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1982 Petrology and alteration in the core of the Bear Lodge Tertiary intrusive complex, Bear Lodge Mountains, Crook County, Wyoming Michael Wilkinson University of North Dakota

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Recommended Citation Wilkinson, Michael, "Petrology and alteration in the core of the Bear Lodge Tertiary intrusive complex, Bear Lodge Mountains, Crook County, Wyoming" (1982). Theses and Dissertations. 323. https://commons.und.edu/theses/323

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THE BEAR LODGr; TERTIARY INTRUSIVE COMPLEX,

BEAR LODGE MOUNTAINS, CROOK COUNTY, WYOUING

by Michael Wilkinson

Bachelor of Science, Oregon State University, 1977

A Thesis

Submitted to the Graduate Faculty

of the

University of North Dakota

in partial fulfillment of the requirements

for the degree of

Master of Science

Grand Forks, North Dakota

August 1982 CEOLOGY LIBRARY lilnrlltJ of Nori- Dd1t1 JL 1'· I • ,./ V'

This thesis submitted by Michael Wilkinson in partial fulfillment of the requirements for the Degree of Master of Science from the Univer­ sity of North Dakota is hereby approved by the Faculty Advisory Committee under whom the work has been done.

~~(Chairman) ~~'64.

This thesis meets the standards for appearance and conforms to the style and format requirements of the Graduate School of the Univer­ sity of North Dakota, and is hereby approved. Permission

PETROLOGY AND ALTERATION IN THE CORE OF THE BEAR LODGE TERTIARY INTRUSIVE COMPLEX, Title BEAR .LODGE MOUNTAINS, CROOK COUNTY, WY~O=M=I=N~G~~~~~~-

Department

In presenting this thesis in partial fulfillment of the requirements for a graduate degree from the University of North Dakota, I agree that the Library of this University shall make it freely available for inspection. I further agree that permission for extensive copying for scholarly purposes may be granted by the professor who supervised my thesis work or, in his absence, by the Chairman of the Department,.or the Dean of the Graduate School. It is under­ stood that any copying or publication or other use of this thesis or part thereof for financial gain shall not be allowed without my written permission. It is ~lso understood that due recognition shall be given to me and to the University of North Dakota in any scholarly use which may be made of any material in my thesis.

Signature ?~~ /0~~ Date ap,2 J.~, /_5J'_f:Z.....c....-__

Hi TABLE OF CONTENTS

LIST OF ILLUSTRATIONS vi

LIST OF TABLES. viii

ACKNOWLEDGMENTS ix

ABSTRACT ..• X

INTRODUCTION. .. 1

Location and General Geology of the Bear Lodge Complex 1 Previous Work . ... 4 Purpose and Methods. 7

PRECAMBRIAN GRANITE 9

Pet rography. • 9 Rock Chemistry 14 Field Relationships. 16

PALEOZOIC SEDIMENTARY ROCKS 18

TERTIARY INTRUSIVE ROCKS. 25

Lattte Porphyry ... 25 Petrography and Mineral Chemistry Rock Chemistry Field Relationships

Alkali Trachyte Porphyry 41 Petrography and Mineral Chemistry Rock Chemistry Field Relationships

Microsyenite Porphyry and Related Rock Types 60

Phonolite Porphyry ..• , •••. , . 63 Petrography and Mineral Chemistry Rock Chemistry Field Relationships

iv Pseudoleucite Trachyte Porphyry ...... ~ . . 71 Petrography and Mineral Chemistry Rock Chemistry Field Relationships

Carbonatite (Sovite) .• 77

Petrography and l!ineral Chemistry Field Relationships

Mafic Xenoliths. . . . . 81

INTERPRETATION OF CHEMICAL TRENDS 83

CARBONATITE COMPLEXES AND ALKALINE MAGMATISM. 92

POTASSIUM FELDSPATHIZATION. 95

REGIONAL SIGNIFICANCE ..• 100

INTERPRETATION AND SEQUENCE OF EVENTS 102

Future Studies ••.•.•.•. 110

ECONOMIC POTENTIAL AND EXPLORATION TECHNIQUES 111

APPENDICES

APPENDIX A. Description of Rock Types in the Bear Lodge Complex . • . . . . • . . . • . • 116

APPENDIX B. Optical Characteristics of Latite Porphyry Minerals 119

SELECTED REFERENCES •....•.. 123

V LIST OF ILLUSTRATIONS

Figure

1. Generalized Geologic Map of the Black Hills Region and Location of the Bear Lodge Complex. . ... 2

2. Photomicrograph of Precambrian Granite Sample BL-584. 11

3. Stratigraphic Section of the Black Hills Region 19

4. Photomicrograph of Latite Porphyry Sample BL-52 26

5. Ternary Diagram for Pyroxenes in Latite Porphyry. 32

6. Outcrop Exposure of Latite Porphyry .. 38

7. Photomicrograph of Altered Mafic Phenocrysts from Alkali Trachyte Porphyry Sample BL-494 ...... •••• 45

8. Microprobe·x-Ray Energy Spectrum for Altered Mafic Minerals in Alkali Trachyte Porphyry Sample BL-584 47

9. Outcrop Exposure of Alkali Trachyte Porphyry, 56

10. Photomicrograph of Phonolite Porphyry Sample BL-12. 64

11. Hand Specimen of Pseudoleucite Trachyte Porphyry Sample BL- 7 36 • • • • • • • • • • • • 72

12. Mic roprobe X-Ray Energy Spectrum Showing Rare Earths in Ca rbona t ite ...... • . • . • . . • . 79

13. Silica-Alkali Variation Diagram for Tertiary Intrusives in Northeast Wyoming • . . • . . . . . 85

14. (Na o + K o + CaO) - (FeO + MgO) - (Si0 + Al o ) Ternary 2 2 2 2 3 Diagram ...... ~ ...... • . . 87

15. Na o - Cao - K o Ternary Diagram. 87 2 2 16. Al o - K o - Si0 Ternary Diagram. 90 2 3 2 2 17. CaO - MgO - FeO Ternary Diagram .. 90

vi ---··------

Figure

LB. Cross Section Through the Ilear Lodge Complex~ . . . . " 106

19. Idealized Cross Section Across the Bear Lodge Mountains 108

Plate

1. Geologic Map of the Warren Peaks Area, Crook County, Wyoming.

vii LIST OF TABLES Table

l. Modal analyses of Precambrian Granite •..... , . 10 2. Normalized Chemical Analyses and Normative Mineralogy of Precambrian Granite ...... ~ .. 1S

3. Modal Analyses of Latite Porphyry 28

4. Normalized Chemical Analyses of a Zoned Oligoclase- Andesine Phenocryst •.... 30

5. Normalized Chemical Analyses of Latite Porphyry Minerals. 31

6. Normalized Chemical Analyses and Normative Mineralogy of Latite Porphyry. • . . • • ...... 36

7. ~odal Analyses of Alkali Trachyte Porphyry. 42

8. Normalized Chemical Analyses of Alkali Trachyte Porphyry Minerals ..... 43

9. Normalized Chemical Analyses and Normative Mineralogy of Alkali Trachyte Porphyry . . . . • ...... • . 50

10. Normalized Chemical Analyses and Normative Mineralogy of Microsyenite Porphyry and Related Rocks. 62

11. Modal Analyses of Phonolite Porphyry ..•.. 66

12. Normalized Chemical Analyses and Normative Mineralogy of Phonolite Porphyry. . . . • •.•... 69

13. Normalized Chemical Analyses of a Zoned Pseudoleucite Phenocryst ...... • 74

14. Normalized Chemical Analyses and Normative Mineralogy of Pseudoleucite Trachyte Porphyry •....•.• 76

1.5. Optical Characteristics of Latite Porphyry Minerals 120

viii_ ACKNOWLEDGEMENTS

The assistance and advice offered to me by the members of my graduate committee has proved to be of great value. Sincere appre­ ciation is extended to Dr. Frank Karner for his guidance and construc­ tive cricitism during the course of this study. I am also indebted to Dr+ Don Halvorson for sharing his ideas and experience~ and to

Dr. John Reid for providing assistance in the preparation of this manuscript. Mr. David Brekke also deserves recognition for bis tech­ nical assistance in acquiring chemical data.

I am especially grateful to Mr. Miguel Lujan, Mr. John Landreth and Nr. Alan Levy' for their expertise and guidance, and to Molycorp,

Inc. for their support.

I also would like to express a warm thanks to Mr. and Mrs. Steve

Hilte, and all of Sundance, Wyomins, for their gracious hospitality.

ix ABSTRACT

The Bear Lodge complex is an elongate, dome-shape uplift about

8.5 km long by 4 km wide, 10 km northwest of Sundance, Wyoming. The study area is centered about Warren Peaks and includes approximately 2 23 km. The igneous intrusive core of the complex is mostly alkali trachyte but latite is abundant in the southern portion of the complex.

Phonolite, pseudoleucite trachyte porphyry, and carbonatite dikes cut across the core. Precambrian granitic rocks and lower Paleozoic sedi­ mentary rocks flank the intrusion and occur as xenoliths.

The latite is relatively unaltered and consists mostly of oligo- clase, andesine, alkali feldspar, salite, and hastingsite phenocrysts in an aphanitic groundmass of alkali feldspar. The alkali trachyte is pervasively altered and exceptionally potassic. It is intensely fractured, brecciated, and stained by limonite. Phenocrysts of sanidine and relict mafic mineral grains are present in an aphanitic groundmass.

Altered mafic minerals are replaced by potassium feldspar. The ground­ mass is also very potassic. Potassium feldspar and limonitic Th-RE veinlets cut the alkali trachyte.

Compared with other Tertiary rocks in northeast Wyoming) the alkali trachyte has anomalously low Na o/K 0 ratios. Chemical changes 2 2 resulting from alteration of the trachyte include increased K 0 and 2 FeO + Fe o , and decreased Na o, Cao, and MgO. Variation in Si0 is a 2 3 2 2 reflection of the degree of silicification.

X Alteration of the alkali trachyte is due to feldspathization, a high level type.of potassic metasomatism found in alkaline carbonatite complexes along the East African rift zone. The Bear Lodge complex is similar to some of the East African complexes and is also feldspathized.

The rocks are hypabyssal and represent an intermediate volcanic level.

Rare earth elements are present, primarily in Th-RE veins and carbonatite. Th-RE veins occur throughout the central portion of the intrusion. Carbonatite may be more abundant at depth. Potential for economic rare earth deposits in the Bear Lodge complex is significant.

xi INTRODUCTION

Location and General Geology_of the Bear Lodge Complex . - The Nortnern Black Hills Tertian, igneous province extehds approximately 105 km in a west-northwest direction, from Bear Butte,

South Dakota, in the east, to Missouri Buttes, Wyoming., in the west.

Thirteen intrusive centers occur within this zone (Figure 1). These include, from east to -west, Bear Butte, Vanocker, Gilt Edge-Galena,

:

Sundance ~!ountain, the Bear Lodge Mountains, and the Devils Tower-

Missouri Buttes (Karner, 1981).

The study area is centered about War'ren Peaks in the southern

Bear Lodge Mountains. It is approximately 10 km northwest of Sundance and 25 km southeast of the Devils Tower, in Crook County, Wyoming.

The map area i.ncludes sections 7, 8. 17, 18, 19, 20, 27, 28, and 33,

T. 52 N., R. 63 W.

The southern Bear Lodge Mountains are formed by an elongate uplift in the shape of a dome with lower Paleozoic strata flanking an intrusive igneous complex. The igneous core is about 8.5 km long by

4 km wide and trends north-northwest.

Intrusive rocks in the complex are hypabyssal in character and include alkali trachyte porphyry. latite porphyry, and microsyenite porphyry. The central portion of the core is formed of alkali trachyte

1 ,-,• . . I;,r,;:,;:,~~;;:_,:- ,,, 2

Fig. 1. Generalized geologic map of the Black Hills region showing the location of the study area. The areal extent of the Black Hills is defined by the top of the Fall River Sandstone. Base map (from Gries and Tullis, 1955, p. 32). 3

I ---- Montana ______j Wyoming j North i I : I 'I .

R~ G3 w. I •• z 7 B I N I ~ H 18 17 I

11 20

29

l 0 2 km

EXPLANATION

~ Cenozoic Igneous Cente~s

~ Precambrian Granitic Rocks

{II!] Precambrian Mecamm:ph{c Rocks 0 1,0 I

'I L__ __ South Dakota ------r- - Nebraska ' 4

porphyry and has been pervasively altered. Later intrusions include latite porphyry in the southeast and southwest, and microsyenite

porphyry in the north. Phonolite porphyry, pseudoleucite trachyte

porphyry and carbanatite dikes cut the complex. Large xenoliths of

Precambrian granitic rocks and lower Paleozoic strata have been incor­

porated into the intrusion (Plate 1). A brief description of these

rock types is provided in Appendix A and is intended to be used in

conjunction with the geologic map.

Several small bodies intrude the Paleozoic strata within a few kilometers of the complex. To the northwest these are similar to the latite porphyry, whereas phonolite porphyry bodies occur just outside

the southern and western borders of the core.

Previous Work

The first major contribution to the geology of the Bear Lodge

Mountains was by Darton (1905a,b). Darton recognized the large intru­ sive mass as being post-Cretaceous and presumably of Eocene(?) age.

By analogy with other Tertiary Black Hills igneous rocks he inter­ preted the Bear Lodge to be a laccalith. The principal rock type was described as syenire porphyry with variations encompassing mon­ zonite and trachyte porphyries. Phonolite porphyry was interpreted to be the youngest igneous rock in the uplift, forming intrusive sheets, laccoliths, and dikes, as it does in much of the Black Hills.

Granite masses were interpreted as xenoliths in later igneous rocks.

Darton (1905a) also reported breccias, formed by successive intrusions, with fragments of previously intruded porphyry, granite, and sedimentary rock. 5 Brown (1952) agreed that the intrusive mass formed a laccolith.

He suggested that the trachytes and phonolites were emplaced from secondary magmas formed by fractional differentiation of a primary magma somewhere at depth.

At least two periods of igneous activity were recognized by

Chenoweth (1955). He described them as older and younger intrusives.

The older intrusives consisted of very altered porphyritic syenites and traehytes. The younger intrusives were less altered syenites, trachytes, and andesites, often containing feldspathoids. Latites and basalts were also considered younger.

The Mineral Hill alkalic ring dike complex is 29 km east of the Bear Lodge and intrudes Precambrian schists. Welch (1974) reported a core consistin~ of feldspathic breccia with an outermost nepheline syenite ring surrounding both the core and a partial inner ring of pyroxenite. Various late magmas of dioritic composition intrude the complex. Three igneous bodies, Sundance Mountain, Sugarfoot, and the

Sundance dike, were studied by Fashbaugh (1979) and are centered about 12 km southeast of Warren Peaks. Sundance Mountain was mapped as massive quartz latite units with layered ash falls and ash flow tuffs above, and abundant breccias in lower parts of the section~

Both Sundance Mountain and Sugarloaf were interpreted as extrusive, mixed volcanic cones. Fashbaugh also studied a phonolitic mass, the

Sundance dike, and classified it as foyaite, thereby emphasizing its

hypabyssal origin. White (1980) mapped several small bodies flanking the intrusion

in the Lytle Creek area just northwest of Warren Peaks. He classified 6

the rock types as calc-alkali phonoli"te, ca 1 c-alk a li trac hyte, trachyte,

ferrohastingsite trachyte, pseudoleucite trachyte porphyry, latite,

syenite, and granite. uost" of the roe k s were reporte d as chemically alkaline and similar to rocks of the potassium alkali olivine basalt series. Assimilation of granitic material, by a potassium alkali olivine basalt series magma, was speculated to be a major factor involved in the formation of these rocks.

Halvorson (1980) studied an area northwest of the Bear Lodge

Hountains which includes the Devils Tower, Missouri Buttest and Barlow

Canyon. The igneous rocks present at Missouri Buttes were classified as £aid-bearing alkali trachyte and analcime phonolite, whereas the

Devils Tower and Barlow Canyon were classified as analcime phonolite.

Alloclastic breccias with a crystal-charged volcanic glass matrix and collapse structures led to the interpretation that both the Devils

Tower and ~issouri Buttes are the erosional remnants of volcanic necks.

Barlow Canyon is considered a small laccolithic dome. Halvorson 1 s st1.1dy indicates a differentiation trend, by fractional crystallization due to flotation and flow differentiation, from trachyte to phonolite.

Phonolite bodies flanking the Bear Lodge intrusive were examined by O'Toole (1981). The rock types there were classified as phonolite, phonolite porphyry, trachyte porphyry, and sodalite-bearing phonolite.

He interpreted these bodies as sills intruding lower Paleozoic sedi­ mentary rocks~ Chemical trends pointed toward the generation of phonolitic magma by partial melting of alkali olivine basaltic material in the lower crust. 7

PuIJ>ose and Methods

This study is designed to develop an understanding of the geo­

logic history and the processes involved in the formation of the Bear

Lodge intrusive complex. An attempt has been made to accomplish the

following: 1) define the petrological variation, 2) develop an insight

into the type of alteration present, 3) establish a sequence of events,

and 4) discuss the economic potential.

Geologic mapping for the project was performed by pace and com­

pass methods. A 1:10,000 scale was used for the base map (Plate 1).

Thin section and sample location numbers are provided on the geologic

map and are preceeded by BL-, denoting Bear Lodge, in tables and illus­

trations. Outcrop locations are indicated by darkened colors.

Igneous rocks were sampled on the basis of lithological variety and availability. Difficulties were sometimes encountered in distin­ guishing the rock types present. These difficulties were a result of

the aphanitic ch_aracter of the rock:, extensive alteration and weather­

ing, and a pronounced lack of outcrop exposure. All contacts on the geological map (Plate 1) are necessarily dashed and in many areas mapping was accomplished exclusively by the use of float. The best exposures are provided by exploratory trenches, prospect pits, and

road cuts. The trenches and pits were bulldozed during the early

1.950s while exploration for thorium was under way.

Interpretation of the geology is based on observati.ons made in

the study area, thin section investigation~ and by chemical and X-ray diffraction analyses. Modal analyses were completed by taking 500 counts per thin section and are expressed in volume percent. Whole 8

rock and mineral analyses were conducted with a JEOL 35C scanning

electron mic.roprobe and are expressed in oxide weight percent. Analy­

ses expressing FeO include FeO + Fe o . X-ray diffraction was performed 2 3 by a Philips-Norelco high-angle diffractometer and Cu K-alpha radiation.

f 1 PRECAMBRIAN GRANITE

Petrography

The granite is a grayish pink, holocrystalline rock which fre­ quently displays a yellowish brown color due to staining by limonite and hematite. It may be medium to coarse-grained but is most often medium-grained and exhibits a xenomorphic-granular texture. Porphyritic varieties, with feldspar phenocrysts as large as 25 nun across, are pre­ sent but not common. The major minerals are quartz, microperthite, microcline, and sodic oligoclase. Accessory minerals include apatite, biotite and pyrite where unaltered. Limonite pseudomorphs after pyrite are common in altered samples. The modal analyses of four granite samples are given in Table 1. The amount of quartz and feldspars substantiates the classification as granite (Streckeisen, 1967)" Thin siliceous veinlets sometimes cut the granite. Sericitic clay, limonlte, hemattte, and calcite are common alteration products. Disseminated fluorite may occasionally be present.

Quartz comprises 28 to 35 percent of the rock. Individual grains are anhedral and ordinarily occur as blebs with well rounded grain boundaries. The quartz is colorless but cloudy and is often intensely shattered (Figure 2). Less altered samples contai.n quartz with inter­ locking grain boundaries.

The most abundant feldspar is microperthite which varies in con­ tent from 19 to 39 percent. Where fresh, it forms colorless, anhedral

g 10

TABLE 1

MODAL ANALYSES OF PRECAMBRIA.~ GRANITE

Sample Number BL-597* BL-568 BL-543 BL-584

Quart2: 27.8 28.4 30.0 35.4

Microperthite 3.8.2 35.8 33.4 19.4

Mic roe.line 15.2 16.6 13. 6 19.0

Sodic oligoclase 11.8 12.4 10.8 20.0

Sericite 5.2 4.8 4.4 3.8

Limonite pesudomorphs 1.8 1.4 2.0 1.6 after pyrite

Apatite tra 0.6 0.8 0.8 Biotite o.o o.o 4.8 tr Calcite o.o tr 0.0 o.o

atr = trace, less than 0.2 percent *See Plate 1 for location and samples. 11

Fig. 2. Photomicrograph of Precambrian granite sample BL-584. Note the shattering of quartz, limonite veinlets, and iron staining of feldspars. Plane polarized light, 25 X.

13 grains. Exsolution laminae are sometimes distorted, suggest i ng tath the rock may have been subjected to strain after its initial formation.

Microcline ranges from 13 to 19 percent. Grains • are an h erad 1 and colorless but are frequently stained by limonite. Combined pericline-albite twin lamellae are sometimes bent and, like the micro­ perthi te ,' suggest that the rock may have been subjected to strain.

Anhedral sodic oligoclase (Ab ) is the most strongly altered 12 feldspar and comprises 10 to 20 percent of the granite. Albite twins may be masked by alteration to clay and limonite staining. Myrmekite intergrowths occur near plagioclase borders.

Biotite, when present, forms anherlral flakes. The flakes are strongly pleochroic from yellow brown to dark brown. Biotite is largely altered to limonite and is absent in most samples. Anhedral apatite is nearly ubiquitous but does not exceed 1 percent. It is very fine to fine-grained. Pyrite, or limonite pseudomorphs after pyrite, forms 1 to 2 percent of the rock and occur as disseminated cubes and irregular grains. Many of the opaque pyrite grains exhibit limonite halos formed by oxidation. Fluorite may be present as rare disseminated crystals or thin veinlets~ The granite has been subjected to extensive alteration, shatter- ing and microfracturing. Dark brown limonite and hematite are the most dominant alteration products. They form in veinlets cutting individual minerals, along crystal boundaries and cleavage planes, and as a stain which imparts a cloudy appearance, especially to the feldspars. Sericite

al.so contributes toward the cloudy appearance but is more prominant

along grain boundaries. 14

The granite may be altered in a manner which gives the rock a partially leached appearance~ Small irregular patches of the rock consist of feldspars and open spaces lined by limonite. Quartz is thought to have formerly occupied the open spaces. In some samples fi.ne-grained silica may occupy microfractures. The combination of siliceous veinlets, quartz blebs, and the absence of quartz, hints toward an episode in which silica was mobilized.

One of the most distinctive features of the granite is the lack of accessory mi.nerals normally found j_n most granites. About 10 per­ cent biotite was found i.n an apparently less altered granite body adjacent to the study area. The sporadic occurrence and strong altera­ tion of biotite suggests that some minerals may be absent due to the alteration.

Rock Chemistry

The chemical analyses and CIPW normative mineralogy for two granite samples are given in Table 2. These are comparable to the average calc-alkali granite from Nockolds (1954). Both samples con­ tain slightly less silica and more alumina than the average granite.

The normative. quartz content, 20 to 25 percent, shows that the granite is considerably ove.rsaturated. It is also strongly peraluminous, with

Al 0 exceeding CaO + Na 0 + K 0 (Shand, 1947). The combined alkalies 2 3 2 2 are somewhat high, as i.s evidenced by 65 to 66 percent normative orthoclase plus albite. Na 0tK 0 ratios are consistent with the average 2 2 granite. The presence of other normative minerals are probably due to clays·and lirnonitic alteration~ 15

TABLE 2

NORMALIZED CHEMICAL ANALYSES AND NORMATIVE MINERALOGY OF PRECAMBRIAN GRANITE ======- Sample Number BL-597 BL-584 A

68.9 70.8 72.1

15.9 IS.I 13. 9

FeO 3.6 2.9 2.5

MgO 0.8 0.6 0.5

CaO 1.0 0.7 1.3

4.0 3.3 3.1

5.5 6.2 5.5

0.3 0.2 0.4 o.o o.o 0.2 MnO 0.1 o.o 0.1 Cl o.o o.o o.o o.o

quartz 20.6 25.3 29.2 corundum LS 1. 7 Q.8 orthoclase 32.6 37.0 32.2 albite 33.5 27.7 26.2 anorthite 5.0 3.5 5.6 nepheline o.o 0.0 o.o enstatite 1.9 1.6 1.3 ferrosilite 1.8 0.4 1.7 fosterite 0.0 o.o fayalite o.o 0.0 magnetite 2.5 2.5 1.4 hematite o.o o.o ilmenite 0.5 0.4 0.8 apatite o.o o.o 0.4

Na 20/Kz0 ratio 0.73 0.53 0.56

A = Average calc-alkali granite, from 72 analyses (fockolds, 1954). 16

Field Relationships

Several granite bodies are present in the Bear Lodge complex and are interpreted as zenoliths. Twelve of these are large enough to be included on the geologic map (Plate 1). Most were mapped on the basis of float but good outcrops are found at the top of Warren

Peaks and in Bear Den Canyon. The granites are the oldest rocks within the complex and have been dated at 2,600 my (Staatzt verbal communication}. Various granites and metamorphic rocks form most of the Precambrian uplift in the Black Hills of South Dakota (Figure 1) and it seems logical they form the Precambrian basement below the

Bear Lodge Mountains. O'Toole (1981) notes that these rocks resemble the Little Elk granite, in the northern Black Hills, which is dated at 2,560 my (Zartman, Norton~ and Stern, 1967). Gneisses and amphibole schist pods are minor but are found with granites.

Granite xenoliths large enough to be included on the map are restricted to the southern half of the complex (Plate 1). Stratigraphi­ cally, the granites lie below the Cambrian Deadwood Formation and in

SE\, NE\, Sec. 33, T. 52 N., R. 63 W., the granite is separated from the Deadwood Formation by only 4 m of latite. The contact here demon­ strates an interdigitating relationship with individual sills, 2 to 5 m wide, cutting the granite and conglomeratic beds of the lower Deadwood

Formation. This same relationship is observed elsewhere in the Deadwood

Formation. Large mappable granites are absent from the northern portion of the complex, where younger Mississippian and Pennsylvanian strata are in contact with igneous rock. 17

Contacts between granite and intrusive rocks are sometimes brecciated. Fragments of granite are included in the trachyte and near the top of Warren Peaks it sometimes appears that trachyte frag­ ments are included in the granite but this is because the trachyte has invaded and brecciated the granite near its contact. A wide range in size distribution of granite xenoliths is apparent. The largest, which forms four of the Warren Peaks, measures approximately 1200 m long by 240 m wide at the surface (Plate 1). Smaller xenoliths have also been observed in trachyte, latlte, and phonolite. It is probable that the granites were rafted up with the successive episodes of intru­ sive activity which formed the Bear Lodge complex. PALEOZOIC SEDIMENTARY ROCKS

The Bear Lodge complex is discontinuously bounded by Paleozoic

sedimentary rocks which dip radially outward from the central core

(Plate 1). Sedimentary rocks in the map area range from Cambrian

through Pennsylvanian in age. The intrusive is in contact with the

Deadwood Formation in the south and southwest, with the Whitewood and Pahasapa Formations in the west, and with the Minnelusa Formation

in the north. The descriptive characteristics of each formation are

given below. These characteristics, along with a generalized strati­

graphic section for the Black Hills (Figure 3), were used as the basis

:or mapping the s~dimentary rocks.

The Deadwood Formation was named by Darton (1901) with the type

section along Whitewood Creek, just north of Deadwood 1 South Dakota.

In the northern Black Hills the Deadwood Formation consists of over 120 m of conglomerate~ sandstone, shale, and limestone, and rests unconformably on Precambrian schists and granites. In the southern Black Hills the

Deadwood Formation thins to less than 15 m (Darton, 1904). Meyerhoff and Loch~~n (1935) have correlated four faunal zones with upper Cambrian strata in Wisconsin, whereas Furnish, Barragy, and Miller (1936) have assigned a Late Cambrian to Early Ordovician age to the Deadwood Forma­ ti.on. Kulik (1965) divided the Deadwood Formation into lower, middle, and upper members. The lower, Dark Canyon Member, consists of a local basal conglomerate and pebble quartzite overlain by massive quartzite beds 2-3 m thick. The quartzite is white to pinkish gray in color and 18 19

Fig. 3. Stratigraphic section of the Black Hills region (from Wyoming Geological Association Guidebook, 1968, p. 5). GENERAL OUTCROP SECTION OF THE BLACK HILLS AR F IOH I RI T , ... u,.. __ """_ PI.UXUC --·-­...,...... '""'"'" ·-· ...,.. -___.._ ._ ,::;--;;;;;:--- t Ol.l®eCNIE-- •;: •w PAl.EOC:Uiff: " i ...... ""'~---- _. - ,______...... ,qc•--"/111 . ... • ... ------­ U..-l'"u-"""' -tMl"-.... ~,..,,, ----1--~·c-=c"""""----.. l---·-··...... -----· fflll tULLI a,.,.IP·-·"'--- -·- ,.,.__... ,,, ...... _... _ __ -·-·--...... -- UPPER --- -_,,1-_.. ----··­

~ t ..... _, __/Q __ ..... ~ 0 w .. . u ---~

~• w ...,. .;;._ ...,, ·-.,., . .,.. -- • ::::::.:=i...:":'1,ti.J;__ :-1<,f:::: __ -· .. 11. - ,:j.---1--"~::.~--.:· - - , H-•·=- _;- ·~~~__ ,,,_._ -:;;;--- __... .;;;; ...... -~. *·---~ ,_.,..,..... --.. ,,. ... n, -.--~"'1·-·••14'-..,. l ...... __ _ ·- ...._ ·-- tw _ .. -In--·I'~--;;;;;;;;;;-

··-·-·-·-· ... 1.. ,._ - ,;"'··-· --! ...... -.. 1... , • ... -· .,;;;;,-..;;,·1'"""'----

P!RIUAH ... "" ...... __ __ l,,~;.::_~n:_.::..~.~. - .-.... ,..,.$,..... ____..... _ P£NNSYt..l/MtAH'...._. -- ______-·- ~,c•cHcu,. ______, ...... ,,.,..,.,_"'_ 11-, .....,.. 1-- .. ,..,,. ~-.... ,..,. '!""If~sio,,. ,-;;;-­ -~.,,., ,,,,_I>• ...- o,1,:;;;...---"-· ----,:~=:. •:: .,... ;..c,.·"" ~..... ·,:--··.. ,--~··· - ""'"'"·__ .1 ..,,,,_,... , ,_.., __.. , - ..- .,.,,,, -·-, - "1 -IU oWH-Ht'°' 21 forms resistant ridges~ It is fine to medium-grained, very well sorted~ and consists of subangular to subrounded quartz grains. The middle member, named the Boxelder shale, contains a sequence of inter­ bedded shales, siltstones, and a glauconitic dolomite. The shales are dark green and form thinly laminated beds up to 1 m thick. Expo­ sures are poor but where metamorphosed they are more resistant and often slaty. The most characteristic feature of the Boxelder }1ember is an intraformational, flat-pebble conglomerate. It consists of shale with elongate pebble and cobble limestone clasts. The upper,

Red Gate Member, consists mostly of a massive red sandstone in the northern Black Hills. In the Bear Lodge, the upper member is very similar to the lower member and consists of massive, white quartzite beds that are well sorted and fine to medium-grained. The upper por­ tion of the Red Gate Member ls characterized by abundant Scolithu'!_ burrows. McCoy (1952) separated the .5colithus zone from the Deadwood

Formation and named these sandstones the Aladdin Formation. For the purpose of field mapping, the Scollthu~ zone is included in the upper

Deadwood Formation.

The contact between the Deadwood Formation and igneous rock may be described as an interdigitating zone. Individual sedimentary beds are separated by sills intruded along bedding planes. This rela­ tjonship is most easily observed in roadcuts in NW~, SW~, Sec. 33. T. 52

N., R~ 63 W~, where massive quartzite beds are intruded by latite, and in NE!,;, SE~, Sec. 33, T. 52 N., R. 63 W., where granite and basal con­ glomerate are intruded by alkali trachyte. Some of these beds have been completely incorporated into the igneous rock and now occur as large xenoliths. 22

The Winnipeg Formation was named by Dowling (1895) for its type locality near Lake Winnipeg, Manitoba. McCoy (1952) divides the

Winnlpeg Formation into two members. The Ice Box shale is the unit overlying the ~colithus_ quartzite and the Roughlock siltstone is the unit underlying the Whitewood Formation. The Ice Box shale consists of 9-10 m of green silty shale and contains a hasal sandstone which lies nonconformably above the Deadwood Formation. The Roughlock silt­ stone consists of 7-9 m of siltstone to fine-grained sandstone~ Furnish,

Barragy, and Miller (1936} have proposed an Ordovician age based on fossil evidence. No occurrence of Winnipeg Formation was observed in the southern Bear Lodge Mountains.

The Whitewood Formation was named by Jagger (from Darton, 1904) for its typical exposures in Whitewood Canyon~ just below Deadwood,

South Dakota. In the Bear Lodge, the Whitewood Formation is best exposed in SW!!, SW~, Sec. 33, T. 52 N., R. 63 W. The rock is a dark gray to buff colored, finely crystalline dolomite or dolomitic lime­ stone. Characteristic dark brown mottles are probably former burrows and help distinguish the Whitewood Formation from the Pahasapa Forma­ tion above. No contact relationships were observed. Fossil evidence for an Ordovician age was ftrst presented by Walcott (1892}.

The Englewood Formation, named by Darton (1901), forms the basal unit of Mississippian strata in the Black Hills area. Its type locality is in the northern Black Hills, near Englewood, South Dakota.

The Englewood Formation consists of thin-bedded, pinkish to buff colored limestone with shale occurring locally near its base~ Corals, brachia~ pods, and shark's teeth help correlate the Englewood Formation with 23

Mississippian strata in the Mississippi Valley (Gries, 1952). The

Englewood Formation lies unconformably above the Whitewood Formation in the northern Black Hills and above the Deadwood Formation j_n the southern Black Hills. Exposures of Englewood Formation are absent in the study area.

Darton (1901) first named the Pahasapa Formation. It consists of a medium gray to beige colored, medium crystalline limestone which weathers to dark gray. Bedding thicknesses range from O.1 to 1 m.

Fossiliferous zones probably represent ancient reefs. One such zone is exposed on the access road, approximately 200 m south of Sec., 33, where it is 7 m thick. Fossil material is broken but many samples are well preserved. Tabulate and rugosa corals, crinoid stems, and brachiopods have been identified. These fauna correlate with lime­ stones of the Madison Formation which is of Mississippian age (Darton,

1909). The limestone is moderately resistant but no contact with other sedimentary rock was recognized. The contact with intrusive rock is sharp and forms an interdigitating relationship, similar to the contact between the Deadwood Formation and intrusive rock. In most areas where the Pahasapa Formation is in contact with intrusive rock there is little evidence of metamorphism. In some areas, however, limestone is extensively recrystallized to marble.

The Minnelusa Formation was named by Winchell (from Ludlow, 1875), originally for a Pennsylvanian sandstone in the western Black Hills. The name now includes the entire sequence of beds between the Pahasapa For- mation below and the Opeche Formation above. In the Bear Lodge the

Minnelusa Formation consists of a white to buff colored, well sorted 24

sandstone which weathers to medium gray~ It is compoSed of medium-­

grained, subangular quartz grains and is extremely friable~ In the

northern portion of the map area the sandstone is metamorphosed to

quartzite~ On the basis of fossil evidence, most of the Minnelusa

is considered Pennsylvanian in age (Foster, 1958). No contacts with

other sedimentary rock was observed.

' 25

TERTIARY INTRUSIVE ROCKS

Latite Porphyry

Petrography and Mineral~ Chemistry

The latite porphyry is a holocrystalline, pale green to light olive, feldspar-rich rock.. It is moderately magnetic and sometimes calcareous. The latite is distinctly porphyritic with phenocrysts of oligoclase, andesine, alkali feldspar, salite, hastingsite, biotite, titaniferous magnetite, apatite, sphene, and melanite. Optic.al char­ acteristics are listed in Appendix~ and a photomicrograph shows some of the phenocrysts present in the rock (Figure 4). Groundmass fonns about half the rock and is usually trachytic.. Common alteration prod­ ucts include limonite, hematite, calcite, and clay minerals. Granite, quartzite, shale, trachyte, sanidine trac.hyte porphyry, and pyroxenite are present as xenoliths.

The modal analyses of four samples are given in Table 3. The lack of a more calcic plagioclase and the alkaline character of the groundmass classifies che rock as latite (Streckeisen, 1967).

The most abundant phenocrysts are plagioclase which vary between oligoclase and andesine, An to An . 'Euhedral to subhedral phenocrysts 26 36 comprise 13 to 25 percent of the rock and are sometimes arranged in glomeroporphyritic clusters. Typical albite twinning is common and

Carlsbad twins may also be present. Zoning is apparent in several phenocrysts- The chemical analyses of four potnts across a zoned .., '

Fig. 4. Photomicrograph of latite porphyry sample BL-52 showing phenocr.ysts of hastingsite, salite, oligoclase-andesine, apatite, and magnetite. Plane polarized light, 25 X.

TABLE 3

MODAL AJIALYSES OF LATITE PORPHYRY ---=-- Sample Number BL-50 BL-100 BL-710 BL-52

Oligoclase-andesine 18.8 16.4 12.6 4.0

Alkali Feldspar 5.4 7.0 5.2 9.0

Salite 11.6 12.6 8.2 14.6

Hastingsite 4.0 5.8 2.4 11.8

Biotite 0.0 1.0 6.2 2.8

Titaniferous magnetite 5.6 5.2 4.2 3.8

Apatite 0.8 0.8 o.o 1.6 Sphene o.o o.o tra tr Melanite 0.0 0.0 tr tr

Quartz 8.0 5.8 0.0 0.0

Groundmass 45.8 45.4 61.2 52.4

atr = trace, less than 0.2 percent 29

plagioclase are given in Table 4. The outer three analyses (A-C)

resemble andesine in composition, whereas the core (D) is more similar

to oligoclase (Deer, Howie, and Zussman, 1963). Na o and CaO values 2 show a sodic core (oligoclase) with a more calcic rim (andesine).

These variations indicate that zoning is reverse but oscillatory.

Plagioclase grains are usually slightly altered to cryptocrystalline

clay minerals. Alteration is strongest in their cores. Calcite may

be observed along microfractures and cleavage planes.

Alkali feldspars occur as phenocrysts and form less than 10

percent of the rock. They are mostly subhedral, nonperthitic, and

twinned according to the Carlsbad law. The lack of polysynthetic

twinning and high 2V angles, 65-80°, indicate thaL these are probably orthoclase.

Salite comprises 8 to 1.5 percent of the latite and occurs as subhedral to euhedral grains. Typically, they occur as slender pris­

~natic crystals and are sometimes twinned. Pleochroism is weak to moderate with X = pale green~ Y ~ green, and Z = yellowish· green. Zon­ ing is moderate with light brown iron-rich rims and light green Mg-rich cores. Analyses of two points are given in Table 5 (A,B) and indicate that zoning is normal. Si0 and MgO increase toward the core. whereas 2 FeO decreases. Small increases in Ti0 and Na 0 inward are also pre­ 2 2 sent. CaO is relatively constant. The pyroxenes are comparable to sali te analyses in Deer, Howie and Znssman (1963}. An FeSi0 - MgSi0 - 3 3 CaSi0 ternarydiagram shows that compositions plot close to the salite 3 field (Figure S). Altered salites usually exhibit limonite deposited about the crystal margins with calcite replacements in their central 30

TABLE 4

NORMALIZED CHEMICAL Ac~ALYSES OF A ZONED OLIGOCLASE- ANDESINE PHENOCRYST =-- A B C D

SiOz 59.8 59.2 60.4 63.4

Alz03 25.0 25.9 24.5 23.2 FeO 0.6 0.5 0.4 0.2

Mg0 0.2 o.o 0.2 0.0 CaO 7.0 8.1 6.5 4.7

Na2o 6.4 5.4 7 .1 7.4 KzO 0.9 0.8 0.8 0.9

TiOz 0.1 0.1 o.o 0.1 ~no o.o 0.0 o.o 0.0 ·----· Explanation

A= Rim, BL-50

B = 0.1 mm from rim, BL-50

C = 0.3 mm from rim, BL-50

D = 0.5 mm from rim, in center of core, BL-50

V. 31

TABLE 5

NORMALIZED CHEMICAL ANALYSES OF LATITE PORPHYRY MINERALS

A B C D E F

SiOz 49.0 51. 7 41.3 38.0 o.o 65.0

A1203 4.4 2.8 12.9 15.1 1.6 19.4 i·eo 12.0 9.4 15.3 17.4 88.0 1.4 MgO 8.4 10.3 9.4 11.9 o.o 0.3 CaO 22.9 23.8 12.2 o.o 0.1 0.9

Na2o 1.3 0.7 2.6 0.9 o.o 5.1 K2D 0.1 o.o 1.9 10.0 o.o 7.7

Ti02 1.2 0.7 3.8 6.4 9.5 0.3

Pz05 o.o 0.0 0.0 o.o o.o o.o MnO o. 7 0.5 0.3 0.4 0.6 0.0

Cl o.o 0.0 0.0 0.0 0.0 0.1

S03 o.o 0.2 0.2 o.o 0.1 o.o

Explanation

A= Salite rim, BL-52, zoned phenocryst

B = Salite core,, BL-52, zoned phenocryst C = Hastingsite, BL-52, phenocryst

D = Biotite, BL-52, phenocryst

E = Titaniferous magnetite, BL-52, opaque

F = Alkali Feldspar, BL-52, groundmass ~l <

32

Fig. 5. Ternary diagram for pyroxenes in latite porphyry shows compositions plot close to salite field.

j 33

A Diopside B Salite C Ferrosalite D Hedenbergite E Augite F Ferroaugite

••• B C

E F

10 20 30 40 50 60 70 80 90 100 Molecular percent FeSi03 34 portions. The ca lei te probably represents secondary fillings of salite casts.

Subhedral to euhedral hastingsite occurs as prismatic crystals with pseudohexagonal cross se.ctions and forms 2 to 12 percent of the latite. Twinning parallel to the {100) plane is common. Pleochroism is strong with X = yellowish brown, Y = light brown, and Z = dark greenish brown. An analysis of hastingsite is included in Table 5 (C) and resembles those provided by Deer, Bowie, and Zussman (1963) but contai.ns slightly more Ti0 • Bastingsite is sometimes replaced by 2 biotite having more than one orientation, as is indicated by optical continuities. Limonite may be deposited around crystal borders and thin calcite-filled microfractures may cut phenocrysts.

Biotite forms up to 7 percent of the latite but is not present in all samples. Subhedral phenocrysts are strongly pleochroic with

X = brownish green, Y = light brown, and Z = dark brown. An analysis

(Table 5, D) indicates a high Ti0 content. Biotite may be altered to 2 limonite.

Characteristically, the latite is weakly magnetic. Fine-grained magnetite occurs as cubic-shape and irregular granules disseminated throughout the groundmass. It may fill microfractures or form along cleavage planes and crystal boundaries with mafic minerals~ Alteration halos are frequently present around magnetite grains and indicate oxida­ tion to limonite. The analysis present in Table 5 (E) shows that the magnetite is titaniferous.

Apatite is present in most samples but forms less than 2 percent of the rock. It occurs as small euhedral grains with hexagonal or 35 prismatic forms. Anhedral grains may be common in some samples.

Apatite is colorless but sometimes cloudy. Sphene may occur in some samples but not in excess of about l percent. Twins are parallel to the length of euhedral rhombic sections. Melanite (titaniferous andradite) garnets are rare~ They are euhedral, very dark brown, and weakly pleochroic.

Small, subrounde.d quartz grains may form as much as 8 percent of the rock but are not always present. Samples with quartz grains occur near contact between latite and Deadwood quartzite. Clusters of cemented grains with sharp borders indicate that these are xenoliths and xenocrysts from the Deadwood Formation. Seconda_ry calcite and clay minerals commonly surround many of these grains~

The groundmass forms 45 to 62 percent of the latite. It is aphanitic and predominantly composed of alkali feldspar microlites arranged in a trachti.c texture~ Less often, microgranular to seriate textures are present. Limonite and hematite staining of the ground­ mass is common. An analysis is given in Table 5 (F) and illustrates an alkaline character~ The groundmass has sometimes invaded and broken earlier-formed phenocrysts of hastingsite.

Rock Chemistry

The chemical analyses and CIPW normative mineralogy is given in

Table 6. The latite is comparable to the average latite provided by

Nockolds (1954) but subtle differences do appear. The Bear Lodge Latite contains somewhat more silica but normative quartz is present only in 36

TABLE 6

NORMALIZED CllE;!ICAL ANALYSES AND NORMATIVE MINERALOGY OF LATITE PORPHYRY . -__ = -~c-::-- Sample Number BL-579 BL-95 BL-100 A --~-----·---·--·--· Si02 57 .5 57.6 59 .5 54.0 Al 0 17.5 16.6 16.5 17.2 2 3 FeO 7.2 6.6 5.2 7.8

MgO 2.4 2.9 3.5 3.9

CaO 4.7 5.9 5.6 6.8

Na o 3.9 4.1 4.4 3.3 2 5.8 4.2 4.4 K20 3. 7 Ti02 0.6 0.8 0.5 1.2

P205 0.0 1.2 1.0 0.5 ;!nO 0.3 0.2 0.1 0.1

Cl o.o 0.0 0.1

so 0.0 0.0 0.0 3 quartz 0.0 3.5 5.6 0.5 corundum 0.0 0.0 0.0 0.0 orthoclase 34.6 24.8 22.0 26.1 albite 33.1 34.3 36 .9 27.8 anorthite 12.9 14.6 14.3 19.2 leucite 0.0 0.0 o.o o.o nepheline o.o o.o 0.0 0.0 halite 0.0 0.0 1.0 0.0 acmite o.o o.o 0.0 o.o wollastonite 4.5 3.0 2.8 4.5 enstatite 2.1 7.2 8.7 9.7 ferrosilite 2.6 5.1 3.4 2.4 fosterite 2.7 0.0 0.0 0.0 fayalite 3. 7 o.o 0.0 o.o magnetite 3.0 3.3 2.9 5.6 hematite 0.0 o.o 0.0 o.o i_lmenite 1.1 1.5 LO 2.3 apatite 0.0 2.8 2.4 1.2 Na 0/K 0 ratio 0.67 0.98 1.19 0.75 2 2

A - Average latite, from 42 selected samples (Nockolds, 1954). 37 samples where quartzite grains are observed. FeO, MgO, CaO, and Ti02 are all slightly low and are accounted for by normative pyroxenes, magnetite, and ilmenite~ The presence of normative wollastonite reflects the calcic nature of salite phenocrysts.

Alkalies are variable and a considerable range in Na 0/K o 2 2 ratios is apparent. High normative orthoclase reflects a higher

K 0 content and normative albite probably reflects Na o in both 2 2 alkali groundmass and sodic plagioclase. No normative feldspathoids were computed~

Field Relationships

The latite porphyry occupies the southern and western portions of the complex. It may be weathered, and forms more prominent outcrops than the trachyfe. Intense fracturing is absent but the latite is sheeted, possibly as a result of cooling (Figure 6).

Contacts between latite and lower Paleozoic sedimentary rocks are sharp and interdigitating. The latite between sedimentary layers is phenocryst-poor relative to latites farther from the igneous­ sedimentary contact. Cambrian and Ordovician sedimentary rocks exposed along the southern and western peripheries of the complex dip more steeply than the Mississippian and Pennsylvanian strata in contact with tracbytes at the northern periphery. This may indicate a period of renewed igneous activity during which the latite was emplaced and sedimentary rocks were further uplifted. Contacts between latite and trachyte were-not recognized.

Tbe latite is occasiona.lly brecciated but not as commonly as the trachyte. Latite breccias contain subrounded to subangular - ,

38

Fig. 6. Outcrop exposure of latite porphyry. Note sheeting, presumably due to cooling. ' 40 fragments of sedimentary rocks, latite, trachyte, phonolite, broken

feldspar xenocrysts, magnetite, and pyroxenite~

Xenoliths in the latite include granite, biotite schist, amphibole schist, sedimentary rocks, trachyte, sanidine trachyte

porphyry, microsyenite, and pyroxenite and other mafic rocks.

These are variable in size and range from large mappable bodies to

~icroscopic xenocrysts. 41

Alkali Trachyte P~!2'

Petrography and Mineral Chemistry

The alkali trachyte porphyry may be light gray to bluish green in color but is most often stained by iron oxide to yellowish brown or reddish brown. It is nonmagnetic, holocrystalline, and porphyritic with sanidine phenocrysts in an aphanitic, trachytic groundmass.

Apatite and limonite pseudomor?hs are also present. Plagioclase is absent. The rock is intensely fractured and pervasive alteration has destroyed the original mafic. mineral grains. Alteration products include limonite~ hematite, clay minerals, calcite, analcime, and potassium feldspar.

Four samples were selected for modal analyses (Table 7) but do not represent the.complete range of variation. Trachytes with greater than 25 percent sanidine phenocrysts, such as BL-584, are designated as sanidine trachyte porphyry and will be discussed further.

Potassium feldspar phenocrysts are sanidine and are the least altered major mineral~ They may comprise as much as 50 to 55 percent of the rock (visual estimation) and range in size from less than 0.1 mm to 70 mm long. Subhedral to euhedral crystal forms are present. No zoning was observed but simple Carlsbad twins are common. lnterference figures are often obscured by alteration to clay minerals, but where observed, the 2V angle is very low (5-10°). The chemical analysis of sanid-ine (Table 8, A), compared with analyses from Deer, Houie., and

Zussman (1963), indicates that the K D content is relatively high and 2 Na O is low with very low Na 0/K O ratios. ' 2 . ' 2 2 1,2

TABLE 7

!!ODAL ANALYSES OF ALKALI TRACHYTE PORPHYRY

Sample :-.:umber BL-226 BL-494 BL-554 BL-584 -·----- Sanidine 16.0 7.2 14.4 29.8 a Relict minerals 19,6 16 .8 17 .4 15.8 b Opaques 0.4 7.4 4.8 0.0

Apatite 1.0 1.8 1.0 0. 2

Groundmass 63.0 66 .8 62.4 54.2

aRelict minerals pervasively altered and replaced by potassium feldspar and limonite-hematite. Relict crystal forms indicate initial pyroxene, sphene, and possibly amphibole.

b Opaque minerals are limonite-hematite pseudomorphs after cubic pyrite~ 43

TABLE 8

NORMALIZED CHEMICAL ANALYSES OF ALKALI TRACHYTE PORPHYRY MINERALS .:=

A B C D

SiOz 63.9 45.5 65.3 64.2

Alz03 18.3 25.8 18.3 18.0 FeO o.o 18. 7 0.3 0.4 MgO o.o 1.9 0.0 o.o CaO o.o 0.4 o.o 0.0

NazO 0.4 0.3 2.8 0.4

K20 17.l 6.6 13.4 16.9 TiOz 0.3 0.1 o.o o.o

Pz05 o.o 0.5 o.o o.o MnO 0.0 0.1 0.0 o.o Cl o.o o.o o.o 0.2

S03 0.0 0.2 o.o o.o ···------Explanation

A= Sanidine, HL-297, phenocryst

B = Altered mafic mineral, BL-297, relict phenocryst

C = Potassium feldspar, BL-297, groundmass

D = Potassium feldspar, BL-731, veinlet 44

Wright's (1968) three-peak method was used to determine the structural state of feldspar phenocrysts. Anomalous cell dimensions were found and the results are therefore uncertain. By plotting 20

(204) against 28 (060), two samples were determined to be maximum orthoclase and one was high sanidine. Composition varies from Or 94 to Or on plots of 28 (201) against orthoclase content. 97 Mafic minerals originally formed 15 to 20 percent, or more, of the trachyte but are now completely altered (Figure 7). Relict euhedral crystal outlines indicate that these were originally mostly pyroxenes

(salite?) with accessory sphene and possibly amphibole. The outer crystal borders are usually dark brown while cores may be· clear or yellowish brown. Two microprobe X-ray energy spectra are given for an altered pyroxene grain with a dark brown border and yellowish brown core (Figure 8). The analysis taken from the dark brown border indi­ cates a high FeO content whereas the other analysi~ with considerable

Kz°, is from the yellowish brown core. The chemical analysis of another relict phenocryst is given in Table 8 (B) and indicates that the grain is replaced by potassium feldspar. Secondary silica and caicite may also occupy mafic casts.

Limonite pseudomorphs after pyrite are often present in acces­ sory or minor quantities .. Cubes, pyrttohedrons, and irregular masses occur disseminated or as thin fracture fillings. Striations are clearly visible in hand specimens and cubic casts are observed in strongly oxidized rock. Unaltered pyrite is rare.

Apatite is an accessory mineral but is ubiquitous. It is anhedral to euhedral, colorless, and typically unaltered. 45

Fig. 7, Photomicrograph of altered alkali trachyte porphyry sample BL-494. _Relict mafic phenocrysts replaced by limonite and potassium feldspar. Trachytic groundmass is limonite-stained. Plane polarized light, 25 X.

47

Fig. 8. Microprobe X-ray energy spectrum for altered roafic minerals in alkali trachyte porphyry sample BL-584. ~,,-,c,,Sl±,:,'>'1,°"'o!>" ~: i:k,:e.1· t' ·· li"i/, .,.~··) i"·•1:Y"' ,. ,"/":'

LT:100 SECS SB0-155 GR B, BROWN T' sao-155 GR B, YELLOW I I lOK-t I I F + E I I I T I I J. I I C I 0 t u I I N s T 5000-+ I s I I J. - I I .. rI I I A" t L ,p • I .,.: .': I M I.•: X .. :.• J. NG,.. •• •.• • I A : • ,..._,,, IC . . :: I .•:":.; l o-t 1-- I,I I ·~ I I I I I I I I I I I I I I I I I 0.000 5.000 10.000 15.000 20.000 ENERGY (KEV) 49

Groundmass forms 62 to 67 percent of the rock, but is less abundant in sanidine trachyte porphyry. It is trachytic and composed of alkali feldspar microlites. Individual microlites may exhibit simple Carlsbad twinning. The chemical analysis of groundmass feldspars

(Table B, C) indicates a potassium-rich composition. A small sodium component is also present and may be due to secondary analcimea Crypto­ crystalline clay minerals, limonite, and hematite are pervasively dis·­ seminated throughout the groundmass. Secondary silica occasionally occupies small open spaces.

Rock_ Chemistry

The chemical analyses of seven alkali tracnyte samples, and CIPW normative mineralogies for four of these, are given in Table 9. These are compared with the average alkali trachyte (Table 9, A) provided by

Nockolds (1954).

Silica and alumina values are quite variable and range from 54.2 to 70.0 percent and 12.3 to 22.1 percent, respectively. Because of this variation no direct comparison with the average alkali trachyte can be made; however, a range in silica saturation from undersaturated to oversaturated is indicated by the normative mineralogies. The extent of this variation suggests different degrees of silicification,

The most striking feature of the Bear Lodge trachyte is the extremely potassic character. Potassium contents range from 10.0 to 13.8 percent, whereas sodium comprises less than 1.5 percent. Anomalously low Na 0/K 0 ratios and normative orthoclase values of 70 percent or 2 2 greater characterize the trachyte. 50

TABLE 9

NORMALIZED CHEMICAL ANALYSES AND NORMATIVE MINERALOGY OF ALKALI TRACl!YTE PORPHYRY

Sample Number BL-219 BL-277 BL-731 BL-325 ~---··- 59.7 Si02 54.2 55.1 57.5 17.1 .. Al203 22.1 17.2 18.2 FeO 9.2 9.7 9.0 8.0

MgO 0.6 2.1 0.0 0.3

cao 0.1 0.2 0.0 0.3

Na20 0.2 0.4 0.6 0.4 12,7 KzO 12,6 13.4 12.5

TiOz 0.3 0.6 1.1 0.9 0.2 P205 o.o o.o o.o MnO 0,6 1.3 0.2 0.3

Cl o.o o.o 0.1 0.1

S03 o.o o.o 0.1 o.o quartz 0.0 o.o 3.5 corundum 7.9 1. 7 4.7 orthoclase 74,9 70.0 73.9 albite 1. 7 o.o 3.8 anorthite 0.7 0.8 o.o leucite o.o 7.4 o.o nepheline 0.0 2.0 0.0 acmite 0.0 0.0 0.0 wollastonite o.o 0.0 o.o enstatite 0.6 0.0 o.o ferrosilite 4,6 o.o 8.1 fosterite o. 7 3.8 0.0 fayalite 6,2 10.3 o.o magnetite 2.6 3.0 3.8 hematite o.o o.o 0.0 ilmenite 0.6 1.1 2.2 apatite o.o 0.0 o.o 0,05 0.03 Naz0/K20 ratio 0.02 0.03 TABLE 9--Continued

sample Number BL-389* BL-220* BL-577* B

60.3 60. 7 61.0 58.4 15.2 16.1 21.2 17 .8 6.6 6.7 s.o 5.1 ::,jgO 3.1 l..7 0.7 0.4 CaO 0.2 0.4 0.1 0.8 0.6 3.0 3.1 0.4 K20 13.1 10.4 8.6 13.9 TiOz o. 7 0.6 0.3 0.3 Pz05 o.o o.o o.o 0.4 Mn0 0.2 0.3 o.o 0.4 o.o o.o o.o o.o o.o o.o quartz 0.0 0.0 6.7 0,5 corundum o.o o.o 6.6 0.6 orthoclase 77 .3 61.8 50.8 82.3 albite 4.7 24.5 26.6 3.1 i anorthite 0.4 o.o 0.5 4.2 leucite 0.0 o.o 0.0 0.0 nepheline o.o o.o o.o acmi te o.o 0,9 0.0 wollastonite 0.3 o. 7 o.o o.o enstatite 6.6 0.4 1. 7 ferrosilite 4.9 0.6 4.2 fosterite 0.7 2.7 o.o fayalite 0.6 4.7 0.0 magnetite 3.2 2.6 2.s 0.5 hematite 0.0 o.o 0.0 4.8 ilmenite 1.3 1.1 o.s 0,6 apatite o.o 0.0 o.o 0.7 hypersthene 1.1 Na20/K20 ratio o.os 0.29 0.36 0.03

*Samples are sanidine trachyte porphyry A; Average alkali trachyte~ from 15 selected samples (Nockolds, 1954) B ~ Potash trachyte, Toror Hills, Uganda (Sutherland, 1965) 52

TABLE 9--Continued

= .. -...... ,::,..·cccc Sample Number BL-726 BL-502 BL-250 A

SiOz 64.1 65.5 70.0 62.0

Al203 16.8 18.3 12.3 18.0 FeO 4.1 2.4 6.4 3.8

MgO o.o 0.7 o.o 0.6 CaO 0.1 o.o o.o 1.9

Na20 0.5 1.4 0.5 6.6

K20 13.8 10.3 10.0 5.5

Ti02 0.6 0.8 0.5 o. 7

Pz05 o.o o.o 0.2 0.2 MnO o.o o.o 0.1 0.1 " Cl a' o.o 0.1 o.o S03 0.1 0.4 0.1 .· quartz 8.3 0.0 corundt.nn 1.1 0.0 ' orthoclase 81.3 32.8 albite 3.5 54.0 anorthite 0.5 3.3 leucite 0.0 0.6 nepheline 0.0 r: ,;ii acmite 0.0 wollastonite o.o 2.1 ·, enstatite 0.0 1.6 ,,_-, ferrosilite 1.1 0,0 ' fosterit:e 0.0 o.o --~ fayalite o.o o.o ) magnetite 3.0 3.3 hematite o.o ilmenite 1.1 1.4 apatite 0.0 0.4

Na20/K20 ratio 0.04 0.14 0.05 l.Z 53

The FeO content varies from 2.4 to 9. 7 percent and is often

considerably greater than values for the average alkali trachyte. Of

the remaining oxides, most are in quantities of less than 1 percent

and are similar to values of the average alkali trachyte. Calcium

is an exception and is consistently low to absent.

Several differences are apparent between the Bear Lodge trachyte

and Nockolds' (1954) average alkali trachyte. The composition of the

Bear Lodge trachyte more closely approximates potash trachyte (Table. 9,

B) from the Toror Hills, Uganda (Sutherland, 1965).

Three analyses of sanidine trachyte porphyry are also given in

Iable 9. These show consistent silica but other elements are variable.

Like the trachyte, sanidine trachyte porphyry contains high potassium

and iron but normative orthoclase values are lower than the trachyte

and Na 0/K 0 ratios are higher. Sodium and magnesium seem to be more 2 2 abundant and are illustrated by higher normative albite and enstatite.

Field Relationships

Alkali trachyte porphyry is the most abundant rock type in the

complex. It occupies the northern, central, and southwestern portions

of the study area. Significant textural variation is exhibited. Con­

tacts may be sharp or brecciated and xenoliths are common. Fracturing and brecciation of the central portion of the trachyte is widespread.

Silicification is extensive but variable in intensity. Veinlets of

potassium feldspar cut the rock.

Much textural variation is observed and pronounced differences

in size and abundance of sanidine phenocrysts are apparent. They are often randomly distributed but occasionally have subparallel 54

orientations which suggest flow. A large scale flow structure, with

sani

Sanidine trachyte porphyry forms sharp contacts with trachvte

and occurs as intrusive sills, lenses, or irregular apophysesw Although

it appears on the geologic map (Plate 1) in one area only, its occur­

rence as float shows it is more widespread than the geologic map indi-

cates. It is absent in the northern and southern parts of the complex

but is present in the centralt more fractured area.

Contacts between trachyte and Precambrian granite or Paleozoic

sedimentary rocks are sharp but sometimes brecciated. Xenoliths of

granite and sedimentary rock are incorporated into the intrusive mass and range in size from the largest granite at Warren Peaks to fragments

less than J. cm in diameter (Plate 1). Small fragments are most abun­ dant near granite or sedimentary contacts but do occur isolated in other parts of the complex. Rounded open spaces, on the order of a few centimeters, are commonly filled with limonite. It is speculated that these were originally mafic xenoliths which have been destroyed by alteration.

Brecciation of the trachyte is widespread but is most intense in the central region. Breccia fragments are usually subangular and variable in size. They include trachyte, trachyte porphyry, microsyenite, and broken sanidine xenocrysts~ The groundmass of trachyte breccias are trachytic to cryptocrystalline. Alteration_, weathering, and intense limonite staining are common. The breccia is sometimes silicified and may contain drusy quartz-filled fractures and voids. 55

The trachyte core is intensely fractured (Figure 9) but no pre­ ferred jointing patterns were recognized. Fractures are Ulled with iron and manganese oxides which may contain thorium and rare earths.

Thorium and rare earth-bearing fractures are radioactive and range from thin, hairline veinlets to large veins over 125 m long and 1.5 m wide. The veins vary in color from dark brown to black, depending on the proportions of iron and manganese (Staatz, Conklin, Bunker, and others, 1980). Staatz reports that the veins consist of goethite, pyrolusite, cryptomelane, and hematite, with thorium and rare earth minerals including brockite, monazite, bastnaesite, and weinschenkite.

Other minerals reported are sanidine, magnetite, barite, rutile, and brookite. Of the 216 analyses included in his paper, K 0 averages 10.4 2 percent and total rare earth values may be as high as 27,145 ppm.

Colloforrn banding with botryoidal surfaces and coarse iron manganese oxide pseudomorphs after pyrite suggest a secondary, hydrothermal origin. Veining may occasionally be intense enough to give the appear­ ance of a breccia with trachyte fragments in an iron manganese oxide matrix.

Three other types of veinlets cut the trachyte in a zone through the NEl,, Sec, 18 and !.'W',., Sec. 17, T. 52 N., R. 63 W. (Plate 1).

Potassium feldspar veinlets are bluish gray in color, monominerallic, and usually not more than 3 to 4 cm wide. They are formed of coarse­ grained feldspars in their centers with fine to medium-grained, sub­ hedral, lath-shaped feldspars perpendicular to their contacts. The contacts are sharp and trachyte is aphanitic but granular. The feldspars are stained by iron oxides, making measurement of the 2V angle uncertain.

Estimates, however, indicate that the ZV is about 15°. The chemical 56

Fig, 9. Outcrop exposure of alkali trachyte porphyry showing intense fracturing and iron staining.

58 analysis of_ one of these feldspars is given in Table 8, D, and approxi­

mates the potassic sanidine phenocrysts in the trachyte (Table 8, A).

Reddish brown chert may be observed in association with the veinlets.

Another type of veinlet consists of isolated, medium-grained, euhedral

feldspars suspended in an iron manganese oxide matrix. These feldspars

are also stained by iron oxides. Both types of vein.lets contain angu­

lar fragments of trachyte wall rock and indicate that material was

injected into the trachyte, probably along previously formed fractures.

A third type of veinlet is micaceous (glimmerite?), occurs with botryoidal

iron manganese oxide; and was found at only two locations~

Various degrees of hardness of trachyte and trachyte breccia

reflects different degrees of silicification and concurs with the large

range of the Sio content in Table 9. Silicification is most intense 2 in the central portion of the core and seems to correspond with the

zone of intense fracturing and iron manganese oxide {Th-RE) veining.

Secondary silica is also observed as cavity fillings and veinlets.

Alteration of country rock in contact with the trachyte is

sporadic. In some areas no alteration is observedt whereas in others

it is extensive. Pahasapa limestone in SW~, SW\, Sec. 27, T. 52 N.,

R. 63 W., is recrystallized to marble. The rock is beige to dark brown

in color and medium to coarse-grained. Small amounts of fluorite are

disseminated throughout the marble and in nearby sanidine trachyte

porphyry. A hard, light purple siliceous rock is found in abundance

near the marble. It is speculated that the purple color of this rock

is attributable to a high fluorite content. Silicified limestone and quartzite are found in SW~, NW~, Sec. 18, T. 52 N., R. 63 w. The lime­ stone is "fossiliferous and belongs to the Pahasapa Formation. Minnelusa 59 sandston

Several fluorite occurrences are present. Sparse disseminations are random throughout the trachyte. A breccia consisting of angular

trachyte fragments in a coarse-grained> purple fluorite is present in

SE\, NW\, Sec. 8, T. 52 N., R. 63 W. Fluorite replacements of lime­ stone clasts in flat-pebble conglomerates of the Deadwood Formation were also observed. 60

Microsyenites and microsyenite porphyries are present in the

northern and northwestern parts of the complex, where exposures are

especially poor. Float consists of both alkali trachyte and micro-

syenitic rocks~ Due to the absence of observable contacts, dashed

contacts on the geologic map are intended to show float distribution

only (Plate 1).

The microsyenites are aphanitic to very fine-grained and light

gray to greenish gray in color. Two compositionally different types

are recognized.

The most abundant type of microsyenite has a seriate-porphyritic

texture. An age of 48.5 my was provided for the microsyenite at loca­

tion 785, NW~, NE~, Sec. 8, T. 52 N., R. 63 W. (Staatz, verbal communi-

cat.ion). The rock 1 s often weakly magnetic and sometimes calcareous.

Alkali feldspar and andesine form phenocrysts but albite twins are

usually masked by alteration. Alkali feldspar is also observed in the

groundmass. Other minerals are salite, titaniferous magnetite,

apatite, sphene, and biotite, and form 10 to 20 percent of the rock.

Silica and calcite occur as secondary minerals. Mafic xenoliths may also be present.

The other type of microsyenite is hypidiomorphic-granular in

texture and formed essentially of interlocking, lath-shaped potassium

feldspar~ Xenomorphic-granular varieties were also observed. Carlsbad

twins are occasionally well defined in the feldspar. Small irregular

grains and rnasse-s of limonite occur disseminated throughout and may

occupy hairline fractures. Interstitial material consists of clay 61 niinernl.s, .Umonite, calcite, and a small groundmass component which is probably potassium feldspar.

Four microsyenites are represented chemically in Table 10. The first three samples, BL-353, BL-774, and BL-785, are seriate-porphyritic.

Their silica contents are variable and are illustrated normatively by a range from undersaturated to oversaturated types~ Secondary silica is present in BL-785. Except for the more potassic BL-774, Na 0/K 0 2 2 ratios are similar to the average alkali trachyte (Nockolds, 1954).

The high normative calcite content of BL-353 reflects an abundance of unaltered salite and secondary calcite. The normative mineralogies illustrate the high feldspar content. These rocks show a strong similar­ ity to the latite.

The analysis of one hypidiomorphic-granular microsyenite, BL-747, contains very high silica and potassium values. This sample is chemically similar to BL-250 (Table 9) and is analogous to the alkali trachyte.

The microsyenites are interpreted as textural variations of alkali trachyte and latite. Microsyenites which intrude the latite

(Plate 1) are analogous to the latite and probably represent later pulses of the same type of magmatism. h2

TABLE 10

NORMALIZE!l CHEMICAL ANALYSES AND NORMATIVE MINERALOGY OF MICROSYENITE PORPHYRY AND RELATED ROCKS

Sample Number BL-353 BL-774 BL-785 A

58. 9 60.2 64.8 69.2 61.9

17.8 18.2 17 .5 13 .9 16.9

FeO 4.8 5.8 4.8 4.8 5.0

MgO 2.8 2.2 0.5 0.0 1.0

cao 6.4 0.2 1.8 0.2 2.5

Na o 4. 7 0.5 5.5 2 s.o 3.9 KzO 5.5 7.6 4.8 9.8 5.9

Ti02 0.7 1.6 0.5 1.1 0.6

P205 0.4 0.3 0.3 0.3 0.2

MnO 0.0 0.1 0.0 0.0 0.1

Cl 0.1 o.o 0.1 o.o 0.0 o.o 0.0 0.0 0.0 a.a quartz 0.0 14.4 1. 7 corundum o.o 1.9 a.a orthoclase 31.6 28.5 35.0 albite 32.9 39.4 46.1 anorthite 10.4 7. 4 4.2 leucite o.o a.a 0.0 nepheline 3.9 0.0 a.a halite 0.2 0.1 0.0 acmite 0.0 o.o 0.0 wollastonite 7.5 o.o 3.0 enstatite 5.3 1.2 2.4 ferrosilite 1.5 2.7 2.1 fosterite 1.1 o.o a.a fayalite 0.3 o.o a.a magnetite 3.1 2.9 3.3 hematite o.o a.a 0.0 ilmenite 1.2 0.9 1.2 apatite 0.9 0.6 0.5

0.91 0.51 0.98 0.05 0.93

A - Average alkali syenite, from 25 selected samples (Nockolds, 1954). 63

Phonolite Porphyry

Petrography_and Mineral Chemistry

The phonolite porphyry is a dark green to greenish gray, holo­ crystalline rock. It is porphyritic with phenocrysts of sanidine, nepheline, salite, magnetite, sphene, apatite, melanite, and biotite

(Figure 10). The groundmass is aphanitic and trachytic in texture.

Calcite veinlets, clays minerals, limonite, and analcime are common alteration products.

The modal analyses of four phonolite samples are given in

Table 11. Sanidine, nepheline, and salite account for 35 to 54 per­ cent of the phenocrysts and, with the exception of BL-563, the ground­ mass accounts for a little less than half the rock.

Subhedral to euhedral sanidine phenocrysts comprise 11 to 24 percent of the phonolite and range in size from less than 0.1 nun to about 30 mm. Tiiese are colorless, nonperthitic and frequently twinned according to the Carlsbad twin law. Salite, apatite, and nepheline are commonly poikilitically enclosed by sanidine.

Salite forms 10 to 20 percent of the phonolite and occurs as subhedral to euhedral prisms which exhibit typical pyroxene cleavage.

These range from less than 0.01 mm to 4 mm in length. Individual crystals are often oriented parallel to the trachytic groundmass or may occur in clusters~ Clusters are variably associated with mag­ netite, apatite, sphene, or more rarely, nepheline and are interpreted as small xenoliths. Pleochroism is weak with X = pale green, Y = green, and z = yellow green. Twin planes are fairly common. Compositional zoning is moderate with light brown rims and light green cores. 64

Fig. 10. Photomicrograph of phonolite porphyry sample BL-12 show:i.ng nepheline cluster (upper right), sanidine, zoned sali te, melanite, and magnetite. Plane polarized light, 25 X. 65

~·,,,~'

<. > ,,,"' .. , . 66

TABLE 11

MODAL ANALYSES OF PHONOLITE PORPHYRY -==c-=--~-======--

Sample Nu.-nber BL-568Q BL-12 BL-563 BL-358

Sanidine 23.6 20.2 12.6 11.0

Nepheline 13.4 17.0 8.4 22.4 -·1 Salite :J 10.8 16.2 14.8 19.6 :, Magnetite ; 2.6 1.8 0.8 o.o Sphene 0.6 0,4 1.2 0.4

Apatite tra 0.4 0.4 0.2 Melanite tr tr o.o o.o Biotite o.o tr o.o o.o

Groundmass 49.0 44.0 61.8 46.4 --··-· atr trace, less than 0.2 percent 67

Microprobe analyses show that zoning is normal with iron-rich rim.sand magnesium-rich cores. Magnetite, apatite, and sphene are often poikilitically enclosed by salite.

Euhedral nepheline forms 8 to 23 percent of the rock. Individual crystals range in size from about 0.1 mm to 4 nun. Nepheline occurs as isolated hexagonal phenocrysts which may be poikilitically enclosed by sanidine. Clusters of nepheline are commonly observed. Some nepheline display, in part, irregular borders which indicates they were broken before crystallization of the groundmass. Most are altered and cloudy but where fresh they are clear and colorless and exhibit exsolution intergrowths ~

Magnetite forms less than 3 percent of the phonolite and commonly occurs as disseminated octahedra or irregular granules. Grains may be as large as 0.3 mm but are usually smaller. Apatite and salite occur poikilitically enclosed by magnetite. Their occurrence with melanite and sphene suggests that they may be titaniferous.

Sphene, apatite, melanite, and biotite, together comprise less than 2 percent of the phonolite~ Sphene occurs as euhedral phenocrysts with twin planes parallel to the length of rhombic sections. Crystals are colorless but cloudy and less than 1 mm in length. Colorless apatite forms isolated euhedral phenocrysts although some may be sub­ hedral. 1bey are less than 0.2 mm in size. Melanite (titaniferous andradite) was found in only two of the samples studied. They are euhe.dral and distinctly zoned. Light brown cores are surrounded by

~edium brown rims and probably indicate normal zoning with increased iron toward the ri.m..s * These isotropic garnets range from less than O.1 i;un to 1 mm and contain about 6 percent Ti0 . Salite and sphene may 2 68

be poikilitically enclosed in melanite rims but were not observed in

their cores. Biotite was found in one sample only. It is strongly

pleochroic, from yellowish green to dark greenish brown.

The groundmass consists predominantly of alkali feldspar micro­

lites which exb.i.bit a strong trachytic texture. Carlsbad twins are

sometimes observed in individual microlites. Other minerals present

in the phonolite occur as grains small enough to be considered as

part of the groundmass and usually conform to the trachytic texture

of the rock. Extensive iron staining of the groundmass, along with --j; disseminated salite grains, accounts for the characteristic dark green

color of the rock.

The phonolite is one of the least altered rocks in the Bear

L,odge complex but nevertheless is not unaltered. The most commonly

altered mineral is nepheline, which is replaced by calcite, clay

minerals, and analcime. Sanidine phenocrysts are altered to clay

minerals along fractures and cleavage planes. Calcite forms thin;-

hairline veinlets or irregular secondary fillings disseminated through­

out the groundmass. Minute grains of magnetite (?) occur along cleav­

age planes in salite and may represent poikilitic inclusions.

Rock Chernis try

Chemical analyses of three phonolite porphyry samples (Table 12)

are compared with the average phonolite compositions provided by

Nockholds (1954). The Bear Lodge phonolites contain high silica and

>t slightly high potassium. This is reflected by a high normative ortho­

clase content. The FeO content is somew~at low, as is evidenced by

lower normative magnetite and ilmenite, although normative hematite 69

TABLE 12

NORMALIZED CHEMICAL ANALYSIS AND NORMATIVE MINERALOGY OF PHONOLITE PORPHYRY .-."'"_c·:c·,- ,-= ·-·. ,":/.~.::--:.- ·=--=------'=-=··==:c, __ •... --=~-----·;-=~c:, .. o::-:-·=="""'~=---"== Sample Number BL-91 BL-544 BL-568Q A --·---· ·----

Si02 58.7 61.8 59.2 56.9

A1 o 20.5 19.0 20.1 20. 2 2 3 FeO 3.3 2.8 2.6 4.1

MgO 1.2 0.0 0.3 0.6

Cao 3.0 1.4 2.9 1.9

Na 0 6.2 8.2 5.5 2 8. 7 K 0 6.0 5.4 5.4 2 7.9 TiO 0.6 0.2 0.5 0.6 2

,• P205 0.4 0.2 0.5 0.2 MnO 0.0 0.3 o.o 0.2 Cl 0.1 0.2 o.o 0.2

so 0.5 0.2 0.1 3 o.o quartz 0.0 0.0 0.0 corundum 0.0 o.o o.o cn:thoclase 35.4 31. 7 46.6 31. 7 albite 39 .1 53.8 31.6 36.2 anorthite 10.8 1.9 7.9 1. 7 nepeline 7 .1 5.7 7.3 18.7 halite 0.1 0.3 0.0 0.4 thenardite 0.0 0.9 0.4 0.3 wollastonite 0.6 1.5 1.2 2.9 enstatite 0.6 0.0 0.7 1.4 ferrosilite 0.0 0.9 0.0 0.9 fosterite 1.6 o.o 0.0 fayalite 0.0 0.0 0.0 magnetite 2.3 2.4 0.8 3.3 hematite 0.5 0.0 1.4 ilmenite 1.1 0.3 0.9 l.J apatite 0.9 0.6 1.3 0.3

Na 0/K D 2 2 1.03 1. 52 0. 70 1.61 ------.. .__.____ 70

is present. Sodiuin is also low and is more consistent with lower norm-

ative nepheline than the average phonolite. High calcium contributes

a higher normative anorthite content~ This is due to the calcic composi­

tion of salite and to secondary calcite. The absence of normative

quartz and presence of minor normative nepheline indicates undersatura­

tion of the magma with respect to silica.

Field Relat ionshi.e_s

Phonolite occurs in the Bear Lodge Mountains as small intrusive

bodies and dikes. The small bodies have been studied by O'Toole (1981)

and Fashbaugh (1979) who interpreted them as sills emplaced within ' lower ~aleozoic sedimentary strataa They are exposed within 1 or 2 km

from the southern and southwestern borders of the core. Only a portion

of one of the sills is present in the study area.

In the Bear Lodge complex phonolite is the most abundant type of dike rock. A date of 50.5 my has been furnished by Staatz (verbal

communication) for the dike exposed 100 m south of the Fire Lookout

(location 12, SE~. SE!,,, Sec. 20, T. 52 N., R. 63 W.). Latite, trachyte, and granite are cut by the dikes but latite is by far the most frequent host. The dikes are restricted to the southern and south central parts of the complex, and are absent in the north (Plate 1).

Phonolite dikes are usually magnetic, but not always, and Th-RE veinlets are lacking. Individual dikes vary in width between 1 and 10 m and contacts are sharp. No chill margins were recognized. Large sani-

dine phenocrysts are often oriented subparallel to the plane of the

dikes. Fragments in various dikes include granite, quartzite, shale,

trachyte) latite, and mafic xenoliths. 71

~seudo_leucite Trachyte Porphyry

P~tr5~~raphy and Mineral Chemistry

Pseudoleucite trachyte porphyry is a light gray to medium gray, nonmagnetic rock. It is holocrystalline and contains phenocrysts of pseudoleucite, sanidine, and relict mafic minerals suspended in an aphanitic groundmass. Groundmass textures are usually trachytic but bostonitic varieties are also common.

Pseudoleucite comprises about 20 to 35 percent of the rock and is easily recognized by its trapezohedral form. Phenocrysts are occasionally clustered but most occur isolated and range from 0.5 mm to 15 mm in diameter (Figure 11). Many pseudoleucites appear zoned with clear rims and cloudy cores. Microprobe analyses of three points across a zoned ps·eudoleucite (Table 13, A-C) indicate silica-rich rims deficient in iron and magnesium. Alumina· increases inward, and a.long with the cloudy appearance of the cores, indicates a high clay content.

The potassium-rich and sodium-poor composition indicates that the pseudoleucites have been replaced by potassium feldspar. Aphanitic anhedra, possibly potassium feldspar, are observed within these crystals.

No nepheline was detected.

Euhedral sanidine phenocrysts are present in some samples but do not form more than 10 percent of the rock. They display simple

Carlsbad twins and resemble sanidine found in trachyte or phonolite.

Relict mafic mineral grains were probably pyroxene and sphene and are recognized by their external forms. They are mostly altered to limonite although some now form voids, Relict mafic minerals are 72

Fig. 11. Hand specimen of pseudoleucite trachyte porphyry sample BL-736. Note eight-sided, t.rapezohedral form. Scale in mm.

74

TABLE 11

NORMALIZED CHEMICAL ANALYSES OF A ZONED PSEUDOLEUCITE PHENOCRYST

A B C D

Si02 66.5 66.1 51.6 64.6

Alz03 17.0 21.0 32.7 18.4 FeO 0.3 3. 7 3.3 0.1 MgO o.o 0.6 0.8 a.a CaO a.a 0.1 0.1 a.a

Na2o 0.4 0.2 0.2 o.s K20 15.6 8.2 11.0 16.8 :!- TiOz o.o 0.1 0.3 o.o

PzOs 0. (J o.o a.a o.o MnO 0.2 o.o o.o o.o ·------Explanation

A= Rim, BL-736

B = 0.8 mm from rim~ BL-736

C = 3. S mm from rim, in center of core, BL-736

D = Groundmass, BL-736 75

as long as 0.5 mm and although disseminated throughout are not pre­

sent in all samples. Magnetite octahedra are altered to limonite and

are disseminated through the groundmass as very fine grains. Thin,

secondary silica veinlets are also observed cutting the rock~

The gr.oundmass consists of potassium feldspar microlites and

is stained by limonite. 1'he groundmass (Table 13, D) is extremely

potassic with Si0 , A1 0 , and K 0 accounting for over 99 percent. 2 2 3 2 Limonite staining is common.

lbe chemical analyses and normative mineralogy for two pseudo­

leucite trachyte porphyry samples are shown in Table 14. These rocks

are ultrapotassic and similar to the alkali trachyte. Very low Na 0/ 2 K 0 ratios and the presence of normative quartz indicates that the rock 2 is oversaturated.

Field Relationships

Pseudoleucite trachyte porphyry occurs as dikes with sharp con- tacts. They cut microsyenite and trachyte and are restricted to the northern portion of the core (Plate 1). Although exposures are poor, it is surmised that these dikes trend dominantly northwest or north- northwest. The subparallel attitude of their outcrop pattern suggests that they were emplaced in previously formed fractures. 76

TABLE 11+

NOR.'!ALIZED CHEMICAL ANALYSES AND NORMATIVE MINERALOGY OF PSEUDOLEUCITE TRACHYTE PORPHYRY -==..... ···-········---··--- Sample Number BL-414 BL-736

... -·····------62.2 66 .6

20.3 16.6

FeO 5.0 2.7

MgO 0.4 o.o

CaO 0.0 0.0

Na o 0.4 0.4 2 K 0 11.3 12.6 2

Ti02 0.3 0.1

P205 0.0 o.o MnO 0.1 0.0

Cl 0.0 0.0

0.0 0.0 quartz 14 .0 15.8 corundum 7.4 2.2 orthoclase 66.9 74.9 albite 3.4 3.7 anorthite leucite nepeline acmite enstatite 1.0 ferro!lilite 4.4 fosterite fayalite magnetite 2.5 hematite 2.2 ilmenite 0.5 1.1 apatite rutile 0.1 77

Carbonatite (Sovite)

The carbonatite is a pale yellowish brown rock which is non­ magnetic and holocrystalline. It is aphanitic to very fine-grained and xenomorphic-granular. The carbonatite consists mostly of calcite and is strongly effervescent with HCI. Carbonatite is exposed at the surface at only two locations (Plate 1) and each contains different accessory minerals. These include biotite, apatite, fluorite, and rare earth oxides. Limonite and hematite staining and limonite pseudomorphs after pyrite are also present.

Approximately 90 percent of the carbonatite is calcite and is

classified as a sOvite (Heinrich, 1980)~ The calcite is equigranular

and anhedral with individual grains varying from less than 0.001 mm

to 0.05 mm in diameter. Calcite grains may be either clear or clouded.

Typical rhombohedral cleavage may be observed. Microprobe analysis

indicates that strontium is present within the calcite.

The carbonatite exposed in NW!i, NE\, Sec., 20, T. 52 N., R. 63

W. consists mostly of calcite but biotite, apatite) limonite pseudomorphs

and rare earth oxides are also present. Biotite occurs in coarse­

grained books, up to 5 cm across, as very fine-graineif disseminated

laths, or poikilitically enclosed in cak.ite. It exhibits moderately

strong pleochroism with X ~ light orange brown, Y = light brown, and

Z dark brown. The distribution of biotite is sporadic due to its

concentration in _coarse books but forms approxim.a.tely 5 to 7 percent

of the rock. These books may largely be replaced by limonite and 78

hematite. Apatite forms fine euhedral prisms less than 0.05 mm in

length and comprises less than 3 percent of the carbonatite. Limonite

pseudomorphs after pyrite occur in thin veinlets and as disseminated

cubes up to 0.2 mm across. These pseudomorphs form 2 to 3 percent of

the rock. Rare earth oxides are yellowish in color, soft, and earthy.

They are found' in very small quantities and occur as cavity and

fracture fillings, in thin bands, or disseminated. The strongest

microprobe X~ray energy spectrum (Figure 12) show that lanthanum,

cerium, and neodymium are the most abundant of the light lanthanides.

1borium is commonly associated with rare earth oxides in carbonatite.

The second exposure of carbonatite is located in SE~, SE~,

Sec. 7, T. 52 N.. , R. 63 W. and contains small amounts of fluorite

and rare earth oxides. Fluorite is purple in col.or, anhedral, and

comprises about 5 percent of the rock. It occurs disseminated and

in thin, irregular veinlets.

Field Relati~E_;,hips

Carbonatite forms dikes cutting trachyte. AT both occurrences

trenches expose the rock but no float is observed outside the trenches.

Dike contacts are sharp and parallel. The fluorite-bearing carbonatite

is exposed in two trenches about 20 m apart and thins from 4 m to 1 m

wide toward the southeast. The biotite-bearing carbonatite is about 9 m wide. Trachyte adjacent to carbonatite dikes is moderately effervescent

with HCl and suggests that the host rock was impregnated, or flooded,

by carbonatitic material. Secondary chert may also he found associated

with carbonatite. 79

Fig. 12. Microprobe X-ray energy spectrum showing rare earths in carbonatite (sovite). Symbols represent chemical elements. 0 0 0 0-

0 0 0 a:,

0 0 " ·. 0 .... •" ...... , .. > ·;."'. w ... • "" ...... •• 0 ..J< .. "' "'z 0 ... 0 0 :,- U<

r<::: .....

fl) u w U) f-<:C -r;, 0 0 0 0 0 • "'II -' ... ,,- .,...... J "'

0 0 -+-~-~-~~-+--+---+---t--~-r-~-+-~-~-1-~--+--+--t- o I t I t 0 0 O oo 0 0 0 "' 0 "' 81

Mafic Xenol~

Many mafic xenoliths are found in the Bear Lodge complex.

Sizes range from l mm to 25 cm in diameter and textures are variable.

They are mostly subrounded and very fine-grained although fine and

medium-grained varieties were also observed. They are most abundant

in microsyenite but are also present in latite and phonalite. The

two major xenoliths types are pyroxenite and biotite-rich syenitic rocks~

Pyroxenite is the more abundant type of mafic xenolith and con-

tains salite, titan1ferous magnetite, apatite, and sphene, although

not all may necessarily be present in each xenolith. Salite forms

subhedral to euhedral prismatic crystals which comprise 75 to 100

percent of the rock. Magnetite may constitute as much as 20 percent

and occurs as globular grains, sometimes enveloping other minerals.

Both sphene and apatite may be either subhedral or euhedral and prob- ably do not exceed 10 percent.

Biotite-rich xenoliths are also common but weather easily to clay. These tend to be coarser grained than pyroxenite and consist of biotite and alkali feldspar, with variable salite, magnetite, apatitet or sphene.

Both types of xenoliths may contain an aphani tic to fine-grained' anhedral feldspar matrix. Contacts of xeno l L'ths are sometimes sharp but other times the gr.;undma.ss material of t he h os tis continuous into the xenolith in a manner which suggests they were impregnated•

This bears a similarity to hastingsite phenocrysts in Ia ti te which also appear to be impregnated by groundmass material. Several 82

microprobe analyses were obtained and show that the feldspathic material may be chemically equivalent to andesine, alkali feldspar, or potassium feldspar. INTERPRETATiml OF CHEMICAL TRENDS

The Bear Lodge intrusive rocks are alkali.ne in character, The

term 'alkaline rock' is described by Sorensen (1974) as being of the

Atlantic or alkaline series, having alkali feldspar as the domi.nant

feldspar, containing feldspathoids and/or soda-pyroxenes and/or soda­ amphiboles, and having an alkali-lime index of less than 51.

The rocks in the Bear Lodge complex are generally similar to the alkali group of the Atlantic series rocks, Le. the basalt-trachyte­ phonolite group. Mineralogically, the latite is similar to alkali basaltic rocks. Salite and titaniferous magnetite are specifically characteristic. Plagioclase in alkali basalts is often in the range of labradorite but crystallization trends involve enrichment in soda and extend through ande.sine and oligoclase. Alkali feldspars in alkali basaltic rocks may be sanidine-cryptoperthite, anorthoclase~ or anorthoclase-cryptoperthite (Wilkinson, 1974). Alkali feldspar in the

Bear Lodge trachyte and phonolite are sanidine. White (1980) identi­ fied anorthoclase from latite in the Lytle Creek area.

The predominance of alkali feldspar in the Bear Lodge rocks is apparent. Modal analyses of the latite (Table 7) shows a greater oligoclase-andesine phenocryst content than alkali feldspar phenocrysts; however, the groundmass is thought to be composed mostly of alkali feldspar. This is supported by normative mineralogies (Table 10). The alkali trachyte is extremely potassic, with greater than 50 percent

83 84 normative orthoclase and nn observed plagioclase.

Feldspathoids are present in same of the Bear Lodge rocks.

Phonolite contains nepheline and pseudoleucite trachyte porphyry originally contained leuc.ite but now even the, pseudoleucite is altered~

The alkali-ltme index for the Bear Lodge intrusive rocks was

not determined. Silicification and the lack of Cao in the trachyte,

and secondary calcitic replacements in the latite has made determina­

tion of the alkali-lime index meaningless.

The analyses of all Tertiary rocks show that the Bear Lodge

complex is chemically miascitic rather than agpaitic. This is indi­

cated by Na o + K 0/A1 o ratios of less than one, the absence of 2 2 2 3 sodic minerals, and the presence of calcic minerals (Heinrich, 1980).

A diagram of silica-alkali variation for Tertiary intrusives in

the area is given in Figure 13. The diagram shows that rocks from

Lytle Creek, the Devils Tower-Missouri Buttes, Tinton, and Sundance

Mountain form a phonolite-trachyte-quartz latite association (Karner,

1981). The Na 0/K 0 ratios of these rocks range from greater than 0.3 2 2 to 2. Latite and phonolite from the core of the Bear Lodge complex,

although not plotted on the diagram, are comparable to similar rocks

from other nearby Tertiary intrusives. The alkali trachyte, however,

has anomalously low Na 0/K 0 ratios and is chemically different from 2 2 other Tertiary rocks.

Five ternary diagrams were plotted from whole rock chemical

data. Figure 14 is designed to show differences between feldspar and

ma.fie mineral contents for the major rock types. Variations in

silification of the alkali trachyte has obscured any significant

differences between rock types. With one exception, however 1 the 85

Fig. 13. Silica-alkali variation diagram for Tertiary intrusives in northeast Wyoming. Note anomalously low Na 0/K 0 ratios of alkali 2 2 trachyte (from Karn"er, 1981). 86

5 PHONOT.1 TE TRACHYTE QUARTZ LATITE

I I Devils Tower­ ----..,... I I • • !I. ' ~ - ~ ) issouri Buttes ... ~ ; / • • ...... , • Tinton I .... \ "- '\, • ~,-,"::} { ol\: _ .. - ·.- -. -.. . • ' I '--.-..., ' \ ..... D~ # 00'\ ,• \ ' // .. l -..... \ ,-.., \ \ I / • 0 • \ ,- '•', ., Sund.a ,•, nee Mtn. I • • • \ .s \ • • • I '\ • ., • '1:lear Lodge .., (Lytle Creek) ' 0 ' ' N -.."' 0 N :z "' -, / -...... 1, ' .1 Bear Lodge ,1 \ I Core / I \ I 1' I I I / I I .05 • I I • I I I I ,. I 1• > • ALKALI TRACHYTE / 1 • PHO"IOLI TE / I • TRACHTY:E \ , / I TRACHYTE- \ ,,. ,,. ,. - _1,., 1 • QUARTZ LATITE I I * QUARTZ LA Tl TE .01 1 2 3 4 5 6 7 8 9 10 87

Figure 14. (Na o + K 0 + CaO) - (FeO + MgO) - (Si0 + Al 0 ) 2 2 2 2 3 ternary diagram for Tertiary intrusive rocks9 Variations in sil1c1fica­ tion have obscured differences between rock types. r

I Fig. 15. Na o - CaO - K 0 ternary diagram for Tertiary intrusive 2 2 rocks. The diagram shows that alkali trachyte is strongly potassic and poor in Na o and- CaO. Differences between latite and microsyenite are r not discernible.2 Sanidine trachyte porphyry is gradational between alkali trachyte and latite or microsyenite. I r I I I I I i I L 88

• Alkali Trachyte Porphyry • San id i ne Trachy te Porphyry Lat ite Porphyry "' Microsyeni te 25

20

l '> ....·, • • • 10

5

FeO + MgO ~---T----..------.----~----.----"' 25 20 15 !O 5

70

50

' .

30

20 • •

KzO 89

latite has more FeO and MgO than the microsyenites and indicates a

slightly highe.r mafic mineral content.

The Na 0 - K 0 - CaO plot (Figure 15) is intended to distinguish 2 2 the feldspar components. The alkali trachyte is strongly potassic and

poor in Na o and CaO. This is illustrated by the clustering of alkali 2 trachyte in the K 0 corner of the diagram. Fields for the latite and 2 microsyenites overlap significantly and differences are not discern­

ible. Sanidine trachyte porphyry is somewhat gradational between

alkali trachyte and latite or microsyenites.

The K 0 - Si0 - Al o ternary diagram (Figure 16) shows that 2 2 2 3 the alkali trachyte ls very potassic, relative to $10 and Al o . 2 2 3 Figure 17, a plot of CaO - MgO - FeO, shows that both the alkali

trachyte and sanidine trachyte porphyry, as well as one of the

microsyenitest are poor in CaO compared to the other rock types. The

alkali trachyte contains abundant FeO and is poor in MgO. Sanidine

trachyte porphyry contains more MgO than the alkali trachyte.

The chemical differences between alkali trachyte and other

Tertiary rock types in the area are interpreted as resulting from an

episode of alteration. The preceding ternary diagrams show that

chemical changes have resulted in an increase in K20 and FeO~ and . ' a depletion of Na 0, CaO, and MgO. 2 90

Fig. 16. Al o - K o - Sio ternary diagram for Tertiary 3 2 2 intrusive rocks. T~e diagram shows that the alkali trachyte is more potassic, relative to Sio and A1 o3' than the other rock types. 2 2

Fig. 17. CaO - MgO - FeO ternary diagram for Tertiary intrusive rocks shows that both the alkali trachyte and sanidine trachyte porphyry are. poor in calcium and rich in FeO, compared to the other rock types. I 91

Al203

• Alkali Trachyte Porphyry • Sanidine Trachyte Porphyry Porphyry • Latite 35 o MLcrosyenite

30

25 • • . • 0 ., • • 0 20 • •• " 15 • 10

5

SiOz KzO 35 30 25 20 15 10 5 cao

50

40 •

• 30

20

10

• • 0 • • FeO MgO " 50 40 30 20 10 CARBONATITE COMPLEXES AND ALKALINE MAGMATISM

Carbonatites usually occur in association with miascitic alkaline igneous rock complexes (Heinrich, 1980). These complexes often form concentric ring structures with complete or partial rings. They may show such features as updoming and cauldron collapse, arcuate structural forms such as ring dikes and cone sheetst linear features such as radial and tangential dike swarms, and shock zones presumably due to an early explosive stage of activity.

It is generally recognized that there is a close connection between alkaline magmatism and major tectonic structures (Bailey, 1974).

This is nowhere more evident than in the rift zones of Africa. Two types of rifting are commonly distinguished, oceanic and continental.

Oceanic rifting gives rise to tholeiitic and oversaturated basaltic magmas. Magmatism assoctated with continental rifting is gene•rally enriched in alkalies and undersaturated with respect to silica.

Alkaline igneous activity is not restricted to rift zones but where continental rifting is known, alkaline magm.atism is strongly in evidence.

In order to develop a coherent synthesis of the rock relationships and sequence of events in the Bear Lodge complex, attention is now focused on carbonatite complexes intruding rift zones along the east side of the African craton. An increase in erosion southward from

Uganda reveals a sequence of levels from upper volcanic, through inter­ mediate volcanic and upper plutonic, down to deep-seated plutonic stems

(Garson, 1966). Except where otherwise noted, the following descriptions 92

7 93 and examples of the different levels are from Garson (1966),

The uppermost volcanic levels consist of flows and ejecta,

Examples include Oldoinyo Lengai and Kerim.asi, in Tanganyika, and

Elgon and Napak, in Uganda. At Oldoinyo Lengai, flows of Na-carbonatite have been recorded several times (Dawson, 1966). Kerimasi consists of carbonatite and nephelinite flows, tuffs, and ejecta. The volcanic piles of Elgon and Napak are tuffs, agglomerates, and breccias of trachyte, phonolite, and nephelinite. Napak has been more deeply dis­ sected and reveals a circular plug with an outer ring of ijolitic rocks and a central carbonatite~

Good examples of the intermediate volcanic level are present in Nyasaland (formerly Malawi) and Zambia. These include Tundulu,

Chilwa Island, and Kangankunde, and are characterized by feldspathic breccias and agglomerates. Nephelinites of the upper volcanic level are represented by feeders comprising cone sheets, dikes, and pipes~

Plutonic stems of more mafic ijolitic rock may also be present.

Carbonatite stocks and dikes range from ankeritic sOvite to sideritic sbvite. Fenites around the carbonatite are dominantly potassic. The

Bear Lodge complex fits into this model at the intermediate volcanic level, just below the cone of volcanic flows and ejecta.

Upper plutonic stems are represented by Dorowa and Shawa, both in Southern Rhodesia, and by Spitskop, in South Africa. Silicate rocks comprise ijolitic and foyaitic varieties, and more mafic or ultramafic types such as pyroxenite, dunite, and serpentine or periodite. Other

rock types include carbonatite 1 which may be sOvitic or dolomitic, with a zone of fenitization in which the introduction of sodium is 94 greater than potassium. No trachytic rocks occur at this level.

Phalaborwa (also spelled Palabora), South Africa, and

Jacupiranga, Brazil, represent deep-seated plutonic stem. Rock types are characteristically plutonic and include .carbonatite in association with syenite, ijolite, peridotite, or pyroxenite.

The diversity of rock types associated with carbonatite and its occurrence with rift faulting suggests the tapping of deep-seated ultramafic magmas which were highly differentiated. This is in agree- . 87 86 ment with low Sr/ Sr ratios (Powell, Hurley, and Fairbairn, 1966).

In the Bear Lodge complex, pyroxenite is a possible parental material. POTASSIUM FELDSPATHIZATION

Many carbonatite complexes are enveloped by a halo of

metasomatically altered or completely metasomatized rocks called

fenites. The classic fenite is a syenite formed by an in situ alkali,

ferric iron metasomatism with requisite desilication, usually of

granitoid wall rocks. Fenitization is one of the most characteristic

features associated with alkaline complexes and occurs in alkaline

complexes with carbonatite, in alkaline complexes 1;,1ithout carbonatite,

and with carbonatite free of alkaline rocks. Fenitization is noted ' along crush zones at Bukusu, Uganda, in tension fractures at Chilwa

Island, Nyasaland, in fault zones in the Wet Mountains, Colorado, and

around Th-RE veins at Iron Hill, Colorado (Heinrich, 1980).

The most common fenitization produces permeation-type fenites

which are rich in potassic feldspar and sodic pyroxene. However,

extensive variation in the chemistry of fen! tes, from ultrapotass.ic

to ultrasodic., is reported in the literature. Certain generalized

mineralogical tendencies have been summarized by Heinrich (1980).

These include: a) concentration of potassium in feldspars and the.ir

recrystallization into larger euhedra, b) oxidation of ferrous iron

to ferric iron and its concentration in sodic pyroxene and as hematite

dispersed in potassium feldspar, c) concentration of available sodium

and calcium in pyroxene, d) formation of variable amounts of sodium

plagioclase with introduced sodium, and e) the elimination of quartz.

95 96

Mineralogical changes during fenitization for a few specific complexes

are reported by MeKie (1966).

Three variations of the metasomatic processes which affect alka­

line carbonatite complexes include fenitization, nephelinization, and

feldspathization (Heinrich, 1980). The chemical changes during

fenitization include a decrease in silica, from oversaturated to

~aturated, with an increase in Na, K, Fe, Mg, Ca, Ti, and P. Nephelini­

zation occurs with a further decrease in silica saturation, to under­

saturation, and is also accompanied by an increase in alkalies.

Feldspathization includes a radical increase in K 0/Na o ratios and 2 2 a decrease in Fe, Mg, and Ca.

Rock types at Tundulu, Chilwa Island, and Toror Hills, in

Nyasaland and Uganda, are analogous to the alkali trachyte in the Bear

Lodge complex. Tundulu is thought to represent a high level type of

fenitization in which potassium metasomatism is dominant (Garson, 1966).

The end produrt of tfl'is mt•tnsomatism is the development of a leucotrachytt"

consisting mostly of sanidine in a trachytic groundmass of potassium

feldspar. Garson (1966) attributes the formation of these rheomorphic

leucotrac.hytes to potassic metasomatism. These rocks were affected

chemically by the removal of silica and an increase in K 0 and Al o , 2 2 3 as well as a decrease in Na o, MgO, FeO, and Fe o • The potassic 2 2 3 emanations responsible for feldspathization are thought to originate

from a carbonatite magma.

Chi.lwa Island represents a somewhat lower erosion level than

Tundulu. In addition to rheomorphic leucotrachytes like those at

' l Tundulu, there are recrystallized feldspathic rocks consisting of 97

orthoclase-cryptoperthite and microcline along with dark brown iron

oxide pseudomorphs after pyroxene and pyrite. Garson (1966) proposes

a similar interpretation for Chilwa island as for Tundulu.

The Toror complex contains fenite recrystallized from baseIIM::nt

gneisses and is intruded by carbonatite. Both biotite and quartz are

sporadically removed . Sutherland (1965) suggested that alteration

of ma fie minerals to limonite may be the result of fenitization. Other

intrusive rocks are hypabyssal and Include dikes, sheets, and intrusions

of phonolite, trachyte, and agglomerate. The trachytes are unusually

potassic and consist of cloudy orthoclase and sanidine phenocrysts in

a fine-grained felsitic groundmass with dispersed flakes of brown

limonite. Sutherland (1965) attributes the formation of these trachytes

to potassic metasomatism associated with the emplacement of carbonatite.

The trachyte at Toror Hills is nearly identical, chemically, to the Bear

Lodge alkali trachyte (Table 5, B). Other complexes in East Africa

reportedly altered by potassic metasomatism include Kangankunde, Mbeya,

Budeda, and Tororo~

Many factors indicate that the alkali trachyte in the Bear Lodge

complex has been subjected to precarbonatite potassic emanations which

caused potassium feldspathization. Evidence for rheomorphism is not

very compelling since latites, also with trachytic textures, have not

been affected by this type of alteration. In addition, distinct contact

relations between sanidine trachyte porphyry and alkali trachyte are

sometimes observed. These contacts would probably have been obliterated I by any rheomorphic event. The following points have contributed toward this interpretation: L) The shattering and cltrnination of quartz and the removal

of biotite from Precambrian basement granite in contact

with the trachyte. This is one of the first indications

of low-grade, permeation-type fenitization. Mobilization

of silica from Precambrian granite may account, at least

in part, for silici fication of the trachyte.

2) The anomalous chemistry of the trachyte which includes:

a) above normal K 0/Na o ratios compared with other intru­ 2 2 sives in the area, as well as with the latites,

b) higher K o and FeO + Fe o as well as lower Na o and 2 2 3 2 MgO contents compared with Nockholds' average alkali

trachyte (Table 5, A), and

c) the. ultrapotassic chemistry of the trachyte components,

i.e. sanidine phenocrysts, altered mafic phenocrysts,

and the groundmass-

3) Extensive oxidation and replacement of mafic minerals by

limonite.

4) The occurrence of potassium feldspar veinlets. These are

chemicall.y similar to the whole rock analyses for alkali

trachyte and are interpreted as rheomorphic fenite which j crystallized in fractures from potassic fluids which I emanated throughout the trachyte. 5) The occurrence of Th-RE veins and veinlets. The presence

of suspended potassium feldspar euhedra in some of the

veinlets suggests these may be genetically related to the

potassium feldspar veinlets and feldspathization. Abundant 99

Th and rare earths indicate a genetic relationship to

carbonatite. Th-RE veins contain an average of 10~4 per­

cent K 0 and may have served as passageways for potassic 2 emanations. The same observations and conclusions were

made by King and Sutherland (1966) for the Budeda complex

·1n Uganda.

6) By analogy with other carbonatite complexes which contain

rocks similar to the Bear Lodge complex. These include

Tundulu, Chilwa Island, Toror Hills, and others.

7) The high level type of potassic feldspathization which is

especially associated with carbonatite complexes which con­

tain volcanic or hypabyssal alkaline rocks. l REGIONAL SIGNIFICANCE

One third of the alkaline rocks in North America lie in the

Precambrian shield. The remainder are in linear belts within and

generally parallel or perpendicular to the axes of Phanerozoic orogens

(Barker, 1974). Major fracture systems within these belts are not

always obvious at the surface.

Alkaline rock complexes in the African rift zones are intruded

in linear belts similar to the northern Black Hills. The Black Hills

may constitute the easternmost evidence of Laramide deformation involv-

ing uplifted Precambrian blocks (Karner, 1981). The possibility of a

former rift zone through the northern Black Hills is suggested by a

linear belt of Tertiary intrusions. This zone is possibly an exten­

sion of the Rio Grande rift which trends towards the Black Hills.

Within the belt of northern Tertiary Black Hills intrusions, alkalinity

increases to the west. Intrusions near the east end are dominantly

rhyolitic. Near the center of the belt, and continuing toward the west,

the rocks are mostly quartz latite, trachyte, and phonolite.

Ring structures are not evident in the Bear Lodge although the

latite gives the appearance of a partial outer ring. The Mineral Hill J; complex, 29 km east of the Bear Lodge, forms a more obvious concentric

ring pattern. It consists of a central feldspathic breccia partially

surrounded by pyroxenite. An outermost nepheline syenite ring surrounds

both the breccia and the pyroxenite. Alkalic lamprophyre and pseudoleucite

porphyry dikes cut across the complex and alkali trachyte sills border it 100 101

(Welch, 1974). Pyroxenites are also present in the nearby Tinton district and are salite-rich (Ray, 1979). These are chemically simi- lar to the pyroxenes and pyroxenite xenoliths in the Bear Lodge com­ plex. The Mineral Hill ring dike probably belongs to the upper plutonic level of Garson's (1966) model. Sundance Mountain and Sugarloaf Mountain represent upper volcanic cones.

Another feature which the Bear Lodge complex shares in common with other alkaline carbonatite complexes in North America is the presence of Th-RE veins. At Iron Hill, Colorado, deposits of thorium and rare earths occur in large veins over 1000 m long and in small, irregular veinlets (Olson and Wallace, 1956). Olson and Wallace note that some of the veins and veinlets have been oxidized, leached, and jasperized, with jasper replacing carbonate vein material~ Extensive silicification results in the destruction _of carbonates and the forma­ tion of a rock consisting chiefly of limonite~ hematite, and quartz.

Heinrich (1980) interprets the veins as late replacements of carbonatite.

Th-RE veins are also reported in the carbonatite district extending from Ravalli County, Montana, to Lemhi Pass, Idaho (Anderson, 1961), at

Salmon Bay, Alaska (Houston, Bates, Velikanje, and others, 1958), the

Iron Mountain-McClure Mountain complex, Colorado» the Wet Mountains~

Colorado, Firesand River, Ontario (Heinrich, 1980), and in the Bear

Lodge Mountains, Wyoming (Staatz, Conklin, Bunker, and others, 1980).

I INTERPRETATION AND SEQcENCE OF EVENTS

The oldest Tertiary rock in the Bear Lodge complex, although not

exposed at the surface, is interpreted as pyroxenite. It occurs as

xenoliths in younger rock types. At riine.ral Hill, South Dakota, the

outcrop pattern of pyroxenite is discontinuous (Welch, 1974) and seems

to be due to disruptions caused by the emplacement of later intrusive

rock types. No Precambrian pyroxenites have been described in the

Black Hills and their occurrence as Tertiary rocks fits Carson's (1966)

model, in which ultramafic rocks represent a more deep-seated environ­

ment. It is believed that pyroxenite is more abundant at depth in the

Bear Lodge complex. The alkali trachyte represents the earliest rock type abundantly

exposed at the surface. Extreme fracturing and brecciation of the cen­

tral core and the extent of textural variation, including sills of

sanidine trachyte porphyry, indicates that emplacement consisted of a

series of repeated intrusive pulses. The fluorite matrix of a trachyte

breccia attests to an episode of explosive activity, probably early in

the history of the complex.

Following the emplacement of these rocks~ a period of alteration,

in which potassium feldspathization occurred, caused a conversion to

alkali trachyte. The source of the feldspathizing fluid is not known

but is probably attributable to metasomatic emanations which originated l below the complex. As with other fenitized alkaline complexes, I i fenitizing fluids probably advanced ahead of carbonatitic magma and

102

Ci 103

preceded its emplacement (Heinrich, 1980). The lack of potassium in the

ca:rbonatite substantiates a precarbonatite episode of potassic alte·ration.

Carbonatite dikes are clearly younger thorn alkali trachyte. An

age of 60 +- 2.5 my has been obtained for the biotite-bearing carbonatite

(Levy, 1982). Differences in the accessory mineral content of the two

exposed dikes could reflect intrusion from different depths or at dif­

ferent times. There is much room for speculation concerning the ultimate

genesis of carbonatite; this subject has been extensively treated by

Tuttle and Gittins (1966) and Heinrich (1980). It is postulated that

carbonatites originate as a concentration of mostly Cao, Fe0 1 MgO, and

volatiles in highly differentiated igneous melts. , Porphyritic textures in the latite, as in the trachyte and other

rocks, indicate at. least two stages of crystallization. Initially,

euhedral and subhedral crystals formed. This was followed by intrusion

to shallow depth with rapid cooling, as is evidenced by aphantic,

volcanic textures. Oscillatory zoning of plagioclase may have been

caused by variations in vapor pressure due to the loss of volatiles

(Smith, 1974). This may have occurred in response to episodic eruptive

activity. Normal zoning in salite suggests a trend toward depletion of

magnesium and enrichment of iron in the magma.

The alkali trachyte is thought to have initially been similar to

the latite. The lack of recognizable contacts has made interpretation

of the relationship difficult. Two choices seem likely: a) the latite

and trachyte were emplaced penecontemporaneously with later alteration

of the central core to trachyte, or b) the latite is intrusive to the

trachyte..

' 104

It is apparent that the trachyte is older than the carbonatite.

Staatz (verbal communication) has provided age dates of 48.5 my for a

microsyenite equivalent to latite (location 785, NE\, NW\, Sec. 8, T. 52

N., R. 63 W.) and 50.5 my for a phonolite dike which cuts trachyte

(location 12, SE\, SE!,;, Sec. 20, T. 52 N., R. 63 W.). Other age dates + + of 48.9 - 1.6 my and 38.8 - 2.1 my have been provided by McDowell (1971)

but it is not known from what rock types these were taken. The dates

show that igneous activity in the Bear Lodge Mountains occurred over a

lengthy time interval and this is consistent with other alkaline com­

plexes (Bailey, 1974). No age dates specific to the latite are known but all the ages reported are younger than the carbonatite. By them- selves, these age dates are inconclusive but other evidence helps sup­

port a post-trachy_te age for the latite. These include: a) Th-RE veinlets which cut trachyte but are absent from latite and phonolite, b) the lack of gradational chemical analyses between highly potassic

trachyte and latite, c) more steeply dipping older sedimentary rocks in contact with the latite, which suggest renewed uplift (NW~, SW\, Sec. 33,

T. 52 N., R. 63 W.), and d) the restriction of phonolite to the southern portion of the complex indicating a spatial and perhaps a time associa­ tion, between latite and phonolite.

Phonolite dikes cut the latite and represent the youngest rocks in the complex. The phonolite is similar to latite but contains high temperature sanidine feldspar and nepheline rather than oligoclase and andesine. Halvorson (1980) concluded that phonolite in the Devils Tower­

Missouri Buttes area may be the product of differentiation by fractional crystallization.

7 105

The origin of psPudoleucite trachyte porphyry dikes is an enigma.

Saharna (1974) proposes that the formation of leucite cannot be attributed

to fractional crystallization or any type of chemical differentiation.

An example is cited in which leucite is formed in the top part of the

magma column at the Katunga volcano, Uganda. Sahama (1974) attributes

this to selective enrichment of potassium with respect to sodium by an

upward gaseous transfer. It is speculated that leucite in the Bear ; Lodge complex may have crystallized in response to the fusion of an

:l undersaturated alkaline magma, i.e. phonolite, with potassic emanations related to carbonatlte activity. The later breakdown to pseudoleucite

was eventually followed by alteration to secondary minerals.

Actual and idealized cross sections through the Bear Lodge complex

are provided. Fig~re 18 is an actual cross section, reduced in scale

from Plate 1. It shows an alkali trachyte-latite-microsyenite core with

extensive fracturing~ Th-RE mineralization, potassium veinlets, and

brecciation of the trachyte. Phonolite dikes cut the latite, and

pseudoleucite trachyte porphyry dikes cut the microsyenite and trachyte.

Precambrian granites are shown as xenoliths. Figure 19 is an idealized

cross section across the Bear Lodge Mountains. It includes Precambrian

granites overlain by Paleozoic sedimentary rocks and intruded by the

Tertiary complex. Trachyte is intruded by latite and carbonatite dikes.

It is speculated that the carbonatite dikes merge from a larger carbon-

atite body and phonolites grade into massive nepheline syenite.

Pyroxenite is shown at depth.

In summary, the interpreted sequence, of events is as follows:

1) intrusion of pyroxenite,

r f ! ' 106

Fig. 18. Cross section through the Bear Lodge complex. Note extensive fracturing~ Th-RE mineralization, potassium feldspar veinlets, and brecciation are restricted to central alkali trachyte~

j,, ,l ! ',f,I''' ;t',r ' 11; '':11, ,---~------,_._ - , 1 ,,,,·,'*<1!&111:lilliJ\lT.

North South A' A" A Warren Peaks ,:; 6600 Tl 1 6400 £d ~ ., , c: 6200 ., ' I 0 ., ;r., ....,_, 6000 ., }pBg I I ! 5800 ? \J ... I- . 7'Tpt w S600 I Tl Tt I ? I Ts

EX PLANA TIO:>! 0 1 2km Scale G Phonolite porphyry 0 Deadwood Formation Pseudoleucite trachyte Ipeg I Granite ~ porphyry Extensive fracturing and ~ RE oxide mineralization G Microsyeni te Th, Zone of potassium feldspar Lat i te porphyry 0 ~ veinlecs !t"achyte porphyry EJ !AAI Brecciation ~ Minnelusa Formation 108

Fig. 19. Idealized cross section across the Bear Lodge Mountains. Shows rock relationships and proposed sequence of intrusive events. .------:,· ·"" >·· 1 ·,·· m'rtrhfi'Nl".¥f11ift-

Bear Lodge Mountains

West East

EXPLANATION

Pb.onolite porphyry- l Tpx] pyroxenite si:g Ne phe 1 ine syenite ~ Paleozoic sedimentary 0 Latite pcirphyry rocks G Carbonatite l p€g l Granite ~ Trachyte porphyry HO

2) emplacement of alkal l trachytP and sanidine trachyte porphyry,

3) potassium feldspathization with the formation of potassium

feldspar veinlets and Th-RE minerali~ation,

4) carbonatite emplacement,

5) intrusion of latite, and

6) intrusion of phonolite and pseugoleucite trachyte porphyry(?).

Futur~.tudies

In order to more fully understand the petrozenesis of the Bear

Lodge complex, the following studies should be conducted:

1. Trace element and isotope analyses to determine age dates and genetic relationships.

2. Study of subsurface rock to determine spatial relationships between rock types poorly exposed at the surface. ECONOMIC POTENTIAL AND EXPLORATION TECHNIQUES

A variety of mineral products may occur associated with carbon­

atite and carbonati te complexes. Phosphates, fluorite, barite,

strontianite, vermiculite, and carbonate rock are nonmetallic products

from various complexes~ The principal metallic elements include

niobiumt copper, thorium, uranium, rare earths, iron, and titanium.

The following section is a composite of carbonatite related economic

mineral occurrences from Heinrich (1980) and Stanton (1972).

Phosphate. may be obtained as a byproduct of niobium or rare

earth production and nearly always occurs in apatite. The apatite

may be derived from carbonatite (Araxa and Jacupiranga, Brazil; Alno,

Sweden; Sukulu Hills, Uganda), precarbonatite magnetite-apatite rocks j (La<'kner Lake, Onlario; Sukulu Hills, Uganda), and phoscorite

(Phalaborwa, South Africa). Apatite may also form post-carbonatite

veins (Magnet Cove, Arkansas; Spitskop, South Africa).

Fluorite, barite, and strontianite deposits are usually post­

carbonatite~ Fluorite occurs as veins and replacement bodies~ mainly

within a complex (Bukusu, Uganda), whereas veins and replacement bodies

of barite may occur within or outside a complex (AlnO, Sweden; Chilwa

Island, Nyasaland; Magnet Cove, Arkansas). Strontianite veins may cut

carbonatite (Kangankunde, Nyasaland). Vermiculite may be derived from

magnetite-apatite-phlogopite rock (Bukusu, Uganda), or from phlogopite

in pyroxenite (Libby, Montana) where it is believed to be of hydro­

thermal origin. Carbonate rock may be mined for cement (Tororo, Uganda). 111 112

Carbonatite complexes can be a major source of niobium.

Pyrochlore, the most abundant niobium-bearing mineral, is found in

nearly all rock types at these complexes (Nemegosencla Lake and

Lackner Lake, Ontario). It may also occur in magnetite-perovskite

ore lenses (Iron Hill, Colorado; Jacupiranga, Brazil), columbite

veins (Iron Hill, Colorado), or in niobian rutile (Magnet Gove,

Arkansas; Ravalli County, Montana).

Copper minerals and other sulfides of iron, zinc, lead, nickel,

and molybdenum commonly occur as trace quantities within carbonatites

and are too low grade to be of economic interest. A notable exception

is Phalaborwa, Brazil, which is a source of copper. The deposit is

low grade with large tonnages, and from a mining point of view is

analogous to the porphyry copper deposits.

Thorium and uranium occur in very small quantities, typically

in pyrochlore or uranothorianite. The principal value of the U-Th

minerals is as indicators of carbonatite rather than as sources of

the radioactive metals. Thorium commonly occurs with rare earths

and forms rnineralogically complex veins and mineralized fractures

which are usually hematitic or limonitic (Iron Hill, Colorado; Bear

Lodge ~ountains, Wyoming; Ghilwa Island, Uganda). i Rare earths appear in greater abundance in carbonatite than in any other type of rock. These metals are of the cerium subgroup I and may form notable concentrations of RE-carbonate minerals 1 {bastnliesite, ancylite, cordylite, etc.) and monazite. The Sulfide Queen carbonate ore body (Mountain Pass, California) consists of 40 I to 75 percent calcite, 15 to 50 percent barite, and 5 to 15 percent I 13

bastnaesite. Several African carbonatites also contain significant

quantities of RE-bearing minerals (Kangankunde, Chilwa Island, and

Tundulu). Iron and/or titanium most commonly occurs in magnetite.

titaniferous magnetite, ilmenite, rutile, and brookite. Lenses and

masses of magnetite-ilmenite (Iron Mountain! Colorado), magnetite­

perovskite (Iron Hill, Colorado), and magnetite-apatite (Sukulu Hills,

Uganda) are usually pre-carbonatite and form in association with mafic

or ultramafic alkaline rocks. Magnetite may occur as disseminations

or bands in sOvite (Bukusu, Uganda). Post-carbonatite veins and

replacement bodies with rutile and brookite occur both within the com­

plex and in metasomatized wall rocks (Magnet Cove, Arkansas).

Tite Bear Lodge complex contains a significant potential for

rare earth deposits. Limonitic Th-RE veins occur in an area covering 2 , approximately 4 km Rare earths also occur in carbonatite dikes exposed at the surface. Other forms of mineralization in the Bear

Lodge complex include fluorite, which occurs as disseminations,

stringers, limestone replacements 1 and breccia fillings, and secondary

chrysocolla deposited in highly sheared trachyte. Neither of these,

however, forms significant quantities at the surface.

Several methods of exploration can be utilized in the search for

carbonatite-related deposits. These include radiometric and magnetic

surveys. Many carbonatite complexes are distinctly radioactive and

radiometric surveys may outline the boundary of a complex. Magnetic

surveys may help locate ores associated with magnetite or ilmenite.

Geochemical surveys may help locate near surface carbon.atite which 114 not exposed or covered. As a result of an aeromagnetic survey the· Big

Beaver House complex and the Schryburt Lake complex (Ontario), and other complexes in northern Ontario and Manitoba, have been discovered. Air­ borne radiometric surveys in Brazil have led to the discovery of ore bodies containing pyrochlore and apati.te. Two of these bodies, Araxa and Tapira, rank among major niobium deposits of the workd. APPENDICES APPENDIX A

Description of rock types in the Bear Lodge Complex. 117

DESCRIPTION OF ROCK TYPES IN THE BEAR LODGE COMPLEX

------··-Precambrian Granite: grayish pink to yellowish brown, holocrystalline, medium to coarse~grained; xenomorphic~granular; minerals include quartz, microperthite, microcline, sodic oligoclase, apatite, biotite, and pyrite; alteration products are iron oxide and clay minerals.

Granitic gneiss, amphibole schist, and biotite schist~

-----Paleozo.ic Cambrian Deadwood quartzite: white to pinkish gray, fine to medium­ grained, well sorted, subangular to subrounded quartz grains; massive, pebble quartzite and conglomerate at base.

Cambrian Deadwood shale: dark green, thinly laminated; slaty where metamorphosed.

Ordovician Whitewood dolomite: dark gray, finely crystalline, mottled; limestone in part.

Mississippian Pahasapa limestone: beige to medium gray, medium crystalline, fossiliferous; marbleized in part~

Pennsylvanian Minnelusa sandstone: white to buff, medium-grained, J well sorted, subangular quartz grains; friable. Tertiary

Alkali trachyte porphyry: light gray to reddish brown, nonmagnetic, holocrystalline, porphyritic; phenocrysts are sanidine, apatite, and iron oxide pseudomorphs after pyroxene, amphibole (?), and sphene; groundmass is aphanitic and trachytic with potassium feldspar microlites; alteration products include iron oxide, calcite, clay minerals, analcime, and potassium feldspar. Sanidine trachyte porphyry: alkali trachyte with greater than 25 per­ cent sanidine phenocrysts.

Latite porphyry: pale green to light olive, moderately magnetic, holocrystalline, porphyritic; phenocrysts include oligvclase, andesine, alkali feldspar, salite, hastingsite, biotite, titaniferous magnetite, apatite, sphene, and melanite; groundmass is aphanitic I and trachytic to microgranular with alkali feldspar microlites; alteration products include iron oxides, _clay minerals, and calcite. , I 118

Microsyenite porphyry and related rock types: light gray to greenish gray, weakly magnetic, holoc.rystalline, porphyritic; minerals include andesine> a1kali feldspar, salite, titaniferous magnetite, tipatite> sphene, and biotite; groundmass is aphanitic to very fine-grained, textures are hypi

granular 1 and seriate-porphyritic; alteration products include iron oxides, clay minerals, and calcite.

Phonolite porphyry: dark green to greenish gray, moderately magnetic, holocrystalline, porphyritic; phenocrysts are sanidine, salite, nepheliOe, biotite, magnetite, apatite, sphene, and melanite; groundrnass is aphanitic and trachytic with alkali feldspar micro.lites; alteration products include iron oxides, clay minerals, c.alctte> and analcime ~ Pseudoleucite trachyte porphyry: light gray to medium gray, nonmagnetic~ holocrystalline, porphyritic; phenocrysts are pseudoleucite, sanidine, and iron oxide pseudomorphs after pyroxene, sphene, and magnetite ('!); groundmass is aphanitic and trachytic to bostonitic with potassium feldspar microlites; alteration products include iron oxides and clay minerals. Carbonatite (sOvite): pale yellowish brown, nonmagnetic, holocrystalline, aphanitic to very fine-grained, xenomorphic-granular; mostly calcite with accessory _biotite, apatite, l~monite pseudomorphs after pyrite, fluorite, and rare earth oxides; iron oxide alteration. APPENDIX JI

Optical characteristics of latite porphyry minerals. TABLE 15

OPTICAL CHARACTERISTICS OF LATITE PORPHYRY MINERALS

-_c:;c =,=---.---~---·-'-----~= -'----'-----·--··------·.------··--- .- =··..:.-- -- 01 i goc lase­ Alkali Mineral Andesine Feldspar Salite Hastingsite

~~~~~~~~~--~~~~~-~~--~~~~~~~~~

Crystal Subhedral to Subbedral to Subhedral to Subhedral to Form Euhedral Euhedral Euhedral Euhedral

Approximate <0.01 mm <0.001 mm <0.001 mm 0.1 mm Crystal Size 15. 0 mm 12 .0 mm 4.0 mm 10.0 mm

Zoning Strong, ***a Normal it** Osei llatory r-' 0""

Color Colorless Colorless Weak, X = pale Strong, X = yellow Pleochroism greenj Y green, brown, Y = light Z = yellow green brown, Z.; dark brown

b 0 Extinction cAZ=30° cAZ=l2-lS

Twinning Common; Al.bite, Cammon; Rare; Common; f 100} Carlsbad Carlsbad Occasional twin planes ------Refractive " = 1. 702 Index 6=1.717 ~ 1. 735 ------

------·-··------ii'lit't/111111111

TABLE 15--Continued =s.--='---c"'-·=-·o:==·-'" ---'-- '-"·=- ·""---'""'-c. =,;c•:=,- Oligoclase­ Alkali Mineral Andesine Feldspar Sa lite Hastingsite ------·------Replacing **'I; "Id,* ~t** *** ------· ·------Alteration Clay Limonite Limonite To Calcite Clay Hematite Hematite Magnetite Magnetite Calcite Biotite ----- ·------· 2V 75 - 90° 65 - 80° 50 - 55° 10 - 15° ------

~ Titaniferous N Mineral Bio ti te Magnetite Apatite Sphene Quartz r' ------Crystal Subhedral Cubes and Anhedral to Euhedral Subrounded Form Irregular Euhedral G:rains Granules ------· Approximate 0.01 mm to <0.001 mm to 0. mm to <0.01 mm to <0.05 mm to Crystal Size 2.5 mm 2 .O mm 2.0 mm 3.0 mm 1.0 mm ------Zoning *** *** *** *** ***

------· ------TABLE 15--Continued

TLtaniferuus !ll.neral Biotite Uagnetite Apatite Sphene Quartz ------Color Strong, X = Opaque Colorless Neutral Colorless Pl eochrolsm Brownish green Y = light brown Z = dark brown

Extinction *** -- Twinning *** Common {100} ·Jdd, *** *** Twin plane ------,..: '" Re frac ti v<: -Index

------Replacing Hastingsite Sali te, 1d,:k *** *** Hastingsite, Biotite ------Alteration Limonite Limonite, *** Caldte *** To Hematite, Hematite Magnetite

2V *** *** *** *** - ~--·------b ***a Optical Characteristic was not detected Optical Characteristic was not determined.

------SEL~CTED REFERENCES

i i I

===------!ID·s·:\-:-_I SELECTED REFERENCES

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Barker, D.S., 1974, Alkaline rocks of North America, in Sorensen, H., ed., The alkaline rocks: New York, John Wiley,-i,. 160-171.

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124 125

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Furnish, W.M.,. Barragy, E.J., and Miller, A.K~, 1936, Ordovician fossils from the upper part of the type section of the Deadwood Formation, South Dakota: American Association of Petroleum Geologists Bulletin, v. 20, no. 10, p. 1329-1341,

Garson, M.S., 1966, Carbonatites in Malawi, in Tuttle, O.F., and Gittins. J~, eds., Carbonatites: New York, John Wiley, P- 33- 71.

Gries, J.P., 1952, Paleozoic stratigraphy of western South Dakota: Billings Geological Society Guidebook, 3rd Annual Field Con­ ference, p. 70-72.

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Halvorson, D.L., 1980, Geology and petrology of the Devils Tower, Missouri Buttest and Barlow Canyon area, Crook County, Wyoming: Unpublished Ph.D. dissertation, University of North Dakota, 163 p.

Heinrich, W.E., 1980, The geology of carbonatites: New York, Rand McNally, 585 p.

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Kulik, J.W., 1965, Stratigraphy of the Deadwood Formation, Black Hills, South Dakota and Wyoming: Unpublished M.S. thesis, South Dakota School of Mines and Technology.

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Powell, J.L~, Hurley, P.M., and Fairbairn, H.W., 1966, The strontium isotope composition and origin of carbonatites, _in Tuttle, O.F., and Gittins, J., eds., Carbonatites: New York, John Wiley, p. 365-378. Ray, J.T., 1979, Petrology of the Cenozoic igneous rocks of the Tinton district, Black Hills, South Dakota-Wyoming: Unpublished M.S. thesis, University of North Dakota, 122 p,

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Wilkinson, J .F .G., 1974, The mineralogy and petrography of alkali basaltic. rocks, i..!! Sorensen, H., ed., The alkaline rocks: New York, John Wiley, p. 67-95. Wright, T,L,, 1968, X-raY and optical study of alkali feldspar, Vol. 2, An x-ray method for determining the composition and structural state from measurement of 29 values for three reflections: American Mineralogist, v. 53, p. 88-104.

Wyoming Geological Association Guidebook, 1968, 20th Field Conference, Black Hills area, p. 6. Zartman, R.E., Norton, J.J., and Stern, T.W., 1967, Ancient granite gneiss in the Black Hills, South Dakota: United States Geologi­ cal Survey professional Paper 575-D, p. 157-163. C 11 --""' ... ---, · - • EXPLANATION 11 ~", I ------· II

Phonolite p orphyry

Pseudoleucite trachyte porphyr y

> Microsyenite and ac: <( microsye nite porphyry t­- ac: Latite porphyry w t- " T \' IC Carbonat 1te (sovite)

\ ' Sanidine trachyt e porphyry \ \ • \ \ Trachyte porphyry

u ~ Minne l usa Formation 0- N Pahasapa Format ion 0 ~ w .... @;] White w ood Formation <( ll.. ~ Deadwood Format io n

z -<( , . I .--- - " Granite, granitic gneiss, and . J I . \ L--....._ I \ amphi b o le schist \ '\ '-~}"' \ I '----~ ,j '\,.. __/

E x tensive fracturing and Th, RE oxide mineralizatio n

B rec c i at ion

Zon e of potass i um feldspar v ei n lets

Geo l ogic contact s (dashed "', \./"-.._ ~ where infe r red ) \ 100 Locatio n and sa mp le number s ..,.._ - - "7 Str ike and d i p of beds ' - X Pros pe ct pit

)- Adil

!I Ver t ical shalt \

I L / '

'PLATE 1

'\~ - \ ', --.... N I \

I ' • \ I , ' \ \ (P£91-- 1 I

\ II I ~ '\

GEOLOGIC MAP OF THE WARREN PEAKS -AREA, ' . .. \ ·cROOK COUNTY, . WYOMING .f ../ .i ' .

1 0 1 km

SCALE 1=10,000

CONTOUR INTERVAL 40 FEET

by Michael W ilkinson. 1981

Base map fro m Sundance and Alva Quadrangles. USGS