\rsenic as an Indicator Element 

or Mineralized Volcanic Pipes 

n the Red Mountains Area, 

N estern SanJuan Mountains, 

Jolorado 

:y WILBUR S. BURBANK, ROBERT G. LUEDKE, and 

REDERICK N. WARD 

~EOLOGICAL SURVEY BULLETIN 1364 

)repared in cooperation with the Colorado 

tate Mining Industrial Development Board 

(rsenic as a geochemical indicator element 

or pipelike mineralized deposits 

n a solfatarically altered environment 

JNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1972 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog card No. 72-600262 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402-Price 25 cents (paper cover) 

Stock Number 2401-2195 
CONTENTS 

Page 

Abstract 1 

Introduction ______ _ __________________ ---------------------------------------------------- 1 

Geologic setting _________ _______ -------------------------------------------------- 4 

Rock alteration and associated ores and gangues ____ _______ _______________________ 4 

Sampling and analytical methods _ _ ______________________ 6 

Sample localities __________ ________________ ___ _____ _ ____ ------------------------------------- 7 

Mineral Creek site (loc. 1) __________________________ ------------------------------------------ 7 

National Belle mine site (loc. 2) ______ _______________________ 9 

Lark mine site (loc. 3) ___ _ _______________________________ ---------------------------------- 16 

Longfellow and St. Paul mine sites (locs. 4 and 5) ______________ ___ 22 

Brooklyn mine site (loc. 6) ____ _ __________________ 23 

Conclusions _________________ ___________________ ___________________________ 28 

References cited ------------------------------------------ 30 

FIGURE 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

ILLUSTRATIONS 

Index map of Colorado showing area of this report 

Map showing geochemical sampling sites in the 

generalized structural setting of the Silverton 

cauldron and vicinity _ __________ _ ____________________ _ 

Graphs showing selected chemical data of a section 

along U.S. Highway 550 in Mineral Creek valley ___ _ 

Topographic map showing sample localities at the 

National Belle mine site _ ___ _ _________ ----------------------------------

Diagrammatic profile and graphs showing selected 

chemical data across the National Belle mine outcrop _____ _ 

Map showing sample localities at the Lark mine site ________ . 

Diagrammatic profile and graphs showing selected 

chemical data along the lower traverse across the 

Lark mine site _____________________________________________________________________ _ 

Diagrammatic profile and graphs showing selected 

chemical data along the upper traverse across the 

Lark mine site --------------------------------------------------------------------------

Geologic sketch map of the Koehler compound volcanic 

pipe and sample localities at the Longfellow and 

St. Paul mine sites __ _ 

Sketch map of sample localities at the Brooklyn 

mine site ------------------------------------------------------------------------------------

Graphs showing comparison of field and laboratory 

determinations for arsenic ______________________________________ _ 

III 

Page 

2 

3 

8 

11 

13 

16 

18 

21 

23 

25 

30 
IV CONTENTS 

TABLES 

Page 

TABLE 1. Chemical analyses of rocks and slope wash along 

U.S. Highway 550 in Mineral Creek valley _____________ ____ 7 

2. Spectrographic analyses of rocks and slope wash along 

U.S. Highway 550 in Mineral Creek valley 10 

3. Chemical analyses of rocks and soil at the National 

Belle mine site _ --------------------------------------------- ____________________________ 14 

4. Spectrographic analyses of rocks and soil at the 

National Belle mine site _ _ _ _ __ _____ _____________________________ 15 

5. Chemical analyses of rocks and gouge at the Lark 

mine site _ ________________ __ _______ _____________ ____________________ 19 

6. Spectrographic analyses of rocks and gouge at the 

Lark mine site ________________________________________ --------------------------------- 20 

7. Spectrographic analyses of rocks at the Longfellow 

and St. Paul mine sites ___ ----------------------------------------- 24 

8. Chemical analyses of rocks, gouge, and undersoil at the 

Brooklyn mine site _________ -------------------------------- ------------------------- 26 

9. Spectrographic analyses of rocks at the 

Brooklyn mine site ------------------------------------- 27 
ARSENIC AS AN INDICATOR ELEMENT 

FOR MINERALIZED VOLCANIC PIPES 

IN THE RED MOUNTAINS AREA, 

WESTERN SAN JUAN MOUNTAINS, 

COLORADO 

By WILBUR S. BuRBANK, RoBERT G. LuEDKE, 

and FREDERICK N. WARD 

ABSTRACT 

In the Red Mountains and adjacent areas, southwestern Colorado, selective 

testing was done for residual arsenic in surface rocks that have undergone 

strong acid-sulfate leaching. Analytical results from samples taken across 

exposed and partly concealed known mineralized volcanic pipes in this 

solfataric-type altered environment show the general usefulness of arsenic 

as a geochemical indicator element. The values of arsenic, enhanced by mercury, silver, lead, and locally bismuth and tin, increase over the ore bodies and 

outline sites for more detailed sampling and elaborate field and laboratory 

tests. This geochemical technique seems more applicable to preliminary testing 

for mineralized pipes than for fissures. Caution must be exercised in sample 

collection and preparation because the presence of pyrite interferes with the 

accuracy of field determinations. 

INTRODUCTION 

The Red Mountains mining district, Ouray and San Juan Counties, southwestern Colorado, lies astride the county line and mostly 

east of U.S. Highway 550 between Ouray and Silverton (figs. 1, 2). 

This district was noted in the 1880's and 1890's for its output of 

chiefly base-metal ores from small pipelike or chimney ore deposits, some of which contained small pockets of ore extremely rich 

in silver. Those early mining operations yielded several million 

dollars, but subsequent search for possible concealed ore bodies 

has been limited and handicapped because of the small irregular 

shape of the ore bodies, the complex structural controls, the intensely altered country rocks, and a large amount of surface overburden. Burbank (1947, p. 431) pointed out that geochemical 

methods of prospecting may prove applicable to finding concealed 

chimney deposits. 

1 
2 ARSENIC. WESTERN SAN JUAN MOUNTAINS, COLORADO 

I 

~·:;···~ -. \I 

. t 

FIGURE 1.-Index map of Colorado showing area of this report. 

I 

I 

I 

Limited and selective geochemical sampling in and near the Red 

Mountains area was undertaken in 1965 to determine whether 

arsenic might serve as an indicator element for the chimney or 

pipelike deposits in an area where the rocks had been subjected to 

acid-sulfate alteration and leaching. The occurrence of arsenical 

minerals in the leached and oxidized ore deposits and the very low 

content of arsenic in acidified mine and surface waters led to the 

selection of arsenic (first suggested by Varnes and Burbank, 1945), 

for these preliminary tests. In this report, the distribution and 

concentration of arsenic and several selected elements are briefly 

summarized for several sites. For these types of pipelike deposits 

in a solfatarically altered environment, geochemical testing for 

arsenic is suggested as a useful prospecting technique. 

The association of anomalous arsenic with certain mineral deposits has long been recognized, and its possible use as a pathfinder 

in geochemical prospecting has long been considered. Recent geochemical investigations in north-central Nevada used the distribution of arsenic and other metals to define low-grade gold deposits 
38'00' 

55' 

37'50' 

0 

107'45' 

INTRODUCTION 

/:) 

(:;./ 

SILVERTON 

CAULDRON BLOCK 

3 

2 MILES 

107' 35' 

FIGURE 2.-The geochemical sampling sites in the generalized structural 

setting of the Silverton cauldron and vicinity. Intrusive bodies, stippled 

pattern; faults, heavy lines; sampling sites, x: 1, Mineral Creek; 2, 

National Belle; 3, Lark; 4, Longfellow; 5, St. Paul; 6, Brooklyn. 

in hydrothermally altered, faulted, and folded sedimentary rocks 

(Erickson and others, 1964, 1966; Wells and others, 1969). To 

our knowledge, however, the use of arsenic as an indicator element 

has not been applied to very restricted chimney-type deposits in 

such intensively acid-sulfate altered, leached, and broken volcanic 
4 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

rocks. Also, the arsenic here is associated principally with silver 

and base metals, not gold. 

The opportunity to carry out these field tests was provided by 

F. C. Canney of the U.S. Geological Survey. The Denver laboratories of the Geological Survey verified field tests and supplied 

other analytical data; E. L. Mosier did the spectrographic analyses 

and J. B. McHugh, Margaret Hinkle, and Doris Price did the other 

chemical analyses. P. A. Burbank and R. A. DeHon assisted in the 

field studies. This work, done in cooperation with the Colorado 

State Mining Industrial Development Board, supplements other 

concurrent geochemical studies being conducted in the area by the 

Geological Survey. 

GEOLOGIC SETTING 

Rocks of the Red Mountains area consist of bedded middle Tertiary volcanic lava flows, breccias, and tuffs of intermediate composition that are intruded by upper Tertiary igneous and clastic 

materials (Burbank and Luedke, 1964). The volcanic rocks are 

part of the Silverton Volcanic Group. This group, which locally is 

several thousand feet thick, overlies Precambrian metamorphic 

and igneous rocks and local Paleozoic sedimentary rocks. These 

volcanic rocks, part of the great San Juan volcanic plateau built 

by middle and late Tertiary volcanism (Larsen and Cross, 1956), 

are spatially related to a nested complex of cauldrons (Luedke 

and Burbank, 1968; Burbank and Luedke, 1969). The Silverton 

cauldron, particularly, has been intruded along its margins and, 

to a lesser extent, in its interior by late igneous stocks, plugs, and 

dikes. The western margin of the Silverton cauldron, where the 

sample sites are located (fig. 2), consists of a fault zone which is 

locally a mile or more wide and which was subjected to hydrothermal alteration and leaching during the postvolcanic episode. 

Parts, and perhaps all, of this fault zone were probably the site of 

a large solfataric field. The rocks at the present erosion surface 

are brilliantly stained yellow, orange, and red, largely as a result 

of the oxidation of pyrite. 

ROCK ALTERATION AND ASSOCIATED ORES 

AND GANGUES 

The nature of the rock alteration, including that resulting from 

hydrothermal activity as well as that from the continued effects 

of acidified surface waters, has an important bearing on the local 

problems of geochemical prospecting. Most of the rocks within and 
ROCK ALTERATION AND ASSOCIATED ORES AND GANGUES 5 

closely surrounding the cauldron area were subject initially to 

pervasive propylitic alteration caused by effusions of water and 

carbon dioxide. The resulting altered rocks range from those 

weakly carbonatized and chloritized to those albitized and epidotized. This type of alteration probably took place during or 

shortly after the late resurgence of magmas that emplaced the 

larger igneous stocks and their associated dikes (Burbank, 1960). 

More intense hydrothermal alteration was localized and coincided with intrusions of porphyritic quartz latite and rhyolite in 

volcanic pipes along the cauldron margin. This type of alteration, 

formerly called solfataric, is actually a surface phenomenon (Burbank, 1950) and is more correctly termed advanced argillic 

(Hemley and Jones, 1964). The highly altered rocks consist of 

quartz, various clay minerals, natroalunite, diaspore, and pyrite; 

most primary minerals were completely destroyed, and the rock 

bases were strongly leached. The association of dickite, pyrophyllite, and alunite is characteristic of acid-surface alteration, which 

generally is considered to be of hypogene origin (Hemley and 

others, 1969). 

The acid-sulfate alteration was superimposed upon the earlier 

propylitic alteration, thereby driving out carbon dioxide from any 

carbonates present and leaching and redistributing the rock bases. 

In extreme examples of leaching, solution channels contain cavities 

and in places have an enclosing envelope of silicified rock. These 

silicified casings contain some residual titania as rutile and some 

residual alumina as clay and usually grade downward and outward 

to argillized and chloritized rock. At those places where large 

volumes of rock have been altered by acid-sulfate solutions, most 

of the original iron has been converted to pyrite. 

The associated sulfide ores commonly fill the cavernous spaces 

in the upper leached parts of the channels or pipes and also 

replace the walls in part ; at depth, less preore leaching is evident, 

and replacement or filling of small fissures is more typical. The 

principal ore minerals are pyrite, enargite, bornite, sphalerite, 

galena, tennantite, and chalcopyrite. Additional minerals identified 

include stromeyerite, chalcocite (massive), arsenian bournonite, 

argentite, proustite, pyrargyrite, covellite, argentiferous cosalite, 

kobellite, guitermanite, and, rarely, free gold. Colusite, a tinbearing complex sulfosalt analogous to tennantite, has also been 

identified in the ores of two pipes. 

Gangues associated more or less closely with the ores include 

dickite, pyrophyllite, illite, zunyite, barite, and fluorite. Quartz is 

not common in massive sulfide ores of the pipes, except in the walls 
6 ARSENIC, WESTERN SANi JUAN MOUNTAINS, COLORADO 

and silicified envelopes or as a local inclusion. Late calcite or a 

maganiferous carbonate mineral occurs locally. 

The surficial oxidation of p;y-rite has acidified the ground waters, 

and the ground waters in turn have attacked minerals and leached 

metals. Mine waters carry cotiderable iron as well as copper and 

zinc locally. Veinlets of gyps m and clays in altered ground may 

be supergene. Native sulfur , as found at one locality in altered 

pyritized ground that was a$sociated with veinlets of unaltered 

pyrite or molybdenite. In plades it becomes difficult to distinguish 

between the recent effects of supergene acidified waters and the 

possible effects of the intermingling of original hypogene solutions 

and shallow meteoric waters. Although galena and other sulfides 

are found at the surface in protected spots, some cavities near the 

surface contain carbonate of; lead, cerussite, anglesite, limonite, 

scorodite, and silver minerals ~uch as cerargyrite and native silver. 

In general, the products of ox~dation were superficial and of little 

economic importance; however, at the Lark mine, sizeable bodies 

of carbonates of lead and zinc occur alongside sulfides and extend 

to considerable depths. Whether these bodies are of hypogene or 

supergene origin is problematical. No residuals of the original ore 

are apparent, and the absence of secondary minerals of other sulfides seems inconsistent with the usual assemblages of base-metal 

ores. 

SAMPLING AND ANALYTICAL METHODS 

Most samples collected were of hydrothermally altered country 

rock, but also included were so~e of the silicified rock in the pipes; 

a few samples were of the propylitically altered country rock. 

Chips of rock a few millimeters across were collected at each 

sample locality; each sample totaled about 100 grams. The sample 

was ground to a fine powder and was then analyzed in the field 

laboratory for arsenic, using the modified Gutzeit apparatus 

method described by Ward, Lakin, Canney, and others (1963, p. 

40-44). The samples were then forwarded to the Denver laboratories of the Geological Survey where they were analyzed chemically for arsenic, mercury, and silver and spectrographically for 

selected elements. Arsenic was determined by the same method 

used in the field. Mercury was determined both chemically by a 

catalytic method (Hinkle and others, 1966) and instrumentally by 

an atomic-absorption technique (Vaughn and McCarthy, 1964). 

Silver also was determined by a catalytic method (Nakagawa and 

Lakin, 1965). The spectrographic values were obtained by using 

a three-step semiquantitative procedure which was devised by 

Myers, Havens, and Dunton (1961). 
ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 7 

SAMPLE LOCALITIES 

Six localities were sampled in the Red Mountains area (fig. 2) 

within the drainages of Red Mountain Creek, Mineral Creek, and 

Cement Creek. Five of the localities selected were individual mineralized deposits; the sixth locality-locality 1-was not. It was 

selected to provide background information to compare with the 

information obtained from the mineralized localities. All localities 

had moderate to intense acid-sulfate-altered country rock. 

MINERAL CREEK SITE (LOC. I) 

To provide background data on altered rocks not directly ·adjacent to major ore deposits, a section along U.S. Highway 550 in 

the valley of Mineral Creek (fig. 2) was selected. This section, 

about 4 miles long, contains only a few minor veins and little other 

indication of significant metallic mineralization. Most of the rocks 

exposed along the southern part of the section (fig. 3) were 

strongly altered by acid-sulfate waters, in part of hypogene origin 

and in part of supergene origin from the oxidation of pyrite. The 

slopes east of the valley are colored by oxides of iron. Rocks along 

the northern part of the valley have been subject to propylitic and 

locally to acid-sulfate alteration. 

With few exceptions, arsenic and mercury did not much exceea 

their normal content in the volcanic rocks, assuming that from 

the least altered rocks (fig. 3; table 1) the background for arsenic 

TABLE 1.-Chemical analyses of rocks and slope wash along U.S. Highway bbU 

in Mineral Creek valley 

Field 

No. 

SM-1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

[Chemical and atomic-absorption analyses by M. E. Hinkle and J. B. McHugh, U.S. Geol. Survey. Sample localities site shown in fig. 2] 

Parts per million Parts per billion 

Description As Ag Hg 

Field Lab Lab Cata- Instru- lytic mental 

Altered shaly rock, much sheared ____ 0 30 1.5 60 40 

Pyri tized rock ------------------------------------ 5 20 1.2 100 40 

Partly silicified rock, iron-stained _ 5 30 .02 <30 60 

Limonite-cemented slope wash __________ 10 20 .1 <30 20 

Pyri tized rock -------------------------------------- 20 30 .1 75 <5 

Silicified rock ----------------------------- ______ 60 30 .1 40 60 

Bleached rock -------------------------------------- 0 10 .01 40 10 

Chloritized-kaolinized rock, gray ______ 10 10 .02 60 80 

Porous bleached rock, gray ---- 15 15 .2 30 <5 

Limonite-stained rock ------------------- 5 10 .1 30 10 

Bleached kaolinized rock ____________________ 0 15 .02 30 30 

Pyri tized -chlori tized rock ------------------ 5 <10 1.2 60 110 

Bleached-kaolinized rock 0 <10 .2 30 20 

Pyritized rhyolitic rock ---------------------- 0 20 .2 30 <5 

Do 0 <10 .02 30 40 

Pyritized rock -- ---------------------- 20 10 .8 60 30 
(/) 

I100 

ppm 

U- -o z...J Ww 

a:u.. "'-

<(-

~ 100 

:::2: ppm 

w 

....J a: w ~ 0 Cl c.. <( 

w 0 w I- u ...J u 

w 

....J 

w 

(/) 

u. 

0 

(/) 

1-

SOUTH NORTH 

- Mostly acid-sulfate altered rocks Mostly bleached and propylitically altered rocks ~ 

100 

ppb 

:J 

< ~ _., f ~ ·-H~~------ "- // 

' :::> w 

~ ~ p..s w a: 0 

~ 4 5 6 t;; 0 1 2 3 z 

·,,..q9 

' 

~ 

7 8 9 10 

1\ 

I \ 

I \ 

I \ / \ A \ 

11 12 13 14 15 16 

ppm · I \ I 

a: . : \ : !Ol ;(\\ ~ /\ w --> . /"' _., I . I > G . q'O.).; -- -- ' : •\ PI- : _J ,..../ .- ' • p - u 

;:;; Z_~/ . ~--!: ~--.. --.---~ 0 I I \ .. _ .. _ .. - .. -·~-. .· j--. j· I --

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

1500 5000 3.0 

2.0 

: t \ I Mq I I • I 

I I 

Sr 

z 1000 w ppm PERCENT 1- I 

z 

0 

u 

w ~ 1000 <I) 

w :::> ppm 

z ;:;; ,---"'----. 

<( W ~ I 

(.!) z :::> z~ 

z (.!) a: 0 :::> <( <( 

~ ~ ;ii ~ f= 

0 

6 2 3 4 

,, 

I I 

I I 

I 

. \ I I I \ 

..... .· ...... .,. _____ _ 

. · I I 

/ \ \ ' I I ·. ·, I \ 

. \~ ·) "··, .. ...._.,,cc·:·:·:;/~;? 

5 6 7 8 9 10 11 

SAMPLE NUMBERS 

I 

"'· 

12 

I 

I 

I 

I / . / I - _.,., )/ ·- /', / .. 

.·\ 

/ \ 

. ' / \ ',/ ·. 

13 14 15 16 

FIGURE 3.-Selected chemical data of a section along U.S. Highway 550 in Mineral Creek valley (fig. 2, loc. 1). 

00 

> 

~ 

r:n 

t?=j 

z 

~ 

p 

:E1 

t?=j 

r:n 

~ 

t?=j 

~ 

z 

r:n 

> z 

<:...; 

c:: 

> z 

~ 

0 

c:: 

z 

~ 

> z 

!Ji 

0 

0 

~ 

0 

~ t:;j 

0 
SAMPLE LOCALITIES 9 

is no more than 20 ppm (parts per million) and for mercury no 

more than 100 ppb (parts per billion). Mercury appears somewhat more sensitive to minor spotty reflections of silver and copper. On the basis of our sampling, the normal background for 

silver in this area appears to be about 1 ppm. The background 

for copper (table 2) ranges from 20 to 50 ppm ; the background 

for lead is about 20 ppm. 

In relatively fresh volcanic rocks of the area, the normal content 

of manganese determined spectrographically ranges from 1,000 

to 1,500 ppm; magnesium ranges from 1.0 to 1.4 percent. These 

elements (fig. 3) appear somewhat more strongly leached and 

transported in the southern part of the traverse, as might be expected from acid-sulfate action, and their higher values in the 

northern part of the traverse perhaps reflect a greater proportion 

of propylitized rocks not subject to leaching by acidified waters. 

In the propylitized rocks, manganese and magnesium are locally 

transported by carbon dioxide solutions. 

Strontium, barium, and other minor elements do not vary significantly. Spotty high values of strontium (for example, the sample at locality 5 in fig. 3) do not correlate with other significant 

elements except for relatively high barium. Further data for these 

samples are given in table 2. The relatively small number of samples collected along U.S. Highway 550 in Mineral Creek valley are 

questionably representative of the altered rocks in the area, but 

comparison of analytical results of these and other samples collected in the area (Fischer and others, 1968; F. S. Fisher, written 

commun. 1971) indicate that the data from the samples taken 

along the highway provide reasonable background information; 

more elaborate analysis does not appear justified. 

NATIONAL BELLE MINE SITE (LOC. 2) 

The National Belle mine was selected as a site for sampling because of its exposure as a prominent knoll that contains a central 

core of silicified and leached rock surrounded by weakly propylitized rocks (figs. 2, 4). Because of strong surficial leaching of the 

exposed parts of this ore deposit, this site should indicate whether 

tests for the presence of any residual indicators, such as arsenic, 

are tenable. The ore-bearing pipelike deposit, or "chimney" as 

locally known, is about 400 by 300 feet in diameter (Burbank, 

1941, p. 185-187). It consists of an outer envelope of strongly 

silicified and locally cavernous rock enclosing a core of partly 

brecciated material replaced by quartz, clay minerals, natroalunite, 

barite, and sulfides. The lead-zinc-silver ore bodies occupied cav-
1-l 

0 

> 

~ 

r:n 

M 

z 

~ 

TABLE 2.-Spectrographic analyses of rocks and slope wash along U.S. Highway 550 in Mineral Creek valley ~0. 

(Spectrographic analyses by E. L. Mosier, U.S. Geol. Survey. n.d., not detected. Ag, As, Cd, Nb, Sb, Sn, W, and Zn were not detected spectrographically; ~ Be < 1 except SM-13 which is 5; Bi < 20 except SM-3 which is 70. Sample localities site shown in fig. 2] M 

r:n 

Field Percent Parts per million ~ 

M No. Fe Mg Ca Ti Mn B Ba Co Cr Cu Ga La Mo Ni Pb Sc Sr v y Zr ~ 

z 

SM-1 5.0 1.0 0.1 10,000 100 10 1,000 10 20 20 30 30 5 5 20 30 200 300 10 200 r:n 2 10 2 5 2,000 1,500 n.d. 200 15 15 50 20 30 n.d. 7 20 10 300 100 10 50 > 3 1 .7 .1 7,000 20 10 1,500 n.d. 7 100 15 70 1 n.d. 50 15 1,500 150 10 150 z 

4 10 .3 .07 5,000 50 10 700 n.d. 10 20 20 30 7 n.d. 30 15 300 100 15 200 ~ 

5 5 .07 .5 5,000 2 10 1,500 n.d. 15 30 15 70 1 1 50 15 5,000 150 10 200 c::: 

6 7 .07 .05 5,000 <2 10 500 15 10 100 10 50 2 7 10 20 200 200 10 300 > 

7 2 .1 .1 10,000 2 10 700 n.d. 15 5 20 70 n.d. n.d. 15 20 700 300 7 150 z 

8 1.5 .01 .1 >10,000 2 10 300 n.d. 15 15 n.d. n.d. n.d. n.d. 30 5 300 50 n.d. 200 ~ 9 .05 .07 .07 10,000 2 10 200 n.d. 20 10 10 n.d. n.d. n.d. n.d. 10 300 200 n.d. 100 0 

10 3 1.5 .1 2,000 150 n.d. 500 n.d. n.d. 2 15 50 1 n.d. 20 7 500 50 10 100 c::: 

11 .1 .1 .07 5,000 <2 10 300 n.d. 5 2 20 n.d. n.d. n.d. 10 10 200 150 5 300 z 

~ 12 15 3 7 5,000 700 n.d. 500 5 5 30 20 50 1 2 n.d. 20 1,000 200 30 200 > 13 .7 1 .05 1,500 150 10 100 n.d. n.d. 5 20 n.d. 2 n.d. n.d. n.d. n.d. 10 n.d. 50 ~ 

14 2 1 .1 3,000 150 10 1,000 n.d. 2 3 20 70 n.d. n.d. 30 10 n.d. 20 20 300 z 

15 2 .5 .05 3,000 70 10 1,500 n.d. n.d. 5 20 50 2 n.d. 20 10 n.d. 20 30 300 ~r:n 

16 5 1 .15 5,000 100 10 500 10 5 30 20 50 3 5 20 15 n.d. 150 20 200 0. 

0 

t-t 

0 

~ 

> t:; 

0 
SAMPLE LOCALITIES 11 

0 500 1000 FEET 

CONTOUR INTERVAL 25and 100 FEET 

FIGURE 4.-Sample localities at the National Belle mine site. Base from 

U.S. Geological Survey advance map of Ironton and vicinity, 1927-29 

and 1980. 

erns or fissures that generally terminated in the outer siliceous 

envelope. Mining and erosion have removed all of any original ore 

from the few near-surface cavernous bodies examined. There are, 

however, small isolated remnants of galena, sphalerite, enargite, 

and pyrite in the massively silicified rock or in the clayey gouges. 

Fragments of ore in the dumps also contain tennantite, chalcopyrite, and colusite. Other silver-bearing sulfosalts common to the 

Red Mountains' pipes also possibly were constituents of the ore. 

The grade of ore from the National Belle mine (Ransome, 1901, 

p. 231-236) ranged from 10 to 40 percent copper and from 5 to 

25 ounces of silver per ton. Lead ranged from 15 to 60 percent in 
12 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

some ores. The gold content was reported at less than 0.1 ounce 

per ton. The better grade of ore appears to have bottomed at about 

the 400-foot level of the mine. Total production was at least several 

hundred thousand dollars prior to the early 1900's, but this mine 

was not one of the larger producers of the Red Mountain mining 

district. 

Examination of some of the caverns above the water level in the 

National Belle mine showed extensive leaching of sulfides. Ransome (1901, p. 233) reported that the oxidized residues consisted 

originally of carbonate of lead, iron oxides, lead sulfate, and arsenates. Identity of the arsenates is uncertain, although scorodite 

has been recognized in some local mines. Material collected by 

W. S. Burbank from the walls of a cave in the silicified rock was 

identified by Charles Milton of the U.S. Geological Survey as pitticite, a hydrated sulfate and arsenate of ferric iron. These occurrences of arsenic minerals in strongly leached ground in part led 

to the belief that arsenic might be a useful indicator element for 

geochemical prospecting. 

Sampling locations and results at the National Belle site are 

shown in figures 4 and 5 and tables 3 and 4. 

The arsenic content is within background limits (20 ppm) 

within short distances outward from the pipe and its peripheral 

altered zone, but its value rises gradually in the bleached and argillized rock. The high arsenic values of samples at localities 9 and 

10 are related to mineralized and silicified fissures in the pipe. At 

sample locality 8 on the top of the knoll, where leaching appears 

most complete, arsenic values attain four times the background 

value in the bleached and silicified rock; the adjoining soil sample 

(NB--8S) has a lower arsenic content. Mercury shows a similar 

variation between bedrock and soil, although the values are of a 

much higher order of magnitude than the background value. Both 

arsenic and mercury have their higher values in samples NB-5 

to 12, which closely define the limits of the pipe. Within these same 

limits, bismuth and tin, both relatively minor constituents of the 

ore, are detectible spectrographically (fig. 5; table 4). Only sample 

NB-10 was above the detection limit (200 ppm) for antimony, 

with a value of 500 ppm. 

Of the common base metals, copper is erratic because of migration during leaching or of inhibition to leaching by the type and 

density of alteration products. Lead is well above the background 

(20 ppm) across the mineralized core, and lead, silver, and bismuth 

together attain maxima at locality 9. This locality is also the only 

place where zinc is detectable (200 ppm). Locality 9 is on a strong 
SAMPLE LOCALITIES 13 

PROPYLITIZED ARGILLIZED SILICIFIED CORE ARGILLIZED PROPYLITIZED 

SAMPLE LOCALITIES i (PROJECTED TO PROFILE) 

40.000 200 ppb ppm 

.:J >- <{ a: 1- 0 z 1- >- w <{ 

a: :2 a: :::>=> 0 ua:: co a: 1- <{ 

wC/l ::! :2 ~ u 6 z w w ::r: (/) 

CJ 

w ~0 

~ 1 

400 200 8 ppm ppm ppm 

2 

3 4 

I 

I 

3 4 5 6 7 8 

\ 

\ 

300 

t 

As 

\ 

\I 

\! 

9 12 11 10 

Ag t16 1 

!: / \ 

i 

I <iJ' 

/ .. ·· \ 

SE 

13 14 

Ag Pb / \ ':~;~~------~ 0~~-~-=-~·=-=-=-=-=-~-·p-~-f/---,-,--,---,----.---.-.-r 

2 

4,000 20 ppm 

w 

(/) 

w 

z 

<{ 

CJ z z 0 <{ 

:2 ~ 

0 

1 2 

3 4 5 6 7 8 

>5,000 

Mn 

~ ~ 

"0 

c: 

"' 

~ '0 

~ 

3 4 5 6 7 8 

9 

I 

I 

I 

I 

I 

121110 13 14 

"0 

"' ·;; 

·~ 

a.. 

,, 

;I 

I I 

I \ 

I 

I 

\ /•"" 

\/ 

9 12 1110 13 14 

SAMPLE LOCALITIES 

FIGURE 5.--Diagrammatic profile and graphs showing selected chemical data 

across the National Belle mine outcrop. Sample localities shown in figure 4. 

northeast-trending fissure; underground in this fissure is a vein 

which extends along the east side of the pipe and which contains 

unoxidized sulfides above the water level. 

Manganese and iron, although largely leached within pipe limits, 

show erratic maxima, owing partly to fixation of iron as pyrite 
14 SAMPLE LOCALITIES 

TABLE 3.-Chemical analyses of rocks and soil at the National Belle mine site 

[Chemical and atomic-absorption analyses by M. E. Hinkle and J. B. McHugh, 

U.S. Geol. Survey. Sample localities shown in fig. 4] 

Parts per million Parts per billion 

Field 

No. 

As Hg 

Description Ag Instru- Instru- Cata- mental mental 

Field Lab lytic (scooped) (weighed) 

NB-1 Chloritized rock _________ __________________ 0 

1S Soil ________ ------------------------------------- 20 

2 Chloritized rock _____ _____________________ 0 

3 Green argillized rock __________________ 10 

4 Red argillized rock ______________________ 0 

5 Leached-argillized rock ___ 60 

6 Clay-silica cavity filling 200 

7 Silicified-argillized rock _ __ _______ 0 

8 Siliceous rock _____________________ 80 

8S Soil --------------------------------- ______________ 20 

9 Sinterous cavity filling with sulfides _____ __________________ _ __ 320 

10 Silicified rock __ _____________________________ 200 

11 Argillized-silicified rock ____________ 70 

12 Silicified breccia __________________________ 180 

13 Oxidized-chloritized rock ____________ 0 

14 Chloritized rock _____________ ____________ 0 

15 Chloritized-argillized rock _____ ____ 20 

16 Bleached argillized rock _ 10 

17 Do __________________________________ <10 

18 Oxidized-argillized rock ____________ 40 

19 Argillized loamy rock __ 30 

20 Oxidized-argillized bleached 

rock _____________ ______________________________ 320 

21 Do __ _ ___________ _______ 30 

10 0.01 75 30 40 

20 .4 <30 40 40 

10 .2 <30 <5 20 

30 .2 <30 20 10 

30 .02 <30 10 40 

40 1.5 1,200 2,500 2,300 

30 1.4 60 250 240 

20 1.5 500 250 325 

80 8.0 8,000 25,000 27,000 

30 1.5 400 250 350 

300 16.0 

150 1.5 

60 3.0 

60 8.0 

<10 1.2 

30 .1 

30 1.5 

40 .2 

30 .1 

30 1.5 

40 .02 

80 5.0 

30 1.5 

1,000 8,000 7,000 

1,500 15,000 11,000 

1,500 9,000 6,000 

3,000 30,000 44,000 

150 150 350 

150 110 90 

100 150 325 

200 250 450 

300 750 1,400 

400 500 725 

300 120 170 

400 

150 

650 

180 

650 

275 

and partly to some migration of manganese and iron.' Bari urn and 

strontium are locally high over the pipe core, reflecting the common 

occurrence of barite -in ores. Of the rarer elements, vanadium, 

nickel, and cobalt are definitely leached along with other base 

elements. 

In general, arsenic, mercury, lead, silver, tin, and bismuth all 

reflect the core and indicate the strongest metallization. As anticipated, arsenic, which is mainly associated with the copper minerals enargite and tennantite, is inconsistent with copper showings 

except at locality 10 where leaching was inhibited. Aresnic is more 

consistent with lead, bismuth, and tin than is mercury; the reasons 

for this are unknown. 

A few samples along an altered streak extending southwest of 

the National Belle pipe (samples NB-15 to 21, fig. 4; table 4) give 

erratic and spotty indications of silver, mercury, and lead, but 

arsenic is only slightly above the background value. However, 

sample NB-20, from a prospect cut at a small iron-stained and 
TABLE 4.-Spectrographic analyses of rocks and soil at the National Belle mine site 

[Spectrographic analyses by E. L. Mosier, U.S. Geol. Survey. n.d., not detected. Sample localities shown in fig. 4] 

Field Percent Parts per million 

No. Fe Mg Ca Ti Mn Ag B Ba Bi Co Cr Cu Ga La Ni Pb Sc Sn Sr v y Zr 

NB-1 15.0 1.5 7.0 10,000 1,500 1.0 50 1,000 n.d. 7 15 30 20 100 10 n.d. 30 n.d. 300 300 30 300 

18 10 1 .7 10,000 2,000 n.d. 50 500 n.d. 15 15 30 20 50 10 20 10 n.d. 150 150 15 300 

2 15 1 7 5,000 1,500 n.d. 50 700 n.d. 10 15 50 20 100 10 n.d. 20 n.d. 200 200 30 300 Ul 

3 15 . 7 .3 3,000 > 5,000 n.d. 70 300 n.d . 15 10 150 20 30 15 10 15 n.d. n.d. 100 30 200 > 

4 10 .7 7 5,000 1,500 n.d. 50 300 n.d. 10 15 100 20 70 15 n.d. 20 n.d. 150 600 30 200 ~ 

5 .7 .015 . 05 3,000 5 3 n.d. >5,000 50 n.d. 10 20 20 30 n.d . 200 5 n.d. 1,500 70 n.d. 30 '"C 

t"'4 6 .2 .02 .07 >10,000 20 3 10 70 n.d. n.d. 7 10 n.d. n.d. n.d. 30 10 50 150 30 n.d. 1,000 l:.%j 

7 5 . 015 .05 10,000 5 1.5 n.d. 150 n.d. n.d . 7 20 10 50 n.d. 50 10 n.d. 1,000 100 10 200 t"'4 8 1.5 .015 .05 3,000 10 15 10 >5,000 50 n.d. 5 30 n.d. n.d. n.d. 100 5 15 500 10 n.d. >1,000 0 

88 5 .7 .7 5,000 200 1.5 50 700 n.d. 5 15 20 10 30 10 20 10 n.d. 200 70 20 700 l..l 

9 2 .02 .07 >10,000 5 30 10 1,000 100 n.d. 10 50 15 n.d. n.d. > 500 15 70 1,500 70 n.d. 300 > t"'4 10 7 . 01 .05 2,000 5 20 n.d. 500 30 10 7 200 70 50 10 >500 5 10 1,500 70 n.d . 30 1-( 

11 5 .015 .1 3,000 5 7 n.d. >5,000 20 n.d. 3 30 n.d. 50 n.d. 200 5 n.d. > 5,000 50 n.d. 50 ~ 12 15 . 01 .07 >10,000 10 7 n.d. 700 70 10 3 50 n.d. 30 15 150 10 n.d . 1,500 30 7 500 l:.%j 

13 15 1 1 5,000 1,000 n.d. 20 500 n.d. 10 7 30 15 n.d. 15 n.d. 15 n.d. 150 200 15 100 Ul 

14 10 1 10 2,000 1,500 n.d. 20 500 n.d. 15 10 20 20 30 15 n.d. 15 n.d. 200 150 15 100 

15 5 .01 .15 10,000 15 1 n.d. 200 n.d. n.d. 7 15 10 50 n.d. 50 15 n.d. 700 100 20 300 

16 1 .5 .15 5,000 100 n.d. 50 700 n.d. n.d. 10 5 20 50 n.d. n.d. 15 n.d. 500 300 20 150 

17 7 . 015 .03 3,000 70 .5 n.d. 700 n.d. n.d. 7 30 10 30 n.d. 20 10 n.d . 150 100 5 200 

18 5 .01 .02 5,000 10 .5 n.d. 500 n.d. n.d. 7 20 10 30 n.d. 10 15 n.d. n.d. 100 10 200 

19 1 . 5 .1 3,000 150 n.d. 70 200 n.d. n.d. 15 22 20 70 n.d. 30 15 n.d . 150 300 30 200 

20 2 .015 .1 5,000 7 10 n.d. >5,000 30 n.d. 10 20 10 70 n.d. 300 10 10 2,000 50 5 200 

21 . 07 .01 .05 3,000 2 5 n.d . 700 n.d. n.d. 10 1 15. 50 n.d. 200 5 n.d. 1,000 30 n.d. 70 

"""" 01 
16 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

silicified knob, gave higher values of arsenic, silver, lead, and 

mercury than the samples from the altered streak. 

LARK MINE SITE (LOC. 3) 

The Lark mine site is of particular interest because it is within 

highly altered rocks, and because the ore body is not marked at 

the present erosion surface by silicified rocks (Varnes and Burbank, 1945). Surface indications of the mineralized pipe consist 

of bleached and red- and brown-stained country rock containing 

some small cavities, probably spaces once occupied by sulfides, and 

some clayey streaks of oxidation products of lead and silver 

minerals. 

The production figures for the Lark mine are unknown, as the 

output was included in the district production figures during World 

37°54' 

1000 2000 FEET 

RED MOUNTAIN 

/ 

I 

/ 

/ 

N0.3~- ':J0o1 

( 12.890 -.........., ___ ~::_,; 

I 

J 

/ 

/ 

/ 

/ 

/ 

/ 

15 

-···~ ... 

FIGURE 6.-Sample localities at the Lark mine site. Intrusive body, v-pattern; pipe, stippled pattern; silicified zone, x-pattern; vein, dash-dot line; 

o, sample locality. 
SAMPLE LOCALITIES 17 

War II. The mine is believed to have produced a substantial 

amount, although this amount was undoubtedly less in dollar value 

than that from several of the larger Red Mountains area mines 

that contained silver-rich ores. 

The lower tunnel of the Lark mine, at an altitude of about 

11,350 feet, is on the southeast slope of Red Mountain No. 3 ridge 

in upper Prospect Gulch, a tributary of Cement Creek (figs. 2, 6). 

A small pit, said to be the discovery cut, is about 675 feet above 

the lower tunnel. A tunnel about 200 feet below the discovery pit 

penetrated an ore body 30-40 feet in diameter, consisting of galena, sphalerite, and enargite with alteration products of the 

country rock ; farther in beyond the main sulfide body, the tunnel 

penetrated another ore body, of unknown size, consisting of lead 

and zinc carbonate and sulfate minerals. At the pit representing 

the presumed outcrop of this body,, a 10-foot channel sample taken 

by A. V. Heyl of the U.S. Geological Survey (written commun. 

1954) gave a content per ton of 0.03 ounce gold, 1.2 ounces silver, 

14.4 percent lead, 2.4 percent zinc, and 66.2 percent silica; the 

sample also contained 17.7 percent carbonate and 2.3 percent sulfate. Some carbonate high in iron content was also present. 

Most shallow sulfide ores of the Red Mountains area show only 

surficial conversion of sulfides to carbonates or sulfates. In some 

pipes, calcite or a manganiferous carbonate mineral are minor late 

products, but carbonate minerals are not abundant. The ore body 

of the Lark mine may be an exception. The primary or secondary 

origin of this carbonate ore body is obscure. The possibility of a 

late hypogene effusion of carbon dioxide in converting sulfides to 

carbonates is conceivable but not established. 

Two lines of samples were taken at the Lark mine site. The lower 

line, samples SL-1 to 14 (fig. 7; tables 5, 6), extended northeast 

through the discovery pit and thence north to a prominent point of 

the ridge heavily stained with iron oxides. The upper line, 400-

500 feet higher on the slope (samples SL-15 to 21), extended 

northeast to the same ridge point. As shown in profile in figure 7 

(see also tables 5, 6), the line through the discovery pit shows 

arsenic and lead values throughout, generally above the background values (20 ppm) obtained in the Mineral Creek section as 

well as above the background values obtained in propylitized rock 

near the National Belle site. Samples 10 and lOG from the discovery 

pit give arsenic values greater than those values obtained above 

the National Belle pipe. Associated with the high lead and silver 

contents in these samples were exceptionally high antimony values, 

probably indicative of antimonial sulfosalts such as tetrahedrite, 
18 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

12 400 -- -- -SOLFATARICALLY ALTERED---

f- ...J ' l >- ~ >- AND MOSTLY PYRITIZED ~ ALTERED AND 

tlj ~ ...J 0 S c.-~ PYRITIZED 

U..wl2,200 ~0 ~ e~: . Z...J S!w w c.:;;;; :-;:;5 f-a: > VOLCANIC FLOWS ;f o ...J 

§ ~ 12,000 ~~ ~ AND BRECCIAS ,.--A---, 

f-> :3<~: ~ 5~ 11,800 a: ~ 

<{ <{ i 0.. r-"----, _.,.,---

11,600 --r----.- 1 ---,--,--1 ~:-..-; -~~-.----,c----.--.-----.---.--~-~~, LOCALITIES 1 2 4 5 6 7 S gF 10 lOG 1

1

1 1

1

2 1

1

3 1

1

4 20F 

(PROJECTED TO PROF! LE) 600 1600 1200 1600 

500 2,500 ppm ppb 

> 

a: 

0 

f-

<{ 

a: 

0 

til 

<{ 

::::; 

<{ 

fz 

LU 

:2: 

::J 

a: 

fC/l 

z 

f •, t I f \/ t As 1

, As I As ~ As 

L 

I 

I 

L 

I 

I 

I 

I 

\ 

I 

I 

As 

:::::! 

S! 

z 

w 

C/l 

a: 

<{ 

>-

a: 

::J 

u 

a: 

w 

:2: 

'?"_/ \ 

__ fi_g \ 4s /'' /!_g,,/ 

0 T--.-~r=~~=r-.--~~r=~--.-~-~\~--=-=-~~-~-~~ 

200 

ppm 

r---"----, 

:r 

100 

ppm 

a: f- a: w 0 ::J w 

~ ;:;5 ~ ~ :; u ...J til f- iii 

2 3 4 5 6 7 8 9F 10 lOG 11 12 13 14 20F 

I 

I 

I 

I 

I >5000 >5000 

,' + "' 1 Pb Pb I 

300 

... 

Ag 

··-~ 

I 

I 

I I 

I Pb I \- __ .J 

>5000 

+ 

Pb 

500 5000 

t + Pb Pb 

500 

Jn 

>If 

Bi 

200 

.f. 

Ag 

0 T--~~,--.--r-.--T--.-~--.----~~~~~~--, 

2 3 4 5 6 7 8 9F 10 lOG 11 12 13 14 20F 

>5000 

20 2.0 1000 Mrl. PERCENT ppm t i~ 

~~\" ',,, 

h ~ \ ',,, 

\ 

z \ 

0 

!E 

w 

C/l 

w 

z 

<{ 

l? 

z 

<{ 

:2: o,_-,--.--.·'_··~~\~/~r=~-==¥~.-~~-~--~~~~-M~n-~~==~ 1 2 3 4 5 6 7 8 9F 10 lOG 11 12 13 14 20F 

SAMPLE LOCALITIES 

FIGURE 7.-Diagrammatic profile and graphs showing selected chemical 

data along the lower traverse (fig. 6) across the Lark mine site. 

in the upper part of the ore body. Sample SL-10 also is relatively 

high in tin and bismuth values. The values of mercury are much 

lower relative to arsenic than those found at the National Belle 

mine. 
SAMPLE LOCALITIES 19 

TABLE 5.-Chemical analyses of rocks and gouge at the Lark mine site 

[Chemical and atomic-absorption analyses by M. E. Hinkle and J. B. McHugh, 

U.S. Geol. Survey. Sample localities sites shown in fig. 6] 

Parts per million Parts per billion 

Field Description As Hg 

No. Field Lab Catalytic Instrumental 

SL--1 Limonite-stained rind on altered flow rock ____ 30 40 300 140 

2 Limonite-stained 

intrusive rock 60 120 <30 70 

3 Bleached intrusive rock 5 40 <30 50 

4 Bleached shaly rock 30 60 <30 50 

5 Bleached shaly and 

sheared rock 60 80 <30 40 

6 Sheared and bleached rock 20 40 100 325 

7 Bleached rock <5 30 75 40 

8 Limonitic rind on altered rock 70 120 30 40 

9F Oxidized and bleached 

rock (local float) ______ 30 120 <30 40 

10 Bleached porous rock 

from pit_ >320 600 3,000 4,800 

lOG Yellow-stained gouge from pit __ >320 1,600 600 1,000 

11 Iron-stained bleached rock_ 20 60 <30 40 

12 Altered rock from 

shear zone 30 40 <30 30 

13 Yell ow limonitic 

much altered rock 200 1,200 400 280 

14 Brown limonite-coated 

much altered rock 65 400 50 110 

15 Oxidized and 

bleached rock _ 30 120 30 50 

16 Silicified iron-stained 

rock ----------------- 60 80 <30 40 

17F Limonitic iron-stained 

rock (local float)_ 30 120 100 150 

18 Oxidized and bleached rock 80 400 75 120 

19 Do ------------------------- 160 600 <30 40 

20F Limonite-stained altered 

rock (local float) 200+ 1,600 150 375 

21 Silicified and limonitestained altered rock ___ 30 150 <30 40 

In the upper line of samples (fig. 8), samples SL-15, 16, and 

17F represent altered rock without appreciable metals; however, 

lead values gradually increase to samples SL-18 and 19 which 

were taken from outcrops of a small vein. This vein heads toward 

the high ridge point at localities 20F and 21. Sample SL-20F, 

shown at the termination of both profiles (figs. 7, 8), represents 

local debris from an outcrop of red and brown iron-stained silicified rock near the crest of the ridge point. This sample is high in 

silver, lead, tin, bismuth, and antimony, but low in mercury as 

compared with arsenic. The significance of the relatively low mer-
~ 

0 

> 

~ 

UJ. 

trj 

z 

~ 

TABLE 6.-Spectrographic analyses of rocks and gouge at the Lark mine site ~0. 

[Spectrographic analyses by E. L. Moiser, U.S. Geol. Survey. n.d., not detected. Sample localities sites shown in fig. 6] ~ trj 

Field Percent Parts per million UJ. 

~ No. Fe Mg Ca Ti Mn Ag As B Ba Bi Cr Cu Ga La Mo Ni Ph Sb Sc Sn Sr v y Zr trj 

~ 

SL-1 10.0 1.5 0.15 >10,000 > 5,000 2 n.d. 100 1,000 n.d. 20 50 30 50 2 n.d. 50 n.d. 20 n.d. 100 300· 30 500 z 2 5 1 .05 10,000 150 3 n.d. 70 500 n.d. 50 20 20 70 10 n.d. 70 n.d. 10 n.d. n.d. 100 20 200 UJ. 3 7 . 7 .3 10,000 150 <1 n.d. 70 500 n.d. 20 20 20 100 2 n.d. 70 n.d. 10 n.d . 1,000 100 30 500 4 7 .7 .1 >10,000 150 <1 n.d. 50 1,000 n.d. 30 30 20 70 2 n.d. 100 n.d. 20 n.d. 200 200 30 300 > 5 20 . 07 .15 3,000 10 <1 n.d . 20 500 50 7 5 20 n.d. 3 n.d. 70 n.d. 10 10 300 100 10 150 z 6 10 . 3 • 3 IO,OOO 20 <I n.d • 20 700 n.d. 15 20 20 50 2 n.d. 30 n.d . 30 n.d. 2.00 200 30 300 ~ 

7 .2 • 05 .2 10,000 5 <I n.d. n.d. 500 n.d. I5 3 2,0 50 n.d. n.d . 70 n.d. 20 n.d. 700 200 20 500 ~ 8 I5 .05 .I IO,OOO 10 <1 n.d. 20 300 n.d. 10 30 15 50 IO n.d. 50 n.d. I5 n.d. >5,000 200 10 300 9F 2 .2 . 5 10,000 I 50 <1 n.d. 20 200 n.d. I5 > 5 I5 70 n.d. n.d. 200 n.d. I5 n.d . 500 200 20 200 IO 1 .02 .5 > IO,OOO 5 300 n.d. n.d. 300 30 20 50 70 150 n.d. n.d. >5,000 7,000 15 70 >5,000 200 7 500 z lOG IO .01 • 07 7,000 n.d. 20 3,000 n.d. 200 n.d. IO 50 100 50 5 n.d . > 5,000 I,OOO 5 15 500 I50 n.d. 300 11 2 • 3 .2 5,000 100 <1 5,000 20 I,500 n.d. 7 15 20 70 2 <1 100 n.d. 15 n.d . I,OOO I 50 IO 200 ~ 

I2 10 .2 .2 5,000 30 1 n.d. 50 700 n.d. 7 50 20 30 I <1 100 n.d. 20 n.d. 500 I 50 I5 200 0 13 15 .05 .07 5,000 5 IO n.d. n.d. 500 50 I5 20 50 30 IO n.d. 5,000 200 5 15 500 70 n.d. 100 ~ I4 2.0 .OI .I 2,000 IO 5 2,000 n.d. 200 n.d. IO 150 15 n.d. 3 <1 500 n.d. 5 10 300 I 50 5 I 50 z I5 10 1.5 .2 IO,OOO 500 <I n.d. 30 1,000 n.d. 20 50 20 I50 2 5 30 n.d. I5 n.d. IOO I 50 20 300 ~ I6 IO • 07 .I IO,OOO 50 <I n.d • 20 300 30 I5 50 I5 50 <I n.d. 150 n.d. I5 n.d. 700 200 20 300 > I7F IO . OI .I 5,000 IOO 3 n.d. IO 200 30 IO 70 15 70 I n.d. 300 n.d . I5 n.d. I,500 200 IO 500 ~ 

I8 I .01 • 05 >IO,OOO n.d. 5 n.d. n.d. 1,000 30 30 IO > IOO 30 1 n.d . '> 5,000 n.d. 10 20 2,000 70 n.d. 200 z 19 I5 .02 .I5 IO,OOO 10 7 n.d. 20 500 50 15 50 20 70 3 n.d. 3,000 n.d. IO n.d. I,OOO 200 IO 300 UJ. 20F 20 • 01 .I5 5,000 5 2.00 7,000 n.d. 700 >100 30 IOO 100 50 5 n.d . 5,000 3,000 IO 500 I,500 300 5 300 ~ 

21 IO . 5 .2 10,000 20 2 n.d. 30 700 50 IO 20 20 50 I5 n.d • I 50 n.d. 20 n.d. 700 200 20 300 0. 

0 

~ 

0 

~ 

> tj 

0 
SAMPLE LOCALITIES 

~~-~ 12,500 --ALTERED ROCK--

·ow 

~CD~ ::::l~-1 

1-1-~ 

i=ww 

-lWCI) 

~u.. 

12,000 

LOCALITIES 15 16 17 F 18 19 20F 21 

(PROJECTED 

TO PROFILE) 600 1,600 

500 2,500 t + ppm ppb As As 

~ 

_J 

~ 

): 1-

z a: w 0 2 (NOTE: High 1- ::::l ~ a: Sb at 20F, a: 1- 0 (/) Table 6) CD ~ ~ 

= >- ~ a: 

::::l z () w a: ,•, (/) w , a: ' 

2 / / ' ~ ' 

0 ---./ '• 

15 16 17 F 18 19 20F 21 

300 >5,000 3,0001 5,000; I 

200 100 + + + , + i I ppm !Jpm Pb Pb Pb j Pb : I~ 

:Ag 200 I \ isn 500 : L 

: + I I ·"P • <0 

I .~ .. \ ...... 

\. i i ..... ,/ ::c a: 

wo::::l 1- a: w .....~·- lt~2 2 > I 

ow~- _J 

()-I COl- Vi 

0 

15 16 17 F 18 19 20F 21 

20 2.0 1,000 PERCENT ppm 

2 w 

(/) 

::::l w 

Vi z w ~ 

z z (!) ,• (!) z 0 ~ ~ I 

I 

~ 2 2 /,~~. 0 

15 16 17F 18 19 20F 21 

SAMPLE LOCALITIES 

FIGURE B.-Diagrammatic profile and graphs 

showing selected chemical data along the 

upper traverse (fig. 6) across the Lark 

mine site. 

21 
22 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

cury showings at the Lark mine site is problematical; we speculate 

that it might relate to a little known but relatively weak late-stage 

gold enrichment. 

In general, the results of sampling at the Lark mine site appear 

to reflect the known pipe and possibly an unknown pipe. Judging 

by comparison of the Lark samples with the samples near other ore 

bodies, it would be worthwhile to take closer grid samples in the 

area of the ridge point to confirm the metal values and to check 

the size of the area of anomalies. 

LONGFELLOW AND ST. PAUL MINE SITES (LOCS. 4 AND 5) 

A few samples were taken from two small mines south of the 

National Belle mine (fig. 2). These two sites may be considered 

typical of chimney ore bodies that occur around the peripheries of 

pipes occupied by intrusive rocks. The results were not enlightening and indicate that more elaborate sampling would be required 

for significant results. 

The Longfellow mine (fig. 9) was started in the late 1940's or 

early 1950's and was worked intermittently for several years. Mine 

development consists of a shaft and several short tunnels on the 

150-foot level. Enargite, galena, and sphalerite in clays occur as 

irregular bunches in two ore bodies. 

Two samples of altered volcanic country rock that were taken 

from a shallow pit south of the Longfellow shaft and near the 

edge of the silicic intrusive body (fig. 9) indicated only moderate 

arsenic, lead, silver, and copper were above background values; 

strontium was fairly high. 

The St. Paul is one of the older mines of the Red Mountains mining district but was never a major producer. According to Ransome (1901, p. 238) the main vertical shaft, which was about 200 

feet deep, had drifts at the 200-foot level. The ore, chiefly enargite 

and galena in irregular bunches, was reported to be low grade and 

was found by following seams of gouge clay. The mine workings 

were opened and prospected for gold-bearing ore in more recent 

years. 

The mineralized fissure at the St. Paul mine, similar to those 

described previously near the National Belle mine and elsewhere, 

has a strike of about N. 30° E. in volcanic flows and breccias. 

Sampling along this fissure gave low values in arsenic except for 

location southwest of the shaft which had 80 ppm arsenic, 500 

ppm lead, and 1 ppm silver (fig. 9; table 7). Enrichment in barium 

and strontium were noticeable in the walls locally; strontium was 

higher closest to the fissure. 
SAMPLE LOCALITIES 23 

I / 

/ 

I 

I 

/ 

'Longfellow shaft 

/ 9 (60, 10~, 20,-, 10, 150, 1000) 

• IO.~Pit 

10 ( 120, 500, 100, 7' 30, 150, 5000) 

0 25 50 FEET 

l_L_j 

EXPLANATION, 

D Intrusive body 

[!] 

Chimneys of silicified rock or ore 

Mineralized fissure 

•9 

Sample locality 

(As, Pb, Cu, Ag, Mn, Ba, Sr in parts per million) 

500 1000 FEET 

107"42' 

1 (<10, 30, 50,-, 200, 150,.....J).}·/ 

2 <<1o,-. 10.-. 30o. 2oo,-) I 3 (10, 10, 20,-, 50, 200,-l . 

I 

I 

I 

8 (<10, 50, 30,-, 200, 500,-) ·'St PaL!I shaft 

I 

I 

6 (<10, 10. 10.-. 30oo. >5ooo. 3om I 7 (10,-, 15,-; 700, 700,-) / 

• 7 6. ~~ . 

}1 YP!t 

4 (80, 500, 1:5, 1' 20, 200, 3000) 

5 (10, 300, 70, 1, 20, 300, 700C) 

50 100 FEET 

FIGURE 9.-Geologic sketch map of the Koehler compound volcanic pipe and 

sample localities at the Longfellow and St. Paul mine sites. From Bu~bank 

(1941, fig. 5). 

BROOKLYN MINE SITE (LOC. 6) 

The Brooklyn mine is north of Browns Gulch on the east wall of 

Mineral Creek valley (figs. 2,10). Its northeast-trending v~in is 

poorly exposed except for a few small cuts at about 11,400 feet 

altitude on a valley-wall bench which is locally mantled with glacial 
~ 

~ 

> 

~ 

r.n TABLE 7.-Spectrographic analyses of rocks at the Longfellow and St. Paul mine sites t::_j 

z [Spectrographic analyses by E. L. Mosier, U.S. Geol. Survey. n.d., not detected. Sample localities shown in fig. 9] ~ 

9 

Field Percent Parts per million :f3 No. Description t::_j Mg Ca Ti Mn Ag B Ba Co Cr Cu Ga La Mo Ni Ph Sc Sr v y Zn Zr r.n 

St. Paul mine t-3 

t::_j 

1 Red rind on ~ 

altered rock _______ 0.2-4 < 1 7,000 200 n.d. 100 150 n.d. 7 50 20 70 2 <1 30 15 n.d. 100 20 n.d. 500 z 

2 Gray rind on r.n 

> altered rock _______ .2-4 < 1 5,000 300 n.d. 50 200 n.d. 5 10 20 50 n.d. 1 n.d. 10 n.d. 70 20 n.d. 150 z 

3 Limonite-stained ~ 

bleached rock ___ .2-4 < 1 5,000 50 n.d. 50 200 n.d. 5 20 20 50 n.d. <1 10 10 n.d. 70 20 n.d. 150 c::: 

4 Oxidized rind on > 

altered rock _______ <·2 <1 5,000 20 1 n.d. 200 n.d. 15 15 10 50 n.d. <1 500 n.d. 3,000 70 10 n.d. 150 z 

5 Do _______________________ <.2 <1 5,000 20 , 1 n.d. 300 n.d. 20 70 n.d. 150 n.d. 5 300 n.d. 7,000 50 10 n.d. 150 a= 

6 Oxidized rind on 0 

chloritized rock .2-4 < 1 7,000 3,000 n.d. 50 >5,000 10 10 70 30 100 n.d. 3 10 15 300 150 30 n.d. 500 c::: 

7 Chloritized rock _ .2-4 < 1 5,000 700 n.d. 30 700 10 7 15 20 50 n.d. 3 n.d. 15 n.d. 100 20 n.d. 200 z 

8 Pyritized and t-3 

> altered rock _______ .2-4 <1 5,000 200 n.d. 50 500 10 5 30 20 50 n.d. 3 50 15 n.d. 100 20 n.d. 200 ~ 

z 

Longfellow mine ~r.n 

9 Oxidized rind on () 

altered rock _______ < .2 <1 5,000 10 n.d. n.d. 150 10 5 20 15 50 1 5 100 15 1,000 100 20 n.d. 150 0 

t"' 

10 Do _____________________ <.2 <1 5,000 30 7 20 150 n.d. 5 100 10 50 n.d. <1 500 n.d. 5,000 50 n.d. 700 100 0 

~ 

> tj 

0 
N 

I 

SAMPLE LOCALITIES 25 

107"43' 

0 1000 FEET 

L._j_J 

26 (20+, 50, 200, 500,-1 

27 (30, 10, 70,-, 21 / 

28(30,15,100,-,21. 

. '\/ 25 (20, 30, 200,-, 1) /' 

'\·/ 

/' 

24 (30+, 10, 150,-, 2~/ / 

/. /. 23 (120, 100, 

')., . >500, 700, 15) . 29 (120, 70, 500,-, 201 I. '·· Cavedshaft.•...._22 (120, 30, >500,100, 15) 

........_ I'· 

. ......__ I . 

,!-Approximate trace of Brooklyn vein 

I 19 (120, 20, >500,-, 3) i 

•• 20 (20, 2, 50,-,-1 

I ·-21 12o+. 5o. 5o, 2oo. 11 

I .......... . 

18 (30, 15, 50, 100, 1·)~. EXPLANATION I . SHAFT I 

' 

Sample locality 

(As, Cu, Pb, Zn, Ag, in parts per million) 

Mineralized fissure and (or) 17 (120, 20,300,-.-1/ i: 16 (60, 50,200,300,11 altered zone 

I 

I 

15 (30, 50, >500, 700, 71 

11 {80, 100, >500, 200, 71 . 

• ./runnel portal 

• 14 (60, 50, >500, 1000, 11 12 (60, 20, 500,-, 21 

13(20+,20,>500, 100,21 

400·FEET 

(Approximate scale) 

FIGURE 10.-Sketch map of sample localities at the Brooklyn mine site. 

debris and slope wash. In addition, the vein walls of altered and 

sheared volcanic rocks are soft; this condition results locally in 

a trenchlike surface expression along strike. Most of the geochemical samples were collected along the vein and crosscutting fissures, 

but some of the samples were taken from the altered wallrocks 

from a few feet to as much as 30 feet on either side of the vein. 

Extension of the vein's course beyond the caved shaft (fig. 10) 

is somewhat uncertain. 

The predominant ore minerals are pyrite, galena, and sphalerite, a little chalcopyrite, and possibly tennantite. Some narrow 

fissures contain native gold associated with a little manganiferous 
26 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

carbonate and pyrite. At one time, small operations recovered substantial gold from these fissures along certain stretches of the vein. 

The results of sampling, shown in figure 10 and tables 8 and 9, 

indicate that trenching of the surface debris and closer spacing of 

samples are required for more satisfactory delineation of anomalies. The relatively high values of arsenic and metals, shown by 

the several samples taken in the walls 15-30 feet from the vein, 

indicate a high permeability of the country rock. This high permeability is possibly a result of preore leaching by solutions of the 

acid-sulfate stage. As shown by the spectrographic analyses (table 

9), calcium, magnesium, and locally manganese are below the normal amounts in fresher rocks. These elements were partially removed by leaching. Iron, on the other hand, is only locally leached 

and, for the most part, has been increased by the addition of pyrite. 

The barium content has a range somewhat like that of the background traverse along the highway in Mineral Creek valley west 

of this site. Because these values are comparable, it is assumed 

that the arsenic background also should correspond roughly to that 

of Mineral Creek (fig. 3). 

Correlation between the moderate arsenic values and the general 

trend of the Brooklyn vein is obscure (fig. 10), possibly because 

of poor sampling conditions and the limited numbers of samples. 

TABLE B.-Chemical analyses of rocks, gouge, and undersoil at the 

Brooklyn mine site 

[Chemical analyses by Doris Price, U.S. Geol. Survey. Sample localities shown in fig. 10] 

Parts per million 

Field 

No. Description Arsenic 

Field Lab Lab 1 

65-11 Chloritized rock _ ___ _ _ _____________________________________ < 10 

12 Gouge ____ ______ ____________ _______ __________________ _____________________ 10 

13 Rind on altered rock_________________________________________ -<10 

14 Oxidized seam in rock_______________ __________ 20 

15 Altered seam in rock ___________ _ 10 

16 Manganese-stained altered rock ______ 80 

17 Iron-stained bleached seam in rock____ 60 

18 Iron-stained seam in fractured rock __ ____ 20 

19 Bleached rock _ ______ _____ ___________ __ ___________ _ _ 80 

20 Clayey undersoil (slope wash?)____ ________ 10 

21 Manganese- and iron-stained chloritized rock_____ 10 

22 Partly oxidized rock __ ___________________________ _____________ 60 

23 Oxidized altered rock _______ 120 

24 Bleached and iron-stained rock _ ___________ ___ 40 

25 Oxidized rock __________ _______ ________ _ _________ < 10 

26 Manganese-stained rock _ ___ __ ____ 30 

27 Limonite-stained rock ______________________ _______________ 30 

28 Bluish gouge ___ ___ _ _ ______________ 10 

29 Silicified rock with limonitic selvage_ 80 

1 Sodium peroxide added. 

80 

60 

30 

60 

20+ 

80 

120 

40 

120 

20 

30 

120 

120 

40 

20 

30 

30 

40 

120 

80 

60 

20+ 

60 

30 

60 

120 

30 

120 

20 

20+ 

120 

120 

30+ 

20 

20+ 

30 

30 
TABLE 9.-Spectrographic analyses of rocks at the Brooklyn mine site 

[Spectrographic analyses by E. L. Mosier, U.S. Geol. Survey. n.d., not detected. Sample localities shown in fig. 10] 

Field Percent Parts per million 

No. Fe Mg Ca Ti Mn Ag B Ba Co Cr Cu Ga La Mo Ni Pb Sc Sr v 

65-11 10 0.7 0.30 5,000 700 7 20 700 n.d. 7 100 20 n.d. 7 2 >500 20 n.d. 200 

12 5 . 5 .10 10,000 150 2 70 1,000 n.d. 10 20 20 100 5 n.d . 500 30 n.d. 500 

13 10 1. .15 7,000 700 2 10 1,500 n.d. 7 20 20 50 2 n.d. >500 20 n.d. 300 

14 10 2. . 5 7,000 2,000 1 10 700 7 10 50 20 50 n.d . 3 >500 20 200 300 

15 15 2. .07 7,000 5,000 7 10 200 5 10 50 20 n.d. 70 n.d. >500 20 n.d. 200 

16 7 1. . 15 5,000 5,000 1 10 700 7 7 50 20 n.d. n.d . 3 200 15 n.d. 200 

17 7 1. . 15 7,000 700 n.d. 20 700 n.d. 10 20 20 70 1 n.d . 300 20 n.d. 300 

18 10 . 5 .15 7,000 500 1 20 1,000 n.d . 10 15 20 70 1 2 50 20 n.d. 200 

19 5 .5 .10 7,000 500 3 20 1,000 n.d. 10 20 20 70 1 n.d. >500 30 n.d. 300 

20 3 .5 . 2 10,000 200 n.d . 30 2,000 n.d. 10 2 20 100 1 n.d. 50 30 200 200 

21 7 1. .2 7,000 5,000 1 30 1,000 10 10 50 20 70 1 5 50 20 n.d. 200 

22 20 .2 .1 7,000 500 15 50 1,000 n.d. 10 30 20 70 2 2 >500 20 300 200 

23 15 .5 .1 10,000 500 15 30 500 n.d. 10 100 20 n.d. 2 n.d. >500 30 n.d. 300 

24 3 .1 .15 5,000 500 2 30 500 n.d. 7 10 20 100 2 n.d. 150 15 200 200 

25 10 .7 .15 7,000 500 1 30 1,000 n.d. 10 30 20 70 1 2 200 20 300.300 

26 10 3 .3 7,000 > 5,000 n.d. 10 700 5 10 50 20 70 n.d. 5 200 30 n.d. 500 

27 3 .5 .1 5,000 500 2 30 1,000 n.d. 5 10 50 70 n.d. n.d. 70 20 200 300 

28 2 .5 .2 10,000 50 2 30 2,000 n.d. 10 15 50 70 2 n.d. 100 50 200 500 

29 2 .3 .1 7,000 500 20 30 1,000 n.d. 5 70 15 n.d. n.d. n.d. 500 15 200 150 

y Zn Zr 

10 200 300 

50 n.d. 1,000 

30 100 300 

30 1,000 300 

30 700 500 

20 300 200 

30 n.d. 200 

30 100 200 

50 n.d. 300 

50 n.d. 300 

30 200 300 

30 100 300 

30 700 500 

30 n.d. 300 

30 n.d. 500 

50 500 300 

50 n.d. 300 

50 n.d. 500 

20 n.d. 300 

r:n 

> 

~ 

~ 

t""t 

t?=_j 

t""t 

0 

0 

> t""t 

~ 

1-3 

~ 

t?=_j 

r:n 

1:..:> 

-=1 
28 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

However, the higher arsenic values and the valuable metals do 

correlate fairly well; copper and lead in the higher grouping average about twice as much as those in the lower group, and silver 

averages three to four times as much. The sodium-peroxide-treated 

laboratory samples (table 8) were used for comparison and plotting (fig. 10) because of the relatively high amount of unoxidized 

pyrite in most of the samples. 

The relatively high amount of zinc in some Brooklyn samples 

contrasts markedly with the generally low amounts noted at all 

other sites. Zinc was present below detection limits in about onethird of the samples. Probably, this variant is caused by the cover 

of slope wash and glacial debris, poor drainage conditions owing to 

very little adjacent surface area from which waters can drain 

across this site, and the incomplete leaching by acid-sulfate waters. 

The Brooklyn vein, although in acid-sulfate altered rocks, must 

be considered possibly to have undergone a type of mineralization 

somewhat different than that of the chimneys; thus, the associated 

geochemical anomalies may be different. This possibility is suggested by the presence of chalcopyrite, by the apparent lack of 

enargite, and by the relatively anomalous occurrence of zinc near 

veins in the area (F. S. Fisher, written commun., 1971). Further 

corroboration of the difference in geochemical anomalies between 

veins and chimneys is provided by the fact that veins are less subject to supergene leaching than chimneys are. 

CONCLUSIONS 

Preliminary field tests for arsenic in areas that have undergone 

acid-sulfate leaching, such as the Red Mountains area of the western San Juan Mountains, seem to be very useful in geochemical 

prospecting, particularly for mineralized chimney deposits. The 

principal advantage is that these tests for arsenic residuals do not 

show anomalous broad halos but define specific areas, such as the 

relatively small pipe localities, for further and more elaborate 

sampling and laboratory tests. In general, residual lead, silver, 

and mercury conform to the residual arsenic values, whereas copper and zinc generally do not because they are easily leached by 

acid surface waters. 

The background values of mercury are somewhat more erratic. 

than those of arsenic but mercury also greatly exceeds arsenic in 

sensitivity to local metal concentrations. This greater sensitivity 

could be of advantage under some conditions. Because the metals 

are widely dispersed in the fractured and porous rocks of the area, 

the useful selectivity of mercury determinations is perhaps not 
CONCLUSIONS 29 

fully established by the limited sampling that was done. However, 

the larger number of samples required may also lessen its value 

as a prospecting technique. 

These field samples were mainly taken from specific localized 

sites rather than from the altered fissures of northerly to northeasterly trend in the Red Mountains area that are characteristic 

of the western rim of the Silverton cauldron and that are related 

to the trend of its boundary faults. Bleached and pyritized rocks 

are typical along the more pronounced fissures between the major 

pipes, but concentrations of ore are spotty and generally weak; a 

few small veins and irregular masses of lead-silver ore have been 

prospected. Many of the pipe deposits at depth tend to assume 

linear forms or else become a series of fissures. The delineation of 

these fissured and mineralized breaks at the surface may of course 

be useful to outline broad patterns of mineral concentrations. 

Whether or not arsenic is sufficiently sensitive for this purpose is 

questionable without further testing. 

Certain precautions in sampling and sample preparation are 

necessary to obtain the full advantage of the arsenic test. The large 

quantities of pyrite in many samples tended to interfere with the 

accuracy of the field determinations. Accordingly, fusion of the 

samples with sodium peroxide prior to field determinations seems 

to be desirable. Figure 11 illustrates the comparison between field 

and laboratory determinations for samples from the Lark and 

Brooklyn mine sites. Most field results were usually lower in value 

than the better controlled laboratory results, but this factor does 

not necessarily reflect the reliability of the field results ; reliability 

was not tested. This difference probably results in part from the 

inexperience of the field operators and in part from the interfering 

effects of pyrite in many samples. 

The results also indicate that somewhat closer spacing of field 

samples would have been helpful in the sites of established deposits. Where the overburden is fairly thick, trenching to the bedrock surface is of course desirable, as the local debris may be 

contaminated with transported clayey material and residual products of oxidation. Rapid erosion on the steep slopes of much of the 

area contributes to this contamination. 

The tests made were intended only to determine if there was 

appreciable residual arsenic in leached and oxidized rocks of the 

area. No attempt was made to define areas for further testing or 

to locate sites for exploration. 
30 ARSENIC, WESTERN SAN JUAN MOUNTAINS, COLORADO 

600 '1200 400 400 6001600 

t t t t t t 

200 

1\ A 

1\ II 

I I I I I 

I I I I I I 

I I I I I I 

1>320 1 I I I 

I I I I t I I I 

100 I I I I I 

z I I I I I 

0 I I I I 

:::i I I I I ...J I I 

~ I I I I 

I I I I cr:: I ~ w ;, I I / I a. I " ,·, V) 

' I "' ~--· ,,,.., '."'FIELD I 1- I cr:: 

<( 0 ·-.... , a. 

~ LARK MINE SITE 

~-

z 

w 

V) 

cr:: 

<( 100 

BROOKLYN MINE SITE 

FIGURE H.-Comparison of field and laboratory determinations for 

arsenic. See text and tables 5 and 8. 

REFERENCES CITED 

Burbank, W. S., 1941, Structural control of ore deposition in the Red Mountain, Sneffels, and Telluride districts of the San Juan Mountains, Colorado: Colorado Sci. Soc. Proc., v. 14, no. 5, p. 141-261. 

--- 1947, Red Mountain district, Ouray County, in Vanderwilt, J. W., and 

others, Mineral resources of Colorado: Denver, Colo., Colorado Mineral 

Resources Board, p. 428-431. 

--- 1950, Problems of wall-rock alteration in shallow volcanic environments, in Van Tuyl, F. M., and Kuhn, T. H., eds., Applied geology, a 

symposium: Colorado School Mines Quart., v. 45, no. 1B, p. 287-319. 

--- 1960, Pre-ore propylitization, Silverton caldera, Colorado, U. S. Geol. 

Survey Prof. Paper 400-B, p. B12-B13. 

Burbank, W. S., and Luedke, R. G., 1964, Geology of the Ironton quadrangle, 

Colorado: U. S. Geol. Survey Geol. Quad. Map GQ-291. 

--- 1969, Geology and ore deposits of the Eureka and adjoining districts, 

San Juan Mountains, Colorado: U. S. Geol. Survey Prof. Paper 535, 73 p. 

Erickson, R. L., Marranzino, A. P., Oda, Uteana, and Janes, W. W., 1964, 

Geochemical exploration near the Getchell mine, Humboldt County, Nevada: U. S. Geol. Survey Bull. 1198-A, 26 p. 
REFERENCES CITED 31 

Erickson, R. L., Van Sickle, G. H., Nakagawa, H. M., McCarthy, J. H., Jr., and 

Leong, K. W., 1966, Gold geochemical anomaly in the Cortez district, 

Nevada: U. S. Geol. Survey Circ. 534, 9 p. 

Fischer, R. F., Luedke, R. G., Sheridan, M. J., and Raabe, R. G., 1968, Mineral 

resources of the Uncompahgre primitive area, Colorado: U. S. Geol. 

Survey Bull. 1261-C, 91 p. 

Remley, J. J., and Jones, W. R., 1964, Chemical aspects of hydrothermal alteration with emphasis on hydrogen metasomatism: Econ. Geology, v. 59, 

no. 4, p. 538-567. 

Remley, J. J., Hostetler, P. B., Gude, A. J., and Mountjoy, W. T., 1969, Some 

stability relations of alunite: Econ. Geology, v. 64, no. 6, p. 599-612. 

Hinkle, Margaret, Leong, K. W., and Ward, F. N., 1966, Field determination 

of nanogram quantities of mercury in soils and rocks, in Geological Survey research 1966: U. S. Geol. Survey Prof. Paper 550-B, p. B135-B137. 

Larsen, E. S., Jr., and Cross, Whitman, 1956, Geology and petrology of the 

San Juan region, southwestern Colorado: U. S. Geol. Survey Prof. Paper 

258, 303 p. 

Luedke, R. G., and Burbank, W. S., 1968, Volcanism and cauldron development 

in the western San Juan Mountains, Colorado: Colorado School Mines 

Quart., v. 63, no. 3, p. 175-208. 

Myers, A. T., Havens, R. G., and Dunton, P. J., 1961, A spectrochemical method 

for the semiquantitative analysis of rocks, minerals, and ores: U. S. Geol. 

Survey Bull. 1084-I, p. 207-229. 

Nakagawa, H. M., and Lakin, H. W., 1965, A field method for the determination of silver in soils and rocks, in Geological Survey research 1965: U. S. 

Geol. Survey Prof. Paper 525-C, p. C172-C175. 

Ransome, F. L., 1901, A report on the economic geology of the Silverton quadrangle, Colorado: U. S. Geol. Survey Bull. 182, 265 p. 

Varnes, D. J., and Burbank, W. S., 1945, Lark mine, Cement Creek area, San 

Juan County, Colorado: Colorado Mining Assoc. Yearbook, p. 36-37. 

Vaughn, W. W., and McCarthy, J. H., Jr., 1964, An instrumental technique 

for the determination of submicrogram concentrations of mercury in soils, 

rocks, and gas: U. S. Geol. Survey Prof. Paper 501-D, p. D123-D127. 

Ward, F. N., Lakin, H. W., Canney, F. C., and others, 1963, Analytical methods 

used in geochemical exploration by the U. S. Geological Survey: U. S. 

Geol. Survey Bull. 1152, 100 p. 

Wells, J. D., Stoiser, L. R., and Elliott, J. E., 1969, Geology and geochemistry 

of the Cortez gold deposit, Nevada: Econ. Geology, v. 64, no. 5, p. 526-537. 

*U.S. GOVERNMENT PRINTING OFFICE: 1972 0-472-568