University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1999 Geology and mineralization of the West Butte intrusive complex Sweet Grass Hills northern-central Montana Jon C. Carlson The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Carlson, Jon C., "Geology and mineralization of the West Butte intrusive complex Sweet Grass Hills northern-central Montana" (1999). Graduate Student Theses, Dissertations, & Professional Papers. 7121. https://scholarworks.umt.edu/etd/7121 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. i I Maureen and Mike MANSFIELD LIBRARY The University of IV ^ O N T A N A Permission is granted by the author to reproduce this material in its entirety, provided that this material is used for scholarly purposes and is properly cited in published works and reports. ** ** Please check "Yes" or "No" and provide signature Yes, I grant permission y No, I do not grant permission Author's Signature Date Any copying for commercial purposes or financial gain may be undertaken only with the author's explicit consent. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Geology and Mineralization of the West Butte Intrusive Complex, Sweet Grass Hills, Northern-Central Montana by Jon C. Carlson February 1999 A thesis presented to the Geology Department of the University of Montana in partial fulfillment of the requirements of a Master's degree in geology ^Approved by: Chairperson Dean, Graduate School 3 - 15 - Date Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: EP37922 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Dissertation UMI EP37922 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQ uesf ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carlson, Jon 0., M.S., May 1999 Geology Geology and Mineralization of the West Butte Intrusive Complex, Sweet Grass Hills, Northern-Central Montana (179 pp.) Thesis Advisor: Ian Lange^^^-^^) West Butte, the western-most of the Sweetgrass Hills, located in North- Central Montana, is composed of a cluster of laccolithic domes, emplaced 53.6 million years ago. The laccoliths intruded the Cretaceous age Marias River and Telegraph Peak Formations at a depth approximately 1200 m below the paleo- surface. The intrusives are composed of equlgranular trachyandesite and lesser amounts of porphyritic trachyte. Rarer igneous rocks include phonolite and tephrite. Petrographical analyses indicate that the trachyandesite. trachyte and tephrite could not have differentiated from a common parent, rather they originated from three different magma sources. Further analyses suggest the phonolite formed from the concentration of volatiles exsolved from the trachyte. The central portions of the igneous complex contains large areas of breccia, interpreted to be breccia pipes that formed in response to venting and subsequent collapse of one or several magma chambers. Propylitic alteration is common throughout the igneous units with weak to strong argillic alteration associated primarily with the breccia units. The strongest anomalous concentrations of base metals and gold occur within the breccias and occur in parallel linear bands trending northeast. Similar, but weaker lineaments trend northwest and north-south. The mineralized lineaments coincide with the dominant, incised drainage patterns which radiate from the central highlands. It is postulated that the same structures that controlled the emplacement of the laccoliths, also controlled the distribution of metals, along pre-existing major fractures and subsequently, the location and trend of the modern drainages. II Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Carlson, Jon 0., M.S., May 1999 Geology Geology and Mineralization of the West Butte Intrusive Complex, Sweet Grass Hills, Northern-Central Montana (179 pp.) Thesis Advisor: Ian Lange^^^^^ West Butte, the western-most of the Sweetgrass Hills, located in North- Central Montana, is composed of a cluster of laccolithic domes, emplaced 53.6 million years ago. The laccoliths intruded the Cretaceous age Marias River and Telegraph Peak Formations at a depth approximately 1200 m below the paleo- surface. The intrusives are composed of equlgranular trachyandesite and lesser amounts of porphyritic trachyte. Rarer igneous rocks include phonolite and tephrite. Petrographical analyses indicate that the trachyandesite, trachyte and tephrite could not have differentiated from a common parent, rather they originated from three different magma sources. Further analyses suggest the phonolite formed from the concentration of volatiles exsolved from the trachyte. The central portions of the igneous complex contains large areas of breccia, interpreted to be breccia pipes that formed in response to venting and subsequent collapse of one or several magma chambers. Propylitic alteration is common throughout the igneous units with weak to strong argillic alteration associated primarily with the breccia units. The strongest anomalous concentrations of base metals and gold occur within the breccias and occur In parallel linear bands trending northeast. Similar, but weaker lineaments trend northwest and north-south. The mineralized lineaments coincide with the dominant, incised drainage patterns which radiate from the central highlands. It is postulated that the same structures that controlled the emplacement of the laccoliths, also controlled the distribution of metals, along pre-existing major fractures and subsequently, the location and trend of the modern drainages. II Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Acknowledgments The list of persons who have contributed to the completion of this thesis is great. Foremost, I like to recognize my parents, Robert and Geri Carlson, and my wife Staci. I Iwould ike to thank my advisor, Ian Lange for his patience. Peter Mitchell, and Ralph Greene, both associated with Coeur Exploration who provided the initial impetus and funding for extended field and lab work. I also want to thank the remainder of my thesis committee, Dave Alt, Don Hyndman, Paul Wilson and the geologic community at Santa Barbara City College, Humboldt State University and the University of Montana. Ill Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table of Contents 1. Introduction ................................................................................ 1 2. Previous R ese a rch ................................................................. 2 3. Current Study.............................................................................. 4 4. Geology Regional Geology and Structure ............................... 5 Local Geology Sedimentary Rocks ........................................... 12 Stratigraphy ...................................................... 14 Blackleaf Foimation ................................ 14 Flood Member ............................ 14 Taft Hill Member .......................... 14 Vaughn Member .......................... 16 Bootlegger Member .................... 16 Marias River Formation ........................... 16 Floweree Member ....................... 17 Cone Member .............................. 17 Ferdig Member ........................... 17 Kevin Shale M em ber .................. 18 Telegraph Creek Formation .................. 19 Igneous Rocks ................................................... 19 Phonolite ...................................... 20 Trachyte ....................................... 20 Trachyandesite. ........................... 21 T ^ h rite ........................................ 21 Breccias..; .......................... ,............................... 21 Quaternary Sediments ............................ 23 Igneous Petrology ........................................................... 24 'Lacccollths ........................................................... 24 Classification of Laccdiths ................................. 25 Depth of Emplacement ........................................ 28 Intrusion Emplacement ................................. 29 Classification of the Igneous Units ...................... 30 Petrogenesis ........................................................ 37 Crystal Fractionation .......................................... 37 Magma Mixing ....................................................