Volcanic Geology in Parts of tHe 

Southern PelonclHo Mountains, 

Arizona and New Mexico 
Volcanic Geology in Parts of the 

Southern Peloncillo Mountains, 

Arizona and New Mexico 

By D. H. MciNTYRE 

U.S. GEOLOGICAL SURVEY BULLETIN 1671 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Any use of trade names is for descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey. 

Library of Congress Cataloging-in-Publication Data 

Mcintyre, David H. 

Volcanic geology in parts of the Southern Peloncillo Mountains, Arizona and New 

Mexico. 

(U.S. Geological Survey bulletin ; 1671 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1671 

1. Volcanic ash, tuff, etc.-Peloncillo Mountains (Ariz. and N.M.) 

2. Volcanism-Peloncillo Mountains (Ariz. and N.M.) 3. Geology-Peloncillo 

Mountains (Ariz. and N.M.) I. Title. II. Series. 

QE5.B9 no. 1671 [QE461] 557.3 s 86-600174 

[551.2'1'09789] 
CONTENTS 

Abstract 1 

Introduction 1 

Volcanic stratigraphy 1 

Tuff of Guadalupe Canyon along Cottonwood Creek 1 

Ash-flow tuff vitrophyre at Cowboy Flats 3 

Intermediate-composition volcanic breccias, lavas, domes, and intrusive 

rocks 3 

Breccia of Hog Canyon and related rocks 5 

Dacite of Outlaw Mountain and related domes and intrusive rocks 8 

Tuff of Skeleton Canyon 9 

Tuff of Whitmire Canyon and associated rocks 10 

Rhyolite of Clanton Draw 10 

Rocks of uncertain correlation at the mouth of Guadalupe Canyon 10 

Bluff Creek(?) Formation 10 

Unit A 14 

Unit B 15 

Unit C 15 

Olivine andesite(?) lava and dike 15 

Rhyolitic and intermediate-composition breccia 15 

Basalt 16 

Geronimo Trail cauldron 16 

Clanton Draw cauldron 16 

Structure 17 

Mineralization 17 

Hydrothermal alteration and quartz veins 17 

Tin 18 

References cited 18 

FIGURES 

1. Map showing locations of figures 2 and 8 and areas underlain by Geronimo Trail 

and Clanton Draw cauldrons 2 

2. Geologic map of Cottonwood Creek area 4 

3. Stratigraphic section in tuff of Guadalupe Canyon southwest of Outlaw Mountain, and histograms showing crystal and lithic-fragment content of samples 7 

4. Histograms showing crystal and lithic fragment content of ash-flow tuff vitrophyre 

at Cowboy Flats and overlying tuff of Guadalupe Canyon 8 

5. Histograms showing crystal and lithic fragment content in matrix of breccia of 

Hog Canyon 8 

6. Stratigraphic section of tuff of Skeleton Canyon in Skeleton Canyon and 

histograms showing crystal content of samples 11 

7. Stratigraphic section of tuff of Whitmire Canyon and tuff of Skeleton Canyon 

and histograms showing crystal and lithic fragment content of samples 12 

8. Geologic map of area near the mouth of Guadalupe Canyon 13 

9. Histograms showing crystal and lithic-fragment content of rocks of uncertain correlation at the mouth of Guadalupe Canyon 14 

Contents Ill 
TABLES 

1. Fission-track ages on zircon, and magnetic polarities for principal volcanic units 3 

2. Chemical analyses and CIPW norms for principal units 6 

3. Chemical analyses and CIPW norms for intermediate-composition intrusive rocks 

and basalt 9 

4. Trace-element analyses of samples from hydrothermally altered zone south of upper Cottonwood Creek 15 

IV Contents 
Volcanic Geology in Parts of the 

Southern Peloncillo Mountains, 

Arizona and New Mexico 

By D. H. Mcintyre 

Abstract 

Two small cauldrons are exposed in the southern Peloncillo Mountains, Arizona and New Mexico. The first, the 

Geronimo Trail cauldron, collapsed during eruption of the 

rhyolitic tuff of Guadalupe Canyon. Resurgence accompanied 

eruption of intermediate-composition lavas and breccias and 

emplacement of intermediate-composition intrusions and 

domes near the ring-fracture zone and on the resurgent dome. 

A 5-km2 area of hydrothermally altered rocks is located on the 

resurgent dome. Initial collapse of the second cauldron, the 

Clanton Draw cauldron, appears to have accompanied eruption of the alkali rhyolitic tuff of Skeleton Canyon. The cauldron 

subsequently was filled to overflowing with tin-bearing alkali 

rhyolitic lava, the rhyolite of Clanton Draw. Intermediatecomposition intrusive masses mark part of the ring-fracture 

zone, which is concealed by the rhyolite lava. 

INTRODUCTION 

This report describes the results of a brief study of 

the volcanic geology of the southern Peloncillo Mountains, Arizona and New Mexico. Field work occupied five 

weeks in October and November of 1983. Scott Birmingham ably assisted the author for several days during that 

period. 

The most comprehensive previous study (Erb, 1979) 

that includes the area described in this report (fig. 1) 

defined the stratigraphic relations in this and nearby 

areas. Erb (1979) also defined several cauldrons, one of 

which, the Geronimo Trail cauldron, is located within the 

study area. Also valuable are a series of reports on the 

Bunk Robinson Peak and Whitmire Canyon Roadless 

Areas. The first of these reports, a geologic map by Hayes 

(1982), draws heavily on the map by Erb (1979) but includes some important additions. A geochemical study 

by Watts and others (1983) and a mineral resource potential map (Hayes and others, 1983) pointed out, for the 

first time, the occurrences of tin in stream sediments in 

part of the area and the presence of a large area underlain 

by hydrothermally altered rocks. 

VOLCANIC STRATIGRAPHY 

The principal volcanic rock units, as defined by Erb 

(1979), include, from oldest to youngest, the tuff of 

Guadalupe Canyon, the breccia of Hog Canyon and 

associated rocks, the dacite of Outlaw Mountain and 

related intrusive rocks and domes, the tuff of Skeleton 

Canyon, the tuff of Whitmire Canyon, and the rhyolite 

of Clanton Draw (table 1). Fission-track age determinations on zircon indicate that this sequence, of late 

Oligocene age, ranges from about 26 to 27 m.y. (Erb, 

1979, table 1); part of the sequence could be about 

30 m.y. (table 1, this report). A small area mapped near 

the mouth of Guadalupe Canyon contains a sequence of 

rocks that could not be readily correlated with rocks exposed elsewhere in the area. 

Tuff of Guadalupe Canyon along 

Cottonwood Creek 

During this study, the tuff of Guadalupe Canyon 

was seen chiefly in exposures along Cottonwood Creek 

and its tributaries (fig. 2). A stratigraphic section 

somewhat more than 200 m thick southwest of Outlaw 

Mountain was examined and sampled (fig. 3). Most of 

the tuff of Guadalupe Canyon exposed there and in the 

surrounding area is nonwelded to moderately welded 

rhyolitic ash-flow tuff (table 2). All the rocks in this unit 

are altered in the Cottonwood Creek exposures. The principal alteration products are opal and zeolites, which have 

replaced glass shards and pumice; the phenocrysts have 

been little affected by alteration. Cementation of the rock 

by the alteration minerals gives the deceptive impression 

that the rock is densely welded. The opalized rock, which 

makes up almost half the stratigraphic thickness exposed 

southwest of Outlaw Mountain, is white to gray on 

weathered surfaces. Partial dehydration of the opal commonly has produced fine networks of shrinkage cracks 

on the weathered surfaces. In exposures along Cottonwood Creek south of Outlaw Mountain, gray, intensely 

Tuff of Guadalupe Canyon 
Area of 

figure 2 

/ 

5 MILES 

EXPLANATION 

D Clanton Draw cauldron D Geronimo Trail cauldron 

10 KILOMETERS 

Figure 1. Map showing locations offigure 2 and figure 8 and 

approximate areas underlain by Geronimo Trail and Clanton 

Draw cauldrons. Southern boundary of Geronimo Trail 

cauldron from Erb (1979, fig. 2). 

opalized rock locally overlies brown, less altered rock 

along a sharp contact. Relict masses of the brown, less 

altered rock within the gray, opalized rock clearly 

demonstrate that the color differences were caused by 

alteration and have no stratigraphic significance. Rock 

containing unweathered olive-green opal is exposed in 

roadcuts southeast of Outlaw Mountain. 

2 Volcanic Geology, Southern Peloncillo Mountains 

The tuff of Guadalupe Canyon in the Cottonwood 

Creek area can be divided into two distinct units based 

on relative proportions of plagioclase, sanidine, and 

quartz. The lower of the two units consistently contains 

more plagioclase than sanidine or quartz; the upper unit 

consistently contains less plagioclase (fig. 3). The break 

between the two units appears to be abrupt. Biotite is the 

only mafic mineral present in most samples. Very small 

amounts of amphibole and (or) pyroxene (less than 1 percent) may have been present in some of the rock prior 

to alteration. 

The tuff of Guadalupe Canyon exposed in Skeleton 

Canyon east of the Arizona-New Mexico boundary (north 

of the area shown in figure 2) has phenocryst proportions 

characteristic of the lower unit. 

Magnetic polarities for the tuff of Guadalupe Canyon measured in the field with a fluxgate magnetometer 

were consistently normal. Some of these measurements 

were made on outcrops of altered rocks, so the reliability of some determinations is doubtful. 

The age of the tuff of Guadalupe Canyon is somewhat uncertain. Erb (1979, table 1) reported a zircon 

fission-track age of 27.1 ± 1.5 m.y. for a sample collected 

from the NWY4 SWY4 sec. 27, T. 22 S., R. 32 E. Deal 

and others (1978) reported a potassium-argon age on 

sanidine of 24.26±0.52 m.y. on a sample collected from 

the same area. This potassium-argon age, significantly 

younger than the fission-track age, also is younger than 

a fission-track age for the rhyolite of Clanton Draw, 

several units higher in the local stratigraphic sequence 

(Erb, 1979). 

The sample locality of Deal and others (1978) (lat 

31 ~9' N., long 109004' 45" W.) was recovered during 

the present study. The rock debris left behind during trimming of the sample for age determination was found and 

sampled. The material is intensely opalized, which casts 

some doubt on its suitability for age determination. 

The imprecise location provided by Erb (1979) 

prevents recovery of his sample locality. However, if the 

locality were along the road up Cottonwood Creek, which 

is permitted by the locality information, his sample 

possibly was less altered than that of Deal and others 

(1978). Most exposures along the road in the area in question are less altered than the one sampled by Deal and 

others (1978) on the slope north of Cottonwood Creek. 

If this inference about the Erb (1979) sample locality is 

correct, it would explain why the fission-track age of Erb 

(1979) is older than the potassium-argon age of Deal and 

others (1978). It seems reasonable to conclude that the 

fission-track age of 27.1 ± 1.5 m.y. is the better age 

estimate for the tuff of Guadalupe Canyon. 
Table 1. Fission-track ages on zircon, and magnetic polarities for principal volcanic units 

in the southern Peloncillo Mountains 

[Magnetic polarity determinations made in the field with a portable fluxgate magnetometer] 

Unit name Fission-track age (m.y.) Magnetic polarity 

Rhyolite of 

Clanton Draw 

25.8± 1.2 Reversed 

Tuff of Whitmire 

Canyon 

Tuff of Skeleton 

Canyon 

Dacite of Outlaw 

Mountain 

Breccia of Hog 

Canyon 

Reversed 

Normal 

Reversed 

Reversed 

Tuff of Guadalupe 

Canyon 

27.1 ± 1.5 Normal 

Ash-flow tuff 

vitrophyre at 

Cowboy Flats 

30.6±3.4 

Ash-Flow Tuff Vitrophyre at Cowboy Flats 

Cowboy Flats, on upper Sycamore Creek south of 

Cottonwood Creek, exposes a mass of vitrophyre at least 

100 m thick that is overlain by devitrified tuff of 

Guadalupe Canyon and separated by a fault from tuff 

of Guadalupe Canyon exposed to the west. The 

vitrophyre crops out as a series of horizontal ledges as 

much as 10m high. 

Embedded in the densely welded, shard-rich matrix 

of the vitrophyre are phenocrysts of quartz, sanidine, 

plagioclase, and biotite in proportions similar to those 

in rocks in the lower part of the tuff of Guadalupe Canyon (fig. 4). In addition, the vitrophyre contains nearly 

4 percent clinopyroxene and amphibole, not noted in the 

tuff of Guadalupe Canyon exposed along Cottonwood 

Creek. The devitrified tuff of Guadalupe Canyon that 

overlies the vitrophyre has phenocryst proportions (fig. 

4) nearly identical to one of the samples collected low in 

the section southwest of Outlaw Mountain (fig. 3). 

Measurements of magnetic polarity on the 

vitrophyre outcrops revealed both normal and reversed 

polarity. Lightning strikes on these very exposed ledges 

may account for the mixed results. 

A fission-track determination on zircon from the 

vitrophyre gave an age of 30.6± 3.4 m.y. (W. E. Brooks, 

written commun., 1984; table 1 ). This result is apparently older than, but statistically not significantly different 

from, the 27.1 ± 1.5 m.y. of Erb (1979) from the somewhat altered exposures of the tuff of Guadalupe Canyon 

along Cottonwood Creek. 

The vitrophyre probably should be considered part 

of the tuff of Guadalupe Canyon. 

Intermediate-Composition Volcanic Breccias, 

Lavas, Domes, and Intrusive Rocks 

Intermediate-composition volcanic breccias, lavas, 

domes, and intrusive rocks, younger than the tuff of 

Guadalupe Canyon, crop out over a broad area in the 

southern Peloncillo Mountains. Similarities in mineralogy 

and compositio·n among those units that crop out in the 

area of figure 2 indicate that they form a cogenetic sequence. These rocks are the breccia of Hog Canyon and 

related rocks and the dacite of Outlaw Mountain and 

related intrusive rocks and domes. 

Intermediate-Composition Volcanic Rocks 3 
3 KILOMETERS 

2 MILES 

Figure 2 (above and facing page). Geologic map of Cottonwood Creek area. Data from Erb (1979), Hays (1982), and Mcintyre 

(unpublished mapping, 1983). 

4 Volcanic Geology, Southern Peloncillo Mountains 
CORRELATION OF MAP UNITS 

Os OJ 

Unconformity QUATERNARY 

orb joT;~-/ 

Unconformity 

TERTIARY 

Oligocene 

Unconformity 

Os 

OJ 

OTb 

OTib 

Ted 

Twc 

Tsc 

Ti 

Tom 

The 

nc 

Tgc 

Tcf 

Kmu 

} Lower } CRETACEOUS 

Cretaceous 

LIST OF MAP UNITS 

Surficial deposits, undivided 

Landslide deposit 

Basalt 

Basalt dike 

Rhyolite of Clanton Draw 

Tuff of Whitmire Canyon 

Tuff of Skeleton Canyon 

Intermediate-composition 

intrusive rocks 

Dacite of Outlaw Mountain 

Breccia of Hog Canyon 

Deposit containing blocks of 

limestone and limestone 

conglomerate 

Tuff of Guadalupe Canyon 

Ash-flow tuff vitrophyre at 

Cowboy Flats 

Mural limestone 

Contact-Dashed where 

approximately located 

Fault-Dashed where approximately 

MD-73 located, queried where uncertain 

• Sample locality-Listed in table 4 

---- Outline of altered area 

Breccia of Hog Canyon and Related Rocks 

The breccia of Hog Canyon and related rocks 

overlie the tuff of Guadalupe Canyon. Erb (1979) 

distinguished several units-the breccia of Hog Canyon, 

the sedimentary breccia of Cottonwood Creek, and the 

breccia of Geronimo Pass. All locally are similar in 

mineralogy and all appear to be genetically related to 

eruptions from vents marked by nearby intermediatecomposition intrusive rocks and domes. Erb's three units 

are treated as a single unit in this report. 

The breccias contain clasts of intermediatecomposition volcanic rock. Locally they also contain 

clasts of rhyolitic ash-flow tuff, especially near the base 

of the sequence. The tuff clasts resemble the tuff of 

Guadalupe Canyon. The crystal contents of breccia 

matrices are surprisingly consistent over a fairly large area 

(fig. 5). Plagioclase is more abundant than sanidine, 

which is more abundant than quartz. Biotite, clinopyroxene, and amphibole are present in varied amounts. 

The breccias exposed in Hog Canyon clearly are 

near their source. One unit that forms a continuous cliff 

about 6 m high contains blocks more than 2 m in 

diameter. Blocks greater than 1 m are common in many 

exposures. At one series of exposures in the canyon, the 

breccia matrix has a strong reversed magnetic polarity, 

which shows that this material was above the Curie 

temperature of its magnetic minerals at the time of 

deposition. The site from which the rocks in the canyon 

were erupted most probably is now marked by an intrusive mass exposed on the ridge west of the canyon. 

Breccia similar to that exposed in Hog Canyon 

crops out in the area northeast of the Silvertip Mine (fig. 

2). Most of these rocks, mapped as tuff of Guadalupe 

Canyon by Erb (1979}, occur within an area of intense 

hydrothermal alteration, which makes identification difficult. However, exposures of little-altered rocks are present, especially north of the Silvertip Mine (figs. 2 and 5). 

Rocks there include, in addition to the breccia, densely 

welded ash-flow tuff that is plagioclase rich and quartz 

poor, similar to the Hog Canyon rocks. Furthermore, in 

the area of intense alteration breccia textures are visible 

in some places, and well-bedded, thin layers of fine tuff 

can be seen separating more massive units. None of these 

features of the altered rocks are present in nearby exposures of the tuff of Guadalupe Canyon. 

Blocks of limestone or limestone conglomerate 

locally are present at the contact of the breccia of Hog 

Canyon and older rocks. They probably represent debris 

slumped from caldera-wall exposures of pre-Tertiary 

rocks. The two largest occurrences noted are near the jeep 

trail leading to the Silvertip Mine (fig. 2). The eastern 

occurrence is characterized by blocks of limestone conglomerate larger than 1 m in diameter. The conglomerate 

contains well-rounded gray fossiliferous limestone cobbles and subordinate black chert pebbles in a reddishorange mud matrix. The western occurrence contains 

blocks of gray and reddish-purple, highly fractured, 

recrystallized limestone. Locally present are blocks of 

conglomerate containing well-rounded clasts of shardrich, densely welded ash-flow tuff, opaque-oxide-rich 

lava, and granitic rock. Single crystals of plagioclase, 

Intermediate-Composition Volcanic Rocks 5 
Table 2. Chemical analyses and CIPW norms for principal units in the southern Peloncillo 

Mountains 

[Analyst: E. E. Erb, using methods described in Erb (1979, p. 6-7). BaO, SrO in original analyses omitted] 

2 3 4 5 6 

Chemical composition (wt percent) 

Si02 

Al20 3 

Fe20 3 

FeO 

MgO 

CaO 

N~O 

K20 

H2o+ 

H2oTi02 

P20s 

MnO 

SUM 

74.83 

12.65 

1.01 

0 .. 12 

0.28 

0.83 

2.69 

4.96 

1.58 

0.62 

0.16 

0.03 

0.04 

99.80 

65.87 63.61 

16.31 16.86 

3.27 3.06 

0.19 1.07 

0.67 1.37 

2.72 3.85 

3.58 3.69 

4.55 3.33 

1.23 1.30 

0.74 0.46 

. 0.46 0.61 

0.12 0.18 

0.04 0.06 

99.75 99.45 

76.74 74.64 74.96 

12.50 13.83 13.09 

0.95 1.23 1.28 

0.08 0.20 0.14 

0.08 0.16 0.12 

0.21 0.42 0.37 

3.87 3.33 3.62 

4.59 5.48 5.21 

0.26 0.58 0.47 

0.00 0.06 0.16 

0.16 0.22 0.15 

0.02 0.03 0.02 

0.07 0.04 0.06 

99.53 100.22 99.65 

Normative composition 

Q 

c 

or 

ab 

an 

hy-en 

mt 

hm 

il 

ap 

39.93 

1.45 

30.03 

23.32 

4.02 

0.71 

0.05 

1.00 

0.31 

0.07 

21.67 

0.86 

27.55 

31.04 

13.02 

1.71 

0.00 

3.35 

0.50 

0.29 

20.08 

0.63 

20.14 

31.96 

18.35 

3.49 

1.92 

1.81 

1.19 

0.44 

36.42 

0.84 

27.32 

32.99 

0.92 

0.20 

0.02 

0.94 

0.31 

0.05 

33.38 

1.74 

32.52 

28.30 

1.90 

0.40 

0.14 

1.14 

0.42 

0.07 

33.38 

0.88 

31.09 

30.93 

1.72 

0.30 

0.21 

1.14 

0.29 

0.05 

Descriptions 

1. Tuff of Guadalupe Canyon (Erb, 1979; average of nos. 2, 3, 4, 5, 6 of table 36). 

2. Breccia of Hog Canyon (Erb, 1979; average of nos. 7, 8, 9 of table 36). 

3. Rhyodacite; "Dacite of Outlaw Mountain" (Erb, 1979; average of nos. 10 and 11 of table 36). 

4. Tuff of Skeleton Canyon (Erb, 1979, table 45). 

5. Tuff of Whitmire Canyon (Erb, 1979, table 36, no. 14). 

6. Rhyolite of Clanton Draw (Erb, 1979; average of nos. 15 and 16 of table 36). 

quartz, sanidine, and microcline occur interstitially. Exposures east of the abundant limestone are of a reddishbrown monolithologic breccia of the breccia of Hog Canyon. The limestone, conglomerate, and breccia of Hog 

Canyon in the western occurrence probably are in a 

downdropped fault block. 

A third example of limestone-block breccia is 

located near the center of sec. 33, T. 22 S., R. 32 E., at 

the breccia of Hog Canyon-tuff of Guadalupe Canyon 

contact west of the fault. The largest block, of 

fossiliferous gray limestone, is about 6 m long by 2 m 

wide. The block contains abundant oyster and gastropod 

fragments. 

6 Volcanic Geology, Southern Peloncillo Mountains 

Figure 3(facing page). Stratigraphic section in tuff of Guadalupe Canyon southwest of Outlaw Mountain, and histograms 

showing crystal and lithic fragment content of samples. Values 

for phenocrysts and lithic fragments are percentages of the 

whole rock. Values for individual minerals are percentages of 

total phenocrysts, not percentages of the whole rock. An 

unlabeled mafic mineral bar means that biotite is the only mafic 

mineral present. Bar is labeled where hornblende (Hb), 

clinopyroxene (Cpx), or orthopyroxene (Opx) are present in 

addition to biotite (B). Where value for a component exceeds 

50 percent, the numerical value is shown. 
0 10 

EXPLANATION 

PERCENT 

20 30 

~ 

~ Lithophysal zone 

40 50 

K·Ar age sample of 

Deal and others 1978 

'lJ~ ~ o.s. Q 

METERS FEET 

100 

300 

200 

100 

Breccia of 

Hog Canyon 

I 

I 

I 

-cl 

~I 

~I 

O.J 

01 

I 

I 

I 

"'0 

C1) 

N 

:~ 

0 

C1) 

N 

I 

I 

I 

I 

I 

I 

I 

I 

I 

-cl 

~I (1) 

0. 

0 

c 

0 

> 

c 

(1) 

u 

C1) 

0. 

..::! 

(1) 

"'0 

(1) 

::J 

(.!) 

0 

--::J 

1-

Intermediate-Composition Volcanic Rocks 7 
Tuff of Guadalupe Canyon 

that overlies vitrophyre 

Ash-flow tuff vitrophyre 

at Cowboy Flats 

55 

Figure 4. Histograms showing crystal and lithic fragment content of ash-flow tuff vitrophyre at Cowboy 

Flats and overlying tuff of Guadalupe Canyon. (For 

explanation of symbols see figure 3.) 

Dacite of Outlaw Mountain and 

Related Domes and Intrusive Rocks 

Erb (1979) used the name dacite of Outlaw Mountain for a sequence of intermediate-composition lavas that 

overlie the breccia of Hog Canyon and related rocks. 

These lavas and related intrusive rocks and domes commonly are porphyritic and contain phenocrysts of 

plagioclase, quartz, and mafic minerals. Of the mafic 

minerals, biotite and amphibole are present everywhere; 

clinopyroxene and orthopyroxene occur in only a few 

specimens. Chemical compositions range from rhyodacite 

for the lavas and most of the intrusive rocks to quartz 

latite for part of the domical mass exposed north of 

Sycamore Creek (tables 2 and 3). According to Erb (1979, 

p. 211) the analyzed sample from Sycamore Creek (table 

3, no. 2) contained sanidine phenocrysts. The sample collected by us from the southeast margin of the mass contained no sanidine. This sequence of rocks has reversed 

magnetic polarity. 

In addition to the intrusive rocks and domes exposed in the western part of the area, numerous small 

intrusive masses occur near the headwaters of Cottonwood Creek. Some of these, too small to be shown on 

the map, are located in the southwest-trending draw in 

the NEY4 sec. 26, T. 22 S., R. 32 E. Most are hydrq_thermally altered. 

8 Volcanic Geology, Southern Peloncillo Mountains 

72 

78 

Hog Canyon Area 

79 

82 

71 

Silvertip Mine Area 

Figure 5. Histograms showing crystal and lithic fragment content in matrix of breccia of Hog Canyon. (For 

explanation of symbols see figure 3.) 
Table 3. Chemical analyses and CIPW norms for intermediate-composition intrusive rocks 

and basalt, southern Peloncillo Mountains 

[Analyst: E. E. Erb, using methods described in Erb (1979, p. 6-7). BaO, SrO in original analyses omitted] 

2 3 4 

Chemical composition (wt percent) 

Si02 65.90 68.33 46.99 47.78 

Al20 3 16.02 14.40 16.39 15.70 

Fe2

0 3 3.50 1.81 3.58 3.63 

FeO 0.23 0.39 7.14 6.81 

MgO 1.20 0.82 7.22 8.24 

CaO 2.64 1.72 9.47 9.36 

Na20 4.75 2.62 2.99 3.42 

K20 2.94 4.90 1.38 0.83 

H2o+ 0.91 3.58 1.71 1.18 

H2o- 0.54 0.72 0.28 0.16 

Ti02 0.48 0.30 2.32 2.22 

PzOs 0.17 0.08 0.59 0.45 

MnO 0.09 0.06 0.17 0.16 

SUM 99.37 99.73 100.23 99.94 

Normative composition 

Q 20.50 31.08 0.00 0.00 

c 0.64 1.94 0.00 0.00 

or 17.76 30.34 8.30 4.97 

ab 41.09 23.23 25.75 29.34 

an 12.26 8.39 27.70 25.39. 

di-wo 0.00 0.00 6.76 7.81 

di-en 0.00 0.00 4.56 5.50 

di-fs 0.00 0.00 1.68 1.65 

hy-en 3.06 2.14 1.37 1.05 

hy-fs 0.00 0.00 0.50 0.31 

ol-fo 0.00 0.00 8.67 9.99 

ol-fa 0.00 0.00 3.53 3.29 

mt 0.00 0.61 5.28 5.34 

hm 3.58 1.48 0.00 0.00 

il 0.69 0.60 4.48 4.28 

ap 0.41 0.20 1.42 1.08 

Descriptions 

l. Rhyodacite of "Spring of Contention member of volcanics of Hog Ranch" of Erb (1979, table 36, 

no. 12). 

2. "Quartz latite of Sycamore Creek" of Erb (1979, table 36, no. 13). 

3. "Basalt of the Peloncillo Mountains" of Erb (1979, table 48, no. 1). 

4. "Basalt of the Peloncillo Mountains" of Erb (1979, table 48, no. 2). 

Tuff of Skeleton Canyon 

The tuff of Skeleton Canyon, named by Erb (1979) 

for exposures within that canyon, is an alkali rhyolitic 

ash-flow tuff (table 2, no. 4). The densely welded part 

of the tuff of Skeleton Canyon commonly is grayish purple and contains gray wisps of flattened pumice. Within 

Skeleton Canyon, the upper part of the zone of dense 

welding contains a zone of closely spaced lithophysae that 

give a Swiss cheese-like aspect to the outcrop. The tuff 

contains phenocrysts of quartz, sanidine, minor pl~gioclase, and biotite. A few crystals of sphene also are present in most samples. The sphene has an unusually small 

optic axial angle and is rimmed by opaque oxides. The 

Tuff of Skeleton Canyon 9 
electron microprobe shows that the sphene has a relatively 

high iron content and contains detectable yttrium and 

neodymium. It does not contain detectable tin (R. P. 

Christian, written commun., 1984). The upper part of the 

zone of dense welding in Skeleton Canyon is more mafic 

than the lower, containing much more plagioclase and 

biotite (fig. 6). The tuff of Skeleton Canyon has normal 

magnetic polarity. 

Tuff of Whitmire Canyon and Associated Rocks 

Only the southernmost exposures of the tuff of 

Whitmire Canyon of Erb (1979) were examined during 

this study (figs. 2 and 7). The unit is a crystal-poor 

rhyolitic ash-flow tuff that varies greatly in degree of 

welding and phenocryst proportions from base to top. 

Welding increases gradationally upward from a nonwelded base to a densely welded, platy-weathering top. 

Samples in the lower part of the unit contain abundant 

sanidine; the sample from the upper, platy part lacks 

sanidine (fig. 7). The unit has reversed magnetic polarity. 

The lower part of the tuff of Whitmire Canyon contains abundant rhyolite blocks and overlies a zone about 

1 m thick in which rhyolite blocks, as much as 1 m in 

diameter, are concentrated. In outcrop, these blocks 

resemble the tuff of Skeleton Canyon; a sample from one 

of them, however, contains no plagioclase. A similar 

block-rich zone occurs still lower in the section (fig. 7). 

The two block-rich zones are separated by an interval of 

nonwelded, pumice-rich tuff. This interval probably is 

related to the tuff of Whitmire Canyon. 

Erb (1979, fig. 2) showed the tuff of Whitmire Canyon as overlying the rhyolite of Clanton Draw. The exposures examined during this study show instead that the 

tuff of Whitmire Canyon overlies the tuff of Skeleton 

Canyon (fig. 7). Talus shed from the ledge of tuff of 

Skeleton Canyon obscured the rocks underlying the tuff 

of Skeleton Canyon at this locality. The tuff of Skeleton 

Canyon overlies the dacite of Outlaw Mountain in exposures a few kilometers to the southwest. 

The exposures examined gave no direct clues concerning how the tuff of Whitmire Canyon is related to 

the rhyolite of Clanton Draw. The tuff of Whitmire Canyon probably is older than at least part of the rhyolite 

of Clanton Draw. The tuff of Whitmire Canyon rests on 

the tuff of Skeleton Canyon, which in nearby exposures 

directly underlies the rhyolite of Clanton Draw. 

Rhyolite of Clanton Draw 

The rhyolite of Clanton Draw is a crystal-poor, 

alkali rhyolite lava (Erb, 1979, table 36). It is a major, 

widespread unit east and north of the area of figure 2. 

Reconnaissance examination of most of the area in which 

these lavas are exposed (mostly east of the area shown 

10 Volcanic Geology, Southern Peloncillo Mountains 

in figure 2) showed them to be remarkably uniform. Most 

exposures are of gray and purplish-gray, flow-laminated 

rock that contains inconspicuous grains of quartz, 

sanidine, plagioclase, and biotite. This rock occurs as 

thick flows and domes. Vitrophyre occurs locally within 

the sequence and at contacts with older rocks. Intercalated within this sequence of flows and domes are local 

accumulations of rhyolitic tephra, which probably are 

located near the vents from which much of the rhyolite 

was erupted. A sequence of rhythmically bedded rhyolitic 

tephra that overlies the rhyolite of Clanton Draw crops 

out east of the area of figure 2, in sec. 17, T. 32 S., R. 

21 W. These bedded rocks, indistinguishable from tephra 

intercalated between flows elsewhere, were mistakenly included by Erb (1979) as part of the unit he termed the 

conglomerate of Animas Valley. 

Zircon from the rhyolite of Clanton Draw gave a 

fission-track age of 25.8 ± 1.2 m.y. (Erb, 1979; see table 

1). The rhyolite of Clanton Draw has reversed magnetic 

polarity. 

Rocks of Uncertain Correlation at the 

Mouth of Guadalupe Canyon 

The hills north of the mouth of Guadalupe Canyon 

(fig. 8) are underlain by a sequence of volcanic rocks that, 

on the basis of available information, cannot be firmly 

correlated with other volcanic units in the region. The 

problem is not that they are unique; it is, rather, that 

several of the units resemble in some respects at least two 

others well known elsewhere. Insufficient information is 

available to make the necessary choices for all but one 

of the units. The units are referred to here as follows: 

Bluff Creek(?) Formation and ash-flow tuff units A, B, 

and C. Olivine andesite(?) lava locally is intercalated between the Bluff Creek Formation and unit A. Breccia 

containing clasts of rhyolite and intermediatecomposition lava locally overlies unit A. 

Bluff Creek(l) Formation 

The Bluff Creek(?) Formation of Zeller and Alper 

(1965) consists chiefly of crystal-poor, nonwelded, 

pumice- and lithic-rich ash-flow t\lff more than 30 m 

thick. The non welded tuff is capped by a zone about 20 m 

thick that grades from a moderately welded base to a 

densely welded top. These rocks are characterized by 

abundant sanidine, no quartz, and little plagioclase (fig. 

9). The upper part of the zone of dense welding contains 

more biotite than the rocks below. 

Erb (1979, p. 42 and 195) tentatively correlated 

the rocks of this unit with the Bluff Creek Formation, 

which was named for exposures near Bluff Creek on the 

east flank of the Animas Mountains. The phenocryst 
69.5 

62.6 

Zone that 

weathers to 

ridges and 

hollows 

METERS FEET 

10 

30 

20 

10 

Figure 6. Stratigraphic section of tuff of Skeleton Canyon in Skeleton Canyon and histograms showing 

crystal content of samples. Section located on north side of canyon in NE 1/4 SW 1/4 sec. 24, T. 31 

S., R. 22 W. (For explanation of symbols see figure 3.) 

Rocks at Gaudalupe Canyon 11 
57.3 

71.6 

55 

Nonwelded 

pumice-rich 

/ Top of densely 

_ .L welded zone 

c: 

0 

>-

c: 

co 

u 

c: 

0 

1ii 

Qi 

.:.! 

(/) 

0 

~ 

::J 

1-

Nonwelded 

pumice-rich 

METERS 

10 

0 

I 

I 

I 

I 

I 

c: 

0 

> 

c: 

co 

u 

~ 

·g 

:c 

S: 

0 

~ 

:l 

7 

I 

I 

I 

FEET 

30 

20 

10 

0 

Figure 7. Stratigraphic section of tuff of Whitmire Canyon and tuff of Skeleton Canyon and histograms showing 

crystal and lithic fragment content of samples. (For explanation of symbols see figure 3.) 

12 Volcanic Geology, Southern Peloncillo Mountains 
MEXICO 

CORRELATION OF MAP UNITS 

} QUATERNARY 

Unconformity 

Tiy 

Oligocene TERTIARY 

Unconformity 

G } Lower 

Cretaceous } CRETACEOUS 

Unconformity 

B } PERMIAN AND 

PENNSYLVANIAN 

R 32E 

2 4 KILOMETERS 

2 MILES 

UST OF MAP UNITS 

Os Surficial deposits, undivided 

Ti lntermediate-,composition dikes 

Tc UnitC 

Tb UnitB 

Tbr Rhyolitic and intermediate- composition breccia 

Toi Olivine andesite(?) dike 

Toa Olivine andesite(?) lava 

Ta UnitA 

Tbc Bluff Creek(?) Formation 

Km Morita Formation-Sandstone, siltstone, 

mudstone, and minor limestone 

PIPn Naco Group-Limestone and dolomite, with 

minor siltstone 

---Contact-Dashed where 

approximately located 

---L.- Fault-Dashed where approximately 

located, dotted where concealed; 

bar and ball on downthrown 

side 

Figure 8. Geologic map of area near the mouth of Guadalupe Canyon. Geology of western part after Hayes (1982); geology 

of volcanic rocks in south-central part by D. H. Mcintyre (unpublished mapping, 1983). 

Rocks at Gaudalupe Canyon 13 
Unit 

c 

Unit 

B 

Unit 

A 

Bluff 

Creek(?) 

Formation 

84 

73 

55 

82 

93 

85.1 

54 

Oak Creek Tuff 

Erb (1979, Table 7, no. 4) 

Figure 9. Histograms showing crystal and lithic fragment content of rocks of uncertain correlation at the mouth 

of Guadalupe Canyon. (For explanation of symbols see figure 3.) 

proportions of the rocks near the mouth of Guadalupe 

Canyon are very close to those in ash-flow tuffs of the 

lower part of the Bluff Creek exposed to the east (Reiter, 

1980, table 5), so this tentative correlation is quite 

reasonable. 

Unit A 

Unit A is a moderately to densely welded, crystalrich, ledge-forming ash-flow tuff. Outcrops along the 

14 Volcanic Geology, Southern Peloncillo Mountains 

road up Guadalupe Canyon expose lenses of white collapsed pumice as much as 30 em long. Phenocryst proportions in unit A (fig. 9) closely resemble both those in 

the Oak Creek Tuff of the Animas Mountains (Zeller and 

Alper, 1965; Erb, 1979) and the lower part of the tuff 

of Guadalupe Canyon as exposed along Cottonwood 

Creek. If the correlation of the underlying unit with the 

Bluff Creek Formation is correct, it would be logical for 

unit A to be Oak Creek Tuff because, in the Animas 

Mountains, Oak Creek Tuff overlies the Bluff Creek Formation. Unit A, however, could as well be tuff of 
Table 4. Trace-element analyses of samples from hydrothermally altered zone south of upper 

Cottonwood Creek 

[Sample locations shown on fig 2. Au, As, Bi, Sb, Te determined by atomic absorption; analyst: T. A. 

Roemer. Se determined by flameless atomic absorption; analyst: E. P. Welsch. Cu, Mo, Pb, Ag determined by semiquantitative spectrographic analysis; analyst: M. S. Erickson. Detection limits for some 

elements shown in parentheses. N = not detected at value shown; L = detected, but below limit of determination. All values are in parts per million (ppm)] 

Sample No. Au As Bi Sb Se Te Cu Mo Pb Ag 

MD-50-B L(.05) L(5) 2 L(2) 0.80 1.4 10 N(5) 10 N(.5) 

MD-68 L(.05) N(5) 4 N(2) 2.20 3.0 15 . N(5) 30 N(.5) 

MD-70-A N(.05) L(5) 1 L(2) 0.65 5.0 15 L(5) N(10) N(.5) 

MD-70-D N(.05) N(5) 2 N(2) 0.20 0.1 30 L(5) 20 N(.5) 

MD-73 .05 L(5) 1 N(2) 0.35 20 L(5) N(5) N(10) N(.5) 

MD-50-B 

MD-68 

MD-70-A 

MD-70-0 

Sample descriptions 

Quartz vein that contains finely disseminated pyrite. 

Quartz vein that contains finely disseminated pyrite. 

Spongy vein quartz float that contains finely disseminated pyrite. 

Unweathered, hydrothermally altered breccia impregnated with silica and disseminated 

pyrite. 

MD-73 Quartz vein that contains sparse pyrite and a few grains of an unidentified dark gray metallic 

mineral. 

Guadalupe Canyon unconformably overlying the Bluff 

Creek(?) Formation. Erb (1979) assigned unit A to the 

tuff of Guadalupe Canyon. Unit A has normal magnetic 

polarity. 

Unit 8 

Unit B is massive, moderately welded, and tends 

to form gray to grayish-purple, rounded bluffs. Much of 

the unit is concealed by debris shed by the more competent overlying unit C. Biotite is conspicuous in hand 

specimen. The rock also contains abundant quartz, 

sanidine, and plagioclase (fig. 9). Phenocryst proportions 

resemble those in the Basin Creek Tuff (Zeller and Alper, 

1965, table 4) and the tuff of Gray Ranch (Erb, 1979, 

table 15; Reiter, 1980, table 11). 

Unit C 

Unit Cis a densely welded, reddish-brown, bluffforming ash-flow tuff that caps a hill north of the mouth 

of Guadalupe Canyon. Unit C is crystal-rich; plagioclase 

and biotite are abundant, and sanidine and quartz are 

sparse or absent (fig. 9). Phenocryst proportions resemble those for the tuff of Black Bill Canyon (Erb, 1979, 

table 11) and the matrix of the breccia of Hog Canyon 

(fig. 5). 

A black vitrophyre occurs at the base of unit C 

where exposed on the southeast side of the hill. Most of 

the lower part of the unit above the vitrophyre, however, 

is best exposed on the north side of the hill. Exposures 

and float there show well-developed vitroclastic texture. 

Higher in the unit the texture is smeared by flowage; near 

the exposed top this texture is wholly obliterated and only 

fine flow laminae are visible. Erb (1979, p. 210) called 

these rocks dacite(?) lava, evidently having seen only the 

flow-laminated rocks in the upper part of the unit. The 

unit has reversed magnetic polarity. 

Olivine Andesite(?) Lava and Dike 

A small patch of olivine-bearing lava crops out 

north of the mouth of Guadalupe Canyon. The lava is 

a phenocryst-poor rock that contains brown 

pseudomorphs after olivine that are as much as 2 mm 

long and sparse plagioclase phenocrysts less than 1 mm 

long, all set in a trachytic matrix of plagioclase and 

opaque oxide grains. The lava rests concordantly on eastdipping rocks of the Bluff Creek(?) Formation and is in 

fault contact with Unit Ta. Local exposure of an oxidized 

flow top shows that these rocks are a lava rather than 

a sill. The lava is cut by a small dike that in outcrop 

resembles the intrusive rocks associated with the dacite 

of Outlaw Mountain. An olivine phenocryst-bearing dike 

exposed in Guadalupe Canyon, in the SW\.4 sec. 15, T. 

23 S., R. 32 E., appears in thin section to be identical 

to the lava. 

Rhyolitic and Intermediate-Composition Breccia 

The southwest slopes of the hill capped by unit C 

are underlain by poor exposures of breccia containing 

clasts chiefly of rhyolite and intermediatecomposition 

lava. Locally, rhyolite blocks more than 1 m in diameter 

litter the slope. One block of gneiss (a few centimeters 

in diameter) was noted. 

Rocks at Gaudalupe Canyon 15 
Rhyolite clasts, although crystal-poor, contain 

phenocrysts as much as 2 mm in length of quartz, 

sanidine, minor plagioclase, and biotite. In thin section, 

some of the larger rhyolite clasts have been molded 

around smaller clasts, indicating that the larger clasts were 

hot and plastic when deposited whereas the smaller ones 

were cool and rigid. 

The intermediate-composition clasts contain phenocrysts of clinopyroxene, plagioclase, and amphibole. The 

matrix of this breccia contains crystals as much as 1 mm 

long of plagioclase, sanidine, quartz, and amphibole. 

Some of the breccia rich in intermediate-composition 

clasts also contains clasts of siltstone that probably were 

derived from pre-Tertiary formations. 

Basalt 

Basalt in the area occurs as lavas that unconformably overlap the older rocks in the southwestern part of 

figure 2 and as dikes cutting the altered rocks in the hills 

northeast of the Silvertip Mine. The lavas and dikes are 

indistinguishable in hand specimen or thin section. 

Although phenocryst-poor, all contain olivine, clinopyroxene, and minor plagioclase phenocrysts as much as 

3 mm in size. The dikes in the altered zone are themselves 

unaltered. Dike rocks are not altered even immediately 

adjacent to the only well-exposed contact found. 

The basalt is part of an extensive field in the San 

Bernardino Valley, west of the area of figure 2. The 

clusters of dikes that occur in the Peloncillo Mountains 

show that basalt eruptions occurred within the mountains 

as well as within the adjacent valley. 

GERONIMO TRAIL CAULDRON 

Erb (1979) recognized a single cauldron in the 

southern Peloncillo Mountains, which he named the 

Geronimo Trail cauldron. Evidence for this cauldron is 

best seen along its southwest margin, where the volcanic 

rocks within the cauldron are downdropped against preTertiary rocks along an arcuate fault (fig. 1; Erb, 1979; 

Hayes, 1982). Most domes and intrusive rocks are 

restricted to a zone a few kilometers wide near this ringfracture fault (Erb, 1979; fig. 2). Collapse of the cauldron 

most likely was caused by eruption.of the tuff of Guadalupe Canyon. 

Erb (1979) recognized that the cauldron collapse 

was followed by resurgence. The fault-bounded area 

northeast of Estes Canyon, including the Silvertip Mine 

area (sec. 26, T. 32 S., R. 32 E.) and Outlaw Mountain, 

are part of a broad, rather flat resurgent dome. Uplift 

of the resurgent dome accompanied invasion by dacitic 

magma of the collapsed block within the cauldron. Much 

16 Volcanic Geology, Southern Peloncillo Mountains 

of the dacitic magma leaked to the surface near the ringfracture zone and formed the domes and intrusive masses 

north of Sycamore Creek and west of Hog Canyon. Small 

leaks on the resurgent dome itself resulted in the intrusive 

bodies near the headwaters of Cottonwood Creek. The 

breccia of Hog Canyon and related rocks and the dacite 

of Outlaw Mountain were erupted from vents marked by 

these intrusive bodies in the ring-fracture zone and on 

the dome. 

Only the southwestern part of the Geronimo Trail 

cauldron has been preserved. The fault marking the 

western margin of cauldron passes beneath younger rocks 

of the valley to the west. The north margin of the 

cauldron probably is marked by a fault that follows the 

north rim of Skeleton Canyon; a vastly different suite 

of rocks occurs north of the fault. The east half of the 

cauldron, including the ring-fracture zone and part of the 

resurgent dome, has been obliterated by collapse of a 

younger cauldron, centered to the northeast, that overlaps 

the Geronimo Trail cauldron. 

CLANTON DRAW CAULDRON 

The name Clanton Draw cauldron is here proposed 

for a structure that truncates the Geronimo Trail 

cauldron. Available evidence suggests that initial collapse 

of this cauldron accompanied eruption of the tuff of 

Skeleton -Canyon. The cauldron subsequently was filled 

to overflowing with the rhyolite of Clanton Draw. 

The topographic wall at the southwest margin of 

the cauldron is exposed near the head of Clanton Draw, 

where the rhyolite of Clanton Draw and older rocks meet 

at a steep contact (fig. 2). Erb (1979), impressed by 

vitrophyre at this contact, believed it to be an intrusive 

contact. However, vitrophyre is common at the contact 

of the rhyolite of Clanton Draw with older rocks, whether 

the contact is steep, as in the example above, or flat and 

clearly extrusive, as in exposures of the contact northeast 

of Outlaw Mountain. The moderately to steeply eastwarddipping contact continues southeastward for several 

kilometers (fig. 2). Toward the northwest, the contact 

flattens abruptly. Farther west and northwest, the rhyolite 

rests either quasiconcordantly on the irregular upper surface of the dacite of Outlaw Mountain or concordantly 

on the tuff of Skeleton Canyon where that tuff is present. The change of contact attitude from steep to flat is 

interpreted here to mark the spillover point, where the 

rhyolite filling the Clanton Draw cauldron overtopped 

the topographic wall. Beyond this point, the rhyolite 

spread westward as outflow upon the rocks outside the 

cauldron. No collapse breccia occurs adjacent to the wall 

because present exposures are high on the wall, in the 

zone from which such deposits are derived and 

topographically above where they accumulate. 
As noted earlier, accumulations of rhyolitic tephra 

east of the area of figure 2 mark the vents within the 

cauldron from which most of the rhyolite lava was 

erupted. The vents probably are located along part of the 

ring-fracture zone. The current position of the topographic wall, several kilometers to the west, probably is 

the result of rapid scarp retreat before or early during 

the lava eruptions. Collapse breccia formed during the 

scarp retreat should be widespread beneath the rhyolite. 

Eruption of the tuff of Skeleton Canyon is proposed as the event triggering collapse of the Clanton Draw 

cauldron because (1) it commonly underlies the rhyolite 

of Clanton Draw, which is the principal unit exposed 

within the cauldron; (2) it is alkali rhyolitic, similar in 

composition to the rhyolite of Clanton Draw; and (3) according to Erb (1979, p. 231), it has a minimum thickness 

of 125 m in Skeleton Canyon-thicker at that place within 

the proposed cauldron than outside it. 

A sequence of events consistent with what currently is known is (1) explosive eruption of a modest volume 

of volatile-charged alkali rhyolitic magma (tuff of 

Skeleton Canyon) accompanied by formation of a 

shallow cauldron and then (2) more quiescent eruption 

of the remaining available alkali rhyolitic magma, filling the shallow cauldron to overflowing with rhyolite 

lava. 

What role, if any, the tuff of Whitmire Canyon 

played in development of the Clanton Draw cauldron is 

unclear. Its chemical composition is virtually identical to 

that of the rhyolite of Clanton Draw (table 2); it thus may 

be an explosively erupted, immediate precursor to the 

rhyolite lava. 

Intermediate-composition intrusions into the rhyolite of Clanton Draw and related rocks northeast of the 

area of figure 2 probably mark part of the ring-fracture 

zone on the southwest side of the Clanton Draw cauldron. 

Flooding of the cauldron by the rhyolite lava following 

collapse has concealed the ring-fracture faults. The intrusive bodies are the analogues of those in the ringfracture zone of the Geronimo Trail cauldron. The 

eastern part of the cauldron is concealed by younger 

deposits in the valley east of the mountain range. 

STRUCTURE 

The structure within the mapped area (figs. 2 and 

8) appears chiefly cauldron related; insufficient area was 

studied to permit identification of those elements that 

might clearly be related to regional tectonism. With the 

possible exception of the fault bounding the southwest 

margin of the Geronimo Trail cauldron, none of the 

faults has large displacement. The faults on the resurgent 

dome in the upper Cottonwood Creek area, which break 

up the rocks into a mosaic of small blocks, form a pattern much like that found on some other resurgent domes 

(see, for example, Carr and Quinlivan, 1966). More 

detailed mapping in this area probably would reveal many 

more such small faults. 

Ml N E RALIZA TION 

Mineralization in and northeast of the area shown 

in figure 2 was of two kinds. Products of the first are 

associated with a 5-km2 area of hydrothermally altered 

rocks and quartz veins located primarily east and northeast of the Silvertip Mine. The second mineralization 

event is indicated by geochemically anomalous tin in 

sediments of streams draining areas underlain by the 

rhyolite of Clanton Draw. 

Hydrothermal Alteration and Quartz Veins 

The area of hydrothermal alteration, outlined on 

figure 2, contains intensely leached, clay-altered rock that 

includes several zones of large quartz veins. One exposure 

of unweathered rock within the area is rich in disseminated pyrite. Much, if not most, of the leaching and clay 

formation elsewhere in the zone evidently has been by 

acid waters derived from weathering of disseminated 

pyrite. Watts and others (1983, table 2) collected acidsulfate water (pH 3.66) from a seep in upper Cottonwood 

Creek that contained large amounts of aluminum, iron, 

manganese, and zinc. This water clearly shows that 

leaching of the rock in the altered area is going on at the 

present time. Much of the leachate evidently is leaving 

the area via the surface drainage rather than being 

reprecipitated and concentrated at depth. 

The intensity of rock alteration varied within the 

zone. In many parts of the zone, especially near the Silvertip Mine, feldspar phenocrysts in the rock are fresh and 

glassy, despite leaching of the matrix. In the northwestern 

part of the zone, alteration was. more intense; feldspars 

commonly have been completely dissolved, leaving empty 

holes in the rock matrix. The altered biotite is silver-green. 

Limonitic coatings on joints commonly are heavy and 

goethite rich, locally with minor hematite. 

The quartz veins within the altered zone trend 

N. 10° W. toN. 15 ° E., are tens of meters long, essentially vertical, and locally attain widths of 6 m. All contain abundant blocks and slabs of country rock. Most 

exposures are deeply weathered and strongly stained by 

goethite and hematite. Fresh vein rock is dark greenishgray and contains finely disseminated pyrite, which in 

some veins alternates with stringers of barren white quartz 

a few centimeters across. 

Hydrothermal Alteration and Quartz Veins 17 
The Silvertip Mine is aS. 40°W.-trending adit, still 

open at the portal, but not entered during this study. The 

adit at the portal follows a steep shear zone about 1 m 

wide that lacks strong silicification. According to Hayes 

and others (1983), the adit is 240 ft (73 m) long and explores a fracture zone that attains a width of 10ft (3m). 

A shallow shaft also is present. The small dump near the 

adit contains a few pieces of sulfide-bearing quartz in addition to altered breccia of Hog Canyon. The quartz, 

unlike that seen in nearby veins, is dark gray and spongy. 

Quartz phenocrysts recognizable within the spongy matrix 

suggest that the spongy quartz may have formed, at least 

in part, by replacement of the volcanic country rock. The 

quartz contains a few scattered crystals of pyrite and (or) 

chalcopyrite. One specimen contained a single grain a 

fraction of a millimeter in diameter of a dark gray, soft 

mineral with a submetallic luster. Similar, spongy quartz 

was seen as float at locality MD-70 (fig. 2). The Silvertip 

Mine has no recorded production. 

The age and genetic affinities of the altered zone 

are not well defined, but field relations permit some 

limited inferences. Alteration affected rocks as young as 

the dacite of Outlaw Mountain. Younger rocks, such as 

the tuff of Skeleton Canyon and the rhyolite of Clanton 

Draw, are not altered, but they do not occur in or very 

near the altered area. Another approach to the problem 

would note that none of the rocks seen during this study 

anywhere in the Clanton Draw cauldron are hydrothermally altered, but that all of the rocks in the Geronimo 

Trail cauldron locally are. This would suggest that the 

alteration was a characteristic of the Geronimo Trail 

cauldron. As noted earlier, the zone of intense alteration 

is located on the cauldron's resurgent dome. 

Unweathered samples from several of the veins and 

one sample of unweathered, altered rock did not contain 

unusual concentrations of any element except that some 

of the veins contain very small amounts of gold. 

Analytical results for several elements are shown in table 

4. Intense leaching of most rocks exposed at the surface 

in the altered zone makes evaluation of its metals content very difficult. It is possible that higher metals values 

may exist in some of the unweathered rock concealed 

beneath the leached surface capping. The results shown 

in table 4, however, are not encou·raging. 

Tin 

Anomalous quantities of tin were found during the 

geochemical survey (Watts and others, 1983) chiefly in 

sediments from streams draining the area underlain by 

the rhyolite of Clanton Draw. The tin anomalies are most 

apparent .in the panned heavy-mineral concentrates, in 

which cassiterite was visually identified (Watts and others, 

1983). Other minerals identified in the concentrates that 

commonly are associated with cassiterite in tin deposits 

18 Volcanic Geology, Southern Peloncillo Mountains 

are fluorite and possible pseudobrookite (Watts and 

others, 1983). Topaz was identified by the author in one 

of the pan concentrates. No cassiterite was noted in any 

of the rock samples collected from the rhyolite of Clanton Draw during this study. No hydrothermal alteration 

was observed in terrain in and around two drainages that 

provided pan concentrates containing 2,000->2,000 ppm 

tin southeast of Clanton Draw east of the area of figure 

2 (Watts and others, 1983). Fresh glass was common at 

the margin of several rhyolite bodies exposed there. 

Although the sources for the tin in the stream sediments 

were not located, the presence of large hydrothermally 

altered areas in which the tin might have been concentrated was ruled out. No hydrothermal alteration was 

noted elsewhere in the outcrop area of the rhyolite of 

Clanton Draw. The cassiterite evidently was supplied to 

the stream sediments from small cassiterite-bearing zones 

within the unaltered rhyolite that were missed during the 

rock sampling. 

RE FE RE NCES CITED 

Carr, W. J., and Quinlivan, W. D., 1966, Geologic map of the 

Timber Mountain quadrangle, Nye County, Nevada: U.S. 

Geological Survey Geologic Quadrangle Map GQ-503. 

Deal, E. G., Elston, W. E., Erb, E. E., Peterson, S. L., Reiter, 

D. E., Damon, P. E., and Shafiqullah, M., 1978, Cenozoic 

volcanic geology of the Basin and Range Province in 

Hidalgo County, southwestern New Mexico: New Mexico 

Geological Society Guidebook, 29th Field Conference, Land 

of Cochise, p. 219-229. 

Erb, E. E., Jr., 1979, Petrologic and structural evolution of ashflow tuff cauldrons and noncauldron-related volcanic rocks 

in the Animas and southern Peloncillo Mountains, Hidalgo 

County, New Mexico: University of New Mexico Ph.D. 

thesis, 286 p. 

Hayes, P. T., 1982, Geologic map of Bunk Robinson Peak and 

Whitmire Canyon Roadless Areas, Coronado National 

Forest, New Mexico and Arizona: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1425-A, scale 1:62,500. 

Hayes, P. T., Watts, K. C., Hassemer, J. R., and Brown, S.D., 

1983, Mineral resource potential map of Bunk Robinson 

Peak and Whitmire Canyon Roadless Areas, Hidalgo County, New Mexico and Cochise County, Arizona: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-1425-B, scale 1:62,500. 

Reiter, D. F., 1980, Geology of the Alamo Hueco and Dog Mountains, Hidalgo County, New Mexico: Albuquerque, University of New Mexico, M.S. thesis, 100 p. 

Watts, K. C., Hassemer, J. R., and Day, G. W., 1983, Geochemical maps of Bunk Robinson Peak and Whitmire Canyon Roadless Areas, Hidalgo County, New Mexico and 

Cochise County, Arizona: U.S. Geological Survey Miscellaneous Field Studies Map MF-1425-C, scale 1:62,500. 

Zeller, R. A., Jr., and Alper, A. M., 1965, Geology of the Walnut 

Wells quadrangle, Hidalgo County, New Mexico: New Mexico Bureau of Mines and Mineral Resources Bulletin 84, 

105 p. 

* U.S. GPO: 1988 r:i13-fJ.I7 166,tm 
~ z 

Vl