Geology and ore deposits of the Landusky Mining Dustrict, Phillips County, Montana Item Type text; Thesis-Reproduction (electronic); maps Authors Richardson, George Lusk, 1942- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 04/10/2021 06:58:58 Link to Item http://hdl.handle.net/10150/554511 £ t ; GEOLOGY AND ORE DEPOSITS OF THE LANDUSKY MINING DISTRICT, PHILLIPS COUNTY, MONTANA by George Lusk Richardson A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE WITH A MAJOR IN GEOLOGY In the Graduate College THE UNIVERSITY OF ARIZONA • 19 7 3 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ ment the proposed use of the material is in the interests of scholar­ ship. In all other instances, however, permission must be obtained from the author. APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: '\ JOHN M./GflLBERT / ] Date ACKNOWLEDGMENTS While I was 9,000 miles away in South Africa, Dr. J„ M. Guilbert reminded me that I had not fulfilled an obligation to myself until I had completed this thesis. For that reminder alone I acknow­ ledge a deep indebtedness. Dr. Guilbert's encouragement and direction as thesis advisor are also sincerely appreciated. Appreciation is ex­ tended to Drs. J. M. Guilbert, D. E. Livingston, W. J. McLean, Mr. J. E. Kinnison, and my father. Dr. J. K. Richardson, who read the manuscript and offered many helpful suggestions. Particular thanks are due Dr. J. J. Durek, Dr. T. F. O'Neill, and Kaiser Exploration and Mining Company for allowing me to expand the Landusky project into a thesis. I also wish to express thanks to Mr. R. E. Legg, president of Niseka Mining Limited of Vancouver, B. C ., who allowed the release of certain material for use in this thesis, and to Edward Wieglanda of Oracle, Arizona, formerly of Lewistown, Montana, who provided samples and information about the Landusky d istric t. An indebtedness is acknowledged to the patient encouragement of my wife, mother, and father. TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS ................... vi LIST OF TABLES ................................ v iii ABSTRACT ......................... ix INTRODUCTION ....................... 1 Methods of Study ................... 3 Topography and Accessibility .............. 4 History and Production ................. 5 Previous Work ..................... 10 REGIONAL GEOLOGIC SETTING. ............... 12 GEOLOGY OF THE LANDUSKY DISTRICT ................................ 15 Rocks ........................ 15 Precambrian Rocks ................. 15 Paleozoic Rocks .................. 16 Flathead Sandstone. .............. 16 Emerson Formation ............... 16 Bighorn Dolomite. ............... 17 Tertiary Rocks . ................. 17 Mission Peak Porphyritic Syenite. ........ 18 Platy Syenite Porphyry ............. 19 Syenite Porphyry ................................ 20 Big Eye Porphyry ................ 22 Possum Porphyry ....................... 22 Structure ....................... 26 VEIN MINERALIZATION OF THE LANDUSKY DISTRICT........................... 29 August-Niseka Vein .................. 29 Mineralogy of the August-Niseka Vein . ....................... 30 New Discovery Zone ................... 30 Mineralogy of the New Discovery Z one ................................. 31 Gold Bug Vein ................... 34 M ineralogy of the Gold Bug Vein ........... 36 Wall-rock Alteration . 36 Paragenesis ........................... 40 SUPERGENE ENRICHMENT . ................. 43 iv V TABLE OF CONTENTS—Continued Page GEOTHERMOMETRY OF THE LANDUSKY DISTRICT . ....... 46 GEOLOGIC HISTORY OF THE LANDUSKY MINING DISTRICT .... 48 A COMPARISON OF THE LANDUSKY, CRIPPLE CREEK, AND BOULDER COUNTY TELLURIDE DISTRICTS .......... 52 ECONOMIC EVALUATION OF THE NEW DISCOVERY ZONE. .... 58 SUMMARY. ........................ 60 REFERENCES. 63 LIST OF ILLUSTRATIONS Figure Page 1 „ Location Map of the Little Rocky Mountains , Phillips County, Montana 2 1 2 „ Geologic Map of. Little Rocky Mountains and Encircling Foothills, Montana ........ in pocket 3. Reconnaissance Geological Map of the Landusky Mining District, , Phillips County, Montana .......... in pocket 4. Geologic Cross Sections, Landusky Mining District, Phillips County, Montana ...... in pocket 5. Geology of the Underground Workings, August—Little Ben Zone, Landusky Mining District, Phillips County, Montana ...... in pocket 6. Geology and Assay Plan of the Niseka Crosscut, Landusky Mining District, Phillips County, Montana .......... in pocket 7. Block Diagram of the New Discovery Zone from Sections B-B' and C -C , Landusky Mining District, Phillips County, Montana ...... in pocket 8 .■ Assay Comparison at the Elevation of the Niseka Tunnel, Landusky Mining District, Phillips County, Montana .......... in pocket 9o Section along D.D.H.-5 Bearing S54E at -45°, Section Looking NE, Landusky Mining District, Phillips County, Montana ...... in pocket 10. Topography of the Landusky Mining District ...... 6 11. Core Specimen and Photomicrograph of Thin Section of Syenite Porphyry (subtype Tsp-1). .... 21 12. Hand Specimen and Photomicrographs of Thin Section of Big Eye Porphyry .................................... 23 13. Photomicrographs of Thin Section of Possum Porphyry ................ 25 vi v ii LIST OF ILLUSTRATIONS— Continued Figure Page 14. Photomicrographs of Polished Section of Vein Filling from the Niseka Crosscut ....... 33 LIST OF TABLES Table Page 1 „ Summary of the Mineralogy of the Landusky Telluride District ............................................... 37 2. Comparison of Characteristics of Cripple Creek, Landusky, and Boulder County Telluride D istricts ..................... 56 I v iii ABSTRACT The Landusky mining district, Phillips County, Montana, is an area containing a series of epithermal gold-silver telluride veins in a syenite and quartz monzonite laccolith of Tertiary age „ The laccolith has domed the overlying Paleozoic and Mesozoic sedimentary strata. Erosion has exposed the alkalic igneous cores of the domes which make up the laccolith„ Vein deposits of the Landusky district occur in min­ eralized reverse faults and shears spatially related to late-stage phono- lite dikes „ All vein deposits of economic importance lie wholly within igneous rocks of the laccolith. Hypogene ore minerals present in the vein deposits of the Lan­ dusky district include: sylvanite, hessite, empressite, native gold, freibergite, acanthite, and minor amounts of galena and sphalerite. The gangue vein minerals present are botryoidal and vein pyrite, calcite, alpha quartz, and earthy to banded purple fluorite. Wall-rock alteration immediately associated with the veins consists of quartz + sericite + kaolinite „ Based on the physical, chemical, and mineralogical character­ istics of the vein deposits and the spatially associated phonolite dikes, it is concluded that the laccolith and its related domes are intrusive, the veins epithermal„ The alkalic rocks and associated telluride min­ eralization of the Little Rocky Mountains are an example of several small ranges in central Montana showing similar Tertiary igneous rock associ­ ations and, in one other range, telluride mineralization. ix INTRODUCTION The Landusky mining district is located in the Little Rocky Mountains in Phillips County, north-central Montana (Fig. 1). It is one mile north of the small settlement of Landusky (Fig. 2, in pocket), 190 road miles northeast of Great Falls, and 160 miles north of Billings, the two largest cities in Montana. The primary purpose of this study is to preserve and consolidate past geologic data on the Landusky mining district and to augment it with the extensive new geologic data‘gathered during 1970 and 1971. From this assembled material a picture emerges of the geologic, geochemical and genetic relationships of the gold and silver deposits in syenite in­ trusions of the Landusky district. Of prime interest is that the Landusky mining district contains classic early Tertiary vein deposits related to subvolcanic mineralization. The relative obscurity of the district as a cited example of telluride vein mineralization is due to a lack of detailed study. This thesis by presenting a better description of the district should help to establish it as typical locality for telluride deposits. Also of interest is the apparent analogy between disseminated porphyry copper deposits and the silver mineralization of the Landusky district. The study develops a coherent picture of epithermal vein min­ eralization in the Landusky district and the relationship of the telluride mineralization to alkalic igneous rocks of the Little Rocky Mountains laccolith. The