GEOLOGICAL SURVEY CIRCULAR 557 

Lead, Copper, Molybdenum, and 

Zinc Geochemical Anomalies 

South of the Summitville District 

Rio Grande County, Colorado 

Lead, Copper, Molybdenum, and 

Zinc Geochemical Anomalies 

South of the Summitville District 

Rio Grande County, Colorado 

By William N. Sharp and James l. Gualtieri 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 557 

Washington 1968 
United States Department of the Interior 

STEWART l. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

Free on application to the U.S. Geological Sunoey, Washington, D.C. 2024 
CONTENTS 

Page 

Abstract ------------------------------------ 1 Geochemical anomalies ____________________ _ 

Introduction _______________________________ _ 1 Comparison with the Summitville district ___ _ 

Geology ------------------------------------ 1 Conclu~ons --------------------------------- Rock alteration ____________________________ _ 1 References cited ____________________________ _ 

ILLUSTRATIONS 

FIGURE 1. Generalized geologic map of the Lookout Mountain-Summitville area 

2. Geochemical map showing areas with molybdenum values >10 ppm in relation to areas with 

Pag-e 

2 

2 

2 

2 

Page 

3 

lead values > 100 ppm ---------------------------------------- ______________________________ 4 

3. Geochemical map showing areas with zinc values > 100 ppm in relation to areas with lead 

values > 100 ppm ___________ ------------------------------------------------------------- 5 

4. Geochemical map showing areas with copper values > 10 ppm in relation to areas with lead 

values >100 ppm -------------------------------- __ ------------------------------------- 6 

5. Combined geochemical map showing relation of anomalous areas ___________________________ _ 7 

III 

LEAD, COPPER, MOLYBDENUM, AND ZINC GEOCHEMICAL ANOMALIES SOUTH OF 

THE SUMMITVILLE DISTRICT, RIO GRANDE COUNTY, COLORADO 

By WILLIAM N. SHARP and JAMES L. GUALTIERI 

Abstract 

An extensive lead geochemical anomaly, as well as 

several smaller anomalies of copper, molybdenum, and 

zinc, have been found in a large area of altered volcanic and intrusive rocks south of the Summitville 

mining district. These geochemical anomalies, together 

with the geologic similarity to the productive Summitville district, make the area attractive for further 

exploration. 

INTRODUCTION 

A geologic and geochemical study of a large 

area of highly altered rocks in the volcanic 

and intrusive center around Sumn1itville and 

Lookout Mountain in the eastern San Juan 

Mountains has revealed a possible exploration 

target near Looko:1t Mountain. This area was 

investigated as part of the Heavy Metals program of the U.S. Geological Survey to assess 

its resource potential for metal deposits. Summitville has produced significant quantities of 

gold, silver, and copper from a mineralized 

volcanic plug and is currently being developed 

for further mining. Other areas to the south 

have been widely prospected, but production 

has been negligible. The geologic similarity of 

the Sumn1itville district and the area around 

Lookout Mountain to the south suggested that 

reconnaissance geochen1ical and hydrothermal 

alteration studies of the Lookout Mountain 

area n1ight reveal evidence for the presence of 

hidden ore deposits. 

GEOLOGY 

The area studied is underlain by a thick 

series of lava flows representing several distinct periods of volcanic activity. The lowermost and most widespread flows are part of the 

Conejos Formation of Oligocene or older age 

1 

(Steven, Mehnert, and Obradovich, 1967). 

These are overlain by later flows of rhyodacite, 

quartz latite, and rhyolite; and at Summitville, 

they are cut by numerous associated plugs 

which were called Fisher Quartz Latite by 

Steven and Ratte (1960). South of Summitville the Conejos Formation has been intruded 

by a large body of quartz monzonite (quartz 

monzonite of Klondike Mountain of Patton, 

1917) and by a large dike of quartz latite 

porphyry and several smaller bodies of coarsely 

porphyritic quartz latite closely similar to the 

Fisher Quartz Latite at Summitville. 

ROCK ALTERATION 

A large mass of altered rock forms the 

northern side of the upper Alamosa Creek 

valley and includes Lookout Mountain (fig. 1). 

It extends over a northeast-trending area 4-5 

n1iles long and about 2 miles wide and lies 

roughly astride the contact between volcanic 

rocks on the northwest and quartz n1onzonite 

to the southeast. The large dike and other 

small bodies of quartz latite porphyry that intrude the volcanic rocks also are altered. The 

altered rocks are exceptionally well exposed on 

the rugged east slope of Lookout Mountain. 

Hydrothermal activity in one or two stages 

has converted the original drab-colored flows 

and intrusive rocks into conspicuous white, 

yellow, and red masses consisting largely of 

quartz, kaolinite, and illite. Rocks along a wide 

belt that includes the large quartz latite 

porphyry dike south and east of Lookout Mountain have been altered predominantly to quartz, 

alunite, and pyrophyllite. This assemblage is 

considered to represent a more advanced stage 

of hydrothermal alteration than the more wide-
spread quartz-kaolinite-illite assemblage, and 

the general parallelism of the quartz-alunitepyrophyllite rocks to the dike suggests that 

they are genetically related to the dike. 

GEOCHEMICAL ANOMALIES 

More than 200 rock samples were collected 

throughout the area of altered rock (fig. 1); 

most were collected from intensely altered 

areas. Chemical analyses of these samples indicate zones of anomalously high content of 

lead, n1olybdenum, copper, and zinc (figs. 2-5). 

Neither gold nor silver was detected in the 

samples from the area around Lookout Mountain; the limits of detection of the analytical 

methods used were 0.1 ppm (part per million) 

for gold and 0.5 ppm for silver. Traces to 

appreciable amounts of both 1netals were detected in samples from the Summitville area. 

The most pronounced geochemical anomalies 

are in a group east of Lookout Mountain. A 

lead anomaly nearly 2 miles long and as much 

as 0.6 mile wide encloses most of the large 

quartz latite porphyry dike and overlaps ento 

the quartz monzonite to the southeast. Within 

this anomalous area, all samples contain > 100 

ppm lead. The lead anomaly covers and is 

roughly parallel to the southeastern part of 

the zone of quartz-alunite-pyrophyllite alteration and extends into adjacent parts of the 

zone of quartz-kaolinite-illite alteration. 

Smaller anomalies of molybdenum, .copper, and 

zinc are peripheral to the lead anomaly. 

This arrangement of metal anomalies suggests that lead, being less mobile in a surface 

environment, may be accumulated more closely 

around the locus of mineralization, whereas the 

copper, molybdenum, and zinc were more easily 

moved through the rocks and thus were deposited farther from the center. 

COMPARISON WITH THE SUMMITVILLE DISTRICT 

The area of altered rocks discussed in this 

report is about 3 miles south of the Summit2 

ville district, and the similarities of the two 

areas in geologic relations, alteration pattern, 

and geochemical anomalies are noteworthy. In 

both areas, rock alteration is concentrated in 

the vicinity of intrusive bodies and volcanic 

plugs. The gold-silver-copper mineralization at 

Summitville was centered in the most strongly 

altered rocks in a plug of coarsely porphyritic 

quartz latite (Steven and Ratte, 1960) that 

lithologically resembles the large dike east of 

Lookout Mountain. \Vithin the plug, mineralization was closely confined to a zone of quartzalunite alteration within a wider body of more 

weakly altered rocks. An area with anomalously high lead content encloses the mineralized zone at Summitville, and local areas with 

high zinc and molybdenum content are peripheral to the area of high lead content. 

CONCLUSIONS 

The geologic relations, the alteration patterns, and the geochemical anomalies indicate 

that the area east of Lookout Mountain 

deserves careful consideration as an exploration target. The area around the northeast end 

of the lead anomaly seems to be the most 

favorable, inasmuch as geochemical anomalies 

of the other metals also occur there. In view 

of the possible secondary migration of metals 

downhill, the topographically higher parts of 

the target area near the porphyry dike should 

be tested first. 

REFERENCES CITED 

Patton, H. B., 1917, Geology and ore deposits of the 

Platoro-Summitville mining district, Colorado: Colorado Geol. Survey Bull. 13, 122 p. 

Steven, T. A., Mehnert, H. H., and Obradovich,' J. D., 

1967, Age of volcanic activity in the San Juan 

Mountains, Colorado, in Geological Survey research 

1967: U.S. Geol. Survey Prof. Paper 575-D, p. 

D47-D55. 

Steven, T. A., and Ratte, J. C., 1960, Geology and 

ore deposits of the Summitville district, San Juan 

Mountains, Colorado: U.S. Geol. Survey Prof. 

Paper 343, 70 p. 
25' 

cr$Jo 

• s oMO~TAI 

0 

0 

• 

Cropsy Peak 8 

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0 

0 

0 0 

•etc» 0 

.. ~ eo 0 

0 

00. 

0 0 

LOOKOUT o 1 

-~\ '1~_; MOUNTAIN-

- i 

= / .=: lf'f\:-

0 0 

Area of this report 

0 1 MILE .._ _ __._ __ ....__ _ ___._ __ .......___________.J 

Quartz latite porphyry 

of Miocene age 

Fault 

EXPLANATION 

Quartz monzonite of 

Miocene age 

Conejos Formation and 

younger volcanic rocks, 

of Oligocene or older age 

All rocks are altered except where stippled; 

stippling indicates unaltered rock 

() 

Alunite-bearing 

• 

Illite-kaolinite -

bearing 

0 

Not differentiated 

Sample localities, showing mineralogy of altered rocks 

FIGURE I.-Generalized geologic map of the Lookout Mountain-Summitville area. 

3 
• 

() 

0 

0 

0 

• 

Cropay Peak 8 

0 

0 

0 

0 

LOOKOUT 

~'' '1. MOUNTAI .. "'~~~' -:: ~ 

-

Area of this report 

0 1 MILE 

EXPLANATION 

Quartz latite porphyry 

of Miocene age 

Quartz monzonite of 

Miocene age 

Conejos Formation and 

younger volcanic rocks, 

of Oligocene or older age 

Fault 

All rocks are altered except where stippled; 

stippling indicates unaltered rock. 

() 

Alunite-bearing 

• 

Illite-kaolinite -

bearing 

0 

Not differentiated 

Sample localities, showing mineralogy of altered rocks 

FIGURE 2.--Geochemical map showing areas with molybdenum values >10 ppm in relation to areas with lead 

values >100 ppm. 

4 
() 

0 cl z \ n 

0 '-~ 

0 0 

0 

• 0 

0 0 

Cropsy Peak 8 o 0 

'\oo/' 

. 0 : 

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0 

0 

0 0 

.. ·~ ~ 0 

eo 0 

0 

~~ 

Area of this report 

0 1 MILE 

EXPLANATION 

Quartz latite porphyry 

of Miocene age 

Conejos Formation and 

younger volcanic rocks, 

of Oligocene or older age 

Fault 

All rocks are altered except where stippled; 

stippling indicates unaltered rock. 

() 

Alunite-bearing 

• 

Illite-kaolinite -

bearing 

0 

Not differentiated 

Sample localities, showing mineralogy of altered rocks 

FIGURE 3.-Geochemical map showing areas with zinc values >100 ppm in relation to areas with lead values 

>100 ppm. 

5 
0 

• 

Cropsy Peak 8 0 

0 

0 

0 0 

• =lao ~--· . eo 0 0 o Cu 

LOOKQUT o~ ... :~\l,,t;.- MOUNTAI - ~ 

0 

EXPLANATION 

Area of this report 

1 MILE 

Quartz latite porphyry 

of Miocene age 

Quartz monzonite of 

Miocene age 

Conejos Formation and 

younger volcanic rocks, 

of Oligocene or older age 

Fault 

All rocks are altered except where stippled; 

stippling indicates unaltered rock 

() 

Alunite- bearing 

• 

Illite-kaolinite -

bearing 

0 

Not differentiated 

Sample localities, showing mineralogy of altered rocks 

FIGURE 4.-Geochemical map showing areas with copper values > 10 ppm in relation to areas with lead values 

>100 ppm. 

6 
Cropsy Peak 8 

LOOKOUT 

~\I,,;. MOUNTAI·~~F' _::: ~ 

Quartz latite porphyry 

of Miocene age 

-

0 

EXPLANATION 

Quartz monzonite of 

Miocene age 

/ \ 

I/Cu l 

?..._./ 

( 

Area of this report 

1 MILE 

Conejos Formation and 

younger volcanic rocks, 

of Oligocene or older age 

All rocks are altered except where stippled; 

stippling indicates unaltered rock. 

Fault 

FIGURE 5.-Combined geochemical map showing relation of anomalous areas. 

7