Petrography and Origin 

of Xenotime and 

Monazite Concentrations 

Central City District 

Colorado

GEOLOGICAL SURVEY BULLETIN 1032-F

Prepared in part on behalf of the U.S. 

Atomic Energy Commission and published with the permission of the Commission

Petrography and Origin 

of Xenotime and 

Monazite Concentrations 

Central City District 

Colorado

By E. J. YOUNG and P. K. SIMS

GEOLOGY AND ORE DEPOSITS, CLEAR CREEK, GILPIN, 

AND LARIMER COUNTIES, COLORADO

GEOLOGICAL SURVEY BULLETIN 1032-F

Prepared in part on behalf of the U.S. 

Atomic Energy Commission and published with the permission of the Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

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

Washington 25, D.C. - Price 20 cents (paper cover)
CONTENTS

Page

Abstract___ --------_-__-_--__--___-__-______-_-_______--_-______ 273

Introduction. _____________________________________________________ 273

Laboratory methods.______________________________________________ 275

Geologic setting.__________________________________________________ 275

Jasper Cuts area._________________________________________________ 277

Fourmile Gulch area.______________________________________________ 287

Illinois Gulch area_________________________________________________ 293

Origin of xenotime and monazite concentrations.______________________ 294

References cited___________________________________________________ 296

Index_ _ __________________________________________________________ 299

ILLUSTRATIONS

FIGURE 66. Generalized geologic map of the Central City district,

Colorado.___________________________________________ 276

67. Geologic map of the Jasper Cuts area.-------------------- 278

68. Exposure of migmatite at pit 1, Jasper Cuts area_______--__ 279

69. Photomicrograph showing monazite (light-gray) and xenotime

(white) in rare-earth zone at the Jasper Cuts area_ _______ 282

70. Zircons from rare-earth zone, Jasper Cuts area, showing

overgrowths __,___________-_____--___---________----_ 283

71. Petrofabric diagrams of xenotime at the Jasper Cuts and

100 random points_-____-----------------__--_-------- 286

72. Photomicrograph showing xenotime and monazite in rareearth zone in biotite gneiss at Fourmile Gulch______--__-_ 290

73. Petrofabric diagrams of xenotime and quartz at Fourmile

Gulch and 200 random points._________________________ 292

TABLES

TABLE 1. Modes (volume percent) of rock types in the Jasper Cuts area_ 280

2. Accessory minerals heavier than methylene iodide in the

various rock types from the Jasper Cuts area. ___________ 281

3. Semiquantitative spectrographic analyses of migmatite

(rare-earth zone) from the Jasper Cuts area____-______--_ 285

4. Modes (volume percent) of biotite gneisses from the Fourmile

Gulch area--._---_-_-_-___--_____-__---_-----_--_--- 288

5. Accessory minerals heavier than methylene iodide in rocks

from the Fourmile Gulch area________________________ 289

6. Semiquantitative spectrographic analyses of rare-earth zone

in biotite gneiss from the Fourmile Gulch area___________ 291

7. Accessory minerals heavier than methylene iodide in migmatitic biotite gneiss from the Illinois Gulch area___________ 293

in

GEOLOGY AND ORE DEPOSITS, CLEAR CREEK, GILPIN, AND LARIMER

COUNTIES, COLORADO

PETROGRAPHY AND ORIGIN OF XENOTIME AND MONAZITE CONCENTRATIONS, CENTRAL CITY DISTRICT, 

COLORADO

By E. J. YOUNG and P. K. SIMS

ABSTRACT

Xenotime and monazite are uncommonly abundant in Precambrian biotite 

gneiss and migmatite at three localities near Central City, Gilpin County, Colo. 

The occurrences are in the lower part of a thick layer of migmatized biotite 

gneiss in a sequence of rocks that have been metamorphosed to the almandine 

amphibolite facies. The zones of concentration are a maximum of about 5 feet 

thick and a few hundred feet long, and contain about 1 to 5 percent by volume 

combined xenotime and monazite.

The rare-earth minerals occur dominantly as aggregates of sand-size crystals 

in thin layers and clots of biotite, which are much coarser than the mica in the 

typical biotite gneiss. Xenotime is more abundant than monazite in 2 of the 3 

occurrences. Both minerals are subrounded to rounded and crystal faces are 

rare. The two minerals appear to have crystallized contemporaneously. Except 

for magnetite, other accessory minerals that are common to the country rock 

are not concentrated with the xenotime and monazite.

The field and laboratory data are consistent with the hypothesis that the 

rare-earth minerals were concentrated at their present sites during migmatization of the biotite gneiss country rock, in a period of Precambrian plastic 

deformation. Presumably, granitic fluids derived during the deformation 

selectively mobilized rare-earth cations and phosphate from the biotite gneiss 

country rock. These ions crystallized with biotite and locally with magnetite 

to form zones of xenotime and monazite concentrations in migmatized parts 

of the gneiss.

INTRODUCTION

Xenotime and monazite, which are common accessory minerals in 

some of the Precambrian crystalline rocks of the Front Range of Colorado, are sufficiently abundant at places near Central City, in Gilpin 

County, to have attracted the interest of private mining companies. 

The occurrences that have been investigated are in migmatized biotite 

gneisses which are within a thick sequence of biotite gneisses that has 

been called the Idaho Springs formation (Ball, 1906; Levering and

273
274 GEOLOGY AND ORE DEPOSITS, COLORADO

Goddard, 1950). They were discovered by radiation-detection instruments because of their high abnormal radioactivity, locally as much 

as 10 times as great as the average radioactivity of the country rock.

Because of the potential economic importance as well as scientific 

interest in these unusual occurrences of rare-earth minerals, they were 

studied as part of the U.S. Geological Survey's geologic investigations 

in the central part of the Front Range. Field studies were done 

largely by Sims in conjunction with geologic mapping of the Central 

City district that was started in 1952; the field studies were done partly 

on behalf of the Division of Raw Materials, U.S. Atomic Energy 

Commission. Laboratory studies were done by Young in 1957-58. 

In this report we describe the petrography of the rock units containing 

the xenotime and monazite occurrences, with particular emphasis on 

these accessory minerals. We also propose a mode of origin for these 

concentrations of rare-earth minerals.

J. C. Hamilton and R. G. Havens of the Geological Survey did 

the semiquantitative spectrographic analyses. J. W. Adams of the 

Geological Survey made a preliminary petrographic study of samples 

containing monazite and xenotime from the region.

Cogeneric monazite and xenotime have been reported in the literature from several localities and various geologic environments, but 

generally xenotime is sparse and the two minerals are less abundant 

than associated rare-earth and multiple-oxide minerals. Most of the 

described occurrences are in pegmatites. Brogger (1885) and 

Bj^rlykke (1935) have described xenotime and monazite from pegmatites in Scandinavia; Scharizer (1888) has reported these same 

minerals from pegmatites in Germany; and Meixner (1938) has 

reported them from pegmatites in Austria. Occurrences of the rareearth minerals in the southeastern United States have been noted by 

Hidden (1893) and Mertie (1953) in pegmatitic and metamorphic 

rocks. Hussak (1899) has described intergrowths of monazite and 

xenotime in diamond-bearing sands in Brazil.

Concentrations of cogeneric monazite and xenotime similar to the 

Gilpin County occurrences in having several percent of the combined 

minerals with virtual exclusion of other accessory minerals are rare, so 

far as known. The only occurrence known to us to be closely similar 

is in a granite augen gneiss from near Mesquite, Nev. H. W. Jaffe of 

the Geological Survey reports (written communication, 1951) that a 

sample from this gneiss contained an estimated 5 percent xenotime and 

2 percent monazite. Hornblende, limonite, magnetite, zircon, and alla.nite (?) are sparse associated accessory minerals in the rock. An 

occurrence in rock similar to the host rock of the deposits in the Central 

City district, but containing xenotime alone in association with garnet
XENiOTIME AND MONAZITE, CENTRAL CITY DISTRICT 275

and biotite, has been described from Charlevoix County, Quebec 

(Shaw, 1957).

Although described occurrences of cogeneric xenotime and monazite 

in high-grade metamorphic rocks are rare, this type of deposit probably is widespread. It is difficult to recognize these deposits in the 

field, for the host rock containing moderately large quantities of the 

rare-earth minerals is similar in physical appearance to the barren 

gneisses. They can best be distinguished in the field by their abnormally high radioactivity.

LABORATORY METHODS

To extract the accessory minerals from the rocks about 50 grams of 

each rock type was crushed to pass through a 20-mesh sieve. The rock 

particles then were separated into four size ranges by sieving to facilitate separation in methylene iodide. Each of the size fractions was 

washed ultrasonically to remove fine objectionable rock dust. Methylene iodide (specific gravity = 3.3) was used to separate xenotime, monazite, and zircon from the other heavy minerals in preference to bromoform to prevent biotite from flooding the desired separation. The 

fraction that floated in methylene iodide was examined for accessory 

minerals such as apatite and allaiiite. Magnetite was removed with a 

hand magnet from all size fractions. The 200- to 270-mesh fraction 

of the minerals heavier than methylene iodide represents about 12 percent of the heavy minerals. This fraction was chosen in each case for 

a quantitative count, made by point counting grain mounts. Oil having an index of refraction of 1.72 was very useful for mounting as the 

w index of xenotime is close to 1.72. An alternative to thin-section 

modal analysis was used to determine the composition of the micro- 

^line-bearing gneiss. A sawn slab of rock was immersed in hydrofluoric acid for 5 seconds, washed in water, stained with sodium cobaltinitrite and washed again. This treatment imparted a deep-yellow 

to the potassium feldspar, a milky-white to the plagioclase, and did 

not affect the color of the quartz. This technique is different from that 

of Gabriel and Cox (1929) in that immersion in hydrofluoric acid is 

used. After the rock slab was stained a point count was easily made 

using reflected light and a petrographic microscope.

GEOLOGIC SETTING

The biotite gneiss and migmatite that contain the abnormal concentrations of xenotime and monazite occur in the lower part of a layer 

of biotite gneisses that overlies a layer of contrasting lithology, composed chiefly of microcline, quartz, plagioclase, and biotite (fig. 66).
276 GEOLOGY AND ORE DEPOSITS, COLORADO

Geology by Paul K Sims and others, 1952-55 

2 MILES

FIGURE 66. Generalized geologic map of the Central City district, Colorado. Outlined

area is shown in figure 67.

The biotite gneiss layer is one of several thick layers of this rock that 

are interlayered with equally thick layers of microcline-quartz-plagioclase-biotite gneiss in this part of the Front Kange. Both the biotite 

gneisses and the microcline-bearing gneiss are interpreted as paragneisses. Previous workers (Bastin and Hill, 1917; Levering and 

Goddard, 1950) referred to the microcline-bearing gneiss as granite 

gneiss and inferred it to be of igneous origin. All the rocks are metamorphosed to the upper range of the almandine amphibolite facies 

(Fyfe, Turner, and Verhoogen, 1958, p. 228).

The paragneisses in this part of the Front Range are intruded by 

granodiorite, correlative with the Boulder Creek granite (Lovering 

and Goddard, 1950), quartz diorite and associated hornblendite, and 

rarely biotite-muscovite granite, correlative with the Silver Plume 

granite at Silver Plume, Colo. Only the granodiorite, however, occurs 

in moderately close proximity to the area containing the rare-earth 

mineral deposits (fig. 66).
XENOTIME AND MONAZITE, CENTRAL CITY DISTRICT 277

The localities where xenotime and monazite concentrations have 

been prospected are on the east side of the Central City anticline (fig. 

66). Deposit 1, at the Jasper Cuts, is in migmatite; deposit 2, at 

Fourmile Gulch, and deposit 3, at Illinois Gulch, are in biotite-quartz 

gneiss. Each occurrence is about 100 feet stratigraphically above the 

base of the biotite gneiss layer; therefore, they are approximately 

in the same part of the layer.

The biotite gneiss layer consists dominantly of biotite-quartz-plagioclase gneiss and silHmanitic biotite-quartz gneiss that are interlayered 

on a scale ranging from an inch or less to several tens of feet, but 

garnet-bearing varieties and quartz-rich varieties are present locally. 

Migmatite, as used in this report, refers to a rock consisting of an intimate interlayering of roughly equal quantities of biotite gneiss and 

pegmatite.

Migmatization of the biotite gneisses took place during a period of 

plastic deformation that produced the major folds of the Precambrian 

rocks in the region. The origin of the pegmatite that constitutes the 

felsic phase of the migmatite is problematic, but possibly it was generated from the country rock by the heat and pressure that accompanied deformation of the deeply buried rocks.

JASPER CUTS AREA

The principal deposits of xenotime and monazite known in the 

region are in the Jasper Cuts area, about a mile south-southeast of 

Central City (fig. 66). The deposits were explored in 1957 by the 

Michigan Chemical Co., and about 100 tons of rock \yere mined as a 

source of yttrium-group rare earths. Previously, other companies 

had explored the area for radioactive minerals. The source of the 

radioactivity was identified by George Phair of the U.S. Geological 

Survey (oral communication, 1952) as mainly thorium in monazite.

OCCURRENCE

The geology of the Jasper Cuts area is shown in figure 67. The 

xenotime and monazite deposits occur in the layer mapped as interlayered biotite gneisses and migmatite. This layer overlies microcline-quartz-plagioclase-biotite gneiss. All the rocks are deformed 

into small, open anticlines and synclines that plunge gently northeast 

or southwest, subparallel to the axis of the Central City anticline.

The host rock of the xenotime and monazite concentrations is migmatite, which lies between 2 layers of pegmatite a few tens of feet 

thick, about 100 feet vertically above the underlying microcline-quartzplagioclase-biotite gneiss (fig. 67). The migmatite is gradational both

576761 O 61 2
278 GEOLOGY AND ORE DEPOSITS, COLORADO

EXPLANATION

Porphyritic igneous rock

Pegmatite

 gnbs, mig

Interlayered biotitite gneisses and migmatite

Microcline-quartz-plagioclase-biotite gneiss. 

Metalliferous vein, approximately located

500

Geology by Paul K. Sims and 

Avery A. Drake, Jr., 1953 

and 1959

500 FEET

Contact

Dashed where approximately located, 

short dashed where inferred or gradational

Plunge of minor anticline 

Strike and dip of foliation

K^J7

Strike and dip of foliation and 

plunge of lineation

 K

Portal of caved adit

CONTOUR INTERVAL 20 FEET 

DATUM IS MEAN SEA LEVEL

B

Vertical shaft

Prospect pit 

Number referred to in text

FIGURE 67. Geologic map of the Jasper Cuts area, Gilpin County, Colo.

vertically and laterally into biotite gneiss on the one hand and pegmatite on the other.

The zone of concentration of xenotime and monazite, herein called 

the rare-earth zone, is about 5 feet thick in pit 1 (fig. 67) and contains 

3 to 5 percent by volume of the rare-earth minerals. A photograph 

that shows the character of the rare-earth zone and the overlying 

migmatized biotite gneiss in pit 1 is shown in figure 68. The rareearth zone is the blocky rock in the lower left part of the photograph. 

The pegmatite layers appear light gray in the photograph; they range 

from less than 1 to 6 inches in thickness.

It is probable that xenotime- and monazite-rich parts of the migmatite at pit 3 (fig. 67) occur at about the same stratigraphic position 

as the occurrences in pit 1, but the rare-earth zone cannot be traced 

continuously between the pits because of folding.
XENOTIME AND MONAZITE, CENTRAL CITY DISTRICT 279

~r** 'wjmn,.

FIGURE 68. Exposure of migmatite at pit 1, Jasper Cuts area. The rare-earth zone is 

in the lower left part of the photograph. The conformable light-gray layers (plagioclase-rich pegmatite) are from less than 1 to about 6 inches thick.

The migmatite in the rare-earth zone is similar in appearance to the 

overlying and underlying migmatite, but can be distinguished from it 

by its high abnormal radioactivity and abundant inch-thick lenses and 

clots rich in biotite. The biotite in these lenses is coarse grained in 

contrast to the typical fine-grained biotite of the barren migmatite, and 

most of it is not foliated, in contrast to the well-oriented biotite of the 

barren gneiss. At places in the rare-earth zone the crystals of xenotime and monazite are sufficiently large and abundant to be visible 

with a hand lens, particularly in the coarse-grained biotite.

PETROGRAPHY OF ROCK UNITS

Modal analyses of the rock units exposed in the Jasper Cuts area and 

of the intrusive grandiorite exposed about half a mile wrest of the area 

(fig. 66) are given in table 1. The locations of the samples relative to 

pit 1 (fig. 67) are also given in the table. Only the range in variation of the minerals is given for migmatite and pegmatite, for the 

essential minerals in these rocks vary from place to place.
280 GEOLOGY AND ORE DEPOSITS, COLORADO

TABLE 1. Modes (volume percent) of rock types in the Jasper Cuts area

Plagioclase _ ____ _ ___

Composition

Quartz_ _ _

Microcline _ _ ___

Biotite _ _ _ _ _

Muscovite ___ __ __.

Hematite

Total accessory minerals

Xenotime _ _ _ _ _

Monazite. __

Zircon _ _

i

Migmatite

20-60

(An17)

10-25

Tr.

10-80

1-10

1-10

Tr.

2

Biotitequartzplagioclase 

gneiss

30. 9

(Anjg)

53. 2

Tr.

15. 2

.6

. 1

3

Pegmatite

5-50

(An17)

10-40

10-50

1 Tr.

(2)

4

Microclinequartzplagioclasebiotite gneiss

44. 6

(An17)

32. 8

18. 1

3. 5

.8

. 2

5

Granodiorite

43. 9

(An43)

20. 7

5.9

21. 3

8. 2

' Tr.=trace.

2 _= not found or not determined in thin section.

1. Migmatite from rare-earth zone in pit 1 (fig. 67). The oligoclase has maximum extinction angles to the 

(010) trace of -5° to -6°; 2V ranees from 75° to>8°(-); a' Qower index of refraction on (001) or (010) cleavage 

flakes suggested by Tsuboi (1923)) is 1.537±0.002. The optic angle indicates that the feldspar is midway 

between a low and a high structural state, according to data by Smith (1958). The biotite has indices of 

refraction OS and 7) ranging from 1.66 to 1.68 and is siderophyllite according to Winchell (1951).

2. Sample taken approximately 350 ft north-northeast of pit 1. 670 point counts were made over an area 

of 240 SQ mm. Gneiss is fine grained. The oligoclase has an a' index of 1.538±0.003; 2V=77°( ); maximum 

extinction angles to the (010) trace are 8° to 9°.

3. Sample taken 150 ft S. 70° W. from pit 1.

4. Sample taken about 550 ft West-Southwest of pit 1. 1,329 point counts were made over an area of 525 

sq mm. Gneiss is fine grained to medium grained. The a' index of oligoclase is 1.537 ±0.003; maximum 

extinction aneles to the (010) trace are 5° to 6°. The heavy accessory minerals, in addition to hematite, are mainly apatite and zircon.

5. Average of 11 modal analyses by R. H. Moench of the U.S. Geological Survey of different specimens 

from granodiorite body shown on figure 66. The pjagioclase in one specimen has an a' index of 1.550±0.003. 

The accessory minerals are mainly magnetite, sphene, and apatite. Neither xenotime nor monazite has 

been noted.

ACCESSORY MINERALS

The data obtained from a quantitative study of the accessory minerals in the different rock types in the area are given in table 2. The 

weight percent of the accessory minerals heavier than methylene iodide and the number percent of the several minerals constituting this 

group are given for the 200-270-mesh fraction of the different rocks 

studied. Quantitative data on other size fractions were not obtained 

during this investigation.

Both xenotime and monazite occur in more than trace amounts in 

the 200-270-mesh fraction (table 2) in biotite gneiss and pegmatite 

country rock as well as in the migmatite of the rare-earth zone. 

Although the microcline-quartz-plagioclase-biotite gneiss contains 

some monazite (3.8 number percent in the 200-270-mesh fraction), 

xenotime is rare. Neither monazite nor xenotime are present in the 

granodiorite. It should be noted that the migmatite has more than 

200 times as much total accessory minerals as the pegmatite. The 

xenotime-monazite ratio (~3:1) within the 200-270-mesh fraction 

of both rocks is similar, however. According to point counts of the
XENiOTIME AND MONAZITE, CENTRAL CITY DISTRICT 281

xenotime-monazite ratio in thin sections from the rare-earth zone 

xenotime is slightly more abundant than monazite on a volumetric 

basis (1.2:1). Where the slightly higher specific gravity of monazite 

is taken into account, the comparison ratio on a weight percent basis 

is slightly less (1.1:1).

TABLE 2. Accessory minerals heavier than methylene iodide in the different rock 

types from the Jasper Cuts area

[ Samples are the same as those described in table 1. The granodiorite sample was collected half a mile

west-northwest of pit 1]

Rock type

Migmatite. __ .. __

Biotite-quartz-plagioclase

gneiss . ___ __ ...

Pegmatite ___ .

Microcline-quartz-plagioclase-biotite gneiss 2 -

Granodiorite 4 . _ - ...

g

1

«*-«

o 'O

%£ & a

8 o

1°

730

439

128

550

284

ofs

1-1 I

fe B M

a g

^ t'.'O £ >s

 Ef 2 g-S ^-c

7 ^t r . o

.05

.03

.95

6.65

"o

"S

S-S

«1

r^3 bfi II

Tr.

0

Tr.

.84

4.73

Number percent in 200-270 mesh fraction minus

magnetite

^

.-§

+3

6

^

1.1

74.8

6.3

73.3

4.3

a

o

N

1.2

14.9

50i9

22.9

8.9

1

1

fl

71.0

1.8

32.9

3Tr.

0

S

1O

S

26.7

8.3

9.9

3.8

0

t5fl

C3

0

0

.2

0

0

0

c

JSa 

cc

0

0

0

0

78.7

a? w'3

_C3

0

0

0

0

6.9

O

 s

"2'a 

K

0

0

0

0

1.2

Tr.=trace.

1 Hematite possibly contains some ilmenite.

2 The. heavy minerals in the 150-270 mesh fraction having sp. gr. >bromoform, but <methylene iodide 

consist of biotite, apatite, anatase (leucoxene), and hematite in decreasing order of abundance. No allanite 

was seen.

3 Two grains of xenotime were seen in a slide of several thousand grains.

4 The heavy minerals in the 150-270 mesh fraction having a sp. gr. >bromoform, but <methylene iodide 

contain apatite and some allanite.

The xenotime from this deposit is easily distinguished from monazite by color. The xenotime is virtually colorless, whereas the monazite is yellow, and the strong color difference is apparent in the photomicrograph in figure 69.

Although monazite and xenotime range in grain size from 0.4 mm 

to slightly less than 0.05 mm, more than 95 percent by weight of these 

accessory minerals are between 0.2 and 0.05 mm (65-270 mesh). In 

general, monazite tends to be somewhat coarser grained than xenotime, 

the mode of the monazite grains being close to 0.12 mm and that of 

the xenotime 0.09 mm. Both minerals are subrounded to rounded 

and crystal faces are rare. Xenotime is uniaxial positive, «= 1.724 ± 

0.003 and c= 1.815 ±0.005, and the birefringence is 0.091 ±0.008. 

Pleochroism is weak with w colorless and e pale yellow. Monazite 

is biaxial positive and 2V ranges from 10°-15° (average 12°), as determined on the universal stage, a, (3 and y are 1.790 ±0.003, 

1.795±0.003 and 1.840 ±0.003 respectively, and the birefringence is 

0.050 ±0.006. No pleochroism was noted between a, fi and y, which 

are all pale yellow.
282 GEOLOGY AND ORE DEPOSITS, COLORADO

FIGURE 69. Photomicrograph showing monazite (light-gray) and xenotime (white) in 

rare-earth zone at the Jasper Cuts area. Both minerals are subrounded. The white 

lathlike mineral in the lower left corner is muscovite. Some of the white irregular 

shapes, as in the upper right corner, are quartz. The black opaque-appearing background is biotite darkened by radiation damage. Plane polarized light.

In contrast to the xenotime and monazite, zircon from the rareearth zone has conspicuous overgrowths, most of which show zoning 

(fig. 70). The cores of zircon grains are rounded and in general 

darker and pinker than the overgrowths; also, they generally contain notably more inclusions than the overgrowths, as shown in figure 

70A. In some zircons (fig. 70A) the cores are cracked whereas the 

rims are not, probably indicating that the cracks occurred before 

overgrowth age. In other zircon grains, however, the overgrowths are 

cracked (fig. 7(15), indicating that the cracking occurred after overgrowth age. Under crossed nicols (not illustrated) optical continuity 

from core to overgrowth is shown for the grain illustrated in figure 

70Z? by the fact that the interference colors cross boundaries between 

core and overgrowth without apparent change. An unusual zircon 

is shown in figure 706r. The two rounded, innermost cores are pink 

and contain abundant tiny inclusions; they are enveloped by paleyellowish-brown, zoned zircon. This is enveloped completely, in turn, 

by an outermost overgrowth of pale pink to colorless zircon.

The paragenesis of the heavy accessory minerals is controversial in 

view of the fact that Bj0rlykke (1935) postulated simultaneous crys-
XENOTIME AND MONAZITE, CENTRAL CITY DISTRICT 283

*

 a

3 

O

 a S
284 GEOLOGY AND ORE DEPOSITS, COLORADO

tallation of monazite and xenotime in granite pegmatites, whereas Carron and others (1958) have indicated that xenotime should fractionally crystallize first. The latter authors, however, do not preclude the 

possibility of prior deposition of monazite. At the Jasper Cuts area 

xenotime and monazite mutually interfere with each others growth, 

that is, xenotime grains truncate adjacent monazite grains, but the 

reverse is equally true, indicating that the two minerals crystallized 

almost simultaneously.

SFECTROCHEMICAL ANALYSIS OF THE RARE-EARTH ZONE

Two samples of migmatite, taken from the rare-earth zone, were 

spectrographically analyzed semiquantitatively (table 3). MCC-1 

represents an average sample from this zone, whereas MCC-2 is 

markedly richer in xenotime and monazite than the average sample.

It can be noted in table 3 that both samples contain about nine times 

as much thorium as uranium. Also, the yttrium-group rare earths 

are more abundant than the cerium-group rare earths, as shown in the 

following tabulation:

MCC-l MCC-2 

(percent) (percent)

Total rare earths____________________________ 3.8 8.0 

Yttrium group (Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) ____ 2.2 4.5 

Cerium group (La, Ce, Nd, Pr, Sm, and Eu)____________ 1. 6 3. 5 

Yttrium group-cerium group ratio___________________ 1. 4 1.3

PETROFABRICS

To determine the degree of orientation of xenotime in the migmatite, 

100 xenotime c axes (optic axes) were measured and plotted on a 

Schmidt equal-area net (fig. TL4, B). So far as known to us, petrof abric diagrams of xenotime have not been published previously. All 

the c axes were measured from a podlike aggregate of several hundred 

grains in an area of 0.5 by 0.5 cm in a thin section of sample MCC-2. 

The pod also contains monazite; these 2 minerals comprise about 80 

percent of the pod, the remainder being biotite. The thin section is 

roughly perpendicular to biotite cleavage, but much of the biotite is 

disoriented with respect to foliation in the rock. Unfortunately orientation of the thin section in space is not known with certainty.

It can be seen from the contoured diagram (fig. 71) that xenotime 

optic axes have no striking preferred orientation. To acquire some 

idea as to the randomness of the xenotime c axis plot, 100 random 

points were chosen from a random number table and plotted similarly 

on a Schmidt equal-area net. Figure 11B illustrates the result. As 

would be expected no preferred orientation is shown. Although the 

xenotime c axes maxima are disposed somewhat symmetrically around
XENiOTIME AND MONAZITE, CENTRAL CITY DISTRICT 285

TABLE 3. Semiquantitative spectrographic analyses of migmatite (rare-earth zone)

from the Jasper Cuts area

[Analysts, J. C. Hamilton and others]

Element MCC-1

(percent)

MCC-2 

(percent)

Radioactive:

U, equivalent 1 _________________________ 0. 065

TJ, chemical 2 _____________________ . 022

Th, radiochemical 3 _________________________ . 18

Th________________________________________ . 15 '

Rare-earth:

¥_________________________________________ 1. 5

La________________________________ . 3

Ce________-_ __________________________ . 7

Pr__________-___.___________________ . 15

Nd__________-__ _______________________ . 3

Sm__________________________ . 15

Gd___________-___-________________________ . 15

Dy__________________________________ . 15

Ho__ ___________________________________ . 07

Er_---_-__-__--_________________________ . 15

Tm _ _ . 015

Yb________________________________________ . 15

Lu___-________-__-____________________ . 015

Major and minor:

Si____________-_.__________________________ M

Al________________________________________ 7

Fe___________-_--_-_-__-_-_.______________ 3

Mg_______________________________________ .7

Ca__v. _________________________ 1. 5

Na ---_--_-_------_-_._____________ 3

K_________________________________________ 3

Ti_____________________________ . 3

P__.____________--________________________ . 3

Trace:

Mn____________-______________________ . 07

Ba_________________________ __ . 03

Be_-___-__--_-_-_-_________-_-___-__---_ . 00015

Co _ ___ _ __ _ _ - 0007

Cr__._____________________________________ . 015

Cu________________________________________ . 015

Ga________________._____________________ . 003

Ni________________________________________ . 007

Pb________________________________________ . 03

Sr_______-_______________________-________ . 015

¥__________________--_____________-_______ . 007

Zr_______________-_-___________________--_ . 07

0. 22

. 070

. 66

. 7

3.0

.7 

1. 5

.3

.7

. 3

. 3

. 3

. 15

.3

.07

. 3

.07

M

7

7

. 7 

1. 5 

3 

3

. 7

.7

.07 

.03 

0

<. 001 

.015 

.015 

.007 

.007 

.07 

.015 

.015 

. 15

NOTES. Looked for but not found: Ag, As, Au, B, Bi, Cd, Eu, Ge, Hf, Hg, In, Ir, Li, Mo, Nb, Os, Pd, 

Pt, Re, Rh, Ru, Sb, Sc, Sn, Ta, Tb, Te, Tl, U, W, and Zn.

Figures are reported to the nearest number in the series 7,3,1.5,0.7,0.3,0.15, in percent. 

Sixty percent of the reported results may be expected to agree with the results of quantitative methods.

M=major constituent, greater than 10 percent.

1 Radiometric analyses by G. S. Erickson.

2 Chemical analyses by D. L. Ferguson and W. W. Niles.

3 Radiochemical analyses by J. N. Rosholt, Jr.

676761 O 61-
286 GEOLOGY AND ORE DEPOSITS, COLORADO

A 100 XENOTIME OPTIC AXES

EXPLANATION

PERCENT

B 100 RANDOM POINTS

FIGURE 71. Petrofabric diagrams of xenotime at the Jasper Cuts and 100 random points. 

(Lower hemisphere projection on Schmidt equal-area net.) A, 100 xenotime optic 

axes ; B, 100 random points.
XENIOTIME AND MONAZITE, CENTRAL CITY DISTRICT 287

the principal S-plane (plane of biotite foliation) of the thin section, 

this may be happenstance, as shown by the following maxima analysis:

Xenotime Random

Clusters (percent) 

6 _ ____ __ _ _______

5 ______ _ __

4 _

3__

2_

C axes points 

_ __ __ __ _ __ 1 0

_ __ ___ __ _ 3

5

__ _ __ 10

_ 9

1 

5 

10 

12

FOURMILE GULCH AREA

Concentrations of xenotime and monazite of a similar order of 

magnitude to those at the Jasper Cuts but in biotite gneiss rather than 

migmatite are present at the Fourmile Gulch area (loc. 2, fig. 66). 

This occurrence is exposed on the steep hill east of Fourmile Gulch, 

about three-fourths of a mile northeast of Black Hawk. It has been 

explored by shallow bulldozer cuts, and small samples have been 

collected for milling and metallurgical tests; however, no rock has 

been mined for shipment as an ore.

OCCURRENCE

The biotite gneiss in the area of the surface cuts varies in mineralogic composition and contains but little pegmatite. One lens of 

pegmatite similar in mineralogy and structure to that in the Jasper 

Cuts area occurs in the biotite gneisses about 15 feet stratigraphically 

above the zone of xenotime and monazite concentrations; other scattered lenses crop out sporadically both stratigraphically below and 

above the occurrence. At the surface cuts, the biotite gneiss strikes 

.about N. 30° E. and dips about 30° SE. Lineations in the rock plunge 

gently northeast or locally southwest.

The exposed zone of rare-earth concentrations is about 3 feet thick 

and has a probable maximum strike length of less than 100 feet. The 

zone is estimated to contain 1 to 3 percent by volume of xenotime and 

monazite. The radioactivity of the rare-earth zone is about 10 times 

that of the surrounding country rock. Similar to the Jasper Cuts 

occurrence, the xenotime and monazite are mainly associated with 

coarse biotite that constitutes thin lenses in the rock; the biotite lenses 

also contain abundant magnetite.

PETROGRAPHY

The host rock containing the concentrations of xenotime and monazite is a fine-grained magnetite-bearing quartz-biotite gneiss (table 4), 

a local variety of the biotite-quartz-plagioclase gneiss. The overlying
288 GEOLOGY AND ORE DEPOSITS, COLORADO

and underlying gneiss is typical biotite-quartz-plagioclase gneiss with 

thin interlayers of sillimanitic biotite-quartz gneiss. A mode of a 

typical specimen (3, table 4) of biotite-quartz-plagioclase gneiss collected near the rare-earth zone is given for comparison with the xenotime-monazite-bearing gneiss.

TABLE 4. Modes (volume percent) of biotite gneisses from the Fourmile Gulch area

Quartz _

Biotite_

Plagioclase

Composition

Magnetite (and hematite)

Muscovite _

Accessory minerals

l

51.0

25. 5

3.6

(An28)

14.2

2. 6

3. 1

2

61. 5

28.6

0

7.3

1. 5

1. 1

3

40. 1

9.2

48. 1

(An30)

1. 4

1.2

1. Sample (SH-IA) taken from rare-earth zone; 1,156 point counts were made over an area of 325 sq mm 

Accessory minerals are xenotime, monazite, and sparse zircon, a' of the plagioclase is 1.543±0.002.

2. Sample (SH-2A) taken from rare-earth zone; 1,143 point counts were made over an area of 300 sq mm. 

Accessory minerals are xenotime, monazite, and sparse zircon.

3. Sample taken about 800 ft. southwest of rare-earth zone. Modal analysis by D. J. Gable of the U.S. 

Geological Survey. Accessory minerals are mainly apatite and allanite.

ACCESSORY MINERALS

The accessory minerals in the rare-earth zone, the enclosing biotite 

gneiss, and the nearby pegmatite were extracted and studied in the 

same manner as those in the Jasper Cuts area. The data are recorded 

in table 5.

Several similarities and some differences can be noted between this 

occurrence and that in the Jasper Cuts area. The biotite gneiss and 

the pegmatite at the two localities are similar in that both contain 

monazite and xenotime; also, the pegmatite has the same preferential 

concentration of xenotime over monazite. The size and shape of xenotime and monazite and the cores of many zircons in the rare-earth 

zone at this locality are similar to those at the Jasper Cuts area. The 

major mineralogic differences between the two localities are the abundance of magnetite at the Fourmile Gulch area, the predominance 

of monazite over xenotime in the rare-earth zone at the Fourmile 

Gulch area, and the presence of more monazite relative to xenotime 

in the biotite gneiss (country rock) at Fourmile Gulch.

Although monazite exceeds xenotime in abundance in the rare-earth 

zone at the Fourmile Gulch area, spectrographic analyses of the rocks 

from this locality indicate that the yttrium-earth elements predominate over the cerium-earth elements, as shown in table 6. Accordingly we conclude that the monazite at Fourmile Gulch contains more 

yttrium than the mona zite at the Jasper Cuts area.

In contrast to the Jasper Cuts area, xenotime and monazite are virtually impossible to distinguish in thin section in the Fourmile Gulch
XEN'OTIME AND MONAZITE, CENTRAL CITY DISTRICT 289

area, both being pale yellow, probably as a result of a higher percentage of yttrium in the Fourmile Gulch monazite. As shown in figure 72, 

a photomicrograph taken in plane polarized light, the xenotime and 

monazite are closely similar in appearance. Under crossed nicols some 

xenotime grains that have their optic axes more nearly in a horizontal 

than a vertical position are distinguishable from monazite, however, 

because of their greater birefringence. Xenotime and monazite can 

be distinguished in thick sections (approximately 0.3 mm) of rocks 

from the rare-earth zone with a calibrated absorption microspectroscope, for the two minerals give completely different absorption spectra. The following absorption spectra are especially diagnostic:

Xenotime at 668mu, a strong, broad line in the red, at 523 and

525m/>t, a strong doublet in the green. 

Monazite 572 to 592m/i, a very strong band in the yellow.

It might be noted that the data for xenotime do not match that given 

by Wherry (1915). Unfortunately the ordinary thin section is too 

thin for this instrument to be used successfully.

The optical properties of the xenotime and monazite are similar but 

differ slightly from those in the Jasper Cuts area. The xenotime from 

Fourmile Gulch is uniaxial positive with <o= 1.723 ±0.004 and e= 1.821 

±0.005; the birefringence is 0.098±0.009; pleochroism is faint with <o 

colorless and e pale yellow. Monazite is biaxial positive and 2F ranges 

from 8° to 11° ; the average of three measurements on the universal 

stage and two measurements using Mallard's constant was 10°. The 

monazite is not perceptibly pleochroic; all orientations appear pale 

yellow, a-1.785±0.004, p= 1.787±0.004, and y= 1.835±0.005; the 

birefringence is 0.050±0.009. These indices are somewhat lower than

TABLE 5. Accessory minerals heavier than methylene iodide in rocks from the Fourmile Gulch area

Rock type

1. Biotite gneiss in the rare-earth

2. Biotite gneiss---- - __ __

Number ofgrains 

counted

352 

395 

256

Weight of percent 

minerals accessory heavier thanmeth- ylene iodide, including magnetite

34.8 

2.12

.28

Weight of percent 

magnetite

'

31.4 

2.03 

.21

Number percent in 200-270 mesh fraction 

minus magnetite

Hematite

7.2 

62.5 

22.0

t* 

Si

4.9 

9.4 

14.0

Xenotime

26.4 

2.3 

33.0

o

61.5 

24.9 

31.0

t: O'5,

0 

.9 

0

NOTES. No apatite or allanite was found in the fractions lighter than methylene iodide. Sillimanite 

is present in the biotite pneiss.

1. Sample taken from same outcrop as sample 1 of table 4.

2. Sample taken 30 ft stratigraphically above sample 1.

3. Sample taken 15 ft stratigraphically above sample 1.
290 GEOLOGY AND ORE DEPOSITS, COLORADO

those of the monazite from Jasper Cuts, possibly because of its higher 

yttria content. Such an interpretation is partly substantiated by monazite from Bolivia, described by Gordon (1939), that has similar indices and contains 5.08 percent Y2O3 and virtually no thoria.

SPECTROCHEMICAL ANALYSIS

Semiquantitative spectrochemical analyses of a selected sample 

(SH-2) from the rare-earth zone and a composite chip sample (4 1) 

over a 5-foot width are given in table 6.

As in the rare-earth zone at Jasper Cuts, yttrium-group rare earths 

predominate over the cerium-group rare earths despite the predominance of monazite at Fourmile Gulch, as mentioned earlier. This is 

shown in tabular form as follows:

SH-2 Jt-l 

(percent) (percent) 

Total rare earths__________________ _________________________ 3.3 0.8

Yttrium group (Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). 

Cerium group (La, Ce, Nd, Pr, Sm, and Eu) . 

Yttrium group-cerium group ratio________-______

2.2 

1.0 

2.2

.4

.3

1.3

FIGURE 72. Photomicrograph showing xenotime and monazite in rare-earth zone in 

biotite gneiss at Fourmile Gulch. Both minerals are rounded, gray, and indistinguishable, unlike those in figure 69. White mineral is quartz. Pale-gray laths are biotite. 

Most of the black mineral is magnetite. Plane polarized light.
XENOTIME AND MONAZITE, CENTRAL CITY DISTRICT 291

TABLE 6. Semiquantitative spectrographic analyses of rare-earth zone in biotite 

gneiss from the Fourmile Gulch area

[Analyst, Raymond G. Havens]

Element SH-2 (percent)

Radioactive:

TJ ___ __ __ ____ _ ___ __ ___ _ iQ

Th____________________________________ . 3

Rare-earth:

Y____________________________._. 1. 5

La___ _________ __ _____ ___ __ _______ _ . 3

Ce_________________________________-__--__ . 3

Pr________________________________________ . 07

Nd _ _ .3

Sm____________.__________________________ . 07

Gd ____________ .15

Tb_ _ ___ ____ _________ _______ _ <. 07

Dy__________________.____________________ . 15

Ho _ _ __.._____ .03

Er_________________-___-________-_________ . 15

Tm _ _ __ _________________ .015

Yb _ _ _ __ .15

Lu_____________________________ . 03

Major and Minor:

Si __ __ __ _ __ -___ M

Al_ _______________________ _ ______________ 3

Fe _ _ __ ______ __ __ _ M

Mg__.____________________________________ 1. 5

Ca __ _ _________________ . 15

Na_________-_______-__---_--__-_-_----____ - 3

K _ __ ______________________________ 3

Ti _ _ _ _____________________ .7

P_______________-__-__---_-_---_------_-_- 2- 15

Trace:

Mn________________-__-_-_--___--___-__--_ . 15

Ba - __ _ _ __ ____ . 03

Co _ ____-._-_-__---_-_____________ -003

Cr_____________________.-_-_-_-_-_-_ -07

Cu _ _ _. _ _ _ _ __ .0015

Ga_._________.___-_ _ _ _ < 007

Nb_ ____ __ __ __ _ _____ ________ .003

Ni______________---____--___-___--_____--_ - 015

Pb ___ _______ __ __ ___________ .03

Sc___ ___ _ __ __ __ _ __- _---_ .003

Sn _ ____ __ _______ .007

Sr _ _____ ___ . 0015

V_ _ _ .03

Zr . __ _ __________ . 15

'0 

.07

. 3

. 07

. 15

. 15

.07

0

.03

0

. 03

.015

.03

.007

.03

.007

M 

M

7

1. 5

1. 5

3

3 

.7

0

. 15

.03

.003

.015

3. 7

. 003

.003

.015

.015

. 003

. 0015

.015

. 015

. 07

NOTE. Looked for but not found: Ag, As, Au, B, Be, Bi, Cd, Eu, Qe, Hf, Hg, In, Ir, Li, Mo, Os, Pd, 

Ft, Re, Rh, Ru, Sb, Ta, Te, Tl, W, and Zn.

Figures are reported to the nearest number in the series 7, 3, 1.5, 0.7, 0.3,0.15, in percent. Sixty percent 

of the reported results may be expected to agree with the results of quantitative methods.

M=major constituent, greater than 10 percent.

1 Less than 0.05 percent U may not be detected with this method.

2 Determined chemically by John P. Schuch, U.S. Geological Survey.

3 The high value for Cu probably represents contamination.
292 GEOLOGY AND ORE DEPOSITS, COLORADO

0. 

X 

UJ

I

a 'o

O cq
XENOTIME AND MONAZITE, CENTRAL CITY DISTRICT 293

PETROFABKICS

Figures 73A and 732? illustrate contoured plots of the c axes maxima 

of xenotime and quartz from thin section SH-2b from Fourmile Gulch, 

which was cut normal to the biotite foliation. The principal /tS'-plane 

of the foliation is shown. Figure 736"1 gives a contoured plot of 200 

random points, selected and contoured as noted previously for figure 

71. Diagrams in figure 73 A, B show slightly more preferred orientation for xenotime and quartz than the diagrams for the Jasper Cuts 

area, owing to a tendency for 4 percent maxima to be close to the 

principal /S'-plane. The maxima in the three diagrams show the following relations:

Clusters (percent) 

4 _ _

3

2

Xenotime Q 

c axes c

_ __ 3

__ ___ __ _ __ 11

14

Hiartz Random 

 axes points

3 1

6 8 

7 11

ILLINOIS GULCH AREA

A third known occurrence, apparently lower in tenor than the Jasper Cuts and Fourmile Gulch deposits, is about a mile south-southwest 

of Central City (fig. 66, loc. 3). It has been prospected by several 

shallow bulldozer cuts. The host rock is a migmatitic garnetiferous 

biotite-quartz gneiss that forms small interlayers in migmatitic sillimanitic biotite-quartz gneiss. These rocks are infolded into the layer 

of microcline-quartz-plagioclase-biotite gneiss along the axis of a small 

syncline on the southeast flank of the Central City anticline (fig. 66), 

and are stratigraphically approximately equivalent to the biotite 

gneiss and migmatite in the Jasper Cuts area.

The accessory minerals heavier than methylene iodide extracted 

from a sample of the host rock are listed in table 7. Garnet is the 

dominant accessory mineral. Xenotime and monazite are not concentrated to the extent they are in the other two occurrences; xenotime 

is nearly twice as abundant as monazite.

TABLE 7. Accessory minerals heavier than methylene iodide in migmatitic biotite 

gneiss from the Illinois Gulch area

Number 

of grains 

counted

300

Weight percent of 

accessory minerals 

heavier than 

methylene iodide, 

including magnetite

3.6

Weight 

percent 

of magnetite

0.06

Number percent in 200-270 mesh fraction minus magnetite

Hematite

2.4

Zircon

3.4

Xenotime

9.3

Monazite

4.8

Garnet

79.1

Epidote

1.0

NOTE. No apatite or allanite was found in the fraction lighter than methylene iodide.
294 GEOLOGY AND ORE DEPOSITS, COLORADO

ORIGIN OF XENOTIME AND MONAZITE 

CONCENTRATIONS

To account for the local occurrences in this region of abnormal 

quantities of combined xenotime and monazite without appreciable 

amounts of other heavy accessory minerals except magnetite, it seems 

that two principal hypotheses must be considered, first, an original 

mechanical concentration such as a placer, and second a selective 

mobilization of rare-earth ions from the country rock and recrystallization of xenotime and monazite in local zones of concentration 

within migmatized biotite gneisses.

The mineral concentrations occur at three widely separated localities 

at approximately the same stratigraphic position within a single layer 

of biotite gneiss. These concentrations could indicate that the minerals 

were concentrated locally at the time of deposition of the original 

sediments, perhaps as small lenticular placers on a moderately uniform 

surface of deposition. Possibly such concentrations could accumulate 

during normal weathering processes. Hence a placer hypothesis 

seems plausible. At least two objections to this hypothesis can be 

raised, however. One objection is that the mineral suite is much 

simpler than that in most known placer deposits. In the Jasper Cuts 

area xenotime and monazite predominate, with much smaller amounts 

of zircon and hematite. The Fourmile Gulch area differs in that magnetite predominates over monazite and xenotime, and zircon and 

hematite -are relatively unimportant. All known unconsolidated 

monazite placer deposits have much more complex mineral suites 

containing rutile, sillimanite, staurolite, and other common heavy 

minerals. A possible exception is a deposit classified as a consolidated 

placer by Vickers (1956) from the Precambrian Goodrich quartzite 

in the Palmer area, Marquette County, Mich. It contains monazite, 

hematite, magnetite, ilmenite, and rutile. The second and more serious 

objection to the placer hypothesis is that a mechanical concentration 

of heavy accessory minerals similar to those contained in the enclosing 

rocks would yield a concentrate richer in zircon than xenotime, for 

the enclosing biotite gneisses both at Jasper Cuts and at Fourmile 

Gulch contain more zircon than xenotime (tables 2 and 5).

Much evidence supports the contention that xenotime and monazite 

were deposited at their present sites from fluids that selectively mobilized rare-earth cations and phosphate from the biotite gneiss country 

rock. The field data clearly indicate that concentration of the rareearth minerals at their present sites took place during migmatization 

of the gneissic country rock. Presumably the agent of concentration 

was the liquid or silicate melt that crystallized as pegmatite; the 

character of this fluid is not known, but certainly its properties were
XENOTIME AND MONAZITE, CENTRAL CITY DISTRICT 295

such that it could intimately diffuse through the country rock, dominantly along the planar structure, in part replace the country rock, 

and in turn incorporate material from it.

The assumption that the pegmatite was the dominant concentrating agent for the rare-earth minerals is based upon two observations: 

(a) pegmatite is closely associated with the occurrences as the felsic 

component in migmatite and as larger distinct bodies; and (b) the 

pegmatite contains xenotime and monazite in approximately the same 

proportion as the host rock in the rare-earth zone, although in much 

smaller amount (tables 2 and 5). Also, pegmatite is closely associated 

with the lenses of coarse biotite that contain the xenotime and monazite. The field relations indicate that the xenotime and monazite 

crystallized with this coarse biotite, presumably from the same fluid 

that yielded the pegmatitic phase of the migmatite. This biotite is 

unoriented, in contrast to the mica in the biotite gneisses, indicating 

that it crystallized relatively late in the deformation of the rocks. 

Zircon must have been a stable mineral in this environment; the 

original mineral seems to have persisted and changed only by the 

development of overgrowths.

The biotite gneiss country rock is adequate as a source for the 

xenotime and monazite, for samples of this rock taken in the vicinity 

of the rare-earth deposits contain both accessory minerals (tables 2 

and 5) ; the country rock at both the Jasper Cuts and Fourmile Gulch 

localities, however, contains more monazite than xenotime. The greater xenotime-monazite ratio in the rare-earth zone and in the pegmatite 

than in the country rock is evidence that xenotime was preferentially 

mobilized with respect to monazite. Along this line of reasoning, 

the greater monazite-xenotiine ratio in the rare-earth zone at Fourmile Gulch than at the same zone at Jasper Cuts is attributable to 

the fact that the country rock at Fourmile Gulch contains more monazite relative to xenotime than the country rock at Jasper Cuts.

It seems improbable that either the microcline-quartz-plagioclasebiotite paragneiss or the intrusive granodiorite could have been the 

ultimate source of the rare-earth elements, for the microcline-bearing 

gneiss contains only traces of xenotime and sparse monazite (table 2) 

and the granodiorite lacks both minerals, so far as known. Also, 

known granodiorite is not present in close proximity to the deposits. 

A biotite-muscovite granite that occurs in this region, particularly 

south of Central City (Harrison and Wells, 1956; 1959), also can be 

ruled out as a potential source even though the rock is known to contain monazite as a common accessory mineral, for this granite is 

absent in this part of the Central City district; also it was emplaced
296 GEOLOGY AND ORE DEPOSITS, COLORADO

after the formation of the migmatite (Moench, Harrison, and Sims, 

1958, p.1737).

One implication of these data is that both xenotime and monazite 

are mobile minerals in environments of high-grade metamorphic rocks 

and migmatite, whereas zircon probably remains as a relic, stable 

mineral but is modified by the development of overgrowths.

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Scharizer, R., 1888, t'ber den Xenotim und uber eine neue Glimmerverwachsung 

von Schiittenhofen, Zeitschr. Kristallographie, v. 13, p. 15-22.
XENOTIME AND MONAZITE, CENTRAL CITY DISTRICT 297

Shaw, D. M., 1957, Xenotime from St. Simeon, Charlevoix County, Quebec: 

Canadian Mineralogist, v. 0, p. 61-67.

Smith, J. R., 1058, The optical properties of heated plagioclases: Am. Mineralogist, v. 43, p. 1179-1194.

Tsuboi, S., 1923, A dispersion method of determining plagioclases in cleavageflakes : Mineralog. Mag., v. 20, p. 108-122.

Vickers, R. C., 1956, Geology and monazite content of the Goodrich quartzite, 

Palmer area, Marquette County, Michigan: U.S. Geol. Survey Bull. 1030-F, 

p. 171-185.

Wherry, E. T., 1915, The microspectroseope in mineralogy: Smithsonian Misc. 

Colln., v. 65, no. 5, p. 1-16.

Winchell, A. N., and Winchell, Horace, 1951, Elements of optical mineralogy an 

introduction to microscopic petrography, pt. 2, Description of the minerals: 

4th ed., New York, John Wiley and Sons, 551 p.

INDEX

Page

Accessory minerals.__........ 280-284,288-290,293

Acknowledgments_______________ 274 

Boulder Creek granite....-_.-.--...--.. 276,280,281

Exploration................................ 277,287

Fourmile Gulch area, geology__...-__.-._ 287 

host rock__.....__......._....... 287

location..__________________ 287

rare-earth zone.....__.._...._...._. 287

Gneiss, biotite.._................... 276,287,288,289

biotite-quartz-plagioclase____ 276,280,281,288 

microcline-quartz-plagioclase-biotite. _ 276,

280,281 

migmatitic biotite..__......__..__ 293

sillimanitic biotite-quartz 277,288

Granodiorite............................ 276,280,281

Illinois Gulch area, host rock................. 293

location....___ _.__ 293

Jasper Cuts area, geology.---.---...__._.__. 277 

host rock..____..............._... 277

location.-...________ ___...._... 277

production.-.---_-.---_._-----__-__--__-- 277

rare-earth zone..__ 278

Methods, laboratory__._._...._....____ 275

Migmatite.................. 277,279,280,281,284,285

Modal analysis___.________... 279-280,288 

Monazite, absorption spectra.-------__...-. 290

color .... . . 281,290

grain size______________ _____ 281 

optical properties_____ . _ 281,290 

paragenesis________________ 282-283

Page

Origin, xenotime and monazite concentrations. 294-

296

Pegmatite - 277,278,279,280,287,295 

Petrofabrics, quartz..__.--._.......__.. 292

random points.._. --- 286,287,292

xenotime-.. . 287,288-289,292

Petrography...--.-..-...-.-..---.--..-- 279,287-288

Quartz, petrofabrics..-.___. -.. 292

Radioactivity-....-.-.-----.- 274,275,277,279,287

Rare-earth zone, location. ---------- 277,279,288,287

mineralogy..--_--._-------------..- 281,284,289

origin 294-296

petrography-.--.--..-----.-----.-.. 280,287-288

spectrochemical analyses..-..-. 284-285,290-291 

tenor. 278,288

xenotime-monazite ratio. _ - - 280,281 

yttrium group-cerium group ratio.-..... 284,290

Spectrochemical analyses. ..._.. 284-285,290-291 

Staining, feldspar....._... ___...__.. 275

Thorium... .... . 277,284,285,291

Uranium...... .-..... ....... 284,285,291

Xenotime, absorption spectra.. .---..- .. 290 

color . 281,290

grain size.-..__._____------_-__.._ _.. 281

optical properties.___--------------_---- 281,290

paragenesis.-.-------------.------------ 282,284

petrofabrics......-.-----------. 284,286,292

Zircon, overgrowths__. . . 282,283,295,296

299

eology and Ore Deposits 

lear Creek, Gilpin, and 

arimer Counties, Colorado

GEOLOGICAL SURVEY BULLETIN 1032

This bulletin was printed as separate 

chapters A-F. Prepared on behalf of the 

U.S. Atomic Energy Commission and in 

part under the auspices of the Defense 

Minerals Exploration Administration 

and published with the permission of the 

Commission

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director
CONTENTS

[The letters In parentheses indicate separately published chapters]

(A) Uranium deposits in the Eureka Gulch area, Central City district,

Gilpin County, Colorado, by P. K. Sims, F. W. Osterwald, and E. Page 

W. Tooker___-_________.__________________________________ 1

(B) Geology and ore deposits of the Freeland-Lamartine district, Clear

Creek County, Colorado, by J. E. Harrison and J. D. Wells. ____ 33

(C) Geology of the Wood and East Calhoun mines, Central City district,

Gilpin County, Colorado, by Avery A. Drake, Jr.--____________ 129

(D) Geology of the Copper King uranium mine, Larimer County, Colorado, P. K. Sims, George Phair, and R. H. Moench___________ 171

(E) Petrography of radioactive Tertiary igneous rocks, Front Range

mineral belt, Colorado, by J. D. Wells. _______________________ 223

(F) Petrography and origin of xenotime and monazite concentrations,

Central City district, Colorado, by E. J. Young and P. K. Sims___ 273

in

U.S. GOVERNMENT PRINTING OFFICE: 1961 O - 576761