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AN ABSTRACT OF THE THESIS OF ROBERT JOSEPH CASACELI for the degree of MASTER OF SCIENCE in GEOLOGY presented on December 14, 1983 Title: THE GEOLOGY AND MINERAL POTENTIAL OV INS PEAK INTRUSIVE PORPHYRY, J?øtYfl C Abstract approved: Redacted for privacy S ii. ne Hahns Peak ia composite laccolithic intrusion of latite and quartz latite porphyry, approximately 12 m.y. old. The country rocks that were domed by the porphyritic intrusions consist of Paleozoic siltstones, sandstones, and claystones; Mesozoic sandstones, shales, and minor limestones; and Cenozoic sandstone and conglomerate. Base- ment rocks consist of Precambrian gneissic granodiorite. A thin, cone-shaped body of intrusive breccia (referred to as a breccia cone sheet) is centered on the intrusive complex and is interpreted to be relatedto ahidden rhyolitic pluton oflate Tertiaryage. The brecciaconesheet consistsof monolithicbreccia, that probably formed by boiling of magmatic fluids, anda more fluidized multi- lithic breccia that probably formed byphreaticexplosionswhen ground water came into contact with super-heated magmatic fluids. Multilithic breccia apparently breached the surface on the northwest flank of Hahns Peak as steam-blast eruptions which formed a stratifiedvent complex at thatlocationand a pyroclastic surge deposit approximately one mile to the west. Silicified fragments of porphyry withstockwork veins of silica, pyrite, and molybdeniteare present in both an early phase of quartz latite porphyry and in thebreccia cone sheet. These fragments sug- gest the presence ofa stockwork molybdenite deposit at depth beneath HahnsPeak. Pyrite,galena,sphalerite,chalcopyrite,and molyb- denite occur locally in the matrixof monolithic breccia. Minor amounts of auriferous pyrite,argentiferous tetrahedrite, and trace covellite are present locally inthe matrix of multilithic breccia. Gold that occurs in placerson the perimeter of the peak is thought to have been derived from theupper-most portion of the breccia cone sheet. Late-stage sphalerite and galena fillopen spaces locally in the breccia cone sheet andare thought to have been derived from Pre- cambrian source rocks thatwere leached by meteoric fluids circulated above a cooling pluton. Hydrothermal alteration consists ofan early phase of pervasive albitization and a later phase ofpropylitic, argillic, phyllic, and advanced argillicmineral assemblages that are zonedaround the breccia cone sheet. Hydrothermal brecciation, mineralization, and alteration at Hahns Peak is interpretedtobe relatedto a porphyry molybdenum system. The location of Hahns Peaknear the intersection of deep- seated Precambrian structures witha possible northward extension of the Rio Grande Rift isa favorable tectonic environment for molyb- denurn mineralization. The Geology arid Mineral Potentialof the Hahns Peak Intrusive Porphyry, Routt County, Colorado by Robert Joseph Casaceli A THESIS submitted to Oregon State University inpartial fulfillment of the requirementsforthe degree of Masterof Science December 14, 1983 Cormnencement June, 1984 APPROVED: Redacted for privacy PTOI tOT t.eoogy 1 rrcnarge & majórJ Redacted for privacy V artment 0 0 gy Redacted for privacy Dean of Gradui School Date thesis is presented December 14, 1983 Typed by Patricia Brioady for Robert Joseph Casaceli ACKNOWLEDGEMENTS I would like to express my sincere appreciation to the Anaconda Minerals Company for providing financial support for thisthesis. In particular,I would like to thank John R. King of Anaconda for intro- ducing the thesis topic to me and for his continued support,encour- agement, and positive criticisms throughout the course of this project. I am very grateful to Bill Bowes and the staff of W. A. Bowes, Inc. for their constant help in a variety of scientific and logis- tical matters. In particular I would like to acknowledge the assist- ance and friendship provided by Steve Aaker and the late Rose Watts. The geochemical sampling and underground mapping was accomplished through the able assistance of Gail Genasci-Wells, R. Wade Holder, and Ruth Starkins. I thank Larry Hillesland for his help in the ground radiometrics survey. Drafting assistance was pro- vided by Mickey Edell, Lucy Chronic, Carol Johnston, MoniFox,and Lynn Schilling. The extensive editing of Dr. C. W. Field greatly improved the quality of the manuscript. In addition, I thank John Doucette and Paul McCarter for critically reading parts of the thesis. I will be forever grateful to Velma Fisher and the late Leonard Fisher for their hospitality and friendship and for allowing me to share with them their wonderful little town of Columbine. Field observations by Dr. W. Atkinson and Dr.J. Kutina proved helpful and are much appreciated. Sincere thanks are due also to Patricia Brioady for typing the final manuscript. Geo1ogic discussions withmy fellow graduate students Gary Sidder, Rich Fifarek, and Larry Hilleslandwere beneficial in devel- oping the conceptual models presented herein. I am deeply indebtedto thelate C. E. uChuckuBeverly for having introduced meto volcanic rocksandthe effectsof hydro- thermal alteration. His enthusiam for geology was contagious and I dedicate this thesis in hismemory. TABLE OF CONTENTS Page INTRODUCTION . 1 Purpose and Method of Study ............................. 1 Physiography and Climate ................................ 3 History of Mining and Land Use .......................... 4 Previous Geologic Investigations ........................ 9 REGIONAL GEOLOGIC SETTING .................................... 13 PrecarnbrianGeology ..................................... 13 PaleozoicGeology ....................................... 15 MesozoicGeology ........................................ 16 Cenozoic Geology ........................................ 17 Regional Geophysics ..................................... 19 Regional Tectonics ...................................... 22 Precambrian Tectonics .............................. 24 Paleozoic Tectonics ................................ 25 Mesozoic Tectonics ................................. 26 Cenozoic Tectonics ................................. 28 Metallogenic - Tectonic Relationships .............. 33 LOCAL GEOLOGIC SETTING ....................................... 36 Age Determination of Intrusive Rocks .................... 37 Geomorphology ........................................... 38 GeophysicalSurveys ..................................... 41 Airborne Magnetics ................................. 41 Ground Radiometrics ................................ 43 Heat Flow .......................................... 45 HAHNS PEAK LITHOLOGIES ....................................... 47 Precambrian Metamorphic Rocks ........................... 47 Paleozoic Sedimentary Rocks ............................. 48 Mesozoic Sedimentary Rocks .............................. 49 JurassicRocks ..................................... 49 Cretaceous Rocks ................................... 50 Cenozoic Sedimentary Rocks .............................. 52 Browns Park Formation .............................. 52 Cenozoic Volcanic Rocks ................................. 53 Vent Complex Pyroclastic Surge .................................. 58 Page Cenozoic IntrusiveRocks 60 Beryl Mountain Porphyry ............................ 61 LittleMountain Porphyry ........................... 64 Columbine Porphyry ................................. 65 SumitPorphyry .................................... 70 7DPorphyry ........................................ 76 Monolithic Breccia ................................. 78 Aplite Dikes ....................................... 80 Multilithic Breccia ................................ 82 Late-Stage Porphyry Dikes .......................... 86 STRUCTURAL GEOLOGY ........................................... 88 Evolution of Structural Dome and Normal Faults .......... 89 Reverse Faults .......................................... 93 Joints.................................................. 95 Structural Evolution of the Breccia Cone Sheet .......... 98 Minor Late Fractures .................................... 102 VOLCANIC ACTIVITY AND INTRUSIVE BRECCIATION .................. 104 Intrusion of Laccolith .................................. 104 Intrusive Brecciation ................................... 106 Pyroclastic Vent Complex ................................ 120 PyroclasticSurgeDeposit ............................... 125 HYDROTHERMAL ALTERATION ...................................... 133 Early-Stage of Alteration ............................... 134 Main-Stage of Alteration ................................ 140 A1tritinn 7nncc ----------------------------------- 141 PropyliticZone ............................... 141 Argillic Zone ................................. 144 MixedPhyllic-ArgillicZone ................... 145 Phyllic Zone .................................. 147 Advanced Argillic Zone ........................ 148 Alteration Processes ............................... 148 Albitic Alteration ............................ 151 PropyliticAlteration ......................... 152 Argillic, Phyllic, andAdvanced Argillic Alteration ........................ 153 Supergene Alteration .................................... 159 Page METALLIZATION ................................................ 161 Trace Element Geochemistry .............................. 163 Sulfide Mineralization .................................. 170 Occurrence and Distribution ........................ 170 Paragenesis ........................................ 174 Origin of Hydrothermal Fluids