Jasperoids '- ';;|3 d:C)" Area , 

Eagle <3p^

G E O L 6 G I C A t ° S IJ II \^ ¥ B XJ L L E TIN 1° 4 7 4

Jasperoids of the Pando Area, 

Eagle County, Colorado

By T. G. LOVERING and A. V. HEYL

GEOLOGICAL SURVEY BULLETIN 1474

Tungsten-rich outcrops may indicate 

concealed tungsten and beryllium 

deposits

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Levering, Tom Gray, 1921-

Jasperoids of the Pando Area, Eagle County, Colorado.

(Geological Survey Bulletin 1474)

Bibliography: p. 35

1. Jasperoid Colorado Eagle Co. 2. Tungsten ores Colorado Eagle Co. 3. Beryllium

ores-Colorado-Eagle Co. I. Heyl, Alien Van, 1918- joint author. II. Title.

HI. Series: United States Geological Survey Bulletin 1474. 

QE475.J3169 552'4 79-11345

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

Washington, D.C. 20402 

Stock No. 024-001-03273-2
CONTENTS

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

Introduction............................................................. 2

Acknowledgments ....................................................... 8

Distribution and appearance of jasperoid................................... 8

Microtexture and mineralogy of jasperoid samples .......................... 12

Minor element distribution in jasperoid .................................... 17

Association of elements .................................................. 24

Genesis of jasperoid ...................................................... 28

Suggestions for prospecting .............................................. 34

References cited ......................................................... 35

ILLUSTRATIONS

Page

FIGURE 1. Index map showing location of Pando area relative to nearby

jasperoid-bearing mineralized areas ........................... 3

2. Generalized geologic map of the Pando area showing distribution

of jasperoid bodies, and sample localities ...................... 6

3. Sample locality map of the Pando area, showing generations of

jasperoid ................................................... 13

4. Geometric means and deviations of selected elements in samples of

three generations of jasperoid from the Pando area ............. 20

5. Sample locality map of Pando area showing tungsten anomalies ... 22

TABLES

TABLE 1. Sedimentary rocks of the Pando area, Colorado.................... 4

2. Appearance, texture, and mineralogy of jasperoid varieties, in the

Pando area................................................... 11

3. Measures of central tendency and dispersion in three generations

of Pando jasperoid ............................................ 18

4. Measures of central tendency and dispersion of selected elements

in jasperoid samples from Pando and adjacent areas ............. 23

5. Associated minor elements in samples of various types from the

Pando area................................................... 25

6. Oxygen isotopic analysis of silica from Pando jasperoid ............ 29

in
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JASPEROIDS OF THE PANDO AREA, 

EAGLE COUNTY, COLORADO

By T. G. LOVERING and A. V. HEYL

ABSTRACT

In the upper valley of the Eagle River, near the old townsite of Pando in central 

Colorado, carbonate rocks of Devonian and Mississippian age have been hydrothermally altered and extensively replaced by bodies of jasperoid. Despite the widespread hydrothermal and limonitic alteration, suggestive of proximity to large ore 

deposits, no important orebodies have been found, although there are abundant prospect pits and a few small abandoned mines in the area.

The jasperoid bodies are complex, and they record a complicated history of at least 

three separate episodes of silicification, each episode reflecting a different environment of formation. The oldest and most abundant generation of jasperoid is a dense 

gray rock, commonly containing sericite and fresh pyrite, with no consistently 

anomalous concentrations of minor elements; jasperoid of this type is widely 

distributed in carbonate rocks for many kilometers to the north, east, and south of 

Pando. The second generation of jasperoid is light colored (in varying shades of pink, 

orange, yellow, and brown) and has less sericite than the oldest jasperoid, some 

limonite pseudomorphous after pyrite, and abundant dust particles of limonite 

clouding the matrix quartz; it commonly contains anomalous concentrations of 

arsenic, beryllium, germanium, manganese, niobium, antimony, scandium, tungsten, 

and yttrium. This second type is restricted to the mid-Paleozoic carbonate rocks exposed on the hills surrounding Pando from about 3 km (kilometers) south to about 

3 km north of the townsite. Jasperoid of the third generation consists of an early deepred variety with abundant hematite, and later reddish-brown to deep-brown varieties, 

which are vuggy and contain relatively clean phaneritic quartz in a matrix of darkbrown limonite, goethite, and manganese oxides. These later jasperoids commonly 

contain highly anomalous concentrations of arsenic, beryllium, germanium, niobium, 

antimony, and tungsten with tungsten concentrations as high as 1 percent and are 

largely restricted to a belt about 4 km long extending southeasterly from the divide 

between Homestake Creek and the Eagle River, across the upper end of the Eagle 

River canyon, to the mouth of Resolution Creek; the highest tungsten concentrations 

occur near the north end of the belt.

Evidence is presented that the oldest generation of jasperoid is unrelated to the 

other two, that it came from a source in the Gore Range to the east, that it formed at 

moderate temperature and depth in. a reducing environment, and that if it was 

genetically related to any large sulfide ore bodies, they have probably been largely 

removed by erosion. The second and third generations of jasperoid are thought to 

have had a magma tic source, beneath the north end of the Eagle River valley and the 

ridge northwest of Pando, that moved upward during genesis of these jasperoids to 

form a shallowly buried intrusive stock. If such a stock is present, it may contain commercial concentrations of tungsten and beryllium minerals in portions of its hood.
2 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

INTRODUCTION

The Pando area is in central Colorado, in the valley of the upper 

Eagle River, north of Tennessee Pass, 25-32 km (kilometers) north of 

Leadville, and about 120 km west-southwest of Denver. It consists of 

a nearly level parklike area approximately 1 km wide and 7 km long 

and the adjacent lower slopes and ridges of the Sawatch Range, 

which rises steeply on the west, and of the Gore Range on the east. 

The site of the former Camp Hale Military Reservation occupies the 

center of this flat area. The Eagle River valley narrows abruptly to a 

steep-walled canyon at the north end of this area. Prominent masses 

of rusty-brown-weathering jasperoid crop out on the hills to the east, 

north, and northwest of the valley flats.

Pando is near the north end of the Colorado jasperoid province 

recognized by Lovering (1972, p. 4, 5, 96); other jasperoid-bearing 

mineralized areas and mining districts lie a few kilometers away to 

the north, east, and south (fig. 1).

The jasperoids near Pando were first discussed by Crawford and 

Gibson (1925, p. 56-59). Tweto (1953), in the text of a geologic map of 

the area, discussed the distribution and characteristics of these 

bodies, their probable genesis, and their economic implications. 

Lovering (1972, p. 99-100) briefly summarized the earlier observations of Crawford and Gibson, and of Tweto, and added some descriptive information on a few samples from the area.

In addition to the jasperoid bodies in this area, there are other indications of widespread hydrothermal alteration and mineralization, 

such as hydrothermal dolomitization, sanding of dolomite in the 

lower Paleozoic carbonate rocks, iron oxide impregnation of the 

underlying quartzite, and the presence of pyritic quartz veins cutting 

the Precambrian basement rocks. Although the area has been extensively prospected and a few small mines in the northern part of it 

have produced a little gold, silver, and copper, no large ore deposits 

have been discovered.

Tweto (1953) suggested that the rock alteration at Pando might 

reflect updip leakage from a major ore deposit to the east or northeast. The present investigation was conducted in the hope that a 

detailed study of the jasperoids could provide evidence of the nature 

and location of any major ore deposits with which they might be 

associated.

Precambrian basement rocks are exposed in the Sawatch Range on 

the west and are overlain by Cambrian to Pennsylvanian sedimentary rocks in the Gore Range on the east. The Precambrian rocks consist largely of schists and gneisses intruded by plutonic granite and 

diorite. These are overlain by about 150 m (meters) of lower and 

middle Paleozoic rocks (table 1) consisting (in ascending order) of the
106°30'

INTRODUCTION

15'

\Southern Gore Range 

Kokomo

10 MILES

FIGURE 1. Location of Pando area relative to nearby jasperoid-bearing mineralized

areas (patterned).
TABLE 1. Sedimentary rocks of the Pando area, C

[Modified from Tweto, 1953, 1974; and Tweto and Levering, 

Series

Middle 

Pennsylvania)!

Lower 

Mississippian

Lower 

Mississippian (?)

and (or)

Upper 

Devonian

Upper 

Devonian

Middle 

Ordovician

Upper 

Cambrian

Formation

Minturn Formation

Belden Formation 

- - UNCONFORMITY - - - - 

Leadville Dolomite

Chaffee Group

Hardin

Peerle

Gilman 

Sandstone

Dyer 

Dolomite

Parting 

Quartzite

3 Quartzite

NFORMITY - - - - 

ss Formation

Sawatch Quartzite

Thickness 

(meters)

1,800+

8-61

20-29

5- 7

5-24

14-17

6- 8

11-34

30-56

Predominantly g

conglomerate, 

few beds and 

Interbedded dar

dark-gray lime

Massive gray to 

dolomite .

Dolomitic sands

and dolomite 

Dark-gray to bla

dolomite.

White and tan q

locally shaly 

Tan, white, and 

quartzite.

Thin-bedded, va

dolomitic san

Fine-grained wh

and top.
INTRODUCTION 5

Upper Cambrian Sawatch Quartzite and Peerless Formation, the 

Middle Ordovician Harding Quartzite, the Upper Devonian and 

Lower Mississippian(?) Chaffee Group (which comprises the Parting 

Quartzite, the Dyer Dolomite, and the Oilman Sandstone), and the 

Lower Mississippian Leadville Dolomite. The maximum thickness of 

this lower and middle Paleozoic carbonate section is about 60 m. This 

section is overlain by approximately 2,000 m of predominantly 

clastic Pennsylvanian sedimentary rocks of the Belden and Minturn 

Formations. The Minturn Formation contains carbonate beds and 

reefs in its upper part.

Monzonitic sills of Late Cretaceous and Tertiary age have intruded 

the sedimentary section (Tweto, 1974) from the base of the Sawatch 

Quartzite to just above the top of the Leadville Dolomite in the 

Pando area. The uppermost sill is remarkably persistent and can be 

found either at the top of the Leadville or just above it (in the black 

shale of the Pennsylvanian Belden Formation) from the east fork of 

the Eagle River northward as far as Oilman. A few small dikes of Tertiary quartz latite porphyry and quartz monzonite porphyry cut both 

the Precambrian rocks and the overlying pre-Pennsylvanian sedimentary rocks near Pando. Many large sills of quartz monzonite porphyry have also been emplaced in the upper part of the Pennsylvanian Minturn Formation, both a few kilometers east of Pando, and 

south of the East Fork of the Eagle River southeast of Pando.

Numerous high-angle normal faults cut the older Paleozoic rocks 

and the underlying basement complex near Pando, and the upper 

part of the Minturn Formation a few kilometers to the east is cut by a 

complex network of large faults.

Jasperoid bodies varying in size from small pods to large irregular 

masses as much as 300 m long and 60 m thick are exposed in the Dyer 

and Leadville Dolomites at invervals from the valley of the East Fork 

of the Eagle River northward for approximately 8 km to an isolated 

outcrop in the Eagle River canyon about \Vz km south of the town of 

Red Cliff (fig. 1). Large masses of jasperoid replacing these same formations are also exposed to the west along the top of the ridge that 

forms the divide between the Eagle River and Homestake Creek; and 

a large isolated mass of jasperoidized Dyer and Leadville Dolomites 

also forms an erosion remnant on the eastern flank of the Sawatch 

Range opposite the southernmost jasperoid body on the east side of 

the Eagle River (fig. 2).
JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.
INTRODUCTION

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8 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

Small bodies of jasperoid also locally replace the carbonate rocks in 

the upper part of the Minturn Formation in the vicinity of faults in an 

easterly trending belt about 3 km wide, high in the Gore Range about 

5Yz km east of the Eagle River valley.

ACKNOWLEDGMENTS

We gratefully acknowledge the assistance of J. M. Brown and J. S. 

Merchant of the New Jersey Zinc Company staff at Oilman in providing us with samples of jasperoid from five diamond drill holes east 

of Pando, which furnished valuable information on the continuity and 

nature of jasperoid in the carbonate section downdip from the outcrops in the Eagle River valley. C. N. Bozion, M. L. Heyl, and David 

Levering helped collect the samples.

Of the many members of the U.S. Geological Survey that assisted 

in this study, the following were most helpful. Irving Friedman made 

oxygen isotope analyses of several jasperoid samples. Quantitative 

gold assays were made by J. A. Thomas and G. D. Shipley using a 

combination of fire assay and atomic absorption methods. Spectrographic analyses were provided by Leon Bradly and J. C. 

Hamilton. Sherman Marsh made mineral separations from several 

samples of igneous intrusive rocks. John Viets extracted limonite 

from a suite of altered rock samples and also made quantitative 

analyses for mercury, gold, tungsten, and fluorine on the leachates, 

the residues, and the whole-rock samples. Spectrographic analyses on 

these leachates were provided by Elwin Mosier. Fieldwork was done 

at intervals during the summers of 1970, 1971, and 1972.

DISTRIBUTION AND APPEARANCE OF JASPEROID

Nearly all the Devonian and Mississippian dolomitic rocks that are 

exposed along the Eagle River valley, from the east fork of the Eagle 

River northward to a point about 1 km north of the junction of Silver 

Creek and the Eagle River, have been partly replaced by jasperoid. In 

some places the entire carbonate section has been silicified through a 

thickness of more than 50 m, and the resulting jasperoid bodies form 

prominent, rusty-brown reefs and monoliths. Crawford and Gibson 

(1925, p. 56-59) first studied and described these bodies, which were 

further studied and mapped by Tweto in 1953. Tweto also found and 

mapped scattered bodies of jasperoid in carbonate rocks in the Minturn Formation to the east. Most of these bodies are high in the Gore 

Range on Pearl Peak, in the upper drainage basins of Pearl and 

Cataract Creeks, and on Sheep Mountain, where they replace the 

Hornsilver and Resolution Dolomite Members of the Minturn Formation, generally in close proximity to large faults. A thin, persistent
DISTRIBUTION AND APPEARANCE OF JASPEROID 9

dolomite bed, informally called the Jack 8 bed, in the lower part of the 

Minturn Formation, about 150 m above the base, has also been largely converted to jasperoid (Tweto, 1974).

We did not study the jasperoid bodies in the upper part of the Minturn Formation, most of which are far above and several kilometers 

to the east of those along the Eagle River valley. They have been 

described and discussed by Tweto (1953) in the text that accompanies 

his map of the area. Spectrographic analyses of some of Tweto's 

jasperoid samples from this area have been used in this paper for 

comparison with those at Pando. Many of them were collected within 

3 km of the old Kokomo mining district east of Pando (fig. 1), and 

they may represent leakage of silica-bearing solutions from that 

center of mineralization.

The largest and most abundant jasperoid bodies in. the Pando area 

are concentrated in two areas: (1) along the crest of the ridge that 

forms the divide between Homestake Creek and the Eagle River 

north of the low saddle crossed by U.S. Highway 24 and opposite this 

ridge on the east side of Eagle River; and (2) at the mouth of Resolution Creek (fig. 2). Jasperoid in this area extends for at least 1.6 km 

back of the outcrop and to a depth of at least 215 m, as shown by drill 

core samples. A large, isolated body of jasperoid partly replaces 

Leadville and Dyer Dolomites to the west of the Eagle River in the 

Sawatch Range on top of the ridge between Rule Gulch and Yoder 

Gulch at an elevation of 3,125-3,275 m.

Although high-angle faults with small displacement are present in 

the vicinity of most of the larger jasperoid bodies, these faults do not 

appear to have greatly influenced the distribution of the jasperoid, 

and in many places such bodies are displaced along small faults. The 

distribution of many smaller jasperoid bodies in the Dyer Dolomite, 

however, does appear to be controlled by joints and fracture zones. 

Tweto (1953) observed that the largest and most persistent jasperoid 

bodies are concentrated in the Chaffee Group, at the stratigraphic 

level of the Gilman Sandstone, between the Leadville Dolomite above 

and the Dyer Dolomite below. This relationship suggests that 

relatively permeable breccia and sandstone of the Gilman was a 

major channel way for the silicifying solutions. Channels in the upper 

part of the Leadville Dolomite, related to a pre-Pennsylvanian karst 

erosion surface, also locally provided conduits for these solutions, 

resulting in the silicification of the adjacent parts of the Leadville 

Dolomite.

Although many of the larger jasperoid bodies appear as rugged, 

homogeneous, rusty-brown masses, standing out in relief above the 

surrounding terrain, on closer inspection these masses are seen to be 

composed of breccia fragments of many different jasperoid types set 

in a siliceous matrix. Some of these bodies are cut by still younger
10 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

jasperoid of a different type, and locally the siliceous matrix forms 

breccia fragments within the younger jasperoid. Both Crawford and 

Gibson (1925, p. 56, 57) and Tweto (1953) recognized the earliest and 

most abundant gray jasperoid as representing the primary siliceous 

replacement of dolomite, of dolomite breccia fragments, and of the 

macerated carbonate matrix around these fragments. Tweto also 

recognized that the presence of fragments of this early type in a 

matrix of younger jasperoid provides evidence that this early 

siliceous replacement breccia was locally shattered and recemented 

by later silica.

Our study has revealed the presence of three major generations and 

types of jasperoid in the area, with each generation consisting of two 

or three subtypes (table 2). The three major types appear to be consistent in their relationship to each other throughout the Pando area, 

although the interrelations of the subtypes within the major types 

are equivocal, one appearing to be older at one locality and younger at 

another.

Type 1 is the oldest jasperoid; it is various shades of gray, generally dense,- and ranges in texture from fine grained to aphanitic. 

Type 2 is generally colored light shades of red, pink, yellow, orange, 

and yellowish brown; it is also generally dense and ranges from very 

fine grained to aphanitic texture. Type 3 is dark red, reddish brown, 

or dark brown; it is commonly porous and vuggy with a fine- to 

medium-grained texture.

Type 1 jasperoid consists of two subtypes: (A) a light-gray to 

medium-gray variety that has a variable texture, grading from a fine 

granular texture resembling quartzite to an aphanitic texture 

resembling chert; and (B) a dark-gray, dense, aphanitic variety. Commonly the lighter gray varieties form breccia fragments in the darkgray matrix, but in some places this relationship is reversed. 

Medium-gray to dark-gray nodules of primary chert preserved in 

unbrecciated light-gray jasperoid may closely resemble the darker 

gray jasperoid in both color and texture.

Type 2 jasperoid includes three main subtypes, each of which 

displays color variations. Type 2A, generally the oldest, varies from 

olive gray through dark yellowish brown to yellowish orange and 

yellowish gray. It is generally dense and aphanitic in texture. In 

some places it forms a matrix for fragments of Type 1 gray jasperoid; 

in others it forms breccia fragments cemented by other Type 2 or 

Type 3 jasperoids. Type 2B consists of pale-red, grayish-red, pink, 

and tan color variants, which commonly form breccia fragments in 

Type 3 jasperoid. This subtype is generally very fine grained and 

dense, though it locally contains sparse tiny vugs. Type 2C is pale 

pinkish or yellowish gray, fine grained, and dense. It is less common
DISTRIBUTION AND APPEARANCE OF JASPEROID 11

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12 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

than the other two subtypes and forms a matrix for fragments of the 

Type 1 and Type 2 A jasperoids. In an isolated hand specimen, it may 

strongly resemble the light-gray Type 1A jasperoid.

Type 3 jasperoid differs strikingly in appearance from the other 

two types. It consists of three subtypes, the relative ages of which 

cannot be firmly established from field evidence. Type 3A is a dark 

brick-red color, is generally dense and fine grained, and locally contains small vugs. Type 3B is dark reddish brown, has an aphanitic 

texture, and is commonly porous and vuggy. Type 3C is dark brown, 

aphanitic, and vuggy; locally, it grades into the reddish-brown variety (3B).

The distribution of sample localities of the three generations of 

jasperoid in the Pando area is shown in figure 3.

Type 1 jasperoids are represented in samples from all areas, though 

they are sparse on the Homestake Creek-Eagle River divide. Type 2 

jasperoids are present in all areas except the two drill hole localities 

farthest up Resolution Creek and are sparsely represented in the 

northernmost localities east of the Eagle River and north of the 

mouth of Silver Gulch. Type 3 jasperoids are abundantly represented 

in samples from near the mouths of Resolution Creek and McAllister 

Gulch and from the Eagle River-Homestake Creek divide, but they 

are very sparse south of Resolution Creek and are largely missing 

east of the Eagle River north of a point midway between McAllister 

and Silver Gulches. They appear to be largely confined to a belt extending northwesterly from near the mouth of Resolution Creek, 

across the nose of the ridge west of the mouth of McAllister Gulch, to 

the northernmost jasperoid outcrops on the Homestake Creek-Eagle 

River divide.

This youngest generation (Type 3) is present in small amounts in 

many of the Type 1 and Type 2 jasperoid bodies on the east side of 

the Eagle River, where it forms fracture fillings in the older types. On 

top of the divide between the Eagle River and Homestake Creek to 

the west, Type 3 is the dominant type.

MICROTEXTURE AND MINERALOGY OF JASPEROID SAMPLES

The three main types of jasperoid in the Pando area can generally 

be distinguished from each other and from primary chert by their accessory minerals and microtextures, as well as by their appearances. 

The mineral assemblages are more diagnostic than the textures (table 

2); only Types 3B and 3C have distinctive textures.

Type 1 jasperoid generally contains both sericite and pyrite. The 

most distinctive feature of Types 2A and 2B jasperoids is the 

prevalence of iron oxides (hematite, goethite, or limonite) in the 

matrix quartz; but Type 2C lacks these features, and the lightyellowish-gray variety of this subtype is readily distinguishable from
MICROTEXTURE AND MINERALOGY OF JASPEROID SAMPLES 13

106°20'

39° 

25'

EXPLANATION 

Generations of jasperoid

+ First

* Second 

o Third

* First and second

First and third 

® Second and third 

® First, second, and third

4 KILOMETERS

2 MILES

FIGURE 3. Sample localities in the Pando area, showing jasperoid generations 1, 2, 3,

and combinations.

chert only where it forms a matrix cement for breccia fragments of 

older jasperoid. Type 3 jasperoid differs strikingly in appearance 

from the older varieties, contains no sericite, and commonly displays
14 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

numerous vugs surrounded by colloform bands of iron and 

manganese oxides; its matrix quartz is also generally coarser grained 

than that of the other types.

Types 1A and IB jasperoid probably represent hypogene silicic 

alteration of dolomite and dolomite breccia. Type 1A varieties appear 

to be products of replacement of massive dolomite and dolomite breccia by silica and commonly preserve the texture of the host rock. The 

Type IB jasperoid seems to largely represent colloidally deposited 

silica that filled interstices between larger fragments in an original 

dolomite breccia when these fragments were replaced by jasperoid. 

Identification of Type IB jasperoid is complicated by the fact that 

primary chert preserved in the jasperopid bodies is indistinguishable 

in both color and texture from the dark-gray jasperoid. Consequently, the true identity of dark-gray siliceous fragments cemented 

by younger jasperoid can often be determined only by exmination 

with a petrographic microscope. Primary chert does not contain 

either sericite or pyrite, which are common accessory minerals in 

Type IB jasperoid.

Type 1A jasperoid exhibits a broad range in both texture and grain 

diameter. Some specimens consist of tiny, sinuously interlocking 

quartz grains (jigsaw puzzle texture) having an average diameter 

<0.01 mm (millimeters). Others are composed of relatively coarse 

grains averaging about 0.1 mm in diameter showing a xenomorphic 

microtexture. Commonly, a single specimen will consist of a mixture 

of both types with local aggregates of coarser quartz grading outward into the aphanitic variety. In some samples isolated grains of 

coarser quartz are scattered through a relatively homogeneous, very 

fine grained matrix. The Type IB jasperoid is much more uniform, 

generally consisting of grains with an average diameter of about 

0.01 mm interlocking in a jigsaw puzzle texture; however, this matrix 

may also contain scattered, isolated grains of coarser quartz.

Most of the samples of both subtypes of Type 1 jasperoids exhibit 

minute carbonate inclusions a few micrometers in diameter in both 

matrix quartz and the coarser quartz grain inclusions. The most common interstitial accessory minerals are sericite and pyrite. In some 

specimens the pyrite is present as tiny pyritohedrons, though more 

commonly it forms cubes or small anhedral grains, generally less 

than half a millimeter in diameter. Barite, jarosite, sphalerite, 

apatite, and fluorite are present as minor accessory minerals in a few 

samples of Type 1 jasperoid.

Type 2 jasperoid contains breccia fragments of the Type 1 

jasperoid, and occurs as breccia fragments in Type 3 jasperoid. It is 

extremely variable both in microtexture and associated accessory 

minerals. The matrix quartz in Type 2A is cherty and homogeneous
MICROTEXTURE AND MINERALOGY OF JASPEROID SAMPLES 15

with an average quartz grain diameter in some samples of about 

0.01 mm. In other samples, the grain size varies from <0.01 mm to as 

much as 0.1 mm, and the texture also varies from jigsaw puzzle 

through xenomorphic to granular. The cementing matrix quartz of 

this subtype is characteristically clouded by tiny, dusty inclusions of 

yellow, yellowish-brown, or yellowish-orange limonite, which impart 

the color to the rock. The matrix quartz also commonly contains carbonate dust inclusions, and interstitial sericite is abundant in some 

specimens. One of the coarser grained samples also contains apatite 

inclusions in large quartz grains.

Type 2B jasperoid consists of both aphanitic homogeneous quartz 

and coarser grained heterogeneous quartz with variable textures, including xenomorphic, granular, and reticulated. Vugs and small open 

fractures are more common in this subtype than in Type 2A. Tiny 

dust particle inclusions of carbonate, hematite, and yellow limonite 

are commonly present in the matrix quartz of the aphanitic variety. 

Interstitial sericite, hematite, and goethite or limonite are also abundant in some specimens. The former presence of accessory pyrite in 

the majority of samples of this subtype is shown by the presence of 

scattered cubes and pyritohedrons of hematite or goethite, a few of 

which contain pyrite cores. Kaolinite, sphene, chalcedony, and late 

barite are less common accessory minerals. A feature that 

distinguishes Type 2B from Type 2A is the presence of primary 

hematite or the oxidation products of primary pyrite. Some samples 

with yellow limonite dust inclusions in the matrix quartz also contain 

oxidized accessory pyrite, but the combination of red hematite dust 

inclusions and oxidized pyrite was not observed.

Type 2C jasperoid is characterized by a relatively homogeneous, 

xenomorphic texture with quartz grains averaging about 0.03 mm. 

Carbonate dust particles are common in the quartz grains. The palebrown variety also has abundant interstitial brown limonite, and 

some specimens contain late barite and goethite pseudomorphs after 

pyrite. The light-yellowish-gray variety shows only sparse carbonate 

and allophane dust particles in the matrix quartz. This subtype differs from Types 2A and 2B in that it contains no iron oxide dust particles in the quartz and no interstitial sericite.

Type 3 jasperoids also show textural variations. Types 3A and 3B 

varieties are dense in some samples and vuggy in others, and Type 

3C, which appears to be the youngest, is characteristically porous 

and vuggy.

Type 3A typically consists of granular to xenomorphic quartz with 

an average grain diameter of about 0.03 mm and has carbonate dust 

and red hematite dust inclusions in the matrix quartz, abundant interstitial red hematite, and local orange goethite. Grains of red
16 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

hematite in the matrix of some samples appear to be pseudomorphous after pyrite. In others, a late stage of hematite forms colloform 

crusts lining vugs.

Type 3B exhibits more variation in texture and grain size than 

Type 3A. Some samples have a jigsaw puzzle texture, and some are 

xenomorphic to granular; vugs filled with coarse, clean xenomorphic 

quartz are common. Grain size of the matrix quartz ranges from 

0.01 mm to 0.1 mm. Orange to reddish-brown accessory limonite occurs as dust particles in the quartz and as an interstitial matrix or 

disseminated grains. Other common accessory minerals include 

disseminated and interstitial hematite, carbonate dust particles in 

coarser quartz grains, and goethite, manganese oxide, and barite as 

late cavity and vug fillings.

Type 3C is the most uniformly coarse grained jasperoid. It has a 

xenomorphic to reticulated texture with an average grain diameter of 

0.05 mm and individual grains as much as 0.25 mm in diameter, commonly containing carbonate dust inclusions. In some samples the 

quartz grains are scattered through an amorphous, dark-brown 

limonite matrix, in others quartz bands alternate with limonite 

bands, and in still others brown or dark-reddish-brown limonite is interstitial to the matrix quartz. Vugs are numerous and are commonly 

lined with limonite, goethite, jarosite, and manganese oxide. Many 

samples have crystals of late barite. Thin crusts of an unknown late 

secondary mineral are commonly observed; the mineral is pleochroic 

from pale yellowish brown to dark yellowish brown, anisotropic with 

moderate birefringence, and has strong relief above quartz. This 

mineral either forms crusts on goethite or thin interstitial selvages on 

late quartz, filling vugs in limonite. It is too fine grained for determination of its optical character, too strongly colored for accurate 

determination of its birefringence, and too sparse to be isolated for 

chemical analysis. It is only present in samples containing more than 

1,000 ppm (parts per million) tungsten, and may be a secondary 

tungsten mineral.

The presence of carbonate dust in the matrix quartz of all three 

generations of jasperoid suggests that all of them formed by replacement of carbonate rock. Iron minerals associated with the jasperoid 

quartz appear to change progressively through the generations of 

jasperoid. Iron occurs as discrete grains, cubes, and pyritohedrons of 

pyrite in Type 1 jasperoid, as opaque yellow limonite dust inclusions 

in Type 2A jasperoid, and as brown limonite or red hematite dust in 

Type 2B jasperoid; Types 2A and 2B also contain iron oxide 

pseudomorphs of original pyrite. However, these types do not represent surficial oxidation of older jasperoids, because unaltered breccia 

fragments of Type 1 jasperoid containing fresh pyrite are commonly
MINOR ELEMENT DISTRIBUTION IN JASPEROID 17

enclosed in a matrix of Type 2 jasperoid. Type 3A jasperoid is 

characterized by abundant red hematite as dusty inclusions in the 

quartz and as an interstitial matrix to the quartz. Type 3C jasperoid 

consists of coarser quartz free of iron oxide inclusions but closely 

associated with hydrous iron oxide as brown limonite or goethite, 

with no pyrite pseudomorphs (table 2).

MINOR ELEMENT DISTRIBUTION IN JASPEROID

Two hundred jasperoid samples from the Pando area were analyzed 

spectrographically for 27 elements by Leon Bradley and J. C. 

Hamilton, and they were also quantitatively assayed for gold, using a 

combination of fire assay and atomic absorption spectroscopy, by 

J. A. Thomas and G. D. Shipley.

At Pando, the three types of jasperoid display differences in concentration of associated minor elements. Although most of the Pando 

samples that were analyzed consist of mixtures of more than one 

generation, a sufficient number of samples composed dominantly of a 

single type of jasperoid representative of each of the three generations were analyzed to illustrate these differences (table 3). The subtypes within each generation weje grouped together in order to obtain sufficient analyses of each generation for comparison.

The median concentrations of all elements except barium in Type 1 

jasperoid are close to normal background values. Barium is abnormally high, but it is also very erratic in its distribution in all three 

jasperoid types. Barite always appears as a late mineral, younger 

than the matrix quartz, in veinlets and lining vugs, so the high Ba 

content of the jasperoid may not reflect the composition of the 

original siliceous replacement of the carbonate rocks.

Type 2 and Type 3 jasperoid samples from Pando show a progressive increase in median concentrations of manganese, arsenic, 

beryllium, germanium, niobium, nickel, antimony, scandium, 

tungsten, and yttrium. Sodium, boron, and zirconium are relatively 

abundant in the Type 2 jasperoid samples. Figure 4 illustrates these 

differences graphically.

Type 2 jasperoid samples exhibit appreciably higher concentrations of all the elements shown in figure 4, except boron and zirconium, than do those of Type 1. The samples of Type 3 jasperoid are 

noticeably depleted in sodium, boron, and zirconium, relative to Type 

2 jasperoid, but they have higher concentrations of the other 11 

elements shown in figure 4.

The concentration of tungsten in Type 3 ferruginous jasperoid on 

the ridge between Homestake Creek and the Eagle River canyon in 

the northern part of the area is illustrated in figure 5, which also
TABLE 3. Measures of central tendency and dispersion in three 

[Med., median; G.M., geometric mean; G.D., geometric deviation; N, not detected at 

parentheses); , could not be determined from avail

Element 1

Fe

Mg

Ti

Mn

As

B

Ba

Be

Co

Cr

Cu

Ga

Ge

Hg

Li

Mo

Nb

Ni

Pb

Sb

Sc

Sr

V

W

Y

Zn

Zr

Normal 2 

background 

Med. (ppm)

7,000

700

200

80

N(<700)

N(<15)

100

N(<.5)

N{<1)

7

15

N(<1)

N(<5)

.2

N(<30)

N(<2)

N(<10)

4

15

N(<70)

N(<3)

17

7

N(<150)

N(<10)

N(<200)

15

Type 1

Med. (ppm)

3,000

700

200

50

N{<700)

15

200

1

N(<1)

5

10

2

N(<5)

.3

N(<30)

N(<2)

N(<10)

7

15

N(<70)

N(<3)

10

7

N(<150)

N(<10)

N(<200)

20

(48 samples)

G.M. (ppm)

3,600

850

220

47

N

15

230

1.1

1.8

6.1

9.9

2.3

5.2

.34

25

1.3

4.4

5.7

13

N

N

13

6.2

N

N

N

15

G.D.

3.7

4.9

3.0

3.6

 

1.4

10.0

2.3

2.6

2:3

2.6

1.5

2.1

6.9

1.9

1.8

1.6

2.8

2.9

 

3.6

2.4

 

 

....

3.4

Type 2

Med. (ppm)

15,000

500

300

100

N(<700)

N(<15)

200

5

2

7

17

N(<1)

15

.15

N(<30)

N(<2)

N(<10)

15

15

N(<70)

N(<3)

15

15

200

N(<10)

N(<200)

20

(43 sa

G.M. (

15,00

64

28

9

46

1

38

1

1

1

1

1

7

1

1

13

1

J Hg by atomic absorption analysis, all other elements by six-step spect

2 From reference suite of >500 jasperoid samples representing >100distri

(T. G. Levering, unpub. data, 1978).
MINOR ELEMENT DISTRIBUTION IN JASPEROID 19

shows a few sample localities of tungsten-bearing fractures in quartzite and Precambrian rocks. All the localities from which multiple 

samples averaged 0.2 percent tungsten or more are within this area. 

Three localities to the southeast yielded samples averaging more 

than 0.05 percent tungsten; one of these is at the mouth of McAllister 

Gulch, one between McAllister Gulch and the mouth of Resolution 

Creek, and one at the mouth of Resolution Creek. Tungsten concentration declines rapidly eastward in jasperoid samples from the 

Resolution Creek area, and tungsten was not detected in core 

samples from the drill hole in McAllister Gulch nor in those from 

upper Resolution Creek.

Anomalous tungsten concentrations are not confined to jasperoid 

replacing carbonate rocks. Samples of siliceous brown limonite in 

veins from fracture zones cutting the Sawatch Quartzite also yielded 

anomalous tungsten values: for example, tungsten anomalies were 

found on (1) the west side of the ridge between Homestake Creek 

and the Eagle River, within and close to the strong northeasterly 

trending fault that crosses this ridge; and (2) east of this ridge in 

the Eagle River canyon between McAllister Gulch and Silver Gulch. 

A sample of limonite-impregnated fault breccia also contains 

anomalous tungsten; it was collected along the northeasterly trending fault where it cuts Precambrian rocks just below the 

Sawatch Quartzite on the southwest side of the ridge. The presence of 

the anomalous tungsten in the fault breccia suggests that fissures in 

the basement, not just in the sediments, were involved in the 

mineralization.

A comparison was made of the distribution of minor elements in 

samples of jasperoid from the Pando area with those in jasperoids 

from the three closest areas to Pando that have numerous jasperoid 

bodies (fig. 1). These are the Red Cliff district, about 3 km to the 

north; a mineralized area north of Tennessee Pass and east of Piney 

Creek, 5 to 7 km to the south; and the high Gore Range from 5 to 

6V& km east (fig. 1; table 4).

The Red Cliff district is a small silver-lead district that has 

jasperoid associated with replacement orebodies in the lower and 

middle Paleozoic carbonate rocks. The area north of Tennessee Pass 

contains a few small gold mines and abundant prospects in this same 

carbonate section, and here also, mineralization appears to be closely
20 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

Concentra 

PPM 

10,000

5.000

1.000 

500

100 

50

10

e 

1st

As Ba

2d 3rd 1st 2d 3rd 1st

i

i i

i i

Sb W

2d 3rd 1st 2d 3rd

i i

i

i I

i I

Concentrate Mn Ge Ni

PPM

1,000 

100

10

1.0

m

1 t 2

i

d 3

i

d 1st 2

"}-|

i

d 3rd

- 4 =f -

I

1s t 2

I

i 3r

I

d 1s 2

i

1

i 3r

1

1

1 1st 2d 3rd

I I

 

I I

FIGURE 4 (above and facing page). Geometric means and deviations of selected 

elements in samples of three generations of jasperoid from the Pando area.

associated with silicification; total production from this area has 

been only about $250,000. Jasperoid in the high Gore Range locally 

replaces dolomite beds in the upper part of the Minturn Formation, 

more than a 1,000 m above the Devonian and Mississippian rocks
MINOR ELEMENT DISTRIBUTION IN JASPEROID 21

Concentrate Be Mb Sc Y

PPM 1st 2d 3rd 1st 2d 3rd 1st 2d 3rd 1st 2d 3rd 

100

1.0

0.1

EXPLANATION

Geometric mean 4> Geometric deviation

Below limit of detection in more than 75 percent of samples

Limit of detection

Value assigned to "not detected"

 Estimated normal background concentration (table 4)

that contain jasperoid in the other two areas and at Pando. There 

are no mines in this part of the Gore Range, although the Kokomo 

mining district, about 3 km farther to the east, has produced lead, 

zinc, and silver from orebodies in carbonate beds of the upper part of 

the Minturn Formation.

Table 4 shows medians (Med), geometric means (G.M.), and 

geometric deviations (G.D.) for 28 elements in the entire group of 

jasperoid samples collected from each of these four areas. A column 

of normal background (median) concentrations of each of these 

elements, derived from a reference file containing the analyses of 

more than 500 jasperoid samples representing more than 100 mining 

districts and mineralized areas in the United States (T. G. Lovering, 

unpub. data, 1978), is also included for comparison.

Median concentrations of titanium, barium, beryllium, antimony, 

tungsten, and zirconium are appreciably higher in the Pando samples 

than in those from the other three areas, and germanium is also high. 

High values for silver, boron, germanium, lead, and zinc characterize 

the Red Cliff area jasperoids. Samples from the area north of Tennessee Pass showed distinctively higher concentrations of gold and 

the rare earth elements lanthanum and yttrium than those from the 

other areas. The median concentration of arsenic is high in samples
22 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

106°20'

EXPLANATION 

o Drill core sample locality 

 Outcrop sample locality

Samples averaging ^0.2 percent W 

) Samples averaging >0.05 percent W

2 MILES

FIGURE 5. Sample locality map of Pando area showing tungsten anomalies.
TABLE 4. Measures of central tendency and dispersion of selected elements injasperoid 

[Med., median; G.M., geometric mean; G.D., geometric deviation; I G.D. Range, I geometric deviation ranges; N, not det

theses); , could not be determined from available data]

Element 1

Ti

Na

Mn

Ag

As

Au

B

Ba

Be

Co

Cr

Cu

Ga

Ge 

La

Li

Mo

Nb

Ni

Pb

Sb

Sc

Sr

V

W

Y

Zn 

Zr

Normal 2 

background 

Med.

150

30

100

1

<100

<.01

5

150

<.3

<1

15

30

<1 

<5

<10

<10

2

<3

6

20

<30

<1

10

10

<50

4

<30 

10

(200

Med. 

(ppm)

250

N{150)

100

N(O.l)

N(100)

N(0.01)

N(7)

200

2

N(l)

7

10

N(1.5)

15

N(10)

N(D

N(3)

7

15

N(30)

N(D

15

10

100

N(2)

N(30) 

20

Pando 

samples, 64

G.M. G 

(ppm)

289 3

229 2

103 4

.11 1

183 3

.011 1

12.4 1

333 6

2.6 4

2.5 3

7.9 2

15.9 2

2.4 1

7Q )

/ . 0 J

7.1 2

17.4 2

1.4 2

4.3 2

7.2 3

13.6 2

63.4 2

1.7 2

15.5 3

7.2 4

98.2 5

5.1 3

CO O 1

Do . J 1

18.0 5

local ities)

I G.O. 

.0. Range 

(ppm)

.2

.5

.1

.7

.4

.6

.8

.6

.6

.0

.5

.8

.9

.9 

.1

.1

.0

.7

.1

.5

.8

.4

.8

.6

.7

.1

.7 

.5

90-925

N-570

30-420

N-.2

N-620

N-.02

>22

50-2,200

0.6-12

N-7.5

3-20

6-45

N-5

N-30 

N-15

N-37

N-3

N-12

N-22

5-34

N-180

N-4

N-60

N-33

N-560

N-16

N-100 

N-100

Red Cliff N. of Te

(31 samples, 14 localities) (32 samples

I G.D. 

Med. G.M. G.D. Range Med. G.M

(ppm) (ppm) (ppm) (ppm) (ppm

150 224 3.5 64-780 150 101 

N 156 2.0 N-310 N 215 

70 66 4.0 15-26 150 171 

2 2.1 14.1 N-30 N .3

N 134 2.5 N-335 N 223 

N .02 3.8 N-.076 N .0

15 16 2.7 N-43 N 9.3 

50 58 1.6 35-93 60 60 

.8 1.0 2.7 N-2.7 N .5 

N 1.8 2.5 N-4. 5 N 2.0 

7 6.6 2.8 2-18 7 4.6 

10 10.0 3.7 3-7 15 17 

N 1.7 2.1 N-3. 5 N 1.9 

N 7.7 2.4 N-18. 5 N N 

N N N 10.4 

N N N N 

N 1.3 1.8 N-2.3 N 1.1 

N N N 4.1 

5 4.8 3.5 N-17 4 5.6 

15 29 7.1 4-210 10 7.5 

N 41 2.7 N-110 N N 

N 1.3 1.9 N-2.4 N 1.3 

10 67 1.9 N-13 7 9.1 

7 6.6 3.8 N-25 8 9.2 

N(50) N N N 

N 3.7 1.5 N-5. 6 10 9.4 

N 92 3.8 N-350 N . N 

7 14 4.5 N-63 8 12.3 

'Au by fire assay and atomic absorption, all other elements by six-step spectrographic analysi

Calculated from data on >500 samples representing 2,100 districts and mineralized areas in th
24 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

from all areas except the Gore Range. The Gore Range samples have 

higher median concentrations of sodium, manganese, and strontium 

than the samples from the other three areas, but are lower in most of 

the other minor elements, particularly titanium. These differences in 

the minor element content of jasperoids from the four areas suggest 

different sources for the silica in each of them; differences caused by 

differing compositions of the host rock are not likely, since all but the 

small group from the Gore Range represent siliceous replacement of 

the same stratigraphic units, and in all four areas the same rock type 

(dolomite) was replaced.

The distribution of elements in Type 1 jasperoid at Pando (table 3) 

strongly resembles that of the suite of samples from the Red Cliff 

district (table 4), except that the Pando samples contain more barium 

and less lead, zinc, and silver than do the Red Cliff samples. Most of 

the samples from Red Cliff also resemble the Type 1 Pando 

jasperoids in appearance, being dominantly dense, medium-gray 

rocks.

ASSOCIATIONS OF ELEMENTS

Four different types of samples from the Pando area were analyzed 

spectrographically. The largest group is composed of jasperoid of 

each of the three major types previously discussed. In addition to the 

jasperoid samples, we have analyzed whole-rock samples of Tertiary 

intrusive dike and sill rocks, samples of mineralized shear and fracture zones in the more siliceous impermeable rocks below the Dyer 

Dolomite, and samples of limonite concentrates extracted from rocks 

of various kinds along and near the northeast-trending fault that 

crosses the Homestake Creek-Eagle River divide. Minor element 

associations that characterize each of these sample types are summarized in table 5.

The 48 Type 1 jasperoid samples show no consistent minor element 

associations. The only elements that displayed anomalously high 

concentrations in more than 10 percent of the samples are barium, 

titanium, mercury, and cobalt. Neither the barium-rich samples nor 

the titanium-rich samples showed any other element consistently 

present in abnormally high amounts, and some high-barium and 

high-titanium samples show no other minor elements present in 

greater than normal amounts. Most of the samples with >1 ppm mercury and 11 of the samples with >5 ppm cobalt are anomalous in at 

least one other element, but neither mercury nor cobalt shows any 

consistent associations. Thus, the most conspicuous feature of the 

group is its general lack of anomalies.
ASSOCIATIONS OF ELEMENTS 25 

TABLE 5. Associated minor elements in samples of various types from the Pando area

Type of sample material Characteristic minor element associations

Type 1 jasperoid - None.

Type 2 jasperoid Fe-Ni-Sc-Sb; Be-Ge-W; Co-Pb; Cu-V. 

Type 3 jasperoid As-Be-Nb-Y; Co-Mo-Mn-Ni; Ge-H; Ni-Co. 

Tertiary igneous Ba-Sr-Pb.

intrusive rocks. 

Veins and shear zones in (A) As-Be-Ge-Sb-W; (B) Cr-La-Sr-Ti;

Precambrian rocks and (C)' Ni-Sc-V-Y.

Sawatch Quartzite. 

Limonite concentrate from (D) As-Be-Ge-Nb-Sb-W; (E) La-Zn;

veins and fractures. (F) Cr-Ni-V; (G) Co-Cr-Ni; (H) Cr-Cu-Ni.

The group of 43 samples of Type 2 jasperoid, in strong contrast to 

the Type 1 jasperoid group, displays high concentrations of many 

minor elements in a large proportion of the samples. Beryllium, germanium, tungsten, nickel, iron, scandium, and antimony are 

anomalous in more than 30 percent of these samples. A statistical 

qualitative test of association (the chi square test) reveals highly 

significant associations among two groups of elements (Fe-Ni-Sc-Sb 

and Be-Ge-W); and also between two pairs of elements (Co-Pb and 

Cu-V) in samples representing Type 2 jasperoid.

The late ferruginous Type 3 jasperoid is represented by 18 samples. 

More than 30 percent of these samples display anomalously high concentrations of tungsten, iron, arsenic, barium, beryllium, cobalt, germanium, niobium, nickel, antimony, and scandium. This suite of 

elements includes all those that are characteristically high in the 

Type 2 jasperoid, with arsenic, barium, cobalt, and niobium in addition. A chi square test of element associations in samples of Type 3 

jasperoid shows a highly significant mutual association within the 

group Be-As-Nb-Y, and also between the element pairs Ge-W and 

Ni-Co. Molybdenum is anomalous in 20 percent of the samples in this 

group, and shows a strong affinity for manganese, cobalt, and nickel. 

Copper and vanadium are present in normal background concentrations in these samples, as contrasted with samples representing 

Type 2 jasperoid, in which these two elements were anomalous and 

strongly associated in several samples. Barium is also abundant in 

the Type 3 jasperoids, but it shows no significant correlation with 

any of the other elements.

Whole-rock analyses were made on 25 samples of Tertiary porphyry dikes and sills that intrude the Precambrian basement rocks, 

the Cambrian Sawatch Quartzite and overlying Peerless Formation, 

and the Pennsylvanian Belden Formation. Many of these intrusives
26 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

exhibit intense sericitic alteration, but they all appear to have consisted originally of quartz, sodic plagioclase, and biotite phenocrysts 

in a groundmass of fine-grained quartz and orthoclase, with minor accessory magnetite, apatite, sphene, and zircon.

The least altered dike samples contained anomalously high concentrations of barium (0.1-0.2 percent), strontium (0.07-0.1 percent), 

and lead (0.005-0.007 percent), although the intensely altered 

samples closest to bodies of mineralized jasperoid showed much 

lower concentrations of these elements. Barium, strontium, and lead 

were probably originally present largely in the feldspars, and the 

hydrothermal sericitization of these minerals may have released 

much of the barium to the altering solutions and may thus account 

for the abundant barite in some of the jasperoid. Biotite separated 

from six of the least altered samples showed high concentrations of 

titanium, lead, and zinc, plus the rare earth elements cerium, lanthanum, neodymium, and yttrium, compared to normal biotite from 

felsic intrusive rocks. Neither the less altered whole-rock samples nor 

their biotite showed anomalous concentrations of tungsten or 

beryllium, or of germanium, niobium, and scandium, with which 

tungsten is closely associated in the mineralized jasperoid. However, 

tungsten, beryllium, and germanium do show up in sericite concentrates from some of the samples of intensely altered intrusive rocks 

in the mineralized area.

Forty samples taken from small veins and mineralized shear zones 

that cut the Sawatch Quartzite and the Precambrian basement rocks 

were also analyzed spectrographically. These samples were taken at 

15 localities in the Eagle River valley and on the ridge between 

Homestake Creek and the Eagle River to provide information on the 

assemblage of elements that had migrated along channelways 

through the impermeable and nonreactive rocks beneath the 

jasperoid bodies in the carbonate section.

Three groups of mutually associated elements appear in these 

samples. The first group (group A) includes arsenic, beryllium, germanium, antimony, and tungsten. The second group (group B) consists of titanium, lanthanum, chromium, and strontium; this group 

shows some affinities with a third group (group C) consisting of 

nickel, scandium, vanadium, and yttrium, but neither group B nor C 

shows any significant association with elements in group A, although 

individual samples contain elements found in the other groups.

Small veins cutting Precambrian rocks on both sides of the Eagle 

River valley at the south end of Camp Hale are characterized by 

anomalous concentrations of the elements in groups B and C. 

Anomalous, as used here, means concentrations in the upper decile of 

the normalized distribution of the reference collection of jasperoid
ASSOCIATIONS OF ELEMENTS 27

samples, median values of which are given in tables 3 and 4. Veins 

cutting Sawatch Quartzite along the Eagle River west of the mouth 

of McAllister Gulch are high in group B elements and slightly high in 

group A elements. A mineralized fracture zone cutting Sawatch 

Quartzite in the Eagle River canyon south of the mouth of Silver 

Gulch is high in group C elements and slightly high in group A 

elements. A fracture zone in Sawatch Quartzite on the west side of 

the Homestake Creek-Eagle River divide approximately IVfc km west 

of this locality, is high only in elements of group A. However, fractures and shear zones cutting both Sawatch Quartzite and Tertiary 

porphyry intrusives about 100 m northwest of this locality are high 

only in elements of groups B and C. (See fig. 2 for sample locations.)

Several elements, although present in anomalous concentrations in 

many samples, show no significant associations with any other 

elements in the vein samples. These independent elements are Mn, 

Ba, Co, Cu, Li, and Nb.

Samples of Tertiary porphyry, Sawatch Quartzite, and Precambrian basement complex were collected along the northeast-trending 

fault zone that crosses the Homestake Creek-Eagle River divide and 

marks the northern boundary of the mineralized area. Samples from 

quartzite and porphyry on the east side of the ridge and across the 

top of it are high in elements of group A (As, Be, Ge, Sb, and W). 

Those from quartzite just above the Precambrian on the west side are 

anomalous in group A elements and group C elements (Ni, Sc, V, and 

Y), and the southwesternmost sample of this series from Precambrian host rock shows anomalous concentrations of elements belonging to group B (Ti, La, Cr, and Sr) and not group A.

Fracture fillings at seven sample localities on the Eagle 

River-Homestake Creek divide at the north end of the mineralized 

area consist largely of siliceous brown limonite very similar in appearance to the ferruginous tungsten-bearing jasperoid that forms 

replacement bodies in the overlying carbonate section. Iron oxide 

was selectively leached from these samples with oxalic acid and 

analyzed separately for comparison of minor element associations 

with those of the Type 3 ferruginous jasperoids.

This limonite concentrate (table 5) shows strong mutual affinities 

among arsenic, beryllium, germanium, niobium, antimony, and 

tungsten (group D). All these elements correlate negatively with lanthanum and zinc (group E), which show a strong positive correlation 

with each other. Chromium, nickel, and vanadium (group F) also 

show a strong mutual affinity, but none of these three elements correlates significantly with those of group D. Cobalt shows significant 

associations with chromium and nickel, but not with vanadium. 

Copper also correlates most strongly with chromium and nickel but
28 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

not with cobalt or vanadium. Lead shows barely significant positive 

correlations with copper and yttrium; and zinc shows a moderately 

strong correlation with nickel in addition to its strong correlation 

with lanthanum.

Although neither the Type 1 jasperoid nor the igneous intrusive 

rock samples shows any geochemical similarities to the other types of 

samples, the other four types do show common affinities, particularly 

in the elements associated with tungsten. The absence of any significant association among the common base metal elements, copper, 

lead, and zinc, in any of these sample groups is unusual, as these 

three elements normally show significant intercorrelations in 

jasperoid or mineralized fracture zones that are related to base metal 

sulfide deposits.

The jasperoid bodies in Leadville and Dyer Dolomites exposed on 

the hills around the Eagle River valley near the townsite of Pando, 

exhibit local variations in color and texture. Although the Type 2 and 

Type 3 jasperoids show certain similarities, the Type 1 jasperoid differs so markedly in distribution, appearance, and composition from 

the other two as to suggest that it was derived from a different 

source. A brief recapitulation of the distribution and distinguishing 

characteristics of each of these types is given in the following 

paragraphs, to provide a basis for an hypothesis concerning their 

origin.

Type 1 jasperoid is widely distributed throughout the surrounding 

region. Bodies of this type are present in the Devonian and Mississippian carbonate rocks in the Eagle River drainage basin, at intervals, 

all the way from the north side of Tennessee Pass to the flanks of the 

Sawatch Range west of Minturn, and also in limestone and dolomite 

beds high in the Minturn Formation in the Gore Range near the 

heads of Pearl and Cataract Creeks.

The distribution of Type 2 jasperoid is shown in figure 3. This type 

of jasperoid is abundantly represented in exposures near the south 

end of the Eagle River canyon and eastward to the mouth of 

McAllister Gulch. It also occurs in large bodies both north and south 

of the mouth of Resolution Creek, although jasperoid recovered from 

a drill core to the east along Resolution Creek is of the early gray 

variety (Type 1, fig. 3).

The late brown variety of jasperoid (Type 3, fig. 3) is still more 

restricted in distribution. It crops out in abundance on the divide between Homes take Creek and the Eagle River from the boundary fault 

southward for about 1 km and is also common in association with in-
GENESIS OF JASPEROID 29

termediate stage jasperoid from the south end of the Eagle River 

Canyon to the mouth of Resolution Creek.

The three generations of jasperoid also show distinctive differences 

in appearance, mineralogy, and composition (tables 2, 4). The progressive increase of arsenic, antimony, beryllium, germanium, and 

tungsten from near normal background values in the Type 1 

jasperoid, through intermediate concentrations in Type 2, to concentrations an order of magnitude or more above background in the 

Type 3 jasperoid suggests that the second and third generations may 

be derived from a common source, different from the one responsible 

for the first generation jasperoid. The decrease in sodium, boron, and 

zirconium in Type 3 jasperoid probably reflects a marked difference 

in depositional environment. The porous, vuggy texture and the 

abundance of manganese oxide and hydrous iron oxide characteristic 

of this late jasperoid suggest deposition under low-pressure, nearsurface conditions.

The presence of limonite veinlets, which are similar both in appearance and composition to Type 3 jasperoid, cutting Sawatch 

Quartzite and Tertiary intrusive rocks well below the base of the carbonate section, suggests that Type 3 jasperoid is not supergene, but 

was formed by solutions moving upward along feeding channels.

Isotopic analysis of oxygen derived from jasperoid representing 

each of the three generations of Pando jasperoid was performed on 

four selected samples (one sample of each type, plus one sample of 

Type 1 breccia fragments and the Type 2 matrix that cements them). 

Such analytical data on a larger group of representative samples of 

each type is needed to firmly establish any conclusions. Nevertheless, 

the results from these four samples, which are presented in table 6, 

suggest that some real differences may exist in the oxygen isotope 

ratios in the jasperoid of each of the three types.

The two samples of Type 1 jasperoid, from localities several 

kilometers apart, yield very similar dO18 values. The two samples of 

Type 2 jasperoid come from within less than two kilometers of each 

other; not only do they yield dO18 values less than half those of the

TABLE 6. Oxygen isotopic analysis of silica from Pando jasperoid

Sample No. Description 60 18 SMOH

20T49

VI21A-1

VI21A-2

52T47

VI34D

Type 1 jasperoid --- --

Type 1 jasperoid in Type 2 matrix-----

Type 2 jasperoid matrix of VI21A -

Type 2 jasperoid----------------------

Type 3 jasperoid -- 

+21.7

+22.7

+ 7.4

+10.1

+ 4.3
30 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

Type 1 samples, but they also have a much greater variation in dO18 

values. Sample 52T47, of Type 2 jasperoid, was taken on the north 

side of the large fault crossing the ridge between Homestake Creek 

and the Eagle River, adjacent to an altered body of Tertiary quartz 

latite porphyry and within 150 meters of the Type 3 sample VI34D, 

which came from a jasperoid body in the Leadville Dolomite on the 

south side of this fault. The Type 3 jasperoid sample has a <50 18 value 

less than half that of the nearby Type 2 sample.

In summary, the Type 1 jasperoid in the Pando area differ so 

strikingly in distribution, appearance, mineralogy, chemical composition, and oxygen isotopic ratios from the Type 2 and Type 3 

jasperoids as to indicate that the source of the first type was separate 

and unrelated to the source of the other two. The Type 2 and Type 3 

jasperoids, on the other hand, show many similarities and may have 

been derived from the same source, but formed in different physical 

chemical environments. The low dO18 ratio of the type 3 sample suggests that the solution that deposited this silica was the hottest. If 

so, this hot solution may have contained a larger proportion of 

magmatic components or may have been exposed to greater heat 

than the fluid that deposited jasperoids Types 1 and 2.

Fluids that formed the Type 1 jasperoid probably emanated from a 

large, regional igneous source many kilometers to the east or northeast, under the Gore Range, possibly the same source from which 

some of the Tertiary porphyry sills in the upper Paleozoic rocks of 

these mountains were derived at a slightly earlier time. The fluids 

may have moved updip in the carbonate section or more directly upward along fault zones for a considerable distance from their source 

until they had cooled sufficiently to replace carbonate rock with 

silica. This process appears to have occurred under stable conditions, 

such that isotopic equilibrium was established between the silica and 

the solutions from which it was deposited. The solutions were probably hot, either neutral or alkaline, and charged with silica, 

potassium, iron, and sulfur, and they may have locally contained base 

and precious metals as well. The jasperoid bodies appear to have 

formed at considerable depth.

Jasperoids in the Oilman district, to the north of Pando, form barren replacement bodies in the carbonate section both updip and 

downdip from the orebodies and also along faults cutting limestone 

beds in the Minturn Formation more than 600 m above an orebody. 

Most of these Oilman jasperoids resemble in appearance and composition the Type 1 jasperoids at Pando, and samples of Oilman 

jasperoid taken from bodies closest to orebodies contain traces of 

silver as well as slightly anomalous concentrations of lead and zinc,
GENESIS OF JASPEROID 31

suggesting that these jasperoid bodies may represent a fringe zone 

type of alteration that is genetically related to mineralization.

The Type 1 jasperoids in the Pando area may have a relation to 

sulfide mineralization similar to that of the jasperoids at Oilman. If 

so, the occurrence of scattered sphalerite in some samples of Type 1 

jasperoid in outcrops along the eastern edge of the valley at Pando, 

considering that the jasperoid both downdip to the east and updip to 

the west is barren, suggests that orebodies once could have existed in 

the carbonate section above the Eagle River valley; and remnants of 

these bodies may still exist in some places, such as the vicinity of 

Ranch Creek, where composite samples of Type 1 and 3 jasperoid 

from the southeasternmost jasperoid body exposed in the Pando area 

(figs. 2, 3) yielded anomalously high concentrations of copper, lead, 

and zinc.

The Type 2 jasperoids appear to have been derived from a local 

source, beneath the valley of the Eagle River near Pando and beneath 

the southern end of the divide between Homestake Creek and the 

Eagle River northwest of Pando. The earlier subtypes of Type 2 

jasperoid contain both sericite and limonite pseudomorphs after 

pyrite, as do the altered Tertiary intrusives beneath the carbonate 

section northwest of Pando. This evidence suggests a common 

source, and also that this source was probably quite deep.

Both pyrite and sericite suggest formation of Type 1 and Type 2 

jasperoid from alkaline or neutral solutions in a reducing environment. However, a change in this environment, before the earliest subtype of Type 2 jasperoid had become completely impermeable by 

lithification, is suggested by the oxidation of early pyrite to hematite 

and by the iron-stained character of the sericite.

During the next stage of formation of Type 2 jasperoid, the hot 

solutions apparently precipitated iron in an oxidizing environment as 

hematite dust in the jasperoid quartz and as irregular masses and 

veinlets of hematite cutting the quartz. This change from pyrite to 

hematite was probably not brought about by a deficiency of sulfur in 

the system, because barite is a common accessory mineral in these 

rocks, although most of the barite is late.

The final stage in formation of Type 2 jasperoid is marked by 

disseminated limonite particles in quartz, imparting pale-yellow, 

pale-orange, and pale-brown colors to the rock. This change from 

anhydrous to hydrous ferric oxides seems to indicate formation at 

lower temperatures.

Changing conditions during the formation of second generation 

jasperoid are reflected not only by these changes in mineralogy and 

by a sporadic and irregular distribution of the various subtypes, but
32 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

also by considerable variation both in the minor element content and 

the dO18 isotope values of these rocks over short distances.

Type 3A jasperoid, which contains abundant red hematite, forms a 

matrix for breccia fragments of second generation jasperoid that contain disseminated limonite in some places. This circumstance suggests that during the initial stage of formation of Type 3 jasperoid, 

hot silica and iron-bearing fluids locally invaded and engulfed masses 

of Type 2 jasperoid that had formed in a cooler environment. 

Elsewhere in the district, fragments of Type 3A jasperoid are 

cemented by a matrix of Type 3C jasperoid. The open vuggy texture, 

colloform banding, and abundance of limonite, goethite, and 

manganese oxides that characterize the youngest, tungsten-rich, 

Type 3C jasperoid all suggest that it must have formed in an epithermal environment.

The anomalously high concentrations of beryllium, niobium, and 

tungsten, elements commonly found in high-temperature deposits, in 

close association with hydrous iron oxide, arsenic, and antimony, 

suggest a very steep temperature gradient in the feeding fissures; 

such conditions are typical of deposition in a shallow epithermal environment. The low dO18 value of silica from a sample of Type 3 

jasperoid (+4.3) may reflect deposition from a very hot solution.

The tungsten-rich jasperoid at Pando is probably not related to the 

Pando porphyry, because veinlets of silicious brown limonite similar 

in composition to this jasperoid cut a sill of hydrothermally altered 

and pyritized Pando porphyry close to the area in which the Type 3 

jasperoid is most abundant. This porphyry has an approximate age

of 70 million years (Pearson and others, 1962); it is the oldest of the 

Laramide intrusives in this area (Tweto, 1974) and is the only porphyry exposed in the immediate vicinity of the area of Type 3 

jasperoid mineralization. The ultimate source of Type 3 jasperoid is 

probably a buried intrusive as yet unknown, which provided some 

elements but also mobilized heated meteoric solutions that supplied 

many of the more common elements, such as silicon, magnesium, 

iron, and potassium.

An Oligocene rhyolite porphyry sill (approximately 29 m.y. old) 

(Tweto, 1974) is exposed at intervals just north of the East Fork of 

the Eagle River. The westernmost outcrop of this porphyry is within 

about 30 m of the southeasternmost exposure of jasperoid shown in 

figure 2. This jasperoid body consists of breccia fragments of Type 1 

jasperoid in a matrix of tungsten-bearing Type 3 jasperoid, but it is 

an isolated occurrence more than 6 km southeast of the main center 

of Type 3 jasperoid mineralization on the ridge between Homestake 

Creek and the Eagle River (fig. 3). Nevertheless, there is a possibility 

of a genetic relationship between the Oligocene magma that formed
GENESIS OF JASPEROID 33

- __

this porphyry and the late jasperoid of Types 2 and 3. Such a relationship is indirectly supported by Tweto's suggestion of an Oligocene 

age for the epithermal, iron-rich tungsten veins of Boulder County 

(Tweto, 1968a, p. 579), and also by the Oligocene age of the Climax 

stock about 16 km southeast of Pando, which is genetically related to 

both molybdenum and tungsten mineralization (Wallace and others, 

1968, p. 615).

If a pluton similar in age and composition to the Climax stock had 

been emplaced in the Precambrian rock beneath the upper valley of 

the Eagle River, siliceous fluids rising from it along northeastwardtrending fractures and shear zones could have formed Type 2 

jasperoid bodies in the carbonate rocks of the Pando area. The period 

of intrusion could also have accounted for the large body of Oligocene 

rhyolite porphyry in the lower part of the Minturn Formation that is 

shown on the geologic map of the Holy Cross quadrangle (Tweto, 

1974) to the southeast of the jasperoid-bearing area.

This genetic hypothesis for the formation of Type 2 and 3 jasperoid 

leaves at least two serious questions unresolved: (1) Why are there no 

dikes or sills of Oligocene porphyry in the vicinity of the areas of 

abundant jasperoid east and north of Pando? (2) How could bodies of 

limonite-bearing jasperoid have formed under the temperatures and 

pressures that must have existed in Oligocene time, when the midPaleozoic carbonate section was presumably buried beneath 2 or 

3 km of younger rock? Wallace and others (1968, p. 639) estimated 

that approximately 9,000 feet (3 km) of post-Oligocene uplift and erosion were required to expose the Climax stock and its associated 

mineral deposits; and this stock is only about 16 km southeast of 

Pando. Although the Type 2 j asperoid bodies might conceivably have 

formed at such a depth, it is unlikely that the open, vuggy, coloformbanded, epithermal jasperoid bodies of Type 3 could have done so!

There is one younger episode of tectonic activity and igneous intrusion in this region to which the formation of Type 3 jasperoid, and 

possibly of Type 2 jasperoid, might be related. Rhyolite dikes and a 

rhyolite breccia plug that postdate oxidation of the orebodies at 

Leadville are present in the southern part of the Holy Cross 

quadrangle, are thought to be of late Miocene or Pliocene age, and are 

intruded at shallow depth (Tweto, 1974). By the end of Miocene time, 

erosion had carved canyons hundreds of meters deep into bedrock 

below Leadville; canyons that were subsequently filled and buried 

beneath younger sediments (Tweto, 1968b, p. 686). It seems probable 

that this Miocene erosion had also removed much of the younger rock 

overlying the carbonate section at Pando, although there is no direct 

evidence of this.
34 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

Although no intrusive as young as Miocene has been recognized at 

the surface in the Pando area, a blind intrusive of this age could be 

present beneath the divide between the Eagle River and Homestake 

Creek. The presence of such an intrusive would provide a logical explanation for the epithermal, ferruginous Type 3 jasperoid, which is 

rich in tungsten, beryllium, and niobium, and such an intrusive could 

be responsible for the Type 2 jasperoid as well. The peculiar suite of 

minor elements in the Type 3 jasperoids could have been remobilized 

from older deep-seated deposits related to an Oligocene pluton. 

Possibly this suite of elements was carried upward in fluids 

generated by the emplacement of a shallow upper Miocene or 

Pliocene intrusive, which lies within a tectonically active belt in the 

northward extension of the San Luis-Arkansas Valley graben. This 

structure, which has been traced northward from the Rio Grande 

graben of New Mexico as far as the Continental Divide north of Leadville, has been marked by recurrent fault movement throughout the 

late Tertiary Period. Epithermal fluorspar deposits, hot springs, and 

geothermal areas are localized along this structural trough (Tweto, 

1968a, p. 566, 582).

An alternative explanation worth considering is that the Type 3 

jasperoids are entirely of supergene origin and represent deposition 

of elements leached from deposits of Oligocene age by ground water. 

However, this hypothesis is somewhat discredited by the complete 

absence of tungsten anomalies to the east of Pando, by the presence 

of what appear to be feeding fractures filled with limonite cutting the 

quartzite beneath the carbonate section, by the isotopic evidence that 

this jasperoid was deposited form hot solutions, and by the general 

lack of mobility of tungsten ions in ground water. Although the 

tungsten oxide ion (WO;) is somewhat soluble in neutral and alkaline 

aqueous solutions at low temperature and pressure, in the presence of 

either iron or calcium, it readily precipitates as insoluble tungstates 

of these elements (Garrels, 1960, p. 184-185).

SUGGESTIONS FOR PROSPECTING

Despite the uncertainty regarding the age and ultimate source of 

the Type 3 tungsten-bearing jasperoid in the Pando area, the 

available evidence suggests that the fluids from which it formed 

moved upward through the lower Paleozoic sedimentary rocks from a 

concealed source below. If this hypothesis is correct, there is a 

reasonable possibility that a large felsic intrusive body, from which 

fluids bearing tungsten, beryllium, germanium, and niobium 

emanated, underlies the southern part of the Eagle 

River-Homestake Creek divide northwest of Pando and the adjacent
SUGGESTIONS FOR PROSPECTING 35

part of the Eagle River valley to the east at least as far south as the 

mouth of Homestake Creek. Mineable veins of ferberite, and possibly 

deposits of beryl, either in pegmatites or disseminated, might 

reasonably be expected to occur either in a greisen zone at the top of 

such an intrusive or in metasomatically altered wallrock adjacent to 

it.

The most favorable site for exploration is the highly altered 

tungsten-rich area on top of the ridge between Homestake Creek and 

the Eagle River, adjacent to the northeasterly trending fault and just 

east of the escarpment formed by the Sawatch Quartzite. Both the 

fault zone and the ground adjacent to it on the southeast could be explored. Drill holes at this locality should penetrate the source of the 

surface mineralization at a fairly shallow depth.

Two alternative drilling sites that are somewhat less favorable 

geologically, but more readily accessible, are (1) at the mouth of 

McAllister Gulch north of the old gravel pit and (2) near the road 

junction at the mouth of Resolution Creek. Masses of tungstenbearing jasperoid in the Leadville Dolomite are exposed near the 

mouth of McAllister Gulch, across the nose of the adjacent ridge to 

the west, and on the north side of the mouth of Resolution Creek. 

Although the tungsten anomalies in jasperoid at these places are 

smaller than the one on the divide between Homestake Creek and the 

Eagle River, they are still appreciable and are highest in the outcrops 

closest to the valley of the Eagle River.

REFERENCES CITED

Crawford, R. D., and Gibson, Russell, 1925, Geology and ore deposits of the Red Cliff 

district, Colorado: Colorado Geological Survey Bulletin 30,89 p.

Garrels, R. M., 1960, Mineral equilibria at low temperature and pressure: New York, 

Harper and Brothers, 254 p.

Levering, T. G., 1972, Jasperoid in the United States its characteristics, origin, and 

economic significance: U.S. Geological Survey Professional Paper 710,164 p.

Pearson, R. C., Tweto, Ogden, Stern, T. W., and Thomas, H. H., 1962, Age of Laramide porphyries near Leadville, Colorado, in Short papers in geology and 

hydrology: U.S. Geological Survey Professional Paper 450-C, p. C78-C80.

Tweto, Ogden, 1953, Geologic map of the Pando area, Eagle and Summit Counties, 

Colorado: U.S. Geological Survey Mineral Investigations Field Studies Map 

MF-12[1954].

___1968a, Geologic setting and interrelationships of mineral deposits in the mountain province of Colorado and South-central Wyoming, in J. D. Ridge, ed., Ore 

deposits of the United States, 1933-1967 (Graton-Sales Volume), v. 1: New York, 

American Institute of Mining, Metallurgical, and Petroleum Engineers, 

p. 551-588. 

_1968b, Leadville district, Colorado, in J. D. Ridge, ed., Ore deposits of the

United States, 1933-1967 (Graton-Sales Volume), v. 1: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, p. 681-705.
36 JASPEROIDS, PANDO AREA, EAGLE COUNTY, COLO.

___1974, Geologic map and sections of the Holy Cross quadrangle, Eagle, Lake, 

Pitkin, and Summit Counties, Colorado: U.S. Geological Survey Miscellaneous 

Geologic Investigations Map 1-830, 2 sheets.

Tweto, Ogden, and Lovering, T. S., 1977, Geology of the Minturn 15-minute quadrangle, Eagle and Summit Counties, Colorado: U.S. Geological Survey Professional Paper 956, 96 p.

Wallace, S. R/., Muncaster, N. K., Jonson, D. C., MacKenzie, W. B., Bookstrom, A. A., 

and Surface, V. E., 1968, Multiple intrusion and mineralization at Climax, Colorado, in J. D. Ridge, ed., Ore deposits of the United States, 1933-1967 (GratonSales Volume): New York, American Institute of Mining, Metallurgical, and 

Petroleum Engineers, p. 605-640.

OU.S. GOVERNMENT PRINTING OFFICE: 1980-677-129/29