Alkaline Rock Complexes in the

Wet Mountains Area,

Custer and Fremont Counties, Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1269
Alkaline Rock Complexes in the

Wet Mountains Area,

Custer and Fremont Counties, Colorado

By THEODORE J. ARMBRUSTMACHER

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1269

Geology and petrology of rocks of the

McClure Mountain Complex, Gem Park Complex,

and complex at Democrat Creek

and associated alkaline rocks

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1984
UNITED STATES DEPARTMENT OF THE INTERIOR 

WILLIAM P. CLARK, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Armbrustmacher, Theodore J., 1946-

Alkaline Rock complexes in the Wet Mountains area, Custer and Fremont Counties, Colorado.

(Geological Survey Professional Paper 1269)

Bibliography: 33 p.

Supt. of Docs. No.: 119.16:1269

1. Alkalic igneous rocks. 2. Petrology Colorado Wet Mountains. I. Title. II. Series.

QE462.A4A76 552'.3 82-600104 AACR2
CONTENTS

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

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

Acknowledgments ........................ 1

Geologic setting ......................... 2

McClure Mountain Complex .................. 2

Mafic-ultramafic rocks ................... 2

Layered series ..................... 2

Discordant intrusions ................. 6

Leucocratic rocks ...................... 6

Nepheline syenite ................... 7

Hornblende-biotite syenite ............... 8

Mafic nepheline-clinopyroxene rocks ............ 10

Gem Park Complex ....................... 11

Mafic-ultramafic rocks ................... 12

Other rocks of the complex ................ 12

Complex at Democrat Creek .................. 12

Mafic-ultramafic rocks ................... 13

Quartz syenite ....................... 13

Carbonatites ........................... 14

Ages of the complexes ..................... 14

Geochemistry ........................... 15

Major elements ....................... 15

Mafic-ultramafic rocks ................. 15

Leucocratic rocks ................... 16

Mafic nepheline-clinopyroxene rocks ......... 19

Additonal observations ................. 19

Minor elements ....................... 22

Mafic-ultramafic rocks ................. 22

Leucocratic rocks ................... 23

Mafic nepheline-clinopyroxene rocks ......... 25

Rubidium-strontium systematics and rare-earthelement distribution patterns ............ 25

Rare-earth elements .................. 25

Initial ^Sr/^Sr ratios ................. 27

Rubidium/strontium ratios ............... 29

Petrologic conclusions ...................... 30

References cited ......................... 32

ILLUSTRATIONS

Page

FIGURE 1. Map of Wet Mountains area showing location of alkaline complexes ........................... 3

2. Geologic map of the Wet Mountains area alkaline complexes ............................... 4

3. Ternary diagram of modal variation in plagioclase, clinopyroxene, and olivine from the McClure Mountain and Gem

	Park Complexes ....................................................... 5

4. Photomicrograph showing adcumulus growth of clinopyroxene on clinopyroxene .................... 5

5. Photomicrograph of crystallographically oriented iron oxide exsolving from clinopyroxene ............... 6

6. Photomicrograph of anorthositic rock with abundant triple-point grain boundaries meeting at 120° .......... 6

7. Photomicrograph of plagioclase with exsolved iron oxide ................................. 6

8. Photomicrograph of abundant cracks between nearby olivine grains ........................... 6

9. Photograph of outcrop of mafic-ultramafic rocks cut by nepheline syenite dikes along McClure Gulch ........ 7

10. Photograph of outcrop of nepheline syenite showing heterogeneous nature of grain size and heterogeneous distribution

	of minerals .......................................................... 7

11. Photograph of xenoliths contained in nepheline syenite .................................. 7

12. Photomicrograph of aegirine-augite rimmed by sodic amphibole in nepheline syenite .................. 8

13. Photomicrograph of myrmekitic intergrowth of climopyroxene and nepheline in nepheline syenite .......... 8

14. Quaternary diagram of modal variation in quartz alkali feldspar, plagioclase, and feldspathoid from the McClure Mountain

	Complex and the complex at Democrat Creek ...................................... 9

15. Ternary diagram of modal variation in nepheline, mafic minerals, and potassic feldspar from the McClure Mountian

	Complex ........................................................... 9

16. Photograph of outcrop of homogeneous hornblende-biotite syenite ............................ 9

17. Photomicrograph showing the alteration of clinopyroxene to green amphibole in hornblende-biotite syenite .... 10

18. Photograph of outcrop of mottled mafic nepheline-clinopyroxene rock .......................... 10

19. Photomicrograph of poikilitic mafic nepheline-clinopyroxene rock ............................. 11

20. Photograph of cumulus layering in mafic-ultramafic rocks of the Gem Park Complex ................. 11

21. Whole-rock isochron of nepheline syenites of McClure Mountain Complex ........................ 15

22. Whole-rock isochron of quartz syenites of the complex at Democrat Creek ....................... 15

23. Plots of weight-percent alkalies verus weight-percent Si02 ................................ 18

24. Plots of agpaitic index versus weight- percent Si02 .................................... 19

25. Plots of differentiation index (DI) versus oxide weight percents ............................. 22

in
IV CONTENTS

Page

FIGURE 26. Diagram of chondrite-normalized rare-earth element data for mafic rocks from McClure Mountain Complex and the

complex at Democrat Creek ................................................ 28

27. Diagram of chondrite-normalized rare-earth element data for liquids from which mafic rocks at the McClure Mountain

Complex and the complex at Democrat Creek crystallized ............................... 28

28. Diagram of rare-earth element patterns for hornblende-biotite syenites from the McClure Mountain Complex ... 28

29. Diagram of rare-earth element patterns for nepheline syenites from the McClure Mountain Complex ........ 29

30. Diagram of rare-earth element patterns for quartz syenites from the complex at Democrat Creek ......... 29

31. Diagram showing distribution of initial ^Sr/^Sr ratios for rocks of the alkaline complexes .............. 29

32. Diagram showing rubidium/strontium ratios of rocks from the McClure Mountain Complex .............. 30

33. Diagram showing rubidium/strontium ratios of rocks from the complex at Democrat Creek .............. 30

34. Summary of rock distributions in the Wet Mountains area ................................ 31

TABLES

Page

TABLE 1. Relative and absolute age relationships of rocks of the Wet Mountains area ...................... 16

2. Chemical and normative data of mafic-ultramafic rocks from the Iron Mountain area of the McClure Mountain Complex . 17

3. Chemical and normative data of mafic-ultramafic rocks from the Gem Park Complex and the complex at Democrat Creek . 18

4. Chemical and normative data of hornblende-biotite syenites, nepheline syenites.and mafic nepheline-clinopyroxene rocks,

McClure Mountian Complex ................................................ 20

5. Chemical and normative data of quartz syenites from the complex at Democrat Creek and nepheline syenite pegmatite

from the Gem Park Complex ............................................... 21

6. Summary of minor-element analyses of samples from the McClure Mountain Complex ................. 23

7. Summary of minor-element analyses of samples from the complex at Democrat Creek and the Gem Park Complex . 24

8. Abundance of elements in several types of rocks ...................................... 25

9. Rubidium and strontium content and several ratios for rocks from the McClure Mountain Complex and the complex

at Democrat Creek ..................................................... 26

10. Rare-earth element content of rocks from the McClure Mountain Complex and the complex at Democrat Creek . . 27
ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, 

CUSTER AND FREMONT COUNTIES, COLORADO

By THEODORE J. ARMBRUSTMACHER

ABSTRACT

Three alkaline intrusive complexes of Cambrian age occur in the 

Wet Mountains area, Colorado. The McClure Mountain Complex consists of mafic-ultramafic cumulates, hornblende-biotite syenites, 

nepheline syenites, and mafic nepheline-clinopyroxene rocks intruded 

by carbonatite, several kinds of syenite, and lamprophyre, mainly 

as dikes, and by thorium-bearing veins. The Gem Park Complex consists chiefly of mafic-ultramafic cumulates intruded by carbonatite, 

lamprophyre, and nepheline syenite pegmatite. The complex at Democrat Creek contains subordinate amounts of mafic-ultramafic rocks 

and abundant quartz syenite bordered partly by a zone of brecciation; 

the complex is intruded by syenite dikes and a quartz-barite-thorite 

vein.

Contents of major and minor elements, including rare-earth elements, rubidium, strontium, and strontium isotopic ratios, show that 

these rocks did not form through fractionation of a single magma, 

but formed as end products of at least three separate magma groups. 

The mafic-ultramafic rocks in the McClure Mountain and Gem Park 

Complexes and the hornblende-biotite syenite in the McClure Mountain Complex appear to have been derived from an alkali basalt parent; the nepheline syenite, mafic nepheline-clinopyroxene rock, and 

carbonatite from the McClure Mountain Complex and the carbonatite 

from the Gem Park Complex appear to have had a more alkaline 

parent rock such as nephelinite; and the quartz syenite and mafic-ultramafic rock of the complex at Democrat Creek appear to have 

evolved from a tholeiitic basalt parent.

INTRODUCTION

Alkaline rocks in the Wet Mountains area of southcentral Colorado, located about 20 km southwest of 

Canon City, Colo., about 5 km south of the Arkansas 

River, and northeast of the Sangre de Cristo Range 

and the Wet Mountain Valley, occur in three distinct 

complexes: the McClure Mountain Complex, the Gem 

Park Complex, and the complex at Democrat Creek. 

Additional spatially and presumably genetically related 

lamprophyre, carbonatite, and red syenite occur mainly 

as dikes. Quartz-barite-thorite veins also appear to be 

related to the episode of alkaline magmatism.

The McClure Mountain Complex contains nepheline 

syenite, hornblende-biotite syenite, mafic nephelineclinopyroxene rocks, and mafic-ultramafic cumulate 

rocks (Shawe and Parker, 1967). The Gem Park Complex contains mafic-ultramafic rocks nearly identical 

to those in the McClure Mountain Complex, and a single 

exposure of nepheline syenite pegmatite (Parker and

Sharp, 1970). The complex at Democrat Creek contains 

quartz syenite and breccia, and mafic-ultramafic rocks 

that were mapped by Brock and Singewald (1968) as 

Precambrian gabbroic gneisses and metamorphosed 

ultramafic rocks, but that were thought by Heinrich 

and Dahlem (1966) to be similar to the gabbros and 

pyroxenites of the Gem Park and McClure Mountain 

Complexes. The rocks at Democrat Creek are less notable for their similarities to the rocks of the other two 

complexes than for their differences.

The intrusive complexes and associated dikes are 520 

m.y. old according to Olson and others (1977). However, their dating of leucocratic rocks from the McClure 

Mountain Complex and the complex at Democrat Creek 

by fission-track, potassium-argon, and rubidium-strontium techniques could not resolve any differences in the 

ages of the various complexes or in the ages of different 

syenites at McClure Mountain. Subsequent rubidium 

and strontium isotopic determinations by C. E. Hedge 

(Armbrustmacher and Hedge, 1982) yielded ages of 

535 ±5 m.y. for syenites at McClure Mountain and 

511 ±8 m.y. for syenites at Democrat Creek.

The petrology of the Wet Mountains alkaline rocks, 

as outlined by major- and minor-element contents, 

suggests the presence of several different rock series. 

Data on rubidium, strontium, rare-earth elements, and 

strontium isotopes show that rocks of the complexes 

did not form through fractionation of a single magma, 

but formed instead as end products of several magmas 

generated from different source materials.

ACKNOWLEDGMENTS

Studies of the Wet Mountains alkaline rocks benefited 

from discussions with U.S. Geological Survey 

geologists, especially R. L. Parker, W. N. Sharp, M. 

R. Brock, and J. C. Olson. Determinations of rubidium 

and strontium isotopes and rare-earth elements by C. 

E. Hedge of the U.S. Geological Survey and discussions 

with Hedge regarding their interpretation resulted in 

invaluable insight into the petrology of the alkaline complexes. I. K. Brownfield assisted in the field and in 

the laboratory.
ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

GEOLOGIC SETTING

The alkaline rocks of the Wet Mountains area intrude 

Proterozoic X metamorphic rocks chiefly layered 

granitic gneisses, hornblende gneisses, and amphibolites and Precambrian intrusive granitic rocks of Boulder Creek or Proterozoic X age (1,720 m.y.) and Silver 

Plume or Proterozoic Y age (1,450 m.y.) (Taylor and 

others, 1975a, b). The western edge of the Gem Park 

Complex is bordered by Tertiary welded tuffs, boulder 

gravels, and water-laid tuffs (Parker and Sharp, 1970). 

According to Scott and Taylor (1975), these rocks are 

correlative with the Oligocene East Gulch, Thorn 

Ranch, and Gribbles Park Tuffs of the Thirtynine Mile 

volcanic field.

The host rocks adjacent to the alkaline complexes, 

carbonatite dikes, and thorium deposits, and parts of 

the complexes themselves, are typically fenitized (Heinrich and Alexander, 1976). Quartzo-feldspathic host 

rocks commonly show loss of quartz; their feldspars are 

replaced by potassic feldspar containing abundant ferric-oxide inclusions, their mafic minerals are destroyed 

or replaced by blue and green sodic amphiboles and 

pyroxenes, and their fractures are lined with these minerals or with epidote. Where mafic- ultramafic host 

rocks are fenitized, the most conspicuous result is the 

replacement of mafic minerals by vermiculite; in some 

places the concentrations are nearly high enough to be 

commercially valuable.

The alkaline complexes, carbonatites, and thorium 

deposits appear to be bounded by the Use fault on the 

east and the Texas Creek and Westcliffe faults on the 

west (fig. 1). Relative movement along the Texas Creek 

fault (Taylor and others, 1975a) and along the Use fault 

(Scott and others, 1976) is east-side upward; this movement suggests that the alkaline intrusive rocks may occupy a different structural level than do the blocks west 

of the Texas Creek fault and east of the Use fault. To 

date, rocks related to the alkaline complexes have not 

been observed in the structural blocks east and west 

of the one containing the complexes. This structural 

block is in turn broken by a set of predominantly northwest-striking vertical faults that likely served as conduits for fluids that formed thorium veins, lamprophyres, red syenite dikes, and carbonatites, especially southeast of the alkaline complexes.

McCLURE MOUNTAIN COMPLEX

The largest of the three complexes, the McClure 

Mountain Complex (fig. 2), consists chiefly of a series 

of mafic-ultramific rocks at Iron Mountain, leucocratic 

hornblende-biotite syenite and nepheline syenite, mafic 

nepheline-clinopyroxene rocks, and various dike rocks

including carbonatites, lamprophyres, and syenites. 

Distance across its widest east-west dimension is about 

11 km and across its north-south dimension is 10 km. 

Exposures of the mafic-ultramafic rocks at Iron Mountain are about 5.8 km long in a northwest-southeast 

direction and 2-3.5 km wide in a northeast-southwest 

direction.

Apparently, Parker and Hildebrand (1963) first mentioned the McClure Mountain Complex and its alkaline 

nature. The mafic-ultramafic part of the complex at 

Iron Mountain has been discussed by Shawe and Parker 

(1967), who also adopted and defined the 

McClure Mountain Complex as a formal term. Although 

the leucocratic part of the complex has been mentioned 

many times (Shawe and Parker, 1967; Parker and 

Sharp, 1970; Heinrich and Dahlem, 1966; Heinrich, 

1966; Heinrich and Moore, 1970), detailed information 

on the rocks is sparse. Rock relationships in the complex have been interpreted by Heinrich and Alexander 

(1979) to represent a "mafic-alkalic ring complex."

The rocks of the McClure Mountain Complex do not 

appear to have much economic potential. Small-scale 

production of iron from titaniferous magnetite in the 

mafic-ultramafic rocks at the Iron Mountain mine occurred as early as 1873 (Becker and others, 1961); however, the high titanium content of the magnetite, as 

much as 14 percent Ti02 , and the low tonnage have 

hindered further development. Several carbonatite 

dikes intruding rocks of the complex have been prospected for thorium and rare-earth elements.

MAFIC-ULTRAMAFIC ROCKS

Mafic and ultramafic rocks of the Iron Mountain part 

of the McClure Mountain Complex form a funnel-shaped 

layered series of rocks intruded by small, discordant 

bodies of similar mafic and ultramafic rocks as well as 

by dikes of carbonatite and syenite (Shawe and Parker, 

1967). Mineral content of rocks in the layered series 

and in the discordant intrusions is similar. The rocks 

differ mainly on the basis of texture; rocks in the 

layered series show stratification, but rocks in the discordant intrusion do not. These two rock types are not 

distinguished on figure 2, but are shown by Shawe and 

Parker (1967, pi. 1).

LAYERED SERIES

The stratified rocks consist of igneous cumulates comprising essentially five cumulus minerals plagioclase, 

a calcium-rich clinopyroxene, olivine, magnetite, and 

spinel that occur in varying proportions. All these 

minerals, except spinel, along with reddish-brown amphibole and red biotite are present as intercumulus material; considerable variations in mineral proportions 

and in grain size can occur within a distance of a few
38' 105°45'

McCLURE MOUNTAIN COMPLEX

30' 15' 105°00'

15 MILES

J

10 15 20 KILOMETERS

EXPLANATION

Tertiary, Mcsozoic, and 

Paleozoic sedimentary 

rocks

Tertiary volcanogenic rocks 

Cambrian alkaline complexes

Precambrian metamorphic 

and igneous rocks

Major fault U, upthrown 

side: D, downthrown side

Contact

FIGURE 1. Map of Wet Mountains area showing location of alkaline complexes, Fremont and Custer Counties,

Colo. Geology modified from Scott and others (1976).

centimeters. These characteristics are typical of igneous 

cumulates (Jackson, 1961). Plots of modal plagioclase, 

clinopyroxene, and olivine for 20 mafic-ultramafic rocks

(fig. 3) show the diversity of rock types found. These 

rock types include clinopyroxene adcumulates, plagioclase adcumulates, and orthocumulates that consist of
ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

105°30' 105°25'

38°20',

380 15i

COUNTY

4 MILES

I I T I I I I 

0123456 KILOMETERS

FIGURE 2. Alkaline complexes of the Wet Mountains, Fremont and Custer Counties, Colo. Geology modified from Taylor and others

(1975a and 1975b) and Olson and others (1977).

the cumulus minerals clinopyroxene, plagioclase, olivine, magnetite, and rarely spinel in various proportions.

Most clinopyroxene appears in thin section as a tanto neutral-colored, unzoned, sometimes pleochroic 

cumulus mineral most likely titaniferous augite; rarely 

does it occur as an intercumulus mineral. In augite-rich 

rocks, adcumulus growth of the augite is sometimes apparent owing to slight differences in optical orientation 

of the augite primocryst and the later, interstitial augite (fig. 4). Augite can also exhibit crystallographically 

controlled, exsolved, opaque iron oxides (fig. 5).

Plagioclase primocrysts commonly exhibit adcumulus 

growth; triple-point junctions near 120° are abundant 

in plagioclase-rich rocks (fig. 6). Most optical measurements place the plagioclase in the labradorite range. 

Clouded plagioclase plagioclase that contains crystallographically controlled, exsolved iron oxide occurs in 

some rocks, especially in those in which the associated 

augite also contains exsolved iron-oxide inclusions (fig. 

7). This exsolution of crystallographically oriented iron 

oxide is due to subsolidus cooling at rates slow enough 

to allow exsolution of structural iron (iron in lattice 

sites; Armbrustmacher and Banks, 1974). Plagioclase 

is not obviously zoned.

In this study, olivine adcumulates have not been observed and intercumulus olivine is not abundant. The 

maximum modal olivine content observed thus far is 

about 58 percent by volume. Some olivine is partly altered to pale yellowish-green serpentine-type minerals
McCLURE MOUNTAIN COMPLEX 

CORRELATION OF MAP UNITS

QUATERNARY 

AND TERTIARY

CAMBRIAN

PRECAMBRIAN

LIST OF MAP UNITS

Quaternary and Tertiary

clastic and volcaniclastic 

deposits

Cambrian (511-535 m.y. 

-old rocks)

Quartz syenite of complex 

at Democrat Creek

Breccia of complex at 

Democrat Creek

Mafic-ultramafic rocks of 

complex at Democrat 

Creek

Mafic nepheline-clinopyroxene 

rocks of McClure Mountain 

Complex

Nepheline syenite of McClure 

Mountain Complex

Hornblende-biotite syenite 

of McClure Mountain 

Complex

Mafic-ultramafic rocks of 

Mountain and 

Gem Park Complexes

Precambrian metamorphic 

and igneous rocks

Fault Dashed where approximately

located; dotted where concealed

Contact

Localities mentioned in text

FIGURE 2. Continued.

and opaque iron oxides. Rocks containing fairly abundant olivine that is partly altered show abundant, 

closely spaced fractures that connect olivine grains (fig. 

8). This feature suggests that the alteration process intro iuced a volume change and that the resultant stress 

is relieved by fracturing.

Magnetite occurs both as cumulus and as intercumulus minerals. Exploitation of layers of nearly 98 

percent cumulus magnetite occurred at the Iron MounEXPLANATION

v McClure Mountain Complex 

v Gem Park Complex

FIGURE 3 Modal variation in plagioclase (P), clinopyroxene (C), 

and olivine (0) of mafic-ultramafic rocks from the McClure Mountain and Gem Park Complexes. Values are in percent by volume.

FIGURE 4. Adcumulus growth of clinopyroxene on clinopyroxene. 

Sample 359; partly crossed nicols; bar is 0.5 mm.

tain mine. Green spinel is invariably associated with 

magnetite, and the spinel/magnetite ratio increases as 

the total magnetite content of a rock increases. Most 

spinel appears to be associated with cumulus magnetite, 

and it is rarely associated with intercumulus magnetite. 

Strongly pleochroic reddish-brown amphibole, which 

has a large optic angle, is most similar to kaersutite 

in optical properties. The amphibole, which may 

poikilitically enclose cumulus augite and magnetite, appears to have formed from intercumulus fluids. Some
ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

FIGURE 5. Crystallographically oriented iron oxide exsolving from 

clinopyroxene. Sample 392; plane light; bar is 0.5 mm.

FIGURE 6. Anorthositic rock with abundant triple-point grain 

boundaries meeting at 120°. Sample 457; partly crossed nicols; 

bar is 0.5 mm.

of the amphibole also forms reaction rims around both 

augite and magnetite. Sparse green amphibole appears 

to be a product of the alteration of clinopyroxene.

Most strongly pleochroic biotite, red to neutral, is 

associated with magnetite. A few rocks contain euhedral apatite, which appears to have coprecipitated with 

magnetise.

The presence of both a single calcium-rich 

clinopyroxene, as opposed to two pyroxenes, and a 

sodic amphibole verifies the alkaline nature of these 

mafic-ultramafic rocks.

DISCORDANT INTRUSIONS

Field evidence for recognizing the ultramafic discordant intrusions include their discordant nature, the

V*

, >

FIGURE 7. Plagioclase with exsolved iron oxide. Sample number 

402Z; plane light; bar is 0.5 mm.

FIGURE 8. Closely spaced fractures in plagioclase (P) connecting 

olivine grains (O). Sample 396; plane light; bar is 0.5 mm.

xenoliths of rocks of the layered complex near contacts, 

and the lack of layering (Shawe and Parker, 1967). 

Rocks of the discordant intrusions consist of minerals 

that commonly are identical to those in rocks of the 

layered intrusions. In thin section, these rocks tend to 

be equigranular, and the distribution of various minerals tends to be more homogeneous than that in the 

layered rocks.

LEUCOCRATIC ROCKS

Two distinct, mappable types of leucocratic rocks, 

hornblende-biotite syenite and younger nepheline syenite, occur in the McClure Mountain Complex; neither 

contains modal or normative quartz. These rocks intrude the mafic-ultramafic rocks at Iron Mountain and
McCLURE MOUNTAIN COMPLEX

are in turn intruded by red syenite dikes, carbonatite 

dikes, and lamprophyre dikes. The nepheline syenite 

intrudes the hornblende-biotite syenite. Precambrian 

host rocks adjacent to syenite contacts are fenitized to 

varying degrees; one of the best examples is at Red 

Mountain (fig. 2), just south of the McClure Mountain 

Complex. Fenitization of mafic-ultramafic rocks at Iron 

Mountain adjacent to the syenites is generally not apparent, probably because of structural complications 

much of that contact is a fault contact. However, at 

McClure Gulch, the mafic-ultramafic rocks are cut by 

nepheline syenite dikes (fig. 9). The syenites are 

fenitized along narrow zones adjacent to later dikes of 

carbonatite and red syenite. The development of ferricoxide-bearing potassic feldspars has turned the lightgray syenite host rock to pink; the original mafic minerals tend to be destroyed or replaced by a vermiculitelike mica. Breccia zones have not been observed in the 

vicinity of rocks of the McClure Mountain Complex, although breccia pipes at nearby Pinon Peak, 5.5 km 

north of the complex (Taylor and others, 1975b), have 

been related to the episode of development of alkaline 

rocks at McClure Mountain by Heinrich and Dahlem 

(1967).

NEPHELINE SYENITE

Nepheline syenite in the McClure Mountain Complex 

is centrally located relative to the hornblende-biotite 

syenite, but is peripheral to the mafic nephelineclinopyroxene rocks (fig. 2). According to Heinrich and 

Alexander (1979), the nepheline syenite occurs as a 

stock that intrudes the central core of an alkaline ring 

complex.

Outcrops of nepheline syenite vary considerably in 

lithologic details (fig. 10). Outcrop colors range from

i I *-, A

.r4''°<>-->** - "k: > jy.: -

.. n

 ViiS

FIGURE 9. Outcrop of mafic-ultramafic rocks cut by nepheline 

syenite dikes (light-colored). This location is along McClure Gulch 

near contact of mafic-ultramafic rocks and hornblende-biotite 

syenite. Hammer within circle is 31 cm long.

medium bluish gray to light gray, and grain sizes range 

from medium to coarse. The medium-grained rocks tend 

to form sharp, angular outcrops, whereas the very 

coarse grained rocks form rounded outcrops because of 

exfoliation, and weather more readily to grus. At a few 

localities, the coarse-grained rocks are foliated; these 

rocks have some of the characteristics of cumulus rocks, 

such as abrupt changes in grain size and mineral proportions. At some localities, the nepheline syenites contain abundant xenoliths, most of which are rounded to 

subangular fragments of medium- to dark gray 

nepheline- and sodic pyroxene-rich rocks (fig. 11).

Petrographic examination of nepheline syenite shows 

several textural types, including hypidiomorphic-granular, allotriomorphic-granular, and porphyritic. In 

hypidiomorphic-granular rocks, the sodic amphiboles 

are typically euhedral; as the amphiboles become anhedral, the texture becomes allotriomorphic-granular. In

FIGURE 10. Outcrop of nepheline syenite showing heterogeneous 

nature of grain size and heterogeneous distribution of minerals. 

Scale is 17.5 cm long.

FIGURE 11. Xenoliths contained in nepheline syenite. Scale is 17.5

cm long.
ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

the porphyritic rocks, phenocrysts are nepheline, microperthite, and sodic amphibole in various combinations. 

In one glomero-porphyritic rock with euhedral 

nepheline phenocrysts, synneusis structures and 

groundmass lath-shaped microperthites that swirl 

around phenocrysts (trachytic texture) give evidence of 

magmatic turbulence.

The major components of nepheline syenites are 

microperthite, nepheline, and sodic amphibole, although 

plagioclase and potassic feldspar are abundant in some 

samples. The major components also vary widely between samples: microperthite ranges between 0 and 84 

percent by volume, nepheline ranges between 0.2 and 

38 percent, and sodic amphibole ranges between 0 and 

26 percent. The inverse relationship of the abundance 

of potassic feldspar to the abundance of microperthite 

suggests that the potassic feldspar may be cryptoperthitic. Sodic pyroxene, which optically best fits 

aegirine-augite, commonly shows a reaction relationship 

with sodic amphibole, which optically best fits eckermannite-arfvedsonite (fig. 12). However, sodic pyroxene also occurs independent of the sodic amphibole, 

commonly in the more nepheline-rich samples. Brown 

to yellow-brown biotite occurs in nearly every sample 

in amounts as much as about 10 percent by volume, 

but averages about 2.5 percent; biotite occurs alone or 

clotted together with sodic amphibole and magnetite. 

Sphene, apatite, and magnetite generally are ubiquitous, but rarely exceed about 2.5 percent by volume; 

they occur separately or clotted together with mafic 

minerals. Apatite may also form poikilitic inclusions in 

sodic amphibole. Carbonate minerals, muscovite, and 

an unidentified isotropic mineral associated with 

nepheline are all present in small amounts and appear 

to have a secondary origin. A few samples contain myrmekitic intergrowths of light-green clinopyroxene and 

nepheline (fig. 13). At Copper Gulch divide (fig. 2), a 

sodalite-bearing variety of nepheline syenite occurs.

Modal components of the nepheline syenites have 

been plotted on a QAPF (quartz-alkali feldspar-plagioclase-feldspathoid) diagram (fig. 14). Most of the samples plot in the alkali syenite and foyaite fields, reflecting their typically low plagioclase content. Those samples containing greater amounts of plagioclase plot in 

the plagifoyaite, essexite, and diorite gabbro fields. 

Modal nepheline-potassic feldspar-mafic minerals of the 

same rocks (fig. 15) fall chiefly in the nepheline syenite 

and adjoining fields. Variation in nepheline/potassic 

feldspar ratios appears to be considerably greater than 

variation in mafic-mineral abundance.

HORNBLENDE-BIOTITE SYENITE

The hornblende-biotite syenite crops out mainly in 

the northern and eastern parts of the McClure MounFIGURE 12. Aegirine-augite rimmed by sodic amphibole in nepheline 

syenite. Sample 394X; plane light; bar is 0.5 mm.

 /v%

FIGURE 13. Myrmekitic intergrowth of clinopyroxene and 

nepheline in nepheline syenite. Sample WM-62-73; plane light; 

bar is 0.5 mm.

tain Complex. In addition, the occurrence of 

hornblende-biotite syenite at the west side of the complex suggests that the syenite forms a nearly circular 

outcrop pattern that surrounds nepheline syenite (fig. 

2).

In the field, the hornblende-biotite syenite is considerably more homogeneous at the outcrop scale than 

nepheline syenite (fig. 16). As discussed later, this 

homogeneous aspect is characteristic of this rock.

Outcrop colors are predominantly light gray and tan, 

although some pink varieties also occur. Almost all the 

rocks are coarse grained, and the outcrops weather to 

rounded boulders that yield abundant grus. The 

hornblende-biotite syenite tends to cause rolling 

meadowland if the land is not dissected locally by 

streams.
McCLURE MOUNTAIN COMPLEX

Foidites

EXPLANATION

McCLURE MOUNTAIN COMPLEX 

D Nepheline syenite 

o Hornblende-biotite syenite 

V Mafic nepheline-clinopyroxene rocks

COMPLEX AT DEMOCRAT CREEK 

 Quartz syenite

FIGURE 14. Modal variation in quartz (Q), alkali feldspar and 

plagioclase (An0.5) (A), plagioclase (An>5) (P), and feldspathoid 

(F) of syenites and mafic nepheline-clinopyroxene rocks from 

the McClure Mountain Complex and quartz syenites from the 

complex at Democrat Creek (modified from Sorensen, 1974a, 

p. 16). Values are in percent by volume.

Petrographic examination shows that the texture of 

most hornblende-biotite syenite is allotriomorphicgranular. Grain sizes range from medium (1-5 mm) to 

coarse (>5 mm). A few samples collected in the vicinity 

of contacts are porphyritic, and the phenocrysts form

Urtite

M

Melteigite 

Feldsparbearing 

melteigite

Leucocratic

nepheline

syenite

Leucocratic

nephelinebearing

syenite

Jacupirangite Melanocratic nephelinebearing syenite

Mesocratic nephelinebearing syenite

EXPLANATION

McCLURE MOUNTAIN COMPLEX 

o Hornblende-biotite syenite 

D Nepheline syenite 

A Mafic nepheline-clinopyroxene rocks

FIGURE 15. Modal variation in nepheline (Ne), mafic minerals (M), 

and potassic feldspar (A) of rocks from the McClure Mountain Complex (from Sorensen, 1974a, p. 17). Values are in percent by volume.

FIGURE 16. Outcrop of homogeneous hornblende-biotite syenite. 

Lens cover is 6 cm in diameter.
10 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

medium-grained microperthite. The allotriomorphicgranular rocks consist of medium- to coarse-grained, 

anhedral, lath-shaped, Carlsbad-twinned microperthite 

with roughly random orientation. Microperthite ranges 

from 48 to 93 percent by volume. Some samples contain 

abundant sodic plagioclase, as much as about 34 percent, but other samples contain no apparent plagioclase 

except that in microperthite. The mafic minerals and 

accessory minerals commonly occur together in clots, 

and these clots may be 10-15 mm across. Clinopyroxene 

rarely exceeds 1 percent by volume; it nearly always 

shows a reaction relationship with sodic amphibole, 

which ranges from 0.3 to 11 percent, and occurs mainly 

as remanent cores within the pleochroic, dark-greenishbrown sodic amphibole (fig. 17). Magnetite in some 

rocks is rimmed by sphene, but euhedral sphene independent of magnetite also is a common constituent. 

Magnetite, apatite, and sphene, which typically occur 

in amounts near 2-3 percent, are ubiquitous; muscovite 

and carbonate appear to be secondary minerals.

Modal components of the hornblende-biotite syenite 

on a QAPF diagram (fig. 14) plot in alkali syenite, syenite, and monzonite fields primarily because of variations 

in the microperthite/plagioclase ratio. Modal components of the nepheline-potassic feldspar-mafic minerals 

(fig. 15) show that the nepheline-bearing samples plot 

in the melanocratic and mesocratic nepheline-bearing 

syenite fields.

FIGURE 17. Alteration of clinopyroxene (Cpx) to green amphibole 

(Am) in hornblende-biotite syenite. Sample 387BX; plane light; bar 

is 0.5 mm.

MAFIC NEPHELINE-CLINOPYROXENE ROCKS

Mafic nepheline-clinopyroxene rocks, termed "ijolite" 

by Heinrich (1966), intrude nepheline syenites in the 

southern part of the McClure Mountain Complex. Outcrops are dark greenish gray in the field and are texturally inhomogeneous. One outcrop found on the southwest side of Elkhorn Mountain (fig. 2) has a mottled 

texture owing to large poikilitic grains of nepheline (fig. 

18). These rocks weather rather readily and yield a 

dark-gray soil mantle.

Petrographic studies show at least two textural 

types, allotriomorphic-granular and poikilitic, and most 

minerals are medium grained. In the poikilitic type, 

large grains of nepheline, with or without potassic feldspar, engulf subhedral sodic clinopyroxenes and amphiboles (fig. 19). Nepheline and sodic clinopyroxene, 

a pale-green aegirine-augite, are the most abundant 

components. Nepheline ranges from 10 to about 45 percent by volume, and sodic clinopyroxene ranges chiefly 

between about 30 and 40 percent. One sample containing nearly 30 percent microperthite, the only sample 

that has identifiable microperthite, contains only 8.4 

percent sodic clinopyroxene. Biotite is present in variable amounts, 5-50 percent, and is thus a major component in some samples. Potassic feldspar and sodic amphibole are present in some samples, whereas sphene, 

magnetite, and apatite are present in all samples 

examined.

Plots of modal components of mafic nephelineclinopyroxene rocks on a QAPF diagram (fig. 14) are 

not too informative because of the lack of plagioclase

FIGURE 18. Outcrop of mottled mafic nepheline-clinopyroxene rocks. 

Approximately 12 cm of scale is shown.
GEM PARK COMPLEX 11

FIGURE 19. Sodic clinopyroxene (Cpx) and nepheline (Ne) in poikilitic mafic nepheline-clinopyroxene rock. Sample 415X; partly crossed 

nicols; bar is 0.5 mm.

in these rocks. The plots show only the variation in 

the sodic feldspar/nepheline ratio, which ranges from 

1:1 to 0:1. Plots of modal nepheline-potassic feldsparmafic minerals (fig. 15) show the paucity of potassic 

feldspar and the greater abundance of mafic minerals 

relative to the nepheline syenites. Most of the rocks 

plot in or near the ijolite and feldspar-bearing ijolite 

fields.

GEM PARK COMPLEX

The oval-shaped Gem Park Complex is a maximum 

of about 2.9 km along a north-south axis and 2.0 km 

along an east-west axis (fig. 2). The first indication that 

the rocks at Gem Park were part of an alkaline complex 

was given by Parker and others (1962); the Gem Park 

Complex was later adopted as a formal term and described in detail by Parker and Sharp (1970). Various 

mineral commodities, including nickel, silver, vermiculite, niobium, magnetite, and ornamental stone, have 

been sought at Gem Park in or adjacent to the complex. 

Recently, diamond drilling has blocked out an area containing economically interesting amounts of Nb205 in 

stockwork carbonatite that intrudes rocks of the complex (D. W. Fieldman, oral commun., 1978).

At Gem Park, the mafic-ultramafic rocks weather at 

a rate greater than that of the surrounding Precambrian granitic gneisses. As a result, rocks tend to be, at 

best, only moderately well exposed, and are mainly restricted to low, rubbly outcrops. The stratiform nature 

of the complex is well exhibited in a small shaft just 

west of the center of the complex (fig. 20) by compositional layering exposed adjacent to a road along Pine 

Gulch at the south end of the complex, and by layering 

in core drilled by several companies. The mineral layerFIGURE 20. Cumulus layering in stratiform mafic-ultramafic rocks 

of the Gem Park Complex. Hammer is 31 cm long.

ing in the mafic-ultramic rocks dipping consistently toward the center of the complex suggests that the Gem 

Park Complex may be a funnel-shaped intrusion similar 

in gross shape to the mafic- ultramafic body at Iron 

Mountain in the McClure Mountain Complex. The Gem 

Park Complex is cut by several high-angle normal faults 

that also cut the volcanic rocks immediately to the west 

of the complex, the age of the faults is post-Oligocene 

(Parker and Sharp, 1970).

At least two general types of fenitization occur in 

the Gem Park Complex: fenitization of enclosing Precambrian granitic gneisses because of metasomatic alteration accompanying intrusion of the rocks of the complex, and fenitization of rocks of the complex because 

of metasomatic alteration accompanying intrusion of 

later rocks, mainly carbonatites. An example of the first 

type of alteration occurs on Democratic Mountain just 

east of the complex, where Precambrian granitic rock 

has been fenitized to a pink granitic rock with abundant 

green, fracture-controlled epidote. This rock has been 

quarried as an ornamental building stone. An example 

of the second type of alteration occurs in the north-central part of Gem Park at the Vermiculite mine, where 

mafic-ultramafic rocks have been metasomatized to an 

assemblage of aegirine, tremolite-actinolite, augite, 

phlogopite, vermiculite, dolomite, calcite, barite, apatite, magnetite, and fibrous, blue, sodic amphibole 

(Parker and Sharp, 1970, p. 10). The source of these 

fenitizing solutions has been postulated by Parker and 

Sharp (1970, p. 23) to be a buried central carbonatite 

core. However, Rock (1976) suggested an incompatibility between calcic plagioclase and carbonatite in alkaline
12 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS A^dA, COLORADO

complexes. This incompatibility would preclude the possibility of a central carbonatite core, but would not explain the occurrence of carbonatite in a calcic plagioclase-rich alkaline-rock complex.

MAFIC-ULTRAMAFIC ROCKS

The mafic-ultramafic rocks in the Gem Park Complex, 

which are predominantly igneous cumulates, comprise 

the cumulus minerals plagioclase, clinopyroxene, olivine, and magnetite. The most abundant rock types are 

plagioclase-clinopyroxene orthocumulates the gabbro 

of Parker and Sharp (1970) and clinopyroxene adcumulates the pyroxenite of Parker and Sharp (1970). 

Locally, plagioclase adcumulates and magnetite orthocumulates are also found. Plagioclase, clinopyroxene, 

and magnetite also form intercumulus material along 

with reddish-brown sodic amphibole and red biotite. 

Modal variation in plagioclase, clinopyroxene, and olivine is shown in the ternary diagram in figure 3. The 

most obvious characteristic of mafic-ultramafic rocks in 

the Gem Park Complex, as compared with the McClure 

Mountain Complex, is the low amounts of olivine. 

Parker and Sharp (1970, table 2) reported as much as 

6 percent olivine by volume in several samples of gabbro, whereas the current study shows 1 percent or less 

olivine in any given rock. In thin section, the cumulus 

minerals clinopyroxene, plagioclase, and olivine appear 

identical to the minerals in the mafic-ultramafic rocks 

in the McClure Mountain Complex at Iron Mountain. 

Pleochroic reddish-brown sodic amphibole, whose optical properties best fit kaersutite, occurs as intercumulus material and appears to have a reaction relationship with magnetite. Pleochroic green amphibole 

also appears to be an alteration product of kaersutite 

and clinopyroxene; it occurs in amounts of less than 

2 percent by volume. Biotite, pleochroic neutral to red, 

does not exceed 1.5 percent by volume and is chiefly 

associated with magnetite. Apatite containing clusters 

of small two-phase fluid inclusions is also intercumulus 

and is typically anhedral. Sphene has been identified 

in only a few rocks.

Drill core from the northern part of the Gem Park 

Complex has intersections of coarse-grained 

clinopyroxene adcumulates that contain abundant 

epigenetic sulfide minerals. These minerals include pyrite, pyrrhotite, and chalcopyrite.

Most of the layered mafic-ultramafic rocks in the Gem 

Park Complex are similar to the layered mafic-ultramafic rocksat Iron Mountain in the McClure Mountain 

Complex. Because of these similarites, the cumulus 

rocks at Gem Park and at Iron Mountain likely share 

a common origin.

OTHER ROCKS OF THE COMPLEX

Lensoid, conformable bodies rich in opaque minerals 

were mapped by Parker and Sharp (1970) along the 

contacts of clinopyroxene adcumulates and plagioclaseclinopyroxene orthocumulates mainly in the southern 

and southeastern parts of the Gem Park Complex. 

These rocks are best described as magnetite orthocumulates and they typically contain more than 95 

percent euhedral magnetite and minor amounts of green 

spinel, pale-yellow-green biotite, and interstitial finegrained aggregates of silicate minerals. These syngenetic rocks are part of the layered sequence and are similar to the magnetite-rich rocks at the Iron Mountain 

mine.

Near the center of the complex a small, shallow pit 

exposes nepheline syenite pegmatite that most likely 

intrudes the mafic-ultramafic rocks (Parker and Sharp, 

1970, p. 6). These coarse-grained rocks contain untwinned and microperthitic potassic feldspar, nepheline, 

aegirine-augite, several amphiboles, including redbrown kaersutite and green hornblende, sphene, muscovite, apatite, opaque minerals, albite, and sodalite(?). 

Parker and Sharp (1970) also noted the presence of natrolite and analcite.

The mafic-ultramafic rocks at Gem Park are intruded 

by several dikes of lamprophyre and syenite porphyry. 

These types of dikes are found throughout the Wet 

Mountains area and have been described by Heinrich 

and Dahlem (1969); dikes at Gem Park were also discussed by Parker and Sharp (1970). Dikes and irregularly shaped intrusions of carbonatite also intrude the 

rocks of the Gem Park Complex as well as the Precambrian host rocks; these intrusions also were discussed 

by Parker and Sharp (1970).

COMPLEX AT DEMOCRAT CREEK

The complex at Democrat Creek is about 14 km east 

of the Gem Park Complex and 7 km southeast of the 

McClure Mountain Complex. It is a maximum of 4.5 

km along a northwest-southeast axis and 3 km along 

a northeast-southwest axis (fig. 2).

Rocks of the complex at Democrat Creek were partly 

mapped by Christman and others (1954), who referred 

to them as "metamorphosed gabbroic and ultramafic 

rocks," "breccia," and "albite syenite." This reference 

appears to be the first mention of the complex at Democrat Creek, although the associated mafic-ultramafic 

rocks were considered to be Precambrian in age and 

therefore not part of the complex. Christman and 

others (1954) reported an age of 595 m.y. by the Larsen 

zircon method. Essentially the same data were pre-
COMPLEX AT DEMOCRAT CREEK 13

sented by Singewald and Brock (1956), Christman and 

others (1959), and Brock and Singewald (1968). Parker 

and Hildenbrand (1963), who mentioned the albite syenite intrusion and the 595-m.y. date, suggested that the 

syenite at Democrat Creek and the rocks of the 

McClure Mountain Complex are probably the same age. 

Heinrich and Dahlem (1966) coined the name "Democrat 

Creek complex" and related the complex to the McClure 

Mountain and Gem Park Complexes. They reinterpreted the "metamorphosed gabbroic and ultramafic 

rocks" of Christman and others (1954) as resembling 

the gabbros of the other complexes and therefore as 

being part of the complex at Democrat Creek in the 

sequence gabbro, breccia, albite syenite, alkalic dikes, 

and thorium veins and carbonatites. Armbrustmacher 

(1979) noted that close examination of rocks of the complex at Democrat Creek reveals more differences than 

similarities between these rocks and those of the 

McClure Mountain and Gem Park Complexes.

Most of the rocks in the complex at Democrat Creek 

are leucocratic, and they show different types of textures and grain sizes. The contact of the leucocratic 

rocks with the Precambrian host rocks is marked by 

a breccia zone in some places. The mafic-ultramafic 

rocks of the complex appear to be restricted to the 

southern part of the complex. Several satellitic intrusions of leucocratic rock also surround the main part 

of the complex. Rocks of the complex are cut by a 

quartz-barite-thorite vein and by several syenitic dikes, 

but neither carbonatite dikes nor red syenite dikes have 

been found within the complex.

MAFIC-ULTRAMAFIC ROCKS

The most significant petrographic aspect of the maficaltramafic rocks, from outcrops south of the large 

quartz syenite intrusion, is the association of two pyroxenes a calcium-rich clinopyroxene and a calcium-poor 

orthopyroxene. According to Wilkinson (1974), this association is characteristic of tholeiitic, not alkaline, 

mafic rocks. On the basis of petrographic data, the 

clinopyroxene is augite and the faintly pleochroic orthopyroxene is hypersthene. Plagioclase either is absent, as in the pyroxenites, or constitutes nearly 25 

percent of the rock, as in the gabbros. The plagioclase 

is clear, but is saussuritized to varying degrees. 

Oriented rodlike inclusions of apatite (?) occur in some 

plagioclase grains. Interstitial to the pyroxenes, and 

having the appearance of intercumulus material, is a 

pale-yellowish-green, faintly pleochroic amphibole that 

has optical and X-ray-diffraction properties similar to 

those of minerals in the tremolite-actinolite group. 

Sparse biotite, pleochroic in shades of green and brown, 

is also interstitial; apatite and magnetite are accessory

minerals. Evidence of stratification is not obvious at 

the outcrop.

Compared with mafic-ultramafic rocks from the 

McClure Mountain and Gem Park Complexes, which 

contain minerals typical of alkaline mafic rocks, the 

mafic-ultramafic rocks from the complex at Democrat 

Creek contain a mineral assemblage characteristic of 

tholeiitic mafic rocks. Chemical data discussed later 

verify the tholeiitic nature of these rocks.

QUARTZ SYENITE

The leucocratic rocks in the complex at Democrat 

Creek have been called "albite syenite" by Christman 

and others (1959) and Heinrich and Dahlem (1966), 

"medium-grained syenite" by Scott and others (1976), 

"fine- to medium-grained syenite" by Taylor and others 

(1975b), and "quartz syenite" by Olson and others 

(1977). In this report, they are termed "quartz syenites." In the field, several varieties of rocks, mainly 

light gray, tan, and pink, are present: medium-grained 

rocks with equigranular textures, coarsely crystalline 

rocks with pegmatitic textures, and medium-grained 

porphyritic rocks, all intermixed at about the 1-10 m 

scale. In hand specimen, the pegmatitic rocks have gradational contacts with the equigranular rocks; in some 

places, the porphyritic rocks cut the pegmatitic rocks. 

Whether several episodes of intrusion are represented 

or whether localized variations in crystallization, such 

as abundances of volatile constituents, are present is 

not clear. Lithologic variations also result from variations in modal mineralogy. The abundance of mafic minerals varies from near zero to moderately abundant at 

the outcrop; in some specimens, the mafic minerals are 

acicular. At some localities, quartz appears to be abundant in the hand specimens, but at other localities 

quartz is difficult to identify. Xenoliths of both mafic 

rocks and granitic gneiss have been observed, sometimes at localities distant from contacts with the country rock. The mafic rocks are altered, and perhaps represent fenitized equivalents of the mafic-ultramafic 

rocks, such as those occurring south of the leucocratic 

part of the complex. Fenitization of the country rock 

Precambrian granitic gneisses adjacent to rocks of the 

complex is not obvious.

Quartz, which usually occurs as an interstitial mineral, ranges from 2 to 32 percent by volume. Small fluid 

inclusions containing daughter minerals are abundant 

in the quartz, which partly replaces the feldspars.

The feldspars, microperthite, microcline, and albite 

are the most abundant constituents of the quartz syenites; they collectively range from 64 to over 93 percent. Microperthite is the most abundant mineral, but
14 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

microcline is absent in some samples. Albite may replace microcline and form discrete grains chiefly interstitial to larger microperthite grains. Microperthite 

ranges from 32 to 83 percent by volume, microcline 

ranges from 0 to 20 percent, and albite ranges from 

6 to nearly 20 percent. Sodic amphibole in the quartz 

syenites ranges from 0 to 10 percent, has a small extinction angle, is biaxial with a moderate 2V, and is pleochroic olive green to dark blue. This mineral appears 

to be a member of the eckermannite-arfvedsonite 

group. Aegirine(?) and riebeckite are sparse. Biotite 

tends to be altered and replaced, and is pleochroic 

through shades of reddish brown. Zircon, fluorite, magnetite, and sparse sphene are accessory minerals that 

occur in amounts of less than 1 percent.

Plots of modal data on a QAPF diagram (fig. 14) show 

that most of the quartz syenites plot in the syenite and 

alkali syenite fields; two of the quartz-rich varieties plot 

in the granite field.

CARBONATITES

Carbonatites associated with alkaline rocks of the 

McClure Mountain and Gem Park Complexes and the 

complex at Democrat Creek can be classified into two 

groups, replacement and primary magmatic, on the 

basis of distinctive petrographic differences (Armbrustmacher, 1979). These two groups also show distinctive differences in mineralogy and geochemistry. 

Textures of replacement carbonatites indicate nearly 

complete pseudomorphous replacement of originally 

porphyritic or hypidiomorphic-granular igneous dike 

rocks by carbonate minerals. They contain an element 

suite characteristic of carbonatites as well as a traceelement signature indicative of a mafic silicate precursor. Minerals that contain thorium, niobium, and rareearth elements as essential constituents generally are 

rare. Primary magmatic carbonatites, which do not 

show the distinctive replacement textures, are also enriched in elements characteristic of carbonatites, but 

contain greater average amounts of these elements than 

do replacement carbonatites. Thorite, bastnaesite, synchysite, ancylite, and monazite are chiefly responsible 

for the high thorium and rare-earth-element content of 

these rocks compared to that in other igneous rocks. 

Primary magmatic carbonatites are spatially more 

closely associated with the alkaline intrusions than are 

replacement carbonatites, which have a somewhat 

wider distribution. Field data and 8 180 and 8 13C values 

support an igneous origin for primary magmatic carbonatites; isotopic values indicate that the carbonatite replacing the earlier alkaline dikes also was derived from 

igneous sources. Exchange of isotopes between both 

types of carbonatite and nonigneous reservoirs appears 

to be minimal.

Abundances of rare-earth .elements and niobium in 

carbonatites of the Wet Mountains area are somewhat 

lower than those at other carbonatite localities that contain economic or near-economic concentrations. Thorium is more abundant in spatially related vein-type deposits than in the carbonatites.

AGES OF THE COMPLEXES

The first known age determinations of Wet Mountains alkaline rocks were reported by Jaffe and others 

(1959). Two samples of nonmetamict zircon from the 

"albite syenite" at Democrat Creek gave ages of 580 

and 601 m.y. by the Larsen zircon method. A K-Ar 

age reported by Brock and Singewald (1968) for biotite 

from the syenite at Democrat Creek is 500 ±25 m.y. 

Fenton and Faure (1970) calculated a whole-rock Rb-Sr 

isochron for rocks from the McClure Mountain Complex 

that indicated an age of 517 ± 14 m.y. and an initial 87Sr/ 

86Sr ratio of 0.7059 ±0.0003. Using a variety of dating 

techniques, Olson and others (1977) reported ages from 

a variety of alkaline rocks. Their results for rocks from 

the McClure Mountain Complex are: 520 m.y. (average 

hornblende K-Ar age), 521 m.y. (Rb-Sr isochron), 508 

m.y. (average biotite K-Ar ages), and 506 m.y. (sphene 

fission-track age); the quartz syenite in the complex at 

Democrat Creek has average K-Ar ages of 512 m.y. 

(biotite) and 534 m.y. (hornblende). Their data showed 

that red syenite dikes associated with rocks of the three 

complexes have an age of 495 m.y. (Rb-Sr isochron), 

but attempts to date rocks from the Gem Park Complex 

gave unsuitable results (Olson and others, 1977, p. 683).

In more recent studies (Armbrustmacher and Hedge, 

1982), whole-rock isochrons were prepared for leucocratic rocks from the McClure Mountain Complex and for 

the complex at Democrat Creek by C. E. Hedge of 

the U.S. Geological Survey. The hornblende-biotite 

syenites and nepheline syenites from the McClure 

Mountain Complex lie along the same whole-rock isochron (fig. 21), and, although field relationships indicate 

that the nepheline syenite is younger than the 

hornblende-biotite syenite, these differences cannot be 

resolved with these data. The age of these syenites is 

535 ±5 m.y.; and the initial 87Sr/86Sr ratio is 

0.7037 ±0.0002. The quartz syenites from the complex 

at Democrat Creek show a greater variation in Rb/Sr 

ratios; the whole-rock isochron gives an age of 511 ±8 

m.y. and an initial 87Sr/86Sr ratio of 0.7032 ±0.0002 (fig. 

22). Thus, an age difference of about 24 m.y. exists 

between the syenites in the McClure Mountain Complex and the quartz syenites in the complex at Democrat Creek. The relative and absolute age relationships 

of the rocks of the Wet Mountains area are given in 

table 1.
GEOCHEMISTRY 15

0.76

0.75

0.74

~C 0.73 to

0.72

0.70-

EXPLANATION

McCLURE MOUNTAIN COMPLEX 

D Nepheline syenite

 Nepheline syenite (Olson and 

others, 1977)

o Hornblende-biotite syenite

 Hornblende-biotite syenite (Olson 

and others, 1977)

0.7100

0.7075

0.7050

0.7025

WM-8-71 

WM-12-71

WM-21-71

wM-4-71

0.2 t.4 0.6 0.8

87Rb/86Sr 

Enlargement of boxed area at left

3

87Rb/86Sr

FIGURE 21. Whole-rock isochron of nepheline syenites of McClure Mountain Complex.

1.4

1.2

1.0

0.8

412

0 20 40 60

87Rb/86Sr

80 100

FIGURE 22. Whole-rock isochron of quartz syenites of the complex 

at Democrat Creek.

GEOCHEMISTRY

Discussion of the geochemistry of the alkaline rock 

complexes is based on data from several sources. Majorelement analyses of mafic-ultramafic rocks from the 

Gem Park Complex are from Parker and Sharp (1970, 

table 1), and analyses of rocks from the Iron Mountain 

part of the McClure Mountain Complex, as well as 

minor- and trace-element contents of some of the rocks

are from Shawe and Parker (1967, table 3). The remaining whole-rock analyses are new. Geochemical data 

based mainly on carbonatites have been presented by 

Armbrustmacher (1979) and Armbrustmacher and 

Brownfield (1978). Radiometric ages of rocks in the 

complexes have been reported by Jaffe and others 

(1959, p. 127), Fenton and Faure (1970), Olson and Marvin (1971), and Armbrustmacher and Hedge (1982). Additonal minor-element and isotopic data have been reported by Roden and Cullers (1976) and 

Armbrustmacher, Brownfield, and Osmonson (1979). 

The major- and minor-element distributions in the alkaline rocks have been summarized by Armbrustmacher (1980).

MAJOR ELEMENTS

MAFIC-ULTRAMAFIC ROCKS

Chemical and normative data of mafic-ultramafic 

rocks from the Iron Mountain area of the McClure 

Mountain Complex are given in table 2; similar data 

for mafic-ultramafic rocks from the Gem Park Complex 

and the complex at Democrat Creek are given in table 

3. Analyses 1-9 in table 2 are from Shawe and Parker 

(1967, table 3), except that the norms have been recalculated from analyses adjusted to 100 percent. Analysis
16 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

TABLE 1. Relative atid absolute age relationships of rocks of the 

Wet Mountains area, Colorado

Geologic Rock types Absolute

ages

is^

0)

s_

.a 

£ 

o

s_

.0

£

£

DVolcaniclastic rocks

Quartz-barite-thorite veins; various leucocratic dikes

Complex at Democrat Creek: quartz syenite

breccia

mafic-ultramafic rocks

Carbonatites, red syenites, lamprophyres (?)

McClure Mountain Complex: nepheline syenite

hornblende-biotite syenite

mafic nephel ine-cl inopyroxene rock

Gem Park Complex: nepheline syenite pegmatite

Gem Park and McClure Mountain cumulates

Silver Plume or Proterozoic Y age: granitic intrusive

rocks

Boulder Creek or Proterozoic X age: granitic

intrusive rocks

Proterozoic X metamorphic rocks

ages

511 m.y.

535 m.y.

535 m.y.

1,450 m.y.

1,720 m.y.

10 (table 2), a plagioclase-clinopyroxene orthocumulate, 

was obtained by Shawe and Parker but was not included in their 1967 Iron Mountain report. In table 3, 

the two analyses of Gem Park rocks are from Parker 

and Sharp (1970, table 1); the norms also have been 

recalculated as in table 2. The four analyses of Democrat Creek maflc-ultramafic rocks are new.

None of the mafic-ultramafic rocks from the McClure 

Mountain and Gem Park Complexes and only one from 

the complex at Democrat Creek contain normative 

quartz. Seven rocks from McClure Mountain, both 

rocks from Gem Park, and no rocks from Democrat 

Creek contain normative nepheline. Only three rocks 

from McClure Mountain contain normative hypersthene, all in amounts of less than 3 percent; neither 

of the rocks from Gem Park contains normative hypersthene, but all rocks from Democrat Creek contain normative hypersthene in amounts exceeding 37 percent. 

All McClure Mountain rocks contain normative olivine 

in variable amounts; Gem Park rocks contain some normative olivine, and Democrat Creek rocks contain normative olivine in amounts of less than 2.1 percent. According to Wilkinson (1974, p. 67), basaltic rocks containing normative nepheline and normative olivine are 

alkali basaltic types, but rocks containing normative hypersthene are saturated tholeiitic basaltic types. Rocks 

from the McClure Mountain and Gem Park Complexes 

appear to be alkali basaltic, whereas rocks from the 

complex at Democrat Creek appear to be tholeiitic 

basaltic. Figure 23, which is modified from Currie 

(1976, fig. 1), incorporates the boundary line conventionally used to partition alkaline basalts from tholeiitic 

or subalkaline basalts (MacDonald and Katsura, 1964). 

The plagioclase-bearing rocks from the McClure Mountain and Gem Park Complexes plot as members of the 

alkali basalt family. Plagioclase-poor pyroxenitic rocks 

plot as subalkaline rocks because of their deficiency in 

Na20 + K20, and plagioclase-rich anorthositic rocks plot 

near the boundary. Mafic-ultramafic rocks from the 

complex at Democrat Creek cluster together, well within the subalkaline rock field. Plots of agpaitic index 

(molecular proportions Na + K/Al) versus weight percent Si02 (fig. 24) of Gem Park rocks place them in 

the alkaline basalt family, whereas McClure Mountain 

rocks occur within the alkaline basalt family and subalkaline rocks fields regardless of lithologic type. The 

Democrat Creek rocks again cluster in the subalkaline 

rock field.

On the basis of whole-rock chemical composition, the 

mafic-ultramafic rocks of the McClure Mountain and 

Gem Park Complexes appear to be similar to and to 

be derived from an alkaline basalt parent. Wager and 

Brown (1967, p. 540) stated that they "* * *are not 

aware of any major layered intrusion likely to have 

stemmed from an alkali-basalt parent magma." Currie 

(1976, p. 5) also stated that "* * * plutonic complexes 

of alkaline basalt composition are rare, although sub volcanic complexes such as the Monteregian Hills are common." However, Brown (1973) pointed out that layered 

intrusions of alkaline basalt parentage, such as Lilloise 

in east Greenland, are not unknown, and Currie (1976) 

described a few occurrences in Canada in addition to 

that in the Monteregian Hills. Mafic-ultramafic rocks 

at Democrat Creek appear to have subalkaline, saturated, tholeiitic affinities.

LEUCOCRATIC ROCKS

Chemical and normative data of hornblende-biotite 

syenites and nepheline syenites from the McClure 

Mountain Complex are given in table 4, and those of 

quartz syenites from the complex at Democrat Creek 

and of nepheline syenite pegmatite from the Gem Park 

Complex are given in table 5. The quartz syenites from 

the McClure Mountain Complex that have the WM sample-number prefix were collected by R. L. Parker and 

W. N. Sharp of the U.S. Geological Survey; the syenites from Democrat Creek were collected for this 

study. None of the analyses has been published previously.
GEOCHEMISTRY 17

TABLE 2. Chemical arid normative data of mafic-ultramafic rocks from the Iron Mountain area of the McClure Mountain

Complex, south-central Colorado

[Values in weight percent. Leaders ( ) indicate no data. Analyses 1-9 are from Shawe and Parker, 1967, table 3; analysis 10 by Shawe and Parker, unpublished data, 1980]

Analysis No.* 1 

Field No. IM-10-64

2 

WM-64-827

3 4 

IM-20-64 WM-62-73

5678 

WM-64-828 WM-64-829 WM-64-824 IM-28-64

9 

IM-31-64

10 

WM-64-896A

Chemical Compositions

Si02

A1 2 03

Fe 20 3

FeO

MgO

CaO

Na 2 0

K20

H 20

Ti02

P 2°5

MnO

C02

Cl

F

40.31

7.31

2.81

15.61

25.22

5.34

.88

.14

1.24

.54

.05

.29

.38

.01

.01

Total-- 100.14

42.81

17.08

5.53

7.49

6.53

13.59

2.18

.22

.69

2.57

.37

.15

.78

.02

.04

100.05

43.28

24.82

1.54

3.22

9.41

12.44

1.55

.19

2.89

.11

.03

.07

.39

.02

.01

99.97

46.

21.

1.

4.

7.

13.

2.

.

.

.

.

.

100.

74

10

51

93

48

62

41

21

74

72

13

10

33

03

02

07

46.88

11.18

3.47

5.12

10.61

19.47

1.07

.09

.36

1.83

.08

.13

.02

.02

100.33

46.99 44.00 46.07

11.47 9.67 8.73

3.09 4.36 4.07

4.73 4.95 3.83

10.68 13.66 11.95

20.02 20.01 21.42

.94 .47 .94

.07 .12 .18

.29 .69 .48

1.71 1.57 1.81

.03 .04 .06

.13 .13 .11

.03 .56 .37

.01 .02

.02 .02 .04

100.20 100.26 100.08

51.

26.

1.

1.

12.

3.

1.

.

.

.

.

99.

05

94

80

08

29

47

78

62

00

28

02

03

48

01

02

87

46

23

1

3

4

15

2

100

.59

.32

.92

.01

.89

.26

.69

.26

.70

.74

.14

.07

.57

.03

.16

.35

Normative Composition

C

Or

Ab

An

Lc

Ne

HI

Mo

En

Fs

Fo

Fa

Cs

Mt

n

Ap

Fr

Cc

Di

Hy

01

0.83

7.36

15.60

 

.02

3.38

3.80

1.57

41.29

18.83

 

4.07

1.02

.12

.01

.86

6.52

2.23

60.12

_._

1.30

18.24

36.23

.03

.03

9.92

6.89

2.21

6.56

2.32

8.01

4.88

.88

.01

1.77

19.02

 

8.88

0.47

1.12

12.97

59.02

___

.03

.28

.06

16.23

3.50

2.23

.21

.07

.02

.89

___

.34

19.73

-

1.

18.

46.

-

1.

.

7.

5.

1.

9.

3.

-

2.

1.

.

.

14.

-

13.

--

24

05

22

--

14

05

64

17

89

42

79

--

19

37

31

02

75

70

22

0.53

4.14

25.35

2.65

 

29.30

22.85

3.25

2.44

.38

5.02

3.46

.19

.03

.04

55.40

 

2.82

0.41

2.66

26.82 23.90 19.13

.56 .83

2.86 2.11 4.22

.02 .03

29.98 26.70 33.53

23.52 21.56 28.34

3.14 1.99 .84

2.12 8.67 .98

.31 .88 .03

2.25 1.16

4.47 6.30 5.90

3.24 2.97 3.44

.07 .09 .14

.04 .03 .07

.07 1.27 .84

56.63 50.25 62.72

2.43 9.55 1.01

-

3.

31.

54.

-

_

.

1.

3.

.

.

-

1.

.

.

1.

3.

2.

--

67

95

82

--

02

60

02

86

14

04

16

53

05

04

09

05

43

18

1

17

50

2

7

5

1

4

1

2

1

1

15

5

.53

.24

.73

.83

.05

.99

.86

.37

.40

.13

.77

.40

.33

.30

.29

.22

.53

*Analysis Nos. are:

1. 

2. 

3. 

4. 

5.

Clinopyroxene-plagioclase-ol i vine orthocumulate. 

PI agi ocl ase-cl i nopyroxene orthocumul ate. 

Plagioclase-oli vine orthocumulate. 

Pi agi ocl ase-cl i nopyroxene-ol i vi ne cumul ate. 

Cl i nopyroxene-pl agi ocl ase cumul ate.

6. 

7. 

8. 

9. 

10.

Cl i nopyroxene-pl agi ocl ase cumulate. 

Pyroxenite from discordant intrusion. 

Pyroxenite from discordant intrusion. 

Anorthosite from discordant intrusion. 

PI agiocl ase-cl i nopyroxene orthocumul ate.
18 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

TABLE 3. Chemical and normative data of mafic-ultramafic rocks

from the Gem Park Complex and the complex at Democrat Creek,

south-central Colorado

[Values in weight percent. Leaders ( ) indicate no data]

Gem Park Complex

Analysis No.* 1 2 

Field No. WM-64-859 WM-64-875

Si02

A1 203

Fe 203

FeO

MgO

CaO

Na 2 0

K20

H 20

Ti02

P 2 05

MnO

C02

Cl

F

Chemical

39.16 45.31

13.72 7.03

7.50 3.98

9.79 6.05

6.68 12.84

12.12 19.99

2.56 .82

.45 .17

1.16 .84

4.42 2.50

1.06 .14

.21 .15

.99 .40

.04 .02

.12 .04

Total-- 99.98 100.28

Q

Or

Ab

An

Ne

HI

Mo

En

Fs

Fo

Fa

Mt

11

Ap

Fr

Cc

Di

Hy

01

Normative

2.66 1.00

18.84 .12

24.78 15.04

1.37 3.60

.07 .03

9.18 33.50

6.49 26.45

1.90 3.29

7.11 3.81

2.30 .52

10.88 5.76

8.40 4.74

2.51 .33

.05 .06

2.25 .91

17.57 63.23

...

9.41 4.33

Complex at

3 4 

75A-299 75A-300

Composition

52.8 52

9.3 4

1.2 3

6.5 6

16.8 22

10.5 8

1.3

.22

.82

.34

.08

.13

.02

 

100.0 99

Composition

0

1.3

11.0 5

18.9 10

 

...

13.6 12

40.0 55

10.1 9

1.3

.4

1.7 4

.6

.2

 

.04

25.9 24

37.8 53

1.7

.6

.8

.0

.5

.2

.2

.59

.15

.57

.39

.05

.17

.01

 

.2

.5

.9

.0

.1

. 

.7

.7

.2

...

.4

.7

.1

 

.02

.1

.5

 

Democrat Creek

5

75A-302

53.1

7.4

.5

7.8

18.2

10.1

.97

.11

.68

.45

.08

.13

...

 

99.5

 

.7

8.2

15.6

...

14.3

44.8

13.3

.5

.2

.7

.9

.2

 

 

27.4

45.0

.7

6 

75A-297

53.

4.

1.

7.

22.

7.

-

-

100.

-

2.

5.

9.

-

_

11.

53.

11.

1.

2.

-

21.

55.

2.

4

8

9

8

5

5

67

34

66

38

09

18

04

-

3

0

7

1

--

_

4

5

9

6

4

7

7

2

...

09

6

2

1

*Analysis Nos. are:

1. 

2. 

3. 

4. 

5. 

6.

Plagioclase-cl inopyroxene orthocu 

Clinopyroxene adcumulate (Parker 

Gabbro (analyst Floyd Brown). 

Pyroxenite (analyst Floyd Brown). 

Gabbro (analyst Floyd Brown). 

Pyroxenite (analyst Z. A. Hamlin)

mulate (Parker 

and Sharp, 1970,

and Sharp, 1970, 

Table 1).

table 1).

All hornblende-biotite syenites and nepheline syenites from McClure Mountain contain normative 

nepheline (less than 1 percent and 9-25 percent respectively). However, none of the rocks contains normative 

quartz. The syenites from Democrat Creek typically 

contain abundant normative quartz and lack normative 

nepheline. Three feldspars exist in the norms of 

hornblende-biotite syenite and nepheline syenite from 

McClure Mountain: normative Ab>Or>An and norma40 50 60 70

SiOc

EXPLANATION

McCLURE MOUNTAIN 

COMPLEX

Nepheline syenite

Hornblende-biotite 

syenite

Mafic nephelineclinopyroxene rocks

V Mafic-ultramafic rocks

GEM PARK COMPLEX

Nepheline syenite pegmatite 

V Mafic-ultramafic rocks

COMPLEX AT DEMOCRAT 

CREEK

 Quartz syenite

V Mafic-ultramafic rocks

FIGURE 23. Plots of weight-percent alkalies versus weight-percent 

silica for samples from the McClure Mountain Complex, Gem Park 

Complex, and complex at Democrat Creek (modified from Currie, 

1976, fig. 1).

tive Or>Ab>An, respectively in most norms. Nearly 

all the quartz syenites from Democrat Creek contain 

two normative feldspars with Ab>0r. The CaO content 

of both hornblende-biotite syenites and nepheline syenites totals several percent by weight. The CaO content 

of quartz syenites, which rarely exceeds 1 percent, is 

so low in several samples that occurs not enough CaO 

to combine with all the fluorine to make normative Fr 

(fluorite) in the norm; therefore, excess fluorine must 

be listed as normative F (fluorine). This relation reflects 

the abundance of modal fluorite and the paucity of 

modal plagioclase in the quartz syenites from Democrat 

Creek. All the hornblende-biotite syenites and 

nepheline syenites are devoid of normative hypersthene, whereas all the quartz syenites contain it. The 

presence of normative olivine as well as normative 

nepheline in the hornblende-biotite syenites and 

nepheline syenites indicates that they are chemically 

undersaturated; neither normative olivine nor normative nepheline occurs in quartz syenites.
GEOCHEMISTRY 19

1.2

Carbonatite 

family >

Miaskitic Agpaitic syenite family 

syenite

family

70

SiO;

EXPLANATION

McCLURE MOUNTAIN 

COMPLEX

D Nepheline syenite

o Hornblende-biotite 

syenite

0 Mafic nephelineclinopyroxene rocks

V Mafic-ultramafic rocks

GEM PARK COMPLEX 

x Nepheline syenite pegmatite 

V Mafic-ultramafic rocks

COMPLEX AT DEMOCRAT 

CREEK

 Quartz syenite

V Mafic-ultramafic rocks

FIGURE 24. Plots of agpaitic index versus weight-percent silica for 

samples from the McClure Mountain Complex, Gem Park Complex, 

and complex at Democrat Creek (modified from Currie, 1976, fig.

2).

Plots of the leucocratic rocks on a Na20 + K20 versus 

Si02 diagram (fig. 23) show that the nepheline syenites 

from McClure Mountain generally contain somewhat 

more Na20 + K20 and slightly less Si02 than do the 

hornblende-biotite syenites; the values overlap slightly. 

The quartz syenites of Democrat Creek, because of 

their higher Si02 content, tend to plot away from the 

syenites from the McClure Mountain Complex. One 

sample of nepheline syenite pegmatite from Gem Park 

(analyzed by Roden, 1977) contains considerably more 

Na20 + K20 than do the other syenites. All the 

hornblende-biotite syenites plot within the alkali basalt 

family field and are homogeneous in distribution; the 

nepheline syenites plot within various fields and are 

somewhat heterogeneous in distribution; and the quartz 

syenites from Democrat Creek plot in the alkaline 

basalt family and subalkaline rocks fields and also are 

heterogeneous. These plots do not provide unique solutions to problems of leucocratic rock genetic affiliations. 

Plots of leucocratic rocks on an agpaitic index versus 

Si02 diagram (fig. 24) provide nearly the same information. The nepheline syenite pegmatite at Gem Park is 

the only rock that plots in the agpaitic syenite family 

field.

These chemical data alone show distinct differences 

between quartz syenites from Democrat Creek and the 

syenites from the McClure Mountain Complex. Although somewhat more subtle, chemical differences also 

exist between the hornblende-biotite syenites and 

nepheline syenites at McClure Mountain. The small exposure of nepheline syenite pegmatite, represented by 

one analysis (table 5, No. 8), shows large differences 

where compared with all other leucocratic rocks.

MAFIC NEPHELINE-CLINOPYROXENE ROCKS

Chemical and normative data of mafic nephelineclinopyroxene rocks are given in table 4. These rocks 

have been found only in the McClure Mountain Complex.

The Si02 content of the mafic nephelineclinopyroxene rocks is similar to that of the maficultramafic rocks (table 2), whereas the Na20 content 

is similar to that of the syenites at McClure Mountain 

(table 4). The P205 and Ti02 content is higher than 

that in most of the other rock types. Comparison of 

the analysis with the average derived from 41 analyses 

of African ijolites (LeBas, 1977, p. 307) shows a number 

of similarities, especially in the amounts of Si02 (40.01 

percent), Ti02 (2.38 percent), Na20 (7.10 percent), and 

Na20/K20 ratio (2.3). The P205 content (0.88 percent) 

is not similar.

Plots of the mafic nepheline-clinopyroxene rocks on 

a diagram of Na20 + K20 versus Si02 (fig. 23) show 

they contain more abundant alkalies but comparable 

amounts of Si02 in comparison to the mafic-ultramafic 

rocks of McClure Mountain. These characteristics place 

these analyses in or near the nephelinite family field. 

Moreover, plots of these rocks on a diagram of agpaitic 

index versus Si02 (fig. 24) also place the analyses in 

the nephelinite family field.

ADDITIONAL OBSERVATIONS

In the diagram of Na20 + K20 versus Si02 (fig. 23), 

rocks of comagmatic series should plot at least subparallel to the boundary separating the alkaline basalt family 

from the subalkaline rocks. If this is true, one can visualize that three separate rock series are illustrated: (1) 

the mafic-ultramafic rocks and the quartz syenites from 

Democrat Creek; (2) the mafic-ultramafic rocks of 

McClure Mountain and Gem Park Complexes and the 

hornblende-biotite syenites of the McClure Mountain 

Complex, and (3) the mafic nepheline-clinopyroxene 

rocks and the nepheline syenite of the McClure Mountain Complex, and perhaps the nepheline syenite pegmatite of the Gem Park Complex.

Plots of all the analyses on a diagram of differentiation index versus oxide weight percent (fig. 25) show
20 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

TABLE 4. Chemical and normative data of hornblende-biotite syenites, nepheline syenites, and mafic nepheline-clinopyroxene

rocks, McClure Mountain Complex, south-central Colorado

[Values in weight percent. Leaders ( ) indicate no data. Analyses 1-7 by Ellen Daniels; Nos. 8-10 by K. Coates and H. Smith]

Analysis No.* 123 

Field No. WM-62-131 WM-62-140 WM-62-142

4 

WM-62-143

5 6 

WM-62-114 WM-62-116

7 

WM-64-896C

8 

75-201X

9 

78-496

10 

78A-498A

Chemical Composition

Si02

A1 2°3

Fe203

FeO

MgO

CaO

Na 2 0

K 20

H 2 0

Ti02

P 2°5

MnO

Co 2

Cl

F

59.57 59.50

20.04 18.64

1.41 1.65

1.80 1.80

.96 1.09

2.69 2.55

5.98 5.37

5.25 6.60

.64 .54

.86 .92

.32 .41

.08 .11

.07 .43

.02 .01

.10 .15

Total-- 99.79 99.77

57.71

19.18

1.86

2.09

1.28

3.34

5.39

5.64

.57

1.15

.50

.10

.45

.02

.14

99.42

59.80

18.64

1.46

1.93

1.11

2.34

5.82

5.91

.76

.88

.31

.12

.50

.01

.15

99.74

55.95 51.70

21.43 21.41

1.28 2.31

2.09 2.34

.69 .67

2.49 3.39

7.36 7.65

6.32 6.54

.67 1.11

.56 .94

.18 .19

.11 .16

.24 1.11

.21 .02

.10 .09

99.68 99.63

57.77

21.29

2.33

1.73

.37

1.63

7.46

5.21

.81

.58

.06

.24

.31

.12

.06

99.97

46.9

16.2

4.7

4.7

3.5

9.9

6.7

3.6

1.04

2.1

1.2

.23

.02

100.8

43.0

16.5

3.1

7.4

5.6

9.6

5.4

3.6

1.18

2.7

1.2

.23

.05

---

99.6

37.7

11.3

6.2

10.6

7.3

12.5

2.4

3.8

2.04

3.4

2.2

.31

.02

---

99.8

Normative Composition

C

Or

Ab

An

Ne

HI

Lc

Wo

En

Fs

Fo

Fa

Cs

Mt

11

Ap

Fr

Cc

Di

Hy

01

*Analysis

1. 

2. 

3.

4. 

5.

0.78 0.32

31.09 39.09

49.33 44.00

10.31 6.44

.66 .80

.03 .02

 

 

 

1.68 1.91

.67 .49

 

2.05 2.40

1.64 1.75

.76 .97

.15 .23

.16 .98

___ __-

 

2.35 2.39

Nos. are:

Hornblende-biotite syenite. 

Hornblende-biotite syenite. 

Hornblende-biotite syenite. 

Hornblende-biotite syenite. 

Nepheline syenite.

0.66

33.52

44.79

9.82

.51

.03

---

2.25

.46

2.71

2.20

1.19

.20

1.03

___

2.70

0.66

35.02

49.11

5.54

.10

.02

 

1.94

.86

 

2.12

1.68

.74

.25

1.14

_.-

2.80

6. 

7. 

8. 

9. 

10.

...

37.47 38.79

28.71 17.51

7.62 4.86

17.45 25.63

.35 .03

.61 1.33

.27 .76

.33 .52

1.02 .64

1.34 .48

1.86 3.36

1.07 1.79

.43 .45

.17 .15

.55 2.53

1.21 2.61

2.36 1.12

1.60

30.81

45.48

5.34

9.11

.20

 

.65

.57

3.38

1.10

.14

.11

.71

__.

1.22

 

21.1

9.7

3.5

25.2

_._

15.6

8.6

1.7

__-

 

6.8

4.0

2.8

.05

21.8

4.1

 

17.2

10.2

24.9

_-_

3.3

12.3

7.7

3.9

4.4

2.4

4.5

5.2

2.9

.1

23.8

6.9

...

8.9

11.0

17.6

15.1

9.4

4.8

6.2

3.5

.8

9.0

6.5

5.2

.05

29.3

9.7

Nepheline syenite. 

Nepheline syenite. 

Mafic nephel ine-cl inopyroxene rock. 

Mafic nepheline-clinopyroxene rock. 

Mafic nepheline-clinopyroxene rock.
GEOCHEMISTRY

TABLE 5. Chemical and normative data of quartz syenites from the complex at Democrat Creek 

and nepheline syenite pegmatite from the Gem, Park Complex, south-central Colorado

[Values in weight percent. Leaders ( ) indicate no data. Analysis 1-7 by Z. A. Hamlin and P. Brown]

21

Analysis No.* 1

Field No. 75A-363

2

75A-369

3

75A-373

4 5

75A-381 75A-382

6

75A-383

7

75A-385

8

GP-4

Chemical Composition

Si02

A1 2 0 3

Fe 2°3

FeO

MgO

CaO

Na 2 0

K2 0

H 2 0

Ti02

P 2°5

MnO

co2

Cl

F

Total 

67.7

15.4

1.3

2.2

.12

.47

6.8

4.9

.52

.08

.03

.12

.25

99.9

74.3

13.4

.88

.72

.01

.39

5.1

4.2

.28

.03

.02

.03

.36

99.7

66.8

15.7

1.6

1.9

.01

.52

6.8

4.7

.41

.12

.04

.12

.02

.01

.37

99.1

74.9 64.

12.7 16.

1.0 1.

.28 3.

.01

.25 1.

4.8 6.

4.1 5.

.47

.02

.02

.02

.12

.02

.1

98.8 99.

1

8

4

1

16

1

7

3

59

25

06

12

--

03

08

8

68.8

16.0

.93

.92

.01

.39

6.6

4.8

.45

.02

.03

.06

.02

.15

99.2

64.3

16.6

1.2

2.5

.11

1.1

6.8

5.3

.61

.22

.06

.12

.16

.01

.14

99.2

59.47

20.53

2.86**

.15

.97

8.11

8.97

3.25

.05

104.36

Normative Composition

Q

Or

Ab

An

Ne

HI

Ac

Ns

Wo

En

Fs

Mt

n

F

Ap

Fr

Pf

Di

Hy

01

*Analysis Nos.

1. Quartz

2. Quartz

3. Quartz

4. Quartz

5. Quartz

6. Quartz

7. Quartz

9.0

29.0

52.0

___

3.8

.3

.1

.3

4.1

.15

.07

.51

.29

4.28

are:

syenite.

syenite.

syenite.

syenite.

syenite.

syenite.

syenite.

8. Nepheline syenite

28.5

25.0

43.0

...

--_

.02

.5

1.4

.06

.1

.03

.7

___

.6

pegmatite

8.6

27.8

57.7

---

_.-

.01

2.3

2.3

.2

.04

.1

.7

__-

2.3

(Roden,

31.6 1.

24.5 31.

40.9 56.

1.

.02

.3 4.

1.4 2.

.05

.4

.03

.2

3.

.3 2.

1977, table 1).

8

4

6

2

--

05

--

8

4

4

0

5

1

2

--

8

8

--

11.6

28.6

56.0

.03

.1

.3

.02

1.0

1.3

.04

.07

.3

.6

.8

2.3

31.6

56.3

.02

1.4

1.3

.3

3.9

1.0

.4

.1

.3

2.7

2.7

52.99

19.93

 

19.32

3.00

1.05

.87

 

5.54

 

.56

.87

1.05

 

**Total iron as Fe 2 0 3 .
22 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

(N

O

CO

65

55

45

OK

-

ocQ, x

- _ D 

r- ^ D -

-vi^ w o

v i 1^1 i i i i i i

o

O(N

<

Oen

2

(0

O

9u9»

o(N

a

m

Ooo

*J\J

20

10

0

30

20

10

0

20

10

0

12

10

8

4

0

8

4

0

8

6 

4

2

0

6

4

2

I I I I I I 

- w ^

V ,-p-.

^ 0 A O ^w A

v^7 A <Q

I I I I I I I I

I i III

V

_ ** X

T V

^7V7 __

 w v 0

^ o A

 v Qocjyn*****-

i l i

II II I

i&y

^/ v^ ° ' o o -

I I I I I nJifalu~ Tfr

VI I I I I I I I

_

0

~ ** v 0

_ T v

v ^ v 0

i i i ^ i i i i D i 

1 ' ' x

 

o0 °a V ,, 

000 *»

^^TT ^7 v

1 1 1 1 1 1 1 1 1

1 1 1 1 1 i lt:thu^ x

~ o °° ° ** V

- v 0 -

 r^Mh^ V

-^/Wty V ^W |^| | | | |

i i

0

"v 0

^ 0

- * v / oo° ° . -

*[ T i v i i ii*

0 10 20 30 40 50 60 70 80 90 1

that the compositions of the rocks in the complexes are 

typically bimodal, with the exception of the mafic 

nepheline-clinopyroxene rocks. This bimodality is 

characteristic of rock sequences that form as a result 

of liquid immiscibility, although other evidence of this 

origin, such as the presence of ocellate structures, has 

not been recognized.

MINOR ELEMENTS

MAFIC-ULTRAMAFIC ROCKS

A summary of minor-element contents of maficultramafic rocks from the three alkaline complexes is 

presented in tables 6 and 7. Nine analyses from the 

Iron Mountain part of the McClure Mountain Complex 

(Shawe and Parker, 1967, table 3) are included in the 

summary.

Abundances of elements in Wet Mountains maficultramafic rocks, alkaline ultramafic rocks of the Kola 

Peninsula (Gerasimovsky, 1974, table 3), and "average" 

ultramafic rocks (Goles, 1967, table 11.1) are compared 

with the abundance of elements in igneous rocks of the 

upper continental crust (Wedepohl, 1971) in table 8. The 

alkaline rocks of the Kola Peninsula are more alkaline 

than are Wet Mountains pyroxenitic rocks, and the "average" ultramafic rock is derived chiefly from tholeiitic 

rocks.

Alkaline ultramafic rocks from the Kola Peninsula 

contain amounts of Ba, Be, Nb, Sc, Sr, V, Zr, Ga, U, 

Th, and other elements greater than the amounts in 

average igneous rocks in the Earth's crust; Cr, Co, Ni, 

Cu, Pb, and other elements occur in amounts less than, 

or nearly equal to, those in average crustal rocks. The 

"average" ultramafic rock (Goles, 1967), on the other 

hand, contains abundant Co, Cr, and Ni, and low conEXPLANATION

McCLURE MOUNTAIN COMPLEX 

D Nepheline syenite 

o Hornblende-biotite syenite 

0 Mafic nepheline-clinopyroxene rocks 

V Mafic-ultramafic rocks

GEM PARK COMPLEX 

x Nepheline syenite pegmatite

v Mafic-ultramafic rocks

COMPLEX AT DEMOCRAT CREEK 

 Quartz syenite 

V Mafic-ultramafic rocks

FIGURE 25. Plots of differentiation index (DI) versus oxide weight 

percent of rocks from the McClure Mountain Complex, Gem Park 

Complex, and complex at Democrat Creek. DI = S(Q + Or + Ab + Ne

+ LC).

DIFFERENTIATION INDEX
GEOCHEMISTRY 23

TABLE 6. Summary of minor-element analyses of samples from the McClure Mountain Complex,

south-central Colorado

[Values in parts per million; n, number of samples; Leaders ( ) indicate no data]

Hornblende-biotite Nepheline syenite, Nepheline-clinopyroxene Mafic-ultramafic 

syenite, n = 44 n = 11 rocks, n = 8 rocks, n = 20

Ba

Be

Co

Cr

Cu

La

Mo

Nb

Ni

Pb

Sc

Sr

V

Y

Zr

Ce

Ga

Yb

Nd

U

Th

Mean

2527

<1.5

<5

<1

4.3

78

<3

46

<3

<10

<5

1559

50

22

158

<100

20

2.2

<70

2.44

9.04

Range

200-7000

<1.5-2

<5-15

<1-50

<1-30

<50-150

10-150

<3-30

<10-20

<5-10

50-7000

<10-200

10-70

10-1000

<100-200

15-30

<1-10

<70-150

0.53-7.06

<2.3-23.4

Mean

2479

<1.5

8.5

15

11

108

<3

88

8

11

<5

2900

103

29

165

155

24

2.8

84

4.01

6.9

Range

70-5000

<1.5-2

<5-20

<1-150

1-50

50-200

<3-5

20-300

<3-50

<10-30

<5-15

200-7000

<7-300

10-70

70-300

<150-500

15-30

1.5-7

<70-150

0.9-22.4

4.0-11.7

Mean

1320

<1.5

27

167

32

142

<3

112

65

<10

13

1712

258

53

219

188

24

5

114

1.4

8.4

Range

300-3000

<1. 5-5.1

15-50

2-700

7-50

100-200

70-220

7-150

<10-10

7-30

700-3000

150-500

30-100

150-390

<100-380

20-30

3-9.6

<70-200

0.9-2.2

6.4-13.4

Mean

111

<1.5

42

285

77

<30

<3

<10

128

<10

43

844

256

12

18

<100

14

<1

<70

0.2

.7

Range

30-200

 

5-150

2-1000

3-200

<30-30

<10-10

<5-500

5-100

50-3000

30-700

<10-20

<10-70

7-20

<l-2

 

<0.2-0.5

<. 1-1.4

centrations of trace elements characteristic of syenitic 

and granitic rocks.

The mafic-ultramafic rocks from the complex at 

Democrat Creek (table 7) contain average amounts of 

Co, Cr, Cu, Ni, and Sc greater than those in average 

crustal rocks, and amounts of Ba, Sr, Y, Zr, Ga, RaelJ, 

and Th less than those in average crustal rocks. Thus, 

these rocks have minor-element signatures more similar 

to the "average" ultramafic rocks derived from tholeitic 

rocks (table 8).

Mafic-ultramafic rocks from the Gem Park Complex 

(table 7) contain amounts of Ba, Co, Cr, Cu, Ni, Sc, 

Sr, V, and Ga greater than those in average crustal 

rocks, and amounts of Y, Zr, RaelJ, and Th less than 

those in average crustal rocks. This suite of minor elements is partly characteristic of the "average" ultramafic rock (table 8) and partly characteristic of Kola 

Peninsula alkaline ultramafic rocks (table 8).

Mafic-ultramafic rocks from the McClure Mountain

Complex (table 6) contain amounts of Co, Cr, Cu, Ni, 

Sc, Sr, and V greater than those in average crustal 

rocks, and amounts of Ba, Y, Zr, RaelJ, and Th less 

than those in average crustal rocks. Except for barium, 

these minor-element signatures are similar to those of 

mafic-ultramafic rocks of the Gem Park Complex (table 

7).

These comparisons suggest that the mafic-ultramafic 

rocks from the complex at Democrat Creek have minorelement abundances more similar to tholeiitic rocks 

than to alkaline rocks. The mafic-ultramafic rocks from 

the McClure Mountain and Gem Park Complexes have 

minor-element abundances similar to each other, but 

the minor elements have characteristics of both alkaline 

and tholeiitic ultramafic rocks.

LEUCOCRATIC ROCKS

A summary of minor-element analyses of quartz syenites from the complex at Democrat Creek is given in
24 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

TABLE 7. Summary of minor-element analyses of samples from the complex at Democrat Creek 

and the Gem Park Complex, south-central Colorado

[Values in parts per million; n, number of samples; leaders ( ) indicate no data]

Complex at Democrat Creek Gem Park Complex

Quartz syenite, Mafic-ultramafic Mafic-ultramafic 

n = 21 rocks, n = 4 rocks, n = 7

Mean Range Mean Range Mean Range

Ba 320 50-2000 72

Be 15 5-50 <1

Co <5 50

Cr <1 <l-2 1675

Cu 8 <1-70 175

La 244 <30-500 <30

Mo <3 <3-5 <3

Nb 340 70-1000 <10

Ni <5 - 412

Pb 40 <10-300 <10

Sc <3 <3-7 45

Sr 30 10-100 135

V <5 <5-10 110

Y 95 30-150 10

Zr 785 200-2000 16

Ce 384 <100-700 <100

Ga 72 50-100 7

Yb 12 3-20 1

Nd 133 <70-200

U 11.8 3-22 0

Th 78.3 50-105 0

table 7. A similar summary for hornblende-biotite syenites and nepheline syenites from the McClure Mountain Complex is given in table 6. Abundance of elements 

in alkali syenite and miaskitic nepheline syenite 

(Lazarenkov, 1978) and average crustal abundance of 

certain elements (Wedepohl, 1971) are given in table 

8.

The quartz syenites of the complex at Democrat 

Creek contain average amounts of Be, La, Nb, Pb, Y, 

Zr, Ce, Ga, Yb, Nd, RaeU, and Th greater than crustal 

abundances, and amounts of Ba, Co, Cr, Cu, Ni, Sc, 

Sr, and V less than crustal abundances. Hornblendebiotite syenites from the McClure Mountain Complex 

contain average amounts of Ba, La, Nb, and Sr greater 

than crustal abundances and amounts of Co, Cr, Cu,

50-100 657 100-1500

.5 <1.5

30-70 73 50-100

700-3000 404 1-1000

150-200 143 20-500

<30 <30-30

<3-7 <3

<10 <10-10

150-700 152 5-200

<10

30-70 49 20-70

70-200 757 300-2000

70-150 586 <7-1000

<10-20 26 20-50

<10-30 98 50-150

<100

<5-10 28 15-50

.2 <l-2 <1

<70 <70-70

.2 0.4 0.2-0.62

.5 2.4 1.0-3.7

Ni, Sc, V, Y, Zr, RaeU, and Th less than crustal abundances. The nepheline syenites from the McClure 

Mountain Complex contain average amounts of Ba, La, 

Nb, Sr, Ce, Ga, and Nd greater than crustal abundances, and amounts of Cr, Cu, Ni, Sc, RaeU, and Th 

less than crustal abundances; some elements occur in 

amounts similar to crustal abundances.

Considerable differences exist in the average content 

of minor elements in the syenites from Democrat Creek 

and McClure Mountain. The quartz syenites from the 

complex at Democrat Creek contain greater average 

amounts of Be, La, Nb, Pb, Y, Zr, Ce, Ga, Yb, Nd, 

RaeU, and Th than do the syenites from the McClure 

Mountain Complex. According to Gerasimovsky (1974, 

p. 406), all these elements, excluding lead, are charac-
GEOCHEMISTRY 25

TABLE 8. Abundance of elements in several types of rocks

[Values in parts per million. Leaders ( -) indicate no data]

Analysis No.* 1

Ba

Be

Co

Cr

Cu

La

Mo

Nb

Ni

Pb

Sc

Sr

V

Y

Zr

Ce

Ga

Yb

Nd

U

Th

590

2

12

70

30

44

1

20

44

15

14

290

95

34

160

75

17

3.4

30

3.5

11

1567

14

11

28

146

70

6.9

25

-__

434

26

64

510

___

27

4.5

20

1388

9.3

16

23

134

89

10

35

___

835

52

62

411

___

30

4.8

13.2

850

7.4

15

30

34

___

300

36

13

24

1300

440

340

___

26

15

90

0.4

.2

110

2400

30

___

.2

1500

.05

16

20

40

5

30-45

___

5

...

.02

.06

*Analysis Nos. are:

1. Abundance of elements in igneous rocks of the upper 

continental crust (Wedepohl, 1971, table 7.3).

2. Alkali syenite (Lazarenkov, 1978, table 1).

3. Miaskitic nepheline syenite (Lazarenkov, 1978, table 1)

4. Alkaline ultramafic rocks, Kola Peninsula. 

(Gerasimovsky, 1974, table 3).

5. "Average" ultramafic rock (Goles, 1967, table 11.1).

teristic of agpaitic nepheline syenites, which by definition (Sorensen, 1974b, p. 23) have the ratio 

(Na20 + K20)/A1203 (molecular proportions) greater 

than one. This agpaitic index (see fig. 24) averages 

1.007 for the seven chemical analyses of quartz syenite 

in table 5, but the quartz syenites possess few additional characteristics of agpaitic nepheline syenites 

(Sorensen, 1974b, p. 24). The quartz syenites also contain less average amounts of Ba, Co, Cr, Ni, Sr, and 

V than do the nepheline syenites. These elements, except barium and strontium, chiefly reside in the mafic 

mineral assemblage; the paucity of mafic minerals in 

the quartz syenites reflects this characteristic. The 

paucity of barium and strontium is usually a function 

of crystallization and removal of feldspar from the magmatic system. This deficiency, especially of strontium, 

in the quartz syenites relative to barium and strontium 

in the associated mafic-ultramafic rocks is perplexing, 

and may suggest a low concentration of strontium and 

probably barium in the crystallizing magma.

The quartz syenites at Democrat Creek contain high 

average abundances of uranium and thorium. Eighteen 

samples contain 12.9 ppm RaeU (15 ppm U308) and 80.4 

ppm thorium (91 ppm Th02). For comparison, the Mesozoic Conway Granite of New Hampshire contains similar abundances of uranium (15 ppm U3 08), but somewhat less thorium (64 ppm Th02).

MAFIC NEPHELINE-CLINOPYROXENE ROCKS

The average composition of eight samples of mafic 

nepheline-clinopyroxene rocks is given in table 5. These 

rocks contain average amounts of Ba, Co, Cr, La, Nb, 

Sr, V, Y, Zr, Ce, Yb, and Nd greater than crustal 

abundance, and average amounts of lead less than crustal abundance (table 8). Be, Cu, Ni, Sc, and Ga occur 

in amounts similar to crustal abundance. These rocks 

possess minor-element signatures similar to those of 

alkaline ultramafic rocks of the Kola Peninsula (table 

8).

RUBIDIUM-STRONTIUM SYSTEMATICS AND

RARE-EARTH-ELEMENT 

DISTRIBUTION PATTERNS

Concentrations of rubidium, strontium, rare-earth 

elements, and strontium isotopes have been determined 

in several alkaline rocks by C. E. Hedge and reported 

in Armbrustmacher, Hedge, and Parker (1979) and in 

Armbrustmacher and Hedge (1982). These values are 

given in tables 9 and 10.

RARE-EARTH ELEMENTS

Chondrite-normalized rare-earth-element (REE) data 

for mafic rocks from the complex at Democrat Creek 

and the McClure Mountain Complex are plotted on figure 26, along with data from Kay and Gast (1973) for 

tholeiitic mid-ocean-ridge basalt, alkali basalt, and 

nephelinite. The gabbroic rock from Democrat Creek 

contains less total REE than do the several types of 

basalts, and it shows some increase in the light- to 

heavy-REE ratio relative to amounts of REE in the 

tholeiitic basalt. The mafic cumulate rocks from the 

McClure Mountain Complex contain total amounts of 

REE comparable to those in tholeiitic basalt, but the 

light REE are enriched and the heavy REE depleted 

relative to the tholeiitic basalt. Total REE content of 

cumulus rocks from the McClure Mountain Complex is 

greater than that in the Democrat Creek mafic rock; 

however, the McClure Mountain cumulates contain less
26 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

TABLE 9. Rubidium ami strontium content (in parts per million) and several ratios for rocks from the McClure Mountain Complex

and the complex at Democrat Creek, south-central Colorado

[Analyses by C. E. Hedge; leaders ( ) indicate no data]

Field No. Rock type Rb Sr Rb/Sr 87 Rb/ 86 Sr 87 Sr/ 86 Sr

Initial

87 Sr/86 Sr

McClure Mountain Complex

WM-64-827

WM-64-829

417

WM-62-116

4

421

WM-62-143

8

12

21

419

419X

496

498A

Plagioclase-clinopyroxene cumulate 3.17

 do 

Nepheline syenite

 do 

 do 

 do 

Hornblende-biotite syenite

 do 

 do 

 do 

Carbonatite

 do--

Mafic nephel ine-cl inopyroxene

 do 

1.69

126

134

161

194

94.6

86.9

98.4

92.0

0.3

0.6

rock 97.1

78.6

1010

370

1725

2939

2266

103

542

487

530

531

4997

11632

1582

1370

0.0031

0.0046

0.0730

0.0456

0.0711

1.883

0.1745

0.1784

0.1857

0.1733

0.00006

0.00008

0.0614

0.0574

0.0091

0.0132

0.2113

0.1318

0.1890

5.470

0.5048

0.5166

0.5371

0.5019

0.00020

0.00016

0.1775

0.1660

0.70484

0.70454

0.70519

0.70497

0.70515

0.74548

0.70856

0.70831

0.70819

0.70821

0.70378

0.70406

0.7050

0.7051

0.70477

0.70444

0.70359

0.70397

0.70372

---

0.70475

0.70441

0.70413

0.70442

0.70378

0.70406

0.7037

0.7038

Complex at Democrat Creek

299

302

359

412

413

414

363

369

382

Gabbro

 do 

Pyroxenite

Quartz syenite

 do 

 do 

 do 

 do 

 do 

5.25

1.32

3.42

96.9

251

174

297

233

172

285

208

162

474

34.1

26.1

11.0

10.4

41.9

0.0184

0.0063

0.0211

0.2044

7.361

6.667

27.00

22.40

4.105

0.0535

0.0183

0.0609

0.5923

21.61

19.54

82.47

67.84

12.00

0.70290

0.70309

0.70384

0.70746

0.86059

0.84414

1.3146

1.1893

0.79088

0.70252

0.70296

0.70340

0.70315

 

abundant REE than do the alkali basalt and the 

nephelinite.

Using the modal analyses of the mafic-ultramafic 

rocks and published crystal-to-liquid distribution coefficients for the component minerals, and assuming that 

the rocks formed by cumulus processes, one can calculate the REE patterns for the liquids from which these 

mafic rocks crystallized. These patterns are shown with 

the REE pattern for alkali basalt on figure 27. The 

close similarity of these REE patterns suggests that 

the liquids from which the gabbro at Democrat Creek 

and the cumulus mafic rocks at McClure Mountain Complex crystallized are similar in REE composition to a 

normal alkali basalt magma. Just as the REE content

of the basalts increases in the order tholeiite-alkali 

basalt-nephelinite (fig. 26), perhaps the liquid from 

which the McClure Mountain cumulates crystallized had 

a slightly higher alkali content than the liquid from 

which the gabbros at Democrat Creek crystallized; the 

result is the more tholeiitic composition of the Democrat 

Creek rocks.

Chondrite-normalized REE data for the various syenites (table 2) are plotted on the diagrams in figures 

28, 29, and 30. The REE patterns of the hornblendebiotite syenites (fig. 28) in the McClure Mountain Complex vary only slightly. This small variation correlates 

with their small variations in modal mineralogy, normative mineralogy, and major-element abundances. Their
GEOCHEMISTRY 27

TABLE 10. Rare-earth-element content (in parts per million) of rocks from the McClure Mountain Complex and the complex at Democrat

Creek, south-central Colorado

[Analyses by C. E. Hedge; leaders ( ) indicate no data]

Field No. Rock type Ce Nd

McClure Mountain

WM-64-827

WM-64-829

417

WM-62-116

421

WM-62-143

8

21

419

419X

299

412

413

Pi agi ocl ase-cl i nopyroxene

 do 

Nepheline syenite

 do 

 do 

Hornblende-biotite syenite

 do 

 do 

Carbonatite

 doGabbro

Quartz syenite

 do 

cumulate 25.0

12.6

108

159

104

119

__-

104

3308

648

Complex at

10.8

116

470

15

13

37

60

32

46

50

39

1346

220

Sin Eu Gd Dy Er Yb

Complex

.7

.0

.8

.5

.7

.2

.0

.0

Democrat

6

45

122

.64

.6

3.71

3.97

5.74

9.38

4.58

7.29

7.95

6.11

177

34.9

Creek

1.71

8.26

18.8

1.36

1.36

2.34

2.78

0.871

2.64

2.87

2.54

51.1

10.3

0.521

2.58

0.966

5.28

5.33

4.34

6.39

4.02

5.37

5.93

4.67

2.03

7.05

17.5

2.

3.

3.

4.

2.

3.

4.

3.

116

22.

1.

6.

19.

71

57

52

98

30

95

38

34

9

43

32

4

1.20

1.57

1.98

2.37

1.32

2.25

2.05

1.69

51.3

13.2

0.787

3.52

12.9

0.860

1.06

1.90

1.95

1.19

1.65

1.64

1.41

38.5

13.9

0.722

3.40

14.1

small positive europium anomalies are due to the presence of sodic plagioclase. The REE patterns of 

nepheline syenites (fig. 29) show small positive and 

negative europium anomalies, which also correlate respectively with more and less modal plagioclase. These 

patterns also correlate with the variability in modal 

mineralogy of these rocks. The REE patterns of the 

quartz syenites of the complex at Democrat Creek (fig. 

30) vary considerably. One pattern has a fairly large 

negative europium anomaly that corresponds to low 

CaO (<1.0 percent) and low strontium contents (34 

parts per million). These data suggest that this rock 

crystallized from a melt from which plagioclase had 

been removed.

In summary, the REE data show that the Democrat 

Creek mafic rocks are slightly, but significantly, different from the McClure Mountain Complex cumulates. 

The liquids from which the Democrat Creek mafic rocks 

crystallized appear to have been less alkaline than the 

liquids from which the McClure Mountain Complex 

cumulates crystallized. The REE patterns for the syenites reflect the homogeneity and apparent lack of evidence of fractionation of the hornblende-biotite syenite, 

the variation in plagioclase content because of some 

fractionation in the nepheline syenites, and the strong 

evidence of fractionation because of loss of plagioclase 

in the quartz syenites at Democrat Creek.

INITIAL 87Sr/86Sr RATIOS

Initial 87Sr/86Sr ratios for alkaline rocks of the Wet 

Mountains area range in value from 0.7025 to 0.7048 

(fig. 31). Uncertainty of these ratios is less than 

±0.0001. Mafic-ultramafic rocks from the complex at 

Democrat Creek have initial values that range from 

0.7025 to 0.7034, and one sample of quartz syenite has 

a value of 0.7032. Mafic-ultramafic rocks from the 

McClure Mountain Complex have values of 0.7044 and 

0.7048, nepheline syenites range from 0.7036 to 0.7040, 

hornblende-biotite syenites range from 0.7041 to 0.7048, 

mafic nepheline-clinopyroxene rocks have values of 

0.7037 and 0.7038, and six carbonatites, including four 

analyzed by Roden (1977), have values of 0.7036 to 

0.7041. The Democrat Creek rocks appear to form a 

distinct group characterized by rather low initial 87Sr/ 

86Sr values. At the McClure Mountain Complex, 

hornblende-biotite syenite has initial 87Sr/86Sr ratios 

similar to those of the mafic-ultramafic cumulate rocks 

but different from the nepheline syenites, the mafic 

nepheline-clinopyroxene rocks, and the carbonatites, 

which constitute a separate group having similar values. The carbonatites that intrude the Gem Park Complex have ratios (Roden, 1977) similar to carbonatites 

that intrude the McClure Mountain Complex.

A summary of strontium isotopic contents of alkaline
28 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

1000 

cc.

Q

z 

o 

x 

o

111 111 

(T

100

10

EXPLANATION

Tholeiitic midocean ridge basalt (Kay 

and Cast, 1973, fig. 1, sample 6}

Alkali basalt (Kay and Gast, 1973, 

table 2F, average of samples 04 and 

K12)

Nephelinite (Kay and Gast, 1973, table 

2B, average of samples K17, K23, 

O23, and O35)

Mafic rock, McClure Mountain Complex 

Mafic rock, complex at Democrat Creek

- 82

Ce Nd Sm Eu Gd Dy Er 

RARE-EARTH ELEMENTS

Yb

FIGURE 26. Chondrite-normalized rare-earth-element data for 

mafic rocks from McClure Mountain Complex, the complex at 

Democrat Creek, and other basalt samples (from Kay and Gast, 

1973).

100

cr Q

z 

O

x 

o

111 

111 

(T

10

EXPLANATION

v Mafic rock, McClure Mountain Complex 

v Mafic rock, complex at Democrat Creek 

o Alkali basalt (Kay and Gast, 1973, fig. 7)

Ce Nd Sm Eu Gd Dy Er Yb

RARE EARTH ELEMENTS

FIGURE 27. Chondrite-normalized rare-earth-element data for 

liquids from which mafic rocks of McClure Mountain Complex and 

complex at Democrat Creek crystallized. Data are calculated from 

published crystal/liquid distribution coefficients.

1000

100

10

J43

Ce Nd Sm Eu Gd Dy Er Yb

RARE-EARTH ELEMENTS

FIGURE 28. Chondrite-normalized rare-earth-element data for 

hornblende-biotite syenite samples from the McClure Mountain 

Complex.

rocks from Australia, Spain, and the Western United 

States (Powell and Bell, 1970) shows initial 87Sr/86Sr 

ratios ranging from 0.7034 to 0.7169; some of the values 

are similar to those of oceanic basaltic rocks. But ratios 

for rocks from the Western United States (from 0.703 

to 0.709) suggested to Powell and Bell that those rocks 

may have formed by partial melting of deep crustal 

rocks that have lower rubidium/strontium ratios than 

those of average crust. Bell and Powell (1970) showed 

a range of initial ^Sr/^Sr ratios between 0.7029 and 

0.7061 for carbonatites and alkaline rocks from eastern 

Uganda; the carbonatites average 0.7034 and the associated alkaline rocks average 0.7045. These differences indicated to them that the rocks are not related 

solely by magmatic differentiation from a single parent 

magma. Compared with these data, the Wet Mountains 

rocks have a considerably narrower range in initial 87Sr/ 

86Sr ratios, and the carbonatites have ratios comparable 

with those of nepheline syenite and mafic nephelineclinopyroxene rock.

In summary, samples from the complex at Democrat 

Creek have distinctly lower initial 87Sr/86Sr ratios than 

do those from the McClure Mountain Complex. Within 

the McClure Mountain Complex, the mafic-ultramafic 

cumulates and the hornblende-biotite syenites have initial 87Sr/86Sr ratios that are similar, and the nepheline 

syenites, the mafic nepheline-clinopyroxene rocks, and 

the carbonatites all have similar ratios. These data
GEOCHEMISTRY 29

1000

DC 

O

z 

o I 

o

LU 

LU 

DC

LU

_l 

D_

100 

10 

Nd Sm Eu Gd Dy Er Yb

RARE-EARTH ELEMENTS

FIGURE 29. Chondrite-normalized rare-earth-element data for 

nepheline syenite samples from the McClure Mountain Complex.

suggest that at least three separate magmatic sources, 

originating in the upper mantle or lower crust, are responsible for the Wet Mountains alkaline rocks.

RUBIDIUM/STRONTIUM RATIOS

Rubidium and strontium concentrations and Rb/Sr 

ratios (table 9) suggest additional petrologic aspects of 

these rocks. The rubidium content of the maficultramafic rocks from the McClure Mountain Complex 

and the complex at Democrat Creek is low. Values 

range from 1.3 to 5.3 ppm. These low values are consistent with a cumulus origin for the mafic-ultramafic 

rocks at Democrat Creek as well as with those from 

the McClure Mountain Complex, because rubidium 

tends to be depleted in plagioclase and pyroxene the 

major minerals in these rocks and is concentrated in 

the residual melt. The highly variable Rb/Sr ratios in 

quartz syenites in the complex at Democrat Creek 

range from 4.1 to 27. This wide range reflects the 

highly fractionated nature of these rocks. The consistent Rb/Sr ratios of the hornblende-biotite syenites in 

the McClure Mountain Complex range from 0.17 to 

0.18, and suggest that these rocks are not fractionated. 

The Rb/Sr ratios of nepheline syenite and mafic 

nepheline-clinopyroxene rocks also tend to be somewhat 

variable.

Figure 32 shows the distribution of rubidium and 

strontium values for rocks of the McClure Mountain

1000

100

10

^413

Ce Nd Sm Eu Gd Dy

RARE-EARTH ELEMENTS

Er Yb

FIGURE 30. Chondrite-normalized rare-earth-element data for quartz 

syenite samples from the complex at Democrat Creek.

1 1 1

4

4

V V V nA 1 1 1 v

1 1

> ^ o

X ^ O o

V

0.7020 0.7030 0.7040 

INITIAL 8?Sr/86Sr RATIO

EXPLANATION

0.7050

COMPLEX AT DEMOCRAT 

CREEK

 Quartz syenite

V Mafic-ultramafic rocks

GEM PARK COMPLEX

^ Carbonatites (from 

Roden, 1977)

McCLURE MOUNTAIN 

COMPLEX

X Carbonatite

D Nepheline syenite

0 Mafic nephelineclinopyroxene rocks

o Hornblende-biotite syenite 

V Mafic-ultramafic rocks

FIGURE 31. Distribution of initial 87Sr/86Sr ratios for alkaline rocks 

of the Wet Mountains area.

Complex. The plotting of the mafic rocks below the RbSr diagonal is a consequence of their cumulus origin 

the liquid remaining after the removal of pyroxenes and 

plagioclase would contain proportionately more rubidium than the resulting cumulus rocks. Figure 32 also 

shows that three of the nepheline syenites and the mafic 

nepheline-clinopyroxene rocks lie on or near the Rb-Sr 

diagonal, indicating that the Rb/Sr ratios of these rocks 

are similar to those of the material from which they 

were derived. The fourth nepheline syenite and a sample of red syenite lie on a line that is a trend of Rb-Sr
30 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

z 

g H 100

=. 10

EXPLANATION

D Nepheline syenite

o Hornblende-biotite syenite 

Mafic nepheline-clinopyroxene rocks 

Mafic-ultramafic rocks J 

Red syenite dike :

0

10 100 1000

STRONTIUM (IN PARTS PER MILLION)

FIGURE 32. Rubidium/strontium ratios of rocks from the McClure 

Mountain Complex. Patterned area represents approximate Rb/Sr 

ratios required to yield observed 87Sr/86Sr ratios.

values best developed by fractionation of nephelinebearing rocks. The hornblende-biotite syenites 

homogeneously possess higher Rb/Sr ratios than their 

calculated parent and do not lie on the Rb-Sr diagonal.

Figure 33 shows the Rb/Sr ratios of rocks from the 

complex at Democrat Creek. The fact that the mafic-ultramafic rocks again plot below the Rb-Sr diagonal 

suggests a cumulus origin for these rocks also. No rocks 

have Rb/Sr ratios that plot near the Rb/Sr diagonal, 

and thus a complicated process is required to increase 

the Rb/Sr ratios of the quartz syenites to more than 

those of the calculated parent magma rocks reflecting 

this process, if present, have not been identified in this 

study. The distribution of Rb/Sr ratios in the quartz 

syenites shows a more or less progressive decrease in 

strontium that suggests fractionation through removal 

of plagioclase. This fractionation as also suggested by 

the REE data (fig. 30).

PETROLOGIC CONCLUSIONS

The mafic-ultramafic cumulus rocks from the McClure 

Mountain and Gem Park Complexes typically contain 

normative nepheline and normative olivine but lack normative quartz and normative hypersthene. This assemblage is characteristic of alkali basalt types. The 

mafic-ultramafic rocks from the complex at Democrat 

Creek contain abundant normative hypersthene but 

lack normative nepheline and normative quartz. This

1000

100

EXPLANATION 

Quartz syenite

Mafic-ultramafic rocks

10 100

STRONTIUM (IN PARTS PER MILLION)

1000

FIGURE 33. Rubidium/strontium ratios of rocks from the complex 

at Democrat Creek. Solid line indicates approximate Rb/Sr ratios 

required to yield observed 87Sr/86Sr ratios. Dashed line indicates 

fractionation.

assemblage is characteristic of saturated tholeiitic 

basalt types. Minor-element contents of maficultramafic rocks of the complex at Democrat Creek are 

similar to those of alpine-type ultramafic rocks and ultramafic rocks of layered calc-alkaline intrusions. 

Minor-element contents of mafic-ultramafic cumulus 

rocks from the McClure Mountain and Gem Park Complexes are similar; they have characteristics of the more 

silica saturated alpine-type ultramafic rocks and ultramafic rocks of layered calc-alkaline intrusions, as 

well as of some of the strongly silica-undersaturated 

alkaline ultramafic rocks.

The hornblende-biotite syenites and the nepheline 

syenites from the McClure Mountain Complex contain 

normative nepheline and thus lack normative quartz. 

The quartz syenites from the complex at Democrat 

Creek contain abundant normative quartz but lack normative nepheline. The hornblende-biotite syenites contain normative Ab>Or>An, the nepheline syenites contain normative Or>Ab>An, and the quartz syenites 

contain only normative Ab>0r. The quartz syenites 

contain normative hypersthene, but the other syenites 

do not. The hornblende-biotite syenites and nepheline
PETROLOGIC CONCLUSIONS 31

COMPLEX AT 

DEMOCRAT CREEK

McCLURE MOUNTAIN AND 

GEM PARK COMPLEXES

Mafic-ultramafic rocks 

(0.7025-0.7034)

Quartz 

syenite 

(0.7032)

McClure Mountain 

and Gem Park: maficultramafic cumulates 

(0.7044-0.7048)

McClure Mountain: 

hornblende-biotite 

syenite (0.7041- 

0.7048)

McClure Mountain: 

nepheline syenite 

(0.7036-0.7040)

X

McClure Mountain: 

mafic nephelineclinopyroxene rocks 

(0.7037-0.7038)

McClure Mountain 

and Gem Park: 

carbonatite 

(0.7036-0.7041)

FIGURE 34. Grouping according to initial 87Sr/86Sr ratios (values in parentheses) of rock types of alkaline complexes in the Wet Mountains

area.

syenites are chemically undersaturated; this is indicated 

by the presence of normative olivine as well as normative nepheline. The quartz syenites lack normative olivine as well as normative nepheline. Minor-element contents are considerably different among the several 

types of syenites. The quartz syenites contain a suite 

of elements characteristic of agpaitic nepheline syenites, even though they possess none of the other traits 

of these rocks. The quartz syenites are also deficient 

in those elements typically concentrated in the maficmineral assemblage of a rock. The major- and minorelement contents of the mafic nepheline-clinopyroxene 

rocks are similar to those of alkaline mafic and ultramafic rocks.

The significant differences in age, mineralogy, 

chemistry, and initial 87Sr/86Sr ratios between the 

McClure Mountain Complex and the complex at Democrat Creek point out the fundamental genetic differences between the two complexes.

How much time might have been involved in the 

emplacement of the various rock types of the McClure 

Mountain Complex is not known, but the differences 

in initial 87Sr/86Sr ratios and minor element geochemistry require at least two source materials. The nepheline 

syenites and the carbonatites have similar 87Sr/86Sr 

ratios; therefore, they possibly were derived from the 

same source, and their profound differences in chemistry and mineralogy are possibly due to some process 

such as liquid immiscibility.

Initial 87Sr/86Sr ratios of the hornblende-biotite syenites and the mafic-ultramafic rocks are similar, but are 

distinct from those of the nepheline syenites. A genetic 

relationship between the hornblende-biotite syenites 

and the mafic-ultramafic rocks is possible, but such a 

hypothesis cannot be tested rigorously because the necessary intermediate rocks are not available for study. 

Mafic-ultramafic rocks are cumulates. Their extremely 

mafic mineralogy and their trace-element geochemistry 

indicate that they are cumulates of a more mafic magma

than are the hornblende-biotite syenites. The 

hornblende-biotite syenites possibly fractionated from 

this mafic magma. The rather high Rb/Sr ratios of the 

hornblende-biotite syenites are consistent with such a 

process, but the positive europium anomalies are not.

The hornblende-biotite syenites have lower strontium 

contents than do the nepheline syenites but higher initial 87Sr/86Sr ratios. Therefore, the hornblende-biotite 

syenites possibly were derived from the nepheline syenites with the addition of radiogenic 87Sr from the Precambrian wall rocks. This hypothesis could explain the 

data, however, only if significant fractional crystallization that lowered the strontium content occurred together with the assimilation that raised the 87Sr/86Sr 

ratio. Fractional crystallization that would greatly 

lower the strontium content would have to involve 

plagioclase; plagioclase removal would produce negative 

europium anomalies, whereas the hornblende-biotite 

syenites actually have positive europium anomalies.

The only mechanism that can be envisioned for the 

genesis of the hornblende-biotite syenites that would 

be compatible with their high Rb/Sr ratios and positive 

europium anomalies is that the syenites represent essentially primary magmas formed by a partial melting 

process that left a residue rich in pyroxene. The pyroxene would retain significant strontium but would not 

have a relative preference for europium.

Apparently a minimum of three distinct source materials and several geologic processes were necessary to 

produce the major alkaline rock types observed in the 

Wet Mountains area. This complicated situation is diagramed in figure 34. Only one source material is necessary to form the rocks of the complex at Democrat 

Creek, and the rock types there might be simply described as products of fractional crystallization. The 

hornblende-biotite syenites of the McClure Mountain 

Complex and the mafic-ultramafic rocks of the McClure 

Mountain and Gem Park Complexes ultimately may 

have been derived from the same source material, but
32 ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO

mafic-ultramafic rocks are cumulates from a basaltic 

magma that was probably not directly related to the 

hornblende-biotite syenite magma. Similarly, the 

nepheline syenites and the carbonatites might have 

been derived from the same source material.

The source materials for the alkaline rocks in the Wet 

Mountains area might have been rocks that differed in 

basaltic compositions. A gabbro of tholeiitic composition 

would be a suitable source material for the rocks of 

the complex at Democrat Creek. A more alkaline gabbro might be a suitable source for the hornblende-biotite syenites of the McClure Mountain Complex, 

whereas a gabbro of nepheline basalt composition may 

have been the source of the nepheline syenites. Although this narrative is admittedly speculative, a history of formation at least this complicated is necessary 

to account for rocks so diverse and so closely related 

spatially.

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U.S. GOVERNMENT PRINTING OFFICE: 1983 776-041/4022 REGION NO. 8