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The Arizona Geological Survey is not responsible for the accuracy of the records, information, or opinions that may be contained in the files. The Survey collects, catalogs, and archives data on mineral properties regardless of its views of the veracity or accuracy of those data. STATE OF MONTANA J. Hugo Aronson, Go1;errwr MEMOIR 36 October 1957 BUREAU OF MINES AND GEOLOGY E. G. Koch, I>i:reeWr

THE MONTANA BUREAU OF MINES AND GEOLOGY IS A PUBLIC SERVICE AGENCY FOR THE STA'l'E OF MONTANA. ITS PURPOSE IS TO ASSIST IN DEVELOPING THE STATE'S MINERAL RESOURCES. IT CONDUCTS FIELD STUDIES OF MONTANA GEOLOGY AND MINERAL DEPOSITS, INCLUDING METALS, OIL AND GAS, COAL, OTHER NONMETALLIC MIN· ERALS, AND GROUND·WATER. IT ALSO CARRIES OUT RE. BEDROCK GEOLOGY OF THE NORTH END SEARCH IN MINERAL BENEFICIATION, EXTRACTIVE METAL­ LURGY, AND ECONOMIC PROBLEMS CONNECTED WITH THE MINERAL INDUSTRY IN MONTANA. THE RESULTS OF THESE OF THE TOBACCO ROOT MOUNTAINS STUDIES ARE PUBLISHED IN REPORTS SUCH AS THIS. FOR FURTHER INFORMATION, ADDRESS THE DIRECTOR. MONTANA BUREAU OF MINES AND GEOLOGY, MONTANA MADISON COUNTY, MONTANA SCHOOL OF MINES, BUTTE. By ROLLAND R. REID ILLUSTRATIONS

Plate Page

1. Geologic map of the north end of the Tobacco Root Mountains, Madison Cou n ty, Montana ______- ______In pocket

2. General view of the topography of the Tobacco Root Mountains ______Frontispiece

3. A, View of characteristic planar foliation in Pony metamorphics. B, View of minor folding in Pony metamorphics. C, view of boudinage-like structure in Pony gneiss. D, View of large garnets in amphibolite ______2

4. A, View of metabasalt in contact with gneiss. B, View of large metabasalt boudin­ age. C, View of bending in foliation in ampr.ibolite. D, View of strong jointing in gran od iori te ______6

5. Aerial view of Madison limestone offset by Pony fault ______

6. Aerial view showing trace of the Mammoth fault ______18

7. Aerial view showing trace of the Bismark fault ______18

8. A, View looking southeast along the trace of Hollowtop fault. B, View of large microcline porphyroblast in Pony Metamorphics. C, View of the hornblende-plagio­ clase swirled gneiss in amphibolite. D, View showing enrichment of dark minerals along a quartzofeldspathic layer in amphibolite ______18

Figure 1. Index map showing location of area mapped ______2

2. Sketch of gneiss cut by metabasalt and pegmatite ______~______8

3. Sketch of xenoliths in Tobacco Root bathol ith - _ ----______9

4. Cross-section at the Pony-Cherry Creek con tact on Spring Branch Creek ______14

5. Recumbent isoclinal folding near axis of Noble Peak anticline ______15

6. Sketch of partly concordant structure of pegmatite II ______21

I ." . I

vi CONTENTS

Foreword ______iii Structural geology ______13 ~ Abstract ______1 General setting ______13 '" ~ ..: Introduction ______1 Metamorphic rock structure ______13 ..:l il< ~ Purpose of report ______1 Pony-Cherry Creek structure ______13 '" 0 z Geo graphy ______-- 1 Pony-Cherry Creek contact relations ______14 ~ (5 ~ Topo gra phy ______1 The Noble Peak anticline ______15 fiI ~ U1 Z The Belle Creek syncline ______16 t-4 Transportation and accessibility ______2 ~ E-< Igneous rock structure ______16 Z Clima te ---- ______2 P Bath 0 1i th ______16 0 Working conditions ______2 -"" Stocks ______16 E-< 0 Previous Work ______2 0""; ~ gj Sills ______16 o ~ Ackno w ledgmen ts ______2 U 0.0 Dikes ______17 U _S ~.!<: Rock types ______3 >:Q 0 Sedimentary rock structure ______17 0.3 E-< ..c:' General remarks ______3 Faul ts ______1 7 0. ~ ro II: '-' E-< 0.0 Metamorphic rocks ______3 0 Description of faults ______17 ~+-' 0] Pony metamorphics ______3 0. Displacement along faults ______19 >-< II: 0; p.. -,... Cherry Creek metamorphics ______4 Relation of faulting to ore deposits ______20 ~ ;;; ~ :;:s Conclusions ______----- 20 c.!J (1) Metabasal t sills and dikes ______5 0 ::l p..-g' 0:0 Distinction between Pony and Cherry Origin of the metamorphic rocks ______20 E-< 0 Creek metamorphics ______6 ~~ Pony-Cherry Creek age relations ______21 II: 0.0 E-< >= Metamorphic pegmatites ______7 ro Structural control of batholithic emplace- ~ ~ 0 0 ment and ore deposition ______21 >< Earlier ideas on the age of C!l ~~ metamorphic rocks ______8 0 ~ Geologic his tory ______21 ..:l >-< 0 ;> fiI Igneous rocks ______8 C!l ~ Suggestions for prospecting ______22 0 ~ Granodiorite ______8 z ~ Magn eti te ______22 ..: ~ tfJ Z fiI A pli te and pegma ti te ______10 ~ Sill imani te ______22 Z c.!J ~ Andesite ______10 .,. Talc ______------23 0 ;:J Lati te ______11 ..: P e gmati tes ______24 fiI ~ ______11 ~ Basalt Metalliferous deposits ______24 ~ ..: Age of igneous rocks ______11 z . Glos sary ______------25 ..: J Eo< Sedimentary rocks ______11 Z Bibliography ______25 0 :<1 r~ Pal eo zo i c ______11 Appendix, Representative Lower Pony- Tertiary ______12 Upper Cherry Creek section ______27

v STATE OF MONTANA

BUREAU OF MINES AND GEOLOGY

E. G. Koch, Director

MEMOIR NO. 36

BEDROCK GEOLOGY OF THE NORTH END OF THE TOBACCO ROOT MOUNTAINS MA.DISON COUNTY, MONTANA

By ROLLAND R. REID

MONTANA SCHOOL OF MINES .. Butte, Montana . 1957

ARTCRArT ~ PRINTERS FOREWORD

This report on the Tobacco Root Mountains is another forward step in the program of the Montana Bureau of Mines and Geology to map the geology and ore deposits of the state. It is largely concerned with the general geology of the area studied and deals only in a general way with ore deposits. Work on ore deposits in the Tobacco Roots is still in progress and will be described in subsequent publications. Only broad structural control for ore deposition is described herein. When the detailed study of the individual districts is completed, valuable guides for prospecting should be forthcoming. For Sale By In addition, this paper contributes to a more complete understanding of early geologic his­ Montana Bureau of Mines and Geology Room 203-B, Main Hall tory of this region. The author has described evidence for two periods of metamorphism in pre­ Montana School of Mines Beltian time, both of which have affected Cherry Creek and Pony gneisses. Butte, Montana Of especial interest is the discovery of hitherto undescribed deposits of magnetite, talc, Price $1.00 graphite, vermiculite, and sillimanite in the pre-Beltian metamorphics of the Tobacco Root Moun­ tains. Prospectors in coming years will do well to examine the metamorphics closely for such de­ posits. As our industrial appetite for such minerals continues to grow, the deposits in the Tobac­ co Roots may ultimately become economic.

Prospectors and mine operators will be particularly interested in the section entitled "Sug­ gestions for prospecting." For these suggestions to be of most value, however, the basic geology of the area should be studied first.

E. G.KOCH, Director

iii 6 BED ROCK GEOLOGY, TOBACCO ROOT MOUNTAINS than the correct geologic name for the rock be­ amphibolites to cut across foliation in the en­ ing described (diopside-garnet ortho-amphibo­ closing gneiss (or other rock) . lite). Tansley and Schafer recognized the meta­ BEDROCK GEOLOGY OF THE NORTH END The metabasalts in both Pony and Cherry basalt sills and described their structure brief­ Creek have identical mineralogy and texture. ly. (1933, p. 9) They are made up predominantly of hornblende OF THE TOBACCO ROOT M01JNTAINS Metabasalt sills and dikes occur scattered and plagioclase andesine) with lesser amounts more or less randomly through both the Pony of diopside and garnet. Sphene and magnetite MADISON COUNTY, MONTANA and the Cherry Creek. They are particularly are the accessories generally present, although abundant near the base of the Pony metamorph­ both may be entirely absent. Foliation is moder­ By ics. A few of the more prominent ones are ately strong, consisting largely of sub-parallel ROLAND R. REID shown on the geologic map (Plate 1). Most of alignment of hornblende grains. As would be them were considered too small to put on the expected, the rock composition, based on the map and too much time would have been requir­ amount of each mineral present, is almost iden­ ed to trace out each of them. tical for each of the sills studied. ABSTRACT Metabasalt sills occur as tabular bodies rang­ DISTINCTION BETW.EEN PONY AND CHERRY Cherry Creek and Pony metamorphic rocks in the Tobacco Roots have undergone the same sequence of meta­ ing from a few feet to a maximum of about 150 CREEK MET AMORPHICS morphic processes. After deposition of both Cherry Creek and Pony sediments (with accompanying subordinate feet thick. They are parallel or nearly parallel amounts of igneous rocks?) a period of high temperature metamorphism (accompanied by introduction of sodium The principal rock types of both Pony and to the foliation in the enclosing gneiss, locally and potassium?) ensued. Structurally, the rocks were deformed into a series of recumbent (?) isoclinal folds Cherry Creek appear to be leptite, gneiss, and cutting it at angles of 1° to 10° (pI. 4, A). In with axial plane foliation. Pegmatites formed in at least some of the major fold crests. This was followed by a amphibolite. In outcrop, in hand specimen, or time in which basalt sills and dikes were injected into both Pony and Cherry Creek rocks. Still later came a outcrop the rock is brown in color and has a under the microscope, Pony leptite, gneiss, or period of metamorphism in which the flat-lying (?) foliation was warped into north-plunging open anticlines and much finer grain size than the enclosing gneiss synclines. Cherry Creek plunges beneath Pony in these folds in the Tobacco Roots. In this period, the basalt sills amphibolite exactly resemble Cherry Creek lep­ or amphibolite. Foliation is moderately strong and dikes acquired a foliation parallel to the foliation of the enclosing gneisses and were converted to diopside­ tite, gneiss, or amphibolite-in texture, in struc­ and parellel to foliation in the enclosing gneiss. garnet orthoamphibolites. Still in pre-Beltian, pegmatites formed which cut the meta-basalts. Following this ture, and in mineralogy. came the development, probably in Precambrian, of northwest faults with vertical displacements of several thou­ The foliation appears to be caused by the pres­ sand feet. Horizontal displacement is not known, but may be of the same order of magnitude. These faults were ence of elongate lenticular grains or grain-clots Because many of the rock types in Pony and reactivated in Laramide, with reversal of direction of movement from Precambrian occurring along some of them. of plagioclase. In outcrop, then, the rock has Cherry Creek are so alike, distinction between Northeast faults of Laramide (?) age are present, but of small size. a very fine flaser structure. Flaser structure them may be difficult, if not impossible, in a particular outcrop. Distinction must rely upon The emplacement of the Tobacco Root batholith was controlled by the northwest faults. Post-batholith ore de­ consist of tiny lenticular feldspar grains orient­ position appears to have been most strongly controlled by the northwest faults and less strongly controlled by the ed parallel to foliation. Locally, xeonliths of the examination of a section 500 to 1,000- feet northeast fault. gneiss or amphibolite are present within the in thickness. Deposits of magnetite, talc, sillimanite, graphite, and vermiculite occur in the metamorphics. sills. In some places the sms have been stretched Gneiss, leptite, and amphibolite intermixed in and form large elongate boudin-like structures layers of different thickness over a total section of 500 to 1,000 feet in thickness will probably (pI. 4, B). Metabasalt dikes have the same ap­ INTRonUCTION GEOGRAPHY belong to the Pony. Subordinate schist, thin pearance as the sills except that they strongly TOPOGRAPHY quartzite layers (1 to 5 feet), serpentine, or PURPOSE crosscut the foliation in the enclosing rock. Foli­ The area mapped, approximately 150 square anorthosite layers may be present. Locally, the ation within the dikes is parallel to the foliation This report represents a step in a project by miles, lies in the north end of the Tobacco Root examination of a still thicker section may be in the enclosing rocks. the Montana Bureau of Mines and Geology to Mountains (fig. 1). Relief in the area is slightly necessary. study the general geology and ore deposits in It is important to recognize the structural dif­ more than a mile, from approximately 5,400 feet the Tobacco Root Mountains. It is felt that ferences between the meta-basalts (fine-grained The same three main rock types intermixed in the north end of the area to approximately such a study may point the way to further ex­ amphibolites) described in the previous para­ with layers of marble, thick greenish quartzite 10,700 feet at M"t. Jefferson. pI ora tion and may aid in the finding of new ore graph and the main amphibolites of the Pony (5 to 50 feet), sillimanite schist (5 to 50 feet), The mountains have undergone rather strong anthophyllite schist (5 to 50 feet), or coarse deposits. and Cherry Creek (those interlayered with the glaciation in their higher parts, although none garnet amphibolite will belong to the Cherry gneisses and other rocks). Metabasalt sills are The geology of the region is emphasized in of the glaciers appear to have extended beyond Creek. Heinrich (1950, p. 8, 9) emphasized that locally discordant-partly parallel to and partly this paper. Work on the ore deposits is still in the range margins. Indeed, most of them term­ greenish quartzite and anthophyllite schists are cutting across foliation in the enclosing rocks. progress. It is hoped that this report will aid inated well within the range. characteristic of Cherry Creek. The coarse-grained amphibolites and horblend­ those engaged in prospecting or mining in recog­ Morainal deposits are rather abundant; some ites of Pony and Cherry Creek are, in contrast, It must be emphasized that the distinction be­ nition of the major rock types and structures of the larger deposits are shown on the geologic completely concordant with the enclosing rocks. tween Pony and Cherry Creek depends not on present as well as in the understanding of their map. Moraines obscured by heavy tree cover and The author has nowhere observed one of these peculiarities visible in every outcrop, but rather significance. moraines of smaller size were not mapped. 2 BED ROCK GEOLOGY, TOBACCO ROOT MOUNTAINS METAMORPHIC ROCKS 5

Timber cover is largely restricted to the north o o o fifteen feet thick and about forty feet long were kyanite, garnet, anthophyllite, plagioclase, mi­ N o CD Q slopes, between the elevations of 5,500 feet and ALBERTA obRerved in a thick quartzite layer near Noble crocline, rutile, or hornblende may be present. 9,500 feet. Lake. The different rock types are intermixed All mineral gradations between leptite and TRANSPORTATION AND ACCESSIBILITY in thick to thin layers in the Cherry Creek meta­ quartzite appear to exist. Certain of the quartz­ morphics, just as in the Pony. ites are rich in sillimanite. Forest Service roads are present in most of the valleys in the range. The area is accessible 480~------~-----U~~~~R-IV-ER------t~ All the maj or rock types that occur in the In the foregoing rocks, the feldspars enclose through Harrison and Pony on the east; White­ "'\~~o Pony metamorphics also occur in the Cherry the other minerals rather commonly and thus hall, Cardwell, and Jefferson Island on the north; • Great Foils Creek: in addition, marble, sillimanite schist, appear to be later. and, Twin Bridges, Silver Star, and Sheridan on anthophyllite schist, and coarse garnet amphibo­ Cherry Creek schists exhibit more mineral­ the west. The Milwaukee and Northern Pacific lite are present. Quartzite is far more abundant ogic variation than any other rock type in the railroads pass through Whitehall. Spur lines MONTANA in the Cherry Creek than in the Pony. Leptite, region. Quartz and mica are the predominant of the Northern Pacific railroad pass through amphibolite, and gneiss of the Cherry Creek are minerals, but lesser amounts of garnet, plagio­ both Twin Bridges and Harrison. indistinguishable in hand specimen and in the clase, sillimanite, anthophyllite, graphite, kyan­ microscope from their counterparts in the Pony. ite, and microcline may occur. Zircon, magnetite, CLIMATE Individual layers seem to be about as continuous rutile. ilmenite, and apatite are the most common Snow generally hampers access to the higher as layers in the Pony. accessory minerals. Abundant sericite and chlor­ parts of the range from November until late The marbles weather to a brownish color in ite occur in certain schists in which cataclastic June. During July, August, and September, the deformation has occurred parallel to the existing area is largely free of snow, although snow may WYOMING outcrop. Quartzite generally weathers to a pale greenish color-when broken it is white to foliation. Much of the biotite in these rocks is fall in the higher parts of the range at any time strongly rutilated. of the year. Snow generally does not fall in suf­ IDAHO brownish-red. Anthophyllite schist, high in an­ Anthophyllite is so abundant in certain schists ficient amounts in October to interfere seriously Figure I.-Index map showing location of area mapped. thophyllite, weathers to a chocolate brown color. that it predominates over the other minerals. with mining operations. PREVIOUS WORK Coarse garnet amphibolite is a black rock with large reddish brown garnets generally half an These schists then must be termed anthophyllite Tansley and Schafer (1933, p. 3) give figures Hayden (1872) did the earliest reconnaissance inch to two inches in diameter (pI. 3, D). schists. of 12.67 inches (1930) for precipitation, and 90° geology in this region. Peal (1896) made a re­ Ultrabasic rocks of the Cherry Creek include a F. and -30° F. for maximum summer and mini­ connaissance study of the Three Forks Quad­ Gneiss of the Cherry Creek is a prominently variety of foliated rocks high in magnetite, horn­ mum winter temperatures. rangle, a small part of which is included within foliated rock in which quartz, plagioclase, and blende, a rare colorless amphibole, talc, antigor­ the area covered by the present report. Winch­ microcline are the main minerals. Lesser a­ WORKING OONDITIONS ite, or (locally) graphite. ell (1914) briefly described the general geology mounts of biotite or hornblende, or both, give This report is based on geologic mapping done in the several mining districts of the Dillon the rock a gray to dark-gray color. The gneiss is, Marble occurs in the Cherry Creek metamor­ in July and August of 1954, 1955, and 1956. Quadrangle. Tansley, Schafer, and Hart (1933) however, widely variable in composition. Epi­ phics within the map area only in the block of Geology east of the 112th Meridian was map­ studied the general geology and ore deposits of dote, actinolite, diopside, or garnet may be pre­ ground between the Bismark and Mammoth ped on enlarged aerial photographs and plotted tte Tobacco Root Mountains. Berry (1943) stud­ sent in small amounts. Common accessory min­ faults. It consists predominantly of calcite, and on a base map made from preliminary sheets of ied the stratigraphy and structure in the vicinity e:als include apatite, zircon, allanite, and mag­ contains also diopside, tremolite, antigorite, or the Harrison Quadrangle (V. S. Geological Sur­ of Three Forks, Montana; the southwest corner netite. (locally) graphite. vey) at a scale of 1 :24,000. of his map overlaps the northeast corner of the As in the Pony metamorphics, feldspar shows area covered by the present report. Amphibolite consists predominantly of horn­ The mapping area was extended west of the blende and plagioclase. However, small amounts a strong preferential distribution according to 112th Meridian midway in the summer of 1955. ACKNOWLEDGMENTS of garnet, diopside, quartz, biotite, tremolite, rock composition. Gneiss and leptite contain The only base readily obtainable for mapping The write is grateful to J. R. Van Pelt, Direc­ or anthophyllite may be present. Accessories in­ both. microcline and sodic plagioclase (oligoclase) (aerial photographs were not then available for tor, during this study, and to W. S. March, Jr., clude rutile, apatite, sphene, and magnetite. with microcline generally predominant. Certain that area) was an enlargement from the V. S. Associate Director of the Montana Bureau of Garnet in particular may exhibit strong chlor­ gneisses may contain plagioclase, but no micro­ Forest Service map of the Beaverhead and Deer Mines and Geology, for helpful counsel given itic alteration. cline. Amphibolites, on the other hand, contain Lodge National Forests. Consequently, the map during the course of the field work. The writer predominantly more calcic plagioclase (ande­ Leptite contains quartz, micro cline, and plagio­ of the area west of the 112th Meridian must be was ably assisted in the field in successive sum­ sine) : microcline is very rarely present and then clase. It is strongly foliated, due largely to the regarded as a geologic sketch map, in no way mers by Ted Eyde, Marvin Lanphere, and Frank only in minor amount. elongation of quartz grains. Very small amounts comparable in accuracy of location and in detail Koucky. Clem Bartzen of the Montana Bureau of biotite, magnetite, hornblende, chlorite, or ap­ to the map of the area east of the 112th Merid­ of Mines and Geology performed the assays. METABASALT SILLS AND DIKES atite may be present. ian. Frank Koucky, Bahngrell W. Brown, and Vuno It must be made clear at the outset that the In the laboratory, more than 420 thin sections Sahinen read the manuscript critically and made Quartzite is composed largely of quartz. Small term "metabasalt" is used in a general sense. of rocks were examined under the microscope. several valuable suggestions. amounts of sericite, chlorite, biotite, apatite, It is a less cumbersome term to use in writing 4 BED ROCK GEOLOGY, TOBACCO ROOT MOUNTAINS MONTANA BUREAU OF MINES AND GEOLOGY ~fEMOIR NO. 36 . PLATE 3 plagioclase are predominant; less abundant are northeast of the upper lake. The layer is about muscovite, biotite, or hornblende. Zircon is the 50 feet thick and has a strike length of about a most common accessory mineral. mile. The rock is made up largely of magnetite, garnet, and pyroxene. Heinrich (1950, p. 8) ob­ Quartzite commonly contains significant a­ served similar rocks in the Ruby Range. mounts of plagioclase and microcline in addition to quartz. It is probably gradational into lep­ Worthy of emphasis is the strong preferen­ tite. Sillimanite occurs loc?.lly in quartzite. tial distribution of feldspar according to rock composition. The amphibolites have rather cal­ Mica schist is variable in composition; contain­ cic plagioclase (andesine) as their principal feld­ ing, in addition to quartz and biotite, small spar. The gneisses and leptite, on the other amounts of plagioclase, sillimanite, kyanite, horn­ hand, contain abundant microcline and sodic blende, diopside, hypersthene, garnet, or cordier­ plagioclase (oligoclase); microcline is generally ite. Common accessories are zircon, apatite, and predominant. Certain gneisses contain only magnetite. Chlorite and sericite may be abund­ plagioclase feldspar; microcline is absent. antly present as alteration products in certain schists in which cataclastic deformation has oc­ In all the feldspar-bearing rocks, the feldsp3rs curred parallel to the existing foliation. Cata­ commonly enclose the other minerals. This may clastic deformation is a purely mechanical de­ be taken to mean that the feldspars are later formation in which the minerals are broken but than the other minerals. Thin section study reveals intermediate rock not recrystallized. A.-CHARACTERISTIC PLANAR FOLIATION B.-MINOR FOLDING IN PONY METAMOR­ The author has observed moderately strong types transitional among all the main rock types OF PONY METAMORPHICS PHICS cataclasis (result of cataclastic deformation) described above. This serves to emphasize the Picture taken looking along strike of alternating Alternating gneiss and amphibolite layers. Picture gneiss and amphibolite layers. taken looking down the plunge of the fold. parallel to the foliation also in a specimen of variability of rock types present in the Pony gneiss collected in an area in the Bridger Range metamorphics. of Montana that has been described by McMan­ CHERRY CREEK MET AMORPHICS nis (1956). This feature indicates low tempera­ The Cherry Creek was first defined by Peale ture deformation parallel to the existing folia­ (J 8913, p. 2). His definition follows: "The Cherry tion. Creek beds consists of a series of marbles, or Hornblendite contains predominantly horn­ crystalline limestones, and interlaminated mica blende with lesser amounts of one or more of schists, quartzites, and gneisses, well shown the following: diopside, quartz, plagioclase, pleo­ southwest of Madison Valley." Winchell (1914, naste, olivine, enstatite, or garnet. p. 29) correlated the marble-bearing metamor­ Serpentine, a soft black rock, contains magne­ phics of the Tobacco Roots with the Cherry tite, talc, antigorite, chlorite, tremolite, and di­ Creek of Peale. "Schists, gneisses, and quartz­ opside, all in widely variable amounts. ites, with interbedded limestones, which are cor­ related with the Cherry Creek of Peale as de­ Anorthosite consists largely of plagioclase, scribed in the Three Forks folio, occupy the although minor amounts of actinolite, horn­ whole southern end of the Tobacco Root Moun­ blende. biotite, or chlorite may be present. The tains ... " plagioclase of anorthosite ranges in composition Within the map area, the Cherry Creek meta­ from sodic (oligoclase) to calcic (bytownite). morphics are confined to the southern and south­ However, at anyone place, the composition of western part, except for the area between the the plagioclase in the anorthosite corresponds to Bismark and Mammoth faults (plate 1). The the composition of the plagioclase in adjacent rocks of the Cherry Creek range as widely in amphibolite. Anorthosite layers are less than composition as those of the Pony, if not more so. D.-LARGE GARNETS (DARK SPOTS) IN AM­ three feet thick and appear to have rather short As in the Pony, the single feature that all of C.-BOUDINAGE-LIKE STRUCTURE IN PONY lateral extent. They are invariably interlayered GNEISS PHIBOLITE OF THE UPPER CHERRY CREEK. the rocks have in common is that of strong plan­ Observe that foliation in the oval rock mass makes with hornblendite or amphibolite. an angle with thc foliation in the enclosing gneiss. ar foliation. Locally, the compositional layers Picture taken looking along strike. This may be a Magnetite schist occurs in upper Dry Boulder are contorted in tight folds. Boudinage struc­ portion of a sheared-off fold crest. Gulch. It is well exposed in the valley wall ture is not uncommon. Boudins of amphibolite METAMORPHIC ROCKS 3

ROCK TYPES The different rock types are intermixed in GENERA.L REMARKS thick to thin layers; for example, dark-gray am­ phibolite and white quartz-feldspar schist (lep­ The area mapped in this study includes meta­ tite) or gneiss may be intermixed in layers rang­ morphic rocks, igneous rocks and sedimentary ing from a fraction of an inch in thickness to rocks. Several varieties of each are present. several hundreds of feet in thickness. Most lay­ The metamorphics include rocks mapped as ers are continuous over considerable distances­ "Pony Series" and "Cherry Creek Series" by those traced extend for distances of a few feet Tansley and Schafer (1933) . The Tobacco Root to distances of two miles. batholith is the principal igneous rock body pres­ The bulk of the Pony metamorphics is made ent. Most of the sedimentary rocks present are up of biotite and hornblende gneiss and leptite; lower Paleozoic-Flathead sandstone of Cambrian amphibolite, commonly present, is quantitatiYely age through the Madison limestone of Mississip­ subordinate. Locally thin layers of quartzite, pian age. Tertiary basin deposits are present in hornblendite, anorthosite, mica schist, serpen­ the northwest part of the area. tine, or magnetite schist are present. Of these, ~. anorthosite appears to occur in discontinuous METAMORPHIC ROCKS lenses rather than in more continuous layers. PONY METAMORPHICS Hornblende gneiss is a dark gray rock; bio­ The "Pony Series" was first defined by Tans­ tite gneiss has about the same color. Their dis­ ley and Schafer (1933, p. 8) ; their definition fol­ tinction depends on recognition of the dark min­ lows: " . . . gneisses and schists . . . well ex­ eral present. Leptite is a white rock, free of posed in the vicinity of Pony ... will be called dark mineral, in which quartz and feldspar are the Pony Series . . . " In this paper, the term about equally abundant. Amphibolite is a gray­ "Pony metamorphics" will be used when refer­ black rock high in plagioclase and hornblende. ring to these rocks. Hornblendite is black and is made up entirely of The Pony metamorphics are the most wide­ coarse hornblende. Serpentine is black, too, and spread rocks in the map area. They lie between can be distinguished from hornblendite by its the batholith on the south and the Paleozoic softness; it can be scratched easily with a knife. rocks on the north (plate 1) and are widespread Anorthosite strongly resembles quartzite-both in the west part of the area. are white and rather massive. Their distinction The rocks of the Pony metamorphics range depends upon recognition of plagioclase (anor­ widely in composition, from ultrabasic to acidic. thosite) or quartz (quartzite). Mica schist is The single feature that most of the rocks have generally brownish to pale green and is rich in mica. All of these rocks are coarse-grained, with in common is that of strong foliation. In most the single exception of serpentine. outcrops the foliation is planar. The term planar foliation as used here implies a tabular struc·· Gneiss consists largely of quartz, microcline, ture-compositional layers in the rock are rela­ and plagioclase with small amounts of biotite tively flat over distances of many feet (pI. 3, A). or hornblende or both. Locally, diopside or gar­ Less commonly, the compositional layers are net may be present. Accessory minerals include contracted in tight folds (pI. 3, B). Flaky or rod­ zircon, magnetite, apatite, allanite, and sphene. s[,aped minerals are aligned in parallel fashion. Alteration products are generally chlorite, seri- cite, epidote, and clay minerals. Foliation is In certain zones, boudinage structure is pres­ prominent (pI. 3, A) . ent. It consists of sausage-shaped or elongate lenticular masses formed by stretching of thin Amphibolite consists mostly of hornblende relatively brittle rock layers during folding. A and plagioclase; subordinate quartz, microcline, similar-appearing structure may be formed by garnet, or diopside are not uncommon. Acces­ shearing-off of fold crests. The two may be dif­ sories include magnetite, apatite, and sphene. ficult to distinguish, particularly if more or less Leptite may be regarded as gneiss virtually rotation of the blocks has occurred (pI. 3, C). free of dark minerals. Quartz, microcline, and 12 BED ROCK GEOLOGY, TOBACCO ROOT MOUNTAINS MONTANA BUREAU OF MINES AND GEOLOGY MEMOIR NO. 36, PLATE 4 and a lower "laminated limestone", after Peale. (6) The Park formation of Cambrian age un­ These units are now known respectively as the derlies the Pilgrim formation. It is about 150 Mission Canyon member and the Lodgepole feet thick and is, in the map area, rather well member. The' Mission Canyon is a thick (600 exposed in part along the road at and near the feet) massive white-weathering limestone that Strawn mine. It is composed of green and black stands up in massive cliffs. Freshly broken sur­ shales that weather to a brownish-gray color. faces are gray to dark gray in color. The Lodge­ (7) The Meagher formation of Cambian age pole is thin-bedded or laminated limestone with underlies the Park shale. It is about 460 feet shaly (?) partings and is also about 600 feet thick. It weathers to a gray color and is darker thick and is also well exposed in cuts along the gray on fresh surfaces. It immediately under­ road leading to the Strawn mine. Much of this formation is mottled in dark-gray to black mat­ lies the Mission Canyon member. rix with small lenticular or irregular patches of The Sappington formation (60 feet) of yellow tan to yellow-"black and gold marble" as it is sandstone was included in the Three Forks form­ commonly called. The upper part of the forma­ ation in this mapping. It is considered to be tion has mottling in two shades of gray, much Mississippian in age, but is lithologically similar like that in the Pilgrim formation. to the Three Forks. (8) The Wolsey formation of Cambrian age (2) The Three Forks formation (200 feet) of underlies the Meagher limestone. It is about 240 Devonian age underlies the Sappington. The feet thick and is composed largely of olive-color­ Three Forks shale is a brownish-weathering rock A.-METABASALT (DARK) IN CONTACT WITH B.-LARGE METABASALT BOUDINAGE (DARK ed shale with thin interbedded layers of brown made up of argillaceous limestone, black carbon­ GNEISS LAYER) IN PONY MET AMORPHICS shaly sandstone and flat-pebble limestone con­ aceous shale and greenish shales. Note that the metabasalt cuts across the foliation The boudinage lies just left of the thick metabasalt glomerates. in the gneiss at a small angle. body in the east wall of Sunrise Cirque, upper Wis­ consin Creek. (3) The Jefferson formation of Devonian age (9) The Flathead formation of Cambrian age underlies the Three Forks formation. The rock underlies the Wolsey shale and forms the bottom is dolomite, and it weathers to a dark grayish­ of the Cambrian sequence. In this area it lies brown color. It is approximately 450 feet thick unconformably on Pony metamorphics. The and is distinguished by the odor of hydrogen sul­ actual contact was nowhere observed in the map fide upon freshly broken surfaces. Fresh sur­ area; however, rock only a few feet below the faces are black to dark brown. Black chert con­ unconformity is fresh and unweathered in ap­ cretions may be present. pearance. Hanson (1952, p. 32) described the Cambrian The Flathead consists of about 135 feet of section in South Boulder Creek, just north of vitreous pink silica-cemented sandstone; accord­ the present map. The following units (listed ingly it is sometimes called the Flathead quartz­ from younger to older) described by him occur ite. It is important when calling it a quartzite in the map area and were utilized as map units. to specify that it is a quartzite of sedimentary (4) The Maywood and Red Lion formations, origin, i. e. an orthoquartzite, to avoid confusion of Devonian and Upper Cambrian ages respec­ with quartzite of metamorphic origin (meta­ tively, were not differentiated in mapping. They quartzite) . underlie the Jefferson formation and are to­ The following table shows graphically the gether approximately 60 feet thick. These two principal sedimentary rock units. formations are composed of cream-colored dolo­ mite, black and greenish shales, gray silstones, TERTIARY and gray, thin-bedded dolomite. Tertiary sediments occur in the northwest C.-BENDING OF FOLIATION IN AMPHIBOLITE D.-STRONG JOINTING IN GRANODIORITE (5) The Pilgrim formation of Cambrian age part of the map area. (Plate 1) In this area, NEAR THE CONTACT OF A PEGMATITE II underlies the Red Lion formation. It is about the Tertiary sediments consist largely of coarse­ DIKE On northeast face of Mt. Jefferson. Joints strike 360 feet thick and is composed predominantly Bending above the dike is in the reverse sense; the northwesterly. to-fine conglomerate sediments deposited as al­ pegmatite has formed along- a low-angle shear zone. of light-gray dolomite with more or less well­ luvial fans during Tertiary uplift of the moun­ West valley wall in uppet· Belle Creek. marked surface mottling in two shades of gray. tain range. SEDIMENTARY ROCKS 11

The andesite occurs mostly as sills in or near AGE OF IGNEOUS ROCKS the contact between the Flathead sandstone and A lead-alpha age determination on zircon from the Wolsey shale. The Wolsey shale exhibits sample 55 RR-1, granite, Tobacco Root batho­ slight baking effect. The large andesite sill in lith, near Pony, Montana, was made by H. W. the north end of the area has abundant quartz Jaffe of the U. S. Geological Survey. The age crystals present within it. These are probably of the zircon was determined to be 66 million foreign crystals taken into the magma from the years indicating that the granite was intruded underlying Flathead sandstone. in Late Cretaceous or early Tertiary time. (H. Andesite also forms a small partly concordant, W. Jaffe, written communication.) partly discordant body in the ridge west of Sail­ The andesite sills have been injected along or Lakes in upper South Boulder Creek. The bedding planes in the Paleozoic rocks and are body cuts Pony metamorphics and contains thus later than Paleozoic. Workers in adjacent abundant xenoliths of gneiss, amphibolite, and areas place andesitic rocks in the Cretaceous. magnetite schist (from the nearby magnetite Latite cuts Paleozoic rocks in several places. schist layer) . Many of the xenoliths are of mar­ ble, a rock not present in the surrounding Pony Berry (1943, p. 23), for example, states that metamorphics. andesites making up the bulk of the Livingston LATITE formation were formed during the Laramide orogeny. Presumably, the andesite sills and Latite occurs on the west side of the map stocks were emplaced during this time. area. (See plate 1). In outcrop, latite is a pinkish-gray rock, mass­ The present writer found andesite to be con­ ive and dense with phenocrysts of orthoclase. cordant and nearby latite to be partly concordant Under the microscope, approximately equal and partly discordant. He found no conclusive amounts of orthoclase and plagioclase pheno­ evidence with regard to age relations between crysts are observed, all strongly altered to seri­ latite and andesite. cite. The matrix is made up of fine-grained Basalt is mentioned by Berry (1943, p. 23) stubby prisms of orthoclase and plagioclase in as occurring both in the upper Livingston form­ which sericite alteration is moderately strong. ation and in the Tertiary basin deposits. The The latite occurs as small partly discordant, basalt dike cutting the Tobacco Root batholith partly concordant bodies that have slightly bak­ is certainly Tertiary. The basalt dikes in the ed or recrystallized the surrounding Paleozoic Mammoth fault zone may be Precambrian, late sedimentary rocks. Latite also cuts the Pony in Cretaceous, or Tertiary. places. SEDIMENTARY ROCKS BASALT PALEOZOIC Basalt has been observed at three localities Most of the Paleozoic rocks within the map within the map area. One small dike (two feet area occur in the north and west portions, i. e. thick) cuts the batholith in the southeast part within the area mapped on Forest Service map of the area. Basalt cuts Pony gneiss and quartz­ sheets. The generalized relationships were microcline pegmatite in the north fork of Dry sketched on the map, but no important degree Boulder Creek. Basalt ·dikes, one of which is of accuracy is claimed. The Paleozoic rocks were shown on the geologic map (plate 1), are present not closely studied. in and near the Mammoth fault in the north­ Berry (1943, p. 10-18) described the Devonian western part of the map area. They are five and Mississippian sedimentary rocks near Three to fifty feet thick. In the batholith and along Forks, Montana. The following units (listed the Mammoth fault the basalt is strongly shear­ from younger to older) described by him occur ed, indicating post-basalt faulting movement. in the map area and were utilized as map units. The basalt is made up of tiny plagioclase laths, stubby pigeonite prisms, somewhat altered to (1) The Madison formation of Mississippian green hornblende, and accessory magnetite. age. He described an upper "massive limestone" 10 BED ROCK GEOLOGY, TOBACCO ROOT MOUNTAINS METAMORPHIC ROCKS 7 ment of magma and as a pathway for the later The igneous pegmatites are in general in dikes depends on the aggregate of rock types present to tensional zones (as fold crests) or to cracks rise of mineralizing solutions. crosscutting the batholith and the country rock. in a rather thick section. Because of possible formed late in the time of metamorphism - The minerals are those of the batholith. lateral variation in rock composition, the criter­ where the surrounding gneisses contain quartz, APLITE AND PEGMATITE However, igneous pegmatites may occur as ia listed above for distinction may be valid only kyanite, and feldspar as in the Mineral Hill peg­ Aplite dikes are confined to the batholith. sills, and metamorphic pegmatites may conceiv­ in the immediate vicinity of the map area. matite. If the minerals are coarse grained, a Pegmatite dikes occur both inside and outside pegmatite has been formed. Further, if a gneiss ably occur as dikes. These remarks give an idea A representative section was measured which the batholith. Neither aplite nor pegmatite is were rich only in quartz and feldspar, it might of the difficulties involved. includes the contact between Pony and Cherry present in sufficiently large bodies to show on then give rise to a quartz-feldspar pegmatite. To illustrate further the difficulties involved, Creek as originally mapped by Tansley and the geologic map; both are uncommon in the If diffusion to a tensional crack had occurred, a brief description is given of the pegmatites Schafer (1933, p. 1) in upper South Boulder-Up­ batholith.. Aplites are sugary textured white the resulting tabular body would be identical that are rather abundant in upper Belle Creek, per Wisconsin Creek divide. This section is by rocks present in thin dikes (one inch to one in structure and appearance to a pegmatite of upper Dry Boulder Creek and upper Bear Gulch. no means complete for either Cherry Creek or foot thick) cutting the batholith rock. Pegma­ igneous origin. tites are coarse-grained white rocks cutting There pegmatites are composed predominantly Pony, but it is a rather short section to show Field study discloses two groups of pegma­ the batholith or the nearby metamorphic rocks. of quartz and microcline, with subordinate plag­ the kinds of rock present just below the contact. tites, shown on the geologic map (plate 1) as Individual crystal masses of pegmatite one to ioclase. They are massive and crosscut the meta­ The section is given in the Appendix. morphics (and the meta-basalts). By the criter­ Pegmatite I and Pegmatite II. Pegmatite I, be­ two feet in diameter are not uncommon. The section may be briefly summarized here. ia listed above, these would be called igneous lieved to be associated with the first period of The lower Pony consists largely of gneiss, and Aplite and pegmatite within the batholith pegmatites. pre-Beltian metamorphism, is best represented metabasalt sills. The upper Cherry Creek con­ have approximately the same composition as the On the ridge north of the Bielenberg and Hig­ by a pegmatite body lying- in section 26, T. 1 S., sists largely of sillimanite quartzite, amphibo­ enclosing granodiorite. In other words, they are gins mine (Bear Gulch), the writer observed R. 3 W., Madison County, Montana. It will be ite, coarse garnet amphibolite, and anthophyll­ composed of quartz, plagioclase, and microcline. xenoliths of massive quartz-microcline pegma­ called the Mineral Hill pegmatite in this report. ite schist. Beneath the measured section, still Garnet is locally present. The plagioclase in one tite in an andesite dike. Inasmuch as the ande­ This body is the largest encountered in field in upper Cherry Creek, is a zone predominantly aplite dike studied has the same composition as site is pre-batholith, the pegmatites, which are mapping. Smaller bodies of similar structure of amphibolite, estimated to be 500 feet thick. plagioclase in the enclosing granodiorite. preandesite, are also pre-batholith, (and prob­ elsewhere were not mapped. Pegmatite II is Beneath this a zone predominantly of gneiss es­ The aplite and pegmatite dikes cut sharply ably pre-Beltian). Pebbles of massive quartz­ represented by a swarm of pegmatites, mostly timated to be 800 to 1,000 feet thick. Still be­ across the granodiorite and appear to be of in­ microcline pegmatite in the Lahood conglomer­ too small to show on the geologic map, occurring neath this is a zone of unknown thickness, ex­ trusive origin. Within the dikes, complex tex­ ate (Belt) in the Jefferson Canyon, a few miles in upper Bear Gulch, upper Dry Boulder Gulch, tending south of the map area, of marble, tural gradations may be observed. For exam­ to the north, exactly resemble the afore-describ­ and upper Belle Creek. The largest body occurs quartzite, gneiss, and amphibolite. ple, aplite may form an outer zone, pegmatite ed pegmatites. These data permit the dating of in the upper north fork of Dry Boulder Gulch an inner zone, and massive white quartz the the pegmatites as pre-Beltian and post-meta­ METAMORPHIC PEGMATITES and is shown on the geologic map (plate 1). central zone. Massive tabular white quartz bod­ basalt. These pegmatites, therefore, are not as­ Pegmatite II is associated with the second per­ Metamorphic pegmatites are those pegmatites ies, not uncommon near the batholith and with­ sociated with the Tobacco Root batholith. iod of pre-Beltian metamorphism; it is included believed by the writer to have been formed by under this heading on scanty evidence. in it, may have had their origin from pegmatite It is not possible to say whether the pegma­ dikes. metamorphic processes. Metamorphic pegma­ tites are definitely igneous or metamorphic. The tites are somewhat controversial; the writer Two large bodies of pegmatite (the larger be­ Pegmatites formed by both igneous and meta­ features shown in figure 6 are in part analogous adopts those views with regard to their origin ing the Mineral Hill pegmatite) occur in the morphic processes are present within the map to the appearance of a porphyroblast in meta­ set forth by Ramberg (1956, p. 209-210). north end of the map area. More accurately, area. Their distinction is not a simple matter; morphic rock. This tends to support a meta­ these bodies consist of an intermixture of peg­ one origin or another cannot be assigned defin­ "This is a metamorphic-metasomatic theory in morphic origin for the pegmatite. On the other which diffusional transfer is essential ... The matitic material, gneiss, amphibolite, biotite itely to every pegmatite. hand, the structural form of the largest body of theory is based upon but two assumptions : schist, and serpentine-pyroxenite layers. They The metamorphic pegmatites in general are this pegmatite in upper Dry Boulder Gulch is (1) That diffusion may occur in rocks during- re­ lie in the crests of two- isoclinal anticlines that in tabular to lenticular bodies parallel to the that of a stock. This tends to support an igne­ gional metamorphism and (2) that, under re­ plunge about 55 0 northwesterly. These folds foliation in the enclosing metamorphic rocks. gional metamorphism, the elements constituting were revealed by detailed mapping in the north ous origin for the pegmatite. quartz and alkali feldspar are generally consid­ They occur in a definite structural position, as erably more diffusive than the elements of ca­ part of the map area where exposures are ex­ in the crest of an isoclinal anticline. The miner­ ANDESITE femic minerals . .. " cellent, mapping more detailed than undertaken als present are those of the surrounding meta­ Andesite occurs in the west and north parts So far as the present author knows, no one in the rest of the area. Similar structures are morphic rock. Leptite, for example, gives rise of the map area. (Plate 1.) It is a reddish-brown questions that quartz-kyanite segregation veins probably present elsewhere in the area, but were to a quartz-plagioclase-microcline pegmatite. rock in outcrop, massive, and has small angular in kyanite schist, for example, are formed by not revealed by reconnaissance-scale mapping. The pegmatite may still retain some of the foli­ phenocrysts of white plagioclase. The plagio­ diffusion of the constituents of these minerals Because of probable faulting the trace of the an­ ation of the host rock. Certain minerals charac­ clase is strongly altered to sericite. Much of to cracks in the rock, formed late in the time of ticlinal axial plane is shown for only a short dis­ teristic of metamorphic rocks, such as kyanite the very fine-grained groundmass is also altered metamorphism. It does not seem illogical to sup­ tance on the map; it probably extends northwest or sillimanite, may be present in the pegmatite. to sericite. pose that quartz-kyanite-feldspar may diffuse beyond the map area.