Geology of the Wood and 

East Calhoun Mines 

Central City District 

Gilpin County, Colorado

GEOLOGICAL SURVEY BULLETIN 1032-C

7 his report concerns work done on behalf 

rfthe U. S. Atomic Energy Commission 

and in part under the auspices of the 

Defense Minerals Exploration Administration and is published with the permission of the Commission
A
Geology of the Wood and 

East Calhoun Mines 

Central City District 

Gilpin County, Colorado

By AVERY A. DRAKE, JR.

GEOLOGY AND ORE DEPOSITS OF CLEAR CREEK, 

GILPIN, AND LARIMER COUNTIES, COLORADO

GEOLOGICAL SURVEY BULLETIN 1032-C

This report concerns work done on behalf 

of the U. S. Atomic Energy Commission 

and in part under the auspices of the 

Defense Minerals Exploration Administration and is published with the permission of the Commission

STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957
UNITED STATES DEPARTMENT OF THE INTERIOR 

FRED A. SEATON, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D. C. - Price $1 (paper cover)
CONTENTS

	Page 

Abstract_______________________________________________ 129

Introduction ______________________________________________________ 129

Previous investigations._-_--_-______-___________------_---_--_- 131

Field work_____________________________________ 131

Acknowledgments _-___________________-_-_-__--_-_-_--_--___-- 132

General geology________________________-_-----_-_--_---_-__.-_---- 132

Rocks of Precambrian age___________-_---___-__-_-___--_----_-_ 132

Biotite-quartz-plagioclase gneiss_____________________________ 134

Amphibolite__________________________.__-_-_____.-_-_ 134

Migmatite__________________________________________ 134

Granite gneiss___________-_______-____-_____--_-__----___ 135

Granite pegmatite.________________________________________ 135

Rocks of Tertiary age__--_______________-_--_____-_--_--__---_- 135

Quartz bostonite porphyry________________________________ 136

Syenitic bostonite porphyry___-_______--__-____--_-_-_---- 136

Structure_____________________________________________ 136

Folds.__________--__-__________________-_____--_-_-__-_ 136

Lineation_________________________________________________ 137

Joints,.________________________________________________ 138

Faults________________________________________- 139

Calhoun fault________________________---___---_-_ 139

Quartz Mill fault_______.____._-___--_-____________ 139

Wood fault____-_-___________-____-_--__------__-_-_ 140

Minor faults..___-______--_-___-__-_-______--------_ 141

Structure of Tertiary dikes.________________________________ 142

Economic geology _________________________________________________ 142

History and exploration______________________________________ 142

Production ___________________________________________________ 144

General character of the veins_______________---____-----__-___ 147

Calhoun vein._________________________________________ 147

Quartz Mill vein__________________________________________ 148

Wood vein ______.-___-_-_____-____.____-_-_______-_ 148

Willowdale vein___________--____________-_----____-__-__ 149

Mineralogy ___________________________________________________ 149

Quartz.___________________________________ 149

Pyrite____.___._..._.._...___.___-___.__.__..___._ .__ 149

Chalcopyrite. _____________________________________________ 150

Sphalerite.-__-_____-________________-___-_--_-_-_----____ 150

Tetrahedrite-tennantite. ___________________________________ 151

Galena.._______________________________________________ 151

Gold and silver__________________________________________ 151

Pitchblende.. _____________________________ 152

Bornite, sooty chalcocite, and covellite_______________________ 153

Paragenesis.__________________________________________________ 153

Results of sampling____________________________________________ 155

Trace elements__________________________________________ 159

Wall-rock alteration._______________________.___ 163

Zoning._____________________________________________________ 164

Ore shoots___________________________________________ 164

Origin.___.,. __ ..._________________________ 166

m
IV CONTENTS

Economic geology Continued Page

Age___________________________________________ 167

Future of the mines__________________________________________ 167

Literature cited.__________________________________________________ 167

Index.____-___.____,___________.________________________ 169

ILLUSTRATIONS

[Plates in pocket]

PLATE 12. Geologic maps of the East Calhoun mine.

13. Geologic map of the Wood crosscut and drift, sixth level, East Calhoun mine.

14. Geologic sections of the East Calhoun mine and detailed section of 

the Wood 520 raise.

15. Vertical longitudinal section of the Wood vein, showing assay values.

Page 

FIGURE 28. Index maps showing location of the Wood and East Calhoun

mines..--_________________________._ 130

29. Geologic map of the Wood-East Calhoun area__.__--__-_ 133

30. Contour diagram of 110 lineations______________________ 137

31. Contour diagram of 386 joints.--..-___________---_ 138

32. Contour diagram of 70 slickensides on the Wood vein..______ 140

33. Longitudinal projection of the East Calhoun mine. _______ 143

34. Vertical longitudinal projection of the Wood mine._________ 145

35. Graph showing the relation of gold to silver in samples from

the Wood vein___________________________________-___ 152

36. Paragenesis of the vein minerals________________________ 154

37. Distribution histograms of some elements in 132 samples

from the Wood vein____-______________________ 160

38. Graph showing the relation of molybdenum and zirconium to

uranium in samples from the Wood vein__._______ 161

39. Graph showing the relation of zinc and cadmium in samples

from the Wood vein______________________ 162

40. Graph showing the relation of copper, arsenic, bismuth, and

antimony in samples from the Wood vein_____________ 163

TABLES

TABLE 1. Production from the Jefferson-Calhoun vein, 1902-15_______ 146

2. Substantiated pitchblende production from the Wood vein. _ _ _ 146

3. Gold and silver content of pyrite, Calhoun and Wood veins__ 150

4. Gold and silver content of chalcopyrite, Calhoun and Wood

veins. ______________________________ 150

5. Analyses of sphalerite, Calhoun and Wood veins.__________ 151

6. Some chemical analyses, fire assays, and semiquantitative

spectrographic analyses of ore from the Wood vein.________ 156

7. Threshold values of elements included in the semiquantitative

method. _____________________________________________ 159
GEOLOGY AND ORE DEPOSITS OF CLEAR CREEK, GILPIN, AND LARIMER

COUNTIES, COLORADO

GEOLOGY OF THE WOOD AND EAST CALHOUN MINES 

CENTRAL CITY DISTRICT, GILPIN COUNTY, COLORADO

By AVERT A. DRAKE, JR.

ABSTRACT

The Wood-East Calhoun area is underlain by complexly folded Precambrian 

gneiss and pegmatite. The major fold is an anticline that trends about N. 60° E. 

The Precambrian rocks are intruded by bostonite porphyry dikes of Tertiary age. 

All the rocks are cut by eastward- to northeastward-trending faults that have 

been filled by precious-metal-sulfide veins which have been worked chiefly for 

gold. The Wood vein, which has produced much of the uranium of the Central 

City district, fills an east-trending fault; the Calhoun vein is in a northeast-trending fault.

The veins consist chiefly of quartz; pyrite is the predominant metallic mineral 

and chalcopyrite ranks next in abundance. Sphalerite, galena, tetrahedritetennantite, and pitchblende occur locally. Alteration-stage quartz and pyrite 

were deposited first, followed in order of deposition by pitchblende, light-yellow 

pyrite, massive quartz, yellow pyrite, sphalerite, comb quartz, chalcopyrite, 

tetrahedrite-tennantite, galena, chalcopyrite, pyrite, and gray to light-brown 

fine-grained quartz. The veins of the district are zoned; quartz-pyrite veins are 

near the center, and galena-sphalerite veins are on the periphery. The pitchblende bodies occur between these veins, but, paragenetically, the pitchblende is 

earlier than all other metallic minerals.

A trace-element study of the ore shows an association of bismuth, antimony, 

and arsenic with copper, and of cadmium with zinc; high-grade uranium samples 

contain unusually large quantities of zirconium and molybdenum.

Pitchblende and other ore minerals are concentrated in ore shoots. These are 

in open spaces controlled by the competency of the wall rocks, the presence of a 

prevailing direction of weakness in the rocks, and changes in strike and dip of the 

vein. The pitchblende is believed to be a local constituent of the quartz-pyrite 

ores and was deposited by residual solutions from the quartz bostonite magma.

INTRODUCTION

The Wood and East Calhoun mines are on the south slope of Quartz 

Hill, near the head of Leavenworth Gulch, in unsurveyed sec. 14, 

T. 3 S., R. 73 E., about 1.5 miles southwest of Central City, Colo. 

(fig. 28). The mines are accessible from State Route 279 by about a 

quarter of a mile of unimproved road.

129
130 GEOLOGY AND ORE DEPOSITS, COLORADO 

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FIGURE 28. Index maps showing location of the Wood and East Calhoun mines, Central City district,

Gilpin County, Colo.

The Wood mine was an intermittent source of pitchblende from 

1872 until about 1916 and the East Calhoun one of the largest gold 

mines in the Central City district, but both properties have been idle 

since World War I. Because of the strategic importance of uranium^
GEOLOGY OF WOOD AND EAST CALHOTJN MINES, GILPIN COUNTY 131

the Defense Minerals Exploration Administration in 1951 negotiated 

a contract with the Denver Realty Co. to explore the Wood vein from 

the East Calhoun mine. As little was known about the distribution 

and paragenetic relations of pitchblende in the veins of the Central 

City district, the Geological Survey, on behalf of the U. S. Atomic 

Energy Commission and in part under the auspices of the Defense 

Minerals Exploration Administration, carried out a program of detailed geologic study and sampling in conjunction with the exploration work.

PREVIOUS INVESTIGATIONS

Pearce (1895, p. 156-158), Moore and Kithil (1913, p. 46), Alsdorf 

(1916, p. 270), and Bastin and Hill (1917, p. 245) have described the 

pitchblende and other ores obtained from the Wood mine. G. B. 

Guillotte (written communication, 1944) examined the Wood and 

East Calhoun dumps in 1943. Moore and Butler (1952) mapped the 

accessible workings of both mines in 1950 as part of the general 

reconnaissance investigation for radioactivity in the Colorado Front 

Range. F. C. Armstrong (written communication, 1956) studied the 

surface geology of Quartz Hill in 1951.

Phair (1952) noted a spatial relationship between uranium deposits 

and the quartz bostonite porphyry rocks of the Tertiary intrusive 

series of the Front Range. As a result of his work, he inferred a 

genetic relationship between the intrusion of these rocks and the 

uranium deposits.

Leonard (1952, p. 1274-1275) found that two main types of ore 

deposits give the Central City district a zonal arrangement in plan 

view: quartz-pyrite veins near the center and galena-sphalerite veins 

on the periphery. He noted that the pitchblende deposits of Quartz 

Hill lie between the zones.

FIELD WORK

The present investigation, part of the Geological Survey's current 

studies in the Front Range, began in July 1952 and continued through 

October 1953. The writer made geologic maps of the accessible 

East Calhoun workings and the exploration headings. The working 

faces were examined and sampled after each round of blasting until 

the last week of January 1953. Thereafter the workings were visited 

at least once a week, and samples were taken from the back of the 

drift at appropriate spacings. Office and laboratory work was completed at the Geological Survey offices in Denver. A total of 605 

samples were cut. They were assayed for uranium, gold, silver, 

copper, lead, and zinc. A semiquantitative spectrographic analysis 

was run on a split of each sample.
132 GEOLOGY AND ORE DEPOSITS, COLORADO

All exploration at the mine, on or from the 600-foot level of the 

East Calhoun shaft, October 1951 to October 1953, was under a 

contract with the Defense Minerals Exploration Administration. 

The U. S. Atomic Energy Commission mapped the exploration 

workings and sampled the uranium-bearing parts of the Wood vein.

ACKNOWLEDGMENTS

It is a pleasure to acknowledge the work of A. E. Dearth, who" 

assisted the writer in the geologic mapping and sampling. Every 

courtesy and consideration were shown the writer by the Denver 

Realty Co., present owners of the mines. The suggestions of P. K. 

Sims of the U. S. Geological Survey are deeply appreciated. All assays, 

chemical analyses, and spectrographic analyses were made by the 

Geological Survey. A. J. Martin of the U. S. Bureau of Mines kindly 

furnished the production data.

GENERAL GEOLOGY

The Wood-East Calhoun area is on a northeast-trending anticline, 

which is the major structural feature of this part of the Central 

City district (fig. 29). The area is underlain by interlayered Precambrian gneiss and granite pegmatite. The gneiss consists of 

layers of granite gneiss, described by Bastin and Hill (1917, p. 30-32), 

with lesser amounts of biotite-quartz-plagioclase gneiss, migmatite, 

and amphibolite. Conformable layers and small irregular-shaped 

pods of granite pegmatite occur within the gneiss. These units 

generally dip gently away from the crest of the anticline and are 

locally deformed into tight drag folds or broad, gentle warps.

The Precambrian rocks are intruded by several quartz bostonite 

porphyry dikes of Tertiary age. The dikes occupy northwest-trending fractures (fig. 29). Steeply dipping faults that trend from east 

to N. 40° E. cut all the rocks (fig. 29). The faults are occupied 

by veins that average about 1 foot in width. The veins contain 

quartz-pyrite-gold-silver in the east and quartz-pyrite-sphaleritechalcopyrite-gray copper-galena-gold-silver in the west.

BOCKS OF PRECAMBRIAN AGE

The Precambrian rocks of the Wood and East Calhoun mines consist of granite gneiss, biotite-quartz-plagioclase gneiss, migmatite, 

amphibolite, and granite pegmatite. All these rocks except some of 

the granite pegmatite have been deformed and recrystallized and 

have a metamorphic texture.

The biotite-quartz-plagioclase gneiss and amphibolite are probably 

of metasedimentary origin. Bastin and Hill (1917, p. 26-30) and 

Lovering and Goddard (1950, p. 19-20) included these rocks in U^[fl
100 200 Feet

FIGURE 29. Geologic map of the Wood-East Calhoun area, Central City 
134 GEOLOGY AND ORE DEPOSITS, COLORADO

Precambrian Idaho Springs formation. The granite gneiss, as described by Bastin and Hill, is a metamorphic rock of uncertain origin. 

Its average composition is quartz monzonite; in this report, however, 

the writer follows the usage of Bastin and Hill and refers to it as 

granite gneiss. The granite pegmatite has a simple composition and 

is younger than the other rocks.

BIOTTTB-QUAKTZ-PLAGIOCIiASE GNEISS

Biotite-quartz-plagioclase gneiss does not crop out in the WoodEast Calhoun area, but it is found in the workings of both mines 

(pi. 12 and 13). The unit occurs principally in a layer between the 

fifth and sixth levels of the East Calhoun mine. Moore and Butler 

(1952) give schist, presumably equivalent to biotite-quartz-plagioclase 

gneiss, as the principal rock type in the upper workings of the Wood 

mine. Many layers of the rock that are too small to be mapped 

occur within the granite gneiss.

The biotite-quartz-plagioclase gneiss is light to dark gray, depending on the biotite content. The rock ranges from fine to medium 

grained; the medium-grained variety predominates. The principal 

minerals in order of abundance are plagioclase (near oligoclase), 

quartz, biotite, and microcline. Accessory minerals include magnetite, zircon, and apatite. In the East Calhoun mine, Wood drift east, 

a somewhat different phase of the unit is found within the granite 

gneiss in layers ranging from an inch to about 1 foot in thickness. It 

is a fine-grained salt-and-pepper rock that contains as much as 5 

percent pink garnet.

The rock is moderately to strongly foliated by mineral orientation 

and compositional banding. Nearly all the rock contains conformable layers of pegmatite from a few inches to several feet thick.

AMPHTBCHLITE

Amphibolite occurs as layers and lenses from 1 to 24 inches thick 

within the granite gneiss. It is a dark- to medium-graj7, medium- to 

coarse-grained equigranular rock composed of nearly equal parts of 

dark-green hornblende and oligoclase-andesine. Accessory minerals 

include biotite, microcline, and some quartz. Many lenses of the 

rock have been altered to a "punky" biotite schist. Subparallel 

layering produces a fair to good foliation. Amphibolite lenses, now 

chiefly biotite schist, occur on the crests and troughs of northwesttrending warps in the more alaskitic granite gneiss.

MIGMATITE

A unit of closely interlayered rocks, termed migmatite in this 

report, was mapped west of the shaft on the fourth level of the East 

Calhoun mine (pi. 12). The unit is composed of biotite-quartz-
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 135

plagioclase gneiss in layers 1-2 inches thick separated by granitic 

layers averaging about 1 inch in thickness. The unit is somewhat 

similar in appearance to the so-called "injection gneisses."

GRANITE GNEISS

The most abundant rock in the area is granite gneiss. It is dominant in the East Calhoun mine above the first and below the fifth 

levels. Several 1-10-foot layers are also present in the metasedimentary body between the first and fifth levels. Contacts of the 

granite gneiss are conformable to the layering of the metasedimentary 

rocks. The granite gneiss is a dark- and light-gray layered, mediumgrained rock composed chiefly of feldspar, quartz, and biotite. Individual layers range from about an inch to several feet in thickness 

and contain different proportions of biotite. The perfection of rock 

foliation varies, depending upon the primary layering and parallel 

orientation of biotite flakes; foliation in the more alaskitic phases of 

the rock is poor.

Bastin and Hill (1917, p. 30-32) classified the rock as granite gneiss, 

but the composition of the specimens and thin sections examined by 

the writer more nearly approximates that of quartz monzonite. The 

average composition is about 44 percent plagioclase (An25), 30 percent 

quartz, 20 percent microcline, and 5 percent biotite. Other minerals, 

not everywhere present, include magnetite (as much as 1 percent in 

some specimens), muscovite, hornblende, apatite, sphene, rutile, and 

epidote. The texture is granoblastic. Discrete layers and lenses of 

metasedimentary rock, commonly only a few feet wide, are scattered 

through the granite gneiss at places. Younger granite pegmatite intrudes the gneiss.

GRANITE PEGMATITE

The granite pegmatite is a medium- to coarse-grained light-gray 

alaskitic rock composed primarily of microcline and quartz. Biotite 

and magnetite are accessory minerals; magnetite is more widespread 

and abundant. Granite pegmatite bodies are generally conformable 

to the foliation of the other Precambrian rocks. Bodies of pegmatite 

within the granite gneiss are discrete layers along the foliation and 

small irregular-shaped pods in the crests and troughs of small folds. 

The pegmatite in the biotite-quartz-plagioclase gneiss occurs as distinct bodies in part large enough to map (pi. 12) and as discontinuous 

thin conformable layers.

BOCKS OF TERTIARY AGE

The bostonite dike rocks in the area mapped belong to the Tertiary 

intrusive sequence of the Front Range and are among the most radioactive igneous series in the world. Phair (1952) has divided the bostonite rocks into three subtypes quartz bostonite porphyry, non-
136 GEOLOGY AND ORE DEPOSITS, COLORADO

porphyritic quartz bostonite, and syenitic bostonite depending on 

the presence or absence of quartz in excess of 5 percent by volume and 

by the presence or absence of megascopic phenocrysts of pink potash 

feldspar. The quartz bostonite subtype of this area contains from 

0.010 to 0.025 percent equivalent uranium and is from 10 to 20 times 

more radioactive than the intruded Precambrian rocks.

QUARTZ BOSTONITE PORPHYRY

Four quartz bostonite dikes were mapped on the surface and underground in the area. The dikes trend northwestward, dip moderately 

to steeply to the northeast, and range in thickness from 1 to perhaps 

10 feet. Quartz bostonite is a lilac-colored fine-grained porphyritic 

rock. Its characteristic texture is trachitoid, resulting from the 

subparallel arrangement of feldspar phenocrysts in a groundmass of 

quartz and feldspar. The margins of the dikes are finer grained than 

the interiors; glass is found at some contacts.

SYENITIC BOSTONTTB PORPHYRY

In the Wood drift east, about 22 feet west of the face (pi. 13), a 

relict 2- to 3-inch bostonite dike was found in what is now a northwest-striking vein. The dike is badly broken and on the north wall 

of the drift it is completely obliterated. A thin-section study shows 

that this dike contains very little quartz, is high in mafic minerals, 

and that the phenocrysts are plagioclase. Based on the classification 

by Phair (1952) this rock is syenitic bostonite porphyry.

STRUCTURE

The principal Precambrian structure in the area is a broad, eastnortheast-trending anticline with gentle to moderate dipping limbs. 

Associated minor fold structures, such as warps and drag folds, essentially parallel the anticlinal axis or transect it at an angle of 60° 

Mineral lineations generally parallel the fold axes.

Steeply dipping faults of Tertiary age trend east, east-northeast, 

and northwest. The northwest faults were loci for dike emplacement 

and the east and east-northeast faults are metallized. The northwest 

faults are cut and displaced by both the east and east-northeast trending faults; the east-trending faults are displaced by the east-northeast 

faults. Some of the joints were formed by the same forces that produced the faults. There is no apparent genetic relationship between 

the Precambrian structures and the faults of Tertiary age.

FOLDS

The major structural feature in the area is the Central City anticline. The trace of its axial plane trends about N. 60° E. (fig. 29),
GEOLOGY OP WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 137

but it averages about N. 30° E. 1 Associated with this major structure 

are many small drag folds which plunge gently to the north-northeast 

or east-northeast. Superposed on the major anticline and its associated structures are a series of warps that trend N. 10°-50° W., 

averaging about N. 30° W., and plunge gently to the northwest or 

southeast (not shown on figures).

IJNEATION

Lineations measured in the East Calhoun and Wood mines include 

mineral alinement, small drag-fold axes, warps, rodding, and corrugations. A lower-hemisphere Scbmidt-net plot of 110 lineations (fig.^30) 

shows four maximums: 10° N. 30° E., 5° S. 60° W., 5° N. 60°tE., and

Plotted on lower hemisphere of Schmidt net

0-2.5 percent 2.5-3.5 percent

EXPLANATION

3.5-4.5 percent 4.5-5.5 percent 5.5-6,5 percent

FIGUBE 30. Contour diagram of 110 lineations in rocks of the East Calhoun and Wood mines plotted on 

the lower hemisphere of the Schmidt net.

1 Sims, P. K., Drake, A. A., Jr., and Moench, R. H., 1953, Preliminary geologic and vein maps of part of 

the Central City district, Gilpin and Clear Creek Counties, Colo.: U. S. Geol. Survey open-file rept.
138 GEOLOGY AND ORE DEPOSITS, COLORADO

15° S. 25° W. These lineations generally parallel the major and 

minor folding in the area studied.

JOINTS

To avoid overcrowding, joints have not been plotted on plates 12 

and 13. Instead, the poles to joint planes were plotted and contoured 

on the upper hemisphere of a Schmidt net (fig. 31). Three strong 

maximums representing joint sets are present: N. 80° E., nearly 

vertical; N. 40° W., nearly vertical; and N. 45° W., 70° NE. The 

steep N. 80° E. set closely approximates the attitude of the Wood 

vein and possibly was formed by the same forces that produced the 

east-west fractures. The two northwest-trending maximums probably represent statistical peak readings on the same joint set.

Plotted on upper hemisphere of Schmidt net

O-r percent 1-2 percent

EXPLANATION

2-"3 percent 3-3.5 percent 3.S.-.4 percent

FIGUBE 31. Contour diagram of 386 joints in the Wood-East Calhoun area plotted on the upper hemisphere

of the Schmidt net.
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 139

FAULTS

The three principal faults in the area are the east-trending Wood 

fault and the east-northeast-trending Calhoun and Quartz Mill faults. 

These faults are metallized and are major veins. Minor east-northeast 

to northeast-trending faults are also metallized. Several barren 

northwest-trending faults were mapped underground (pi. 13). The 

faults hi order of age from oldest to youngest are northwest-, east-, 

and east-northeast-trending.

CALHOUN FAULT

The Calhoun fault strikes about N. 65° E. and dips an average of 

70° SE. It is the most persistent fault on the south slope of Quartz 

Hill and has been traced on the surface for 4,000 feet.2 Minor ore 

brecciation and the formation of gouge indicate that some postmineral 

movement took place along the break. Neither the true direction 

nor the amount of movement on the fault is known, but on the fourth 

level of the East Calhoun mine a bostonite dike contains an apparent 

horizontal separation of 10 feet, the south wall having moved west 

with respect to the north wall. On the first level of the East Calhoun 

mine the Wood vein is displaced by the Calhoun vein, the south wall 

having moved west horizontally about 4 feet. On the same level, 

the south segment of a bostonite dike has moved west a few inches. 

It therefore seems that movement on the Calhoun fault was strikeslip with a normal type of displacement.

QUARTZ MILL FAULT

The Quartz Mill fault crops out south of the Calhoun fault and 

intersects it just above the sixth level of the East Calhoun mine 

(pi. 12). The fault strikes about N. 65° E. and dips 70°-75° N. It 

is similar in character to the Calhoun fault. In the Bezant mine, 

which exploits the Quartz Mill vein, the Quartz Mill fault displaces 

the south segment of a bostonite dike about 7 feet horizontally to the 

west. Therefore, it appears that the movement along the fault was 

similar to that along the Calhoun fault. The Quartz Mill-Calhoun 

fault crossing can no longer be seen, but Sanderson 3 observed no 

displacement at the intersection. The attitude of the two veins is 

such that their intersection is essentially horizontal except for local 

flat plunges to the east or west.

* Sims, P. K., Drake, A. A., Jr., and Moench, R. H., op. cit.

' Sanderson, H. S., 1909, Report on the properties of the Bezant Mining Co.: Unpublished private mining report.
140 GEOLOGY AND ORE DEPOSITS, COLORADO

WOOD FAULT

The Wood fault has been traced on the surface for about 1,000 

feet.4 It trends eastward and dips steeply either side of vertical. 

Abundant gouge, intravein slip planes, and numerous slickensides 

indicate that, in contrast to the Calhoun fault, there was much recurrent movement along the break. The attitudes of 70 slickensides 

were plotted and contoured on the lower hemisphere of a Schmidt 

net (fig. 32). This diagram shows nearly horizontal masdmums in an 

east-west direction, indicating that the movement along the fault 

was essentially horizontal. Several steep slickensides, too few to 

show a statistical maximum, plunge vertically down the dip of the 

fault. Where measured, these dip-slip slickensides are later than the

Plotted on lower hemisphere of Schmidt net

0-9 percent 9-12 percent

EXPLANATION

12-15 percent IS-18 percent 18-21 percent

FIGUBE 32. Contour diagram of 70 slickensides on the Wood vein plotted on the lower hemisphere of the

Schmidt net.

4 Sims, P. K., Drake, A. A., Jr., and Moench, R. H., op. cit.
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 141

strike-slip slickensides. The direction and amount of displacement 

on the Wood fault are not known definitely, but it appears probable 

that the movement was principally horizontal, the south wall having 

moved east with respect to the north wall. Detailed geologic mapping 

of the surface shows that the south segments of two bostonite dikes 

have shifted 30-40 feet east along the Wood fault. The horizontal 

displacement may appear exaggerated, however, because of the poor 

exposures on Quartz Hill. On the first level of the East Calhoun 

mine, the south segment of a bostonite dike is displaced perhaps 3 

inches to the east. As the Wood fault is distinguished by lateral 

movement, is nearly vertical, and varies little in strike and much in 

dip, it is probably a wrench fault (Anderson, 1951, p. 59-60).

MINOR FAULTS

The Wood exploration drift east (pi. 14) does not follow the Wood 

vein but is instead on a northeasterly split that dips steeply either 

side of vertical. This fault alines well with the Willowdale patented 

claim, and accordingly the writer has named this split the Willowdale 

vein. Moore and Butler (1952) show a split off the Wood vein near 

the shaft on the 135, 197, and 275 levels of the Wood mine that bears 

S. 75°-80° W. and dips nearly vertical. On the 275 level west, about 

200 feet from the Wood shaft, they show another southwestwardtrending split from the Wood fault. The writer's mapping shows 

many minor splits from the Wood fault that trend either northeastward or southwestward. Many branches connecting parallel to subparallel veins have a general northeasterly strike. Although these 

minor faults have the appearance of splays from the Wood fault, the 

presence of gouge and abundant slickensides characteristic of shearing 

complicates the theory of fault movement. This is mechanically 

resolved in the following interpretation of the order of faulting:

(1) formation of the Wood fault and associated tension fractures, the 

movement being south wall east with respect to the north wall;

(2) formation of the Calhoun fault, the movement being south wall 

west with respect to the north wall; (3) stress relieved by movement 

along the Wood fault and shearing along the previously formed 

tension fractures, such movement tending to realine displaced units 

along the Wood fault and accounting for the apparent lack of displacement. If this type of fracturing took place, one would expect 

strong fracturing in the vicinity of the Wood fault-tension fracture 

junctions. Such loci are in fact strongly fractured.

The Mayflower fault, mapped on the surface in the Wood-East 

Calhoun area, strikes about N. 50° E. and dips nearly vertical. Its 

relation to the Wood fault is obscure. Armstrong (written com413512 O 57 3
142 GEOLOGY AND ORE DEPOSITS, COLORADO

munication, 1956) believes that this fault extends north and south of 

the Wood fault, but the writer mapped no structure underground 

that correlates with the Mayflower fault.

STRUCTURE OP TERTIARY DIKES

The Tertiary dikes in the Wood-East Calhoun area fill steeply 

dipping fractures in the Precambrian rocks. The attitude of the 

dikes is essentially the same as the northwest-trending joint set and 

barren northwest-trending faults. This evidence indicates, at least 

for the small area studied, that the dikes were emplaced in northwesttrending fractures. Because the dikes are cut and displaced by the 

veins, it is inferred that they fill a fracture system older than the veins.

ECONOMIC GEOLOGY

The gold, silver, uranium, copper, lead, and zinc deposits of the 

area are found in veins probably formed at moderate temperatures. 

These deposits are early Tertiary in age and have been related genetically to the intrusion of porphyritic dike rocks (Lovering and Goddard, 

1950, p. 170-191; Phair, 1952). Gold, silver, and uranium provided 

most of the dollar value of the ore produced.

Two main types of ore deposits give the Central City district a 

zonal arrangement in plan view (Leonard, 1952). A core, about 2 

miles in diameter, of quartz-pyrite veins is surrounded by a wide 

outer zone of lead-zinc-silver veins. The area of overlap of these two 

zones includes the Wood and East Calhoun mines where transition 

veins contain pitchblende in addition to gold, silver, copper, lead, 

and zinc ores.

The uranium deposits of the area contain pitchblende and occur 

at places along the metalliferous veins as pods and lenses. These 

pods and lenses occur in the same westerly raking ore shoots as the 

other metals. Pitchblende was deposited earlier than sphalerite, 

galena, chalcopyrite, gray copper, and most of the pyrite and contains 

high trace amounts of zirconium and molybdenum.

HISTORY AND EXPLORATION

EAST CALHOUN MINE

The East Calhoun mine (pi. 12) was probably opened in the 1860's. 

The mine was operated intermittently until World War I. It was 

reopened in 1949, and a Defense Minerals Exploration Administration 

project was approved in 1951.

The East Calhoun shaft is inclined about 70° south from the surface 

to just above the sixth level, where the Calhoun vein intersects the 

Quartz Mill vein. The original shaft was continued on the Calhoun
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 143

vein for perhaps another 30 feet,5 but as the vein was valueless another 

shaft was sunk on the Quartz Mill vein which dips to the north. This 

vein was exploited to a depth of about 980 feet. In 1954 the mine 

was inaccessible below the sixth level. The positions of drifts and 

stopes are shown on figure 33.

9200-

9100-

8600-

8500-

8400-

NE

LEVEL 2

LEVEL 3

LEVEL 5

Crosscut south to Bezant mine'

JZL

Collor of Eosl Colhoun shoft

Crosscut northx' II 

to Wood vein

LEVEL '£ 6

^Crosscut north to Wood vein

EXPLANATION

Mine' workings 

Dusted where approximate

Inaccessible workings

«^

Stops

Modified from Moore and Butler, I9SZ

100 200 feet

Datum /s mean'sf a lev*! 

FiGtJEE|33. Longitudinal projection of the East Calhoun mine, Central City district, Gilpin County, Colo.

* Sanderson, H. S., op. cit.
144 GEOLOGY AND ORE DEPOSITS, COLORADO

The Wood mine is one of the oldest patented locations on Quartz

Hill, No 

to the p:

WOOD MINE

232. It has been worked intermittently from the late 1860's 

 esent. Pitchblende was first noted on the dump of the Old

Wood si aft in 1871 (Pearce, 1895, p. 157-158).

The T food vein is exploited by the Wood and Old Wood shafts on

the west) (fig. 29) and by the Ross shaft that is 320 feet east of the

junction of the Wood and Calhoun veins. Moore and Butler (1952) 

mapped the 135, 197, and 275 levels of the Wood mine (fig. 34); other 

levels th it are inaccessible supposedly are present at vertical depths of 

400, 500 and 600 feet. Neither the depth nor the amount of working 

from the Old Wood shaft is known. The Ross shaft is about 120 feet 

deep and is inclined about 85° south. The vein has been stoped from 

the surface to a point about 70 feet below the collar of the shaft. 

These workings were inaccessible in 1954. The Wood vein is accessible from] the first level of the East Calhoun mine by a crosscut north 

and a sh^rt drift (pi. 12) and by the exploration workings on the sixth 

level of {,he East Calhoun mine (pi. 13).

PRODUCTION

EAST CAUIOtlNr MINE

The t:>tal production of ore from the East Calhoun mine is not 

known. Table 1 shows the production from the Jefferson-Calhoun 

vein from 1902 to 1915, including production from the East Calhoun, 

West Calhoun, Kemp Calhoun, and Jefferson mines. Residents of the

district generally assign a value of about $1,500,000 to the ore produced from the Calhoun vein. Little ore was produced from the 

Calhoun! vein in the East Calhoun mine as most of the stopes are on 

the Quartz Mill vein. So far as known, no pitchblende has been 

produce^ from the East Calhoun mine.

WOOD MINE

are no records of the total production of ore from the Wood 

"able 2 shows the substantiated pitchblende production, 

found record of only one shipment of gold and silver, but 

of the district assign a value of about $600,000 to the ore 

the Wood vein. Bastin and Hill (1917, p. 245) state that 

extracted in driving the 200 level to the east averaged $10 per 

value in 1917: $20.67 per ounce) and that the gold content 

high, one assay showing 35 ounces.

There 

mine. T. The writer 

residents 

mined 

the ore 

ton (gold 

was exceptionally

from
Feet 

9300-

9200-

9100-

9000-

8900-

8800-

8700-

WOOD SHAFT

West

OLD WOOD SHAFT

1 [ EAST CAL

\\ \\ 

\ 

\\ 

\\

Crosscut south 

East Colhoun 

80160 

Datum is mean sea level * 

FIGUEE 34. Vertical longitudinal projection of the Wood mine, Central 
146 GEOLOGY AND ORE DEPOSITS, COLORADO

1. Production from the Jefferson-Calhoun vein, 1902-15 l

Yepr

1902___.________

1903-

1904_. _________

1905--___-___-_-

1906. -. --_-___

1907. ___________

1907____________

1908-_--l----__-

1908--__, _______

1909____________

1910___. ________

1910_

1912... _________

1912____. _______

1913_-__------_-

1913_____--__-_-

1914__-_________

1914_

1915_.__----___-

Total_.__._

Crude ore 

shipped 

(tons)

219

313

60

1,510

30

2,280

5,843

38

198

171

1,406

561

5

12, 634

Concentrates 

shipped 

(tons)

382

695

11

27

130

62

1,307

Gold (ounces)

179. 00

225. 37

90.00

755. 00

96. 75

901. 16

265. 55

1 724 34

472. 29

28.00

74. 89

3. 98

49.06

20.40

592. 53

39.44

342. 98

36.97

3. 86

5, 901. 57

Silver 

(ounces)

1,020

1,051

900

750

746

1,944

3,021

4,079

3,828

304

550

49

70

48

1,577

517

1,012

225

29

22, 720

Copper 

(pounds)

5,829

3,533

3,429

10, 786

2,043

9,676

4,630

19, 373

3,333

660

7,014

2,027

6,338

569

181

79, 421

Lead 

(pounds)

5,476

1, 125

330

6,931

i A. J. Martin, Metals Economics Branch, XJ. S. Bureau of Mines, Denver, Oolo. Published with 

permission ofj the Bureau of Mines.

2. Substantiated pitchblende production from the Wood vein 

[From Armstrong, F. C., written communication, 1966]

Year

1871_________

1872_________

1873--__-____

1884. ________

1894_________

1897_--_^__--

1898--__ .__-

1913----^-_--

1916-_-.J____

1940. ________

Total. __

Crude 

ore 

shipped 

(tons)

0. 10

3. 10

2. 70

3. 00

(?)

17.00

33. 00

.20

10.00

.50

69. 60

Concentrates 

shipped 

(tons)

'35

35

Gold 

(ounces)

1 53.60

53. 60

Silver 

(ounces)

i 77

77

U308 

(pounds)

120

3, 720

2,254

4,200

(?)

27, 000

7,920

200

1,200

135

46, 749

Remarks

Hand sorted.

Do.

Pearce purchased a

quantity of ore. 

Not fully substantiated.

' A. J. Martin, Metals Economics Branch, U. S. Bureau of Mines, Denver, Colo. Published with permission of the Bureau of Mines.
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GJLPIN COUNTY 147

GENERAL CHARACTER OF THE VEINS

The veins in the area are for the most part single well-defined fault 

fissures ranging from % inch to 24 inches in width containing from a 

few to many subparallel veinlets and slips which form complex lodes. 

Outward from these fissures the wall rocks are replaced for a few inches 

to a few feet. The wall rock is exceptionally silicified and pyritized to 

widths as much as 5 feet; on the average, the width of the altered zone 

is about 18 inches. The east-trending veins, such as the Wood, show a 

marked tendency to form looping and "horsetail type" branches. 

These branches generally are more common in areas of metasedimentary rocks. "Horses" of rock between splits and parallel veinlets are 

strongly shattered, silicified, pyritized, arid in some places nearly 

completely replaced by ore minerals. The lode zones are generally 

uniform in strike, but highly variable in dip. Ore shoots commonly 

occur on the steeper parts of the veins. All the veins show repeated 

fracturing, open filling, and replacement to a greater or lesser degree. 

Breccia fragments of both altered wall rock and vein filling, granulation 

of minerals, particularly pyrite, and many sets of crosscutting veinlets 

and slip planes indicate repeated movement. Deposition in open 

cavities is shown by the pronounced formation of comb structure in 

the vein quartz and abundant crystals and encrustations of ore 

minerals on the walls of openings. The veins are stronger in the 

granite gneiss and in places pinch to a barren slip in the biotite-quartzplagioclase gneiss and amphibolite.

CALHOUN VEIN

The Calhoun vein strikes about N. 60° E. and for the most part 

is a single well-defined fault fissure that ranges in width from ^ to 18 

inches. It consists of pyritized silicified wall-rock fragments, gouge, 

pyrite, and quartz, with spotty chalcopyrite and sphalerite and, rarely, 

a little sooty pitchblende. Individual streaks of quartz and ore 

minerals seldom exceed 3 inches in width. The silicified and pyritized 

halo may be as much as 5 feet in width but averages about 18 inches. 

The vein is tight much more so than the Wood and shows little 

evidence of much postmineral movement; however, 1-3 inches of 

gouge is present along the walls and along a few oblique postmineral 

slips, and ore minerals, particularly pyrite, are granulated. Deposition in open spaces is indicated by the presence of comb quartz, 

pyrite, chalcopyrite, and sphalerite crystals in a few vugs. Silicified 

and pyritized wall rock, relict islands of pyrite surrounded by chalcopyrite and sphalerite, and the corrosion of quartz crystal faces by 

sulfides indicate that the vein-forming minerals formed, in part at 

least, by replacement.
148 GEOLOGY AND ORE DEPOSITS, COLORADO

The Calhoun vein generally is narrow and sparsely filled where it 

cuts metasedimentary rocks, except where these rocks were strongly 

migmatized. The vein seldom splits or loops except on the fourth and 

sixth levels west, where the vein is in biotite-quartz-plagioclase gneiss. 

For the most part the vein is stronger on the "steeps." The vein 

steepens in dip to the west and is nearly vertical at the West Calhoun 

shaft (Moore and Butler, 1952); all the major stopes are to the west 

of the area mapped.

QUARTZ MILL VEIN

The Quartz Mill vein strikes about N. 65° E. and dips 70°-75° N. 

and ranges in width from about 3 inches to 3 feet. Where present in 

the East Calhoun and Bezant mines the vein has been stoped and 

little is known about its character. It consists of silicified and 

pyritized wall-rock fragments, quartz, and pyrite. Presumably, 

where stoped, the vein also contained chalcopyrite and possibly some 

gray copper, for samples cut by Sanderson 6 show moderate amounts 

of copper. In the area studied, most of the stoping was done on the 

steeper parts of the vein.

WOOD VEIN

The Wood vein trends nearly east-west and is highly variable in dip 

(pi. 14). It consists of one principal vein-filled fissure with many 

parallel to subparallel fractures and veinlets; therefore, it more 

properly should be termed a lode (Lindgren, 1933, p. 157-158). The 

principal vein-filled fissure ranges from 2 to about 24 inches in width. 

Discontinuous streaks of sulfides and quartz seldom exceed 6 inches in 

width. The vein shows abundant evidence of repeated fracturing, 

open filling, and replacement. Breccia fragments of altered wall rock, 

granulation of pyrite and pitchblende, many crosscutting veinlets and 

slip planes, abundant slickensides, and strong gouge indicate repeated 

movement (pi. 14). Deposition in open cavities is shown by the 

colloform structure in the pitchblende, a well-defined comb structure 

in vein quartz, and the abundant deposition of quartz, pyrite, and 

sphalerite crystals as encrustations in vugs. Replacement is indicated 

by silicified and pyritized wall rock and by the texture of the ore 

minerals. The vein is thickened near acute fissure intersections because of the shattering of the intervening wedge of rock. Parts of the 

vein are similarly thickened between parallel fissures. The vein is 

stronger on strike changes to the northeast, indicating that wider 

fissures were produced by movement of the south wall east. No dip 

control was noted; however, the vein appears to be stronger on steep 

dips to the'north.

'Sanderson, H. S., op. cit.
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 149

WTLLiOWDALE VEIN

The Willowdale vein, a hanging-wall branch of the Wood vein (pi. 

13), strikes about N. 45° E. and is highly variable in dip north and 

south. The vein is similar to the Wood in that it shows abundant 

evidence of recurrent movement. However, it is a much weaker 

structure and carries sparse amounts of ore minerals.

MINERALOGY

The veins in the Wood and East Calhoun mines contain in order of 

decreasing abundance: quartz, pyrite, chalcopyrite, sphalerite, tetrahedrite-tennantite, galena, pitchblende, bornite, sooty chalcocite, and 

covellite. The suite is generally typical of those deposits that probably 

formed at moderate temperature and pressure.

QUARTZ

The gangue mineral in all the veins is quartz. At least four types of 

quartz are recognized. The first type is a light-gray very fine grained 

cherty material that replaces the wall rock. In polished surface 

observed under the microscope, a similar-appearing fine-grained 

quartz cements fragments of pitchblende. White massive mediumgrained quartz replaces the wall rock and fills fractures. White to 

clear, terminated quartz crystals, as much as half an inch long, grow 

outward from the massive quartz, line vugs, and form comb structure 

in the veins. The fourth type of quartz, the last to form, is a brown 

to gray fine-grained, sometimes banded, variety (the horn quartz of 

the miners) which fills open spaces in the veins and forms thin coatings 

on crystals in vugs.

PYRITE

Pyrite is the most abundant mineral in the veins and four types are 

recognized. The first type is yellow finely crystalline cubic pyrite 

that impregnates the wall rock. Another type pale-yellow pyrite 

is associated paragenetically and spatially with pitchblende in the 

Wood vein. This pyrite occurs in veinlets and aggregates and is 

brecciated or fractured. Where unbroken, it is chiefly cubic in habit. 

Nearly everywhere observed, this pyrite veins pitchblende or pitchblende-quartz breccia; but in a few polished surfaces it appears also to 

be intergrown with pitchblende. The third type of pyrite, a yellow 

generally crystalline (mostly cubic) variety, is the principal veinfilling mineral. A fourth type of pyrite forms cubic crystals and 

encrustations on quartz and other crystals in vugs, and is in turn 

coated by gray to brown fine-grained quartz. Gold and silver occur 

in the third type of pyrite (table 3). The other types of pyrite also 

may carry gold and silver, but no data are available.
150 GEOLOGY AND ORE DEPOSITS, COLORADO

TABLE 3. Gold and silver content of pyrite, Calhoun and Wood veins, Central City 

district, Gilpin County, Colo.

Locality

East Calhoun mine, 6th level: 

Calhoun vein, 800 feet west of shaft l _ ___________________

Wood exploration drift, Wood vein, 500 feet west of crosscut 2 _

Ounces per ton

Au

1.24 

.04

Ag

2.46 

3.02

1 Armstrong (written communication, 1956).

2 Sized, superpanned, and hand-picked. Analysis by U. S. Geological Survey.

CHAIX30PYRITE

Chalcopyrite is the second most abundant ore mineral in the Wood 

and Calhoun veins. Veinlets of chalcopyrite cut sphalerite, pyrite, 

quartz, and galena. Under the reflecting microscope, a replacement 

texture composed of relict islands of sphalerite and pyrite was noted 

in chalcopyrite. Relict islands of chalcopyrite were noted in galena. 

Two analyses of chalcopyrite (table 4) show a high silver and a moderate gold content.

TABLE 4. Gold and silver content of chalcopyrite, Calhoun and Wood veins, Central 

City district, Gilpin County, Colo.

Ounces per ton

East Calhoun mine, 6th level: 

Calhoun vein, 800 feet west of shaft 1 _____--_-_--------__-

Wood exploration drift, Wood vein, 500 feet west of crosscut 2 _

Au

0.90

.34

Ag

33.80

17.60

1 Armstrong (written communication, 1956). 

* Analysis by U. S. Geological Survey.

SPHALERITE

Sphalerite veins, which commonly consist of the dark iron-rich 

variety, marmatite (table 5), cut quartz and pyrite; sphalerite crystals 

fill fractures and vugs and corrode pyrite crystals along mutual 

boundaries. "Resin jack" is also found in vugs as small tetrahedrons 

that have been deposited on dark sphalerite. Chalcopyrite veinlets 

traverse sphalerite, and tiny blebs and needles are distributed through 

the dark sphalerite. Most of this chalcopyrite probably is of replacement origin, but some may have been exsolved. The sphalerite 

contains small amounts of gold and silver (table 5).
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 151

TABLE 5. Analyses of sphalerite, Calhoun and Wood veins, Central City district,

Gilpin County, Colo.

Locality

East Calhoun mine, 6th level: 

Calhoun vein, 800 feet west of shaft J _ 

Wood exploration drift, Wood vein, 

500 feet west of crosscut 2 __________

Ounces per ton

Au

0. 16

Ag

0.76

Percent

Fe

17.03

Cu

4. 26

Pb

0. 30

 Armstrong (written communication, 1966).

* Analysis by U. S. Geological Survey.

TETRAHEDRITE-TENNANTITE

Gray copper is moderately abundant in both the Calhoun and Wood 

veins. It is steel gray, has a red streak, and may be mistaken for 

hematite. In polished section the mineral has a greenish cast indicating that it may be near the tennantite end of the solid solution. 

Bastin and Hill (1917, p. 99) found that most of the gray copper of 

the Central City district was tennantite. In polished section, 

tetrahedrite-tennantite veins chalcopyrite and sphalerite. Good "sea 

and island" textures of gray copper in both sphalerite and chalcopyrite 

indicate replacement.

GALENA

Galena is sparse in both the Wood and Calhoun veins. Generally it 

is strongly crystalline and fills open spaces in the veins. Under the 

reflecting microscope galena veins sphalerite, tetrahedrite-tennantite, 

and chalcopyrite, and it is in turn cut by chalcopyrite veinlets. 

Relict islands of pyrite, sphalerite, and chalcopyrite occur in the galena. 

Galena streaks were also noted along the cleavage of pyrite, sphalerite, 

and chalcopyrite. Relict islands of galena also were seen in chalcopyrite.

GOLD ANX> SILVER

Gold and silver are carried by pyrite, chalcopyrite and sphalerite, 

and probably also by tetrahedrite-tennantite and galena. It is not 

known, however, whether they are carried mechanically or in solid 

solution. Free gold is also present. Miners at the East Calhoun 

mine have panned gold from ore from stopes above the 520 level 

(fig. 34 and pi. 13). The gold is a "flour" type which is lost by normal 

milling practice. No gold was observed in polished section. The 

gold-silver ratio in samples from the Wood exploration drift is 1:5 

(fig. 35).
152 GEOLOGY AND ORE DEPOSITS, COLORADO

\.ZS, 28.7Z

0 I 2 3 * 

SILVER, IN OUNCES PER TON

FIGURE 35. Graph showing the relation of gold to silver in samples from the Wood vela. 

PITCHBLENDE

Hard, lustrous pitchblende occurs as small discontinuous streaks 

and lenses on the footwall of the Wood vein. The maximum thickness of individual lenses observed by the writer is 4 inches; however, 

Bastin and Hill (1917, p. 245) reported ore bodies as much as 2 feet 

thick on the upper levels of the Wood mine. The pitchblende lenses 

are separated from other parts of the vein by a strong, heavily slickensided fracture (pis. 13 and 14). Sooty pitchblende was observed 

in postmineral slips near the hard pitchblende and on fracture surfaces at the shaft on the first level of the East Calhoun mine. On 

the same level sooty pitchblende coats the vein wall just east of the 

intersection of the Wood and Calhoun veins.

Megascopically, the pitchblende is hard, black, and lustrous. It 

appears massive but shows a colloform structure under a reflecting 

microscope. It has been brecciated and it occurs in veinlets and as 

aggregates of spheroidal grains. Most of the grains have rounded 

margins that are in part spheroidal, and many have been rotated. 

All spheroidal grains have both radial (syneresis) and circumferential 

cracks and some sections contain tiny, apparently massive, grains of 

pitchblende. They probably represent breccia fragments of larger 

areas of nearly uniform appearance. Most cracked grains are predominantly light gray, in part healed by a darker gray pitchblende,
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 153

which indicates brecciation during deposition. Almost all the grains 

have a slight color banding parallel to the colloform layering; darker 

material occurs on the outside and is probably the result of the difference in oxidation. A little pale-yellow pyrite is intergrown with 

the pitchblende, but pitchblende is veined by pyrite in most sections 

studied. In some places the brecciated pitchblende is healed and 

cemented by quartz. Some pitchblende appears to replace quartz, 

but for the most part it does not. In the pitchblende-bearing sections 

studied, no minerals other than pitchblende, quartz, and pyrite were 

noted.

An analysis of pitchblende from the Wood mine (Hillebrand, 1891, 

p. 65-66) showed 58.51 percent UO2 and 25.25 percent UO3 . The 

specific gravity is 8.068. Lead-uranium age determinations on two 

specimens from the Wood mine (Stieff and Stern, written communication, 1951) gave absolute ages of 57.3 and 60 million years.

BORNITE, SOOTY CHALCOCITE, AND COVEULITE

Minor amounts of bornite were observed in the Calhoun vein west 

of the East Calhoun shaft; none was detected under the microscope, 

however. The bornite is probably closely related to the chalcopyrite.

Sooty chalcocite was observed in both veins in extremely wet areas. 

It is probably supergene.

Thin brilliant-blue covellite coatings were noted on galena, pyrite, 

and chalcopyrite in the Calhoun and Wood veins. This supergene 

coating is apparently a district-wide phenomenon which has been 

observed by the writer in other mines in the district.

PABAGENESIS

The sequence of events in the formation of the veins is shown in 

figure 36. This sequence was determined from a megascopic study 

of the veins and examination of 20 polished sections.

Mineralization began with the deposition of gray, fine-grained quartz 

which silicified the wall rock. Essentially contemporaneous with the 

silicification, finely crystalline pyrite was formed in the wall rock, 

possibly from iron ions liberated from the rock by the altering solutions. Colloform. masses of pitchblende were then deposited in open 

spaces. Movements along the vein fractured the pitchblende, and 

the fractures were healed by fine-grained quartz and pitchblende. 

Pale-yellow pyrite was deposited with the younger pitchblende. Strong 

movement in the vein brecciated the quartz-healed pitchblende and 

the pale-yellow pyrite. Subsequently, white massive quartz, yellow 

pyrite, comb quartz, and sphalerite with exsolved chalcopyrite were 

deposited in open spaces. Sphalerite replaced pyrite to a limited extent. Later movement along the vein fractured the minerals and
154 GEOLOGY AND ORE DEPOSITS, COLORADO

TIME

Primary minerals

Gray, fine 

^grained

Quartz

Pyrite

Pitchblende

Sphalerite

Chalcopyrite

Tetrahedrite-tennantite

Galena

Covet lite

Chalcacite

White.mossive comb

Crystalline !r| Rale Yellow,granulated 

fM yellow '

Fine groined

Crystalline

J

Exsolutlon

Secondary 

minerals

FIGURE 36. Paragenesis of the vein minerals.

opened fissures in which chalcopyrite, tetrahedrite-tennantite, and 

galena were deposited. Chalcopyrite replaced sphalerite, pyrite, and, 

to a limited extent, quartz. Tetrahedrite-tennantite closely followed 

chalcopyrite, replacing it and the sphalerite. Galena filled open 

spaces surrounding other mineral grains and replaced sphalerite and 

gray copper. Galena was then replaced by late chalcopyrite.

Coarse pyrite crystals were then deposited on earlier minerals in 

vugs. The sequence ended when gray to brown fine-grained quartz 

coated ore minerals in vugs. The final event in the formation of the 

veins was the coating of ore minerals by covellite and the deposition 

of sooty chalcocite. These secondary minerals were precipitated from 

cupriferous supergene solutions.
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 155 

RESULTS OF SAMPLING

Plate 15 shows in graphic form the results of assays for uranium, 

gold, silver, copper, lead, and zinc in samples taken from the Wood 

vein. Table 6 shows assay data from 132 individual samples.

It was formerly thought that pitchblende occurred in small bodies 

randomly distributed through the vein. Plate 15 shows that uranium 

values are concentrated in one limited area of the vein, near the 520 

raise. Negligible quantities are detected elsewhere. The uraniumbearing zone lies on the projection of the Wood ore shoot (fig. 34) 

as outlined by Moore and Butler (1952). The ore-shoot relationship 

is discussed on page 164.

The highest gold and silver assays are from samples cut in the 

uranium-bearing zone. The gold-uranium relation is of considerable 

interest in the Central City district. Many workers in the area have 

remarked on the inverse relation between gold and uranium content 

of the veins. Rickard (1913, p. 853) states:

* * * it is axiomatic in these mines that as the pitchblende comes in the gold 

goes out, and, as a matter of fact, the pitchblende ores seldom contain more than 

$2 to $4 in gold.

This reported inverse relation does not occur in the Wood vein, at 

least in that, part sampled in this study. Perhaps the miners sampled 

pitchblende and obtained poor gold assays. Another possible explanation is that pitchblende, which was deposited before the principal 

gold-bearing minerals, almost completely filled available open spaces 

where it was deposited, leaving little room for subsequent gold-bearing 

minerals. As less pitchblende was deposited at depth more space 

was available for later minerals.

Both pyrite and chalcopyrite contain gold and silver, as shown in 

tables 3 and 4. The pyrite apparently does not contain sufficient 

gold to constitute ore. Many samples of almost solid pyrite vein 

material showed very low gold assays. If pyrite contained enough 

gold to constitute ore, one would expect samples largely of pyrite to 

show ore-grade assays. It has been long known that gold and copper 

values are found together (Bastin and Hill, 1917, p. 109). This is 

generally true for the Wood vein, but the analysis of nearly clean 

chalcopyrite from the Wood vein (table 4) shows a low gold content. 

The writer believes that free gold may have been deposited almost 

contemporaneously with chalcopyrite and that the copper-gold 

relation may be more apparent than real.

Gray copper and chalcopyrite both contain silver. Galena is 

presumably argentiferous but is too sparse to be of economic interest 

either for lead or for silver. Moderate quantities of zinc are found, 

but at present zinc is of little economic importance.
TABLE 6. Some chemical analyses, fire assays, and semiquantitative spectrogra

[All assays and spectrographic analyses made by U. S. Geological Survey. Au and Ag in ounces par ton. U. el

by group-number method. Zero (0) indicates that the element was looked 

Sample W 

2 -

31... .

36. 

39 

42. 

44 . 

48 

50 

53. 

58 

59.. 

61 

62 

66.. ..

70. 

76 

82 

84. 

89..--..

98.......

102 

112 

117 

122 

125

139 

142 

147 .

1 RA

156 

160 

178 

179

180 

182 .

187 

192 .

196 

Width 

(inches)

3

3

7

6

6

4

2

3

3.5

2.5

18

12

2

3

6

2.5

1.5

12

4

4

4

3 

12

6

2

3.5

7

8.5

7

7

10

5

8

8

8

12

8

4

U

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

eTJ

0.003

.002

.002

.000

.002

.002

.002

.001

.002

.001

.003

.002

.002

001

.001

001

.001

.001

.001

.003

.001

.002

.002 

.003

.001

.001

.001

.002

.002

.002

.002

004

.003

.001

000

.003

nno

004

.001

.001

003

.000

Au

0.12

<.10

Tr.

Tr.

1.00

Tr.

.26

Tr.

Tr.

.88

.22

Tr.

Tr.

Tr.

Tr.

.02

.04

Tr.

Tr.

Tr.

.28

Tr.

Tr. 

Tr.

Tr.

Tr.

04

Tr.

.70

Tr.

.16

Tr.

.06

Tr.

Tr.

Tr.

Tr.

Tr.

Tr.

Tr.

Tr.

.72

Ag

1.20

0

.20

1.10

1.74

.92

.76

Tr.

2.12

.44

.44

Tr.

Tr.

.12

.28

.42

.10

.64

.52

3.02

.24

.28 

.12

.40

.04

.42

.14

12.58

.60

5.94

Tr.

.20

.44

.48

.66

Tr.

Tr.

.36

.28

.16

1.04

Cu

0.35

<.05

.12

0

.57

0

.22

.11

0

1.11

.17

1.46

0

0

0

0

.12

0

1.09

0

0

0

0

0

0

1 93

0

1.37

0

0

0

.11

0

0

0

0

Pb

2.12

<.01

.09

.77

.22

0

.10

.81

0

.43

.04

.13

0

.03

0

.16

0

.51

.59

.64

0

0 

0

0

0

0

3.31

.70

2.10

0

0

0

0

0

0

Zn

2.03

0

.35

.13

0

0

0

.42

0

1.00

0

1.16

.26

0

0

.10

0

.74

.17

2.03

.14

0 

.08

0

0

0

0

O 77

1 43

9.60

.11

0

0

.03

OK

.38

0

.03

.17

0

Fe2Os

42.04

21.63

12.57

24.35

23.93

15.67

ia98

53.78

5.96

53.79

15.85

7.77

18.44

18.56

33.18

31.61

19.03

27.56

29.95

13.97

26.72

35.57 

16.12

17.50

21.22

11 04

OK QA

1 QQ

22.17

c 7Q

31.29

on AO

4.82

14 Qrt

14.11

32.62

28 74

17 7t\

31.11

Ti

4

4

3

4

4

3

4

4

3

4

3

3

4

3

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

4

3

4

4

3

4

Mn

4

4

4

4

4

4

3

4

3

4

4

4

4

3

4

4

4

3

4

4

3

4

4

4

4

4

4

4

4

4

3

3

4

3

As

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0 

0

0

0

0

3

0

0

0

0

0

0

0

0

0

Ba

5

5

4

fi

4

4

4

6

4

S

4

3

5

5

5

4

4

5

4

4

5

5

5

4

5

4

fi

4

4

4

5

4

5

Be

0

0

Tr

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

Tr

0

0 

Tr,

0

0

0

0

0

0

0

0

Tr

6

6

Tr

0

6

0

Bi

5

0

6

5

4

0

5

5

0

4

5

5

5

5

5

5

5

5

4

5

5 

6

5

0

5

4

0

0

0

5

0

4

Co

0

fi

fi

0

0

5

fi

fi

fi

fi

fi

fi

fi

fi

fi

fi

fi

fi

fi

6 

fi

0

fi

fi

0

0

0

6

fi

fi

fi

fi

g

g

fi

fi

fi

Or

6

fi

4

5

5

5

fi

5

4

5

6

5

5

6

6 

6

6

5

6

6

6

6

6

5

6

5

4

4

6

6

4

6
CTI en Cn Cn c?»OiCjiCj» Ot Of Of Cn 

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OOOOOrft-OOOtf^OOOOtf^tf^OOOOOOOOOOOOOOOOOOOOOOOOOOOOOtf^Orfk-OOO

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000. no 

I+IIII+ l+l+ll 

OCJ»OOOOOOOOCJ»OOOOOOOO?OOOOOOOOOOCJ»OOOC7iC7iC7iOOOOOOOOOOtf^OOOO

OOO OO OOOOOO OOOOOOO OOOOOOO OOOOO OOO O OOOOO OO OO OOOO OOO O 

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TABLE 6. Some chemical analyses, fire assays, and semiquantitative spectrographic a

Sample W524..

533-.

534- 

535.. 

537- 

553- 

561..

562-

563- 

568-

572-

573-

574-

575..

576.. 

577- 

578..

579-

580-

581-.

582-

583-

584- 

585..

586-

587-

588-

589-

590-

591-

592- 

593..

594-

595-

596-

597- 

598..

599-

600-

Width 

(inches)

3

2

3

6

12

6

24

14

12

12

2

10

8

6

20

21

4 

20 

15 

12

6

3 

17 

(") 

20 

13 

24 

12

2.5

0

0

0

0

0

0

0

0

0

0

0

.15

.23

.006

.003

9.94

.010

.002

.021

.010

.006

3.76

.008

.006

.003

.003

.006

.002

.002

.002

0

0

o'

0

0

.01

.002

.002

.004

eU

0.000

.089

.02

.01

.002

.002

.004

.001

.002

.004

.01

.13

.25

.012

.006

9.3

.021

.004

.026

.017

.006

3.60

.012

.006

.004

,005

.008

.006

.005

.005

.003

.002

.009

.001

.003

.004

.005

.005

.007

Au

0.04 

0

Tr.

Tr.

Tr.

.06

Tr.

Tr.

.04

.18

.82

.30

Tr.

Tr.

Tr.

.16 

1.21

Tr.

Tr.

Tr. 

1.10

Tr.

Tr.

Tr.

.30

Tr.

Tr. 

1.28

Tr.

Tr.

Tr. 

0

.16

.44

Tr.

Tr.

Tr.

Tr.

Tr.

Ag

0.50

0

.16

.04

.18

.70

.12

.26

.36

1.64

2.90 

3.36

.08 

4.44 

2.54 

1.64

Tr.

.08

.14 

5.02

.54

Tr.

Tr. 

2.00

.10

.10 

28.72

Tr.

.40

.20 

0

.44 

2.06 

0 

0 

0 

0 

0

Cu

0

0

0

0

0

.27

0

0

0

1.03

4.67

2.81

7.40

0

9.27

1.55

.97

0

0

0

8.15

0

0

0

.29

0

0

6.71

0

0

.07

0

.22

1.82

0

0

0

0

0

Pb

0

0

0

0

0

0

.42

0

0

.22

.30

.11

.08

0

0

0

0

0

0

0

.19

.05

0

0

.17

0

0

1.67

0

.05

.05

Zn

.50

.07

.14

.26

.21

.87

0

1.66

0

.19

0

0

.23

3.32

.03

0

0

.16

.14

.07

.55

0

1.57

.12

.47 

0 

.22

FesOi

41.75

2.30

16.72

18.02

23.04

18.75

12.34

30.70

25.82

23.16

45.82

41.43

37.13

8.64

28.77

36.96

19.50

3.03

8.56

11.49

39.87

27.26

9.17

10.55

41.46

9.12

12.74

23.69

4.65

13.07

8.33

3.49

9.86

51.46

6.61

8.07

2.92

5.58

6.84

Ti Mn As

6+

Tr.

Tr. 

5+Tr.

Be

0 

Tr.

Tr.

0 

6 

0 

0

6- 

Tr.

Tr.

Bi

5 

0

5- 

0

5- 

5

6+ 

5 

5

5+ 

3

4- 

5+ 

0 

5

5+ 

5+ 

0

6+ 

5-

4-

5-

0

0

4- 

Tr.

5-

5-

0

5

4-

5- 

o* 

o 

o

0

Co

5- 

Tr. 

Tr.

Tr. 

6+

6+Tr.

Tr. 

Tr.

6+Tr. 

5-

6- 

6+

Or

6 

6+ 

5- 

4+

4-

5- 

6+ 

6+ 

6+ 

5- 

6+ 

6+

» DDH-2-Core. * Grab. « Grab (rock). < Grab (gouge). « Core.
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 159

TRACE ELEMENTS

Distribution patterns of trace elements in the ore from the Wood 

exploration drift were made from semiquantitative spectrographic 

analyses to guide uranium prospecting and to aid in the interpretation 

of ore genesis. Table 6 presents the data obtained from the analysis 

of 132 samples. Results of this study show an association of bismuth, 

antimony, and arsenic with copper, of cadmium with zinc, and indicate 

an association of zirconium and molybdenum with uranium.

Threshold values, shown in table 7, are those attained in the spectrographic laboratories of the Geological Survey. A "group-number" 

method of reporting results of spectrographic analyses (Riley and 

Shoemaker, written communication, 1952) is used in this report and 

is shown by the following data:

Group no. 

1__ ___ ---_ __ __ ___

2-..---.------ _ -. __ .-.

3. ________________________

4___________-__-____ ____

5_ --------- _ _-__-.____

6_____---_-_---_--_---___-

Percent 

_____._____-_._-_ 10 -100

_______---_-_--__ 1 - 10

_ ______________ .1 - 1

_______.__-__--_- .01 - .1

_________________ .001- .01

_________________ .0001- .001

TABLE 7. Threshold values of elements included in the semiquantitative method

Element

Ti _

Mn

As_ _ - _ . ___

Ba.

Be__

Bi___ - _ _ _ . -

Co__

Cr

Ga_._ _ - ... _ _ _ Mo__

Ni_.__- _ _ _ _._ ___ _ ___ _ ___

Sb___ _ _ -

Sc__--___._______________ _________________

Sr_____ ___________________________________

V __ _

Y> _ _ _

Zr_

Cd. ______________________________________

B.___ ___ _ __ _ _ __ _

Sn___

Ce..______________. _______________________

Percent

0.001

.0005

. 1

.0001

.0001

.001

.0005

.0005

.001

.001

.0005

.05

.001

.0001

.001

.001

.001

.005

.005

.001

.05

Group no.

5

6

3

6

6

5

6

6

5

5

6

4

5

6

5

5

5

5

5

5

4

' Parts per 

million

1

5

1,000

1

1

10

5

5

10

10

5

500

10

5

10

10

10

50

50

10

500
160 GEOLOGY AND ORE DEPOSITS, COLORADO

In the tabulations of the elements in samples (table 6), plus and 

minus signs after group numbers show the relative position within 

the group: plus, toward the upper limit of the range indicated; and 

minus, toward the lower limit. A zero indicates that the element 

was looked for but not found.

The most marked apparent variation in trace-element content 

occurs, as one would expect, between samples of strong vein and 

samples of altered and sparsely mineralized rock. The vein samples 

contain a nearly uniform suite of trace elements, and, except for 

arsenic, antimony, bismuth, cadmium, zirconium, and molybdenum, 

show no appreciable variation in quantity (fig. 37).

120

60

to 0065432 

a- Arsenic

0654. 0654 0654 

Molybdenum Nickel Cadmium

120

60

006 5 4 3

Bismuth

5 4 0 6

Antimony Zirconium 

AMOUNT PRESENT IN GROUP NUMBERS

06-5 

Cobalt

FIGURE 37. Distribution histograms of some elements in 132 samples from the Wood vein.

The two samples having the highest uranium content also show a 

high zirconium content (group no. 3, table 6, and fig. 38). A picked 

sample of pitchblende (eU, 13 percent) contained X.O percent zirconium (group no. 2). The average zirconium content of all samples is 

within group 4 or 5 (fig. 37). A chemical analysis of pitchblende 

from the Wood mine showed 5.47 percent zirconium (Phair, written 

communication, 1953). Hillebrand (1891, p. 65-66) found that a 

sample of pitchblende from the Central City district contained 7.59
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 161

Molybdenum

T

60 80

SAMPLE NUMBERS

FIGURE 38. Graph showing the relation of molybdenum and zirconium to uranium in samples from the

Wood vein.

percent zirconium. R. H. Campbell (oral communication, 1954) 

found that samples from Gold Hill, Colo., containing appreciable 

uranium also were high in zirconium. T. G. Levering (oral communication, 1954) has found similar uranium-zirconium relations in the 

spectrographic study of pitchblende from several localities.

The suggested uranium-and-zirconium relation is of economic 

interest. Phair (1952, p. 45) believes that most of the pitchblende 

now found in veins of the Central City district had its source in latestage uranium-rich differentiates of quartz bostonite magma. The 

solutions mingled with regional solutions and rose along the planes of 

weakness provided by the porphyry dikes; they reacted with and 

leached part of the uranium. If, as Phair (1952, p. 22) believes, the 

uranium in the quartz bostonite is associated with zircon, it is not 

unreasonable to assume that zirconium would also be picked up by 

the solutions and deposited with the pitchblende.

A tenuous relationship between molybdenum and uranium exists 

in samples from the Wood vein. Samples of the uranium-bearing part 

of the vein near the 520 raise contain more molybdenum than normal 

(figs. 37 and 38). A molybdenum-uranium relation has been noted in 

other areas: at Climax (King, oral communication, 1954), in the 

Henry Mountains (Riley and Shoemaker, written communication, 

1952), and at the Happy Jack mine,Utah (Lovering, oral communication,
162 GEOLOGY AND ORE DEPOSITS, COLORADO

1954). The writer has noted the same relationship in British Columbia. Sparse molybdenite has been found in the Central City district 

(Bastin and Hill, 1917, p. 105), and, where present, was the first mineral to crystallize. The writer can only point out the molybdenumuranium association and note that both were deposited early in the 

paragenetic sequence.

Cadmium is locally found in the Wood ores (table 6) and is correlated with zinc (fig. 39). Sphalerite contains a large proportion of

Cadmium

H_l

Zinc

1^

4O

I 

60 80

SAMPLE NUMBERS

>v\^/

I 

too

I

132

FIGURE 39. Graph showing the relation of zinc and cadmium in samples from the Wood vein.

cadmium, as much as 4.5 percent (Rankama and Sahama, 1950, 

p. 708). Sphalerite in the Wood ores probably contains this proportion of cadmium.

In ores from the Wood vein arsenic, antimony, and bismuth vary 

in quantity (fig. 37) and generally correlate well with copper (fig. 40). 

Inasmuch as tetrahedrite-tennantite occurs in the vein, it seems 

reasonable to correlate the amount of arsenic and antimony in samples 

directly to the amount of gray copper present. Bismuth is grouped 

with arsenic and antimony and, in general, behaves in a similar manner 

(Rankama and Sahama, 1950, p. 738). Tetrahedrite may contain 

bismuth, commonly less than 2 percent, whereas bismuthian tennantite may contain as much as 13 percent bismuth (Palache, Berman, 

and Frondel, 1944, p. 379). It is believed, therefore, that the bismuth 

in the ore from the Wood vein is in tetrahedrite-tennantite.

Nickel and cobalt minerals frequently are associated with uranium 

deposits (Everhart and Wright, 1953, p. 88). Such minerals have not 

been identified in the Central City district, however. Samples of ore 

from the Wood vein contain (reported by group no.) 6 to 4 nickel, 

averaging 5, and 6 to 5 cobalt, averaging 6 (fig. 37). The four high-
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 163

Antimony

60 80 

SAMPLE NUMBERS

FIGURE 40. Graph showing the relation of copper, arsenic, bismuth, and antimony in samples from the

Wood vein.

nickel samples were taken from altered rock that contained hornblende. It is not certain that all the reported nickel and cobalt was 

originally in the samples because the samples were pulverized in steelgrinding machines and the amounts of nickel and cobalt they contain 

are within the limits of contamination set by Myers and Barnett 

(1953, p. 828).

WALL-BOCK ALTERATION

Particular effort was made to detect a difference in the wall-rock 

alteration between pitchblende- and nonpitchblende-bearing parts of 

the Wood vein, but no difference was detected.

Megascopically, the wall rocks are silicified, pyritized, and altered 

to argillic minerals. The width of alteration varies directly with the 

vein strength, ranging from about 6 inches to 5 feet, and averaging 

about 18 inches. The alteration progresses generally outward from a 

pyritized and silicified zone through a silicified to an argillic zone. 

At most places, pyritization is stronger where the wall rock originally 

contained a large proportion of mafic minerals, indicating that at 

least part of the pyrite was formed from iron ions liberated by the 

altering solutions.

Microscopic study shows that the plagioclase feldspars were generally the first to be affected; they are sericitized and in part al-
164 GEOLOGY AND ORE DEPOSITS, COLORADO

tered to green montmorillonite. Mafic minerals are bleached and 

altered to chlorite(?). Microcline is last to be affected and is in part 

sericitized. Some kaolinite is present and appears to be late.

ZONING

The mineralogy of the Wood, Calhoun, Willowdale, and Quartz Mill 

veins changes from essentially quartz-pyrite in the east to quartzpyrite - chalcopyrite - sphalerite - galena - tetrahedrite - tennantite in 

the west. This mineralogic change along strike is part of the larger 

hypogene zoning pattern of the Central City district (Leonard, 1952, 

p. 1274-1275). In simplified form, it is a core of quartz-pyrite veins 

surrounded by a shell of galena-sphalerite veins. The veins mapped 

in this report lie in the transitional area between these major zones.

It is also expected that mineralogic changes should occur vertically 

along the veins. Evidence indicates that such a change takes place 

in the Wood-Calhoun group of veins. The Calhoun vein, on the upper 

levels of the East Calhoun mine, is largely filled by quartz and pyrite. 

Chalcopyrite, gray copper, galena, and sphalerite are moderately 

abundant in the Calhoun vein below the fourth level. Similarly, the 

Wood vein above the 197 level consists principally of quartz and pyrite 

(Moore and Butler, 1952); galena and sphalerite are below that level. 

This change in vein mineralogy with depth indicates that the transition zone at this locality dips to the east.

Leonard (1952, p. 1274-1275) found that the major pitchblende 

occurrences of the Central City district are concentrated in the transition zone; Wallace and Campbell (oral communication, 1954) believe 

that pitchblende is genetically related to a zoning sequence. Emmons 

(1927, p. 35), in his classic zoning sequence, placed uranium above 

copper and below zinc, overlapping both; consequently, it is to be 

expected that pitchblende will be closely related to copper-bearing 

minerals and sphalerite in space and time. In the Wood vein, the 

spatial relationship generally holds true. In specimens studied by 

the writer, however, the pitchblende is earlier in the paragenetic sequence than chalcopyrite and sphalerite. Bastin and Hill (1917, 

p. 123-124) report that chalcopyrite is intergrown with pitchblende in 

specimens from the Wood vein, indicating that they were deposited 

almost contemporaneously. The questionable relation between pitchblende and zoning is open for further study.

ORE SHOOTS

Mapping and sampling in the East Calhoun and Wood mines show 

that minable ore is not uniformly distributed throughout the veins. 

Most of the ore bodies occur in shoots that apparently are the result 

of deposition within open spaces along premineral faults. Factors in-
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 165

fluencing the formation of open spaces include the competency of the 

wall rock, a northwest-trending joint set, changes in strike and dip, 

and intersections and near-intersections of individual fractures.

Sharply defined openings occurred along the fractures where the 

faults intersected relatively brittle wall rocks. At the Wood and 

East Calhoun mines the most favorable wall rocks are granite gneiss, 

pegmatite, migmatite, bostonite, amphibolite, and biotite-quartz-plagioclase gneiss, in that order.

Another feature favorable for the formation of openings is a strong 

northwest-trending joint set (fig. 31) in the country rock. The intersection of the veins with joints of this set plunges about 70° S. 80° W. 

This approximates the rake of the shoots shown by the stope outlines 

on figures 33 and 34. Theoretically, the rocks should be intensely 

broken at the intersection of the two fractures. Favorable openings 

should occur along the plunge of the intersection.

A relationship between the steepness of dip and strength of vein 

was noted, particularly on the Calhoun and Quartz Mill veins. In 

general, the steeper the dip, the stronger the vein. The Wood vein 

is stronger and richer where it changes strike toward the northeast.

Small, local ore bodies are found near the intersections and nearintersections of branching and subparallel fissures. These ore bodies 

are attributed to the open spaces produced by the shattering of the 

rock between the fissures.

The principal ore shoot in the East Calhoun mine, between the 6th 

and 10th levels, is on the Quartz Mill vein. The stope outlines on 

the longitudinal projection (fig. 33) indicate that the ore body raked 

about 75°-80°, S. 70° W. The stope length was 100-300 feet, and 

the rake length, 450 feet. Miners at the property generally ascribe 

this ore shoot to the intersection of the Calhoun and Quartz Mill 

veins. It is difficult to imagine how such a flat intersection could 

create a steeply raking open space. Sanderson, 7 in a report on the 

Bezant mine, outlined an ore shoot in the Quartz Mill vein having the 

same general attitude as the ore shoot mined in the East Calhoun 

mine. As previously mentioned, this ore shoot alines with the stoped 

ground below the East Calhoun sixth level. The writer believes 

that the ore mined below the East Calhoun sixth level was a continuation of the Bezant ore shoot.

The Wood vein has been stoped on the upper levels throughout 

most of its explored length. Much of this stoping probably was 

done in the oxidized zone and the ore here was mechanically enriched 

in gold. Moore and Butler (1952) outlined a pitchblende ore shoot 

on the upper levels of the Wood mine. Pitchblende ore bodies found

7 Sanderson, H. S., op. cit.
166 GEOLOGY AND ORE DEPOSITS, COLORADO

in the exploration done during the writer's study are well alined with 

the projected rake of this ore shoot (fig. 34). As previously mentioned, 

gold, silver, and other minerals in this area were of greatest commercial value. Therefore, i t is believed that the pitchblende outlines a general ore shoot. It must be remembered that pitchblende occurs as relatively small pods, lenses, and kidney-shaped ore bodies scattered 

through the vein; therefore, the pitchblende ore shoot only outlines 

ground favorable for the occurrence of pitchblende.

ORIGIN

The Wood, Calhoun, Quartz Mill, and Willowdale veins were filled 

by minerals deposited from hydrothermal solutions. Repeated 

movement along the veins throughout the period of ore deposition 

reopened channels through which the vein-forming solutions could 

migrate. Deposition of minerals by fracture filling and by replacement of earlier minerals was generally restricted to the zone of fracturing, although the wall rock is silicified and pyritized for widths as 

much as 5 feet. Mineral deposition occurred in two general stages: 

quartz-pyrite and galena-sphalerite.

Locally veins of pitchblende apparently were formed during an 

early phase of the general quartz-pyrite stage of mineralization. 

Alsdorf (1916, p. 270, 273) believes that these pitchblende veins 

were cut by subsequent faults and obliterated by formation of precious-metal-sulfide veins. The pitchblende ore bodies observed in 

this study certainly indicate deposition before the main preciousmetal-sulfide filling, although they lie in the same vein and even 

in the same ore shoot as the later minerals. Therefore, the writer 

concurs with Alsdorf that the pitchblende was deposited locally in 

the quartz-pyrite stage of mineralization. It is difficult to reconcile 

the observed field and paragenetic relationship with the hypogenezoning theory of Leonard (1952). Such factors as telescoping can 

be applied to show that it is possible for pitchblende to be deposited 

as the first metallic mineral and still be genetically related to a 

zoning sequence. No evidence of telescoping was noted hi the mines.

The writer prefers the hypothesis of Phair (1952) that residual 

solutions from the differentiation of a quartz bostonite magma 

mingled locally with regional hydrothermal solutions and rose along 

the faults. Pitchblende was deposited when the temperature was 

lowered sufficiently to allow chemical reduction. The pitchblende 

was preceded in deposition by quartz and a little pyrite. Recurrent 

movement reopened the veins to allow the deposition of the later 

precious-metal-sulfide ores.
GEOLOGY OF WOOD AND EAST CALHOUN MINES, GILPIN COUNTY 167

It is difficult to assign these deposits to one of the usual pressuretemperature classifications. Armstrong (written communication, 

1956) classified the ore deposits of Quartz Hill as xenothermal, largely 

on an inferred temperature of formation. His temperature-of-formation inferences were based on the presence of exsolved chalcopyrite in 

sphalerite (chalcopyrite and sphalerite supposedly unmix at about 

350°-400°C, Edwards, 1947, p. 98) and on the U02-U03 ratio of the 

pitchblende (pitchblende with a relatively high U02 percentage presumably indicates a high-temperature origin, Tomkeieff, 1946). The 

writer prefers to classify the deposits as leptothermal (Graton, 1913, 

p. 536-540). The deposits have many characteristics of the mesothermal zone, yet plentiful vugs and the growth of comb structure in 

some places indicate that the conditions of formation were somewhat 

shallower than most mesothermal types.

AGE

Bastin and Hill (1917, p. 93) believe the ore deposits of the Central 

City district to be of early Tertiary age. Recent absolute age determinations on two pitchblende specimens from the Wood mine by the 

Pb206-!!238 ratio method gave ages of 57.3 and 60 million years, respectively (Stieff and Stern, written communication, 1951). These data 

confirm the early Tertiary age postulated by Bastin and Hill.

FUTURE OF THE MINES

The exploration during this investigation failed to develop economic 

quantities of pitchblende in the Wood vein. Pitchblende apparently 

decreases in abundance with depth, and, as the upper parts of the 

Wood vein have been well explored, probably there is little likelihood 

of substantial new uranium production. The ore shoot outlined in 

this report contains moderate gold and silver values, but the vein 

material generally is not now economic. If conditions for gold mining 

become favorable in the future, small quantities of pitchblende probably will be produced as a byproduct.

LITERATURE CITED

Alsdorf, P. R., 1916, Occurrence, geology, and economic value of the pitchblende 

deposits of Gilpin County, Colo.: Econ. Geology, v. 11, p. 266-275.

Anderson, E. M., 1951, The dynamics of faulting and dyke formation with applications to Britain: Edinburgh [Scotland], Oliver and Boyd, Ltd., 206 p.

Bastin, E. S., and Hill, J. M., 1917, Economic geology of Gilpin County and 

adjacent parts of Clear Creek and Boulder Counties, Colo.: U. S. Geol. 

Survey Prof. Paper 94, 379 p.

Edwards, A. B., 1947, Textures of the ore minerals: Melbourne, Australasian 

Inst. Min. and Metallurgy (Inc.), 185 p.
168 GEOLOGY AND ORE DEPOSITS, COLORADO

Emmons, W. H., 1927, Relation of metalliferous lode systems to igneous intrusions: Am. Inst. Min. Metall. Eng. Trans., v. 74, p. 29-70.

Everhart, D. L., and Wright, R. J., 1953, The geologic character of typical pitchblende veins: Econ. Geology, v. 48, p. 77-96. 

Graton, L. C., 1913, The depth zones in ore deposition: Econ. Geology, v. 28,

p. 513-555. 

Hillebrand, W. F., 1891, On the occurrence of nitrogen in uraninite and on the

composition of uraninite in general: U. S. Geol. Survey Bull. 78, p. 65-66. 

Leonard, B. F., 1952, Relation of pitchblende deposits to hypogene zoning in the

Front Range mineral belt, Colorado (abs.): Geol. Soc. America Bull., v. 63,

no. 12, pt. 2, p. 1274-1275. 

Lindgren, Waldemar, 1933, Mineral deposits, 4th ed., New York, McGraw-Hill,

p. 157-158. 

Levering, T. S., and Goddard, E. N., 1950, Geology and ore deposits of the

Front Range, Colorado: U. S. Geol. Survey Prof. Paper 223, 319 p. 

Moore, F. B., and Butler, C. R., 1952, Pitchblende deposits at the Wood and

Calhoun mines, Central City mining district, Gilpin County, Colo.: U. S.

Geol. Survey Circ. 186, 8 p. 

Moore, R. B., and Kithil, K. L., 1913, A preliminary report on uranium, radium,

and vanadium: U. S. Bur. Mines Bull. 70, p. 46. 

Myers, A. T., and Barnett, P. R., 1953, Contamination of rock samples during

grinding as determined spectrographically: Am. Jour. Sci., v. 251, no. 11,

p. 814-830. 

Palache, Charles, Berman, Harry, and Frondel, Clifford, 1944, Dana's system of

mineralogy, 7th ed., v. 1, New York, John Wiley and Sons, p. 374-384. 

Pearce, Richard, 1895, Some notes on the occurrence of uraninite in Colorado:

Colo. Sci. Soc. Proc., v. 5, p. 156-158. 

Phair, George, 1952, Radioactive tertiary porphyries in the Central City district,

Colo., and their bearing upon pitchblende deposition: U. S. Geol. Survey

Trace Elements Inv. Rept. 247, U. S. Atomic Energy Comm., Tech Inf.

Service Extension, Oak Ridge, Tenn. 

Rankama, Kalervo, and Sahama, Th. G., 1950, Geochemistry: Chicago, Univ.

Chicago Press, p. 708-738. 

Rickard, Forbes, 1913, Pitchblende from Quartz Hill, Gilpin County, Colo.:

Min. and Sci. Press [San Francisco, Calif.], v. 106, no. 23, p. 851-856. 

Tomkeieff, S. I., 1946, The geochemistry of uranium: Sci. Progress, London, v.

34, p. 696-712.
INDEX

Page 

Abstract --._ 129

Acknowledgments-.____.__.__.__ 132 

Age determinations_.._.. ...____ 153,167 

Age of ore._.___ __________.-. 167 

Amphibolite.---------____. 134,165

Analyses, chemical_.__._._. _. __ 156-158

semiquantitative spectrographic.__ __ 156-158

Antimony________.______.__ 160,162

Apatite- ---. -- 134,135

Argillic zone ------------_---.._------ 163

Arsenic --_ . ..._..-._........ 160,162

Assay data... -.______________ 155 

S, fire 156-158

Bezant mine.---------.--- ---.-----__. 139,148

Biotite - . ......... 134,135

Biotite-quartz-plagioclase gneiss........ __ 148,165

Biotite schist ... .......... ... 134

Bismuth-____-_..................._ 160,162

Bornite ........ ................ 149

Bostonite dike rocks____________ 135,136 

Breccia fragments______________ 148,152 

Brecciation___________________ 153

Cadmium.----..---.--.- ......._ 160,162

Calhoun mine_________________ 148

Calhoun vein, general character of_____ 147-148

ore production,-... .................... 144

Central City anticline-_-_ .....__.. 136

Chalcocite ....... ..................... 149,154

Chalcopyrite....... 142,147,149,151,154,155,164,167

Chlorite 164

Cobalt- . . . 162

Comb structure-- ______ - _____ 147-149,167 

Copper. . 142,151,155,162,164

Copper-gold relation..____.___...__ 155 

Covellite.. --- 149,154

Defense Minerals Exploration Administration_--_---._-----_......_.____ 142

Dip-slip slickensides --_ _________ 140

East Calhoun mine_ - _ - _-__ 148

production_________________ 144

Economic considerations. ___. _______ 167

Epidote.. ............ -.. 135

Faults. ... 136,139

Feldspar..--.-_...._ ...... 135,136

Folds --- 136,137

Foliation - .- 134,135

Formation of veins.______________ 153 

Front Range Tertiary intrusive sequence__ 135 

Future of mines. _______________ 167

Galena....-- . . 142,149,150,154,155,164

Garnet---.---...---__.__......____ 134

Glass.------..---____---.-.._.___ 136

Gneiss------- .._ ___ 132

Page 

Gold. --------- 142,144,149-151,155,166,167

Gold-silver ratio ... . 151 

Gold-uranium relation_. __. __. ____ 155 

Granite gneiss .. 134,135,165 

Group-number method of reporting rest Its_ 159

Hornblende-.-----..-.--.--- 134,135,163 

Hydrothermal solutions._..._..-..-.-.-.- 166 

Hypogene-zoning theory.______-___.. 166

Idaho Springs formation___. ____ - __ 134 

Injection gneisses____________ . 135

Jefferson-Calhoun veto, production_____ 144 

Jefferson mine, production...... __. ___. 144

Jornts~ ~l 138,165

Kaolinite_....._________-___-- 164

Kemp Calhoun mine, production. _ _ __. 144

Lead. - . i 142,155 

Lead-uranium age determinations _ . . 153 

Leptothermal deposits____________. 167 

Ltoeation.._ 4 137,138

Literature cited-.. L... 167,168

Mafic minerals.. ____ _______ 164 

Magnetite _ L 134,135

Marmatite._-___ ......... | .... 150

Mayflower fault. - 141,142 

Mesothermal deposits..______-. . 167 

Microcline. - . . 134,135,164 

Migmatlte 165

Mineable ore____________-- . 164 

Mineral deposition____._... .. 166 

Mineralization.... ..j . . 153

Molybdenum.. I- 142,160,161

Molybdenum-uranium relation__...j 161,162 

Montmorillonite-.. ------ . 164

Movement.. --------- 139,140,147-149,153,166

Muscovite___-_._.. . 135

Nickel. . i . 162

Old Wood shaft | 144

Oligoclase-andestoe. ... . ... 134

Ore deposits .. . .. ! . 142

production-. .--.-__. ... 144

shoots.. I 164-166

Origin of ore ...... i 166,167

Paragenesis.___...._------

Pegmatite__-----. ...

Pitchblende . . .

144,147-149,152,153-155,: 

Plagioclase__,___-.- 

Precious-metal-sulfide ores.....

Pyrite.. - 142,147-149,

Pyritized zone.. .............

150,

150,

153,154 

.- 156 

130,142, 

i, 164,166,167 

134-136 

- 166 

i, 153,154,164 

163

169
170 INDEX

Page 

Quartz. ............ 134-136, 147-149, 150, 153, 154, 164

Quartz bostonite. ___ . __ ........... __ 136

Quartz bostonite magma. ___ -.....---.... 161

Quartz bostonite porphyry dikes. . . _ ...... 132

Quartz, horn ____ .. ___ .. ___ ... _ ... 149 

Quartz Mill vein, general character of_ - . . . _ 148 

Quartz monzonite. ...... ___ ..... _ ...... 135

..--.---.---.-...-.- 147,148,150,151 

Ross shaft..... _ --.-.... _ ................. 144

Rutile ..................................... 135

Sanderson, H. S., cited.... ......... 139,143,148,165

Schist - . . .... ...... 134

Semiquantitative spectrographic analyses _ 156-159 

Silicified zone............ .................... 163

Silver... ............ 142,144,149-151,155,166,167

Sphalerite ....... 142, 147, 149, 151, 154, 162, 164, 167

Sphene..... ----.-----..----.---.-.-..... __ 135

Sulfides. ... ... 148

Syenitic bostonite porphyry. ............ _ . . 136

Tertiary dikes _ . ___ . __________ .. 142

Page

Tetrahedrite-tennantite.....-..... .- 149,164,162 

Trace elements..__._--..-.. ... 160

Uranium. - 130,136,142,160,161,164,167 

Uranium-zirconium relations.__ .. 161

Vein formation_________..___ 153

Wallrock...... . .... 153

Wall-rock alteration___________ 163,164 

West Calhoun mine, production. . 144 

Willowdale vein______._.___.. . 141 

general character of_ . .. 149 

Wood exploration drift______._.. 151 

Wood fault, displacement- 141 

Wood mine. . . . 153 

Wood vein, general character of 148,149

Xenothermal deposits.._ 167

Zinc. . - - 142,155,162,164 

Zircon____________-....__.___ 134 

Zirconium... - - 142,160,161

o