Rutile and Topaz in 

Precambrian Gneiss 

Jefferson and Clear 

Creek Counties, Colorado 

By Douglas M. Sheridan, Richard B. Taylor, and 

Sherman P. Marsh 

G E 0 L 0 G I C A L 5 U R V E Y C I R C U L A R 567 

Washington 1968 
United States Department of the Interior 

WALTER. J. HICKEL, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing ·1968 

Second printing 1969 

Free on applkation to the U.S. Geologkal Survey, Washington, D.C. 20242 
CONTENTS 

Page 

Abstract - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Geologic environment - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Stratigraphy of the gneissic rocks - - - - - - - - - - - - - - - - - - - - - - 1 

Rutile-bearing sillimanitic topaz-quartz gneiss - - - - - - - - - - - - - - - 3 

Petrography - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 3 

Rutile and topaz content- - - - - - - - - - - - - - - - - - - - - - - - - - - - 5 

Chemistry - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

Geologic interpretation - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

Geophysical data- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

Economic implications - - - - - - - - - - - - - - - - - - - - - - - - - - - - 7 

References cited- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 7 

ILLUSTRATIONS 

Page 

Figure 1. Preliminary geologic map showing principal rutile and topaz 

occurrence, Jefferson and Clear Creek Counties, Colo- - - - - 2 

2. Photomicrograph showing fibrolitic sillimanite replaced in part by 

topaz - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 4 

3. Photomicrograph showing rutile grains molded around topaz 

grains - - - - - - - - - - - .:. - - - - - - - - - - - - - - - - - - 5 

4. Photomicrograph showing prismatic sillimanite intergrown with 

rutile- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 5 

TABLES 

Page 

Table 1. Modal analyses of samples from the rutile-bearing sillimanitic 

topaz-quartz gneiss unit and its stratigraphic equivalent - - - - 4 

2. Mineralogic composition based on mineral separates from 

rutile-bearing topaz-quartz gneiss- - - - - - - - - - - - - - - - 6 

3. Semiquantitative spectrographic analyses of rutile concentrates 

from the sillimanitic topaz-quartz gneiss unit- - - - - - - - - - 6 

·rn 

Rutile and Topaz in Precambrian Gneiss 

Jefferson and Clear Creek Counties, Colorado 

By Douglas M. Sheridan, Richard B. Taylor, and 

Sherman P. Marsh 

Abstract 

Disseminated rutile and major amounts of topaz have been identified in Precambrian topaz-quartz gneiss northwest of Evergreen, 

Colo. The rutile occurs in quartz-topaz-sillimanite gneiss that 

forms a stratigraphic unit which is 11 to 100 feer thick and i,.; 

identified along strike for more than 7,000 feet. Thre. :: rr:posite 

ch1p samples taken across this unit contain 2.2 to 4.2 oercent of 

rutile, by weight, in grains averaging from 0.1 to 0. '3 millimeter 

in size. The topaz content, by weight, in the same samples ranges 

from 23 to 67 percent. 

INTRODUCTION 

Significant amounts of rutile and topaz have been 

found in an unusual sillimanitic topaz-quartz gneiss 

in the central part of the Front Range, west of Denver, 

Colo. This gneiss forms an apparent stratigraphic 

unit in a metasedimentary sequence of strikingly varied 

lithology. The principal known occurrence is about 6 

miles northwest of Evergreen, Colo., in the Squaw 

Pass 7!-2-minute quadrangle (fig. 1). Other gneiss layers 

in this immediate area contain rutile in small amounts, 

though regionally sphene and ilmenite are the principal titanium-rich accessory minerals. 

The rutile was discovered by microscopic examination of thin sections by D. M. Sheridan in March 1968 in 

the course of detailed studies of Precambrian rock as a 

part of a program of regional mapping of the central 

Front Range by the U.S. Geological Survey, and identification was confirmed by X-ray diffraction study by 

W. N. Sharp andJ. W. Adams. Thepreliminary geologic investigation of this deposit has been made in cooperation with S. P. Marsh under a comprehensive research program authorized by the Office of Emergency 

P Ianning and undertaken by the Department oflnterior 

under the Defense Production Act for the purpose of developing a domestic source of rutile. 

GEOLOGIC ENVIRONMENT 

The area of rutile occurrence lies within the block 

of uplifted Precambrian rock that forms the core of 

the Front Range of the Rocky Mountains. The bedrock 

is composed of folded gneisses of high metamorphic 

grade and of granitic plutons intruded into the gneisses 

during a complex history of Precambrian tectonism 

and metamorphism. 

The structural framework in the metasedimentary 

gneisses was largely developed by plastic folding dur1 

ing regional metamorphism that recryf'~allized the 

rucks t<:-> 1 mineral assemhlage characteristic of the 

upper p'l:·t lJ;· the amphibolitefacies.Duringthismetamorphic episode, broad folds of westerly to northwester1v ~rend were initia1lv formed and were sub--

sequently modified in certain areas by folds of more 

northedv direction. The intrusion of bodies of Boulder 

Creek dranodiorite late in this period of tectonism 

has been iRotopically dated (Peterman, Hedge, and 

Braddock, 1 Q68) at about 1. 7 b.y. (billion years); this 

date in effect marks the end of the major regional metamorphic period. Structural complexitief' introduced 

by folding and cataclastic deformation about 1.45 b.y. 

ago, at abot:t the time of intrusion ofthe Silver Plume 

Granite and related rocks, are regionall:· important 

but have not been recognized in the part of the Squaw 

Pass quadrangle under consideration here. Major 

faulting of northwest trend, the ''breccia reef" faults 

of Lovermg and Goddard (1950, p. 79), has a Precambrian ancestry, and movement recurred intermitrer,tly in!i"J the Tertiary. 

The rutile-bearing rocks are a part of a gneissic 

sequence of east-west strike and near -Yertical dip. 

This belt, apparently homoclinal in structure, extends 

more than 5 miles from Santa Fe Mountain, 2~ miles 

west of the report area, eastward towards the mountain front, though the stratigraphic contimtity of lithologic units within the belt is broken by northwesttrending faults. Minor folds and mineral lineations in 

this area trend N. 50° W. toN. 85° W., which indicates 

that the dominant folding took place during the early 

period of Precambrian deformation. Elsewhere in the 

Squaw Pass quadrangle large-scale isoclinal folding 

is suggested by the distribution of marker units in the 

gneissic sequence. Major isoclinal folds t'ave not been 

recognized in the area of this report (fig. 1 ). Five 

miles to the southwest, the metasedimentcry sequence 

is intruded by the Mount Evans batholith of Boulder 

Creek Granodiorite. Crowding during intrusion may 

have contributed to the folding of the gneisses and perhaps may have steepened the dip of the layers, but it 

modified the shape of folds rather than C"'used them. 

STRATIGRAPHY OF THE GNEISSIC ROCKS 

The rutile-bearing sillimanitic topaz-quartz gneiss 

is one of a series of distinctive lithologic units that 
CONTOUR INTERVAL 40 FEET 

EXPLANATION 

Colluvium and alluvium 

z 

::; 

cr 

Ill 

Gneissic rocks ~ 

gn1, interlauered garnet(fernus hiotite gneiss, hornblende gneiss, <( 

and migmatite ~ gns, .~i/limanitic quartzite fHirl si/limanitic hiotite-quarto gneiss (X 

gnh, hornblende gneiss with inte1·/ayered calc-silicate gneiss 0.. 

gnq, ndile-beal'ing sillimanitit• topaz-qlU!I'fz gneiss 

gn m, m icmcline-q uartz-plagiaclase-biotite gneiss 

gnb, si/limonitir biotite-quartz-plagior/a.•e gneiss 

----------------? Contact 

A.pproximatelu /orated; dotted 1chere concealed 

Fault 

A.ppro.rinwte/y located; dotted whue concealed 

50~ 

Strike and dip of foliation and bearins 

and plunge of lineation 

+A 

Sample locality 

Figure 1.-Preliminary geologic map showing principal rutile and.topaz occurrence, Jefferson and Clear Cree'-: 

Counties, Colo. 

have been mapped within the report area. The stratigraphic sense-top versus bottom-of the sequence 

can be best inferred from broad fold patterns outside 

of this area, and on this basis the tops of the beds are 

believed to lie to the north. 

The lowest stratigraphic unit, the sillimanitic biotite-quartz-plagioclase gneiss along the southern edge 

of the area ( gnb, fig. 1), is a part of a major unit about 

5,000 feet thick. The rock is generally sillimanitic 

and is dominantly composed of quartz, plagioclase, 

and biotite, and variable amounts of micro c 1 in e, 

muscovite, and garnet. Lenses of calc-silicate rock 

and hornblende gneiss are contained within the unit. 

Thin sections of microcline-bearing varieties commonly show microcline and sillimanite as an appar2 

ently stable pair, and this is interpreted as indicating 

that the regional metamorphic grade reached the 

uppermost level of the amphibolite facief'. 

The hornblende gneiss unit ( gnh, fig. 1) that lies 

next in the sequence is of varied mineral composition 

but is characterized by hornblende, plagioclase, and 

clinopyroxene. The unit is about 1,500 feet thick in 

the eastern part of the area, if it is assumed that the 

bedding-plane fault contained within the unit has minor 

stratigraphic displacement. Layers containing about 

equal amounts of hornblende and plagioclase are abundant and intergrade with units containing clinopyroxene 

and varying amounts of epidote, sphene, quartz, and 

other minerals. A distinctive stratigraohic horizon 
about 100 feet below the top of the unit is marked by 

lenses of anthophyllite-garnet-co;rdierite rock or by 

layers of cordierite-bearing s i 11 i manit e-b i o tit e 

gneiss. The uppermost part of the unit is a garnetiferous sillimanitic biotite-quartz gneiss. 

To the west, the hornblende gneiss unit interfingers 

and intergrades with a microcline-quartz-plagioclasebiotite gneiss unit (gnm, fig. 1). This microcline gneiss 

unit is about 1,000 feet thick in the western part of the 

area and can be traced westward for more than 4 

miles. The unit is more granitic in appearance than 

the other gneisses, and foliation is defined by oriented 

biotite flakes in a medium-grained quartz-feldspar 

rock, rather than by marked compositional banding. 

The sillimanitic topaz-quartz gneiss unit that contains the principal rutile concentrations (gnq, fig. 1) 

is a lenticular layer within the hornblende gneiss unit. 

The layer is about 100 feet thick from sample locality 

D to locality G (fig. 1), but it thins to about 11 feet 

at locality B and grades westward into a feldspathic 

gneiss with high quartz content. The topaz-quartz 

gneiss layer is recognized over a strike length of 

more than 7,000 feet and is 80 feet thick at the point 

where it is covered by surficial materials. The layer 

may continue eastward under the alluvium as much 

as 5,500 feet before it intersects a major fault of 

northwest trend. 

A unit 30 to 500 feet thick (gns, fig. 1), which 1 i e s 

stratigraphically above the hornblende gneiss unit, 

is composed of sillimanitic quartzite and sillimanitic 

biotite-quartz gneiss and has been traced about half 

a mile east of the report area. Discontinuous lenses 

of these rocks persist westward at this stratigraphic 

position for almost a mile west of the report area. 

Fibrolitic needles of sillimanite form interlensing 

laminae between granular quartz septa. Sillimanite 

makes up about 20 to 30 percent of the quartzite and 

about 10 to 20 percent of the biotite-quartz gneiss. 

Variable amounts of biotite, feldspar, and muscovite 

are found. Rutile is a characteristic accessory mineral in amounts of a few tenths of a percent. 

Above the unit of sillimanitic quartzite and gneiss 

are interlayered gneisses of varied mineralogy that 

have been grouped into a composite unit ( gni, fig. 1) 

for purposes of mapping. The lowermost lr.yer, immediately above the quartzite, is composed of 

garnetiferous sillimanitic biotite-quartz gneiss and 

is about 200 feet thick. Above this are 1 ayers of 

complexly interbedded hornblende-plagioclase gneiss; 

calc-silicate gneiss containing clinopyroxene, epidote, 

hornblende, plagioclase, and other minerals; biotitequartz-plagioclase gneiss; and migmatitic masses 

rich in feldspar and quartz. This unit of interlayered 

gneisses is at least 5,000 feet thick and is the uppermost lithologic unit in the report area. 

RUTILE-BEARING SILLIMANITIC 

TOPAZ-QUARTZ GNEISS 

The principal known concentrations of rutile and 

topaz are contained within the sillimanitic topaz3 

quartz gneiss unit (gnq, fig. 1). The rock is almost 

white, with creamy or red tones. Medium-grained 

white or glassy quartz 1 aye r s 2 millimeters to 4 

centimeters thick alternate with discontinuous thinner 

layers of finer grained granular topaz-rutile rock and 

with radiating aggregates of prismatic or fibrous sillimanite. The rock closely resembles a sillimanitic 

quartzite in appearance and the topaz layers are not 

readily dis tin g u ish e d in the field except by their 

greater density. The rutile is easily seen with a hand 

lens and is concentrated in thin layers colored p a 1 e 

red by the tiny splendent rutile crystals. 

All the rock types in this unit are very light colored 

and are white, with glistening grains, where unweathered. Other than the red to orange rutile grains 

and weathered pale yellow apatite grains, all the minerals are white or light gray. The rock lacks the blacksand laminae (magnetite, ilmenite, garnet) characteristic of quartzites in the gneissic succession; it lacks 

the dark brown to black biotite flakes that dot almost 

all the quartz-feldspar rocks; it lacks even the feldspar grains common to all the quartz-rich rocks. 

PETROGRAPHY 

White, fine- to medium-grained gneisses composed 

chiefly of quartz and topaz and lesser amounts of sillimanite, rutile, and apatite are common (modes 2, 3, 

4, table 1) in the topaz-quartz gneiss unit. Thin sections show that these gneisses are com p o s e d of 

alternating quartz-rich and topaz-rich layers. Quartz 

and apatite occur in irregular anhedral grains 1 to 

5 mm across; the topaz characteristically occurs in 

slightly smaller rounded grains flattened parallel to 

the foliation. The topaz grains commonly exhibit 

healed cross fractures marked by bubbles concentrated in planar zones that are parallel to the basal 

cleavage of the topaz grains. These fractures are 

rudely perpendicular to the foliation of the rock. The 

rutile grains are commonly anhedral or subhedral, 

and some are intergrown with topaz or sillimanite. 

The largest quartz grains contain rutile needles, and 

a few tiny equant euhedral rutile crystals. 

The topaz-quartz gneisses intergrade with interlayered s i 11 i manit i c topaz-quartz gneiss, characterized by prismatic sillimanite (mode 5, table 1 ). The 

prismatic sillimanite crystals are as much as 3 em 

Ion~ but are slender and generally are only 1 or 2 

mm in cross-section area. These crystals form irregular to radiating aggregates flattened parallel to 

the foliation, but they lack linear orientation. Certain 

sillimanitic quartz gneiss laminae (mode 6, table 1) 

contain fibrolitic sillimanite with linear orientation. 

These needles are replaced in part by topaz and, less 

commonly, by randomly oriented prismatic sillimanite. 

Pegmatitic stringers or segregations within the 

quartz gneiss (mode 7, table 1) are composed mainly 

of quartz and coarsely prismatic sillimanite. Much 

of the quartz is coarse-grained and rutilated and is 
Table 1.--Modal analyses, in volume percent, of samples from the rutile-bearing sillimanitic topaz - quartz 

gneiss un it and its stratigraphic equivalent 

[Modes calculated from 1 1 000 points each, Tr, trace , Sample localities shown in f ig . l] 

l 

Sample locality -- A 

Quartz ----------- 41.2 

Topaz ------------

Sillimanite------

Rutile----------- .6 

Apatit e----------

Zircon----------- Tr 

Muscovite -------- . 5 

Biotite----------

Kaolinite-- ------

Plagioclase--- --- 57 . 6 

Hematite ----- ---- Tr 

l . Plagioclase-quartz gneiss. 

2, Topaz -quartz gneiss. 

3. Quartz-topaz gneiss, 

4 , Quartz -topaz gneiss, 

2 3 

B B 

10 .6 61 . 2 

70,8 35.8 

1.0 

2. 7 2. J 

13.0 ,? 

. 3 

l.l 

• 5 

segregated into lenses as much as 5 em thick. Bent 

sillimanite prisms 5 mm across and as much as 3 

em long occur in bundles curving around the quartz 

lenticles. Rutile is intergrown with the sillimanite 

and is especially abundant in apatite-rich stringers. 

West of the pinchout of the topaz-quartz gneiss unit, 

a light-gray plagioclase-quartz gneiss is found at the 

same stratigraphic position and seems to lack both 

topaz and sillimanite but contains 0.6 percent of rutile (mode 1, table l ). This lithologic type has not 

been mapped as a part of the topaz-quartz gneiss unit. 

Certain mineralogic details observed in thin section seem to be critical in the interpretation of the 

origin of the rutile-bearing sillimanitic topaz-quartz 

gneiss. 

l. Fibrolitic sillimanite is absent, is partly replaced 

by topaz (fig. 2), or is partly recrystallized to 

prismatic sillimanite. As the fibrolitic sillimanite is linearly oriented parallel to small-s c a 1 e 

folds related to the early period of folding, it is 

believed to have formed during the regiona l metamorphism. The topaz and prismatic sillimanite 

are younger than the fibrolitic sillimanite and 

therefore are interpreted as being younger than 

the regional metamor phism. 

2, The rutile grains are molded around topaz (fig. 3) 

and are intergrown with prismatic sillimanite 

(fig. 4). The rutile therefore seems to have crystallized after the p r inc i p a 1 regional metamorphism, at the same time as the topaz and prismatic sillimanite. The rock cannot represent a 

4 

6 7 

D E G G 

68,c 5. 5 49 .4 84 , 4 

30 . 1 71.9 . 4 .6 

18. 2 44.8 12. 0 

l. - 3. 1 2.7 1. 7 

.4 1.3 .5 

Tr Tr Tr Tr 

. 9 1.4 • 5 

. 3 

5. Sillimanite-topaz -quartz gneiss, 

6 . Sillimanite -quartz gneiss. 

7 , Pegmatitic quart z~sillimanite rock . 

Figure 2,-Fibrolitic sillimanite replaced in part by topaz. Magnification·, X 40. Q, quartz; s, sillimanite; t. topaz. 
_,:.-. -:_-~ / -

.. -· -~ \. ·\ ... , .· ~,~:;..'4'.::- :"""---..1 

j' . ·~~ .. <1 ·:._. . .,..~ .. _.,"'- • 

· .... / . I~ . '- ~ --.... . <\/'""' 

.r .· - ,_ ...... :( ... \ 

-·--.... - ' ' . 

- ' 

F igur e 3. -Rutile grains molded around topaz grains. Magnifica -

tion, )( 40. Q, quartz; R, rutile; t, topaz. 

fossil rutile placer, a lthough the rutile may have 

formed from titanium alr eady within the rock at 

the time of r egional metamorphism. 

3. Although magnetite, ilmenite, and sphene occur t.'1 

rocks bordering the topaz-quartz gneiss, th<::!se 

minerals do not occur in the same rock WJth rutile. The rutile-bearing rocks are essentially 

free of ferromagnesian minerals. 

RUTILE AND TOPAZ CONTENT 

A series of samples was taken to evaluate the r utile and topaz content of the principal outcrops (table 

2). Samples R-1, R-3, and R-6 are composites of a 

large number of rock chips taken across the entire 

layer at the only three localities where it crops out 

fully (fig. 1). Samples R-2A, R-4, and R-7 are grab 

samples taken at localities where outcrops are sparse, 

and they may not represent the average composition 

of the unit. 

Each sample was crushed, ground, and screened, 

and the part between 60 and 150 mesh was washed 

and sonically cleaned to remove adhering particles 

of dust. One hundred and fift y grams of this material 

was split into heavy and light fractions in bromoform 

5 

Figure 4.-Prismatic sillimanite imergrown with rutile. Magnification, )( 40. 8, biotite; Q, quartz; R, rutile; 5, sillimanite. 

(tlensity 2.8). The weights of the heavy and light fractions were recorded, and percentages were calculated 

(table 2 ). Further mineral s eparation using the Frantz 

isudynamic separator and a laboratory micro pan 

de vi ce yielded an almost pure rutile fraction. Other 

md.gnetic concentrates also contained small amounts 

of rutile, and the weight of this rutile was added to 

that of the pure fraction to calculate the rutile percentages given in table 2. The topaz percentages were 

obtained by a similar procedure. Because of losses 

of material to other fractions of the sample during 

separation, the percentages quoted are conservative 

estimates of the actual rutile and topaz contents of 

each sample processed. The major source of error 

in evaluating the rutile and topaz contents of the topazquartz gneiss unit almost certainly lies in the field 

sampling procedure. Core drilling or other physical 

exploration would be required to adequately sample 

this unit. 

Semiquantitative spectrographic analyses of rutile 

concentrates from the sillimanitic topaz-quartz gneiss 

unit show that the content of niobium and iron in the 

rutile is low. The rutile samples contained less than 

5 percent impurities, and these impurities were 

almost entirely made up of sillimanite and topaz. 
Table 2.--Mineralogic composition, in weight percent, based on mineral separates from rutile-bearing topzaquartz gneiss 

[Determinations by S. P. Marsh] 

Sample Locality 

(in fig. 1) 

Heavy fraction 

(density >2.8) 

Light fraction 

(density <2.8) 

Topaz Rutile Thickness 

of unit 

sampled 

(feet) 

I Composite chip samples 

R-1 B 74 26 67 4.2 11 

R-3 G 36 64 23 3.9 So 

R:..6 E 50 50 36 2.2 100 

Grab samples 

R-2A 

R-4 

R-7 

c 

F 

D 

65 

43 

52 

Table 3.--Semiquantitative spectrographic analyses of 1 

rutile concentrates from the sillimanitic topazquartz gneiss unit 

[A. L. Sutton, Jr., analyst] 

Sample Locality Nb Fe 

(ppm) (percent) 

l B 5.00 0.3 

2A c 700 .5 

3 G 500 .5 

4 F 500 .5 

6 E 500 .5 

7 D 1500 .7 

CHEMISTRY 

Semiquantitative spectrographic an a 1 y s e s were 

made by J. C. Hamilton of heavy (density ,.z. 8) and 

light (density <2.8) fractions separated from the 

s i 11 i manit i c topaz-quartz gneiss. The rock is 

abnormally low in iron, calcium, sodium, potassium, 

and it contains less than 1 part per million of beryllium, an element that is frequently concentrated in 

topaz greisens. Small amounts of tin (0-lSppm), 

niobium (0-15 ppm), molybdenum (0-7 ppm), and rareearth elements were detected. 

GEOLOGIC INTERPRETATION 

The topaz-quartz gneiss is mineralogically simple, 

chemically unusual, and rather undistinguished in appearance. Quartz, topaz, and sillimanite are the only 

abundant minerals and rutile, apatite, and zircon are 

the only characteristic accessory minerals. These 

minerals clearly formed at the expense of the previously formed regional metamorphic suite. Chemi6 

35 

57 

48 

51 

22 

42 

0.6 

1.4 

1.0 

cally, the rock has a very high fluorine content and 

in comparison with other quartz-rich gneisses in the 

Front Range is unusually low in the alkali metals, 

alkaline earths, and iron. The chemically resistant 

character of the miner a 1 s in the rock, the high 

fluorine content, and the depletion of several common 

elements suggest that the topaz-quartz gneiss is the 

product of fluorine metasomatism. The high fluorine, 

aluminum, and titanium contents suggest that these 

elements were added or enriched during the metasomatism. According to this hypothesis, a gneissic 

progenitor, probably a sillimanitic quartz-rich gneiss, 

was attacked by hot fluorine-rich solutions that swept 

the rock clean of many of the common elements and 

minerals, left a resistant assemblage of zircon, 

quartz, and sillimanite, and added rutile and the topaz 

and apatite. 

Rutile is apparently stable in this gneissic sequence 

only in rocks deficient in both iron and c a 1 c i u m. 

Gneisses adjacent to the topaz-quartz gneiss contain both ilmenite and sphene, but no rutile. Rutile 

is an accessory mineral (}2 percent) in several other 

gneiss layers, notably the sillimanitic quartzite unit 

(gns , fig. 1 ). This unit probably also underwent limited 

metasomatic alteration, as it contains no magnetite, . 

ilmenite, or garnet but does contain rutile and small 

amounts of topaz. 

GEOPHYSICAL DATA 

Preliminary aeromagnetic data show that the report 

area lies on a strong linear east-west magnetic gradient. The anomaly is interpreted (Peter Popenoe, 

oral commun., 1968) as indicating a steeply dipping 

contact or a major fault that marks the north boundary of a relatively magnetic block. The anomaly extends from the mountain front about 20 miles west 
into the range. The occurrence of plutonic rocks to 

the south of the report area suggests that the anomaly marks a steep contact between p 1 u tonic and 

metasedimentary rocks at depth. The geophysical data 

are the strongest direct evidence for a major subjacent igneous mass from which fluorine-rich solution 

could have been derived. 

ECONOMIC IMPLICATIONS 

Rutile is an important industrial mineral and source 

of titanium, and topaz and sillimanite are raw materials for refractories and ceramics. Most rutile is 

obtained from beach sands, and mostofitis imported. 

Topaz and sillimanite are members of a group of 

aluminosilicate minerals-also including kyanite, andalusite, and dumortierite-that has steady demand 

for production of mullite and other uses. Minerals 

of this group are produced domestically and also are 

imported. 

Considering the content of rutile in the topaz-quartz 

gneiss as determined thus far, and considering the 

purity of the rutile, the gneiss warrants investigation 

as an ore of rutile, especially since it might also 

yield a saleable topaz product. Tests to determine the 

7 

recoverability of the rutile and topaz wo·Jld be required; but first, much more extensive investigations 

than the preliminary studies reported here would be 

required to determine the character and c'stribution 

of the rutile and topaz and their average content in 

the gneiss unit. Further study of the derosit to the 

extent permitted by the limited exposures is planned, 

but the grade of the deposit probably could be determined satisfactorily only by means of core drill holes 

through the gneiss unit. 

The discovery of rutile in the topaz-quartz gneiss 

suggests that a part of the central Front Range is 

favorable for prospecting for rutile and that the search 

should be concentrated on white sillimanitic quartzrich gneisses. 

REFERENCES CITED 

Lovering, T. S., and Goddard, E. N., 1950, Geology 

and ore deposits of the Front Range, Co 1 or ado: 

U.S. Geol. Survey Prof. Paper 223, 319 p. 

Peterman, Z. E., Hedge, C. E., and Braddock, W. A., 

1968, Age of Precambrian events in the northeastern Front Range, Colorado: Jour. Geophys. 

Research, v. 73, no. 6, p. 2277-2296. 

INTERIOR-GEOLOGICAL SURVEY WASHINGTON, D C -151168 

f:r U. S. GOVERNMENT PTINTING OFFICE: 1969 0- 367-861