GEOLOGY OF THE SOUTHERN PART OF THE NEVER SUMMER MOUNTAINS, COLORADO
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GEOLOGY OF THE SOUTHERN PART OF THE NEVER SUMMER MOUNTAINS, COLORADO
by Tracy V* Buckwalter, Jr.
A dissertation submitted In partial fulfillment of the requirements for the degree of Doctor of Philosophy in the University of Michigan 1950
Committee in charge: Assistant Professor Professor Professor Assooiate
Professor E. William Helnrloh, Chairman Edwin N. Goddard Walter F. Hunt Lewis B. Kellum Professor Frederick 3, Turneaure
CONTENTS . . . . . . . .
Page ix
Introduction • • • • • • • • • • • • • • . . • • •
1
Abstract
1
Geographic setting........... Location and accessibility ............
1
Topography, drainage,
2
and vegetation • •
Geologic setting.
••••
Previous w o r k ........................
4 6
Field and laboratory work and acknowledgments
7
Pre-Cambrian rocks.................
9
General statement.............
9
Idaho Springs formation............ '........
11
Occurrence in Never Summer Mountains. . •
11
Distribution • • • • • • • • • • • •
11
Petrology. • • • • • •
............
11
Megascopic features • • • • • •
11
Microscopic features..........
15
Origin.................
22
Regional correlations in Colorado.
••• •
Introduction • • • • • • • • • • • • Problems of correlation
29 29
•
29
Previous regional correlations . . .
33
Type locality. • • • • • •
34
........
Front Range mineral belt locations •
36
Montesuma quadrangle. • • . • •
36
Breckenridge district • . • • •
37
Central City quadrangle . . . .
37
ii
CONTENTS (continued) Page Boulder County tungsten bait . . 38 Ward region...................... 38 Lake Albion area . . . . . . . .
39
Jamestown district • • • • • • •
39
Coal Creek area.................. 40 Front Range localities outside the mineral belt. • • • • . • • • • • • •
41
Vasquez Mountains. . . . . . . .
41
Never Summer Mountains . . • . .
42
Big Thompson Canyon, Rocky Moun tain National Park • • • • • • •
43
Bergen Park. • • • • • • • • • •
43
South Park • • . • • • • • • • •
44
Pikes Peak - Colorado Springs Cripple Creek area • • • • • • •
45
Bight Mile Park.................. 46 Wet Mountains.............
46
Localities outside the Front Range. •
47
Dillon area • • • • • • • • • • Tenmlle district
.......... 48
Mosquito Range • • • • • • • • • Sawatch Range.
47
48
............ 49
Gunnison Canyon. • • • • • • • •
51
Sangre de Crlsto Range ........
52
Conclusions . . • • • • • • • • • • • Hornblende Gneiss . . Distribution • • • • • 111
. . . . . . • • ........
• • • • •
53 55 55
CONTENTS (continued) Fetrology..............................
Page 55
Megascopic features................
55
Microscopic features . . • • • . • •
56
Origin..................................
60
Blotlte gneiss ..............................
67
Distribution............................
67
Petrology..............................
67
Megascopic features...............
67
Microscopic features ..............
70
Origin.......................
74
Blotlte-garnet amphlbollte ..................
81
Intrusive rocks. . . • • . • • •
83
..........
•
G r a n i t e ............. Distribution
85 ...........
83
Petrology.........................
83
Megascopic features ..........
83
Microscopic features..........
84
Pegmatites.......... • • • • • ..........
85
Distribution and structure........
85
Petrology..........................
86
Megascopic features ..........
86
Microscopic features..........
87
Origin • • • • • • • • • • • • • • •
87
Tertiary Igneous rocks............................ Introduction • • • • • • • Intrusive rocks............. Distribution and age........... iv.
89
•
89
. • .
89
. •
39
CONTENTS (continued) . . . . .
Page 91
Porphyry Peaks trachyte porphyry........
92
Quartz latite porphyry..................
93
Latlte p o r p h y r y ............
95
Bhyolite................................
96
Basalt. .
97
Rhyollte porphyry ............
............
98
Extrusive rocks................ Distribution and age....................
98
Rhyollte. .
98
................
• • • • • •
Basalt................................... Sedimentary rOcka . . .
........
* . . . . . . .
•
99 101
Pierre shale (Cretaceous).......................101 Distribution and stratigraphlc relations.
101
Petrology............
101
Correlation............................... 103 Thickness ..........
• • • • • • • . • •
105
Middle Park formation (Paleocene).............. 106 Distribution and stratigraphlc relations. Petrology
106
...........................108
Volcanic member. • • • • • • • • • •
109
Upper clastic b e d s ............
HO
Origin.....................................113 Miocene (?) conglomerate
..........
Location and general features . . . . . .
115 115
Correlation.............................. 117 Origin. .
. . . . .
118
CONTENTS (continued) Relation to RockyMountain peneplain. . • Quaternary deposits. . .
....................
Page 119 119
Structure................... • ................... 122 Pre-Cambrian structure. . . . .
.............
Regional............. ..
122 122
L o c a l ................................... 123 Laramide structure
................ 125
Regional................................. 125 Local
................................. 128
Geologic history. . . . . .. ......................
134
Economic geology............... ................. 141 Wolverine m i n e ....................... • • • • Bibliography.............
.141 146
ILLUSTRATIONS Plate 1.
Geologic map of the southern part of the Never Slimmer Mountain, Colorado In pocket
2.
Geologic structure sections of the southern part of the Never Summer Mountains, Colorado. In pocket
3.
Map of central Colorado showing areal extent of the Idaho Springs formation and location of writer's area . . . • • • • • • • • • • •
Figure
In pocket
Following page
1.
The Never Summer Mountains from Bowen Mountain. • . 1
2.
East side of the Never Summer Mountains from Trail Ridge Road .....................................
3.
Cirque and arete between Baker Mountain and Mount Nimbus.......................... .......... .
2
Cirque on east side of Cascade Mountain ..........
2
4.
5. .^Cirque on northwest side of Cascade Mountain and unglaclated west slope.............. • • • • • • •
2
6. .-Glaciated valley of Bowen Gulch viewed toward the east.............. .................. .
2
7.
Bowen Lake and glaciated valley of Bowen Gulch viewed toward the west. . . . . . ................
2
8.
Outcrop of Injected Idaho Springs schist. • • • . • 14
9.
Photomicrograph of knotted schist . .............. 18 ................ 18
10.
Photomicrograph of schist . . . .
11.
Photomicrograph of schist ........................ 19
12.
Photomicrograph of metasomatized schist .......... 19
13.
Photomicrograph of quartzite...................... 28
14.
Photomicrograph of migmatized schist.............. 28
15.
Photomicrograph of hornblende gneiss. • • ........ 57
16.
Photomicrograph of hornblende gneiss.
57
17.
Photomicrograph of hornblende gneiss. • • • • • • •
58
•
'vii.
ILLUSTRATIONS (continued) Figure
Following page
18,
Photomicrograph of hornblende gneiss . . . • • • • 5 8
19.
Photomicrograph of hornblende gneiss . . . • • • • 6 9
20•
Outcrop of mlgmatltlc biotite gneiss • • • • • • •
21.
Contact of biotite gneiss and hornblende gneiss. • 67
22.
Photomicrogx*aph of biotite paragnelss............ 70
23.
Photomicrograph of biotite paragnelss. • • • • • •
24.
Photomicrograph of mlgmatltlc biotite gneiss • • , 7 3
25.
Photomicrograph of, mlgmatltlc biotite gneiss • . • 73
26.
Photomicrograph of mlgmatltlc biotite gneiss • • • 73
27.
Photomicrograph of mlgmatltlc biotite gneiss • • . 7 3
28.
Network of pegmatite dikes on Bowen
Mountain • • • 86
29.
Network of pegmatite dikes on Baker
Mountain . • • 86
30.
Porphyry Peaks viewed from
Blue Ridge. • • •• • • 89
31.
West face of the northeast
PorphyryPeak • •• • • 89
67
70
32.
Weathered rhyollte porphyry dike on Bowen Mountain 91
33.
Columnar jointing in dike of rhyollte. • • • • • •
91
34.
Photomicrograph of quartz-latlte porphyry. • . . .
94
35.
Photomicrograph of quartz^latite porphyry......... 94
36.
Photomicrograph of rhyollte. • • • • • • • • • • • 9 7
37*
Photomicrograph of basalt. • • • • • • • • • • • • 9 7
38.
Tectonic map of north-central Colorado • • • • •
.125
59.
Fault zone In Baker Guloh* • • • • • • • • • • •
.131
40.
Plan of Wolverine mine, Bowen Mountain • • • . . .141
vlli
INTRODUCTION GEOGRAPHIC SETTING Location and Accessibility The Never Summer Mountains (figs. 1,2) comprise a high range which forms the southern continuation of the Medicine BoW Mountains of north-central Colorado and south-central Wyoming.
The area mapped extends for about eight miles
along the southern part of this range.
The Never Summer
Mountains are separated from the Colorado Front Range to the east by the headwaters of the Colorado River and are bordered on the west by Middle Park.
A narrow strip of
the Park bordering the mountains Is Included in the area mapped.
The range Is about 17 miles long and extends south-
southwest from Cameron Pass on the north,, where the Medicine Bows end, to the Porphyry Peaks on the south.
The Contin
ental Divide follows the crest line of the northern twothirds of the Never Summer Mountains, then veers to the west.
The divide forms a natural boundary between the two
counties in which the area lies, Jackson County to the north and Grand County to the south. The area is in T. 4 and 6 N., R* 76 a n d -77 W.
On
the east it borders Rocky Mountain National Park and its northeast corner Includes a little more than’two square miles of the park. The nearest settlements are the villages: of Grand Lake, two miles east; Granby, twelve miles south; and Rand, twelve miles northwest.
A few ruins are left of
two ghost towns in Bowen Gulch and Baker Gulch In the 1
.
Figure 1.
Southern part of the Never Summer Mountains from Bowen Mountain. Baker Mountain in right foreground; Mt. Nimbus just behind it
Figure 2.
East side of the Never Summer Mountains from the Trail Ridge Road, Rooky Mountain National Park. Red Mountain in foreground; Baker Moun tain at far left; Howard Mountain at for right.
writer’s area, and only some buildings remain of the aban doned mining camp of Teller City, two miles to the north. The interior of the area is rather inaccessible.
The
Trail Ridge Road, U. S. Highway 34, parallels the east edge, and two lumber roads connecting with It lead Into the southeast portion.
A dirt road diverging from Colorado
Highway 125, which connects Granby and Rand, enters the; west side.
The Fort Collins Water Supply Board has built
a ditch and narrow private road, which lead Into the upper part of Baker Gulch in the heart of the range.
The ditch
is a prominent feature of the landscape both from the main highway and from the air.
Several trails are maintained
bj the U. S. Forest Service. Topography, Drainage, and Vegetation The map area Includes much high and rugged country. Altitudes range from 8,650 feet in the Colorado River valley to 12,541 feet at the summit of Bowen Mountain. Parika Peak, Baker, Cascade and Bowen Mountains and several unnamed peaks rise above 12,000 feet.
The Never Summer
Mountains are strongly glaciated (figs. 3,4,5) as far south as Cascade Mountain.
Baker Gulch, Bowen Gulch (figs.
6,7) and the valley of Illinois Creek, all within the map area, are typical U-shaped valleys.
The divides between
the glacial valleys are very sharp in a few places.1 Gorton (1941) noted these knife-edged aretes, especially at the Nckhu Crags in the northern part of the Never Summers. Related glacial forms in this vicinity were studied by
Figure S. Cirque and arete between Baker Mountain and Mt* Minibus. Typloal of the creatllne of the Never Simmer a.
Figure 5.
Figure 4« Large cirque on east aide of Cascade Moun tain*
Cirque on northwest aide of Cascade Mountain and unglaclated gentler west slop*.
Figure 6*
Glaciated Valley of Bbwen Gulch viewed toward the east from Cascade Mountain
Figure 7.
Bowen Lake and the glaciated valley of Bowen Guloh viewed toward the west from Cascade Moun tain. The moralnal dean of this cirque lake Is visible In the lower right.
3. Ives (1946)•
South of Cascade Mountain the range Is un
glaciated, and the topography is therefore much smoother. The crest line from Cascade Mountain to the Porphyry Peaks, a distance of about three miles, is generally unbroken at an altitude of about 11,000 feet and is known as Blue Ridge. Marvine (1874) of the Hayden Survey aptly described this change in topography:
"At the north this crest (Never
Summers) is a sharp and ragged ridge, but southward it becomes comparatively even and rounded in outline, a mas sive ridge, falling gradually until covered with lake beds near the junction of the East Fork(Arapaho Creek) with the Main Grand (Colorado)". The tributary glaciers of Bowen Gulch, Baker Gulch, and others flowed into the main glacier of the Colorado River valley, which deposited high lateral moraines along the valley sides and prominent terminal moraines near Grand Lake.
Some of these rise about 300 feet above the valley
floor and one has dammed Grand Lake• considerable detail by Marvine (1874).
They were mapped in The valley flat’
of the Colorado River ranges from one to two miles in width north of Grand Lake. The lakes include four which fill rock basins in cirques (fig. 7), two in the Colorado valley that are of morainal origin, and one in Willow Creek valley which is dammed by a rock glncier descending from Porphyry Peaks. Beaver ponds are common in most of the creek valleys. The entire east slope of the Never Summer Mountains
is drained to the Pacific by the upper Colorado Elver and its tributaries.
The west slope as far south as Cascade
Mountain, where the Continental Divide turns west, is drained to the Atlantic by the North Platte and its trib utaries, which include Illinois Creek.
The west slope
south of Cascade Mountain Is drained by Willow Creek, which empties Into the Colorado River near Granby. Almost all of the country below timberline Is covered with a heavy growth of pine and 3pruce except for park-like meadows along some of the streams.
Timberline varies
somewhat in altitude but in most places lies at about 11,500 feet.
Dense forest has obscured outcrops In many
places. GEOLOGIC SETTING Structurally and stratigraphically, the Never Summer Mountains are part of the Front Range.
Ihe distinction
between the two ranges Is solely topographic, for the intervening Colorado River valley Is, as far as can be determined, an eroslonal and not a structural feature. The Never Summers, like the Front Range, are composed largely of pre-Cambrian rocks, and one of the main pur poses of this study was to determine the origin of these rocks and their structural and stratigraphic relationships. The western part of the area lies In Middle Park which Is a broad, rather Irregular, structural and topographic basin bounded on the east by the Front Range and Never Summer Mountains, and on the west by the Park flange {pi. 3).
5. Late Cretaceous marine beds, Eocene continental deposits and volcanics, and late Tertiary lake beds occur in the park.
Although the basin is structurally continuous to
the north with North Park, it is separated from it topo graphically by,a high ridge, locally known as the Rabbit Ears Mountains, whioh consists largely of late Tertiary volcanics and early Tertiary continental deposits.
This
ridge, which throughout its length forms the Continental Divide, joins the Never Summers at the north end of the area.
The Vasquez Mountains and Williams River Mountains,
both high, northwest-trending spurs of the Front Range, jut into the southern part of Middle Park, dividing it into three long strips which follow southward the courses of the Blue, Williams, and Fraser Rivers.
Most of these
regional features are well shown on the geologic map of Colorado (1935) and also on the tectonic map of the United States (1944). The northeast border of the North Park-Middle Park basin is known from previous work (Gorton, 1941) to have been formed by a great overthrust of pre—Cambrian rocks of the Never Summers onto marine Cretaceous'sediments.of North Park (fig. 38).
A second principal purpose of this work
was to determine whether this thrust continued southward along the edge of Middle Park or whether the sediments, lapped normally onto the pre—Cambrian rocks of the moun tains .
6. PREVIOUS WORK No detailed mapping had been done In this area prior to 1946.
Spock in 1928 made a brief reconnaissance cover
ing most of the Never Summer range and Immediate vicinity, including the map area.
He did not separate the pre-Cam
brian formations, but described the petrography of the Tertiary igneous rocks in considerable detail.
Brief men
tion is made of erosion surfaces In the area by Van Tuyl and Loverlng (1935) In their paper on the physiographic history of the Front Range. A moderate amount of geological work has been done In nearby regions.
Gorton (1941) mapped the Cameron Pass
area, which Includes the southern part of the Medicine Bow Mountains and the northern part of the Never Summers.
Dur
ing this work he made a reconnaissance to the south, which was joined to the present work.
Wahlstrom (1941, 1944)
studied the structure, petrology and hydrothermal deposits of Specimen Mountain, a Tertiary volcano In the northwest part of Rocky Mountain National Park.
Loverlng (1930)
made a reconnaissance map of an area near Granby In Middle Park during his study of the Granby anticline. naissance joins at the north with the map area.
This recon Beekly
(1915) mapped most of North Park during his study of its coal resources.
Grout, Worcester, and Henderson (1913)
mapped the Rabbit Ears region between North and South Parks.
This area lies about 15 miles west of the area
mapped by the writer.
Tweto (1947) made a comprehensive
study of the Vasquez Mountains in southeastern Middle Park
7. in which, he discussed the Laramide.geology and the petrol ogy and genesis of the pre-Cambrian rocks, FIELD AND LABORATORY WORK AND ACKNOWLEDGMENTS The surface and underground geology were mapped in the summer of 1946 and three weeks of the summer of 1947, All the mapping was done by foot traverses, and locations were made by compass triangulations.
The Rocky Mountain
National Park topographic map and the U, S, Forest Service map of the Kremmling Quadrangle were used as base maps and were enlarged to a scale of two inches to the mile.
Aerial
photos made by the Forest Service were used Intermittently In the field. Eighty-nine thin sections were studied and a number of Rosiwal analyses were made.
The refractive indices of
several specimens of biotite were determined for use In cal culating oxide analyses of schists.
Several minerals, par
ticularly from pegmatites, were determined by crushed frag ments methods. Dr. T. S. Loverlng suggested the area to be studied, and the writer Is further Indebted to him for aid and advice In the field and laboratory.
Thanks are due to
Professor E. Wm. Heinrich who guided the writer during the preparation of the report.
Professors W. F. Hunt, A. J.
Eardley, F. S. Turneaure, Lewis B. Kellum, and E^N. Goddard read part or all of the manuscript and made many helpful suggestions and criticisms.
Dr. Kellum checked the Identi— «
fication-. of the fossils from the Pierre.
Dr. Norman Snively
8. kindly gave the writer his rock specimens from his area at Bergen Park.
His doctoral dissertation and that of
Dr. Ogden Tweto, whom the writer assisted in the Fraser area, served as inspiring guides in the present study. Thanks are also due to Mssrs. Gerald Cooley and Eugene A. Chavez who acted as field assistants in the 1946 season and to Mr. Harold Meadow who assisted in the 1947 season. The writer is indebted to his wife for assistance in pre paring the manuscript. The unrestricted use of the facilities of the Depart ment of Mineralogy is gratefully acknowledged. The National Research Council rendered financial assistance through a Predoctoral Fellowship during the time the work was in progress. And finally, the writer wishes to acknowledge the assistance rendered by residents in and near the area, especially Mr. A. B. Apperson of Cascade Ranch and Mr. and Mrs. Carl Plock, whose hospitality was of material aid in furthering the field work.
PRE-CAMBRIAN ROCKS GENERAL STATEMENT The pre-Cambrian rocks of the Front Range consist principally of a thick series of schists and gneisses intruded by a succession of granites and closely related rocks probably belonging to one great period of batholithic invasion.
A generalized grouping of these and other
pre-Cambrian rocks of Colorado was made by Van Hise and Leith (1909, p. 824), and a detailed sequence was worked out by Ball (1908) in the Georgetown quadrangle.
This was
refined by Lovering, mainly on the basis of his work in the Montezuma quadrangle, and his sequence, which follows, has received general recognition over much of the Front Range: Silver Plume group of granites (Includes Cripple Creek, Coal Creek, and Mt. Olympus granites) Pikes Peak group of granites (Includes Rosalie granite, granite gneiss quartz diorite and hornblendite, and Boulder Creek quartz monzonite) Quartz monzonite gneiss Hornblende gneiss Idaho Springs formation Lovering (1929, p. 64) believes that the Idaho Springs formation is of sedimentary origin, and that the horn blende gneiss was originally intruded into the Idaho Springs sediments as a great series of basic sills.
Portions of
the gneiss may represent intercalated surface flows.
He
believes that the major batholithic cycle commenced with 9.
10. the intrusion of the quartz monzonite gneiss, in part as lit-par-lit injections into the schists, was culminated by the invasion of the Pikes Peak and related granites, and concluded by the intrusion of the Silver Plume group of granites. Most workers agree on the sedimentary origin of the Idaho Springs formation, but opinion is divided, as noted later, on the origin of hornblende gneiss and quartz monzon ite gneiss.
Several workers consider one or both to be
partly or wholly sedimentary and approximately the same age as the Idaho Springs. In the southern Never Summer Mountains the Idaho Springs formation, hornblende gneiss, and "quartz monzonite gneiss" (biotite gneiss) are all well represented, but as in the Vasquez Mountains (Tweto, 1947, p. 14) very little granite is exposed.
Presumably the small stocks that are present
correlate with the Silver Plume group. The evidence in this region suggests that the Idaho Springs formation, horn blende gneiss, and much of the "quartz monzonite gneiss" are all of sedimentary origin, and that parts of the latter are of migmatitic and also of truly igneous origin.
IDAHO SPRINGS FORMATION OCCURRENCE IN NEVER SUMMER MOUNTAINS Distribution The outcrop area of the Idaho Springs formation exceeds that of any other formation in the Never Summer Mountains. The largest exposure is in the central part of the area mapped in the vicinity of Cascade and Bowen Mountains. Smaller exposures occur in the north end near Parika Peak and the south end in sections 28 and 33, T. 4 N., R. 76 W. The details of its occurrence and structure are shown on the geologic map and section (pi. 1). Petrology Megascopic features: The Idaho Springs formation consists almost entirely of various types of schist whose distinguishing schistosity is due to many flakes of oriented biotite.
Some of the schists are so strongly injected in
a lit-par-lit manner by granitic pegmatite and aplite that they somewhat resemble granite gneiss, but wherever the original schistose structure of the rocks is still apparent they are included in the formation. The principal types are quartz-biotite schist, quartzbiotite-sillimanite schist, and injection gneiss.
Muscovite,
garnet, and especially feldspar are locally nearly as abun dant as quartz and biotite, and form distinct lithologic facies of the principal types. The typical schists are dark gray to black, mediumto coarse-grained, strongly biotitic, and well-foliated. 11.
12. Most of the outcrops are rather sharp and jagged and many of them weather brown or reddish-brown because of the for mation of Iron oxides on the biotite.
Some show a rather
weak banding in which dark layers of biotite, ranging in thickness from a knife edge to about 2 millimeters, alter nate with light-colored layers of quartz and feldspar of similar thickness.
In many of the schists quartz and
feldspar occur in ill-defined small lenses and stringers \
encased in biotite instead of occurring in bands.
In some
of the feldspathio schists, biotite and the quartz and feldspar present a "salt and pepper" appearanoe on a fresh fracture. Smoky quartz was observed in a small lens in one schist.
Sinimanlte occurs In very thin, gray to white
layers parallel to the schistosity.
It Is widespread in
minor amounts but in larger amounts it is confined to quartzbiotite-sillimanite schist and the knotted schist mentioned below.
These beds are distinct llthologic units Inter
layered with other varieties of schist.
In places along
the strike, slllimanite diminishes in amount or disappears and the rock grades into an ordinary qusrtz-biotite schist. Most schists containing abundant slllimanite carry some garnet, and the converse Is also true.
Garnet is more abun
dant in the injection gneiss and biotite gneiss than in the typical schists, in which it occurs rather sparingly In this area.
Some of the garnet has crystallized as Imperfect
dodecahedrons as much as half an inch in diameter;
Black
tourmaline occurs in schists on Blue Ridge as small clusters
and radiating suns about an inch across.
Muscovite locally
occurs In flakes as much as 7 millimeters in length.
Very
small grains of magnetite are scattered sparingly through out some of the schists. A variety of schist found only on the south slopes of peak 11,501, east of Bowen Mountain, Is the rock called "ellipsoidal masses” by Ball (1908, p. 41), "pebble-bearing gneiss" by Spnrr (1908, p. 177), and "knotted schist" by Loverlng (1935, p. 8).
The chief characteristic of this
sohist Is the presence of small ellipsoidal nodules of quartz and slllimanite scattered throughout the rock which give it a nearly conglomeratic appearance.
The dimensions
of these nodules may be as much as 2js Inches by 3/4 inch by l/4 inch.
Some are encased In thin layers of muscovite
and biotite.
They are more resistant than the matrix of
the schist and project either as knobs on the outcrop or weather out as pebbles onto the hill slope. Impure quartzite crops out In a single locality in the southeast part of section 24, T. 5 N., R. 77 W.
It is a
very dense, light brown, fine-grained rock with a weak foliated structure Imparted by chlorltized biotite and elon gated quartz grains.
A little biotite, feldspar, garnet,
and muscovite are the chief accessories.
Chloritization
of biotite and sericitlzatlon of feldspar have been extreme In this rock.
These processes probably accompanied unusually
pronounced retrograde metamorphism (see page 28) along the major thrust fault near which* the outcrop Is located. A variety of impure quartzite, resembling somewhat the
14. one described, but showing a clearer foliate structure, occurs only sparingly In the float on the south side of Bowen Gulch in section 9, T. 4 N., R. 76 W.
Muscovite is
the chief accessory In this rock and forms the foliate structure.
Although lying in the plane of foliation it
shows no preferred orientation intthls plane.
Minor acces
sories Include sodic oligoclase, slllimanite, and a few crystals of magnetite as much as 3 centimeters long. Marble and lime-silicate rocks, which are fairly common elsewhere in the Idaho Springs formation, do not occur in this area except for a small oriented lens of lime-silicate gneiss in hornblende gneiss.
The near-
absence of these facies of the formation la somewhat tinusual since they are associated in numerous places with the schist in areas closer to the Idaho Springs type local ity.
The geographical occurrence'of these members is dis
cussed in a later section.Injection gneiss, formed by lit-par-lit injection of the schists by seams of granitic pegmatite and aplite,* is very abundant.(fig• 8).
It underlies most of Cascade Moun
tain and the adjacent high ridges and part of peak 11,501 near Bowen Mountain.
As Loverlng (1935, p. 8) notes, it
is almost impossible to find any mass of schist 25 feet thick that is not seamed by one or more of the granitic rocks.
All gradations exist from uninjected schist through
Injection gneiss to biotite gneiss.
The Injecting material
is white to pinkish and fine- to coarse-grained.
The
seams range in thickness from a knife edge to about 1
Figure 8.
Outcrop or Idaho Springs quarts»«»lotIt* achi at strongly Injsotad by peguetlte la Boven Gulch.
centimeter.
Some velnlets pinch and swell.
Where injec
tion has been intense the volume of rock has evidently been greatly increased.
Had the volume remained the same,
the innumerable veinlets must have metasomatically replaced the schist.
The many knife-edge contacts of veinlets and
schist and the scarcity of unroplaced relicts of schist in the veinlet3 argue against more than minor replacement. Also, if replacement had commonly occurred, iron and mag nesia from biotlte of the schist must in some way be accoun ted for, but the veinlets rarely contain ferromagnesian minerals or exhibit minute selvage zones. Hornblende gneiss is considered part of the Idaho Springs formation, but as the evidence for sedimentary origin is much less certain than for the Idaho Springs, and as many other nearly identical hornblende gneisses in central Colorado are igneous or of uncertain origin, it Is discussed separately In a later section. Mloroscoplo features: The sohistose texture is formed principally by the parallel orientation of biotlte flakes.
In
some schists muscovite and silllmanlte are filso abundant enough to aid in forming the schistose structure.
Soma re-
crystallized quartz and a little,feldspar also are elongated parallel to the schiatoelty.
Tourmaline needles have grown
in the plane of sohistoslty but show no preferred orientation. Quartz and biotito are everywhere major minerals and feldspar occurs in Important amounts In many,places. .Mus covite, sillimanite, and garnet are essential minerals in some varieties but occur mainly as accessory constituents. Microscopic accessory minerals are zircon, apatite, and allanite; microecopic alteration products are epidote.
16. chlorite, sericite, and kaolinlte.
The modes of three
typical schists determined by the Rosiwal method are shown below. Modes of schist Quartz-biotitemuscovite schist #46 Quartz 31.5 Biotlte 30.2 Muscovite 13.3 Oligoclase 22.7 Sillimanite .4 Accessories 1.9
Feldspathic quartz-biotite schist #44 43.1 22.6 .9 32.7 .6
Quartz-biotite aIllimanite schist #43 46.5 24.7 12.6 15.9 .4
Accessory minerals in order of abundanoe are magnetite chlorite, zircon, and apatite. #46
Quartz-biotite-muscovite schist from south side of peak 11,501 in sec. 34, T. 5 N., R. 76
#44
Feldspathic quartz-biotite schist. 500 feetwest peak 11,501 in sec. 27, T. 5 N., R. 76
#43
W. of
W.
Quartz-biotite-sillimaniteschist from outcrop along Bowen Quloh trail in northwest quarter of sec. 4, T. 4 N., R. 76 W. Quartz is the most abundant mineral in most schists. '
• ■-
r
Much of it occurs with feldspars in the light-colored thin bands or in discontinuous stringers separated by bands of biotlte, but it is also plentifully scattered throughout the schist.
Quartz has been strongly recrystallized ,in
places as indicated by its pronounced elongation parallel with the schistosity and by its sutured, irregular con tacts.
Some of the coarser_grains of quartz, which have
been introduced into the schist in the litrpar-lit vein-
17
.
lets, are also somewhat elongated and show a pronounced wavy extinction, but the vein quartz exhibits this to the strongest degree.
Some grains have been granulated at
their borders, and the granulated material has recrystal lized.
A myrmekitic intergrowth consisting of rows of
blebs of sodic oligoclase in quartz was observed.
In a
few places, lines of fluid inclusions run through adjacent quartz grains.
These lines were noted by Ball (1908, p.
39) who suggests that they are probably secondary and dis tributed along minute cracks. Quartz embays and replaces biotite in many places. Some of this replacing quartz has clearly been introduced as it occurs in lit-par-lit veinlets, but some is from the original sediments and has recrystallized and replaced biotite.
In one locality a veinlet of late quartz has re
placed oligoclase. Biotlte is almost everywhere well oriented in the plane of schistosity.
Most of it occurs in rude bands
separated by light-colored bands composed chiefly of quartz ' and feldspar and in some places muscovite, but it also occurs sparingly in the lighter bands.
In its typical occurrence
it is somewhat embayed and replaced by quartz and the ifeldspars.
As the schists grade Into biotite gneiss^ the re-
placement by quartz and feldspars becomes stronger, until much of the biotlte occurs only as deeply embayed scraps and shreds.
(fig. 21). In many places fine-grained chlor
ite, sericite, and magnetite replace biotite along Its" ... ,?
cleavage; coarser aggregates of these minerals replace
\
-t
18« large parts of biotlte flakes.
Some sericite and chlorite
developed from biotlte along narrow shear planes.
Some secon
*
dary magnetite has a fine ddsty appearance.
Sillimanite al
so has developed at the expense of biotite.
Biotite appears
to be the earliest major mineral formed in the schist as all other major minerals replace it to at least some extent. Almost all the blotites are pleochroic from light yellowish-brown to very deep brown.
Biotlte from one
locality showed light yellow green to dark yellowgreen ' /V
pleochrolsm.
* •
-1
The indices range from approximately 1*635
to 1.645 and 2V ranges from about 5 to 15 degrees. Sillimanite is locally one of the major minerals and is widespread in minor amounts.
Where abundant, it tends to
occur in swarms of thick, felt-like clusters composed of thin rods, needles, and hair-like crystals (fig. 9).
These
clusters are associated with all the other minerals of the schist.
Where less abundant, it occurs principally as slen
der needles in a matrix of quarts or, in a few places, in plagloclase.
Except for the micas it is the mineral,largely
responsible for the sohiatose structure (flg.10).
It is
fairly well lineated in the plane Of schistoslty, although a number of clusters are not.
The replacement relationships
of sillimanite and muscovite are not clear.
Where silliman
ite is associated with muscovite, sillimanite is usually larger and coarser (fig. 10).
Snively (1948, p. 10) states
that In the Bergen Park area sillimanite becomes coarser In texture In the presence of igusoovite and finally disappears. Ho definite evidence Indicating this replacement of silli manite by muscovite was noted In this area.
This common
Figure 9*
Figure 10.
Edge of a knot composed of allllmanlte and quartz In knotted schist* Sillimanite replaces biotlte in the center. The thin elear needle-1 Ike crys tals Ore silllmanlce; the darker bladed crystals are biotlte; the greundmass is quarts* X 104*
Strongly oriented biotlte (b), muscovite (m), and sillimanite (a) in schist. Hote that sillimanite is coarser where associated with muscovite. X 40.
coarsening of the sillimanite crystals does In fact suggest replacement of muscovite by sillimanite, although It is not conclusive.
Loverlng (1935, p. 8 ) noted that In the Monte
zuma quadrangle sillimanite commonly replaced muscovite. Some sillimanite has clearly formed from the destruction of biotlte (fig. 9).
Euhedral needles of sillimanite out across
or occur within all the other minerals of the sohlst exoept serlclte and chlorite, suggesting that it was the latest mineral formed during the main period of metamorphlsm.
This
evidence of late formation is in accord with Harker*s view (1932, p. 227) of the sillimanite stage as the latest most in tense stage of regional metamorphism of argillaceous sediments Muscovite is locally very abundant.
A few medium-
sized flakes occur in the rouks from most localities, al though at some places it is absent.
Some is well oriented,
but most shows either a weak orientation or none at all. Muscovite exhibits varied modes of occurrence.
Where abun
dant, it tends to oollect in bands oriented parallel to the biotlte and quartz-feldspar bands.
In some places biotite
is also found in these muscovite bands.
It also occurs > •i
abundantly in the light-colored quartz-feldspar bands and in places in narrow’ ,- very fine-grained injection seams with quartz.
It replaces, biotite particularly along Its cleav
age (fig. 11) and also forms’rims along Its edges.
In
some large muscovite flakes, only faint yellow-brown streaks remain of the replaced biotite.
Some large flakes
poikllltically enclose quartz and oligoclase, suggesting * the later formation of muscovite. clearly replaced oligoclase.
In other places it has
The evidence of replacement
Figure ll.
Muscovite replacing biotlte (b) in a quartsblotite-muaoovite sohlst. Some quarts grains (q) are elongated parallel to the sohlstoslty. A few slender needles of sillimanite (») are aligned in muscovite (»). X40*
Figure 12.
Tourmaline (t) and unorlented muscovite.(m) in metasomatized quarts-blotite-muscovite schist, q, quartz. X 39.
20
.
of sillimanite, as discussed earlier, is not clear.
Tweto
(1947, p. 22) believed that in the Fraser area muscovite was formed after the main period of metamorphism when the stresses that controlled development of the schist had relaxed, but when the rocks were still heated and were permeated by solu tions residual from the main period of metamorphism and in jection.
Muscovite was thus a metamorphic equivalent of a
deuterlc or hydrothermal mineral.
He based this conclusion
principally on the lack of orientation of muscovite and its replacement of all other minerals•
Since some of the mus
covite in the writer’s area,is oriented, it formed during the main period of metamorphism, probably before the silli manite stage, because sillimanite seems to replace it* • Some muscovite with n o .orientation and doubtful replacement rela tions
with sillimanite may well have formed 3a ter, in the
manner Tweto indicates. Serlcite,1s present in almost all sections in at least minor amounts.
Its most frequent occurrence is as a replace
ment of plagioclase.
This type of replacement can be very
selective, as serlcite in some sections replaces only one set of the albite twins, leaving the other clear, or It may de velop only along cleavage directions.
Where sericltlzatlon
Is more Intense, nearly all the plagioclase has been replaced. This occurs particularly in the quartzite along the main Never Summer thrust.
Sericlte and associated chlorite both re-
plaoe ga.rnet and biotlte in many places.
Some serlcite and
chlorite are found along slip,planes Which are mainly parallelito the aohistosity.
These planes are post-metamor-
phlc, for the metamorphic minerals are granulated along
21
them.
.
The abundance of serlcite and chlorite in these
fracture zones and their replacement of other minerals show that they are the latest minerals to form.
Their
widespread occurrence and association with slip planes favors an origin by retrogressive metamorphism rather than by hydrothermal alteration. Feldspars are present in nearly all the schist3 in at least accessory amounts, and in many places constitute 20 to 30 per cent of the rock.
Where feldspar is abundant,
it exhibits a mosaic texture with quartz in many places and also raxdlyoccurs as Interstitial grains between larger grains of quartz. potash feldspar.
Plagioclase Is much more common than Of the latter, microcllne Is more abun
dant and orthoclase Is quite scanty.
Mlcrecline Is more
abundant in the schists which are transitional to biotlte gneiss than In the typical schist Itself.
Plagioclase
ranges in composition'from calcic alblte (Abg4 Ang) to to sodic andesine (AbesA*^) with the range from sodic to . medium oligoclase (AbgQAnio to AbgQA^o) most common. Much of It is either poorly twinned or untwinned, and is at least slightly sericitized. Pale pink to colorless almandite garnet Is sparingly present In a number of places and only locally abundant. The grains In many places carry numerous Inclusions - prin cipally of quartz and rarely of biotite. always anhedral and Irregular In shape. many of the grains.
They~ are almost Fractures cross
As biotlte schist passes into biotite
gneiss, garnet Is corroded and embayed by quartz and micro-
22
.
cllne and replaced along the fractures and at the edges by chlorite, serlcite, and a second generation of fine-grained biotlte.
Microscopic grains of garnet do not affect the
lineation of biotite as do the megascopic crystals. In addition to its occurrence as megascopic grains, magnetite Is a rather common microscopic accessory and al teration product.
Zircon occurs mainly as fine- to medium-
sized grains in biotlte, where It Is characterized by Its prominent pleoohroic halos.
Small crystals of apatite
are present in most localities.
In the very few places
where it Is moderately abundant, It may represent a phos phate content In the original sediments.
Fine-grained
aggregates of epidote replace biotite along its cleavage and some plagioclase.
A single grain of allanite occurred
in a strongly Injected schist. Origin Field relations indicate a sedimentary origin for the schists.
The several lithologic varieties are closely Inter
layered in numerous places like sediments. of schist anywhere transect other beds.
None of the beds
In some parts of •
the area where the schist Is well exposed, It Is folddd into anticlines and syncllnes just as sediments or extrusives would be, rather than arranged In Igneous types of struc ture.
This folded pattern Is exhibited in greater detail
In some better exposed areas of the Idaho Springs formation, particularly near Idaho Springs, where Snively (1948) mapped much of this type of structure.
Tweto (1947, p. 25)'alsA
has noted evidence of old bedding planes and has even recog-
23. nized cross-bedding. Chemical evidence also points to a sedimentary origin of the Idaho Springs.
Calculated analyses of three typical
schists are shown below.
The alumina content of the silli-
Calculated Analyses of Schist and Composite Analysis of Shale
TiOg ZrOg *2°5 BaO COg SO3 C
#44
#46
Shale
65.85 21.31 1.81 3.03 2.60 4.07 1.08
72.20 13.04 1.40
61.44 16.24 4.29 5.06 3.59 .72 2.07 4.77 1.23
60.15 16.45 4.04 2.90 2.32 1.41
.13
.13 .05
.16
-
H O.
Si02 A 1203 Feg°3 FeO MgO CaO NagO KgO HgO
#43
-
• -
-
-
-
.11
2.88
2.60 1.68
2.77 2.61 .60
#43
3.60 3,82 .89 .76 .15 .04 1.46 .58
.11
—
-
-
-
-
-
99. 6*7
1.01
$£.68
.88
100.46
Qnartz-biotite-silllmanlte schist from outcrop along Bowen Gulch trail in; the NW^ sec. 4, T. 4 N., R. 76 W.
#44
Feldspathic quartz-biotite schist.
500 feet west of
peak 11,501 and in sec. 27, T. 5 N., R. 76' W. #46
Quartz-blotlte-muscovite schist from south side of peak 11,501 in sec. 34, 1. 5 N., R. 76 W.
Shale - Composite analysis of fifty-one Paleozoic shales, by H. N. Stokes. 1924, p. 552.
Clarke, F. W., Data of Geochemistry
24. manite-rich schists in particular is higher than that of most igneous rocks.
The composition of the schist also
satisfies, with a minor exception, three principal chemical criteria (Bastin 1909, p. 472) for distinguishing metamorphic rocks of sedimentary origin from those of igneous origin. Rocks of sedimentary origin should have the content of MgO greater than CaO; K 2 O greater than Na2 0 ; and AI2 O5 should be in considerable excess over the 1:1 ratio necessary to satisfy the lime and alkalies.
It should be emphasized
.
that analyses of this sort calculated fro;m Rosiwal micrometric analyses are at best approximations to the true composition, especially if the rock contains minerals with complex chemical compositions.
Also the Rosiwal method
itself is subject to certain inaccuracies (Larsen and Miller, 1935, p. 260).
Biotlte here is the only complex
mineral and its composition was approximated from the in dices by use of the charts prepared by Heinrich (1946, pp. 844, 847).
It is thought that the inaccuracies resulting
from the calculation of biotite composition do not influ ence the rock analyses to the extent of invalidating the above criteria.
These analyses are compared with the aver
age analysis of 58 Paleozoic shales shown in Clarke»s Data of Geochemistry.
The similarity of the analyses, especially
of #46, to the average shale analysis suggests that at least some of the schists were originally shales.
Rocks
like the impure quartzite, which contain nearly the same minerals as the schist but with a preponderance of quartz, were probably slightly shaly sandstones.
25. Further evidence of the sedimentary origin of the schist is afforded In other areas of the Idaho Sprint formation where the schists are interbedded with quartzItes, marbles, and llme-silicate gneisses which, from their composition, are almost certainly of sedimentary origin. The formation was recognized to be of sedimentary origin by Ball (1908), who named It, Bastln (1917), Lowering (1929), Tweto (1947), and Snively (1948).
Hunter (1925),
who described the schists of Gunnison Canyon that bear some resemblance to the Idaho Springs, Indicated an Igneous ori gin, but on rather limited data. The origin of the knotted schist has been discussed In some detail by Ball and Spurr (1908), Lovering (1935), Tweto (1947), and Snively (1948).
Ball and Spurr considered
It to be metamorphosed conglomerate, but this view is no longer tenable.
Lovering noted that it Is moat abundant
near the large Intrusive masses of pre-Cambrian granite and that the region In which It Is most common coincides with that In which the most aluminous pegmatites are found. He believed the k*. )ts are the result of extreme metamor phism attended by the migration of siliceous and aluminous material. In part magmatic and In part derived from the schist Itself.
Tweto believed the knotted schist had
formed by a sort of "metacrystic” segregation of the re sultant orthoclase and sillimanite produced In the course of strong metamorphism of a highly sericitic original rock. He based this conclusion in part on the rather abundant sericitic aggregates partly replaced by quartz and feld-
26. spar, and also on the sharp segregation of sillimanite in the nodules and feldspar in the matrix.
Migrant pegroatitio
fluids were not believed essential to the formation of the knotted schist.
Snively attributed the nodules to metamor-
phic differentiation or segregation which proceeded under conditions of slow recrystallization.
He found them local
ized fairly near intrusive bodies and near or just within the biotite gneiss which he considered to be of hydrother mal origin.
The presence of veinlets of sillimanite in the
schist and proximity to intrusives and to the biotite gneis suggested that its formation was aided by heat and the pres ence of solutions from the cooling magma. In the writer^ area the knotted schist occurs about a mile from the biotlte gneiss contact, although as ex plained farther on, this contact itself is very Indefinite. It Is about half a mile from a small granite body on hill 10,650.
Its location then Is fairly near an intrusive 'and
near an area of strong Injection1 and possible hydrothOrriial activity, somewhat similar to those noted by Lovering bhd Snively.
However, no sillimanite or quartz veinlets'Hike
the ones they observed connecting the nodules were'sben: either In the field or In thin:section.
This wouldrsug
gest, as in Tweto*s area, that igneous solutions did not by Injection
add to the consitituents of the nodules. 'The
sericitic aggregates noted by Tweto do not occur In*these rooks, hence the original highly sericitic source rocfe can not be inferred.
TheJidea proposed by Snively seems most
applicable here, namely that slow recrystallization’main-
27. tained by sources of magmatic heat favored a metamorphic segregation of the quartz and sillimanite from the matrix. As he states, under conditions of slow crystallization, as in intrusives, or slow recrystallization as in a rock at high temperature and pressure and containing abundant pore liquids, the individual elements tend to aggregate into large units such as the coarse grains of bathollths or the large garnet porphyroblasts which occur in the lime-silicate gneisses.
The process of segregation, however precisely
achieved, apparently ran farther to completion in the rocks described by Tweto, for there sillimanite is almost com pletely segregated into the nodules whereas in the area described by the writer and in Snively*s area sillimanite is still abundant in the matrix. Since the schists were very probably sediments, the large lithologic units or bands they exhibit were probably inherited from the original sediments•
The planes of conr
tact between the lithologic units would then correspond to the original bedding planes.
As the schistosity nearly,
everywhere is parallel to these planes,of contact between the lithologic units, it is presumably parallel to the original bedding planes and was thus apparently developed during the folding of the original sediments. Lovering (1955, p* 9) arrived at this conclusion partly by his observations that the elongate minerals follow the minor crumpling3 in the schist without granulation and that these minerals were thus developed during the period of folding approximately parallel to the original bedding*
28
.
The writer noted this relationship in many places/in the Never Summer Mountains.
Lovering recognized furthermore a
"concordance between the occurrence of certain recognizable •
. - V . a
sedimentary zones and the planes of schistosity”. After the main period of ' •\ the schists owe their origin,
regional metamorphism to which iT *"%' \}/*-■-'' they were subjected to a lim
ited retrogressive metamorphism.
This is indicated chiefly
by the abundant serlcite and chlorite which have formed in part along post-metamorphlo slip planes.
Here aerioite
and chlorite have strongly replaced biotite, and se.riolte has abundantly replaced feldspars, especially plagioclases• Sericitlzation and chloritization were particularly intense in the quartzite (fig. 13) situated near the major over thrust of the area.
Since this overthrust formed in Lara-
mide time, the retrogressive metamorphism there ooourred at least that late. The schists on Blue Ridge were slightly metasomatized after the regional metamorphism.
The evidence for this
is the introduction of soattered grains of tourmaline and the association with it of .some unoriented, late muscovite (fig. 1 2 ).
.
‘
;
J'O
.
Figure 13*
Retrogreaaively metamorphosed Impure quartzite containing strongly serioitized feldspar (f) and some ehlorltized biotlte (b). Crossed nloola* X 102«
Figure 14*
Mlorooline (m) and quartz (q) replacing sodic andeslne ;(a) in a strongly mignatised quartzbiotite schist transitional to biotlte gneiss. Crossed nieols. X 189.
REGIONAL CORRELATIONS IN COLORADO Introduction In recent years considerable literature has accumu lated relating to the Idaho Springs formation of central Colorado.
Metamorphlc rocks from localities in the main
Front Range as far apart as Cameron Pass near the Wyoming line and Canon City at the southern end have been corre lated with the formation.
Several suggested correlations
have been made in the Park, Sawatch, and Sangre de Cristo Ranges and other localities to the west and south.
The
term "Idaho Springs formation" has been somewhat loosely used and in a few places has designated rooks whose equiv alence to those at the type locality is doubtful.
The
purpose of this section is to examine the problems in volved in correlations with the Idaho Springs formation, to determine the present extent of the formation so far as is possible from a review of the existing literature and from some field observations, and to note any regional lith ologic changes in the members of the formation. Problems of correlation Similarity of origin and similar age relations to other rocks must clearly be established in order to corre late metamorphlc rocks from different localities.
Correla
tion of the Idaho Springs formation In the areas discussed below Is thus somewhat simplified at the outset, as the metamorphlc rocks of this formation In practically all these localities are believed to be of sedimentary origin. 29.
30
.
In contrast, hornblende gneisses and biotlte gneisses In several localities are of uncertain origin and hence almost impossible to correlate. The criteria that should be applied in correlating exposures of pre-Cambrian metasedimentary formations are 1 ) petrologic and petrographic similarity to the rocks of
the type section, 2 ) similar stratigraphic position in the entire seotion, 3) continuity of outcrops from one area to another, and 4) similarity in history subsequent to deposi tion. The criterion of similar stratigraphic position has little application as the Idaho Springs formation itself in some places comprises the whole known metasedimentary section.
The base of the formation is nowhere exposed nor
is the top clearly defined anywhere.
Locally, as in part
of the Montezuma quadrangle, the formation is overlain by hornblende gneiss apparently of igneous origin, but in the adjoining Fraser quadrangle, hornblende paragneisses ooour at several horizons in the section.
Also, in many places
the intense isoclinal folding and lit-par-lit injection render determination of stratigraphic position of beds;, nearly impossible.
Ball(tl90$, p. 44), who first described
the formation, wrote that ”no regular stratigraphic sequence of the various members of the formation can now.be worked out'! . As he noted, this lack of regularity is probably, partly original for some members have been deposited in~ lenses, but It is ..largely due to ”the intense folding,-., faulting, shearing, and injection to which the formation has been subjected”.
Correlation by continuity of beds is locally of value near the type area at Idaho Springs from which specific beds have been traced into adjacent areas.
How
ever the complex folding, lenticular!ty of some of the beds, local intense Injection, and heavy forest cover in many places precludes tracing individual beds more than a few miles.
The Front Range mineral belt map (Lovering and God
dard, 1938b) shows that broad structural trends in the metamorphics are continuous over long distances, but the infer ence should not be drawn that individual beds are likewise necessarily continuous as innumerable minor plunging folds destroy the continuity.
Finally, several of the areas dis
cussed below are separated from the main metamorphlc area of the Front Range In which the type section is located by wide exposures of much younger sediments or volcanics. Similarities In post-depositional history of metasediments are of some indirect value In correlation.
All
the metasediments reviewed below bear at least one point of similarity in that they are the oldest rocks exposed. In widely separated areas, suoh as the Sawatoh and eastern Front Ranges, the sequence of injection of Intrusions into the metasediments Ip strikingly similar.
Deformation Of
the metasediments by both pro-Cambrian and La'ramide orog enies is demonstrable in many places.
Such similarities
In history at least indicate similar upper age limits of the metasediments and suggest that they could be correlated. Proof of equivalence, however, must be furnished by>s6me other method.
-
;‘
-
32. The chief means of correlation is by petrologic and petrographic similarity to the metasediments at the type area.
Thus the rocks at many localities, especially those
within about 30 miles of the type area, show such striking similarities that correlations are readily made* A factor which limits correlation by this method is the lenticularity of the llme-silicate rocks, marbles, and quartzites.
These limy and sandy beds of the Idaho Springs
formation vary in thickness even within the type area, and some lens out away from Idaho Springs, although they reap pear elsewhere*
Thus llme-silicate gneisses almost disappear
to the west in the adjacent Montezuma quadrangle, but reap pear in considerable abundance in the Dillon quadrangle far ther west.
This irregular distribution and lenticularity
over wide areas renders correlation more difficult, Inasmuch a 3 some members may be very thin or lacking at some local ities*
However, since the type section does contain consid
erable amounts of these minor types, and since they are wide spread, although spotty, in distribution, any metasedimen tary section should contain most of them in order to be corre lated with confidence.
A thick section consisting only of
biotite schists similar to those of the type area, especially if remote from the type locality, probably could not be defi nitely correlated, as such a section suggests solely shale deposition probably In a marine environment, whereas the Idaho Springs section with Its lenticular sandy and limy members suggests that at least temporary near shore, del taic, or even continental conditions prevailed during depo-
sition.
However, such a thick section of biotlte schist
could represent only a lateral facies change in the for mation. In several localities hornblende gneiss appears to be a metasediment interlayered with and presumably part of the Idaho Springs formation.
However, at the type locality It
self and In a few other areas (Lovering, 1935, p. 11), field evidence suggests It Is a meta-igneous rock which cuts the Idaho Springs metasediments.
It has also been interk.
preted as sheets Intruded concordantly into the upper beds of the Idaho Springs formation and as lava flows interbedded with and overlying the formation.
At most localities It
shows parallel structure to the Idaho Springs even where not Interlayered.
Hence there is some doubt whether hornblende
gneiss is a part of the formation and therefore its asso61ation with other metasediments in any given area.'; does not lend unqualified support to a possible correlation with the Idaho Springs formation. Previous regional correlations Lovering (1929, pp. 64-65) in writing on the geologic history of the Front Range notes that the Idaho Springs for mation is widely distributed in the Range.
However Its
areal extent is not Indicated and specific localities are not mentioned In the short description. Goddard (1936, p. 22) also refers to its wide distrlbution in the Front Range and briefly notes that "It is also found in the mountain ranges of central and south ern Colorado, notably in the Mosquito, the Sawatoh, the
Gore, and the Sangre de Cristo Ranges". A detailed map on a scale of one inch to one mile of the Front Range mineral belt and a brief explanatory text were compiled In 1938 by Lovering and Goddard.
The map
covers an area about 20 miles in breadth and extends across the range from Dillon to Boulder.
The writers indicate that
the formation is widely distributed throughout the belt and is predominant in a broad zone that extends from Web ster Pass, about 4 miles south of Montezuma, northward through Idaho Springs to Ward.
The principal varieties
described by Ball at the type locality (pp. 34-36fraactcrarr throughout the area although the distribution of the quartzitos and lime-silicate rocks Is sporadic.
In view of the
petrologic similarities of the rockB' throughout the belt ’ to those of the type locality and the writers* wide field experience In the mineral belt, there Is little doubt of the validity of the correlation Of these metasedimentB with the Idaho Springs.
The petrology of the formation in the
several areas of the belt’for which data a r e >available is briefly summarized below In order to show1 more fully the petrologic similarities on which the correlations are prin cipally, based and to trace any regional variations in the formation. . Type locality
.nv-
Ball (1908, pp. 37-44). named and first described the Idaho Springs formation at the type section near Idaho Springs in tha Georgetown quadrangle in the central part ’• of the Front Range, about 30 miles west of Denver.
He■
states that the formation Includes "four Intensely meta-
morphosed crystalline members, three of which, the biotitesillimanlte schist, the biotite schist and the quartz gneiss are interbedded with and grade into one another, while the fourth, consisting, of the lime-s*ilicate rocks, although interbedded with the others, appears to grade into the quartz gneiss only"• Associated principally with biotlte schist is the rock he terms "ellipsoidal massesn, prevously referred to in this report as "knotted schist". He recognized four main intergrading types of llme-silicate rocks, which include quartz-magnetite gneiss, horriblendediopslde gneiss, and "two massive rocks, the one composed essentially of quartz, epidote, and garnet, the other of calcite and lime-slllcates".
Of all these rocks, biotite-
sillimanite schist is the most widely distributed, biotite schist less, and quartz gneiss and llme-silicate rooks least. Except for three lime-silicate rooks, the petrology and petrography of the formation is similar to that already described for the Never Summer Mountains.
Hie most abun
dant of the lime-silicate rocks is hornblende-diopside gneiss which is a "well-banded, fine-grained rook composed of alternating bands of white quartz and feldspar and black or greenish-black hornblende and pyroxene".
Quartz-magne
tite gneiss, which is not widespread, contains also abun dant brown or pink garnet and hornblende as essential con stituents.
Quartz-epidote-garnet rock, abundant through
out the Georgetown quadrangle, is partly an alteration of hornblende-diopside gneiss and partly "contemporaneous in
36. origin with that rock".
The calcitic lime-silicate rock
is a locally occurring rather fine-grained aggregate of calcite, scapollte, diopside, grossularite, quartz, tremolite, titanite, and ilmenlte. Spurr (1908, p. 176), writing in the same report on the Georgetown quadrangle, described the formation in the Silver Plume district about lo miles west of Idaho Springs. He separated it into three lithologic classes.
They in
clude the knotted schi3t, a highly siliceous "granitic gneiss", which he considered a metamorphosed arkose, and "black biotltlc gneiss" somewhat similar to Ballfs biotite schist.
Lime-silloate rocks, blotito-sIllimanite schist,
and true quartzite are apparently lacking although all re appear to the west. Front Range mineral belt localities Montezuma quadrangle.
Thi3 quadrangle, mapped by
Lovering (1935), adjoins the Georgetown quadrangle to the west.
Some exposures of the formation mapped by Ball in
the western part of, the Georgetown quadrangle continue into the Montezuma quadrangle.
Three of Ball,s four major groups -
biotite schist, biotite-sillimanlte schist, and "quartz schist and gneiss" - are present, but lime-silicate rocks are almost lacking.
Knotted schist occurs at several places.
Lovering
notes the prevalence of quartz-biotite^garnet schist and in jection gneiss.
Both were observed by Ball, but apparently
are not abundant in the Georgetown quadrangle•
Injeqtlon
gneiss is of little correlative value, however, as it is
37. partly igneous and may also be developed from several types of metasediments.
Lenticular beds of "quartz schist and
gneiss", probably better termed quartzite, attain a maxi mum thickness of 1,500 feet.
However, they make up only a
small part of the entire formation.
Quartz-blotite schist
and gneiss are most common and grade into each other. Breckenrldge district.
This district, mapped by Ran-
aome (1911) and Lovering (19o4) adjoins the Montezuma quad rangle to the southwest.
Pre-Cambrian exposures are so
small that little study has been made of them.
The most
abundant rock type Is a gray, fissile, quartz-blotlteserlclte schist which Ransome (1911, p. 26) considered part of the Idaho Springs formation.
In view of the occur
rence of the formation immediately to the northwest near Dillon, and to the northeast near Montezuma, this corre lation seemB justified despite the poor exposures. Central City quadrangle.
Most of the southern and
eastern parts of this quadrangle, which Is immediately north of the Georgetown quadrangle, was mapped by Bastln and Hill (1917).
The Idaho Springs type locality borders
on the south edge of this quadrangle.
Although Ball*a
four principal lithologlc types are present, only biotite schist Is abundant, the others being confined to small local areas.
Knotted schist, found in all the other areas
near Idaho Springs, is absent.
The lime-silicate rock in
most places is of massive character, composed mainly of quartz, epidote, and garnet..
B&s^tln considers the horn
blende gneiss of the quadrangle to be, part of the Idaho
38
.
Springs formation, and believes it was formed from shales by contact-metamorphic processes. Boulder County tungsten belt.
This belt, which Is
about 20 miles north of Idaho Springs, extends across south eastern Boulder County and alsofoverlaps the north end of the Central City quadrangle.
It has been Investigated by
George (1908), Lovering (1940), and Tweto (1947).
Pre-
Caxribrian metasediments are exposed only in the western part of the belt where they border and interfinger with the 1 0 0 square mile Boulder Creek batholith.
Lovering (1940, p.
135) and Tweto (1947, p. 328), who correlate them with the Idaho Springs, state that they consist of biotite schist, quartz-biotite schist, and associated quartzite.
George
(1908, p. 18) also notes biotite-sillimanite gneiss and "hornblende gnelss-schlst"•
Although the quartzites and
biotite-sillimanite schists are of minor occurrence, three of Ball’s four principal members of the formation are thus present, only the lime-silicates being absent. Ward region.
This area of about 25 square miles,
mapped by Worcester (1920), is about 8 miles north of the western end of the tungsten belt.
The general trend of
the metasediments Inmost of the area is west except in the southern part where the northwesterly trends of the schists of the tungsten belt gradually merge into'the wes terly trend.
Worcester (1920, pp. 21-23) notes that quartz-
biotite—garnet schist Is most common and that quartz—bio— tite-slllimanlte schist Is abundant.
Quartz-biotite schist
and hornblende sohlst are of only local occurrence.
He
states that the foliation planes of hornblende schist ' do not correspond In attitude with those of the other schists and that It may therefore be metamorphosed dlorite or gabbro.
Q,uartzite3, which are present only a few
miles south, are missing, as are the lime-sillcates.
How
ever as the two commonest lithologic varieties of the type section are present, correlation of these bedsw ith the Idaho Springs seems Justified,
Also, structural continuity
with the Idaho Springs beds to the south lends some support to the correlation. Lake Albion area. Metasediments assigned to the Idaho Springs formation are briefly noted by Wahlstrom (1940 a,b) in the vicinity of Lake Albion, 25 miles northwest of Idaho Springs and 6 miles west of Ward.
He states that the for
mation "consists predominantly of biotite-sillimanite gneiss, but many other rock types, such as garnetifeous mica schist, biotite schist,'quartzite, amphibolite, and’ pyroxene gneiss are present",
The formation "originally-
consisted of a thick series of shales, sandstones and im pure limestones".
Of the major varieties of the Idaho ■
Springs formation, only lime-silicate gneiss Is not noted. Jamestown district.
Metasediments classified'as Idaho
Springs exposed in the Jamestown district at the north east end of the mineral be It *have been described In";detail by Goddard (1936, pp. 22-34) .
The most abundant type is.
biotite schist, but there are local areas of blotlte-sillimanite schist, quartz-biotite schist, injection gneiss^, and lime-silicate rocks.
The lime-silicate rocks include1len-
40
.
ticular masses principally composed of varying amounts of hornblende, diopside, calcite, garnet, epidote, and feld spar similar to those described by Ball.
The only variety
at the type locality which is missing here is quartzite. However original sandy sediments are represented by quartzschists which contain as much as 50 per cent quartz.
The
petrography of the metasediments exposed here is very simi lar to that described for the type area and other adjacent localities.
In view of this similarity and the presence of
all members of the Idaho Springs formation but quartzite, there can be no reasonable doubt that the correlation is valid. The Gold Hill mining distriot, described by Goddard (1940) is 3 miles south of Jamestown and 3 miles north of the east end of the tungsten belt.
Although only quartz-
biotite schist and injection gneiss are noted here, they are undoubtedly part of the Idaho Springs formation, as it occurs In the Jamestown district only a mile to the north east.
The structural trends of the exposures In the two
districts are interrupted only by a tongue of the Boulder batholith. Coal Creek area. :
The metasediments exposed at Coal
'
.
i
’
i
. V
'
•'
■
Creek 10 miles southeast Of the tungsten belt have been discussed by several writers, including Lovering and God dard (1938, pp. 10-11), Adler (19ol), Lovering (1929, pp. 72,73), George (1908, p. 20), Van HIse and Leith (1909, pp. 804-5), and Lakes (1390).
The rocks are composed of
a series of quartzites, quartz—biotite—muscovite schists,
41
.
and conglomerates at least 2,000 feet thick.
The exposures
are Isolated In a large southwest-trending syncline which Is cut on the north, south and west by Boulder Creek gran ite, granite gneiss, and gneisslc aplite, and on the east Is faulted against Paleozoic and Hesozoic sediments.
The
nearest Idaho Springs metasediments crop out a mile to the south and are separated structurally from the Coal Creek syncline by a pronounced shear zone.
The correlation of
these Coal Creek rocks Is still In doubt but it is generally agreed because of petrologic dissimilarities and lack of structural continuity that they are not equivalent to the Idaho Springs formation.
Thus Lovering and Goddard (1938,
p. 10) write that they are "unlike any member of the Idaho Springs . . . and that in most places the schists are phyllitic and therefore show a lower grade of metamorphism than do those of the Idaho Springs formation to the south". Front Range localities outside the mineral belt Vasquez Mountains.
This range in the northwestern
part of the Fraser quadrangle, mapped by Tweto (1947), Is 7 miles north of the nearest exposures of definite Idaho Springs formation In the Montezuma quadrangle and 15 miles northwest of the type area.
The typical quartz-biotite
schist and quartz—biotite—sllllmanlte schist are most abun dant, and quartz-biotite-garnet schist and knotted schist occur much less widely.
Quartzite is much less abundant
than to the south as it occurs only In thin beds seldom extending more than a few hundred feet along the strike.
■
42. Several varieties of lime-silicate rock are abundant, in cluding hornblende-diopside-feldspar gneisses, and calcitic lime-silicate rocks which grade into impure marble.
Tweto
gives abundant evidence that hornblende gneiss is a meta sediment interbedded with the Idaho Springs and accordingly includes it in the formation.
He further includes as a dis
tinct variety of the formation thick quartz-biotite-feldspar paragneisses which resemble the Idaho Springs biotite gneiss noted by Lovering (1935, p. 6 ) in the Montezuma area.
The occurrence of all the major petrologic types
noted by Ball together with the proximity of this area to the type locality leaves no doubt that Tweto*s correlation is completely justified. Never Summer Mountains.
The metasediments of the
northern part of the Never Summer Mountains, mapped by Gorton (1941) and assigned by him to the Idaho Springs for mation, are similar to those described by the writer In the southern part of the range.
However quartzite and lime-
silicate rocks are completely lacking In the northern part. The most abundant types are quartz-biotite schist and gneiss, and biotite-sillimanite schist.
The quartz-biotite schist
locally contains abundant muscovite and garnet as In the southern Never Summers.
The marked petrographic and petro
logic^ similarity of these rocks to those in the southern Never Summer Mountains suggests that they are also part of the Idaho Springs formation, but the complete absence of quartzites and llme-silicates somewhat weakens the correlation (see conclusions, p. 53).
43. Big Thompson Canyon. Rocky Mountain National Park.
Ex
posed In the canyon of the Big Thompson River 18 miles north of the Jamestown district Is a group of metamorphic rocks which have been termed the Big Thompson schist by Boos (1924, p. 52).
They grade from quartz schist, quartz-bio
tite schist, and chlorite schist on the east near the moun tain front to dense black biotite-sillimanite schist on the west near Estes Park.
Boos* map (1934, p. 308) shows that
they are exposed over a large area extending to Grand Lake on the west and nearly to the mineral belt on the south. The west and south edges of this area thu 3 border and partly overlap regions In which rooks Assigned to the Idaho Springs formation are exposed.
There appears to be little reason
for differentiating the Big Thompson schist from the Idaho Springs.
The lithologic types are similar and there Is no
stratigraphic or structural evidence to Indicate that they are separate formations.
Rocks at the borders of the two
formations can not reasonably be placed in’one in preference to the other on the basis of llthology•
Thus the rooks
near Grand Lake termed Big Thompson schist by Boos strong ly resemble typical Idaho Springs formation and are so’cor related by the writer.
It Is believed that the older for
mation name, the Idaho Springs, should prevail, and hence the-rocks termed Big Thompson schist are here considered part of the Idaho Springs formation. Bergen Park.
-
Snively (1948) mapped In great detail
an area of about 18 square miles near Bergen Park In the Denver Mountain Parks, the western end of which Is two miles
east of the ©astern margin of the Georgetown type area quadrangle.
All the principal members in the type area
are present here in about the same relative abundance, ex cept that quartzite is somewhat less common.
The lime-
silicate rocks, as in the Vasquez Mountains, are chiefly hornblende-diopslde gneisses and calcltlo lime-silicate rocks which grade into impure marble,
Snlvely found some
evidence Indicating that hornblende gneiss is a metased iment interbedded and intergrading with the members of the Idaho Springs formation, but he does not correlate It with the Idaho Springs because at a single locality he observed a cros3 -outting relationship suggesting an igneous origin. As in the other areas close to the type locality, the occur rence of every rock type noted by Ball except the minor quartz-magnetlte gneiss allows no question of the correct ness of the correlation of these beds with the Idaho Springs formation. In an area about 15 miles east and south of Bergen Park, Boos (1940, p, 698) mapped metasedimentary schists and gneisses which she states are equivalent to the Idaho Springs formation in the Georgetown quadrangle. criptions of the rocks are given, South Park.
No des
■.-» .
A large part of South Park extending
nearly from the south edges of the>Georgetown and Monter zuma quadrangles to the latitude, of. Cripple Creek was mapped by Stark and others (1949).
Schists "composed
dominantly of quartz, feldspar,, and biotite, with varying amounts of hornblende and sillimanite” are exposed along
45. the eastern edge.
Stark states they are "similar in many
respects to the Idaho Springs complex of the Georgetown quadrangle" and correlates them with it.
In many places
they have been strongly injected and migmatized by solu tions from the Pikes Peak and Silver Plume granites into which they grade imperceptibly.
Q,uartzites and lime-sili-
cates are not mentioned, although they are found a few miles south of this area. Pikes Peak - Colorado Springs - Cripple Creek area. Schists similar to the above as well as quartzite are briefly noted by Cross (1894, p. 1) in the vicinity of Pikes Peak. Peak granite.
They occur as large inclusions in the Pikes Llndgren, Rarisome, and Graton (1906, pp.
50-53) In their.study of the Cripple Creek district des cribe quartz-muscovlte schists which contain variable amounts of sillimanite and biotite*
The petrographlc des
criptions are much like those for schists at and near Idaho Springs.
A chemical analysis is given, which in its high
alumina and potash content, resembles those calculated by Snively (1948,table 1), Tweto (1947, p. 24) and the writer. Finlay (1916, p. 4) described Inclusions of similar schist in the Colorado Springs area. did not map them separately.
They are so scarce that he Laughlin and Kosohman (1935,
pp. 5 4 , 232) briefly refer to a considerable mass of mica schist in the granite around Cripple Creek.
All these meta-
morphios probably belong to the Idaho Springs formation In view of their lithologic similarity and their proximity to the schists of South Park, referred to above.
Eight Mile Park,
At Eight Mil© Park 15 miles south
of Cripple Creek, Heinrich (1948, p. 424) mapped an area Including varied metasediments which he correlates with the Idaho Springs formation.
He notes that "generally
minor amounts of biotite schists, biotlte-muscovite schist, biotite-sillimanite schist, quartz-mlca schist, quartzite and exceedingly rare layers of quartz-epidote rock" are present.
These varieties correspond to each major rock
type noted by Ball at Idaho Springs,
The chief differences
between these rocks and those at the type locality are that at Eight Mile Park muscovite schist is more abundant than biotite schist and lime-silicates are far less abundant. The correlation is undoubtedly valid because of the lithologic similarity and the probable occurrence of Idaho Springs formation in South Park about 15 miles to the north. Wet Mountains.
This range, which is the southern con
tinuation of the Front Range, extends about 40 miles south ward from the vicinity of Canon City to Greenhorn Mountain, Hills (1900, p, 1) states that "the principal mass of Green horn Mountain consists of coarse- and fine-grained granites and gneisses, hornblende-, mica- and chlorite-schlst, and subordinate masses of garnet- and epidote-sohist".
As he
did not describe these rocks, correlation can not be at tempted, although mica schist and garnet and epidote schist resemble some of the Idaho Springs members. At Silver Cliff and Rosita Hills, on the central-west side of the range, Cross (1896, pp. 276-79) found a group of variable gneisses of which "a gray, rather flne-graiiied
47. biotite gneiss is perhaps more common than any other type*1. He states that “quartz, biotite, hornblende, and muscovite are frequently segregated in thin layers or streaks in the predominantly feldspathic mass”.
This description suggests
injection gneiss so commonly developed from the Idaho Springs schist throughout the Front Range, although Cross does not emphasize injection by the local granites.
Similarity to
the Idaho Springs formation is suggested also by “silli manite gneiss or schist ... of very limited extent" as well as "garnetiferous strata”.
However he states that distinct
ly schistose rocks do not appear anywhere in the observed section.
Their minor importance, coupled with the absence
of quartzite and lime-silicates which Heinrich notes a few miles north, renders correlation with the Idaho Springs rather doubtful. Localities outside of the Front Range Dillon area.
Metasediments exposed near Dillon, in
the southern Gore and northern Tenmile Ranges, were mapped by the writer in 1940.
They are separated from the near
est outcrops of the Idaho Springs formation in the Monte zuma quadrangle about 6 miles east by a thick section of Jurassic and Cretaceous rocks which form the southwestern extension of Middle Park.
Correlations are thus more un
certain than In the Front Range where the metamorphic areas are continuous.
However, all the principal varieties of
the type locality are present, although the beds of quartz ite, lime-silicate, and biotite-sillimanite schist are thin
48
.
and of limited occurrence.
Quartz-biotite-gamet schist
is fairly abundant, but the knotted schist Is absent.
As
In the areas to the east, muscovite Is locally abundant In the schist.
The north-south structural trends of these
metasediments parallel those In the Montezuma quadrangle and in general they continue southward in the Tenmile and Mosquito Ranges nearly to Leadville without much change. Because of the lithologic and petrographle similarity of the rocks of this area to the Idaho Springs at Montezuma the correlation is very probable despite the break in con tinuity of outcrop. Tenmile district.
Four miles south of the Dillon-
area is the Tenmile district, mapped by S. F. Emmons (1898)•
He did not systematically study the pre-Cambrian
rocks, and notes only that they consist of granites, granite gneisses, mica-schists, and amphibolites similar to those at Leadville.
All these types also occur to the north ■
near Dillon and probably include the Idaho Springs for mation. Mosquito Range.
Directly south of the Tenmile- dis
trict is the Mosquito Range, much of which was mapped' by Emmons (1886).
Within the range smaller localities In
which the metasediments have been described include the Climax district (Butler and Vanderwilt, 1950), Alma dis trict (Patton, et al, 1912; Slngewald and Butler, 1953'), and the west slope of the range near Leadville (Behre,-1939). According to Patton, who discussed the metasediments In most detail, the pre-Cambrian metamorphics Include granitic
49
.
gnoiss and schists of several varieties, all of which grade into and are interbedded with each other.
The gran
ite gneiss is petrographically much like the biotite paragneiss from the Vasquez Mountains and the Never Summer Mountains, and the field relations are similar.
The vari
eties of schist are 1 ) biotite-hornblende schist, 2 ) gray biotite-muscovite schist, and b) blotite-muscovite-sillimanite schist.
The latter two closely resemble schists
from the Never Summer Mountains.
Emmons (1885) has des
cribed "amphibolites” which resemble the hornblende gneisses near Dillon and in the Front Range.
Like many of the horn
blende gneisses, these amphibolites ”occur interstratified with them [schists]
in layers of varying thickness and some
times in large lenticular bodies”.
Injection gneiss has
been noted by nearly all the writers.
Slngewald (1933,
p. 92) has tentatively correlated the schists in the Alma district with the Idaho Springs,' and Butler and Vanderwilt (1930, p. 324) have similarly correlated those at Cllmfcx. The strong petrographlc similarity to. definite Idaho Springs schists, as well as similar association with hornblende gneiss, suggests that the correlation is probable, but it is somewhat weakened by the complete absence o f 5two types prominent at Idaho Springs - quartzite and lime—silicate rooks• Sawatch Range.
The pre-Cambrian metamorphlos of the
Sawatch Range have been described in considerable detail by several writers.
Stark and Barnes (1934) mapped the
entire range in reconnaissance, Howell (1919) wrote on the
50
.
Twin Lakes ar«a in the central part of the range, Goddard (1936) on the Tincup district on the southwestern part, and Crawford on the Monarch and Tomichi districts (1910, 1913) and the Gold Brick district (1916) at the southern end, and also on Redcliff (1924) in the northeastern part.
Stark
and Barnes (1934, p. 470) recognized three principal types, which are the Sawatch and Holy Cross schists, silicified limestone and marble, and quartzite.
The schists, which
are by far the most abundant rocks of the range, "are sep arated more on the basis of their field appearance than on any difference of origin or composition".
They consist
largely of quartz and feldspar with local facies rich in biotite, garnet, sillimanite, and hornblende.
Like the
schists of the Front Range they have been Injected strongly by the Pikes Peak and Silver Plume groups of granites and grade through banded injection gneiss into granitoid rooks with only faint traces of schist remnants.
Stark (1935,
p. 23) states that these mlgmatitlc rocks are "similar In character and probably continuous" with those In Tenmile Canyon to the east, which are correlated above with the Idaho Springs formation.
He also states that "the Idaho
Springs schist of the eastern edge of South Park strongly resembles many facies of the granitized schists of the ranges named (Sawatch and Tenmile) both mogaacopically and microscopically"•
Goddard (1936, p. 557) .notes thatethe
schist at Tincup "resembles that of the Idaho Springs:for mation of the Front Range".
The petrographic descriptions
of these schists at Twin Lakes by Howell (1921, pp. 38-42)
51
.
are much like those of Idaho Springs schists in many parts of the Front Range.
He notes further that some dlopslde
occurs in the marble as -In the Front Range.
Crawford,
writing on the Monarch (1910,p. 12) and the Gold Brick districts (1916,pp. 22-31), described biotite-sillimanite schists which he states are "of similar character" to the knotted schist described by Ball.
He also noted horn
blende gnels3 which is Interbedded with the schists as in the Front Range, and "epidote rock" resembling a similar Front Range tlime-silioate rock.
Although the metasediments
of the range are from 50 to 100 miles west of the Idaho Springs type locality and about 12 miles west of the near est probable Idaho Springs outcrops at Dillon, they are believed to be equivalent.
Every rock type described by
Ball at the type locality, excepting the minor quartzmagnetite schist, has Its counterpart In the Sawatch Range. Indeed, similar types are more numerous in this range than at the eastern end of the Front Range mineral belt, 25 miles north of the type locality.
A similar history of injec
tion by both Pikes Peak and Silver Plume granites supports the correlation. Gunnison Canyon.
In the canyon of the Gunnison
River and Its tributary, Tomichl Creek, pre-Cambrian rooks are exposed almost continuously for 60 miles from the Sawatch Range to the vicinity of Montrose.
The western
40 miles of these exposures were mapped by Hunter (1925), but unfortunately a wide gap exists between his work and that of Crawford at Gold Brick and Monarch.
52
.
Hunter divides the metamorphics into three groups, the Black Canyon schist, River Portal mica schist, and Dubois greenstone, each of which contains several members. Although the Black Canyon schist, where seen by the writer, somewhat resembles the Idaho Springs biotite schists megascopically and petrographically, the other members, espec ially the
River Portal mica schist, bear little resemblance
to the other varieties of the Idaho Springs formation.
The
River Portal schist has a much finer texture than the Black (■ Canyon schist, shows poorer foliation and less abundant mica.
Quartaites and lime-silicate rocks are absent.
In
view of the limited lithologic similarity to the Idaho Springs formation, the large unmapped region between this area and the Sawatch Range, and the remoteness from the type locality, these rocks should not for the present be correlated with the Idaho Springs formation. Sangre de Crlsto Range.
The pre-Cambrian metamor
phics of this range, which extends southeastward from the Sawatch Range Into New Mexico, are little known and only a small amount of literature relating to them has appeared. Burbank and Goddard (1957) briefly described the great' massifs along the west border of the range, Burbank (1932, pp. 5-7) and ?atton (1916, pp. 55-57) the small exposures at Bonanza on the northwest side of the range, Patton (1910) those at Russell (Grayback) in the central part, and Gun ther (1906, p. 143) those at Plomo near the New Mexico bor der.
None of the rooks described show much resemblance to
the members of the Idaho Springs formation except some minor
53. quartz-biotite schist at Bonanza and possibly some schist in the massifs.
Nearly all the major llthologlo varieties
of the formation are apparently absent.
With the present
Information there is little justification for correlating these rocks with the Idaho Springs formation. Conclusions Plate 3 shows the areal extent of the Idaho Springs formation as suggested by the above study.
The boundaries
in several places are tentative and have been placed as shown because insufficient data In outlying regions pre clude even tentative correlations.
Thus the northern
Gore Range, Sangre de Crlsto Range, and northernmost Front Range may well Include the formation, but the scattered small areas studied within the ranges do not afford suf ficient Information to include them within the boundaries of the Idaho Springs formation at present. In general the members of the formation with the ex ception of the ubiquitous biotite schist show erratic and spotty distribution throughout the entire area of outcrop, but a few regional variations may be briefly summarized. Quartzites, Including quartz-sohist, thin out and finally disappear toward the northwest from the vicinity of the type locality.
Thus, in the Montezuma quadrangle, they are
about 1,500 feet thick; In the Fraser quadrangle they occur only In a few thin beds; In the southern Never Summer Moun tains they crop out at a single locality; and in the northern Never Summar Mountains they are absent.
Boos (1924, p. 52)
notes a similar thinning and disappearance of "quartz-sohist"
54
.
westward from Big Thompson Canyon to western Rooky Mountain National Park.
Lime-slllcates also thin out and disappear
between the Vasquez Mountains and the northern Never Summer Mountains.
Tweto (1947, p. 6 6 ) notes that there are hun *-1
dreds of lenses in the Vasquez Mountains, but only one was noted In the southern Never Summer Mountains and none in the northern part of the range. of the mineral belt.
They are also soarce south
No mention is made of them in all of
South Park, but they recur sparingly in the Eight Mile Park area.
HORNBLENDE GNEISS Distribution Hornblende gneiss is one of* the abundant pre-Cambrian rocks of the southern Never Summer Mountains as well as of the Front Range.
The largest single exposure is on Bowen
Mountain and peak 12,449, but several other thick bands occur nearby on Parika Peak and Baker Mountain.
Thinner
bands are interlayered with biotite gneiss and schist In many localities as shown on plate 1.
Many are too thin
to map separately. Petrology Megascopic features:
Hornblende gneiss Includes a
variety of mafic rocks, composed principally of hornblende aind andesine or labradorlte.
Many of the outcrops are
massive and blocky (fig. 28), unlike the sharp jagged ones characteristic of schist.
Some outcrops weather to
shades of brown, and those with abundant garnet show a pinkish cast on weathering.
Typical hornblende gneiss is
a dark gray to black or greenish black, fine- to mediumgrained rock with a strongly gneissic structure, but rather massive, weakly foliated gneisses are known locally;
Some
strongly feldspathic varieties present a salt and pepper appearance on fresh fracture.
A few types show Irregularly
spaced bands of plagioclase and quartz about two millimeters wide, but generally banding Is weak or lacking.
The light
colored minerals tend to collect In places In narrow lenses rarely more than a few millimeters long. 55.
Pink almandite
56. garnet is locally abundant and occurs in.irregular grains and subhedral crystals as large as six millimeters in diam eter.
Quartz occurs sparingly except in injected gneiss. The contact of hornblende gneiss with schist and with
biotite paragneiss is usually fairly sharp, but at two localities on Baker Mountain, hornblende gneiss grades into biotite schist through a transition zone several feet wide.
At a few localities thin beds of slightly schistose,
unusually biotitic, quartzose hornblende gneiss interbedded with paragneiss and schist appear to be intermediate in composition between typical hornblende gneiss and biotite paragneiss.
At one place on the water supply ditch horn
blende gneiss containing a little oalcite in small string ers grades into a small lens of lime-silicate gneiss with foliate structure oriented parallel to that of hornblende gneiss. Injection by pegmatite and aplite is by no means' afit widespread as in schist but i3 nevertheless common.
Some
of the dikes and sills cut across the foliation and some occur as lenses or large knots In the planes of foliation. On Bowen Mountain injection has locally strongly contorted the gneiss.
Intrusion of hornblende gneiss by biotite
orthogneiss is discussed on page 79. Microscopic features:
The foliate structure.is caused
mainly by the orientation of hornblende crystals In the plane of foliation and in part by mineralofelpAd;..! banding. Although the long axes of hornblende generally lie In the foliate planes, they are not everywhere parallel to each
57
other.
.
Many plagioclase crystals are somewhat elongated
In the plane of foliation and some are oriented parallel to the hornblende llneatlon. Gneisses from a number of localities exhibit imper fect microscopic banding in which dark, layers of horn blende alternate with*light-colored ones of plagioclase and quartz (fig.15).
The bands are-not everywhere sharply
delineated, as each type contalhs small amounts of the min erals of the other., Tremollte and minor plagioclase com pose a conspicuous band in one. gneiss .
;
The main constituents in nearly all gneisses are horn blende and plagioclase, but diopside, tremolite, biotite, hypersthene, quartz, garnet, epidote, cllnozoisite, and calcite locally may be moderately abundant.
Common acces
sory minerals include apatite, sphene, magnetite, and zir con; whereas sillimanite, rutile, and llmenite occur spar ingly.
Sericlte, chlorite, and fine-grained epidote are
common alteration products.
Modes of four gneisses, deter
mined by the Rosiwal method, follow on page 58. Hypersthene has not been found in hornblende gneiss from nearby areas, although Tweto (1947, p.’8 8 ) notes its occurrence in the Vasquez Mountains in thermally metamor phosed paragneiss adjacent to hypersthene gabbro.
It
occurs in two localities in the Never Summer fountains in scattered non-pleochrolc, somewhat fibrous crystals traversed by many fraotures and replaced by hornblende along the cleavages and borders.
Diopside is also re
placed by hornblende in the same manner.
Diopside seems
Figure 15.
Rude bending In hornblende gneiss« Dark mineral is hornblende, light minerals ere sndeslne (a) and quarts (q). X 25,
Figure 16.
Garnet (g) metaoryst In hornblende gneiss loaded with Inclusions of hornblende (h), bio tite (b), and quarts (q)• The foliation of the gneiss bends around the metacrys't. X100.
58
.
Modes of hornblende gneiss #15
#17
#18
#21
Hornblende
60.7
62.2
27.9
62.8
Plagioclase
25.6 (Anso)
29.1 (An46r51)
17.8 (AU7g) 29.4
30.7 r (An28_35)
11.1
.8
Quartz
2.0
Biotite
6.5
.3
Diopside
4.3
2.4
-
-
Hypersthene
-
5.1
-
-
Titanite
-
-
-
3.4
Garnet
-
-
Magnetite
.3
.6
1.6
11.2
-
2.4
-
#15
Diopsidic hornblende gneiss, southeast side of Baker Mountain, sec. 27, T. 5 N., R. 76 W.
#17
Hypersthenic hornblende gneiss, southeast side of Baker Mountain, sec. 26, T. 5 N., R. 76 W.
#18
Garnetiferous, biotltic hornblende gneiss, northeast side of Bowen Mountain, sec. 28, T. 4 N., R. 76 W.
#21
Hornblende gneiss, south side of ridge between Bowen Mountain and peak 11,501, sec. 33, T. 4 N., R. 76 W.
Accessory minerals of less than 1 per cent Include In order of abundance: apatite, cllnozolalte, epidote, and tremollte; the alteration minerals lnolude chlorite and serlclte. ^ to replace garnet, but the evidence Is Inconclusive.
Trem
ollte occurs partly as slender prismatic crystals replacing both hornblende and biotite (figs. 17,28).
Biotite Is
found mainly as well-developed crystals in marked contrast to its shred—like, strongly corroded form In most biotite gneiss.
Large blades cut across hornblende in places, and
fine-grained velnlets replace hornblende and garnet along cleavage and fracture planes.
Biotite is itself replaced
Figure 17.
Figure 18.
i» Velnl*t
^
*‘JFi*oi«« feoi^jbiendt4/h.
Tremolite (t) replacing hornblende (h) In hornblende gnelas. X 89.
59
.
along Its cleavage by sericite.
Some large garnet meta-
crysts are loaded with unoriented inclusions of hornblende, biotite, and quartz, and, as in schist, the folia bend around the garnet grains (fig. 16)•
Quartz occurs mainly
in a mosaic texture with plagioclase and hornblende (fig. 19), and also in narrow injection seams by itself or with andesine.
In some localities quartz replaces biotite,
hornblende, garnet, and plagioclase, but the replacement relations are usually obscure.
Some strained quartz is
biaxial with an optic angle of about five degrees* Hornblende has locally been bleached, so that it ex hibits only a weak pleochroism.
In some rocks it poikilit-
ically encloses small quartz grains, and the mosaic inter growths with quartz are so fine-grained.that they resemble a partly recrystallized crushed zone* The composition of plagioclase ranges widely from basic oligoclase, An2 8 , to sodlc bytownlte, An72*
Of
twelve specimen^ eight fall within the range of medium andesine to medium labradorite, AJI4 Q to Ango*
Plagioclase
from a single locality may show a slight variation In com position, and as in biotite gneiss, a few anomalous results were obtained In determinations by the Fouquft method.
Most
crystals show albite twinning and many exhibit pericline twinning as well.
Deformation has bent twinning lamellae
and granulated the borders of grains In several places. Much of it Is mildly sericitlzed, and that from one locality has been replaced almost completely.
Some grains replace
biotite and hornblende, but most of them show mutually
Figure 19.
Mosaic texture exhibited by quartz and hornblende In hornblende gneiea* X 89.
60
embayed contacts.
.
In one locality a second generation
of clear, slightly more sodic plagioclase seems to replace older labradorlte.
Microcllne is associated abundantly
with andesine in one slightly Injected gneiss, and also exhibits a myrmekitic Intergrowth with quartz.
Lovering
(1936) notes its frequent occurrence in the hornblende gneiss of the Montezuma quadrangle. The lens of lime-silicate gneiss Inclosed in horn blende gneiss is composed of medium-grained dlopside, sericitized plagioclase ranging from medium andesine to sodic labradorlte, actinollte, cllnozolslte, and accessory;quartz and hornblende.
Hornblende has been largely replaced by
actinollte and partly by epidote and magnetite.
This
gneiss resembles some of the common lime-silicate rocks described by several workers in other parts of the Front Range, as for example by Snively (1948, p. 12) near Bergen Park, Tweto (1947, p. 66) in the Vasquez Mountains, and Ball (1908, p. 42) in the Georgetown quadrangle. Origin Field relations offer the best evidence of a sedimen tary origin of hornblende gneiss.
The foliate structure
and contacts are everywhere parallel to that of the enclosing schists or paragneiss. J. .
.
.
.
I
This parallelism, however, ‘
1
W
-
•
would also be expected if the gneiss represents Interlayered flows or sills.
Ho indications whatever were found
of the cross-cutting relationships typical of Igneous rooks, such as those mentioned by Ball (1908), Lovering and Goddard
61
.
(1938) and Snively (1948), nor of other igneous features like the possible pillow lava structures referred to by Snively (1948, p. 52). Hornblende gneiss is interbedded with and Ideally grades into the metasediments, as though all were part of an original sedimentary sequence.
The occurrence of many
lenses of hornblende gneiss in schist and paragneiss and of occasional lenses of 3Chist in hornblende gneiss is in accord with this hypothesis.
The gradation of one lens of
oriented lime-silicate gneiss into hornblende gneios does not by itself lend great support to the hypothesis, but in other areas of the Front Range where lime-silicate gneiss crops out abundantly, wide zones of transition into horn blende gneiss are common.
Further, the minerals of both
rocks may be identical, the relative proportions; alone being different.
Goddard (1936, p. 37) believed similar
lenses of lime-silicate material in hornblende gneiss-at Jamestown were formed by met amorphi sin of the gneiss by "pegmatite juices".
Pegmatites do not'inject the gneiss
which encloses the lens in the writer1s area; although they have somewhat Injected some biotite. paragneiss several feet away.
••••"' In an original sedimentary section beds might well
have existed with compositions between those of the shales which gave rise to schist and the more calcareous sediments which gave rise to typical hornblende gneiss; and indeed’ the quartzose, garnetiferous, biotitic hornblende gneiss, like #18 in the table of modes* (p. 58), probably isuthe
62
.
metamorphosed equivalent of these beds. Petrographic evidence is not particularly convincing either of sedimentary or of igneous origin, although it is not incompatible with the sedimentary origin suggested by field relations.
The wide variety of plagioclases, dif
ferent in almost every outcrop, fits well into the sedimen tary hypothesis, but does not preclude the alternative hy pothesis of several original varieties of basic igneous rocks, possibly related differentiates.
This variation
in the plagioclase is rapid in places, as on the north side of Bowen Mountain, where one narrow band in schist contains sodic bytownite, and another band 250 feet higher contains medium andesine.
These bands/could represent sills of rock
types as far apart in composition as monzonite and basic gabbro, formed under rather special conditions of differ entiation; but it would be simpler to explain them as common variations in sediments.
Hornblende also shows a consider
able variation in composition as reflected by its optical properties.
Snively (1947, p. 52) studied the composition
of hornblendes from gneisses, some of which from field rela tions seemed igneous and some sedimentary', and concluded that "there is no justification for making a distinction between sedimentary and igneous hornblende gneiss on the basis of the refractive Indices (which are a function of the chemical composition) ” of the: hornblende.
Finally, the
presence of calcite lends some support to an origin in limy sediments •
.
On the other hand, the occurrence of hypersthene.
65 • might indicate original igneous rocks, possibly like the hypersthene gabbro described by Tweto (1947, p. 86) in the Vasquez Mountains.
However, at both localities of
hypersthene occurrence, hornblende gneiss is intruded by biotite orthogneiss, which suggests that hypersthene may be a product of local thermal metamorphism. No attempt is made here to determine the origin of the gneiss by comparison of its calculated composition with chemical analyses of possible source rocks as was done with schist.
First, the minerals of hornblende
gneiss are all more complex, except plagioclase and quartz, and it is not definitely possible to obtain by calculation from optical properties the accuracy necessary to make meaningful comparisons.
Second, the occasional variation
in plagioclase wlthin^a single specimen would also intro duce a considerable error.
Third, the chemical cOm£6si-
tlon of some metamorphic rocks, like the highly aluminous schists or the iime-s'ilicate rocks and marbles of the Idaho Springs formation, show conclusively whether the aource rock was sedimentary or igneous; but a comparison of accurate analyses of hornblende gneiss with those of pos sible igneous or sedimentary source rocks does hot prove which type was the source, for the compositions of mlany hornblende gneisses are similar both to gabbros and to certain impure calcareous sediments.
Source rooks of
widely different types are capable of r'ecrystallizing during metamorphism to petrographlcally similar hornblende gneisses, as several authors have shown. Including‘Adams
64
.
(1909), Buddington (1939, pp. 11-14), Harker (1939, pp. 268, 311-312), and Turner (1948, p. 82). Lovering (1935, p. 11) believed hornblende gneiss in the Montezuma quadrangle represents a great series of flows Interbedded with the Idaho Springs formation.
Several of
the lines of evidence already cited suggest that this ori gin does not apply in the Never Summer Mountains.
In addi
tion, as Tweto (1947) notes, a minimum thickness might be expected in flows, and many lenses of gneiss seem too thin to be ascribed to this cause.
Some of the lenses appear
to be completely enclosed by metasediments, hence out off from any pragmatic source, although admittedly outcrops afford only a sectional and. not a three dimensional view. The locally rapid variation In composition and texture and the inclusion, of oriented lenses of schist and lime-sllioate which grade into hornblende gneiss are probably the most cogent arguments against this origin*. If a sedimentary origin Is assumed, the problem remains whether the sediments merely recrystallized to:gneiss or whether they were transformed by hydrothermal solutions, as dlscussed under blotlte gneiss*. The latter Is not probable because hornblende gneiss occurs in areas of schist which have not been noticeably hydrothermally altered.
Snively
(1948, p.55) notes alao that in the Bergen Park area, where metasomatism was important in .biotite gneiss formation,, limesilicate gneiss occurring In the metasomatlzed areas, shows no sign of conversion to hornblende gneiss, as might be expected If original- limy sediments had been converted. Finally, if metasomatism were dominant, an Irregular dla-
65. tribution of the lithologic varieties of hornblende gneiss, possibly around some magmatic center, would reasonably be expected, instead of the actual widespread bedded arrange ment. Since the original sediments recrystallized to horn blende gneiss, presumably without notable addition of mate rial, they must have been relatively high In lime, magnesia, and iron oxides.
Possible sediments which have been sug
gested are glauconitic greensand, calcareous ohlorltic shale, and dolomitic shale.
The first two are too res
tricted to be likely source beds, whereas dolomitic shale is not uncommon In existing stratigraphic sections, for ex ample In the lower Paleozoic of the Michigan basin or In the Pennsylvanian of central Colorado.
Quartzose horn
blende gneiss, like # 18 In the modal table, probably rep resents sandier beds. Hornblende gneiss Is believed to be a part of the Idaho Springs formation In view of Its probable sedimen tary origin and Interbedding with the metasediments of that formation.
However, its origin Is by no means as clear as
that of the other members of the Idaho Springs.
It would
presumably correlate with the hornblende gneiss of Gilpin County (Bastln, 1917), the Vasquez Mountains (Tweto, 1947), and with part of the gneiss at Bergen Park (Snively, 1948); but probably not with the Swandyke hornblende gneiss at Montezuma (Lovering, 1935) and Jamestown (Goddard, 1936) or the gneiss at Georgetown (Ball, 1908), all of which seem to be In part Igneous.
As In schist and biotite
66
•
gneiss, limited retrogressive metamorphism of hornblende gneiss is indicated by mild sericitization of plagioclase and chloritization of hornblende and biotite.
Some additional
evidence is provided by local veinlets of fine-grained bio tite which replace hornblende along slip planes, replace ment of diopside by hornblende, and replacement of horn blende by tremolite veinlets, one of which is localized along a slip plane.
BIOTITB GNEISS
"
v
Distribution
•
vy-y/^ " ••
The area underlain by biotite gneiss is second in size to that of the Idaho Springs formation.
The gneiss occurs .
only in the northern part where it underlies much of Baker Mountain and parts'of Bowen Mountain and Parika Peak. *
:f
Its occurrence and structure are shown on.the geologic map and section (pi* 1*2), and also are discussed in the. a fair
ly wide, variety of migmatite, paragneiss, cued orthogneiss characterized by conspicuous banding•(figi BO) and by; hiotite as the major mafic mineral.
It resembles rather:blosely
the igneous quartz monzonlte gpeiss .described, by Gorton. (1941, p • 13) in the *northernt.SeVor^Snitoer^'U6untftid9^&^': by lovering (1955) and others elsewhere in .the -Front Ranged / s the migmatite and paragneiss ("quartz monzonite gneiss n) inf, eluded by Tweto (1947) in the Idahb^Springs foii^ion in 'y the Fraser,area> and the biotite gneiss' described b y Snively .• (1948) in the Bergen Park area.
Although this rock was re^
ferred to in the field as nquartz monzonlte gneiaa11 because V of the strong lithologio similarity^ the term wasyabandoned because of. •
•
*/
' *
*
its tw
.
strictly i^eous 'CoimotatiOnfcfuld because the., ■
megasoopically rather simill^’ ^roojes^of diverse origin* -j
.«
1; ,*
Figure 20.
Banded mlgmatltlc biotite gneiss at top of 12,300 foot peak Just east of Baker Mountain. Bark bands at upper and lover right are remnants of sehlst; thin white bands are stringers of pegmatite and apllte.
Figure 21.
Sharp contact between mlgmatltlc biotite gneiss and hornblende gneiss. A pegmatite dike (lower right) cuts across the foliation of both gneisses.
Typical biotite gneiss, which Includes most migmatite, orthogneiss, and light-colored paragneiss, Is fine- to med ium-grained, light to dark gray, and weathers in places to brown or reddish brown.
The light bands are composed of
quartz and feldspar and range In thickness from one-eighth inch to about an Inch. than feldspar.
Usually they contain more quartz
The dark bands, consisting of biotite with
minor quartz and feldspar, range in thickness from a knifeedge to about one-fourth inch.
Some of the blotitlc bands
are persistent along the strike for many feet.
In a few
places the banding is contorted, but generally the atti tude of the gneissic structure is regular.
Fink almandlte
garnet, occurring as irregular grains and as imperfect dodecahedrons as much as half an lnoh in diameter, is locally abundant, especially In paragneiss.
Muscovite'
and sllllmanlte occur sparingly. A note on the use of terms may clarify the following discussion.
"Injection gneiss" refers to a rock composed
of alternating distinguishable bands of schist and ofsgran itic pegmatite, aplite, and minor granitic materials, In which the igneous bands have been injected in a lit-parlit manner.
The mineral composition of the igneous:bands
is stated on page 75.
The term "migmatitic' biotite gneiss",
used bynonymously with "migmatite" and "migmatitic gneiss", is restricted to gneissio rock so strongly injected o i ^ partly assimilated by the above-mentioned Igneous!seamsr that the original texture and composition of the schist are completely obscured.
It looks like a gnelssold igneous
69
.
rock, but is of "migmatitic11 or mixed metamorphic and ig neous origin.
"Biotite paragneiss" refers to a gneiss of
sedimentary origin, much of which Is identical megascopically with "migmatite" and with "biotite orthogneiss", which is of igneous origin. Biotite gneiss grades into most of the other preCambrian rocks through rather narrow zones, but all phases of gradation of mlgmatltlc biotite gneiss into schist are well exposed over broad belts.
The transition zone between
schist and gneiss consists of rocks ranging from schist only slightly Injected in a lit-par-lit manner, through injection gnelss((p. 14), to mlgmatltlc gneiss so strongly Injected that the original schist is no longer evident. This zone may be as much as half a mile in width.
The
definition of a contact between biotite gneiss and schist is, as Snively (1948, p. 19) has remarked, a somewhat arbi trary process.
The writer*s praotloe has been to map as
schist all rooks in which the remnants of schist are still apparent, and to restrict the mlgmatltlc biotite gneiss in the manner stated above.
Howhere was biotite gneiss ob
served to out aoross schist. The oontaet between biotite gneiss and hornblende gneiss Is sharp In most places (fig. 22), although in a few localities the two rocks ihtergrade over a narrow transition zone.
Hornblende gneiss is more massive than
schist and exhibits a weaker foliate structure ■, hence it has been less receptive to the injected material, which has readily transformed schist.
70
.
Typical biotite gneiss grades Into darker fine grained garnetlferous biotite-plagioclase gneiss which is either unbanded or poorly banded.
Xt is believed to be
paragneiss (see page 74). • Although a considerable part of biotite gneiss has been formed by Intense Injection of sctilSt, the gneiss itself has been out' and Injected later by sills and dikes of pegmatite and apllte (fig.2l). Mloroecoplo features?
*
Because the petrography of
biotite gneiss is generally similar to that of the schists, only significant differences are described.
The gneisslo
structure is caused by the alternation of quartz-feldspar and biotite bands (figs. 22, 23) and by the parallel orien tation of plates of biotite within the plane of the bands. As in schist, biotite is oriented In the foliation planes. In some gneisses reorystalllzed quartz and feldspar are elongated parallel to the foliate structure. The chief mineralogical distinctions between gneiss and schist are that gneiss contains more feldspar and less biotite, slllimanlte, and muscovite.
Modes of four gneisses
determined by the Roslwal method are shown on page 71. The composition ranges from granitic to quartz dlorltio. Most mlgmatltes are granitio or quartz monzonitlc, and most paragneisses are more basic.
The ratio of potash feldspar
to plagioclase feldspar varies widely, as well as the per centage of total feldspar.
The gneiss Is not subdivided -
on the basis of feldspars, since variations In their relative abundance and composition reflect only local changes in the
Figure 2,2,,
Rudely banded biotite paragneiss. Dark bands are biotite; the light ones quarts (q) and serioltleed labradorlte (1)# X 80.
Figure 23.
Same view as figure 22 under crossed nicols. Hote the sutured contacts of some quarts grains (q) and the strongly sericltised feldspar (f).
71
.
Modes of biotite gneiss #24
#27
#30
#31
Quartz
47.5
42.6
52.8
32.1
Biotite
12.1
9.9
13.8
15.0
Plagioclase
39.1(An3o) 37.0(An4o) 31.7(An3o)
Potash feldspar
.3
Garnet
-
Accessories
1.1
-
-
9.8
1.3
.8
.4
.4 51.8 .5
#24
Biotite paragneiss, southeast side of Bowen Mountain, sec. 5, T. 4 N. , R, 76 W.
#27
Biotite paragneiss, southeast side of Baker Mountain, sec. 26, T. 5 N., R. 76 W.
#50
Biotite orthogneiss, southwest side of Baker Mountain, sec. 21, T. 6 N., R. 76 W.
#31
Migmatitic biotite gneiss, south side of Bowen Moun tain, sec. 32, T. 5 N., R. 76 W.
Plagioclase in #24 varies from Anggto An^g, and in #27, from An4Q^° An52« Ttie compositions listed above are most abundant. Accessory minerals in order of abundance are apatite, magnetite, pyrlte, tremollte, zircon, sllllmanite, and allanite; alteration products are serlcite, magnetite, and hematite. original sediments, which take place over short distances, or in the amount of introduced material.
Also, some of the
varieties occur locally in beds too thin to be mapped sep arately. Plagioclase ranges in composition from sodic oligoclase (An^2)
oodic labradorlte (Angj), with andesine
and labradorlte more common.
In most places plagioclase
is less abundant in migmatite than in paragneiss and orthogneiss.
Paragneiss from a single outcrop may contain
72
.
several kinds of plagioclase which are as far apart in composition as the end members in the above range.
An
unusual feature of a few plagioclases is the anomalous result obtained in determination by the Fouqu^ method. The angle between the fast ray and twinning in some crys tals that show a negative bisectrix figure is much greater than the standard charts indicate even for the most basic plagioclases.
These crystals are apparently more basic
than sodic labradorlte, but the determination of composition can not be made any more exact by this method.
Albite twin
ning is common in most gneisses and perlcllne twinning is also locally abundant.
However in a few localities, nearly
all plagioclase 1b untwlnned.
Deformation has bent some
albite twinning lamellae, and in some grains has probably produced wavy extinction resembling that of quartz.
The
borders of plagioclase and quartz have been granulated in a few gneisses. Potash feldspar occurs mainly in migmatite and only sparingly in paragneiss and orthogneiss.
Microcline is
the most abundant potash feldspar, but orthoclase and perthite are both common locally.
Perthite consists of
large grains of orthoclase with parallel rows of minute lens-shaped blebs of albite.
Antlperthlte has developed
in minor amounts in a few places. Paragenetic relationships of the feldspars are evi dent in several gneisses.
Microcline has clearly replaced
plagioclase in a few mlgmatltes (fig. 14), but the relation ships of microcline to plagioclase are obscure in most of
73. gneisses.
Microcline also locally replaces sericltized
orthoclase.
In one paragneiss a little andesine is replaced
toy somewhat more sodic plagioclase.
Locally quartz has
replaced some plagioclase in all three varieties of bio tite gneiss.
Quartz has also replaced a little microcline,
tout generally quartz and feldspars are intergrown'In a mosaic texture without obvious age differences.
Narrow
veinlets of quartz and microcline cut older microcline grains in a migmatite.
A little muscovite has replaCed:
microcline. Most plagioclase has been slightly sericltized and in some gneisses it has been almost completely replaced. Potash feldspars, however, are rarely strongly sericltized. Sericitization has been very selective, as In schists, for some gneisses which are otherwise relatively unaltered, contain plagioolase grains so strongly replaced that:they resemble sericltized grains of original.sediments (fig. 24)• Indeed they have been, so Interpreted by Tweto (1947, pp. * ^ 36, 40), but early serlclte would probably not be stable throughout the Intense metamorphism, imposed on the original sediments. The occurrence of biotite is similar to that in the schists, although in the migmatitic gneisses, it Isgenerally found as smaller blades, scraps., and shreds (fig.< 26) • Sllllmanite commonly is present as a few scattered thtnv, rods, usually oriented, in quartz and feldspars.
Inrone
migmatite It occurs In garnet as ncedle-llke inclusions' oriented parallel to the foliation of the gneiss.(fig.28-) .
Figure 24•
Selectively aerloltlsed plagieolaae (p) in otherwise relatively unaltered migmatitic bio tite gneiss. Crossed nlcols. «, microcline; b, blotltei q, quartz. X 102.
Figure 25.
Carnet (g) containing oriented sllllmanlte (s) needles in migmatitic biotite gneiss1. Quartz (q) has cut garnet. X 100*
Figure 26.
Figure 27.
Scraps and shreds of biotite in mlgmatltlc bio tite gneiss. The light bands are mainly quartz and mieroollne. X 102.
Shear zone in mlgmatltlc biotite gneiss marked by strong sericitization of feldspar (f) and of some biotite (b). q, quartz. X 101.
74
.
This occurrence suggests that garnet formed hers later than sillimanite, and also that the formation of the garnet metacrysts required no alumina from aIllimanite.
These garnets
with their oriented inclusions also provide a petrographlc criterion by which It is possible to distinguish migmatite and paragneiss from megascopically similar orthogneiss. Garnet is more abundant than in schist but in other res pects is similar.
The accessories include those of schist
and also pyrite, clinozoisite, and tremolite; alteration products are the same. Origin Several lines of evidence indicate a sedimentary ori gin for part of the biotite gneiss.
Some thin sohist
layers are Interlaminated with gneiss as though all were part of the same older sedimentary sequence.
Tweto (1947,
p. 42) states that in the Fraser area similar paragnelsses are interbedded with and grade into bands of quartzlte and limy rocks as well as schist.
Gorton (1941, p. 10) notes
in the northern Never Summer Mountains that biotite schist and gneiss grade into each other, and that nthere is little difference in the two types except in the relative amounts of quartz and biotite”. metasediments.
He places both in the Idaho Springs
Lovering (1935, p. 6) also describes quartz—
biotite paragnelsses in the Montezuma quadrangle interbedded with Idaho Springs metaeediments.
The dark obscurely banded
or unbanded plagioclase gneisses (page 70) show no com pelling evidence of lit-par-lit,injection or other igneous activity.
The long persistent stringers of injected mate
75. rial characteristic of injection gneiss and much migmatite are absent.
The composition cf the injected material In
migmatite is granitic, the most common minerals being quartz, orthoclase, microcline, and oligoclase; yet some gneisses, which are not demonstrably Igneous, contain none of the feldspars listed, but Instead contain plagioclase as basic as calcic andesine and labradorlte.
Finally the wide var
iation in the plagioclases In a few gneisses suggests a sedimentary origin. The predominance of quartz, andesine, and labradorlte in paragneiss along with garnet and biotite suggests that the source rock was probably a graywacke or an arkose. Migmatitic biotite gneiss is believed to have formed by an extremely large injection and partial assimilation of schist which resulted in an indistinguishable mixture of igneous and schist material.
Locally on Baker Mountain,
hornblende gneiss was also attacked.
This origin has been
suggested by Tweto (1947, p. 4b) for the migmatitic "quartz monzonlte gneiss" of the Vasquez Mountains.
Lovering (1929,
p. 67) in discussing quartz monzonlte gneiss of the Front Range states that
• it is difficult to escape the con
clusion that assimilation (of schist and hornblende gneiss), resulting from extreme development of lit-par-llt injection, played an important part in the formation of the gneiss". In the Eight Mile Park area Heinrich (1948, p. 452) notes a broad exposure of injection gneiss somewhat resembling biotite gneiss, and states it was formed by granitization of Idaho Springs schist through intense lit-par-lit injec
76
.
tlon by pegmatite, apllte, and granite. The chief evidence for this origin here la the com plete gradation of schist to mlgmatltlc biotite gneiss (see page 69).
Thoroughly applicable is Tweto*s statement (1947,
p. 44) that "the gradation is so complete and the associa tion of injected schist with migmatite so constant, that there can be little doubt that mlgmatl2 ation and lit-parlit injection are but degrees of the same process".
As
discussed previously injection Is believed to have been accompanied by an increase in volume and is not considered to be entirely a metasoma tic process, although replacement did occur to some extent.
Despite the extreme injection
and partial assimilation, the schistose origin of part of the rock Is clearly established.
Gfce mineral content of
.
migmatite is much like that of the injected granitic mate rial except for a greater content of biotite and other reliot minerals from schist. At four localities within the migmatltes, thin beds of only slightly or moderately Injected schist have es caped attack by the Injection solutions.
Their sohistosity
is parallel with the foliate structure of the migmatite, and where well exposed, they grade into migmatite over a narrow transition zone. Snively (1948) ascribes a hydrothermal origin to rocks he oalls "biotite gneiss", which closely resemble migmatitic biotite gneiss of the area mapped.
Although he also noted
the same wide transition zone, he believes the gneiss never passed through a semi—Igneous stage by Injection, but was converted metasomatically while In a nearly solid
77
.
condition by hydrothermal emanations from underlying gran ite.
Hla moat convincing evidence, as he also states, is
the field evidence of replacement, such as folds in schist passing on into gneiss, similarity in appearance of out crops of schist and gneiss, abrupt change from schist to gneiss along strike, the preference of the replacing solu tions for schist and lime-Billoate gneiss to hornblende gneiss and the abundance of various sized oriented inclu sions of schist and hornblende gneiss in biotite gneiss* Most of this evldenoe is lacking in the Never Summer Mountains with the exception of the relict layers of schist referred to above* are not found.
Abundant smaller oriented inclusions
Folds in schist could not clearly be demon**
strated to continue into gneiss.
On the contrary, on Bowen
Mountain, where schist and gneiss are best exposed together, both comprise the east limb of a major anticline*
Similar
relations occur on Farika Peak and north of Illinois Creek* Between Bowen and Baker Mountains schist grades into gneiss along the strike, but the change is far from abrupt.
Tweto
(1947, p. 44) also noted this parallelism in struoture of schist and gneiss and the gradation along'the strike of gneiss into strongly injected schist. Microscopic evidences ,of replacement are stated by Snively to be much less satisfactory than,the field rela tionships*
He has listed a number of evidences of replace
ment, but notes that it, is much less;than.might be expected, and that .many of the textures, such as graphic, myrmekitic, and perthitic intergrowtha do not exclude an igneous dOs-
cent.
As previously discussed on page 72, some replacement
clearly occurred during the formation of gneiss, but as in Snively*s area microscopic evidence of replacement is not abundant enough to indicate that the entire rock formed in this fashion.
Some replacement is undoubtedly inevitable
during large scale injection and partial assimilation, where granitic material of somewhat different composition is in troduced into schists (fig, 14).
However, the abundance
of older biotite and minor amounts of older plagioclase, garnet, and slllimanlte show that replacement was by no means complete*
The obscurity of replacement relations in
some gneisses suggests that mechanical mixing of the in jected material and the original minerals of schist occurred without much replacement. Comparison of chemical analyses affords little sup port to the replacement hypothesis, as Snively (1948, pp. 66-7u) has shown.
Hot the least reason is the inaeouraey
of chemical compositions calculated from Rosiwal analyses. The chief reason is the absence of a constant factor in the metamorphism of schist to gneiss; thus only variables exist and meaningful comparison is Impossible, Head, 1915, p, 287),
(Leith and
Constant volume can not be assumed
iaee page 15)» nor is there evidence that either total weight or the weight of any one chemical constituent re mained constant. The Igneous origin of a few rather small outcrops of biotite gneiss is indicated almost entirely by field rela tions, as the rocks closely resemble some paragnelsses
79
.
megascopically and microscopically*
Orthogneiss even con
tains scattered needles of sillimanlte and rather abun dant almandite garnet, obtained from partial assimilation of schist during Intrusion,
However, despite the petro-
graphlc similarity to paragneiss, the evidence of igneous origin Is conclusive, as orthogneiss cuts across and con tains xenoliths of hornblende gneiss along the water supply ditch on Baker Mountain,
The xenoliths range in size from
minute fragments to blocks several feet across.
The foliate
structure of a few xenoliths near the contacts is parallel to that of the host rock, but most are unoriented.
On
Parika Peak the foliate structure Is contorted to such an extent that turbulence in a magma seems the only reasonable explanation. Preliminary study suggested that this gneiss repre sented the end member of the sequence, schist, injection gneiss, migmatitlc blotite gneiss, and Intrusive biotite gneiss, which reflects the succeeding rock types developed from the schist host during the rise of a batholith or large stock.
In the sequence mildly Injected schist would
be expected at a considerable distance from the Intrusive, injection gneiss somewhat closer where Igneous activity was more Intense, and migmatite In a relatively narrow zone around the Intrusive.
According to Clooa (1948) all these
rocks formed under deep-seated conditions, injection being uncommon at shallow depths.
Some Igneous-looking gneisses
may be part of this intrusive end member, but those examined under the microscope contained feldspars considerably moro
80
.
basic (andeslne) than the more sodic feldspars of Injec tion gneiss and migmatite.
They are thus apparently local
intrusivea unrelated to the larger more acidic intrusive which supplied the Igneous materials which partly comprise migmatite and injection gneiss.
This intrusive does not
crop out in the area, but probably underlies migmatitlc biotite gneiss at a relatively shallow depth.
The minor
granite stocks on hill 10,650 and near South Supply Greek are too small to be considered the sources, and also they do not visibly inject the adjacent schists. Retrograde metamorphism affected gneiss as well as schist, and resulted in widespread mild sericitization and chloritizatlon, which in a few places is localized along slip planes (fig. 27).
Slight metasomatism after
the regional metamorphism is indicated, as in schist, by the local introduction of scattered grains of tourmaline.
81
.
BIOTITE-GARNET AMPKIBOLITB An unusual layer of black coarse-grained biotitegarnet amphibolite about 10 feet thick, between layers of strongly Injected, locally garnetlferous schist and hornblende gneiss, crops out In a single locality on the southeast side of Bowen Mountain.
The contacts are sharp
and parallel to the foliation of the enclosing metasediments.
Megascoplcally, the rock seems composed of unori
ented hornblende, abundant biotlte, and numerous grains of pink garnet.
The blades of biotite are about 5 millimeters
In diameter and grains of garnet as large as 1 centimeter. A modal analysis determined by the Roslwal method shows the following percentages:
hornblende 49.1, biotite
38.4, garnet 9.7, plagloclase 2.0, oalcite 0.5, and apatite, magnetite, and epidote 0.3.
Plagloclase is rather strongly
serlcitized, but it appears to be untwinned basic oligoclase or andeslne.
Tt occurs as Inclusions and lenses In
biotite and hornblende and as small interstitial grains. Most biotite and hornblende exhibit an Interlocking tex ture with no evidence of lineation, but a few small veins of biotite cut hornblende and also garnet in a few places. Garnet contains numerous small unoriented Inclusions of hornblende and biotite.
Veins of calcite cut across horn
blende and replace biotite along Its cleavage. Field relations are not especially helpful in deter mining the origin of the rock, as either a sill or a sedi ment could be interbedded with the enclosing metasediment.
82
.
The absence of a gradational zone might suggest a sill, but on the other hand the association with metasediments, especially hornblende gneiss, which contains different proportions of the same minerals, suggests that it repre sents only a sediment of different composition. Harker (1932, p. 284) describes garnet-amphibolites derived from basic igneous rocks which somewhat resemble this amphibolite, but plagloclase occurs in moderate amounts in all of them.
Strong metamorphism of feldspar-free ultra-
basic rocks has given rise to massive hornblende-garnet rocks (Harker, p. 278), but an abundance of biotite would hardly be expected in such rocks, as hornblende rwould prob ably account for the low original potash, alumina, and hy droxyl content.
Since the chief difference between the com
position of this rock and garnetiferous hornblende gneiss is the low plagloclase content, it may have originated in a sedimentary bed relatively richer in iron, magnesia, and lime, and lower in alumina that the dolomitic shales which presumably gave rise to hornblende gneiss.
Such a sediment,
possibly a calcareous, strongly chloritic shale, is uncommon but the amphibolite is itself rare.
INTRUSIVE ROCKS GRANITE Distribution Granite comprises only a small part of the pre-Cambrian rocks of the area mapped*
Two small stocks are poorly ex
posed in section 3, T. 4 N., R. 76 W. on hill 10650 and in section 33, T. 4 N., R. 76 W, on South Supply Creek near the south edge.
A somewhat larger Intrusive is located
east of Apiatan Mountain in section 9, T* 3 N., R. 76 W. about two miles south of the area.
The outcrop of the
stock on hill 10650 Is separated into two parts by a narrow band of hornblende gneiss and schist.
The larger part has
an irregular shape (pi. 1) and an area of about a quarter of a square mile.
The smaller part has a roughly oval shape
and an area about a quarter of the larger part. Petrology Megascopic features:
The granite of hill 10650 is
a slightly pink, rather even-grained, granular rock consis ting mostly of quartz and feldspar.
It weathers red or
brown and gives rise to a sandy mantle.
The outcrops are
rounded in most places, rather strongly weathered, and the *•
!
' i
-
‘ /
_
' ,
>
topography of most of the granite area is aubdued.
Bio
tite and muscovite are sprinkled abundantly enough through out to give a salt and pepper appearance, but neither con stitute more than a few per cent of the rock.
Most of the •
-
:
;
-i •
grains are a millimeter or so in diameter with a maximum of •
about three millimeters.
,i • -4i•. ■1
An outcrop near the margin showed 83
.
84
.
weak foliation, but otherwise the rock is without foliation and is uniformly granular.
Two fairly large pegmatites,
one of coarse-grained graphic granite, occur in and at the margins, but the contacts are covered.
The abrupt changes
in float suggest that pegmatite cuts rather than grades into granite.
The granites of South Supply Creek and
Apiatan Mountain are a little coarser but otherwise are similar in most respects. Microscopic features; A modal analysis shows that the granite of hill 10650 consists of quartz 50 per cent, miorocline 26 per cent, oligoclase (Anll-13) 32 per cent, biotite 5 per cent, muscovite 6 per cent, and less than 1 per cent total of magnetite, epidote, zircon, rutile, and apatite. The composition is thus near quartz monzonite according to Grout's classification (1932, p. 48), but the plagloclase Is a little too sodic.
The mineral content of the granite
on Supply Creek is about the same, and that of Apiatan Mountain contains orthoclase as well as microcline. Micrographic intergrowths of quartz and orthoclase, and of quartz and microcline are present, as well as micro cline perthite and •islands" of microclinelln plagloclase, i
which somewhat resemble antiperthite.
Plagloclase is mod
erately to strongly sericltized and much of it is poorly twinned or untwinned. by microcline. biotite.
It appears to be locally replaced
Both quartz and feldspars corrode and embay
A slight degree of metamorphism is indicated by
granulation of the borders of some grains,by strained quartz,
85
.
and by bending of the twinning lamellae of a few plagloclase grains. Correlation of these granites with others of the Front Range is difficult to establish.
They resemble the border
phases of the pegmatitic granite described by Gorton (1941, p. 13) in the northern Never Summer Mountains, but plagioclase is more abundant and the grain size is smaller.
The
Silver Plume granite (Goddard, 1936, p. 43), of the common granites in the central part of the Front Range, seems to resemble them closest in mineral content, but the texture and grain size are far different. PEGMATITES Distribution and structure Pegmatites and associated aplites are widely distri buted throughout the entire pre-Cambrian terrain.
The bulk
of them occurs as relatively thin lit-par-lit Injection seams in schist, but numerous dikes, sills and irregular masses crop out, which range in thickness from about one foot to several hundred feet and In length from a few feet to three thousand feet.
The largest pegmatite Is an irreg
ular northeast-trending mass of graphic granite which appears to cut the granite of hill 10650 near the mouth of Bowen Gulch (page 84). The pegmatites were evidently Intruded over a long period of time.
On Baker Mountain three generations are
In evidence In one outcrop, but over the whole area, at least two broad age groups can be determined.
The older
86. group Includes the llt-par-lit Injection seams in schist, which in most places do not cut across the schistosity. The younger group outs across the foliate structure of all the metamorphics, although locally it also injects them condordantly.
Crosscutting pegmatites are conspicuous in
hornblende gneiss, which with its imperfect foliate struc ture is less susceptible to lit-par-lit Injection.
On the
north face of Bowen Mountain and the east face, of Baker Mountain, they form a lacy network of dikes and sills (figs. 28, 29).
Some of the pegmatites that cut hornblende gneiss
are possibly contemporaneous with the lit-par-lit seams, but many are younger, because they cut biotite orthogneiss which itself intrudes hornblende gneiss and injected schist. Petrology Megascopic features;
The mineralogy of nearly all
the pegmatites is'"fairly simple; all but one can be put in the "simple” cl^ss " ■ the Tertiary Igneous rocks, are up^to three-eighths inch in
diameter, andVtibeVfeldspars may reach three-fourths inch in
length.
The feldspars include abundant sodlc albite and
some orthoclase and sanldlne. pyrite, and a little sphene.
The accessoriesare apatite, Sericlte and calcite are
common alteration products of the feldspars and in the groundmass.
The groundmass is composed of very minute
Figure 32.
Dike of soft, weathered rhyolite porphyry about 200 feet wide near top of Bowen Mountain.
Figure 33.
Hexagonal columnar Jointing In a large dike of rhyolite on upper Bill Greek.
A
92. feldspar grains with indices less than balsam and without unusual texture. Porphyry Peaks trachyte porphyry The trachyte porphyry which underlies the Porphyry Peaks consists of phenocrysts of sanidine, sodlc oligoolase (An-^g) 9 orthoclase, and biotite in a finely crystal line groundmass composed of potash feldspar and locally a little quartz and streaks of glass.
Some of the sani-
dine crystals from the southwest peak are as much as 2 ^ inches long, but those from the northeast peak are rarely £ inch in length.
Most of them are clear and glassy, un
like the ollgoclase phenocrysts which are slightly cloudy. All biotite crystals are partly resorbed and altered to limonite and magnetite.
Yellow and red stains of the iron
oxides are common throughout the rock and impart the colors to the weathered outcrops. present.
A few crystals of zircon are
Feldspar of the groundmass consists mainly of
microlites and innumerable minute irregular grains.
Some
tiny anhedral quartz grains occur in the groundmassof the porphyry from the southwest Porphyry Peak, but quartz is absent from the outcrops on the northeast peak 3,500 feet away. This rock was termed rhyolite porphyry by Spock (1928, p. 2 2 1 ), but quartz is so scarce in the outcrops sampled by the writer that the rock should be termed quartz-bearing trachyte porphyry.
Spock states that it is a flow capping
the Porphyry Peaks, but several determinations of the lineati*on of sanidine phenocrysts suggest that the magma was
93
.
intruded almost vertically upward and then flattened out toward the east at an angle of about 50 degrees. Quartz latite porphyry Light brown to dark gray quartz latite porphyry crops out in four localities in sections 14, 15, 22, and 23, T. 4 Icii., R. 76 W. between Bowen Gulch and North Supply Creek (pi. 1).
The two exposures at the edge of the Colo
rado valley flat in sections 14 and 2o are completely sur rounded by drift.
The porphyry at these localities was in
truded as groups of dikes separated by fairly sharp con tact planes which dip from 40 to 60 degrees to the south west.
A few of these dikes are as thin as 6 inches and
many are from 1 to 3 feet thick.
They break down during
weathering either into sand or into sharp angular fragments about a quarter inch long.
Locally differential weathering
has produced small pockets a foot deep in the steep faces of the bluffs at the edge of.the valley.
All the porphyry
was intruded near the surface, as the groundmass is either glassy, partly glassy, or cryptocrystalline.
The hypoth
esis that the bodies represent flows is improbably because of the steep dips unassooiated with visible faulting and the complete absence of vesicular and amygdaloidal struc tures or fragmental tops at the contacts between the layers. Megascopically the quartz latite porphyry somewhat resembles the. latite and trachyte porphyries of Supply Greek and the Porphyry Peaks, but it differs mainly in the absence of the (large sanidine phenocrysts.
The largest
feldspar phenocrysts in this rock are little over a quar—
94
ter Inch long.
•
They consist of orthoclase, sanidine, and
plagloclase, nearly all of which ranges in composition from basic ollgoclase (An 2 7 ) to medium andeslne (An^g)•
At one
locality abundant crystals of medium andeslne and a very few of sodlc ollgoclase occur together suggesting prog ressive differentiation during the intratelluric stage. Zoning of the plagioclases is inconspicuous.
Most of the
feldspar phenocrysts are somewhat rounded and corroded, and a few are shattered, the fractures having been filled by the groundmass.
Quartz is plentiful, occurring as
corroded phenocrysts, the largest of which are 2 to 3 mil limeters in diameter.
Biotite is the ferromagnesian mineral
and, as usual in other porphyries, is strongly resorbed and partly replaced by magnetite dust.
At one locality
it shows a spongy texture due' to these alterations; tite, as usual, is a fairlycommon accessory.
Apa
ThJe’porphyry
exposed in section 15,' T . 4 N •, Hi 76 W. includes a little greenish-brown hornblende Jand also a few brown microscopic fragments of a basalt consisting largely of labradorite microlites. The groundmass of the porphyry of section 15 is composed almost entirely of glass with innumerable longulites exhibiting^flow structure (fig. 34)•
Uie indices
are less thanbalsam, hence the composition of the glass is probably equivaient to silicic feldspar. mass of the porphyry from section
23
The ground
aLong the Colorado
valley is very finely crystalline and contains numerous spherulites of feldspar and small cavities lined with
Figure 94.
Flow structure oaused by longolltas la a glassy base in quafts-latite porphyry. The phenoeryst is orthoclase. Z 89.
Figure 96.
Ifloro.spherulltlo groundmass of quarts-latite porphyry. The phenoeryst is sodio andeslne. Mote the orosa.es Is the top oenter and lower right formed toy extinction of the individual fibers of the mloroapherulites. Crossed nlools. X 105•
95
fibrous chalcedony.
.
The porphyry exposed in section 14
has a dirty brown groundmass composed almost entirely of microspherulites.
Under crossed nicols most of them show
a black cross formed by extinction of the individual fibers (fis• 35)
•
Latite porphyry Latite porphyry, which crops out in South Supply Creek valley in sections 33 and 34, T. 4 N., R. 76 W., closely resembles megascopically the trachyte porphyry of Porphyry Peaks and, like it, weathers into large, roughsurfaced boulders stained brown from iron oxides.
In
fresh exposures its light gray groundmass is mottled by . abundant light-colored feldspar and dark ferromagnesian phenocrysts.
Sanidine Is the most conspicuous mineral,
occurring In large phenocrysts as long as 1^- inches. The microscope shows that medium andesine, occur ring as partly embayed phenocrysts about one-fourth inch long, is the most abundant feldspar, and that sanidine is uncommon except as macroscopic phenocrysts.
Two moderate
sized grains of antiperthite are composed of numerous blebs of orthoclase distributed throughout andeslne in a sievelike pattern.
Other andesine crystals also contain less
conspicuous small Intergrowths of orthoclase.
Small crys
tals of pale greenish augite and deep brown hexagonal bio tite crystals are fairly common.
Biotite is strongly
resorbed and In places only small portions of the original crystals remain. magnetite.
Much of It has been replaced by dusty^
Small prisms and hexagonal crystals of apatite,
96
.
some of which are pleochroic as described below, are scattered throughout the rock.
The groundmass is composed
of untwinned microlites and irregular tiny grains of silicic feldspars. Rhyolite Rhyolite occurs in small dikes at numerous localities In Willow and Bill Creek valleys and In the pre-Cambrian terrain In the northern part of the area.
However, on the
west side of upper Bill Creek In section 35, T. 5 N., R. 77 W. a group of adjoining large dikes which cut the Middle Park sediments is over 200 feet thick.
The dikes in general
are cliff-forming and have given rise to a large talus of sharp angular fragments.
Some of the Individual dikes
formed prominent hexagonal joints on cooling (fig. 33). The hexagonal "posts" which break out at the surface are about a foot In diameter and from 6 Inches to 6 feet in length. All the rhyolites are dense rocks of light shades:of gray, green, and brown with only a sprinkling of small phenocrysts.
In most of them quartz occurs as locally
euhedral phenocrysts 1 to 2 millimeters in diameter. Some crystals are biaxial with a small optic angle and a few contain inclusions of the groundmass.
Quartz is
generally fairly abundant in the groundmass either In granophyric intergrowths or as numerous individual grains• The feldspars include orthoclase, sanidine, and plagloclase ranging In composition from albite (A114) to sodlc oligo-
97
.
clase (An-^g),-. .The relative amounts of the feldspar pheno crysts are quite variable, as one dike on the west side of upper Bill Greek valley carries, .only plagloclase, whereas another nearby contains only orthoclase.
Some plagloclase
Is untwinned, but most of it shows either albite or Carls bad twinning.
Ferromagnesian minerals are nearly absent
except for a few flakes of biotite.
Xn several of these
rhyolites both the feldspar phenocrysts and much of the groundmass have been replaced by sericlte and calcite, and biotite has been partly replaced by chlorite, magnetite, and iron oxides.
The groundmass is usually very finely crys
talline and nowhere contains more than a small amount of glass.
The most common texture is a very fine-grained
granophyric intergrowth especially characteristic of the of thick group/dikes mentioned above. This intergrowth locally is spherulitio, but in moai, places presents a feathery con fused appearance (fig. 36).
One rhyolite shows a weakly
developed flow structure. Basalt A dike about 5 feet wide of dense black basalt crops out on the high ridge between Bowen Mountain and the 12,&00 foot peak 2,000 feet east of it.
The scattered phenocrysts
are principally small crystals of olivine which are locally almost completely replaced by a confused aggregate of fine grained magnetite and carbonate, probably calcite.
Basic
labradorite occurs mainly as numerous microlites, and also as a few phenocrysts little larger than the microlites. The texture of the groundmass is hyalopilitic.
Sericite
Figure 36.
Granophyrlc Intergrowths in groundmass of rhyolite. The slightly aerloltlsed phenocryst is sodic ollgoclase. Crossed nieols. X 162.
Figure 37.
Pilotaxitic structure in groundmass of basalt. The phenocrysts and microlites are medium labra dorlte; the scattered dark grains are lddlngslte (?). Crossed nieols. X 89.
98
.
and calcite have abundantly developed on labradorlte.
The
rock contains many fragments of foreign material such as large grains of orthoclase and irregular small fragments of quartz, microcline, aplite, and another type of basalt with closely packed microlites of sodlc labradorlte. EXTRUSIVE ROCKS Distribution and age The extrusive rocks, which Include rather thin rhyo lite and basalt flows, occur on the north side of south Supply Creek in sections 27, 28, 33, and 34, T. 4 N. , R. 76 W.
The basal flows, which are basalt, overlie large
outcrops of latite porphyry.
Field relations afford little
evidence of their age, other than that they are younger than the latite porphyry intrusive which had been exposed before the flows were formed.
The flows may be Miocene In
view of the widespread flows of this age In nearby regions. Thus, near Granby, a few miles south, Lovering (1930, p. 73) has described andeslte and basalt flows interbedded with Arikaree (Lower Miocene) sediments, and in eastern Horth Park Gorton (1941, p. 43) states that flows of several types are Interbedded with and rest on sediments of the Miocene Horth Park formation. Rhyolite A flow of rhyolite not over 50 feet thick, interlayered between basalt flows, is poorly exposed In sections 27 and 28, T. 4 N., R. 76 W.
This is a white, somewhat porous rook
with flow structure, which on weathering gives a pitted sur face.
It contains numerous phenocrysts, none over 3 milll-
99
*
meters long, which. In order of abundance are sodlc oligoclase
* orthoclase, quartz, and biotite.
A few
plagioclase crystals are zoned and show distinct growth lines.
Magnetite has completely replaced some biotite.
A
few small crystals of apatite and inclusions of basalt are present.
The groundmass Is mainly glass but contains numer
ous longulites, a few orthoclase microlites, and some lens like stringers of cryptocrystalline material, all of which are arranged in flow structures. Basalt Dark brown vesicular, scoriaceous, and amygdaloidal basalt flows form the bulk of the extrusives in South Sup ply Creek valley.
The rock, which shows pilotaxitic tex
ture (fig. 37), consists almost entirely of microlites, a few small phenocrysts of medium labradorite and a reddishbrown mineral closely resembling iddingsite.
Some iddings-
ite (?) has crystal outlines like olivine, suggesting that the latter has been completely replaced, but a few long blade-like crystals bear little resemblance to olivine forms.
The microlites range in size from exceedingly small
laths to some large enough to clearly show the albite twin ning.
A few aggregates of magnetite and chlorite occur
throughout the groundmass.
Long thin needles and shorter
prisms of light brownish, somewhat pleochroic apatite are sparingly scattered throughout the rock.
The color is
caused by innumerable tiny inclusions which are not uni formly distributed throughout some of the crystals.
Thus
the ends of long prisms may be clear whereas the rest of the
100. crystal is colored.
The absorption is strongest when the
£ axis is parallel to the plane of* vibration of* the lower nlcol.
The amygdules of some flows are composed of calcite,
one grain of which shows a biaxial figure with a very small optic angle.
SEDIMENTARY ROCKS PIERRE SHALE', (CRETACEOUS) Distribution and stratigraphic relations The Pierre shale crops out or occurs beneath a few Inches of soil at several localities In the western part of the map area, but nowhere more than 200 feet of section Is exposed.
The two best exposures are both along Willow
Creek near Lake Solitaire (Lost Lake).
Both contacts of
the Pierre are covered, but in other parts of central Colorado the formation Is conformable with the underlying Niobrara'limestone and shale.
The upper contact Is dis
cussed in the section on the Middle Park formation. Petrology Throughout most of central Colorado the Pierre Is predominantly gray shale with a few thin beds of sandstone and limestone.
Most of the smaller exposures In the southern
Never Summer Mountains are dark gray to black, non-calcareous, fiddile, clay shales that weather into fine flakes which form a thin mantle over the outcrop.
The following
two partial sections illustrate the petrology of both the shale and limestone members of the Pierre. Partial section of the Pierre shale along Willow Creek In S/2 S/2 sec. lb, T. 4 N., R. 77 W. about 2,500 feet northwest of Lake Solitaire (Lost Lake) from top to bottoms 101.
102. Bed n o .
Feet
Covered 6.
5.
4.
3.
Shale, dark gray, fossiliferous; in beds £ in. thick with a few sandstone layers about 6 in. thick. Pterla nebrascana Whitfield
10
Sandstone, shaly, light brown. Weathers some what like shale into dark brown to black, limonite-stained fragments 1 to 2 in. long.
3
Sandstone, very light brown, fine-rgrained. Con tains carbonaceous seams and lenses ranging from a knife edge to -J- in. thick and much dis seminated carbonaceous material* Weathers nearly black in places; some limonite stains. Upper 12 feet in beds 1 to 6 in., blocky; con tains calcareous crusts between beds. Lower 6 feet in beds 1 to 2 in.
18
Sandstone, dark gray, fine-grained fossiliferous; in beds about 2 in. thick. Contains carbon aceous material disseminated and in lenses. Weathers into blocky fragments. Fossils are limonitic. Pterla nebrascana Whitfield.
4
2.
Sandstone, very carbonaceous, non-calcareous, light brown, fine-grained; in beds 1 to 4 in. thick. Numerous limonitic fossils. Carbon aceous material in lenses and thin stringers. Occasional thin calcareous crusts between beds possibly from leaching of shells. Weathers slabby and in red-brown streaks. Pteria nebras cana Whitfield. 16
1.
Sandstone, calcareous, almost quartzitic, bluegray, fine-grained, dense, hard, fossiliferous, in beds 2 to 3 feet thick. Weathers into sharp, angular fragments; weathered surface to depth of -J- in. light brown and softer. Some thin beds composed almost entirely of fossils. Interbedded with abundant light-brown softer sandstone, weathers irregularly and less sharply, fewer fossils, in beds in. to 1 foot; contains abun dant calcareous crusts and some thin shale strin gers. Camptonectes n. sp., Ostrea congeata Con rad, Liopistha undata Meek and Hayden, Pteria nebrascana Whitfield, Pteria sp., Baculltes compressus Say, Belemnitella sp. Covered
103
.
Partial section of the Pierre shale along east fork of upper Willow Creek in SE/4 SE/4 SE/4 sec. 13, T. 4 N., R. 77 W. about 800 feet north of Lake Solitaire (Lost Lake) from top to bottoms Bed no.
Feet
Covered 3.
Shale, calcareous, dark gray to nearly black, some what fissile, fairly well consolidated, in beds in. to 1 in. thick. 7/eathers into fragments J- to 2 in. long, light gray to brown when dry, darker brown when wet. Hydrochloric acid effer vesces, producing a. green scum. A few beds 2 to1 3 in. thick thick are harder. 10
2.
Sandstone, calcareous, light gray with shale par tings 1 to 2 in. thick, and thin conspicuous carbonaceous films and partings. Weathers into angular faces and in relief in contrast to shale. 36
1.
Shale, same as is Bed no. 3. Contains a 4 in. limestone bed 5 feet from top, weathers in moderate relief.
119 165
The first section probably occurs about 1,000 feet below the top of the formation, but as no outcrops are present between this one and the nearest lower Middle Park volcanics about 1,700 feet to the west, the exact position is indetermimable.
Further, the axis of a fold
lies between these outcrops.
The second section appears
to be in about the same position in the formation accor ding to the structural interpretation (pi. 2 ). Correlation The basis for correlation of these 3 hale with the Pierre Is the similarity of fauna coupled with lithologic
104
resemblance.
.
The table on the following pages shows the
ranges and distribution of the fossils collected from the first section and the sources of this Information.
A
small outcrop of shale In SE/4 sec. 35, T. 5 N., R. 77 W., adjacent to the thrust, also contains Liopiatha undata, which occurs in the above section, and another specimen was found In sandstone from the float about 2,000 feet north of this outcrop.
All of these fossils are conclu
sively Upper Cretaceous and of the four identified by species, all have been found in the Pierre and at least two In the Fox Hills.
The two Identified by genus only
resemble species found also In the Pierre and Fox Hills, although the new species of Camptonecte3 resembles a Benton form.
Reference of this fossiliferous horizon to
the Pierre rather than Fox Hills is suggested by the occur rence of Lioplstha undata, Baculites compressus, Pterla nebrascana, and Ostrea s p . 700 to 1,000 feet below the top of the Pierre at a locality (no. 7) noted by Beekly (1915, p. 47) about thirty miles to the northwest In North Park. Lithologically, the relatively thick sandstones: of the first section somewhat resemble some fossiliferous sandstones of comparable thickness in the upper Pierre sections measured by Richards'(1941) and Loverlng (1934, pp. 12,13) In western and extreme southern Middle Park. These sections were tentatively correlated by Qrlffitts (1941) with the Pierre of eastern Colorado and Kansas, which has been divided into zones based bh:lithology and
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Ov vl Paleocene age. Thick andesltic volcanics were extruded along the borders of the central Front Range, Including this area, in early Middle Park time, and continued to accumulate in the range itself throughout much of the period of Middle Park deposi
138. tion.
This andesitlc mantle covered the pre-Cambrian core
of the range and prevented the accumulation of pre-Cambrian detritus in the lower part of the formation.
Toward North
Park and also In western Middle Park along the east slope of the Park Range, the lower Middle Park becomes much more shaly and contains coal seams.
Beekly (1915, p. 51) found
the coal-bearing shales and sands in the lower Middle Park at the easternmost outcrops near the edge of the Medicine Bow Mountains.
This suggests that the thick Pierre shale
was still exposed on the northern Front Range and the Park Range at the beginning of Middle Park time and was reworked into the lower Middle Park shales.
Both Richards (1941,
p. 21) in the Kremmling area and Beekly (1915, p. 51) in North Park note the resemblance of these shales to the Pierre. Loverlng (1929, p. 93) writes that these "changes in charac ter of the sediments suggest that coarse alluvial deposits at the base of the arching highland graded out Into swampy stretches on the level plains a few miles away."
The
Pierre was certainly removed farther south on the Front Range by lower Middle Park time because the first detritus obtained, after the andesitlc mantle was breached, wqs preCambrian. The most Intense phase of the orogeny followed the deposition of the Middle Park and at first strong, resistant elastics were cast into sharp folds.
Compression was even
tually relieved by thrusting, during which the pre-Cambrian rocks were pushed at least seven miles over the sediments.
139
.
The final phase of the revolution in this region Is believed to be the Intrusion of small stocks, dikes and sills of rhyolite porphyry and hypabyssal rocks, and the formation of the steep faults.
These Intrusions can not be dated
more accurately than post-thrusting and pre-Miocene, but they probably belong to the widespread series of Eocene In trusives which marked the waning stages of the revolution (Loverlng and Goddard, 1938a, pp. 48-50).
Intrusives of
probable Eocene age occur in the northern Never Summer Mountains (Gorton, 1941, p. 36).
The steep faults were
probably formed both during and after the intrusion of the above igneous rocks. The next recorded event Is the deposition of the con glomerates on Blue Ridge, which apparently are part of a group of alluvial fans formed during rejuvenation of the Front Range In the Miocene.
Similar deposits are exposed
near Granby and also in the North Park formation west of the northern Never Summer Mountains.
This rejuvenation began
the cycle of erosion which formed the Rocky Mountain pene plain, probably of Pliocene age, which Is believed to lie near an elevation of 11,500 feet in this area.
Rejuvenation
was accompanied by vulcanism, for lava flows are lnterbedded in the conglomerates near
Granby (Loverlng, 1930, p. 74).
and In the North Park formation (Gorton, 1941, p. 4a). Flows that cap the pre-Cambrian along South Supply Creek probably belong to this volcanic period.
Since the Supply
Creek lavas dip gently to the north, It Is concluded that the>Never Summer Mountains were tilted slightly after the
140. extrusion of the flows. Early Pleistocene glaciation ismarked by many high level tills and high shallow valleys In various parts of the Front Range and northern Never Summer Mountains, but I3 represented in the southern Never Summers only by a doubtful remnant of a high valley on the side of Baker Gulch.
During the following Interglacial stage and cer
tainly during the Wisconsin glaciation the early valleys were lowered as much as 1,000 feet.
The deposits and
eroslonal features left by the Wisconsin glaciers, which filled these deepened valleys, are obvious and make up most of the scenic elements of the landscape.
In Recent times
erosion has continued rather strongly as shown by the coarse gravels and fills in the valleys of the major streams •
ECONOMIC GEOLOGY WOLVERINE MINE Although a number of small prospect holes have been sunk In the area mapped, only one mine, the Wolverine, is known to have been operated.
It is on the south side
of Bowen Mountain at an altitude of about 10,400 feet and is within a half mile of an old wagon road up Bowen Gulch.
It Is presumably included In the Grand Lake (Wol
verine) district, which was Indefinitely located by Vanderwilt and Fischer (1947, pp. 96-97, pi.4) and Henderson (1926, p. 122, pi. 1) In Rocky Mountain Park near the west border. Local residents report the mine was worked for lead and silver as early as the eighties, was abandoned for many years, arid was reopened shortly before World War II. Lindgren, writing in the 1909 Mineral Resources (p. 201), refers to the "
old Wolverine and other silver-lead
deposits, located 12 miles northwest of Grand Lake ---"• It was abandoned at the time of the writer's visit in 1946, and only one level and sublevel (fig. 40) were accessible. The only available references to production were found in Mineral Resources (19CQ) and Minerals Yearbooks for 1939, 1940, and 1941.
In 1908 the Grand Lake district produced
1,561 pounds of copper, according to Mineral Resources. The Minerals Yearbook for 1941 (p. 275) states that "small lots of silver-lead ore were shipped to the Leadville smel ter from the Wolverine about 22 miles north of Granby". 141.
.,-y W
True
.'V
;v:
North
v ' * ;•1 ■ ■
i4
\ w '
No, I Level el. 10400'
\
I '' '
wimejb' . • \ jTftar W , } / Al - ,..7 tf^mrrmabz, ,
CperseiStrongly injected : ' biotite gneiss /m
W
Mainkvei
m , barren qtz. 75'
’ a vein, bar mw 45' q t i//lit t le p y r it e
injected tiot
a
, u 2nd level 50' above mein level diSSemir levels inaccessible IKi55^r~3 ■ V v ein . . : ?vein ypyrite'&ore Disseminated i" ■ \ 2 qtz,calote, ora \ barite ■' ' '
barren q ti
' i ,
PLAN OF WOLVERINE MINE BOWEN MOUNTAIN
EXPLANATION
,
■ ': % //
Barren vein , I io j2 in wide '•^ v Barren vein ,more than 12 in, wide :■i. ■/ Mineralized vein w ip h ile s s ify ri/ V'1..4 I in, o f ore, .J
—
Scale
Ll.llllL. 40 20 0
.
:v,
, ., .
40 ft.
m -J1
Mineralized v e in /b o le in io fi^ i Downthrown aide o f fault ] Strike and .dip ■ X ' Timbered d rift
. /.
, ,
Compass and papa skated mh
i 1,.'
.
142. The Yearbook for 1940 (p. 299) notes that 28 tons of silver-lead-gold ore was shipped from the Wolverine group, and the one for 1939 states that the Wolverine Syndicate shipped 25 tons of lead-silver-gold ore from the Wolverine group which was operated one month In 1939. The ore deposit Is a lead-zinc vein that strikes generally north and dips 40 to 45 degrees west.
The vein
carries macroscopic amounts of pyrite, chalcopyrite, galena, and sphalerite in a gangue of quartz, calcite, and barite. In addition the microscope discloses tetrahedrite and arsenopyrite.
Some of the ore is open-textured, and vugs
lined with crystals of gangue minerals are not uncommon. No evidence of supergene enrichment is present. A paragenetic study of the ore from the accessible small pockets and from the dump suggests the following sequence of deposition; pyrite, arsenopyrite, quartz, tetrahedrite, sphalerite and chalcopyrite, galena, and late quartz.
As samples could not be obtained from the
bulk of the ore In the upper Inaccessible workings, this sequence must be considered tentative.
Pyrite, the most
abundant sulphide, occurs as irregular grains, aggregates, and subhedral crystals, all of which have been brecciated locally and filled by quartz.
Pyrite is also disseminated
in the country rock as much as a foot from the veins. senopyrite Is generally associated with pyrite.
Ar
It cuts
pyrite and completely encloses "Islands" of pyrite separated from the"mainland".
Lath-shaped and diamond-shaped crys
tals of arsenopyrite are common.
Early quartz occurs as
143. veinlets cutting across pyrite and arsenopyrite and as euhedral crystals which In turn are corroded and embayed by the other ore minerals, especially galena.
Late quartz,
which is less abundant than early quartz, cuts galena as veinlets.
Tetrahedrite is Inconspicuous and 13 found prin
cipally as small grains enclosed and corroded by galena, sphalerite, and chalcopyrite.
Simultaneous deposition of
chalcopyrite and sphalerite is suggested by Innumerable tiny blebs of chalcopyrite scattered without definite arrangement in sphalerite.
Galena is the most abundknt
ore mineral and embays and corrodes all other sulphides. The silver reported In the ore probably Is carried by galena, inasmuch as specific silver minerals are not present in the ore examined. A few unusual botryoidal nodules were found In the dump.
These were composed of an outermost layer of barite
about -jjf Inch thick, a band of the same thickness of color less quartz, and an Inner band of gray quartz and pyrite about a core of sericitized country rock.
These nodules
were probably deposited within a large brecclated zone with much open space. The country rock Is biotite gneiss, which In most places has been Injected by pegmatite.
Immediately ad
jacent to the vein the wall rock has been slightly slllcified, but chiefly It has been strongly sericitized and calcified.
Except for quartz, all the minerals of the
gneiss have been partly or completely replaced by a con
144 • fused aggregate of fine-grained sericite and calcite. Biotite, in particular, has been almost completely replaced by sericite.
Much feldspar has been entirely replaced,
but some mlcroclino has been only mildly sericitized along the cleavages.
Unoriented blades of muscovite have formed
locally in the above aggregates.
In addition to its occur
rence with sericite, calcite has also been introduced into the wall rock in minute veinlets.
Thin sections of the
country rock 3 feet from the vein show no appreciable dim inution of the alteration. The width of the vein varies from 2 inches to 2^ feet, but in few places is more than 2 inches of solid ore ex posed.
The vein is barren of sulphides throughout much of
its length and so are the two intersecting veins developed by a crosscut and a branch drift. The features controlling ore deposition are not evi dent from these small workings.
The abrupt thickening of
the vein where it changes strike near the winze might be explained by filling of open space created by horizontal movement along a premineral fault, the hanging wall of which moved south. In the Front Range mineral belt, veins of this type, formed during the main period of metallization, are ?shown by Lovering and Goddard (1938, p. 50, fig. 2) and others to have followed the intrusion of a group of Eocene (?) acidic porphyries and related rocks.
Thus, at Jamestown,
one of the nearest large mining districts, Goddard (1936,
p. 103) writes that fluorspar, lead-silver, and low-grade pyrltic gold ores are later than and bear a close genetic relationship to an Eocene (?) sodic granite stock.
In the
Montezuma quadrangle Lovering (1935, p. 60) states that the Montezuma quartz monzonite stock was the center of mineralization for many lead-zlnc-sliver veins.
In the
southern Never Summer Mountains small Eocene (?) plugs, chiefly of rhyolite porphyry, crop out near the Porphyry Peaks 5 miles south of the mine, and small rhyolite and trachyte dikes and sills are exposed near the mine, but none of these show a demonstrable genetic relationship to the veins.
The nearest known Eocene (?) intrusive more
closely resembling the mineral belt intrusives is the Lake Agnes quartz monzonite stock mapped by Gorton 6 miles north.
This stock is.rather far from the mine to be a cen
ter of mineralization* but closer similar intrusives or extensions of the stock b&low the surface may occur In the unmapped area between thq stock and mine.
Also, the
deposits near old Teller City, 7 miles northwest of the mine, suggest that the center of mineralization lay.to the north rather than to the south near the Porphyry Peaks. The association of galena,sphalerite, and tetrahedrlte in a quartz, carbonate, and barite gangue, coupled with the open texture of part of the veins suggest that It is mesothermal.
Further, many similar deposits in the
mineral belt are believed by LIndgren (1933, p. 594) to be mesothermal.
;
BIBLIOGRAPHY Adams, P* D. (1909) On the origin of the amphibolites of the Laurentlan area of Canadas
Jour, Geology, vol. 17,
pp. 1-18. Adler, Joseph (1931) Geologic relations of the Coal Creek quartzite In Colorado, Univ. of Chicago doctoral thesis Atwood,WiWVrand Atwood, W. W. Jr. (1938) Working hypothesis for the physiographic history of the Rooky Mountain region:
Geol. Soc. America Bull., vol. 49, pp. 957-980
Ball, S. H. (1908) General geology of the Georgetown quad rangle, Colorado.
In: Geology of the Georgetown quad
rangle, by J. E. Spurr, G. H. Garrey, and S. H. Ball: U. S. Geol. Survey Prof. Paper 63. Bastin,
E. S. (1909) Chemioal composition as a criterion
in identifying metamorphosed sediments: Jour. Geology, vol. 17, pp. 445-472. , and Hill, J. M. (1917) Economic geology of Gilpin County and adjacent parts of Clear Creek and Boulder Counties, Colorado: U. S. Geol. Survey Prof• Paper 94. Beekly,
A. L. (1915) Geology and coal resources of North
Park, Colorado: U. S. Geol. Survey Bull. 596. Behre, C. H. Jr. (1939) Preliminary geological report on the west slope of the Mosquito Range in the vicinity of Leadville, Colorado:
Colorado Sci. Soc. Proc.,
“
vol. 14, no. 2, pp. 50-79. Boos, M. P. (1924) General features of pre-Cambrian struc ture along the Big Thompson River valley in-Colorado; Jour. Geology, vol. 32, no. 1, pp. 49-63.
147. , and Aberdeen, E. J. (1940) Granites of the Front Range, Colorado; the Indian Creek plutons: Geol. Soc. America Bull., vol. 51, no. 5, pp. 695-730. , and Boos, C. M. (1934) Granites of the Front Range the Longs Peak - St. Vrain batholith; Geol.Soc. Amer ica Bull., vol. 45, no. 2, pp. 303-322. Brown, R. W. (1943) Cretaceous - Tertiary boundary in the Denver basin, Colorado: Geol. Soc. America Bull., vol. 54, pp. 65-86. Buckwalter, T. V. (1946) Geology of a part of the Dillon quadrangle, Colorado, Univ. Michigan master*s thesis. Buddington, A. F. (,\9o9) Adirondack igneous rocks and their metamorphism: Geol.Soc. America Memoir 7. Burbank, ¥/. S. (1932) Geology, and ore deposits of the Bonanza mining district, Colorado: U. S. Geol. Survey Prof. Paper 169. , and Goddard, E. N. (1937) Thrusting In Huerfano Park, Colorado, and related problems of orogeny In the Sangre de Cristo Mountains: Geol. Soc. America Bull., vol. 48, no. 7, pp. 931-976. Butler, B. S. and Vanderwilt, J. W. (1930) The Climax mblybdenum deposit of Colorado: Colorado Sci. Soc. Proc., vol. 12, no. 19, pp. 309-353. Clarke, F. W. (1924) the data of geochemistry, (fifth edi tion): U. S. Geol. Survey, Bull. 770. Cloos, Hans (1948) Granltizatlon and the structural behavior of Igneous rocks, Lecture at Univ., Michigan, Nov. 23, 1948.
148
.
Crawford, R. D. (1910) Preliminary report on the geology of Monarch mining district, Chaffee Co., Colorado: Colorado Geol. Survey Bull. 1. , (1913) Geology and ore deposits of the Monarch?and Tomichi districts, Colorado: Colorado Geol. Survey Bull. 4. —
, (1916) Geology and ore deposits of the Gold Brick dis trict, Gunnison Co.: Colorado Geol.Survey Bull. 10 , (1924) Geology and ore deposits of the Redcliff mining district: Colorado Geol. Survey Bull. 30.
Cross, Whitman (1892) Post-Laramie deposits of Colorado: Am. Jour. Sci., 3rd ser., vol. 44, pp. 19-42. -■— , (1894) Description of the Pikes Peak sheet, Colorado: U. S. Geol. Survey, Geol. Atlas, Pikes Peak folio (no. 7). , (1896) Geology of Silver Cliff and the Rosita Hills, Colorado: U. S. Geol. Surv. 17th Ann. Rept., pt. 2; pp. 263-403. Eardley, A. J. (1949) Paleoteotonlo and paleogeologic maps of central and western North America: Am. Assoc. Pet roleum Geologists Bull., vol. 33, no. 5, pp. 655-682. Emmons, S. F. (1886) Geology and mining industry of Leadville, Colorado: U. S. Geol, Survey Monograph 12. —
, (1898) Description of the Tenmile district'quadrangle, Colorado: U. S. Geol. Surveyj Geologic Atlas, Tenmile fdlio (no. 48).
Fenneman, N. M. (19ol)
Physiography'
McGraw-Hill Book Co.
of western United States,
Finlay, G. I. (1916) Description of* the Colorado Springs quadrangle, Colorado: U. S. Geol. Survey, Geologic Atla3, Colorado Springs folio (no. 203). Fischer, R. P. (1947) Metallic mineral deposits of Colorado, plate 4.
In: Mineral resources of Colorado, Colorado
Mineral Resources Board. Gazin, C. L. (1941) Paleocene mammals from the Denver basin, Colorado: Jour. Washington Acad. Sci., vol. 31, pp. 289-285. Geologic Map of Colorado (1935) U. S. Geol. Survey in coop eration
with Colorado State Geol. Board and Colorado
Met. Mining Fund. George, R. D. (1908) The main tungsten area, Boulder County, Colorado: Colorado Geol. Survey, First Report, pp. 7-103 Goddard, E. N., (1936) The geology and ore deposits of the Tlncup mining district, Gunnison County, Colorado: Colorado Sci. Soc. Proc., vol. 13, no. 10, pp. 551-595. —
, (1936) Geology and ore deposits of the Jamestown dis trict, Boulder County, Colorado, Univ. Michigan doc toral thesis•
—
(1940) Preliminary report on the Gold Hill mining dis trict, Boulder County, Colorado: Colorado Sci. Soc. Proc., vol. 14, no. 4, pp. 103-139.
Gorton, Km A. (1941) Geology of the Cameron Pass area, Grand, Jackson, and Larimer Counties, Colorado, Univ. Michigan doctoral thesis. Griffitts, M. 0. (1941) Fauna of the Pierre shale In the Kremmling area, Colorado, Univ. Michigan master*s. thesis -— , (1949) Zones of Pierre formation of Colorado: Am.
150. Assoc. Petroleum Geologists Bull., vol. 33, no. 12, pp. 2011-2028. Grout, F. F. (1932) Petrography and petrology, McGraw-Hill Book Co. New York. , Worcester, P. G., and Henderson, J. (1913) Recon naissance of the Rabbit Ears region, Routt, Grand, and Jackson Counties, Colorado: Colorado Geol. Survey Bull. 5, pt. 1• Gunther, C. G. (1906) The gold deposits of Plomo, San Luis Park, Colorado: Econ. Geol., vol. 1, pp. 143-154. Harker, Alfred (1939) Metamorphism (second edition), Methuen & Co., London. Heinrich, E. Wm. (1946) Studies in the mica group; the biotite-phlogopite series: Am. Jour. Sci., 5th ser., no. 12, pp. 836-849. , (1948) Pegmatites of Eight Mile Park, Fremont County, Colorado: Am. Mineralogist, vol. 33, no. 7, pp. 420-448. Henderson, C. W. (1926) Mining in Colorado: a history of discovery, development, and production: U. S. Geol. Survey, Prof. Paper 138. Henderson, Junius (1913) Section on stratigraphy in Recon naissance of the geology of the Rabbit Ears region by Grout, F. F., Worcester, P. G., and Henderson, Junius; Colorado Geol. Survey, Bull. 5, part 1, pp. 24-42. Hills, R. 0. (1900) Description of the Walsenburg quadrangle, Colorado: U. S. Geol. Survey, Geologic Atlas, Walseni
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Plate I
E XP LAN A TIO N
| r tIBW^ \' n Od 1
l o t i
i1 J
AIIuhvt
0
Poet
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mt
W «(0l Sk'Hi, Rilkj Mcuntjm NHwol Port »5nl
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1946-41
P late. 2
A
A’ C o lo ra d o
R iv e r
Qlm Volley
•
• / /'
Qqrri
/ //"~l
pCbg ,
1rF
12000
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pC p
on ^ ' / / B o w e n G u l ch
10000
\
\ \\ Y ' '
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Q lm
pCbg
\A
10000
12 0 0 0
10000
BOOO
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Blue W il lo w
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Ridge
SfcKp
Qal hTmp
GEOLOGIC
STRUCTURE N EVE R
SECTIO NS
OF THE
SOUTHERN
SU MMER M O U N T A IN S ,
COLORADO
Qa l, A l l u v i u m ; Q r l , r o c k g t o d t r s or id l o t u s ; Q l m , l o t e r a l
m araine; Qgm , ground m o ra in e ; Tc. conglom erale; T r , rh y o lite ;
h T m p , M id d le P o r k fo rm a tio n ; K 7 v , M id d l e P o rk v o l c a n i c s ; p C h g , h o r n b ’tn de gn eiss;
PART OF TH E
K p , Pierre
shale, p C p , p e g m c tite
p C ig , Id ah o S p r i n g s i n je c tio n g n e i s s ; p C i s ,
o n d ap lite ; p £ & 9 . b i o t i l e gn eis s;
Id ah o S p r i n g s schist.
Plate 3
-o
o
c—r
m
cr
O
IO
20
30
M a p of c e n t r a l Colorado showinq area I e x t e n t of the Idaho S pr inas formation and location of fhe w r i t e r ' s a rea (stippled)