48±

3.1 29.08±

28.3 1±

28.22 ±

.12

.12

.12

.05

.05

.05

.04

.06

.06

.06

.07

.07

22 Western San Juan Mountains, Colo., Lake City Caldera Area
Table 4—Continued. 40 Ar/39 Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic8 40Ar 

(°C) (V)

K-derived 

39Ar (v)

Sample: SM-5 Sapinero Mesa Tuff 

Measured 40Ar/36Ar = 297.0 

700 0.07708 0.03586

800

900

1000

1050

1100

1150

1200

1250

1300

1350

1450

Total

0.21841

0.73010

1.51451

1.26352

1.49738

1.80917

2.52249

2.09532

2.04206

1.06195

0.09050

-Gas

0.09933

0.34111

0.71750

0.59787

0.71118

0.86120

1.19734

0.99518

0.96537

0.49744

0.04415

40Ar/39Ar 

ratio

2.150

2.199

2.140

2.111

2.113

2.105

2.101

2.107

2.105

2.115

2.135

2.050

2.113

39Ar/37Ar 

ratio

19.07

19.84

25.88

32.40

37.25

42.14

46.83

52.28

56.61

58.21

57.99

29.55

Radiogenic 

yield (%)

33.9

46.7

95.4

96.3

96.8

98.1

98.2

98.5

98.3

98.3

98.0

79.0

%39Ar Apparent age and error5 

(Ma at la)

Sanidine; 65.8 mg 

J - value = .007465 ± .1% (lo) 

0.5 28.72 ± .58

1.4

4.8

10.2

8.5

10.1

12.2

17.0

14.1

13.7

7.0

0.6

Plateau date (1000°- 1300° C)

29.37 ±

28.60 ±

28.20 ±

28.24 ±

28.13 ±

28.07 ±

28.15 ±

28.13 ±

28.26 ±

28.52 ±

28.39 ±

28.23 ±

28.17 ±

.27

.06

.06

.09

.05

.04

.04

.04

.11

.06

.71

.07

.08

Sample: SM-6 Sapinero Mesa Tuff 

Measured '"Ar/^Ar = 298.9

Sample: CL-6 Crystal Lake Tuff 

Measured 40Ar/3SAr = 297.0

Sanidine; 66.6 mg 

J- value = .007498 ± .1% (lo)

900

1000

1100

1200

1250

1300

1350

1400

1450

1500

1600

Total

Plate

0.51454

1.88658

2.60077

2.74205

2.77353

2.42268

2.46556

3.40411

3.54420

2.49060

1.17288

-Gas

audate(1100°-1400°C

0.22367

0.88331

1.24099

1.30962

1.31806

1.15420

1.17275

1.62027

1.67881

1.18041

0.55744

:)

2.300

2.136

2.096

2.094

2.104

2.099

2.102

2.101

2.111

2.110

2.104

2.108

25.87

19.80

25.49

31.38

36.61

38.94

39.56

39.57

39.10

37.12

27.67

70.9

77.0

96.1

96.5

97.5

97.4

97.7

97.7

98.0

97.5

95.6

1.8

7.2

10.1

10.6

10.7

9.4

9.5

13.1

13.6

9.6

4.5

30.85 ±

28.66 ±

28.13 ±

28.10±

28.24 ±

28.17±

28.22 ±

28.20 ±

28.33 ±

28.32 ±

28.24 ±

28.30 ±

28.18 ±

.17

.10

.04

.04

.06

.04

.09

.04

.04

.06

.04

.04

.05

Sanidine; 73.8 mg

J- value = .007053 ± .1% (lo)

700

800

900

1000

1050

1100

1150

1200

1250

1300

1350

1400

Total -

0.03055

0.25705

0.67206

1.24321

1.18334

1.22996

1.79874

1.75943

2.82580

2.93237

2.12447

0.42564

Gas

0.01118

0.11088

0.30704

0.56760

0.54206

0.56240

0.82263

0.80323

1.28855

1.33420

0.96482

0.19180

2.733

2.318

2.189

2.190

2.183

2.187

2.187

2.190

2.193

2.198

2.202

2.219

2.196

36.49

45.36

56.87

61.56

65.73

68.47

73.61

72.90

74.44

75.36

77.15

69.73

22.0

50.3

93.5

96.4

96.3

97.7

98.4

98.7

98.9

98.9

98.7

95.4

0.1

1.5

4.1

7.6

8.2

8.5

11.0

10.7

17.2

17.8

12.9

2.6

Plateau date (900°- 1200° C)

34.44 ±

29.26 ±

27.64 ±

27.66 ±

27.56 ±

27.61 ±

27.61 +

27.66 ±

27.69 ±

27.75 ±

27.80 ±

28.02 ±

27.72 ±

27.62 ±

5.48

.34

.14

.07

.04

.05

.07

.06

.05

.06

.04

.05

.07

.07

Appendix 23
Table 4—Continued. 40 Ar/39 Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic" ""AT K-derived 

(°C) (V) 39Ar(V)

Sample: CL-7 Crystal Lake Tuff

Measured 40Ar/36Ar = 297.0

700 0.02293 0.01029

800 0.23120 0.10029

900 0.54264 0.24573

1000 1.20938 0.55225

1050 1.11390 0.51069

1100 1.46228 0.67549

1150 1.65114 0.76310

1200 1.63159 0.74962

1250 1.87044 0.86397

1300 2.07067 0.94669

1350 1.55915 0.71000

1400 0.18818 0.08893

1450 0.13694 0.06441

Total - Gas

Plateau date (1 100°-1250° C)

Sample: SM-1 Sultan Mountain Stock

Measured 40Ar/36Ar = 298.9

800 0.22794 0.1224&

900 0.10540 0.08493

1000 0.39958 0.20272

1100 1.94902 0.93841

1200 1.97061 0.94589

1250 1.32046 0.62991

1300 1.57494 0.74461

1350 2.36161 1.12322

1400 3.53391 1.70137

1450 3.45471 1.65910

1550 1.31994 0.62770

Total - Gas

Weighted mean date (1100°- 1550° C)

Sample: SM-2 Sultan Mountain Stock

Measured 40Ar/36Ar = 298.9

800 0.88567 0.46229

900 0.24570 0.12794

950 0.18006 0.09076

1000 0.48369 0.24300

1100 1.43960 0.71790

1150 1.02607 0.51668

1200 1.32755 0.65887

1250 1.24586 0.62722

1300 1.38120 0.70244

1350 1.34623 1.68894

1400 2.88092 1.46493

1450 1.4704 0.71099

1550 0.42435 0.21713

Total - Gas

Weighted mean date (1350M5500 C)

40Ar/39Ar 

ratio

2.229

2.305

2.208

2.190

2.181

2.165

2.164

2.177

2.165

2.187

2.196

2.116

2.126

2.180

1.861

1.241

1.971

2.074

2.083

2.096

2.115

2.103

2.077

2.082

2.103

2.075

1.916

1.920

1.984

1.991

2.005

1.986

2.015

1.986

1.966

1.981

1.967

1.979

1.954

1.978

39Ar/37Ar 

ratio

19.61

30.52

44.39

52.63

57.70

65.03

69.59

72.08

73.76

75.53

75.61

74.56

63.57

26.65

10.08

48.23

150.06

137.47

92.59

52.15

57.88

80.71

92.34

15.45

38.12

38.45

39.68

52.84

58.02

89.81

90.98

83.32

74.27

77.23

84.96

62.59

10.60

Radiogenic 

yield (%)

14.6

45.9

94.2

97.3

97.5

97.9

98.3

98.9

98.4

98.8

98.1

91.5

90.4

66.3

52.4

38.3

85.5

94.5

94.8

94.5

95.6

97.0

97.1

96.7

84.3

62.7

29.2

85.5

93.4

93.8

95.5

94.7

94.1

95.7

95.2

95.0

87.3

%39Ar Apparent age and error" 

(Ma at la)

Sanidine; 59.4 mg

J- value = . 007147 ±.1 (la)

0.2 28.51 +4.15

1.6 29.48 ± .24

3.9 28.25 ± .15

8.8 28.02 ± .04

8.1 27.91 ± .06

10.8 27.70 ± .05

12.1 27.68 ± .04

11.9 27.85 ± .04

13.8 27.70 ± .05

15.1 27.98 ± .04

11.3 28.09 ± .14

1.1 27.08 ± .27

1.0 27.21 ± .38

27.88 ± .07

27.62 ± 07

Sanidine; 53.4mg

J- value = . 007098 ±.1 (la)

1.4 23.67+ .45

1.0 15.821 .72

2.3 25.06 ± .13

10.7 26.36 ± .04

10.8 26.48 ± .05

7.2 26.64 ± .04

8.5 26.88 ± .06

12.8 26.72 ± .04

19.4 26.40 ± .04

18.9 26.47 ± .04

7.1 26.73 ± .04

26.37 ± .06

26.56 ± .04

Biotite; 47.8 mg

J- value = .007543 ± .1% (la)

5.6 25.88 ± .13

1.6 25.94 ± .21

1.1 26.80 ± .54

3.0 26.88 ± .28

8.7 27.08 ± .04

6.3 26.82 ± .10

8.0 27.21 ± .04

7.6 26.83 ± .08

8.5 26.56 ± .05

20.5 26.76 ± .04

17.8 26.56 ± .04

8.6 26.73 ± .04

2.6 26.40 ± .05

26.71 ± .07

26.66 ± .06

24 Western San Juan Mountains, Colo., Lake City Caldera Area
Table 4—Continued. 40Ar/39Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic" 40Ar 

(°C) (V)

Sample: MP-8 

Measured *°Ar/*6Ar = 296.6 

650 0.05293

750 0.68721

850 3.50545

950 1.46535

1000 0.48788

1050 0.71651

1150 1.49848

1350 1.03653

Total - Gas

Plateau date (750°- 1050° C)

K-derived 

39Ar (v)

0.04122

0.33905

1.74244

0.72520

0.23879

0.35380

0.73782

0.48897

Sample: UPL-1 Uncompahgre Peak Volcanics 

Measured 40Ar/36Ar = 298.9 

700 0.01409 0.00653

800 0.03111

900 0.09703

1000 0.28699

1100 0.78449

1200 1.52569

1250 1.56426

1300 1.84449

1350 2.61909

1400 1.55860

1450 4.57149

1500 3.14272

1550 4.38158

1650 1.31119

Total - Gas

Plateau date (1300°-1500° C)

0.01108

0.05077

0.13937

0.39326

.077394

0.79407

0.93184

1.32197

0.78766

2.30020

1.58420

2.20231

0.65481

Sample: UHT-10 Ute-Hidden Treasure vein 

Measured "Ar/^Ar = 298.9 

400 1.54336 0.75315

500 1.17534

600 4.91006

650 7.82130

700 5.29088

750 1.76903

800 0.41276

900 0.05887

1000 0.03890

Total - Gas

0.57671

2.21342

3.34640

2.24264

0.75854

0.17310

0.02679

0.00696

"Ar^Ar 

ratio

1.284

2.027

2.012

2.021

2.043

2.025

2.031

2.120

2.025

1.607

2.809

1.911

2.059

1.995

1.971

1.970

1.979

1.981

1.979

1.987

1.984

1.990

2.002

1.985

2.049

2.038

2.218

2.337

2.359

2.332

2.385

2.198

5.590

2.280

wAr/"Ar 

ratio

n.d.c

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

10.50

12.98

20.43

20.12

21.12

23.24

26.84

33.53

42.32

49.30

55.60

59.97

62.46

31.90

268.56

186.52

296.88

311.82

384.27

392.02

222.06

41.74

4.23

Radiogenic 

yield (%)

13.3

52.1

87.4

93.8

90.2

90.7

92.9

90.1

29.4

81.5

68.4

43.7

83.1

95.3

96.0

96.8

97.2

97.6

98.1

98.1

98.3

96.9

85.6

93.0

98.3

98.6

92.8

93.8

88.8

39.1

18.3

%39Ar Apparent age and error" 

(Maatla)

Biotite; 96.6 mg 

J- value = .007299± .1% (la) 

0.9 16.83 ± .58

7.3

37.3

15.5

5.1

7.6

15.8

10.5

26.49 ±

26.30 ±

26.41 ±

26.70 ±

26.47 ±

26.55 ±

27.70 ±

26.47 ±

26.39 ±

.13

.05

.08

.08

.17

.09

.05

.08

.10

Sanidine; 53.4 mg 

J- value -. 007098 ±.I (la) 

0.1 20.80+ 6.52

0.1

0.4

1.2

3.3

6.5

6.6

7.8

11.1

6.6

19.2

13.3

18.4

5.5

36.20 ±

24.71 ±

26.61 ±

25.79 ±

25.48 ±

25.47 ±

25.59 ±

25.61 ±

25.58 ±

25.69 ±

25.64 ±

25.72 ±

25.88 ±

25.67 ±

25.63 ±

3.07

1.31

.27

.05

.05

.04

.05

.05

.04

.04

.04

.04

.06

.06

.04

Sericite; 96.6 mg 

J- value = .006248 ± .1% (la) 

7.5 22.95 ± .12

5.7

21.9

33.1

22.2

7.5

1.7

0.3

0.1

22.83 ±

24.83 ±

26.15 ±

26.40 ±

26.10 ±

26.68 ±

24.60 ±

.13

.04

.04

.04

.04

.29

.84

61. 94 ±13.26

25.51 ± .06

Appendix 25
Table 4—Continued. 40Ar/39Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic" 40Ar 

(°C) (V)

Sample: LV-9 Lilly vein

Measured "Ar/^Ar = 298.9

400 0.21777

500 0.27176

600 0.97027

650 2.18909

700 2.41222

750 5.16248

800 5.72740

850 1.91043

900 0.46721

950 0.13267

1050 0.10862

Total - Gas

Plateau date (700°-800° C)

K-derived 

39Ar(v)

0.12015

0.13992

0.52423

1.19033

1.30444

2.79019

3.10158

1.03986

0.25185

0.07282

0.06427

ratio

1.812

1.942

1.851

1.839

1.849

1.850

1.847

1.837

1.855

1.822

1.690

1.846

3*Ar/37Ar 

ratio

208.03

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d

n.d.

52.37

Radiogenic 

yield (%)

21.3

39.9

72.0

85.9

95.2

93.6

97.5

96.4

83.2

46.0

27.7

Sample: SP-1 1 Sunshine Peak Tuff; silicic pumice

Measured "Ar/^Ar = 298.9

800 0.07435

950 2.62038

1100 5.37049

1150 3.38696

1200 0.97807

1250 0.26786

1450 0.42953

Total - Gas

Plateau date (950°-1250° C)

0.04785

1.55207

3.18308

2.00157

0.57623

0.15789

0.26499

1.554

1.688

1.687

1.692

1.697

1.697

1.621

1.687

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

25.5

72.5

89.3

88.2

68.1

27.7

14.4

Sample: SP-1 2 Sunshine Peak Tuff; mafic pumice

Measured ""Ar/^Ar = 298.9

650 0.16346

800 0.34135

950 1.20614

1025 1.19869

1100 2.75274

1150 2.29205

1200 2.26634

1250 0.84743

1450 0.82346

Total - Gas

Plateau date (950°-1200° C)

0.08336

0.19596

0.71543

0.71083

1.62518

1.35480

1.33556

0.49659

0.48281

1.961

1.742

1.686

1.686

1.694

1.692

1.697

1.707

1.706

1.699

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

18.9

44.2

10.2

10.2

23.2

91.1

88.3

71.1

40.7

%39Ar Apparent age and error" 

(Ma at 10)

Sericite; 151.8mg

J- value - .007636 ± .1% (10)

1.1 24.80 ± .43

1.3 26.56 ± .46

4.9 25.32 + .07

11.2 25.16 ± .04

12.3 25.30 ± .06

26.3 25.31 ± .04

29.3 25.26 ± .04

9.8 25.13 ± .04

2.4 25.38 ± .18

0.7 24.93 ± 1.20

0.6 23.13 ± .31

25.26 ± .06

25.29 ± .05

Sanidine; 100.7 mg

J- value = .007574 ± .1% (10)

0.6 21.11± .81

19.9 22.92 ± .05

40.9 22.91 ± .04

25.7 22.97 ± .04

7.4 23.04 ± .10

2.0. 23.03 ± .27

3.4 22.01 ± .23

22.90 ± .06

22.94 ± .05

Sanidine; 98.6 mg

J- value = .007562 ± .1% (10)

1.2 26.55 ± .48

2.8 23.61 ± .12

10.2 22.85 ± .05

10.2 22.86 ± .08

23.2 22.96 ± .04

19.4 22.93 ± .04

19.1 23.00 ± .04

7.1 23.13 ± .07

6.9 23.12± .07

23.03 ± .06

22.94 ± .10

26 Western San Juan Mountains, Colo., Lake City Caldera Area
Table 4—Continued. 40 Ar/39Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic3 40Ar 

(°C) (V)

K-derived 

MAr(v)

40Ar/39Ar 

ratio

39Ar/37Ar 

ratio

Radiogenic 

yield (%)

Sample: DGM-13 Dacite of Grassy Mountain lava

Measured ^Ar/^Ar = 94.8

800 0.86641

850 0.81766

900 0.92674

950 1.30628

1000 2.17434

1050 2.52265

1100 2.35585

1150 1.99166

1250 0.44339

Total - Gas

Plateau date (850°- 1 100° C)

0.49882

0.48207

0.54324

0.77217

1.28272

1.48910

1.38670

1.16533

0.26252

1.737

1.696

1.706

1.692

1.695

1.694

1.699

1.709

1.689

1.701

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

23.1

25.1

29.4

38.0

56.2

73.2

77.4

69.7

23.6

Sample: DRM- 14 Dacite of Red Mountain south dike

Measured ^Ar/^Ar = 298.9

500 0.11688

600 0.06289

700 0.53759

750 0.48993

800 0.68113

850 0.61579

900 0.59597

950 0.87486

1000 1.37027

1050 2.13208

1150 5.96794

1250 4.06666

1400 0.27981

1450 0.07038

Total Gas

0.06395

0.03657

0.32193

0.29245

0.40526

0.36493

0.35479

0.51895

0.81221

1.26626

3.50172

2.39778

0.15619

0.03015

1.828

1.720

1.670

1.675

1.681

1.687

1.680

1.686

1.687

1.684

1.704

1.696

1.792

2.334

1.697

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

22.3

55.3

3.1

61.2

78.9

83.9

82.5

74.8

60.4

58.8

70.5

79.2

64.6

64.4

%3yAr Apparent age and error" 

(Ma at 10)

Sanidine; 94.8 mg

J - value

6.3

6.1

6.9

9.8

16.3

18.9

17.6

14.8

3.3

Sericite;

J - value

0.6

0.3

3.1

2.8

3.9

3.5

3.4

4.9

7.7

12.0

33.3

22.8

1.5

0.3

Average weighted mean age (700°- 1050° C)

Sample: DRM- 15 Dacite of Red Mountain north dike

Measured ^Ar/^Ar = 298.9

700 0.66614

800 0.56278

850 1.00985

900 0.79283

950 1.94647

1050 3.21170

1150 7.57970

1400 6.09049

Total Gas

Plateau date (950°- 1 400° C)

0.38812

0.33798

0.61164

0.48125

0.63016

1.93506

4.56325

3.65451

1.716

1.665

1.651

1.647

1.661

1.660

1.661

1.667

1.663

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

39.3

83.8

91.9

92.5

91.6

91.4

88.2

84.1

= . 007534 ±.1% (10)

23.45 ±

22.91 ±

23.04 ±

22.85 ±

22.89 ±

22.88 ±

22.94 ±

23.08 ±

22.81 ±

22.97 ±

22.91 ±

96.6mg

.10

.09

.07

.04

.04

.04

.04

.04

.13

.05

.05

= . 007625 ±.1% (la)

24.97 ±

23.50 ±

22.83 ±

22.90 ±

22.97 ±

23.06 ±

22.96 ±

23.04 ±

23.06 ±

23.01 ±

23.29 ±

23.18 ±

24.48 ±

31.83 ±

23.03 ±

23.01 ±

.35

1.35

.19

.31

.12

.17

.14

.08

.06

.04

.04

.04

.18

1.88

.07

.10

Sanidine; 98.6 mg

J- value

3.1

2.7

4.9

3.8

5.0

15.4

36.2

29.0

= . 007675 ±.1% (10)

23.61 ±

22.91 ±

22.72 ±

22.67 +

22.85 ±

22.84 ±

22.85 ±

22.93 ±

22.88 ±

22.87 ±

.09

.09

.15

.25

.11

.05

.07

.04

.07

.07

Appendix 27
Table 4—Continued. 40Ar/39Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic* "° AT 

CC) (V)

K-derived 

39Ar(v)

'"Ar^Ar 

ratio

Sample: DRM-16 Dacite of Red Mountain post-alunite dike 

Measured 40Ar/36Ar - 298.9 

600 0.08937 0.05572 1.604

800 1.11923

850 0.90892

900 0.73385

950 1.15751

1000 1.50515

1050 1.56763

1100 2.00859

1150 2.59668

1250 0.55580

1400 0.12080

Total Gas

0.63868

0.54107

0.43973

0.69183

0.89791

0.92980

1.19461

1.53820

0.32353

0.06656

1.752

1.680

1.669

1.673

1.676

1.686

1.681

1.688

1.718

1.815

1.690

Sample: DRM-17 Dacite of Red Mountain post-alunite dike 

Measured "Ar/^Ar = 298.9 

500 0.02774 0.01763 1.573

600 0.04282

650 0.04106

700 0.06525

750 0.15741

800 0.40456

850 0.93166

900 0.84302

950 1.12973

1000 1.28113

1050 2.48423

1150 4.12228

1300 0.53857

Total Gas

Plateau age (1050°- 11 50° C)

0.02669

0.02137

0.02598

0.07412

0.21452

0.53000

0.47765

0.64564

0.74263

1.45273

2.41476

0.30793

1.604

1.921

2.512

2.124

1.886

1.758

1.765

1.750

1.725

1.710

1.707

1.749

1.736

Sample: CA- 18 Coarse-grained vein alunite; Red Mountain 

Measured 40Ar/36Ar = 298.9 

400 0.07126 0.03875 1.839

450 0.01255

500 0.03620

600 3.11894

650 6.82897

700 6.10068

750 0.26300

800 0.02004

Total Gas

Plateau age (650°-700° C)

0.00680

0.02183

1.83436

3.94701

3.53057

0.12615

0.00381

1.844

1.659

1.700

1.730

1.728

2.085

5.265

1.730

39Ar/37Ar 

ratio

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

Radiogenic 

yield (%)

4.7

57.6

88.7

90.3

91.2

91.0

88.5

80.8

78.9

68.4

50.8

3.7

10.2

17.9

15.5

26.4

45.5

67.2

82.3

85.5

76.8

73.7

73.6

80.2

7.8

14.1

50.4

79.8

51.7

52.1

8.4

2.7

%39Ar Apparent age and errorb 

(Ma at 10)

Sanidine; 101.9g 

J- value = .007557 ± .1% (la) 

0.8 21.74 ± .59

8.7 23.73 ± .05

7.4 22.76 ± .11

6.0 22.61 ± .10

9.5 22.67 ± .04

12.3 22.71 ± .04

12.7 22.84 ± .05

16.3 22.78 ± .04

21.0 22.87 ± .04

4.4 23.27 ± .12

0.9 24.57 ± .45

22.89 ± .06

Biotite; 88 mg 

J- value = . 00759 ±.1% (la) 

0.3 21.41 ±2.78

0.4 21.83 ±2.21

0.3 26.11 ±2.22

0.4 34.07 ± 1.88

1.1 28.85 ± .71

3.1 25.64 ± .21

7.6 23.91 ± .08

6.9 24.01 ± .09

9.3 23.80 ± .13

10.7 23.47 ± .05

20.9 23.26 ± .04

34.7 23.22 ± .04

4.4 23.79 ± .23

23.62 ± .10

23.24 ± .04

Alunite; 108 mg 

J- value = .007455 ± .1% (la) 

0.4 24.56 ± 2.28

0.1 24.63 ± 7.33

0.2 22.17 ± 1.64

19.3 22.72 ± .04

41.5 23.12 ± .04

37.1 23.09 ± .04

1.3 27.82 ± .27

0.0 69.45 ±11.87

23.12 ± .06

23.11 ± .04

28 Western San Juan Mountains, Colo., Lake City Caldera Area
Table 4—Continued. 40Ar/39Ar data for mineral separates from rocks and mineralized material from western San Juan Mountains.

Temp. Radiogenic* ""AT 

(°C) (V)

K-derived 

39Ar(v)

Sample: BWV-19 Black Wonder vein 

Measured *°Ax/*6Ai = 298.9 

500 0.10627 0.05281

600 0.17164

650 0.31946

700 0.44707

750 0.53482

800 1.05704

850 1.09813

900 1.62924

950 2.37365

1000 2.28044

1050 2.69076

1100 2.63784

1150 2.91681

1200 3.03192

1250 6.45907

1300 12.12783

1350 1.36364

1400 0.16648

1450 0.07849

Total Gas

Plateau date (800°-1200°C)

0.12173

0.24091

0.35307

0.42204

0.84149

0.87470

1.30016

1.89530

1.81954

2.14736

2.10314

2.32595

2.41245

5.12884

9.56362

1.04195

0.12667

0.06090

ratio

2.012

1.410

1.326

1.266

1.267

1.256

1.255

1.253

1.252

1.253

1.253

1,254

1.254

1.257

1.259

1.268

1.309

1.314

1.289

1.264

39Ar/37Ar ratio

74.81

93.23

21.49

15.07

63.86

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

Radiogenic 

yield (%)

7.4

22.0

40.1

42.3

48.4

81.1

89.2

93.1

92.9

87.9

88.3

89.0

86.5

74.5

64.2

62.5

53.7

51.5

47.2

%39Ar Apparent age and error15 

(Ma at la)

Adularia; 134 mg 

J- value = .007341± .1% (la) 

0.2 26.46 ± .35

0.4

0.7

1.1

1.3

3.6

3.7

4.0

5.8

D.5

6.5

6.4

7.1

10.3

15.6

25.1

3.2

0.4

0.2

18.58 ± .31

17.48 ± .19

16.69 ± .06

16.70 ± .03

16.56 ± .04

16.55 ± .04

16.52 ± .03

16.51 ± .03

16.52 ± .03

16.52 ± .03

16.53 + .03

16.53 ± .03

16.57 ± .03

16.60 + .03

16.72 ± .03

17.25 ± .04

17.32 ± .11

16.99 ± .33

16.66 ± .03

16.52 ± .03

* 40ArR/39ArK has been corrected for all interfering isotopes including atmospheric argon. The measured production ratios for samples SP11, SP-12, DGM-13, DRM-14, DRM-15, DRM-16, DRM-17, and CA-18 are (^Ar/^Ar^SJOxlO'3 , (^Ar/^Ar^l.SlOxlO'2, 

(37Ar/39Ar)K=l. llxlO'4, (36Ar/37Ar)Ca=2.69xlO-4, (39Ar/37Ar)Ca=6.55xlO-4, and ("Ar^Ar^S.SOxlO'5 . The measured production ratios for 

sample UHT-10 are (40Ar/39Ar)K=1.01xlO-2, ("Ar^Ar^LSlOxlQ-2, ("Ar/^Ar^.SSxlO'4, (^Ar^Ar^^JOxlO'4, 

(39Ar/37Ar)Cf =6.48xlO-4, and (38Ar/37Ar)Ca=3.70xlO-5 . The measured production ratios for samples MP-8 and LV-9 are 

(40Ar/39Ar)K=9.184xlO-3 , (^Ar^Ar^l.SOexlO'2, (37Ar/39Ar)K=8.20xlO-5, (^Ar/^Ar^^JOxlO-4, (^Ar^Ar^^.SexlQ-4, and 

C8Ar/37Ar)Ca=3.17xlO-s. The measured production ratios for sample BWV-19 are (^Ar/^Ar^^SOxK)-3, (38Ar/39Ar)K=1.308xlO-2, 

(37Ar/39Ar)K=1.50xlO'4, C6Ar/37Ar)Ca=2.66xlO-4, C'Ar/^Ar^.SSxlO-4, and (38Ar/37Ar)Ca=4.50xlO-5 . The measured production ratios for 

samples UR-1, BM-2, BM-3, DM-3, DM-4, SM-1, SM-2, UPL-1, SM-4, SM-5, SM-6, CL-6, and CL-7 are (40Ar/39Ar)K=9.350xlO-3 , 

(38Ar/39Ar)K=1.304xlO-2, (37Ar/39Ar)K=3.0xlO-4, (36Ar/37Ar)Ca=2.70xlO-4, (39Ar/37Ar)Ca=7.20xlO-4, andC'Ar^^c^JOxlQ-5 . Irradiation 

of each group of samples was for 30 hours at 1 megawatt in the TRIGA reactor at the U.S. Geological Survey, Denver, Colo. 

b 1 o error. 

°n.d. not determined.

Appendix 29
Table 5. K-Ar analytical data for rhyolitic rocks in Lake City area.

[Analytical details are in Hon and Mehnert (1983). Map figure number locations are shown in figure 2A]

Map Sample 

Figure No. description

Field No. Mineral Na2O% K,O% * 40Ar(10"10 % 40Ar Age Ma

moles/gram) ± 1 o

26

27

28

29

30

31

Dolly Varden 80L-106A Biotite 0.59,0.58 9.14,9.12 2.464

(Rhyolite

vitrophyre)

Mary Alice 1-6-ID Biotite 0.58,0.57 8.92,8.89 2.423

Creek (Rhyolite

vitrophyre)

Broken Hill Sill 80L-103 Biotite 0.56,0.54 9.12,9.20 2.510

(Rhyolite

vitrophrye)

Nellie Creek 80L-111A Biotite 0.63,0.65 8.71,8.70 2.353

(Rhyolite

vitrophyre)

Larson Creek 80L-110 Biotite 0.57,0.57 9.09,9.08 2.449 

(Rhyolite

felsite)

Cuba Gulch RC-5-2A Sanidine 0.12,0.14 15.62,15.60 3.865 

(Rhyolite dike)

71.1 18.7 ±0.4

70.1 18.8 ±0.4

62.5 18.9 ± 0.4

65.9 18.7 ± 0.4

50.4 18.6 ±0.4

79.3 17.11 ±0.3

30 Western San Juan Mountains, Colo., Lake City Caldera Area

* U.S. GOVERNMENT PRINTING OFFICE: 2001 — 673-049 / 30048 Rei 'on No. 8