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Graduate Student Theses, Dissertations, & Professional Papers Graduate School
1979
Petrology and origin of the Camp Creek corundum deposit southwest Ruby Range Montana
Eric R. Haartz The University of Montana
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Recommended Citation Haartz, Eric R., "Petrology and origin of the Camp Creek corundum deposit southwest Ruby Range Montana" (1979). Graduate Student Theses, Dissertations, & Professional Papers. 7124. https://scholarworks.umt.edu/etd/7124
This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. PETROLOGY AND ORIGIN OF THE CAMP CREEK
CORUNDUM DEPOSIT, SOUTHWEST
RUBY RANGE, MONTANA
by
Eric R. Haartz
B.A., New England College, 1976
Presented in partial fulfillment of the requirements for the degree of
Master of Science
UNIVERSITY OF MONTANA
1979
Approved by:
Chairman, Board of Examiners
Dean, Graduate Softool
Date UMl Number: EP37925
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ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 ABSTRACT
Haartz, Eric R., M.S., Winter, 1979 Geology
The Petrology and Origin of the Camp Creek Corundum Deposit, Southwest Ruby Range, Montana u (60 pp.)
Director: David Alt
The Camp Creek corundum deposit occurs on a ridge crest in the WSW corner of the Ruby Range, about one kilometer west of the Crystal Graphite Mine. Pre-Beltian rocks, mostly of the metasedimentary and metavolcanic Cherry Creek group underlie the area and have a depositional age probably exceeding 2.5 billion years. The rocks at the study area underwent at least one episode of high-grade regional metamorphism. Maximum pressure/temperature conditions varied be tween the uppermost amphibolite facies and lower granulite facies in a system undersaturated with respect to water. A subsequent retro grade metamorphism in the greenschist facies, accompanied by the introduction of water, partially altered the high-grade rocks. Foliation dominates the structure of the area. It strikes northeasterly and dips moderately NW. Two sets of folds deform the foliation, but not extensively. The sequence at the study area consists of the following lithologles in order from structurally lowest (SE) to highest (NW): 1) granitic gneiss with minor biotite; 2) biotite gneiss, locally garnetiferous; 3) marble, pure and impure; 4) corundum gneiss; 5) amphibolite; 6) marble; 7) hornblende gneiss. Detailed mapping on 1:400 scale shows that the corundum gneiss forms a discrete, localized layer, concordant with surrounding layers and foliation. The contacts between the gneiss and adjacent units are sharp. The unit probably underwent isochemical metamorphism except for possible boron metasomatism. Modal analyses and oxide estimations indicate compositions similar to those of kaolinitie to bauxitic clays. The corundum gneiss probably developed as a clay layer in a small topographic basin on a carbonate rock surface during an interval of nondeposition and subaerial exposure of Cherry Creek sediments.
11 ACKNOWLEDGMENTS
Special thanks go to Dr. Dave Alt and Dr. Jack Wehrenberg for their interest and guidance during the study. I also thank
James Bielak for suggesting the problem, and Dr. David Fountain and others for helpful discussions of various aspects of the study.
m TABLE OF CONTENTS
Page
ABSTRACT...... il
ACKNOWLEDGMENTS ...... i i i
LIST OF TABLES ...... vi
LIST OF ILLUSTRATIONS...... vii
CHAPTER
I. INTRODUCTION ...... 1
Location ...... 2
Access ...... 2
Previous W o r k ...... 2
Present W ork ...... 5
Economie Potential ...... 5
II. CHERRY CREEK GROUP AND OTHER ROCKS ...... 6
Granitic Gneiss ...... 6
Biotite Gneiss ...... 9
M arble ...... 10
Hornblende Gneiss ...... 13
Amphibolite ...... 14
Q u a r t z it e ...... 15
D ia b a s e ...... 15
Mafic and Ultramafic R o c k s ...... 16
IV CHAPTER Page
I I I . FIELD DESCRIPTION AND PETROGRAPHY OF THE CORUNDUM GNEISS ...... 18
IV. ORIGIN OF THE CORUNDUM GNEISS...... 27
V. METAMORPHISM...... 40
Prograde Metamorphism ...... 40
Retrograde Metamorphism ...... 43
VI. DEFORMATION...... 44
V II. SUMMARY AND GEOLOGIC HISTORY ...... 47
REFERENCES CITED ...... 50
APPENDIX...... 53 LIST OF TABLES
Table Page
1. Modal analyses of corundum rocks ...... 29
2. Petrographically estimated oxide compositions of corundum r o c k s ...... 31
3. Comparison of petrographically estimated oxide compositions of rocks from the Camp Creek corundum deposit to chemical analyses of corundum gneisses from the Madison Range ...... 32
4. Oxide comparison of corundum gneiss to aluminous clays ...... 33
VI LIST OF ILLUSTRATIONS
Plate Page
1. Geologic map of the Camp Creek corundum deposit: 1:400 s c a le ...... pocket
2. Geologic map of the Camp Creek corundum deposit and vicinity: 1:2500 s c a l e ...... pocket
Figure
1. Study area location map ...... 3
2. Map of study area showing topography ...... 4
3. Sketch of granulitic texture in granitic gneiss .... 8
4. Outcrop view of contorted compositional layering in m arble ...... 11
5. Photomicrograph of diabase ...... 17
6A. Block of plagioclase-corundum-biotite gneiss ...... 19
6B. Exposure of plagioclase-corundum-biotite gneiss in small prospect pit ...... 20
7. Photomicrograph of corundum crystal showing twinning . 22
8A. Photomicrograph of zoning in corundum crystal in plane light ...... 23
8B. Photomicrograph of zoning in corundum crystal in plane l i g h t ...... 24
9. Cross section through ridge at the study area ...... 37
10. Stability field for Cherry Creek rocks during upper amphibolite-lower granulite facies metamorphism . 42
11. Contact between hornblende gneiss and marble ...... 46
V I1 CHAPTER I
INTRODUCTION
A series of pre-Beltian crystalline rocks underlies most of the Ruby
Range of southwestern Montana. The major grain of these northwest-dipping rocks parallels the northeasterly trend of the range. Three groups com prise the sequence:
1) Pre-Cherry Creek gneisses and schists form the structurally
lowest group and underlie the southeast flank of the range:
2) An extensive granitic gneiss unit, the so-called "Dillon
Granite gneiss" underlies the crest of the range. While the
origin of the gneiss remains in question, field relationships
suggest that they are metasediments (Garihan and Williams 1976);
3) The structurally highest Cherry Creek group, outcropping on
the northwest flank of the range, consists of a variety of
metasediments and metavolcanics.
Chronology of the pre-Belt metamorphics remains inconclusive.
G iletti (1966) presents thirty-six metamorphic cooling dates, mostly from micas, which show three age clusters near 1.6 billion years (b .y .),
2.6 b.y. and 3.2 b.y. He concludes that at least the pre-Cherry Creek gneisses may exceed three billion years in age. King (1976, p. 48) suggests that both groups represent the same original terrane of
Precambrian W age (^ 2 .5 b.y.) with younger ages resulting from a
Precambrian X (Hudsonian) age metamorphic event. 1 The Camp Creek corundum deposit occurs within the metasediments and metavolcanics of the Cherry Creek group. Detailed mapping and pétrographie work shows that the corundum occurs in a discrete local layer of sedimentary origin later subjected to deformation and isochemical meta morphism (except for possible boron metasomatism). Modal analyses and oxide calculations reveal compositions that range from impure kaolin itie to bauxitic clays.
Location
The study area lies in the southeast quarter of Section 25 and the northeast quarter of Section 36 of T.8S., R.8W. at the southwest end of the Ruby Range, Beaverhead County Montana (Ashbough Canyon 7 1/2 mimute quadrangle) (Fig. 1). The corundum deposit straddles a northeast trending ridge at 2170 meters (7100 f t .) elevation, just 600 meters southwest of the Crystal Graphite mill site (Fig. 2).
Access
A rough but serviceable road provides access to the graphite mill from the Blacktail Valley. A recent bulldozer road extends to the corundum deposit. It is necessary to obtain trespass permission from local land owners.
Previous Work
Peale (1896) defined the Cherry Creek group in the Gravelly Range.
Although the Crystal Graphite mine and surroundings were studied in the R.9W.R. 8 W. R .7 W. R.6 W.
• DILLON
6 0 0 d
.2
SCAJ.E IN KM.
Figure 1. Location map of study area. 1 = Camp Creek corundum deposit surrounded by area of Crys talPlate 2. 2 = CrystalPlate Graphite Mine. R. 8 W. R.7W.
Sec. 3 0 Sec. 25
T 8 T X 8 GRAPHITE S MINE
Sec. 36 7 0 0 0
Sec. 31
I 1/2 o I I I I I I__ _L_ Scale in kilometers Figure 2. Map of study area showing topography. Larger outline is area of Plate 2 while the smaller, stippled area indicates the extent of Plate 1. Contour interval is 100 feet. Modified from U.S.G.S. Ashbough Canyon 7 1/2 minute quadrangle. early 1900's (Winchell 1914), the corundum deposit remained undiscovered until 1949 when Heinrich (1950) described and mapped its occurrence.
Heinrich (1960) mapped the pre-Beltian rocks of the Ruby Range including the study area and published a short article on the thulite in the deposit
(Heinrich 1964). Okuma (1971) examined the structure and petrology of the southwestern Ruby Range.
Present Work
During the summer of 1978 I remapped the corundum deposit on a scale of 1 :400 (Plate 1) and a surrounding area of about one square kilometer at 1:2500 (Plate 2) scale using an enlarged portion of the U.S.G.S. 7 1/2 minute map as a base for the la tte r. Sampling for pétrographie work was done in October, 1977. I examined eighty-seven thin sections and made point counts on slabs of eight representative samples of corundum gneiss for modal analyses and oxide calculations.
Economic Potential
The corundum deposit was in itia lly examined as a potential source of natural industrial abrasive but its small size and the availability of synthetic abrasives render i t uneconomical. Recently fu tile attempts have been made to extract gem quality corundum from the gneiss, however none exists. CHAPTER II
CHERRY CREEK GROUP AND OTHER ROCKS
A variety of lithologles comprise the Cherry Creek group. The major types include:
1) hornblende gneiss and amphibolite
2) marble and calc-silicate gneiss
3) sillimanite gneiss and schist
4) pyroxene gneiss and schist
5) biotite gneiss
6) mica schist
7) quartzite
8) banded iron formation.
The group also hosts an assortment of minor lithologies including the corundum gneiss. Garnet-bearing biotite gneiss, hornblende gneiss, marble and calc-silicate gneiss dominate the Cherry Creek lithologies in the study area. Granitic gneiss ("Dillon Granite gneiss"), diabase and ultramafic rocks also occur in the study area.
Granitic Gneiss
Granitic gneiss ("Dillon Granite gneiss" of Heinrich 1960; Okuma,
1971; Garihan 1973) dominates the sequence structurally beneath the corundum gneiss. It also occurs in small lenticular bodies within the hornblende gneiss to the northwest. It forms sporadic but prominent
6 outcrops which are commonly well foliated. Color varies from light gray to orange-brown depending on the content of mafic minerals. Two lithologies predominate: a) pink to orange quartzofeldspathic gneiss; b) biotite-quartz-feldspar gneiss. They correspond to Okuma's (1971) quartz-feldspar gneiss and biotite-feldspar-quartz gneiss respectively.
Thin (< 5mm) but prominent gneissic banding dominates the textures of the rocks.
Within the detailed study area the gneiss is represented by a white to light-gray, coarse-grained quartzofeldspathic gneiss. In outcrop, the large feldspar (usually orthoclase) blebs and strung-out quartz masses are distinctive. Because i t grades into biotite gneiss near the top, I did not map i t as a separate unit on the large-scale map (Plate 1).
Thin section study of the light gray gneiss reveals that it consists chiefly of orthoclase, plagioclase (An^g with variance between An^^ and
An^g) and quartz. Also present are biotite, garnet and, rarely, hornblende
Accessories include apatite, ru tile , zircon and iron and titanum oxides.
White mica, zoisite and penninite comprise the alteration minerals,
Microperthite has developed in the orthoclase as thin strings or, less commonly, as blebs.
On microscopic scale, quartz occurs as strung-out masses about one to five millimeters thick, wrapping around feldspar masses (Fig. 3). Ovoid porphyroblasts of orthoclase punctuate the rock, reaching four centimeters in the elongate direction and a thickness of two centimeters. Relatively few garnets exist in the granitic gneiss and do not exceed eight m illi meters in diameter. 8
SPAR
- p /c 7/K SPAR :V,-.
PLA6
Figure 3. Sketch of granulite texture in granite gneiss. K-spar (orthoclase) is stippled and plagioclase is unshaded. Surrounding rockmass consists of strung-out quartz, minor biotite and some minute feldspar grains. Approximately full size. Biotite Gneiss
Garnet-bearing biotite gneiss structurally underlies the corundum layer and adjacent marble. Its exposure consists of scattered small outcrops with very prominent foliation. Although dominantly gneissic, this unit commonly contains well developed schistosity. The color of the unit varies from black or dark purple on clean or fresh surfaces to light brown on some weathered surfaces. Porphyryblasts of feldspar and garnet dot the foliation surfaces, the feldspar knots imparting a mottled appearance
Quartz, orthoclase,, plagioclase (An^g to Ao^q ) and biotite dominate the rockmass while garnet, sillim anite and rarely hornblende and muscovite occur in minor amounts. The ubiquitous retrograde minerals white mica, zoisite (and some clinozoisite) and chlorite (as both clinochlore and penninite) also occur as minor phases. Apatite, iron and titanium oxides, sphene and zircon* comprise the accessory minerals. Orthoclase usually contains string microperthite but a few grains contain larger perthitic blebs. In one sample, these blebs show twofold alignment, probably de veloped along crystallographic directions.
Fracturing and S-shaped trains of inclusions in garnet and trains of white mica in rounded feldspar porphyroblasts attest to the rotation of these minerals. Since white mica commonly develops firs t along particular crystallographic planes (such as twinning planes in plagioclase), I infer that the trains of white mica developed along crystallographic planes of
*Zircon in the granitic gneiss, biotite gneiss and corundum gneiss is anhedral and shows no overgrowths. 10 porphyroblasts which grew during deformation and rotation. Garnets, where not badly corroded, contain poikiloblastic inclusions of quartz and feldspar, in some cases throughout the porphyroblasts, in others concen trated in the center. These features apparently developed from rotation of the porphyroblasts between fo lia during shear-deformation of the biotite gneiss.
Marble
Marbles of varied mineralogy form a prominent unit within the Cherry
Creek sequence. They occur adjacent to the corundum gneiss, both structurally above and below and are extensively interlayered with hornblende gneisses to the northwest of the study area. Although marble tends to form long bands of outcrop, its susceptibility to deformation resul ts in highly variable thicknesses (Fig. 4). Although it rarely forms prominent outcrops, the marble does form distinctive light-colored rubble and low outcrop, especially on ridge crests. The rubble consists of rather granular masses as much as several centimeters across. Well developed compositional banding probably represents original bedding (Heinrich, 1960; Okuma, 1971).
On the ridge northwest of the corundum deposit a sugary, fine-grained marble several tens of meters thick forms an obvious white band which also outcrops along the access road to the graphite mine and study area.
Brown, punky, talc-bearing marble forms the large body about 25 to
40 meters below the corundum gneiss. This unit contains abundant evidence of the extent of deformation in the marble. The rock forms extensive. 11
>*
\
, ^ > ÿ»
Figure 4. Outcrop view of contorted compositional layering in marble. Wrench is about 15 cm long. 12 rounded outcrop. As a result of weathering it is so rotten that samples suitable for thin section study cannot be obtained.
The marble adjacent to the corundum gneiss varies from carbonate to calc-silicate rock. In most of the thin sections I examined, calcite forms the dominant carbonate phase. Although not common in the Cherry
Creek marbles (Heinrich, 1960), the calcite occurs in medium to coarsely granular masses and rhombs which become three centimeters thick. In the calc-silicate rocks diopside forms the major silicate phase. Amphibole, usually tremolite, occurs in subordinate quantities. Quartz and plagio clase are subordinate phases in a few samples. Sphene and zircon comprise the most common accessories. One sample contains the remnants of garnet
(probably grossularite). Iron and iron-titanium oxides are common.
The marbles and calc-silicate rocks host a variety of alteration minerals. In addition to the ever-present white mica and epidote (mostly ziosite), the rocks contain at least two types of chlorite: penninite and clinochlore. Both chlorites may exist within one thin section.
Locally, talc forms an important alteration phase. Serpentine occurs in patches and veinlets, the la tte r developed only adjacent to the diabase dikes. According to Okuma (1971, p. 18) the serpentine represents contact alteration resulting from intrusion of the diabase dikes which may account for the veinlets but doesn't explain the patches. Some patches clearly have developed by alteration of diopside.
The well developed compositional layering seen in outcrops, and some times in hand sample, usually does not show in thin section. Carbonate 13 grains usually occur in an equigranular interlocking mass. Diopside and amphibole rarely impart a foliation on microscopic scale.
Hornblende Gneiss
Hornblende gneiss occurs extensively in the sequence structurally
above the corundum gneiss and dominates the northwest half of the 1:2500 map area. At the corundum deposit, i t forms a continuous unit underlying
the northwest flank of the ridge. It also occurs as lenticular bodies
throughout the granitic gneiss (while granitic gneiss occurs in some lenses
in areas mapped as hornblende gneiss). The dominant rock type consists
of stubby black crystals of hornblende, plagioclase (An^Q to An^o), biotite
and, rarely, quartz. Secondary minerals include chlorite, white mica,
epidote and calcite. Apatite, magnetite, sphene and zircon comprise the accessory minerals. In general the alteration to retrograde metamorphic minerals is not extensive.
Hornblende gneiss occurs as blocky to slabby outcrops. Exposure is
good on ridge crests and south-facing slopes where prominent outcrops
commonly exist. Although homogeneous and well foliated on outcrop scale,
these characteristics change on small scale (of less than a few centi meters), where pinch and swell and minor offset of folia developed.
Hornblende-rich and plagioclase-rich layers impart foliation, some of which contain abundant biotite. Locally the gneiss contains up to five percent garnet which is also concentrated in particular fo lia . The thick ness of hornblende gneiss bodies varies considerably but ranges to more than 100 meters. 14
Amphiboli te
Amphibolite lenses structurally overlie the corundum layer and nowhere exceed one meter in thickness. An amphibolite layer in the south west half of the study area grades into hornblende gneiss toward the southwest, before pinching out. The lenses over the corundum gneiss may
represent the same unit in spite of the discontinuity between them. The
amphibolite forms low but distinctive bands of well foliated black rock with parting parallel or subparallel to foliation. Grain size ranges
from fine to medium. Minerals recognizable in hand sample include black
hornblende, plagioclase, and calcite as fracture filling. Biotite is
visible locally. Hornblende dominates the rock (about 70% to 90%) while
subordinate plagioclase, biotite and calcite occur in varying amounts up
to ten percent. Anhedral zircon grains range from trace amounts to five
percent of the rockmass. Magnetite, apatite and quartz form the usual
accessory minerals. The ubiquitous retrograde minerals, white mica and
clinochlore are developed locally. The primary equilibrium minerals range
from sub- to anhedral with irregular to straight grain boundaries. Triple
junctions are common. Contacts of the amphibol i tes are sharp and con
cordant to surrounding foliation. Bielak (1978) discusses the amphibolites
of the Winnipeg Creek area in detail. 15
Quartzite
Quartzite crops out in low, discontinuous ribs in a lens-shaped body about 500 meters NNE of the study area. The rock is dark gray to gray- green and contains minor amounts of muscovite, magnetite and some felds par. Foliation may coincide with bedding surfaces.
Within the study area, near the northeast end of the corundum gneiss layer a small lens of quartzite occurs within biotite gneiss. It forms a ten centimeter thick rib of ground-level outcrop for several meters.
Total strike length of the unit is less than fifteen meters. In addition to quartz, the rock contains muscovite and feldspar. The mica imparts a foliation while the quartz gives the rock a translucent gray appearance.
The unit is too small to map.
Diabase
At least two diabase dikes cut across the ridge and corundum gneiss within the study area. Small branches splay off the main dikes and may anastomose between them but are too obscure to map. The rock weathers easily forming a rubble of angular to spherical blocks which rarely exceed ten centimeters in any dimension. A few fractured outcrops occur on the southeast flank of the ridge. The larger dike strikes N50°W and is vertical with a thickness not exceeding five meters. The smaller dike, about two meters thick, strikes N50°W and dips steeply northeast.
The brown-weathering diabase consists of plagioclase, augite and subordinate amounts of magnetite and calcite. Chlorite and white mica formed at the expense of augite and plagioclase respectively during 16 retrograde metamorphism. The fine to medium-grained rock displays an ophitic texture. Plagioclase varies from euhedral laths to anhedral grains. Augite grains are anhedral. Magnetite occurs as sub-to euhedral grains and as needles forming a prominent latticework in the rock in most of the samples examined (Fig. 5). Minerals present in minor or trace amounts include apatite, quartz and hematite.
Mafic and Ultramafic Rocks
Mafic and ultramafic rocks crop out in pod-like masses in several locations in the 1:2500 map area. They usually occur as black or dark brown, coarsely granular rocks lacking foliation. They were probably tectonically emplaced into the Cherry Creek metasedimentary-metavolcanic sequence during prograde metamorphism (Desmarais, 1978). 17
Figure 5, Photomicrograph of diabase (sample #4828) showing g rid -lik e network of magnetite needles. Plane lig h t. Field of view is 0.9 mm across. CHAPTER I I I
FIELD DESCRIPTION AND PETROGRAPHY OF THE CORUNDUM GNEISS
In spite of its resistance to weathering, the corundum gneiss forms only low broken outcrops. Large massive float blocks of the gneiss mark the area of exposure, especially along the fenceline marking the section boundary. Many float blocks have rolled down slope southeast of the outcrop area. In outcrop, the rock varies from massive to gneissic
(Fig. 6). Locally, large amounts of biotite impart schistosity. Al though the biotite-poor gneiss appears massive in hand sample and outcrop, close examination reveals poorly developed foliation due to compositional layering and orientation of corundum and minor biotite within the folia tion plane. Usually only larger float blocks and outcrops display the foliation. The color of the gneiss varies from milky white to lavender or brown (with abundant bio tite). On weathered surfaces, the rock may become yellow to brown. Where larger crystals occur, the gneiss is studded with the blue to gray corundum. The gneiss consists principally of plagioclase, bio tite, corundum, tourmaline, and associated retrograde metamorphic minerals.
The plagioclase is labradorite (An^y) with a range from An^^ to An^g and commonly shows albite twinning or, less commonly, combined albite- carlsbad twinning. The grains are usually equidimensional and range in size from 0.5mm to 1mm. Grain boundaries range from straight or gently curved to irregular. 18 19
rtZ a k '"* ■S>T'
Figure 6A. Block of plagioclase-corundum-biotlte gneiss with well developed corundum crystals. 20
f-' ."5L «
Figure 6B. Exposure of piagloclase-corundum-blotite gneiss in small prospect pit. Aluminum beer can for scale. 21
Biotite in the feldspar-rich rock type occurs in random and en echelon masses. In the corundum-biotite schist it assumes its normal habit with grains up to 0.5mm by 2mm in size. Pleochroism is: X = colorless or pale yellow; Y = orange or light brown; Z = brown or deep red.
Corundum occurs in abundanteuhedral crystals and, less comonly, as partially altered, subhedral grains. They are usually fractured and uncommonly contain inclusions of magnetite or alteration minerals. Al though barrel-shaped crystals predominate, long thin crystals exist locally
Many crystals are bent or otherwise deformed. Parting on [0001] is visible macroscopically but not in thin section while parting on [ l o T l ] shows up only under magnification. Many crystals also show twinning which usually parallels [1011] parting (Figure 7). Many corundum crystals contain wel1-developed zoning caused by the presence of brown, hair-like inclusions oriented in three directions parallel to prism faces (Fig. 8).
The inclusions, probably ru tile, impart a purple to reddish color to the otherwise blue corundum. Some crystals give a biaxial negative inter ference figure (2V < 10*) rather than the usual uniaxial negative figure.
Twinning of the crystal may cause the biaxial character (Deer and others
1962, Vol. 5, p. 14). A few crystals contain inky blue blotches or zones.
Tourmaline comprises a minor but prominent phase in the feldspar- rich corundum rock. It occurs as sporadic, large crystals and crystal masses (often tabular) 10 cm or more long and up to 2 cm thick. The crystals are usually sub-to anhedral with pleochroism: £' = pale green to yellow;-ur' = green to dark yellow. Cross fractures break the elongate prisms of tourmaline. 22
y * . LT
&
Figure 7, Photomicrograph of corundum crystal showing twinning. Crossed nichols. Field of view is 3.6 mm across. 23
r
t
Figure 8A. Photomicrograph of zoning in corundum crystal in plane lig h t. Plane of thin section is parallel to [0001]. Field of view is 3.6 mm across. 24
Figure 8B. Photomicrograph of zoning in corundum crystal in plane lig h t. Plane of thin section is normal to [0001]. Field of view is 2.25 mm across. 25
Several accessories exist in the corundum gneiss, some of which vary considerably in amount. Apatite forms a common but inconspicuous con stituent. Iron and iron-titanium oxides exist in all samples and varied amounts of rutile needles occur in close association with biotite in many samples. Subhedral sphene grains form a common accessory throughout most of the corundum-bearing u n it. In the thin portion of the corundum gneiss, toward the southwest end of the layer, sphene lo cally becomes a major phase. I t occurs as subhedral to euhedral, wedge-shaped crystals
1mm to 3mm long embayed and corroded by adjacent plagioclase and biotite. Zircon is a ubiquitous accessory throughout the corundum gneiss.
Where it occurs within biotite, an olive green pleochroic halo (due to radiation damage) commonly surrounds i t .
Along contacts between the corundum-biotite gneiss and the garnet- biotite gneiss, the mineralogy is gradational between the two over a few centimeters. Garnet occurs in the same rock with corundum in such cases.
Three samples contain minor amounts (<5%) of fine-grained sillim an ite which exists in patches or fo lia tio n planes as discrete grains or seg mented trains.
The corundum gneiss has suffered extensive alteration to a variety of hydrous minerals during retrograde metamorphism. Fine, scaly white mica (sericite), irregular grains and masses of zoisite (with some thulite) and small, irreg u lar patches of ca lc ite a ll formed from plagioclase.
Biotite altered to clinochlore (rarely to penninite), magnetite and leucoxene. Margarite formed at the expense of corundum. In thin section. 26 margarite is difficult to distinguish from white mica. Where present,
i t forms pronounced halos around corundum grains and has s lig h tly higher
r e lie f than the otherwise sim ilar white mica.
Differing abundances of the major minerals impart a lithologie
variation to the corundum gneiss. Corundum-plagioclase gneiss with minor
biotite contains the largest and most abundant corundum crystals. Locally
this rock type contains large masses of tourmaline. Corundum concentration
varies laterally as well as between folia. Some layers contain as much
as seventy-five percent corundum while in others the content is five per
cent or less. The corundum crystals commonly measure 4mm by 10mm but may
reach 10mm by 40mm. Feldspathic gneiss lacking corundum occurs irreg u larly
on the edges and along the thin portions of the corundum gneiss layer.
Much of the unit consists of piagioclase-corundum-biotite gneiss or schist
of which b io tite may comprise more than fo rty percent of the rock. Again
corundum concentration varies between folia but with less dramatic
difference than in the corundum-plagioclase gneiss. CHAPTER IV
ORIGIN OF THE CORUNDUM GNEISS
Aluminous rocks occur throughout the pre-Beltian metamorphic
complexes. Within the Ruby Range, widespread s illim a n ite schists com prise a distinctive unit (Heinrich, 1960). Although the Camp Creek deposit
is the only known exposure of corundum bearing rock in the range, identical
placer corundum occurs at one lo c a lity in the Sweetwater Basin where i t undoubtedly comes from the metamorphics. Corundum-(and s illim a n ite ) bearing rocks are well known from three localities in the pre-Beltian crystallin e rocks of the northern Madison Range (Clabaugh and Armstrong,
1950; Clabaugh, 1952). Root (1965) describes corundum-bearing pegmatitic and gneissic rocks near the Indiana University Field Station in the Pony
area. S il1im anite-rich rocks are common throughout these crystallin e
sequences.
The following observations lead me to believe that the corundum gneiss
underwent isochemical metamorphism;
1) The gneiss occurs in a discrete layer concordant with surrounding
foliation and layers (Plate 1). Internal foliation also parallels
the trend. The map pattern shows that interfingering of the units
clearly results from folding.
2) Contacts, where exposed, are sharp.
27 28
3) There is no d efin ite evidence of major metasomatism.
The presence of boron complicates the concept of an isochemical metamorphic development of the corundum gneiss. Boron, a d istin ctive constituent of tourmaline, is characteristic only in moderately aluminous marine clays, shales, saline and hypersaline evaporites, volcanic gasses and pegmatites (Krauskopf, 1967). Even the most abundant such occurrences would barely account for the boron present in tourmaline in the corundum gneiss. Shales typically contain 100 parts per million (ppm) boron
(Krauskopf, 1967, p. 592) while the corundum gneiss contains up to a few thousand ppm boron (Table 2). However, boron is very mobile and may have been introduced metasomatically during prograde metamorphism. I f so, the corundum gneiss evidently provided the only chemically suitable host for the boron, in contrast to the other units where little or no tourmaline
(or any other boron-bearing phase) developed.
Table 1 contains modal analyses of eight representative samples of corundum gneiss. The percentages of the four major minerals were obtained by point-counting slab surfaces. Percentages of accessory minerals repre sent visual estimates from thin sections aided by the use of a grid. In compiling the analyses, retrograde minerals were assigned to the primary minerals from which they altered . The average of samples probably does not represent a weighted sample with respect to the relative amounts of plagioclase-corundum gneiss and plagioclase-corundum-biotite gneiss.
Such an average cannot be accurately determined from the available exposure 29
Table 1. Modal Analyses of Corundum Rocks
4819 4820 4821 4822 4823 4824 4825 4826 AVG.
Plagioclase 19.6 65.7 41.8 41.7 53.5 61 .5 85.0 1 0 . 8 47.5
Biotite 59.0 15.7 5.3 4.4 4.1 18.2 1 2 . 8 42.0 2 0 . 2
Corundum 20.2 16.1 37.4 47.3 39.2 19.8 1 .1 46.3 28.4
Toumaline — — 14.3 5.2 2.1 — — — — — 2.7
Magnetite 0.6 0.1 0.5 0.6 0.4 0.1 0.1 0.4 0.4
Ilmenite 0.1 0.2 0.3 0.3 — — 0 . 6 0.4 0 . 2
Zircon 0.5 0 . 2 0.4 0.4 0 . 2 0.3 0 . 2 0.1 0.3
Rutile 0.1 t r . 0 . 2 0.1 0.1 0 . 2 0 . 2 t r . 0-1
Sample descriptions:
4819: PIagioclase-corundum-biotite gneiss. 4820: Biotite-corundum-plagioclase gneiss. 4821: Corundum-plagioclase gneiss. 4822: Corundum-plagioclase gneiss. 4823: Corundum-plagioclase gneiss 4824: Plagioclase-corundum-biotite and corundum-plagioclase gneiss • 4825: Corundum-bearing plagioclase rock. 4826: Plagioclase-biotite-corundum gneiss.
All samples taken from main part of corundum deposit in vicinity of prospect pits (along fenceline). 30
To fa c ilita te comparison of the corundum gneiss to other aluminous rock types I estimated the percentages of major oxides using the same method followed by Bielak (1978). Using an appropriate chemical analysis
for plagioclase, obtained from Deer and others (1962) the major oxides were proportionally reduced. A fter following the procedure for the re maining minerals each oxide was totalled. As noted by Bielak (1978),
the result probably approximates a chemical analysis to within five to eight percent or better.
Table 2 contains the calculated oxide data of the corundum gneiss.
Alumina, silica and iron vary considerably, as indicated. Foster (1962)
presents chemical analyses of corundum rocks from the northern Madison
Range. Table 3 compares some of his analyses with data from Table 2.
Alumina and s ilic a vary within sim ilar ranges as does sodium. The com
positional estimates of the Camp Creek rocks indicate considerably higher
iron and calcium values and less potassium and titanium than the rocks from
the Madison Range.
Table 4 compares the corundum gneiss oxide data with chemical analyses
of aluminous clays. They are arranged in order of increasing silica and
decreasing alumina. Many of the oxides of corundum gneiss vary from those
of the clays. The locally high concentrations of Ti 02 in corundum and b io tite , not accounted fo r in the pétrographie estimates of those minerals, may compensate for the paucity of that oxide in the gneiss values relative to those for the clays. 31
Table 2. Petrographically estimated oxide
Avg. of Modes 4822 4825 4826
SiOg 34.7 26.7 51 .7 21 .4
AI2 O3 46.1 60.8 27.1 57.0
FezOs 1 .2 1.1 1 .3 1 .1
FeO 3.4 1.1 2 . 2 6 . 8
MgO 2 . 6 1 .1 1 .5 4.8
B2O3 0.3 0.5 0 . 0 0 . 0
CaO 5.3 4.8 9.1 1 .4
K2O 1.9 0.5 1 .3 3.8
NagO 2 . 6 2.3 4.6 0.7
TiOg 0.9 0 . 6 0.9 1 .5
HgO 0 . 8 0.3 0.5 1 .6 32
Table 3. Comparison of petrographically estimated oxide compositions of rocks from the Camp Creek corundum deposit to chemical ______analyses of corundum gneisses from the Madison Range. ______
Av. of th i s Sample Report B49-13A B50-4E B50-4B 4825 B50-1/
SiOg 34.7 32.6 45.6 43.1 51.7 52.5
46.1 59.3 41.3 46.9 27.1 30.1
FezOs 1 .2 t r t r t r 1,3 0.4
FeO 3.4 t r 0.1 t r 2 . 2 0.5
MgO 2 . 6 0 . 2 0.5 0 . 2 1 .5 4.8
B2 O3 0.3 — — — — — — —— — —
CaO 5.3 0 . 2 0.4 t r 9.1 1.1
KgO 1 .9 3.7 4.8 6.1 1 .3 4.8
Na^O 2 . 6 2 . 8 4.5 0 . 2 4.6 4.5
I i 02 0.9 1 .1 2.7 2.4 0.9 2.7
HgO 0 . 8 0.1 0.1 1 .0 0.5 0.1
Sample descriptions and locations from Foster (1962).
B49-13A Feldspathic gneiss (orbicular), Bozeman deposit. B50-4E Fedspathic gneiss. Elk Creek deposit. B50-4B Sillimanite schist. Elk Creek deposit. B50-1A Biotite syenite gneiss (orbicular). Elk Creek deposit. 33
Table 4. Oxide comparison of curundum gneiss to aluminous clays
Clay Clay Clay Clay 5 5 4822 B-1 4826 B-5 Kaol . 18 14 4825
SiO^ 4.1 1 1 .2 26.7 23.0 21 .4 32.5 45.7 43.3 53.6 51 .7
AI2O3 75.5 67.9 60.8 58.5 57.0 46.5 39.8 38.2 29.2 27.1
FCgOs 1 .9 1 .7 1 .1 0 . 6 1 .1 0.4 0.1 1 .6 2 . 2 1.3
FeO — — — — 1.1 —— 6 . 8 2 . 2
MgO 0.1 0.1 1 .1 1 .0 4.8 0.7 — — 0.1 0.5 1 .5
CaO 0 . 2 0 . 2 4.8 0.4 1 .4 0.7 — — 0.4 0.5 9.1
%20 1 . 0 1.7 0.5 1.7 3.8 1 .4 0.4 0.5 1 .2 1.3
NSfO 0.3 0.5 2.3 0.7 0.7 2.1 0 . 2 0.1 0.1 4.6
TiO^ 3.1 3.1 0 . 6 2.1 1 .5 2.4 0.4 1 .6 1 .6 0.9
H2 O 0.1 0.1 0.3 1 .2 1 .6 0.3 14.2 0.4 0.4 0.5 (14.3) (13.7) (13.5) (13.6) (13.5) (10.5)
H2 O values in parentheses are H 2 O values before ignition.
Sample descriptions of clays:
Clay 5: Diaspore clay. Gasconade Co., Mo., R. T. Rolufs, analyst (McQueen 1948). Clay 4: O o litic burley clay. Gasconade Co., Mo., R.T. Rolufs analyst (McQueen, 1948). B-1 : Burley clay. Gasconade Co., Mo., H. W, Mundt, analyst (McQueen, 1948). B-5: Burley clay. Gasconade Co., Mo., H. W. Nundt, analyst (McQueen, 1948). Kaol.: Kaolinite (Kerr and others, 1950).
Clay 18: F lin t clay Gasconade Co., Mo., R. T. Rolufs, analyst (McQueen, 1948). Clay 14: "Semi-plastic" clay, Audrain Co., Mo., R. T. Rolufs analyst (McQueen, 1948). 34
The potassium content in the clays varies considerably but the value for sample 4826 clearly exceeds the normal range. The potassium occurs prim arily in the abundant b io tite of that sample. Whereas most of the iron oxide values compare well with the iron content of the clays, the biotite-rich gneiss contains excess iron in the ferrous state.
Aluminous clays commonly contain more iron than indicated by the analyses shown here, however. CaO, MgO and Na 20 in the gneiss sig n ifican tly exceed the range of those oxides in the clays. The high calcium and magnesium concentrations may result from the proximity of carbonate rocks
(marble) but bauxitic clays associated with carbonate rocks usually con tain no enrichment of either oxide.
In spite of the discrepancies in CaO, MgO and Na^O the composition of the gneiss strongly resembles that of aluminum-rich clay. Plagioclase hosts most of the calcium and sodium while magnesium occurs chiefly in b io tite and tourmaline. Corundum-rich fo lia that contain re la tiv e ly little plagioclase or biotite contain much less of the anomalous trio of oxides than the calculations indicate and better approximate a bauxitic clay such as diaspore. Both diaspore clays and corundum-rich folia may contain up to eighty percent AI 2 O3 . Corundum-poor gneiss, exemplified by sample 4825, contains amounts of s ilic a and alumina characteristic of kaolinitie clay.
Bauxites form by weathering and chemical breakdown of minerals in aluminum-bearing rocks. The formation of hydrated aluminum oxide minerals
(gibbsite, boehmite, diaspore) permits the retention of aluminum while 35 other constituents leach out of the residual deposit. Stamper and
Kurtz (1978, p. 10) summarize the prerequisites of bauxite formation:
Conditions favorable for the formation of bauxite most frequently occurred in areas and geological periods which provided warm, wet climates, parent aluminous rocks with high permeability and easily soluble minerals, good subsurface drainage, and long periods of tectonic stability that permitted deep weathering and preservation of land surfaces.
Bauxitic clays develop on a wide variety of bedrock types ranging from mafic to alkalin e igneous rocks, volcanic and aluminous metamorphic rocks.
They also form on clastic and carbonate sediments (Valeton, 1972).
Several large bauxites of Tertiary age occur in association with kaolinitie clays (Valeton, 1972) and the Wiepa bauxite deposit in Australia occurs with, and probably developed from, kaolin itie and arkosic sands (Stamper and Kurtz, 1978).
The specific stratigraphie setting fo r aluminous clays in the Cherry
Creek group remains in question. The succession lacks d e fin itiv e evidence indicating the direction of stratigraphie younging. However, most pre-
Belt researchers surmise that the sequence in the Ruby Range becomes younger towards the northwest (Heinrich, 1960; Okuma, 1971; Garihan, 1973).
Granitic gneiss of great thickness underlies the area southeast of the study area. Folding prevents accurate determination of its thickness, but i t may exceed a kilometer. Heinrich (1950, p. 10) estimates a thick ness of about one-half mile (0.8 kilom eter). The top of the gneiss occurs within a few tens of meters of the corundum gneiss (35 to 45 meters).
The origin of the g ran itic gneiss has generated abundant debate but much 36 of the fie ld and pétrographie evidence supports a sedimentary origin
for the unit. Compositional determinations indicate an arkosic sediment
but it occurs in apparently unlikely association with carbonate rocks,
thus throwing the exact nature of such a sediment into question (Garihan
and Williams, 1976). If we can assume a sedimentary protolith for the granitic gneiss, the unit fits the setting regardless of the precise nature of the sediment.
Biotite gneiss, locally garnetiferous, dominates the top of the graniti
gneiss. Garihan (1973) proposes that i t represents metamorphosed impure
psammites or fine-grained clayey sandstones (A.G.I., 1972). The thickness
of this unit ranges from 15 to 25 meters. A layer of talcose marble
fingers into b io tite gneiss near the top of the la tte r . I t extends north
east maintaining a fairly steady thickness of about thirty meters.
Marble, varying in character from carbonate to calc-si1icate, overlies
the b io tite gneiss on a sharp contact and discontinuously underlies the
corundum gneiss. Thickness varies from zero to twenty meters. A similar
marble overlies the gneiss with a maximum thickness of twelve meters.
Composition of the marble varies laterally and vertically.
About 75 meters of hornblende gneiss overlies the marble above the
corundum gneiss. A thin layer of amphibolite directly overlies parts of
the corundum layer.
The corundum gneiss forms a re la tiv e ly thin and localized unit between
marble layers (Fig. 9). I propose that i t represents a metamorphosed, ELEV. IN METERS
MARBLE
^^MARBLE GNEISS
OTITE GNEISS - m g # # # "
CORUNDUM BIOTITE GNEISS MARBLE -{'.GNEISS.':'::
Figure 9. Cross section through ridge at the study area. The structurally competent corundum gneiss (and hornblende gneiss) behaved as b rittle units and folded while the marble underwent plastic deformation, typical of an incompetent unit Shear deformation predominated in the biotite gneiss. 38 impure, aluminous clay layer of varied composition. The map pattern suggests a localized body developed in a small topographic basin or an irregular surface on carbonate rocks. Corundum occurs in greatest abun dance in the thicker parts of the layer while the thinner parts contain l i t t l e or none. In modern bauxitic deposits of similar nature the greatest aluminum enrichment occurs in the thicker, well-drained portions
(Valeton, 1972). Corundum-rich fo lia may represent an analog to bauxitic clays while fo lia composed primarily of plagioclase approximate impure kaolinitie clays. The plagioclase rock-type predominates in the thinner and marginal areas of the corundum layer. That compositional layering is a relict sedimentary feature is possible. Heinrich (1960) and Okuma
(1971) suggest that such layering in marble is sedimentary. Bielak (1978) noted distin ct quartzite pebble layers in a block of hornblende gneiss.
As noted e a rlie r, the age of the Cherry Creek sequence remains in question, but the depositional age certainly exceeds 2.5 billion years.
The atmosphere and weathering conditions then differed considerably from those of today, distinguished primarily by little or no atmospheric oxygen
(Cloud, 1968; Siever, 1977). Before the appearance of abundant oxygen- releasing biological forms, only minor amounts of O 2 became available through reactions such as photolysis of H 2O in the atmosphere (Siever, 1977)
Vigorous and e ffic ie n t oxygen-consuming reactions in the lithosphere consumed the small amounts of O 2 then available (Schidlowski and Eichman,
1977). Reduced carbon in the form of graphite in gneissic rocks and the presence of the banded iron formation (in the Christensen Ranch quadrangle) 39
(Heinrich, 1960) clearly indicate that primitive life was well established. Climatic and geochemical conditions during much of Archean time probably remained monotonous, resulting in common features among contemporaneous supercrustal complexes (Salop, 1964). In the absence of vegetation, chemical weathering and soil development occurred only on flat-lying areas protected from erosion and deposition of clastic terri genous sediments. To form the high alumina clays of the corundum-gneiss protolith, the climate must have been wet and warm for an extended period.
The rate of development of such a clay deposit cannot be determined since chemical weathering rates may have varied significantly in a different atmospheric regime and absence of vegetation.
The formation of the aluminous clays would have proceeded much like analogous deposits of the Tertiary and Quaternary. The weathering process would retain aluminum and titanium while iron and silica would leach through the layer (Lepp and Goldich, 1959). However, the b io tite-rich corundum gneiss contains abundant iron. This may represent high iron in flux from the source rocks (such as the iron-rich biotite gneiss) or actual iron retention in the clay through the formation of an Fe** - clay mineral, similar to nontronite. Influx of minor clastic sediments or later lack of drainage might explain the excess of CaO, MgO and Na20.
Schwab (1978) notes the Archean sediments in general contain comparatively more AlgO^, MgO and Na20 than younger sediments. CHAPTER V
METAMORPHISM
Proqrade Metamorphism
Cherry Creek group rocks underwent at least one high-grade regional metamorphism accompanied by deformation. Most workers (Okuma, 1971;
Garihan, 1973; Dahl, 1977) estimate that the rocks equilibrated in the
upper amphibolite facies (system of Hyndman, 1972). I determined the
equilibrium assemblages in the rocks of the corundum deposit on the
basis of simple and consistent mineralogy in each unit and lack of textural
or chemical evidence of in s ta b ility between coexisting phases.
The equilibrium assemblages are as follows (with respective units
in parentheses):
1 ) hornblende + plagioclase + biotite + quartz(rare) (hbld. gneiss)
2 ) calcite dolomite (marble)
3) calcite + diopside tremolite (marble)
4) plagioclase + b io tite + corundum (corundum gneiss)
5) plagioclase + K-feldspar + biotite + quartz (granitic gneiss)
6 ) plagioclase + K-feldspar + biotite + quartz ^sillimanite (biotite gneiss).
Of these assemblages only that of the b io tite gneiss gives a precise
indication of metamorphic grade. The assemblage may occur in the range
from the s il 1 imanite-muscovite zone of the amphibolite facies to the
hornblende-orthopyroxene zone of the granulite facies but the presence
40 41 of microperthite in the orthoclase indicates that conditions straddled
the boundary between amphibolite facies and granulite facies. Figure 10
shows the s ta b ility fie ld during maximum metamorphic conditions. Tem
peratures ranged between 550® and 750® C while pressure varied
between 3 1/2 and 8 kilobars.
The garnets which occur in the granitic and b io tite gneisses de
veloped before maximum metamorphic conditions. In a ll but one case
garnet prophyroblasts have embayed and some corroded looking boundaries.
Biotite grains commonly pierce the porphyroblasts. At some point during
prograding metamorphism, the garnets became unstable with respect to the
developing assemblage. In the Shuswap complex of the Canadian cordillera
Hyndman (1968, p. 35) notes that the pair biotite-sil1imanite developed
rather than garnet-muscovite or garnet-orthoclase due to a relatively low
FeO/MgO ratio in high grade rocks.
Although amphiboles and b io tite retained water throughout metamorphism
most factors indicate undersaturation with respect to water in the over
all chemical system. I f P^^Q did equal ^total'^^^^^^^ melting should have taken place in the granitic gneiss. No partial melting features
exist in or adjacent to the study area. Relatively high calcium content
in the plagioclase (compared with that of "common granite" (see Hyndman,
1972, Fig. 1-9), increases the melting temperature somewhat but in
formation provided by Winkler (1974, Table 18-3) indicates that the
difference is less than 20°C. Even minor undersaturation (with respect
to H2O) could raise the melting temperature enough to prevent melting of W*- Z a: ÜJ co z o 9 - w tu Lu o Z m m, o -JIUJ üJ m o
7 - CL O
g < OCD
O «M
4 0 0 500 6 0 0 7 0 0 8 0 0 9 0 0 1000 TEMPERATURE (°C )
Figure 10. Stability field (stippled) for Cherry Creek rocks during upper amphibolite-lower granulite facies metamorphism. The homogeneous K-spar to perthite line approximates the boundary between amphibolite and granulite facies conditions. After Hyndman (1972, p. 313). 43 the granitic gneiss, I estimate (from Figure 3-3, Hyndman, 1972, p. 70) that a difference of one kilobar, or less, between and Ptotal would raise the melting temperature of the gneiss to conditions beyond the s ta b ility fie ld in Figure 10. The presence of scapolite in two samples of marble indicate that high conditions prevailed, at least locally
(Deer and others, 1966, p. 38).
Retrograde Metamorphism
A retrograde metamorphism altered previous metamorphic minerals to more hydrated phases. The retrograde minerals represent an assemblage of the greenschist facies which encompasses temperatures of 400*C to
500*C at pressures of 3 to 8 kilobars P^^^. Muscovite (white mica or s e ric ite ), chlorite and zoisite dominate the retrograde assemblages.
Margarite developed only in association with corundum. The extent of a l teration varies widely. Some samples underwent complete alteration while in others only trace amounts of such minerals exist. Such variation occurs even within the area of a thin section. The availability of water was the dominant control on the development of greenschist facies minerals, and produced the variation of alteration throughout the rocks.
The lack of metamorphism of lower Paleozoic sediments in the area demonstrates that the retrograde metamorphism occurred during the
Precambrian. King (1976) and Mueller and Cordua (1976) suggest that this event accounts for the 1 . 6 to 1 . 7 b.y. metamorphism dates. CHAPTER VI
DEFORMATION
Compositional and gneissic layering and schistosity define the fo liatio n which dominates the structure of the study area. The corundum deposit occurs within structural domain VI of Okuma (1971) who makes the following statement on the area (p. 6 6 ).
With the exception of a few minor folds in marble, gneiss and schist, most rocks in domain VI are characterized by a homoclinal structure. Careful fie ld observations has [sic] also revealed folds that have tightly appressed hinges. The trend of fo liatio n in this domain ranges from N45° E to N55° E, and the rocks dip to the northwest at 45 to 70 degrees.
The sixty-six foliation attitudes that I measured vary more than
Okuma indicates but essentially agree with his data.
At least two sets of folds deform foliation. The earlier folds are close to tig h t, assymetric, sim ilar folds (Ramsay, 1967, p. 349). The axial planes of these folds strike parallel to prevailing foliation but dip more steeply. Such folds in shallow-dipping to horizontal foliation produce the striking pattern in the central part of the map (1:400)
(Plate 1). Fold hinges fractured, perhaps an axial plane cleavage, making preservation of the hinges rare. Wavelengths of these folds range from one to several meters in the corundum gneiss. Later folding along
NNW to NW bearing axes gently warps fo liatio n and early folds throughout the area. The scale of these folds ranges from tens to even hundreds of meters. 44 45
The various rock units behaved in d ifferen t manners during deformation.
The marble, being the most incompetent unit, folded readily and thickened or thinned to accommodate the deformation of other units. Biotite and granitic gneiss deformed by shearing parallel to fo liatio n . The shearing rolled and fractured feldspar and garnet, bent biotite and smeared out quartz. The corundum gneiss behaved as a rather b rittle unit by folding with minimum attenuation of thickness on fold limbs. Hornblende gneiss remained a fa ir ly competent unit throughout deformation.
Contacts between units, rarely preserved in surface exposure, are generally sharp. Where actually exposed, contacts show concordant to slightly discordant foliation. An irregular contact (Fig. n ) between marble and hornblende gneiss (structurally above) has a pronounced discordancy of foliation on small (centimeter) scale but on a larger scale (meters) is concordant.
A small fault terminates at the ridge crest on the southwest side of the swale. The fault extends northwesterly for less than a kilometer. By sighting along its strike to the next ridge I estimate its attitude to be about N45°W / 73*NE. It apparently terminates on the ridge at the study area with rotational movement. On the southeast flank of the ridge the fault manifests itself as a splay of vertical joints which parallel and occur on strike with the fault. The joints show well in the marble and especially in the granitic gneiss. This fa u lt probably developed during late Mesozoic to Tertiary block faulting. 46
5 cm. MARBLE
Figure 11. Contact between hornblende gneiss and marble with approximate trends of fo liatio n indicated by lines. This exposure occurs below the southwest end of the corundum gneiss. CHAPTER VII
SUMMARY AND GEOLOGIC HISTORY
Field relationships and petrology of the Camp Creek corundum deposit indicate that an aluminous sedimentary protolith underwent nearly isochemical metamorphism to form the corundum gneiss. The gneiss forms an irregular, concordant layer with sharp contacts. It consists primarily of plaqioclase, corundum, biotite and minor tourmaline in widely varying proportions.
Adjacent b io tite gneiss samples indicate that the rocks underwent metamorphism in the uppermost amphibolite facies to lower granulite facies.
Estimates of major oxide compositions indicate a range of SiOg and
AI2O3 values comparable to those of kaolin itie to bauxitic (diaspore) clays. Corundum-deficient plagioclase rock represents the former while the la tte r formed corundum-rich fo lia . Corundum-biotite gneiss represents iron- rich clays. Excess CaO, MgO and NagO probably resulted from impurities in the clays which developed in a small, irregular depression on the surface of carbonate sediments. Major geochemical mechanisms would be similar to those of the present in spite of d ifferen t atmospheric and weathering conditions in the Precambrian atmosphere.
The following geologic history is based on field and pétrographie in formation from the study area.
47 48
Precambrian W 1) Deposition of arkosic composition sediments late r (Archean) to become "Dillon Granite gneiss" (assuming unproven
sedimentary origin. Garihan and Williams, 1976).
2) Deposition of Cherry Creek group sediments and vol-
canics. Basalt flows in sequence (amphibolites - see
Bielak, 1978). The development of clays (corundum gneiss;
also si 1 1 imanite-rich schists?) records a period of
subaerial exposure in a warm, wet climate, with local
erosion or non-deposition and tectonic stability as
aluminous clays developed.
3) Renewed periods of deposition producing a sequence
dominated by carbonates and mafic sediments (?) (marbles
and hornblende gneisses respectively).
Precambrian W 4) Deformation and metamorphism. The sediments may have to X (Archean to Aphebian) suffered multiple metamorphism. Most of the deformation
probably preceded the peak of metamorphism which produced
temperatures of 550° to 750° C at pressures of 3 1/2 to
8 kilobars. Minor deformation followed maximum meta
morphism. A long period of very gradual cooling and
pressure reduction may have followed. Ultramafic rocks
may have been tectonically emplaced in the Cherry Creek
sequence during the early stage deformation (Desmarais,
1978). 49
5) Retrograde metamorphism. Maximum conditions reached the
greenschist facies. The corundum gneiss underwent ex
tensive alteration.
6 ) U p lift of Dillon block to become the source of Beltian
sediments. Erosion of pre-Beltian rocks at considerably
higher structural level than Camp Creek corundum gneiss.
Precambrian Y 7) Intrusion of diabase dikes. The dominantly northwesterly (Helikian) trend probably resulted from intrusion along pre-existing
planes of weakness.
Latest Precam- 8 ) Erosion followed by deposition of Paleozoic sediments brian to Mesozoic unconformably on Cherry Creek group. (Klepper, 1950;
Tysdal, 1976).
Late Mesozoic 9) Laramide orogeny. Block faulting raised the Ruby and and Tertiary adjacent ranges (Klepper, 1950; Tysdal, 1976). Erosion.
Post- 10) Erosion of overlying Phanerozoic rocks and pre-Belt Tertiary metamorphics to the present landscape. REFERENCES CITED
A .G .I. (1972), "Glossary of Geology and Related Science", Amer. Geol. In s t., 857 p.
Bielak, J ., (1978), "The Origin of Cherry Creek Amphibolites from the Winnipeg Creek Area of the Ruby Range, Southwestern Montana", unpublished M.S. thesis, Univ. of Montana, 46 p.
Clabaugh, S.E. (1952), "Corundum Deposits of Montana", U.S.G.S. Bull. 983
and Armstrong, F.C., (1950), "Corundum Deposits of Gallatin and Madison Counties, Montana", U.S.G.S. Bull. 969-B.
Cloud, P.E., (1968), "Atmospheric and Hydrospheric Evolution of the Primitive Earth", Science, v. 160, p. 729-736.
(1972), "A working model of the primitive earth". Amer. Journ. S ci., V. 272, no. 6, p. 537-548.
Dahl, P.S., (1977), "The Mineralogy and Petrology of Precambrian Metamorphic Rocks from the Ruby Mountains, Southwestern Montana", Ph.D. thesis, Indiana Univ., 280 p.
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List of hand specimens and thin sections on file in the University of Montana, Department of Geology pétrographie collection.
U.M. No. Rock Type Thin Section
4819 Corundum-biotite gneiss X
4820 Corundum-plagioclase gneiss X
4821 Corundum-plagioclase gneiss X
4822 Corundum-plagioclase gneiss
4823 Corundum-plagioclase gneiss
4824 Corundum-plagioclase gneiss
4825 Feldspathic (plagioclase) gneiss X
4826 Corundum-biotite gneiss X
4827 Corundum-plagioclase gneiss X
4828 Diabase X
4829 Carbonate marble X
4830 Calc-silicate (diopside) marble X
4831 Serpentinlzed diopside marble X
4832 Garnet-biotite-gneiss X
4833 Granitic gneiss X
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