The Whitehorn Granodiorite 

Of the Arkansas Valley 

In Central Colorado

GEOLOGICAL SURVEY BULLETIN 1394 H

The Whitehorn Granodiorite 

Of the Arkansas Valley 

In Central Colorado

By CHESTER T. WRUCKE

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1394-H

Description of a new formation of 

Cretaceous age in central Colorado

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1974
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress catalog card No. 74-600082

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

Washington, D. C. 20402 - Price 25 cents (paper cover)

Stock Number 2401-02514
CONTENTS

Page

Abstract ________________________________________ 1

Name, type area, and previous work ____________________ 1

Geologic setting, distribution, and form ____________________ 3

Megascopic character _______________________________ 3

Microscopic character ____________________________ 4

Modal and chemical data____________________________ 5

Age___________________________________________________________ 6

References cited _______________________________ 6

ILLUSTRATIONS

Page

FIGURE 1. Map of part of central Colorado showing the geology in the vicinity 

of Cameron Mountain and the location of the Calumet mine, 

Salida, and Whitehorn _______________________ 2

2. Plot of modal quartz, plagioclase, and K-feldspar in 12 specimens

of the Whitehorn Granodiorite _______________________ 5

TABLE

Page

TABLE!. Chemical and modal data of the Whitehorn Granodiorite ___ 7

CONTRIBUTIONS TO STRATIGRAPHY

THE WHITEHORN GRANODIORITE

OF THE ARKANSAS VALLEY

IN CENTRAL COLORADO

By CHESTER T. WRUCKE

ABSTRACT

The Whitehorn Granodiorite is a new formation on the east side of the Arkansas 

Valley in the vicinity of Salida, Colo. It ranges in composition from syenogabbro to 

quartz monzonite and forms a pluton and three small satellitic bodies. The main body, 25 

km long and as much as 8 km wide, intrudes Precambrian and Paleozoic rocks; it is 

interpreted as having a subhorizontal floor on upper Paleozoic strata and a steeply 

inclined feeder conduit. One of the satellitic bodies is a plug, one is sill-like, and the other 

is of complex form.

Most of the granodiorite, including rock at the type locality on Cameron Mountain, is 

fine to medium grained and medium gray. It contains 12-27 percent dark minerals and 

medium-gray plagioclase. A few light-colored varieties occur locally that contain only a 

few percent dark minerals. The syenogabbro is dark olive gray and forms small bodies at 

the margin of the plug.

The granodiorite is typically a hypidiomorphic seriate rock consisting of quartz, 

plagioclase, orthoclase, augite, hornblende, biotite, and a few accessory minerals. The 

plagioclase is as calcic as Aries- Chemical analyses show that the rock is lower in SiO2 , 

NaO2 , and higher in Fe^C^, MgO, and CaO than average granodiorite.

Potassium-argon ages range from 69.4 to 70.4 million years, suggesting that the 

granodiorite was intruded in Late Cretaceous (early Maestrichtian) time.

NAME, TYPE AREA, AND PREVIOUS WORK

The name Whitehorn Granodiorite is given here to the rock that 

forms a pluton and several small satellitic bodies (fig. 1) on the east 

side of the Arkansas Valley in central Colorado. This name is taken 

from the ghost mining town of Whitehorn founded in 1897 (Eberhart, 

1959) at the east margin of the body. Names previously used for the 

pluton include Whitehorn stock (Burbank and Levering, 1933, p. 290), 

Calumet Granodiorite (Behre and others, 1936, p. 786), Calumet stock 

(Behre and others, 1936, p. 790), Cameron Mountain stock (Young, 

1972, p. 118), and Salida stock (McDowell, 1971). The name "Calumet" 

has been preempted (Keroher, 1966) for rocks in Michigan. Cameron 

Mountain, near the center of the pluton, is designated the type locality. 

It is accessible from a gravel road that extends northeast from Salida

and crosses the west and north sides of the mountain.

l
CONTRIBUTIONS TO STRATIGRAPHY

106 00 105 45

38 45'

38 30'

EXPLANATION

Fault

-tAxis of anticline 

Axis of syncline

Locality of chemically 

analyzed or potassiumargon-dated sample

COLORADO

10MILES

10 KILOMETERS

FIGURE 1. Geology in the vicinity of Cameron Mountain and the location of the 

Calumet mine, Salida, and Whitehorn.

Burbank and Levering (1933) mention the granodiorite at 

Whitehorn in a study of the stratigraphic and structural setting of ore 

deposits in Colorado. Behre, Osborn, and Rainwater (1936) describe 

the granodiorite in a discussion of the Calumet mine (fig. 1), and 

Harris and Wilbanks (1966) report on zircons in fresh and weathered 

samples of the rock. Data by McDowell (1971) on the age of the 

granodiorite and by Youn&(1972) on chemical analyses of the rock are
WHITEHORN GRANODIORITE, COLORADO 3

discussed subsequently. Theses that contain maps showing large areas 

of the granodiorite or important contact relations are by Bhutta 

(1954), Normand (1968), Lowell (1969), and Pierce (1969).

GEOLOGIC SETTING, DISTRIBUTION, AND FORM

The granodiorite, which ranges in composition from syenogabbro to 

quartz monzonite, intrudes Precambrian igneous and metamorphic 

rocks and nearly flat lying Paleozoic sedimentary rocks (fig. 1). The 

main body of the granodiorite, 25 km (15.5 mi) long and as wide as 8 km 

(5 mi), lies almost completely in Paleozoic rocks. Exposures of this body 

are separated into two parts by a narrow band of Tertiary volcanic 

rocks deposited in an east-trending paleovalley (Chapin and others, 

1970) 2.4 km (1.5 mi) wide across the central part of the pluton (fig. 1). 

The contact around the south and west sides dips inward subparallel to 

the nearly flat lying host sandstone and shale of Pennsylvanian and 

Permian age. Along the southeast side, the contact is steep, and the 

granodiorite invades Precambrian rocks that were upthrown against 

the Paleozoic rocks to the west before emplacement of the pluton. 

Around the east side of the northern part of the intrusion, the contact is 

vertical to steeply inward dipping and entirely in rocks of Pennsylvanian age. Although known imperfectly, the pluton is interpreted as 

having a subhorizontal floor on Paleozoic strata and to have been fed 

by a steeply inclined conduit. Roof rocks have been eroded away.

Three satellitic bodies crop out west of the main intrusion. The 

northern body is sill-like and intrudes faulted Paleozoic rocks. Of the 

other two bodies, the eastern mass is of complex form and has a 

subhorizontal eastern contact but a steep western contact. The western body is a plug that cuts Precambrian and Paleozoic rocks.

MEGASCOPIC CHARACTER

Most of the granodiorite, including rock at the type locality, is fine to 

medium grained and medium gray on fresh and weathered surfaces. It 

has medium-gray plagioclase and 12-27 percent dark minerals. At 

many places it contains very dark gray angular to subrounded biotiterich inclusions less than 2.5 cm to about 30 cm across. Inclusions of 

carbonate rocks and sandstone or quartzite are reported by Behre, 

Osborn, and Rainwater (1936). Light-gray varieties of the granodiorite containing only a few percent dark minerals crop out locally, but 

aplitic and pegmatitic dikes are sparse. The distribution of these 

leucocratic rocks and the nature of the contacts with the more common 

medium-gray granodiorite are poorly known. A dark-olive-gray 

medium-grained syenogabbro occurs locally at the margin of the plug 

southwest of the main pluton.

A weak flow foliation occurs near the margin of the northern part of
4 CONTRIBUTIONS TO STRATIGRAPHY

the main pluton. It dips gently eastward along the west side but 

steeply inward to almost vertical at the east margin. Lowell (1969) 

shows foliation with random attitudes in a small area in the southern 

part of the main intrusive, but most of this part of the pluton has not 

been studied in detail.

MICROSCOPIC CHARACTER

As seen in thin section, most of the granodiorite is a hypidiomorphic 

seriate rock in which plagioclase is conspicuous as euhedral to subhedral crystals generally 0.2-3 mm long. The plagioclase crystals have 

cores of labradorite as calcic as An6a and rims of oligoclase; many 

crystals have oscillatory zoning. Orthoclase ranges from small interstitial grains to poikilitic crystals 3 mm across. A few K-feldspar 

crystals are faintly perthitic. Most quartz is interstitial, but a few 

grains as large as 1 mm across are poikilitic. Subhedral to anhedral 

augite crystals 0.1-1 mm long occur as isolated crystals and in aggregates associated with green hornblende, brown biotite, and magnetite. 

Apatite is a common accessory mineral; hypersthene, sphene, and 

zircon are sparse. Porphyritic varieties of granodiorite at the border of 

the large pluton have blocky plagioclase phenocrysts in a microgranitic groundmass.

Examination of a few specimens of the light-colored rocks reveals 

that most are altered granodiorite in which feldspars are dusted with 

fine alteration products and the mafic grains are largely altered to 

chlorite. A relatively common leucocratic quartz monzonite in the 

southern part of the pluton has no more than 5 percent mafic grains, 

now largely chlorite. This rock is porphyritic and contains abundant 

quartz and partly sericitized plagioclase phenocrysts in a microgranitic groundmass of K-feldspar and quartz.

The syenogabbro border zone of the plug is a labradorite-Kfeldspar-pyroxene-olivine rock containing both augite and hypersthene. Olivine has been converted to serpentine, clays, and magnetite. This rock has a higher color index (about 40) and a much larger 

grain size than the granodiorite. Plagioclase crystals are mostly 1-3 

mm long, but some reach 6 mm; augite grains commonly are 0.5-2.5 

mm across.

The K-feldspar in the granodiorite and syenogabbro was identified 

by X-ray examination using the 20 value of the 060 and 204 diffraction 

peaks, according to the method described by Wright (1968). Two samples of granodiorite, one collected near the south end of the main 

pluton and the other from the north part, contain K-feldspar close to 

the orthoclase end member of the orthoclase-albite series. Optic axial 

angles of this orthoclase are 50°-58°. K-feldspar from the syenogabbro 

is sanidine according to the method of Wright. Optic axial angles of the 

sanidine are 43°-48°.
WHITEHORN GRANODIORITE, COLORADO 5

MODAL AND CHEMICAL DATA

The composition of the Whitehorn Granodiorite ranges from 

syenogabbro to quartz monzonite as determined from modal analyses 

of 12 specimens (fig. 2). Modes of nine specimens of granodiorite collected from scattered localities throughout the main pluton show that 

the composition, in volume percent, is quartz, 9-20; plagioclase, 45-55; 

orthoclase, 9-18; biotite, 2-12; augite, 0-13; hornblende, 0-10; and 

accessory hypersthene, apatite, sphene, and magnetite. Although the 

anorthite content of the plagioclase in most specimens studied averages less than 50 percent, it is greater than 50 percent in a few 

samples. The rocks with high anorthite contents would be classified as 

granogabbro by Johannsen (1931). According to recommendations of 

the International Union of Geological Sciences (1973), some of the 

granodiorite with less than 20 percent quartz would be quartz monzodiorite and some would be quartz monzogabbro. Using available 

data, the common rock of the Whitehorn Granodiorite might best be 

termed augite granodiorite, except that some of it lacks augite. Quartz 

diorite and quartz monzonite in the Whitehorn differ from its 

granodiorite in the proportions of the feldspars.

Granodiorite Quartz monzonite

Syenogabbro

Plagioclase 10 35 

EXPLANATION

65 K-feldspar

Mode of quartz-bearing rocks Mode of border fades of plug

FIGURE 2. Modal quartz, plagioclase, and K-feldspar in 12 specimens of the Whitehorn

Granodiorite.
6 CONTRIBUTIONS TO STRATIGRAPHY

Chemical analyses of four samples of the rock are given in table 1. 

Specimens 1 and 2 have compositions similar to average hypersthene-bearing granodiorite of Nockolds (1954). Modes of these two 

rocks plot on the quartz side of the granodiorite field of figure 2. 

However, these granodiorites are unusual in containing plagioclase 

with labradorite cores, which may account in part for the higher CaO 

and lower Na2 O than in Nockolds' average hypersthene-bearing 

granodiorite. Specimens 3 and 4 have low contents of SiO2 , reflecting 

their mafic character. Modes of these rocks plot on the quartz-poor side 

of the granodiorite field of figure 2.

The modal composition of the syenogabbro, in volume percent, is 

plagioclase, 36; sanidine, 22; augite, 22; hypersthene, 5; clays after 

olivine, 3; biotite, 2; magnetite, 6; apatite, 2; calcite and other alteration products, 2. In the International Union of Geological Sciences 

(1973) classification, this rock could be termed a monzogabbro.

AGE

Biotite from a specimen collected near the north end of the large 

pluton (fig. 1, loc. 4) gave a potassium-argon age of 70.0±2.6 million 

years (R. F. Marvin, written commun., 1971). McDowell (1971) reports 

that a sample collected west of Cameron Mountain had concordant 

ages from biotite and hornblende of 70.4±2.1 and 69.4±2.1 million 

years, respectively. These radiometric ages suggest that the 

granodiorite was intruded in Late Cretaceous (early Maestrichtian) 

time.

REFERENCES CITED

Behre, C. H., Osborn, E. F., and Rainwater, E. H., 1936, Contact ore deposition at the 

Calumet iron mine, Colorado: Econ. Geology, v. 31, no. 8, p. 781-804.

Bhutta, M. A., 1954, Geology of the Salida area, Chaffee and Fremont Counties, Colorado: Colorado School Mines, Golden, D. Sc. thesis, 173 p.

Burbank, W. S., and Levering, T. S., 1933, Relation of stratigraphy, structure, and 

igneous activity to ore deposition of Colorado and southern Wyoming, in Ore 

deposits of the western states (Lindgren volume): Am. Inst. Mining Metall. Engineers, p. 272-316.

Chapin, C. E., Epis, R. C., and Lowell, G. R., 1970, Late Eocene paleovalleys and 

Oligocene volcanic rocks along the upper Arkansas Valley segment of the Rio 

Grande rift zone in Colorado [abs.], in Woodward, L. A., ed., Tyrone-Big Hatchet 

Mountains-Florida Mountains region: New Mexico Geol. Soc. Guidebook 21st Field 

Conf., p. 159-160.

Eberhart, Perry, 1959, Guide to the Colorado ghost towns and mining camps: Denver, 

Colo., Sage Books, 479 p.

Harris, R. L., Jr., and Wilbanks, J. R., 1966, Variations in zircon crop measurements 

caused by weathering, Whitehorn stock, central Colorado: Mtn. Geologist, v. 3, no. 

3, p. 135-138.

International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks, 1973, Plutonic rocks: Geotimes, v. 18, no. 10, p. 26-30.

Johannsen, Albert, 1931, A descriptive petrography of igneous rocks, v. 1: Chicago, 111., 

Chicago Univ. Press, 318 p.
WHITEHORN GRANODIORITE, COLORADO 

TABLE 1. Chemical and modal data of the Whitehorn Granodiorite

[Rapid chemical analyses of specimens 1 and 3 by P. L. D. Elmore, James Kelsey, Hezekiah Smith, 

Roosevelt Moore, and J. L. Glenn and of specimens 2 and 4 by P. L. D. Elmore. Spectrographic 

analyses of specimens 1 and 3 by semiquantitative methods by Norma Rait and of specimens 2 

and 4 by quantitative methods by Chris Heropoulos. Looked for but not found in specimens 1 

and 3: Ag, As, Au, B, Bi, Cd, Eu, Ge, Hf, In,Li,Mo, Nd, Pd, Pr, Pt, Re, Sb, Sn, Ta, Te, Th, Tl, U, W, 

Zn]

Locality number

.SiOz _________

AkOs _________

Fe2Qi _________

FeO ___________

MgO ______ _

CaO___________

NaaO _________

R.O___________

HzO- _____

HiO + _______

TiO2_.__ _ ____

PaOs ________

MnO _________

C02 ___________

Total

 (fig. !)______-______ 1 2

Chemical analyses (weight percent)

______ _ __ _ 62.4 60.2

_____________________ 15.4 17.6

__ __ _ __ __ __ __ 3.4 2.8

_____________________ 3.5 3.3

____________________ 2.1 2.0

_____________ ____ _ 5.9 5.6

2.9 3.4

__ __ _ __ ________ 2.3 2.4

_____________________ .18 .12

__ __ _______________ .66 .88

_ __ __ ___ __ _ .68 .79

,__ __ _______________ .50 .41

__ _____ .10 .20

______________ __ ___ <,05 .15

_ _____ __ __ ___ __ 100.07 99.85

3

i

57.6 

15.6 

3.6 

4.9 

3.0 

6.6 

3.1 

2.6 

.10 

.42 

1.0 

.68 

.18 

<.05

99.43

4

55.6 

16.9 

3.5 

5.2 

3.2 

7.1 

3.0 

2.4 

.06 

.64 

1.2 

.48 

.20 

<.05

99.53

Spectrographic analyses (weight percent)2

Ba __________

Be __ _ ______

Co ___________

Cr __ _________

Cu _____ ___ _

Ga ___________

La ___________

Mn _______ _ _

Nb ___________

Ni_____________

Sc____.______._

Sr_______ ___

V__ _____

Y _____________

Yb ___________

Zr _ ______

_____ _ _____________ 0.07 0.062

_ _ ______ .0015 .0007

.__ __ _ __ __ .002 .0008

_______________ __ __ .002 .0009

_ _ -. _ .001 .0022

________ __ ____ _ ._ ____ .0095

_____________________ .07 .012

________ _ _______ _ L __ _____

.____ ____ _ _____ _ .0015 .0013

. __ __ __ _ _ _ .07 .07

_ . _ _____ .015 .0078

. _ _ _ _ _ _ .0003 .004

._. __ ______________ .0003 .0003

.015 .019

0.15

.002 

.003 

.007 

.0015

.1

.003 

.002 

.07 

.02 

.0003 

.0003 

.015

0.044

.0016 

.0018 

.005 

.0022 

.0052 

.12

.0008 

.0024 

.14 

.02 

.0046 

.0004 

.019

Modes (volume percent)

Biotite _

Other _

Total ._.

._ __ ___________ _ - 17 20

_______ __ ___________ 45 55

_ _ _ 12 13

___________ _ ___,_.. 11 5

10 4

1

.__ _ ____-_._____ . 3 2

__ -___- _ ___ 1 1

9 

51 

17 

9 

7 

4 

2 

1

.__ __ ________. 100 100 100

13 

52 

13 

10 

2 

5 

3 

2

100

'Methods used are those of Shapiro and Brannock (1962) supplemented by atomic absorption. 

2Results of semiquantitative analyses of specimens 1 and 3 are reported in percent to the 

nearest number in the series 1,0.7,0.5,0.3,0.2,0.15, and 0.1, etc., which represent approximate 

midpoints of interval data on a geometric scale. The assigned interval for semiquantitative 

results will include the quantitative value 30 percent of the time. Results for quantitative 

analyses of specimens 2 and 4 are reported to two significant figures and have an overall 

accuracy of ±15 percent, except that they are less accurate near the limits of detection where 

only one digit is intended. L=detected but below limit of determination.

1. Granodiorite from west margin of main intrusive body. Collected by E. J. Young in quarry on 

Ute Trail Road. Analyses from Young (1972), sample 16-71.

2. Granodiorite from south-central part of main intrusive body at Federal quarry, SEVi sec. 20, T. 

50 N., R. 10 E.

3. Granodiorite from central part of main intrusive body. Collected by E. J. Young at quarry, SEVi 

sec. 6, T. 50 N., R. 10 E. Analyses from Young (1972), sample 15-71.

4. Augite granodiorite from north-central part of main intrusive body at quarry, SEYt sec. 1, T. 51 

N., R. 9 E.
8 CONTRIBUTIONS TO STRATIGRAPHY

Keroher, G. C., and others, 1966, Lexicon of geologic names of the United States for

1936-60: U.S. Geol. Survey Bull. 1200, 4341 p. 

Lowell, G. R., 1969, Geologic relationships of the Salida area to the Thirtynine Mile

volcanic field of central Colorado: New Mexico Inst. Mining and Technology,

Socorro, Ph. D. thesis, 113 p. 

McDowell, F. W., 1971, K/Ar ages of igneous rocks from western United States:

Isochron/West, no. 2, p. 1-16. 

Nockolds, S. R., 1954, Average chemical compositions of some igneous rocks: Geol. Soc.

America Bull., v. 65, no. 10, p. 1007-1032. 

Normand, D. E., 1968, The geology of the Whitehorn stock area, Chaffee, Fremont and

Park Counties, Colorado: Texas Technol. Coll., Lubbock, M.S. thesis, 61 p. 

Pierce, W. H., 1969, Geology and Pennsylvanian-Permian stratigraphy of Howard area,

Fremont County, Colorado: Colorado School Mines, Golden, M.S. thesis, 129 p. 

Shapiro, Leonard, and Brannock, W. W., 1962, Rapid analyses of silicate, carbonate, and

phosphate rocks: U.S. Geol. Survey Bull. 1144-A, 56 p. 

Wright, T. L., 1968, X-ray and optical study of alkali feldspar: II, An X-ray method for

determining the composition and structural state from measurement of 26 values

for three reflections: Am. Mineralogist, v. 53, nos. 1-2, p. 88-104. 

Young, E. J., 1972, Laramide-Tertiary intrusive rocks of Colorado: U.S. Geol. Survey

open-file report, 206 p.

* U.S. GOVERNMENT PRINTING OFFICE: 1974 543 5S5/13 1