Redacted for privacy The Early Jurassic "Grade Creek conglomerate" has amini- mum thickness of 400 feetand unconformably overlies the Seven Devils Volcanics.Unconformably (?) overlying the conglomerate unit is the "Camp Creek latite" which is 300-400feet thick and Early Jurassic in age.Tectonicly overlying the latite is more than 1,000 feet of gray to black shales and interbedded conglomeraticschists of the Early Jurassic "Brownlee Canyon series." Flows ofMiocene to Early Pliocene Columbia River Basalthave a maximum thickness of 1, 000 feet and upconformably overlie all pre-Tertiaryrocks. Major phases of the Cuddy Mountain intrusive complex are porphyritic granodiorite (200 ± 5 m. y.), hornblende gabbro, and leucocratic quartz diorite in order of decreasing age. Two periods of alteration are present. The first episode altered part ofthe porphyritic granocliorite to a rock that is chemically and mineralogi- cally similar to the albite granites near Sparta, Oregon.The last stage represents hydrothermal alteration related tointrusion of leucocratic quartz diorite. Regional, contact, and dislocation metamorphism are present in this area.All pre-Tertiary stratigraphic units and the porphyritic granodiorite have been subjected to greenschist regional meta- morphism during Middle Jurassic time. The hornblende gabbro and leucocratic quartz diorite are believed to postdate regional meta- morphism. Parts of the porphyritic granodiorite and hornblende gabbro have been contact metamorphosed to the hornblende hornfels facies by the leucocratic quartz diorite.Dislocation metamorphism related to intrusion of the quartz dioritehas produced a secondary lineation and foliation in the Rush Peak gabbro. Magmatic, contact metasomatic, andhydrothermal mineral deposits occur in this area.Magmatic concentrations of rnagnetite and chalcopyrite occur in the East Brownleegabbro.Contact meta- somatic deposits of pyrite, chalcopyrite,magnetite, and specular hematite are located in calcareous units ofthe Seven Devils Volcanics adjacent to the intrusive complex.Hydrothermal deposits of chalco- pyrite with subordinate molybdenite,pyrrhotite, and bornite occur in the leucocratic quartz diorite whereasargentiferous galena and manganese oxides are located nearthe contact of the "Grade Creek conglomerate" and the "Camp Creek latite." Geology and Mineralization of the Southern Cuddy Mountains, Washington County, Idaho by Robert Eugene Fankhauser A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science June 1969 APPROVED: Redacted for Privacy Professo Geology in charge of major Redacted for Privacy Chairmajf the Department of Geology Redacted for Privacy Dean of Graduate School Date thesis is presented Ma.y 1i, 196 Typed by Gwendolyn Hansen for Robert Eugene Fankhauser ACKNOW LEDGEMENTS The writer is especially grateful for the guidance,manuscript criticism, and constant encouragement given by his majorprofessor, Dr. Cyrus W. Field.Dr. Field and Mr. Wayne R. Bruce (Doctors thesis in progress at Oregon State University) are membersof the joint study of the Cuddy Mountain region.Discussions of the geology in their respective areas have been especiallyhelpful in solving regional problems. Thanks are due Dr. David A. Bostwick for the identificationof fossils.Sincere appreciation is extended to Professor Richard L. Armstrong of Yale University for potassium-argon agedeterminations. The writer is indebted to land owners in the area of study for their courtesy and cooperation, especially Mr. and Mrs. Carson Wells.Also, the writer is deeply grateful to Rose Fankhauser for her patience, encouragement, and assistance throughall aspects of this study. TABLE OF CONTENTS Page INTRODUCTION I Location and Accessibility 1 Topography 3 Drainage 3 Climate 5 Vegetation 6 Purposes and Scope of Study 7 Methods of Investigation 7 Previous Investigations 8 STRATIGRAPHY 10 Introduction 10 Seven Devil Volcanics 10 Distribution and Physiographic Expression 10 Stratigraphic Relationships and Thickness 12 Lithology 14 Petrography 15 Keratophyre 15 Meta-andesite 18 Spilite 19 Metadacite Porphyry 19 Tuffaceous Sediments 20 Depositional Environment 21 Age and Regional Correlation 22 "Cuddy Mountain Limestone" 24 Distribution and Physiographic Expression 24 Stratigraphic Relationships and Thickness 24 Lithology and Petrography 24 Origin and Depositional Environment 25 Age and Regional Correlation 26 "Grade Creek Conglomerate" 27 Distribution and Topographic Expression 27 Stratigraphic Relationships and Thickness 27 Lithology 28 Petrography 30 Provenance 31 Origin and Depositional Environment 32 Age and Regional Correlation 32 Page "Camp Creek Latite" 33 Distribution and Physiographic Expression 33 Stratigraphic R elationships and Thickness 35 Lithology and Petrography 35 Age and Regional Correlation 36 "Brownlee Canyon Series" 37 Introduction 37 Distribution and Physiographic Expression 37 Stratigraphic Relationships and Thickness 38 Lithology 38 Slates and Shales 38 Conglomeratic Schists 39 Origin and Depositional Environment 41 Age and Regional Correlation 42 Columbia River Basalt 43 Distribution and Phys iographic Expression 43 Stratigraphic Relationships and Thickness 43 Lithology and Petrography 44 Age 46 Quaternary Unconsolidated Deposits 46 INTRUSIVE ROCKS 49 Introduction 49 Porphyritic Granodiorite 49 Distribution 49 Lithology 50 Petrography 51 Relative Age 55 Absolute Age 56 Altered Porphyritic Granodiorite (Albite Granite) 57 Introduction 57 Lithology 57 Petrography 58 Chemical Analyses 60 Relative Age of Alteration 62 Conclusions 62 Hornblende Gabbro 63 Distribution 63 Lithology 63 Petrography 64 Discussion 68 Relative Age 69 Leucocratic Quartz Diorite 69 Distribution 69 Page Lithology 71 Petrography 71 Relative Age 74 Dikes 77 General Nature of the Intrusive Complex 80 Form 80 Emplacement of the Stock 80 Correlation and Genetic Implications 82 METAMORPHISM 84 Introduction 84 Regional Metamorphism 84 Conditions of Regional Metamorphism 84 Significance of Regional Metamorphism 86 Contact Metamorphism 86 Dislocation Metamorphism 88 Introduction 88 Nature of Foliation 90 Conclusion 92 STRUCTURE 93 Introduction 93 Late Triassic Deformation 93 Triassic-Jurassic Unconformity 94 Post-Early Jurassic-Pre-Miocene Deformation 95 Introduction 95 Faulting 95 'Overthrust" 98 Mesozoic-Tertiary Unconformity 99 Post-Miocene Deformation 99 General Statement 99 Anticlinal Hypothesis 101 GEOMORPHOLOGY 103 Classification of Stream Valleys 103 Rejuvenation 103 Resurrected Topography 104 Glacial Features 108 MINERAL DEPOSITS 109 History of Mining 109 Page Distribution of Mineral Deposits 110 Classification of Deposits 110 Introduction 110 Magmatic Concentrations 111 Contact Metasomatic Deposits 112 Hydrothermal Deposits 113 Mineralogy 115 Pyrite 115 Chalcopyrite 116 Magnetite 11 6 Specular Hematite 11 7 Argentiferous Galena 118 Molybdenite, Pyrrhotite, and Bornite 118 Manganese Oxides 119 Gold 119 Tourmaline 120 Barite 120 Garnet 120 Biotite 122 Epidote and Chlorite 122 Sericite 123 Oxidation and Secondary Enrichment 123 Structural Controls 124 Mineralization Sequence 125 Interrelationships 125 Paragenesis and Zoning 126 Hydrothermal Alteration 127 Introduction 127 Stages of Alteration 128 Late Stage Hydrothermal Alteration 128 GEOLOGIC HISTORY 130 BIBLIOGRAPHY 134 LIST OF TABLES Table Page 1.Generalized Columnar Section of the Stratigraphic Units in the Map Area 11 2.Modal Analyses of Porphyritic Granodiorite 52 3.Chemical Analyses of Porphyritic Granodiorite, Average Granodiorite, and Average Quartz Diorite 53 4.Modal Analyses of Altered Porphyritic Granodiorite 59 5.Chemical Analyses of Porphyritic Granodiorite from Cuddy Mountain and Albite Granite from Sparta,Oregon 61 6.Modal Analyses of Hornblende Gabbros 65 7.Modal Analyses of Leucocratic Quartz Diorite 72 8.Petrographic Distinction Between Primary and Recrystallized Plagioclase in the Contact Aureole 89 9.Features that Characterize the Zonal Change in Rock Texture from Areas of Foliation into Adjacent Areas Lacking Preferred Orientation 91 10.Geologic History of the Southern Cuddy Mountains 131 LIST OF FLAT ES Plate Page 1.Geologic Map of the Southern Cuddy Mountains 139 2.Mineralization Overlay to Accompany Geologic Map 140 LIST OF FIGURES Figure Page I.Index Map of Idaho 2 2.View of Rush Peak Showing Basalt Plateau 4 3.Typical Topography Near Flanks of the Southern Cuddy Mountains 4 4.Seven Devils Volcanics Exposed at the Head of Camp Creek 13 5.Bedded Tuffaceous Sandstones and Siltstones in East Brownlee Creek 16 6.Massive Tuffaceous Siltstones Near East Brownlee Creek 17 7.Stretched Pebble Conglomerate Near the Head of Grade Creek 17 8.Photograph of "Grade Creek Conglomerate" Showing its Poor Sorting 29 9. 'Grade Creek Conglomerate" 29 10.Latite Flow Exposed in Grade Creek 34 11.Slates and Shales of the "Brownlee Canyon Series" 40 12.Fine-Grained Sandstone Member of the "Brownlee Canyon Series" 40 13.Columbia River Basalt at the Head of East Brownlee Creek 45 14.Irregular Surface Between Two Basalt Flows 45 15.Cross-Section of an S-Shaped Deposit of Unconsolidated Material 48 Figure Page 16.Offshoot of Hornblende Gabbro from a LargerDike Cutting the Porphyritic Granodiorite 70 17.Dike of Leucocratic Quartz Diorite Cutting the Horn-. blende Gabbro 76 18.Quartz Diorite Containing Several SmallInclusions of Hornblende Gabbro 76 19.Aplite Dikes Cutting the Granodiorite 79 20.Feeder Dike of Columbia River Basalt 79 21.Mafic Inclusion in Leucocratic Quartz Diorite Near the IXL