Brigham Young University BYU ScholarsArchive Theses and Dissertations 2019-07-01 Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting,Mineralization, and Mass Movement Collin G. Jensen Brigham Young University Follow this and additional works at: https://scholarsarchive.byu.edu/etd BYU ScholarsArchive Citation Jensen, Collin G., "Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting,Mineralization, and Mass Movement" (2019). Theses and Dissertations. 7552. https://scholarsarchive.byu.edu/etd/7552 This Thesis is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting, Mineralization, and Mass Movement Collin G. Jensen A thesis submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of Master of Science Jeffrey D. Keith, Chair Eric H. Christiansen Michael Dorais Department of Geological Sciences Brigham Young University Copyright © 2019 Collin G. Jensen All Rights Reserved ABSTRACT Multi-Stage Construction of the Little Cottonwood Stock, Utah: Origin, Intrusion, Venting, Mineralization, and Mass Movement Collin G. Jensen Department of Geological Sciences, BYU Master of Science The Little Cottonwood stock in central Utah, USA, is a composite granitic pluton that hosts the White Pine porphyry Mo-W deposit towards its northeast margin. The deposit is centered on the smaller White Pine intrusion, and associated igneous units include the Red Pine porphyry, phreatomagmatic pebble dikes, and rhyolite dikes. Twelve new U-Pb zircon LA-ICP- MS ages, for samples from this deposit and in pebble dikes from the nearby East Traverse Mountains, give peak ages of about 30 Ma and 27 Ma for the Little Cottonwood stock and White Pine intrusion, respectively, which correlate well with ages from previous studies. Ages of about 26 Ma were obtained for the previously undated Red Pine porphyry. The ages of the Little Cottonwood stock, White Pine intrusion, and Red Pine porphyry, as well as disparities in whole rock elemental differentiation trends, suggest that these units are magmatically distinct, and are not simply derivatives of one another with varying degrees of differentiation. Quench textures and resorbed quartz in the Red Pine porphyry are evidence that the magma system vented, which probably produced volcanic eruptions and emplacement of pebble dikes nearly synchronously with quartz-sericite-pyrite alteration and Mo-W mineralization. The mineralogy and geochemistry of these units imply that the magmas formed in a subduction-related magmatic arc setting rather than in an extensional basin related to orogenic collapse. Pebble dikes in the East Traverse Mountains 17 km away contain igneous clasts that resemble the units in the White Pine deposit in texture, mineralogy, and in U-Pb zircon ages. This supports other recent studies that suggest that the East Traverse Mountains rested atop the White Pine deposit prior to being displaced in a mega-landslide, and the pebble dikes in both locations are the top and bottom of the same mineralized phreatomagmatic system. The construction of the pluton began with intrusion of the Little Cottonwood stock, then the White Pine and Red Pine magmas. Fluid exsolution from the Red Pine magma accompanied venting, inception of the mineralizing hydrothermal system, and quenching to a porphyritic stock. Pebble dikes intruded into the overlying East Traverse Mountain block, which catastrophically failed millions of years later and was emplaced in its current location. Keywords: Little Cottonwood stock, granites, porphyry Mo deposits, U/Pb geochronology, hydrothermal alteration, pebble dikes, landslides ACKNOWLEDGMENTS Funding for this study came from the BYU College of Physical and Mathematical Sciences, in part from the College High-Impact Research Program. Many thanks go to the many professors and students who helped this project become a thesis. First, my thesis advisor Jeff Keith, who first came up with the idea that the East Traverse Mountains are a landslide, and who provided constant encouragement as I worked on the project. I also extend thanks to the others of my committee: Eric Christiansen, who gave me direction and helped me work through many of the ideas in this paper, and Mike Dorais, a source of help with much of the petrography and mineralogy. Thanks also go to others who provided valuable thoughts and insight or field expertise on the project: Ron Harris, Bob Biek, Ken Krahulec, and Tim Thompson. Scott Thayne took us on a tour of the Geneva Rock gravel pit on Traverse Mountain to collect samples. Several people helped with analytical lab work both at BYU and at the University of Utah, including Mike Stearns, Diego Fernandez, Grant Rea, Mike Standing, David Tomlinson, and Tober Dyorich. And much thanks for many hours of field work and lab work go to the students of the Traverse Mountain Army, including Ryan Chadburn, Bret Young, Spencer Burr, Cameron Harrison, Chris Perfili, Lars Jordan, Thane Kindred, Riley Dunkley, Tia Misuraca, Alec Hoopes, Sam Martin, Alec Martin, Joseph Tolworthy, and Steven Hood. And, of course, tech support, emotional support, and financial support of my friends and my family allowed me to finish this intriguing study, for which they will always have my gratitude. TABLE OF CONTENTS List of Figures .................................................................................................................... vi List of Tables .................................................................................................................... vii Introduction ......................................................................................................................... 1 Geologic History of the Region ...................................................................................... 3 Geology of the Wasatch Igneous Belt ............................................................................ 5 Methods............................................................................................................................... 7 Geology ............................................................................................................................... 9 Zircon U-Pb Ages ........................................................................................................... 9 Unit Descriptions ......................................................................................................... 11 Little Cottonwood Stock ........................................................................................... 12 White Pine Intrusion ................................................................................................. 17 Red Pine Porphyry .................................................................................................... 23 Pebble Dikes and Breccia Pipe ................................................................................. 24 Rhyolite Dikes .......................................................................................................... 29 Geochemistry ................................................................................................................ 31 Discussion ......................................................................................................................... 34 Origin and Evolution of the Magmatic Units ............................................................... 34 Tectonic Setting ........................................................................................................ 34 Origin of the White Pine and Red Pine Units ........................................................... 38 Y and the “Adakitic” Signature ................................................................................ 39 iv Rhyolite Dikes .......................................................................................................... 41 Construction of the Pluton ............................................................................................ 42 Evidence for Venting .................................................................................................... 43 Molybdenum-Tungsten Mineralization ........................................................................ 45 Connection to the East Traverse Mountain Landslide .................................................. 51 Pebble Dike Evidence Supporting the Landslide Hypothesis................................... 51 Possible Alteration Contribution to the East Traverse Mountain Landslide ............ 52 Sequence of Events ....................................................................................................... 53 Conclusions ....................................................................................................................... 55 References Cited ............................................................................................................... 57 Appendices ........................................................................................................................ 65 v LIST OF FIGURES Figure 1. Geologic map of the Little Cottonwood Stock and East Traverse Mountains ................ 2 Figure 2. Geologic index