Precambrian Deposits of 

Zinc-Copper-Lead Sulfides 

and Zinc Spinel (Gahnite) 

in Colorado 

'l 

U,S. GEOLOGICAL SURVEY BULLETIN 1550 , 

Precambrian Deposits of 

Zinc-Copper-Lead Sulfides 

and Zinc Spinel (Gahnite) 

in Colorado 

By DOUGLAS M. SHERIDAN and WILLIAM H. RAYMOND 

U.S. GEOLOGICAL SURVEY BULLETIN 1550 

An interim report evaluating these 

Precambrian deposits as potential 

resources of base and precious metals 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984 
DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress Cataloging in Publication Data 

Sheridan, Douglas M., 1921-

Precambrian deposits of zinc-copper-lead sulfides and zinc spinel (gahnite) in Colorado. 

(Geological Survey bulletin ; 1550) 

Bibliography: 31 p. 

Supt. of Docs. No.: I 19.3 

1. Sulfides. 2. Geology, Stratigraphic-Precambrian. 3. Geology-Colorado. I. Raymond, 

William H. II. Title. III. Series. 

QE75.B9 no. 1550 557.3s [553'.09788] 82-600116 

[QE389.2] 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 
Abstract 

Introduction 

History ... 

CONTENTS 

Geologic setting and general character of the ores 

Deposits in rocks metamorphosed to lower amphibolite facies 

Deposits in the Gunnison area . . . . . . . . . . . 

Economic potential . . . . . . . . . . . . . . . . 

Deposits in rocks metamorphosed to upper amphibolite facies 

Deposits in the vicinity of Salida 

Gahnite and its significance . . . 

Economic potential . . . . . . . . 

Deposits in other areas in Colorado 

Distribution and significance of gahnite 

Economic potential 

References cited . . . . . . . . . . . . . . . . . . 

ILLUSTRATIONS 

FIGURE 1. Index map of western Colorado showing distribution of Precambrian 

Page 

1 

4 

5 

6 

6 

8 

9 

11 

15 

18 

21 

23 

27 

28 

Page 

rocks and Precambrian sulfide deposits mentioned in text . . 2 

2. Geologic map showing distribution of Precambrian sulfide deposits 

in the Gunnison area . . . . . . . . . . . . . . . . . . . . . . . 7 

3. Geologic map showing distribution of Precambrian sulfide deposits 

in the Salida area . . . . . . . . . . . . . . . . . . . . . . . 11 

4. Geologic map of the Sedalia mine area . . . . . . . . . . . . . . 13 

5. Photomicrograph of gahnite-bearing ore from the Sedalia mine . 14 

6. Photograph of polished slab of gahnite-biotite-quartz gneiss from the 

Bon Ton mine . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

7. Photograph of polished slab of gahnite-rich rock from the Bon Ton 

mine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 

8. Photomicrograph of gahnite-bearing garnet-biotite-quartz gneiss 

from the Bon Ton mine . . . . . . . . . . . . . . . . . . . . . 19 

9. Photomicrograph of gahnite-amphibole rock from Mine Gin the Wet 

Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

III 
IV CONTENTS 

TABLES 

Page 

TABLE 1. Analyses of Precambrian ores from the Powderhorn quadrangle . 9 

2. Analyses of Precambrian ores from the vicinity of Salida . . . . 16 

3. Zinc content of gahnite-bearing rocks from the vicinity of Salida . 20 

4. Analyses of Precambrian ores from other areas in Colorado . . 24 

5. Zinc content of gahnite-bearing rocks from other areas in Colorado . 25 
PRECAJMBRIAN DEPOSITS OF 

ZINC-COPlf»ER-LEAD SULFIDES AND 

ZINC SPINEl~ (GAHNITE) IN COLORADO 

By DOUGLAS M. SHERIDAN and WILLIAM H. RAYMOND 

ABSTRACT 

Precambrian sulfide deposits in the Southern Rocky Mountains in Colorado are being 

studied and reevaluated by the U.S. Geological Survey according to geologic concepts 

concerning ore-host rock associations, stratigraphic controls, and other factors. These 

concepts were developed elsewhere in recent years during successful research regarding 

economic massive sulfide deposits. Our work, initiated in 1974, has indicated that a 

new look at areas containing long-dormant mines and prospects may well lead to the 

discovery of economic Precambrian sulfide deposits. The zinc, copper, and lead contents 

of ores investigated to date, supplemented by silver and gold contents, indicate that 

many long-forgotten deposits may be minable today. The deposits occur in Precambrian 

rocks metamorphosed to the lower amphibolite facies in one major region and to the 

upper amphibolite facies in other regions. Field studies have provided ample evidence 

indicating that the search for commercial tonnages can be facilitated by using newer 

concepts of economic geology regarding ore-host rock associations as prospecting guides 

and by using structural considerations aimed toward learning how metamorphism and 

folding have modified the shapes and distribution of the ore bodies. Our field and laboratory data indicate that gahnite, a zinc spinel, can be used by geologists and prospectors 

as a guide to ore and can be considered by mining engineers as a potential major 

ore mineral, contributing significant amounts of zinc to the sulfide ores in many of 

the deposits. Our work to date indicates that the areas near Salida, Guffey, and Gunnison and several areas in the northern Park Range, the Front Range, and the Wet 

Mountains have particularly favorable ground for the occurrence of Precambrian sulfide 

deposits. 

INTRODUCTION 

Precambrian sulfide deposits occur at numerous localities in the 

Southern Rocky Mountains in Colorado (fig. 1). Earlier studies of some 

of these deposits (Lindgren, 1908; Boyd, 1932, 1934; Lovering and 

Goddard, 1950), like similar earlier studies conducted elsewhere in 

the world, generally tended to evaluate the ores in terms of hydrothermal and magmatic concepts of origin, which were then conventional. 

During the last two decades, interest throughout the world has increased tremendously in massive sulfide deposits ranging in age from 

1 
2 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

40°1- 1 

I 

I 

t 

I 

I 

I 

I 

I 

___ 108° 1 ___ _ 107° 

intrusive rocks 

37°L__-- __ l ____ _j_ ____ J __ _ 

50 100 KILOMETERS 

DENVER 

0 

River 

FIGURE 1.-Index map of western Colorado showing distribution of Precambrian 

rocks (shaded areas) and Precambrian sulfide deposits (numbered localities) mentioned in text: 1, Vulcan mine; 2, Bon Ton mine; 3, Cinderella No. 7 mine; 4, 

Sedalia mine; 5, Ace High and Jackpot prospect; 6, Independence mine; 7, mines 

and prospects near Cleora; 8, High Lonesome mine; 9, Hosa Lodge mine; 10, Creswell mine; 11, F. M. D. mine; 12, shafts at Wilkerson Pass; 13, Betty (Lone Chimney) mine; 14, Mill Gulch mine; 15, Isabel mine; 16, Cotopaxi mine; 17, Green 

Mountain mine; 18, mines and prospects in Grape Creek area; 19, Marion mine; 

20, Greenville mines; 21, Slavonia mine (lower); 22, Wolverine mines; 23, Swede 

Group mines; 24, Slavonia mine (upper). 

Precambrian to Tertiary. Much careful new work has been done concerning exhalative processes of ore formation and the relation of deposits to certain types of host-rock lithology (King and Thompson, 

1953; Horikoshi, 1969; Anderson, 1969; Matsukuma and Horikoshi, 

1970; Sangster, 1972; Stanton, 1972; Hutchinson, 1973). Many massive 

sulfide deposits in various parts of the world now are considered to 

be volcanogenic. Using the newer geologic concepts and newer, more 
INTRODUCTION 3 

refined guides to ore, mining companies have made many new discoveries, including, for example, the recent finding of two large Precambrian deposits: more than 7 million tons1 of zinc-copper-lead ore 

at Izok Lake in the Northwest Territories, Canada (Money and Heslop, 1976, p. 24-25), and 60 million tons of zinc-copper ore at Crandon, 

Wis. (Eyde, 1977, p. 51). 

Recognizing the importance of the newer concepts of economic geology and responding to increasing numbers of inquiries from the mining 

industry, the U.S. Geological Survey initiated a project in 1974 entitled "Precambrian sulfide deposits in Colorado." An earlier open-file 

report (Sheridan and Raymond, 1977) and the present report summarize the work that has been done and the results that have been 

obtained on this project through June 1977. 

During the reconnaissance studies that were the basis for this interim report, sampling of the Precambrian sulfide ores and their host 

rocks was done at many old mines and prospects. Access to underground workings was not possible or was not considered safe at any 

of these old mines and prospects with the exception of the Sedalia 

mine near Salida, but even there access to unoxidized primary ore 

is extremely limited. Consequently, most of the sampling was conducted in the following manner from the materials available- usually 

those found on the dumps. Sampling of host rocks was done by searching for varieties presently available on dumps and in nearby outcrops 

and by selecting representative grab samples for petrographic studies 

to determine their lithology. Samples of sulfide-bearing materials, 

however, had to be collected with the realization that the miners and 

prospectors had already shipped the bulk of their ores long ago to 

whatever ore-buying and ore-testing facilities were available. At each 

mine and prospect, therefore, a diligent search was made to find and 

collect representative samples of sulfide-bearing materials that had 

been overlooked and discarded in the dumps. Such materials commonly represent only the minimal base- and precious-metal contents 

of the ores present in these deposits. Our sampling was done by collecting large composite samples of these sulfide-bearing materials. At 

some of the mines and prospects only one large composite sample 

was obtained. At others where a possible zoning of ores was recognized, large composite samples of each of the varieties were obtained. 

Splits of each of these samples were then made, with one part reserved for mineralogic and petrographic studies and the other part 

for analyses. In this interim report, the analytical data concerning 

the samples are tabulated as ranges and averages of the base- and 

precious-metal contents for various mining areas. 

1 Figures cited from published sources are in the units originally reported. 
4 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

The significance of gahnite, the zinc spinel, as a prospector's guide 

to base-metal sulfide deposits in Colorado is described both in this 

report and in the earlier open-file report (Sheridan and Raymond, 

1977). Although we had arrived at this conclusion independently 

through our own field and laboratory studies, we have learned subsequently that this same concept had been recognized much earlier 

in Australia. Paul G. Spry (written commun., 1982) informed us that 

gahnite has been used by mining companies for many years as an 

exploration guide for deposits of Broken Hill-type mineralization in 

the Willyama Complex in New South Wales. In his letter, Spry kindly 

referred us to three reports from Australian sources that are especially pertinent. Reports by Dewar (1968), Forwood (1968), and Stevens (1974) contain information indicating the usefulness of gahnite 

as an indicator of base-metal sulfide deposits of the Broken Hill type. 

We gratefully acknowledge the advice and help given at various 

stages of this project by J. M. Franklin of the Geological Survey of 

Canada, by P. G. Spry of the University of Toronto, Canada, by J. 

E. Bever of Miami University, Ohio, and by our U.S. Geological Survey colleagues: R. B. Taylor, A. V. Heyl, Ogden Tweto, J. C. Olson, 

D. C. Hedlund, G. A. Desborough, W. J. Hasler, P. K. Sims, B. 

R. Doe, and G. L. Snyder. 

HISTORY 

Many of the old mines and prospects in Precambrian sulfide deposits 

in Colorado date back to the last two decades of the nineteenth century. Most of them did not progress much beyond the prospecting 

and initial development stages, probably because of their low content 

of precious metals. The Precambrian sulfide ores generally contain 

less than 10 g (grams) of gold per metric ton (0.3 oz (ounce) per 

ton) and less than 70 g of silver per metric ton (2 oz per ton). Such 

contents of the precious metals were not sufficient at that time to 

support mining that was directed toward gold and silver, and other 

mines that opened during this early period were better sources of 

copper and zinc ores. The Sedalia mine near Salida has been the 

largest producer of ore from Precambrian sulfide deposits in Colorado. 

Heyl (1964, p. C83) estimated that nearly 100,000 tons of copper-zinc 

ore was produced from the Sedalia mine. According to J. V. Dodge 

(written commun., 1975), the Sedalia mine produced ores until the 

end of 1918 when copper ore prices dropped. The production from 

the Sedalia mine was principally from the oxidized zone of the deposit. 

A mining engineer's report (C. H. Swanton, written commun., 1922), 

given to us by J. V. Dodge, indicated that the sulfide ore was too 

hard to be worked by hand-mining, the only method ever used on 
GEOLOGIC SETTING AND GENERAL CHARACTER OF ORES 5 

the property, and that, consequently, little of the sulfide ore had been 

mined. Other mines, such as the Betty (Lone Chimney) mine in the 

Guffey area, managed to keep operating intermittently into the 1950's 

(Buford Dell, oral commun., 1975). According to G. L. Snyder (oral 

commun., 1977), production of ore at the Greenville mine in the Park 

Range occurred as recently as the World War II era. In general, 

however, activity at mines and prospects in Precambrian sulfide deposits has been long dormant in most areas in Colorado. The search 

for minable Precambrian sulfide deposits has become quite active in 

recent years in numerous areas in Colorado, New Mexico, and southern Wyoming. Currently, many of the known deposits are being 

reexamined closely by the U.S. Geological Survey, other governmental agencies, and mining companies. Some of the initial stimulus for 

this renewed interest most likely was given by Giles' suggestion (1974) 

that a significant part of the Precambrian terrane in northern New 

Mexico and southern Colorado may be a "previously unrecognized volcanogenic massive sulfide metallogenic province." 

GEOLOGIC SETTING AND 

GENERAL CHARACTER OF THE ORES 

Precambrian sulfide deposits occur at many localities in Colorado 

(fig. 1) in metamorphic terranes that are known to be about 1,800 

million years (m.y.) in age (Tweto, 1980, p. 37), that is, Proterozoic 

X. The progenitors of the metamorphic rocks were sedimentary, volcanic, and subvolcanic intrusive rocks (Tweto, 1977, p. D3). A major 

period of Precambrian folding and regional metamorphism reached its 

peak in the period 1, 700-1,775 m.y. ago, according to radiometric age 

determinations (Hedge and others, 1967; Hedge and others, 1968; 

Hansen and Peterman, 1968; Silver and Barker, 1968; Stern and 

others, 1971). Hedge (1969) has suggested that the available isotopic 

data indicate that no great amount of time elapsed between the original deposition of the rocks and the time of metamorphism. Lead 

isotope studies on galena samples from Precambrian sulfide deposits 

in Colorado indicate an age falling in the same 1, 700-1,800 m.y. bracket (Bruce Doe, oral commun., 1979). Syntectonic intrusive igneous 

rocks ranging in age from 1,650 to 1,730 m.y. are present in many 

areas and commonly are granodiorite and quartz monzonite of the 

Boulder Creek type (Tweto, 1977, p. D12). 

Precambrian sulfide deposits occur in rocks metamorphosed principally to the lower amphibolite facies in the Gunnison area and in rocks 

metamorphosed tQ the upper amphibolite fades (sillimanite zone of 

regional metamorphism) in most other areas in Colorado. Whereas 

garnet is present in the lower grade terranes, sillimanite, cordierite, 
6 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

garnet, and, locally, andalusite are characteristic of the higher grade 

terranes. 

The Precambrian sulfide deposits contain sphalerite, chalcopyrite, 

and galena as the principal ore minerals, but silver and gold also are 

present consistently in noteworthy amounts. Gahnite is present in 

many of the deposits, ranging in abundance from accessory amounts 

to ore quantities. Locally, molybdenum, tungsten, titanium, and nickel 

are present. Some of the ores consist of base-metal sulfides in a matrix 

composed predominantly of pyrite and (or) pyrrhotite; this type of 

ore is "massive sulfide," as defined by Sangster (1972, p. 11). More 

commonly, the ore specimens noted on dumps are of the disseminated 

type, characterized by probable minable-grade concentrations of basemetal sulfides complexly intergrown with a matrix of silicate minerals. 

The textural features observed in the ores indicate that the sulfide 

minerals and the silicates and other minerals of the matrix recrystallized together at the time of regional metamorphism. 

DEPOSITS IN ROCKS METAMORPHOSED 

TO LOWER AMPHIBOLITE FACIES 

The Gunnison area is one of few areas in Colorado where Precambrian metamorphism has not been severe enough to obliterate original 

textures and structures, which can be recognized in many of the outcrops. According to J. C. Olson and D. C. Hedlund (oral commun., 

1977), most of the Precambrian rocks in this area are in the lower 

amphibolite facies, although the metamorphic grade in the western 

part is locally higher where rocks contain staurolite, kyanite, or andalusite; they noted that the rocks in the easternmost part of the 

area are in the greenschist facies. 

DEPOSITS IN THE GUNNISON AREA 

The Gunnison area contains a belt of Precambrian greenstone, long 

known as "the Gunnison gold belt" (Lakes, 1896). This belt of metavolcanic rocks (the Proterozoic X Dubois Greenstone), together with related intrusive rocks, trends northeasterly (fig. 2) for about 50 km 

(kilometers) and is as much as 10 km wide. J. C. Olson and D. C. 

Hedlund recognized that gold deposits in the area "are spatially and 

probably genetically related to a belt of Precambrian metavolcanic 

rocks" (U.S. Geological Survey, 1970, p. A3). 

We consider this belt of metavolcanic rocks to be especially favorable for discovery of minable Precambrian sulfide deposits, because 

the geologic environment is similar to that in productive areas in some 

parts of Canada and the United States. In addition to mafic metavolcanic rocks, the belt contains large volumes of felsic metavolcanics 

and numerous thin layers of quartzite that are believed to have originated as seafloor chert beds. Most of the presently known Precambri-
DEPOSITS METAMORPHOSED TO LOWER AMPHIBOLITE FACIES 7 

Geology simplified from 

Tweto and others 119761 

Montrose P x 2° quadrangle 

PRECAMBRIAN 

10J000' 

10 20 KI LOMETERS 

EXPLANATION 

PHANEROZOIC ROCKS UNDIFFERENTIATED 

GRANITIC INTRUSIVE ROCKS 

METASEDIMENTARY ROCKS 

DUBOIS GREENSTONE-METAVOLCANIC ROCKS 

CONTACT 

FAULT -Dotted where concealed, bar and ball on downthrown 

side 

SULFIDE DEPOSIT 

NAMES OF MINES 

1 White Iron 

2 Headlight 

3 Anaconda 

4 lroncap 

5 Good Hope 

6 Vulcan 

7 Midland 

8 Denver City 

9 Yukon 

FIGURE 2.-Distribution of Precambrian sulfide deposits in the Gunnison area, Colorado. 

an sulfide deposits occur within the belt of metavolcanic rocks (fig. 

2), but one deposit occurs in metasedimentary terrane. Geologic maps 

that provide coverage of the entire greenstone belt have been pub-
8 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

lished in recent years by the U.S. Geological Survey on 71f2-minute 

topographic quadrangle bases (Olson, 1974, 1976a,b; Olson and Hedlund, 1973; Olson and Steven, 1976a,b; Olson and others, 1975; Hedlund, 1974; Hedlund and Olson, 1973, 1974, 1975). 

We examined and sampled dumps and outcrops at numerous mines 

and prospects in the greenstone belt-mostly in the Powderhorn quadrangle (Hedlund and Olson, 1975), including the Vulcan mine (Lakes, 

1896), and to a lesser extent elsewhere in the belt. The Precambrian 

sulfide deposits are elongate parallel to gradational and interfingering 

contacts between mafic and felsic metavolcanic rocks. Groups of deposits appear to be strung out parallel to contacts. Some groups of 

deposits are adjacent to thin beds of magnetite-bearing quartzite. Sulfides commonly occur in laminae or elongate aggregates parallel to 

layering and foliation. Some specimens of ores are crudely banded. 

The ores are characterized by abundant pyrite accompanied by variable amounts of sphalerite and chalcopyrite. The sulfides generally 

are fine grained ( < 1 mm-millimeter) but locally are medium grained 

(1-5 mm). Locally, as at the Headlight mine in the Powderhorn quadrangle, the zinc spinel, gahnite, also is present in the ore; the terrane 

in the vicinity of the Headlight mine has been metamorphosed to a 

somewhat higher grade than the lower amphibolite facies. 

The base-metal and precious-metal contents of samples of ore from 

five mines in the Powderhorn quadrangle are summarized in table 

1. Zinc is the principal base metal, followed in abundance by copper 

and a minor amount of lead. 

At the time of our reconnaissance studies in 1975, exploration for 

Precambrian massive sulfide deposits was being conducted in the Powderhorn quadrangle by two mining companies. In 1976, one company 

also conducted some exploration in a small area near Cochetopa Canyon in the eastern part of the greenstone belt. We understand that 

these exploratory efforts revealed deposits of minable grade but, in 

light of only scattered drilling, of insufficient size to mount a major 

mining effort. 

ECONOMIC POTENTIAL 

Despite the fact that limited drilling has revealed no minable deposits thus far, we believe that the Gunnison belt of metavolcanic 

rocks has good potential for the discovery of large economic Precambrian sulfide deposits. The geologic evidence is compelling that the 

deposits are volcanogenic. The large size of this belt of metavolcanic 

rocks, the generally favorable geologic setting in the belt, the existence of ores previously mined economically, and the fact that clusters 

of old mines and prospects are present in many parts of the belt 

suggest that additional search may lead to the discovery of large con-
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 9 

TABLE I.-Analyses of Precambrian ores from the Powderhorn quadrangle, Colorado 

[Data for copper, zinc, lead, and silver from semiquantitative spectrographic analyses by N. M. Conklin, U.S. 

Geological Survey; data for gold from analyses by fire assay and atomic absorption methods by J. G. Crock, 

A. W. Haubert, and Joseph Haffty, U.S. Geological Survey] 

Copper (percent)---------------------

Zinc (percent)-------------~--------­

Lead (percent)-----------------------

Silver (grams/metric ton)------------

Silver (troy ounces/short ton)-------

Gold (grams/metric ton)--------------

Gold (troy ounces/short ton)---------

5 samples from dumps of 5 mines 

Range Average 

0.07-7.0 

.1-)10 

o-.3 

3-70 

.09-2.06 

.13-2.11 

.004-.061 

2.3 

>6 

.13 

30 

.94 

• 71 

.021 

cealed deposits or to the discovery that some known deposits may 

extend down-plunge a considerable distance from their surface showings. 

Search can be continued in various parts of the greenstone belt, 

both in areas where deposits are clustered and in areas extending 

laterally from such clusters. Commonly used Canadian guides to ore 

(lithologic and stratigraphic) can be used to search for favorable areas 

for exploration along contacts between felsic and mafic metavolcanics 

and along the thin layers of metachert. A search for so-called "millrock" (pyroclastic breccias), as advocated by Sangster (1972, p. 3) 

for Canadian terranes, may prove useful in prospecting for areas in 

which massive sulfide deposits are likely to occur. In addition to advocating use of these lithologic and stratigraphic guides to ore, we 

also urge strongly that special attention be paid to folds and lineations. 

Folds are present in this region, and they probably formed at the 

time of regional metamorphism. The deformation may well have modified considerably the original (presumably tabular) shapes of Precambrian sulfide deposits. The possibility should be recognized that deposits may be rod-shaped rather than discoidal or tabular, and a search 

for fold-axes and other b-lineations defining the dominant plunge direction is pertinent. Although plunge directions may not form a consistent 

regional pattern, the directions found in local areas could well be 

highly significant in determining the extent, shape, and size of ore 

bodies. 

DEPOSITS IN ROCKS METAMORPHOSED 

TO UPPER AMPHIBOLITE FACIES 

With the exception of a small area of rocks belonging to the lower 

amphibolite facies southeast of Salida, the Precambrian rocks in all 

other areas examined have been metamorphosed to the upper am-
10 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

phibolite facies (the sillimanite zone of regional metamorphism) and 

locally even higher. In or close to all the areas examined, sillimanite 

is present in layers of pelitic gneiss or schist, and cordierite is common 

in the host rocks of the deposits. Original textures and structures 

have been modified so greatly by intense regional metamorphism and 

deformation that the original nature of many rocks is difficult to determine. Despite these difficulties, the combination of adequate prior 

study with mapping in many of these areas is sufficient to establish 

that the Precambrian metamorphic complex consists of interlayered 

metasedimentary and metavolcanic rocks. In some areas the rocks 

have been modified by retrograde processes, but the changes have 

been only moderate. 

In the areas studied, amphibolites and feldspar-rich gneisses are 

abundant and are interlayered with varying amounts of sillimanitic 

gneiss or schist, calc-silicate gneiss, and, locally, impure marble. Many 

of the amphibolitic rocks are interpreted as having been derived from 

mafic volcanic rocks, and many of the feldspar-rich gneisses are interpreted as having been derived from felsic volcanic rocks. The sillimanitic schists and gneisses, the calc-silicate gneisses, and the impure marbles probably were originally shales and limestones. 

R. B. Taylor (oral commun., 1975) acquainted us in the field with 

a small area southeast of Salida. The rocks there are lower in 

metamorphic grade, having been metamorphosed only as high as the 

lower amphibolite facies. They resemble some of the rocks in the Gunnison greenstone belt. They include breccia units with basaltic porphyry fragments and a felsic unit containing phenocrysts and flattened 

pumice fragments; present also are fine-grained quartzitic layers. 

North and northwest from this area, however, the metamorphic grade 

changes to that of the upper amphibolite facies, and most of the original textural and structural features have been obliterated. In describing this same general area, Boardman (1976, p. 96) reported a gradual 

south-to-north transformation of welded tuff units into quartz-feldspar 

gneisses. 

Precambrian sulfide deposits occur in rocks of the upper amphibolite 

facies in most of the Salida area and in the Front Range, Wet Mountains, and Park Range (fig. 1). Locally in parts of the Wet Mountains, 

the metamorphic grade reached the amphibolite-granulite transitional 

facies (Raymond and others, 1980, p. 19-20). The principal ore minerals are sphalerite, chalcopyrite, and lesser amounts of silver-bearing 

galena. Gahnite, the zinc spinel, occurs in most of these sulfide deposits; and, in some, it is so abundant that it merits consideration 

as an ore mineral. 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 11 

DEPOSITS IN THE VICINITY OF SALIDA 

Numerous Precambrian sulfide deposits are clustered in the vicinity 

of Salida (fig. 3), including the Sedalia mine, northwest of Salida; the 

Independence mine (Lindgren, 1908, p. 166) and the Ace High and 

PRECAMBRIAN 

10 15 KILOMETERS 

EXPLANATION 

YOUNGER ROCKS UNDIFFERENTIATED 

GRANITIC INTRUSIVE ROCKS 

METAGABBRO 

METAVOLCANIC AND METASEDIMENTARY ROCKS 

CONTACT 

_1._ FAULT ~Bar and ball on downthrown side; dotted where concealed 

e SULFIDE DEPOSIT 

FIGURE 3.-Distribution of Precambrian sulfide deposits in the Salida area, Colorado. 

Geology modified from Tweto and others (1976) and Scott and others (1975). 
12 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

Jackpot prospect (Van Alstine, 1969, p. 43-44) in the vicinity of Turret, an abandoned mining town north of Salida; and the Bon Ton mine 

(Crawford, 1913, p. 280), Cinderella No. 7, and several other deposits 

in the area south of Maysville, a small community west of Salida. 

In addition, Precambrian quartz veins containing copper and tungsten 

are present near Cleora, 3 km southeast of Salida (Tweto, 1960, p. 

1420-1422). The general geology of the region is shown on a map 

of the Salida area (Boardman, 1976, fig. 1) and on maps of the Poncha 

Springs quadrangle (Scott and others, 1975; Van Alstine, 1969, 1974). 

In 1975 and 1976, all the main mines and numerous other prospects 

were visited, and samples of ores and host rocks were collected. The 

area surrounding the Sedalia mine was mapped on large-scale aerial 

photographs, and the geology of accessible parts of the mine was 

mapped. 

The layered succession of metamorphic rocks in the Sedalia mine 

area (fig. 4) trends northeast and dips steeply southeast. Major 

lithologic units mapped in the area are amphibolite, feldspathic gneiss, 

mica schist, and garnet-cordierite-amphibole gneiss and garnetiferous 

schist. These units were intruded by small bodies of metagabbro and 

pegmatite. The Precambrian sulfide deposit at the mine is associated 

with and is alined along medium- to coarse-grained garnet-cordieriteamphibole gneiss. Present locally in the ore zone is a carbonate-bearing calc-silicate rock. Sphalerite, chalcopyrite, and lesser amounts of 

silver-bearing galena are the major primary ore minerals in the deposit. Although pyrite, pyrrhotite, and magnetite are present also, 

the iron sulfides do not form a dominating matrix. Instead, the basemetal sulfides commonly are contained in ore-grade amounts in a rock 

rich in pale-gray magnesian amphiboles. Common also in this same 

type of rock are green grains and aggregates of gahnite, the zinc 

spinel (fig. 5). The sulfide minerals occur both as disseminated grains 

and aggregates and as crude laminations parallel to the lithologic 

layering. Evidence for textural parallelism between ore minerals and 

silicate minerals is abundant, although locally some of the sulfide minerals appear to have been mobilized to form discordant irregular 

stringers. The ore minerals, iron sulfides, gahnite, and magnetite are 

commonly medium grained, although locally some fine-grained and 

coarse-grained textures are present also. Scheelite has been reported 

at the Sedalia mine (Tweto, 1960, p. 1420-1421). The uppermost 100 

m (meters) of the ore body is partly oxidized and contains various 

secondary zinc and copper minerals, which supplied most of an estimated 100,000 tons of ore previously mined. Much tonnage of such 

supergene ore still remains and includes chalcocite, hydrozincite, 

malachite, smithsonite, hemimorphite, willemite, aurichalcite, rosasite(?), chalcanthite, cerussite, goslarite, anglesite, linarite, and cuprite (Heyl, 1964, p. C31; written commun., 1981). 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 13 

PRECAMBRIAN 

0 5 

EXPLANATION 

PEGMATITE 

METAGABBRO 

AMPHIBOLITE 

FELDSPATHIC GNEISS 

MICA SCHIST 

1 KILOMETER 

GARNET-CORDIERITE-AMPHIBOLE GNEISS 

AND GARNETIFEROUS SCHIST 

---- CONTACT 

----,--FAULT -Arrows show apparent horizontal movement; bar and ball on downthrown side 

>----

STRIKE AND DIP OF COMPOSITIONAL 

LAYERING AND FOLIATION 

ADIT 

OPEN-PIT MINE 

FIGURE 4.- Geologic map of the Sedalia mine area, Colorado. Geology by 

D. M. Sheridan and W. H. Raymond, 1976. 
14 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

FIGURE 5.-Photomicrograph of gahnite-bearing ore from the Sedalia mine, Colorado. 

Gahnite (gh) is intergrown with sphalerite (sl) in a matrix of light-colored amphibole 

(am). Other minerals are magnetite (mt) and chalcopyrite (cp). Plane-polarized light. 

At the other mines and prospects in the vicinity of Salida, the ore 

minerals are associated with rocks similar to those forming the layered 

succession at the Sedalia mine. Garnetiferous biotite-quartz gneiss, 

various kinds of amphibole-rich rocks, and cordierite-biotite-amphibole 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 15 

gneiss are commonly the main host rocks. In the area south of Maysville, a layer of banded magnetite-quartz iron-formation forms part 

of the layered succession near the sulfide deposits. Sphalerite and 

chalcopyrite are the principal sulfide minerals present in deposits 

throughout the Salida region. In the Maysville area, galena was observed at one mine and scheelite at another mine. In the Turret area, 

molybdenite was noted at one mine and noteworthy amounts of rutile 

were observed at another mine. Textural features and grain size of 

the ores are similar to those at the Sedalia mine. 

The clustering of numerous deposits in the Salida region and the 

association of ore minerals with magnesium-rich amphiboles and garnetiferous gneiss suggest a common mode of origin. Although the 

shapes of the ore bodies are not known in detail, they clearly are 

alined approximately parallel to the lithologic layering. A logical assumption, therefore, is that the ore bodies were originally stratiform 

deposits that have been recrystallized and deformed during high-grade 

regional metamorphism to their present mineralogy and distribution. 

In the light of recent studies in Canada and elsewhere, the combination of evidence in the Salida area suggests that the deposits originally 

may have been products of some sort of exhalative process, although 

high-grade metamorphism makes it difficult to prove that they are 

volcanogenic. 

The base-metal and precious-metal contents of ore samples from 

the vicinity of Salida are summarized in table 2. The summary indicates that the ores contain appreciable amounts of zinc and copper 

and that silver averages between 20 and 30 g/metric ton (between 

0.5 and 1 oz/ton). 

GAHNITE AND ITS SIGNIFICANCE 

During field studies in the vicinity of Salida, we found that gahnite, 

the zinc spinel, occurs at all the mines and at most of the prospects 

in Precambrian sulfide deposits. Although Lindgren (1908, p. 165) had 

noted the presence of spinel at the Sedalia mine and Crawford (1913, 

p. 205) had reported the presence of gahnite at the Bon Ton mine 

in the area south of Maysville, the zinc spinel apparently has been 

considered merely as a mineralogic curiosity during the ensuing years. 

Instead of finding it as a rare accessory, we found that gahnite is 

characteristic of the ores and is present in noteworthy amounts at 

some of the deposits. At the Bon Ton mine, for example, gahnite-biotite-quartz gneiss (figs. 6 and 7) and gahnite-bearing garnet-biotitequartz gneiss (fig. 8) are abundant. Moreover, we also found gahnite 

to be megascopically identifiable in lithologic units adjacent to and 

along strike from some of the known sulfide deposits. 

Because gahnite occurs in such abundance in several of the deposits, 
TABLE 2.-Analyses of Precambrian ores from the vicinity of Salida, Colo. 

[Data for copper, zinc, lead, and silver from semiquantitative spectrographic analyses by J. C. Hamilton, N. M. Conklin, and M. W. Solt, U.S. Geological Survey; data for gold from 

analyses by fire assay and atomic absorption methods by J. G. Crock, A. W. Haubert, and Joseph Haffty, U.S. Geological Survey] 

Copper (percent)----------------

Zinc (percent)------------------

Lead (percent)------------------

Silver (grams/metric ton)-------

Silver (troy ounces/short ton)--

Gold (grams/metric ton)---------

Gold (troy ounces/short ton)----

1Average of 8 analyses. 2Average of 2 analyses. 3Average of 2 analyses. 

Sedalia mine 

(5 samples from dumps 

and 5 samples from 

underground workings) 

Range 

0.15->10 

.3->10 

.001-5.0 

3-70 

.10-2.06 

.08-.64 

.002-.019 

Average 

>3.8 

>5.2 

.5 

30 

.79 

1.24 

1.007 

Turret area 

(4 samples from 2 

mine dumps) 

---

Range Average 

o.o5-3.o 0.8 

1.0-5.0 3.5 

o-.002 Trace 

0-70 20 

0-2.06 .65 

.07-2.00 2 1.04 

.002-.058 2 .030 

Maysville area 

(5 samples from dumps 

of 3 mines and 1 

prospect) 

Range Average 

0.07-3.0 1.3 

1.5->10 >6.9 

.002-.1 .03 

7-70 30 

0-2.06 • 91 

.12-.68 3.40 

.004-.020 3.012 

~ 

0";) 

N 

z 

a 

n 

0 

"0 

"0 

tr:l 

;:a 

~ 

tr:l 

> tJ 

UJ. 

d 

t"' 

"%j 

..... 

tJ 

tr:l 

UJ. 

> z 

tJ 

0 

> 

~ 

z 

..... 

!--3 

tr:l 

a 

0 

L-4 

0 

;:a 

> tJ 

0 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 17 

gh 

gh 

FIGURE 6.-Photograph of polished slab of gahnite-biotite-quartz gneiss from the Bon 

Ton mine, Colorado, showing layers of gahnite-rich rock (gh) alternating with thinner 

sulfide-bearing layers (s). Many of the larger grains of gahnite, 3-10 mm in size, 

are clearly visible as prominent dark grains in the photograph, but many smaller 

grains of gahnite are interspersed among the biotite and quartz of the matrix. Sulfide 

minerals in the thinner layers are sphalerite, chalcopyrite, and minor pyrite and 

galena. 

we collected samples to determine analytically how much zinc such 

rocks contain. Although gahnite occurs in varying proportions with 

the base-metal sulfides, the samples chosen for this study are gahnitebearing rocks containing only minor amounts of sulfides; this was done 

in order to ascertain the amounts of zinc contributed principally by 

gahnite. Analyses of gahnite-bearing rocks from several mines and 

prospects are reported in table 3. 

In light of these analytical results and the large quantities of 

gahnite-bearing material observed on some of the dumps, gahnite 

must be considered a potentially significant ore mineral in these deposits. Note, in this regard, that franklinite (another zinc-bearing spinel), willemite, tephroite, and gahnite-all highly refractory multiple 

oxides and silicates-are components of ores that have been mined 

and smelted successfully for many years from the deposits at Franklin 

and Sterling Hill, N.J. Therefore, modified beneficiation processes 

and metallurgical methods may make possible the utilization of both 

the gahnite and the sulfide components of the ores of the Salida region. 

Shown also in table 3 are the results of analytical work on two 

samples obtained from outcrops of gahnite-bearing rocks. Analysis 10, 

giving 5 percent zinc, is a gahnite-bearing cordierite-biotite gneiss 
18 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

FIGURE 7.-Photograph of polished slab of gahnite-rich rock from the Bon Ton mine, 

Colorado. A layer of gahnite-quartz rock in the lower part of the photograph contains 

subhedral to euhedral grains of gahnite as much as 6 mm across. The finer-grained 

layer comprising the upper part of the photograph contains equally abundant but 

finer grains of gahnite interspersed in a matrix of biotite and quartz containing abundant chalcopyrite. 

that crops out along the southeast side of the ore zone at the Sedalia 

mine. Analysis 11, giving 1.25 percent zinc, is a gahnite-bearing 

quartz-mica schist that occurs along the northern margin of the layer 

of garnet-cordierite-amphibole gneiss and garnetiferous schist in an 

area 2 km northeast of the Sedalia mine (fig. 4). Although not delineated separately on figure 4, the gahnite-bearing schist crops out 

intermittently for 240 m along strike and is as much as 40 m thick. 

In addition to its occurrence in outcrops in the Sedalia mine area, 

gahnite also can be traced along the trend of layering in the cluster 

of deposits south of Maysville. The presence of gahnite in outcrops 

where sulfides are sparse or even absent may be a significant and 

useful clue in searching for concealed Precambrian sulfide deposits. 

ECONOMIC POTENTIAL 

Field and laboratory studies to date suggest that the Salida region 

has good potential for workable deposits. The clustering in this region 

of numerous deposits containing materials of probable economic grade 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 19 

FIGURE 8.-Photomicrograph of gahnite-bearing garnet-biotite-quartz gneiss from the 

Bon Ton mine, Colorado. Gahnite (gh) and garnet (G) are in large subhedral to euhedral grains set in a matrix of biotite (B) and quartz (Q). Other mineral shown is 

chalcopyrite (cp). Plane-polarized light. 

(table 2), the significant quantities of gahnite (table 3), and the association of the ores with garnetiferous and magnesium- rich rocks suggest 

both a common mode of origin for all these deposits and the favorabil-
TABLE 3.-Zinc content of gahnite-bearing rocks from the vicinity of Salida, Colo. 

[C, chemical analyses for zinc (in percent) determined by N A20 2 fusion-atomic absorption method by J. G. Crock, U.S. Geolgoical Survey; S, analyses for zinc (in percent) 

converted from semiquantitative spectrographic analyses (in parts per million) by J. C. Hamilton and M. W. Solt, U.S. Geological Survey] 

Analysis 

Number Locality 

1. Sedalia mine, dump--------------

2. Sedalia mine, Dewey level-------

3. Mine A, dump (Turret area)------

4. Mine A, dump (Turret area)------

5. Mine B, dump (Turret area)------

6. Mine C, dump (Maysville area)---

7. Mine C, dump (Maysville area)---

8. Prospect, dump (Maysville area)-

9. Mine D, dump (Maysville area)---

10. Outcrop, east side of Sedalia 

mine. 

11. Outcrop 2 km northeast of 

Sedalia mine. 

Lithology 

Gahnite-amphibole rock-----------------

-----do--------------------------------

Gahnite-bearing amphibole-mica rock----

-----do--------------------------------

Gahnite-biotite-amphibole rock---------

Gahnite-bearing garnetiferous gneiss---

Gahnite-rich gneiss--------------------

Gahnite-quartz gneiss------------------

Gahnite-quartz-mica gneiss-------------

Gahnite-bearing cordierite-biotite 

gneiss. 

Gahnite-bearing quartz-mica schist-----

Zinc Type of 

(percent) analysis 

7.0 c 

7.7 c 

3.5 c 

4.2 c 

7.3 c 

7.8 c 

12.5 c 

10 s 

10 s 

5 s 

1.25 c 

1:\:) 

0 

N 

z 

n 

(J 

0 

"0 

"0 

M 

~ 

t-< 

M 

> t:i 

UJ 

e 

t"' 

'7j 

1-< 

tJ 

M 

UJ 

> z 

tJ 

(:) 

> :I: 

z 

-

~ 

M 

n 

0 

t"' 

0 

~ 

> 

t:i 

0 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 21 

ity of this region for additional search. Exploration might lead to the 

discovery of large concealed deposits along the trend of favorable 

lithologic units and to the discovery that some of the known deposits 

have a large down-plunge dimension. 

Exploration for Precambrian sulfide deposits in this region can utilize the lithologic affinities noted in this report, together with the 

occurrence of gahnite as an ore-guide. Because the grade of 

metamorphism is high and because evidence for folding was noted 

by us in our studies and by Van Alstine (1974, p. 11, plate 1) and 

Boardman (1976, fig. 1), an important possibility to consider is that 

the shapes of Precambrian sulfide deposits, whatever their original 

shapes, have been modified considerably by folding and metamorphism. Sangster and Scott (1976, p. 189-190) have summarized changes 

in form that can occur during metamorphism, noting that the ore 

bodies at Balmat, N.Y. (also described by Lea and Dill, 1968), and 

at Chisel Lake, Manitoba (also described by Martin, 1966), have been 

changed from a presumably oval form to a linear- or rod-shaped form 

during medium- to high-grade metamorphism. Even more complex 

forms of ore bodies, including pinching and swelling (possibly related 

to boudinage), are reported by Sangster and Scott (1976, p. 190). 

Money and Heslop (1976, p. 26--27) noted that some thickening and 

thinning of Canadian ore bodies associated with cordierite- and 

gahnite-bearing rocks may have been partly an original feature and 

(or) possibly related to boudinage during deformation. Most massive 

and disseminated sulfide bodies seen by A. V. Heyl, J. L. Jolly, and 

C. N. Bozion in the Appalachians in 1966--1968 are elongated along 

the lineations as boudins (A. V. Heyl, oral commun., 1981). Due consideration must be given therefore to the probability that ore bodies 

may have been modified considerably in form and distribution by folding and high-grade regional metamorphism. Local evidence of plunge 

directions could be of great value in planning exploration programs, 

although complexly folded, thinned, thickened, and even dislocated 

ore bodies are all possible in terrane that has been so highly 

metamorphosed. Due allowance for the potential economic value of 

gahnite as an ore mineral should be made in any evaluation of the 

Precambrian sulfide deposits in the Salida region. 

DEPOSITS IN OTHER AREAS IN COLORADO 

During the field seasons of 1975 and 1976, we visited and sampled 

ores and host rocks at numerous Precambrian sulfide deposits in 

widely scattered areas in the Front Range, Wet Mountains, and Park 

Range. Mines visited in the Front Range (fig. 1) include the High 

Lonesome mine (Lovering and Goddard, 1950, p. 71); the Creswell 
22 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

mine (Eckel, 1961, p. 22) and F. M. D. and Hosa Lodge mines (Lovering and Goddard, 1950, p. 67, 70), all in the east-central Front Range; 

mine shafts at Wilkerson Pass; the Betty (Lone Chimney) and Mill 

Gulch mines (Lovering and Goddard, 1950, p. 69) and others in the 

Guffey area; the Isabel mine (Lovering and Goddard, 1950, p. 68); 

and the Cotopaxi mine (Lindgren, 1908, p. 166-167; Salotti, 1965, p. 

1179-1212). Mines visited in the Wet Mountains (fig. 1) include the 

Green Mountain mine, numerous mines and prospects in the Grape 

Creek area, and the Marion mine (Salotti, 1965, p. 1179). Preliminary 

studies made late in the 1976 field season in the Park Range (fig. 

1) included brief visits to the Greenville mines, the Slavonia mine 

(lower), and several mines near the old mining community of Pearl, 

including the Wolverine mines and the Swede Group mines. In addition to these reconnaissance studies, a more detailed study was made 

of the Betty mine area southwest of Guffey; this study was aided 

by field visits with James E. Bever, who also kindly provided geologic 

maps (Bever, 1954). 

Host rocks in these widely scattered localities are more diverse 

in lithology than those in the Salida area and range in character from 

garnetiferous gneisses and amphibole-rich rocks to calc-silicate gneisses and impure marbles. Deposits such as the F. M. D. mine in the 

east-central Front Range and some of the prospects in the Grape 

Creek area in the Wet Mountains have garnetiferous biotitic gneiss 

and dark amphibolites as the hosts of the sulfide minerals. At the 

other extreme, the deposit at the Creswell mine in the east-central 

Front Range is associated with a calc-silicate gneiss gradational locally 

to impure marble. Other deposits, such as the Betty mine in the Guffey area, have several interlayered host rocks, including cordierite-sillimanite-quartz gneiss and amphibole-rich rocks of various types; calcsilicate rocks are also present in the mine area. At the Marion mine 

in the Wet Mountains, the host rocks are interlayered amphibole-rich 

gneiss, calc-silicate gneiss, and impure marble. Sapphirine, a rare 

magnesium-iron-aluminum silicate, has been identified (Raymond and 

others, 1980) in the host rocks of Precambrian sulfide deposits at three 

mines and prospects in the Marion mine area. 

The ores noted on dumps at these various localities range from 

typical massive sulfide, containing greater than 50 percent sulfide minerals (Sangster, 1972, p. 11), to disseminated types containing oregrade concentrations of base-metal sulfides in a matrix of silicate minerals. Some specimens from the F. M. D. mine and mines in the 

Grape Creek area consist of base-metal sulfides in a matrix composed 

almost entirely of pyrite or pyrrhotite. Some ore specimens at the 

Betty mine and the Marion mine contain more than 50 percent copper 

sulfide or combined copper and zinc sulfide. Ores of the disseminated 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 23 

type are more common, however. In these the ore minerals are commonly alined along the layering and foliation. In the impure marbles 

and calc-silicate rocks, the sulfides are disseminated more randomly, 

with galena, sphalerite, and chalcopyrite occurring in irregularly scattered clots and aggregates. The ore minerals in most of these deposits 

are medium to coarse grained. Grains or aggregates of galena and 

sphalerite as large as 5 em occur in skarn-like calc-silicate rocks. 

The deposits in the Front Range, Wet Mountains, and Park Range 

are varied and all may not be of the same origin. Although some 

may be of an exhalative seafloor origin, some, particularly the disseminated deposits associated with calc-silicate rocks and impure marbles, 

possibly may have been syngenetic or epigenetic stratiform ore deposits in sedimentary rocks before metamorphism. 

The base-metal and precious-metal contents of ore samples from 

the Front Range and the Wet Mountains are summarized in table 

4. As in the Salida region, zinc is again the chief base metal. Lead 

is somewhat more abundant than in the Salida region. The ores average more than 40 g of silver per metric ton (more than 1.2 oz of 

silver per ton). 

DISTRIBUTION AND SIGNIFICANCE OF GAHNITE 

The zinc spinel, gahnite, is present in varying amounts in numerous 

localities in the Wet Mountains, Front Range, and Park Range. Table 

5 presents several analyses of gahnite-bearing rocks from this broad 

region. Analyses 1 through 3 from the Betty mine and another mine 

in the Guffey area of the Front Range indicate that gahnite could 

make a noteworthy contribution to the total zinc content of sulfide 

ores from this area. Analysis 4 from Mine F in the Wet Mountains 

suggests that gahnite might be useful as a prospecting guide in that 

area. Analysis 5 from Mine Gin the Wet Mountains is of a gahnite-rich 

amphibole rock (fig. 9) that could contribute significant amounts of 

zinc to the ore. Analysis 6 is of a gahnite-bearing gneiss that crops 

out north of the Cotopaxi mine. Megascopically identifiable gahnite 

in these outcrops, confirmed by this analysis, suggest again that 

gahnite can be used as a prospector's guide to ore, even where sulfide 

minerals or their oxidation products are not seen in the outcrops. 

Our preliminary work in the Park Range indicates that gahnite also 

may be significant in that region. Gahnite is present in samples we 

obtained from the Greenville mines and is present also in certain 

layers of Precambrian metamorphic rocks in the Pearl area. 

Analysis 7 in table 5 represents a gahnite-bearing layer of sillimanite-quartz gneiss that occurs 0.25 km north of the Creswell mine 

in the east-central Front Range. The Creswell mine is shown (but 
TABLE 4.-Analyses of Precambrian ores from other areas in Colorado 

[Data for copper, zinc, lead, and silver from semiquantitative spectrographic analyses by J. C. Hamilton, N. M. Conklin, and M. W. Solt, U.S. Geological Survey; data for gold from 

analyses by fire assay and atomic absorption methods by J. G. Crock, A. W. Haubert, and Joseph Haffty, U.S. Geological Survey] 

Copper (percent)------------------

Zinc (percent)--------------------

Lead (percent)--------------------

Silver (grams/metric ton)---------

Silver (troy ounces/short ton)----

Gold (grams/metric ton)-----------

Gold (troy ounces/short ton)------

1Average of 6 analyses. 2 Average of 9 analyses. 

Front Range 

(9 samples from dumps of 6 mines) 

Range 

0.3-5.0 

.2-)10 

.002-1.5 

3-150 

.09-4.41 

.05-3.58 

.001-.105 

Average 

1. 8 

>4.7 

.3 

70 

1.97 

1 .98 

1.029 

Wet Mountains 

(12 samples from dumps of 4 mines) 

Range 

0.15-7.0 

.10->10 

.001-10 

1.5-300 

.04-8.82 

<.05-9.24 

.001-.272 

Average 

2.1 

>4.4 

1.1 

45 

1.26 

2 1.6 

2 .048 

~ 

N 

z 

0 

0 

0 

""'C 

""'C 

~ 

~ 

~ ~ 

> t:l 

UJ. 

~ 

~ 

"%j 

1--4 

t:l 

~ 

UJ. 

> z 

t:l 

0 

> 

= z 

1--4 

1-3 

~ 

0 

0 

t"' 

0 

~ 

> t::l 

0 
TABLE 5.-Zinc content of gahnite-bearing rocks from other areas in Colorado 

[C, chemical analyses for zinc (in percent) determined by Na20 2 fusion-atomic absorption method by J. G. Crock, U.S. Geological Survey; S, analyses for zinc (in percent) converted 

from semiquantitative spectrographic analyses (in parts per million) by J. C. Hamilton and M. W. Solt, U.S. Geological Survey] 

Analysis 

Number Locality 

1. Betty mine, dump (Front Range)---

2. Betty mine, dump (Front Range)---

3. Mine E, dump (Front Range)-------

4. Mine F, dump (Wet Mountains)-----

5. Mine G, dump (Wet Mountains)-----

6. Outcrop, 90 m north of 

Cotopaxi mine (Front Range). 

7. Outcrop, 0.25 km north of 

Creswell mine (Front Range). 

Lithology 

Gahnite-amphibole rock----------------- -

Gahnite-amphibole-cordierite rock-------

Oxidized copper ore containing gahnite--

Gahnite-bearing impure marble-----------

Gahnite-amphibole rock------------------

Gahnite-bearing quartz-sillimanitemica gneiss. 

Gahnite-bearing sillimanite-quartz 

gneiss. 

Zinc Type of 

(percent) analysis 

8.0 c 

19.1 c 

10 s 

1.25 c 

7 s 

1.08 c 

3.9 c 

t::l 

tzj 

~ 

0 

r:n 

-

1-3 

r:n 

~ tzj 

1-3 

> 

~ 

0 

~ 

t-cJ 

::X:: 

0 

r:n 

tzj 

t::l 

1-3 

0 

c::: 

t-cJ 

~ 

tzj 

~ 

> 

~ 

~ 

::X:: 

63 

0 

~ 

-

1-3 

tzj 

~ 

> 0 

sa 

r:n 

~ 

Ol 
26 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

FIGURE 9.-Photomicrograph of gahnite-amphibole rock from Mine Gin the Wet Mountains, Colo. Gahnite (gh) and light-colored amphibole (am) form a foliated intergrowth 

in this specimen. Other mineral shown is chalcopyrite (cp). Plane-polarized light. 
DEPOSITS METAMORPHOSED TO UPPER AMPHIBOLITE FACIES 27 

not by name) by symbols for mine shafts (recently bulldozed and 

backfilled) on the geologic map of the Evergreen 1V2-minute quadrangle (Sheridan and others, 1972) in the SW% sec. 17, T. 4 S., R. 

71 W. The gahnite-bearing layer crops out near the fault shown on 

this map north of the mine shafts within a map unit (map symbol 

hcs) of interlayered hornblende gneiss, calc-silicate gneiss, and amphibolite. As shown on a geologic map of the adjacent Squaw Pass 

quadrangle (Sheridan and Marsh, 1976), similar layers of light-colored 

gneiss containing small but variable amounts of gahnite and rutile 

(map symbol Xrg) extend for long distances subparallel to the main 

rutile-bearing layers of light-colored Precambrian gneiss reported in 

previous studies (Sheridan and others, 1968; Marsh and Sheridan, 

1976). Gahnite also occurs with galena, sphalerite, chalcopyrite, calcite, and calc-silicate minerals in the sulfide-bearing zone at the Creswell mine. 

ECONOMIC POTENTIAL 

Our field and laboratory studies to date in the Front Range, Wet 

Mountains, and Park Range suggest that certain areas are especially 

promising. The Betty mine area near Guffey in the Front Range (fig. 

1) is considered one of the better areas in which to search for extensions of known deposits as well as to search for concealed deposits. 

Clustering of deposits in this area, ample evidence of ores of economic 

grade, and presence of anomalous amounts of zinc in lithologic units 

along strike from known sulfide deposits are evidence favoring the 

economic potential. Local clusters of deposits of minable grade in the 

Grape Creek area and in the vicinity of the Green Mountain mine 

in the Wet Mountains (fig. 1) also may be considered favorable for 

further economic consideration. Mines such as the Cotopaxi and Marion sometimes have been considered less favorably because they occur 

in xenoliths of metamorphic rocks engulfed in Precambrian granitic 

plutons. Our observations, however, suggest that the size of these 

xenoliths is so great that they could contain major ore bodies. Our 

studies in the Park Range (fig. 1) are barely started, but preliminary 

indications are that minable deposits also may occur in this region. 

Exploration in these areas in Colorado can utilize the various 

lithologic affinities noted in this report, together with the presence 

of gahnite as an indicator mineral in many areas. As we emphasized 

in the discussion of the economic potential of the Salida region, due 

allowance must be made for the possible deformation of ore bodies; 

they may range from rod-shaped to complexly thinned, thickened, 

folded, and even dislocated, because of the intensity of metamorphism 
28 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

and folding in these areas. Such structural considerations are mandatory in terrane like this that has been subject not only to high-grade 

regional metamorphism but also to several episodes of folding (Moench 

and others, 1962; Sheridan and others, 1967, p. 57-70; Taylor, 1976). 

Attention to the significance of gahnite, both as a prospecting tool 

and as a potential ore mineral, is urged for this entire region. 

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30 ZINC-COPPER-LEAD SULFIDES AND GAHNITE, COLORADO 

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cc U.S GOVERNMENT PRINTING OFFICE: 1984 - 461-431 - 226/10092