I 

u
The Boulder Creek Batholith, 

Front Range, Colorado

By DOLORES J. GABLE

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1101

A study of differentiation, assimilation, 

and origin of a granodiorite batholith 

showing interrelated differences in 

chemistry and mineralogy in the batholith 

and cogenetic rock types

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

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Gable, Dolores J. 1922-

The Boulder Creek batholith, Front Range, Colorado

(Geological Survey Professional Paper 1101)

Bibliography: p. 85

Supt. of Docs. No.: I 19.16:1101

1. Batholiths Colorado Boulder region. I. Title. II. Series: United States Geological Survey Professional

Paper 1101. 

QE611.5.U6G3 551.8; 8 78-24482

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

Washington, D.C. 20402
CONTENTS

Page

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

Introduction ............................................ 1

Previous work........................................... 2

Techniques used in this study ............................ 2

Geologic setting ......................................... 3

The batholith ........................................... 3

Constitution of the batholith.......................... 4

Contact relations and mode of emplacement............ 6

Constituent rocks of the batholith ..................... 7

Boulder Creek Granodiorite ....................... 7

Inclusions in Boulder Creek Granodiorite .......... 17

Mafic plutonic rocks ............................. 20

Gabbro and pyroxenite ....................... 23

Hornblendite, hornblende diorite, quartz

diorite ..................................... 25

Twin Spruce Quartz Monzonite ................... 26

Granite gneiss, gneissic aplite, and pegmatite ...... 34

Ages of batholithic rocks ......................... 35

Structure ............................................... 35

Geochemistry ........................................... 38

Boulder Creek Granodiorite........................... 38

Chemical trends within the batholith .............. 38

Mineralogy in relation to chemical trends .......... 45

Chemical trends in mafic inclusions ................ 48

Chemical trends in gabbro, pyroxenite, and hornblende diorite ................................. 50

Chemical and mineralogical trends in the Twin

Spruce Quartz Monzonite ...................... 50

Chemical equilibrium in the Boulder Creek Granodiorite .... 51

Assimilation and differentiation in the Boulder Creek Granodiorite ............................................ 52

Page

Origin of the Boulder Creek Granodiorite and the Twin

Spruce Quartz Monzonite .......................... 62

Mineralogy, petrology, and chemistry of minerals in the

batholith.......................................... 64

Biotite ...'........................................... 64

Boulder Creek Granodiorite ....................... 64

Twin Spruce Quartz Monzonite ................... 69

Mafic inclusions and lamprophyre dikes in the

Boulder Creek Granodiorite ..................... 69

Petrogenesis .................................... 69

Hornblende ......................................... 71

Boulder Creek Granodiorite ....................... 71

Mafic inclusions and lamprophyre dikes in the

Boulder Creek Granodiorite ..................... 73

Hornblende diorite and hornblendite............... 73

Petrogenesis .................................... 73

Plagioclase .......................................... 73

Boulder Creek Granodiorite ....................... 73

Twin Spruce Quartz Monzonite ................... 77

Mafic inclusions in the Boulder Creek Granodiorite . 77

Potassium feldspar .................................. 77

Boulder Creek Granodiorite ....................... 77

Mafic inclusions and lamprophyre dikes in the

Boulder Creek Granodiorite ..................... 79

Twin Spruce Quartz Monzonite ................... 79

Quartz .............................................. 79

Accessory minerals .................................. 80

Alteration minerals .................................. 83

References cited ......................................... 85

Index................................................... 87

ILLUSTRATIONS

Page

PLATE 1. Generalized geologic map of the Boulder Creek batholith with emphasis on cogenetic mafic rocks in the vicinity of the

Boulder Creek batholith, central Colorado ................................................................ In pocket

2. Geologic map of the Boulder Creek batholith .................................................................. In pocket

FIGURE 1. Index map of the Boulder Creek batholith ....................................................................... 2

2. Photographs of typical weathered bouldery outcrops of Boulder Creek Granodiorite ................................. 4

3. Map of Boulder Creek batholith showing rock types .............................................................. 5

4. Photographs of typical Boulder Creek Granodiorite in hand specimen .............................................. 8

5. Photomicrographs of Boulder Creek Granodiorite ................................................................ 9

6. Maps showing modal distribution of quartz, ores, sphene, allanite, hornblende, and potassium feldspar: plagioclase

	ratios and An content of plagioclase in Boulder Creek Granodiorite............................................. 10

7. Map and diagrams showing modal variation of quartz, plagioclase, and potassium feldspar for Boulder Creek

	Granodiorite .............................................................................................. 21

8. Photographs of exposures of mafic inclusions in Boulder Creek Granodiorite........................................ 22

9. Photographs of hand specimens of mafic inclusions in Boulder Creek Granodiorite .................................. 23

10. Photograph of oval-shaped lenses of Boulder Creek Granodiorite in mafic inclusion .................................. 25

11. Photomicrographs of mafic inclusions in Boulder Creek Granodiorite............................................... 26

ill
IV CONTENTS

Page

FIGURE 12. Photographs of outcrops of layered and foliated biotitic hornblende diorite and massive pyroxenite ................... 27

13. Photomicrographs of Twin Spruce Quartz Monzonite ............................................................. 29

14. Ternary diagrams of modal variation of quartz-plagioclase-potassium feldspar for Twin Spruce Quartz Monzonite ..... 35

15. Ternary diagram of modal variation in quartz-plagioclase-potassium feldspar for granite gneiss from the granite gneiss

	and pegmatite unit......................................................................................... 37

16. Diagram of more prominent joint sets, Tungsten and Gold Hill quadrangles ........................................ 37

17. Sample locality map for rock and mineral analyses shown in tables in this report.................................... 49

18. Graphs of minor elements for rock units included in text.......................................................... 50

19. Variation diagrams of common oxides in Boulder Creek Granodiorite plotted against SiO2 ........................... 52

20. Variation diagrams of CaO plotted against other oxides and fluorine for Boulder Creek Granodiorite .................. 54

21. Ternary diagrams of K2O-Na2O-CaO chemical variation of Boulder Creek Granodiorite............................... 56

22. Ternary diagram of chemical variation of Boulder Creek Granodiorite expressed in terms of normative Q-Ab+An-Or... 56

23. Mole percent variation in an AFM diagram for Boulder Creek Granodiorite and mafic rocks .......................... 57

24. Diagrams of CaO content plotted against weight percent of rock-forming minerals in the Boulder Creek Granodiorite .. 58

25. Ternary plot of K2O-Na2O-CaO showing chemical variation of Twin Spruce Quartz Monzonite ........................ 59

26. Ternary plot of normative Q-Ab-f An-Or showing chemical variation of Twin Spruce Quartz Monzonite ............... 59

27. AFM diagram showing molar variation of Twin Spruce Quartz Monzonite .......................................... 59

28. Diagram of sample variation of plagioclase in relation to CaO in Twin Spruce Quartz Monzonite ...................... 59

29. Diagram of distribution of manganese in biotite and hornblende in Boulder Creek Granodiorite and hornblende diorite . 60

30. Diagram of phase relations for Boulder Creek Granodiorite in system SiO2 -NaAlSi3O8 -KAlSi3O8 -CaAl2Si2O8 -H2O ....... 62

31. Graph of strontium, rubidium, and potassium feldspar plotted against Or/Ab in Boulder Creek Granodiorite .......... 63

32. Modal biotite plotted against potassium feldspar for the batholith ................................................. 65

33. Graph of percent biotite in Boulder Creek Granodiorite and Twin Spruce Quartz Monzonite in relation to percent SiO2

	in rock .................................................................................................... 65

34. Plot of compositional variations of K2 O, MgO, FeO, A12O3 , and SiO2 in biotites ...................................... 72

35. Graph of fluorine in biotite plotted against d(005 ) spacing of biotite.................................................. 72

36. Ternary diagram of relation of Fe+3-Fe+2-Mg between biotites ...................................................... 72

37. Photomicrograph of allanite and monazite crystals ............................................................... 82

TABLES

TABLE 1. Modes for Boulder Creek Granodiorite from the batholith ......................................................... 12

2. Modes for Boulder Creek Granodiorite from small plutons and small lenses in biotite gneiss and schist................ 18

3. Modes for Boulder Creek Granodiorite from contact area of batholith .............................................. 19

4. Modes for Boulder Creek Granodiorite adjacent to known faults ................................................... 19

5. Summary of modal data for the Boulder Creek Granodiorite ....................................................... 20

6. Modes for mafic inclusions and metamorphosed lamprophyre dikes in Boulder Creek Granodiorite.................... 24

7. Modes for gabbro, hornblendite, hornblende diorite, quartz diorite, and hornblende pyroxenite ....................... 28

8. Modes for Twin Spruce Quartz Monzonite ....................................................................... 30

9. Modes for aplite, aplitic pegmatite, and granitic gneiss ............................................................ 36

10. Chemical and spectrographic analyses and modes for Boulder Creek Granodiorite................................... 40

11. Chemical and spectrographic analyses and modes for mafic inclusions and lamprophyre dikes in Boulder Creek

Granodiorite .............................................................................................. 44

12. Chemical and spectrographic analyses for mafic rocks ............................................................ 46

13. Chemical and spectrographic analyses and modes for Twin Spruce Quartz Monzonite ................................ 47

14. Total iron expressed as FeO in biotite and hornblende ............................................................. 60

15. Comparison of sillimanite-biotite gneiss and schist country rocks with averaged compositions of Boulder Creek

Granodiorite .............................................................................................. 61

16. Rubidium, strontium, and potassium analyses of Boulder Creek Granodiorite samples ............................... 63

17. Mode summary of major minerals in the Boulder Creek batholith .................................................. 65

18. Chemical and spectrographic analyses and mineral formula for biotite from Boulder Creek Granodiorite, hornblende

diorite, mafic inclusions, and a lamprophyre dike ............................................................. 66

19. Chemical and spectrographic analyses and mineral formula for biotite from Twin Spruce Quartz Monzonite ........... 70

20. Chemical and spectrographic analyses and mineral formula for hornblende from Boulder Creek Granodiorite, mafic

inclusions, a lamprophyre dike, hornblende diorite, and hornblendite ........................................... 74

21. Comparison of partial hornblende lattice structures .............................................................. 77

22. Chemical and semiquantitative spectrographic analyses of potassium feldspar from Boulder Creek Granodiorite ....... 78

23. Chemical and semiquantitative spectrographic analyses of potassium feldspar from mafic inclusions and a lamprophyre

dike in Boulder Creek Granodiorite .......................................................................... 79

24. Chemical and semiquantitative spectrographic analyses of potassium feldspar in Twin Spruce Quartz Monzonite ..... 80
CONTENTS V

Page

TABLE 25. Chemical and semiquantitative spectrographic analyses of sphene in Boulder Creek Granodiorite, inclusions, in

granodiorite and Twin Spruce Quartz Monzonite ............................................................. 81

26. Chemical and spectrographic analyses of allanite in Boulder Creek Granodiorite, inclusions, and biotitic hornblende

diorite .................................................................................................... 84

THE BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

By DOLORES J. GABLE

ABSTRACT

The Boulder Creek batholith is the best known of several large 

Precambrian batholiths of similar rock composition that crop out 

across central Colorado. The rocks in the batholith belong to the 

calc-alkaline series and range in composition from granodiorite 

through quartz diorite (tonalite) to gneissic aplite. Two rock types 

dominate: the Boulder Creek Granodiorite, the major rock unit, and 

a more leucocratic and slightly younger unit herein named Twin 

Spruce Quartz Monzonite. Besides mafic inclusions, which occur 

mainly in hornblende-bearing phases of the Boulder Creek 

Granodiorite, there are cogenetic older and younger lenses, dikes, 

and small plutons of hornblende diorite, hornblendite, gabbro, and 

pyroxenite. Pyroxenite is not found in the batholith. The Boulder 

Creek Granodiorite in the batholith represents essentially two contemporaneous magmas, a northern body occurring in the Gold Hill 

and Boulder quadrangles and a larger southern body exposed in the 

Blackhawk and the greater parts of the Tungsten and Eldorado 

Springs quadrangles. The two bodies are chemically and 

mineralogically distinct. The northern body is richer in CaO and 

poorer in K2O, is more mafic, and has a larger percentage of 

plagioclase than the southern body.

A crude sequence of rock types occurs from west to east in the 

batholith accompanied by a change in plagioclase composition from 

calcic plagioclase on the west to sodic on the east. Ore minerals tend 

to decrease, and the ratio potassium feldspanplagioclase increases 

inward from the western contact of the batholith, indicating that 

the Boulder Creek batholith is similar to granodiorite batholiths the 

world over. Emplacement of the Boulder Creek batholith was contemporaneous with plastic deformation and high-grade regional 

metamorphism that folded the country rock and the batholith contact along west-northwest and north-northwest axes. Also, smaller 

satellitic granodiorite bodies tend to conform to the trends of foliation and fold axes in the country rock, suggesting that emplacement 

was controlled by preexisting structures in the country rock.

On a gross scale, chemical equilibrium in the Boulder Creek 

Granodiorite is expressed by a near 1:1 ratio, or straight-line relationship in the distribution of iron, magnesium, and manganese in 

biotite and hornblende. General mineralogical trends in the Boulder 

Creek Granodiorite indicate that modal biotite, hornblende, and 

plagioclase tend to increase and quartz and microcline tend to 

decrease as CaO increases. These trends were not found in the Twin 

Spruce Quartz Monzonite.

Differentiation is believed to have played a major role and 

assimilation a minor role in the development of the Boulder Creek 

batholith. The Boulder Creek Granodiorite is of probable mantle or 

lower crust origin, and, based on the scant data available, the Twin 

Spruce Quartz Monzonite may be of crustal origin, but the magma 

was extensively altered by contaminants of ambiguous origin. 

Mafic inclusions, possibly derived from a dioritic magma which was 

an early differentiate associated temporally with the Boulder Creek 

Granodiorite and (or) the Twin Spruce Quartz Monzonite, were injected into the Boulder Creek Granodiorite during the mush stage 

and before the batholith was completely crystallized.

Biotite, hornblende, and potassium feldspar were studied extensively. Their chemistry and petrology indicate a homogeneity 

throughout the batholith not believed possible by a casual observance of the batholithic rocks in the field. The accessory minerals, 

where investigated, also tend to indicate this same pervasive 

homogeneity.

INTRODUCTION

Geologic investigations, of which this report is a 

product, have been carried out in the Front Range of 

Colorado since the early 1950's when detailed geologic 

studies of the crystalline rocks west of Denver were 

begun. The Boulder Creek batholith, just west of 

Boulder, Colo., occupies an area 14 km by 27 km (fig. 

1). The batholith is the type area of similar masses of 

granodiorite that constitute a major part of the 

Precambrian intrusive rocks of Colorado.

This report is a composite of some old and much new 

work, both published and unpublished. The Boulder 

Creek batholith was first briefly described by Boos and 

Boos (1934); in 1950, Lovering and Goddard mapped 

and presented a short description of the batholith; in 

1953, Lovering and Tweto presented a more detailed 

description. More recently, mineralogic and 

geochemical aspects of the batholith have been 

described by George Phair and colleagues of the U.S. 

Geological Survey; these are discussed later in this 

report. This paper summarizes the field data concerning the batholith, gained by the quadrangle mapping 

of others as well as myself, and presents much new 

data on the overall mineralogy, geochemistry, and 

origin of the batholithic rocks.

Chemical, mineralogical, and structural data indicate that the Boulder Creek batholith is represented 

by two similar but chemically different granodiorite 

magmas and by a distinctly different quartz monzonite unit, the Twin Spruce Quartz Monzonite, that is 

in part the same age and in part younger than the 

Boulder Creek Granodiorite. Like the Boulder 

batholith in Montana (Tilling, 1973), the two similar 

but chemically different granodiorite magmas were
BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

105°30'

40° 

00'

Jamestown

Boulder

Nederland o BOULDER CREEK 

BATHOLITH

10 KILOMETERS

FIGURE 1. Index map of the Boulder Creek batholith, Front Range, 

Colorado.

emplaced nearly contemporaneously. The Boulder 

Creek batholith was emplaced during the major 

regional metamorphism and sufficiently early for parts 

of the batholith to reflect this regional deformation.

PREVIOUS WORK

The early work of Boos and Boos (1934), Lovering 

and Goddard (1950), and Lovering and Tweto (1953) on 

the batholith was followed by detailed quadrangle 

mapping and by reports on various parts of the 

batholith. These reports are: Boulder quadrangle 

(Wrucke and Wilson, 1967); Nederland and Tungsten 

quadrangles (Gable, 1969, 1972); Ward quadrangle 

(Gable and Madole, 1976); Gold Hill quadrangle 

(Gable, 1977); Blackhawk quadrangle (Taylor, 1976);

Eldorado Springs quadrangle (Wells, 1967); and 

Ralston Buttes quadrangle (Sheridan and others, 

1967). In addition, Sims and Gable (1964, 1967) 

described a few of the satellitic plutons in the 

metasedimentary rocks west and southwest of the 

batholith, and Pearson and Speltz (1975) mapped 

Boulder Creek Granodiorite in the Indian Peaks 

Wilderness area west of the Ward quadrangle.

Recent topical studies of minerals from the batholith 

include those by Wrucke (1965) on prehnite and 

hydrogarnet; Hickling, Phair, Moore, and Rose (1970) 

on allanite; Stern, Phair, and Newell (1971) on isotopic 

ages and morphology of zircon; and Phair, Stern, and 

Gottfried (1971) on fingerprinting discordant zircon 

ages.

TECHNIQUES USED IN THIS STUDY

The compositional classification and patterns of 

mineral distribution described in this report are based 

on modal analyses in which point counts of sufficient 

number (800-1,000 counts per thin section) were made 

to accurately represent the rock composition. Modes 

rounded to the nearest percent reflect less accuracy 

than those carried to the nearest tenth. Modal analyses 

of very coarse-grained rocks, such as were found in the 

central part of the batholith, were made using stained 

rock slabs, but a relatively small number of modes 

were made by this method. Only quartz, plagioclase, 

potassium feldspar, and total mafic minerals were 

counted on these slabs; the total mafic minerals were 

broken down by using a proportion determined by 

thin-section counts of the mafic minerals. Boundaries 

between composition fields in a triangular diagram 

(quartz, plagioclase, potassium feldspar) were determined as follows: granodiorite 5 to 20 percent dark 

minerals, more than 5 percent quartz, soda-lime 

feldspar at least equal to double the amount of the 

potassium feldspar; potassium feldspar may vary by 8 

to 20 percent in rock of 60 percent feldspar. Below the 

granodiorite field the rock is a quartz diorite (tonalite); 

above it, the rock is a quartz morizonite. In quartz 

monzonite, alkali feldspar (microcline plus microperthite) ranges from 20 to 40 percent of a total of 60 percent feldspar. Rock units were named by using an 

average composition for the unit. Due to the large 

amount of quartz in these rocks the quartz monzonites 

generally fall in the granite or granitoid group of rocks 

according to Streckeisen's (1976) classification. 

Almost none of the rocks described in this report 

would be classified as quartz monzonite according to 

the classification of Streckeisen. Therefore only the 

more mafic rocks within this report generally are 

classified by the system of Streckeisen.
THE BATHOLITH

A total of 78 samples representative of all the rock 

types discussed here, excluding aplite and pegmatite, 

were chemically analyzed for both major and minor 

elements; 44 of the analyses have never been published 

before. Whole-rock samples were analyzed by standard 

(Peck, 1964) and rapid rock (Shapiro and Brannock, 

1962) techniques. Minor elements were determined by 

semiquantitative spectrographic methods, and each 

result reported is the standard deviation of any single 

determination and should be taken as plus 50 percent 

and minus 33 percent.

Analyzed mineral concentrates were purified by first 

subjecting them to ultrasonic vibrations, then centrifuging in adjusted mixtures of methylene iodide and 

bromoform, and finally passed through an isodynamic 

separator. Samples were then examined under a 

binocular microscope. This process was repeated until 

only 1-2 percent impurities remained. Feldsparconcentrate powders were X-rayed for purity. 

Analytical procedures used for each mineral are given 

in the chemical tables (tables 10-13, 16, 18-20 and 

22-26). Other procedures not applicable to all samples 

have been included at appropriate places in the text.

Composition of plagioclase was determined by 

measuring the refractive indices in index oils. Accuracy using this method is in the range of ±003. 

Another method that is less time consuming but not 

quite as accurate used extinction angles in conjunction 

with albite twinning (Deer and others, 1963, 

p. 135-139). Figure 6G was mostly compiled by this 

last method.

GEOLOGIC SETTING

Precambrian igneous and metamorphic rocks account for the greater part of the Colorado Front Range 

basement and are representative of the crystalline 

basement underlying the entire southern Rocky Mountain region.

The Boulder Creek batholith is the oldest (1,700 

m.y.) of three extensive intrusive episodes of plutonicrock emplacement in the Front Range. Its emplacement was syntectonic with the earlier major regional 

metamorphism. The Silver Plume Quartz Monzonite, a 

l,450-m.y.-old intrusive suite, is exposed extensively 

north and west of the batholith but only occurs as 

small dikes and lenses within the batholith. Cataclasis 

affecting the batholith along the southeast contact (pi. 

1), however, is related to the Silver Plume period of 

magmatism. The Pikes Peak Granite, representing the 

youngest Precambrian intrusive episode (1,040 m.y.), 

lies well outside the area of main concern and except 

for a few lamprophyre dikes in the batholith that are

perhaps of Pikes Peak Granite age, the batholith was 

not involved in that period of metamorphism.

Paleozoic and younger rocks upturned during the 

Laramide orogeny (Tweto and Sims, 1963) overlap the 

Boulder Creek batholith on the east. Elsewhere, 

Precambrian rocks representing a high-grade 

metamorphic gneiss terrane constitute the country 

rocks into which the batholith was emplaced. The 

country rock consists of biotite-sillimanite gneiss and 

schist for the most part, but layers of microcline gneiss 

and quartzite occur along the southeast contact. Interlayered with the schist and gneiss, but at some 

distance from the batholith, are several layers of hornblende gneiss that include some calc-silicate rocks and 

impure quartzite. Other than the mafic rocks shown on 

plate 1, only sparse, small dikes or sills of quartz 

diorite and hornblendite cut the schist and gneiss 

country rock.

Intrusive dikes and stocks of Cretaceous to Eocene 

age that are coextensive with the Colorado mineral 

belt occur mostly to the west of the batholith; a few 

smaller stocks and dikes occur along the contact, but 

only dikes are found within the batholith.

THE BATHOLITH

The Boulder Creek batholith crops out typically as 

pictured in figure 2, as a north-south-trending body 

along the eastern side of the Front Range just west of 

Boulder, Colo., and within the Colorado mineral belt. 

Extending from this centrally located batholith, both 

to the northwest and southwest across central Colorado, are lenses, plutons, and even other batholiths of 

similar composition.

The batholith was emplaced in the catazone, roughly 

at a depth of 12-20 km. As defined by Buddington 

(1959), the catazone is represented by an environment 

of intense pressure-temperature conditions. The 

batholith was emplaced partly by pushing aside the 

surrounding country rocks, as seen in outcrops on the 

west side of the batholith, partly by assimilation, 

and partly by passive magma emplacement into fold 

structures.

To the west of the batholith is a series of 

granodioritic and comagmatic mafic rocks that are 

crudely alined forming a continuous pattern of outcrop 

from the Strawberry Lake batholith to the northwest 

of the Boulder Creek batholith to the Mt. Evans 

batholith southeast of the Boulder Creek batholith (pi. 

1). This pattern reflects a common tendency for 

associated mafic rocks to be satellitic to a major mass 

of granodiorite. Emplacement of the batholithic rocks 

apparently followed definite compositional trends as 

delineated in figure 3. The presumed concentric order
BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

FIGURE 2. Typical weathered bouldery outcrops of Boulder Creek 

Granodiorite, Pinecliffe area, Coal Creek Canyon, Colorado. A, 

view looking north across South Boulder Creek from turnout on 

Colorado Highway 72. B, an outcrop along dirt road leading to 

Gross Reservoir from Colorado Highway 72.

of emplacement was (1) quartz diorite, predominantly 

on the west; (2) quartz monzonite, to the east and adjacent to the batholith contact, followed in the batholith 

proper at the contact by mafic quartz diorite; inward in 

the batholith the quartz diorite grades into

granodiorite, quartz monzonite, and some granite. The 

eastern part of the batholith is buried, so it is not 

possible to comment on that part of the batholith. 

From figure 3 it appears that the three major rock 

types are nearly equally represented in the exposed 

part of the batholith. This crude zonal arrangement 

resembles that in plutons of similar composition the 

world over (Whitten, 1962; Nilssen and Smithson, 

1965; Bateman and others, 1963).

CONSTITUTION OF THE BATHOLITH

The Boulder Creek batholith consists mainly of two 

rock types (pi. 2): (1) the Boulder Creek Granodiorite, 

generally the coarser grained, more mafic rock most 

often referred to in the literature and formally named 

by Sims and Gable (1964), and (2) quartz monzonite, a 

finer grained, often thinly slabbed, leucocratic rock 

that first was recognized as a distinct rock unit by 

Wells (1967, p. D24-D27) but not formally defined. In 

order to identify this rock type specifically and 

distinguish it from another variety of quartz monzonite in the Boulder Creek Granodiorite from the 

same area, the name Twin Spruce Quartz Monzonite is 

formally proposed for the unit later in this report.

Also included in the batholith are scattered smaller 

masses of darker plutonic rock that are both older and 

younger than the granodiorite, profuse large and small 

pegmatites, numerous aplites, and remnants of 

metamorphic rocks.

Approximately three-fourths of the Boulder Creek 

batholith consists of Boulder Creek Granodiorite, 

which is the name given a series of rocks that have the 

same macroscopic and microscopic characteristics and 

vary in composition from quartz diorite to quartz monzonite but average granodiorite. The granodiorite in 

the batholith appears to be related to two magma intrusions. A smaller northern intrusion is separated 

from a larger southern intrusion by quartz diorite 

similar in composition to that in the contact zones of 

the batholith. This area of quartz diorite is north of 

Magnolia (Magnolia is in the northeast quarter of section 6 in the Eldorado Springs quadrangle) and trends 

diagonally across the batholith in an east-southeast 

direction along the 40 ° latitude (fig. 3).

Twin Spruce Quartz Monzonite occurs predominantly in the central part of the batholith and along the 

southeast contact. Its composition is much less 

variable than that of the Boulder Creek Granodiorite. 

The extent to which the Twin Spruce Quartz Monzonite accompanies the Boulder Creek Granodiorite in 

plutons and batholiths other than the Boulder Creek 

batholith is unknown. It is found in the Mt. Evans 

batholith; the Rosalie lobe, studied by Bryant and 

Hedge, (1978), appears to be Twin Spruce Quartz Mon-
THE BATHOLITH

105°37'30"

40° 

07' 

30'

30' 22'30" 105°14'30"

EXPLANATION

Quartz diorite (tonalite)

Granodiorite

Quartz monzonite

Twin Spruce Quartz Monzonite

R.72 W R.71 W.

10 KILOMETERS

J__L

APPROXIMATE SCALE

FIGURE 3. Boulder Creek batholith showing rock types as indicated from modes. Dots are mode localities; dashes represent areas of

insufficient data to place boundaries accurately.
BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

zonite (pi. 1). The Boulder Creek Granodiorite on 

Rollins Pass (an area between the Boulder Creek 

batholith and the Strawberry Lake batholith) is also 

accompanied by large lenses of Twin Spruce Quartz 

Monzonite. Further detailed mapping, however, is 

needed to determine the extent of the Twin Spruce in 

all areas of Boulder Creek Granodiorite.

Gabbro, pyroxenite, hornblendite, and hornblende 

diorite (pi. 1) are believed to be associated in time and 

space with the Boulder Creek Granodiorite. Some of 

these mafic rocks appear to be older than the Boulder 

Creek Granodiorite because of their intense alteration; 

probably they belong to the same igneous event but 

were emplaced earlier than the granodiorite, as also 

suggested by Braddock in the Empire area (1969, 

p. 18). Other mafic bodies are probably the same age as 

the granodiorite, and still others probably are younger 

(Sims and Gable, 1967, p. E41).

CONTACT RELATIONS AND MODE OF EMPLACEMENT

Contacts of the Boulder Creek Granodiorite in the 

batholith with the country rocks are grossly sharp, and 

dips are steep to nearly vertical, although there are 

local variations. Foliations in the contact zone of the 

granodiorite are defined principally by the segregation 

of minerals into darker biotite-rich and lighter quartzplagioclase-rich layers, and foliations in country rock 

and batholith are conformable, especially on the west 

side of the batholith. Forceful emplacement of the 

batholithic rocks is evident both along the west and 

south sides of the batholith; on the west, whereas foliations in the schist parallel the batholithic contact on a 

large scale, the schists are tightly folded on the scale of 

a hand specimen. Often chevron-type folding occurs at 

the contact with more open folds replacing the chevron 

folding out from the contact. Along the southern 

border of the batholith, as described by Taylor (1976), 

forceful emplacement of the Boulder Creek 

Granodiorite is suggested by the warping of adjacent 

gneiss into folds having overturned limbs and by the 

bending of the axial surfaces of older folds. To the 

north the schist-granodiorite contact from the Peak to 

Peak highway to just south of Gold Hill is folded along 

northwest-trending axes, and the granodiorite has 

been conformably emplaced along these fold axes giving the batholith contact an irregular or scalloped appearance (pi. 2). North of Gold Hill the trend of foliation in the batholith and in the gneiss and schist at the 

contact are parallel; however, the batholith and the adjoining metasedimentary rocks are folded along an 

east-northeast synclinal axis.

Interlensing of metasedimentary and batholithic 

rocks is common locally; at Nederland, for example, it 

is difficult to recognize and map a distinct, definite

contact for the batholith. Along most of the western 

margin, however, the contact is relatively sharp, 

although bodies of pegmatite and aplite occur in the 

contact zone. The south and southeast contacts lie in 

the Idaho Springs-Ralston cataclastic zone (pi. 1); 

shearing in this zone produced a shear foliation in the 

rocks that extends into the batholith and masks 

original irregularities of the contact zone. On the north 

margin between the Peak to Peak highway and 

Sugarloaf Mountain (pi. 2), the contact zone is not 

nearly so foliated as on the west flank. Dips also vary 

only from 50° to 80° to the north, away from the 

batholith, whereas elsewhere dips are very steep. As 

the predominant country rock adjacent to the 

batholith is biotite schist and gneiss, it stands to 

reason most inclusions in the contact zone of the 

batholith are predominantly schist and gneiss; 

however, along the southeast batholith contact, there 

are inclusions of quartzite and microcline gneiss.

Contacts between Boulder Creek Granodiorite and 

the finer grained Twin Spruce Quartz Monzonite 

within the batholith are both gradational and sharp or 

they are locally separated by thin pegmatite lenses or 

mafic (biotitic) layers; also, occasional shearing may 

occur along the contact. Granodiorite in contact with 

quartz monzonite is commonly foliated, but elsewhere 

the contact may be blurred due to structural 

similarities between granodiorite and quartz monzonite. In the southern part of the batholith, north of 

Tremont Mountain and along South Boulder Creek, 

irregular-shaped inclusions of granodiorite are found in 

the finer grained quartz monzonite. Dips of the 

granodiorite-quartz monzonite contacts vary from 

near horizontal, as in the central part of the batholith, 

to vertical in the southern part.

The mineralogy of biotite gneiss and schist country 

rock is normally quite variable but from observations 

made while walking over contacts of metasedimentary 

rock and granodiorite it appears all gneiss and schist 

adjacent to the batholith are sillimanite bearing and 

abnormally rich in biotite. Along the west contact, the 

metasedimentary rocks in thin section are very rich in 

biotite, sillimanite, and quartz; plagioclase is very corroded, poorly twinned (the twins are indistinct and 

poorly developed), and very sparse as much as several 

tens of meters from the contact. Also, these rocks are 

ptygmatically folded. At Nederland, both plagioclase 

and potassium feldspar are noticeably absent, but as 

much as 10 or 15 percent kaolinite is present in the contact rocks. This alteration does not appear to be 

related to emplacement of the batholith but instead to 

processes associated with the Laramide mineralization 

in the Nederland-Tungsten district. Northward from 

Nederland, plagioclase in gneiss adjacent to the contact is very corroded, poorly twinned, and commonly
THE BATHOLITH

zoned (normal zoning), but is not diminished in quantity; potassium feldspar in the same rocks as indicated 

by modes varies no more than between samples from 

outcrops at a distance from the batholith.

In the Caribou pluton, a satellitic body between 

Boulder County Hill, Nederland quadrangle, and 

Mount Albion in the Ward quadrangle (pi. 1), 

granodiorite on the southeast side bears profuse 

biotite gneiss and schist inclusions. West of Overland 

Mountain in the Gold Hill quadrangle the Overland 

pluton is very coarse grained similar to coarse-grained, 

nonfoliated granodiorite from the central part of the 

batholith proper; it is less foliated than any of the 

other satellitic plutons. It also has a pronounced lineation due to the alinement and increase in length of the 

feldspar crystals in the granodiorite. In the Overland 

pluton biotite gneiss and schist inclusions occur 

throughout but are more profuse along the contact. In 

both these satellitic plutons, some gneiss and schist inclusions show alteration several millimeters in from 

the contact, especially where contacts are indistinct 

due to assimilation at the boundary of inclusion and 

granodiorite. South of the Ward and Gold Hill 

quadrangles and west of the batholith proper the 

metasedimentary rocks are pierced by many bodies of 

Boulder Creek Granodiorite. At Mt. Pisgah (Central 

City quadrangle), the Pisgah pluton has sharp contacts 

with the adjacent metasedimentary rocks except along 

the northeast margin where again assimilation of the 

gneiss by granodiorite is evident. At this locality 

biotite stringers derived from the gneiss occur abundantly in the granodiorite across a distance of several meters.

Emplacement of batholithic rocks, in general, appears to have been from west to east although nothing 

is known about the easternmost edge of the batholith. 

Most satellitic plutons and batholithic contact rocks 

were probably emplaced almost simultaneously. The 

contact magma was followed by core magma that is 

represented predominantly by hornblende-bearing 

rocks (fig. QA). This core magma broke through the 

contact rocks at Gold Hill and just north and east of 

Nederland. The tongue of magma to the north of 

Nederland moved west and cut into the southern part 

of the Caribou pluton.

The larger structural features represented by foliation trends throughout the batholith (pi. 2) have been 

deflected to varying degrees by high-grade regional 

metamorphism. Lineations for the greater part of the 

batholith have conformed to foliation trends; however, 

lineation trends do not always agree with foliation 

trends especially in the area including Pinecliffe, Tremont Mountain, and Gross Reservoir. That lineations 

are askew of foliations may be because of late magma 

adjustments or there may be some other reason not

understood. These structural trends in the batholith 

led Lovering (Lovering and Tweto, 1953) to believe the 

magma for the entire batholith welled up from a conduit just east of Gold Hill and spread southward, rising at an angle of about 50 ° with the horizontal. It is 

possible that Lovering's conduit represents the final 

emplacement of magma for the smaller northern mass 

whose east-to-west compositional trend is not nearly 

as consistent as the trend in the southern part of the 

batholith (fig. 3).

Chemical and structural relationships tend to indicate that two magma intrusions may have been 

emplaced almost simultaneously in the batholith: a 

smaller northern mass that has not differentiated to 

the extent of a larger southern mass.

CONSTITUENT ROCKS OF THE BATHOLITH

BOULDER CREEK GRANODIORITE

The Boulder Creek Granodiorite in the batholith is a 

medium- to very coarse-grained (0.6-2.0+ mm), locally 

porphyritic rock that has both massive and foliated 

facies (fig. 4). Porphyritic granodiorite generally is 

found in the more mafic parts of the batholith or in 

those parts of the batholith and adjoining plutons that 

have been extensively sheared and perhaps locally 

metasomatized. The massive and coarser grained rocks 

are generally found near the center of the batholith 

(fig. 5D). Such an area lies north and east of Pinecliffe, 

where the rock is very coarse and generally nonfoliated, but the feldspars show a good lineation that 

appears to be a primary structure. The feldspars in the 

batholithic rocks also may be alined because of 

recrystallization accompanying shearing, but these 

textures are distinctly different because alined 

feldspars are accompanied by a foliation not found in 

the primary structure. Foliated, medium-grained, and 

nearly equigranular granodiorite, having perhaps both 

primary and secondary structures but chiefly secondary ones, generally occurs in marginal zones of the 

batholith, in lenses and plutons in adjacent 

metasedimentary rocks, or along the many faults 

within the batholith proper. This foliation results from 

oriented alined feldspar phenocrysts, stringers of 

biotite and less commonly hornblende, and a weak 

compositional layering due to segregation of biotite, 

quartz, and the feldspars. While the granodiorite appears to vary in texture and composition within a 

single outcrop, it has a surprising characteristic uniformity in texture and in mineral composition across 

the entire batholith.

The granodiorite is typically a mottled grayish-white 

and black rock on weathered surfaces but appears dark 

bluish gray on freshly broken surfaces. Weathered sur-
BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

A

tfytr^ry' fcsT"~ *

/^^^r^*:-^ "Sv^X tfa&zf* &!^>£%<&!?r

<*>.*"••<*-•* *- ' • V» "4. ' &^~~~ 

' •«» »«•*.. ' ' '•^P*' -t'S-s -, .^- -r'aa?"^- - _ " :*. ^ '"' * 

^ZL^-gS^X. At4 .'* 'y>x*a& -^' i •^^cr^-. T^^-r' •

" ; 

 a-;J* - ^ :r " *""". ^ -

- > * r v , H. ' ' * i

'i3*

FIGURE 4. Typical Boulder Creek Granodiorite. A, foliated 

granodiorite from west contact (29). B, biotitic, slightly porphyritic 

granodiorite from north contact (50). C, slightly foliated, mediumgrained granodiorite with good lineation (129). D, medium-coarsefaces are commonly pitted by bowl-shaped depressions 

from 1 cm to 1 m in diameter and from several centimeters to several tens of centimeters in depth. 

Weathering also produces bouldery outcrops and a 

knobby topography, especially in areas where the finer 

grained, more easily weathered quartz monzonite has 

been weathered out of granodiorite. Jointing also 

causes differential weathering and is partly responsible for the development of the bouldery topography 

from which Boulder Creek derives its name.

The dominant minerals in the Boulder Creek 

Granodiorite include plagioclase, potassium feldspar 

(microcline and microperthite), quartz, and biotite. 

Hornblende is either disseminated throughout the 

rock, as is common in the central and northern part of 

the batholith (fig. 6), or occurs in spotty distribution as

^v*; &•-••

grained, slightly porphyritic granodiorite (41). Photographs by 

Louise Hedricks, U.S. Geological Survey. Numbers in parentheses 

are sample localities (tables 1 and 10). Scale in centimeters.

in the granodiorite in the southern part of the 

batholith. In figure 6 it can be seen that the ores, mainly magnetite, are more profuse to the west of the 

batholith as is to be expected because the mafic rocks 

related to the Boulder Creek batholith occur dominantly in this area. Probably both the ores and the 

mafic rocks represent an early phase of a differentiating magma. This trend also coincides with the 

trend in plagioclase composition and with the ratio 

potassium-feldspanplagioclase. Alternatively, the 

north to northwest modal trend of the ores, quartz, 

feldspar proportions, and sphene also may have 

developed during emplacement of the batholith and 

represent a regional dynamic imprint for the batholith. 

Modes representative of the Boulder Creek 

Granodiorite in the area of this report are given in
THE BATHOLITH

FIGURE 5. Photomicrographs of Boulder Creek Granodiorite. A, 

near west contact with metasedimentary rocks (389), x-polarizers, 

X10. B, shearing along crystal boundaries in granodiorite (47), 

x-polarizers X12. C, granodiorite, medium grained from interior of 

batholith (50), x-polarizers, X12. D, same section as in C; note

tables 1-5. In the tables, the modes are grouped in 

order to show variations in mineralogy to best advantage. A summary of the modal data is here, in two 

categories: (1) lenses and small plutons within 

metasedimentary rocks that are adjacent (contact 

zone) to the batholith; and (2) the batholith proper, 

with three groups; granodiorite from the contact zone,

clustering of mafic minerals, plain light, XI2. Photographs by 

Louise Hedricks, U.S. Geological Survey. Ap, apatite; E, epidote; B, 

biotite; H, hornblende; Me, microcline; Mu, muscovite;O, ores; P, 

plagioclase; Q, quartz; S, sphene.

sheared and faulted segments, and the main bulk of the 

batholith.

From the summation above, the following conclusions appear valid: (1) the granodiorite in lenses and 

plutons in the metasedimentary rocks and in the 

batholith proper is similar in composition, except that 

ores are more common in the satellitic bodies and
10 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

R.72W. R.71W.

10 KILOMETERS

R.72 W. R.71 W.

10 KILOMETERS

APPROXIMATE SCALE APPROXIMATE SCALE

EXPLANATION

BO 0.1-0.5

0.5-1.0 

1.0-7.0

R.72 W. R.71 W.

10 KILOMETERS

EXPLANATIO 

i——I trace-0.2 

0.2-0.4 

>0.4

R.72W. R.71W.

10 KILOMETERS

APPROXIMATE SCALE APPROXIMATE SCALE

FIGURE 6. Modal distribution of quartz, ores, sphene, allanite, hornblende, and potassium 

f eldspanplagioclase ratios and An content of plagioclase in Boulder Creek Granodiorite. Modal distribution, in 

percent, of A, hornblende; B, ores (mainly magnetite); C, sphene; D, allanite; E, quartz; F, apatite; G, composi-
THE BATHOLITH 11

R.72 W. R.71 W.

10 KILOMETERS

EXPLANATION

Oligoclase 

Andesine 

Labradorite

R.72 W. R.71 W.

10 KILOMETERS

APPROXIMATE SCALE APPROXIMATE SCALE

EXPLANATION 

CD 0.2-0.6 

0.6-0.9 

>0.9

R.72 W. R.71 W.

10 KILOMETERS

EXPLANATION

ma 0.2-0.5

0.5-0.9 

>0.9

R.72 W. R.71 W.

10 KILOMETERS

APPROXIMATE SCALE APPROXIMATE SCALE

tions of plagioclase in percent An; H, ratio of potassium feldspar to plagioclase. Hornblende in southeast part 

of batholith approximate, plotted from modes and text in Wells (1967). Areas shaded gray represent Twin 

Spruce Quartz Monzonite; dots indicate sample localities.
12 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 1.— Modes (volume percent) for Boulder Creek Granodiorite, from the batholith, Front Range, Colo.

[(-), not found; Tr., trace; Tr.?, may be present; ores include all opaque minerals. Samples 1-76 and 192-217, Tungsten quadrangle: 99-131, Gold Hill quadrangle; 132-169, Boulder

quadrangle; 170-191, Nederland quadrangle; 403-407, Ward quadrangle]

Sample No. ----- — ——

Potassium feldspar — 

Plagioclase ——————

Biotite ————————

Muscovite —— —— ———

Allanite ———— ————

Apatite — —— ——— ——

Palf-ll-o — — —— — —

Epidote-clinozoisite 

Rutile ———— — ——— —

Pat-notSphene ———————— 

Kaolinite ————————

Pt-aVirfi f- o _

To tal ——————

Sample No. — — —— ——

Potassium feldspar — 

Plagioclase ——————

Biotite — —— —— ——— 

Muscovite —— — ——— —

Ores —————————— 

Hornblende — — — — — 

Allenite ——————— 

Apatite ——————— 

Zircon ——— ——— —— ——

Calcite —— —— ——— —— 

Chlor ite —— ——— ——— - 

Epidote-clinozoisite

RnHlo —

Sphene — — ——— ——— 

Kaolinite ————————

Total —— ———— —

1

26.1 

33.4 

30.7

8

1

0.1 

0.5

Tr. 

0.2

Tr.

0.4

0.2 

0.3

—

100.0

15

15.3 

49.3

17.9 

13.4

2.0

0.5 

0.1

0.6 

0.6

0.3

100.0

2

10.8 

41.1 

33.1

12.9

0.1 

1.2

0.2

0.1

0.5

100.0

16

15.0 

41.8

11.8 

0.1

0.1

0.6 

0.4

Tr.

0.2

0.2

100.0

3

8.2 

53.0 

17.7

17.3

0_8

0.3

Tr.

0.1

0.8

100.0

17

13.1 

46.7

11.5

2.4 

0.3

0.6 

0.1

0.4 

0.1 

1.0

0.5

100.0

4

8.2 

43.4

15.5

0.1 

0.8

0.6 

0.7

0.1

0.1 

0.1

Tr.

Tr.

100.0

18

22.5 

35.1

30.7 

9.1 

0.1

1.2

0.3 

0.6

Tr.

0.1 

0.1

Tr.

Tr.

0.2

100.0

5

5.2 

47.0

31.3

15.0 

0.6

0.6

Tr.

Tr.

0.3

100.0

19

.26.8 

33.6

5.5 

1.8

1.1

0.8 

Tr.

Tr. 

Tr.

—

100.0

6

12.0 

46.3 

25.2

15.3 

0.1

0.5

0.1 

0.3

0.1

—

0.1

100.0

20

27.4 

28.0

25.7 

14.1 

0.1

0.7 

2.8 

0.1 

0.7 

0.1

0.2

0.1

100.0

7

11.1 

41.5

30.1 

15.2

0.2 

0.8

0.4

Tr.

—

0.7

100.0

21

17.5 

40.3

10.4

0.5 

1.9 

0.1 

0.5 

Tr.

0.9

1.1

100.0

8

15.3 

45.1

17.5 

i o o

0.1 

1.0

Tr. 

0.5

Tr.

0.1

Tr.

0.6

100.0

22

13.8 

37.7

30.7 

13.2

0.6 

1.3 

0.1 

0.1

Tr.

1.1

1.4

100.0

9

22.3 

36.6 

29.1

9.1

0.4

Tr. 

0.3 

0.1

1.5

0.6

100.0

23

11.3 

46.5

13.2

0.3 

1.9 

0.3 

0.3 

0.1

0.2 

0.2

1.3

100.0

10

20.0 

41.4

19 £

13.3

0.5 

0.5

0.2 

0.3

Tr.

0.8

0.4

100.0

24

10.6 

46.6

12.8 

0.3

0.1

Tr. 

0.2 

Tr.

0.3 

0.2 

0.6

0.1

100.0

11

5.5 

46.0 

19.0

1C 7

1.0

8

r

0.1 

0.5 

0.2

—

0.5

100.0

25

24.4 

36.3

30.2 

6.7 

0.2

1.1

0.7 

Tr.

0.1

0.3

100.0

12

40.2 

32.0 

6.0

4.5

0.3 

14.6

0.2 

0.6 

0.1

0.4

1.1

100.0

26

20.7 

41.9

24.8 

9.9

0.4 

0.2

0.6 

0.1

0.1 

0.7

0.6

100.0

13

13.2 

39.0 

33.7

13.5

0.1

0.3 

0.1

0.1

Tr.

100.0

27

14.5 

44.4

21.3 

16.0

0.4 

0.5 

Tr. 

0.7 

Tr.

0.1 

1.4

0.7

100.0

14

16.7 

47.0

nr\ o

10.6

1.0

Tr. 

0.1 

0.5

Tr.

0.4 

2.0

0.9

100.0

28

27.1 

36.3

24.4 

10.6

0.8

0.2 

0.2 

Tr.

—

0.4

100.0
THE BATHOLITH 13 

TABLE 1.—Modes (volume percent) for Boulder Creek Granodiorite, from the batholith, Front Range, Goto.—Continued

Potassium feldspar —

Allanite —— —— —— —— 

Apatite — —— ——— —— 

Zircon ————————

Chlorite —— —— —— —— 

Epidote-clinozoisite 

Rutile — —— —— —— ——

Sphene ————————

Va *i"l -i ri -i f-o— —

PvoVin-i f-o «

Total —— ——— ——

29 30

18.4 8.1 

37.8 48.2 

29.0 26.9 

14.0 15.7 

0.1 0.2

0.5 0.3

0.1 

0.2 Tr. 

Tr . Tr .

0.1 

0.2

Tr. 0.2

100.0 100.0

31

36.4 

27.4 

25.9 

8.1 

0.3

1.0

0.3 

0.1

0.3 

0.2

Tr.

100.0

32

8.3 

47.4 

23.2 

14.5

0.2 

5.3 

Tr. 

0.2 

0.1

0.4

0.6

100.0

33

16.7 

36.1 

23.3 

15.8

0.8 

4.7 

0.1 

0.5 

0.1

0.1 

1.5

0.3

100.0

34

17.8 

46.6 

22.7 

10.5

0.2 

0.9 

Tr. 

0.1 

0.1

0.9

0.2

100.0

35

13.6 

47.5 

25.1 

11.8

0.7

0.2 

0.4 

Tr.

0.3

0.4

100.0

36

17.4 

42.1 

21.8 

15.9 

0.1

0.9

0.3 

0.8 

0.1

—

0.6

100.0

37

0.3 

47.2 

32.3 

18.2 

0.1

0.8

0.1 

0.7

—

0.3

100.0

38

18.2 

38.3 

31.6 

9.8 

0.5

0.4

0.4 

0.3 

Tr.

0.4 

Tr. 

0.1

—

100.0

39

10.8 

48.3 

27.8 

11.2

0.9

0.1 

0.7 

Tr.

—

0.2

100.0

40

16.9 

37.5 

26.6 

17.3 

0.1

0.5

0.6 

Tr.

Tr. 

Tr.

0.4

100.0

41

16.0 

43.0 

22.0 

14.7

0.9 

1.4 

0.4 

0.5 

Tr.

0.2 

0.5

0.4

100.0

42

1.7 

62.1 

11.1 

15.9

0.5 

7.4 

Tr. 

0.6 

Tr.

0.3

0.4

100.0

Potassium feldspar — 

Plagioclase — —— —— 

Quartz ————————

Muscovite — — —— ———

Allanite —— —— — —— 

Apatite — —— —— — — 

Zircon — —— —— — ——

Chlorite ————————— 

Epidote-clinozoisite

Rnt--i 1 o

Sphene ———————— 

Kaolinite ————————

Total —— — —— ——

43 44

37.6 23.4 

36.8 25.7 

7.1 9.1 

13.6 13.3

0.4 0.9 

3.7 23.3 

Tr. 0.2 

0.4 0.9 

0.1 0.1

0.2

2.3

0.3 0.6

100.0 100.0

45

34.5 

32.4 

24.4 

7.0

0.5

Tr. 

0.5 

0.1

0.1 

0.3

0.2

100.0

46

21.1 

33.7 

38.4 

5.5 

0.2

0.1

Tr. 

0.1

0.2 

0.1 

0.5

0.1

100.0

47

10.6 

48.5 

24.9 

12.9

0.6 

0.7 

Tr. 

0.4 

0.1

0.2 

0.2

0.9

100.0

48

16.6 

40.7 

18.7 

14.6

0.6 

7.8

0.5 

Tr.

0.4

0.1

100.0

49

19.0 

34.3 

27.6 

14.4

0.7 

0.2 

0.3 

0.7 

Tr.

0.5 

1.7

0.6

100.0

50

2.5 

46.7 

14.1 

17.9

0.4 

14.1

1.1 

Tr.

0.4 

0.1 

1.6

1.1

100.0

51

19.4 

40.8 

26.5 

10.4 

0.5

0.4

0.5 

0.2 

Tr.

0.2 

0.2

0.9

100.0

52

17.1 

30.9 

27.8 

8.2

1.0 

12.4

0.3 

0.1

Tr. 

1.8

0.4

100.0

53

11.8 

41.1 

25.0 

17.3 

0.1

0.5 

1.3

0.6 

0.1

1.6

0.6

100.0

54

14.0 

34.8 

34.4 

12.3

0.7 

2.3 

0.4 

0.2 

Tr.

Tr. 

0.6

0.3

100.0

55

9.2 

44.7 

30.3 

12.5 

0.4

1.0

0.3 

0.5 

Tr.

0.2 

0.3

0.6

100.0

56

25.3 

41.2 

23.0 

8.4

0.5

0.4 

0.4 

Tr.

Tr. 

0.2 

0.3

0.3

100.0
14 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 1.—Modes (volume percent) for Boulder Creek Granodiorite, from the batholith, Front Range, Goto.—Continued

Potassium feldspar —

Plagioclase

Allanite ——————— 

Apatite ——————————

r*K 1 J-.T--I *- o_ —».__.— _

Epidote-clinozoisite 

Rutile —— — — —— ——

57

6.2 

53.0 

3.0 

12.8

0.2 

o o a

Tr. 

Tr.

0.1

1.4

0.7

100.0

58

0.7 

56.7 

17.3 

22.9 

0.6

0.5

Tr. 

0.2 

0.1

—

1.0

100.0

59

27.6 

37.2 

28.7 

5.1 

1.3

0.1

Tr. 

Tr.

Tr.

Tr.

100.0

60

27.8 

35.5 

31.5 

3.6 

1.2

0.4

Tr. 

Tr.

Tr.

—

100.0

61 62

18.6 14.0 

39.9 42.5 

33.0 31.6 

4.8 10.9 

2.6

1.1 0.1

Tr. 

Tr. 0.1 

Tr. Tr.

Tr. 

0.2 

Tr. 0.4

0.2

100.0 100.0

63

5.6 

48.9 

17.5 

20.4

0.5 

4.4 

0.2 

0.9

Tr.

Tr. 

0.7

0.9

100.0

64

15.2 

42.8 

24.9 

14.7 

0.1

0.8

0.2 

0.2

Tr.

0.2

Tr.

0.9

100.0

65

30.5 

34.4 

24.8 

8.2 

0.3

1.5

Tr. 

0.2 

Tr.

0.1 

Tr,

Tr.

Tr.

100.0

66

25.6 

42.8 

20.4 

9.6 

0.4

0.5

0.5

Tr.

0.1 

0.1

Tr.

Tr.

100.0

67 68

31.9 12.1 

35.1 50.0 

26.8 19.3 

4.6 13.6 

0.7

0.8 0.6 

2.7 

Tr. 0.3 

0.1 0.6 

Tr. 0.1

0.3

0.4

100.0 100.0

69

29.7 

34.7 

25.0 

7.9 

1.1

0.4

0.3 

0.5

Tr.

0.3

0.1

100.0

70

26.7 

27.6 

31.3 

12.0

0.9

0.3 

0.6

Tr.

~

0.3

100.0

Potassium feldspar — 

Plag ioclas e —— —— ——

Muscovite —— —— ———

Allanite -—————__ 

Apatite ——————————

Monazite-xenotime —

Chlor i t e ----- -—— —— 

Epidote-clinozoisite 

Rutile ————————

Sphene ——— — —— ———

Total -— — — ——

71

18.8 

43.5 

27.2 

10.0

0.3

0.1 

0.1

Tr.

Tr.

100.0

72

20.3 

36.3 

29.0 

12.7 

0.1

0.5

0.4

Tr.

0.1

Tr.

0.6

100.0

73

10.7 

52.9 

11.1 

14.0 

0.1

1.0 

9.6

0.1

Tr.

0.1

0.4

100.0

74

13.0 

42.2 

24.3 

17.3 

0.6

0.8

0.1 

0.7

Tr.

Tr.

1.0

100.0

75 76

9.3 27.4 

51.6 31.1 

27.3 32.4 

9.5 6.9 

0.3

0.5 0.5

0.3 Tr . 

1.1 1.0 

Tr . Tr .

0.1

Tr. 

0.3

0.3 0.1

100.0 100.0

99

0.5 

59.6 

16.9 

19.4

0.6

Tr. 

1.3

0.1 

0.3 

0.5

0.8

100.0

100

0.3 

47.3 

18.2 

21.7

Tr. 

7.6 

0.1 

0.8 

Tr.

3.7

0.3

100.0

101

41.6 

34.4 

21.5

0.7 

0.2 

Tr. 

0.4 

0.2

Tr. 

0.7

0.3

100.0

102

23.6 

38.2 

24.4 

10.4

1.7

0.1 

0.1

Tr.

0.6

0.9

100.0

103 104

0.5 0.7 

44.5 50.1 

25.9 20.1 

27.5 19.3

0.3 0.5 

7.8 

0.1 

0.6 1.0

0.1 0.1

0.6 0.3

100.0 100.0

105

3.7 

52.6 

21.0 

13.3

0.6 

5.0 

Tr. 

0.7 

0.3

1.2

1.2 

0.4

100.0

106

2.0 

54.7 

21.3 

15.3

0,5 

5.1 

Tr. 

0.3 

Tr.

0.3

—

0.5

100.0
THE BATHOLITH 

TABLE 1.—Modes (volume percent) for Boulder Creek Granodiorite, from the batholith, Front Range, Cofo.—Continued

15

Potassium feldspar —

Quartz ———————— 

Bio t it e —— — — — ——

Hornblende ——— —— ——

ralf-il-a— —— — ——— — . _

Chlorite ————————— 

Epidote-clinozoisite

•D..4- -I 1 rt __ ——

Sillimanite ——— ———

Kaolinite ————————

PT> £»KTI-I t-£* — «_»

Total ——— ——— —

107

23.5 

39.0 

24.6 

0.5 

1.6

0.8

0.3 

Tr.

Tr. 

Tr. 

0.3

0.6 

S8.8

100.0

108

0.3 

51.6 

16.6 

23.5

0.8 

3.8

0.8 

0.2

0.2 

1.7

0.5

100.0

109

1.4 

46.1 

27.0 

18.7

1.8 

4.4 

0.1 

0.5 

Tr.

Tr.

—

100.0

110

0.4 

52.9 

19.5 

19.9

1.6

5.0 

Tr. 

0.4 

Tr.

Tr. 

Tr. 

0.3

Tr.

100.0

111 112

0.2 25.3 

51.5 32.6 

12.2 30.6 

36.5 9.5

0.9 0.5 

0.5 

0.2 0.1 

1.5 0.1 

Tr . Tr .

Tr. 0.7

0.1

100.0 100.0

113

17.5 

38.0 

13.6 

18.9 

0.1

0.1 

8.5

1.4 

Tr.

0.1 

Tr. 

0.9

0.9

100.0

114

7.8 

46.0 

20.0 

18.7

0.6 

4.8

0.7 

Tr.

0.8

0.6

100.0

115

7.1 

50.9 

20.5 

14.6

0.4 

4.2 

0.1 

0.4

Tr.

0.1 

0.3 

Tr.

1.4

100.0

116

57.0 

4.4 

17.2

1.1 

17.6 

0.1 

0.6 

Tr.

0.3 

0.6 

0.8

0.3

100.0

117

28.0 

25.6 

34.5 

5.0 

4.6

2.2

Tr. 

0.1

Tr.

Tr.

—

100.0

118

37.4 

24.0 

32.0 

1.0 

1.9

1.0

0.3 

Tr.

2.2 

0.1

Tr. 

0.1

100.0

119

53.9 

22.6 

21.0

1.2

Tr. 

0.7 

Tr.

0.1 

0.5

—

100.0

120

17.8 

47.3 

16.4

0.4 

17.0 

Tr. 

0.1 

Tr.

0.6

0.4

100.0

Potassium feldspar — 

Plagioclase —— — ——

Biotite — ———— —— —

Zircon —— -—— ———— —

*"» n 1 - J 4- -._ _ ___....

Chlor it e —— —— — —— 

Epidote-clinozoisite 

Rutile ——— ——— — —

Sillimanite — —————

Garnet —— ——— ——— — 

Sphene — ——— — — ——

T?flrt1 -1 TI-I *-£»— ____

PyiaVin -i f-*a« «.«,««• __«

Total ———— — ———

121

44.4 

18.5 

24.5

0.7 

8.9 

0.2 

1.2

Tr.

0.8

0.8

Tr.?

100.0

122

55.2 

24.6 

19.5

0.3

Tr. 

0.3 

Tr.

Tr. 

0.1

—

—

100.0

123

47.5 

19.6 

30.4

1.3

0.1 

Tr. 

Tr.

0.2 

0.1

0.8

—

100.0

124

37.5 

25.8 

24.3 

10.7

0.2

Tr. 

0.1 

0.1

0.4 

0.6

0.3

—

100.0

125 126

Tr. 11.0 

48.6 37.0 

24.7 29.0 

23.6 18.8 

0.4

1.8 0.4 

2.9 

0.1 

0.8 0.1 

Tr . Tr .

0.3 

0.1 0.4

—

—

100.0 100.0

127

53.6 

15.0 

22.8

0.3 

7.0 

0.3 

0.4 

Tr.

Tr. 

0.3 

Tr.

0.3

Tr.?

100.0

128

22.9 

33.3 

26.8 

13.3 

0.6

2.3

0.8 

Tr.

Tr. 

Tr.

—

100.0

129

25.9 

36.8 

23.2 

11.0 

0.6

1.8

Tr. 

0.3 

Tr.

0.4

—

100.0

130

17.6 

34.6 

35.4 

9.7 

0.7

1.4

0.6 

Tr.

Tr.

Tr. 

Tr.

—

100.0

131

11.8 

40.6 

23.0 

13.3

0.4 

6.2 

Tr. 

0.6

0.2 

0.6 

2.2

m

100.0

132

4.9 

46.0 

21.3 

20.3

0.5 

5.0 

0.1 

0.4 

Tr.

Tr. 

1.1

0.4

100.0

133

10.0 

44.3 

25.8 

16.4 

0.1

0.3 

1.0 

0.1 

0.1

1.5

0.4

100.0

134

2.3 

55.7 

21.1 

3.1

2.4 

1.9 

0.1 

0.5

Tr. 

9.3 

3.1

0.5

100.0
16 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 1.—Modes (volume percent) for Boulder Creek Granodiorite, from the batholith, Front Range, Cofo.—Continued

Potassium feldspar — 

Plagioclase — — —— —

Biotite ——— — — ———

Hornblende —— — ——— 

Allanite ———— —— — 

Apatite —— — — —— — 

Zir con ——————————

Epidote-clinozoisite

•p., f--i 1 Q

Sphene ——— ——— ———

Total —— ——— ——

135

24.0 

29.4 

29.6 

7.9

1.1 

4.2

0.3

0.1 

0.7 

1.1

1.2

0.4

100.0

136 137

0.1 4.3 

53.3 43.8 

20.3 19.7 

22.5 20.6

0.9 1.1 

6.8 

Tr . Tr . 

1.0 0.6 

Tr.

1.0 1.4

0.9 0.7

100.0 100.0

138

6.4 

46.9 

25.4 

12.2

0.8 

5.4 

0.1 

0.9

0.1 

0.3 

0.9

0.5

0.1

100.0

139

0.3 

39.5 

25.3 

24.3

0.9 

6.5

Tr. 

0.8 

Tr.

1.5

0.9

—

100.0

140

0.1 

61.6 

31.0 

5.8

0.5

Tr. 

0.1 

Tr.

Tr. 

0.6 

0.3

Tr.

Tr.

100.0

141

7.7 

35.4 

32.1 

18.5 

b 5.8

Tr.

0.1 

0.3

Tr. 

0.1

—

—

100.0

142

2.1 

40.4 

32.5 

22.8 

2.1

0.1

Tr. 

Tr.

Tr. 

Tr.

—

—

100.0

143

15.4 

40.3 

24.1 

8.6

0.3 

8.4

0.7 

Tr.

0.4

1.8 

Tr.

Tr.

100.0

144

24.6 

36.2 

26.1 

0.7 

0.1

0.3 

0.9 

Tr. 

Tr. 

0.1

6.6 

1.2

0.3 

0.3

2.6

100.0

145

15.3 

37.5 

27.1 

9.9

1.6

2.8 

0.1 

0.7 

Tr.

1.2 

0.4 

3.2

Tr. 

0.1

0.1

100.0

146

22.8 

38.8 

20.3 

13.3

Tr. 

2.3 

0.1 

0.1 

Tr.

0.3 

1.6

0.4

—

100.0

147

13.3 

35.5 

26.8 

21.1

0.2

0.2 

0.2 

0.1

Tr. 

0.2 

0.9

0.3 

1.2

100.0

148

14.0 

40.5 

33.4 

10.5 

0.3

0.1

0.1 

0.1 

0.3

0.3

0.4

—

100.0

Potassium feldspar —

Quartz ————————

Apatite —— —— — ——— 

Zircon — ——— ——— ———

Epidote-clinozoisite

PllH 1 0 _ _ —

S illimani te ———————

149

0.3 

46.0 

31.8 

19.8

Tr. 

0.5

0.5 

0.3

0.3

~

0.5

100.0

150 151

27.0 8,3 

30.3 48.5 

31.7 24.2 

8.8 17.2

1.3 0.7

Tr. 0.7 

0.7 0.1 

0.3

0.1 

Tr.

0.1 Tr.

100.0 100.0

152

18.1 

35.5 

34.6 

9.2

0.2 

1.1 

0.1 

0.2 

Tr.

0.5

0.5

Tr.?

100.0

153

12.8 

43.8 

23.8 

14.8

0.4 

1.7 

0.1 

0.7 

Tr.

0.3 

0.9

0.7

100.0

154

8.8 

40.6 

26.9 

16.8

0.9 

4.4 

Tr. 

0.4 

Tr.

0.4 

0.1

0.7

100.0

155

17.3 

35.0 

32.0 

12.5 

1.0

0.6 

Tr. 

Tr. 

Tr. 

Tr.

0.7 

0.3 

0.6

0.2

100.0

156

16.2 

37.0 

23.8 

18.3 

Tr.

0.2 

1.7 

Tr. 

0.6 

Tr.

0.1 

0.4 

1.1

0.6

100.0

157

9.4 

46.5 

23.0 

16.0

0.1 

1.6 

0.1 

1.3 

Tr.

1.4

0.6

100.0

158

5.6 

43.0 

31.0 

16.2

0.4 

1.2 

0.4 

0.1 

Tr.

0.1 

1.6

0.4

100.0

159

15.2 

38.5 

33.0 

10.1 

0.7

1.3

0.1 

Tr.

0.1 

Tr.

Tr.

0.6

100.0

160

12.9 

37.4 

32.3 

15.3

0.1

Tr. 

0.3 

Tr.

0.4

1.3

100.0

161

14.6 

42.3 

30.2 

11.2 

0.6

0.4

0.3 

Tr.

0.3 

0.1

—

100.0

162

7.6 

49.7 

20.5 

18.6 

Tr.

0.2 

1.3 

0.3 

0.3 

Tr.

0.8

0.7

—

100.0
THE BATHOLITH 

TABLE 1.—Modes (volume percent) for Boulder Creek Granodiorite, from the batholith, Front Range, Co/o.—Continued

17

Potassium feldspar —

T>1 • 1

Quartz ——— —— ——— -

Biotite —— —— — — ——

Muscovite ——— —— ——

Ores — —— —— ——— ———

Hornblende ——— —— ——

Allanite — ——— —— _

Chlorite —————————

Epidote-clinozoisite

163

20.3

40.8

24.8

11.1

1.5

0.4

—

Tr.

0.4

Tr.

—

0.4

164

11.2

42.3

26.3

14.9

—

0.4

3.7

Tr.

Tr.

Tr.

--

0.7

165

5.1

43.2

20.8

21.9

Tr.

0.3

5.7

0.5

0.6

_—

0.4 

0.1

0.8

166

11.0

43.5

23.2

18.1

Tr.

0.2

1.3

Tr.

0.6

Tr.

0.1

1.7

167

21.2

39.5

21.0

15.4

—

Tr.

1.0

Tr.

0.5

Tr.

Tr. 

0.2

0.5

168

19.3

43.1

23.3

10.4

Tr.

Tr.

2.6

—

0.6

Tr.

—

0.7

169

—

49.7

23.8

21.4

—

0.4

1.7

0.1

0.6

Tr.

Tr. 

0.3

1.6

403

Tr.

34.4

49.2

14.9

Tr.

1.5

—

—

Tr.

Tr.

—

—

404

30.0

35.0

27.0

6.0

—

1.0

—

0.5

0.5

~

—

405

42.1

9.9

21.0

26.5

0.2

0.3

—

—

--

Tr.

—

—

406

24.4

22.1

32.9

18.3

—

1.3

—

0.1

0.6

~

—

--

407

10.3

42.8

28.3

6.8

—

1.8

—

--

0.5

Tr.

—

8.3

SillimaniteGarnetSphene—————————

Kaolinite———————

Prehnite———————

0.3 0.5 0.5 0.3 0.7 Tr. 0.4 Tr. Tr. 0.3 1.2

Tr. Tr2

Total———————— 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Includes some chlorite. Sillimanite altered to muscovite.

potassium feldspar and hornblende are more abundant 

within the batholith; (2) the contact zone and segments 

adjacent to faults are similar compositionally, suggesting that rocks in both environments have been 

recrystallized; the potassium went into biotite and the 

rest of the constituents into plagioclase.

The distribution of the major minerals of the 

Boulder Creek Granodiorite within the batholith and 

adjacent satellitic bodies may also be shown by the use 

of quartz-plagioclase-potassium feldspar triangular 

diagrams in figure 7. These diagrams show that the 

southeast part of the batholith is the most siliceous 

and the central part of the batholith the least siliceous. 

The abundance of quartz diorite in the triangular 

diagrams (fig. 7) for the Blackhawk, Tungsten, and 

Gold Hill quadrangles reflects border areas of the 

batholith. Most modes fall in the granodiorite-quartz 

monzonite fields; however, those for the Ward 

quadrangle plot on the granitic side of the quartz monzonite field.

Boulder Creek Granodiorite has a predominant 

hypidiomorphic granular texture in which the mafic 

minerals tend to cluster. The mafic clusters consist of 

biotite, hornblende, apatite, sphene, allanite, zircon, 

and the alteration mineral epidote. Plagioclase, quartz,

and some potassium feldspar form the largest crystals. 

Biotite and hornblende are the most prominent mafic 

minerals but only biotite is ubiquitous in granodiorite. 

Potassium feldspar, represented chiefly by microcline, 

forms both large crystals similar in size to plagioclase 

and smaller interstitial crystals.

Rocks of the batholith have been sheared and 

recrystallized and tend to have a preferred mineral 

alteration adjacent to faults and along much of the 

contact zone. Serrate quartz and anhedral feldspar 

(mostly microcline) occur interstitial to larger mineral 

grains in these rocks.

INCLUSIONS IN BOULDER CREEK GRANODIORITE

Inclusions in the Boulder Creek Granodiorite are 

mainly of three types: (1) lenses, some quite large, of 

biotite and sillimanite-biotite gneiss and schist; (2) 

massive to foliated mafic inclusions in discrete clots, 

spindles, and lenses consisting essentially of biotite, 

hornblende, and plagioclase and containing generally 

less than 10 percent quartz and 2 percent or less of 

potassium feldspar; (3) very old foliated and altered 

lamprophyres in eroded sills that are now rounded 

elongate bodies so altered that they often resemble 

mafic inclusions of type 2.
18 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 2.—Modes (volume percent) for Boulder Creek Granodiorite from small plutons and small lenses in biotite gneiss and schist, Front

Range, Colo.

[(-), not found; Tr., trace; ores include all opaque minerals. Samples 170-185 from Nederland quadrangle; 186-191 from Tungsten quadrangle]

Potassium feldspar —

Biotite -—————-——

Hornblende ——————— 

Actinolite ———————

Al lan-i i-o — — —— —

Chlorite ————---——

Epidote ————————

T?ii<--{ 1 <->___ .._

170

3.7 

43.9 

29.8 

a!9.3 

0.3

2.0

0.3

Tr.

0.7

—

100.0

171

11.0 

46.5 

29.1 

a!1.3 

0.4

1.6

0.1 

Tr.

100.0

172

7.0 

40.8 

20.7 

22.3

4.2

1.8 

0.1

Tr.

3.1

100.0

173

0.1 

34.6 

25.6 

30.0 

0.4

4.8

1.7

—

—

2.8

100.0

174

12.5 

44.9 

33.1 

a5.7 

1.1

1.3

0.1

Tr. 

Tr.

1.3

Tr.

100.0

175

23.3 

36.4 

23.0 

a!1.4 

1.1

1.9

0.1 

0.7

Tr.

2.1

100.0

176

17.0 

44.0 

23.2 

11.0 

0.1

2.0

0.1 

0.8

0.1

Tr.

0.1 

1.6

100.0

177

49.2 

32.7 

17.9

0.1

0.1

Tr.

Tr.

—

100.0

178

0.3 

54.8 

22.1 

al8.3

2.7

0.4 

0.3

Tr.

0.7 

0.4

100.0

179

0.1 

48.8 

17.4 

17.1

5.6 

6,4

0.7 

2.2

0.4 

0.3

1.0

100.0

180

6.2 

49.6 

23.8 

13.6

bi.o

3.1

1.3 

Tr.

1.4

Tr.

100.0

Potassium feldspar —

Allanite ---——-——— 

Apatite ——————————

Pal r>-T i-o — —

Chloirite —— — —— —— 

Epidote ——————————

Total ———-—-

181

Tr. 

46.8 

23.5 

19.0

bi.o

2.2 

1.4

0.1 

1.8

Tr.

0.1 

2.0

2.1

100.0

182

1.7 

53.5 

19.1 

17.7 

b 1.9

1.0 

3.0 

Tr. 

Tr. 

0.1

0.3 

0.4 

1.0

0.3

100.0

183

Tr. 

40.3 

16.3 

21.8

b / / 4.4

5.2 

8.2

0.1 

2.0

0.1

0.1 

0.8

0.7

100.0

184

0.1 

46.2 

20.8 

25.7

3.5

1.4

0.3 

1.3

0.3

0.4

100.0

185

24.3 

34.8 

31.3 

5.1 

1.7

2.7

0.1

Tr.

100.0

186

28.8 

24.9 

28.0 

12.6 

2.9

2.0

0.5 

0.2

Tr. 

0.1

100.0

187

22.1 

30.3 

30.2 

10.4 

4.2

2.3

0.3 

0.2

100.0

188

22.9 

30.6 

30.0 

11.0 

3.2

1.6

0.1 

0.4

0.1 

0.1

100.0

189

21.4 

36.2 

25.9 

9.8 

3.7

1.8

0.1 

0.9

0.1 

0.1

100.0

190

26.9 

28.5 

30.2 

11.0 

0.9

1.2

0.5 

0.6

Tr.

0.1 

0.1

100.0

191

17.5 

38.1 

27.2 

13.2 

1.5

1.6

0.6 

0.3

100.0

a. K Includes some chlorite. Includes sericite.
THE BATHOLITH 19 

TABLE 3.—Modes (volume percent) for Boulder Creek Granodiorite from contact area of batholith, Front Range, Colo.

[(-), not found; Tr., trace; ores include all opaque minerals. Samples 192-199 from Tungsten quadrangle; 200-204 from Gold Hill quadrangle]

Potassium feldspar —

Biotite ——————————

Zircon ——————————

Chlor i te —————————

Sphene ——————————— 

Total ———————

193

4.0 

44.7 

28.1 

17.5 

1.5

2.7

0.1 

1.1 

0.3

—

100.0

194

0.2 

57.5 

5.0 

23.6 

0.2

0.5 

11.2 

0.1 

0.8 

Tr.

0.3 

0.6

—

100.0

195 196

35.7 46.0 

23.3 25.0 

28.6 17.0 

0.4 1.0

7.0 3.0

0.5 

0.1 Tr. 

1.4 Tr. 

0.1 Tr.

0.2 5.0 

0.6 3.0

2.1

100.0 100.0

197 198 199

7.7 0.2 

37.8 48.8 55.1 

38.6 31.8 11.8 

15.0 18.5 23.1

. t- U . _L

0.6 0.3 0.2

O . H

0.3 

0.1 0.3 0.2 

Tr . Tr . Tr .

0.1

\J • J.

0.7

100.0 100.0 100.0

200

0.5 

50.8 

18.8 

22.8 

a0.7

1.8 

1.7

1.2 

0.2

0.5

0.5 

Tr. 

0.5

201 202

58.1 50.2 

22.1 23.4 

18.8 23.0 

0.1

0.4 1.8

Tr. 0.1 

0.4 0.1 

0.1

Tr. 

0.1 0.3 

0.1 0.9 

Tr . Tr . 

— Tr.

100.0 100.0 100.0

2<B3

44.4 

24.7 

23.4 

Tr.

3.1

Tr. 

0.5

0.1 

1.8 

2.0 

Tr.

100.0

204

1.3 

49.8 

25.8 

16.5

0.6 

3.4 

0.2 

0.7 

Tr.

0.5

1.2

100.0

Kaolin 0.2 percent of total muscovite. 

TABLE 4.— Modes (volume percent) for Boulder Creek Granodiorite adjacent to known faults, Front Range, Colo.

[(-), not found; Tr., trace; ores include all opaque minerals. All samples are from Tungsten quadrangle]

Potassium feldspar —

Pa 1 r»-i f~o _

Chlorite ————————— 

Epidote-clinozoisite

Tr\t-a1 _ __ 1

205

0.3 

59.3 

21.6 

13.1

1.4

0.6 

0.7

Tr. 

1.7 

1.3

.00.0

206

55.2 

16.0 

27.8

0.2

0.1 

0.5 

0.1

0.1

207 208 209 210 211 212

0.3 0.2 1 

52.0 54.4 38 

24.0 26.3 24 

21.1 16.9 18

1.1 0.2 5 

1.5 7

0.6 0.2 1 

Tr. 0.1 0

Tr . 

0.3 0.2 

0.6 — 3

100.0 100.0 100.0 100

.3 3.5 0.8 2. 

.3 51.9 57.8 49. 

.4 23.3 22.6 20. 

.7 14.9 13.1 23. 

0.

.6 0.1 1.1 1. 

.1 5.1 3.0 

0.2 0.2

.5 0.5 0.4 0. 

.1 Tr. 0.5 Tr

0.2 0.

0.1 0.1 

0.2 0.2 2.

.0 0.2 «-- 0.

.0 100.0 100.0 100.

213

2 5.3 

0 42.8 

1 25.4 

9 16.8 

2

2 0.4 

5.9 

0.3 

7 0.7 

Tr.

2 

0.1 

3 1.5 

2 0.8

0 100.0

214 215

9.6 

63.3 47.7 

9.3 28.5 

25.4 12.6

0.8 0.5

Tr. 0.2 

0.6 0.1 

Tr.

0.6 

0.3 0.1 

0.3 0.1

100.0 100.0

216

8.1 

45.6 

31.8 

12.7 

1.2

0.2

0.1 

0.2 

Tr.

0.1

100.0

217

0.4 

48.2 

26.8 

23.2

0.2

0.1 

1.1

Tr.

Tr.

100.0
20 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 5.—Summary of modal data (volume percent) for the Boulder 

Creek Granodiorite, Front Range, Colo.

[Ores include all opaque minerals]

Granodiorite in batholith

Granodiorite

rocks

Contact Known

areas

1.0 2.0 

47.0 51.0 

23.0 23.0 

21.0 19.0 

2.0 1.7

1.7 1.1 

0.1 0.1 

0.6 0.7 

0.6 0.5 

0.3 0.5 

0.3 0.1

Remainder

batholith

16.1 

40.5 

25.0 

13.6 

2.1

0.7 

0.2 

0.5 

0.5 

0.4 

0.4

The mafic inclusions of type 2 (figs. 8 and 9 and 

samples 318-327, table 6) are believed to be plutonic in 

origin because of their physical characteristics and 

composition, especially trace-element composition, 

that is more nearly related to diorite-quartz diorite. 

These inclusions are found in the hornblende-bearing 

part of the batholith and are common in the roadcuts 

of Boulder Creek Granodiorite along Middle Boulder 

Creek, along the slopes north and south of Pinecliffe, 

and in roadcuts of the Salina-Gold Hill (County 89) 

and Gold Hill-Bighorn (County 52) mountain roads. 

While these mafic inclusions are widely scattered 

throughout the granodiorite, they are, or appear to be, 

more profuse in the central part of the batholith, extending from Thorodin Mountain to Bighorn Mountain.

The inclusions of type 2 occur in discrete clots, 

spindles, and lenses oriented either along the regional 

foliation or at an angle to it; their plunge is moderate 

to steep. Small inclusions as much as 0.5 m in size have 

blunt ends or rounded terminations and in cross section most are ellipsoidal; the larger inclusions range 

from a few meters to more than several hundred 

meters in length and have widths of as much as several 

tens of meters. Smaller inclusions can often be picked 

out with a hammer, leaving a somewhat smooth 

depression, especially in intensely weathered 

granodiorite. The larger inclusions are nearly always 

foliated and occur in irregular masses or lenticular 

bodies along foliation planes; some enclose small areas 

of granodiorite, forming augenlike segregations of the 

granodiorite (fig. 10). The larger inclusions are composite rocks that have a composition of diorite, quartz 

diorite, and may include some granodiorite; they are 

coarse to fine grained, dark gray to almost black. The 

smaller inclusions are finer grained and have a mottled 

texture due to a finer grained salt-and-pepper groundmass having large hornblende or feldspar crystals and 

clots of mafic minerals either alined or in random orientation throughout the groundmass. Whereas some inclusions have a mafic rim or a border of pegmatite, 

others grade into granodiorite with no distinct border. 

The larger inclusions are generally gradational with 

the Boulder Creek Granodiorite and often appear to interlens with the granodiorite for some distance along 

the contact.

Biotite and hornblende make up 26-65 percent by 

volume of the mafic inclusions (table 6). Quartz rarely 

exceeds 10 percent and generally is in the range of 2-8 

percent. Apatite and ores each are in the range of 1-2 

percent. In thin section the mafic inclusions have a 

xenomorphic granular texture (fig. 11).

The third type of inclusion is lamprophyre (samples 

328, 329, 331, 332, table 6); it is not an inclusion like 

types 1 and 2 but occurs as eroded sills and dikes and 

is referred to as sills and dikes in the remainder of this 

report. The sills and dikes are of two ages. The older 

ones are generally found only in the central part of the 

batholith as altered and well-foliated stubby lenses 

from several meters to 300-400 m in length and are 

probably comagmatic with the Boulder Creek 

Granodiorite. The younger lamprophyre occurs as 

dikes and sills, is fresh looking, and may belong to the 

Pikes Peak Granite plutonism. The younger lamprophyre is of no further concern to this paper and is 

thus discussed no further.

The metamorphosed lamprophyre of concern to this 

report varies in composition from syenodiorite to melasyenodiorite, according to modes in samples 328, 329, 

331, and 332 (table 6), and samples 387 and 388 (table 

11). Biotite, hornblende, and pyroxene make up 48-86 

percent by volume of the lamprophyre; a few bodies are 

pyroxene rich, and quartz is nearly absent. Allanite, 

apatite, and sphene average 6 percent. These altered 

and well-foliated lamprophyric dikes often take on the 

appearance of porphyritic microgranular inclusions of 

type 2, and the distinction between types 2 and 3 is 

made on texture and the presence or absence of 

potassium feldspar.

In thin section, the lamprophyre has a diabasic to 

xenomorphic texture depending on degree of metamorphism. Figure 11C shows a lamprophyre with 

xenomorphic texture.

MAFIC PLUTONIC ROCKS

The mafic intrusive rocks, pyroxenite, gabbro, hornblendite, hornblende diorite, and quartz diorite, and 

related aplites and pegmatites, occur mainly in 

metasedimentary rocks as satellitic bodies, although a 

few scattered lenses of all rock types except pyroxenite 

occur within the batholith itself. Radiometric age 

determinations have not been made on any of the mafic 

intrusives, but field relations suggest that they are approximately the same age as the Boulder Creek
THE BATHOLITH 21

105°30' 105°22'30"

FIGURE 7.—Modal variation (in volume percent) of quartz, plagio- potassium feldspar. Distribution of modes across the batholith

clase, and potassium feldspar for Boulder Creek Granodiorite in and in lenses indicated in figure 3. Each diagram occurs with its

the batholith and adjacent lenses. Q, quartz; P, plagioclase; K, corresponding 7 Ms-minute quadrangle or quadrangles.
22 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

JPCI* ,-f.

FIGURE 8.—Photographs of exposures of mafic inclusions in Boulder 

Creek Granodiorite, mainly from Boulder Canyon. A, mafic stringers in a biotitic phase of Boulder Creek. B, mafic clots in a 

more leucocratic Boulder Creek Granodiorite; note the leucocratic 

halo between granodiorite and inclusion. C, a foliated lens of

dense biotite-quartz diorite and small pegmatite veins in 

granodiorite. D, a remanent, dense mafic lens; note the ghostlike 

leucocratic areas segregating darker granodiorite into patchy 

areas.

Granodiorite although some appear both older and 

younger (Sims and Gable, 1967, p. E29-E39; Braddock, 1969, p. 18). Most were emplaced in a period of 

deformation and metamorphism that attained 

sillimanite grade, and, accordingly, they were to some

extent deformed and metamorphosed. Most plutonic 

bodies are massive to strongly foliated (fig. 12) and occur as stocks or blunt, narrow dikes and sills or lenses 

along Precambrian structures, especially in fold axes. 

Although they locally transect structures in the gneiss
THE BATHOLITH 23

FIGURE 9.—Photographs of hand specimens of mafic inclusions in 

Boulder Creek Granodiorite. A, small bleb, regular outline with indentations of leucocratic minerals, biotite clusters at contact. B, 

inclusion, part of a larger mass of hornblende-bearing quartz 

diorite; assimilation is quite advanced. Photographs by Louise 

Hedricks, U.S. Geological Survey.

and schist country rock, they are subconformable to 

the regional foliation. Lineations within the rock are 

due to alined hornblende and biotite and are generally 

parallel to that in the enclosing gneiss. These mafic 

rocks compose a distinct belt west of the Boulder 

Creek batholith that is semicircular, as can be seen in 

plates 1 and 2. The bodies are small; the largest is less 

than 2 km in length. Most of the mafic bodies that 

were plotted on 7 1/2-minute quadrangle maps are 

shown on plate 1, but hundreds of bodies observed in 

the field are too small to plot at the scale of a

quadrangle map. The distribution of these rocks as 

shown on plate 1, however, is representative of their 

outcrop pattern.

GABBRO AND PYROXENITE

Gabbro is best known from its outcrop in the Elk 

Creek pluton in the northwest part of the Central 

City quadrangle (Taylor and Sims, 1962, p. D118; Sims 

and Gable, 1967, p. E35), but it also occurs in the 

southwest corner of the Nederland quadrangle—an extension of a lens from the Elk Creek pluton—and in 

two small bodies west of Gold Hill, both of which are 

strongly altered by a Tertiary diabase dike. Also, 

metagabbro was mapped by Wrucke and Wilson (1967) 

in the northern part of the Boulder Creek batholith.

The gabbro is a dark-gray, medium- to coarsegrained, massive, nearly equigranular rock that has a 

mottled texture due to large poikiloblastic mafic 

clusters. The matrix consists of interlocking plagioclase and pyroxene crystals. Typically the rock contains roughly 50 percent plagioclase. Where in contact 

with schist and gneiss, the smaller lenses are biotite 

rich and commonly weakly foliated. Samples 335-337 

in table 7 are from a lens in the southwest corner of the 

Nederland quadrangle and vary little mineralogically 

from the Elk Creek pluton itself. Gabbro is characterized by calcic plagioclase (An41 _54), orthopyroxene, 

clinopyroxene, hornblende, biotite, ores, and about 5 

percent quartz.

The pyroxenites are best developed in the Los Lagos 

lens, north of Rollinsville (pi. 2), in the southeast half of 

the Nederland quadrangle. Here contacts between 

pyroxenite and hornblende diorite with country rock or 

hornblende gneiss, where exposed, are indistinct, 

probably in part because of contact heating and in part 

because of retrograde metamorphism that occurred 

during or shortly after their emplacement. Another 

pyroxenite occurs west of Lakewood Reservoir and 

north of Nederland. It shows extensive alteration in 

thin section, in part due to thermal metamorphism 

caused by emplacement of younger hornblendite. The 

rock originally consisted almost wholly of pyroxene 

but now is composed of 50 percent pyroxene, about 40 

percent hornblende, and 10 percent ores. Skeletal 

crystals in the rock consisting of serpentine and calcite 

probably were originally olivine. Olivine, however, 

never accounts for more than 2 percent of the total 

rock.

Hornblende pyroxenite is a grayish-black, mediumgrained, massive rock that breaks with a glistening 

hackly surface. It occurs as clots in diorite or is gradational into hornblende diorite. The Los Lagos lens is 

representative of these gradational features. The
TABLE 6.— Modes (volume percent) for mafic inclusions and metamorphosed lamprophyre dikes in Boulder Creek Granodiorite, Front Range, 

Colo.

[(-•), not found; Tr., trace; n.d., not determined; ores include all opaque minerals. Sample numbers 318-321, 323-331 from Tungsten quadrangle; 322, 332, 333 from Gold Hill quadrangle]

Potassium feldspar — Quartz —————————— 

Biotite —————————— All am* t-a— — — — — — — — — — — Sphene —————————— Chlorite ————————— Epidote-clinozoisite 

Rutile —————————— Composition of

318 319 320 321 322 323

0.8 

35.3 29.0 36.7 50.7 53.8 46.1 

8.5 6.3 7.9 5.3 14.3 1.5 

19.6 17.3 12.1 32.6 26.5 40.4

1 o Q o/.Q /. i o ~! ^ co

9.5

0.7 0.4 Tr. 1.0 1.1 1.3

Tr. Tr.

2.1 Tr. 0.1 0.9 1.3 1.3 

1.2 1.0 2.0

Tr. — Tr. Tr. — Tr. 

in n s n i

1.7 1.1 1.2 1.1 2.2

Tr.

100.0 100.0 100.0 100.0 100.0 100.0 

An n o An i n An / _ n.d. n.d. An _ « Lo tZ 4 / JZ

324

53.3 

1.2 

25.6 11.7

1.3 

0.2 

0.41.7 

0.14.5

100.0 

n.d.

325 326 327

0.1 2.0 11.4 

48.4 55.8 33.3 

4.8 0.6 2.7 

27.5 21.0 31.1 

16.1 16.9 11.1

Tr. 0.6 

0.2 0.1 0.2 

0.8 0.3 1.01.3 2.3 1.4 0.1 0.2 0.10.7 0.2 7.7

100.0 100.0 100.0

AnOQ An 9 , An__ z,o /j /o

a328

26.0 

8.0

48.0

1.0 1.0 

1.0 

4.0

2.0

8.0 

1.0

100.0 

n.d.

a329

19.1 

0.2 0.2 

45.3 

3.1

26.3 

Tr. 

1.3 

2.31.8 Tr.

0.4 

Tr.

100.0 

n.d.

330

15.2 

26.0 

7.7 

17.9 32.6

Tr. 

Tr. 

Tr.

0.5 

0.1Tr.

100.0 

n.d.

a331

13.017.0 

55.0

3.0 

1.0 1.0 

4.0

1.0 

1.0

4.0

100.0 

n.d.

a332 333

27.9 4.3 

0.7 

0.9 

31.6 46.6 33.3 12.2

28.0

0.2 0.6 1.6 

3.2 1.9

2.3 1.5 Tr. 

Tr. 

3.2

100.0 100.0 

n.d. n.d.

24 BOULDER CREEKBATHOLITH, FRONTRANGE, COLORADO

Metamorphosed lamprophyres.
THE BATHOLITH 25

FIGURE 10.—Oval-shaped lenses of Boulder Creek Granodiorite in 

mafic inclusion. Pegmatite occurs around periphery of 

granodiorite, especially at each end; roadcut, middle of Boulder 

Canyon, Tungsten quadrangle.

Severance lens (fig. 12; pi. 2), in the west half of the 

Tungsten quadrangle, was perhaps originally in part a 

pyroxenite; in a roadcut adjacent to the Bureau of 

Standards building on Colorado Highway 119, the 

more mafic part of the Severance lens is too crumbly 

and weathered to sample, but from outward appearances it is very probably an altered pyroxenite. If 

so, it too is gradational into hornblende-bearing quartz 

diorite and its outer border of biotitic hornblendebearing quartz diorite is gradational into mafic 

Boulder Creek Granodiorite. These altered pyroxenites 

have a hornblende to pyroxene ratio that is quite 

variable, ranging from 1:1 to 1:4, and a few are 

pyroxene-bearing hornblendites (table 7, samples 

347-355). The pyroxene minerals in pyroxenite are 

augite and bronzite. Biotite forms small slender laths 

and is reddish brown. Plagioclase is interstitial, and 

crystals are normally zoned and poorly twinned. In 

thin section, pyroxenites have a texture similar to the 

gabbro described earlier but are somewhat finer 

grained.

HORNBLENDITE, HORNBLENDE DIORITE, AND QUARTZ DIORITE

Black to mottled black and white, medium-grained, 

rarely slightly porphyritic rocks consisting principally 

of hornblende or of hornblende and plagioclase were

mapped as hornblendite and diorite (pi. 2). The rocks 

are massive to foliated but lack the strong foliation 

characteristic of amphibolite of the area. Foliated 

diorite generally occurs at the margins of the larger 

dioritic bodies. Lighter colored, finer grained, foliated 

hornblende diorite has a salt-and-pepper appearance 

due predominantly to plagioclase in the rock. Quartz 

diorite, similar in appearance to hornblende diorite, 

consists of hornblende and plagioclase and at least 10 

percent quartz.

The hornblendite in samples 338-340 (table 7) contains more than 70 percent hornblende; pyroxene and 

plagioclase account for most of the remaining 30 percent. Sample 339 contains 27 percent pyroxene and 

may be altered pyroxenite because its composition approaches that in the Severance lens, samples 352-354, 

table 7. In hornblendite, plagioclase is interstitial and 

is invariably altered to an aggregate of muscovitesericite. The hornblende in thin section is generally 

brown, but some hornblendes are green; there is also 

some alteration of hornblende to actinolite or tremolite 

along cleavage. From optical data the pyroxene is 

probably diopside. Most pyroxene crystals are 

subrounded and are mantled somewhat by hornblende. 

The accessories are much the same as for the diorites 

and include ores, apatite, sphene, allanite, and zircon.

The diorites are principally hornblende and 

plagioclase bearing and contain less than 15 percent of 

other minerals including quartz, biotite, pyroxene, 

ores, and accessory minerals. Samples 341-344 (table 

7) are representative of the diorites. Most diorites contain some chlorite, calcite, and secondary epidote.

The diorites have a hypautomorphic to allotriomorphic texture, depending upon degree of schistosity. 

The mafic minerals generally display the better 

developed crystal faces. The potassium feldspar in 

hornblende diorite is orthoclase. It forms small, 

subrounded, nontwinned, clear to partly cloudy grains 

that are easily misidentified as quartz. Plagioclase 

ranges in composition from andesine to labradorite 

and is normally zoned. Plagioclase from quartz diorite 

tends to be less calcic than from diorite and poikilitic 

with subrounded quartz inclusions. Plagioclase twinning includes Carlsbad, albite, pericline, and complex 

twins, with albite and pericline dominant. Alteration 

of plagioclase varies between diorites; in some the 

plagioclase is fresh, whereas in others it is completely 

altered to sericite and muscovite. Mafic minerals tend 

to cluster, similar to the texture in other Precambrian 

intrusive rocks in the area. Hornblende appears black 

in hand specimen and varies from grayish green to 

olive brown in thin section. It is often anhedral, and in 

a few sections traces of actinolite occur along grain 

boundaries and cleavage. In others, a twinning 

characteristic of cummingtonite indicates cum-
26 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

•5*"

FIGURE 11.—Mafic inclusions in Boulder Creek Granodiorite. A, 

type 2 inclusion, biotitic quartz diorite inclusion (322); typical 

elongated clot but a little more leucocratic than in figure 8, 

x-polarizers, X16. B, same section, plain light, XI2. C, type 3 inclusion, lamprophyre (331), plain light X16. A, allanite; Ap, 

apatite; B, biotite; E, epidote; H, hornblende; O, ores; Or, orthoclase; P, plagioclase; Q, quartz; S, sphene. Photographs by 

Louise Hedricks, U.S. Geological Survey; sample numbers in 

parentheses.

mingtonite may be replacing hornblende on a small 

scale. Biotite varies in amount by as much as 20 percent in the more schistose diorites and is more prominent in the outer zones of the larger mafic bodies. It is 

reddish brown and is commonly of two generations: 

one generation forms stubby crystals that are in 

mutual contact with the other mafic minerals, whereas 

the other is more acicular and rarely forms radiating 

crystals that penetrate or sever hornblende and 

plagioclase crystals. Pyroxene in diorites occurs as 

relict grains that have been extensively replaced by 

hornblende. Where relict pyroxene exists, ores are in 

greater abundance and biotite is less common than in 

nonpyroxene-bearing diorites.

TWIN SPRUCE QUARTZ MONZONITE

The Twin Spruce Quartz Monzonite is herein named 

for the small settlement of Twin Spruce in the 

Eldorado Springs quadrangle. The type locality is just
THE BATHOLITH 27

FIGURE 12.—Outcrops of layered and foliated biotitic hornblende 

diorite and massive pyroxenitel?): A, layered biotite-hornblende 

diorite in roadcut, Colorado 72 south of Rollinsville. B, same location as A, weathered rubble roadcut with resistant pyroxenite(?) 

cut by feldspar-rich pegmatites.

northeast of Twin Spruce where typical quartz monzonite crops out in Coal Creek Canyon. Twin Spruce 

Quartz Monzonite is a finer grained and more 

leucocratic rock than the Boulder Creek Granodiorite 

with which it is commonly associated, and although its

composition ranges from granite to quartz monzonite, 

it is chiefly quartz monzonite. The unit occupies nearly 

a quarter of the batholith and is more extensive and 

definitely more massive in the southern part of the 

batholith (pi. 2) adjacent to the schist and gneiss contact than farther north in the central part of the 

Tungsten quadrangle. Aplite, pegmatite, and rarely 

trondhjemite accompany the quartz monzonite. 

Although most bodies of quartz monzonite in the 

Boulder Creek Granodiorite are generally mappable, 

local thin lenses commonly are intermixed with the 

granodiorite or are gradational into it and cannot be 

mapped separately at a scale of 1:24,000.

Twin Spruce Quartz Monzonite was mapped by 

Levering and Goddard (1950) as granite gneiss and 

gneissic aplite; however, recent mapping in the Front 

Range has shown that the granite gneiss and gneissic 

aplite unit of Levering and Goddard includes more 

than one rock unit. In addition to the quartz monzonite 

delineated by Wells (1967), thick layers of older 

Precambrian microcline gneiss (microcline-quartzplagioclase-biotite gneiss) conformably interlayered 

with biotite gneiss can also be distinguished locally 

(Moench and others, 1962; Sims and Gable, 1964; 

Hawley and Moore, 1967). Geologic mapping indicates 

the Twin Spruce Quartz Monzonite is certainly 

younger than the Boulder Creek Granodiorite in some 

places; elsewhere the field evidence is not as conclusive. Rb/Sr determinations on samples of this rock 

from the Eldorado Springs quadrangle indicate the 

ages isotopically fit the 1,710±40 m.y. isochron of the 

Boulder Creek Granodiorite (Hedge 1969). Weighing 

all evidence, it appears safe to say that the Twin 

Spruce Quartz Monzonite is in part late Boulder Creek 

in age and in part younger than Boulder Creek.

Quartz monzonite is generally gray where fresh but 

has a characteristic zonal weathering pattern adjacent 

to joints that broke the rock into rough blocks that 

now weather into orange-brown and light-brown concentric zones. This form of weathering is typical of the 

quartz monzonite found in roadcuts in Middle Boulder 

Creek canyon. Joints or weathering also produce slabby rocks that break into thin layers much like shale, 

and these thin slabs tend to spread over weathered surfaces, obscuring contacts. One such area occurs in the 

extreme southeast corner of the Tungsten quadrangle; 

another is in the central part of the Tungsten mining 

district. Both areas are covered with pieces of quartz 

monzonite that give the ground a littered appearance. 

The monzonite in the batholith proper typically has a 

weak foliation and lineation produced by laths of 

biotite and tabular feldspar. In adjacent metasedimentary rocks, bodies of quartz monzonite have a pronounced gneissic structure; this structure is especially 

noticeable in the lens north and west of Nederland.
Sample No. -—— Potassium

Plagioclase —— Hornblende —— Actinolitetremolite —— ClinopyroxeneOrthopyroxeneBiotite ————— 

Chlorite ———— Muscovitesericite —— - Zircon ——-—-- Allanite ———— Apatite ————— Rutile —— ——— 

Prehnitea ——— Epidote ————— Calcite ————— Total —— —— TABLE 7.— Modes (volume percent) for gabbro, hornblendite, hornblende diorite, quartz diorite, and hornblende pyroxenite, Boulder Creek batholith area, Front Range, Colo [(--), not found; Tr., trace; ores include all opaque minerals. Samples 336, 338, 341, 343, 348, 349, 351, 352-354 from Tungsten quadrangle; 335, 337, 339, 340, 342, 343, 344, 345, 346, 347, 350, 355 from Nederland quadrangle] Quartz Hornblende pyroxenite to biotite-hornblende diorite Gabbro Hornblendite Hornblende diorite diorite Los Lagos lens Severance lens 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 49 57 55 1 1 10 35.4 46 28.5 Tr. 29 39 5 Tr. 1 49 36 29.2 0.8 36.2 3 5 Tr. 3 Tr. 1 2 — 3 5.3 2 17 17 — — — Tr. 2 3.8 1.1 0.3 2 22 7 14 83 71 81 56.0 40 56.5 54 27 33 33 17 52 40 53 42.9 78.0 37.2 42 2 Tr. — — — Tr. — — — — Tr. 6 15 6 5 27 1 , 2.1 — — — Tr. — 43 33 9 Tr. 1 — 3.9 0.3/, n I > 49 — -•— — — ±\j — — j_ — — — — _» « 9 6 14 — Tr. 1 4.9 10 — 7 12 8 2 — Tr. 5 Tr. 20.8 — 21.4 2 4 1 1 ~ — 5 3 Tr. — — 1 3 — — — Tr. — — 1 8 — 4 8.2 29 8 — — 6 — 4 3 — 4.3 — 1 Tr. — Tr. — — — — — — — — — — — 0.3 Tr. Tr. __ _ _ __ _ __ TV — — — — — — — - Tr — — — — — — • — — Tr . — Tr . — — 3 1 o T1 *- is- TH- no i TV- TV» *} *} i 9 •« i n "^ n 7 -*— TT* Tr. — — Tr. Tr. Tr. 0.1 Tr. 0.1 — -- — — — — Tr. Tr. 1.9 3.3 2.0 1 Tr. 1 3 — Tr. 0.6 Tr. 0.8 3 3 — 2 — 4 Tr. 1 1.0 1.5 2.6 ~ 0.1 — Tr. _ n i o i triw 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 28 BOULDER CREEKBATHOLITH, FRONT RANGE,COLORADO

Uncertain.
THE BATHOLITH 29

Twin Spruce Quartz Monzonite is chiefly a finegrained rock having an average grain size of 4-5 mm 

(fig. ISA), but a medium-grained (0.6-0.8 mm) phase 

(fig. 13J3) and a speckled phase occur locally. The 

speckled phase, not shown in figure 13, is due to clots 

of minerals surrounded by lighter haloes that are scattered randomly throughout the rock. These phases 

have not been mapped separately, especially as contacts are in general gradational.

Twin Spruce Quartz Monzonite is predominantly a 

microcline-, plagioclase-, and quartz-bearing rock with 

biotite, muscovite, and ores making up approximately 

15 percent of the total rock (table 8). Accessory 

minerals are the same but are scarcer than in the 

Boulder Creek Granodiorite. Modally, quartz monzonite is restricted within the quartz monzonite field of 

a ternary quartz-potassium feldspar-plagioclase 

diagram; only a few modes plot in the granodiorite and 

granite fields (fig. 14). Quartz monzonite in the 

Eldorado Springs (Wells, 1967) and Ralston Buttes

FIGURE 13.—Photomicrographs of Twin Spruce Quartz Monzonite. 

A, fine-grained quartz monzonite (sample 87, table 8), 

x-polarizers, X12. B, coarse-grained quartz monzonite, slightly 

sheared (sample 380, table 13), x-polarizers, X12. C, same section 

as B, except plain light. Ap, apatite; B, biotite; Me, microcline; 

Mu, muscovite; O, ores; P, plagioclase; Q, quartz; Z, zircon. 

Photographs by Louise Hedricks, U.S. Geological Survey.

,*• X

/^ > w ->- . *"•?:*- JS

. y /i.. ;:^v..,^%^x,^^i^

>r
30 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 8.—Modes (volume percent) for Twin Spruce Quartz Monzonite, Front Range, Colo.

[(-), not found; Tr., trace; ores include all opaque minerals. Samples 77-98 and 218-289 from Tungsten quadrangle; 290-303 from Nederland quadrangle; 304-311 from Ward quadrangle;

312-317 from Gold Hill quadrangle]

Sample No. — — —— — —

Potassium feldspar —

Apatite ———————————

AT 1 QTTI \~ o —

Zircon ——————————— 

C'n.lcitG'TM"

Epidote ———————————

Chlorite —— —— —— ——

Total ———————

77

47.8 

21.6 

28.8 

1.3 

0.1

0.4 

Tr. 

Tr. 

Tr.

—

—

100.0

78

35.5 

26.6 

27.5 

7.0 

1.0

1.1 

0.3 

0.6 

0.1

Tr.

0.3

100.0

79

37.7 

28.8 

26.6 

1.8 

3.4

1.3

0.1

—

0.3

100.0

80

36.8 

29.2 

28.1 

3.2 

1.3

0.9 

0.2 

Tr. 

0.1

0.1

0.1

100.0

81

44.8 

18.4 

35.8 

0.1 

0.2

0.5

Tr.

—

0.2

100.0

82

36.5 

28.2 

29.5 

1.0 

2.2

2.5

Tr.

Tr.

—

0.1

100.0

83

38.5 

25.5 

32.4 

0.1 

1.8

1.6

Tr.

—

—

0.1

100.0

84

35.8 

31.4 

31.2 

0.7 

0.5

0.2 

0.1

Tr. 

0.1

Tr.

100.0

86

42.6 

23.4 

24.1 

4.9 

1.4

2.2 

0.4 

0.6

0.1 

0.3

Tr.

100.0

87

40.4 

23.8 

29.5 

2.6 

2.9

0.4 

0.4 

Tr.

—

Tr.

100.0

88

34.5 

32.3 

27.3 

4.5

0.9 

0.4 

Tr.

—

0.1

100.0

89

33.6 

30.0 

30.6 

4.2 

0.9

0.7

Tr.

Tr.

Tr.

100.0

90

28.6 

37.4 

29.1 

3.4 

1.1

0.4 

Tr.

Tr.

Tr.

Tr.

100.0

91

21.8 

40.3 

27.4 

6.1 

3.0

0.6 

0.1 

0.4

0.1 

0.1

0.1

100.0

Sample No. —— — — ——

Potassium feldspar — 

Plagioclase

All a-n-i 1-o —

PaT r«-i 1-o

Chlorite -——————- 

Sillimanite ———————

Monazite-xenotime ——

Total ———— — ——

92

17.8 

41.1 

24.9 

10.7 

2.4

1.9 

0.2 

0.2

Tr.

Tr.

0.8

100.0

93

33.6 

29.0 

20.2 

10.6 

3.0

2.9 

0.4 

0.3

Tr.

Tr.

—

100.0

94

42.7 

23.9 

29.4 

1.0 

0.4

1.2

0.2 

0.2

Tr. 

1.0

—

100.0

95

31.3 

31.8 

32.6 

2.8 

0.7

0.8 

Tr.

Tr.

Tr.

Tr.

100.0

96

37.8 

30.2 

30.0 

1.5 

0.3

0.1 

0.1

Tr.

—

Tr.

100.0

97

5.9 

45.7 

40.9 

5.1 

0.8

1.0 

0.4

Tr. 

Tr.

—

0.2

100.0

98

37.3 

27.0 

26.5 

6.2 

0.7

2.3

Tr.

Tr.

Tr.

—

100.0

218

32.7 

35.1 

27.7 

3.4 

0.1

0.9 

0.1

Tr.

Tr.

—

100.0

219

48.0 

21.6 

26.7 

1.1 

1.9

0.7

Tr. 

Tr. 

Tr.

Tr. 

Tr. 

Tr.

Tr.

100.0

220

32.1 

30.6 

25.7 

5.1 

2.5

2.1 

0.7

Tr. 

Tr.

0.1 

1.1

Tr.

100.0

221

37.8 

21.5 

35.3 

4.9

Tr.

0.4 

0.1

Tr.

Tr. 

Tr.

—

100.0

222

33.7 

22.0 

38.4 

4.8 

0.6

0.5

Tr.

—

Tr.

100.0

223

7.8 

13.9 

67.6 

8.9 

0.3

1.5

Tr.

—

Tr.

100.0

224

26.5 

37.2 

26.9 

7.2 

1.0

1.1 

0.1

Tr. 

Tr. 

Tr.

Tr. 

Tr. 

Tr.

Tr.

100.0
THE BATHOLITH 

TABLE 8.—Modes (volume percent) for Twin Spruce Quartz Monzonite, Front Range, CoZo.—Continued

31

Potassium feldspar —

Rutile —— — —— —— ——

Chlorite — —— — — ——

Monazite-xenotime ——

225

29.2 

28.2 

27.7 

6.8 

6.2

1.1 

0.1 

0.5 

0.1

Tr.

0.1

Tr.

100.0

226

34.1 

24.8 

29.2 

7.5 

3.0

1.0 

0.3 

0.1

Tr.

Tr. 

Tr.

Tr.

100.0

227

39.4 

24.0 

24.8 

7.2 

2.9

1.3 

0.3

0.1

Tr. 

Tr.

Tr.

100.0

228

36.4 

24.7 

28.4 

6.9 

2.2

1.2 

0.1

0.1

—

Tr.

100.0

229

35.8 

12.9 

40.3 

6.2 

3.5

1.1

0.2

Tr.

Tr.

100.0

230

4.1 

14.3 

69.0 

7.5 

3.8

Tr.

Tr.

Tr.

—

100.0

231

32.0 

30.3 

27.0 

6.4 

2.1

1.8 

Tr.

0.2

Tr.

0.2 

Tr.

Tr.

100.0

232

23.5 

18.5 

51.6 

3.3

2.2

0.8

Tr.

Tr.

0.1

Tr.

100.0

233

29.7 

32.4 

26.4 

5.9 

2.2

1.9 

0.2 •

Tr.

Tr. 

1.3

Tr.

100.0

234

14.1 

27.7 

51.9 

2.8 

2.5

1.0

Tr.

—

—

100.0

235

30.6 

31.0 

27.1 

1.1 

1.8

1.5 

0.7 

0.3 

Tr.

0.2 

5.7

—

100.0

236

34.6 

27.5 

27.7 

6.0 

1.9

1.2 

0.4 

0.1 

0.2

Tr. 

0.4

Tr.

100.0

237

31.5 

16.2 

46.6 

3.1 

1.6

1.0 

Tr.

—

Tr.

100.0

238

31.3 

28.4 

26.7 

9.5 

1.9

1.9 

0.3

Tr.

Tr.

Tr.

100.0

Potassium feldspar —

Biotite ——————————

Sillimanite —— —— ——

Monazite-xenotime ——

Total —— — — ——

239

45.5 

21.4 

26.3 

3.5 

1.4

1.3

Tr.

0.6

Tr. 

Tr.

Tr.

100.0

240

42.1 

25.2 

25.1 

2.0 

1.4

1.3

0.1 

0.1 

0.1

2.6

—

100.0

241

44.5 

22.8 

28.0 

2.0 

1.2

1.0 

Tr. 

0.3 

Tr.

0.2

—

100.0

242

33.2 

25.3 

30.3 

7.7 

0.4

2.1 

0.3 

0.6 

Tr.

0.1

—

100.0

243

36.9 

27.6 

23.4 

8.5 

2.1

0.9 

0.4 

Tr. 

Tr.

0.2

Tr.

100.0

244

26.0 

39.4 

27.1 

4.9 

0.6

1.2 

0.1

Tr.

0.7

—

100.0

245

45.5 

23.4 

26.8 

2.2 

1.0

0.8 

Tr.

Tr. 

0.3

—

100.0

246

33.2 

29.7 

26.2 

7.2 

1.2

1.6 

0.7 

Tr. 

0.2

Tr.

Tr.

100.0

247

31.3 

31.1 

26.9 

6.4 

2.5

1.6 

0.2

Tr.

—

Tr.

100.0

248

36.5 

24.8 

31.7 

4.1 

0.3

1.0 

0.2

0.1

0.7

0.6

—

100.0

249

52.3 

16.2 

20.8 

4.0 

1.9

3.3

Tr. 

0.3 

Tr.

—

1.2

~

100.0

250

35.6 

25.5 

29.4 

7.4 

1.8

0.2 

0.1

Tr.

—

—

—

100.0

251

37.8 

21.1 

29.2 

3.1 

7.7

0.7 

0.3

0.1

—

—

_

100.0

252

43.3 

22.4 

30.4 

1.5 

2.0

0.1

—

Tr. 

0.3

Tr.

100.0
32 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 8.—Modes (volume percent) for Twin Spruce Quartz Monzonite, Front Range, Co/o.—Continued

Potassium feldspar —

Monazite-xenotime ——

Total — —— — ——

253

28.8 

36.1 

30.3 

2.5 

1.9

0.3

Tr.

0.1

Tr.

Tr.

100.0

254

35.3 

26.8 

34.9 

2.5 

0.1

0.4

Tr.

Tr.

—

Tr.

100.0

255

38.9 

22.4 

28.9 

6.2 

1.8

1.6 

Tr. 

0.1

Tr.

0.1

100.0

256

23.9 

39.3 

25.3 

6.7 

1.9

2.4 

0.5

Tr.

Tr.

100.0

257

32.4 

33.4 

28.4 

2.0 

3.0

0.8 

Tr.

Tr.

—

100.0

258

38.9 

25.8 

30.2 

0.9 

1.8

0.6

Tr. 

0.1

Tr.

0.2 

1.5

100.0

259

48.2 

17.9 

32.8

0.6

0.1 

0.1

Tr.

0.3

100.0

260

30.2 

33.4 

30.2 

5.4 

0.1

0.6 

0.1

Tr.

—

100.0

261

41.4 

19.6 

30.0 

5.0 

2.7

0.6 

0.3

Tr. 

Tr.

0.4

Tr.

100.0

262

41.7 

32.5 

21.7 

1.6 

1.9

0.4 

Tr. 

Tr. 

0.1

Tr. 

0.1

—

100.0

263

33.3 

30.6 

31.2 

2.4 

1.2

0.8 

Tr. 

Tr. 

0.2

Tr. 

0.3

Tr.

100.0

264

34.8 

32.2 

28.0 

0.4 

1.6

1.3

Tr. 

0.1 

Tr.

0..1 

Tr. 

1.5

Tr.

100.0

265

34.7 

25.8

25.7 

7.5 

3.7

2.1 

0.4

0.1

—

—

Tr.

100.0

266

34.5 

29.2 

30.6 

2.5 

1.8

0.8 

0.1 

Tr. 

Tr.

0.4 

0.1

Tr.

100.0

Potassium feldspar —

P agioclase

Biotite ——————————

Rutile ———————————

Chlorite —————————

Total —— —— —— -

267

47.8 

21.6 

28.8 

1.3 

0.1

0.4 

Tr.

Tr..

Tr.

100.0

268

35.5 

26.6 

27.5 

7.0 

1.0

1.1 

0.3 

0.6 

0.1

Tr.

0.3

100.0

269

37.7 

28.8 

26.6 

1.8 

3.4

1.3 

0.1

—

0.3

100.0

270

36.8 

29.2 

28.1 

3.2 

1.3

0.9 

0.2 

Tr. 

0.1

0.1

0.1

100.0

271

44.8 

18.4 

35.8 

0.1 

0.2

0.5 

Tr.

—

0.2

100.0

272

36.5 

28.2 

29.5 

1.0 

2.2

2.5 

Tr.

Tr.

—

0.1

100.0

273

38.5 

25.5 

32.4 

0.1 

1.8

1.6 

Tr.

—

—

0.1

100.0

274

35.8 

31.4 

31.2 

0.7 

0.5

0.2 

0.1

Tr. 

0.1

Tr.

100.0

275

42.6 

23.4 

24.1 

4.9 

1.4

2.2 

0.4 

0.6

0.1

0.3

Tr.

100.0

276

40.4 

23.8 

29.5 

2.6 

2.9

0.4 

0.4 

Tr.

—

—

100.0

277

34.5 

32.3 

27.3 

4.5

0.9 

0.4 

Tr.

—

—

0.1

100.0

278

33.6 

30.0 

30.6 

4.2 

0.9

0.7

Tr.

—

—

100.0

279

28.6 

37.4 

29.1 

3.4 

1.1

0.4

Tr.

Tr.

—

—

100.0

280"

21.8 

40.3 

27.4 

6.1 

3.0

0.6 

0.1 

0.4

0.1

0.1

0.1

100.0
THE BATHOLITH 

TABLE 8.—Modes (volume percent) for Twin Spruce Quartz Monzonite, Front Range, Co/o.—Continued

33

Potassium feldspar —

Plagioclase —— ------ 

Quartz ———————— 

Biotite ——— —— — ——

Muscovite — —— —— ——

Apatite —————————

Zircon ———————————

Epidote ———————— 

Rutile ——————————

*s-i 1 1 -f man-i 1-o __

Sphene -——-—————

Tni-al — —

281

28.1 

33.2 

23.9 

9.7 

2.0

1.7 

0.7 

0.4 

0.2

Tr. 

0.1

—

100.0

282

17.8 

41.1 

24.9 

10.7 

2.4

1.9 

0.2 

0.2 

Tr.

Tr.

0.8

100.0

283

33.6 

29.0 

20.2 

10.6 

3.0

2.9 

0.4 

0.3 

Tr.

Tr.

100.0

284

42.7 

23.9 

29.4 

1.0 

0.4

1.2 

0.2 

0.2

Tr. 

1.0

100.0

285

31.3 

31.8 

32.6 

2.8 

0.7

0.8 

Tr.

Tr.

Tr.

100.0

286

37.8 

30.2 

30.0 

1.5 

0.3

0.1 

0.1

Tr.

—

100.0

287

3.0 

48.6 

42.8 

Tr.

3.0 

0.4 

1.1 

1.1

—

100.0

288

5.9 

45.7 

40.9 

5.1 

0.8

1.0 

0.4

0.2

—

100.0

289

37.3 

27.0 

26.5 

6.2 

0.7

2.3 

Tr. 

Tr. 

Tr.

Tr.

100.0

290

43.3 

23.9 

29.3 

a2.0 

0.7

0.8 

Tr.

0.1

Tr.

100.0

291 292

49.0 26.9 

21.0 35.6 

29.0 32.4 

a1.0 3.4 

Tr. 0.9

0.5

Tr. Tr.

—

0.3

100.0 100.0

293

27.4 

32.8 

29.2 

8.6 

0.8

0.9 

0.3

Tr.

—

100.0

294

31.2 

29.3 

38.6 

0.9 

Tr.

Tr.

Tr.

—~

Tr.

100.0

Potassium feldspar — 

Plagioclase ——————

Biotite —————————

A~l1ar»Tl-Q _ ___

Calcite —— — ——— ——

PiiHlo _ .— — _

Chlor ite —— -— —— ——

Sphene -———-———— 

Monazite-xenotime ——

295

31.5 

19.7 

39.4 

7.4 

1.0

1.0

Tr.

—

Tr.

100.0

296

43.0 

17.4 

36.5 

Tr. 

2.4

0.6

0.1

—

—

Tr.

100.0

297

22.7 

19.2 

54.0 

3.1 

0.1

b0.9

Tr.

—

—

100.0

298

24.7 

22.2 

47.7 

3.8 

0.6

0.7

Tr.

—

Tr.

—

100.0

299

35.3 

32.7 

27.7 

1.3 

1.0

1.0

Tr.

1.0

Tr.

100.0

300

24.9 

36.0 

27.9 

&5.4 

1.8

3.6 

0.3

0.1

—

Tr.

100.0

301

44.4 

26.0 

24.6 

4.1 

0.6

0.3 

Tr.

Tr.

—

—

Tr.

100.0

302

43.6 

26.3 

26.8 

1.5 

0.2

Tr. 

Tr. 

Tr.

Tr.

—

—

100.0

303

31.9 

24.8 

38.6 

3.5 

0.6

0.6 

Tr.

Tr.

Tr.

100.0

304

41.9 

25.4 

23.5 

6.4 

0.1

2.0

0.3 

Tr.

0.3 

0.1

—

100.0

305 306

47.9 41.1 

22.2 21.8 

26.2 26.3 

0.6 3.2 

0.6 6.4

0.8 0.2 

0.4

0

1 —

Tr. 

1.2 1.0 

Tr.

Tr.

100.0 100.0

307

34.6 

28.6 

26.6 

0.4 

0.4

1.4 

0.6

0.1

0.1

7.2 

Tr. 

Tr. 

Tr.

100.0

308

43.7 

22.3 

26.5 

5.9

1.3

Tr.

Tr.

Tr. 

0.3

Tr.

100.0
34 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 8.— Modes (volume percent) for Twin Spruce Quartz Monzonite, Front Range, Co/o.—Continued

Potassium feldspar —

Biotite ——————————

All am' f-a— — — .

T?iit"i 1 a»« « _

Monazite-xenotime ——

Total — —— —— —

309

42. 

22. 

27. 

4. 

1.

1. 

0. 

0. 

Tr

Tr

0.

Tr

100.

2 

0 

5 

8 

3

3 

2 

2

5

0

310

42.2 

29.1 

21.5 

3.9 

0.4

1.6 

0.9 

Tr. 

0.1

Tr. 

Tr. 

0.3

Tr. 

Tr.

100.0

311

35. 

31. 

26. 

4. 

0.

1. 

0. 

0. 

Tr

—

0.

Tr

100.

5 

4 

4 

6

1

5

1

1

3

0

312

27.3 

34.7 

26.2 

6.9 

1.8

1.4 

0.4

Tr.

Tr.

1.3

100.0

313

34.0 

34.0 

25.9 

1.9 

3.3

0.7 

Tr. 

Tr.

0.1

0.1

Tr.

100.0

314

42.6 

27.8 

24.8 

3.2 

1.0

0.1 

0.1

0.3

0.1

—

100.0

315

26.9 

30.2 

32.4 

7.5 

0.7

1.5 

0.3 

0.1 

Tr.

Tr.

Tr. 

0.1 

0.3

100.0

316

35.3 

34.3 

28.3 

1.3

0.7 

0.1

Tr.

Tr.

100.0

317

14.6 

44.8 

31.7 

7.0 

0.5

0.3 

0.3

Tr.

0.3

0.5 

Tr.

100.0

Biotite and chlorite undifferentiated. Traces of green spinel.

areas (Sheridan and others, 1967), in the southeastern 

part of the batholith, contains not only more quartz 

but also more plagioclase than similar rocks within the 

batholith. To the west of the batholith, within 

metasedimentary units, the quartz monzonite lenses 

also have a greater proportion of quartz, probably as a 

result of local contamination from the biotite gneiss 

and schist. In general, the speckled and fine-grained 

quartz monzonite phases have a greater spread in 

modal composition than does medium-grained quartz 

monzonite. Medium-grained quartz monzonite clusters 

imperfectly between 24 and 40 percent quartz, 35 and 

45 percent microcline, and 20 and 35 percent 

plagioclase.

GRANITE GNEISS, GNEISSIC APLITE, AND PEGMATITE

Granite gneiss, gneissic aplite, and pegmatite occur 

as dikes and lenses in the batholith and in the adjacent 

metasedimentary rocks. Whereas these dikes and 

lenses are nearly all of Boulder Creek age (1,700 m.y.), 

it is possible some are of Silver Plume age (1,450 m.y.). 

Generally, but not always, they can be identified with 

one intrusive or the other.

Granite gneiss and pegmatite form topographically 

high ridges that can be easily mapped on the north and 

south sides of Lefthand Canyon, northeast of Gold Hill 

(pi. 2). At Gold Hill granite gneiss, gneissic aplite, and

pegmatite form profuse lenticular masses, dikes, and 

thin lenses that are generally associated with Boulder 

Creek Granodiorite adjacent to the batholith as well as 

in the batholith. An area of extensive pegmatite occurrence is north of Golden Gate Canyon for perhaps 2 or 

3 m adjacent to the batholith in an area of large 

satellitic plutons. Granite gneiss and pegmatite are 

also present in the Central City quadrangle, especially 

the southern half.

Granite gneiss is a leucocratic, generally medium- to 

coarse-grained rock that is locally gradational into 

pegmatite and aplite. It is predominantly a feldspar 

rock containing 20-35 percent quartz and having less 

than 5 percent accessory minerals, including biotite, 

muscovite, and the ores (table 9). Locally, granite 

gneiss and pegmatite bear stringers of biotite gneiss or 

of biotite, muscovite, and sillimanite.

In thin section granite gneiss and pegmatite are 

leucocratic rocks having an allotriomorphic granular 

texture. The predominant minerals, microcline, 

plagioclase, and quartz, are equigranular in some of 

the finer grained granite gneisses, but more commonly 

microcline occurs as distinctly larger crystals. The 

microcline generally appears fresh, but the plagioclase 

is characteristically altered, and twinning is commonly 

indistinct and discontinuous. In a ternary quartzplagioclase-potassium feldspar diagram (fig. 15) modal
STRUCTURE 35

distribution of granite gneiss extends form the quartz 

monzonite field into the granite field with a quartz 

range between 20 and 40 percent.

EXPLANAflON 

0 Tungsten

• Eldorado Springs 

A Gold Hill

* Black Hawk 

x Ralston Buttes

EXPLANATION

* Ward

0 Nederland

• Tungsten

V____V

B

EXPLANATION 

Fine-grained 

Medium-coarse 

Speckled

v

FIGURE 14.—Modal variation in quartz, plagioclase, and potassium 

feldspar (volume percent) for Twin Spruce Quartz Monzonite. A, 

composite of all Twin Spruce Quartz Monzonite modes for the 

batholith. B, variation in lens west of the Boulder Creek 

batholith. C, modes for the Tungsten quadrangle showing 

distribution of medium-grained quartz monzonite. Dashed area 

encloses medium-coarse-grained quartz monzonite. P, plagioclase; 

K, potassium feldspar; Q, quartz.

AGES OF BATHOLITHIC ROCKS

Both the Boulder Creek Granodiorite and the Twin 

Spruce Quartz Monzonite have been dated by determining Pb207/Pb206 in zircons from the rock samples and 

by rubidium-strontium whole-rock methods. Perhaps 

the earliest reliable dates for the batholithic rocks were 

made by the U.S. Geological Survey (1964) on zircons 

and indicated an average age for the batholith of 1,730 

m.y. Later, Stern, Phair, and Newell (1971) indicated 

an average age of the batholith based on six zircon 

samples to be 1,714 m.y. and emplacement age of the 

batholith, 1,725 m.y. Peterman, Hedge, and Braddock

(1968) dated 13 samples from the batholith and smaller 

plutons of granodiorite to the north of the batholith. 

The results of their rubidium-strontium age determinations indicated the batholith was 1,700 m.y. years old. 

This age is in agreement with the zircon age of 1,714 

m.y. obtained by Stern, Phair, and Newell (1971). Also, 

Peterman, Hedge, and Braddock (1968) indicated that 

a postcrystallization event was superimposed on the 

batholith as suggested by a Sr87/Sr86 age of 1,340 m.y. 

Sr87 is suggested to have partly or completely reequilibrated at that time. Several samples of Twin 

Spruce Quartz Monzonite from near the type area in 

the Eldorado Springs quadrangle were dated by Hedge

(1969). He obtained rubidium-strontium whole-rock 

ages that isotopically fit the 1,700-m.y. isochron of the 

Boulder Creek Granodiorite.

Geologic mapping, principally by Wells (1967) and 

Gable (1972), indicated that Twin Spruce Quartz Monzonite is in part younger than the Boulder Creek 

Granodiorite. The field evidence indicates that the 

Twin Spruce Quartz Monzonite is in part the same age 

as the Boulder Creek Granodiorite, but the greater 

part of it is younger; the younger age is still within the 

limits of the isochron date (Hedge, 1969) determined 

for the quartz monzonite.

STRUCTURE

The Boulder Creek batholith was emplaced into a 

generally conformable sequence of high-grade 

metamorphic rocks that were folded into a series of 

complex antiforms and synforms and, locally, 

cataclastically deformed and faulted. Intrusion of the 

batholithic rock, metamorphism, folding, cataclasis, 

and major faulting in the area all took place during 

Precambrian time. Whereas Cretaceous to Tertiary intrusive stocks pierced the metasedimentary rocks to 

the north and northwest of the batholith, only dikes, 

sills, and small lenses occur within the batholith.

The Boulder Creek batholith and its satellitic 

plutons of similar composition and age shown on plate 

1 compose a semicircular structural feature that is
36 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 9.—Modes (volume percent) for aplite, aplitic pegmatite, and granite gneiss from the map unit Granite Gneiss and Pegmatite (pi 1),

Boulder Creek bathottth, Front Range, Colo

[(--), not found; Tr., trace; ores include all opaque minerals. Sample 356 from Nederland quadrangle; 357-360 from Tungsten quadrangle; 361 from Ward quadrangle; 362-374 from Gold

Hill quadrangle]

Potassium feldspar —

Biotite ——————————

Zircon ———————————

PT-il nt-T i-o — -

Monazite-xenotime ——

CaTf-ii-o — — — .

Sillimanite ———————

Trti-ol _ _ _

356 357 358 359 360 361 362 363 364 365 366

33.4 52.9 34.6 56.4 38.7 52.0 46.0 38.6 45.3 39.0 35.4 

30.2 16.6 31.4 14.8 24.8 32.0 22.0 38.6 22.1 17.0 24.7 

32.7 29.3 30.9 26.9 33.7 12.0 30.0 22.8 31.9 34.0 33.9 

0.7 0.6 — 3.0 — Tr. -- 2.0 4.4

2.2 a0.3 1.4 0.5 — 1.0 2.0 — 0.4 8.0 1.3

1.5 — 0.4 — 0.1 — Tr. Tr. — Tr. 0.3

Tr. Tr. Tr. 0.4 Tr. Tr.

Tr. — — Tr. — Tr. 

0.6 0.8 1.1

i.o

n i — — — — —

_ n i _ _ - — \J . _L

100.0 100.0 100.0 100.0 99.3 100.0 100.0 100.0 100.0 100.0 100.0

Sample No. ———————

Potassium feldspar —

"Ri ni"i -ho— — -. _ _«__

Chlor it e —————————

Epidote —————————— 

Rutile ———————————

Monazite-xenotime ——

Pal r»i 1-o —— — — —

Sillimanite ——————

Total ————————

367 368 369 370 371 372 373 374

55.5 40.8 44.6 38.7 40.1 28.0 27.1 50.9 

17.8 25.5 30.1 25.7 20.4 34.4 33.9 20.5 

26.0 29.0 25.2 34.0 36.4 32.7 33.8 26.5 

0.9 — — Tr. 4.0

0.3 2.4 — 0.8 2.3 0.1 4.4 1.5 

0.3 1.2 0.1 0.8 0.1 Tr. Tr. 0.1

Tr. — — Tr. Tr. 0.1

Tr.

— \J • £. \J . / \J . / JLI-.

J.J7 . -LIT . -LIT.

0.1 Tr.

Tr. --• — — — — 0.8 0.5

100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

a. Includes sericite.
STRUCTURE 37

FIGURE 15.—Modal variation in quartz, plagioclase, and potassium 

feldspar (volume percent) for granite gneiss from the granite 

gneiss and pegmatite unit. P, plagioclase; K, potassium feldspar;

r\. i

midway between the Strawberry and Mt. Evans 

batholiths. In addition to Boulder Creek Granodiorite, 

ultramafic, mafic, and distinctly leucocratic rocks were 

emplaced as plugs, dikes, sills, lenses, and plutons 

along this semicircular feature. The smaller bodies of 

intrusive rocks roughly conform to the trends of foliation and fold axes in the older country rocks, suggesting that their emplacement was controlled, to a 

large degree at least, by preexisting structures in the 

country rocks.

Regional mapping has shown that the Precambrian 

country rocks of the batholith were deformed and 

metamorphosed during at least two episodes of 

regional metamorphism and deformation and a later 

episode of cataclastic deformation. Emplacement of 

the granodiorite apparently began late in the first 

period of synmetamorphic deformation and 

culminated during the second period. The first deformation formed tight to isoclinal folds that apparently 

have dominantly west- and northwest-trending axial 

surfaces. This fold set is represented by the overturned 

Buckeye Mountain anticline and the overturned Jenny 

Lind and Nederland synclines in the vicinity of Eldora 

and south of Gold Hill (pi. 2), where the granodiorite is 

subconformable to deformed country rocks that were 

folded on northwest-trending axes. Subsequently, the 

folds were refolded during a deformation that produced north- to northeast-trending folds, which are the 

dominant structures in the region (Sims and Gable, 

1967; Gable, 1969; Gable and Madole 1976; Taylor, 

1976). Both the northern and southern parts of the 

batholith have foliations that have been folded on eastnortheast-trending axes that extend from the country 

rock into the batholith (pi. 2). Foliations along the west 

contact parallel the contact and vary sharply from 

those in the interior of the batholith where foliations 

undulate from northeast to northwest. Both trends in 

foliation are probably primary, that is, they were 

formed through flowage of magma that was still

plastic during the time of metamorphic deformation 

and presumably were caused by protoclastic deformation or by the differential flow of magma before complete consolidation. Continued shearing produced 

secondary foliations and recrystallization in the 

granodiorite along fault zones, particularly along the 

southern margin of the batholith. Both deformations 

took place under temperature-pressure conditions 

represented by 300-500 MPa [3-5 kb] at 620-710 °C 

(Gable and Sims, 1969) and are characteristic of a 

sillimanite-potassium feldspar grade metamorphism. 

The later and more restricted cataclastic deformation 

was confined mainly to a relatively narrow zone passing through the southeast part of the map area (pi. 1) 

and the southernmost part of the batholith and was accompanied by retrograde metamorphism. The 

cataclastic deformation has been dated at about 1,440 

m.y. by Hedge (1969), which is approximately the time 

of emplacement of the Silver Plume Quartz Monzonite.

Jointing is well developed throughout the batholith 

but data are available for only the western half of the 

batholith. Joints mapped in the Tungsten and Gold 

Hill quadrangles are fairly uniform; figure 16 synthesizes the similarities.

Flat-lying joints (not shown in fig. 16) in the 

northern part of the batholith trend N. 72° E. and dip 

approximately 10° NW; this same joint set to the 

south has about the same trend, N. 80° E., and dips 

15 ° SE. If these primary flat-lying joints formed when 

magmatic pressure decreased, they probably indicate a

FIGURE 16.—Synthesis of the more prominent joint sets in the 

Tungsten and Gold Hill quadrangles, showing similarities in 

orientation; 963 poles represented. No, north half of batholith; So, 

south half of batholith.
38 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

doming of the batholith; another possibility is that the 

joints were related to contractions of the batholithic 

mass on cooling, or alternatively, represent fractures 

formed by release of pressure as the batholith was uncovered. Of the three possibilities, the last seems 

preferable, especially if the batholith does consist of 

two magma series. A joint set that dips to the west on 

the west slope of the southern part of the batholith 

(Gable, 1973) and a complementary set that dips east 

may also be related to the release of pressure.

GEOCHEMISTRY

Geochemical data indicate that the Boulder Creek 

Granodiorite consists of two magma sequences both 

regulated in part at least by CaO and SiO2 : (1) a 

northern sequence that has more SiO2 , CaO, and Na2O 

than (2), a southern sequence that is richer in A12O3 , 

FeO, MgO, and K2O. In the Twin Spruce Quartz Monzonite, however, CaO and SiO2 do not appear to have 

influenced distribution of the oxides.

Chemical data were obtained for Boulder Creek 

Granodiorite and each of the significant rock types 

associated with the granodiorite. Major- and minorelement analyses of these rocks are given in tables 10, 

11, 12, and 13. Data from analyses published since 

1950, when usable, have also been used in many of the 

figures (for example, figs. 18-27), especially those from 

the Eldorado Springs quadrangle, in order to have 

more complete coverage of the batholith. Localities of 

chemically analyzed samples in this report are plotted 

in figure 17.

BOULDER CREEK GRANODIORITE

The Boulder Creek Granodiorite has two distinct 

chemical compositions based on the presence or 

absence of hornblende. Biotitic Boulder Creek 

Granodiorite generally has higher SiO2 and CaO and 

lower TiO2, FeO, and MgO than biotite-hornblende 

granodiorite. Chemically, Boulder Creek Granodiorite, 

on the average, shows the same variability in composition, based on the minerals biotite and hornblende, as 

do equivalent analyses of biotitic granodiorite and 

biotite-hornblende granodiorite published by Nockolds 

(1954, p. 1014); the Boulder Creek Granodiorite on the 

average, however, contains less SiO2 , Fe2O3 , and Na2O 

and more A12O3 , total iron as FeO, and MgO.

Chemical data on the mafic inclusions in the Boulder 

Creek Granodiorite are given in table 11. The small 

spindle type of mafic inclusion associated with the 

granodiorite, shown in figures SA and 9 was not 

chemically analyzed; it is too greatly altered. All 

analyses are of larger dioritic inclusions (samples 334,

385, and 386, table 11). Trace-element data for the 

mafic inclusions are quite similar to data for Twin 

Spruce Quartz Monzonite (fig. 18). Trace elements for 

hornblende gneiss and amphibolite (fig. 18, col. 1), a 

rock type suggested as a source for the mafic inclusions and trace elements in the inclusions, vary widely.

Gabbro, hornblende pyroxenite, pyroxene-bearing 

hornblendite, hornblende diorite, and biotitic hornblende diorite, which are all spatially associated with 

the Boulder Creek Granodiorite, were also analyzed 

(table 12). Modes for these rocks are a part of table 7. 

Chemical analyses and modes indicate that these mafic 

rocks are composite and partly gradational into one 

another and even into granodiorite.

Twin Spruce Quartz Monzonite is a high-SiO2 , lowMgO rock, as indicated by 13 analyses from localities 

in the Nederland and Tungsten quadrangles (table 13). 

Average Twin Spruce Quartz Monzonite does not vary 

much from Nockolds' (1954) biotite adamellite or from 

his muscovite-biotite quartz monzonite. The Twin 

Spruce Quartz Monzonite has higher Fe2O3 , due to oxidation, than Nockolds' average quartz monzonite. 

Amounts of K2O in the Twin Spruce Quartz Monzonite 

are extremely variable between samples.

CHEMICAL TRENDS WITHIN THE BATHOLITH

Chemical trends occur from east to west across the 

southern part of the batholith as represented by 

samples from the Tungsten and Eldorado Springs 

quadrangles. However, in the northern part of the 

batholith, no east-west trend is indicated by the 

samples from the Gold Hill and Boulder quadrangles. 

Trends across the southern part of the batholith are 

defined by the percent of SiO2 and (or) CaO in the rock 

in relation to most major oxides. In the northern part 

of the batholith the single east-to-west trend is replaced by two separate trends in which SiO2 for both 

the border and interior of the batholith lie in exactly 

the same range. In general, the southern part of the 

batholith has more FeO, MgO, and K2O and less CaO, 

P2O5 , and possibly less Na2O than the smaller northern 

part. Modal and chemical trends seem to indicate that 

there were two magmas involved in the batholith, the 

northern magma showing less differentiation from contact to central part of the batholith.

Chemical trends of CaO and SiO2 to the major oxides 

in granodiorite from the Boulder Creek batholith are 

indicated in figures 19 and 20, and in general, as TiO2 , 

MgO, and FeO representing total iron increase, CaO increases, whereas K2O and SiO2 decrease as CaO increases. P2O5 and Na2O show no direct relationship to 

CaO; no trend as such is indicated.

Chemical trends of calcic, sodic, and femic oxides 

relative to SiO2 in the batholith (fig. 19) show con-
GEOCHEMISTRY 39

siderable scatter except for total iron, which has a 

nearly straight-line relationship to SiO2 , even within 

the satellitic plutons of granodiorite. Silica ranges 

from about 60-70 percent, and, as can be seen in figure 

19, there is a distinct break in distribution of the oxides in the southern part of the batholith at about 64 

weight-percent SiO2 , the greater part of the Eldorado 

Springs quadrangle samples (interior of the batholith) 

plotting between 60 and 65 percent SiO2 , and Tungsten 

samples (batholith border) plotting above 64 percent 

SiO2 . In the northern part of the batholith, Gold Hill 

and Boulder quadrangle samples each cover the entire 

range from 60 to 72 percent SiO2 . In the batholith, K2O 

in relation to SiO2 is steplike. Gold Hill quadrangle 

samples occupy mostly the 1-2.5 percent range of K2O, 

Tungsten quadrangle samples occupy the 2.5-4.0 percent range, and Eldorado Springs quadrangle samples 

occupy the 3-4.5 percent range, but Boulder 

quadrangle samples cover nearly the entire range from 

1.5 to nearly 5 percent. CaO and Na2O in the northern 

part of the batholith represented by Boulder and Gold 

Hill quadrangle samples plot mostly above the 

Tungsten-Eldorado Springs samples from the 

southern part of the batholith, indicating greater CaO 

and Na2O in the northern part of the batholith. CaO is 

the only oxide in which trends are readily defined in 

the two parts of the batholith and whereas the 

southern batholithic samples divide readily along the 

CaO-SiO2 trend, those in the northern part of the 

batholith are separated by differences in amounts of 

SiO2 rather than CaO.

Oxide distribution in relation to the percent of 

calcium present, especially between the northern and 

southern parts of the batholith, is expressed in figure 

20. In the southern part of the batholith, analyses 

having greater than 4 percent CaO are represented predominantly by Tungsten quadrangle plots, and less 

than 4 percent CaO by Eldorado Springs quadrangle 

plots. In the northern part of the batholith no such 

zoning occurs; as the contact is approached, there is an 

increase in both sodium and calcium. Calcium apparently regulates a number of oxides, especially TiO2 

and MgO, and perhaps FeO, A12O3 , and fluorine, but 

virtually not Na2O and P2O5 at all. Na2O is relatively 

stable throughout the batholith and varies mostly between 3 and 4 percent of the total oxides. MgO in the 

southern part of the batholith follows a single trend, 

whereas in the northern part, represented by 

Boulder-Gold Hill quadrangle samples, it occurs in 

two separate trends, one representing the border area 

and the other the interior of the batholith. MgO trends 

are very distinct in comparison with trends for the 

other oxides in relation to CaO. Fluorine in the 

batholith is abnormally high for a granitic rock, but a 

relatively high fluorine content is characteristic of

almost all granitic Precambrian Front Range rocks 

(Shawe, 1976, p. 25). Fluorine in at least the northern 

part of the batholith shows a regular distribution trend 

with calcium that indicates that calcium increases as 

fluorine increases. Lee and Van Loenen (1971, p. 21) 

noted also that with an increase in calcium, fluorine 

also increased in gneissic granitoid rocks in the 

southern Snake Range of Nevada. Unfortunately, in 

the southern part of the Boulder Creek batholith there 

are insufficient analyses to come to any conclusion.

In a ternary K2O-Na2O-CaO diagram (fig. 21), the 

trend (A) for the northern, part of the batholith 

represented by the Boulder-Gold Hill quadrangle 

plots (non-shaded area) tends to increase in Na2O as 

CaO increases, whereas in the Tungsten and Eldorado 

Springs quadrangles, the trend (B) shows a decrease in 

Na2O as CaO increases. The Tungsten and Eldorado 

Springs plots from the southern part of the batholith 

are not only widely scattered but split at about 38 percent (on the basis of 100 percent) CaO and 32 percent 

for both K2O and Na2O. Tungsten quadrangle samples 

plot on the sodic side and those from the Eldorado 

Springs quadrangle on the potassic side of the 

diagram. The northern part of the batholith tends to be 

more calcic, the trend is well defined, and the scatter of 

points is small. The tendency of Na2O to vary only between 25 and 35 percent on a K2O-Na2O-CaO ternary 

diagram is consistent with observations of many 

others. Bowen (1928, p. 100) was the first to note this 

close approach to a constant value for sodium in calcalkaline rocks containing 55 to 75 percent SiO2 .

On a Q-Ab+An-Or (quartz-albite plus anorthiteorthoclase) diagram (fig. 22), samples from the Gold 

Hill quadrangle in the northern part of the batholith 

plot dominantly on the Ab+An side and to the left of 

the Tungsten and Eldorado Springs quadrangle 

samples from the southern part of the batholith. 

Samples from the Boulder quadrangle plot dominantly 

in the same field as those from the Tungsten and 

Eldorado Springs quadrangles. The trend of the 

granodiorite field is extended to the Ab+An corner by 

plotting the more mafic rocks associated with the 

granodiorite.

On a ternary variation diagram (fig. 23) the 

Tungsten and Eldorado Springs quadrangle samples 

fall closest to a line from the Na2O-K2O corner to the 

midpoint between the MgO, FeO+Fe2O3 +MnO side of 

the diagram, for a 1:1 ratio of total iron to magnesium. 

Differentiation trends for both the northern and 

southern parts of the batholith are indicated; however, 

the trend for the northern part plots closer to the 

FeO+Fe2O3 +MnO corner. Both trend lines flatten between 30 and 35 percent (based on 100 percent) 

FeO+Fe2O3 +MnO as MgO increases.
TABLE 10.—Chemical and spectrographic analyses and modes for Boulder Creek Granodiorite, Front Range, Col

[Rapid-rock chemical analyses for samples 106, 129, 132, 138, 140, 148, 150, 156, 162, 167, 394-402 by Hezekiah Smith; semiquantitative spectrographic analyses b

L. A. Bradley. Standard rock chemical analyses and spectrographic analyses for samples 4, 41, 44, 46-51, 192, 389 from Gable and Smith (1975). Standard rock che

spectrographic analyses for samples 390-393 from Sims and Gable (1964, 1967). Sample 4 has a trace of rutile. N.d., not determined; (--) not found; Tr., trace; L, pres

of determination; <, less than; ores include all opaque minerals]

South half of Boulder Creek batholith Satellitic p

Sample No.————— 4 41 44 46 47 48 49 50 51 192 389 390 391 

Chemical analyses , in weight percent

C-JA ___ _ _ _ DlUrt ———————————————

AJ.2 U3 

TTo 0 _ ___ _ ______ Fe2°3

TToA—— .._«_

MoH- ——— ————————

66.40

1 ^ QQ

1.39

i ?•;

62.25 61.3

1 £ QQ -1C Q

1.67 2.6

3 m T i

9 RR T.A

74.5

12.9

. DO

1.2

n.QQ

62.99 63.5

17.05 16.7

1.77 1.9 

2.98 3.0

?.1fi 9.fi

64.9

•] /• Q

2.4

7.1

61.8

1.5

3.0

?. S

67.9

15.7 

1.3

2.1

1 .6

61.2

16.9 

2.5

4.6

O Q

60.8

17.1

1

Q

5.0

2.9

54.29 54.41 

13.53 17.12

5.28 3.14

5.10 5.60

2.42 5.18

CaO——————————— 3.14 4.30 4.5 2.3 4.37 4.3 3.6 5.4 3.0 4.2 4.5 7.22 7.25 

Na 2 o——————————— 3.29 3.43 2.7 2.6 3.79 3.2 3.0 3.4 3.1 2.7 2.6 2.64 2.98 

K2o——————————— 3.12 3.84 3.9 3.6 2.95 3.0 3.6 3.1 3.7 2.7 2.7 2.40 1.40 

H2 0+—————————— 0.57 0.65 0.86 0.70 0.48 0.70 0.73 0.88 0.71 1.0 1.2 0.72 1.23 

H0 0~——————————— 0.04 0.07 0.24 0.04 0.02 0.03 0.05 0.06 0.07 0.15 0.08 0.22 0.07 

2 

Tin——————————— 0.51 0.59 0.64 0.28 0.63 0.62 0.49 0.75 0.46 0.93 0.95 2.37 0.85 

P o_—————————— 0.25 0.29 0.38 0.11 0.26 0.28 0.36 0.53 0.20 0.23 0.13 2.02 0.07 

2 5 

MnO——————————— 0.05 0.07 0.13 0.04 0.07 0.09 0.09 0.09 0.07 0.12 0.10 0.13 0.15 

C0 —————————— 0.0.1 0.14 Q.ll 0.10 0.06 0.05 <0.05 0.11 0.18 <0.05 <0.05 0.44 0.08 

0.01 0.04 n.d. n.d. 0.03 n*d. n.d. n.d. n.d. 0.03 0.02 n.d. 0.07 

0.16

n.d.

n.d.

0.15

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.13

n.d

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.14

n.d.

n.d.

0.16 0.43 0.07 

n.d. 0.36 0.10 

n.d. 0.17 0.06 

Total—————— 99.61 99.93 100.0 100.0 99.94 100.0 100.0 100.0 100.0 100.0 100.0 99.74 99.83 

Spectrographic analyses , in parts per million

Ag

1,500 

3

150

10

1,500 

2

L

15

2,000 

3

500

30

1,000

3

_

700

2

L

15

1,000

3 

300

20

1

1,000

3

20

1,000

3 

300

20

L

1,500 500

15 30

L

500

20 34 

Cr———————————— 30 70 70 7 30 50 30 70 20 70 70 — 220 

Cu———————————— — 7 20 15 15 20 7 30 57 15 — 73 

Ga———————————— 30 30 15 7 30 15 10 10 10 15 15 — 25 

La——————————— 100 50 70 — 50 70 — 70 -- 30 — — <100 
GEOCHEMISTRY 41

1 1 1 1 1 1111 1

1 1 O O CO O C

1 1 CM in rH CM 0

\D

1 u-> O 00 O C

1 00 CO CM -* \1

v m c

1 1 1 1 1 1 1 1 1 1 1 1

r> 1 O r^ O O C

i u-> CM o c

CO r

-• 1 O r-. O O C

I in CM o c

CO r

O O t-» CO O C 

1 CO O "

0 1 O t^ 0 O C

l in rH oro

f\

rH

0 1 O r-» t-> O C

1 in o rn l o r^ o o c

1 U1 rH O f

H

o o m m o c

J P*. CO rH rH O f 

O

rH

O 1 O O CO O C

1 CO rH O 0

v m

- 1 o r-» co o c

1 r-x 0 C

O r

rH

o o m m o c

J t^ CO rH rH O rrH

D O O O O O C

H O CM CO rH O C

rH Or

rH

i

| |

1

1 

|

1 1 

3 T3 iH ,0 y M

Z S & PM co co i;

1 1

3 O -* O

3 m o

^

3 O CO O

D CO CO

•J rH

1 1

3 O CO O

3 CO O

H CO

3 O CO O

3 CO u"j

H H

3 O rH O 

1 rH CM

3 o m o

»m co

3 o co m

*• CO rH

3 O CO O

"CO rH

3 in co o

•* rH r--

3 O rH m

1 rH

3 O t^ O

3 t-N CO

H

3 O CO O

- co m

rH

3 O CO O

3 CO O

H CO

,0 M

> ^-t ^-t t*3

4J 

g

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M

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|

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^

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;. . . 1 | . Ill nil

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^r <y co <y. o o <N sr • co

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CO CO rH rH

cMr~,o\cMco CM* • •• -in

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II • 1 M 1 II

rHf^HCMI 1 00 IHI II

rH CO CM CM .Q

rHCMinO OCO .r-» rH rH

• 1 | . . | M -III- • II o-*r^mi ICMOIH 01110 o 11

-* CM CM

\O O O\ •* rH • «

1 • • • 1 1 • • 1 M t-l 1 1 1 1 II 

ICOCMCMI IrHOIH Hill 1 II

«^" CO CM

-*oom-* -*mo\.cMin CM CM

O\OvOOI IOOOH OOIO O II

rH -* CM rH

mf^rHOlrH •-* rH. rH« rH-* VD

eMvD-tft^-* HOIrHH rHHIOO rH II

-* rH rH rH

OfOvD-tfCM | r-x | VD^. ^ ^ 1 1 "2 ^ II

0\-*r-~-*O IOIOH OOIIO rH II

rH CO CM rH

vDOOO-*r-> IOIOH Ollll O II

rH -* rH rH

vDinCxC\r*» VD CXrH*^". CM CM

OOO-*CMO IOIOO OHI O O II

rH -* CM rH

rHt^-*m rHCMrHrH •. rHCM m

1 | .... M M 1 ' II Hcoaomi 10000 HHIOO o 11

CM CO CO

-*t^rHCOCO CTi \DrHCTiCM CM CO

coino\coco .10100 00110 CM 11

CM CM rH CM

cOcocO-*iH IOIOH OOIOO O II

H -* CM rH

CM-*-*U-> OOrH-rHt^vD rH. rH

1 | . . 'M • . . | . M • 1 oocoomi IOOHO OOIOH o i

-* CO rH

II 1 III II 1 II 1

II 1 II 1 II 1 II II

II 1 1 (U 1

II 1 i 1 1

II (U II Ml 

1 (U 4J II O 1 M CO O 1 -H || T (U G n) n) -o <u i— i a) I I <u I <o G

9(XrH fl-UO 4J | (1) 4J <U | 4J -H D 

•H ra U DO) T-l C T-l <U4J-H4J<U (U -H 4J 4J 

OI T3 O N 4J t-l rHl-l > 0) 3 4_) T-l C -rl 4J 4J C C O T-l

COrH-H4J-H43 O4J OfiO -Hfi-HM-H OTOfifi cdtuaoM-ug g o ra o <u u ucdrHoy >ar n <o f •UM-icdcdOM 3 n) a) ra ^3 t-i cdi-HOrHrH -H a x <D

O rH3l-IO M (-1 3 O. T-l (XrHM^Cd Ot O M

PM PnO-pqW H OSWN «J«J(2oo W S PH

O

o

o

H

O

o

o

rH

O

o

o

H

O

o

o

rH

O

o

o

rH

O

o

o

rH

O

srH

O

o 

o

rH

O

o 

o

rH

O

o 

o

rH

O

srH

O

o

o

rH

O

o 

o

rH

O

o

o

rH

O

o

o

H

H 

n)

4J

O

H

•a

a

t^.

CM

ti <5

•a

a

•a

a

«M

£i

<JJ

mfi"*

•<J

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<r

f

00 f

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00 

CM

00

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rH

f

CO 

CM

s<i

00

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fl

1

1

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sition gloclas

O R) 

(XrH 

g 0,

O
TABLE 10.—Chemical and spectrographic analyses and modes for Boulder Creek Granodiorite, Front Range, Coto.—Co

North half of Boulder Creek

394 395 396 397 398 399 400 106 401 402 129 132

batholith

138 140 148 15

Chemical analyses, in weight percent

q-«ri ________ ___ ___

Fe2°3

FeO — ——— ———— ———

1J a* TM

H

n ___ _—_, _ .— _-

H

n"~ __-.

TM n _—_-_._>__ __ _____ __ JUJrt __-.__— ._—___

P2°5 ———————————

BaO —— —— — ——— ——

o a

63.5 

14.4 

3.7 

2.6 

1.5

4.6 

3.1 

3.1 

0.48 

0.26

1.1 

0.82 

0.08 

0.13 

0.01

0.31 

n.d. 

n.d.

99.00

61.1 

14.9 

4.2 

3.4 

1.0

6.3 

3.2 

2.3 

0.68 

0.29

1.7 

1.1 

0.08 

0.08 

0.02

0.40 

n.d. 

n.d.

100.00

62.3 65.6 71.0 

15.5 16.2 14.8 

3.5 1.7 1.0 

2.2 3.2 1.0 

1.1 1.6 0.72

4.4 4.4 3.2 

3.2 3.0 3.1 

3.4 1.8 3.7 

0.94 1.0 0.40 

0.16 0.16 0.18

0.93 0.68 0.43 

0.74 0.44 0.52 

0.06 0.05 0.04 

0.08 0.06 0.04 

0.02 0.01 0.01

0.27 0.13 0.04 

n.d. n.d. n.d. 

n.d. n.d. n.d.

99.00 100.00 100.00

65.1 

15.8 

1.8 

1.8 

1.5

5.7 

3.5 

1.8 

0.76 

0.10

0.67 

0.63 

0.06 

0.08 

0.02

0.09 

n.d. 

n.d.

99.00

65.2 

17.3 

1.5 

1.7 

1.3

5.9 

4.0 

1.3 

0.78 

0.12

0.59 

0.78 

0.04 

0.12 

0.01

0.12 

n.d. 

n.d.

101.00

62.3 

17.0 

1.8 

2.0

1.5

5.8 

3.8 

2.2 

0.60 

0.10

0.71 

0.66 

0.06 

0.08 

0.02

0.09 

n.d. 

n.d.

99.00

69.2 

15.4 

1.6 

1.3 

1.2

4.1 

3.2 

3.1 

0.50 

0.07

0.52 

0.43 

0.05 

0.03 

0.01

0.07 

n.d. 

n.d.

101.00

Spectrographic

Af>___ ___ .___. __ __

•D g__ _ _________ _ .____

700 

2 

300 

10

20 

20 

7 

100 

i

700 

2 

300 

10

35 

30 

15 

100 

i

700 700 700 

2 n.d. 1 

150 L L 

10 7 2

20 20 20 

30 70 n.d. 

15 7 7 

50 30 L

700 

2 

L

7

30 

20

7 

30

r> A

700 

2 

L

7

20 

300 

15 

30

r> A

700 

3 

150 

7

30 

15 

15 

50

r> A

61.1 

16.3 

3.5 

3.8 

2.4

6.2 

3.0 

1.3 

1.2 

0.09

0.83 

0.68 

0.08 

0.15 

0.03

0.11 

n.d. 

n.d.

101.00

68.1 

15.6 

2.0 

2.3 

0.84

3.3 

3.0 

4.3 

0.38 

0.18

0.81 

0.46 

0.06 

0.08 

0.01

0.12 

0.01 

n.d.

101.00

analyses, in

700 700 

2 1 

L L 

L 10

20 70 

n.d. 50 

7 7 

n.d. L

2,000 

2 

300

7

5 

10 

20 

150 

i

62.8 

15.4 

2.1 

2.8 

3.0

4.9 

3.4 

3.1 

1.1 

0.32

0.79 

0.70 

0.08 

0.22 

0.01

0.15 

0.01 

n.d.

101.00

parts

700 

3 

n.d. 

15

70 

20 

20 

L

7

63.1 

16.2 

1.8 

2.7 

2.2

5.4 

3.8 

2.6 

0.65 

0.12

0.67 

0.54 

0.06 

0.08 

0.02

0.07 

0.01 

n.d.

100.00

63.5 

16.4 

1.0 

3.0 

2.4

5.1 

3.7 

1.9 

0.74 

0.31

0.65 

0.50 

0.05 

0.36 

0.02

0.12 

0.01 

n.d.

100.00

69. 

14. 

0. 

1. 

1.

3. 

3. 

4. 

0. 

0.

0. 

0. 

0. 

0. 

0.

0. 

0. 

n.

101.

7 68

4 15

74 

9 

5 

2 2

0 3

5 4

79 0

24 0

49 0

44 0

02 0

25 0

01 0

01 — 

01 0

d. n

00 101

per million

700 

2 

L 

15

70 

20 

20 

50 

^

1,000 1 

2 

L 

15

70 

2 

20 

70

";

,500 

2 

L

7

50 

2 

15 

50

1,50

L 

3

1

7
GEOCHEMISTRY 43

o

o 1-^

o

o

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c l-x

l-x

r~

1 C

H IT

r-

> _

1

. C

r>

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1 C

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If

T-

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H C

Q T

5 t

O O O C 

ro CM rH c

C

O O O C 

ro CM rH c

1-

O O O C 

ro ro rH c

C

""

O 0 C 

ro ro hJ c

C

r1

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ro rH r-» c

C

ro m m c

rO rH rH C

1-

O 0 O C 

ro CM CM c

C

ro o m c 

ro ro rH c

C

•-

moo c 

ro rH 2

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CM rH C

it

m o in c

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if

o o o c

CM rH CM c

r-

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- CM rH C

C

o o m c

3 CM rH i-H C

r-

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3 rH C

• If

o in o c

3 CM rH i-H C

i/

3 m in i~- c

3 rH -c

4 i/

3 o o m c

1 CM CM rH C

H l/

3 m o m e

3 rH C

H u

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O O CM O 

r-~ rH o

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3 o m CM o

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3 r-~ rH r-» i

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CM

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CM

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3 0 0 CM 0 

3 m CM O

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1 1 • 1 • I 'III! I II r^oo-*vDl iroirHirHillI 1 II

-* CM rH

rHvror^ r^-a-vo o rH 1 1 ... 1 .III* 1 II

CMvOrOCMI ICMOrHlrHIIIO 1 II

CM rO CM rH

1 1 1 1 1 1 III Illl 1 II 1 1 1 1 1 1 III Illl 1 II 1 1 1 1 1 1 III Illl 1 II 1 1 1 1 1 1 III Illl 1 II 1 1 1 1 1 1 III Illl 1 II

1 1 1 1 1 1 III Illl 01 II

1 1 1 1 1 1 III Illl 4-1 II 

1 1 1 1 1 tU III Illl -H II

MMIIUIlH 

1 (U 1 1 1 4-1 III Illl Ulrll)

III Illl -H O (U I

EmmiiTjQjrH tu i i i tu i i o 4-i|

3 CL r-l | | (5 4-1 O 4-l|| (U4-IOJI 1 N (U -H (U

•H (0 O 1 Q) Q) -HC -HI 1 (U4-I-H4J1U d)OEN4J 

0)T3ON4-lrH -H-H >1UG 4-I-HC-H4J 4JC-HC8.H

4-ll*-IC8C8Orl Sc8(UWj3M CBrHOrHrH -HOCEtU

PL, pLiO'PQBC H OSCOtO <<j!^CJCJ W X PL,

O

O

o

rH

o

O

O 

rH

0

O 

O

rH

O

C3

0 

rH

O

C3

O

rH

O

O

0 

rH

O

C3

O

rH

0

O

O 

rH

O

o'

0 

rH

0

0

0

rH

O

O

0

rH

0

O

O 

rH

o

0

O 

rH

o

0

O

rH

O

0

0

rH

0

O

O 

rH

o

0

0

rH

0

o

0

rH

o

0

0 

rH

,1

1

1

1

rHttl

4-1

0

H

00 

CM

vO

CM

*"

rH

f

CO

CM

CM

CM

C

O 

ro

^

00 

CM

**

T3

C

00 

ro

-o

C

T3

S

rH

ro

C

T3

C

m

CM

CM

ro

00

CM

^

CM

ro

£j *fl

m

ro

C

1

1

1

O (U 

0)

c a

O rH 

•H O 

4-1 O

•rl -H 

0) 00 o m

O.1-H

E ^

u

in CM in

CM ro ro
44 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 11.—Chemical and spectrographic analyses and modes for mafic inclusions and lamprophyre 

dikes in Boulder Creek Granodiorite, Front Range, Colo.

[Rapid-rock chemical analyses by P. L. D. Elmore, Gillison Chloe, Hezekiah Smith, James Kelsey, J. L. Glenn, S. t>. Botts, 

J. W. Budinsky; semiquantitative spectrographic analyses by J. L. Harris. N.d., not determined; (-), not found; Tr., trace;

<, less than; ores include all opaque minerals]

Lab. No. —————————

o-fn — — — — . _ __

AT n _ _ ._ _ _ . _ __ AJ- 2U3

T7o n __________ _ Fe2°3

MgO ——————————————

Pan —— —— — — .... __ _

Ma H— ——————————— -_

K

n_ — —— — _ _ _»__ ... _ _

T-I -L J.WQ n __ _ «_ _

P n — _ _ __ „_ ___._,__ P2°5

Mr»n————— —————— ————___

pn «_. —_.—__—— _____

F ____ __. ————.. ______ 

n

iOtflJ.TM"^^^TM"^"TM""1*"^

Powder density ——— - ——

334 

W171193

52.6 

16.6 

4.7 

5.0 

3.0

5.4 

3.3 

3.7 

0.11 

1.1

2.7 

1.1 

0.12 

<0.05 

0.20 

0.03

99.00 

n.d.

Mafic inclusions

385 

W185231

53.6 

15.6 

2.9 

5.5 

4.4

7.9 

3.1 

2.6 

1.3 

0.09

0.64 

0.71 

0.18 

0.02 

n.d. 

n.d.

99.00 

n.d.

386 

W171834

52.2 

19.0 

2.6 

4.9 

5.0

7.4 

3.9 

2.4 

1.2 

0.14

0.82 

0.23 

0.15 

<0.05 

0.19 

0.04

100.00 

3.00

Semiquantitative spectrographic analyses, in parts

PO.... _.__.-.«— _._._,——„„

Co~* ——————————————

Pa _____ _ _„_. _

Mr\— — — ————_———_——

T^Tk— _—_____. — —— — ^

JJjJ ___________ ___________

^j^^^»_^__^__ ..»»_>_____»_._._.___

p-u _ _ _, _______ ^

rj__

Cv

y —— » __ mm—— ^«* ^—— ^^—— ————— —— -»••—— __

V—— ——«————-—— _-—— — »

2,000 

1 

2,000 

20 

70

20 

20 

500 

5 

20

500 

30

10

1,000 

150 

Rn

1,120 

4 

191 

31 

153

123 

36 

80

102

46 

14

660 1 

136

fi7

500 1 

1 

500 1 

30 

150

200 

15 

70 

10 

100

__ i

100 

7 

20

,000 2 

150 

sn

Lamprophyre dikes

387 388 

W171832 W171833

43.8 

12.5 

2.7 

5.2 

8.4

9.6 

1.5 

5.9 

0.14 

1.3

1.7 

1.2 

0.09 

<0.05 

0.62 

0.04

99.00 

3.00

per million

,000 1, 

5 

,500 3, 

30 

200

20 

10 

500 1, 

5 

15

,000 1, 

300

15 

100

,000 2, 

150 

inn

50.7 

13.3 

3.0 

5.7 

7.9

7.8 

1.7 

6.6 

0.15 

0.40

1.3 

1.2 

0.16 

<0.05 

0.59 

0.04

100.00 

3.00

000 

5 

000 

30 

200

30 

10 

000 

5 

15

500 

200

15 

100

000 

150 

inn
GEOCHEMISTRY

TABLE 11.—Chemical and spectrographic analyses and modes for mafic inclusions and lamprophyre 

dikes in Boulder Creek Granodiorite, Front Range, Coto.—Continued

45

Mafic inclusions

Lab. No. —————————

Yb——————————___

Zr ——————————————

Potassium feldspar ——

Plagioclase ————— - —

Quartz ———————————

Biotite ——————————

Hornblende —————— —

Pyroxene ———————— —

Ap£ltitGTM~~~~~~~TM~~~""~

Chlorite —————————

Prehnite —————————

Epidoteclinozoisite —————

Total ———————

Composition of 

plagioclase —————

334 

W171193

5

1,000

Modes ,

15.6

42.7

15.5

19.0

—

3.0 

0.3 

0.9

2.6 

0.2

0.2

100.0

^26

385 

W185231

9

734

in volume

1.9

46.7

7.7

14.8

23.5

Tr. 

1.2

0.5

0.3 

1.3

Tr. 

2.1

100.0

An3Q

386 

W171834

5

150

percent

6.2

53.0

3.0

12.8

22.6

0.2

Tr. 

Tr.

0.7

Tr.

0.1

1.4

100.0

An31

Lamprophyre dikes

387 388 

W171832 W171833

10

700

31.5

0.2

—

19.6

41.9

0.2

0.6 

2.8

3.2

Tr.

100.0

n.d.

10

500

39.2

0.6

0.2

14.5

37.0

0.9 

1.1 

2.4

3.9

—

0.2

100.0

An43

Whereas the major elements vary rather systematically with SiO2 or CaO, this is not particularly 

true for the trace elements. Of 20 minor elements 

only chromium, nickel, and vanadium appear to vary 

directly with CaO or SiO2 content. Chromium, nickel, 

and vanadium increase as CaO increases and all three 

decrease as SiO2 increases.

MINERALOGY IN RELATION TO CHEMICAL TRENDS

Chemical trends in the granodiorite (fig. 20) are more 

definitely related to calcium than they are to silicon 

and are better defined than those related to silicon (fig. 

19); therefore, weight percents of essential minerals 

were plotted against calcium (fig. 24) to determine 

related mineralogical trends. Modes are not available 

for chemically analyzed samples from the Eldorado

Springs quadrangle, and accordingly these samples do 

not appear on any of the plots. All trends have widely 

scattered points except biotite in biotitic granodiorite 

and hornblende in biotite-hornblende granodiorite.

Trends for modal potassium feldspar and quartz are 

inversely related to CaO in the rock, and from figure 

24, those trends for potassium feldspar are more consistent. Microcline accounts for the greater part of the 

potassium feldspar in the granodiorite, and, accordingly, trends approximate those of quartz; however, with 

an increase in the amount of biotite, the K2O is used 

first to form biotite and less K2O is left to form 

potassium feldspar, causing some scattering of plots. 

Modal biotite, hornblende, and plagioclase increase as 

CaO increases (although biotite uses no CaO), and their 

trends are opposite to those of quartz and microcline.
46 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 12.—Chemical and spectrographic analyses for gabbro, hornblende, pyroxenite, pyroxene-bearing hornblendite, hornblende diorite,

biotitic hornblende diorite, Boulder Creek batholith area, Front Range, Colo.

[Rapid-rock analyses by Joseph Budinsky, P. L. D. Elmore, Lowell Artis, J. L. Glenn, Gillison Chloe, Hezekiah Smith, and James Kelsey for sample 343, all other samples by Leung Mei. 

Spectrographic analysis for sample 343 by J. L. Harris; all other samples by H. W. Worthing. (-), not found]

Lab. No. ———————

QT H — —

A1203

Fe203—— —— —— ——

FeO —— —— — —— ——

MgO ———————————

fi-sCl

K n i;. __ __ —— —— _ — . ———— __

H 0+

H

n"~

Tz02

P2°5

Gabbro

336 

W179527

53.3 

17.8 

1.4 

6.4 

7.8

7.6 

3.4 

0.63 

0.77 

0.07

o.4y

0.24 

0.11 

0.01 

0.04 

0.05

100.00

Hornblende pyroxenite

343 

W179530

52.9 

5.8 

2.0 

6.3 

17.0

13.2 

0.43 

0.16 

0.40 

0.02

0.31 

0.10 

0.17 

0.08 

0.02

99.00

349 

W179528

Chemical

50.1 

5.4 

4.8 

5.6 

17.3

12.1 

0.53 

0.20 

1.1 

0.02

0.35 

0.24 

0.21 

0.08 

0.02 

0.01

99.00

Pyroxene-bearing 

hornblendite

353 

W179533

composition,

50.5 

8.8 

2.9 

5.8 

13.4

12.4 

0.73 

1.6 

0.61 

0.09

0.84 

1.10 

0.21 

0.03 

0.03 

0.24

99.00

Semiquantitative spectrographic analyses

Be —— —— —— —— ——

Co ———————————

Ga ———————————

T „ _

Pb ——————————————

Sn —— —— —— —— —— 

Sr ———————————

V _

700 

1 

70 

30 

300

70 

20

150 

5 

15

1,000 

100 

15 

1.5 

50

1,500 

1

70 

1,500

200 

7

500 

5 

70

7 

200 

150 

15 

1.5 

50

70 1 

1 

70 

70 

2,000 1

200

7

700 

5 

70

10 

100 1 

150 

20 

2 

50

,500 

1 

200 

30 

,500

20 

10 

200 

200 

5 

70

7 

,500 1 

100 

50 

5 

200

333 

W179529

in weight

50.2 

9.9 

2.9 

5.9 

13.1

13.2 

1.2 

0.79 

0.59 

0.01

0.82 

0.16 

0.16 

0.01 

0.01 

0.17

99.00

, in parts

200 

1 

100 

50 

500

50 

10 

50 

500 

5 

50

,000 

150 

30 

3 

100

Biotitic hornblende 

Hornblende diorite diorite

S351 

W179531

percent

50.0

2.8 

6.7 

7.5

10.0 

2.7 

0.56 

1.3 

0.08

0.95 

0.11 

0.15 

0.05 

0.04 

0.53

100.00

per million

150

60 

175

100 

12

135 

5 

30

300 

200 

25 

2 

40

343 

W173309

55.1 

13.4 

3.8 

6.1 

7.7

8.6 

1.9 

1.1 

0.51 

0.10

0.55 

0.09 

0.24 

0.05 

0.03 

0.13

99.00

150

50 

500

10 

10 

30 

150 

5 

70

300 

150 

30 

3 

150

352 

W179532

47.8 

14.3 

3.6 

7.0 

9.7

9.8 

2.0 

2.5 

1.1 

0.16

1.1 

0.86 

0.18 

0.05 

0.09 

0.32

100.00

500 

1 

325 

40 

300

7 

17 

125 

200 

5 

60

700 

150 

60 

6 

125

Average of two analyses.
GEOCHEMISTRY 47 

TABLE 13.—Chemical and spectrographic analyses and modes for Twin Spruce Quartz Monzonite, Front Range, Colo.

[Wet chemical analyses for samples 1 and 2 by C. L. Parker; rapid-rock analyses for samples 3, 4, and 5 by P. L. D. Elmore, Lowell Artis, Gillison Chloe, J. L. Glenn, S. D. Botts, Hezekiah

Smith, James Kelsey; spectrographic analyses for samples 1 and 2 by G. W. Sears, Jr., for samples 3-5, J. L. Harris, for samples 9-13, H. W. Worthing. N.d., not determined; <, less

than; (-), not found; Tr., trace; L, present but below limit of determination; ores include all opaque minerals unless stated otherwise]

Lab. No. ——————

2 3

MgO ——————————

MnO ——————————

Subtotal ——

Less 0 -----

Powder density —

a 232 

D101719

75.06 

12.27 

1.34 

1.71 

0.67

0.93 

2.85 

3.9 

0.34 

0.03 

0.35

0.04 

0.05 

0.05 

0.01 

0.06 

n.d.

99.66 

0.03

99.63 

2.69

a 235 

D10172D

68.91 

14.72 

1.62 

1.96 

0.84

1.6 

2.57 

5.89 

0.51 

0.02 

0.6

0.26 

0.04 

0.02 

0.02 

0.12 

n.d.

99.70 

0.05

99.65 

2.70

375 

W170101

69.4 

15.7 

0.89 

0.92 

0.38

1.5 

3.4 

6.2 

0.48 

0.04 

0.18

0.09 

0.04 

0.09 

n.d. 

n.d. 

n.d.

100.0 

n.d.

100.0 

C2.63

85 376 

W171192 W171191

71. 

14. 

0. 

1. 

0.

1. 

2. 

6. 

0. 

0. 

0.

0. 

0. 

0. 

0. 

0. 

n.

100. 

n.

100. 

n.

9 62.0 

9 16.9 

42 3.2 

2 2.7 

38 1.6

4 3.2 

3 3.0 

6 5.3 

39 0.51 

07 0.1 

19 0.96

06 0.34 

04 0.09 

05 <0.05 

001 0.02 

02 0.12 

d. n.d.

0 100.0 

d. n.d.

0 100.0 

d. n.d.

377 

W173306

72.4 

14.0 

0.34 

1.9 

0.45

0.92 

3.0 

5.3 

0.69 

0.17 

0.36

0.07 

0.07 

<0.05 

0.011 

0.06 

n.d.

100.0 

n.d.

100.0 

C 2.56

378 379 

W173307 W173308

71.5 72.4 

14.5 14.8 

0.62 0.93 

1.8 0.88 

0.53 0.38

1.9 1.7 

3.8 3.7 

4.2 4.5 

0.41 

0.03 0.1 

0.31 0.24

0.09 0.05 

0.06 0.05 

<0.05 <0.05 

0.006 0.004 

0.05 0.04 

n.d. n.d.

100.0 100.0

100.0 100.0 

C 2.56 C2.56

380 

W177080

69.4 

14.9 

2.0 

1.3 

0.54

1.7 

2.7 

5.5 

0.8 

0.07 

0.53

0.33 

0.05 

<0.05 

<0.001 

0.05 

0.02

100.0 

n.d.

100.0 

n.d.

a381 a 382 

W177081 W 177083

68.7 78.9 

16.1 9.7 

2.6 1.8 

1.1 2.5 

0.24 1.1

2.4 1.3 

4.4 1.7 

2.4 1.3 

0.66 0.84 

0.08 0.05 

0.21 0.57

0,03 0.09 

0.09 0.03 

<0.05 <0.05 

<0.001 0.012 

0.01 0.01

99.0 100.0 

n.d. n.d.

99.0 100.0 

n.d. n.d.

a 383

W177084

70.9 

13.3 

1.8 

3.3 

1.0

1.1 

2.3 

3.6 

1.1 

0.11 

0.75

0.04 

0.12 

<0.05 

0.010 

0.03

99.0 

n.d.

99.0 

n.d.

W177082

69.7 

14.2 

2.4 

2.1 

0.5

1.5 

2.5 

5.3 

0.78 

0.08 

0.56

0.12 

0.09 

<0.05 

0.010 

0.02 

0.02

100.0

n.H.

100.0 

n.d.

Spectrographic analyses, in parts per million

Ce ——————————

C

«

Ga ——————————

Mi ——————————

V ___————————

7r- - __________

1,500

<150 

5 

30

1 

30 

70

10

70 

10 

50

10

300 

30 

70 

10 

";nn

1,500

700 

5 

10

30 

30 

300

10

300 

5 

70 

<100 

5

500 

70 

50 

3 

";nn

2,000 

2 

500

3

10 

10 

150

3

30

3

300 

10 

10

1 

i ";

2,000 

1 

500

3

7 

15 

150

30

3

3 000 

20 

20 

2 

9nn

2,000 

1 

1,000 

10 

20

560 

20 

500

5

300 

L 

20

10

1,000 

70 

30 

3 

700

700 

500

7

50 

15 

150 

3 

20

50

10

200 

30 

20 

2 

7nn

700 700 

200 500

10 10

2 3 

15 15 

70 100 

3 3 

7 10

L 

30 50

7 5

200 200 

30 20 

20 20 

2 2 

•*nn i ";n

1,500

500 

5 

15

50 

20 

500

20

500 

5 

50 

50 

10

300 

70 

50 

3 

i nnn

1,000 300 

1 <1 

100 100 

3 10 

1 310

3 3 

30 20 

100 70 

5

30

15 10

5 10

300 100 

10 100 

100 50 

15 5 

7nn 9nn

1,500

150 

10 

50

3 

20 

100 

3 

7

10 

20

15

200 

70 

100 

10 

?nn

2,000

200 

5 

5

5 

20 

200 

L 

7

200 

7 

20

7

500 

30 

70 

5 

i _nnn
48 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 13.—Chemical and spectrographic analyses and modes for Twin Spruce Quartz Monzonite, Front Range, Colo.—Continued

Sample No. ———— E232

Lab. No. —————— D101719

Potassium

Plagioclase ——— 18.5

Quartz ——————— 51.6

Muscovite ————— 2. 2

Sillimanite ——— 0 . 1

Monazite — —— ——

Chlorite ——————

Calcite ------ ——

Allanite —— — ——

Rutile —————

Sphene — — —— ——

Garnet —————

Total ———— 100.0

Composition of 

plagioclase —— ^n-ii

a235 

D10172D

34.6 

27.5

27.7 

6.0 

1.9

1.2

0.4 

Tr.

0.2 

0.4

—

0.1

Tr.

—

100.0

375 

W170101

34.3 

31.5

31.3 

0.5 

0.1

0.5 

0.1

Tr. 

1.6

0.1

Tr.

—

—

100.0

85 

W171192

45.5 

21.2

27.0 

3.6 

1.5

0.4 

Tr.

—

0.3

—

0.1 

0.4

100.0

376 377 

W171191 W173306

Modes , in

31.6 31.2 

39.3 29.1

13.6 32.6 

10.3 4.4 

0.6 2.0

2.4 0.7 

0.4

Tr.

Tr. 

Tr.

__

0.1

—

1.6

100.0 100.0

378 

W173307

379 

W173308

380 

W177080

a381 E382 a383 a' b 384 

W177081 W177083 W177084 W177082

volume percent

28.4 

33.4

30.0 

6.4 

0.8

0.8 

0.1

0.1

—

—

—

—

100.0 

n.d.

34.3 

33.9

25.2 

2.9 

2.9

0.4 

Tr.

Tr.

Tr. 

0.4

—

—

—

—

—

100.0 

n.d.

35.1 

31.4

22.6 

8.4 

0.8

1.2

0.1 

Tr.

Tr. 

0.3

—

0.1

--

—

100.0

15.1 0.9 18.0 36.2 

51.8 23.4 28.3 27.4

27.9 62.5 39.3 28.1 

3.2 12.0 11.1 5.3 

0.3 1.2

1.5 1.0 0.8 1.7 

Tr. 

Tr. 0.1 — Tr.

Tr. — Tr. 

Tr. 0.1 0.1 0.1

—

Tr.

~

—

0.5 -- 2.1

100.0 100.0 100.0 100.0 -

Samples 232, 235, 381, 382, 383, 384 are from quartz monzonite in metasedimentary rocks.

Ores in separate include molybdenum, pyrite, chalcopyrite, and magnetite.

Bulk density.

Biotite and chlorite undifferentiated.

The spread in points, however, is so great that 

systematic relationships are nearly impossible to 

define except for hornblende in biotite-hornblende 

granodiorite and biotite in biotite granodiorite (fig. 24). 

Chemical differences between the granodioritic rocks 

in the northern and southern parts of the batholith, 

especially in calcium, potassium, and to a lesser degree 

sodium, are indicated by larger percentages of 

plagioclase, biotite, hornblende, and apatite in the 

northern part. In the chemical variation diagram (fig. 

23) the ratio of iron to magnesium in the northern part 

of the batholith is expressed in a differentiation trend 

that distinctly differs from the differentiation trend for 

the southern part of the batholith. The presence of 

more biotite and hornblende and less potassium 

feldspar in the northern part of the batholith reflects 

this chemical difference as indicated by the following 

modes (in percent): for the southern part of the 

batholith, potassium feldspar, 20; plagioclase, 39; 

quartz, 26; biotite, 11; hornblende, 1.4; for the northern

part of the batholith, potassium feldspar, 10; 

plagioclase, 44; quartz, 24; biotite, 15; hornblende, 2.9.

CHEMICAL TRENDS IN MAFIC INCLUSIONS

The mafic inclusions (table 11, nos. 334, 385, 386) 

are, according to their major oxides, allied to the hornblende diorite, but the inclusions have larger amounts 

of sodium and potassium and less calcium and 

magnesium than the hornblende diorite. The lamprophyre represented by samples 387 and 388 in table 

11 is chemically distinct from the mafic inclusions in 

that it has low aluminum and high magnesium, 

calcium, and potassium.

The major chemical characteristics of inclusions are 

compared with the Boulder Creek Granodiorite and 

associated rocks in a Q-Ab+An-Or variation diagram 

(fig. 22). The analyzed inclusions plot with diorite in 

the Ab+An part of the diagram. On an AFM diagram 

(fig. 23) of the same rocks the inclusions plot farther
GEOCHEMISTRY 49

105°37'30" 105°22'30"

EXPLANATION 

Twin Spruce Quartz Monzonite

0

R. 72 W. R. 71 W. 

10 KILOMETERS

APPROXIMATE SCALE

FIGURE 17.—Sample localities for rock and mineral analyses shown in tables in this report. Batholith covered by grid showing 

section, township, and range. Dots without numbers indicate samples for which modes have been made but which have no 

chemical data.
50 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

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FIGURE 18.—Abundances of minor elements for 1, hornblende 

gneiss and amphibolite, average of 20 analyses; 2, Boulder Creek 

Granodiorite (white bar, northern part of batholith, average of 15 

analyses; black bar, rest of batholith, average of 16 analyses); 3, 

inclusions in Boulder Creek Granodiorite, average of three

from the M corner and closer to the F corner of the 

diagram than do other major rock types, indicating 

less MgO and more FeO than in the Boulder Creek 

Granodiorite or Twin Spruce Quartz Monzonite. Inclusions, however, plot closer to the quartz monzonite 

trend with one inclusion plotting on the quartz monzonite trend (see fig. 27).

The trace elements in the inclusions are more nearly 

allied to hornblende diorite and the Twin Spruce 

Quartz Monzonite (fig. 18) than to the Boulder Creek 

Granodiorite. The mafic inclusions are richer in cerium, 

copper, lanthanum, nickel, niobium, neodymium, and 

zirconium than are granodiorite rocks.

CHEMICAL TRENDS IN GABBRO, 

PYROXENITE, AND HORNBLENDE DIORITE

The composition of gabbro (sample 336, table 12) is 

similar to that of hornblende diorite (samples 351, 343) 

and probably reflects a common origin for the two.

analyses excluding the lamprophyres; 4, mafic rocks, average of 

eight analyses; 5, Twin Spruce Quartz Monzonite, average of 13 

analyses; 6, biotite gneiss, average of 27 analyses. Number beside 

bar indicates length bar should have been.

Biotitic hornblende diorite (sample 352) is an altered 

hornblende diorite; potassium apparently was introduced at the time of alteration. Hornblende pyroxenite and the pyroxene-bearing hornblendite (sample 

338, table 7; and samples 348, 349, and 353, table 12) 

are chemically similar and reflect degrees of alteration 

indicated by the amount of hornblende present.

Except for copper, cobalt, chromium, nickel, and 

scandium, the trace elements in gabbro, pyroxenite, 

and diorite are very similar to those in the Boulder 

Creek Granodiorite and differ greatly from the mafic 

inclusions in the granodiorite (fig. 18). Chromium and 

nickel are especially abundant in these mafic rocks 

while all are devoid of niobium and neodymium.

CHEMICAL AND MINERALOGICAL TRENDS 

IN THE TWIN SPRUCE QUARTZ MONZONITE

In the Twin Spruce Quartz Monzonite, MgO, FeO, 

SiO2 , and Na2O generally increase as K2O and A12O3 

decrease. Most oxide relationships are graphically
CHEMICAL EQUILIBRIUM IN THE BOULDER CREEK GRANODIORITE 51

shown in: (1) a K2O-Na2O-CaO diagram where most 

quartz monzonite samples plot on the K2O side of the 

Boulder Creek Granodiorite samples and Na2O in 

quartz monzonite samples is more variable than in 

granodiorite samples (fig. 25); (2) a Q-Ab+An-Or 

diagram where quartz monzonite samples plot in the 

same quartz range as Boulder Creek Granodiorite 

samples (figs. 22, 26); and (3) an AFM diagram where 

as iron decreases MgO also decreases forming an orderly trend toward the Na2O+K2O corner of the diagram 

(fig. 27).

Chemical trends are difficult to define in the quartz 

monzonite because neither SiO2 nor CaO appear to 

have controlled oxide distribution. In a ternary plot of 

K2O-Na2O-CaO (fig. 25), Na2O was more important in 

the distribution of other oxides than it was in the 

Boulder Creek Granodiorite. The general trend indicates that as Na2O increases, CaO also increases, and 

points are widely scattered on the K2O side of the 

diagram and to the left of those for Boulder Creek 

Granodiorite.

In the Q-Ab+An-Or ternary diagram (fig. 26), Twin 

Spruce Quartz Monzonite points are scattered but do 

tend top cluster around 25-40 percent for normative 

quartz and are centered between the normative orthoclase and plagioclase fields. Normative values 

agree quite well with modal values (fig. 14) for the 

same areas. On the AFM diagram (fig. 27), all quartz 

monzonite samples plot close to a curved trend line 

that is parallel to that of the Boulder Creek 

Granodiorite in figure 23, especially the trend line of 

samples from the Boulder and Gold Hill quadrangles 

in the northern part of the batholith. The quartz monzonite samples, however, plot closer to the 

FeO+Fe2O3 +MnO side, reflecting the low MgO content of quartz monzonite.

Plagioclase having an anorthite content averaging 

21-22 percent is the only major mineral in quartz monzonite that plots in a trend that is characteristic of a 

differentiated intrusion. Plagioclase generally increases as CaO increases (fig. 28).

In Twin Spruce Quartz Monzonite, minor elements, 

especially barium, cerium, lanthanum, strontium, and 

zirconium, are surprisingly abundant. In addition, 

quartz monzonite lenses in schist and gneiss also appear to have high amounts of chromium, yttrium, and 

nickel (table 13). Analyses indicate that quartz monzonite is poorer in the trace elements beryllium, cobalt, 

chromium, and strontium than the Boulder Creek 

Granodiorite, but quartz monzonite is richer in barium, 

lanthanum, lead, and zirconium. Quartz monzonite and 

the mafic inclusions in granodiorite have similar 

amounts of trace elements (fig. 18), but quartz monzonite contains more copper, vanadium, and strontium, and, to a lesser extent, more chromium. Lead

does not occur in the mafic inclusions but does in 

quartz monzonite.

CHEMICAL EQUILIBRIUM IN THE 

BOULDER CREEK GRANODIORITE

On a gross scale the major minerals hornblende, 

biotite, potassium feldspar, and perhaps plagioclase 

are in imperfect chemical equilibrium with one 

another, as indicated by similar mineral compositions 

in samples throughout the batholith (see mineral 

descriptions in the section on "Mineralogy, petrology, 

and chemistry of minerals in the batholith"). On a 

microscale, however, inequilibrium apparently exists 

locally between most major minerals, because some 

plagioclase is normally zoned, myrmekite appears between plagioclase and potassium feldspar crystals, and 

hornblende has bleached contacts and is corroded and 

replaced by biotite and iron oxides. Where altered, 

biotite contains irregular crystals of allanite that commonly are rimmed by epidote; also, chlorite, sphene, 

and tiny red-brown blebs (probably hematite) occur 

along cleavage and grain boundaries. Hornblende, 

biotite, allanite, and plagioclase have been locally 

replaced by some epidote. Muscovite forms rims on 

magnetite, occurs in veins, or is associated with 

sericite in replacing plagioclase. Sparse muscovite occurs in hornblende-bearing granodiorite, even where 

sheared or foliated and considerably altered; under the 

same circumstances, biotite-bearing granodiorite may 

contain substantial muscovite. As suggested by Kretz 

(1959, p. 374), primary muscovite does not occur with 

hornblende; the only muscovite present occurs as an 

alteration mineral in biotite or plagioclase, and in 

quartz. In the recrystallized border zones of the 

batholith muscovite in non-hornblende-bearing 

granodiorite has cross-cutting characteristics of a 

primary mineral. Most indications of inequilibrium 

found between minerals, however, are believed to be 

related to retrograde and, locally, later hydrothermal 

metamorphism.

Gable and Smith (1975) determined manganese, iron, 

and magnesium contents of hornblende and biotite 

while making a study of hornblende coexisting with 

biotite in a part of the Boulder Creek batholith. The 

amount of iron in both biotite and hornblende was 

nearly identical in the batholith and mafic inclusion 

samples, as shown in table 14. Manganese in both 

minerals, expressed as Mn/Fe+2 +Mg+Mn+Ti, is in 

equilibrium as shown in figure 29. Magnesium in 

biotite and hornblende has a straight-line relationship 

to ferric iron; as magnesium increases, ferric iron 

decreases. Alkali feldspar from the same samples also
52 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

* t

o

North part of batholith 

Gold Hill trend

Tungsten-EldoradoSprings trend °

South part of batholith

!=? 1

55 60 65

SiO2

70 75

o" 18

50 55

0 •

* * !

o*" .:

4 0 •«

60 65 70

SiO,

75

FIGURE 19 (above and right).—Variation diagrams of common oxides plotted against SiO2 for Boulder Creek Granodiorite (in weight

percent).

has a range in composition of Ab, An, and Or that is 

very similar (see table 22). Ab, An, and Or contents for 

alkali feldspar from the Twin Spruce Quartz Monzonite, on the other hand, are quite variable (see table 

24). Judged by the limited data available (see tables 25 

and 26), sphene and allanite do not vary noticeably in 

composition in the Boulder Creek Granodiorite but do 

vary between it and associated rock types. Plagioclase 

is the single mineral, according to our data, that 

changes in composition zonally across the batholith 

(fig. 6G) from oligoclase in the central part of the

batholith to andesine along its contacts with 

metasedimentary rocks to labradorite in plutons of 

granodiorite in the metasedimentary rocks west of the 

batholith.

ASSIMILATION AND DIFFERENTIATION 

IN THE BOULDER CREEK GRANODIORITE

Geochemical data, as presented, indicates that differentiation and not large-scale assimilation of country 

rock was responsible for producing the Boulder Creek
ASSIMILATION AND DIFFERENTIATION 53

10

o o 

*•

0 0 

* 0

EXPLANATION

oGold Hill

• Boulder

* Tungsten

o Eldorado Springs

x Satellitic plutons

50 60 70

SiO2

80

Granodiorite. Despite variations within the 

granodiorite, apparent even within a single outcrop, 

definite chemical trends occur, indicating considerable

orderliness in the distribution of the major oxides 

within the batholithic rocks.

In an attempt to determine whether assimilation or 

contamination by country rock played a major role in 

the makeup of the batholith, samples of metasedimentary rock west of the batholith, and granodiorite at the 

contact, within the batholith, and across the batholith 

were collected and chemically analyzed. A summation 

of these data is given in table 15. The biotitesillimanite gneiss and schist samples (col. 1, table 15), 

typical of the gneiss and schist west of the batholith, 

were taken at some distance from the batholith. The 

granodiorite samples listed in column 2 were taken 

from 100 to 1,000 m from the contact. Columns 3 and 4 

are averaged analyses for granodiorite samples 

grouped into biotite-bearing and biotite-hornblendebearing granodiorite from the central part of the 

batholith. Columns 5 through 7 are averaged 

granodiorite samples that are grouped to facilitate 

comparison of compositional variations within the 

batholith.

From the data in table 15 it would seem that the role 

assimilation played in the generation of Boulder Creek 

Granodiorite in the batholith is mainly confined to 

local border phases. Sillimanite-biotite gneiss, the 

country rock to the west of the batholith, is typically 

impoverished in both CaO and Na2O with respect to 

rocks of the batholith. Both oxides occur in amounts of 

less than 1 percent in the gneiss and schist country 

rock and contrast sharply with amounts of these oxides in granodiorite from the contact and the batholith 

proper (table 15). Also, the contact zone of the 

batholith has less K2O than the main part of the 

batholith or the average sillimanite-biotite gneiss 

country rock. Whereas both FeO and MgO are higher 

in the contact zone than in the rest of the batholith, 

only FeO is considerably higher in comparison with the 

rest of the batholith. In the satellitic plutons on the 

west (col. 7, table 15) FeO is high as in the contact 

zone, but MgO is low. A transfer of iron to the 

batholith from the contact rocks cannot be ruled out, 

but the data in figure 6 suggest that the high amounts 

of iron may be due in large part to an early, more mafic 

magma. Granodiorite from the contact zone along the 

west side of the batholith in the Tungsten quadrangle 

(col. 2, table 15) is quite different from averaged 

granodiorite samples from the Tungsten and Eldorado 

Springs quadrangles in the south-central part of the 

batholith (col. 5) and the Boulder quadrangle samples 

from the north-central part of the batholith (col. 6b).

This lack of large-scale assimilation may be explained if, as suggested by Vance (1961), crystallization of a granite pluton begins from the border against
54 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

» ELDORADO SPRINGS

South part of batholith

part of batholith

TUNGSTEN

4 

CaO

1.0

0.8

0.6

0.4

0.2

* / 

X

/

/ Tungsten-Eldorado Springs trend 

/

/o 

I— /oo

EXPLANATION 

A Ward 

O Gold Hill

• Boulder

* Tungsten

0 Eldorado Springs

CaO

1.0

0.8

0.6

0.4

0.2 hNorth part of batholith

0 * South part of batholith

CaO

FIGURE 20.—Variation diagrams of CaO plotted against other oxides
ASSIMILATION AND DIFFERENTIATION

6

55

Tungsten-Eldorado Springs trend

/o ^^^< Boulder _J 

*/^ ^*s* trend |

CaO

0.40

0.30

0.20

0.10

•0

Boulder-Gold Hill trend

4 6 

CaO

CaO

Tungsten-Eldorado / 

Springs trend ^ ,&

• North part of batholith

CaO

and fluorine for Boulder Creek Granodiorite (in weight percent).
56

K2 0

BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

Na,0

A »o*~

EXPLANATION 

O Gold Hill 

» Tungsten 

0 Eldorado Sprin 

• Boulder

FIGURE 21.—Ternary diagrams of K2O-Na2O-CaO chemical variation of Boulder Creek Granodiorite (recalculated to 100 percent). Dashed 

outline enclosed total area covered by Boulder-Gold Hill area; shaded area represents total area analyses cover for Tungsten and 

Eldorado Springs; A, trend for Boulder-Gold Hill analyses. B, trend for Tungsten and Eldorado Springs analyses.

Ab+An

Tungsten-Eldorado Springs EXPLANATION

Ward

Gold Hill

Tungsten

Eldorado Springs

Boulder

Inclusion

Diorite

FIGURE 22.—Ternary diagrams of chemical variation of Boulder Creek Granodiorite expressed in terms of normative Q-Ab+An-Or

(recalculated to 100 percent).

cooler country rock that tends to seal in released 

volatiles. Accordingly, other elements will be sealed in 

and out preventing an interchange of elements between country rock and the batholith magma. In the 

Boulder Creek batholith, there may have been some 

movement of oxides into the country rock from the
ASSIMILATION AND DIFFERENTIATION 57

batholith because migmatite and pegmatite are more 

prevalent near the batholith contact and appear to be 

related to the Boulder Creek Granodiorite, indicating 

that the magma, at least in its later stages, was probably saturated in H2O that partly escaped into the 

country rock. Except for water, mobilization and 

recrystallization in the Boulder Creek Granodiorite 

along the contact may be only slightly related to 

chemical exchanges such as iron between the batholith 

and country rock. Assimilation of quartz from the 

quartzite unit (pi. 2) in the southeastern part of the 

batholith is possible and was likely caused by 

cataclasis and recrystallization due to proximity to the 

large Idaho Springs-Ralston shear zone. This assimilation of quartz by the granodiorite is related to 

retrograde metamorphism rather than to processes in 

the batholith itself.

Differentiation trends in figures 19, 20, 21, and 23 

for the Boulder Creek Granodiorite, based on majoroxide chemistry, are typical for a calc-alkaline series 

and suggest that the analyzed rocks are comagmatic. 

Mineralogical data, especially for biotite and hornblende, in the section on "Mineralogy, petrology, and 

chemistry of minerals in the batholith," support this

assumption. Fractionation trends in the Boulder Creek 

Granodiorite, expressed by fractionation curves (Tuttle and Bowen, 1958) and phase relationships (Winkler 

and others, 1975) of crystallizing granitic and 

granodioritic melts in the system SiO2-NaAlSi3O8- 

KAlSi3O8-CaAl2Si2O8-H2O are analyzed below. Both 

fractionation trends and phase relationships are important in the interpretation of the origin of the Boulder 

Creek Granodiorite.

Comparisons of Boulder Creek Granodiorite with 

Tuttle and Bowen's (1958) fractionation curves that 

are based on a rock containing 80 percent or more AbOr-Q (albite-orthoclase-quartz) are risky, especially 

because Boulder Creek samples have a fairly large An 

content and Ab-Or-Q totals less than 80 percent. 

However, representative Boulder Creek samples from 

the interior of the batholith concentrate in Bowen's 

(1937) thermal valley in the area represented by the 

lower phase boundary (fig. 30A). Here the two 

feldspars, plagioclase and potassium feldspar, are 

dominant, but because of the high-An content in 

plagioclase, plots occur predominantly in the 

plagioclase field rather than in the orthoclase field of 

granite. Analyzed samples from the Ward quadrangle 

and from the contact areas of the Tungsten, Boulder,

FeO+Fe203+MnO 

(F)

^Tungsten-Eldorado

Springs trend,

south part

of batholith

EXPLANATION 

0 Eldorado Springs 

» Tungsten

* Boulder

X Inclusions

O Gold Hill

A Ward

O Hornblende diorite

* Hornblendite 

A Pyroxenite

* Gabbro

(M) 

MgO

FIGURE 23.—Mole precent variation in an AFM diagram for Boulder Creek Granodiorite and the more mafic rocks associated with the

granodiorite.
58 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

dU

oc

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(/) 20

o

_j

QJ

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<£ 10

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60

50

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5 40

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30

20

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20

QJ

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00

10

0

9

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»

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^

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»

»

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0

0

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I ———————— I ———————— I — , —— , ———— L. ——————— L_o —

I "

0

— • • • ~~

4 ^

*

» 0

^

0 »

0 •

-

I

4

- _

o

» »0

— * —

4*

Biotite granodiorite

JU

20

i i i

^~

P

0

00

10

0

40

30

NJ ^—

oc 

^20

O

10

0 

30

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Z

QJ

00

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I

»,

__ 4 »

» 0 »

O t •

o • »

_ • • • o _

0

» o

0

"

I

I I I I

EXPLANATION

o Gold Hill

• Boulder

» Tungsten 

~ o Eldorado Springs ~~

.

— —

4

• • o

Biotite-hornblende granodiorite ?

* 4 • °

• * *

« 0 »

23 45 6234 56

CaO CaO

FIGURE 24.—CaO content (in weight percent) plotted against weight percent of rock-forming minerals (weight percent calculated from

modal percent) in the Boulder Creek Granodiorite.
ASSIMILATION AND DIFFERENTIATION 59

Na 2 0

K 2 0

/Boulder Creek < — — • 

\ Granodiorite ^

° ~~^

CaO

FIGURE 25.—Ternary plot of the oxides K2O-Na2O-CaO showing 

chemical variation for Twin Spruce Quartz Monzonite (open 

circle). Dashed outline encloses field occupied by Boulder Creek 

Granodiorite samples.

and Gold Hill quadrangles, from within the batholith, 

do not fall in the thermal valley, but most do define differentiation trends (figs. 19 and 20). That the older contact samples fall on the Ab side, or plagioclase side, of 

figure 30A, and outside the thermal valley, is to be expected in samples that contain less K2O than CaO. 

Contact rocks (table 14) are in this category, and, as indicated by Chayes (1952, p. 243), "crystallization of a 

liquid whose initial composition lies in the diagram 

[Tuttle and Bowen's equilibrium diagram] generates a 

liquid residue which approaches and finally enters the 

thermal valley; and unless heat is added, the liquid 

cannot escape the valley, once it has entered. The composition of adequate samples of such a crystallizing 

mass would necessarily fall either in the valley or outside it on the side nearest the initial composition."

Ab+An

EXPLANATION 

oNederland 

* Tungsten 

0 Eldorado Springs

V__v \/

FIGURE 26.—Ternary plot of normative Q-Ab+An-Or showing 

chemical variation for Twin Spruce Quartz Monzonite 

(recalculated to 100 percent).

FeO,

EXPLANATION 

0 Eldorado Springs 

» Tungsten 

• Nederland 

o Inclusion 

V V V V

Na 2 0+K2 0 MgO

FIGURE 27.—AFM diagram showing molar variation for Twin Spruce 

Quartz Monzonite (in weight percent). Dashed lines are trends for 

Boulder Creek Granodiorite from figure 23; trend A represents 

the southern part of the batholith, trend B represents the northern part of the batholith.

New research on low-temperature granitic melts has 

been carried out in the system SiO2-NaAlSi3O8- 

KAlSi3O8-CaAl2Si2O8-H2O (Winkler and others, 1975; 

Winkler, 1976). Winkler, Boese, and Marcopoulos

o 

o

50 -

40

30 -

20 -

10

1.0 2.0 

CaO

3.0 4.0

FIGURE 28.—Sample variation of plagioclase (calculated in weight 

percent from the mode) in relation to CaO (weight percent) in 

Twin Spruce Quartz Monzonite. Straight line is statistical trend 

of data.
60 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 14.— Total iron expressed as FeO (in percent) in biotite and 

hornblende, Boulder Creek batholith area, Front Range, Colo.

Hornblende --— -

Boulder Creek 

Granodiorite

41 48 50

17.3 17.0 17.1 

17.0 17.1 17.2

Mafic 

inclusions

385 386

17.0 16.8 

17.6 16.8

Hornblende 

diorite

352

14.6 

15.7

(1975) have done considerable research on phase relations in the system Q-Ab-Or-An-H2O, especially on 

low-temperature granitic melts, and they believe from 

their data that two fundamentally different processes 

of granitic magma generation—magma tic or ana tectic—can be recognized, although Winkler (1976) now 

believes all granitic-granodiorite rocks were derived by 

anatexis. Tuttle and Bowen's research (1958) ignored 

the An content of feldspar. Winkler, Boese, and Marcopoulos (1975), took into consideration the An con0.18

c/)

1

LLJ

Q•z.

LLJ

_J 

CO•z.

DC 

O

I

0.16 —

0.14

0.12

0.10

0.08

0.06

48

50 D

,386

352,

0.02 0.04 0.06 0.08

Mn/Fe+2+Mg + Mn+Ti IN BIOTITE (ATOMS)

0.10

FIGURE 29.—Distribution of manganese in biotite and hornblende 

in Boulder Creek Granodiorite and hornblende diorite. Square, 

Boulder Creek Granodiorite; solid triangle, hornblende diorite 

from Gable and Smith (1975, table 9); square with dot, mafic inclusion (this report); straight line is statistical trend of data (table 

14).

tent of the feldspar, and, by making certain assumptions, presented a Q-Ab-Or diagram (fig. 305) in which 

the An content in weight percent is shown as a radial 

projection onto the Q-Ab-Or surface.

Nine samples of Boulder Creek Granodiorite, where 

data are complete, have been plotted on the 500 MPa 

(mega pascals) phase diagram presented by Winkler, 

(1976) (fig. 305). In application of the system to 

natural rocks, Winkler, Boese, and Marcopoulos 

(1975), and Winkler (1976) ignored the potassium in 

biotite and the calcium in hornblende because, according to their experiments, it did not affect the 

overall conclusions concerning the crystallization or 

melting history of plagioclase, alkali feldspar, or 

quartz. The data, as plotted in figure 305, are from 

modes, An was determined on plagioclase in thin section, and the alkali-feldspar minerals were chemically 

analyzed. Normative data derived from whole-rock 

chemical analyses indicate that plagioclase seldom 

contains more than a small amount of potassium so 

that points as plotted would only shift slightly to the 

Or side of the diagram, and the An content would 

decrease only slightly. In applying the 500 MPa water 

pressure Q-Ab-Or diagram to Boulder Creek rocks it 

must be remembered that Winkler (1976) found that 

the compositions determined at 500 MPa water 

pressure were also a good approximation of conditions 

at somewhat lower (300-400 MPa) and higher (700 

MPa) pressures. Temperatures at 300-400 MPa 

pressure were about 5°-10°C higher and the An composition was 1 or 2 percent lower; the opposite was true 

at 700 MPa pressure. Boulder Creek Granodiorite 

plots in the plagioclase-quartz field of the Q-Ab-Or 

diagram (fig. 305) and pierces the cotectic surface 

quartz+plagioclase+liquid+vapor with a somewhat 

higher An content than that shown on the isotherms. 

For larger departures from the cotectic plane, as in 

Boulder Creek rocks, there is a marked increase in 

temperature. Most Boulder Creek plots lie between 

660° and 685 °C (as projected onto the isotherms) 

where the An on the isotherms is between 4 and 6 percent. However, as all but one Boulder Creek sample 

has more than 10 percent An above the An indicated 

on the nearest isotherm, nearly all crystallized at 

higher temperatures than the 660°-685°C indicated. 

From the experiments of Winkler, Boese, and Marcopoulos (1975, fig. 11, p. 264) the temperature for 

crystallization of Boulder Creek plagioclase appears to 

be in the range of or above 750 °C at 500 MPa water 

pressure. This temperature suggests that plagioclase 

in the Boulder Creek batholith is of magmatic origin 

and it tends to support the data in figure 3QA wherein 

most samples lie in Bowen's thermal valley. From 

figure 305, the stages of crystallization for 

plagioclase, alkali feldspar, and quartz appear to be:
ASSIMILATION AND DIFFERENTIATION 61

.20O

oT

bo

53 5eefc.

•SF 'C

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Q> **O

•oo

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oo

tsC

53

oo

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bo

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53 •!

ooOeo

oo 'C

53&s0

CJ1.

I— 1 W

CO

r TM

r^

vO

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CM

rH1111|

f£g

rHO

CJ

rj

4-1

•HO^

PP

r;

4JO

r;

4J

3 O

C/l

cu

CJ 4-1

•H -H

4-1 CO J-I

•H ti O 

H O -H

f~H 4-J *X3

CU 3 0

4-> H C

td P- cd

Cfl J-I

WO

CU

CU 4-1

J-I rH -rl 

CU &0 J-i

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PP fd C

3 td

C7* J-I

00

cu

rH CU 4-1

T-H T-H *rl

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crj td 'H 

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rH T3 0

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00

C CU

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J-l T-H -rl

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OCMT-HLOOOr>- OOrHrHT-HOH

T-HCTiCMvOCMO O<-OrHT-HO

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1 1

1 1

1 1

1 1

1 1

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on on ii

CM O O 1 O + CM LO

o CNJCMOOO CNJOOOOO

•HrHCUCUbOtd td CMC CNJ-H CM c/i<3ii<&4Su jgfc^SfflE-ipH

r^

oo r->

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vO

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rH

LO

^D r^^* 0

rH0

0 0

0 rH

T~H

r^-

a\ o

O\ T-H o

O vD

O T-H

rH

a\

o\ o

O\ rH

c^

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T-H T3 CU cd cu co

4-1 t>0 S*>

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CU C

<3
62 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

first, plagioclase, then plagioclase+quartz, and then 

plagioclase+quartz+alkali feldspar; this is essentially 

the paragenetic sequence observed in thin section.

Fractionation trends based on strontium and 

rubidium in granodiorite rocks are inconclusive. Fractionation as indicated by a decrease in strontium 

values as normative Or/Ab increases does not exist in 

the Boulder Creek Granodiorite (table 16, fig. 31), but 

because strontium is much more mobile than the major 

mineral-forming elements, this reversal of trend could 

be expected. Rubidium trends are normal; as normative Or/Ab increases so does rubidium, but this increase could also have occurred during alkali exchange 

in which rubidium accompanied potassium in the 

feldspar.

ORIGIN OF THE BOULDER CREEK

GRANODIORITE AND THE 

TWIN SPRUCE QUARTZ MONZONITE

Data from the Boulder Creek Granodiorite are consistent with the theory of a calc-alkaline magma 

derivation from the mantle or lower crust, as suggested by modeling and experimental studies of 

Cawthorn and Brown (1976) and Green and Ringwood 

(1968) for the genesis of calc-alkaline magmas. The 

Twin Spruce Quartz Monzonite probably is of crustal 

origin, but the magma was greatly contaminated 

either by a deep-seated source or by country rock, as 

suggested by the lack of differentiation trends. The 

mafic inclusions in the Boulder Creek Granodiorite 

could perhaps have been refractory material that remained after partial fusion of mantle or lower crust 

and that was reconstituted as it was carried upward as 

part of the magma, as suggested by Presnall and 

Bateman (1973, p. 3197) in the Sierra Nevada 

batholith. Trace elements in the Twin Spruce Quartz 

Monzonite and in the inclusion are similar. Data suggest that the refractory material was injected into the 

Boulder Creek Granodiorite during the mush stage and 

before the batholith was completely crystallized.

Evidence supporting the above conclusions, especially for the Boulder Creek Granodiorite, is as follows:

1. Analyses from the major part of the batholith (in 

the Boulder, Eldorado Springs, and Tungsten 

quadrangles) plot in Bowen's (1937) lowtemperature thermal valley for granites, suggesting fractional crystallization in a system

FIGURE 30.—Phase relations for Boulder Creek Granodiorite in the 

system SiO2 -NaAlSi3O8-KAlSi3O8-CaAl2Si2O8-H2O, where modes 

and chemical and optical data are available. A, Ternary diagram 

of normative Q, Ab, and Or in Boulder Creek Granodiorite from 

the Boulder Creek batholith and adjacent plutons. Dotted area, 

position of thermal valley (Bowen, 1937, Chayes, 1952); dashed 

lines represent isotherms, in °C, on liquidus in the system albiteorthoclase-quartz at PR Q =2,000 kg/cm2 (Tuttle and Bowen, 

1958, p. 55). Open circle, analyzed samples from main part of the 

batholith; solid dot, analyzed samples from the batholith contact; 

solid square, analyzed samples from plutons in metasedimentary 

rocks. B, Isobaric system, Q-Ab-Or-An-H2O with excess H2O-rich 

vapor at PH o=5 Kb (500 MPa or megapascals) for lowtemperature granitic melts from Winkler (1976). Projection of 

cotectic line P,-P2 and isotherms, in °C, (showing weight percent 

An where it pierces isotherms), on cotectic surfaces: 

quartz+plagioclase+liquid+vapor, plagioclase+alkali feldspar + liquid + vapor. Figures in parentheses adjacent to solid dot, 

calculated weight percent An (this report). Open circle, location of 

experimentally determined temperature.
ORIGIN, BOULDER CREEK GRANODIORITE, TWIN SPRUCE QUARTZ MONZONITE 63

TABLE 16.—Rubidium, strontium, and potassium analyses (inppm) of Boulder Creek Granodiorite samples, Front Range, Colo.

[Rb analyses by flame emission spectrography; Sr analyses by atomic-absorption spectrography. Analysts: L. P. Greenland, Roosevelt Moore, and M. M. Schnepfe; K values from table 10]

Sample No. ----- ——

Lab. No. ———————

•pi.

Sr

K _ _ _ _ _ _

K/Rb —— —— —— —— —

Rb/Sr ——————————

——— S129

——— D173346W

——— 170

——— 300

——— 31,000

——— 182

——— 0.57

132

D173347W

120

c on

22,000

183

0.20

138

D173348W

74

710

18,000

243

0.10

140

D173349W

95 

670

13,000

137

0.14

148

D173350W

120

690

32,000

267

0.17

150

D173351W

150

630

oo nnn

220

0.24

156

ID173352W

120

con

27,000

225

0.19

162

D173353W

92

510

21,000

228

0.18

167

D173354W

QO

550

24,000

245

0.18

From pluton in metasedimentary rocks northwest of batholith.

similar to that used by Tuttle and Bowen (1958), 

Na2AlSi3O3-KAlSi3O8-SiO2-H2O (fig. 3(L4). In the 

experimental ternary phase diagram of Winkler 

(1976) and Winkler, Boese, and Marcopoulos 

(1975) (fig. 305) CaAl2Si2O8 is also considered in 

the system, and the sequence in the crystallization 

history of plagioclase in the Boulder Creek 

Granodiorite appears to have begun with high 

temperatures, thus further substantiating fractional crystallization.

2. In their model for the formation of corundumnormative calc-alkaline magmas, Cawthorn and

800

700

600

500

0.2 0.4 0.6 0.8 

NORMATIVE Or/Ab

1.0

300

200

100

FIGURE 31.—Strontium (circle), rubidium (solid dot), and potassium feldspar analyses (in ppm) of Boulder Creek Granodiorite 

samples from the northern part of the batholith; data from table 

16. Lines represent median of values as plotted.

Brown (1976) showed a trend in the calc-alkaline 

suite from diopside normative to corundum normative as SiO2 increases. Whereas the magmas 

they used in their model frequently contained 

almandine-spessartine garnet, garnet of 

almandine-spessartine composition is known from 

only one locality in the Boulder Creek 

Granodiorite, and that is a pegmatite, an association not unlike that described by them (1976, p. 

474). A most interesting relationship described by 

them can be related to the Boulder Creek 

Granodiorite as well and that is the indication that 

the formation and crystallization of corundumnormative calc-alkaline magmas through amphibole fractionation may be explained by the 

crystallization of hornblende in a hydrous magma. 

They interpret the crystallization trend from 

diopside-normative to corundum-normative rocks 

as the result of fractionation (crystallization or 

melting) of hornblende from a diopside-normative 

basic magma; normative-corundum trends depend 

in part on the Na/(Na+K) ratio of the magma. 

Both the Boulder Creek Granodiorite rocks and 

the Sierra Nevada batholithic rocks (Bateman and 

others, 1963) became corundum-normative at 

about 60 percent SiO2 .

3. Experimentally determined fractionation trends 

in an AFM plot for a calc-alkaline province at 

2,700-3,600 MPa under dry conditions determined by Green and Ringwood (1968, p. 153) are 

similar to the one determined in this report for 

Boulder Creek Granodiorite (fig. 23). In both plots 

there is no marked iron enrichment relative to 

magnesium in the more basic compositions. Alternatively, according to Green and Ringwood, the 

same fractionation trend may be obtained experimentally at 900-1,000 MPa under wet conditions (PH2O<P£°od). Calculated liquid fractionates 

at 900-1,000 MPa follow the same calc-alkaline
64 BOULDER CREEK GRANODIORITE, FRONT RANGE, COLORADO

trends whereby partial melting of amphibolite at 

lower crustal depths under wet conditions yields 

rocks of the calc-alkaline series.

4. Initial Sr87/Sr86 ratios of 0.7034±0.0010 for the 

Boulder Creek Granodiorite (Peterman and others, 

1968) fall in the oceanic volcanic field. Such 

moderately low initial strontium-isotope ratios 

could be derived from short-lived crustal material 

or from the mantle. Green and Ringwood (1968, p. 

156) indicated that there is increasing evidence 

that initial Sr87/Sr86 ratios found in the calcalkaline series characteristically fall in the range 

of about 0.704-0.708 owing to derivation from 

basaltic material having low initial Sr87/Sr86 ratios. 

Melting of old crystalline or miogeosynclinal 

sedimentary rocks would produce higher strontium ratios and greater variability than possessed 

by Boulder Creek rocks. Thus, an origin of upper 

mantle or lower crust seems plausible for the 

Boulder Creek Granodiorite, especially in relation 

to data presented here.

The Twin Spruce Quartz Monzonite in many 

respects appears to be a later differentiate of the 

magma that produced the Boulder Creek Granodiorite, 

but additional data may be needed to confirm this 

opinion. In figure 25, variations in K2O, Na2O, and CaO 

suggest a differentiation trend; also, AFM diagrams 

(figs. 23 and 27) show that iron-magnesium trends are 

complementary for both rock types. In biotite analyses 

from both rock types (see fig. 34), the major oxides 

MgO, FeO, A12O3 , and SiO2 show definite compositional relationships. For example, all Boulder Creek 

Granodiorite biotite analyses contain greater than 12 

percent MgO, and Twin Spruce Quartz Monzonite 

biotite analyses contain less than 12 percent MgO. 

Boulder Creek Granodiorite biotite has more MgO and 

SiO2 and less A12O3 and FeO than Twin Spruce Quartz 

Monzonite. K2O, however, is just as variable in one 

rock type as in the other. Variation diagrams for Twin 

Spruce Quartz Monzonite appear to be meaningless 

because of contamination; SiO2 appears to have exerted no control over the other oxides and CaO very little control. Plagioclase, however, has a definite trend 

relative to the CaO content in rock (fig. 28); calcium increases directly as weight percent of plagioclase increases. Other than the plagioclase and potassium 

feldspar, no major calcium-bearing mineral occurs in 

the Twin Spruce Quartz Monzonite; thus, as in the 

Boulder Creek Granodiorite, crystallization was 

dominated by plagioclase; potassic feldspar crystallized later for both rock types. It may be that the Twin 

Spruce Quartz Monzonite is a differentiate of the 

Boulder Creek Granodiorite magma, but the variation 

indicated by oxides in relation to SiO2 and calcium suggests that the magma was contaminated. In some instances contamination appears local in origin, whereas 

in other instances the contaminants are of ambiguous 

origin and may be from a deep-seated source.

MINERALOGY, PETROLOGY, AND CHEMISTRY 

OF MINERALS IN THE BATHOLITH

BIOTITE

BOULDER CREEK GRANODIORITE

General description. Biotite, along with hornblende 

and, locally, plagioclase and quartz, defines the foliation in the Boulder Creek batholith. Biotite in 

recrystallized contact rocks, unlike in the central part 

of the batholith where it clusters with the other mafic 

and accessory minerals, may form stringers and thin 

wispy layers with the mafic and accessory minerals. 

Biotite in the batholith proper averages slightly 

greater than 15 percent; the west contact and sheared 

and recrystallized areas adjacent to faults average 22 

percent. In granodiorite, potassium feldspar decreases 

as biotite increases (fig. 32 and table 17).

Biotite from granodiorite lenses in schist and gneiss 

country rock is grayish red brown in color, always ragged, and commonly oxidized to an orangish color along 

grain boundaries and cleavage. At the contact in 

gneiss and schist, biotites are moderate reddish brown 

and within the batholith are a dark reddish brown or 

dark yellowish brown. Biotite cuts and embays hornblende but also appears frayed, ragged, and sometimes 

zoned. Inclusions consist of ores, monazite and (or) 

xenotime, and allanite. Biotite typically alters to 

chlorite, iron oxides, epidote, sphene, and allanite. In 

the northern part of the batholith biotite is part of a 

reaction in which hydrogarnets formed locally.

Chemistry. In a very general way biotite can be 

related to the bulk chemistry of the rock. Modal biotite 

increases as CaO in the host rock increases. The CaO, 

however, is localized in the plagioclase and (or) hornblende and as hornblende increases in the rock, biotite 

decreases. Also, biotite decreases as SiO2 increases 

(fig. 33), but there are many exceptions and the trend is 

quite general.

Major elements were determined by standard wet 

chemical analyses described by Peck (1964) and structural formulas were calculated (tables 18 and 19) on the 

basis of 24 (O, OH, Cl, F) atoms to the general mica formula X2Y4 _6Z8O20 (OH, F, Cl)4 , using a computer program described by Jackson, Stevens, and Bowen 

(1967). Totals for the octahedral sites are 5.67-5.85 out 

of a possible 6 atoms per formula unit and the large cations (Ca, Na, K) in the X group range from 1.88 to
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 65

30 i———

25

20

15

10

A o

EXPLANATION 

A Border granodiorite 

o Granodiorite adjacent to fault 

• Batholith interior granodiorite

Batholith border Batholith interior

20 25 30 

POTASSIUM FELDSPAR (MODAL PERCENT)

35 40 45

FIGURE 32.—Modal biotite plotted against potassium feldspar for the border and the interior of the batholith. No linear distance

across batholith indicated. Curve represents statistical trend of data.

1.99, fairly close to the ideal 2.00. Chemical variations 

between granodiorite biotites are quite similar if sample 392, a biotite from the satellitic Pisgah pluton, is 

excluded. The Pisgah biotite is higher in FeO and lower 

in MgO than batholithic biotites. Chemical trends in 

biotite are rather inconsistent due to the presence of 

hornblende in many samples. Trends for Na2O in 

biotite relate in a general way to the anorthite content 

of plagioclase coexisting with biotite, but the presence 

of hornblende seems to interfere in the relationship 

because it too is a calcium and sodium-bearing mineral.

TABLE 17.—Mode summary (volume percent) of major minerals in 

the Boulder Creek batholith, Front Range, Colo.

Contact 

area

Potassium feldspar -—

Plagioclase — —— — ——

Biotite —————— - —— -

Hornblende —— —— —— —

1

47 

23 

21

2

Adjacent 

to 

faults

2

51 

23 

19

2

Entire 

batholith

14

42 

25 

15

2

Batholith 

south of 

lat 40 N.

18

40 

26 

14

2

Batholith 

north of 

lat 40 N.

11

46 

24 

15

3

50 60 70 80

SiO2 (WEIGHT PERCENT) IN ROCK

EXPLANATION

• Twin Spruce Quartz ° Boulder Creek 

Monzonite Granodiorite

FIGURE 33.—Percent biotite in Boulder Creek Granodiorite and Twin Spruce Quartz Monzonite in relation to percent SiO2 in rock. Curve drawn on median of plots.
TABLE 18.—Chemical and spectrographic analyses and mineral formula for biotite from Boulder Creek Granodiorite, hornblende 

ic inclusions, and a lamprophyre dike, Boulder Creek batholith area, Front Range, Colo.

[Sampled areas located on figure 17. (--), not looked for; N, not detected; L, detected but below the limit of determination; N.d., not determined. Standard rock analyses b

cept sample 385 by E. L. Brandt. Spectrographic analyses by L. A. Bradley except sample 385 by Ray Havens. Results are based on their identity with geometric brack

aries are 1.2, 0.83, 0.56, 0.38, 0.26, 0.18, 0.12, and so forth, and are reported arbitrarily as midpoints of these brackets: 1, 0.7, 0.5, 0.3, 0.2, 0.15, and 0.1, respective

reported value is approximately plus or minus one bracket at 68-percent confidence, or two brackets at 95-percent confidence. Samples 41, 48, 49, 50, 352, 386 from 

(1975), 392 from Sims and Gable (1967)]

a

4 47

Boulder

392

Creek Granodiorite

49 50 41

Chemical composition, in

D J.U n

AT n — _ A1 2°3

* G 2U 3

TTort

MgO ———————————

Pafi — —

Na 20 ——————————

v o ________ K2 0

u n _ ____

H n~ ____________

Ti02 ————————— 

•p ft ___ _ _ _ P 2°5 

MnO ———————————

pi _ _ _ _

p ——————————————

Subtotal ————

37.49 

16.19 

2.53 

14.81 

11.72

0.49 

0.33 

9.36 

3.07 

0.15

2.61 

0.05 

0.30 

0.07 

0.98

100.20 

0.43

QQ 77

37.38 

15.82 

2.79 

14.59 

12.96

0.13 

0.22 

9.57 

3.07 

0.05

2.27 

0.02 

0.31 

0.14 

0.68

100.00 

0.32

QQ £R

35.60 

17.74 

2.28 

17.68 

9.78

0.18 

0.22 

9.22 

3.32 

0.04

2.96 

0.04 

0.22 

n.d. 

0.59

99.87 

0.25

QQ £0

37.67 

15.53 

3.16 

14.34 

13.05

0.25 

0.28 

9.43 

3.02 

0.10

1.90 

0.02 

0.34 

0.12 

0.58

99.79 

0.27

QQ t;o

37.50 

15.32 

2.21 

15.21 

13.21

0.52 

0.29 

9.04 

3.41 

0.11

1.95 

0.01 

0.26 

0.14 

0.56

99.74 

0.27

QQ /, 7

37.88 

15.08 

2.78 

14.51 

12.83

1.34 

0.24 

8.79 

2.85 

0.15

2.23 

0.05 

0.30 

0.13 

0.81

99.97 

0.37

QQ £n

Hornblende 

diorite

48 352

Mafic inclusio

334 385 

weight percent

37.34 

15.55 

2.86 

14.52 

12.67

0.47 

0.22 

9.29 

2.98 

0.04

2.84 

0.07 

0.26 

0.15 

0.61

99.87 

0.29

QQ ^8

36.97 

15.09 

3.30 

12.69 

14.10

0.41 

0.13 

8.87 

3.31 

0.23

3.34 

0.10 

0.17 

0.15 

0.67

99.53 

0.31

QQ OO

36.31 

16.06 

2.41 

16.15 

11.75

0.52 

0.22 

9.00 

3.19 

0.05

2.99 

0.09 

0.25 

0.21 

0.63

99.83 

0.32

QQ m

36.81 

16.08 

2.57 

15.30 

12.86

0.67 

0.14 

8.64 

3.96 

0.04

2.19 

0.05 

0.29 

0.13 

0.45

100.18 

0.22

QQ Q£
Number of atoms on the basis

Octahedral Al IV —— 

Y Al VI ——

Fe+3-~ - 

Fe+2—— - 

Mn ————

Mg ————

X pa_ ___ __

Ca+Na=2 Na ————

IT __._•.__ 

pi _____

OH— ———

b !00 Fe+/Fe++Mg=

5.669 

2.331

8.00

0.555 

0.297 

0.294 

1.873 

0.038 

2.642

5.70

0.079 

0.097 

1.806

1.98

0.469 

0.018 

2.945

3.43

45.1

5.649 

2.351

8.00

0.468 

0.258 

0.317 

1.844 

0.040 

2.920

5.85

0.021 

0.065 

1.845

1.93

0.325 

0.036 

3.045

3.41

42.5

5.44 

2.56

8.00

0.062 

0.34 

0.26 

2.26 

0.02 

2.22

5.72

0.02 

0.06 

1.80

1.88

0.28 

3.34

3.62

53.2

of 24 (0,

5.700 

2.300

8.00

0.470 

0.216 

0.360 

1.815 

0.044 

2.943

5.85

0.041 

0.082 

1.820

1.94

0.278 

0.031 

3.048

3.36

42.5

OH, F, Cl) in the general formula X-Y-Z

5.669 

2.331

8.00

0.399 

0.222 

0.251 

1.923 

0.033 

2.977

5.81

0.084 

0.085 

1.743

1.91

0.268 

0.036 

3.328

3.63

42.2

5.751 

2.249

8.00

0.449 

0.255 

0.318 

1.842 

0.039 

2.903

5.81

0.218 

0.071 

1.702

1.99

0.389 

0.033 

2.734

3.16

42.7

5.655 

2.345

8.00

0.431 

0.323 

0.326 

1.839 

0.033 

2.860

5.81

0.076 

0.065 

1.795

1.94

0.292 

0.039 

2.970

3.30

43.1

5.560 

2.440

8.00

0.235 

0.378 

0.374 

1.596 

0.022 

3.161

5.77

0.066 

0.038 

1.702

1.81

0.319 

0.038 

3.320

3.68

38.4

8 (0, OH, F, Cl) 

5.523 

2.477

8.00

0.420 

0.342 

0.276 

2.054 

0.032 

2.664

5.79

0.085 

0.065 

1.746

1.90

0.303 

0.054 

3.237

3.59

46.7

5.4

2.

8.

0.3

0.2

0.2

1.9

0.0

2.8

5.6

0.1

0.0

1.6

1.7

0.2

0.0

3.9

4.1

43.6
TABLE 18—Chemical and spectrographic analyses and mineral formula for biotite from Boulder Creek Granodiorite, hornblende d

inclusions, and a lamprophyre dike, Boulder Creek batholith area, Front Range, Colo.—Continued

Boulder Creek Granodiorite

a - 4 47 392 49 50 41

Semiquantitative spectrographic

Co ————————————

OT- __ ___ _, _-__

Cu ———————————

Ga ————————————

Mo ————————————

ftTU_ ______ _ ____ _.

_

P>._ _ ___ _____

<3o __ _

Cy ___ ______

v —————————

Yb ————————————

Zn ————————————

7y ___ __ ___

Lab. numbers not

b_, + _, +2 . _ +3 

Fe =Fe +Fe

.,000 

50 

100 

10 

50

30 

5 

20 

70 

20 

50

20 

300

N 

3 

N 

100

published

700 1, 

70 

100 

50 

50

N 

20 

N 

70 

N 

20

15 

300 

N 

2 

N 

50

previously:

500 1,000 1 

30 100 

100 100 

41 20 

50 50

N N 

N 5 

90 N 

80 100 

N N 

60 20

20 20 

280 300 

30 N 

N 1 

N N 

380 50

sample 4, D102146

,000 

100 

200 

150 

50

N 

20 

N 

100 

N 

10

15 

300 

N 

2 

N 

30

; 47,

1,000 

100 

150 

15 

50

N 

5 

N 

100 

N 

30

20 

300 

20 

2 

N 

100

D102147;

Hornblende 

diorite

48

analyses, in

1,000 

100 

150 

100 

50

N 

5 

10 

100 

N 

20

20 

300

N 

N 

N 

70

334,D102519;

Mafic inclu

352 334 385 

parts per million

5,000 3, 

70 

30 

200 

50

N 

7 

20 

500

N 

7

70 

300 

L 

N 

300 

N

385, D103822;

000 

70 

N 

30 

100

N 

7 

L 

150 

10 

30

30 

300 

N 

N 

700 

300

388,

700 

50 

200 

150 

50

50

N 

N 

150 

N 

15

30 

300 

N 

N 

300 

30

D102520
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 69

Minor elements in biotite were determined by semiquantitative spectrographic analysis and it was found 

that the trace elements in biotites were fairly consistent from one sample to another. Trace amounts of zirconium are probably due to tiny zircons not completely 

removed from the samples during separation and 

purification.

TWIN SPRUCE QUARTZ MONZONITE

General description. Biotite in the Twin Spruce 

Quartz Monzonite occurs mostly as an interstitial 

mineral to plagioclase, microcline, and quartz, and, 

unlike biotite laths in granodiorite, the laths are 

generally smaller and quite ragged. Their color is 

similar to that of biotite in Boulder Creek 

Granodiorite. Accessory minerals in the Boulder Creek 

Granodiorite tend to cluster with the biotite and other 

mafic minerals whereas in the quartz monzonite accessories occur both with biotite or completely 

separated but in foliation trends with biotite. Accessories associated with biotite in quartz monzonite 

include apatite, ores, zircon, monazite or xenotime and 

allanite. Biotite alters somewhat to chlorite, contains 

many opaque blebs, and enters into a reaction forming 

allanite. Biotite in quartz monzonite varies in abundance between outcrops but is rarely more than 10 percent of the rock and averages close to 6 percent.

Chemistry. Nine biotite samples separated from 

quartz monzonite were chemically analyzed (table 19) 

by the same methods used for the biotite from 

granodiorite. Quartz monzonite biotites are richer in 

A12O3 , TiO2 , and FeO but contain less MgO and SiO2 

than Boulder Creek Granodiorite biotites (fig. 34; TiO2 

not shown). CaO, K2O, and Na2O are nearly constant in 

biotites from both rock types. Biotites from Twin 

Spruce Quartz Monzonite also appear to be much more 

uniform in major-element composition than the hostrock composition would indicate; this uniformity was 

also true for biotites in the Boulder Creek 

Granodiorite.

The minor elements cerium, neodymium, and zinc, 

though occurring in quartz monzonite biotites, are not 

found in the granodiorite biotites. Lead and tin are 

reported in most quartz monzonite biotites and rarely 

in granodiorite biotites; chromium, however, is consistently higher in Boulder Creek Granodiorite biotite 

than in biotite in quartz monzonite.

X-ray of the basal (005) interplanar spacing of biotite 

from quartz monzonite indicates an increase in fluorine 

as doos spacings decrease that is similar to the findings 

of Dodge, Smith, and Mays (1969), and Lee and Van

Loenen (1971). In figure 35 biotite in quartz monzonite 

occurs in two trends that are subparallel. Both trends 

definitely show an increase in fluorine as do05 spacings 

increase. Why the quartz monzonite biotites plot in 

two distinct trends is not obvious. The only consistent 

variation is in the number of potassium atoms in 

biotite (table 19); those biotite samples having the 

higher potassium values plot in a lower range of doOB 

spacings, but this may not be a complete explanation. 

Biotite from the younger Silver Plume Quartz Monzonite was plotted and the doos spacings also formed 

a single straight line indicating again that the do06 

spacings increase as fluorine increases. Biotite in the 

Boulder Creek Granodiorite is plotted on the same 

figure but it shows no such relationship; points cluster 

between 0.20140 and 0.20180 run (nanometers) for 0.5 

to 1.0 weight percent fluorine.

MAFIC INCLUSIONS AND LAMPROPHYRE DIKES 

IN THE BOULDER CREEK GRANODIORITE

General description. Thin sections show that biotite 

and hornblende do not tend to cluster in most inclusions as they do in the Boulder Creek Granodiorite. 

Biotite replaces both hornblende and pyroxene and it 

can be generally assumed that even though some 

biotite may be primary, the greater the percentage of 

biotite the greater the alteration of the inclusion. Some 

biotite is reddish and some is greenish brown in color, 

is frayed and eroded, and, in general, its habit and form 

of alteration is similar to that of biotite in the Boulder 

Creek Granodiorite.

Chemistry. The distribution of the major oxides in 

biotite from the syenodiorite or lamprophyre (sample 

388, table 18) is not much different from that in biotite 

from mafic inclusions; however, TiO2 , P2O5 , and fluorine do not follow this distribution. Both P2O5 and 

fluorine amounts are higher in the lamprophyre; TiO2 

is lower. Other oxides vary as much between the 

various mafic inclusions as between rock types. Biotite 

from inclusions tends to contain more nickel, zinc, and 

lithium than granodiorite biotite.

X-ray determination of the doOB spacing (fig. 35) indicates that biotites from inclusions plot near biotites 

from the Boulder Creek Granodiorite and Twin Spruce 

Quartz Monzonite whereas the biotite from a lamprophyre is isolated owing to its high fluorine content.

PETROGENESIS

Figure 36 shows that biotites from the Boulder 

Creek Granodiorite, the Twin Spruce Quartz Monzonite, and the inclusions all belong to the assemblage 

magnetite-potassium feldspar-biotite and plot along a
70 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 19.—Chemical and spectrographic analyses and mineral formula for biotite from Twin Spruce Quartz Monzonite, Front Range, Colo.

[Standard rock wet chemical analyses by V. C. Smith; spectrographic analyses by L. A. Bradley; (-), not found; N.d., not determined]

T ah TJ/-i — — — . _ _ T

232 235 35 

)102154 D102153 D102513

376 380 

D102514 D102155

Chemical analyses

o.?n _ _ . _ _ _

Ai n — — — — _ _

T?FeO 2°3n _. _ ._

FeO ----——-—— —_—_—————_

MgO ————————————————————

p-n — — _— _. __ — __ — . _

MO n_ __ _ _ _ _

K o _ _ _ _

H n+ _ - _ - -

H

n~_ _ _ _ _ _

T-if) _ __ __ _ _ __

p P 2°5rv _ _ _ _ _ _

MnO ————————————————

£^ __ __

Subtotal ————————

Total ———————————

Number of

Z OJ__ __ __ ._

35. 

18. 

2. 

17. 

8.

0. 

0. 

9. 

2. 

0.

3. 

0. 

0. 

0. 

0.

99.

99. 

n.

79 

47 

63 

44 

36

00 

15 

41 

84 

02

22 

01 

32 

07 

73

96 

33

63 

d.

atoms on

5. 

2.

8.

476 

524

00

35.74 

16.52 

2.70 

18.10 

10.06

0.61 

0.24 

8.52 

3.08 

0.20

2.50 

0.04 

0.16 

0.19 

1.41

100.07 

.63

99.44 

n.d.

the basis

5.496 

2.504

8.00

36.90 

16.52 

2.81 

17.10 

10.17

0.73 

0.32 

8.51 

3.75 

0.09

2.10 

0.02 

0.33 

0.05 

0.47

99.87 

.21

99.66 

3.05

of 24

5.574 

2.426

8.00

36.99 

16.31 

2.57 

15.82 

11.42

0.38 

0.24 

9.36 

3.13 

0.07

2.41 

0.04 

0.38 

0.08 

0.83

100.03 

.37

99.66 

3.04

(0, OH, F,

5.607 

2.393

8.00

381 382 383 

D102157 D102156 D102152

384 

D102158

, in weight percent

36.62 

16.63 

4.32 

15.51 

9.99

0.00 

0.28 

9.28 

3.21 

0.13

2.76 

0.04 

0.16 

0.18 

1.11

100.22 

.51

99.71 

n.d.

Cl) in

5.561 

2.439

8.00

35. 

16. 

2. 

18. 

9.

0. 

0. 

8. 

3. 

0.

3. 

0. 

0. 

0. 

1.

100.

99.

n.

56 

19 

85 

94 

03

00 

22 

85 

09 

15

50 

03 

39 

14 

23

17 

55

62 

d.

the general

5. 

2.

8.

486 

514

00

35.50 

17.16 

2.58 

17.89 

9.27

0.00 

0.19 

9.44 

2.89 

0.05

4.15 

0.03 

0.22 

0.10 

0.47

99.93 

.22

99.71 

n.d.

formula

5.463 

2.537

8.00

35.86 

18.35 

2.36 

17.91 

8.30

0.00 

0.19 

9.10 

2.86 

0.08

3.78 

( a) 

0.08 

0.11 

0.88

99.86 

.39

99.47 

n.d.

X?Y Z (0 Zoo

5.493 

2.507

8.00

36.69 

17.19 

2.38 

17.26 

9.63

0.19 

0.27 

9.23 

2.89 

0.11

2.88 

( a) 

0.38 

n.d. 

1.12

100.22 

.47

99.75 

n.d.

,OH,F,C1) 24

5.612 

2.388

8.00

V A1_ ___ _ _

T7 +3

T7 +2

Mn ——— ———— —

Mg _____________

0. 

0. 

0. 

2. 

0. 

2.

5.

806 

371 

303 

231 

042 

020

77

0.490 

0.289 

0.312 

2.328 

0.021 

2.306

5.75

0.515 

0.239 

0.319 

2.160 

0.042 

2.290

5.57

0.520 

0.275 

0.293 

2.005 

0.049 

2.580

5.72

0.538 

0.275 

0.494 

1.970 

0.021 

2.261

5.60

0. 

0. 

0. 

2. 

0. 

2.

5.

429 

406 

331 

443 

051 

076

74

0.576 

0.479 

0.299 

2.302 

0.029 

2.127

5.81

0.806 

0.435 

0.272 

2.294 

0.010 

1.895

5.71

0.693 

0.330 

0.275 

2.2QO 

0.046 

2.200

5.75

X Ca ——— ——— —— -

Ma __ __ __ ___ _ 0. 

1.

1.

044 

836

88

0.101 

0.072 

1.671

1.84

0.118 

0.094 

1.640

1.85

0.062 

0.071 

1.810

1.94

0.082 

1.798

1.88

0. 

1.

1.

066 

741

81

0.057 

1.853

1.91

0.056 

1.778

1.83

0.028 

0.073 

1.797

1.89

F ———————————

PI _«.-. -.„. __ _._ —

f\U

100 (Fe+/Fe+4Mg) —— — —— —— ——

0. 

0. 

2.

3. 

55.

353 

018 

878

25 

60

0.686 

0.049 

2.954

3.69 

53.40

0.225 

0.013 

3.778

4.02 

52.00

0.398 

0.021 

3.165

3.58 

47.00

0.533 

0.046 

3.120

3.70 

52.10

0. 

0. 

3.

3.

57.

600 

037 

025

66 

20

0.229 

0.026 

2.916

3.17 

55.00

0.426 

0.029 

2.841

3.30 

57.50

0.541 

2.824

3.36 

52.90
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 71

TABLE 19.—Chemical and spectrographic analyses and mineral formula for biotite from Twin Spruce Quartz Monzonite, Front Range, Colo.—

Continued

Lab No. ———————

_____ _ .___ OQO

————————— D102154

235 

D102153

85 

D102513

376 

D102514

380 

D102155

381 

D102157

382 

D102156

383 

D102152

384 

D102158

Spectrographic analyses, in parts per million

Ba --—-————_—

Ce— — — — — ———

Co ______________

Cf ______

Pll— _____ _

Ga -———————__

T a_ __ . ___ _

Li ______________

Mo ______________

Hb -__——___—___

Nd ————————

Hi ______________

pv,_ __ _____

C-n

Qv— __ _ _

V _

Y ————————————

Vk— __» _

Zn ———— ——— ——

Zr —— —— _———— _

__ _ . _ _ 7nn

—————————— 10

_ _ __ on

__ __ -in

___ _ cr\

_ __. _ _ _ on

————————— 20

————————— 15

————————— 10Q

— — —-._ onn

————————— 1,000 

_ _ _ _ onn

700 

300 

50 

50 

150

100 

200

10 

20

200 

50 

50 

50 

50

70 

300 

20 

2 

1,000 

50

700 

1 

100 

10 

50

70 

70 

700 

10 

20

70 

50 

30 

15

70 

300

15

500 

300

1,000

100 

30 

700

100

300

7

100 

30

30 

300

700 

500

700

50 

50 

200

100 

70

5 

20

150 

30 

20 

50 

50

15 

300

2 

1,000 

50

2,000

50 

50 

20

50

5 

10

10 

10 

30 

20

20 

300

5 

500 

50

1,000

70 

500 

10

50

5 

10

100 

10 

50 

20

10 

300 

20 

5 

500 

70

1,000

70 

100 

10

100 

30

7 

20

100 

50

50 

20

20 

200 

20 

5 

500 

100

700 

200 

50 

10 

30

100 

200

5 

10

150 

20 

20 

50 

20

15 

100 

30 

5 

500 

200

P20 included in ^-.

trend paralleling the compositions of "buffered" 

biotites in the ternary system KFe3+3 AlSi3 Ol2 (H-1 )- 

KFe3+2AlSi3O10(OH)2 (Wones and Eugster, 1965, p. 1232). 

This trend suggests that oxygen fugacities in biotite of 

the Boulder Creek Granodiorite are slightly greater 

than those of the Ni-NiO buffer. One granodiorite sample was not plotted on the ternary diagram; it, too, 

would have plotted in the concentration of points for 

granodiorite.

In the presence of magnetite acting as a buffer, 

oxygen fugacities in a crystallizing magma decrease as 

temperatures decrease and generally as FeT:FeT+Mg 

ratios increase. Boulder Creek Granodiorite biotites 

have FeT:FeT+Mg values of 42.5-45.1 (the Mt. 

Pisgah pluton sample having a ratio of 53.2 is excluded 

because the granodiorite lies outside the batholith), inclusions have values of 38.4-46.7, and Twin Spruce 

Quartz Monzonite from the batholith only, 47.0-52.1; 

these values appear consistent for the Boulder Creek

Granodiorite and the Twin Spruce Quartz Monzonite 

because the quartz monzonite was emplaced later in 

the sequence, after the Boulder Creek Granodiorite, 

when temperatures would be expected to be somewhat 

lower. Biotites in the inclusions have Fe:Fe+Mg 

values that indicate some inclusions are younger and 

some older than Boulder Creek Granodiorite. These 

data substantiate the belief stated earlier that some of 

the inclusions are younger and some older than the 

granodiorite.

HORNBLENDE

BOULDER CREEK GRANODIORITE

General description. The approximate extent of 

hornblende-bearing granodiorite is outlined in figure 

6A Hornblende is not common in Boulder Creek 

Granodiorite lenses and plutons that are satellitic to
72 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

IU

° 9

^

14

012

O) ^ 10

19

18

17

LI15

5 n

__

1 ——— J

—

-

~

—

•

,

L

.

1

1

•

I

L

L

•

i

»

t

i

»

»

»

k

t

i 1 I 1 1 ! 1 l 1 

* . * _

i r r i i i .

1 1 ! 1 1 III

—

f 1 1 1 1 1 i

1 I T 1 1

• _

*

•

—

1 III 1

018

CM

< 16 

14 

39 

38

t/J 36

35

1A

1

1 I

I

• '

I

1

—

—

I I I I

I I

• • *****

-

' * . 

* * • • *

I

4 47 49 50 41 48 386 232 235 85 376 380 381 382 383 384

SAMPLE NUMBERS

BIOTITE FROM BOULDER BIOTITE FROM TWIN SPRUCE

CREEK GRANODIORITE QUARTZ MONZONITE

FIGURE 34.—Compositional variations of K2O, MgO, FeO, A12O3 , 

and SiO2 in biotites from Boulder Creek Granodiorite and Twin 

Spruce Quartz Monzonite. Horizontal lines show median separation of each element, except K2O, between Boulder Creek 

Granodiorite and Twin Spruce Quartz Monzonite. Oxides in 

weight percent.

the batholith proper; one exception lies in the 

southernmost part of the map area (pi. 1). Here the 

northeast-trending Bald Mountain pluton, if extended 

northeastward from its outcrop, lies along a trend of 

hornblende-rich granodiorite which extends into the 

batholith proper (fig. 6A). Another exception lies north 

of Nederland in the easternmost part of the Caribou 

pluton.

Hornblende in the batholith proper occurs as 

disseminated crystals in granodiorite and in clusters 

containing, besides hornblende, apatite, allanite, 

biotite, and ores; the occurrence in clusters is more 

common. About half of the granodiorite samples collected were hornblende bearing and contained as much 

as 23.3 percent hornblende (table 1).

In thin section, the hornblende is green or bluish

Area represented by Boulder 

Creek Granodiorite biotites

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 

FLUORINE IN BIOTITE, IN WEIGHT PERCENT

2.0

FIGURE 35.—Fluorine in biotite plotted against do06 spacing of biotite.

green and crystals are as much as 1.5 cm in length, but 

most hornblende is in smaller crystals, some of which 

are slightly zoned (zoning is principally by color and index difference) and often eroded by other minerals. 

Magnetite, sphene, calcite, and allanite have replaced 

hornblende, in some areas extensively. Physical and 

chemical properties for most of these hornblendes have 

already been published (Gable and Smith, 1975) and 

will not be included here except for pertinent chemical 

data. 

Chemistry. The hornblendes analyzed are calcic, and

Fe+3

Fe+2

Fe2SiO4-SiO2-Fe3O4

FIGURE 36.—Relation of Fe+3-Fe+2-Mg (atomic ratios) between 

biotites of Boulder Creek Granodiorite and Twin Spruce Quartz 

Monzonite. Dashed lines represent compositions of buffered biotites in ternary system KFe3+3 AlSiA^KFea+'AlSiO^OH),- 

KMg3AlSi3O10 (OH)2 depicted by Wones and Eugster (1965, fig. 1). 

Solid circle, Boulder Creek Granodiorite; solid circle with tail, 

inclusion; X, Twin Spruce Quartz Monzonite. Line through 

plotted biotites represents composition of buffered biotites. 

Buffers are: Fe3O4-Fe2O3,Ni-NiO,Fe2SiO4-Fe3O4-SiO2 .
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 73

chemical variations among hornblendes in the 

granodiorite are small, except for SiO2 , FeO, A12O3 , and 

MgO. FeO and A12O3 , in general, increase as MgO and 

SiO2 decrease. Table 20 has been reproduced in part 

from Gable and Smith (1975). 1 The same trace 

elements, except for beryllium occur in both biotite 

and hornblende in granodiorite rocks. Beryllium, 

however, occurs only in hornblende.

MAFIC INCLUSIONS AND LAMPROPHYRE DIKES 

IN THE BOULDER CREEK GRANODIORITE

General description. Hornblende-rich mafic inclusions are particularly abundant in the south-central 

part of the batholith but are present nearly everywhere 

in the northern half. The west contact and the Caribou 

pluton in the Nederland and Ward quadrangles are 

nearly free of hornblende. Mafic inclusions in the 

Caribou pluton are biotitic due to alteration and rarely 

contain hornblende.

Hornblende in the lamprophyric rocks is generally 

poikilitic and has inclusions of potassium feldspar (orthoclase), apatite, ores, biotite, and allanite. Pyroxene 

in the lamprophyre is in small subrounded crystals 

that accompany hornblende or occur as inclusions in 

hornblende. In the foliated and porphyritic 

microgranular mafic inclusions, the occurrence of hornblende is not much different from that in hornblende 

diorite. In pyroxene-bearing hornblende diorite, 

however, hornblendes are generally very large and 

typically poikilitic, containing subrounded pyroxene 

crystals.

Chemistry. The hornblende from the mafic inclusion 

has a composition similar to hornblendes from the 

Boulder Creek Granodiorite and hornblende diorite 

(table 20) except for fluorine, which is a little higher in 

inclusion hornblendes. Cerium and zirconium found in 

the hornblende inclusion may be due to small inclusions of allanite and zircon crystals within the hornblende. Hornblendes in the inclusions also contain 

more chromium, nickel, and strontium than hornblendes from the granodiorite.

HORNBLENDE DIORITE AND HORNBLENDITE

Hornblendes from hornblende diorite and hornblendite are also calcic. These hornblendes are generally 

higher in SiO2 than Boulder Creek Granodiorite hornblendes or inclusion hornblendes. This higher SiO2 content may be due to a small excess amount of SiO2 occurring in the hornblende initially and then not being 

available to form quartz or enter into reactions forming other minerals. In hornblendite, high SiO2 is ac1 Hornblende from sample 386 (Gable and Smith, 1975, sample T 170-68) was originally 

included with hornblende of granodiorite but is now believed to be part of a very mafic inclusion that is gradational into normal Boulder Creek Granodiorite.

companied by low A12O3 , a combination discussed 

below. Of the minor elements, chromium and nickel are 

both higher in hornblende from hornblende diorite and 

hornblendite rocks whereas yttrium is lower.

PETROGENESIS

Hornblende from granodiorite, inclusions, and hornblende diorite is chemically similar. Hornblende from 

lamprophyre and hornblendite contains less A12O3 than 

the more leucocratic rocks; hornblende from hornblendite carries greater SiO2 and MgO but less 

FeO+Fe2O3 .

In an investigation of aluminum replacement of 

silicon in the amphibole lattice, Harry (1950) indicated 

that the higher the temperature of formation, the 

higher the solid solubility, and at high temperatures 

aluminum substitutes for silicon and the substitution 

is controlled by magmatic temperatures at time of 

crystallization. On the basis of tetrahedrally and octahedrally coordinated aluminum, early formed hornblendes from rocks of appinitic type belong to Harry's 

(1950) type I, hornblendes in diorites and trondhjemites to type II and hornblendes of late crystallization to type III (table 21).

Silicon-aluminum relationships in hornblendes from 

the Boulder Creek Granodiorite, mafic inclusions, and 

hornblende diorite place the hornblende composition of 

the rock in this report between Harry's types I and II, 

which are primary hornblendes; hornblendes from 

hornblendites of this report belong to type III, which 

are hornblendes of late crystallization.

In a study of Caledonian amphiboles, Nockolds and 

Mitchell (1946) found that rocks rich in amphibole, 

that is, hornblendite, have different trace elements in 

hornblendes in comparison to hornblendes from hornblende diorites, quartz diorites, and granodiorites. 

Trace-element abundances vary between hornblendes 

in this report but not significantly; perhaps more 

analyses are needed to identify real differences. Lead 

occurs only in hornblendite; and lanthanum, niobium, 

and neodymium, although not found in hornblendites, 

do occur in the hornblendes of other rocks (table 20).

PLAGIOCLASE

BOULDER CREEK GRANODIORITE

Plagioclase is neither as fresh nor as free of inclusions as accompanying potassium feldspars, and eroded hemihedral to anhedral crystals of plagioclase are 

about the same size as the potassium feldspars. 

Plagioclase may be antiperthitic and normally zoned; 

the zoned crystals have rims ranging in composition 

from oligoclase to albite. Reaction rims are broader
TABLE 20.—Chemical and spectrographic analyses and mineral formula for hornblende from Boulder Creek Granodiorite, ma

lamprophyre dike, hornblende diorite, and hornblendite, Boulder Creek batholith area, Front Range, Colo.

[Analyst for standard rock analyses on hornblende in sample 385, E. L. Brandt; 388, V. C. Smith; all others credited in Gable and Smith (1975). Spectrographic analyst for 

Ray Havens; all others credited in Gable and Smith (1975). N.d., not determined; (--), looked for but not found]

Boulder Creek Granodiorite

q-Jn — __ —

A12 U 3

e2 U 3

MgO ——————————

IsdU

"NTa n — — — —

H2 0+__ __ _

tr n _ _ _ _ _ H2°

Ti02 —————————

p ft _______ __ P2°5

MnO ——————————

PI

F __

Subtotal —— 

Less 0 ———

T«<-al ————————

49

45.2 

11.4

\8.6 

10.0

11.6 

1.16 

1.34 

n.d. 

n.d.

n.d. 

n.d. 

n.d. 

n.d. 

n.d.

n.d. 

n.d.

n A

50

43.43 

10.29 

5.93 

11.80 

10.79

11.85 

1.20 

1.17 

1.70 

0.02

1.13 

0.02 

0.37 

0.14 

0.25

100.09 

0.14

QQ Qq

41

42.84 

10.44 

6.19 

11. 71 

10.37

11.83 

1.19 

1.34 

1.64 

0.03

1.20 

0.31 

0.46 

0.13 

0.35

100.03 

0.18

QQ Rq

48

Chemical

43.13 

10.57 

5.99 

11.62 

10.66

11.83 

1.09 

1.31 

1.57 

0.08

1.24 

0.06 

0.40 

0.14 

0.27

99.96 

0.14

QQ R9

Mafic Lamprophyre 

Inclusions dike

a385

analyses,

44.03 

10.04 

5.70 

11.88 

10.71

11.92 

1.02 

1.07 

1.75 

0.00

1.10 

0.12 

0.45 

0.12 

0.22

100.13 

0.12

inn m

386

a388

Hornblende 

diorite

343 352

in x^eight percent

43.85 

10.12 

5.80 

11.61 

10.92

11.98 

1.10 

1.14 

1.69 

0.05

0.96 

0.08 

0.31 

0.10 

0.32

100.03 

0.15

QQ ftft

46.08 

8.39 

4.82 

10.33 

12.69

12.01 

1.33 

0.95 

1.56 

0.01

0.75 

0.19 

0.30 

0.07 

0.66

100.14 

0.30

QQ «/,

46.42 

10.14 

4.09 

10.09 

13.40

11.26 

1.16 

0.53 

1.84 

0.00

0.83 

0.01 

0.30 

0.06 

0.17

100.30 

0.08

inn 99

44.48 

10.27 

4.83 

10.22 

12.34

12.10 

1.17 

1.07 

1.62 

0.02

1.06 

0.11 

0.30 

0.17 

0.30

100.06 

0.17

QQ «Q
Number of atoms on

Tetrahedral 

(D

Octahedral

X=Ca+Na=2 . 0

Al IV ——

Al VI ——

Ti ————

F.+3———

Mg —— —

Fe+2—— -

Mn ————

Na ———— 

Ca ————

K__

OH — ———

F —————

Cl —— ——

6.645 

1.355

8.00

0.620

0.133

°0.658

2.191

°1.402

—

5.00

0.331 

1.827 

0.251

2.41

—

—

—

—

6.497 

1.503

8.00

0.311

0.127

0.668

2.406

1.476

0.047

5.04

0.348 

1.899 

0.223

2.47

1.676

0.118

0.036

1.83

the basis

6.455 

1.545

8.00

0.309

0.136

0.702

2.329

1.476

0.059

5.01

0.348 

1.910 

0.258

2.52

1.618

0.167

01033

1.82

of 24 (0,

6.486 

1.514

8.00

0.359

0.140

0.678

2.389

1.461

0.051

5.08

0.318 

1.906 

0.251

2.48

1.495

0.128

0.036

1.66

OH, F, Cl) in the general formula Ao_iX2Y5Z8°22

6.567 

1.433

8.00

0.332

0.123

0.640

1.482

0.057

2.381

5.02

1.905 

0.295 

0.204

2.40

0.104

0.030

1.741

1.88

6.549 

1.451

8.00

0.331

0.108

0.652

2.431

1.450

0.039

5.01

0.318 

1.917 

0.217

2.45

1.684

0.151

0.025

1.86

6.808 

1.192

8.00

0.269

0.083

0.536

2.795

1.276

0.038

5.00

0.381 

1.901 

0.179

2.46

1.537

0.308

0.018

1.86

6.741 

1.259

8.00

0.476

0.091

0.447

2.900

1.225

0.037

5.18

0.327 

1.752 

0.098

2.18

1.782

0.078

0.015

1.88

6.583 

1.417

8.00

0.375

0.118

0.538

2.722

1.265

0.038

5.06

0.336 

1.919 

0.202

2.46

1.599

0.140

0.043

1.78

Ratio:

+2

lOOMg/ (Mg+Fe +Fe J+Mn) 51.5 52.3 51.0 52.2 52.2 53.2 60.2 62.9 59.6 
TABLE 20.-Chemical and spectrographic analyses and mineral formula for hornblende from Boulder Creek Granodiorite, mafic 

lamprophyre dike, hornblende diorite, and hornblendite, Boulder Creek batholith area, Front Range, Coto. -Contin

Boulder Creek Granodiorite

49 50 41 48

Mafic 

Inclusions

a385

386

Lamprophyre Hornblende 

dike diorite

a388

343 352

Semiquantitative spectrographic analyses, in parts per million

Ba —————————— -

Be ———————————

fv _ __ — _

Ga —————————— -

T a ________ ____

t3A

______ ___ __ 

pb ———————————————— .

Y _

vu

- — 50

- — 50

- — 70

or>

on

10

- — 100

70

70

200

_ _ i en

10

- — 50

50 

5

50 

100

2 

20

10

100

150 

100

300 

300 

20

50

50 

2

50 

100

10 

20 

70 

7 

10

200 

100

150 

100

300 

300 

20

100

30 

3

50 

150

20 

30 

50

10

150 

100

150 

70

300 

150 

15 

200 

70

70 

2

70 

150

30 

30

10

150

100 

100

500 

30 

3

70

100 

7 

150 

50 

500

7 

30 

100

100 

300

70 

300

200 

30 

2

150

50

70 

300

20 

20

10

300

70 

50

300 

30 

3

30

150 

2

50 

200

15 

30 

70

10

200 

300

100 

200

200 

100 

7

70

Lab. number for sample 3S5 is D103823; for sample 388, D1025116. 

Total iron reported as FeO. 

"Used sample 50 hornblende composition for breakdown of FeO and from total reported FeO.
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 77

TABLE 21.—Comparison of partial hornblende lattice structures

[Columns 1, 5, and 6 from Harry (1950)]

Type

I

Granodiorite

Hornblende 

Inclusions diorite

Type 

II

Type 

III Hornblendlte

Z group

Y group

Si ——

Al — —

Al — —

1

6.31

1.69

0.22

2

6.52

1.48

0.40

6,

1.

0.

3

.55

.51

.33

4

6.64

1.34

0.43

6,

1.

0.

5

.79

.21

.11

6

7.12

0.88

0.15

1.

0.

0.

7

.23

.74

.30

and mild cataclasis prevails in plagioclase from rocks 

in which muscovite occurs. Albite twinning is predominant, but Carlsbad, pericline, and minor complex twins 

were also observed in thin section. Composition varies 

from labradorite to oligoclase, but labradorite occurs 

only in a few lenses satellitic to the batholith, east of 

the zone of more mafic rocks. Plagioclase in the 

batholith is of oligoclase-andesine composition (fig. 

6G). Figure 6G indicates widespread erratic zoning of 

plagioclase in the granodiorite from west to east. 

Plagioclase at the batholith contact is andesine and 

plagioclase in the main part of the batholith is 

oligoclase. Plagioclase in a northwest-trending wide 

band in the central northern half of the batholith, like 

the border rocks, is andesine. It appears that 

plagioclase in the batholith has had a long crystallization history because subrounded crystals of 

plagioclase appear in both biotite and hornblende, suggesting that plagioclase may have been the earliest 

mineral to crystallize or that it formed nearly 

simultaneously with hornblende and biotite. Locally, 

smaller and more altered plagioclase crystals occur in 

younger, larger, and clearer plagioclase crystals. Twinning in smaller altered crystals and newer crystals is 

different; twinning by the same law is disoriented between the older and younger crystals, and twinning by 

two different laws occurs in the older and host 

crystals. The younger crystals contain biotite, quartz, 

and other inclusions, and crystal shape is well defined 

to subrounded.

TWIN SPRUCE QUARTZ MONZONITE

Plagioclase in the quartz monzonite is more altered 

than plagioclase in the granodiorite and composition is 

much more limited, ranging from sodic to calcic 

oligoclase. Zoning is normal, and the thin outer zone 

ranges from calcic albite to sodic oligoclase. Generally 

plagioclase is the host mineral for myrmekite that is 

well developed adjacent to potassium feldspar 

crystals, but myrmekite forms adjacent to potassium 

feldspars in the absence of plagioclase.

MAFIC INCLUSIONS 

IN THE BOULDER CREEK GRANODIORITE

Plagioclase is both fresh looking and sericitized, and 

crystals are generally subordinate in size to those of 

the mafic minerals, but the rocks generally contain a 

few scattered porphyritic plagioclase crystals. Twinning is predominantly albite and complex albitepericline types. Plagioclase unzoned to normally zoned 

has a composition that varies from calcic oligoclase to 

calcic andesine.

POTASSIUM FELDSPAR

BOULDER CREEK GRANODIORITE

General description. The alkali feldspars in the 

granodiorite consist of microcline and microperthite 

crystals that are slightly smaller to equal in size to the 

larger plagioclase crystals. Microcline crystals are 

generally subhedral to anhedral and show good 

microcline grid twinning. Large crystals are often 

poikilitic, having inclusions of zoned plagioclase, 

subhedral quartz, anhedral ores, and frayed, somewhat 

altered biotite. Myrmekitic intergrowths are particularly common near fault zones or wherever rocks 

have been sheared and recrystallized; muscovite may 

also occur in these rocks. Microperthite is more local 

and seems to be more prevalent near the more mafic 

lenses that are gradational with granodiorite. Some 

perthites, as in the southeast part of the batholith, 

have clear polysynthetic twinned albite rims that 

formed where two or more potassium feldspars have 

common grain boundaries.

Chemistry. Six potassium feldspars were chemically 

analyzed (table 22). Feldspar composition ranges from 

Orse.s-sT.gAb!0.7-1 i.9An1 . 2-2.i. Six of the seven analyses 

have a compositional range of only 1 percent for any 

one component, indicating a quite uniform composition of potassium feldspar throughout the batholith. 

The minor elements are also fairly consistent in 

amounts between potassium feldspars with only 

barium and strontium in more than trace amounts. 

Determination of triclinicity from X-ray diffraction 

powder patterns using the 131-131 peaks (Goldsmith 

and Lavas, 1954) indicates that all chemically analyzed 

potassium feldspars from the Boulder Creek 

Granodiorite have a triclinicity (obliquity) of 

0.60-0.82±0.05. Nilssen and Smithson (1965) found 

that low triclinicity values are generally from more 

mafic rocks. This relationship is true for the Boulder 

Creek Granodiorite as well; potassium feldspar in the 

more mafic granodiorite adjacent to the larger mafic 

inclusions is often a microcline-microperthite having a 

triclinicity of 0.60-0.65. Elsewhere in the granodiorite
78 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 22. — Chemical and semiquantitative spectrographic analyses of potassium feldspar from Boulder Creek Granodiorite,

Front Range, Colo.

[Total Fe and CaO determined by atomic absorption methods, Na2O and K2O by flame photometric methods; Wayne Mountjoy, analyst. SiO2 and A12OS determined by 

rapid-rock techniques, Sam Botts, analyst; semiquantitative spectrographic analyses by L. A. Bradley. N, not found; L, detected but below limit of determination; N.d.,

not donel

Sample N 

Lab. No. a m ^s

W173

1923 

1436

Chemical analyses,

Si0 2 ——— 

A1AI 2°3 n

FeOb -

fop)

Na2 0 ——— 

Kn _

_ _ _ ___ AA

1 Q

.

.

——————————————————— !.

—————————————————— 14>

Tnr-al — QQ

40 

60 

05 

11 

31 

80

27

Semiquantitative spectrographic

B o

rv

Cu ————

Ga ————

Ni ————

Pb ————

c v __

————————————————— 5> 000

—————————————— N

—————————————————— 7

_ _ _ _ on

———————————————————————— N

—————————————————— 70

—————————————————— 1>000

41 

D138924 

W173441

in weight

64. 

18. 

0. 

0. 

1. 

14.

98.

40 

60 

09 

17 

26 

10

62

analyses,

5,000 

N 

7 

20 

L 

70 

1,500

3

1

47 

D138925 

W173437

percent

64.40 

18.40 

0.05 

0.24 

1.34 

14.40

98.83

in parts

,000 5, 

N 

15 

20 

5 

70 

,000 1,

48 49 50 

D138926 D138927 D138928 

W173431 W173442 W173439

64 

18 

0

0 

1 

14

98

per

000 

N 

100 

20 

5 

70 

000

.40 64.40 

.60 18.60 

.05 0.05 

.17 0.13 

.26 1.25 

.50 14.40

.98 98.83

million

5,000 

N 

5 

20 

N 

50 

1,000

64. 

18. 

0. 

0. 

1. 

14.

98.

5,000 

N 

10 

20 

N 

100 

1,000

40 

60 

05 

18 

34 

10

67

Composition ratios

AVi

An -

Or ————

—————————————————— 10.

—————————————————— 1.

07

96 

17 

87

10. 

1. 

87.

97 

91 

12

11.40 

2.10 

86.50

10

1 

87

.71 10.74 

.60 1.33 

.69 87.93

11. 

1.

86.

59 

70 

71

o

"W" numbers for A102 ~ and Si02 ; "D" numbers for all other oxides. Total Fe as FeO.

the triclinicity averages 0.80-0.85, maximum 

microcline (microcline having maximum obliquity). Investigators (Kuroda, 1958; Marmo and others, 1963; 

Nilssen and Smithson, 1965) have concluded that low 

triclinicity or low-delta potassium feldspars may be 

related to porphyroblastesis and metasomatism. They 

suggest that rocks containing feldspars having low 

triclinicity may have undergone a change in bulk composition due to introduced potassium, and the process 

was arrested before the potassium feldspars were completely inverted to the higher delta values. This explanation may be valid for the Boulder Creek 

Granodiorite as well; it explains the large potassium

feldspar crystals often found in the vicinity of mafic 

areas. It is possible that remobilized potassium, due to 

shearing, caused this local metasomatism.

Variation in distribution of potassium feldspars for 

the batholith is summarized in figure 6H. The ratio of 

potassium feldspar to plagioclase in granodiorite has 

been plotted and contoured for the batholith and a 

zonal arrangement of areas lacking alkali feldspars and 

areas rich in potassium feldspars became apparent. 

The absence of potassium feldspar along the western 

contact of the batholith and metasedimentary rocks 

and along a west-northwest trending area between the 

northern and southern parts of the batholith is evident
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 79

in figure GH. From areas devoid of potassium feldspar 

there is an increase in potassium feldspar toward the 

centers of the north and south halves of the batholith. 

An Ab content of potassium feldspars of less than 15 

percent is generally believed to indicate that the rocks 

completed crystallization or were recrystallized at low 

temperatures (Tuttle and Bowen, 1958, p. 129). Granodiorite having an 0.80-0.85 triclinicity is believed by 

some to have crystallized in the presence of excess 

water vapor. The presence of excess water vapor during the latter stages of crystallization is possible for 

the Boulder Creek Granodiorite, especially because 

large pegmatites accompany the batholithic rocks.

MAFIC INCLUSIONS AND LAMPROPHYRE DIKES 

IN THE BOULDER CREEK GRANODIORITE

Potassium feldspars in the mafic inclusions are 

either orthoclase or microcline, and in the lamprophyre 

are orthoclase. These feldspars are generally anhedral, 

poorly twinned to nontwinned, cloudy in appearance, 

and contain profuse small apatite crystals. Potassium 

feldspars also occur in rounded blebs as inclusions in 

other minerals, including hornblende. Analyses of the 

feldspars (table 23) indicate a composition range of 

Orse.B-Bs.sAbg.T-ii.TAnLs-LB in the mafic inclusions. A 

potassium feldspar from lamprophyre has a composition of Or77Ab18An5 . The composition of potassium 

feldspar from the mafic inclusions is more nearly that 

of the feldspars from more mafic Boulder Creek 

Granodiorite.

TWIN SPRUCE QUARTZ MONZONITE

General description. Microcline occurs throughout 

the Twin Spruce Quartz Monzonite in subhedral to 

anhedral grid-twinned crystals. Medium-grained 

quartz monzonite has a variable grain size and chiefly 

bears large Carlsbad-twinned poikilitic crystals that 

show good grid twinning or are perthitic. The poikilitic 

crystals contain inclusions of predominantly zoned 

plagioclase, biotite, subrounded quartz, and eroded 

perthitic microcline crystals. Alkali feldspar in finegrained and speckled quartz monzonite is more nearly 

equigranular, and plagioclase in the rock appears more 

altered.

Chemistry. The composition of the potassium 

feldspars in quartz monzonite is more variable than in 

the Boulder Creek Granodiorite (tables 22 and 24). The 

potassium feldspar in sample 384 (table 24) is orthoclase and in sample 381 microcline microperthite; 

all other analyzed alkali feldspars from quartz monzonite are microcline. The range in composition is from 

Or84.7_89. 6Ab9.5-1 4. 6Ano.9-2 .2. Triclinicity of the analyzed

TABLE 23.—Chemical and semiquantitative spectrographic analyses of potassium feldspar from mafic inclusions and a lamprophyre dike in Boulder Creek Granodiorite, Front Range, Colo.

[Total Fe and CaO determined by atomic absorption methods, Na2O and K2O determined 

by flame photometric methods, Wayne Mountjoy, analyst; SiO2 and A12O3 determined by 

rapid-rock techniques, Sam Botts, analyst; semiquantitative spectrographic analyses by 

L. A. Bradley. L, detected but below limit of determination; N, not found; N.d., not done]

Mafic inclusions

Sample No.- 

Lab. No. a—

Orthoclase

334

D138909 

W173440

386 

D138929

Lamprophyre 

Microperthite

388

D139810 

W173444

Chemical analyses, in weight percent

D-LUn —————

"fr—

FeOb——-

CaO——— 

Na,0-—— 

K0 0———

63.80

18.60

0.05

0.12

1.12

14.10

n.d. 

n.d. 

0.05 

0.15 

1.38 

14.10

62.50

19.20

0.09

0.28

2.09

12.40

97.79 96.56

Semiquantitative spectrographic analyses, in parts per million

BaCrCuGaNiPbSrZr10,000

N

15

30

5

70

2,000 

100

10,000 

1 

15 

20 

N 

70

1,000 

N

15,000

N

10

30

L

100

7,000

50

Composition ratios

9.71

1.78

88.51

11.74

1.46

86.80

17.59

5.03

77.38

"W" numbers for Al-0- and SiO,; "D" numbers for all other oxides and

Total Fe as FeO. 

Analysis incomplete.

feldspars varies from 0.65 to 0.82±0.05. Minor 

elements, especially lead and strontium, are more 

variable in potassium feldspars from quartz monzonite 

than in those from granodiorite.

QUARTZ

Most quartz and feldspars in the Boulder Creek 

Granodiorite are nearly equigranular; however, larger 

feldspar crystals are common throughout the 

granodiorite. In areas of cataclasis smaller quartz 

crystals cluster in rounded or serrate contact with 

larger quartz crystals or with other minerals. In 

general, quartz is conspicuously strained and generally 

free of inclusions. Inclusions of quartz, especially in 

the feldspars, are subrounded, clear, and locally abundant. As in many of the isopleth maps of other 

minerals in figure 6 (B, C, E, H), quartz amounts show
80 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 24.—Chemical and semiquantitative analyses of potassium feldspar in Twin Spruce Quartz Monzonite, Front Range, Colo.

[Total Fe and CaO determined by atomic absorption methods, Na2O and K2O by flame photometric methods, Wayne Mountjoy, analyst; SiO2 and A12O3 determined by rapid-rock techniques, H. H. Lipp, analyst for sample 384, Sam Botts for all others; semiquantitative spectrographic analyses, L. A. Bradley, analyst. N, not detected]

Sample No. ———

T Lab. 1 No. HT "• ————

b-g.

—————— D13891 

W17344

383 

2 D138913 

8 W173443

384 380 232 

D138717 D138915 

D138914 W173435 W173434

Chemical analyses, in weight

1 2 

A1AI 2°3 n _____

FeO° ————————

wa2u 

Kn __

Total —

Ba ——————————

Ni ——————————

CT-

/- o

1 Q

——————— 0.

—————— 0.

•

——————— 14.

—————— 97.

Semiquantitative

—————— 7,000

—————— 15

——————— N

—————— 1,000

—————— 150

80 64.40 

60 18.80 

07 0.05 

13 0.13 

13 1.51 

20 14.10

93 98.99

67.90 

17.20 

0.05 

0.15 

1.54 

13.60

100.44

65 

18 

0 

0 

1 

13

99

spectrographic analyses,

3,000 

30 

20 

N 

100 

500 

100

3,000 10,000 

5 15 

30 20 

5 N 

50 70 

500 2,000 

70 20

percent

.70 64.40 

.40 18.60 

.11 0.04 

.18 0.16 

.14 1.64 

.80 14.10

.33 99.24

in parts per

2,000 

15 

30 

N 

100 

500 

50

235 

D138916 

W173438

65.10 

18.20 

0.07 

0.11 

1.24 

14.50

99.22

million

3,000 

15 

30 

N 

100 

500 

N

85 

D138918 

W173445

64.40 

18.80 

0.05 

0.10 

1.18 

14.70

99.23

5,000 

2 

20 

N 

70 

1,000 

70

376 

D138921 

W173446

64.40 

18.60 

0.05 

0.10 

1.11 

14.80

99.06

3,000 

10 

20 

N 

70 

500 

50

Composition ratios

AT-, _ __

AT, ____ __

____ ____ Q

. 

OQ

84 13.02 

35 1.14 

81 85.84

13.63 

1.32 

85.05

10 

2 

87

.01 14.00 

.21 1.34 

.78 84.66

10.69 

0.99 

88.32

10.03 

1.10 

88.87

9.52 

0.92 

89.56

"W" numbers for Al

there are no "W" numbers.

and Si02 ; "D" numbers for all other oxides and for Al-O- and SiO~ where

Sample has low total oxides; SiO? may be low. 

'Total Fe reported as FeO.

a strong west to northwest regional percent distribution pattern in the central and southwestern part of 

the batholith (fig. 6E).

In the Twin Spruce Quartz Monzonite, quartz and 

the feldspars are equigranular. Quartz contacts are 

mutual, serrate, or embayed; it is always strained, and 

small blebs of quartz along mineral boundaries are particularly noticeable in strained and altered rocks. 

Quartz averages about 29 percent of the quartz monzonite, except in the Nederland quadrangle and in the

lens in schist and gneiss adjacent to the west contact 

of the Boulder Creek Granodiorite (pi. 2) where it 

averages 35 percent.

ACCESSORY MINERALS

Accessory minerals including the ores, allanite, 

apatite, and sphene, are common and characteristic of 

the Boulder Creek Granodiorite and associated mafic 

inclusions but are relatively sparse in the Twin Spruce
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 81

Quartz Monzonite and the more mafic rocks. Apatite, 

sphene, allanite, zircon, and monazite and (or) 

xenotime are abundant in that order.

Ores. Ores include iron oxides (magnetite, hematite, 

ilmenite) and the sulfides (pyrite, chalcopyrite, 

molybdenite). In the Boulder Creek Granodiorite the 

ores are predominantly magnetite and some hematite 

and pyrite; other ore minerals are extremely rare. 

Hematite is generally in the range of 15-20 percent of 

the ores in Twin Spruce Quartz Monzonite; it occurs as 

individual grains, as an intergrowth in magnetite, or as 

an overgrowth on the magnetite. Iron oxides in the 

Twin Spruce Quartz Monzonite are more complex 

because ilmenite is also present. Ilmenite is less abundant than magnetite or hematite, but, where present, it 

is associated with magnetite or hematite as single 

grains or intergrown with hematite. The speckled appearance of quartz monzonite is caused by the clustering of ores and adjacent leucocratic haloes. The dominant iron oxide in the clusters is hematite.

Sphene. Sphene in the Boulder Creek Granodiorite 

occurs predominantly along the trend of hornblendebearing Boulder Creek Granodiorite (fig. 6A and Q. 

Sphene in the batholith proper averages 1.5 percent 

but along the western contact it is nearly absent. 

Granodiorite lenses within the metasedimentary rocks, 

along the western contact, bear almost no sphene but 

further to the west in lenses of granodiorite associated 

with the more mafic rocks, sphene averages 3 percent; 

these lenses are also rich in ores. The occurrence of 

sphene is twofold: (1) as primary sphene in hemihedral 

to anhedral crystals clustered with the mafic minerals 

and the accessory minerals apatite, allanite, and zircon; or (2) as secondary sphene, which rims magnetite, 

forms small exsolved blebs in biotite and replaces 

biotite along cleavage and grain boundaries. Sphene is 

often bleached along magnetite-sphene contacts.

Sphene in the Twin Spruce Quartz Monzonite is 

generally anhedral and is dominantly a product of the 

alteration of some plagioclase, magnetite, and biotite; 

it corrodes magnetite grains and is often in bleached 

contact with magnetite.

Both a light greenish-yellow and a grayish-yellow 

sphene can be found in a single hand specimen. No effort was made to separate the two for chemical 

analysis but the greenish-yellow variety is more common and is represented in the chemical analysis. According to partial chemical analysis of CaO and TiO2 

(table 25), all sphene samples are very similar in regard 

to CaO and TiO2 . Spectrographic analyses, however, 

indicate that the sphene from Twin Spruce Quartz 

Monzonite is richer in the trace elements cerium, copper, lead, yttrium, zirconium, and neodymium than 

sphene in Boulder Creek Granodiorite (table 25). SpecTABLE 25.—Chemical and semiquantitative spectrographic analyses 

of sphene in Boulder Creek Granodiorite, inclusions in granodiorite, 

and Twin Spruce Quartz Monzonite, Front Range, Colo.

[CaO determined by atomic absorption methods, TiO2 colorimetrically determined, John 

Gardner, analyst; spectrographic analyses by L. A. Bradley. N, not determined]

Boulder Creek 

Granodiorite

Twin Spruce 

Quartz Monzonite

Sample No. — — —

Lab. No. —— —— ~

47 

D138953

49 

D138956

386 

D138954

334 

D138955

376 

D138957

Chemical analyses, in weight percent

CaO— 

TiO~-

28.8

35.4

29.8

35.4

27.9

36.0

28.3

35.4

Semiquantitative spectrographic analyses, in parts per million

Ba——-

Cr——

Cu—— 

D>———

Ga—— 

La—— 

Mo——— 

Nb——— 

Nd———

Ni——- 

Pb——— 

Pr——— 

Sc———

Sm——-

Sr———

y_____

Yb——— 

Zr———

———— 10

——— 2,000

——— 150

——— 30

——— 200

——— 300

——— 15

——— 300

——— 3,000

N 

20

500 

20

700

200

70

700

1,500

70

300

30

2,000

100

20

200

N 

200

30

200

3,000

N 

70

200 

15

700

150

100

700

1,500

70

500

30

3,000

150

150

200

N 

700

15

100

3,000

5

50

500

20

500

100

70

700

1,500

70

200

10 150

7,000 5,000

50 50

20 1,000

300 150

10

2,000

20

300

7,000

N 

50

1,500 

20

1,500

200

150

300

2,000

50

500

7

1,500

15

300

5,000

N

150 

1,000

50 

1,000

150

150

500

3,000

70

1,000

trographic analyses of trace elements in sphene from 

inclusions indicate that some sphene has trace 

elements similar to sphene in Twin Spruce Quartz 

Monzonite and other sphene is more closely allied to 

Boulder Creek Granodiorite.

Apatite. Apatite occurs as small euhedral to anhedral crystals that, according to X-ray powder patterns, 

are fluorapatite. It is very pale greenish in hand 

specimen, colorless in thin section, and is predominantly associated with the mafic clusters in the granodiorite. Its modal distribution indicates that apatite is 

partly controlled by rock type. (Compare figs. 3 and 

6E). In the Boulder Creek Granodiorite apatite tends 

to concentrate in the rocks of quartz diorite (tonalite) 

or granodiorite composition and is nearly absent in 

those of quartz monzonite composition.

Allanite. Allanite is rare in the mafic rocks to the 

west of the batholith proper, including the Los Lagos 

and Severance lenses (table 7) but is a common accessory mineral in mafic inclusions in the batholith 

(table 6, samples 318-327, 333) and also in the lam-
82 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

prophyre (table 6, samples 328, 329, 331, 332). Allanite 

in the Twin Spruce Quartz Monzonite, however, rarely 

exceeds 0.2 modal percent and normally if present is in 

trace amounts only. The distribution of allanite in the 

Boulder Creek Granodiorite is outlined in fi'gure 6D. 

The distribution pattern in figure 6D does not agree 

with the one presented by Hickling, Phair, Moore, and 

Rose (1970, p. 1976). Their pattern was based on fewer 

than one-third of the observation points used here. 

Areas where allanite is more abundant, greater than 

0.2 percent, correspond to areas that are in the trend of 

hornblende-bearing Boulder Creek Granodiorite. 

Allanite, however, is not confined to hornblendebearing rocks, but where it does occur without hornblende it is found always in less than 0.2 percent 

amounts. This relationship is also true of the scattered 

plutons to the west of the batholith. The Pisgah pluton 

is a good example; large allanite crystals occur in it but 

no hornblende thus far has been found in the pluton. 

Physical properties of allanite in the Boulder Creek 

batholith have been studied extensively by Hickling, 

Phair, Moore, and Rose (1970) and will only be mentioned briefly here for consistency in mineral description. Allanite in thin section is yellowish brown to reddish brown and commonly is zoned and rimmed by 

epidote. The centers of crystals are commonly isotopic, 

and epidote usually occurs between biotite-allanite 

contacts. Hickling, Phair, Moore, and Rose (1970) 

reported high-birefringent allanite in the northwest 

part of the batholith but high-birefringent allanite is 

also found sparsely scattered throughout the central 

part of the batholith extending to the southern border. 

In separating allanite from rock samples, it was found 

that allanite is also locally associated with pyrite. 

Pyrite attached to or as an inclusion in allanite is 

euhedral to anhedral. This association is rarely seen in 

thin section. An explanation may be that the small 

allanite and still smaller pyrite crystals are plucked 

out in grinding of the section.

It is here proposed that both primary and secondary 

allanite exists in these rocks. This interpretation differs from that of Hickling, Phair, Moore, and Rose 

(1970), who proposed that allanite was all of secondary 

origin, principally an alteration product of biotite. 

True primary allanite is not readily distinguishable 

because of the large amount of allanite replacing 

biotite, but small euhedral crystals of allanite in 

plagioclase and hornblende, hemihedral allanite 

crystals cutting biotite at an angle, and large allanite 

crystals in granodiorite plutons well out from the 

batholith all suggest that some of the allanite is 

primary. It is suspected that a considerable amount of 

the larger allanite crystals in the batholith is also 

primary. The modal distribution pattern of allanite,

which follows that of hornblende in a general way, suggests that primary allanite is characteristic of the 

batholith.

Allanites in Twin Spruce Quartz Monzonite are often 

zoned (sometimes as many as five or more distinct 

zones can be counted) but have sharp contacts against 

adjacent minerals (fig. 37). Almost no epidote occurs at 

the contact of allanite and biotite as is commonly

•/^•^.».v\J^X: ." S'^^****'*?*-*^*^ &,t>*>#£-Nr % •>•"—4 r ~ -.-|^-->^ If^i^;^ l£?% * •:', ^ 1

FIGURE 37.—Allanite and monazite in Twin Spruce Quartz Monzonite. A, Allanite crystal slightly altered to epidote with growth 

pressure cracks radiating away from it. Allanite may be late 

primary crystal in Twin Spruce Quartz Monzonite (sample 85, 

table 13). Plain light, X100. B, Monazite crystal with brown, 

almost isotropic alteration zone that may be allanite. Epidote occurs in radiating growths on allanite (sample 89, table 8). Plain 

light, X100. A, allanite; B, biotite; E, epidote; M, monzonite; Mu, 

muscovite; My, myrmekite; P, zoned plagioclase; Me, microcline; 

Q, quartz. Photographs by Louise Hedricks, U.S. Geological 

Survey.
MINERALOGY, PETROLOGY, AND CHEMISTRY OF MINERALS 83

observed in the Boulder Creek Granodiorite. In quartz 

monzonite, monazite locally alters to what appears to 

be allanite (fig. 31B). It appears that allanite from 

hornblende diorite may be richer in copper, yttrium, 

neodymium, samarium, gadolinium, and dysprosium 

than allanite from either granodiorite or its inclusions 

(table 26).

Allanite in a quartz monzonite lens along the crest of 

the ridge north of Boulder County Hill in the Caribou 

pluton (pi. 1) appears to have formed from the reaction 

of monazite and biotite. Within the batholith proper, 

quartz monzonite lenses locally bear monazite that has 

coronas of what may be secondary allanite (fig. 31B). 

This secondary allanite is mantled by epidote crystals 

formed by the reaction biotite+plagioclase?+monazite-*allanite+epidote+iron oxides. Dietrich (1961) 

noted similar coronas of monazite altering to epidote in 

the Mount Airy Granite.

Monazite and (or) xenotime. Both monazite and 

xenotime are rare accessory minerals in the Boulder 

Creek Granodiorite and Twin Spruce Quartz Monzonite. Both minerals are difficult to identify in thin 

section because they usually occur as very small 

subhedral to subrounded crystals that are associated 

with apatite, biotite, allanite, and ores. Xenotime has a 

faint yellow pleochroism and monazite a faint brown 

pleochroism, but this pleochroism is difficult to 

observe unless the minerals are isolated in leucocratic 

areas of the thin section.

ALTERATION MINERALS

Muscovite-sericite, Muscovite never occurs with 

hornblende except as small flakes totally within 

plagioclase crystals. Wells (1967, p. D22) also noted 

this characteristic from his work in the Eldorado 

Springs quadrangle.

Muscovite laths in Boulder Creek Granodiorite 

lenses adjacent to the batholith are at large angles to 

biotite with no evident reaction between them. This 

muscovite is older than most microcline and quartz 

and is perhaps a primary muscovite as both microcline 

and quartz embay it. In addition to this early 

muscovite, a later muscovite occurs in large lathlike 

patches that extend across microcline crystals. Within 

the batholith, all muscovite is clearly secondary; it 

forms thin rims on the ores, and occurs as overgrowths 

on biotite, as growths along biotite cleavage, or forms 

in plagioclase where plagioclase is intensely altered. 

Epidote and (or) calcite may accompany the muscovite 

in recrystallized parts of the batholith.

Muscovite appears to be both primary and secondary in the Twin Spruce Quartz Monzonite. Primary 

muscovite appears to be the same age as biotite and

the ores, or younger, and has had a long and complex 

history. In the southeast part of the batholith, along 

the granodiorite-quartz monzonite contact, rocks that 

have been recrystallized contain secondary musocovite 

laths that are larger than anywhere else in the 

batholith. In the field muscovite is plainly visible and 

often profuse. In thin section the large laths appear 

eroded again by potassium feldspar and quartz. The 

laths are characteristically bent and strained and 

dendritic-like. Muscovite having a single orientation 

has grown in the host mineral, feldspar.

Prehnite and hydrogarnet. Both minerals were first 

described by Wrucke (1965) as occurring in the northeast part of the exposed batholith. This study confirms 

that this is also the only area of the batholith in which 

they occur. Prehnite alone has been found only along 

the northwest contact of the batholith and along its 

most northern contact in Left Hand Canyon. Growth 

of prehnite is due to the release of calcium in the alteration of plagioclase and hornblende. Prehnite in lensshaped aggregates parallel to the (001) cleavage in 

biotite is perhaps epitaxial; the 001 surface provides 

suitable nucleation sites (Moore, 1976, p. 527). 

Prehnite was associated with quartz in only one thin 

section and this was in a small vein cutting other 

minerals, indicating mobility of Ca2+ ions. 

Hydrogarnet replaces prehnite but is more likely part 

of the reaction involving hornblende and (or) 

biotite+plagioclase.

The experiments of Coombs, Ellis, Fyfe, and Taylor 

(1959) showed that prehnite is not stable above 450 °C 

at water pressure of as much as 500 MPa; thus a 

magmatic origin is not likely. A calcium or potassium 

metasomatism accompanying faulting, shearing, and 

recrystallization may be possible.

Epidote, clinozoisite, and piedmontite. Very little 

clinozoisite has been identified in the southern twothirds of the batholith because of its similar occurrence 

to epidote and the difficulty in distinguishing it from 

epidote; therefore the two are reported together in 

modes. Piedmontite was only recognized at one locality, north of Wallstreet, Gold Hill quadrangle, in a 

pegmatite outcrop. The piedmontite gives the rock a 

very pink color. Crystallization was probably from 

hydrothermal solutions.

Like sphene, epidote is nearly absent from granodiorite lenses in metasedimentary rocks adjacent to 

the Boulder Creek batholith and from the batholith 

near its contact, except in areas where hornblendebearing rocks occur along the contact (fig. 6-A). 

Epidote chiefly replaces biotite or hornblende 

associated with biotite; hornblende only weakly alters 

to epidote. Replacement is along crystal boundaries or 

in aggregates along biotite cleavage. Also, epidote
84 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

TABLE 26.—Chemical and spectrographic analyses ofattanite in Boulder Creek Granodiorite, inclusions, and biotitic

hornblende diorite, Boulder Creek batholith area, Front Range, Colo.

[Oxides determined by lithium borate fusion, X-ray methods by J. S. Wahlberg; spectrographic analysis by J. C. Hamilton. >, greater than, H. not found;

N.D. not done]

Boulder Creek 

Granodiorite

________ *49

—————— D138960

47 

D138961

Inclusion

385 334 

D172033 D138962

Hornblende 

diorite

352 

D138963

Chemical analyses, in weight percent

Paf) ___ . . __ _

Ce203

FeO —————

T LaQ 2°3f) _____ __

MnO —— — ——

B _ _

•RQ _ _

Po — — —

Cu ———————

fly —————————————

TT-ii

Ga ———————

f A

Un . __ . _

Mo ——————— 

Nd ———————

pv, _ _ _ _

Pr ——————

Sm ——————— •

C-r — J L

•Hi __ _____ . v ————— .

YH- _ _ X U 

*7v

11 C

—————— 9.0 

—————— 12.0

—————— 5.5

—————— 0.2

Spectrographic

————— n.d.

————— n.d.

—————— n.d.

—————— n.d. 

—————— n.d.

—————— n.d.

—————— n.d. 

—————— n.d.

—————— n.d. 

—————— n.d.

—————— n.d.

—————— n.d. 

—————— n .d. >2Q

—————— n.d. 

—————— n.d.

—————— n .d. >io

—————— n.d.

—————— n.d. 3 

—————— n.d.

11 • VJ. * ^f

————— n.d. 

—————— n.d.

—————— n.d.

—————— n.d.

12.1 

9.0 

12.2 

5.5 

1.3

analyses,

70 

20 

150 

100

70 

70 

150 

50 

150

100 

15 

,000 

20 

300

,000 

500 

,000 

700 

,000

700 

500 

50 

300

17.0 

12.0 

16.2 

3.4 

0.2

in parts

50 

500 

20 

700 

100

100 

50 

300

70

30,000 

50 

900

10,000 

1,000 

2,000 

1,500 

10,000

500 

700 

50 

150

12.1 

10.5 

12.0 

7.2 

0.3

per million

150 

15 

70 

70

70 

70 

150 

30 

200

70

30,000 

7 

150

10,000 

700 

3,000 

1,000 

2,000

700 

700 

15 

300

10.6 

8.9 

11.5 

4.3 

0.3

30 

300 

30 

70 

1,500

500 

100 

150 

30 

1,500

100 

70,000

500

15,000 

1,000 

7,000 

1,500 

5,000

500 

2,000 

70 

500

limits of detection do not apply due to dilution technique. bxotal Fe as FeO.
REFERENCESCITED 85

replaces allanite, sometimes leaving only an allanite 

core. Allanite-biotite contacts are generally separated 

by a zone or layer of epidote, and incomplete reactions 

involving epidote are: biotite-*-epidote+sphene; hornblende +biotite-*-sphene+epidote+magnetite; hornblende-*epidote± magnetite; allanite+biotite-*-epidote. 

Epidote in the batholith modally occurs in numerous 

small epidote islands that are difficult to correlate with 

any of the other mineral trends, but if allanite and 

epidote modes are combined, there is a distinct pattern 

that again mimics the distribution of hornblendebearing rocks in the batholith. The higher values of 

allanite are not in epidote-rich areas but appear to be 

coincident with the more mafic inclusions in the 

Boulder Creek batholith.

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recent work across the central part: U.S. Geological Survey Professional Paper 414-D, p. D1-D46.

Boos, M. F., and Boos, C. M., 1934, Granites of the Front Rangethe Longs Peak-St. Vrain batholith: Geological Society of 

America Bulletin, v. 45, no. 2, p. 302-322.

Bowen, N. L., 1928, The evolution of the igneous rocks: Princeton, 

Princeton University Press, 332 p.

___1937, Recent high-temperature research on silicates and its 

significance in igneous geology: American Journal of Science, 

5th ser., v. 33, no. 193, p. 1-21.

Braddock, W. A., 1969, Geology of the Empire quadrangle, Grand, 

Gilpin, and Clear Creek Counties, Colorado: U.S. Geological 

Survey Professional Paper 616, 56 p.

Bryant, Bruce, and Hedge, C. E., 1978, Granite of Rosalie Peak, a 

phase of the 1,700 m.y. old Mount Evans pluton, Front Range, 

Colo.: U.S. Geological Survey Journal of Research, v. 6, no. 4, 

p. 447-452.

Buddington, A. F., 1959, Granite emplacement with special reference to North America: Geological Society of America Bulletin, 

v. 70, no. 6, p. 671-747.

Cawthorn, R. G., and Brown, P. A., 1976, A model for the formation 

and crystallization of corundum-normative calc-alkaline 

magmas through amphibole fractionation: Journal of Geology, 

v. 84, no. 4, p. 467-476.

Chayes, Felix, 1952, The finer-grained calc-alkaline granites of New 

England: Journal of Geology, v. 60, no. 3, p. 207-254.

Coombs, D. S., Ellis, A. J., Fyfe, W. S., and Taylor, A. M., 1959, The 

zeolite facies, with comments on the interpretation of 

hydrothermal syntheses: Geochimica et Cosmochimica Acta, v. 

17, no. 1-2, p. 53-107.

Deer, W. A., Howie, R. A., and Zussman, J., 1963, Framework silicates, v. 4 of Rock forming minerals: New York, John Wiley and 

Sons, 435 p.

Dietrich, R. V., 1961, Petrology of the Mount Airy "granite": Virginia Polytechnic Institute Bulletin, Engineering Experiment 

Station Series 144, v. 54, no. 6,63 p.

Dodge, F. C. W., Smith, V. C., and Mays, R. E., 1969, Biotites from 

granitic rocks of the central Sierra Nevada batholith, California: 

Journal of Petrology, v. 10, no. 2, p. 250-271.

Gable, D. J., 1969, Geologic map of the Nederland quadrangle, Boulder and Gilpin Counties, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-833, scale 1:24,000.

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86 BOULDER CREEK BATHOLITH, FRONT RANGE, COLORADO

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INDEX

[Italic page numbers indicate major references]

Abstract...................................

Actinolite ..................................

Age .................................... 3, 20,

Alkali feldspar, Boulder Creek Granodiorite...

Twin Spruce Quartz Monzonite ..........

Allanite ............................ 25, 78, 81,

Boulder Creek Granodiorite ...... 17, 51, 64,

in lamprophyre .........................

Twin Spruce Quartz Monzonite ..........

Aluminum .................................

in lamprophyre .........................

Analyses, techniques .......................

Andesine, Boulder Creek Granodiorite........

Apatite................................ 25, 48,

Boulder Creek Granodiorite............. 17,

in lamprophyre .........................

in mafic inclusions ......................

Twin Spruce Quartz Monzonite ..........

Aplite .....................................

gneissic ................................

Twin Spruce Quartz Monzonite ..........

Assimilation, in Boulder Creek Granodiorite .. 

Augite.....................................

B

Bald Mountain pluton.................

Barium, Boulder Creek Granodiorite ..

Twin Spruce Quartz Monzonite .... 

Beryllium, Boulder Creek Granodiorite .

in quartz monzonite...............

Bibliography

Bighorn Mountain, mafic inclusions ..........

Biotite ........................ 6, 23, 24, 34, 45,

Boulder Creek Granodiorite ............. 7,

51,64,

in lamprophyres ...................../..

in mafic inclusions ......................

Twin Spruce Quartz Monzonite ......... 28,

Biotite gneiss ..............................

Biotite schist...............................

Biotite-sillimanite gneiss .................... 3,

Blackhawk quadrangle, quartz diorite ........

Boulder batholith, Montana .................

Boulder Creek, Middle, mafic inclusions ......

Boulder Creek Granodiorite, constituent rocks

defined ................................

geochemistry ...........................

Bronzite ...................................

Buckeye Mountain anticline, folds ...........

1

25

27,35

51

51

83,85

72,73

20

69

73

48

2

52

81, 83

72,73

20

20, 79

69

6, 20

34

27

52

25

72

77

51

73

51

85

20

48,83

17,38,

72,73

20

20

69, 79

17

6

17,53

17

1

20

7

4

38

25

37

Calcite.................................... 23, 25, 83

Boulder Creek Granodiorite.............. 72

Calcium ................................... 48, 83

in lamprophyre ......................... 48

relation to oxides ....................... 39

Caribou pluton ......................... 7, 72, 73, 83

Central City quadrangle, gabbro ............

granite gneiss .........................

pegmatite .............................

Pisgah pluton .........................

Cerium ..................................

Boulder Creek Granodiorite............

in mafic inclusions....................

...... 23

...... 34

...... 34

....... 7

....... 81

....... 72

....... 50

Twin Spruce Quartz Monzonite .......... 51, 69

Page

Chalcopyrite ............................... 81

Chemical equilibrium ....................... 51

Chemical trends ............................ 38

Chlorite.................................... 25

Boulder Creek Granodiorite.............. 51, 64

Twin Spruce Quartz Monzonite .......... 69

Chromium ................................ 45, 50, 73

Boulder Creek Granodiorite .............. 73

in quartz monzonite ..................... 51

Twin Spruce Quartz Monzonite .......... 69

Classification, rock ......................... 2

Clinopyroxene.............................. 23

Clinozoisite ................................ 83

Coal Creek Canyon, quartz monzonite ........ 27

Cobalt ..................................... 50

in quartz monzonite ..................... 51

Colorado mineral belt....................... 3

Constitution ............................... 4

Contact relations ........................... 6

Copper ................................... 50, 81, 83

in mafic inclusion ....................... 50

Country rock .............................. 23, 37,53

Cummingtonite ............................ 25

D

Deformation ............................... 37

Differentiation, in Boulder Creek Granodiorite 52

Dikes ...................................... 20, 34

Diopside ................................... 25

Diorite .................................... 20, 23

Dysprosium................................ 83

E

Eldora, folds ............................... 37

Eldorado Springs, quartz monzonite.......... 29

Elk Creek pluton, gabbro.................... 23

Eldorado Springs quadrangle ................ 4, 39, 83

Emplacement, mode ........................ 6

Epidote................................... 25, 82,83

Boulder Creek Granodiorite............. 17, 51,64

Feldspar ................................... 8

lineation ............................... 7

Fluorine ................................... 39, 73

Foliation................................... 6, 37,64

Formation ................................. 3

Gabbro .................................... 6, 20, 23

chemical trends .....................

Gadolinium ............................

Garnet, Boulder Creek Granodiorite......

Geochemistry ..........................

Gold Hill, contact relations ..............

folds ...............................

gabbro .............................

gneissic aplite ......................

granite gneiss ......................

pegmatite ..........................

Gold Hill-Bighorn road, mafic inclusions . 

Gold Hill quadrangle....................

quartz diorite.......................

50

83

63

38

6

37

23

34

34

34

20

7

17

Page

Golden Gate Canyon, pegmatite ............. 34

Granite .................................... 4

Granite gneiss ...................••-..•••••• 34

Granodiorite ..................••••••••••••• 4, 20

Gross Reservoir, lineation trends ............. 7

H

Hematite .......................•••.••••••• 81

Boulder Creek Granodiorite.............. 51

Hornblende ............................. 6, 23, 48, 64

Boulder Creek Granodiorite........ 8,17,38, 51, 71

in lamprophyres ........................ 20

in mafic inclusions ...................... 20

Hornblende diorite......................... 20, 23, 25

chemical trends ......................... 50

Hornblende gneiss .......................... 3, 23

Hornblendite............................... 6, 20, 25

Hydrogarnet ............................... 83

Idaho Springs-Ralston cataclastic zone, contact relations ................... 6

Idaho Springs-Ralston shear zone ........... 57

Ilmenite ................................... 81

Inclusions, Boulder Creek Granodiorite ....... 17

mafic .................................. 17, 38

allanite............................. 81

Boulder Creek Granodiorite ...... 69, 73, 77, 79

chemical trends ..................... 48

Iron oxides................................. 81

Boulder Creek Granodiorite.............. 51, 64

J, K

Jenny Lind syncline, folds . 

Jointing .................

Kaolinite

Labradorite ................................ 52, 77

Lakewood Reservoir, pyroxenite ............. 23

Lamprophyre .............................. 17, 20

allanite ................................ 82

Lamprophyre dike. Boulder Creek Granodiorite .69,73,79

Lanthanum, in mafic inclusion ............... 50

Twin Spruce Quartz Monzonite .......... 51

Lead ...................................... 81

in quartz monzonite ..................... 51

Twin Spruce Quartz Monzonite .......... 69, 79

Lefthand Canyon, granite gneiss .

pegmatite ..................

prehnite....................

Lenses .........................

Lineations, mafic rocks..........

Los Lagos lens .................

pyroxenite .................

M

Magma, emplacement..........

Boulder Creek Granodiorite. 

Magnesium, in lamprophyre ....

34

34

83

34

22

81

23

87
88 INDEX

	Page

Magnetite ................................. 81

Boulder Creek Granodiorite.............. 51, 72

Magnetite ore .............................. 8

Magnolia .................................. 4

Manganese, Boulder Creek Granodiorite...... 51

Metagabbro................................ 23

Metamorphism ............................. 37

hydrothermal........................... 51

Microcline ................................. 34, 45

Boulder Creek Granodiorite.............. 8,17, 77

Twin Spruce Quartz Monzonite .......... 69, 79

Microcline gneiss ........................... 3, 6

Microperthite, Boulder Creek Granodiorite.... 8, 77

Molybdenite ............................... 81

Monazite .................................. 81, 83

Boulder Creek Granodiorite.............. 64, 69

Twin Spruce Quartz Monzonite .......... 69

Mt. Evans batholith ........................ 3, 37

Mt. Pisgah, contact relations ................ 7

Muscovite ................................ 25, 34, 83

Boulder Creek Granodiorite.............. 51, 77

Twin Spruce Quartz Monzonite .......... 29

Myrmekite, Boulder Creek Granodiorite ...... 51, 77

Twin Spruce Quartz Monzonite .......... 77

N

Nederland, contact relations ................. 6

quartz monzonite ....................... 27

Nederland quadrangle, gabbro ............... 23

pyroxenite ............................. 23

quartz ................................. 80

Nederland syncline, folds .................... 37

Nederland-Tungsten district, Laramide mineralization ...................... 6

Neodymium ................................ 81, 83

in mafic inclusions ...................... 50

Twin Spruce Quartz Monzonite .......... 69

Nickel.................................... 45, 50, 73

in mafic inclusions ...................... 50, 73

in quartz monzonite ..................... 51

O

Oligoclase, Boulder Creek Granodiorite ....... 52

Olivine .................................... 23

Ores ............................... 23, 25, 34, SO, 83

Boulder Creek Granodiorite............. 64, 72, 73

Twin Spruce Quartz Monzonite .......... 29, 69

Origin ..................................... 62

Orthopyroxene ............................. 23

Overland Mountain ......................... 7

Overland pluton, described .................. 7

Oxides.................................... 38, 64,69

Page

Peak to Peak highway, contact relations ...... 6

Pegmatite.................................. 6, 20, 34

Twin Spruce Quartz Monzonite .......... 27

Piedmontite................................ 83

Pikes Peak Granite ......................... 3

Pinecliffe, lineation trends ................... 7

mafic inclusions ........................ 20

Pisgah pluton .............................. 7, 65, 82

Plagioclase ................ 6, 23, 25, 34, 48, 57, 64, 81

Boulder Creek Granodiorite........... 8,17, 51, 73

in quartz monzonite ..................... 51

Twin Spruce Quartz Monzonite ......... 69, 77, 79

Potassium ................................. 48

in lamprophyre ......................... 48

Potassium feldspar ......................... 7, 45, 57

Boulder Creek Granodiorite.... 8, 17, 51, 64, 73, 77

Twin Spruce Quartz Monzonite .......... 79

Prehnite ................................... 83

Pyrite ..................................... 81, 82

Pyroxene ................................. 23, 25, 69

in lamprophyre ......................... 20, 73

Pyroxenite .............................. 6, 20, 23, 25

chemical trends......................... 50

Q, R

Quartz ........................... 6, 23, 34, 64, 79, 83

Boulder Creek Granodiorite.............. 8, 17

in lamprophyre ......................... 20

in mafic inclusions ...................... 20

Twin Spruce Quartz Monzonite .......... 69, 79

Quartz diorite .............................. 4, 20, 25

Quartz monzonite .......................... 4, 26

Quartzite .................................. 3, 6

Ralston Buttes, quartz monzonite. 

Rollins Pass .....................

Rosalie lobe .....................

29

6

4

Salina-Gold Hill road, mafic inclusions ....... 20

Samarium ................................. 83

Scandium .................................. 50

Sericite .................................... 25, 83

Boulder Creek Granodiorite .............. 51

Serpentine ................................. 23

Severance lens ............................. 81

pyroxenite ............................. 25

Sillimanite ................................. 6, 34

Sillimanite-biotite gneiss .................... 3, 17

Sills ....................................... 20

Silver Plume Quartz Monzonite ..............

biotite .................................

Sodium ....................................

South Boulder Creek, contact relations .......

Sphene .................................... 8,

Boulder Creek Granodiorite ......... 17, 51,

in lamprophyre .........................

Strawberry batholith .......................

Strawberry Lake batholith ..................

Strontium, Boulder Creek Granodiorite.......

Twin Spruce Quartz Monzonite ..........

Structure ..................................

Studies, previous ...........................

Sugarloaf Mountain, contact relations ........

Summary ..................................

Thorodin Mountain, mafic inclusions .........

Tremolite ..................................

Tremont Mountain, contact relations .........

lineation trends .........................

Trondhjemite ..............................

Twin Spruce Quartz Monzonite ..........

Tungsten mining district, quartz monzonite... 

Tungsten quadrangle .......................

quartz diorite...........................

quartz monzonite .......................

Severance lens..........................

Twin Spruce Quartz Monzonite, chemical 

trends ..........................

defined ................................

mineralogical trends ....................

V, W, X

Vanadium .............

in quartz monzonite .

Page

3

69

48

6

25, 81

64,72

20

37

3

73, 77

51,79

35

1,2

6

1

20

25

6

7

73

27

27

39

17

27

25

50

4

50

Ward quadrangle ........................... 7, 17

Xenotime .............................. 64, 69, 81,83

Y, Z

Yttrium .................................. 73, 81, 83

in quartz monzonite ..................... 51

Zinc, Twin Spruce Quartz Monzonite ......... 69

Zircon ..................................... 25, 81

Boulder Creek Granodiorite.............. 17

Twin Spruce Quartz Monzonite .......... 69

Zirconium.................................. 81

in mafic inclusions ...................... 50, 73

Twin Spruce Quartz Monzonite .......... 51

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