Alkaline Rock Complexes in the

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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 Ages of the complexes ..................... 14
Introduction ........................... 1 Geochemistry ........................... 15
Acknowledgments ........................ 1 Major elements ....................... 15
Geologic setting ......................... 2 Mafic-ultramafic rocks ................. 15
McClure Mountain Complex .................. 2 Leucocratic rocks ................... 16
Mafic-ultramafic rocks ................... 2 Mafic nepheline-clinopyroxene rocks ......... 19
Layered series ..................... 2 Additonal observations ................. 19
Discordant intrusions ................. 6 Minor elements ....................... 22
Leucocratic rocks ...................... 6 Mafic-ultramafic rocks ................. 22
Nepheline syenite ................... 7 Leucocratic rocks ................... 23
Hornblende-biotite syenite ............... 8 Mafic nepheline-clinopyroxene rocks ......... 25
Mafic nepheline-clinopyroxene rocks ............ 10 Rubidium-strontium systematics and rare-earth-
Gem Park Complex ....................... 11 element distribution patterns ............ 25
Mafic-ultramafic rocks ................... 12 Rare-earth elements .................. 25
Other rocks of the complex ................ 12 Initial ^Sr/^Sr ratios ................. 27
Complex at Democrat Creek .................. 12 Rubidium/strontium ratios ............... 29
Mafic-ultramafic rocks ................... 13 Petrologic conclusions ...................... 30
Quartz syenite ....................... 13 References cited ......................... 32
Carbonatites ........................... 14
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
(cid:9)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
(cid:9)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
(cid:9)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
(cid:9)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 Sharp, 1970). The complex at Democrat Creek contains
quartz syenite and breccia, and mafic-ultramafic rocks
Three alkaline intrusive complexes of Cambrian age occur in the
that were mapped by Brock and Singewald (1968) as
Wet Mountains area, Colorado. The McClure Mountain Complex con
sists of mafic-ultramafic cumulates, hornblende-biotite syenites, Precambrian gabbroic gneisses and metamorphosed
nepheline syenites, and mafic nepheline-clinopyroxene rocks intruded ultramafic rocks, but that were thought by Heinrich
by carbonatite, several kinds of syenite, and lamprophyre, mainly and Dahlem (1966) to be similar to the gabbros and
as dikes, and by thorium-bearing veins. The Gem Park Complex con
pyroxenites of the Gem Park and McClure Mountain
sists chiefly of mafic-ultramafic cumulates intruded by carbonatite,
Complexes. The rocks at Democrat Creek are less nota
lamprophyre, and nepheline syenite pegmatite. The complex at Demo
crat Creek contains subordinate amounts of mafic-ultramafic rocks ble for their similarities to the rocks of the other two
and abundant quartz syenite bordered partly by a zone of brecciation; complexes than for their differences.
the complex is intruded by syenite dikes and a quartz-barite-thorite The intrusive complexes and associated dikes are 520
vein.
m.y. old according to Olson and others (1977). How
Contents of major and minor elements, including rare-earth ele
ever, their dating of leucocratic rocks from the McClure
ments, rubidium, strontium, and strontium isotopic ratios, show that
these rocks did not form through fractionation of a single magma, Mountain Complex and the complex at Democrat Creek
but formed as end products of at least three separate magma groups. by fission-track, potassium-argon, and rubidium-stronti
The mafic-ultramafic rocks in the McClure Mountain and Gem Park um techniques could not resolve any differences in the
Complexes and the hornblende-biotite syenite in the McClure Moun
ages of the various complexes or in the ages of different
tain Complex appear to have been derived from an alkali basalt par
syenites at McClure Mountain. Subsequent rubidium
ent; the nepheline syenite, mafic nepheline-clinopyroxene rock, and
carbonatite from the McClure Mountain Complex and the carbonatite and strontium isotopic determinations by C. E. Hedge
from the Gem Park Complex appear to have had a more alkaline (Armbrustmacher and Hedge, 1982) yielded ages of
parent rock such as nephelinite; and the quartz syenite and mafic-ul 535 ±5 m.y. for syenites at McClure Mountain and
tramafic rock of the complex at Democrat Creek appear to have
511 ±8 m.y. for syenites at Democrat Creek.
evolved from a tholeiitic basalt parent.
The petrology of the Wet Mountains alkaline rocks,
as outlined by major- and minor-element contents,
INTRODUCTION
suggests the presence of several different rock series.
Alkaline rocks in the Wet Mountains area of south- Data on rubidium, strontium, rare-earth elements, and
central Colorado, located about 20 km southwest of strontium isotopes show that rocks of the complexes
Canon City, Colo., about 5 km south of the Arkansas did not form through fractionation of a single magma,
River, and northeast of the Sangre de Cristo Range but formed instead as end products of several magmas
and the Wet Mountain Valley, occur in three distinct generated from different source materials.
complexes: the McClure Mountain Complex, the Gem
Park Complex, and the complex at Democrat Creek. ACKNOWLEDGMENTS
Additional spatially and presumably genetically related
lamprophyre, carbonatite, and red syenite occur mainly Studies of the Wet Mountains alkaline rocks benefited
as dikes. Quartz-barite-thorite veins also appear to be from discussions with U.S. Geological Survey
related to the episode of alkaline magmatism. geologists, especially R. L. Parker, W. N. Sharp, M.
The McClure Mountain Complex contains nepheline R. Brock, and J. C. Olson. Determinations of rubidium
syenite, hornblende-biotite syenite, mafic nepheline- and strontium isotopes and rare-earth elements by C.
clinopyroxene rocks, and mafic-ultramafic cumulate E. Hedge of the U.S. Geological Survey and discussions
rocks (Shawe and Parker, 1967). The Gem Park Com with Hedge regarding their interpretation resulted in
plex contains mafic-ultramafic rocks nearly identical invaluable insight into the petrology of the alkaline com
to those in the McClure Mountain Complex, and a single plexes. I. K. Brownfield assisted in the field and in
exposure of nepheline syenite pegmatite (Parker and the laboratory.
ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO
GEOLOGIC SETTING including carbonatites, lamprophyres, and syenites.
Distance across its widest east-west dimension is about
The alkaline rocks of the Wet Mountains area intrude
11 km and across its north-south dimension is 10 km.
Proterozoic X metamorphic rocks chiefly layered
Exposures of the mafic-ultramafic rocks at Iron Moun
granitic gneisses, hornblende gneisses, and amphibo-
tain are about 5.8 km long in a northwest-southeast
lites and Precambrian intrusive granitic rocks of Boul
direction and 2-3.5 km wide in a northeast-southwest
der Creek or Proterozoic X age (1,720 m.y.) and Silver
direction.
Plume or Proterozoic Y age (1,450 m.y.) (Taylor and
Apparently, Parker and Hildebrand (1963) first men
others, 1975a, b). The western edge of the Gem Park
tioned the McClure Mountain Complex and its alkaline
Complex is bordered by Tertiary welded tuffs, boulder
nature. The mafic-ultramafic part of the complex at
gravels, and water-laid tuffs (Parker and Sharp, 1970).
Iron Mountain has been discussed by Shawe and Parker
According to Scott and Taylor (1975), these rocks are
(1967), who also adopted and defined the
correlative with the Oligocene East Gulch, Thorn
McClure Mountain Complex as a formal term. Although
Ranch, and Gribbles Park Tuffs of the Thirtynine Mile
the leucocratic part of the complex has been mentioned
volcanic field.
many times (Shawe and Parker, 1967; Parker and
The host rocks adjacent to the alkaline complexes, Sharp, 1970; Heinrich and Dahlem, 1966; Heinrich,
carbonatite dikes, and thorium deposits, and parts of 1966; Heinrich and Moore, 1970), detailed information
the complexes themselves, are typically fenitized (Hein- on the rocks is sparse. Rock relationships in the com
rich and Alexander, 1976). Quartzo-feldspathic host plex have been interpreted by Heinrich and Alexander
rocks commonly show loss of quartz; their feldspars are (1979) to represent a "mafic-alkalic ring complex."
replaced by potassic feldspar containing abundant fer The rocks of the McClure Mountain Complex do not
ric-oxide inclusions, their mafic minerals are destroyed
appear to have much economic potential. Small-scale
or replaced by blue and green sodic amphiboles and production of iron from titaniferous magnetite in the
pyroxenes, and their fractures are lined with these min mafic-ultramafic rocks at the Iron Mountain mine oc
erals or with epidote. Where mafic- ultramafic host
curred as early as 1873 (Becker and others, 1961); how
rocks are fenitized, the most conspicuous result is the
ever, the high titanium content of the magnetite, as
replacement of mafic minerals by vermiculite; in some
much as 14 percent Ti02, and the low tonnage have
places the concentrations are nearly high enough to be
hindered further development. Several carbonatite
commercially valuable.
dikes intruding rocks of the complex have been pros
The alkaline complexes, carbonatites, and thorium pected for thorium and rare-earth elements.
deposits appear to be bounded by the Use fault on the
east and the Texas Creek and Westcliffe faults on the MAFIC-ULTRAMAFIC ROCKS
west (fig. 1). Relative movement along the Texas Creek
Mafic and ultramafic rocks of the Iron Mountain part
fault (Taylor and others, 1975a) and along the Use fault
of the McClure Mountain Complex form a funnel-shaped
(Scott and others, 1976) is east-side upward; this move
layered series of rocks intruded by small, discordant
ment suggests that the alkaline intrusive rocks may oc
bodies of similar mafic and ultramafic rocks as well as
cupy a different structural level than do the blocks west
by dikes of carbonatite and syenite (Shawe and Parker,
of the Texas Creek fault and east of the Use fault. To
1967). Mineral content of rocks in the layered series
date, rocks related to the alkaline complexes have not
and in the discordant intrusions is similar. The rocks
been observed in the structural blocks east and west
differ mainly on the basis of texture; rocks in the
of the one containing the complexes. This structural
layered series show stratification, but rocks in the dis
block is in turn broken by a set of predominantly north
cordant intrusion do not. These two rock types are not
west-striking vertical faults that likely served as con
distinguished on figure 2, but are shown by Shawe and
duits for fluids that formed thorium veins, lam-
Parker (1967, pi. 1).
prophyres, red syenite dikes, and carbonatites, espe
cially southeast of the alkaline complexes. LAYERED SERIES
The stratified rocks consist of igneous cumulates com
McCLURE MOUNTAIN COMPLEX prising essentially five cumulus minerals plagioclase,
a calcium-rich clinopyroxene, olivine, magnetite, and
The largest of the three complexes, the McClure spinel that occur in varying proportions. All these
Mountain Complex (fig. 2), consists chiefly of a series minerals, except spinel, along with reddish-brown am-
of mafic-ultramific rocks at Iron Mountain, leucocratic phibole and red biotite are present as intercumulus ma
hornblende-biotite syenite and nepheline syenite, mafic terial; considerable variations in mineral proportions
nepheline-clinopyroxene rocks, and various dike rocks and in grain size can occur within a distance of a few
McCLURE MOUNTAIN COMPLEX
105°45' 30' 15' 105°00'
38'
15 MILES
J
10 15 20 KILOMETERS
EXPLANATION
Tertiary, Mcsozoic, and Precambrian metamorphic
Paleozoic sedimentary and igneous rocks
rocks
Tertiary volcanogenic rocks Major fault U, upthrown
side: D, downthrown side
Cambrian alkaline complexes
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 (fig. 3) show the diversity of rock types found. These
cumulates (Jackson, 1961). Plots of modal plagioclase, rock types include clinopyroxene adcumulates, plagio
clinopyroxene, and olivine for 20 mafic-ultramafic rocks clase adcumulates, and orthocumulates that consist of
ALKALINE ROCK COMPLEXES IN THE WET MOUNTAINS AREA, COLORADO
105°30' 105°25'
38°20',
COUNTY
38015i
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, oli- ments place the plagioclase in the labradorite range.
vine, magnetite, and rarely spinel in various propor Clouded plagioclase plagioclase that contains crystal
tions. lographically controlled, exsolved iron oxide occurs in
Most clinopyroxene appears in thin section as a tan- some rocks, especially in those in which the associated
to neutral-colored, unzoned, sometimes pleochroic augite also contains exsolved iron-oxide inclusions (fig.
cumulus mineral most likely titaniferous augite; rarely 7). This exsolution of crystallographically oriented iron
does it occur as an intercumulus mineral. In augite-rich oxide is due to subsolidus cooling at rates slow enough
rocks, adcumulus growth of the augite is sometimes ap to allow exsolution of structural iron (iron in lattice
parent owing to slight differences in optical orientation sites; Armbrustmacher and Banks, 1974). Plagioclase
of the augite primocryst and the later, interstitial au is not obviously zoned.
gite (fig. 4). Augite can also exhibit crystallographically In this study, olivine adcumulates have not been ob
controlled, exsolved, opaque iron oxides (fig. 5). served and intercumulus olivine is not abundant. The
Plagioclase primocrysts commonly exhibit adcumulus maximum modal olivine content observed thus far is
growth; triple-point junctions near 120° are abundant about 58 percent by volume. Some olivine is partly al
in plagioclase-rich rocks (fig. 6). Most optical measure tered 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
EXPLANATION
Breccia of complex at v McClure Mountain Complex
Democrat Creek v Gem Park Complex
Mafic-ultramafic rocks of FIGURE 3 Modal variation in plagioclase (P), clinopyroxene (C),
complex at Democrat
and olivine (0) of mafic-ultramafic rocks from the McClure Moun
Creek
tain and Gem Park Complexes. Values are in percent by volume.
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 4. Adcumulus growth of clinopyroxene on clinopyroxene.
Sample 359; partly crossed nicols; bar is 0.5 mm.
FIGURE 2. Continued.
tain mine. Green spinel is invariably associated with
and opaque iron oxides. Rocks containing fairly abun magnetite, and the spinel/magnetite ratio increases as
dant olivine that is partly altered show abundant, the total magnetite content of a rock increases. Most
closely spaced fractures that connect olivine grains (fig. spinel appears to be associated with cumulus magnetite,
8). This feature suggests that the alteration process in- and it is rarely associated with intercumulus magnetite.
tro iuced a volume change and that the resultant stress Strongly pleochroic reddish-brown amphibole, which
is relieved by fracturing. has a large optic angle, is most similar to kaersutite
Magnetite occurs both as cumulus and as inter- in optical properties. The amphibole, which may
cumulus minerals. Exploitation of layers of nearly 98 poikilitically enclose cumulus augite and magnetite, ap
percent cumulus magnetite occurred at the Iron Moun- pears to have formed from intercumulus fluids. Some