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Petrology and Alteration in the Core of the Bear Lodge Tertiary Intrusive

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Petrology and Alteration in the Core of the Bear Lodge Tertiary Intrusive University of North Dakota UND Scholarly Commons Theses and Dissertations Theses, Dissertations, and Senior Projects 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 Follow this and additional works at: https://commons.und.edu/theses Part of the Geology Commons 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 This Thesis is brought to you for free and open access by the Theses, Dissertations, and Senior Projects at UND Scholarly Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of UND Scholarly Commons. For more information, please contact [email protected]. PETROLOGY AND ALTERATION IN THE CORE OF 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
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